Fluctuations in cellular proliferation across the light/dark cycle in the subgranular zone of the dentate gyrus in the adult male Syrian hamster

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.

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.

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.

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.

Circadian Clock Dysfunction and Psychiatric Disease: Could Fruit Flies have a Say?

There is evidence of a link between the circadian system and psychiatric diseases. Studies in humans and mammals suggest that environmental and/or genetic disruption of the circadian system leads to an increased liability to psychiatric disease. Disruption of clock genes and/or the clock network might be related to the etiology of these pathologies; also, some genes, known for their circadian clock functions, might be associated to mental illnesses through clock-independent pleiotropy. Here, we examine the features which we believe make Drosophila melanogaster a model apt to study the role of the circadian clock in psychiatric disease. Despite differences in the organization of the clock system, the molecular architecture of the Drosophila and mammalian circadian oscillators are comparable and many components are evolutionarily related. In addition, Drosophila has a rather complex nervous system, which shares much at the cell and neurobiological level with humans, i.e., a tripartite brain, the main neurotransmitter systems, and behavioral traits: circadian behavior, learning and memory, motivation, addiction, social behavior. There is evidence that the Drosophila brain shares some homologies with the vertebrate cerebellum, basal ganglia, and hypothalamus-pituitary-adrenal axis, the dysfunctions of which have been tied to mental illness. We discuss Drosophila in comparison to mammals with reference to the: organization of the brain and neurotransmitter systems; architecture of the circadian clock; clock-controlled behaviors. We sum up current knowledge on behavioral endophenotypes, which are amenable to modeling in flies, such as defects involving sleep, cognition, or social interactions, and discuss the relationship of the circadian system to these traits. Finally, we consider if Drosophila could be a valuable asset to understand the relationship between circadian clock malfunction and psychiatric disease.

The primate seahorse rhythm

The main Zeitgeber, the day-night cycle, synchronizes the central oscillator which determines behaviors rhythms as sleep-wake behavior, body temperature, the regulation of hormone secretion, and the acquisition and processing of memory. Thus, actions such as acquisition, consolidation, and retrieval performed in the hippocampus are modulated by the circadian system and show a varied dependence on light and dark. To investigate changes in the hippocampus' cellular mechanism invoked by the day and night in a diurnal primate, this study analyzed the expression of PER2 and the calcium binding proteins (CaBPs) calbindin, calretinin and parvalbumin in the hippocampus of Sapajus apella, a diurnal primate, at two different time points, one during the day and one during the dark phase. The PER2 protein expression peaked at night in the antiphase described for the suprachiasmatic nucleus (SCN) of the same primate, indicating that hippocampal cells can present independent rhythmicity. This hippocampal rhythm was similar to that presented by diurnal but not nocturnal rodents. The CaBPs immunoreactivity also showed day/night variations in the cell number and in the cell morphology. Our findings provide evidence for the claim that the circadian regulation in the hippocampus may involve rhythms of PER2 and CaBPs expression that may contribute to the adaptation of this species in events and activities relevant to the respective periods.

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

The subgranular zone of the hippocampal formation gives rise to new neurons that populate the dentate gyrus throughout life. Cells in the hippocampus exhibit rhythmic clock gene expression and the circadian clock is known to regulate the cycle of cell division in other areas of the body. These facts suggest that the circadian clock may regulate adult neurogenesis in the hippocampus as well. In the present study, neurogenesis in the hippocampal subgranular zone was examined in arrhythmic Bmal1 knockout (-KO) mice and their rhythmic heterozygous and wildtype littermates. Proliferation and survival of newly generated subgranular zone cells were examined using bromodeoxyuridine labelling, while pyknosis (a measure of cell death) and hippocampal volume were examined in cresyl violet stained sections. There was no significant difference in cellular proliferation between any of the groups, yet survival of proliferating cells, 6 weeks after the bromodeoxyuridine injection, was significantly greater in the BMAL1-KO animals. The number of pyknotic cells was significantly decreased in Bmal1-KO animals, yet hippocampal volume remained the same across genotypes. These findings suggest that while a functional circadian clock is not necessary for normal proliferation of neuronal precursor cells, the normal pruning of newly generated neurons in the hippocampus may require a functional circadian clock.

A time to remember: the role of circadian clocks in learning and memory

The circadian system has pronounced influence on learning and memory, manifesting as marked changes in memory acquisition and recall across the day. From a mechanistic perspective, the majority of studies have investigated mammalian hippocampal-dependent learning and memory, as this system is highly tractable. The hippocampus plays a major role in learning and memory, and has the potential to integrate circadian information in many ways, including information from local, independent oscillators, and through circadian modulation of neurogenesis, synaptic remodeling, intracellular cascades, and epigenetic regulation of gene expression. These local processes are combined with input from other oscillatory systems to synergistically augment hippocampal rhythmic function. This overview presents an account of the current state of knowledge on circadian interactions with learning and memory circuitry and provides a framework for those interested in further exploring these interactions.

