Distinct calcium-independent and calcium-dependent adhesion systems of chicken embryo cells

Communication between bone marrow mesenchymal stem cells and multiple myeloma cells: Impact on disease progression

Multiple myeloma (MM) is a hematological malignancy characterized by the accumulation of immunoglobulin-secreting clonal plasma cells at the bone marrow (BM). The interaction between MM cells and the BM microenvironment, and specifically BM mesenchymal stem cells (BM-MSCs), has a key role in the pathophysiology of this disease. Multiple data support the idea that BM-MSCs not only enhance the proliferation and survival of MM cells but are also involved in the resistance of MM cells to certain drugs, aiding the progression of this hematological tumor. The relation of MM cells with the resident BM-MSCs is a two-way interaction. MM modulate the behavior of BM-MSCs altering their expression profile, proliferation rate, osteogenic potential, and expression of senescence markers. In turn, modified BM-MSCs can produce a set of cytokines that would modulate the BM microenvironment to favor disease progression. The interaction between MM cells and BM-MSCs can be mediated by the secretion of a variety of soluble factors and extracellular vesicles carrying microRNAs, long non-coding RNAs or other molecules. However, the communication between these two types of cells could also involve a direct physical interaction through adhesion molecules or tunneling nanotubes. Thus, understanding the way this communication works and developing strategies to interfere in the process, would preclude the expansion of the MM cells and might offer alternative treatments for this incurable disease.

Adhesion molecules in multiple myeloma oncogenesis and targeted therapy

Every day we march closer to finding the cure for multiple myeloma. The myeloma cells inflict their damage through specialized cellular meshwork and cytokines system. Implicit in these interactions are cellular adhesion molecules and their regulators which include but are not limited to integrins and syndecan-1/CD138, immunoglobulin superfamily cell adhesion molecules, such as CD44, cadherins such as N-cadherin, and selectins, such as E-selectin. Several adhesion molecules are respectively involved in myelomagenesis such as in the transition from the precursor disorder monoclonal gammopathy of undetermined significance to indolent asymptomatic multiple myeloma (smoldering myeloma) then to active multiple myeloma or primary plasma cell leukemia, and in the pathological manifestations of multiple myeloma.

Historical review of the discovery of cadherin, in memory of Tokindo Okada

The cadherin family of cell-cell adhesion molecules plays a pivotal role in animal tissue formation. Discovery of this molecular family can be traced back to some unexpected observations of strange cell behavior that were made around 1970 in the Kyoto University laboratory of Tokindo Okada, and then in the Department of Embryology at the Carnegie Institution of Washington (currently the Carnegie Institution for Science). This article looks back on these discoveries, and recalls how these observations led to the identification of important cell-cell adhesion molecules known as cadherins.

Deep transcriptome profiling sheds light on key players in nucleus implantation induced immune response in the pearl oyster Pinctada martensii

Immunological rejection of the pearl oysters following nucleus implantation is a major issue limiting the successful rate of cultured pearls. To date, the molecular mechanism of immune tolerance during pearl formation in the pearl oysters is still largely unknown. Through the RNA sequencing platform and comparative transcriptomic analysis, we investigated the chronic gene expression changes at seven time points (0, 5, 10, 15, 20, 30, 60 days post implantation or dpi) over a period of 60 days following nucleus implantation in the pearl oyster Pinctada martensii. A total of 81,390 unique transcripts (or unigenes) with a combined length of 96.8 million bp and a N50 value of 2227 bp were obtained. When compared with sequences in the nr, nt, Swiss-Prot, KEGG, COG and GO databases, 36,380 unigenes can find homologous genes. Pairwise comparison of gene expression among all the samples showed that the largest number (or 6846) of differentially expressed genes was observed at 10 dpi. The number then decreased to below 5000 at 15, 20 and 30 dpi and increased again to 6679 at 60 dpi. PCA analysis further showed that the seven time points can be roughly divided into four groups. Comparative transcriptomic analysis between the four groups identified a variety of genes showing differential expression at different time points, including many immune-related genes such as those encoding for toll-like receptor, lectin, scavenger receptor, and peroxidase. In addition, GO and KEGG enrichment analysis revealed that these differentially expressed genes were mainly associated with metabolism, ribosome function, immune response, signaling transduction, and cytoskeleton organization. Notably, two KEGG pathways, namely "cell adhesion molecules" and "primary immunodeficiency" were significantly enriched during the whole process. This finding indicates that genes in these pathways are likely to play critical roles in the immune tolerance of the pearl oysters. To conclude, the data obtained contribute to a better understanding of the molecular mechanisms of nucleus implantation induced immune response in the pearl oysters, and will facilitate the development of effective measures to improve the performance of pearl culture.

The mechanism for keratinocyte detaching from pH-responsive chitosan

In this study, we compared the detachment ratio of HaCaT and Hs68 cells from pH-responsive chitosan surface by raising medium pH from 7.20 to 7.65 for 60 min. The detachment ratio of elongated Hs68 cells was over 75%, but that of round-shaped HaCaT cells was less than 50%, even extending the incubation time to 6 h or enhancing the cytoskeletal contractile force with the Rho activator CN01. However, the addition of 2 mm of EDTA into the medium at pH 7.65 could effectively detach HaCaT cells (detachment ratio > 90%), indicating that the calcium ion played an important role in the detachment process. Therefore, the family of Ca(+2)-dependent integrin receptors was examined by RT-PCR, real-time PCR and immunocytochemistry. It was found the expression of integrin β4 (ITGb4) was HaCaT cell-specific and the mRNA level of ITGb4 in undetached HaCaT cells was significantly higher than that in detached ones. By modulating ITGb4 activity with specific functional blocking antibody ASC-8, the detachment ratio of HaCaT cells could be increased to be greater than 85%. Conversely, the addition of the ligand of ITGb4 laminin into the culture system decreased the medium pH-induced detachment ratio for HaCaT cells, but not for Hs68 cells. Further addition of ASC-8 could rescue the effect of laminin on preventing the detachment of HaCaT cells from pH-sensitive chitosan surface. Therefore, this study demonstrated the interaction of ITGb4 and laminin played an important role in controlling the detachment of HaCaT cells on pH-responsive chitosan.

Evolution of the cadherin-catenin complex

Adherens junctions are the most common junction type found in animal epithelia. Their core components are classical cadherins and catenins, which form membrane-spanning complexes that mediate intercellular binding on the extracellular side and associate with the actin cytoskeleton on the intracellular side. Junctional cadherin-catenin complexes are key elements involved in driving animal morphogenesis. Despite their ubiquity and importance, comparative studies of classical cadherins, catenins and their related molecules suggest that the cadherin/catenin-based adherens junctions have undergone structural and compositional transitions during the diversification of animal lineages. This chapter describes the molecular diversities related to the cadherin-catenin complex, based on accumulated molecular and genomic information. Understanding when and how the junctional cadherin-catenin complex originated, and its subsequent diversification in animals, promotes a comprehensive understanding of the mechanisms of animal morphological diversification.

