RhoGEF Solo regulates cell density-dependent keratin remodeling and desmosome maturation
Desmosomes (DSMs) are specialized cell-cell adhesion structures that anchor keratin intermediate filaments and play a crucial role in maintaining the mechanical integrity of epithelial tissues. Keratin filaments associate with DSMs in two characteristic patterns: radial spokes extending from DSMs into the cytoplasmic keratin network and circumferential rims connecting neighboring DSMs. Solo is a Rho-guanine nucleotide exchange factor (RhoGEF) involved in cellular responses to mechanical stress and keratin network reorganization. Here, we investigated the role of Solo in regulating keratin-DSM interactions in response to cell density. The number of radial keratin spokes at cell-cell adhesion sites was markedly reduced at high cell density, concomitant with increased Solo localization at these sites. Conversely, Solo knockout (KO) increased the number of radial keratin spokes. Expression of wild-type Solo, but not a GEF-inactive Solo mutant, rescued the increase in radial keratin spokes in confluent Solo-KO cells, indicating that Solo RhoGEF activity is required for this regulation. Inhibition of actin polymerization or ROCK activity also suppressed the reduction in radial spoke density. Furthermore, expansion microscopy and electron microscopy revealed that DSMs in Solo-KO cells remained small and immature even under confluent conditions. Solo was also partially required for the localization of plakoglobin (PG), a core DSM protein, whereas PG-KO markedly increased the number of radial keratin spokes. These findings suggest that increased cell density promotes DSM maturation accompanied by a reduction in radial keratin spokes, and Solo contributes to both processes, potentially through facilitating PG localization at DSMs and regulating Rho-ROCK-dependent actin cytoskeletal remodeling.