Unconventional functions of miRNAs

The existence of extracellular miRNAs (ex-miRNAs) was first described in plasma and soon demonstrated in virtually all biological fluids. Ex-miRNAs are remarkably stable in harsh conditions, including high nuclease concentrations and extreme pH variations. This largely depends on their association with proteins protecting them from degradation, such as those of the AGO family, and/or on their loading into extracellular vesicles (EV-miRNAs) and apoptotic bodies. Ex-miRNAs can be released upon cell damage or death, so possibly representing remnants devoid of any particular function. However, the fact that cells can also operate specific sorting of secreted miRNAs, suggested that ex-miRNAs may work as soluble messengers regulating cell-to-cell communications. Indeed, ample experimental evidence demonstrates that EV-miRNAs penetrate into target cells, where they exert posttranscriptional regulation of specific target messenger RNAs. In addition, we and others have proposed another regulatory function of EV-miRNAs, namely, the triggering of innate immune receptors. In this scenario, ex-miRNAs would regulate the responses of neighboring cells by inducing cytokine secretion which, in turn, would trigger "sterile" inflammation and immune activation. The main focus of this chapter is to convey a comprehensive view of ex-miRNAs as ligands of innate immune receptors. Specific paragraphs will cover the biology of the main innate RNA sensors, ex-miRNA biogenesis, uptake by recipient cells and delivery to innate receptors, as well as experimental evidence of ex-miRNAs regulating the pathogenesis of inflammation-dependent diseases via stimulation of RNA sensors. In addition, the chapter will also provide a brief outline of noncanonical subcellular locations and of other unconventional functions attributed to miRNAs, both suggesting that we have only begun to touch upon the complexity and versatility of the regulatory functions exerted by these short noncoding RNAs.