Adhesive properties of connexin hemichannels

Gap junctions are intercellular channels formed by hemichannels (or connexons) from two neighboring cells. Hemichannels, which are composed of proteins called connexins, can function as conduits of ATP and glutamate, and interact with adhesion molecules and other signaling elements. As a result, their functional repertoire is expanding into other roles, such as control of cell growth or cell migration. Here we further elucidate the involvement of hemichannels in cell-cell adhesion by analyzing how connexins regulate cell adhesion without the need of gap junction formation. Using a short-term aggregation assay with C6-glioma and HeLa cells stably transfected with connexin (Cx) 43 or Cx32, we found that the connexin type dictates the ability of these cells to aggregate, even though these two cell types do not usually adhere to each other. We have also found that high expression of Cx43, but not Cx32 hemichannels, can drive adhesion of cells expressing low levels of Cx43. Aggregation was not dependent on high levels of extracellular Ca(2+), as Ca(2+) removal did not change the aggregation of Cx43-expressing cells. Our data confirm that connexin hemichannels can establish adhesive interactions without the need for functional gap junctions, and support the concept that connexins act as adhesion molecules independently of channel formation.

Cadherins in development and cancer

Proper embryonic development is guaranteed under conditions of regulated cell-cell and cell-matrix adhesion. The cells of an embryo have to be able to distinguish their neighbours as being alike or different. Cadherins, single-pass transmembrane, Ca(2+)-dependent adhesion molecules that mainly interact in a homophilic manner, are major contributors to cell-cell adhesion. Cadherins play pivotal roles in important morphogenetic and differentiation processes during development, and in maintaining tissue integrity and homeostasis. Changes in cadherin expression throughout development enable differentiation and the formation of various organs. In addition to these functions, cadherins have strong implications in tumourigenesis, since frequently tumour cells show deregulated cadherin expression and inappropriate switching among family members. In this review, I focus on E- and N-cadherin, giving an overview of their structure, cellular function, importance during development, role in cancer, and of the complexity of Ecadherin gene regulation.

Pharmacology of cell adhesion molecules of the nervous system

Cell adhesion molecules (CAMs) play a pivotal role in the development and maintenance of the nervous system under normal conditions. They also are involved in numerous pathological processes such as inflammation, degenerative disorders, and cancer, making them attractive targets for drug development. The majority of CAMs are signal transducing receptors. CAM-induced intracellular signalling is triggered via homophilic (CAM-CAM) and heterophilic (CAM - other counter-receptors) interactions, which both can be targeted pharmacologically. We here describe the progress in the CAM pharmacology focusing on cadherins and CAMs of the immunoglobulin (Ig) superfamily, such as NCAM and L1. Structural basis of CAM-mediated cell adhesion and CAM-induced signalling are outlined. Different pharmacological approaches to study functions of CAMs are presented including the use of specific antibodies, recombinant proteins, and synthetic peptides. We also discuss how unravelling of the 3D structure of CAMs provides novel pharmacological tools for dissection of CAM-induced signalling pathways and offers therapeutic opportunities for a range of neurological disorders.

Single-molecule analysis of cadherin-mediated cell-cell adhesion

Cadherins are ubiquitous cell surface molecules that are expressed in virtually all solid tissues and localize at sites of cell-cell contact. Cadherins form a large and diverse family of adhesion molecules, which play a crucial role in a multitude of cellular processes, including cell-cell adhesion, motility, and cell sorting in maturing organs and tissues, presumably because of their different binding capacity and specificity. Here, we develop a method that probes the biochemical and biophysical properties of the binding interactions between cadherins expressed on the surface of living cells, at the single-molecule level. Single-molecule force spectroscopy reveals that classical cadherins, N-cadherin and E-cadherin, form bonds that display adhesion specificity, and a pronounced difference in adhesion force and reactive compliance, but not in bond lifetime. Moreover, their potentials of interaction, derived from force-spectroscopy measurements, are qualitatively different when comparing the single-barrier energy potential for the dissociation of an N-cadherin-N-cadherin bond with the double-barrier energy potential for an E-cadherin-E-cadherin bond. Together these results suggest that N-cadherin and E-cadherin molecules form homophilic bonds between juxtaposed cells that have significantly different kinetic and micromechanical properties.

Similarities between heterophilic and homophilic cadherin adhesion

The mechanism that drives the segregation of cells into tissue-specific subpopulations during development is largely attributed to differences in intercellular adhesion. This process requires the cadherin family of calcium-dependent glycoproteins. A widely held view is that protein-level discrimination between different cadherins on cell surfaces drives this sorting process. Despite this postulated molecular selectivity, adhesion selectivity has not been quantitatively verified at the protein level. In this work, molecular force measurements and bead aggregation assays tested whether differences in cadherin bond strengths could account for cell sorting in vivo and in vitro. Studies were conducted with chicken N-cadherin, canine E-cadherin, and Xenopus C-cadherin. Both qualitative bead aggregation and quantitative force measurements show that the cadherins cross-react. Furthermore, heterophilic adhesion is not substantially weaker than homophilic adhesion, and the measured differences in adhesion do not correlate with cell sorting behavior. These results suggest that the basis for cell segregation during morphogenesis does not map exclusively to protein-level differences in cadherin adhesion.

Mechanism and dynamics of cadherin adhesion

Cadherins are essential cell adhesion molecules involved in tissue morphogenesis and the maintenance of tissue architecture in adults. The adhesion and selectivity functions of cadherins are located in their extracellular regions. Biophysical studies show that the adhesive activity is not confined to a single interface. Instead, multiple cadherin domains contribute to binding. By contrast, the specificity-determining site maps to the N-terminal domains, which adhere by the reciprocal binding of Trp2 residues from opposing proteins. Structural cooperativity can transmit the effects of subtle structural changes or ligand binding over large distances in the protein. Increasingly, studies show that differential cadherin-mediated adhesion, rather than exclusive homophilic binding between identical cadherins, direct cell segregation and the organization of tissue interfaces during morphogenesis. Force measurements quantified both kinetic and strength differences between different classical cadherins that may underlie cell sorting behavior. Despite the complex adhesion mechanisms and differences in binding properties, cadherin-mediated cell adhesion is also regulated by many other biochemical processes. Elucidating the mechanisms by which cadherins organize cell junctions and tissue architecture requires not only quantitative, mechanistic investigations of cadherin function but also investigations of the biochemical and cellular processes that can modulate those functions.

Neural cell adhesion molecule (N-CAM) homophilic binding mediated by the two N-terminal Ig domains is influenced by intramolecular domain-domain interactions

The mechanism by which the neural cell adhesion molecule, N-CAM, mediates homophilic interactions between cells has been variously attributed to an isologous interaction of the third immunoglobulin (Ig) domain, to reciprocal binding of the two N-terminal Ig domains, or to reciprocal interactions of all five Ig domains. Here, we have used a panel of recombinant proteins in a bead binding assay, as well as transfected and primary cells, to clarify the molecular mechanism of N-CAM homophilic binding. The entire extracellular region of N-CAM mediated bead aggregation in a concentration- and temperature-dependent manner. Interactions of the N-terminal Ig domains, Ig1 and Ig2, were essential for bead binding, based on deletion and mutation experiments and on antibody inhibition studies. These findings were largely in accord with aggregation experiments using transfected L cells or primary chick brain cells. Additionally, maximal binding was dependent on the integrity of the intramolecular domain-domain interactions throughout the extracellular region. We propose that these interactions maintain the relative orientation of each domain in an optimal configuration for binding. Our results suggest that the role of Ig3 in homophilic binding is largely structural. Several Ig3-specific reagents failed to affect N-CAM binding on beads or on cells, while an inhibitory effect of an Ig3-specific monoclonal antibody is probably due to perturbations at the Ig2-Ig3 boundary. Thus, it appears that reciprocal interactions between Ig1 and Ig2 are necessary and sufficient for N-CAM homophilic binding, but that maximal binding requires the quaternary structure of the extracellular region defined by intramolecular domain-domain interactions.

