KCC2 overexpression prevents the paradoxical seizure-promoting action of somatic inhibition

Network models incorporating chloride dynamics predict optimal strategies for terminating status epilepticus

Status epilepticus (SE), seizures lasting beyond five minutes, is a medical emergency commonly treated with benzodiazepines which enhance GABAA receptor (GABAAR) conductance. Despite widespread use, benzodiazepines fail in over one-third of patients, potentially due to seizure-induced disruption of neuronal chloride (Cl-) homeostasis. Understanding these changes at a network level is crucial for improving clinical translation. Here, we address this using a large-scale spiking neural network model incorporating Cl- dynamics, informed by clinical EEG and experimental slice recordings. Our simulations confirm that the GABAAR reversal potential (EGABA) dictates the pro- or anti-seizure effect of GABAAR conductance modulation, with high EGABA rendering benzodiazepines ineffective or excitatory. We show SE-like activity and EGABA depend non-linearly on Cl- extrusion efficacy and GABAAR conductance. Critically, cell-type specific manipulations reveal that pyramidal cell, not interneuron, Cl- extrusion predominantly determines the severity of SE activity and the response to simulated benzodiazepines. Leveraging these mechanistic insights, we develop a predictive framework mapping network states to Cl- extrusion capacity and GABAergic load, yielding a proposed decision-making strategy to guide therapeutic interventions based on initial treatment response. This work identifies pyramidal cell Cl- handling as a key therapeutic target and demonstrates the utility of biophysically detailed network models for optimising SE treatment protocols.

Potassium KCa3.1 channel overexpression deteriorates functionality and availability of channels at the outer cellular membrane

The engineered expression of K+ channels has been proposed as a potential treatment for epilepsy due to their exceptional ability to hyperpolarize neurons. A number of rodent models of gene therapy have yielded promising outcomes. However, the prevailing viral delivery methods for transgenes lack external control over expression, which may lead to the overproduction of K+ channel subunits and subsequent adverse effects. AAV-based expression of the KCNN4 gene in excitatory neurons has recently been demonstrated to suppress seizures by decreasing neuronal spiking activity. In this study, we examine the effects of overexpression of KCNN4, a gene encoding a pore-forming subunit of KCa3.1 channels, in neurons and HEK293 cells at the cellular and subcellular levels. We employ patch-clamp electrophysiology, immunocytochemistry, and imaging of tagged channel subunits to gain insights into the consequences of KCNN4 overexpression. Our results show that at higher expression levels, the number of channels at the cell membrane decreases, while the engineered expression of the KCa3.1 channel shows a peak in efficiency. Furthermore, our experiments demonstrate that KCNN4 overexpression results in decreased availability of other channels on the membrane and compromised functionality of other channels of the cells. These findings raise an important issue regarding the potential side effects of channel-based gene therapy for neurological disorders. It is critical to consider these side effects in order to successfully translate animal models into clinical trials.

Development of KCC2 therapeutics to treat neurological disorders

KCC2 is CNS neuron-specific chloride extruder, essential for the establishment and maintenance of the transmembrane chloride gradient, thereby enabling synaptic inhibition within the CNS. Herein, we highlight KCC2 hypofunction as a fundamental and conserved pathology contributing to neuronal circuit excitation/inhibition (E/I) imbalances that underly epilepsies, chronic pain, neuro-developmental/-traumatic/-degenerative/-psychiatric disorders. Indeed, downstream of both acquired and genetic factors, multiple pathologies (e.g., hyperexcitability and inflammation) converge to impair KCC2-dependent inhibition in CNS. When KCC2 hypofunction occurs, affected neurons are disinhibited due to impaired inhibitory responses to GABA/glycine. This causes neuronal hyperexcitability, disinhibition within neuron circuits, and disrupted neurological functions. More recently, KCC2 was identified as a genetically-validated target for epilepsy, intellectual disability, and autism spectrum disorder, and pathogenic mutations in human SLC12A5 gene were linked to psychiatric/mood disorders. The broad therapeutic utility of KCC2-upmodulating drugs relates to its critical role in determining inhibitory activity of GABAergic neurotransmission, a mechanism widely targeted by several drugs. However, in cases of KCC2 hypofunction GABAergic neurotransmission can be depolarizing/excitatory, thereby impairing endogenous neuronal inhibition while also limiting the effectiveness of existing therapeutics targeting/requiring GABAergic pathway inhibition. Several preclinical reports have shown that KCC2 upmodulating treatments rescue and increase the efficacy of anti-seizure and analgesic medications. Thus, a first-in-class KCC2-potentiating therapy would provide a novel mechanism for restoring physiological CNS inhibition and addressing drug resistance in patients with E/I imbalance pathologies. Herein, we discuss progress toward and further work needed to develop the first-in-class KCC2 therapeutics to treat neurological disorder patients.

Expansion of epileptogenic networks via neuroplasticity in neural mass models

Neuroplasticity refers to functional and structural changes in brain regions in response to healthy and pathological activity. Activity dependent plasticity induced by epileptic activity can involve healthy brain regions into the epileptogenic network by perturbing their excitation/inhibition balance. In this article, we present a new neural mass model, which accounts for neuroplasticity, for investigating the possible mechanisms underlying the epileptogenic network expansion. Our multiple-timescale model is inspired by physiological calcium-mediated synaptic plasticity and pathological extrasynaptic N-methyl-D-aspartate (NMDA) dependent plasticity dynamics. The model highlights that synaptic plasticity at excitatory connections and structural changes in the inhibitory system can transform a healthy region into a secondary epileptic focus under recurrent seizures and interictal activity occurring in the primary focus. Our results suggest that the latent period of this transformation can provide a window of opportunity to prevent the expansion of epileptogenic networks, formation of an epileptic focus, or other comorbidities associated with epileptic activity.

Adeno-Associated Viral Vectors in the Treatment of Epilepsy

Epilepsy is a brain disorder characterized by a persistent predisposition to epileptic seizures. With various etiologies of epilepsy, a significant proportion of patients develop pharmacoresistance to antiepileptic drugs, which necessitates the search for new therapeutic methods, in particular, using gene therapy. This review discusses the use of adeno-associated viral (AAV) vectors in gene therapy for epilepsy, emphasizing their advantages, such as high efficiency of neuronal tissue transduction and low immunogenicity/cytotoxicity. AAV vectors provide the possibility of personalized therapy due to the diversity of serotypes and genomic constructs, which allows for increasing the specificity and effectiveness of treatment. Promising orientations include the modulation of the expression of neuropeptides, ion channels, transcription, and neurotrophic factors, as well as the use of antisense oligonucleotides to regulate seizure activity, which can reduce the severity of epileptic disorders. This review summarizes the current advances in the use of AAV vectors for the treatment of epilepsy of various etiologies, demonstrating the significant potential of AAV vectors for the development of personalized and more effective approaches to reducing seizure activity and improving patient prognosis.

Dual effects of ARX poly-alanine mutations in human cortical and interneuron development

Infantile spasms, with an incidence of 1.6 to 4.5 per 10,000 live births, are a relentless and devastating childhood epilepsy marked by severe seizures but also leads to lifelong intellectual disability. Alarmingly, up to 5% of males with this condition carry a mutation in the Aristaless-related homeobox ( ARX ) gene. Our current lack of human-specific models for developmental epilepsy, coupled with discrepancies between animal studies and human data, underscores the gap in knowledge and urgent need for innovative human models, organoids being one of the best available. Here, we used human neural organoid models, cortical organoids (CO) and ganglionic eminences organoids (GEO) which mimic cortical and interneuron development respectively, to study the consequences of PAE mutations, one of the most prevalent mutation in ARX . ARX PAE produces a decrease expression of ARX in GEOs, and an enhancement in interneuron migration. That accelerated migration is cell autonomously driven, and it can be rescued by inhibiting CXCR4. We also found that PAE mutations result in an early increase in radial glia cells and intermediate progenitor cells, followed by a subsequent loss of cortical neurons at later timepoints. Moreover, ARX expression is upregulated in COs derived from patients at 30 DIV and is associated with alterations in the expression of CDKN1C . Furthermore, ARX PAE assembloids had hyperactivity which were evident at early stages of development. With effective treatments for infantile spasms and developmental epilepsies still elusive, delving into the role of ARX PAE mutations in human brain organoids represents a pivotal step toward uncovering groundbreaking therapeutic strategies.

