Astrocytic Activation Generates De Novo Neuronal Potentiation and Memory Enhancement

Cortical astrocyte activation triggers meningeal nociception and migraine-like pain

Migraine attacks are believed to originate in the brain, but the exact mechanisms by which the brain generates peripheral nociceptive signals that drive migraine pain remain unclear. Sensory cortex hyperexcitability has been observed consistently across different migraine subtypes. Astrocytes detect aberrant increases in cortical activity via their Gq-coupled receptors and respond by releasing gliotransmitters and other factors with pro-inflammatory and nociceptive properties. In the present study, we used a rat model to investigate whether heightened cortical astrocyte Gq-coupled signaling is sufficient to drive peripheral trigeminal meningeal nociceptive responses linked to the generation of migraine headaches. We used an AAV-based chemogenetic approach that allows selective activation of cortical astrocyte Gq-GPCR signaling. We targeted astrocytes in the visual cortex as hyperexcitability in this region has been implicated in migraine. Furthermore, the meninges overlying the visual cortex are densely innervated by nociceptive fibers. We combined this chemogenetic approach with in vivo single-unit recording of meningeal nociceptors to assess changes in their ongoing activity and mechanosensitivity, along with testing of migraine-like behaviors. We further targeted calcitonin gene-related peptide (CGRP), using a monoclonal antibody (anti-CGRP mAb), to assess the relevance of cortical astrocyte activation to migraine. We discovered that heightened activation of Gq-coupled signaling in visual cortex astrocytes drives persistent discharge and increased mechanosensitivity of meningeal nociceptors. Cortical astrocytic activation also generated cephalic mechanical pain hypersensitivity, reduced exploratory behavior, and anxiety-like behaviors linked to migraine headaches. Blocking calcitonin gene-related peptide signaling suppressed astrocyte-mediated increases in meningeal nociceptor discharge and alleviated associated migraine-related behaviors. Our findings reveal a previously unappreciated role for augmented visual cortex astrocyte signaling as a triggering factor sufficient to generate meningeal nociception and migraine pain and greatly expand our understanding of migraine pathophysiology.

Early-life stress induces persistent astrocyte dysfunction associated with fear generalisation

Early-life stress can have lifelong consequences, enhancing stress susceptibility and resulting in behavioural and cognitive deficits. While the effects of early-life stress on neuronal function have been well-described, we still know very little about the contribution of non-neuronal brain cells. Investigating the complex interactions between distinct brain cell types is critical to fully understand how cellular changes manifest as behavioural deficits following early-life stress. Here, using male and female mice we report that early-life stress induces anxiety-like behaviour and fear generalisation in an amygdala-dependent learning and memory task. These behavioural changes were associated with impaired synaptic plasticity, increased neural excitability, and astrocyte hypofunction. Genetic perturbation of amygdala astrocyte function by either reducing astrocyte calcium activity or reducing astrocyte network function was sufficient to replicate cellular, synaptic, and fear memory generalisation associated with early-life stress. Our data reveal a role of astrocytes in tuning emotionally salient memory and provide mechanistic links between early-life stress, astrocyte hypofunction, and behavioural deficits.

Astrocytic determinant of the fate of long-term memory

While some vivid memories are unyielding and unforgettable, others fade with time. Astrocytes are recognized for their role in modulating the brain's environment and have recently been considered integral to the brain's information processing and memory formation. This suggests their potential roles in emotional perception and memory formation. In this study, we delve into the impact of amygdala astrocytes on fear behaviors and memory, employing astrocyte-specific optogenetic manipulations in mice. Our findings reveal that astrocytic photoactivation with channelrhodopsin-2 (ChR2) provokes aversive behavioral responses, while archaerhodopsin-T (ArchT) photoactivation diminishes fear perception. ChR2 photoactivation amplifies fear perception and fear memory encoding but obstructs its consolidation. On the other hand, ArchT photoactivation inhibits memory formation during intense aversive stimuli, possibly due to weakened fear perception. However, it prevents the decay of remote fear memory over three weeks. Crucially, these memory effects were observed when optogenetic manipulations coincided with the aversive experience, indicating a deterministic role of astrocytic states at the exact moment of fear experiences in shaping long-term memory. This research underscores the significant and multifaceted role of astrocytes in emotional perception, fear memory formation, and modulation, suggesting a sophisticated astrocyte-neuron communication mechanism underlying basic emotional state transitions of information processing in the brain.

Ezrin-mediated astrocyte-synapse signaling regulates cognitive function via astrocyte morphological changes in fine processes in male mice

Astrocytes, which actively participate in cognitive processes, have a complex spongiform morphology, highlighted by extensive ramified fine processes that closely enwrap the pre- and post-synaptic compartments, forming tripartite synapses. However, the role of astrocyte morphology in cognitive processes remains incompletely understood and even controversial. The actin-binding protein Ezrin is highly expressed in astrocytes and is a key structural determinant of astrocyte morphology. Here, we found that Ezrin expression and astrocyte fine process volume in the hippocampus of male mice increased after learning but decreased after lipopolysaccharide injection and in a mouse model of postoperative cognitive dysfunction, both of which involved models with impaired cognitive function. Additionally, astrocytic Ezrin knock-out led to significantly decreased astrocytic fine process volumes, decreased astrocyte-neuron proximity, and induced anxiety-like behaviors and cognitive dysfunction. Astrocytic Ezrin deficiency in the hippocampus was achieved by using a microRNA silencing technique delivered by adeno-associated viruses. Down-regulation of Ezrin in hippocampal astrocytes led to disrupted astrocyte-synapse interactions and impaired synaptic functions, including synaptic transmission and synaptic plasticity, which could be rescued by exogenous administration of D-serine. Remarkably, decreased Ezrin expression and reduced astrocyte fine processes volumes were also observed in aged mice with decreased cognitive function. Moreover, overexpression of astrocytic Ezrin increased astrocyte fine process volumes and improved cognitive function in aged mice. Overall, our results indicate Ezrin-mediated astrocyte fine processes integrity shapes astrocyte-synapse signaling contributing to cognitive function.

Hydrogen Sulfide (H2S) Generated in the Colon Induces Neuropathic Pain by Activating Spinal NMDA Receptors in a Rodent Model of Chronic Constriction Injury

Neuropathic pain (NP) imposes a significant burden on individuals, manifesting as nociceptive anaphylaxis, hypersensitivity, and spontaneous pain. Previous studies have shown that traumatic stress in the nervous system can lead to excessive production of hydrogen sulfide (H2S) in the gut. As a toxic gas, it can damage the nervous system through the gut-brain axis. However, whether traumatic stress in the nervous system leading to excessive production of H2S in the gut can ultimately cause neuropathic pain through the gut-brain axis remains to be investigated. This study established a model of chronic constriction injury (CCI) in mice to determine its effects on gut H2S production, the associated damage via the gut-brain axis, the potential neuropathic pain, as well as the probable mechanism. A CCI mouse model was developed using a spinal nerve ligation approach. Subsequently, the mechanical withdrawal threshold (MWT) and thermal withdrawal latency (TWL) were used to determine the mice's pain thresholds. A variety of assays were performed, including immunofluorescence, western blotting, real-time quantitative Polymerase Chain Reaction (PCR), and membrane clamp whole-cell recordings. Mice subjected to CCI showed decreased MWT and TWL, decreased ZO-1 staining, decreased HuD staining, increased Glial fibrillary acidic protein (GFAP) staining, increased expression of tumor necrosis factor-alpha (TNF-α) protein and interleukin-6 (IL-6) protein, increased expression of NMDAR2B (NR2B) protein and NR2B mRNA, increased colocalization of vGlut2- and c-fos-positive cells, and a higher amplitude of evoked excitatory postsynaptic potential (EPSP) compared to Sham group. These changes were significantly reversed by H2S inhibitor treatment, and the specific NMDA receptor inhibitor MK-801 effectively restored the neurotoxicity of H2S. H2S is involved in CCI-induced neuropathic pain in mice, which might be mediated by the activation of the NMDA signaling pathway.

