Long-chain inorganic polyphosphate is highly enriched in osteoblastic matrix vesicles

Abstract

Evidence is emerging that long-chain inorganic polyphosphates play a variety of critical roles in mammalian cells that remain little understood. In particular it has been posited that polyphosphate is found at a relatively high concentration in osteoblasts but many unanswered questions remain regarding polyphosphate function and synthesis. One significant challenge is the lack of tools to observe and analyse polyphosphate function. Here, we report evidence that polyphosphate is highly enriched in SaOS-2 osteosarcoma osteoblastic matrix vesicles. In one line of evidence, we use various chromatographic approaches coupled to use of DAPI as a wavelength-dependent polyphosphate specific dye. In a second line of evidence, we use a heavy metal labelled polyphosphate binding domain (PPBD) of Escherichia coli exopolyphosphatase (PPX) as a tool for the observation of polyphosphate by transmission electron microscopy. Results correlated in observing a particularly strong enrichment of polyphosphate in osteoblastic matrix vesicles. Polyphosphates likely play important roles in mineralization in matrix vesicles, suggesting polyphosphate could be an overlooked key molecule in extracellular matrix biology.