Ca(2+) signaling in astrocytes from Ip3r2(-/-) mice in brain slices and during startle responses in vivo

The Paradox of Astroglial Ca2 + Signals at the Interface of Excitation and Inhibition

Astroglial networks constitute a non-neuronal communication system in the brain and are acknowledged modulators of synaptic plasticity. A sophisticated set of transmitter receptors in combination with distinct secretion mechanisms enables astrocytes to sense and modulate synaptic transmission. This integrative function evolved around intracellular Ca²⁺ signals, by and large considered as the main indicator of astrocyte activity. Regular brain physiology meticulously relies on the constant reciprocity of excitation and inhibition (E/I). Astrocytes are metabolically, physically, and functionally associated to the E/I convergence. Metabolically, astrocytes provide glutamine, the precursor of both major neurotransmitters governing E/I in the central nervous system (CNS): glutamate and γ-aminobutyric acid (GABA). Perisynaptic astroglial processes are structurally and functionally associated with the respective circuits throughout the CNS. Astonishingly, in astrocytes, glutamatergic as well as GABAergic inputs elicit similar rises in intracellular Ca²⁺ that in turn can trigger the release of glutamate and GABA as well. Paradoxically, as gliotransmitters, these two molecules can thus strengthen, weaken or even reverse the input signal. Therefore, the net impact on neuronal network function is often convoluted and cannot be simply predicted by the nature of the stimulus itself. In this review, we highlight the ambiguity of astrocytes on discriminating and affecting synaptic activity in physiological and pathological state. Indeed, aberrant astroglial Ca²⁺ signaling is a key aspect of pathological conditions exhibiting compromised network excitability, such as epilepsy. Here, we gather recent evidence on the complexity of astroglial Ca²⁺ signals in health and disease, challenging the traditional, neuro-centric concept of segregating E/I, in favor of a non-binary, mutually dependent perspective on glutamatergic and GABAergic transmission.

A Role for Astroglial Calcium in Mammalian Sleep and Sleep Regulation

Mammalian sleep expression and regulation have historically been thought to reflect the activity of neurons. Changes in other brain cells (glia) across the sleep-wake cycle and their role in sleep regulation are comparatively unexplored. We show that sleep and wakefulness are accompanied by state-dependent changes in astroglial activity. Using a miniature microscope in freely behaving mice and a two-photon microscope in head-fixed, unanesthetized mice, we show that astroglial calcium signals are highest in wake and lowest in sleep and are most pronounced in astroglial processes. We also find that astroglial calcium signals during non-rapid eye movement sleep change in proportion to sleep need. In contrast to neurons, astrocytes become less synchronized during non-rapid eye movement sleep after sleep deprivation at the network and single-cell level. Finally, we show that conditionally reducing intracellular calcium in astrocytes impairs the homeostatic response to sleep deprivation. Thus, astroglial calcium activity changes dynamically across vigilance states, is proportional to sleep need, and is a component of the sleep homeostat.

Astrocyte-mediated spike-timing-dependent long-term depression modulates synaptic properties in the developing cortex

Astrocytes have been shown to modulate synaptic transmission and plasticity in specific cortical synapses, but our understanding of the underlying molecular and cellular mechanisms remains limited. Here we present a new biophysicochemical model of a somatosensory cortical layer 4 to layer 2/3 synapse to study the role of astrocytes in spike-timing-dependent long-term depression (t-LTD) in vivo. By applying the synapse model and electrophysiological data recorded from rodent somatosensory cortex, we show that a signal from a postsynaptic neuron, orchestrated by endocannabinoids, astrocytic calcium signaling, and presynaptic N-methyl-D-aspartate receptors coupled with calcineurin signaling, induces t-LTD which is sensitive to the temporal difference between post- and presynaptic firing. We predict for the first time the dynamics of astrocyte-mediated molecular mechanisms underlying t-LTD and link complex biochemical networks at presynaptic, postsynaptic, and astrocytic sites to the time window of t-LTD induction. During t-LTD a single astrocyte acts as a delay factor for fast neuronal activity and integrates fast neuronal sensory processing with slow non-neuronal processing to modulate synaptic properties in the brain. Our results suggest that astrocytes play a critical role in synaptic computation during postnatal development and are of paramount importance in guiding the development of brain circuit functions, learning and memory.

Infrared Excitation Induces Heating and Calcium Microdomain Hyperactivity in Cortical Astrocytes

Unraveling how neural networks process and represent sensory information and how these cellular signals instruct behavioral output is a main goal in neuroscience. Two-photon activation of optogenetic actuators and calcium (Ca²⁺) imaging with genetically encoded indicators allow, respectively, the all-optical stimulation and readout of activity from genetically identified cell populations. However, these techniques locally expose the brain to high near-infrared light doses, raising the concern of light-induced adverse effects on the biology under study. Combining 2P imaging of Ca²⁺ transients in GCaMP6f-expressing cortical astrocytes and unbiased machine-based event detection, we demonstrate the subtle build-up of aberrant microdomain Ca²⁺ transients in the fine astroglial processes that depended on the average rather than peak laser power. Illumination conditions routinely being used in biological 2P microscopy (920-nm excitation, ∼100-fs, and ∼10 mW average power) increased the frequency of microdomain Ca²⁺ events but left their amplitude, area, and duration largely unchanged. Ca²⁺ transients in the otherwise silent soma were secondary to this peripheral hyperactivity that occurred without overt morphological damage. Continuous-wave (nonpulsed) 920-nm illumination at the same average power was as damaging as femtosecond pulses, unraveling the dominance of a heating-mediated damage mechanism. In an astrocyte-specific inositol 3-phosphate receptor type-2 knockout mouse, near-infrared light-induced Ca²⁺ microdomains persisted in the small processes, underpinning their resemblance to physiological inositol 3-phosphate receptor type-2-independent Ca²⁺ signals, whereas somatic hyperactivity was abolished. We conclude that, contrary to what has generally been believed in the field, shorter pulses and lower average power can help to alleviate damage and allow for longer recording windows at 920 nm.

Context-Specific Striatal Astrocyte Molecular Responses Are Phenotypically Exploitable

Astrocytes tile the central nervous system and are widely implicated in brain diseases, but the molecular mechanisms by which astrocytes contribute to brain disorders remain incompletely explored. By performing astrocyte gene expression analyses following 14 experimental perturbations of relevance to the striatum, we discovered that striatal astrocytes mount context-specific molecular responses at the level of genes, pathways, and upstream regulators. Through data mining, we also identified astrocyte pathways in Huntington's disease (HD) that were reciprocally altered with respect to the activation of striatal astrocyte G protein-coupled receptor (GPCR) signaling. Furthermore, selective striatal astrocyte stimulation of the G_i-GPCR pathway in vivo corrected several HD-associated astrocytic, synaptic, and behavioral phenotypes, with accompanying improvement of HD-associated astrocyte signaling pathways, including those related to synaptogenesis and neuroimmune functions. Overall, our data show that astrocytes are malleable, using context-specific responses that can be dissected molecularly and used for phenotypic benefit in brain disorders.

