Phosphatidylserine-enriched monolayers controls calcium phosphate nucleation and growth: a physicochemical understanding of matrix vesicles mineralization

Bone biomineralization is an exquisite process by which cells control the synthesis and organization of a hierarchically mineralized matrix. Growing evidence has uncovered the involvement of one class of extracellular vesicles, the matrix vesicles (MVs), in the formation and delivery of the first mineral nuclei to the bone growth front. MVs are nanoreactors equipped with specific biochemical machinery and released by mineral-competent cells and to initiate mineral formation. However, little is known about the pathways by which MVs can template and trigger this process. Here, we present a combination of in situ investigations and ex vivo analysis of MVs extracted from growing-femurs of chicken embryos to investigate the role played by phosphatidylserine (PS) in the formation of the mineral nuclei. By using self-assembled Langmuir monolayers, we reconstructed the nucleation core- a PS-enriched motif present inside MVs and already thought to be the entity responsible to trigger mineral formation during mineralization. In situ infrared spectroscopy of Langmuir monolayers and ex situ analysis by transmission electron microscopy evidenced that mineralization was achieved within 240 min on supersaturated solutions only when PS was present. Amorphous calcium phosphate nucleated by PS was converted into biomimetic apatite after 24 h of incubation. By using monolayers containing lipids extracted from native MVs, mineral formation was also achieved after 240 min in a manner that resembles the artificial monolayers containing PS. We raise the possibility that PS-mediated nucleation could be a predominant pathway to produce the very first mineral in the bone/cartilage mineralization.