The molecular mechanisms underlying exocytosis


We have a long-standing interest in defining the critical control points for exocytosis. Our work in this area has led us to study two proteins classically viewed to operate at different ends of storage granule trafficking at the membrane – dynamin and synaptotagmin. A more current perspective is that both protein families operate at or near the time of fusion, perhaps in a spatially overlapping manner. Multiple isoforms of dynamin and synaptotagmin co-exist in most cell types. There is strong evidence that the isoforms perform non-redundant roles in secretion, serving, for example, to “tune” neurotransmitter or hormone release to the physiological or metabolic demands of organ system or organism. In addition to delineating the roles of synaptotagmin isoforms in secretory cells, we would also like to discover the basis for its sorting to distinct intracellular organelles. Experiments in reduced systems have allowed us to begin to develop a model for protein function based on its known structure and biochemistry. These latter studies, in particular, have led us to create novel approaches for assaying protein function and to employ computer-based simulations of protein-membrane interactions (in collaboration with Dr. Jefferson Knight at UC Denver).

Insulin hormone sorting, trafficking, and secretion



Exocytosis of insulin secretory granules from pancreatic beta cells in the islets of Langerhans is required for control of blood glucose — both in basal conditions to maintain normoglycemia, and even more importantly, post-prandially when insulin exocytosis helps to limit excessive excursion of blood glucose into the diabetic range.  Through a collaboration with Dr. David Castle (UVA), we have started investigating mechanisms that are important for proper sorting, trafficking, and secretion of the insulin hormone. We have found that the formation and function of insulin storage granules depend critically on the regulation of cholesterol dynamics. Two proteins of particular interest to us are the ATP binding cassette proteins ABCG1 and ABCA1, which work to maintain appropriate levels of cholesterol in intracellular membranes. We have shown that genetic suppression of ABC protein expression severely disrupts both insulin sorting and granule fusion. Our work in this area has direct implications for not only type-2 diabetes, but a wider range of disorders associated with dysregulation of cholesterol dynamics.

Behavioral and metabolic regulation of stress hormone release

Adrenomedullary chromaffin cells serve as a key effector arm of the sympathetic nervous system in the periphery. These cells synthesize, store, and secrete a complex cocktail of bioactive agents, including opioid peptides, and stress hormones critical for the "fight-or-flight" response. By design, the secretion process is mutable so that release can be rapidly tuned to match physiological demands. These demands have the potential to vary tremendously, depending on a variety of environmental or metabolic factors, including stress, pain, and diet. Unfortunately, the mechanisms by which this tuning is achieved remain unclear. Because the sympatho-adrenal system is known to modify the function of nearly every organ in the human body, this conceptual gap is a significant issue. Within this context, projects are designed to: 1) Understand how excitation/inhibition balance in sympathetic nerves projecting to the adrenal medulla is determined; 2) Define how stress modifies activity of adrenal tissue and the adrenal gland, in vivo; and, 3) Understand how hormone/peptide secretion and signaling are regulated (and dysregulated by stress) at the sympatho-adrenal synapse.