Survival of adult generated hippocampal neurons is altered in circadian arrhythmic mice

Lack of Bmal1 leads to changes in rhythmicity and impairs motivation towards natural stimuli

Maintaining proper circadian rhythms is essential for coordinating biological functions in mammals. This study investigates the effects of daily arrhythmicity using Bmal1-knockout (KO) mice as a model, aiming to understand behavioural and motivational implications. By employing a new mathematical analysis based on entropy divergence, we identified disrupted intricate activity patterns in mice derived by the complete absence of BMAL1 and quantified the difference regarding the activity oscillation's complexity. Changes in locomotor activity coincided with disturbances in circadian gene expression patterns. Additionally, we found a dysregulated gene expression profile particularly in brain nuclei like the ventral striatum, impacting genes related to reward and motivation. Further investigation revealed that arrhythmic mice exhibited heightened motivation for food and water rewards, indicating a link between circadian disruptions and the reward system. This research sheds light on how circadian clock alterations impact the gene expression regulating the reward system and how this, in turn, can lead to altered seeking behaviour and motivation for natural rewards. In summary, the present study contributes to our understanding of how reward processing is under the regulation of circadian clock machinery.

Associations of Circadian Rest-Activity Rhythms With Affect and Cognition in Community-Dwelling Stroke Survivors: An Ambulatory Assessment Study

BACKGROUND

Rest-activity rhythm (RAR) is a modifiable behavioral factor associated with affect and cognition. Identifying RAR characteristics associated with affect and cognition among stroke survivors provides insight into preventing poststroke affective and cognitive impairment.

OBJECTIVE

To examine the associations of RAR characteristics with affect and cognition among community-dwelling stroke survivors.

METHODS

Forty participants with mild stroke (mean age = 52.8; 42.5% female; 55% White) reported their affect and cognitive complaints using ecological momentary assessment and wore an accelerometer for 7 consecutive days and completed the National Institutes of Health Toolbox Cognition Battery. RAR characteristics were extracted using parametric and non-parametric approaches. Multivariable linear regressions were used to identify RAR characteristics associated with affect and cognition.

RESULTS

Later onset of rest (B = 0.45; P = .008) and activity (B = 0.36; P = .041) were positively associated with depressed affect. These associations were reversed for cheerful effect (rest onset: B = -0.42; P = .017; activity onset: B = -0.39; P = .033). Cheerful affect was also positively associated with relative amplitude (ie, distinctions in activity levels between rest and activity; B = .39; P = .030). Intra-daily variability (ie, RAR fragmentation; B = 0.35; P = .042) and later onset of activity (B = .36; P = .048) were positively associated with cognitive complaints. Less erratic RAR was positively associated with fluid cognition (B = 0.29; P = .036); RAR fragmentation was positively associated with crystallized cognition (B = 0.39; P = .015).

CONCLUSIONS

We identified RAR correlates of affect and cognition among stroke survivors, highlighting the value of managing RAR and sleep in stroke rehabilitation. Future studies should test whether advancing the onset of rest and activity, promoting a regular active lifestyle, and improving rest and sleep in the nighttime protect stroke survivors from affective and cognitive impairment.

Microglia activation mediates circadian rhythm disruption-induced cognitive impairment in mice

Alzheimer's disease (AD) is the leading cause of dementia and there are no effective treatments for this disease currently. Circadian rhythm disruption (CRD) is a hallmark of modern society that appears to be on the rise. It is well reported that AD is associated with disrupted circadian functioning and CRD can impair cognitive function. However, the cellular mechanisms underlying CRD-associated cognitive decline remain elusive. In this study, we investigated whether microglia are involved in CRD-induced cognitive decline. We established experimental 'jet lag' (phase delay of the light/dark cycles)-induced CRD mouse model and observed significant impairment of spatial learning and memory function in these mice. In the brain, CRD resulted in neuroinflammation, which was characterized by microglia activation and increased pro-inflammatory cytokine production, impairments in neurogenesis and reduction of synaptic proteins in the hippocampus. Interestingly, elimination of microglia with the colony stimulating factor-1 receptor inhibitor PLX3397 prevented CRD-induced neuroinflammation, cognitive decline, impairment of neurogenesis and loss of synaptic proteins. These findings collectively suggest that microglia activation plays a key role in CRD-induced cognitive deficit most likely through neuroinflammation-mediated impairments in adult neurogenesis and synapses.

