Light-regulated translational control of circadian behavior by eIF4E phosphorylation

A Pilot Urinary Proteome Study Reveals Widespread Influences of Circadian Rhythm Disruption by Sleep Deprivation

It is widely accepted that circadian rhythm disruption caused short- or long-term adverse effects on health. Although many previous studies have focused on exploration of the molecular mechanisms, there is no rapid, convenient, and non-invasive method to reveal the influence on health after circadian rhythm disruption. Here, we performed a high-resolution mass spectrometry-based data-independent acquisition (DIA) quantitative urinary proteomic approach in order to explore whether urine could reveal stress changes to those brought about by circadian rhythm disruption after sleep deprivation. After sleep deprivation, the subjects showed a significant increase in both systolic and diastolic blood pressure compared with routine sleep. More than 2000 proteins were quantified and they contained specific proteins for various organs throughout the body. And a total of 177 significantly up-regulated proteins and 68 significantly down-regulated proteins were obtained after sleep deprivation. These differentially expressed proteins (DEPs) were associated with multiple organs and pathways, which reflected widespread influences of sleep deprivation. Besides, machine learning identified a panel of five DEPs (CD300A, SCAMP3, TXN2, EFEMP1, and MYH11) that can effectively discriminate circadian rhythm disruption. Taken together, our results validate the value of urinary proteome in predicting and diagnosing the changes by circadian rhythm disruption.

Translation machinery: the basis of translational control

Messenger RNA (mRNA) translation consists of initiation, elongation, termination, and ribosome recycling, carried out by the translation machinery, primarily including tRNAs, ribosomes, and translation factors (TrFs). Translational regulators transduce signals of growth and development, as well as biotic and abiotic stresses, to the translation machinery, where global or selective translational control occurs to modulate mRNA translation efficiency (TrE). As the basis of translational control, the translation machinery directly determines the quality and quantity of newly synthesized peptides and, ultimately, the cellular adaption. Thus, regulating the availability of diverse machinery components is reviewed as the central strategy of translational control. We provide classical signaling pathways (e.g., integrated stress responses) and cellular behaviors (e.g., liquid-liquid phase separation) to exemplify this strategy within different physiological contexts, particularly during host-microbe interactions. With new technologies developed, further understanding this strategy will speed up translational medicine and translational agriculture.

Circadian neurogenetics and its implications in neurophysiology, behavior, and chronomedicine

The circadian rhythm affects multiple physiological processes, and disruption of the circadian system can be involved in a range of disease-related pathways. The genetic underpinnings of the circadian rhythm have been well-studied in model organisms. Significant progress has been made in understanding how clock genes affect the physiological functions of the nervous system. In addition, circadian timing is becoming a key factor in improving drug efficacy and reducing drug toxicity. The circadian biology of the target cell determines how the organ responds to the drug at a specific time of day, thus regulating pharmacodynamics. The current review brings together recent advances that have begun to unravel the molecular mechanisms of how the circadian clock affects neurophysiological and behavioral processes associated with human brain diseases. We start with a brief description of how the ubiquitous circadian rhythms are regulated at the genetic, cellular, and neural circuit levels, based on knowledge derived from extensive research on model organisms. We then summarize the latest findings from genetic studies of human brain disorders, focusing on the role of human clock gene variants in these diseases. Lastly, we discuss the impact of common dietary factors and medications on human circadian rhythms and advocate for a broader application of the concept of chronomedicine.

Autistic-like behavior and cerebellar dysfunction in Bmal1 mutant mice ameliorated by mTORC1 inhibition

Although circadian and sleep disorders are frequently associated with autism spectrum disorders (ASD), it remains elusive whether clock gene disruption can lead to autistic-like phenotypes in animals. The essential clock gene Bmal1 has been associated with human sociability and its missense mutations are identified in ASD. Here we report that global Bmal1 deletion led to significant social impairments, excessive stereotyped and repetitive behaviors, as well as motor learning disabilities in mice, all of which resemble core behavioral deficits in ASD. Furthermore, aberrant cell density and immature morphology of dendritic spines were identified in the cerebellar Purkinje cells (PCs) of Bmal1 knockout (KO) mice. Electrophysiological recordings uncovered enhanced excitatory and inhibitory synaptic transmission and reduced firing rates in the PCs of Bmal1 KO mice. Differential expression of ASD- and ataxia-associated genes (Ntng2, Mfrp, Nr4a2, Thbs1, Atxn1, and Atxn3) and dysregulated pathways of translational control, including hyperactivated mammalian target of rapamycin complex 1 (mTORC1) signaling, were identified in the cerebellum of Bmal1 KO mice. Interestingly, the antidiabetic drug metformin reversed mTORC1 hyperactivation and alleviated major behavioral and PC deficits in Bmal1 KO mice. Importantly, conditional Bmal1 deletion only in cerebellar PCs was sufficient to recapitulate autistic-like behavioral and cellular changes akin to those identified in Bmal1 KO mice. Together, these results unveil a previously unidentified role for Bmal1 disruption in cerebellar dysfunction and autistic-like behaviors. Our findings provide experimental evidence supporting a putative role for dysregulation of circadian clock gene expression in the pathogenesis of ASD.

Opto4E-BP, an optogenetic tool for inducible, reversible, and cell type-specific inhibition of translation initiation

The protein kinase mechanistic target of rapamycin complex 1 (mTORC1) is one of the primary triggers for initiating cap-dependent translation. Amongst its functions, mTORC1 phosphorylates eIF4E-binding proteins (4E-BPs), which prevents them from binding to eIF4E and thereby enables translation initiation. mTORC1 signaling is required for multiple forms of protein synthesis-dependent synaptic plasticity and various forms of long-term memory (LTM), including associative threat memory. However, the approaches used thus far to target mTORC1 and its effectors, such as pharmacological inhibitors or genetic knockouts, lack fine spatial and temporal control. The development of a conditional and inducible eIF4E knockdown mouse line partially solved the issue of spatial control, but still lacked optimal temporal control to study memory consolidation. Here, we have designed a novel optogenetic tool (Opto4E-BP) for cell type-specific, light-dependent regulation of eIF4E in the brain. We show that light-activation of Opto4E-BP decreases protein synthesis in HEK cells and primary mouse neurons. In situ , light-activation of Opto4E-BP in excitatory neurons decreased protein synthesis in acute amygdala slices. Finally, light activation of Opto4E-BP in principal excitatory neurons in the lateral amygdala (LA) of mice after training blocked the consolidation of LTM. The development of this novel optogenetic tool to modulate eIF4E-dependent translation with spatiotemporal precision will permit future studies to unravel the complex relationship between protein synthesis and the consolidation of LTM.

eIF4E phosphorylation recruits β-catenin to mRNA cap and promotes Wnt pathway translation in dentate gyrus LTP maintenance

The mRNA cap-binding protein, eukaryotic initiation factor 4E (eIF4E), is crucial for translation and regulated by Ser209 phosphorylation. However, the biochemical and physiological role of eIF4E phosphorylation in translational control of long-term synaptic plasticity is unknown. We demonstrate that phospho-ablated Eif4e^S209A Knockin mice are profoundly impaired in dentate gyrus LTP maintenance in vivo, whereas basal perforant path-evoked transmission and LTP induction are intact. mRNA cap-pulldown assays show that phosphorylation is required for synaptic activity-induced removal of translational repressors from eIF4E, allowing initiation complex formation. Using ribosome profiling, we identified selective, phospho-eIF4E-dependent translation of the Wnt signaling pathway in LTP. Surprisingly, the canonical Wnt effector, β-catenin, was massively recruited to the eIF4E cap complex following LTP induction in wild-type, but not Eif4e^S209A, mice. These results demonstrate a critical role for activity-evoked eIF4E phosphorylation in dentate gyrus LTP maintenance, remodeling of the mRNA cap-binding complex, and specific translation of the Wnt pathway.

d-Glucosamine induces circadian phase delay by promoting BMAL1 degradation through AMPK/mTOR pathway

Circadian rhythms are closely linked to the metabolic network through circadian feedback regulation. The hexosamine biosynthetic pathway (HBP) is a branch of glucose metabolism that affects circadian rhythms through the O-linked N-acetylglucosamine modification (O-GlcNAcylation) of clock proteins. Here, we found out that, among the downstream metabolites regulated by d-glucosamine (GlcN) in HBP salvage pathway, only GlcN is able to induce circadian phase delay both in vitro and in vivo. Mechanistic studies indicated that the phase-shift induced by GlcN is independent of O-GlcNAcylation. Instead, GlcN selectively up-regulates p-AMPK activity, leading to the inhibition of mTOR signaling pathway, and thus down-regulation of p-BMAL1 both in human cell line and mouse tissues. Moreover, GlcN promoted BMAL1 degradation via proteasome pathway. These findings reveal a novel molecular mechanism of GlcN in regulating clock phase and suggest the therapeutic potential of GlcN as new use for an old drug in the future treatment of shift work and circadian misalignment.

