Astrocytes selectively enhance N-type calcium current in hippocampal neurons

Epigenetic Alterations of Brain Non-Neuronal Cells in Major Mental Diseases

The tissue-specific expression and epigenetic dysregulation of many genes in cells derived from the postmortem brains of patients have been reported to provide a fundamental biological framework for major mental diseases such as autism, schizophrenia, bipolar disorder, and major depression. However, until recently, the impact of non-neuronal brain cells, which arises due to cell-type-specific alterations, has not been adequately scrutinized; this is because of the absence of techniques that directly evaluate their functionality. With the emergence of single-cell technologies, such as RNA sequencing (RNA-seq) and other novel techniques, various studies have now started to uncover the cell-type-specific expression and DNA methylation regulation of many genes (e.g., TREM2, MECP2, SLC1A2, TGFB2, NTRK2, S100B, KCNJ10, and HMGB1, and several complement genes such as C1q, C3, C3R, and C4) in the non-neuronal brain cells involved in the pathogenesis of mental diseases. Additionally, several lines of experimental evidence indicate that inflammation and inflammation-induced oxidative stress, as well as many insidious/latent infectious elements including the gut microbiome, alter the expression status and the epigenetic landscapes of brain non-neuronal cells. Here, we present supporting evidence highlighting the importance of the contribution of the brain's non-neuronal cells (in particular, microglia and different types of astrocytes) in the pathogenesis of mental diseases. Furthermore, we also address the potential impacts of the gut microbiome in the dysfunction of enteric and brain glia, as well as astrocytes, which, in turn, may affect neuronal functions in mental disorders. Finally, we present evidence that supports that microbiota transplantations from the affected individuals or mice provoke the corresponding disease-like behavior in the recipient mice, while specific bacterial species may have beneficial effects.

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.

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.

Astrocyte Infection Is Required for Retrovirus-Induced Spongiform Neurodegeneration Despite Suppressed Viral Protein Expression

The ability of retroviruses (RVs) to cause neurodegeneration is critically dependent upon two activities of the envelope protein (Env). First, Env facilitates viral genome delivery to CNS target cells through receptor binding and membrane fusion. Second, Env expression within one or more targets indirectly alters the physiology of certain neurons. Although the major Env expressing CNS cell types have been identified for many neurovirulent RVs, it remains unresolved, which targets play a causal role in neuropathogenesis. Moreover, this issue is complicated by the potential for post-infection virus suppression. To address these questions we explored herein, whether and how cryptic neurotropism differences between ecotropic and amphotropic murine leukemia viruses (MLVs) impacted neurovirulence. Neurotropism was first explored ex vivo using (1) acute primary glial cell cultures and (2) neural progenitor cell (NPC)- neural stem cell (NSC) neural sphere (NPH) chimeras. These experiments indicated that primary astrocytes and NPCs acutely restrict amphotropic but not ecotropic virus entry. CNS tropism was investigated using NSC transplant-based Cre-vector pseudotyping wherein mTmG transgenic fluorescent protein reporter mice revealed both productive and suppressed infection. Cre-pseudotyping with FrCasE, a prototypic neurovirulent ecotropic virus, identified glia and endothelia, but not neurons, as targets. Almost two-thirds (62%) of mGFP+ cells failed to show Env expression, suggesting widespread virus suppression. To circumvent RV superinfection interference confounds, targets were also identified using ecotropic packaging NSCs. These experiments identified known ecotropic targets: microglia, oligodendrocyte progenitor cells (OPCs) and endothelia. Additionally, one third of mGFP+ cells were identified as protoplasmic astrocytes, cells that rarely express virus in vivo. A CNS targeting comparison between isogenic ecotropic (FrCasE) and amphotropic (FrAmE) viruses showed a fourfold higher astrocyte targeting by FrCasE. Since ecotropic Env pseudotyping of amphotropic virus in the CNS dramatically exacerbates neurodegeneration, these results strongly suggest that astrocyte infection is a major disease requirement. Moreover, since viral Env protein expression is largely subdetectable in astrocytes, minimal viral protein expression appears sufficient for affecting neuronal physiology. More broadly, these findings raise the specter that subdetectable astrocyte expression of exogenous or endogenous RVs could play a major role in human and animal neurodegenerative diseases.