The shared molecular mechanisms underlying aging of the brain, major depressive disorder, and Alzheimer's disease: The role of circadian rhythm disturbances

An association with circadian clock function and pathophysiology of aging, major depressive disorder (MDD), and Alzheimer's disease (AD) is well established and has been proposed as a factor in the development of these diseases. Depression and changes in circadian rhythm have been increasingly suggested as the two primary overlapping and interpenetrating changes that occur with aging. The relationship between AD and depression in late life is not completely understood and probably is complex. Patients with major depression or AD suffer from disturbed sleep/wake cycles and altered rhythms in daily activities. Although classical monoaminergic hypotheses are traditionally proposed to explain the pathophysiology of MDD, several clinical and preclinical studies have reported a strong association between circadian rhythm and mood regulation. In addition, a large body of evidence supports an association between disruption of circadian rhythm and AD. Some clock genes are dysregulated in rodent models of depression. If aging, AD, and MDD share a common biological basis in pathophysiology, common therapeutic tools could be investigated for their prevention and treatment. Nitro-oxidative stress (NOS), for example, plays a fundamental role in aging, as well as in the pathogenesis of AD and MDD and is associated with circadian clock disturbances. Thus, development of therapeutic possibilities with these NOS-related conditions is advisable. This review describes recent findings that link disrupted circadian clocks to aging, MDD, and AD and summarizes the experimental evidence that supports connections between the circadian clock and molecular pathologic factors as shared common pathophysiological mechanisms underlying aging, AD, and MDD.

Environmental Enrichment Engages Vesicular Zinc Signaling to Enhance Hippocampal Neurogenesis

Zinc is highly concentrated in synaptic vesicles throughout the mammalian telencephalon and, in particular, the hippocampal dentate gyrus. A role for zinc in modulating synaptic plasticity has been inferred, but whether zinc has a particular role in experience-dependent plasticity has yet to be determined. The aim of the current study was to determine whether vesicular zinc is important for modulating adult hippocampal neurogenesis in an experience-dependent manner and, consequently, hippocampal-dependent behaviour. We assessed the role of vesicular zinc in modulating hippocampal neurogenesis and behaviour by comparing ZnT3 knockout (KO) mice, which lack vesicular zinc, to wild-type (WT) littermates exposed to either standard housing conditions (SH) or an enriched environment (EE). We found that vesicular zinc is necessary for a cascade of changes in hippocampal plasticity following EE, such as increases in hippocampal neurogenesis and elevations in mature brain-derived neurotrophic factor (mBDNF), but was otherwise dispensable under SH conditions. Using the Spatial Object Recognition task and the Morris Water task we show that, unlike WT mice, ZnT3 KO mice showed no improvements in spatial memory following EE. These experiments demonstrate that vesicular zinc is essential for the enhancement of adult hippocampal neurogenesis and behaviour following enrichment, supporting a role for zincergic neurons in contributing to experience-dependent plasticity in the hippocampus.

Vesicular Zinc Modulates Cell Proliferation and Survival in the Developing Hippocampus

In the brain, vesicular zinc, which refers to a subset of zinc that is sequestered into synaptic vesicles by zinc transporter 3 (ZnT3), has extensive effects on neuronal signalling and modulation. Vesicular zinc-focused research has mainly been directed to its role in the hippocampus, particularly in adult neurogenesis. However, whether vesicular zinc is involved in modulating neurogenesis during the early postnatal period has been less studied. As a first step to understanding this, we used ZnT3 knockout (KO) mice, which lack ZnT3 and, thus, vesicular zinc, to evaluate cell proliferation at three different age points spanning postnatal development (P6, P14, and P28). The survival and the neuronal phenotype of these cells was also assessed in adulthood. We found that male ZnT3 KO mice exhibited lower rates of cell proliferation at P14, but a greater number of these cells survived to adulthood. Additionally, significantly more cells labelled on P6 survived to adulthood in male and female ZnT3 KO mice. We also found sex-dependent differences, whereby male mice showed higher levels of cell proliferation at P28, as well as higher levels of cell survival for P14-labelled cells, compared to female mice. However, female mice showed greater percentages of neuronal differentiation for P14-labelled cells. Finally, we found significant effects of age of BrdU injections on cell proliferation, survival, and neuronal differentiation. Collectively, our results suggest that the loss of vesicular zinc affects normal proliferation and survival of cells born at different age points during postnatal development and highlight prominent sex- and age-dependent differences. Our findings provide the foundation for future studies to further probe the role of vesicular zinc in the modulation of developmental neurogenesis.

