Temporal lobe epilepsy (TLE), the most common form of adult focal epilepsy, is highly resistant to current medical therapy. Cytokine signaling, including via tumor necrosis factor-alpha (TNFα), has been implicated not only as an accompanying factor but as a key aspect in the initiation of the disease. TNFα, through its type-1 receptor (TNFR1) in astrocytes, controls excitatory circuits in the hippocampus, yet it remains unknown whether this astrocyte pathway specifically contributes to epilepsy. Here, we used a conditional cell-specific knockout mouse line to induce TNFR1 deletion selectively in astrocytes and test its roles in the initiation and progression of TLE. Mice lacking astrocyte TNFR1 showed decreased basal spectral power, longer latency to first seizure following treatment with kainic acid, and a less severe phenotype up to 4 weeks later. Thus, abrogation of astrocyte TNFR1 signaling may be beneficial in TLE and could provide a new therapeutic target for this disease.

LRRC8 channels are volume-regulated anion channels (VRACs) activated by cellular swelling, which mediate regulatory volume decrease in many cell types. Recently, it has been shown that these channels contribute to the release of glutamate from astrocytes. Since enhanced extracellular glutamate concentrations produce hyperexcitability, and microdialysis revealed elevated levels of the transmitter in the brains of epileptic patients, we asked whether astroglial glutamate release through LRRC8/VRACs might contribute to the initiation of experimental temporal lobe epilepsy (TLE). Patch clamp, pharmacological, and single-cell transcript analyses were performed in the hippocampus of controls and mice with inducible deletion of LRRC8a in astrocytes. In addition, these mice were exposed to our unilateral intracortical kainate model of TLE. Tonic currents were recorded from CA1 pyramidal neurons as a measure of glutamate release. Our data show that neither expression of LRRC8a nor the amplitude of tonic currents was altered 4 h after status epilepticus-induced TLE. These findings do not suggest that increased astroglial glutamate release through LRRC8 channels contributes to the initiation of experimental TLE.

Temporal lobe epilepsy (TLE) is the most common and severe form of adult focal epilepsy and is frequently associated with hippocampal sclerosis (HS). Accumulating evidence suggests that brain inflammation plays an important pathophysiological role in TLE. A key pro-inflammatory mediator is tumour necrosis factor α (TNFα), which can trigger necroptosis pathways regulated by RIPK1, RIPK3 and MLKL through binding to the TNF receptor 1 (TNFR1). Previously, we detected activation of RIPK1, RIPK3 and MLKL in hippocampal CA1 astrocytes, along with a reduction in astrocyte density, during the early stages of experimentally induced TLE-HS, providing strong evidence for necroptotic astrocytic cell death. Using immunohistochemistry, pharmacology and long-term EEG recording, here we unravel the mechanisms underlying necroptosis induction in astrocytes and its role in epileptogenesis. The results show that pharmacological inhibition of necroptosis using Nec-1s, a specific inhibitor of RIPK1, or selective inhibition of soluble TNFα by XPro1595, as well as genetic knockout of TNFR1, effectively rescued CA1 astrocyte loss caused by kainate-induced status epilepticus. Furthermore, targeting necroptosis by Nec-1s administration attenuated CA1 astrogliosis, degeneration of pyramidal neurons, granular cell dispersion and shrinkage of the CA1 subfield. In contrast, Nec-1s did not affect acute and chronic epileptic activity in the TLE-HS model. Our findings demonstrate that TNFα-induced necroptotic astrocyte death is involved in the pathogenesis of TLE. KEY POINTS: In the early stage of experimental temporal lobe epilepsy with hippocampal sclerosis (TLE-HS), we observed activation of RIPK1, RIPK3 and MLKL in CA1 astrocytes, along with a reduction in astrocyte density, providing evidence for necroptotic cell death. Pharmacological inhibition of necroptosis (Nec-1s), blockade of soluble tumour necrosis factor α (XPro1595) or genetic knockout of TNFR1 prevented astrocyte loss after status epilepticus. Continuous telemetric EEG recordings showed that Nec-1s has no effect on acute or chronic epileptic activity in the TLE-HS model. Immunohistochemical analysis revealed that Nec-1s treatment reduced the extent of hippocampal sclerosis in experimental TLE-HS. Our results provide further insights into the molecular mechanisms underlying the development and progression of TLE.

