Interaction of Circadian Clock Proteins CRY1 and PER2 Is Modulated by Zinc Binding and Disulfide Bond Formation

Zinc is an important inter-kingdom signal between the host and microbe

Zinc (Zn) is an essential trace element in living organisms and plays a vital role in the regulation of both microbial virulence and host immune responses. A growing number of studies have shown that zinc deficiency or the internal Zn concentration does not meet the needs of animals and microbes, leading to an imbalance in zinc homeostasis and intracellular signalling pathway dysregulation. Competition for zinc ions (Zn²⁺) between microbes and the host exists in the use of Zn²⁺ to maintain cell structure and physiological functions. It also affects the interplay between microbial virulence factors and their specific receptors in the host. This review will focus on the role of Zn in the crosstalk between the host and microbe, especially for changes in microbial pathogenesis and nociceptive neuron-immune interactions, as it may lead to new ways to prevent or treat microbial infections.

Circadian hepatocyte clocks keep synchrony in the absence of a master pacemaker in the suprachiasmatic nucleus or other extrahepatic clocks

In this study, Sinturel et al. sought to show that the suprachiasmatic nucleus (SCN) synchronizes peripheral circadian oscillators. By using long-term bioluminescence recordings in freely moving mice, they show that the SCN is indeed required for maintaining synchrony between organs and that circadian oscillations persist in the livers of mice devoid of an SCN or oscillators in cells other than hepatocytes.

It has been assumed that the suprachiasmatic nucleus (SCN) synchronizes peripheral circadian oscillators. However, this has never been convincingly shown, since biochemical time series experiments are not feasible in behaviorally arrhythmic animals. By using long-term bioluminescence recording in freely moving mice, we show that the SCN is indeed required for maintaining synchrony between organs. Surprisingly, however, circadian oscillations persist in the livers of mice devoid of an SCN or oscillators in cells other than hepatocytes. Hence, similar to SCN neurons, hepatocytes can maintain phase coherence in the absence of Zeitgeber signals produced by other organs or environmental cycles.

Photopharmacological Manipulation of Mammalian CRY1 for Regulation of the Circadian Clock

CRY1 and CRY2 proteins are highly conserved components of the circadian clock that controls daily physiological rhythms. Disruption of CRY functions are related to many diseases, including circadian sleep phase disorder. Development of isoform-selective and spatiotemporally controllable tools will facilitate the understanding of shared and distinct functions of CRY1 and CRY2. Here, we developed CRY1-selective compounds that enable light-dependent manipulation of the circadian clock. From phenotypic chemical screening in human cells, we identified benzophenone derivatives that lengthened the circadian period. These compounds selectively interacted with the CRY1 photolyase homology region, resulting in activation of CRY1 but not CRY2. The benzophenone moiety rearranged a CRY1 region called the “lid loop” located outside of the compound-binding pocket and formed a unique interaction with Phe409 in the lid loop. Manipulation of this key interaction was achieved by rationally designed replacement of the benzophenone with a switchable azobenzene moiety whose cis–trans isomerization can be controlled by light. The metastable cis form exhibited sufficiently high half-life in aqueous solutions and structurally mimicked the benzophenone unit, enabling reversible period regulation over days by cellular irradiation with visible light. This study revealed an unprecedented role of the lid loop in CRY-compound interaction and paves the way for spatiotemporal regulation of CRY1 activity by photopharmacology for molecular understanding of CRY1-dependent functions in health and disease.

The tail of cryptochromes: an intrinsically disordered cog within the mammalian circadian clock

Cryptochrome (CRY) proteins play an essential role in regulating mammalian circadian rhythms. CRY is composed of a structured N-terminal domain known as the photolyase homology region (PHR), which is tethered to an intrinsically disordered C-terminal tail. The PHR domain is a critical hub for binding other circadian clock components such as CLOCK, BMAL1, PERIOD, or the ubiquitin ligases FBXL3 and FBXL21. While the isolated PHR domain is necessary and sufficient to generate circadian rhythms, removing or modifying the cryptochrome tails modulates the amplitude and/or periodicity of circadian rhythms, suggesting that they play important regulatory roles in the molecular circadian clock. In this commentary, we will discuss how recent studies of these intrinsically disordered tails are helping to establish a general and evolutionarily conserved model for CRY function, where the function of PHR domains is modulated by reversible interactions with their intrinsically disordered tails.

The human CRY1 tail controls circadian timing by regulating its association with CLOCK:BMAL1

Circadian rhythms are generated by interlocked transcription-translation feedback loops that establish cell-autonomous biological timing of ∼24 h. Mutations in core clock genes that alter their stability or affinity for one another lead to changes in circadian period. The human CRY1Δ11 mutant lengthens circadian period to cause delayed sleep phase disorder (DSPD), characterized by a very late onset of sleep. CRY1 is a repressor that binds to the transcription factor CLOCK:BMAL1 to inhibit its activity and close the core feedback loop. We previously showed how the PHR (photolyase homology region) domain of CRY1 interacts with distinct sites on CLOCK and BMAL1 to sequester the transactivation domain from coactivators. However, the Δ11 variant alters an intrinsically disordered tail in CRY1 downstream of the PHR. We show here that the CRY1 tail, and in particular the region encoded by exon 11, modulates the affinity of the PHR domain for CLOCK:BMAL1. The PHR-binding epitope in exon 11 is necessary and sufficient to disrupt the interaction between CRY1 and the subunit CLOCK. Moreover, PHR-tail interactions are conserved in the paralog CRY2 and reduced when either CRY is bound to the circadian corepressor PERIOD2. Discovery of this autoregulatory role for the mammalian CRY1 tail and conservation of PHR-tail interactions in both mammalian cryptochromes highlights functional conservation with plant and insect cryptochromes, which also utilize PHR-tail interactions to reversibly control their activity.

Bile acid metabolism and circadian rhythms

Circadian rhythms are biological systems that synchronize cellular circadian oscillators with the organism's daily feeding-fasting or rest-activity cycles in mammals. Circadian rhythms regulate nutrient absorption and utilization at the cellular level and are closely related to obesity and metabolic disorders. Bile acids are important modulators that facilitate nutrient absorption and regulate energy metabolism. Here, we provide an overview of the current connections and future perspectives between the circadian clock and bile acid metabolism as well as related metabolic diseases. Feeding and fasting cycles influence bile acid pool size and composition, and bile acid signaling can respond to acute lipid and glucose utilization and mediate energy balance. Disruption of circadian rhythms such as shift work, irregular diet, and gene mutations can contribute to altered bile acid metabolism and heighten obesity risk. High-fat diets, alcohol, and gene mutations related to bile acid signaling result in desynchronized circadian rhythms. Gut microbiome also plays a role in connecting circadian rhythms with bile acid metabolism. The underlying mechanism of how circadian rhythms interact with bile acid metabolism has not been fully explored. Sustaining bile acid homeostasis based on circadian rhythms may be a potential therapy to alleviate metabolic disturbance.