Adult Neurogenesis Is Altered by Circadian Phase Shifts and the Duper Mutation in Female Syrian Hamsters

Cell birth and survival in the adult hippocampus are regulated by a circadian clock. Rotating shift work and jet lag disrupt circadian rhythms and aggravate disease. Internal misalignment, a state in which abnormal phase relationships prevail between and within organs, is proposed to account for adverse effects of circadian disruption. This hypothesis has been difficult to test because phase shifts of the entraining cycle inevitably lead to transient desynchrony. Thus, it remains possible that phase shifts, regardless of internal desynchrony, account for adverse effects of circadian disruption and alter neurogenesis and cell fate. To address this question, we examined cell birth and differentiation in the duper Syrian hamster (Mesocricetus auratus), a Cry1-null mutant in which re-entrainment of locomotor rhythms is greatly accelerated. Adult females were subjected to alternating 8 h advances and delays at eight 16 d intervals. BrdU, a cell birth marker, was given midway through the experiment. Repeated phase shifts decreased the number of newborn non-neuronal cells in WT, but not in duper hamsters. The duper mutation increased the number of BrdU-IR cells that stained for NeuN, which marks neuronal differentiation. Immunocytochemical staining for proliferating cell nuclear antigen indicated no overall effect of genotype or repeated shifts on cell division rates after 131 days. Cell differentiation, assessed by doublecortin, was higher in duper hamsters but was not significantly altered by repeated phase shifts. Our results support the internal misalignment hypothesis and indicate that Cry1 regulates cell differentiation. Phase shifts may determine neuronal stem cell survival and time course of differentiation after cell birth. Figure created with BioRender.

Differentiated Embryo-Chondrocyte Expressed Gene1 and Parkinson's Disease: New Insights and Therapeutic Perspectives

Differentiated embryo-chondrocyte expressed gene1 (DEC1), an important transcription factor with a basic helix-loop-helix domain, is ubiquitously expressed in both human embryonic and adult tissues. DEC1 is involved in neural differentiation and neural maturation in the central nervous system (CNS). Recent studies suggest that DEC1 protects against Parkinson's disease (PD) by regulating apoptosis, oxidative stress, lipid metabolism, immune system, and glucose metabolism disorders. In this review, we summarize the recent progress on the role of DEC1 in the pathogenesis of PD and provide new insights into the prevention and treatment of PD and neurodegenerative diseases.

The Relevance of Circadian Clocks to Stem Cell Differentiation and Cancer Progression

The molecular mechanism of circadian clocks depends on transcription-translation feedback loops (TTFLs) that have known effects on key cellular processes. However, the distinct role of circadian TTFLs in mammalian stem cells and other less differentiated cells remains poorly understood. Neural stem cells (NSCs) of the brain generate neurons and glia postnatally but also may become cancer stem cells (CSCs), particularly in astrocytomas. Evidence indicates clock TTFL impairment is needed for tumor growth and progression; although, this issue has been examined primarily in more differentiated cancer cells rather than CSCs. Similarly, few studies have examined circadian rhythms in NSCs. After decades of research, it is now well recognized that tumors consist of CSCs and a range of other cancer cells along with noncancerous stromal cells. The circadian properties of these many contributors to tumor properties and treatment outcome are being widely explored. New molecular tools and ones in development will likely enable greater discrimination of important circadian and non-circadian cells within malignancies at multiple stages of cancer progression and following therapy. Here, we focus on adult NSCs and glioma CSCs to address how cells at different stages of differentiation may harbor unique states of the molecular circadian clock influencing differentiation and cell fate.