Minimal upstream open reading frame of Per2 mediates phase fitness of the circadian clock to day/night physiological body temperature rhythm

Body temperature in homeothermic animals does not remain constant but displays a regular circadian fluctuation within a physiological range (e.g., 35°C-38.5°C in mice), constituting a fundamental systemic signal to harmonize circadian clock-regulated physiology. Here, we find the minimal upstream open reading frame (uORF) encoded by the 5' UTR of the mammalian core clock gene Per2 and reveal its role as a regulatory module for temperature-dependent circadian clock entrainment. A temperature shift within the physiological range does not affect transcription but instead increases translation of Per2 through its minimal uORF. Genetic ablation of the Per2 minimal uORF and inhibition of phosphoinositide-3-kinase, lying upstream of temperature-dependent Per2 protein synthesis, perturb the entrainment of cells to simulated body temperature cycles. At the organismal level, Per2 minimal uORF mutant skin shows delayed wound healing, indicating that uORF-mediated Per2 modulation is crucial for optimal tissue homeostasis. Combined with transcriptional regulation, Per2 minimal uORF-mediated translation may enhance the fitness of circadian physiology.

A circadian clock translational control mechanism targets specific mRNAs to cytoplasmic messenger ribonucleoprotein granules

Phosphorylation of Neurospora crassa eukaryotic initiation factor 2 α (eIF2α), a conserved translation initiation factor, is clock controlled. To determine the impact of rhythmic eIF2α phosphorylation on translation, we performed temporal ribosome profiling and RNA sequencing (RNA-seq) in wild-type (WT), clock mutant Δfrq, eIF2α kinase mutant Δcpc-3, and constitutively active cpc-3c cells. About 14% of mRNAs are rhythmically translated in WT cells, and translation rhythms for ∼30% of these mRNAs, which we named circadian translation-initiation-controlled genes (cTICs), are dependent on the clock and CPC-3. Most cTICs are expressed from arrhythmic mRNAs and contain a P-body (PB) localization motif in their 5' leader sequence. Deletion of SNR-1, a component of cytoplasmic messenger ribonucleoprotein granules (cmRNPgs) that include PBs and stress granules (SGs), and the PB motif on one of the cTIC mRNAs, zip-1, significantly alters zip-1 rhythmic translation. These results reveal that the clock regulates rhythmic translation of specific mRNAs through rhythmic eIF2α activity and cmRNPg metabolism.

The trilateral interactions between mammalian target of rapamycin (mTOR) signaling, the circadian clock, and psychiatric disorders: an emerging model

Circadian (~24 h) rhythms in physiology and behavior are evolutionarily conserved and found in almost all living organisms. The rhythms are endogenously driven by daily oscillatory activities of so-called "clock genes/proteins", which are widely distributed throughout the mammalian brain. Mammalian (mechanistic) target of rapamycin (mTOR) signaling is a fundamental intracellular signal transduction cascade that controls important neuronal processes including neurodevelopment, synaptic plasticity, metabolism, and aging. Dysregulation of the mTOR pathway is associated with psychiatric disorders including autism spectrum disorders (ASD) and mood disorders (MD), in which patients often exhibit disrupted daily physiological rhythms and abnormal circadian gene expression in the brain. Recent work has found that the activities of mTOR signaling are temporally controlled by the circadian clock and exhibit robust circadian oscillations in multiple systems. In the meantime, mTOR signaling regulates fundamental properties of the central and peripheral circadian clocks, including period length, entrainment, and synchronization. Whereas the underlying mechanisms remain to be fully elucidated, increasing clinical and preclinical evidence support significant crosstalk between mTOR signaling, the circadian clock, and psychiatric disorders. Here, we review recent progress in understanding the trilateral interactions and propose an "interaction triangle" model between mTOR signaling, the circadian clock, and psychiatric disorders (focusing on ASD and MD).

Haploinsufficiency of a Circadian Clock Gene Bmal1 (Arntl or Mop3) Causes Brain-Wide mTOR Hyperactivation and Autism-like Behavioral Phenotypes in Mice

Approximately 50-80% of children with autism spectrum disorders (ASDs) exhibit sleep problems, but the contribution of circadian clock dysfunction to the development of ASDs remains largely unknown. The essential clock gene Bmal1 (Arntl or Mop3) has been associated with human sociability, and its missense mutation is found in ASD. Our recent study found that Bmal1-null mice exhibit a variety of autism-like phenotypes. Here, we further investigated whether an incomplete loss of Bmal1 function could cause significant autism-like behavioral changes in mice. Our results demonstrated that heterozygous Bmal1 deletion (Bmal1+/-) reduced the Bmal1 protein levels by ~50-75%. Reduced Bmal1 expression led to decreased levels of clock proteins, including Per1, Per2, Cry 1, and Clock but increased mTOR activities in the brain. Accordingly, Bmal1+/- mice exhibited aberrant ultrasonic vocalizations during maternal separation, deficits in sociability and social novelty, excessive repetitive behaviors, impairments in motor coordination, as well as increased anxiety-like behavior. The novel object recognition memory remained intact. Together, these results demonstrate that haploinsufficiency of Bmal1 can cause autism-like behavioral changes in mice, akin to those identified in Bmal1-null mice. This study provides further experimental evidence supporting a potential role for disrupted clock gene expression in the development of ASD.

Circadian activities of the brain MNK-eIF4E signalling axis contribute to diurnal rhythms of some cognitive functions

Although it is well recognized that the circadian timing system profoundly influences cognitive performance, the underlying molecular mechanisms remain poorly defined. Our previous work has found that the mitogen-activated protein kinase-interacting kinase (MNK)-eukaryotic translation initiation factor 4E (eIF4E) axis, a conserved cellular signalling pathway regulating mRNA translation, modulates the function of the suprachiasmatic nucleus (SCN), the master circadian clock. Here, with the use of a combination of genetic, biochemical and behavioural approaches, we investigated the distribution and temporal regulation of eIF4E phosphorylation in the brain and its role in regulating the diurnal oscillations of some aspects of cognition in mice. We found that activities of the MNK-eIF4E axis, as indicated by the level of eIF4E phosphorylation at Ser209, exhibited significant circadian oscillations in a variety of brain regions, including but not limited to the prefrontal cortex, the hippocampus, the amygdala and the cerebellum. Phosphorylated eIF4E was enriched in neurons but not in astrocytes or microglia. Mice lacking eIF4E phosphorylation (eIF4E^S209A/S209A ) or the MNKs (Mnk^1-/-,2-/- ), the kinases that phosphorylate eIF4E, exhibited impaired diurnal variations of novel object recognition, object location memory, Barnes maze learning and ambulatory activities. Together, these results suggest that circadian activities of the MNK-eIF4E axis contribute to the diurnal rhythms of some cognitive functions, highlighting a role for rhythmic translational control in circadian regulation of cognitive performance.

