Cell sorting-out is modulated by both the specificity and amount of different cell adhesion molecules (CAMs) expressed on cell surfaces

Cell adhesion molecules (CAMs) are cell surface glycoproteins that may play a variety of roles in morphogenesis and histogenesis, particularly in defining borders of discrete cell populations. To examine the influence of CAM expression on such cell segregation events in vitro, we have transfected cells with cDNAs coding for two calcium-dependent CAMs of different specificity, the liver CAM (L-CAM) and the structurally related molecule N-cadherin. The cDNAs were introduced separately or together into murine sarcoma S180 cells, which normally do not express these molecules, to produce cell lines denoted S180L, S180cadN, and S180L/cadN, respectively. A number of cell lines of each type were produced that differed in their levels of CAM expression. In adhesion assays, S180L and S180cadN cells aggregated specifically via their respective CAMs, and S180L cells did not appear to adhere to S180cadN cells. Cells expressing high levels of each CAM aggregated more rapidly than cells expressing low levels. Segregation between two cell types occurred when they expressed CAMs of different specificity or different levels of the same CAM. S180L and S180cadN cells both sorted out from untransfected cells, and cells expressing high levels of either L-CAM or N-cadherin segregated from cells expressing low levels of the same CAM; in all cases segregation was inhibited by antibodies specific for the transfected CAM. S180L cells sorted out from S180cadN cells, but this segregation was inhibited only when antibodies to both CAMs were applied together. Doubly transfected S180L/cadN cells also sorted out from S180L cells and from S180cadN cells, and the process was inhibited by antibodies to the unshared CAM (N-cadherin or L-CAM, respectively). Cytochalasin D and nocodazole inhibited sorting-out, consistent with the probable role of microfilaments and microtubules in cell movement and in accord with evidence that the action of these CAMs depends on interactions with cortical cytoplasmic components. Using cDNAs for only two CAMs in these studies, we could distinguish at least eight cell lines by their behavior in sorting-out assays. This suggests that qualitative and quantitative differences in the expression in vivo of a relatively small number of CAMs can lead to a large variety of patterns among cell collectives and their borders during tissue formation.

cDNAs of cell adhesion molecules of different specificity induce changes in cell shape and border formation in cultured S180 cells

The liver cell adhesion molecule (L-CAM) and N-cadherin or adherens junction-specific CAM (A-CAM) are structurally related cell surface glycoproteins that mediate calcium-dependent adhesion in different tissues. We have isolated and characterized a full-length cDNA clone for chicken N-cadherin and used this clone to transfect S180 mouse sarcoma cells that do not normally express N-cadherin. The transfected cells (S180cadN cells) expressed N-cadherin on their surfaces and resembled S180 cells transfected with L-CAM (S180L cells) in that at confluence they formed an epithelioid sheet and displayed a large increase in the number of adherens and gap junctions. In addition, N-cadherin in S180cadN cells, like L-CAM in S180L cells, accumulated at cellular boundaries where it was colocalized with cortical actin. In S180L cells and S180cadN cells, L-CAM and N-cadherin were seen at sites of adherens junctions but were not restricted to these areas. Adhesion mediated by either CAM was inhibited by treatment with cytochalasin D that disrupted the actin network of the transfected cells. Despite their known structural similarities, there was no evidence of interaction between L-CAM and N-cadherin. Doubly transfected cells (S180L/cadN) also formed epithelioid sheets. In these cells, both N-cadherin and L-CAM colocalized at areas of cell contact and the presence of antibodies to both CAMs was required to disrupt the sheets of cells. Studies using divalent antibodies to localize each CAM at the cell surface or to perturb their distributions indicated that in the same cell there were no interactions between L-CAM and N-cadherin molecules. These data suggest that the Ca(++)-dependent CAMs are likely to play a critical role in the maintenance of epithelial structures and support a model for the segregation of CAM mediated binding. They also provide further support for the so-called precedence hypothesis that proposes that expression and homophilic binding of CAMs are necessary for formation of junctional structures in epithelia.

