Disrupted sleep-wake regulation in the MCI-Park mouse model of Parkinson's disease

Disrupted sleep has a profound adverse impact on lives of Parkinson's disease (PD) patients and their caregivers. Sleep disturbances are exceedingly common in PD, with substantial heterogeneity in type, timing, and severity. Among the most common sleep-related symptoms reported by PD patients are insomnia, excessive daytime sleepiness, and sleep fragmentation, characterized by interruptions and decreased continuity of sleep. Alterations in brain wave activity, as measured on the electroencephalogram (EEG), also occur in PD, with changes in the pattern and relative contributions of different frequency bands of the EEG spectrum to overall EEG activity in different vigilance states consistently observed. The mechanisms underlying these PD-associated sleep-wake abnormalities are poorly understood, and they are ineffectively treated by conventional PD therapies. To help fill this gap in knowledge, a new progressive model of PD - the MCI-Park mouse - was studied. Near the transition to the parkinsonian state, these mice exhibited significantly altered sleep-wake regulation, including increased wakefulness, decreased non-rapid eye movement (NREM) sleep, increased sleep fragmentation, reduced rapid eye movement (REM) sleep, and altered EEG activity patterns. These sleep-wake abnormalities resemble those identified in PD patients. Thus, this model may help elucidate the circuit mechanisms underlying sleep disruption in PD and identify targets for novel therapeutic approaches.

Ruminiclostridium 5, Parabacteroides distasonis, and bile acid profile are modulated by prebiotic diet and associate with facilitated sleep/clock realignment after chronic disruption of rhythms

Chronic disruption of rhythms (CDR) impacts sleep and can result in circadian misalignment of physiological systems which, in turn, is associated with increased disease risk. Exposure to repeated or severe stressors also disturbs sleep and diurnal rhythms. Prebiotic nutrients produce favorable changes in gut microbial ecology, the gut metabolome, and reduce several negative impacts of acute severe stressor exposure, including disturbed sleep, core body temperature rhythmicity, and gut microbial dysbiosis. In light of previous compelling evidence that prebiotic diet broadly reduces negative impacts of acute, severe stressors, we hypothesize that prebiotic diet will also effectively mitigate the negative impacts of chronic disruption of circadian rhythms on physiology and sleep/wake behavior. Male, Sprague Dawley rats were fed diets enriched in prebiotic substrates or calorically matched control chow. After 5 weeks on diet, rats were exposed to CDR (12 h light/dark reversal, weekly for 8 weeks) or remained on undisturbed normal light/dark cycles (NLD). Sleep EEG, core body temperature, and locomotor activity were recorded via biotelemetry in freely moving rats. Fecal samples were collected on experimental days -33, 0 (day of onset of CDR), and 42. Taxonomic identification and relative abundances of gut microbes were measured in fecal samples using 16S rRNA gene sequencing and shotgun metagenomics. Fecal primary, bacterially modified secondary, and conjugated bile acids were measured using liquid chromatography with tandem mass spectrometry (LC-MS/MS). Prebiotic diet produced rapid and stable increases in the relative abundances of Parabacteroides distasonis and Ruminiclostridium 5. Shotgun metagenomics analyses confirmed reliable increases in relative abundances of Parabacteroides distasonis and Clostridium leptum, a member of the Ruminiclostridium genus. Prebiotic diet also modified fecal bile acid profiles; and based on correlational and step-wise regression analyses, Parabacteroides distasonis and Ruminiclostridium 5 were positively associated with each other and negatively associated with secondary and conjugated bile acids. Prebiotic diet, but not CDR, impacted beta diversity. Measures of alpha diversity evenness were decreased by CDR and prebiotic diet prevented that effect. Rats exposed to CDR while eating prebiotic, compared to control diet, more quickly realigned NREM sleep and core body temperature (ClockLab) diurnal rhythms to the altered light/dark cycle. Finally, both cholic acid and Ruminiclostridium 5 prior to CDR were associated with time to realign CBT rhythms to the new light/dark cycle after CDR; whereas both Ruminiclostridium 5 and taurocholic acid prior to CDR were associated with NREM sleep recovery after CDR. These results support our hypothesis and suggest that ingestion of prebiotic substrates is an effective strategy to increase the relative abundance of health promoting microbes, alter the fecal bile acid profile, and facilitate the recovery and realignment of sleep and diurnal rhythms after circadian disruption.

Fundamental Biological Features of Spaceflight: Advancing the Field to Enable Deep-Space Exploration

Research on astronaut health and model organisms have revealed six features of spaceflight biology that guide our current understanding of fundamental molecular changes that occur during space travel. The features include oxidative stress, DNA damage, mitochondrial dysregulation, epigenetic changes (including gene regulation), telomere length alterations, and microbiome shifts. Here we review the known hazards of human spaceflight, how spaceflight affects living systems through these six fundamental features, and the associated health risks of space exploration. We also discuss the essential issues related to the health and safety of astronauts involved in future missions, especially planned long-duration and Martian missions.

Resource competition shapes biological rhythms and promotes temporal niche differentiation in a community simulation

Competition for resources often contributes strongly to defining an organism's ecological niche. Endogenous biological rhythms are important adaptations to the temporal dimension of niches, but how other organisms influence such temporal niches has not been much studied, and the role of competition in particular has been even less examined. We investigated how interspecific competition and intraspecific competition for resources shape an organism's activity rhythms.To do this, we simulated communities of one or two species in an agent-based model. Individuals in the simulation move according to a circadian activity rhythm and compete for limited resources. Probability of reproduction is proportional to an individual's success in obtaining resources. Offspring may have variance in rhythm parameters, which allow for the population to evolve over time.We demonstrate that when organisms are arrhythmic, one species will always be competitively excluded from the environment, but the existence of activity rhythms allows niche differentiation and indefinite coexistence of the two species. Two species which are initially active at the same phase will differentiate their phase angle of entrainment over time to avoid each other. When only one species is present in an environment, competition within the species strongly selects for niche expansion through arrhythmicity, but the addition of an interspecific competitor facilitates evolution of increased rhythmic amplitude when combined with additional adaptations for temporal specialization. Finally, if individuals preferentially mate with others who are active at similar times of day, then disruptive selection by intraspecific competition can split one population into two reproductively isolated groups separated in activity time.These simulations suggest that biological rhythms are an effective method to temporally differentiate ecological niches and that competition is an important ecological pressure promoting the evolution of rhythms and sleep. This is the first study to use ecological modeling to examine biological rhythms.