Frequency-dependent seizure-suppressing effects of optogenetic activation of septal inhibitory cells in mesial temporal lobe epilepsy

Mesial temporal lobe epilepsy (MTLE) is characterized by recurring focal seizures that arise from limbic areas and are often refractory to pharmacological interventions. We have reported that optogenetic stimulation of PV-positive cells in the medial septum at 0.5 Hz exerts seizure-suppressive effects. Therefore, we compared here these results with those obtained by optogenetic stimulation of medial septum PV-positive neurons at 8 Hz in male PV-ChR2 mice (P60-P100) undergoing an initial, pilocarpine-induced status epilepticus (SE). Optogenetic stimulation (5 min ON, 10 min OFF) was performed from day 8 to day 12 after SE at a frequency of 8 Hz (n = 6 animals) or 0.5 Hz (n = 8 animals). Surprisingly, in both groups, no effects were observed on the occurrence of interictal spikes and interictal high frequency oscillations (HFOs). However, 0.5 Hz stimulation induced a significant decrease of seizure occurrence (p < 0.05). Such anti-ictogenic effect was not observed in the 8 Hz protocol that instead triggered seizures (p < 0.05); these seizures were significantly longer under optogenetic stimulation compared to when optogenetic stimulation was not implemented (p < 0.05). Analysis of ictal HFOs revealed that in the 0.5 Hz group, but not in the 8 Hz group, seizures occurring under optogenetic stimulation were associated with significantly lower rates of fast ripples compared to when optogenetic stimulation was not performed (p < 0.05). Our results indicate that activation of GABAergic PV-positive neurons in the medial septum exerts seizure-suppressing effects that are frequency-dependent and associated with low rates of fast ripples. Optogenetic activation of medial septum PV-positive neurons at 0.5 Hz is efficient in blocking seizures in the pilocarpine model of MTLE, an effect that did not occur with 8 Hz stimulation.

The critical dynamics of hippocampal seizures

Epilepsy is defined by the abrupt emergence of harmful seizures, but the nature of these regime shifts remains enigmatic. From the perspective of dynamical systems theory, such critical transitions occur upon inconspicuous perturbations in highly interconnected systems and can be modeled as mathematical bifurcations between alternative regimes. The predictability of critical transitions represents a major challenge, but the theory predicts the appearance of subtle dynamical signatures on the verge of instability. Whether such dynamical signatures can be measured before impending seizures remains uncertain. Here, we verified that predictions on bifurcations applied to the onset of hippocampal seizures, providing concordant results from in silico modeling, optogenetics experiments in male mice and intracranial EEG recordings in human patients with epilepsy. Leveraging pharmacological control over neural excitability, we showed that the boundary between physiological excitability and seizures can be inferred from dynamical signatures passively recorded or actively probed in hippocampal circuits. Of importance for the design of future neurotechnologies, active probing surpassed passive recording to decode underlying levels of neural excitability, notably when assessed from a network of propagating neural responses. Our findings provide a promising approach for predicting and preventing seizures, based on a sound understanding of their dynamics.

Intraparenchymal delivery of adeno-associated virus vectors for the gene therapy of neurological diseases

INTRODUCTION

In gene therapy with adeno-associated virus (AAV) vectors for diseases of the central nervous system, the vectors can be administered into blood vessels, cerebrospinal fluid space, or the brain parenchyma. When gene transfer to a large area of the brain is required, the first two methods are used, but for diseases in which local gene transfer is expected to be effective, vectors are administered directly into the brain parenchyma.

AREAS COVERED

Strategies for intraparenchymal vector delivery in gene therapy for Parkinson's disease, aromatic l-amino acid decarboxylase (AADC) deficiency, and epilepsy are reviewed.

EXPERT OPINION

Stereotactic intraparenchymal injection of AAV vectors allows precise gene delivery to the target site. Although more surgically invasive than intravascular or intrathecal administration, intraparenchymal vector delivery has the advantage of a lower vector dose, and preexisting neutralizing antibodies have little effect on the transduction efficacy. This approach improves motor function in AADC deficiency and led to regulatory approval of an AAV vector for the disease in the EU. Although further validation through clinical studies is needed, direct infusion of viral vectors into the brain parenchyma is expected to be a novel treatment for Parkinson's disease and drug-resistant epilepsy.

Light-Driven Sodium Pump as a Potential Tool for the Control of Seizures in Epilepsy

The marine flavobacterium Krokinobactereikastus light-driven sodium pump (KR2) generates an outward sodium ion current under 530 nm light stimulation, representing a promising optogenetic tool for seizure control. However, the specifics of KR2 application to suppress epileptic activity have not yet been addressed. In the present study, we investigated the possibility of KR2 photostimulation to suppress epileptiform activity in mouse brain slices using the 4-aminopyrindine (4-AP) model. We injected the adeno-associated viral vector (AAV-PHP.eB-hSyn-KR2-YFP) containing the KR2 sodium pump gene enhanced with appropriate trafficking tags. KR2 expression was observed in the lateral entorhinal cortex and CA1 hippocampus. Using whole-cell patch clamp in mouse brain slices, we show that KR2, when stimulated with LED light, induces a substantial hyperpolarization of entorhinal neurons. However, continuous photostimulation of KR2 does not interrupt ictal discharges in mouse entorhinal cortex slices induced by a solution containing 4-AP. KR2-induced hyperpolarization strongly activates neuronal HCN channels. Consequently, turning off photostimulation resulted in HCN channel-mediated rebound depolarization accompanied by a transient increase in spontaneous network activity. Using low-frequency pulsed photostimulation, we induced the generation of short HCN channel-mediated discharges that occurred in response to the light stimulus being turned off; these discharges reliably interrupt ictal activity. Thus, low-frequency pulsed photostimulation of KR2 can be considered as a potential tool for controlling epileptic seizures.

The suppressive effect of the specific KCC2 modulator CLP290 on seizure in mice

Epilepsy is a chronic neurological disorder characterized by episodic dysfunction of central nervous system. The most basic mechanism of epilepsy falls to the imbalance between excitation and inhibition. In adults, GABAA receptor (GABAAR) is the main inhibitory receptor to prevent neurons from developing hyperexcitability, while its inhibition relies on the low intracellular chloride anion concentration ([Cl-]i). Neuronal-specific electroneutral K+-Cl- cotransporter (KCC2) can mediate chloride efflux to lower [Cl-]i for GABAAR mediated inhibition. Our previous study has revealed that the coordinated downregulation of KCC2 and GABAAR participates in epilepsy. According to a high-throughout screen for compounds that reduce [Cl-]i, CLP290 turns out to be a specific KCC2 functional modulator. In current study, we first confirmed that CLP290 could dose-dependently suppress convulsant-induced seizures in mice in vivo as well as the epileptiform burst activities in cultured hippocampal neurons in vitro. Then, we discovered that CLP290 functioned through preventing the downregulation of the KCC2 phosphorylation at Ser940 and hence the KCC2 membrane expression during convulsant stimulation, and consequently restored the GABA inhibition. In addition, while CLP290 was given in early epileptogenesis period, it also effectively decreased the spontaneous recurrent seizures. Generally, our current results demonstrated that CLP290, as a specific KCC2 modulator by enhancing KCC2 function, not only inhibits the occurrence of the ictal seizures, but also suppresses the epileptogenic process. Therefore, we believe KCC2 may be a suitable target for future anti-epileptic drug development.

