Vesicle trafficking is a critical component of the dynamic processes which maintain the intracellular milieu and mediate communication between and among cells. Intracellular membrane vesicles shuttle proteins between intracellular compartments and into and out of the cell. A knowledge of these processes is critical for the understanding of alterations in ionic transport and secretion which are manifest in many pathophysiological processes involving polarized epithelial and mucosal cells. The gastric parietal cell redistributes more membrane during its secretory process than any other mammalian cell. This massive coordinated fusion of intracellular tubulovesicular membranes with a specific canalicular target membrane surface provides a unique model in which to delineate the critical mechanisms responsible for stimulus- secretion coupling. Although considerable information is available concerning the activation of membrane receptors in parietal cells, comparatively little is known of the intracellular signal transducing events coupled to secretion. Of particular relevance is the recent recognition in a number of systems of a novel family of small GTP-binding proteins (smGTPBPs), which have been implicated in the mediation and modulation of membrane vesicle trafficking. We have recently demonstrated that the smGTPBPs rab2 and rab11 are strategically positioned on the tubulovesicles, which contain the proton pump. In addition, we have identified a novel smGTPBP, rab25, whose expression appears to be limited to polarized epithelial and mucosal cells. Therefore, we now seek to investigate these putative regulators of vesicle trafficking in the modulation of stimulated secretion and other dynamic processes in gastric parietal cells as well as other mucosal and epithelial systems. To define these critical mechanisms responsible for the dynamic movement of intracellular membranes, we will firstly investigate the relationship of secretory stimulation or inhibition on the distribution and expression of rab2 and rab11 in parietal cells. Secondly, we will seek to define the distribution of rab25 in parietal cells and in other epithelial tissues. Thirdly, we will identify and characterize the putative targets of rab 2, rab11, and rab 25, by isolation and characterization of associated proteins (rabphilins). Fourthly, we will investigate the genomic structure of the rab 25 gene in order to define the tissue specific regulation of rab25 expression. Finally, we will investigate the possible phosphorylation of rab2 and rab11 in gastric parietal cells. Overall these investigations seek to identify the intracellular components responsible for the membrane trafficking dynamics required for stimulus-secretion coupling in the parietal cell, as well as other polarized epithelial cells. Given the importance of secretory processes in the pathobiology of many gastrointestinal maladies, information garnered from the parietal cell system may provide critical insights into the general mechanisms involved in the maintenance of ordered secretory processes.
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