Contributions of astrocytes to epileptogenesis following status epilepticus: opportunities for preventive therapy?

Parkinson's disease, epilepsy, and amyotrophic lateral sclerosis-emerging role of AMPA and kainate subtypes of ionotropic glutamate receptors

Ionotropic glutamate receptors (iGluRs) mediate the majority of excitatory neurotransmission and are implicated in various neurological disorders. In this review, we discuss the role of the two fastest iGluRs subtypes, namely, α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) and kainate receptors, in the pathogenesis and treatment of Parkinson's disease, epilepsy, and amyotrophic lateral sclerosis. Although both AMPA and kainate receptors represent promising therapeutic targets for the treatment of these diseases, many of their antagonists show adverse side effects. Further studies of factors affecting the selective subunit expression and trafficking of AMPA and kainate receptors, and a reasonable approach to their regulation by the recently identified novel compounds remain promising directions for pharmacological research.

The Role of Glutamate Receptors in Epilepsy

Glutamate is an essential excitatory neurotransmitter in the central nervous system, playing an indispensable role in neuronal development and memory formation. The dysregulation of glutamate receptors and the glutamatergic system is involved in numerous neurological and psychiatric disorders, especially epilepsy. There are two main classes of glutamate receptor, namely ionotropic and metabotropic (mGluRs) receptors. The former stimulate fast excitatory neurotransmission, are N-methyl-d-aspartate (NMDA), α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA), and kainate; while the latter are G-protein-coupled receptors that mediate glutamatergic activity via intracellular messenger systems. Glutamate, glutamate receptors, and regulation of astrocytes are significantly involved in the pathogenesis of acute seizure and chronic epilepsy. Some glutamate receptor antagonists have been shown to be effective for the treatment of epilepsy, and research and clinical trials are ongoing.

Neuroelectric Mechanisms of Delayed Cerebral Ischemia after Aneurysmal Subarachnoid Hemorrhage

Delayed cerebral ischemia (DCI) remains a challenging but very important condition, because DCI is preventable and treatable for improving functional outcomes after aneurysmal subarachnoid hemorrhage (SAH). The pathologies underlying DCI are multifactorial. Classical approaches to DCI focus exclusively on preventing and treating the reduction of blood flow supply. However, recently, glutamate-mediated neuroelectric disruptions, such as excitotoxicity, cortical spreading depolarization and seizures, and epileptiform discharges, have been reported to occur in high frequencies in association with DCI development after SAH. Each of the neuroelectric disruptions can trigger the other, which augments metabolic demand. If increased metabolic demand exceeds the impaired blood supply, the mismatch leads to relative ischemia, resulting in DCI. The neuroelectric disruption also induces inverted vasoconstrictive neurovascular coupling in compromised brain tissues after SAH, causing DCI. Although glutamates and the receptors may play central roles in the development of excitotoxicity, cortical spreading ischemia and epileptic activity-related events, more studies are needed to clarify the pathophysiology and to develop novel therapeutic strategies for preventing or treating neuroelectric disruption-related DCI after SAH. This article reviews the recent advancement in research on neuroelectric disruption after SAH.

Baicalin suppresses neuron autophagy and apoptosis by regulating astrocyte polarization in pentylenetetrazol-induced epileptic rats and PC12 cells

Epilepsy is a common chronic neurological disorder worldwide, but its entire pathology remains unknown. The purpose of this study was to explore the antiepileptic effect of baicalin (BAL), the main bioactive component of scutellaria. We isolated astrocytes from neonatal rats and astrocytes were identified by glial fibrillary acidic protein (GFAP) immunostaining. The viability and phenotype of astrocytes were determined by Cell Counting Kit-8 (CCK-8) and immunofluorescence staining, respectively. For investigating the effect of BAL on the autophagy in A1 astrocytes treated PC12 cells, expression of light chain 3B (LC3-B) and sequestosome 1 (P62) was analyzed by immunofluorescence staining and apoptosis by acridine orange/ethidium bromide (AO/EB) staining, respectively. For animal experiments, pentylenetetrazol (PTZ)-induced epileptic model was used to explore the antiepileptic effect of BAL. The results showed that BAL reduced lipopolysaccharide (LPS)-induced complement C3 (C3, a marker of A1 astrocytes) + A1 cells and decreased autophagy and apoptosis in PC12 cells. Further findings showed seizure grade and latency were positively correlated with GFAP+/C3 + A1 cells' infiltration in interstitial astrocytes. After BAL treatment, epileptogenesis was ameliorated with decreased A1 astrocytes in the brain and improved behavioral performance. The enzyme-linked immunosorbent assay (ELISA) showed that the levels of interleukin-1α (IL-1α) and tumor necrosis factor-α (TNF-α) were reduced in the cerebral interstitial site in the BAL group compared to the PTZ group. Western blotting analysis showed that BAL treatment reduced expression of C3, inward rectifier potassium channel Kir4.1, aquaporin-4 (AQP4) in the frontal cortex and Caspase-3, BCL2-associated X protein (Bax) in the hippocampus. In conclusion, these findings suggest that BAL can prevents cognitive and emotional disorders and has antiepileptic effects in rats, which may be associated with suppresses neuron autophagy and apoptosis in the hippocampus via regulate astrocyte phenotypes.

