microRNAs (miRNAs) are small, noncoding, single-stranded RNAs that control diverse key cellular pathways at the posttranscriptional level. Their mode of action is translational repression or degradation of target mRNAs containing complementary sequences. As many miRNAs act in crucial cellular pathways, their dysregulation can result in various diseases including cancer. Here, we summarize recent insights into the complex processing pathway generating the mature, functional miRNA. Cleavage of the primary miRNA transcripts (pri-miRNAs) by the microprocessor complex Drosha-DGCR8/Pasha releases 60-70 nt hairpin structures, the pre-miRNAs. After export to the cytoplasm mediated by Exportin-5, the RNase III-like Dicer completes processing in conjunction with its dsRBD partner protein (TRBP in mammals and LOQS in flies). The small RNA duplexes are unwound, and one strand, the guide strand, is incorporated together with Argonaute proteins into the RNA-induced silencing complex (RISC). Multiple studies in recent years have revealed that every step of this processing pathway can be regulated and that certain miRNAs do not follow this general processing pathway but use a variety of other processing and regulatory options for their maturation. Importantly, the miRNA processing and effector proteins also provide the essential machinery for RNA interference (RNAi). While ectopically delivered RNA (like dsRNA, siRNA, or shRNA) functions as specificity component to knockdown target genes, the processing and effector machinery has to be contributed by the targeted cell. Also, several miRNA processing factors can be used to enhance RNAi. Thus, a deeper understanding of miRNA processing, regulation, and function is an essential prerequisite to optimize experimental RNAi and enable therapeutic RNAi approaches.
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