Microglia dynamics in sleep/wake states and in response to sleep loss

NAD+ supplement relieved chronic sleep restriction (CSR)-induced microglial proinflammation in vivo and in vitro

Sleep insufficiency is a significant health problem worldwide and can induce multiple neurodevelopmental disorders in the central nervous system (CNS). Sleep deprivation (SD), especially chronic SD, leads to cognition and memory loss and worsens neurodegenerative disease liability. Microglia are the main inflammation-dominant glia and play a crucial role in SD-induced neurological impairments. Nicotinamide adenine dinucleotide (NAD+) is a redox reaction coenzyme that exerts anti-inflammatory and mitochondria-protective effects in microglia. Whether NAD+ mitigated SD-induced neurological disorders by regulating microglial functions is still unknown. In the current study, we designed an in vivo and in vitro model to evaluate the neuroprotective effect of NAD+ on chronic sleep restriction (CSR) and further investigate the underlying mechanisms. Behavioral tests and immunofluorescence staining were applied to investigate the cognition impairments and microglial activation. Biochemical experiments were tested to analyze the oxidative status and possible mechanism. In vitro data were used to verify the in vivo data. Our results displayed that NAD+ supplement mitigated CSR-induced cognitive decline and microglial activation response by suppressing the expression of pro-inflammatory cytokines in vivo. NAD+ administration also decreased oxidative stress and mitochondrial impairments in microglia. In vitro results showed that NAD+ treatment inhibited ROS production and prompted M1 conversion to M2 phenotype. cGAS-STING/NF-κB pathways were significantly activated but down-regulated by NAD+ administration. H151, a STING antagonist, was applied to validate that NAD+ treatment alleviates neuroinflammation partially by regulating cGAS-STING pathways in microglia. Our findings suggest that NAD+ supplement is a promising therapy for sleep disorders-induced neurological problems, and cGAS-STING pathway may act as a critical regulator in microglial proinflammation.

Current perspectives on microglia-neuron communication in the central nervous system: Direct and indirect modes of interaction

BACKGROUND

The incessant communication that takes place between microglia and neurons is essential the development, maintenance, and pathogenesis of the central nervous system (CNS). As mobile phagocytic cells, microglia serve a critical role in surveilling and scavenging the neuronal milieu to uphold homeostasis.

AIM OF REVIEW

This review aims to discuss the various mechanisms that govern the interaction between microglia and neurons, from the molecular to the organ system level, and to highlight the importance of these interactions in the development, maintenance, and pathogenesis of the CNS.

KEY SCIENTIFIC CONCEPTS OF REVIEW

Recent research has revealed that microglia-neuron interaction is vital for regulating fundamental neuronal functions, such as synaptic pruning, axonal remodeling, and neurogenesis. The review will elucidate the intricate signaling pathways involved in these interactions, both direct and indirect, to provide a better understanding of the fundamental mechanisms of brain function. Furthermore, gaining insights into these signals could lead to the development of innovative therapies for neural disorders.

Sleep disorders and risk of alzheimer's disease: A two-way road

Substantial sleep impairment in patients with Alzheimer's disease (AD) is one of the emerging points for continued efforts to better understand the disease. Individuals without cognitive decline, an important marker of the clinical phase of AD, may show early alterations in the sleep-wake cycle. The objective of this critical narrative review is to explore the bidirectional pathophysiological correlation between sleep disturbances and Alzheimer's Disease. Specifically, it examines how the disruption of sleep homeostasis in individuals without dementia could contribute to the pathogenesis of AD, and conversely, how neurodegeneration in individuals with Alzheimer's Disease might lead to dysregulation of the sleep-wake cycle. Recent scientific results indicate that sleep disturbances, particularly those related to impaired glymphatic clearance, may act as an important mechanism associated with the genesis of Alzheimer's Disease. Additionally, amyloid deposition and tau protein hyperphosphorylation, along with astrocytic hyperactivation, appear to trigger changes in neurotransmission dynamics in areas related to sleep, which may explain the onset of sleep disturbances in individuals with AD. Disruption of sleep homeostasis appears to be a modifiable risk factor in Alzheimer's disease. Whenever possible, the use of non-pharmacological strategies becomes important in this context. From a different perspective, additional research is needed to understand and treat the dysfunction of the sleep-wake cycle in individuals already affected by AD. Early recognition and correction of sleep disturbances in this population could potentially mitigate the progression of dementia and improve the quality of life for those with AD.

Subclinical brain manifestations of repeated mild traumatic brain injury are changed by chronic exposure to sleep loss, caffeine, and sleep aids

INTRODUCTION

After mild traumatic brain injury (mTBI), the brain is labile for weeks and months and vulnerable to repeated concussions. During this time, patients are exposed to everyday circumstances that, in themselves, affect brain metabolism and blood flow and neural processing. How commonplace activities interact with the injured brain is unknown. The present study in an animal model investigated the extent to which three commonly experienced exposures-daily caffeine usage, chronic sleep loss, and chronic sleep aid medication-affect the injured brain in the chronic phase.

METHODS

Subclinical trauma by repeated mTBIs was produced by our head rotational acceleration injury model, which causes brain injury consistent with the mechanism of concussion in humans. Forty-eight hours after a third mTBI, chronic administrations of caffeine, sleep restriction, or zolpidem (sedative hypnotic) began and were continued for 70 days. On Days 30 and 60 post injury, resting state functional magnetic resonance imaging (fMRI) and diffusion tensor imaging (DTI) were performed.

RESULTS

Chronic caffeine, sleep restriction, and zolpidem each changed the subclinical brain characteristics of mTBI at both 30 and 60 days post injury, detected by different MRI modalities. Each treatment caused microstructural alterations in DTI metrics in the insular cortex and retrosplenial cortex compared with mTBI, but also uniquely affected other gray and white matter regions. Zolpidem administration affected the largest number of individual structures in mTBI at both 30 and 60 days, and not necessarily toward normalization (sham treatment). Chronic sleep restriction changed local functional connectivity at 30 days in diametrical opposition to chronic caffeine ingestion, and both treatment outcomes were different from sham, mTBI-only and zolpidem comparisons. The results indicate that commonly encountered exposures modify subclinical brain activity and structure long after healing is expected to be complete.

CONCLUSIONS

Changes in activity and structure detected by fMRI are widely understood to reflect changes in the functions of the affected region which conceivably underlie mTBI neuropathology and symptomatology in the chronic phase after injury.