Cadherin expression is required for the spread of Shigella flexneri between epithelial cells

Shigella flexneri, a gram-negative pathogen, invades the human colonic epithelium. After entering epithelial cells, bacteria escape into the cytoplasm, move intracellularly, and pass from cell to cell. The bacterium diverts actin and associated actin-binding proteins to generate a cytoskeleton-based motor that pushes forward the bacterium. As the moving bacterium reaches the inner face of the host-cell cytoplasmic membrane, a protrusion forms that allows passage of this bacterium into a neighboring cell. We show here that components of the intermediate junction are used by the bacterium to allow this passage. Using S180, a mouse fibroblastic sarcoma cell line that does not produce cell adhesion molecules (CAM), and S180L and S180cadN, the same cell line transfected with L-CAM and N-cadherin cDNA, respectively, we demonstrate that expression of a cadherin is required for cell-to-cell spread to occur.

Immobilized dimers of N-cadherin-Fc chimera mimic cadherin-mediated cell contact formation: contribution of both outside-in and inside-out signals

Cell adhesion receptors of the cadherin family are involved in various developmental processes, affecting cell adhesion and migration, and also cell proliferation and differentiation. In order to dissect the molecular mechanisms of cadherin-based cell-cell adhesion and subsequent signal transduction to the cytoskeleton and/or cytoplasm leading to adapted cell responses, we developed an approach allowing us to mimic and control cadherin activation. We produced a dimeric N-cadherin-Fc chimera (Ncad-Fc) which retains structural and functional properties of cadherins, including glycosylation, Ca(2+)-dependent trypsin sensitivity and the ability to mediate Ca(2+)-dependent self-aggregation of covered microbeads. Beads covered with either Ncad-Fc or anti-N-cadherin antibodies specifically bound to N-cadherin expressing cells. Both types of beads induced the recruitment of N-cadherin, beta-catenin, alpha-catenin and p120, by lateral mobilization of preexisting cell membrane complexes. Furthermore, cadherin clustering elicited by Ncad-Fc beads triggered local accumulations of tyrosine phosphorylated proteins, a recruitment and redistribution of actin filaments, as well as local membrane remodeling. These results support a model where the adhesion of cadherin ectodomains is followed by clustering of cadherin/catenin complexes allowing signal transduction affecting both cytoskeletal reorganization and cytoplasmic signal mobilization (outside-in signaling). Interestingly, bead-cell binding was altered by agents promoting microfilament and microtubule depolymerization or tyrosine phosphorylation, indicating a possible regulation of the adhesive properties of the extracellular domain of N-cadherin by intracellular factors (inside-out signaling).

The mechanotransduction machinery at work at adherens junctions

The shaping of a multicellular body, and the maintenance and repair of adult tissues require fine-tuning of cell adhesion responses and the transmission of mechanical load between the cell, its neighbors and the underlying extracellular matrix. A growing field of research is focused on how single cells sense mechanical properties of their micro-environment (extracellular matrix, other cells), and on how mechanotransduction pathways affect cell shape, migration, survival as well as differentiation. Within multicellular assemblies, the mechanical load imposed by the physical properties of the environment is transmitted to neighboring cells. Force imbalance at cell-cell contacts induces essential morphogenetic processes such as cell-cell junction remodeling, cell polarization and migration, cell extrusion and cell intercalation. However, how cells respond and adapt to the mechanical properties of neighboring cells, transmit forces, and transform mechanical signals into chemical signals remain open questions. A defining feature of compact tissues is adhesion between cells at the specialized adherens junction (AJ) involving the cadherin super-family of Ca(2+)-dependent cell-cell adhesion proteins (e.g., E-cadherin in epithelia). Cadherins bind to the cytoplasmic protein β-catenin, which in turn binds to the filamentous (F)-actin binding adaptor protein α-catenin, which can also recruit vinculin, making the mechanical connection between cell-cell adhesion proteins and the contractile actomyosin cytoskeleton. The cadherin-catenin adhesion complex is a key component of the AJ, and contributes to cell assembly stability and dynamic cell movements. It has also emerged as the main route of propagation of forces within epithelial and non-epithelial tissues. Here, we discuss recent molecular studies that point toward force-dependent conformational changes in α-catenin that regulate protein interactions in the cadherin-catenin adhesion complex, and show that α-catenin is the core mechanosensor that allows cells to locally sense, transduce and adapt to environmental mechanical constrains.

