MicroRNAs (miRNAs) are key regulators of the transcriptome and thereby as potential disease biomarkers as well as putative new drug targets. Following their discovery some 2,000 distinct miRNAs have now been identified in the humane genome and evidence continues to accumulate on the central role that miRNAs play in regulating cellular pathways in health and disease. The majority of present knowledge regarding the roles of miRNAs in human physiology and pathology concerns effects of intracellular miRNAs on mRNA stability and translation efficacy—and hence protein levels—by the same cells where they are transcribed from DNA and processed from pre-miRNAs.
In recent years, however, a new role for miRNA has emerged, suggesting that these short noncoding RNAs can be secreted by cells to their surrounding extracellular fluids, packed inside exosomes. The latter can be transported by blood or lymph and taken-up by other cells, including other tissues, where their miRNA cargo can affect the cellular transcriptome in essence functioning as endocrine modulators. In this scenario, miRNAs may mediate tissue crosstalk, somewhat resembling the role of hormones in tissue communication. Indeed, both hormones and miRNAs are unique to multicellular organisms, and are present in both animals and plants, attesting their fundamental roles in cellular function.