Environmental Factors Such as Noise and Air Pollution and Vascular Disease

Significance: According to the World Health Organization, noncommunicable diseases are the globally leading cause of mortality. Recent Advances: About 71% of 56 million deaths that occurred worldwide are due to noncommunicable cardiovascular risk factors, including tobacco smoking, unhealthy diets, lack of physical activity, overweight, arterial hypertension, diabetes, and hypercholesterolemia, which can be either avoided or substantially reduced. Critical Issues: Thus, it is estimated that 80% of premature heart disease, stroke, and diabetes can be prevented. More recent evidence indicates that environmental stressors such as noise and air pollution contribute significantly to the global burden of cardiovascular disease. In the present review, we focus primarily on important environmental stressors such as transportation noise and air pollution. We discuss the pathophysiology of vascular damage caused by these environmental stressors, with emphasis on early subclinical damage of the vasculature such as endothelial dysfunction and the role of oxidative stress. Future Directions: Lower legal thresholds and mitigation measures should be implemented and may help to prevent vascular damage.

Galvanization of Protein-Protein Interactions in a Dynamic Zinc Interactome

The presence of Zn²⁺ at protein-protein interfaces modulates complex function, stability, and introduces structural flexibility/complexity, chemical selectivity, and reversibility driven in a Zn²⁺-dependent manner. Recent studies have demonstrated that dynamically changing Zn²⁺ affects numerous cellular processes, including protein-protein communication and protein complex assembly. How Zn²⁺-involved protein-protein interactions (ZPPIs) are formed and dissociate and how their stability and reactivity are driven in a zinc interactome remain poorly understood, mostly due to experimental obstacles. Here, we review recent research advances on the role of Zn²⁺ in the formation of interprotein sites, their architecture, function, and stability. Moreover, we underline the importance of zinc networks in intersystemic communication and highlight bioinformatic and experimental challenges required for the identification and investigation of ZPPIs.

Cryptochrome 1 Alleviates the Antiproliferative Effect of Isoproterenol on Human Gastric Cancer Cells

Background:

Cryptochrome 1 (CRY1) is a key protein that regulates the feedback loop of circadian clock. The abnormal expression of CRY1 was reported in numerous cancers, and contributed to tumorigenesis and progression. But the underlying mechanism remains undefined.

Methods:

CRY1 overexpression was constructed by lentivirus vector. Gene and protein expression was detected by reverse transcription quantitative polymerase chain reaction and Western blot. Cell proliferation was analyzed by CCK-8 assay. Cell migration ability was analyzed by scratch assay and transwell migration assay. The cAMP concentration was measured by intracellular cAMP assay.

Results:

Overexpression of CRY1 showed slightly effect on the proliferation and migration of HGC-27 cells. Upon exposure to isoproterenol (ISO), a β-adrenergic receptor agonist, cell proliferation, and migration were inhibited while the cAMP/PKA pathway was activated and ERK1/2 phosphorylation was suppressed. CRY1 overexpression reduced cAMP accumulation, retained ERK1/2 phosphorylation level and alleviated the antiproliferative effect upon exposure to ISO. However, CRY1 overexpression was inoperative on the antiproliferative effect of forskolin (FSK), a direct activator of adenyl cyclase (AC), or 3-isobutyl-1-methylxanthine (IBMX), a phosphodiesterase (PDE) inhibitor.

Conclusions:

Our results suggest CRY1 overexpression may protect cells from the antiproliferative effects via activation of the cAMP/PKA pathway through interrupting signal transduction from G protein-coupled receptors to AC.

The oligomeric structures of plant cryptochromes

Cryptochromes (CRYs) are a group of evolutionarily conserved flavoproteins found in many organisms. In plants, the well-studied CRY photoreceptor, activated by blue light, plays essential roles in plant growth and development. However, the mechanism of activation remains largely unknown. Here, we determined the oligomeric structures of the blue-light-perceiving PHR domain of Zea mays CRY1 and an Arabidopsis CRY2 constitutively active mutant. The structures form dimers and tetramers whose functional importance is examined in vitro and in vivo with Arabidopsis CRY2. Structure-based analysis suggests that blue light may be perceived by CRY to cause conformational changes, whose precise nature remains to be determined, leading to oligomerization that is essential for downstream signaling. This photoactivation mechanism may be widely used by plant CRYs. Our study reveals a molecular mechanism of plant CRY activation and also paves the way for design of CRY as a more efficient optical switch.

Mechanisms of Cryptochrome-Mediated Photoresponses in Plants

Cryptochromes are blue-light receptors that mediate photoresponses in plants. The genomes of most land plants encode two clades of cryptochromes, CRY1 and CRY2, which mediate distinct and overlapping photoresponses within the same species and between different plant species. Photoresponsive protein-protein interaction is the primary mode of signal transduction of cryptochromes. Cryptochromes exist as physiologically inactive monomers in the dark; the absorption of photons leads to conformational change and cryptochrome homooligomerization, which alters the affinity of cryptochromes interacting with cryptochrome-interacting proteins to form various cryptochrome complexes. These cryptochrome complexes, collectively referred to as the cryptochrome complexome, regulate transcription or stability of photoresponsive proteins to modulate plant growth and development. The activity of cryptochromes is regulated by photooligomerization; dark monomerization; cryptochrome regulatory proteins; and cryptochrome phosphorylation, ubiquitination, and degradation. Most of the more than 30 presently known cryptochrome-interacting proteins are either regulated by other photoreceptors or physically interactingwith the protein complexes of other photoreceptors. Some cryptochrome-interacting proteins are also hormonal signaling or regulatory proteins. These two mechanisms enable cryptochromes to integrate blue-light signals with other internal and external signals to optimize plant growth and development.

Thermal stability analyses of human PERIOD-2 C-terminal domain using dynamic light scattering and circular dichroism

At the molecular level, the circadian clock is regulated by a time delayed transcriptional-translational feedback loop in which the core proteins interact with each other rhythmically to drive daily biological rhythms. The C-terminal domain of a key clock protein PER2 (PER2c) plays a critically important role in the loop, not only for its interaction with the binding partner CRY proteins but also for the CRY/PER complex’s translocation from the cytosol to the nucleus. Previous circular dichroism (CD) spectroscopic studies have shown that mouse PER2c (mPER2c) is less structured in solution by itself but folded into stable secondary structures upon interaction with mouse CRYs. To understand the stability and folding of human PER2c (hPER2c), we expressed and purified hPER2c. Three oligomerization forms of recombinant hPER2c were identified and thoroughly characterized through a combination of biochemical and biophysical techniques. Different to mPER2c, both thermal unfolding DLS and CD analyses suggested that all forms of hPER2c have very stable secondary structures in solution by themselves with melting temperatures higher than the physiological body temperature, indicating that hPER2c does not require CRY to fold. Furthermore, we examined the effects of EDTA, salt concentration, and a reducing agent on hPER2c folding and oligomerization. The ability of hPER2c forming oligomers reflects the potential role of hPER2c in the assembly of circadian rhythm core protein complexes.

