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|Title: ||The Role of Munc18 Proteins in Physiologic and Pathologic Exocytoses in the Pancreatic Acinar Cell|
|Authors: ||Lam, Patrick Pak Ling|
|Advisor: ||Gaisano, Herbert Young|
|Issue Date: ||18-Feb-2010|
|Abstract: ||Distinct membrane fusion events in the polarized pancreatic acinar cell involve highly specific interactions between distinct sets of SNARE proteins forming exocytotic complexes, whose assembly is modulated by distinct Munc18 proteins. The Munc18 isoform responsible for these exocytotic events in the acinar cell is unknown. Here, I postulate Munc18b to regulate apical exocytosis in the acinar cell. Current dogma for the pathogenesis of acute pancreatitis, including alcoholic pancreatitis, is mis-targeting and deregulated fusion of zymogen granules with lysosomal bodies in the acinar cells. This derangement results in premature activation of proteolytic zymogens and autophagic digestion of cellular contents. I have hypothesized an alternate mechanism, which is pathologic exocytosis occurring at the basolateral plasma membrane, and further propose Munc18c to mediate this process in alcoholic pancreatitis. The aims of this thesis are to demonstrate the roles of Munc18b and Munc18c in regulated apical exocytosis and pathologic basolateral exocytosis underling alcoholic pancreatitis, respectively.
In Chapter Three, using both real-time and static imaging techniques and biochemical tools, I demonstrated that Munc18c is dissociated from the acinar basal plasma membrane (BPM) when stimulated with postprandial CCK8 preceding preincubation of acini with postprandial 20-50mM ethanol concentrations. This activated Syntaxin (Syn)-4 and SNAP-23 on the BPM to complex with VAMP proteins on the granule to form the exocytotic SNARE complex that triggered basolateral exocytosis. This molecular mechanism of pathologic basolateral exocytosis was recapitulated in an Ethanol-diet rat model of pancreatitis. In Chapter Four, I determined Munc18b to be in the apical pole of the acinar cell to appropriately bind cognate Syn-2 and Syn-3 in the apical PM and ZGs. Here, I examined the structure-function of Munc18b on amylase secretion by employing Munc18b mutants with distinct affinities to Syn-2 and Syn-3. In Chapter Five, I discovered a novel EF-hand Ca2+-binding protein called Cab45b, which binds Munc18b to regulate its membrane targeting and interactions with Syntaxins in the acinar cell in a manner that influenced Ca2+-induced amylase release.
Taken together, these studies clarify our understanding of the role of Munc18 proteins involved in regulated and pathologic membrane fusion events underlying physiologic digestive enzyme secretion and clinical alcoholic pancreatitis.|
|Appears in Collections:||Doctoral|
Department of Physiology - Doctoral theses
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