WNT Takes Two to Tango: Molecular Links between the Circadian Clock and the Cell Cycle in Adult Stem Cells

Abstract

Like two dancers, the circadian clock and cell cycle are biological oscillators engaged in bidirectional communication, resulting in circadian clock–gated cell division cycles in species ranging from cyanobacteria to mammals. The identified mechanisms for this phenomenon have expanded beyond intracellular molecular coupling components to include intercellular connections. However, detailed molecular mechanisms, dynamics, and physiological functions of the circadian clock and cell cycle as coupled oscillators remain largely unknown. In this review, we discuss current understanding of this connection in light of recent findings that have uncovered intercellular coupling between the circadian clock in Paneth cells and the cell cycle in intestinal stem cells via WNT signaling. This extends the impact of circadian rhythms regulating the timing of cell divisions beyond the intracellular domain of homogenous cell populations into dynamic, multicellular systems. In-depth understanding of the molecular links and dynamics of these two oscillators will identify potential targets and temporal regimens for effective chronotherapy.

Keywords

circadian clock cell cycle WNT signaling adult stem cells coupled oscillators

Molecular Mechanisms of Circadian Rhythms

Most organisms exhibit circadian rhythms, reflecting an internal biological clock with an approximate period of 24 h, to anticipate and align their physiological functions with daily environmental changes. Autonomous oscillations of circadian rhythms are generated at single-cell level by transcriptional-translational feedback loops (TTFLs), which consist of positive and negative elements (Hurley et al., 2016; Lowrey and Takahashi, 2011) (Fig. 1A). In mammals, the positive elements consist of heterodimeric basic helix loop helix-Per/Arnt/Sim (bHLH-PAS) transcription factors, Circadian Locomotor Output Cycles Kaput (CLOCK) and Brain and Muscle ARNT-Like 1 (BMAL1), which regulate the expression of negative elements, Period (Per1, Per2, and Per3) and Cryptochrome (Cry1 and Cry2) genes. PER and CRY proteins form complexes and directly inhibit CLOCK/BMAL1 transcriptional activity and repress their own mRNA expression forming a time-delayed negative feedback loop. In addition, CLOCK/BMAL1 controls the expression of nuclear receptors, Retinoic acid–related Orphan Receptor (ROR) and Rev-erbα, whose proteins activate and repress the expression of Bmal1, respectively, forming additional positive and negative feedback loops.

Figure 1.

Molecular connections between the circadian clock and the cell cycle. (A) In mammalian somatic cells, the circadian clock gates the cell cycle by intracellular molecular connections. (B) The circadian clock in Paneth cells regulates the timing of divisions in intestinal stem cells and progenitor cells through rhythmic secretion of WNT.

Efficient operation of the above interlocked feedback loops that sustain autonomous circadian oscillations requires posttranslational regulation of the core clock proteins. Casein Kinase 2α (CK2α) rhythmically phosphorylates BMAL1, which determines the rhythmic accumulation of CLOCK/BMAL1 in the nucleus (Kondratov et al., 2003; Tamaru et al., 2009). Casein Kinase 1ε (CK1ε) phosphorylates CRY1, CRY2, and BMAL1, and CKIε-dependent phosphorylation of BMAL1 positively regulates its transcriptional activity (Eide et al., 2002). Importantly, the negative elements undergo extensive phosphorylation that regulates the function and degradation of these proteins. PER2 in mammals and its analogous protein, FREQUENCY (FRQ) in Neurospora crassa, have over 21 and 75 phosphorylated residues, respectively, which regulate the proteins’ half-life, subcellular localization, and circadian period (Baker et al., 2009; Vanselow et al., 2006). Specifically, CK1ε and CK1δ phosphorylate PER proteins and regulate subcellular localization and subsequent ubiquitin-mediated degradation (Akashi et al., 2002; Lee et al., 2009; Vanselow et al., 2006). Phosphorylated PER proteins by CK1ε/δ become targets of the E3 ubiquitin ligase complex that contains beta-transducin repeat–containing protein 1 (β-TrCP1) (Reischl et al., 2007). F-Box and Leucine Rich Repeat Protein 3 (FBXL3) forms a Skp1-Cul1-F-box protein (SCF) E3 ligase complex that polyubiquitinates the CRY proteins for degradation by the proteasomes (Busino et al., 2007; Godinho et al., 2007; Siepka et al., 2007). In contrast, FBXL21, a paralog of FBXL3, forms a SCF E3 ligase complex and stabilizes CRY by protecting it from FBXL3-mediated degradation in the nucleus while destabilizing it in the cytoplasm (Hirano et al., 2013; Yoo et al., 2013). REV-ERBα is ubiquitinated by two E3 ligases, ADP-Ribosylation Factor Binding Protein 1 (ARF-BP1) and Protein Associated with MYC (PAM), which leads to subsequent degradation of REV-ERBα (Yin et al., 2010). BMAL1 is also ubiquitinated by another E3 ligase, Ubiquitin Protein Ligase E3A (UBE3A), and RNA interference (RNAi) of UBE3A disrupts circadian rhythmicity in NIH3T3 cells (Gossan et al., 2014). These findings demonstrate that core clock protein-specific ubiquitin-mediated degradation is important for circadian rhythms. Intriguingly, recent findings from the Neurospora circadian clock have shown unexpected circadian oscillations in the absence of F-box and WD40 repeat-containing protein 1 (FWD-1), an ortholog of β-TrCP1, which determines the stability of the negative element, FRQ (Larrondo et al., 2015). This revealed that there are distinct phosphorylation events that determine the circadian period independent of half-life of FRQ. For more information on detailed molecular underpinnings of circadian rhythms, we refer to comprehensive reviews (Hurley et al., 2016; Lowrey and Takahashi, 2011).

