Regulation of Progenitor Cell Proliferation and Granulocyte Function by microRNA-223.

MicroRNAs are abundant in animal genomes and have been predicted to have important roles in a broad range of gene expression programs. Despite this prominence, there is a dearth of functional knowledge regarding individual mammalian microRNAs. Using a loss-of-function allele in mice, we report here that the myeloid-specific microRNA-223 (miR-223) negatively regulates progenitor proliferation and granulocyte differentiation and activation. miR-223 mutant mice have an expanded granulocytic compartment resulting from a specific and cell-autonomous increase in the number of granulocyte-monocyte progenitors (GMPs). We show that Mef2c, a transcription factor that promotes GMP renewal, is a target of miR-223 and that genetic ablation of Mef2c suppresses progenitor expansion and corrects the neutrophilic phenotype in miR-223 null mice. In addition, granulocytes lacking miR-223 are hypermature, hypersensitive to activating stimuli and display increased fungicidal activity. Consequent to this neutrophil hyperactivity, miR-223 mutant mice spontaneously develop inflammatory lung pathology and exhibit exaggerated tissue destruction following endotoxin challenge. Our results present a new paradigm in miRNA-mediated gene regulation, in which a microRNA can negatively modulate the proliferation and differentiation program of the lineage to which it is most strongly associated.