Recent advances in the neuropathology of focal lesions in epilepsy

From bedside to bench: New insights in epilepsy-associated tumors based on recent classification updates and animal models on brain tumor networks

Low-grade neuroepithelial tumors (LGNTs), particularly those with glioneuronal histology, are highly associated with pharmacoresistant epilepsy. Increasing research focused on these neoplastic lesions did not translate into drug discovery; and anticonvulsant or antitumor therapies are not available yet. During the last years, animal modeling has improved, thereby leading to the possibility of generating brain tumors in mice mimicking crucial genetic, molecular and immunohistological features. Among them, intraventricular in utero electroporation (IUE) has been proven to be a valuable tool for the generation of animal models for LGNTs allowing endogenous tumor growth within the mouse brain parenchyma. Epileptogenicity is mostly determined by the slow-growing patterns of these tumors, thus mirroring intrinsic interactions between tumor cells and surrounding neurons is crucial to investigate the mechanisms underlying convulsive activity. In this review, we provide an updated classification of the human LGNT and summarize the most recent data from human and animal models, with a focus on the crosstalk between brain tumors and neuronal function.

The role of astrocytes in epileptic disorders

Epilepsy affects about 1% of the population and approximately 30% of epileptic patients are resistant to current antiepileptic drugs. As a hallmark in epileptic tissue, many of the epileptic patients show changes in glia morphology and function. There are characteristic changes in different types of glia in different epilepsy models. Some of these changes such as astrogliosis are enough to provoke epileptic seizures. Astrogliosis is well known in mesial temporal lobe epilepsy (MTLE), the most common form of refractory epilepsy. A better understanding of astrocytes alterations could lead to novel and efficient pharmacological approaches for epilepsy. In this review, we present the alterations of astrocyte morphology and function and present some instances of targeting astrocytes in seizure and epilepsy.

The Neuroprotective Effect of miR-136 on Pilocarpine-Induced Temporal Lobe Epilepsy Rats by Inhibiting Wnt/β-Catenin Signaling Pathway

Objective

To explore the effect of miR-136 on temporal lobe epilepsy (Ep) and its mechanism of action.

Methods

30 male rats were injected intraperitoneally with 30 mg/kg pilocarpine to construct a rat temporal lobe epilepsy model, and they were randomly divided into 5 groups (n = 6 per group): control group, Ep group, agomir NC group, miR-136 agomir group, and miR-136+LiCl group. The brain tissues of the rats were collected 7 days after the treatment. The expression of miR-136 in the hippocampus tissue was detected by qRT-PCR. H&E and Nissl staining were used to observe the histopathological changes and neuron damage in the hippocampus tissue. IL-1β, IL-6, and TNF-α levels in the hippocampus tissue were detected by ELISA. Flow cytometry was used to detect the apoptosis rate in the hippocampus tissue. Western blot was used to detect the expression levels of c-Caspase-3, Bcl-2, β-catenin, Cyclin D1, and c-myc protein in the hippocampus.

Results

The expression of miR-136 was significantly downregulated in the hippocampus tissue of epileptic rats. After overexpression of miR-136, the number of seizures and the duration of epilepsy in rats were significantly reduced. At the same time, hippocampal tissue damage was improved considerably, and the degree of neuronal damage decreased. Overexpression of miR-136 also significantly reduced the apoptosis rate in the hippocampus tissue and inhibited the levels of inflammatory factors. Meanwhile, miR-136 downregulates the expression of Wnt/β-catenin signaling pathway-related proteins. However, Wnt pathway activator LiCl could destroy the protective effect of miR-136.

Conclusion

miR-136 could exert its neuroprotective influence on temporal lobe epilepsy rats by inhibiting the Wnt/β-catenin signaling pathway.

Study of Neuronal Apoptosis ceRNA Network in Hippocampal Sclerosis of Human Temporal Lobe Epilepsy by RNA-Seq

Hippocampal sclerosis (HS) is one of the most common pathological type of intractable temporal lobe epilepsy (TLE), often characterized by hippocampal atrophy, neuronal apoptosis, and gliogenesis. However, the molecular mechanisms of neuronal apoptosis in patients with HS are still not fully understood. We therefore conducted a pilot study focusing on the neuronal apoptosis ceRNA network in the sclerotic hippocampus of intractable TLE patients. In this research, RNA sequencing (RNA-seq) was utilized to quantify the expression levels of lncRNAs, miRNAs, and mRNAs in TLE patients with HS (HS-TLE) and without HS (non-HS-TLE), and reverse transcription-quantitative PCR (qRT-PCR). The interactions of differential expression (DE) lncRNAs-miRNAs or DEmiRNAs-mRNAs were integrated by StarBase v3.0, and visualized using Cytoscape. Subsequently, we annotate the functions of lncRNA-associated competitive endogenous RNA (ceRNA) network through analysis of their interactions with mRNAs. RNA-seq analyses showed 381 lncRNAs, 42 miRNAs, and 457 mRNAs were dysregulated expression in HS-TLE compared to non-HS-TLE. According to the ceRNA hypothesis, 5 HS-specific ceRNA network were constructed. Among them, the core ceRNA regulatory network involved in neuronal apoptosis was constituted by 10 DElncRNAs (CDKN2B-AS1, MEG3, UBA6-AS1, etc.), 7 DEmiRNAs (hsa-miR-155-5p, hsa-miR-195-5p, hsa-miR-200c-3p, etc.), and 3 DEmRNAs (SCN2A, DYRK2, and MAPK8), which belonging to apoptotic and epileptic terms. Our findings established the first ceRNA network of lncRNA-mediated neuronal apoptosis in HS-TLE based on transcriptome sequencing, which provide a new perspective on the disease pathogenesis and precise treatments of HS.