Combination administration of alprazolam and N-Ethylmaleimide synergistically enhances sleep behaviors in mice with no potential CNS side effects

Background

N-Ethylmaleimide (NEM), an agonist of the potassium chloride cotransporters 2 (KCC2) receptor, has been correlated with neurosuppressive outcomes, including decreased pain perception and the prevention of epileptic seizures. Nevertheless, its relationship with sleep-inducing effects remains unreported.

Objective

The present study aimed to investigate the potential enhancement of NEM on the sleep-inducing properties of alprazolam (Alp).

Methods

The test of the righting reflex was used to identify the appropriate concentrations of Alp and NEM for inducing sleep-promoting effects in mice. Total sleep duration and sleep quality were evaluated through EEG/EMG analysis. The neural mechanism underlying the sleep-promoting effect was examined through c-fos immunoreactivity in the brain using immunofluorescence. Furthermore, potential CNS-side effects of the combination Alp and NEM were assessed using LABORAS automated home-cage behavioral phenotyping.

Results

Combination administration of Alp (1.84 mg/kg) and NEM (1.0 mg/kg) significantly decreased sleep latency and increased sleep duration in comparison to administering 1.84 mg/kg Alp alone. This effect was characterized by a notable increase in REM duration. The findings from c-fos immunoreactivity indicated that NEM significantly suppressed neuron activation in brain regions associated with wakefulness. Additionally, combination administration of Alp and NEM showed no effects on mouse neural behaviors during automated home cage monitoring.

Conclusions

This study is the first to propose and demonstrate a combination therapy involving Alp and NEM that not only enhances the hypnotic effect but also mitigates potential CNS side effects, suggesting its potential application in treating insomnia.

Multi-scale modelling of the epileptic brain: advantages of computational therapy exploration

Objective: Epilepsy is a complex disease spanning across multiple scales, from ion channels in neurons to neuronal circuits across the entire brain. Over the past decades, computational models have been used to describe the pathophysiological activity of the epileptic brain from different aspects. Traditionally, each computational model can aid in optimizing therapeutic interventions, therefore, providing a particular view to design strategies for treating epilepsy. As a result, most studies are concerned with generating specific models of the epileptic brain that can help us understand the certain machinery of the pathological state. Those specific models vary in complexity and biological accuracy, with system-level models often lacking biological details.Approach: Here, we review various types of computational model of epilepsy and discuss their potential for different therapeutic approaches and scenarios, including drug discovery, surgical strategies, brain stimulation, and seizure prediction. We propose that we need to consider an integrated approach with a unified modelling framework across multiple scales to understand the epileptic brain. Our proposal is based on the recent increase in computational power, which has opened up the possibility of unifying those specific epileptic models into simulations with an unprecedented level of detail.Main results: A multi-scale epilepsy model can bridge the gap between biologically detailed models, used to address molecular and cellular questions, and brain-wide models based on abstract models which can account for complex neurological and behavioural observations.Significance: With these efforts, we move toward the next generation of epileptic brain models capable of connecting cellular features, such as ion channel properties, with standard clinical measures such as seizure severity.

Overexpression of KCNN4 channels in principal neurons produces an anti-seizure effect without reducing their coding ability

Gene therapy offers a potential alternative to the surgical treatment of epilepsy, which affects millions of people and is pharmacoresistant in ~30% of cases. Aimed at reducing the excitability of principal neurons, the engineered expression of K+ channels has been proposed as a treatment due to the outstanding ability of K+ channels to hyperpolarize neurons. However, the effects of K+ channel overexpression on cell physiology remain to be investigated. Here we report an adeno-associated virus (AAV) vector designed to reduce epileptiform activity specifically in excitatory pyramidal neurons by expressing the human Ca2+-gated K+ channel KCNN4 (KCa3.1). Electrophysiological and pharmacological experiments in acute brain slices showed that KCNN4-transduced cells exhibited a Ca2+-dependent slow afterhyperpolarization that significantly decreased the ability of KCNN4-positive neurons to generate high-frequency spike trains without affecting their lower-frequency coding ability and action potential shapes. Antiepileptic activity tests showed potent suppression of pharmacologically induced seizures in vitro at both single cell and local field potential levels with decreased spiking during ictal discharges. Taken together, our findings strongly suggest that the AAV-based expression of the KCNN4 channel in excitatory neurons is a promising therapeutic intervention as gene therapy for epilepsy.

Extracellular glutamate and GABA transients at the transition from interictal spiking to seizures

Focal epilepsy is associated with intermittent brief population discharges (interictal spikes), which resemble sentinel spikes that often occur at the onset of seizures. Why interictal spikes self-terminate whilst seizures persist and propagate is incompletely understood. We used fluorescent glutamate and GABA sensors in an awake rodent model of neocortical seizures to resolve the spatiotemporal evolution of both neurotransmitters in the extracellular space. Interictal spikes were accompanied by brief glutamate transients which were maximal at the initiation site and rapidly propagated centrifugally. GABA transients lasted longer than glutamate transients and were maximal ∼1.5 mm from the focus where they propagated centripetally. Prior to seizure initiation GABA transients were attenuated, whilst glutamate transients increased, consistent with a progressive failure of local inhibitory restraint. As seizures increased in frequency, there was a gradual increase in the spatial extent of spike-associated glutamate transients associated with interictal spikes. Neurotransmitter imaging thus reveals a progressive collapse of an annulus of feed-forward GABA release, allowing seizures to escape from local inhibitory restraint.

KCC2 reverse mode helps to clear postsynaptically released potassium at glutamatergic synapses

Extracellular potassium [K+]o elevation during synaptic activity retrogradely modifies presynaptic release and astrocytic uptake of glutamate. Hence, local K+ clearance and replenishment mechanisms are crucial regulators of glutamatergic transmission and plasticity. Based on recordings of astrocytic inward rectifier potassium current IKir and K+-sensitive electrodes as sensors of [K+]o as well as on in silico modeling, we demonstrate that the neuronal K+-Cl- co-transporter KCC2 clears local perisynaptic [K+]o during synaptic excitation by operating in an activity-dependent reversed mode. In reverse mode, KCC2 replenishes K+ in dendritic spines and complements clearance of [K+]o, therewith attenuating presynaptic glutamate release and shortening LTP. We thus demonstrate a physiological role of KCC2 in neuron-glial interactions and regulation of synaptic signaling and plasticity through the uptake of postsynaptically released K+.

Microscale Neuronal Activity Collectively Drives Chaotic and Inflexible Dynamics at the Macroscale in Seizures

Neuronal activity propagates through the network during seizures, engaging brain dynamics at multiple scales. Such propagating events can be described through the avalanches framework, which can relate spatiotemporal activity at the microscale with global network properties. Interestingly, propagating avalanches in healthy networks are indicative of critical dynamics, where the network is organized to a phase transition, which optimizes certain computational properties. Some have hypothesized that the pathologic brain dynamics of epileptic seizures are an emergent property of microscale neuronal networks collectively driving the brain away from criticality. Demonstrating this would provide a unifying mechanism linking microscale spatiotemporal activity with emergent brain dysfunction during seizures. Here, we investigated the effect of drug-induced seizures on critical avalanche dynamics, using in vivo whole-brain two-photon imaging of GCaMP6s larval zebrafish (males and females) at single neuron resolution. We demonstrate that single neuron activity across the whole brain exhibits a loss of critical statistics during seizures, suggesting that microscale activity collectively drives macroscale dynamics away from criticality. We also construct spiking network models at the scale of the larval zebrafish brain, to demonstrate that only densely connected networks can drive brain-wide seizure dynamics away from criticality. Importantly, such dense networks also disrupt the optimal computational capacities of critical networks, leading to chaotic dynamics, impaired network response properties and sticky states, thus helping to explain functional impairments during seizures. This study bridges the gap between microscale neuronal activity and emergent macroscale dynamics and cognitive dysfunction during seizures.SIGNIFICANCE STATEMENT Epileptic seizures are debilitating and impair normal brain function. It is unclear how the coordinated behavior of neurons collectively impairs brain function during seizures. To investigate this we perform fluorescence microscopy in larval zebrafish, which allows for the recording of whole-brain activity at single-neuron resolution. Using techniques from physics, we show that neuronal activity during seizures drives the brain away from criticality, a regime that enables both high and low activity states, into an inflexible regime that drives high activity states. Importantly, this change is caused by more connections in the network, which we show disrupts the ability of the brain to respond appropriately to its environment. Therefore, we identify key neuronal network mechanisms driving seizures and concurrent cognitive dysfunction.