Astrocytic Calcium Dynamics Along the Pain Pathway

Astrocytes, once thought to be passive cells merely filling the space between neurons in the nervous system, are receiving attention as active modulators of the brain and spinal cord physiology by providing nutrients, maintaining homeostasis, and modulating synaptic transmission. Accumulating evidence indicates that astrocytes are critically involved in chronic pain regulation. Injury induces astrocytes to become reactive, and recent studies suggest that reactive astrocytes can have either neuroprotective or neurodegenerative effects. While the exact mechanisms underlying the transition from resting astrocytes to reactive astrocytes remain unknown, astrocytic calcium increase, coordinated by inflammatory molecules, has been suggested to trigger this transition. In this mini review article, we will discuss the roles of astrocytic calcium, channels contributing to calcium dynamics in astrocytes, astrocyte activations along the pain pathway, and possible relationships between astrocytic calcium dynamics and chronic pain.

Astrocytes: From the Physiology to the Disease

Astrocytes are key cells for adequate brain formation and regulation of cerebral blood flow as well as for the maintenance of neuronal metabolism, neurotransmitter synthesis and exocytosis, and synaptic transmission. Many of these functions are intrinsically related to neurodegeneration, allowing refocusing on the role of astrocytes in physiological and neurodegenerative states. Indeed, emerging evidence in the field indicates that abnormalities in the astrocytic function are involved in the pathogenesis of multiple neurodegenerative diseases, including Alzheimer's Disease (AD), Parkinson's Disease (PD), Huntington's Disease (HD) and Amyotrophic Lateral Sclerosis (ALS). In the present review, we highlight the physiological role of astrocytes in the CNS, including their communication with other cells in the brain. Furthermore, we discuss exciting findings and novel experimental approaches that elucidate the role of astrocytes in multiple neurological disorders.

Duplication of Symbiotic Lysin Motif Receptors Predates the Evolution of Nitrogen-Fixing Nodule Symbiosis

Rhizobium nitrogen-fixing nodule symbiosis occurs in two taxonomic lineages: legumes (Fabaceae) and the genus Parasponia (Cannabaceae). Both symbioses are initiated upon the perception of rhizobium-secreted lipochitooligosaccharides (LCOs), called Nod factors. Studies in the model legumes Lotus japonicus and Medicago truncatula showed that rhizobium LCOs are perceived by a heteromeric receptor complex of distinct Lys motif (LysM)-type transmembrane receptors named NOD FACTOR RECEPTOR1 (LjNFR1) and LjNFR5 (L. japonicus) and LYSM DOMAIN CONTAINING RECEPTOR KINASE3 (MtLYK3)-NOD FACTOR PERCEPTION (MtNFP; M. truncatula). Recent phylogenomic comparative analyses indicated that the nodulation traits of legumes, Parasponia spp., as well as so-called actinorhizal plants that establish a symbiosis with diazotrophic Frankia spp. bacteria share an evolutionary origin about 110 million years ago. However, the evolutionary trajectory of LysM-type LCO receptors remains elusive. By conducting phylogenetic analysis, transcomplementation studies, and CRISPR-Cas9 mutagenesis in Parasponia andersonii, we obtained insight into the origin of LCO receptors essential for nodulation. We identified four LysM-type receptors controlling nodulation in P. andersonii: PanLYK1, PanLYK3, PanNFP1, and PanNFP2 These genes evolved from ancient duplication events predating and coinciding with the origin of nodulation. Phylogenetic and functional analyses associated the occurrence of a functional NFP2-orthologous receptor to LCO-driven nodulation. Legumes and Parasponia spp. use orthologous LysM-type receptors to perceive rhizobium LCOs, suggesting a shared evolutionary origin of LCO-driven nodulation. Furthermore, we found that both PanLYK1 and PanLYK3 are essential for intracellular arbuscule formation of mutualistic endomycorrhizal fungi. PanLYK3 also acts as a chitin oligomer receptor essential for innate immune signaling, demonstrating functional analogy to CHITIN ELECITOR RECEPTOR KINASE-type receptors.

Interstitial ions: A key regulator of state-dependent neural activity?

Throughout the nervous system, ion gradients drive fundamental processes. Yet, the roles of interstitial ions in brain functioning is largely forgotten. Emerging literature is now revitalizing this area of neuroscience by showing that interstitial cations (K+, Ca2+ and Mg2+) are not static quantities but change dynamically across states such as sleep and locomotion. In turn, these state-dependent changes are capable of sculpting neuronal activity; for example, changing the local interstitial ion composition in the cortex is sufficient for modulating the prevalence of slow-frequency neuronal oscillations, or potentiating the gain of visually evoked responses. Disturbances in interstitial ionic homeostasis may also play a central role in the pathogenesis of central nervous system diseases. For example, impairments in K+ buffering occur in a number of neurodegenerative diseases, and abnormalities in neuronal activity in disease models disappear when interstitial K+ is normalized. Here we provide an overview of the roles of interstitial ions in physiology and pathology. We propose the brain uses interstitial ion signaling as a global mechanism to coordinate its complex activity patterns, and ion homeostasis failure contributes to central nervous system diseases affecting cognitive functions and behavior.

Vasculo-Neuronal Coupling and Neurovascular Coupling at the Neurovascular Unit: Impact of Hypertension

Components of the neurovascular unit (NVU) establish dynamic crosstalk that regulates cerebral blood flow and maintain brain homeostasis. Here, we describe accumulating evidence for cellular elements of the NVU contributing to critical physiological processes such as cerebral autoregulation, neurovascular coupling, and vasculo-neuronal coupling. We discuss how alterations in the cellular mechanisms governing NVU homeostasis can lead to pathological changes in which vascular endothelial and smooth muscle cell, pericyte and astrocyte function may play a key role. Because hypertension is a modifiable risk factor for stroke and accelerated cognitive decline in aging, we focus on hypertension-associated changes on cerebral arteriole function and structure, and the molecular mechanisms through which these may contribute to cognitive decline. We gather recent emerging evidence concerning cognitive loss in hypertension and the link with vascular dementia and Alzheimer’s disease. Collectively, we summarize how vascular dysfunction, chronic hypoperfusion, oxidative stress, and inflammatory processes can uncouple communication at the NVU impairing cerebral perfusion and contributing to neurodegeneration.

