Patent Abstract:
the invention relates to a method for at least in part inhibiting differentiation of stem cells in a population of mammalian cells comprising up - regulating a wnt - signaling pathway to a differentiation - inhibiting level in the population of cells . the invention increases at the same time the number of stem cells in a population of mammalian cells compared to a reference population , and induces , at least in part , apoptosis in mesenchymal cells in a population of mammalian cells . the invention also discloses a method for selective differentiation of a stem cell , comprising controlling the level of wnt pathway activation . the invention is used for the proliferation and subsequent differentiation of embryonic stem cells and lung stem cells , and for ex vivo lung explant cultivation .

Detailed Description:
“ lung tissue ”, as used herein , comprises at least two or more of the following compartments : the alveolar compartment , the bronchiolar compartment , the bronchial compartment and the tracheal compartment . a “ lung cell ” is defined herein as an epithelial cell of at least one of these four compartments . a population of lung cells comprises at least two lung cells . in this invention in a preferred embodiment , a population of lung cells together is capable of functioning in the exchange of gasses between the organism &# 39 ; s blood compartment and the atmosphere . in the lung , epithelial cells and supporting tissue are present . the supporting tissue comprises connective tissue , nerves and lymph and blood vessels . in this application , connective tissue is described by the term “ mesenchyme .” omnipotent stem cells are cells that possess the ability to proliferate indefinitely and posses the capacity to differentiate into any possible cell type that is present in the full - grown organism . pluripotent stem cell are cells that possess the ability to proliferate indefinitively and possess the capacity to differentiate into a large but limited number of cell types that , in general , belong to a certain tissue , such as the lung , or a certain cell lineage , such as the endoderm , ectoderm or mesoderm . stem cells of the invention may be genetically modified and / or they may be obtained by nuclear transfer . the canonical wnt - signaling pathway is a signaling cascade in which a collection of proteins controls the proteolytic breakdown of beta - catenin . some proteins in this pathway are chemically modified by other proteins and / or are capable of inducing other molecules , such as , for instance , sugars and lipids , that often have an influence on the proteolytic breakdown of beta - catenin . wnt3a protein is part of a large family of proteins that have a close resemblance to each other in structure and function . wnt proteins are strongly conserved during evolution in structure and function and are interchangeable between species . the invention , therefore , comprises all wnt proteins or a functional part , derivative or analogue thereof , that , like wnt3a , are capable of inducing the canonical wnt pathway . other components of the wnt - signaling pathway or fragments of these components , or compounds mimicking the function of these components , that in the end reduce the proteolytic breakdown of beta - catenin will have the same effect as wnt3a in kind , not necessarily in amount , and are also within the scope of the present invention . the terms “ wnt ,” “ wnt gene product ” or “ wnt polypeptide ,” when used herein , encompass native sequence wnt polypeptides , wnt polypeptide variants , wnt polypeptide fragments and chimeric wnt polypeptides . optionally , the wnt polypeptide is not associated with native glycosylation or palmitolyation . “ native glycosylation ” refers to the carbohydrate moieties that are covalently attached to wnt polypeptide when it is produced in the metazoan cell from which it is derived in nature . native palmitolyation in its turn refers to the covalent attachment of lipid derivatives to a wnt polypeptide when it is produced in the metazoan cell that is derived in nature ( willert et al ., 2003 ). accordingly , a human wnt polypeptide produced in a non - metazoan cell is an example of a wnt that is “ not associated with native glycosylation or palmitolyation .” sometimes , the wnt polypeptide is unglycosylated or unpalmitolyated ( e . g ., as a result of being produced recombinantly in a prokaryote ). a “ native sequence ” polypeptide is a polypeptide that has the same primary amino acid sequence as a polypeptide ( e . g ., wnt polypeptide ) derived from nature . such native sequence polypeptide is , for instance , isolated from nature or is produced by recombinant or synthetic means . a native sequence polypeptide has the amino acid sequence of naturally occurring human polypeptide , murine polypeptide , or polypeptide from any other mammalian species . the term “ native sequence wnt polypeptide ” includes wnt polypeptides from any animal species ( e . g ., human , murine , rabbit , cat , cow , sheep , chicken , porcine , equine , etc .) as occurring in nature . the term “ native sequence wnt protein ” includes the native proteins with or without the initiating n - terminal methionine ( met ) and with or without native signal sequence . the native sequence human and murine wnt polypeptides known in the art are from about 348 to about 389 amino acids long in their unprocessed form reflecting variability ( particularly at the poorly conserved amino - terminus and several internal sites ), contain 21 conserved cysteines , and have the features of a secreted protein ( see , e . g ., wnt polypeptides as in gavin et al ., supra ; lee et al ., supra ; christiansen et al ., supra ; pct / us94 / 14708 ( wo 95 / 17416 )). the molecular weight of a wnt polypeptide is about 38 to 42 kd in a monomeric form . a “ functional part , derivative or analogue ” of a wnt polypeptide means a biologically active polypeptide as defined below having less than 100 % sequence identity with a native sequence wnt polypeptide and having the same activity in kind , not in amount . such functional parts , derivatives or analogues include polypeptides , wherein one or more amino acid residues are added at the n - or c - terminus of , or within , the native sequence ; polypeptides , wherein from about one to forty amino acid residues are deleted and optionally substituted by one or more amino acid residues ; and derivatives of the above polypeptides , wherein an amino acid residue has been covalently modified so that the resulting product has a non - naturally occurring amino acid . preferably , a biologically active wnt variant has an amino acid sequence having at least about 90 % amino acid sequence identity with a native sequence wnt polypeptide , preferably , at least about 95 %, more preferably , at least about 99 %. a “ functional part ” means having an effector function that is directly or indirectly caused or performed by native sequence wnt polypeptide , such as wnt3a . effector functions of native sequence wnt polypeptides preferably include inhibition of differentiation and / or enhancement of proliferation and / or induction of apoptosis . “ functional part or derivatives ” include , but are not limited to , fragments of a native wnt polypeptide sequence and derivatives of a native sequence wnt polypeptide and its fragments , provided that they have a biological activity in common with a corresponding native sequence wnt polypeptide . the term “ derivative ” encompasses both amino acid sequence variants of wnt polypeptide and covalent modifications thereof . “ isolated ” wnt polypeptide has been purified from a wnt source or has , for instance , been prepared by recombinant or synthetic methods and is sufficiently free of other peptides or proteins ( 1 ) to show homology for at least 15 and preferably 20 amino acid residues of the n - terminal or of an internal amino acid wnt polypeptide sequence , or ( 2 ) to homogeneity by sds - page under non - reducing or reducing conditions using coomassie blue or , preferably , silver stain . the term “ antibody ” is used for binding molecules in the broadest sense and , amongst other things , covers monoclonal antibodies , antibody compositions with poly - epitope specificity , bispecific antibodies , diabodies , and single - chain molecules , as well as antibody fragments ( e . g ., fab , f ( ab ′). sub . 2 , and fv ), so long as they exhibit the desired biological activity . the term “ monoclonal antibody ,” as used herein , refers to an antibody obtained from a population of substantially homogeneous antibodies , i . e ., the individual antibodies constituting the population are identical except for possible naturally occurring mutations that may be present in minor amounts . monoclonal antibodies are highly specific , being directed against a single antigenic site . furthermore , in contrast to conventional ( polyclonal ) antibody preparations that typically include different antibodies directed against different determinants ( epitopes ), each monoclonal antibody is directed against a single determinant on the antigen . in addition to their specificity , the use of monoclonal antibodies synthesized by a hybridoma culture is advantageous in that they are uncontaminated by other immunoglobulins . the modifier “ monoclonal ” indicates the character of the antibody as being obtained from a substantially homogeneous population of antibodies , and is not to be construed as requiring production of the antibody by any particular method . for example , the monoclonal antibodies to be used in accordance with the present invention are in one embodiment made by the hybridoma method first described by kohler et al ., nature 256 : 495 ( 1975 ), and are in another embodiment made by recombinant dna methods ( see , e . g ., u . s . pat . no . 