The overall goal of this project is to establish the systems physiology of glucoregulation in which alpha 7 nicotinic acetylcholine receptors (?7nAChR) regulate the incretin hormone action of Glucagon-Like Peptide-1 (GLP-1). This project may reveal novel features of glucoregulation that are of relevance to our understanding of type 2 diabetes mellitus (T2DM) since we find that the ?7nAChR agonist GTS-21 stimulates intestinal GLP-1 secretion, raises circulating levels of GLP-1, and improves glucose tolerance in mice. Thus, we propose a novel and important role for the ?7nAChR in the control of glucose homeostasis by virtue of its ability to exert inter-organ control over GLP-1 secretion and GLP-1 action. To test this Hypothesis, our aims are as follows:
Aim 1 : We will use Cre/lox technology in combination with gene knockout or knock-in technology to determine how ?7nAChR agonists lower levels of blood glucose in healthy mice or db/db and ob/ob mouse models of diabetes. A first goal is to evaluate baseline alterations of glucoregulation in tissue-specific ?7nAChR knockout mice, while also assessing how ?7nAChR agonist action is modified. A second goal is to identify which cell types express the GLP-1 receptor (GLP-1R) that mediates glucoregulatory actions of ?7nAChR agonists. Four possibilities exist: 1) when mice are administered GTS-21 in combination with a DPP-4 inhibitor, the concentration of GLP-1 in the blood will reach high levels so that GLP-1 will act at the pancreatic ?-cell GLP-1R to enhance insulin secretion, or 2) GLP-1 released from L-cells in response to GTS-21 might exert a local effect in the intestinal wall to initiate neural reflexes that stimulate insulin secretion, or 3) GTS-21 might act in the hindbrain nucleus tractus solitarius to stimulate GLP-1 release so that glucose homeostasis is improved, or 4) GTS-21 might stimulate GLP-1 release from pancreatic ?-cells so that intra-islet GLP-1 will exert a paracrine hormone action at the ?-cell GLP-1R.
Aim 2 : We will perform in vitro studies to test if GTS-21 acts exclusively at L-cells, or if it also acts at other types of enteroendocrine cells that may express the ?7nAChR. For example, GTS-21 might lower levels of blood glucose by stimulating the release of GIP from K-cells so that GIP will then act at the ?-cell GIP receptor to stimulate insulin secretion. Potentially, this action of GIP will be enhanced by a DPP-4 inhibitor. Since Peptide YY (PYY) is co-secreted with GLP-1 from L-cells, we may find that its release is also stimulated by GTS-21. This would be significant because PYY is important to the suppression of food intake in obesity-related T2DM. Finally, we will test for actions of GTS-21 to stimulate GLP-1 biosynthesis in L-cells and pancreatic ?-cells. This possibility exists because we find that GTS-21 upregulates expression of a prohormone convertase (PC1/3) that liberates GLP-1 from proglucagon, while also upregulating expression of GPR119, a GPCR that stimulates glucagon gene transcription. It will be especially interesting to determine if the ?7nAChR in pancreatic islets regulates coordinate expression of PC1/3 and GPR119 so that ?-cells acquire the ability to secret GLP-1 under conditions of T2DM. Summary: Our long-term goal is to establish the systems physiology of glucoregulation under ?7nAChR control.
Type 2 diabetes mellitus (T2DM) is an increasingly prevalent metabolic disorder that leads to progressive deterioration of health, increased rates of mortality, and that is epidemic to many age groups and cultures worldwide. Studies proposed here seek to facilitate the diagnosis, treatment, and eventual cure of T2DM by providing key insights concerning inter-organ communication that enables the body to effectively maintain normal blood glucose homeostasis. In this regard, we provide a plan of action in which we focus on the ?7 nicotinic acetylcholine receptor (?7nAChR) and the incretin hormone GLP-1 as important molecular determinants of pancreatic insulin secretion in the healthy and also T2DM states.