Reproducible changes in the gut microbiome suggest a shift in microbial and host metabolism during spaceflight

BACKGROUND

Space environment imposes a range of challenges to mammalian physiology and the gut microbiota, and interactions between the two are thought to be important in mammalian health in space. While previous findings have demonstrated a change in the gut microbial community structure during spaceflight, specific environmental factors that alter the gut microbiome and the functional relevance of the microbiome changes during spaceflight remain elusive.

METHODS

We profiled the microbiome using 16S rRNA gene amplicon sequencing in fecal samples collected from mice after a 37-day spaceflight onboard the International Space Station. We developed an analytical tool, named STARMAPs (Similarity Test for Accordant and Reproducible Microbiome Abundance Patterns), to compare microbiome changes reported here to other relevant datasets. We also integrated the gut microbiome data with the publically available transcriptomic data in the liver of the same animals for a systems-level analysis.

RESULTS

We report an elevated microbiome alpha diversity and an altered microbial community structure that were associated with spaceflight environment. Using STARMAPs, we found the observed microbiome changes shared similarity with data reported in mice flown in a previous space shuttle mission, suggesting reproducibility of the effects of spaceflight on the gut microbiome. However, such changes were not comparable with those induced by space-type radiation in Earth-based studies. We found spaceflight led to significantly altered taxon abundance in one order, one family, five genera, and six species of microbes. This was accompanied by a change in the inferred microbial gene abundance that suggests an altered capacity in energy metabolism. Finally, we identified host genes whose expression in the liver were concordantly altered with the inferred gut microbial gene content, particularly highlighting a relationship between host genes involved in protein metabolism and microbial genes involved in putrescine degradation.

CONCLUSIONS

These observations shed light on the specific environmental factors that contributed to a robust effect on the gut microbiome during spaceflight with important implications for mammalian metabolism. Our findings represent a key step toward a better understanding the role of the gut microbiome in mammalian health during spaceflight and provide a basis for future efforts to develop microbiota-based countermeasures that mitigate risks to crew health during long-term human space expeditions.

Genetics of Circadian Rhythms

In mammals, genetic influences of circadian rhythms occur at many levels. A set of core "clock genes" have been identified that form a feedback loop of gene transcription and translation. The core genetic clockwork generates circadian rhythms in cells throughout the body. Polymorphisms in both core clock genes and interacting genes contribute to individual differences in the expression and properties of circadian rhythms. The circadian clock profoundly influences the patterns of gene expression and cellular functions, providing a mechanistic basis for the impact of the genetic circadian system on normal physiological processes as well as the development of diseases.

The NASA Twins Study: A multidimensional analysis of a year-long human spaceflight

To understand the health impact of long-duration spaceflight, one identical twin astronaut was monitored before, during, and after a 1-year mission onboard the International Space Station; his twin served as a genetically matched ground control. Longitudinal assessments identified spaceflight-specific changes, including decreased body mass, telomere elongation, genome instability, carotid artery distension and increased intima-media thickness, altered ocular structure, transcriptional and metabolic changes, DNA methylation changes in immune and oxidative stress-related pathways, gastrointestinal microbiota alterations, and some cognitive decline postflight. Although average telomere length, global gene expression, and microbiome changes returned to near preflight levels within 6 months after return to Earth, increased numbers of short telomeres were observed and expression of some genes was still disrupted. These multiomic, molecular, physiological, and behavioral datasets provide a valuable roadmap of the putative health risks for future human spaceflight.

Light/Dark Shifting Promotes Alcohol-Induced Colon Carcinogenesis: Possible Role of Intestinal Inflammatory Milieu and Microbiota

Background: Colorectal cancer (CRC) is associated with the modern lifestyle. Chronic alcohol consumption—a frequent habit of majority of modern societies—increases the risk of CRC. Our group showed that chronic alcohol consumption increases polyposis in a mouse mode of CRC. Here we assess the effect of circadian disruption—another modern life style habit—in promoting alcohol-associated CRC. Method: TS4Cre × adenomatous polyposis coli (APC)^lox468 mice underwent (a) an alcohol-containing diet while maintained on a normal 12 h light:12 h dark cycle; or (b) an alcohol-containing diet in conjunction with circadian disruption by once-weekly 12 h phase reversals of the light:dark (LD) cycle. Mice were sacrificed after eight weeks of full alcohol and/or LD shift to collect intestine samples. Tumor number, size, and histologic grades were compared between animal groups. Mast cell protease 2 (MCP2) and 6 (MCP6) histology score were analyzed and compared. Stool collected at baseline and after four weeks of experimental manipulations was used for microbiota analysis. Results: The combination of alcohol and LD shifting accelerated intestinal polyposis, with a significant increase in polyp size, and caused advanced neoplasia. Consistent with a pathogenic role of stromal tryptase-positive mast cells in colon carcinogenesis, the ratio of mMCP6 (stromal)/mMCP2 (intraepithelial) mast cells increased upon LD shifting. Baseline microbiota was similar between groups, and experimental manipulations resulted in a significant difference in the microbiota composition between groups. Conclusions: Circadian disruption by Light:dark shifting exacerbates alcohol-induced polyposis and CRC. Effect of circadian disruption could, at least partly, be mediated by promoting a pro-tumorigenic inflammatory milieu via changes in microbiota.

