Myosin-9b Controls Epithelial Brush Border Architecture through Motility-Dependent RhoA Signaling

Genetic variations in the MYO9B gene have been associated with Crohn's disease, celiac disease, and ulcerative colitis. These diseases have been characterized as primarily immune disorders. However, the overall molecular basis for the influence of Myo9b in these diseases remains poorly understood. Using in vivo small intestine ileum and human cell culture models, we identify a molecular function for Myo9b in the regulation of epithelial brush border microvilli. Using live-cell super-resolution microscopy, we characterize the motility of Myo9b as it moves toward enriched puncta at the tips of microvilli and visualize its direct regulation of small GTPase signaling using an active RhoA biosensor. In Myo9b knockout cells, microvilli abundance and dynamics are altered, but the cells ultimately maintain the presence of microvilli and the appropriate incorporation of microvilli specific cytoskeletal to membrane regulators such as Ezrin. Alternatively, expression of the Myo9b-S1011A disease variant as the only genetic copy in human cells results in a total loss of microvilli and Ezrin apical localization. These results indicate that Myo9b is a critical regulator of epithelial cell morphology and microvilli. Further, our data establish that the S1011A disease variant disrupts microvilli in human cells, suggesting a potential mechanistic link to its involvement in disease states.

Inflammatory bowel diseases (IBD) are characterized by intestinal barrier dysfunction. However, it is unknown whether the barrier dysfunction is a primary cause of the disease or a result of the disease-induced immune response. Here, we show that the protein Myosin 9b is required to regulate the epithelial brush border microvilli in human cell culture models, where its motile properties localize its signaling domain to microvilli tips. Introducing Myo9b disease variants into cells results in the loss of proper epithelial cell morphology. Our data suggest a potential mechanism where disease causing Myo9b mutants produce a functionally disruptive protein, contributing to the destruction of the intestinal barrier in disease states.