Astrocyte Viability and Functionality in Spatially Confined Microcavitation Zone

Blast-induced traumatic brain injury (bTBI) can result in cell/tissue damage and lead to clinical and neuropsychiatric symptoms. Shock waves from a blast propagate through the brain and initiate cascades of mechanical and physiological events that can adversely affect the brain function. Although studies using animal models and brain slices have shown macroscale changes in the brain tissue in response to blast, systematic elucidation of coupling mechanisms is currently lacking. One mechanism that has been postulated and demonstrated repeatedly is the blast-induced generation and subsequent collapse of micron-size bubbles (i.e., microcavitation). Using a custom-designed exposure system, we have previously reported that upon collapsing of microbubbles, astrocytes exhibited changes in the cell viability, cellular biomechanics, production of reactive oxygen species, and activation of apoptotic signaling pathways. In this paper, we have applied microfabrication techniques and seeded astrocytes in a spatially controlled manner to determine the extent of cell damage from the site of the collapse of microbubbles. Such a novel experimental design is proven to facilitate our effort to examine the altered cell viability and functionality by monitoring the transient calcium spiking activity in real-time. We now report that the effect of microcavitation depends on the distance from which cells are seeded, and the cell functionality assessed by calcium dynamics is significantly diminished in the cells located within ∼800 μm of the collapsing microbubbles. Both calcium influx across the cell membrane via N-type calcium channels and intracellular calcium store are altered in response to microcavitation. Finally, the FDA-approved poloxamer 188 (P188) was used to reconstitute the compromised cell membrane and restore the cell's reparative capability. This finding may lead to a feasible treatment for partially mitigating the tissue damage associated with bTBI.

Improving and accelerating the differentiation and functional maturation of human stem cell-derived neurons: role of extracellular calcium and GABA

Neurons differentiated from pluripotent stem cells using established neural culture conditions often exhibit functional deficits. Recently, we have developed enhanced media which both synchronize the neurogenesis of pluripotent stem cell-derived neural progenitors and accelerate their functional maturation; together these media are termed SynaptoJuice. This pair of media are pro-synaptogenic and generate authentic, mature synaptic networks of connected forebrain neurons from a variety of induced pluripotent and embryonic stem cell lines. Such enhanced rate and extent of synchronized maturation of pluripotent stem cell-derived neural progenitor cells generates neurons which are characterized by a relatively hyperpolarized resting membrane potential, higher spontaneous and induced action potential activity, enhanced synaptic activity, more complete development of a mature inhibitory GABA_A receptor phenotype and faster production of electrical network activity when compared to standard differentiation media. This entire process - from pre-patterned neural progenitor to active neuron - takes 3 weeks or less, making it an ideal platform for drug discovery and disease modelling in the fields of human neurodegenerative and neuropsychiatric disorders, such as Huntington's disease, Parkinson's disease, Alzheimer's disease and Schizophrenia.

Astrocyte transforming growth factor beta 1 promotes inhibitory synapse formation via CaM kinase II signaling

The balance between excitatory and inhibitory synaptic inputs is critical for the control of brain function. Astrocytes play important role in the development and maintenance of neuronal circuitry. Whereas astrocytes-derived molecules involved in excitatory synapses are recognized, molecules and molecular mechanisms underlying astrocyte-induced inhibitory synapses remain unknown. Here, we identified transforming growth factor beta 1 (TGF-β1), derived from human and murine astrocytes, as regulator of inhibitory synapse in vitro and in vivo. Conditioned media derived from human and murine astrocytes induce inhibitory synapse formation in cerebral cortex neurons, an event inhibited by pharmacologic and genetic manipulation of the TGF-β pathway. TGF-β1-induction of inhibitory synapse depends on glutamatergic activity and activation of CaM kinase II, which thus induces localization and cluster formation of the synaptic adhesion protein, Neuroligin 2, in inhibitory postsynaptic terminals. Additionally, intraventricular injection of TGF-β1 enhanced inhibitory synapse number in the cerebral cortex. Our results identify TGF-β1/CaMKII pathway as a novel molecular mechanism underlying astrocyte control of inhibitory synapse formation. We propose here that the balance between excitatory and inhibitory inputs might be provided by astrocyte signals, at least partly achieved via TGF-β1 downstream pathways. Our work contributes to the understanding of the GABAergic synapse formation and may be of relevance to further the current knowledge on the mechanisms underlying the development of various neurological disorders, which commonly involve impairment of inhibitory synapse transmission.