Increased immunoreactivity of glutamate receptors, neuronal nuclear protein and glial fibrillary acidic protein in the hippocampus of epileptic rats with fast ripple activity

Epilepsy is a neurological disorder in which an imbalance between excitatory and inhibitory transmission is observed. Glutamate is the principal excitatory neurotransmitter that acts through ionic and metabotropic receptors; both types of receptors are involved in temporal lobe epilepsy (TLE). High frequency oscillations called fast ripples (FR, 250-600 Hz) have been observed, particularly in the hippocampus, and they are involved in epileptogenesis. The present study analyzed the immunoreactivity of the principal glutamate receptors associated with epilepsy in epileptic animals with FR activity. Male Swiss-Wistar rats (210-250 gr) were injected with pilocarpine (2.4 mg/2 µl) and were video monitored (24/7) until the appearance of spontaneous and recurrent seizures. Then, a deep microelectrode implantation surgery was performed in the DG, CA3 and CA1 regions, and FR activity was observed 1-, 2-, 3-, 7-, and 14-day postsurgery. The animals were sacrificed on day 15, and fluorescence immunohistochemistry was carried out in the hippocampus for the α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA), N-methyl-D-aspartate (NMDA) and mGlu-R5 glutamate receptors as well as Neuronal Nuclear Protein (NeuN) and Glial Fibrillary Acidic Protein (GFAP). An increase in the immunoreactivity for the three receptors was found. However, the AMPA receptor showed an increase in the three regions analyzed (i.e., DG, CA1 and CA3). The findings showed a decrease of NeuN in the DG and an increase of GFAP. These results suggest an important role of glutamate receptors in the hippocampus of epileptic rats with FR activity.

Possible interplay between the theories of pharmacoresistant epilepsy

Epilepsy is one of the oldest known neurological disorders and is characterized by recurrent seizure activity. It has a high incidence rate, affecting a broad demographic in both developed and developing countries. Comorbid conditions are frequent in patients with epilepsy and have detrimental effects on their quality of life. Current management options for epilepsy include the use of anti-epileptic drugs, surgery, or a ketogenic diet. However, more than 30% of patients diagnosed with epilepsy exhibit drug resistance to anti-epileptic drugs. Further, surgery and ketogenic diets do little to alleviate the symptoms of patients with pharmacoresistant epilepsy. Thus, there is an urgent need to understand the underlying mechanisms of pharmacoresistant epilepsy to design newer and more effective anti-epileptic drugs. Several theories of pharmacoresistant epilepsy have been suggested over the years, the most common being the gene variant hypothesis, network hypothesis, multidrug transporter hypothesis, and target hypothesis. In our review, we discuss the main theories of pharmacoresistant epilepsy and highlight a possible interconnection between their mechanisms that could lead to the development of novel therapies for pharmacoresistant epilepsy.

Reactivity and increased proliferation of NG2 cells following central nervous system infection with Theiler's murine encephalomyelitis virus

BACKGROUND

Neuron-glial antigen 2 (NG2) cells are a glial cell type tiled throughout the gray and white matter of the central nervous system (CNS). NG2 cells are known for their ability to differentiate into oligodendrocytes and are commonly referred to as oligodendrocyte precursor cells. However, recent investigations have begun to identify additional functions of NG2 cells in CNS health and pathology. NG2 cells form physical and functional connections with neurons and other glial cell types throughout the CNS, allowing them to monitor and respond to the neural environment. Growing evidence indicates that NG2 cells become reactive under pathological conditions, though their specific roles are only beginning to be elucidated. While reactive microglia and astrocytes are well-established contributors to neuroinflammation and the development of epilepsy following CNS infection, the dynamics of NG2 cells remain unclear. Therefore, we investigated NG2 cell reactivity in a viral-induced mouse model of temporal lobe epilepsy.

METHODS

C57BL6/J mice were injected intracortically with Theiler's murine encephalomyelitis virus (TMEV) or PBS. Mice were graded twice daily for seizures between 3 and 7 days post-injection (dpi). At 4 and 14 dpi, brains were fixed and stained for NG2, the microglia/macrophage marker IBA1, and the proliferation marker Ki-67. Confocal z stacks were acquired in both the hippocampus and the overlying cortex. Total field areas stained by each cell marker and total field area of colocalized pixels between NG2 and Ki67 were compared between groups.

RESULTS

Both NG2 cells and microglia/macrophages displayed increased immunoreactivity and reactive morphologies in the hippocampus of TMEV-injected mice. While increased immunoreactivity for IBA1 was also present in the cortex, there was no significant change in NG2 immunoreactivity in the cortex following TMEV infection. Colocalization analysis for NG2 and Ki-67 revealed a significant increase in overlap between NG2 and Ki-67 in the hippocampus of TMEV-injected mice at both time points, but no significant differences in cortex.

CONCLUSIONS

NG2 cells acquire a reactive phenotype and proliferate in response to TMEV infection. These results suggest that NG2 cells alter their function in response to viral encephalopathy, making them potential targets to prevent the development of epilepsy following viral infection.

