Diversity of the cadherin family: evidence for eight new cadherins in nervous tissue

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.

Endothelial cells: adhesion and tight junctions

This article reviews recent discoveries concerning the identity of endothelial cell adhesion molecules and their participation in intercellular junction formation. Observations relating to the formation of high-resistance tight junctions between brain endothelial cells are emphasized.

A novel endothelial-specific membrane protein is a marker of cell-cell contacts

mAbs were raised in mice against cultured human endothelial cells (EC) and screened by indirect immunofluorescence for their ability to stain intercellular contacts. One mAb denoted 7B4 was identified which, out of many cultured cell types, specifically decorated cultured human EC. The antigen recognized by mAb 7B4 is bound at the appositional surfaces of cultured EC only as they become confluent and is stably expressed at intercellular boundaries of confluent monolayers. EC recognition specificity was maintained when the antibody was assayed by immuno-histochemistry in tissue sections of many normal and malignant tissues and in blood vessels of different size and type. The antigen recognized by 7B4 was enriched at EC intercellular boundaries similarly in vitro and in situ. In vitro, addition of mAb 7B4 to confluent EC increased permeation of macromolecules across monolayers even without any obvious changes of cell morphology. In addition, when EC permeability was increased by agents such as thrombin, elastase, and TNF/gamma IFN, its distribution pattern at intercellular contact rims was severely altered. mAb 7B4 immunoprecipitated a major protein of 140 kD from metabolically and surface-labeled cultured EC extracts which appeared to be an integral membrane glycoprotein. On the basis of its distribution in cultured cells and in tissues in situ, 7B4 antigen is distinct from other described EC proteins enriched at intercellular contacts. NH2-terminal sequencing of the antigen, immunopurified from human placenta, and sequencing of peptides from tryptic peptide maps revealed identity to the cDNA deduced sequence of a recently identified new member of the cadherin family (Suzuki, S., K. Sano, and H. Tanihara. 1991. Cell Regul. 2:261-270.) These data indicate that 7B4 antigen is an endothelial-specific cadherin that plays a role in the organization of lateral endothelial junctions and in the control of permeability properties of vascular endothelium.

The vertebrate adhesive junction proteins beta-catenin and plakoglobin and the Drosophila segment polarity gene armadillo form a multigene family with similar properties

Three proteins identified by quite different criteria in three different systems, the Drosophila segment polarity gene armadillo, the human desmosomal protein plakoglobin, and the Xenopus E-cadherin-associated protein beta-catenin, share amino acid sequence similarity. These findings raise questions about the relationship among the three molecules and their roles in different cell-cell adhesive junctions. We have found that antibodies against the Drosophila segment polarity gene armadillo cross react with a conserved vertebrate protein. This protein is membrane associated, probably via its interaction with a cadherin-like molecule. This cross-reacting protein is the cadherin-associated protein beta-catenin. Using anti-armadillo and antiplakoglobin antibodies, it was shown that beta-catenin and plakoglobin are distinct molecules, which can coexist in the same cell type. Plakoglobin interacts with the desmosomal glycoprotein desmoglein I, and weakly with E-cadherin. Although beta-catenin interacts tightly with E-cadherin, it does not seem to be associated with either desmoglein I or with isolated desmosomes. Anti-armadillo antibodies have been further used to determine the intracellular localization of beta-catenin, and to examine its tissue distribution. The implications of these results for the structure and function of different cell-cell adhesive junctions are discussed.

Specificity of cell adhesion in development: the cadherin superfamily

Cadherins are major cell-surface receptors involved in specific cell adhesion during development. Recent results reveal the existence of a growing array of related molecules involved in various forms of cell-cell adhesion, including that mediated by desmosomes. Comparisons with other families of adhesion receptors suggest testable models for functions of the emerging cadherin superfamily in development and disease.

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.

