Role of phosphate sensing in bone and mineral metabolism

Abstract

Inorganic phosphate (P_i) is essential for signal transduction and cell metabolism, and is also an essential structural component of the extracellular matrix of the skeleton. P_i is sensed in bacteria and yeast at the plasma membrane, which activates intracellular signal transduction to control the expression of P_i transporters and other genes that control intracellular P_i levels. In multicellular organisms, P_i homeostasis must be maintained in the organism and at the cellular level, requiring an endocrine and metabolic P_i-sensing mechanism, about which little is currently known. This Review will discuss the metabolic effects of P_i, which are mediated by P_i transporters, inositol pyrophosphates and SYG1-Pho81-XPR1 (SPX)-domain proteins to maintain cellular phosphate homeostasis in the musculoskeletal system. In addition, we will discuss how P_i is sensed by the human body to regulate the production of fibroblast growth factor 23 (FGF23), parathyroid hormone and calcitriol to maintain serum levels of P_i in a narrow range. New findings on the crosstalk between iron and P_i homeostasis in the regulation of FGF23 expression will also be outlined. Mutations in components of these metabolic and endocrine phosphate sensors result in genetic disorders of phosphate homeostasis, cardiomyopathy and familial basal ganglial calcifications, highlighting the importance of this newly emerging area of research.

Fig. 1 |. P_i sensing pathways.

a | P_i sensing in bacteria. High P_i levels are sensed by PstS. Then, together with the Pst–ABC complex (PstA, PstB and PstC), it forms a plasma membrane protein complex in the bacterial inner membrane that stimulates the binding of PhoU to PhoB and PhoR. This process inactivates the transcription factor PhoB and the Pho regulon. The low-affinity P_i transporters 1 and 2 (PitA and PitB) facilitate uptake of P_i for cellular metabolism. Under low P_i conditions, PhoR is autophosphorylated and phosphorylates PhoB and activates the Pho box, allowing downstream activation of the Pho regulon. b | In yeast, high P_i levels activate the Pho80 and Pho85 cyclin–cyclin-dependent kinase complex (1), which results in the phosphorylation and export of Pho4 into the cytosol (2) and inactivation of the yeast Pho regulon. The high-affinity P_i transporter and sensor Pho84 is internalized and degraded (3), whereas the low-affinity P_i transporters Pho87 and Pho90 are responsible for P_i uptake in high P_i conditions (4). Low P_i levels stimulate synthesis of Ip7 by the yeast inositol hexakisphosphate (Ip6) kinase 1 (Kcs1) and Vip (5), which activates Pho81 (6). Pho81 inhibits Pho80 and Pho85, preventing phosphorylation of Pho4, resulting in the association of Pho4 with Pho2 (7) in the nucleus to activate the Pho regulon. Ip7 also stimulates Vtc proteins 1–4, which stimulate polyphosphate synthesis from ATP (8) and the conversion of polyphosphate into P_i by endopolyphosphatase (Phm5). P_i is transported by Pho91 from the vacuole to the cytosol (9), thereby indirectly using ATP to supply P_i for metabolic processes. Pho84 and Pho89 are responsible for P_i uptake in low P_i condition, whereas Pho87 and Pho90 are internalized and degraded. c | Metabolic P_i sensing in mammalian cells is mediated by PIT1 and/or PIT2, resulting in activation of the ERK1 and ERK2 pathway, which might also have a role in endocrine P_i sensing but could require co-receptors (single sensor hypothesis), possibly FGFR1, which was shown to be activated by P_i and might regulate FGF23 secretion by osteocytes. Alternatively, endocrine P_i sensing might involve the calcium-sensing receptor (CaSR) or other molecules as a second sensor (multiple sensor hypothesis), which might mediate secondary hyperparathyroidism in the parathyroids. Question marks indicate unknown mechanisms or sensors. P, phosphate; PTH, parathyroid hormone; Ub, ubiquitin.

