Skip to main content
NCBI home page
NIHPA Author Manuscripts logoLink to NIHPA Author Manuscripts
. Author manuscript; available in PMC: 2011 Jul 1.
Published in final edited form as: Chronobiol Int. 2010 Jul;27(6):1317–1328. doi: 10.3109/07420528.2010.489166

Altered body mass regulation in male mPeriod mutant mice on high fat diet

Robert Dallmann 1,*, David R Weaver 1
PMCID: PMC2911971  NIHMSID: NIHMS194482  PMID: 20653457

Abstract

The circadian clock orchestrates most physiological processes in mammals. Disruption of circadian rhythms appears to contribute to the development of obesity and metabolic syndrome. The Period genes mPer1 and mPer2 but not mPer3 are essential for core clock function in mice. To assess the impact of mPer genes on body mass regulation, mPer mutant and control mice were fed a high-fat diet. Here we report that male mPer1/2/3 triple deficient mice gain significantly more body mass than wild-type controls on high fat diet. Surprisingly, mPer3 single deficient animals mimicked this phenotype suggesting a previously unrecognized role for mPer3 in body mass regulation.

Keywords: mPeriod, diet induced obesity, mouse, circadian rhythms, metabolism

Introduction

Most organisms possess an internal biological clock that is capable of measuring time in 24-hour increments, and is reset by the daily light-dark cycle. In mammals, this circadian clock is composed of two interlocked transcriptional/(post-)translational feedback loops (Weaver and Reppert 2008). Two mammalian Period (mPer) genes, mPer1 and mPer2, are essential parts of the negative arm of this core clock mechanism (Reppert and Weaver 2002; Shearman et al. 2000b). In contrast, another member of the same gene family, mPer3, is not necessary for the core clock mechanism; mPer3 deficient mice show only a subtle circadian phenotype (Shearman et al. 2000a; van der Veen and Archer). Nevertheless, polymorphisms in human hPer3 have been associated with diurnal preference, sleep structure, and altered responses to sleep deprivation (Dijk and Archer 2009).

Numerous phenotypic abnormalities have been described in mice with alterations in the core clock genes, i.e. essential parts of the molecular feedback loops. These changes include alterations in many different physiological systems as well as pathological responses (Kondratov et al. 2007), and, most importantly for the present study, alterations in body mass regulation and sleep (see below). Many of these phenotypes have been demonstrated in mice housed in light:dark cycles, where the masking influence of the lighting cycle will drive rhythmicity in several physiological functions. Thus, it is not always clear whether the phenotype is due to disruption of circadian clock function, or to another (pleiotropic) role of the clock gene being studied. These considerations led us to examine body mass regulation in mice with disruption of the Period genes.

Various mouse lines with mutations in core clock genes have been linked to altered metabolic pathways, metabolic syndrome and obesity phenotypes (Dallmann et al. 2006; Rudic et al. 2004; Turek et al. 2005). Through which output mechanism the core circadian clockwork influences these pathways is still unclear. Recently, it has been acknowledged that metabolic syndrome and obesity are causally linked to disruption of rhythms (Bechtold 2008) and sleep (Laposky et al. 2008; Wolk and Somers 2007). Notably, high fat diet and/or diet-induced obesity have been shown to alter physiological rhythms and also to alter the amplitude and phase of clock gene and clock-controlled gene expression patterns (Barnea et al. 2009; Hsieh et al, 2010; Kohsaka et al. 2007). Even mice with targeted disruptions of mPer1 and mPer2 have been shown to be slightly lighter and heavier, respectively, than wild-type controls (Dallmann et al. 2006; Pilorz and Steinlechner 2008). Dependent on the genetic background of the animals used, however, results can vary between experimenters (Kennaway et al. 2007).

Here, we set out to investigate the impact of disruption of mPer genes on body mass development and food intake in mice under two different feeding regimes, i.e. high fat vs. control chow. Period triple deficient animals on high fat diet showed a late onset obesity phenotype. Most surprisingly, however, we found that even the single PER3 deficient mice have this late onset obesity; indicating an unexpected role for mPer3 in body mass regulation.