A genetically targeted ion sensor reveals distinct seizure-related chloride and pH dynamics in GABAergic interneuron populations

Intracellular chloride and pH play fundamental roles in determining a neuron's synaptic inhibition and excitability. Yet it has been difficult to measure changes in these ions during periods of heightened network activity, such as occur in epilepsy. Here we develop a version of the fluorescent reporter, ClopHensorN, to enable simultaneous quantification of chloride and pH in genetically defined neurons during epileptiform activity. We compare pyramidal neurons to the major GABAergic interneuron subtypes in the mouse hippocampus, which express parvalbumin (PV), somatostatin (SST), or vasoactive intestinal polypeptide (VIP). Interneuron populations exhibit higher baseline chloride, with PV interneurons exhibiting the highest levels. During an epileptiform discharge, however, all subtypes converge upon a common elevated chloride level. Concurrent with these dynamics, epileptiform activity leads to different degrees of intracellular acidification, which reflect baseline pH. Thus, a new optical tool for dissociating chloride and pH reveals neuron-specific ion dynamics during heightened network activity.

GABAergic circuits drive focal seizures

Epilepsy is based on abnormal neuronal activities that have historically been suggested to arise from an excess of excitation and a defect of inhibition, or in other words from an excessive glutamatergic drive not balanced by GABAergic activity. More recent data however indicate that GABAergic signaling is not defective at focal seizure onset and may even be actively involved in seizure generation by providing excitatory inputs. Recordings of interneurons revealed that they are active at seizure initiation and that their selective and time-controlled activation using optogenetics triggers seizures in a more general context of increased excitability. Moreover, GABAergic signaling appears to be mandatory at seizure onset in many models. The main pro-ictogenic effect of GABAergic signaling is the depolarizing action of GABA_A conductance which may occur when an excessive GABAergic activity causes Cl^- accumulation in neurons. This process may combine with background dysregulation of Cl^-, well described in epileptic tissues. Cl^- equilibrium is maintained by (Na⁺)/K⁺/Cl^- co-transporters, which can be defective and therefore favor the depolarizing effects of GABA. In addition, these co-transporters further contribute to this effect as they mediate K⁺ outflow together with Cl^- extrusion, a process that is responsible for K⁺ accumulation in the extracellular space and subsequent increase of local excitability. The role of GABAergic signaling in focal seizure generation is obvious but its complex dynamics and balance between GABA_A flux polarity and local excitability still remain to be established, especially in epileptic tissues where receptors and ion regulators are disrupted and in which GABAergic signaling rather plays a 2 faces Janus role.

LINCs Are Vulnerable to Epileptic Insult and Fail to Provide Seizure Control via On-Demand Activation

Temporal lobe epilepsy (TLE) is notoriously pharmacoresistant, and identifying novel therapeutic targets for controlling seizures is crucial. Long-range inhibitory neuronal nitric oxide synthase-expressing cells (LINCs), a population of hippocampal neurons, were recently identified as a unique source of widespread inhibition in CA1, able to elicit both GABAA-mediated and GABAB-mediated postsynaptic inhibition. We therefore hypothesized that LINCs could be an effective target for seizure control. LINCs were optogenetically activated for on-demand seizure intervention in the intrahippocampal kainate (KA) mouse model of chronic TLE. Unexpectedly, LINC activation at 1 month post-KA did not substantially reduce seizure duration in either male or female mice. We tested two different sets of stimulation parameters, both previously found to be effective with on-demand optogenetic approaches, but neither was successful. Quantification of LINCs following intervention revealed a substantial reduction of LINC numbers compared with saline-injected controls. We also observed a decreased number of LINCs when the site of initial insult (i.e., KA injection) was moved to the amygdala [basolateral amygdala (BLA)-KA], and correspondingly, no effect of light delivery on BLA-KA seizures. This indicates that LINCs may be a vulnerable population in TLE, regardless of the site of initial insult. To determine whether long-term circuitry changes could influence outcomes, we continued testing once a month for up to 6 months post-KA. However, at no time point did LINC activation provide meaningful seizure suppression. Altogether, our results suggest that LINCs are not a promising target for seizure inhibition in TLE.

Chloride ion dysregulation in epileptogenic neuronal networks

GABA is the major inhibitory neurotransmitter in the mature CNS. When GABA_A receptors are activated the membrane potential is driven towards hyperpolarization due to chloride entry into the neuron. However, chloride ion dysregulation that alters the ionic gradient can result in depolarizing GABAergic post-synaptic potentials instead. In this review, we highlight that GABAergic inhibition prevents and restrains focal seizures but then reexamine this notion in the context of evidence that a static and/or a dynamic chloride ion dysregulation, that increases intracellular chloride ion concentrations, promotes epileptiform activity and seizures. To reconcile these findings, we hypothesize that epileptogenic pathologically interconnected neuron (PIN) microcircuits, representing a small minority of neurons, exhibit static chloride dysregulation and should exhibit depolarizing inhibitory post-synaptic potentials (IPSPs). We speculate that chloride ion dysregulation and PIN cluster activation may generate fast ripples and epileptiform spikes as well as initiate the hypersynchronous seizure onset pattern and microseizures. Also, we discuss the genetic, molecular, and cellular players important in chloride dysregulation which regulate epileptogenesis and initiate the low-voltage fast seizure onset pattern. We conclude that chloride dysregulation in neuronal networks appears to be critical for epileptogenesis and seizure genesis, but feed-back and feed-forward inhibitory GABAergic neurotransmission plays an important role in preventing and restraining seizures as well.

SCN1A channelopathies: Navigating from genotype to neural circuit dysfunction

The SCN1A gene is strongly associated with epilepsy and plays a central role for supporting cortical excitation-inhibition balance through the expression of Na_V1.1 within inhibitory interneurons. The phenotype of SCN1A disorders has been conceptualized as driven primarily by impaired interneuron function that predisposes to disinhibition and cortical hyperexcitability. However, recent studies have identified SCN1A gain-of-function variants associated with epilepsy, and the presence of cellular and synaptic changes in mouse models that point toward homeostatic adaptations and complex network remodeling. These findings highlight the need to understand microcircuit-scale dysfunction in SCN1A disorders to contextualize genetic and cellular disease mechanisms. Targeting the restoration of microcircuit properties may be a fruitful strategy for the development of novel therapies.

On-demand cell-autonomous gene therapy for brain circuit disorders

Several neurodevelopmental and neuropsychiatric disorders are characterized by intermittent episodes of pathological activity. Although genetic therapies offer the ability to modulate neuronal excitability, a limiting factor is that they do not discriminate between neurons involved in circuit pathologies and "healthy" surrounding or intermingled neurons. We describe a gene therapy strategy that down-regulates the excitability of overactive neurons in closed loop, which we tested in models of epilepsy. We used an immediate early gene promoter to drive the expression of Kv1.1 potassium channels specifically in hyperactive neurons, and only for as long as they exhibit abnormal activity. Neuronal excitability was reduced by seizure-related activity, leading to a persistent antiepileptic effect without interfering with normal behaviors. Activity-dependent gene therapy is a promising on-demand cell-autonomous treatment for brain circuit disorders.

