COMPOUNDS RELATED TO OR DERIVED FROM GFR alpha 4 AND THEIR USE

Abstract: The present invention discloses purified and isolated nucleic acid sequences encoding polypeptides having a structure substantially similar to that of splicing forms of mammalian GFR alpha 4 comprising the amino acid sequence (SEQ ID NO:1:) - (SEQ ID NO:6:). The preferred sequences comprise cDNAs having the sequence (SEQ ID NO:7:) - (SEQ ID NO :13:). The present invention is also related to purified and isolated polypeptides comprising the amino acid sequence and or substantially similar splicing forms of mammalian GFRà4. Furthermore, the invention is related to substances capable of specifically recognizing said polypeptides and including both antibodies and receptors. The active compounds of the present invention including cDNAS, polypeptides, binding substances, and antibodies. The invention is useful not only for producing cell-lines and/or transgenic non-human animal but also for diagnosing and treating neuronal disorders or endocrine tumours as well as other related diseases by enabling measurements of GFR alpha 4-mediated signalling in endocrine cells and neurons.

Description: COMPOUNDS RELATED TO OR DERIVED FROM GFRx4 AND THEIR USE The Technical Field of the Invention The present invention discloses compounds related to, derived from or interacting with mammalian GFRcϋ4 , a glycosyl-phosphatidylinositol (GPI) -linked GDNF family α-receptor. The invention relates to the mammalian protein as such or derivatives (homologs) thereof, nucleic acid sequences encoding said protein or derivatives thereof as well as modulators or specific binding substances of said GFRα4 or GFRo;4 -related substances. The compounds are useful especially for diagnosing and treating neuronal or neurological disorders, endocrine cell related diseases as well as tumours by recording GFRc4-mediated signalling on said cells. The GFRo;4 or GFRα4-like molecules or compounds interacting with said compounds are also useful for screening mimetics potentially useful as drugs. The Background of the Invention The GDNF family of neurotrophic factors includes four members : . glial cell line-derived neurotrophic factor (GDNF) , neurturin, arte in, and persephin (PSPN) . GDNF family ligands signal through receptors consisting of a GPI-linked GFRα; subunit and the transmembrane receptor tyrosine kinase RET. In order to activate the transmembrane receptor tyrosine kinase Ret, each of the GDNF family neurotrophic factors binds preferentially to one of the glycosyl-phosphatidylinositol (GPI) -linked GDNF family α-receptors (GFRαl-4) (Airaksinen et al . , 1999). GFRα;4, the preferential receptor for persephin (PSPN), has so far been described only from chicken (Enokido et al . , 1998; Thompson et al . , 1998). GDNF signals via GFRαl, neurturin via GFRα2 , artemin via GFRα3 , whereas the mammalian GFRα receptor for persephin (PSPN) and the biological role of GFRα4 has so far remained unclear. RET-mutations are known to be related with inherited cancer syndrome (MEN2) , characterized by medullary thyroid carcinoma (MTC) and with sporadic forms of MTC and pheochromocytoma, but it is not clear why different specific RET mutations associate with the different disease phenotypes, for example, why hyperparathyroidism and familial MTC associate with certain types of mutations whereas sporadic MTC is associated with other types of mutations. For example Ret is expressed in malignant thyroid C-cells and adrenal chromaffin cells but also in normal cells non-affected in MEN2, leaving the cause for the cell specificity of the MEN2 cancer syndrome unclear. Studies of expression and splicing patterns with mouse and human GFRα4 , surprisingly provided a response to the previous unclarities . The Summary of the Invention The novel cDNA sequences encoding GFRα4 and corresponding to the mammalian splicing forms of mRNA enable the manufacture of new useful tools for diagnosis and/or treatment of certain endocrine tumours and other diseases related to endocrine cells and/or neurons in mammals, especially humans as well as for identifying and producing new products mimicking the compounds of the present invention. The present invention is related to an isolated and purified nucleic acid sequence, which is a cDNA encoding a splicing isoform of mammalian GFRc having the amino acid sequence (SEQ ID NO:l:), (SEQ ID NO : 2 : ) , (SEQ ID NO : 3 : ) , which are mouse derived or (SEQ ID NO:4:), (SEQ ID NO : 5 : ) or (SEQ ID NO: 6:), which are human derived. More specifically the cDNA sequences encoding human GFRα4 molecules are characterized by comprising the nucleic acid sequence (SEQ ID NO:10:), (SEQ ID NO: 11:), (SEQ ID NO: 12:) or (SEQ ID NO: 13:). The genomic sequence (SEQ ID NO: 13:) and fragments thereof including at least one of the exons (SEQ ID NO: 29:), (SEQ ID NO: 30:), (SEQ ID NO: 31:), (SEQ ID NO: 33: ), (SEQ ID NO: 35:), (SEQ ID NO: 37:) or (SEQ ID NO:39:) or any combination thereof with or without introns or parts of the introns (SEQ ID NO: 32:), (SEQ ID NO: 34:), (SEQ ID NO: 36:) or (SEQ ID NO: 38:) still encoding a polypeptide having a functional activity substantially similar to that of human GFRαi . The present invention is also related to the use of said nucleic acid sequence according to claims 1-5 for recording GFRα4-mediated signalling in neurons or endocrine cells. The present invention is related to the use of nucleic acid sequence according to claim 1-3 for manufacturing polypeptides useful for diagnosing and/or treating tumours in parathyroid gland cells, adrenal chromaffin cells, cells of pituitary intermediate lobe, neoplasia, endocrine tumours, medullary thyroid carcinoma and pheochromocytoma or for treating neuronal or neurological disorders. The present invention is also related to purified and/or isolated polypeptides having the amino acid sequence (SEQ ID NO:l:), (SEQ ID NO : 2 : ) , (SEQ ID NO : 3 : ) , (SEQ ID NO : 4 : ) , (SEQ ID NO:5:) or (SEQ ID NO : 6 : ) or other peptides, which have a structure substantially similar to an isoform of mouse GFRo;4 or of human GFROJ4. The present invention is related to the use of said polypeptides for determining GFRα4 -mediated signalling in neurons or endocrine cells and for the use of said polypeptides for treating disorders and disease in the endocrine cells, such as thyroid calcitonin-producing C-cells, parathyroid gland cells, adrenal chromaffin cells or cells of pituitary intermediate lobe, such as medullary thyroid carcinoma including pheochromocytoma and parathyroid hyperplasia, as well as neuronal disorders. The present invention aslo include the use of said polypeptides for manufacturing a composition for diagnosing and/or treating neuronal or neuronic disorders, osteoporosis, cancer in parathyroid gland cells, adrenal chromaffin cells and cells of pituitary intermediate lobe, in neoplasia, including treatment of endocrine tumours, including medullary thyroid carcinoma, pheochromocytoma and parathyroid hyperplasia. The nucleic acid sequences and polypeptides of the present invention are also useful for recognizing substances which are mimetics of said sequences and are potentially useful as medicines for the diseases mentioned above. Naturally the present invention is also related to methods for obtaining a cell-line (clone) useful in the production of cells, cell-lines and transgenic non-human animal. The characteristics of the present invention are as defined in the claims. Deposition: The sequences reported in this paper have been deposited in the GenBank database and the accession numbers are listed below. Mus: AJ276870, AJ276871, AJ276872, AJ276514, AJ276515, AJ276516. Human: AJ291673, AJ291674, AJ291675 The Detailed Description of the Invention Abbreviations ARTN, artemin; FTA, follicular thyroid adenoma; FTC, follicular thyroid carcinoma; GDNF, glial cell line -derived neurotrophic factor; GFR, GDNF family receptor; MEN, multiple endocrine neoplasia; MTC, medullary thyroid carcinoma; NRTN, neurturin; PSPN, persephin; PTC, papillary thyroid carcinoma, SCG, superior cervical ganglion Definitions In the present invention the terms used have the meaning they generally have in the fields of biochemistry, pharmacology, recombinant DNA technology, including production of transgenic animal but some terms are used with a somewhat deviating or broader meaning than in the normal context. Accordingly, in order to avoid uncertainty caused by terms with unclear meaning some of the terms used in this specification and in the claims are defined in more detail below. As used herein, "a neurotrophic factor" refers to a protein that modulates a biological activity of a cell, particularly neurons, but also other cells, through a neurotrophic factor signalling pathway, such as the RET signalling pathway, serine/threonine kinase Akt-mediated pathway or the RAS/MEK/MAPK pathway. Examples of biological activities in the present invention include, but are not limited to persephin. As used herein, "RET signalling pathway" includes a cell, e.g., neural cell signalling pathway which involves the tyrosine kinase receptor RET. Said pathway includes the GDNF, neurturin, artemin and persephin signalling pathways, which neurotrophic factors bind specific GFRα coreceptors, GFRαl-GFRoi4, which in turn bind and activate RET. As used herein "neurotrophic factor signalling pathway" includes a ligand, e.g. persephin (PSPN) independent signalling by GFRα4/RET or GFRα4 without RET. As used herein, "a neurotrophic factor responsive cell" includes a cell which has a biological activity that can be modulated (e.g., stimulated or inhibited) by a neurotrophic factor. Examples of such functions include mobilization of intracellular molecules, which participate in a signal transduction pathway, production or secretion of molecules, alteration in the structure of a cellular component, cell proliferation, cell migration, cell differentiation, and cell, survival. Cells responsive to neurotrophic factors preferably express a neurotrophic factor receptor, e.g., a GFRc receptor, such as GFRo;4, and/or tyrosine kinase receptor, e.g., the tyrosine kinase receptor RET. Examples of neurotrophic factor responsive cells of the present invention include neurons, endocrine cells, such as thyroid C-cells and cells in adrenal medulla expressing GFRα4. Depending on the type of cell, the response elicited by neurotrophic factors is different. For example, in neuronal cells, neurotrophic factors regulate neuron survival and neuronal function. Abnormal or aberrant activity of proteins involved in specific trophic signalling pathways, can lead to a variety of disorders, for example in the endocrine system, e.g., abnormal growth (dysplasia or hyperplasia) as well as abnormal development, differentiation or function of the cells . Abnormal or aberrant activity of a GFRo!4-protein (or abnormal or aberrant nucleic acid expression of the nucleic acid encoding said protein in a neurotrophic signalling factor pathway, in the ' nervous system or the endocrine system includes the above mentioned disorders in cells expressing or missexpressing GFRα4. In addition, neurotrophic factors, as such or combined with GDNF, artemin and neurturin and the interaction with GFRα-receptors also promote proper development of a variety of cell types. For example, neurotrophic factor/receptor interactions promote development, differentiation or function of certain peripheral organs and cells of the nervous system, innervation associated with said organs, tissues or cells. Activity of GFRα4 signalling pathway in endocrine systems can lead to disorders including tumours associated with cellular development of cells of these organs as shown in the present RET invention. For example, axonal sprouting in epilepsy due to compensatory neurotrophic factor production after seizures may aggravate the disease process . Augmenting GFRα;4 type of signaling in such conditions could be useful by preventing neuronal death without producing aberrant sprouting. The term "GFRθ!4" in the present invention means mammalian GFRo;4 and/or GFRα4-like or GFRα4-derived protein molecules or polypeptides, i.e. soluble receptor analogues expressed by and being substantially homologous to GFRα4 at amino acid level. Said GFRo;4-like or GFRα4 -derived molecules defined in the claims are obtainable by isolation from natural mammalian, e.g. human or murine sources as different splicing forms. Said splicing forms lack the first Cys-rich domain characteristic of other GFRα receptors or said domain is easily spliced off during transcription or translation. The GFRα4-like molecules of the present invention can also be produced by synthetic, semisynthetic, enzymatic and other biochemical or chemical methods including recombinant DNA techniques . The amino acid sequences of the splicing isoforms or variants of mammalian GFRo!4 are listed below: (SEQ ID NO:l:) murine GPI-anchored isoform al Fig 18B; (SEQ ID NO:2:) murine putative transmembrane isoform a2 Fig 19B (SEQ ID NO: 3 ) murine secreted isoform a3/4 Fig 20B (SEQ ID NO: 4 ) human GPI-anchored isoform a Fig 21B (SEQ ID NO: 5 ) human putative GPI-anchored isoform b Fig 22B; (SEQ ID NO:6:) human putative soluble isoform c Fig 23B. Polypeptide fragments useful for identifying and isolating nucleotide sequences encoding the above amino acid sequences are (SEQ ID NO: 14:), (SEQ ID NO: 15:), (SEQ ID NO: 16:) and (SEQ ID NO: 17:) . The "GFRθ!4-like or GFRα4-derived molecules" are substantially homologous with the amino acid sequences of mouse origin (SEQ ID NO:l:), (SEQ ID N0:2:),(SEQ ID NO : 3 : ) as well as of human origin (SEQ ID NO : 4 : ) , (SEQ ID NO : 5 : ) or (SEQ ID NO : 6 : ) . They all lack the first Cys-rich domain characteristic of other GFRc receptors and chicken GFRα4. The "GFRα4-like or GFRα4-derived molecules" are characterized by comprising polypeptides such as the amino acid sequences (SEQ ID NO:_^4:), (SEQ ID NO: 15:), (SEQ ID NO: 16:) and/or (SEQ ID NO: 17:) or amino acid sequences being truncated at the N-terminal or C-terminal end of the murine or human amino acid sequences (SEQ ID NO:l:) , (SEQ ID NO : 2 : ) , (SEQ ID NO : 3 : ) , (SEQ ID NO : 4 : ) , (SEQ ID NO.-5:), (SEQ ID NO : 6 : ) , but still covering the contagious and/or overlapping regions of the mammalian sequences (SEQ ID NO : 1 : ) , (SEQ ID NO : 2 : ) , (SEQ ID NO : 3 : ) , (SEQ ID NO:4:) , (SEQ ID NO : 5 : ) , (SEQ ID NO : 6 : ) . The term "substantially homologous" at amino acid level means that the GFRα4-like protein molecules have a significant similarity or identity of at least 80%, preferably 85 %, most preferably more than 90 % with proteins encoded by human genom- ic sequence GFRα4 (SEQ ID NO: 13:) In the present invention "homology" is determined e.g. using BLAST analysis combined with database searches matching the above defined expressed sequence tags (ESTs) with known sequences in the data bases. Nucleotide and amino acid sequence comparisons' are also performed with (BestFit) , alignment (PileUp) , and phylogenetic tree (GrowTree) . Also used are as signal sequence and transmembrane hydrophobicity plots (PeptideStructure) with the SeqWeb program package (Genetics Computer Group, Inc.). The term "GFRo;4-like molecules and derivatives thereof" comprise polypeptides having the structure, properties and functions characteristic of GFRα;4-like molecules. Thus, the term "GFRo?4-like molecules and derivatives thereof" includes GFRo;4-like molecules, wherein one or more amino acid residues are replaced by another amino acid residue. Also truncated, complexed or chemically substituted, forms of said GFRc -like molecules are included in the term "GFRo!4-like molecules and derivatives thereof" . Chemically substituted forms include for example, alkylated, esterified, etherified or amidized forms preferably with a low substitution degree. Especially, useful are substitutions with small molecules, such as methyl or ethyl and a low substitution degree, because said small molecules and/or low substitution degrees, while stabilizing and modifying the GFRαi4-molecules of the present invention, are not prone to change or disturb the properties and functions of the GFRα4-like molecules. The truncated, complexed and/or substituted variants of said GFRα4-like polypeptides can be produced by synthetic or semisynthetic methods, including enzymatic and recσmbinant DNA techniques. The only other prerequisite is that the derivatives still are substantially homologous with and have the properties and/or express the functions of the domains of GFRo!4 and can be used for screening compounds mimicking the binding substances of GFRα4. The isolated GFRα4 molecules should preferably comprise amino acid sequences which are at least about 70-80% or more homologous to the murine and human amino acid sequences defined above and should be characterized by at least one but preferably several of the following activities: 1) it can interact with (e.g. bind to) a neurotrophic factor, e.g. PSPN. 2) it can interact (e.g. bind to) a tyrosine kinase receptor, e.g., the tyrosine kinase receptor RET; 3) it can modulate the activity of a tyrosine kinase receptor, e.g., the tyrosine kinase receptor RET; and 4) it can bind a neurotrophic factor and modulate a response in endocrine cells, e.g., thyroid C-cells The term "GFRα;4-like molecules" otherwise covers all possible splice variants of GFRα4 expressed by mammalian tissues possibly excluding certain rat sequences, which are the thyroid, especially thyroid C-cells, and parathyroid glands and cells in adrenal medulla and the pituitary intermediate lobe, and possibly cells of the nervous system. As a conclusion "GFRα4-like molecules" in its broadest aspect in the present invention, covers not only GFRα4-like molecules derived from nature, including their isoforms of different origin, but also synthetically, semisynthetically, enzymatically produced GFRα4-like molecules including molecules produced by recombinant DNA techniques. Said GFRc4-like molecules can be used either as separate entities or in any combinations. The term "isoform" refers to the different forms of the same protein, which originate from different sources, e.g. different mammalian species, e.g. human and murine sources in the present case. In the present invention the term, thus, includes fragments, complexes and their derivatives. For example, GFRo;4-like molecules can be generated by the cleavage of the corresponding preproprotein. Different reactions, including different enzymatic and non-enzymatic reactions, proteolytic and non-proteolytic, are also capable of creating truncated, derivatized or complexed forms of the GFRα-molecules . Preferably, all "GFRc.4-like molecules and their derivatives" should be recognizable using binding substances capable of recognizing the native mammalian, including human and murine GFRα;4 or GFRo;4-like molecules. Such binding substances are for example the neurotrophic factor PSPN and/or fragments thereof and antibodies specifically recognizing GFRα4. In the present invention the term "nucleic acid sequence" means any isolated and purified nucleic acid sequences encoding mammalian GFRoi4 and/or GFRo!4-like substances or nucleic acid sequences with substantial similarity still encoding GFRα4-like molecules having an amino acid sequence substantially homologous with the murine sequences (SEQ ID N0:1:), (SEQ ID NO : 2 : ) or (SEQ ID NO : 3 : ) and/or the human sequences (SEQ ID NO : 4 : ) , (SEQ ID NO : 5 : ) or (SEQ ID NO : 6 : ) , lacking the first Cys-rich domain characteristic of other GFRα receptors (GFRαl, GFRc2 , GFRα3 ) . Preferably the nucleic acid sequence should encode or express a polypeptide having the characteristics described above and at least one contiguous amino acid sequence (SEQ ID NO : 14 : ) , (SEQ ID NO: 15:), (SEQ ID NO: 16:) and/or (SEQ ID NO: 17:). ' The nucleic acids sequences of the present invention belong to the functionally active GFRα4 -related compounds of the present invention and they can be used as such or they can be introduced into suitable transformation or expression vectors, which in turn can be introduced into suitable host organisms to provide procaryotic, eukaryotic organisms capable of expressing altered levels of GFRα4 as well as transgenic animals, e.g. knockout mice useful as test animals. The "nucleic acid sequences" are selected from a group consisting of murine and human cDNAs listed below: (SEQ ID NO: 7 ) murine cDNA al Fig 18A (SEQ ID NO: 8 ) murine cDNA a2 Fig 19A (SEQ ID NO: 9 ) murine cDNA a3/4 Fig 20A (SEQ ID NO: 10 :) human cDNA (GPIa) Fig 21A (SEQ ID NO: 11 :) human cDNA (GPIb) Fig 22A (SEQ ID NO: 12 :) human cDNA c Fig 23A (SEQ ID NO: 13 :) human genomic Fig 24 Nucleotide fragments useful as probes or primers in hybridization or PCR-techniques and for identifying and synthetically preparing the human genomic sequence are (SEQ ID NO: 18:), (SEQ ID NO: 19:), (SEQ ID NO: 20:), (SEQ ID NO: 21:), (SEQ ID NO: 22:), (SEQ ID NO: 23:), (SEQ ID NO: 24:), (SEQ ID NO: 25:), (SEQ ID NO: 26:), (SEQ ID NO:27:),(SEQ ID NO: 28:), (SEQ ID NO: 29:), (SEQ ID NO: 30:), (SEQ ID NO:31:), (SEQ ID NO: 33:), (SEQ ID NO: 35:), (SEQ ID NO: 37:), (SEQ ID NO:39:),(SEQ ID NO:40:). The introns of the human genomic sequence include 5 introns, the first being an unsequenced about 500 bp long sequence situated between the nucleotides 97 and 98 in (SEQ ID NO: 13:), (SEQ ID NO: 41:), (SEQ ID NO: 42:), (SEQ ID NO:43:), (SEQ ID NO: 44:), (SEQ ID NO: 45:), (SEQ ID NO:46:), (SEQ ID NO:47:), (SEQ ID NO:48:), (SEQ ID NO:49:) and/or (SEQ ID NO: 50:). The second intron has the sequence (SEQ ID NO.