Rapid-acting antidepressants and the circadian clock

A growing number of epidemiological and experimental studies has established that circadian disruption is strongly associated with psychiatric disorders, including major depressive disorder (MDD). This association is becoming increasingly relevant considering that modern lifestyles, social zeitgebers (time cues) and genetic variants contribute to disrupting circadian rhythms that may lead to psychiatric disorders. Circadian abnormalities associated with MDD include dysregulated rhythms of sleep, temperature, hormonal secretions, and mood which are modulated by the molecular clock. Rapid-acting antidepressants such as subanesthetic ketamine and sleep deprivation therapy can improve symptoms within 24 h in a subset of depressed patients, in striking contrast to conventional treatments, which generally require weeks for a full clinical response. Importantly, animal data show that sleep deprivation and ketamine have overlapping effects on clock gene expression. Furthermore, emerging data implicate the circadian system as a critical component involved in rapid antidepressant responses via several intracellular signaling pathways such as GSK3β, mTOR, MAPK, and NOTCH to initiate synaptic plasticity. Future research on the relationship between depression and the circadian clock may contribute to the development of novel therapeutic strategies for depression-like symptoms. In this review we summarize recent evidence describing: (1) how the circadian clock is implicated in depression, (2) how clock genes may contribute to fast-acting antidepressants, and (3) the mechanistic links between the clock genes driving circadian rhythms and neuroplasticity.

Modelling of plant circadian clock for characterizing hypocotyl growth under different light quality conditions

To meet the ever-increasing global food demand, the food production rate needs to be increased significantly in the near future. Speed breeding is considered as a promising agricultural technology solution to achieve the zero-hunger vision as specified in the United Nations Sustainable Development Goal 2. In speed breeding, the photoperiod of the artificial light has been manipulated to enhance crop productivity. In particular, regulating the photoperiod of different light qualities rather than solely white light can further improve speed breading. However, identifying the optimal light quality and the associated photoperiod simultaneously remains a challenging open problem due to complex interactions between multiple photoreceptors and proteins controlling plant growth. To tackle this, we develop a first comprehensive model describing the profound effect of multiple light qualities with different photoperiods on plant growth (i.e. hypocotyl growth). The model predicts that hypocotyls elongated more under red light compared to both red and blue light. Drawing similar findings from previous related studies, we propose that this might result from the competitive binding of red and blue light receptors, primarily Phytochrome B (phyB) and Cryptochrome 1 (cry1) for the core photomorphogenic regulator, CONSTITUTIVE PHOTOMORPHOGENIC 1 (COP1). This prediction is validated through an experimental study on Arabidopsis thaliana. Our work proposes a potential molecular mechanism underlying plant growth under different light qualities and ultimately suggests an optimal breeding protocol that takes into account light quality.

Minding the message: tactics controlling RNA decay, modification, and translation in virus-infected cells

With their categorical requirement for host ribosomes to translate mRNA, viruses provide a wealth of genetically tractable models to investigate how gene expression is remodeled post-transcriptionally by infection-triggered biological stress. By co-opting and subverting cellular pathways that control mRNA decay, modification, and translation, the global landscape of post-transcriptional processes is swiftly reshaped by virus-encoded factors. Concurrent host cell-intrinsic countermeasures likewise conscript post-transcriptional strategies to mobilize critical innate immune defenses. Here we review strategies and mechanisms that control mRNA decay, modification, and translation in animal virus-infected cells. Besides settling infection outcomes, post-transcriptional gene regulation in virus-infected cells epitomizes fundamental physiological stress responses in health and disease.

Role of Sleep Restriction in Daily Rhythms of Expression of Hypothalamic Core Clock Genes in Mice

Lack of sleep time is a menace to modern people, and it leads to chronic diseases and mental illnesses. Circadian processes control sleep, but little is known about how sleep affects the circadian system. Therefore, we performed a 28-day sleep restriction (SR) treatment in mice. Sleep restriction disrupted the clock genes’ circadian rhythm. The circadian rhythms of the Cry1 and Per1/2/3 genes disappeared. The acrophase of the clock genes (Bmal1, Clock, Rev-erbα, and Rorβ) that still had a circadian rhythm was advanced, while the acrophase of negative clock gene Cry2 was delayed. Clock genes’ upstream signals ERK and EIFs also had circadian rhythm disorders. Accompanied by changes in the central oscillator, the plasma output signal (melatonin, corticosterone, IL-6, and TNF-α) had an advanced acrophase. While the melatonin mesor was decreased, the corticosterone, IL-6, and TNF-α mesor was increased. Our results indicated that chronic sleep loss could disrupt the circadian rhythm of the central clock through ERK and EIFs and affect the output signal downstream of the core biological clock.

The sleep-wake distribution contributes to the peripheral rhythms in PERIOD-2

In the mouse, Period-2 (Per2) expression in tissues peripheral to the suprachiasmatic nuclei (SCN) increases during sleep deprivation and at times of the day when animals are predominantly awake spontaneously, suggesting that the circadian sleep-wake distribution directly contributes to the daily rhythms in Per2. We found support for this hypothesis by recording sleep-wake state alongside PER2 bioluminescence in freely behaving mice, demonstrating that PER2 bioluminescence increases during spontaneous waking and decreases during sleep. The temporary reinstatement of PER2-bioluminescence rhythmicity in behaviorally arrhythmic SCN-lesioned mice submitted to daily recurring sleep deprivations substantiates our hypothesis. Mathematical modeling revealed that PER2 dynamics can be described by a damped harmonic oscillator driven by two forces: a sleep-wake-dependent force and an SCN-independent circadian force. Our work underscores the notion that in peripheral tissues the clock gene circuitry integrates sleep-wake information and could thereby contribute to behavioral adaptability to respond to homeostatic requirements.

Circadian rhythms are daily cycles in behavior and physiology which repeat approximately every 24 hours. The master regulator of these rhythms is located in a small part of the brain called the supra-chiasmatic nucleus. This brain structure regulates the timing of sleep and wakefulness and is also thought to control the daily rhythms of cells throughout the body on a molecular level. It does this by synchronizing the activity of a set of genes called clock genes.

Under normal conditions, the levels of proteins coded for by clock genes change throughout the day following a rhythm that matches sleep-wake patterns. However, keeping animals and humans awake at their preferred sleeping times affects the protein levels of clock genes in many tissues of the body. This suggests that, in addition to the supra-chiasmatic nucleus, sleep-wake cycles may also influence clock-gene rhythms throughout the body.

To test this theory, Hoekstra, Jan et al. measured the levels of PERIOD-2, a protein coded for by the clock gene Period-2, while tracking sleep-wake states in mice. They did this by imaging a bioluminescent version of the PERIOD-2 protein in the brain and the kidneys, at the same time as they recorded the brain activity, movement and muscle response of animals. Results showed that PERIOD-2 increased on waking and decreased when mice fell asleep. Additionally, in mice lacking a circadian rhythm in sleep-wake behavior – whose changes in PERIOD-2 levels with respect to time were greatly reduced – imposing a regular sleep-wake cycle restored normal PERIOD-2 rhythmicity.

Next, Hoekstra, Jan et al. developed a mathematical model to understand how sleep-wake cycles together with circadian rhythms affect clock-gene activity in the brain and kidneys. Computer simulations suggested that sleep-wake cycles and circadian factors act as forces of comparable strength driving clock-gene dynamics. Both need to act in concert to keep clock-genes rhythmic. The model also predicted the large and immediate effects of sleep deprivation on PERIOD-2 levels, giving further credence to the idea that waking accelerated clock-gene rhythms while sleeping slowed them down. Modelling also suggested that having regular clock-gene rhythms protects against sleep disturbances.

In summary, this work shows how sleep patterns contribute to the daily rhythms in clock genes in the brain and body. The findings support the idea that well-timed sleep-wake schedules could help people to adjust to new time zones. It might also be useful to inform other strategies to reduce the health impacts of shift work.

