Project Summary Phosphate levels are critical for cellular energy and skeletal development and mineralization. FGF23 is a key phosphaturic hormone, dysregulated actions/levels of which are the cause of several inherited bone diseases. FGF23 elevation in response to phosphate retention also plays a key role in the pathogenesis of and complications related to renal failure. The mechanisms underlying the production of FGF23 are poorly understood, limiting the ability to develop effective treatments for these disorders. XL?s is a G protein alpha-subunit, genetic alterations of which are associated with multiple human diseases. The goal of this proposal is to investigate the regulatory function of XL?s in FGF23 production and its role in diseases with dysregulated FGF23 production. Studying XL?s knockout (XLKO) mice as a model to explore the function of XL?s in humans, we found that XL?s plays a critical role in phosphate homeostasis during early postnatal development. We also showed that XL?s mediates renal PTH actions immediately after birth via the PLC/PKC pathway. Our recent work showed that XL?s ablation results in hyperphosphatemia with significantly reduced serum FGF23. Our preliminary results strongly indicate that XL?s ablation leads to diminished FGF23 and Fgfr1 mRNA levels, as well as reduced PLC/PKC and ERK1/2 signaling in a bone cell-autonomous manner. In addition, our preliminary results show that XL?s ablation mitigates the rise of FGF23 induced by acute kidney injury in mice. Therefore, we hypothesize that XL?s plays a critical role in the production of FGF23 in bone. In Aim 1, we will determine (a) the role of XL?s/PLC/PKC signaling in FGF23 production, and (b) the mechanism by which XL?s regulates ERK1/2 activation. In Aim 2, we will test the hypothesis that XL?s contributes to PTH-induced FGF23 production via the PLC/PKC and/or ERK1/2 pathway. We will also address the hypothesis that the interaction of XL?s with SNX9, dynamin, RACK1 and/or IQGAP1 is critical for XL?s- mediated FGF23 production. In Aim 3, we will determine whether XL?s ablation mitigates the elevated serum FGF23 levels in a mouse model of (a) acute kidney injury and (b) chronic kidney disease. Results of the proposed experiments will provide novel mechanistic insights into the regulation of FGF23 production and will also identify potential drug targets for treating patients with dysregulated FGF23 levels. Moreover, our studies will provide mechanistic insights into the cellular actions of XL?s in bone and enhance the knowledge of its roles in human diseases.