Development 102. 639-655 (1988) Review Article 639 Printed in Great Britain © The Company of Biologists Limited 1988 The cadherins: cell-cell adhesion molecules controlling animal morphogenesis MASATOSHI TAKEICHI Department of Biophysics, Faculty of Science, Kyoto University, Kitashirakawa, Sakyo-ku, Kyoto 606, Japan Summary Cadherins are a family of glycoproteins involved in the compact type, providing direct evidence for a key role Ca2+-dependent cell-cell adhesion mechanism which of cadherins in cell-cell adhesion. It has been sugges- is detected in most kinds of tissues. Inhibition of the ted that cadherins bind cells by their homophilic cadherin activity with antibodies induces dissociation interactions at the extracellular domain and are as- of cell layers, indicating a fundamental importance of sociated with actin bundles at the cytoplasmic domain. these molecules in maintaining the multicellular struc- It appears that each cadherin subclass has binding ture. Cadherins are divided into subclasses, including specificity and this molecular family is involved in E-, N- and P-cadherins. While all subclasses are selective cell-cell adhesion. In development, the ex- similar in molecular weight, Ca2+- and protease- pression of each cadherin subclass is spatiotemporally sensitivity, each subclass is characterized by a unique regulated and associated with a variety of morphogen- tissue distribution pattern and immunological speci- etic events; e.g. the termination or initiation of ex- ficity. Analysis of amino acid sequences deduced from pression of a cadherin subclass in a given cell collective cDNA encoding these molecules showed that they are is correlated with its segregation from or connection integral membrane proteins of 723-748 amino acids with other cell collectives. Antibodies to cadherins long and share common sequences; similarity in the were shown to perturb the morphogenesis of some sequences between subclasses is in a range of 50-60 % embryonic organs in vitro. These observations suggest when compared within a single animal species. that cadherins play a crucial role in construction of L cells, with very little endogenous cadherin ac- tissues and the whole animal body. tivity, transfected with the cadherin cDNA acquired high cadherin-mediated aggregating activity. Their colony morphology was altered by the ectopic ex- Key words: cadherins, cell adhesion molecule, CAM, pression of cadherins from the dispersed type to the calcium, protein, antibody. Introduction own type when mixed with others (e.g. Roth & Weston, 1967). Such selectivity in cell-cell adhesion Cells of dissociated animal tissues can assemble probably has a key role in the organization of tissues autonomously and reform the original tissue-like comprising multiple cell types. Therefore, it is im- structures (Moscona & Moscona, 1952; Townes & portant to elucidate the molecular basis of selective Holtfreter, 1955; Weiss & Taylor, 1960). In some cell adhesiveness in order to understand tissue con- animal species, dispersed embryonic cells can even struction mechanisms. reconstruct the complete embryonic body (Guidice, To this end, a variety of cell adhesion molecules 1962; Spiegel & Spiegel, 1975; Dan-Sohkawa et al. have been identified (see review by Damsky et al. 1986). The construction of tissues, thus, seems to 1984). Various models have also been proposed to depend at least partly upon the intrinsic morphogen- explain the mechanism of selective cell adhesion (see etic capacity of individual cells. An important prop- review by Curtis, 1967). In this essay, I focus on a erty of cells associated with their morphogenetic particular class of cell-cell adhesion molecules, capacity is their ability to recognize identical or termed 'cadherins', and discuss their role in animal different cell types, adhering preferentially to their morphogenesis. They display properties which can be 640 M. Takeichi implicated in a variety of morphogenetic behaviours Schachner, 1984), G4 (Rathjien etal. 1987) belong to of cells including selective adhesion. the CIDS group, as they do not require Ca2+. Some molecules that can be classified into CIDS require Two distinct cell-cell adhesion mechanisms Mg2"1"; e.g. LFA-1, a member of the integrin super- family involved in leukocyte cell-cell adhesion, re- Cell-cell adhesion is a complex system in both its quires Mg2"1" (Rothlein etal. 1986). Determination of structural and functional aspects. Ultrastructural the amino acid sequence for each molecule will allow studies revealed that cells are connected with multiple further classification of CIDS; N-CAM is now classi- types of junction, such as tight junctions, adherens fied as a member of the immunoglobulin superfamily junctions, gap junctions and desmosomes. Takeichi (Cunningham et al. 1987). Cadherins are the major (1977) found that cell-cell adhesion mechanisms are component of CADS, as described in detail below. functionally divided into two systems, the Ca2+- 2+ dependent and the Ca -independent systems. These Identification of Ca2+-dependent cell-cell two systems coexist on single cells, and can be adhesion molecules differentially removed by trypsin treatments. The 2+ Ca -dependent system (CADS) is highly sensitive to Definition of the Ca2+-dependent cell-cell adhesion 2+ trypsin, but can be protected by Ca against proteol- system (CADS) 2+ ysis. In contrast, the Ca -independent system Ca2+ is an essential ion for cell-cell adhesion in all (CIDS) is inactivated only with high concentrations of animal species; generally, incubation of tissues in trypsin and the proteolytic degradation cannot be 2+ 2+ Ca -free media facilitates their dissociation. Since protected by Ca . Therefore, if cells are treated with Ca2+ could be involved in multiple processes of cell a high concentration of trypsin in the presence of 2+ adhesion, we define 'CADS' as a mechanism whose Ca (TC-treatment), CADS is left intact but CIDS is components are exposed on cell surfaces, require removed. If cells are treated with a low concentration 2+ 2+ Ca for cell-cell binding action and are protected by of trypsin in the absence of Ca (LTE-treatment), Ca2+ against proteolytic cleavage. It was found that a CIDS is left intact but CADS is inactivated (Urushi- large variety of cell lines and cells freshly collected hara et al. 1979). Reaggregation of these treated cells from tissues display this type of aggregating property. thus can be mediated only by CADS or CIDS. A key property of CADS is its resistance to trypsin Treatment of cells with a high concentration of 2+ 2+ treatment in the presence of Ca (TC-treatment). trypsin in the absence of Ca (TE-treatment) causes This effect of Ca2+ on CADS was observed in disappearance of both CADS and CIDS from cell resistance not only to trypsin but also to many kinds surfaces, rendering cells completely nonadhesive to of proteolytic enzymes (Takeichi et al. 1981). This each other. unique character has been utilized as a marker for Protection against proteolysis of a cell aggregation 2+ CADS in its molecular identification. In principle, mechanism by Ca was first observed by Steinberg et cell surface proteins present on TC-treated cells but al. (1973) and the above findings were confirmed not on LTE- or TE-treated cells are regarded as using different cell types (Urushihara et al. 1977; candidates for molecules of CADS. Surface proteins Ueda et al. 1980; Grunwald et al. 1980; Brackenbury with such protease sensitivity have been, in fact, etal. 1981; Magnani etal. 1981; Thomas & Steinberg, found using fibroblasts and teratocarcinoma cells 1981; Thomas et al. 1981; Gibralter & Turner, 1985; (Takeichi, 1977; Takeichi etal. 1981). Knudsen, 1985; Nomura et al. 1986). CADS and CIDS are entirely independent systems; TC-treated Immunological identification of CA DS molecules in cells (with CADS only) of a given type cannot adhere teratocarcinoma to LTE-treated cells (with CIDS only) of the same The 'Fab strategy' has often been used for the type (Takeichi etal. 1979; Gibralter & Turner, 1985). identification of cell adhesion molecules (e.g. Miiller They are immunologically distinguished (Urushihara & Gerisch, 1978; Brackenbury et al. 1977). Antisera et al. 1979) and have physiologically distinct proper- raised against whole cells or their cell membranes ties; e.g. activity of CADS is temperature-dependent sometimes contain antibodies to cell adhesion mol- while that of CIDS is not (Takeichi, 1977). Generally, ecules. Fab preparations of such antisera are cells establish tighter connections with CADS than expected to inhibit cell-cell adhesion. If antisera of with CIDS (Atsumi & Takeichi, 1980). this activity are obtained, it should be possible to Cell-cell adhesion molecules thus far identified can identify molecules that can absorb the adhesion- be classified into either CADS or CIDS. For example, inhibitory effect of the antibodies; these molecules 3 the 125xlO Mr (125K) glycoprotein (Urushihara & are candidates for adhesion molecules. Takeichi, 1980), N-CAM (see review by Rutishauser, Fab preparations of an antibody, obtained by 1984), Ng-CAM (Grumet etal. 1984), LI (Rathjien & injecting teratocarcinoma F9 cells into a rabbit, Cadherins in morphogenesis 641 inhibited the CADS mediated aggregation of these Other approaches have reached a similar con- cells (Takeichi et al. 1981). This inhibitory activity of clusion. Human mammary carcinoma cells spon- the antibody (anti-F9) was fully absorbed with TC- taneously release an 80K peptide into the serum-free treated F9 cells