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The suprachiasmatic nucleus (SCN) of the hypothalamus is the principal clock driving circadian rhythms of physiology and behavior that adapt mammals to environmental cycles. Disruption of SCN-dependent rhythms compromises health, and so understanding SCN time keeping will inform management of diseases associated with modern lifestyles. SCN time keeping is a self-sustaining transcriptional/translational delayed feedback loop (TTFL), whereby negative regulators inhibit their own transcription. Formally, the SCN clock is viewed as a limit-cycle oscillator, the simplest being a trajectory of successive phases that progresses through two-dimensional space defined by two state variables mapped along their respective axes. The TTFL motif is readily compatible with limit-cycle models, and in Neurospora and Drosophila the negative regulators Frequency (FRQ) and Period (Per) have been identified as state variables of their respective TTFLs. The identity of state variables of the SCN oscillator is, however, less clear. Experimental identification of state variables requires reversible and temporally specific control over their abundance. Translational switching (ts) provides this, the expression of a protein of interest relying on the provision of a noncanonical amino acid. We show that the negative regulator Cryptochrome 1 (CRY1) fulfills criteria defining a state variable: ts-CRY1 dose-dependently and reversibly suppresses the baseline, amplitude, and period of SCN rhythms, and its acute withdrawal releases the TTFL to oscillate from a defined phase. Its effect also depends on its temporal pattern of expression, although constitutive ts-CRY1 sustained (albeit less stable) oscillations. We conclude that CRY1 has properties of a state variable, but may operate among several state variables within a multidimensional limit cycle.

The ∼20,000 cells of the suprachiasmatic nucleus (SCN), the master circadian clock of the mammalian brain, coordinate subordinate cellular clocks across the organism, driving adaptive daily rhythms of physiology and behavior. The canonical model for SCN timekeeping pivots around transcriptional/translational feedback loops (TTFL) whereby PERIOD (PER) and CRYPTOCHROME (CRY) clock proteins associate and translocate to the nucleus to inhibit their own expression. The fundamental individual and interactive behaviors of PER and CRY in the SCN cellular environment and the mechanisms that regulate them are poorly understood. We therefore used confocal imaging to explore the behavior of endogenous PER2 in the SCN of PER2::Venus reporter mice, transduced with viral vectors expressing various forms of CRY1 and CRY2. In contrast to nuclear localization in wild-type SCN, in the absence of CRY proteins, PER2 was predominantly cytoplasmic and more mobile, as measured by fluorescence recovery after photobleaching. Virally expressed CRY1 or CRY2 relocalized PER2 to the nucleus, initiated SCN circadian rhythms, and determined their period. We used translational switching to control CRY1 cellular abundance and found that low levels of CRY1 resulted in minimal relocalization of PER2, but yet, remarkably, were sufficient to initiate and maintain circadian rhythmicity. Importantly, the C-terminal tail was necessary for CRY1 to localize PER2 to the nucleus and to initiate SCN rhythms. In CRY1-null SCN, CRY1Δtail opposed PER2 nuclear localization and correspondingly shortened SCN period. Through manipulation of CRY proteins, we have obtained insights into the spatiotemporal behaviors of PER and CRY sitting at the heart of the TTFL molecular mechanism., Competing Interests: The authors declare no competing interest., (Copyright © 2022 the Author(s). Published by PNAS.)

