Journal of Cell Science 109, 2609-2611 (1996) 2609 Printed in Great Britain © The Company of Biologists Limited 1996 JCS4298

COMMENTARY and diversity of the cadherin superfamily

Shintaro T. Suzuki* Doheny Eye Institute and Departments of Ophthalmology and Microbiology, University of Southern California School of Medicine, 1450 San Pablo Street, DVRC-309, Los Angeles, CA 90033, USA *Present address: Department of Developmental Biology, Institute for Developmental Research, Aichi Human Service Center, 713-8 Kamiya-cho, Kasugai-shi, Aichi 480-03, Japan

SUMMARY

Recent cadherin studies have revealed that many cadherins logical role of protocadherins is elusive. Circumstantial and cadherin-related proteins are expressed in various evidence, however, suggests that protocadherins are tissues of different multicellular organisms. These proteins involved in a variety of cell-cell interactions. Since proto- are characterized by the multiple repeats of the cadherin cadherins, and many other new cadherins as well, have motif in their extracellular domains. The members of the unique properties, studies of these cadherins may provide cadherin superfamily are divided into two groups: classical insight into the structure and biological role of the cadherin cadherin type and type. The current superfamily. cadherins appear to have evolved from a protocadherin type. Recent studies have proved the role of classical cadherins in embryogenesis. In contrast, the bio- Key words: Cadherin, Protocadherin,

INTRODUCTION terized by their unique extracellular domains, which are primarily composed of multiple repeats (cadherin repeats) of a Cell-cell interaction is one of the fundamental processes required cadherin specific motif (cadherin motif). The motif is about for development. It is natural to postulate that certain molecules 100 amino acids in length and consists of multiple, highly should mediate this interaction. Cell adhesion molecules are conserved amino acids and short amino acid sequences. The good candidates to act as mediators of at least some parts of the three-dimensional structural model has recently been deter- interaction, and a large number of studies on cell adhesion mined (Overduin et al., 1995; Shapiro et al., 1995). One inter- molecules have been carried out to elucidate their developmen- esting result of the structural studies is that cadherins are clas- tal roles extensively (for review see Hynes and Lauder, 1992). sified into two groups based on their extracellular domain Cadherins are a group of cell adhesion proteins that mediate features (Fig. 1): the classical cadherin type, including classical Ca2+-dependent cell-cell adhesion. The adhesion properties Ð cadherins, desmosomal cadherins and HPT/LI-cadherin; and the homophilic nature of the activity, the high specificity of the the protocadherin type, including vertebrate protocadherins, interaction and the tissue specificity of the expression – led to Drosophila fat and Drosophila DE-cadherin. Cadherins of the the hypothesis that various cadherins might mediate specific classical cadherin type share essentially the same extracellular cell-cell adhesion and play a pivotal role in the formation and domain structure, consisting of five cadherin repeats, each with maintenance of tissues (for review see Takeichi, 1991). Indeed, its own characteristic features. Importantly, these cadherins many cadherins and cadherin-related proteins have been iden- share the cadherin repeats that show the characteristic features tified in various tissues of different organisms (for review see of the third and fifth repeats (EC3 and EC5) of classical Suzuki, 1996). The initial studies appeared to support this cadherins: EC3 has one amino acid deletion near the C hypothesis, but subsequent studies have indicated that the story terminus and the DRE sequence in the middle of the repeat is is not as simple as expected. Nevertheless, the features of these replaced by a DFE or DYE sequence; EC5 contains the char- new proteins are very intriguing, and they appear to play a acteristic four cysteine residues. Cadherins of the protocad- variety of important roles during and other bio- herin type, on the other hand, contain more than five cadherin logical processes. I will describe some of the recent findings repeats in their extracellular domains: the sequences are very and discuss their potential implications. In this commentary, I similar to each other, and none of them contains the charac- will focus on the recently identified protocadherins. teristic features of the EC3 or EC5 of the classical type cadherins (Sano et al., 1993). STRUCTURAL PROPERTIES OF PROTOCADHERINS The cytoplasmic domains of protocadherins, but not AND THE CADHERIN SUPERFAMILY classical cadherins, are variable. Especially, the protocadherin type are highly variable and contain various cytoplasmic Members of the cadherin superfamily (cadherins) are charac- sequences. There has even been a report of a protocadherin 2610 S. T. Suzuki