Inhibition of miR-181a-5p reduces astrocyte and microglia activation and oxidative stress by activating SIRT1 in immature rats with epilepsy

MicroRNAs regulate gene expression at the posttranscriptional level, and this process has been shown to be implicated in the pathological processes of temporal lobe epilepsy. At present, studies about the impact of microRNA-181a (miR-181a) on epilepsy have focused on hippocampal neurons, and the effect of miR-181a on other cells in the hippocampus remains poorly understood. Herein, we explored the role of miR-181a-5p in a lithium-pilocarpine model of epilepticus in immature rats. We found that the hippocampal expression level of miR-181a-5p was increased. Inhibition of miR-181a-5p protected the hippocampus against epilepsy, including hippocampal insults, neuronal apoptosis, astrocyte and microglia activation, neuroinflammation, oxidative stress, mitochondrial function, and cognitive dysfunction. Moreover, miR-181a-5p inhibition exerted a seizure-suppressing effect via SIRT1 upregulation. Overall, our findings reveal the potential role of the miR-181a-5p/SIRT1 pathway in the development of temporal lobe epilepsy, and this pathway may represent a novel target for ameliorating epilepsy and its sequelae.

MicroRNA-15a-5p serves as a potential biomarker and regulates the viability and apoptosis of hippocampus neuron in children with temporal lobe epilepsy

BACKGROUND

Temporal lobe epilepsy (TLE) is the most common type of epilepsy, usually starting in childhood. The dysregulation of microRNAs (miRNAs) has been identified in neurological disorders. The current study investigated the expression level and clinical significance of miR-15a-5p in TLE children, and explored its function in regulating cell behaviors of hippocampal neurons.

METHODS

The expression level of miR-15a-5p was examined in the serum of 63 TLE children. Primary hippocampal cells were cultured in magnesium-free medium to mimic TLE condition in children. The effect of miR-15a-5p on the viability and apoptosis of hippocampus neuron was assessed using MTT or flow cytometric apoptosis assay.

RESULTS

TLE children had significantly low expression of miR-15a-5p. MiR-15a-5p was of great value for the diagnosis of TLE in children, with high specificity and sensitivity. The expression level of miR-15a-5p was decreased significantly in hippocampal cells treated in the magnesium-free medium. Overexpression of miR-15a-5p attenuated TLE-induced reduction for cell viability, and reversed the cell apoptosis induced by TLE.

CONCLUSIONS

MiR-15a-5p is downregulated in children with TLE, and overexpression of miR-15a-5p promoted the viability and inhibited the apoptosis of hippocampal neuron. MiR-15a-5p may be a promising biomarker for the diagnosis of children TLE.

Coding and non-coding transcriptome of mesial temporal lobe epilepsy: Critical role of small non-coding RNAs

Our understanding of mesial temporal lobe epilepsy (MTLE), one of the most common form of drug-resistant epilepsy in humans, is derived mainly from clinical, imaging, and physiological data from humans and animal models. High-throughput gene expression studies of human MTLE have the potential to uncover molecular changes underlying disease pathogenesis along with novel therapeutic targets. Using RNA- and small RNA-sequencing in parrallel, we explored differentially expressed genes in the hippocampus and cortex of MTLE patients who had undergone surgical resection and non-epileptic controls. We identified differentially expressed genes in the hippocampus of MTLE patients and differentially expressed small RNAs across both the cortex and hippocampus. We found significant enrichment for astrocytic and microglial genes among up-regulated genes, and down regulation of neuron specific genes in the hippocampus of MTLE patients. The transcriptome profile of the small RNAs reflected disease state more robustly than mRNAs, even across brain regions which show very little pathology. While mRNAs segregated predominately by brain region for MTLE and controls, small RNAs segregated by disease state. In particular, our data suggest that specific miRNAs (e.g., let-7b-3p and let-7c-3p) may be key regulators of multiple pathways related to MTLE pathology. Further, we report a strong association of other small RNA species with MTLE pathology. As such we have uncovered novel elements that may contribute to the establishment and progression of MTLE pathogenesis and that could be leveraged as therapeutic targets.

MiR-153 regulates expression of hypoxia-inducible factor-1α in refractory epilepsy

Mesial temporal lobe epilepsy (mTLE), the most common type of temporal lobe epilepsy (TLE), is particularly relevant due to its high frequency of therapeutic resistance of anti-epileptic therapies. MicroRNAs (miRNAs) have been shown to be dysregulated in epilepsy and neurodegenerative diseases, and we hypothesized that miRNAs could be involved in the pathogenesis of MTLE. The present study aimed to explore the expression and functions of miRNA-153 in mTLE. The expression levels of miRNA-153 in refractory TLE patients were evaluated. The bioinformatics analysis showed that the potential target genes of miR-153 were involved in biological processes, molecular functions, and cellular components. miRNA-153 is significantly dysregulated in temporal cortex and plasma of mTLE patients. We identify HIF-1α as a direct target of miRNA-153, and luciferase reporter assays demonstrated that miR-153 could regulate the HIF-1αexpression via 3'-UTR pairing. These data suggest that miR-153 might represent a useful biomarker and treatment target for patients with mTLE.

Seizure activity per se does not induce tissue damage markers in human neocortical focal epilepsy

OBJECTIVE

The contribution of recurring seizures to the progression of epileptogenesis is debated. Seizure-induced brain damage is not conclusively demonstrated either in humans or in animal models of epilepsy. We evaluated the expression of brain injury biomarkers on postsurgical brain tissue obtained from 20 patients with frequent seizures and a long history of drug-resistant focal epilepsy.

