MEPE Effects on Hydroxyapatite Proliferaton Depend on Post-translational Modifications

OBJECTIVES: Matrix extracellular phosphoglycoprotein (MEPE) is a member of the SIBLING family of extracellular matrix proteins. These proteins are multifunctional having both signaling properties and being involved in the regulation of mineralization. MEPE is cleaved in situ by PHEX yielding an ASARM peptide that inhibits mineralization in cell culture [Martin et al, Endocrin, 2008; Addison et al, JBMR, 2008] and when administered to a mouse [Rowe et al, Bone, 2004]. METHODS: To test the hypothesis that post translational modification of MEPE is important in this process we used the gelatin gel diffusion system [Silverman & Boskey, CTI, 2004] to compare the effects of intact MEPE (with and without phosphorylation) and the ASARM peptide (with or without phosphorylation) in the presence or absence of fibrillar collagen. Effects were also examined in the presence of 0.5mg/ml hydroxyapatite (+HA) seed crystals. RESULTS: As reported in culture, in the gel diffusion system the phosphorylated, and to a lesser extent the dephosphorylated ASARM peptide inhibited HA formation and growth. In contrast, as shown in the figure, the intact MEPE promoted HA formation in a concentration dependent manner. The dephosphorylated intact protein had no effect. Data in the figure are compared to the control (dashed line=1, * p<0.05). CONCLUSIONS: These results demonstrate that post-translational modification of MEPE determines its ability to promote or inhibit hydroxyapatite formation and growth. SUPPORT: NIH grant DE04141.