A global transcriptional atlas of the effect of acute sleep deprivation in the mouse frontal cortex

Sleep deprivation (SD) has negative effects on brain and body function. Sleep problems are prevalent in a variety of disorders, including neurodevelopmental and psychiatric conditions. Thus, understanding the molecular consequences of SD is of fundamental importance in biology. In this study, we present the first simultaneous bulk and single-nuclear RNA sequencing characterization of the effects of SD in the male mouse frontal cortex. We show that SD predominantly affects glutamatergic neurons, specifically in layers 4 and 5, and produces isoform switching of over 1500 genes, particularly those involved in splicing and RNA binding. At both the global and cell-type specific level, SD has a large repressive effect on transcription, downregulating thousands of genes and transcripts. As a resource we provide extensive characterizations of cell-types, genes, transcripts, and pathways affected by SD. We also provide publicly available tutorials aimed at allowing readers adapt analyses performed in this study to their own datasets.

Geraniol (GER) attenuated chronic sleep restriction (CSR)-induced neuroinflammation in adolescent mice

Sleep insufficiency is a significant health problem worldwide, and adolescent sleep restriction (SR) could induce multiple neurodevelopmental disorders in the central nervous system (CNS). Microglial-mediated neuroinflammation plays a vital role in multiple neurological diseases, and recent research showed the regulation effect of immunoproteasome on microglia functions. Geraniol (GER), an important ingredient in many essential oils, possesses diverse pharmacological properties like anti-inflammatory and antioxidant. The present study was designed to evaluate the neuroprotective effect of GER on SR in adolescent mice and further investigate the underlying mechanisms. Our results displayed that 14 days of chronic sleep restriction (CSR) induced cognitive decline, and anxiety-like and attention-deficit behaviors, which were mitigated by GER pretreatment. GER administration also reversed microglial pro-inflammatory response under CSR stimulation in the anterior cingulate cortex (ACC) regions by reducing the expression and secretion of cytokines like IL-1β and TNF-α. Mechanism research showed that LMP7 mRNA was selectively up-regulated under CSR treatment but down-regulated by GER administration. Proteasome activity and protein expression of LMP7 were consistent with mRNA data. ONX-0914 was applied to inhibit LMP7 selectively, and data validated that GER might alleviate CSR-induced neuroinflammation by regulating LMP7. Our study provides evidence that LMP7 is a critical regulator of CSR-induced proinflammation, and geraniol might be a promising therapy against CSR-induced neurodevelopmental disorders.

Microglia undergo molecular and functional adaptations to dark and light phases in male laboratory mice

Microglia are increasingly recognized to contribute to brain health and disease. Preclinical studies using laboratory rodents are essential to advance our understanding of the physiological and pathophysiological roles of these cells in the central nervous system. Rodents are nocturnal animals, and they are mostly maintained in a defined light-dark cycle within animal facilities, with many laboratories investigating the molecular and functional profiles of microglia exclusively during the animals' light (sleep) phase. However, only a few studies have considered possible differences in microglial functions between the active and sleep phases. Based on initial evidence suggesting that microglial intrinsic clock genes can affect their phenotypes, we sought to investigate differences in transcriptional, proteotype and functional profiles of microglia between light (sleep) and dark (active) phases, and how these changes are affected in pathological models. We found marked transcriptional and proteotype differences between microglia harvested from male mice during the light or dark phase. Amongst others, these differences related to genes and proteins associated with immune responses, motility, and phagocytosis, which were reflected by functional alterations in microglial synaptic pruning and response to bacterial stimuli. Possibly accounting for such changes, we found RNA and protein regulation in SWI/SNF and NuRD chromatin remodeling complexes between light and dark phases. Importantly, we also show that the time of microglial sample collection influences the nature of microglial transcriptomic changes in a model of immune-mediated neurodevelopmental disorders. Our findings emphasize the importance of considering diurnal factors in studying microglial cells and indicate that implementing a circadian perspective is pivotal for advancing our understanding of their physiological and pathophysiological roles in brain health and disease.

Effects of sleep deprivation on anxiety-depressive-like behavior and neuroinflammation

BACKGROUND

Depression is defined by a persistent low mood and disruptions in sleep patterns, with the WHO forecasting that major depression will rank as the third most prevalent contributor to the global burden of disease by the year 2030. Sleep deprivation serves as a stressor that triggers inflammation within the central nervous system, a process known as neuroinflammation. This inflammatory response plays a crucial role in the development of depression by upregulating the expression of inflammatory mediators that contribute to symptoms such as anxiety, hopelessness, and loss of pleasure.

METHODS

In this study, sleep deprivation was utilized as a method to induce anxiety and depressive-like behaviors in mice. The behavioral changes in the mice were then evaluated using the EZM, EPM, TST, FST, and SPT. H&E staining and Nissl staining was used to detect morphological changes in the medial prefrontal cortical (mPFC) regions. Elisa to assess serum CORT levels. Detection of mRNA levels and protein expression of clock genes, high mobility genome box-1 (Hmgb1), silent message regulator 6 (Sirt6), and pro-inflammatory factors by RT-qPCR, Western blotting, and immunofluorescence techniques.

RESULTS

Sleep deprivation resulted in decreased exploration of unfamiliar territory, increased time spent in a state of despair, and lower sucrose water intake in mice. Additionally, sleep deprivation led to increased secretion of serum CORT and upregulation of clock genes, IL6, IL1β, TNFα, Cox-2, iNOS, Sirt6, and Hmgb1. Sleep.

CONCLUSIONS

Sleep deprivation induces anxiety-depressive-like behaviors and neuroinflammation in the brain. Transcription of clock genes and activation of the Sirt6/Hmgb1 pathway may contribute to inflammatory responses in the mPFC.

Sleep rebound leads to marked recovery of prolonged sleep deprivation-induced adversities in the stress response and hippocampal neuroplasticity of male rats

BACKGROUND

Sleep disturbances are not only frequent symptoms, but also risk factors for major depressive disorder. We previously reported that depressed patients who experienced "Hypersomnia" showed a higher and more rapid response rate under paroxetine treatment, but the underlying mechanism remains unclear. The present study was conducted to clarify the beneficial effects of sleep rebound through an experimental "Hypersomnia" rat model on glucocorticoid and hippocampal neuroplasticity associated with antidepressive potency.