Night workers with circadian misalignment are susceptible to alcohol-induced intestinal hyperpermeability with social drinking

Alcohol-induced intestinal hyperpermeability (AIHP) is a known risk factor for alcoholic liver disease (ALD), but only 20-30% of heavy alcoholics develop AIHP and ALD. The hypothesis of this study is that circadian misalignment would promote AIHP. We studied two groups of healthy subjects on a stable work schedule for 3 mo [day workers (DW) and night workers (NW)]. Subjects underwent two circadian phase assessments with sugar challenge to access intestinal permeability between which they drank 0.5 g/kg alcohol daily for 7 days. Sleep architecture by actigraphy did not differ at baseline or after alcohol between either group. After alcohol, the dim light melatonin onset (DLMO) in the DW group did not change significantly, but in the NW group there was a significant 2-h phase delay. Both the NW and DW groups had no change in small bowel permeability with alcohol, but only in the NW group was there an increase in colonic and whole gut permeability. A lower area under the curve of melatonin inversely correlated with increased colonic permeability. Alcohol also altered peripheral clock gene amplitude of peripheral blood mononuclear cells in CLOCK, BMAL, PER1, CRY1, and CRY2 in both groups, and inflammatory markers lipopolysaccharide-binding protein, LPS, and IL-6 had an elevated mesor at baseline in NW vs. DW and became arrhythmic with alcohol consumption. Together, our data suggest that central circadian misalignment is a previously unappreciated risk factor for AIHP and that night workers may be at increased risk for developing liver injury with alcohol consumption.

Osteoarthritis-like pathologic changes in the knee joint induced by environmental disruption of circadian rhythms is potentiated by a high-fat diet

A variety of environmental factors contribute to progressive development of osteoarthritis (OA). Environmental factors that upset circadian rhythms have been linked to various diseases. Our recent work establishes chronic environmental circadian disruption - analogous to rotating shiftwork-associated disruption of circadian rhythms in humans - as a novel risk factor for the development of OA. Evidence suggests shift workers are prone to obesity and also show altered eating habits (i.e., increased preference for high-fat containing food). In the present study, we investigated the impact of chronic circadian rhythm disruption in combination with a high-fat diet (HFD) on progression of OA in a mouse model. Our study demonstrates that when mice with chronically circadian rhythms were fed a HFD, there was a significant proteoglycan (PG) loss and fibrillation in knee joint as well as increased activation of the expression of the catabolic mediators involved in cartilage homeostasis. Our results, for the first time, provide the evidence that environmental disruption of circadian rhythms plus HFD potentiate OA-like pathological changes in the mouse joints. Thus, our findings may open new perspectives on the interactions of chronic circadian rhythms disruption with diet in the development of OA and may have potential clinical implications.

Environmental disruption of circadian rhythm predisposes mice to osteoarthritis-like changes in knee joint

Circadian rhythm dysfunction is linked to many diseases, yet pathophysiological roles in articular cartilage homeostasis and degenerative joint disease including osteoarthritis (OA) remains to be investigated in vivo. Here, we tested whether environmental or genetic disruption of circadian homeostasis predisposes to OA-like pathological changes. Male mice were examined for circadian locomotor activity upon changes in the light:dark (LD) cycle or genetic disruption of circadian rhythms. Wild-type (WT) mice were maintained on a constant 12 h:12 h LD cycle (12:12 LD) or exposed to weekly 12 h phase shifts. Alternatively, male circadian mutant mice (Clock(Δ19) or Csnk1e(tau) mutants) were compared with age-matched WT littermates that were maintained on a constant 12:12 LD cycle. Disruption of circadian rhythms promoted osteoarthritic changes by suppressing proteoglycan accumulation, upregulating matrix-degrading enzymes and downregulating anabolic mediators in the mouse knee joint. Mechanistically, these effects involved activation of the PKCδ-ERK-RUNX2/NFκB and β-catenin signaling pathways, stimulation of MMP-13 and ADAMTS-5, as well as suppression of the anabolic mediators SOX9 and TIMP-3 in articular chondrocytes of phase-shifted mice. Genetic disruption of circadian homeostasis does not predispose to OA-like pathological changes in joints. Our results, for the first time, provide compelling in vivo evidence that environmental disruption of circadian rhythms is a risk factor for the development of OA-like pathological changes in the mouse knee joint.

Validation of video motion-detection scoring of forced swim test in mice

BACKGROUND

The forced swim test (FST) is used to predict the effectiveness of novel antidepressant treatments. In this test, a mouse or rat is placed in a beaker of water for several minutes, and the amount of time spent passively floating is measured; antidepressants reduce the amount of such immobility. Though the FST is commonly used, manually scoring the test is time-consuming and involves considerable subjectivity.

NEW METHOD

We developed a simple MATLAB-based motion-detection method to quantify mice's activity in videos of FST. FST trials are video-recorded from a side view. Each pixel of the video is compared between subsequent video frames; if the pixel's color difference surpasses a threshold, a motion count is recorded.

RESULTS

Human-scored immobility time correlates well with total motion detected by the computer (r=-0.80) and immobility time determined by the computer (r=0.83). Our computer method successfully detects group differences in activity between genotypes and different days of testing. Furthermore, we observe heterosis for this behavior, in which (C57BL/6J×A/J) F1 hybrid mice are more active in the FST than the parental strains.

COMPARISON WITH EXISTING METHODS

This computer-scoring method is much faster and more objective than human scoring. Other automatic scoring methods exist, but they require the purchase of expensive hardware and/or software.

CONCLUSION

This computer-scoring method is an effective, fast, and low-cost method of quantifying the FST. It is validated by replicating statistical differences observed in traditional visual scoring. We also demonstrate a case of heterosis in the FST.

The circadian clock gene Csnk1e regulates rapid eye movement sleep amount, and nonrapid eye movement sleep architecture in mice

STUDY OBJECTIVES

Efforts to identify the genetic basis of mammalian sleep have included quantitative trait locus (QTL) mapping and gene targeting of known core circadian clock genes. We combined three different genetic approaches to identify and test a positional candidate sleep gene - the circadian gene casein kinase 1 epsilon (Csnk1e), which is located in a QTL we identified for rapid eye movement (REM) sleep on chromosome 15.

MEASUREMENTS AND RESULTS

Using electroencephalographic (EEG) and electromyographic (EMG) recordings, baseline sleep was examined in a 12-h light:12-h dark (LD 12:12) cycle in mice of seven genotypes, including Csnk1e(tau/tau) and Csnk1e(-/-) mutant mice, Csnk1e (B6.D2) and Csnk1e (D2.B6) congenic mice, and their respective wild-type littermate control mice. Additionally, Csnk1e(tau/tau) and wild-type mice were examined in constant darkness (DD). Csnk1e(tau/tau) mutant mice and both Csnk1e (B6.D2) and Csnk1e (D2.B6) congenic mice showed significantly higher proportion of sleep time spent in REM sleep during the dark period than wild-type controls - the original phenotype for which the QTL on chromosome 15 was identified. This phenotype persisted in Csnk1e(tau/tau) mice while under free-running DD conditions. Other sleep phenotypes observed in Csnk1e(tau/tau) mice and congenics included a decreased number of bouts of nonrapid eye movement (NREM) sleep and an increased average NREM sleep bout duration.

