Expression of M-cadherin, a member of the cadherin multigene family, correlates with differentiation of skeletal muscle cells

Fusion from myoblasts to myotubes is dependent on the rolling stone gene (rost) of Drosophila

The development and differentiation of the body wall musculature in Drosophila are accompanied by changes in gene expression and cellular architecture. We isolated a Drosophila gene, termed rolling stone (rost), which, when mutated, specifically blocks the fusion of mononucleated cells to myotubes in the body wall musculature. beta 3 tubulin, which is an early marker for the onset of mesoderm differentiation, is still expressed in these cells. Gastrulation and mesoderm formation, as well as the development of the epidermis and of the central and peripheral nervous systems, appear quite normal in homozygous rolling stone embryos. Embryonic development stops shortly before hatching in a P-element-induced mutant, as well as in 16 EMS-induced alleles. In mutant embryos, other mesodermal derivatives such as the visceral mesoderm and the dorsal vessel, develop fairly normally and defects are restricted to the body wall musculature. Myoblasts remain as single mononucleated cells, which express muscle myosin, showing that the developmental program of gene expression proceeds. These myoblasts occur at positions corresponding to the locations of dorsal, ventral and pleural muscles, showing that the gene rolling stone is involved in cell fusion, a process that is independent of cell migration in these mutants. This genetic analysis has set the stage for a molecular analysis to clarify where the rolling stone action is manifested in the fusion process and thus gives insight into the complex regulating network controlling the differentiation of the body wall musculature.

Contactus adherens, a special type of plaque-bearing adhering junction containing M-cadherin, in the granule cell layer of the cerebellar glomerulus

In the glomeruli of the granule cell layer of mammalian cerebellum, neuronal extensions are interconnected by numerous small, nearly isodiametric (diameters up to 0.1 micron), junctions previously classified as puncta adherentia related to the vinculin-containing, actin microfilament-anchoring junctions of the zonula adherens of epithelial and certain other cells. Using immunofluorescence and immunoelectron microscopy, we have found, however, that these junctions are negative for E- and VE-cadherin, for desmosomal cadherins, and also for vinculin, alpha-actinin, and desmoplakin, but they do contain, in addition to the protein plakoglobin common to all forms of adhering junctions, the plaque proteins alpha- and beta-catenin and the transmembrane glycoprotein M-cadherin previously found as a spread--i.e., not junction bound--plasma membrane protein in certain fetal and regenerating muscle cells and in satellite cells of adult skeletal muscle. We conclude that these M-cadherin-containing junctions of the granule cell layer represent a special type of adhering junction, for which we propose the term contactus adherens (from the Latin contactus, for touch, site of bordering upon, also influence), and we discuss the differences between the various adhering junctions on the basis of their molecular constituents.

The role of cadherin in the generation of multinucleated osteoclasts from mononuclear precursors in murine marrow

A critical step in bone resorption is the fusion of mononuclear osteoclast precursors to form multinucleated osteoclasts. However, little is known of the molecular mechanisms that are responsible for this important process. Since the expression of proteins in the cadherin family of homophilic calcium-dependent cell adhesion molecules is involved in the fusion process for certain other cells, we examined their role in osteoclast formation. Immunohistochemical examination of human and mouse bone using monoclonal antibodies to human and mouse E-cadherin clearly demonstrated positive staining in osteoclasts. N- and P-cadherin were not detected. In cultures of murine marrow mononuclear cells in which osteoclasts form by cell fusion, E-cadherin expression determined by Western blotting reached the highest levels as fusion was taking place. Expression of E-cadherin gene fragment was also detected in the marrow cultures by polymerase chain reaction. To study the functional role of E-cadherin expression in osteoclastic differentiation, neutralizing monoclonal antibodies were examined for their effects on osteoclast formation. The antibodies decreased the number of tartrate-resistant acid phosphatase (a marker of murine osteoclast)-positive multinucleated cell (TRAP-positive MNC) by inhibiting the fusion of mononuclear osteoclast precursors, but not proliferation of these cells or their attachment to plastic dish surfaces. This inhibitory effect was reversible. Furthermore, synthetic peptides containing the cell adhesion recognition sequence of cadherins also decreased TRAP-positive MNC formation. The antibodies and peptides inhibited not only osteoclast formation but also bone resorption. Antibodies to other types of cadherins and control rat IgG had no effects in these culture systems. Our findings suggest that E-cadherin expression may be involved in fusion (differentiation) of hemopoietic osteoclast precursors into mature multinucleated osteoclasts.

Phylogenetic analysis of the cadherin superfamily

Cadherins are a multigene family of proteins which mediate homophilic calcium-dependent cell adhesion and are thought to play an important role in morphogenesis by mediating specific intercellular adhesion. Different lines of experimental evidence have recently indicated that the site responsible for mediating adhesive interactions is localized to the first extracellular domain of cadherin. Based upon an analysis of the sequence of this domain, I show that cadherins can be classified into three groups with distinct structural features. Furthermore, using this sequence information a phylogenetic tree relating the known cadherins was assembled. This is the first such tree to be published for the cadherins. One cadherin subtype, neural cadherin (N-cadherin), shows very little sequence divergence between species, whereas all other cadherin subtypes show more substantial divergence, suggesting that selective pressure upon this domain may be greater for N-cadherin than for other cadherins. Phylogenetic analysis also suggests that the gene duplications which established the main branches leading to the different cadherin subtypes occurred very early in their history. These duplications set the stage for the diversified superfamily we now observe.

Cadherins, steroids and cancer

The cadherins are a family of calcium-dependent cell adhesion molecules which are thought to be key regulators of morphogenesis. This review contains a discussion of the structure, function and regulation of these cell adhesion molecules. In particular, we discuss recent studies that demonstrate the ability of steroids to modulate cadherin levelsin vivo. We speculate that steroids and estrogenic organochlorines exert their diverse morphoregulatory actions on tissues by altering cadherin levels.

