Local disruption of glial adenosine homeostasis in mice associates with focal electrographic seizures: a first step in epileptogenesis?

Deep Brain Stimulation Inhibits Epileptic Seizures via Increase of Adenosine Release and Inhibition of ENT1, CD39, and CD73 Expression

Deep brain stimulation (DBS) of the anterior nucleus of the thalamus is an efficacious treatment option for patients with refractory epilepsy. Our previous study demonstrates that adenosine is a potential target of DBS for the treatment of epilepsy. Equilibrative nucleoside transporters-1 (ENT1) and ectonucleotidases (CD39, CD73) function as regulators of extracellular adenosine in the brain. It is unclear whether ENT1, CD39, and CD73 are involved in the mechanism of DBS for epilepsy. A total of 48 SD male rats were divided into four groups: control (naïve rats), Pilo (pilocarpine induced rats with epilepsy), DBS (rats with epilepsy treated with DBS for 8 weeks), and sham. In the present study, video electroencephalogram monitoring, Morris water maze assays, in vivo measurements of adenosine using fiber photometry, histochemistry, and western blot were performed on the hippocampus. DBS markedly attenuated spontaneous recurrent seizures (SRSs) and enhanced spatial learning in rats with epilepsy, assessed through video-EEG and water maze assays. Fibred photometry measurements of an adenosine sensor revealed dynamic increase in extracellular adenosine during DBS. The expressions of ENT1, CD39, and CD73 in Pilo group and sham group increased compared with the control group, while the expressions of ENT1, CD39, and CD73 in DBS group decreased compared to that of Pilo group and sham group. The findings indicate that DBS reduces the number of SRSs and improves spatial memory in rats with epilepsy with concomitant decrease of ENT1, CD39, and CD73 expressions. Adenosine-modulating enzymes might be the potential targets of DBS for the treatment of epilepsy.

Stabilization of mitochondrial function by chlorogenic acid protects against kainic acid-induced seizures and neuronal cell death in rats

The current study investigated the effect of chlorogenic acid, a polyphenolic compound found in numerous plant products, on a kainic acid-induced seizure rat model and its potential mechanism. Rats were administered chlorogenic acid (10 and 50 mg/kg) intraperitoneally for 30 min before kainic acid (15 mg/kg) intraperitoneal administration. Pretreatment with chlorogenic acid decreased the seizure score, increased the latency to onset of the first seizure, and decreased the mortality rate. Chlorogenic acid pretreatment also resulted in a significant reduction in glutamate elevation and neuronal death in the hippocampus of kainic acid-treated rats. In addition, electron microscopy revealed that kainic acid-induced changes in hippocampal mitochondrial structure were prevented by chlorogenic acid pretreatment. Additionally, the levels of mitochondrial function-related proteins, including sirtuin 3, Complex I, glutamate dehydrogenase 1 and ATP synthase, were increased, and the level of the mitochondrial damage marker cytochrome C was decreased in the hippocampus of chlorogenic acid/kainic acid rats. Furthermore, the expression of mitochondrial biogenesis-related proteins [AMP-activated protein kinase (AMPK), sirtuin1, and peroxisome proliferator-activated receptor γ-coactivator-1α (PGC-1α)] and mitophagy-related proteins [phosphatase and tensin homolog (PTEN)-induced putative kinase 1 (PINK1), Parkin, and microtubule-associated protein 1 light chain 3 (LC3)] was decreased in the hippocampus of kainic acid-treated rats, which was reversed by chlorogenic acid pretreatment. These observations reveal the marked neuroprotective potential of chlorogenic acid against kainic acid-induced neurotoxicity and seizures through prevention of glutamate increase and preservation of AMPK/sirtuin 1/PGC-1α-mediated mitochondrial biogenesis and PINK1/Parkin-induced mitophagy to maintain adequate mitochondrial homeostasis and function.

Aberrant adenosine signaling in patients with focal cortical dysplasia

Focal cortical dysplasia (FCD), a common malformation of cortical development, is frequently associated with pharmacoresistant epilepsy in both children and adults. Adenosine is an inhibitory modulator of brain activity and a prospective anti-seizure agent with potential for clinical translation. Our previous results demonstrated that the major adenosine-metabolizing enzyme adenosine kinase (ADK) was upregulated in balloon cells (BCs) within FCD type IIB lesions, suggesting that dysfunction of the adenosine system is implicated in the pathophysiology of FCD. In our current study, we therefore performed a comprehensive analysis of adenosine signaling in surgically resected cortical specimens from patients with FCD type I and type II via immunohistochemistry and immunoblot analysis. Adenosine enzyme signaling was assessed by quantifying the levels of the key enzymes of adenosine metabolism, i.e., ADK, adenosine deaminase (ADA), and ecto-5'-nucleotidase (CD73). Adenosine receptor signaling was assessed by quantifying the levels of adenosine A2A receptor (A2AR) and putative downstream mediators of adenosine, namely, glutamate transporter-1 (GLT-1) and mammalian target of rapamycin (mTOR). Within lesions in FCD specimens, we found that the adenosine-metabolizing enzymes ADK and ADA, as well as the adenosine-producing enzyme CD73, were upregulated. We also observed an increase in A2AR density, as well as a decrease in GLT-1 levels and an increase in mTOR levels, in FCD specimens compared with control tissue. These results suggest that dysregulation of the adenosine system is a common pathologic feature of both FCD type I and type II. The adenosine system might therefore be a therapeutic target for the treatment of epilepsy associated with FCD.

Reactive A1 Astrocyte-Targeted Nucleic Acid Nanoantiepileptic Drug Downregulating Adenosine Kinase to Rescue Endogenous Antiepileptic Pathway

Resistance to traditional antiepileptic drugs is a major challenge in chronic epilepsy treatment. MicroRNA-based gene therapy is a promising alternative but has demonstrated limited efficacy due to poor blood-brain barrier permeability, cellular uptake, and targeting efficiency. Adenosine is an endogenous antiseizure agent deficient in the epileptic brain due to elevated adenosine kinase (ADK) activity in reactive A1 astrocytes. We designed a nucleic acid nanoantiepileptic drug (tFNA-ADK^ASO@AS1) based on a tetrahedral framework nucleic acid (tFNA), carrying an antisense oligonucleotide targeting ADK (ADK^ASO) and A1 astrocyte-targeted peptide (AS1). This tFNA-ADK^ASO@AS1 construct effectively reduced brain ADK, increased brain adenosine, mitigated aberrant mossy fiber sprouting, and reduced the recurrent spontaneous epileptic spike frequency in a mouse model of chronic temporal lobe epilepsy. Further, the treatment did not induce any neurotoxicity or major organ damage. This work provides proof-of-concept for a novel antiepileptic drug delivery strategy and for endogenous adenosine as a promising target for gene-based modulation.

Characterization of the anticonvulsant effect of dapsone on metabolic activity assessed by [18F]FDG -PET after kainic acid-induced status epilepticus in rats

BACKGROUND

Development of effective drugs for epilepsy are needed, as nearly 30 % of epileptic patients, are resistant to current treatments. This study is aimed to characterize the anticonvulsant effect of dapsone (DDS), in the kainic acid (KA)-induced Status Epilepticus (SE) by recording the brain metabolic activity with an [¹⁸F]FDG-PET analysis.

METHODS

Wistar rats received KA (10 mg/kg, i.p., single dose) to produce sustained seizures. [¹⁸F]FDG-PET and electroencephalographic (EEG) studies were then performed. DDS or vehicle were administered 30 min before KA. [¹⁸F]FDG uptake and EEG were evaluated at baseline, 2 and 25 h after KA injection. Likewise, caspase-8, 3 hippocampal activities and Fluoro-Jade B neuronal degeneration and Hematoxylin-eosin staining were measured 25 h after KA.

RESULTS

PET data evaluated at 2 h showed hyper-uptake of [¹⁸F]FDG in the control group, which was decreased by DDS. At 25 h, hypo-uptake was observed in the control group and higher values due to DDS effect. EEG spectral power was increased 2 h after KA administration in the control group during the generalized tonic-clonic seizures, which was reversed by DDS, correlated with [¹⁸F]FDG-PET uptake changes. The values of caspases-8 activity decreased 48 and 43 % vs control group in the groups treated with DDS (12.5 y 25 mg/kg respectively), likewise; caspase-3 activity diminished by 57 and 53 %. Fewer degenerated neurons were observed due to DDS treatments.

CONCLUSIONS

This study pinpoints the anticonvulsant therapeutic potential of DDS. Given its safety and effectiveness, DDS may be a viable alternative for patients with drug-resistant epilepsy.