Anakinra Reduces Epileptogenesis, Provides Neuroprotection, and Attenuates Behavioral Impairments in Rats in the Lithium-Pilocarpine Model of Epilepsy

Temporal lobe epilepsy is a widespread chronic disorder that manifests as spontaneous seizures and is often characterized by refractoriness to drug treatment. Temporal lobe epilepsy can be caused by a primary brain injury; therefore, the prevention of epileptogenesis after a primary event is considered one of the best treatment options. However, a preventive treatment for epilepsy still does not exist. Neuroinflammation is directly involved in epileptogenesis and neurodegeneration, leading to the epileptic condition and cognitive decline. In the present study, we aimed to clarify the effect of treatment with a recombinant form of the Interleukin-1 receptor antagonist (anakinra) on epileptogenesis and behavioral impairments in rats using the lithium–pilocarpine model. We found that anakinra administration during the latent phase of the model significantly suppressed the duration and frequency of spontaneous recurrent seizures in the chronic phase. Moreover, anakinra administration prevented some behavioral impairments, including motor hyperactivity and disturbances in social interactions, during both the latent and chronic periods. Histological analysis revealed that anakinra administration decreased neuronal loss in the CA1 and CA3 areas of the hippocampus but did not prevent astro- and microgliosis. The treatment increased the expression level of the solute carrier family 1 member 2 gene (Slc1a2, encoding excitatory amino acid transporter 2 (EAAT2)) in the hippocampus, potentially leading to a neuroprotective effect. However, the increased gene expression of proinflammatory cytokine genes (Interleukin-1β (Il1b) and tumor necrosis factor α (Tnfa)) and astroglial marker genes (glial fibrillary acidic protein (Gfap) and inositol 1,4,5-trisphosphate receptor type 2 (Itpr2)) in experimental rats was not affected by anakinra treatment. Thus, our data demonstrate that the administration of anakinra during epileptogenesis has some beneficial disease-modifying effects.

Kainate Receptor Activation Enhances Amyloidogenic Processing of APP in Astrocytes

Kainic acid (KA) is an analogue of the excitatory neurotransmitter glutamate that, when injected systemically into adult rats, can trigger seizures and progressive neuronal loss in a manner that mirrors the neuropathology of human mesial temporal lobe epilepsy. However, biomolecular mechanisms responsible for the neuronal loss that occurs as a consequence of this treatment remains elusive. We have recently reported that toxicity induced by KA can partly be mediated by astrocyte-derived amyloid β (Aβ) peptides, which are critical in the development of Alzheimer's disease (AD). Nonetheless, little is known how KA can influence amyloid precursor protein (APP) levels and processing in astrocytes. Thus, in the present study using human U-373 astrocytoma and rat primary astrocytes, we evaluated the role of KA on APP metabolism. Our results revealed that KA treatment increased the levels of APP and its cleaved products (α-/β-CTFs) in cultured U-373 astrocytoma and primary astrocytes, without altering the cell viability. The cellular and secretory levels of Aβ_1-40/Aβ_1-42 were markedly increased in KA-treated astrocytes. We also demonstrated that the steady-state levels of APP-secretases were not altered but the activity of γ-secretase is enhanced in KA-treated U-373 astrocytoma. Furthermore, using selective receptor antagonists, we showed that the effects of KA is mediated by activation of kainate receptors and not NMDA or AMPA receptors. These results suggest that KA can enhance amyloidogenic processing of APP by activating its own receptor leading to increased production/secretion of Aβ-related peptides from activated astrocytes which may contribute to the pathogenesis of temporal lobe epilepsy.

Perampanel but Not Amantadine Prevents Behavioral Alterations and Epileptogenesis in Pilocarpine Rat Model of Status Epilepticus

Pilocarpine-induced status epilepticus (SE), which results in the development of spontaneous recurrent seizures (SRSs) activates glutamatergic receptors that contribute to seizure sustenance and neuronal cell death. In the current study, we evaluate whether the exposure to perampanel, an α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor blocker, or amantadine, a N-methyl-D-aspartic acid (NMDA) receptor blocker would reduce the SE-induced long-term consequences. SE was induced in adult male Sprague Dawley rats with pilocarpine. Perampanel or amantadine was injected 10 or 60 min after SE onset. The efficacy of either, in overcoming pilocarpine-induced SE was assessed using electroencephalogram (EEG) recordings. In addition, alterations in cognitive function, development of spontaneous recurrent seizures (SRSs), and hippocampal damage that are generally encountered after SE were also assessed at 72 h and 5 weeks after the induction of SE. Our results indicate that both early and late treatment with perampanel but not amantadine significantly reduced seizure activity. Furthermore, perampanel but not amantadine, reversed the memory deficits in Y-maze and novel object recognition (NOR) tests and retarded the appearance of SRSs. Moreover, perampanel treatment led to reduced SE-induced caspase-3 activation in the hippocampal lysates. Taken together, the data obtained from the study reveals that blocking AMPA receptors by perampanel can modify SE-induced long-term consequences. Our results may provide a proof of principle for the potential therapeutic application of perampanel in clinical use for status epilepticus in future.