Adult Neurogenesis under Control of the Circadian System

The mammalian circadian system is a hierarchically organized system, which controls a 24-h periodicity in a wide variety of body and brain functions and physiological processes. There is increasing evidence that the circadian system modulates the complex multistep process of adult neurogenesis, which is crucial for brain plasticity. This modulatory effect may be exercised via rhythmic systemic factors including neurotransmitters, hormones and neurotrophic factors as well as rhythmic behavior and physiology or via intrinsic factors within the neural progenitor cells such as the redox state and clock genes/molecular clockwork. In this review, we discuss the role of the circadian system for adult neurogenesis at both the systemic and the cellular levels. Better understanding of the role of the circadian system in modulation of adult neurogenesis can help develop new treatment strategies to improve the cognitive deterioration associated with chronodisruption due to detrimental light regimes or neurodegenerative diseases.

P2Y2 deficiency impacts adult neurogenesis and related forebrain functions

Adult neurogenesis occurs particularly in the subgranular zone (SGZ) of the hippocampus and the subventricular zone (SVZ) of the lateral ventricle. This continuous addition of neurons to pre-existing neuronal networks is essential for intact cognitive and olfactory functions, respectively. Purinergic signaling modulates adult neurogenesis, however, the role of individual purinergic receptor subtypes in this dynamic process and related cognitive performance is poorly understood. In this study, we analyzed the role of P2Y₂ receptor in the neurogenic niches and in related forebrain functions such as spatial working memory and olfaction using mice with a targeted deletion of the P2Y₂ receptor (P2Y2^-/- ). Proliferation, migration, differentiation, and survival of neuronal precursor cells (NPCs) were analyzed by BrdU assay and immunohistochemistry; signal transduction pathway components were analyzed by immunoblot. In P2Y2^-/- mice, proliferation of NPCs in the SGZ and the SVZ was reduced. However, migration, neuronal fate decision, and survival were not affected. Moreover, p-Akt expression was decreased in P2Y2^-/- mice. P2Y2^-/- mice showed an impaired performance in the Y-maze and a higher latency in the hidden food test. These data indicate that the P2Y₂ receptor plays an important role in NPC proliferation as well as in hippocampus-dependent working memory and olfactory function.

The circadian system: From clocks to physiology

The circadian system, composed of the central autonomous clock, the suprachiasmatic nucleus (SCN), and systems of the body that follow the signals of the SCN, continuously change the homeostatic set points of the body over the day-night cycle. Changes in the body's physiological state that do not agree with the time of the day feedback to the hypothalamus, and provide input to the SCN to adjust the condition, thus reaching another set point required by the changed conditions. This allows the adjustment of the set points to another level when environmental conditions change, which is thought to promote adaptation and survival. In fasting, the body temperature drops to a lower level only at the beginning of the sleep phase. Stressful conditions raise blood pressure relatively more during the active period than during the rest phase. Extensive, mostly reciprocal SCN interactions, with hypothalamic networks, induce these physiological adjustments by hormonal and autonomic control of the body's organs. More importantly, in addition to SCN's hormonal and autonomic influences, SCN induced behavior, such as rhythmic food intake, induces the oscillation of many genes in all tissues, including the so-called clock genes, which have an essential role as a transcriptional driving force for numerous cellular processes. Consequently, the light-dark cycle, the rhythm of the SCN, and the resulting rhythm in behavior need to be perfectly synchronized, especially where it involves synchronizing food intake with the activity phase. If these rhythms are not synchronous for extended periods of times, such as during shift work, light exposure at night, or frequent night eating, disease may develop. As such, our circadian system is a perfect illustration of how hypothalamic-driven processes depend on and interact with each other and need to be in seamless synchrony with the body's physiology.

Regulation of tissue regeneration by the circadian clock

Circadian rhythms are regulated by a highly conserved transcriptional/translational feedback loop that maintains approximately 24-hr periodicity from cellular to organismal levels. Much research effort is being devoted to understanding how the outputs of the master clock affect peripheral oscillators, and in turn, numerous biological processes. Recent studies have revealed roles for circadian timing in the regulation of numerous cellular behaviours in support of complex tissue regeneration. One such role involves the interaction between the circadian clockwork and the cell cycle. The molecular mechanisms that control the cell cycle create a system of regulation that allows for high fidelity DNA synthesis, mitosis and apoptosis. In recent years, it has become clear that clock gene products are required for proper DNA synthesis and cell cycle progression, and conversely, elements of the cell cycle cascade feedback to influence molecular circadian timing mechanisms. It is through this crosstalk that the circadian system orchestrates stem cell proliferation, niche exit and control of the signalling pathways that govern differentiation and self-renewal. In this review, we discuss the evidence for circadian control of tissue homeostasis and repair and suggest new avenues for research.