Epilepsy and Autism Spectrum Disorder (ASD): The Underlying Mechanisms and Therapy Targets Related to Adenosine

Epilepsy and autism spectrum disorder (ASD) are highly mutually comorbid, suggesting potential overlaps in genetic etiology, pathophysiology, and neurodevelopmental abnormalities. Adenosine, an endogenous anticonvulsant and neuroprotective neuromodulator of the brain, has been proved to affect the process of epilepsy and ASD. On the one hand, adenosine plays a crucial role in preventing the progression and development of epilepsy through adenosine receptordependent and -independent ways. On the other hand, adenosine signaling can not only regulate core symptoms but also improve comorbid disorders in ASD. Given the important role of adenosine in epilepsy and ASD, therapeutic strategies related to adenosine, including the ketogenic diet, neuromodulation therapy, and adenosine augmentation therapy, have been suggested for the arrangement of epilepsy and ASD. There are several proposals in this review. Firstly, it is necessary to further discuss the relationship between both diseases based on the comorbid symptoms and mechanisms of epilepsy and ASD. Secondly, it is important to explore the role of adenosine involved in epilepsy and ASD. Lastly, potential therapeutic value and clinical approaches of adenosine-related therapies in treating epilepsy and ASD need to be emphasized.

Deep brain stimulation of the anterior thalamus attenuates PTZ kindling with concomitant reduction of adenosine kinase expression in rats

BACKGROUND

Deep brain stimulation (DBS) of the anterior nucleus of the thalamus (ANT) is an emerging therapy to provide seizure control in patients with refractory epilepsy, although its therapeutic mechanisms remain elusive.

OBJECTIVE

We tested the hypothesis that ANT-DBS might interfere with the kindling process using three experimental groups: PTZ, DBS-ON and DBS-OFF.

METHODS

79 male rats were used in two experiments and exposed to chemical kindling with pentylenetetrazole (PTZ, 30 mg/kg i.p.), delivered three times a week for a total of 18 kindling days (KD). These animals were divided into two sets of three groups: PTZ (n = 26), DBS-ON (n = 28) and DBS-OFF (n = 25). ANT-DBS (130 Hz, 90 μs, and 200 μA) was paired with PTZ injections, while DBS-OFF group, although implanted remained unstimulated. After KD 18, the first set of PTZ-treated animals and an additional group of 11 naïve rats were euthanized for brain extraction to study adenosine kinase (ADK) expression. To observe possible long-lasting effects of ANT stimulation, the second set of animals underwent a 1-week treatment and stimulation-free period after KD 18 before a final PTZ challenge.

RESULTS

ANT-DBS markedly attenuated kindling progression in the DBS-ON group, which developed seizure scores of 2.4 on KD 13, whereas equivalent seizure scores were reached in the DBS-OFF and PTZ groups as early as KD5 and KD6, respectively. The incidence of animals with generalized seizures following 3 consecutive PTZ injections was 94%, 74% and 21% in PTZ, DBS-OFF and DBS-ON groups, respectively. Seizure scores triggered by a PTZ challenge one week after cessation of stimulation revealed lasting suppression of seizure scores in the DBS-ON group (2.7 ± 0.2) compared to scores of 4.5 ± 0.1 for the PTZ group and 4.3 ± 0.1 for the DBS-OFF group (P = 0.0001). While ANT-DBS protected hippocampal cells, the expression of ADK was decreased in the DBS-ON group compared to both PTZ (P < 0.01) and naïve animals (P < 0.01).

CONCLUSIONS

Our study demonstrates that ANT-DBS interferes with the kindling process and reduced seizure activity was maintained after a stimulation free period of one week. Our findings suggest that ANT-DBS might have additional therapeutic benefits to attenuate seizure progression in epilepsy.

Genetic variations of adenosine kinase as predictable biomarkers of efficacy of vagus nerve stimulation in patients with pharmacoresistant epilepsy

OBJECTIVE

Vagus nerve stimulation (VNS) is an alternative treatment option for individuals with refractory epilepsy, with nearly 40% of patients showing no benefit after VNS and only 6%-8% achieving seizure freedom. It is presently unclear why some patients respond to treatment and others do not. Therefore, identification of biomarkers to predict efficacy of VNS is of utmost importance. The objective of this study was to explore whether genetic variations in genes involved in adenosine kinase (ADK), ecto-5'-nucleotidase (NT5E), and adenosine A1 receptor (A1R) are linked to outcome of VNS in patients with refractory epilepsy.

METHODS

Thirty single-nucleotide polymorphisms (SNPs), including 9 in genes encoding ADK, 3 in genes encoding NT5E, and 18 in genes encoding A1R, were genotyped in 194 refractory epilepsy patients who underwent VNS. The chi-square test and binary logistic regression were used to determine associations between genetic differences and VNS efficacy.

RESULTS

A significant association between ADK SNPs rs11001109, rs7899674, and rs946185 and seizure reduction with VNS was found. Regardless of sex, age, seizure frequency and type, antiseizure drug use, etiology, and prior surgical history, all patients (10/10 patients [100%]) with minor allele homozygosity at rs11001109 (genotype AA) or rs946185 (AA) achieved > 50% seizure reduction and 4 patients (4/10 [40%]) achieved seizure freedom. VNS therapy demonstrated higher efficacy among carriers of minor allele rs7899674 (CG + GG) (68.3% vs 48.8% for patients with major allele homozygosity).

CONCLUSIONS

Homozygous ADK SNPs rs11001109 (AA) and rs946185 (AA), as well as minor allele rs7899674 (CG + GG), may serve as useful biomarkers for prediction of VNS therapy outcome.

Animal Models of Epilepsy: A Phenotype-oriented Review

Epilepsy is a serious neurological disorder characterized by abnormal, recurrent, and synchronous discharges in the brain. Long-term recurrent seizure attacks can cause serious damage to brain function, which is usually observed in patients with temporal lobe epilepsy. Controlling seizure attacks is vital for the treatment and prognosis of epilepsy. Animal models, such as the kindling model, which was the most widely used model in the past, allow the understanding of the potential epileptogenic mechanisms and selection of antiepileptic drugs. In recent years, various animal models of epilepsy have been established to mimic different seizure types, without clear merits and demerits. Accordingly, this review provides a summary of the views mentioned above, aiming to provide a reference for animal model selection.

Life and death in the hippocampus: What's bad?

The hippocampal formation is crucial for the generation and regulation of several brain functions, including memory and learning processes; however, it is vulnerable to neurological disorders, such as epilepsy. Temporal lobe epilepsy (TLE), the most common type of epilepsy, changes the hippocampal circuitry and excitability, under the contribution of both neuronal degeneration and abnormal neurogenesis. Classically, neurodegeneration affects sensitive areas of the hippocampus, such as dentate gyrus (DG) hilus, as well as specific fields of the Ammon's horn, CA3, and CA1. In addition, the proliferation, migration, and abnormal integration of newly generated hippocampal granular cells (GCs) into the brain characterize TLE neurogenesis. Robust studies over the years have intensely discussed the effects of death and life in the hippocampus, though there are still questions to be answered about their possible benefits and risks. Here, we review the impacts of death and life in the hippocampus, discussing its influence on TLE, providing new perspectives or insights for the implementation of new possible therapeutic targets. This article is part of the Special Issue "NEWroscience 2018".

ATP and adenosine-Two players in the control of seizures and epilepsy development

Despite continuous advances in understanding the underlying pathogenesis of hyperexcitable networks and lowered seizure thresholds, the treatment of epilepsy remains a clinical challenge. Over one third of patients remain resistant to current pharmacological interventions. Moreover, even when effective in suppressing seizures, current medications are merely symptomatic without significantly altering the course of the disease. Much effort is therefore invested in identifying new treatments with novel mechanisms of action, effective in drug-refractory epilepsy patients, and with the potential to modify disease progression. Compelling evidence has demonstrated that the purines, ATP and adenosine, are key mediators of the epileptogenic process. Extracellular ATP concentrations increase dramatically under pathological conditions, where it functions as a ligand at a host of purinergic receptors. ATP, however, also forms a substrate pool for the production of adenosine, via the action of an array of extracellular ATP degrading enzymes. ATP and adenosine have assumed largely opposite roles in coupling neuronal excitability to energy homeostasis in the brain. This review integrates and critically discusses novel findings regarding how ATP and adenosine control seizures and the development of epilepsy. This includes purine receptor P1 and P2-dependent mechanisms, release and reuptake mechanisms, extracellular and intracellular purine metabolism, and emerging receptor-independent effects of purines. Finally, possible purine-based therapeutic strategies for seizure suppression and disease modification are discussed.