CONCLUSIONS

These results demonstrate a role for Csnk1e in regulating not only the timing of sleep, but also the REM sleep amount and NREM sleep architecture, and support Csnk1e as a causal gene in the sleep QTL on chromosome 15.

Disruption of the Circadian Clock in Mice Increases Intestinal Permeability and Promotes Alcohol-Induced Hepatic Pathology and Inflammation

The circadian clock orchestrates temporal patterns of physiology and behavior relative to the environmental light:dark cycle by generating and organizing transcriptional and biochemical rhythms in cells and tissues throughout the body. Circadian clock genes have been shown to regulate the physiology and function of the gastrointestinal tract. Disruption of the intestinal epithelial barrier enables the translocation of proinflammatory bacterial products, such as endotoxin, across the intestinal wall and into systemic circulation; a process that has been linked to pathologic inflammatory states associated with metabolic, hepatic, cardiovascular and neurodegenerative diseases – many of which are commonly reported in shift workers. Here we report, for the first time, that circadian disorganization, using independent genetic and environmental strategies, increases permeability of the intestinal epithelial barrier (i.e., gut leakiness) in mice. Utilizing chronic alcohol consumption as a well-established model of induced intestinal hyperpermeability, we also found that both genetic and environmental circadian disruption promote alcohol-induced gut leakiness, endotoxemia and steatohepatitis, possibly through a mechanism involving the tight junction protein occludin. Circadian organization thus appears critical for the maintenance of intestinal barrier integrity, especially in the context of injurious agents, such as alcohol. Circadian disruption may therefore represent a previously unrecognized risk factor underlying the susceptibility to or development of alcoholic liver disease, as well as other conditions associated with intestinal hyperpermeability and an endotoxin-triggered inflammatory state.

Environmental perturbation of the circadian clock disrupts pregnancy in the mouse

Background

The circadian clock has been linked to reproduction at many levels in mammals. Epidemiological studies of female shift workers have reported increased rates of reproductive abnormalities and adverse pregnancy outcomes, although whether the cause is circadian disruption or another factor associated with shift work is unknown. Here we test whether environmental disruption of circadian rhythms, using repeated shifts of the light:dark (LD) cycle, adversely affects reproductive success in mice.

Methodology/Principal Findings

Young adult female C57BL/6J (B6) mice were paired with B6 males until copulation was verified by visual identification of vaginal plug formation. Females were then randomly assigned to one of three groups: control, phase-delay or phase-advance. Controls remained on a constant 12-hr light:12-hr dark cycle, whereas phase-delayed and phase-advanced mice were subjected to 6-hr delays or advances in the LD cycle every 5–6 days, respectively. The number of copulations resulting in term pregnancies was determined. Control females had a full-term pregnancy success rate of 90% (11/12), which fell to 50% (9/18; p<0.1) in the phase-delay group and 22% (4/18; p<0.01) in the phase-advance group.

Conclusions/Significance

Repeated shifting of the LD cycle, which disrupts endogenous circadian timekeeping, dramatically reduces pregnancy success in mice. Advances of the LD cycle have a greater negative impact on pregnancy outcomes and, in non-pregnant female mice, require longer for circadian re-entrainment, suggesting that the magnitude or duration of circadian misalignment may be related to the severity of the adverse impact on pregnancy. These results explicitly link disruptions of circadian entrainment to adverse pregnancy outcomes in mammals, which may have important implications for the reproductive health of female shift workers, women with circadian rhythm sleep disorders and/or women with disturbed circadian rhythms for other reasons.

High-resolution mapping of a novel genetic locus regulating voluntary physical activity in mice

Both human beings and animals exhibit substantial inter-individual variation in voluntary physical activity, and evidence indicates that a significant component of this variation is because of genetic factors. However, little is known of the genetic basis underlying central regulation of voluntary physical activity in mammals. In this study, using an F(2) intercross population and interval-specific congenic strains (ISCS) derived from the C57BL/6J strain and a chromosome 13 substitution strain, C57BL/6J-Chr13A/J/NA/J, we identified a 3.76-Mb interval on chromosome 13 containing 25 genes with a significant impact on daily voluntary wheel running activity in mice. Brain expression and polymorphisms between the C57BL/6J and A/J strains were examined to prioritize candidate genes. As the dopaminergic pathway regulates motor movement and motivational behaviors, we tested its function by examining cocaine-induced locomotor responses in ISCS with different levels of activity. The low-activity ISCS exhibited a significantly higher response to acute cocaine administration than the high-activity ISCS. Expression analysis of key dopamine-related genes (dopamine transporter and D1, D2, D3, D4 and D5 receptors) revealed that expression of D1 receptor was higher in the low-activity ISCS than in the high-activity ISCS in both the dorsal striatum and nucleus accumbens. Pathway analysis implicated Tcfap2a, a gene found within the 3.76-Mb interval, involved in the D1 receptor pathway. Using a luciferase reporter assay, we confirmed that the transcriptional factor, Tcfap2a, regulates the promoter activity of the D1 receptor gene. Thus, Tcfap2a is proposed as a candidate genetic regulator of the level of voluntary physical activity through its influence on a dopaminergic pathway.

Rhythmic leptin is required for weight gain from circadian desynchronized feeding in the mouse

The neuroendocrine and metabolic effects of leptin have been extensively researched since the discovery, and the later identification, of the leptin gene mutated within the ob/ob mouse. Leptin is required for optimal health in a number of physiological systems (e.g. fertility, bone density, body weight regulation). Despite the extensive leptin literature and many observations of leptin's cyclical pattern over the 24-hour day, few studies have specifically examined how the circadian rhythm of leptin may be essential to leptin signaling and health. Here we present data indicating that a rhythmic leptin profile (e.g. 1 peak every 24 hours) leads to excessive weight gain during desynchronized feeding whereas non-rhythmic leptin provided in a continuous manner does not lead to excessive body weight gain under similar feeding conditions. This study suggests that feeding time can interact with leptin's endogenous rhythm to influence metabolic signals, specifically leading to excessive body weight gains during 'wrongly' timed feeding.