The meninges host a distinct compartment of regulatory T cells that preserves brain homeostasis

Our understanding of the meningeal immune system has recently burgeoned, particularly regarding how innate and adaptive effector cells are mobilized to meet brain challenges. However, information on how meningeal immunocytes guard brain homeostasis in healthy individuals remains limited. This study highlights the heterogeneous, polyfunctional regulatory T cell (Treg) compartment in the meninges. A Treg subtype specialized in controlling interferon-gamma (IFN-γ) responses and another dedicated to regulating follicular B cell responses were substantial components of this compartment. Accordingly, punctual Treg ablation rapidly unleashed IFN-γ production by meningeal lymphocytes, unlocked access to the brain parenchyma, and altered meningeal B cell profiles. Distally, the hippocampus assumed a reactive state, with morphological and transcriptional changes in multiple glial cell types. Within the dentate gyrus, neural stem cells underwent more death and were blocked from further differentiation, which coincided with impairments in short-term spatial-reference memory. Thus, meningeal Tregs are a multifaceted safeguard of brain homeostasis at steady state.

Brain Plasticity and Cell Competition: Immediate Early Genes Are the Focus

Brain plasticity is at the basis of many cognitive functions, including learning and memory. It includes several mechanisms of synaptic and extrasynaptic changes, neurogenesis, and the formation and elimination of synapses. The plasticity of synaptic transmission involves the expression of immediate early genes (IEGs) that regulate neuronal activity, thereby supporting learning and memory. In addition, IEGs are involved in the regulation of brain cells' metabolism, proliferation, and survival, in the establishment of multicellular ensembles, and, presumably, in cell competition in the tissue. In this review, we analyze the current understanding of the role of IEGs (c-Fos, c-Myc, Arg3.1/Arc) in controlling brain plasticity in physiological and pathological conditions, including brain aging and neurodegeneration. This work might inspire new gene therapy strategies targeting IEGs to regulate synaptic plasticity, and potentially prevent or mitigate neurodegenerative diseases.

Anti-seizure effects of norepinephrine-induced free fatty acid release

The brain's ability to rapidly transition between sleep, quiet wakefulness, and states of high vigilance is remarkable. Cerebral norepinephrine (NE) plays a key role in promoting wakefulness, but how does the brain avoid neuronal hyperexcitability upon arousal? Here, we show that NE exposure results in the generation of free fatty acids (FFAs) within the plasma membrane from both astrocytes and neurons. In turn, FFAs dampen excitability by differentially modulating the activity of astrocytic and neuronal Na+, K+, ATPase. Direct application of FFA to the occipital cortex in awake, behaving mice dampened visual-evoked potentials (VEPs). Conversely, blocking FFA production via local application of a lipase inhibitor heightened VEP and triggered seizure-like activity. These results suggest that FFA release is a crucial step in NE signaling that safeguards against hyperexcitability. Targeting lipid-signaling pathways may offer a novel therapeutic approach for seizure prevention.

Astrocytes phenomics as new druggable targets in healthy aging and Alzheimer's disease progression

For over a century after their discovery astrocytes were regarded merely as cells located among other brain cells to hold and give support to neurons. Astrocytes activation, "astrocytosis" or A1 functional state, was considered a detrimental mechanism against neuronal survival. Recently, the scientific view on astrocytes has changed. Accumulating evidence indicate that astrocytes are not homogeneous, but rather encompass heterogeneous subpopulations of cells that differ from each other in terms of transcriptomics, molecular signature, function and response in physiological and pathological conditions. In this review, we report and discuss the recent literature on the phenomic differences of astrocytes in health and their modifications in disease conditions, focusing mainly on the hippocampus, a region involved in learning and memory encoding, in the age-related memory impairments, and in Alzheimer's disease (AD) dementia. The morphological and functional heterogeneity of astrocytes in different brain regions may be related to their different housekeeping functions. Astrocytes that express diverse transcriptomics and phenomics are present in strictly correlated brain regions and they are likely responsible for interactions essential for the formation of the specialized neural circuits that drive complex behaviors. In the contiguous and interconnected hippocampal areas CA1 and CA3, astrocytes show different, finely regulated, and region-specific heterogeneity. Heterogeneous astrocytes have specific activities in the healthy brain, and respond differently to physiological or pathological stimuli, such as inflammaging present in normal brain aging or beta-amyloid-dependent neuroinflammation typical of AD. To become reactive, astrocytes undergo transcriptional, functional, and morphological changes that transform them into cells with different properties and functions. Alterations of astrocytes affect the neurovascular unit, the blood-brain barrier and reverberate to other brain cell populations, favoring or dysregulating their activities. It will be of great interest to understand whether the differential phenomics of astrocytes in health and disease can explain the diverse vulnerability of the hippocampal areas to aging or to different damaging insults, in order to find new astrocyte-targeted therapies that might prevent or treat neurodegenerative disorders.

Learning-associated astrocyte ensembles regulate memory recall

The physical manifestations of memory formation and recall are fundamental questions that remain unresolved1. At the cellular level, ensembles of neurons called engrams are activated by learning events and control memory recall1-5. Astrocytes are found in close proximity to neurons and engage in a range of activities that support neurotransmission and circuit plasticity6-10. Moreover, astrocytes exhibit experience-dependent plasticity11-13, although whether specific ensembles of astrocytes participate in memory recall remains obscure. Here we show that learning events induce c-Fos expression in a subset of hippocampal astrocytes, and that this subsequently regulates the function of the hippocampal circuit in mice. Intersectional labelling of astrocyte ensembles with c-Fos after learning events shows that they are closely affiliated with engram neurons, and reactivation of these astrocyte ensembles stimulates memory recall. At the molecular level, learning-associated astrocyte (LAA) ensembles exhibit elevated expression of nuclear factor I-A, and its selective deletion from this population suppresses memory recall. Taken together, our data identify LAA ensembles as a form of plasticity that is sufficient to provoke memory recall and indicate that astrocytes are an active component of the engram.

Modulating Anxiety-Like Behaviors in Neuropathic Pain: Role of Anterior Cingulate Cortex Astrocytes Activation

AIMS

The comorbidity of anxiety-like symptoms in neuropathic pain (NP) is a significant yet often overlooked health concern. Anxiety sufferers may have a lower tolerance for pain, but which is difficult to treat. Accumulating evidence suggests a strong link between astrocytes and the manifestation of NP with concurrent anxiety-like behaviors. And the anterior cingulate cortex (ACC) has emerged as a key player in pain modulation and related emotional processing. However, the complex mechanisms that astrocytes in ACC influence anxiety behavior in mouse models of NP remain largely unexplored.

METHODS

Utilizing the traditional spared nerve injury (SNI) surgical model, we employed chemogenetic approaches, immunofluorescence, and western blot to investigate the functional significance and interactive dynamics between ACC astrocytes and excitatory neurons.

RESULTS

Our results revealed that SNI surgery induces NP and delayed anxiety-like behaviors, accompanied by increased astrocyte activity in the ACC. Chemogenetic manipulation demonstrated that inhibiting astrocytes alleviates anxiety symptoms, while activating them exacerbates anxiety-like behaviors, affecting local excitatory neurons and synapse density. Direct manipulation of ACC excitatory neurons also significantly impacted anxiety-like behaviors.

CONCLUSION

Our results highlight the pivotal role of ACC astrocytes in modulating anxiety-like behavior, suggesting a novel therapeutic strategy for anxiety associated with NP by targeting astrocyte function.