The Kainic Acid Models of Temporal Lobe Epilepsy

Experimental models of epilepsy are useful to identify potential mechanisms of epileptogenesis, seizure genesis, comorbidities, and treatment efficacy. The kainic acid (KA) model is one of the most commonly used. Several modes of administration of KA exist, each producing different effects in a strain-, species-, gender-, and age-dependent manner. In this review, we discuss the advantages and limitations of the various forms of KA administration (systemic, intrahippocampal, and intranasal), as well as the histologic, electrophysiological, and behavioral outcomes in different strains and species. We attempt a personal perspective and discuss areas where work is needed. The diversity of KA models and their outcomes offers researchers a rich palette of phenotypes, which may be relevant to specific traits found in patients with temporal lobe epilepsy.

PKA-RIIβ autophosphorylation modulates PKA activity and seizure phenotypes in mice

Temporal lobe epilepsy (TLE) is one of the most common and intractable neurological disorders in adults. Dysfunctional PKA signaling is causally linked to the TLE. However, the mechanism underlying PKA involves in epileptogenesis is still poorly understood. In the present study, we found the autophosphorylation level at serine 114 site (serine 112 site in mice) of PKA-RIIβ subunit was robustly decreased in the epileptic foci obtained from both surgical specimens of TLE patients and seizure model mice. The p-RIIβ level was negatively correlated with the activities of PKA. Notably, by using a P-site mutant that cannot be autophosphorylated and thus results in the released catalytic subunit to exert persistent phosphorylation, an increase in PKA activities through transduction with AAV-RIIβ-S112A in hippocampal DG granule cells decreased mIPSC frequency but not mEPSC, enhanced neuronal intrinsic excitability and seizure susceptibility. In contrast, a reduction of PKA activities by RIIβ knockout led to an increased mIPSC frequency, a reduction in neuronal excitability, and mice less prone to experimental seizure onset. Collectively, our data demonstrated that the autophosphorylation of RIIβ subunit plays a critical role in controlling neuronal and network excitabilities by regulating the activities of PKA, providing a potential therapeutic target for TLE.

Rasmussen's encephalitis: mechanisms update and potential therapy target

Rasmussen’s encephalitis (RE) is rare neurological diseases characterized as epilepsia partialis continua, invariably hemiparesis, and cognitive impairment. This disease is encountered frequently in childhood and presents with progressive atrophy of the unilateral hemisphere, and there are also sustained neurological complications. Owing to uncertain pathogenesis, the most effective way to limit the influence of seizures currently is cerebral hemispherectomy. In this review, we focus on four main lines of pathogenesis: virus infection, antibody-mediated, cell-mediated immunity, and microglia activation. Although one or more antigenic epitopes may give rise to infiltrating T cell responses in RE brain tissue, no exact antigen was confirmed as the definite cause of the disease. On the other hand, the appearance of antibodies related with RE seem to be a secondary pathological process. Synthetic studies have suggested an adaptive immune mechanism mediated by CD8⁺ T cells and an innate immune mechanism mediated by activated microglia and neuroglia. Accordingly, opinions have been raised that immunomodulatory treatments aimed at initial damage to the brain that are induced by cytotoxic CD8⁺ T cell lymphocytes and microglia in the early stage of RE slow down disease progression. However, systematic exploration of the theory behind these therapeutic effects based on multicenter and large sample studies are needed. In addition, dysfunction of the adenosine system, including the main adenosine removing enzyme adenosine kinase and adenosine receptors, has been demonstrated in RE, which might provide a novel therapeutic target for treatment of RE in future.

Adenosine and Ketogenic Treatments

It is well known that the neuromodulator adenosine, acting through the adenosine A₁ receptor subtype, can limit or stop seizures. In 2008, adenosine was proposed as a key component of the anticonvulsant mechanism of the ketogenic diet (KD), a very low carbohydrate diet that can be highly effective in drug-refractory epilepsy. In this study, we review the accumulated data on the intersection among adenosine, ketosis, and anticonvulsant/antiepileptogenic effects. In several rodent models of epilepsy and seizures, antiseizure effects of ketogenic treatments (the KD itself, exogenous ketone bodies, medium-chain triglycerides or fatty acids) are reversed by administration of an adenosine A₁ receptor antagonist. In addition, KD treatment elevates extracellular adenosine and tissue adenosine content in brain. Efforts to maintain or mimic a ketogenic milieu in brain slices reveal a state of reduced excitability produced by pre- and postsynaptic adenosine A₁ receptor-based effects. Long-lasting seizure reduction may be due to adenosine-based epigenetic effects. In conclusion, there is accumulating evidence for an adenosinergic anticonvulsant action in the ketogenic state. In some cases, the main trigger is mildly but consistently lowered glucose in the brain. More research is needed to investigate the importance of adenosine in the antiepileptogenic and neuroprotective effects of these treatments. Future research may begin to investigate alternative adenosine-promoting strategies to enhance the KD or to find use as treatments themselves.

Role of Adenosine in Epilepsy and Seizures

Adenosine is an endogenous anticonvulsant and neuroprotectant of the brain. Seizure activity produces large quantities of adenosine, and it is this seizure-induced adenosine surge that normally stops a seizure. However, within the context of epilepsy, adenosine plays a wide spectrum of different roles. It not only controls seizures (ictogenesis), but also plays a major role in processes that turn a normal brain into an epileptic brain (epileptogenesis). It is involved in the control of abnormal synaptic plasticity and neurodegeneration and plays a major role in the expression of comorbid symptoms and complications of epilepsy, such as sudden unexpected death in epilepsy (SUDEP). Given the important role of adenosine in epilepsy, therapeutic strategies are in development with the goal to utilize adenosine augmentation not only for the suppression of seizures but also for disease modification and epilepsy prevention, as well as strategies to block adenosine A2A receptor overfunction associated with neurodegeneration. This review provides a comprehensive overview of the role of adenosine in epilepsy.

A face-to-face comparison of the intra-amygdala and intrahippocampal kainate mouse models of mesial temporal lobe epilepsy and their utility for testing novel therapies

OBJECTIVE

Intracranial (intrahippocampal or intra-amygdala) administration of kainate in rodents leads to spatially restricted brain injury and development of focal epilepsy with characteristics that resemble mesial temporal lobe epilepsy. Such rodent models are used both in the search for more effective antiseizure drugs (ASDs) and in the development of antiepileptogenic strategies. However, it is not clear which of the models is best suited for testing different types of epilepsy therapies.

METHODS

In the present study, we performed a face-to-face comparison of the intra-amygdala kainate (IAK) and intrahippocampal kainate (IHK) mouse models using the same mouse inbred strain (C57BL/6). For comparison, some experiments were performed in mouse outbred strains.

RESULTS

Intra-amygdala kainate injection led to more severe status epilepticus and higher mortality than intrahippocampal injection. In male C57BL/6 mice, the latent period to spontaneous recurrent seizures (SRSs) was short or absent in both models, whereas a significantly longer latent period was determined in NMRI and CD-1 outbred mice. When SRSs were recorded from the ipsilateral hippocampus, relatively frequent electroclinical seizures were determined in the IAK model, whereas only infrequent electroclinical seizures but extremely frequent focal electrographic seizures were determined in the IHK model. As a consequence of the differences in SRS frequency, prolonged video-electroencephalographic monitoring and drug administration were needed for testing efficacy of the benchmark ASD carbamazepine in the IAK model, whereas acute drug testing was possible in the IHK model. In both models, carbamazepine was only effective at high doses, indicating ASD resistance to this benchmark drug.

SIGNIFICANCE

We found a variety of significant differences between the IAK and IHK models, which are important when deciding which of these models is best suited for studies on novel epilepsy therapies. The IAK model appears particularly interesting for studies on disease-modifying treatments, whereas the IHK model is well suited for studying the antiseizure activity of novel ASDs against difficult-to-treated focal seizures.

The role of adenosine in epilepsy

Adenosine is a well-characterized endogenous anticonvulsant and seizure terminator of the brain. Through a combination of adenosine receptor-dependent and -independent mechanisms, adenosine affects seizure generation (ictogenesis), as well as the development of epilepsy and its progression (epileptogenesis). Maladaptive changes in adenosine metabolism, in particular increased expression of the astroglial enzyme adenosine kinase (ADK), play a major role in epileptogenesis. Increased expression of ADK has dual roles in both reducing the inhibitory tone of adenosine in the brain, which consequently reduces the threshold for seizure generation, and also driving an increased flux of methyl-groups through the transmethylation pathway, thereby increasing global DNA methylation. Through these mechanisms, adenosine is uniquely positioned to link metabolism with epigenetic outcome. Therapeutic adenosine augmentation therefore not only holds promise for the suppression of seizures in epilepsy, but moreover the prevention of epilepsy and its progression overall. This review will focus on adenosine-related mechanisms implicated in ictogenesis and epileptogenesis and will discuss therapeutic opportunities and challenges.