A role for astrocyte-derived amyloid β peptides in the degeneration of neurons in an animal model of temporal lobe epilepsy

Kainic acid, an analogue of the excitatory neurotransmitter glutamate, can trigger seizures and neurotoxicity in the hippocampus and other limbic structures in a manner that mirrors the neuropathology of human temporal lobe epilepsy (TLE). However, the underlying mechanisms associated with the neurotoxicity remain unclear. Since amyloid-β (Aβ) peptides, which are critical in the development of Alzheimer's disease, can mediate toxicity by activating glutamatergic NMDA receptors, it is likely that the enhanced glutamatergic transmission that renders hippocampal neurons vulnerable to kainic acid treatment may involve Aβ peptides. Thus, we seek to establish what role Aβ plays in kainic acid-induced toxicity using in vivo and in vitro paradigms. Our results show that systemic injection of kainic acid to adult rats triggers seizures, gliosis and loss of hippocampal neurons, along with increased levels/processing of amyloid precursor protein (APP), resulting in the enhanced production of Aβ-related peptides. The changes in APP levels/processing were evident primarily in activated astrocytes, implying a role for astrocytic Aβ in kainic acid-induced toxicity. Accordingly, we showed that treating rat primary cultured astrocytes with kainic acid can lead to increased Aβ production/secretion without any compromise in cell viability. Additionally, we revealed that kainic acid reduces neuronal viability more in neuronal/astrocyte co-cultures than in pure neuronal culture, and this is attenuated by precluding Aβ production. Collectively, these results indicate that increased production/secretion of Aβ-related peptides from activated astrocytes can contribute to neurotoxicity in kainic acid-treated rats. Since kainic acid administration can lead to neuropathological changes resembling TLE, it is likely that APP/Aβ peptides derived from astrocytes may have a role in TLE pathogenesis.

Metabolomic studies identify changes in transmethylation and polyamine metabolism in a brain-specific mouse model of tuberous sclerosis complex

Tuberous sclerosis complex (TSC) is an autosomal dominant neurodevelopmental disorder and the quintessential disorder of mechanistic Target of Rapamycin Complex 1 (mTORC1) dysregulation. Loss of either causative gene, TSC1 or TSC2, leads to constitutive mTORC1 kinase activation and a pathologically anabolic state of macromolecular biosynthesis. Little is known about the organ-specific metabolic reprogramming that occurs in TSC-affected organs. Using a mouse model of TSC in which Tsc2 is disrupted in radial glial precursors and their neuronal and glial descendants, we performed an unbiased metabolomic analysis of hippocampi to identify Tsc2-dependent metabolic changes. Significant metabolic reprogramming was found in well-established pathways associated with mTORC1 activation, including redox homeostasis, glutamine/tricarboxylic acid cycle, pentose and nucleotide metabolism. Changes in two novel pathways were identified: transmethylation and polyamine metabolism. Changes in transmethylation included reduced methionine, cystathionine, S-adenosylmethionine (SAM-the major methyl donor), reduced SAM/S-adenosylhomocysteine ratio (cellular methylation potential), and elevated betaine, an alternative methyl donor. These changes were associated with alterations in SAM-dependent methylation pathways and expression of the enzymes methionine adenosyltransferase 2A and cystathionine beta synthase. We also found increased levels of the polyamine putrescine due to increased activity of ornithine decarboxylase, the rate-determining enzyme in polyamine synthesis. Treatment of Tsc2+/- mice with the ornithine decarboxylase inhibitor α-difluoromethylornithine, to reduce putrescine synthesis dose-dependently reduced hippocampal astrogliosis. These data establish roles for SAM-dependent methylation reactions and polyamine metabolism in TSC neuropathology. Importantly, both pathways are amenable to nutritional or pharmacologic therapy.

Astroglial role in the pathophysiology of status epilepticus: an overview

Status epilepticus is a medical emergency with elevated morbidity and mortality rates, and represents a leading cause of epilepsy-related deaths. Though status epilepticus can occur at any age, it manifests more likely in children and elderly people. Despite the common prevalence of epileptic disorders, a complete explanation for the mechanisms leading to development of self-limited or long lasting seizures (as in status epilepticus) are still lacking. Apart from neurons, research evidence suggests the involvement of immune and glial cells in epileptogenesis. Among glial cells, astrocytes represent an ideal target for the study of the pathophysiology of status epilepticus, due to their key role in homeostatic balance of the central nervous system. During status epilepticus, astroglial cells are activated by the presence of cytokines, damage associated molecular patterns and reactive oxygen species. The persistent activation of astrocytes leads to a decrease in glutamate clearance with a corresponding accumulation in the synaptic extracellular space, increasing the chance of neuronal excitotoxicity. Moreover, major alterations in astrocytic gap junction coupling, inflammation and receptor expression, facilitate the generation of seizures. Astrocytes are also involved in dysregulation of inhibitory transmission in the central nervous system and directly participate in ionic homeostatic alterations during status epilepticus. In the present review, we focus on the functional and structural changes in astrocytic activity that participate in the development and maintenance of status epilepticus, with special attention on concurrent inflammatory alterations. We also include potential astrocytic treatment targets for status epilepticus.

Dynamic Changes of Astrocytes and Adenosine Signaling in Rat Hippocampus in Post-status Epilepticus Model of Epileptogenesis