-32:), the third intron has the sequence (SEQ ID NO: 34:), the forth intron has the sequence (SEQ ID NO: 36:) and the fifth intron has the sequence (SEQ ID NO: 38:) - It is to be noted that the splicing variants may comprise introns or parts thereof. The "nucleic acid sequences" of the present invention are not in their natural state but are isolated from their natural environment as transiently expressed mRNAs from different mammalian tissue. Thereafter, the mRNAs are purified and multiplied in vi tro in order to provide by technical means new copies, which are capable of encoding said mammalian GFRα4 or substantially homologous "GFRα4-like molecules" of the present invention. Also the genomic sequence (SEQ ID NO:13:) and its complementary sequence are reprosuced by copying the original strand or overlapping fragments thereof. The isolated nucleic acid sequences of the present invention also include the human GFRα4 encoding nucleic acid sequences SEQ ID NO: 10:, SEQ ID NO: 11: or SEQ ID NO : 12 : , which are a cDNAs identified by aid of the mRNAs from cDNA libraries from human thyroid and adrenal tissues as well as the genomic sequence (SEQ ID NO: 13:) encoding and expressing a GFRα4 protein or fragments thereof, which act as a receptor of the corresponding ligand, e.g. persephin. The term "genomic sequence" means the corresponding sequence present in the nucleus of the mammalian cells and comprising introns as well as exons . In the present invention the genomic sequence is included as such and/or as separate exons or introns or combinations thereof. The splicing variants for example may contain exons and part of introns as shown in the examples . In the present context the term "cDNA" means a DNA sequence obtainable by reversed transcription of mRNA transcribed from the genomic DNA sequence . In the present invention the term cDNA also include the complementary strand of the original cDNA. Thus, nucleotide sequences hybridizing with the cDNA include both sequences substantially identical with the cDNA made as a copy of the respective mRNA but also the complementary strands and/or antisense strands. Genomic sequences copied are also included in the term cDNA. The term "nucleic acid sequence encoding or expressing GFRα4-like molecules" means nucleic acid sequences as well as substantially homologous nucleic acid sequences, which comprise at least one contiguous nucleic acid sequence selected from a group consisting of murine sequences (SEQ ID NO:7:), (SEQ ID NO:8:) or (SEQ ID NO : 9 : ) as well as the human sequences (SEQ ID NO: 10:), (SEQ ID NO:ll:), (SEQ ID NO: 12:) or (SEQ ID NO: 13 :) Among especially preferred nucleic acid sequences the contiguous sequences (SEQ ID NO: 10:), (SEQ ID NO: 11:) or (SEQ ID NO: 12:) deduced from human cDNA can be mentioned. This sequence or their complementary sequences or nucleic acid sequences comprising parts thereof, e.g. fragments of said nucleotide sequences truncated at the 3 ' -terminal or 5 '-terminal end, as well as sequences containing point mutations or SNPs caused by single nucleotide polymorphism are especially useful as probes for detecting nucleic acid sequences of the present invention. Specific nucleic acid sequences useful as primers and/or probes are the nucleotide sequences (SEQ ID NO: 18:), (SEQ ID NO: 19:), (SEQ ID NO: 20:), (SEQ ID NO:21:), (SEQ ID NO: 22:), (SEQ ID NO: 23:), (SEQ ID NO: 24:), (SEQ ID NO: 25:), (SEQ ID NO: 26:), (SEQ ID NO: 27:) and/or (SEQ ID NO: 28:), (SEQ ID NO: 29:), (SEQ ID NO: 30:), (SEQ ID NO: 31:), (SEQ ID NO:33:), (SEQ ID NO:35:), (SEQ ID NO:37:), (SEQ ID NO:39:), (SEQ ID NO:40:), (SEQ ID NO.41:), (SEQ ID NO:42:), (SEQ ID NO:43:), (SEQ ID NO: 44:), (SEQ ID NO: 45:), (SEQ ID NO: 46:), (SEQ ID NO: 47:), (SEQ ID NO : 48 : ) , (SEQ ID NO : 49 : ) or (SEQ ID NO : 50 : ) . It is, however, clear for those skilled in the art that other nucleic acid sequence capable of encoding GFRα4-like molecules and useful for their production can be prepared based on this information. The nucleic acid sequences encoding GFRc4-like molecules should be capable of hybridizing under stringent condition varying e.g between 58°C, 2xSSC and 65°C, 0. lxSSC (Sambrook, J. , et al . , Molecular Cloning: A Laboratory Manual., Cold Spring Harbor, NY: Cold Spring Harbor Laboratory, 1989) with sequence encoding the domain of the GFRα4 or parts thereof . The nucleic acid sequences of the present invention should have a substantial similarity with the SEQ ID NO:10:, SEQ ID NO: 11: or SEQ ID NO : 12. "Substantial similarity" in this context means that the nucleotide sequences fulfill the prerequisites defined above and have a significant similarity, i.e. a sequence identity of at least 60 %, preferably 70 %, most preferably more than 80 % with the nucleotide sequences encoding the murine amino acid sequences (SEQ ID NO : 1 : ) , (SEQ ID NO:2:) or (SEQ ID NO : 3 : ) as well as the human amino acid sequences (SEQ ID NO:4:), (SEQ ID NO : 5 : ) or (SEQ ID NO : 6 : ) . The term "nucleic acid sequences encoding human GFRα4 or GFRα4-like molecules" include their truncated or complexed forms as well as point mutations of said nucleic acid sequences as long as the GFRo!4-like molecules are capable of encoding amino acid sequences having the essential structural features as well as the properties and/or functions of GFRα4-like molecules. The functions of the GFRo;4 or GFRα4-like molecules or the compounds interacting with said compounds can be measured with methods generally used for measuring the GDNF family receptor-GFRα mediated signalling including the method disclosed in the example (s) below. The nucleic acid sequences are useful as such, for preparing transformation vectors and/or expression vectors as well as host cells carrying nucleic acid sequences capable of expressing the desired protein. They are capable of encoding/expressing GFRα4 or GFRα4-like proteins or fragments thereof. The GFRΛ4 or GFRα4-like molecules or compounds interacting with said molecules are useful for identifying or recognizing compounds mimicking said compounds, respectively. Binding substances are for example antibodies, receptors as well as shorter binding peptides, which specifically bind and recognize GFRα4 or GFRα4-like molecules or compounds interacting with said compounds. Said substances are useful especially for screening mimetics which may be useful in therapeutic or diagnostic applications. Such methods are well known to those skilled in the art . Examples of such methods it is referred to the following patent applications EP 639 584, WO 9638553, US 5,571,506, WO 200018790, WO 9950439, WO 9945930, US 6,030,619 and/or WO 9906599 which by reference are incorparated into the specification of the present invention. The GFRα4-like molecules can be identified and their amounts can be measured using "binding substances". The term "binding substance" means any substances capable of specifically recognizing and binding the GFRo;4-like molecule or derivatives thereof or at least one specific portion of said GFRc4 molecules. Such substances are, ligands of the GFRα4 receptor or other binding proteins or peptides, comprising e.g. specific portions of the GDNF family neurotrophic factor, especially persephin (PSPN) , capable of specifically binding GFRα4-like molecules and taking part in the signalling specific for GFRc4. Above all binding substances mean antibodies capable of specifically recognizing one or more GFRα4-like molecules alone or in any combination. The antibodies include both poly- clonal and/or monoclonal antibodies as well as fragments thereof. Preferably, such binding substances recognize and bind to specific epitopes or active sites of the GFRα4-like molecules. Said binding substances can be produced using specific domains of GFRoi4 or any GFRc4-like molecules, their isoforms as well as their fragments, derivatives or complexes with the prerequisite that they are capable of acting as "antigens" . The antigens include any compositions or materials capable of eliciting an antibody response specific to said GFRα;4 or GFRα>4-like molecules or compounds interacting with said molecules. Ligands of GFRα4 -receptors, in other words specific neurotrophic factors are also included in the term binding substances. Said binding substances, preferably antibodies can be produced by conventional techniques for producing polyclonal antibodies as well as monoclonal antibodies. The methods for preparing monoclonal antibodies include hybridoma techniques. Fragments of antibodies or other binding proteins such as specific binding peptides can be developed by phage display techniques and produced by recombinant DNA techniques . All methods are well known by those skilled in the art and described in laboratory handbooks. The term "screening" means that arrays of substances produced or recognized e.g. by combinatorial chemistry, with genomics or proteomics can be screened against the GFRα4 compounds of the present invention or their binding substances in order to find compounds, mimicking the properties or effects of said GFRα4 or GFRα/4-like molecules and/or their binding substances are potentially useful as drugs or as models for developing further compounds . The results can be analyzed by methods provided by efficient tools in modern bioinformatics . The term "diagnosing" means judging, predicting, assessing and/or evaluating as well as identifying and characterizing, including screening, whether a person is susceptible of or suffers from endocrine cell related diseases or disorders in neurons, e.g. neurite outgrowth leading to neuronal death or aberrant sprouting. The diagnoses also enable evaluation of the severity of the condition, therapy required as well as the efficacy of treatment modalities or medical treatments needed. Especially, early identification of the disease, e.g. neuronal or neurological disorders in order to start prophylactic and/or other treatments before the onset of the actual disease is a desirable feature, enabled by the present invention. The results are recordable with means for performing immu- noassays using GFRc4-like molecules and/or their binding substances as well as parts thereof. Means for performing amplification and hybridization methods using sequence specific probes or primers, which can be selected from the parts of the nucleic acid sequence fragments encoding suitable domains of mammalian, especially human GFR 4 . The term "screening" may also mean the screening of a population for the presence or absence e.g. of autoantibod- ies . This screening means that based on the fact that people or subjects susceptible of or suffering from complications related to a certain said disease caused by GFRc4 or GFRo!4-like molecules or compounds interacting with said compounds suffer from aberrant expression patterns which may elicit antibody production not occuring in subjects not suffering from the disorder. These so called autoantibodies can be determined using GFRc4-like molecules. It is easy to screen a large population for the susceptibility of a certain diseases from blood or serum samples obtained from inflowing serum samples taken during routine diagnostic tests. If such autoantibodies can be detected in the serum of a person, it is a clear indication that the person in question is a potential patient and might be susceptible also to endocrine cell-related diseases or neurologic disorders and should be treated appropriately. The term "immunoassay" refers to a immunochemical method or procedure capable of detecting and/or measuring at least one substance, either GFRα4 or GFRo;4-like molecules or compounds interacting with said molecules or autoantibodies against said molecules using per se known means for performing an immunoassay, which means include a substance capable of specifically recognizing the substance to be determined, i.e. either at least one binding substance or a GFRα4 or GFRoj4-like molecules or compounds interacting with said molecules or fragments thereof, for the desired application, respectively. Well known examples of immunoassays are radioimmunoassays (RIA) , radioimmunometric assays (IRMA) , fluoroimmunometric assays (IFMA) enzyme immunoassays (EIA) , enzyme-linked immu- nosorbent assays (ELISA) , fluoroimmmunoassays (FIA) , luminescence immunoassays, immunoagglutination assays, turbidimetric immunoassays, nephelometric immunoassays, etc. All methods are well known by those skilled in the art and described in laboratory handbooks . The preparation and development of methods and means for measuring different antigens and antibodies, which can be applied also to the determination of GFRα4-like molecules and their autoantibodies have been described for example in the following patent publications applicable as convenient kits US 5,591,645, US 5,712,170, US 5,602,040, US 5,622,871, US 5,656,503, EP 149 168, US 4,552,839, US 4,361,537, US 4,373,932, WO 86/04683, EP 154 749, EP 7654, WO 86/03839, EP 191 640, EP 212 599, US 4,552,839, EP 158 746, EP 225 054 and which are herewith incorporated by reference . Even if said patents are restricted to the development of test kits for diagnosing other diseases by aid of binding substance recognizing the respective active molecule, the man skilled in the art can use the information for developing corresponding test kits for measuring GFRc4-like molecules of the present invention. Any immunochemical test methods can in principle be used for diagnosing endocrine diseases as well as for longitudinal or latitudinal screening of the progress of disease and effect of medical treatment. However, visual agglutination, flow-through and immunochromatographic methods are best suited for rapid assays or tests. The term "prophylactic treatment" includes specific measures and/or precautions, e.g. control including possible medication, before the onset of disease. After the on-set of disease, prophylactic treatment requires therapeutic treatment as a precaution in order to avoid further complications. The term "therapeutic treatment" includes methods for treating persons by administrating a compositions comprising GFRoi4 or GFRα4-like molecules or compounds interacting with said compounds, including GFRc -like polypeptides or GFRα4-binding substances . Nucleic acid sequences encoding the same are useful in gene therapy or for preventing the genes causing the disease from expressing the gene products causing the diseases. The term "preparation or compositions" means the active ingredients of the present invention including the nucleic acid molecules encoding GFRα4 , GFRα4-like proteins and/or polypeptides, GFRo;4 modulators and antiGFRα4 antibodies in combination with at least one pharmaceutically acceptable carrier, which is compatible with the active ingredient, i.e. GFRo!4 or GFRc4-like molecules or compounds interacting with said compounds and the route of administration. The nucleic acid molecules of the present invention can be used in gene therapy by introducing the molecules to suitable vectors or other per se known delivery systems. The pharmaceutical compositions can be included in a container, pack or dispenser together with instructions for use. The nucleic acid molecules, proteins, protein homologues, modulators and antibodies described can be used in drug-screening assay, diagnostic assays, methods of treatment, pharmacogenomics, proteonomics, bioinformatics and/or monitoring of effects during clinical trials. The nucleic acids of the present invention can be used to provide mammalian vectors which in turn can be introduced into mammalian cells. Suitable expression systems for pro- and eukaryotic cells are described in Sambrook et al . , 1989. The host cells can be used for expressing the GFRα4-like proteins of the present invention. The DNA constructs, vector construct and/or host cells of the present invention can also be used to produce non-human transgenic animals useful in screening assays designed to identify agents or compounds, drugs, pharmaceuticals, etc., which are capable of ameliorating detrimental systems related to specific neurons and/or endocrine cells. Especially useful are knockout mice capable of expressing altered levels of GFRα4. The molecules, including nucleic acid sequences, GFRα4 or GFRθ!4-like molecules interacting with said compounds and the products developed, characterized and/or produced based on said molecules or compounds of the present invention are useful for studying the role of GFRα4-mediated signalling in neurons, in neoplasia, in endocrine cells and in thyroid calcitonin-producing C-cells (correlated with osteoporosis) , parathyroid gland cells, adrenal chromaffin cells, cells of pituitary intermediate lobe, endocrine tumours, medullary thyroid carcinoma, pheochromocytoma, parathyroid hyperplasia and association with specific mutations in the RET oncogene as well as in nervous system- and testis. The General Description of the Invention The present invention is related to GFRα4 or GFR< -like molecules or compounds interacting with said molecules which GFRα4-molecules are members of the GDNF protein family including glial-cell derived neurothrophic factor (GDNF) , neurturin, artemin and persephin signal through receptors consisting of GPI-linked GFRα subunits and the transmembrane tyrosine kinase Ret, but also other signalling pathways. The present invention is based on preliminary studies made with mouse Gfra.4 which was characterized and shown to -undergo developmentally regulated alternative splicing in several tissues . The results also indicated that mammalian GFRα4 receptor lacked the first Cys-rich domain characteristic of the other GFRo; receptors . The glial cell line-derived neurotrophic factor (GDNF) family ligands GDNF, neurturin (NRTN) , artemin (ARTN) and persephin (PSPN) are structurally related neurotrophic factors that signal through a multicomponent receptor composed of the transmembrane receptor tyrosine kinase RET and high affinity glycosyl-phosphatidylinositol (GPI) -anchored proteins, the GDNF family alpha receptors 1-4 reviewed by Airaksinen, et al., 1999 and Baloh, et al . , 2000. GFRα4 was first described from chicken and shown to be the preferential receptor for PSPN (Enokido, et al . , 1998; Thompson, et al . , 1998). The mouse GFRoi4 receptor described herein was also disclosed by Lindahl, et al . , 2000 after the filing of the priority application. It differs from all other GFRc receptors, including chicken GFRc4 by being smaller in size and lacking the first Cys-rich " domain. Mouse Gfra4 transcripts were shown to be expressed in many embryonic and adult tissues but efficient splicing leading to a functional GPI-linked isoform, as well as putative transmembrane and soluble isoforms, occured only in thyroid and adrenal medulla and in pituitary intermedi- ate lobe. In mouse, Gfra4 and Ret were co-expressed only in the thyroid C-cells and adrenal chromaffin cells. In chicken, Gfra4 mRNA was broadly expressed during embryonic development, including the spinal motoneurons and kidney (Thompson, et al . , 1998) . Chicken GFRα4 has been shown to bind mouse PSPN and to confer a survival response to PSPN in the presence of Ret (Enokido, et al . (1998). However, due to different structures of chicken and mammalian GFRo;4 , as well as the lack of information about the existence of chicken GFRc3 , ligand-specificity of mammalian GFRc4 cannot be directly extrapolated from experiments with chicken GFRα4. It is known that PSPN mRNA is expressed at low levels in many rat tissues, where two transcripts, an unspliced and a functional spliced form are produced (Milbrandt, et al . , 1998; Jaszai, et al . , 1998). PSPN is known to promote the survival of embryonic motor neurons in vi tro and to rescue nigral dopamine neurons following neurotoxic injury in vivo but not to support the survival of any peripheral neurons tested (Milbrandt, et al . , 1998). However, unlike GDNF and NRTN, PSPN does not induce neurite outgrowth of rat P8 motor axons in organotypic cultures (Ho, et al., 2000). Somatic rearrangements were known to cause chromosomal inversions which activate the oncogenic potential of RET in human papillary thyroid carcinomas, whereas germline point mutations are known to be responsible of multiple endocrine neoplasia type 2 (MEN2; reviewed in Edery, et al . , 1997; Eng, et al., (1999); Ponder, et al . , 1999). The malignancies associated with these syndromes are known to arise in several neural crest-derived cell populations. MEN2A is characterized by medullary thyroid carcinoma (MTC) , pheochromocytomas and parathyroid hyperplasia, whereas MEN2B is characterized by MTC associated with pheochromocytomas, enteric ganglioneuromas, skeletal abnormalities and mucosal neuromas. MTC is the only phenotype in familial medullary thyroid carcinoma (FMTC) . In MEN2A, cysteine substitutions in the extracellular domain of RET cause aberrant homodimerization and ligand-independent constitutive activation of RET (Santoro, et al . , 1995; Iwashita, et al . , 1996) , which alone or together with unknown factors cause the malignant proliferation of cells. FMTC mutations are more evenly distributed among the extracellular cysteines but are also found in the intracellular part of RET. It is known that most cases of MEN2B are caused by a specific mutation in the tyrosine kinase domain, which is also frequently found in sporadic MTC. MEN2B is caused by constitutive activation of RET with altered substrate specificity in downstream signalling pathway (Santoro, et al . , 1995); Iwashita, et al . , 1996) . However, in MEN2B-RET transfected cells, ligand binding has been reported to increase the intensity of RET signalling (Carlomangno, et al . , 1998; Bongarzone, et al . , 1998). It has been shown that transgenic mouse models of these mutations cause some of the malignant phenotypes found in human MEN2 syndrome (Michiels, et al . , 1997; Smith-Hicks, et al . , 2000). Phenotypic variability within the same MEN2 family and between different families carrying the same mutation in RET, suggests that further genetic events or modifier genes are required to induce the tumor phenotype in MTC. The mouse Gfra4 cDNA and genomic DNA sequence described in the present invention was characterized by having an unusual structure. The Gfra.4 pre-mRNA was alternatively spliced in tissue-specific manner that was developmentally regulated. A splice variant of Gfra.4 encoding GPI-linked isoform of GFRα4 was selectively expressed in juvenile thyroid C-cells and parathyroid gland. In contrast, newborn and adult thyroid, parathyroid, pituitary and adrenal glands produced another Gfra.4 mRNA species, which encodes a putative transmembrane-anchored isoform. i?et-deficient mice were shown to have significantly less C-cells than their wild type littermates suggesting that thyroid C-cells and possibly adrenal chromaffin cells require GFRo;4-Ret-signalling, which may specify tissues affected by the MEN2 cancer syndromes. The nucleic acid sequences of the present invention encode an isoform of mouse or human GFRc4 comprising the murine amino acid sequence (SEQ ID NO : 1 : ) , (SEQ ID NO : 2 : ) or (SEQ ID NO : 3 : ) as well as the human amino acid sequences (SEQ ID NO:4:), (SEQ ID NO:5:) or (SEQ ID NO : 6 : ) . Preferably, the sequences are cDNAs of murine origin having the sequence (SEQ ID NO : 7 : ) , (SEQ ID NO:8:) or (SEQ ID NO : 9 : ) as well as the cDNAs of human origin (SEQ ID NO: 10:), (SEQ ID NO: 11:) or (SEQ ID NO: 12:) encoding isoforms of mammalian GFRα4. Also included among the nucleic acid sequences of the present invention are the genomic sequence of human GFRα4 locus (SEQ ID NO: 13:) . Originally, a murine GFRα4 nucleic acid sequences was identified by the aid of available genomic sequences in formation of mouse mahogany gene locus (Gunn et al . , 1999, Genebank AF155960) . Within this genomic region, an "open reading frame" was identified that encoded a protein that showed sequence homology to chicken GFRα>4. Probes were generated based on portions of the genomic sequence and cDNA libraries were screened with said probes . Nucleotide sequences were determined and assembled and various methods such as RACE and genomic sequence analysis were used to extend the 5 ' sequence. Examples of the mammalian nucleotide sequences which were found and which encoded GFRcϋ4 were the mouse cDNAs (SEQ ID NO:7:), (SEQ ID NO : 8 : ) , (SEQ ID NO : 9 : ) as well as the human CDNAs (SEQ ID NO: 10:), (SEQ ID NO: 11:), (SEQ ID NO: 12:) or (SEQ ID NO: 13:) and the predicted amino acid-sequences were the mouse originated sequences (SEQ ID NO : 1 : ) , (SEQ ID NO : 2 : ) or (SEQ ID NO : 3 : ) as well as the human originated sequences (SEQ ID N0:4:), (SEQ ID NO : 5 : ) or (SEQ ID NO : 6 : ) also shown in Figures 18-24. The present inventors also isolated and characterized the human GFRo;4 receptor and showed that it is a functional co-receptor for PSPN, which mediates RET activation. Selective co-expression of GFRA4 and RET in normal and malignant C-cells and developing adrenal gland suggested that GFRA4 is a candidate modifier gene in medullary thyroid carcinoma and pheochromocytoma associated with the MEN syndromes . The present invention is based on the amino acid and/or nucleic acid sequence defined above and in the claims, especially, the GFR .4 cDNA, that encodes a specific member of the Glial Derived Neurotrophic Factor-Alpha Family of Receptors, i.e. the GFRα!4 protein, which functions in neurotrophic factor signalling pathways . The first mammalian members of the GFR 4 subfamily were identified, as described herein. Gfra4 was expressed in many mouse tissues including nervous system, where intron retention leads to a putative secreted GFRc protein. Efficient splicing was shown to occur only in thyroid, parathyroid, pituitary, and less in adrenal glands. A splice form leading to a GPI-linked GFRcϋ4 receptor was shown to be expressed in juvenile thyroid as well as in parathyroid and pituitary glands . Significant loss of thyroid C-cells in Ret-deficient mice suggested that C-cells, and cells in adrenal medulla also express Ret and may require signalling via GFRα4-i?et receptor. Finally, GFRα4 expression may restrict the inherited cancer syndrome multiple endocrine type 2, associated with mutations in RET, to these cells. These preliminary hypothesis were supported by further observations as disclosed below. The first GFRo;4 proteins isolated were expected to bind to neurotrophic factors, especially GDNF-family ligands, and mediate signals within cells expressing the GFRc4 protein. As expected, it was shown that the GFRc4 protein transmitted a signal to the interior of the cell by activation of the RET protein tyrosine kinase signalling pathway. Neurotrophic factors are known to promote survival and function of neural cells of both the central and peripheral nervous systems . Thus, modulation of the activity of a molecule involved in transmitting a neurotrophic factor signal to a cell results in modulation of the neurotrophic factor initiated cell function. Consequently, the ability of GFRα4 to modulate neurotrophic factor initiated cell functions were tested. It was shown that GFRa4 can be used to modulate neurotrophic factor action and/or activity and thereby GFRα4 is useful for treating disorders associated with such functions (or lack thereof) . In addition, GFRc4 mRNAs were ' expressed in a variety of tissues, especially, during embryogenesis, including, for example, cells of the pituitary gland and the intermediate lobe. Thus, modulators of GFRoi4 can be used to modulate development of these tissues and thereby to treat disorders associated with abnormal or aberrant development of these various tissues. The molecular mechanisms by which the mutated Ret alleles contribute to the development of neuroendocrine neoplasms in MEN-2 syndromes is largely unknown. The effects of tyrosine kinase receptors are known to be mediated by the concerted activation of several signalling pathways including those of phospholipase C-, phosphatidylinositol 3 -kinase, and the Ras/mitogen-activated protein (MAP) kinase .(also known as ERK) (Kazlauskas, et al . , 1994). In a rat pheochromocytoma cell line, PC12 , activation of the Ras signalling cascade is a prerequisite for nerve growth factor (NGF) -induced cell differentiation (D 1 Arcangelo, et al . , 1993). Although Ras is implicated in Ret-induced neuronal differentiation, little is known about the contribution of this pathway to the biological effects triggered by Ret mutants in neuroectoder al cells (Rossel et al., 1997). For instance, the expression of active Ret mutants and their causal function in neuroendocrine tumors, associated with MEN-2 syndromes, are difficult to reconcile with the dramatic differentiating effects observed when the same mutants are overexpressed either in PC12 or in human neuroblastoma cells (Borrello et al . , 1995); Califano et al . , 1995; Rossel et al . , 1997) . In PC12 cells, the Ret-induced molecular and morphological changes leading to differentiation are both mediated by Ras-dependent pathways (Califano et al . , 2000). The studies of the present inventors using the claimed GFRα?4 molecules provided useful new information to this field. It was generally believed that the serine/threonine kinase Akt-mediated pathway is responsible for most of the neurotrophin-regulated cell survival (for review see Kaplan and Miller, 2000) . The evidence for the contribution of Ras/MEK/MAPK pathway to neuronal survival is rather conflicting. While neurotrophins induce a strong and sustained activation of MAPK in sympathetic neurons and PC12 cells, most studies have found that inhibition of MEK has minimal effects on neurotrophin-dependent neuronal survival. The major role for MAPK-induced survival pathways may be to protect neurons from death due to injury or toxicity, rather than from trophic factor withdrawal (Kaplan and Miller, 2000) . Recent work of Ashcroft et al . , 1999) has shown that while PI-3 kinase can contribute in 'part to neurite initiation processes, its selective activation and subsequent signalling to downstream effectors such as Akt, functions mainly to promote cell survival in PC12 cells. The Ras/MAPK pathway, on the other hand, is mainly responsible for neurite outgrowth and differentiation (Perron and Bixby, 1999), Ashcroft et al . , 1999) . Also in this respect the results of the present invention provided new information as regards GFRα4s. The transforming ability of several viral and cellular oncoproteins depends on their capacity to activate PI3-K. a) Ret-MEN2A and MEN2B mediate enhanced PI3-K activity and high phosphorylation state of its downstream signalling molecules ; b) Ret-mediated cell-transforming effect is known to be critically dependent on the activation of PI3-K/Akt pathway (Murakami et al . , 1999; Segouffin-Cariou and Billaud 2000). In medullary thyroid carcinoma cell line TT, which carries Ret-MEN2A mutation, introduction of v-Ha-ras oncogene or activation of exogenous Raf (a kinase of MEK) leads to TT cell differentiation, a process opposite to oncogenic transformation (Nakagawa, et al . , 1987; Carson-Walter, et al . , 1998) . Also as regards to these articles the present invention provides new useful data. All members of GDNF family have shown to protect against excitotoxic motor neuron degeneration. In contrast, only GDNF and NRTN but not PSPN are able to induce neurite outgrowth in the organotypic culture of the same motoneuronal population. A specific inhibitor of MAP kinase , PD98059, inhibits the motor axon outgrowth-promoting activity of the GDNF but not the neuroprotective activity. (Ho, et al . , 2000). Also as regards neurite outgrowth new data is provided by the present invention. Because Ret in vivo is never expressed alone but always with a GFRα co-receptor (Golden, J. et al . , 1999) it has been suggested that there is a difference in the activation of Ret signalling cascades depending on Ret interaction with one of the GFRo! co-receptors. The signalling events might be different also when constitutively activated oncogenic Ret acts alone, without a co-receptor. This difference may lead to the predominance of either differentiating or surviving/ proliferating cellular response. The present invention provides information suggesting that the Ras/MAPK pathway, being responsible mostly for cells differentiation in this paradigm, may undermine the action of PI3-K/Akt cascade, which can lead to the oncogenic transformation. The absence or inhibition of Ras/MAPK pathway on top of PI3-K/Akt activation might be causative in the development of Ret-mediated MEN2 cancer syndrome, which is also shown below in the present invention. The present inventor showed that unlabelled PSPN displaces [ 125 j] PSPN from GFRA4-transfected Neuro-2a cells, which express endogenous Ret. PSPN can be specifically cross-linked to mammalian GFRo;4 and Ret, and is able to promote autophosporylation of Ret in GFRA4-transfected cells. PSPN, but not other GDNF family ligands, promotes the survival of cultured sympathetic neurons microinjected with GFRA4 and RET. Three different splice forms of human GFRA4 mRNA encoding for two GPI-linked and one putative soluble isoform identified by the present inventors were expressed exclusively in the thyroid gland. Overlapping expression of RET and GFRA4 but not other GFRA mRNAs in normal and malignant thyroid medullary cells suggests that GFRα4 may restrict the MEN2 syndrome to these cells . As already described above the present inventors in their preliminary studies identified from a putative mouse the Gfra4 gene localized near the mahogany locus (Gunn et al . , 1999) putative exons of the Gfra4 gene homologous with other Gfra receptor genes and identical to several expressed sequence tags (see the Examples) . The present inventors used RACE-cloning and RT-PCR of Gfra4 cDNA from various tissues to identify the 5' and 3' sequences and splice variants of mouse Gfra4 mRNA. All 5 ' RACE clones from the thyroid (including parathyroid) glands contained the signal sequence encoded by exon la (Fig 1A, 1C) . One 5 'RACE clone contained an alternatively spliced exon lb (Figure 1C) , which would be translated into a weak putative signal sequence . All cDNA clones isolated from brain included a 53 bp intron between exons II and III, and the 5' extended up to 100 bp into the first intron upstream of exon II (Figure 1C) . Translation of these mRNAs gave rise to a protein without a signal sequence (not shown) (Figure 18, SEQ ID NO : 1 : and SEQ ID NO : 7 : ) . The predicted amino-acid sequence of full-length mouse GFRc4 from juvenile thyroid gland consisted of 263 amino-acids with a hydrophobic signal and C-terminal sequences typical for a putative GPI-anchored protein (Udenfriend and Kodukula, 1994) . Mouse GFRα4 showed higher amino-acid identity with chicken GFRO4 (53%) than with chicken GFRαl (40%) and GFRα2 (40%) . However, the identity between mouse and chicken GFRo;4 was lower than the identity between mouse and chicken GFRoll (80%) or between the mouse and chicken GFRc2 (75%) . The alignment of mouse GFRcl, GFRα2 , GFRα3 and GFRα4 , as well as chicken GFRα4 is shown Figure 1A. The sequence of the GPI-anchored isoform of human GFR (Figures 21B and 22B, SEQ ID NO : 4 : or SEQ ID NO : 5 : ) showed 79% amino-acid identity with the mouse GFRα4 (SEQ ID NO:l:). Phylogenetic comparison indicated that the mouse GFRo;4 is more closely related to mouse GFRcl and GFRo!2 than to GFRα3 (Figure IB) . Amino-acid sequence alignment (Figure 1A) showed that mouse GFRc protein is smaller than the other GFRc-receptors. The mouse Gfra4 gene (Figure 1C) contains at least six exons but it lacks the sequences corresponding to exons 2 and 3 of the other Gfra genes (Angrist, et al . , 1998; Baloh, et al . , 1998), which encode for the first Cys-rich domain (Dl) and the hinge region in the putative GFRo domain structure (Airaksinen et al . , 1999). Thus in contrast to other GFRα-receptors including chicken GFRα4 that have three domains, mouse and human GFRc4 has only two Cys-rich domains (Figure ID) . Northern blot analysis of Gfra4 expression in different tissues showed high mRNA levels in the developing and mature nervous system, thyroid gland and testis (Figure 2A) . Moderate expression was seen in the adrenal gland and in several embryonic tissues (Figure 2A and not shown) . The size of the major Gfra4 transcripts in testis (approx. 1.2 kb) shorter than in other tissues, such as l-week-old brain (approx. 1.4kb) . Longer transcripts (approx. 2.5 kb) were also present. (Figure 2A) . Gfra4 mRNA levels were low or not detectable in developing and adult kidney, muscle, spleen, liver, and salivary gland (Figure 2A and not shown) . RT-PCR was used to analyse Gfra4 expression and to distinguish between the different splice forms in various tissues using primers spanning different exons . Gfra4 mRNA comprising exon la was prominently expressed in the thyroid, parathyroid, and pituitary glands, and less in adrenal gland (Figure. 