Redundant Function of REV-ERBα and β and Non-Essential Role for Bmal1 Cycling in Transcriptional Regulation of Intracellular Circadian Rhythms

We first examined expression of the Rors and Rev-erbs in various tissues (Figure S1A, S1B, S1C). In contrast to the ubiquitous expression of Bmal1, Rev-erbα and Rev-erbβ in all the tissues examined, expression and rhythmicity of the RORs are more restrictive. Rora expression is ubiquitous, but its circadian cycling is restricted to SCN. Rorb is expressed in the SCN, hypothalamus, cerebral cortex and retina, but not in the liver. Conversely, Rorc is rhythmically expressed in the liver, but not detected in the SCN or other brain regions. Expression patterns of the Ror genes in the lung were similar to those in the liver (data not shown). The tissue-specific expression patterns of the RORs are consistent with previous reports [6], [25]–[28].

In this study, we extensively used fibroblasts derived from mice as a cell-based clock model. Of the three Rors, only Rora is highly expressed in mouse fibroblasts, but no distinct mRNA rhythm was detected (Figure S1A); Rorb and Rorc were not detected in fibroblasts (Figure S1A and S1B). Differential tissue distribution and rhythmicity of the Rors suggests that they may have different functions in clock mechanisms.

Rora and Rorb have been characterized as clock components, functioning to regulate Bmal1 expression in the SCN (Figure S1C) [6],[10],[11],[25]. However, since RORc is not expressed in the SCN, it should not affect function of the SCN pacemaker, which drives circadian locomotor behavior. We tested this hypothesis in a mouse line deficient in Rorc function. Deletion of Rorc results in reduced survival of thymocytes and abnormal lymphoid organ development, but Bcl-xL transgene (Bcl-xL^Tg) expression restored most aspects of normal thymocyte development and significantly improved animal survival [29]. Compared to Bcl-xL^Tg control (period length τ = 23.42 hr±0.08, n = 5), Rorc^−/−:Bcl-xL^Tg mice displayed normal circadian wheel-running activity under constant darkness (τ = 23.34 hr±0.2, n = 8). These mice also showed a normal response to a light pulse at CT16 (Figure S1D). We further examined the dynamics of molecular rhythms in the SCN and showed that SCN explants from Rorc^−/−:Bcl-xL^Tg mice also displayed similar mPer2^Luc bioluminescence rhythms to control mice (data not shown). Thus, consistent with the absence of Rorc gene expression in the SCN, these results confirm that RORc plays no role in SCN pacemaker function.

Based on the ability of RORc to activate a Bmal1-Luc reporter in vitro and its strong rhythmic expression in many peripheral tissues including the liver and lung [6],[9],[25], we hypothesized that RORc, like RORa and RORb in the SCN, may play an important role as an activator of Bmal1 in peripheral oscillators. We tested this hypothesis by analyzing Bmal1 expression in the mouse liver. In Bcl-xL^Tg control mice, Bmal1 expression peaked at CT24 (Figure 1A). In contrast, Bmal1 expression at CT28, CT44 and CT48 in the liver of Rorc^−/−:Bcl-xL^Tg mice was significantly reduced, compared to those of Bcl-xL^Tg siblings (Figure 1A). These results suggest that RORc activates Bmal1 transcription in the positive arm of the ROR/REV/Bmal1 loop, functioning to maintain normal amplitude of Bmal1 rhythmicity.

Although Bmal1 peak expression levels are reduced in the absence of RORc, Bmal1 mRNA still retains a rhythm with fairly high amplitude, indicative of functional redundancy from RORa and/or contributions from the REV-ERBs. RORc also regulates transcription of Cry1, Clock and Npas2, all of which are considered RORE-containing genes [30],[31], and their mRNAs were also reduced during peaking hours (Figure 1A). Despite the blunted rhythm amplitudes for Bmal1, Clock, Npas2 and Cry1, cyclic expression of Per2 and Dbp, however, was not dramatically affected by Rorc deletion, similar to observations in Rev-erbα^−/− mice [5].

As the SCN clock functions normally in the absence of Rorc, we assessed the effect of Rorc deletion on peripheral clock function in tissue-autonomous preparations in which confounding influences from the SCN are eliminated. Tissue explants of the lung from Bcl-xL^Tg control mice displayed persistent mPer2^Luc rhythms (τ = 24.00 hr±0.33, n = 4). Rorc^−/−:Bcl-xL^Tg lung explants exhibited rhythmic mPer2^Luc expression with comparable period lengths to controls (τ = 24.15±0.49, n = 4) (Figure 1B). Rorc^−/−:Bcl-xL^Tg liver explants also displayed persistent mPer2^Luc rhythms (τ = 22.59 hr±1.54, n = 5), similar to controls (τ = 22.22 hr±0.71, n = 3). Surprisingly, no significant differences in circadian amplitude or damping rate were observed between controls and Rorc^−/− mice. The normal bioluminescence rhythms are consistent with unaltered molecular phenotypes of Per2 expression (Figure 1A). Moreover, we observed normal rhythms in fibroblasts, in which Rorc expression is not detectable (data not shown), further confirming results from liver and explants. In fibroblasts, over-expression of Rorc did not affect Bmal1 rhythms (data not shown). These results demonstrate that RORc does not play an essential role in maintaining circadian oscillation and suggest that a high-amplitude Bmal1 rhythm may not be critically required for basic clock operation, similar to phenotypes observed for Rev-erbα deficiency [5].

