Calcium signalling in astroglia

Store-operated calcium entry in the satellite glial cells of rat sympathetic ganglia

Satellite glial cells (SGCs), a major type of glial cell in the autonomic ganglia, closely envelop the cell body and even the synaptic regions of a single neuron with a very narrow gap. This structurally unique organization suggests that autonomic neurons and SGCs may communicate reciprocally. Glial Ca2+ signaling is critical for controlling neural activity. Here, for the first time we identified the machinery of store-operated Ca2+ entry (SOCE) which is critical for cellular Ca2+ homeostasis in rat sympathetic ganglia under normal and pathological states. Quantitative realtime PCR and immunostaining analyses showed that Orai1 and stromal interaction molecules 1 (STIM1) proteins are the primary components of SOCE machinery in the sympathetic ganglia. When the internal Ca2+ stores were depleted in the absence of extracellular Ca2+, the number of plasmalemmal Orai1 puncta was increased in neurons and SGCs, suggesting activation of the Ca2+ entry channels. Intracellular Ca2+ imaging revealed that SOCE was present in SGCs and neurons; however, the magnitude of SOCE was much larger in the SGCs than in the neurons. The SOCE was significantly suppressed by GSK7975A, a selective Orai1 blocker, and Pyr6, a SOCE blocker. Lipopolysaccharide (LPS) upregulated the glial fibrillary acidic protein and Toll-like receptor 4 in the sympathetic ganglia. Importantly, LPS attenuated SOCE via downregulating Orai1 and STIM1 expression. In conclusion, sympathetic SGCs functionally express the SOCE machinery, which is indispensable for intracellular Ca2+ signaling. The SOCE is highly susceptible to inflammation, which may affect sympathetic neuronal activity and thereby autonomic output.

Astroglial S100B Secretion Is Mediated by Ca2+ Mobilization from Endoplasmic Reticulum: A Study Using Forskolin and DMSO as Secretagogues

S100B, a homodimeric Ca2+-binding protein, is produced and secreted by astrocytes, and its extracellular levels have been used as a glial marker in brain damage and neurodegenerative and psychiatric diseases; however, its mechanism of secretion is elusive. We used primary astrocyte cultures and calcium measurements from real-time fluorescence microscopy to investigate the role of intracellular calcium in S100B secretion. In addition, the dimethyl sulfoxide (DMSO) effect on S100B was investigated in vitro and in vivo using Wistar rats. We found that DMSO, a widely used vehicle in biological assays, is a powerful S100B secretagogue, which caused a biphasic response of Ca2+ mobilization. Our data show that astroglial S100B secretion is triggered by the increase in intracellular Ca2+ and indicate that this increase is due to Ca2+ mobilization from the endoplasmic reticulum. Also, blocking plasma membrane Ca2+ channels involved in the Ca2+ replenishment of internal stores decreased S100B secretion. The DMSO-induced S100B secretion was confirmed in vivo and in ex vivo hippocampal slices. Our data support a nonclassic vesicular export of S100B modulated by Ca2+, and the results might contribute to understanding the mechanism underlying the astroglial release of S100B.

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

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

Mechanisms of Organophosphate Toxicity and the Role of Acetylcholinesterase Inhibition

Organophosphorus compounds (OPs) have applications in agriculture (e.g., pesticides), industry (e.g., flame retardants), and chemical warfare (nerve agents). In high doses or chronic exposure, they can be toxic or lethal. The primary mechanism, common among all OPs, that initiates their toxic effects is the inhibition of acetylcholinesterase. In acute OP exposure, the subsequent surge of acetylcholine in cholinergic synapses causes a peripheral cholinergic crisis and status epilepticus (SE), either of which can lead to death. If death is averted without effective seizure control, long-term brain damage ensues. This review describes the mechanisms by which elevated acetylcholine can cause respiratory failure and trigger SE; the role of the amygdala in seizure initiation; the role of M1 muscarinic receptors in the early stages of SE; the neurotoxic pathways activated by SE (excitotoxicity/Ca++ overload/oxidative stress, neuroinflammation); and neurotoxic mechanisms linked to low-dose, chronic exposure (Ca++ dyshomeostasis/oxidative stress, inflammation), which do not depend on SE and do not necessarily involve acetylcholinesterase inhibition. The evidence so far indicates that brain damage from acute OP exposure is a direct result of SE, while the neurotoxic mechanisms activated by low-dose chronic exposure are independent of SE and may not be associated with acetylcholinesterase inhibition.

Astroglial Cell-to-Cell Interaction with Autoreactive Immune Cells in Experimental Autoimmune Encephalomyelitis Involves P2X7 Receptor, β3-Integrin, and Connexin-43

In multiple sclerosis (MS), glial cells astrocytes interact with the autoreactive immune cells that attack the central nervous system (CNS), which causes and sustains neuroinflammation. However, little is known about the direct interaction between these cells when they are in close proximity in the inflamed CNS. By using an experimental autoimmune encephalomyelitis (EAE) model of MS, we previously found that in the proximity of autoreactive CNS-infiltrated immune cells (CNS-IICs), astrocytes respond with a rapid calcium increase that is mediated by the autocrine P2X7 receptor (P2X7R) activation. We now reveal that the mechanisms regulating this direct interaction of astrocytes and CNS-IICs involve the coupling between P2X7R, connexin-43, and β3-integrin. We found that P2X7R and astroglial connexin-43 interact and concentrate in the immediate proximity of the CNS-IICs in EAE. P2X7R also interacts with β3-integrin, and the block of astroglial αvβ3-integrin reduces the P2X7R-dependent calcium response of astrocytes upon encountering CNS-IICs. This interaction was dependent on astroglial mitochondrial activity, which regulated the ATP-driven P2X7R activation and facilitated the termination of the astrocytic calcium response evoked by CNS-IICs. By further defining the interactions between the CNS and the immune system, our findings provide a novel perspective toward expanding integrin-targeting therapeutic approaches for MS treatment by controlling the cell-cell interactions between astrocytes and CNS-IICs.

Role of glia and extracellular matrix in controlling neuroplasticity in the central nervous system

Neuronal plasticity is critical for the maintenance and modulation of brain activity. Emerging evidence indicates that glial cells actively shape neuroplasticity, allowing for highly flexible regulation of synaptic transmission, neuronal excitability, and network synchronization. Astrocytes regulate synaptogenesis, stabilize synaptic connectivity, and preserve the balance between excitation and inhibition in neuronal networks. Microglia, the brain-resident immune cells, continuously monitor and sculpt synapses, allowing for the remodeling of brain circuits. Glia-mediated neuroplasticity is driven by neuronal activity, controlled by a plethora of feedback signaling mechanisms and crucially involves extracellular matrix remodeling in the central nervous system. This review summarizes the key findings considering neurotransmission regulation and metabolic support by astrocyte-neuronal networks, and synaptic remodeling mediated by microglia. Novel data indicate that astrocytes and microglia are pivotal for controlling brain function, indicating the necessity to rethink neurocentric neuroplasticity views.