Epithelial-mesenchymal transitions in development and tumor progression

Epithelial-mesenchymal transitions play important roles in development and malignancy. Here we discuss molecular events in the control of such transitions: changes in cellular adhesion components, action of oncogenes and tyrosine kinase receptors, as well as activation of transcription factors. In development, epithelial-mesenchymal transitions take place in a temporally and spatially controlled manner, whereas in tumors these changes are highly uncontrolled. Loss of epithelial character is typically observed late in progression of human carcinomas, and correlates there with the acquisition of invasive and metastatic potential.

Structure and turnover of junctional complexes between principal cells of the rat epididymis

The epididymal junctional complex between adjacent principal cells is composed of apically located gap, adherens and tight junctions. Tight junctions between adjacent epithelial cells lead to the formation of the blood-epididymal barrier. The objectives of this study were to examine the structure of the epididymal junctional complex in the different regions of the epididymis and to review the regulation of epithelial cadherin in the rat epididymis. Changes in the structure of the junctional complex, at the level of the electron microscope, were evident when comparing the initial segment to other regions of the epididymis. In the initial segment, the tight junction spanned a considerable length of the apical plasma membrane but had few desmosomes. In the other regions of the epididymis, the span of merging plasma membranes was considerably reduced, but in these regions, numerous desmosomes were present in the apical region. Several examples of what appeared to be a loss of portions of the plasma membrane of adjacent principal cells were evident along the entire epididymis. Such images as the invagination of a portion of the lateral plasma membrane of one principal cell into another, constriction of the invaginated area and eventual detachment leading to the formation of annular junctions suggest that there is a turnover of plasma membranes. The formation of cellular junctions involves the interactions of cell adhesion proteins followed by the addition of junctional proteins which assemble into tight and gap junctions. Epithelial cadherin (E-Cad), a calcium-dependent cell adhesion protein, was localized to the principal cells of the epididymis. Immunocytochemistry at the level of the electron microscope showed that E-Cad was present between the lateral plasma membranes of adjacent principal cells, both in the region of the junctional complex and in the deeper lying areas. E-Cad was also present in annular junctions located in close proximity to the junctional complex, indicating that these structures were related to the plasma membrane. E-Cad mRNA levels are regulated during postnatal epididymal development. In the caput-corpus epididymidis, E-Cad mRNA concentrations increase to peak at 42 days of age. This is well correlated with the conversion of testosterone to dihydrotestosterone in the epididymis. In the cauda epididymidis, however, E-Cad mRNA concentrations do not increase as a function of age, indicating that this protein is regulated in a segment-specific manner.(ABSTRACT TRUNCATED AT 400 WORDS)

Similarities in structure and expression between mouse P-cadherin, chicken B-cadherin and frog XB/U-cadherin

By immunological methods, we show that the monoclonal antibody 6D5 which reacts specifically with Xenopus laevis XB/U-cadherin, also binds to mouse P-cadherin and to chicken B-cadherin but not to the respective E-cadherins (L-CAM) or other "classical" cadherins in these species. In the first extracellular domain, three amino acid residues are identified that are shared by frog XB/U-cadherin, chicken B-cadherin and mammalian P-cadherins but not by the other "classical" cadherins. With few exceptions, the other cadherins possess residues at these positions that are also characteristic of each type of cadherin. Moreover, the expression patterns of P-, B-, and XB/U-cadherin in mouse, chicken and frog are more similar to each other than they are to those of the E-cadherins, L-CAM or other classical cadherins. Taken together, our results suggest that mammalian P-cadherins, chicken B-cadherin and frog XB/U-cadherin are closely related, if not homologous, molecules. A number of differences in the expression patterns between P-, B-, and XB/U-cadherin indicate that these molecules assume differential morphogenetic roles in different species.

Expression of M-cadherin protein in myogenic cells during prenatal mouse development and differentiation of embryonic stem cells in culture

Molecules regulating morphogenesis by cell-cell interactions are the cadherins, a class of calcium-dependent adhesion molecules. One of its members, M-cadherin, has been isolated from a myoblast cell line (Donalies et al. [1991] Proc. Natl. Acad. Sci. U.S.A. 88:8024-8028). In mouse development, expression of M-cadherin mRNA first appears at day 8.5 of gestation (E8.5) in somites and has been postulated to be down-regulated in developing muscle masses (Moore and Walsh [1993] Development 117:1409-1420). Affinity-purified polyclonal M-cadherin antibodies, detecting a protein of approximately 120 kDa, were used to study the cell expression pattern of M-cadherin protein. It was first visualized in somites at E10 1/3 and could be confined to desmin positive, myotomal cells. At all subsequent prenatal stages, M-cadherin was only found in myogenic cells of somitic origin. The detection of the protein at E10 1/3 suggests a translational delay of M-cadherin mRNA of 1 to 2 days (E8.5 vs. E10 1/3). This was further supported by the finding that during differentiation of ES cell line BLC6 into skeletal muscle cells in culture, expression of M-cadherin mRNA can be detected 2 days prior to M-cadherin protein. During prenatal development, the pattern of M-cadherin expression changes: In E10 1/3 embryos and also in myotomal cells of later stages, M-cadherin is evenly distributed on the cell surface. In developing muscle masses (tested at E16 to E18), however, M-cadherin protein becomes clustered most likely at sites of cell-cell contact as indicated by double-labelling experiments: M-cadherin-staining is the positive image of laminin negative areas excluding the presence of a basal lamina at M-cadherin positive sites. Furthermore, M-cadherin is coexpressed with the neuronal cell adhesion molecule N-CAM which has been shown to mediate cell-cell contact in myogenic cells. In summary, our results are in line with the idea that M-cadherin might play a central role in myogenic morphogenesis.