EC1 EC2 EC3 EC4 EC5 TM CP experiment has proved the role of classical cadherins in embryogenesis (Larue et al., 1994). In contrast, the biological (A) AAAAAAAAAAAAAAAAAA ᭹ ᭹᭹᭹ role of protocadherins is elusive, although a large number of protocadherins are expressed in a variety of organisms. Trans- EC1 EC2 EC3 ECn TM CP fection experiments have indicated that protocadherins were AAAAAAAAAAA AAAA localized at cell-cell contact sites in a Ca2+-dependent manner (B) AAAAAAAAAAA AAAA and showed cell aggregation activity similar to the classical Fig. 1. Structure of the two types of cadherins. Two types of cadherins, indicating that protocadherins have homophilic cadherins are schematically shown. Cadherins of the classical interaction activity. Many cell adhesion properties of the trans- cadherin type (A) contain characteristic EC3 and EC5 repeats. EC3 fectants are similar to those of classical cadherins, but proto- has a DYE or DFE sequence instead of a DRE sequence in the cadherins also show unique properties that have not been middle of the repeat (᭺) and one amino acid deletion near the end of reported for the classical cadherins (Obata et al., 1995; Sago the repeat (᭝), whereas EC5 contains the characteristic four cysteine et al., 1995). One noticeable difference is that the cell adhesion residues (᭹). In contrast, cadherins of the protocadherin type (B) activity of protocadherins appears to be weaker than that of contain more than five repeats in their extracellular domains, which are similar to each other. classical cadherins. The weak cell adhesion activity is not a characteristic property of protocadherins; some classical cadherins do not show strong cell adhesion activity (Tanihara containing the sequence that interacts with the SH3 domain et al., 1994). To date, no one has shown directly the homophilic (Kohmura et al., 1995). In contrast, the classical cadherins interaction of cadherins in vitro, and this has been an enigma contain conserved sequences, to which the protocadherins for cadherin research for many years. However, a consensus is show no homology. now emerging in this field that the homophilic interaction The gene structures of several classical cadherins have been activity of the cadherin extracellular domains is intrinsically reported (Sorkin et al., 1991; Miyatani et al., 1992; Huber et very weak and that cadherins need to make clusters on the cell al., 1996); these results indicate that the overall gene structures surface to generate strong cell adhesion activity. Recently of classical cadherins are essentially the same. Interestingly, Shapiro et al. (1995) even proposed a zipper model based on although classical cadherin genes have two introns in the the crystal structure of a cadherin repeat. region corresponding to each cadherin repeat, the positions of Classical cadherins directly or indirectly associate with the introns are not the same among the repeats. The gene struc- and other proteins (Ozawa et al., 1989; Reynolds et tures of protocadherins, on the other hand, appear to be very al., 1994; Hoschuetzky 1994; Brady-Kalnay et al., 1995). The different from those of classical cadherins. The regions of pro- interaction appears to play a major role in cadherin clustering, tocadherins corresponding to cadherin repeats have fewer which generates the strong cell adhesion activity and may par- introns than classical cadherins. Indeed, the genes of proto- ticipate in other activities (Matsunaga et al., 1988; Dantzig et cadherin 1 (Pcdh1) and protocadherin-2 (Pcdh2) do not contain al., 1994). Protocadherins, on the other hand, do not interact introns in their extracellular domains (unpublished observa- strongly with cytoskeletal proteins; thus, protocadherins are tion). Furthermore, protocadherins make a cluster on mouse easily extracted with detergent and the localization at cell-cell chromosome 18 that is different from the clusters