Intracellular Molecular Links between the Cell Cycle and the Circadian Clock

Previously, several circadian clock–regulated cell cycle components have been identified. Matsuo et al. (2003) reported that the expression of a G2/M checkpoint kinase, Wee1, is directly regulated by the heterodimeric circadian clock transcription factor, CLOCK/BMAL1, resulting in rhythmic gene expression and WEE1 kinase activity, which are abolished in Cry-deficient mice. In contrast, RORs and REV-ERBα/β antagonistically regulate the rhythmic expression of p21, a cyclin-dependent kinase inhibitor, which demonstrates that the circadian clock regulates G1/S transition in addition to G2/M (Grechez-Cassiau et al., 2008). Circadian gene expression of p21 is abolished in Bmal1-deficient mice with increased abundance of p21 and delayed cell proliferation (Grechez-Cassiau et al., 2008). Furthermore, PER proteins interact with a nuclear protein, NONO, to regulate rhythmic binding and activation of another cyclin-dependent kinase inhibitor, p16, which shows an additional link to the G1/S transition (Kowalska et al., 2013). Intriguingly, PER1 physically interacts with the checkpoint kinase proteins, Ataxia Telangiectasia Mutated (ATM) and Checkpoint Kinase 2 (CHK2), and overexpression of PER1 leads to a reduction of cell growth and increased susceptibility to DNA damage–induced apoptosis, which reveals a connection between circadian rhythms and DNA damage response (Gery et al., 2006). However, recent findings indicate that the cell cycle machinery also influences the circadian clock. Bieler et al. (2014) demonstrated that the cell division cycle influences period and phase of the circadian clock in NIH3T3 fibroblasts, and DNA replication is necessary for rhythmic access of the frq promoter via the histone chaperon, FACT complex, in N. crassa(Liu et al., 2017).

Importantly, core circadian clock elements regulate key components that control cell proliferation and tumorigenesis. Gotoh et al. reported a series of studies of PER2 interaction with a tumor suppressor, p53. The studies revealed that the physical interaction between PER2 and p53 results in (1) stabilization of p53 from Murine Double Minute-2 (MDM2)–mediated ubiquitination and degradation (Gotoh et al., 2014), (2) inactivation of the transcriptional activity of p53 (Gotoh et al., 2015), and (3) nuclear translocation of p53 in human colon cancer HCT116 cells (Gotoh et al., 2016). A proto-oncogene, c-Myc, and its downstream genes, Cyclin D1 and a tumor suppressor, Growth Arrest and DNA-Damage-Inducible 45 (GADD45), show rhythmic gene expression in mouse liver (Fu et al., 2002). The expression of these genes was altered in the Per2 mutant mouse (mPer2^m/m) and showed dramatically increased expression of c-Myc mRNA, which correlates with radiation-induced tumorigenesis in mPer2^m/m mice (Fu et al., 2002). In addition, CRY2 regulates the stability of c-Myc by promoting the ubiquitination and degradation of c-Myc (Huber et al., 2016). To add to this complexity, overexpression of c-Myc disrupts circadian rhythms by inducing REV-ERBα, which reduces the expression of Bmal1 (Altman et al., 2015; Shostak et al., 2016), establishing a bidirectional communication between circadian rhythms and cell proliferation. Together, the aforementioned molecular connections (summarized in Table 1) orchestrate intracellular coupling of the circadian clock and the cell cycle in mammalian somatic cells.

Table 1

Molecular connection between the circadian clock and the cell cycle.

Cell Type (Tissue)	Molecular Connection	Potential Cell Cycle Phase to Regulate	Regulation	Reference
Mouse hepatocytes (mouse liver after partial hepatectomy)	CLOCK/BMAL1 regulates expression of Wee1	G2/M	Inhibition	Matsuo et al. (2003)
Mouse myoblasts (C2C12), mouse fibroblasts (NIH3T3), and mouse primary hepatocytes	RORs and REB-ERBs regulate expression of p21	G1/S and G2/M	Inhibition	Grechez-Cassiau et al. (2008)
Mouse fibroblasts	PER proteins interact with NONO to regulate p16	G1/S	Inhibition	Kowalska et al. (2013)
Human colon carcinoma cells (HCT116)	PER1 interacts with ATM and CHK2	G1/S and G2/M	Inhibition	Gery et al. (2006)
Human colon carcinoma cells (HCT116)	PER2 interacts with p53 and regulates half-life, transcriptional activity, and nuclear localization	G1/S and G2/M	Inhibition	Gotoh et al. (2014, 2015, 2016)
Mouse liver	Rhythmic expression of c-Myc and CyclinD1	G1/S	Promotion	Fu et al. (2002)
Mouse liver	Rhythmic expression of GADD45	G2/M	Inhibition
Mouse embryonic fibroblasts (MEFs) and mouse lymphoma	CRY2 works as a component of ubiquitin ligase complex to degrade c-Myc	G1/S	Promotion	Huber et al. (2016)
Mouse hepatocellular carcinoma cells (mHCC 3-4) and human bone osteosarcoma epithelial cells (U2OS)	c-Myc disrupts circadian rhythms through inducing REV-ERBα expression	NA	NA	Altman et al. (2015), Shostak et al. (2016)
Mouse intestinal stem/progenitor cells (mouse intestinal organoids)	Circadian clock regulated WNT production/secretion	G1/S	Promotion	Matsu-Ura et al. (2016)

