7. PROJECT SUMMARY The distal convoluted tubule (DCT) of the kidney expresses the thiazide-sensitive Na-Cl cotransporter, NCC, a mechanism essential to Na+ reabsorption, control of blood pressure, and K+ secretion. NCC is activated by phosphorylation, which is mediated by a pair of protein kinases: SPAK, a Ste20p-like kinase which directly binds and phosphorylates the cotransporter, and WNK4, an upstream kinase, which phosphorylates and activates SPAK. Increasing consensus is developing that the function of the DCT is to sense and regulate plasma K+ through the modulation of K+ secretion in the aldosterone-sensitive distal nephron (ASDN). The precise mechanism(s) by which the DCT fulfills this function is still not well-understood. Preliminary data collected in the previous funding cycle indicated that even in the presence of a constitutively-active SPAK kinase, increase in plasma K+ still decreases phosphorylation of NCC leading to a first hypothesis that the K+ activated phosphatase, calcineurin, mediates inhibition of the cotransporter (Aim 1). Based on preliminary work that highlights the role of Cab39 adaptor proteins (Cab39 and Cab39l) in facilitating SPAK and WNK function we hypothesize that these proteins play a critical role in the pathway regulating NCC phosphorylation. In Aim 2, the molecular interaction between Cab39 and SPAK/WNK, the relationship with the Cl- sensitivity of WNK4, and the relevance of the adaptor proteins at the animal level will all be addressed. To account for the relationship between Na+ reabsorption in DCT and K+ secretion in the ASDN, we hypothesize that on top of the well-accepted view that Na+ delivery inversely modulates epithelial Na+ channel and ROMK, there is paracrine communication between the two nephron segments. This novel idea will be addressed in Aim 3. Finally, longer term coupling between the DCT and the ASDN involves significant remodeling for which we now have a precise signaling pathway to study. Through lineage mapping studies and Jagged-1 knockout studies we will test the hypothesis that NOTCH signaling increases the number of principal cells in the connecting segment (Aim 4). These new studies will provide a greater integrated view of the role of Na+ transport in the DCT in renal function.