Two regions of cadherin cytoplasmic domains are involved in suppressing motility of a mammary carcinoma cell line

E-cadherin has been termed an "invasion suppressor," yet the mechanism of this suppression is not known. In contrast, several reports indicate N-cadherin does not suppress but, rather, promotes cell motility and invasion. Here, by characterizing a series of chimeric cadherins we defined a previously uncharacterized region consisting of the transmembrane domain and an adjacent portion of the cytoplasmic segment that is responsible for the difference in ability of E- and N-cadherin to suppress movement of mammary carcinoma cells, as quantified from time-lapse video recordings. A mutation in this region enabled N-cadherin to suppress motility, indicating that both E- and N-cadherin can suppress, but the activity of N-cadherin is latent, presumably repressed by binding of a specific inhibitor. To define regions common to E- and N-cadherin that are required for suppression, we analyzed a series of deletion mutants. We found that suppression of movement requires E-cadherin amino acids 699-710. Strikingly, beta-catenin binding is not sufficient for and p120ctn is not involved in suppression of these mammary carcinoma cells. Furthermore, the comparable region of N-cadherin can substitute for this required region in E-cadherin and is required for suppression by the mutant form of N-cadherin that is capable of suppressing. Variations in expression of factors that bind to the two regions we have identified may explain previously observed differences in response of tumor cells to cadherins.

Galpha12 and Galpha13 negatively regulate the adhesive functions of cadherin

Cadherins function to promote adhesion between adjacent cells and play critical roles in such cellular processes as development, tissue maintenance, and tumor suppression. We previously demonstrated that heterotrimeric G proteins of the G12 subfamily comprised of Galpha12 and Galpha13 interact with the cytoplasmic domain of cadherins and cause the release of the transcriptional activator beta-catenin (Meigs, T. E., Fields, T. A., McKee, D. D., and Casey, P. J. (2001) Proc. Natl. Acad. Sci. U. S. A. 98, 519-524). Because of the importance of beta-catenin in cadherin-mediated cell-cell adhesion, we examined whether G12 subfamily proteins could also regulate cadherin function. The introduction of mutationally activated G12 proteins into K562 cells expressing E-cadherin blocked cadherin-mediated cell adhesion in steady-state assays. Also, in breast cancer cells, the introduction of activated G12 proteins blocked E-cadherin function in a fast aggregation assay. Aggregation mediated by a mutant cadherin that lacks G12 binding ability was not affected by activated G12 proteins, indicating a requirement for direct G12-cadherin interaction. Furthermore, in wound-filling assays in which ectopic expression of E-cadherin inhibits cell migration, the expression of activated G12 proteins reversed the inhibition via a mechanism that was independent of G12-mediated Rho activation. These results validate the G12-cadherin interaction as a potentially important event in cell biology and suggest novel roles for G12 proteins in the regulation of cadherin-mediated developmental events and in the loss of cadherin function that is characteristic of metastatic tumor progression.

E-cadherin mediates adhesion and suppresses cell motility via distinct mechanisms

Expression of the calcium-dependent adhesion molecule E-cadherin suppresses the invasion of cells in vitro, but the mechanism of this effect is unknown. To investigate this mechanism, we analyzed the effects of expressing E-cadherin in mouse L-cells and rat astrocyte-like WC5 cells. Increased cellular adhesion mediated by E-cadherin reduced invasion in WC5 cells and in some L-cells, but not in others. In all cases, suppression of invasion was correlated with decreased cell movement as assessed in an in vitro wound-filling assay and a transwell motility assay. To define the relationship between adhesion mediated by E-cadherin and suppression of motility, we analyzed the effects of deleting different regions of the E-cadherin cytoplasmic domain. E-cadherin lacking the entire cytoplasmic domain did not mediate calcium-dependent adhesion and did not reduce cell motility when expressed in WC5 cells. E-cadherin lacking a portion of the catenin-binding domain did not associate with the cytoskeleton and did not promote adhesion, yet still suppressed the motility of WC5 cells. In addition, E-cadherin that retains an intact catenin-binding domain, but lacks a juxtamembrane portion of the cytoplasmic domain, mediated effective adhesion, but did not suppress motility. These results indicate E-cadherin mediates adhesion and suppresses cell motility via distinct of E-cadherin plays a key role in suppressing motility.

Regulated binding of PTP1B-like phosphatase to N-cadherin: control of cadherin-mediated adhesion by dephosphorylation of beta-catenin

Cadherins are a family of cell-cell adhesion molecules which play a central role in controlling morphogenetic movements during development. Cadherin function is regulated by its association with the actin containing cytoskeleton, an association mediated by a complex of cytoplasmic proteins, the catenins: alpha, beta, and gamma. Phosphorylated tyrosine residues on beta-catenin are correlated with loss of cadherin function. Consistent with this, we find that only nontyrosine phosphorylated beta-catenin is associated with N-cadherin in E10 chick retina tissue. Moreover, we demonstrate that a PTP1B-like tyrosine phosphatase associates with N-cadherin and may function as a regulatory switch controlling cadherin function by dephosphorylating beta-catenin, thereby maintaining cells in an adhesion-competent state. The PTP1B-like phosphatase is itself tyrosine phosphorylated. Moreover, both direct binding experiments performed with phosphorylated and dephosphorylated molecules, and treatment of cells with tyrosine kinase inhibitors indicate that the interaction of the PTP1B-like phosphatase with N-cadherin depends on its tyrosine phosphorylation. Concomitant with the tyrosine kinase inhibitor-induced loss of the PTP1B-like phosphatase from its association with N-cadherin, phosphorylated tyrosine residues are retained on beta-catenin, the association of N-cadherin with the actin containing cytoskeleton is lost and N-cadherin-mediated cell adhesion is prevented. Tyrosine phosphatase inhibitors also result in the accumulation of phosphorylated tyrosine residues on beta-catenin, loss of the association of N-cadherin with the actin-containing cytoskeleton, and prevent N-cadherin mediated adhesion, presumably by directly blocking the function of the PTP1B-like phosphatase. We previously showed that the binding of two ligands to the cell surface N-acetylgalactosaminylphosphotransferase (GalNAcPTase), the monoclonal antibody 1B11 and a proteoglycan with a 250-kD core protein, results in the accumulation of phosphorylated tyrosine residues on beta-catenin, uncoupling of N-cadherin from its association with the actin containing cytoskeleton, and loss of N-cadherin function. We now report that binding of these ligands to the GalNAcPTase results in the absence of the PTP1B-like phosphatase from its association with N-cadherin as well as the loss of the tyrosine kinase and tyrosine phosphatase activities that otherwise co-precipitate with N-cadherin. Control antibodies and proteoglycans have no such effect. This effect is similar to that observed with tyrosine kinase inhibitors, suggesting that the GalNAcPTase/proteoglycan interaction inhibits a tyrosine kinase, thereby preventing the phosphorylation of the PTP1B-like phosphatase, and its association with N-cadherin. Taken together these data indicate that a PTP1B-like tyrosine phosphatase can regulate N-cadherin function through its ability to dephosphorylate beta-catenin and that the association of the phosphatase with N-cadherin is regulated via the interaction of the GalNAcPTase with its proteoglycan ligand. In this manner the GalNAcPTase-proteoglycan interaction may play a major role in morphogenetic cell and tissue interactions during development.