Dysregulated cellular proliferation is a characteristic property of cancer. Oscillations of circadian clock genes were reported in cancer cell lines including osteosarcoma cells (U2OS) (Hughes et al., 2009), breast cancer cells (MCF10A) (Xiang et al., 2012), and colorectal cancer cells (HCT116 and Caco2) (Gotoh et al., 2016; Moore et al., 2014). In contrast, it has been shown that circadian clock–related genes are impaired in most human cancers, suggesting that cancer cells target the circadian clock machinery to achieve uncontrolled growth and proliferation (Davidson et al., 2006). In fact, the number of rhythmic genes is dramatically reduced in cancers and immortalized cell lines cultured in vitro (percentage of rhythmic genes: 1.5% in U2OS [Krishnaiah et al., 2017]; 2.6% in NIH3T3 [Menger et al., 2007]; and 1.9% in Rat-1 [Duffield et al., 2002]) compared with liver and other organs (10%-40%) (Panda et al., 2002; Vollmers et al., 2009; Zhang et al., 2014). Although the difference in the number of rhythmic genes between cell lines and mouse organs may be due to differences in conditions in vitro and in vivo, these results suggest a disruption of the molecular clockworks in cancer and immortalized cells. In 2007, the International Agency for Research on Cancer (IARC) categorized shiftwork that involves circadian disruption as carcinogenic to humans (Straif et al., 2007). Per gene expression is associated with tumor development in γ-irradiated mice (Fu et al., 2002) and human cancers (Zhao et al., 2014). BMAL1 induces apoptosis in pancreatic cancer cells via p53 signaling (Jiang et al., 2016), and downregulation of Bmal1 expression results in increased cell proliferation and tumor growth of colon cancer (Zeng et al., 2010). This evidence supports the view that defective circadian clock gene expression and disruption of circadian rhythms correlate with tumor development and tumor progression in various human cancers (Savvidis and Koutsilieris, 2012).

The Circadian Clock in Adult Stem Cells and Its Connection to the Cell Cycle

Many of the previous studies used cancer or immortalized cell lines to investigate molecular coupling between the circadian clock and cell cycle/proliferation in the mammalian system. However, it is important to note that adult stem cells are one of the major proliferating cell types in our body, and the circadian clock appears to operate differently depending on the context of stem cells. Therefore, it is critical to investigate the roles of circadian rhythms in stem cell regeneration and proliferation. Yagita et al. (2010) reported the lack of circadian rhythms in mouse embryonic stem cells and demonstrated the initiation of circadian rhythms during differentiation. In contrast, human epidermal stem cells possess canonical TTFL of clock components with similar robustness as differentiated keratinocytes, with stemness, differentiation, epidermal aging, and tumorization regulated by the circadian clock (Janich et al., 2011; Janich et al., 2013). The circadian clock gates the cell cycle in epidermal stem cells, which controls the timing of S-phase at around the late afternoon to provide protection against DNA damage from UV irradiation (Geyfman et al., 2012; Plikus et al., 2013). In addition, quiescent neural stem/progenitor cells in the subgranular zone of adult mouse hippocampus possess rhythmic Per2 gene expressing cells and demonstrate time-of-day-dependent proliferation (Bouchard-Cannon et al., 2013). Interestingly, both BMAL1 and PER2 levels are strongly suppressed in the stem cells during the cell cycle, which releases the stem cells from inhibitory effects of cell cycle progression by the circadian clock (see Table 1) (Bouchard-Cannon et al., 2013).

In contrast to the aforementioned direct/intracellular coupling and regulation of stem/progenitor cell proliferation by the circadian clock, adult stem cells are also regulated indirectly by circadian rhythms. Hematopoietic stem cells and progenitor cells (HSPCs) circulate in the bloodstream, and the release of HSPCs from bone marrow is rhythmically regulated by noradrenaline secretion from the sympathetic nervous system that is under the control of the master clock (Mendez-Ferrer et al., 2008). Drosophila intestinal stem cells (ISCs) have a circadian rhythm of PER protein expression, and RNAi of per gene abolishes the daily proliferation rhythm of ISCs in damaged intestine (Karpowicz et al., 2013). Importantly, the daily proliferation rhythm in Drosophila ISCs is disrupted by enterocyte-specific per gene knockdown by RNAi (Karpowicz et al., 2013), suggesting indirect regulation of the cell cycle in ISCs via enterocytes. Recently, we reported a novel intercellular coupling mechanism between the circadian clock and the cell cycle in mouse enteroids (intestinal organoids), which possess ISCs, progenitor cells (PCs), and differentiated epithelial cells including Paneth cells. First, we noticed that the circadian rhythm of PER2 expression is weak or absent in the dividing ISCs/PCs compared with terminally differentiated intestinal epithelial cells in mouse enteroids (Matsu-Ura et al., 2016). Then we discovered that the circadian clock in Paneth cells regulates rhythmic WNT expression and secretion, which controls the circadian clock–gated cell divisions of ISCs and PCs, establishing an intercellular connection between these two oscillators (Fig. 1B). These findings indicate that tissue-specific adult stem cells in different tissues are in different stages of circadian clock development, which results in distinct circadian clock phenotypes depending on the types of adult stem cells.