Astrocytic gap junctional communication plays a critical role in regulating neuronal activity and network synchronization, yet its precise contributions to brain function and the pathogenesis of neurological disorders remains incompletely understood. To address this, we generated a transgenic mouse line with inducible, astrocyte-specific overexpression of the gap junction protein connexin43 (Cx43). In these mice, hippocampal astrocytes exhibited markedly elevated Cx43 protein levels and a 20

OBJECTIVE:Neuronal cell death and neuroinflammation are characteristic features of epilepsy, but it remains unclear whether neuronal cell death as such is causative for the development of epileptic seizures. To test this hypothesis, we established a novel mouse line permitting inducible ablation of pyramidal neurons by inserting simian diphtheria toxin (DT) receptor (DTR) cDNA into the Ccl17 locus. The chemokine CCL17 is expressed in pyramidal CA1 neurons in adult mice controlling microglial quiescence. METHODS:Seizure activity in CCL17-DTR mice was analyzed by electroencephalographic recordings following treatment with DT for 3 consecutive days. Neuroinflammation and neuronal cell death were evaluated by (immuno)histochemistry. Pharmacological inhibition of TNFR1 signaling was achieved by treatment with XPro1595, a dominant-negative inhibitor of soluble tumor necrosis factor. RESULTS:Neuronal cell death was detectable 7 days (d7) after the first DT injection in heterozygous CCL17-DTR mice. Spontaneous epileptic seizures were observed in the vast majority of mice, often with an initial peak at d6-9, followed by a period of reduced activity and a gradual increase during the 1-month observation period. Microglial reactivity was overt from d5 after DT administration not only in the CA1 region but also in the CA2/CA3 area, shortly followed by astrogliosis. Reactive microgliosis and astrogliosis persisted until d30 and, together with neuronal loss and stratum radiatum shrinkage, reflected important features of human hippocampal sclerosis. Granule cell dispersion was detectable only 3 months after DT treatment. Application of XPro1595 significantly reduced chronic seizure burden without affecting the development of hippocampal sclerosis. SIGNIFICANCE:In conclusion, our data demonstrate that sterile pyramidal neuronal death is sufficient to cause epilepsy in the absence of other pathological processes. The CCL17-DTR mouse line may thus be a valuable model for further mechanistic studies on epilepsy and assessment of antiseizure medication.

Abstract A growing body of evidence suggests that astrocytes are crucial actors in the initiation and progression of epilepsy. Reactive astrocytes in human and experimental epilepsy display marked morphological, transcriptional, and functional changes, but it is still often unclear whether this represents a causative factor, a consequence, or an adaptive response in epileptogenesis. A distinctive feature of astrocytes is their extensive intercellular coupling via gap junction channels. This allows them to form large syncytium-like functional networks that play essential roles in ion and neurotransmitter homeostasis, gliotransmission, nutrient supply to neurons, and regulation of the extracellular space volume. The first part of this chapter summarizes current knowledge on the role of gap junction channels in epilepsy, their expression and activity in human and experimental epilepsy, and the consequences of their genetic and pharmacological modulation on neuronal excitability and epileptogenesis. Astrocytes also regulate and respond to extracellular glutamate levels in the central nervous system via the Na+-dependent glutamate transporters glutamate transporter-1 (GLT-1) and glutamate aspartate transporter (GLAST) and the metabotropic glutamate receptors (mGluR) 3 and mGluR5. Both impaired astrocytic glutamate clearance and changes in mGluR signaling could contribute to the development of epilepsy. The second part of this chapter summarizes the changes in astrocyte glutamate receptors and transporters in epilepsy. Overall, both astrocytic gap junction channels and glutamate transporters and receptors could serve as novel therapeutic targets for epilepsy.

Increasing evidence suggests that inflammation promotes epileptogenesis. TAK1 is a central enzyme in the upstream pathway of NF-κB and is known to play a central role in promoting neuroinflammation in neurodegenerative diseases. Here, we investigated the cellular role of TAK1 in experimental epilepsy. C57Bl6 and transgenic mice with inducible and microglia-specific deletion of Tak1 (Cx3cr1CreER:Tak1fl/fl) were subjected to the unilateral intracortical kainate mouse model of temporal lobe epilepsy (TLE). Immunohistochemical staining was performed to quantify different cell populations. The epileptic activity was monitored by continuous telemetric electroencephalogram (EEG) recordings over a period of 4 weeks. The results show that TAK1 was activated predominantly in microglia at an early stage of kainate-induced epileptogenesis. Tak1 deletion in microglia resulted in reduced hippocampal reactive microgliosis and a significant decrease in chronic epileptic activity. Overall, our data suggest that TAK1-dependent microglial activation contributes to the pathogenesis of chronic epilepsy.

