Grant: Project Grant

Pancreatic beta cells secrete insulin in biphasic manner, an initial peak phase (~15 min) followed by a sustained phase (several hours), to ensure continuous assimilation of circulating nutrients into tissues. In type-2 diabetes, both phases are perturbed resulting in deficient insulin release causing hyperglycemia. Half of initial peak phase is attributed to secretory granules (SGs) predocked on calcium channels (Cav1.2/1.3) on the plasma membrane where calcium influx induces SG fusion to release insulin. Newcomer SGs (NSG) are a newly discovered SG population that account for the entire sustained phase and half of first-phase secretion. We discovered the SNARE (syntaxin-3, VAMP8) and associated proteins responsible for NSG fusion; but unlike predocked SGs, little else is known about the NSG secretory machinery. Our first hypothesis (Aim 1) is that NSGs specifically land on and undergo fusion at a new set of calcium channels (Cav2.1/2.3) which bind syntaxin-3. An undefined reserve pool must exist to continually replenish NSGs after depletion from stimulated release. We found potassium channel (Kv2.1) clusters on plasma membrane that bind NSG-syntaxin-3. Our second hypothesis (Aim 2) is that syntaxin-3/Kv2.1 clusters form complexes that serve as an accessible reserve pool for NSGs to park prior to their relocation to adjacent calcium channels. This provides a unique architecture that couples the reserve pool (Kv2.1 clusters) of SGs to SG release sites (Cav), a process that has long eluded previous investigation. This work has broad application to many endocrine cells that exhibit sustained secretion required to meet varying metabolic needs. Insights into these novel fundamental steps of secretion are a basis for development of treatment strategies to improve secretion. As proof, we will express (with virus) Cav and Kv2.1 into pancreatic islets of diabetic humans (in vitro) and rats (in vivo) to improve the deficient insulin secretion and restore glucose homeostasis.