Removal of KCNQ2 from parvalbumin-expressing interneurons improves anti-seizure efficacy of retigabine

Anti-seizure drug (ASD) targets are widely expressed in both excitatory and inhibitory neurons. It remains unknown if the action of an ASD upon inhibitory neurons could counteract its beneficial effects on excitatory neurons (or vice versa), thereby reducing the efficacy of the ASD. Here, we examine whether the efficacy of the ASD retigabine (RTG) is altered after removal of the Kv7 potassium channel subunit KCNQ2, one of its drug targets, from parvalbumin-expressing interneurons (PV-INs). Parvalbumin-Cre (PV-Cre) mice were crossed with Kcnq2-floxed (Kcnq2fl/fl) mice to conditionally delete Kcnq2 from PV-INs. In these conditional knockout mice (cKO, PV-Kcnq2fl/fl), RTG (10 mg/kg, i.p.) significantly delayed the onset of either picrotoxin (PTX, 10 mg/kg, i.p)- or kainic acid (KA, 30 mg/kg, i.p.)-induced convulsive seizures compared to vehicle, while RTG was not effective in wild-type littermates (WT). Immunostaining for KCNQ2 and KCNQ3 revealed that both subunits were enriched at axon initial segments (AISs) of hippocampal CA1 PV-INs, and their specific expression was selectively abolished in cKO mice. Accordingly, the M-currents recorded from CA1 PV-INs and their sensitivity to RTG were significantly reduced in cKO mice. While the ability of RTG to suppress CA1 excitatory neurons in hippocampal slices was unchanged in cKO mice, its suppressive effect on the spike activity of CA1 PV-INs was significantly reduced compared with WT mice. In addition, the RTG-induced suppression on intrinsic membrane excitability of PV-INs in WT mice was significantly reduced in cKO mice. These findings suggest that preventing RTG from suppressing PV-INs improves its anticonvulsant effect.

Regulation of Potassium and Chloride Concentrations in Nervous Tissue as a Method of Anticonvulsant Therapy

Abstract

Under some pathological conditions, such as pharmacoresistant epilepsy, status epilepticus or certain forms of genetic abnormalities, spiking activity of GABAergic interneurons may enhance excitation processes in neuronal circuits and provoke the generation of ictal discharges. As a result, anticonvulsants acting on the GABAergic system may be ineffective or even increase seizure activity. This paradoxical effect of the inhibitory system is due to ionic imbalances in nervous tissue. This review addresses the mechanisms of ictal discharge initiation in neuronal networks due to the imbalance of chloride and potassium ions, as well as possible ways to regulate ionic concentrations. Both the enhancement (or attenuation) of the activity of certain neuronal ion transporters and ion pumps and their additional expression via gene therapy can be effective in suppressing seizure activity caused by ionic imbalances. The Na+–K+-pump, NKCC1 and KCC2 cotransporters are important for maintaining proper K+ and Cl– concentrations in nervous tissue, having been repeatedly considered as pharmacological targets for antiepileptic exposures. Further progress in this direction is hampered by the lack of sufficiently selective pharmacological tools and methods for providing effective drug delivery to the epileptic focus. The use of the gene therapy techniques, such as overexpressing of the KCC2 transporter in the epileptic focus, seems to be a more promising approach. Another possible direction could be the use of optogenetic tools, namely specially designed light-activated ion pumps or ion channels. In this case, photon energy can be used to create the required gradients of chloride and potassium ions, although these methods also have significant limitations which complicate their rapid introduction into medicine.

GABAB Receptors: are they Missing in Action in Focal Epilepsy Research?

GABA, the key inhibitory neurotransmitter in the adult forebrain, activates pre- and postsynaptic receptors that have been categorized as GABAA, which directly open ligand-gated (or receptor-operated) ion-channels, and GABA_B, which are metabotropic since they operate through second messengers. Over the last three decades, several studies have addressed the role of GABA_B receptors in the pathophysiology of generalized and focal epileptic disorders. Here, we will address their involvement in focal epileptic disorders by mainly reviewing in vitro studies that have shown: (i) how either enhancing or decreasing GABA_B receptor function can favour epileptiform synchronization and thus ictogenesis, although with different features; (ii) the surprising ability of GABA_B receptor antagonism to disclose ictal-like activity when the excitatory ionotropic transmission is abolished; and (iii) their contribution to controlling seizure-like discharges during repetitive electrical stimuli delivered in limbic structures. In spite of this evidence, the role of GABA_B receptor function in focal epileptic disorders has been attracting less interest when compared to the numerous studies that have addressed GABA_A receptor signaling. Therefore, the main aim of our mini-review is to revive interest in the function of GABA_B receptors in focal epilepsy research.

Focal seizures are organized by feedback between neural activity and ion concentration changes

Human and animal EEG data demonstrate that focal seizures start with low-voltage fast activity, evolve into rhythmic burst discharges and are followed by a period of suppressed background activity. This suggests that processes with dynamics in the range of tens of seconds govern focal seizure evolution. We investigate the processes associated with seizure dynamics by complementing the Hodgkin-Huxley mathematical model with the physical laws that dictate ion movement and maintain ionic gradients. Our biophysically realistic computational model closely replicates the electrographic pattern of a typical human focal seizure characterized by low voltage fast activity onset, tonic phase, clonic phase and postictal suppression. Our study demonstrates, for the first time in silico, the potential mechanism of seizure initiation by inhibitory interneurons via the initial build-up of extracellular K+ due to intense interneuronal spiking. The model also identifies ionic mechanisms that may underlie a key feature in seizure dynamics, i.e., progressive slowing down of ictal discharges towards the end of seizure. Our model prediction of specific scaling of inter-burst intervals is confirmed by seizure data recorded in the whole guinea pig brain in vitro and in humans, suggesting that the observed termination pattern may hold across different species. Our results emphasize ionic dynamics as elementary processes behind seizure generation and indicate targets for new therapeutic strategies.

Molecular Mechanisms of Epilepsy: The Role of the Chloride Transporter KCC2

Epilepsy is a neurological disease characterized by abnormal or synchronous brain activity causing seizures, which may produce convulsions, minor physical signs, or a combination of symptoms. These disorders affect approximately 65 million people worldwide, from all ages and genders. Seizures apart, epileptic patients present a high risk to develop neuropsychological comorbidities such as cognitive deficits, emotional disturbance, and psychiatric disorders, which severely impair quality of life. Currently, the treatment for epilepsy includes the administration of drugs or surgery, but about 30% of the patients treated with antiepileptic drugs develop time-dependent pharmacoresistence. Therefore, further investigation about epilepsy and its causes is needed to find new pharmacological targets and innovative therapeutic strategies. Pharmacoresistance is associated to changes in neuronal plasticity and alterations of GABA_A receptor-mediated neurotransmission. The downregulation of GABA inhibitory activity may arise from a positive shift in GABA_A receptor reversal potential, due to an alteration in chloride homeostasis. In this paper, we review the contribution of K⁺-Cl^--cotransporter (KCC2) to the alterations in the Cl^- gradient observed in epileptic condition, and how these alterations are coupled to the increase in the excitability.