Disruption of Circadian Rhythms by Ambient Light during Neurodevelopment Leads to Autistic-like Molecular and Behavioral Alterations in Adult Mice

Although circadian rhythms are thought to be essential for maintaining body health, the effects of chronic circadian disruption during neurodevelopment remain elusive. Here, using the "Short Day" (SD) mouse model, in which an 8 h/8 h light/dark (LD) cycle was applied from embryonic day 1 to postnatal day 42, we investigated the molecular and behavioral changes after circadian disruption in mice. Adult SD mice fully entrained to the 8 h/8 h LD cycle, and the circadian oscillations of the clock proteins, PERIOD1 and PERIOD2, were disrupted in the suprachiasmatic nucleus and the hippocampus of these mice. By RNA-seq widespread changes were identified in the hippocampal transcriptome, which are functionally associated with neurodevelopment, translational control, and autism. By western blotting and immunostaining hyperactivation of the mTOR and MAPK signaling pathways and enhanced global protein synthesis were found in the hippocampi of SD mice. Electrophysiological recording uncovered enhanced excitatory, but attenuated inhibitory, synaptic transmission in the hippocampal CA1 pyramidal neurons. These functional changes at synapses were corroborated by the immature morphology of the dendritic spines in these neurons. Lastly, autistic-like animal behavioral changes, including impaired social interaction and communication, increased repetitive behaviors, and impaired novel object recognition and location memory, were found in SD mice. Together, these results demonstrate molecular, cellular, and behavioral changes in SD mice, all of which resemble autistic-like phenotypes caused by circadian rhythm disruption. The findings highlight a critical role for circadian rhythms in neurodevelopment.

Circadian Clock Control of Translation Initiation Factor eIF2α Activity Requires eIF2γ-Dependent Recruitment of Rhythmic PPP-1 Phosphatase in Neurospora crassa

The circadian clock controls the phosphorylation and activity of eukaryotic translation initiation factor 2α (eIF2α). In Neurospora crassa, the clock drives a daytime peak in the activity of the eIF2α kinase CPC-3, the homolog of yeast and mammalian GCN2 kinase. This leads to increased levels of phosphorylated eIF2α (P-eIF2α) and reduced mRNA translation initiation during the day. We hypothesized that rhythmic eIF2α activity also requires dephosphorylation of P-eIF2α at night by phosphatases. In support of this hypothesis, we show that mutation of N. crassa PPP-1, a homolog of the yeast eIF2α phosphatase GLC7, leads to high and arrhythmic P-eIF2α levels, while maintaining core circadian oscillator function. PPP-1 levels are clock-controlled, peaking in the early evening, and rhythmic PPP-1 levels are necessary for rhythmic P-eIF2α accumulation. Deletion of the N terminus of N. crassa eIF2γ, the region necessary for eIF2γ interaction with GLC7 in yeast, led to high and arrhythmic P-eIF2α levels. These data supported that N. crassa eIF2γ functions to recruit PPP-1 to dephosphorylate eIF2α at night. Thus, in addition to the activity of CPC-3 kinase, circadian clock regulation of eIF2α activity requires dephosphorylation by PPP-1 phosphatase at night. These data show how the circadian clock controls the activity a central regulator of translation, critical for cellular metabolism and growth control, through the temporal coordination of phosphorylation and dephosphorylation events.

Translating around the clock: Multi-level regulation of post-transcriptional processes by the circadian clock

The endogenous circadian clock functions to maintain optimal physiological health through the tissue specific coordination of gene expression and synchronization between tissues of metabolic processes throughout the 24 hour day. Individuals face numerous challenges to circadian function on a daily basis resulting in significant incidences of circadian disorders in the United States and worldwide. Dysfunction of the circadian clock has been implicated in numerous diseases including cancer, diabetes, obesity, cardiovascular and hepatic abnormalities, mood disorders and neurodegenerative diseases. The circadian clock regulates molecular, metabolic and physiological processes through rhythmic gene expression via transcriptional and post-transcriptional processes. Mounting evidence indicates that post-transcriptional regulation by the circadian clock plays a crucial role in maintaining tissue specific biological rhythms. Circadian regulation affecting RNA stability and localization through RNA processing, mRNA degradation, and RNA availability for translation can result in rhythmic protein synthesis, even when the mRNA transcripts themselves do not exhibit rhythms in abundance. The circadian clock also targets the initiation and elongation steps of translation through multiple pathways. In this review, the influence of the circadian clock across the levels of post-transcriptional, translation, and post-translational modifications are examined using examples from humans to cyanobacteria demonstrating the phylogenetic conservation of circadian regulation. Lastly, we briefly discuss chronotherapies and pharmacological treatments that target circadian function. Understanding the complexity and levels through which the circadian clock regulates molecular and physiological processes is important for future advancement of therapeutic outcomes.

Biological Timing and Neurodevelopmental Disorders: A Role for Circadian Dysfunction in Autism Spectrum Disorders

Autism spectrum disorders (ASDs) are a spectrum of neurodevelopmental disorders characterized by impaired social interaction and communication, as well as stereotyped and repetitive behaviors. ASDs affect nearly 2% of the United States child population and the worldwide prevalence has dramatically increased in recent years. The etiology is not clear but ASD is thought to be caused by a combination of intrinsic and extrinsic factors. Circadian rhythms are the ∼24 h rhythms driven by the endogenous biological clock, and they are found in a variety of physiological processes. Growing evidence from basic and clinical studies suggest that the dysfunction of the circadian timing system may be associated with ASD and its pathogenesis. Here we review the findings that link circadian dysfunctions to ASD in both experimental and clinical studies. We first introduce the organization of the circadian system and ASD. Next, we review physiological indicators of circadian rhythms that are found disrupted in ASD individuals, including sleep-wake cycles, melatonin, cortisol, and serotonin. Finally, we review evidence in epidemiology, human genetics, and biochemistry that indicates underlying associations between circadian regulation and the pathogenesis of ASD. In conclusion, we propose that understanding the functional importance of the circadian clock in normal and aberrant neurodevelopmental processes may provide a novel perspective to tackle ASD, and clinical treatments for ASD individuals should comprise an integrative approach considering the dynamics of daily rhythms in physical, mental, and social processes.

The role of cap-dependent translation in aged-related changes in neuroimmunity and affective behaviors

Translation regulation in the context of aged-associated inflammation and behavioral impairments is not well characterized. Aged individuals experience lower life quality due to behavioral impairments. In this study, we used young and aged transgenic mice that are unable to activate the cap-binding protein, eukaryotic translation initiation factor 4E (eIF4E) to examine the role of protein translation control in aging, memory, depression, and anxiety. To determine how products of cap-dependent translation play a permissive role in aged-associated inflammation, we assessed levels of pro-inflammatory cytokines in various brain regions involved in the above-mentioned behaviors. We found that functional eIF4E is not necessary for age-related deficits in spatial and short-term memory but is important for depressive and anxiety-like behavior and this is correlated with pro-inflammatory cytokines in discrete brain regions. Thus, we have begun to elucidate a role for eIF4E phosphorylation in the context of aged-related behavioral impairments and chronic low-grade inflammation that may help identify novel immune modulators for therapeutic targets and decrease the burden of self-care among the geriatric population.

New insights into non-transcriptional regulation of mammalian core clock proteins

Mammalian circadian rhythms drive ∼24 h periodicity in a wide range of cellular processes, temporally coordinating physiology and behaviour within an organism, and synchronising this with the external day-night cycle. The canonical model for this timekeeping consists of a delayed negative-feedback loop, containing transcriptional activator complex CLOCK-BMAL1 (BMAL1 is also known as ARNTL) and repressors period 1, 2 and 3 (PER1, PER2 and PER3) and cryptochrome 1 and 2 (CRY1 and CRY2), along with a number of accessory factors. Although the broad strokes of this system are defined, the exact molecular mechanisms by which these proteins generate a self-sustained rhythm with such periodicity and fidelity remains a topic of much research. Recent studies have identified prominent roles for a number of crucial post-transcriptional, translational and, particularly, post-translational events within the mammalian circadian oscillator, providing an increasingly complex understanding of the activities and interactions of the core clock proteins. In this Review, we highlight such contemporary work on non-transcriptional events and set it within our current understanding of cellular circadian timekeeping.