Stimulation of GABA-induced Ca2+ influx enhances maturation of human induced pluripotent stem cell-derived neurons

Optimal use of patient-derived, induced pluripotent stem cells for modeling neuronal diseases is crucially dependent upon the proper physiological maturation of derived neurons. As a strategy to develop defined differentiation protocols that optimize electrophysiological function, we investigated the role of Ca²⁺ channel regulation by astrocyte conditioned medium in neuronal maturation, using whole-cell patch clamp and Ca²⁺ imaging. Standard control medium supported basic differentiation of induced pluripotent stem cell-derived neurons, as assayed by the ability to fire simple, single, induced action potentials. In contrast, treatment with astrocyte conditioned medium elicited complex and spontaneous neuronal activity, often with rhythmic and biphasic characteristics. Such augmented spontaneous activity correlated with astrocyte conditioned medium-evoked hyperpolarization and was dependent upon regulated function of L-, N- and R-type Ca²⁺ channels. The requirement for astrocyte conditioned medium could be substituted by simply supplementing control differentiation medium with high Ca²⁺ or γ-amino butyric acid (GABA). Importantly, even in the absence of GABA signalling, opening Ca²⁺ channels directly using Bay K8644 was able to hyperpolarise neurons and enhance excitability, producing fully functional neurons. These data provide mechanistic insight into how secreted astrocyte factors control differentiation and, importantly, suggest that pharmacological modulation of Ca²⁺ channel function leads to the development of a defined protocol for improved maturation of induced pluripotent stem cell-derived neurons.

Astroglial networks and implications for therapeutic neuromodulation of epilepsy

Epilepsy is a common chronic neurologic disorder affecting approximately 1% of the world population. More than one-third of all epilepsy patients have incompletely controlled seizures or debilitating medication side effects in spite of optimal medical management. Medically refractory epilepsy is associated with excess injury and mortality, psychosocial dysfunction, and significant cognitive impairment. Effective treatment options for these patients can be limited. The cellular mechanisms underlying seizure activity are incompletely understood, though we here describe multiple lines of evidence supporting the likely contribution of astroglia to epilepsy, with focus on individual astrocytes and their network functions. Of the emerging therapeutic modalities for epilepsy, one of the most intriguing is the field of neuromodulation. Neuromodulatory treatment, which consists of administering electrical pulses to neural tissue to modulate its activity leading to a beneficial effect, may be an option for these patients. Current modalities consist of vagal nerve stimulation, open and closed-loop stimulation, and transcranial magnetic stimulation. Due to their unique properties, we here present astrocytes as likely important targets for the developing field of neuromodulation in the treatment of epilepsy.

A mathematical model of the tripartite synapse: astrocyte-induced synaptic plasticity

In this paper, we present a biologically detailed mathematical model of tripartite synapses, where astrocytes modulate short-term synaptic plasticity. The model consists of a pre-synaptic bouton, a post-synaptic dendritic spine-head, a synaptic cleft and a peri-synaptic astrocyte controlling Ca(2 + ) dynamics inside the synaptic bouton. This in turn controls glutamate release dynamics in the cleft. As a consequence of this, glutamate concentration in the cleft has been modeled, in which glutamate reuptake by astrocytes has also been incorporated. Finally, dendritic spine-head dynamics has been modeled. As an application, this model clearly shows synaptic potentiation in the hippocampal region, i.e., astrocyte Ca(2 + ) mediates synaptic plasticity, which is in conformity with the majority of the recent findings (Perea and Araque (Science 317, 1083-1086, 2007); Henneberger et al. (Nature 463, 232-236, 2010); Navarrete et al. (PLoS Biol. 10, e1001259, 2012)).

A mathematical model for astrocytes mediated LTP at single hippocampal synapses

Many contemporary studies have shown that astrocytes play a significant role in modulating both short and long form of synaptic plasticity. There are very few experimental models which elucidate the role of astrocyte over Long-term Potentiation (LTP). Recently, Perea and Araque (Science 317:1083-1086, 2007) demonstrated a role of astrocytes in induction of LTP at single hippocampal synapses. They suggested a purely pre-synaptic basis for induction of this N-methyl-D-Aspartate (NMDA) Receptor-independent LTP. Also, the mechanisms underlying this pre-synaptic induction were not investigated. Here, in this article, we propose a mathematical model for astrocyte modulated LTP which successfully imitates the experimental findings of Perea and Araque (Science 317:1083-1086, 2007). Our study suggests the role of retrograde messengers, possibly Nitric Oxide (NO), for this pre-synaptically modulated LTP.