The emerging science of Glioception: Contribution of glia in sensing, transduction, circuit integration of interoception

Interoception is the process by which the nervous system regulates internal functions to achieve homeostasis. The role of neurons in interoception has received considerable recent attention, but glial cells also contribute. Glial cells can sense and transduce signals including osmotic, chemical, and mechanical status of extracellular milieu. Their ability to dynamically communicate "listening" and "talking" to neurons is necessary to monitor and regulate homeostasis and information integration in the nervous system. This review introduces the concept of "Glioception" and focuses on the process by which glial cells sense, interpret and integrate information about the inner state of the organism. Glial cells are ideally positioned to act as sensors and integrators of diverse interoceptive signals and can trigger regulatory responses via modulation of the activity of neuronal networks, both in physiological and pathological conditions. We believe that understanding and manipulating glioceptive processes and underlying molecular mechanisms provide a key path to develop new therapies for the prevention and alleviation of devastating interoceptive dysfunctions, among which pain is emphasized here with more focused details.

Chronic Stress Impairs the Structure and Function of Astrocyte Networks in an Animal Model of Depression

Now astrocytes appear to be the key contributors to the pathophysiology of major depression. Evidence in rodents shows that chronic stress is associated with a decreased expression of astrocytic GFAP-immunoreactivity within the cortex in addition to changes in the complexity and length of astrocyte processes. Furthermore, postmortem brains of individuals with depression have revealed a decrease in astrocyte density. Notably, astrocytes are extensively coupled to one another through gap junctions to form a network, or syncytium, and we have previously demonstrated that syncytial isopotentiality is a mechanism by which astrocytes function as an efficient system with respect to brain homeostasis. Interestingly, the question of how astrocyte network function changes following chronic stress is yet to be elucidated. Here, we sought to examine the effects of chronic stress on network-level astrocyte (dys)function. Using a transgenic aldh1l1-eGFP astrocyte reporter mouse, a six-week unpredictable chronic mild stress (UCMS) paradigm as a rodent model of major depression, and immunohistochemical approaches, we show that the morphology of individual astrocytes is altered by chronic stress exposure. Additionally, in astrocyte syncytial isopotentiality measurement, we found that UCMS impairs the syncytial coupling strength of astrocytes within the hippocampus and prefrontal cortex-two brain regions that have been implicated in the regulation of mood. Together, these findings reveal that chronic stress leads to astrocyte atrophy and impaired gap junction coupling, raising the prospect that both individual and network-level astrocyte functionality are important in the etiology of major depression and other neuropsychiatric disorders.

Nanotopography and Microconfinement Impact on Primary Hippocampal Astrocyte Morphology, Cytoskeleton and Spontaneous Calcium Wave Signalling

Astrocytes' organisation affects the functioning and the fine morphology of the brain, both in physiological and pathological contexts. Although many aspects of their role have been characterised, their complex functions remain, to a certain extent, unclear with respect to their contribution to brain cell communication. Here, we studied the effects of nanotopography and microconfinement on primary hippocampal rat astrocytes. For this purpose, we fabricated nanostructured zirconia surfaces as homogenous substrates and as micrometric patterns, the latter produced by a combination of an additive nanofabrication and micropatterning technique. These engineered substrates reproduce both nanotopographical features and microscale geometries that astrocytes encounter in their natural environment, such as basement membrane topography, as well as blood vessels and axonal fibre topology. The impact of restrictive adhesion manifests in the modulation of several cellular properties of single cells (morphological and actin cytoskeletal changes) and the network organisation and functioning. Calcium wave signalling was observed only in astrocytes grown in confined geometries, with an activity enhancement in cells forming elongated agglomerates with dimensions typical of blood vessels or axon fibres. Our results suggest that calcium oscillation and wave propagation are closely related to astrocytic morphology and actin cytoskeleton organisation.

GABAergic Regulation of Astroglial Gliotransmission through Cx43 Hemichannels

Gamma-Aminobutyric acid (GABA) is the primary inhibitory neurotransmitter in the brain. It is produced by interneurons and recycled by astrocytes. In neurons, GABA activates the influx of Cl- via the GABAA receptor or efflux or K+ via the GABAB receptor, inducing hyperpolarization and synaptic inhibition. In astrocytes, the activation of both GABAA and GABAB receptors induces an increase in intracellular Ca2+ and the release of glutamate and ATP. Connexin 43 (Cx43) hemichannels are among the main Ca2+-dependent cellular mechanisms for the astroglial release of glutamate and ATP. However, no study has evaluated the effect of GABA on astroglial Cx43 hemichannel activity and Cx43 hemichannel-mediated gliotransmission. Here we assessed the effects of GABA on Cx43 hemichannel activity in DI NCT1 rat astrocytes and hippocampal brain slices. We found that GABA induces a Ca2+-dependent increase in Cx43 hemichannel activity in astrocytes mediated by the GABAA receptor, as it was blunted by the GABAA receptor antagonist bicuculline but unaffected by GABAB receptor antagonist CGP55845. Moreover, GABA induced the Cx43 hemichannel-dependent release of glutamate and ATP, which was also prevented by bicuculline, but unaffected by CGP. Gliotransmission in response to GABA was also unaffected by pannexin 1 channel blockade. These results are discussed in terms of the possible role of astroglial Cx43 hemichannel-mediated glutamate and ATP release in regulating the excitatory/inhibitory balance in the brain and their possible contribution to psychiatric disorders.

A single cell death is disruptive to spontaneous Ca2+ activity in astrocytes

Astrocytes in the brain are rapidly recruited to sites of injury where they phagocytose damaged material and take up neurotransmitters and ions to avoid the spreading of damaging molecules. In this study we investigate the calcium (Ca2+) response in astrocytes to nearby cell death. To induce cell death in a nearby cell we utilized a laser nanosurgery system to photolyze a selected cell from an established astrocyte cell line (Ast1). Our results show that the lysis of a nearby cell is disruptive to surrounding cells' Ca2+ activity. Additionally, astrocytes exhibit a Ca2+ transient in response to cell death which differs from the spontaneous oscillations occurring in astrocytes prior to cell lysis. We show that the primary source of the Ca2+ transient is the endoplasmic reticulum.

Distinct roles of astroglia and neurons in synaptic plasticity and memory

Long-term potentiation (LTP) in the hippocampus is the most studied form of synaptic plasticity. Temporal integration of synaptic inputs is essential in synaptic plasticity and is assumed to be achieved through Ca²⁺ signaling in neurons and astroglia. However, whether these two cell types play different roles in LTP remain unknown. Here, we found that through the integration of synaptic inputs, astrocyte inositol triphosphate (IP₃) receptor type 2 (IP₃R2)-dependent Ca²⁺ signaling was critical for late-phase LTP (L-LTP) but not early-phase LTP (E-LTP). Moreover, this process was mediated by astrocyte-derived brain-derived neurotrophic factor (BDNF). In contrast, neuron-derived BDNF was critical for both E-LTP and L-LTP. Importantly, the dynamic differences in BDNF secretion play a role in modulating distinct forms of LTP. Moreover, astrocyte- and neuron-derived BDNF exhibited different roles in memory. These observations enriched our knowledge of LTP and memory at the cellular level and implied distinct roles of astrocytes and neurons in information integration.

Neural Stem Cells in Adult Mammals are not Astrocytes

At the turn of the 21st century studies of the cells that resided in the adult mammalian subventricular zone (SVZ) characterized the neural stem cells (NSCs) as a subtype of astrocyte. Over the ensuing years, numerous studies have further characterized the properties of these NSCs and compared them to parenchymal astrocytes. Here we have evaluated the evidence collected to date to establish whether classifying the NSCs as astrocytes is appropriate and useful. We also performed a meta-analysis with 4 previously published datasets that used cell sorting and unbiased single-cell RNAseq to highlight the distinct gene expression profiles of adult murine NSCs and niche astrocytes. On the basis of our understanding of the properties and functions of astrocytes versus the properties and functions of NSCs, and from our comparative transcriptomic analyses we conclude that classifying the adult mammalian NSC as an astrocyte is potentially misleading. From our vantage point, it is more appropriate to refer to the cells in the adult mammalian SVZ that retain the capacity to produce new neurons and macroglia as NSCs without attaching the term "astrocyte-like."