4 , 816 , 567 ( cabilly et al .)). in yet another embodiment , the “ monoclonal antibodies ” are isolated from phage antibody libraries using the techniques in clackson et al ., nature 352 : 624 - 628 ( 1991 ), and marks et al ., j . mol . biol . 222 : 581 - 597 ( 1991 ), for example . the monoclonal antibodies as provided herein also comprise “ chimeric ” antibodies ( immunoglobulins ) in which a portion of the heavy and / or light chain is identical with or homologous to corresponding sequences in antibodies derived from a particular species or identical with or homologous to corresponding sequences in antibodies derived from another species or belonging to another antibody class or subclass , as well as fragments of such antibodies , so long as they exhibit desired biological activity ( cabilly et al ., supra ; morrison et al ., proc . natl . acad . sci . usa , 81 : 6851 - 6855 ( 1984 )). the phrase “ enhancement of proliferation of a cell ” encompasses the step of increasing the extent of growth and / or reproduction of the cell , relative to an untreated cell either in vitro or in vivo . an increase in cell proliferation in cell culture is , for instance , detected by counting the number of cells before and after exposure to a molecule of interest . the extent of proliferation is alternatively quantified via microscopic examination of the degree of confluency . cell proliferation is also quantified using a thymidine or brdu incorporation assay . by “ controlling differentiation of a cell ” is meant the act of increasing the extent of the acquisition or possession of one or more characteristics or functions that differ from that of the original cell ( i . e ., cell specialization ). this is , for instance , detected by screening for a change in the phenotype of the cell ( e . g ., identifying morphological changes in the cell and / or surface markers on the cell ). a “ lung stem cell or lung progenitor cell ” or “ primitive lung cell ” is a cell that is able to differentiate to form a more committed or mature lung cell type . “ mammal ” refers to a human or non - human mammal , including , a domestic and farm animal , and a zoo , a sports , or a pet animal , such as a dog , a horse , a cat , a cow , etc . preferably , the mammal is human . the invention is further described with the aid of the following illustrative examples . preservation and potential growth of alveolar tissue in a murine lung by culture treatment with recombinant wnt3a polypeptide to provide proof of evidence , an in vitro explant lung culture ( obtained from a mouse ) is used . generation of new alveolar tissue in patients is stimulated by activation or re - activation of alveolar bud formation . growth and branching of alveolar buds , followed by transformation into alveolar ducts , sacs and pouches , results in the establishment of pulmonary acini . this is a general differentiation principle in both the fetal and the adult mammal . as a proof of evidence , it is , therefore , shown that treatment with selected molecules involved in a wnt pathway inhibits the differentiation process of alveolar differentiation . inhibition of this differentiation process is achieved by applying a purified wnt3a polypeptide to an in vitro lung explant culture of 13 - day - old ( e13 ) mouse embryos . the ( fetal ) murine lung explant culture was generated according to our standard protocol . briefly , complete lungs or individually dissected lung lobes were isolated from fetal mice and cultured for one to five days in a hanging drop culture system . the wnt3a polypeptide was administered to the culture medium in different concentrations , ranging from 0 to 4000 ng / ml . the effect of this treatment was monitored by stereomicroscopy . the lung explant cultures were subsequently fixed for 30 minutes in 10 % paraformaldehyde ( pfa ), 30 minutes in 2 % pfa and 30 minutes in 4 % pfa . after fixation , the lung explants were dehydrated through an increasing series of 2 - isopropanol and subsequently embedded in paraffin . for the histological analysis , 4 μm thick sections were cut and stained with hematoxilin and eosin using standard techniques . for the immunohistochemical analysis , comparable sections were used and treated with an anti - pcna monoclonal antibody followed by a goat anti - mouse horseradish peroxidase conjugate . detection of the pcna epitopes was carried out by treating the sections with di - aminobenzidine ( dab ) for 15 minutes . primary mouse fibroblasts isolated from e12 . 