Upregulation of adenosine A2A receptor and downregulation of GLT1 is associated with neuronal cell death in Rasmussen's encephalitis

Rasmussen encephalitis (RE) is a severe pediatric inflammatory brain disease characterized by unilateral inflammation and atrophy of the cerebral cortex, drug-resistant focal epilepsy and progressive neurological and cognitive deterioration. The etiology and pathogenesis of RE remain unclear. Our previous results demonstrated that the adenosine A1 receptor (A1R) and the major adenosine-removing enzyme adenosine kinase play an important role in the etiology of RE. Because the downstream pathways of inhibitory A1R signaling are modulated by stimulatory A2AR signaling, which by itself controls neuro-inflammation, glial activation and glial glutamate homeostasis through interaction with glutamate transporter GLT-1, we hypothesized that maladaptive changes in adenosine A2A receptor (A2AR) expression are associated with RE. We used immunohistochemistry and Western blot analysis to examine the expression of A2ARs, glutamate transporter-I (GLT-1) and the apoptotic marker Bcl-2 in surgically resected cortical specimens from RE patients (n = 18) in comparison with control cortical tissue. In lesions of the RE specimen we found upregulation of A2ARs, downregulation of GLT-1 and increased apoptosis of both neurons and astroglia. Double staining revealed colocalization of A2ARs and Bcl-2 in RE lesions. These results suggest that maladaptive changes in A2AR expression are associated with a decrease in GLT-I expression as a possible precipitator for apoptotic cell loss in RE. Because A2AR antagonists are already under clinical evaluation for Parkinson's disease, the A2AR might likewise be a tractable target for the treatment of RE.

Research progress on adenosine in central nervous system diseases

As an endogenous neuroprotectant agent, adenosine is extensively distributed and is particularly abundant in the central nervous system (CNS). Under physiological conditions, the concentration of adenosine is low intra- and extracellularly, but increases significantly in response to stress. The majority of adenosine functions are receptor-mediated, and primarily include the A1, A2A, A2B, and A3 receptors (A1R, A2AR, A2BR, and A3R). Adenosine is currently widely used in the treatment of diseases of the CNS and the cardiovascular systems, and the mechanisms are related to the disease types, disease locations, and the adenosine receptors distribution in the CNS. For example, the main infarction sites of cerebral ischemia are cortex and striatum, which have high levels of A1 and A2A receptors. Cerebral ischemia is manifested with A1R decrease and A2AR increase, as well as reduction in the A1R-mediated inhibitory processes and enhancement of the A2AR-mediated excitatory process. Adenosine receptor dysfunction is also involved in the pathology of Alzheimer's disease (AD), depression, and epilepsy. Thus, the adenosine receptor balance theory is important for brain disease treatment. The concentration of adenosine can be increased by endogenous or exogenous pathways due to its short half-life and high inactivation properties. Therefore, we will discuss the function of adenosine and its receptors, adenosine formation, and metabolism, and its role for the treatment of CNS diseases (such as cerebral ischemia, AD, depression, Parkinson's disease, epilepsy, and sleep disorders). This article will provide a scientific basis for the development of novel adenosine derivatives through adenosine structure modification, which will lead to experimental applications.

Ripple-related firing of identified deep CA1 pyramidal cells in chronic temporal lobe epilepsy in mice

OBJECTIVE

Temporal lobe epilepsy (TLE) is often associated with memory deficits. Reactivation of memory traces in the hippocampus occurs during sharp-wave ripples (SWRs; 140-250 Hz). To better understand the mechanisms underlying high-frequency oscillations and cognitive comorbidities in epilepsy, we evaluated how rigorously identified deep CA1 pyramidal cells (dPCs) discharge during SWRs in control and TLE mice.

METHODS

We used the unilateral intraamygdala kainate model of TLE in video-electroencephalography (EEG) verified chronically epileptic adult mice. Local field potential and single-cell recordings were performed using juxtacellular recordings from awake control and TLE mice resting on a spherical treadmill, followed by post hoc identification of the recorded cells.

RESULTS

Hippocampal SWRs in TLE mice occurred with increased intraripple frequency compared to control mice. The frequency of SWR events was decreased, whereas the overall frequency of SWRs, interictal epileptiform discharges, and high-frequency ripples (250-500 Hz) together was not altered. CA1 dPCs in TLE mice showed significantly increased firing during ripples as well as between the ripple events. The strength of ripple modulation of dPC discharges increased in TLE without alteration of the preferred phase of firing during the ripple waves.

SIGNIFICANCE

These juxtacellular electrophysiology data obtained from identified CA1 dPCs from chronically epileptic mice are in general agreement with recent findings indicating distortion of normal firing patterns during offline SWRs as a mechanism underlying deficits in memory consolidation in epilepsy. Because the primary seizure focus in our experiments was in the amygdala and we recorded from the CA1 region, these results are also in agreement with the presence of altered high-frequency oscillations in areas of secondary seizure spread.

Role of DNA Methylation and Adenosine in Ketogenic Diet for Pharmacoresistant Epilepsy: Focus on Epileptogenesis and Associated Comorbidities

Epilepsy is a neurological disorder characterized by a long term propensity to produce unprovoked seizures and by the associated comorbidities including neurological, cognitive, psychiatric, and impairment the quality of life. Despite the clinic availability of several novel antiepileptic drugs (AEDs) with different mechanisms of action, more than one-third of patients with epilepsy suffer with pharmacoresistant epilepsy. Until now, no AEDs have been proven to confer the efficacy in alteration of disease progression or inhibition of the development of epilepsy. The ketogenic diet, the high-fat, low-carbohydrate composition is an alternative metabolic therapy for epilepsy, especially for children with drug-resistant epilepsy. Recently clinical and experimental results demonstrate its efficacy in ameliorating both seizures and comorbidities associated with epilepsy, such as cognitive/psychiatric concerns for the patients with refractory epilepsy. Of importance, ketogenic diet demonstrates to be a promising disease-modifying or partial antiepileptogenesis therapy for epilepsy. The mechanisms of action of ketogenic diet in epilepsy have been revealed recently, such as epigenetic mechanism for increase the adenosine level in the brain and inhibition of DNA methylation. In the present review, we will focus on the mechanisms of ketogenic diet therapies underlying adenosine system in the prevention of epileptogenesis and disease modification. In addition, we will review the role of ketogenic diet therapy in comorbidities associated epilepsy and the underlying mechanisms of adenosine.

Protective effect of compound Danshen (Salvia miltiorrhiza) dripping pills alone and in combination with carbamazepine on kainic acid-induced temporal lobe epilepsy and cognitive impairment in rats

CONTEXT

Temporal lobe epilepsy (TLE) is resistant to antiepileptic drugs (AEDs) and is associated with cognitive impairment. The modern Chinese medicine, compound Danshen dripping pills (CDDP), is clinically effective in treating epilepsy and improving cognitive impairment.

OBJECTIVE

This study evaluated the protective effects of CDDP alone and in combination with carbamazepine (CBZ) on kainic acid-induced TLE and cognitive impairment in rats.

MATERIALS AND METHODS

Sprague-Dawley rats were randomly divided into five groups: control (sham operated), model, CDDP, CBZ and combined. A TLE model was then created via bilateral intrahippocampal injection of 0.35 μg kainic acid (KA). Rats received CDDP (85 mg/kg), CBZ (100 mg/kg) or combined (85 mg/kg CDDP +100 mg/kg CBZ) via intragastric administration for 90 d, respectively. Seizure intensity, apoptosis and glial cell line-derived neurotrophic factor (GDNF) were measured. Furthermore, the improvement in cognitive impairment and hippocampal neuronal damage was evaluated.

RESULTS

CDDP combined with CBZ significantly decreased seizure severity and frequency (p < 0.05) and ameliorated cognitive impairment (p < 0.05). The model group showed a significant reduction of neurons and Bcl-2/Bax expression in the hippocampus CA3 area (p < 0.01), the combined groups significantly reversed these change (p < 0.01). GDNF expression in the combined groups showed a clear increase over the model group (p < 0.05).