Sigma-1 receptor increases intracellular calcium in cultured astrocytes and contributes to mechanical allodynia in a model of neuropathic pain

Recent studies have revealed that glial sigma-1 receptor (Sig-1R) in the spinal cord may be a critical factor to mediate sensory function. However, the functional role of Sig-1R in astrocyte has not been clearly elucidated. Here, we determined whether Sig-1Rs modulate calcium responses in primary cultured astrocytes and pathological changes in spinal astrocytes, and whether they contribute to pain hypersensitivity in naïve mice and neuropathic pain following chronic constriction injury (CCI) of the sciatic nerve in mice. Sig-1R was expressed in glial fibrillary acidic protein (GFAP)-positive cultured astrocytes. Treatment with the Sig-1R agonist, PRE-084 or neurosteroid dehydroepiandrosterone (DHEA) increased intracellular calcium responses in cultured astrocytes, and this increase was blocked by the pretreatment with the Sig-1R antagonist, BD-1047 or neurosteroid progesterone. Intrathecal administration of PRE-084 or DHEA for 10 days induced mechanical and thermal hypersensitivity and increased the number of Sig-1R-immunostained GFAP-positive cells in the superficial dorsal horn (SDH) region of the spinal cord in naïve mice, and these changes were inhibited by administration with BD-1047 or progesterone. In CCI mice, intrathecal administration of BD-1047 or progesterone at post-operative day 14 suppressed the developed mechanical allodynia and the number of Sig-1R-immunostained GFAP-positive cells that were increased in the SDH region of the spinal cord following CCI of the sciatic nerve. These results demonstrate that Sig-1Rs play an important role in the modulation of intracellular calcium responses in cultured astrocytes and pathological changes in spinal astrocytes and that administration of BD-1047 or progesterone alleviates the Sig-1R-induced pain hypersensitivity and the peripheral nerve injury-induced mechanical allodynia.

Astrocytes in cocaine addiction and beyond

Drug addiction remains a key biomedical challenge facing current neuroscience research. In addition to neural mechanisms, the focus of the vast majority of studies to date, astrocytes have been increasingly recognized as an "accomplice." According to the tripartite synapse model, astrocytes critically regulate nearby pre- and postsynaptic neuronal substrates to craft experience-dependent synaptic plasticity, including synapse formation and elimination. Astrocytes within brain regions that are implicated in drug addiction exhibit dynamic changes in activity upon exposure to cocaine and subsequently undergo adaptive changes themselves during chronic drug exposure. Recent results have identified several key astrocytic signaling pathways that are involved in cocaine-induced synaptic and circuit adaptations. In this review, we provide a brief overview of the role of astrocytes in regulating synaptic transmission and neuronal function, and discuss how cocaine influences these astrocyte-mediated mechanisms to induce persistent synaptic and circuit alterations that promote cocaine seeking and relapse. We also consider the therapeutic potential of targeting astrocytic substrates to ameliorate drug-induced neuroplasticity for behavioral benefits. While primarily focusing on cocaine-induced astrocytic responses, we also include brief discussion of other drugs of abuse where data are available.

Ketamine Action on Astrocytes Provides New Insights into Rapid Antidepressant Mechanisms

Ketamine, a non-competitive N-methyl-D-aspartate receptor (NMDAR) antagonist, exerts rapid, potent and long-lasting antidepressant effect already after a single administration of a low dose into depressed individuals. Apart from targeting neuronal NMDARs essential for synaptic transmission, ketamine also interacts with astrocytes, the principal homoeostatic cells of the central nervous system. The cellular mechanisms underlying astrocyte-based rapid antidepressant effect are incompletely understood. Here we overview recent data that describe ketamine-dependent changes in astrocyte cytosolic cAMP activity ([cAMP]_i) and ketamine-induced modifications of stimulus-evoked Ca²⁺ signalling. The latter regulates exocytotic release of gliosignalling molecules and stabilizes the vesicle fusion pore in a narrow configuration that obstructs cargo discharge or vesicle membrane recycling. Ketamine also instigates rapid redistribution of cholesterol in the astrocyte plasmalemma that may alter flux of cholesterol to neurones, where it is required for changes in synaptic plasticity. Finally, ketamine attenuates mobility of vesicles carrying the inward rectifying potassium channel (K_ir4.1) and reduces the surface density of K_ir4.1 channels that control extracellular K⁺ concentration, which tunes the pattern of action potential firing in neurones of lateral habenula as demonstrated in a rat model of depression. Thus, diverse, but not mutually exclusive, mechanisms act synergistically to evoke changes in synaptic plasticity leading to sustained strengthening of excitatory synapses necessary for rapid antidepressant effect of ketamine.

Astroglial Serotonin Receptors as the Central Target of Classic Antidepressants

Major depressive disorder (MDD) presents multiple clinical phenotypes and has complex underlying pathological mechanisms. Existing theories cannot completely explain the pathophysiological mechanism(s) of MDD, while the pharmacology of current antidepressants is far from being fully understood. Astrocytes, the homeostatic and defensive cells of the central nervous system, contribute to shaping behaviors, and regulating mood and emotions. A detailed introduction on the role of astrocytes in depressive disorders is thus required, to which this chapter is dedicated. We also focus on the interactions between classic antidepressants and serotonin receptors, overview the role of astrocytes in the pharmacological mechanisms of various antidepressants, and present astrocytes as targets for the treatment of bipolar disorder. We provide a foundation of knowledge on the role of astrocytes in depressive disorders and astroglial 5-HT_2B receptors as targets for selective serotonin reuptake inhibitors in vivo and in vitro.

Principles of Astrogliopathology

The role of astrocytes in the nervous system pathology was early on embraced by neuroscientists at end of the nineteenth and the beginning of the twentieth century, only to be pushed aside by neurone-centric dogmas during most of the twentieth century. However, the last decade of the twentieth century and the twenty-first century have brought the astroglial "renaissance", which has put astroglial cells as key players in pathophysiology of most if not all disorders of the nervous system and has regarded astroglia as a fertile ground for therapeutic intervention.Astrocytic contribution to neuropathology can be primary, whereby cell-autonomous changes, such as mutations in gene encoding for glial fibrillary acidic protein, can drive the pathologic progression, in this example, Alexander disease. They can also be secondary, when astrocytes respond to a variety of insults to the nervous tissue. Regardless of their origin, being cell-autonomous or not, changes in astroglia that occur in pathology, that is, astrogliopathology, can be contemporary and arbitrary classified into four forms: (i) reactive astrogliosis, (ii) astrocytic atrophy with loss of function, (iii) pathological remodelling of astrocytes and (iv) astrodegeneration morphologically manifested as clasmatodendrosis. Inevitably, as with any other classification, this classification of astrogliopathology awaits its revision that shall be rooted in new discoveries and concepts.