Differential perturbations in the morphogenesis of anterior structures induced by overexpression of truncated XB- and N-cadherins in Xenopus embryos

Cadherins, a family of Ca-dependent adhesion molecules, have been proposed to act as regulators of morphogenetic processes and to be major effectors in the maintenance of tissue integrity. In this study, we have compared the effects of the expression of two truncated cadherins during early neurogenesis in Xenopus laevis. mRNA encoding deleted forms of XB- and N-cadherin lacking most of the extracellular domain were injected into the four animal dorsal blastomeres of 32-cell stage Xenopus embryos. These truncated cadherins altered the cohesion of cells derived from the injected blastomeres and induced morphogenetic defects in the anterior neural tissue to which they chiefly contributed. Truncated XB-cadherin was more efficient than N-cadherin in inducing these perturbations. Moreover, the coexpression of both truncated cadherins had additive perturbation effects on neural development. The two truncated cadherins can interact with the three known catenins, but with distinct affinities. These results suggest that the adhesive signal mediated by cadherins can be perturbed by overexpressing their cytoplasmic domains by competing with different affinity with catenins and/or a common anchor structure. Therefore, the correct regulation of cadherin function through the cytoplasmic domain appears to be a crucial step in the formation of the neural tissue.

Developmental regulation of M-cadherin in the terminal differentiation of skeletal myoblasts

Cadherins form a large family of membrane glycoproteins which mediate homophilic calcium-dependent cell adhesion. They are thought to mediate the initial calcium-dependent cell adhesion which precedes the plasma membrane fusion of skeletal myoblasts. Two cadherin subtypes are known to be expressed in mammalian skeletal myoblasts: muscle cadherin (M-cadherin) and neural cadherin (N-cadherin). In the present study we demonstrate that 1) the expression of M- and N-cadherin is differentially regulated during myoblast differentiation in vitro, 2) the expression of M-cadherin but not N-cadherin is inhibited by 5-bromo-2'-deoxyuridine (BUdR), an agent which selectively inhibits skeletal myoblast differentiation, and 3) fusion and differentiation-competent rat L6 myoblasts do not express detectable levels of N-cadherin mRNA. In vivo, M-cadherin mRNA was detectable exclusively in skeletal muscle. M-cadherin mRNA levels peaked during the secondary myogenic wave in rat hindlimb muscle, becoming barely detectable in 1-week-old and adult rats. These observations indicate that M-cadherin is unique in two ways: It is the first cadherin to be included in the family of skeletal muscle-specific genes, and it shows peak levels of expression in developing skeletal muscle tissue. Taken together, these results suggest that M-cadherin plays an important role in skeletal myogenesis.

Complexus adhaerentes, a new group of desmoplakin-containing junctions in endothelial cells: II. Different types of lymphatic vessels

In diverse mammalian species, including (man, cow and rat) the very flat endothelial cells of lymphatic vessels of various organs, including the retothelial meshwork of sinus of lymph nodes, are connected by zonula-like plaque-bearing junctions which differ from the similarly structured junctions of blood vessel endothelia by the presence of desmoplakin or an as yet unknown but closely related plaque protein. These extended junctions, which also contain plakoglobin but none of the presently known desmogleins and desmocollins, are therefore different from the spot-like desmosomes (maculae adhaerentes) present in epithelia, myocardium and dendritic reticulum cells of lymphatic follicles, and are collectively subsumed under the new category of complexus adhaerentes, including the 'syndesmos' connecting the processes of the retothelial cells. The lymphatic endothelial cells possessing these special desmoplakin-containing junctions also contain the calcium-dependent transmembrane glycoproteins, V-cadherin and cadherin 5, of which the latter has also been partly localized to regions with desmoplakin-positive junctions. Possible functional reasons for the formation and maintenance of complexus adhaerentes are discussed as well as the potential value of reagents which allow their identification in relation to physiology and pathology.

Conformational changes of the recombinant extracellular domain of E-cadherin upon calcium binding

The cell-adhesion protein E-cadherin/uvomorulin exhibits a calcium-dependent homoassociation. The effect of Ca2+ on the extracellular fragment of E-cadherin was studied using the recombinant protein expressed in the baculovirus expression system. The recombinant and native fragment of E-cadherin were found to be similar by many biochemical criteria [Herrenknecht, K. & Kemler, R. (1993) J. Cell Sci. 17, 147-154]. A large and reversible conformational transition was observed upon Ca2+ depletion. A change from a rod-like structure, 22 nm in length, to a more globular assembly of the five subdomains became evident by electron-microscopical analysis. In the presence of Ca2+, the circular dichroic spectra indicated predominantly beta-structure but a more negative ellipticity was observed in the absence of Ca2+. The intrinsic tryptophan fluorescence decreased by 12% upon Ca2+ depletion. Both effects were used for calcium titrations which indicated calcium binding to several sites with average K(d) values of 45-150 microM. Cleavage of the protein fragment by trypsin occurred only at low Ca2+ concentrations and from the calcium-dependence of cleavage rates, a K(d) value of 24 microM was derived. The major site of cleavage was identified by partial sequencing to be located between the two putative calcium-binding sites in the third subdomain from the N-terminus. In agreement with earlier results with the native fragment, the recombinant protein did not associate in the presence or absence of Ca2+. We suggest the calcium-dependent homoassociation therefore depends on additional effects connected with the cell surface association of E-cadherin.

Expression and localization of N- and E-cadherin in the human testis and epididymis

Cellular interactions in the testis and epididymis are an important prerequisite for spermatogenesis and sperm maturation, and involve a well-developed complex of intercellular junctions. Cadherins are cell surface proteins which mediate intercellular Ca(2+)-dependent adhesion and are believed to be fundamentally important for maintaining multicellular structures. In the present study we report the expression of a 135 kDa N-cadherin polypeptide in the human seminiferous epithelium by immunoblotting. The presence of N-cadherin was demonstrated by immunohistochemistry on the surface of spermatogonia and primary spermatocytes, and possibly also around some early spermatids, whereas late spermatids were always negative. Endothelial cells also stained for N-cadherin, whereas peritubular cells and Leydig cells did not. No expression of E-cadherin could be demonstrated in the human testis. In the human epididymis E-cadherin, but not N-cadherin, was expressed and localized to the surface of the principal epithelial cells as shown by immunohistochemistry. These observations indicate that cadherins play an important role in the organization of the seminiferous and epididymal epithelium.

