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New insights into G protein coupled receptor regulation of glucose metabolism by β-cells, skeletal muscle and liver hepatocytes identify GPRC6A as a potential therapeutic target for treating type 2 diabetes mellitus (T2D). Activating GPRC6A with a small molecule drug represents a potential paradigm-shifting opportunity to make significant strides in regulating glucose homeostasis by simultaneously correcting multiple metabolic derangements that underlie T2D, including abnormalities in β-cell proliferation and insulin secretion and peripheral insulin resistance. Using a computational, structure-based high-throughput screening approach, we identified novel tri-phenyl compounds predicted to bind to the venus fly trap (VFT) and 7-transmembrane (7-TM) domains of GPRC6A. Experimental testing found that these compounds dose-dependently stimulated GPRC6A signaling in a heterologous cell expression system. Additional chemical modifications and functional analysis identified one tri-phenyl lead compound, DJ-V-159 that demonstrated the greatest potency in stimulating insulin secretion in β-cells and lowering serum glucose in wild-type mice. Collectively, these studies show that GPRC6A is a "druggable" target for developing chemical probes to treat T2DM.

A postnatal role of fibroblast growth factor receptor-1 (FGFR1) in the kidney is suggested by its binding to α-Klotho to form an obligate receptor for the hormone fibroblast growth factor-23 (FGF-23). FGFR1 is expressed in both the proximal and distal renal tubular segments, but its tubular specific functions are unclear. In this study, we crossed Fgfr1flox/flox mice with either gamma-glutamyltransferase-Cre (γGT-Cre) or kidney specific-Cre (Ksp-Cre) mice to selectively create proximal tubule (PT) and distal tubule (DT) Fgfr1 conditional knockout mice (designated Fgfr1PT-cKO and Fgfr1DT-cKO, respectively). Fgfr1PT-cKO mice exhibited an increase in sodium-dependent phosphate co-transporter expression, hyperphosphatemia, and refractoriness to the phosphaturic actions of FGF-23, consistent with a direct role of FGFR1 in mediating the proximal tubular phosphate responses to FGF-23. In contrast, Fgfr1DT-cKO mice unexpectedly developed hypercalciuria, secondary elevations of parathyroid hormone (PTH), hypophosphatemia and enhanced urinary phosphate excretion. Fgfr1PT-cKO mice also developed a curly tail/spina bifida-like skeletal phenotype, whereas Fgfr1DT-cKO mice developed renal tubular micro-calcifications and reductions in cortical bone thickness. Thus, FGFR1 has dual functions to directly regulate proximal and distal tubule phosphate and calcium reabsorption, indicating a physiological role of FGFR1 signaling in both phosphate and calcium homeostasis.

Elevations of circulating Fibroblast growth factor 23 (FGF23) are associated with adverse cardiovascular outcomes and progression of renal failure in chronic kidney disease (CKD). Efforts to identify gene products whose transcription is directly regulated by FGF23 stimulation of fibroblast growth factor receptors (FGFR)/α-Klotho complexes in the kidney is confounded by both systemic alterations in calcium, phosphorus and vitamin D metabolism and intrinsic alterations caused by the underlying renal pathology in CKD. To identify FGF23 responsive genes in the kidney that might explain the association between FGF23 and adverse outcomes in CKD, we performed comparative genome wide analysis of gene expression profiles in the kidney of the Collagen 4 alpha 3 null mice (Col4a3(-/-)) model of progressive kidney disease with kidney expression profiles of Hypophosphatemic (Hyp) and FGF23 transgenic mouse models of elevated FGF23. The different complement of potentially confounding factors in these models allowed us to identify genes that are directly targeted by FGF23. This analysis found that α-Klotho, an anti-aging hormone and FGF23 co-receptor, was decreased by FGF23. We also identified additional FGF23-responsive transcripts and activation of networks associated with renal damage and chronic inflammation, including lipocalin 2 (Lcn2), transforming growth factor beta (TGF-β) and tumor necrosis factor-alpha (TNF-α) signaling pathways. Finally, we found that FGF23 suppresses angiotensin-converting enzyme 2 (ACE2) expression in the kidney, thereby providing a pathway for FGF23 regulation of the renin-angiotensin system. These gene products provide a possible mechanistic links between elevated FGF23 and pathways responsible for renal failure progression and cardiovascular diseases.

Pkd1 localizes to primary cilia in osteoblasts and osteocytes. Targeted deletion of Pkd1 in osteoblasts results in osteopenia and abnormalities in Runx2-mediated osteoblast development. Kif3a, an intraflagellar transport protein required for cilia function, is also expressed in osteoblasts. To assess the relationship between Pkd1 and primary cilia function on bone development, we crossed heterozygous Pkd1- and Kif3a-deficient mice to create compound Pkd1 and Kif3a-deficient mice. Pkd1 haploinsufficiency (Pkd1(+/Δ)) resulted in osteopenia, characterized by decreased bone mineral density, trabecular bone volume, and cortical thickness. In addition, deficiency of Pkd1 resulted in impaired osteoblastic differentiation and enhanced adipogenesis in both primary osteoblasts and/or bone marrow stromal cell cultures. These changes were associated with decreased Runx2 expression, increased PPARγ expression, and impaired hedgehog signaling as evidenced by decreased Gli2 expression in bone and osteoblast cultures. In contrast, heterozygous Kif3a(+/Δ) mice display no abnormalities in skeletal development or osteoblast function, but exhibited decreased adipogenic markers in bone and impaired adipogenesis in vitro in association with decreased PPARγ expression and upregulation of Gli2. Superimposed Kif3a deficiency onto Pkd1(+/Δ) mice paradoxically corrected the effects of Pkd1 deficiency on bone mass, osteoblastic differentiation, and adipogenesis. In addition, Runx2, PPARγ and Gli2 expression in bone and osteoblasts were normalized in compound double Pkd1(+/Δ) and Kif3a(+/Δ) heterozygous mice. The administration of sonic hedgehog, overexpression of Gli2, and the PC1 C-tail construct all increased Gli2 and Runx2-II expression, but decreased PPARγ2 gene expression in C3H10T1/2 cells. Our findings suggest a role for Pkd1 and Kif3a to counterbalance the regulation of osteogenesis and adipogenesis through differential regulation of Runx2 and PPARγ by Gli2.