Sulfasalazine decreases mouse cortical hyperexcitability

Implication of system xc- in neuroinflammation during the onset and maintenance of neuropathic pain

BACKGROUND

Despite the high prevalence of neuropathic pain, treating this neurological disease remains challenging, given the limited efficacy and numerous side effects associated with current therapies. The complexity in patient management is largely attributed to an incomplete understanding of the underlying pathological mechanisms. Central sensitization, that refers to the adaptation of the central nervous system to persistent inflammation and heightened excitatory transmission within pain pathways, stands as a significant contributor to persistent pain. Considering the role of the cystine/glutamate exchanger (also designated as system xc-) in modulating glutamate transmission and in supporting neuroinflammatory responses, we investigated the contribution of this exchanger in the development of neuropathic pain.

METHODS

We examined the implication of system xc- by evaluating changes in the expression/activity of this exchanger in the dorsal spinal cord of mice after unilateral partial sciatic nerve ligation. In this surgical model of neuropathic pain, we also examined the consequence of the genetic suppression of system xc- (using mice lacking the system xc- specific subunit xCT) or its pharmacological manipulation (using the pharmacological inhibitor sulfasalazine) on the pain-associated behavioral responses. Finally, we assessed the glial activation and the inflammatory response in the spinal cord by measuring mRNA and protein levels of GFAP and selected M1 and M2 microglial markers.

RESULTS

The sciatic nerve lesion was found to upregulate system xc- at the spinal level. The genetic deletion of xCT attenuated both the amplitude and the duration of the pain sensitization after nerve surgery, as evidenced by reduced responses to mechanical and thermal stimuli, and this was accompanied by reduced glial activation. Consistently, pharmacological inhibition of system xc- had an analgesic effect in lesioned mice.

CONCLUSION

Together, these observations provide evidence for a role of system xc- in the biochemical processes underlying central sensitization. We propose that the reduced hypersensitivity observed in the transgenic mice lacking xCT or in sulfasalazine-treated mice is mediated by a reduced gliosis in the lumbar spinal cord and/or a shift in microglial M1/M2 polarization towards an anti-inflammatory phenotype in the absence of system xc-. These findings suggest that drugs targeting system xc- could contribute to prevent or reduce neuropathic pain.

Pharmacological profiling identifies divergent chemosensitivities of differentiating and maturing iPSC-derived human cortical neuron populations

Neuronal differentiation and maturation are extended developmental processes. To determine whether neurons at different developmental stages have divergent chemosensitivities, we screened differentiating and maturing neuronal populations using a small compound library comprising FDA-approved and investigational drugs. Using a neurotoxicity assay format, both respective neuronal population-based screening campaigns performed robustly (Z-factors = 0.7-0.8), although the hit rate for the differentiating neurons (2.8%) was slightly higher than for maturing neurons (1.9%). While the majority of hits were toxic to both neuronal populations, these hits predominantly represented promiscuous drugs. Other drugs were selectively neurotoxic, with receptor tyrosine kinase inhibitors disproportionally represented after confirmation. Ponatinib and amuvatinib were neuroinhibitory for differentiating and maturing neurons, respectively. Chemoinformatic analyses confirmed differences in potential drug targets that may be differentially expressed during neuronal development. Subsequent studies demonstrated neuronal expression of AXL, an amuvatinib target, in both neuronal populations. However, functional AXL activity was confirmed only in the maturing neuronal population as determined by AXL phosphorylation in response to GAS6, the cognate ligand of AXL, and concurrent STAT3Y705 phosphorylation. Differentiating neurons were unresponsive to the effects of GAS6 suggesting that the AXL-STAT3 signaling axis was nonfunctional. Amuvatinib treatment of maturing neuronal cultures significantly reduced pAXL levels. These studies indicate that neuronal developmental states may exhibit unique chemosensitivities and that drugs may have different neuro-inhibitory effects depending upon the developmental stage of the neuronal population.