Astroglial FMRP modulates synaptic signaling and behavior phenotypes in FXS mouse model

Fragile X syndrome (FXS) is one of the most common inherited intellectual disability (ID) disorders, in which the loss of FMRP protein induces a range of cellular signaling changes primarily through excess protein synthesis. Although neuron-centered molecular and cellular events underlying FXS have been characterized, how different CNS cell types are involved in typical FXS synaptic signaling changes and behavioral phenotypes is largely unknown. Recent evidence suggests that selective loss of astroglial FMRP is able to dysregulate glutamate uptake, increase spine density, and impair motor-skill learning. Here we investigated the effect of astroglial FMRP on synaptic signaling and FXS-related behavioral and learning phenotypes in astroglial Fmr1 cKO and cON mice in which FMRP expression is selectively diminished or restored in astroglia. We found that selective loss of astroglial FMRP contributes to cortical hyperexcitability by enhancing NMDAR-mediated evoked but not spontaneous miniEPSCs and elongating cortical UP state duration. Selective loss of astroglial FMRP is also sufficient to increase locomotor hyperactivity, significantly diminish social novelty preference, and induce memory acquisition and extinction deficits in astroglial Fmr1 cKO mice. Importantly, re-expression of astroglial FMRP is able to significantly rescue the hyperactivity (evoked NMDAR response, UP state duration, and open field test) and social novelty preference in astroglial Fmr1 cON mice. These results demonstrate a profound role of astroglial FMRP in the evoked synaptic signaling, spontaneously occurring cortical UP states, and FXS-related behavioral and learning phenotypes and provide important new insights in the cell type consideration for the FMRP reactivation strategy.

Physiology of Astroglial Excitability

Classic physiology divides all neural cells into excitable neurons and nonexcitable neuroglia. Neuroglial cells, chiefly responsible for homeostasis and defense of the nervous tissue, coordinate their complex homeostatic responses with neuronal activity. This coordination reflects a specific form of glial excitability mediated by complex changes in intracellular concentration of ions and second messengers organized in both space and time. Astrocytes are equipped with multiple molecular cascades, which are central for regulating homeostasis of neurotransmitters, ionostasis, synaptic connectivity, and metabolic support of the central nervous system. Astrocytes are further provisioned with multiple receptors for neurotransmitters and neurohormones, which upon activation trigger intracellular signals mediated by Ca²⁺, Na⁺, and cyclic AMP. Calcium signals have distinct organization and underlying mechanisms in different astrocytic compartments thus allowing complex spatiotemporal signaling. Signals mediated by fluctuations in cytosolic Na⁺ are instrumental for coordination of Na⁺ dependent astrocytic transporters with tissue state and homeostatic demands. Astroglial ionic excitability may also involve K⁺, H^+, and Cl^-. The cyclic AMP signalling system is, in comparison to ions, much slower in targeting astroglial effector mechanisms. This evidence review summarizes the concept of astroglial intracellular excitability.

The molecular mechanism of synaptic activity-induced astrocytic volume transient

KEY POINTS

Neuronal activity causes astrocytic volume change via K⁺ uptake through TREK-1 containing two-pore domain potassium channels. The volume transient is terminated by Cl^- efflux through the Ca²⁺ -activated anion channel BEST1. The source of the Ca²⁺ required to open BEST1 appears to be the stretch-activated TRPA1 channel. Intense neuronal activity is synaptically coupled with a physical change in astrocytes via volume transients.

ABSTRACT

The brain volume changes dynamically and transiently upon intense neuronal activity through a tight regulation of ion concentrations and water movement across the plasma membrane of astrocytes. We have recently demonstrated that an intense neuronal activity and subsequent astrocytic AQP4-dependent volume transient are critical for synaptic plasticity and memory. We have also pharmacologically demonstrated a functional coupling between synaptic activity and the astrocytic volume transient. However, the precise molecular mechanisms of how intense neuronal activity and the astrocytic volume transient are coupled remain unclear. Here we utilized an intrinsic optical signal imaging technique combined with fluorescence imaging using ion sensitive dyes and molecular probes and electrophysiology to investigate the detailed molecular mechanisms in genetically modified mice. We report that a brief synaptic activity induced by a train stimulation (20 Hz, 1 s) causes a prolonged astrocytic volume transient (80 s) via K⁺ uptake through TREK-1 containing two-pore domain potassium (K2P) channels, but not Kir4.1 or NKCC1. This volume change is terminated by Cl^- efflux through the Ca²⁺ -activated anion channel BEST1, but not the volume-regulated anion channel TTYH. The source of the Ca²⁺ required to open BEST1 appears to be the stretch-activated TRPA1 channel in astrocytes, but not IP₃ R2. In summary, our study identifies several important astrocytic ion channels (AQP4, TREK-1, BEST1, TRPA1) as the key molecules leading to the neuronal activity-dependent volume transient in astrocytes. Our findings reveal new molecular and cellular mechanisms for the synaptic coupling of intense neuronal activity with a physical change in astrocytes via volume transients.

Neuronal Network Topology Indicates Distinct Recovery Processes after Stroke

Despite substantial recent progress in network neuroscience, the impact of stroke on the distinct features of reorganizing neuronal networks during recovery has not been defined. Using a functional connections-based approach through 2-photon in vivo calcium imaging at the level of single neurons, we demonstrate for the first time the functional connectivity maps during motion and nonmotion states, connection length distribution in functional connectome maps and a pattern of high clustering in motor and premotor cortical networks that is disturbed in stroke and reconstitutes partially in recovery. Stroke disrupts the network topology of connected inhibitory and excitatory neurons with distinct patterns in these 2 cell types and in different cortical areas. These data indicate that premotor cortex displays a distinguished neuron-specific recovery profile after stroke.

Microglial calcium signaling is attuned to neuronal activity in awake mice

Microglial calcium signaling underlies a number of key physiological and pathological processes in situ, but has not been studied in vivo in awake mice. Using multiple GCaMP6 variants targeted to microglia, we assessed how microglial calcium signaling responds to alterations in neuronal activity across a wide range. We find that only a small subset of microglial somata and processes exhibited spontaneous calcium transients in a chronic window preparation. However, hyperactive shifts in neuronal activity (kainate status epilepticus and CaMKIIa Gq DREADD activation) triggered increased microglial process calcium signaling, often concomitant with process extension. Additionally, hypoactive shifts in neuronal activity (isoflurane anesthesia and CaMKIIa Gi DREADD activation) also increased microglial process calcium signaling. Under hypoactive neuronal conditions, microglia also exhibited process extension and outgrowth with greater calcium signaling. Our work reveals that microglia have highly distinct microdomain signaling, and that processes specifically respond to bi-directional shifts in neuronal activity through increased calcium signaling.

Astrocytic release of ATP through type 2 inositol 1,4,5-trisphosphate receptor calcium signaling and social dominance behavior in mice

Brain mitochondrial function is critical for numerous neuronal processes. We recently identified a link between brain energy and social dominance, where higher levels of mitochondrial function resulted in increased social competitive ability. The underlying mechanism of this link, however, remains unclear. Here, we investigated the contribution of astrocytic release of adenosine triphosphate (ATP) through the type 2 inositol 1,4,5-triphosphate receptor to social dominance behavior. Mice lacking the type 2 inositol 1,4,5-triphosphate receptor were characterized for their social dominance behavior, as well as their performance on a nonsocial task, the Morris Water Maze. In parallel, we also examined mitochondrial function in the medial prefrontal cortex, nucleus accumbens, and hippocampus to investigate how deficiencies in astrocytic ATP could modulate overall mitochondrial function. While knockout mice showed similar competitive ability compared with their wild-type littermates, dominant knockout mice exhibited a significant delay in exerting their dominance during the initial encounter. Otherwise, there were no differences in anxiety and exploratory traits, spatial learning and memory, or brain mitochondrial function in either light or dark circadian phases. Our findings point to a marginal role of astrocytic ATP through IP₃ R2 in social competition, suggesting that, under basal conditions, the neuronal compartment is predominant for social dominance exertion.