Long-term modifications of astrocyte function and morphology are well known to occur in epilepsy. They are implicated in the development and manifestation of the disease, but the relevant mechanisms and their pathophysiological role are not firmly established. For instance, it is unclear how quickly the onset of epileptic activity triggers astrocyte morphology changes and what the relevant molecular signals are. We therefore used two-photon excitation fluorescence microscopy to monitor astrocyte morphology in parallel to the induction of epileptiform activity. We uncovered astrocyte morphology changes within 10-20 min under various experimental conditions in acute hippocampal slices. In vivo, induction of status epilepticus resulted in similarly altered astrocyte morphology within 30 min. Further analysis in vitro revealed a persistent volume reduction of peripheral astrocyte processes triggered by induction of epileptiform activity. In addition, an impaired diffusion within astrocytes and within the astrocyte network was observed, which most likely is a direct consequence of the astrocyte remodeling. These astrocyte morphology changes were prevented by inhibition of the Rho GTPase RhoA and of the Rho-associated kinase (ROCK). Selective deletion of ROCK1 but not ROCK2 from astrocytes also prevented the morphology change after induction of epileptiform activity and reduced epileptiform activity. Together these observations reveal that epileptic activity triggers a rapid ROCK1-dependent astrocyte morphology change, which is mechanistically linked to the strength of epileptiform activity. This suggests that astrocytic ROCK1 signaling is a maladaptive response of astrocytes to the onset of epileptic activity.

The gap-junction-coupled astroglial network plays a central role in the regulation of neuronal activity and synchronisation, but its involvement in the pathogenesis of neuronal diseases is not yet understood. Here, we present the current state of knowledge about the impact of impaired glial coupling in the development and progression of epilepsy and discuss whether astrocytes represent alternative therapeutic targets. We focus mainly on temporal lobe epilepsy (TLE), which is the most common form of epilepsy in adults and is characterised by high therapy resistance. Functional data from TLE patients and corresponding experimental models point to a complete loss of astrocytic coupling, but preservation of the gap junction forming proteins connexin43 and connexin30 in hippocampal sclerosis. Several studies further indicate that astrocyte uncoupling is a causal event in the initiation of TLE, as it occurs very early in epileptogenesis, clearly preceding dysfunctional changes in neurons. However, more research is needed to fully understand the role of gap junction channels in epilepsy and to develop safe and effective therapeutic strategies targeting astrocytes.

Astrocytes play a dual role in the brain. On the one hand, they are active signaling partners of neurons and can for instance control synaptic transmission and its plasticity. On the other hand, they fulfill various homeostatic functions such as clearance of glutamate and K + released from neurons. The latter is for instance important for limiting neuronal excitability. Therefore, an impairment or failure of glutamate and K + clearance will lead to increased neuronal excitability, which could trigger or aggravate brain diseases such as epilepsy, in which neuronal hyperexcitability plays a role. Experimental data indicate that astrocytes could have such a causal role in epilepsy, but the role of astrocytes as initiators of epilepsy and the relevant mechanisms are under debate. In this overview, we will discuss the potential mechanisms with focus on K + clearance, glutamate uptake and homoeostasis and related mechanisms, and the evidence for their causative role in epilepsy.

Extensive microglia reactivity has been well described in human and experimental temporal lobe epilepsy (TLE). To date, however, it is not clear whether and based on which molecular mechanisms microglia contribute to the development and progression of focal epilepsy. Astroglial gap junction coupled networks play an important role in regulating neuronal activity and loss of interastrocytic coupling causally contributes to TLE. Here, we show in the unilateral intracortical kainate (KA) mouse model of TLE that reactive microglia are primary producers of tumor necrosis factor (TNF)α and contribute to astrocyte dysfunction and severity of status epilepticus (SE). Immunohistochemical analyses revealed pronounced and persistent microglia reactivity, which already started 4 h after KA-induced SE. Partial depletion of microglia using a colony stimulating factor 1 receptor inhibitor prevented early astrocyte uncoupling and attenuated the severity of SE, but increased the mortality of epileptic mice following surgery. Using microglia-specific inducible TNFα knockout mice we identified microglia as the major source of TNFα during early epileptogenesis. Importantly, microglia-specific TNFα knockout prevented SE-induced gap junction uncoupling in astrocytes. Continuous telemetric EEG recordings revealed that during the first 4 weeks after SE induction, microglial TNFα did not significantly contribute to spontaneous generalized seizure activity. Moreover, the absence of microglial TNFα did not affect the development of hippocampal sclerosis but attenuated gliosis. Taken together, these data implicate reactive microglia in astrocyte dysfunction and network hyperexcitability after an epileptogenic insult.

