Control of mammalian circadian rhythm by CKIepsilon-regulated proteasome-mediated PER2 degradation

Spatiotemporal Control Over Circadian Rhythms With Light

Circadian rhythms are endogenous biological oscillators that synchronize internal physiological processes and behaviors with external environmental changes, sustaining homeostasis and health. Disruption of circadian rhythms leads to numerous diseases, including cardiovascular and metabolic diseases, cancer, diabetes, and neurological disorders. Despite the potential to restore healthy rhythms in the organism, pharmacological chronotherapy lacks spatial and temporal resolution. Addressing this challenge, chrono-photopharmacology, the approach that employs small molecules with light-controlled activity, enables the modulation of circadian rhythms when and where needed. Two approaches-relying on irreversible and reversible drug activation-have been proposed for this purpose. These methodologies are based on photoremovable protecting groups and photoswitches, respectively. Designing photoresponsive bioactive molecules requires meticulous structural optimization to obtain the desired chemical and photophysical properties, and the design principles, detailed guidelines and challenges are summarized here. In this review, we also analyze all the known circadian modulators responsive to light and dissect the rationale following their construction and application to control circadian biology from the protein level to living organisms. Finally, we present the strength of a reversible approach in allowing the modulation of the circadian period and the phase.

Whole genome sequencing revealed genetic structure, domestication, and selection of Chinese indigenous ducks

The genetic architecture and domestication history of Chinese indigenous ducks, particularly those with distinct traits like the Longsheng duck (LSD), are not well characterized. This study used whole-genome resequencing data from 540 ducks across 30 populations to explore the genetic diversity and structure of these ducks. Our findings suggest extensive interspecific hybridization between mallard and spot-billed duck. Comparing with other Chinese ducks, LSD is a distinct breed with moderate genetic diversity. Selective signal analysis identified several key genes impacting neural development, fat metabolism, immunity, and circadian rhythms like SLC25A20 and PER2. These genes showed strong selective pressures that parallelled other domestication processes. Additionally, EDNRB2 was identified as a potential gene influencing the unique coloration of LSD's plumage, bill, and webbed feet, associated with distinct mutations in non-coding regions. Comparative analysis with other indigenous breeds further pinpointed genes associated with LSD-specific traits, including plumage color, reproductive capabilities, and fat deposition, such as MITF, SPATA2, EIF2S2, PLIN3, ATP1B1, and CCDC80. Our findings clarify the population genetics of Chinese indigenous ducks. They also highlight key genes and mutations that shape the unique characteristics of LSD. These insights pave the way for further research into the genetic resources and domestication patterns of Chinese ducks.

Identification of potential casein kinase I isoform epsilon inhibitors from phytoconstituents: implications for targeted anticancer therapeutics

Casein kinase I isoform epsilon (CK1ε) demonstrates significant implications in cancer pathogenesis, influencing key cellular processes linked to oncogenesis. Its regulatory roles in cell survival, proliferation, and modulation of oncogenic pathways highlight CK1ε as a potential target for therapeutic strategies in diverse cancer types. In this research, a virtual screening of phytoconstituents from the IMPPAT2.0 database was conducted to find potential inhibitors targeting CK1ε. Initially, compounds adhering to Lipinski's rule of five were retrieved, followed by filtering based on binding affinities and subsequent interaction analyses to refine the selection. Finally, two compounds, Chrysin-7-O-Glucuronide and Rhodiolin, demonstrated considerable affinities with specific interactions at the CK1ε ATP binding site (involving SER17, SER19, and LYS38), forming hydrogen bonds, and were identified for further analysis via PASS server. Employing all-atom molecular dynamic (MD) simulations for 200 ns, structural deviation, residual fluctuation, compactness by radius of gyration, solvent accessible surface area calculation, principal component analysis, and free energy landscapes, were conducted. These findings suggest that Chrysin-7-O-Glucuronide and Rhodiolin warrant further investigation in experimental and clinical research as potential candidates for developing anticancer therapeutics targeting CK1ε kinase.

A wrinkle in timers: evolutionary rewiring of conserved biological timekeepers

Biological timing mechanisms are intrinsic to all organisms, orchestrating the temporal coordination of biological events through complex genetic networks. Circadian rhythms and developmental timers utilize distinct timekeeping mechanisms. This review summarizes the molecular basis for circadian rhythms in mammals and Drosophila, and recent work leveraging these clocks to understand temporal regulation in Caenorhabditis elegans development. We describe the evolutionary connections between distinct timing mechanisms and discuss recent insights into the rewiring of core clock components in development. By integrating findings from circadian and developmental studies with biochemical and structural analyses of conserved components, we aim to illuminate the molecular basis of nematode timing mechanisms and highlight broader insights into biological timing across species.

Beyond vision: effects of light on the circadian clock and mood-related behaviours

Light is a crucial environmental factor that influences various aspects of life, including physiological and psychological processes. While light is well-known for its role in enabling humans and other animals to perceive their surroundings, its influence extends beyond vision. Importantly, light affects our internal time-keeping system, the circadian clock, which regulates daily rhythms of biochemical and physiological processes, ultimately impacting mood and behaviour. The 24-h availability of light can have profound effects on our well-being, both physically and mentally, as seen in cases of jet lag and shift work. This review summarizes the intricate relationships between light, the circadian clock, and mood-related behaviours, exploring the underlying mechanisms and its implications for health.

Circadian rhythms in stem cells and their therapeutic potential

Circadian rhythms are present in almost all cells, but their existence in stem cells has remains not well established. Circadian clock appears to be closely associated with differentiated mature cells and rarely detected in immature embryonic stem cells. Recent evidence reveals the presence of circadian genes and rhythmic physiologic activities in stem cells as well as stem cell-derived extracellular vesicle (EV) characteristics. The circadian clock entails diverse physiologic and pathological mechanisms underlying cell fate. Integration of circadian rhythm to clinical applications, such as chronotherapy, chrono-biomarker, and environment modification, may facilitate therapeutic outcomes of stem cell-based regenerative medicine. Understanding circadian rhythms in stem cells can optimize stem cell-based therapies by determining the best times for harvesting and administering stem cells, thereby enhancing therapeutic efficacy. Further research into the circadian properties of stem cells will refine stem cell-based therapies, contributing to advancements in regenerative medicine.

PERspectives on circadian cell biology

Daily rhythms in the activities of PERIOD proteins are critical to the temporal regulation of mammalian physiology. While the molecular partners and genetic circuits that allow PERIOD to effect auto-repression and regulate transcriptional programmes are increasingly well understood, comprehension of the time-resolved mechanisms that allow PERIOD to conduct this daily dance is incomplete. Here, we consider the character and controversies of this central mammalian clock protein with a focus on its intrinsically disordered nature.This article is part of the Theo Murphy meeting issue 'Circadian rhythms in infection and immunity'.