Phosphorylation of the mRNA cap-binding protein eIF4E and cancer

Dysregulated protein synthesis is frequently involved in oncogenesis and cancer progression. Translation initiation is thought to be the rate-limiting step in protein synthesis, and the mRNA 5' cap-binding protein eukaryotic translation initiation factor 4E (eIF4E) is a pivotal factor that initiates translation. The activities of eIF4E are regulated at multiple levels, one of which is through its phosphorylation at Serine 209 by the mitogen-activated protein kinase-interacting kinases (MNKs, including MNK1 and MNK2). Benefiting from novel mouse genetic tools and pharmacological MNK inhibitors, our understanding of a role for eIF4E phosphorylation in tumor biology and cancer therapy has greatly evolved in recent years. Importantly, recent studies have found that the level of eIF4E phosphorylation is frequently upregulated in a wide variety of human cancer types, and phosphorylation of eIF4E drives a number of important processes in cancer biology, including cell transformation, proliferation, apoptosis, metastasis and angiogenesis. The MNK-eIF4E axis is being assessed as a therapeutic target either alone or in combination with other therapies in different cancer models. As novel MNK inhibitors are being developed, experimental studies bring new hope to cure human cancers that are not responsive to traditional therapies. Herein we review recent progress on our understanding of a mechanistic role for phosphorylation of eIF4E in cancer biology and therapy.

Physiological and cognitive consequences of a daily 26 h photoperiod in a primate: exploring the underlying mechanisms of the circadian resonance theory

The biological clock expresses circadian rhythms, whose endogenous period (tau) is close to 24 h. Daily resetting of the circadian clock to the 24 h natural photoperiod might induce marginal costs that would accumulate over time and forward affect fitness. It was proposed as the circadian resonance theory. For the first time, we aimed to evaluate these physiological and cognitive costs that would partially explain the mechanisms of the circadian resonance hypothesis. We evaluated the potential costs of imposing a 26 h photoperiodic regimen compared to the classical 24 h entrainment measuring several physiological and cognitive parameters (body temperature, energetic expenditure, oxidative stress, cognitive performances) in males of a non-human primate (Microcebus murinus), a nocturnal species whose endogenous period is about 23.5 h. We found significant higher resting body temperature and energy expenditure and lower cognitive performances when the photoperiodic cycle length was 26 h. Together these results suggest that a great deviation of external cycles from tau leads to daily greater energetic expenditure, and lower cognitive capacities. To our knowledge, this study is the first to highlight potential mechanisms of circadian resonance theory.

Disabling MNK protein kinases promotes oxidative metabolism and protects against diet-induced obesity

Objectives

Diet-driven obesity is increasingly widespread. Its consequences pose major challenges to human health and health care systems. There are MAP kinase-interacting kinases (MNKs) in mice, MNK1 and MNK2. Studies have demonstrated that mice lacking either MNK1 or MNK2 were partially protected against high-fat diet (HFD)-induced weight gain and insulin resistance. The aims of this study were to evaluate the phenotype of mice lacking both MNKs when given an HFD, to assess whether pharmacological inhibition of MNK function also protects against diet-induced obesity (DIO) and its consequences and to probe the mechanisms underlying such protection.

Methods

Male wild-type (WT) C57Bl6 mice or mice lacking both MNK1 and MNK2 (double knockout, DKO) were fed an HFD or control diet (CD) for up to 16 weeks.

In a separate study, WT mice were also given an HFD for 6 weeks, after which half were treated with the recently-developed MNK inhibitor ETC-206 daily for 10 more weeks while continuing an HFD. Metabolites and other parameters were measured, and the expression of selected mRNAs and proteins was assessed.

Results

MNK-DKO mice were almost completely protected from HFD-induced obesity. Higher energy expenditure (EE) in MNK-DKO mice was observed, which probably reflects the changes in a number of genes or proteins linked to lipolysis, mitochondrial function/biogenesis, oxidative metabolism, and/or ATP consumption. The MNK inhibitor ETC-206 also prevented HFD-induced weight gain, confirming that the activity of the MNKs facilitates weight gain due to excessive caloric consumption.

Conclusions

Disabling MNKs in mice, either genetically or pharmacologically, strongly prevents weight gain on a calorie-rich diet. This finding likely results from increased energy utilisation, involving greater ATP consumption, mitochondrial oxidative metabolism, and other processes.

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Knockout of MNK1/MNK2 protects mice against diet-induced obesity.

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MNK1/2 DKO mice have higher energy expenditure.

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MNK1/2 DKO increases the expression of genes of lipid and mitochondrial metabolism.

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Pharmacological inhibition of MNKs has similar effects.

Genome-wide circadian regulation: A unique system for computational biology

Circadian rhythms are 24-hour oscillations affecting an organism at multiple levels from gene expression all the way to tissues and organs. They have been observed in organisms across the kingdom of life, spanning from cyanobacteria to humans. In mammals, the master circadian pacemaker is located in the hypothalamic suprachiasmatic nuclei (SCN) in the brain where it synchronizes the peripheral oscillators that exist in other tissues. This system regulates the circadian activity of a large part of the transcriptome and recent findings indicate that almost every cell in the body has this clock at the molecular level. In this review, we briefly summarize the different factors that can influence the circadian transcriptome, including light, temperature, and food intake. We then summarize recently identified general principles governing genome-scale circadian regulation, as well as future lines of research. Genome-scale circadian activity represents a fascinating study model for computational biology. For this purpose, systems biology methods are promising exploratory tools to decode the global regulatory principles of circadian regulation.

Inefficient thermogenic mitochondrial respiration due to futile proton leak in a mouse model of fragile X syndrome

Fragile X syndrome (FXS) is the leading known inherited intellectual disability and the most common genetic cause of autism. The full mutation results in transcriptional silencing of the Fmr1 gene and loss of fragile X mental retardation protein (FMRP) expression. Defects in neuroenergetic capacity are known to cause a variety of neurodevelopmental disorders. Thus, we explored the integrity of forebrain mitochondria in Fmr1 knockout mice during the peak of synaptogenesis. We found inefficient thermogenic respiration due to futile proton leak in Fmr1 KO mitochondria caused by coenzyme Q (CoQ) deficiency and an open cyclosporine-sensitive channel. Repletion of mitochondrial CoQ within the Fmr1 KO forebrain closed the channel, blocked the pathological proton leak, restored rates of protein synthesis during synaptogenesis, and normalized the key phenotypic features later in life. The findings demonstrate that FMRP deficiency results in inefficient oxidative phosphorylation during the neurodevelopment and suggest that dysfunctional mitochondria may contribute to the FXS phenotype.

Suprachiasmatic function in a circadian period mutant: Duper alters light-induced activation of vasoactive intestinal peptide cells and PERIOD1 immunostaining

Mammalian circadian rhythms are entrained by photic stimuli that are relayed by retinal projections to the core of the suprachiasmatic nucleus (SCN). Neuronal activation, as demonstrated by expression of the immediate early gene c-fos, leads to transcription of the core clock gene per1. The duper mutation in hamsters shortens circadian period and amplifies light-induced phase shifts. We performed two experiments to compare the number of c-FOS immunoreactive (ir) and PER1-ir cells, and the intensity of staining, in the SCN of wild-type (WT) and duper hamsters at various intervals after presentation of a 15-min light pulse in the early subjective night. Light-induced c-FOS-ir within 1 hr in the dorsocaudal SCN of duper, but not WT hamsters. In cells that express vasoactive intestinal peptide (VIP), which plays a critical role in synchronization of SCN cellular oscillators, light-induced c-FOS-ir was greater in duper than WT hamsters. After the light pulse, PER1-ir cells were found in more medial portions of the SCN than FOS-ir, and appeared with a longer latency and over a longer time course, in VIP cells of duper than wild-type hamsters. Our results indicate that the duper allele alters SCN function in ways that may contribute to changes in free running period and phase resetting.

