Regulation of the epithelial Ca2+ channels TRPV6 and TRPV5

ABSTRACT Transient Receptor Vanilloid 6 (TRPV6) and its close relative TRPV5 are Ca2+ selective epithelial ion channels. The membrane phospholipid phosphatidylinositol 4,5-bisphosphate [PI(4,5)P2] is the endogenous ligand of these channels that is required for their activity. These channels also undergo Ca2+-induced inactivation which is mediated by binding of Ca2+-calmodulin (CaM) to the channel. TRPV6 and TRPV5 are constitutively active, and their level of activity is determined by the balance between the activating PI(4,5)P2 and the inhibitory CaM. In the previous funding period, we identified the PI(4,5)P2 binding site in TRPV6 using homology modeling and site directed mutagenesis, which was essentially identical to the experimentally determined PI(4,5)P2 binding site in the closely related TRPV5. CryoEM structures for CaM and TRPV6 or TRPV5 also became available, showing a highly consistent picture of one CaM molecule binding to the channel tetramer, and blocking the pore in a fashion consistent with our experimental results. An important development during the previous funding period was the finding that loss of function mutations in TRPV6 are associated with chronic pancreatitis. The likely mechanism is reduced Ca2+ removal from pancreatic fluid in the acini or the ducts leading to increased Ca2+ and premature activation of pancreatic digestive enzymes. Given the crucial role of Ca2+ in digestive enzyme activation, small molecules that increase the activity of TRPV6 can, in principle, be used as novel therapeutic approach to treat pancreatitis. The overall goal of this application is to gain molecular insight into how the two key endogenous regulators PI(4,5)P2 and CaM gate TRPV6 and to use our molecular level understanding of their regulation to identify small molecules that increase their activity. We will use a combination of computational and experimental approaches to decipher the molecular mechanism of PI(4,5)P2 activation of TRPV6 and TRPV5 in Aim1, to determine the relationship between CaM and PI(4,5)P2 regulation of TRPV6 in Aim 2, and to identify small molecules that increase TRPV6 and/or TRPV5 activity by interfering with CaM inhibition in Aim 3. This work will further our understanding of TRPV6 and TRPV5 gating, and small molecules that enhance the activity of TRPV6 or TRPV5 can be used in future experiments to explore the possibility of using this approach to treat or prevent pancreatitis and kidney stones.