of classical contact sites is very labile. Therefore, the cell adhesion activity cadherins (Obata et al., 1995). These results further support the of protocadherins is predicted to be weaker than that of notion that the two types of cadherins are very different groups. classical cadherins, and indeed, the activity is weak as Cadherins have been found in a variety of multicellular described above. The chimeric Pcdh2 with E-cadherin cyto- organisms such as planaria, hydra, Drosophila, C. elegans, and plasmic domain showed stronger cell aggregation activity various mammals. Interestingly, cadherins of the classical (Obata et al., 1995), which is consistent with this notion. cadherin type have been found only in vertebrates so far, It appears that these cadherins do not have a role in typical whereas cadherins of the protocadherin type have been identi- cell-cell adhesion because of their weak cell adhesion activity fied in a variety of multicellular organisms and the number is and labile localization at cell-cell contact sites. However, con- much larger than that of the classical cadherin type. Recently, sidering other features such as the capability of homophilic however, cadherins of an intermediate type have been reported interaction and the expression of many protocadherins with in Drosophila and sea urchin (Oda et al., 1994; Miller and different cytoplasmic sequences in various organisms, proto- McClay, personal communication). The extracellular domains cadherins may have a role in more general cell-cell interac- of these proteins show the properties of the protocadherins, tions. In this context, the interaction between the cytoplasmic whereas the cytoplasmic domains show high homology to domains of cadherins and the cytoplasmic proteins is very those of classical cadherins. When these findings are taken interesting. If protocadherins play an important role as together, it appears that the primordial cadherins might be the predicted, it should be exerted through the interaction between protocadherin type with different cytoplasmic sequences and the cytoplasmic domains and the cytoplasmic proteins. Our that the classical cadherins found in current vertebrates have recent results suggest that the cytoplasmic domains of proto- evolved from one of the protocadherin type proteins. cadherins interact with several cytoplasmic proteins that are different from the known catenins (Sago et al., 1995). Charac- terization of the proteins should provide useful information FUNCTIONAL PROPERTIES OF PROTOCADHERINS about the biological role of protocadherins. Furthermore, it may be noteworthy that heterophilic inter- Classical cadherins were initially identified as the molecules action of E-cadherin has recently been reported by Cepek et al. that mediate cell-cell adhesion, and a recent knockout mouse (1994). If heterophilic interaction is a common property among Cadherin superfamily 2611 cadherins, the whole story of cadherin function should be chromosomal mapping of the mouse VE-cadherin gene (Cdh5). Genomics rewritten. 32, 21-28. Hynes, R. O. and Lander, A. D. (1992). Contact and adhesive specificities in the associations, migrations, and targeting of cells and axons. Cell 68, 303- 322. CONCLUDING REMARKS Kohmura, N., Kai, N. and Yagi, T. (1995). Diversity of cadherin-like receptor (CNR). Annual Meeting of the Molecular Biology Society of Japan. We now know that a variety of cadherins are expressed in Abstract, p. 240. various tissues of different organisms and that their properties Larue, L., Ohsugi, M., Hirchenhain, J. and Kemler, R. (1994). E-cadherin null mutant embryos fail to form a trophectoderm . Proc. Nat. are highly divergent. It is evident that classical cadherins Acad. Sci. USA 91, 8263-8267. including E-cadherin are just a part of the cadherin superfam- Matsunaga, M., Hatta, K. and Takeichi, M. (1988). Role of N-cadherin cell ily. The major unanswered questions regarding protocadherins, adhesion molecules in the histogenesis of neural retina. Neuron 1, 289-295. and many other new cadherins as well are: (1) what activity do Miyatani, S., Copeland, N. G., Gilbert, D. J., Jenkins, N. A. and Takeichi, M. (1992). Genomic structure and chromosomal mapping of the mouse N- they actually exert in vivo? and (2) what processes are they cadherin gene. Proc. Nat. Acad. Sci. USA 89, 8443-8447. involved in? Circumstantial evidence suggests that they play Obata, S., Sago, H., Mori, N., Rochelle, J. M., Seldin, M. F., Davidson, M., an