Desmosome Assembly Regulates Apical-Basal Polarization and Nuclear Shape in Simple Epithelial Cells

In epithelia, the desmosome is a cell-cell adhesive junction that anchors keratin filaments, thereby providing tissue integrity and mechanical resistance. Compared with considerable study on the desmosome in stratified epithelia, the mechanism of desmosome assembly and its functional roles remain incompletely understood in single-layered (simple) epithelia. Here we show that not only E-cadherin (E-cad) but also cadherin-6 (Cdh6) participates in recruitment of plakoglobin (Pg) from E-cad-containing cytoplasmic vesicles to cell-cell junctional regions during Ca²⁺-induced desmosome formation in simple epithelial MDCK cells. Subsequently, Pg cooperates with its related proteins plakophilin-2 (PKP2) and plakophilin-3 (PKP3), being recruited independently of E-cad/Cdh6, in junctional localization of the desmosomal cadherins desmoglein-2 (Dsg2) and desmocollin-2 (Dsc2) and the keratin-anchoring protein desmoplakin (DP), leading to functional keratin filament network formation. We also demonstrate that DP contributes to establishment of apical-basal cell polarity during cystogenesis in 3D culture of MDCK cells; depletion of DP results in formation of aberrant cysts containing cells with inverted polarity. Furthermore, the morphology of the nucleus in MDCK cells appears to be regulated by proper desmosome assembly and subsequent keratin network formation, because depletion of DP as well as that of E-cad/Cdh6, Pg, or PKP2/3 results in an irregular nuclear shape.