Mesenchymal stromal/stem cell-derived extracellular vesicles in tissue repair: challenges and opportunities

Abstract

Mesenchymal stem/stromal cells (MSCs) are important players in tissue homeostasis and regeneration owing to their immunomodulatory potential and release of trophic factors that promote healing. They have been increasingly used in clinical trials to treat multiple conditions associated with inflammation and tissue damage such as graft versus host disease, orthopedic injuries and cardiac and liver diseases. Recent evidence demonstrates that their beneficial effects are derived, at least in part, from their secretome. In particular, data from animal models and first-in-man studies indicate that MSC-derived extracellular vesicles (MSC-EVs) can exert similar therapeutic potential as their cells of origin. MSC-EVs are membranous structures loaded with proteins, lipids, carbohydrates and nucleic acids, which play an important role in cell-cell communication and may represent an attractive alternative for cell-based therapy. In this article we summarize recent advances in the use of MSC-EVs for tissue repair. We highlight several isolation and characterization approaches used to enrich MSC-derived EVs. We discuss our current understanding of the relative contribution of the MSC-EVs to the immunomodulatory and regenerative effects mediated by MSCs and MSC secretome. Finally we highlight the challenges and opportunities, which come with the potential use of MSC-EVs as cell free therapy for conditions that require tissue repair.

Keywords: extracellular vesicles; in vivo; isolation; mesenchymal stromal/stem cells; tissue repair.

Figure 1

Schematic representation of EV biogenesis, secretion and uptake. Exosomes (40-140 nm) are intraluminal vesicles (ILV) formed by the inward budding of endosomal membrane during maturation of multivesicular body (MVB), which are secreted upon fusion of the MVBs, with the plasma membrane. Microvesicles (50-1000 nm) comprise large and heterogeneous group of vesicles with different membranes depending on their origin and morphology. Apoptotic bodies are shedding vesicles derived from apoptotic cells. After the release into the extracellular space, EVs can bind to the cell surface receptors and can initiate intracellular signaling pathways. EVs can also be internalized through processes such as macropinocytosis, phogocytosis or can fuse with the plasma membrane and release their content in the intracellular space. The cargo consisting of proteins, RNA's and lipids are released in the intracellular space or taken up by the ensosomal system of the recipient cell.

Figure 2

Schematic classification of common methods of EVs isolation, characterization and quantification. (Left panel) MSCs can be isolated from various tissues such as umbilical cord, bone marrow, placenta or adipose tissue. MSCs are cultured in vitro and the conditioned medium is collected to enrich for EV. Middle panel depicts different strategies for EV isolation and different EV properties used as a base for the isolation protocols are indicated in colours. Right panel illustrates strategies such as electron microspopy, nanoparticle tracking (NTA), dynamic light scattering (DLS), flow cytometry or western blot, which are typically used for EV quantification or characterization.

Figure 3

Schematic representation of the components of MSC-derived EVs. The molecules present in MSC-EVs can be categorized into sixteen groups based on their molecular and cellular function. These are: -transcription factors, -extracellular matrix proteins, -chemokines, cytokines, -enzymes, -growth factors, RNA binding molecules, -miRNAs, -molecules involved in angionenesis, -cell adhesion, -development, -degradation, -protein folding, -immunomodulation, -regulation of apoptosis and survival, and -adipogenesis. In green are depicted MSC hallmark proteins. In red are depicted molecules, which role in the therapeutic effect of MSC-EVs was proven by knocking them down/out in MSCs. In blue are depicted molecules, which role in the therapeutic effect of MSC-EVs was proven by overexpressing them in MSCs. In violet are depicted molecules, which normally are not present in MSC-EVs, but upon overexpression they induce the therapeutic effect of MSC-EVs.