Physiology of ClC-K2/b Cl- channel in the collecting duct

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Abundant basic and clinical evidence identify a critical role of the augmented epithelial Na+ channel (ENaC) mediated reabsorption in the collecting duct (CD) principal cells in the pathology of salt-sensitive and Angiotensin II (Ang II)-dependent hypertension. On the contrary, aldosterone-mediated increases in ENaC activity during hyperkalemia are necessary to stimulate potassium secretion with no sign of volume retention. Here, we propose that the unique mosaic structure of the CD, consisting of electrically uncoupled principal and intercalated cells, is instrumental for separate control of Na+, K+ and Cl- fluxes to determine its transport profile from volume retention (Na+ and Cl- reabsorption) to Na+/K+ exchange (coupling Na+ reabsorption to K+ secretion). We further generated strong preliminary evidence supporting essential role of ClC-K2/b channel mediated Cl- flux in intercalated cells in this process. Using freshly isolated murine CDs, we found that ClC-K2/b activity and expression is inversely related to dietary Cl- but not K+ intake. Moreover, Ang II exhibits multicomponent stimulatory effects on ClC-K2/b activity, trafficking and channel abundance in the CD intercalated cells implicating its important role in ClC-K2/b activation during low Cl- diet and volume depletion. Consistently, mice lacking Angiotensin type 1 receptors (AT1R) are hypovolemic and have reduced renal ClC-K2/b expression. Overall, we hypothesize that ClC-K2/b function in intercalated cells is primarily regulated by dietary Cl- intake likely in an Ang II-dependent manner. This discretional anionic ClC-K2/b-mediated Cl- influx reduces the ENaC- generated driving force for K+ secretion enabling to tune CD transport profile from Na+/K+ exchange during hyperkalemia (only aldosterone is elevated) to NaCl reabsorption during hypovolemia (both aldosterone and Ang II are increased). Concomitant over-stimulation of ENaC and ClC-K2/b shifts the CD operation to NaCl reabsorptive mode, thereby contributing to the pathology of Ang II-induced hypertension. To address this central hypothesis, we developed 3 specific aims: SA1: Examine regulation of ClC-K2/b activity and expression in the CD by dietary cues. Establish the contribution and supremacy of Ang II and aldosterone in this process. SA2: Define the mechanism of action and delineate the cellular signaling pathway of Ang II regulation of ClC- K2/b activity in the CD. SA3: Establish pathophysiological ramifications of augmented ClC-K2/b activity in the CD in the pathology of Ang II-dependent hypertension using transgenic mice with targeted channel deletion. In summary, this proposal seeks to greatly expand our understanding how electrolyte transport in principal and intercalated cells integrate for proper CD response to dietary and endocrine inputs. Moreover, it also provides physiologically relevant means to target ClC-K2/b-dependent Cl- reabsorption in the CD as a tool to fight salt-sensitive and Ang II-dependent hypertension in clinical setting.