Role of Renal and Intestinal AC6 and NHE3 for Phosphate Homeostasis

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The precise regulation of the body's phosphate level is a critical task. Nearly all patients with chronic kidney disease (CKD) exhibit hyperphosphatemia which is associated with increased cardiovascular mortality. Renal reabsorption of Pi in the proximal tubule is hormonally regulated and requires fibroblast growth factor 23 (FGF23) and parathyroid hormone (PTH). The latter has been described to signal via cyclic adenosine monophosphate (cAMP), generated by adenylyl cyclases (AC), and retrieve Na+-Pi cotransporters 2a and 2c (Npt2a and Npt2c) and Na+/H+ exchanger 3 (NHE3) from the apical cell membrane. The overarching goal of this proposal is to determine the roles of adenylyl cyclase 6 (AC6) and NHE3 in Pi homeostasis by analyzing the intestine-kidney axis. We identified that AC6 is the most important isoform for PTH-mediated cAMP formation and Pi homeostasis. In contrast to the expected pathophysiology resulting in impaired Pi excretion, lack of AC6 causes renal Pi wasting with 80% of Npt2a residing in lysosomes. To avoid further Pi loss, PTH and FGF23 levels would be expected to be suppressed; however, lack of AC6 is associated with significantly elevated levels of both hormones indicating that this Pi loss cannot be countered hormonally. While regulation of the milieu intérieur would require intestinal Pi uptake to be enhanced or unchanged, we found that lack of AC6 causes an almost complete absence of intestinal Npt2b. This paradox highlights that AC6 plays a role in a so far unidentified negative feedback loop that suppresses Pi regulating hormones. Since PTH also targets NHE3, we generated a novel kidney-specific NHE3 knockout mouse to determine the contribution of NHE3 for Pi homeostasis. While this model has normal Npt2a abundance, Npt2c abundance is diminished, providing a novel link between NHE3 and Npt2c that has never been shown before. In Aim 1, we will determine the role of renal AC6 in Pi homeostasis under normal conditions and CKD. In Aim 2, we will delineate the contribution of 3 specific signaling pathways for Pi homeostasis: i) Gαs protein coupled AC6/cAMP/protein kinase A; ii) Gαq/11 protein coupled phospholipase C(PLC)/inositol triphosphate/Ca2+/protein kinase C; and iii) FGF23. A novel mouse model with defective PLC (named DSEL mouse) and AC6 signaling will allow us to study the contribution of each of these pathways in regulating expression of Npt2a/c in the proximal tubule. To determine the contribution of FGF23 signaling we will pharmacologically antagonize FGF23 via a novel neutralizing antibody. Aim 3 will determine if there is a linkage between NHE3 and Npt2c for Pi homeostasis. Based on the hypothesis that NHE3 is regulated by PTH, we will use our kidney-specific NHE3 knockout mouse to study Npt2a/c trafficking and colocalization and determine to which extent NHE3 is required for renal Pi homeostasis. Aim 4 will determine if AC6 and/or NHE3 play a role in intestinal Pi uptake and Pi homeostasis by utilizing novel intestinal mucosa-specific AC6 and intestinal mucosa-specific NHE3 knockout mice. Modulating PTH, FGF23 and active vitamin D levels will test for the regulation of Npt2b.