The behavioral processes of learning and memory are reflected at the cellular level by changes in synaptic plasticity. In simple terms, synapses are not static, but rather subject to rapid modulation in strength and/or structure. Among many mechanisms that may alter synaptic performance, those that affect the probability and/or amount of neurotransmitter release at the active zone (AZ) are widespread in the CNS. Indeed, it is well established that the rate and extent to which the AZ reloads and primes vesicles to fusion competency dictates the rate at which neurons may sustain normal action potential mediated synaptic transmission. Sustained secretion from neuroendocrine cells is also defined by the rate of recruitment, docking, and priming of dense core secretory granules. Remarkably, similar sets of proteins have been identified in neurons and neuroendocrine cells that act to replenish and prime secretory vesicles to fusion competency. Yet, an understanding of the spatio-temporal dynamics, functional inter-relationships, and structural arrangement of these proteins remains poorly understood. Our laboratory applies a multidisciplinary approach towards developing a mechanistic understanding of the molecular pathways that control regulated exocytotic events at synapses and in neuroendocrine cells. Our long-term goal is use the information to identify deficits in the signaling pathways and exocytotic process that underlie specific neurodegenerative disease states and of pathophysiological imbalances in neurohormone secretion.
Currently under intense investigation are projects to identify and understand signaling processes that control the formation & disassembly of those molecular complexes that regulate neurotransmitter release. Among the molecular assemblies critically important to neurotransmitter release is the formation of SNARE core complexes composed of a vesicle delimited R-SNARE protein (VAMP) with plasma membrane localized Q-SNARE proteins (SNAP25, & Syntaxin). Controlling the rate or extent of SNARE core complex formation exerts a profound influence on neurotransmitter and neurohormone release. We work on different identifying and defining the spatial and temporal dynamics, as well as mechanism(s) of action, of specific regulators of SNARE complex formation. One of these is a protein termed Tomosyn, which is a soluble R-SNARE protein that when activated reduces formation of fusion competent SNARE core complexes and neurotransmitter release. Additional regulators under investigation include a number of active zone proteins known to strongly influence the probability of vesicle release. We use combinations of state of the art molecular, imaging and electrophysiological methodology to image protein and vesicle mobility in living cells, to quantify the dynamic assembly/dissolution and functional consequences of protein complexes.
An additional important focus of research in the laboratory centers on understanding regulatory controls on insulin secretion from pancreatic β-cells. Secretion of insulin is essential for the maintenance of glucose homeostasis and for tissue development and growth. Insulin is normally secreted at rates and times appropriate to maintain blood glucose levels within a narrow range. Type-2 diabetes occurs when the ability of β-cells to release insulin is insufficient for proper regulation of glucose and lipid metabolism. This problem is further complicated by an excessive demand for insulin, which may arise, in part, as a result of insulin-resistance of peripheral tissues. Our studies focus on defining the role and regulation of Rab proteins that are involved in glucose induced insulin secretion (GSIS). Among the Rab proteins in β-cells, Rab27A is critically important in regulating the terminal events in GSIS. Yet, to date little is known on regulation of RAb27A, the role of specific effectors (many effectors have been identified), or the specific molecular mechanisms by which Rab27A tunes secretory throughput.