Isoform-selective regulation of mammalian cryptochromes

CRY1 and CRY2 are essential components of the circadian clock controlling daily physiological rhythms. Accumulating evidences indicate distinct roles of these highly homologous proteins, in addition to redundant functions. Therefore, the development of isoform-selective compounds represents an effective approach towards understanding the similarities and differences of CRY1 and CRY2 by controlling each isoform individually. We conducted phenotypic screenings of circadian clock modulators, and identified KL101 and TH301 that selectively stabilize CRY1 and CRY2, respectively. Crystal structures of CRY-compound complexes revealed conservation of compound-binding sites between CRY1 and CRY2. We further discovered a unique mechanism underlying compound selectivity in which the disordered C-terminal region outside the pocket was required for the differential effects of KL101 and TH301 against CRY isoforms. By using these compounds, we found a new role of CRY1 and CRY2 as enhancers of brown adipocyte differentiation, providing the basis of CRY-mediated regulation of energy expenditure.

Light entrainment of retinal biorhythms: cryptochrome 2 as candidate photoreceptor in mammals

The mechanisms that synchronize the biorhythms of the mammalian retina with the light/dark cycle are independent of those synchronizing the rhythms in the central pacemaker, the suprachiasmatic nucleus. The identity of the photoreceptor(s) responsible for the light entrainment of the retina of mammals is still a matter of debate, and recent studies have reported contradictory results in this respect. Here, we suggest that cryptochromes (CRY), in particular CRY 2, are involved in that light entrainment. CRY are highly conserved proteins that are a key component of the cellular circadian clock machinery. In plants and insects, they are responsible for the light entrainment of these biorhythms, mediated by the light response of their flavin cofactor (FAD). In mammals, however, no light-dependent role is currently assumed for CRY in light-exposed tissues, including the retina. It has been reported that FAD influences the function of mammalian CRY 2 and that human CRY 2 responds to light in Drosophila, suggesting that mammalian CRY 2 keeps the ability to respond to light. Here, we hypothesize that CRY 2 plays a role in the light entrainment of retinal biorhythms, at least in diurnal mammals. Indeed, published data shows that the light intensity dependence and the wavelength sensitivity commonly reported for that light entrainment fits the light sensitivity and absorption spectrum of light-responsive CRY. We propose experiments to test our hypothesis and to further explore the still-pending question of the function of CRY 2 in the mammalian retina.

Dynamics at the serine loop underlie differential affinity of cryptochromes for CLOCK:BMAL1 to control circadian timing

Mammalian circadian rhythms are generated by a transcription-based feedback loop in which CLOCK:BMAL1 drives transcription of its repressors (PER1/2, CRY1/2), which ultimately interact with CLOCK:BMAL1 to close the feedback loop with ~24 hr periodicity. Here we pinpoint a key difference between CRY1 and CRY2 that underlies their differential strengths as transcriptional repressors. Both cryptochromes bind the BMAL1 transactivation domain similarly to sequester it from coactivators and repress CLOCK:BMAL1 activity. However, we find that CRY1 is recruited with much higher affinity to the PAS domain core of CLOCK:BMAL1, allowing it to serve as a stronger repressor that lengthens circadian period. We discovered a dynamic serine-rich loop adjacent to the secondary pocket in the photolyase homology region (PHR) domain that regulates differential binding of cryptochromes to the PAS domain core of CLOCK:BMAL1. Notably, binding of the co-repressor PER2 remodels the serine loop of CRY2, making it more CRY1-like and enhancing its affinity for CLOCK:BMAL1.

Influence of mental stress and environmental toxins on circadian clocks: Implications for redox regulation of the heart and cardioprotection

Risk factors in the environment such as air pollution and mental stress contribute to the development of chronic non-communicable disease. Air pollution was identified as the leading health risk factor in the physical environment, followed by water pollution, soil pollution/heavy metals/chemicals and occupational exposures, however neglecting the non-chemical environmental health risk factors (e.g. mental stress and noise). Epidemiological data suggest that environmental risk factors are associated with higher risk for cardiovascular, metabolic and mental diseases, including hypertension, heart failure, myocardial infarction, diabetes, arrhythmia, stroke, depression and anxiety disorders. We provide an overview on the impact of the external exposome comprising risk factors/exposures on cardiovascular health with a focus on dysregulation of stress hormones, mitochondrial function, redox balance and inflammation with special emphasis on the circadian clock. Finally, we assess the impact of circadian clock dysregulation on cardiovascular health and the potential of environment-specific preventive strategies or "chrono" therapy for cardioprotection. LINKED ARTICLES: This article is part of a themed issue on Risk factors, comorbidities, and comedications in cardioprotection. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v177.23/issuetoc.

Circadian Clocks Make Metabolism Run

Most organisms adapt to the 24-h cycle of the Earth's rotation by anticipating the time of the day through light-dark cycles. The internal time-keeping system of the circadian clocks has been developed to ensure this anticipation. The circadian system governs the rhythmicity of nearly all physiological and behavioral processes in mammals. In this review, we summarize current knowledge stemming from rodent and human studies on the tight interconnection between the circadian system and metabolism in the body. In particular, we highlight recent advances emphasizing the roles of the peripheral clocks located in the metabolic organs in regulating glucose, lipid, and protein homeostasis at the organismal and cellular levels. Experimental disruption of circadian system in rodents is associated with various metabolic disturbance phenotypes. Similarly, perturbation of the clockwork in humans is linked to the development of metabolic diseases. We discuss recent studies that reveal roles of the circadian system in the temporal coordination of metabolism under physiological conditions and in the development of human pathologies.

Molecular-genetic Manipulation of the Suprachiasmatic Nucleus Circadian Clock

Circadian (approximately daily) rhythms of physiology and behaviour adapt organisms to the alternating environments of day and night. The suprachiasmatic nucleus (SCN) of the hypothalamus is the principal circadian timekeeper of mammals. The mammalian cell-autonomous circadian clock is built around a self-sustaining transcriptional-translational negative feedback loop (TTFL) in which the negative regulators Per and Cry suppress their own expression, which is driven by the positive regulators Clock and Bmal1. Importantly, such TTFL-based clocks are present in all major tissues across the organism, and the SCN is their central co-ordinator. First, we analyse SCN timekeeping at the cell-autonomous and the circuit-based levels of organisation. We consider how molecular-genetic manipulations have been used to probe cell-autonomous timing in the SCN, identifying the integral components of the clock. Second, we consider new approaches that enable real-time monitoring of the activity of these clock components and clock-driven cellular outputs. Finally, we review how intersectional genetic manipulations of the cell-autonomous clockwork can be used to determine how SCN cells interact to generate an ensemble circadian signal. Critically, it is these network-level interactions that confer on the SCN its emergent properties of robustness, light-entrained phase and precision- properties that are essential for its role as the central co-ordinator. Remaining gaps in knowledge include an understanding of how the TTFL proteins behave individually and in complexes: whether particular SCN neuronal populations act as pacemakers, and if so, by which signalling mechanisms, and finally the nature of the recently discovered role of astrocytes within the SCN network.