WNT Signaling in Adult Stem Cells and Its Interaction with the Circadian Clock

WNT signaling is known to regulate the self-renewal and differentiation of various adult stem cells. In the small intestine, secreted WNT from Paneth cells promotes self-renewal of ISCs (Clevers et al., 2014). The activity of WNT is meticulously controlled for tissue patterning in the stomach. The homeodomain transcription factor, BarH-Like Homeobox 1 (BARX1), is abundantly expressed in embryonic mesenchymal cells in the stomach and regulates the expression of WNT inhibitor, Secreted Frizzled Related Protein 1 (SFRP), resulting in the development of gastric epithelium (Kim et al., 2005). In addition, gastric and colon cancers are associated with abnormality of canonical WNT signaling (Polakis, 2000; Vogiatzi et al., 2007). The importance of WNT signaling for self-renewal of adult stem cells in the gastrointestinal tract is also highlighted by the requirement of exogenous WNT for the growth and maintenance of organoid cultures from mammalian taste bud (Aihara et al., 2015), stomach (Barker et al., 2010), intestine (Sato et al., 2011), and colon (Sato et al., 2011). As well, WNT signaling is critical for the niche of adult stem cells in other tissues. Adult rat hippocampal neural progenitor cells (AHPs) express various types of Wnt and Frizzled (Fzd) genes, and WNT signaling maintains multipotency of AHPs and controls their differentiation (Wexler et al., 2009). Identical to AHPs, epidermal stem cells self-renew by autocrine WNT (Lim et al., 2013). WNT signaling is essential for proper embryonic cardiac differentiation, progenitor expansion, and myocardial growth (Klaus et al., 2007). Modulation of WNT activity enhances directed cardiac differentiation of human pluripotent stem cells (Lian et al., 2013), and WNT signaling is suggested to increase cardiac repair after ischemic damage of myocardium and epicardium (Duan et al., 2012). These results suggest that the link between the circadian clock and cell cycle through WNT signaling has an extensive impact on various adult stem cells.

Interactions of the circadian clock and WNT signaling have been described in other cells, tissues, and organisms. Numerous genes in WNT signaling are under circadian regulation, which highlights the impact of circadian rhythms on WNT signaling. Approximately 50 genes that are involved in WNT signaling demonstrate rhythmic expression profiles in 39 microarray data sets from 19 distinct mammalian tissues (Sotak et al., 2014). ChIP-Seq analysis using mouse liver (Rey et al., 2011) revealed rhythmic occupancy of BMAL1 on the promoters of the canonical WNT signaling pathway including Wnt10a, β-catenin, Fzd5, Dishevelled Segment Polarity Protein 2 (Dvl2), and Transcription Factor 3 (TCF3) (Guo et al., 2012), which highlights a direct link between the circadian clock and WNT signaling. Furthermore, a negative WNT signaling regulator, Axis Inhibition Protein 2 (AXIN-2), and WNT antagonist, Dickkopf WNT Signaling Pathway Inhibitor 3 (DKK-3), show rhythmic gene expression in arcuate nucleus in the brain of female Djungarian hamster (Boucsein et al., 2016).

Genetic and physiological disturbances of circadian rhythms affect the expression of genes in WNT signaling. Xenografted cancer cells proliferate faster in nude mice exposed to a “circadian disrupted” constant light condition compared with nude mice exposed to a “normal” 12-h light-dark cycle, secondary to increased expression of WNT10A in dermal fibroblasts due to disruption of the circadian clock (Yasuniwa et al., 2010). Furthermore, overexpression of Bmal1 increased the expression of β-catenin and cell proliferation in NIH3T3 cells (Lin et al., 2013). In contrast, shRNA knockdown of Bmal1 decreased the expression of genes in the canonical WNT pathway, Axin2, Wnt10a, β-catenin, Fzd5, Dvl2, and TCF3, during adipogenesis of 10T1/2 mouse embryonic cells (Guo et al., 2012). Last, in zebrafish, mRNAs of wnt1, wnt3a, and wnt5a were down-regulated in neurons of per1 mutants in constant dark conditions (Huang et al., 2015). Together, these data indicate a robust connection between the circadian clock and WNT signaling, which widens the impact of circadian rhythms in cell proliferation, self-renewal and differentiation of stem cells, and tissue patterning.

Conclusion

Three and a half centuries following Christiaan Huygens’ classic description of coupled oscillations between two pendulum clocks in 1665 (Huygens, 1893), coupled oscillators continue to be identified throughout the natural world. In biology, coupled oscillators generate diverse dynamic behaviors such as the synchronous flashing of fireflies (Blair, 1915), circadian rhythms in the SCN (Barker et al., 2007), frog choruses (Aihara et al., 2014), myocardial cell contractions (Mirollo and Strogatz, 1990), polyrhythmic interactions among critical brain areas during working memory maintenance (Fujisawa and Buzsaki, 2011), and time-of-day proliferation of somatic cells including adult stem/progenitor cells. These observations indicate the ubiquitous nature of coupled oscillators, but molecular mechanisms that determine diverse dynamic behaviors remain largely unexplored. The circadian clock and the cell cycle use both intra- and intercellular coupling mechanisms to establish their connections, which may vary depending on the cell types and multicellular environment.