Homophilic adhesion mediated by the neural cell adhesion molecule involves multiple immunoglobulin domains

The neural cell adhesion molecule (N-CAM) mediates homophilic binding between a variety of cell types including neurons, neurons and glia, and neurons and muscle cells. The mechanism by which N-CAM on one cell interacts with N-CAM on another, however, is unknown. Attempts to identify which of the five immunoglobulin-like domains (Ig I-V) and the two fibronectin type III repeats (FnIII 1-2) in the extracellular region of N-CAM are involved in this process have led to ambiguous results. We have generated soluble recombinant proteins corresponding to each of the individual immunoglobulin domains and the combined FnIII 1-2 and prepared polyclonal antibodies specific for each. The purified proteins and antibodies were used in aggregation experiments with fluorescent microspheres and chicken embryo brain cells to determine possible contributions of each domain to homophilic adhesion. The recombinant domains were tested for their ability to bind to purified native N-CAM, to bind to each other, and to inhibit the aggregation of N-CAM on microspheres and the aggregation of neuronal cells. Each of the immunoglobulin domains bound to N-CAM, and in solution all of the immunoglobulin domains inhibited the aggregation of N-CAM-coated microspheres. Soluble Ig II, Ig III, and Ig IV inhibited neuronal aggregation; antibodies against whole N-CAM, the Ig III domain, and the Ig I domain all strongly inhibited neuronal aggregation, as well as the aggregation of N-CAM-coated microspheres. Of all the domains, the third immunoglobulin domain alone demonstrated the ability to self-aggregate, whereas Ig I bound to Ig V and Ig II bound to Ig IV. The combined FnIII 1-2 exhibited a slight ability to self-aggregate but did not bind to any of the immunoglobulin-like domains. These results suggest that N-CAM-N-CAM binding involves all five immunoglobulin domains and prompt the hypothesis that in homophilic cell-cell binding mediated by N-CAM these domains may interact pairwise in an antiparallel orientation.

Expression of N-cadherin and alpha-catenin in astrocytomas and glioblastomas

We examined levels of mRNA and protein for N-cadherin, the predominant cadherin in neural tissues, and mRNA levels for the cadherin-associated protein, alpha-catenin, in a series of gliomas and in glioblastoma cell lines. mRNA levels for N-cadherin and alpha-catenin were significantly higher in glioblastomas than in low-grade astrocytomas or normal brain, while the levels of intact N-cadherin protein were similar in glioblastomas, low-grade astrocytomas and brain. In addition, there was no consistent relationship between invasiveness and expression of N-cadherin and alpha-catenin in highly invasive vs minimally invasive tumours within the same histopathological grade. To assess further the relationship between cadherin expression and neural tumour invasion, we measured N-cadherin expression, calcium-dependent cell adhesion and motility of several glioblastoma cell lines. While all N-cadherin-expressing lines were adhesive, no correlation was seen between the level of N-cadherin expression and cell motility. Together, these findings imply that, in contrast to the role played by E-cadherin in carcinomas, N-cadherin does not restrict the invasion of glioblastomas.

Receptor protein tyrosine phosphatase PTPmu associates with cadherins and catenins in vivo

The extracellular segment of the receptor-type type protein tyrosine phosphatase PTPmu, possesses an MAM domain, an immunoglobulin domain, and four fibronectin type-III repeats. It binds homophilically, i.e., PTPmu on the surface of one cell binds to PTPmu on an apposing cell, and the binding site lies within the immunoglobulin domain. The intracellular segment of PTPmu has two PTP domains and a juxtamembrane segment that is homologous to the conserved intracellular domain of the cadherins. In cadherins, this segment interacts with proteins termed catenins to mediate association with the actin cytoskeleton. In this article, we demonstrate that PTPmu associates with a complex containing cadherins, alpha- and beta-catenin in mink lung (MvLu) cells, and in rat heart, lung, and brain tissues. Greater than 80% of the cadherin in the cell is cleared from Triton X-100 lysates of MvLu cells after immunoprecipitation with antibodies to PTPmu; however, the complex is dissociated when lysates are prepared in more stringent, SDS-containing RIPA buffer. In vitro binding studies demonstrated that the intracellular segment of PTPmu binds directly to the intracellular domain of E-cadherin, but not to alpha- or beta-catenin. Consistent with their ability to interact in vivo, PTPmu, cadherins, and catenins all localized to points of cell-cell contact in MvLu cells, as assessed by immunocytochemical staining. After pervanadate treatment of MvLu cells, which inhibits cellular tyrosine phosphatase activity including PTPmu, the cadherins associated with PTPmu are now found in a tyrosine-phosphorylated form, indicating that the cadherins may be an endogenous substrate for PTPmu. These data suggest that PTPmu may be one of the enzymes that regulates the dynamic tyrosine phosphorylation, and thus function, of the cadherin/catenin complex in vivo.

The interaction of the retina cell surface N-acetylgalactosaminylphosphotransferase with an endogenous proteoglycan ligand results in inhibition of cadherin-mediated adhesion

We have previously shown that the binding to cells of a monoclonal antibody directed against the chick neural retina N-acetylgalactosaminylphosphotransferase (GalNAcPTase) results in inhibition of cadherin-mediated adhesion and neurite outgrowth. We hypothesized that the antibody mimics the action of an endogenous ligand. Chondroitin sulfate proteoglycans (CSPGs) are potential ligands because they inhibit adhesion and neurite outgrowth and are present in situ at barriers to neuronal growth. We therefore assayed purified CSPGs for their ability to inhibit homophilic cadherin-mediated adhesion and neurite outgrowth, as well as their ability to bind directly to the GalNAcPTase. A proteoglycan with a 250-kD core protein following removal of chondroitin sulfate chains (250-kD PG) inhibits cadherin-mediated adhesion and neurite outgrowth whether presented as the core protein or as a proteoglycan monomer bearing chondroitin sulfate. A proteoglycan with a 400-kD core protein is not inhibitory in either core protein or monomer form. Treatment of cells with phosphatidylinositol-specific phospholipase C, which removes cell surface GalNAcPTase, abolishes this inhibitory effect. Binding of the 250-kD core protein to cells is competed by the anti-GalNAcPTase antibody 1B11, suggesting that 1B11 and the 250-kD core protein bind to the same site or in close proximity. Moreover, soluble GalNAcPTase binds to the immobilized 250-kD core protein but not to the immobilized 400-kD core protein. Concomitant with inhibition of cadherin mediated adhesion, binding of the 250-kD core protein to the GalNAcPTase on cells results in the enhanced tyrosine phosphorylation of beta-catenin and the uncoupling of N-cadherin from its association with the cytoskeleton. Moreover, the 250-kD PG is present in embryonic chick retina and brain and is associated with the GalNAcPTase in situ. We conclude that the 250-kD PG is an endogenous ligand for the GalNAcPTase. Binding of the 250-kD PG to the GalNAcPTase initiates a signal cascade, involving the tyrosine phosphorylation of beta-catenin, which alters the association of cadherin with the actin-containing cytoskeleton and thereby inhibits adhesion and neurite outgrowth. Regulation of the temporal and spatial expression patterns of each member of the GalNacPTase/250-kD PG interactive pair may create opportunities for interaction that influence the course of development through effects on cadherin-based morphogenetic processes.