It is of great importance to explore the development of epileptogenesis, and the adenosine and adenosine kinase (ADK) system seems to play a key role in this process. The aim of this study is to explore the dynamic changes of astrocytes and adenosine signaling during epileptogenesis in rat hippocampus in a post-status epileptogenesis (SE) model. Rat SE models were built and killed for experiments at 1 day (acute phase of epileptogenesis), 5 days (latent phase), 4 weeks (chronic phase), and 8 weeks (late chronic phase of epileptogenesis) after SE induction. Immunofluorescence staining, high-performance liquid chromatography, and Western blotting were performed to assess changes of astrocytes, adenosine, ADK, and ADK receptors (including A1R, A2aR, A2bR, and A3R) in hippocampus. The expression level of glial fibrillary acidic protein significantly increased from latent to late chronic phase. The concentration of adenosine sharply increased in acute phase and gradually decreased in the remaining phases of post-SE, being significantly lower than in the control group in late chronic phase. Protein levels of A1R and A2aR in post-SE models increased in acute phase, whereas A2bR and A3R protein expression decreased in latent phase, chronic phase, and late chronic phase following post-SE epileptogenesis. Protein expression of ADK significantly increased during latent phase, chronic phase, and late chronic phase of post-SE epileptogenesis. In conclusion, the levels of adenosine and protein expression of A1R and A2R significantly increased during acute phase of post-SE. During the remaining phases of post-SE epileptogenesis, there was imbalance among astrocytes, adenosine, adenosine receptors, and ADK. Regulation of the ADK/adenosine system may provide potential treatment strategies for epileptogenesis.

LncRNA H19 contributes to hippocampal glial cell activation via JAK/STAT signaling in a rat model of temporal lobe epilepsy

Background

Astrocyte and microglia activation are well-known features of temporal lobe epilepsy that may contribute to epileptogenesis. However, the mechanisms underlying glia activation are not well understood. Long non-coding RNA (lncRNA) H19 has diverse functions depending on physiological or pathological state, and its role in epilepsy is unknown. We previously demonstrated that H19 was significantly upregulated in the latent period of epilepsy and may be associated with cell proliferation and immune and inflammatory responses. We therefore speculated that H19 is involved in the hippocampal glial cell activation during epileptogenesis.

Methods

H19 was overexpressed or knocked down using an adeno-associated viral vector delivery system. A rat status epilepticus model was induced by intra-amygdala kainic acid injection. Astrocyte and microglia activation were assessed by immunofluorescence and western blot analyses. Expression of proinflammatory cytokines and components of the Janus kinase (JAK)/signal transducer and activator of transcription (STAT) signaling pathways were evaluated with western blotting.

Results

H19 overexpression induced the activation of astrocytes and microglia and the release of proinflammatory cytokines (interleukin-1β and interleukin-6 and tumor necrosis factor-α) in the hippocampus, whereas H19 knockdown inhibited status epilepticus-induced glial cell activation. Moreover, H19 activated JAK/STAT signaling by promoting the expression of Stat3 and c-Myc, which is thought to be involved in astrocyte activation.

Conclusions

LncRNA H19 contributes to hippocampal glial cell activation via modulation of the JAK/STAT pathway and could be a therapeutic tool to prevent the development of epilepsy.

Electronic supplementary material

The online version of this article (10.1186/s12974-018-1139-z) contains supplementary material, which is available to authorized users.

Seizure frequency correlates with loss of dentate gyrus GABAergic neurons in a mouse model of temporal lobe epilepsy

Epilepsy occurs in one of 26 people. Temporal lobe epilepsy is common and can be difficult to treat effectively. It can develop after brain injuries that damage the hippocampus. Multiple pathophysiological mechanisms involving the hippocampal dentate gyrus have been proposed. This study evaluated a mouse model of temporal lobe epilepsy to test which pathological changes in the dentate gyrus correlate with seizure frequency and help prioritize potential mechanisms for further study. FVB mice (n = 127) that had experienced status epilepticus after systemic treatment with pilocarpine 31-61 days earlier were video-monitored for spontaneous, convulsive seizures 9 hr/day every day for 24-36 days. Over 4,060 seizures were observed. Seizure frequency ranged from an average of one every 3.6 days to one every 2.1 hr. Hippocampal sections were processed for Nissl stain, Prox1-immunocytochemistry, GluR2-immunocytochemistry, Timm stain, glial fibrillary acidic protein-immunocytochemistry, glutamic acid decarboxylase in situ hybridization, and parvalbumin-immunocytochemistry. Stereological methods were used to measure hilar ectopic granule cells, mossy cells, mossy fiber sprouting, astrogliosis, and GABAergic interneurons. Seizure frequency was not significantly correlated with the generation of hilar ectopic granule cells, the number of mossy cells, the extent of mossy fiber sprouting, the extent of astrogliosis, or the number of GABAergic interneurons in the molecular layer or hilus. Seizure frequency significantly correlated with the loss of GABAergic interneurons in or adjacent to the granule cell layer, but not with the loss of parvalbumin-positive interneurons. These findings prioritize the loss of granule cell layer interneurons for further testing as a potential cause of temporal lobe epilepsy.

Ablation of Newly Generated Hippocampal Granule Cells Has Disease-Modifying Effects in Epilepsy

Hippocampal granule cells generated in the weeks before and after an epileptogenic brain injury can integrate abnormally into the dentate gyrus, potentially mediating temporal lobe epileptogenesis. Previous studies have demonstrated that inhibiting granule cell production before an epileptogenic brain insult can mitigate epileptogenesis. Here, we extend upon these findings by ablating newly generated cells after the epileptogenic insult using a conditional, inducible diphtheria-toxin receptor expression strategy in mice. Diphtheria-toxin receptor expression was induced among granule cells born up to 5 weeks before pilocarpine-induced status epilepticus and these cells were then eliminated beginning 3 d after the epileptogenic injury. This treatment produced a 50% reduction in seizure frequency, but also a 20% increase in seizure duration, when the animals were examined 2 months later. These findings provide the first proof-of-concept data demonstrating that granule cell ablation therapy applied at a clinically relevant time point after injury can have disease-modifying effects in epilepsy.

