Effects of constant light on circadian rhythmicity in mice lacking functional cry genes: dissimilar from per mutants

Genetics and functional significance of the understudied methamphetamine sensitive circadian oscillator (MASCO)

The last 50 years have witnessed extraordinary discoveries in the field of circadian rhythms. However, there are still several mysteries that remain. One of these chronobiological mysteries is the circadian rhythm that is revealed by administration of stimulant drugs to rodents. Herein we describe the discovery of this circadian rhythm and its underlying oscillator, which is frequently called the methamphetamine-sensitive circadian oscillator, or MASCO. This oscillator is distinct from canonical circadian oscillators because it controls robust activity rhythms independently of the suprachiasmatic nucleus and circadian genes are not essential for its timekeeping. We discuss these fundamental properties of MASCO and integrate studies of strain, sex, and circadian gene mutations on MASCO. The anatomical loci of MASCO are not known, so it has not been possible thus far to discover its novel molecular timekeeping mechanism or its functional significance. However, studies in mutant mice suggest that genetic approaches can be used to identify the neural network involved in the rhythm generation of MASCO. We also discuss parallels between human and rodent studies that support our working hypothesis that a function of MASCO may be to regulate sleep-wake cycles.

Constant light enhances synchrony among circadian clock cells and promotes behavioral rhythms in VPAC2-signaling deficient mice

Individual neurons in the suprachiasmatic nuclei (SCN) contain an intracellular molecular clock and use intercellular signaling to synchronize their timekeeping activities so that the SCN can coordinate brain physiology and behavior. The neuropeptide vasoactive intestinal polypeptide (VIP) and its VPAC₂ receptor form a key component of intercellular signaling systems in the SCN and critically control cellular coupling. Targeted mutations in either the intracellular clock or intercellular neuropeptide signaling mechanisms, such as VIP-VPAC₂ signaling, can lead to desynchronization of SCN neuronal clocks and loss of behavioral rhythms. An important goal in chronobiology is to develop interventions to correct deficiencies in circadian timekeeping. Here we show that extended exposure to constant light promotes synchrony among SCN clock cells and the expression of ~24 h rhythms in behavior in mice in which intercellular signaling is disrupted through loss of VIP-VPAC₂ signaling. This study highlights the importance of SCN synchrony for the expression of rhythms in behavior and reveals how non-invasive manipulations in the external environment can be used to overcome neurochemical communication deficits in this important brain system.

Cry1-/- circadian rhythmicity depends on SCN intercellular coupling

In mammals, the suprachiasmatic nucleus (SCN) is the central pacemaker organizing circadian rhythms of behavior and physiology. At the cellular level, the mammalian clock consists of autoregulatory feedback loops involving a set of "clock genes," including the Cryptochrome (Cry) genes, Cry1 and Cry2. Experimental evidence suggests that Cry1 and Cry2 play distinct roles in circadian clock function. In mice, Cry1 is required for sustained circadian rhythms in dissociated SCN neurons or fibroblasts but not in organotypic SCN slices or at the behavioral level, whereas Cry2 is not required at any of these levels. It has been argued that coupling among SCN cellular oscillators compensates for clock gene defects to preserve oscillatory function. Here we test this hypothesis in Cry1(-/-) mice by first disrupting intercellular coupling in vivo using constant light (resulting in behavioral arrhythmicity) and then examining circadian clock gene expression in SCN slices at the single cell level. In this manner, we were able to test the role of intercellular coupling without drugs and while preserving tissue organization, avoiding the confounding influences of more invasive manipulations. Cry1(-/-) mice (as well as control Cry2(-/-) mice) bearing the PER2::LUC knock-in reporter were transferred from a standard light:dark cycle to constant bright light (~650 lux) to induce arrhythmic locomotor patterns. In SCN slices from these animals, we used bioluminescence imaging to monitor PER2::LUC expression in single cells. We show that SCN slices from rhythmic Cry1(-/-) and Cry2(-/-) mice had similarly high percentages of functional single-cell oscillators. In contrast, SCN slices from arrhythmic Cry1(-/-) mice had significantly fewer rhythmic cells than SCN slices from arrhythmic Cry2(-/-) mice. Thus, constant light in vivo disrupted intercellular SCN coupling to reveal a cell-autonomous circadian defect in Cry1(-/-) cells that is normally compensated by intercellular coupling in vivo.