Astrocytes as a target for therapeutic strategies in epilepsy: current insights

Astrocytes are specialized non-neuronal glial cells of the central nervous system, contributing to neuronal excitability and synaptic transmission (gliotransmission). Astrocytes play a key roles in epileptogenesis and seizure generation. Epilepsy, as a chronic disorder characterized by neuronal hyperexcitation and hypersynchronization, is accompanied by substantial disturbances of glial cells and impairment of astrocytic functions and neuronal signaling. Anti-seizure drugs that provide symptomatic control of seizures primarily target neural activity. In epileptic patients with inadequate control of seizures with available anti-seizure drugs, novel therapeutic candidates are needed. These candidates should treat epilepsy with anti-epileptogenic and disease-modifying effects. Evidence from human and animal studies shows that astrocytes have value for developing new anti-seizure and anti-epileptogenic drugs. In this review, we present the key functions of astrocytes contributing to neuronal hyperexcitability and synaptic activity following an etiology-based approach. We analyze the role of astrocytes in both development (epileptogenesis) and generation of seizures (ictogenesis). Several promising new strategies that attempted to modify astroglial functions for treating epilepsy are being developed: (1) selective targeting of glia-related molecular mechanisms of glutamate transport; (2) modulation of tonic GABA release from astrocytes; (3) gliotransmission; (4) targeting the astrocytic Kir4.1-BDNF system; (5) astrocytic Na+/K+/ATPase activity; (6) targeting DNA hypo- or hypermethylation of candidate genes in astrocytes; (7) targeting astrocytic gap junction regulators; (8) targeting astrocytic adenosine kinase (the major adenosine-metabolizing enzyme); and (9) targeting microglia-astrocyte communication and inflammatory pathways. Novel disease-modifying therapeutic strategies have now been developed, such as astroglia-targeted gene therapy with a broad spectrum of genetic constructs to target astroglial cells.

Loss of functional System x-c uncouples aberrant postnatal neurogenesis from epileptogenesis in the hippocampus of Kcna1-KO mice

Mutations in Kv1.1 (Kcna1) voltage-gated potassium channels in humans and mice generate network hyperexcitability, enhancing aberrant postnatal neurogenesis in the dentate subgranular zone, resulting in epilepsy and hippocampal hypertrophy. While Kcna1 loss stimulates proliferation of progenitor cell subpopulations, the identity of extrinsic molecular triggers linking network hyperexcitability to aberrant postnatal neurogenesis remains incomplete. System x-c (Sxc) is an inducible glutamate/cysteine antiporter that regulates extracellular glutamate. Here, we find that the functional unit of Sxc, xCT (Slc7a11), is upregulated in regions of Kcna1 knockout (KO) hippocampus, suggesting a contribution to both hyperplasia and epilepsy. However, Slc7a11 KO suppressed and rescued hippocampal enlargement without altering seizure severity in Kcna1-Slc7a11-KO mice. Microglial activation, but not astrocytosis, was also reduced. Our study identifies Sxc-mediated glutamate homeostasis as an essential non-synaptic trigger coupling aberrant postnatal neurogenesis and neuroimmune crosstalk, revealing that neurogenesis and epileptogenesis in the dentate gyrus are not mutually contingent events.

Sulfasalazine decreases astrogliosis-mediated seizure burden

Objective

Previously, we reported that inhibition of the astrocytic cystine/glutamate antiporter system xc‐ (SXC), using sulfasalazine (SAS), decreased evoked excitatory signaling in three distinct hyperexcitability models ex vivo. The current study expands on this work by evaluating the in vivo efficacy of SAS in decreasing astrogliosis‐mediated seizure burden seen in the beta‐1 integrin knockout (B1KO) model.

Methods

Video‐EEG (electroencephalography) monitoring (24/7) was obtained using Biopac EEG acquisition hardware and software. EEG spectral analysis was performed using MATLAB. SAS was used at an equivalence of doses taken by Crohn’s disease patients. Whole‐cell patch‐clamp recordings were made from cortical layer 2/3 pyramidal neurons.

Results

We report that 100% of B1KO mice that underwent 24/7 video‐EEG monitoring developed spontaneous recurrent seizures and that intraperitoneal administration of SAS significantly reduced seizure frequency in B1KOs compared to B1KOs receiving sham saline. Spectral analysis found an acute reduction in EEG power following SAS treatment in B1KOs; however, this effect was not observed in nonepileptic control mice receiving SAS. Finally, whole‐cell recordings from SXC knockout mice had hyperpolarized neurons and SXC‐B1 double knockouts fired significantly less action potentials in response to current injection compared to B1KOs with SXC.

Significance

To devise effective strategies in finding relief for one‐in‐three patients with epilepsy who experience drug‐resistant epilepsy we must continue to explore the mechanisms regulating glutamate homeostasis. This study explored the efficacy of targeting an astrocytic glutamate antiporter, SXC, as a novel antiepileptic drug (AED) target and further characterized a unique mouse model in which chronic astrogliosis is sufficient to induce spontaneous seizures and epilepsy. These findings may serve as a foundation to further assess the potential for SAS or inform the development of more potent and specific compounds that target SXC as a novel treatment for epilepsy.