Materials and Methods

In this study we used mice homozygous for alleles with targeted disruptions of mPer1, mPer2, and mPer3 (Per1tm1Drw;Per2tm1Drw;Per3tm1Drw, henceforth: Per/1/2/3tm1Drw) or only mPer3 (abbreviated Per3tm1Drw) and congenic 129/Sv wild-type control mice. Generation and circadian phenotype of all mutant lines have been described elsewhere (Bae et al. 2001; Shearman et al. 2000a).

Unless otherwise noted, animals were single housed in individually ventilated cages (Allentown, NJ, USA, 24×16×13 cm) maintained in light tight (lights on from 06:00 h to 18:00 h each day) and climate controlled closets (21±1°C) with gamma irradiated food and tab-water available ad libitum. As specified below, animals were either provided with control diet (CD, Prolab 5P76 IsoPro RMH 3000, Labdiet, St. Louis, MO, USA) or high fat diet (HFD, D12451i, Research Diets, New Brunswick, NJ, USA), with 4.1 kcal/g (%kcal from carbohydrates 60, protein 26, fat 14) or 4.73 kcal/g (%kcal from carbohydrates 20, protein 35, fat 45), respectively. During the light phase, light was provided by fluorescent bulbs (ca. 100 lux) and a dim red light (>600nm) was present at all times. Animals were weighted (CS200, Ohaus, Pine Brook, NJ, USA) 2–3 hours before lights off. The content of the food hopper was determined (HH120D, Ohaus, Pine Brook, NJ, USA) at the same time and refilled. In a pilot experiment, amount of spilled food in the cage was determined for all mice of Experiment 1, but no differences for genotypes nor gender were detected (data not shown) and so spillage was not quantified in the experiments, i.e. food intake is defined as the amount of food that disappeared from the food hopper. All animal experiments were approved by the Institutional Animal Care and Use Committee at the University of Massachusetts Medical School, and were in accordance with international standards (Portaluppi et al. 2008).

Experiment 1

As baseline, mPer1/2/3tm1Drw (n=9 males, n=10 females) and mPer3tm1Drw (n=7 males, n=5 females) single deficient animals, with age matched (3–5 month) wild-type controls (n=10 males, n=9 females) were monitored for body mass and food intake on control diet (CD) for 8 weeks. Then, the animals were switched to high fat diet (HFD) and body mass and food intake was recorded for 21 weeks at irregular intervals. At the end of the experiment, subcutaneous inguinal and epididymal (male) or parametrial (female) fat pads, and liver were dissected and their mass determined (HH120D, Ohaus, Pine Brook, NJ, USA). Body mass gain is defined as the increase in mass for each mouse from baseline (CD) week 8 and after 21 weeks on HFD. Experiment 1 was conducted between March and October 2006. High fat diet was introduced on 9 May 2006 and the experiment was terminated 21 weeks later.

Experiment 2

Male mPer3tm1Drw (n=19) were compared to wild-type control (n=16) mice to confirm the unexpected finding of altered body mass regulation in males of this genotype as observed in Experiment 1. (Female mice did not show diet induced obesity in Experiment 1, and so were not included in Experiment 2). All animals were weaned and single-housed at 3 weeks of age and kept on control diet for 1 week. Then, mice in the high fat diet (HFD) group were switched to the new diet while the control mice remained on control diet (CD) until the end of the experiment.

Body mass and food intake were monitored weekly. At 8 month of age, animals were sacrificed and fat pads and liver were dissected and weighed as in Experiment 1.

Experiment 2 was carried out between December 2006 and November 2007.

Data Analysis

All data are given as mean ± standard error of the mean (SEM). Data were compared using repeated measures or one way ANOVA and Scheffé’s post-hoc test where appropriate (StatView 5.0, SAS Institute, Cary, NC). Statistical tests were 2-tailed and the level of significance was set to α = 0.05.