Discovery of Small Molecule KCC2 Potentiators Which Attenuate In Vitro Seizure-Like Activity in Cultured Neurons

KCC2 is a K⁺-Cl^- cotransporter that is expressed in neurons throughout the central nervous system. Deficits in KCC2 activity have been implicated in a variety of neurological disorders, including epilepsy, chronic pain, autism spectrum disorders, and Rett syndrome. Therefore, it has been hypothesized that pharmacological potentiation of KCC2 activity could provide a treatment for these disorders. To evaluate the therapeutic potential of pharmacological KCC2 potentiation, drug-like, selective KCC2 potentiators are required. Unfortunately, the lack of such tools has greatly hampered the investigation of the KCC2 potentiation hypothesis. Herein, we describe the discovery and characterization of a new class of small-molecule KCC2 potentiator. This newly discovered class exhibits KCC2-dependent activity and a unique mechanistic profile relative to previously reported small molecules. Furthermore, we demonstrate that KCC2 potentiation by this new class of KCC2 potentiator attenuates seizure-like activity in neuronal-glial co-cultures. Together, our results provide evidence that pharmacological KCC2 potentiation, by itself, is sufficient to attenuate neuronal excitability in an in vitro model that is sensitive to anti-epileptic drugs. Our findings and chemical tools are important for evaluating the promise of KCC2 as a therapeutic target and could lay a foundation for the development of KCC2-directed therapeutics for multiple neurological disorders.

Bilateral optogenetic activation of inhibitory cells favors ictogenesis

Mesial temporal lobe epilepsy (MTLE) is the most common type of focal refractory epilepsy and is characterized by recurring seizures that are often refractory to medication. Since parvalbumin-positive (PV) interneurons were recently shown to play significant roles in ictogenesis, we established here how bilateral optogenetic stimulation of these interneurons in the hippocampus CA3 regions modulates seizures, interictal spikes and high-frequency oscillations (HFOs; ripples: 80-200 Hz, fast ripples: 250-500 Hz) in the pilocarpine model of MTLE. Bilateral optogenetic stimulation of CA3 PV-positive interneurons at 8 Hz (lasting 30 s, every 2 min) was implemented in PV-ChR2 mice for 8 consecutive days starting on day 7 (n = 8) or on day 13 (n = 6) after pilocarpine-induced status epilepticus (SE). Seizure occurrence was higher in both day 7 and day 13 groups of PV-ChR2 mice during periods of optogenetic stimulation ("ON"), compared to when stimulation was not performed ("OFF") (day 7 group = p < 0.01, day 13 group = p < 0.01). In the PV-ChR2 day 13 group, rates of seizures (p < 0.05), of interictal spikes associated with fast ripples (p < 0.01), and of isolated fast ripples (p < 0.01) during optogenetic stimulations were significantly higher than in the PV-ChR2 day 7 group. Our findings reveal that bilateral activation of PV-interneurons in the hippocampus (leading to a presumptive increase in GABA signaling) favors ictogenesis. These effects may also mirror the neuropathological changes that occur over time after SE in this animal model.

Gene Therapy for Neurological Disease: State of the Art and Opportunities for Next-generation Approaches

Gene therapy for rare monogenetic neurological disorders is reaching clinics and offering hope to families affected by these diseases. There is also potential for gene therapy to offer new and effective treatments for common, non-genetic disorders. Treatments for Parkinson's Disease are in clinical trials, and treatments for refractory epilepsies are due to enter first-in-human clinical trials in 2022. Gene therapies for these disorders are based on delivering genes that address the mechanism of the disease, not repairing a mutated gene. Similar 'mechanistic' gene therapies could offer treatments to a wide range of neurological and neuropsychiatric diseases where there is a known mechanism that could be restored using gene therapy. However, the permanent nature of most gene therapies is a serious drawback for translation of gene therapies to a wide-range of diseases because it could present risk of irreversible adverse effects. Several lines of research are aimed at developing gene therapy approaches that allow for the treatment to be turned on and off, including: using proteins activated by exogenous ligands, and promoters turned on by activators. We review these approaches and propose an overall de-risking strategy for gene therapy for common neurological and psychiatric diseases. This approach is based on using a temporary mRNA-based treatment to initially assess efficacy and safety of the planned manipulation, and only following with permanent, virally-delivered treatment if the approach appears safe and effective.

Why won't it stop? The dynamics of benzodiazepine resistance in status epilepticus

Status epilepticus is a life-threatening neurological emergency that affects both adults and children. Approximately 36% of episodes of status epilepticus do not respond to the current preferred first-line treatment, benzodiazepines. The proportion of episodes that are refractory to benzodiazepines is higher in low-income and middle-income countries (LMICs) than in high-income countries (HICs). Evidence suggests that longer episodes of status epilepticus alter brain physiology, thereby contributing to the emergence of benzodiazepine resistance. Such changes include alterations in GABA_A receptor function and in the transmembrane gradient for chloride, both of which erode the ability of benzodiazepines to enhance inhibitory synaptic signalling. Often, current management guidelines for status epilepticus do not account for these duration-related changes in pathophysiology, which might differentially impact individuals in LMICs, where the average time taken to reach medical attention is longer than in HICs. In this Perspective article, we aim to combine clinical insights and the latest evidence from basic science to inspire a new, context-specific approach to efficiently managing status epilepticus.

Seizures initiate in zones of relative hyperexcitation in a zebrafish epilepsy model

Seizures are thought to arise from an imbalance of excitatory and inhibitory neuronal activity. While most classical studies suggest excessive excitatory neural activity plays a generative role, some recent findings challenge this view and instead argue that excessive activity in inhibitory neurons initiates seizures. We investigated this question of imbalance in a zebrafish seizure model with two-photon imaging of excitatory and inhibitory neuronal activity throughout the brain using a nuclear-localized calcium sensor. We found that seizures consistently initiated in circumscribed zones of the midbrain before propagating to other brain regions. Excitatory neurons were both more prevalent and more likely to be recruited than inhibitory neurons in initiation as compared with propagation zones. These findings support a mechanistic picture whereby seizures initiate in a region of hyper-excitation, then propagate more broadly once inhibitory restraint in the surround is overcome.

Disruption of KCC2 in Parvalbumin-Positive Interneurons Is Associated With a Decreased Seizure Threshold and a Progressive Loss of Parvalbumin-Positive Interneurons

GABA_A receptors are ligand-gated ion channels, which are predominantly permeable for chloride. The neuronal K-Cl cotransporter KCC2 lowers the intraneuronal chloride concentration and thus plays an important role for GABA signaling. KCC2 loss-of-function is associated with seizures and epilepsy. Here, we show that KCC2 is expressed in the majority of parvalbumin-positive interneurons (PV-INs) of the mouse brain. PV-INs receive excitatory input from principle cells and in turn control principle cell activity by perisomatic inhibition and inhibitory input from other interneurons. Upon Cre-mediated disruption of KCC2 in mice, the polarity of the GABA response of PV-INs changed from hyperpolarization to depolarization for the majority of PV-INs. Reduced excitatory postsynaptic potential-spike (E-S) coupling and increased spontaneous inhibitory postsynaptic current (sIPSC) frequencies further suggest that PV-INs are disinhibited upon disruption of KCC2. In vivo, PV-IN-specific KCC2 knockout mice display a reduced seizure threshold and develop spontaneous sometimes fatal seizures. We further found a time dependent loss of PV-INs, which was preceded by an up-regulation of pro-apoptotic genes upon disruption of KCC2.

Global transgenic upregulation of KCC2 confers enhanced diazepam efficacy in treating sustained seizures

Reduced anticonvulsant efficacy of benzodiazepines is a problem in the treatment of status epilepticus, with up to 50% of patients failing to respond to their first dose. KCC2 is a neuronal K⁺ -Cl^- co-transporter that helps set and maintain intracellular Cl^- concentrations. KCC2 functional downregulation is a potential contributor to benzodiazepine resistance. We tested this idea using male and female doxycycline-inducible, conditional transgenic mice to increase the functional expression of KCC2 in pyramidal neurons. We administered mice with two doses of the chemoconvulsant kainic acid (5 mg/kg, i.p.) 60 min apart and quantified the resultant seizures with electroencephalography (EEG) recordings. Overexpression of KCC2 prior to the chemoconvulsant challenge did not affect seizure latency or other measures of seizure severity, but it did increase diazepam's efficacy in stopping EEG seizures. Spike rate, time in seizure, and EEG spectral power following diazepam (5 mg/kg, i.p) were all significantly lower in KCC2 overexpression mice as compared to control mice. Our results indicate that, in the context of benzodiazepine resistance during sustained seizures, addressing impaired Cl^- homeostasis alone appreciably improves the efficacy of γ-aminobutyric acid (GABA)ergic inhibition. We therefore suggest the simultaneous targeting of KCC2 and GABA_A receptors as a pathway for improving current anticonvulsant therapeutic strategies.

