# Patent

## Title
Tools for the identification of Lingo-1, Lingo-2, Lingo-3 and Lingo-4 ligands, and uses thereof

## Abstract

The present invention relates to a system comprising coupling products formed by a monomer of a protein chosen from Lingo-1, Lingo-2, Lingo-3 and Lingo-4 and by a probe emitting a signal when said monomer undergoes conformational changes, and to a screening method using said system, enabling ligands of a protein chosen from Lingo-1, Lingo-2, Lingo-3 and Lingo-4 to be identified. The present invention is industrially applicable in the field of methods for detecting molecules, for detecting interaction between molecules and for molecular screening, and also in the medical field.

## Background
CROSS-REFERENCE TO RELATED APPLICATION
This Application is a National Stage entry of International Application No. PCT/FR2011/050743, having an international filing date of Apr. 1, 2011; which claims priority to French Application No.: 1001374, filed Apr. 1, 2010; the disclosure of each of which is hereby incorporated in its entirety by reference.
SEQUENCE LISTING
The instant application contains a Sequence Listing which has been submitted in ASCII format via EFS-Web and is hereby incorporated by reference in its entirety. Said ASCII copy, created on Nov. 21, 2012, is named 02130500.txt and is 101,368 bytes in size.
TECHNICAL FIELD
The present invention relates to a coupling product comprising a monomer of a protein chosen from Lingo-1, Lingo-2, Lingo-3 and Lingo-4 and a probe emitting a signal when said monomer undergoes conformational changes, to nucleic acid sequences encoding this product, to vectors enabling the preparation thereof, to a system comprising it and to a screening method which makes it possible to identify ligands of a protein chosen from Lingo-1, Lingo-2, Lingo-3 and Lingo-4.
The present invention is industrially applicable in the field of methods for detecting molecules, for detecting interaction between molecules and for molecular screening, and also in the medical field.
In the description below, the references between square brackets ([ ]) refer to the list of references given after the examples.
PRIOR ART
Neurodegenerative diseases are characterized by the degeneration of certain neurons and the destruction of the nervous system. Parkinson's disease and multiple sclerosis are two of these diseases.
Parkinson's disease affects 1 to 2 individuals in 1000. The degeneration involves dopaminergic neurons which produce a neurotransmitter, dopamine, which is involved in particular in the control of body movements. Apparently, the treatments developed against the disease, essentially dopaminergics and dopamine agonists, enable an improvement in the problems by slowing down the development of the disease. However, these treatments do not make it possible to cure the disease.
Multiple sclerosis is an autoimmune disease which has a prevalence of 1 in 1000 and in which the myelin of the axons of the brain and of the spinal cord is destroyed. This causes difficulties in conduction of nerve influx, thus affecting virtually all biological functions. The current treatments call upon immunosuppressants or immunomodulators. However, once again, these treatments only manage to slow down the progression of the disease and not to cure it.
One and the same protein, called Lingo-1, is at the intersection of these two diseases. Moreover, the Lingo-4 protein has a role in diseases involving myelin (WO 2009/061500). Lingo-1 is a transmembrane protein of 580 residues (without the signal peptide). It has a significant extracellular region (516 amino acids) consisting of 12 LRR (leucine-rich repeat) motifs and of an Ig (immunoglobulin) domain. The cytoplasmic portion, comprising 38 residues, contains a canonical site of phosphorylation by EGFR (Epidermal Growth Factor Receptor). Lingo-1 belongs to the family of LRR proteins which play key roles in the biology of the central nervous system (CNS) and constitute attractive targets for the treatment of neurological and neurodegenerative diseases.
The expression profile of Lingo-1 is nervous system-specific, with abundant expression in the CNS and more particularly in the neurons and the oligodendrocytes. This transmembrane protein exercises several inhibitory functions: that of neuron survival, that of axon and neurite regeneration after damage and that of oligodendrocyte maturation and of myelination.
Lingo-1 is a component of the NgR1/p75 complex and its inhibition promotes neurite and axon growth. Indeed, the gene encoding Lingo-1 was cloned in 2003 by Carim-Todd et al., (“LRRN6A/LERN1 (leucine-rich repeat neuronal protein 1), a novel gene with enriched expression in limbic system and neocortex”. 2003, Eur. J; Neurosci. 18 (12):3167-82, [1]) and it is in 2004 that Mi et al. (Mi et al., “LINGO-1 is a component of the Nogo-66 receptor/p75 signaling complex”. 2004, Nat Neurosci. 7(3):221-8, [2]) showed that Lingo-1 was an integral part of the NgR1/p75 and NgR1/TROY receptor complex. These complexes, located in the membrane of neurons, inhibit neurite and axon regeneration in the presence of myelin constituents (OMGp, Nogo and MAG) via the activation of the RhoA kinase. This has been confirmed in an in vivo experimental model of axonal regeneration in rat. In this model, the spinal cord is injured and this is reflected by an increase in the expression of Lingo-1 and strong apoptosis of the oligodendrocytes and of the neurons surrounding the injury. Treatment of the injured rats with a Lingo-1 function antagonist, Lingo-1-Fc (a soluble form of Lingo-1 capable of competing with endogenous Lingo-1) makes it possible to obtain significant improvements in their mobility, sizeable regeneration of the axons and an increase in the survival of the neurons and oligodendrocytes around the injury (Ji et al., “LINGO-1 antagonist promotes functional recovery and axonal sprouting after spinal cord injury”, 2006, Mol Cell Neurosci. 33(3):311-20, [3]).
Moreover, Lingo-1, expressed in the oligodendrocytes or the axons, inhibits oligodendrocyte differentiation and myelination (Mi et al., “Lingo-1 negatively regulates myelination by oligodendrocytes”, 2005, Nature Neuroscience 8: 745-751, [4]). This has subsequently been confirmed by Lee et al. (“NGF regulates the expression of axonal LINGO-1 to inhibit oligodendrocyte differentiation and myelination”. 2007, J Neurosci. 27(1):220-5, [5]) and Zhao et al. (“An in vitro study on the involvement of LINGO-1 and Rho GTPases in Nogo-A regulated differentiation of oligodendrocyte precursor cells”, 2007, Mol Cell Neurosci. 36(2):260-9, [6]). Indeed, although the use of Lingo-1 antagonists (Lingo-1-Fc, an anti-Lingo-1 antibody, DN-Lingo-1 (dominant negative) corresponding to Lingo-1 without its cytoplasmic domain, Lingo-1 RNAi, Lingo-1 knockout) induces increased differentiation of oligodendrocytes and the formation of abundant myelin sheets, the overproduction of whole Lingo-1 on the other hand causes opposite effects. Lee et al. (2007) ([5]) have, furthermore, shown that the expression of the Lingo-1 protein in the axons plays just as important a role in the differentiation of the adjacent oligodendrocytes and myelination as a Lingo-1 protein expressed in the oligodendrocytes. Lingo-1 expression in the axons is activated by NGF and its receptor, TrkA. In order to test the role that the inhibition of Lingo-1 might play in diseases affecting myelination, such as multiple sclerosis, an experimental animal model of the disease, called “MOG-induced murine experimental autoimmune encephalomyelitis” (EAE) has been tested. The use of an anti-Lingo-1 antibody (Lingo-1 antagonist) and of Lingo-1-knockout mice in this model has made it possible to demonstrate that decreased Lingo-1 function is associated with better axon integrity and increased remyelination thereof (Mi et al., “Lingo-1 antagonist promotes spinal cord remyelination and axonal integrity in MOG-induced experimental autoimmune encephalomyelitis”, 2007, Nature medicine 13: 1228-1233, [7]).
Moreover, Lingo-1 inhibits the survival of dopaminergic (DA) neurons and of retinal ganglion cells (RGCs). Several studies carried out in animal models or cell cultures using Lingo-1 antagonists show that Lingo-1 plays an important role in neuronal survival and more particularly in the survival of retinal ganglion cells (RGCs) (destroyed in glaucoma) (Fu et al., “Blocking LINGO-1 function promotes retinal ganglion cell survival following ocular hypertension and optic nerve transection”. Invest Ophthalmol Vis Sci. 2008, 49(3):975-85, [8]; Fu et al., “Combined effect of brain-derived neurotrophic factor and Lingo-1 fusion protein on long-term survival of retinal ganglion cells in chronic glaucoma”, 2009, Neuroscience 162: 375-382, [9]), in cerebellar neurons (Zhao et al., “Inactivation of glycogen synthase kinase-3beta and up-regulation of LINGO-1 are involved in LINGO-1 antagonist regulated survival of cerebellar granular neurons”, 2008, Cell Mol Neurobiol. 28(5):727-35, [10]), and also in DA neurons (involved in Parkinson's disease) (Inoue et al., “Inhibition of the leucine-rich repeat protein Lingo-1 enhances survival, structure, and function of dopaminergic neurons in Parkinson's disease models”, 2007, PNAS 104: 14430-14435, [11]). In the DA neurons of patients suffering from Parkinson's disease and in mouse experimental models, it appears that Lingo-1 is overproduced. Since Lingo-1 inhibits EGFR in these neurons, and since EGFR activates the PI3k/Akt pathway, the action of Lingo-1 on neuronal survival could come from an inhibition of the PI3k/Akt pathway.
In the animal model for glaucoma, it is known that BDNF (Brain-derived neurotrophic factor) is an important survival factor for RGCs which delays their death. Fu et al. (2009) ([9]) have recently established that the combined effect of Lingo-Fc and BDNF is much more effective than BDNF alone and allows increased long-term survival of RGCs. Since Lingo-1 is capable of interacting with the BDNF receptor, TrkB, and of inhibiting it, this molecular mechanism could be that involved in the long-term protection of RGCs.
The Lingo proteins, and in particular Lingo-1, constitute a new, particularly advantageous therapeutic target against these diseases, since inhibition thereof generates neuroprotective effects and stimulates myelin synthesis. Experiments carried out in vitro and in vivo on experimental animal models have validated the hypothesis that Lingo-1 inhibition is an innovative and promising approach for curing several nervous system diseases, in particular Parkinson's disease and multiple sclerosis.
The natural ligands of the Lingo proteins, and in particular of the Lingo-1 protein, are currently unknown. The identification of novel chemical molecules which are Lingo-1 antagonists would therefore make it possible to develop novel therapies against these two nervous system diseases. However, no method which makes it possible to measure the binding of Lingo-1 ligands is currently available. Furthermore, no method of identification that can be automated and that can be adapted to the high-throughput of Lingo-1 ligands and antagonists currently exists. The only current means for identifying Lingo-1 ligands or antagonists are biological tests that are laborious to implement on cerebellar granular neurons (Mi et al., 2004, [2]; Zho at al., 2008, [10]), retinal ganglion cells (Fu et al., 2009, [9]), or oligodendrocytes (Mi et al., 2005, [4]) or in experimental murine models of autoimmune encephalomyelitis (Mi et al., 2007, [7]) or of Parkinson's disease (Inoue et al., 2007, [11]). These tests cannot be adapted to high-throughput.
For this reason, the applicant has sought to develop a method for identifying ligands, and in particular antagonists, of Lingo, and in particular Lingo-1, that can be automated and can be adapted to high-throughput.
DESCRIPTION OF THE INVENTION
The invention actually makes it possible to overcome the drawbacks of the prior art and to meet these needs.
The applicant has observed, surprisingly and after considerable research, that Lingo-1 forms a dimer in vivo. It was described in the literature that the extracellular part of Lingo-1 was capable of tetramerization (Mosyak et al., “The structure of Lingo-1 ectodomain, a module implicated in CNS repair inhibition”, 2006, J. Biol. Chem. 281: 36378-36390, [12]). The applicant has shown, for the first time, that the whole and membrane form of Lingo-1 forms a dimer in vivo.
The applicant has thus developed a method for identifying a ligand of a protein chosen from Lingo-1, Lingo-2, Lingo-3 and Lingo-4, which can be easily automated and adapted to high-throughput, and which will make it possible to screen large libraries of chemical molecules in order to discover the most suitable inhibitors and exploit them for therapeutic purposes.
This method is based on the principle that, when Lingo, and in particular Lingo-1, interacts with a ligand, the conformation of the Lingo-1 dimer changes. The method of identification makes it possible to detect this conformational change and to identify Lingo-1 ligands.
The method of the invention comprises in particular the following steps:
a) incubating a system comprising coupling products formed by a monomer of a protein chosen from Lingo-1, Lingo-2, Lingo-3 and Lingo-4 and by a probe capable of emitting a detectable signal when said monomer undergoes conformational changes, and a candidate molecule,
b) detecting a modification of the signal emitted by at least one of the probes, the modification of the signal revealing the binding of said candidate molecule to at least one of said coupling products.
The invention relates to a coupling product comprising a monomer of a protein chosen from Lingo-1, Lingo-2, Lingo-3 and Lingo-4, and a probe capable of emitting a signal when the monomer undergoes conformational changes.
For the purpose of the present invention, the term “monomer” means a protein composed of a single polypeptide molecule. In other words, it is a three-dimensional arrangement of atoms in a single polypeptide chain. Advantageously, the monomers are capable of forming dimers. In other words, the monomers are capable of associating with one another so as to form a molecule composed of two monomers, i.e. of two monomeric subunits. It may, for example, be a homodimer, i.e. two monomers of the same protein bonded to one another. Alternatively, it may be a heterodimer, i.e. two monomers of two different proteins bonded to one another. Advantageously, the dimer may be formed by polymerization of the monomers in vivo, under appropriate conditions. For example, these conditions may be suitable for the formation of disulfide bridges or to that of hydrophobic interactions.
For the purpose of the present invention, the term “Lingo-1, Lingo-2, Lingo-3 and Lingo-4” means any protein which has substantially the sequence of the wild-type protein. It may be a protein which has all or part of the wild-type protein, or a protein which has one or more mutations, for example by addition, deletion or substitution. It may be a human or non-human protein, for example a murine, equine, for example horse, porcine, rabbit, rat, ovine, for example sheep, bovine, for example cow, monkey, for example macaque, protein.
When the protein is Lingo-1, it may be the wild-type human protein, in particular composed of 580 residues, comprising an extracellular region (516 amino acids) consisting of 12 LRR (leucine-rich repeat) motifs and of an Ig (immunoglobulin) domain, and a cytoplasmic portion of 38 residues. Advantageously, the sequence of the Lingo-1 monomer may be the sequence SEQ ID No. 1. It may be the sequence described in Genbank (http://www.ncbi.nlm.nih.gov/Genbank/) by accession number NP_116197.4. It may also be the protein resulting from the polynucleotide sequence corresponding to the mRNA described by the sequence SEQ ID No. 5, or by the sequence NM_032808 in Genbank.
When the protein is Lingo-2, it may be the wild-type human protein, in particular composed of 606 amino acids, and comprising an extracellular domain composed of 515 amino acids, and a cytoplasmic portion of 40 residues. Advantageously, the sequence may be the sequence SEQ ID No. 2. It may be the sequence described in Genbank by accession number NP_689783.1. It may also be the protein resulting from the polynucleotide sequence corresponding to the mRNA described by the sequence SEQ ID No. 6, or by the sequence NM_152570 in Genbank.
When the protein is Lingo-3, it may be the wild-type human protein. Advantageously, the sequence may be the sequence SEQ ID No. 3. It may be the sequence described in Genbank by accession number NP_001094861. It may also be the protein resulting from the polynucleotide sequence corresponding to the mRNA described by the sequence SEQ ID No. 7, or by the sequence NM_001101391 in Genbank.
When the protein is Lingo-4, it may be the wild-type human protein, in particular composed of 593 amino acids. Advantageously, the sequence may be the sequence SEQ ID No. 4. It may also be the protein resulting from the polynucleotide sequence corresponding to the mRNA described by the sequence SEQ ID No. 8, or by the sequence NM_001004432 (human sequence), or the murine sequence NP 796224 in Genbank.
Advantageously, the interaction of the monomer or of the dimer with a ligand of the protein can cause a change in a physical characteristic of the monomer and/or of the dimer. It may, for example, be the spatial conformation of the monomer and/or dimer.
For the purpose of the present invention, the expression “conformational change in the monomer” means any spatial modification of the monomer. Advantageously, this modification may result from the interaction of the monomer and/or of the dimer with a ligand.
For the purpose of the present invention, the expression “probe capable of emitting a signal” means any molecule capable of producing an effect when a physical characteristic of the monomer undergoes a change, in particular when the monomer undergoes a conformational change. Advantageously, it may be a conformationally sensitive detectable label. Advantageously, the interaction of the ligand with the monomer or the dimer causes a conformational change in the monomer or in the dimer, this change involving the emission of a detectable signal.
Advantageously, the probe may be a molecule chosen from a chemical tag, for example an antibody, a luminescent molecule or a fluorescent molecule, a fragment integrated into the monomer, for example a protease cleavage site, or an immunodetectable fragment.
For the purpose of the present invention, the term “luminescent molecule” means any molecule which has the property of releasing, in the form of photons with an energy of nonthermal origin, a part of the energy absorbed during an excitation. It therefore involves the deactivation of an excited molecule toward a lower energy state. In other words, a luminescent molecule is a molecule capable of acting on an appropriate substance in order to generate luminescence.
The luminescent molecule may, for example, be a protein, or a chemical compound. Advantageously, the luminescent protein has the property of emitting a blue, yellow or green light. The protein may be chosen from those which are known to those skilled in the art, described for example in the documents Kamal et al., “Improved donor/acceptor BRET couples for monitoring beta-arrestin recruitment to G protein-coupled receptors”. Biotechnol J. 2009 September; 4(9):1337-44, [17]; Kocan M et al., “Demonstration of improvements to the bioluminescence resonance energy transfer (BRET) technology for the monitoring of G protein-coupled receptors in live cells”, J Biomol Screen. 2008 October; 13(9):888-98, [18]; Michelini E et al., “Luminescent probes and visualization of bioluminescence”, Methods Mol Biol. 2009; 574:1-13, [19]). For example, the luminescent protein may be luciferase. The luciferase can be chosen from molecules known to those skilled in the art, for example those cited in WO 01/046691. It may, for example, be Renilla luciferase, RLuc2, RLuc8, firefly luciferase, Gaussia luciferase or Aequorin, and also mutants or derivatives thereof. If it is Renilla luciferase, it may be a protein encoded by the sequence SEQ ID No. 12.
For example, the luminescent protein may be Renilla luciferase, and the monomer may be Lingo-1. In this case, the coupling product can have the peptide sequence SEQ ID No. 14.
For the purpose of the present invention, the term “signal” means any physical or chemical effect. It may, for example, be a luminous signal, for example fluorescent, luminescent, colorimetric, electric, this list not being limiting.
For the purpose of the present invention, the term “fluorescent molecule” means any molecule which has the property of absorbing light energy (excitation light) and of rapidly releasing it in the form of fluorescent light, by emission of a photon very rapidly (emission light). Once the energy of the photon has been absorbed, the molecule is then generally in an electronically excited state. In other words, it may be a fluorophore or a fluorochrome.
Advantageously, the luminescent or fluorescent protein can be a protein capable of emitting a blue, yellow or green light.
Particularly advantageously, the luminescent or fluorescent protein can be a fluorescent protein capable of emitting a yellow or green light. The fluorescent protein can, for example, be chosen from GFP (“Green Fluorescent Protein”), YFP (“Yellow Fluorescent Protein”), Enhanced Yellow Fluorescent Protein (eYFP), eGFP, GFP2, GFP10, RGFP (Renilla Green Fluorescent Protein) and YPet, and mutants or derivatives thereof as described in document WO 01/46691. If it is eYFP, it can be a protein encoded by the sequence SEQ ID No. 9.
For example, the fluorescent protein may be eYFP, and the monomer may be Lingo-1. In this case, the coupling product can have the peptide sequence SEQ ID No. 11.
For the purpose of the present invention, the term “coupling product” means any product in which the monomer and the probe are bonded to one another. In other words, the coupling product can be a combination of the monomer and of the probe with one another. Again in other words, the coupling product can be a conjugate of the monomer and of the probe with one another.
Advantageously, the probe and the monomer can be coupled via one or more bonds that are stable in biological media. In other words, the probe and the monomer can be bonded via one or more bonds which do not alter at the time in the medium in which the coupling product is stored or produced. Advantageously, the medium can be a cell medium, or a medium used in the context of in vitro or in vivo operations.
The probe and the monomer can be covalently bonded. In this regard, the bond can be a bond between two atoms resulting from the sharing of two electrons originating separately from each of them.