2B,C). The expression pattern of transcripts containing exon lb was similar to those containing exon la. In testis and brain, in contrast transcripts containing exon la or lb were barely detectable or undetectable, respectively (Figure 2B,C). This suggests that other exons encoding the N-terminal part of GFRc4 protein are used in brain and testis. Of all tissue examined, only thyroid, parathyroid and pituitary glands expressed major subset of Gfra4 transcripts in which the short 53 bp intron separating exons II and III was correctly spliced (Figure 2B) . Splicing of this intron occured also in the adrenal gland but not in the brain or the testis. Gfra4 transcripts with inclusion of the short 53 bp intron separating exons II and III were present in all tissues examined, including thyroid, pituitary and adrenal glands (Figures 2B and 2C) . Translation of this intron causes a frame shift and premature stop codon producing a truncated protein with domain-2 (D2, Figure ID) followed by a 53 amino-acid hydrophilic tail (SEQ ID NO.-15:). This suggests that, regardless of possible signal sequence, testis and brain tissues examined would produce little, if any, membrane anchored GFRα4. These results were confirmed by RNAse protection assay using a probe that spans from exon II to exon IV (see Experimental Methods, data not shown) . Translation of the transcripts that contain exon la and the intron (a3 and a4 in Figure 2C) would lead to production of a putative truncated isoform of GFRc4 with 190 amino acids and strong signal sequence (SEQ ID NO:3:). In contrast, possible translation products of Gfra4 transcripts that contain exon lb and the intron (b3 and b4 in Figure 2C) might be poorly secreted. Another alternative splicing event, which was ' shown to be developmentally regulated, involved the last exon (exon IV) . A splice form that encodes the putative GPI-linked receptor was selectively expressed in the thyroid and parathyroid gland of juvenile 3 -week-old animals (Figure 2B, transcripts al and bl in Figure 2C.) Surprisingly, in newborn and 6-week-old mice, the thyroid and parathyroid glands preferably produced a longer splice form of Gfra4 mRNA with an alternative usage of splice acceptor site in exon IV (Figure 2B, transcript a2 and b2 in Figure 2C) . This exon IVb would translate into a putative transmembrane (TM) isoform of GFRc4 (SEQ ID NO : 2 : ) with and over 20 amino-acid hydrophobic strech followed by a nine amino acid hydrophobic tail (SEQ ID NO: 16:) instead of a putative GPI -anchor signal sequence (SEQ ID NO: 17:) encoded by exon Via. This TM splice variant was also expressed in adrenal and pituitary glands (Figure 2B) . In conclusion, the splicing of Gfra4 gene is both tissue-specific (with splicing of the short intron between exons II and III, and use of the signal sequence exon la) and also developmentally regulated (with alternative usage of different 5' splice sites of exon VI) . Splice site sequences of the short 53 bp intron are weak (Figure 3) with sub-optimal 3' polypyrimidine tract of the intron between exons V and Via is also non-canonical. Weak splice sites are typically found at alternatively spliced sites, for example, in the calcitonin/CGRP gene that produces calcitonin in thyroid C-cells and CGRP in neuronal tissue. According to one model, additional RNA processing factors that help to recognize weak splicing sequences in calcitonin/GCRP gene are present in the C-cells (Lou and Gagel, 1998) . It is possible that the tissue-specific and developmentally regulated splicing of the Gfra4 gene uses these same factors. However, although highly specific alternative splicing factors cannot be ruled out, the evidence found suggested the developmental and tissue specificity may arise from different ratios of general antagonistic and cooperating splicing factors as discussed by Lopez, 1998; Grabowski, 1998. In si tu hybridization was used to study the distribution of Gfra4 and Ret mRNAs in mouse tissues (summarized in Table 1) . Gfra was highly expressed in the intermediate lobe of developing (E14, E16 and P0) and mature pituitary gland (Figure 4A and Table 1) , while Ret was expressed in the neurohypophysis and not in the intermediate lobe (Figure 4B) . Moderate levels of Gfra4 transcripts were found in several tissues at different embryonic and postnatal stages examined such as the condensing mesenchyme of developing bones (Figure 4C) and developing nervous system (Table 1) . In adult spinal cord, Gfra4 mRNA was present in most principal neurons including motor neurons (Figure 4D) . In adult brain, Gfra4 was expressed in neurons, with higher levels in the cerebral cortex and hippocampus than in the brainstem and cerebellum (Figure 4E) . This expression pattern was strikingly different from that of Ret (Figure 4F) . Moderate Gfra4 expression was seen in the gut circular muscle and myenteric ganglia (Figure 4G,H) , as well as in other peripheral ganglia, including the sensory doral root and trigeminal as well as superior cervical and sympathetic chain ganglia (Table 1) . In testis, Gfra4 mRNA was expressed in seminiferous tubules (Figures 41 and 4J) . Gfra4 and Ret were co-expressed in the neural crest-derived ultimobranchial body at E12 (Figures 5A-5C) , E14, and also E16, when it starts to fuse with the thyroid diverticulum (Table 1) . In newborn mouse, highest expression of Gfra4 was detected in distinct cells in thyroid medulla where calcitonin-producing C-cells are located (Figure 5D) . Hybridization of adjacent sections with Ret probe (Figures 5E and 5F) showed a similar pattern to Gfra4 . Thus, the thyroid C-cells, which express Ret (Tsuzuki et al . , 1995; Belluardo et al . , 1999), co-express Gfra4 . In contrast, other Gfra ( Gfral -2) mRNAs were not detected in the thyroid C-cells (data not shown) consistent with previous data) (Belluardo, et al . , 1999; Golden, et al . , 1999) . In postnatal thyroid gland, Gfra4 (Figure 5G) and Ret (Figures 5H and 51) co-expression continued in the C-cells. Gfra4 mRNA levels remained abundant at least up to 17 weeks while Ret levels appeared to decrease (Table 1) . Interestingly, the parathyroid gland, which expressed little if any Gfra4 at birth (Figure 5D) , expressed highly in 3- and 6-week old mice (Figure 5G and Table 1) . However, we could not detect Ret expression in the parathyroid gland by in si tu hybridization at any developmental stage examined (Figures 5E and 51 as well as and Table 1) . Gfr ' a4 and Ret were also expressed by newborn, postnatal and adult adrenal medulla (Figures 5J-5L and Table 1) at P8 while all cells in the medulla appeared to express Gfra4 (Figure 5J) , only a subpopulation of them, presumably chromaffin cells (Nakamura, et al . , 1994; Tsuzuki, et al . , 1995), contained Ret mRNA (Figure 5L) . The tissue-specific splicing, leading to putative GFRα4 isoforms with either GPI- or transmembrane anchor, and co-expression of Gfra4 and Ret in embryonic and postnatal thyroid C-cells suggested a physiological role for GFRc in the development of these tissues . To test this possibility, we studied the number of C-cells in thyroid glands from Ret-deficient (-/-) mice. The number of calcitonin-immunoreactive cells in E18 Ret -/- mice appeared clearly reduced compared to control (wild-type or heterozygous) littermates (Figures 6A and 6B) . Counting of immunoreactive profiles from serial reactions throughout the thyroid gland indicated a significant loss of about 37% (control 2540+240, Ret -/- 1600 ± 100, n=4 littermate pairs, P<0.005 using t-test) . Thus, some thyroid C-cells seem to require Ret-signalling already during embryonic development. The selective expression of Gfra4 in these cells suggests that GFRα4 mediates the Ret-signalling in C-cell development. Other C-cells may require GFRc4/Ret-signalling postnatally, as Gfra4 and Ret are co-expressed in these cells up to adulthood. The adrenal gland has been reported to be unaffected in newborn Ret-deficient mice (Durbec, et al . , 1996). However, the expression of Gfra4 and Ret in postnatal adrenal medulla suggests that signalling via GFRα;4-Ret receptor complex may be important in postnatal development of adrenal medullary cells . Finally, the expression of Gfra4 and Ret in chromaffin and C-cells suggests that the putative ligand PSPN should have trophic or other biological effects on these cells . Efficiently spliced Gfra4 transcripts in mouse are expressed in thyroid, parathyroid and adrenal glands, that is, in tissues affected in humans with MEN2 syndromes, suggesting a causal link. GFRA4 is expressed and efficiently spliced also in human thyroid gland. In MEN2 cancer cells, deregulated RNA processing (Lou and Gagel, 1998) could lead to altered expression of Gfra4 mRNA splice forms. GFRα4 -receptors may bind and modulate specific Ret mutants and could be essential for their oncogenic activity. In the present invention the structure and expression patterns of human GFRα4 are described as well as the cloning of human GFRθ!4 cDNAs encoding three different isoforms of human GFRα4 , two GPI-linked and one putative soluble isoform, from adult human thyroid gland. The cloning and expression of various mouse GFRo;4 CDNAS encoding different protein isoforms have been described above. As in mouse, all human GFRc4 isoforms lack the first Cys-rich domain (Dl) common for all known GFRα-receptors, including chicken GFRα4 (Thompson, et al . , 1998) . Of 25 different adult and two fetal tissues studied, GFRA4 transcripts were detected only in the adult thyroid gland. However, it cannot be excluded that GFRA4 could be expressed in other tissues (such as pituitary intermediate lobe, which expresses high levels of Gfra4 in mouse) or at earlier developmental stages (such as developing adrenal medulla) not analyzed here. However, this apparently exclusive expression of human GFRA4 in thyroid suggests a more tissue-specific transcriptional regulation and possibly a more restricted role for GFRo!4 in human, than in chicken and rodents, where Gfra4 is expressed in many tissues (Thompson, et al . , 1998). For example, the lack of GFRA4 in developing and adult human nervous system does not support a physiological role for PSPN as dopamine or motor neuron survival factor as was initially suggested using rat neurons (Milbrandt, et al . , 1998) . Consistent with previous results disclosed by Nakamura, et al., 1994, Lorenzo, et al . , 1995; Tsuzuki, et al . , 1995) RET mRNA was expressed at variable levels in many adult human tissues, including the thyroid and adrenal glands. According to Milbrandt, et al . , 1998 functional spliced PSPN mRNA is not present in humans. However, the present inventors found unspliced transcript of PSPN in all human tissues examined, whereas only low levels of spliced mRNA were detected in human adrenal gland, cerebellum, spinal cord and testis. In contrast, similar levels of both PSPN transcripts are present in most tissues in rat (Milbrandt, et al . , 1998; Jaszai, et al., 1998). Although the spliced transcript encoding functional PSPN is expressed in the rat thyroid (not shown) , its expression in either adult human (Figure 10) or postnatal mouse (not shown) thyroid gland could not be detected. Adrenal gland was the only tissue where the spliced PSPN transcript was detected in all three species analyzed (mouse, rat and human) . The source of ligand for GFRcϋ4 in human thyroid remains elusive, although secretion of PSPN (from an unknown source) into the blood circulation can be speculated because PSPN triggered Ret autophosphorylation in cells was detected only after serum deprivation (Poteriaev, D., unpublished data) . It is also possible that functional PSPN is expressed in the thyroid region at earlier developmental stages not analyzed here . Masure, et al . , 2000 have reported a Gfra4 cDNA encoding a different GFROJ4 isoform. It was cloned from rat brain and reported as a GPI-linked receptor for PSPN. However, the signal sequence encoded by the rat Gfra4 cDNA in that report is very weak and differs from the GFRα>4 signal sequence disclosed in the present invention. Sequence information from the preliminary research works using mouse Gfra4 cDNA and gene was used to identify human GFRA4 cDNA and genomic clones which is the main subject of the present invention. First, the genomic region covering putative exon 2 to exon 4 was cloned by PCR from human genomic DNA. Next, the structure of GFRA4 cDNAs and gene were characterized by 5 • and 3 ' RACE of adult human thyroid cDNA and PCR of genomic DNA using primers corresponding to different regions of the putative exons of human GFRA4 as disclosed in the examples above . The sequence of human GFRA4 exon 1 encoding the signal sequence showed high homology to the mouse Gfra4 exon la. cDNAs encoding proteins with an alternative signal sequence homologous to the lb signal sequence found in mouse, or putative transmembrane and soluble isoforms found in mouse, were not identified from human thyroid. Instead, three different alternatively spliced GFRA4 cDNAs were identified (GFRA4a, GFRA4b and GFRA4C, (SEQ ID NO:10), (SEQ ID NO: 11) (SEQ ID NO: 12), Figure 9B) : (i) GFRA4a (810 bp) corresponds to the mouse Gfra4 transcript al . The predicted protein (GFRα4a, 290 amino acids, Figure 9C) contains a putative N-terminal hydrophobic signal, one N-linked glycosylation site (NVSA) at position 178, and a hydrophobic stretch of amino acids in the C-terminus, preceded by a hydrophilic linker region, consistent with a GPI-anchor signal sequence (Udenfried, et al . , 1995). The amino acid identity between mouse and human GFRc4a is 76%, whereas the identity between human and chicken GFRα4 is 54% (covering amino acids 143-340 in chicken GFRα4) . The other alternative human GFRo!4 transcripts are: (ii) GFRA4b (900 bp) , in which the small intron (79 bp) separating exons 2 and 3 (Figure 9A) is included in the transcript, and the 3' splice site of exon 4 is located 11 bp upstream of the respective splice site used in the GFRA4a transcript. The small intron between exons 2 and 3 is also inefficiently spliced in majority of mouse tissues. Inclusion of this intron in mouse Gfra4 transcripts with exon la or exon lb leads to putative soluble protein isoform. In contrast, translation of the transcript GFRA4b in human would lead to a GPI-linked protein of 299 amino acids. In this protein isoform, the N-and C-terminal ends are identical to the respective regions of GFRA4a, but the middle region consists of a stretch of 66 amino acids translated in different frame from intron 2 and exon 3. This sequence is not homologous to any protein in public databases. (iii) GFRA4C (867 bp) , in which the introns between exons 2 and 3 , and between exons 3 and 4 are included in the transcript . These introns would lead to a frameshift with a stop codon located inside exon 5 and production of a putative soluble isoform of 236 amino acids. It is interesting to note that the sequences of the 3 ' splice sites of both mouse and human exon 3 and exon 4 contain a short polypyrimidine-tract interrupted with purines, which is characteristic to alternatively spliced exons (Lou, et al . , 1994) . The present invention is also related to expression of GFRA4 , PSPN and RET in different human tissues. Expression of GFRA4 , PSPN arid RET was analyzed by RT-PCR in 25 different adult and two fetal tissues (Figure 10) . Expression of GFRA4 was only detected in the adult thyroid gland (Figure 10) using primers P2 and P3 (see Figure 9) . Similar results were obtained using different combinations of primers (data not shown) . The smallest band of 699 bp corresponds to the GFRA4a cDNA where all introns are spliced out, whereas the 789 bp band corresponds to the intron 2-containing GFRA4b transcript. The 863 bp PCR fragment corresponds to the cDNA containing both introns 2 and 3 encoding the putative soluble isoform (GFRA4c, Figure 9B) . The unspliced transcript (approx. 589 bp) of PSPN was expressed in all human tissues examined using primers detecting the full-length PSPN transcript (Figure 10, row 2) . No transcripts were observed when RNA was used as a template for PCR, which shows that the cDNA samples did not contain chromosomal DNA. Low levels of spliced PSPN transcripts (476 bp) encoding the functional protein were present in human adrenal gland, cerebellum, spinal cord and testis, and were not detected in the thyroid gland. RET mRNA was expressed at variable levels in most tissues examined (Figure 10, row3) . The present invention is related to activation of human GFRα4/Ret receptor complex, which is unique for PSPN. To study the binding of human PSPN to human GFRo!4 , we used GFRA4a-transfected mouse neuroblastoma Neuro-2a cells. (hGFRα4/Neuro-2a) , which endogenously express Ret. These cells strongly bound [^25ι] PSPN, while mock-transfected Neuro-2a cells did not (Figure 11C) . Low concentrations (Approx. 1 nM) of unlabeled PSPN effectively displaced [ 125 I] PSPN from the GFRA4-expressing cells, whereas GDNF (up to 300 nM) , NRTN (up to 200 nM) and ARTN (up to 200 mM) were ineffective (Figure 11A) . Thus, in the presence of ^ Ret human GFRα4 binds specifically PSPN, but not the other GDNF family ligands . PSPN binds to human GFRc4 with IC50 of 1 nM, and a dissociation constant (KD) of approximately 100 pM (Figure 11A) . Next we studied the binding of PSPN to GFRA4a-transfected CHO cells, which do not express Ret. Treatment of the cells with [ 125 I]PSPN, followed by chemical cross-linking and analysis by SDS-PAGE, resulted in a major band of approx. 46 kDa and minor bands of approx. 62 and 92 kDa (Figure 11B) . The approx. 46 kDa band corresponds a PSPN monomer (approx. 14 kDa) cross-linked to monomeric GFRo;4 (30 kDa) . The approximately 62 and 92 kDa bands most probably correspond to PSPN dimer cross-linked to GFRo;4 monomer and dimer, respectively. Additional 16 and 32 kDa bands represent PSPN monomer and dimer, respectively (Figure 11B, lane 6) . No specific bands were detected from mock-transfected cells (Figure 11B, lanes 1 and 2) . The amount of cell-bound complexes was greatly reduced by adding unlabeled PSPN or by removal of the GPI-anchored proteins by phosphoinositide-specific phospholipase C (PI-PLC) after cross-linking (Figure 11B, lane 4 and 5) . Combination of these two treatments further reduced the yield of cross-linked products to undetectable levels (Figure 11B, lane 3) . Thus, the human GFRα4 protein, encoded by the GFRA4a transcript indeed contains a GPI anchor. Cross-linked complexes of the same sizes were also identified from hGFRo;4/Neuro-2a cells (Figure 11C) . In addition, a minor band of about 200 kDa was observed (Figure 11C) that could be a complex of PSPN/GFRo;4 with Ret . To study the interaction of PSPN with Ret, proteins cross-linked to [ 125 I] PSPN in hGFRo;4/Neuro-2a cells were precipitated with Ret antibodies (Figure 11D, lanes 2 and 4) . The major cross-linked complexes of approximately 196 and 232 kDa, as well as minor complexes of about 62 and 92 kDa (same size as bands in Figure 11C) , and also of approx. 400 kDa were obtained under reducing conditions (Figure 11D, lane 2) . Under non-reducing conditions, the approx. 