Diet1, a new player in intestine nutrient and renal protein absorption
The Diet1 gene was identified in an inbred mouse strain that is resistant to diet-induced elevations in plasma cholesterol and atherosclerosis. Diet1 (also known as Malrd1: MAM and LDL Receptor Class A Domain Containing 1) is expressed nearly exclusively in two tissues, the small intestine and kidney. Previous studies identified a role for Diet1 in the regulation of an intestinal hormone that restrains cholesterol conversion to bile acids in the liver. This dissertation further elucidates the role of Diet1 in the absorptive epithelium of the small intestine and provides the initial characterization of Diet1 function in kidney proximal tubules. Following an introductory chapter, Chapter 2 reports the parallels between Diet1-deficiency in the mouse and human primary bile acid diarrhea. These include elevated bile acid levels, increased gastrointestinal motility, and reduced levels of the intestinal hormone fibroblast growth factor 15 (FGF19 in humans). This work also identifies a genetic variant in the coding sequence of human DIET1 that regulates FGF15/19 secretion and is associated with bile acid diarrhea. Chapter 3 further explores the role of Diet1 in intestinal lipid homeostasis and identifies a role for Diet1 in the adaptation to the increased demands for nutrient absorption that occur in mice fed a lipid-enriched diet. Chapter 4 describes the first studies of Diet1 function in the kidney and identifies a role in proximal tubule epithelial cell function. Following the identification of genetic variants in human DIET1 that are associated with albuminuria, we determined that Diet1-deficient mice develop albuminuria due to proximal tubular endocytic dysfunction with intact glomerular function. Transcriptome profiling of primary proximal tubule cells revealed that Diet1 influences the expression of genes associated with extracellular matrix, focal adhesion, and tight junction complexes, many of which are substrates for the meprin metalloproteases. We demonstrated that Diet1 and meprin proteins interact, and that Diet1 deficiency alters meprin protein levels and subcellular localization, which likely contributes to impaired proximal tubule barrier function and paracellular flux of proteins such as albumin in Diet1-deficient mice. The concluding chapter synthesizes our findings that establish a role for Diet1 in the absorptive properties and barrier function of epithelial cells of the small intestine and kidney proximal tubule.