Functional changes in glutamate transporters and astrocyte biophysical properties in a rodent model of focal cortical dysplasia

Inhibition of astroglial hemichannels ameliorates infrasonic noise induced short-term learning and memory impairment

As a kind of environmental noise, infrasonic noise has negative effects on various human organs. To date, research has shown that infrasound impairs cognitive function, especially the ability for learning and memory. Previously, we demonstrated that impaired learning and memory induced by infrasound was closely related with glia activation; however, the underlying mechanisms remain unclear. Connexin 43 hemichannels (Cx43 HCs), which are mainly expressed in hippocampal astrocytes, are activated under pathological conditions, lending support to the hypothesis that Cx43 HCs might function in the impaired learning and memory induced by infrasound. This study revealed that that blocking hippocampal Cx43 HCs or downregulating hippocampal Cx43 expression significantly alleviated impaired learning and memory induced by infrasound. We also observed that infrasound exposure led to the abundant release of glutamate and ATP through Cx43 HCs. In addition, the abundant release of glutamate and ATP depended on proinflammatory cytokines. Our finds suggested that the enhanced release of ATP and glutamate by astroglial Cx43 HCs may be involved in the learning and memory deficits caused by infrasound exposure.

Expression profiles of α-synuclein in cortical lesions of patients with FCD IIb and TSC, and FCD rats

Background

Focal cortical dysplasia (FCD) IIb and tuberous sclerosis complex (TSC) are common causes of drug-resistant epilepsy in children. However, the etiologies related to the development of FCD IIb and TSC are not fully understood. α-synuclein (α-syn) is a member of synucleins family that plays crucial roles in modulating synaptic transmission in central nervous system. Here, we explored the expression profiles and potential pathogenic functions of α-syn in cortical lesions of epileptic patients with FCD IIb and TSC.

Methods

Surgical specimens from epileptic patients with FCD IIb and TSC, as well as FCD rats generated by in utero X-ray-radiation were adopted in this study and studied with immunohistochemistry, immunofluorescence, western blotting, and co-immunoprecipitation etc. molecular biological techniques.

Result

Our results showed that α-syn expression was reduced in FCD IIb and TSC lesions. Specifically, α-syn protein was intensely expressed in dysplastic neurons (DNs) and balloon cells (BCs) in FCD IIb lesions, whereas was barely detected in DNs and giant cells (GCs) of TSC lesions. Additionally, p-α-syn, the aggregated form of α-syn, was detected in DNs, BCs, GCs, and glia-like cells of FCD IIb and TSC lesions. We previous showed that the function of N-methyl-D-aspartate receptor (NMDAR) was enhanced in FCD rats generated by X-ray-radiation. Here, we found the interaction between α-syn and NMDAR subunits NMDAR2A, NMDAR2B were augmented in cortical lesions of FCD patients and FCD rats.

Conclusion

These results suggested a potential role of α-syn in the pathogenesis of FCD IIb and TSC by interfering with NMDAR.

Vascular endothelial growth factor-C modulates cortical NMDA receptor activity in cortical lesions of young patients and rat model with focal cortical dysplasia

Emergence of dysmorphic neurons is the primary pathology in focal cortical dysplasia (FCD) associated pediatric intractable epilepsy; however, the etiologies related to the development and function of dysmorphic neurons are not fully understood. Our previous studies revealed that the expression of vascular endothelial growth factor-C (VEGF-C) and corresponding receptors VEGFR-2, VEGFR-3 was increased in the epileptic lesions of patients with tuberous sclerosis complex or mesial temporal lobe epilepsy. Here, we showed that the expression of VEGF-C, VEGFR-2, and VEGFR-3 was increased at both mRNA and protein levels in patients with cortical lesions of type I, IIa, and IIb FCD. The immunoreactivity of VEGF-C, VEGFR-2 and VEGFR-3 was located in the micro-columnar neurons in FCD type I lesions, dysplastic neurons (DNs) in FCD type IIa lesions, balloon cells (BCs) and astrocytes in FCD type IIb lesions. Additionally, the amplitude of evoked-EPSCs (eEPSC) mediated by NMDA receptor, the ratio of NMDA receptor- and AMPA receptor-mediated eEPSC were increased in the dysmorphic neurons of FCD rats established by prenatal X-ray radiation. Furthermore, NMDA receptor mediated current in dysmorphic neurons was further potentiated by exogenous administration of VEGF-C, however, could be antagonized by ki8751, the blocker of VEGFR-2. These results suggest that VEGF-C system participate in the pathogenesis of cortical lesions in patients with FCD in association with modulating NMDA receptor-mediated currents.