SIGNIFICANCE STATEMENT

These findings support the long-standing hypothesis that newly generated dentate granule cells are pro-epileptogenic and contribute to the occurrence of seizures. This work also provides the first evidence that ablation of newly generated granule cells can be an effective therapy when begun at a clinically relevant time point after an epileptogenic insult. The present study also demonstrates that granule cell ablation, while reducing seizure frequency, paradoxically increases seizure duration. This paradoxical effect may reflect a disruption of homeostatic mechanisms that normally act to reduce seizure duration, but only when seizures occur frequently.

VEGF treatment during status epilepticus attenuates long-term seizure-associated alterations in astrocyte morphology

Vascular endothelial growth factor (VEGF) treatment during pilocarpine-induced status epilepticus (SE) causes sustained preservation of behavioral function in rats in the absence of enduring neuroprotection (Nicoletti et al., 2010), suggesting the possibility that other cells or mechanisms could be involved in the beneficial effects of VEGF during SE. Astrocytes have been reported to contribute to epileptiform discharges in the hippocampus (Tian et al., 2005; Kang et al., 1998) and to express VEGF receptors (Krum & Rosenstein, 2002). We report here that VEGF treatment significantly alters multiple astrocyte parameters. This study investigated astrocyte morphology one month after SE in animals treated with VEGF or inactivated VEGF control protein during SE. Individual GFAP-immunostained astrocytes from CA1 and dentate gyrus hilus were traced and morphologically quantified, and both somatic and process structures were analyzed. VEGF treatment during SE significantly prevented post-SE increases in number of branch intersections, process length, and node count. Furthermore, analysis of distance to nearest neighboring astrocytes revealed that VEGF treatment significantly increased inter-astrocyte distance. Overall, VEGF treatment during SE did not significantly alter the shape of the astrocytes, but did prevent SE-induced changes in branching complexity, size, and spatial patterning. Because astrocyte morphology may be related to astrocyte function, it is possible that VEGF's enduring effects on astrocyte morphology may impact the functioning of the post-seizure hippocampus.

Astrocytic Acid-Sensing Ion Channel 1a Contributes to the Development of Chronic Epileptogenesis

Unraveling mechanisms underlying epileptogenesis after brain injury is an unmet medical challenge. Although histopathological studies have revealed that reactive astrogliosis and tissue acidosis are prominent features in epileptogenic foci, their roles in epileptogenesis remain unclear. Here, we explored whether astrocytic acid-sensing ion channel-1a (ASIC1a) contributes to the development of chronic epilepsy. High levels of ASIC1a were measured in reactive astrocytes in the hippocampi of patients with temporal lobe epilepsy (TLE) and epileptic mice. Extracellular acidosis caused a significant Ca²⁺ influx in cultured astrocytes, and this influx was sensitive to inhibition by the ASIC1a-specific blocker psalmotoxin 1 (PcTX1). In addition, recombinant adeno-associated virus (rAAV) vectors carrying a GFAP promoter in conjunction with ASIC1a shRNA or cDNA were generated to suppress or restore, respectively, ASIC1a expression in astrocytes. Injection of rAAV-ASIC1a-shRNA into the dentate gyrus of the wide type TLE mouse model resulted in the inhibition of astrocytic ASIC1a expression and a reduction in spontaneous seizures. By contrast, rAAV-ASIC1a-cDNA restored astrocytic ASIC1a expression in an ASIC1a knock-out TLE mouse model and increased the frequency of spontaneous seizures. Taken together, our results reveal that astrocytic ASIC1a may be an attractive new target for the treatment of epilepsy.

Impact of rapamycin on status epilepticus induced hippocampal pathology and weight gain

Growing evidence implicates the dentate gyrus in temporal lobe epilepsy (TLE). Dentate granule cells limit the amount of excitatory signaling through the hippocampus and exhibit striking neuroplastic changes that may impair this function during epileptogenesis. Furthermore, aberrant integration of newly-generated granule cells underlies the majority of dentate restructuring. Recently, attention has focused on the mammalian target of rapamycin (mTOR) signaling pathway as a potential mediator of epileptogenic change. Systemic administration of the mTOR inhibitor rapamycin has promising therapeutic potential, as it has been shown to reduce seizure frequency and seizure severity in rodent models. Here, we tested whether mTOR signaling facilitates abnormal development of granule cells during epileptogenesis. We also examined dentate inflammation and mossy cell death in the dentate hilus. To determine if mTOR activation is necessary for abnormal granule cell development, transgenic mice that harbored fluorescently-labeled adult-born granule cells were treated with rapamycin following pilocarpine-induced status epilepticus. Systemic rapamycin effectively blocked phosphorylation of S6 protein (a readout of mTOR activity) and reduced granule cell mossy fiber axon sprouting. However, the accumulation of ectopic granule cells and granule cells with aberrant basal dendrites was not significantly reduced. Mossy cell death and reactive astrocytosis were also unaffected. These data suggest that anti-epileptogenic effects of mTOR inhibition may be mediated by mechanisms other than inhibition of these common dentate pathologies. Consistent with this conclusion, rapamycin prevented pathological weight gain in epileptic mice, suggesting that rapamycin might act on central circuits or even peripheral tissues controlling weight gain in epilepsy.