E-cadherin expression in a particular subset of sensory neurons

We found that the dorsal root ganglia (DRG) and trigeminal ganglia of mouse embryos express the E-cadherin cell-cell adhesion molecule and analyzed its expression profile. E-cadherin expression began around Embryonic Day 12 (E12) in these ganglia, thereafter increased, and persisted to the adult stage. This cadherin was expressed by 10 and 30% of DRG neurons in E17 and postnatal animals, respectively, as well as by satellite cells and some Schwann cells. E-cadherin-positive primary sensory fibers terminated only in a narrow region of the dorsal horn of the spinal cord, which was identified as part of lamina II by double-staining for E-cadherin and substance P or somatostatin. This E-cadherin expressing area of the spinal cord extended to part of the trigeminal nucleus in the medulla. These results showed that E-cadherin is expressed in a particular subset of primary sensory neurons which may have specific functional properties. We suggest that this adhesion molecule may play a role in the selective adhesion of sensory neuronal fibers.

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.

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.

Expression of cadherin and NCAM in human small cell lung cancer cell lines and xenografts

Tumour cell adhesion, detachment and aggregation seem to play an important part in tumour invasion and metastasis, and numerous cell adhesion molecules are expressed by tumour cells. Several families of cell-cell adhesion molecules have been described, of which two groups are particularly well characterised, the cadherin family and the Ig superfamily member, neural cell adhesion molecule (NCAM). We investigated expression of these two adhesion molecule families in small cell lung cancer (SCLC) cell lines and xenografts by immunoblotting. Nineteen tumours established from 15 patients with SCLC were examined. All tumours but one expressed both cadherin and NCAM. The tumours expressed one, two or rarely three cadherin bands, and different combinations of two major isoforms of NCAM with M(r)'s of approximately 190,000 and 135,000. Polysialylation of NCAM, a feature characteristic of NCAM during embryonic development, which may play a role in connection with tumour invasion and metastasis, was found in 14/18 NCAM expressing SCLC tumours. Individual tumours grown as cell lines and as nude mouse xenografts showed no qualitative differences in cadherin or NCAM expression.

Diversity of axonal growth-promoting receptors and regulation of their function

Growth-promoting receptors for substrate-bound molecules are usually found to belong to the integrin, immunoglobulin, or cadherin families of glycoproteins. New members of each of these families have been identified in the past year, and advances have been made in our understanding of their functional regulation.

Pathway recognition by neuronal growth cones: genetic analysis of neural cell adhesion molecules in Drosophila

Genetic analysis has finally come of age in the study of neural cell adhesion molecules and their function during growth cone guidance in Drosophila. Recent studies have shown that fasciclin II, a neural cell adhesion molecule of the immunoglobulin superfamily, functions as a recognition molecule for the MP1 axon pathway, thus serving as the first molecular confirmation for the existence of functional labels on specific axon pathways in the developing organism.

Genes for two calcium-dependent cell adhesion molecules have similar structures and are arranged in tandem in the chicken genome

Genomic sequences immediately upstream of the translational start site for the chicken liver cell adhesion molecule (L-CAM) gene contain a second closely related gene, which, because of its location, we have designated the K-CAM gene. Less than 700 base pairs separate the presumed poly(A) site in the K-CAM gene from the translation initiation site for L-CAM. The sizes of exons 4-15 of the K-CAM gene are almost identical to those in the L-CAM gene and the exon/intron junctions occur at exactly equivalent positions in both genes. Exon 16, which includes the 3' untranslated region, is much shorter in the K-CAM gene and intron sizes and sequences are not generally conserved between the two genes. Probes from the K-CAM gene hybridized to a 3-kilobase mRNA that was present at high levels in embryonic skin, at lower levels in kidney, heart, and gizzard, and at still lower levels in brain and liver, as determined by Northern blotting. The sequence of the predicted gene product was nearly identical to that of the chicken B-cadherin cDNA, although the distribution of the K-CAM gene transcript differed from that reported for the cadherin. The proximity and identical overall structure of the K-CAM and L-CAM genes strongly suggest that they arose by gene duplication and raise the possibility that genes for other calcium-dependent CAMs may be located in clusters. Moreover, the tandem arrangement of the genes may have important implications for the regulation of their expression.