OXTR-mediated signaling in astrocytes contributes to anxiolysis

Astrocytes are an indispensable part of signal processing within the mammalian brain. Thus, the mode of action of a neuropeptide such as oxytocin (OXT) can only be fully understood considering this integral part of the CNS. Here, we show that OXT regulates astrocytic gene expression, intracellular signaling and specific proteins both in vitro and in vivo. This translates into rapid regulation of astroglial structural and functional properties including cytoskeletal plasticity, coverage of synapses and gap-junction coupling. At the molecular level, we identify the previously undescribed Sp1-Gem signaling cascade as the key driver for these cell type-specific OXT effects. Finally at the behavioral level, we found in vivo that OXT requires astrocytes to exert its well described anxiolytic properties within the hypothalamic paraventricular nucleus. Thus, our study points to OXT receptor-expressing astrocytes as a critical component of the brain OXT system.

A spatial threshold for astrocyte calcium surge

Astrocytes are active cells involved in brain function through the bidirectional communication with neurons, in which astrocyte calcium plays a crucial role. Synaptically evoked calcium increases can be localized to independent subcellular domains or expand to the entire cell, i.e., calcium surge. Because a single astrocyte may contact ~100,000 synapses, the control of the intracellular calcium signal propagation may have relevant consequences on brain function. Yet, the properties governing the spatial dynamics of astrocyte calcium remains poorly defined. Imaging subcellular responses of cortical astrocytes to sensory stimulation in mice, we show that sensory-evoked astrocyte calcium responses originated and remained localized in domains of the astrocytic arborization, but eventually propagated to the entire cell if a spatial threshold of >23% of the arborization being activated was surpassed. Using Itpr2-/- mice, we found that type-2 IP3 receptors were necessary for the generation of astrocyte calcium surge. We finally show using in situ electrophysiological recordings that the spatial threshold of the astrocyte calcium signal consequently determined the gliotransmitter release. Present results reveal a fundamental property of astrocyte physiology, i.e., a spatial threshold for astrocyte calcium propagation, which depends on astrocyte intrinsic properties and governs astrocyte integration of local synaptic activity and subsequent neuromodulation.

The effects of locus coeruleus ablation on mouse brain volume and microstructure evaluated by high-field MRI

The locus coeruleus (LC) produces most of the brain's noradrenaline (NA). Among its many roles, NA is often said to be neuroprotective and important for brain upkeep. For this reason, loss of LC integrity is thought to impact brain volume and microstructure as well as plasticity broadly. LC dysfunction is also a suspected driver in the development of neurodegenerative diseases. Nevertheless, the impact of LC dysfunction on the gross structure and microstructure of normal brains is not well-studied. We employed high-field ex vivo magnetic resonance imaging (MRI) to investigate brain volumetrics and microstructure in control (CON) mice and mice with LC ablation (LCA) at two ages, representing the developing brain and the fully matured brain. These whole-brain methods are known to be capable of detecting subtle morphological changes and brain microstructural remodeling. We found mice behavior consistent with histologically confirmed LC ablation. However, MRI showed no difference between CON and LCA groups with regard to brain size, relative regional volumes, or regional microstructural indices. Our findings suggest that LC-NA is not needed for postnatal brain maturation and growth in mice. Nor is it required for maintenance in the normal adult mouse brain, as no atrophy or microstructural aberration is detected after weeks of LC dysfunction. This adds clarity to the often-encountered notion that LC-NA is important for brain "trophic support" as it shows that such effects are likely most relevant to mechanisms related to brain plasticity and neuroprotection in the (pre)diseased brain.

Activation of Bradykinin B2 Receptors in Astrocytes Stimulates the Release of Leukemia Inhibitory Factor for Autocrine and Paracrine Signaling

Communications between different cell types within a tissue are often critical for the proper functioning of an organ. In the central nervous system, interactions among neurons and glial cells are known to modulate neurotransmission, energy metabolism, extracellular ion homeostasis, and neuroprotection. Here we showed that bradykinin, a proinflammatory neuropeptide, can be detected by astrocytes, resulting in the secretion of cytokines that act on neurons. In astrocytic cell lines and primary astrocytes, bradykinin and several other ligands acting on Gq-coupled receptors stimulated Ca2+ mobilization, which subsequently led to the release of leukemia inhibitory factor (LIF) and interleukin-6 (IL-6). The bradykinin B2 receptor antagonist, HOE-140, effectively blocked the ability of bradykinin to mobilize Ca2+ and stimulate mitogen-activated protein kinases (MAPKs) in astrocytes. Interestingly, incubation of neuronal cell lines and primary cortical neurons with conditioned media from bradykinin-treated astrocytes resulted in the activation of STAT3, a key component downstream of LIF and IL-6 receptors. LIF was apparently the major active factor in the conditioned media as the STAT3 response was almost completely neutralized by an anti-LIF antiserum. The presence of kininogen and kallikrein transcripts in neuronal cells but not in astrocytic cells indicates that neurons can produce bradykinin. Correspondingly, conditioned media from neuronal cells stimulated MAPKs in astrocytes in a HOE-140-sensitive manner. These studies demonstrate that paracrine signaling between neurons and astrocytes may involve ligands of Gq-coupled receptors and cytokines such as LIF.

Astrocytic Ca2+ activation by chemogenetics mitigates the effect of kainic acid-induced excitotoxicity on the hippocampus

Astrocytes play a multifaceted role regulating brain glucose metabolism, ion homeostasis, neurotransmitters clearance, and water dynamics being essential in supporting synaptic function. Under different pathological conditions such as brain stroke, epilepsy, and neurodegenerative disorders, excitotoxicity plays a crucial role, however, the contribution of astrocytic activity in protecting neurons from excitotoxicity-induced damage is yet to be fully understood. In this work, we evaluated the effect of astrocytic activation by Designer Receptors Exclusively Activated by Designer Drugs (DREADDs) on brain glucose metabolism in wild-type (WT) mice, and we investigated the effects of sustained astrocyte activation following an insult induced by intrahippocampal (iHPC) kainic acid (KA) injection using 2-deoxy-2-[18F]-fluoro-D-glucose (18F-FDG) positron emission tomography (PET) imaging, along with behavioral test, nuclear magnetic resonance (NMR) spectroscopy and histochemistry. Astrocytic Ca2+ activation increased the 18F-FDG uptake, but this effect was not found when the study was performed in knock out mice for type-2 inositol 1,4,5-trisphosphate receptor (Ip3r2-/-) nor in floxed mice to abolish glucose transporter 1 (GLUT1) expression in hippocampal astrocytes (GLUT1ΔGFAP). Sustained astrocyte activation after KA injection reversed the brain glucose hypometabolism, restored hippocampal function, prevented neuronal death, and increased hippocampal GABA levels. The findings of our study indicate that astrocytic GLUT1 function is crucial for regulating brain glucose metabolism. Astrocytic Ca2+ activation has been shown to promote adaptive changes that significantly contribute to mitigating the effects of KA-induced damage. This evidence suggests a protective role of activated astrocytes against KA-induced excitotoxicity.

On the functions of astrocyte-mediated neuronal slow inward currents

Slow inward currents are known as neuronal excitatory currents mediated by glutamate release and activation of neuronal extrasynaptic N-methyl-D-aspartate receptors with the contribution of astrocytes. These events are significantly slower than the excitatory postsynaptic currents. Parameters of slow inward currents are determined by several factors including the mechanisms of astrocytic activation and glutamate release, as well as the diffusion pathways from the release site towards the extrasynaptic receptors. Astrocytes are stimulated by neuronal network activity, which in turn excite neurons, forming an astrocyte-neuron feedback loop. Mostly as a consequence of brain edema, astrocytic swelling can also induce slow inward currents under pathological conditions. There is a growing body of evidence on the roles of slow inward currents on a single neuron or local network level. These events often occur in synchrony on neurons located in the same astrocytic domain. Besides synchronization of neuronal excitability, slow inward currents also set synaptic strength via eliciting timing-dependent synaptic plasticity. In addition, slow inward currents are also subject to non-synaptic plasticity triggered by long-lasting stimulation of the excitatory inputs. Of note, there might be important region-specific differences in the roles and actions triggering slow inward currents. In greater networks, the pathophysiological roles of slow inward currents can be better understood than physiological ones. Slow inward currents are identified in the pathophysiological background of autism, as slow inward currents drive early hypersynchrony of the neural networks. Slow inward currents are significant contributors to paroxysmal depolarizational shifts/interictal spikes. These events are related to epilepsy, but also found in Alzheimer's disease, Parkinson's disease, and stroke, leading to the decline of cognitive functions. Events with features overlapping with slow inward currents (excitatory, N-methyl-D-aspartate-receptor mediated currents with astrocytic contribution) as ischemic currents and spreading depolarization also have a well-known pathophysiological role in worsening consequences of stroke, traumatic brain injury, or epilepsy. One might assume that slow inward currents occurring with low frequency under physiological conditions might contribute to synaptic plasticity and memory formation. However, to state this, more experimental evidence from greater neuronal networks or the level of the individual is needed. In this review, I aimed to summarize findings on slow inward currents and to speculate on the potential functions of it.