METHODS

Thirty-four male Sprague-Dawley rats were subjected to sham treatment, 72-h sleep deprivation, or sleep deprivation and subsequent follow-up for one week. Approximately half of the animals were sacrificed to evaluate adrenal weight, plasma corticosterone level, hippocampal content of mRNA isoforms, and protein of the brain-derived neurotrophic factor (Bdnf) gene. In the other half of the rats, Ki-67- and doublecortin (DCX)-positive cells in the hippocampus were counted via immunostaining to quantify adult neurogenesis.

RESULTS

Prolonged sleep deprivation led to adrenal hypertrophy and an increase in the plasma corticosterone level, which had returned to normal after one week follow-up. Of note, sleep deprivation-induced decreases in hippocampal Bdnf transcripts containing exons II, IV, VI, and IX and BDNF protein levels, Ki-67-(+)-proliferating cells, and DCX-(+)-newly-born neurons were not merely reversed, but overshot their normal levels with sleep rebound.

LIMITATIONS

The present study did not record electroencephalogram or assess behavioral changes of the sleep-deprived rats.

CONCLUSIONS

The present study demonstrated that prolonged sleep deprivation-induced adversities are reversed or recovered by sleep rebound, which supports "Hypersomnia" in depressed patients as having a beneficial pharmacological effect.

Quantifying microglial morphology: an insight into function

Microglia are specialized immune cells unique to the central nervous system (CNS). Microglia have a highly plastic morphology that changes rapidly in response to injury or infection. Qualitative and quantitative measurements of ever-changing microglial morphology are considered a cornerstone of many microglia-centric research studies. The distinctive morphological variations seen in microglia are a useful marker of inflammation and severity of tissue damage. Although a wide array of damage-associated microglial morphologies has been documented, the exact functions of these distinct morphologies are not fully understood. In this review, we discuss how microglia morphology is not synonymous with microglia function, however, morphological outcomes can be used to make inferences about microglial function. For a comprehensive examination of the reactive status of a microglial cell, both histological and genetic approaches should be combined. However, the importance of quality immunohistochemistry-based analyses should not be overlooked as they can succinctly answer many research questions.

Microglia regulate sleep through calcium-dependent modulation of norepinephrine transmission

Sleep interacts reciprocally with immune system activity, but its specific relationship with microglia-the resident immune cells in the brain-remains poorly understood. Here, we show in mice that microglia can regulate sleep through a mechanism involving Gi-coupled GPCRs, intracellular Ca2+ signaling and suppression of norepinephrine transmission. Chemogenetic activation of microglia Gi signaling strongly promoted sleep, whereas pharmacological blockade of Gi-coupled P2Y12 receptors decreased sleep. Two-photon imaging in the cortex showed that P2Y12-Gi activation elevated microglia intracellular Ca2+, and blockade of this Ca2+ elevation largely abolished the Gi-induced sleep increase. Microglia Ca2+ level also increased at natural wake-to-sleep transitions, caused partly by reduced norepinephrine levels. Furthermore, imaging of norepinephrine with its biosensor in the cortex showed that microglia P2Y12-Gi activation significantly reduced norepinephrine levels, partly by increasing the adenosine concentration. These findings indicate that microglia can regulate sleep through reciprocal interactions with norepinephrine transmission.

Microglia and Sleep Disorders

Sleep is a physiological state that is essential for maintaining physical and mental health. Sleep disorders and sleep deprivation therefore have many adverse effects, including an increased risk of metabolic diseases and a decline in cognitive function that may be implicated in the long-term development of neurodegenerative diseases. There is increasing evidence that microglia, the resident immune cells of the central nervous system (CNS), are involved in regulating the sleep-wake cycle and the CNS response to sleep alteration and deprivation. In this chapter, we will discuss the involvement of microglia in various sleep disorders, including sleep-disordered breathing, insomnia, narcolepsy, myalgic encephalomyelitis/chronic fatigue syndrome, and idiopathic rapid-eye-movement sleep behavior disorder. We will also explore the impact of acute and chronic sleep deprivation on microglial functions. Moreover, we will look into the potential involvement of microglia in sleep disorders as a comorbidity to Alzheimer's disease and Parkinson's disease.

Microglial Regulation of Sleep and Wakefulness

Sleep serves a multitude of roles in brain maturation and function. Although the neural networks involved in sleep regulation have been extensively characterized, it is increasingly recognized that neurons are not the sole conductor orchestrating the rhythmic cycle of sleep and wakefulness. In the central nervous system, microglia have emerged as an important player in sleep regulation. Within the last two decades, microglia have gained substantial attention for carrying out numerous nonimmune tasks that are crucial for brain development and function by co-opting similar mechanisms used in their conventional immune functions. Here, we highlight the importance of microglia in sleep regulation with recent findings reporting an arrhythmic sleep/wake cycle in the absence of microglia. Although the underlying mechanisms for such regulation are still being uncovered, it is likely that microglial contributions to the rhythmic control of the sleep/wake cycle come from their influence on synaptic strength and neuronal activity. We review the current literature to provide speculative signaling pathways and suggest key questions for future research. Advancing our knowledge of the mechanistic contribution of microglia to sleep regulation will not only further our insight into this critical biological process but also be instrumental in providing novel therapeutic strategies for sleep disorders.

Imaging microglia surveillance during sleep-wake cycles in freely behaving mice

Microglia surveillance manifests itself as dynamic changes in cell morphology and functional remodeling. Whether and how microglia surveillance is coupled to brain state switches during natural sleep-wake cycles remains unclear. To address this question, we used miniature two-photon microscopy (mTPM) to acquire time-lapse high-resolution microglia images of the somatosensory cortex, along with EEG/EMG recordings and behavioral video, in freely-behaving mice. We uncovered fast and robust brain state-dependent changes in microglia surveillance, occurring in parallel with sleep dynamics and early-onset phagocytic microglial contraction during sleep deprivation stress. We also detected local norepinephrine fluctuation occurring in a sleep state-dependent manner. We showed that the locus coeruleus-norepinephrine system, which is crucial to sleep homeostasis, is required for both sleep state-dependent and stress-induced microglial responses and β2-adrenergic receptor signaling plays a significant role in this process. These results provide direct evidence that microglial surveillance is exquisitely tuned to signals and stressors that regulate sleep dynamics and homeostasis so as to adjust its varied roles to complement those of neurons in the brain. In vivo imaging with mTPM in freely behaving animals, as demonstrated here, opens a new avenue for future investigation of microglia dynamics and sleep biology in freely behaving animals.

