Control Mechanisms of Acid Secretion (Drs. Alexander and Schwartz)

Renal inner medullary collecting duct cells transport protons, mediated by an H+-ATPase, and H2O, mediated by aquaporin-2 (AQP2) across their apical membrane. In our cultured line of IMCD cells, as in the kidney both of these processes are controlled by regulated exocytic insertion and endocytic retrieval of vesicles that carry either an H+-ATPase or AQP2 as cargo in their membranes, but not both. The targeting and fusion of these vesicles to the apical membrane may be mediated by SNARE proteins, the same proteins that mediate exocytosis at the synaptic membrane. However, despite the similarity of the postulated targeting-fusion system, exocytosis of H+-ATPase and AQP2 are independently regulated. Our group is evaluateing how a polar renal epithelial cell target two distinct cargo-laden vesicles to the apical membrane utilizing similar docking-fusion proteins. We propose the following hypotheses which will be the focus of our current studies: 1) the minimal machinery, the SNAREpin, required for targeting and fusion of H+-ATPase or AQP2 vesicle subtypes to the apical membrane consists of a distinct set of v & t-SNAREs; 2) the cargo proteins (H+-ATPase and AQP2), per se, participate in the regulation, targeting and exocytic insertion of their carrier vesicles and 3) regulated exocytosis of these vesicles is not only initiated but proceeds by different signal cascades that modify (phosphorylate) dissimilar proteins in either the vesicles or target (apical) membrane.

References

  1. Li G, Alexander EA, Schwartz JH. Syntaxin isoform specificity in the regulation of renal H+-ATPase exocytosis. J Biol Chem 278: 19791-19797, 2003.
  2. Julie A Nicoletta, Jonathan J Ross, Guangmu Li, Qingzhang Cheng, John H Schwartz, Edward A Alexander, , and John H Schwartz. MUNC 18-2 regulates the exocytosis of H+-ATPase in the rat inner medullary collecting duct cell. Am J Physiol Cell Physiol 287: C1366-2374, 2004.
  3. Li, G., Yang, Q., Alexander EA, and Schwartz JH, J.H. Syntaxin 1A has a specific binding site in the H3 domain that is critical for targeting of H+-ATPase to apical membrane of renal epithelial cells. Am J Physiol Cell Physiol 289:C665-672, 2005
  4. Li, G, Yang, Q., Krishnan, S, Alexander EA, Borkan, SC & Schwartz JH, JH: A novel cellular survival factor – the B2 subunit of vacuolar H+-ATPase inhibits apoptosis. Cell Death Differ, 2006.
  5. Yang, Q, Li, G, Singh, SK, Alexander EA, & Schwartz JH, JH: Vacuolar H+-ATPase B1 Subunit Mutations that Cause Inherited Distal Renal Tubular Acidosis Affect Proton Pump Assembly and Trafficking in Inner Medullary Collecting Duct Cells. J Am Soc Nephrol 17:1858-1866, 2006.
  6. Alexander, E.A. Non-Anion Gap Acidosis. Hyper., Dialy., and Clin. Neph. 2006
  7. Schwartz, J.H., G. Li, V. Suri, J.J. Ross and E.A. Alexander. Role of SNAREs and H+-ATPase in the Targeting and Insertion of Proton Pump Coated Vesicles into the Apical Membrane of Collecting Duct Cells. Kid. Int. 72: 1310-1315, 2007.
Primary teaching affiliate
of BU School of Medicine