Liver-intestine cadherin: molecular cloning and characterization of a novel Ca(2+)-dependent cell adhesion molecule expressed in liver and intestine

A novel member of the cadherin family of cell adhesion molecules has been characterized by cloning from rat liver, sequencing of the corresponding cDNA, and functional analysis after heterologous expression in nonadhesive S2 cells. cDNA clones were isolated using a polyclonal antibody inhibiting Ca(2+)-dependent intercellular adhesion of hepatoma cells. As inferred from the deduced amino acid sequence, the novel molecule has homologies with E-, P-, and N-cadherins, but differs from these classical cadherins in four characteristics. Its extracellular domain is composed of five homologous repeated domains instead of four characteristic for the classical cadherins. Four of the five domains are characterized by the sequence motifs DXNDN and DXD or modifications thereof representing putative Ca(2+)-binding sites of classical cadherins. In its NH2-terminal region, this cadherin lacks both the precursor segment and the endogenous protease cleavage site RXKR found in classical cadherins. In the extracellular EC1 domain, the novel cadherin contains an AAL sequence in place of the HAV sequence motif representing the common cell adhesion recognition sequence of E-, P-, and N-cadherin. In contrast to the conserved cytoplasmic domain of classical cadherins with a length of 150-160 amino acid residues, that of the novel cadherin has only 18 amino acids. Examination of transfected S2 cells showed that despite these structural differences, this cadherin mediates intercellular adhesion in a Ca(2+)-dependent manner. The novel cadherin is solely expressed in liver and intestine and was, hence, assigned the name LI-cadherin. In these tissues, LI-cadherin is localized to the basolateral domain of hepatocytes and enterocytes. These results suggest that LI-cadherin represents a new cadherin subtype and may have a role in the morphological organization of liver and intestine.

Transformation by myc prevents fusion but not biochemical differentiation of C2C12 myoblasts: mechanisms of phenotypic correction in mixed culture with normal cells

To study the effects of myc oncogene on muscle differentiation, we infected the murine skeletal muscle cell line C2C12 with retroviral vectors encoding various forms of avian c- or v-myc oncogene. myc expression induced cell transformation but, unlike many other oncogenes, prevented neither biochemical differentiation, nor commitment (irreversible withdrawal from the cell cycle). Yet, myotube formation by fusion of differentiated cells was strongly inhibited. Comparison of uninfected C2C12 myotubes with differentiated myc-expressing C2C12 did not reveal consistent differences in the expression of several muscle regulatory or structural genes. The present results lead us to conclude that transformation by myc is compatible with differentiation in C2C12 cells. myc expression induced cell death under growth restricting conditions. Differentiated cells escaped cell death despite continuing expression of myc, suggesting that the muscle differentiation programme interferes with the mechanism of myc-induced cell death. Cocultivation of v-myc-transformed C2C12 cells with normal fibroblasts or myoblasts restored fusion competence and revealed two distinguishable mechanisms that lead to correction of the fusion defect.

Immunocytochemistry of M-cadherin in mature and regenerating rat muscle

BACKGROUND

Cadherins are transmembrane proteins mediating calcium-dependent cell-cell adhesion in a cell type-specific manner by means of homophilic binding. M(muscle)-cadherin is a recently detected member of the cadherin family.

METHODS

We have investigated the localization of M-cadherin in normal and aneurally regenerating skeletal muscle of rat by means of pre-embedding immunocytochemistry. The antibody was directed against the extra-cellular domain of M-cadherin.

RESULTS

Myoblasts and myotubes in regenerating muscles tended to be arranged in clusters enclosed by a common basal lamina. Satellite cells of mature muscle fibers were attached to the underlying fiber without separating basal lamina. Reactivity for M-cadherin was restricted to the plasma membranes of myoblasts and satellite cells, and was most intense at the membrane areas facing adjacent myotubes or myofibers. Myoblasts interposed between two myotubes were stained on the entire surface. The adjacent plasma membrane of the otherwise negative myotube or muscle fiber, was stained as well, and extracellular reaction product was in the gap between the cells.

CONCLUSION

The cellular localization of M-cadherin may indicate that M-cadherin is involved in calcium-dependent adhesion between satellite cells and muscle fibers or, by means of interposed myoblasts, between the clustered myotubes in a regenerating muscle.

Correlation of E- and P-cadherin expression with differentiation grade and mode of invasion in gingival carcinoma

The expression pattern of two Ca(2+)-dependent cell-cell adhesion molecules, E- and P-cadherin (CD), in 25 primary gingival squamous cell carcinomas (SCC) was examined immunohistochemically. The occurrence of reduced-type expression of both E- and P-CD increased significantly with the grade of carcinoma differentiation, culminating in a complete loss of P-CD in poorly differentiated SCC. The occurrence of reduced-type P-CD expression also increased significantly with the mode of invasion, as was the case with E-CD. Furthermore, no P-CD molecules were detected in one of the six SCC having a diffuse, cord-like invasion and in three of the six having a diffuse type of invasion. These findings suggest that the down-regulation of these cell adhesion molecules closely correlates with the differentiation grade and mode of invasion of gingival SCC.

Cloning of five human cadherins clarifies characteristic features of cadherin extracellular domain and provides further evidence for two structurally different types of cadherin

The entire coding sequences for five possible human cadherins, named cadherin-4, -8, -11, -12 and -13, were determined. The deduced amino acid sequences of cadherin-4 and cadherin-13 showed high homology with those of chicken R-cadherin or chicken T-cadherin, suggesting that cadherin-4 and cadherin-13 are mammalian homologues of the chicken R-cadherin or T-cadherin. Comparison of the extracellular domain of these proteins with those of other cadherins and cadherin-related proteins clarifies characteristic structural features of this domain. The domain is subdivided into five subdomains, each of which contains a cadherin-specific motif characterized by well-conserved amino acid residues and short amino acid sequences. Moreover, each subdomain has unique features of its own. The comparison also provides additional evidence for two structurally different types of cadherins: the first type includes B-, E-, EP-, M, N-, P- and R-cadherins and cadherin-4; the second type includes cadherin-5 through cadherin-12. Cadherin-13 lacks the sequence corresponding to the cytoplasmic domain of typical cadherins, but the extracellular domain shares most of the features common to the extracellular domain of cadherins, especially those of the first type of cadherins, suggesting that cadherin-13 is a special type of cadherin. These results, and those of other recent cloning studies, indicate that many cadherins with different properties are expressed in various tissues of different organisms.

