Combinatorial effect of miR-486-5p & miR-22-3p mimics thrombopoietin's impact on hematopoietic stem & progenitor cells

Megakaryocytes shed and release submicron size microparticles (MkMPs), the most abundant microparticle in circulation. We have previously reported that MkMPs target peripheral-blood CD34+ hematopoietic stem/progenitor cells (HSPCs) to induce megakaryocytic differentiation and proliferation, and that small RNAs delivered to HSPCs via MkMPs play an important role in the development of this phenotype. Here, using single-molecule real-time (SMRT) RNA sequencing (RNAseq), we identify the top seven most abundant microRNAs (miRs) in MkMPs as potential candidates in mediating the effect of MkMPs on HSPCs. Using miR mimics, we demonstrate that among the seven most abundant miRs, two, miR-486-5p and miR-22-3p, are able to drive the Mk differentiation of HSPCs in the absence of thrombopoietin (TPO). The effect of these two miRs is comparable to the TPO- or MkMP-induced megakaryocytic differentiation of HSPCs, thus suggesting that these two miRs are responsible for this MkMP-induced phenotype. To probe the signaling through which MkMPs might enable this phenotype, we used kinase inhibitors of potential signaling pathways engaged in megakaryocytic differentiation. Our data suggest that MkMP-induced Mk differentiation of HSPCs is enabled through JNK and PI3K/Akt/mTOR signaling. Our data show that MkMPs activate Akt and mTOR phosphorylation. Furthermore, MkMPs downregulate PTEN expression, a direct target of miR-486-5p and a negative regulator of PI3K/Akt signaling, via JNK signaling. Taken together, our data provide a mechanistic understanding of the biological effect of MkMPs in inducing megakaryocytic differentiation of HSPCs, which, as was previously suggested, is a phenotype of potential physiological significance in stress megakaryopoiesis.