Adverse Cardiovascular Effects of Traffic Noise with a Focus on Nighttime Noise and the New WHO Noise Guidelines

Exposure to traffic noise is associated with stress and sleep disturbances. The World Health Organization (WHO) recently concluded that road traffic noise increases the risk for ischemic heart disease and potentially other cardiometabolic diseases, including stroke, obesity, and diabetes. The WHO report focused on whole-day noise exposure, but new epidemiological and translational field noise studies indicate that nighttime noise, in particular,is an important risk factor for cardiovascular disease (CVD) through increased levels of stress hormones and vascular oxidative stress, leading to endothelial dysfunction and subsequent development of various CVDs. Novel experimental studies found noise to be associated with oxidative stress-induced vascular and brain damage, mediated by activation of the NADPH oxidase, uncoupling of endothelial and neuronal nitric oxide synthase, and vascular/brain infiltration with inflammatory cells. Noise-induced pathophysiology was more pronounced in response to nighttime as compared with daytime noise. This review focuses on the consequences of nighttime noise.

The importance of being rhythmic: Living in harmony with your body clocks

Circadian rhythms have developed in all light-sensitive organisms, including humans, as a fundamental anticipatory mechanism that enables proactive adaptation to environmental changes. The circadian system is organized in a highly hierarchical manner, with clocks operative in most cells of the body ensuring the temporal coordination of physiological processes. Circadian misalignment, stemming from modern life style, draws increasing attention due to its tight association with the development of metabolic, cardiovascular, inflammatory and mental diseases as well as cancer. This review highlights recent findings emphasizing the role of the circadian system in the temporal orchestration of physiology, with a particular focus on implications of circadian misalignment in human pathologies.

Structural Biology of CRL Ubiquitin Ligases

Cullin-RING ubiquitin ligases (CRLs) represent the largest superfamily of multi-subunit E3s conserved in all eukaryotes. Soon after the discovery of these important ubiquitin ligase machineries, structural studies have made tremendous contributions to our understanding of their functions. Identification of the key components of CRLs by early studies raised immediate questions as to how these multi-subunit complexes assemble to promote the polyubiquitination of substrates. Specifically, how do the CRL subunits interact with each other to form a versatile E3 platform? How do they recognize specific substrates? How are the CRL-substrate interactions regulated in response to upstream signals? How are the CRL E3s themselves activated and deactivated, and how are substrate receptor subunits of CRLs exchanged in the cell? Even though we might not yet have complete answers to these questions, extensive structural analyses of CRL complexes in the past two decades have begun to unveil the themes and variations of CRL biology. In this chapter we will discuss both classic and emerging structures that help elucidate the overall architecture of CRLs, their substrate recognition modes, and regulatory mechanism of CRLs by NEDD8 modification.

A missense variant in PER2 is associated with delayed sleep-wake phase disorder in a Japanese population

Delayed sleep-wake phase disorder (DSWPD) is a subtype of circadian rhythm sleep-wake disorders, and is characterized by an inability to fall asleep until late at night and wake up at a socially acceptable time in the morning. The study aim was to identify low-frequency nonsense and missense variants that are associated with DSWPD. Candidate variants in circadian rhythm-related genes were extracted by integration of genetic variation databases and in silico assessment. We narrowed down the candidates to six variants. To examine whether the six variants are associated with DSWPD, we performed an association study in 236 Japanese patients with DSWPD and 1436 controls. A low-frequency missense variant (p.Val1205Met) in PER2 showed a significant association with DSWPD (2.5% in cases and 1.1% in controls, P = 0.026, odds ratio (OR) = 2.32). The variant was also associated with idiopathic hypersomnia known to have a tendency toward phase delay (P = 0.038, OR = 2.07). PER2 forms a heterodimer with CRY, and the heterodimer plays an important role in the regulation of circadian rhythms. Val1205 is located in the CRY-binding domain of PER2 and was hypothesized to interact with CRY. The p.Val1205Met substitution could be a potential genetic marker for DSWPD.

Structure of the bifunctional cryptochrome aCRY from Chlamydomonas reinhardtii

Photolyases and cryptochromes form an almost ubiquitous family of blue light photoreceptors involved in the repair and maintenance of DNA integrity or regulatory control. We found that one cryptochrome from the green alga Chlamydomonas reinhardtii (CraCRY) is capable of both, control of transcript levels and the sexual cycle of the alga in a positive (germination) and negative manner (mating ability), as well as catalyzing the repair of UV-DNA lesions. Its 1.6 Å crystal structure shows besides the FAD chromophore an aromatic tetrad that is indispensable in animal-like type I cryptochromes for light-driven change of their signaling-active redox state and formation of a stable radical pair. Given CraCRY’s catalytic activity as (6-4) photolyase in vivo and in vitro, we present the first co-crystal structure of a cryptochrome with duplex DNA comprising a (6-4) pyrimidine–pyrimidone lesion. This 2.9 Å structure reveals a distinct conformation for the catalytic histidine His₁, H357, that challenges previous models of a single-photon driven (6-4) photolyase mechanism.

A two genes - for - one gene interaction between Leptosphaeria maculans and Brassica napus

Interactions between Leptosphaeria maculans, causal agent of stem canker of oilseed rape, and its Brassica hosts are models of choice to explore the multiplicity of 'gene-for-gene' complementarities and how they diversified to increased complexity in the course of plant-pathogen co-evolution. Here, we support this postulate by investigating the AvrLm10 avirulence that induces a resistance response when recognized by the Brassica nigra resistance gene Rlm10. Using genome-assisted map-based cloning, we identified and cloned two AvrLm10 candidates as two genes in opposite transcriptional orientation located in a subtelomeric repeat-rich region of the genome. The AvrLm10 genes encode small secreted proteins and show expression profiles in planta similar to those of all L. maculans avirulence genes identified so far. Complementation and silencing assays indicated that both genes are necessary to trigger Rlm10 resistance. Three assays for protein-protein interactions showed that the two AvrLm10 proteins interact physically in vitro and in planta. Some avirulence genes are recognized by two distinct resistance genes and some avirulence genes hide the recognition specificities of another. Our L. maculans model illustrates an additional case where two genes located in opposite transcriptional orientation are necessary to induce resistance. Interestingly, orthologues exist for both L. maculans genes in other phytopathogenic species, with a similar genome organization, which may point to an important conserved effector function linked to heterodimerization of the two proteins.

Cryptochromes-Mediated Inhibition of the CRL4Cop1-Complex Assembly Defines an Evolutionary Conserved Signaling Mechanism

In plants, cryptochromes are photoreceptors that negatively regulate the ubiquitin ligase CRL4^Cop1. In mammals, cryptochromes are core components of the circadian clock and repressors of the glucocorticoid receptor (GR). Moreover, mammalian cryptochromes lost their ability to interact with Cop1, suggesting that they are unable to inhibit CRL4^Cop1. Contrary to this assumption, we found that mammalian cryptochromes are also negative regulators of CRL4^Cop1, and through this mechanism, they repress the GR transcriptional network both in cultured cells and in the mouse liver. Mechanistically, cryptochromes inactivate Cop1 by interacting with Det1, a subunit of the mammalian CRL4^Cop1 complex that is not present in other CRL4s. Through this interaction, the ability of Cop1 to join the CRL4 complex is inhibited; therefore, its substrates accumulate. Thus, the interaction between cryptochromes and Det1 in mammals mirrors the interaction between cryptochromes and Cop1 in planta, pointing to a common ancestor in which the cryptochromes-Cop1 axis originated.

