Evidence that dorsal-ventral differences in gap junctional communication in the early Xenopus embryo are generated by β-catenin independent of cell adhesion effects

Abstract

Gap junctional communication (GJC) is regulated in the early Xenopus embryo and quantitative differences in junctional communication correlate with the specification of the dorsal-ventral axis. To address the mechanism that is responsible for regulating this differential communication, we investigated the function of β-catenin during the formation of the dorsal- ventral axis in Xenopus embryos by blocking its synthesis with antisense oligodeoxynucleotides. This method has previously been shown to reduce the level of β-catenin in the early embryo, prior to zygotic transcription, and to inhibit the formation of the dorsal axis (Heasman et al., 1994, Cell 79, 791-803). We show here that antisense inhibition of β-catenin synthesis also reduces GJC among cells in the dorsal hemisphere of 32-cell embryos to levels similar to those observed among ventral cells. Full-length β-catenin mRNA can restore elevated levels of dorsal GJC when injected into β-catenin- deficient oocytes, demonstrating the specificity of the β-catenin depletion with the antisense oligonueleotides. Thus, endogenous β-catenin is required for the observed differential GJC. This regulation of GJC is the earliest known action of the dorsal regulator, β-catenin, in Xenopus development. Two lines of evidence, presented here, indicate that β-catenin acts within the cytoplasm to regulate GJC, rather than through an effect on cell adhesion. First, when EP-cadherin is overexpressed and increased adhesion is observed, embryos display both a ventralized phenotype and reduced dye transfer. Second, a truncated form of β-catenin (i.e., the ARM region), that lacks the cadherin-binding domain, restores dorsal GJC to β-catenin-depleted embryos. Thus, β-catenin appears to regulate GJC independent of its role in cell- cell adhesion, by acting within the cytoplasm through a signaling mechanism.