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The mechanism of β‐glycerophosphate action in mineralizing chick limb‐bud mesenchymal cell cultures

Differentiating chick limb‐bud mesenchymal cells plated in micromass culture form a cartilage matrix that can be mineralized in the presence of 4 mM inorganic phosphate (Pi) and 1 mM calcium. Previous studies showed that when β‐glycerophosphate (βGP) is used in place of Pi, the mineral crystals formed are larger and differ in distribution. The present study shows that the difference in distribution is not associated with alterations in cell proliferation, protein synthesis, or with collagen, proteoglycan core protein, or alkaline phosphatase gene expression. Cultures with 2.5, 5, and 10 mM βGP did show different levels of alkaline phosphatase activity, and in the presence of low (0.3 mM) Ca had different Pi contents (4, 6, and 9 mM, respectively), indicating that the increase in CaxP product may in part be responsible for the altered pattern of mineralization. However, cultures with βGP in which alkaline phosphatase activity was inhibited with levamisole still had an altered mineral distribution as revealed by Fourier transform‐infrared (FT‐IR) microspectroscopy. The presence of a casein kinase II‐like activity in the mineralizing cultures, the ability of specific inhibitors of this enzyme to block mineralization, and the known ability of βGP to block phosphoprotein phosphatase activity suggests that altered patterns of matrix protein phosphorylation may influence mineral deposition in these cultures. (J Bone Miner Res 1996;11:1694‐1702)