Establishment of Neurotoxicity Assessment Using Microelectrode Array (MEA) with hiPSC-Derived Neurons and Evaluation of New Psychoactive Substances (NPS)

Background and Objectives

Currently, safety pharmacological tests for the central nervous system depend on animal behavioral analysis. However, due to the subjectivity of behavioral analysis and differences between species, there is a limit to appropriate nervous system toxicity assessment, therefore a new neurotoxicity assessment that can simulate the human central nervous system is required.

Methods and Results

In our study, we developed an in vitro neurotoxicity assessment focusing on neuronal function. To minimize the differences between species and fast screening, hiPSC-derived neurons and a microelectrode array (MEA) that could simultaneously measure the action potentials of the neuronal networks were used. After analyzing the molecular and electrophysiological characters of our neuronal network, we conducted a neurotoxicity assessment on neurotransmitters, neurotoxicants, illicit drugs, and new psychoactive substances (NPS). We found that most substances used in our experiments responded more sensitively to our MEA-based neurotoxicity assessment than to the conventional neurotoxicity assessment. Also, this is the first paper that evaluates various illicit drugs and NPS using MEA-based neurotoxicity assessment using hiPSC-derived neurons.

Conclusions

Our study expanded the scope of application of neurotoxicity assessment using hiPSC-derived neurons to NPS, and accumulated evaluation data of various toxic substances for hiPSC-derived neurons.

Development of astrocyte morphology and function in mouse visual thalamus

The rodent visual thalamus has served as a powerful model to elucidate the cellular and molecular mechanisms that underlie sensory circuit formation and function. Despite significant advances in our understanding of the role of axon-target interactions and neural activity in orchestrating circuit formation in visual thalamus, the role of non-neuronal cells, such as astrocytes, is less clear. In fact, we know little about the transcriptional identity and development of astrocytes in mouse visual thalamus. To address this gap in knowledge, we studied the expression of canonical astrocyte molecules in visual thalamus using immunostaining, in situ hybridization, and reporter lines. While our data suggests some level of heterogeneity of astrocytes in different nuclei of the visual thalamus, the majority of thalamic astrocytes appeared to be labeled in Aldh1l1-EGFP mice. This led us to use this transgenic line to characterize the neonatal and postnatal development of these cells in visual thalamus. Our data show that not only have the entire cohort of astrocytes migrated into visual thalamus by eye-opening but they also have acquired their adult-like morphology, even while retinogeniculate synapses are still maturing. Furthermore, ultrastructural, immunohistochemical, and functional approaches revealed that by eye-opening, thalamic astrocytes ensheathe retinogeniculate synapses and are capable of efficient uptake of glutamate. Taken together, our results reveal that the morphological, anatomical, and functional development of astrocytes in visual thalamus occurs prior to eye-opening and the emergence of experience-dependent visual activity.

Neuronal Network Excitability in Alzheimer's Disease: The Puzzle of Similar versus Divergent Roles of Amyloid β and Tau

Alzheimer’s disease (AD) is the most frequent neurodegenerative disorder that commonly causes dementia in the elderly. Recent evidence indicates that network abnormalities, including hypersynchrony, altered oscillatory rhythmic activity, interneuron dysfunction, and synaptic depression, may be key mediators of cognitive decline in AD. In this review, we discuss characteristics of neuronal network excitability in AD, and the role of Aβ and tau in the induction of network hyperexcitability. Many patients harboring genetic mutations that lead to increased Aβ production suffer from seizures and epilepsy before the development of plaques. Similarly, pathologic accumulation of hyperphosphorylated tau has been associated with hyperexcitability in the hippocampus. We present common and divergent roles of tau and Aβ on neuronal hyperexcitability in AD, and hypotheses that could serve as a template for future experiments.

DNA methylation: A mechanism for sustained alteration of KIR4.1 expression following central nervous system insult

Kir4.1, a glial-specific inwardly rectifying potassium channel, is implicated in astrocytic maintenance of K+ homeostasis. Underscoring the role of Kir4.1 in central nervous system (CNS) functioning, genetic mutations in KCNJ10, the gene which encodes Kir4.1, causes seizures, ataxia and developmental disability in humans. Kir4.1 protein and mRNA loss are consistently observed in CNS injury and neurological diseases linked to hyperexcitability and neuronal dysfunction, leading to the notion that Kir4.1 represents an attractive therapeutic target. Despite this, little is understood regarding the mechanisms that underpin this downregulation. Previous work by our lab revealed that DNA hypomethylation of the Kcnj10 gene functions to regulate mRNA levels during astrocyte maturation whereas hypermethylation in vitro led to decreased promoter activity. In the present study, we utilized two vastly different injury models with known acute and chronic loss of Kir4.1 protein and mRNA to evaluate the methylation status of Kcnj10 as a candidate molecular mechanism for reduced transcription and subsequent protein loss. Examining whole hippocampal tissue and isolated astrocytes, in a lithium-pilocarpine model of epilepsy, we consistently identified hypermethylation of CpG island two, which resides in the large intronic region spanning the Kcnj10 gene. Strikingly similar results were observed using the second injury paradigm, a fifth cervical (C5) vertebral hemi-contusion model of spinal cord injury. Our previous work indicates the same gene region is significantly hypomethylated when transcription increases during astrocyte maturation. Our results suggest that DNA methylation can bidirectionally modulate Kcnj10 transcription and may represent a targetable molecular mechanism for the restoring astroglial Kir4.1 expression following CNS insult.