Is intercellular communication via gap junctions required for myoblast fusion?

Fusion of myoblasts to form syncitial muscle cells results from a complex series of sequential events including cell alignment, cell adhesion and cell communication. The aim of the present investigation was to assess whether intercellular communication through gap junctions would be required for subsequent membrane fusion. The presence of the gap junction protein connexin 43 at areas of contact between prefusing rat L6 myoblasts was established by immunofluorescent staining. These myoblasts were dye-coupled, as demonstrated by the use of the scrape-loading/dye transfer technique. L6 myoblast dye coupling was reversibly blocked by heptanol in short term experiments as well as after chronic treatment. After a single addition of 3.5 mM heptanol, gap junctions remained blocked for up to 8 hours, then this inhibitory effect decreased gradually, likely because the alcohol was evaporated. Changing heptanol solutions every 8 hours during the time course of L6 differentiation resulted in a lasting drastic inhibition of myoblast fusion. We further investigated the effect of heptanol and of other uncoupling agents on the differentiation of primary cultures of embryonic chicken myoblasts. These cells are transiently coupled by gap junctions before myoblast fusion and prolonged application of heptanol, octanol and 18-beta-glycyrrhetinic acid also inhibited their fusion. The effect of heptanol and octanol was neither due to a cytotoxic effect nor to a modification of cell proliferation. Moreover, heptanol treatment did not alter myoblast alignment and adhesion. Taken together these observations suggest that intercellular communication might be a necessary step for myoblast fusion.

M-cadherin localization in developing adult and regenerating mouse skeletal muscle: possible involvement in secondary myogenesis

In this work, we investigated the distribution of the Ca(2+)-dependent cell adhesion molecule, M-cadherin, in mouse limb muscle during normal development and regeneration. Using two unrelated anti-M-cadherin peptide antibodies, we found scarce M-cadherin immunostaining during primary myogenesis (E12-E14) with no accumulation at areas of cell-cell contact. In contrast, the staining sharply increased in intensity at E16, remained high during secondary myogenesis (E16-P0) but disappeared soon after birth. During secondary myogenesis, M-cadherin was specifically accumulated at the characteristic sites of insertion of secondary myotubes in neighbouring primary myotubes. M-cadherin was also accumulated at the areas of contact between fusing secondary myoblasts and myotubes in vitro. In the adult normal and regenerating muscle, we did not detect M-cadherin accumulations at the surface of myofibres. All together, these observations suggest that M-cadherin is specifically involved in secondary myogenesis.

Growth and cell density-dependent expression of stathmin in C2 myoblasts in culture

Stathmin is a 19-kDa, ubiquitous cytoplasmic phosphoprotein whose expression is strongly regulated during tissue development and maturation and which was proposed as a general relay integrating diverse intracellular signaling pathways. Since myoblasts tend to align and differentiate in vitro toward myotubes above a certain density in culture, we examined the expression of stathmin as a function of cell density in the C2 myogenic cell line. Whereas stathmin was hardly detectable in low-to medium-density cultures corresponding to less than 1 microgram soluble protein/cm2, it became expressed to a stable level above this threshold of cell density. This cell density effect on stathmin expression was not mediated by a diffusible factor, since myogenic C2 or fibroblastic 3T3 cells grown at low and high density within the same culture flask displayed the same pattern of density-dependent stathmin expression, significant stathmin levels being observed only in the dense moiety of the flask. Interestingly, culture conditions which indirectly perturb cell-cell contacts, such as low Ca2+ or incubation with cytoskeleton disrupting agents such as nocodazole or cytochalasin D, prevented the expression of stathmin in C2 cells even at high density. More directly, anti-E-cadherin immunoglobulins, interfering with direct cell-cell contacts of the E-cadherin expressing S180 sarcoma-derived 2B2 cells, also prevented the expression of stathmin in these cells even at high density. Altogether, our results indicate that cell-cell contacts, probably mediated by adhesion molecules such as cadherins, are responsible for the high-density-induced expression of stathmin, which might then participate, in particular in the case of myogenic cells, in the control of the proliferation of cells and of their entry into the differentiation process.