The effect of sulfasalazine in pentylenetetrazole-induced seizures in rats

We aimed to reveal the anti-convulsant effects sulfasalazine and its mechanism in pentylenetetrazole (PTZ)-induced seizures in rats. Forty-eight male Wistar albino rats (200-250 g) were randomly divided into two groups: 24 for electroencephalography (EEG) recording (group A) and 24 for behavioral studies (group B). About 70 mg/kg PTZ was used for behavioral studies after sulfasalazine administration and 35 mg/kg PTZ was used for EEG recording after sulfasalazine administration. Electrodes were implanted on the dura mater over the left frontal cortex and the reference electrode was implanted over the cerebellum for EEG recording. Racine's convulsion scale, first myoclonic jerk onset time, spike percentages, brain malondialdehyde (MDA), superoxide dismutase (SOD), and prostaglandin F2α (PGF2α) levels were evaluated between the groups. First myoclonic jerk onset time was significantly shorter in the saline group than both 250 and 500 mg/kg sulfasalazine groups (P<0.05). Racine's convulsion scores were significantly lower in the 250 and 500 mg/kg sulfasalazine groups than the saline group (P<0.05, P<0.001). The two sulfasalazine groups had lower spike percentages than the saline group (P<0.05). Significantly lower MDA and PGF2α levels were observed in the 250 and 500 mg/kg sulfasalazine groups compared with the saline group (P<0.05, P<0.001, respectively). SOD increased significantly in both sulfasalazine groups compared with the PTZ+saline group (P<0.05). Our study demonstrated that sulfasalazine had protective effects on PTZ-induced convulsions by protecting against oxidative and inflammatory damage associated with PTZ.

Neuroinflammation in neurological disorders: pharmacotherapeutic targets from bench to bedside

Neuroinflammation is one of the host defensive mechanisms through which the nervous system protects itself from pathogenic and or infectious insults. Moreover, neuroinflammation occurs as one of the most common pathological outcomes in various neurological disorders, makes it the promising target. The present review focuses on elaborating the recent advancement in understanding molecular mechanisms of neuroinflammation and its role in the etiopathogenesis of various neurological disorders, especially Alzheimer's disease (AD), Parkinson's disease (PD), and Epilepsy. Furthermore, the current status of anti-inflammatory agents in neurological diseases has been summarized in light of different preclinical and clinical studies. Finally, possible limitations and future directions for the effective use of anti-inflammatory agents in neurological disorders have been discussed.

Novel test strategies for in vitro seizure liability assessment

INTRODUCTION

The increasing incidence of mental illnesses and neurodegenerative diseases results in a high demand for drugs targeting the central nervous system (CNS). These drugs easily reach the CNS, have a high affinity for CNS targets, and are prone to cause seizures as an adverse drug reaction. Current seizure liability assessment heavily depends on in vivo or ex vivo animal models and is therefore ethically debated, labor intensive, expensive, and not always predictive for human risk.

AREAS COVERED

The demand for CNS drugs urges the development of alternative safety assessment strategies. Yet, the complexity of the CNS hampers reliable detection of compound-induced seizures. This review provides an overview of the requirements of in vitro seizure liability assays and highlights recent advances, including micro-electrode array (MEA) recordings using rodent and human cell models.

EXPERT OPINION

Successful and cost-effective replacement of in vivo and ex vivo models for seizure liability screening can reduce animal use for drug development, while increasing the predictive value of the assays, particularly if human cell models are used. However, these novel test strategies require further validation and standardization as well as additional refinements to better mimic the human in vivo situation and increase their predictive value.

Gut metabolite S-equol ameliorates hyperexcitability in entorhinal cortex neurons following Theiler murine encephalomyelitis virus-induced acute seizures

Objective

A growing body of evidence indicates a potential role for the gut–brain axis as a novel therapeutic target in treating seizures. The present study sought to characterize the gut microbiome in Theiler murine encephalomyelitis virus (TMEV)‐induced seizures, and to evaluate the effect of microbial metabolite S‐equol on neuronal physiology as well as TMEV‐induced neuronal hyperexcitability ex vivo.

Methods

We infected C57BL/6J mice with TMEV and monitored the development of acute behavioral seizures 0–7 days postinfection (dpi). Fecal samples were collected at 5–7 dpi and processed for 16S sequencing, and bioinformatics were performed with QIIME2. Finally, we conducted whole‐cell patch‐clamp recordings in cortical neurons to investigate the effect of exogenous S‐equol on cell intrinsic properties and neuronal hyperexcitability.