Sleep restoration by optogenetic targeting of GABAergic neurons reprograms microglia and ameliorates pathological phenotypes in an Alzheimer's disease model

BACKGROUND

Alzheimer's disease (AD) patients exhibit memory disruptions and profound sleep disturbances, including disruption of deep non-rapid eye movement (NREM) sleep. Slow-wave activity (SWA) is a major restorative feature of NREM sleep and is important for memory consolidation.

METHODS

We generated a mouse model where GABAergic interneurons could be targeted in the presence of APPswe/PS1dE9 (APP) amyloidosis, APP-GAD-Cre mice. An electroencephalography (EEG) / electromyography (EMG) telemetry system was used to monitor sleep disruptions in these animals. Optogenetic stimulation of GABAergic interneurons in the anterior cortex targeted with channelrhodopsin-2 (ChR2) allowed us to examine the role GABAergic interneurons play in sleep deficits. We also examined the effect of optogenetic stimulation on amyloid plaques, neuronal calcium as well as sleep-dependent memory consolidation. In addition, microglial morphological features and functions were assessed using confocal microscopy and flow cytometry. Finally, we performed sleep deprivation during optogenetic stimulation to investigate whether sleep restoration was necessary to slow AD progression.

RESULTS

APP-GAD-Cre mice exhibited impairments in sleep architecture including decreased time spent in NREM sleep, decreased delta power, and increased sleep fragmentation compared to nontransgenic (NTG) NTG-GAD-Cre mice. Optogenetic stimulation of cortical GABAergic interneurons increased SWA and rescued sleep impairments in APP-GAD-Cre animals. Furthermore, it slowed AD progression by reducing amyloid deposition, normalizing neuronal calcium homeostasis, and improving memory function. These changes were accompanied by increased numbers and a morphological transformation of microglia, elevated phagocytic marker expression, and enhanced amyloid β (Aβ) phagocytic activity of microglia. Sleep was necessary for amelioration of pathophysiological phenotypes in APP-GAD-Cre mice.

CONCLUSIONS

In summary, our study shows that optogenetic targeting of GABAergic interneurons rescues sleep, which then ameliorates neuropathological as well as behavioral deficits by increasing clearance of Aβ by microglia in an AD mouse model.

Mechanisms of Activation of Brain's Drainage during Sleep: The Nightlife of Astrocytes

The study of functions, mechanisms of generation, and pathways of movement of cerebral fluids has a long history, but the last decade has been especially productive. The proposed glymphatic hypothesis, which suggests a mechanism of the brain waste removal system (BWRS), caused an active discussion on both the criticism of some of the perspectives and our intensive study of new experimental facts. It was especially found that the intensity of the metabolite clearance changes significantly during the transition between sleep and wakefulness. Interestingly, at the cellular level, a number of aspects of this problem have been focused on, such as astrocytes-glial cells, which, over the past two decades, have been recognized as equal partners of neurons and perform many important functions. In particular, an important role was assigned to astrocytes within the framework of the glymphatic hypothesis. In this review, we return to the "astrocytocentric" view of the BWRS function and the explanation of its activation during sleep from the viewpoint of new findings over the last decade. Our main conclusion is that the BWRS's action may be analyzed both at the systemic (whole-brain) and at the local (cellular) level. The local level means here that the neuro-glial-vascular unit can also be regarded as the smallest functional unit of sleep, and therefore, the smallest functional unit of the BWRS.

Hsp70 ameliorates sleep deprivation-induced anxiety-like behavior and cognitive impairment in mice

BACKGROUND

Many neurobehavioral processes, including psychomotor, cognitive, and affection are negatively impacted by sleep deprivation (SD), which may be harmful to a person's physical and mental health. Heat shock proteins (Hsps) have been demonstrated to play a protective role in a number of neurodegenerative diseases and are essential for maintaining intracellular protein homeostasis, but their roles in SD remain elusive.

METHODS

A mouse SD model was constructed using a modified multi-platform water environment method. The cognitive function was tested by novel object recognition test and Y-maze test, and anxiety-like behaviors were assessed by open field test (OFT). Protein expression was determined by Western blotting assay and ELISA assay.

RESULTS

We found that SD could profoundly enhance anxiety levels and impair cognitive function in mice. SD also reduced the expression levels of p-cAMP-response element binding protein (CREB) and brain-derived neurotrophic factor (BDNF) and increased microglial activation and neuroinflammatory response in the hippocampus of mice. The intranasal injection of human recombinant Hsp70 protein could alleviate SD-induced anxiety and cognitive impairment, as well as restore pCREB and BDNF levels and reduce microglia-induced neuroinflammation in the hippocampus of SD mice.

CONCLUSIONS

Hsp70 treatment might serve as a potential treatment for mitigating SD-related unfavorable symptoms.

Sleep deprivation enhances amyloid beta peptide, p-tau and serotonin in the brain: Neuroprotective effects of nanowired delivery of cerebrolysin with monoclonal antibodies to amyloid beta peptide, p-tau and serotonin

Sleep deprivation is quite frequent in military during combat, intelligence gathering or peacekeeping operations. Even one night of sleep deprivation leads to accumulation of amyloid beta peptide burden that would lead to precipitation of Alzheimer's disease over the years. Thus, efforts are needed to slow down or neutralize accumulation of amyloid beta peptide (AβP) and associated Alzheimer's disease brain pathology including phosphorylated tau (p-tau) within the brain fluid environment. Sleep deprivation also alters serotonin (5-hydroxytryptamine) metabolism in the brain microenvironment and impair upregulation of several neurotrophic factors. Thus, blockade or neutralization of AβP, p-tau and serotonin in sleep deprivation may attenuate brain pathology. In this investigation this hypothesis is examined using nanodelivery of cerebrolysin- a balanced composition of several neurotrophic factors and active peptide fragments together with monoclonal antibodies against AβP, p-tau and serotonin (5-hydroxytryptamine, 5-HT). Our observations suggest that sleep deprivation induced pathophysiology is significantly reduced following nanodelivery of cerebrolysin together with monoclonal antibodies to AβP, p-tau and 5-HT, not reported earlier.

