PROJECT SUMMARY Normal mineral homeostasis is regulated by the actions of parathyroid hormone (PTH), fibroblast growth factor 23 (FGF23), and vitaminD on the intestine,kidney andbone. Vitamin D is counter regulatedby PTHand FGF23: PTH increases andFGF23 decreases circulatinglevels of thebiologically active formof vitamin D, 1,25-dihydroxyvitamin D (1,25D), via opposing actions on the 1,25D-synthetic enzyme, CYP27B1 (1α-hydroxylase), and the 1,25D- catabolic enzyme, CYP24A1 (24-hydroxylase). Despite compelling genetic and physiological evidence in humans and animals that point to the critical importance of CYP24A1, most prior research of vitamin D in health and in chronic kidney disease (CKD) focused on 1,25D synthesis by CYP27B1; much less attention has been devoted to 1,25D catabolism by CYP24A1. To advance research of CYP24A1, we developed new lox-P mice that enable tissue-selective and inducible deletion of Cyp24a1. We will use this novel mouse line to test our overarching hypothesis that tissue-specific effects of CYP24A1 in the kidney and intestinecontribute to normal and disordered mineral homeostasis. Since the kidney is the main regulator of circulating 1,25D, we hypothesize that kidney- specific Cyp24a1 deletion will increase serum 1,25D and downstream vitamin D receptor (VDR) activity in all tissues, including the intestine and kidney. In CKD, this will cause hypercalcemia that suppresses PTH, but will further harm kidney function due to 1,25D intoxication. In contrast, since the intestine does not contribute meaningfully to circulating1,25D, we expect intestine-specific Cyp24a1 deletion to increase1,25D and VDR activity only in the intestine. Resultant increases in intestinal calcium absorption will suppress PTH, which will lead to decreased serum 1,25D and thus, decreased VDR activity in the kidney. As a result, we hypothesize that inhibiting intestinal CYP24A1 in CKD will attenuate secondary hyperparathyroidismwithout causing hypercalcemia or further kidney toxicity. In Aim 1, we will investigate the effects of CYP24A1 in normal mineral homeostasis by studying mice with deletion of Cyp24a1 from the kidney (Six2Cre-Cyp24flox), intestine (VillinCreERT2-Cyp24flox) and globally (UBCCreERT2-Cyp24flox). In Aim 2, we will test our hypothesis that inhibiting intestinal CYP24A1 will attenuate secondary hyperparathyroidism and downstream complications of CKD without worsening CKD. In Aims 1 and 2, readouts of effect will include longitudinal physiological measures of mineral homeostasis and kidney function, and expression of VDR target genes in the kidney and intestine; in Aim 2 we will also assess cardiac structure and function, and kidney, bone and vascular histology. In Aim 3, we will define the molecular mechanisms of CYP24A1 regulationin enterocytes usingpharmacological and genetic approaches in mouseand humanintestinal organoids. Readouts of effect will include cell signaling assays and expressionof Cyp24a1 and VDR target genes. By defin...