Are collagen and Annexin V responsible for the control in the biomineralization process?

Abstract

The biomineralization process occurs through the accumulation of mineral constituted mainly by phosphate and calcium ions which form the calcium phosphate salt with the structure of the hydroxiapatite. This process is mediated by osteoblasts, cells responsible for the beginning of the biomineralization process, mediated by release of the matrix vesicles (MVs). These vesicles arise from the cell's surface by budding and are secreted in the specific place in the beginning of biomineralization in the bone tissue matrix. MVs contain high concentrations of Ca2+ and inorganic phosphate (Pi) ions providing an adequate microenvironment for the initial formation and propagation of the hydroxiapatite crystals. Special attention should be given to some proteins present in the MVs: Annexin V (AnxA5), alkaline phosphatase (TNAP) and ectonucleotide pyrophosphatase/phosphodiesterase-1 (NPP1). These proteins regulate the formation of calcium phosphate crystals, thus acting directly on the bone mineralization process. Among the annexins, the Annexin V specifically (protein with ~35 kDa) is responsible for the formation of calcium channel through the association of this protein with both inner and outer layer of the MVs membrane. The annexins are also responsible for the disorganization of cellular membrane resulting in the apoptosis process. TNAP is a nonspecific phosphomonohydrolase able to hydrolyze phosphate monoesters, pyrophosphate, phosphate diesters, as well as catalyze the transphosphorylation reactions. It's inserted in the plasmatic membrane of the MVs through the glycosylphosphatidylinositol anchor (GPI) and it is called "alkaline" for its ability to perform the substrate hydrolysis reactions more efficiently at pH above neutral (pH 8-11). Furthermore, TNAP has a crucial role in limiting the extracellular inorganic pyrophosphate concentration (ePPi), a potent inhibitor of mineralization, in order to maintain the Pi/PPi ratio adequate for a normal bone mineralization. The primary function of the TNAP is to degrade ePPi, which is produced extracellularly by NPP1. We intend to study the interactions of proteins (AnxA5, TNAP e NPP1), lipids and collagen (type II and X) in the biomineralization process, using the vesicular systems of proteoliposomes in order to form mimetic systems of MVs and study how these protein/protein and protein/collagen interactions can regulate and modulate the mineralization process mediated by MVs. (AU)