MicroRNAs as novel players in skin development, homeostasis and disease.

MicroRNAs (miRNAs) are a group of newly discovered small (∼19-24 nucleotides), noncoding RNAs that modulate gene expression by interacting with the 3' untranslated region of the corresponding target gene messenger RNA (mRNA). miRNAs have been estimated to regulate more than one-third of protein-encoding mRNAs. As a consequence, cellular protein expression and a large number of biological processes are influenced by miRNA-mediated post-transcriptional regulation of gene expression. The severe phenotype of mice lacking key enzymes of the miRNA biogenesis pathway (Dgcr8 and Dicer) in the skin confirmed the essential function of miRNAs in this tissue. In addition, a growing number of reports has identified miRNAs as regulators of the morphogenesis and homeostasis of the skin and its appendages, and miRNA deregulation was shown to be associated or even causally related to several skin diseases. Profiling studies have identified numerous differentially regulated miRNAs associated with physiological (e.g. keratinocyte differentiation) and pathological (e.g. psoriasis, melanoma) processes. These data bear enormous potential for further studies. Because of the easy accessibility of the skin, it is plausible to anticipate that, once efficient and safe methods for the topical delivery of substances mimicking or modulating miRNA activity become available, skin diseases will be among the first to be approached with miRNA-based therapies. This review article gives a short introduction to miRNA biology and summarizes and discusses existing evidence for a role of these molecules in the skin.