Evidence and potential in vivo functions for biofluid miRNAs: From expression profiling to functional testing

A controversial hypothesis in RNA biology is that extracellular microRNAs (miRNAs), including those in biofluids, have non‐cell‐autonomous activities. Several studies have characterized biofluid miRNA profiles in healthy or diseased individuals but generally have failed to identify distinct disease signatures. It remains unclear whether alterations in fluid miRNA levels are simply indicators of physiological change or whether miRNAs are taken up by new cells at concentrations sufficient to affect gene expression. There are limitations to biofluid miRNA studies performed to date: methodology for isolating and quantifying biofluid miRNAs is not standardized across studies; mechanistic details of miRNA release and uptake are incomplete; and efforts to assess non‐cell‐autonomous effects of extracellular miRNAs have employed predominantly in vitro strategies. We describe controversies and questions that need to be addressed to test possible in vivo roles of extracellular miRNAs and propose model organisms with rich genetic toolkits for carrying out in vivo functional analyses. Also watch the Video . The biological importance of biofluid miRNAs remains elusive. Mechanistic details of miRNA release and uptake are unclear, and evidence for miRNA functionality in cells that take them up has mostly come from in vitro studies. Standardized protocols and additional in vivo studies are needed to clarify roles of biofluid miRNAs.