Rates of both obesity and diabetes continue to escalate concomitant with economic and health burdens. Increasing evidence suggests that gut-secreted hormones regulate energy homeostasis. Glucagon-like peptide-2 (GLP-2) is a gut-secreted hormone, which is co-secreted with GLP-1 from endocrine L cells in the gut in response to food intake. Through a specific G protein-coupled receptor (GLP-2R), GLP-2 stimulates intestinal crypt cell proliferation and mucosal blood flow to promote nutrient absorption, while it also decreases gastric emptying and gut motility to inhibit food intake. Moreover, GLP-2R global knockout mice shows reduced intestinal adaptive response following changes in energy homeostasis. However, mechanistically it is difficult to interpret this effect because the GLP-2R is expressed in both brain and gut. The hypothalamus integrates central and peripheral signals and exerts homeostatic control over food intake and energy expenditure. In fact, GLP-2R-mediated physiological significance and signaling network in vivo are largely unknown in the central nervous system (CNS). Our preliminary studies show that [1] GLP-2R is expressed in anorexigenic proopiomelanocortin (POMC) neurons in the mouse hypothalamus, suggesting that GLP-2 may play a direct role in the regulation of food intake and glucose homeostasis;[2] central administration of GLP-2 improves glucose tolerance and inhibits food intake, associated with increased expression of pomc mRNA in the arcuate nucleus;[3] central administration of GLP-2 enhances small bowel growth with increased rate of crypt cell proliferation;and [4] GLP-2 increased L-type Ca2+ channel activity in primary Hippocampal neurons, suggesting that GLP-2 may act in both paracrine and neuroendocrine ways via Ca2+-mediated neurotransmitter release. Thus, this proposal will critically test two hypotheses: (1) CNS GLP-2R is required for maintaining energy homeostasis in the mouse;and (2) GLP-2 excites hypothalamic POMC neurons by enhancing protein kinase A-mediated L-type Ca2+ channel activity.
Specific aim 1 will define the physiological role of the CNS GLP-2R in energy homeostasis using our newly generated glp2r-floxed conditional knockout mouse model. We will determine whether CNS GLP-2R deficiency is crucial for regulating food intake, energy expenditure, glucose homeostasis, intestinal growth and blood flow in both fasting and refeeding conditions.
Specific aim 2 will determine if GLP-2 acutely excites anorexigenic POMC neurons in the hypothalamus using whole-cell patch-clamp. We will identify cellular and molecular mechanisms by which GLP-2 activates POMC neurons.
This proposal will be the first to elucidate the physiological role of CNS GLP-2R at functional and signaling levels in a tissue- and cell-specific manner, and to establish if CNS GLP-2R is a key mediator involved in regulation of peripheral energy homeostasis and intestinal growth. This project will advance our understanding of the physiological role of gut hormones in regulation of food intake centrally and energy expenditure peripherally, which will have profound implications for the development of distinct diets for prevention of human obesity and diabetes.