Dysfunctional synaptic pruning by microglia correlates with cognitive impairment in sleep-deprived mice: Involvement of CX3CR1 signaling

Microglia are involved in sleep/wake cycles and the response to sleep loss. Synaptic pruning by microglia is necessary for central nervous system circuit refinement and contributes to cognitive function. Here, we investigated whether and how microglia-mediated synaptic pruning may be involved in cognitive deficits induced by sleep deprivation in mice. Mice were deprived of sleep by leaving them in a spontaneously rotating rod for 72 h, after which their cognitive function was assessed using an object location test, Y maze, and novel object recognition test. Sleep deprivation lowered the discrimination index for familiar locations in the object location test and Y maze. Microglial morphology was assessed using immunostaining Iba1, while microglia-mediated synaptic pruning was examined based on immunostaining PSD95, CD68, and Iba1. Sleep deprivation also activated microglial cells in the hippocampus, as reflected in bigger soma as well as fewer and shorter branches than normal sleep. Sleep deprivation downregulated phagocytic markers and internalization of postsynaptic protein 95 (PSD95), suggesting impaired synaptic pruning. CX3C motif chemokine receptor 1 (CX3CR1) signaling was detected in in vitro experiments. Sleep deprivation also downregulated CX3CR1. Activation of CX3CR1 signaling increased phagocytosis activity of BV2 microglia in vitro. Sleep deprivation dysregulates microglial CX3CR1 signaling and inhibits synaptic pruning, contributing to associated cognitive deficits. These findings identify CX3CR1-dependent synaptic pruning as a potential therapeutic target in which sleep deprivation causes recognition impairments.

Alcohol induces apoptosis and autophagy in microglia BV-2 cells

Alcohol (ethanol) has proven to be toxic to nearly all organs, with the brain being one of the principal targets. As one of the important components of the brain's blood-brain barrier (BBB) and central nervous system, the state of microglia may be associated with some symptoms of alcohol intoxication. In the present study, microglia BV-2 cells were exposed to various concentrations of alcohol for 3 or 12 h, imitating different stages of drunkenness after alcohol use, respectively. From the perspective of the autophagy-phagocytosis axis, our findings show that alcohol alters autophagy levels or promotes apoptosis in BV-2 cells. The current study adds to the understanding of the action mechanisms of alcohol neurotoxicity. We anticipate that this study will increase public awareness of alcohol's negative effects and contribute to the creation of novel alcoholism treatment approaches.

Understanding neuron-glia crosstalk and biological clocks in insomnia

According to the World Health Organization, about one-third of the population experiences insomnia symptoms, and about 10-15% suffer from chronic insomnia, the most common sleep disorder. Sleeping difficulties associated with insomnia are often linked to chronic sleep deprivation, which has a negative health impact partly due to disruption in the internal synchronisation of biological clocks. These are regulated by clock genes and modulate most biological processes. Most studies addressing circadian rhythm regulation have focused on the role of neurons, yet glial cells also impact circadian rhythms and sleep regulation. Chronic insomnia and sleep loss have been associated with glial cell activation, exacerbated neuroinflammation, oxidative stress, altered neuronal metabolism and synaptic plasticity, accelerated age-related processes and decreased lifespan. It is, therefore, essential to highlight the importance of glia-neuron interplay on sleep/circadian regulation and overall healthy brain function. Hence, in this review, we aim to address the main neurobiological mechanisms involved in neuron-glia crosstalk, with an emphasis on microglia and astrocytes, in both healthy sleep, chronic sleep deprivation and chronic insomnia.

Myeloid deficiency of the intrinsic clock protein BMAL1 accelerates cognitive aging by disrupting microglial synaptic pruning

Aging is associated with loss of circadian immune responses and circadian gene transcription in peripheral macrophages. Microglia, the resident macrophages of the brain, also show diurnal rhythmicity in regulating local immune responses and synaptic remodeling. To investigate the interaction between aging and microglial circadian rhythmicity, we examined mice deficient in the core clock transcription factor, BMAL1. Aging Cd11bcre;Bmallox/lox mice demonstrated accelerated cognitive decline in association with suppressed hippocampal long-term potentiation and increases in immature dendritic spines. C1q deposition at synapses and synaptic engulfment were significantly decreased in aging Bmal1-deficient microglia, suggesting that BMAL1 plays a role in regulating synaptic pruning in aging. In addition to accelerated age-associated hippocampal deficits, Cd11bcre;Bmallox/lox mice also showed deficits in the sleep-wake cycle with increased wakefulness across light and dark phases. These results highlight an essential role of microglial BMAL1 in maintenance of synapse homeostasis in the aging brain.

The Putative Role of Neuroinflammation in the Interaction between Traumatic Brain Injuries, Sleep, Pain and Other Neuropsychiatric Outcomes: A State-of-the-Art Review

Sleep disturbances are widely prevalent following a traumatic brain injury (TBI) and have the potential to contribute to numerous post-traumatic physiological, psychological, and cognitive difficulties developing chronically, including chronic pain. An important pathophysiological mechanism involved in the recovery of TBI is neuroinflammation, which leads to many downstream consequences. While neuroinflammation is a process that can be both beneficial and detrimental to individuals' recovery after sustaining a TBI, recent evidence suggests that neuroinflammation may worsen outcomes in traumatically injured patients, as well as exacerbate the deleterious consequences of sleep disturbances. Additionally, a bidirectional relationship between neuroinflammation and sleep has been described, where neuroinflammation plays a role in sleep regulation and, in turn, poor sleep promotes neuroinflammation. Given the complexity of this interplay, this review aims to clarify the role of neuroinflammation in the relationship between sleep and TBI, with an emphasis on long-term outcomes such as pain, mood disorders, cognitive dysfunctions, and elevated risk of Alzheimer's disease and dementia. In addition, some management strategies and novel treatment targeting sleep and neuroinflammation will be discussed in order to establish an effective approach to mitigate long-term outcomes after TBI.

Disturbance of REM sleep exacerbates microglial activation in APP/PS1 mice

Although both nonrapid eye movement (NREM) sleep loss and rapid eye movement (REM) sleep loss exacerbate Alzheimer's disease (AD) progression, they exert different effects. Microglial activation can be beneficial or detrimental to AD patients under different conditions. However, few studies have investigated which sleep stage is the main regulator of microglial activation or the downstream effects of this activation. We aimed to explore the roles of different sleep phases in microglial activation and to investigate the possible effect of microglial activation on AD pathology. In this study, thirty-six 6-month-old APP/PS1 mice were equally divided into 3 groups: the stress control (SC), total sleep deprivation (TSD), and REM deprivation (RD) groups. All mice underwent a 48-hour intervention before their spatial memory was assessed using a Morris water maze (MWM). Then, microglial morphology, activation- and synapse-related protein expression, and inflammatory cytokine and amyloid β (Aβ) levels in hippocampal tissues were measured. We found that the RD and TSD groups exhibited worse spatial memory in the MWM tests. In addition, the RD and TSD groups showed greater microglial activation, higher inflammatory cytokine levels, lower synapse-related protein expression and more severe Aβ accumulation than the SC group, but there were no significant differences between the RD and TSD groups. This study demonstrates that disturbance of REM sleep may activate microglia in APP/PS1 mice. These activated microglia may promote neuroinflammation and engulf synapses but show a weakened ability to clear plaques.