Deciphering the star codings: astrocyte manipulation alters mouse behavior

Astrocytes occupy a vast area within the central nervous system (CNS). Despite their abundance, the functional role of astrocytes in vivo has only begun to be uncovered. Astrocytes were typically thought to be involved in pathophysiological states. However, recent studies have shown that astrocytes are actively involved in cell signaling in normal physiological states; manipulating various aspects of astrocytic cell signaling in vivo has revealed that astrocytes are key players in controlling healthy behavior in the absence of pathophysiology. Unfortunately, the study of astrocyte function is often limited by the number of approaches available due to our lack of understanding of cell physiology. This review summarizes recent studies in which altered astrocyte signaling capacity resulted in dramatic changes in behavior. We not only discuss the methodologies available to manipulate astrocytes but also provide insights into the behavioral roles of astrocytes in the CNS.

Genetic studies provide increased evidence that astrocytes, star-shaped cells in the central nervous system, play important roles affecting behavior in mammals. Although they are just as abundant as neurons, astrocytes are not excited by electrical signals. For this reason they have traditionally been regarded simply as ‘support cells’ for neurons, but recent evidence suggests that they can significantly modulate neuron signals. A review paper by Keebum Park and Sung Joong Lee at Seoul National University in South Korea highlights improved methods for monitoring the signaling processes related to astrocytes, which manifest most notably through sharp changes in calcium levels. Several studies have used genetic knockout mice, designer drugs and light-sensitive proteins to change astrocyte activity, affecting a diverse range of behaviors including sleeping and feeding patterns, memory formation, depression and obsessive compulsive disorder.

Melanopsin for Time-Controlling Activation of Astrocyte -Neuron Networks

Melanopsin, a mammalian G-protein-coupled photopigment, is a novel optical tool which enables studying astrocyte-neuron networks. Here, we describe the required guidelines to take advantage of this promising optical tool for functional neuron-glia studies. The selective expression of melanopsin in astrocytes allows triggering astrocytic Ca²⁺ signaling, changes in synaptic transmission, and modifying behavioral responses.

Astrocytic Ca2+ signaling is reduced during sleep and is involved in the regulation of slow wave sleep

Astrocytic Ca2+ signaling has been intensively studied in health and disease but has not been quantified during natural sleep. Here, we employ an activity-based algorithm to assess astrocytic Ca2+ signals in the neocortex of awake and naturally sleeping mice while monitoring neuronal Ca2+ activity, brain rhythms and behavior. We show that astrocytic Ca2+ signals exhibit distinct features across the sleep-wake cycle and are reduced during sleep compared to wakefulness. Moreover, an increase in astrocytic Ca2+ signaling precedes transitions from slow wave sleep to wakefulness, with a peak upon awakening exceeding the levels during whisking and locomotion. Finally, genetic ablation of an important astrocytic Ca2+ signaling pathway impairs slow wave sleep and results in an increased number of microarousals, abnormal brain rhythms, and an increased frequency of slow wave sleep state transitions and sleep spindles. Our findings demonstrate an essential role for astrocytic Ca2+ signaling in regulating slow wave sleep.

Mapping astrocyte activity domains by light sheet imaging and spatio-temporal correlation screening

Astrocytes are a major type of glial cell in the mammalian brain, essentially regulating neuronal development and function. Quantitative imaging represents an important approach to study astrocytic signaling in neural circuits. Focusing on astrocytic Ca²⁺ activity, a key pathway implicated in astrocye-neuron interaction, we here report a strategy combining fast light sheet fluorescence microscopy (LSFM) and correlative screening-based time series analysis, to map activity domains in astrocytes in living mammalian nerve tissue. Light sheet of micron-scale thickness enables wide-field optical sectioning to image astrocytes in acute mouse brain slices. Using both chemical and genetically encoded Ca²⁺ indicators, we demonstrate the complementary advantages of LSFM in mapping Ca²⁺ domains in astrocyte populations as compared to epifluorescence and two-photon microscopy. Our approach then revealed distinct kinetics of Ca²⁺ signals between cortical and hypothalamic astrocytes in resting conditions and following the activation of adrenergic G protein coupled receptor (GPCR). This observation highlights the activity heterogeneity across regionally distinct astrocyte populations, and indicates the potential of our method for investigating dynamic signals in astrocytes.

Region-Specific Transcriptional Control of Astrocyte Function Oversees Local Circuit Activities

Astrocytes play essential roles in brain function by supporting synaptic connectivity and associated circuits. How these roles are regulated by transcription factors is unknown. Moreover, there is emerging evidence that astrocytes exhibit regional heterogeneity, and the mechanisms controlling this diversity remain nascent. Here, we show that conditional deletion of the transcription factor nuclear factor I-A (NFIA) in astrocytes in the adult brain results in region-specific alterations in morphology and physiology that are mediated by selective DNA binding. Disruptions in astrocyte function following loss of NFIA are most pronounced in the hippocampus, manifested by impaired interactions with neurons, coupled with diminution of learning and memory behaviors. These changes in hippocampal astrocytes did not affect basal neuronal properties but specifically inhibited synaptic plasticity, which is regulated by NFIA in astrocytes through calcium-dependent mechanisms. Together, our studies reveal region-specific transcriptional dependencies for astrocytes and identify astrocytic NFIA as a key transcriptional regulator of hippocampal circuits.

Epileptic Seizures: Glia-Neuron Interactions For Better or For Worse

Investigations of the mechanisms generating epileptic seizures have primarily focused on neurons. However, more systemic research of brain circuits has highlighted an important role of non-neuronal cells such as glia in the genesis and spreading of generalized seizures in the brain.

The Emerging Nature of Astrocyte Diversity

Astrocytes are morphologically complex, ubiquitous cells that are viewed as a homogeneous population tiling the entire central nervous system (CNS). However, this view has been challenged in the last few years with the availability of RNA sequencing, immunohistochemistry, electron microscopy, morphological reconstruction, and imaging data. These studies suggest that astrocytes represent a diverse population of cells and that they display brain area- and disease-specific properties and functions. In this review, we summarize these observations, emphasize areas where clear conclusions can be made, and discuss potential unifying themes. We also identify knowledge gaps that need to be addressed in order to exploit astrocyte diversity as a biological phenomenon of physiological relevance in the CNS. We thus provide a summary and a perspective on astrocyte diversity in the vertebrate CNS.

