microRNA-25 as a novel modulator of circadianPeriod2gene oscillation

Circadian rhythm: microRNAs keep the clock in order A newly identified microRNA plays a key role in fine-tuning the genetic interactions governing the circadian rhythms in mammals, according to researchers in South Korea. Numerous studies have suggested that thePeriodgenes, which negatively regulate theCLOCKandBMAL1genes to produce a 24-hour feedback loop, may be further modified by microRNAs after they are transcribed. Kyungjin Kim at Daegu Gyeongbuk Institute of Science and Technology, South Korea, and co-workers confirmed that a novel microRNA, miR-25-3p, reduces the expression of aPeriodgene,Per2, in mice. When miR-25-3p is over-expressed, it dampens and shortens the oscillations ofPer2levels. Interestingly, the researchers showed that natural miR-25-3p expression levels varied across different parts of the brain, supporting the theory that different tissues of the body maintain their own unique circadian cycles. Circadian clock controls an organism's biological rhythm and regulates its physiological processes in response to external time cues. Most living organisms have their own time-keeping mechanism that is maintained by transcriptional-translational autoregulatory feedback loops involving several core clock genes, such asPeriod. Recent studies have found the relevance between the modulation of circadian oscillation and posttranscriptional modifications by microRNAs (miRNAs). However, there are limited studies on candidate miRNAs that regulate circadian oscillation. Here, we characterize the functions of novel miRNA-25 regulating circadianPeriod2(Per2) expression. Using several in silico algorithms, we identified novel miR-25-3p that, together with miR-24-3p, targets thePer2gene. Luciferase reporter assays validated that miR-25-3p and miR-24-3p repressedPer2expression and confirmed their predicted binding sites in the 3 '-untranslated region (UTR) ofPer2mRNA. Real-time bioluminescence analyses using Per2::Luc mouse embryonic fibroblasts confirmed that PER2 protein oscillation patterns were responsive to miR-25-3p and miR-24-3. The overexpression of miR-25-3p or miR-24-3p resulted in the dampening and period shortening of the PER2::LUC oscillation, while inhibition of either miRNA increased the relative amplitude of the PER2::LUC oscillation. Notably, endogenous miR-25-3p expression in the suprachiasmatic nucleus (SCN) showed no circadian rhythmicity, but the expression levels differed in various brain regions and peripheral tissues. These results suggest that the posttranscriptional regulation of miR-25-3p and miR-24-3p may differ according toPer2gene expression in different tissue regions. In summary, we found that novel miR-25-3p was involved in fine-tuning circadian rhythmicity by regulatingPer2oscillation at the posttranscriptional level and that it functioned synergistically with miR-24-3p to affectPer2oscillation.