CONCLUSION

These findings support the use of CDDP as an adjuvant drug for the treatment of TLE and cognitive deficit. Its mechanism might be related to an anti-apoptosis effect and up-regulation of GDNF.

Commonalities in epileptogenic processes from different acute brain insults: Do they translate?

The most common forms of acquired epilepsies arise following acute brain insults such as traumatic brain injury, stroke, or central nervous system infections. Treatment is effective for only 60%-70% of patients and remains symptomatic despite decades of effort to develop epilepsy prevention therapies. Recent preclinical efforts are focused on likely primary drivers of epileptogenesis, namely inflammation, neuron loss, plasticity, and circuit reorganization. This review suggests a path to identify neuronal and molecular targets for clinical testing of specific hypotheses about epileptogenesis and its prevention or modification. Acquired human epilepsies with different etiologies share some features with animal models. We identify these commonalities and discuss their relevance to the development of successful epilepsy prevention or disease modification strategies. Risk factors for developing epilepsy that appear common to multiple acute injury etiologies include intracranial bleeding, disruption of the blood-brain barrier, more severe injury, and early seizures within 1 week of injury. In diverse human epilepsies and animal models, seizures appear to propagate within a limbic or thalamocortical/corticocortical network. Common histopathologic features of epilepsy of diverse and mostly focal origin are microglial activation and astrogliosis, heterotopic neurons in the white matter, loss of neurons, and the presence of inflammatory cellular infiltrates. Astrocytes exhibit smaller K+ conductances and lose gap junction coupling in many animal models as well as in sclerotic hippocampi from temporal lobe epilepsy patients. There is increasing evidence that epilepsy can be prevented or aborted in preclinical animal models of acquired epilepsy by interfering with processes that appear common to multiple acute injury etiologies, for example, in post-status epilepticus models of focal epilepsy by transient treatment with a trkB/PLCγ1 inhibitor, isoflurane, or HMGB1 antibodies and by topical administration of adenosine, in the cortical fluid percussion injury model by focal cooling, and in the albumin posttraumatic epilepsy model by losartan. Preclinical studies further highlight the roles of mTOR1 pathways, JAK-STAT3, IL-1R/TLR4 signaling, and other inflammatory pathways in the genesis or modulation of epilepsy after brain injury. The wealth of commonalities, diversity of molecular targets identified preclinically, and likely multidimensional nature of epileptogenesis argue for a combinatorial strategy in prevention therapy. Going forward, the identification of impending epilepsy biomarkers to allow better patient selection, together with better alignment with multisite preclinical trials in animal models, should guide the clinical testing of new hypotheses for epileptogenesis and its prevention.

Epilepsy and astrocyte energy metabolism

Epilepsy is a complex neurological syndrome characterized by neuronal hyperexcitability and sudden, synchronized electrical discharges that can manifest as seizures. It is now increasingly recognized that impaired astrocyte function and energy homeostasis play key roles in the pathogenesis of epilepsy. Excessive neuronal discharges can only happen, if adequate energy sources are made available to neurons. Conversely, energy depletion during seizures is an endogenous mechanism of seizure termination. Astrocytes control neuronal energy homeostasis through neurometabolic coupling. In this review, we will discuss how astrocyte dysfunction in epilepsy leads to distortion of key metabolic and biochemical mechanisms. Dysfunctional glutamate metabolism in astrocytes can directly contribute to neuronal hyperexcitability. Closure of astrocyte intercellular gap junction coupling as observed early during epileptogenesis limits activity-dependent trafficking of energy metabolites, but also impairs clearance of the extracellular space from accumulation of K⁺ and glutamate. Dysfunctional astrocytes also increase the metabolism of adenosine, a metabolic product of ATP degradation that broadly inhibits energy-consuming processes as an evolutionary adaptation to conserve energy. Due to the critical role of astroglial energy homeostasis in the control of neuronal excitability, metabolic therapeutic approaches that prevent the utilization of glucose might represent a potent antiepileptic strategy. In particular, high fat low carbohydrate "ketogenic diets" as well as inhibitors of glycolysis and lactate metabolism are of growing interest for the therapy of epilepsy.

Establishment of a rhesus monkey model of chronic temporal lobe epilepsy using repetitive unilateral intra-amygdala kainic acid injections

OBJECTIVE

Temporal lobe epilepsy (TLE) is a common type of acquired epilepsy refractory to medical treatment. As such, establishing animal models of this disease is critical to developing new and effective treatment modalities. Because of their small head size, rodents are not suitable for comprehensive electroencephalography (EEG) evaluation via scalp or subdural electrodes. Therefore, a larger primate model that closely recapitulates signs of TLE is needed; here we describe a rhesus monkey model resembling chronic TLE.

METHODS

Eight monkeys were divided into two groups: kainic acid (KA) group (n=6) and saline control group (n=2). Intra-amygdala KA injections were performed biweekly via an Ommaya device until obvious epileptiform discharges were recorded. Video-EEG recording was conducted intermittently throughout the experiment using both scalp and subdural electrodes. Brains were then analyzed for Nissl and glial fibrillary acid protein (GFAP) immunostaining.

RESULTS

After 2-4 injections of KA (approximately 1.2-2.4mg, 0.12-0.24mg/kg), interictal epileptiform discharges (IEDs) were recorded in all KA-treated animals. Spontaneous recurrent seizures (SRSs) accompanied by symptoms mimicking temporal lobe absence (undetectable without EEG recording), but few mild motor signs, were recorded in 66.7% (four of six) KA-treated animals. Both IEDs and seizures indicated a primary epileptic zone in the right temporal region and contralateral discharges were later detected. Segmental pyramidal cell loss and gliosis were detected in the brain of a KA-treated monkey.

CONCLUSIONS

Through a modified protocol of unilateral repetitive intra-amygdala KA injections, a rhesus monkey model with similar behavioral and brain electrical features as TLE was developed.

Role of the purinergic signaling in epilepsy

Adenine nucleotides and adenosine are signaling molecules that activate purinergic receptors P1 and P2. Activation of A1 adenosine receptors has an anticonvulsant action, whereas activation of A2A receptors might initiate seizures. Therefore, a significant limitation to the use of A1 receptor agonists as drugs in the CNS might be their peripheral side effects. The anti-epileptic activity of adenosine is related to its increased concentration outside the cell. This increase might result from the inhibition of the equilibrative nucleoside transporters (ENTs). Moreover, the implantation of implants or stem cells into the brain might cause slow and persistent increases in adenosine concentrations in the extracellular spaces of the brain. The role of adenosine in seizure inhibition has been confirmed by results demonstrating that in patients with epilepsy, the adenosine kinase (ADK) present in astrocytes is the only purine-metabolizing enzyme that exhibits increased expression. Increased ADK activity causes intensified phosphorylation of adenosine to 5'-AMP, which therefore lowers the adenosine level in the extracellular spaces. These changes might initiate astrogliosis and epileptogenesis, which are the manifestations of epilepsy. Seizures might induce inflammatory processes and vice versa. Activation of P2X7 receptors causes intensified release of pro-inflammatory cytokines (IL-1β and TNF-α) and activates metabolic pathways that induce inflammatory processes in the CNS. Therefore, antagonists of P2X7 and the interleukin 1β receptor might be efficient drugs for recurring seizures and prolonged status epilepticus. Inhibitors of ADK would simultaneously inhibit the seizures, prevent the astrogliosis and epileptogenesis processes and prevent the formation of new epileptogenic foci. Therefore, these drugs might become beneficial seizure-suppressing drugs.

Translational potential of astrocytes in brain disorders

Fundamentally, all brain disorders can be broadly defined as the homeostatic failure of this organ. As the brain is composed of many different cells types, including but not limited to neurons and glia, it is only logical that all the cell types/constituents could play a role in health and disease. Yet, for a long time the sole conceptualization of brain pathology was focused on the well-being of neurons. Here, we challenge this neuron-centric view and present neuroglia as a key element in neuropathology, a process that has a toll on astrocytes, which undergo complex morpho-functional changes that can in turn affect the course of the disorder. Such changes can be grossly identified as reactivity, atrophy with loss of function and pathological remodeling. We outline the pathogenic potential of astrocytes in variety of disorders, ranging from neurotrauma, infection, toxic damage, stroke, epilepsy, neurodevelopmental, neurodegenerative and psychiatric disorders, Alexander disease to neoplastic changes seen in gliomas. We hope that in near future we would witness glial-based translational medicine with generation of deliverables for the containment and cure of disorders. We point out that such as a task will require a holistic and multi-disciplinary approach that will take in consideration the concerted operation of all the cell types in the brain.