Astrocytes: The Housekeepers and Guardians of the CNS

Astroglia are a diverse group of cells in the central nervous system. They are of the ectodermal, neuroepithelial origin and vary in morphology and function, yet, they can be collectively defined as cells having principle function to maintain homeostasis of the central nervous system at all levels of organisation, including homeostasis of ions, pH and neurotransmitters; supplying neurones with metabolic substrates; supporting oligodendrocytes and axons; regulating synaptogenesis, neurogenesis, and formation and maintenance of the blood-brain barrier; contributing to operation of the glymphatic system; and regulation of systemic homeostasis being central chemosensors for oxygen, CO₂ and Na⁺. Their basic physiological features show a lack of electrical excitability (inapt to produce action potentials), but display instead a rather active excitability based on variations in cytosolic concentrations of Ca²⁺ and Na⁺. It is expression of neurotransmitter receptors, pumps and transporters at their plasmalemma, along with transports on the endoplasmic reticulum and mitochondria that exquisitely regulate the cytosolic levels of these ions, the fluctuation of which underlies most, if not all, astroglial homeostatic functions.

Astrocyte Bioenergetics and Major Psychiatric Disorders

Ongoing research continues to add new elements to the emerging picture of involvement of astrocyte energy metabolism in the pathophysiology of major psychiatric disorders, including schizophrenia, mood disorders, and addictions. This review outlines what is known about the energy metabolism in astrocytes, the most numerous cell type in the brain, and summarizes the recent work on how specific perturbations of astrocyte bioenergetics may contribute to the neuropsychiatric conditions. The role of astrocyte energy metabolism in mental health and disease is reviewed on the organism, organ, and cell level. Data arising from genomic, metabolomic, in vitro, and neurobehavioral studies is critically analyzed to suggest future directions in research and possible metabolism-focused therapeutic interventions.

The Ryanodine receptor stabilizer S107 fails to support motor neuronal neuritogenesis in vitro

Calcium homeostasis is essential for neuronal cell survival/differentiation. Imbalance of the Ca2+ homeostasis due to excessive Ca2+ overload is essential for spinal cord injury (SCI). The overload resulted from Ca2+ flux across the plasma membrane and from internal Ca2+ store release (mitochondria, endoplasmic reticulum, ER). Inositol trisphosphate receptors (IP3R) and ryanodine receptors (RyR) are involved in releasing Ca2+ from ER contributing to axonal degeneration following SCI. In turn, block of both receptors is axoprotective. The calstabin RyR subunit, stabilizing the channel in a state of reduced activity, prevents pathological Ca2+ release too. We investigated whether S107, a RyR-stabilizing compound (Rycal), is beneficial for survival and neuritogenesis of spinal cord motor neurons in vitro. We used a spinal cord slice model and the motor neuron-like NSC-34 cell line. Effects of S107 were tested by propidium iodide/fluorescein diacetate vital staining, mitotic index determination via BrdU-incorporation, and neurite sprouting parameters. Results showed that S107 (i) had no effect on gliosis resulting from slices preparation; (ii) had no effect on motor neuronal survival and proliferation; and (iii) impaired neurite sprouting, no matter whether it was a differentiation (NSC-34 cells) or regeneration (spinal cord slices) process. The results underline the need for a flexible Ca2+homeostasis provided by the ER for re-initiation of neuritogenesis.

Two Metabolic Fuels, Glucose and Lactate, Differentially Modulate Exocytotic Glutamate Release from Cultured Astrocytes

Astrocytes have a prominent role in metabolic homeostasis of the brain and can signal to adjacent neurons by releasing glutamate via a process of regulated exocytosis. Astrocytes synthesize glutamate de novo owing to the pyruvate entry to the citric/tricarboxylic acid cycle via pyruvate carboxylase, an astrocyte specific enzyme. Pyruvate can be sourced from two metabolic fuels, glucose and lactate. Thus, we investigated the role of these energy/carbon sources in exocytotic glutamate release from astrocytes. Purified astrocyte cultures were acutely incubated (1 h) in glucose and/or lactate-containing media. Astrocytes were mechanically stimulated, a procedure known to increase intracellular Ca2+ levels and cause exocytotic glutamate release, the dynamics of which were monitored using single cell fluorescence microscopy. Our data indicate that glucose, either taken-up from the extracellular space or mobilized from the intracellular glycogen storage, sustained glutamate release, while the availability of lactate significantly reduced the release of glutamate from astrocytes. Based on further pharmacological manipulation during imaging along with tandem mass spectrometry (proteomics) analysis, lactate alone, but not in the hybrid fuel, caused metabolic changes consistent with an increased synthesis of fatty acids. Proteomics analysis further unveiled complex changes in protein profiles, which were condition-dependent and generally included changes in levels of cytoskeletal proteins, proteins of secretory organelle/vesicle traffic and recycling at the plasma membrane in aglycemic, lactate or hybrid-fueled astrocytes. These findings support the notion that the availability of energy sources and metabolic milieu play a significant role in gliotransmission.

A Functional Signature in the Developing Cerebellum: Evidence From a Preclinical Model of Autism

Autism spectrum disorders (ASD) are pervasive neurodevelopmental conditions detected during childhood when delayed language onset and social deficits are observed. Children diagnosed with ASD frequently display sensorimotor deficits associated with the cerebellum, suggesting a dysfunction of synaptic circuits. Astroglia are part of the tripartite synapses and postmortem studies reported an increased expression of the glial fibrillary acidic protein (GFAP) in the cerebellum of ASD patients. Astroglia respond to neuronal activity with calcium transients that propagate to neighboring cells, resulting in a functional response known as a calcium wave. This form of intercellular signaling is implicated in proliferation, migration, and differentiation of neural precursors. Prenatal exposure to valproate (VPA) is a preclinical model of ASD in which premature migration and excess of apoptosis occur in the internal granular layer (IGL) of the cerebellum during the early postnatal period. In this study we tested calcium wave propagation in the IGL of mice prenatally exposed to VPA. Sensorimotor deficits were observed and IGL depolarization evoked a calcium wave with astrocyte recruitment. The calcium wave propagation, initial cell recruitment, and mean amplitude of the calcium transients increased significantly in VPA-exposed mice compared to the control group. Astrocyte recruitment was significantly increased in the VPA model, but the mean amplitude of the calcium transients was unchanged. Western blot and histological studies revealed an increased expression of GFAP, higher astroglial density and augmented morphological complexity. We conclude that the functional signature of the IGL is remarkably augmented in the preclinical model of autism.

Astrocytic processes: from tripartite synapses to the active milieu

We define a new concept of 'active milieu' that unifies all components of nervous tissue (neuronal and glial compartments, extracellular space, extracellular matrix, and vasculature) into a dynamic information processing system. Within this framework, we focus on the role of astrocytic processes, classified into organelle-containing branches and organelle-free leaflets. We argue that astrocytic branches with emanating leaflets are homologous to dendritic shafts with spines. Within the active milieu, astrocytic processes are engaged in reciprocal interactions with neuronal compartments and communication with other cellular and non-cellular elements of the nervous tissue.