The timing and quality of sleep-wake cycles are regulated by interacting circadian and homeostatic mechanisms. Although the suprachiasmatic nucleus (SCN) is the principal clock, circadian clocks are active across the brain and the respective sleep-regulatory roles of SCN and local clocks are unclear. To determine the specific contribution(s) of the SCN, we used virally mediated genetic complementation, expressing Cryptochrome1 (Cry1) to establish circadian molecular competence in the suprachiasmatic hypothalamus of globally clockless, arrhythmic male Cry1/Cry2 -null mice. Under free-running conditions, the rest/activity behavior of Cry1/Cry2 -null controls expressing EGFP (SCN Con ) was arrhythmic, whereas Cry1-complemented mice (SCN Cry1 ) had coherent circadian behavior, comparable to that of Cry1,2-competent wild types (WTs). In SCN Con mice, sleep-wakefulness, assessed by electroencephalography (EEG)/electromyography (EMG), lacked circadian organization. In SCN Cry1 mice, however, it matched WTs, with consolidated vigilance states [wake, rapid eye movement sleep (REMS) and non-REMS (NREMS)] and rhythms in NREMS δ power and expression of REMS within total sleep (TS). Wakefulness in SCN Con mice was more fragmented than in WTs, with more wake-NREMS-wake transitions. This disruption was reversed in SCN Cry1 mice. Following sleep deprivation (SD), all mice showed a homeostatic increase in NREMS δ power, although the SCN Con mice had reduced NREMS during the inactive (light) phase of recovery. In contrast, the dynamics of homeostatic responses in the SCN Cry1 mice were comparable to WTs. Finally, SCN Con mice exhibited poor sleep-dependent memory but this was corrected in SCN Cry1 mice. In clockless mice, circadian molecular competence focused solely on the SCN rescued the architecture and consolidation of sleep-wake and sleep-dependent memory, highlighting its dominant role in timing sleep. SIGNIFICANCE STATEMENT The circadian timing system regulates sleep-wake cycles. The hypothalamic suprachiasmatic nucleus (SCN) is the principal circadian clock, but the presence of multiple local brain and peripheral clocks mean the respective roles of SCN and other clocks in regulating sleep are unclear. We therefore used virally mediated genetic complementation to restore molecular circadian functions in the suprachiasmatic hypothalamus, focusing on the SCN, in otherwise genetically clockless, arrhythmic mice. This initiated circadian activity-rest cycles, and circadian sleep-wake cycles, circadian patterning to the intensity of non-rapid eye movement sleep (NREMS) and circadian control of REMS as a proportion of total sleep (TS). Consolidation of sleep-wake established normal dynamics of sleep homeostasis and enhanced sleep-dependent memory. Thus, the suprachiasmatic hypothalamus, alone, can direct circadian regulation of sleep-wake., (Copyright © 2021 Maywood et al.)

Circadian rhythms are a pervasive property of mammalian cells, tissues and behaviour, ensuring physiological adaptation to solar time. Models of cellular timekeeping revolve around transcriptional feedback repression, whereby CLOCK and BMAL1 activate the expression of PERIOD (PER) and CRYPTOCHROME (CRY), which in turn repress CLOCK/BMAL1 activity. CRY proteins are therefore considered essential components of the cellular clock mechanism, supported by behavioural arrhythmicity of CRY-deficient (CKO) mice under constant conditions. Challenging this interpretation, we find locomotor rhythms in adult CKO mice under specific environmental conditions and circadian rhythms in cellular PER2 levels when CRY is absent. CRY-less oscillations are variable in their expression and have shorter periods than wild-type controls. Importantly, we find classic circadian hallmarks such as temperature compensation and period determination by CK1δ/ε activity to be maintained. In the absence of CRY-mediated feedback repression and rhythmic Per2 transcription, PER2 protein rhythms are sustained for several cycles, accompanied by circadian variation in protein stability. We suggest that, whereas circadian transcriptional feedback imparts robustness and functionality onto biological clocks, the core timekeeping mechanism is post-translational., (© 2021 MRC Laboratory of Molecular Biology. Published under the terms of the CC BY 4.0 license.)

The suprachiasmatic nucleus (SCN) is the principal circadian clock of mammals, coordinating daily rhythms of physiology and behavior. Circadian timing pivots around self-sustaining transcriptional-translational negative feedback loops (TTFLs), whereby CLOCK and BMAL1 drive the expression of the negative regulators Period and Cryptochrome (Cry). Global deletion of Cry1 and Cry2 disables the TTFL, resulting in arrhythmicity in downstream behaviors. We used this highly tractable biology to further develop genetic code expansion (GCE) as a translational switch to achieve reversible control of a biologically relevant protein, Cry1, in the SCN. This employed an orthogonal aminoacyl-tRNA synthetase/tRNA CUA pair delivered to the SCN by adeno-associated virus (AAV) vectors, allowing incorporation of a noncanonical amino acid (ncAA) into AAV-encoded Cry1 protein carrying an ectopic amber stop codon. Thus, translational readthrough and Cry1 expression were conditional on the supply of ncAA via culture medium or drinking water and were restricted to neurons by synapsin-dependent expression of aminoacyl tRNA-synthetase. Activation of Cry1 translation by ncAA in neurons of arrhythmic Cry-null SCN slices immediately and dose-dependently initiated TTFL circadian rhythms, which dissipated rapidly after ncAA withdrawal. Moreover, genetic activation of the TTFL in SCN neurons rapidly and reversibly initiated circadian behavior in otherwise arrhythmic Cry-null mice, with rhythm amplitude being determined by the number of transduced SCN neurons. Thus, Cry1 does not specify the development of circadian circuitry and competence but is essential for its labile and rapidly reversible activation. This demonstrates reversible control of mammalian behavior using GCE-based translational switching, a method of potentially broad neurobiological interest., Competing Interests: The authors declare no conflict of interest., (Copyright © 2018 the Author(s). Published by PNAS.)

