Mesenchymal Stem Cell-Derived Extracellular Vesicle Therapy for Stroke: Challenges and Progress

Abstract

Stroke is the leading cause of physical disability among adults. Stem cells such as mesenchymal stem cells (MSCs) secrete a variety of bioactive substances, including trophic factors and extracellular vesicles (EVs), into the injured brain, which may be associated with enhanced neurogenesis, angiogenesis, and neuroprotection. EVs are circular membrane fragments (30 nm-1 μm) that are shed from the cell surface and harbor proteins, microRNAs, etc. Since 2013 when it was first reported that intravenous application of MSC-derived EVs in a stroke rat model improved neurological outcomes and increased angiogenesis and neurogenesis, many preclinical studies have shown that stem cell-derived EVs can be used in stroke therapy, as an alternative approach to stem cell infusion. Although scientific research regarding MSC-derived EV therapeutics is still at an early stage, research is rapidly increasing and is demonstrating a promising approach for patients with severe stroke. MSC therapies have already been tested in preclinical studies and clinical trials, and EV-mediated therapy has unique advantages over cell therapies in stroke patients, in terms of biodistribution (overcoming the first pass effect and crossing the blood-brain-barrier), cell-free paradigm (avoidance of cell-related problems such as tumor formation and infarcts caused by vascular occlusion), whilst offering an off-the-shelf approach for acute ischemic stroke. Recently, advances have been made in the understanding of the function and biogenesis of EVs and EVs therapeutics for various diseases. This review presents the most recent advances in MSC-derived EV therapy for stroke, focusing on the application of this strategy for stroke patients.

Keywords: extracellular vesicles; ischemic stroke; mesenchymal stem cells; microRNA; stem cells; stroke.

Figure 1

Biogenesis of extracellular vesicles. EVs are released through two different pathways. When extracellular adenosine triphosphate (ATP) increases in response to external stimuli, the P2X7 receptor opens and calcium ions enter the cell. Membrane-associated proteins, tetraspanins, and cytoplasmic cargos are clustered in discrete membrane of the plasma membrane for microvesicles. The cargo of MVs are composed of cytoplasmic proteins, mRNAs, miRNAs, and DNAs. Similar to exosomes, RAS-related protein (RAB), actin, the endosomal sorting complex required for transport (ESCRT), ADP ribosylation factor 6 (ARF6) and phospholipase D2 (PLD2), and soluble N-ethylmaleimide-sensitive protein receptor (SNARE) proteins play important roles in MV release. However, MVs differ from exosomes in that they bud directly through flipping of lipid from the plasma membrane. The cargo of multivesicular bodies (MVBs) are either derived from endocytosis of the plasma membrane or from the trans-Golgi network. The reverse flow in the direction of the Golgi or recirculation to the plasma membrane is controlled by various Rab GTPases. Once MVB has matured, it is transported to the plasma membrane along the microtubule, and not by lysosomes. As a final step in exosome release, MVBs are docked and fused with the plasma membrane. Rab, actin, and SNARE proteins play important roles in these exosome release steps.