The mouse Clock mutation reduces circadian pacemaker amplitude and enhances efficacy of resetting stimuli and phase-response curve amplitude

The mouse Clock gene encodes a basic helix-loop-helix-PAS transcription factor, CLOCK, that acts in concert with BMAL1 to form the positive elements of the circadian clock mechanism in mammals. The original Clock mutant allele is a dominant negative (antimorphic) mutation that deletes exon 19 and causes an internal deletion of 51 aa in the C-terminal activation domain of the CLOCK protein. Here we report that heterozygous Clock/+ mice exhibit high-amplitude phase-resetting responses to 6-h light pulses (Type 0 resetting) as compared with wild-type mice that have low amplitude (Type 1) phase resetting. The magnitude and time course of acute light induction in the suprachiasmatic nuclei of the only known light-induced core clock genes, Per1 and Per2, are not affected by the Clock/+ mutation. However, the amplitude of the circadian rhythms of Per gene expression are significantly reduced in Clock homozygous and heterozygous mutants. Rhythms of PER2::LUCIFERASE expression in suprachiasmatic nuclei explant cultures also are reduced in amplitude in Clock heterozygotes. The phase-response curves to changes in culture medium are Type 0 in Clock heterozygotes, but Type 1 in wild types, similar to that seen for light in vivo. The increased efficacy of resetting stimuli and decreased PER expression amplitude can be explained in a unified manner by a model in which the Clock mutation reduces circadian pacemaker amplitude in the suprachiasmatic nuclei.

Large-scale mutagenesis and phenotypic screens for the nervous system and behavior in mice

Significant developments have occurred in our understanding of the mammalian genome thanks to informatics, expression profiling and sequencing of the human and rodent genomes. However, although these facets of genomic analysis are being addressed, analysis of in vivo gene function remains a formidable task. Evaluation of the phenotype of mutants provides powerful access to gene function, and this approach is particularly relevant to the nervous system and behavior. Here, we discuss the complementary mouse genetic approaches of gene-driven, targeted mutagenesis and phenotype-driven, chemical mutagenesis. We highlight an NIH-supported large-scale effort to use phenotype-driven mutagenesis screens to identify mouse mutants with neural and behavioral alterations. Such single-gene mutations can then be used for gene identification using positional candidate gene-cloning methods.

Utilization of a whole genome SNP panel for efficient genetic mapping in the mouse

Phenotype-driven genetics can be used to create mouse models of human disease and birth defects. However, the utility of these mutant models is limited without identification of the causal gene. To facilitate genetic mapping, we developed a fixed single nucleotide polymorphism (SNP) panel of 394 SNPs as an alternative to analyses using simple sequence length polymorphism (SSLP) marker mapping. With the SNP panel, chromosomal locations for 22 monogenic mutants were identified. The average number of affected progeny genotyped for mapped monogenic mutations is nine. Map locations for several mutants have been obtained with as few as four affected progeny. The average size of genetic intervals obtained for these mutants is 43 Mb, with a range of 17-83 Mb. Thus, our SNP panel allows for identification of moderate resolution map position with small numbers of mice in a high-throughput manner. Importantly, the panel is suitable for mapping crosses from many inbred and wild-derived inbred strain combinations. The chromosomal localizations obtained with the SNP panel allow one to quickly distinguish between potentially novel loci or remutations in known genes, and facilitates fine mapping and positional cloning. By using this approach, we identified DNA sequence changes in two ethylnitrosourea-induced mutants.

Implementing large-scale ENU mutagenesis screens in North America

A step towards annotating the mouse genome is to use forward genetics in phenotype-driven screens to saturate the genome with mutations. The purpose of this article is to highlight the new projects in North America that are focused on isolating mouse mutations after ENU mutagenesis and phenotype screening.

Results from screening over 9000 mutation-bearing mice for defects in the electroretinogram and appearance of the fundus

Random mutagenesis combined with phenotypic screening using carefully crafted functional tests has successfully led to the discovery of genes that are essential for a number of functions. This approach does not require prior knowledge of the identity of the genes that are involved and is a way to ascribe function to the nearly 6000 genes for which knowledge of the DNA sequence has been inadequate to determine the function of the gene product. In an effort to identify genes involved in the visual system via this approach, we have tested over 9000 first and third generation offspring of mice treated with the mutagen N-ethyl-N-nitrosourea (ENU) for visual defects, as evidenced by abnormalities in the electroretinogram and appearance of the fundus. We identified 61 putative mutations with this procedure and outline the steps needed to identify the affected genes.

Effects of age on circadian rhythms are similar in wild-type and heterozygous Clock mutant mice

The amplitudes of many circadian rhythms, at the behavioral, physiological, cellular, and biochemical levels, decrease with advanced age. Previous studies suggest that the amplitude of the central circadian pacemaker is decreased in old animals. Recently, it has been reported that expression of several circadian clock genes, including Clock, is lower in the master circadian pacemaker of old rodents. To test the hypothesis that decreased activity of a circadian clock gene renders animals more susceptible to the effects of aging, we analyzed the circadian rhythm of locomotor activity in young and old wild-type and heterozygous Clock mutant mice. We found that the effects of age and the Clock mutation were additive. These results indicate that age-related changes in circadian rhythmicity occur equally in wild-type and heterozygous Clock mutants, suggesting that the Clock mutation does not render mice more susceptible to the effects of age on the circadian pacemaker.