Interactions of the chondroitin sulfate proteoglycan phosphacan, the extracellular domain of a receptor-type protein tyrosine phosphatase, with neurons, glia, and neural cell adhesion molecules

Phosphacan is a chondroitin sulfate proteoglycan produced by glial cells in the central nervous system, and represents the extracellular domain of a receptor-type protein tyrosine phosphatase (RPTP zeta/beta). We previously demonstrated that soluble phosphacan inhibited the aggregation of microbeads coated with N-CAM or Ng-CAM, and have now found that soluble 125I-phosphacan bound reversibly to these neural cell adhesion molecules, but not to a number of other cell surface and extracellular matrix proteins. The binding was saturable, and Scatchard plots indicated a single high affinity binding site with a Kd of approximately 0.1 nM. Binding was reduced by approximately 15% after chondroitinase treatment, and free chondroitin sulfate was only moderately inhibitory, indicating that the phosphacan core glycoprotein accounts for most of the binding activity. Immunocytochemical studies of embryonic rat spinal phosphacan, Ng-CAM, and N-CAM have overlapping distributions. When dissociated neurons were incubated on dishes coated with combinations of phosphacan and Ng-CAM, neuronal adhesion and neurite growth were inhibited. 125I-phosphacan bound to neurons, and the binding was inhibited by antibodies against Ng-CAM and N-CAM, suggesting that these CAMs are major receptors for phosphacan on neurons. C6 glioma cells, which express phosphacan, adhered to dishes coated with Ng-CAM, and low concentrations of phosphacan inhibited adhesion to Ng-CAM but not to laminin and fibronectin. Our studies suggest that by binding to neural cell adhesion molecules, and possibly also by competing for ligands of the transmembrane phosphatase, phosphacan may play a major role in modulating neuronal and glial adhesion, neurite growth, and signal transduction during the development of the central nervous system.

The neuronal chondroitin sulfate proteoglycan neurocan binds to the neural cell adhesion molecules Ng-CAM/L1/NILE and N-CAM, and inhibits neuronal adhesion and neurite outgrowth

We have previously shown that aggregation of microbeads coated with N-CAM and Ng-CAM is inhibited by incubation with soluble neurocan, a chondroitin sulfate proteoglycan of brain, suggesting that neurocan binds to these cell adhesion molecules (Grumet, M., A. Flaccus, and R. U. Margolis. 1993. J. Cell Biol. 120:815). To investigate these interactions more directly, we have tested binding of soluble 125I-neurocan to microwells coated with different glycoproteins. Neurocan bound at high levels to Ng-CAM and N-CAM, but little or no binding was detected to myelin-associated glycoprotein, EGF receptor, fibronectin, laminin, and collagen IV. The binding to Ng-CAM and N-CAM was saturable and in each case Scatchard plots indicated a high affinity binding site with a dissociation constant of approximately 1 nM. Binding was significantly reduced after treatment of neurocan with chondroitinase, and free chondroitin sulfate inhibited binding of neurocan to Ng-CAM and N-CAM. These results indicate a role for chondroitin sulfate in this process, although the core glycoprotein also has binding activity. The COOH-terminal half of neurocan was shown to have binding properties essentially identical to those of the full-length proteoglycan. To study the potential biological functions of neurocan, its effects on neuronal adhesion and neurite growth were analyzed. When neurons were incubated on dishes coated with different combinations of neurocan and Ng-CAM, neuronal adhesion and neurite extension were inhibited. Experiments using anti-Ng-CAM antibodies as a substrate also indicate that neurocan has a direct inhibitory effect on neuronal adhesion and neurite growth. Immunoperoxidase staining of tissue sections showed that neurocan, Ng-CAM, and N-CAM are all present at highest concentration in the molecular layer and fiber tracts of developing cerebellum. The overlapping localization in vivo, the molecular binding studies, and the striking effects on neuronal adhesion and neurite growth support the view that neurocan may modulate neuronal adhesion and neurite growth during development by binding to neural cell adhesion molecules.

Cell adhesion alters gene transcription in chicken embryo brain cells and mouse embryonal carcinoma cells

To determine whether changes in gene expression occur in embryonic cells as a consequence of changes in cellular aggregation, chicken embryo brain (CEB) cells isolated from 8-day embryos were allowed to aggregate or prevented from aggregating by treatment with anti-neural cell adhesion molecule (N-CAM) Fab' fragments. A subtractive hybridization cloning strategy was employed to identify genes that might show different levels of expression in the two populations of cells. In addition, the transcription rates of a number of genes specifying CAMs and transcription factors were directly estimated by using nuclear run-off transcription assays. The transcription rates of several genes, including those encoding N-CAM, Ng-CAM, alpha-N-catenin, HoxA4 (Hox1.4), a fatty acid-binding protein, and a subunit of the mitochondrially encoded cytochrome-c oxidase enzyme decreased upon CEB cell aggregation. The transcription rates of several previously unidentified genes either increased or decreased upon aggregation, while the transcription of other genes remained unchanged. The transcription rate of the N-CAM gene was 3.3-fold higher in dissociated than in aggregated CEB cells. This rate of transcription also increased when the brain tissue was dissociated into single cells and the increased rate was maintained by keeping the cells dissociated in the presence of Fab' fragments of antibodies to N-CAM. Decreased transcription rates of the N-CAM gene were also observed upon aggregation of P19 cells, a mouse embryonal carcinoma cell line. Primary chicken embryo liver cells, which aggregate primarily by calcium-dependent adhesion mechanisms, did not show changes in the N-CAM gene or in the other genes whose transcription rates changed in CEB cells and P19 cells. These observations suggest that the types of genes regulated by cell aggregation include those for CAMs themselves as well as for transcription factors that may control the expression of CAMs and other molecules significant for morphogenesis.

Increasing N-CAM-mediated cell-cell adhesion does not reduce invasion of RSV-transformed WC5 rat cerebellar cells

The WC5 rat cerebellar cell line, infected with a Rous sarcoma virus (RSV) that is temperature-sensitive for pp60v-src transformation, expresses high levels of the neural cell adhesion molecule, N-CAM, when grown at the non-permissive temperature for pp60v-src activity. At the permissive temperature, N-CAM expression is 4- to 10-fold reduced and the cells aggregate poorly. To evaluate the effects of variations in N-CAM expression, we compared the invasive ability of transformed WC5 cells that express low levels of N-CAM with transformed cells in which N-CAM-mediated adhesion was restored. WC5 cells were transfected with expression vectors containing cDNAs encoding the 120 or 180 kDa forms of chicken N-CAM linked to constitutive promoters. Several permanently transfected lines that expressed chicken N-CAM at the cell surface were isolated. These cell lines showed enhanced aggregation at the permissive temperature relative to untransfected WC5 cells or cells transfected with control constructs. By comparing the ability of control and transfected WC5 cells to invade reconstituted extracellular matrix, we tested the effect of variations in N-CAM-mediated adhesion on invasion. Clones that expressed high levels of N-CAM showed invasion rates that were similar to control cells, indicating that increasing N-CAM-mediated adhesion does not inhibit the invasiveness of RSV-transformed WC5 cells.