Ca(2+) channel currents of cortical neurons from pure and mixed cultures

Voltage-gated Ca(2+) channels (VGCCs) are key regulators of many neuronal functions, and involved in multiple central nervous system diseases. In the last 30 years, a large number of injury and disease models have been established based on cultured neurons. Culture with serum develops a mixture of neurons and glial cells, while culture without serum develops pure neurons. Both of these neuronal-culture methods are widely used. However, the properties of Ca(2+) currents in neurons from these two cultures have not been compared. In this study, we cultured rat cortical neurons in serum-containing or -free medium and then recorded the Ca(2+) channel currents using patch-clamp technique. Our results showed that there were significant differences in the amplitude and activation properties of whole-cell Ca(2+) channel currents, and of non-L-type Ca(2+) channel currents between the neurons from these two culture systems. Our data suggested that the difference of whole-cell Ca(2+) currents may result from the differences in non-L-type currents. Understanding of these properties will considerably advance studies of VGCCs in neurons from pure or mixed culture.

Three-dimensional relationships between perisynaptic astroglia and human hippocampal synapses

Perisynaptic astroglia are critical for normal synaptic development and function. Little is known, however, about perisynaptic astroglia in the human hippocampus. When mesial temporal lobe epilepsy (MTLE) is refractory to medication, surgical removal is required for seizure quiescence. To investigate perisynaptic astroglia in human hippocampus, we recovered slices for several hours in vitro from three surgical specimens and then quickly fixed them to achieve high-quality ultrastructure. Histological samples from each case were found to have mesial temporal sclerosis with Blumcke Type 1a (mild, moderate) or 1b (severe) pathology. Quantitative analysis through serial section transmission electron microscopy in CA1 stratum radiatum revealed more synapses in the mild (10/10 microm(3)) than the moderate (5/10 microm(3)) or severe (1/10 microm(3)) cases. Normal spines occurred in mild and moderate cases, but a few multisynaptic spines were all that remained in the severe case. Like adult rat hippocampus, perisynaptic astroglial processes were preferentially associated with larger synapses in the mild and moderate cases, but rarely penetrated the cluster of axonal boutons surrounding multisynaptic spines. Synapse perimeters were only partially surrounded by astroglial processes such that all synapses had some access to substances in the extracellular space, similar to adult rat hippocampus. Junctions between astroglial processes were observed more frequently in moderate than mild case, but were obscured by densely packed intermediate filaments in astroglial processes of the severe case. These findings suggest that perisynaptic astroglial processes associate with synapses in human hippocampus in a manner similar to model systems and are disrupted by severe MTLE pathology.

Developmental expression of FMRP in the astrocyte lineage: implications for fragile X syndrome

One of the most common causes of mental retardation in humans, Fragile X syndrome, results from the absence of FMRP, the protein product of the FMR1 gene. In the nervous system, expression of FMRP has been thought to be confined mainly to neurons as little research has examined FMRP expression in non-neuronal lineages. We present evidence that, in addition to neuronal expression, FMRP is expressed in developing CNS glial cells in vitro and in vivo. The neurosphere assay was used to establish cultures of stem and progenitor cells from the brains of wildtype and FMRP knockout (B6.129.FMR1/FvBn) mouse pups. When the neurospheres were differentiated in vitro, approximately 50% of the FMRP positive cells also expressed GFAP. Immunocytochemical studies of the embryonic and postnatal mouse brain revealed coexpression of FMRP and GFAP in the developing hippocampus. Prominent coexpression was also observed in ependymal cells surrounding the third ventricle and astrocytes of the glia limitans. No double-labeled cells were evident in the brains of young adult mice. Cells coexpressing FMRP and the oligodendrocyte precursor marker NG2 were also identified in the hippocampus and corpus callosum of the early postnatal brain. Our results suggest that FMRP is expressed in cells of non-neuronal lineage(s) during development. This represents potential involvement of glial cells in the neural development of fragile X syndrome.