METHODS

The expression patterns of specific glial, neuronal, and inflammatory molecules were evaluated by immunohistochemistry in the core of type II focal cortical dysplasias (FCD-II), at the FCD boundary (perilesion), and in the adjacent normal-appearing area included in the epileptogenic region. We also analyzed surgical specimens from cryptogenic patients not presenting structural alterations at imaging.

RESULTS

Astroglial and microglial activation, reduced neuronal density, perivascular CD3-positive T-lymphocyte clustering, and fibrinogen extravasation were demonstrated in the core of FCD-II lesions. No pathological immunoreactivity was observed outside the FCD-II or in cryptogenetic specimens, where the occurrence of interictal and ictal epileptiform activity was confirmed by either stereo-electroencephalography or intraoperative electrocorticography.

INTERPRETATION

Recurrent seizures do not induce the expression of brain damage markers in nonlesional epileptogenic cortex studied in postsurgical tissue from cryptogenic and FCD patients. This evidence argues against the hypothesis that epileptiform activity per se contributes to focal brain injury, at least in the neocortical epilepsies considered here. Ann Neurol 2017;82:331-341.

Neuropathology of epilepsy

Epilepsy is one of the most common neurologic disorders, affecting about 50 million people worldwide. The disease is characterized by recurrent seizures, which are due to aberrant neuronal networks resulting in synchronous discharges. The term epilepsy encompasses a large spectrum of syndromes and diseases with different etiopathogenesis. The recent development of imaging and epilepsy surgery techniques is now enabling the identification of structural abnormalities that are part of the epileptic network, and the removal of these lesions may result in control of seizures. Access of this clinically well-characterized neurosurgical material has provided neuropathologists with the opportunity to study a variety of structural brain abnormalities associated with epilepsy, by combining traditional routine histopathologic methods with molecular genetics and functional analysis of the resected tissue. This approach has contributed greatly to a better diagnosis and classification of these structural lesions, and has provided important new insights into their pathogenesis and epileptogenesis. The present chapter provides a detailed description of the large spectrum of histopathologic findings encountered in epilepsy surgery patients, addressing in particular the nonneoplastic pathologies, including hippocampal sclerosis, malformations of cortical development, Sturge-Weber syndrome, and Rasmussen encephalitis, and reviews current knowledge regarding the underlying molecular pathomechanisms and cellular mechanisms mediating hyperexcitability.

Modulation of miR-146a/complement factor H-mediated inflammatory responses in a rat model of temporal lobe epilepsy

Increasing evidence supports the involvement of inflammatory and immune processes in temporal lobe epilepsy (TLE). miRNAs represent small regulatory RNA molecules that have been shown to act as negative regulators of gene expression controlling different biological processes, including immune system homoeostasis and function. We investigated the expression and cellular distribution of miRNA-146a (miR-146a) in a rat model of TLE. Prominent up-regulation of miR-146a activation was evident in 1 week after status epilepticus (SE) and persisted in the chronic phase. The predicted miR-146a's target complement factor H (CFH) mRNA and protein expression was also down-regulated in TLE rat model. Furthermore, transfection of miR-146a mimics in neuronal and glial cells down-regulated CFH mRNA and protein levels respectively. Luciferase reporter assays demonstrated that miR-146a down-regulated CFH mRNA expression via 3'-UTR pairing. Down-regulating miR-146a by intracerebroventricular injection of antagomir-146a enhanced the hippocampal expression of CFH in TLE model and decreased seizure susceptibility. These findings suggest that immunopathological deficits associated with TLE can in part be explained by a generalized miR-146a-mediated down-regulation of CFH that may contribute to epileptogenesis in a rat model of TLE.

Differential expression of miR-184 in temporal lobe epilepsy patients with and without hippocampal sclerosis - Influence on microglial function

Epilepsy is one of the most common neurological disorders characterized by recurrent seizures due to neuronal hyperexcitability. Here we compared miRNA expression patterns in mesial temporal lobe epilepsy with and without hippocampal sclerosis (mTLE + HS and mTLE −HS) to investigate the regulatory mechanisms differentiating both patient groups. Whole genome miRNA sequencing in surgically resected hippocampi did not reveal obvious differences in expression profiles between the two groups of patients. However, one microRNA (miR-184) was significantly dysregulated, which was confirmed by qPCR. We observed that overexpression of miR-184 inhibited cytokine release after LPS stimulation in primary microglial cells, while it did not affect the viability of murine primary neurons and primary astrocytes. Pathway analysis revealed that miR-184 is potentially involved in the regulation of inflammatory signal transduction and apoptosis. Dysregulation of some the potential miR-184 target genes was confirmed by qPCR and 3′UTR luciferase reporter assay. The reduced expression of miR-184 observed in patients with mTLE + HS together with its anti-inflammatory effects indicate that miR-184 might be involved in the modulation of inflammatory processes associated with hippocampal sclerosis which warrants further studies elucidating the role of miR-184 in the pathophysiology of mTLE.

SOX11 identified by target gene evaluation of miRNAs differentially expressed in focal and non-focal brain tissue of therapy-resistant epilepsy patients

