BACKGROUND AND AIMS - Inactivating mutations in MYO5B cause severe neonatal diarrhea in Microvillus Inclusion Disease. Loss of active MYO5B causes the formation of pathognomonic inclusions and aberrations in brush border enzymes.
METHODS - We developed three mouse models of germline, constitutively intestinal targeted and inducible intestinal targeted deletion of MYO5B. The mice were evaluated for enterocyte cellular morphology.
RESULTS - Germline MYO5B KO mice showed early diarrhea and failure to thrive with evident microvillus inclusions and loss of apical transporters in the duodenum. IgG was present within inclusions. Apical transporters were lost and inclusions were present in the duodenum, but were nearly absent in the ileum. VillinCre;MYO5B mice showed similar pathology and morphological changes in duodenal enterocytes. In contrast, when MYO5B KO was induced with tamoxifen treatment at 8 weeks of age, VillinCre;MYO5B mice developed severe diarrhea with loss of duodenal brush border enzymes, but few inclusions were observed in enterocytes. However, if tamoxifen is administered to 2-day-old VillinCre;MYO5B mice, prominent microvillus inclusions were observed.
CONCLUSIONS - The microvillus inclusions that develop after MYO5B loss reveal the presence of an unrecognized apical membrane trafficking pathway in neonatal duodenal enterocytes. However, the diarrheal pathology after MYO5B loss is due to deficits in transporter presentation at the apical membrane in duodenal enterocytes.