Homophilic and heterophilic binding activities of Nr-CAM, a nervous system cell adhesion molecule

Nr-CAM is a membrane glycoprotein that is expressed on neurons. It is structurally related to members of the N-CAM superfamily of neural cell adhesion molecules having six immunoglobulin-like domains and five fibronectin type III repeats in the extracellular region. We have found that the aggregation of chick brain cells was inhibited by anti-Nr-CAM Fab' fragments, indicating that Nr-CAM can act as a cell adhesion molecule. To clarify the mode of action of Nr-CAM, a mouse fibroblast cell line L-M(TK-) (or L cells) was transfected with a DNA expression construct encoding an entire chicken Nr-CAM cDNA sequence. After transfection, L cells expressed Nr-CAM on their surface and aggregated. Aggregation was specifically inhibited by anti-Nr-CAM Fab' fragments. To check the specificity of this aggregation, a fusion protein (FGTNr) consisting of glutathione S-transferase linked to the six immunoglobulin domains and the first fibronectin type III repeat of Nr-CAM was expressed in Escherichia coli. Addition of FGTNr to the transfected cells blocked their aggregation. Further analysis using a combination of cell aggregation assays, binding of cells to FGTNr-coated substrates, aggregation of FGTNr-coated Covaspheres and binding of FGTNr-coated Covaspheres to FGTNr-coated substrates revealed that Nr-CAM mediates two types of cell interactions: a homophilic, divalent cation-independent binding, and a heterophilic, divalent cation-dependent binding. Homophilic binding was demonstrated between transfected L cells, between chick embryo brain cells and FGTNr, and between Covaspheres to which FGTNr was covalently attached. Heterophilic binding was shown to occur between transfected and untransfected L cells, and between FGTNr and primary chick embryo fibroblasts; in all cases, it was dependent on the presence of either calcium or magnesium. Primary chick embryo glia or a human glial cell line did not bind to FGTNr-coated substrates. The results indicate that Nr-CAM is a cell adhesion molecule of the nervous system that can bind by two distinct mechanisms, a homophilic mechanism that can mediate interactions between neurons and a heterophilic mechanism that can mediate binding between neurons and other cells such as fibroblasts.

Identification of a peptide sequence involved in homophilic binding in the neural cell adhesion molecule NCAM

The neural cell adhesion molecule NCAM is capable of mediating cell-cell adhesion via homophilic interactions. In this study, three strategies have been combined to identify regions of NCAM that participate directly in NCAM-NCAM binding: analysis of domain deletion mutations, mapping of epitopes of monoclonal antibodies, and use of synthetic peptides to inhibit NCAM activity. Studies on L cells transfected with NCAM mutant cDNAs using cell aggregation and NCAM-covasphere binding assays indicate that the third immunoglobulin-like domain is involved in homophilic binding. The epitopes of four monoclonal antibodies that have been previously shown to affect cell-cell adhesion mediated by NCAM were also mapped to domain 3. Overlapping hexapeptides were synthesized on plastic pins and assayed for binding with these monoclonal antibodies. One of them (PP) reacted specifically with the sequence KYSFNY. Synthetic oligopeptides containing the PP epitope were potent and specific inhibitors of NCAM binding activity. A substratum containing immobilized peptide conjugates also exhibited adhesiveness for neural retinal cells. Cell attachment was specifically inhibited by peptides that contained the PP-epitope and by anti-NCAM univalent antibodies. The shortest active peptide has the sequence KYSFNYDGSE, suggesting that this site is directly involved in NCAM homophilic interaction.

Evidence for heterophilic adhesion of embryonic retinal cells and neuroblastoma cells to substratum-adsorbed NCAM

The adhesion of embryonic chicken retinal cells and mouse N2A neuroblastoma cells to purified embryonic chicken retinal NCAM adsorbed on a solid substratum was examined using a quantitative centrifugal adhesion assay. Both cell types adhered to NCAM and the adhesion was specifically inhibited by monovalent anti-NCAM antibody fragments. N2A cell adhesion depended on the amount of NCAM applied to the substratum, was cation independent, and was insensitive to treatment with the cytoskeletal perturbing drugs colchicine and cytochalasin D. These results indicated that the tubulin and actin cytoskeletons were not critically required for adhesion to NCAM and make it unlikely that the cell surface ligand for NCAM is an integrin. Adhesion was however temperature dependent, strengthening greatly after a brief incubation at 37 degrees C. CHO cells transfected with NCAM cDNAs did not adhere specifically to substratum-bound NCAM and pretreatment of N2A cells and retinal cells with anti-NCAM antibodies did not inhibit adhesion to substratum-bound NCAM. These results suggest that a heterophilic interaction between substratum-adsorbed NCAM and a non-NCAM ligand on the surface of the probe cells affects adhesion in this system and support the possibility that heterophilic adhesion may be a function of NCAM in vivo.

Cytotactin binding: inhibition of stimulated proliferation and intracellular alkalinization in fibroblasts

Cytotactin is an extracellular matrix protein that is dynamically and transiently expressed in a place-dependent fashion during development by glial cells, fibroblasts, and several other cell types. In the present study, the effects of cytotactin on cell proliferation were examined in fibroblastic cells in culture. NIH 3T3 mouse cells plated on tissue culture substrata in the presence of soluble cytotactin remained rounded for longer periods than untreated control cells, similar to their response to cytotactin-coated substrates. These rounding effects could be prevented by pretreatment of the cells with nocodazole, a microtubule-disrupting agent. Cytotactin inhibited the proliferation of fibroblasts in culture in a dose- and time-dependent manner, and this inhibition occurred even after nocodazole treatment. In addition, the presence of cytotactin inhibited proliferation stimulated by growth factors or tumor promoter. These effects on cell growth were accompanied by an early inhibition of the intracellular alkalinization that normally occurs upon mitogenic stimulation by a number of growth-promoting agents. Together these observations suggest that cytotactin is an endogenous cell surface modulatory protein and provide a possible mechanism whereby cytotactin may contribute to pattern formation during development, regeneration, tumorigenesis, and wound healing.

Acetylcholine receptor clustering is triggered by a change in the density of a nonreceptor molecule

Acetylcholine receptors become clustered at the neuromuscular junction during synaptogenesis, at least in part via lateral migration of diffusely expressed receptors. We have shown previously that electric fields initiate a specific receptor clustering event which is dependent on lateral migration in aneural muscle cell cultures (Stollberg, J., and S. E. Fraser. 1988. J. Cell Biol. 107:1397-1408). Subsequent work with this model system ruled out the possibility that the clustering event was triggered by increasing the receptor density beyond a critical threshold (Stollberg, J., and S. E. Fraser. 1990. J. Neurosci. 10:247-255). This leaves two possibilities: the clustering event could be triggered by the field-induced change in the density of some other molecule, or by a membrane voltage-sensitive mechanism (e.g., a voltage-gated calcium signal). Electromigration is a slow, linear process, while voltage-sensitive mechanisms respond in a rapid, nonlinear fashion. Because of this the two possibilities make different predictions about receptor clustering behavior in response to pulsed or alternating electric fields. In the present work we have studied subcellular calcium distributions, as well as receptor clustering, in response to such fields. Subcellular calcium distributions were quantified and found to be consistent with the predicted nonlinear response. Receptor clustering, however, behaves in accordance with the predictions of a linear response, consistent with the electromigration hypothesis. The experiments demonstrate that a local increase in calcium, or, more generally, a voltage-sensitive mechanism, is not sufficient and probably not necessary to trigger receptor clustering. Experiments with slowly alternating electric fields confirm the view that the clustering of acetylcholine receptors is initiated by a local change in the density of some non-receptor molecule.

