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BACKGROUND & AIMS - Inactivating mutations in MYO5B cause microvillus inclusion disease (MVID), but the physiological cause of the diarrhea associated with this disease is unclear. We investigated whether loss of MYO5B results in aberrant expression of apical enterocyte transporters.
METHODS - We studied alterations in apical membrane transporters in MYO5B-knockout mice, as well as mice with tamoxifen-inducible, intestine-specific disruption of Myo5b (VilCre;Myo5b mice) or those not given tamoxifen (controls). Intestinal tissues were collected from mice and analyzed by immunostaining, immunoelectron microscopy, or cultured enteroids were derived. Functions of brush border transporters in intestinal mucosa were measured in Ussing chambers. We obtained duodenal biopsy specimens from individuals with MVID and individuals without MVID (controls) and compared transporter distribution by immunocytochemistry.
RESULTS - Compared to intestinal tissues from littermate controls, intestinal tissues from MYO5B-knockout mice had decreased apical localization of SLC9A3 (also called NHE3), SLC5A1 (also called SGLT1), aquaporin (AQP) 7, and sucrase isomaltase, and subapical localization of intestinal alkaline phosphatase and CDC42. However, CFTR was present on apical membranes of enterocytes from MYO5B knockout and control mice. Intestinal biopsies from patients with MVID had subapical localization of NHE3, SGLT1, and AQP7, but maintained apical CFTR. After tamoxifen administration, VilCre;Myo5b mice lost apical NHE3, SGLT1, DRA, and AQP7, similar to germline MYO5B knockout mice. Intestinal tissues from VilCre;Myo5b mice had increased CFTR in crypts and CFTR localized to the apical membranes of enterocytes. Intestinal mucosa from VilCre;Myo5b mice given tamoxifen did not have an intestinal barrier defect, based on Ussing chamber analysis, but did have decreased SGLT1 activity and increased CFTR activity.
CONCLUSIONS - Although trafficking of many apical transporters is regulated by MYO5B, trafficking of CFTR is largely independent of MYO5B. Decreased apical localization of NHE3, SGLT1, DRA, and AQP7 might be responsible for dysfunctional water absorption in enterocytes of patients with MVID. Maintenance of apical CFTR might exacerbate water loss by active secretion of chloride into the intestinal lumen.
Copyright © 2018 AGA Institute. Published by Elsevier Inc. All rights reserved.
To gain insight regarding myosin-1A (M1A) function, we expressed a dominant negative fragment of this motor in the intestinal epithelial cell line, CACO-2BBE. Sucrase isomaltase (SI), a transmembrane disaccharidase found in microvillar lipid rafts, was missing from the brush border (BB) in cells expressing this fragment. Density gradient centrifugation, affinity purification, and immunopurification of detergent-resistant membranes isolated from CACO-2BBE cells and rat microvilli (MV) all indicate that M1A and SI reside on the same population of low density (approximately 1.12 g/ml) membranes. Chemical cross-linking of detergent-resistant membranes from rat MV indicates that SI and M1A may interact in a lipid raft complex. The functional significance of such a complex is highlighted by expression of the cytoplasmic domain of SI, which results in lower levels of M1A and a loss of SI from the BB. Together, these studies are the first to assign a specific role to M1A and suggest that this motor is involved in the retention of SI within the BB.
Copyright the Rockefeller University Press
We estimated in vivo turnover rates of sucrase-isomaltase and lactase-phlorizin hydrolase in adult rats. Fed animals received a primed continuous infusion of phenylalanine (300 microCi, 150 mumol Phe/100 g of body weight for 30 s, then 7.5 microCi, 3.75 mumol Phe/min for 10 to 140 min). Sucrase-isomaltase and lactase-phlorizin hydrolase were immunoprecipitated from jejunal mucosal membranes; isoforms were separated by SDS-polyacrylamide gel electrophoresis. Endoglycosidase H digestions and (for lactase-phlorizin hydrolase) N-terminal amino acid sequencing were performed on all isoforms. Specific radioactivity of prosucrase-isomaltase and prolactase-phlorizin hydrolase isoforms reached isotopic equilibrium by 60 and 90 min, respectively. Specific radioactivity of brush border sucrase and lactase did not reach steady state. The isotope kinetic, N-terminal amino acid sequencing, and endoglycosidase H digestion data suggested that one of the high molecular weight lactase isoforms is a dimer of mature lactase. Compartmental modeling of specific radioactivity demonstrated that mean intracellular residence time is 59 min for prosucrase-isomaltase isoforms and 68 min for prolactase-phlorizin hydrolase isoforms. Mean residence time in the brush border was 5.8 h for sucrase and 7.8 h for lactase. Fractional synthesis rates were 414%/day for sucrase and 307%/day for lactase. Thus, in vivo brush border sucrase and lactase turn over at similar rates in the adult rat.