MicroRNAs (miRNAs) are small non-coding RNAs that post-transcriptionally control the expression of their target genes via RNA interference. There is increasing evidence that expression of miRNAs is dysregulated in neuronal disorders, including epilepsy, a chronic neurological disorder characterized by spontaneous recurrent seizures. Mesial temporal lobe epilepsy (MTLE) is a common type of focal epilepsy in which disease-induced abnormalities of hippocampal neurogenesis in the subgranular zone as well as gliosis and neuronal cell loss in the cornu ammonis area are reported. We hypothesized that in MTLE altered miRNA-mediated regulation of target genes could be involved in hippocampal cell remodeling. A miRNA screen was performed in hippocampal focal and non-focal brain tissue samples obtained from the temporal neocortex (both n=8) of MTLE patients. Out of 215 detected miRNAs, two were differentially expressed (hsa-miR-34c-5p: mean increase of 5.7 fold (p=0.014), hsa-miR-212-3p: mean decrease of 76.9% (p=0.0014)). After in-silico target gene analysis and filtering, reporter gene assays confirmed RNA interference for hsa-miR-34c-5p with 3'-UTR sequences of GABRA3, GRM7 and GABBR2 and for hsa-miR-212-3p with 3'-UTR sequences of SOX11, MECP2, ADCY1 and ABCG2. Reporter gene assays with mutated 3'-UTR sequences of the transcription factor SOX11 identified two different binding sites for hsa-miR-212-3p and its primary transcript partner hsa-miR-132-3p. Additionally, there was an inverse time-dependent expression of Sox11 and miR-212-3p as well as miR-132-3p in rat neonatal cortical neurons. Transfection of neurons with anti-miRs for miR-212-3p and miR-132-3p suggest that both miRNAs work synergistically to control Sox11 expression. Taken together, these results suggest that differential miRNA expression in neurons could contribute to an altered function of the transcription factor SOX11 and other genes in the setting of epilepsy, resulting not only in impaired neural differentiation, but also in imbalanced neuronal excitability and accelerated drug export.

Mapping of cognitive functions in chronic intractable epilepsy: Role of fMRI

Background:

Functional magnetic resonance imaging (fMRI), a non-invasive technique with high spatial resolution and blood oxygen level dependent (BOLD) contrast, has been applied to localize and map cognitive functions in the clinical condition of chronic intractable epilepsy.

Purpose:

fMRI was used to map the language and memory network in patients of chronic intractable epilepsy pre- and post-surgery.

Materials and Methods:

After obtaining approval from the institutional ethics committee, six patients with intractable epilepsy with an equal number of age-matched controls were recruited in the study. A 1.5 T MR scanner with 12-channel head coil, integrated with audio-visual fMRI accessories was used. Echo planar imaging sequence was used for BOLD studies. There were two sessions in TLE (pre- and post-surgery).

Results:

In TLE patients, BOLD activation increased post-surgery in comparison of pre-surgery in inferior frontal gyrus (IFG), middle frontal gyrus (MFG), and superior temporal gyrus (STG), during semantic lexical, judgment, comprehension, and semantic memory tasks.

Conclusion:

Functional MRI is useful to study the basic concepts related to language and memory lateralization in TLE and guide surgeons for preservation of important brain areas during ATLR. This will help in understanding future directions for the diagnosis and treatment of such disease.

Hippocampus, hippocampal sclerosis and epilepsy

Hippocampal sclerosis (HS) is considered one of the major pathogenic factors of drug-resistant temporal lobe epilepsy. HS is characterized by selective loss of pyramidal neurons - especially of sectors CA1 and CA3 of the hippocampus - pathological proliferation of interneuron networks, and severe glia reaction. These changes occur in the course of long-term and complex epileptogenesis. The authors, on the basis of a review of the literature and own experience, present the pathomechanisms leading to hippocampal sclerosis and epileptogenesis, including various morphological and functional elements of this structure of the brain and pharmacological possibilities of preventing these processes.

Genome-wide microRNA profiling of rat hippocampus after status epilepticus induced by amygdala stimulation identifies modulators of neuronal apoptosis

MicroRNAs (miRNAs) are small and endogenously expressed non-coding RNAs that negatively regulate the expression of protein-coding genes at the translational level. Emerging evidence suggests that miRNAs play critical roles in central nervous system under physiological and pathological conditions. However, their expression and functions in status epilepticus (SE) have not been well characterized thus far. Here, by using high-throughput sequencing, we characterized miRNA expression profile in rat hippocampus at 24 hours following SE induced by amygdala stimulation. After confirmation by qRT-PCR, six miRNAs were found to be differentially expressed in brain after SE. Subsequent Kyoto Encyclopedia of Genes and Genomes pathway analysis indicated that most of the predicted target genes for these six miRNAs were related to neuronal apoptosis. We then investigated the dynamic changes of these six miRNAs at different time-point (4 hours, 24 hours, 1 week and 3 weeks) after SE. Meanwhile, neuronal survival and apoptosis in the hippocampus after SE were evaluated by Nissl staining and terminal deoxynucleotidyl transferase-mediated dUTP end-labeling assay. We found that the expression of miR-874-3p, miR-20a-5p, miR-345-3p, miR-365-5p, and miR-764-3p were significantly increased from 24 hours to 1 week, whereas miR-99b-3p level was markedly decreased from 24 hours to 3 weeks after SE. Further analysis revealed that the levels of miR-365-5p and miR-99b-3p were significantly correlated with neuronal apoptosis after SE. Taken together, our data suggest that miRNAs are important modulators of SE-induced neuronal apoptosis. These findings also open new avenues for future studies aimed at developing strategies against neuronal apoptosis after SE.