Astrocytes release ATP/ADP and glutamate in flashes via vesicular exocytosis

Astrocytes regulate brain functions through gliotransmitters like ATP/ADP and glutamate, but their release patterns and mechanisms remain controversial. Here, we visualized ATP/ADP and glutamate response following astrocyte activation and investigated their mechanisms in vivo. Employing cOpn5-mediated optogenetic stimulation, genetically encoded fluorescent sensors, and two-photon imaging, we observed ATP/ADP released as temporally prolonged and spatially extended flashes that later converted to adenosine. This release occurs via Ca2+ and VNUT-dependent vesicular exocytosis. Additionally, astrocytes also release glutamate in flashes through TeNT-sensitive exocytosis, independent of ATP/ADP release. ATP/ADP released by astrocytes triggers further ATP/ADP release from microglia through P2Y12- and VNUT-dependent mechanisms. VNUT in astrocytes and microglia also contributes to ATP/ADP release under LPS-induced brain inflammation. These findings establish Ca2+-dependent vesicular exocytosis as a key mode of action, reveal intricate astrocyte-microglia interactions, and suggest a role for gliotransmission in brain inflammation. Furthermore, the methodologies may provide valuable tools for deciphering glial physiology and pathophysiology.

Whole-body vibration elicits 40 Hz cortical gamma oscillations and ameliorates age-related cognitive impairment through hippocampal astrocyte synapses in male rats

Age-related cognitive impairment is a prevalent issue in developed societies. Gamma oscil2lations at 40 Hz have been identified as a potential therapeutic approach for age-related cognitive decline and can be induced through various modalities, including auditory, visual, electrical, and magnetic stimulation. In this study, we investigated a novel modality of stimulation: whole-body vibration at 40 Hz. We examined the effects of 40 Hz vibration on cognitive performance and associated neuronal activity in the brains of aged male rats. Our findings revealed that only vibration at 40 Hz, rather than 20 Hz or 80 Hz, elicited cortical gamma oscillations in aged male rats. Additionally, following 8 weeks of prolonged treatment, the implementation of 40 Hz whole-body vibration significantly augmented the cognitive function of aged male rats as evidenced by behavioral assessments. Mechanistic studies demonstrated that these beneficial effects were attributed to the reduction of neuronal apoptosis in hippocampal CA1 through regulation of synaptic connections between astrocytes and neurons via 40 Hz gamma oscillations. Collectively, this suggests a promising intervention for age-related cognitive decline and identifies neuron-astrocyte synapses as potential therapeutic targets.

Astrocytes contribute to the functional differentiation of the hippocampal longitudinal axis during reward and aversion processing in the adult male rat

This study aims to investigate whether glial cells, in particular putative astrocytes, contribute to functional distinctions between the dorsal (DH), intermediate (IH), and ventral (VH) hippocampus. To evaluate this, we performed three different behavioral tasks (i.e., Morris water maze; MWM, Passive avoidance; PA, T-maze place preference; TPP) to determine whether the DH, IH, and VH are necessary for each task. Sensitivity of behavioral tasks was confirmed using lidocaine (2 %, 1 μl) reversible inactivation. Subsequently, we examined the effects of silencing astrocytes, using fluorocitrate (FC, 1 mM/1 μl), into the DH, IH, and VH on these tasks. The effects of drugs were examined separately. We observed that injection of FC into the DH resulted in a significant impairment in MWM performance. In contrast, while FC injections into the IH or VH did not prevent platform localization during the acquisition phase, rats showed difficulty recalling the target zone during the retrieval phase. In the PA test, FC injection into the VH impaired task learning and memory. During the acquisition phase, FC injection into the DH or IH did not differ from the control in the number of shocks; however, during retrieval, there was a significant decrease in the latency before entering the dark chamber. The TPP test performance was impaired by FC injection in the IH. In sum, we show that glial cells, especially astrocytes in specific functional regions of the hippocampus, play distinct roles in processing aversive and rewarding experiences and contribute to the functional organization of the hippocampal longitudinal axis.

Revisiting astrocytic calcium signaling in the brain

Astrocytes, characterized by complex spongiform morphology, participate in various physiological processes, and abnormal changes in their calcium (Ca2+) signaling are implicated in central nervous system disorders. However, medications targeting the control of Ca2+ have fallen short of the anticipated therapeutic outcomes in clinical applications. This underscores the fact that our comprehension of this intricate regulation of calcium ions remains considerably incomplete. In recent years, with the advancement of Ca2+ labeling, imaging, and analysis techniques, Ca2+ signals have been found to exhibit high specificity at different spatial locations within the intricate structure of astrocytes. This has ushered the study of Ca2+ signaling in astrocytes into a new phase, leading to several groundbreaking research achievements. Despite this, the comprehensive understanding of astrocytic Ca2+ signaling and their implications remains challenging area for future research.

Melatonin's role in the timing of sleep onset is conserved in nocturnal mice

Melatonin supplementation strengthens non-restorative sleep rhythms and its temporal alignment in both humans and night-active rodents. Of note, although the sleep cycle is reversed in day-active and night-active (nocturnal) mammals, both, produce melatonin at night under the control of the circadian clock. The effects of exogenous melatonin on sleep and sleepiness are relatively clear, but its endogenous role in sleep, particularly, in timing sleep onset (SO), remains poorly understood. We show in nocturnal mice that the increases in mid-nighttime sleep episodes, and the mid-nighttime decline in activity, are coupled to nighttime melatonin signaling. Furthermore, we show that endogenous melatonin modulates SO by reducing the threshold for wake-to-sleep transitioning. Such link between melatonin and SO timing may explain phenomena such as increased sleep propensity in circadian rhythm sleep disorders and chronic insomnia in patients with severely reduced nocturnal melatonin levels. Our findings demonstrate that melatonin's role in sleep is evolutionarily conserved, effectively challenging the argument that melatonin cannot play a major role in sleep regulation in nocturnal mammals, where the main activity phase coincides with high melatonin levels.

Identifying dysfunctional cell types and circuits in animal models for psychiatric disorders with calcium imaging

A central goal of neuroscience is to understand how the brain transforms external stimuli and internal bodily signals into patterns of activity that underlie cognition, emotional states, and behavior. Understanding how these patterns of activity may be disrupted in mental illness is crucial for developing novel therapeutics. It is well appreciated that psychiatric disorders are complex, circuit-based disorders that arise from dysfunctional activity patterns generated in discrete cell types and their connections. Recent advances in large-scale, cell-type specific calcium imaging approaches have shed new light on the cellular, circuit, and network-level dysfunction in animal models for psychiatric disorders. Here, we highlight a series of recent findings over the last ~10 years from in vivo calcium imaging studies that show how aberrant patterns of activity in discrete cell types and circuits may underlie behavioral deficits in animal models for several psychiatric disorders, including depression, anxiety, autism spectrum disorders, and schizophrenia. These advances in calcium imaging in pre-clinical models demonstrate the power of cell-type-specific imaging tools in understanding the underlying dysfunction in cell types, activity patterns, and neural circuits that may contribute to disease and provide new blueprints for developing more targeted therapeutics and treatment strategies.