Circadian rhythms in mammals are coordinated by the suprachiasmatic nucleus (SCN). SCN neurons define circadian time using transcriptional/posttranslational feedback loops (TTFL) in which expression of Cryptochrome (Cry) and Period (Per) genes is inhibited by their protein products. Loss of Cry1 and Cry2 stops the SCN clock, whereas individual deletions accelerate and decelerate it, respectively. At the circuit level, neuronal interactions synchronize cellular TTFLs, creating a spatiotemporal wave of gene expression across the SCN that is lost in Cry1/2-deficient SCN. To interrogate the properties of CRY proteins required for circadian function, we expressed CRY in SCN of Cry-deficient mice using adeno-associated virus (AAV). Expression of CRY1::EGFP or CRY2::EGFP under a minimal Cry1 promoter was circadian and rapidly induced PER2-dependent bioluminescence rhythms in previously arrhythmic Cry1/2-deficient SCN, with periods appropriate to each isoform. CRY1::EGFP appropriately lengthened the behavioral period in Cry1-deficient mice. Thus, determination of specific circadian periods reflects properties of the respective proteins, independently of their phase of expression. Phase of CRY1::EGFP expression was critical, however, because constitutive or phase-delayed promoters failed to sustain coherent rhythms. At the circuit level, CRY1::EGFP induced the spatiotemporal wave of PER2 expression in Cry1/2-deficient SCN. This was dependent on the neuropeptide arginine vasopressin (AVP) because it was prevented by pharmacological blockade of AVP receptors. Thus, our genetic complementation assay reveals acute, protein-specific induction of cell-autonomous and network-level circadian rhythmicity in SCN never previously exposed to CRY. Specifically, Cry expression must be circadian and appropriately phased to support rhythms, and AVP receptor signaling is required to impose circuit-level circadian function.

The suprachiasmatic nucleus (SCN) coordinates circadian rhythms that adapt the individual to solar time. SCN pacemaking revolves around feedback loops in which expression of Period (Per) and Cryptochrome (Cry) genes is periodically suppressed by their protein products. Specifically, PER/CRY complexes act at E-box sequences in Per and Cry to inhibit their transactivation by CLOCK/BMAL1 heterodimers. To function effectively, these closed intracellular loops need to be synchronized between SCN cells and to the light/dark cycle. For Per expression, this is mediated by neuropeptidergic and glutamatergic extracellular cues acting via cAMP/calcium-responsive elements (CREs) in Per genes. Cry genes, however, carry no CREs, and how CRY-dependent SCN pacemaking is synchronized remains unclear. Furthermore, whereas reporter lines are available to explore Per circadian expression in real time, no Cry equivalent exists. We therefore created a mouse, B6.Cg-Tg(Cry1-luc)01Ld, carrying a transgene (mCry1-luc) consisting of mCry1 elements containing an E-box and E'-box driving firefly luciferase. mCry1-luc organotypic SCN slices exhibited stable circadian bioluminescence rhythms with appropriate phase, period, profile, and spatial organization. In SCN lacking vasoactive intestinal peptide or its receptor, mCry1 expression was damped and desynchronized between cells. Despite the absence of CREs, mCry1-luc expression was nevertheless (indirectly) sensitive to manipulation of cAMP-dependent signaling. In mPer1/2-null SCN, mCry1-luc bioluminescence was arrhythmic and no longer suppressed by elevation of cAMP. Finally, an SCN graft procedure showed that PER-independent as well as PER-dependent mechanisms could sustain circadian expression of mCry1. The mCry1-luc mouse therefore reports circadian mCry1 expression and its interactions with vasoactive intestinal peptide, cAMP, and PER at the heart of the SCN pacemaker.