Molecular Interactions between Pathogens and the Circadian Clock

The daily periodicity of the Earth's rotation around the Sun, referred to as circadian (Latin "circa" = about, and "diem" = day), is also mirrored in the behavior and metabolism of living beings. The discovery that dedicated cellular genes control various aspects of this periodicity has led to studies of the molecular mechanism of the circadian response at the cellular level. It is now established that the circadian genes impact on a large network of hormonal, metabolic, and immunological pathways, affecting multiple aspects of biology. Recent studies have extended the role of the circadian system to the regulation of infection, host-pathogen interaction, and the resultant disease outcome. This critical review summarizes our current knowledge of circadian-pathogen interaction at both systemic and cellular levels, but with emphasis on the molecular aspects of the regulation. Wherever applicable, the potential of a direct interaction between circadian factors and pathogenic macromolecules is also explored. Finally, this review offers new directions and guidelines for future research in this area, which should facilitate progress.

The eIF2α Kinase GCN2 Modulates Period and Rhythmicity of the Circadian Clock by Translational Control of Atf4

The integrated stress response (ISR) is activated in response to diverse stress stimuli to maintain homeostasis in neurons. Central to this process is the phosphorylation of eukaryotic translation initiation factor 2 alpha (eIF2α). Here, we report a critical role for ISR in regulating the mammalian circadian clock. The eIF2α kinase GCN2 rhythmically phosphorylates eIF2α in the suprachiasmatic circadian clock. Increased eIF2α phosphorylation shortens the circadian period in both fibroblasts and mice, whereas reduced eIF2α phosphorylation lengthens the circadian period and impairs circadian rhythmicity in animals. Mechanistically, phosphorylation of eIF2α promotes mRNA translation of Atf4. ATF4 binding motifs are identified in multiple clock genes, including Per2, Per3, Cry1, Cry2, and Clock. ATF4 binds to the TTGCAGCA motif in the Per2 promoter and activates its transcription. Together, these results demonstrate a significant role for ISR in circadian physiology and provide a potential link between dysregulated ISR and circadian dysfunction in brain diseases.

Systems approach reveals photosensitivity and PER2 level as determinants of clock-modulator efficacy

In mammals, the master circadian clock synchronizes daily rhythms of physiology and behavior with the day-night cycle. Failure of synchrony, which increases the risk for numerous chronic diseases, can be treated by phase adjustment of the circadian clock pharmacologically, for example, with melatonin, or a CK1δ/ε inhibitor. Here, using in silico experiments with a systems pharmacology model describing molecular interactions, and pharmacokinetic and behavioral experiments in cynomolgus monkeys, we find that the circadian phase delay caused by CK1δ/ε inhibition is more strongly attenuated by light in diurnal monkeys and humans than in nocturnal mice, which are common preclinical models. Furthermore, the effect of CK1δ/ε inhibition strongly depends on endogenous PER2 protein levels, which differs depending on both the molecular cause of the circadian disruption and the patient's lighting environment. To circumvent such large interindividual variations, we developed an adaptive chronotherapeutics to identify precise dosing regimens that could restore normal circadian phase under different conditions. Our results reveal the importance of photosensitivity in the clinical efficacy of clock-modulating drugs, and enable precision medicine for circadian disruption.

Phosphorylation and Signal Transduction Pathways in Translational Control

Protein synthesis, including the translation of specific messenger RNAs (mRNAs), is regulated by extracellular stimuli such as hormones and by the levels of certain nutrients within cells. This control involves several well-understood signaling pathways and protein kinases, which regulate the phosphorylation of proteins that control the translational machinery. These pathways include the mechanistic target of rapamycin complex 1 (mTORC1), its downstream effectors, and the mitogen-activated protein (MAP) kinase (extracellular ligand-regulated kinase [ERK]) signaling pathway. This review describes the regulatory mechanisms that control translation initiation and elongation factors, in particular the effects of phosphorylation on their interactions or activities. It also discusses current knowledge concerning the impact of these control systems on the translation of specific mRNAs or subsets of mRNAs, both in physiological processes and in diseases such as cancer.

mTOR Senses Intracellular pH through Lysosome Dispersion from RHEB

Acidity, generated in hypoxia or hypermetabolic states, perturbs homeostasis and is a feature of solid tumors. That acid peripherally disperses lysosomes is a three-decade-old observation, yet one little understood or appreciated. However, recent work has recognized the inhibitory impact this spatial redistribution has on mechanistic target of rapamycin complex 1 (mTORC1), a key regulator of metabolism. This finding argues for a paradigm shift in localization of mTORC1 activator Ras homolog enriched in brain (RHEB), a conclusion several others have now independently reached. Thus, mTORC1, known to sense amino acids, mitogens, and energy to restrict biosynthesis to times of adequate resources, also senses pH and, via dampened mTOR-governed synthesis of clock proteins, regulates the circadian clock to achieve concerted responses to metabolic stress. While this may allow cancer to endure metabolic deprivation, immune cell mTOR signaling likewise exhibits pH sensitivity, suggesting that suppression of antitumor immune function by solid tumor acidity may additionally fuel cancers, an obstacle potentially reversible through therapeutic pH manipulation.

The Role of the Eukaryotic Translation Initiation Factor 4E (eIF4E) in Neuropsychiatric Disorders

Protein synthesis in eukaryotic cells is a complex, multi-step and tightly regulated process. Translation initiation, the rate limiting step in protein synthesis, is dependent on the activity of eukaryotic translation Initiation Factor 4E (eIF4E). eIF4E is the cap-binding protein which, in synergy with proteins such as the helicase eIF4A and the scaffolding protein eIF4G, binds to mRNA, allowing the recruitment of ribosomes and translation initiation. The function of eIF4E is tightly regulated in cells under normal physiological conditions and can be controlled by post-translational modifications, such as phosphorylation, and by the binding of inhibitory proteins, including eIF4E binding proteins (4E-BPs) and CYFIP1. Recent studies have highlighted the importance of eIF4E in normal or aberrant function of the nervous system. In this mini-review, we will highlight the role of eIF4E function and regulation in the pathophysiology of neurodevelopmental and neuropsychiatric disorders.

Time-restricted feeding of a high-fat diet in male C57BL/6 mice reduces adiposity but does not protect against increased systemic inflammation

Time-restricted feeding (TRF) limits the duration of food availability without altering diet composition and can combat obesity in humans and mice. For this study we evaluated the effect of timing of food access during a TRF protocol on weight gain, adiposity, and inflammation. Young male C57BL/6 mice were placed on a high-fat (HF) diet (45% fat) for 8 weeks. Food access was unrestricted (HF) or restricted to 6 h per day, either for the first half (HF-early) or the second half (HF-late) of the active phase to resemble a window of time for food consumption early or late in the day in a human population. Weight, obesity-associated parameters, and inflammation were measured. TRF reduced weight gain over the 8-week period in mice consuming the same high-fat diet. Consistent with decreased weight gain in the TRF groups, body fat percentage, liver triglycerides, and plasma leptin and cholesterol levels were reduced. Adipose tissue inflammation, measured by CD11b+F4/80+ macrophage infiltration, was reduced in both TRF groups, but systemic tumor necrosis factor-α was increased in all groups consuming the high-fat diet. The HF-late group gained more weight than the HF-early group and had increased insulin resistance, while the HF-early group was protected. Therefore, a TRF protocol is beneficial for weight management when a high-fat diet is consumed, with food consumption earlier in the day showing greater health benefits. However, increased inflammatory markers in the TRF groups suggest that diet components can still increase inflammation even in the absence of overt obesity.

mTOR Signaling, Translational Control, and the Circadian Clock

Almost all cellular processes are regulated by the approximately 24 h rhythms that are endogenously driven by the circadian clock. mRNA translation, as the most energy consuming step in gene expression, is temporally controlled by circadian rhythms. Recent research has uncovered key mechanisms of translational control that are orchestrated by circadian rhythmicity and in turn feed back to the clock machinery to maintain robustness and accuracy of circadian timekeeping. Here I review recent progress in our understanding of translation control mechanisms in the circadian clock, focusing on a role for the mammalian/mechanistic target of rapamycin (mTOR) signaling pathway in modulating entrainment, synchronization and autonomous oscillation of circadian clocks. I also discuss the relevance of circadian mTOR functions in disease.

