Comparative study of cellular and synaptic abnormalities in brain tissue samples from pediatric tuberous sclerosis complex and cortical dysplasia type II

Iconography of abnormal non-neuronal cells in pediatric focal cortical dysplasia type IIb and tuberous sclerosis complex

Once believed to be the culprits of epileptogenic activity, the functional properties of balloon/giant cells (BC/GC), commonly found in some malformations of cortical development including focal cortical dysplasia type IIb (FCDIIb) and tuberous sclerosis complex (TSC), are beginning to be unraveled. These abnormal cells emerge during early brain development as a result of a hyperactive mTOR pathway and may express both neuronal and glial markers. A paradigm shift occurred when our group demonstrated that BC/GC in pediatric cases of FCDIIb and TSC are unable to generate action potentials and lack synaptic inputs. Hence, their role in epileptogenesis remained obscure. In this review, we provide a detailed characterization of abnormal non-neuronal cells including BC/GC, intermediate cells, and dysmorphic/reactive astrocytes found in FCDIIb and TSC cases, with special emphasis on electrophysiological and morphological assessments. Regardless of pathology, the electrophysiological properties of abnormal cells appear more glial-like, while others appear more neuronal-like. Their morphology also differs in terms of somatic size, shape, and dendritic elaboration. A common feature of these types of non-neuronal cells is their inability to generate action potentials. Thus, despite their distinct properties and etiologies, they share a common functional feature. We hypothesize that, although the exact role of abnormal non-neuronal cells in FCDIIb and TSC remains mysterious, it can be suggested that cells displaying more glial-like properties function in a similar way as astrocytes do, i.e., to buffer K+ ions and neurotransmitters, while those with more neuronal properties, may represent a metabolic burden due to high energy demands but inability to receive or transmit electric signals. In addition, due to the heterogeneity of these cells, a new classification scheme based on morphological, electrophysiological, and gene/protein expression in FCDIIb and TSC cases seems warranted.

The mTOR pathway genes MTOR, Rheb, Depdc5, Pten, and Tsc1 have convergent and divergent impacts on cortical neuron development and function

Brain somatic mutations in various components of the mTOR complex 1 (mTORC1) pathway have emerged as major causes of focal malformations of cortical development and intractable epilepsy. While these distinct gene mutations converge on excessive mTORC1 signaling and lead to common clinical manifestations, it remains unclear whether they cause similar cellular and synaptic disruptions underlying cortical network hyperexcitability. Here, we show that in utero activation of the mTORC1 activator genes, Rheb or MTOR, or biallelic inactivation of the mTORC1 repressor genes, Depdc5, Tsc1, or Pten in the mouse medial prefrontal cortex leads to shared alterations in pyramidal neuron morphology, positioning, and membrane excitability but different changes in excitatory synaptic transmission. Our findings suggest that, despite converging on mTORC1 signaling, mutations in different mTORC1 pathway genes differentially impact cortical excitatory synaptic activity, which may confer gene-specific mechanisms of hyperexcitability and responses to therapeutic intervention.

Reduced evoked cortical beta and gamma activity and neuronal synchronization in succinic semialdehyde dehydrogenase deficiency, a disorder of γ-aminobutyric acid metabolism

Succinic semialdehyde dehydrogenase deficiency is a rare autosomal recessively inherited metabolic disorder of γ-aminobutyric acid catabolism manifested by intellectual disability, expressive aphasia, movement disorders, psychiatric ailments and epilepsy. Subjects with succinic semialdehyde dehydrogenase deficiency are characterized by elevated γ-aminobutyric acid and related metabolites, such as γ-guanidinobutyric acid, and an age-dependent downregulation of cerebral γ-aminobutyric acid receptors. These findings indicate impaired γ-aminobutyric acid and γ-aminobutyric acid sub-type A (GABAA) receptor signalling as major factors underlying the pathophysiology of this neurometabolic disorder. We studied the cortical oscillation patterns and their relationship with γ-aminobutyric acid metabolism in 18 children affected by this condition and 10 healthy controls. Using high-density EEG, we recorded somatosensory cortical responses and resting-state activity. Using electrical source imaging, we estimated the relative power changes (compared with baseline) in both stimulus-evoked and stimulus-induced responses for physiologically relevant frequency bands and resting-state power. Stimulus-evoked oscillations are phase locked to the stimulus, whereas induced oscillations are not. Power changes for both evoked and induced responses as well as resting-state power were correlated with plasma γ-aminobutyric acid and γ-guanidinobutyric acid concentrations and with cortical γ-aminobutyric acid measured by proton magnetic resonance spectroscopy. Plasma γ-aminobutyric acid, γ-guanidinobutyric acid and cortical γ-aminobutyric acid were higher in patients than in controls (P < 0.001 for both). Beta and gamma relative power were suppressed for evoked responses in patients versus controls (P < 0.01). No group differences were observed for induced activity (P > 0.05). The mean gamma frequency of evoked responses was lower in patients versus controls (P = 0.002). Resting-state activity was suppressed in patients for theta (P = 0.011) and gamma (P < 0.001) bands. Evoked power changes were inversely correlated with plasma γ-aminobutyric acid and with γ-guanidinobutyric acid for beta (P < 0.001) and gamma (P < 0.001) bands. Similar relationships were observed between the evoked power changes and cortical γ-aminobutyric acid for all tested areas in the beta band (P < 0.001) and for the posterior cingulate gyrus in the gamma band (P < 0.001). We also observed a negative correlation between resting-state activity and plasma γ-aminobutyric acid and γ-guanidinobutyric acid for theta (P < 0.001; P = 0.003), alpha (P = 0.003; P = 0.02) and gamma (P = 0.02; P = 0.01) bands. Our findings indicate that increased γ-aminobutyric acid concentration is associated with reduced sensory-evoked beta and gamma activity and impaired neuronal synchronization in patients with succinic semialdehyde dehydrogenase deficiency. This further elucidates the pathophysiology of this neurometabolic disorder and serves as a potential biomarker for therapeutic trials.