Tackling mitochondrial diversity in brain function: from animal models to human brain organoids

Mitochondria exhibit high degree of heterogeneity within various tissues, including differences in terms of morphology, quantity, or function. Mitochondria can even vary among distinct sub-compartments of the same cell. Emerging evidence suggest that the molecular diversity of mitochondria can influence the identity and functionality of a given cell type. Human pathologies affecting mitochondria typically cause tissue and cell-type-specific impairment. Mitochondrial diversity could thus play a contributing role not only in physiological cell fate specification but also during pathological disease development. In this review, we discuss the role of mitochondrial diversity in brain function during health and disease. Recent advances in induced pluripotent stem cells (iPSCs) research and the derivation of cerebral organoids could provide novel opportunities to unveil the role of mitochondrial heterogeneity for the function of the human brain. Mitochondrial diversity might be at the bases of the cell-type-specific vulnerability of mitochondrial disorders and may represent an underappreciated target of disease intervention.

Structural basis of astrocytic Ca2+ signals at tripartite synapses

Astrocytic Ca²⁺ signals can be fast and local, supporting the idea that astrocytes have the ability to regulate single synapses. However, the anatomical basis of such specific signaling remains unclear, owing to difficulties in resolving the spongiform domain of astrocytes where most tripartite synapses are located. Using 3D-STED microscopy in living organotypic brain slices, we imaged the spongiform domain of astrocytes and observed a reticular meshwork of nodes and shafts that often formed loop-like structures. These anatomical features were also observed in acute hippocampal slices and in barrel cortex in vivo. The majority of dendritic spines were contacted by nodes and their sizes were correlated. FRAP experiments and Ca²⁺ imaging showed that nodes were biochemical compartments and Ca²⁺ microdomains. Mapping astrocytic Ca²⁺ signals onto STED images of nodes and dendritic spines showed they were associated with individual synapses. Here, we report on the nanoscale organization of astrocytes, identifying nodes as a functional astrocytic component of tripartite synapses that may enable synapse-specific communication between neurons and astrocytes.

Astrocytic Ca²⁺ signals can be fast and local, supporting the idea that astrocytes have the ability to regulate single synapses. Here, the authors report the organization of astrocytes at nanoscale level and identify nodes as a functional astrocytic component of tripartite synapses.

IP3 R2 null mice display a normal acquisition of somatic and neurological development milestones

Astrocytes are key players in the regulation of brain development and function. They sense and respond to the surrounding activity by elevating their intracellular calcium (Ca²⁺ ) levels. These astrocytic Ca²⁺ elevations emerge from different sources and display complex spatio-temporal properties. Ca²⁺ elevations are spatially distributed in global (soma and main processes) and/or focal regions (microdomains). The inositol 1,4,5-trisphosphate receptor type 2 knockout (IP₃ R2 KO) mouse model lacks global Ca²⁺ elevations in astrocytes, and it has been used by different laboratories. However, the constitutive deletion of IP₃ R2 during development may trigger compensating phenotypes, which could bias the results of experiments using developing or adult mice. To address this issue, we performed a detailed neurodevelopmental evaluation of male and female IP₃ R2 KO mice, during the first 21 days of life, as well as an evaluation of motor function, strength and neurological reflexes in adult mice. Our results show that male and female IP₃ R2 KO mice display a normal acquisition of developmental milestones, as compared with wild-type (WT) mice. We also show that IP₃ R2 KO mice display normal motor coordination, strength and neurological reflexes in adulthood. To exclude a potential compensatory overexpression of other IP₃ Rs, we quantified the relative mRNA levels of all 3 subtypes, in brain tissue. We found that, along with the complete deletion of Itpr2, there is no compensatory expression of Itpr1 or Itrp3. Overall, our results show that the IP₃ R2 KO mouse is a reliable model to study the functional impact of global IP₃ R2-dependent astrocytic Ca²⁺ elevations.

Neuronal Activity Drives Astroglial Connexin 30 in Perisynaptic Processes and Shapes Its Functions

Astrocytes play key roles in brain functions through dynamic interactions with neurons. One of their typical features is to express high levels of connexins (Cxs), Cx43 and Cx30, the gap junction (GJ)-forming proteins. Cx30 is involved in basic cognitive processes and shapes synaptic and network activities, as shown by recent studies in transgenic animals. Yet it remains unknown whether astroglial Cx30 expression, localization, and functions are endogenously and dynamically regulated by neuronal activity and could therefore play physiological roles in neurotransmission. We here show that neuronal activity increased hippocampal Cx30 protein levels via a posttranslational mechanism regulating lysosomal degradation. Neuronal activity also increased Cx30 protein levels at membranes and perisynaptic processes, as revealed by superresolution imaging. This translated at the functional level in the activation of Cx30 hemichannels and in Cx30-mediated remodeling of astrocyte morphology independently of GJ biochemical coupling. Altogether, these data show activity-dependent dynamics of Cx30 expression, perisynaptic localization, and functions.

High-Frequency Microdomain Ca2+ Transients and Waves during Early Myelin Internode Remodeling

Ensheathment of axons by myelin is a highly complex and multi-cellular process. Cytosolic calcium (Ca²⁺) changes in the myelin sheath have been implicated in myelin synthesis, but the source of this Ca²⁺ and the role of neuronal activity is not well understood. Using one-photon Ca²⁺ imaging, we investigated myelin sheath formation in the mouse somatosensory cortex and found a high rate of spontaneous microdomain Ca²⁺ transients and large-amplitude Ca²⁺ waves propagating along the internode. The frequency of Ca²⁺ transients and waves rapidly declines with maturation and reactivates during remyelination. Unexpectedly, myelin microdomain Ca²⁺ transients occur independent of neuronal action potential generation or network activity but are nearly completely abolished when the mitochondrial permeability transition pores are blocked. These findings are supported by the discovery of mitochondria organelles in non-compacted myelin. Together, the results suggest that myelin microdomain Ca²⁺ signals are cell-autonomously driven by high activity of mitochondria during myelin remodeling.

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Developing myelin sheaths show high rates of calcium transients and calcium waves

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Myelin calcium transients are independent from neuronal activity

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Adaxonal and paranodal myelin contained mitochondria

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Calcium transients require opening of mitochondrial permeability transition pores

Local Resting Ca2+ Controls the Scale of Astroglial Ca2+ Signals

Astroglia regulate neurovascular coupling while engaging in signal exchange with neurons. The underlying cellular machinery is thought to rely on astrocytic Ca2+ signals, but what controls their amplitude and waveform is poorly understood. Here, we employ time-resolved two-photon excitation fluorescence imaging in acute hippocampal slices and in cortex in vivo to find that resting [Ca2+] predicts the scale (amplitude) and the maximum (peak) of astroglial Ca2+ elevations. We bidirectionally manipulate resting [Ca2+] by uncaging intracellular Ca2+ or Ca2+ buffers and use ratiometric imaging of a genetically encoded Ca2+ indicator to establish that alterations in resting [Ca2+] change co-directionally the peak level and anti-directionally the amplitude of local Ca2+ transients. This relationship holds for spontaneous and for induced (for instance by locomotion) Ca2+ signals. Our findings uncover a basic generic rule of Ca2+ signal formation in astrocytes, thus also associating the resting Ca2+ level with the physiological "excitability" state of astroglia.

