Serum and Glucocorticoid-Inducible Kinase 1 (SGK1): An Important Contributor to Diarrhea and Malabsorption in Microvillus Inclusion Disease (MVID).

Microvillus Inclusion Disease (MVID) is a lethal congenital diarrheal disease resulting from loss of function mutations in the actin motor myosin VB (MYO5B) that regulates apical traffic. MVID remains without treatment to reverse the severe diarrhea that leads to death. MVID diarrhea results from both increased fluid secretion and malabsorption of ions and carbohydrates in the small intestine. Serum and Glucocorticoid-inducible kinase 1 (SGK1), is a potent regulator of ion transporters including cystic fibrosis transmembrane conductance regulator (CFTR) in the intestine. Since SGK1 upregulates CFTR function in the intestine, we hypothesized that loss of SGK1 could potentially reduce MVID diarrhea by decreasing CFTR fluid secretion. Using CRISPR-Cas 9 approaches, we first generated MYO5B-floxed mice and to generate conditional MYO5B-KO (R26 ER;MYO5B ) mice, we cross bred with R26 ER mice. Tamoxifen-treated R26 ER;MYO5B mice resulted in characteristic features of human MVID including severe diarrhea, Microvillus Inclusions (MIs), defective apical traffic, depolarization of proteins, and ion transporters including CFTR, NHE3 and DRA. MYO5B-KO mice also showed increased phosphorylation of SGK1 and PDK1 in the intestine. SGK1-floxed (SGK1 )mice were crossed with R26 ER;MYO5B mice to generate conditional MYO5B/SGK1 DcKO (R26 ER;MYO5B ;SGK1 ) upon Tamoxifen induction. Surprisingly, tamoxifen treatment (3 day) of MYO5B/SGK1 DcKO mice resulted in more severe diarrhea compared to MYO5B (R26 ER;MYO5B ) mice. Immunoblots of intestinal lysates revealed decreased CFTR, increased alpha- and beta-ENaC, increased phosphorylation of PDK1, Nedd4-2 and pPKCι in MYO5B/SGK1 DcKO vs. MYO5B ScKO mice. Finally, fecal glucose loss was higher with reduced SGLT1 and GLUT2 by immunoblot in MYO5B/SGK1 DcKO vs MYO5B ScKO mice. We conclude that activation of SGK1 pathway and CFTR contribute to worsening of diarrhea and carbohydrate malabsorption in MVID.