The probe and the monomer can form a fusion protein, i.e. an artificial protein obtained through the combination of different proteins, or parts of proteins.
The coupling product can be obtained by means of techniques known to those skilled in the art, for example by expression by a host organism of a nucleic acid molecule encoding the coupling product, or by in vitro synthesis of the peptide and coupling after chemical ligation. For example, if it is a fusion protein, the coupling product can be obtained following the creation, by DNA recombination, of a gene comprising the open reading frames corresponding to the probe and to the monomer, insertion of the gene into an expression vector, insertion of the expression vector into a host cell, production of the corresponding fusion protein by the host cell and purification of the fusion protein.
The invention also relates to an isolated and purified nucleic acid molecule encoding a coupling product as previously defined.
For the purpose of the present invention, the term “nucleic acid molecule” means any molecule consisting of a series of nucleotides. It may, for example, be deoxyribonucleic acid (DNA), ribonucleic acid (RNA), messenger RNA (mRNA) or complementary DNA (cDNA).
For the purpose of the present invention, the term “isolated” means being separated from the natural environment. Advantageously, the nucleic acid can undergo the routine manipulations carried out in the context of recombinant DNA technology, for example sequencing, digestion with restriction enzymes, mutagenesis, and cloning into expression vectors.
For the purpose of the present invention, the term “purified” means the characteristic of being substantially separated from the contaminants.
The nucleic acid molecule can comprise a nucleic acid sequence encoding a luminescent or fluorescent protein as previously defined, and a nucleic acid sequence encoding a monomer as previously defined. Advantageously, the nucleic acid molecule can comprise the sequence SEQ ID No. 10 (the monomer being Lingo-1 and the protein being eYFP) or the sequence SEQ ID No. 13 (the monomer being Lingo-1 and the protein being Renilla luciferase).
The nucleic acid molecule can be obtained by means of techniques known to those skilled in the art. It can in particular be constructed artificially and obtained by chemical synthesis or genetic engineering, for example as described in Lewin B.: Genes VI, De Boeck University, 6th edition, Chapter 20, 1999. It can, for example, be obtained by recombination of the DNA of a gene comprising the open reading frames corresponding to the probe and to the monomer. By way of indication, the recombinant DNA can be inserted into a host expression organism, such as in particular a bacterium, by means of an expression vector, in particular a bacterial plasmid or a bacteriophage.
The invention also relates to an expression vector comprising a nucleic acid molecule as previously defined.
For the purpose of the present invention, the term “expression vector” means any molecule which has a structure that allows a nucleic acid molecule having a coding sequence to be transcribed or translated into a protein. Advantageously, it can comprise at least one of the following sequences: a promoter, a transcription start sequence, a transcription stop sequence, such as a polyadenylation sequence, a selectable gene, an enhancer, a regulatory sequence and an inducible sequence.
The vector can be any vector known to those skilled in the art, for instance a virus, such as an adenovirus or a retrovirus, a plasmid, a bacteriophage, a cosmid or a phagemid. It can, for example, be a commercially available vector, such as the plasmids p3xFlag (Sigma), pcDNA3 (Invitrogen) or peYFP-N1 (Clontech).
Such a vector can be prepared according to techniques known to those skilled in the art, as described for example by Lewin B.: Genes VI, De Boeck University, 6th edition, Chapter 20, 1999.
The invention also relates to a host cell comprising at least one nucleic acid molecule as previously defined or an expression vector as previously defined.
For the purpose of the present invention, the term “host cell” means any organism which has the capacity to produce a protein when an expression vector encoding the protein is introduced therein. Advantageously, the host cell can express the coupling product when an expression vector encoding the coupling product is introduced therein.
Advantageously, the host cell can express at least one coupling product as previously defined. For example, the host cell can express a coupling product comprising Renilla luciferase and Lingo-1, or a coupling product comprising eYFP and Lingo-1.
Advantageously, the host cell can comprise two nucleic acid molecules or an expression vector. For example, the host cell can comprise and express a nucleic acid molecule encoding a coupling product comprising a luminescent protein and a monomer, and a nucleic acid molecule encoding a coupling product comprising a fluorescent protein and a monomer. For example, the host cell can comprise and express a nucleic acid molecule encoding a coupling product comprising Renilla luciferase and a Lingo-1 monomer, and a nucleic acid molecule encoding a coupling product comprising eYFP and a Lingo-1 monomer.
Advantageously, the coupling products can form dimers in the host cell. This is because the applicant has shown that, in vivo, the monomers expressed in the cells form dimers.
The host cell can be any organism suitable for the production of recombinant proteins that is known to those skilled in the art. It may be a eukaryotic or prokaryotic cell. For example, it may be YB2/0 (ATCC CRL-1662), CHO-K1 (ATCC CCL-61), for example HEK 293 (ATCC CRL-1573), BHK (ATCC CRL-12072) or COS (ATCC CRL-1650), PC12 (ATCC CRL-1721) or SH-SY5Y (ATCC CRL-2266) or Hela (ATCC CCL-2).
The host cell can be produced by insertion of an expression vector as previously defined or of a nucleic acid molecule as previously defined, into the host cell, by means of techniques known to those skilled in the art. Mention may be made, for example, of electroporation, or calcium phosphate, described in the document “Molecular Cloning: A Laboratory Manual 2nd edition” (1989), Cold Spring Harbor Laboratory Press, “Current Protocols in Molecular Biology” (1987), John Wiley & Sons, Inc. ISBNO-471-50338-X, and the like, an electroporation method described in “Methods in Electroporation: Gene Pulser/E. coli Pulser System” Bio-Rad Laboratories (1993).
The host cell can be cultured in any suitable medium known to those skilled in the art. Mention may be made, for example, of DMEM medium (Dulbecco's Modified Eagle Medium, InvitrogenTM), Ham's F-12 medium, Eagle's Minimum Essential Medium (MEM), and Neuronal Base Medium (PAA Laboratories).
The coupling product can be purified by any means known to those skilled in the art, for example by immunopurification or by affinity chromatography.
A first subject of the invention relates to a system comprising:
a) a first coupling product comprising a monomer of a protein chosen from Lingo-1, Lingo-2, Lingo-3 and Lingo-4, and a probe capable of emitting a detectable signal which is modified when the monomer undergoes conformational changes, as previously defined,
b) a second coupling product comprising a monomer of a protein chosen from Lingo-1, Lingo-2, Lingo-3 and Lingo-4, and a probe capable of emitting a detectable signal which is modified when the monomer undergoes conformational changes, as previously defined,
wherein the first coupling product and the second coupling product form a dimer, and in which a change in interaction between the monomers causes a change in signal.
For the purpose of the present invention, the term “system” means a dimer (i.e. a complex of proteins made of two subunits very close to one another owing to the intermolecular interaction) resulting from couplings formed by a monomer of a protein chosen from Lingo-1, Lingo-2, Lingo-3 and Lingo-4 and by a probe capable of emitting a signal which is detectable, said signal being modified when said monomer and/or dimer undergo conformational changes, by any suitable technique known to those skilled in the art, for example a system suitable for detection by bioluminescence resonance energy transfer (BRET), by fluorescence resonance energy transfer (FRET), by TR-FRET (time-resolved-FRET) or by HTRF (homogeneous time resolved fluorescence). Preferably, it is a system of detection by bioluminescence resonance energy transfer (BRET). Said system of the invention can be in the form of a cell suspension, of a culture of adherent cells, of membranes or tissues, etc., expressing the donor/acceptor couple.
For the purpose of the present invention, the term “dimer” means any molecule composed of two monomers. Advantageously, the distance between the monomers making up the dimer is less than 150 Å, for example less than 100 Å, advantageously less than 75 Å or 50 Å.
For the purpose of the invention, the term “change in interaction” means any physical or structural modification between the monomers. It may in particular be a spatial or conformational modification of the monomers. Advantageously, this change can result from the binding of a ligand to the dimer or to at least one of the monomers, or from the proximity of a ligand on the dimer or at least one of the monomers.
For the purpose of the present invention, the term “ligand” means any molecule which binds to the coupling product. Advantageously, this binding can take place when the coupling product is in dimer form. The ligand can be a hormone, a neurotransmitter, a chemical compound, a medicament, a diagnostic agent, an antibody or a peptide sequence. It can be a total or partial agonist or a total or partial antagonist. Advantageously, a ligand can be capable of binding a monomer or a dimer with an affinity of between 1 μM and 1 pM, preferentially between 1 nM and 1 pM. Advantageously, the ligands can treat or prevent Parkinson's disease or multiple sclerosis, glaucomas or diseases involving myelin.
The applicant has in fact shown, firstly, that the coupling product, and more particularly the monomer, dimerizes, in particular in vivo, and secondly, that the binding of the ligand to the dimer can cause a conformational change in the dimer.
Advantageously, the conformational change causes a change in emission of a signal by a probe. Advantageously, the signal can be detected by any suitable technique known to those skilled in the art. Advantageously, the system can be suitable for detection by bioluminescence resonance energy transfer (BRET), a fluorescence resonance energy transfer (FRET) system, TR-FRET (time-resolved-FRET) or HTRF (homogeneous time resolved fluorescence).
For example, the system of the invention can be implemented in the context of detection by BRET, according to a protocol known to those skilled in the art. Mention may be made, for example, of Xu et al., “A bioluminescence resonance energy transfer (BRET) system: application to interacting circadian clock proteins” (1999) Proc. Natl. Acad. Sci. USA. 96, 151-156 [13]; Angers et al., “Detection of beta 2-adrenergic receptor dimerization in living cells using bioluminescence resonance energy transfer (BRET)” (2000) Proc. Natl. Acad. Sci. USA 97: 3684-3689 [14]). In particular, BRET can be implemented by means of a coupling product comprising Rluc and Lingo-1, and a coupling product comprising eYFP and Lingo-1. Rluc is an enzyme which produces blue light in the presence of a substrate, for example coelenterazine or a commercially available derivative thereof (Molecular Probes Eugene Oreg., Biosynth Napierville Ill.). When the spatial orientation is appropriate, eYFP is capable of absorbing a portion of blue light and emits as a consequence a light with a different wavelength, in the yellow range. In other words, in the presence of coelenterazine, eYFP emits a yellow light if Rluc and eYFP are sufficiently close to one another. For the purpose of the present invention, the term “sufficiently close” means at a distance of less than 150 Å, or else less than 100 Å, and advantageously less than 75 Å or 50 Å. If Rluc and eYFP are too far apart, the energy is not efficiently transferred and only the blue light of Rluc is detected. It so happens that the spatial disposition of Rluc and eYFP with respect to one another is dependent on the spatial arrangement of the monomers with respect to one another. If a molecule binds to a monomer or to the dimer, then the spatial conformation of the dimer changes. In particular, the monomers move close together or further apart. When the two monomers, and in particular Rluc and eYFP, are sufficiently close, eYFP can absorb a portion of the blue light emitted by Rluc, and can emit a wavelength in the yellow range. Thus, in order to detect whether a molecule binds to the dimer or to the monomer, the efficiency of the energy transfer is determined by measuring the luminous efficiency at the wavelength of the donor (Rluc) and at the wavelength of the acceptor (eYFP) after addition of coelenterazine. Moreover, the “luminous efficiency of the acceptor/luminous efficiency of the donor” ratio is calculated in order to quantify the BRET.
Alternatively, the system of the invention can be implemented in the context of detection by FRET, according to a protocol known to those skilled in the art. Mention may, for example, be made of U.S. Pat. No. 7,183,066, (Kroeger et al., “Constitutive and agonist-dependent homo-oligomerization of the thyrotropin-releasing hormone receptor” (2001) J Biol Chem, 276, 12736-12743, [20]). For example, a donor fluorophore and an acceptor chromophore known to those skilled in the art can be used, for example by means of the fluorescein/rhodamine or CFP (cyan fluorescent protein)/YFP couples.
Advantageously, the system of the invention can be implemented in the context of detection by TR-FRET, according to a protocol known to those skilled in the art, for example described in Maurel D et al., “Cell-surface protein-protein interaction analysis with time-resolved FRET and snap-tag technologies: application to GPCR oligomerization” (2008), Nat Methods (6):561-7, [21]). For example, it is possible to improve the signal-to-noise of the FRET by dispensing with a part of the parasitic signals by virtue of a time-resolved reading. This is possible through the use of long-life fluorophores such as rare earth cryptates or chelates.
Advantageously, the system of the invention can be implemented in the context of detection by HTRF®, according to a protocol known to those skilled in the art, for instance that of Hermand P et al., “Functional adhesiveness of the CX3CL1 chemokine requires its aggregation. Role of the transmembrane domain” (2008) J Biol Chem 283(44):30225-34; ([22]), or Whitfield J et al., “High-throughput methods to detect dimerization of Bcl-2 family proteins” (2003), Anal Biochem, 322(2):170-8 ([23]). For example, antibodies labeled with a fluorescence donor such as europium cryptate or a fluorescence acceptor such as a pigment of a phycobiliprotein purified from a red alga (for example, XL665) recognize tags on each of the monomers. The interaction of the monomers is detected by energy transfer, for example by excitation at 337 nm and emission at 665 nm. The system can be obtained by cotransfection of a cell with two expression vectors as previously defined. The system can, for example, be implemented in vitro, for example on plates.
Another subject of the invention relates to the use of a system as previously defined, for identifying ligands of a protein chosen from Lingo-1, Lingo-2, Lingo-3 and Lingo-4.
Another subject of the invention relates to the use of a system as previously defined, for detecting conformational changes in a protein chosen from Lingo-1, Lingo-2, Lingo-3 and Lingo-4.
Another subject of the invention relates to a method for identifying a ligand of a protein chosen from Lingo-1, Lingo-2, Lingo-3 and Lingo-4, comprising the following steps:
a) incubating a system as previously defined and a candidate molecule, said system comprising:
(i) a first coupling product comprising a monomer of a protein chosen from Lingo-1, Lingo-2, Lingo-3 and Lingo-4, and a probe capable of emitting a detectable signal which is modified when said monomer undergoes conformational changes,
(ii) a second coupling product comprising a monomer of a protein chosen from Lingo-1, Lingo-2, Lingo-3 and Lingo-4, and a probe capable of emitting a detectable signal which is modified when said monomer undergoes conformational changes,
b) detecting a signal emitted by at least one of the probes,
the signal revealing the binding of said candidate molecule to at least one of the coupling products.
For the purpose of the present invention, the term “incubating” means bringing together the first coupling product, the second coupling product and the candidate molecule. Advantageously, the incubation is carried out under conditions which allow the dimerization of the monomers of the first and second coupling products. Advantageously, the incubation is carried out under known conditions which allow the binding of the candidate molecule to at least one of the monomers or to the dimer, if the candidate molecule is a ligand of the protein. The term “candidate molecule” means any test molecule. It may be, for example, a protein, a peptide, a chemical molecule, an antibody, this list not being limiting. The molecule can, for example, be derived from a protein library, a cell suspension, a chemical library, this list not being limiting. The molecule can be a ligand of Lingo-1, Lingo-2, Lingo-3 or Lingo-4.
Advantageously, when the candidate molecule is not a ligand of the protein, the BRET signal, resulting from the formation of the dimer, is not modified following the addition of the candidate molecule. Advantageously, there is, in this case, no modification of a physical property of the monomers, of the dimer, of the probes or of the coupling products.
Advantageously, the variations in the BRET signal after the addition of a ligand may be monitored over time, for 30 minutes for example.
Advantageously, when the candidate molecule is a ligand of the protein, the BRET signal, resulting from the formation of the dimer, is modified, for example increased or decreased, following the addition of the candidate molecule. Advantageously, there is, in this case, a modification of at least one physical property of at least one monomer, of the dimer, of the probes or of the coupling products. Advantageously, the binding of the candidate molecule causes a conformational modification of at least one monomer, of the dimer, of the probes or of the coupling products.
Advantageously, the signal can be detected in the context of a BRET, FRET, TR-FRET or HTRF system.
Advantageously, the method of the invention can also comprise a step of adding a substrate which makes it possible for the signal to occur. It may, for example, be coelenterazine or a coelenterazine derivative, for example coelenterazine 400a, or DeepBlueC. The substrate can be added in any way known to those skilled in the art, for example exogenously, or added in the form of a nucleic acid encoding the substrate.
Advantageously, the method can comprise any step or condition required for implementing a BRET, FRET, TR-FRET or HTRF process. These steps are well known to those skilled in the art, and can comprise a step of measuring the signal, of calculating the emission and absorption wavelengths, of using filters in order to distinguish the wavelengths, of calculating the wavelength intensity by means, for example, of a photomultiplier tube or of a CDD (Charged Coupled Device) camera, of calculating energy transfer efficiency, of detecting the emission peaks, and of using instruments such as BRETCount, Mithras (Berthold), microplate scintillation and luminescence counters (Berthold, Packard Instruments).
The method can be implemented in vitro or in vivo. Advantageously, the method can be implemented on live cells. For example, the method can be implemented with stably or transiently transfected cell lines, under physiological conditions.
Advantageously, the method can be implemented on preparations of membranes, optionally prepared in advance and frozen, or on purified proteins.
Advantageously, the method can also comprise a high-throughput screening system. For the purpose of the present invention, the term “high-throughput screening system” means any means of automation which makes it possible to accelerate the identification method. It may be any means known to those skilled in the art, involving, for example, information technology, bioinformatics, genomics, proteomics, robotics, and sometimes nanotechnologies, this list not being limiting.
Advantageously, the method can also comprise a test which makes it possible to identify the agonist (activator) ligands and the antagonist (inhibitor) ligands of the proteins. It may, for example, be a biological test. It may be any test which makes it possible to identify the impact of a candidate molecule on at least one signaling pathway in which the protein plays a role, for example oligodendrocyte differentiation, as implemented, for example, in Mi et al., 2005, ([4]), axonal myelination, or the survival of dopaminergic neurons and of retinal ganglion cells, as implemented, for example, in Fu et al., 2009, ([9]).
Advantageously, the method can make it possible to determine whether, downstream, cell signaling pathways involving the RhoA or Akt protein kinases are activated or inhibited by the ligand. The effect of the ligands can, for example, be tested on the Rho pathway (RhoA test sold by Pierce) or on the Akt pathway (test using AlphaScreen from Perkin or by Western blotting analysis using specific antibodies from Cell Signaling).
Advantageously, the method can make it possible to determine whether ligands disrupt the interaction between the protein and the membrane receptors NgR1, EGFR and TrkB. In this regard, the interaction between Lingo-1 and NgR, Lingo-1-EGFR and Lingo-1/TrkB can be measured.
The invention also relates to a device, also called kit, for implementing the method as defined above, which can make it possible to study at least one interaction described above. The kit can comprise all or a part of the elements required for implementing the method, for example the coupling products, substrates, at least one means for detecting or measuring the BRET signal, this list not being limiting. The kit can, for example, contain the plasmids of the fusion proteins, for example Lingo-1-RLuc, Lingo-1-YFP, Lingo-2-RLuc, Lingo-2-YFP, Lingo-3-RLuc, Lingo-3-YFP, Lingo-4-RLuc, Lingo-4-YFP, NgR-RLuc, NgR-YFP, EGFR-RLuc, EGFR-YFP, TrkB-RLuc and TrkB-YFP, and also a substrate, for example coelenterazine.