Potassium and glutamate transport is impaired in scar-forming tumor-associated astrocytes

Unprovoked recurrent seizures are a serious comorbidity affecting most patients who suffer from glioma, a primary brain tumor composed of malignant glial cells. Cellular mechanisms contributing to the development of recurrent spontaneous seizures include the release of the excitatory neurotransmitter glutamate from glioma into extracellular space. Under physiological conditions, astrocytes express two high affinity glutamate transporters, Glt-1 and Glast, which are responsible for the removal of excess extracellular glutamate. In the context of neurological disease or brain injury, astrocytes become reactive which can negatively affect neuronal function, causing hyperexcitability and/or death. Using electrophysiology, immunohistochemistry, fluorescent in situ hybridization, and Western blot analysis in different orthotopic xenograft and allograft models of human and mouse gliomas, we find that peritumoral astrocytes exhibit astrocyte scar formation characterized by proliferation, cellular hypertrophy, process elongation, and increased GFAP and pSTAT3. Overall, peritumoral reactive astrocytes show a significant reduction in glutamate and potassium uptake, as well as decreased glutamine synthetase activity. A subset of peritumoral astrocytes displayed a depolarized resting membrane potential, further contributing to reduced potassium and glutamate homeostasis. These changes may contribute to the propagation of peritumoral neuronal hyperexcitability and excitotoxic death.

Ceftriaxone Treatment Affects EAAT2 Expression and Glutamatergic Neurotransmission and Exerts a Weak Anticonvulsant Effect in Young Rats

Epilepsy is a common neurological disorder. Despite the availability of a wide range of antiepileptic drugs, these are unsuccessful in preventing seizures in 20–30% of patients. Therefore, new pharmacological strategies are urgently required to control seizures. Modulation of glutamate uptake may have potential in the treatment of pharmacoresistant forms of epilepsy. Previous research showed that the antibiotic ceftriaxone (CTX) increased the expression and functional activity of excitatory amino acid transporter 2 (EAAT2) and exerted considerable anticonvulsant effects. However, other studies did not confirm a significant anticonvulsant effect of CTX administration. We investigated the impacts of CTX treatment on EAAT expression and glutamatergic neurotransmission, as well its anticonvulsant action, in young male Wistar rats. As shown by a quantitative real-time polymerase chain reaction (qPCR) assay and a Western blot analysis, the mRNA but not the protein level of EAAT2 increased in the hippocampus following CTX treatment. Repetitive CTX administration had only a mild anticonvulsant effect on pentylenetetrazol (PTZ)-induced convulsions in a maximal electroshock threshold test (MEST). CTX treatment did not affect the glutamatergic neurotransmission, including synaptic efficacy, short-term facilitation, or the summation of excitatory postsynaptic potentials (EPSPs) in the hippocampus and temporal cortex. However, it decreased the field EPSP (fEPSP) amplitudes evoked by intense electrical stimulation. In conclusion, in young rats, CTX treatment did not induce overexpression of EAAT2, therefore exerting only a weak antiseizure effect. Our data provide new insight into the effects of modulation of EAAT2 expression on brain functioning.

The Impact of the Ketogenic Diet on Glial Cells Morphology. A Quantitative Morphological Analysis

Ketogenic diet is reported to protect against cognitive decline, drug-resistant epilepsy, Alzheimer's Disease, damaging effect of ischemic stroke and many neurological diseases. Despite mounting evidence that this dietary treatment works, the exact mechanism of its protective activity is largely unknown. Ketogenic diet acts systemically, not only changing GABA signaling in neurons, but also influencing the reliance on mitochondrial respiration, known to be disrupted in many neurological diseases. Normally, human body is driven by glucose while ketogenic diet mimics starvation and energy required for proper functioning comes from fatty acids oxidation. In the brain astrocytes are believed to be the sole neural cells capable of fatty oxidation. Here we try to explain that not exclusively neurons, but also morphological changes of astroglia and/or microglia due to different metabolic state are important for the mechanism underlying the protective role of ketogenic diet. By quantifying different parameters describing cellular morphology like ramification index or fractal dimension and using Principal Component Analysis to discover the regularities between them, we demonstrate that in normal adult rat brain, ketogenic diet itself is able to change glial morphology, indicating an important role of these underappreciated cells in the brain metabolism.