400 kDa band was greatly intensified, which indicates the presence of S-S-bound complexes. No cross-linked cell-bound products were detected using parental Neuro-2a cells (Figure 11D, lanes 1 and 3) . The bands of about 196 and 232 kDa correspond to the complexes of PSPN/Ret (186 kDa) and PSPN/GFRα4/Ret (216 kDa) , respectively. The components of the complex of about 400 kDa could be a dimer of the 186 kDa PSPN/Ret complex. To determine whether PSPN binding to human GFRo;4 mediates Ret autophosphorylation, hGFRα4/Neuro-2a cells treated with PSPN were used. Gfral expressing Neuro-2a cells treated with GDNF served as a positive control. PSPN induced Ret tyrosine autophosphorylation in hGFRα>4/Neuro-2a cells, which showed a phosphorylated band of 170 kDa, corresponding to the active form of Ret (Figure HE) . No phosphorylation of Ret was observed in vector-transfected Neuro-2a cells (Figure HE, mock) . Stimulation of Ret phosphorylation was dose-dependent starting at 0.1 ng/ml of PSPN (Figure 11F) . The present invention is related to PSPN which specifically promotes survival of neurons ectopically expressing GFRα4 and RET. In order to study whether binding of PSPN to mammalian GFRα4/Ret complex triggers functional cellular programs, as described for chick GFRα4 (Enokido, et al . , 1998). The present inventors expressed the GPI-linked mouse Gfra4 or human GFRA4a together with human RET in neonatal mouse SCG neurons and maintained the neurons further with PSPN for three days. SCG neurons are trophically dependent on NGF, do not express full-length Gfra4 (not shown) and cannot be trophically supported by PSPN (Enokido, et al . , 1998; Lindahl, et al . , 1999; Milbrandt, et al . , 1998). Significant portion of neurons expressing mouse GFRα4 and RET was maintained by PSPN (60% versus 16% without PSPN, p<0.001), whereas omission of GFRα4 abolished this trophic effect (Figure 12) . Similar results were obtained with human GFRA4a (not shown) . Also GDNF and NRTN maintained part of neurons expressing RET and Gfra4, but these effects were not reduced by omission of Gfra4 (Figure 12) . Moreover, both factors maintained also uninj ected neurons similarly to receptor-injected neurons (Figure 12) . ARTN promoted survival of neither GFRα4/RET injected nor uninj ected mouse SCG neurons (Figure 12) . Thus, GFRα4/RET complex requires PSPN, but not other GDNF family members, to activate a survival-promoting program in SCG neurons. The present invention is related to selective expression of GFRA4 and RET mRNAs solely in medullary thyroid tumours . Co-localization of Gfra4 and Ret in developing and mature mouse thyroid C-cells and their co-expression in adult human thyroid tissue (Figure 10) , prompted us to study the expression of GFRA4, RET and PSPN mRNAs in human primary thyroid tumors. RT-PCR analysis of GFRA4 expression, using primers generating full-length transcript, showed that similar levels of GFRA4 mRNAs encoding the two GPI-linked isoforms and lower levels of transcripts encoding the soluble isoform, are expressed in all eight MTC samples analyzed (Figure 13, row 1) . GFRA4 mRNA was not detected in any of the follicular thyroid adenomas (FTA) , follicular thyroid carcinomas (FTC) or papillary thyroid carcinomas (PTC) analyzed. Cellular localization of GFRA4 mRNA was studied by in si tu hybridization in the same thyroid tumor samples (Figures 14A and 14B) . GFRA4 was highly expressed by virtually all the malignant C-cells in the MTC samples analyzed, but not in the accompanying connective tissue and blood vessels. No expression of GFRA4 was detected in the adjacent apparently normal follicle cells found in some samples. Other thyroid tumors, including FTA, FTC and PTC did not express GFRA4. None of the thyroid tumors expressed detectable levels of spliced PSPN mRNA, whereas the unspliced form of PSPN transcript was present in all tumor samples analyzed (Figure 13, row 2) . High levels of RET expression was seen in all MTC tumors (Figure 13, row 3 and Figure 14) . Tumor samples were further analyzed for GFRA1, GFRA2 and GFRA3 mRNA expression by RT-PCR and in si tu hybridization (Figure 14A and Table 2) . In some tumor samples, low levels of GFRA1, GFRA2 and GFRA3 mRNAs were present in subsets of the tumor cells. To study if GFRA4 gene is mutated in the MTC tumors, full-length GFRA4 cDNA clones from eight MTC samples, the TT cell-line, and normal thyroid were sequenced. Except for a few polymorphisms in the coding region, which do not change the amino acid composition of GFRα4 , no mutations were found in the MTC samples or in the TT cell-line. Thus, the coding region of GFRA4 gene is not mutated in MTC tumors . The present invention is related to a PSPN-GFRα4-Ret complex inducing only the PI3-K/Akt pathway. To test the possible difference between GDNF-GFRαl-Ret and PSPN-GFRα4-Ret signalling mouse neuroblastoma Neuro-2a cell lines stable transfected with either GFRαl or hGFRα . Neuro-2a cells express endogenous Ret. Stimulation of Neuro-2a/GFRcl cells with GDNF lead to the activation of MAP (Figure 16A) as well as Akt kinases (Figure 15A) . Stimulation of Neuro-2a/hGFRα4 cells with PSPN also lead to the Akt activation (Figure 15B) , however, the level of MAPK phosphorylation remained unchanged (Figure 16B) . This difference between GDNF/GFRoil and PSPN/hGFRo!4 evoked responses appeared also at the physiological level . GDNF and PSPN, acting via their respective co-receptors, both were able to stimulate Neuro-2a cell survival in serum-free conditions (Figure 17A) . In another assay, GDNF but not PSPN was able to induce the neurite outgrowth from the Neuro-2a cells transfected with either GFRoϋ or hGFRα4 , respectively (Figure 17B) . These results suggest that while GDNF-GFRcl-Ret system activates both Ras/MAPK and PI3-K/Akt cascades, the PSPN-GFRα4-Ret complex can induce only the PI3-K/Akt pathway. PSPN-GFRo!4 -Ret signalling may be unable to activate the MAPK activity or it may be suppressed during this signalling. Consequently, GFRcl- and GFRα4-specific pathways lead to either mainly differentiation or survival. In the present invention it was also shown that unlabelled PSPN displaces [ 125 I] PSPN from GFRA4-transfected Neuro-2a cells, which express endogenous Ret. PSPN can be specifically cross-linked to mammalian GFRo;4 and Ret, and is able to promote autophosporylation of Ret in GFRA4 -transfected cells. Furthermore, it is shown that PSPN, but not other GDNF family ligands, promotes the survival of cultured sympathetic neurons microinj ected with GFRA4 and RET. In the present invention it has further been shown that GFRα4 is the functional receptor for PSPN. The binding and cross-linking studies demonstrated that association of PSPN with the Ret receptor protein tyrosine kinase is mediated by GFRα4. Ligand displacement binding showed that in the presence of Ret, only PSPN but not GDNF, NRTN or ARTN was effective in displacing [ 125 I]PSPN from GFRA4-expressing Neuro-2a cells. The dissociation constant (KD) of about 100 pM found here is ten times lower than the KD of about 1 nM reported for mouse PSPN binding to chicken GFRα4 (Enokido, et al . , 1998), and 60 times lower than the about 6 nM reported for rat PSPN binding to immobilized rat GFRce4 fusion protein reported by Masure, et al . , 2000, but similar to those reported for GFRαl, GFRo!2 and GFRc3 and their cognate ligands (Klein, et al . , 1997; Trupp, et al., 1998; Baloh, et al . , 2000). The lower binding of mammalian PSPN to chicken GFRα4 probably reflects the species difference in GFRα4 structure. PSPN has not yet been characterized from chicken and, if it exists, might differ significantly from the mammalian PSPN. The cross-linking studies show that PSPN is able to bind GFRα4 also in the absence of Ret. This is consistent with the model that a GDNF family ligand first binds to the corresponding GFRα receptor and subsequently the ligand-GFRo; complex binds to Ret (Jing, et al . , 1996) However, our results also agree with the alternative model in which the ligand binds a preformed GFRα/Ret complex (Sanicola, et al . , 1997). Although PSPN binding to chicken GFRα!4 has been shown earlier (Enokido, et al. , 1998) . In the present invention it is demonstrated for the first time that the association of PSPN with GFRα4 results in activation of the Ret tyrosine kinase. PSPN treatment of cells expressing GFRα4 rapidly induced Ret autophosphorylation in a dose-dependent manner. PSPN is unable to stimulate Ret autophosphorylation in cells that do not express GFRc4. Thus, the GFRo;4a isoform characterized in the present invention is a functional receptor for PSPN in triggering Ret activation. In contrast to this, the rat GFRα4 isoform with a poor signal sequence described by Masure et al . , 2000 bound PSPN only as soluble fusion protein and did not lead to Ret activation. In the present invention it has also been demonstrated that PSPN but not other GDNF family ligands, can promote the survival of cultured SCG neurons ectopically expressing Gfra4 and RET. In our assay, neurons co-expressing RET plus GFRα4 showed an elevated response to GDNF and NRTN but not to ARTN, however, also the uninjected neurons responded to GDNF and NRTN. Therefore, it seems that GDNF and NRTN maintain these neurons via activation of endogenous binding sites and not via GFRα4. Taken together, the binding of PSPN to GFRα4/Ret complex triggers functional cellular responses . In the present invention the putative role for GFRcv4 in medullary thyroid carcinoma is demonstrated. As the RET proto-oncogene plays an important role in the oncogenesis of MTC, a logical step was to examine GFRc4 expression in these tumors. GFRA4 transcripts encoding the two GPI-anchored isoforms were expressed at high level in all MTC samples, whereas no GFRA4 expression was detected in any other type of thyroid tumor analyzed. In addition, moderate levels of GFRA4c encoding the putative soluble form of GFRα4 were found in all MTC samples. Taken together, the data provided by the present inventors suggest that MTC-specific expression of GFRA4 could have diagnostic value. Strong RET expression was also localized to the malignant C-cells consistent with previous reports (Nakamura, et al . , 1994; Santoro, et al . , 1990). Low levels of GFRA1, GFRA2 and GFRA3 transcripts were present in subsets of tumor cells, indicating that their expression is not specific to particular thyroid tumor cell type. This result is in line with a recent report showing GFRA1 and GFRA2 expression in some MTC tumor cells (Frisk, et al . , 2000). However, our results showing low GFRA1 and GFRA2 expression also in normal thyroid, in other thyroid tumor types, and not in all MTC samples, does not support the idea that GFRαl or GFRα2 play a role in the primary pathogenesis of MTC. Phenotypic variability within the same MEN2 family or between different families carrying the same mutation in RET suggest that further genetic events or modifier genes are required to induce the tumor phenotype in MTC. The results of the present invention indicate that co-expression of GFRo;4 with mutated RET may be necessary for the initial hyperplasia of C-cells occurring in MTC. It should be possible to test this hypothesis by examining whether the development of C-cell hyperplasia and subsequently MTC, pheochromocytoma and ganglioneuromas are suppressed in Gfra4-deficient mice crossed with MEN2 transgenic mice (Smith-Hicks, et al . , 2000; Acton, et al . , 2000; Laurikainen, et al. , 2000) . Further studies with metastases derived from MTC tumors and other endocrine tumors such as GFRc4 expressing pheocromocytomas provide more evidence as regards these questions. Further in vi tro studies on cell lines and primary thyroid C-cells should also be informative to reveal the roles for PSPN-signalling via different GFRα4 isoforms through normal and mutant RET. Similar results were obtained from all thyroid tumours analyzed: GFRA4 was highly expressed in all tumour cells of 6 medullary carcinoma samples but not detectable in cells from other thyroid tumours including tissue samples from 4 follicular adenomas, 4 follicular carcinomas and 5 papillary carcinomas, samples. The six medullary carcinomas analyzed included both sporadic MTC, as well as tumors from patients with inherited MEN2 syndrome (carrying different mutations in RET) . As a conclusion the present invention is related to mammalian, but especially human GFRα4 and GFRα-like molecules and/or compounds interacting with said compounds, where GFRcκ4 is the ligand-binding subunit required together with RET for PSPN signalling. The invention provides isolated and purified nucleic acid sequences (e.g. cDNAs) comprising a nucleotide sequence encoding a GFRα4 protein or a fragment thereof. Also described are fragments of said nucleotide sequences, which are suitable as primers or hybridization probes for the detection of GFRo;4-encoding nucleic acid sequences (e.g. mRNA) . The isolated nucleic acid sequences encode the murine amino acid sequences (SEQ ID NO : 1 : ) , (SEQ ID NO:2:) or (SEQ ID NO:3:) and the human amino acid sequences (SEQ ID NO : 4 : ) , (SEQ ID NO:5:) or (SEQ ID NO : 6 : ) , particularly the murine nucleotide sequences (SEQ ID NO:7), (SEQ ID NO : 8 : ) or (SEQ ID NO:9:) and the human nucleotide sequences (SEQ ID NO:10:), (SEQ ID NO: 11:) , (SEQ ID NO: 12:) , (SEQ ID NO : 13 : ) . The isolated nucleic acid sequences of the present invention hybridize under stringent conditions to sequences which are at least about 60%, 65%, 70%, 75%, 80%, 85%, 90%, 98% or more homologous to the nucleic acid sequences defined in the claims . The GFRα4 protein or polypeptide fragments or a biologically active portion thereof can be operatively linked to a non-GFRα;4 polypeptide to form a fusion protein. The isolated and purified nucleic acid sequences encode proteins or fragments thereof including amino acid sequences which are sufficiently homologous to a protein comprising the murine amino acid sequences (SEQ ID NO : 1 : ) , (SEQ ID NO : 2 : ) or (SEQ ID NO:3:) as well as the human amino acid sequences (SEQ ID NO:4:), (SEQ ID NO : 5 : ) or (SEQ ID NO : 6 : ) as long as the protein or the fragment thereof maintains the specific GFRα4 activities described above. Preferably, the GFRc4-protein or fragments should maintain the ability to bind to a neurotrophic factor and modulate a cellular response in the way defined in the claims and as described above. The protein encoded by the nucleic acid sequences should be at least about 60%, 65%, 70%, 75%, 80%, 85%, 90%, 98% or more homologous to the murine amino acid sequences (SEQ ID NO:l:), (SEQ ID NO : 2 : ) or (SEQ ID NO : 3 : ) or the human amino acid sequences (SEQ ID N0:4:), (SEQ ID NO : 5 : ) or (SEQ ID NO : 6 : ) or the fragments (SEQ ID NO: 14:), (SEQ ID NO: 15:), (SEQ ID NO: 16:) or (SEQ ID NO: 17:) . The protein should preferably be a full length protein which is substantially homologous to the entire murine amino acid sequences (SEQ ID NO:l:), (SEQ ID NO : 2 : ) or (SEQ ID NO : 3 : ) or the entire human amino acid sequences (SEQ ID NO : 4 : ) , (SEQ ID NO:5:), (SEQ ID NO : 6 : ) or the fragments (SEQ ID NO: 14:), (SEQ ID NO: 15:), (SEQ ID NO: 16:) or (SEQ ID NO: 17:) and includes all naturally occurring full length proteins and allelic variants and splice variants of human or murine GFRα4. The isolated GFRα4s should preferably comprise amino acid sequences which are at least about 70-80% or more homologous to the murine and human amino acid sequences defined above and should be characterized by at least one but preferably several of the following activities as well as those described in the discussion of the experiments disclosed above: 1) it can interact with (e.g. bind to) a neurotrophic factor. 2) it can interact (e.g. bind to) a tyrosine kinase receptor, e.g., the tyrosine kinase receptor RET; 3) it can modulate the activity of a tyrosine kinase receptor, e.g., the tyrosine kinase receptor RET; and 4) it can bind a neurotrophic factor and modulate a response in endocrine cells, e.g., thyroid C-cells. The isolated nucleic acid sequence should comprise at least 15-20 nucleotides capable of hybridizing under stringent conditions to a nucleic acid sequence comprising the one of the murine nucleotide sequences (SEQ ID NO:7:), (SEQ ID NO : 8 : ) or (SEQ ID NO : 9 : ) or the human nucleotide sequences (SEQ ID NO: 10:), (SEQ ID NO:ll:), (SEQ ID NO : 12 : ) or (SEQ ID NO: 13:) or parts thereof. Preferably, the isolated and/or purified nucleic acid sequence corresponds to the naturally occurring nucleic acid sequence. More preferably, the isolated or purified nucleic acid sequences should encode naturally-occurring alleles and splice variants of human GFRc4. Moreover, the GFRα4-encoding murine cDNA sequences (SEQ ID NO : 7 : ) , (SEQ ID NO : 8 : ) or '(SEQ ID NO:9:) or the human cDNA sequences (SEQ ID NO: 10:), (SEQ ID NO: 11:) or (SEQ ID NO : 12 : ) or (SEQ ID NO: 13:) or cDNA fragments (SEQ ID NO: 18:), (SEQ ID NO: 19:), (SEQ ID NO: 20:), (SEQ ID NO: 21:), (SEQ ID NO: 22: ), (SEQ ID NO: 23:), (SEQ ID NO: 24:), (SEQ ID NO: 25:), (SEQ ID NO: 26:), (SEQ ID NO: 27:), (SEQ ID NO:28:), (SEQ ID NO: 29:), (SEQ ID NO: 30:), (SEQ ID NO: 31:), (SEQ ID NO: 33:), (SEQ ID NO: 35:), (SEQ ID NO: 37:), (SEQ ID NO: 39:), (SEQ ID NO: 40:), (SEQ ID NO: 41:), (SEQ ID NO: 42:), (SEQ ID NO: 43:), (SEQ ID NO: 44:), (SEQ ID NO: 45:), (SEQ ID NO: 46:), (SEQ ID NO: 47:), (SEQ ID NO: 48:), (SEQ ID NO: 48:) or (SEQ ID NO: 50:), as well as strands which are complementary to the coding strand of the GFRα4 cDNA sequences . The invention is also related to recombinant transformation or expression vectors or constructs as well as procaryotic or eucaryotic, including mammal host cells comprising one or more of the nucleic acid sequences defined in the claims. Such host cells may be used to produce GFRα4 proteins by culturing the host cell in a suitable medium. If desired, the GFRcϋ4 -protein can then be isolated from the host cell. The invention is also related to transgenic non-human animals in which a GFRaA gene has been introduced or altered. In such cases the genome of the non-human animal, e.g. a mouse, rat, guinea pig, etc., can be altered by introduction of a nucleic acid sequences of the invention encoding GFRo!4 as a transgene . An endogenous GFR .