A Robust Model for Circadian Redox Oscillations

The circadian clock is an endogenous oscillator that controls daily rhythms in metabolism, physiology, and behavior. Although the timekeeping components differ among species, a common design principle is a transcription-translation negative feedback loop. However, it is becoming clear that other mechanisms can contribute to the generation of 24 h rhythms. Peroxiredoxins (Prxs) exhibit 24 h rhythms in their redox state in all kingdoms of life. In mammalian adrenal gland, heart and brown adipose tissue, such rhythms are generated as a result of an inactivating hyperoxidation reaction that is reduced by coordinated import of sulfiredoxin (Srx) into the mitochondria. However, a quantitative description of the Prx/Srx oscillating system is still missing. We investigate the basic principles that generate mitochondrial Prx/Srx rhythms using computational modeling. We observe that the previously described delay in mitochondrial Srx import, in combination with an appropriate separation of fast and slow reactions, is sufficient to generate robust self-sustained relaxation-like oscillations. We find that our conceptual model can be regarded as a series of three consecutive phases and two temporal switches, highlighting the importance of delayed negative feedback and switches in the generation of oscillations.

Chronodentistry: the role & potential of molecular clocks in oral medicine

Molecular clocks help organisms to adapt important physiological functions to periodically changing conditions in the environment. These include the adaption of the 24 h sleep-wake rhythm to changes of day and night. The circadian clock is known to act as a key regulator in processes of health and disease in different organs. The knowledge on the circadian clock led to the development of chronopharmacology and chronotherapy. These fields aim to investigate how efficiency of medication and therapies can be improved based on circadian clock mechanisms. In this review we aim to highlight the role of the circadian clock in oral tissues and its potential in the different fields of dentistry including oral and maxillofacial surgery, restorative dentistry, endodontics, periodontics and orthodontics to trigger the evolving field of chronodentistry.

Metal binding properties of zinc fingers with a naturally altered metal binding site

Zinc fingers (ZFs) are among the most abundant motifs found in proteins, and are commonly known for their structural role. Classical ZFs (CCHH) are part of the transcription factors that participate in DNA binding. Although biochemical studies of classical ZFs have a long history, there is limited knowledge about the sequential and structural diversity of ZFs. We have found that classical ZFs, with metal binding sites consisting of amino acids other than conserved Cys or His residues, are frequently encoded in the human genome, and we refer to these peptides as ZFs with a naturally altered metal binding site. The biological role of the altered ZFs remains undiscovered. In this study, we characterized nine natural XCHH, CXHH, CCXH and CCHX ZFs in terms of their Zn(ii) and Co(ii) binding properties, such as complex stoichiometry, spectroscopic properties and metal-to-peptide affinity. We revealed that XCHH and CXHH ZFs form ML complexes that are 4-5 orders of magnitude weaker in comparison to CCHH ZFs. Nevertheless, spectroscopic studies demonstrate that, depending on the altered position, they may adopt an open coordination geometry with one or two water molecules bound to a central metal ion, which has not been demonstrated in natural ZFs before. Stability data show that both CCXH and CCHX peptides have high Zn(ii) affinity (with a K_d of 10^-9 to 10^-11 M), suggesting their potential biological function. This study is a comprehensive overview of the relationship between the sequence, structure, and stability of ZFs.

Zinc deficiency induces hypertension by promoting renal Na+ reabsorption

Zn²⁺ deficiency (ZnD) is a common comorbidity of many chronic diseases. In these settings, ZnD exacerbates hypertension. Whether ZnD alone is sufficient to alter blood pressure (BP) is unknown. To explore the role of Zn²⁺ in BP regulation, adult mice were fed a Zn²⁺-adequate (ZnA) or a Zn²⁺-deficient (ZnD) diet. A subset of ZnD mice were either returned to the ZnA diet or treated with hydrochlorothiazide (HCTZ), a Na⁺-Cl^- cotransporter (NCC) inhibitor. To reduce intracellular Zn²⁺ in vitro, mouse distal convoluted tubule cells were cultured in N,N,N',N'-tetrakis(2-pyridylmethyl)ethylenediamine (TPEN, a Zn²⁺ chelator)- or vehicle (DMSO)-containing medium. To replete intracellular Zn²⁺, TPEN-exposed cells were then cultured in Zn²⁺-supplemented medium. ZnD promoted a biphasic BP response, characterized by episodes of high BP. BP increases were accompanied by reduced renal Na⁺ excretion and NCC upregulation. These effects were reversed in Zn²⁺-replete mice. Likewise, HCTZ stimulated natriuresis and reversed BP increases. In vitro, Zn²⁺ depletion increased NCC expression. Furthermore, TPEN promoted NCC surface localization and Na⁺ uptake activity. Zn²⁺ repletion reversed TPEN effects on NCC. These data indicate that 1) Zn²⁺ contributes to BP regulation via modulation of renal Na⁺ transport, 2) renal NCC mediates ZnD-induced hypertension, and 3) NCC is a Zn²⁺-regulated transporter that is upregulated with ZnD. This study links dysregulated renal Na⁺ handling to ZnD-induced hypertension. Furthermore, NCC is identified as a novel mechanism by which Zn²⁺ regulates BP. Understanding the mechanisms of ZnD-induced BP dysregulation may have an important therapeutic impact on hypertension.

Karyopherin Alpha 2-Expressing Pancreatic Duct Glands and Intra-Islet Ducts in Aged Diabetic C414A-Mutant-CRY1 Transgenic Mice

Our earlier studies demonstrated that cysteine414- (zinc-binding site of mCRY1-) alanine mutant mCRY1 transgenic mice (Tg mice) exhibit diabetes characterized by the reduction of β-cell proliferation and by β-cell dysfunction, presumably caused by senescence-associated secretory phenotype- (SASP-) like characters of islets. Earlier studies also showed that atypical duct-like structures in the pancreas developed age-dependently in Tg mice. Numerous reports have described that karyopherin alpha 2 (KPNA2) is highly expressed in cancers of different kinds. However, details of the expression of KPNA2 in pancreatic ductal atypia and in normal pancreatic tissues remain unclear. To assess the feature of the expression of KPNA2 in the development of the ductal atypia and islet architectures, we scrutinized the pancreas of Tg mice histopathologically. Results showed that considerable expression of KPNA2 was observed in pancreatic β-cells, suggesting its importance in maintaining the functions of β-cells. In mature stages, the level of KPNA2 expression was lower in islets of Tg mice than in wild-type controls. At 4 weeks, the expression levels of KPNA2 in islets of Tg mice were the same as those in wild-type controls. These results suggest that the reduction of KPNA2 might contribute to β-cell dysfunction in mature Tg mice. Additionally, the formation of mucin-producing intra-islet ducts, islet fibrosis, and massive T cell recruitment to the islet occurred in aged Tg mice. In exocrine areas, primary pancreatic intraepithelial neoplasias (PanINs) with mucinous pancreatic duct glands (PDGs) emerged in aged Tg mice. High expression of KPNA2 was observed in the ductal atypia. By contrast, KPNA2 expression in normal ducts was quite low. Thus, upregulation of KPNA2 seemed to be correlated with progression of the degree of atypia in pancreatic ductal cells. The SASP-like microenvironment inside islets might play stimulatory roles in the formation of ductal metaplasia inside islets and in islet fibrosis in Tg mice.