Depressive-like behavior and stress reactivity are independent traits in a Wistar Kyoto x Fisher 344 cross

Depression is a heritable disorder that is often precipitated by stress. Abnormalities of the stress-reactive hypothalamic-pituitary-adrenal (HPA) axis are also common in depressed patients. In animal models, the forced swim test (FST) is the most frequently used test of depressive-like behavior. We have used a proposed animal model of depression, the Wistar Kyoto (WKY) rat, to investigate the relationship as well as the mode of inheritance of FST behaviors and HPA measures. Through reciprocal breeding of WKY and F344 parent strains and brother-sister breeding of the F1 generation, we obtained 486 F2 animals. Parent, F1 and F2 animals were tested in the FST. Blood samples were collected for determination of basal and stress (10-min restraint) plasma corticosterone (CORT) levels, and adrenal weights were measured. We found that all measures were heritable to some extent and that this heritability was highly sex dependent. Both correlation and factor analyses of the F2 generation data demonstrate that FST behavior and HPA axis measures are not directly related. Thus, the underlying genetic components of depressive-like behavior and HPA axis abnormalities are likely to be disparate in the segregating F2 generation of a WKY x F344 cross.

Genome-wide epistatic interaction analysis reveals complex genetic determinants of circadian behavior in mice

Genetic heterogeneity underlies many phenotypic variations observed in circadian rhythmicity. Continuous distributions in measures of circadian behavior observed among multiple inbred strains of mice suggest that the inherent contributions to variability are polygenic in nature. To identify genetic loci that underlie this complex behavior, we have carried out a genome-wide complex trait analysis in 196 (C57BL/6J X BALB/cJ)F(2) hybrid mice. We have characterized variation in this panel of F(2) mice among five circadian phenotypes: free-running circadian period, phase angle of entrainment, amplitude of the circadian rhythm, circadian activity level, and dissociation of rhythmicity. Our genetic analyses of these phenotypes have led to the identification of 14 loci having significant effects on this behavior, including significant main effect loci that contribute to three of these phenotypic measures: period, phase, and amplitude. We describe an additional locus detection method, genome-wide genetic interaction analysis, developed to identify locus pairs that may interact epistatically to significantly affect phenotype. Using this analysis, we identified two additional pairs of loci that have significant effects on dissociation and activity level; we also detected interaction effects in loci contributing to differences of period, phase, and amplitude. Although single gene mutations can affect circadian rhythms, the analysis of interstrain variants demonstrates that significant genetic complexity underlies this behavior. Importantly, most of the loci that we have detected by these methods map to locations that differ from the nine known clock genes, indicating the presence of additional clock-relevant genes in the mammalian circadian system. These data demonstrate the analytical value of both genome-wide complex trait and epistatic interaction analyses in further understanding complex phenotypes, and point to promising approaches for genetic analysis of such phenotypes in other mammals, including humans.

Overview of circadian rhythms

The daily light-dark cycle governs rhythmic changes in the behavior and/or physiology of most species. Studies have found that these changes are governed by a biological clock, which in mammals is located in two brain areas called the suprachiasmatic nuclei. The circadian cycles established by this clock occur throughout nature and have a period of approximately 24 hours. In addition, these circadian cycles can be synchronized to external time signals but also can persist in the absence of such signals. Studies have found that the internal clock consists of an array of genes and the protein products they encode, which regulate various physiological processes throughout the body. Disruptions of the biological rhythms can impair the health and well-being of the organism.

The circadian clock mutation alters sleep homeostasis in the mouse

The onset and duration of sleep are thought to be primarily under the control of a homeostatic mechanism affected by previous periods of wake and sleep and a circadian timing mechanism that partitions wake and sleep into different portions of the day and night. The mouse Clock mutation induces pronounced changes in overall circadian organization. We sought to determine whether this genetic disruption of circadian timing would affect sleep homeostasis. The Clock mutation affected a number of sleep parameters during entrainment to a 12 hr light/dark (LD 12:12) cycle, when animals were free-running in constant darkness (DD), and during recovery from 6 hr of sleep deprivation in LD 12:12. In particular, in LD 12:12, heterozygous and homozygous Clock mutants slept, respectively, approximately 1 and approximately 2 hr less than wild-type mice, and they had 25 and 51% smaller increases in rapid eye movement (REM) sleep during 24 hr recovery, respectively, than wild-type mice. The effects of the mutation on sleep are not readily attributable to differential entrainment to LD 12:12 because the baseline sleep differences between genotypes were also present when animals were free-running in DD. These results indicate that genetic alterations of the circadian clock system and/or its regulatory genes are likely to have widespread effects on a variety of sleep and wake parameters, including the homeostatic regulation of sleep.

Differential regulation of mammalian period genes and circadian rhythmicity by cryptochromes 1 and 2

Cryptochromes regulate the circadian clock in animals and plants. Humans and mice have two cryptochrome (Cry) genes. A previous study showed that mice lacking the Cry2 gene had reduced sensitivity to acute light induction of the circadian gene mPer1 in the suprachiasmatic nucleus (SCN) and had an intrinsic period 1 hr longer than normal. In this study, Cry1(-/-) and Cry1(-/-)Cry2(-/-) mice were generated and their circadian clocks were analyzed at behavioral and molecular levels. Behaviorally, the Cry1(-/-) mice had a circadian period 1 hr shorter than wild type and the Cry1(-/-)Cry2(-/-) mice were arrhythmic in constant darkness (DD). Biochemically, acute light induction of mPer1 mRNA in the SCN was blunted in Cry1(-/-) and abolished in Cry1(-/-)Cry2(-/-) mice. In contrast, the acute light induction of mPer2 in the SCN was intact in Cry1(-/-) and Cry1(-/-)Cry2(-/-) animals. Importantly, in double mutants, mPer1 expression was constitutively elevated and no rhythmicity was detected in either 12-hr light/12-hr dark or DD, whereas mPer2 expression appeared rhythmic in 12-hr light/12-hr dark, but nonrhythmic in DD with intermediate levels. These results demonstrate that Cry1 and Cry2 are required for the normal expression of circadian behavioral rhythms, as well as circadian rhythms of mPer1 and mPer2 in the SCN. The differential regulation of mPer1 and mPer2 by light in Cry double mutants reveals a surprising complexity in the role of cryptochromes in mammals.