Circadian Biology and Stroke

Circadian biology modulates almost all aspects of mammalian physiology, disease, and response to therapies. Emerging data suggest that circadian biology may significantly affect the mechanisms of susceptibility, injury, recovery, and the response to therapy in stroke. In this review/perspective, we survey the accumulating literature and attempt to connect molecular, cellular, and physiological pathways in circadian biology to clinical consequences in stroke. Accounting for the complex and multifactorial effects of circadian rhythm may improve translational opportunities for stroke diagnostics and therapeutics.

REV-ERB Agonist SR9009 Regulates the Proliferation and Neurite Outgrowth/Suppression of Cultured Rat Adult Hippocampal Neural Stem/Progenitor Cells in a Concentration-Dependent Manner

REV-ERBs are heme-binding nuclear receptors that regulate the circadian rhythm and play important roles in the regulation of proliferation and the neuronal differentiation process in neuronal stem/progenitor cells in the adult brain. However, the effects of REV-ERB activation in the adult brain remain unclear. In this study, SR9009, a synthetic REV-ERB agonist that produces anxiolytic effects in mice, was used to treat undifferentiated and neuronally differentiated cultured rat adult hippocampal neural stem/progenitor cells (AHPs). The expression of Rev-erbβ was upregulated during neurogenesis in cultured rat AHPs, and Rev-erbβ knockdown analysis indicated that REV-ERBβ regulates the proliferation and neurite outgrowth of cultured rat AHPs. The application of a low concentration (0.1 µM) of the REV-ERB agonist SR9009 enhanced neurite outgrowth during neurogenesis in cultured rat AHPs, whereas the addition of a high concentration (2.5 µM) of SR9009 suppressed neurite outgrowth. Further examination of the SR9009 regulatory mechanism showed that the expressions of downstream target genes of REV-ERBβ, including Ccna2 and Sez6, were modulated by SR9009. The results of this study indicated that REV-ERBβ activity in cultured rat AHPs was regulated by SR9009 in a concentration-dependent manner. Furthermore, SR9009 inhibited the growth of cultured rat AHPs through various pathways, which may provide insight into the multifunctional mechanisms of action associated with SR9009. The findings of this study may provide an improved understanding of proliferation and neuronal maturation mechanisms in cultured rat AHPs through SR9009-regulated REV-ERBβ signaling pathways.

Circadian Regulation of Cochlear Sensitivity to Noise by Circulating Glucocorticoids

The cochlea possesses a robust circadian clock machinery that regulates auditory function. How the cochlear clock is influenced by the circadian system remains unknown. Here, we show that cochlear rhythms are system driven and require local Bmal1 as well as central input from the suprachiasmatic nuclei (SCN). SCN ablations disrupted the circadian expression of the core clock genes in the cochlea. Because the circadian secretion of glucocorticoids (GCs) is controlled by the SCN and GCs are known to modulate auditory function, we assessed their influence on circadian gene expression. Removal of circulating GCs by adrenalectomy (ADX) did not have a major impact on core clock gene expression in the cochlea. Rather it abolished the transcription of clock-controlled genes involved in inflammation. ADX abolished the known differential auditory sensitivity to day and night noise trauma and prevented the induction of GABA-ergic and glutamate receptors mRNA transcripts. However, these improvements were unrelated to changes at the synaptic level, suggesting other cochlear functions may be involved. Due to this circadian regulation of noise sensitivity by GCs, we evaluated the actions of the synthetic glucocorticoid dexamethasone (DEX) at different times of the day. DEX was effective in protecting from acute noise trauma only when administered during daytime, when circulating glucocorticoids are low, indicating that chronopharmacological approaches are important for obtaining optimal treatment strategies for hearing loss. GCs appear as a major regulator of the differential sensitivity to day or night noise trauma, a mechanism likely involving the circadian control of inflammatory responses.