We recently demonstrated that the circadian clock in Paneth cells regulates rhythmic expression and secretion of WNT, which coordinates the timing of cell divisions of adult stem and progenitor cells resulting in different coupling ratios depending on the inherent frequency of cell cycle times (Matsu-Ura et al., 2016). Importantly, we identified rhythmic intestinal crypt formation following the coordinated cell division cycles, as well as a reduced number of crypts in the small intestine of circadian arrhythmic, Per1/2-double-knockout, mice. These data suggest that the circadian clock–dependent gating of cell division cycles affects proper timing and formation of intestinal crypts. Importantly, our data enable us to hypothesize that gastrointestinal stem and progenitor cells will likely respond differently to chemotherapeutic drugs such as 5-Fluorouracil depending on the time of administration due to clock-gated cell cycle progression.

Unintended damage to healthy stem and progenitor cells remains a major limitation of cancer chemotherapy (Davidson et al., 2006; Savvidis and Koutsilieris, 2012). Further, disruption of circadian rhythms is associated with a higher risk of various human cancers (Savvidis and Koutsilieris, 2012), and the circadian clock is dysregulated in most human neoplasms (Davidson et al., 2006). Therefore, it will be important to characterize cell cycle profiles of different adult stem cells and cancer cells with respect to the circadian cycle and determine whether those cells show time-of-day-specific responses to chemotherapy. This information could then be exploited to design novel treatment regimens to administer chemotherapeutic drugs at the trough of cell divisions of healthy stem/progenitor cells to reduce the risk of unintended damage to healthy cells, without reducing the efficacy of drugs to eliminate cancerous cells. Further investigation of the molecular coupling components and the intimate dance between the cell cycle and the circadian clock in both normal and cancer cells will be critical to effectively use circadian molecular signatures to treat cancer.

Footnotes

Acknowledgements

This work was supported by Department of Interior grant D12AP00005 (to S.R.M. and C.I.H.) and NIH grant 1U19AI116491 (to S.R.M. and C.I.H.). All authors contributed to writing this manuscript and generating figures.

Conflict of Interest Statement

The author(s) have no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

ORCID iD

Toru Matsu-ura

References

Aihara

Mahe

Schumacher

Matthis

Feng

Ren

Noah

Matsu-ura

Moore

Hong

et al . (2015) Characterization of stem/progenitor cell cycle using murine circumvallate papilla taste bud organoid. Sci Rep 5:17185.

Aihara

Mizumoto

Otsuka

Awano

Nagira

Okuno

Aihara

(2014) Spatio-temporal dynamics in collective frog choruses examined by mathematical modeling and field observations. Sci Rep 4:3891.

Akashi

Tsuchiya

Yoshino

Nishida

(2002) Control of intracellular dynamics of mammalian period proteins by casein kinase I epsilon (CKIepsilon) and CKIdelta in cultured cells. Mol Cell Biol 22:1693-1703.

Altman

Hsieh

Sengupta

Krishnanaiah

Stine

Walton

Gouw

Venkataraman

Goraksha-Hicks

et al . (2015) MYC Disrupts the circadian clock and metabolism in cancer cells. Cell Metab 22:1009-1019.

Baker

Kettenbach

Loros

Gerber

Dunlap

(2009) Quantitative proteomics reveals a dynamic interactome and phase-specific phosphorylation in the Neurospora circadian clock. Mol Cell 34:354-363.

Barker

Huch

Kujala

van de Wetering

Snippert

van Es

Sato

Stange

Begthel

et al . (2010) Lgr5(+ve) stem cells drive self-renewal in the stomach and build long-lived gastric units in vitro. Cell Stem Cell 6:25-36.

Barker

van Es

Kuipers

Kujala

van den Born

Cozijnsen

Haegebarth

Korving

Begthel

Peters

et al . (2007). Identification of stem cells in small intestine and colon by marker gene Lgr5. Nature 449:1003-1007.

Bieler

Cannavo

Gustafson

Gobet

Gatfield

Naef

(2014) Robust synchronization of coupled circadian and cell cycle oscillators in single mammalian cells. Mol Syst Biol 10:739.

Blair

(1915) Luminous insects. Nature 96:411-415.

10.

Bouchard-Cannon

Mendoza-Viveros

Yuen

Kaern

Cheng

(2013) The circadian molecular clock regulates adult hippocampal neurogenesis by controlling the timing of cell-cycle entry and exit. Cell Rep 5:961-973.

11.

Boucsein

Benzler

Hempp

Stohr

Helfer

Tups

(2016) Photoperiodic and diurnal regulation of WNT signaling in the arcuate nucleus of the female Djungarian hamster, Phodopus sungorus. Endocrinology 157:799-809.

12.

Busino

Bassermann

Maiolica

Lee

Nolan

Godinho

Draetta

Pagano

(2007) SCFFbxl3 controls the oscillation of the circadian clock by directing the degradation of cryptochrome proteins. Science 316:900-904.

13.

Clevers

Loh

Nusse

(2014) Stem cell signaling: an integral program for tissue renewal and regeneration: Wnt signaling and stem cell control. Science 346:1248012.