BRIEF DESCRIPTION OF THE FIGURES
FIG. 1 represents coimmunoprecipitation experiments showing that Lingo-1 forms a dimer in HEK-293 cells. The HEK-293 cells are cotransfected (+ sign) with 5 μg of the pcDNA3, Lingo-1-HA or Lingo-1-YFP plasmids. A coimmunoprecipitation is carried out with agarose beads coupled to an anti-HA antibody (cf. part a, showing the anti-HA Western Blot or WB) or an anti-YFP antibody (cf. part b, showing the anti-YFP Western Blot or WB). The incubation of the corresponding membrane with an anti-HA antibody shows that the Lingo-HA protein has indeed been immunoprecipitated (a). Two immunoreactive bands appear specifically at approximately 120 kDa and 140 kDa and attest to the presence of Lingo-YFP in the immunoprecipitate (b).
FIG. 2 represents the validation of the anti-Lingo-1 antibody. The cell lysates originating from HEK-293 cells transfected with a control plasmid, pcDNA3 (mock), or the plasmid encoding the Lingo-1 protein fused at the N-terminal to an HA tag are analyzed on an acrylamide gel (8%). The use of an anti-HA antibody makes it possible to detect two immunoreactive bands at 80 and 100 kDa, probably corresponding to two different glycosylation states (cf. part a, showing the anti-HA Western Blot or WB). The presence of these same two immunoreactive bands can also be observed when the cell lysates are incubated with a Lingo-1 specific antibody (cf. part b, showing the anti-YFP Western Blot or WB).
FIG. 3 represents the detection of Lingo-1 dimers in rat cortical neurons in culture. Twelve days after placing in culture, the cortical neurons put out extensions and form a network (cf. part a). The neurons are treated (+DSP) or not treated (−DSP) with a bridging agent, DSP (cf part b: anti-Lingo-1 Western Blot or WB b). Lingo-1 is indeed expressed in the cortical neurons, as attested to by the presence of an immunoreactive band at 90 kDa when the Western blot is incubated in the presence of the anti-Lingo-1 antibody (lane: −DSP). When the neurons are treated with DSP, an additional immunoreactive band appears at 180 kDa corresponding to the dimeric form of Lingo-1, this being to the detriment of the monomer at 90 kDa, which disappears (lane: +DSP).
FIG. 4 represents the detection of the oligomerization of the Lingo-1 protein in live cells using the BRET technique. HEK-293 (parts a and c) and SHSY-5Y (parts b and d) cells were transfected with a fixed concentration of donor (100 ng of Lingo-1-RLuc plasmid) and increasing concentrations of acceptors, Lingo-1-YFP or the YFP fluorescent protein alone (from 50 ng to 4000 ng of YFP-N1 plasmids). The BRET measurement is carried out 48 hours after transfection on the live cells using the sets of old filters (parts a and b) and of new filters (parts c and d). The BRET signal is measured in mBu, for the Lingo-1-RLuc/Lingo-1-YFP product (squares) and the Lingo-1-RLuc/YFP-N1 product (circles).
FIG. 5 represents the specificity of the interaction between Lingo-1-RLuc and Lingo-1-YFP. HEK-293 cells are transfected with the Lingo-1-RLuc and Lingo-1-YFP fusion proteins (with a constant donor/acceptor ratio) and increasing concentrations of the whole Lingo-1 protein (amino acids 1-620, “Lingo-HA” lane) or its dominant negative (amino acids 1-577, “DN-HA” lane) tagged with HA. The BRET measurement (mBu) is carried out 48 hours after the transfection on live cells. The HA-tagged constructs significantly reduce the BRET signal obtained between Lingo-1-RLuc and Lingo-1-YFP (“Control” lane) (p<0.001 vs control, ANOVA, NewMan Keuls).
FIG. 6 shows the detection of the oligomerization of the Lingo-1, Lingo-2, Lingo-3 and Lingo-4 proteins in live cells using the BRET technique in 96-well plates (a) and in 384-well plates (b). HEK-293 cells are transfected with a fixed concentration of donor (100 ng of Lingo-1-RLuc, Lingo-2-RLuc or Lingo-3-RLuc or Lingo-4-RLuc plasmid) and increasing concentrations of acceptors (from 50 ng to 4000 ng of the Lingo-1-YFP, Lingo-2-YFP or Lingo-3-YFP or Lingo-4-YFP plasmids). (a) The BRET signal is measured (mBU) for Lingo-2-RLuc/Lingo-2-YFP (circles), Lingo-3-RLuc/Lingo-3-YFP (triangles) and Lingo-4-RLuc/Lingo-4-YFP (diamonds). (b) The BRET signal is measured (mBU) for Lingo-1-RLuc/Lingo-1-YFP. The BRET measurement is carried out 48 hours after transfection on the live cells. The BRET ratio was calculated with a set of filters, using the 540±40 nm filter for the light emitted by eYFP and the 480±20 nm filter for the light emitted by RLuc.
FIG. 7 represents stable HEK-293 cell lines expressing hLingo-1-RLuc or hLingo-1-YFP.
FIG. 8 represents the validation and the specificity of the screening test using the system of the invention with a known inhibitor of Lingo-1 (DN-Lingo-1).
FIG. 9 represents (a, b) the identification and the validation of potential ligands of Lingo-1 (dimerization inhibitors) using the system of the invention, (c) the kinetics of the effect of the specific ligand B27 thus identified.
FIG. 10 represents the study of the specificity of B27 in a counter-screening test (study of the signal originating from the dimerization of the (β2 adrenergic receptor, (β2AR).



