The role of extracellular vesicles in biomineralisation: current perspective and application in regenerative medicine

Abstract

Extracellular vesicles comprise a heterogenous population of exosomes and microvesicles that have critical roles in intercellular signalling and tissue development. These complex particles have been implicated as mediators of the therapeutic effects of stem cells via the transfer of an assorted cargo of proteins and nucleic acids, which can modulate inflammation and enhance endogenous regeneration in a range of tissues. In addition, extracellular vesicles have the capacity to be loaded with therapeutic molecules for targeted delivery of pharmaceuticals. The versatility, biostability and biocompatibility of extracellular vesicles make them appealing for regenerative medicine and may endow considerable advantages over single molecule approaches. Furthermore, since production can be optimised and assessed ex vivo, extracellular vesicles present a decreased risk of neoplastic transformation when compared with cell-based methods. To date, the contribution of vesicles to tissue development has perhaps been most comprehensively defined within hard tissues, such as endochondral bone, where they were first identified in 1969 and henceforth referred to as matrix vesicles. Within developing bone, vesicles function as vehicles for the delivery of pro-osteogenic factors and initiate early nucleational events necessary for matrix mineralisation. However, advancement in our understanding of the biogenesis and characterisation of matrix vesicles has occurred largely in parallel to associated developments in wider extracellular vesicle biology. As such, there is a requirement to align current understanding of matrix vesicle-mediated mineralisation within the context of an evolving literature surrounding exosomes and microvesicles. In this review, we present an overview of current progress and opinion surrounding the application of vesicles in regenerative medicine with a primary focus on their potential as an acellular approach for enhancing hard tissue regeneration. This is balanced with an assessment of areas where further development is required to maximise their application for regenerative medicine.

Keywords: Extracellular vesicles; exosomes; matrix vesicles; mineralisation; pathological calcification; regenerative medicine.

Figure 1.

Publication trend of extracellular vesicles. Data were exported from Web of Science using the following criteria: (1) all databases, keywords (exosomes OR extracellular vesicle) and year range (1930–2017); (2) all databases, keywords (exosomes OR extracellular vesicle AND therapy) and year range (1930–2017).

Figure 2.

Matrix vesicle (MV) cargo. Graphical representation of the matrix vesicle contents and membrane orientation of proteins, lipids and nucleic acids. Some of the listed components may be present in some matrix vesicles but not in others. For instance, in our previous study, we did not detect MHC complexes.

Figure 3.

Schematic diagram of the mineralisation process. NPP1 inhibits mineralisation by generating PPi by catalysing extracellular ATP. TNAP promotes mineralisation by hydrolysing PPi into inorganic phosphate ions, which are in turn transported to the matrix vesicle (MV) through phosphate transporters such as Pit1. Conversely, ANK transports PPi from the MV into the developing ECM. Annexins function as calcium channels, transporting ${\mathrm{Ca}}_2^{+}$ inside the MV and localise ${\mathrm{Ca}}_2^{+}$ and ${\mathrm{PO}}_4^{3-}$ in a nucleational core complex, which facilitates mineral nucleation and transition to a crystalline hydroxyapatite. This is hypothesised to eventually rupture the vesicle membrane and propagate within the collagenous extracellular matrix.