Human bone marrow-and adiposemesenchymal stem cells secrete exosomes enriched in distinctive miRNA and tRNA species

Introduction: Administration of mesenchymal stem cells (MSCs) represents a promising treatment option fo patients suffering from immunological and degenerative disorders. Accumulating evidence indicates that the healin effects of MSCs are mainly related to unique paracrine properties, opening opportunities for secretome-based therapies Apart from soluble factors, MSCs release functional small RNAs via extracellular vesicles (EVs) that seem to conve essential features of MSCs. Here we set out to characterize the full small RNAome of MSC-produced exosomes Methods: We set up a protocol for isolating exosomes released by early passage adipose-(ASC) and bon marrow-MSCs (BMSC) and characterized them via electron microscopy, protein analysis and small RNA-sequencing We developed a bioinformatics pipeline to define the exosome-enclosed RNA species and performed the firs complete small RNA characterization of BMSCs and ASCs and their corresponding exosomes in biological replicates Results: Our analysis revealed that primary ASCs and BMSCs have highly similar small RNA expression profile dominated by miRNAs and snoRNAs (together 64-71 %), of which 150-200 miRNAs are present at physiological levels In contrast, the miRNA pool in MSC exosomes is only 2-5 % of the total small RNAome and is dominated by a mino subset of miRNAs. Nevertheless, the miRNAs in exosomes do not merely reflect the cellular content and a defined se of miRNAs are overrepresented in exosomes compared to the cell of origin. Moreover, multiple highly expresse miRNAs are precluded from exosomal sorting, consistent with the notion that these miRNAs are involved in functiona repression of RNA targets. While ASC and BMSC exosomes are similar in RNA class distribution and composition, w observed striking differences in the sorting of evolutionary conserved tRNA species that seems associated with th differentiation status of MSCs, as defined by Sox2, POU5F1A/B and Nanog expression Conclusions: We demonstrate that primary MSCs release small RNAs via exosomes, which are increasingly implicate in intercellular communications.TRNAs species, and in particular tRNA halves, are preferentially released and thei specific sorting into exosomes is related to MSC tissue origin and stemness. These findings may help to understan how MSCs impact neighboring or distant cells with possible consequences for their therapeutic usag.