^. gene within the genome of the non-human animal can be altered or made functionally incompetent by homologous recombination or by using the knock-out technique. The invention is further related to isolated GFRα4 or GFRα4-like molecules or compounds interacting with said molecules, e.g. biologically active fragments thereof. The isolated GFRα4s or fragments can bind to a specific neurotrophic factor and stimulate a response in a neurotrophic factor responsive cell. The invention also provides preparations or compositions of GFRα4s which comprise amino acid sequences of the GFRo;4s defined above in combination with carriers or diluents which can be pharmaceutically acceptable or not depending upon the application. The GFRα4s of the present invention can be used to prepare anti-GFRα4 antibodies. Accordingly, the invention also provides antigenic peptides of GFRα4. These should preferably comprise at least 8 amino acid residues of the murine or human amino acid sequences defined above and in the claims . These should encompass at least one epitope of GFRα4 against which an antibody may be raised. The peptide may form a specific immune complex with GFRo?4. Preferably, the antigenic peptide comprises at least 10, more preferably 15, at least 20 or more amino acid residues providing a sufficiently high antigenicity index. Thus the present invention provides antibodies which specially bind GFRα4 or GFRα4-like molecules or compounds interacting with said molecules. The antibody is preferably monoclonal, but naturally polyclonal antibodies may be used. The antibody may be coupled to a detectable or recordable substance and it may be incorporated into a pharmaceutical composition comprising the antibody and a pharmaceutically acceptable carrier. Furthermore, the invention pertains to methods for modulating the GFRα4 mediated cell activity or signalling including cell proliferation and/or differentiation. Such methods include contacting the cell with a compound or agent capable of modulating the activity of a GFRα4 protein or the expression of GFR .4 nucleic acid sequence in such a way that a cell associated activity is measurably altered in relation to the cell associated activity in the absence of the agent. The cell, e.g. a neuron or endocrine cell should be capable or responding to a neurotrophic factor through a specific signalling pathway involving the GFRc-4 protein, as described in the examples . The agents, which modulate GFRc4 protein can be agonists, which stimulate GFRα4 activity or the expression of the GFRoι4 nucleic acid sequences or antagonists, which inhibit GFRo;4 protein activity or expression of GFRα4 nucleic acid sequence . Examples of agents which stimulate GFRα4 protein activity or GFRα4 nucleic acid expression include small molecules and nucleic acid sequences encoding GFRc4 which can be or have been introduced into the cell. Examples of agents which inhibit GFRc4 activity or expression include small molecules, antisense GFRaA nucleic acid sequences, antibodies specifically binding to GFRα4. The agent is administered to a cell, such as a neuron or an endocrine cell present in the subject . The present invention also pertains to methods for making diagnoses and/or treating subjects having disorders mediated by abnormal GFRα4 activity and/or expression. For example, the invention pertains to methods for treating a subject having a disorder characterized by aberrant GFRα4 protein activity or nucleic acid expression. Examples of such disorders are neuronal disorders, endocrine system disorders, e.g., multiple endocrine neoplasia type 2 (MEN2) syndrome, disorders associated with abnormal or aberrant cell development, differentiation and function. For example, axonal sprouting in epilepsy due to compensatory neurotrophic factor production after seizures may aggravate the disease process. Augmenting GFRα4 type of signaling in such conditions could be useful by preventing neuronal death without producing aberrant sprouting. The methods of treatment include administering to the subject GFRα4 or GFRα4-like molecules or compounds interacting (modulators or mimetics) with said molecules in such amounts that treatment of the subject occurs. The invention also pertains to methods for detecting genetic mutations in a GFRa gene, thereby determining if a subject with a mutated gene is at risk of having or is predisposed to have a disorder characterized by aberrant or abnormal GFRα4 nucleic acid expression or GFRα4 activity, such as an endocrine system disorder, rare types of MEN2-like syndromes without Ret mutations or MEN2-like syndromes with additional phenotype, e.g., pituitary tumours. Activity of GFROJ4 signalling pathway in endocrine systems can lead to disorders including tumours associated with cellular development of cells of these organs as shown in the present RET invention. For example, axonal sprouting in epilepsy due to compensatory neurotrophic factor production after seizures may aggravate the disease process. Augmenting GFRoi4 type of signaling in such conditions could be useful by preventing neuronal death without producing aberrant sprouting. The methods include detecting, in a sample of cells from the subject, the presence or absence of a genetic mutation characterized by an alteration affecting the integrity of a gene encoding a GFRα4 protein, or the misexpression of the GFRoi gene . The invention further pertains to methods for detecting the presence of GFRo;4 or fragment thereof, in a biological sample. The methods involve contacting a biological sample, e.g. any (metastatic) tumour sample, with a compound or an agent capable of detecting GFRα4 protein or GFRα4 encoding mRNA in such away that the presence or abscence of GFRα4 can be detected in the biological sample. The compound or agent can be, for example, a nucleic acid probe which is or can be labelled and is capable of hybridizing to GFROJ4 encoding mRNA or an antibody which is labelled or can be labelled with a suitable detectable marker and is capable of binding to a GFRα4 protein. The invention further provides methods for diagnosing the subject with, for example, an endocrine system disorder or neuron outgrowth disorder based on detection of GFRα4 protein or its mRNA as well as the GFRc4 mediated signalling. The method involves for example the contacting of a cell, tissue, or fluid sample (e.g., a tumour sample) from the subject with an agent capable of detecting GFRα4 protein or its mRNA and thereafter determining the amount of GFRα4 protein or mRNA expressed in the sample to a control sample and forming a diagnosis based on the amount of GFRα4 protein or mRNA expressed in the sample or its signalling as compared to the control sample. Kits for detecting GFRc4 , or fragments thereof by immunoassays or hybridization or microarray techniques from a biological sample are thus also within the scope of the invention. The present invention also relates to methods, especially, screening assays for identifying compounds or identifying disorders characterized by aberrant GFRα4 nucleic acid expression or protein activity. Activity of GFRα4 signalling pathway in endocrine systems can lead to disorders including tumours associated with cellular development of cells of these organs as shown in the present RET invention. For example, axonal sprouting in epilepsy due to compensatory neurotrophic factor production after seizures may aggravate the disease process. Augmenting GFRα4 type of signaling in such conditions could be useful by preventing neuronal death without producing aberrant sprouting. These methods typically include assaying the ability of a compound or agent to modulate the expression of the GFRoι4 gene or the activity of the GFRo;4 mediated signalling and thereby identifying compounds which are useful for treating a disorder characterized by aberrant GFRα4 nucleic acid expression or protein activity. The method involves contacting a biological sample obtained from a subject having the disorder with the compound or agent, determining the amount of GFRα4 protein expressed and/or measuring the activity of the GFRα4 protein in the biological sample, comparing the amount of GFR4 protein expressed in the biological sample and/or the measurable GFR4 biological activity in the cell to that of a control sample. An alteration in the GFRα4 expression or GFRα4 mediated signalling in a cell exposed to a compound or agent in comparison to the control is indicative of a modulation of GFRoi4 expression and/or GFRo!4 activity. The invention also pertains to methods for identifying compounds or agents which interacts with (e.g., binds to) or mimicks a compound binding to or vice versa a GFRc4 protein. These methods can include the steps of contacting the GFRc4 protein or fragments thereof or alternatively a cell expressing GFRo;4 with the compound or agent under conditions which allow binding of the compound to the GFRα!4 protein to form a complex. Thereafter, the formation of the complex comprising the GFRα4 protein and the compound having the ability of specifically binding to the GFRα4 protein is measured. The formation of the complex is an indication of the presence of a potentially useful interacting compound. Applicable methods are described in the following patents and patent applications EP 639 584, WO 9638553, US 5,571,506, WO 200018790, WO 9950439, WO 9945930, US 0,630,619 and WO 9906599. The invention further pertains to methods for identifying a compound or agent which can modulate, stimulate or inhibit the interaction of the GFRc4 protein with a target molecule, such as persephin or other neurotrophic factors and the tyrosine kinase receptor RET or other components of signalling pathways and their complexes. In these methods, the GFRα4 protein is contacted, in the presence of the compound or agent, with the target molecule under conditions which allow binding of the target molecule to the GFRα4 protein to form a complex. An alteration, e.g., an increase or decrease, in complex formation between the GFRα4 protein and the target molecule as compared to the amount of the complex formed in the absence of the compound or agent is indicative of the ability of the compound or agent to modulate the interaction of the GFRc4 protein with a target molecule. The invention is also related to substances capable of specifically recognizing the GFRα4 polypeptides defined in above and in the claims. Such substances are for example antibodies, including polyclonal, monoclonal antibodies produced by recombinant DNA techniques or hybridoma techniques as well as ligands capable of binding specifically to the mammalian GFRo;4-receptor . Fragments of antibodies and receptors are also useful as specifically binding substances . The present invention also enables the production of transgenic non-human animal lacking a functional GFRα4 receptor and which is useful as a test animal obtainable from a genetically modified compatible cell-line (clone) containing a genomic gene coding for a functional GFRoi4 receptor, which genomic gene may be functionally inactivated or conditionally inactivatable . Said transgenic non-human animal is useful as a test animal or model for studying GFRo;4 -mediated signalling in neuronal cells, endocrine cells, especially in thyroid calcitonin-producing C-cells, parathyroid gland cells, adrenal chromaffin cells, cells of pituitary intermediate lobe, nervous system and testis, neoplasia, endocrine tumours, medullary thyroid carcinoma, pheochromocytoma and parathyroid hyperplasia and for diagnosing and/or treating osteoporosis, endocrine tumours, cancer in parathyroid gland cells, adrenal chromaffin cells, cells of pituitary intermediate lobe, neoplasia, endocrine tumours, medullary thyroid carcinoma, pheochromocytoma and parathyroid hyperplasia. Activity of GFRo!4 signalling pathway in endocrine systems can lead to disorders including tumours associated with cellular development of cells of these organs as shown in the- present RET invention. For example, axonal sprouting in epilepsy due to compensatory neurotrophic factor production after seizures may aggravate the disease process. Augmenting GFRα4 type of signalling in such conditions could be useful by preventing neuronal death without producing aberrant sprouting. The present invention also provides a method for obtaining a cell-line (clone) useful in the production of the transgenic non-human animal characterized by having the genomic gene encoding a functional GFRα4 receptor inactivated by introducing into stem cells a nucleic acid sequence, which is capable of integrating to said genomic Gfra4 gene and by said integration functionally inactivating said genomic gene or making it conditionally inactivatable . The cell-line (clone) lacking a functional GFRc4 receptor is obtainable by established gene targeting methods, by: (a) providing a nucleic acid sequence defined in the claims, which sequence is capable of being integrated into the genomic gene coding for a functional GFRα4 receptor; (b) preparing a targeting construct by combining the nucleic acid sequence according to step (a) , with a selectable nucleic acid sequence acting as . a marker which is capable of functionally inactivating and by said integration functionally inactivating the genomic gene or making it conditionally inactivatable; (c) introducing the targeting construct or vector of step (b) into stem cells or cell lines (clones) of the selected animal ; (d) selecting a cell line (clone) containing the nucleic acid sequence or part thereof according to step (a) which has been integrated into the genomic gene of said stem cell or cell line (clone) using a probe, which is capable of recognizing the selectable nucleic acid sequence acting as a marker. A cell line (clone) obtained by the method described above is lacking a functional GFRα4 receptor and produces consequently altered levels of GFRα4. The cell lines (clones) of the present invention are useful not only for producing a transgenic non-human animal but also for studying the effect of different compounds, e.g. obtained by combinatorial chemistry, on GFRα4-mediated signalling in neurons, endocrine cells, especially in thyroid calcitonin-producing C-cells, parathyroid gland cells, adrenal chromaffin cells, cells of pituitary intermediate lobe, nervous system and testis, neoplasia, endocrine tumours, medullary thyroid carcinoma, pheochromocytoma and parathyroid hyperplasia and for diagnosing and/or treating osteoporosis, endocrine tumours, cancer in parathyroid gland cells, adrenal chromaffin cells, cells of pituitary intermediate lobe, neoplasia, endocrine tumours, medullary thyroid carcinoma, pheochromocytoma and parathyroid hyperplasia. The invention is described in more detail in the following examples which are intended to illustrate the invention not to limit its scope. Example 1 Sequence analysis Several putative exon sequences homologous to known Gfra genes were identified by BLAST analysis of a 47887 bp sequence containing the putative mouse Gfra4 locus (Gunn et al . , 1999) (Genebank accession number AF155960) . Database search using these sequences as query detected three mouse expressed sequence tags (ESTs) matching the 3 ' end of the putative Gfra4 gene (AA823200 or AI1553571, a spliced EST from mammary gland; AU035938, an unspliced one from the brain; and Al845209 a short one from the spinal cord) . Nucleotide and amino acid sequence comparisons (BestFit) , alignment (PileUp) , and phylogenetic tree (GrowTree) , as well as signal sequence and transmembrane hydrophobicity plots (PeptideStructure) were analyzed using the SeqWeb program package (Genetics Computer Group , Inc . ) . Example 2 RACE cloning cDNA clones representing the 5 ' -region of Gfra4 transcript were isolated from adult mouse brain, testis and thyroid gland, as well as from E17 embryo, using the 5 ' -RACE System (Life Technologies) and the GC-RICH PCR kit (Roche) according to the manufacturer's instructions. Total RNA was extracted with Trizol reagent (Life Technologies) . The anti-sense Gfra4 specific primers used in 5' RACE were: 5 ' -TTC AGC TCA GTG AGC AGT CAT CG-3' (SEQ ID NO:18:) and second nested primer 5'-CAG GTT GTC CAG GTA GTT GG-3 ' (SEQ ID NO: 19:) or 5 '-CAT CGT GCC ACG TAC TCA GA-3'(SEQ ID NO: 20:). The 3' region of the Gfra4 was cloned with 3 ' RACE method using Clontech Marathon Ready cDNA from E15 mouse embryos according to the manufacturer ' s instructions. The primers used in 3 ' -RACE were: 5 ' -TAC AAG CCT TTG ACA GCT TGC AGC-3 ' (SEQ ID NO: 21:) and a nested one 5 ' -AGA GCT GGA GGC AGA AAC AGT CC-3 ' (SEQ ID NO: 22:) . Human GFRA4 cDNAs were identified from a human thyroid cDNA (Clontech Marathon Ready cDNA) by 3 ' and 5 ' RACE method using the GC-rich PCR kit (Roche) according to the manufacturer's instructions. Reverse transcription reactions were performed using Superscript II (Life Technologies) . PCR systems were GC-RICH PCR kit (Roche) for GFRA4 and PSPN, Expand Long Template PCR system (Roche) for GFRA1, GFRA2 , GFRA3 and RET and Dynazyme II (Finnzymes) for PPIA (alias cyclophilin) . PCR was run 40 cycles for GFRA4 , 35 cycles for PSPN, GFRA1 and GFRA2, and 30 cycles for GFRA3 , RET and PPIA using annealing temperatures of 55-60 (C. Full-length human GFRA4 cDNA was obtained using Expand High Fidelity system PCR enzyme (Roche) together with GC- RICH PCR system buffers (Roche) . PCR products were cloned into mammalian expression vector pCR3.1 (Invitrogen) . Example 3 Northern Blot and Reverse Transcription (RT) -PCR Norther blot analysis of Gfra4 expression was carried out from total RNA essentially as described (Rossi et al . , 1999). Analysis of the splicing pattern of Gfra4 was performed by RT-PCR using Superscript II (Life Technologies) for RT and the GC-RICH PCR kit (Roche) for PCR. The following primers were used (see Figure 2C) : PI, 5 ' -CCA CCA TGG CCC ACT GCA TGG AGTC-3' (SEQ ID NO: 23.-); P2 , 5' -CCA CCA TGT TGA GAA GAG CA-3 ' (SEQ ID NO: 24:); P3 , 5 ' -GTG TAC AGC AGA CGA GCG GT- ' 3 ' (SEQ ID NO: 25:); P5 , 5 ' -ATA CAA GCC TTT GAC AGC TTG C-3 ' (SEQ ID NO: 26:); and reverse primers P4, 5 ' -GTT CCT TGT AAA GAG CTT GCG-3' (SEQ ID NO: 27:); and P6, 5 ' -TGG ACA AGA TGC CTA CTG ACG-3' (SEQ ID NO:28:). The amounts of total RNA was run 35-40 cycles using annealing temperatures of 55-58°C. All the PCR products of different length were sequenced to verify their identity. Example 4 RNAse Protection Analysis RNAse protection assays were performed as described (Timmusk et al., 1993) using the RPAII kit of Ambion according to manufacturer's instructions. The template for Gfra4 specific cRNA probe was a 550 bp PCR fragment generated using primers P3 and P4. It yielded a fully protected fragment corresponding to Gfra4 mRNA that have spliced both introns between exons II and IV, as well as 235 bp and 315 bp fragment corresponding to Gfra4 mRNA that is unspliced between exons II and III. Example 5 In Situ Hybridization In si tu hybridizations for Ret, Gfral and Gfra2 was performed as described Rossi et al . , 1999). A mouse full-length Gfra3 cDNA (AF051766) and a bp cDNA fragment of the 3 ' -end of the mouse Gfra4 was used as template for sense and anti-sense RNA probes. Control sections hybridized with sense probe did not show labeling above background (not shown) . In si tu hybridizations for RET, GFRA1, GFRA2 , GFRA3 and GFRA4 were performed principally in the same way as described above on cryosections of the same thyroid tumor samples analyzed by RT-PCR. A 206 bp cDNA fragment of the 5 ' -end of human GFRA4 was used as template for sense and antisense RNA probes . PCR fragments of GFRA1, GFRA2 , GFRA3 and RET generated with the same primers as in RT-PCR were cloned and sequenced and used as templates for antisense and sense RNA probes. Control sections hybridized with sense probe did not show labeling above background (not shown) . Dark-field and corresponding bright-field images of Nissl counter-stained sections were digitized and processed using Adobe PhotoShop software. Example 6 Immunohistochemistry Ret-deficient (-/-) and control littermate mice were genotyped as described (Durbec et al., 1996). Anesthetized day E18 embryos were perfused transcardially with 4% paraformaldehyde in PBS, postfixed for 2 hours and cryoprotected overnight in 25% sucrose. Horizontal cryosections of the thyroid gland, serially sectioned at 15μm, were stained and analyzed by standard immunofluorescence techniques. DAPI (Sigma) was included in embedding medium to stain the cell nuclei. The primary goat polyclonal antibody against calcitonin (sc-7784) was from Santa Cruz Biotechnology. All immunopositive profiles with a clear nucleus were counted in every section through the thyroid glands. The mean nuclear diameter of the C-cells was similar (<1% difference) between the Ret -/- and control groups (not shown) . Raw profile numbers per animal without correction are given as mean ± S.D. Example 7 RNA isolation RNA from different human tissues was purchased from Clontech (Human Total RNA Panel I-V) . Total RNA from frozen human thyroid tissue was isolated using UltraspecTM-II RNA isolation system (Biotecx Laboratories, Inc.). Total RNA from the TT cell-line (ATCC#CRL1803) was isolated with Trizol reagent (Life Technologies) . The RET mutation of five MTC patients had been analyzed: one from a MEN2A family member contained the germline C634R mutation, one patient was positive for the M918T but the germline situation was unknown, and three were sporadic MTCs of which one was positive for the M918T mutation. Example 8 RT-PCR The antisense primers used in 5 ' RACE were mouse specific Gfra4 primer 5' -CAC GTT GTC CAG GTA GTT GG-3 ' (SEQ ID NO: 41:) and a second human specific GFRA4 nested primer 5 ' -GCA CTG CGC CAC ATA CTC GGA-3' (SEQ ID NO: 42:). The 3 ' region of GFRA4 was cloned in two steps, first using a GFRA4 specific sense primer P2 (see Figure 1A) 5 ' -GCT CCG AGT ATG TGG CGC AGT-3 ' (SEQ ID NO: 43:) and a nested primer 