Translational control of depression-like behavior via phosphorylation of eukaryotic translation initiation factor 4E

Translation of mRNA into protein has a fundamental role in neurodevelopment, plasticity, and memory formation; however, its contribution in the pathophysiology of depressive disorders is not fully understood. We investigated the involvement of MNK1/2 (MAPK-interacting serine/threonine-protein kinase 1 and 2) and their target, eIF4E (eukaryotic initiation factor 4E), in depression-like behavior in mice. Mice carrying a mutation in eIF4E for the MNK1/2 phosphorylation site (Ser209Ala, Eif4e ki/ki), the Mnk1/2 double knockout mice (Mnk1/2^-/-), or mice treated with the MNK1/2 inhibitor, cercosporamide, displayed anxiety- and depression-like behaviors, impaired serotonin-induced excitatory synaptic activity in the prefrontal cortex, and diminished firing of the dorsal raphe neurons. In Eif4e ki/ki mice, brain IκBα, was decreased, while the NF-κB target, TNFα was elevated. TNFα inhibition in Eif4e ki/ki mice rescued, whereas TNFα administration to wild-type mice mimicked the depression-like behaviors and 5-HT synaptic deficits. We conclude that eIF4E phosphorylation modulates depression-like behavior through regulation of inflammatory responses.

Circadian Posttranscriptional Regulatory Mechanisms in Mammals

The circadian clock drives rhythms in the levels of thousands of proteins in the mammalian cell, arising in part from rhythmic transcriptional regulation of the genes that encode them. However, recent evidence has shown that posttranscriptional processes also play a major role in generating the rhythmic protein makeup and ultimately the rhythmic physiology of the cell. Regulation of steps throughout the life of the messenger RNA (mRNA), ranging from initial mRNA processing and export from the nucleus to extensive control of translation and degradation in the cytosol have been shown to be important for producing the final rhythms in protein levels critical for proper circadian rhythmicity. These findings will be reviewed here.

Characterization of basal gene expression trends over a diurnal cycle in Xiphophorus maculatus skin, brain and liver

Evolutionarily conserved diurnal circadian mechanisms maintain oscillating patterns of gene expression based on the day-night cycle. Xiphophorus fish have been used to evaluate transcriptional responses after exposure to various light sources and it was determined that each source incites distinct genetic responses in skin tissue. However, basal expression levels of genes that show oscillating expression patterns in day-night cycle, may affect the outcomes of such experiments, since basal gene expression levels at each point in the circadian path may influence the profile of identified light responsive genes. Lack of knowledge regarding diurnal fluctuations in basal gene expression patterns may confound the understanding of genetic responses to external stimuli (e.g., light) since the dynamic nature of gene expression implies animals subjected to stimuli at different times may be at very different stages within the continuum of genetic homeostasis. We assessed basal gene expression changes over a 24-hour period in 200 select Xiphophorus gene targets known to transcriptionally respond to various types of light exposure. We identified 22 genes in skin, 36 genes in brain and 28 genes in liver that exhibit basal oscillation of expression patterns. These genes, including known circadian regulators, produced the expected expression patterns over a 24-hour cycle when compared to circadian regulatory genes identified in other species, especially human and other vertebrate animal models. Our results suggest the regulatory network governing diurnal oscillating gene expression is similar between Xiphophorus and other vertebrates for the three Xiphophorus organs tested. In addition, we were able to categorize light responsive gene sets in Xiphophorus that do, and do not, exhibit circadian based oscillating expression patterns.

mTOR signaling regulates central and peripheral circadian clock function

The circadian clock coordinates physiology and metabolism. mTOR (mammalian/mechanistic target of rapamycin) is a major intracellular sensor that integrates nutrient and energy status to regulate protein synthesis, metabolism, and cell growth. Previous studies have identified a key role for mTOR in regulating photic entrainment and synchrony of the central circadian clock in the suprachiasmatic nucleus (SCN). Given that mTOR activities exhibit robust circadian oscillations in a variety of tissues and cells including the SCN, here we continued to investigate the role of mTOR in orchestrating autonomous clock functions in central and peripheral circadian oscillators. Using a combination of genetic and pharmacological approaches we show that mTOR regulates intrinsic clock properties including period and amplitude. In peripheral clock models of hepatocytes and adipocytes, mTOR inhibition lengthens period and dampens amplitude, whereas mTOR activation shortens period and augments amplitude. Constitutive activation of mTOR in Tsc2-/-fibroblasts elevates levels of core clock proteins, including CRY1, BMAL1 and CLOCK. Serum stimulation induces CRY1 upregulation in fibroblasts in an mTOR-dependent but Bmal1- and Period-independent manner. Consistent with results from cellular clock models, mTOR perturbation also regulates period and amplitude in the ex vivo SCN and liver clocks. Further, mTOR heterozygous mice show lengthened circadian period of locomotor activity in both constant darkness and constant light. Together, these results support a significant role for mTOR in circadian timekeeping and in linking metabolic states to circadian clock functions.

mTOR signaling in VIP neurons regulates circadian clock synchrony and olfaction

The mammalian/mechanistic target of rapamycin (mTOR) kinase resides at the crux of an intracellular signaling network that controls fundamental biological processes. Dysregulation of mTOR signaling is linked to neurological and psychiatric diseases. However, the physiological functions of mTOR signaling in the adult brain are not fully understood. In the current study, we discovered that mTOR in vasoactive intestinal peptide (VIP) neurons plays a key role in regulating neurophysiology in the brain circadian clock and the olfactory system. The conditional mTOR knockout mouse will be a useful model for future investigations of mTOR and/or VIP.

Mammalian/mechanistic target of rapamycin (mTOR) signaling controls cell growth, proliferation, and metabolism in dividing cells. Less is known regarding its function in postmitotic neurons in the adult brain. Here we created a conditional mTOR knockout mouse model to address this question. Using the Cre-LoxP system, the mTOR gene was specifically knocked out in cells expressing Vip (vasoactive intestinal peptide), which represent a major population of interneurons widely distributed in the neocortex, suprachiasmatic nucleus (SCN), olfactory bulb (OB), and other brain regions. Using a combination of biochemical, behavioral, and imaging approaches, we found that mice lacking mTOR in VIP neurons displayed erratic circadian behavior and weakened synchronization among cells in the SCN, the master circadian pacemaker in mammals. Furthermore, we have discovered a critical role for mTOR signaling in mediating olfaction. Odor stimulated mTOR activation in the OB, anterior olfactory nucleus, as well as piriform cortex. Odor-evoked c-Fos responses along the olfactory pathway were abolished in mice lacking mTOR in VIP neurons, which is consistent with reduced olfactory sensitivity in these animals. Together, these results demonstrate that mTOR is a key regulator of SCN circadian clock synchrony and olfaction.

Overexpression of TFEB Drives a Pleiotropic Neurotrophic Effect and Prevents Parkinson's Disease-Related Neurodegeneration

The possible implication of transcription factor EB (TFEB) as a therapeutic target in Parkinson's disease has gained momentum since it was discovered that TFEB controls lysosomal biogenesis and autophagy and that its activation might counteract lysosomal impairment and protein aggregation. However, the majority of putative direct targets of TFEB described to date is linked to a range of biological processes that are not related to the lysosomal-autophagic system. Here, we assessed the effect of overexpressing TFEB with an adeno-associated viral vector in mouse substantia nigra dopaminergic neurons. We demonstrate that TFEB overexpression drives a previously unknown bona fide neurotrophic effect, giving rise to cell growth, higher tyrosine hydroxylase levels, and increased dopamine release in the striatum. TFEB overexpression induces the activation of the mitogen-activated protein kinase 1/3 (MAPK1/3) and AKT pro-survival pathways, phosphorylation of mTORC1 effectors 4E-binding protein 1 (4E-BP1) and S6 kinase B1 (S6K1), and increased protein synthesis. We show that TFEB overexpression prevents dopaminergic cell loss and counteracts atrophy and the associated protein synthesis decline in the MPTP mouse model of Parkinson's disease. Our results suggest that increasing TFEB activity might prevent neuronal death and restore neuronal function in Parkinson's disease and other neurodegenerative diseases through different mechanisms.

