Editors' ChoiceCircadian Rhythms

Buying Time with miRNAs

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Sci. Signal.  29 Oct 2013:
Vol. 6, Issue 299, pp. ec259
DOI: 10.1126/scisignal.2004844

Circadian clocks regulate daily patterns of behavior, metabolism, and immune system function. At the core of the clock are transcriptional and translational feedback loops that generate periodic, oscillating changes in the abundance of clock components. One of these loops involves the action of the transcription factors CLOCK and BMAL1, heterodimers of which stimulate the expression of genes encoding the transcription factors Period 1 and Period 2 (PER1 and PER2). PER1 and PER2 accumulate in the cytoplasm, translocate into the nucleus, and inhibit transcription of Clock and Bmal1. The timing of transcription and translation of clock components is important for determining the clock's periodicity, and Chen et al. report that microRNAs (miRNAs) play a key role in this timing mechanism. Because the circadian clock is essential for early development, the authors used mice harboring a conditional knockout of Dicer, which encodes a critical component of the miRNA processing machinery, to assess the involvement of miRNAs in adult mice. Removing Dicer function in adult mice caused the period of the animals' daily behavior patterns to gradually shorten and resulted in death within a few weeks. In cultured mouse embryonic fibroblasts (MEFs), removal of Dicer function shortened the period but had little effect on the magnitude of oscillations of a fluorescent clock reporter. PER1 and PER2 proteins accumulated faster in the cytoplasm, and Per2 mRNA was somewhat increased, in Dicer mutant MEFs compared to control cells, suggesting increased translation of PER1 and PER2 and, to a lesser extent, increased half-life of Per2 transcripts, in the mutant cells. Overexpression of the 3′-untranslated regions (UTRs) of Per1 and Per2 in MEFs also reduced circadian periodicity, presumably by competing with endogenous Per1 and Per2 transcripts for binding to miRNAs. Based on sequence analysis, miRNAs miR-24, miR-29a, and miR-30a were predicted to target the 3′ UTRS of Per1 and Per2, and experiments with mutated Per1 and Per2 3′ UTRs coupled to fluorescent reporters were consistent with this prediction. Simultaneously knocking down these three miRNAs in MEFs shortened the clock period similarly to Dicer mutation. These findings imply that miRNA-mediated inhibition of translation of PER1 and PER2 may slow cytoplasmic accumulation of these proteins, thereby accounting for the observed delay between the accumulation of Per transcripts and PER proteins.

R. Chen, M. D’Alessandro, C. Lee, miRNAs are required for generating a time delay critical for the circadian oscillator. Curr. Biol. 23, 1959–1968 (2013). [PubMed]