Neuroreceptor kinetics in rats repeatedly exposed to quinpirole as a model for OCD

Background

Obsessive-compulsive disorder (OCD) is a chronic, incapacitating, early onset psychiatric disorder that is characterized by obsessions and compulsions originating from a disturbance in the cortico-striato-thalamico-cortical circuit. We implemented the preclinical quinpirole (QP) rat model for compulsive checking in OCD to analyse the behaviour and visualize the D2R, mGluR5 and GLT1 density in order to contribute to the understanding of the neuroreceptor kinetics.

Methods

Animals (n = 14) were exposed to either saline (1 mL/kg) or QP (dopamine D2-agonist, 0.5 mg/kg) twice-weekly during 7 weeks. After each injection animals were placed on an open field test. After model setup, animals were placed in a behavioural cage equipped with tracking software and hardware in order to analyse the behaviour. Subsequently, sagittal slides were made of the CP in the right hemisphere and a staining was done with the D2R, mGluR5 and GLT-1 antibody to visualize the corresponding receptor.

Results

The QP animals displayed a strong increase in travelled distance (+596.70%) and in the number of homebase visits (+1222.90%) compared to the control animals. After chronic exposure to QP, animals had a significantly (p < 0.05) higher percentage of D2R density in the CP (7.92% ± 0.48%) versus 6.66% ± 0.28% in animals treated with saline. There were no differences for mGluR5 and GLT1 receptor density.

Conclusions

Chronic exposure to QP leads to hyperlocomotion and an increase in D2R density. Furthermore, as mGluR5 and GLT1 density did not seem to be directly affected, decreased levels of glutamate might have influenced the binding potential in earlier reports.

Single Synapse Indicators of Impaired Glutamate Clearance Derived from Fast iGlu u Imaging of Cortical Afferents in the Striatum of Normal and Huntington (Q175) Mice

Changes in the balance between glutamate (Glu) release and uptake may stimulate synaptic reorganization and even synapse loss. In the case of neurodegeneration, a mismatch between astroglial Glu uptake and presynaptic Glu release could be detected if both parameters were assessed independently and at a single-synapse level. This has now become possible due to a new imaging assay with the genetically encoded ultrafast Glu sensor iGlu _u We report findings from individual corticostriatal synapses in acute slices prepared from mice of either sex that were >1 year of age. Contrasting patterns of short-term plasticity and a size criterion identified two classes of terminals, presumably corresponding to the previously defined IT (intratelencephalic) and PT (pyramidal tract) synapses. The latter exhibited a higher degree of frequency potentiation/residual Glu accumulation and were selected for our first iGlu _u single-synapse study in Q175 mice, a model of Huntington's disease (HD). In HD mice, the decay time constant of the perisynaptic Glu concentration (TauD), as an indicator of uptake, and the peak iGlu _u amplitude, as an indicator of release, were prolonged and reduced, respectively. Treatment of WT preparations with the astrocytic Glu uptake blocker TFB-TBOA (100 nm) mimicked the TauD changes in homozygotes. Considering the largest TauD values encountered in WT, ∼40% of PT synapses tested in Q175 heterozygotes can be classified as dysfunctional. Moreover, HD but not WT synapses exhibited a positive correlation between TauD and the peak amplitude of iGlu _u Finally, EAAT2 (excitatory amino acid transport protein 2) immunoreactivity was reduced next to corticostriatal terminals. Thus, astrocytic Glu transport remains a promising target for therapeutic intervention.SIGNIFICANCE STATEMENT Alterations in astrocytic Glu uptake can play a role in synaptic plasticity and neurodegeneration. Until now, the sensitivity of synaptic responses to pharmacological transport block and the resulting activation of NMDA receptors were regarded as reliable evidence for a mismatch between synaptic uptake and release. But the latter parameters are interdependent. Using a new genetically encoded sensor to monitor extracellular glutamate concentration ([Glu]) at individual corticostriatal synapses, we can now quantify the time constant of perisynaptic [Glu] decay (as an indicator of uptake) and the maximal [Glu] elevation next to the active zone (as an indicator of Glu release). The results provide a positive answer to the hitherto unresolved question of whether neurodegeneration (e.g., Huntington's disease) associates with a glutamate uptake deficit at tripartite excitatory synapses.

