MicroRNA (miRNA) are small (~22nt) non-coding RNAs that are involved in post-transcriptional regulation of gene expression. miRNAs bind to complementary sequences in the 3’ UTR of target mRNAs, resulting in either degradation or translation repression. Over one thousand unique miRNAs have been identified in a variety of tissues and species, making this class of regulator one of the most abundant in cells. Given their abundance and versatility, it is not surprising that miRNAs play critical roles during development and homeostasis, as well as in disease states.
The biogenesis of miRNAs begins with transcription by RNA polymerase II or III to generate a long primary miRNA (pri-miRNA). The pri-miRNA is then cleaved by Drosha, a ribonuclease III enzyme, within the nucleus to release a ~70nt stem-loop precursor called pre-miRNA. This pre-miRNA is exported to the cytoplasm via Exportin 5 where it is further processed by Dicer, another ribonuclease III enzyme, to yield the mature miRNA. Mature miRNAs exist predominantly as part of RISC complexes (either MiRNP1 or MiRNP2), which contain Argonaute proteins. These complexes mediate target mRNA binding and cleavage/repression depending on perfect or imperfect base pairing between the seed region of the mature miRNA (nucleotides 2-8 from the 5’ end) and its target mRNA 3’UTR .
Depending on sequence complementarity, binding can lead to either translational repression or mRNA degradation: perfect complementarity results inseed destabilizationand rapid turnover; while nearperfect/imperfectcomplementarity promotes slowedturnover with gradualdecreases intargetm RNA levels over time through deadenylationand decay mechanisms. In general however, most targets tendtoundergo at least some formof translational suppression(usually 20–40%). Although many details regarding RISC loading onto specific mRNAs remain unclear due largelyto stochastic effects inherentto such low abundance molecules(~30 copies per cell), certain assembly factorshave beendemonstratedtocontribute including Ago2for mammalian cellsor VIGfor flycells. In additionto these corecomponents howeverothers suchas piwi familyproteins may also playa roleparticularlyin germline developmentwhere they areresponsible for protectingpiwi -interacting RNAs(piRNAs). Once loadedonto their respective RISCs these smallregulatory RNPsscan their environment for potentialtargets through bothsequence -specificand structure -specific interactionswith partialbase pairing beingfavored dueto thermalstability considerationsat 37°C .
It should benoted that off -target effectsare always apossibilitywith any sg RNAbased systembut given therather short lengthof most micro RNAsthey would beexpectedto bediscrete eventslikely limited tomismatches localizednearthe seedregionif they occur at all . Regardlessof mechanismhoweverit remains critically importantto performthorough targetpredictionanalysesbefore embarkingon any large scaleexperimentation utilizingthese powerfulmoleculeslest unanticipatedconsequencesarise .
In mammals three different classesof nuclear hormone receptorshave beendescribed each capableof directbindingand functionalinteractionwith micro RN As: type IARNRswhich directly recruitmicroprocessorcomplexescontainingDGCR8;type IIARNRswithinthe cytoplasmthat function cooperativelywith CNOT4L topromote zygotictranscription silencingduring embryogenesis ;and type IB NRswhose precise modeof actionremains lessclear but appearsto require interactionwith membersof the exportin5 complex possiblyfor efficientcytoplasmictransport outofthe nucleus . Interestinglyenoughseveral recentstudies have implicatedlet -7 miro RNAin negativefeedbackregulation oftissue growthby targetingmultiple key componentswithin varioussignalingpathways importantfor cell cyclecontrol tumorigenesisand embryonicdevelopmentmaking itoneoffew exampleswhere amicro RNAis known totarget numerousgenes simultaneously acrossmultiple pathways .