Role of mouse cryptochrome blue-light photoreceptor in circadian photoresponses

Cryptochromes are photoactive pigments in the eye that have been proposed to function as circadian photopigments. Mice lacking the cryptochrome 2 blue-light photoreceptor gene (mCry2) were tested for circadian clock-related functions. The mutant mice had a lower sensitivity to acute light induction of mPer1 in the suprachiasmatic nucleus (SCN) but exhibited normal circadian oscillations of mPer1 and mCry1 messenger RNA in the SCN. Behaviorally, the mutants had an intrinsic circadian period about 1 hour longer than normal and exhibited high-amplitude phase shifts in response to light pulses administered at circadian time 17. These data are consistent with the hypothesis that CRY2 protein modulates circadian responses in mice and suggest that cryptochromes have a role in circadian photoreception in mammals.

Mammalian circadian autoregulatory loop: a timeless ortholog and mPer1 interact and negatively regulate CLOCK-BMAL1-induced transcription

We report the cloning and mapping of mouse (mTim) and human (hTIM) orthologs of the Drosophila timeless (dtim) gene. The mammalian Tim genes are widely expressed in a variety of tissues; however, unlike Drosophila, mTim mRNA levels do not oscillate in the suprachiasmatic nucleus (SCN) or retina. Importantly, hTIM interacts with the Drosophila PERIOD (dPER) protein as well as the mouse PER1 and PER2 proteins in vitro. In Drosophila (S2) cells, hTIM and dPER interact and translocate into the nucleus. Finally, hTIM and mPER1 specifically inhibit CLOCK-BMAL1-induced transactivation of the mPer1 promoter. Taken together, these results demonstrate that mTim and hTIM are mammalian orthologs of timeless and provide a framework for a basic circadian autoregulatory loop in mammals.

Automated measurement of mouse freezing behavior and its use for quantitative trait locus analysis of contextual fear conditioning in (BALB/cJ x C57BL/6J)F2 mice

The most commonly measured mouse behavior in fear conditioning tests is freezing. A technical limitation, particularly for genetic studies, is the method of direct observation used for quantifying this response, with the potential for bias or inconsistencies. We report the use of a computerized method based on latency between photobeam interruption measures as a reliable scoring criterion in mice. The different computer measures obtained during contextual fear conditioning tests showed high correlations with hand-scored freezing; r values ranged from 0.87 to 0.94. Previously reported strain differences between C57BL/6J and DBA/2J in context-dependent fear conditioning were also detected by the computer-based system. In addition, the use of computer-scored freezing of 199 (BALB/cJ x C57BL/6J)F2 mice enabled us to detect a suggestive gender-dependent chromosomal locus for contextual fear conditioning on distal chromosome 8 by QTL analysis. Automation of freeze scoring would significantly increase efficiency and reliability of this learning and memory test.

Automated Measurement of Mouse Freezing Behavior and its Use for Quantitative Trait Locus Analysis of Contextual Fear Conditioning in (BALB/cJ × C57BL/6J)F2 Mice

The most commonly measured mouse behavior in fear conditioning tests is freezing. A technical limitation, particularly for genetic studies, is the method of direct observation used for quantifying this response, with the potential for bias or inconsistencies. We report the use of a computerized method based on latency between photobeam interruption measures as a reliable scoring criterion in mice. The different computer measures obtained during contextual fear conditioning tests showed high correlations with hand-scored freezing; r values ranged from 0.87 to 0.94. Previously reported strain differences between C57BL/6J and DBA/2J in context-dependent fear conditioning were also detected by the computer-based system. In addition, the use of computer-scored freezing of 199 (BALB/cJ × C57BL/6J)F2 mice enabled us to detect a suggestive gender-dependent chromosomal locus for contextual fear conditioning on distal chromosome 8 by QTL analysis. Automation of freeze scoring would significantly increase efficiency and reliability of this learning and memory test.

Rapid photoperiod-induced increase in detectable GnRH mRNA-containing cells in Siberian hamster

To determine whether changes in gonadotropin-releasing hormone (GnRH) neurons are early indicators of photostimulation, Siberian hamsters were placed in short days (6:18-h light-dark) at 3 (experiment 1) or 6 (experiment 2) wk of age where they were held for 3 (experiment 1) or 4 (experiment 2) wk. Hamsters were then moved to long photoperiod (16:8-h light-dark). In experiment 1, brains were collected 1-21 days after transfer from short to long days. In experiment 2, brains were collected only on the second morning of long day exposure. Long and short day controls were included in both experiments. Cells containing GnRH mRNA, as visualized by in situ hybridization, were counted. As expected, there were no differences in the number of detectable GnRH mRNA-containing cells among animals chronically exposed to long or short photoperiods. However, on the second morning after transfer from short to long photoperiod, a positive shift in the distribution of GnRH mRNA-containing cells occurred relative to the respective controls in the two experiments. Increases in follicle-stimulating hormone secretion and gonadal growth occurred days later. In conclusion, a rapid but transient increase in the distribution of detectable GnRH mRNA-containing cells is an early step in the photostimulation of the hypothalamic-pituitary-gonadal axis.

The mouse Clock mutation behaves as an antimorph and maps within the W19H deletion, distal of Kit

Clock is a semidominant mutation identified from an N-ethyl-N-nitrosourea mutagenesis screen in mice. Mice carrying the Clock mutation exhibit abnormalities of circadian behavior, including lengthening of endogenous period and loss of rhythmicity. To identify the gene affected by this mutation, we have generated a high-resolution genetic map (> 1800 meioses) of the Clock locus. We report that Clock is 0.7 cM distal of Kit on mouse chromosome 5. Mapping shows that Clock lies within the W19H deletion. Complementation analysis of different Clock and W19H compound genotypes indicates that the Clock mutation behaves as an antimorph. This antimorphic behavior of Clock strongly argues that Clock defines a gene centrally involved in the mammalian circadian system.

Functional identification of the mouse circadian Clock gene by transgenic BAC rescue

As a complementary approach to positional cloning, we used in vivo complementation with bacterial artificial chromosome (BAC) clones expressed in transgenic mice to identify the circadian Clock gene. A 140 kb BAC transgene completely rescued both the long period and the loss-of-rhythm phenotypes in Clock mutant mice. Analysis with overlapping BAC transgenes demonstrates that a large transcription unit spanning approximately 100,000 base pairs is the Clock gene and encodes a novel basic-helix-loop-helix-PAS domain protein. Overexpression of the Clock transgene can shorten period length beyond the wild-type range, which provides additional evidence that Clock is an integral component of the circadian pacemaking system. Taken together, these results provide a proof of principle that "cloning by rescue" is an efficient and definitive method in mice.