Neuronal hyperactivity accelerates depletion of neural stem cells and impairs hippocampal neurogenesis

Adult hippocampal neurogenesis is believed to maintain a range of cognitive functions, many of which decline with age. We recently reported that radial neural stem cells (rNSCs) in the hippocampus undergo activation-dependent conversion into astrocytes, a mechanism that over time contributes to a reduction in the rNSC population. Here, we injected low and high levels of kainic acid (KA) in the dentate gyrus to assess whether neuronal hyperexcitation, a hallmark of epileptic disorders, could accelerate this conversion. At low levels of KA, generating epileptiform activity without seizures, we indeed found increased rNSC activation and conversion into astrocytes. At high levels, generating sustained epileptic seizures, however, we find that rNSCs divide symmetrically and that both mother and daughter cells convert into reactive astrocytes. Our results demonstrate that a threshold response for neuronal hyperexcitation provokes a dramatic shift in rNSC function, which impairs adult hippocampal neurogenesis in the long term.

Infections, inflammation and epilepsy

Epilepsy is the tendency to have unprovoked epileptic seizures. Anything causing structural or functional derangement of brain physiology may lead to seizures, and different conditions may express themselves solely by recurrent seizures and thus be labelled "epilepsy." Worldwide, epilepsy is the most common serious neurological condition. The range of risk factors for the development of epilepsy varies with age and geographic location. Congenital, developmental and genetic conditions are mostly associated with the development of epilepsy in childhood, adolescence and early adulthood. Head trauma, infections of the central nervous system (CNS) and tumours may occur at any age and may lead to the development of epilepsy. Infections of the CNS are a major risk factor for epilepsy. The reported risk of unprovoked seizures in population-based cohorts of survivors of CNS infections from developed countries is between 6.8 and 8.3 %, and is much higher in resource-poor countries. In this review, the various viral, bacterial, fungal and parasitic infectious diseases of the CNS which result in seizures and epilepsy are discussed. The pathogenesis of epilepsy due to brain infections, as well as the role of experimental models to study mechanisms of epileptogenesis induced by infectious agents, is reviewed. The sterile (non-infectious) inflammatory response that occurs following brain insults is also discussed, as well as its overlap with inflammation due to infections, and the potential role in epileptogenesis. Furthermore, autoimmune encephalitis as a cause of seizures is reviewed. Potential strategies to prevent epilepsy resulting from brain infections and non-infectious inflammation are also considered.

Glia as drivers of abnormal neuronal activity

Reactive astrocytes have been proposed to become incompetent bystanders in epilepsy as a result of cellular changes rendering them unable to perform important housekeeping functions. Indeed, successful surgical treatment of mesiotemporal lobe epilepsy hinges on the removal of the glial scar. New research now extends the role of astrocytes, suggesting that they may drive the disease process by impairing the inhibitory action of neuronal GABA receptors. Here we discuss studies that include hyperexcitability resulting from impaired supply of astrocytic glutamine for neuronal GABA synthesis, and epilepsy resulting from genetically induced astrogliosis or malignant transformation, both of which render the inhibitory neurotransmitter GABA excitatory. In these examples, glial cells alter the expression or function of neuronal proteins involved in excitability. Although epilepsy has traditionally been thought of as a disease caused by changes in neuronal properties exclusively, these new findings challenge us to consider the contribution of glial cells as drivers of epileptogenesis in acquired epilepsies.

Altered structure and function of astrocytes following status epilepticus

Temporal lobe epilepsy (TLE) is a devastating seizure disorder that is often caused by status epilepticus (SE). Temporal lobe epilepsy can be very difficult to control with currently available antiseizure drugs, and there are currently no disease-modifying therapies that can prevent the development of TLE in those patients who are at risk. While the functional changes that occur in neurons following SE and leading to TLE have been well studied, only recently has research attention turned to the role in epileptogenesis of astrocytes, the other major cell type of the brain. Given that epilepsy is a neural circuit disorder, innovative ways to evaluate the contributions that both neurons and astrocytes make to aberrant circuit activity will be critical for the understanding of the emergent network properties that result in seizures. Recently described approaches using genetically encoded calcium-indicating proteins can be used to image dynamic calcium transients, a marker of activity in both neurons and glial cells. It is anticipated that this work will lead to novel insights into the process of epileptogenesis at the network level and may identify disease-modifying therapeutic targets that have been missed because of a largely neurocentric view of seizure generation following SE. This article is part of a Special Issue entitled "Status Epilepticus".

Glutamatergic Mechanisms Associated with Seizures and Epilepsy

Epilepsy is broadly characterized by aberrant neuronal excitability. Glutamate is the predominant excitatory neurotransmitter in the adult mammalian brain; thus, much of past epilepsy research has attempted to understand the role of glutamate in seizures and epilepsy. Seizures induce elevations in extracellular glutamate, which then contribute to excitotoxic damage. Chronic seizures can alter neuronal and glial expression of glutamate receptors and uptake transporters, further contributing to epileptogenesis. Evidence points to a shared glutamate pathology for epilepsy and other central nervous system (CNS) disorders, including depression, which is often a comorbidity of epilepsy. Therapies that target glutamatergic neurotransmission are available, but many have met with difficulty because of untoward adverse effects. Better understanding of this system has generated novel therapeutic targets that directly and indirectly modulate glutamatergic signaling. Thus, future efforts to manage the epileptic patient with glutamatergic-centric treatments now hold greater potential.