So far, data suggest a functional redundancy among RORa, RORb and RORc. In the liver and fibroblasts of both Rora^sg/sg[6],[11] and Rorc^−/− mice, Bmal1 peak expression is reduced, but the mRNA rhythm is largely retained and Per2 oscillation is not altered. Although Rora does not show strong rhythmicity in the liver, its expression alone could partially complement the loss of Rorc. To study the ROR redundancy genetically, a mouse line deficient in both Rora and Rorc would represent an ideal reagent. However, such a line is extremely difficult to obtain because Rora^sg/sg mutant mice display cerebellar ataxia and mostly infertile [18] and Rorc^−/− mice also have strongly abnormal phenotypes [29]. Therefore, we decided to address the ROR redundancy using Rora^sg/sg fibroblasts. Because Rorb and Rorc are also not expressed in Rora^sg/sg fibroblasts as determined by Q-PCR (data not shown), thus excluding the possibility of a compensation mechanism, the positive arm of the ROR/REV/Bmal1 loop is essentially missing in cells lacking Rora function.

To monitor the function of the core loop and the ROR/REV/Bmal1 loop in parallel, we generated two lentivirus-mediated circadian reporters, pLV6-Per2-dLuc and pLV6-Bmal1-dLuc, designed to report the E-box and RORE-regulated rhythms, respectively. As expected, WT cells displayed persistent Bmal1-dLuc rhythms (τ = 24.44±1.55 hr, n = 17 culture dishes from 2 independent cell lines). Importantly, Rora^sg/sg fibroblasts also displayed rhythmic Bmal1-dLuc oscillations (τ = 24.34±0.95 hr, n = 30 from 3 lines), comparable to WT cells (Figure 1C). Not surprisingly, these cells also exhibited Per2-dLuc rhythms similar to those of WT cells (Figure 1C). Our results demonstrate that the ROR activators contribute to Bmal1 rhythm amplitude, but are clearly not required for Bmal1 rhythmicity and core clock function in fibroblasts.

Next, we examined the consequence of disrupting the negative arm of the ROR/REV/Bmal1 loop. Bmal1 expression is significantly higher in the liver [5] and fibroblasts of Rev-erbα^−/− mice than in WT (data not shown). Given the abnormal Bmal1 expression in the liver and fibroblasts, we expected that deletion of Rev-erbα would dramatically compromise the Bmal1 rhythm, as previously suggested from mRNA analysis [5]. Surprisingly, however, Rev-erbα^−/− fibroblasts displayed rhythmic Bmal1-dLuc expression (Figure 2A). The period lengths for Rev-erbα^−/− fibroblasts harboring Per2-dLuc reporter were determined to be 26.59±0.29 hr (n = 10) for cell line-1 and 24.25±0.72 hr (n = 10) for cell line-2, and the corresponding WT fibroblasts exhibited a periodicity of 24.88±1.48 hr (n = 7). Thus, as expected, real-time longitudinal bioluminescence recording reveals the dynamics of gene expression, while mRNA profiling lacks temporal resolution and is generally more subject to noise. Given the apparent redundant contribution from Rev-erbβ, Bmal1 rhythms in the liver and lung of Rev-erbα^−/− mice are also likely to be rhythmic, similar to that observed in fibroblasts.

We assessed any redundant contribution from Rev-erbβ using small hairpin RNAs (shRNA). We designed and tested nine shRNA constructs against different regions of the Rev-erbβ gene, and three of them (shRNA-β1, β2 and β3) were found to be functional in efficiently knocking down Rev-erbβ expression (Figure 2C). We introduced Rev-erbβ-shRNA constructs into WT fibroblasts harboring Bmal1-dLuc reporter. Knockdown of Rev-erbβ resulted in higher Bmal1 mRNA expression, with shRNA-β1 being the most potent (Figure 2C); these cells displayed rhythmic Bmal1-dLuc expression (Figure 2A), similar to effects of Rev-erbα-knockout. Thus, Rev-erbα and Rev-erbβ are functionally redundant and disruption of either one alone is not sufficient to disrupt Bmal1 rhythms.

