PARACRINE FEEDBACK BY PANCREATIC DELTA CELLS TO CONTROL GLUCAGON AND INSULIN RELEASE AND MANAGE DIABETES

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Half of US adults have diabetes or pre-diabetes, illustrating a critical need for novel diabetes treatments. There is a fundamental gap in the understanding of how paracrine feedback in the islet controls insulin and glucagon. The long-term goal is to elucidate the key players in (patho)physiological crosstalk within pancreatic islets in order to identify novel therapeutic targets. The overall objective in this application is to understand delta cell-mediated feedback control. The peptide hormone urocortin3 (Ucn3) promotes somatostatin (Sst) secretion from delta cells in order to attenuate insulin and glucagon. This feedback determines the set-point for plasma glucose, but is perturbed in diabetes and contributes to its pathophysiology. The central hypothesis is that the pancreatic delta cell is a local control hub that determines the homeostatic set point for glucose and can be targeted to rebalance glucagon and insulin in diabetes. This hypothesis was formulated on the basis of preliminary data produced in the applicants' laboratory. The rationale for the proposed research is to understand how Ucn3 and Sst control insulin and glucagon, thus identifying delta cell-dependent feedback as a novel target for diabetes treatment. This hypothesis will be tested in 3 specific aims. Aim 1 tests the hypothesis that delta cells secrete most of their Sst under control of GPCR activation by Ucn3, which assures delayed, beta cell-dependent Sst secretion. Insulin and Sst release will be measured in parallel by islet perfusion, and the mechanistic similarities and differences of delta and beta cell activation will be investigated following the selective expression of the GCaMP6 calcium indicator in both cell types. Aim 2 tests the hypothesis that beta cell-derived Ucn3 promotes Sst release to inhibit alpha cells under high glucose, while Ucn3 from human alpha cells reflects a separate feedback loop to attenuate glucagon at low glucose. This hypothesis is supported by the applicants' preliminary observations that Ucn3 promotes Sst secretion and inhibits alpha cell activity and glucagon release. Glucagon secretion and calcium responses of alpha cells within intact islets will be measured following genetic and pharmacological inhibition of Ucn3. We will also test the effect of `humanizing' feedback control in mice by the inducible expression of Ucn3 in mouse alpha cells. Aim 3 tests the hypothesis that loss of Ucn3-dependent negative feedback aggravates diabetes by allowing inappropriate glucagon release. We will measure alpha and beta cell responses in islets of diabetes models using GCaMP6 in the presence and absence of Ucn3, and the restoration of delta cell feedback using chemicogenetics in vivo secondary to STZ-induced diabetes. The research is conceptually and technically innovative, in the applicants' opinion, as it evaluates the important physiological role of delta cells within intact islets in attenuating alpha and beta cell activity. It attains this by applying new technologies to overcome the hurdles that have previously precluded such studies. This is significant because delta cell control over glucagon and insulin secretion has broad translational importance and could uncover novel strategies to curb the diabetes epidemic in the US.