SummaryMatrix vesicle-mediated mineralization is an orchestrated sequence of ultrastructural and biochemical events that lead to crystal nucleation and growth. The influx of phosphate ions into the matrix vesicle is mediated by several proteins such as TNAP, ENPP1, Pit1, annexin and so forth. The catalytic activity of ENPP1 generates pyrophosphate (PPi) using extracellular ATPs as a substrate, and the resultant PPi prevents crystal overgrowth. However, TNAP hydrolyzes PPi into phosphate ion monomers, which are then transported into the matrix vesicle through Pit1. Accumulation of Ca2+ and PO43− inside matrix vesicles then induces crystalline nucleation, with calcium phosphate crystals budding off radially, puncturing the matrix vesicle’s membrane and finally growing out of it to form mineralized nodules.

To elucidate which of elevated serum concentration of inorganic phosphate (Pi) or disrupted signaling linked to αklotho/fibroblast growth factor 23 (FGF23) is a predominant regulator for senescence-related degeneration seen in αKlotho-deficient mice, we have examined histological alteration of the periodontal tissues in the mandibular interalveolar septum of αKlotho-deficient mice fed with Pi-insufficient diet. We prepared six groups of mice: wild-type, kl/kl, and αKlotho mice with normal diet or low-Pi diet. As a consequence, kl/kl and αKlotho mice showed the same abnormalities in periodontal tissues: intensely stained areas with hematoxylin in the interalveolar septum, dispersed localization of alkaline phosphatase-positive osteoblasts and tartrate-resistant acid phosphatase-reactive osteoclasts, and accumulation of dentin matrix protein 1 in the osteocytic lacunae. Although kl/kl mice improved these histological abnormalities, αKlotho mice failed to normalize those. Gene expression of αKlotho was shown to be increased in kl/kl specimens. It seems likely that histological abnormalities of kl/kl mice have been improved by the rescued expression of αKlotho, rather than low concentration of serum Pi. Thus, the histological malformation in periodontal tissues in αKlotho-deficient mice appears to be due to not only increased concentration of Pi but also disrupted αklotho/FGF23 signaling.