Markovian State Models uncover Casein Kinase 1 dynamics that govern circadian period

Circadian rhythms in mammals are tightly regulated through phosphorylation of Period (PER) proteins by Casein Kinase 1 (CK1, subtypes δ and ε). CK1 acts on at least two different regions of PER with opposing effects: phosphorylation of phosphodegron (pD) regions leads to PER degradation, while phosphorylation of the Familial Advanced Sleep Phase (FASP) region leads to PER stabilization. To investigate how substrate selectivity is encoded by the conformational dynamics of CK1, we performed a large set of independent molecular dynamics (MD) simulations of wildtype CK1 and the tau mutant (R178C) that biases kinase activity toward a pD. We used Markovian State Models (MSMs) to integrate the simulations into a single model of the conformational landscape of CK1 and used Gaussian accelerated molecular dynamics (GaMD) to build the first molecular model of CK1 and the unphosphorylated FASP motif. Together, these findings provide a mechanistic view of CK1, establishing how the activation loop acts as a key molecular switch to control substrate selectivity. We show that the tau mutant favors an alternative conformation of the activation loop and significantly accelerates the dynamics of CK1. This reshapes the binding cleft in a way that impairs FASP binding and would ultimately lead to PER destabilization and shorter circadian periods. Finally, we identified an allosteric pocket that could be targeted to bias this molecular switch. Our integrated approach offers a detailed model of CK1's conformational landscape and its relevance to normal, mutant, and druggable circadian timekeeping.

The role of circadian rhythm regulator PERs in oxidative stress, immunity, and cancer development

The complex interaction between circadian rhythms and physiological functions is essential for maintaining human health. At the heart of this interaction lies the PERIOD proteins (PERs), pivotal to the circadian clock, influencing the timing of physiological and behavioral processes and impacting oxidative stress, immune functionality, and tumorigenesis. PER1 orchestrates the cooperation of the enzyme GPX1, modulating mitochondrial dynamics in sync with daily rhythms and oxidative stress, thus regulating the mechanisms managing energy substrates. PERs in innate immune cells modulate the temporal patterns of NF-κB and TNF-α activities, as well as the response to LPS-induced toxic shock, initiating inflammatory responses that escalate into chronic inflammatory conditions. Crucially, PERs modulate cancer cell behaviors including proliferation, apoptosis, and migration by influencing the levels of cell cycle proteins and stimulating the expression of oncogenes c-Myc and MDM2. PER2/3, as antagonists in cancer stem cell biology, play important roles in differentiating cancer stem cells and in maintaining their stemness. Importantly, the expression of Pers serve as a significant factor for early cancer diagnosis and prognosis. This review delves into the link between circadian rhythm regulator PERs, disruptions in circadian rhythm, and oncogenesis. We examine the evidence that highlights how dysfunctions in PERs activities initiate cancer development, aid tumor growth, and modify cancer cell metabolism through pathways involved in oxidative stress and immune system. Comprehending these connections opens new pathways for the development of circadian rhythm-based therapeutic strategies, with the aims of boosting immune responses and enhancing cancer treatments.

A conserved chronobiological complex times C. elegans development

The mammalian PAS-domain protein PERIOD (PER) and its C. elegans orthologue LIN-42 have been proposed to constitute an evolutionary link between two distinct, circadian and developmental, timing systems. However, while the function of PER in animal circadian rhythms is well understood molecularly and mechanistically, this is not true for LIN-42's function in timing rhythmic development. Here, using targeted deletions, we find that the LIN-42 PAS domains are dispensable for the protein's function in timing molts. Instead, we observe arrhythmic molts upon deletion of a distinct sequence element, conserved with PER. We show that this element, designated CK1δ-binding domain (CK1BD), mediates stable binding to KIN-20, the C. elegans CK1δ/ε orthologue. We demonstrate that CK1δ phosphorylates LIN-42 and define two conserved helical motifs in the CK1BD, CK1BD-A and CK1BD-B, that have distinct roles in controlling CK1δ-binding and kinase activity in vitro. KIN-20 and the LIN-42 CK1BD are required for proper molting timing in vivo, and loss of LIN-42 binding changes KIN-20 subcellular localization. The interactions mirror the central role of a stable circadian PER-CK1 complex in setting a robust ~24-hour period. Hence, our results establish LIN-42/PER - KIN-20/CK1δ/ε as a functionally conserved signaling module of two distinct chronobiological systems.

Circadian control of polycyclic aromatic hydrocarbon-induced dysregulation of endothelial tight junctions and mitochondrial bioenergetics

The study evaluates the impact of environmental toxicants, such as polycyclic aromatic hydrocarbons (PAHs), on circadian regulations and functions of brain endothelial cells, which form the main structural element of the blood-brain barrier (BBB). PAH are lipophilic and highly toxic environmental pollutants that accumulate in human and animal tissues. Environmental factors related to climate change, such as an increase in frequency and intensity of wildfires or enhanced strength of hurricanes or tropical cyclones, may lead to redistribution of these toxicants and enhanced human exposure. These natural disasters are also associated with disruption of circadian rhythms in affected populations, linking increased exposure to environmental toxicants to alterations of circadian rhythm pathways. Several vital physiological processes are coordinated by circadian rhythms, and disruption of the circadian clock can contribute to the development of several diseases. The blood-brain barrier (BBB) is crucial for protecting the brain from blood-borne harmful substances, and its integrity is influenced by circadian rhythms. Exposure of brain endothelial cells to a human and environmentally-relevant PAH mixture resulted in dose-dependent alterations of expression of critical circadian modulators, such as Clock, Bmal1, Cry1/2, and Per1/2. Moreover, silencing of the circadian Clock gene potentiated the impact of PAHs on the expression of the main tight junction genes and proteins (namely, claudin-5, occludin, JAM-2, and ZO-2), as well as mitochondrial bioenergetics. Findings from this study contribute to a better understanding of pathological influence of PAH-induced health effects, especially those related to circadian rhythm disruption.

The clockwork of champions: Influence of circadian biology on exercise performance

Exercise physiology and circadian biology are distinct and long-standing fields. Recently they have seen increased integration, largely due to the discovery of the molecular components of the circadian clock and recognition of human exercise performance differences over time-of-day. Circadian clocks, ubiquitous in cells, regulate a daily tissue specific program of gene expression that contribute to temporal patterns of physiological functions over a 24-h cycle. Understanding how circadian clock function in skeletal muscle, as well as other tissues contribute to exercise performance is still in the very early stages. This review provides background on this emerging field with a review of early exercise and time-of-day studies in both human and animals. We then move into the role of the circadian clock and its daily program of gene expression in skeletal muscle with a focus on specific metabolic and physiological outputs that vary over time-of-day. Lastly, we discuss the recognition that the timing of exercise communicates with the skeletal muscle circadian clock to adjust its phase settings and why this maybe important for performance and health.