Arousal-induced cortical activity triggers lactate release from astrocytes

It has been suggested that, in states of arousal, release of noradrenaline and β-adrenergic signalling affect long-term memory formation by stimulating astrocytic lactate production from glycogen. However, the temporal relationship between cortical activity and cellular lactate fluctuations upon changes in arousal remains to be fully established. Also, the role of β-adrenergic signalling and brain glycogen metabolism on neural lactate dynamics in vivo is still unknown. Here, we show that an arousal-induced increase in cortical activity triggers lactate release into the extracellular space, and this correlates with a fast and prominent lactate dip in astrocytes. The immediate drop in astrocytic lactate concentration and the parallel increase in extracellular lactate levels underline an activity-dependent lactate release from astrocytes. Moreover, when β-adrenergic signalling is blocked or the brain is depleted of glycogen, the arousal-evoked cellular lactate surges are significantly reduced. We provide in vivo evidence that cortical activation upon arousal triggers lactate release from astrocytes, a rise in intracellular lactate levels mediated by β-adrenergic signalling and the mobilization of lactate from glycogen stores.

Emerging technologies to study glial cells

Development, physiological functions, and pathologies of the brain depend on tight interactions between neurons and different types of glial cells, such as astrocytes, microglia, oligodendrocytes, and oligodendrocyte precursor cells. Assessing the relative contribution of different glial cell types is required for the full understanding of brain function and dysfunction. Over the recent years, several technological breakthroughs were achieved, allowing "glio-scientists" to address new challenging biological questions. These technical developments make it possible to study the roles of specific cell types with medium or high-content workflows and perform fine analysis of their mutual interactions in a preserved environment. This review illustrates the potency of several cutting-edge experimental approaches (advanced cell cultures, induced pluripotent stem cell (iPSC)-derived human glial cells, viral vectors, in situ glia imaging, opto- and chemogenetic approaches, and high-content molecular analysis) to unravel the role of glial cells in specific brain functions or diseases. It also illustrates the translation of some techniques to the clinics, to monitor glial cells in patients, through specific brain imaging methods. The advantages, pitfalls, and future developments are discussed for each technique, and selected examples are provided to illustrate how specific "gliobiological" questions can now be tackled.

Genetic targeting of astrocytes to combat neurodegenerative disease

Astrocytes, glial cells that interact extensively with neurons and other support cells throughout the central nervous system, have recently come under the spotlight for their potential contribution to, or potential regenerative role in a host of neurodegenerative disorders. It is becoming increasingly clear that astrocytes, in concert with microglial cells, activate intrinsic immunological pathways in the setting of neurodegenerative injury, although the direct and indirect consequences of such activation are still largely unknown. We review the current literature on the astrocyte’s role in several neurodegenerative diseases, as well as highlighting recent advances in genetic manipulation of astrocytes that may prove critical to modulating their response to neurological injury, potentially combatting neurodegenerative damage.

A roadmap to integrate astrocytes into Systems Neuroscience

Systems neuroscience is still mainly a neuronal field, despite the plethora of evidence supporting the fact that astrocytes modulate local neural circuits, networks, and complex behaviors. In this article, we sought to identify which types of studies are necessary to establish whether astrocytes, beyond their well-documented homeostatic and metabolic functions, perform computations implementing mathematical algorithms that sub-serve coding and higher-brain functions. First, we reviewed Systems-like studies that include astrocytes in order to identify computational operations that these cells may perform, using Ca2+ transients as their encoding language. The analysis suggests that astrocytes may carry out canonical computations in a time scale of subseconds to seconds in sensory processing, neuromodulation, brain state, memory formation, fear, and complex homeostatic reflexes. Next, we propose a list of actions to gain insight into the outstanding question of which variables are encoded by such computations. The application of statistical analyses based on machine learning, such as dimensionality reduction and decoding in the context of complex behaviors, combined with connectomics of astrocyte-neuronal circuits, is, in our view, fundamental undertakings. We also discuss technical and analytical approaches to study neuronal and astrocytic populations simultaneously, and the inclusion of astrocytes in advanced modeling of neural circuits, as well as in theories currently under exploration such as predictive coding and energy-efficient coding. Clarifying the relationship between astrocytic Ca2+ and brain coding may represent a leap forward toward novel approaches in the study of astrocytes in health and disease.

Spontaneous Ultraslow Na+ Fluctuations in the Neonatal Mouse Brain

In the neonate forebrain, network formation is driven by the spontaneous synchronized activity of pyramidal cells and interneurons, consisting of bursts of electrical activity and intracellular Ca²⁺ oscillations. By employing ratiometric Na⁺ imaging in tissue slices obtained from animals at postnatal day 2-4 (P2-4), we found that 20% of pyramidal neurons and 44% of astrocytes in neonatal mouse hippocampus also exhibit transient fluctuations in intracellular Na⁺. These occurred at very low frequencies (~2/h), were exceptionally long (~8 min), and strongly declined after the first postnatal week. Similar Na⁺ fluctuations were also observed in the neonate neocortex. In the hippocampus, Na⁺ elevations in both cell types were diminished when blocking action potential generation with tetrodotoxin. Neuronal Na⁺ fluctuations were significantly reduced by bicuculline, suggesting the involvement of GABA_A-receptors in their generation. Astrocytic signals, by contrast, were neither blocked by inhibition of receptors and/or transporters for different transmitters including GABA and glutamate, nor of various Na⁺-dependent transporters or Na⁺-permeable channels. In summary, our results demonstrate for the first time that neonatal astrocytes and neurons display spontaneous ultraslow Na⁺ fluctuations. While neuronal Na⁺ signals apparently largely rely on suprathreshold GABAergic excitation, astrocytic Na⁺ signals, albeit being dependent on neuronal action potentials, appear to have a separate trigger and mechanism, the source of which remains unclear at present.

Macrophage Migration Inhibitory Factor Alters Functional Properties of CA1 Hippocampal Neurons in Mouse Brain Slices

Neuroinflammation is implicated in a host of neurological insults, such as traumatic brain injury (TBI), ischemic stroke, Alzheimer's disease, Parkinson's disease, and epilepsy. The immune response to central nervous system (CNS) injury involves sequelae including the release of numerous cytokines and chemokines. Macrophage migration inhibitory factor (MIF), is one such cytokine that is elevated following CNS injury, and is associated with the prognosis of TBI, and ischemic stroke. MIF has been identified in astrocytes and neurons, and some of the trophic actions of MIF have been related to its direct and indirect actions on astrocytes. However, the potential modulation of CNS neuronal function by MIF has not yet been explored. This study tests the hypothesis that MIF can directly influence hippocampal neuronal function. MIF was microinjected into the hippocampus and the genetically encoded calcium indicator, GCaMP6f, was used to measure Ca²⁺ events in acute adult mouse brain hippocampal slices. Results demonstrated that a single injection of 200 ng MIF into the hippocampus significantly increased baseline calcium signals in CA1 pyramidal neuron somata, and altered calcium responses to N-methyl-d-aspartate (NMDA) + D-serine in pyramidal cell apical dendrites located in the stratum radiatum. These data are the first to show direct effects of MIF on hippocampal neurons and on NMDA receptor function. Considering that MIF is elevated after brain insults such as TBI, the data suggest that, in addition to the previously described role of MIF in astrocyte reactivity, elevated MIF can have significant effects on neuronal function in the hippocampus.