EXAMPLES
Materials and Methods
1. Cells
The HEK-293 (human embryonic kidney fibroblasts, ATCC CRL-1573) and SH-SY5Y (human neuroblastomas, ATCC CRL-2266) cell lines are cultured in a DMEM medium (Dulbecco's Modified Eagle Medium) supplemented with fetal calf serum (10%, BioWest, France), penicillin (100 U/ml, Eurobio), streptomycin (100 μg/ml, Eurobio, France) and L-glutamine (2 mM, Eurobio). The primary neuron cultures are prepared from the cortex of rat embryos taken at E18 (embryonic day 18). The neurons are cultured in NeurobasalTM (Gibco®, Invitrogen, composed of glycine, L-alanine, L-arginine hydrochloride, L-asparagine-H2O, L-cysteine, L-histidine hydrochloride-H2O, L-isoleucine, L-leucine, L-lysine hydrochloride, L-methionine, L-phenylalanine, L-proline, L-serine, L-threonine, L-tryptophan, L-tyrosine, L-valine, choline chloride, D-calcium pantothenate, folic acid, niacinamide, pyridoxine hydrochloride, riboflavin, thiamine hydrochloride, vitamin B12, i-inositol, calcium chloride, ferric nitrate, magnesium chloride, potassium chloride, sodium bicarbonate, sodium chloride, monobasic sodium phosphate, zinc sulfate, HEPES, phenol red) supplemented with B27 (composed in particular of d-biotin, bovine serum albumin, free of fatty acid, catalase, L-carnitine HCl, corticosterone, ethanolamine HCl, D-galactose, glutathione, insulin, linoleic acid, linolenic acid, progesterone, putrescine.2HCl, sodium selenite, superoxide dismutase, T-3/albumin, DL-alpha-tocopherol, DL-alpha-tocopherol acetate, transferrin, vitamin A acetate, Gibco®), penicillin (100 U/ml) and streptomycin (100 μg/ml). The cells are cultured in incubators at 37° C. under a humid atmosphere, at 5% CO2.