Expression pattern of M-cadherin in normal, denervated, and regenerating mouse muscles

Following muscle damage in adult vertebrates, myofibers can be regenerated from muscle precursor cells (satellite cells). During this process, prenatal myogenesis is recapitulated to a large extent, both morphologically and molecularly. A putative morphoregulatory molecule involved in myogenesis is M-cadherin (Mcad), a calcium-dependent cell adhesion protein. The expression of Mcad was studied by immunofluorescence in regenerating, denervated, and normal mouse muscles. Our results demonstrate that Mcad is present in satellite cells in normal muscle. Enhanced staining at sites of contact between satellite cells and the parent muscle fiber suggests an additional, spatially restricted expression of Mcad in muscle fibers. Mcad positive cells in normal and denervated muscles did not incorporate bromodeoxyuridine within 24 hr after injection in vivo, indicating that Mcad is expressed on mitotically quiescent satellite cells. Neural cell adhesion molecule (NCAM) co-localized with Mcad in nearly all satellite cells in denervated muscles but rarely in intact muscles. At early stages of regeneration, Mcad was exclusively and strongly expressed in myoblasts. After fusion of myoblasts into myotubes, Mcad was down-regulated and was barely detectable on more mature myotubes surrounded by distinct basal lamina sheaths. These observations are in line with the idea that Mcad plays a crucial role in myogenesis. In intact muscle, Mcad might function as a molecular link between satellite cell and muscle fiber.

N-cadherin expression in developing, adult and denervated chicken neuromuscular system: accumulations at both the neuromuscular junction and the node of Ranvier

N-cadherin, a member of the Ca(2+)-dependent cell adhesion molecule family plays essential roles in morphogenesis and histogenesis. N-cadherin has been shown in vitro to promote myoblast fusion and neurite outgrowth. We report here the cellular localization of N-cadherin during development and regeneration of the chick neuromuscular system. N-cadherin was uniformly expressed along the surface of myoblasts and myotubes of E6 limb muscles. Later, as synaptogenesis and secondary myogenesis proceeded, N-cadherin expression was down-regulated and restricted to some large-diameter fibres, then to the areas of contact between few myofibres and subsequently disappeared by embryonic day 17, suggesting that this cadherin may be implicated predominantly in fusion of primary myoblasts and, at lower degree, of secondary myoblasts. The presence of N-cadherin in muscle during the period of nerve trunk ingrowth and its down-regulation after synaptogenesis suggests that this molecule might be implicated in both processes. N-cadherin became accumulated at the neuromuscular junction only a few days after the first synaptic contacts were established and remained at the adult neuromuscular junction, suggesting a role of this molecule in the stabilization of the mature neuromuscular junction. In sciatic nerve, the level of N-cadherin expression remained unchanged from hatching to adult life. N-cadherin was widely distributed on the surface of myelinated fibres and on myelinating Schwann cells: in addition, it was concentrated at the node of Ranvier. At the ultrastructural level, the molecule was detected inside, at the surface and in the basal lamina of Schwann cells and also associated with endoneurial collagen. These observations suggest a role of N-cadherin in the structuring and stabilization of the myelin sheaths. After nerve injury, N-cadherin continued to be expressed by proliferating Schwann cells in the distal stump providing a substratum for regenerating axons. N-cadherin reappeared at the surface of denervated muscle fibres without disappearing from the former synaptic sites. It was detected not only in the sarcoplasm and on sarcolemma of denervated muscle fibres, but also in the basal lamina and in the extracellular matrix. The reexpression of N-cadherin at the surface of denervated muscle fibres suggests a role for this molecule in muscle reinnervation. The presence of N-cadherin in basal lamina and its association with collagen fibres raise questions about the release of N-cadherin in the extracellular space and the existence of a putative heterophilic ligand for N-cadherin.

Regulation and activity-dependence of N-cadherin, NCAM isoforms, and polysialic acid on chick myotubes during development

Muscle development in vivo involves a complex sequence of cell-cell interactions in which secondary myotubes first form in association with primary myotubes and subsequently separate from them. We show here that during this process N-cadherin and the different structural forms of NCAM are regulated in a pattern that involves both temporal changes in expression and localization to particular regions of the muscle cell surface. In particular, levels of N-cadherin on maturing myotubes are decreased, and the form of NCAM synthesized by the muscle changes from a transmembrane non-polysialylated to a lipid-linked polysialylated membrane protein. Moreover, while NCAM was distributed on all myotube surfaces, the polysialyated form of NCAM was restricted to regions of the myotube surface that had recently separated from neighboring cells. We previously found that blockade of nerve-induced activity by d-Tubocurarine perturbed muscle cell interactions, resulting in a failure of myotubes to separate. We now show that this activity blockade also alters adhesion molecule expression. First, N-cadherin was no longer down-regulated in maturing myotubes, and its persistence on the surfaces of mature myotubes may partly explain their failure to separate. Secondly, the developmental switch from transmembrane to lipid-linked NCAM did not occur, and polysialylated NCAM was no longer formed. As the unusual physical properties of PSA have been proposed to impede cell-cell interactions, this alteration would also be expected to compromise cell separation. Together, these results suggest that the regulated expression of both N-cadherin and NCAM isoforms including their polysialylation, is an essential mechanism for the normal separation of secondary myotubes from primary myotubes.