Epilepsy affects ~65 million people worldwide. First-line treatment options include >20 antiseizure medications, but seizure control is not achieved in approximately one-third of patients. Antiseizure medications act primarily on neurons and can provide symptomatic control of seizures, but do not alter the onset and progression of epilepsy and can cause serious adverse effects. Therefore, medications with new cellular and molecular targets and mechanisms of action are needed. Accumulating evidence indicates that astrocytes are crucial to the pathophysiological mechanisms of epilepsy, raising the possibility that these cells could be novel therapeutic targets. In this Review, we discuss how dysregulation of key astrocyte functions — gliotransmission, cell metabolism and immune function — contribute to the development and progression of hyperexcitability in epilepsy. We consider strategies to mitigate astrocyte dysfunction in each of these areas, and provide an overview of how astrocyte activation states can be monitored in vivo not only to assess their contribution to disease but also to identify markers of disease processes and treatment effects. Improved understanding of the roles of astrocytes in epilepsy has the potential to lead to novel therapies to prevent the initiation and progression of epilepsy.

Connexin gap junctions (Cx GJs) enable the passage of small molecules and ions between cells and are therefore important for cell-to-cell communication. Their dysfunction is associated with diseases, and small molecules acting as modulators of GJs may therefore be useful as therapeutic drugs. To identify GJ modulators, suitable assays are needed that allow compound screening. In the present study, we established a novel assay utilizing HeLa cells recombinantly expressing Cx43. Donor cells additionally expressing the Gs protein-coupled adenosine A(2A) receptor, and biosensor cells expressing a cAMP-sensitive GloSensor luciferase were established. Adenosine A(2A) receptor activation in the donor cells using a selective agonist results in intracellular cAMP production. The negatively charged cAMP migrates via the Cx43 gap junctions to the biosensor cells and can there be measured by the cAMP-dependent luminescence signal. Cx43 GJ modulators can be expected to impact the transfer of cAMP from the donor to the biosensor cells, since cAMP transit is only possible via GJs. The new assay was validated by testing the standard GJ inhibitor carbenoxolon, which showed a concentration-dependent inhibition of the signal and an IC50 value that was consistent with previously reported values. The assay was demonstrated to be suitable for high-throughput screening.

Blood–brain barrier (BBB) dysfunction following brain insults has been associated with the development and progression of focal epilepsy, although the underlying molecular mechanisms are not fully elucidated yet. Activation of transforming growth factor beta (TGFβ) signaling in astrocytes by extravasated albumin impairs the ability of astrocytes to properly interact with neurons, eventually leading to epileptiform activity. We used the unilateral intracortical kainate mouse model of temporal lobe epilepsy (TLE) with hippocampal sclerosis (HS) to gain further insights into the role of BBB leakage in status epilepticus (SE)-induced epileptogenesis. Immunohistochemical examination revealed pronounced albumin extravasation already 4 h after SE induction. Astrocytes were virtually devoid of albumin immunoreactivity (IR), indicating the lack of uptake by this time point. Inhibition of the TGFβ pathway by the specific TGFβ receptor 1 (TGFβR1) kinase inhibitor IPW-5371 did not prevent seizure-induced reduction of astrocytic gap junction coupling. Thus, loss of coupling, which is thought to play a causative role in triggering TLE-HS, is most likely not mediated by extravasated albumin. Continuous telemetric EEG recordings and video monitoring performed over a period of 4 weeks after epilepsy induction revealed that inhibition of the TGFβ pathway during the initial phase of epileptogenesis slightly attenuated acute and chronic epileptiform activity, but did not reduce the extent of HS. Together, these data indicate that albumin extravasation due to increased BBB permeability and TGFβ pathway activation during the first hours after SE induction are not significantly involved in initiating TLE.