Results

Experiment 1

The age of the mice at the beginning of the experiment was counter-balanced between genotypes, but was quite variable, spanning 2 months. Body mass did not differ between males of the three genotypes at the beginning of the experiment. After 21 weeks on HFD, body mass of mPer1/2/3tm1Drw and mPer3tm1Drw male mice was significantly higher than wild-type controls (Figure 1). The females did not show body mass differences between genotypes although there was a trend toward elevated body mass in the two mutant lines, relative to wild-type females. In fact, females did not show a significant diet induced body mass gain in any genotype even after 21 weeks on HFD (Figure 1, Table 1).

Figure 1.

Figure 1

Open in a new tab

Body mass development in male (closed symbols) and female (open symbols) Per1/2/3tm1Drw, Per3tm1Drw and wild-type (WT) mice in Experiment 1. See also Table 1.

Table 1.

Body mass gain on HFD, mean daily food intake on control (CD) and high fat diet (HFD), subcutaneous and perigonadal fat and liver mass of mice used in Experiment 1 (mean±SEM, in g or g/30g mouse).

Genotype N Body mass gain CD food intake HFD food intake Subcutaneous fat Perigonadal fat Liver
Abs. Rel. Abs. Rel. Abs. Rel. Abs. Rel. Abs. Rel.
Males
Per1/2/3tm1Drw 9 9.4±1.9*** 4.8±0.1 6.0±0.1*** 2.3±0.1* 2.4±0.1 1.1±0.2*** 0.9±0.1*** 2.1±0.3*** 1.8±0.2*** 1.6±0.2 1.3±0.1
Per3tm1Drw- 7 13.1±1.7*** 4.9±0.2 5.6±0.1 2.3±0.1 2.4±0.1 1.5±0.2*** 1.1±0.1*** 2.6±0.3*** 1.8±0.1*** 1.9±0.2 1.4±0.1
wild-type 10 1.4±1.5 5.2±0.3 5.4±0.1 2.7±0.1 2.5±0.1 0.6±0.1 0.5±0.1 1.0±0.2 1.0±0.1 1.7±0.1 1.7±0.1
Females
Per1/2/3tm1Drw 10 2.5±0.7 5.0±0.1 6.8±0.1*** 2.1±0.1 2.7±0.1 0.5±0.1 0.6±0.1 1.0±0.2 1.3±0.2 1.1±0.1 1.4±0.1*
Per3tm1Drw 5 2.3±1.1 4.8±0.3 5.9±0.1 2.2±0.1 2.4±0.1 0.5±0.1 0.6±0.1 1.2±0.1* 1.4±0.1* 1.1±0.1 1.3±0.1*
wild-type 9 0.0±0.7 5.2±0.3 6.1±0.1 2.4±0.1 2.6±0.1 0.3±0.1 0.4±0.1 0.7±0.1 0.8±0.1 1.5±0.1 1.8±0.1
Open in a new tab

Abbreviations: M = male, F = female, Abs. = absolute per mouse, Rel. = relative per 30 g mouse.

*

p<0.05 and

***

p<0.001 vs. wild-type of same sex.

Food intake did not differ between mPer3tm1Drw animals and wild-type controls, within either sex (Table 1). In mPer1/2/3tm1Drw mice, however, a slightly higher intake of CD was observed in both males and females, relative to the wild-type controls. This difference in food intake disappeared during HFD. All groups reduced their food intake after the transition from CD to HFD diet (Table 1).

Body composition, judged by the mass of subcutaneous and perigonadal fat pads, was quite different between genotypes; male mPer1/2/3tm1Drw and mPer3tm1Drw mice had more fat than controls. Differences in female mice were more subtle. Nevertheless, perigonadal fat pads of mPer3tm1Drw females were heavier than those of the two other lines. In addition, relative liver mass of the mutant lines was decreased compare to wild-type mice (Table 1).