MicroRNA regulation and dysregulation in epilepsy

Epilepsy, one of the most frequent neurological disorders, represents a group of diseases that have in common the clinical occurrence of seizures. The pathogenesis of different types of epilepsy involves many important biological pathways; some of which have been shown to be regulated by microRNAs (miRNAs). In this paper, we will critically review relevant studies regarding the role of miRNAs in epilepsy. Overall, the most common type of epilepsy in the adult population is temporal lobe epilepsy (TLE), and the form associated with mesial temporal sclerosis (MTS), called mesial TLE, is particularly relevant due to the high frequency of resistance to clinical treatment. There are several target studies, as well few genome-wide miRNA expression profiling studies reporting abnormal miRNA expression in tissue with MTS, both in patients and in animal models. Overall, these studies show a fine correlation between miRNA regulation/dysregulation and inflammation, seizure-induced neuronal death and other relevant biological pathways. Furthermore, expression of many miRNAs is dynamically regulated during neurogenesis and its dysregulation may play a role in the process of cerebral corticogenesis leading to malformations of cortical development (MCD), which represent one of the major causes of drug-resistant epilepsy. In addition, there are reports of miRNAs involved in cell proliferation, fate specification, and neuronal maturation and these processes are tightly linked to the pathogenesis of MCD. Large-scale analyzes of miRNA expression in animal models with induced status epilepticus have demonstrated changes in a selected group of miRNAs thought to be involved in the regulation of cell death, synaptic reorganization, neuroinflammation, and neural excitability. In addition, knocking-down specific miRNAs in these animals have demonstrated that this may consist in a promising therapeutic intervention.

Refractory epilepsy associated with microglial activation

BACKGROUND

Experimental and clinical studies support a pathogenic role of microglial activation and proliferation (MAP) in epileptogenesis.

METHODS

From a consecutive series of 319 surgically treated epilepsy cases, we retrospectively reviewed the histopathological sections of 92 cases to define the prevalence and severity of MAP after excluding the other 227 because of coexisting disorders that might contribute to MAP. Severity of MAP was compared with underlying abnormalities. We assessed the response to intravenous immunoglobulin and plasmapheresis in one patient with severe MAP who had failed multiple antiepileptic drugs and epilepsy surgery.

RESULTS

MAP was detected with routine (hematoxylin and eosin) stain in 46 of 92 cases (50%). MAP was mild in 32 cases (69.6%), moderate in 12 (26.1%), and severe in 2 (4.3%). The prevalence and severity of MAP were independent of underlying abnormalities. Immunomodulatory therapy was followed by a greater than 90% reduction in seizure activity in the treated patient.

CONCLUSION

MAP is prevalent in resected human epilepsy tissue. Failure to down-regulate MAP contributes to chronic neuronal hyperexcitability. We hypothesize that MAP initiates a cycle of inflammation-induced seizures and seizure-induced inflammation. Microglia-driven epilepsy may be a primary pathogenic process in a small number of cases, as suggested by the pathology and therapeutic response in our patient, but may contribute to epileptogenesis in many more.

Focal epilepsy associated with glioneuronal tumors

Glioneuronal tumors are an increasingly recognized cause of partial seizures that occur primarily in children and young adults. Focal epilepsy associated with glioneuronal tumors is often resistant to pharmacological treatment. The cellular mechanisms underlying the epileptogenicity of glioneuronal tumors remain largely unknown. The involved mechanisms are certain to be multifactorial and depend on specific tumor histology, integrity of the blood-brain barrier, characteristics of the peritumoral environment, circuit abnormalities, or cellular and molecular defects. Glioneuronal tumors presenting with epilepsy were observed to have relatively benign biological behavior. The completeness of the tumor resection is of paramount importance in avoiding tumor progression and malignant transformation, which are rare in cases of epileptogenic glioneuronal tumors. An evolving understanding of the various mechanisms of tumor-related epileptogenicity may also lead to a more defined surgical objective and effective therapeutic strategies, including antiepileptogenic treatments, to prevent epilepsy in at-risk patients.

Temporal lobe epilepsy induces differential expression of hippocampal miRNAs including let-7e and miR-23a/b

To understand the role of miRNAs in the molecular mechanisms of temporal lobe epilepsy (TLE), we investigated the changes in microRNA (miRNA) expression profiles of chronic TLE rat models. MiRNAs microarray analysis results showed that 125 miRNAs were detected in the hippocampus of lithium-pilocarpine-induced TLE rats and normal rats. Compared with normal rats (control group), 23 of the 125 miRNAs were expressed differentially in TLE rats including 5 down-regulated miRNAs (let-7 e included) and 18 up-regulated miRNAs (miR-23 a/b included). Furthermore, let-7 e and miR-23 a/b analysis in rat hippocampus were performed by real-time quantitative polymerase chain reaction at 0 h, 1h, 6h, 12h, 24h, 2 days, 7 days,10 days, 30 days,50 days after induction of status epilepticus (SE). let-7 e was detected down-regulated expression at 0 h, 1h, 6h, 2 days, 7 days, 50 days after SE and up-regulated expression at 12h, 24h, 10 days, 30 days after SE, which was significantly up-regulated expression at 24h after SE (10.49 folds, P<0.01). miR-23 a/b was detected down-regulated at 0 h, 1h, 6h, 12h, 2 days, 7 days, 10 days, 30 days after SE and significantly up-regulated at 24h (4.49 folds P<0.01), 50 d (2.4 folds, P<0.01) after SE. TLE alters the expression levels of a subset of miRNAs in the hippocampus and these deregulated miRNAs may be involved in the pathogenesis of epilepsy directly or indirectly. Also the temporal change of the let-7 e and miR-23 a/b expression in the epileptogenesis indicated their underlying functions on TLE.

Evaluation of the innate and adaptive immunity in type I and type II focal cortical dysplasias

PURPOSE

Induction of inflammatory pathways has been reported in epileptic patients with focal malformations of cortical development. In the present study we examined the innate and adaptive immune responses in focal cortical dysplasia (FCD) with different histopathologic and pathogenetic features.

METHODS

The inflammatory cell components and the induction of major proinflammatory pathways and molecules [complement pathway, interleukin (IL)-1β, and chemokine monocyte chemotactic protein-1 (MCP1)] was investigated in surgical specimens of sporadic type IA and type IIB FCD using immunocytochemical methods.