Positional cloning of the mouse circadian clock gene

We used positional cloning to identify the circadian Clock gene in mice. Clock is a large transcription unit with 24 exons spanning approximately 100,000 bp of DNA from which transcript classes of 7.5 and approximately 10 kb arise. Clock encodes a novel member of the bHLH-PAS family of transcription factors. In the Clock mutant allele, an A-->T nucleotide transversion in a splice donor site causes exon skipping and deletion of 51 amino acids in the CLOCK protein. Clock is a unique gene with known circadian function and with features predicting DNA binding, protein dimerization, and activation domains. CLOCK represents the second example of a PAS domain-containing clock protein (besides Drosophila PERIOD), which suggests that this motif may define an evolutionarily conserved feature of the circadian clock mechanism.

Pharmacological and genetic approaches for the study of circadian rhythms in mammals

Two different approaches have been utilized to study the controlling mechanisms that underlie the generation and entrainment of circadian rhythms in mammals. The use of specific drugs to alter the period and/or the phase of circadian rhythms has provided new insights into both the pathways by which environmental information reaches the mammalian circadian pacemaker in the suprachiasmatic nuclei (SCN) and the cellular and neurochemical events within the SCN itself which are involved in circadian rhythmicity. A second approach, which seeks to exploit genetic differences in the properties of the circadian system, holds the promise of eventually defining the cellular and molecular events that are part of the clock itself, the events that underlie the entrainment of the circadian clock by environmental factors, and the expression of overt rhythms driven by the clock. It is anticipated that the pharmacological and genetic approaches to the study of circadian rhythms will complement each other as the underlying physiological mechanisms of the circadian clock system become defined.

Forward and reverse genetic approaches to behavior in the mouse

Modern molecular genetic and genomic approaches are revolutionizing the study of behavior in the mouse. "Reverse genetics" (from gene to phenotype) with targeted gene transfer provides a powerful tool to dissect behavior and has been used successfully to study the effects of null mutations in genes implicated in the regulation of long-term potentiation and spatial learning in mice. In addition, "forward genetics" (from phenotype to gene) with high-efficiency mutagenesis in the mouse can uncover unknown genes and has been used to isolate a behavioral mutant of the circadian system. With the recent availability of high-density genetic maps and physical mapping resources, positional cloning of virtually any mutation is now feasible in the mouse. Together, these approaches permit a molecular analysis of both known and previously unknown genes regulating behavior.

Mutagenesis and mapping of a mouse gene, Clock, essential for circadian behavior

In a search for genes that regulate circadian rhythms in mammals, the progeny of mice treated with N-ethyl-N-nitrosourea (ENU) were screened for circadian clock mutations. A semidominant mutation, Clock, that lengthens circadian period and abolishes persistence of rhythmicity was identified. Clock segregated as a single gene that mapped to the midportion of mouse chromosome 5, a region syntenic to human chromosome 4. The power of ENU mutagenesis combined with the ability to clone murine genes by map position provides a generally applicable approach to study complex behavior in mammals.

Photoperiodic responses differ among inbred strains of golden hamsters (Mesocricetus auratus)

Inbred strains of golden hamsters differ in both the free-running period of the circadian rhythm of locomotor activity in constant darkness, and in the phase angle of entrainment of activity to a 14L:10D cycle. To determine whether these differences in circadian entrainment affect photoperiodic time measurement, we measured the critical photoperiod for maintaining testicular function as well as the rate of response for four different inbred strains (MHA/SsLak, LSH/SsLak, BIO 1.5, and BIO 87.20) and an outbred stock (Lak:LVG(SYR)) of golden hamsters. Hamsters of each group were maintained for 12 wk under one of five different LD cycles. Animals of all groups maintained testis size in 14L:10D and 12.5L:11.5D. Significant strain differences were observed in the critical photoperiod for maintaining testis size after 12 wk; the LSH/SsLak inbred strain showed complete testicular regression during exposure to 12L:12D, while little change was observed in any of the other strains under this photoperiod. Some degree of testicular regression was observed in all groups exposed to 11.5L:12.5D, while complete regression was observed in all animals exposed to 6L:18D. The rate of testicular regression differed markedly between the different groups under both 6L:18D and 11.5L:12.5D. The differences in critical photoperiod and rate of testicular regression observed between the various strains could not be correlated with the known strain differences in entrainment or circadian period, indicating that genetic differences in photoperiodic response are not related to the genetic differences in circadian rhythmicity.(ABSTRACT TRUNCATED AT 250 WORDS)

Reduced light sensitivity of the circadian clock in a hypopigmented mouse mutant

Pink-eyed dilution (p/p) is a recessive mutation in mice which results in reduced pigmentation of the retinal pigment epithelium, as well as alterations in visual pathways and function. We investigated whether this mutation also affects light information reaching the circadian clock. Entrainment to a 12 h light 12 h dark cycle and the free-running period in constant darkness were not affected by this mutation. Phase shifts in response to 1 h light pulses consisting of bright white light at either circadian time 16 or 24 also did not differ between mutant and wild-type C57BL/6J mice. However, when 5 min, 502 nm light pulses of 1.2 x 10(-1) microW/cm2 or 4 x 10(-2) microW/cm2 were given at circadian time 16, the mutant mice responded with significantly smaller phase shifts than the wild-type mice. When animals were transferred to constant light, the free-running period of wild-type mice was longer than that of mutant mice, a finding which is consistent with a sensitivity difference between mutant and wild-type mice. Horseradish peroxidase tracing of retinal innervation of the hypothalamic suprachiasmatic nuclei (SCN)--the location of a circadian pacemaker--revealed a reduced innervation of the SCN in mutant mice compared with wild-type mice. The total volume of the SCN, as determined by neutral red stain, was also reduced in mutant mice, although not to as great an extent as the retinal innervation. Taken together, these results indicate that while basic characteristics of circadian clock function are not altered by the pink-eyed dilution mutation, the sensitivity of the clock to light is reduced.(ABSTRACT TRUNCATED AT 250 WORDS)