Upregulation of HMGB1, toll-like receptor and RAGE in human Rasmussen's encephalitis

Rasmussen encephalitis (RE) is a rare neurological disorder of childhood characterized by uni-hemispheric inflammation, progressive neurological deficits and intractable focal epilepsy. The pathogenesis of RE is still enigmatic. Activation of endogenous high-mobility group box-1 (HMGB1) and Toll-like receptor (TLR) has been proved to be with pro-inflammatory as well as pro-convulsant effects. We hypothesized that the epileptogenic mechanisms underlying RE are related to activation of HMGB1/TLR signaling. Immunnohistochemistry approach was used to examine the expression of HMGB1, TLR2, TLR4, receptor for advanced glycation end products (RAGE) in surgically resected human epileptic cortical specimens from RE (n=12), and compared that with control cortical issue (n=6). HMGB1 was ubiquitously detected in nuclei of astrocytes while its receptors were not detected in control cortex specimens. Marked expression of the receptors were observed in the lesions of RE. In particular, HMGB1 was in stead detected in cytoplasm of reactive astrocytes in RE cortex, predictive its release from glial cells. Significant greater HMGB1 and its receptors expression in RE vs. control was demonstrated by western blot. These results provide the novel evidence of intrinsic activation of these pro-inflammation pathways in RE, which suggest the specific targets in the treatment of epilepsy associated with RE.

The Biochemistry and Epigenetics of Epilepsy: Focus on Adenosine and Glycine

Epilepsy, one of the most prevalent neurological conditions, presents as a complex disorder of network homeostasis characterized by spontaneous non-provoked seizures and associated comorbidities. Currently used antiepileptic drugs have been designed to suppress neuronal hyperexcitability and thereby to suppress epileptic seizures. However, the current armamentarium of antiepileptic drugs is not effective in over 30% of patients, does not affect the comorbidities of epilepsy, and does not prevent the development and progression of epilepsy (epileptogenesis). Prevention of epilepsy and its progression remains the Holy Grail for epilepsy research and therapy development, requiring novel conceptual advances to find a solution to this urgent medical need. The methylation hypothesis of epileptogenesis suggests that changes in DNA methylation are implicated in the progression of the disease. In particular, global DNA hypermethylation appears to be associated with chronic epilepsy. Clinical as well as experimental evidence demonstrates that epilepsy and its progression can be prevented by biochemical manipulations and those that target previously unrecognized epigenetic functions contributing to epilepsy development and maintenance of the epileptic state. This mini-review will discuss, epigenetic mechanisms implicated in epileptogenesis and biochemical interactions between adenosine and glycine as a conceptual advance to understand the contribution of maladaptive changes in biochemistry as a major contributing factor to the development of epilepsy. New findings based on biochemical manipulation of the DNA methylome suggest that: (i) epigenetic mechanisms play a functional role in epileptogenesis; and (ii) therapeutic reconstruction of the epigenome is an effective antiepileptogenic therapy.

Comorbidities in Neurology: Is adenosine the common link?

Comorbidities in Neurology represent a major conceptual and therapeutic challenge. For example, temporal lobe epilepsy (TLE) is a syndrome comprised of epileptic seizures and comorbid symptoms including memory and psychiatric impairment, depression, and sleep dysfunction. Similarly, Alzheimer's disease (AD), Parkinson's disease (PD), and Amyotrophic Lateral Sclerosis (ALS) are accompanied by various degrees of memory dysfunction. Patients with AD have an increased likelihood for seizures, whereas all four conditions share certain aspects of psychosis, depression, and sleep dysfunction. This remarkable overlap suggests common pathophysiological mechanisms, which include synaptic dysfunction and synaptotoxicity, as well as glial activation and astrogliosis. Astrogliosis is linked to synapse function via the tripartite synapse, but astrocytes also control the availability of gliotransmitters and adenosine. Here we will specifically focus on the 'adenosine hypothesis of comorbidities' implying that astrocyte activation, via overexpression of adenosine kinase (ADK), induces a deficiency in the homeostatic tone of adenosine. We present evidence from patient-derived samples showing astrogliosis and overexpression of ADK as common pathological hallmark of epilepsy, AD, PD, and ALS. We discuss a transgenic 'comorbidity model', in which brain-wide overexpression of ADK and resulting adenosine deficiency produces a comorbid spectrum of seizures, altered dopaminergic function, attentional impairment, and deficits in cognitive domains and sleep regulation. We conclude that dysfunction of adenosine signaling is common in neurological conditions, that adenosine dysfunction can explain co-morbid phenotypes, and that therapeutic adenosine augmentation might be effective for the treatment of comorbid symptoms in multiple neurological conditions.

Adenosinergic signaling in epilepsy

Despite the introduction of at least 20 new antiepileptic drugs (AEDs) into clinical practice over the past decades, about one third of all epilepsies remain refractory to conventional forms of treatment. In addition, currently used AEDs have been developed to suppress neuronal hyperexcitability, but not necessarily to address pathogenic mechanisms involved in epilepsy development or progression (epileptogenesis). For those reasons endogenous seizure control mechanisms of the brain may provide alternative therapeutic opportunities. Adenosine is a well characterized endogenous anticonvulsant and seizure terminator of the brain. Several lines of evidence suggest that endogenous adenosine-mediated seizure control mechanisms fail in chronic epilepsy, whereas therapeutic adenosine augmentation effectively prevents epileptic seizures, even those that are refractory to conventional AEDs. New findings demonstrate that dysregulation of adenosinergic mechanisms are intricately involved in the development of epilepsy and its comorbidities, whereas adenosine-associated epigenetic mechanisms may play a role in epileptogenesis. The first goal of this review is to discuss how maladaptive changes of adenosinergic mechanisms contribute to the expression of seizures (ictogenesis) and the development of epilepsy (epileptogenesis) by focusing on pharmacological (adenosine receptor dependent) and biochemical (adenosine receptor independent) mechanisms as well as on enzymatic and transport based mechanisms that control the availability (homeostasis) of adenosine. The second goal of this review is to highlight innovative adenosine-based opportunities for therapeutic intervention aimed at reconstructing normal adenosine function and signaling for improved seizure control in chronic epilepsy. New findings suggest that transient adenosine augmentation can have lasting epigenetic effects with disease modifying and antiepileptogenic outcome. This article is part of the Special Issue entitled 'Purines in Neurodegeneration and Neuroregeneration'.

Genetic variation in the adenosine regulatory cycle is associated with posttraumatic epilepsy development

OBJECTIVE

Determine if genetic variation in enzymes/transporters influencing extracellular adenosine homeostasis, including adenosine kinase (ADK), [ecto-5'-nucleotidase (NT5E), cluster of differentiation 73 (CD73)], and equilibrative nucleoside transporter type-1 (ENT-1), is significantly associated with epileptogenesis and posttraumatic epilepsy (PTE) risk, as indicated by time to first seizure analyses.

METHODS

Nine ADK, three CD73, and two ENT-1 tagging single nucleotide polymorphisms (SNPs) were genotyped in 162 white adults with moderate/severe traumatic brain injury (TBI) and no history of premorbid seizures. Kaplan-Meier models were used to screen for genetic differences in time to first seizure occurring >1 week post-TBI. SNPs remaining significant after correction for multiple comparisons were examined using Cox proportional hazards analyses, adjusting for subdural hematoma, injury severity score, and isolated TBI status. SNPs significant in multivariate models were then entered simultaneously into an adjusted Cox model.

RESULTS

Comparing Kaplan-Meier curves, rs11001109 (ADK) rare allele homozygosity and rs9444348 (NT5E) heterozygosity were significantly associated with shorter time to first seizure and an increased seizure rate 3 years post-TBI. Multivariate Cox proportional hazard models showed that these genotypes remained significantly associated with increased PTE hazard up to 3 years post-TBI after controlling for variables of interest (rs11001109: hazard ratio (HR) 4.47, 95% confidence interval (CI) 1.27-15.77, p = 0.020; rs9444348: HR 2.95, 95% CI 1.19-7.31, p = 0.019) .

SIGNIFICANCE

Genetic variation in ADK and NT5E may help explain variability in time to first seizure and PTE risk, independent of previously identified risk factors, after TBI. Once validated, identifying genetic variation in adenosine regulatory pathways relating to epileptogenesis and PTE may facilitate exploration of therapeutic targets and pharmacotherapy development.