Results

We demonstrated that gut microbiota diversity is significantly altered in TMEV‐infected mice at 5–7 dpi, exhibiting separation in beta diversity in TMEV‐infected mice dependent on seizure phenotype, and lower abundance of genus Allobaculum in TMEV‐infected mice regardless of seizure phenotype. In contrast, we identified specific loss of S‐equol‐producing genus Adlercreutzia as a microbial hallmark of seizure phenotype following TMEV infection. Electrophysiological recordings indicated that exogenous S‐equol alters cortical neuronal physiology. We found that entorhinal cortex neurons are hyperexcitable in TMEV‐infected mice, and exogenous application of microbial‐derived S‐equol ameliorated this TMEV‐induced hyperexcitability.

Significance

Our study presents the first evidence of microbial‐derived metabolite S‐equol as a potential mechanism for alteration of TMEV‐induced neuronal excitability. These findings provide new insight for the novel role of S‐equol and the gut–brain axis in epilepsy treatment.

Dysregulation of Ambient Glutamate and Glutamate Receptors in Epilepsy: An Astrocytic Perspective

Given the important functions that glutamate serves in excitatory neurotransmission, understanding the regulation of glutamate in physiological and pathological states is critical to devising novel therapies to treat epilepsy. Exclusive expression of pyruvate carboxylase and glutamine synthetase in astrocytes positions astrocytes as essential regulators of glutamate in the central nervous system (CNS). Additionally, astrocytes can significantly alter the volume of the extracellular space (ECS) in the CNS due to their expression of the bi-directional water channel, aquaporin-4, which are enriched at perivascular endfeet. Rapid ECS shrinkage has been observed following epileptiform activity and can inherently concentrate ions and neurotransmitters including glutamate. This review highlights our emerging knowledge on the various potential contributions of astrocytes to epilepsy, particularly supporting the notion that astrocytes may be involved in seizure initiation via failure of homeostatic responses that lead to increased ambient glutamate. We also review the mechanisms whereby ambient glutamate can influence neuronal excitability, including via generation of the glutamate receptor subunit GluN2B-mediated slow inward currents, as well as indirectly affect neuronal excitability via actions on metabotropic glutamate receptors that can potentiate GluN2B currents and influence neuronal glutamate release probabilities. Additionally, we discuss evidence for upregulation of System x c - , a cystine/glutamate antiporter expressed on astrocytes, in epileptic tissue and changes in expression patterns of glutamate receptors.

Quantification and Metabolite Identification of Sulfasalazine in Mouse Brain and Plasma Using Quadrupole-Time-of-Flight Mass Spectrometry

Sulfasalazine (SAS), an anti-inflammatory drug with potent cysteine/glutamate antiporter system xc-(SXC) inhibition has recently shown beneficial effects in brain-related diseases. Despite many reports related to central nervous system (CNS) effect of SAS, pharmacokinetics (PK) and metabolite identification studies in the brain for SAS were quite limited. The aim of this study was to investigate the pharmacokinetics and metabolite identification of SAS and their distributions in mouse brain. Using in vivo brain exposure studies (neuro PK), the PK parameters of SAS was calculated for plasma as well as brain following intravenous and oral administration at 10 mg/kg and 50 mg/kg in mouse, respectively. In addition, in vivo metabolite identification (MetID) studies of SAS in plasma and brain were also conducted. The concentration of SAS in brain was much lower than that in plasma and only 1.26% of SAS was detected in mouse brain when compared to the SAS concentration in plasma (brain to plasma ratio (%): 1.26). In the MetID study, sulfapyridine (SP), hydroxy-sulfapyridine (SP-OH), and N-acetyl sulfapyridine (Ac-SP) were identified in plasma, whereas only SP and Ac-SP were identified as significant metabolites in brain. As a conclusion, our results suggest that the metabolites of SAS such as SP and Ac-SP might be responsible for the pharmacological effect in brain, not the SAS itself.

Corticostriatal dysfunction and social interaction deficits in mice lacking the cystine/glutamate antiporter