Cadherin-dependent cell aggregation is affected by decapeptide derived from rat extracellular super-oxide dismutase

A synthetic HAV-containing decapeptide homologous to the amino acid sequence 44R-Q53 in rat extracellular superoxide dismutase B affects cadherin-dependent cell aggregation. Cell lines, some of them transfected, expressing different types of cadherins were tested using in vitro cell aggregation and cell dissociation assays. A concentration-dependent inhibition of aggregation by the EC-SOD-derived HAV-containing peptide was detected only in N-cadherin expressing cells. These results suggest the localisation and possible protective role of EC-SOD B for cells expressing N-cadherin.

Cytotactin is involved in synaptogenesis during regeneration of the frog neuromuscular system

The expression of cytotactin, an extracellular matrix glycoprotein involved in morphogenesis and regeneration, was determined in the normal and regenerating neuromuscular system of the frog Rana temporaria. Cytotactin was expressed in adult brain and gut as two major components of Mr 190,000 and 200,000 and a minor form of higher molecular weight, but was almost undetectable in skeletal muscle extract. However, cytotactin was concentrated at the neuromuscular junctions as well as at the nodes of Ranvier. After nerve transection, cytotactin staining increased in the distal stump along the endoneurial tubes. In preparations of basal lamina sheaths of frog cutaneous pectoris muscle obtained by inducing the degeneration of both nerve and muscle fibers, cytotactin was found in dense accumulations at original synaptic sites. In order to define the role of cytotactin in axonal regeneration and muscle reinnervation, the effect of anti-cytotactin antibodies on the reinnervation of the basal lamina sheaths preparations was examined in vivo. In control preparations, regenerating nerve terminals preferentially reinnervate the original synaptic sites. In the presence of anti-cytotactin antibodies, axon regeneration occurred with normal fasciculation and branching but with altered preterminal nerve fibers pathways. Ultrastructural observations showed that synaptic basal laminae reinnervation was greatly delayed or inhibited. These results suggest that cytotactin plays a primordial role in synaptogenesis, at least during nerve regeneration and reinnervation in the adult neuromuscular system.

N-cadherin and N-CAM-mediated adhesion in development and regeneration of skeletal muscle

In this review, the experimental evidence supporting the fact that the cell adhesion molecules N-CAM and N-cadherin are involved in myogenesis has been surveyed. In order to give access to the function of these molecules, a strategy of in vivo localization and in vitro perturbation of their adhesive function by interfering antibodies and peptides was applied. Both molecules are expressed at the surface of myogenic cells during myogenesis in vivo and in vitro. The blockade of the N-CAM adhesion function leads to a mild reduction of the rate of myoblast fusion, while the inhibition of the N-cadherin function induces a drastic inhibition of fusion suggesting that N-cadherin-mediated adhesion is a critical step in the process of myoblast fusion. Both molecules are re-expressed during muscle regeneration suggesting that adult myogenesis is under the control of the same adhesive systems as embryonic and foetal myogenesis.

M-cadherin distribution in the mouse adult neuromuscular system suggests a role in muscle innervation

