Abstract
Inorganic phosphate (Pi) homeostasis is crucial to organismal health, yet the mechanisms underlying the regulation of it remain unclear. Critically, we lack a clear understanding of the Pi response circuitry in osteogenic cells that identifies altered serum Pi levels and transmits this information to changes in serum FGF23 levels, a key hormone regulating circulating Pi. We utilized genome-wide CRISPR screens in osteogenic Pi-responsive fluorescent reporter cell lines to identify regulators of the response to high phosphate, intersecting those results with loci associated with circulating FGF23 levels by genome-wide association studies (GWAS) and identified a potential role for TGF-β2. We found that each of the three ligands (TGF-β1, 2, 3) can enhance the response to Pi in osteogenic cell lines and ex vivo cultures of calvariae, while inhibitors of TGFβ receptor signaling dampen it. Co-treatment of Pi with TGFβ ligands led to an elevated, synergistic transcriptional induction of Slc20a1 , which encodes a key Pi importer, which corresponded with an increased intracellular uptake of phosphate. Furthermore, in mice, blocking TGFβ signaling disrupted the induction of FGF23 in mice on a high phosphate diet, resulting in disrupted downstream endocrine control of phosphate homeostasis. Together, these findings reveal a role for TGFβ signaling in the regulation of phosphate homeostasis in osteogenic cells through regulation of cellular phosphate uptake, which in turn contributes to the maintenance of organismal phosphate homeostasis.