STIMULUS-EVENT COUPLING IN CHROMAFFIN SECRETORY GRANULES

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DESCRIPTION (adapted from the application) In secretory granules of sympathoadrenal chromaffin cells, co-stored catecholamines and peptides are cosecreted by exocytosis in response to acetylcholine or to the novel non-cholinergic neurotransmitter pituitary adenylyl cyclase-activating polypeptide (PACAP). The chromogranins/ secretogranins (Cg/Sg), a family of acidic, soluble proteins found in the cores of amine and peptide hormone and neurotransmitter secretory vesicles, have a virtually ubiquitous occurrence in such vesicles throughout the neuroendocrine system. Their proteolytic fragments exert effects on hormone or neurotransmitter release as well as target cells at a variety of sites. For example, chromogranin A (CgA) fragments vasostatin and catestatin control vasoreactivity and catecholamine release, and CgA fragment pancreastatin elevates blood glucose. This proposal develops 3 specific Aims in seeking to understand secretory granule events which occur in response to secretory stimuli in neuroendocrine cells such as the chromaffin cell. The responses to be investigated are secretion and gene expression (transcription). In Aim 1, we will investigate how catecholamine secretion and CgA gene transactivation responses desensitize following pre-ganglionic stimulation of chromaffin cells by PACAP and whether desensitization of these responses involves G protein-coupled receptor kinases (GRK), beta-arrestins and/or second messenger-activated kinases. In Aim 2, we will use the fluorescence of a novel CgA-EGFP chimera (Enhanced Green Fluorescent Protein) to determine in situ whether discrete CgA primary structural domains are responsible for the trafficking of CgA into the regulated pathway of secretion, and how CgA interacts with other secreted or membrane proteins of the secretory granule. Finally, we will use this chimeric CgA-EGFP protein as a calibrated H+ (pH) sensor (Aim 3) to investigate in situ, the dynamics of intravesicular pH and its role in the secretory process triggered by the physiologic secretagogues, such as PACAP or acetylcholine, or by sympathomimetic amines.