Periodicity, repression, and the molecular architecture of the mammalian circadian clock

Large molecular machines regulate daily cycles of transcriptional activity and help generate rhythmic behavior. In recent years, structural and biochemical analyses have elucidated a number of principles guiding the interactions of proteins that form the basis of circadian timing. In its simplest form, the circadian clock is composed of a transcription/translation feedback loop. However, this description elides a complicated process of activator recruitment, chromatin decompaction, recruitment of coactivators, expression of repressors, formation of a repressive complex, repression of the activators, and ultimately degradation of the repressors and reinitiation of the cycle. Understanding the core principles underlying the clock requires careful examination of molecular and even atomic level details of these processes. Here, we review major structural and biochemical findings in circadian biology and make the argument that shared protein interfaces within the clockwork are critical for both the generation of rhythmicity and timing of the clock.

Distinct control of PERIOD2 degradation and circadian rhythms by the oncoprotein and ubiquitin ligase MDM2

The circadian clock relies on posttranslational modifications to set the timing for degradation of core regulatory components, which drives clock progression. Ubiquitin-modifying enzymes that target clock components for degradation mainly recognize phosphorylated substrates. Degradation of the circadian clock component PERIOD 2 (PER2) is mediated by its phospho-specific recognition by β-transducin repeat-containing proteins (β-TrCPs), which are F-box-containing proteins that function as substrate recognition subunits of the SCF^β-TRCP ubiquitin ligase complex. However, this mode of regulating PER2 stability falls short of explaining the persistent oscillatory phenotypes reported in biological systems lacking functional elements of the phospho-dependent PER2 degradation machinery. We identified PER2 as a previously uncharacterized substrate for the ubiquitin ligase mouse double minute 2 homolog (MDM2) and found that MDM2 targeted PER2 for degradation in a manner independent of PER2 phosphorylation. Deregulation of MDM2 plays a major role in oncogenesis by contributing to the accumulation of genomic and epigenomic alterations that favor tumor development. MDM2-mediated PER2 turnover was important for defining the circadian period length in mammalian cells, a finding that emphasizes the connection between the circadian clock and cancer. Our results not only broaden the range of specific substrates of MDM2 beyond the cell cycle to include circadian components but also identify a previously unknown regulator of the clock as a druggable node that is often found to be deregulated during tumorigenesis.

Neuronal circadian clock protein oscillations are similar in behaviourally rhythmic forager honeybees and in arrhythmic nurses

Internal clocks driving rhythms of about a day (circadian) are ubiquitous in animals, allowing them to anticipate environmental changes. Genetic or environmental disturbances to circadian clocks or the rhythms they produce are commonly associated with illness, compromised performance or reduced survival. Nevertheless, some animals including Arctic mammals, open sea fish and social insects such as honeybees are active around-the-clock with no apparent ill effects. The mechanisms allowing this remarkable natural plasticity are unknown. We generated and validated a new and specific antibody against the clock protein PERIOD of the honeybee Apis mellifera (amPER) and used it to characterize the circadian network in the honeybee brain. We found many similarities to Drosophila melanogaster and other insects, suggesting common anatomical organization principles in the insect clock that have not been appreciated before. Time course analyses revealed strong daily oscillations in amPER levels in foragers, which show circadian rhythms, and also in nurses that do not, although the latter have attenuated oscillations in brain mRNA clock gene levels. The oscillations in nurses show that activity can be uncoupled from the circadian network and support the hypothesis that a ticking circadian clock is essential even in around-the-clock active animals in a constant physical environment.

Cryptochrome: The magnetosensor with a sinister side?

Over the last three decades, evidence has emerged that low-intensity magnetic fields can influence biological systems. It is now well established that migratory birds have the capacity to detect the Earth's magnetic field; it has been reported that power lines are associated with childhood leukemia and that pulsed magnetic fields increase the production of reactive oxidative species (ROS) in cellular systems. Justifiably, studies in this field have been viewed with skepticism, as the underlying molecular mechanisms are unknown. In the accompanying paper, Sherrard and colleagues report that low-flux pulsed electromagnetic fields (PEMFs) result in aversive behavior in Drosophila larvae and ROS production in cell culture. They further report that these responses require the presence of cryptochrome, a putative magnetoreceptor. If correct, it is conceivable that carcinogenesis associated with power lines, PEMF-induced ROS generation, and animal magnetoreception share a common mechanistic basis.

Time zones of pancreatic islet metabolism

Most living beings possess an intrinsic system of circadian oscillators, allowing anticipation of the Earth's rotation around its own axis. The mammalian circadian timing system orchestrates nearly all aspects of physiology and behaviour. Together with systemic signals originating from the central clock that resides in the hypothalamic suprachiasmatic nucleus, peripheral oscillators orchestrate tissue-specific fluctuations in gene transcription and translation, and posttranslational modifications, driving overt rhythms in physiology and behaviour. There is accumulating evidence of a reciprocal connection between the circadian oscillator and most aspects of physiology and metabolism, in particular as the circadian system plays a critical role in orchestrating body glucose homeostasis. Recent reports imply that circadian clocks operative in the endocrine pancreas regulate insulin secretion, and that islet clock perturbation in rodents leads to the development of overt type 2 diabetes. While whole islet clocks have been extensively studied during the last years, the heterogeneity of islet cell oscillators and the interplay between α- and β-cellular clocks for orchestrating glucagon and insulin secretion have only recently gained attention. Here, we review recent findings on the molecular makeup of the circadian clocks operative in pancreatic islet cells in rodents and in humans, and focus on the physiologically relevant synchronizers that are resetting these time-keepers. Moreover, the implication of islet clock functional outputs in the temporal coordination of metabolism in health and disease will be highlighted.

Ascorbic acid may not be involved in cryptochrome-based magnetoreception

Seventeen years after it was originally suggested, the photoreceptor protein cryptochrome remains the most probable host for the radical pair intermediates that are thought to be the sensors in the avian magnetic compass. Although evidence in favour of this hypothesis is accumulating, the intracellular interaction partners of the sensory protein are still unknown. It has been suggested that ascorbate ions could interact with surface-exposed tryptophan radicals in photoactivated cryptochromes, and so lead to the formation of a radical pair comprised of the reduced form of the flavin adenine dinucleotide cofactor, FAD^•-, and the ascorbate radical, Asc^•- This species could provide a more sensitive compass than a FAD-tryptophan radical pair. In this study of Drosophila melanogaster cryptochrome and Erithacus rubecula (European robin) cryptochrome 1a, we use molecular dynamics simulations to characterize the transient encounters of ascorbate ions with tryptophan radicals in cryptochrome in order to assess the likelihood of the [FAD^•- Asc^•-]-pathway. It is shown that ascorbate ions are expected to bind near the tryptophan radicals for periods of a few nanoseconds. The rate at which these encounters happen is low, and it is therefore concluded that ascorbate ions are unlikely to be involved in magnetoreception if the ascorbate concentration is only of the order of 1 mM or less.