Plasticity of perisynaptic astroglia during synaptogenesis in the mature rat hippocampus

Astroglia are integral components of synapse formation and maturation during development. Less is known about how astroglia might influence synaptogenesis in the mature brain. Preparation of mature hippocampal slices results in synapse loss followed by recuperative synaptogenesis during subsequent maintenance in vitro. Hence, this model system was used to discern whether perisynaptic astroglial processes are similarly plastic, associating more or less with recently formed synapses in mature brain slices. Perisynaptic astroglia was quantified through serial section electron microscopy in perfusion-fixed or sliced hippocampus from adult male Long-Evans rats that were 65-75 days old. Fewer synapses had perisynaptic astroglia in the recovered hippocampal slices (42.4% +/- 3.4%) than in the intact hippocampus (62.2% +/- 2.6%), yet synapses were larger when perisynaptic astroglia was present (0.055 +/- 0.003 microm2) than when it was absent (0.036 +/- 0.004 microm2) in both conditions. Importantly, the length of the synaptic perimeter surrounded by perisynaptic astroglia and the distance between neighboring synapses was not proportional to synapse size. Instead, larger synapses had longer astroglia-free perimeters where substances could escape from or enter into the synaptic clefts. Thus, smaller presumably newer synapses as well as established larger synapses have equal access to extracellular glutamate and secreted astroglial factors, which may facilitate recuperative synaptogenesis. These findings suggest that as synapses enlarge and release more neurotransmitter, they attract astroglial processes to a discrete portion of their perimeters, further enhancing synaptic efficacy without limiting the potential for cross talk with neighboring synapses in the mature rat hippocampus.

Regional variations in the glial influence on synapse development in the mouse CNS

There is increasing evidence that synapse function depends on interactions with glial cells, namely astrocytes. Studies on specific neurons of the central nervous system (CNS) indicated that glial signals also control synapse development, but it remained unclear whether this is a general principle that applies to other neuronal cell types. To address this question, we developed new methods to immunoisolate neurons from different brain regions of postnatal mice and to culture them in a chemically defined medium. Electrophysiological recordings and immunocytochemical staining revealed vigorous synaptogenesis in hippocampal and cerebellar neurons, but not in retinal ganglion cells (RGCs) in the absence of glial cells. Co-culture with glia promoted synapse formation in RGCs as indicated by a strong increase in the incidence and frequency of action potential-independent miniature synaptic currents, but showed no such effects in hippocampal or cerebellar neurons. On the other hand, glial signals promoted the efficacy of excitatory synapses in all regions as indicated by an increase in the size of spontaneous synaptic events in cerebellar cultures and of miniature synaptic currents in hippocampal neurons and RGCs. Inhibitory synaptic currents remained largely unaffected by glia. Our results indicate that in the mammalian CNS, the way that glial signals promote the development of excitatory synapses depends on the type of neuron.

Neurons set the tone of gap junctional communication in astrocytic networks

A number of studies have contributed to demonstrate that neurons and astrocytes tightly and actively interact. Indeed, the presence of astrocytes in neuronal cultures increases the number of synapses and their efficiency, and thanks to enzymatic and uptake processes, astrocytes play a role in neuroprotection. A typical feature of astrocytes is that they establish cell-cell communication in vitro, as well as in situ, through intercellular channels forming specialized membrane areas defined as gap junctions. These channels are composed of junctional proteins termed connexins (Cxs): in astrocytes connexin 43 (Cx43) and 30 (Cx30) have been shown to prevail. Several recent works indicate that gap junctional communication (GJC) and/or connexin expression in astrocytes are controlled by neurons. Altogether, these observations lead to the concept that neuronal and astrocytic networks interact through mutual setting of their respective mode of communication and that astrocyte gap junctions represent a target in neuroglial interaction.

New roles for astrocytes: regulation of CNS synaptogenesis

The notion that astrocytes have a profound influence on the function of synapses between CNS neurons implies that the development of synaptic connections and their glial neighbors are controlled by reciprocally acting signals. Currently, however, synaptogenesis is considered a purely neuronal affair. This article summarizes recent experimental evidence suggesting that this may not be the case. Astrocytes may indeed regulate the formation, maturation and maintenance of synapses. The recent advances caution that synapses cannot develop correctly without astrocytes. Further progress on this issue requires new experimental models to identify signaling pathways and to scrutinize the relevance of glia-synapse interactions in vivo.

Glial Cells and Neurotransmission

Glial cells throughout the nervous system are closely associated with synapses. Accompanying these anatomical couplings are intriguing functional interactions, including the capacity of certain glial cells to respond to and modulate neurotransmission. Glial cells can also help establish, maintain, and reconstitute synapses. In this review, we discuss evidence indicating that glial cells make important contributions to synaptic function.