Adenosine kinase expression in cortical dysplasia with balloon cells: analysis of developmental lineage of cell types

Focal cortical dysplasia type IIB (FCDIIB) is a developmental malformation of the cerebral cortex that is associated with pharmacoresistant epilepsy. Overexpression of adenosine kinase (ADK) has been regarded as a pathologic hallmark of epilepsy. We hypothesized that the epileptogenic mechanisms underlying FCDIIB are related to abnormal ADK expression. We used immunohistochemistry to examine the expression of ADK and of heterogeneous cell population markers of astrocytes (glial fibrillary acidic protein), immature glia (vimentin), immature neurons (neuronal class III beta-tubulin, TUJ1), multipotential progenitor cells (nestin), mature neurons (microtubule-associated protein 2), and antiapoptotic gene products (Bcl-2) in surgically resected human epileptic cortical specimens from FCDIIB patients (n = 20). Expression patterns were compared with those in normal autopsy (n = 6) and surgical control (n = 6) brain samples. Balloon cells in FCDII lesions were immunoreactive for ADK (77%) and balloon cells expressing the different cell markers expressing different degrees of ADK. Adenosine kinase expression assessed by Western blot and enzymatic activity were also greater in FCD versus control samples. These results suggest that upregulation of ADK is a common pathologic component of FCDIIB. Adenosine kinase might, therefore, be a target in the treatment of epilepsy associated with FCD.

Adenosine kinase, glutamine synthetase and EAAT2 as gene therapy targets for temporal lobe epilepsy

Astrocytes are an attractive cell target for gene therapy, but the validation of new therapeutic candidates is needed. We determined whether adeno-associated viral (AAV) vector-mediated overexpression of glutamine synthetase (GS) or excitatory amino-acid transporter 2 (EAAT2), or expression of microRNA targeting adenosine kinase (miR-ADK) in hippocampal astrocytes in the rat brain could modulate susceptibility to kainate-induced seizures and neuronal cell loss. Transgene expression was found predominantly in astrocytes following direct injection of glial-targeting AAV9 vectors by 3 weeks postinjection. ADK expression in miR-ADK vector-injected rats was reduced by 94-96% and was associated with an ~50% reduction in the duration of kainate-induced seizures and greater protection of dentate hilar neurons but not CA3 neurons compared with miR-control vector-injected rats. In contrast, infusion of AAV-GS and EAAT2 vectors did not afford any protection against seizures or neuronal damage as the level of transcriptional activity of the glial fibrillary acidic promoter was too low to drive any significant increase in transgenic GS or EAAT2 relative to the high endogenous levels of these proteins. Our findings support ADK as a prime therapeutic target for gene therapy of temporal lobe epilepsy and suggest that alternative approaches including the use of stronger glial promoters are needed to increase transgenic GS and EAAT2 expression to levels that may be required to affect seizure induction and propagation.

Ginseng Rb fraction protects glia, neurons and cognitive function in a rat model of neurodegeneration

The loss and injury of neurons play an important role in the onset of various neurodegenerative diseases, while both microgliosis and astrocyte loss or dysfunction are significant causes of neuronal degeneration. Previous studies have suggested that an extract enriched panaxadiol saponins from ginseng has more neuroprotective potential than the total saponins of ginseng. The present study investigated whether a fraction of highly purified panaxadiol saponins (termed as Rb fraction) was protective for both glia and neurons, especially GABAergic interneurons, against kainic acid (KA)-induced excitotoxicity in rats. Rats received Rb fraction at 30 mg/kg (i.p.), 40 mg/kg (i.p. or saline followed 40 min later by an intracerebroventricular injection of KA. Acute hippocampal injury was determined at 48 h after KA, and impairment of hippocampus-dependent learning and memory as well as delayed neuronal injury was determined 16 to 21 days later. KA injection produced significant acute hippocampal injuries, including GAD67-positive GABAergic interneuron loss in CA1, paralbumin (PV)-positive GABAergic interneuron loss, pyramidal neuron degeneration and astrocyte damage accompanied with reactive microglia in both CA1 and CA3 regions of the hippocampus. There was also a delayed loss of GAD67-positive interneurons in CA1, CA3, hilus and dentate gyrus. Microgliosis also became more severe 21 days later. Accordingly, KA injection resulted in hippocampus-dependent spatial memory impairment. Interestingly, the pretreatment with Rb fraction at 30 or 40 mg/kg significantly protected the pyramidal neurons and GABAergic interneurons against KA-induced acute excitotoxicity and delayed injury. Rb fraction also prevented memory impairments and protected astrocytes from KA-induced acute excitotoxicity. Additionally, microglial activation, especially the delayed microgliosis, was inhibited by Rb fraction. Overall, this study demonstrated that Rb fraction protected both astrocytes and neurons, especially GABAergic interneurons, and maintained microglial homeostasis against KA-induced excitotoxicity. Therefore, Rb fraction has the potential to prevent and treat neurodegenerative diseases.

Upregulation of adenosine kinase in Rasmussen encephalitis

Rasmussen encephalitis (RE) is a rare neurologic disorder of childhood characterized by unihemispheric inflammation, progressive neurologic deficits, and intractable focal epilepsy. The pathogenesis of RE is still enigmatic. Adenosine is a key endogenous signaling molecule with anticonvulsive and anti-inflammatory effects, and our previous work demonstrated that dysfunction of the adenosine kinase (ADK)–adenosine system and astrogliosis are the hallmarks of epilepsy. We hypothesized that the epileptogenic mechanisms underlying RE are related to changes in ADK expression and that those changes might be associated with the development of epilepsy in RE patients. Immunohistochemistry was used to examine the expression of ADK and glial fibrillary acidic protein in surgically resected human epileptic cortical specimens from RE patients (n = 12) and compared with control cortical tissues (n = 6). Adenosine kinase expression using Western blot and enzymatic activity for ADK were assessed in RE versus control samples. Focal astrogliosis and marked expression of ADK were observed in the lesions of RE. Significantly greater ADK expression in RE versus controls was demonstrated by Western blot, and greater enzymatic activity for ADK was demonstrated using an enzyme-coupled bioluminescent assay. These results suggest that upregulation of ADK is a common pathologic hallmark of RE and that ADK might be a target in the treatment of epilepsy associated with RE.

Glial adenosine kinase--a neuropathological marker of the epileptic brain

Experimental research over the past decade has supported the critical role of astrocytes activated by different types of injury and the pathophysiological processes that underlie the development of epilepsy. In both experimental and human epileptic tissues astrocytes undergo complex changes in their physiological properties, which can alter glio-neuronal communication, contributing to seizure precipitation and recurrence. In this context, understanding which of the molecular mechanisms are crucially involved in the regulation of glio-neuronal interactions under pathological conditions associated with seizure development is important to get more insight into the role of astrocytes in epilepsy. This article reviews current knowledge regarding the role of glial adenosine kinase as a neuropathological marker of the epileptic brain. Both experimental findings in clinically relevant models, as well as observations in drug-resistant human epilepsies will be discussed, highlighting the link between astrogliosis, dysfunction of adenosine homeostasis and seizure generation and therefore suggesting new strategies for targeting astrocyte-mediated epileptogenesis.

Overexpression of adenosine kinase in cortical astrocytes and focal neocortical epilepsy in mice

OBJECT

New experimental models and diagnostic methods are needed to better understand the pathophysiology of focal neocortical epilepsies in a search for improved epilepsy treatment options. The authors hypothesized that a focal disruption of adenosine homeostasis in the neocortex might be sufficient to trigger electrographic seizures. They further hypothesized that a focal disruption of adenosine homeostasis might affect microcirculation and thus offer a diagnostic opportunity for the detection of a seizure focus located in the neocortex.

METHODS

Focal disruption of adenosine homeostasis was achieved by injecting an adeno-associated virus (AAV) engineered to overexpress adenosine kinase (ADK), the major metabolic clearance enzyme for the brain's endogenous anticonvulsant adenosine, into the neocortex of mice. Eight weeks following virus injection, the affected brain area was imaged via optical microangiography (OMAG) to detect changes in microcirculation. After completion of imaging, cortical electroencephalography (EEG) recordings were obtained from the imaged brain area.

RESULTS

Viral expression of the Adk cDNA in astrocytes generated a focal area (~ 2 mm in diameter) of ADK overexpression within the neocortex. OMAG scanning revealed a reduction in vessel density within the affected brain area of approximately 23% and 29% compared with control animals and the contralateral hemisphere, respectively. EEG recordings revealed electrographic seizures within the focal area of ADK overexpression at a rate of 1.3 ± 0.2 seizures per hour (mean ± SEM).

