Proteomic characterisation of matrix vesicles from primary osteoblasts indicates a mixed population with both exosomal and ectosomal routes of biogenesis.

Matrix Vesicles are a crucial step in producing a mineralised, healthy skeleton. Released from chondrocytes and osteoblasts, they concentrate calcium and phosphate to establish the deposition of hydroxyapatite around and within the collagen fibrils of the extracellular matrix, becoming embedded in the matrix in the process. In the 55 years since H. Clarke Anderson first described them, blebbing from the surface of chondrocytes, a consensus on their role, contents and their biogenesis, has yet to be reached. This is in part due to the range of cell types from which they are released and the multitude techniques that can be employed to isolate them. In this study, we, for the first time, characterise the proteome of matrix vesicles isolated from primary osteoblasts. By focussing on those vesicles that have become embedded within the matrix, we are able to avoid bias for specific modes of biogenesis and focus only on osteoblast-released vesicles that are associated with the matrix. Moreover, by studying these vesicles over a time course of mineralising activity, we are able to identify changes in the properties of these vesicles, and develop a more accurate picture of which proteins are involved specifically in mineralisation. In particular we identify the presence of markers associated with the ectosomal and exosomal release of matrix vesicles, as well as identifying proteins required for the maintenance and mineralisation of the extracellular matrix. These data portray a heterogeneous population of matrix vesicles, with different roles to play in bone development.