Mineralization Profile of Annexin A6-Harbouring Proteoliposomes: Shedding Light on the Role of Annexin A6 on Matrix Vesicle-Mediated Mineralization

The biochemical machinery involved in matrix vesicles-mediated bone mineralization involves a specific set of lipids, enzymes, and proteins. Annexins, among their many functions, have been described as responsible for the formation and stabilization of the matrix vesicles′ nucleational core. However, the specific role of each member of the annexin family, especially in the presence of type-I collagen, remains to be clarified. To address this issue, in vitro mineralization was carried out using AnxA6 (in solution or associated to the proteoliposomes) in the presence or in the absence of type-I collagen, incubated with either amorphous calcium phosphate (ACP) or a phosphatidylserine-calcium phosphate complex (PS–CPLX) as nucleators. Proteoliposomes were composed of 1,2-dipalmitoylphosphatidylcholine (DPPC), 1,2-dipalmitoylphosphatidylcholine: 1,2-dipalmitoylphosphatidylserine (DPPC:DPPS), and DPPC:Cholesterol:DPPS to mimic the outer and the inner leaflet of the matrix vesicles membrane as well as to investigate the effect of the membrane fluidity. Kinetic parameters of mineralization were calculated from time-dependent turbidity curves of free Annexin A6 (AnxA6) and AnxA6-containing proteoliposomes dispersed in synthetic cartilage lymph. The chemical composition of the minerals formed was investigated by Fourier transform infrared spectroscopy (FTIR). Free AnxA6 and AnxA6-proteoliposomes in the presence of ACP were not able to propagate mineralization; however, poorly crystalline calcium phosphates were formed in the presence of PS–CPLX, supporting the role of annexin-calcium-phosphatidylserine complex in the formation and stabilization of the matrix vesicles’ nucleational core. We found that AnxA6 lacks nucleation propagation capacity when incorporated into liposomes in the presence of PS–CPLX and type-I collagen. This suggests that AnxA6 may interact either with phospholipids, forming a nucleational core, or with type-I collagen, albeit less efficiently, to induce the nucleation process.

Lipid microenvironment affects the ability of proteoliposomes harboring TNAP to induce mineralization without nucleators

Tissue-nonspecific alkaline phosphatase (TNAP), a glycosylphosphatidylinositol-anchored ectoenzyme present on the membrane of matrix vesicles (MVs), hydrolyzes the mineralization inhibitor inorganic pyrophosphate as well as ATP to generate the inorganic phosphate needed for apatite formation. Herein, we used proteoliposomes harboring TNAP as MV biomimetics with or without nucleators of mineral formation (amorphous calcium phosphate and complexes with phosphatidylserine) to assess the role of the MVs' membrane lipid composition on TNAP activity by means of turbidity assay and FTIR analysis. We found that TNAP-proteoliposomes have the ability to induce mineralization even in the absence of mineral nucleators. We also found that the addition of cholesterol or sphingomyelin to TNAP-proteoliposomes composed of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine reduced the ability of TNAP to induce biomineralization. Our results suggest that the lipid microenvironment is essential for the induction and propagation of minerals mediated by TNAP.