Fig. 2 |. Regulation of bone cell function and matrix mineralization by P_i.

a | In chondrocytes, P_i stimulates the expression of hypertrophic chondrocyte markers (that is, collagen X) and induces apoptosis via the mitochondrial caspase 3 pathway in an ERK1-dependent and ERK2-dependent fashion. P_i also stimulates PIT1-dependent matrix vesicle mineralization. b | In osteoblasts, P_i induces the expression of osteopontin (OPN) through an ERK1-dependent and ERK2-dependent mechanism to support the formation of bone matrix. P_i also stimulates IGF1 secretion, which increases osteoblast proliferation in an autocrine fashion. c | In osteocytes, DMP1, ENPP1 and PHEX expression are stimulated by P_i (REF.), which also induces the secretion of bioactive intact FGF23 (REF.). d | In osteoclasts, P_i reduces gene expression of RANKL and thereby suppresses RANK, which results in the inhibition of osteoclastogenesis and bone resorption. P_i also induces the production of ROS, possibly through proton (H+) channels, which increases osteoclast function and survival. e | In myocytes, P_i is important for the function of the mitochondrial respiratory chain and ATP synthesis. This process is possibly due to the function of the muscle-specific isoform of PIC, which mediates mitochondrial uptake of P_i (REF.), and PIT1 (REF.). ↑, upregulation; ↓, downregulation.

Fig. 3 |. Endocrine regulation of P_i homeostasis.

Serum P_i stimulates secretion of bioactive FGF23 in osteoblasts and osteocytes (blue arrow), which directly or indirectly acts at the proximal tubule of the kidneys to inhibit synthesis of calcitriol and the function of NPT2A and NPT2C (red arrows). Inhibition of calcitriol reduces absorption of P_i from the diet in the gut (grey arrow) and mobilization of P_i from bone mineral. Downregulation of NPT2A and NPT2C reduces renal phosphate reabsorption (black arrows). The net effect of FGF23 action is to lower blood levels of P_i. Similar to FGF23, parathyroid hormone (PTH) downregulates NPT2A and NPT2B and reduces renal phosphate reabsorption (green arrows). However, different from FGF23, PTH induces calcitriol and bone turnover, which increase blood P_i (green arrows). However, the net effect of PTH is to lower blood levels of P_i. Although not completely understood, FAM20C, DMP1, ENPP1 and PHEX reduce FGF23 expression or secretion whereas phosphate, iron deficiency, erythropoietin (EPO), GALNT3 and GNAS1 stimulate it (black arrows)^,. In addition, sclerostin (SOST) seems to negatively regulate FGF23 (black arrows). Furthermore, EPO might directly upregulate FGF23 gene expression in myeloid lineage stem cells of the spleen, providing a link to iron homeostasis (black arrows). PTH is suppressed by FGF23 in rodents but not in humans (red arrow).

Fig. 4 |. Diseases of phosphate homeostasis organized by organ system.

Linkage analysis in human disorders of inorganic phosphate (P_i) homeostasis, cardiomyopathy and familial basal ganglial calcifications identified several novel genes important for the regulation of P_i homeostasis. DMP1, dentin matrix acidic phosphoprotein 1; ENPP1, ectonucleotide pyrophosphatase-phosphodiesterase family member 1; FAM20C, Golgi-associated secretory pathway kinase; FGF23, fibroblast growth factor 23; FGFR1, FGF receptor 1; GALNT3, polypeptide N-acetylgalactosaminyltransferase 3; GNAS1, guanine nucleotide-binding protein G(s), subunit α; NPT1, sodium-dependent phosphate transport protein 1; NPT2A, sodium-dependent phosphate transport protein 2A; PHEX, phosphate-regulating endopeptidase homologue, X-linked; PIT, sodium-dependent P_i transporter; SLC25A3, solute carrier family 25 member 3; XPR1, xenotropic and polytropic retrovirus receptor 1. Question mark indicates unknown.