Increased interaction between endoplasmic reticulum and mitochondria following sleep deprivation

BACKGROUND

Prolonged cellular activity may overload cell function, leading to high rates of protein synthesis and accumulation of misfolded or unassembled proteins, which cause endoplasmic reticulum (ER) stress and activate the unfolded protein response (UPR) to re-establish normal protein homeostasis. Previous molecular work has demonstrated that sleep deprivation (SD) leads to ER stress in neurons, with a number of ER-specific proteins being upregulated to maintain optimal cellular proteostasis. It is still not clear which cellular processes activated by sleep deprivation lead to ER- stress, but increased cellular metabolism, higher request for protein synthesis, and over production of oxygen radicals have been proposed as potential contributing factors. Here, we investigate the transcriptional and ultrastructural ER and mitochondrial modifications induced by sleep loss.

RESULTS

We used gene expression analysis in mouse forebrains to show that SD was associated with significant transcriptional modifications of genes involved in ER stress but also in ER-mitochondria interaction, calcium homeostasis, and mitochondrial respiratory activity. Using electron microscopy, we also showed that SD was associated with a general increase in the density of ER cisternae in pyramidal neurons of the motor cortex. Moreover, ER cisternae established new contact sites with mitochondria, the so-called mitochondria associated membranes (MAMs), important hubs for molecule shuttling, such as calcium and lipids, and for the modulation of ATP production and redox state. Finally, we demonstrated that Drosophila male mutant flies (elav > linker), in which the number of MAMs had been genetically increased, showed a reduction in the amount and consolidation of sleep without alterations in the homeostatic sleep response to SD.

CONCLUSIONS

We provide evidence that sleep loss induces ER stress characterized by increased crosstalk between ER and mitochondria. MAMs formation associated with SD could represent a key phenomenon for the modulation of multiple cellular processes that ensure appropriate responses to increased cell metabolism. In addition, MAMs establishment may play a role in the regulation of sleep under baseline conditions.

Melatonin upregulates BMAL1 to attenuate chronic sleep deprivation-related cognitive impairment by alleviating oxidative stress

PURPOSE

To investigate the mechanism underlying the regulatory effect of melatonin on chronic sleep deprivation-related cognitive impairment.

METHODS

Chronic sleep deprivation (CSD) model was established using the MMPM method. After the model was established, melatonin receptor agonist and inhibitor were given, respectively. Water maze was conducted to record the escape latency and the duration of crossing the platform of space exploration. The concentration of TNF-α, IL-6, MDA, and SOD was measured by ELISA. Immunofluorescence was used to determine the expression level of CD86 and CD206, while the mRNA expression of Bax, Bcl-2, P65, IκB, and BMAL1 was detected by qPCR. Western blotting assay was utilized to determine the protein expression of Bax, Bcl-2, P65, p-P65, IκB, p-I κB, and BMAL1.

RESULTS

Compared with the control, the escape latency was greatly increased on the second and third day, accompanied by the increased expression of TNF-α, IL-6, MDA, and SOD in serum. Furthermore, dramatically upregulated Bax, Bcl-2, P65, IκB, and CD86 were observed in the model group, accompanied by the declined expression level of BMAL1 and CD206. Compared with the model group, the escape latency was declined, the concentration of TNF-α, IL-6, MDA, and SOD was decreased, the expression level of Bax, Bcl-2, P65, IκB, and CD86 was declined, and the level of BMAL1 and CD206 was promoted by the treatment of the melatonin agonist, while the opposite results were observed under the treatment of the melatonin inhibitor.

CONCLUSION

Melatonin upregulates BMAL1 to attenuate chronic sleep deprivation-related cognitive impairment by alleviating oxidative stress.

Reactogenic sleepiness after COVID-19 vaccination. A hypothesis involving orexinergic system linked to inflammatory signals

Coronavirus disease 2019 (COVID-19) represents a global healthcare crisis that has led to morbidity and mortality on an unprecedented scale. While studies on COVID-19 vaccines are ongoing, the knowledge about the reactogenic symptoms that can occur after vaccination and its generator mechanisms can be critical for healthcare professionals to improve compliance with the future vaccination campaign. Because sleep and immunity are bidirectionally linked, sleepiness or sleep disturbance side effects reported after some of the COVID-19 vaccines advise an academic research line in the context of physiological or pathological neuroimmune interactions. On the recognized basis of inflammatory regulation of hypothalamic neurons in sickness behavior, we hypothesized that IL-1β, INF-γ and TNF-α pro-inflammatory cytokines inhibit orexinergic neurons promoting sleepiness after peripheral activation of the innate immune system induced by the novel COVID-19 vaccines. In addition, based on knowledge of previous vaccines and disease manifestations of SARS-CoV-2 infection, it also suggests that narcolepsy must be included as potential adverse events of particular interest to consider in pharmacovigilance studies.

Cognitive dysfunction in SLE: An understudied clinical manifestation

Neuropsychiatric lupus (NPSLE) is a debilitating manifestation of SLE which occurs in a majority of SLE patients and has a variety of clinical manifestations. In the central nervous system, NPSLE may result from ischemia or penetration of inflammatory mediators and neurotoxic antibodies through the blood brain barrier (BBB). Here we focus on cognitive dysfunction (CD) as an NPSLE manifestation; it is common, underdiagnosed, and without specific therapy. For a very long time, clinicians ignored cognitive dysfunction and researchers who might be interested in the question struggled to find an approach to understanding mechanisms for this manifestation. Recent years, however, propelled by a more patient-centric approach to disease, have seen remarkable progress in our understanding of CD pathogenesis. This has been enabled through the use of novel imaging modalities and numerous mouse models. Overall, these studies point to a pivotal role of an impaired BBB and microglial activation in leading to neuronal injury. These insights suggest potential therapeutic modalities and make possible clinical trials for cognitive impairment.