Genetic redundancy strengthens the circadian clock leading to a narrow entrainment range

Circadian clocks are internal timekeepers present in almost all organisms. Driven by a genetic network of highly conserved structure, they generate self-sustained oscillations that entrain to periodic external signals such as the 24 h light-dark cycle. Vertebrates possess multiple, functionally overlapping homologues of the core clock genes. Furthermore, vertebrate clocks entrain to a range of periods three times as narrow as that of other organisms. We asked whether genetic redundancies play a role in governing entrainment properties and analysed locomotor activity rhythms of genetically modified mice lacking one set of clock homologues. Exposing them to non-24 h light-dark cycles, we found that the mutant mice have a wider entrainment range than the wild types. Spectral analysis furthermore revealed nonlinear phenomena of periodically forced self-sustained oscillators for which the entrainment range relates inversely to oscillator amplitude. Using the forced oscillator model to explain the observed differences in entrainment range between mutant and wild-type mice, we sought to quantify the overall oscillator amplitude of their clocks from the activity rhythms and found that mutant mice have weaker circadian clocks than wild types. Our results suggest that genetic redundancy strengthens the circadian clock leading to a narrow entrainment range in vertebrates.

Distinct patterns of Period gene expression in the suprachiasmatic nucleus underlie circadian clock photoentrainment by advances or delays

The circadian clock in the mammalian hypothalamic suprachiasmatic nucleus (SCN) is entrained by the ambient light/dark cycle, which differentially acts to cause the clock to advance or delay. Light-induced changes in the rhythmic expression of SCN clock genes are believed to be a critical step in this process, but how the two entrainment modalities--advances vs. delays--engage the molecular clockwork remains incompletely understood. We investigated molecular substrates of photic entrainment of the clock in the SCN by stably entraining hamsters to T cycles (non-24-h light/dark cycles) consisting of a single 1-h light pulse repeated as either a short (23.33-h) or a long (24.67-h) cycle; under these conditions, the light pulse of the short cycle acts as "dawn," whereas that of the long cycle acts as "dusk." Analyses of the expression of the photoinducible and rhythmic clock genes Period 1 and 2 (Per1 and Per2) in the SCN revealed fundamental differences under these two entrainment modes. Light at dawn advanced the clock, advancing the onset of the Per1 mRNA rhythm and acutely increasing mRNA transcription, whereas light at dusk delayed the clock, delaying the offset of the Per2 mRNA rhythm and tonically increasing mRNA stability. The results suggest that the underlying molecular mechanisms of circadian entrainment differ with morning (advancing) or evening (delaying) light exposure, and such differences may reflect how entrainment takes place in nocturnal animals under natural conditions.

Lab mice in the field: unorthodox daily activity and effects of a dysfunctional circadian clock allele

Daily patterns of animal behavior are potentially of vast functional importance. Fitness benefits have been identified in nature by the association between individual timing and survival or by the fate of individuals after experimental deletion of their circadian pacemaker. The recent advances in unraveling the molecular basis of circadian timing enable new approaches to natural selection on timing. The investigators report on the effect and fate of the mutant Per2(Brdm1) allele in 4 replicate populations of house mice in a seminatural outside environment over 2 years. This allele is known to compromise circadian organization and entrainment and to cause multiple physiological disturbances. Mice (N=250) bred from Per2(Brdm1) heterozygotes were implanted subcutaneously with transponders and released in approximately Mendelian ratios in four 400 m(2) pens. An electronic system stored the times of all visits to feeders of each individual. The study first demonstrates that mice are not explicitly nocturnal in this natural environment. Feeding activity was predominantly and sometimes exclusively diurnal and spread nearly equally over day and night under the protective snow cover in winter. The effect of Per2(Brdm1) on activity timing is negligible compared to seasonal changes in all genotypes. Second, the Per2(Brdm1) allele did not have persistent negative effects on fitness. In the first year, the allele gradually became less frequent by reducing survival. New cohorts captured had the same Per2(Brdm1) frequency as the survivors from previous cohorts, consistent with an absence of an effect on reproduction. In the second year, the allele recovered to about its initial frequency (0.54). These changes in selective advantage were primarily due to female mice, as females lived longer and the sex ratio dropped to about 25% males in the population. While it is unknown which selective advantage led to the recovery, the results caution against inferences from laboratory experiments on fitness consequences in the natural environment. It also demonstrates that the activity of mice, while strictly nocturnal in the laboratory, may be partially or completely diurnal in the field. The new method allows assessment of natural selection on specific alleles on a day-today basis.