To disrupt the function of REV-ERBα and β simultaneously, Rev-erbβ-shRNA constructs were stably introduced into Rev-erbα^−/− fibroblasts harboring the Bmal1-dLuc reporter to obtain Rev-erbα^−/−:Rev-erbβ-shRNA:Bmal1-dLuc cell lines. In striking contrast to rhythmic Bmal1-dLuc expression in Rev-erbα-knockout or Rev-erbβ-knockdown fibroblasts, cells deficient in both Rev-erbα and β function displayed significantly higher levels but largely arrhythmic Bmal1-dLuc expression (Figure 2B). For cell line-2, 15/18 dishes of Rev-erbα^−/− cells expressing control shRNA displayed rhythmic Bmal1-dLuc expression (FFT spectral amplitude = 0.80±0.08, n = 15), but only 6/19 of Rev-erbα^−/−:Rev-erbβ-shRNA-β1 showed any rhythms, and those that were rhythmic showed significantly lower spectral amplitude (FFT spectral amplitude = 0.50±0.08, n = 6). The weak rhythms may likely result from residual levels of REV-ERBβ expression in these knockdown cells. Similar results were observed in cell line-1 (data not shown). These results demonstrate that the REV-ERBα and β are required for rhythmic Bmal1 expression in fibroblasts. The finding that cells lacking ROR function retain Bmal1-dLuc rhythms whereas those deficient in REV-ERB function are arrhythmic, suggests that the REV-ERB repressors play more prominent roles than the ROR activators in the ROR/REV/Bmal1 loop.

Given that the Bmal1-dLuc reporter is rhythmic in Rev-erbα^−/− fibroblasts, it is not surprising to observe that the Per2-dLuc reporter was also rhythmic (Figure 2D). However, it was not known whether disrupting both Rev-erbα and β would affect the core feedback loop function. We thus introduced Rev-erbβ-shRNA constructs into Rev-erbα^−/−:Per2-dLuc fibroblasts and demonstrated that Rev-erbα^−/−:Rev-erbβ-shRNA cells also displayed rhythmic patterns of Per2-dLuc expression (τ = 25.99±0.40 hr, n = 7 for cell line-1; τ = 25.12±0.60 hr, n = 22 for cell line-2), similar to cells expressing control shRNA (τ = 26.48±0.27 hr, n = 7 for cell line-1; τ = 25.31±0.52 hr, n = 23 for cell line-2) (Figure 2D).

We also examined effects of Rev-erbβ-knockdown on the expression of other clock genes. In shRNA control cells, peaks of Rev-erbβ and Per2 mRNAs (CT40–48) were almost anti-phasic to Bmal1 (CT32–36). Bmal1 mRNA was effectively de-repressed, especially at CT46–52 when Bmal1 was at its nadir in control cells (Figure 2C). Consistent with rhythmic Per2-dLuc bioluminescence expression, the Per2 mRNA expression pattern was essentially the same in Rev-erbα^−/− cells expressing control shRNA and in those expressing shRNA against Rev-erbβ.

Given that Cry1 is under combinatorial regulation by both BMAL1/CLOCK and REV-ERBs [5],[30],[31], we expected that disruption of REV-ERB function would alter the Cry1 expression pattern. Indeed, compared to WT cells, Cry1 mRNA levels were higher in Rev-erbα^−/− fibroblasts (data not shown), and even higher in Rev-erbα^−/−:Rev-erbβ-shRNA fibroblasts (Figure 2C), all consistent with REV-ERB proteins being repressors. Although interference with the REV-ERBs clearly disrupted the Bmal1 rhythm, it did not seem to substantially alter the rhythm of Cry1 mRNA. Cry1 mRNA remained to be rhythmic, reaching its nadir at CT36–40 and peaking at CT46–50, illustrating the resilience of the intracellular clock mechanism. It is possible that, even though the Bmal1 rhythm is abolished, the residual level of REV-ERBβ in the cells was sufficient for combinatorial regulation of Cry1. It is also possible that other unknown mechanisms contribute to Cry1 regulation. This ambiguity can be resolved in future studies by examining cells completely deficient in both Rev-erbα and β function. Nevertheless, our results suggest that REV-ERBα and β are required for rhythmic expression of Bmal1, but REV-ERB function and the Bmal1 rhythm are not required for normal oscillations of Per and Cry.

To further test the role of RORE-mediated Bmal1 regulation, we eliminated all influences of the RORs and REV-ERBs on Bmal1 expression in cell-based genetic complementation experiments. Fibroblasts derived from Bmal1^−/−:mPer2^Luc mice displayed arrhythmic patterns of bioluminescence expression, demonstrating that Bmal1 is an essential clock component for cellular rhythmicity in fibroblasts (Figure 3A). We asked whether constitutively expressed BMAL1 in Bmal1^−/− fibroblasts could restore circadian rhythmicity. This approach precludes residual REV-ERBβ function from shRNA knockdown and circumvents any off-target effects.

To manipulate Bmal1 expression, we used three promoters: Bmal1(WT) contains a 526-bp DNA fragment from the Bmal1 promoter encompassing ROREs, Bmal1(Mut) is identical to Bmal1(WT) except that the RORE sites are mutated to prevent ROR/REV-ERB from binding, and UbC is a commonly used constitutive promoter from the UbC gene. We showed that WT fibroblasts transduced with a lentiviral Bmal1(WT)-dLuc reporter displayed rhythmic bioluminescence expression, but Bmal1(Mut) or UbC promoters did not confer rhythmicity in these cells (Figure 3B).