Differential effects of over-expressed neural cell adhesion molecule isoforms on myoblast fusion

We have used a transfection based approach to analyze the role of neural cell adhesion molecule (NCAM) in myogenesis at the stage of myoblast fusion to form multinucleate myotubes. Stable cell lines of myogenic C2 cells were isolated that express the transmembrane 140- or 180-kD NCAM isoforms or the glycosylphosphatidylinositol (GPI) linked isoforms of 120 or 125 kD. We found that expression of the 140-kD transmembrane isoform led to a potent enhancement of myoblast fusion. The 125-kD GPI-linked NCAM also enhanced the rate of fusion but less so when a direct comparison of cell surface levels of the 140-kD transmembrane form was carried out. While the 180-kD transmembrane NCAM isoform was effective in promoting C2 cell fusion similar to the 140-kD isoform, the 120-kD isoform did not have an effect on fusion parameters. It is possible that these alterations in cell fusion are associated with cis NCAM interactions in the plane of the membrane. While all of the transfected human NCAMs (the transmembrane 140- and 180-kD isoforms and the 125- and 120-kD GPI isoforms) could be clustered in the plane of the plasma membrane by species-specific antibodies there was a concomitant clustering of the endogenous mouse NCAM protein in all cases except with the 120-kD human isoform. These studies show that different isoforms of NCAM can undergo specific interactions in the plasma membrane which are likely to be important in fusion. While the transmembrane and the 125-kD GPI-anchored NCAMs are capable of enhancing fusion the 120-kD GPI NCAM is not. Thus it is likely that interactions associated with NCAM intracellular domains and also the muscle specific domain (MSD) region in the extracellular domain of the GPI-linked 125-kD NCAM are important. In particular this is the first role ascribed to the O-linked carbohydrate containing MSD region which is specifically expressed in skeletal muscle.

E-cadherin in the mature and developing organ of Corti of the mouse

The reticular lamina of the mammalian cochlea is formed by the tightly joined apical surfaces of hair cells and supporting cells. This lamina creates a barrier separating endolymph and perilymph, two fluids of different composition. The preservation of this fluid barrier is a requirement of cochlear function. This study aimed to determine whether the calcium-dependent, cell adhesion molecule, E-cadherin was appropriately placed both temporally and spatially to contribute to the formation and maintenance of the reticular lamina. Cochleas aged E15 to P31 were stained immunocytochemically for E-cadherin. In the P31 organ of Corti, E-cadherin is localized to the apical intercellular junctions of supporting cells, but is absent from supporting cell-hair cell borders. During development, E-cadherin is present only on the apices and lateral edges of those cells which will eventually lie adjacent to fluid spaces--pillar, outer hair and Deiters cells. The molecule disappears from the lateral cell membranes at about the time in development that fluid spaces form. These data suggest that E-cadherin plays a role in maintaining the reticular lamina by mediating inter-supporting cell adhesion and raise the possibility that fluid space opening in the organ of Corti is facilitated by the down-regulation or redistribution of E-cadherin.

Use of p112-deficient myoblasts to determine the temporal order of the in vitro expression of myogenic components

The present investigation examines the function and site(s) of involvement of an ecto-protein kinase and its substrate protein (a cell surface 112 kDa protein) in the in vitro myogenic pathway. The phosphorylated 112 kDa protein (p112) has recently been shown to be involved in myogenesis. Not much information is currently available on the role of the ecto-protein kinase and the 112 kDa protein in modulating the expression of the myogenic factors and various muscle-specific proteins. Five different p112-deficient rat myoblasts were used to examine the temporal order of the in vitro expression of the myogenic components; namely, L6 myoblasts treated with BrdUrd or phloretin, a conditional p112-defective mutant (clone D1), an ecto-protein kinase-deficient mutant (clone F72), and a mutant defective in the 112 kDa protein (clone D1/S4). All these p112-deficient myoblasts were also impaired in myogenesis. The absence of p112, ecto-protein kinase, and/or the 112 kDa protein was found to have no effect on the Myf-5 mRNA level. However, the expected increase in NCAM and Myf-4 mRNAs was not observed in any of the p112-deficient myoblasts examined. This suggests that the p112 site of action is probably located upstream of the Myf-4 and NCAM sites in the myogenic pathway. While 7-28 fold increases in the MLC, MHC, and TnT transcripts were observed during myogenesis, such increases were not observed in the p112-deficient myoblasts. However, when mutant D1/S4 was transfected with the myf-4 cDNA, expression of Myf-4 in the transfectant resulted in increased level of the MLC, MHC, and TnT mRNAs, and in myotube formation, even though the Myf-5 and NCAM mRNA levels and p112 were not altered. This suggests that p112 may function by activating transcription of Myf-4, which will subsequently promote the expression of muscle-specific proteins and myotube formation. In the absence of p112, Myf-5 cannot activate the expression of Myf-4, NCAM, MLC, MHC, TnT, and myotube formation. If all these components are involved in the same myogenic pathway, then p112 may be acting downstream from Myf-5, and upstream from NCAM and Myf-4.

Molecular biology of cadherins in the nervous system

Cadherins are cell-cell adhesion molecules belonging to the Ca(2+)-dependent cadherin superfamily. In the last few years the number of cadherins identified in the nervous system has increased considerably. Cadherins are integral membrane glycoproteins. They are structurally closely related and interspecies homologies are high. The function is mediated through a homophilic binding mechanism, and intracellular proteins, directly or indirectly connected to the cadherins and the cytoskeleton, are necessary for cadherin activity. Cadherins have been implicated in segregation and aggregation of tissues at early developmental stages and in growth and guidance of axons during nervous system development. These functions are modified by changes in type(s) and amount of cadherins expressed at different developmental stages. The regulatory elements guiding cadherin expression are currently being elucidated.

Modulation of myogenic determination factor 1 expression by cell-cell contact

Myogenic determination factor 1 (MyoD1) expression is modulated by a variety of agents including growth factors and activated cellular proto-oncogenes. However, little is known about the effect of cell-cell contact, which increases as myoblasts terminally differentiate, on the control of MyoD1 expression. Steady-state levels of MyoD1 transcripts decline over a 6-12 hour time period when myoblasts plated at a single cell density are incubated in media supplemented with 0.2% serum; by 48 hours MyoD1 mRNA levels have returned to the initial basal level. The decline in MyoD1 transcripts is diminished, but not prevented in myoblasts which maintain cell-cell contacts (at least 50% of cells with two or more sites of contact). MyoD1 transcript levels do not change if single cell cultures are maintained in 10% serum or are cocultured with fibroblasts. Analysis of conditioned media revealed that myoblasts plated at the single cell density or at a density which allowed multiple sites of cell-cell contact are not producing an activity(s) responsible for modulating MyoD1 mRNA levels. The changes in MyoD1 expression are mediated at the transcriptional level. Thus changes in the degree of cell-cell contact in cultures of myogenically determined cell lines effect changes in MyoD1 gene expression. Consequently when the influence of cytokines or other pharmacological agents on commitment to terminal myogenic differentiation is examined, the degree of cell-cell contact within the culture system may affect the response elicited.