Circadian processes in the RNA life cycle

The circadian clock drives daily rhythms of multiple physiological processes, allowing organisms to anticipate and adjust to periodic changes in environmental conditions. These physiological rhythms are associated with robust oscillations in the expression of at least 30% of expressed genes. While the ability for the endogenous timekeeping system to generate a 24-hr cycle is a cell-autonomous mechanism based on negative autoregulatory feedback loops of transcription and translation involving core-clock genes and their protein products, it is now increasingly evident that additional mechanisms also govern the circadian oscillations of clock-controlled genes. Such mechanisms can take place post-transcriptionally during the course of the RNA life cycle. It has been shown that many steps during RNA processing are regulated in a circadian manner, thus contributing to circadian gene expression. These steps include mRNA capping, alternative splicing, changes in splicing efficiency, and changes in RNA stability controlled by the tail length of polyadenylation or the use of alternative polyadenylation sites. RNA transport can also follow a circadian pattern, with a circadian nuclear retention driven by rhythmic expression within the nucleus of particular bodies (the paraspeckles) and circadian export to the cytoplasm driven by rhythmic proteins acting like cargo. Finally, RNA degradation may also follow a circadian pattern through the rhythmic involvement of miRNAs. In this review, we summarize the current knowledge of the post-transcriptional circadian mechanisms known to play a prominent role in shaping circadian gene expression in mammals. This article is categorized under: RNA Processing > Splicing Regulation/Alternative Splicing RNA Processing > RNA Editing and Modification RNA Export and Localization > Nuclear Export/Import.

eIF4E Phosphorylation Influences Bdnf mRNA Translation in Mouse Dorsal Root Ganglion Neurons

Plasticity in dorsal root ganglion (DRG) neurons that promotes pain requires activity-dependent mRNA translation. Protein synthesis inhibitors block the ability of many pain-promoting molecules to enhance excitability in DRG neurons and attenuate behavioral signs of pain plasticity. In line with this, we have recently shown that phosphorylation of the 5′ cap-binding protein, eIF4E, plays a pivotal role in plasticity of DRG nociceptors in models of hyperalgesic priming. However, mRNA targets of eIF4E phosphorylation have not been elucidated in the DRG. Brain-derived neurotrophic factor (BDNF) signaling from nociceptors in the DRG to spinal dorsal horn neurons is an important mediator of hyperalgesic priming. Regulatory mechanisms that promote pain plasticity via controlling BDNF expression that is involved in promoting pain plasticity have not been identified. We show that phosphorylation of eIF4E is paramount for Bdnf mRNA translation in the DRG. Bdnf mRNA translation is reduced in mice lacking eIF4E phosphorylation (eIF4E^S209A) and pro-nociceptive factors fail to increase BDNF protein levels in the DRGs of these mice despite robust upregulation of Bdnf-201 mRNA levels. Importantly, bypassing the DRG by giving intrathecal injection of BDNF in eIF4E^S209A mice creates a strong hyperalgesic priming response that is normally absent or reduced in these mice. We conclude that eIF4E phosphorylation-mediated translational control of BDNF expression is a key mechanism for nociceptor plasticity leading to hyperalgesic priming.

Loss of eIF4E Phosphorylation Engenders Depression-like Behaviors via Selective mRNA Translation

The MAPK/ERK (mitogen-activated protein kinases/extracellular signal-regulated kinase) pathway is a cardinal regulator of synaptic plasticity, learning, and memory in the hippocampus. One of major endpoints of this signaling cascade is the 5′ mRNA cap binding protein eIF4E (eukaryotic Initiation Factor 4E), which is phosphorylated on Ser 209 by MNK (MAPK-interacting protein kinases) and controls mRNA translation. The precise role of phospho-eIF4E in the brain is yet to be determined. Herein, we demonstrate that ablation of eIF4E phosphorylation in male mice (4Eki mice) does not impair long-term spatial or contextual fear memory, or the late phase of LTP. Using unbiased translational profiling in mouse brain, we show that phospho-eIF4E differentially regulates the translation of a subset of mRNAs linked to inflammation, the extracellular matrix, pituitary hormones, and the serotonin pathway. Consequently, 4Eki male mice display exaggerated inflammatory responses and reduced levels of serotonin, concomitant with depression and anxiety-like behaviors. Remarkably, eIF4E phosphorylation is required for the chronic antidepressant action of the selective serotonin reuptake inhibitor fluoxetine. Finally, we propose a novel phospho-eIF4E-dependent translational control mechanism in the brain, via the GAIT complex (gamma IFN activated inhibitor of translation). In summary, our work proposes a novel translational control mechanism involved in the regulation of inflammation and depression, which could be exploited to design novel therapeutics.

SIGNIFICANCE STATEMENT We demonstrate that downstream of the MAPK (mitogen-activated protein kinase) pathway, eukaryotic Initiation Factor 4E (eIF4E) Ser209 phosphorylation is not required for classical forms of hippocampal LTP and memory. We reveal a novel role for eIF4E phosphorylation in inflammatory responses and depression-like behaviors. eIF4E phosphorylation is required for the chronic action of antidepressants, such as fluoxetine in mice. These phenotypes are accompanied by selective translation of extracellular matrix, pituitary hormones, and serotonin pathway genes, in eIF4E phospho-mutant mice. We also describe a previously unidentified translational control mechanism in the brain, whereby eIF4E phosphorylation is required for inhibiting the translation of gamma IFN activated inhibitor of translation element-containing mRNAs. These findings can be used to design novel therapeutics for depression.

Circadian control of pain and neuroinflammation

The importance of a neuroinflammatory response to the development and maintenance of inflammatory and neuropathic pain have been highlighted in recent years. Inflammatory cells contributing to this response include circulating immune cells such as monocytes, T and B lymphocytes, and neutrophils, as well as microglia in the central nervous system. Pain signals are transmitted via sensory neurons in the peripheral nervous system, which express various receptors and channels that respond to mediators secreted from these inflammatory cells. Chronobiological rhythms, which include the 24-hr circadian cycle, have recently been shown to regulate both nervous and immune cell activity and function. This review examines the current literature on chronobiological control of neuroinflammatory processes, with a focus on inflammatory and neuropathic pain states. While the majority of this work has stemmed from observational studies in humans, recent advances in using animal models have highlighted distinct mechanisms underlying these interactions. Better understanding interactions between the circadian and neuroimmune systems can help guide the development of new treatments and provide improved care for patients suffering from acute and chronic pain.

Aberrant Proteostasis of BMAL1 Underlies Circadian Abnormalities in a Paradigmatic mTOR-opathy

Tuberous sclerosis complex (TSC) is a neurodevelopmental disorder characterized by mutations in either the TSC1 or TSC2 genes, whose products form a critical inhibitor of the mechanistic target of rapamycin (mTOR). Loss of TSC1/2 gene function renders an mTOR-overactivated state. Clinically, TSC manifests with epilepsy, intellectual disability, autism, and sleep dysfunction. Here, we report that mouse models of TSC have abnormal circadian rhythms. We show that mTOR regulates the proteostasis of the core clock protein BMAL1, affecting its translation, degradation, and subcellular localization. This results in elevated levels of BMAL1 and a dysfunctional clock that displays abnormal timekeeping under constant conditions and exaggerated responses to phase resetting. Genetically lowering the dose of BMAL1 rescues circadian behavioral phenotypes in TSC mouse models. These findings indicate that BMAL1 deregulation is a feature of the mTOR-activated state and suggest a molecular mechanism for mitigating circadian phenotypes in a neurodevelopmental disorder.