5 lung explants , the human epithelial lung carcinoma cell line a549 cells ( atcc ccl - 185 ), the human epithelial lung adenocarcinoma cell line calu - 3 ( atcc_htb - 55 ) and the normal human fetal lung fibroblast cell line hfl - 1 cells ( atcc — 153 ) were cultured in 96 - well plates and transfected with the topflash and fopflash reporter constructs using lipofectamine 2000 according to the manufacturer &# 39 ; s instructions . as an internal control for transfection efficiency , a constitutive renilla luciferase reporter ( promega ) was cotransfected . the cells were transfected with the above - mentioned reporter constructs and simultaneously treated with different concentrations of wnt3a for 20 hours , after which the cells were lysed and luciferase expression was measured using a berthold luminometer for two seconds ( v . korinek et al ., mol . cell . biol . 18 : 1248 - 1256 , 1998 ). the lipofectamine 2000 transfection reagent was purchased from invitrogen life technologies . the wild - type beta - catenin / tcf reporter plasmid ( topflash ) and mutated reporter plasmid ( fopflash ) were obtained from upstate biotechnology ( ny , usa ). the number of cells undergoing mitosis was measured using immunohistochemical detection of phosphorylated histon h3 , which is present only on condensed chromosomes during mitosis . per condition , at least four different lung explants were treated and in total , 20 sections per condition were counted for phospho histon h3 - positive cells by two different investigators . the primary antibody against phopho histon h3 ( ser10 , clone 6g3 ) was purchased from cell signaling technology and used according to the recommendations of the supplier . the induction of apoptosis was detected using the “ in situ cell death detection kit ,” of roche ( cat . no . 11 684 817 001 ) according to the manufacturer &# 39 ; s instructions . as a source for wnt3a , recombinant mouse wnt3a from r & amp ; d systems ( catalog nr . 1324 - wn , r & amp ; d systems , inc . minneapolis usa ) was used . as culture medium , dmem / ham &# 39 ; s f12 ( 1 : 1 ) from biochrom ag , catalog nr . fg 4815 , was used . the antibody against pcna was purchased from zymed laboratories inc . ( cat nr . 18 - 0110 ). the secondary conjugate used was from jackson laboratories , inc . ( catalog nr . 115 - 056 - 062 ). the primary antibodies used for the immunohistochemical detection was for beta - catenin ; anti beta - catenin igg , mab , clone 14 from bd - transduction laboratories , for tubulin - beta mab clone tbn05 ( tub2 . 1 ) from neomarkers , for brdu ; anti - bromodeoxyuridine ( brdu ), mab clone zbu30 from zymed laboratories inc . the effect of 500 ng / ml to 4000 ng / ml wnt3a on the growth of fetal lung explants results : treatment of the lung cultures of embryonic day 13 ( e13 ) lung explants with 1000 ng / ml wnt3a polypeptide resulted in a total block of lung differentiation after 16 hours ( fig1 , panel e ). even after five days culture under these conditions , no significant differentiation and alveolar development could be observed by stereomicroscopy ( fig1 , panel f ). histological analysis of these five - day - old cultures revealed that the differentiation of the primordial lung epithelium was arrested at a stage comparable to that of the start of the culture ( e13 ). furthermore , most of the mesenchymal cells surrounding the epithelium displayed picnotic nuclei , indicating that these cells had undergone apoptosis . this was in sharp contrast with the epithelium that contained no apoptotic cells ( fig1 , panel h ). to analyze the viability of the epithelium and the surrounding mesenchyme , the sections were stained for the proliferation marker “ proliferating cell nuclear antigen ” ( pcna , fig1 , panels i - j ). this immunohistochemical staining showed that the epithelium was still proliferating whereas most of the mesenchymal cells were inactive or dead ( fig1 , panels k and l ). this is in contrast to the untreated lung explant cultures where both the epithelium and the mesenchyme were strongly proliferating ( fig1 , panels i and j ). the above results indicate that the wnt3a polypeptide has a strong inhibitory effect on the development and differentiation of the fetal lung . in order to determine the concentration range in which the wnt3a polypeptide is effective , total lung explants were cultured in the presence of wnt3a in a range of 0 ng / ml , 500 ng / ml , 750 ng / ml , 1000 ng / ml , 2000 ng / ml and 4000 ng / ml ( fig2 , panels a - r ). stereomicroscopic analysis showed already after one day a deleterious effect on lung development with the wnt3a concentrations , 2000 ng / ml and 4000 ng / ml ( fig2 , panels m and p ). after two days of culture , these lung explants were fixed and embedded in paraffin for immunohistological analysis . the histological analysis of h / e stained sections showed that