Epigenetic modifications regulate developmental genes involved in stem cell identity and lineage choice. NFI-A is a posttranscriptional microRNA-223 (miR-223) target directing human hematopoietic progenitor lineage decision: NFI-A induction or silencing boosts erythropoiesis or granulopoiesis, respectively. Here we show that NFI-A promoter silencing, which allows granulopoiesis, is guaranteed by epigenetic events, including the resolution of opposing chromatin “bivalent domains,” hypermethylation, recruitment of polycomb (PcG)–RNAi complexes, and miR-223 promoter targeting activity. During granulopoiesis, miR-223 localizes inside the nucleus and targets the NFI-A promoter region containing PcGs binding sites and miR-223 complementary DNA sequences, evolutionarily conserved in mammalians. Remarkably, both the integrity of the PcGs-RNAi complex and DNA sequences matching the seed region of miR-223 are required to induce NFI-A transcriptional silencing. Moreover, ectopic miR-223 expression in human myeloid progenitors causes heterochromatic repression of NFI-A gene and channels granulopoiesis, whereas its stable knockdown produces the opposite effects. Our findings indicate that, besides the regulation of translation of mRNA targets, endogenous miRs can affect gene expression at the transcriptional level, functioning in a critical interface between chromatin remodeling complexes and the genome to direct fate lineage determination of hematopoietic progenitors.

MicroRNAs (miRNAs) are small non-coding RNAs which contribute to the regulation of many physiological and pathological processes. Conventionally, miRNAs perform their activity in the cytoplasm where they regulate gene expression by interacting in a sequence-specific manner with mature messenger RNAs. Recent studies point to the presence of mature miRNAs in the nucleus. This review summarizes current findings regarding the molecular activities of nuclear miRNAs. These molecules can regulate gene expression at the transcriptional level by directly binding DNA on the promoter or the enhancer of regulated genes. miRNAs recruit different protein complexes to these regions, resulting in activation or repression of transcription, through a number of molecular mechanisms. Hematopoiesis is presented as a paradigmatic biological process whereby nuclear miRNAs possess a relevant regulatory role. Nuclear miRNAs can influence gene expression by affecting nuclear mRNA processing and by regulating pri-miRNA maturation, thus impacting the biogenesis of miRNAs themselves. Overall, nuclear miRNAs are biologically active molecules that can be critical for the fine tuning of gene expression and deserve further studies in a number of physiological and pathological conditions.

Retinoic acid controls hematopoietic differentiation through the transcription factor activity of its receptors. They act on specific target genes by recruiting protein complexes that deacetylate or acetylate histones and modify chromatin status. The regulation of this process is affected by histone methyltransferases, which can inhibit or activate transcription depending on their amino acid target. We show here that retinoic acid treatment of hematopoietic cells induces the expression of BTG2. Overexpression of this protein increases RARalpha transcriptional activity and the differentiation response to retinoic acid of myeloid leukemia cells and CD34+ hematopoietic progenitors. In the absence of retinoic acid, BTG2 is present in the RARalpha transcriptional complex, together with the arginine methyltransferase PRMT1 and Sin3A. Overexpressed BTG2 increases PRMT1 participation in the RARalpha protein complex on the RARbeta promoter, a target gene model, and enhances gene-specific histone H4 arginine methylation. Upon RA treatment Sin3A, BTG2, and PRMT1 detach from RARalpha and thereafter BGT2 and PRMT1 are driven to the cytoplasm. These events prime histone H4 demethylation and acetylation. Overall, our data show that BTG2 contributes to retinoic acid activity by favoring differentiation through a gene-specific modification of histone H4 arginine methylation and acetylation levels.