RESULTS

FCD II but not FCD I cases exhibit activation of the mammalian target of rapamycin (mTOR) cascade with strong neuronal expression of the phosphorylated isoform of S6 protein. Microglia reactivity was increased in all lesions (FCD I and II) compared to control tissue; however, the number of HLA-DR-positive cells was significantly higher in FCD II than in FCD I. In FCD II specimens we also observed perivascular and parenchymal T lymphocytes (CD3(+) ), with a predominance of CD8(+) T-cytotoxic/suppressor lymphocytes, as well as a few dendritic cells. Expression of components of the complement cascade, IL-1β, and MCP1 was prominent in FCD II cases.

DISCUSSION

Our findings indicate a prominent activation of both innate and adaptive immunity, with involvement of different inflammatory pathways in FCD II cases, supporting the possible involvement of inflammation in the epileptogenesis of these lesions, as well as the notion that FCD II is pathologically distinct from FCD I.

Expression pattern of miR-146a, an inflammation-associated microRNA, in experimental and human temporal lobe epilepsy

Increasing evidence supports the involvement of inflammatory and immune processes in temporal lobe epilepsy (TLE). MicroRNAs (miRNA) represent small regulatory RNA molecules that have been shown to act as negative regulators of gene expression controlling different biological processes, including immune-system homeostasis and function. We investigated the expression and cellular distribution of miRNA-146a (miR-146a) in a rat model of TLE as well as in human TLE. miR-146a analysis in rat hippocampus was performed by polymerase chain reaction and immunocytochemistry at 1 week and 3-4 months after induction of status epilepticus (SE). Prominent upregulation of miR-146a activation was evident at 1 week after SE and persisted in the chronic phase. The miR-146a expression was confirmed to be present in reactive astrocytes. In human TLE with hippocampal sclerosis, increased astroglial expression of miR-146a was observed mainly in regions where neuronal cell loss and reactive gliosis occurred. The increased and persistent expression of miR-146a in reactive astrocytes supports the possible involvement of miRNAs in the modulation of the astroglial inflammatory response occurring in TLE and provides a target for future studies aimed at developing strategies against pro-epileptogenic inflammatory signalling.

Tissue plasminogen activator and urokinase plasminogen activator in human epileptogenic pathologies

A growing body of evidence demonstrates the involvement of plasminogen activators (PAs) in a number of physiologic and pathologic events in the CNS. Induction of both tissue plasminogen activator (tPA) and urokinase plasminogen activator (uPA) has been observed in different experimental models of epilepsy and tPA has been implicated in the mechanisms underlying seizure activity. We investigated the expression and the cellular distribution of tPA and uPA in several epileptogenic pathologies, including hippocampal sclerosis (HS; n=6), and developmental glioneuronal lesions, such as focal cortical dysplasia (FCD, n=6), cortical tubers in patients with the tuberous sclerosis complex (TSC; n=6) and in gangliogliomas (GG; n=6), using immuno-cytochemical, western blot and real-time quantitative PCR analysis. TPA and uPA immunostaining showed increased expression within the epileptogenic lesions compared to control specimens in both glial and neuronal cells (hippocampal neurons in HS and dysplastic neurons in FCD, TSC and GG specimens). Confocal laser scanning microscopy confirmed expression of both proteins in astrocytes and microglia, as well as in microvascular endothelium. Immunoblot demonstrated also up-regulation of the uPA receptor (uPAR; P<0.05). Increased expression of tPA, uPA, uPAR and tissue PA inhibitor type mRNA levels was also detected by PCR analysis in different epileptogenic pathologies (P<0.05). Our data support the role of PA system components in different human focal epileptogenic pathologies, which may critically influence neuronal activity, inflammatory response, as well as contributing to the complex remodeling of the neuronal networks occurring in epileptogenic lesions.

Neuropathology of focal epilepsies: a critical review

A broad spectrum of structural lesions can be histopathologically identified in surgical brain specimens obtained from patients with focal, therapy-refractory epilepsies. In our experience with 4512 tissue samples collected at the German Neuropathological Reference Center for Epilepsy Surgery, three clinicopathological entities are most common: mesial temporal sclerosis (40%), long-term epilepsy-associated tumors (27%), and malformations of cortical development (13%). Notwithstanding, a systematic histopathological and molecular-genetic analysis is mandatory to unravel the underlying pathogenic mechanism of epilepsy-associated lesions and may contribute to our current understanding of pharmacoresistance and epileptogenesis. However, an interdisciplinary approach is necessary to further explore predictive parameters with respect to postsurgical seizure relief and memory impairment, and also to identify new pharmacological targets.

Expression patterns of synaptic vesicle protein 2A in focal cortical dysplasia and TSC-cortical tubers

PURPOSE

Synaptic vesicle protein 2A (SV2A), the binding site for the antiepileptic drug (AED) levetiracetam, has been shown to be involved in the control of neuronal excitability. The aim of the study was to define the expression and cell-specific distribution of SV2A in developmental focal lesions associated with medically intractable epilepsy.

METHODS

SV2A immunocytochemistry and Western blotting was performed in focal cortical dysplasia (FCD type IIB) and cortical tubers from patients with tuberous sclerosis complex (TSC).

RESULTS

Autopsy and surgical control neocortical specimens were characterized by strong SV2A immunoreactivity throughout all cortical layers, with punctate labeling around the somata and dendrites of neurons. In FCD and cortical tuber specimens less intense, SV2A immunoreactivity was observed in the neuropil. The reduction in expression was confirmed by Western blot analysis. In both FCD and tuber specimens, clusters of punctate labeling were detected along cell borders and processes (perisomatic synapses) of dysplastic neuronal cells localized in both gray and white matter. The large majority of balloon cells in FCD, or giant cells in tubers, did not show punctate labeling around their somata. SV2A immunoreactivity was observed occasionally within the neuronal perikarya.