Experiment 2

Due to the lack of major differences in body mass between the genotypes in female mice, we conducted Experiment 2 with male mice only. Experiment 2 was also modified to make the initial age at transfer to HFD more consistent. The results of Experiment 2 confirmed that male mPer3tm1Drw mice had a delayed onset obesity phenotype when fed a high-fat diet, relative to wild-type controls (Table 2).

Table 2.

Body mass gain on HFD, subcutaneous and epididymal fat and liver mass of male mice used in Experiment 2 (mean±SEM, in g or g/30g mouse).

Genotype Diet N Body mass gain Subcutaneous fat Epididymal fat Liver
Abs. Rel. Abs. Rel. Abs. Rel.
Per3tm1Drw HFD 10 22.6±1.3*** 1.6±0.2*** 1.2±0.1*** 2.8±0.2*** 2.1±0.1*** 1.6±0.1* 1.1±0.1
CD 9 14.8±1.2 0.7±0.1*** 0.5±0.1*** 1.2±0.1*** 0.8±0.2*** 1.3±0.1 1.5±0.1
wild-type HFD 7 9.6±1.8 0.5±0.2 0.6±0.1 0.8±0.3 1.1±0.1 1.3±0.1 1.3±0.1
CD 9 10.3±0.8 0.3±0.1 0.3±0.1 0.5±0.1 0.5±0.1 1.4±0.1 1.5±0.1
Open in a new tab

Abbreviations: HFD = high fat diet, CD = control, Abs. = absolute per mouse, Rel. = relative per 30 g mouse,

*

p<0.05 and

***

p<0.01 Per3tm1Drw vs. wild-type on same diet.

Body mass of mPer3tm1Drw mice was indistinguishable from controls until 20 weeks of age, although they were already on HFD for 16 weeks at that time. Only subsequently, the mutant mice in the HFD group became heavier, and at 35 weeks of age the body mass difference between mPer3tm1Drw and control mice was 13.1±1.7 g (p < 0.001). In the CD group, the difference between the genotypes was much smaller (3.5±1.0 g, p = 0.08, n.s.). Interestingly, only the mutant animals showed a difference between the two feeding conditions (7.7 g, p < 0.01); wild-type HFD mice did not differ significantly in body mass from those fed CD (2.1 g, p = 0.23).

Food intake did not differ between the genotypes until 20 weeks of age. Counter-intuitively, however, the mPer3tm1Drw mutants had lower food intake, both in the HFD and CD groups. Also, both genotypes had lower food intake on the HFD compared to CD (Figure 3). Interestingly, body mass differences became apparent shortly after the animals reached 20 weeks of age.

Figure 3.

Figure 3

Open in a new tab

Monthly mean values (±SEM) for daily food intake of male Per3tm1Drw and wild-type mice on different diets in Experiment 2 (HFD = High fat diet, CD = Control diet). Where SEM bars are not visible, they fall within the symbol.

At the end of the experiment (35 weeks of age), fat pads of mPer3tm1Drw mice in the HFD group weighed more than double that of wild-type controls. Even in the CD group, mutant mice had significantly heavier fat pads than controls. This finding was present both when data were compared as absolute amount (per mouse) or relative (per 30 g mouse). Liver mass, however, did not differ between groups or genotypes (Table 2).

Discussion

Our data indicate that mPer genes are involved in body mass control in male mice. More specifically, we found that disrupting the circadian clock in mPer1/2/3tm1Drw male mice led to higher body mass on a high fat diet. Surprisingly, however, mPer3tm1Drw mice had a similar diet-induced obesity phenotype as the triple mutant mice. This was unexpected because mPer3tm1Drw mice do not have any overt disruption of circadian rhythms (Shearman et al. 2000a). It has been proposed that mPer3 plays a role in the output pathway of the core oscillator (Bae et al. 2001; Shearman et al. 2000a). Downstream targets of this output have not yet been identified, however. Nevertheless, subtle effects on the core circadian clock could translate to sizeable downstream effects on whole body physiology.