the mesenchymal cell population undergoes a rapid induction of apoptosis within the range 1000 to 4000 ng / ml wnt3a . apoptosis is also induced in the epithelium at a concentration of 2000 to 4000 ng / ml but is not detectable at a concentration of 1000 ng / ml . the mesenchyme , however , at a concentration higher than 750 ng / ml , becomes clearly apoptotic , indicating that the induction threshold for apoptosis is different for the epithelium and the mesenchyme . at the concentration of 750 ng / ml , apoptosis in the mesenchyme is clearly detectable but not yet abundant , whereas the epithelium showed no sign of apoptosis whatsoever fig2 , panels h and i ). immunohistochemical staining of pcna in addition showed a wnt3a - dependent proliferation upon the lung explant cultures . without addition of wnt3a , the periphery of the lung containing the growing lung buds show a stronger staining for pcna than the more centrally located bronchial epithelium , indicating that the periphery of the lung is stronger proliferating than the central part ( fig2 , panels s and t ). addition of 500 ng / ml wnt3a to the medium , however , shows a marked overall increase in the intensity and frequency of pcna staining in the centrally located bronchial epithelium , indicating that proliferation is enhanced by wnt3a at a dosage of 500 ng / ml wnt3a ( fig2 , panels u and v ). a further increase of the wnt3a concentration to 750 ng / ml ( fig2 , panels w and x ) does not lead to a further increase of the intensity and frequency of the pcna staining , but instead , is comparable to the control situation ( fig2 , panels s and t ). addition of 1000 ng / ml wnt3a reduces the pcna staining below the control situation ( fig2 , panels y and z ), whereas at 2000 and 4000 ng / ml wnt3a ( fig2 , panels aa and ad ), pcna staining was completely negative indicating that proliferation was completely blocked , probably due to the apoptosis induced in the mesenchyme and epithelium ( fig2 ). to further determine the short - term effect of different wnt3a concentrations upon the induction of proliferation , lung explants ( e12 . 5 ) were cultured for one day with 0 , 30 , 125 and 500 ng / ml wnt3a . the lung explants were processed for immunohistochemical analysis and stained for histon h3 phosphorylation , which can be used as a quantifiable cell proliferation marker as it is present in a fixed amount on condensed chromosomes only during active cell division ( material and methods ). the results of this quantification show that wnt3a has a significant stimulatory effect upon proliferation . with more than a 20 % increase overall , proliferation was observed in the 30 , 125 and 500 ng / ml - treated wnt3a samples . however , most of the increase in proliferation could be accounted to the mesenchyme , where an increase in 40 to 60 % of cells undergoing apoptosis was observed . little difference was found between the different wnt3a concentrations , indicating that the maximum stimulatory effect of wnt3a was already reached with 30 ng / ml wnt3a ( fig3 ). the effect of 4 ng / ml to 500 ng / ml wnt3a on fetal lung growth over four days to more exactly determine the concentration of wnt3a that is effective upon lung development , differentiation and apoptosis , fetal lung explants were cultured for a longer period ( four days ) in the presence of lower concentrations of wnt3a , i . e ., 0 ng / ml , 4 ng / ml , 20 ng / ml , 100 ng / ml and 500 ng / ml ( fig4 , panels a - o ). none of these concentrations was effective in completely blocking the differentiation of fetal lung development , although differentiation at a concentration of 500 ng / ml ( fig4 , panels n and o ) was delayed as was displayed by the primordial morphology of the epithelium compared to the control situation ( fig4 , panels b and c ). between 100 and 500 ng / ml , the mesenchyme showed an increasing level of apoptosis , whereas in the epithelium , no sign of apoptosis could be detected ( fig4 , panels k , l , n and o ). between 4 and 20 ng / ml , no clear effects were visible at the level of apoptosis and differentiation compared to the control situation ( fig4 , panels b , c , e , f , h , and i ). analysis of apoptosis by the tunel assay and active proliferation by brdu incorporation on lung explants treated with 4 ng / ml to 1000 ng / ml wnt3a during four days the above - described experiments show that the visible biological effect of wnt3a upon proliferation , apoptosis , and differentiation lies between a concentration range of 1 and 2000 ng / ml . a concentration of 2000 ng / ml is not preferred since it affects the viability of all cells , and thus lacks specificity . a concentration below 1 ng / ml does not show clear morphological changes , although a concentration of 30 ng / ml markedly increases proliferation