Targeting the circadian modulation: novel therapeutic approaches in the management of ASD

Circadian dysfunction is prevalent in neurodevelopmental disorders, particularly in autism spectrum disorder (ASD). A plethora of empirical studies demonstrate a strong correlation between ASD and circadian disruption, suggesting that modulation of circadian rhythms and the clocks could yield satisfactory advancements. Research indicates that circadian dysfunction associated with abnormal neurodevelopmental phenotypes in ASD individuals, potentially contribute to synapse plasticity disruption. Therefore, targeting circadian rhythms may emerge as a key therapeutic approach. In this study, we did a brief review of the mammalian circadian clock, and the correlation between the circadian mechanism and the pathology of ASD at multiple levels. In addition, we highlight that circadian is the target or modulator to participate in the therapeutic approaches in the management of ASD, such as phototherapy, melatonin, modulating circadian components, natural compounds, and chronotherapies. A deep understanding of the circadian clock's regulatory role in the neurodevelopmental phenotypes in ASD may inspire novel strategies for improving ASD treatment.

Arginine vasopressin: Critical regulator of circadian homeostasis

Circadian rhythms optimally regulate numerous physiological processes in an organism and synchronize them with the external environment. The suprachiasmatic nucleus (SCN), the center of the circadian clock in mammals, is composed of multiple cell types that form a network that provides the basis for the remarkable stability of the circadian clock. Among the neuropeptides expressed in the SCN, arginine vasopressin (AVP) has attracted much attention because of its deep involvement in the function of circadian rhythms, as elucidated in particular by studies using genetically engineered mice. This review briefly summarizes the current knowledge on the peptidergic distribution and topographic neuronal organization in the SCN, the molecular mechanisms of the clock genes, and the relationship between the SCN and peripheral clocks. With respect to the physiological roles of AVP and AVP-expressing neurons, in addition to a sex-dependent action of AVP in the SCN, studies using AVP receptor knockout mice and mice genetically manipulated to alter the clock properties of AVP neurons are summarized here, highlighting its importance in maintaining circadian homeostasis and its potential as a target for therapeutic interventions.

Clock gene homologs lin-42 and kin-20 regulate circadian rhythms in C. elegans

Circadian rhythms are endogenous oscillations in nearly all organisms, from prokaryotes to humans, allowing them to adapt to cyclical environments for close to 24 h. Circadian rhythms are regulated by a central clock, based on a transcription-translation feedback loop. One important protein in the central loop in metazoan clocks is PERIOD, which is regulated in part by Casein kinase 1ε/δ (CK1ε/δ) phosphorylation. In the nematode Caenorhabditis elegans, period and casein kinase 1ε/δ are conserved as lin-42 and kin-20, respectively. Here, we studied the involvement of lin-42 and kin-20 in the circadian rhythms of the adult nematode using a bioluminescence-based circadian transcriptional reporter. We show that mutations of lin-42 and kin-20 generate a significantly longer endogenous period, suggesting a role for both genes in the nematode circadian clock, as in other organisms. These phenotypes can be partially rescued by overexpression of either gene under their native promoter. Both proteins are expressed in neurons and epidermal seam cells, as well as in other cells. Depletion of LIN-42 and KIN-20, specifically in neuronal cells after development, was sufficient to lengthen the period of oscillating sur-5 expression. Therefore, we conclude that LIN-42 and KIN-20 are critical regulators of the adult nematode circadian clock through neuronal cells.

Hierarchical and scaffolded phosphorylation of two degrons controls PER2 stability

The duration of the transcription-repression cycles that give rise to mammalian circadian rhythms is largely determined by the stability of the PERIOD (PER) protein, the rate-limiting components of the molecular clock. The degradation of PERs is tightly regulated by multisite phosphorylation by casein kinase 1 (CK1δ/ε). In this phosphoswitch, phosphorylation of a PER2 degron [degron 2 (D2)] causes degradation, while phosphorylation of the PER2 familial advanced sleep phase (FASP) domain blocks CK1 activity on the degron, stabilizing PER2. However, this model and many other studies of PER2 degradation do not include the second degron of PER2 that is conserved in PER1, termed degron 1 (D1). We examined how these two degrons contribute to PER2 stability, affect the balance of the phosphoswitch, and how they are differentiated by CK1. Using PER2-luciferase fusions and real-time luminometry, we investigated the contribution of both D2 and of CK1-PER2 binding. We find that D1, like D2, is a substrate of CK1 but that D1 plays only a 'backup' role in PER2 degradation. Notably, CK1 bound to a PER1:PER2 dimer protein can phosphorylate PER1 D1 in trans. This scaffolded phosphorylation provides additional levels of control to PER stability and circadian rhythms.

A circadian clock regulates the blood-brain barrier across phylogeny

As the central regulatory system of an organism, the brain is responsible for overseeing a wide variety of physiological processes essential for an organism's survival. To maintain the environment necessary for neurons to function, the brain requires highly selective uptake and elimination of specific molecules through the blood-brain barrier (BBB). As an organism's activities vary throughout the day, how does the BBB adapt to meet the changing needs of the brain? A mechanism is through temporal regulation of BBB permeability via its circadian clock, which will be the focal point of this chapter. To comprehend the circadian clock's role within the BBB, we will first examine the anatomy of the BBB and the transport mechanisms enabling it to fulfill its role as a restrictive barrier. Next, we will define the circadian clock, and the discussion will encompass an introduction to circadian rhythms, the Transcription-Translation Feedback Loop (TTFL) as the mechanistic basis of circadian timekeeping, and the organization of tissue clocks found in organisms. Then, we will cover the role of the circadian rhythms in regulating the cellular mechanisms and functions of the BBB. We discuss the implications of this regulation in influencing sleep behavior, the progression of neurodegenerative diseases, and finally drug delivery for treatment of neurological diseases.

Biological clock regulation by the PER gene family: a new perspective on tumor development

The Period (PER) gene family is one of the core components of the circadian clock, with substantial correlations between the PER genes and cancers identified in extensive researches. Abnormal mutations in PER genes can influence cell function, metabolic activity, immunity, and therapy responses, thereby promoting the initiation and development of cancers. This ultimately results in unequal cancers progression and prognosis in patients. This leads to variable cancer progression and prognosis among patients. In-depth studies on the interactions between the PER genes and cancers can reveal novel strategies for cancer detection and treatment. In this review, we aim to provide a comprehensive overview of the latest research on the role of the PER gene family in cancer.

Circadian rhythm regulates the function of immune cells and participates in the development of tumors

Circadian rhythms are present in almost all cells and play a crucial role in regulating various biological processes. Maintaining a stable circadian rhythm is essential for overall health. Disruption of this rhythm can alter the expression of clock genes and cancer-related genes, and affect many metabolic pathways and factors, thereby affecting the function of the immune system and contributing to the occurrence and progression of tumors. This paper aims to elucidate the regulatory effects of BMAL1, clock and other clock genes on immune cells, and reveal the molecular mechanism of circadian rhythm's involvement in tumor and its microenvironment regulation. A deeper understanding of circadian rhythms has the potential to provide new strategies for the treatment of cancer and other immune-related diseases.