14.

Davidson

Straume

Block

Menaker

(2006) Daily timed meals dissociate circadian rhythms in hepatoma and healthy host liver. Int J Cancer 118:1623-1627.

15.

Duan

Gherghe

Liu

Hamlett

Srikantha

Rodgers

Regan

Rojas

Willis

Leask

et al . (2012) Wnt1/betacatenin injury response activates the epicardium and cardiac fibroblasts to promote cardiac repair. EMBO J 31:429-442.

16.

Duffield

Best

Meurers

Bittner

Loros

Dunlap

(2002) Circadian programs of transcriptional activation, signaling, and protein turnover revealed by microarray analysis of mammalian cells. Curr Biol 12:551-557.

17.

Eide

Vielhaber

Hinz

Virshup

(2002) The circadian regulatory proteins BMAL1 and cryptochromes are substrates of casein kinase Iepsilon. J Biol Chem 277:17248-17254.

18.

Pelicano

Liu

Huang

Lee

(2002) The circadian gene Period2 plays an important role in tumor suppression and DNA damage response in vivo. Cell 111:41-50.

19.

Fujisawa

Buzsaki

(2011) A 4 Hz oscillation adaptively synchronizes prefrontal, VTA, and hippocampal activities. Neuron 72:153-165.

20.

Gery

Komatsu

Baldjyan

Koo

Koeffler

(2006) The circadian gene per1 plays an important role in cell growth and DNA damage control in human cancer cells. Mol Cell 22:375-382.

21.

Geyfman

Kumar

Liu

Ruiz

Gordon

Espitia

Cam

Millar

Smyth

Ihler

et al . (2012) Brain and muscle Arnt-like protein-1 (BMAL1) controls circadian cell proliferation and susceptibility to UVB-induced DNA damage in the epidermis. Proc Natl Acad Sci USA 109:11758-11763.

22.

Godinho

Maywood

Shaw

Tucci

Barnard

Busino

Pagano

Kendall

Quwailid

Romero

et al . (2007) The after-hours mutant reveals a role for Fbxl3 in determining mammalian circadian period. Science 316:897-900.

23.

Gossan

Zhang

Guo

Jin

Yoshitane

Yao

Glossop

Zhang

Fukada

Meng

(2014) The E3 ubiquitin ligase UBE3A is an integral component of the molecular circadian clock through regulating the BMAL1 transcription factor. Nucleic Acids Res 42:5765-5775.

24.

Gotoh

Kim

Liu

Vila-Caballer

Stauffer

Tyson

Finkielstein

(2016) Model-driven experimental approach reveals the complex regulatory distribution of p53 by the circadian factor Period 2. Proc Natl Acad Sci U S A 113:13516-13521.

25.

Gotoh

Vila-Caballer

Liu

Schiffhauer

Finkielstein

(2015) Association of the circadian factor Period 2 to p53 influences p53’s function in DNA-damage signaling. Mol Biol Cell 26:359-372.

26.

Gotoh

Vila-Caballer

Santos

Liu

Yang

Finkielstein

(2014) The circadian factor Period 2 modulates p53 stability and transcriptional activity in unstressed cells. Mol Biol Cell 25:3081-3093.

27.

Grechez-Cassiau

Rayet

Guillaumond

Teboul

Delaunay

(2008) The circadian clock component BMAL1 is a critical regulator of p21WAF1/CIP1 expression and hepatocyte proliferation. J Biol Chem 283:4535-4542.

28.

Guo

Chatterjee

Kim

Lee

Yechoor

Minze

Hsueh

(2012) The clock gene, brain and muscle Arnt-like 1, regulates adipogenesis via Wnt signaling pathway. FASEB J 26:3453-3463.

29.

Hirano

Yumimoto

Tsunematsu

Matsumoto

Oyama

Kozuka-Hata

Nakagawa

Lanjakornsiripan

Nakayama

Fukada

(2013) FBXL21 regulates oscillation of the circadian clock through ubiquitination and stabilization of cryptochromes. Cell 152:1106-1118.

30.

Huang

Zhong

Wang

Chen

Tan

Zhang

Huang

et al . (2015) Circadian modulation of dopamine levels and dopaminergic neuron development contributes to attention deficiency and hyperactive behavior. J Neurosci 35:2572-2587.

31.

Huber

Papp

Chan

Henriksson

Jordan

Kriebs

Nguyen

Wallace

Metallo

et al . (2016) CRY2 and FBXL3 cooperatively degrade c-MYC. Mol Cell 64:774-789.

32.

Hughes

DiTacchio

Hayes

Vollmers

Pulivarthy

Baggs

Panda

Hogenesch

(2009) Harmonics of circadian gene transcription in mammals. PLoS Genet 5:e1000442.

33.

Straif

Baan

Grosse

Secretan

Ghissassi

Bouvard

Altieri

Benbrahim-Tallaa

Cogliano

(2007) Carcinogenicity of shift-work, painting, and fire-fighting. Lancet Oncol 8:1065-1066.

34.

Hurley

Loros

Dunlap

(2016) Circadian oscillators: around the transcription-translation feedback loop and on to output. Trends Biochem Sci 41:834-846.

35.