CONCLUSIONS

The pattern of SV2A immunoreactivity with reduced neuropil expression and altered cellular and subcellular distribution suggests a possible contribution of SV2A to the epileptogenicity of these malformations of cortical development. Knowledge of the expression pattern of SV2A in epilepsy-associated pathologies may be valuable for the evaluation of the effectiveness of AEDs targeting this protein.

Cellular distribution of vascular endothelial growth factor A (VEGFA) and B (VEGFB) and VEGF receptors 1 and 2 in focal cortical dysplasia type IIB

Members of the vascular endothelial growth factor (VEGF) family are key signaling proteins in the induction and regulation of angiogenesis, both during development and in pathological conditions. However, signaling mediated through VEGF family proteins and their receptors has recently been shown to have direct effects on neurons and glial cells. In the present study, we immunocytochemically investigated the expression and cellular distribution of VEGFA, VEGFB, and their associated receptors (VEGFR-1 and VEGFR-2) in focal cortical dysplasia (FCD) type IIB from patients with medically intractable epilepsy. Histologically normal temporal cortex and perilesional regions displayed neuronal immunoreactivity (IR) for VEGFA, VEGFB, and VEGF receptors (VEGFR-1 and VEGFR-2), mainly in pyramidal neurons. Weak IR was observed in blood vessels and there was no notable glial IR within the grey and white matter. In all FCD specimens, VEGFA, VEGFB, and both VEGF receptors were highly expressed in dysplastic neurons. IR in astroglial and balloon cells was observed for VEGFA and its receptors. VEGFR-1 displayed strong endothelial staining in FCD. Double-labeling also showed expression of VEGFA, VEGFB and VEGFR-1 in cells of the microglia/macrophage lineage. The neuronal expression of both VEGFA and VEGFB, together with their specific receptors in FCD, suggests autocrine/paracrine effects on dysplastic neurons. These autocrine/paracrine effects could play a role in the development of FCD, preventing the death of abnormal neuronal cells. In addition, the expression of VEGFA and its receptors in glial cells within the dysplastic cortex indicates that VEGF-mediated signaling could contribute to astroglial activation and associated inflammatory reactions.

Group I metabotropic glutamate receptors: a role in neurodevelopmental disorders?

Group I metabotropic glutamate receptors (mGlu1 and mGlu5) are coupled to polyphosphoinositide hydrolysis and are involved in activity-dependent forms of synaptic plasticity, both during development and in the adult life. Group I mGlu receptors can also regulate proliferation, differentiation, and survival of neural stem/progenitor cells, which further support their role in brain development. An exaggerated response to activation of mGlu5 receptors may underlie synaptic dysfunction in Fragile X syndrome, the most common inherited form of mental retardation. In addition, group I mGlu receptors are overexpressed in dysplastic neurons of focal cortical dysplasia and hemimegaloencephaly, which are disorders of cortical development associated with chronic epilepsy. Drugs that block the activity of group I mGlu receptors (in particular, mGlu5 receptors) are potentially helpful for the treatment of Fragile X syndrome and perhaps other neurodevelopmental disorders.

Gene expression profile analysis of epilepsy-associated gangliogliomas

Gangliogliomas (GG) constitute the most frequent tumor entity in young patients undergoing surgery for intractable epilepsy. The histological composition of GG, with the presence of dysplastic neurons, corroborates their maldevelopmental origin. However, their histogenesis, the pathogenetic relationship with other developmental lesions, and the molecular alterations underlying the epileptogenicity of these tumors remain largely unknown. We performed gene expression analysis using the Affymetrix Gene Chip System (U133 plus 2.0 array). We used GENMAPP and the Gene Ontology database to identify global trends in gene expression data. Our analysis has identified various interesting genes and processes that are differentially expressed in GG when compared with normal tissue. The immune and inflammatory responses were the most prominent processes expressed in GG. Several genes involved in the complement pathway displayed a high level of expression compared with control expression levels. Higher expression was also observed for genes involved in cell adhesion, extracellular matrix and proliferation processes. We observed differential expression of genes as cyclin D1 and cyclin-dependent kinases, essential for neuronal cell cycle regulation and differentiation. Synaptic transmission, including GABA receptor signaling was an under-expressed process compared with control tissue. These data provide some suggestions for the molecular pathogenesis of GG. Furthermore, they indicate possible targets that may be investigated in order to dissect the mechanisms of epileptogenesis and possibly counteract the epileptogenic process in these developmental lesions.

Cell specificity of altered cation–chloride cotransporter expression and GABAergic innervation in the epileptic cerebral cortex

Evaluation of: Aronica E, Boer K, Redeker S et al.: Differential expression patterns of chloride transporters, Na+–K+–2Cl-cotransporter and K+–Cl- cotransporter (KCC2), in epilepsy-associated malformations of cortical development. Neuroscience 145(1), 185–196 (2007). The study by Aronica and colleagues used immunocytochemistry to investigate the changes in the expression and distribution of cation–chloride cotransporters in the cerebral cortex of epileptic patients when compared with age-matched controls. The aim was to determine whether or not the deregulation of cation–chloride cotransporter expression might contribute to the hyperexcitability that leads to seizures. The authors studied brain tissue from patients with medically intractable epilepsy associated with different types of malformations, including focal cortical dysplasia type IIB, hemimegalencephaly and ganglioglioma. They found weak neuronal and glial expression of the Na+–K+–2 Cl- cotransporter (NKCC1) in the normal control adult cortex, whereas NKCC1 was strongly expressed in different cell types from most patients, including large dysplastic neurons, balloon cells and astrocytes. In addition, the K+–Cl- cotransporter (KCC2) was reported to be diffusely distributed in the neuropil of control tissue, although more intense somatic KCC2 immunostaining was associated with large dysplastic neurons but not balloon cells in epileptic patients. The intense expression of NKCC1 in dysplastic neurons and the altered subcellular distribution of KCC2 were compared with the situation in the immature normal cortex. The authors concluded that altered expression of cation–chloride cotransporters in epileptic patients, in conjunction with malformations in cortical development, may contribute to epileptogenicity.

