The precise regulation of the body's phosphate level is a critical task. Nearly all patients with chronic kidneydisease (CKD) exhibit hyperphosphatemia which is associated with increased cardiovascular mortality. Renalreabsorption 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 adenosinemonophosphate (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 ofthis proposal is to determine the roles of adenylyl cyclase 6 (AC6) and NHE3 in Pi homeostasis by analyzingthe intestine-kidney axis. We identified that AC6 is the most important isoform for PTH-mediated cAMPformation 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, PTHand FGF23 levels would be expected to be suppressed; however, lack of AC6 is associated with significantlyelevated levels of both hormones indicating that this Pi loss cannot be countered hormonally. While regulationof the milieu int rieur would require intestinal Pi uptake to be enhanced or unchanged, we found that lack ofAC6 causes an almost complete absence of intestinal Npt2b. This paradox highlights that AC6 plays a role in aso far unidentified negative feedback loop that suppresses Pi regulating hormones. Since PTH also targetsNHE3, we generated a novel kidney-specific NHE3 knockout mouse to determine the contribution of NHE3 forPi homeostasis. While this model has normal Npt2a abundance, Npt2c abundance is diminished, providing anovel link between NHE3 and Npt2c that has never been shown before.
In Aim 1, we will determine the role ofrenal AC6 in Pi homeostasis under normal conditions and CKD.
In Aim 2, we will delineate the contribution of 3specific signaling pathways for Pi homeostasis: i) G?s protein coupled AC6/cAMP/protein kinase A; ii) G?q/11protein coupled phospholipase C(PLC)/inositol triphosphate/Ca2+/protein kinase C; and iii) FGF23. A novelmouse model with defective PLC (named DSEL mouse) and AC6 signaling will allow us to study thecontribution of each of these pathways in regulating expression of Npt2a/c in the proximal tubule. To determinethe contribution of FGF23 signaling we will pharmacologically antagonize FGF23 via a novel neutralizingantibody.
Aim 3 will determine if there is a linkage between NHE3 and Npt2c for Pi homeostasis. Based on thehypothesis that NHE3 is regulated by PTH, we will use our kidney-specific NHE3 knockout mouse to studyNpt2a/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 novelintestinal mucosa-specific AC6 and intestinal mucosa-specific NHE3 knockout mice. Modulating PTH, FGF23and active vitamin D levels will test for the regulation of Npt2b.
Increased plasma phosphate; alone or combination with chronic kidney disease (CKD); is associated with a27% higher cardiovascular mortality. The proposed studies will provide a better understanding of phosphatehomeostasis by studying the intestine-kidney axis. Our results may lead to development of novel therapeutic orpreventative strategies; which is of major health impact considering 23 million adults being affected by CKD inthe US alone.
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