Stress-induced c-fos expression in the medial prefrontal cortex differentially affects the main residing cell phenotypes

Stress poses a challenge to the body's equilibrium and triggers a series of responses that enable organisms to adapt to stressful stimuli. The medial prefrontal cortex (mPFC), particularly in acute stress conditions, undergoes significant physiological changes to cope with the demands associated with cellular activation. The proto-oncogene c-fos and its protein product c-Fos have long been utilized to investigate the effects of external factors on the central nervous system (CNS). While c-Fos expression has traditionally been attributed to neurons, emerging evidence suggests its potential expression in glial cells. In this study, our main objective was to explore the expression of c-Fos in glial cells and examine how acute stress influences these activity patterns. We conducted our experiments on male Wistar rats, subjecting them to acute stress and sacrificing them 2 h after the stressor initiation. Using double-labelling fluorescent immunohistochemistry targeting c-Fos, along with markers such as GFAP, Iba-1, Olig2, NG2, and NeuN, we analyzed 35 μm brain slices obtained from the mPFC. Our findings compellingly demonstrate that c-Fos expression extends beyond neurons and is present in astrocytes, oligodendrocytes, microglia, and NG2 cells-the diverse population of glial cells. Moreover, we observed distinct regulation of c-Fos expression in response to stress across different subregions of the mPFC. These results emphasize the importance of considering glial cells and their perspective in studies investigating brain activity, highlighting c-Fos as a response marker in glial cells. By shedding light on the differential regulation of c-Fos expression in response to stress, our study contributes to the understanding of glial cell involvement in stress-related processes. This underscores the significance of including glial cells in investigations of brain activity and expands our knowledge of c-Fos as a potential marker for glial cell responses.

Daily oscillations of neuronal membrane capacitance

Capacitance of biological membranes is determined by the properties of the lipid portion of the membrane as well as the morphological features of a cell. In neurons, membrane capacitance is a determining factor of synaptic integration, action potential propagation speed, and firing frequency due to its direct effect on the membrane time constant. Besides slow changes associated with increased morphological complexity during postnatal maturation, neuronal membrane capacitance is considered a stable, non-regulated, and constant magnitude. Here we report that, in two excitatory neuronal cell types, pyramidal cells of the mouse primary visual cortex and granule cells of the hippocampus, the membrane capacitance significantly changes between the start and the end of a daily light-dark cycle. The changes are large, nearly 2-fold in magnitude in pyramidal cells, but are not observed in cortical parvalbumin-expressing inhibitory interneurons. Consistent with daily capacitance fluctuations, the time window for synaptic integration also changes in pyramidal cells.

Astrocytic GLUT1 reduction paradoxically improves central and peripheral glucose homeostasis

Astrocytes are considered an essential source of blood-borne glucose or its metabolites to neurons. Nonetheless, the necessity of the main astrocyte glucose transporter, i.e., GLUT1, for brain glucose metabolism has not been defined. Unexpectedly, we found that brain glucose metabolism was paradoxically augmented in mice with astrocytic GLUT1 reduction (GLUT1ΔGFAP mice). These mice also exhibited improved peripheral glucose metabolism especially in obesity, rendering them metabolically healthier. Mechanistically, we observed that GLUT1-deficient astrocytes exhibited increased insulin receptor-dependent ATP release, and that both astrocyte insulin signaling and brain purinergic signaling are essential for improved brain function and systemic glucose metabolism. Collectively, we demonstrate that astrocytic GLUT1 is central to the regulation of brain energetics, yet its depletion triggers a reprogramming of brain metabolism sufficient to sustain energy requirements, peripheral glucose homeostasis, and cognitive function.

Astrocytic neuroligin 3 regulates social memory and synaptic plasticity through adenosine signaling in male mice

Social memory impairment is a key symptom of many brain disorders, but its underlying mechanisms remain unclear. Neuroligins (NLGs) are a family of cell adhesion molecules essential for synapse development and function and their dysfunctions are linked to neurodevelopmental and neuropsychiatric disorders, including autism and schizophrenia. Although NLGs are extensively studied in neurons, their role in glial cells is poorly understood. Here we show that astrocytic deletion of NLG3 in the ventral hippocampus of adult male mice impairs social memory, attenuates astrocytic Ca2+ signals, enhances the expression of EAAT2 and prevents long-term potentiation, and these impairments are rescued by increasing astrocyte activity, reducing EAAT2 function or enhancing adenosine/A2a receptor signaling. This study has revealed an important role of NLG3 in astrocyte function, glutamate homeostasis and social memory and identified the glutamate transporter and adenosine signaling pathway as potential therapeutic strategies to treat brain disorders.

Glial Control of Cortical Neuronal Circuit Maturation and Plasticity

The brain is a highly adaptable organ that is molded by experience throughout life. Although the field of neuroscience has historically focused on intrinsic neuronal mechanisms of plasticity, there is growing evidence that multiple glial populations regulate the timing and extent of neuronal plasticity, particularly over the course of development. This review highlights recent discoveries on the role of glial cells in the establishment of cortical circuits and the regulation of experience-dependent neuronal plasticity during critical periods of neurodevelopment. These studies provide strong evidence that neuronal circuit maturation and plasticity are non-cell autonomous processes that require both glial-neuronal and glial-glial cross talk to proceed. We conclude by discussing open questions that will continue to guide research in this nascent field.

Neuron-Glial Interactions: Implications for Plasticity, Behavior, and Cognition

The traditional view of glial cells as mere supportive tissue has shifted, due to advances in technology and theoretical conceptualization, to include a diversity of other functions, such as regulation of complex behaviors. Astrocytes, the most abundant glial cells in the central nervous system (CNS), have been shown to modulate synaptic functions through gliotransmitter-mediated neurotransmitter reuptake, influencing neuronal signaling and behavioral functions. Contemporary studies further highlight astrocytes' involvement in complex cognitive functions. For instance, inhibiting astrocytes in the hippocampus can lead to memory deficits, suggesting their integral role in memory processes. Moreover, astrocytic calcium activity and astrocyte-neuron metabolic coupling have been linked to changes in synaptic strength and learning. Microglia, another type of glial cell, also extend beyond their supportive roles, contributing to learning and memory processes, with microglial reductions impacting these functions in a developmentally dependent manner. Oligodendrocytes, traditionally thought to have limited roles postdevelopment, are now recognized for their activity-dependent modulation of myelination and plasticity, thus influencing behavioral responses. Recent advancements in technology and computational modeling have expanded our understanding of glial functions, particularly how astrocytes influence neuronal circuits and behaviors. This review underscores the importance of glial cells in CNS functions and the need for further research to unravel the complexities of neuron-glia interactions, the impact of these interactions on brain functions, and potential implications for neurological diseases.

Astrocytes in the Ventral Hippocampus Bidirectionally Regulate Innate and Stress-Induced Anxiety-Like Behaviors in Male Mice

The mechanisms of anxiety disorders, the most common mental illness, remain incompletely characterized. The ventral hippocampus (vHPC) is critical for the expression of anxiety. However, current studies primarily focus on vHPC neurons, leaving the role for vHPC astrocytes in anxiety largely unexplored. Here, genetically encoded Ca2+ indicator GCaMP6m and in vivo fiber photometry calcium imaging are used to label vHPC astrocytes and monitor their activity, respectively, genetic and chemogenetic approaches to inhibit and activate vHPC astrocytes, respectively, patch-clamp recordings to measure glutamate currents, and behavioral assays to assess anxiety-like behaviors. It is found that vHPC astrocytic activity is increased in anxiogenic environments and by 3-d subacute restraint stress (SRS), a well-validated mouse model of anxiety disorders. Genetic inhibition of vHPC astrocytes exerts anxiolytic effects on both innate and SRS-induced anxiety-related behaviors, whereas hM3Dq-mediated chemogenetic or SRS-induced activation of vHPC astrocytes enhances anxiety-like behaviors, which are reversed by intra-vHPC application of the ionotropic glutamate N-methyl-d-aspartate receptor antagonists. Furthermore, intra-vHPC or systemic application of the N-methyl-d-aspartate receptor antagonist memantine, a U.S. FDA-approved drug for Alzheimer's disease, fully rescues SRS-induced anxiety-like behaviors. The findings highlight vHPC astrocytes as critical regulators of stress and anxiety and as potential therapeutic targets for anxiety and anxiety-related disorders.

