Abstract
Matrix vesicles (MVs) in the growth plate bind to cartilage collagens and initiate mineralization of the extracellular matrix. Native MVs have been shown to contain a nucleational core responsible for mineral formation that is comprised of Mg(2+)-containing amorphous calcium phosphate and lipid-calcium-phosphate complexes (CPLXs) and the lipid-dependent Ca(2+)-binding proteins, especially annexin-5 (Anx-5), which greatly enhances mineral formation. Incorporation of non-Ca(2+)-binding MV lipids impedes mineral formation by phosphatidylserine (PS)-CPLX. In this study, nucleators based on amorphous calcium phosphate (with or without Anx-5) were prepared with PS alone, PS + phosphatidylethanolamine (PE), or PS + PE and other MV lipids. These were incubated in synthetic cartilage lymph containing no collagen or containing type II or type X collagen. Dilution of PS with PE and other MV lipids progressively retarded nucleation. Incorporation of Anx-5 restored nucleational activity to the PS:PE CPLX; thus PS and Anx-5 proved to be critical for nucleation of mineral. Without Anx-5, induction of mineral formation was slow unless high levels of Ca(2+) were used. The presence of type II collagen in synthetic cartilage lymph improved both the rate and amount of mineral formation but did not enhance nucleation. This stimulatory effect required the presence of the nonhelical telopeptides. Although type X collagen slowed induction, it also increased the rate and amount of mineral formation. Both type II and X collagens markedly increased mineral formation by the MV-like CPLX, requiring Anx-5 to do so. Thus, Anx-5 enhances nucleation by the CPLXs and couples this to propagation of mineral formation by the cartilage collagens.