Glitches in the brain: the dangerous relationship between radiotherapy and brain fog

Up to approximately 70% of cancer survivors report persistent deficits in memory, attention, speed of information processing, multi-tasking, and mental health functioning, a series of symptoms known as "brain fog." The severity and duration of such effects can vary depending on age, cancer type, and treatment regimens. In particular, every year, hundreds of thousands of patients worldwide undergo radiotherapy (RT) for primary brain tumors and brain metastases originating from extracranial tumors. Besides its potential benefits in the control of tumor progression, recent studies indicate that RT reprograms the brain tumor microenvironment inducing increased activation of microglia and astrocytes and a consequent general condition of neuroinflammation that in case it becomes chronic could lead to a cognitive decline. Furthermore, radiation can induce endothelium reticulum (ER) stress directly or indirectly by generating reactive oxygen species (ROS) activating compensatory survival signaling pathways in the RT-surviving fraction of healthy neuronal and glial cells. In particular, the anomalous accumulation of misfolding proteins in neuronal cells exposed to radiation as a consequence of excessive activation of unfolded protein response (UPR) could pave the way to neurodegenerative disorders. Moreover, exposure of cells to ionizing radiation was also shown to affect the normal proteasome activity, slowing the degradation rate of misfolded proteins, and further exacerbating ER-stress conditions. This compromises several neuronal functions, with neuronal accumulation of ubiquitinated proteins with a consequent switch from proteasome to immunoproteasome that increases neuroinflammation, a crucial risk factor for neurodegeneration. The etiology of brain fog remains elusive and can arise not only during treatment but can also persist for an extended period after the end of RT. In this review, we will focus on the molecular pathways triggered by radiation therapy affecting cognitive functions and potentially at the origin of so-called "brain fog" symptomatology, with the aim to define novel therapeutic strategies to preserve healthy brain tissue from cognitive decline.

Identification of the Relationship Between DNA Methylation of Circadian Rhythm Genes and Obesity

In children, teenagers, and young adults, environmental factors and genetic modifications have contributed to the development of obesity. There is a close relationship between obesity and circadian rhythm. To understand the role of CLOCK and BMAL1 in obesity, we analyzed the methylation status of CLOCK and BMAL1 in obese and control subjects. In this paper, we analyzed the methylation status of the CLOCK and BMAL1 genes by using MS-HRM in a total of 55 obese and 54 control subjects. In our study, we demonstrated that the level of fasting glucose and the level of HDL-cholesterol were associated with CLOCK methylation in obesity. We also showed a significant association between BMAL1 gene methylation and waist and hip circumference in obese subjects. This is the first study that shows the methylation of BMAL1 is associated with the obese phenotype. However, we could not show a direct association between CLOCK methylation and the obese phenotype. In this paper, a novel epigenetic interaction between circadian clock genes and obesity was demonstrated.

Mucosal Genes Encoding Clock, Inflammation and Their Mutual Regulators Are Disrupted in Pediatric Patients with Active Ulcerative Colitis

Patients with active ulcerative colitis (UC) display a misalignment of the circadian clock, which plays a vital role in various immune functions. Our aim was to characterize the expression of clock and inflammation genes, and their mutual regulatory genes in treatment-naïve pediatric patients with UC. Using the Inflammatory Bowel Disease Transcriptome and Metatranscriptome Meta-Analysis (IBD TaMMA) platform and R algorithms, we analyzed rectal biopsy transcriptomic data from two cohorts (206 patients with UC vs. 20 healthy controls from the GSE-109142 study, and 43 patients with UC vs. 55 healthy controls from the GSE-117993 study). We compared gene expression levels and correlation of clock genes (BMAL1, CLOCK, PER1, PER2, CRY1, CRY2), inflammatory genes (IκB, IL10, NFκB1, NFκB2, IL6, TNFα) and their mutual regulatory genes (RORα, RORγ, REV-ERBα, PGC1α, PPARα, PPARγ, AMPK, SIRT1) in patients with active UC and healthy controls. The clock genes BMAL1, CLOCK, PER1 and CRY1 and the inflammatory genes IκB, IL10, NFκB1, NFκB2, IL6 and TNFα were significantly upregulated in patients with active UC. The genes encoding the mutual regulators RORα, RORγ, PGC1α, PPARα and PPARγ were significantly downregulated in patients with UC. A uniform pattern of gene expression was found in healthy controls compared to the highly variable expression pattern in patients with UC. Among the healthy controls, inflammatory genes were positively correlated with clock genes and they all showed reduced expression. The difference in gene expression levels was associated with disease severity and endoscopic score but not with histological score. In patients with active UC, clock gene disruption is associated with abnormal mucosal immune response. Disrupted expression of genes encoding clock, inflammation and their mutual regulators together may play a role in active UC.

Regulation of the circadian clock in C. elegans by clock gene homologs kin-20 and lin-42

Circadian rhythms are endogenous oscillations present in nearly all organisms from prokaryotes to humans, allowing them to adapt to cyclical environments close to 24 hours. Circadian rhythms are regulated by a central clock, which is based on a transcription-translation feedback loop. One important protein in the central loop in metazoan clocks is PERIOD, which is regulated in part by Casein kinase 1 ε/δ (CK1 ε/δ ) phosphorylation. In the nematode Caenorhabditis elegans , period and casein kinase 1ε/δ are conserved as lin-42 and kin-20 , respectively. Here we studied the involvement of lin-42 and kin-20 in circadian rhythms of the adult nematode using a bioluminescence-based circadian transcriptional reporter. We show that mutations of lin-42 and kin-20 generate a significantly longer endogenous period, suggesting a role for both genes in the nematode circadian clock, as in other organisms. These phenotypes can be partially rescued by overexpression of either gene under their native promoter. Both proteins are expressed in neurons and seam cells, a population of epidermal stem cells in C. elegans that undergo multiple divisions during development. Depletion of LIN-42 and KIN-20 specifically in neuronal cells after development was sufficient to lengthen the period of oscillating sur-5 expression. Therefore, we conclude that LIN-42 and KIN-20 are critical regulators of the adult nematode circadian clock through neuronal cells.

Urbanization is associated with non-coding polymorphisms in candidate behavioural genes in the Eurasian coot

Extensive transformation of natural land cover into urbanized areas enhances accumulation of phenotypic differences between animals from urban and nonurban populations, but there is little information on whether these changes, especially in terms of animal behaviour and circadian rhythm, have a genetic basis. The aim of this study was to investigate genetic background of behavioural differences between four pairs of urban and nonurban populations of a common waterbird, the Eurasian coot Fulica atra. For this purpose, we quantified polymorphisms in personality-related candidate genes, previously reported to be associated with avian circadian rhythms and behavioural traits that may be crucial for urban life. We found general associations between landscape urbanization level and polymorphisms in 3'UTR region of CREB1 gene encoding transcriptional factor, which participates in development of cognitive functions and regulation of circadian rhythm. We also found significant differentiation between urban and nonurban populations in the intronic region of CKIɛ gene responsible for regulation of circadian clock. Although we lacked evidence for linkage of this intronic variation with coding polymorphisms, genetic differentiation between urban populations was significantly stronger at CKIɛ intron compared with neutral microsatellite markers, suggesting possible local adaptations of CKIɛ expression regulation to specific urban sites. Our results indicate that behavioural differentiation between urban and nonurban coot populations may be the effect of habitat-specific selective pressure resulting in genetic adaptations to urban environment and supporting the microevolutionary scenario. These adaptations, however, prevailed in non-coding regulatory rather than coding gene regions and showed either general or local patterns, revealing high complexity of associations between behaviour and landscape urbanization in birds.