Certain allelic variations of the hPer3 gene are associated with delayed sleep phase and sleep architecture (Archer et al. 2003; Ebisawa et al. 2001; Pereira et al. 2005; Viola et al. 2007). Moreover, significant differences in hPer3 allelic frequencies between human populations have been reported (Ciarleglio et al. 2008; Nadkarni et al. 2005). In the case of the bona fide clock gene, Clock, an association between polymorphisms in Clock and obesity has been reported (Scott et al. 2008; Sookoian et al. 2008). In light of our data, it would be interesting to assess the association of polymorphisms in hPer3 with obesity and other metabolic endpoints.

A link between sleep and metabolic syndrome has been established (Laposky et al. 2008; Wolk and Somers 2007). Notably, however, Per3tm1Drw mice do not have an overt sleep phenotype (Shiromani et al. 2004). It thus seems that the propensity to obesity in this strain of mice is not secondary to a gross alteration in sleep.

In a recent publication, disruption of mPer3 was shown to influence the period and phase of rhythms recorded in vitro from lung and pituitary explants, but SCN rhythms were unaffected (Pendergast et al. 2009). This finding raises the possibility that altered body mass regulation in mPER3-deficient mice is the result of “internal misalignment” in which the relative phasing of peripheral organs differs from wild-type (Scheer et al. 2009). Indeed, it has been shown that phase of food intake matters; day-fed mice gain more weight than night-fed mice (Arble et al. 2009). The results of Pendergast et al. (2009) were obtained with mPER3 deficient animals backcrossed extensively to C57BL/6 mice. Here, we used mice on a 129 genetic background. This particular strain, however, is not widely used in obesity research and is rather resistant to diet induced obesity (Fink et al. 2007; Kokkotou et al. 2005; Su et al. 2008; present results).

In Experiment 1, there was a trend toward higher body mass in the female mPer mutant mice, but this was not significant (Table 1). The mPer3 mutant females did have heavier perigonadal fat pads and reduced relative liver mass, compared to wild-type controls, but even the wild-type females did not develop obesity on HFD. In this regard, the female mice appear more resistant to HFD-induced obesity than male mice on this genetic background. Some studies find that females tend to be more resistant to the effects of high fat diet (e.g., Shockley et al. 2009), but others do not find such sex dimorphism (e.g. Su et al. 2008). It may simply be that the increase in body mass with HFD is generally higher in males, and that in some studies, this increase is below the threshold of significance. While this sexual dimorphism is interesting, we focused our efforts on male mice in Experiment 2, providing an independent confirmation of an effect of mPer3 disruption on diet-induced obesity.

Also, the differential effect on body mass became apparent only late in the experiments. Future studies should use mPER3 deficient mice backcrossed to C57BL/6J mice, in which diet-induced obesity is of larger magnitude, and address important questions beyond the scope of this report, and on which we do not have any data at this time, such as the time-course of alterations in body composition, metabolic rate, and energy balance, as well as biochemical parameters such as glucose and fatty acid metabolism. Metabolic rate measurements should be especially interesting in view of the paradoxical reduction in food intake but a higher body mass gain in Per3tm1Drw mutant mice, compared to wild-type animals.

Figure 2.

Figure 2

Open in a new tab

Body mass development of male Per3tm1Drw and wild-type (WT) mice in Experiment 2.

Acknowledgments

We thank Christopher Lambert for technical assistance. Financial support came from the DFG (DA525/2-1 to R.D.) and the National Institutes of Health (NS056125 to D.R.W.). The content is solely the responsibility of the authors and does not necessarily represent the official views of the sponsors.