A diversity of paracrine signals sustains molecular circadian cycling in suprachiasmatic nucleus circuits

The suprachiasmatic nucleus (SCN) is the principal circadian pacemaker of mammals, coordinating daily rhythms of behavior and metabolism. Circadian timekeeping in SCN neurons revolves around transcriptional/posttranslational feedback loops, in which Period (Per) and Cryptochrome (Cry) genes are negatively regulated by their protein products. Recent studies have revealed, however, that these "core loops" also rely upon cytosolic and circuit-level properties for sustained oscillation. To characterize interneuronal signals responsible for robust pacemaking in SCN cells and circuits, we have developed a unique coculture technique using wild-type (WT) "graft" SCN to drive pacemaking (reported by PER2::LUCIFERASE bioluminescence) in "host" SCN deficient either in elements of neuropeptidergic signaling or in elements of the core feedback loop. We demonstrate that paracrine signaling is sufficient to restore cellular synchrony and amplitude of pacemaking in SCN circuits lacking vasoactive intestinal peptide (VIP). By using grafts with mutant circadian periods we show that pacemaking in the host SCN is specified by the genotype of the graft, confirming graft-derived factors as determinants of the host rhythm. By combining pharmacological with genetic manipulations, we show that a hierarchy of neuropeptidergic signals underpins this paracrine regulation, with a preeminent role for VIP augmented by contributions from arginine vasopressin (AVP) and gastrin-releasing peptide (GRP). Finally, we show that interneuronal signaling is sufficiently powerful to maintain circadian pacemaking in arrhythmic Cry-null SCN, deficient in essential elements of the transcriptional negative feedback loops. Thus, a hierarchy of paracrine neuropeptidergic signals determines cell- and circuit-level circadian pacemaking in the SCN.

Photic entrainment of period mutant mice is predicted from their phase response curves

A fundamental property of circadian clocks is that they entrain to environmental cues. The circadian genes, Period1 and Period2, are involved in entrainment of the mammalian circadian system. To investigate the roles of the Period genes in photic entrainment, we constructed phase response curves (PRC) to light pulses for C57BL/6J wild-type, Per1(-/-), Per2(-/-), and Per3(-/-) mice and tested whether the PRCs accurately predict entrainment to non-24 light-dark cycles (T-cycles) and constant light (LL). The PRCs of wild-type and Per3(-/-) mice are similar in shape and amplitude and have relatively large delay zones and small advance zones, resulting in successful entrainment to 26 h T-cycles (T26), but not T21, with similar phase angles. Per1(-/-) mice have a high-amplitude PRC, resulting in entrainment to a broad range of T-cycles. Per2(-/-) mice also entrain to a wide range of T-cycles because the advance portion of their PRC is larger than wild types. Period aftereffects following entrainment to T-cycles were similar among all genotypes. We found that the ratio of the advance portion to the delay portion of the PRC accurately predicts the lengthening of the period of the activity rhythm in LL. Wild-type, Per1(-/-), and Per3(-/-) mice had larger delay zones than advance zones and lengthened (>24 h) periods in LL, whereas Per2(-/-) mice had delay and advance zones that were equal in size and no period lengthening in LL. Together, these results demonstrate that PRCs are powerful tools for predicting and understanding photic entrainment of circadian mutant mice.

CRY2 is associated with rapid cycling in bipolar disorder patients

Background

Bipolar disorder patients often display abnormalities in circadian rhythm, and they are sensitive to irregular diurnal rhythms. CRY2 participates in the core clock that generates circadian rhythms. CRY2 mRNA expression in blood mononuclear cells was recently shown to display a marked diurnal variation and to respond to total sleep deprivation in healthy human volunteers. It was also shown that bipolar patients in a depressive state had lower CRY2 mRNA levels, nonresponsive to total sleep deprivation, compared to healthy controls, and that CRY2 gene variation was associated with winter depression in both Swedish and Finnish cohorts.

Principal Findings

Four CRY2 SNPs spanning from intron 2 to downstream 3′UTR were analyzed for association to bipolar disorder type 1 (n = 497), bipolar disorder type 2 (n = 60) and bipolar disorder with the feature rapid cycling (n = 155) versus blood donors (n = 1044) in Sweden. Also, the rapid cycling cases were compared with bipolar disorder cases without rapid cycling (n = 422). The haplotype GGAC was underrepresented among rapid cycling cases versus controls and versus bipolar disorder cases without rapid cycling (OR = 0.7, P = 0.006−0.02), whereas overrepresentation among rapid cycling cases was seen for AAAC (OR = 1.3−1.4, P = 0.03−0.04) and AGGA (OR = 1.5, P = 0.05). The risk and protective CRY2 haplotypes and their effect sizes were similar to those recently suggested to be associated with winter depression in Swedes.