Microglia Are Necessary to Regulate Sleep after an Immune Challenge

Microglia play a critical role in the neuroimmune response, but little is known about the role of microglia in sleep following an inflammatory trigger. Nevertheless, decades of research have been predicated on the assumption that an inflammatory trigger increases sleep through microglial activation. We hypothesized that mice (n = 30) with depleted microglia using PLX5622 (PLX) would sleep less following the administration of lipopolysaccharide (LPS) to induce inflammation. Brains were collected and microglial morphology was assessed using quantitative skeletal analyses and physiological parameters were recorded using non-invasive piezoelectric cages. Mice fed PLX diet had a transient increase in sleep that dissipated by week 2. Subsequently, following a first LPS injection (0.4 mg/kg), mice with depleted microglia slept more than mice on the control diet. All mice were returned to normal rodent chow to repopulate microglia in the PLX group (10 days). Nominal differences in sleep existed during the microglia repopulation period. However, following a second LPS injection, mice with repopulated microglia slept similarly to control mice during the dark period but with longer bouts during the light period. Comparing sleep after the first LPS injection to sleep after the second LPS injection, controls exhibited temporal changes in sleep patterns but no change in cumulative minutes slept, whereas cumulative sleep in mice with repopulated microglia decreased during the dark period across all days. Repopulated microglia had a reactive morphology. We conclude that microglia are necessary to regulate sleep after an immune challenge.

Possible Neuropathology of Sleep Disturbance Linking to Alzheimer's Disease: Astrocytic and Microglial Roles

Sleep disturbances not only deteriorate Alzheimer’s disease (AD) progress by affecting cognitive states but also accelerate the neuropathological changes of AD. Astrocytes and microglia are the principal players in the regulation of both sleep and AD. We proposed that possible astrocyte-mediated and microglia-mediated neuropathological changes of sleep disturbances linked to AD, such as astrocytic adenosinergic A1, A2, and A3 regulation; astrocytic dopamine and serotonin; astrocyte-mediated proinflammatory status (TNFα); sleep disturbance-attenuated microglial CX3CR1 and P2Y12; microglial Iba-1 and astrocytic glial fibrillary acidic protein (GFAP); and microglia-mediated proinflammatory status (IL-1b, IL-6, IL-10, and TNFα). Furthermore, astrocytic and microglial amyloid beta (Aβ) and tau in AD were reviewed, such as astrocytic Aβ interaction in AD; astrocyte-mediated proinflammation in AD; astrocytic interaction with Aβ in the central nervous system (CNS); astrocytic apolipoprotein E (ApoE)-induced Aβ clearance in AD, as well as microglial Aβ clearance and aggregation in AD; proinflammation-induced microglial Aβ aggregation in AD; microglial-accumulated tau in AD; and microglial ApoE and TREM2 in AD. We reviewed astrocytic and microglial roles in AD and sleep, such as astrocyte/microglial-mediated proinflammation in AD and sleep; astrocytic ApoE in sleep and AD; and accumulated Aβ-triggered synaptic abnormalities in sleep disturbance. This review will provide a possible astrocytic and microglial mechanism of sleep disturbance linked to AD.

Microglia are involved in the protection of memories formed during sleep deprivation

Sleep deprivation can generate inflammatory responses in the central nervous system. In turn, this inflammation increases sleep drive, leading to a rebound in sleep duration. Microglia, the innate immune cells found exclusively in the CNS, have previously been found to release inflammatory signals and exhibit altered characteristics in response to sleep deprivation. Together, this suggests that microglia may be partially responsible for the brain's response to sleep deprivation through their inflammatory activity. In this study, we ablated microglia from the mouse brain and assessed resulting sleep, circadian, and sleep deprivation phenotypes. We find that microglia are dispensable for both homeostatic sleep and circadian function and the sleep rebound response to sleep deprivation. However, we uncover a phenomenon by which microglia appear to be essential for the protection of fear-conditioning memories formed during the recovery sleep period following a period of sleep deprivation. This phenomenon occurs potentially through the upregulation of synaptic-homeostasis related genes to protect nascent dendritic spines that may be otherwise removed or downscaled during recovery sleep. These findings further expand the list of known functions for microglia in synaptic modulation.

sTREM-1 promotes the phagocytic function of microglia to induce hippocampus damage via the PI3K-AKT signaling pathway

Soluble triggering receptor expressed on myeloid cells-1 (sTREM-1) is a soluble form of TREM-1 released during inflammation. Elevated sTREM-1 levels have been found in neuropsychiatric systemic lupus erythematosus (NPSLE) patients; yet, the exact mechanisms remain unclear. This study investigated the role of sTREM-1 in brain damage and its underlying mechanism. The sTREM-1 recombinant protein (2.5 μg/3 μL) was injected into the lateral ventricle of C57BL/6 female mice. After intracerebroventricular (ICV) injection, the damage in hippocampal neurons increased, and the loss of neuronal synapses and activation of microglia increased compared to the control mice (treated with saline). In vitro. after sTREM-1 stimulation, the apoptosis of BV2 cells decreased, the polarization of BV2 cells shifted to the M1 phenotype, the phagocytic function of BV2 cells significantly improved, while the PI3K-AKT signal pathway was activated in vivo and in vitro. PI3K-AKT pathway inhibitor LY294002 reversed the excessive activation and phagocytosis of microglia caused by sTREM-1 in vivo and in vitro, which in turn improved the hippocampus damage. These results indicated that sTREM-1 activated the microglial by the PI3K-AKT signal pathway, and promoted its excessive phagocytosis of the neuronal synapse, thus inducing hippocampal damage. sTREM-1 might be a potential target for inducing brain lesions.

