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Matrix vesicles mimetic systems: multi-enzymatic proteoliposome systems for biomineralization studies


Bone and cartilage mineralization occur by a series of physical-chemical and biochemical processes that together favor the hydroxyapatite (HA) deposition on specific areas of extracellular matrix (MEC). Experimental evidences show the presence of HA crystals together with collagen fibrils on MEC and also on the interior of matrix vesicles lumen (MVs) derived from chondroblasts and osteoblasts. Researchers in the bone mineralization field normally are divided between those who support the MVs mediated mineralization mechanism and those mediated by collagen.We do not see any incompatibility between these mechanisms . We believe that bone mineralization is initiated primarily inside MVs' lumen and, in a second stage , HA crystals grow beyond the frontiers of the MVs and become exposed to the extracellular medium, where they continue to propagate along with the collagen fibrils. The Tissue Non-specific Alkaline Phosphatase (TNAP) has a crucial role in limiting the extracellular Inorganic Pyrophosphate (PPI) concentration, a mineralization inhibitor, so as to keep an adequate Pi/PPi ratio for normal bone mineralization.The primary function of TNAP is to degrade the extracellular inorganic pyrophosphate (EPPI), a potent mineralization inhibitor, which is produced ectoplasmically by the enzymatic activity of the pyrophosphatase/ phosphodiesterase-1 nucleotide (NPP1). Both enzymes act at membrane osteoblasts level and of MVs derived from osteoblasts.Then, this project, that will be done in collaboration with Professor Dr. José Luis Millan, from Burnham Institute for Medical Research, La Jolla, California, EUA, and will allow us to compare the behavior of liposome containing on its surface TNAP, NPP1 or both enzymes in combination, as well as to compare the kinetic properties and calcification abilities of these simplified and artificial vesicles with those of wild MVs. Such studies will help develop and validate the liposome system as and alternative in vitro system to study the initial events that introduce the biological calcification process.The development of such nonovesicular artificial system which can mime the biological calcification can potentially be useful to the repairing/treatment of craniofacial defects and other skeletal defects, as well as to make titanium based dental implants mineralization easier. (AU)

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