Expression of chicken liver cell adhesion molecule fusion genes in transgenic mice

The tissue-specific expression of the chicken liver cell adhesion molecule (L-CAM) was studied by generating transgenic mice. The rat insulin II promoter was fused to a chicken L-CAM cDNA or to chicken genomic L-CAM sequences. Mice carrying the cDNA showed no expression of L-CAM. Mice carrying L-CAM genomic sequences showed expression in the beta cells of the pancreas, suggesting that sequences in introns or in flanking regions are required for expression. Murine L-CAM was undetectable in the beta cells of the pancreas of those transgenic mice expressing chicken L-CAM and thus appeared to be down-regulated, but expression of the mouse protein was not altered at other sites. Chicken L-CAM was also found in extrapancreatic tissues such as skin, kidney, liver, lung, intestine, blood vessels, and the choroid plexus and leptomeninges of the central nervous system. These findings raised the possibility that the chicken L-CAM gene contains cis regulatory elements that interfere with the specificity of a tissue-specific promoter such as the rat insulin promoter. To test this hypothesis, transgenic mice were produced with a construct containing the murine neurofilament promoter fused to genomic chicken L-CAM sequences. Chicken L-CAM was expressed in the brain and spinal cord, where L-CAM is not normally found, but it was also found in some nonneural tissues (kidney, liver, intestine, lung) in which L-CAM is normally expressed. The combined results suggest that tissue-specific cis-acting elements in the chicken L-CAM gene, when combined with heterologous promoters/enhancers, can generate novel patterns of gene expression.

Identification of a Ca2(+)-dependent cell-cell adhesion molecule in endothelial cells

Confluent cultures of aortic endothelial cells contain two different cell-cell adhesion mechanisms distinguished by their requirement for calcium during trypsinization and adhesion. A hybridoma clone was isolated producing a monoclonal antibody Ec6C10, which inhibits Ca2(+)-dependent adhesion of endothelial cells. There was no inhibition of Ca2(+)-independent adhesion of endothelial cells and only a minor effect on Ca2(+)-dependent adhesion of smooth muscle cells. Immunoblotting analysis shows that the antibody Ec6C10 recognizes a protein in endothelial but not epithelial cells with an apparent molecular weight of 135,000 in reducing conditions and 130,000 in non-reducing conditions. Monoclonal antibody Ec6C10 reacts with an antigen at the cell surface as shown by indirect immunofluorescence of confluent endothelial cells in a junctional pattern outlining the cobblestone morphology of the monolayer. Removal of extracellular calcium increased the susceptibility of the antigen recognized by antibody Ec6C10 to proteolysis by trypsin. The role of the Ca2(+)-dependent cell adhesion molecule in organization of the dense peripheral microfilament band in confluent endothelial cells was examined by adjusting the level of extracellular calcium to modulate cell-cell contact. Addition of the monoclonal antibody Ec6C10 at the time of the calcium switch inhibited the extent of formation of the peripheral F-actin band. These results suggest an association between cell-cell contact and the peripheral F-actin band potentially through the Ca2(+)-dependent CAM.

Intercellular adhesion mediated by human muscle neural cell adhesion molecule: effects of alternative exon use

Mouse 3T3 fibroblasts were permanently transfected with cDNAs encoding isoforms of the neural cell adhesion molecule (N-CAM) present in human skeletal muscle and brain. Parental and transfected cells were then used in a range of adhesion assays. In the absence of external shear forces, transfection with cDNAs encoding either transmembrane or glycosylphosphatidylinositol (GPI)-linked N-CAM species significantly increased the intercellular adhesiveness of 3T3 cells in suspension. Transfection of a cDNA encoding a secreted N-CAM isoform was without effect on adhesion. Cells transfected with cDNAs containing or lacking the muscle-specific domain 1 sequence, a four-exon group spliced into the muscle but not the brain GPI-linked N-CAM species, were equally adhesive in the assays used. We also demonstrate that N-CAM-mediated intercellular adhesiveness is inhibited by 0.2 mg/ml heparin; but, at higher concentrations, reduced adhesion of parental cells was also seen. Coaggregation of fluorescently labeled and unlabeled cell populations was performed and measured by comparing their distribution within aggregates with distributions that assume nonspecific (random) aggregation. These studies demonstrate that random aggregation occurs between transfected cells expressing the transmembrane and GPI-linked N-CAM species and between parental cells and those expressing the secreted N-CAM isoform. Other combinations of these populations tested exhibited partial adhesive specificity, indicating homophilic binding between surface-bound N-CAM. Thus, the approach exploited here allows for a full analysis of the requirements, characteristics, and specificities of the adhesive behavior of individual N-CAM isoforms.

Involvement of cell surface phosphatidylinositol-anchored glycoproteins in cell-cell adhesion of chick embryo myoblasts

During myogenesis myoblasts fuse to form multinucleate cells that express muscle-specific proteins. A specific cell-cell adhesion process precedes lipid bilayer union during myoblast fusion (Knudsen, K. A., and A. F. Horwitz. 1977. Dev. Biol. 58:328-338) and is mediated by cell surface glycoproteins (Knudsen, K. A., 1985. J. Cell Biol. 101:891-897). In this paper we show that myoblast adhesion and myotube formation are inhibited by treating fusion-competent myoblasts with phosphatidylinositol-specific phospholipase C (PI-PLC). The effect of PI-PLC on myoblast adhesion is dose dependent and inhibited by D-myo-inositol 1-monophosphate and the effect on myotube formation is reversible, suggesting a specific, nontoxic effect on myogenesis by the enzyme. A soluble form of adhesion-related glycoproteins is released from fusion-competent myoblasts by treatment with PI-PLC as evidenced by (a) the ability of phospholipase C (PLC)-released material to block the adhesion-perturbing activity of a polyclonal antiserum to intact myoblasts; and (b) the ability of PLC-released glycoprotein to stimulate adhesion-perturbing antisera when injected into mice. PI-PLC treatment of fusion-competent myoblasts releases an isoform of N-CAM into the supernate, suggesting that N-CAM may participate in mediating myoblast interaction during myogenesis.

