Long-chain polyphosphate is enriched in mineralized osteoblasts and matrix vesicles

Abstract

Polyphosphate (polyP) is a linear polymer consisting of tens to hundreds of phosphate residues linked by high-energy phosphoanhydride bonds. This fundamental molecule is ubiquitous in nature yet unanswered questions of polyP remain in higher eukaryotes including how it is synthesized and its downstream metabolism. PolyP has been previously reported to be at a relatively higher level in osteoblasts compared to other cells. Here, using SaOS-2 cell line, we provide evidence that polyP level dramatically increased during cell differentiation and mineralization in SaOS-2 cells. The increased polyP was determined under confocal microscopy with DAPI staining. Matrix vesicles (MVs) isolated from mineralized SaOS-2 cells were highly enriched in long chain polyP. The localization of polyP in MVs was demonstrated by two specific polyP staining approaches under TEM. Moreover, extracellular polyP was found to be able to bind to MVs and inhibit MV-induced mineralization indicating a possible pathway of polyP in regulating bone mineralization in physiological condition. Whether polyP in MVs was synthesized after MV formation or provided by another cell compartment during vesicle formation remains unclear.