Exploring the role of lipid-protein-mineral matrix on bone biomineralization: a biophysical approach using self-assembled films

Abstract

Bone biomineralization is an exquisite process mediated and controlled by cells that culminate in the building of a highly hierarchical hybrid matrix. The building block of this matrix is the mineralized collagen fibrils, where hydroxyapatite (HAp) crystals are infiltrated in the fibril. The origin of these HAp crystals is regarded to the action of Matrix Vesicles (MV). These lipid vesicles are excreted by osteogenic cells and harbor the biochemical machinery required to produce and transport Ca2+ and PO43- ions for the HAp precipitation. Inside the MV, the lipid matrix act as a nucleation core to initiate the HAp crystallization. HAp crystals are them released to the extracellular medium and propagate the mineralization of collagen fibrils. Besides of large knowledge about the MV`s role as the start of bone mineralization and how interactions between collagen and HAp dictates the bone structure, little is known about the interface of MV and collagen mineralization. Our hypothesis is that after the MV`s rupture, lipid-mineral complexes are exposed to the extracellular medium and interact with collagen fibrils to propagate the intrafibrillar mineralization. To access this hypothesis, we will use state-of-art biophysical techniques to characterize the structure of complexes formed between lipids and calcium phosphates and we will explore the ability of these lipid-mineral complexes in propagating the collagen mineralization. We will use Langmuir monolayers composed of lipid found in the MV (phosphatidylserine, sphingomyelin, and cholesterol) to investigate the formation of lipid-mineral complexes. The composition and structure of the organic matrixes will be interconnected with the ability in induce mineral crystallization. We hope to identify with resolution at the molecular level the pathways induced by the lipid matrix in the HAp crystallization. The thermodynamic and kinetic behavior of lipid-induced HAp crystallization will be determined. Once we have identified the nature and ability of lipid-mineral complexes in inducing and control the HAp crystallization, we will evaluate the role of lipid-mineral complexes in propagating the collagen mineralization. For this, we will use collagen hierarchical matrixes mimetizing the structure found in the bone tissue. The creation of hierarchical collagen structures will be done in collaboration with Professor Seung-Wuk Lee (University of California at Berkeley, USA). Our aim with this project is to propose a new vision for bone mineralization, with applications emerging from clinics to the design of bio-inspired synthetic materials. (AU)