5 ' -GCT CAC CCA CGC ACT GCT CTT CTG-3 ' (SEQ ID NO: 44:). The 3 ' region containing the stop codon and 3'UTR sequence was cloned using a sense primer 5 ' -CCT AAC TAC GTG GAC AAC GTG AGC-3 ' (SEQ ID NO-.45:) and a second nested primer 5 ' -ATG GTG CCA TTC AGG CCT TTG CCA G-3 ' (SEQ ID NO:46:) or 5'-GCA GGT GTC CTC CAC AGG CAG-3 ' (SEQ ID NO: 47:). RT-PCR of genomic human DNA was used to locate the intronic nucleotide sequences between exons 4 and 5, and between exons 5 and 6, using a sense primer 5' -CCT AAC TAC GTG GAC AAC GTG AGC-3' (SEQ ID NO:45:) and an antisense primer 5 ' -GAA GTA TGG AGA GCA GGG AGC GTC-3 ' (SEQ ID NO: 48:). Full-length human GFRA4 cDNAs were obtained using a sense primer PI (see Figure 9A) ; 5' -CCA CCA TGG TCC GCT GCC TGG-3 ' (SEQ ID NO:49:) and an antisense primer P3 ; 5 ' -GAG GTC GCT GTC CTA ATC AGA G-3' (SEQ ID NO: 50:) . Primers used in RT-PCR for human, mouse and rat PSPN were as described in (Milbrandt, et al . , 1998). Primers used in RT-PCR for human GFRA1 flanked nucleotides 491-870 of GenBank sequence AF042080, GFRA2 flanked nucleotides 148-427 of GenBank sequence U93703, GFRA3 flanked nucleotides 574-1203 of GenBank sequence NM001496 and RET flanked nucleotides 833-1114 of GenBank sequence X12949. The amounts of total RNA in samples were normalized by amplification of a PPIA fragment. Example 9 125 I-labelled PSPN Binding Murine neuroblastoma Neuro-2a cells were transfected (FuGene6, Roche) with full-length GFRA4a cDNA in pCR3.1 vector (Invitrogen) , and bulk-selected with 400 (g/ml of G418. PSPN was enzymatically iodinated by lactoperoxidase, up to a specific activity of 100.000 cpm/ng. Binding assays were performed essentially as described (Laurikainen, et al . , 2000) with 0.9 nM [ 125 I] PSPN in binding buffer (DMEM containing 0.2% BSA and 15 mM Hepes, pH 7.5) for 4 hrs on ice, either in the presence or absence of different concentrations of unlabeled human PSPN (PeproTech EC Ltd.), rat GDNF (Cephalon, Inc.), human NRTN (PeproTech EC Ltd.) or human ARTN (a gift from Drs. J. Milbrandt and E.M. Johnson, Jr.). The amount of [125j] psPN bound to mock-transfected cells was at the background level. KD was calculated using Cheng-Prusoff equation (McGonigle, et al . , 1994). Example 10 Chemical cross-linking and immunoprecipitation CHO cells were transfected (FuGene 6) with full-length human GFRA4a cDNA, grown for two days, washed and incubated with 0.9 nM [ 125 I]PSPN in binding buffer (see above) on ice for 4 hrs. After washing the cells were incubated with 1 mM Bis suberate (Pierce) in PBS at RT for 25 min. Following washes, some samples were treated with 0.5 U/ml of PI-PLC (Sigma) at +37 °C for 30 min. Cells were lysed in Laem li buffer containing β-mercaptoethanol . The cross- linking reaction for human GFRA4a and mock transfected (pcDNA3 ; Invitrogen) Neuro-2a cells was done as described above but the cells were lysed in NP-40 lysis buffer (IX Tris-borate saline, 2 mM EDTA, 1% Nonidet P-40, 1% Triton X-100, 1 mM PMSF, CompleteTM protease inhibitor cocktail, Roche), clarified by centrifugation at 14 000 rpm for 20 min., immunoprecipitated with 1 (g/ml of Ret antibodies (Santa Cruz Biotechnology) and collected with Protein A Sepharose. The immunocomplexes were washed with NP-40 buffer, and separated by 7.5% SDS-PAGE. Example 11 Ret Phosphorylation Assay Semiconfluent human GFRA4a-, rat Gfral- or vector-expressing Neuro-2a cells were maintained with 0.5% serum for 24 hr and then in serum free medium for 4 hr prior to stimulation. After stimulation with PSPN (0.01-100 ng/ml) or GDNF (100 ng/ml) , cells were lysed and precipitated with agarose-conjugated anti-phosphotyrosine antibodies (4G10, Upstate Biotechnology) overnight at +4 °C on ice. The membranes were probed with 60 ng/ml anti-Ret antibodies (Santa Cruz) . Example 12 Neuronal Microinjections Postnatal day 1-2 superior cervical ganglion (SCG) neurons were grown 5-6 days on polyornithine-laminin-coated dishes with NGF (30 ng/ml). 50:50 ng/μl cDNA mixtures of short isoform of human RET (gift from Dr. M. Billaud) and either mouse Gfra4al (in pcDNA3 , Invitrogen) or human GFRA4a (in pCR3.1, not shown) or empty pcDNA3 vector,- were pressure-microinjected into nuclei. To find successfully injected neurons later, a plasmid encoding green fluorescent protein (10 ng/μl) was included in every injection mixture. Neurons were grown overnight with NGF and thereafter in NGF-free medium containing blocking anti-NGF antibodies (Boehringer-Mannheim) and PSPN, GDNF, NRTN or ARTN, all at 100 ng/ml. Number of living fluorescent neurons or uninj ected control neurons was then counted (initial neurons) . 30-80 initial neurons were successfully injected for every treatment group. Healthy fluorescent neurons with intact nuclei and phase-bright cytoplasm were counted 70-75 h later and expressed as percent of initial neurons. Experiments were repeated on independent cultures: n=7 for PSPN, n=4 for GDNF and NRTN, n=3 for ARTN. Significance of the differences between means was estimated by one-way ANOVA followed by Tuckey's post-hoc test at the significance level of α=0.05. Example 13 Analyzing changes in phosphorylation status To analyze changes in the phosphorylation status of MAPK (ERK1 and ERK2) and Akt (PKB) in Neuro-2a cell lines stable transfected with either hGFRα4 , GFRαl or empty vector, the semiconfluent cells were kept for 18 hrs in the culture medium containing 0.5% fetal calf serum and then were starved for 3 hours in serum-free medium. After stimulation with GDNF or PSPN, the cells were briefly washed with PBS containing sodium vanadate and sodium fluoride and lysed in NP-40 lysis buffer (Tris-buffered saline; 2 mM EDTA; 1% NP-40; 1% Triton X-100; 1 mM PMSF; 1 mM Na 3 V0 4 ; 25 mM NaF; Complete™ protease inhibitors cocktail (Boehringer Mannheim) . The proteins were separated by 10% SDS-polyacrylamide gel electrophoresis and transferred to Hybond ECL (Amersham) membrane. Immunoblotting was carried out using either MAPK (ERKl/2) Anti-Active™ (Promega) or anti-Akt antibodies (New England Biolabs) recognizing phosphorylated forms of the kinases, and detected with ECL system (Amersham) . The blots were then reprobed with MAPK or Akt phosphorylation-independent antibodies. Example 14 Neurite outgrowth assay To assay neurite outgrowth, cells were plated at 0.5 10 5 cells/cm 2 and maintained in RPMI-1640 containing 1 fetal calf serum supplemented with or without 100 ng/ml either GDNF or PSPN. After 72 hours the percentage of cells with neurites (i.e. bearing processes longer than cell body diameter) was determined. To assay cell survival, cells were washed with serum-free medium, detached and plated in 24-well tissue culture dishes (at approximately 0.5 10 5 cells/cm 2 ) containing serum-free medium supplemented with either GDNF or PSPN at different concentrations. Cell viability was determined at day 3 after plating, using Trypan Blue dye exclusion method. The results of the experiments carried out during the studies are disclosed in more detail below. The Description of the Drawings Figure 1. Sequence of the predicted GPI-linked mouse GFRc4 protein (SEQ ID NO : 2 : ) and structure of mouse Gfra4 gene. Figure 1A. Alignment of mouse GFRαtl, GFRα2 , GFRα3 , and GFRc4 and chicken GFRα;4 amino-acid sequences. Predicted signal sequences are underlined. Cysteines are highlighted. The sequence was divided between the predicted hinge regions into four parts and the two cysteins in the homologous Cys-rich domains (D1-D3, see Figure ID) were aligned manually. The last two amino-acids coded by exons II and exon V are marked by asterisk. Figure IB. Phylogenetic tree of the GFRα family. Comparisons were made by using the protein encoding region of mouse Gfra 1 - Gfra 4 cDNAs . Figure IC. Structure of mouse Gfra4 gene (upper) as compared to mouse Gfra2 gene (lower) that has alternatively spliced exons 2 and 3 (Baloh et al . , 1998). The structures of mouse Gfra3 (Baloh et al . , 1998) and human GFRA1 genes (Angrist et al., 1998) are similar to the mouse Gfra2 gene, except for an additional short exon that encodes for hinge region in GFRA1 and lack of exon 8 in Gfra3 . The intron separating exons II and III is very short (53 bp) in Gfra4 as compared to this intron in other Gfra genes (>2 kb) . Exon grey shades refer to the homologous Cys-rich domains (D1-D3) and hinge regions in the domain model. N- and C-terminal signal sequences are lined. The lengths of the 5' UTR sequences of exons la and lb (broken line) were not determined. Figure ID. Proposed domain structure of mouse GFRα4 (left) and GFRα2 receptor (right) (Airaksinen et al . , 1999). Mouse GFRα<4 lacks the first domain (Dl) , which is present in other GFRα-receptors. Putative N-glycosylation sites are also indicated. (>-) Figure 2. Expressions and developmentally regulated tissue-specific splicing of Gfra4 gene. Figure 2A. Northern blot analysis of Gfra4 expression in various mouse tissues (upper) . The major transcript in testis appears smaller (appr. 1.4 kb) . Longer transcripts (appr. 1.6 kb and 2,5 kb) are also seen. Rehybridization with GAPDH probe (below) demonstrates equal loding. Figure 2B. Semiquantitative RT-PCR analysis of Gfra4 transcripts from various tissues and postnatal stages. Locations of PCR primers are indicated on the right and shown in Figure 2C. Upper panel : Transcripts containing exon la and the spliced intron between exons II and III (Ia/Δi 703 bp) are produced in adrenal, pituitary and thyroid glands but not in brain or testis. 2 n & upper panel : mRNAs containing exon lb (Ib/Δi 676 bp) show a expression pattern similar to those containing exon la. Middle panel : The short intron between exons II and III is efficiently spliced (Δi 550 bp) in thyroid and pituitary glands but not (i 603bp) in brain and testis. 2 d ι OW er panel : 3 -week-old thyroid glands express the shorter (GPI, 315 bp) splice form, whereas newborn and 6-week-old thyroid glands, preferably as well as postnatal adrenal and pituitary glands, produce the longer (TM 380 bp) splice form. Note that the testis and brain also express their TM or GPI splice forms, but protein synthesis would produce a putative soluble form due to intron inclusion. Lower panel : The lines contain similar levels of control Ppian transcript. Figure 2C. Structures of different Gfra4 mRNA splice forms containing exon la (al-a4) and exon lb (bl-b4) . Primers P1-P6 used in Figure 2B are indicated below the gene structure. Asterisks denote stop codons . The exon colours are the same as in Figures IC and ID. Transcript al (SEQ ID NO : 7 : ) is predicted to encode for the GPI-linked isoform of GFRo;4 (SEQ ID NO:7:), whereas a2 (SEQ ID NO : 8 : ) encodes for a putative transmembrane (TM) -anchored receptor (SEQ ID NO : 2 : ) . Transcripts a3 and a4 (SEQ ID NO : 9 : ) encode for truncated and secreted molecule (domain-2 only) (SEQ ID N0:3:). Transcripts bl-b4 are similar to the transcripts al-a4 except for usage of exon lb, which encodes for a putative weak N-terminal signal sequence instead of exon la. Shown in Figures 2A and 2B are representative results from one of three independent experiments. Abbreviations: ad, adult; pit, pituitary gland. Figure 3. Intron-exon junctions in the coding region of mouse Gfra4 gene. Consensus splicing sequences are indicated (upper line) . There are more non-canonical nucleotides (marked in bold) in the splicing signals of the alternatively spliced introns. Splice acceptor and donor sites are shaded. Figure 4. Localization of Gfra4 and Ret mRNAs in developing and mature mouse tissues by in si tu hybridization. Figure 4A. Gfra4 is highly expressed in the intermediate lobe (Iml) of E18 pituitary gland (sagittal section) , while moderate Ret expression is seen in the neurohypophysis (Nh) . Ah, adenohypophysis . Figure 4B. Gfra4 is highly expressed in the intermediate lobe (Iml) of E18 pituitary gland (sagittal section) , while moderate Ret expression is seen in the neurohypophysis (Nh) . Ah, adenohypophysis. Figure 4C. The condensing mesenchyme of developing limbs (arrows) shows moderate Gfra4 expression at E16. Figure 4D. Many neurons of adult spinal cord (coronal section) show moderate to high Gfra4 expression. Figure 4E. In adult brain (horizontal section) , the distribution of Gfra4 and Ret mRNAs are not overlapping. Gfra4 is highest in the hippocampus (He) . Cb, cerebellum. Figure 4F. In adult brain (horizontal section) , the distribution of Gfra4 and Ret mRNAs are not overlapping. Gfra4 is highest in the hippocampus (He) . Cb, cerebellum. Figure 4G. The circular muscle layer and the myenteric ganglia of the adult colon express Gfra4. H & E staining. Figure 4H. The circular muscle layer and the myenteric ganglia of the adult colon express Gfra4. Dark field image. Figure 41. Seminiferous tubules in adult testis express Gf a4. H & E staining. Figure 4J. Seminiferous tubules in adult testis express Gfra4. Dark field image. Figure 5. Gfra4 and Ret are co-expressed in mouse thyroid C-cells and adrenal chromaffin cells. Figure 5A. In si tu hybridization of adjacent sagittal sections of E12 mouse embryos for Gfra4 (Figure 5A) and Ret (Figure 5C) . Arrowheads point to the ultimobranchial body, drg dorsal root ganglia. Figure 5B. In si tu hybridization of adjacent sagittal sections of E12 mouse embryos for Gfra4 (Figure 5A) and Ret (Figure 5C) . Arrowheads point to the ultimobranchial body, drg dorsal root ganglia. Figure 5C. In si tu hybridization of adjacent sagittal sections of E12 mouse embryos for Gfra4 (Figure 5A) and Ret (Figure 5C) . Arrowheads point to the ultimobranchial body, drg dorsal root ganglia. Figure 5D. Adjacent sagittal sections of E18 embryos hybridized for Gfra4 (Figure 5D) and Ret (Figure 5F) . Both Gfra4 and Ret are expressed in the C-cells but not in the parathyroid gland (broken line) . Figure 5E. Adjacent sagittal sections of E18 embryos hybridized for Gfra4 (Figure 5D) and Ret (Figure 5F) . Both Gfra4 and Ret are expressed in the C-cells but not in the parathyroid gland (broken line) . Figure 5F. Adjacent sagittal sections of E18 embryos hybridized for Gfra.4 (Figure 5D) and Ret (Figure 5F) . Both Gfra4 and Ret are expressed in the C-cells but not in the parathyroid gland (broken line) . Figure 5G. In 6-week-old mice, Gfra4 is highly expressed in the thyroid parafollicular C-cells (arrow) , but also in the parathyroid gland (broken line) . In contrast, Ret expression is detectable only in the C-cells (arrow) but not in the parathyroid gland. Figure 5H. In 6-week-old mice, Gfra4 is highly expressed in the thyroid parafollicular C-cells (arrow) , but also in the parathyroid gland (broken line) . In contrast, Ret expression is detectable only in the C-cells (arrow) but not in the parathyroid gland. Figure 51. In 6-week-old mice, Gfra4 is highly expressed in the thyroid parafollicular C-cells (arrow) , but also in the parathyroid gland (broken line) . In contrast, Ret expression is detectable only in the C-cells (arrow) but not in the parathyroid gland. Figure 5 . Gfra4 is highly expressed by all cells in adrenal medulla at postnatal day P8 , while only a subpopulation of them express Ret. Figure 5K. Gfra4 is highly expressed by all cells in adrenal medulla at postnatal day P8, while only a subpopulation of them express Ret. Figure 5L. Gfra4 is highly expressed by all cells in adrenal medulla at postnatal day P8, while only a subpopulation of them express Ret. The bright-field images shown in Figures 5B, 5E, 5H, and 5K correspond to those in 5A, 5F, 51 and 5L, respectively. Figure 6. Ret-deficient mice have less thyroid calcitonin-cells . Figure 6A. E18 thyroid glands from wild-type (Figure 6A) and Ret-deficient (Figure 6B) mice are immunostained for calcitonin. Shown are representative sections containing the highest number of labeled C-cells through the middle part of the thyroid gland. The remaining C-cells in Ret-deficient mice appear morphologically normal. Figure 6B. E18 thyroid glands from wild-type (Figure 6A) and Ret-deficient (Figure 6B) mice are immunostained for calcitonin. Shown are representative sections containing the highest number of labeled C-cells through the middle part of the thyroid gland. The remaining C-cells in Ret-deficient mice appear morphologically normal. Figure 7. RT-PCR analysis of GFRA4 expression in human thyroid tumors. Each line represents analysis of a tumor from a different patient. The specific 264 bp PCR product representing GFRA4 (exon la) expression is high in all medullary carcinoma samples (3/3) but not detectable in other thyroid tumours tested including, 4 follicular adenoma, 4 follicular carcinoma and 5 papillary carcinoma samples. Figure 8. Localization of human GFRA4 mRNA expression in medullary thyroid carcinoma but not in other thyroid tumors. In si tu hybridization was performed using a human GFRA4 specific antisense cRNA probe on frozen sections. Figure 8A. GFRA4 is highly expressed by virtually all cells in the medullary thyroid carcinoma (MTC) but not in the surrounding connective tissue (dark field image) . Figure 8B. Light-field image shows the Nissl-stained structure of tissue. Figure 8C. Other thyroid tumors, including follicular adenoma (FTA) (Figures 8C and 8D) and papillary carcinoma (PTC) (Figures 8E and 8F) show no GFRa4 expression above sense control levels (not shown) . Figure 8D. Other thyroid tumors, including follicular adenoma (FTA) (Figures 8C and 8D) and papillary carcinoma (PTC) (Figures 8E and 8F) show no GFRa4 expression above sense control levels (not shown) . Figure 8E. Other thyroid tumors, including follicular adenoma (FTA) (Figures 8C and 8D) and papillary carcinoma (PTC) (Figures 8E and 8F) show no GFRa4 expression above sense control levels (not shown) . Figure 8F. Other thyroid tumors, including follicular adenoma (FTA) (Figure 8C and 8D) and papillary carcinoma (PTC) (Figures 8E and 8F) show no GFRa4 expression above sense control levels (not shown) . Figure 9. Structure of the human GFRA4 gene compared to mouse Gfra4 and different splice forms of human GFRA4 mRNA. Figure 9A. Structure of the human GFRA4 gene (top) compared to mouse Gfra4 gene (bottom) . Both contain at least 6 exons but no alternative exon lb has been found in human. Also the putative transmembrane form encoded by the alternatively spliced exon 6 found in mouse is absent in human. Primers P1-P3, used in RT-PCR in Figures 2 and 5 are marked by arrowheads . Figure 9B. Splicing of GFRA4 gene in human thyroid gland. Two GPI-linked isoforms ( GFRA4a and GFRA4b) , and one putative soluble isoform { GFRA4c) are produced by alternative splicing. In transcript encoding for GFRc4a all the introns are spliced, whereas GFRo;4b contains the intron between exon 2 and exon 3. Transcript encoding GFRα4c contain introns between exons 2 and 3 and between exons 3 and 4. Asterisk marks the stop codon. Figure 9C. Proposed schematic domain structure of human GFRα4 isoforms . Figure 10. RT-PCR analysis of GFRA4 , PSPN and RET transcripts in various human tissues. Top row: GFRA4 expression is detected only in thyroid gland using primers P2 and P3 (see Figure 15A) . All the introns in the transcript corresponding to 699 bp are spliced. The 789 bp transcript contains a small intron between exon 2 and exon 3 and corresponds to the GFRα4b form. The 863 bp fragment contains introns between exons 2 and 3 and between exons 3 and 4, and corresponds to the soluble GFRα4c form. Second row: An unspliced transcript of the human PSPN is expressed in all tissues (approximately 590 bp, intron size not determined) , whereas the spliced transcript (476 bp) , which corresponds to the functional PSPN, is weakly expressed in cerebellum, adrenal gland and testis and