Ketamine Alters Functional Plasticity of Astroglia: An Implication for Antidepressant Effect

Ketamine, a non-competitive N–methyl–d–aspartate receptor (NMDAR) antagonist, exerts a rapid, potent and long-lasting antidepressant effect, although the cellular and molecular mechanisms of this action are yet to be clarified. In addition to targeting neuronal NMDARs fundamental for synaptic transmission, ketamine also affects the function of astrocytes, the key homeostatic cells of the central nervous system that contribute to pathophysiology of major depressive disorder. Here, I review studies revealing that (sub)anesthetic doses of ketamine elevate intracellular cAMP concentration ([cAMP]_i) in astrocytes, attenuate stimulus-evoked astrocyte calcium signaling, which regulates exocytotic secretion of gliosignaling molecules, and stabilize the vesicle fusion pore in a narrow configuration, possibly hindering cargo discharge or vesicle recycling. Next, I discuss how ketamine affects astrocyte capacity to control extracellular K⁺ by reducing vesicular delivery of the inward rectifying potassium channel (K_ir4.1) to the plasmalemma that reduces the surface density of Kir4.1. Modified astroglial K⁺ buffering impacts upon neuronal firing pattern as demonstrated in lateral habenula in a rat model of depression. Finally, I highlight the discovery that ketamine rapidly redistributes cholesterol in the astrocyte plasmalemma, which may alter the flux of cholesterol to neurons. This structural modification may further modulate a host of processes that synergistically contribute to ketamine’s rapid antidepressant action.

Iron induces two distinct Ca2+ signalling cascades in astrocytes

Iron is the fundamental element for numerous physiological functions. Plasmalemmal divalent metal ion transporter 1 (DMT1) is responsible for cellular uptake of ferrous (Fe2+), whereas transferrin receptors (TFR) carry transferrin (TF)-bound ferric (Fe3+). In this study we performed detailed analysis of the action of Fe ions on cytoplasmic free calcium ion concentration ([Ca2+]i) in astrocytes. Administration of Fe2+ or Fe3+ in μM concentrations evoked [Ca2+]i in astrocytes in vitro and in vivo. Iron ions trigger increase in [Ca2+]i through two distinct molecular cascades. Uptake of Fe2+ by DMT1 inhibits astroglial Na+-K+-ATPase, which leads to elevation in cytoplasmic Na+ concentration, thus reversing Na+/Ca2+ exchanger and thereby generating Ca2+ influx. Uptake of Fe3+ by TF-TFR stimulates phospholipase C to produce inositol 1,4,5-trisphosphate (InsP3), thus triggering InsP3 receptor-mediated Ca2+ release from endoplasmic reticulum. In summary, these findings reveal the mechanisms of iron-induced astrocytic signalling operational in conditions of iron overload.

Calcium signaling in neuroglia

Glial cells exploit calcium (Ca²⁺) signals to perceive the information about the activity of the nervous tissue and the tissue environment to translate this information into an array of homeostatic, signaling and defensive reactions. Astrocytes, the best studied glial cells, use several Ca²⁺ signaling generation pathways that include Ca²⁺ entry through plasma membrane, release from endoplasmic reticulum (ER) and from mitochondria. Activation of metabotropic receptors on the plasma membrane of glial cells is coupled to an enzymatic cascade in which a second messenger, InsP₃ is generated thus activating intracellular Ca²⁺ release channels in the ER endomembrane. Astrocytes also possess store-operated Ca²⁺ entry and express several ligand-gated Ca²⁺ channels. In vivo astrocytes generate heterogeneous Ca²⁺ signals, which are short and frequent in distal processes, but large and relatively rare in soma. In response to neuronal activity intracellular and inter-cellular astrocytic Ca²⁺ waves can be produced. Astrocytic Ca²⁺ signals are involved in secretion, they regulate ion transport across cell membranes, and are contributing to cell morphological plasticity. Therefore, astrocytic Ca²⁺ signals are linked to fundamental functions of the central nervous system ranging from synaptic transmission to behavior. In oligodendrocytes, Ca²⁺ signals are generated by plasmalemmal Ca²⁺ influx, or by release from intracellular stores, or by combination of both. Microglial cells exploit Ca²⁺ permeable ionotropic purinergic receptors and transient receptor potential channels as well as ER Ca²⁺ release. In this contribution, basic morphology of glial cells, glial Ca²⁺ signaling toolkit, intracellular Ca²⁺ signals and Ca²⁺-regulated functions are discussed with focus on astrocytes.

Caloric restriction modifies spatiotemporal calcium dynamics in mouse hippocampal astrocytes

We analysed spatiotemporal properties of Ca²⁺ signals in protoplasmic astrocytes in the CA1 stratum radiatum of hippocampal slices from young (2-3 months old) mice housed in control conditions or exposed to a caloric restriction (CR) diet for one month. The astrocytic Ca²⁺ events became shorter in duration and smaller in size; they also demonstrated reduced velocity of expansion and shrinkage following CR. At the same time, Ca²⁺ signals in the astrocytes from the CR animals demonstrated higher amplitude and the faster rise and decay rates. These changes can be attributed to CR-induced morphological remodelling and uncoupling of astrocytes described in our previous study. CR-induced changes in the parameters of Ca²⁺ activity were partially reversed by inhibition of gap junctions/hemichannels with carbenoxolone (CBX). The effect of CBX on Ca²⁺ activity in CR-animals was unexpected because the diet already decreases gap junctional coupling in astrocytic syncytia. It may reflect the blockade of hemichannels also sensitive to this drug. Thus, CR-induced morphological remodelling of astrocytes is at least partly responsible for changes in the pattern of Ca²⁺ activity in the astrocytic network. How such changes in spatiotemporal Ca²⁺ landscape can translate into astrocytic physiology and neuron-glia interactions remains a matter for future studies.

Norepinephrine-Induced Calcium Signaling and Store-Operated Calcium Entry in Olfactory Bulb Astrocytes

It is well-established that astrocytes respond to norepinephrine with cytosolic calcium rises in various brain areas, such as hippocampus or neocortex. However, less is known about the effect of norepinephrine on olfactory bulb astrocytes. In the present study, we used confocal calcium imaging and immunohistochemistry in mouse brain slices of the olfactory bulb, a brain region with a dense innervation of noradrenergic fibers, to investigate the calcium signaling evoked by norepinephrine in astrocytes. Our results show that application of norepinephrine leads to a cytosolic calcium rise in astrocytes which is independent of neuronal activity and mainly mediated by PLC/IP3-dependent internal calcium release. In addition, store-operated calcium entry (SOCE) contributes to the late phase of the response. Antagonists of both α1- and α2-adrenergic receptors, but not β-receptors, largely reduce the adrenergic calcium response, indicating that both α-receptor subtypes mediate norepinephrine-induced calcium transients in olfactory bulb astrocytes, whereas β-receptors do not contribute to the calcium transients.

Modeling of Astrocyte Networks: Toward Realistic Topology and Dynamics

Neuronal firing and neuron-to-neuron synaptic wiring are currently widely described as orchestrated by astrocytes—elaborately ramified glial cells tiling the cortical and hippocampal space into non-overlapping domains, each covering hundreds of individual dendrites and hundreds thousands synapses. A key component to astrocytic signaling is the dynamics of cytosolic Ca²⁺ which displays multiscale spatiotemporal patterns from short confined elemental Ca²⁺ events (puffs) to Ca²⁺ waves expanding through many cells. Here, we synthesize the current understanding of astrocyte morphology, coupling local synaptic activity to astrocytic Ca²⁺ in perisynaptic astrocytic processes and morphology-defined mechanisms of Ca²⁺ regulation in a distributed model. To this end, we build simplified realistic data-driven spatial network templates and compile model equations as defined by local cell morphology. The input to the model is spatially uncorrelated stochastic synaptic activity. The proposed modeling approach is validated by statistics of simulated Ca²⁺ transients at a single cell level. In multicellular templates we observe regular sequences of cell entrainment in Ca²⁺ waves, as a result of interplay between stochastic input and morphology variability between individual astrocytes. Our approach adds spatial dimension to the existing astrocyte models by employment of realistic morphology while retaining enough flexibility and scalability to be embedded in multiscale heterocellular models of neural tissue. We conclude that the proposed approach provides a useful description of neuron-driven Ca²⁺-activity in the astrocyte syncytium.