after one day . drastic effects were observed at the level of apoptosis , proliferation and differentiation . to further analyze the effect of wnt3a upon apoptosis and proliferation , lung explants were cultured for four days in the presence of 0 , 4 , 20 , 100 , 250 , 500 , 750 and 1000 ng / ml . in order to identify whether after wnt3a treatment cells are viable and active , proliferating brdu was added to the culture medium , which is incorporated in newly synthesized dna of active proliferating cells , two hours before the fixation of the lung explants . histological examination at day 4 showed , as before , that the process of differentiation was strongly retarded and revealed many picnotic cells at wnt3a concentrations above 100 ng / ml . to better investigate this effect , histological sections were analyzed with the tunel assay , which detects fragmented dna present in apoptotic cells ( material and methods ). a low background of tunel - positive cells was detected in the sham - treated lung explants , however , a dramatic increase in tunel - positive cells was observed at concentrations above 100 ng / ml wnt3a , which is proportional to the number of picnotic cells found in these samples ( fig5 ). at 1000 ng / ml , almost 100 % of the mesenchymal cells were picnotic and were also tunel positive , indicating that they had undergone apoptosis ( fig4 , panel h ). in contrast , almost none of the epithelial cells were tunel positive , or contained picnotic nuclei , indicating that these cells are much less sensitive for apoptosis compared to the mesenchymal cells ( fig5 ). in order to identify whether the epithelial cells were still actively proliferating after four days of wnt3a treatment , the histological sections were immunostained for incorporated brdu ( fig6 ). at concentrations of 500 ng / ml and lower , both the mesenchyme and epithelium contained many brdu - positive , thus active , proliferating cells . at 1000 ng / ml , almost all brdu - positive cells were present in the epithelium and only a very few were present in the mesenchyme , indicating that despite the massive induction of apoptosis in the mesenchyme , the epithelium is viable and proliferating . at 750 ng / ml wnt3a , an intermediate effect was observed between the 500 and 1000 ng / ml wt3a - treated lung explants ( fig6 ). long - term treatment of fetal lung explant cultures with wnt3a induces a dosage - dependent shift of distal ( alveolar ) lung differentiation to proximal ( bronchial / tracheal ) lung differentiation to further determine the effect of wnt3a upon the development and differentiation of the lung over time , fetal lung explants of isolated e12 . 5 were cultured with 0 , 10 , 50 , 250 , and 500 ng / ml wnt3a for eight days . two hours before the fixation of the lung explants , brdu was added to the culture medium , which is incorporated in newly synthesized dna of active proliferating cells . after the different time points , the lung explants were processed for histological analysis and immuno staining of marker genes . the histological analysis showed that the long - term lung development in the hanging drop culture system , without wnt3a addition to the medium , is comparable to in vivo lung development . the lung explants without wnt3a could be cultured for eight days without the sign of necrosis or apoptosis . contraction of the bronchial epithelium is visible after one to two days of culture , indicating that smooth muscle cells develop along the conducting airways . this was confirmed with immunohistochemical staining for smooth - muscle actin ( data not shown ). several differentiated cell types , such as cubical type - ii cells , secretory clara cells , cartilage and the alveolar - type - i cells could be clearly distinguished after eight days ( fig7 , panel a ). this is not much delayed from the in vivo situation where alveolar - type - i cells differentiate between e17 . 5 and e19 . 5 , thus corresponding to days five and seven of the in vitro culture . treatment of the e12 . 5 day lung explants showed a clear change in morphology , which became more and more apparent over time , indicating that a wnt3a concentration as little as 10 ng / ml has profound effects on lung development . histological analysis of these explants after eight days showed a decreasing alveolarization of the distal epithelium . the normal flattening of the primordial epithelium to the type - i cell that occurs during normal distal lung differentiation is inhibited with increasing wnt3a concentration and , at 250 ng / ml , it is almost completely suppressed ( fig7 , panels b - e ). distal lung development is thus inhibited by wnt3a treatment in a concentration range of at least 10 ng / ml to 250 ng / ml wnt3a . addition of wnt3a has a striking effect on the final differentiation of e12 . 