The circadian clock in adult neural stem cell maintenance

Neural stem cells persist in the adult central nervous system as a continuing source of astrocytes, oligodendrocytes and neurons. Various signalling pathways and transcription factors actively maintain this population by regulating cell cycle entry and exit. Similarly, the circadian clock is interconnected with the cell cycle and actively maintains stem cell populations in various tissues. Here, we discuss emerging evidence for an important role of the circadian clock in neural stem cell maintenance. We propose that the NAD⁺-dependent deacetylase SIRT1 exerts control over the circadian clock in adult neural stem cell function to limit exhaustion of their population. Conversely, disruption of the circadian clock may compromise neural stem cell quiescence resulting in a premature decline of the neural stem cell population. As such, energy metabolism and the circadian clock converge in adult neural stem cell maintenance.

Involvement of Nuclear Receptor REV-ERBβ in Formation of Neurites and Proliferation of Cultured Adult Neural Stem Cells

Neural stem cells (NSCs) serve as the source of both neurons and support cells, and neurogenesis is reportedly linked to the circadian clock. This study aimed to clarify the functional role of the circadian rhythm-related nuclear receptor, REV-ERBβ, in neurogenesis of NSCs from adult brain. Accordingly, Rev-erbβ expression and the effect of Rev-erbβ gene-specific knockdown on neurogenesis in vitro was examined in adult rodent NSCs. Initial experiments confirmed REV-ERBβ expression in cultured adult NSCs, while subsequent gene expression and gene ontogeny analyses identified functional genes upregulated or downregulated by REV-ERBβ. In particular, expression levels of factors associated with proliferation, stemness, and neural differentiation were affected. Knockdown of Rev-erbβ showed involvement of REV-ERBβ in regulation of cellular proliferation and self-renewal of cultured adult NSCs. Moreover, Rev-erbβ-knockdown cells formed neurons with a slightly shrunken morphology, fewer new primary neurites, and reduced length and branch formation of neurites. Altogether, this suggests that REV-ERBβ is involved in neurite formation during neuronal differentiation of cultured adult NSCs. In summary, REV-ERBβ is a known circadian regulatory protein that appears to be involved in neurogenesis via regulation of networks for cell proliferation and neural differentiation/maturation in adult NSCs.

Modulation of Adult Hippocampal Neurogenesis by Sleep: Impact on Mental Health

The process of neurogenesis has been demonstrated to occur throughout life in the subgranular zone (SGZ) of the hippocampal dentate gyrus of several mammals, including humans. The basal rate of adult hippocampal neurogenesis can be altered by lifestyle and environmental factors. In this perspective review, the evidence for sleep as a modulator of adult hippocampal neurogenesis is first summarized. Following this, the impacts of sleep and sleep disturbances on hippocampal-dependent functions, including learning and memory, and depression are critically evaluated. Finally, we postulate that the effects of sleep on hippocampal-dependent functions may possibly be mediated by a change in adult hippocampal neurogenesis. This could provide a route to new treatments for cognitive impairments and psychiatric disorders.

Central and peripheral circadian clocks and their role in Alzheimer's disease

Molecular and cellular oscillations constitute an internal clock that tracks the time of day and permits organisms to optimize their behaviour and metabolism to suit the daily demands they face. The workings of this internal clock become impaired with age. In this review, we discuss whether such age-related impairments in the circadian clock interact with age-related neurodegenerative disorders, such as Alzheimer's disease. Findings from mouse and fly models of Alzheimer's disease have accelerated our understanding of the interaction between neurodegeneration and circadian biology. These models show that neurodegeneration likely impairs circadian rhythms either by damaging the central clock or by blocking its communication with other brain areas and with peripheral tissues. The consequent sleep and metabolic deficits could enhance the susceptibility of the brain to further degenerative processes. Thus, circadian dysfunction might be both a cause and an effect of neurodegeneration. We also discuss the primary role of light in the entrainment of the central clock and describe important, alternative time signals, such as food, that play a role in entraining central and peripheral circadian clocks. Finally, we propose how these recent insights could inform efforts to develop novel therapeutic approaches to re-entrain arrhythmic individuals with neurodegenerative disease.