Huygens

(1893) Letter to de Sluse. In: Oeuveres Completes de Christian Huygens (letters; no. 1333 of 24 February 1665, no. 1335 of 26 February 1665, no. 1345 of 6 March 1665). The Hague: Martinus Nijhoff. Societe Hollandaise Des Sciences.

36.

Janich

Pascual

Merlos-Suarez

Batlle

Ripperger

Albrecht

Cheng

Obrietan

Di Croce

Benitah

(2011) The circadian molecular clock creates epidermal stem cell heterogeneity. Nature 480:209-214.

37.

Janich

Toufighi

Solanas

Luis

Minkwitz

Serrano

Lehner

Benitah

(2013) Human epidermal stem cell function is regulated by circadian oscillations. Cell Stem Cell 13:745-753.

38.

Jiang

Zhao

Jiang

Zhang

Zheng

Xiao

et al . (2016) The circadian clock gene Bmal1 acts as a potential anti-oncogene in pancreatic cancer by activating the p53 tumor suppressor pathway. Cancer Lett 371:314-325.

39.

Karpowicz

Zhang

Hogenesch

Emery

Perrimon

(2013) The circadian clock gates the intestinal stem cell regenerative state. Cell Rep 3:996-1004.

40.

Kim

Buchner

Miletich

Sharpe

Shivdasani

(2005) The stomach mesenchymal transcription factor Barx1 specifies gastric epithelial identity through inhibition of transient Wnt signaling. Dev Cell 8:611-622.

41.

Klaus

Saga

Taketo

Tzahor

Birchmeier

(2007) Distinct roles of Wnt/beta-catenin and Bmp signaling during early cardiogenesis. Proc Natl Acad Sci USA 104:18531-18536.

42.

Kondratov

Chernov

Kondratova

Gorbacheva

Gudkov

Antoch

(2003) BMAL1-dependent circadian oscillation of nuclear CLOCK: posttranslational events induced by dimerization of transcriptional activators of the mammalian clock system. Genes Dev 17:1921-1932.

43.

Kowalska

Ripperger

Hoegger

Bruegger

Buch

Birchler

Mueller

Albrecht

Contaldo

Brown

(2013) NONO couples the circadian clock to the cell cycle. Proc Natl Acad Sci USA 110:1592-1599.

44.

Krishnaiah

Altman

Growe

Rhoades

Coldren

Venkataraman

Olarerin-George

Francey

Mukherjee

et al . (2017). Clock regulation of metabolites reveals coupling between transcription and metabolism. Cell Metab 25:1206.

45.

Larrondo

Olivares-Yanez

Baker

Loros

Dunlap

(2015) Circadian rhythms: decoupling circadian clock protein turnover from circadian period determination. Science 347:1257277.

46.

Lee

Chen

Lee

Yoo

Lee

(2009) Essential roles of CKIdelta and CKIepsilon in the mammalian circadian clock. Proc Natl Acad Sci USA 106:21359-21364.

47.

Lian

Zhang

Azarin

Zhu

Hazeltine

Bao

Hsiao

Kamp

Palecek

(2013) Directed cardiomyocyte differentiation from human pluripotent stem cells by modulating Wnt/beta-catenin signaling under fully defined conditions. Nat Protoc 8:162-175.

48.

Lim

Tan

Koh

Chau

Yan

Kuo

van Amerongen

Klein

Nusse

(2013) Interfollicular epidermal stem cells self-renew via autocrine Wnt signaling. Science 342:1226-1230.

49.

Lin

Chen

Zhao

Tan

(2013) Over-expression of circadian clock gene Bmal1 affects proliferation and the canonical Wnt pathway in NIH-3T3 cells. Cell Biochem Funct 31:166-172.

50.

Liu

Dang

Matsu-Ura

Hong

Liu

(2017) DNA replication is required for circadian clock function by regulating rhythmic nucleosome composition. Mol Cell 67:203-213 e204.

51.

Lowrey

Takahashi

(2011) Genetics of circadian rhythms in mammalian model organisms. Adv Genet 74:175-230.

52.

Matsu-Ura

Dovzhenok

Aihara

Rood

Ren

Rosselot

Zhang

Lee

Obrietan

et al . (2016) Intercellular coupling of the cell cycle and circadian clock in adult stem cell culture. Mol Cell 64:900-912.

53.

Matsuo

Yamaguchi

Mitsui

Emi

Shimoda

Okamura

(2003) Control mechanism of the circadian clock for timing of cell division in vivo. Science 302:255-259.

54.

Mendez-Ferrer

Lucas

Battista

Frenette

(2008) Haematopoietic stem cell release is regulated by circadian oscillations. Nature 452:442-447.

55.

Menger

Allen

Neuendorff

Nahm

Thomas

Cassone

Earnest

(2007) Circadian profiling of the transcriptome in NIH/3T3 fibroblasts: comparison with rhythmic gene expression in SCN2.2 cells and the rat SCN. Physiol Genomics 29:280-289.

56.

Mirollo

Strogatz

(1990). Synchronization of pulse-coupled biological oscillators. SIAM J Appl Math 50:1645-1662.

57.

Moore

Pruszka

Vallance

Aihara

Matsuura

Montrose

Shroyer

Hong

(2014) Robust circadian rhythms in organoid cultures from PERIOD2::LUCIFERASE mouse small intestine. Dis Model Mech 7:1123-1130.

58.