5 lung explants . previous cultures had already shown the short - term inhibitory effect upon branching of 500 ng / ml wnt3a after one day . after eight days treatment with 500 ng / ml wnt3a , branching was still severely suppressed and in morphology , resembled the original isolated e12 . 5 lung explants . however , histological analysis and immunostaining with an anti - tubulin iv antibody that marks ciliated epithelium , showed that these explants consist completely of proximal structures comprising bronchial epithelium composed of ciliated cells and secretory cells , cartilage and mesenchyme . no type - i or type - ii cells could be observed indicating that the differentiation of distal epithelium comprising the alveolar and bronchiolar compartments was completely inhibited by the treatment with 500 ng / ml wnt3a ( fig7 , panel f ). the administration of concentrations higher than 250 ng / ml wnt3a thus restricts lung differentiation to cell types found in the upper airways and is thus suitable for selectively growing proximal lung cell types from primordial lung tissue , an application which is very useful in generating tissue to treat diseases or injuries of the upper airways . wnt3a induces the accumulation of nuclear beta - catenin in lung mesenchymal cells in order to determine whether wnt3a actually elevates the wnt - signaling pathway in these explants , histological sections were stained for beta - catenin , the intracellular factor , which , together with members of the high mobility group ( hmg ) of transcription factors such as tcf1 and lef , transducer the wnt signal to the nucleus . the results showed that there is a wnt3a dosage - dependent increase in nuclear beta - catenin within the first two days , which was especially prominent in the mesenchymal cells surrounding the growing and branching lung buds in the distal lung compartment . this effect was maximal in the 250 and 500 ng / ml wnt3a - treated lungs ( fig8 , panels d and e ), and had disappeared in the four - and eight - day cultures ( data not shown ). wnt3a induces a dosage - dependent activation of wnt - reporter expression in fetal lung tissue the above results indicate that fetal lung tissue is capable of transducing the wnt signal by addition of wnt3a to the culture . to further prove that wnt3a is capable of inducing the wnt - signaling pathway in lung tissue , we conducted the topflash / fopflash reporter assay on dissociated lung explants isolated from e12 . 5 embryos . the dissociated lungs were co - transfected with the topflash reporter construct that is activated by nuclear beta - catenin transcription complexes . this was compared to the activation of a fopflash reporter construct that is mutated in the beta - catenin / tcf - binding sites and measures only the background expression from the minimal promoter ( v . korinek et al ., mol . cell . biol . 18 : 1248 - 1256 , 1998 ). the ratio topflash expression versus fopflash expression is a measure for the induction of the wnt - signaling pathway by wnt3a . the transfected cells were treated with different concentrations of wnt3a ranging between 0 . 4 and 500 ng / ml wnt . a clear wnt3a dosage - dependent response was observed , indicating that the fetal lung cells have all the necessary components to receive and transduce the wnt signal - transduction pathway ( fig9 a ). in order to analyze if wnt3a is able to activate the wnt signal - transduction pathway in humans , we stimulated three different cell lines from human origin with wnt3a and measured the response with the topflash / fopflash reporter assay . the three human lung - derived cell lines analyzed were the lung epithelial carcinoma cell line a549 ( fig9 b ), the normal lung fibroblast line hfl - 1 ( fig9 c ), and the epithelial adenocarcinoma cell line calu - 3 ( fig9 d ). the results revealed that all three cell lines respond to wnt3a in a dosage - dependent manner , indicating that human and mouse lung cells are able to receive and transduce the wnt signal upon stimulation with wnt3a . expression profiling demonstrates that wnt3a inhibits the differentiation of fetal lung explants to further investigate the effect of wnt3a inhibition on lung differentiation , we analyzed the expression profiles of wnt3a - treated lung explants with microarray analysis using the affymetrix chip set moe430ab , containing the complete mouse transcriptome . the lung explants , per condition , ten complete fetal lungs isolated from e12 . 