Protocadherins: a large family of cadherin-related molecules in central nervous system

Using the polymerase chain reaction, we have isolated numerous rat and human cDNAs of which the deduced amino acid sequences are highly homologous to the sequences of the extracellular domain of cadherins. The entire putative coding sequences for two human proteins defined by two of these cDNAs have been determined. The overall structure of these molecules is very similar to that of classic cadherins, but they have some unique features. The extracellular domains are composed of six or seven subdomains that are very similar to those of cadherins, but have characteristic properties. The cytoplasmic domains, on the other hand, have no significant homology with those of classic cadherins. Since various cDNAs with almost identical features were obtained also from Xenopus, Drosophila and Caenorhabditis elegans, it appears that similar molecules are expressed in a variety of organisms. We have tentatively named these proteins protocadherins. They are highly expressed in brain and their expression appears to be developmentally regulated. The proteins expressed from the two full-length cDNAs in L cells were approximately 170 or 150 kDa in size, and were localized mainly at cell-cell contact sites. Moreover, the transfectants showed cell adhesion activity.

The cell adhesion molecule M-cadherin is specifically expressed in developing and regenerating, but not denervated skeletal muscle

The spatiotemporal distribution of M-cadherin mRNA has been determined by in situ hybridization in the mouse embryo and in adult skeletal muscle following experimental regeneration and denervation. M-cadherin mRNA is highly tissue specific and is found only in developing skeletal muscle. In contrast, N-cadherin mRNA has a broader tissue distribution in the embryo, being found on both neural elements and skeletal and cardiac muscle. M-cadherin is expressed in the myotomes shortly after they form, along with the myogenic regulatory factor myogenin. M-cadherin is expressed in muscles derived from the myotomes and is detected in forelimb bud precursor cells at embryonic day 11.5. In the latter case M-cadherin expression appears co-ordinately with that of myogenin and cardiac alpha-actin. Shortly before birth, M-cadherin expression is down regulated. M-cadherin can, however, be re-expressed following experimental regeneration of skeletal muscle. Here M-cadherin is transiently expressed on regenerating myoblasts but not myotubes. Following muscle denervation no evidence was found for re-expression of M-cadherin under conditions where there was strong expression of the nicotinic acetylcholine receptor on myofibres. The highly specific tissue distribution and unique developmental profile distinguishes M-cadherin from other cadherins and suggests a role in cell surface events during early myogenesis.

Adhesion of epidermal Langerhans cells to keratinocytes mediated by E-cadherin

Langerhans cells (LC) are the principal accessory cells present in epidermis. Because LC have limited capacity for self-renewal, epidermis is continually repopulated by as-yet uncharacterized bone marrow-derived LC progenitors. In addition, although LC persist in epidermis for extended periods, LC are induced to migrate from skin to regional lymph nodes after antigen exposure. To begin to elucidate mechanisms involved in LC trafficking, we characterized LC-keratinocyte (KC) interactions. Here we report that fresh murine LC express cadherins, and that LC adhere to KC in vitro through E-cadherin. Cultured LC (which may bear a phenotypic and functional relationship to LC that have migrated to lymph nodes) express lower levels of E-cadherin and exhibit decreased affinity for KC. These results suggest that expression of E-cadherin by LC promotes persistence of these cells in epidermis, and that cadherins may play important and unanticipated roles in interactions between leukocytes and epithelia.

N-cadherin and N-CAM in myoblast fusion: compared localisation and effect of blockade by peptides and antibodies

The expression and distribution of two cell adhesion molecules, N-cadherin and N-CAM, at the surface of cultured leg muscle cells from 11-day-old chicken embryos were studied and compared. N-cadherin, which was expressed by fusing myoblasts, was down-regulated on old myotubes while N-CAM was still present. Both molecules, as viewed by confocal microscopy, appeared to have coaccumulated at the areas of contact between fusing myoblasts. However, immunogold electron microscopy did not reveal significant colocalization of N-cadherin and N-CAM, and their segregation after antibody-induced patching suggested the absence of direct interactions between N-cadherin and N-CAM. The role of the Ca2+ dependent cell adhesion molecule N-cadherin in myogenesis was investigated. Myoblast fusion was inhibited (1) with a synthetic peptide containing the H-A-V sequence and (2) with a monoclonal anti-N-cadherin antibody, demonstrating that N-cadherin-mediated cell adhesion is required for myoblast fusion. Under the same conditions no effect of anti-N-CAM antibodies was observed. Taken together these observations suggest that N-cadherin, acting independently from N-CAM, is a major cell adhesion molecule involved in embryonic myoblast fusion in vitro.

Glycosyl phosphatidylinositol--anchored T-cadherin mediates calcium-dependent, homophilic cell adhesion

Cadherins are a family of cell adhesion molecules that exhibit calcium-dependent, homophilic binding. Their function depends on both an HisAlaVal sequence in the first extracellular domain, EC1, and the interaction of a conserved cytoplasmic region with intracellular proteins. T-cadherin is an unusual member of the cadherin family that lacks the HisAlaVal motif and is anchored to the membrane through a glycosyl phosphatidylinositol moiety (Ranscht, B., and M. T. Dours-Zimmermann. 1991. Neuron. 7:391-402). To assay the function of T-cadherin in cell adhesion, we have transfected T-cadherin cDNA into CHO cells. Two proteins, mature T-cadherin and the uncleaved T-cadherin precursor, were produced from T-cadherin cDNA. The T-cadherin proteins differed from classical cadherins in several aspects. First, the uncleaved T-cadherin precursor was expressed, together with mature T-cadherin, on the surface of the transfected cells. Second, in the absence of calcium, T-cadherin was more resistant to proteolytic cleavage than other cadherins. Lastly, in contrast to classical cadherins, T-cadherin was not concentrated into cell-cell contacts between transfected cells in monolayer cultures. In cellular aggregation assays, T-cadherin induced calcium-dependent, homophilic adhesion which was abolished by treatment of T-cadherin-transfected cells with phosphatidylinositol-specific phospholipase C. These results demonstrate that T-cadherin is a functional cadherin that differs in several properties from classical cadherins. The function of T-cadherin in homophilic cell recognition implies that the mechanism of T-cadherin-induced adhesion is distinct from that of classical cadherins.