Metabolic and chemical architecture of the mammalian circadian clock

Circadian rhythms are endogenous periodic biological processes that occur on a daily timescale. These rhythms are generated by a transcriptional/translational feedback loop that consists of the CLOCK-BMAL1 heterodimeric transcriptional activator complex and the PER1/2-CRY1/2-CK1δ/ε repressive complex. The output pathways of this molecular feedback loop generate circadian rhythmicity in various biological processes. Among these, metabolism is a primary regulatory target of the circadian clock which can also feedback to modulate clock function. This intertwined relationship between circadian rhythms and metabolism makes circadian clock components promising therapeutic targets. Despite this, pharmacological therapeutics that target the circadian clock are relatively rare. In this review, we hope to stimulate interest in chemical chronobiology by providing a comprehensive background on the molecular mechanism of mammalian circadian rhythms and their connection to metabolism, highlighting important studies in the chemical approach to circadian research, and offering our perspectives on future developments in the field.

Circadian Dysfunction in Adipose Tissue: Chronotherapy in Metabolic Diseases

Essential for survival and reproduction, the circadian timing system (CTS) regulates adaptation to cyclical changes such as the light/dark cycle, temperature change, and food availability. The regulation of energy homeostasis possesses rhythmic properties that correspond to constantly fluctuating needs for energy production and consumption. Adipose tissue is mainly responsible for energy storage and, thus, operates as one of the principal components of energy homeostasis regulation. In accordance with its roles in energy homeostasis, alterations in adipose tissue's physiological processes are associated with numerous pathologies, such as obesity and type 2 diabetes. These alterations also include changes in circadian rhythm. In the current review, we aim to summarize the current knowledge regarding the circadian rhythmicity of adipogenesis, lipolysis, adipokine secretion, browning, and non-shivering thermogenesis in adipose tissue and to evaluate possible links between those alterations and metabolic diseases. Based on this evaluation, potential therapeutic approaches, as well as clock genes as potential therapeutic targets, are also discussed in the context of chronotherapy.

Exposure to polychlorinated biphenyls selectively dysregulates endothelial circadian clock and endothelial toxicity

Polychlorinated biphenyls (PCBs) are lipophilic and persistent environmental toxicants, which pose health threats to the exposed population. Among several organs and cell types, vascular tissue and endothelial cells are especially prone to PCB-induced toxicity. Exposure to PCBs can exert detrimental impacts on biological pathways, expression of transcription factors, and tight junction proteins that are integral to the functionality of endothelial cells. Because biological and cellular processes are tightly regulated by circadian rhythms, and disruption of the circadian system may cause several diseases, we evaluated if exposure to PCBs can alter the expression of the major endothelial circadian regulators. In addition, we studied if dysregulation of circadian rhythms by silencing the brain and muscle ARNT-like 1 (Bmal1) gene can contribute to alterations of brain endothelial cells in response to PCB treatment. We demonstrated that diminished expression of Bmal1 enhances PCB-induced dysregulation of tight junction complexes, such as the expression of occludin, JAM-2, ZO-1, and ZO-2 especially at pathologically relevant longer PCB exposure times. Overall, the obtained results imply that dysregulation of the circadian clock is involved in endothelial toxicity of PCBs. The findings provide new insights for toxicological studies focused on the interactions between environmental pollutants and regulation of circadian rhythms.

PERIOD phosphorylation leads to feedback inhibition of CK1 activity to control circadian period

PERIOD (PER) and Casein Kinase 1δ regulate circadian rhythms through a phosphoswitch that controls PER stability and repressive activity in the molecular clock. CK1δ phosphorylation of the familial advanced sleep phase (FASP) serine cluster embedded within the Casein Kinase 1 binding domain (CK1BD) of mammalian PER1/2 inhibits its activity on phosphodegrons to stabilize PER and extend circadian period. Here, we show that the phosphorylated FASP region (pFASP) of PER2 directly interacts with and inhibits CK1δ. Co-crystal structures in conjunction with molecular dynamics simulations reveal how pFASP phosphoserines dock into conserved anion binding sites near the active site of CK1δ. Limiting phosphorylation of the FASP serine cluster reduces product inhibition, decreasing PER2 stability and shortening circadian period in human cells. We found that Drosophila PER also regulates CK1δ via feedback inhibition through the phosphorylated PER-Short domain, revealing a conserved mechanism by which PER phosphorylation near the CK1BD regulates CK1 kinase activity.

The proteasome: A key modulator of nervous system function, brain aging, and neurodegenerative disease

The proteasome is a large multi-subunit protease responsible for the degradation and removal of oxidized, misfolded, and polyubiquitinated proteins. The proteasome plays critical roles in nervous system processes. This includes maintenance of cellular homeostasis in neurons. It also includes roles in long-term potentiation via modulation of CREB signaling. The proteasome also possesses roles in promoting dendritic spine growth driven by proteasome localization to the dendritic spines in an NMDA/CaMKIIα dependent manner. Proteasome inhibition experiments in varied organisms has been shown to impact memory, consolidation, recollection and extinction. The proteasome has been further shown to impact circadian rhythm through modulation of a range of 'clock' genes, and glial function. Proteasome function is impaired as a consequence both of aging and neurodegenerative diseases. Many studies have demonstrated an impairment in 26S proteasome function in the brain and other tissues as a consequence of age, driven by a disassembly of 26S proteasome in favor of 20S proteasome. Some studies also show proteasome augmentation to correct age-related deficits. In amyotrophic lateral sclerosis Alzheimer's, Parkinson's and Huntington's disease proteasome function is impaired through distinct mechanisms with impacts on disease susceptibility and progression. Age and neurodegenerative-related deficits in the function of the constitutive proteasome are often also accompanied by an increase in an alternative form of proteasome called the immunoproteasome. This article discusses the critical role of the proteasome in the nervous system. We then describe how proteasome dysfunction contributes to brain aging and neurodegenerative disease.