Ca2+ Signals in Astrocytes Facilitate Spread of Epileptiform Activity

Epileptic seizures are associated with increased astrocytic Ca²⁺ signaling, but the fine spatiotemporal kinetics of the ictal astrocyte–neuron interplay remains elusive. By using 2-photon imaging of awake head-fixed mice with chronic hippocampal windows we demonstrate that astrocytic Ca²⁺ signals precede neuronal Ca²⁺ elevations during the initial bout of kainate-induced seizures. On average, astrocytic Ca²⁺ elevations preceded neuronal activity in CA1 by about 8 s. In subsequent bouts of epileptic seizures, astrocytes and neurons were activated simultaneously. The initial astrocytic Ca²⁺ elevation was abolished in mice lacking the type 2 inositol-1,4,5-trisphosphate-receptor (Itpr2^−/−). Furthermore, we found that Itpr2^−/− mice exhibited 60% less epileptiform activity compared with wild-type mice when assessed by telemetric EEG monitoring. In both genotypes we also demonstrate that spreading depression waves may play a part in seizure termination. Our findings imply a role for astrocytic Ca²⁺ signals in ictogenesis.

Synchronized Astrocytic Ca2+ Responses in Neurovascular Coupling during Somatosensory Stimulation and for the Resting State

The role of astrocytes in neurovascular coupling (NVC) is unclear. Here, we applied a multimodality imaging approach to concomitantly measure synchronized neuronal or astrocytic Ca²⁺ and hemodynamic changes in the mouse somatosensory cortex at rest and during sensory electrical stimulation. Strikingly, we found that low-frequency stimulation (0.3-1 Hz), which consistently evokes fast neuronal Ca²⁺ transients (6.0 ± 2.7 ms latency) that always precede vascular responses, does not always elicit astrocytic Ca²⁺ transients (313 ± 65 ms latency). However, the magnitude of the hemodynamic response is increased when astrocytic transients occur, suggesting a facilitatory role of astrocytes in NVC. High-frequency stimulation (5-10 Hz) consistently evokes a large, delayed astrocytic Ca²⁺ accumulation (3.48 ± 0.09 s latency) that is temporarily associated with vasoconstriction, suggesting a role for astrocytes in resetting NVC. At rest, neuronal, but not astrocytic, Ca²⁺ fluctuations correlate with hemodynamic low-frequency oscillations. Taken together, these results support a role for astrocytes in modulating, but not triggering, NVC.

NCX activity generates spontaneous Ca2+ oscillations in the astrocytic leaflet microdomain

The synergy between synaptic Glu release and astrocytic Glu-Na⁺ symport is essential to the signalling function of the tripartite synapse. Here we used kinetic data of astrocytic Glu transporters (EAAT) and the Na⁺/Ca²⁺ exchanger (NCX) to simulate Glu release, Glu uptake and subsequent Na⁺ and Ca²⁺ dynamics in the astrocytic leaflet microdomain following single release event. Model simulations show that Glu-Na⁺ symport differently affect intracellular [Na⁺] in synapses with different extent of astrocytic coverage. Surprisingly, NCX activity alone has been shown to generate markedly stable, spontaneous Ca²⁺ oscillation in the astrocytic leaflet. These on-going oscillations appear when NCX operates either in the forward or reverse direction. We conjecture that intrinsic NCX activity may play a prominent role in the generation of astrocytic Ca²⁺ oscillations.

Physiology of Astroglia

Astrocytes are principal cells responsible for maintaining the brain homeostasis. Additionally, these glial cells are also involved in homocellular (astrocyte-astrocyte) and heterocellular (astrocyte-other cell types) signalling and metabolism. These astroglial functions require an expression of the assortment of molecules, be that transporters or pumps, to maintain ion concentration gradients across the plasmalemma and the membrane of the endoplasmic reticulum. Astrocytes sense and balance their neurochemical environment via variety of transmitter receptors and transporters. As they are electrically non-excitable, astrocytes display intracellular calcium and sodium fluctuations, which are not only used for operative signalling but can also affect metabolism. In this chapter we discuss the molecules that achieve ionic gradients and underlie astrocyte signalling.

Accurate quantification of astrocyte and neurotransmitter fluorescence dynamics for single-cell and population-level physiology

Recent work examining astrocytic physiology centers on fluorescence imaging, due to development of sensitive fluorescent indicators and observation of spatiotemporally complex calcium activity. However, the field remains hindered in characterizing these dynamics, both within single cells and at the population level, because of the insufficiency of current region-of-interest (ROI)-based approaches to describe activity that is often spatially unfixed, size-varying, and propagative. Here, we present an analytical framework that releases astrocyte biologists from ROI-based tools. The Astrocyte Quantitative Analysis (AQuA) software takes an event-based perspective to model and accurately quantify complex calcium and neurotransmitter activity in fluorescence imaging datasets. We apply AQuA to a range of ex vivo and in vivo imaging data, and uncover novel physiological phenomena. Since AQuA is data-driven and based on machine learning principles, it can be applied across model organisms, fluorescent indicators, experimental modes, and imaging resolutions and speeds, enabling researchers to elucidate fundamental neural physiology.

Astrocytes in chronic pain and itch

Astrocytes are critical for maintaining the homeostasis of the CNS. Increasing evidence suggests that a number of neurological and neuropsychiatric disorders, including chronic pain, may result from astrocyte 'gliopathy'. Indeed, in recent years there has been substantial progress in our understanding of how astrocytes can regulate nociceptive synaptic transmission via neuronal-glial and glial-glial cell interactions, as well as the involvement of spinal and supraspinal astrocytes in the modulation of pain signalling and the maintenance of neuropathic pain. A role of astrocytes in the pathogenesis of chronic itch is also emerging. These developments suggest that targeting the specific pathways that are responsible for astrogliopathy may represent a novel approach to develop therapies for chronic pain and chronic itch.

A stocked toolbox for understanding the role of astrocytes in disease

Our understanding of astrocytes and their role in neurological diseases has increased considerably over the past two decades as the diverse roles of these cells have become recognized. Our evolving understanding of these cells suggests that they are more than support cells for neurons and that they play important roles in CNS homeostasis under normal conditions, in neuroprotection and in disease exacerbation. These multiple functions make them excellent candidates for targeted therapies to treat neurological disorders. New technological advances, including in vivo imaging, optogenetics and chemogenetics, have allowed us to examine astrocytic functions in ways that have uncovered new insights into the dynamic roles of these cells. Furthermore, the use of induced pluripotent stem cell-derived astrocytes from patients with a host of neurological disorders can help to tease out the contributions of astrocytes to human disease. In this Review, we explore some of the technological advances developed over the past decade that have aided our understanding of astrocyte function. We also highlight neurological disorders in which astrocyte function or dysfunction is believed to have a role in disease pathogenesis or propagation and discuss how the technological advances have been and could be used to study each of these diseases.

Versatile control of synaptic circuits by astrocytes: where, when and how?