2. Construction of the Expression Vectors:

Five expression vectors were used: Lingo-1-HA: the sequence encoding the human Lingo-1 protein (620 amino acids, accession number NP_116197) followed by a hemagglutinin epitope tag (HA) was amplified by PCR (for “Polymerase Chain Reaction”). The fragment obtained was introduced into the pcDNA3 plasmid (Invitrogen). The Lingo-1-HA plasmid has the sequence SEQ ID No. 19.
DN-Lingo-1-HA: the sequence encoding the extracellular part of Lingo and also its transmembrane domain (amino acids 1-577), followed by a hemagglutinin epitope tag (HA), was amplified by PCR. The fragment obtained was introduced into the pcDNA3 plasmid (supplied by Invitrogen, 5446 bp in size, containing in particular a cytomegalovirus promoter, a T7 sequencing primer, an ampicillin resistance gene and the neomycin selection gene).

Lingo-1-RLuc: contains the sequence encoding the whole human Lingo-1 protein (620 amino acids) 3′ tagged with the sequence of a luminescence label (Renilla luciformis). The Lingo-1-RLuc plasmid has the sequence SEQ ID No. 17.

Lingo-1-eYFP: contains the sequence encoding the whole human Lingo-1 protein (620 amino acids) 3′ tagged with the sequence of a yellow fluorescence label (eYFP: enhanced yellow fluorescent protein), the physical characteristics of which, in particular its fluorescence intensity, have been improved compared with YFP. The Lingo-1-eYFP plasmid has the sequence SEQ ID No. 18.

pRLuc-N1: contains the coding sequence of Renilla luciferase (RLuc) downstream of a multiple cloning site. This plasmid was constructed from the commercially available plasmid pEYFP-N1 by removing the sequence of eYFP and replacing it with the sequence of RLuc. The pRLuc-N1 plasmid has the sequence SEQ ID No. 15.

