Contributions of the enterocyte brush border to intestinal health and disease

The luminal surface of each epithelial cell is covered by several thousand bristles, known individually as a microvillus and collectively as the brush border. The brush border promotes efficient digestion and absorption of nutrients by increasing intestinal surface area and by acting as a scaffold for enzymes, transporters, and host defense factors. Our proposal is significant because few studies address how microvillus structures are regulated in homeostasis or intestinal disease, despite the critical importance of the intestinal brush border to normal intestinal physiology. Monogenetic diseases with disruption of the brush border present with severe diarrhea and intestinal pathology. We have reported that abnormally short ileal microvillus length is present in a subset of histologically uninflamed Crohn’s disease (CD) ileal tissue samples. This phenotype identifies CD patients at-risk for treatment-refractory disease and future development of strictures, features associated with poor clinical outcome in CD. The major objective of this application is to define mechanisms that regulate microvillus length and brush border maturation. This knowledge could help us understand how to reverse pathological changes that occur at the brush border in Crohn’s disease and other pathologic conditions of the intestine. The key preliminary data supporting this application include 1) high fat diet feeding or acute loss of Ppara in intestinal epithelial cells of mice results in short microvillus length; 2) overexpression of Ppara in intestinal epithelial cells rescues high fat diet-induced microvillus shortening; 3) experimental injury of the duodenum or distal colon of mice cause microvillus length shortening in the ileum; and 4) altered brush border ultrastructure and protein expression are observed in Crohn’s disease vs. control patient ileal biopsy tissues. Our central hypothesis is that microvillus length and brush border maturity are suppressed following high fat diet or intestinal injury and can be restored by PPARα signaling activation within intestinal epithelial cells. In Aim 1, we will determine how high fat diet and intestinal epithelial PPARα signaling alter brush border morphology, ultrastructure, and protein expression using novel conditional mouse models with Ppara gain-of-function or loss-of-function specifically in the intestinal epithelium and air-liquid interface epithelial monolayer cultures. We will also use untargeted and targeted approaches to investigate the cellular mechanisms linking microvillus length changes to PPARα signaling. In Aim 2, we will use Ppara conditional mice to determine if intestinal epithelial PPARα signaling influences inflammatory and tissue injury outcomes and injury-associated ileal brush border changes. We anticipate that completion of the proposed studies will produce the following deliverable: identification of a biological pathway and cellular mechanism that integrates host and environmental influences to modify intestinal brush border morphology and function.