important role, but we do not know much about their St John, T., Taketani, S. and Suzuki, S. T. (1995). Protocadherin Pcdh2 function. I think several promising approaches are available shows properties similar to, but distinct from, those of classical cadherins. J. now to elucidate their role. The knockout mouse and transgenic Cell Sci. 108, 3765-3773. Oda, H., Uemura, T., Harada, Y., Iwai, Y. and Takeichi, M. (1994). A mouse approaches may provide an important clue. Indeed, our Drosophila homolog of cadherin associated with armadillo and essential for recent preliminary results indicated that ectopic expression of embryonic cell-cell adhesion. Dev. Biol. 165, 716-726. Pcdh2 resulted in the deformation of retinal tissue structure. Overduin, M., Harvey, T. S., Bagby, S., Tong, K. I., Yau, P., Takeichi, M. Furthermore, characterization of the molecules that interact and Ikura, M. (1995). Solution structure of the epithelial cadherin domain responsible for selective cell adhesion. Science 267, 386-389. with the cytoplasmic domains may yield interesting informa- Ozawa, M., Baribault, H. and Kemler, R. (1989). The cytoplasmic domain of tion. the uvomorulin associates with three independent Recent studies on classical cadherins have revealed funda- proteins structurally related in different species. EMBO J. 8, 1711-1717. mental information regarding the structural and functional Reynolds, A. B., Daniel, J., McCrea, P. D., Wheelock, M. J., Wu, J. and properties of cadherins. A structural and functional model of Zhang, Z. (1994). Identification of a new catenin: the tyrosine kinase substrate p120cas associates with E-cadherin complexes. Mol. Cell. Biol. 14, clasical cadherins is now available, but we still have many 8333-8342. important unanswered questions. Sago, H., Kitagawa, M., Obata, S., Mori, N., Taketani, S., Rochelle, J. M., Seldin, M. F., Davidson, M., St John, T. and Suzuki, S. T. (1995). Cloning, I thank Ms Susan Clarke for editorial assistance and Ms Fay Sim expression, and chromosomal localization of a novel cadherin-related for preparing the manuscript. protein, protocadherin-3. Genomics 29, 631-640. Sano, K., Tanihara, H., Heimark, R. L., Obata, S., Davidson, M., St John, T., Taketani, S. and Suzuki, S. (1993). Protocadherins: a large family of cadherin-related molecules in central nervous system. EMBO J. 12, 2249- REFERENCES 2256. Shapiro, L., Fannon, A. M., Kwong, P. D., Thompson, A., Lehmann, M. S., Brady-Kalnay, S. M., Rimm, D. L. and Tonks, N. K. (1995). Receptor Grubel, G., Legrand, J.-F., Als-Nielsen, J., Colman, D. R. and protein tyrosine phosphatase PTP associates with cadherins and catenins in Hendrickson, W. A. (1995). Structural basis of cell-cell adhesion by vivo. J. Cell Biol. 130, 977-986. cadherins. Nature 374, 327-337. Cepek, K. L., Shaw, S. K., Parker, C. M., Russell, G. J., Morrow, J. S., Sorkin, B. C., Gallin, W. J., Edelman, G. M. and Cunningham, B. A. Rimm, D. L. and Brenner, M. B. (1994). Adhesion between epithelial cells (1991). Genes for 2 calcium-dependent cell adhesion molecules have similar E and T lymphocytes mediated by E-cadherin and the 7 . Nature 372, structures and are arranged in tandem in the chicken genome. Proc. Nat. 190-193. Acad. Sci. USA 88, 11545-11549. Dantzig, A. H., Hoskins, J. A., Tabas, L. B., Bright, S., Shepard, R. L., Suzuki, S. T. (1996). Structural and functional diversity of cadherin Jenkins, I. L., Duckworth, D. C., Sportsman, J. R., Mackensen, D., superfamily: Are new members of cadherin superfamily involved in signal Rosteck, P. R. Jr and Skatrud, P. L. (1994). Association of intestinal transduction pathway? J. Cell. Biochem. 60, 1-12. peptide transport with a protein related to the cadherin superfamily. Science Takeichi, M. (1991). Cadherin cell adhesion receptors as a morphogenetic 264, 430-433. regulator. Science 251, 1451-1455. Hoschuetzky, H., Aberle, H. and Kemler, R. (1994). β-catenin mediates the Tanihara, H., Kido, M., Obata, S., Heimark, R. L., Davidson, M., St John interaction of the cadherin-catenin complex with epidermal growth factor T. and Suzuki, S. (1994). Characterization of cadherin-4 and cadherin-5 receptor. J. Cell Biol. 127, 1375-1380. reveals new aspects of cadherins. J. Cell Sci. 107, 1697-1704. Huber, P., Dalmon, J., Engiles, J., Breviario, F., Gory, S., Siracusa, L. D., Buchberg, A. M. and Dejana, E. (1996). Genomic structure and (Received 5 July 1996 - Accepted 12 August 1996)