The Lingo-1-RLuc and Lingo-1-eYFP fusion proteins were obtained by conventional subcloning techniques (digestion of DNA fragments or plasmids with the restriction enzymes Sal I, BamH I, NotI, Hind III, EcoR I, Nhe I; ligation of DNA fragments in a plasmid; transformation of DH5α competent bacteria, Invitrogen), using a commercially available expression vector (peYFP-N1, Clonetech) or an expression vector available in the laboratory (pRLuc-N1) having multiple cloning sites adjacent to the coding sequences for Renilla luciferase (Rluc) or the eYFP fluorescent protein.
The Lingo-1-Rluc and Lingo-1-eYFP fusion proteins were obtained by fusing RLuc and eYFP to the C-terminal end of the human form of Lingo (620 amino acids, accession number NP_116197). The sequence of Lingo-1 devoid of its stop codon was amplified by PCR. The fragment obtained was introduced into the peYFP-N1 plasmid (supplied by Clontech, of sequence SEQ ID No. 16), approximately 4700 bp in size, containing in particular a CMV promoter, an EGFP-N sequencing primer of sequence 5′d[CGTCGCCGTCCAGCTCGACCAG]3′ (SEQ ID NO: 20), a kanamycin resistance gene, a neomycin resistance gene for selection (with G418 or geneticin) of mammalian cells, in order to obtain the construct Lingo-1-eYFP, or into the pRLuc-N1 plasmid in order to obtain the construct Lingo-1-Rluc.
3. Transfection of the Constructs
For the Western Blot and coimmunoprecipitation experiments (FIG. 1), the HEK-293 cells are transfected using a transfection lipid: LipofectamineTM LTX (InvitrogenTM).
The day before the transfection, the cells are distributed, in their culture medium without antibiotics, onto collagen-coated dishes (collagen type I, Serva).
The HEK-293 cells are cotransfected with 5 μg of various plasmids. Two days after transfection, the cells are lysed in a buffer containing 0.5% of triton X100. On the day of the transfection, the plasmid DNA (5 to 10 μg) diluted in Optimem medium (3 ml, Cat No. 31985-047 Gibco®, Invitrogen, composed in particular of L-glutamine, 2400 mg/L of sodium bicarbonate, HEPES, sodium pyruvate, hypoxanthine, thymidine, growth factors, mg/L phenol red) is incubated for 5 minutes in the presence of Plus ReagentTM (10 μl, Cat No. 11514015, InvitrogenTM). The lipofectamine LTX (20 μl, Cat No. 15338-500, InvitrogenTM) is then added to the mixture and incubated for 30 minutes. After the medium has been changed (7 ml/dish), the complex formed by the DNA and the lipofectamine is added dropwise to the cells. In order to avoid a toxic effect of the lipofectamine, the transfection medium is drawn off after 5 hours and it is replaced with fresh medium without antibiotics.
For the BRET experiments, the HEK-293 cells cultured in 6-well plates, pre-treated with collagen, are transiently transfected using the calcium phosphate technique. Briefly, the plasmid DNA is mixed into a phosphate buffer (2×BBS, composed in particular of 50 mM BES (Cat No. B4554, Sigma), 280 mM NaCl, 1.5 mM Na2HPO4, adjusted to pH 6.95) with calcium chloride (2.5 M). After incubation for 15 minutes, the precipitate obtained is applied to the cells for 24 hours. The cells are then rinsed and incubated for a further 24 hours in a culture medium without antibiotics.
For the BRET experiments, the SH-SY5Y cells cultured in 6-well plates, pre-treated with collagen, are transiently transfected using a transfection lipid: lipofectamine 2000 (Cat No. 11668019, InvitrogenTM). On the day of the transfection, the plasmid DNA diluted in Optimem medium (500 μl) is mixed with the lipofectamine 2000 (5 μl) and then incubated for 20 minutes. After the medium has been changed (2 ml of fresh medium without antibiotics, per well), the complex is applied to the cells for 24 hours. The cells are then rinsed and incubated for a further 24 hours in the culture medium without antibiotics.
4. Preparation of the Lysates, Chemical Bridging, Western Blotting
When chemical bridging is carried out, it is performed on the live cells before lysis. After having rinsed the cells with PBS, the cells are incubated with DSP (Lomant's Reagent, dithiobis[succinimidyl propionate], Cat No. 22585, Thermo Scientific) (160 μg/ml, diluted in an HBSS buffer) for 10 minutes at ambient temperature. After three rinses with PBS (“phosphate buffered saline”), the cells are then lysed. The lysis is carried out on ice. After three rinses with cold PBS, the cells are incubated in the lysis buffer for 10 minutes (50 mM Tris/HCl, pH 7.5, Triton X100 from 0.5 to 1% depending on the experiments, 100 mM NaCl, 50 mM NaF, 5 mM EDTA, 10 μg/ml of aprotinin, 10 mM sodium pyrophosphate), to which a cocktail of protease and phosphatase inhibitors is added. The cells are then recovered using a scraper and then centrifuged (15 minutes at 22 000 g at 4° C.). The supernatant is stored at −20° C., loaded onto a gel or used directly for the coimmunoprecipitation experiments. To do this, a fixed amount of the cell lysate is incubated with the beads of agarose coupled to the anti-HA antibody (40 μl of beads, EZview, Sigma), at 4° C., on a wheel overnight. After 5 successive washes with the lysis buffer, the immunoprecipitate is recovered by adding 40 μl of 4× Laemmli buffer (200 mM Tris, pH 6.8, 4% SDS, 40% glycerol, 0.5 mM β-mercaptoethanol, 0.02% bromophenol blue). The cell lysates and the immunoprecipitates are then loaded onto an 8% acrylamide gel under denaturing and reducing conditions. The electrophoresis is carried out in a Tris/glycine migration buffer, and then the proteins are transferred onto a PVDF Hybond-P membrane (Amersham Biosciences). The membrane is saturated in a solution of TBS-T (0.1% Tween 20)-5% skimmed milk for 1 hour. The primary antibodies are incubated in the same solution for 2 hours at ambient temperature: anti-Lingo (antibodies directed against amino acids 40 to 556 of the Lingo-1 sequence; accession number AAH11057, Cat No. AF3086, R &D Systems) at 1/1000th, anti-HA (Roche, rat) at 1/5000th, Living Colors® antibody (Clontech) at 1/5000th. Several washes in TBS-T (Tris buffered saline—0.1% Tween 20) are carried out before incubating the membrane with the corresponding secondary antibodies coupled to peroxydase at a 1/33000th dilution in TBS-T (0.1% Tween 20)—5% skimmed milk for 1 hour. Once again, after several washes, the membrane is incubated with the substrate (SuperSignal West Dura, Pierce) for 5 minutes before exposure on film (CL-Xposure, Amersham Biosciences).
5. Measurement of BRET, Luminescence and Fluorescence
The cells are resuspended in a physiological buffer (HBSS for Hank's Buffered Salt Solution (Cat No. 14025100, Gibco, composed in particular of calcium chloride, magnesium chloride, magnesium sulfate, potassium chloride, monobasic potassium phosphate, sodium bicarbonate, sodium chloride, anhydrous dibasic sodium phosphate)) 48 hours after transfection and distributed into 96-well plates (100 000 to 200 000 cells per well) or into 384-well plates (20 000 to 40 000 cells per well). The fluorescence and luminescence readings are carried out at ambient temperature on a microplate reader, Mithras LB 940 (Berthold). The yield from transfection of the Lingo-1-eYFP, Lingo-2-eYFP, Lingo-3-eYFP or Lingo-4-eYFP construct is evaluated by reading the fluorescence of the eYFP emitted at 530-540 nm after excitation at 480 nm. The yield from transfection of the Lingo-1-RLuc, Lingo-2-RLuc, Lingo-3-RLuc or Lingo-4-RLuc construct is evaluated by reading the luminescence (1 second) obtained immediately after the addition of the coelenterazine substrate (5 μM).
The measurement of the BRET signal is carried out immediately after the addition of coelenterazine. This causes the appearance of a bioluminescence signal with an emission peak at 480 nm, in agreement with the spectral properties of the Rluc. The energy transfer which takes place in the fusion protein between the Rluc and the eYFP causes the appearance of a fluorescence signal with a maximum emission at 530-540 nm, characteristic of the spectral properties of eYFP. The “BRET ratio” is defined as the ratio of the value of light emitted by the eYFP (530-540 nm) to the value of the light emitted by the RLuc (480 nm) after subtraction of the ratio obtained when the RLuc construct is expressed alone. Two sets of BRET filters were used: the first pair of filters (termed old filters) corresponds to a 480 nm filter, bandwidth of 20 nm, light transmittance of 60% and to a 530 nm filter, bandwidth of 25 nm, light transmittance of 60%. The second set of filters (termed new filters) corresponds to a 480 nm filter, bandwidth of 20 nm, light transmittance of 75% and to a 540 nm filter, bandwidth of 40 nm, light transmittance of 75%. The results are expressed in milliBRET (mBu) which corresponds to the value of the BRET ratio multiplied by 1000.
The BRET measurement can also be carried out on adherent cells. In this case, the substrate is preincubated for 15 minutes before the luminescence reading.
6. Method of Screening for Lingo-1 Ligands
HEK-293 cells cultured in dishes (10 cm diameter) were transfected with Lingo-1-RLuc (250 ng) and Lingo-1-YFP (1500 ng) so as to reach approximately 50-60% of the BRETmax. The cells were resuspended in a physiological buffer (HBSS) 48 hours after transfection and distributed in 80 μl into 96-well plates (100 000 to 200 000 cells per well).
Prior to the addition of the cells, the various ligands (generally used at a final concentration of 20 μM) were distributed into the 96-well plates (in 10 μl).
After an incubation time of 15 minutes, coelenterazine h (Interchim) was added (in 10 μl) so as to obtain a final concentration of 5 μM.
The BRET reading was carried out at ambient temperature on a microplate reader, Mithras LB 940 (Berthold), using the new BRET filters. These experiments are also carried out using a stable line expressing Lingo-1-RLuc and Lingo-1-YFP.
7. Counter-Screening Test
The specificity of the effect of the molecules identified by means of our method was evaluated on the basis of the BRET signal originating from the formation of dimers of the β2 adrenergic receptor (counter-screening test).
To do this, HEK-293 cells were transfected with the β2 adrenergic receptor fused at the C-terminal with Renilla luciferase (β2AR-RLuc, 50 ng) acting as donor, and increasing concentrations of β2 adrenergic receptor fused at the C-terminal with the yellow fluorescent protein (β2AR-YFP, from 50 to 1000 ng), acting as acceptor. The cells were resuspended in a physiological buffer (HBSS) 48 hours after transfection and distributed into 96-well plates, in 80 μl (100 000 to 200 000 cells per well).
Prior to the addition of the cells, the products were distributed into the 96-well plates (in 10 μl).
After an incubation time of 15 minutes, coelenterazine h (Interchim) was added (in 10 μl) so as to obtain a final concentration of 5 μM. The BRET reading was carried out at ambient temperature on a microplate reader, Mithras LB 940 (Berthold), using the new BRET filters.
Example 1
Lingo-1 Forms a Dimer In Vivo
1/Lingo-1 Forms a Dimer in HEK-293 Cells
HEK-293 cells were first of all cotransfected with two plasmids which allow the expression of the Lingo-1 protein tagged with HA and Lingo-1 tagged with a fluorescent protein, YFP. Communoprecipitation experiments showed that, when the HA-tagged Lingo-1 protein is immunoprecipitated with agarose beads coupled to an anti-HA antibody (FIG. 1a), the YFP-tagged Lingo-1 protein is coimmunoprecipitated (FIG. 1b). These results show, for the first time, that Lingo-1 formed a dimer in vivo. These results also show for the first time that the whole and membrane form of Lingo-1 was capable of forming dimers in HEK-293 cells.
2/Lingo-1 Forms a Dimer in Neurons
After validation of the anti-Lingo-1 antibody (FIG. 2), existence of dimers in cortical neurons in culture was tested (FIG. 3a). Given that certain detergents used during the lysis can induce the formation of protein aggregates and result in the artefactual formation of dimers, the effect of chemical bridging before lysis of the cells was tested (FIG. 3b).
Twelve days after having been placed in culture, cortical neurons put out extensions and formed a network (cf. FIG. 3, part a). The neurons were treated (+DSP) or not treated (−DSP) with a bridging agent, DSP, which acts at the level of the amine functions, before being lysed in a buffer containing 1% of triton X-100. Lingo-1 was indeed expressed in the cortical neurons, as attested to by the presence of an immunoreactive band at 90 kDa when the Western blot is incubated in the presence of the anti-Lingo-1 antibody (cf. FIG. 3, part b). When the neurons were treated with DSP, Lingo-1 indeed appeared in the form of dimers at 180 kDa (additional immunoreactive band appears at 180 kDa), this being to the detriment of the monomer at 90 kDa, which disappeared (cf. FIG. 3, part b).
Example 2
Detection of the Oligomerization of the Lingo-1 Protein in Live Cells Using the Bret Technique
HEK-293 or SHSY-5Y cells were transfected with a fixed concentration of donor (100 ng of Lingo-1-RLuc plasmid) and increasing concentrations of acceptors, Lingo-1-YFP or the YFP fluorescent protein alone (from 50 ng to 4000 ng of plasmids). The BRET measurement was carried out 48 hours after transfection on live cells using sets of old filters and of new filters.
The BRET signal observed with the old filters corresponding to the oligomerization of Lingo-1 is clearly specific since, when Lingo-1-RLuc and Lingo-1-YFP were coexpressed, the signal increased according to a hyperbolic curve and reached an asymptote in the two cell types (cf. FIGS. 4a and 4b). Furthermore, when Lingo-1-Rluc was coexpressed with the YFP protein alone, the BRET signal was much weaker (<20 mBU) and increased in a linear manner (cf. FIG. 4a, insert); thereby attesting to the appearance of nonspecific and random interactions.
These results show that the Lingo-1-RLuc and Lingo-1-YFP fusion proteins were capable of interacting so as to form dimers (or even oligomers) in live cells in culture. Indeed, with the old filters, a very strong BRET signal (of 100 to 800 mBu) was observed after coexpression of Lingo-1-Rluc and Lingo-1-YFP in HEK-293 cells (cf. FIG. 4a). Since Lingo-1 is exclusively expressed in the nervous system, it was also investigated whether Lingo-1 was capable of dimerizing in a more physiological environment, i.e. in human neuroblastoma cultures (cf. FIG. 4b). Once again, with the old filters, a very strong BRET signal (of 100 to 600 mBu) was observed. This signal is specific since, when saturation curves were plotted, the BRET signal reached a plateau both in the HEK-293 cells (cf. FIG. 4a) and in the SH-SY5Y cells (cf. FIG. 4b). In conclusion, the dimerization of Lingo-1 induced a strong and specific BRET signal.
In addition, the results obtained with a BRET measurement carried out under the same conditions with the set of new filters in the HEK-293 cells (cf. FIG. 4c) and in the SHSY-5Y human neuroblastoma cells (cf. FIG. 4d) coexpressing Lingo-1-Rluc and Lingo-1-YFP, made it possible to optimize the test, with a BRET signal that was twice as strong as previously being obtained.
Example 3
The Interaction Between Lingo-1-RLuc and Lingo-1-YFP is Specific and can be Modulated
HEK-293 cells were transfected with the Lingo-1-RLuc and Lingo-1-YFP fusion proteins (the donor/acceptor ratio remains constant) and increasing concentrations of the whole Lingo-1 protein (Lingo-1-HA, amino acids 1-620) or the dominant negative thereof (DN-HA, amino acids 1-577) tagged with HA. The BRET measurement was carried out 48 hours after the transfection on live cells. The HA-tagged constructs significantly decreased the BRET signal obtained between Lingo-1-RLuc and Lingo-1-YFP (p<0.001 vs control, ANOVA, NewMan Keuls). The results are shown in FIG. 5.
The BRET signal obtained between the Lingo-1-RLuc and Lingo-1-YFP proteins is therefore specific and can be modulated. Indeed, when the HA-tagged whole Lingo-1 protein (620 amino acids) or the HA-tagged dominant negative of Lingo-1 (577 amino acids, without the cytoplasmic portion of the protein) was overexpressed at 800 ng, the BRET signal obtained between Lingo-1-RLuc and Lingo-1-YFP decreased significantly (FIG. 5a). In addition, when increasing concentrations of HA-tagged Lingo-1 protein were added (Lingo-1-HA at 0, 200, 400, 800, 1600 and 2600 ng), the BRET signal decreased (FIG. 5b); thereby making it possible to plot inhibition curves.
Thus, the results show that the HA-tagged Lingo-1 proteins prevent, by competition, the interaction of Lingo-1-RLuc with Lingo-1-YFP.
Example 4
Detection of the Oligomerization of the Lingo-1, Lingo-2, Lingo-3 and Lingo-4 Proteins in Live Cells Using the BRET Technique
HEK-293 cells were transfected with a fixed concentration of donor (100 ng of Lingo-1-RLuc, Lingo-2-RLuc or Lingo-3-RLuc or Lingo-4-RLuc plasmid) and increasing concentrations of acceptors, Lingo-2-YFP or Lingo-3-YFP or Lingo-4-YFP (from 50 ng to 4000 ng of plasmids). The BRET measurement was carried out 48 hours after the transfection on live cells in 96-well and 384-well plates.
The BRET signal observed in 96-well plates, corresponding to the oligomerization of Lingo-2 and Lingo-3 and Lingo-4, is clearly specific since it increased according to a hyperbolic curve and reached an asymptote for each pair of proteins (cf. FIG. 6a).
In addition, the results show that a BRET signal obtained with Lingo-1 could be detected without any difficulty when the measurements were carried out in 384-well plates (high/medium-throughput). Moreover, the calculation of Z′ (value p>0.85) shows how robust the test is (FIG. 6b).
The applicant therefore showed that it is possible, by measuring the luminescence transfer (BRET) between each modified monomer, to detect conformational variations in the dimer in vivo, in live cells in real time. In addition, it showed that the BRET measurement can be carried out on adherent cells or cells in suspension in 96-well plates and that it is adaptable to high/medium-throughput.
Example 5
Construction of Stable Lines
In order to ensure reproducibility of the high/medium-throughput BRET measurements, stable lines of HEK-293 cells expressing Lingo-1-RLuc or Lingo-1-YFP were developed.
The HEK-293 cells were transfected as previously described with Lingo-1-RLuc or Lingo-1-YFP plasmids carrying in particular neomycin resistance, as previously described. The cells having integrated the plasmid DNA were selected 48 hours after the transfection by adding the selection antibiotic (G-418 or geneticin at 2 mg/ml) to the culture medium. After two to three weeks of selection, several tens of antibiotic-resistant clones were recovered. Each clone was subsequently amplified so as to obtain a sufficient amount of cells to carry out the Lingo-1 expression test. This test consists in reading fluorescence in the case of clones of cells expressing Lingo-1-YFP or in reading luminescence after the addition of coelenterazine in the case of clones of cells expressing Lingo-1-RLuc, as previously described.
The results show that, among the clones tested (approximately a hundred or so clones tested) expressing Lingo-1-RLuc, several clones (for example C5, D1 and D2) express hLingo-1-RLuc at a satisfactory level (FIG. 7a) and were frozen. In addition, clone C5, which exhibits a good luminescence signal with good cell viability, was deposited, on 31 Mar., 2011, with the CNCM [National Collection of Microorganism Cultures] (Institut Pasteur, Paris) under number I-4463.
The results show a clone Y1 expressing hLingo-1-YFP at a satisfactory level, namely exhibiting a good fluorescence signal with good cell viability (FIG. 7b). In addition, clone Y1 was deposited, on 31 Mar., 2011, with the CNCM (Institut Pasteur, Paris) under number 1-4462.
Likewise, stable lines of HEK-293 cells coexpressing Lingo-1-RLuc and Lingo-1-YFP are developed. The clones coexpressing the two proteins at a satisfactory level are frozen and deposited with the CNCM.
Example 6
Method for Identifying Lingo-1 Ligands Using the System of the Invention
Validation of the Method of Identification of the Invention
HEK-293 cells were transfected with a fixed concentration of Lingo-1-RLuc (250 ng) and Lingo-1-YFP (1500 ng) so as to achieve approximately 50-60% of the BRETmax. The BRET measurement using the set of new filters was carried out 48 hours after the transfection on live cells in 96-well plates, in the presence of increasing concentrations of an HA-tagged known Lingo-1 inhibitor (DN-Lingo-1-HA at 0, 200, 400, 800, 1600 and 2600 ng) or of an HA-tagged neuronal protein which does not interact with Lingo-1 (Larp6-HA at 0, 200, 400, 800, 1600 and 2600 ng).
The results show that the HA-tagged known Lingo-1 inhibitor (DN-Lingo-1-HA) significantly decreased the BRET signal obtained between the Lingo-1-RLuc and Lingo-1-YFP proteins (FIG. 8a). In addition, the results show that this inhibitory effect was specific since the neuronal protein which does not interact with Lingo-1, called HA-tagged Larp6, did not modify the BRET signal (FIG. 8b).
Identification and Validation of a New Lingo-1 Dimerization Inhibitor: B27
HEK-293 cells were transfected with a fixed concentration of Lingo-1-RLuc (250 ng) and Lingo-1-YFP (1500 ng) so as to achieve approximately 50-60% of the BRETmax. The BRET measurement using the set of new filters was carried out 48 hours after the transfection on live cells in 96-well plates, in the presence of a fixed concentration of ligands (BDNF, N2, Fsk, pDBU at a final concentration of 20 μM) or of a solution of B27 (50×B27, Gibco, reference 17-504-044, 10 μl of pure solution), which are capable of modifying the BRET signal obtained between the Lingo-1-RLuc and Lingo-1-YFP proteins.
The results show that the B27 compound significantly decreased the dimerization of Lingo-1 in the HEK-293 cells (FIG. 9a). The effect of the B27 compound was reproduced at various levels of BRET and with various batches of B27: B27 without antioxidant (B27AO), B27 without insulin (B27ins) or without vitamin A (B27VitA) (FIG. 9b).
A kinetic study of the effect of B27 was also carried out, and revealed that the decrease caused by B27 was gradual and reached a plateau (−15%) after 10 minutes (FIG. 9c).
Counter-Screening Test: Specificity of B27
The specificity of B27 was evaluated in a counter-screening test (study of the signal originating from the dimerization of the β2-adrenergic receptor, β2AR), this dimerization having been previously described by Angers et al. [14].
HEK-293 cells were transfected with (i) a dimer of the β2AR receptor: the β2 adrenergic receptor fused at the C-terminal with Renilla luciferase (β2AR-RLuc, 50 ng) acting as donor, and increasing concentrations of β2 adrenergic receptor fused at the C-terminal with the yellow fluorescent protein (β2AR-YFP, from 100 to 2000 ng) acting as acceptor, or (ii) a dimer of Lingo-1: Lingo-1-RLuc acting as donor, and increasing concentrations of Lingo-1-YFP (from 100 to 2000 ng) acting as acceptor. The BRET measurement using the set of new filters was carried out 48 hours after the transfection on live cells in 96-well plates, in the absence or presence of a fixed concentration of B27 (50×B27, Gibco, reference 17-504-044, 10 μl of pure solution).
The results show that the BRET signal originating from the dimerization of the β2-adrenergic receptor was not modified in the counter-screening test (FIG. 10b), unlike the BRET signal originating from the dimerization of Lingo-1 (FIG. 10a).
The applicant thus showed that the screening method using the system of the invention makes it possible to identify specific ligands of Lingo-1.
REFERENCES


