More than half of Crohn?s disease (CD) patients develop stricture induced intestinal obstruction and ultimately requiring surgery. A mechanistic understanding of intestinal stricture formation is mandatory to develop novel preventive and therapeutic approaches. Hyperplasia of the muscularis propria (MP) rather than extracellular matrix (ECM) deposition is a major contributor to intestinal wall thickening and hence gut luminal narrowing. In intestinal segments affected by CD, wrapping of mesenteric fat around the bowel is typically observed, called ?creeping fat?. This is specific for CD and highly associated with the presence of MP hyperplasia and stricturing disease (with or without internal penetrating disease). There are essentially no mechanistic data linking creeping fat with intestinal stricture formation or explaining creeping fat formation. Preliminary results show that mesenteric fat derived lipids selectively induce remarkable proliferation of human intestinal MP muscle cells (HIMC) via long-chain free fatty acids (LC-FFAs) metabolism and uptake into mitochondria through the transporter carnitine palmitoyltransferase 1A (CPT-1A). ECM released by activated HIMC, predominantly fibronectin (FN), selectively promotes migration of primary human mesenteric adipocytes (Ad) and Pre-Ad. This resembles formation of creeping fat around intestinal segments. Hence, we propose the following hypothesis: stricture formation in CD is the result of a feedback loop where creeping fat non-immune cell-derived factors induce smooth muscle cell hyperplasia leading to increased secretion of ECM which promotes further creeping fat formation. This hypothesis will be tested by three specific aims:
Specific Aim 1. Define the mechanisms of creeping fat-induced smooth muscle cell hyperplasia. We will identify creeping fat derived mediators and their cellular source responsible for HIMC proliferation, focusing on FFA signaling pathways, proliferation, mitochondrial function and modulation of the proliferation pathways.
Specific Aim 2. Determine the role of HIMC-derived ECM molecules in integrin-mediated adipocyte migration. We will investigate mechanisms of HIMC-derived ECM leading to fat migration using a loss-of-function and gain-of-function approach with the goal to identify specific integrin signaling pathways.
Specific Aim 3. Investigate the effect of mesenteric fat deletion and mitochondrial muscle metabolism on intestinal smooth muscle hyperplasia in vivo. We will induce experimental fibrosis in two transgenic mouse strains that 1) exhibit fat deletion that can be temporally controlled and 2) delete the mitochondrial transporter CPT-1 prior to and after induction of experimental intestinal fibrosis specifically in ?-SMA positive muscle cells. In addition, we present the first mouse model for creeping fat, developing after repeated intestinal injury. We will assess creeping fat formation and resolution by temporally controlled deletion of FN selectively in ?-SMA positive muscle cells. If successful, this proposal will challenge the paradigm of purely immune-driven ECM deposition driving stricture formation and provide novel mechanisms to prevent or treat stricture associated intestinal obstruction in CD patients.
This project offers a brand-new paradigm on Crohn?s disease (CD)-associated intestinal strictures that will on the short term delve into novel cellular and molecular mechanisms of intestinal stricture formation and on the long-term will eventually foster the development of specific new drugs. Thus, this proposal is directly relevant to IBD and the goals of the National Institute of Health by benefitting more than half of Crohn?s disease (CD) patients and having a tangible impact on ways to prevent intestinal obstruction and avoid surgical resections. By investigating a previously unidentified role of creeping fat-derived factors promoting smooth muscle hyperplasia via increasing smooth muscle cell proliferation, we will deviate from the traditional notion of immune cell-driven ECM deposition, and instead focus on extraluminal factors responsible for fibrosis and stricture formation.