Adrenergic signaling gates astrocyte responsiveness to neurotransmitters and control of neuronal activity

How astrocytes regulate neuronal circuits is a fundamental, unsolved question in neurobiology. Nevertheless, few studies have explored the rules that govern when astrocytes respond to different neurotransmitters in vivo and how they affect downstream circuit modulation. Here, we report an unexpected mechanism in Drosophila by which G-protein coupled adrenergic signaling in astrocytes can control, or "gate," their ability to respond to other neurotransmitters. Further, we show that manipulating this pathway potently regulates neuronal circuit activity and animal behavior. Finally, we demonstrate that this gating mechanism is conserved in mammalian astrocytes, arguing it is an ancient feature of astrocyte circuit function. Our work establishes a new mechanism by which astrocytes dynamically respond to and modulate neuronal activity in different brain regions and in different behavioral states.

GLT-1 downregulation in hippocampal astrocytes induced by type 2 diabetes contributes to postoperative cognitive dysfunction in adult mice

AIMS

Type 2 diabetes mellitus (T2DM) is related to an increased risk of postoperative cognitive dysfunction (POCD), which may be caused by neuronal hyperexcitability. Astrocyte glutamate transporter 1 (GLT-1) plays a crucial role in regulating neuron excitability. We investigated if T2DM would magnify the increased neuronal excitability induced by anesthesia/surgery (A/S) and lead to POCD in young adult mice, and if so, determined whether these effects were associated with GLT-1 expression.

METHODS

T2DM model was induced by high fat diet (HFD) and injecting STZ. Then, we evaluated the spatial learning and memory of T2DM mice after A/S with the novel object recognition test (NORT) and object location test (OLT). Western blotting and immunofluorescence were used to analyze the expression levels of GLT-1 and neuronal excitability. Oxidative stress reaction and neuronal apoptosis were detected with SOD2 expression, MMP level, and Tunel staining. Hippocampal functional synaptic plasticity was assessed with long-term potentiation (LTP). In the intervention study, we overexpressed hippocampal astrocyte GLT-1 in GFAP-Cre mice. Besides, AAV-Camkllα-hM4Di-mCherry was injected to inhibit neuronal hyperexcitability in CA1 region.

RESULTS

Our study found T2DM but not A/S reduced GLT-1 expression in hippocampal astrocytes. Interestingly, GLT-1 deficiency alone couldn't lead to cognitive decline, but the downregulation of GLT-1 in T2DM mice obviously enhanced increased hippocampal glutamatergic neuron excitability induced by A/S. The hyperexcitability caused neuronal apoptosis and cognitive impairment. Overexpression of GLT-1 rescued postoperative cognitive dysfunction, glutamatergic neuron hyperexcitability, oxidative stress reaction, and apoptosis in hippocampus. Moreover, chemogenetic inhibition of hippocampal glutamatergic neurons reduced oxidative stress and apoptosis and alleviated postoperative cognitive dysfunction.

CONCLUSIONS

These findings suggest that the adult mice with type 2 diabetes are at an increased risk of developing POCD, perhaps due to the downregulation of GLT-1 in hippocampal astrocytes, which enhances increased glutamatergic neuron excitability induced by A/S and leads to oxidative stress reaction, and neuronal apoptosis.

Astrocytic glutamate transport is essential for the memory-enhancing effects of 17β-estradiol in ovariectomized mice

Infusion of 17β-estradiol (E2) into the dorsal hippocampus (DH) of ovariectomized (OVX) mice enhances memory consolidation, an effect that depends on rapid phosphorylation of extracellular signal-regulated kinase (ERK) and Akt. Astrocytic glutamate transporter 1 (GLT-1) modulates neurotransmission via glutamate uptake from the synaptic cleft. However, little is known about the contribution of DH astrocytes, and astrocytic glutamate transport, to the memory-enhancing effects of E2. This study was designed to test whether DH astrocytes contribute to estrogenic modulation of memory consolidation by determining the extent to which DH GLT-1 is necessary for E2 to enhance memory in object recognition and object placement tasks and trigger rapid phosphorylation events in DH astrocytes. OVX female mice were bilaterally cannulated into the DH or the DH and dorsal third ventricle (ICV). Post-training DH infusion of the GLT-1 inhibitor dihydrokainic acid (DHK) dose-dependently impaired memory consolidation in both tasks. Moreover, the memory-enhancing effects of ICV-infused E2 in each task were blocked by DH DHK infusion. E2 increased p42 ERK and Akt phosphorylation in DH astrocytes, and these effects were blocked by DHK. Results suggest the necessity of DH GLT-1 activity for object and spatial memory consolidation, and for E2 to enhance consolidation of these memories and to rapidly activate cell signaling in DH astrocytes. Findings indicate that astrocytic function in the DH of OVX females is necessary for memory formation and is regulated by E2, and suggest an essential role for DH astrocytic GLT-1 activity in the memory-enhancing effects of E2.

High-Frequency Imaging Reveals Synchronised Delta- and Theta-Band Ca2+ Oscillations in the Astrocytic Soma In Vivo

One of the major breakthroughs of neurobiology was the identification of distinct ranges of oscillatory activity in the neuronal network that were found to be responsible for specific biological functions, both physiological and pathological in nature. Astrocytes, physically coupled by gap junctions and possessing the ability to simultaneously modulate the functions of a large number of surrounding synapses, are perfectly positioned to introduce synchronised oscillatory activity into the neural network. However, astrocytic somatic calcium signalling has not been investigated to date in the frequency ranges of common neuronal oscillations, since astrocytes are generally considered to be slow responders in terms of Ca2+ signalling. Using high-frequency two-photon imaging, we reveal fast Ca2+ oscillations in the soma of astrocytes in the delta (0.5-4 Hz) and theta (4-8 Hz) frequency bands in vivo in the rat cortex under ketamine-xylazine anaesthesia, which is known to induce permanent slow-wave sleep. The high-frequency astrocytic Ca2+ signals were not observed under fentanyl anaesthesia, excluding the possibility that the signals were introduced by motion artefacts. We also demonstrate that these fast astrocytic Ca2+ signals, previously considered to be exclusive to neurons, are present in a large number of astrocytes and are phase synchronised at the astrocytic network level. We foresee that the disclosure of these high-frequency astrocytic signals may help with understanding the appearance of synchronised oscillatory signals and may open up new avenues of treatment for neurological conditions characterised by altered neuronal oscillations.

A spatial threshold for astrocyte calcium surge

Astrocytes are active cells involved in brain function through the bidirectional communication with neurons, in which the astrocyte calcium signal plays a crucial role. Synaptically-evoked calcium increases can be localized to independent subcellular domains or expand to the entire cell, i.e., calcium surge. In turn, astrocytes may regulate individual synapses by calcium-dependent release of gliotransmitters. Because a single astrocyte may contact ∼100,000 synapses, the control of the intracellular calcium signal propagation may have relevant consequences on brain function by regulating the spatial range of astrocyte neuromodulation of synapses. Yet, the properties governing the spatial dynamics of the astrocyte calcium signal remains poorly defined. Imaging subcellular responses of cortical astrocytes to sensory stimulation in mice, we show that sensory-evoked astrocyte calcium responses originated and remained localized in domains of the astrocytic arborization, but eventually propagated to the entire cell if a spatial threshold of >23% of the arborization being activated was surpassed. Using transgenic IP 3 R2 -/- mice, we found that type-2 IP 3 receptors were necessary for the generation of the astrocyte calcium surge. We finally show using in situ electrophysiological recordings that the spatial threshold of the astrocyte calcium signal consequently determined the gliotransmitter release. Present results reveal a fundamental property of astrocyte calcium physiology, i.e., a spatial threshold for the astrocyte intracellular calcium signal propagation, which depends on astrocyte intrinsic properties and governs the astrocyte integration of local synaptic activity and the subsequent neuromodulation.

