Proteolytic Potential of the MSC Exosome Proteome: Implications for an Exosome-Mediated Delivery of Therapeutic Proteasome

TLDR

Mesenchymal stem cells are widely used in clinical trials, and their therapeutic effects are increasingly attributed to paracrine factor secretion. The study proposes that the therapeutic benefit arises from a synergistic combination of specific exosome components. Proteomic profiling of MSC exosomes identified the 20S proteasome as a candidate protein. Mass spectrometry confirmed the presence of a functional 20S proteasome in MSC exosomes, which correlated with reduced infarct size and oligomerized protein in mouse myocardial injury models, indicating its potential to synergize with other exosomal constituents to mitigate tissue damage.

Abstract

Mesenchymal stem cells (MSCs) are used in many of the current stem cell-based clinical trials and their therapeutic efficacy has increasingly been attributed to secretion of paracrine factors. We have previously demonstrated that a therapeutic constituent of this secretion is exosome, a secreted bilipid membrane vesicle of ~50-100 nm with a complex cargo that is readily internalized by H9C2 cardiomyocytes. It reduces infarct size in a mouse model of myocardial ischemia/reperfusion (MI/R) injury. We postulate that this therapeutic efficacy is derived from the synergy of a select permutation of individual exosome components. To identify protein candidates in this permutation, the proteome was profiled and here we identified 20S proteasome as a protein candidate. Mass spectrometry analysis detected all seven α and seven β chains of the 20S proteasome, and also the three beta subunits of "immunoproteasome" with a very high confidence level. We demonstrated that a functional proteasome copurified with MSC exosomes with a density of 1.10-1.18 g/mL, and its presence correlated with a modest but significant reduction in oligomerized protein in a mouse model of myocardial infarction. Circulating proteasomes in human blood also copurified with exosomes. Therefore, 20S proteasome is a candidate exosome protein that could synergize with other constituents to ameliorate tissue damage.