Nonictal EEG biomarkers for diagnosis and treatment

There are no reliable nonictal biomarkers for epilepsy, electroencephalography (EEG) or otherwise, but efforts to identify biomarkers that would predict the development of epilepsy after a potential epileptogenic insult, diagnose the existence of epilepsy, or assess the effects of antiseizure or antiepileptogenic interventions are relying heavily on electrophysiology. The most promising EEG biomarkers to date are pathologic high‐frequency oscillations (pHFOs), brief EEG events in the range of 100 to 600 Hz, which are believed to reflect summated action potentials from synchronously bursting neurons. Studies of patients with epilepsy, and experimental animal models, have been based primarily on direct brain recording, which makes pHFOs potentially useful for localizing the epileptogenic zone for surgical resection, but application for other diagnostic and therapeutic purposes is limited. Consequently, recent efforts have involved identification of HFOs recorded with scalp electrodes, and with magnetoencephalography, which may reflect the same pathophysiologic mechanisms as pHFOs recorded directly from the brain. The search is also on for other EEG changes that might serve as epilepsy biomarkers, and candidates include arcuate rhythms, which may reflect repetitive pHFOs, reduction in theta rhythm, which correlates with epileptogenesis in several rodent models of epilepsy, and shortened sleep spindles that correlate with ictogenesis.

α2δ-1 Signaling Drives Cell Death, Synaptogenesis, Circuit Reorganization, and Gabapentin-Mediated Neuroprotection in a Model of Insult-Induced Cortical Malformation

Developmental cortical malformations (DCMs) result from pre- and perinatal insults, as well as genetic mutations. Hypoxia, viral infection, and traumatic injury are the most common environmental causes of DCMs, and are associated with the subsyndromes focal polymicrogyria and focal cortical dysplasia (FCD) Type IIId, both of which have a high incidence of epilepsy. Understanding the molecular signals that lead to the formation of a hyperexcitable network in DCMs is critical to devising novel treatment strategies. In a previous study using the freeze-lesion (FL) murine model of DCM, we found that levels of thrombospondin (TSP) and the calcium channel auxiliary subunit α2δ-1 were elevated. TSP binds to α2δ-1 to drive the formation of excitatory synapses during development, suggesting that overactivation of this pathway may lead to exuberant excitatory synaptogenesis and network hyperexcitability seen in DCMs. In that study, antagonizing TSP/α2δ-1 signaling using the drug gabapentin (GBP) reduced many FL-induced pathologies. Here, we used mice with a genetic deletion of α2δ-1 to determine how α2δ-1 contributes to cell death, elevated excitatory synapse number, and in vitro network function after FL and to examine the molecular specificity of GBP's effects. We identified a critical role for α2δ-1 in FL-induced pathologies and in mediating the neuroprotective effects of GBP. Interestingly, genetic deletion of α2δ-1 did not eliminate GBP's effects on synaptogenesis, suggesting that GBP can have α2δ-1-independent effects. Taken together these studies suggests that inhibiting α2δ-1 signaling may have therapeutic promise to reduce cell death and network reorganization associated with insult-induced DCMs.

Functional Indicators of Glutamate Transport in Single Striatal Astrocytes and the Influence of Kir4.1 in Normal and Huntington Mice

This study evaluates single-cell indicators of glutamate transport in sulforhodamine 101-positive astrocytes of Q175 mice, a knock-in model of Huntington's disease (HD). Transport-related fluorescent ratio signals obtained with sodium-binding benzofuran isophtalate (SBFI) AM from unperturbed or voltage-clamped astrocytes and respective glutamate transporter currents (GTCs) were induced by photolytic or synaptic glutamate release and isolated pharmacologically. The HD-induced deficit ranged from -27% (GTC maximum at -100 mV in Ba(2+)) to -41% (sodium transients in astrocytes after loading SBFI-AM). Our specific aim was to clarify the mechanism(s) by which Kir4.1 channels can influence glutamate transport, as determined by either Na(+) imaging or transport-associated electrical signals. A decrease of Kir4.1 conductance was mimicked with Ba(2+) (200 μm), and an increase of Kir4.1 expression was obtained by intravenous administration of AAV9-gfaABC1D-Kir4.1-EGFP. The decrease of Kir4.1 conductance reduced the sodium transients but increased the amplitudes of somatic GTCs. Accordingly, after genetic upregulation of Kir4.1, somatic GTCs were found to be decreased. In individual cells, there was a negative correlation between Kir4.1 currents and GTCs. The relative effect of the Kir4.1 conductance was higher in the astrocyte periphery. These and other results suggest that the Kir4.1 conductance affects glutamate transporter activity in a dual manner: (1) by providing the driving force (voltage dependency of the transport itself) and (2) by limiting the lateral charge transfer (thereby reducing the interference with other electrogenic transporter functions). This leads to the testable prediction that restoring the high conductance state of passive astrocytes will not only normalize glutamate uptake but also restore other astrocytic transporter activities afflicted with HD.