Influence of Fasting until Noon (Extended Postabsorptive State) on Clock Gene mRNA Expression and Regulation of Body Weight and Glucose Metabolism

The trend of fasting until noon (omission or delayed breakfast) is increasingly prevalent in modern society. This eating pattern triggers discordance between endogenous circadian clock rhythms and the feeding/fasting cycle and is associated with an increased incidence of obesity and T2D. Although the underlying mechanism of this association is not well understood, growing evidence suggests that fasting until noon, also known as an "extended postabsorptive state", has the potential to cause a deleterious effect on clock gene expression and to disrupt regulation of body weight, postprandial and overall glycemia, skeletal muscle protein synthesis, and appetite, and may also lead to lower energy expenditure. This manuscript overviews the clock gene-controlled glucose metabolism during the active and resting phases and the consequences of postponing until noon the transition from postabsorptive to fed state on glucose metabolism, weight control, and energy expenditure. Finally, we will discuss the metabolic advantages of shifting more energy, carbohydrates (CH), and proteins to the early hours of the day.

Malpighian tubules of Rhodnius prolixus: More than post-prandial diuresis

Rhodnius prolixus, a major vector of Chagas disease, may be considered the model upon which the foundations of insect physiology and biochemistry were built. It is an obligate blood feeder in which the blood meal triggers growth, development and reproduction. The blood meal also triggers a post-prandial diuresis to maintain osmotic homeostasis. In R. prolixus, as with other insects, the Malpighian tubules play a critical role in this diuresis, and much has been learned about diuresis in R. prolixus, and in other model insects. But the post-genomic era has brought new insights, identifying functions quite apart from diuresis for Malpighian tubules. Indeed, microarrays, transcriptomes, and proteomics have revealed the major roles that Malpighian tubules play in immunity, detoxification, pesticide resistance, and in tolerance to overall stress. This is particularly relevant to R. prolixus since gorging on blood creates several challenges in addition to osmotic balance. Xenobiotics may be present in the blood or toxins may be produced by metabolism of blood; and these must be neutralized and excreted. These processes have not been well described at the molecular level for Malpighian tubules of R. prolixus. This paper will review the involvement of Malpighian tubules in immunity and detoxification, identifying new aspects for Malpighian tubule physiology of R. prolixus by virtue of a transcriptome analysis. The transcriptome analysis indicates the potential of Malpighian tubules of R. prolixus to mount a robust innate immune response, and to contribute to antioxidant production and heme detoxification.

Circadian Oscillations in Skin and Their Interconnection with the Cycle of Life

Periodically oscillating biological processes, such as circadian rhythms, are carefully concerted events that are only beginning to be understood in the context of tissue pathology and organismal health, as well as the molecular mechanisms underlying these interactions. Recent reports indicate that light can independently entrain peripheral circadian clocks, challenging the currently prevalent hierarchical model. Despite the recent progress that has been made, a comprehensive overview of these periodic processes in skin is lacking in the literature. In this review, molecular circadian clock machinery and the factors that govern it have been highlighted. Circadian rhythm is closely linked to immunological processes and skin homeostasis, and its desynchrony can be linked to the perturbation of the skin. The interplay between circadian rhythm and annual, seasonal oscillations, as well as the impact of these periodic events on the skin, is described. Finally, the changes that occur in the skin over a lifespan are presented. This work encourages further research into the oscillating biological processes occurring in the skin and lays the foundation for future strategies to combat the adverse effects of desynchrony, which would likely have implications in other tissues influenced by periodic oscillatory processes.

Circadian disruption alters gut barrier integrity via a ß-catenin-MMP-related pathway

We evaluated the mechanistic link between circadian rhythms and gut barrier permeability. Mice were subjected to either constant 24-h light (LL) or 12-h light/dark cycles (LD). Mice housed in LL experienced a significant increase in gut barrier permeability that was associated with dysregulated ß-catenin expression and altered expression of tight junction (TJ) proteins. Silencing of ß-catenin resulted in disruption of barrier function in SW480 cells, with ß-catenin appearing to be an upstream regulator of the core circadian components, such as Bmal1, Clock, and Per1/2. In addition, ß-catenin silencing downregulated ZO-1 and occludin TJ proteins with only limited or no changes at their mRNA levels, suggesting post transcriptional regulation. Indeed, silencing of ß-catenin significantly upregulated expression of matrix metallopeptidase (MMP)-2 and MMP-9, and blocking MMP-2/9 activity attenuated epithelial disruption induced by ß-catenin silencing. These results indicate the regulatory role of circadian disruption on gut barrier integrity and the associations between TJ proteins and circadian rhythms, while demonstrating the regulatory role of ß-catenin in this process.

Endogenous circadian reporters reveal functional differences of PERIOD paralogs and the significance of PERIOD:CK1 stable interaction

Adverse consequences from having a faulty circadian clock include compromised sleep quality and poor performance in the short-term, and metabolic diseases and cancer in the long-term. However, our understanding of circadian disorders is limited by the incompleteness of our molecular models and our dearth of defined mutant models. Because it would be prohibitively expensive to develop live animal models to study the full range of complicated clock mechanisms, we developed PER1-luc and PER2-luc endogenous circadian reporters in a validated clock cell model, U-2 OS, where the genome can be easily manipulated, and functional consequences of mutations can be accurately studied. When major clock genes were knocked out in these cells, circadian rhythms were modulated similarly compared with corresponding mutant mice, validating the platform for genetics studies. Using these reporter cells, we uncovered critical differences between two paralogs of PER. Although PER1 and PER2 are considered redundant and either one can serve as a pacemaker alone, they were dramatically different in biochemical parameters such as stability and phosphorylation kinetics. Consistently, circadian phase was dramatically different between PER1 and PER2 knockout reporter cells. We further showed that the stable binding of casein kinase1δ/ε to PER is not required for PER phosphorylation itself, but is critical for delayed timing of phosphorylation. Our system can be used as an efficient platform to study circadian disorders associated with pathogenic mutations and their underlying molecular mechanisms.

Differential Effect of Fructose in the Presence or Absence of Fatty Acids on Circadian Metabolism in Hepatocytes

We aimed to explore whether fructose in the absence or presence of fatty acids modulates circadian metabolism in AML-12 hepatocytes. Fructose treatment under steatosis conditions (FruFA) led to fat synthesis resulting in increased triglycerides and cholesterol content. Fructose led to reduced activity of the AMPK and mTOR-signaling pathway. However, FruFA treatment led to inhibition of the AMPK signaling pathway but activation of the mTOR pathway. Fructose also increased the expression of inflammatory markers, whereas the addition of fatty acids dampened their circadian expression. At the clock level, fructose or FruFA altered the expression of the core clock. More specifically, fructose led to altered expression of the BMAL1-RORα-REV-ERBα axis, together with reduced phosphorylated BMAL1 levels. In conclusion, our results show that hepatocytes treated with fructose respond differently if fatty acids are present, leading to a differential effect on metabolism and circadian rhythms. This is achieved by modulating BMAL1 activity and expression.

