Glucocorticoids and myosin5b loss of function induce heightened PKA signaling in addition to membrane traffic defects.

Loss-of-function mutations in the nonconventional myosin Vb (Myo5b) result in microvillus inclusion disease (MVID) and massive secretory diarrhea that often begins at birth. Myo5b mutations disrupt the apical recycling endosome (ARE) and membrane traffic, resulting in reduced surface expression of apical membrane proteins. ARE disruption also results in constitutive phosphoinositide-dependent kinase 1 gain of function. In MVID, decreased surface expression of apical anion channels involved in Cl^- extrusion, such as cystic fibrosis transmembrane conductance regulator (CFTR), should reduce fluid secretion into the intestinal lumen. But the opposite phenotype is observed. To explain this contradiction and the onset of diarrhea, we hypothesized that signaling effects downstream from Myo5b loss of function synergize with higher levels of glucocorticoids to activate and CFTR. Data from intestinal cell lines, human MVID, and Myo5b KO mouse intestine revealed changes in the subcellular redistribution of duanyu1529 activity to the apical pole, increased CFTR phosphorylation, and establishment of apical cAMP gradients in Myo5b-defective cells exposed to physiological levels of glucocorticoids. These cells also displayed net secretory fluid fluxes and transepithelial currents mainly from Cl^- secretion. We conclude that Myo5b defects result in duanyu1529 stimulation that activates residual channels on the surface when intestinal epithelia are exposed to glucocorticoids at birth.

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