We next determined the ability of the promoters to regulate the expression of Bmal1. In lieu of Western blot analysis of BMAL1, we monitored the bioluminescence expression of BMAL1::LUC fusion protein. We demonstrated that BMAL1::LUC cycled only when it is driven by Bmal1(WT), and that UbC and Bmal1(Mut) promoters did not confer rhythmic fusion protein expression (Figure 3C). Thus, BMAL1 protein itself does not cycle in the absence of a RORE-containing circadian promoter.

To carry out genetic complementation, we generated a lentiviral expression vector Bmal1(WT)-Bmal1·Flag, in which Bmal1 cDNA is under the control of WT Bmal1 promoter. When this construct was introduced into Bmal1^−/−:mPer2^luc fibroblasts, circadian rhythmicity was restored (τ = 22.02±0.68 hr, n = 25 cultured dishes) (Figure 3D), but not in cells expressing a Bmal1(WT)-GFP control construct (data not shown). Importantly, non-cyclically expressed BMAL1 under the control of either UbC or Bmal1(Mut) also effectively restored circadian mPer2^Luc rhythmicity in Bmal1^−/− fibroblasts (τ = 22.08±0.46 hr, n = 20 for UbC-Bmal1; τ = 22.61±0.60 hr, n = 27 for Bmal1(Mut)-Bmal1) (Figure 3D). Taken together, these results demonstrate that rhythmic expression of BMAL1 protein is not essential for the basic functioning of the intracellular clock. These results provide the cellular basis for the finding that constitutive Bmal1 expression was able to rescue circadian behavioral rhythms in Bmal1^−/− mice [24].

The Rorc gene has at least two E-boxes within the promoter region, and its circadian expression pattern is similar to Cry1 in the liver. In vitro studies suggest that Rorc transcription is regulated by BMAL1/CLOCK [31]. To verify the in vitro results, we demonstrated that, similar to the expression patterns of other BMAL1/CLOCK-regulated clock components, the Rorc mRNA rhythm was abolished in the Bmal1^−/− mouse liver, confirming that Rorc is regulated by the core loop (Figure 4A).

Interestingly, however, we observed that mRNA levels of Rorc as well as Cry1 are clearly elevated rather than reduced in Bmal1^−/− liver. This was surprising at first given that BMAL1 is a known activator of Cry1 and Rorc expression. However, it should not be so surprising given the complexity of transcriptional circuitry of the clock. Similarly, higher Cry1 mRNA levels were also reported previously in Bmal1^−/−, Clock^m/m and Clock^−/− mice [32],[33]. A recent in silico study showed that Cry1 and Rorc genes contain two types of circadian regulatory elements, the E-box and the RORE [31]. In vitro and in vivo evidence also supports the presence of RORE sites within the Cry1 gene [5],[30]. In the absence of E-box regulation, factors acting through the RORE, namely the RORs and REV-ERBs, are likely to govern Cry1 and Rorc transcription. In line with this notion, Clock mRNA is also higher in Bmal1^−/− liver (Figure 4A), and Bmal1 mRNA is higher in Clock^−/− mouse liver [33].

A recent study proposed dual activator and repressor functions of BMAL1/CLOCK, in which its repressor function explains the elevated Cry1 expression in the absence of Bmal1[32]. However, that study did not take into consideration Cry1 gene regulation through the ROREs. In both WT and Bmal1^−/− mouse liver, there exists a strong inverse correlation between Rev-erbα and Cry1/Rorc mRNA levels: when Rev-erbα is high, Cry1/Rorc is low, and vice versa (Figure 4A). Similar expression patterns were also observed in fibroblasts (Figures 1B and 5B) and in Rev-erbα^−/− mice [5], and suggested from in silico and in intro studies [30],[31]. Thus, the elevated Rorc and Cry1 expression in the absence of Bmal1 may be regulated primarily by the REV-ERBs rather than the repressor function of BMAL1. We therefore sought to experimentally demonstrate this notion. We hypothesized that over-expression of Rev-erbα in Bmal1^−/− cells would bring down the expression levels of Cry1 and Rorc. Because Cry1 and Rorc genes are regulated similarly but Rorc is not expressed in fibroblasts, we focused our analysis on the Cry1 gene in this cell type. To test this idea, we introduced Rev-erbα into Bmal1^−/− cells by lentivirus-mediated delivery and obtained a Bmal1^−/−:Rev-erbα-OX fibroblast cell line. Indeed, over-expressed REV-ERBα in Bmal1^−/− cells efficiently repressed Cry1 mRNA to levels similar to those in WT cells (Figure 4C). Taken together, we provide direct in vivo genetic and molecular evidence to support the notion that Cry1 and Rorc are regulated not only by BMAL1/CLOCK but also directly by the REV-ERBs (Figure 5A), which is the underlying molecular mechanism for elevated Cry1 expression in Bmal1^−/− cells.