Cell adhesion to fibronectin and tenascin: quantitative measurements of initial binding and subsequent strengthening response

Cell-substratum adhesion strengths have been quantified using fibroblasts and glioma cells binding to two extracellular matrix proteins, fibronectin and tenascin. A centrifugal force-based adhesion assay was used for the adhesive strength measurements, and the corresponding morphology of the adhesions was visualized by interference reflection microscopy. The initial adhesions as measured at 4 degrees C were on the order of 10(-5)dynes/cell and did not involve the cytoskeleton. Adhesion to fibronectin after 15 min at 37 degrees C were more than an order of magnitude stronger; the strengthening response required cytoskeletal involvement. By contrast to the marked strengthening of adhesion to FN, adhesion to TN was unchanged or weakened after 15 min at 37 degrees C. The absolute strength of adhesion achieved varied according to protein and cell type. When a mixed substratum of fibronectin and tenascin was tested, the presence of tenascin was found to reduce the level of the strengthening of cell adhesion normally observed at 37 degrees C on a substratum of fibronectin alone. Parallel analysis of corresponding interference reflection micrographs showed that differences in the area of cell surface within 10-15 nm of the substratum correlated closely with each of the changes in adhesion observed: after incubation for 15 min on fibronectin at 37 degrees C, glioma cells increased their surface area within close contact to the substrate by integral to 125-fold. Cells on tenascin did not increase their surface area of contact. The increased surface area of contact and the inhibitory activity of cytochalasin b suggest that the adhesive "strengthening" in the 15 min after initial binding brings additional adhesion molecules into the adhesive site and couples the actin cytoskeleton to the adhesion complex.

Functional mapping of cytotactin: proteolytic fragments active in cell-substrate adhesion

Cytotactin is an extracellular matrix glycoprotein with a restricted distribution during development. In electron microscopic images, it appears as a hexabrachion with six arms extending from a central core. Cytotactin binds to other extracellular matrix proteins including a chondroitin sulfate proteoglycan (CTB proteoglycan) and fibronectin. Although cytotactin binds to a variety of cells including fibroblasts and neurons, in some cases it causes cells in culture to round up and it inhibits their migration. To relate these various effects of cytotactin on cell behavior to its binding regions, we have examined its ability to support cell-substrate adhesion and have mapped its cell-binding function onto its structure. In a cell-substrate adhesion assay, fibroblasts bound to cytotactin but remained round. In contrast, they both attached and spread on fibronectin. Neither neurons nor glia bound to cytotactin in this assay. In an assay in which cell-substrate contact was initiated by centrifugation, however, neurons and glia bound well to cytotactin; this binding was blocked by specific anti-cytotactin antibodies. The results suggest that neurons and glia can bind to cytotactin-coated substrates and that these cells, like fibroblasts, possess cell surface ligands for cytotactin. After applying methods of limited proteolysis and fractionation, these assays were used to map the binding functions of cytotactin onto its structure. Fragments produced by limited proteolysis were fractionated into two major pools: one (fraction I) contained disulfide-linked oligomers of a 100-kD fragment and two minor related fragments, and the second (fraction II) contained monomeric 90- and 65-kD fragments. The 90- and 65-kD fragments in fraction II were closely related to each other and were structurally and immunologically distinct from the fragments in fraction I. Only components in fraction I were recognized by mAb M1, which binds to an epitope located in the proximal portion of the arms of the hexabrachion and by a polyclonal antibody prepared against a 75-kD CNBr fragment of intact cytotactin. A mAb (1D8) and a polyclonal antibody prepared against a 35-kD CNBr fragment of cytotactin only recognized components present in fraction II. In cell-binding experiments, fibroblasts, neurons, and glia each adhered to substrates coated with fraction II, but did not adhere to substrates coated with fraction I. Fab fragments of the antibody to the 35-kD CNBr fragment strongly inhibited the binding of cells to cytotactin, supporting the conclusion that fraction II contains a cell-binding region. In addition, Fab fragments of this antibody inhibited the binding of cytotactin to CTB pr

Requirements for the Ca2+-independent component in the initial intercellular adhesion of C2 myoblasts

Using a sensitive and quantitative adhesion assay, we have studied the initial stages of the intercellular adhesion of the C2 mouse myoblast line. After dissociation in low levels of trypsin in EDTA, C2 cells can rapidly reaggregate by Ca2+-independent mechanisms to form large multicellular aggregates. If cells are allowed to recover from dissociation by incubation in defined media, this adhesive system is augmented by a Ca2+-dependent mechanism with maximum recovery seen after 4 h incubation. The Ca2+-independent adhesion system is inhibited by preincubation of cell monolayers with cycloheximide before dissociation. Aggregation is also reduced after exposure to monensin, implicating a role for surface-translocated glycoproteins in this mechanism of adhesion. In coaggregation experiments using C2 myoblasts and 3T3 fibroblasts in which the Ca2+-dependent adhesion system was inactivated, no adhesive specificity between the two cell types was seen. Although synthetic peptides containing the RGD sequence are known to inhibit cell-substratum adhesion in various cell types, incubation of C2 myoblasts with the integrin-binding tetrapeptide, RGDS, greatly stimulated the Ca2+-independent aggregation of these cells while control analogs had no effect. These results show that a Ca2+-independent mechanism alone is sufficient to allow for the rapid formation of multicellular aggregates in a mouse myoblast line, and that many of the requirements and perturbants of the Ca2+-independent system of intercellular myoblast adhesion are similar to those of the Ca2+-dependent adhesion mechanisms.

Expression of neural cell adhesion molecules in normal and pathologic tissues

Because cell-cell and cell-matrix interactions directly affect the growth, differentiation, and morphogenesis of neural tissue, abnormal changes in these processes could have severe pathologic consequences. Over the last few years, it has become possible to investigate these interactions at the molecular level due to advances in the identification and characterization of matrix components and receptors and cell adhesion molecules (CAMs). Emerging evidence suggests that two broad classes of CAMs are represented by the neural CAM, N-CAM, and the epithelial CAM, L-CAM. N-CAM and several other neural adhesion molecules contain immunoglobulin-like domains and do not require calcium for binding. In contrast, L-CAM, N-cadherin, and P-cadherin depend on calcium for activity and share structural features that differ from those of the N-CAM family. All of these CAMs are expressed in early embryos and in a variety of tissues throughout development, although each has a characteristic pattern. Initial studies suggest that injury, oncogenic transformation, and some genetic neurologic disorders are accompanied by changes in CAM expression that alter the adhesive or migratory behavior of cells.

Acetylcholine receptors and concanavalin A-binding sites on cultured Xenopus muscle cells: electrophoresis, diffusion, and aggregation

Using digitally analyzed fluorescence videomicroscopy, we have examined the behavior of acetylcholine receptors and concanavalin A binding sites in response to externally applied electric fields. The distributions of these molecules on cultured Xenopus myoballs were used to test a simple model which assumes that electrophoresis and diffusion are the only important processes involved. The model describes the distribution of concanavalin A sites quite well over a fourfold range of electric field strengths; the results suggest an average diffusion constant of approximately 2.3 X 10(-9) cm2/s. At higher electric field strengths, the asymmetry seen is substantially less than that predicted by the model. Acetylcholine receptors subjected to electric fields show distributions substantially different from those predicted on the basis of simple electrophoresis and diffusion, and evidence a marked tendency to aggregate. Our results suggest that this aggregation is due to lateral migration of surface acetylcholine receptors, and is dependent on surface interactions, rather than the rearrangement of microfilaments or microtubules. The data are consistent with a diffusion-trap mechanism of receptor aggregation, and suggest that the event triggering receptor localization is a local increase in the concentration of acetylcholine receptors, or the electrophoretic concentration of some other molecular species. These observations suggest that, whatever mechanism(s) trigger initial clustering events in vivo, the accumulation of acetylcholine receptors can be substantially enhanced by passive, diffusion-mediated aggregation.