MicroRNA in Platelet Production and Activation

Abstract SCI-39

Alterations in gene expression are at the heart of both megakaryocytopoiesis and interindividual variation in platelet reactivity. Over the past decade there has been an increasing awareness of the important role played by microRNAs (miRNAs) in these diverse cell biologic and physiologic processes. MiRNAs are noncoding RNAs that target complementary sequences in mRNAs, leading to mRNA degradation or inhibition of translation. More than 1000 miRNAs have been identified, which are estimated to regulate 30%-90% of all coding genes. Expression of miRNAs is cell and developmental stage specific. MiRNAs regulate hematopoietic lineage commitment, as well as B lymphopoiesis, granulopoiesis, erythropoiesis and monocytopoiesis. Recent data also demonstrate a clear role for miRNAs in megakaryocytopoiesis (Meg-poiesis). The molecular genetic basis for the transition from one stage of Meg-poiesis to another requires fine-tuning of the various control elements, and miRNAs act as “rheostats” in this process. MiRNA-150 has a strong effect on Meg-erythroid progenitor cells, inducing commitment to the Meg lineage at the expense of erythropoiesis. MiRNA-150 directly down-regulates MYB, a transcription factor important in thrombopoiesis. Over-expressed miRNA-146a knocks down CXCR4 (receptor for SDF-1), reducing Meg proliferation and maturation. MiR-146a gene expression is negatively regulated by the promyelocytic zinc finger protein, PLZF. Thus, miRNAs act as intermediaries of transcription factors that control Meg-poiesis. An increasing number of genetic diseases are being described due to mutations in miRNA genes or their mRNA targets. Deletion of miR-145 and miR-146a on 5q is sufficient to cause the 5q- phenotype; miR-125b-2 up-regulates Meg-poiesis and may be involved in megakaryoblastic leukemia. Platelets appear to be a particularly rich source of miRNAs, many of which are expressed at very high levels. Notably, platelets contain Dicer, TRBP2 and Ago2, and are capable of synthesizing miRNAs from pre-miRNAs. MiRNAs regulate Bcl-xL and Bak, raising the possibility that platelet miRNAs affect platelet lifespan. Because miRNAs are very stable, they also represent a potential biomarker, and we have found sets of miRNAs that appear to classify platelet reactivity to epinephrine. Using bioinformatic approaches, we have identified miRNA-mRNA pairs differentially expressed in platelets with differing phenotypes. This approach permits functional characterization of novel platelet mRNAs, and elucidation of a potential genetic mechanism for adjusting megakaryocyte/platelet mRNA expression. Thus, information extracted from these RNA networks continue to provide insights into systems biology of higher organisms.