Chondrogenesis mediated by PPi depletion promotes spontaneous aortic calcification in NPP1-/- mice

Objective - We recently linked human arterial media calcification of infancy to heritable PC-1/nucleotide pyrophosphatase phosphodiesterase 1 (NPP1) deficiency. NPP1 hydrolyzes ATP to generate PPi, a physicochemical inhibitor of hydroxyapatite crystal growth. But pathologic calcification in NPP1 deficiency states is tissue-restricted and in perispinal ligaments is endochondral differentiation - mediated rather than simply a dystrophic process. Because ectopic chondro-osseous differentiation promotes artery calcification in atherosclerosis and other disorders, we tested the hypothesis that NPP1 and PPi deficiencies regulate cell phenotype plasticity to promote artery calcification. Methods and Results - Using cultured multipotential NPP1-/- mouse bone marrow stromal cells, we demonstrated spontaneous chondrogenesis inhibitable by treatment with exogenous PPi. We also demonstrated cartilage-specific gene expression, upregulated alkaline phosphatase, decreased expression of the physiological calcification inhibitor osteopontin, and increased calcification in NPP1 -/- aortic smooth muscle cells (SMCs). Similar changes were demonstrated in aortic SMCs from ank/ank mice, which are extracellular PPi - depleted because of defective ANK transmembrane PPi transport activity. Moreover, NPP1 -/- and ank/ank mice demonstrated aortic media calcification by von Kossa staining, and intra-aortic cartilage-specific collagen gene expression was demonstrated in situ in NPP1 -/- mice. Conclusions - NPP1 and PPi deficiencies modulate phenotype plasticity in artery SMCs and chondrogenesis in mesenchymal precursors, thereby stimulating artery calcification by modulating cell differentiation.