References

  1. Arble DM, Bass J, Laposky AD, Vitaterna MH, Turek FW. Circadian timing of food intake contributes to weight gain. Obesity. 2009;17:2100–2102. doi: 10.1038/oby.2009.264. [DOI] [PMC free article] [PubMed] [Google Scholar]
  2. Archer SN, Robilliard DL, Skene DJ, Smits M, Williams A, Arendt J, von Schantz M. A length polymorphism in the circadian clock gene Per3 is linked to delayed sleep phase syndrome and extreme diurnal preference. Sleep. 2003;26:413–415. doi: 10.1093/sleep/26.4.413. [DOI] [PubMed] [Google Scholar]
  3. Bae K, Jin X, Maywood ES, Hastings MH, Reppert SM, Weaver DR. Differential functions of mPer1, mPer2, and mPer3 in the SCN circadian clock. Neuron. 2001;30:525–536. doi: 10.1016/s0896-6273(01)00302-6. [DOI] [PubMed] [Google Scholar]
  4. Barnea M, Madar Z, Froy O. High-fat diet delays and fasting advances the circadian expression of adiponectin signaling components in mouse liver. Endocrinology. 2009;150:161–168. doi: 10.1210/en.2008-0944. [DOI] [PubMed] [Google Scholar]
  5. Bechtold DA. Energy-responsive timekeeping. J Genet. 2008;87:447–458. doi: 10.1007/s12041-008-0067-6. [DOI] [PubMed] [Google Scholar]
  6. Ciarleglio CM, Ryckman KK, Servick SV, Hida A, Robbins S, Wells N, Hicks J, Larson SA, Wiedermann JP, Carver K, Hamilton N, Kidd KK, Kidd JR, Smith JR, Friedlaender J, McMahon DG, Williams SM, Summar ML, Johnson CH. Genetic differences in human circadian clock genes among worldwide populations. J Biol Rhythm. 2008;23:330–340. doi: 10.1177/0748730408320284. [DOI] [PMC free article] [PubMed] [Google Scholar]
  7. Dallmann R, Touma C, Palme R, Albrecht U, Steinlechner S. Impaired daily glucocorticoid rhythm in Per1Brd mice. J Comp Physiol A. 2006;192:769–775. doi: 10.1007/s00359-006-0114-9. [DOI] [PubMed] [Google Scholar]
  8. Dijk DJ, Archer SN. PERIOD3, circadian phenotypes, and sleep homeostasis. Sleep Med Rev. 2009 doi: 10.1016/j.smrv.2009.07.002. [DOI] [PubMed] [Google Scholar]
  9. Ebisawa T, Uchiyama M, Kajimura N, Mishima K, Kamei Y, Katoh M, Watanabe T, Sekimoto M, Shibui K, Kim K, Kudo Y, Ozeki Y, Sugishita M, Toyoshima R, Inoue Y, Yamada N, Nagase T, Ozaki N, Ohara O, Ishida N, Okawa M, Takahashi K, Yamauchi T. Association of structural polymorphisms in the human period3 gene with delayed sleep phase syndrome. EMBO Rep. 2001;2:342–346. doi: 10.1093/embo-reports/kve070. [DOI] [PMC free article] [PubMed] [Google Scholar]
  10. Fink BD, Herlein JA, Almind K, Cinti S, Kahn CR, Sivitz WI. Mitochondrial proton leak in obesity-resistant and obesity-prone mice. Am J Physiol Reg-I. 2007;293:R1773–1780. doi: 10.1152/ajpregu.00478.2007. [DOI] [PubMed] [Google Scholar]
  11. Hsieh MC, Yang SC, Tseng HL, Hwang LL, Chen CT, Shieh KR. Abnormal expressions of circadian-clock and circadian clock-controlled genes in the livers and kidneys of long-term, high-fat-diet-treated mice. Int J Obes. 2010;34:227–239. doi: 10.1038/ijo.2009.228. [DOI] [PubMed] [Google Scholar]
