MicroRNAs are shaping the hematopoietic landscape.

Hematopoiesis is regulated by microRNAs (miRNAs). These small regulatory RNAs are master regulators of developmental processes that modulate expression of several target genes post-transcriptionally. Various miRNAs are up-regulated at specific stages during hematopoietic development and the functional relevance of miRNAs has been proven at many different stages of lineage specification. Knockout of specific miRNAs can produce dramatic phenotypes leading to severe hematopoietic defects. Furthermore, several studies demonstrated that specific miRNAs are differentially expressed in hematopoietic stem cells. However, the emerging picture is extremely complex due to differences between species, cell type dependent variation in miRNA expression and differential expression of diverse target genes that are involved in various regulatory networks. There is also evidence that miRNAs play a role in cellular aging or in the inter-cellular crosstalk between hematopoietic cells and their microenvironment. The field is rapidly evolving due to new profiling tools and deep sequencing technology. The expression profiles of miRNAs are of diagnostic relevance for classification of different diseases. Recent reports on the generation of induced pluripotent stem cells with miRNAs have fuelled the hope that specific miRNAs and culture conditions facilitate directed differentiation or culture expansion of the hematopoietic stem cell pool. This review summarizes our current knowledge about miRNA expression in hematopoietic stem and progenitor cells, and their role in the hematopoietic stem cell niche.