Interestingly, the mRNA levels of other clock genes in the liver of Bmal1^−/− mice are also very different (Figure 4A): Dbp and Rev-erbα expression is dramatically reduced, and Per1 and Per2 are expressed at constant intermediate levels, consistent with sustained mPer2^Luc expression in Bmal1^−/− cells (Figure 3A), whereas Rorc, Cry1, Clock and E4bp4 are clearly de-repressed. Based on mRNA expression patterns in both WT and Bmal1^−/− cells (Figure 4A), we suggest the following transcriptional regulatory scheme for clock gene expression (Figure 5A): Dbp and Rev-erbα are activated primarily by BMAL1/CLOCK via the E-boxes, and that this E-box-mediated circadian regulation is essentially eliminated in the absence of BMAL1 (and thus PER/CRY-mediated repression via the E-box is no longer relevant). Per1 and Per2 are activated by BMAL1/CLOCK and other non-circadian mechanisms, accounting for the intermediate mRNA levels of Per1 and Per2 in Bmal1^−/− mice. Rorc and Cry1 are regulated not only by BMAL1/CLOCK but also by RORs/REV-ERBs via the RORE. Bmal1, Clock and E4bp4 are regulated by RORs/REV-ERBs via the RORE.

The different regulatory mechanisms offer mechanistic explanations for distinct phases of clock gene expression rhythms observed in vivo (Figure 5A). Dbp is controlled by BMAL1/CLOCK via the E-box, while E4bp4 is primarily regulated via RORE, explaining why the E4bp4 rhythm is in phase with Bmal1 and Clock, but is antiphasic to Dbp. Rev-erbα and Rorc are both activated by BMAL1/CLOCK, but Rorc is also repressed by REV-ERBs, explaining how Rorc mRNA accumulation is phase-delayed compared to that of Rev-erbα. Additional regulation of Per1 and Per2 by non-circadian factors (and possibly also by E4bp4) may cause a phase-delay compared to Dbp and Rev-erbα. In summary, our data provides novel mechanistic insights into how the genes in the clock circuitry are regulated in vivo [31].

The RORs appear to regulate the amplitude of target gene expression, while the REV-ERBs regulate the rhythmic expression of Bmal1 and also participate in combinatorial regulation of Cry1. As these regulatory mechanisms are not required for basic clock function, we suggest that the ROR/REV/Bmal1 loop and its constituents provide additional opportunities to control time-specific expression of output genes in local clock physiology, especially in peripheral tissues. In this context, the differential tissue expression patterns of the RORs also provide additional opportunities for tissue-specific local circadian biology (Figure 5B).

The combinatorial regulatory mechanism provides a novel strategy for identifying and validating target genes of the RORs and REV-ERBs, as well as differentiating RORE-containing genes from those containing both RORE and E-boxes (Figure 4A). Here we examined several of the genes that exhibit phases similar to Bmal1 or Cry1 in the liver and contain potential RORE sequences [25],[26]. For example, mRNAs of heat-shock protein 60 (Hsp60), arginine vasopressin receptor 1A (Avp-V1a) and Apoc3 were reduced in the liver of Rorc^−/− mice, especially at peak time (CT40–48), reflecting reduction of RORE-mediated activation, but their mRNA levels were up-regulated in Bmal1^−/− mice at CT28–36, corresponding to the trough time in WT, reflecting loss of E-box-mediated REV-ERB expression with subsequent relief of RORE-mediated repression. Tubulin beta 5 (Tubb5) and peptidyl-prolyl cis-trans isomerase FK506 binding protein 4 (Fkbp4) also exhibited significantly higher mRNA levels at CT28–36 in Bmal1^−/− mice, but their expression levels were not affected in Rorc^−/− mice. Thus, cyclic RORE-mediated activation and/or repression may modulate expression patterns of specific target genes involved in important biological processes in a tissue-specific manner.

Previous studies using mice deficient in Rora, Rorb or Rev-erbα function strongly suggested functional redundancy among the ROR and REV-ERB family members [5],[6],[10],[11]. Mutation of Rora was shown to reduce Bmal1 mRNA amplitude both in the SCN [6] and in fibroblasts [11], and Rev-erbα deletion resulted in much higher levels of Bmal1 transcription [5], but Bmal1 rhythms were still retained despite either deficiency.

While null mutations in core clock genes typically lead to severe impairment of clock function (see below), deficiencies in clock components within the ROR/REV/Bmal1 loop only produce modest clock phenotypes [5]–[7],[10],[11]. The ROR/REV/Bmal1 loop is thus thought to provide a “stabilizing” function. However, mice deficient in core clock components (e.g., Per1^−/−, Per2^−/− or Clock^m/m mice) also similarly show less precise or less persistent circadian rhythms [19]–[22].