SIGNIFICANCE STATEMENT

Insufficiency of astrocytic glutamate uptake is a major element in the pathophysiology of neurodegenerative diseases. Considering the heterogeneity of astrocytes and their differential susceptibility to therapeutic interventions, it becomes necessary to evaluate the determinants of transport activity in individual astroglial cells. We have examined intracellular Na(+) transients and glutamate transporter currents as the most telling indicators of glutamate clearance after synaptic or photolytic release of glutamate in striatal slices. The results show that, in Huntington's disease, glutamate uptake activity critically depends on Kir4.1. These channels enable the high conductance state of the astrocytic plasma membrane, which ensures the driving force for glutamate transport and dumps the transport-associated depolarization along the astrocyte processes. This has significant implications for developing therapeutic targets.

Acute death of astrocytes in blast-exposed rat organotypic hippocampal slice cultures

Blast traumatic brain injury (bTBI) affects civilians, soldiers, and veterans worldwide and presents significant health concerns. The mechanisms of neurodegeneration following bTBI remain elusive and current therapies are largely ineffective. It is important to better characterize blast-evoked cellular changes and underlying mechanisms in order to develop more effective therapies. In the present study, our group utilized rat organotypic hippocampal slice cultures (OHCs) as an in vitro system to model bTBI. OHCs were exposed to either 138 ± 22 kPa (low) or 273 ± 23 kPa (high) overpressures using an open-ended helium-driven shock tube, or were assigned to sham control group. At 2 hours (h) following injury, we have characterized the astrocytic response to a blast overpressure. Immunostaining against the astrocytic marker glial fibrillary acidic protein (GFAP) revealed acute shearing and morphological changes in astrocytes, including clasmatodendrosis. Moreover, overlap of GFAP immunostaining and propidium iodide (PI) indicated astrocytic death. Quantification of the number of dead astrocytes per counting area in the hippocampal cornu Ammonis 1 region (CA1), demonstrated a significant increase in dead astrocytes in the low- and high-blast, compared to sham control OHCs. However only a small number of GFAP-expressing astrocytes were co-labeled with the apoptotic marker Annexin V, suggesting necrosis as the primary type of cell death in the acute phase following blast exposure. Moreover, western blot analyses revealed calpain mediated breakdown of GFAP. The dextran exclusion additionally indicated membrane disruption as a potential mechanism of acute astrocytic death. Furthermore, although blast exposure did not evoke significant changes in glutamate transporter 1 (GLT-1) expression, loss of GLT-1-expressing astrocytes suggests dysregulation of glutamate uptake following injury. Our data illustrate the profound effect of blast overpressure on astrocytes in OHCs at 2 h following injury and suggest increased calpain activity and membrane disruption as potential underlying mechanisms.

"Symptomatic" infection-associated acute encephalopathy in children with underlying neurological disorders

BACKGROUND

Development of infection-associated acute encephalopathy (AE) is precipitated by several factors, including viral agents, age, and genetic polymorphisms. In addition, children with prior underlying neurological disorders can also present with AE.

METHOD

We reviewed 55 children with AE who were referred to hospitals participating in the Status Epilepticus Study Group from 1988 to 2013. AE was classified into eight subtypes: acute encephalopathy with biphasic seizures and late reduced diffusion (AESD); hemiconvulsion-hemiplegia syndrome (HH); acute necrotizing encephalopathy; hemorrhagic shock and encephalopathy syndrome (HSES); clinically mild encephalitis/encephalopathy with a reversible splenial lesion; acute encephalitis with refractory, repetitive partial seizures; Reye-like syndrome; and unclassified.

RESULT

Of the 55 AE cases, 14 (25.4%) had underlying neurological disorders, including perinatal insults (n=6) and genetic syndrome and/or brain malformations (n=8). These preceding morbidities were relatively common in AESD (6/18, 33.3%), HH (3/9, 33.3%), and HSES (3/6, 50.0%). History of epilepsy or febrile seizures were frequent in HH cases (4/9, 44.4%), whereas they were rare in other AE subtypes.

CONCLUSION

Among the AE subgroups, HH, HSES, and AESD frequently emerged in preceding etiologies with augmented neuronal excitability. These subgroups may have distinct pathomechanism from the "cytokine storm" mediated AEs during childhood.

Network abnormalities and interneuron dysfunction in Alzheimer disease

The function of neural circuits and networks can be controlled, in part, by modulating the synchrony of their components' activities. Network hypersynchrony and altered oscillatory rhythmic activity may contribute to cognitive abnormalities in Alzheimer disease (AD). In this condition, network activities that support cognition are altered decades before clinical disease onset, and these alterations predict future pathology and brain atrophy. Although the precise causes and pathophysiological consequences of these network alterations remain to be defined, interneuron dysfunction and network abnormalities have emerged as potential mechanisms of cognitive dysfunction in AD and related disorders. Here, we explore the concept that modulating these mechanisms may help to improve brain function in these conditions.

