Antagonism of Cell Adhesion by an a -Catenin Mutant, and of the Wnt-signaling Pathway by a -Catenin in Xenopus Embryos

In Xenopus laevis development, b -catenin plays an important role in the Wnt-signaling pathway by establishing the Nieuwkoop center, which in turn leads to specification of the dorsoventral axis. Cadherins are essential for embryonic morphogenesis since they mediate calcium-dependent cell–cell adhesion and can modulate b -catenin signaling. a -catenin links b -catenin to the actin-based cytoskeleton. To study the role of endogenous a -catenin in early development, we have made deletion mutants of a N-catenin. The binding domain of b -catenin has been mapped to the NH 2 terminal 210 amino acids of a N-catenin. Overexpression of mutants lacking the COOH-terminal 230 amino acids causes severe developmental defects that reflect impaired calcium-dependent blastomere adhesion. Lack of normal adhesive interactions results in a loss of the blastocoel in early embryos and ripping of the ectodermal layer during gastrulation. The phenotypes of the dominant-negative mutants can be rescued by coexpressing full-length a N-catenin or a mutant of b -catenin that lacks the internal armadillo repeats. We next show that coexpression of a N-catenin antagonizes the dorsalizing effects of b -catenin and Xwnt-8. This can be seen phenotypically, or by studying the effects of expression on the downstream homeobox gene Siamois. Thus, a -catenin is essential for proper morphogenesis of the embryo and may act as a regulator of the intracellular b -catenin signaling pathway in vivo. C ells in a developing organism depend on various forms of adhesion for their proper morphogenetic movements. Several types of adhesion molecules have been described during the early development of the frog Xenopus laevis. Widely studied are the cadherins, which are transmembrane glycoproteins that mediate calcium-dependent cell–cell adhesion (for review see Huber et al., 1996 a ). The cadherins are in turn linked through their cytoplasmic domains to several cytoplasmic proteins called catenins. b -catenin (and its homologue plakoglobin) has been shown to bind directly to the cadherin cytoplasmic domain, and a -catenin has been shown to bind to the NH 2 terminus of b -catenin (and plakoglobin) (Hülsken et al., 1994; Funayama et al., 1995; Jou et al., 1995; Aberle et al., 1996). In turn, a -catenin appears to bind to a -actinin and actin, establishing a link between the cadherins and the cytoskeleton (Knudsen et al., 1995; Rimm et al., 1995). The regulation of interactions mediated by these molecules is crucial for proper embryogenesis. a -catenin is a 102-kD protein with homology to vinculin (Herrenknecht et al., 1991; Nagafuchi et al., 1991). Two genes encoding isoforms of this protein have been described with a N-catenin having 81.6% identity to the originally described a -catenin (Hirano et al., 1992). a N-catenin has similar properties to a -catenin: both bind to the cadherin complex, but a N-catenin is more prevalent in the nervous system (Hirano et al., 1992). PC9 cells that normally lack a -catenin exhibit cadherin-dependent aggregation upon introduction of a -catenin, identifying the latter as a molecule that promotes cadherin-dependent adhesion (Hirano et al., 1992). In addition, reintroduction of a -catenin or a N-catenin into the same cell line has been shown to induce a polarized phenotype typical of epithelial cells and has also been shown to alter the growth rate (Watabe et al., 1994). Expression of fusion proteins between E-cadherin and the COOH terminus of a -catenin circumvents the requirement of b -catenin for cell adhesion but reduces cell migration, suggesting that b -catenin functions in the cadherin–catenin complex as a regulatable linker between the cytoplasmic domain of cadherins and a -catenin (Nagafuchi et al., 1994). Recently, a gene trap screen in mice identified a fusion between the NH 2 -terminal 632 amino acids of a -catenin and b -geo reporter. Embryos homozygous for this mutant allele were shown to exhibit deficits in cell adhesion resulting in embryonic lethality (Torres et al., 1997). Thus, a -catenin appears to be essential both for Address all correspondence to Louis Reichardt, HHMI, Room U-426, UCSF, Box 0724, San Francisco, CA 94143-0724. Tel.: (415) 476-3976. Fax: (415) 476-9914. E-mail: [email protected] on Jauary 7, 2011 jcb.rress.org D ow nladed fom Published November 17, 1997