PITPNA in pancreatic beta-cell dysfunction and diabetes pathogenesis

PROJECT SUMMARY Critical to successful innovation in treating diabetes is the development of strategies for promoting insulin release and preventing pancreatic beta-cell destruction. Chronic demand for insulin production during insulin resistance and diabetes exacerbates cell dysfunction and this is compounded by ER and oxidative stress. This results in beta-cell death and loss of insulin production. Recent studies have highlighted defects in insulin processing, insulin granule maturation, and granule docking that are also linked to all major forms of diabetes; however conceptual gaps remain in understanding the causes of beta-cell failure and developing methods to reverse or prevent beta-cell dysfunction. Our preliminary studies establish Phosphatidylinositol transfer protein alpha (referred to as human PITPNA and mouse Pitpna), as a major regulator of insulin granule formation and secretion. PITPNA shuttles phosphatidylinositol (PI) from the endoplasmic reticulum (ER) to the trans-Golgi network (TGN) for phosphorylation by Phosphatidylinositol 4-kinase (PI4-K) conversion to phosphatidylinositol-4 phosphate (PtdIns-4-P), an abundant membrane phospholipid involved in insulin granule docking and exocytosis. Our preliminary data shows: 1) PITPNA expression is dramatically silenced in beta-cells of human T2D subjects, 2) reduction of PITPNA in human islets both lowered cellular PI4-P levels and insulin granule maturation and increased accumulation of proinsulin, and 3) conditional beta-cell specific deletion of Pitpna in mice (Ins-Cre; Pitpnaflox/flox) results in decreased insulin secretion and beta-cell mass, random-fed hyperglycemia, and increased expression of ER stress proteins in beta cells. Based on these data, we hypothesize that decreased PITPNA in beta-cells during T2D leads to lower PI4-P for distribution by the TGN as well as incorporation into insulin granules, thereby disrupting granule maturation, docking and secretion. We further hypothesize the reduced granule formation results in accumulation of proinsulin in the ER, leading to ER stress and ultimately beta-cell death. We propose that restoration of PITPNA in beta-cells of T2D individuals will reverse these aspects of cellular dysfunction. We expect these studies will demonstrate that promoting PITPNA function and PI4-P formation is a novel strategy for reversing beta-cell dysfunction in several subcellular compartments including the ER, mitochondria, and the TGN. These studies aim to highlight restoration of PI4-P between intracellular membranes as an innovative approach for increasing granule maturation and secretion as well as reversing beta-cell failure in major forms of diabetes.