Mesenchymal stem cell derived extracellular vesicles enhance myelin plasticity in rhesus monkeys

Abstract

Cortical injury, such as injury from stroke, results in a cascade of events that includes cell death, inflammation and disruption of myelin. To date, there are no highly effective treatments for reducing the deficits that occur after injury. Recently, we have demonstrated that extracellular vesicles (EVs) harvested from rhesus monkey bone marrow derived cells when given 1 day and 14 days following injury facilitate recovery of function in aged rhesus monkeys within the first 3–5 weeks after cortical injury. Based on these findings and current proteomic literature of MSC-EVs, we hypothesized that MSC-EVs enhance myelin plasticity by limiting damage to oligodendrocytes and stimulating remyelination. To assess general myelin integrity after injury, we used Spectral Confocal Reflectance Microscopy (SCoRe) to image myelinated axons and found an increase in the density of myelinated axons in the EV group (p < 0.05). To assess whether the difference was due to reduced damage or remyelination, in sublesional white matter we assessed immunohistochemical labeling of Olig2, a general oligodendrocyte marker, and 8OHdG, a marker for DNA damage. We found reduced densities of Olig2 colocalized with 8OHdG in the EV group (p<0.05). As a marker of active demyelination and myelin debris clearance, we measured Myelin Basic Protein (MBP) concentrations in CSF and found a longitudinal reduction in the EV animals. To assess remyelination, we measured expression of MBP, a gene for myelination in mature oligodendrocytes, Myelin Regulatory Factor (MyRF), a gene for oligodendrocyte differentiation and maintenance, and Breast Carcinoma Amplified Sequence 1 (BCAS1), a gene for newly myelinating oligodendrocytes. Interestingly, we found a 4 fold increase in MyRF expression, and a 1.5 fold increase in MBP and BCAS1 in the EV animals relative to the vehicle control animals in perilesional brain tissue. Consistent with these gene expression differences associated with re-myelination, we found that the densities of newly-myelinating oligodendrocytes immune-labeled with BCAS1, as well as mature oligodendrocytes expressing CC1, exhibited a trend towards an increase in the EV group (p = 0.09). These results suggest that EV treatment reduces myelin damage, while also stimulating myelin repair. Finally, these results correlated with enhanced motor recovery, suggesting that EV-mediated white matter plasticity is a critical component for recovery after cortical injury.