Plakoglobin, a protein belonging to the Armadillo-repeat gene family, is th
e only component that adherens junctions and desmosomes have in common. Pla
koglobin null-mutant mouse embryos die because of severe heart defects and
may exhibit an additional skin phenotype, depending on the genetic backgrou
nd. Lack of plakoglobin affects the number and structure of desmosomes, res
ulting in visible defects when cells are subjected to increasing mechanical
stress, e,g, when embryonic blood starts circulating or during skin differ
entiation. By analysing plakoglobin-negative embryonic skin differentiation
in more detail, we show here that, in the absence of plakoglobin, its clos
est homologue, beta-catenin, becomes localized to desmosomes and associated
with desmoglein, This substitution may account for the relatively late app
earance of the developmental defects seen in plakoglobin null-mutant embryo
s.
beta-catenin cannot, however, fully compensate a lack of plakoglobin, In th
e absence of plakoglobin, there was reduced cell-cell adhesion, resulting i
n large intercellular spaces between keratinocytes, subcorneal acantholysis
and necrosis in the granular layer of the skin. Electron microscopic analy
sis documented a reduced number of desmosomes, and those present lacked the
inner dense plaque and had fewer keratin filaments anchored. Our analysis
underlines the central role of plakoglobin for desmosomal assembly and func
tion during embryogenesis.