  12. Kennaway DJ, Owens JA, Voultsios A, Boden MJ, Varcoe TJ. Metabolic homeostasis in mice with disrupted Clock gene expression in peripheral tissues. Am J Physiol Reg-I. 2007;293:R1528–1537. doi: 10.1152/ajpregu.00018.2007. [DOI] [PubMed] [Google Scholar]
  13. Kohsaka A, Laposky AD, Ramsey KM, Estrada C, Joshu C, Kobayashi Y, Turek FW, Bass J. High-fat diet disrupts behavioral and molecular circadian rhythms in mice. Cell Metab. 2007;6:414–421. doi: 10.1016/j.cmet.2007.09.006. [DOI] [PubMed] [Google Scholar]
  14. Kokkotou E, Jeon JY, Wang X, Marino FE, Carlson M, Trombly DJ, Maratos-Flier E. Mice with MCH ablation resist diet-induced obesity through strain-specific mechanisms. Am J Physiol Reg-I. 2005;289:R117–124. doi: 10.1152/ajpregu.00861.2004. [DOI] [PubMed] [Google Scholar]
  15. Kondratov RV, Gorbacheva VY, Antoch MP. The role of mammalian circadian proteins in normal physiology and genotoxic stress responses. Cur Top Dev Bio. 2007;78:173–216. doi: 10.1016/S0070-2153(06)78005-X. [DOI] [PubMed] [Google Scholar]
  16. Laposky AD, Bass J, Kohsaka A, Turek FW. Sleep and circadian rhythms: key components in the regulation of energy metabolism. FEBS Lett. 2008;582:142–151. doi: 10.1016/j.febslet.2007.06.079. [DOI] [PubMed] [Google Scholar]
  17. Nadkarni NA, Weale ME, von Schantz M, Thomas MG. Evolution of a length polymorphism in the human PER3 gene, a component of the circadian system. J Biol Rhythm. 2005;20:490–499. doi: 10.1177/0748730405281332. [DOI] [PubMed] [Google Scholar]
  18. Pendergast JS, Nakamura W, Friday RC, Hatanaka F, Takumi T, Yamazaki S. Robust food anticipatory activity in BMAL1-deficient mice. PloS One. 2009;4:e4860. doi: 10.1371/journal.pone.0004860. [DOI] [PMC free article] [PubMed] [Google Scholar]
  19. Pereira DS, Tufik S, Louzada FM, Benedito-Silva AA, Lopez AR, Lemos NA, Korczak AL, D’Almeida V, Pedrazzoli M. Association of the length polymorphism in the human Per3 gene with the delayed sleep-phase syndrome: does latitude have an influence upon it? Sleep. 2005;28:29–32. [PubMed] [Google Scholar]
  20. Pilorz V, Steinlechner S. Low reproductive success in Per1 and Per2 mutant mouse females due to accelerated ageing? Reproduction. 2008;135:559–568. doi: 10.1530/REP-07-0434. [DOI] [PubMed] [Google Scholar]
  21. Portaluppi F, Touitou Y, Smolensky MH. Ethical and methodological standards for laboratory and medical biological rhythm research. Chronobiol Int. 2008;25:999–1016. doi: 10.1080/07420520802544530. [DOI] [PubMed] [Google Scholar]
  22. Reppert SM, Weaver DR. Coordination of circadian timing in mammals. Nature. 2002;418:935–941. doi: 10.1038/nature00965. [DOI] [PubMed] [Google Scholar]
  23. Rudic RD, McNamara P, Curtis AM, Boston RC, Panda S, Hogenesch JB, Fitzgerald GA. BMAL1 and CLOCK, two essential components of the circadian clock, are involved in glucose homeostasis. PLoS Biol. 2004;2:e377. doi: 10.1371/journal.pbio.0020377. [DOI] [PMC free article] [PubMed] [Google Scholar]
  24. Scheer FA, Hilton MF, Mantzoros CS, Shea SA. Adverse metabolic and cardiovascular consequences of circadian misalignment. PNAS. 2009;106:4453–4458. doi: 10.1073/pnas.0808180106. [DOI] [PMC free article] [PubMed] [Google Scholar]