The astrocytic cystine/glutamate antiporter system x_c^- represents an important source of extracellular glutamate in the central nervous system, with potential impact on excitatory neurotransmission. Yet, its function and importance in brain physiology remain incompletely understood. Employing slice electrophysiology and mice with a genetic deletion of the specific subunit of system x_c^-, xCT (xCT^-/- mice), we uncovered decreased neurotransmission at corticostriatal synapses. This effect was partly mitigated by replenishing extracellular glutamate levels, indicating a defect linked with decreased extracellular glutamate availability. We observed no changes in the morphology of striatal medium spiny neurons, the density of dendritic spines, or the density or ultrastructure of corticostriatal synapses, indicating that the observed functional defects are not due to morphological or structural abnormalities. By combining electron microscopy with glutamate immunogold labeling, we identified decreased intracellular glutamate density in presynaptic terminals, presynaptic mitochondria, and in dendritic spines of xCT^-/- mice. A proteomic and kinomic screen of the striatum of xCT^-/- mice revealed decreased expression of presynaptic proteins and abnormal kinase network signaling, that may contribute to the observed changes in postsynaptic responses. Finally, these corticostriatal deregulations resulted in a behavioral phenotype suggestive of autism spectrum disorder in the xCT^-/- mice; in tests sensitive to corticostriatal functioning we recorded increased repetitive digging behavior and decreased sociability. To conclude, our findings show that system x_c^- plays a previously unrecognized role in regulating corticostriatal neurotransmission and influences social preference and repetitive behavior.

Interactions between Tumor Cells, Neurons, and Microglia in the Glioma Microenvironment

Despite significant strides made in understanding the pathophysiology of high-grade gliomas over the past two decades, most patients succumb to these neoplasias within two years of diagnosis. Furthermore, there are various co-morbidities associated with glioma and standard of care treatments. Emerging evidence suggests that aberrant glutamate secretion in the glioma microenvironment promotes tumor progression and contributes to the development of co-morbidities, such as cognitive defects, epilepsy, and widespread neurodegeneration. Recent data clearly illustrate that neurons directly synapse onto glioma cells and drive their proliferation and spread via glutamatergic action. Microglia are central nervous system-resident myeloid cells, modulate glioma growth, and possess the capacity to prune synapses and encourage synapse formation. However, current literature has yet to investigate the potential role of microglia in shaping synapse formation between neurons and glioma cells. Herein, we present the literature concerning glutamate's role in glioma progression, involving hyperexcitability and excitotoxic cell death of peritumoral neurons and stimulation of glioma proliferation and invasion. Furthermore, we discuss instances in which microglia are more likely to sculpt or encourage synapse formation during glioma treatment and propose studies to delineate the role of microglia in synapse formation between neurons and glioma cells. The sex-dependent oncogenic or oncolytic actions of microglia and myeloid cells, in general, are considered in addition to the functional differences between microglia and macrophages in tumor progression. We also put forth tractable methods to safely perturb aberrant glutamatergic action in the tumor microenvironment without significantly increasing the toxicities of the standard of care therapies for glioma therapy.

Sulfasalazine decreases mouse cortical hyperexcitability

Objective

Currently prescribed antiepileptic drugs (AEDs) are ineffective in treating approximately 30% of epilepsy patients. Sulfasalazine (SAS) is an US Food and Drug Administration (FDA)–approved drug for the treatment of Crohn disease that has been shown to inhibit the cystine/glutamate antiporter system xc‐ (SXC) and decrease tumor‐associated seizures. This study evaluates the effect of SAS on distinct pharmacologically induced network excitability and determines whether it can further decrease hyperexcitability when administered with currently prescribed AEDs.

Methods

Using in vitro cortical mouse brain slices, whole‐cell patch‐clamp recordings were made from layer 2/3 pyramidal neurons. Epileptiform activity was induced with bicuculline (bic), 4‐aminopyridine (4‐AP) and magnesium‐free (Mg²⁺‐free) solution to determine the effect of SAS on epileptiform events. In addition, voltage‐sensitive dye (VSD) recordings were performed to characterize the effect of SAS on the spatiotemporal spread of hyperexcitable network activity and compared to currently prescribed AEDs.

Results

SAS decreased evoked excitatory postsynaptic currents (eEPSCs) and increased the decay kinetics of evoked inhibitory postsynaptic currents (eIPSCs) in layer 2/3 pyramidal neurons. Although application of SAS to bic and Mg²⁺‐free–induced epileptiform activity caused a decrease in the duration of epileptiform events, SAS completely blocked 4‐AP–induced epileptiform events. In VSD recordings, SAS decreased VSD optical signals induced by 4‐AP. Co‐application of SAS with the AED topiramate (TPM) caused a significantly further decrease in the spatiotemporal spread of VSD optical signals.

Significance

Taken together this study provides evidence that inhibition of SXC by SAS can decrease network hyperexcitability induced by three distinct pharmacologic agents in the superficial layers of the cortex. Furthermore, SAS provided additional suppression of 4‐AP–induced network activity when administered with the currently prescribed AED TPM. These findings may serve as a foundation to assess the potential for SAS or other compounds that selectively target SXC as an adjuvant treatment for epilepsy.