The circadian clock of the suprachiasmatic nucleus (SCN) drives daily rhythms of behavior. Cryptochromes (CRYs) are powerful transcriptional repressors within the molecular negative feedback loops at the heart of the SCN clockwork, where they periodically suppress their own expression and that of clock-controlled genes. To determine the differential contributions of CRY1 and CRY2 within circadian timing in vivo, we exploited the N-ethyl-N-nitrosourea-induced afterhours mutant Fbxl3(Afh) to stabilize endogenous CRY. Importantly, this was conducted in CRY2- and CRY1-deficient mice to test each CRY in isolation. In both CRY-deficient backgrounds, circadian rhythms of wheel-running and SCN bioluminescence showed increased period length with increased Fbxl3(Afh) dosage. Although both CRY proteins slowed the clock, CRY1 was significantly more potent than CRY2, and in SCN slices, CRY1 but not CRY2 prolonged the interval of transcriptional suppression. Selective CRY-stabilization demonstrated that both CRYs are endogenous transcriptional repressors of clock-controlled genes, but again CRY1 was preeminent. Finally, although Cry1(-/-);Cry2(-/-) mice were behaviorally arrhythmic, their SCN expressed short period (~18 h) rhythms with variable stability. Fbxl3(Afh/Afh) had no effect on these CRY-independent rhythms, confirming its circadian action is mediated exclusively via CRYs. Thus, stabilization of both CRY1 and CRY2 are necessary and sufficient to explain circadian period lengthening by Fbxl3(Afh/Afh). Both CRY proteins dose-dependently lengthen the intrinsic, high-frequency SCN rhythm, and CRY2 also attenuates the more potent period-lengthening effects of CRY1. Incorporation of CRY-mediated transcriptional feedback thus confers stability to intrinsic SCN oscillations, establishing periods between 18 and 29 h, as determined by selective contributions of CRY1 and CRY2.

Circadian pacemaking in the suprachiasmatic nucleus (SCN) revolves around a transcriptional/posttranslational feedback loop in which period (Per) and cryptochrome (Cry) genes are negatively regulated by their protein products. Genetically specified differences in this oscillator underlie sleep and metabolic disorders, and dictate diurnal/nocturnal preference. A critical goal, therefore, is to identify mechanisms that generate circadian phenotypic diversity, through both single gene effects and gene interactions. The individual stabilities of PER or CRY proteins determine pacemaker period, and PER/CRY complexes have been proposed to afford mutual stabilization, although how PER and CRY proteins with contrasting stabilities interact is unknown. We therefore examined interactions between two mutations in male mice: Fbxl3(Afh), which lengthens period by stabilizing CRY, and Csnk1ε(tm1Asil) (CK1ε(Tau)), which destabilizes PER, thereby accelerating the clock. By intercrossing these mutants, we show that the stabilities of CRY and PER are independently regulated, contrary to the expectation of mutual stabilization. Segregation of wild-type and mutant alleles generated a spectrum of periods for rest-activity behavior and SCN bioluminescence rhythms. The mutations exerted independent, additive effects on circadian period, biased toward shorter periods determined by CK1ε(Tau). Notably, Fbxl3(Afh) extended the duration of the nadir of the PER2-driven bioluminescence rhythm but CK1ε(Tau) reversed this, indicating that despite maintained CRY expression, CK1ε(Tau) truncated the interval of negative feedback. These results argue for independent, additive biochemical actions of PER and CRY in circadian control, and complement genome-wide epistatic analyses, seeking to decipher the multigenic control of circadian pacemaking.

