Mesenchymal stem cell secretes microparticles enriched in pre-microRNAs

Intercellular exchange of protein and RNA-containing microparticles is an increasingly important mode of cell-cell communication. Here we investigate if mesenchymal stem cells (MSCs) known for secreting therapeutic paracrine factors also secrete RNA-containing microparticles. We observed that human embryonic stem cell (hESC)-derived MSC conditioned medium contained small RNAs (less than 300 nt) encapsulated in cholesterol-rich phospholipid vesicles as evidenced by their RNase sensitivity only in the presence of a sodium dodecyl sulfate-based cell lysis buffer, phospholipase A2 and a chelator of cholesterol, cyclodextrin and the restoration of their lower than expected density by detergent or phospholipase A2 treatment. MicroRNAs (miRNAs) such as hsa-let-7b and hsa-let-7g were present in a high precursor (pre)- to mature miRNA ratio by microarray analysis and quantitative reverse transcription-polymerase chain reaction. The pre-miRNAs were cleaved to mature miRNA by RNase III in vitro. High performance liquid chromatography-purified RNA-containing vesicles have a hydrodynamic radius of 55-65 nm and were readily taken up by H9C2 cardiomyocytes. This study suggests that MSCs could facilitate miRNA-mediated intercellular communication by secreting microparticles enriched for pre-miRNA.