Roles of Phosphate in Skeleton

Area of Science:

• Biochemistry
• Cell Biology
• Skeletal Biology

Background:

• Phosphate is essential for skeletal mineralization, with deficiency causing rickets and osteomalacia.
• Skeletal mineralization initiates in matrix vesicles (MVs) produced by osteoblasts and chondrocytes.
• Tissue non-specific alkaline phosphatase (TNSALP) in MVs generates inorganic orthophosphate (Pi) for hydroxyapatite formation.

Purpose of the Study:

• To investigate the role of extracellular inorganic phosphate (Pi) in skeletal cell function and signaling.
• To explore the involvement of Pi in cellular processes beyond mineralization, such as apoptosis and gene expression.
• To examine the potential link between Pi sensing, fibroblast growth factor receptor (FGFR) signaling, and phosphate homeostasis disorders like X-linked hypophosphatemic rickets (XLH).

Main Methods:

• Review of existing literature on phosphate metabolism and skeletal cell biology.
• Analysis of *in vitro* studies demonstrating Pi-induced cellular responses.
• Examination of signaling pathways, including the Raf/MEK/ERK pathway, activated by extracellular Pi.

Main Results:

• Extracellular Pi regulates hypertrophic chondrocyte apoptosis, a key step in endochondral ossification.
• Pi influences the expression of genes involved in osteoblast proliferation, differentiation, and mineralization.
• Elevated extracellular Pi activates FGFR and the Raf/MEK/ERK pathway, potentially involving PiT-1 cotransporters.

Conclusions:

• Skeletal cells possess mechanisms to sense and respond to extracellular Pi levels, influencing cellular functions.
• FGFR signaling activation by Pi may contribute to FGF23 overproduction in XLH models.
• Impaired Pi sensing is a potential pathogenic factor in XLH that warrants further investigation.