One-Sentence Summary

There is a spatial threshold for the astrocyte intracellular calcium signal propagation that is determined by astrocyte intrinsic properties and controls gliotransmission.

A Novel Approach to Increase Glial Cell Populations in Brain Microphysiological Systems

Brain microphysiological systems (bMPS) recapitulate human brain cellular architecture and functionality more closely than traditional monolayer cultures and have become increasingly relevant for the study of neurological function in health and disease. Existing 3D brain models vary in reflecting the relative populations of different cell types present in the human brain. Most models consist mainly of neurons, while glial cells represent a smaller portion of the cell populations. Here, by means of a chemically defined glial-enriched medium (GEM), an improved method to expand the population of astrocytes and oligodendrocytes without compromising neuronal differentiation in bMPS, is presented. An important finding is that astrocytes also change in morphology when cultured in GEM, more closely recapitulating primary culture astrocytes. GEM bMPS are electro-chemically active and show different patterns of calcium staining and flux. Synaptic vesicles and terminals observed by electron microscopy are also present. No significant changes in neuronal differentiation are observed by gene expression, however, GEM enhanced neurite outgrowth and cell migration, and differentially modulated neuronal maturation in two different cell lines. These results have the potential to significantly improve functionality of bMPS for the study of neurological diseases and drug discovery, contributing to the unmet need for safe human models.

Scaled Complexity of Mammalian Astrocytes: Insights From Mouse and Macaque

Astrocytes intricately weave within the neuropil, giving rise to characteristic bushy morphologies. Pioneering studies suggested that primate astrocytes are more complex due to increased branch numbers and territory size compared to rodent counterparts. However, there has been no comprehensive comparison of astrocyte morphology across species. We employed several techniques to investigate astrocyte morphology and directly compared them between mice and rhesus macaques in cortical and subcortical regions. We assessed astrocyte density, territory size, branching structure, fine morphological complexity, and interactions with neuronal synapses using a combination of techniques, including immunohistochemistry, adeno-associated virus-mediated transduction of astrocytes, diOlistics, confocal imaging, and electron microscopy. We found significant morphological similarities between primate and rodent astrocytes, suggesting that astrocyte structure has scaled with evolution. Our findings show that primate astrocytes are larger and more numerous than those in rodents but contest the view that primate astrocytes are morphologically far more complex.

The ISR downstream target ATF4 represses long-term memory in a cell type-specific manner

The integrated stress response (ISR), a pivotal protein homeostasis network, plays a critical role in the formation of long-term memory (LTM). The precise mechanism by which the ISR controls LTM is not well understood. Here, we report insights into how the ISR modulates the mnemonic process by using targeted deletion of the activating transcription factor 4 (ATF4), a key downstream effector of the ISR, in various neuronal and non-neuronal cell types. We found that the removal of ATF4 from forebrain excitatory neurons (but not from inhibitory neurons, cholinergic neurons, or astrocytes) enhances LTM formation. Furthermore, the deletion of ATF4 in excitatory neurons lowers the threshold for the induction of long-term potentiation, a cellular model for LTM. Transcriptomic and proteomic analyses revealed that ATF4 deletion in excitatory neurons leads to upregulation of components of oxidative phosphorylation pathways, which are critical for ATP production. Thus, we conclude that ATF4 functions as a memory repressor selectively within excitatory neurons.

Astrocytes enhance plasticity response during reversal learning

Astrocytes play a key role in the regulation of synaptic strength and are thought to orchestrate synaptic plasticity and memory. Yet, how specifically astrocytes and their neuroactive transmitters control learning and memory is currently an open question. Recent experiments have uncovered an astrocyte-mediated feedback loop in CA1 pyramidal neurons which is started by the release of endocannabinoids by active neurons and closed by astrocytic regulation of the D-serine levels at the dendrites. D-serine is a co-agonist for the NMDA receptor regulating the strength and direction of synaptic plasticity. Activity-dependent D-serine release mediated by astrocytes is therefore a candidate for mediating between long-term synaptic depression (LTD) and potentiation (LTP) during learning. Here, we show that the mathematical description of this mechanism leads to a biophysical model of synaptic plasticity consistent with the phenomenological model known as the BCM model. The resulting mathematical framework can explain the learning deficit observed in mice upon disruption of the D-serine regulatory mechanism. It shows that D-serine enhances plasticity during reversal learning, ensuring fast responses to changes in the external environment. The model provides new testable predictions about the learning process, driving our understanding of the functional role of neuron-glia interaction in learning.

Activity-induced reactivity of astrocytes impairs cognition

Glial cells such as astrocytes can modulate neuronal signaling. Astrocytes can also acquire a reactive phenotype that correlates with cognitive impairments in brain diseases. A study in PLOS Biology shows that prolonged activation of astrocytes can trigger both cognitive impairments and a reactive astrocyte phenotype.

Aberrant activation of hippocampal astrocytes causes neuroinflammation and cognitive decline in mice

Reactive astrocytes are associated with neuroinflammation and cognitive decline in diverse neuropathologies; however, the underlying mechanisms are unclear. We used optogenetic and chemogenetic tools to identify the crucial roles of the hippocampal CA1 astrocytes in cognitive decline. Our results showed that repeated optogenetic stimulation of the hippocampal CA1 astrocytes induced cognitive impairment in mice and decreased synaptic long-term potentiation (LTP), which was accompanied by the appearance of inflammatory astrocytes. Mechanistic studies conducted using knockout animal models and hippocampal neuronal cultures showed that lipocalin-2 (LCN2), derived from reactive astrocytes, mediated neuroinflammation and induced cognitive impairment by decreasing the LTP through the reduction of neuronal NMDA receptors. Sustained chemogenetic stimulation of hippocampal astrocytes provided similar results. Conversely, these phenomena were attenuated by a metabolic inhibitor of astrocytes. Fiber photometry using GCaMP revealed a high level of hippocampal astrocyte activation in the neuroinflammation model. Our findings suggest that reactive astrocytes in the hippocampus are sufficient and required to induce cognitive decline through LCN2 release and synaptic modulation. This abnormal glial-neuron interaction may contribute to the pathogenesis of cognitive disturbances in neuroinflammation-associated brain conditions.

Role of amygdala astrocytes in different phases of contextual fear memory

Growing evidence indicates a critical role of astrocytes in learning and memory. However, little is known about the role of basolateral amygdala complex (BLA-C) astrocytes in contextual fear conditioning (CFC), a paradigm relevant to understand and generate treatments for fear- and anxiety-related disorders. To get insights on the involvement of BLA-C astrocytes in fear memory, fluorocitrate (FLC), a reversible astroglial metabolic inhibitor, was applied at critical moments of the memory processing in order to target the acquisition, consolidation, retrieval and reconsolidation process of the fear memory. Adult Wistar male rats were bilaterally cannulated in BLA-C. Ten days later they were infused with different doses of FLC (0.5 or 1 nmol/0.5 µl) or saline before or after CFC and before or after retrieval. FLC impaired fear memory expression when administered before and shortly after CFC, but not one hour later. Infusion of FLC prior and after retrieval did not affect the memory. Our findings suggest that BLA-C astrocytes are critically involved in the acquisition/early consolidation of fear memory but not in the retrieval and reconsolidation. Furthermore, the extinction process was presumably not affected (considering that peri-retrieval administration could also affect this process).