Sleep and adult neurogenesis: implications for cognition and mood

The hippocampal dentate gyrus plays a critical role in learning and memory throughout life, in part by the integration of adult-born neurons into existing circuits. Neurogenesis in the adult hippocampus is regulated by numerous environmental, physiological, and behavioral factors known to affect learning and memory. Sleep is also important for learning and memory. Here we critically examine evidence from correlation, deprivation, and stimulation studies that sleep may be among those factors that regulate hippocampal neurogenesis. There is mixed evidence for correlations between sleep variables and rates of hippocampal cell proliferation across the day, the year, and the lifespan. There is modest evidence that periods of increased sleep are associated with increased cell proliferation or survival. There is strong evidence that disruptions of sleep exceeding 24 h, by total deprivation, selective REM sleep deprivation, and chronic restriction or fragmentation, significantly inhibit cell proliferation and in some cases neurogenesis. The mechanisms by which sleep disruption inhibits neurogenesis are not fully understood. Although sleep disruption procedures are typically at least mildly stressful, elevated adrenal corticosterone secretion is not necessary for this effect. However, procedures that prevent both elevated corticosterone and interleukin 1β signaling have been found to block the effect of sleep deprivation on cell proliferation. This result suggests that sleep loss impairs hippocampal neurogenesis by the presence of wake-dependent factors, rather than by the absence of sleep-specific processes. This would weigh against a hypothesis that regulation of neurogenesis is a function of sleep. Nonetheless, impaired neurogenesis may underlie some of the memory and mood effects associated with acute and chronic sleep disruptions.

Direction-dependent effects of chronic "jet-lag" on hippocampal neurogenesis

Disruptions in circadian rhythms, as seen in human shift workers, are often associated with many health consequences including impairments in cognitive functions. However, the mechanisms underlying these affects are not well understood. The objective of the present study is to explore the effects of circadian disruption on hippocampal neurogenesis, which has been implicated in learning and memory and could serve as a potential pathway mediating the cognitive consequences associated with rhythm disruption. Circadian rhythm disruptions were introduced using a weekly 6 h phase shifting paradigm, in which male Wistar rats were subjected to either 6 h phase advances (i.e. traveling eastbound from New York to Paris) or 6 h phase delays (i.e. traveling westbound from Paris to New York) in their light/dark schedule every week. The effects of chronic phase shifts on hippocampal neurogenesis were assessed using doublecortin (DCX), a microtubule binding protein expressed in immature neurons. The results revealed that chronic disruption in circadian rhythms inhibits hippocampal neurogenesis, and the degree of reduction in neurogenesis depends upon the direction and duration of the shifts. In two cohorts of animals that experienced phase shifts for either 4 or 8 weeks, a greater decrease in neurogenesis was observed when the phase was advanced versus delayed in both groups. The direction-dependent effect mirrors the findings on clock gene expression in the SCN, suggesting a causal link between the reduction in hippocampal neurogenesis and a disrupted SCN circadian clock.

Inhibition of hippocampal neurogenesis by sleep deprivation is independent of circadian disruption and melatonin suppression

Procedures that restrict or fragment sleep can inhibit neurogenesis in the hippocampus of adult rodents, although the underlying mechanism is unknown. We showed that rapid-eye-movement (REM) sleep deprivation (RSD) by the platform-over-water method inhibits hippocampal cell proliferation in adrenalectomized rats with low-dose corticosterone clamp. This procedure also greatly disrupts daily behavioral rhythms. Given recent evidence for circadian clock regulation of cell proliferation, we asked whether disruption of circadian rhythms might play a role in the anti-neurogenic effects of sleep loss. Male Sprague-Dawley rats were subjected to a 4-day RSD procedure or were exposed to constant bright light (LL) for 4 days or 10 weeks, a non-invasive procedure for eliminating circadian rhythms of behavior and physiology in this species. Proliferating cells in the granule cell layer of the dentate gyrus were identified by immunolabeling for the thymidine analogue 5-bromo-2-deoxyuridine. Consistent with our previous results, the RSD procedure suppressed cell proliferation by ∼50%. By contrast, although LL attenuated or eliminated daily rhythms of activity and sleep-wake without affecting daily amounts of REM sleep, cell proliferation was not affected. Melatonin, a nocturnally secreted neurohormone that is inhibited by light, has been shown to promote survival of new neurons. We found that 3-weeks of LL eliminated daily rhythms and decreased plasma melatonin by 88% but did not significantly affect either total cell survival or survival of new neurons (doublecortin+). Finally, we measured cell proliferation rates at the beginning and near the end of the daily light period in rats entrained to a 12:12 light/lark (LD) cycle, but did not detect a daily rhythm. These results indicate that the antineurogenic effect of RSD is not secondary to disruption of circadian rhythms, and provide no evidence that hippocampal cell proliferation and survival are regulated by the circadian system or by nocturnal secretion of pineal melatonin.