The suprachiasmatic nucleus (SCN) is the master circadian oscillator encoding time-of-day information. SCN timekeeping is sustained by a cell-autonomous transcriptional-translational feedback loop, whereby expression of the Period and Cryptochrome genes is negatively regulated by their protein products. This loop in turn drives circadian oscillations in gene expression that direct SCN electrical activity and thence behavior. The robustness of SCN timekeeping is further enhanced by interneuronal, circuit-level coupling. The aim of this study was to combine pharmacological and genetic manipulations to push the SCN clockwork toward its limits and, by doing so, probe cell-autonomous and emergent, circuit-level properties. Circadian oscillation of mouse SCN organotypic slice cultures was monitored as PER2::LUC bioluminescence. SCN of three genetic backgrounds--wild-type, short-period CKlεTau/Tau mutant, and long-period Fbxl3Afh/Afh mutant--all responded reversibly to pharmacological manipulation with period-altering compounds: picrotoxin, PF-670462 (4-(l-Cyclohexyl-4-(4-fluorophenyl)-lH-imidazol-5-yl]-2-pyrimidinamine dihydrochloride), and KNK437 (N-Formyl-3,4-methylenedioxy-benzylidine-gamma-butyrolactam). This revealed a remarkably wide operating range of sustained periods extending across 25 h, from ≤ 17 h to > 4 2 h. Moreover, this range was maintained at network and single-cell levels. Development of a new technique for formal analysis of circadian waveform, first derivative analysis (FDA), revealed internal phase patterning to the circadian oscillation at these extreme periods and differential phase sensitivity of the SCN to genetic and pharmacological manipulations. For example, FDA of the CK1εTau/Tau mutant SCN treated with the CK1ε-specific inhibitor PF-4800567 (3-[(3-Chlorophenoxy)methyl]-l-(tetrahydro-2H-pyran-4-yl)-lH-pyrazolo[3,4-d]pyrimidin-4-amine hydrochloride) revealed that period acceleration in the mutant is due to inappropriately phased activity of the CKlε isoform. In conclusion, extreme period manipulation reveals unprecedented elasticity and temporal structure of the SCN circadian oscillation. [ABSTRACT FROM AUTHOR]

The suprachiasmatic nucleus (SCN) defines 24 h of time via a transcriptional/posttranslational feedback loop in which transactivation of Per (period) and Cry (cryptochrome) genes by BMAL1-CLOCK complexes is suppressed by PER-CRY complexes. The molecular/structural basis of how circadian protein complexes function is poorly understood. We describe a novel N-ethyl-N-nitrosourea (ENU)-induced mutation, early doors (Edo), in the PER-ARNTSIM (PAS) domain dimerization region of period 2 (PER2) (I324N) that accelerates the circadian clock of Per2Edo/Edo mice by 1.5 h. Structural and biophysical analyses revealed that Edo alters the packing of the highly conserved interdomain linker of the PER2 PAS core such that, although PER2Edo complexeswith clock proteins, its vulnerability to degradation mediated by casein kinase 1e (CSNK1E) is increased. The functional relevance of this mutation is revealed by the ultrashort (<19 h) but robust circadian rhythms in Per2Edo/Edo; Csnk1eTau/Tau mice and the SCN. These periods are unprecedented inmice. Thus, Per2Edo reveals a direct causal link between the molecular structure of the PER2 PAS core and the pace of SCN circadian timekeeping. [ABSTRACT FROM AUTHOR]

The suprachiasmatic nucleus (SCN) coordinates circadian rhythms that adapt the individual to solar time. SCN pacemaking revolves around feedback loops in which expression of Period (Per) and Cryptochrome (Cry) genes is periodically suppressed by their protein products. Specifically, PER/CRY complexes act at E-box sequences in Per and Cry to inhibit their transactivation by CLOCK/BMAL1 heterodimers. To function effectively, these closed intracellular loops need to be synchronized between SCN cells and to the light/dark cycle. For Per expression, this is mediated by neuropeptidergic and glutamatergic extracellular cues acting via cAMP/calcium-responsive elements (CREs) in Per genes. Cry genes, however, carry no CREs, and how CRY-dependent SCN pacemaking is synchronized remains unclear. Furthermore, whereas reporter lines are available to explore Per circadian expression in real time, no Cry equivalent exists. We therefore created a mouse, B6.Cg-Tg(Cry1-luc)01Ld, carrying a transgene (mCry1-luc) consisting of mCry1 elements containing an E-box and E'-box driving firefly luciferase. mCry1-luc organotypic SCN slices exhibited stable circadian bioluminescence rhythms with appropriate phase, period, profile, and spatial organization. In SCN lacking vasoactive intestinal peptide or its receptor, mCry1 expression was damped and desynchronized between cells. Despite the absence of CREs, mCry1-luc expression was nevertheless (indirectly) sensitive to manipulation of cAMP-dependent signaling. In mPer1/2-null SCN, mCry1-luc bioluminescence was arrhythmic and no longer suppressed by elevation of cAMP. Finally, an SCN graft procedure showed that PER-independent as well as PER-dependent mechanisms could sustain circadian expression of mCry1. The mCry1-luc mouse therefore reports circadian mCry1 expression and its interactions with vasoactive intestinal peptide, cAMP, and PER at the heart of the SCN pacemaker. [ABSTRACT FROM AUTHOR]