Don't just say no: Differential pathways and pharmacological responses to diverse nitric oxide donors

Nitric oxide (NO) is a gaseous free radical molecule with a short half-life (∼1 s), which can gain or lose an electron into three interchangeable redox-dependent forms, the radical (NO), the nitrosonium cation (NO⁺), and nitroxyl anion (HNO). NO acts as an intra and extracellular signaling molecule regulating a wide range of functions in the cardiovascular, immune, and nervous system. NO donors are collectively known by their ability to release NOin vitro and in vivo, being proposed as therapeutic pharmacological tools for the treatment of several pathologies, such as cardiovascular disease. The highly reactive NO molecule is easily oxidized under physiological conditions to N-oxides, nitrate/nitrite and nitrogen dioxide. Different cellular responses are triggered depending on: 1) NO concentration [e.g., nanomolar for heme coordination in the allosteric site of guanylate cyclase (sGC) enzyme]; 2) the type of chemical bound to the nitrosated group (i.e., bound to nitrogen, N-nitro, or bound to sulphur atom, S-nitro) determining post-translational cysteine nitrosation; 3) the time-dependent availability of molecular targets. Classic NO donors are: organic nitrates (e.g., nitroglycerin, or glyceryl trinitrate, GTN; isosorbide mononitrate, ISMN), diazeniumdiolates having a diolate group [or NONOates, e.g., 2-(N,N-diethylamino)-diazenolate-2-oxide], S-nitrosothiols (e.g., S-nitroso glutathione, GSNO; S-nitroso-N-acetylpenicillamine, SNAP) or the organic salt sodium nitroprusside (SNP). In addition, nitroxyl (HNO) donors such as Piloty's acid and Angeli's salt can also be considered. The specific NO form released, as well as its differential reactivity to thiols, could act on different molecular targets and should be discussed in the context of: a) the type and amount of NO species determining the sensitivity of molecular targets (e.g., heme coordination, or S-nitrosation); b) the cellular redox state that could gate different effects. Experimental designs should take special care when choosing which NO donors to use, since different outcomes are to be expected. This article will comment recent findings regarding physiological responses involving NO species and their pharmacological modulation with donor drugs, especially in the context of the photic transduction pathways at the hypothalamic circadian clock.

Inhibiting effects of common trivalent metal ions on transmembrane-type 2 matrix metalloproteinase

Transmembrane-type 2 matrix metalloproteinase (MT2-MMP) degrades connective extracellular matrix between cells and enables tumor cells to migrate and metastasize, making this substance a potential therapeutic target in various diseases. In this work, the interactions between MT2-MMP and common trivalent metal ions, including aluminum (Al³⁺) and ferrum (Fe³⁺) ions, were investigated. Enzymatic detection revealed that Al³⁺ and Fe³⁺ strongly inhibited the MT2-MMP. Fluorescence spectrography elucidated a static quenching interaction between the negatively charged amino acids on MT2-MMP and the inhibitory trivalent metal ions, indicating that a stable complex was formed between MT2-MMP and metal ions. In addition, fluorescence data and molecular modeling analysis of the binding characteristics revealed that one trivalent metal ion bound with a protein in the stable complex formation process. The potential inhibitory effect of Al³⁺ on MT2-MMP was further examined in an MT2-MMP-overexpressed cell line, HT1080, by using flow cytometry. As a result, Al³⁺ can promote HT1080 cell apoptosis in a micromolar concentration-dependent manner. This work illustrated that common trivalent metal ions can potentially inhibit MT2-MMP-related tumors.

Mutations in blind cavefish target the light-regulated circadian clock gene, period 2

Light represents the principal signal driving circadian clock entrainment. However, how light influences the evolution of the clock remains poorly understood. The cavefish Phreatichthys andruzzii represents a fascinating model to explore how evolution under extreme aphotic conditions shapes the circadian clock, since in this species the clock is unresponsive to light. We have previously demonstrated that loss-of-function mutations targeting non-visual opsins contribute in part to this blind clock phenotype. Here, we have compared orthologs of two core clock genes that play a key role in photic entrainment, cry1a and per2, in both zebrafish and P. andruzzii. We encountered aberrantly spliced variants for the P. andruzzii per2 transcript. The most abundant transcript encodes a truncated protein lacking the C-terminal Cry binding domain and incorporating an intronic, transposon-derived coding sequence. We demonstrate that the transposon insertion leads to a predominantly cytoplasmic localization of the cavefish Per2 protein in contrast to the zebrafish ortholog which is distributed in both the nucleus and cytoplasm. Thus, it seems that during evolution in complete darkness, the photic entrainment pathway of the circadian clock has been subject to mutation at multiple levels, extending from opsin photoreceptors to nuclear effectors.

Metformin and AMP Kinase Activation Increase Expression of the Sterol Transporters ABCG5/8 (ATP-Binding Cassette Transporter G5/G8) With Potential Antiatherogenic Consequences

OBJECTIVE

The mechanisms underlying the cardiovascular benefit of the anti-diabetic drug metformin are poorly understood. Recent studies have suggested metformin may upregulate macrophage reverse cholesterol transport. The final steps of reverse cholesterol transport are mediated by the sterol transporters, ABCG5 (ATP-binding cassette transporter G5) and ABCG8 (ATP-binding cassette transporter G8), which facilitate hepato-biliary transport of cholesterol. This study was undertaken to assess the possibility that metformin induces Abcg5 and Abcg8 expression in liver and to elucidate the underlying mechanisms.

APPROACH AND RESULTS

Metformin-treated mouse or human primary hepatocytes showed increased expression of Abcg5/8 and the bile salt export pump, Bsep. Administration of metformin to Western-type diet-fed mice showed significant upregulation of Abcg5/8 and Bsep. This resulted in increased initial clearance of ³H-cholesteryl ester HDL (high-density lipoprotein) from plasma. However, fecal ³H-cholesterol output was only marginally increased, possibly reflecting increased hepatic Ldlr (low-density lipoprotein receptor) expression, which would increase nonradiolabeled cholesterol uptake. Abcg5/8 undergo strong circadian variation. Available chromatin immunoprecipitation-Seq data suggested multiple binding sites for Period 2, a transcriptional repressor, within the Abcg5/8 locus. Addition of AMPK (5' adenosine monophosphate-activated protein kinase) agonists decreased Period 2 occupancy, suggesting derepression of Abcg5/8. Inhibition of ATP citrate lyase, which generates acetyl-CoA from citrate, also decreased Period 2 occupancy, with concomitant upregulation of Abcg5/8. This suggests a mechanistic link between feeding-induced acetyl-CoA production and decreased cholesterol excretion via Period 2, resulting in inhibition of Abcg5/8 expression.