Sleep and hippocampal neurogenesis: Implications for Alzheimer's disease

Alzheimer's disease (AD) is the most common cause of dementia and currently there are no effective disease-modifying treatments available. Hallmark symptoms of AD include impaired hippocampus-dependent episodic memory and disrupted sleep and circadian rhythms. The pathways connecting these symptoms are of particular interest because it is well established that sleep and circadian disruption can impair hippocampus-dependent learning and memory. In rodents, these procedures also markedly suppress adult hippocampal neurogenesis, a form of brain plasticity that is believed to play an important role in pattern separation, and thus episodic memory. A causal role for sleep disruptions in AD pathophysiology is suggested by evidence for sleep-dependent glymphatic clearance of metabolic waste products from the brain. This review explores a complementary hypothesis that sleep and circadian disruptions in AD contribute to cognitive decline by activating neuroendocrine and neuroinflammatory signaling pathways that suppress hippocampal neurogenesis. Evidence for this hypothesis underscores the promise of sleep, circadian rhythms, and neurogenesis as therapeutic targets for remediation of memory impairment in AD.

Circadian Kinetics of Cell Cycle Progression in Adult Neurogenic Niches of a Diurnal Vertebrate

The circadian system may regulate adult neurogenesis via intracellular molecular clock mechanisms or by modifying the environment of neurogenic niches, with daily variation in growth factors or nutrients depending on the animal's diurnal or nocturnal lifestyle. In a diurnal vertebrate, zebrafish, we studied circadian distribution of immunohistochemical markers of the cell division cycle (CDC) in 5 of the 16 neurogenic niches of adult brain, the dorsal telencephalon, habenula, preoptic area, hypothalamus, and cerebellum. We find that common to all niches is the morning initiation of G₁/S transition and daytime S-phase progression, overnight increase in G₂/M, and cycle completion by late night. This is supported by the timing of gene expression for critical cell cycle regulators cyclins D, A2, and B2 and cyclin-dependent kinase inhibitor p20 in brain tissue. The early-night peak in p20, limiting G₁/S transition, and its phase angle with the expression of core clock genes, Clock1 and Per1, are preserved in constant darkness, suggesting intrinsic circadian patterns of cell cycle progression. The statistical modeling of CDC kinetics reveals the significant circadian variation in cell proliferation rates across all of the examined niches, but interniche differences in the magnitude of circadian variation in CDC, S-phase length, phase angle of entrainment to light or clock, and its dispersion. We conclude that, in neurogenic niches of an adult diurnal vertebrate, the circadian modulation of cell cycle progression involves both systemic and niche-specific factors.

SIGNIFICANCE STATEMENT This study establishes that in neurogenic niches of an adult diurnal vertebrate, the cell cycle progression displays a robust circadian pattern. Common to neurogenic niches located in diverse brain regions is daytime progression of DNA replication and nighttime mitosis, suggesting systemic regulation. Differences between neurogenic niches in the phase and degree of S-phase entrainment to the clock suggest additional roles for niche-specific regulatory mechanisms. Understanding the circadian regulation of adult neurogenesis can help optimize the timing of therapeutic approaches in patients with brain traumas or neurodegenerative disorders and preserve neural stem cells during cytostatic cancer therapies.

Mechanisms linking circadian clocks, sleep, and neurodegeneration

Disruptions of normal circadian rhythms and sleep cycles are consequences of aging and can profoundly affect health. Accumulating evidence indicates that circadian and sleep disturbances, which have long been considered symptoms of many neurodegenerative conditions, may actually drive pathogenesis early in the course of these diseases. In this Review, we explore potential cellular and molecular mechanisms linking circadian dysfunction and sleep loss to neurodegenerative diseases, with a focus on Alzheimer's disease. We examine the interplay between central and peripheral circadian rhythms, circadian clock gene function, and sleep in maintaining brain homeostasis, and discuss therapeutic implications. The circadian clock and sleep can influence a number of key processes involved in neurodegeneration, suggesting that these systems might be manipulated to promote healthy brain aging.