Graphene glial-interfaces: challenges and perspectives

Graphene nanosheets are mechanically strong but flexible, electrically conductive and bio-compatible. Thus, due to these unique properties, they are being intensively studied as materials for the next generation of neural interfaces. Most of the literature focused on optimizing the interface between these materials and neurons. However, one of the most common causes of implant failure is the adverse inflammatory reaction of glial cells. These cells are not, as previously considered, just passive and supportive cells, but play a crucial role in the physiology and pathology of the nervous system, and in the interaction with implanted electrodes. Besides providing structural support to neurons, glia are responsible for the modulation of synaptic transmission and control of central and peripheral homeostasis. Accordingly, knowledge on the interaction between glia and biomaterials is essential to develop new implant-based therapies for the treatment of neurological disorders, such as epilepsy, brain tumours, and Alzheimer's and Parkinson's disease. This work provides an overview of the emerging literature on the interaction of graphene-based materials with glial cells, together with a complete description of the different types of glial cells and problems associated with them. We believe that this description will be important for researchers working in materials science and nanotechnology to develop new active materials to interface, measure and stimulate these cells.

Connexins as therapeutic targets in neurological and neuropsychiatric disorders

Astrocytes represent the reticular part of the central nervous system; gap junctions formed by connexins Cx43, Cx30- and Cx26 provide for homocellular astrocyte-astrocyte coupling, whereas connexins Cx30, Cx32, Cx43, and Cx47 connect astrocytes and oligodendrocytes. Astroglial networks are anatomically and functionally segregated being homologous to neuronal ensembles. Connexons, gap junctions and hemichannels (unpaired connexons) are affected in various neuropathologies from neuropsychiatric to neurodegenerative diseases. Manipulation of astrocytic connexins modulates the size and outreach of astroglial syncytia thus affecting astroglial homeostatic support. Modulation of astrocytic connexin significantly modifies pharmacological profile of many CNS drugs, which represents an innovative therapeutic approach for CNS disorders; this approach is now actively tested in pre-clinical and clinical studies. Wide combination of connexin modulators with CNS drugs open new promising perspectives for fundamental studies and therapeutic strategies.

The Association Between Antidepressant Effect of SSRIs and Astrocytes: Conceptual Overview and Meta-analysis of the Literature

Major depressive disorders (MDD) a worldwide psychiatric disease, is yet to be adequately controlled by therapies; while the mechanisms of action of antidepressants are yet to be fully characterised. In the last two decades, an increasing number of studies have demonstrated the role of astrocytes in the pathophysiology and therapy of MDD. Selective serotonin reuptake inhibitors (SSRIs) are the most widely used antidepressants. It is generally acknowledged that SSRIs increase serotonin levels in the central nervous system by inhibiting serotonin transporters, although the SSRIs action is not ideal. The SSRIs antidepressant effect develops with considerable delay; their efficacy is low and frequent relapses are common. Neither cellular nor molecular pharmacological mechanisms of SSRIs are fully characterised; in particular their action on astrocytes remain underappreciated. In this paper we overview potential therapeutic mechanisms of SSRIs associated with astroglia and report the results of meta-analysis of studies dedicated to MDD, SSRIs and astrocytes. In particular, we argue that fluoxetine, the representative SSRI, improves depressive-like behaviours in animals treated with chronic mild stress and reverses depression-associated decrease in astrocytic glial fibrillary acidic protein (GFAP) expression. In addition, fluoxetine upregulates astrocytic mRNA expression of 5-hydroxytriptamin/serotonin_2B receptors (5-HT_2BR). In summary, we infer that SSRIs exert their anti-depressant effect by regulating several molecular and signalling pathways in astrocytes.

Satellite Glial Cells and Astrocytes, a Comparative Review

Astroglia are neural cells, heterogeneous in form and function, which act as supportive elements of the central nervous system; astrocytes contribute to all aspects of neural functions in health and disease. Through their highly ramified processes, astrocytes form close physical contacts with synapses and blood vessels, and are integrated into functional syncytia by gap junctions. Astrocytes interact among themselves and with other cells types (e.g., neurons, microglia, blood vessel cells) by an elaborate repertoire of chemical messengers and receptors; astrocytes also influence neural plasticity and synaptic transmission through maintaining homeostasis of neurotransmitters, K⁺ buffering, synaptic isolation and control over synaptogenesis and synaptic elimination. Satellite glial cells (SGCs) are the most abundant glial cells in sensory ganglia, and are believed to play major roles in sensory functions, but so far research into SGCs attracted relatively little attention. In this review we compare SGCs to astrocytes with the purpose of using the vast knowledge on astrocytes to explore new aspects of SGCs. We survey the main properties of these two cells types and highlight similarities and differences between them. We conclude that despite the much greater diversity in morphology and signaling mechanisms of astrocytes, there are some parallels between them and SGCs. Both types serve as boundary cells, separating different compartments in the nervous system, but much more needs to be learned on this aspect of SGCs. Astrocytes and SGCs employ chemical messengers and calcium waves for intercellular signaling, but their significance is still poorly understood for both cell types. Both types undergo major changes under pathological conditions, which have a protective function, but an also contribute to disease, and chronic pain in particular. The knowledge obtained on astrocytes is likely to benefit future research on SGCs.

Cryptococcus neoformans-astrocyte interactions: effect on fungal blood brain barrier disruption, brain invasion, and meningitis progression

Cryptococcus neoformans is an opportunistic, neurotropic, and encapsulated fungus that causes life-threatening cryptococcal meningitis (CM), especially in regions of the world where AIDS is endemic. The polysaccharide capsule of C. neoformans is the fungus major virulent factor, being copiously released during infection and causing immunosuppressive defects in the host. Although the capsular material is commonly associated with reactive astrocytes in fatal CM, little is known about the molecular and cellular interactions among astroglia and C. neoformans. As astrocytes also make up the neurovascular unit at the blood-brain barrier (BBB), which C. neoformans must transverse to colonize the central nervous system and cause CM; these cells may play a significant regulatory role in the prevention and progression of infection. For example, astrocytes are implicated in neurological disease including the regulation of cerebral intracranial pressure, immune function, and water homeostasis. Hence, in this review, we provide a general overview of astroglia biology and discuss the current knowledge on C. neoformans-astrocyte interactions including their involvement in the development of CM. This "gliocentric view" of cerebral cryptococcosis suggests that therapeutic interventions particularly targeting at preserving the neuroprotective function of astrocytes may be used in preventing and managing C. neoformans BBB transmigration, brain invasion, colonization, and meningitis.

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.