Defective glutamate and K+ clearance by cortical astrocytes in familial hemiplegic migraine type 2

Migraine is a common disabling brain disorder. A subtype of migraine with aura (familial hemiplegic migraine type 2: FHM2) is caused by loss‐of‐function mutations in α₂ Na⁺,K⁺ATPase (α₂NKA), an isoform almost exclusively expressed in astrocytes in adult brain. Cortical spreading depression (CSD), the phenomenon that underlies migraine aura and activates migraine headache mechanisms, is facilitated in heterozygous FHM2‐knockin mice with reduced expression of α₂NKA. The mechanisms underlying an increased susceptibility to CSD in FHM2 are unknown. Here, we show reduced rates of glutamate and K⁺ clearance by cortical astrocytes during neuronal activity and reduced density of GLT‐1a glutamate transporters in cortical perisynaptic astrocytic processes in heterozygous FHM2‐knockin mice, demonstrating key physiological roles of α₂NKA and supporting tight coupling with GLT‐1a. Using ceftriaxone treatment of FHM2 mutants and partial inhibition of glutamate transporters in wild‐type mice, we obtain evidence that defective glutamate clearance can account for most of the facilitation of CSD initiation in FHM2‐knockin mice, pointing to excessive glutamatergic transmission as a key mechanism underlying the vulnerability to CSD ignition in migraine.

Astroglial Scarring and Seizures

Epilepsy is among the most prevalent chronic neurological diseases and affects an estimated 2.2 million people in the United States alone. About one third of patients are resistant to currently available antiepileptic drugs, which are exclusively targeting neuronal function. Yet, reactive astrocytes have emerged as potential contributors to neuronal hyperexcitability and seizures. Astrocytes react to any kind of CNS insult with a range of cellular adjustments to form a scar and protect uninjured brain regions. This process changes astrocyte physiology and can affect neuronal network function in various ways. Traumatic brain injury and stroke, both conditions that trigger astroglial scar formation, are leading causes of acquired epilepsies and surgical removal of this glial scar in patients with drug-resistant epilepsy can alleviate the seizures. This review will summarize the currently available evidence suggesting that epilepsy is not a disease of neurons alone, but that astrocytes, glial cells in the brain, can be major contributors to the disease, especially when they adopt a reactive state in response to central nervous system insult.

Peripherally restricted viral challenge elevates extracellular glutamate and enhances synaptic transmission in the hippocampus

Peripheral infections increase the propensity and severity of seizures in susceptible populations. We have previously shown that intraperitoneal injection of a viral mimic, polyinosinic-polycytidylic acid (PIC), elicits hypersusceptibility of mice to kainic acid (KA)-induced seizures. This study was undertaken to determine whether this seizure hypersusceptibility entails alterations in glutamate signaling. Female C57BL/6 mice were intraperitoneally injected with PIC, and after 24 h, glutamate homeostasis in the hippocampus was monitored using the enzyme-based microelectrode arrays. PIC challenge robustly increased the level of resting extracellular glutamate. While pre-synaptic potassium-evoked glutamate release was not affected, glutamate uptake was profoundly impaired and non-vesicular glutamate release was augmented, indicating functional alterations of astrocytes. Electrophysiological examination of hippocampal slices from PIC-challenged mice revealed a several fold increase in the basal synaptic transmission as compared to control slices. PIC challenge also increased the probability of pre-synaptic glutamate release as seen from a reduction of paired-pulse facilitation and synaptic plasticity as seen from an enhancement of long-term potentiation. Altogether, our results implicate a dysregulation of astrocytic glutamate metabolism and an alteration of excitatory synaptic transmission as the underlying mechanism for the development of hippocampal hyperexcitability, and consequently seizure hypersusceptibility following peripheral PIC challenge. Peripheral infections/inflammations enhance seizure susceptibility. Here, we explored the effect of peritoneal inflammation induced by a viral mimic on glutamate homeostasis and glutamatergic neurotransmission in the mouse hippocampus. We found that peritoneal inflammation elevated extracellular glutamate concentration and enhanced the probability of pre-synaptic glutamate release resulting in hyperexcitability of neuronal networks. These mechanisms are likely to underlie the enhanced seizure propensity.