Close structural and functional interactions of astrocytes with synapses play an important role in brain function. The repertoire of ways in which astrocytes can regulate synaptic transmission is complex so that they can both promote and dampen synaptic efficacy. Such contrasting effects raise questions regarding the determinants of these divergent astroglial functions. Recent findings provide insights into where, when and how astroglial regulation of synapses takes place by revealing major molecular and functional intrinsic heterogeneity as well as switches in astrocytes occurring during development or specific patterns of neuronal activity. Astrocytes may therefore be seen as boosters or gatekeepers of synaptic circuits depending on their intrinsic and transformative properties throughout life.

Astroglial dysfunctions drive aberrant synaptogenesis and social behavioral deficits in mice with neonatal exposure to lengthy general anesthesia

Lengthy use of general anesthetics (GAs) causes neurobehavioral deficits in the developing brain, which has raised significant clinical concerns such that the United States Food and Drug Administration (FDA) is warning on the use of GAs in children younger than 3 years. However, the molecular and cellular mechanisms for GAs-induced neurotoxicity remain largely unknown. Here, we report that sevoflurane (Sevo), a commonly used GA in pediatrics, caused compromised astrocyte morphogenesis spatiotemporally correlated to synaptic overgrowth, with reduced synaptic function in developing cortex in a regional-, exposure-length-, and age-specific manner. Sevo disrupted astrocyte Ca²⁺ homeostasis both acutely and chronically, which led to the down-regulation of Ezrin, an actin-binding membrane-bound protein, which we found was critically involved in astrocyte morphogenesis in vivo. Importantly, overexpression of astrocyte Ezrin rescued astrocytic and neuronal dysfunctions and fully corrected deficits in social behaviors in developing mice with lengthy Sevo exposure. Our data uncover that, in addition to neurons, astrocytes may represent important targets for GAs to exert toxic effects and that astrocyte morphological integrity is crucial for synaptogenesis and neurological behaviors.

The extended use of general anesthetics can cause neurobehavioral deficits in the developing brain, leading to clinical concerns regarding their use in children younger than 3 years. This study shows that general anesthetics target glial cells to disrupt neural circuit formation in the developing brain, an effect that may underlie the observed learning, cognitive, or emotional deficits.

Simulation of calcium signaling in fine astrocytic processes: Effect of spatial properties on spontaneous activity

Astrocytes, a glial cell type of the central nervous system, have emerged as detectors and regulators of neuronal information processing. Astrocyte excitability resides in transient variations of free cytosolic calcium concentration over a range of temporal and spatial scales, from sub-microdomains to waves propagating throughout the cell. Despite extensive experimental approaches, it is not clear how these signals are transmitted to and integrated within an astrocyte. The localization of the main molecular actors and the geometry of the system, including the spatial organization of calcium channels IP3R, are deemed essential. However, as most calcium signals occur in astrocytic ramifications that are too fine to be resolved by conventional light microscopy, most of those spatial data are unknown and computational modeling remains the only methodology to study this issue. Here, we propose an IP3R-mediated calcium signaling model for dynamics in such small sub-cellular volumes. To account for the expected stochasticity and low copy numbers, our model is both spatially explicit and particle-based. Extensive simulations show that spontaneous calcium signals arise in the model via the interplay between excitability and stochasticity. The model reproduces the main forms of calcium signals and indicates that their frequency crucially depends on the spatial organization of the IP3R channels. Importantly, we show that two processes expressing exactly the same calcium channels can display different types of calcium signals depending on the spatial organization of the channels. Our model with realistic process volume and calcium concentrations successfully reproduces spontaneous calcium signals that we measured in calcium micro-domains with confocal microscopy and predicts that local variations of calcium indicators might contribute to the diversity of calcium signals observed in astrocytes. To our knowledge, this model is the first model suited to investigate calcium dynamics in fine astrocytic processes and to propose plausible mechanisms responsible for their variability.

Astroglial endfeet exhibit distinct Ca2+ signals during hypoosmotic conditions

Astrocytic endfeet cover the brain surface and form a sheath around the cerebral vasculature. An emerging concept is that endfeet control blood-brain water transport and drainage of interstitial fluid and waste along paravascular pathways. Little is known about the signaling mechanisms that regulate endfoot volume and hence the width of these drainage pathways. Here, we used the genetically encoded fluorescent Ca²⁺ indicator GCaMP6f to study Ca²⁺ signaling within astrocytic somata, processes, and endfeet in response to an osmotic challenge known to induce cell swelling. Acute cortical slices were subjected to artificial cerebrospinal fluid with 20% reduction in osmolarity while GCaMP6f fluorescence was imaged with two-photon microscopy. Ca²⁺ signals induced by hypoosmotic conditions were observed in all astrocytic compartments except the soma. The Ca²⁺ response was most prominent in subpial and perivascular endfeet and included spikes with single peaks, plateau-type elevations, and rapid oscillations, the latter restricted to subpial endfeet. Genetic removal of the type 2 inositol 1,4,5-triphosphate receptor (IP3R2) severely suppressed the Ca²⁺ responses in endfeet but failed to affect brain water accumulation in vivo after water intoxication. Furthermore, the increase in endfoot Ca²⁺ spike rate during hypoosmotic conditions was attenuated in mutant mice lacking the aquaporin-4 anchoring molecule dystrophin and after blockage of transient receptor potential vanilloid 4 channels. We conclude that the characteristics and underpinning of Ca²⁺ responses to hypoosmotic stress differ within the astrocytic territory and that IP3R2 is essential for the Ca²⁺ signals only in subpial and perivascular endfeet.

Astroglial Calcium Signaling in Aging and Alzheimer's Disease

Astrocytes are the homeostatic and protective cells of the central nervous system (CNS). In neurological diseases, astrocytes undergo complex changes, which are subclassified into (1) reactive astrogliosis, an evolutionary conserved defensive rearrangement of cellular phenotype aimed at neuroprotection; (2) pathological remodeling, when astrocytes acquire new features driving pathology; and (3) astrodegeneration, which is manifested by astroglial atrophy and loss of homeostatic functions. In aging brains as well as in the brains affected by Alzheimer's disease (AD), astrocytes acquire both atrophic and reactive phenotypes in a region- and disease-stage-dependent manner. Prevalence of atrophy overreactivity, observed in certain brain regions and in terminal stages of the disease, arguably facilitates the development of neurological deficits. Astrocytes exhibit ionic excitability mediated by changes in intracellular concentration of ions, most importantly of Ca²⁺ and Na⁺, with intracellular ion dynamics triggered by the activity of neural networks. AD astrocytes associated with senile plaques demonstrate Ca²⁺ hyperactivity in the form of aberrant Ca²⁺ oscillations and pathological long-range Ca²⁺ waves. Astroglial Ca²⁺ signaling originating from Ca²⁺ release from the endoplasmic reticulum is a key factor in initiating astrogliotic response; deficient Ca²⁺ signaling toolkits observed in entorhinal and prefrontal cortices of AD model animals may account for vulnerability of these regions to the pathology.