5 embryos , were treated for four hours with 0 and 250 ng / ml wnt3a ( wnt0 / 4 hr and wnt250 / 4 hr , respectively ), and for 24 hours with 0 , 10 , 50 and 250 ng / ml wnt3a ( wnt10 / 24 hr , wnt10 / 24 hr , wnt50 / 24 hr , wnt250 / 24 hr , respectively ). the 60 lung explants were isolated together from six pregnant swiss mice and randomly distributed over the six different conditions . after the indicated time period , total rna was isolated , which was processed for the synthesis of a probe for hybridization onto the moe - 430ab chip sets . the results show a strong effect of wnt3a upon gene expression in the lung explants . a rapid response was observed when the lungs were treated for four hours with 250 ng / ml wnt3a . a large number of genes were differentially expressed more than two - fold between the wnt250 / 4 hr group and wnt0 / 4 hr group , i . e ., 446 up - regulated genes and 1165 down - regulated genes . in accordance with the expectations , many known wnt - responsive genes were found in the up - regulated gene pool , again showing that the wnt pathway was activated in these lung explants . in the 24 - hour - treated explants , a less pronounced effect was observed . in the ng / ml wnt3a - treated group , 209 genes were up - regulated and 223 genes were down - regulated more than two - fold . in the 50 ng / ml - treated group , 95 and 92 genes were up - regulated and down - regulated , respectively . in the 250 ng / ml - treated group , 195 and 152 genes were up - and down - regulated more than two - fold , respectively . however , it should be noted that a concentration as low as 10 ng / ml wnt3a is capable of inducing significant changes in gene expression . there is a strong correlation in the behavior of the genes in the 24 - hour wnt3a - treated groups . genes up - ( or down -) regulated in one of these groups were also up - ( or down -) regulated in the other two groups . interestingly , there exists an opposite correlation between the genes up - or down - regulated in the four - hour - treated group compared to the genes in the 24 - hour - treated group . in other words , genes up - regulated in the four - hour - treated group were down - regulated in the 24 - hour - treated group and visa versa . cluster analysis of the expression profiles of genes significantly different in one of the six groups ( 4500 genes ), in which genes with a comparable expression level are grouped , clearly shows that the three groups treated with 10 , 50 and 250 ng / ml wnt3a during 24 hours , respond in a similar way ( fig1 ). importantly , the cluster analysis of 4500 significantly changed genes ( p & lt ; 0 . 000001 ) shows that the expression profiles of these three 24 - hour wnt3a - treated lungs are strongly related to the four - hour - untreated lungs and these four conditions resemble each other more than the 24 - hour - untreated lungs . this shows that the transcriptome of 24 - hour - cultured lungs remains younger when cultured in the presence of wnt3a . in other words , wnt3a inhibits , at least partially , the differentiation of fetal lung tissue . the cluster analysis also shows that the four - hour wnt3a - treated group has the most different expression profile of all six groups . furthermore , it shows that this profile is for the majority of genes ( circa 75 %) opposite from the profiles of the 24 - hour wnt3a - treated groups . apparently , a strong feedback mechanism occurs upon wnt3a stimulation after 24 hours , a mechanism which ultimately results in the inhibition of the differentiation and development of the lung . in this example , we have shown that the addition of wnt3a induces the nuclear accumulation of beta - catenin in primordial lung tissue , the hallmark of wnt - signal transduction . in addition , we show that wnt3a administration induces the expression of a wnt - reporter construct transfected in fetal lung tissue , at least between 10 and 1000 ng / ml wnt3a . the primordial lung thus has all the necessary components of the wnt - signal transduction pathway to recognize the wnt3a polypeptide and to transduce its signal intracellularly . the addition of wnt3a has a profound effect upon fetal lung development . differentiation , cell proliferation and apoptosis are greatly altered upon addition of wnt3a to the culture medium . the effective concentration depends on the cell type and the cellular function . using morphological criteria , marker gene analysis and microarray analysis , we show that differentiation of the primordial lung epithelium is effectively inhibited between a concentration of 10 and 2000 ng / ml wnt3a , with an optimum between 50 and 500 ng / ml . proliferation of the bronchial and alveolar epithelium is markedly enhanced between 30 and 500 ng wnt3a / ml culture medium , whereas at concentrations above 2000 ng / ml , proliferation is inhibited . apoptosis in the mesenchyme is induced at concentrations greater than 100 ng / ml , whereas apoptosis in the epithelium is induced at concentrations higher than 1000 ng / ml . the mesenchyme is thus much more sensitive for wnt3a - 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