Panda

Antoch

Miller

Schook

Straume

Schultz

Kay

Takahashi

Hogenesch

(2002) Coordinated transcription of key pathways in the mouse by the circadian clock. Cell 109:307-320.

59.

Plikus

Vollmers

de la Cruz

Chaix

Ramos

Panda

Chuong

(2013) Local circadian clock gates cell cycle progression of transient amplifying cells during regenerative hair cycling. Proc Natl Acad Sci USA 110:E2106-2115.

60.

Polakis

(2000) Wnt signaling and cancer. Genes Dev 14:1837-1851.

61.

Reischl

Vanselow

Westermark

Thierfelder

Maier

Herzel

Kramer

(2007) Beta-TrCP1-mediated degradation of PERIOD2 is essential for circadian dynamics. J Biol Rhythms 22:375-386.

62.

Rey

Cesbron

Rougemont

Reinke

Brunner

Naef

(2011) Genome-wide and phase-specific DNA-binding rhythms of BMAL1 control circadian output functions in mouse liver. PLoS Biol 9:e1000595.

63.

Sato

van Es

Snippert

Stange

Vries

van den Born

Barker

Shroyer

van de Wetering

Clevers

(2011) Paneth cells constitute the niche for Lgr5 stem cells in intestinal crypts. Nature 469:415-418.

64.

Savvidis

Koutsilieris

(2012) Circadian rhythm disruption in cancer biology. Mol Med 18:1249-1260.

65.

Shostak

Ruppert

Bruns

Toprak

,ICGC MMML-Seq Project, Eils

Schlesner

Diernfellner

Brunner

(2016) MYC/MIZ1-dependent gene repression inversely coordinates the circadian clock with cell cycle and proliferation. Nat Commun 7:11807.

66.

Siepka

Yoo

Park

Song

Kumar

Lee

Takahashi

(2007) Circadian mutant Overtime reveals F-box protein FBXL3 regulation of cryptochrome and period gene expression. Cell 129:1011-1023.

67.

Sotak

Sumova

Pacha

(2014) Cross-talk between the circadian clock and the cell cycle in cancer. Ann Med 46:221-232.

68.

Tamaru

Hirayama

Isojima

Nagai

Norioka

Takamatsu

Sassone-Corsi

(2009) CK2alpha phosphorylates BMAL1 to regulate the mammalian clock. Nat Struct Mol Biol 16:446-448.

69.

Vanselow

Westermark

Reischl

Maier

Korte

Herrmann

Herzel

Schlosser

Kramer

(2006) Differential effects of PER2 phosphorylation: molecular basis for the human familial advanced sleep phase syndrome (FASPS). Genes Dev 20:2660-2672.

70.

Vogiatzi

Vindigni

Roviello

Renieri

Giordano

(2007) Deciphering the underlying genetic and epigenetic events leading to gastric carcinogenesis. J Cell Physiol 211:287-295.

71.

Vollmers

Gill

DiTacchio

Pulivarthy

Panda

(2009) Time of feeding and the intrinsic circadian clock drive rhythms in hepatic gene expression. Proc Natl Acad Sci USA 106:21453-21458.

72.

Wexler

Paucer

Kornblum

Palmer

Geschwind

(2009) Endogenous Wnt signaling maintains neural progenitor cell potency. Stem Cells 27:1130-1141.

73.

Xiang

Mao

Duplessis

Yuan

Dauchy

Blask

Frasch

Hill

(2012) Oscillation of clock and clock controlled genes induced by serum shock in human breast epithelial and breast cancer cells: regulation by melatonin. Breast Cancer (Auckl) 6:137-150.

74.

Yagita

Horie

Koinuma

Nakamura

Yamanaka

Urasaki

Shigeyoshi

Kawakami

Shimada

Takeda

et al . (2010) Development of the circadian oscillator during differentiation of mouse embryonic stem cells in vitro. Proc Natl Acad Sci USA 107:3846-3851.

75.

Yasuniwa

Izumi

Wang

Shimajiri

Sasaguri

Kawai

Kasai

Shimada

Miyake

Kashiwagi

et al . (2010) Circadian disruption accelerates tumor growth and angio/stromagenesis through a Wnt signaling pathway. PLoS One 5:e15330.

76.

Yin

Joshi

Tong

Lazar

(2010) E3 ligases Arf-bp1 and Pam mediate lithium-stimulated degradation of the circadian heme receptor Rev-erb alpha. Proc Natl Acad Sci USA 107:11614-11619.

77.

Yoo

Mohawk

Siepka

Shan

Huh

Hong

Kornblum

Kumar

Koike

et al . (2013) Competing E3 ubiquitin ligases govern circadian periodicity by degradation of CRY in nucleus and cytoplasm. Cell 152:1091-1105.

78.

Zeng

Sun

Cai

Huang

Xian

(2010) Effects of the biological clock gene Bmal1 on tumour growth and anti-cancer drug activity. J Biochem 148:319-326.

79.

Zhang

Lahens

Ballance

Hughes

Hogenesch

(2014) A circadian gene expression atlas in mammals: implications for biology and medicine. Proc Natl Acad Sci USA 111:16219-16224.

80.

Zhao

Zeng

Yang

Jin

Qiu

Han

Liu

Liao

et al . (2014) Prognostic relevance of Period1 (Per1) and Period2 (Per2) expression in human gastric cancer. Int J Clin Exp Pathol 7:619-630.