Hippocampal astrocytes induce sex-dimorphic effects on memory

Astrocytic receptors influence cognitive function and can promote behavioral deficits in disease. These effects may vary based on variables such as biological sex, but it is not known if the effects of astrocytic receptors are dependent on sex. We leveraged in vivo gene editing and chemogenetics to examine the roles of astrocytic receptors in spatial memory and other processes. We show that reductions in metabotropic glutamate receptor 3 (mGluR3), the main astrocytic glutamate receptor in adults, impair memory in females but enhance memory in males. Similarly, increases in astrocytic mGluR3 levels have sex-dependent effects and enhance memory in females. mGluR3 manipulations also alter spatial search strategies during recall in a sex-specific manner. In addition, acute chemogenetic stimulation of Gi/o-coupled or Gs-coupled receptors in hippocampal astrocytes induces bidirectional and sex-dimorphic effects on memory. Thus, astrocytes are sex-dependent modulators of cognitive function and may promote sex differences in aging and disease.

The meninges host a unique compartment of regulatory T cells that bulwarks adult hippocampal neurogenesis

Our knowledge about the meningeal immune system has recently burgeoned, particularly our understanding of how innate and adaptive effector cells are mobilized to meet brain challenges. However, information on how meningeal immunocytes guard brain homeostasis in healthy individuals remains sparse. This study highlights the heterogeneous and polyfunctional regulatory-T (Treg) cell compartment in the meninges. A Treg subtype specialized in controlling Th1-cell responses and another known to control responses in B-cell follicles were substantial components of this compartment, foretelling that punctual Treg-cell ablation rapidly unleashed interferon-gamma production by meningeal lymphocytes, unlocked their access to the brain parenchyma, and altered meningeal B-cell profiles. Distally, the hippocampus assumed a reactive state, with morphological and transcriptional changes in multiple glial-cell types; within the dentate gyrus, neural stem cells showed exacerbated death and desisted from further differentiation, associated with inhibition of spatial-reference memory. Thus, meningeal Treg cells are a multifaceted bulwark to brain homeostasis at steady-state.

One sentence summary

A distinct population of regulatory T cells in the murine meninges safeguards homeostasis by keeping local interferon-γ-producing lymphocytes in check, thereby preventing their invasion of the parenchyma, activation of hippocampal glial cells, death of neural stem cells, and memory decay.

Astroglial Dysfunctions in Mood Disorders and Rodent Stress Models: Consequences on Behavior and Potential as Treatment Target

Astrocyte dysfunctions have been consistently observed in patients affected with depression and other psychiatric illnesses. Although over the years our understanding of these changes, their origin, and their consequences on behavior and neuronal function has deepened, many aspects of the role of astroglial dysfunction in major depressive disorder (MDD) and post-traumatic stress disorder (PTSD) remain unknown. In this review, we summarize the known astroglial dysfunctions associated with MDD and PTSD, highlight the impact of chronic stress on specific astroglial functions, and how astroglial dysfunctions are implicated in the expression of depressive- and anxiety-like behaviors, focusing on behavioral consequences of astroglial manipulation on emotion-related and fear-learning behaviors. We also offer a glance at potential astroglial functions that can be targeted for potential antidepressant treatment.

Activation of Gs Signaling in Cortical Astrocytes Does Not Influence Formation of a Persistent Contextual Memory Engram

Formation and retrieval of remote contextual memory depends on cortical engram neurons that are defined during learning. Manipulation of astrocytic Gq and Gi associated G-protein coupled receptor (GPCR) signaling has been shown to affect memory processing, but little is known about the role of cortical astrocytic Gs-GPCR signaling in remote memory acquisition and the functioning of cortical engram neurons. We assessed this by chemogenetic manipulation of astrocytes in the medial prefrontal cortex (mPFC) of male mice, during either encoding or consolidation of a contextual fear memory, while simultaneously labeling cortical engram neurons. We found that stimulation of astrocytic Gs signaling during memory encoding and consolidation did not alter remote memory expression. In line with this, the size of the mPFC engram population and the recall-induced reactivation of these neurons was unaffected. Hence, our data indicate that activation of Gs-GPCR signaling in cortical astrocytes is not sufficient to alter memory performance and functioning of cortical engram neurons.

Astrocyte ryanodine receptors facilitate gliotransmission and astroglial modulation of synaptic plasticity

Intracellular Ca2+-signaling in astrocytes is instrumental for their brain "housekeeping" role and astroglial control of synaptic plasticity. An important source for elevating the cytosolic Ca2+ level in astrocytes is a release from endoplasmic reticulum which can be triggered via two fundamental pathways: IP3 receptors and calcium-induced calcium release (CICR) mediated by Ca2+-sensitive ryanodine receptors (RyRs). While the physiological role for glial IP3 became a focus of intensive research and debate, ryanodine receptors received much less attention. We explored the role for ryanodine receptors in the modulation of cytosolic Ca2+-signaling in the cortical and hippocampal astrocytes, astrocyte-neuron communication and astroglia modulation of synaptic plasticity. Our data show that RyR-mediated Ca2+-induced Ca2+-release from ER brings substantial contribution into signaling in the functional microdomains hippocampal and neocortical astrocytes. Furthermore, RyR-mediated CICR activated the release of ATP and glutamate from hippocampal and neocortical astrocytes which, in turn, elicited transient purinergic and tonic glutamatergic currents in the neighboring pyramidal neurons. The CICR-facilitated release of ATP and glutamate was inhibited after intracellular perfusion of astrocytes with ryanodine and BAPTA and in the transgenic dnSNARE mice with impaired astroglial exocytosis. We also found out that RyR-mediated amplification of astrocytic Ca2+-signaling enhanced the long-term synaptic potentiation in the hippocampus and neocortex of aged mice. Combined, our data demonstrate that ryanodine receptors are essential for astrocytic Ca2+-signaling and efficient astrocyte-neuron communications. The RyR-mediated CICR contributes to astrocytic control of synaptic plasticity and can underlie, at least partially, neuroprotective and cognitive effects of caffein.

Targeting cathepsin S promotes activation of OLF1-BDNF/TrkB axis to enhance cognitive function

BACKGROUND

Cathepsin S (CTSS) is a cysteine protease that played diverse roles in immunity, tumor metastasis, aging and other pathological alterations. At the cellular level, increased CTSS levels have been associated with the secretion of pro-inflammatory cytokines and disrupted the homeostasis of Ca2+ flux. Once CTSS was suppressed, elevated levels of anti-inflammatory cytokines and changes of Ca2+ influx were observed. These findings have inspired us to explore the potential role of CTSS on cognitive functions.

METHODS

We conducted classic Y-maze and Barnes Maze tests to assess the spatial and working memory of Ctss-/- mice, Ctss+/+ mice and Ctss+/+ mice injected with the CTSS inhibitor (RJW-58). Ex vivo analyses including long-term potentiation (LTP), Golgi staining, immunofluorescence staining of sectioned whole brain tissues obtained from experimental animals were conducted. Furthermore, molecular studies were carried out using cultured HT-22 cell line and primary cortical neurons that treated with RJW-58 to comprehensively assess the gene and protein expressions.

RESULTS

Our findings reported that targeting cathepsin S (CTSS) yields improvements in cognitive function, enhancing both working and spatial memory in behavior models. Ex vivo studies showed elevated levels of long-term potentiation levels and increased synaptic complexity. Microarray analysis demonstrated that brain-derived neurotrophic factor (BDNF) was upregulated when CTSS was knocked down by using siRNA. Moreover, the pharmacological blockade of the CTSS enzymatic activity promoted BDNF expression in a dose- and time-dependent manner. Notably, the inhibition of CTSS was associated with increased neurogenesis in the murine dentate gyrus. These results suggested a promising role of CTSS modulation in cognitive enhancement and neurogenesis.

CONCLUSION

Our findings suggest a critical role of CTSS in the regulation of cognitive function by modulating the Ca2+ influx, leading to enhanced activation of the BDNF/TrkB axis. Our study may provide a novel strategy for improving cognitive function by targeting CTSS.