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. 2016 Jul 25;26(14):1880-6.
doi: 10.1016/j.cub.2016.05.018. Epub 2016 Jun 30.

Visualizing and Quantifying Intracellular Behavior and Abundance of the Core Circadian Clock Protein PERIOD2

Free PMC article

Visualizing and Quantifying Intracellular Behavior and Abundance of the Core Circadian Clock Protein PERIOD2

Nicola J Smyllie et al. Curr Biol. .
Free PMC article


Transcriptional-translational feedback loops (TTFLs) are a conserved molecular motif of circadian clocks. The principal clock in mammals is the suprachiasmatic nucleus (SCN) of the hypothalamus. In SCN neurons, auto-regulatory feedback on core clock genes Period (Per) and Cryptochrome (Cry) following nuclear entry of their protein products is the basis of circadian oscillation [1, 2]. In Drosophila clock neurons, the movement of dPer into the nucleus is subject to a circadian gate that generates a delay in the TTFL, and this delay is thought to be critical for oscillation [3, 4]. Analysis of the Drosophila clock has strongly influenced models of the mammalian clock, and such models typically infer complex spatiotemporal, intracellular behaviors of mammalian clock proteins. There are, however, no direct measures of the intracellular behavior of endogenous circadian proteins to support this: dynamic analyses have been limited and often have no circadian dimension [5-7]. We therefore generated a knockin mouse expressing a fluorescent fusion of native PER2 protein (PER2::VENUS) for live imaging. PER2::VENUS recapitulates the circadian functions of wild-type PER2 and, importantly, the behavior of PER2::VENUS runs counter to the Drosophila model: it does not exhibit circadian gating of nuclear entry. Using fluorescent imaging of PER2::VENUS, we acquired the first measures of mobility, molecular concentration, and localization of an endogenous circadian protein in individual mammalian cells, and we showed how the mobility and nuclear translocation of PER2 are regulated by casein kinase. These results provide new qualitative and quantitative insights into the cellular mechanism of the mammalian circadian clock.