Circadian and ultradian glucocorticoid rhythmicity: Implications for the effects of glucocorticoids on neural stem cells and adult hippocampal neurogenesis

Psychosocial stress, and within the neuroendocrine reaction to stress specifically the glucocorticoid hormones, are well-characterized inhibitors of neural stem/progenitor cell proliferation in the adult hippocampus, resulting in a marked reduction in the production of new neurons in this brain area relevant for learning and memory. However, the mechanisms by which stress, and particularly glucocorticoids, inhibit neural stem/progenitor cell proliferation remain unclear and under debate. Here we review the literature on the topic and discuss the evidence for direct and indirect effects of glucocorticoids on neural stem/progenitor cell proliferation and adult neurogenesis. Further, we discuss the hypothesis that glucocorticoid rhythmicity and oscillations originating from the activity of the hypothalamus-pituitary-adrenal axis, may be crucial for the regulation of neural stem/progenitor cells in the hippocampus, as well as the implications of this hypothesis for pathophysiological conditions in which glucocorticoid oscillations are affected.

Circadian Clock Genes Are Essential for Normal Adult Neurogenesis, Differentiation, and Fate Determination

Adult neurogenesis creates new neurons and glia from stem cells in the human brain throughout life. It is best understood in the dentate gyrus (DG) of the hippocampus and the subventricular zone (SVZ). Circadian rhythms have been identified in the hippocampus, but the role of any endogenous circadian oscillator cells in hippocampal neurogenesis and their importance in learning or memory remains unclear. Any study of stem cell regulation by intrinsic circadian timing within the DG is complicated by modulation from circadian clocks elsewhere in the brain. To examine circadian oscillators in greater isolation, neurosphere cultures were prepared from the DG of two knockout mouse lines that lack a functional circadian clock and from mPer1::luc mice to identify circadian oscillations in gene expression. Circadian mPer1 gene activity rhythms were recorded in neurospheres maintained in a culture medium that induces neurogenesis but not in one that maintains the stem cell state. Although the differentiating neural stem progenitor cells of spheres were rhythmic, evidence of any mature neurons was extremely sparse. The circadian timing signal originated in undifferentiated cells within the neurosphere. This conclusion was supported by immunocytochemistry for mPER1 protein that was localized to the inner, more stem cell-like neurosphere core. To test for effects of the circadian clock on neurogenesis, media conditions were altered to induce neurospheres from BMAL1 knockout mice to differentiate. These cultures displayed unusually high differentiation into glia rather than neurons according to GFAP and NeuN expression, respectively, and very few BetaIII tubulin-positive, immature neurons were observed. The knockout neurospheres also displayed areas visibly devoid of cells and had overall higher cell death. Neurospheres from arrhythmic mice lacking two other core clock genes, Cry1 and Cry2, showed significantly reduced growth and increased astrocyte proliferation during differentiation, but they generated normal percentages of neuronal cells. Neuronal fate commitment therefore appears to be controlled through a non-clock function of BMAL1. This study provides insight into how cell autonomous circadian clocks and clock genes regulate adult neural stem cells with implications for treating neurodegenerative disorders and impaired brain functions by manipulating neurogenesis.

Premature aging of the hippocampal neurogenic niche in adult Bmal1-deficient mice

Hippocampal neurogenesis undergoes dramatic age-related changes. Mice with targeted deletion of the clock geneBmal1 (Bmal1(-/-)) show disrupted regulation of reactive oxygen species homeostasis, accelerated aging, neurodegeneration and cognitive deficits. As proliferation of neuronal progenitor/precursor cells (NPCs) is enhanced in young Bmal1(-/-) mice, we tested the hypothesis that this results in premature aging of hippocampal neurogenic niche in adult Bmal1(-/-) mice as compared to wildtype littermates. We found significantly reduced pool of hippocampal NPCs, scattered distribution, enhanced survival of NPCs and an increased differentiation of NPCs into the astroglial lineage at the expense of the neuronal lineage. Immunoreaction of the redox sensitive histone deacetylase Sirtuine 1, peroxisomal membrane protein at 70 kDa and expression of the cell cycle inhibitor p21(Waf1/CIP1) were increased in adult Bmal1(-/-) mice. In conclusion, genetic disruption of the molecular clockwork leads to accelerated age-dependent decline in adult neurogenesis presumably as a consequence of oxidative stress.