Conclusions

We propose that the circadian gene CRY2 is associated with rapid cycling in bipolar disorder. This is the first time a clock gene is implicated in rapid cycling, and one of few findings showing a molecular discrimination between rapid cycling and other forms of bipolar disorder.

The Mammalian Circadian Timing System: Organization and Coordination of Central and Peripheral Clocks

Most physiology and behavior of mammalian organisms follow daily oscillations. These rhythmic processes are governed by environmental cues (e.g., fluctuations in light intensity and temperature), an internal circadian timing system, and the interaction between this timekeeping system and environmental signals. In mammals, the circadian timekeeping system has a complex architecture, composed of a central pacemaker in the brain's suprachiasmatic nuclei (SCN) and subsidiary clocks in nearly every body cell. The central clock is synchronized to geophysical time mainly via photic cues perceived by the retina and transmitted by electrical signals to SCN neurons. In turn, the SCN influences circadian physiology and behavior via neuronal and humoral cues and via the synchronization of local oscillators that are operative in the cells of most organs and tissues. Thus, some of the SCN output pathways serve as input pathways for peripheral tissues. Here we discuss knowledge acquired during the past few years on the complex structure and function of the mammalian circadian timing system.

CRY2 is associated with depression

Background

Abnormalities in the circadian clockwork often characterize patients with major depressive and bipolar disorders. Circadian clock genes are targets of interest in these patients. CRY2 is a circadian gene that participates in regulation of the evening oscillator. This is of interest in mood disorders where a lack of switch from evening to morning oscillators has been postulated.

Principal Findings

We observed a marked diurnal variation in human CRY2 mRNA levels from peripheral blood mononuclear cells and a significant up-regulation (P = 0.020) following one-night total sleep deprivation, a known antidepressant. In depressed bipolar patients, levels of CRY2 mRNA were decreased (P = 0.029) and a complete lack of increase was observed following sleep deprivation. To investigate a possible genetic contribution, we undertook SNP genotyping of the CRY2 gene in two independent population-based samples from Sweden (118 cases and 1011 controls) and Finland (86 cases and 1096 controls). The CRY2 gene was significantly associated with winter depression in both samples (haplotype analysis in Swedish and Finnish samples: OR = 1.8, P = 0.0059 and OR = 1.8, P = 0.00044, respectively).

Conclusions

We propose that a CRY2 locus is associated with vulnerability for depression, and that mechanisms of action involve dysregulation of CRY2 expression.

Does the Morning and Evening Oscillator Model Fit Better for Flies or Mice?

The morning and evening dual oscillator model can explain the adaptation of animals to different photoperiods and other phenomena as bimodal activity patterns, aftereffects, and internal desynchronization of the activity rhythm into 2 free-running components. This review summarizes evidence for and against the existence of morning and evening oscillator cells in the core circadian pacemaker centers of mice and fruit flies.

SCN-AVP release of mPer1/mPer2 double-mutant mice in vitro

Background

Circadian organisation of behavioural and physiological rhythms in mammals is largely driven by the clock in the suprachiasmatic nuclei (SCN) of the hypothalamus. In this clock, a molecular transcriptional repression and activation mechanism generates near 24 hour rhythms. One of the outputs of the molecular clock in specific SCN neurons is arginine-vasopressin (AVP), which is responsive to transcriptional activation by clock gene products. As negative regulators, the protein products of the period genes are thought to repress transcriptional activity of the positive limb after heterodimerisation with CRYPTOCHROME. When both the Per1 and Per2 genes are dysfunctional by targeted deletion of the PAS heterodimer binding domain, mice lose circadian organization of behaviour upon release into constant environmental conditions. To which degree the period genes are involved in the control of AVP output is unknown.

Methods

Using an in vitro slice culture setup, SCN-AVP release of cultures made of 10 wildtype and 9 Per1/2 double-mutant mice was assayed. Mice were sacrificed in either the early light phase of the light-dark cycle, or in the early subjective day on the first day of constant dark.

Results

Here we report that in arrhythmic homozygous Per1/2 double-mutant mice there is still a diurnal peak in in vitro AVP release from the SCN similar to that of wildtypes but distinctively different from the release pattern from the paraventricular nucleus. Such a modulation of AVP release is unexpected in mice where the circadian clockwork is thought to be disrupted.

Conclusion

Our results suggest that the circadian clock in these animals, although deficient in (most) behavioural and molecular rhythms, may still be (partially) functional, possibly as an hourglass mechanism. The level of perturbation of the clock in Per1/2 double mutants may therefore be less than was originally thought.