How phosphorylation impacts intrinsically disordered proteins and their function

Phosphorylation is the most common post-translational modification (PTM) in eukaryotes, occurring particularly frequently in intrinsically disordered proteins (IDPs). These proteins are highly flexible and dynamic by nature. Thus, it is intriguing that the addition of a single phosphoryl group to a disordered chain can impact its function so dramatically. Furthermore, as many IDPs carry multiple phosphorylation sites, the number of possible states increases, enabling larger complexities and novel mechanisms. Although a chemically simple and well-understood process, the impact of phosphorylation on the conformational ensemble and molecular function of IDPs, not to mention biological output, is highly complex and diverse. Since the discovery of the first phosphorylation site in proteins 75 years ago, we have come to a much better understanding of how this PTM works, but with the diversity of IDPs and their capacity for carrying multiple phosphoryl groups, the complexity grows. In this Essay, we highlight some of the basic effects of IDP phosphorylation, allowing it to serve as starting point when embarking on studies into this topic. We further describe how recent complex cases of multisite phosphorylation of IDPs have been instrumental in widening our view on the effect of protein phosphorylation. Finally, we put forward perspectives on the phosphorylation of IDPs, both in relation to disease and in context of other PTMs; areas where deep insight remains to be uncovered.

Deciphering clock genes as emerging targets against aging

The old people often suffer from circadian rhythm disturbances, which in turn accelerate aging. Many aging-related degenerative diseases such as Alzheimer's disease, Parkinson's disease, and osteoarthritis have an inextricable connection with circadian rhythm. In light of the predominant effects of clock genes on regulating circadian rhythm, we systematically present the elaborate network of roles that clock genes play in aging in this review. First, we briefly introduce the basic background regarding clock genes. Second, we systemically summarize the roles of clock genes in aging and aging-related degenerative diseases. Third, we discuss the relationship between clock genes polymorphisms and aging. In summary, this review is intended to clarify the indispensable roles of clock genes in aging and sheds light on developing clock genes as anti-aging targets.

Mood phenotypes in rodent models with circadian disturbances

Many physiological functions with approximately 24-h rhythmicity (circadian rhythms) are generated by an internal time-measuring system of the circadian clock. While sleep/wake cycles, feeding patterns, and body temperature are the most widely known physiological functions under the regulation of the circadian clock, physiological regulation by the circadian clock extends to higher brain functions. Accumulating evidence suggests strong associations between the circadian clock and mood disorders such as depression, but the underlying mechanisms of the functional relationship between them are obscure. This review overviews rodent models with disrupted circadian rhythms on depression-related responses. The animal models with circadian disturbances (by clock gene mutations and artifactual interventions) will help understand the causal link between the circadian clock and depression.

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The molecular mechanisms of the mammalian circadian rhythm are systematically overviewed.

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We overview how genetic and pharmacological manipulations of clock (related) genes are linked to mood phenotypes.

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We overview how artificial perturbations, such as SCN lesions and aberrant light, affect circadian rhythm and mood.

Death of a Protein: The Role of E3 Ubiquitin Ligases in Circadian Rhythms of Mice and Flies

Circadian clocks evolved to enable organisms to anticipate and prepare for periodic environmental changes driven by the day–night cycle. This internal timekeeping mechanism is built on autoregulatory transcription–translation feedback loops that control the rhythmic expression of core clock genes and their protein products. The levels of clock proteins rise and ebb throughout a 24-h period through their rhythmic synthesis and destruction. In the ubiquitin–proteasome system, the process of polyubiquitination, or the covalent attachment of a ubiquitin chain, marks a protein for degradation by the 26S proteasome. The process is regulated by E3 ubiquitin ligases, which recognize specific substrates for ubiquitination. In this review, we summarize the roles that known E3 ubiquitin ligases play in the circadian clocks of two popular model organisms: mice and fruit flies. We also discuss emerging evidence that implicates the N-degron pathway, an alternative proteolytic system, in the regulation of circadian rhythms. We conclude the review with our perspectives on the potential for the proteolytic and non-proteolytic functions of E3 ubiquitin ligases within the circadian clock system.

The secondary pocket of cryptochrome 2 is important for the regulation of its stability and localization

Human clock-gene variations contribute to the phenotypic differences observed in various behavioral and physiological processes, such as diurnal preference, sleep, metabolism, mood regulation, addiction, and fertility. However, little is known about the possible effects of identified variations at the molecular level. In this study, we performed a functional characterization at the cellular level of rare cryptochrome 2 (CRY2) missense variations that were identified from the Ensembl database. Our structural studies revealed that three variations (p.Pro123Leu, p.Asp406His, and p.Ser410Ile) are located at the rim of the secondary pocket of CRY2. We show that these variants were unable to repress CLOCK (circadian locomotor output cycles kaput)/BMAL1 (brain and muscle ARNT-like-1)-driven transcription in a cell-based reporter assay and had reduced affinity to CLOCK-BMAL1. Furthermore, our biochemical studies indicated that the variants were less stable than the WT CRY2, which could be rescued in the presence of period 2 (PER2), another core clock protein. Finally, we found that these variants were unable to properly localize to the nucleus and thereby were unable to rescue the circadian rhythm in a Cry1-/-Cry2-/- double KO mouse embryonic fibroblast cell line. Collectively, our data suggest that the rim of the secondary pocket of CRY2 plays a significant role in its nuclear localization independently of PER2 and in the intact circadian rhythm at the cellular level.

Circadian Rhythm Dysregulation and Leukemia Development: The Role of Clock Genes as Promising Biomarkers

The circadian clock (CC) is a daily system that regulates the oscillations of physiological processes and can respond to the external environment in order to maintain internal homeostasis. For the functioning of the CC, the clock genes (CG) act in different metabolic pathways through the clock-controlled genes (CCG), providing cellular regulation. The CC’s interruption can result in the development of different diseases, such as neurodegenerative and metabolic disorders, as well as cancer. Leukemias correspond to a group of malignancies of the blood and bone marrow that occur when alterations in normal cellular regulatory processes cause the uncontrolled proliferation of hematopoietic stem cells. This review aimed to associate a deregulated CC with the manifestation of leukemia, looking for possible pathways involving CG and their possible role as leukemic biomarkers.

Decoupling PER phosphorylation, stability and rhythmic expression from circadian clock function by abolishing PER-CK1 interaction

Robust rhythms of abundances and phosphorylation profiles of PERIOD proteins were thought be the master rhythms that drive mammalian circadian clock functions. PER stability was proposed to be a major determinant of period length. In mammals, CK1 forms stable complexes with PER. Here we identify the PER residues essential for PER-CK1 interaction. In cells and in mice, their mutation abolishes PER phosphorylation and CLOCK hyperphosphorylation, resulting in PER stabilization, arrhythmic PER abundance and impaired negative feedback process, indicating that PER acts as the CK1 scaffold in circadian feedback mechanism. Surprisingly, the mutant mice exhibit robust short period locomotor activity and other physiological rhythms but low amplitude molecular rhythms. PER-CK1 interaction has two opposing roles in regulating CLOCK-BMAL1 activity. These results indicate that the circadian clock can function independently of PER phosphorylation and abundance rhythms due to another PER-CRY-dependent feedback mechanism and that period length can be uncoupled from PER stability.