Summary Objective Growing evidence suggests that dysfunctional astrocytes are crucial players in the development of mesial temporal lobe epilepsy (MTLE). Using a mouse model closely recapitulating key alterations of chronic human MTLE with hippocampal sclerosis, here we asked whether death of astrocytes contributes to the initiation of the disease and investigated potential underlying molecular mechanisms. Methods Antibody staining was combined with confocal imaging and semiquantitative real‐time polymerase chain reaction analysis to identify markers of different cellular death mechanisms between 4 h and 3 days after epilepsy induction. Results Four hours after kainate‐mediated induction of status epilepticus (SE), we found a significant reduction in the density of astrocytes in the CA1 stratum radiatum (SR) of the ipsilateral hippocampus. This reduction was transient, as within the next 3 days, astrocyte cell numbers recovered to the initial values, which was accompanied by enhanced proliferation. Four hours after SE induction, a small proportion of astrocytes in the ipsilateral CA1 SR expressed autophagy‐related genes and proteins, whereas we did not find astrocytes positive for cleaved caspase 3 or terminal deoxynucleotide transferase–mediated deoxyuridine triphosphate nick‐end labeling, ruling out apoptosis‐related astrocytic death. Importantly, at the same early time point post‐SE, many astrocytes in the ipsilateral CA1 SR showed strong expression of genes encoding pro‐necroptosis factors, including receptor‐interacting protein kinase 3 ( RIPK3 ) and mixed lineage kinase domain‐like protein ( MLKL ). Phosphorylation of MLKL (pMLKL), formation of necrosome complexes composed of RIPK3 and pMLKL, and translocation of pMLKL to the nucleus and to the plasma membrane were often observed in astrocytes of the ipsilateral hippocampus 4 h post‐SE. Significance The present study revealed that astrocytes die shortly after induction of SE. Our expression data and immunohistochemistry suggest that necroptosis and autophagy contribute to astrocytic death. These findings help to better understand how dysfunctional and pathological remodeling of astrocytes contributes to the initiation of temporal lobe epilepsy.

EpilepsiaVolume 62, Issue 9 p. 2299-2300 LETTER Response: Astrocytes as alternative targets for more efficient antiepileptogenic drugs Peter Bedner, Peter Bedner Institute of Cellular Neurosciences, Medical Faculty, University of Bonn, Bonn, GermanySearch for more papers by this authorZhou Wu, Zhou Wu Institute of Cellular Neurosciences, Medical Faculty, University of Bonn, Bonn, GermanySearch for more papers by this authorChristian Steinhäuser, Corresponding Author Christian Steinhäuser cste@uni-bonn.de orcid.org/0000-0003-2579-8357 Institute of Cellular Neurosciences, Medical Faculty, University of Bonn, Bonn, Germany Correspondence Christian Steinhäuser, Institute of Cellular Neurosciences, Medical Faculty, University of Bonn, Venusberg-Campus 1, D-53127 Bonn, Germany. Email: cste@uni-bonn.deSearch for more papers by this author Peter Bedner, Peter Bedner Institute of Cellular Neurosciences, Medical Faculty, University of Bonn, Bonn, GermanySearch for more papers by this authorZhou Wu, Zhou Wu Institute of Cellular Neurosciences, Medical Faculty, University of Bonn, Bonn, GermanySearch for more papers by this authorChristian Steinhäuser, Corresponding Author Christian Steinhäuser cste@uni-bonn.de orcid.org/0000-0003-2579-8357 Institute of Cellular Neurosciences, Medical Faculty, University of Bonn, Bonn, Germany Correspondence Christian Steinhäuser, Institute of Cellular Neurosciences, Medical Faculty, University of Bonn, Venusberg-Campus 1, D-53127 Bonn, Germany. Email: cste@uni-bonn.deSearch for more papers by this author First published: 24 June 2021 https://doi.org/10.1111/epi.16965Read the full textAboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat No abstract is available for this article. Volume62, Issue9September 2021Pages 2299-2300 RelatedInformation

The Alzheimer disease-associated multifunctional low-density lipoprotein receptor-related protein-1 is expressed in the brain. Recent studies uncovered a role of this receptor for the appropriate functioning of neural stem cells, oligodendrocytes, and neurons. The constitutive knock-out (KO) of the receptor is embryonically lethal. To unravel the receptors' role in the developing brain we generated a mouse mutant by specifically targeting radial glia stem cells of the dorsal telencephalon. The low-density lipoprotein receptor-related protein-1 lineage-restricted KO female and male mice, in contrast to available models, developed a severe neurological phenotype with generalized seizures during early postnatal development. The mechanism leading to a buildup of hyperexcitability and emergence of seizures was traced to a failure in adequate astrocyte development and deteriorated postsynaptic density integrity. The detected impairments in the astrocytic lineage: precocious maturation, reactive gliosis, abolished tissue plasminogen activator uptake, and loss of functionality emphasize the importance of this glial cell type for synaptic signaling in the developing brain. Together, the obtained results highlight the relevance of astrocytic low-density lipoprotein receptor-related protein-1 for glutamatergic signaling in the context of neuron-glia interactions and stage this receptor as a contributing factor for epilepsy.