    1. Carim-Todd et al., “LRRN6A/LERN1 (leucine-rich repeat neuronal protein 1), a novel gene with enriched expression in limbic system and neocortex”. 2003, Eur. J; Neurosci. 18 (12):3167-82.
    2. Mi et al., “LINGO-1 is a component of the Nogo-66 receptor/p75 signaling complex”. 2004, Nat Neurosci. 7(3):221-8.
    3. Ji et al., “LINGO-1 antagonist promotes functional recovery and axonal sprouting after spinal cord injury”, 2006, Mol Cell Neurosci. 33(3):311-20.
    4. Mi et al., “Lingo-1 negatively regulates myelination by oligodendrocytes”, 2005, Nature Neuroscience 8: 745-751.
    5. Lee et al. “NGF regulates the expression of axonal LINGO-1 to inhibit oligodendrocyte differentiation and myelination”. 2007, J Neurosci. 27(1):220-5.
    6. Zhao et al. “An in vitro study on the involvement of LINGO-1 and Rho GTPases in Nogo-A regulated differentiation of oligodendrocyte precursor cells”, 2007, Mol Cell Neurosci. 36(2):260-9.
    7. Mi et al., “Lingo-1 antagonist promotes spinal cord remyelination and axonal integrity in MOG-induced experimental autoimmune encephalomyelitis”, 2007, Nature medicine 13: 1228-1233.
    8. Fu et al., “Blocking LINGO-1 function promotes retinal ganglion cell survival following ocular hypertension and optic nerve transection”. Invest Ophthalmol Vis Sci. 2008, 49(3):975-85.
    9. Fu et al., “Combined effect of brain-derived neurotrophic factor and Lingo-1 fusion protein on long-term survival of retinal ganglion cells in chronic glaucoma”, 2009, Neuroscience 162: 375-382.
    10. Zhao et al. “Inactivation of glycogen synthase kinase-3beta and up-regulation of LINGO-1 are involved in LINGO-1 antagonist regulated survival of cerebellar granular neurons”, 2008, Cell Mol Neurobiol. 28(5):727-35.
    11. Inoue et al., “Inhibition of the leucine-rich repeat protein Lingo-1 enhances survival, structure, and function of dopaminergic neurons in Parkinson's disease models”, 2007, PNAS 104: 14430-14435.
    12. Mosyak et al., “The structure of Lingo-1 ectodomain, a module implicated in CNS repair inhibition”, 2006, J. Biol. Chem. 281: 36378-36390.
    13. Xu et al., “A bioluminescence resonance energy transfer (BRET) system: application to interacting circadian clock proteins” (1999) Proc. Natl. Acad. Sci. USA. 96, 151-156.
    14. Angers 2000, “Detection of beta 2-adrenergic receptor dimerization in living cells using bioluminescence resonance energy transfer (BRET)” (2000) Proc. Natl. Acad. Sci. USA 97: 3684-3689.
    15. Mi et al., “Lingo-1 and its role in CNS repair”, 2008, The Int. J. Bioch. Cell. Biol. 40: 1971-1978.
    16. Rudick et al., “Lingo-1 antagonists as therapy for multiple sclerosis: in vitro and in vivo evidence”, 2008, Expert Opin. Biol. Ther. 8: 1561-1570.
    17. Kamal et al., “Improved donor/acceptor BRET couples for monitoring beta-arrestin recruitment to G protein-coupled receptors”. Biotechnol J. 2009 September; 4(9):1337-44.
    18. Kocan M et al., “Demonstration of improvements to the bioluminescence resonance energy transfer (BRET) technology for the monitoring of G protein-coupled receptors in live cells”, J Biomol Screen. 2008 October; 13(9):888-98.
    19. Michelini E et al., “Luminescent probes and visualization of bioluminescence”, Methods Mol Biol. 2009; 574:1-13.
    20. Kroeger et al., “Constitutive and agonist-dependent homo-oligomerization of the thyrotropin-releasing hormone receptor” (2001) J Biol Chem, 276, 12736-12743.
    21. Maurel D et al., “Cell-surface protein-protein interaction analysis with time-resolved FRET and snap-tag technologies: application to GPCR oligomerization” (2008), Nat Methods (6):561-7.
    22. Hermand P et al., “Functional adhesiveness of the CX3CL1 chemokine requires its aggregation. Role of the transmembrane domain” (2008) J Biol Chem 283(44):30225-34.
    23. Whitfield J et al., “High-throughput methods to detect dimerization of Bcl-2 family proteins” (2003), Anal Biochem, 322(2):170-8.

