Pilot 21 – Impact of Intra-Islet GLP-1 Signaling on ß-cell Function in CFRD

P.I.: Sakeneh Zraika, PhD
Research Associate Professor, Medicine, Metabolism

Cystic fibrosis-related diabetes (CFRD) occurs in ~50% of adults with CF, and is distinct from type 1 and type 2 diabetes, though its underlying pathophysiology is not completely understood. Within the pancreas, a reduction in insulin-producing ß cells is observed, however the degree of loss is unlikely to explain the deficit in insulin release. This suggests other factors contribute to insulin insufficiency in CFRD.

In contrast to reduced ß-cell mass in CFRD, there is an increase in α-cell mass in both CF and CFRD. It is increasingly being recognized that signaling from α-cell secretory products, including glucagon and glucagon-like peptide-1 (GLP-1), is required for maintenance of normal insulin secretion. Thus, an important unanswered question is whether this paracrine signaling is defective in CFRD, thereby contributing to insulin secretory dysfunction. Moreover, given that islets comprise a greater proportion of α cells in CF/CFRD, an unexplored area is how this could be leveraged to augment ß-cell function. Paracrine signaling from α to ß cells occurs, in part, via activation of the GLP-1 receptor (GLP-1R), which in islets is predominantly expressed in ß cells. While existing data are mixed regarding levels of circulating active GLP-1 in patients with CF and CFRD, no studies have evaluated whether intra-islet production of GLP-1 or its signaling to the ß cell is impaired in CF/CFRD. Our proposal addresses this knowledge gap, and seeks to determine whether interventions that enhance GLP-1R signaling within the islet may improve ß-cell function in CF/CFRD.

Our preliminary data show that in isolated islets, ablation of GLP-1R signaling reduces glucose-stimulated insulin secretion (GSIS). Conversely, by inhibiting peptidases that degrade active GLP-1 within islets, we show that GLP-1’s insulinotropic action is preserved. Together, these data support the idea that intra-islet GLP-1 action contributes to normal ß-cell function. Further, we have developed strategies to increase α cell-derived GLP-1 release in vitro, with the goal of enhancing insulin secretion in CF/CFRD. Based on these data, we hypothesize that α cell-derived GLP-1 ameliorates impaired ß-cell function in CFRD via its ability to enhance insulin secretion in a GLP-1R-dependent manner. To start, we will quantify islet GLP-1 immunoreactivity in autopsy pancreas specimens from humans with CF and CFRD. Then, using a novel in vitro model of reduced insulin secretion induced by knockdown of CFTR, we will explore whether increasing GLP-1R signaling improves ß-cell function, and thus has therapeutic potential. We propose the following aims:

Aim 1: To determine whether GLP-1 immunoreactivity is reduced in islets from humans with CF and/or CFRD versus non-CF controls. Immunohistochemistry will be performed on human pancreas samples from the CFRTC biorepository to quantify islet immunoreactivity of GLP-1 and GLP-1R (with Host Microbe Core). Given that both glucagon and GLP-1 are cleavage products of proglucagon, immunoreactivity of glucagon and processing enzymes, prohormone convertase 1/3 and 2, will also be determined. Pancreas sections (from clinical and Host Microbe Cores) will be co-labeled with insulin for quantitative morphometric analyses of ß cells. To ascertain whether islet immunoreactivity of GLP-1-degrading enzymes is altered in CF and/or CFRD, immunohistochemistry will be performed to quantify dipeptidyl peptidase-4 (DPP-4) and neprilysin (NEP).

Aim 2: To determine whether α cell-derived GLP-1 enhances ß-cell function in an in vitro model of CFTR deficiency. Using approaches developed by J Engelhardt (U. Iowa), human islets will be treated with a CFTR-shRNA expressing adenovirus to knockdown CFTR and induce a GSIS defect. Thereafter, we will determine whether interventions that stimulate release of α-cell GLP-1 or reduce its degradation rescue the defect in GSIS. To establish the contribution of GLP-1 to enhanced insulin secretion, we will co-culture islets with the well-characterized competitive GLP-1R antagonist, exendin-(9-39). Should these studies reveal a beneficial effect of increased GLP-1 on ß-cell function, future studies will include in vivo studies in ferret models of CF, which are known to develop exocrine pancreatic disease as seen in human CF.

In summary, the proposed studies are innovative as they are the first to interrogate the role of α cell-derived GLP-1 in regulating ß-cell function in CF/CFRD. Also, they are the first to exploit the elevated α-cell mass in CF and CFRD by exploring therapeutic avenues targeting the α cell. This line of research is significant because it addresses the deficit in ß-cell function in CF/CFRD, and will inform interventions aimed at enhancing GLP-1R signaling within the islet could be useful in treating the ß-cell defect. Thus, we expect our findings will have translational potential.