Disrupting Epileptiform Activity by Preventing Parvalbumin Interneuron Depolarization Block

Inhibitory synaptic mechanisms oppose epileptic network activity in the brain. The breakdown in this inhibitory restraint and propagation of seizure activity has been linked to the overwhelming of feedforward inhibition, which is provided in large part by parvalbumin-expressing (PV) interneurons in the cortex. The underlying cellular processes therefore represent potential targets for understanding and preventing the propagation of seizure activity. Here we use an optogenetic strategy to test the hypothesis that depolarization block in PV interneurons is a significant factor during the loss of inhibitory restraint. Depolarization block results from the inactivation of voltage-gated sodium channels and leads to impaired action potential firing. We used focal NMDA stimulation to elicit reproducible epileptiform discharges in hippocampal organotypic brain slices from male and female mice and combined this with targeted recordings from defined neuronal populations. Simultaneous patch-clamp recordings from PV interneurons and pyramidal neurons revealed epileptiform activity that was associated with an overwhelming of inhibitory synaptic mechanisms and the emergence of a partial, and then complete, depolarization block in PV interneurons. To counteract this depolarization block, we developed protocols for eliciting pulsed membrane hyperpolarization via the inhibitory opsin, archaerhodopsin. This optical approach was effective in counteracting cumulative inactivation of voltage-gated channels, maintaining PV interneuron action potential firing properties during the inhibitory restraint period, and reducing the probability of initiating epileptiform activity. These experiments support the idea that depolarization block is a point of weakness in feedforward inhibitory synaptic mechanisms and represents a target for preventing the initiation and spread of seizure activity.SIGNIFICANCE STATEMENT GABA_A receptor-mediated synaptic transmission opposes seizure activity by establishing an inhibitory restraint against spreading excitation. Parvalbumin-expressing (PV) interneurons contribute significantly to this inhibitory restraint, but it has been suggested that these cells are overwhelmed as they enter a state of "depolarization block." Here we test the importance of this process by devising an optogenetic strategy to selectively relieve depolarization block in PV interneurons. By inducing brief membrane hyperpolarization, we show that it is possible to reduce depolarization block in PV interneurons, maintain their action potential firing in the face of strong excitation, and disrupt epileptiform activity in an in vitro model. This represents a proof of principle that targeting rate-limiting processes can strengthen the inhibitory restraint of epileptiform activity.

Targeting ischemia-induced KCC2 hypofunction rescues refractory neonatal seizures and mitigates epileptogenesis in a mouse model

[Figure: see text].

Gene therapy in epilepsy

Gene therapy may constitute a promising alternative to conventional pharmacological tools and surgeries for epilepsy. For primary epilepsy, a single variant leading to a significant effect is relatively rare, while other forms are considered complex in inheritances with multiple susceptible mutations and impacts from the environment. Gene therapy in preclinical models of epilepsy has attempted to perform antiepileptogenic, anticonvulsant, or disease-modifying effects during epileptogenesis or after establishing the disease. Creating gene vectors tailored for different situations is the key to expanding gene therapy, and choosing the appropriate therapeutic target remains another fundamental problem. A variety of treatment strategies, from overexpressing inhibitory neuropeptides to modulating the expression of neurotransmitters or ion channels, have been tested in animal models. Additionally, emerging new approaches of optogenetics and chemogenetics, as well as genome-editing tools will further boost the prosperity of gene therapy. This review summarizes the experience obtained to date and discusses the challenges and opportunities in clinical translations.

The pilocarpine model of mesial temporal lobe epilepsy: Over one decade later, with more rodent species and new investigative approaches

Fundamental work on the mechanisms leading to focal epileptic discharges in mesial temporal lobe epilepsy (MTLE) often rests on the use of rodent models in which an initial status epilepticus (SE) is induced by kainic acid or pilocarpine. In 2008 we reviewed how, following systemic injection of pilocarpine, the main subsequent events are the initial SE, the latent period, and the chronic epileptic state. Up to a decade ago, rats were most often employed and they were frequently analysed only behaviorally. However, the use of transgenic mice has revealed novel information regarding this animal model. Here, we review recent findings showing the existence of specific neuronal events during both latent and chronic states, and how optogenetic activation of specific cell populations modulate spontaneous seizures. We also address neuronal damage induced by pilocarpine treatment, the role of neuroinflammation, and the influence of circadian and estrous cycles. Updating these findings leads us to propose that the rodent pilocarpine model continues to represent a valuable tool for identifying the basic pathophysiology of MTLE.

Impaired KCC2 Function Triggers Interictal-Like Activity Driven by Parvalbumin-Expressing Interneurons in the Isolated Subiculum In Vitro

The decreased expression of the KCC2 membrane transporter in subicular neurons has been proposed to be a key epileptogenic event in temporal lobe epilepsy (TLE). Here, we have addressed this question in a reduced model in vitro and have studied the properties and mechanistic involvement of a major class of interneurons, that is, parvalbumin-expressing cells (PVs). When exposed to the KCC2 blocker VU0463271, mouse subicular slices generated hypersynchronous discharges that could be recorded electrophysiologically and visualized as clusters of co-active neurons with calcium imaging. The pharmacological profile of these events resembled interictal-like discharges in human epileptic tissue because of their dependence on GABAA and AMPA receptors. On average, PVs fired before pyramidal cells (PCs) and the area of co-active clusters was comparable to the individual axonal spread of PVs, suggesting their mechanistic involvement. Optogenetic experiments confirmed this hypothesis, as the flash-stimulation of PVs in the presence of VU0463271 initiated interictal-like discharges, whereas their optogenetic silencing suppressed network hyper-excitability. We conclude that reduced KCC2 activity in subicular networks in vitro is sufficient to induce interictal-like activity via altered GABAergic signaling from PVs without other epilepsy-related changes. This conclusion supports an epileptogenic role for impaired subicular KCC2 function during the progression of TLE.

High-Frequency Oscillations on Interictal Epileptiform Discharges in Routinely Acquired Scalp EEG: Can It Be Used as a Prognostic Marker?

Introduction

Despite all the efforts for optimizing epilepsy management in children over the past decades, there is no clear consensus regarding whether to treat or not to treat epileptiform discharges (EDs) after a first unprovoked seizure or the optimal duration of therapy with anti-seizure medication (ASM). It is therefore highly needed to find markers on scalp electroencephalogram (EEG) that can help identify pathological EEG discharges that require treatment.

Aim of the study

This retrospective study aimed to identify whether the coexistence of ripples/high-frequency oscillations (HFOs) with interictal EDs (IEDs) in routinely acquired scalp EEG is associated with a higher risk of seizure recurrence and could be used as a prognostic marker.

Methods

100 children presenting with new onset seizure to Children’s Medical Center- Dallas during 2015–2016, who were not on ASM and had focal EDs on an awake and sleep EEG recorded with sample frequency of 500 HZ, were randomly identified by database review. EEGs were analyzed blinded to the data of the patients. HFOs were visually identified using review parameters including expanded time base and adjusted filter settings.