In this study, we investigated the redundancy of functions among the ROR and REV-ERB family members and clarified their roles in regulating Bmal1 expression. To circumvent pleiotropic effects of gene deletion, we directly tested this “stabilization function” hypothesis in cell-autonomous clock models by perturbing the BMAL1 rhythm. We demonstrated that cells with Rev-erbα-knockout or Rev-erbβ-knockdown still rhythmically express Bmal1. The Bmal1-dLuc rhythm could be abolished only when both Rev-erbα and β were disrupted (Figure 2B). Thus, REV-ERBα and REV-ERBβ are required for Bmal1 rhythmicity, and they are functionally redundant. In contrast, the RORs are not required for Bmal1 rhythmicity (Figure 1). Thus, the REV-ERBs play a more prominent role than the RORs in regulating the rhythmic expression of Bmal1.

The current models for mouse and fly circadian clocks indicate that the process of evolution has produced a genetic circuitry substantially more complex than a simple transcriptional feedback scheme. Presumably, robustness is a key feature of circadian control that is likely to be under selective pressure, as it would underlie the adaptive significance of a particular physiological rhythm. Robustness is the ability of a system to maintain essential properties despite internal noise and external perturbations, a property which is prevalent in biological control circuits [34]. From a circadian clock perspective, the key measures of robustness are precision (period stability over time), persistence (how long a given clock system sustains rhythm amplitude without a resetting signal), and accuracy (period consistency of cells, tissues, or organisms). It should be noted, however, that period variation and alteration may be an indicator of robustness, not necessarily instability. Mechanisms contributing to the robustness of the clock system include additional interlocking loops, gene redundancy, maintenance of amplitude, and intercellular coupling.

In contrast to the proposed “stabilizing” role of the ROR/REV/Bmal1 loop, we found that Per2-dLuc expression is rhythmic even in cells deficient in both REV-ERBα and β function (Figure 2D) or expressing constitutive BMAL1 protein (Figure 3D). This provides unambiguous evidence from cell-autonomous preparations that Bmal1 mRNA and protein rhythms are not essential for the basic operation of the intracellular clock. In accord with our findings, constitutively expressed Bmal1 in the SCN of Bmal1^−/− mice was able to rescue circadian behavioral rhythmicity [24].

Using real-time bioluminescence imaging to monitor Per2 gene expression in tissues and cells from mutant mice [23], we recently found that both Per1 and Per2 are required for sustained cell-autonomous rhythms in individual cells. Importantly, intercellular coupling in the SCN can compensate for clock gene deficiency, preserving sustained cellular rhythmicity in mutant SCN slices and behavior. Thus, SCN intercellular coupling is essential not only to synchronize component cellular oscillators but also for robustness against genetic perturbations. In this context, it is reasonable to presume that, owing to intercellular coupling, an SCN ensemble that expresses non-cyclic Bmal1 mRNA/proteins would still exhibit robust Per2 and Cry1 rhythms. However, Rora^sg/sg, Rorb^−/− and Rev-erbα^−/− mice exhibit circadian period defects in behavior, albeit very mild. Thus, to address the cellular basis of circadian behavior, future studies using real-time bioluminescence technology are needed to examine the molecular dynamics of circadian rhythmicity in the SCN ensemble as well as in dissociated SCN neurons of single and double loss-of-function mutants of the Ror and Rev-erb genes.

However, the nonessential ROR/REV/Bmal1 loop in the basic intracellular clock mechanism clearly regulates expression rhythm and amplitude of many output genes. Maintaining a biologically relevant high-amplitude rhythm of gene expression also contributes to the robustness of the clock system. The significance of amplitude in clock function is supported by a recent study showing that Clock^m/m mice exhibited increased efficacy in response to resetting stimuli due to reduced circadian amplitude in the SCN pacemaker [35]. Similarly, the ROR/REV/Bmal1 loop may also benefit organismal survival in the natural environment by contributing to robust high-amplitude rhythms [13]. Furthermore, this interlocking loop may contribute to transduction of environmental cues to the core loop [13]. In line with this notion, behavioral studies have implicated Rev-erbα and Rorb in photic responses [5],[36]. It is interesting to note that there appears to be a delayed phase of Per2 oscillation in cells that express arrhythmic Bmal1 mRNA and protein (Figures 3D and 4D). As Per2 induction may be involved in synchronization [37], it is possible that the ROR/REV/Bmal1 loop plays an important role in circadian entrainment of peripheral oscillators.