N-cadherin transcripts in Xenopus laevis from early tailbud to tadpole

Cadherins are Ca(++)-dependent cell adhesion molecules which play a key role in morphogenesis and histogenesis. Two mRNAs clones (8 and 9) corresponding to two N-cadherin pseudo-allelic genes are present in Xenopus laevis. We report here that these transcripts share a highly homologous coding region but diverge in the non-coding region. We have determined the pattern of N-cadherin expression at the mRNA level by in situ hybridization with a riboprobe complementary to the EC5 domain of Xenopus N-cadherin clone 8. This part of the sequence is the least conserved in the cadherin gene family, minimizing the risk of cross-hybridization to other cadherins. N-cadherin transcripts are not detectable in the first stages of development. Expression first appears in the neural plate and reaches its maximum level in the CNS at tailbud stage. From early tadpole, it diminishes, so that a very weak signal is detected in the premetamorphic frog brain. N-cadherin expression is not uniform within the CNS, with some areas such as the roof of the rhombencephalon and the olfactory bulbs expressing higher levels of the transcripts. N-cadherin is present in several mesodermal derivatives such as the notochord, the pronephros, and the heart. It is, however, virtually absent from the myotomes and appears in skeletal muscles at later stages of differentiation. All placodes express high levels of N-cadherin. The non-neural ectoderm and the endoderm are always negative. In the brain and the heart, high levels of hybridization are observed with probes corresponding to both copies of the N-cadherin pseudo-allelic genes in their 5' non-coding region, indicating that both alleles are transcribed.

Structure and interactions of desmosomal and other cadherins

The cadherin superfamily of cell-cell adhesion molecules is now known to include proteins of the desmosome as well as of the adherens type of junction. The desmosomal cadherins consist of two families of proteins, the desmocollins and the desmogleins, both of which are represented by different isoforms which are differentially expressed in epidermis. The desmocollins are quite similar to the classic cadherins in overall structure, but with alternatively spliced variants; the desmogleins have extra cytoplasmic sequences added onto the basic cadherin structure. The cytoplasmic domains are specialized for binding to 'mediator' proteins, such as plakoglobin, which interconnect to the intermediate filament system rather than the actin filaments as do the classic cadherins.

Expression of NCAM isoforms during skeletal myogenesis in the mouse embryo

We have examined the developmental patterns of neural cell adhesion molecule (NCAM) gene expression in embryonic mouse skeletal muscle cells by in situ hybridization. Moreover, by utilising exon-specific cRNA probes, we have examined tissue specific splicing of the NCAM gene. We show that there is a distinct sequence of NCAM isoform expression during skeletal muscle development. Since NCAMs are also expressed in other cell types, particularly neurons, NCAM mRNAs have been colocalised with acetylcholine receptor alpha (AChR alpha) gene transcripts to identify muscle-specific expression. NCAM is first detected in somites as they first form, prior to their differentiation into muscle and nonmuscle compartments. Myotomes, the first skeletal muscle masses to form in the embryo, express mRNAs for the transmembrane 180 and 140 kDa isoforms of NCAM. Both of these transcripts are also detected in the neural tube, and their spatial pattern of expression changes with development. Transcripts containing the muscle-specific domain (MSD) of the NCAM gene are not detected prior to 11 days postcoitum (p.c.), at a time when rostral somites already contain well-developed myotomes. As the level of MSD mRNAs increases at 12 days p.c., the 140 and 180 kDa transcript levels decrease in skeletal muscle masses. The level of all NCAM isoform transcripts declines between 13 and 15 days p.c. in muscle. However, the 180 and 140 kDa NCAM isoforms are expressed at a high level in neural tissue and in other locations in the developing embryo such as in smooth muscle, around vibrissae follicles, and in the perichondrial zone of digits.

Classical cadherins

Cadherins represent a gene family of Ca(2+)-dependent cell adhesion molecules (CAMs) identified during development and in adult organs. They generally mediate cell-cell adhesion by homotypic interaction, although heterotypic binding between different cadherin molecules is possible. Molecular cloning and sequence comparison has led to the characterization of a highly homologous group of 'classical' cadherins and more distantly related members, together composing a gene superfamily. The classical cadherins are transmembrane glycoproteins which exhibit, in addition to the structural homologies, a very similar overall protein topology. Protein sequence comparison has led to the identification of domains of common functional importance. The cytoplasmic domains of cadherins associate with peripheral cytoplasmic proteins termed catenin alpha, beta and gamma with molecular weights of 102, 88 and 80 kDa respectively. This complex formation seems to regulate the adhesive function of cadherins, most likely by connecting cadherins with actin microfilaments. Possible implications of catenins for cadherin function are discussed.

Molecular organization of the uvomorulin-catenin complex

The Ca(2+)-dependent cell adhesion molecule uvomorulin is a member of the cadherin gene family. Its cytoplasmic region complexes with structurally defined proteins termed alpha-, beta-, and gamma-catenins. Here we show that A-CAM (N-cadherin), another member of this gene family, also associates with catenins suggesting that this complex formation may be a general property of the cadherins. For uvomorulin it has been found that this association with catenins is of crucial importance for the adhesive function, but little is known about the molecular organization of the uvomorulin-catenin complex. Using a combination of biochemical analyses we show that a single complex is composed of one molecule of uvomorulin, one or two molecules of beta-catenin, and one molecule of alpha-catenin. Furthermore, beta-catenin seems to interact more directly with uvomorulin. In pulse-chase experiments beta-catenin is already associated with the 135-kD uvomorulin precursor molecule but the assembly of the newly synthesized alpha-catenin into the complex is only detected around the time of endoproteolytic processing.