Results

The average age of patients was 6.3 years (±4.35 SD). HFOs were visually identified in 19% of the studied patients with an inter-rater reliability of 99% for HFO negative discharges and 78% agreement for identification of HFOs. HFOs were identified more often in the younger age group; however, they were identified in 11% of patients >5 years old. They were more frequently associated with spikes than with sharp waves and more often with higher amplitude EDs. Patients with HFOs were more likely to have a recurrence of seizures in the year after the first seizure (P < 0.05) and to continue to have seizures after 2 years (P < 0.0001). There was no statistically significant difference between the two groups with regards to continuing ASM after 2 years.

Conclusion

Including analysis for HFOs in routine EEG interpretation may increase the yield of the study and help guide the decision to either start or discontinue ASM. In the future, this may also help to identify pathological discharges with deleterious effects on the growing brain and set a new target for the management of epilepsy.

Probing the polarity of spontaneous perisomatic GABAergic synaptic transmission in the mouse CA3 circuit in vivo

The hypothesis that reversed, excitatory GABA may be involved in various brain pathologies, including epileptogenesis, is appealing but controversial because of the technical difficulty of probing endogenous GABAergic synaptic function in vivo. We overcome this challenge by non-invasive extracellular recording of neuronal firing responses to optogenetically evoked and spontaneously occurring inhibitory perisomatic GABAergic field potentials, generated by individual parvalbumin interneurons on their target pyramidal cells. Our direct probing of GABAergic transmission suggests a rather anecdotal participation of excitatory GABA in two specific models of epileptogenesis in the mouse CA3 circuit in vivo, even though this does not preclude its expression in other brain areas or pathological conditions. Our approach allows the detection of distinct alterations of inhibition during spontaneous activity in vivo, with high sensitivity. It represents a promising tool for the investigation of excitatory GABA in different pathological conditions that may affect the hippocampal circuit.

Medial septal GABAergic neurons reduce seizure duration upon optogenetic closed-loop stimulation

Seizures can emerge from multiple or large foci in temporal lobe epilepsy, complicating focally targeted strategies such as surgical resection or the modulation of the activity of specific hippocampal neuronal populations through genetic or optogenetic techniques. Here, we evaluate a strategy in which optogenetic activation of medial septal GABAergic neurons, which provide extensive projections throughout the hippocampus, is used to control seizures. We utilized the chronic intrahippocampal kainate mouse model of temporal lobe epilepsy, which results in spontaneous seizures and as is often the case in human patients, presents with hippocampal sclerosis. Medial septal GABAergic neuron populations were immunohistochemically labelled and were not reduced in epileptic conditions. Genetic labelling with mRuby of medial septal GABAergic neuron synaptic puncta and imaging across the rostral to caudal extent of the hippocampus, also indicated an unchanged number of putative synapses in epilepsy. Furthermore, optogenetic stimulation of medial septal GABAergic neurons consistently modulated oscillations across multiple hippocampal locations in control and epileptic conditions. Finally, wireless optogenetic stimulation of medial septal GABAergic neurons, upon electrographic detection of spontaneous hippocampal seizures, resulted in reduced seizure durations. We propose medial septal GABAergic neurons as a novel target for optogenetic control of seizures in temporal lobe epilepsy.

Potassium channels as prominent targets and tools for the treatment of epilepsy

INTRODUCTION

K⁺ channels are of great interest to epilepsy research as mutations in their genes are found in humans with inherited epilepsy. At the level of cellular physiology, K+ channels control neuronal intrinsic excitability and are the main contributors to membrane repolarization of active neurons. Recently, a genetically modified voltage-dependent K⁺ channel has been patented as a remedy for epileptic seizures.

AREAS COVERED

We review the role of potassium channels in excitability, clinical and experimental evidence for the association of potassium channelopathies with epilepsy, the targeting of K+ channels by drugs, and perspectives of gene therapy in epilepsy with the expression of extra K+ channels in the brain.

EXPERT OPINION

Control over K+ conductance is of great potential benefit for the treatment of epilepsy. Nowadays, gene therapy affecting K+ channels is one of the most promising approaches to treat pharmacoresistant focal epilepsy.

KCC2 Chloride Transport Contributes to the Termination of Ictal Epileptiform Activity

Recurrent seizures intensely activate GABAA receptors (GABAA-Rs), which induces transient neuronal chloride ([Cl-]i) elevations and depolarizing GABA responses that contribute to the failure of inhibition that engenders further seizures and anticonvulsant resistance. The K+-Cl- cotransporter KCC2 is responsible for Cl- extrusion and restoration of [Cl-]i equilibrium (ECl) after synaptic activity, but at the cost of increased extracellular potassium which may retard K+-Cl- extrusion, depolarize neurons, and potentiate seizures. Thus, KCC2 may either diminish or facilitate seizure activity, and both proconvulsant and anticonvulsant effects of KCC2 inhibition have been reported. It is now necessary to identify the loci of these divergent responses by assaying both the electrographic effects and the ionic effects of KCC2 manipulation. We therefore determined the net effects of KCC2 transport activity on cytoplasmic chloride elevation and Cl- extrusion rates during spontaneous recurrent ictal-like epileptiform discharges (ILDs) in organotypic hippocampal slices in vitro, as well as the correlation between ionic and electrographic effects. We found that the KCC2 antagonist VU0463271 reduced Cl- extrusion rates, increased ictal [Cl-]i elevation, increased ILD duration, and induced status epilepticus (SE). In contrast, the putative KCC2 upregulator CLP257 improved chloride homeostasis and reduced the duration and frequency of ILDs in a concentration-dependent manner. Our results demonstrate that measuring both the ionic and electrographic effects of KCC2 transport clarify the impact of KCC2 modulation in specific models of epileptiform activity. Anticonvulsant effects predominate when KCC2-mediated chloride transport rather than potassium buffering is the rate-limiting step in restoring ECl and the efficacy of GABAergic inhibition during recurrent ILDs.

The Multifaceted Roles of KCC2 in Cortical Development

KCC2, best known as the neuron-specific chloride-extruder that sets the strength and polarity of GABAergic currents during neuronal maturation, is a multifunctional molecule that can regulate cytoskeletal dynamics via its C-terminal domain (CTD). We describe the molecular and cellular mechanisms involved in the multiple functions of KCC2 and its splice variants, ranging from developmental apoptosis and the control of early network events to the formation and plasticity of cortical dendritic spines. The versatility of KCC2 actions at the cellular and subcellular levels is also evident in mature neurons during plasticity, disease, and aging. Thus, KCC2 has emerged as one of the most important molecules that shape the overall neuronal phenotype.

In vivo CRISPRa decreases seizures and rescues cognitive deficits in a rodent model of epilepsy

Epilepsy is a major health burden, calling for new mechanistic insights and therapies. CRISPR-mediated gene editing shows promise to cure genetic pathologies, although hitherto it has mostly been applied ex vivo. Its translational potential for treating non-genetic pathologies is still unexplored. Furthermore, neurological diseases represent an important challenge for the application of CRISPR, because of the need in many cases to manipulate gene function of neurons in situ. A variant of CRISPR, CRISPRa, offers the possibility to modulate the expression of endogenous genes by directly targeting their promoters. We asked if this strategy can effectively treat acquired focal epilepsy, focusing on ion channels because their manipulation is known be effective in changing network hyperactivity and hypersynchronziation. We applied a doxycycline-inducible CRISPRa technology to increase the expression of the potassium channel gene Kcna1 (encoding K_v1.1) in mouse hippocampal excitatory neurons. CRISPRa-mediated K_v1.1 upregulation led to a substantial decrease in neuronal excitability. Continuous video-EEG telemetry showed that AAV9-mediated delivery of CRISPRa, upon doxycycline administration, decreased spontaneous generalized tonic-clonic seizures in a model of temporal lobe epilepsy, and rescued cognitive impairment and transcriptomic alterations associated with chronic epilepsy. The focal treatment minimizes concerns about off-target effects in other organs and brain areas. This study provides the proof-of-principle for a translational CRISPR-based approach to treat neurological diseases characterized by abnormal circuit excitability.