The circadian clock of the suprachiasmatic nucleus (SCN) drives daily rhythms of behavior. Cryptochromes (CRYs) are powerful transcriptional repressors within the molecular negative feedback loops at the heart of the SCN clockwork, where they periodically suppress their own expression and that of clock-controlled genes. To determine the differential contributions of CRY1 and CRY2 within circadian timing in vivo, we exploited the N-ethyl-N-nitrosourea-induced afterhours mutant Fbxl3Afh to stabilize endogenous CRY. Importantly, this was conducted in CRY2- and CRYl-deficient mice to test each CRY in isolation. In both CRY-deficient backgrounds, circadian rhythms of wheel-running and SCN bioluminescence showed increased period length with increased Fbxl3Afh dosage. Although both CRY proteins slowed the clock, CRY1 was significantly more potent than CRY2, and in SCN slices, CRY1 but not CRY2 prolonged the interval of transcriptional suppression. Selective CRY-stabilization demonstrated that both CRYs are endogenous transcriptional repressors of clock-controlled genes, but again CRY1 was preeminent. Finally, although Cryl-/-;Cry2-/- mice were behaviorally arrhythmic, their SCN expressed short period (~18 h) rhythms with variable stability. Fbxl3Afh/Afh had no effect on these CRY-independent rhythms, confirming its circadian action is mediated exclusively via CRYs. Thus, stabilization of both CRY1 and CRY2 are necessary and sufficient to explain circadian period lengthening by Fbxl3Afh/Afh. Both CRY proteins dose-dependently lengthen the intrinsic, high-frequency SCN rhythm, and CRY2 also attenuates the more potent period-lengthening effects of CRY 1. Incorporation of CRY-mediated transcriptional feedback thus confers stability to intrinsic SCN oscillations, establishing periods between 18 and 29 h, as determined by selective contributions of CRY1 and CRY2. [ABSTRACT FROM AUTHOR]

Circadian pacemaking in the suprachiasmatic nucleus (SCN) revolves around a transcriptional/posttranslational feedback loop in which period (Per) and cryptochrome (Cry) genes are negatively regulated by their protein products. Genetically specified differences in this oscillator underlie sleep and metabolic disorders, and dictate diurnal/nocturnal preference. A critical goal, therefore, is to identify mechanisms that generate circadian phenotypic diversity, through both single gene effects and gene interactions. The individual stabilities of PER or CRY proteins determine pacemaker period, and PER/CRY complexes have been proposed to afford mutual stabilization, although how PER and CRY proteins with contrasting stabilities interact is unknown. We therefore examined interactions between two mutations in male mice: Fbxl3Afh, which lengthens period by stabilizing CRY, and Csnk1ϵtm1Asil (CK1ϵTau), which destabilizes PER, thereby accelerating the clock. By intercrossing these mutants, we show that the stabilities of CRY and PER are independently regulated, contrary to the expectation of mutual stabilization. Segregation of wild-type and mutant alleles generated a spectrum of periods for rest-activity behavior and SCN bioluminescence rhythms. The mutations exerted independent, additive effects on circadian period, biased toward shorter periods determined by CK1ϵTau. Notably, Fbxl3Afh extended the duration of the nadir of the PER2-driven bioluminescence rhythm but CK1ϵTau reversed this, indicating that despite maintained CRY expression, CK1ϵTau truncated the interval of negative feedback. These results argue for independent, additive biochemical actions of PER and CRY in circadian control, and complement genome-wide epistatic analyses, seeking to decipher the multigenic control of circadian pacemaking. [ABSTRACT FROM AUTHOR]

Summary 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. [ABSTRACT FROM AUTHOR]