barely detectable in spinal cord. To exclude DNA contamination in RT-PCR reactions, controls containing RNA were also subjected to PCR (not shown) . Third row: RET expression (282 bp) is detectable in almost all human tissues examined but the expression levels are strongest in adult brain, cerebellum, spinal cord, salivary gland, adrenal gland and prostate. Bottom row: Control RT-PCR with PPIA transcript showing equal loading of cDNA. In all PCR experiments, negative water controls were included (not shown) . Figure 11. Binding and Chemical Cross-linking of [ 125 I] PSPN to Human GFRα4 and Ret . Figure HA. Displacement binding of [ 125 I]PSPN by PSPN, NRTN, GDNF and ARTN from human GFRoi4a-expressing Neuro-2a cells, shown as percentage of [ 125 I]PSPN bound in the absence of a cold ligand (100%) . One representative experiment out of four is shown. Figure 11B. Chemical cross-linking of [ 125 I] PSPN to vector- (lanes 1, 2) or hGFRA4a transfected (lanes 3-6) CHO cells, in the presence (lanes 2, 3, 5) or absence (lanes 1, 4, 6) of unlabeled PSPN, treated (lanes 3, 4) or not treated (lanes 1, 2, 5, 6) with PI-PLC. Bands are designated as follows: arrowhead, approximately 46 kD; arrow, approximately 62 kD; asterisk, approximately 92 kD . Figure 11C. Proteins cross-linked to [ 125 I] PSPN in the presence (lanes 1, 3) or absence (lanes 2, 4) of unlabeled PSPN in vector- (lanes 1, 2) or GFRA4a- (lanes 3, 4) transfected Neuro-2a cells. Bands are designated as in Figure 11B. Figure 11D. Immunoprecipitation of Ret from vector- (lanes 1, 3) or GFRA4a-transfected (lanes 2, 4) Neuro-2a cells after chemical cross-linking of [ 1 5 I] PSPN, separated in the presence (lanes 1, 2) or absence (lanes 3, 4) of (β-mercaptoethanol (β-Me) . Bands are designated as follows: arrowhead, approximately 46 kD; arrow, approximately 196 kD; diamond approximately 232 kD; asterisk, approximately 400 kD. Figure HE. Phosphorylation of Ret in mock- (lanes 1, 2) , GFRA4a~ (lanes 3, 4, 5) or Gfral-transfected Neuro-2a cells upon stimulation with 100 ng/ml of PSPN (lanes 2, 4, 5) or 100 ng/ml of GDNF (lane 7) . Figure HF. Dose-dependent phosphorylation of Ret with PSPN in hGFRA4a-transfected Neuro-2a cells. Figure 12. PSPN specifically promotes survival of RET/ Gfra4- inj ected SCG neurons. Neonatal mouse SCG neurons were grown five days with NGF, injected with indicated plasmid mixtures (all 50:50 ng/μl) , grown overnight with NGF, then NGF was changed for indicated factors (all 100 ng/ml) . Numbers of viable neurons after 3-day treatment period are expressed relative to the number of initial neurons. Means ± s.e.m of three to seven independent repeats for each factor is shown. p<0.05 (*) ; p<0.001 (***). Figure 13. Semiquantitative RT-PCR analysis of GFRA4 , PSPN and RET in different human thyroid tumors. Top row: GFRA4 mRNA is selectively expressed in medullary thyroid carcinomas (MTC) . No GFRA4 transcripts are detectable in follicular thyroid adenomas (FTA) , follicular thyroid carcinomas (FTC) or papillary thyroid carcinomas (PTC) . Each lane represents tumor cDNA from different patient. Normal thyroid gland, including medullary tissue (NT) also expresses GFRA4. Two major transcripts of 831 and 910 bp seen in MTC and TT-cells correspond to GFRα;4a and GFRα4b, respectively (primers PI and P3, see Figure 15A) . The minor transcript of 999 bp corresponds to the soluble GFRo;4c. Second row: No spliced form of the PSPN transcript is detectable using primers covering the initiation and the stop codons . Third row: RET mRNA is expressed in all MTC samples. Bottom row: Control RT-PCR with PPIA transcript showing equal loading of cDNA. All experiments contained negative water controls (not shown) . Figure 14. Localization of GFRAl , GFRA2 GFRA3 , GFRA4 and RET mRNAs in thyroid tumors by in si tu hybridization. Figure 14A. Typical representative examples of autoradiography films showing GFRAl -GFRA4 and RET cRNA hybridization to adjacent frozen sections of different thyroid tumors. Of the four types of tumors represented, only the MTC sections were strongly stained for GFRA4 and RET expression. GFRAl , GFRA2 , GFRA3 mRNA levels were very low or undetectable in all samples including MTC. Figure 14B. GFRA4 and RET probes were hybridized to frozen sections representing follicular thyroid adenoma (FTA) , follicular thyroid carcinoma (FTC) , papillary thyroid carcinoma (PTC) , and medullary thyroid carcinoma (MTC) samples and sections were counter-stained by Nissl-substance . GFRA4 and RET is highly expressed in all malignant cells of the MTC sample but not in the surrounding connective tissue. Bar in A= 0.5 cm, in B=100 μm. Figure 15. Activation of Akt in Neuro-2a cells expressing either GFRcel or hGFRα4 with their respective ligands. Figure 15A. Neither GDNF nor PSPN (native or heat-inactivated) stimulate Akt phosphorylation in ock-transfected Neuro-2a cells. In Neuro-2a cells expressing GFRαl, GDNF, but not PSPN evokes Akt phosphorylation. Figure 15B. PSPN, but not GDNF, stimulates Akt phosphorylation in Neuro-2a cells expressing GFRα4. Upper panels in Figure 15A and Figure 15B show immunoblotting of the cell lysates with anti phospho-Akt (Ser 473) antibodies. Lower panels show the re-probing of the same blots with phosphorylation state-independent anti Akt antibodies. Figure 16. MAP kinases are activated in response to GDNF but not to PSPN in the Neuro-2a cells. Figure 16A. Neither GDNF, nor PSPN (native or heat-inactivated) , stimulate MAPK phosphorylation in mock-transfected Neuro-2a cells. In Neuro-2a/GFRcl cells GDNF, but not PSPN, activates MAPK. Figure 16B. In Neuro-2a/hGFRo!4 cells PSPN is unable to stimulate phosphorylation of MAPK. Upper panels in Figure 16A and Figure 16B show immunoblotting of the cell lysates with anti phospho-MAPK antibodies. Lower panels show the re-probing of the same blots with phosphorylation state-independent anti MAPK antibodies. Figure 17A. GDNF and PSPN both stimulate cell survival in dose-dependent manner via their respective receptors in serum-free conditions. Figure 17B. PSPN, in contrast to GDNF, cannot induce neurite outgrowth in Neuro-2a/hGFRα4 cells. Shown are mean values with SEM. ** - P<0.01; *** - P<0.001. Figure 18. cDNA and amino-acid sequences of mouse GFRα4 GPI-isoform al . The GPI-linked isoform, (cf . Figure 1A) corresponds to transcript al in Figure 2C. The signal sequence encoded by exon la is underlined. Figure 19. The putative TM-anchored isoform, GFRα!4-a2, encoded by transcript a2 in Figure 2C. Figure 20. cDNA and amino-acid sequence of GFRα4-a3/4, a soluble isoform of mouse GFRα4. The signal sequence encoded by exon la is underlined. GFRα4-a3/4 is encoded by transcripts a3-a4 as shown in Figure 2C. Figure 21. Shown are the cDNA and amino-acid sequence of human GFRo;4 a GPI-linked isoform a. Figure 22. Human GFRo;4 b, putative GPI-linked isoform b. Figure 23. Human GFRα;4 c, a putative secreted isoform, formed by alternative splicing of the second short intron. Figure 24. Genomic sequence of human GFRA4 locus. The coding sequences are in uppercase . Intronic sequences are in lowercase and bold. The alternatively spliced 3 ' -end of the 2 n °- short intron is shown in italics . Table 1 Expression of Gfra.4 and Ret mRNAs in Selected Mouse Tissues and Cells Age E12 E16 P0 P20 adult Area Gfra4 Ret Gfra.4 Ret Gfra Ret Gfra4 Ret Gfra4 Ret NC + _ 4-4- _ 4-4- - nd nd + 4- - HC nd nd nd nd 4-4- - nd nd 4-4-4- 4- VM 4- 4-4-4 - nd nd + + + + nd nd 4- 4-4-4- VSC + + + -| - 4-4- + + -1 - + + + + nd nd 4- 4-4-4- PNS + 4-4-4 - 4- 4-4H - + + + + + + + + + + 4- + 4-4-4- PI nd nd 4-4-4- - + + + - + + + nd 4-4-4- - TC 4-4- 4-4- 4-4-4- 4-4- + + + + + + + + 4-4- 4-4-4- 4- PT nd nd - - ± - + + + - 4-4-4- - AdM nd nd nd nd + + + +* + + + 4-4- +* Kidney 4- 4-4-- 1- 4- 4-4-- 1- + + + + - 4- - ± Lung 4- 4- 4- 4- ± - nd nd - - Liver - - - - - - nd nd - - Testis nd nd nd nd + ++* + + ± 4-4- ± mRNA expression was graded from -, no labeling above background, to +4-4-, highest detected expression level based on visual inspection of in situ hybridization sections. Expression of Gfra4 and Ret at E14 and E16 as well as P8 and P20 were overlapping. ""Expression differs from that reported by Golden et al. , 1999. Abbreviations IML, intermediate lobe; nd, not determined; PNS, peripheral nervous system including sensory (DRG), sympathetic (SGG) and enteric ganglia; NC, Neocortex; HC, Hippocampus; VM, Ventral Midbrain; VSC, Ventral Spinal Cord; PNS, PNS ganglia; PI, Pituitary (IML); TC, Thyroid (C-cells); PT, Parathyroid; AdM, Adrenal Medulla; nd, not determinated. Table 2 Expression of human GFRAl, GFRAl, GFRA3, GFRA4 and RET in thyroid tumours and normal thyroid Tumour GFRAl GFRAl GFRA3 GFRA4 RET RT In situ RT In Situ RT In situ RT In situ RT In situ FTA 4- 4- 4- 4- 4- 4- 1/4 1/2 3/4 2/2 4/4 2/2 0/4 0/3 0/4 0/2 FTC 4- 4- 4- 4- 4- 1/4 0/1 2/4 1/1 2/4 1/2 0/4 0/3 0/4 0/1 PTC 4- 4- 4- + 4- 2/5 0/2 3/5 2/2 2/5 1/3 0/5 0/4 0/5 0/2 MTC 4- 4- 4- 4- + 4- 4- 4- 4- 4- 4- 4- 4- 4- 4- 4- 4- 4- 5/8 4/6 7/8 4/6 4/8 2/6 8/8 6/6 8/8 6/6 NT 4- Nd 4- Nd 4- Nd 4- 4- Nd 4- 4- Nd 1/1 1/1 2/3 1/3 1/1 TT cells 4- Nd - Nd - Nd 4- 4- 4- Nd 4- 4- 4- Nd mRNA expression is graded from -, no labelling above background, to 4- 4- 4-, highest detected expression based on visual inspection. The numbers below the tissue gradings denote the number of samples positive/total number of samples analyzed. Abbreviations RT, RT-PCR; FTA, follicular thyroid adenoma; FTC, follicular thyroid carcinoma; PTC, papillary thyroid carcinoma; MTC, medullary thyroid carcinoma; NT, normal thyroid; Nd. not determined. References Acton, D. S. et al . , 2000. Oncogene 19: 3121-3125 Airaksinen, M.S. et al . , 1999. Mol . Cell. Neurosci . 13: 313-325 Angrist, M. et al . , 1998. Geno ics 48: 354-362 D' Arcangelo, G. et al . , 1993. Mol. Cell. Biol . 13: 3146-3155 Ashcroft et al . , 1999. Oncogene 18: 4586-4597 Baloh, R.H. et al . , 1998. Proc . Natl. Acad. Sci . USA 95:5801-5806 Baloh, R. H. et al . , 1998. Neuron 21: 1291-1302 Baloh, R. H. et al . , 2000. Curr. Opin. Neurobiol. 10: 103-110 Belluardo, N. et al . , 1999. Cell Tissue Res 295: 467-475 Bilak, M.M. et al . , 1999. Mol. Cell. Neurosci. 13: 326-336 Bongarzone, I. et al . , 1998. Oncogene 16: 2295-2301 Borrello et al . , 1995. Oncogene 11: 2419-24177 Califano et al . , 1995. Oncogene 11: 107-112 Califano et al . , 2000. J Biol Chem 275: 19297-305 Carlomagno, F. et al . , 1998. Endocrinology 139: 3613-3619 Carson-Walter et al . , 1998. Oncogene 17: 367-376 Durbec, P.L. et al . , 1996. Development 122: 349-358 Edery, P. et al . 1997. Bioassays 19: 385-395 Eng, C, 1999. J. Clin. Oncol. 17: 380-394 Enokido, Y. et al . , 1998. Curr. Biol. 8: 1019-1022 Frisk, T. et al . , 2000. Eur. J. Endocrinol . 142: 643-649 Golden, J.P. et al . , 1999. Exp . Neurol . 158: 504-528 Grabowski, P.J., 1998. 92: 709-712 Gunn, T.M. et al . , 1999. Nature 398: 152-156 Ho, T. W. et al., 2000. Exp. Neurol. 161: 664-675 Iwashita, T. et al . , 1996. Oncogene 12: 481-487 Jaszai, J. et al . , 1998. J. Neurosci. Res. 53: 494-501 Jing, S. et al . , 1996. Cell 85: 1113-1124 Kaplan, D.R. and Miller, F.D. 2000. Curr. Opin. Neurobiol. 10 (3) :381-391 Kazlauskas, A. 1994. Curr. Opin. Genet. Dev. 4: 5-14 Klein, R. D. et al . , 1997. Nature 387: 717-721 Laurikainen, A. et al . , 2000. J. Neurobiol. 43: 198-205 Leitner, M.L. et al . , 1999. J. Neurosci. 19: 9322-9331 Lindahl, M. et al . , 2000. Mol. Cell. Neurosci. 15: 522-533 Lopez, A.J., 1998. Annu. Rev. Genet. 32: 279-305 Lorenzo, M. J. et al . , 1995. Oncogene 10: 1377-1383 Lou, H. et al., 1998. J. Endocrinol . 156: 401-405 Masure, S. et al, . 2000. J. Biol. Chem. (in press). McGonigle, P. et al . , 1994. Basic Neurochemistry, 5th Ed. Ed. Siegel, G. J. Raven Press, NY: 209-230 Michiels, F.-M. et al . , 1997. Proc . Natl. Acad. Sci . USA 94: 3330-3335 Milbrandt, J. et al . , 1998. Neuron 20: 245-253 Murakami et al . , 1999. Biochem Biophys Res Commun 262: 68-75 Nakagawa et al . , 1987. Proc Natl Acad Sci USA 84: 5923-5927 Naka ura, T. et al . , 1994. J. Pathol . 172: 255-260 Pachnis, V. et al . , 1993. Development 119: 1005-1017 Pearse, A.G. et al . , 1976. Nature 214: 929-930 Pearse, A.G. et al . , 1999. Histochemie 27: 96-102 Perron et al . , 1999. Mol Cell Neurosci 13: 362-378 Ponder, B.A., 1999. Cancer Res. 59: 1736s-1741s Rossel et al . , 1997. Oncogene 14: 265-275 Rossi, J. et al., 1999. Neuron 22: 243-252 Sanicola, M. et al . , 1997. Proc. Natl. Acad. Sci. 94: 6238-6243 Santoro, M. et al . , 1990. Oncogene 5: 1595-1598 Santoro, M. et al . , 1995. Science 267: 381-383 Segouffin-Cariou et al . , 2000. J Biol Chem 275: 3568-3576 Smith-Hicks, C. L. et al . , 2000. EMBO J. 19: 612-622 Thompson, J. et al . , 1998. Neurosci. 11: 117-126 Timmusk, T. et al . , 1993. Eur. J. Neurosci. 5: 605-613 Trupp, M. et al., 1998. Mol. Cell. Neurosci. 11: 47-63 Tsuzuki, T. et al . , 1995. Oncogene 10: 191-198 Udenfriend, S. et al . , 1995. Annu. Rev. Biochem. 64: 563-591 Widenfalk, J. et al . , 1998. Eur. J. Neurosci. 10: 1508-1517

Claims: What is claimed: 1. An isolated and purified nucleic acid sequence, c h a r a c t e r i z e d in that the nucleic acid sequence is a cDNA encoding a splicing isoform of mammalian GFRα;4 comprising the amino acid sequence (SEQ ID NO:l:), (SEQ ID N0:2:), (SEQ ID NO : 3 : ) , (SEQ ID NO: 4:), (SEQ ID NO : 5 : ) or (SEQ ID NO : 6 : ) or fragments thereof. 2. The nucleic acid sequence according to claim 1, c h a r a c t e r i z e d in that the nucleic acid sequence is a cDNA encoding a splicing isoform of murine GFRo?4 comprising the amino acid sequences (SEQ ID NO:l:), (SEQ ID NO:2:) or (SEQ ID NO : 3 : ) or fragments thereof. 3. The nucleic acid sequence according to claim 1, c h a r a c t e r i z e d in that the nucleic acid sequence is a cDNA encoding a splicing isoform of human GFRα;4 comprising of the amino acid sequence (SEQ ID NO:4:), (SEQ ID NO:5:) or (SEQ ID NO : 6 : ) or fragments thereof. 4. The nucleic acid sequence according to claim 1, c h a r a c t e r i z e d in that the nucleic acid sequence is a cDNA having the sequence (SEQ ID NO : 7 : ) , (SEQ ID NO:8:), (SEQ ID NO:9:), (SEQ ID NO: 10:), (SEQ ID NO: 11:), (SEQ ID NO: 12:) or (SEQ ID NO: 13:) or fragments thereof, encoding a polypeptide having substantially similar functions as human or mouse GFRα4. 5. The nucleic acid sequence according to claim 1, c h a r a c t e r i z e d in that the nucleic acid sequence is a cDNA encoding a human GFRα4 characterized by comprising the nucleic acid sequence (SEQ ID NO:10:), (SEQ ID NO:ll:), (SEQ ID NO: 12:) or (SEQ ID N0.13:), said (SEQ ID NO: 13:) being a copy of the genomic sequence without or with introns or fragments of the introns with the prerequisite that it includes at least one of the exons or parts thereof (SEQ ID NO: 29:), (SEQ ID NO: 30:), (SEQ ID NO: 31:), (SEQ ID NO:33:), (SEQ ID NO: 35:), (SEQ ID NO: 35:), (SEQ ID NO: 37: ) or (SEQ ID NO: 39:), fragments or any combination thereof having fragments of the introns (SEQ ID NO: 32:), (SEQ ID NO: 34:), (SEQ ID NO: 36) or (SEQ ID NO: 38) but still encoding a polypeptide having a functional activity substantially similar to that of human GFRo!4. 6. The use of nucleic acid sequence according to claims 1-5 for recording GFRo;4 -mediated signalling in neurons or endocrine cells. 7. The use of nucleic acid sequence according to claims 6, wherein the endocrine cells are thyroid calcitonin-producing C-cells, parathyroid gland cells, adrenal chromaffin cells or cells of pituitary intermediate lobe. 8. The use of nucleic acid sequence according to claim 1-5 for manufacturing ' polypeptides useful for diagnosing and/or treating tumours in parathyroid gland cells, adrenal chromaffin cells, cells of pituitary intermediate lobe, neoplasia, endocrine tumours, medullary thyroid carcinoma and pheochromocytoma, parathyroid hyperplasia, neuronal disorders or for preventing neuronal death or aberrant axonal sprouting. 9. A purified and isolated polypeptide, c h a r a c t e r i z e d in that it comprises a polypeptide having the amino acid sequence (SEQ ID NO:l:), (SEQ ID NO : 2 : ) , (SEQ ID NO:3:) (SEQ ID NO : 4 : ) , (SEQ ID N0:5:) or (SEQ ID NO : 6 : ) . 10. The polypeptide according to claim 9, c h a r a c t e r i z e d in that it has a structure substantially similar to an isoform of mouse GFRα4. 11. The polypeptide according to claim 9, c h a r a c t e r i z e d in that it has a structure substantially similar to an isoform of human GFRo;4. 12. The use of the polypeptides according to claims 9-11 for determining GFRQJ4-mediated signalling in neurons, endocrine cells, neoplasia, endocrine tumours, medullary thyroid carcinoma . 13. The use of the polypeptides according to claims 12, wherein the endocrine cells are thyroid calcitonin-producing C-cells, parathyroid gland cells, adrenal chromaffin cells or cells of pituitary intermediate lobe. 14. The use of the polypeptides according to claims 12, wherein the medullary thyroid carcinoma comprises pheochromocytoma and parathyroid hyperplasia. 15. The use of polypeptides according to claims 9-11 for manufacturing a composition for diagnosing and/or treating neuronal disorders, osteoporosis, cancer in parathyroid gland cells, adrenal chromaffin cells and cells of pituitary intermediate lobe, in neoplasia, treatment of endocrine tumours, including medullary thyroid carcinoma, pheochromocytoma and parathyroid hyperplasia, for preventing axonal sprouting in epilepsy due to compensatory neurotrophic factor production after seizures, preventing neuronal death without producing aberrant sprouting. 16. A method for obtaining a cell-line (clone) useful in the production of the transgenic non-human animal according to claims, c h a r a c t e r i z e d in that the genomic gene encoding a functional GFRα4 receptor is inactivated by introducing into stem cells or cell lines (clones) a nucleic acid sequence or a part thereof, which sequence or part is capable of integrating to said genomic gene and by said integration functionally inactivates said genomic gene or makes it conditionally inactivatable . 17. A cell line (clone) obtainable by the method of claim 16, c h a r a c t e r i z e d in that the cell-line (clone) is lacking a functional GFRo;4 receptor and producing altered levels of GFRα4. 18. The use of the cell line (clone) of claim 17, c h a r a c t e r i z e d in that the cell lines are used for producing a transgenic non-human animal and/or diagnosing and/or treating osteoporosis, endocrine tumours, adrenal chromaffin cells, cells of pituitary intermediate lobe, neoplasia, endocrine tumours, medullary thyroid carcinoma, pheochromocytoma and parathyroid hyperplasia by recording GFRα?4-mediated signalling in endocrine cells, thyroid calcitonin-producing C-cells, parathyroid gland cells, adrenal chromaffin cells, cells of pituitary intermediate lobe, neoplasia, endocrine tumours, medullary thyroid carcinoma, pheochromocytoma and parathyroid hyperplasia, neuronal disorders, for preventing axonal sprouting in epilepsy due to compensatory neurotrophic factor production after seizures or preventing neuronal death without producing aberrant sprouting.

Inventors: AIRAKSINEN MATTI, LINDAHL MARIA, POTERIAEV DIMITRI, ROSSI JARI, SAARMA MART, TIMMUSK TOENIS, AIRAKSINEN, MATTI, LINDAHL, MARIA, POTERIAEV, DIMITRI, ROSSI, JARI, SAARMA, MART, TIMMUSK, TOENIS
Applicants: AIRAKSINEN MATTI, LICENTIA LTD, LINDAHL MARIA, POTERIAEV DIMITRI, ROSSI JARI, SAARMA MART, TIMMUSK TOENIS, AIRAKSINEN, MATTI, LICENTIA LTD., LINDAHL, MARIA, POTERIAEV, DIMITRI, ROSSI, JARI, SAARMA, MART, TIMMUSK, TOENIS
Application year: 2000
Main classification: C07K14
IPC codes: A61K38, A61K48, A61P19, A61P25, A61P35, C07K14, C12N5, C12N15, C12P21, C12Q1