Transmembrane molecular assemblies regulated by the greater cadherin family

The cadherin family of cell-cell adhesion molecules is turning out to be much more diverse than previously thought, with members involved in several kinds of intercellular junctions. The adhesive specificity and cytoskeletal interaction of these members varies. Their cytoplasmic terminals are specialized for binding several families of 'mediator' proteins which interconnect to the actin or intermediate filament systems. These multi-molecular complexes have roles not only in cell-cell adhesion, but also in intracellular signalling of developmental information.

Facilitatory and inhibitory effects of glial cells and extracellular matrix in axonal regeneration

Recent studies have shown that Schwann cells stimulate nerve regeneration by producing nerve growth factor in response to macrophage activation as well as by mediating growth through cell-surface and extracellular matrix adhesion molecules. Neurons sprouting in the central nervous system, however, encounter a hostile environment including mature oligodendrocytes with contact inhibitors of growth cone motility, masses of proliferating astrocytes with surface properties that may block regeneration, and an extracellular environment relatively rich in chondroitin sulfate and tenascin forming a matrix less permissive for regeneration than that found in the peripheral nervous system. In addition, as neurons mature, integrins and cell adhesion molecules are reduced in number (transcriptionally) or in efficacy (post-translationally).

Genomic organization and chromosomal mapping of the mouse P-cadherin gene

Cadherins are a family of Ca(2+)-dependent cell adhesion molecules, that includes P-cadherin, E-cadherin, N-cadherin and L-CAM. In this study, the genomic organization of the mouse P-cadherin gene was determined by analyzing overlapping DNA clones obtained from a mouse genomic library. The results showed that this gene spans over 45 kb and consists of 15 exons. A marked feature of this gene is that the first intron is 23 kbp long accounting for half its length. Comparisons of this structure with that of L-CAM, a chicken cadherin, revealed that the exon-intron boundaries are conserved between the two genes except that the P-cadherin first exon includes the correspoding first and second exons of the L-CAM gene. This gene was also similar to the other in that the second intron, which corresponds to the P-cadherin first intron, is exceptionally longer than other introns. These results suggest that the exon-intron pattern conserved in these genes is of significance for generation of domain structure of cadherin molecules or for their transcriptional regulation. We also determined the chromosomal localization of the P-cadherin gene by interspecific backcross analysis, and found that this gene is located in the central region of mouse chromosome 8 and linked with the E-cadherin locus. This is the first evidence for the linkage of different cadherin genes.

R-cadherin: a novel Ca(2+)-dependent cell-cell adhesion molecule expressed in the retina

cDNAs encoding a novel member of the cadherin cell adhesion receptor family were cloned. This cadherin is expressed in the retina of the chicken and is termed R-cadherin. It is similar to other cadherins in its primary structure, but most resembles N-cadherin, showing 74% amino acid identity. Cells expressing R-cadherin can adhere to those expressing N-cadherin when mixed, but they form homotypic clusters within their chimeric aggregates. In the development of the neural retina, R-cadherin begins to be expressed around embryonic day 8 in both neuronal and glial cells, and this expression continues up to the hatching stage. The pattern of the expression of R-cadherin was different from that of N-cadherin, suggesting distinctive roles in retinal morphogenesis.

Cadherin cell adhesion receptors as a morphogenetic regulator

Cadherins are a family of cell adhesion receptors that are crucial for the mutual association of vertebrate cells. Through their homophilic binding interactions, cadherins play a role in cell-sorting mechanisms, conferring adhesion specificities on cells. The regulated expression of cadherins also controls cell polarity and tissue morphology. Cadherins are thus considered to be important regulators of morphogenesis. Moreover, pathological examinations suggest that the down-regulation of cadherin expression is associated with the invasiveness of tumor cells.