Casein Kinase 1 and Human Disease: Insights From the Circadian Phosphoswitch

Biological systems operate in constant communication through shared components and feedback from changes in the environment. Casein kinase 1 (CK1) is a family of protein kinases that functions in diverse biological pathways and its regulation is beginning to be understood. The several isoforms of CK1 take part in key steps of processes including protein translation, cell-cell interactions, synaptic dopaminergic signaling and circadian rhythms. While CK1 mutations are rarely the primary drivers of disease, the kinases are often found to play an accessory role in metabolic disorders and cancers. In these settings, the dysregulation of CK1 coincides with increased disease severity. Among kinases, CK1 is unique in that its substrate specificity changes dramatically with its own phosphorylation state. Understanding the process that governs CK1 substrate selection is thus useful in identifying its role in various ailments. An illustrative example is the PERIOD2 (PER2) phosphoswitch, where CK1δ/ε kinase activity can be varied between three different substrate motifs to regulate the circadian clock.

PERIOD Phosphoclusters Control Temperature Compensation of the Drosophila Circadian Clock

Ambient temperature varies constantly. However, the period of circadian pacemakers is remarkably stable over a wide-range of ecologically- and physiologically-relevant temperatures, even though the kinetics of most biochemical reactions accelerates as temperature rises. This thermal buffering phenomenon, called temperature compensation, is a critical feature of circadian rhythms, but how it is achieved remains elusive. Here, we uncovered the important role played by the Drosophila PERIOD (PER) phosphodegron in temperature compensation. This phosphorylation hotspot is crucial for PER proteasomal degradation and is the functional homolog of mammalian PER2 S478 phosphodegron, which also impacts temperature compensation. Using CRISPR-Cas9, we introduced a series of mutations that altered three Serines of the PER phosphodegron. While all three Serine to Alanine substitutions lengthened period at all temperatures tested, temperature compensation was differentially affected. S44A and S45A substitutions caused undercompensation, while S47A resulted in overcompensation. These results thus reveal unexpected functional heterogeneity of phosphodegron residues in thermal compensation. Furthermore, mutations impairing phosphorylation of the per s phosphocluster showed undercompensation, consistent with its inhibitory role on S47 phosphorylation. We observed that S47A substitution caused increased accumulation of hyper-phosphorylated PER at warmer temperatures. This finding was corroborated by cell culture assays in which S47A slowed down phosphorylation-dependent PER degradation at high temperatures, causing PER degradation to be excessively temperature-compensated. Thus, our results point to a novel role of the PER phosphodegron in temperature compensation through temperature-dependent modulation of the abundance of hyper-phosphorylated PER. Our work reveals interesting mechanistic convergences and differences between mammalian and Drosophila temperature compensation of the circadian clock.

Circadian clock synchrony and chronotherapy opportunities in cancer treatment

Cell-autonomous, tissue-specific circadian rhythms in gene expression and cellular processes have been observed throughout the human body. Disruption of daily rhythms by mistimed exposure to light, food intake, or genetic mutation has been linked to cancer development. Some medications are also more effective at certain times of day. However, a limited number of clinical studies have examined daily rhythms in the patient or drug timing as treatment strategies. This review highlights advances and challenges in cancer biology as a function of time of day. Recent evidence for daily rhythms and their entrainment in tumors indicate that personalized medicine should include understanding and accounting for daily rhythms in cancer patients.

Protein interaction networks of the mammalian core clock proteins

Circadian rhythm is a 24-h cycle that regulates the biochemical and behavioral changes of organisms. It controls a wide range of functions, from gene expression to behavior, allowing organisms to anticipate daily changes in their environment. In mammals, circadian rhythm is generated by a complex transcriptional and translational feedback loop mechanism. The binding of CLOCK/BMAL1 heterodimer to the E-box of DNA located within the promoter region initiates transcription of clock control genes including the transcription of the other two core clock genes of Periods (Pers) and Cryptochromes (Crys). Then PERs and CRYs along with casein kinase 1ɛ/Δ translocate into the nucleus where they suppress CLOCK/BMAL1 transactivation and, in turn, clock-regulated gene expression. Various clock components must be operational to aid in their stabilization and period extension in circadian rhythm. In this review, we have highlighted the recent progress for the core clock interacting proteins to maintain and to stabilize circadian rhythm in mammals.

Circadian clocks, cognition, and Alzheimer's disease: synaptic mechanisms, signaling effectors, and chronotherapeutics

Modulation of basic biochemical and physiological processes by the circadian timing system is now recognized as a fundamental feature of all mammalian organ systems. Within the central nervous system, these clock-modulating effects are reflected in some of the most complex behavioral states including learning, memory, and mood. How the clock shapes these behavioral processes is only now beginning to be realized. In this review we describe recent findings regarding the complex set of cellular signaling events, including kinase pathways, gene networks, and synaptic circuits that are under the influence of the clock timing system and how this, in turn, shapes cognitive capacity over the circadian cycle. Further, we discuss the functional roles of the master circadian clock located in the suprachiasmatic nucleus, and peripheral oscillator populations within cortical and limbic circuits, in the gating of synaptic plasticity and memory over the circadian cycle. These findings are then used as the basis to discuss the connection between clock dysregulation and cognitive impairments resulting from Alzheimer's disease (AD). In addition, we discuss the conceptually novel idea that in AD, there is a selective disruption of circadian timing within cortical and limbic circuits, and that it is the disruption/desynchronization of these regions from the phase-entraining effects of the SCN that underlies aspects of the early- and mid-stage cognitive deficits in AD. Further, we discuss the prospect that the disruption of circadian timing in AD could produce a self-reinforcing feedback loop, where disruption of timing accelerates AD pathogenesis (e.g., amyloid deposition, oxidative stress and cell death) that in turn leads to a further disruption of the circadian timing system. Lastly, we address potential therapeutic approaches that could be used to strengthen cellular timing networks and, in turn, how these approaches could be used to improve cognitive capacity in Alzheimer's patients.

The role of cell-autonomous circadian oscillation of Cry transcription in circadian rhythm generation

The current model of the mammalian circadian clock describes cell-autonomous and negative feedback-driven circadian oscillation of Cry and Per transcription as the core circadian rhythm generator. However, the actual contribution of this oscillation to circadian rhythm generation remains undefined. Here we perform targeted disruption of cis elements indispensable for cell-autonomous Cry oscillation. Mice lacking overt cell-autonomous Cry oscillation show robust circadian rhythms in locomotor activity. In addition, tissue-autonomous circadian rhythms are robust in the absence of overt Cry oscillation. Unexpectedly, although the absence of overt Cry oscillation leads to severe attenuation of Per oscillation at the cell-autonomous level, circadian rhythms in Per2 accumulation remain robust. As a mechanism to explain this counterintuitive result, Per2 half-life shows cell-autonomous circadian rhythms independent of Cry and Per oscillation. The cell-autonomous circadian clock may therefore remain partially functional even in the absence of overt Cry and Per oscillation because of circadian oscillation in Per2 degradation.