  25. Scott EM, Carter AM, Grant PJ. Association between polymorphisms in the Clock gene, obesity and the metabolic syndrome in man. Int J Obesity. 2008;32:658–662. doi: 10.1038/sj.ijo.0803778. [DOI] [PubMed] [Google Scholar]
  26. Shearman LP, Jin X, Lee C, Reppert SM, Weaver DR. Targeted disruption of the mPer3 gene: subtle effects on circadian clock function. Mol Cell Biol. 2000a;20:6269–6275. doi: 10.1128/mcb.20.17.6269-6275.2000. [DOI] [PMC free article] [PubMed] [Google Scholar]
  27. Shearman LP, Sriram S, Weaver DR, Maywood ES, Chaves I, Zheng B, Kume K, Lee CC, van der Horst GT, Hastings MH, Reppert SM. Interacting molecular loops in the mammalian circadian clock. Science. 2000b;288:1013–1019. doi: 10.1126/science.288.5468.1013. [DOI] [PubMed] [Google Scholar]
  28. Shiromani PJ, Xu M, Winston EM, Shiromani SN, Gerashchenko D, Weaver DR. Sleep rhythmicity and homeostasis in mice with targeted disruption of mPeriod genes. Am J Physiol Reg-I. 2004;287:R47–57. doi: 10.1152/ajpregu.00138.2004. [DOI] [PubMed] [Google Scholar]
  29. Shockley KR, Witmer D, Burgess-Herbert SL, Paigen B, Churchill GA. Effects of atherogenic diet on hepatic gene expression across mouse strains. Physiol Genomics. 2009;39:172–182. doi: 10.1152/physiolgenomics.90350.2008. [DOI] [PMC free article] [PubMed] [Google Scholar]
  30. Sookoian S, Gemma C, Gianotti TF, Burgueno A, Castano G, Pirola CJ. Genetic variants of Clock transcription factor are associated with individual susceptibility to obesity. Am J Clin Nutr. 2008;87:1606–1615. doi: 10.1093/ajcn/87.6.1606. [DOI] [PubMed] [Google Scholar]
  31. Su Z, Korstanje R, Tsaih SW, Paigen B. Candidate genes for obesity revealed from a C57BL/6J × 129S1/SvImJ intercross. Int J Obes. 2008;32:1180–1189. doi: 10.1038/ijo.2008.56. [DOI] [PMC free article] [PubMed] [Google Scholar]
  32. Turek FW, Joshu C, Kohsaka A, Lin E, Ivanova G, McDearmon E, Laposky A, Losee-Olson S, Easton A, Jensen DR, Eckel RH, Takahashi JS, Bass J. Obesity and metabolic syndrome in circadian Clock mutant mice. Science. 2005;308:1043–1045. doi: 10.1126/science.1108750. [DOI] [PMC free article] [PubMed] [Google Scholar]
  33. van der Veen DR, Archer SN. Light-dependent behavioral phenotypes in PER3-deficient mice. J Biol Rhythm. 25:3–8. doi: 10.1177/0748730409356680. [DOI] [PubMed] [Google Scholar]
  34. Viola AU, Archer SN, James LM, Groeger JA, Lo JC, Skene DJ, von Schantz M, Dijk DJ. PER3 polymorphism predicts sleep structure and waking performance. Curr Biol. 2007;17:613–618. doi: 10.1016/j.cub.2007.01.073. [DOI] [PubMed] [Google Scholar]
  35. Weaver DR, Reppert SM. Circadian Timekeeping. In: Squire LR, et al., editors. Fundamental Neuroscience. 3. Elsevier/Academic Press; Amsterdam; Boston: 2008. pp. 931–958. [Google Scholar]
  36. Wolk R, Somers VK. Sleep and the metabolic syndrome. Exp Physiol. 2007;92:67–78. doi: 10.1113/expphysiol.2006.033787. [DOI] [PubMed] [Google Scholar]

RESOURCES