Minocycline inhibits brain inflammation and attenuates spontaneous recurrent seizures following pilocarpine-induced status epilepticus

Mounting evidence suggests that brain inflammation mediated by glial cells may contribute to epileptogenesis. Minocycline is a second-generation tetracycline and has potent antiinflammatory effects independent of its antimicrobial action. The present study aimed to investigate whether minocycline could exert antiepileptogenic effects in a rat lithium-pilocarpine model of temporal lobe epilepsy. The temporal patterns of microglial and astrocytic activation were examined in the hippocampal CA1 and the adjacent cortex following pilocarpine-induced status epilepticus (SE). These findings displayed that SE caused acute and persistent activation of microglia and astrocytes. Based on these findings, Minocycline was administered once daily at 45 mg/kg for 14 days following SE. Six weeks after termination of minocycline treatment, spontaneous recurrent seizures (SRS) were recorded by continuous video monitoring. Minocycline inhibited the SE-induced microglial activation and the increased production of interleukin-1β and tumor necrosis factor-α in the hippocampal CA1 and the adjacent cortex, without affecting astrocytic activation. In addition, Minocycline prevented the SE-induced neuronal loss in the brain regions examined. Moreover, minocycline significantly reduced the frequency, duration, and severity of SRS during the two weeks monitoring period. These results demonstrated that minocycline could mitigate SE-induced brain inflammation and might exert disease-modifying effects in an animal model of temporal lobe epilepsy. These findings offer new insights into deciphering the molecular mechanisms of epileptogenesis and exploring a novel therapeutic strategy for prevention of epilepsy.

Expression of astrocyte-related receptors in cortical dysplasia with intractable epilepsy

Epilepsy is one of the major neurologic diseases, and astrocytes play important roles in epileptogenesis. To investigate possible roles of astrocyte-related receptors in patients with intractable epilepsy associated with focal cortical dysplasia (FCD) and other conditions, we examined resected epileptic foci from 31 patients, including 23 with FCD type I, IIa, or IIb, 5 with tuberous sclerosis complex, and 3 with low-grade astrocytoma. Control samples were from 21 autopsied brains of patients without epilepsy or neurologic deficits and 5 patients with pathologic gliosis without epilepsy. Immunohistochemical and immunoblot analyses with antibodies against purinergic receptor subtypes P2RY1, P2RY2, P2RY4, potassium channels Kv4.2 and Kir4.1, and metabotropic receptor subtypes mGluR1 and mGluR5 were performed. Anti-glial fibrillary acidic protein, anti-NeuN, and anti-CD68 immunostaining was used to identify astrocytes, neurons, and microglia, respectively. Most glial fibrillary acidic protein-immunopositive astrocyte cells in the brain samples from patients with epilepsy were P2RY1-, P2RY2-, P2RY4-, Kv4.2-, Kir4.1-, mGluR1-, and mGluR5-positive, whereas samples from controls and pathologic gliosis showed lower expression levels of these astrocyte-related receptors. Our findings suggest that, although these receptors are necessary for astrocyte transmission, formation of the neuron-glia network, and other physiologic functions, overexpression in the brains of patients with intractable epilepsy may be associated with activation of intracellular and glio-neuronal signaling pathways that contribute to epileptogenesis.

Does brain inflammation mediate pathological outcomes in epilepsy?

Inflammation in the central nervous system (CNS) is associated with epilepsy and is characterized by the increased levels of a complex set of soluble molecules and their receptors in epileptogenic foci with profound neuromodulatory effects. These molecules activate receptor-mediated pathways in glia and neurons that contribute to hyperexcitability in neural networks that underlie seizure generation. As a consequence, exciting new opportunities now exist for novel therapies targeting the various components of the immune system and the associated inflammatory mediators, especially the IL-1β system. This review summarizes recent findings that increased our understanding of the role of inflammation in reducing seizure threshold, contributing to seizure generation, and participating in epileptogenesis. We will discuss preclinical studies supporting the hypothesis that pharmacological inhibition of specific proinflammatory signalings may be useful to treat drug-resistant seizures in human epilepsy, and possibly delay or arrest epileptogenesis.

The possible role of GABAA receptors and gephyrin in epileptogenesis

The term epileptogenesis refers to a dynamic alteration in neuronal excitability that promotes the appearance of spontaneous seizures. Temporal lobe epilepsy, the most common type of acquired epilepsy, often develops after an insult to the brain such as trauma, febrile seizures, encephalitis, or status epilepticus. During the pre-epileptic state (also referred as latent or silent period) there is a plethora of molecular, biochemical, and structural changes that lead to the generation of recurrent spontaneous seizures (or epilepsy). The specific contribution of these alterations to epilepsy development is unclear, but a loss of inhibition has been associated with the increased excitability detected in the latent period. A rapid increase in neuronal hyperexcitability could be due, at least in part, to a decline in the number of physiologically active GABAA receptors (GABAAR). Altered expression of scaffolding proteins involved in the trafficking and anchoring of GABAAR could directly impact the stability of GABAergic synapses and promote a deficiency in inhibitory neurotransmission. Uncovering the molecular mechanisms operating during epileptogenesis and its possible impact on the regulation of GABAAR and scaffolding proteins may offer new targets to prevent the development of epilepsy.