The resilience of the intracellular core clock function without inputs from the ROR/REV/Bmal1 loop indicates that general cellular mechanisms must play important roles in attaining robust clock function, including particularly post-translational modifications and protein turnover affecting subcellular translocation and activities of clock components. In particular, results from this study strongly suggested the involvement of tonic signaling in clock function (Figure 5A). In Bmal1^−/− cells, transcription of Rev-erbα and β is completely abolished, whereas Per1 and Per2 maintain intermediate transcription levels throughout the day. Any contribution from Dbp/E4bp4 is minimal in these cells, as the level of the DBP activator is too low and the E4BP4 repressor is constantly high (Figure 4A). Rather, it is likely that, without BMAL1/CLOCK activators, Per1 and Per2 transcription is maintained through a non-circadian, tonic signal input such as the cyclic AMP response element-binding (CREB) signal transduction cascade. Similarly, presence of tonic signaling and lack of repression by the REV-ERBs are the primary cause for the constantly high levels of Cry1, Rorc and Clock expression in Bmal1^−/− cells. It is conceivable that the activating tonic signal input also explains why the ROR activators are dispensable for driving rhythmic transcription of Bmal1 provided that the REV-ERBs are present in the cells. It is likely that the balance between positive and negative regulators as well as tonic signaling determines clock gene expression at any given circadian time. The tonic signal input is usually overlooked in the WT genetic background, but is uncovered when the functions of positive and/or negative regulators are blocked (Figure 5A). Tonic signaling is also important to consider in interpreting effects of Per or Cry mutations on cellular rhythms in the SCN [2],[23].

In addition to the ROR/REV/Bmal1 loop, other known interlocking loops or components include Dbp/E4bp4, Pparα, and Dec1/Dec2 (Figure 5B). These secondary loops are directly regulated by the core loop through the E-boxes [31]. E4BP4 and DBP, analogs of dVRI and dPDP1 in flies [16], form an oscillatory loop by feeding back to regulate Per2 transcription [31], [38]–[40]. DEC1 and DEC2 form another feedback loop, functioning to repress E-box-mediated transcription [41]. Very recently, clockwork orange (cwo), a Dec homolog, has been identified in Drosophila and shown to regulate rhythm amplitude [42]–[44]. The PPARα loop, on the other hand, feeds back to activate Bmal1 expression through potential PPAR response elements in the Bmal1 gene [45]–[47]. Interestingly, peroxisome proliferator-activated receptor-gamma coactivator 1 alpha (PGC-1α) has recently also been shown to activate Bmal1 expression by acting as a ROR activator [48].

Unlike the requirement for core clock components—PER/CRY [23], CLOCK [49], and BMAL1 (Figure 3A in this study), none of the interlocking loops discussed above appears to be required for basic clock function (in this study) [47], [48], [50]–[52]. In addition, the ROR/REV/Bmal1 loop function is not conserved between mammals and flies [7],[16]. Rather, it's conceivable that the major function of the interlocking loops is to transmit circadian signals to control local output genes at different times during the day, as required for circadian behavior and physiology. Direct transduction of circadian information to local output rhythms can be more efficiently accomplished through first-order clock-controlled genes (1^st order CCGs) that are directly regulated by the core loop, which subsequently regulate expression of 2^nd and 3^rd order CCGs (Figure 5B). In this context, the interlocking loops and their constituents serve as the 1^st order CCGs. Most CCGs exhibit tissue-specific expression patterns, and many are involved in rate-limiting steps of reactions important for the main functions of their respective tissues [3],[26],[53],[54]. The core loop components are well conserved among various tissues, while the 1^st order CCGs such as the RORs may be highly tissue-specific. The 1^st order CCGs could also establish crosstalk with other tissue-specific circadian or non-circadian factors. Thus, the multiple interlocking loops provide an efficient means not only to amplify circadian signals but also to provide additional phase information for local outputs (Figure 5B). For instance, the antiphasic expression of Dbp and E4bp4 is known to regulate the rhythmic production of many proteins involved in bile acid production, drug metabolism, and xenobiotic detoxification in liver and kidney [3],[38],[55].

The RORs and REV-ERBs are known to be involved in many cellular, physiological, and pathological processes [4], [56]–[60]. For example, RORa and RORc regulate phase I and II metabolism [61]. RORa activates and REV-ERBα represses genes encoding apolipoprotein C-III (ApoC-III) and ApoA1, key proteins in plasma triglyceride and lipoprotein metabolism [60]. RORa and REV-ERBβ also regulate many genes involved in lipid homeostasis in skeletal muscle cells [62],[63]. REV-ERBα was shown to regulate circadian expression of plasminogen activator inhibitor type 1 (PAI-1), suggesting a role in thrombosis [64]. Interestingly, crosstalk exists between REV-ERBα and PPAR nuclear receptors, which are important factors regulating lipid and glucose homeostasis and inflammation [60]. Recently, anatomical expression profiling of nuclear receptors has revealed significant metabolic implications of peripheral clock biology [27],[28],[65].

With the interlocking loops as entry points, future studies should focus on more detailed characterization of the transcriptional circuitry regulating time- and tissue-specific outputs involved in circadian behavior, physiology, and pathology. Knowledge of circadian signaling and clock-regulated local biology will likely have important implications for the pathogenesis and treatment of diseases such as metabolic syndrome, heart disease, diabetes, and obesity.

Bmal1^−/− mouse line was obtained from Chris Bradfield at the University of Wisconsin, Rorc^−/− line from Dan Littman at New York University, and mPer2^Luc transgenic reporter line from Joe Takahashi at Northwestern University. Knockout mice were bred with mPer2^Luc reporter mice to obtain homozygous knockouts harboring the mPer2^Luc reporter. Wheel-running assays were performed and analyzed as described previously [23]. Behavioral phenotypes of these mice were similar to the respective knockout animals not carrying the reporter. All animal studies were conducted in accordance with the regulations of the Committees on Animal Care and Use at The Scripps Research Institute.