Improved Sleep, Memory, and Cellular Pathological Features of Tauopathy, Including the NLRP3 Inflammasome, after Chronic Administration of Trazodone in rTg4510 Mice

Several cellular pathways contribute to neurodegenerative tauopathy-related disorders. Microglial activation, a major component of neuroinflammation, is an early pathologic hallmark that correlates with cognitive decline, while the unfolded protein response (UPR) contributes to synaptic pathology. Sleep disturbances are prevalent in tauopathies and may also contribute to disease progression. Few studies have investigated whether manipulations of sleep influence cellular pathologic and behavioral features of tauopathy. We investigated whether trazodone, a licensed antidepressant with hypnotic efficacy in dementia, can reduce disease-related cellular pathways and improve memory and sleep in male rTg4510 mice with a tauopathy-like phenotype. In a 9 week dosing regimen, trazodone decreased microglial NLRP3 inflammasome expression and phosphorylated p38 mitogen-activated protein kinase levels, which correlated with the NLRP3 inflammasome, the UPR effector ATF4, and total tau levels. Trazodone reduced theta oscillations during rapid eye movement (REM) sleep and enhanced REM sleep duration. Olfactory memory transiently improved, and memory performance correlated with REM sleep duration and theta oscillations. These findings on the effects of trazodone on the NLRP3 inflammasome, the unfolded protein response and behavioral hallmarks of dementia warrant further studies on the therapeutic value of sleep-modulating compounds for tauopathies.SIGNIFICANCE STATEMENT Dementia and associated behavioral symptoms such as memory loss and sleep disturbance are debilitating. Identifying treatments that alleviate symptoms and concurrently target cellular pathways contributing to disease progression is paramount for the patients and their caregivers. Here we show that a chronic treatment with trazodone, an antidepressant with positive effects on sleep, has beneficial effects on several cellular pathways contributing to neuroinflammation and tau pathology, in tauopathy-like rTg4510 mice. Trazodone also improved rapid eye movement (REM) sleep, the slowing of brain oscillations, and olfactory memory disturbances, which are all early symptoms observed in Alzheimer's disease. Thus, trazodone and compounds with REM sleep-promoting properties may represent a promising treatment approach to reduce the early symptoms of tauopathy and slow down disease progression.

Lonicerae Japonicae Flos Extract Promotes Sleep in Sleep-Deprived and Lipopolysaccharide-Challenged Mice

Lonicerae Japonicae Flos (LJF) is commonly used in Chinese herbal medicines and exhibits anti-viral, anti-oxidative, and anti-inflammatory properties. The reciprocal relationship between sleep, the immune system and the central nervous system is well-established in the animal models. In this study, we used the mouse model to analyze the beneficial effects of the LJF on the dysregulated sleep-wakefulness cycle in response to acute sleep deprivation and lipopolysaccharide (LPS)-induced inflammation and the potential underlying mechanisms. Polysomnography data showed that LJF increased the time spent in non-rapid eye movement (NREM) sleep during the day under basal conditions. Furthermore, latency to sleep was reduced and the time spent in rapid eye movement (REM) sleep was increased during recovery from acute sleep deprivation. Furthermore, LJF-treated mice showed increased REM sleep and altered electroencephalogram (EEG) power spectrum in response to intra-peritoneal injection of LPS. LJF significantly reduced the levels of proinflammatory cytokines such as IL-6, TNF-α, and IL-1β in the blood serum as well as hippocampus, and medial prefrontal cortex (mPFC) tissues in the LPS-challenged mice by inhibiting microglial activation. Moreover, LJF increased the time spent in REM sleep in the LPS-challenged mice compared to the control mice. These results suggested that LJF stimulated the sleep drive in response to acute sleep deprivation and LPS-induced inflammation, thereby increasing REM sleep for recovery and neuroprotection. In conclusion, our findings demonstrate that the clinical potential of LJF in treating sleep disorders related to sleep deprivation and neuro-inflammation.

Sleep Disruption Worsens Seizures: Neuroinflammation as a Potential Mechanistic Link

Sleep disturbances, such as insomnia, obstructive sleep apnea, and daytime sleepiness, are common in people diagnosed with epilepsy. These disturbances can be attributed to nocturnal seizures, psychosocial factors, and/or the use of anti-epileptic drugs with sleep-modifying side effects. Epilepsy patients with poor sleep quality have intensified seizure frequency and disease progression compared to their well-rested counterparts. A better understanding of the complex relationship between sleep and epilepsy is needed, since approximately 20% of seizures and more than 90% of sudden unexpected deaths in epilepsy occur during sleep. Emerging studies suggest that neuroinflammation, (e.g., the CNS immune response characterized by the change in expression of inflammatory mediators and glial activation) may be a potential link between sleep deprivation and seizures. Here, we review the mechanisms by which sleep deprivation induces neuroinflammation and propose that neuroinflammation synergizes with seizure activity to worsen neurodegeneration in the epileptic brain. Additionally, we highlight the relevance of sleep interventions, often overlooked by physicians, to manage seizures, prevent epilepsy-related mortality, and improve quality of life.

Primate-specific retrotransposons and the evolution of circadian networks in the human brain

The circadian rhythm of the human brain is attuned to sleep-wake cycles that entail global alterations in neuronal excitability. This periodicity involves a highly coordinated regulation of gene expression. A growing number of studies are documenting a fascinating connection between primate-specific retrotransposons (Alu elements) and key epigenetic regulatory processes in the primate brain. Collectively, these studies indicate that Alu elements embedded in the human neuronal genome mediate post-transcriptional processes that unite human-specific neuroepigenetic landscapes and circadian rhythm. Here, we review evidence linking Alu retrotransposon-mediated posttranscriptional pathways to circadian gene expression. We hypothesize that Alu retrotransposons participate in the organization of circadian brain function through multidimensional neuroepigenetic pathways. We anticipate that these pathways are closely tied to the evolution of human cognition and their perturbation contributes to the manifestation of human-specific neurological diseases. Finally, we address current challenges and accompanying opportunities in studying primate- and human-specific transposable elements.

Microglia and the Aging Brain: Are Geriatric Microglia Linked to Poor Sleep Quality?

Poor sleep quality and disrupted circadian behavior are a normal part of aging and include excessive daytime sleepiness, increased sleep fragmentation, and decreased total sleep time and sleep quality. Although the neuronal decline underlying the cellular mechanism of poor sleep has been extensively investigated, brain function is not fully dependent on neurons. A recent antemortem autographic study and postmortem RNA sequencing and immunohistochemical studies on aged human brain have investigated the relationship between sleep fragmentation and activation of the innate immune cells of the brain, microglia. In the process of aging, there are marked reductions in the number of brain microglial cells, and the depletion of microglial cells disrupts circadian rhythmicity of brain tissue. We also showed, in a previous study, that pharmacological suppression of microglial function induced sleep abnormalities. However, the mechanism underlying the contribution of microglial cells to sleep homeostasis is only beginning to be understood. This review revisits the impact of aging on the microglial population and activation, as well as microglial contribution to sleep maintenance and response to sleep loss. Most importantly, this review will answer questions such as whether there is any link between senescent microglia and age-related poor quality sleep and how this exacerbates neurodegenerative disease.