Figure 1
Figure 1
PER2::VENUS Fusion Protein Is a Competent Circadian Clock Protein Suitable for Real-Time Imaging (A) PER2::VENUS fluorescence across the mouse brain, at the peak time of SCN expression (ZT12). Inset shows a close up of the SCN. Scale bar, 1 mm. (B) Bright-field and fluorescence confocal images show Per2Venus lung fibroblasts. Scale bar, 20 μm. (C) Representative, double-plotted wheel-running actograms for Per2WT (left) and Per2Venus (right) animals. Mice were entrained on a 12:12 LD cycle, followed by a schedule of constant conditions (dim red light, represented by shaded gray). (D) Representative, de-trended Per1-luc bioluminescence rhythms of SCN slices from Per2WT (left) and Per2Venus (right) mice are shown. (E) Mean ± SEM circadian periods for wheel-running are shown (nWT = 6; nWT/V = 8; nV/V = 7). (F) Mean ± SEM circadian periods for SCN slices (nWT = 6; nWT/V = 7; nV/V = 9). One-way ANOVA revealed no significant effect for either measure. (G) Snapshots from confocal real-time imaging show PER2::VENUS fluorescence in representative Per2Venus (top panel), Per1 null (middle panel), and CK1εTau (lower panel) in SCN slices. (H) Snapshots from confocal real-time imaging show PER2::VENUS in fibroblasts. Scale bar, 20 μm. (I) Mean fluorescence measures from recordings in (G) are shown. (J) Mean fluorescence measures from recordings in (H) are shown. See also Figure S1, Table S1, and Movie S1.
Figure 2
Figure 2
Circadian Subcellular Localization of PER2::VENUS (A) Representative confocal images of SCN neurons from brain sections taken from Per2Venus animals across the LD cycle. PER2::VENUS -positive (green) localization was compared with cytoplasmic immunostaining for AVP (red) and nuclear staining of DAPI (blue). Scale bar, 20 μm. (B) Co-localization of PER2::VENUS and DAPI assessed by Mander’s coefficient analysis (M1, blue, co-localization of PER2::VENUS with DAPI; M2, yellow, co-localization of DAPI with PER2::VENUS). Note that M2 changes across the day because the overall level of PER2::VENUS changes; thus, the proportion of DAPI nuclei containing PER2::VENUS changes. (C) Per2Venus SCN slices were fixed, counterstained with DAPI (blue), and imaged at different time points during the rising phase (CT0–CT9) of the PER2 circadian cycle. Each image shows representative neurons found at that time point, rather than a representative field of view at that time point. See also Figure S2.
Figure 3
Figure 3
FCS of PER2::VENUS Protein in Fibroblasts (A) Schematic diagram illustrating the FCS procedure. PER2::VENUS fluorescence was monitored within a small confocal volume. Individual fluorescent molecules passed through the volume at a given rate. The fluorescence signal of all of the molecules in the volume was followed through time. The concentration and rate of movement of molecules were calculated by auto-correlating the fluorescence signal. (B) Fluorescence images and FCS-calibrated quantification of PER2::VENUS concentration in skin fibroblasts are shown. (C) Circadian variation of nuclear concentration of PER2::VENUS over time for eight representative cells. Images were collected every 6 min but every tenth image was analyzed; thus, there is a data point every 1.12 hr. (D) PER2::VENUS concentration in the nucleus at CT12 (4–6 hr after temperature synchronization) is shown (mean ± SEM). (E) FCS data fit to a two-component diffusion model, color coded by component (fast, light blue; slow, dark blue), are shown. See also Figures S3 and S4.
Figure 4
Figure 4
FRAP Reveals Role of CK1 in Regulating PER2 Mobility in SCN Neurons (A) A schematic diagram illustrating FRAP as follows: (1) select region and measure pre-bleach fluorescence, (2) photobleach region, and (3) monitor the recovery of fluorescence. The T1/2 is calculated from the recovery curve. (B) Snapshots from a FRAP experiment show cytoplasmic photobleaching and fluorescence recovery in neurons in an SCN slice. (C) Fluorescence recovery for a bleached cytoplasmic region (red) and unbleached nuclear region (blue). The latter shows no change in fluorescence over the time course. (D) FRAP-derived diffusion coefficients (mean ± SEM) within SCN nucleus and cytoplasm were comparable at both peak and trough (CT2 and CT12) of the cycle (nCT2,cyto = 3 slices, nCT12,cyto = 4, nCT2,nuc = 3, and nCT12,nuc = 4; two-way ANOVA). Diffusion coefficients were estimated from FRAP-derived T1/2 measures. (E) FRAP-derived T1/2 (mean ± SEM) calculated from total cytoplasm (Nuc to Cyto) and total nucleus (Cyto to Nuc) FRAP of SCN neurons at CT2 (left) and CT12 (right). Treatment with PF670462 (1 μM) significantly decreased T1/2 times for PER2::VENUS (nCT2,WT,cyto = 4 slices, nCT2,WT,nuc = 4, nCT2,PF,cyto = 5, nCT2,PF,nuc = 5, nCT12,WT,cyto = 5, nCT12,WT,nuc = 6, nCT12,PF,cyto = 13, and nCT12,PF,nuc = 12; two-way ANOVA with Tukey’s comparison, ∗∗∗∗p < 0.0001). See also Figure S4 and Movie S2.

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    1. Shearman L.P., Sriram S., Weaver D.R., Maywood E.S., Chaves I., Zheng B., Kume K., Lee C.C., van der Horst G.T.J., Hastings M.H., Reppert S.M. Interacting molecular loops in the mammalian circadian clock. Science. 2000;288:1013–1019. - PubMed
    1. Koike N., Yoo S.H., Huang H.C., Kumar V., Lee C., Kim T.K., Takahashi J.S. Transcriptional architecture and chromatin landscape of the core circadian clock in mammals. Science. 2012;338:349–354. - PMC - PubMed
    1. Meyer P., Saez L., Young M.W. PER-TIM interactions in living Drosophila cells: an interval timer for the circadian clock. Science. 2006;311:226–229. - PubMed
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