Astrocyte membrane properties are altered in a rat model of developmental cortical malformation but single-cell astrocytic glutamate uptake is robust

Developmental cortical malformations (DCMs) are linked with severe epilepsy and are caused by both genetic and environmental insults. DCMs include several neurological diseases, such as focal cortical dysplasia, polymicrogyria, schizencephaly, and others. Human studies have implicated astrocyte reactivity and dysfunction in the pathophysiology of DCMs, but their specific role is unknown. As astrocytes powerfully regulate glutamate neurotransmission, and glutamate levels are known to be increased in human epileptic foci, understanding the role of astrocytes in the pathological sequelae of DCMs is extremely important. Additionally, recent studies examining astrocyte glutamate uptake in DCMs have reported conflicting results, adding confusion to the field. In this study we utilized the freeze lesion (FL) model of DCM, which is known to induce reactive astrocytosis and cause significant changes in astrocyte morphology, proliferation, and distribution. Using whole-cell patch clamp recording from astrocytes, we recorded both UV-uncaging and synaptically evoked glutamate transporter currents (TCs), widely accepted assays of functional glutamate transport by astrocytes. With this approach, we set out to test the hypothesis that astrocyte membrane properties and glutamate transport were disrupted in this model of DCM. Though we found that the developmental maturation of astrocyte membrane resistance was disrupted by FL, glutamate uptake by individual astrocytes was robust throughout FL development. Interestingly, using an immunolabeling approach, we observed spatial and developmental differences in excitatory amino acid transporter (EAAT) expression in FL cortex. Spatially specific differences in EAAT2 (GLT-1) and EAAT1 (GLAST) expression suggest that the relative contribution of each EAAT to astrocytic glutamate uptake may be altered in FL cortex. Lastly, we carefully analyzed the amplitudes and onset times of both synaptically- and UV uncaging-evoked TCs. We found that in the FL cortex, synaptically-evoked, but not UV uncaging-evoked TCs, were larger in amplitude. Additionally, we found that the amount of electrical stimulation required to evoke a synaptic TC was significantly reduced in the FL cortex. Both of these findings are consistent with increased excitatory input to the FL cortex, but not with changes in how individual astrocytes remove glutamate. Taken together, our results demonstrate that the maturation of astrocyte membrane resistance, local distribution of glutamate transporters, and glutamatergic input to the cortex are altered in the FL model, but that single-cell astrocytic glutamate uptake is robust.

Effects of adenosine metabolism in astrocytes on central nervous system oxygen toxicity

Hyperbaric oxygen (HBO) is widely used in military operations, especially underwater missions. However, prolonged and continuous inhalation of HBO can cause central nervous system oxygen toxicity (CNS-OT), which greatly limits HBO's application. The regulation of astrocytes to the metabolism of adenosine is involved in epilepsy. In our study, we aimed to observe the effects of HBO exposure on the metabolism of adenosine in the brain. Furthermore, we aimed to confirm the possible mechanism underlying adenosine's mediation of the CNS-OT. Firstly, anesthetized rats exposed to 5 atm absolute HBO for 80 min. The concentrations of extracellular adenosine, ATP, ADP, and AMP were detected. Secondly, free-moving rats were exposed to HBO at the same pressure for 20 min, and the activities of 5'-nucleotidase and ADK in brain tissues were measured. For the mechanism studies, we observed the effects of a series of different doses of drugs related to adenosine metabolism on the latency of CNS-OT. Results showed HBO exposure could increase adenosine content by inhibiting ADK activity and improving 5'-nucleotidase activity. And adenosine metabolism during HBO exposure may be a protective response against HBO-induced CNS-OT. Moreover, the improvement of adenosine concentration, activation of adenosine A1R, or suppression of ADK and adenosine A2AR, which are involved in the prevention of HBO-induced CNS-OT. This is the first study to demonstrate HBO exposure regulated adenosine metabolism in the brain. Adenosine metabolism and adenosine receptors are related to HBO-induced CNS-OT development. These results will provide new potential targets for the termination or the attenuation of CNS-OT.

Astrogliosis: An integral player in the pathogenesis of Alzheimer's disease

Alzheimer's disease is the main cause of dementia in the elderly and begins with a subtle decline in episodic memory followed by a more general decline in overall cognitive abilities. Though the exact trigger for this cascade of events remains unknown the presence of the misfolded amyloid-beta protein triggers reactive gliosis, a prominent neuropathological feature in the brains of Alzheimer's patients. The cytoskeletal and morphological changes of astrogliosis are its evident features, while changes in oxidative stress defense, cholesterol metabolism, and gene transcription programs are less manifest. However, these latter molecular changes may underlie a disruption in homeostatic regulation that keeps the brain environment balanced. Astrocytes in Alzheimer's disease show changes in glutamate and GABA signaling and recycling, potassium buffering, and in cholinergic, purinergic, and calcium signaling. Ultimately the dysregulation of homeostasis maintained by astrocytes can have grave consequences for the stability of microcircuits within key brain regions. Specifically, altered inhibition influenced by astrocytes can lead to local circuit imbalance with farther reaching consequences for the functioning of larger neuronal networks. Healthy astrocytes have a role in maintaining and modulating normal neuronal communication, synaptic physiology and energy metabolism, astrogliosis interferes with these functions. This review considers the molecular and functional changes occurring during astrogliosis in Alzheimer's disease, and proposes that astrocytes are key players in the development of dementia.