CONCLUSIONS

Our findings provide partial support for the concept that metformin may provide cardiovascular benefit via increased reverse cholesterol transport but also indicate increased Ldlr expression as a potential additional mechanism. AMPK activation or ATP citrate lyase inhibition may mediate antiatherogenic effects through increased ABCG5/8 expression.

Circadian toxicity of environmental pollution. Inhalation of polluted air to give a precedent

Exposures to environmental stressors that derive from pollution (e.g. air, light) or lifestyle choices (e.g. diet, activity, 24-hour-×-7-day) are associated with adverse human health outcomes. For instance, there is evidence that air pollution exposure and changes in sleep/wake pattern increase the risk for vascular and cardiometabolic disorders. Interestingly, air pollution exposure affects pulmonary and cardiovascular functions that follow circadian rhythmicity and increases the risk for pulmonary and cardiovascular events that occur in diurnal patterns suggesting a link between air pollution induced cardiovascular and pulmonary injury and changes in circadian rhythm. Indeed, recent research identified circadian rhythm as an air pollution target and circadian rhythm as factor that increases air pollution sensitivity. Using air pollution exposure as precedent, this review highlights research on how environmental pollution affect circadian rhythm and how circadian rhythm affects the toxicity of environmental stressors.

Cellular Timekeeping: It's Redox o'Clock

Mounting evidence in recent years supports the extensive interaction between the circadian and redox systems. The existence of such a relationship is not surprising because most organisms, be they diurnal or nocturnal, display daily oscillations in energy intake, locomotor activity, and exposure to exogenous and internally generated oxidants. The transcriptional clock controls the levels of many antioxidant proteins and redox-active cofactors, and, conversely, the cellular redox poise has been shown to feed back to the transcriptional oscillator via redox-sensitive transcription factors and enzymes. However, the circadian cycles in the S-sulfinylation of the peroxiredoxin (PRDX) proteins constituted the first example of an autonomous circadian redox oscillation, which occurred independently of the transcriptional clock. Importantly, the high phylogenetic conservation of these rhythms suggests that they might predate the evolution of the transcriptional oscillator, and therefore could be a part of a primordial circadian redox/metabolic oscillator. This discovery forced the reappraisal of the dogmatic transcription-centered view of the clockwork and opened a new avenue of research. Indeed, the investigation into the links between the circadian and redox systems is still in its infancy, and many important questions remain to be addressed.

An evolutionary hotspot defines functional differences between CRYPTOCHROMES

Mammalian circadian clocks are driven by a transcription/translation feedback loop composed of positive regulators (CLOCK/BMAL1) and repressors (CRYPTOCHROME 1/2 (CRY1/2) and PER1/2). To understand the structural principles of regulation, we used evolutionary sequence analysis to identify co-evolving residues within the CRY/PHL protein family. Here we report the identification of an ancestral secondary cofactor-binding pocket as an interface in repressive CRYs, mediating regulation through direct interaction with CLOCK and BMAL1. Mutations weakening binding between CLOCK/BMAL1 and CRY1 lead to acceleration of the clock, suggesting that subtle sequence divergences at this site can modulate clock function. Divergence between CRY1 and CRY2 at this site results in distinct periodic output. Weaker interactions between CRY2 and CLOCK/BMAL1 at this pocket are strengthened by co-expression of PER2, suggesting that PER expression limits the length of the repressive phase in CRY2-driven rhythms. Overall, this work provides a model for the mechanism and evolutionary variation of clock regulatory mechanisms.

The molecular mechanisms that define the periodicity or rate of the circadian clock are not well understood. Here the authors use a multidisciplinary approach and identify a mechanism for period regulation that depends on the affinity of the core clock proteins for one another.

Mammalian Circadian Period, But Not Phase and Amplitude, Is Robust Against Redox and Metabolic Perturbations

AIMS

Circadian rhythms permeate all levels of biology to temporally regulate cell and whole-body physiology, although the cell-autonomous mechanism that confers ∼24-h periodicity is incompletely understood. Reports describing circadian oscillations of over-oxidized peroxiredoxin abundance have suggested that redox signaling plays an important role in the timekeeping mechanism. Here, we tested the functional contribution that redox state and primary metabolism make to mammalian cellular timekeeping.

RESULTS

We found a circadian rhythm in flux through primary glucose metabolic pathways, indicating rhythmic NAD(P)H production. Using pharmacological and genetic perturbations, however, we found that timekeeping was insensitive to changes in glycolytic flux, whereas oxidative pentose phosphate pathway (PPP) inhibition and other chronic redox stressors primarily affected circadian gene expression amplitude, not periodicity. Finally, acute changes in redox state decreased PER2 protein stability, phase dependently, to alter the subsequent phase of oscillation.

INNOVATION

Circadian rhythms in primary cellular metabolism and redox state have been proposed to play a role in the cellular timekeeping mechanism. We present experimental data testing that hypothesis.

CONCLUSION

Circadian flux through primary metabolism is cell autonomous, driving rhythmic NAD(P)⁺ redox cofactor turnover and maintaining a redox balance that is permissive for circadian gene expression cycles. Redox homeostasis and PPP flux, but not glycolysis, are necessary to maintain clock amplitude, but neither redox nor glucose metabolism determines circadian period. Furthermore, cellular rhythms are sensitive to acute changes in redox balance, at least partly through regulation of PER protein. Redox and metabolic state are, thus, both inputs and outputs, but not state variables, of cellular circadian timekeeping. Antioxid. Redox Signal. 28, 507-520.

The non-classical nuclear import carrier Transportin 1 modulates circadian rhythms through its effect on PER1 nuclear localization

Circadian clocks are molecular timekeeping mechanisms that allow organisms to anticipate daily changes in their environment. The fundamental cellular basis of these clocks is delayed negative feedback gene regulation with PERIOD and CRYPTOCHROME containing protein complexes as main inhibitory elements. For a correct circadian period, it is essential that such clock protein complexes accumulate in the nucleus in a precisely timed manner, a mechanism that is poorly understood. We performed a systematic RNAi-mediated screen in human cells and identified 15 genes associated with the nucleo-cytoplasmic translocation machinery, whose expression is important for circadian clock dynamics. Among them was Transportin 1 (TNPO1), a non-classical nuclear import carrier, whose knockdown and knockout led to short circadian periods. TNPO1 was found in endogenous clock protein complexes and particularly binds to PER1 regulating its (but not PER2's) nuclear localization. While PER1 is also transported to the nucleus by the classical, Importin β-mediated pathway, TNPO1 depletion slowed down PER1 nuclear import rate as revealed by fluorescence recovery after photobleaching (FRAP) experiments. In addition, we found that TNPO1-mediated nuclear import may constitute a novel input pathway of how cellular redox state signals to the clock, since redox stress increases binding of TNPO1 to PER1 and decreases its nuclear localization. Together, our RNAi screen knocking down import carriers (but also export carriers) results in short and long circadian periods indicating that the regulatory pathways that control the timing of clock protein subcellular localization are far more complex than previously assumed. TNPO1 is one of the novel players essential for normal circadian periods and potentially for redox regulation of the clock.