CONCLUSIONS

The findings of this study suggest that focal adenosine deficiency is sufficient to generate a neocortical focus of hyperexcitability, which is also characterized by reduced vessel density. The authors conclude that their model constitutes a useful tool to study neocortical epilepsies and that OMAG constitutes a noninvasive diagnostic tool for the imaging of seizure foci with disrupted adenosine homeostasis.

The kainic acid model of temporal lobe epilepsy

The kainic acid model of temporal lobe epilepsy has greatly contributed to the understanding of the molecular, cellular and pharmacological mechanisms underlying epileptogenesis and ictogenesis. This model presents with neuropathological and electroencephalographic features that are seen in patients with temporal lobe epilepsy. It is also characterized by a latent period that follows the initial precipitating injury (i.e., status epilepticus) until the appearance of recurrent seizures, as observed in the human condition. Finally, the kainic acid model can be reproduced in a variety of species using either systemic, intrahippocampal or intra-amygdaloid administrations. In this review, we describe the various methodological procedures and evaluate their differences with respect to the behavioral, electroencephalographic and neuropathological correlates. In addition, we compare the kainic acid model with other animal models of temporal lobe epilepsy such as the pilocarpine and the kindling model. We conclude that the kainic acid model is a reliable tool for understanding temporal lobe epilepsy, provided that the differences existing between methodological procedures are taken into account.

Ceftriaxone treatment after traumatic brain injury restores expression of the glutamate transporter, GLT-1, reduces regional gliosis, and reduces post-traumatic seizures in the rat

Excessive extracellular glutamate after traumatic brain injury (TBI) contributes to excitotoxic cell death and likely to post-traumatic epilepsy. Glutamate transport is the only known mechanism of extracellular glutamate clearance, and glutamate transporter 1 (GLT-1) is the major glutamate transporter of the mammalian brain. We tested, by immunoblot, in the rat lateral fluid percussion injury TBI model whether GLT-1 expression is depressed in the cortex after TBI, and whether GLT-1 expression after TBI is restored after treatment with ceftriaxone, a well-tolerated β-lactam antibiotic previously shown to enhance GLT-1 expression in noninjured animals. We then tested whether treatment with ceftriaxone mitigates the associated regional astrogliosis, as reflected by glial fibrillary acid protein (GFAP) expression, and also whether ceftriaxone treatment mitigates the severity of post-traumatic epilepsy. We found that 7 days after TBI, GLT-1 expression in the ipsilesional cortex was reduced by 29% (n=7/group; p<0.01), relative to the contralesional cortex. However, the loss of GLT-1 expression was reversed by treatment with ceftriaxone (200 mg/kg, daily, intraperitoneally). We found that ceftriaxone treatment also decreased the level of regional GFAP expression by 43% in the lesioned cortex, relative to control treatment with saline (n=7 per group; p<0.05), and, 12 weeks after injury, reduced cumulative post-traumatic seizure duration (n=6 rats in the ceftriaxone treatment group and n=5 rats in the saline control group; p<0.001). We cautiously conclude that our data suggest a potential role for ceftriaxone in treatment of epileptogenic TBI.

Adenosine kinase: exploitation for therapeutic gain

Adenosine kinase (ADK; EC 2.7.1.20) is an evolutionarily conserved phosphotransferase that converts the purine ribonucleoside adenosine into 5'-adenosine-monophosphate. This enzymatic reaction plays a fundamental role in determining the tone of adenosine, which fulfills essential functions as a homeostatic and metabolic regulator in all living systems. Adenosine not only activates specific signaling pathways by activation of four types of adenosine receptors but it is also a primordial metabolite and regulator of biochemical enzyme reactions that couple to bioenergetic and epigenetic functions. By regulating adenosine, ADK can thus be identified as an upstream regulator of complex homeostatic and metabolic networks. Not surprisingly, ADK dysfunction is involved in several pathologies, including diabetes, epilepsy, and cancer. Consequently, ADK emerges as a rational therapeutic target, and adenosine-regulating drugs have been tested extensively. In recent attempts to improve specificity of treatment, localized therapies have been developed to augment adenosine signaling at sites of injury or pathology; those approaches include transplantation of stem cells with deletions of ADK or the use of gene therapy vectors to downregulate ADK expression. More recently, the first human mutations in ADK have been described, and novel findings suggest an unexpected role of ADK in a wider range of pathologies. ADK-regulating strategies thus represent innovative therapeutic opportunities to reconstruct network homeostasis in a multitude of conditions. This review will provide a comprehensive overview of the genetics, biochemistry, and pharmacology of ADK and will then focus on pathologies and therapeutic interventions. Challenges to translate ADK-based therapies into clinical use will be discussed critically.

Ketogenic diet reduces Smac/Diablo and cytochrome c release and attenuates neuronal death in a mouse model of limbic epilepsy

The ketogenic diet (KD) is effective in the treatment of refractory epilepsy, yet the molecular mechanisms underlying its antiepileptic effects have not been determined. There is increasing evidence that neuronal cell death induced by seizures via mitochondrial pathway and seizures can lead to mitochondrial release of cytochrome c, and we have shown previously that translocation of Smac/DIABLO into the cytosol play a role in the brain damage in a model of limbic seizure. In the present study, we explored the neuroprotective effect of KD in C57BL/6 mice with seizures induced by kainic acid (KA). Status epilepticus triggered by intra-amygdaloid microinjection of KA lead to neuronal death in the selective ipsilateral CA3 subfield of the hippocampus and mitochondrial release of Smac/DIABLO and cytochrome c. We found that KD significantly decreased neuronal death in the ipsilateral CA3 at 24h after KA-induced seizures. Furthermore, KD reduced Smac/DIABLO and cytochrome c release from mitochondria, attenuated activation of casepase-9 and caspase-3 following seizures. These results demonstrate that the neuroprotective effect of KD against brain injury induced by limbic seizures, at least partially, is associated with inhibition of mitochondrial release of Smac/DIABLO and cytochrome c.

Neuronal adenosine release, and not astrocytic ATP release, mediates feedback inhibition of excitatory activity

Adenosine is a potent anticonvulsant acting on excitatory synapses through A1 receptors. Cellular release of ATP, and its subsequent extracellular enzymatic degradation to adenosine, could provide a powerful mechanism for astrocytes to control the activity of neural networks during high-intensity activity. Despite adenosine's importance, the cellular source of adenosine remains unclear. We report here that multiple enzymes degrade extracellular ATP in brain tissue, whereas only Nt5e degrades AMP to adenosine. However, endogenous A1 receptor activation during cortical seizures in vivo or heterosynaptic depression in situ is independent of Nt5e activity, and activation of astrocytic ATP release via Ca(2+) photolysis does not trigger synaptic depression. In contrast, selective activation of postsynaptic CA1 neurons leads to release of adenosine and synaptic depression. This study shows that adenosine-mediated synaptic depression is not a consequence of astrocytic ATP release, but is instead an autonomic feedback mechanism that suppresses excitatory transmission during prolonged activity.

Adenosine dysfunction in epilepsy

Extracellular levels of the brain's endogenous anticonvulsant and neuroprotectant adenosine largely depend on an astrocyte-based adenosine cycle, comprised of ATP release, rapid degradation of ATP into adenosine, and metabolic reuptake of adenosine through equilibrative nucleoside transporters and phosphorylation by adenosine kinase (ADK). Changes in ADK expression and activity therefore rapidly translate into changes of extracellular adenosine, which exerts its potent anticonvulsive and neuroprotective effects by activation of pre- and postsynaptic adenosine A(1) receptors. Increases in ADK increase neuronal excitability, whereas decreases in ADK render the brain resistant to seizures and injury. Importantly, ADK was found to be overexpressed and associated with astrogliosis and spontaneous seizures in rodent models of epilepsy, as well as in human specimen resected from patients with hippocampal sclerosis and temporal lobe epilepsy. Several lines of evidence indicate that overexpression of astroglial ADK and adenosine deficiency are pathological hallmarks of the epileptic brain. Consequently, adenosine augmentation therapies constitute a powerful approach for seizure prevention, which is effective in models of epilepsy that are resistant to conventional antiepileptic drugs. The adenosine kinase hypothesis of epileptogenesis suggests that adenosine dysfunction in epilepsy undergoes a biphasic response: an acute surge of adenosine that can be triggered by any type of injury might contribute to the development of astrogliosis via adenosine receptor-dependent and -independent mechanisms. Astrogliosis in turn is associated with overexpression of ADK, which was shown to be sufficient to trigger spontaneous recurrent electrographic seizures. Thus, ADK emerges as a promising target for the prediction and prevention of epilepsy.