Explants of SCN and peripheral tissues were dissected and cultured as previously described [23],[66]. Primary mouse fibroblasts were generated from tails by a standard enzymatic digestion procedure [67]. Fibroblasts that spontaneously overcame replicative senescence (immortalization) were used in this study. All fibroblasts were cultured in DMEM supplemented with 10% fetal bovine serum and antibiotics, and grown to confluence prior to bioluminescence recording or harvesting for mRNA time courses. MMH-D3 hepatocytes were cultured as described previously [68].

Recombinant lentiviruses were produced by transient transfection in 293T cells using the calcium-phosphate method as previously described [23]. Infectious lentiviruses were harvested at 48 hr post-transfection and used to infect various cells. Cells infected with pLL3.7(GW)-shRNA constructs were sorted by FACS for the highest (10%) GFP-expressing cells. Cells infected with pLV156-Per2-dLuc reporter [23] were sorted by FACS for GFP expression as described therein. Cells infected with pLenti6-B4B2 constructs expressing proteins including Per2-dLuc and Bmal1-dLuc reporters were selected with 10 µg/ml Blasticidin and further propagated for further study.

For cDNA expression constructs, DNA sequences including GFP, Bmal1, Rev-erbα, Rev-erbβ, the firefly Luciferase gene (Luc), the rapidly degradable Luciferase gene (dLuc), and Bmal1::Luc, were first cloned into pENTR/D-TOPO entry vector (Invitrogen). All promoter sequences including Bmal1(WT), Bmal1(Mut), UbC, Per2, and the composite CAG promoter were first cloned into pENTR-5′-TOPO vector (Invitrogen). The pENTR/D-TOPO-cDNA and pENTR-5′-promoter plasmid DNAs were then recombined with pLenti6/R4R2/V5-DEST destination vector (Invitrogen) in a MultiSite Gateway recombination reaction to generate expression constructs (see Text S1).

For shRNA expression constructs, we first designed and generated nine 29-bp long oligo-nucleotides against different regions of the Rev-erbβ gene. Synthetic oligonucleotides were annealed and cloned into pENTR/U6 (Invitrogen) according to manufacture's instruction and subsequently cloned into the pLL3.7GW vector which harbors a CMV-driven GFP gene [69]. Among the tested nine shRNA constructs against Rev-erbβ, three (designated β1, β2 and β3) were found that were non-overlapping and efficiently depleted over-expressed REV-ERBβ protein in transfected HEK293T cells as tested by Western blot analysis (data not shown) and knocked down Rev-erbβ mRNA expression in fibroblasts as tested by Q-PCR. The parental pLL3.7GW empty vector and a nonspecific shRNA construct were used as controls (see Text S1).

For liver and lung, mice were first entrained to regular light-dark cycles and then released to constant darkness, and peripheral tissue samples were harvested 28 hr later. Total RNAs from liver and lung were first prepared using Trizol reagents (Invitrogen) followed by further purification using RNeasy mini kit (Qiagen). For fibroblasts, cell growth, serum shock were performed as previously described [5]–[7],[10],[23]. Total RNAs from fibroblasts were prepared using RNeasy mini kit.

Total RNAs were transcribed to cDNA using 1^st strand SuperScript III reverse transcriptase (Invitrogen). Q-PCR was performed using an iCycler thermal cycler with the MyiQ optical module (BioRad) as described previously [6],[23]. Transcript levels for each gene were normalized to Gapdh. Average relative expression ratios for each gene were expressed as a percentage of the maximum ratio at peak expression (see Text S1).

Bioluminescence patterns were monitored using a LumiCycle luminometer (Actimetrics) as previously described [23]. Briefly, after change to fresh explant medium at ambient temperature, culture dishes containing cells or explants were sealed and placed into the luminometer, which was kept inside a standard tissue culture incubator at 36°C. Bioluminescence from each dish was continuously recorded with a photomultiplier tube (PMT) for ∼70 sec at intervals of 10 minutes. Raw data (counts/sec) were plotted against time (days) in culture. For analysis of rhythm parameters, we used the LumiCycle Analysis program (Actimetrics). Raw data were baseline fitted, and the baseline-subtracted data were fitted to a sine wave (damped), from which the period was determined. For samples that showed persistent rhythms, goodness-of-fit of >80% was usually achieved. Due to high transient luminescence upon medium change, the first cycle was usually excluded from rhythm analysis. For FFT spectral analysis (RelAmp) of Bmal1-dLuc oscillations, LumiCycle Analysis version 2.10 was used, in which polynomial order was set at 3 for background subtraction, the first cycle of data was usually excluded, Blackman-Harris windowing was checked (power spectrum unchecked), and circadian range was defined at 20–30 hr.