Alpha7 Nicotinic Acetylcholine Receptor Regulation of Glucagon-Like Peptide-1 Incretin Hormone Action.

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The overall goal of this project is to establish the systems physiology of glucoregulation in which alpha 7 nicotinic acetylcholine receptors (α7nAChR) 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 α7nAChR 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 α7nAChR 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 α7nAChR 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 α7nAChR knockout mice, while also assessing how α7nAChR 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 α7nAChR 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 β-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 α-cells so that intra-islet GLP-1 will exert a paracrine hormone action at the β-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 α7nAChR. 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 β-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 α-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 α7nAChR in pancreatic islets regulates coordinate expression of PC1/3 and GPR119 so that α-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 α7nAChR control.