Gene expression profile in temporal lobe epilepsy

Epilepsy is one of the most common neurological disorders. Temporal lobe epilepsy (TLE) represents the most frequent epilepsy syndrome in adult patients with resistance to pharmacological treatment. In TLE, the origin of seizure activity typically involves the hippocampal formation, which displays major neuropathological features, described with the term hippocampal sclerosis (HS). The expansion of neurosurgical epilepsy programs has offered the possibility of disposing of clinically well-characterized hippocampal tissue, so that the analysis of molecular mechanisms underlying the structural and functional reorganization occurring in the hippocampus and neighboring areas in TLE patients can be done on a large scale. The recent development of molecular biological technologies permits the analysis of changes in the expression of a large number of genes. This has opened new perspectives for epilepsy research. However, the hippocampal specimens obtained from patients with TLE most often represent an advanced stage of the pathology. For this reason, animal models that reproduce the clinical and histopathological features of TLE are helpful in detecting the early development of the pathological cascade leading to TLE with HS. An overview of recent data of gene expression profiles in human and experimental TLE is presented along with a discussion of the relevance of functional genomics, to develop new hypotheses and to detect likely candidate genes involved in epileptogenesis, as well as possible target molecules for new therapeutic approaches.

Differential expression patterns of chloride transporters, Na+-K+-2Cl--cotransporter and K+-Cl--cotransporter, in epilepsy-associated malformations of cortical development

Malformations of cortical development are recognized causes of chronic medically intractable epilepsy. An increasing number of observations suggests an important role for cation-chloride co-transporters (CCTs) in controlling neuronal function. Deregulation of their expression may contribute to the mechanisms of hyperexcitability that lead to seizures. In the present study the expression and cell-specific distribution of Na+-K+-2Cl--cotransporter (NKCC1) and K+-Cl--cotransporter (KCC2) were studied immunocytochemically in different developmental lesions, including focal cortical dysplasia (FCD) type IIB (n=9), hemimegalencephaly (HMEG, n=6) and ganglioglioma (GG, n=9) from patients with medically intractable epilepsy and in age-matched controls. In normal control adult cortex, NKCC1 displayed low neuronal and glial expression levels. In contrast KCC2 showed strong and diffuse neuropil staining. Notable glial immunoreactivity (IR) was not found for KCC2. NKCC1 was highly expressed in the majority of FCD, HMEG and GG specimens. NKCC1 IR was observed in neurons of different size, including large dysplastic neurons, in balloon cells (in FCD and HMEG cases) and in glial cells with astrocytic morphology. The immunoreactivity pattern of KCC2 in FCD, HMEG and GG specimens was characterized by less neuropil staining and more intrasomatic IR compared with control. KCC2 IR was observed in neurons of different size, including large dysplastic neurons, but not in balloon cells or in glial cells with astrocytic morphology. Double-labeling experiments confirmed the differential cellular distribution of the two CCTs and their expression in GABA(A) receptor (alpha1 subunit)-positive dysplastic neurons. The cellular distribution of CCTs, with high expression of NKCC1 in dysplastic neurons and altered subcellular distribution of KCC2 resembles that of immature cortex and suggests a possible contribution of CCTs to the high epileptogenicity of malformations of cortical development.

Enhanced capacity of epilepsy in brain malformation produced during early development

This study investigates the clinical features of epilepsy in 20 patients with brain malformation. Epileptic seizures were recognized in 15 patients, 12 of whom had their first seizure by 1 year of age. Partial seizure was the initial seizure type in 10 patients. Epileptic seizures were controlled in only four patients. Patients with holoprosencephaly and lissencephaly had seizure onset by 3 months of age, resulting in the most severe neurologic outcome. Only two patients with porencephaly had epileptic seizures, and in one of those patients the seizures were well controlled. A wide variety of clinical features of epilepsy in patients with brain malformation was found. More immature anomalous brain lesions may be associated with an enhanced capacity of epilepsy and resultant refractory seizures.

Evidence of activated microglia in focal cortical dysplasia

Focal cortical dysplasia (FCD), which is caused by malformations of cortical development, is known to be a major cause of intractable epilepsy. Cortical laminar disorganization and the presence of abnormal neuronal and astroglial cell types are histological characteristics of FCD. Though, little information is known about the microglia/macrophage cell system in FCD and its possible contribution to the high epileptogenesis of this disorder. In the present study, the distribution of cells of the microglia/macrophage lineage was studied in 20 specimens of FCD (type II) by immunocytochemistry for CD68 and human HLA-DR. A significant number of microglial cells and macrophages were observed within the dysplastic cortex. The mean number of CD68- and HLA-DR-positive cells was significantly higher in FCD specimens than in normal-appearing control cortex obtained at autopsy. HLA-DR-positive cells, which represent activated microglia, were localized around blood vessels and also clustered around dysplastic neuronal cells. The density of these activated HLA-DR-positive microglial cells correlated with the duration of epilepsy, as well as with the frequency of seizures prior to surgical resection. CD68-positive macrophages were mainly located around vessels and the number of these cells did not correlate with seizure frequency, neither with the duration of symptoms prior to surgical resection. In conclusion, our findings demonstrate a specific and persistent increase in the numerical density of HLA-DR-positive activated microglia within the dysplastic region, supporting the contribution of the inflammatory response and proinflammatory molecules to the epileptogenicity of FCD.