Objective: Elevated cartilage inorganic pyrophosphate (PPi) production and PPi-generating nucleoside triphosphate pyrophosphohydrolase (NTPPPH) activity are strongly linked with aging-related cartilage calcification in meniscal and articular cartilages. We hypothesized that there were divergent relationships of 3 NTPPPH isozymes with cartilage matrix calcification and sought to identify them.
Methods: We studied knee medial meniscal expression in situ of 3 NTPPPH isozymes of the phosphodiesterase nucleotide pyrophosphatase (PDNP) family: plasma cell membrane glycoprotein 1 (PC-1, or PDNP1), autotaxin (ATX, or PDNP2), and B10/PDNP3. We also used complementary DNA transfection to assess differential functions in matrix calcification of each NTPPPH isozyme in vitro in meniscal cells.
Results: We observed diffuse cell-associated ATX and B10/PDNP3 expression in central (chondrocytic) and, to a lesser degree, peripheral (fibroblastic) regions of normal, degenerative uncalcified, and degenerative calcified menisci. In contrast, PC-1 expression was only robust at sites of apoptotic cells and calcification in central regions of degenerative menisci. Only PC-1 was abundant at the perimeter of meniscal cells and in association with meniscal cell-derived matrix vesicles (MVs). Because each PDNP-family isozyme was expressed by cells near calcifications, we transfected the isozymes in nonadherent knee meniscal cells cultured with ascorbic acid, beta-glycerophosphate, and dexamethasone supplementation to stimulate them to calcify the matrix. PC-1, but not ATX or B10/PDNP3, consistently promoted increased MV NTPPPH, MV-associated PPi, and extracellular PPi. PC-1 also increased matrix calcification (with hydroxyapatite crystals) by meniscal cells. ATX uniquely induced alkaline phosphatase activity, but promoted only moderately increased matrix calcification.
Conclusion: We identified divergent effects of 3 PDNP-family NTPPPH isozymes on meniscal cell matrix calcification. Increased expression of PC-1 is both a marker and a potential pathogenic factor for knee meniscal cartilage matrix calcification.