M-cadherin belongs to the Ca(2+)-dependent cadherin family of cell adhesion molecules and was first isolated from a mouse muscle cell line cDNA library. It is specifically expressed in muscle tissue during development and is supposed to play an important role in secondary myogenesis. In the present study the expression of M-cadherin mRNA and protein and its localization were investigated in adult mouse skeletal muscle and peripheral nerve. The mRNA was abundant in embryonic legs from embryonic day (E)14 to E18. It remained expressed in new-born and adult muscles. In the adult muscle M-cadherin immunoreactivity was only detected at the neuromuscular junction, associated with perijunctional mononucleated cells and on intramuscular nerves. Peripheral nerves were also M-cadherin-positive. The molecule was found at the surface of myelinated nerve fibres where it was concentrated at the node of Ranvier. When a nerve was crushed and allowed to regenerate, M-cadherin was over-expressed at the site of nerve injury and in the distal stump. M-cadherin was also upregulated on the sarcolemma of denervated muscle fibres. Taken together, these observations point toward a much wider tissue distribution of M-cadherin than previously thought. M-cadherin might be involved not only in specific steps of myogenesis but also in some aspects of synaptogenesis, axon/Schwann cell interactions and node of Ranvier structural maintenance.

Distinct location and prevalence of alpha-, beta-catenins and gamma-catenin/plakoglobin in developing and denervated skeletal muscle

We studied the distribution of alpha-catenin, beta-catenin and gamma-catenin/plakoglobin in developing, adult and denervated mouse skeletal muscle. During primary myogenesis, all three catenins present a subsarcolemmal distribution within primary myotubes. During secondary myogenesis they accumulate at myotube-myotube contacts. In contrast to the other catenins, gamma-catenin is strongly expressed in the sarcoplasm. In adult muscle, all three catenins are localized on the presynaptic elements of the neuromuscular junction. In denervated muscles, alpha- and beta-catenins are upregulated like N- and M-cadherin, while the levels of gamma-catenin/plakoglobin remain unchanged. The developmental changes in localization and regulation of alpha- and beta-catenins in muscle compared to gamma-catenin/plakoglobin are suggestive of a privileged association of alpha- and beta-catenins with N- and M-cadherins, while gamma-catenin/plakoglobin appears to be expressed quite independently and must assume a different role during myogenesis.

[Cell adhesion and development of skeletal muscle]

Cell adhesion is a cell autonomous property of pluricellular organisms at the basis of tissues and organs formation. Thus, adhesive processes must be considered as key features of the development of skeletal muscle as well as of other tissues. We present here the actual knowledge on cell adhesion molecules in skeletal muscle morphogenesis. The spatio-temporal expression patterns of N-CAM, N-cadherin, M-cadherin, VLA-4 and VCAM-1 during chicken and mouse myogenesis suggest that these cell adhesion molecules are differentially involved in myoblast-myoblast, myoblast-myotube and myotube-myotube interactions. These molecules link myogenic cells before they are separated by their basal laminae. They can potentially induce preferential cell adhesion and sorting-out as it has been described by Holtfreter. This differential adhesion may lead either to myoblast fusion or to preferential association between primary and secondary myotubes.

Expression and localization of RAP1 proteins during myogenic differentiation

The RAP1 subfamily of small GTPases has been involved in various differentiation programs. In skeletal muscle, several lines of evidence suggest that various small GTPases could be implicated in muscle development. This raised the question of whether the RAP1 proteins (RAP1A and/or RAP1B) could be involved in myogenesis. In the present study, we report on the regulation of RAP1 transcripts and proteins during myogenic differentiation. Northern blot analysis performed with differentiated and undifferentiated C2 myogenic cells pointed out that both genes undergo specific regulation during myogenesis in vitro since differentiation of C2 cells was accompanied by a down-regulation of RAP1B gene transcription and continuous expression of the RAP1A mRNA. In addition, immunofluorescence experiments revealed the accumulation of the RAP1 proteins in differentiated C2 cells and in primary culture of mouse myotubes. Investigation of the intracellular location of RAP1 proteins in undifferentiated and differentiated C2 cells showed that the proteins were associated with the late endocytic compartments. To verify that the build-up of RAP1 proteins had a relevance for developmental mechanisms in vivo, we studied their expression and localization at different stages of skeletal muscle development. We found that RAP1 proteins accumulated in specialized muscle cell domains undergoing important modifications during early and late myogenesis: these were the neuromuscular and myotendinous junctions, respectively. Altogether, our data indicate that RAP1 proteins are regulated during myogenic differentiation.