## Claims

1. A detection system comprising:
a) a first coupling product (i) comprising a monomer (A) of a whole or membrane form of a protein selected from the group consisting of: Lingo-1, Lingo-2, Lingo-3 and Lingo-4, and (ii) a first probe which is a fluorescent protein selected from the group consisting of GFP (“Green Fluorescent Protein”), YFP (“Yellow Fluorescent Protein”), Enhanced Yellow Fluorescent Protein (eYFP), eGFP, GFP2, GFP10, RGFP (Renilla Green Fluorescent Protein”) and YPet, said first probe being capable of emitting a detectable signal when said monomer (A) undergoes conformational changes,
b) a second coupling product comprising (i) a monomer (B) of a whole or membrane form of a protein selected from the group consisting of: Lingo-1, Lingo-2, Lingo-3 and Lingo-4, and (ii) a second probe which is a luciferase selected from the group consisting of Renilla luciferase, RLuc8, firefly luciferase, Gaussia luciferase and Aequorin, said second probe being capable of emitting a detectable signal when said monomer (B) undergoes conformational changes,
wherein said first coupling product and said second coupling product form a dimer by interaction between said monomers (A) and (B) and,
said dimer undergoes a conformational change upon binding to a candidate molecule, and said conformational change causes a change of the detectable signal emitted by said first and second probes, generated by a transfer of energy from the first probe to the second probe,
and wherein said system is in the form of a cell suspension, or a culture of adherent cells, membranes or tissues.

2. The detection system as claimed in claim 1, said system being a bioluminescence resonance energy transfer system.

3. A method for identifying a ligand of a protein chosen from Lingo-1, Lingo-2, Lingo-3 and Lingo-4, comprising the following steps:
a) incubating a system as defined in claim 1 and a candidate molecule,
b) detecting a modification of the signal emitted by the second probe,
said modification of the signal revealing the binding of said candidate molecule to at least one of said coupling products.

4. The method as claimed in claim 3, performed with a high-throughput screening system.

5. The system as in claim 1, comprising a high-throughput screening system.