Astroglial Regulation of Magnocellular Neuroendocrine Cell Activities in the Supraoptic Nucleus

Studies on the interactions between astrocytes and neurons in the hypothalamo-neurohypophysial system have significantly facilitated our understanding of the regulation of neural activities. This has been exemplified in the interactions between astrocytes and magnocellular neuroendocrine cells (MNCs) in the supraoptic nucleus (SON), specifically during osmotic stimulation and lactation. In response to changes in neurochemical environment in the SON, astrocytic morphology and functions change significantly, which further modulates MNC activity and the secretion of vasopressin and oxytocin. In osmotic regulation, short-term dehydration or water overload causes transient retraction or expansion of astrocytic processes, which increases or decreases the activity of SON neurons, respectively. Prolonged osmotic stimulation causes adaptive change in astrocytic plasticity in the SON, which allows osmosensory neurons to reserve osmosensitivity at new levels. During lactation, changes in neurochemical environment cause retraction of astrocytic processes around oxytocin neurons, which increases MNC's ability to secrete oxytocin. During suckling by a baby/pup, astrocytic processes in the mother/dams exhibit alternative retraction and expansion around oxytocin neurons, which mirrors intermittently synchronized activation of oxytocin neurons and the post-excitation inhibition, respectively. The morphological and functional plasticities of astrocytes depend on a series of cellular events involving glial fibrillary acidic protein, aquaporin 4, volume regulated anion channels, transporters and other astrocytic functional molecules. This review further explores mechanisms underlying astroglial regulation of the neuroendocrine neuronal activities in acute processes based on the knowledge from studies on the SON.

Neuron-derived factors negatively modulate ryanodine receptor-mediated calcium release in cultured mouse astrocytes

Changes in intracellular Ca²⁺ concentration ([Ca²⁺]_i) produced by ryanodine receptor (RyR) agonist, caffeine (caf), and ionotropic agonists: N-methyl-d-aspartate (NMDA) receptor (NMDAR) agonist, NMDA and P2X7 receptor (P2X7R) agonist, 3'-O-(4-benzoyl)benzoyl adenosine 5'-triphosphate (BzATP) were measured in cultured mouse cortical astrocytes loaded with the fluorescent calcium indicator Fluo3-AM in a confocal laser scanning microscope. In mouse astrocytes cultured in standard medium (SM), treatment with caf increased [Ca²⁺]_i, with a peak response occurring about 10 min after stimulus application. Peak responses to NMDA or BzATP were observed about <1 min and 4.5 min post stimulus, respectively. Co-treatment with NMDA or BzATP did not alter the peak response to caf in astrocytes cultured in SM, the absence of the effects being most likely due to asynchrony between the response to caf, NMDA and BzATP. Incubation of astrocytes with neuron-condition medium (NCM) for 24 h totally abolished the caf-evoked [Ca²⁺]_i increase. In NCM-treated astrocytes, peak of [Ca²⁺]_i rise evoked by NMDA was delayed to about 3.5 min, and that induced by BzATP occurred about three minutes earlier than in SM. The results show that neurons secrete factors that negatively modulate RyR-mediated Ca²⁺-induced Ca²⁺ release (CICR) in astrocytes and alter the time course of Ca²⁺ responses to ionotropic stimuli.

Calcium Dynamics in Astrocytes During Cell Injury

The changes in intracellular calcium concentration ([Ca²⁺]) following laser-induced cell injury in nearby cells were studied in primary mouse astrocytes selectively expressing the Ca²⁺ sensitive GFAP-Cre Salsa6f fluorescent tandem protein, in an Ast1 astrocyte cell line, and in primary mouse astrocytes loaded with Fluo4. Astrocytes in these three systems exhibit distinct changes in [Ca²⁺] following induced death of nearby cells. Changes in [Ca²⁺] appear to result from release of Ca²⁺ from intracellular organelles, as opposed to influx from the external medium. Salsa6f expressing astrocytes displayed dynamic Ca²⁺ changes throughout the phagocytic response, including lamellae protrusion, cytosolic signaling during vesicle formation, vesicle maturation, and vesicle tract formation. Our results demonstrate local changes in [Ca²⁺] are involved in the process of phagocytosis in astrocytes responding to cell corpses and/or debris.

Altered serum protein levels in frontotemporal dementia and amyotrophic lateral sclerosis indicate calcium and immunity dysregulation

Frontotemporal dementia (FTD) and amyotrophic lateral sclerosis (ALS) are neurodegenerative diseases that are considered to be on the same disease spectrum because of overlapping genetic, pathological and clinical traits. Changes in serum proteins in FTD and ALS are poorly understood, and currently no definitive biomarkers exist for diagnosing or monitoring disease progression for either disease. Here we applied quantitative discovery proteomics to analyze protein changes in FTD (N = 72) and ALS (N = 28) patient serum compared to controls (N = 22). Twenty three proteins were significantly altered in FTD compared to controls (increased-APOL1, C3, CTSH, EIF5A, MYH2, S100A8, SUSD5, WDR1; decreased-C1S, C7, CILP2, COMP, CRTAC1, EFEMP1, FBLN1, GSN, HSPG2, IGHV1, ITIH2, PROS1, SHBG, UMOD, VASN) and 14 proteins were significantly altered in ALS compared to controls (increased-APOL1, CKM, CTSH, IGHG1, IGKC, MYH2; decreased-C7, COMP, CRTAC1, EFEMP1, FBLN1, GSN, HSPG2, SHBG). There was substantial overlap in the proteins that were altered in FTD and ALS. These results were validated using western blotting. Gene ontology tools were used to assess functional pathways potentially dysregulated in the two diseases, and calcium ion binding and innate immunity pathways were altered in both diseases. When put together, these results suggest significant overlap in pathophysiological peripheral changes in FTD and ALS. This study represents the first proteomics side-by-side comparison of serum changes in FTD and ALS, providing new insights into under-recognized perturbed pathways and an avenue for biomarker development for FTD and ALS.

Axon growth and synaptic function: A balancing act for axonal regeneration and neuronal circuit formation in CNS trauma and disease

Axons in the adult mammalian central nervous system (CNS) fail to regenerate inside out due to intrinsic and extrinsic neuronal determinants. During CNS development, axon growth, synapse formation, and function are tightly regulated processes allowing immature neurons to effectively grow an axon, navigate toward target areas, form synaptic contacts and become part of information processing networks that control behavior in adulthood. Not only immature neurons are able to precisely control the expression of a plethora of genes necessary for axon extension and pathfinding, synapse formation and function, but also non-neuronal cells such as astrocytes and microglia actively participate in sculpting the nervous system through refinement, consolidation, and elimination of synaptic contacts. Recent evidence indicates that a balancing act between axon regeneration and synaptic function may be crucial for rebuilding functional neuronal circuits after CNS trauma and disease in adulthood. Here, we review the role of classical and new intrinsic and extrinsic neuronal determinants in the context of CNS development, injury, and disease. Moreover, we discuss strategies targeting neuronal and non-neuronal cell behaviors, either alone or in combination, to promote axon regeneration and neuronal circuit formation in adulthood.

Chemically Functionalized Water-Soluble Single-Walled Carbon Nanotubes Obstruct Vesicular/Plasmalemmal Recycling in Astrocytes Down-Stream of Calcium Ions

We used single-walled carbon nanotubes chemically functionalized with polyethylene glycol (SWCNT-PEG) to assess the effects of this nanomaterial on astrocytic endocytosis and exocytosis. We observed that the SWCNT-PEG do not affect the adenosine triphosphate (ATP)-evoked Ca²⁺ elevations in astrocytes but significantly reduce the Ca²⁺-dependent glutamate release. There was a significant decrease in the endocytic load of the recycling dye during constitutive and ATP-evoked recycling. Furthermore, SWCNT-PEG hampered ATP-evoked exocytotic release of the loaded recycling dye. Thus, by functionally obstructing evoked vesicular recycling, SWCNT-PEG reduced glutamate release from astrocytes via regulated exocytosis. These effects implicate SWCNT-PEG as a modulator of Ca²⁺-dependent exocytosis in astrocytes downstream of Ca²⁺, likely at the level of vesicle fusion with/pinching off the plasma membrane.

Astrocytes in rapid ketamine antidepressant action

Ketamine, a general anaesthetic and psychotomimetic drug, exerts rapid, potent and long-lasting antidepressant effect, albeit the cellular and molecular mechanisms of this action are yet to be discovered. Besides targeting neuronal NMDARs fundamental for synaptic transmission, ketamine affects the function of astroglia the key homeostatic cells of the central nervous system that contribute to pathophysiology of psychiatric diseases including depression. Here we review studies revealing that (sub)anaesthetic doses of ketamine elevate intracellular cAMP concentration ([cAMP]_i) in astrocytes, attenuate stimulus-evoked astrocyte calcium signalling, which regulates exocytotic secretion of gliosignalling molecules, and stabilize the vesicle fusion pore in a narrow configuration possibly hindering cargo discharge or vesicle recycling. Next we discuss how ketamine affects astroglial capacity to control extracellular K⁺ by reducing cytoplasmic mobility of vesicles delivering the inward rectifying potassium channel (Kir4.1) to the plasmalemma. Modified astroglial K⁺ buffering impacts upon neuronal excitability as demonstrated in the lateral habenula rat model of depression. Finally, we highlight the recent discovery that ketamine rapidly redistributes cholesterol in the plasmalemma of astrocytes, but not in fibroblasts nor in neuronal cells. This alteration of membrane structure may modulate a host of processes that synergistically contribute to ketamine's rapid and prominent antidepressant action.

Glial Connexins and Pannexins in the Healthy and Diseased Brain

Over the past several decades a large amount of data have established that glial cells, the main cell population in the brain, dynamically interact with neurons and thus impact their activity and survival. One typical feature of glia is their marked expression of several connexins, the membrane proteins forming intercellular gap junction channels and hemichannels. Pannexins, which have a tetraspan membrane topology as connexins, are also detected in glial cells. Here, we review the evidence that connexin and pannexin channels are actively involved in dynamic and metabolic neuroglial interactions in physiological as well as in pathological situations. These features of neuroglial interactions open the way to identify novel non-neuronal aspects that allow for a better understanding of behavior and information processing performed by neurons. This will also complement the "neurocentric" view by facilitating the development of glia-targeted therapeutic strategies in brain disease.

Direct monitoring of ER Ca2+ dynamics reveals that Ca2+ entry induces ER-Ca2+ release in astrocytes

Excitability in astroglia is controlled by Ca²⁺ fluxes from intracellular organelles, mostly from the endoplasmic reticulum (ER). Astrocytic ER possesses inositol 1,4,5-trisphosphate receptors (InsP₃R) that can be activated upon stimulation through a vast number of metabotropic G-protein-coupled receptors. By contrast, the role of Ca²⁺-gated Ca²⁺ release channels is less explored in astroglia. Here we address this process by monitoring Ca²⁺ dynamics directly in the cytosol and the ER of astroglial cells. Cultured astrocytes exhibited spontaneous and high-K-evoked cytosolic Ca²⁺ transients, both of them reversibly abolished by external Ca²⁺ removal, addition of plasma membrane channel blockers or ER Ca²⁺ depletion with SERCA inhibitors. Resting astrocyte [Ca²⁺]_ER averaged 400 μM and maximal stimulation with ATP provoked a complete and reversible ER discharge. Direct monitoring of Ca²⁺ in the lumen of ER showed that high-K induced a Ca²⁺ release from the ER, and its amplitude was proportional to the [K]. Furthermore, by combining the low affinity GAP3 indicator targeted to the ER with the high affinity cytosolic Rhod-2, we simultaneously imaged ER- and cytosolic-Ca²⁺ signals, in astrocytes in culture and in situ. Plasma membrane Ca²⁺ entry triggered a fast ER Ca²⁺ release coordinated with an increase in cytosolic Ca²⁺. Thus, we identify a Ca²⁺-induced Ca²⁺-release (CICR) mechanism that is likely to participate in spontaneous astroglial oscillations, providing a graded amplification of the cytosolic Ca²⁺ signal.

Nestin affects fusion pore dynamics in mouse astrocytes

AIM

Astrocytes play a homeostatic role in the central nervous system and influence numerous aspects of neurophysiology via intracellular trafficking of vesicles. Intermediate filaments (IFs), also known as nanofilaments, regulate a number of cellular processes including organelle trafficking and adult hippocampal neurogenesis. We have recently demonstrated that the IF protein nestin, a marker of neural stem cells and immature and reactive astrocytes, is also expressed in some astrocytes in the unchallenged hippocampus and regulates neurogenesis through Notch signalling from astrocytes to neural stem cells, possibly via altered trafficking of vesicles containing the Notch ligand Jagged-1.

METHODS

We thus investigated whether nestin affects vesicle dynamics in astrocytes by examining single vesicle interactions with the plasmalemma and vesicle trafficking with high-resolution cell-attached membrane capacitance measurements and confocal microscopy. We used cell cultures of astrocytes from nestin-deficient (Nes^-/- ) and wild-type (wt) mice, and fluorescent dextran and Fluo-2 to examine vesicle mobility and intracellular Ca²⁺ concentration respectively.

RESULTS

Nes^-/- astrocytes exhibited altered sizes of vesicles undergoing full fission and transient fusion, altered vesicle fusion pore geometry and kinetics, decreased spontaneous vesicle mobility and altered ATP-evoked mobility. Purinergic stimulation evoked Ca²⁺ signalling that was slightly attenuated in Nes^-/- astrocytes, which exhibited more oscillatory Ca²⁺ responses than wt astrocytes.

CONCLUSION

These results demonstrate at the single vesicle level that nestin regulates vesicle interactions with the plasmalemma and vesicle trafficking, indicating its potential role in astrocyte vesicle-based communication.

Dopamine-induced calcium signaling in olfactory bulb astrocytes

It is well established that astrocytes respond to the major neurotransmitters glutamate and GABA with cytosolic calcium rises, whereas less is known about the effect of dopamine on astroglial cells. In the present study, we used confocal calcium imaging in mouse brain slices of the olfactory bulb, a brain region with a large population of dopaminergic neurons, to investigate calcium signaling evoked by dopamine in astrocytes. Our results show that application of dopamine leads to a dose-dependent cytosolic calcium rise in astrocytes (EC₅₀ = 76 µM) which is independent of neuronal activity and mainly mediated by PLC/IP₃-dependent internal calcium release. Antagonists of both D₁- and D₂-class dopamine receptors partly reduce the dopaminergic calcium response, indicating that both receptor classes contribute to dopamine-induced calcium transients in olfactory bulb astrocytes.

Na+-dependent transporters: The backbone of astroglial homeostatic function

Astrocytes are the principal homeostatic cells of the central nerves system (CNS) that support the CNS function at all levels of organisation, from molecular to organ. Several fundamental homeostatic functions of astrocytes are mediated through plasmalemmal pumps and transporters; most of which are also regulated by the transplasmalemmal gradient of Na⁺ ions. Neuronal activity as well as mechanical or chemical stimulation of astrocytes trigger plasmalemmal Na⁺ fluxes, which in turn generate spatio-temporally organised transient changes in the cytosolic Na⁺ concentration, which represent the substrate of astroglial Na⁺ signalling. Astroglial Na⁺ signals link and coordinate neuronal activity and CNS homeostatic demands with the astroglial homeostatic response.

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.