Basal ganglia dysplasia and mTORopathy: A potential cause of postoperative seizures in focal cortical dysplasia

Pathogenic somatic MTOR variants in the cerebral cortex are a frequent cause of focal cortical dysplasia (FCD). We describe a child with drug and surgery-resistant focal epilepsy due to FCD type II who developed progressive enlargement and T2 signal hyperintensity in the ipsilateral caudate and lentiform nuclei. Histopathology of caudate nucleus biopsies showed dysmorphic neurons, similar to those in resected cortex. Genetic analysis of frontal and temporal cortex and caudate nucleus identified a pathogenic somatic MTOR variant [NM_004958.4:c.4375G > C (p.Ala1459Pro)] that was not present in blood-derived gDNA. The mean variant allele frequency ranged from 0.4% to 3.2% in cerebral cortex and up to 5.4% in the caudate nucleus. The basal ganglia abnormalities suggest more widespread, potentially hemispheric dysplasia in this patient, consistent with the pathogenic variant occurring in early cerebral development. This finding provides a potential explanation for persistent seizures in some patients with seemingly complete resection of FCD or disconnection of a dysplastic hemisphere.

Intrinsic and secondary epileptogenicity in focal cortical dysplasia type II

OBJECTIVE

Favorable seizure outcome is reported following resection of bottom-of-sulcus dysplasia (BOSD). We assessed the distribution of epileptogenicity and dysplasia in and around BOSD to better understand this clinical outcome and the optimal surgical approach.

METHODS

We studied 27 children and adolescents with magnetic resonance imaging (MRI)-positive BOSD who underwent epilepsy surgery; 85% became seizure-free postresection (median = 5.0 years follow-up). All patients had resection of the dysplastic sulcus, and 11 had additional resection of the gyral crown (GC) or adjacent gyri (AG). Markers of epileptogenicity were relative cortical hypometabolism on preoperative 18 F-fluorodeoxyglucose (FDG) positron emission tomography (PET), and spiking, ripples, fast ripples, spike-high-frequency oscillation cross-rate, and phase amplitude coupling (PAC) on preresection and postresection electrocorticography (ECoG), all analyzed at the bottom-of-sulcus (BOS), top-of-sulcus (TOS), GC, and AG. Markers of dysplasia were increased cortical thickness on preoperative MRI, and dysmorphic neuron density and variant allele frequency of somatic MTOR mutations in resected tissue, analyzed at similar locations.

RESULTS

Relative cortical metabolism was significantly reduced and ECoG markers were significantly increased at the BOS compared to other regions. Apart from spiking and PAC, which were greater at the TOS compared to the GC, there were no significant differences in PET and other ECoG markers between the TOS, GC, and AG, suggesting a cutoff of epileptogenicity at the TOS rather than a tapering gradient on the cortical surface. MRI and tissue markers of dysplasia were all maximal in the BOS, reduced in the TOS, and mostly absent in the GC. Spiking and PAC reduced significantly over the GC after resection of the dysplastic sulcus.

SIGNIFICANCE

These findings support the concept that dysplasia and intrinsic epileptogenicity are mostly limited to the dysplastic sulcus in BOSD and support resection or ablation confined to the MRI-visible lesion as a first-line surgical approach. 18 F-FDG PET and ECoG abnormalities in surrounding cortex seem to be secondary phenomena.

Synaptic hyperexcitability of cytomegalic pyramidal neurons contributes to epileptogenesis in tuberous sclerosis complex

Tuberous sclerosis complex (TSC) is a developmental disorder associated with epilepsy, autism, and cognitive impairment. Despite inactivating mutations in the TSC1 or TSC2 genes and hyperactive mechanistic target of rapamycin (mTOR) signaling, the mechanisms underlying TSC-associated neurological symptoms remain incompletely understood. Here we generate a Tsc1 conditional knockout (CKO) mouse model in which Tsc1 inactivation in late embryonic radial glia causes social and cognitive impairment and spontaneous seizures. Tsc1 depletion occurs in a subset of layer 2/3 cortical pyramidal neurons, leading to development of cytomegalic pyramidal neurons (CPNs) that mimic dysplastic neurons in human TSC, featuring abnormal dendritic and axonal overgrowth, enhanced glutamatergic synaptic transmission, and increased susceptibility to seizure-like activities. We provide evidence that enhanced synaptic excitation in CPNs contributes to cortical hyperexcitability and epileptogenesis. In contrast, astrocytic regulation of synapse formation and synaptic transmission remains unchanged after late embryonic radial glial Tsc1 inactivation, and astrogliosis evolves secondary to seizures.

Wu et al. demonstrate that Tsc1 inactivation in late embryonic radial glial cells (RGCs) produces cytomegalic pyramidal neurons that mimic TSC-like dysplastic neurons. They find that enhanced excitatory synaptic transmission in Tsc1-null cytomegalic pyramidal neurons contributes to cortical hyperexcitability and epileptogenesis.

Interweaving epilepsy and neurodegeneration: Vitamin E as a treatment approach

Epilepsy is the most common neurological disorder, affecting nearly 50 million people worldwide. The condition can be manifested either due to genetic predisposition or acquired from acute insult which leads to alteration of cellular and molecular mechanisms. Evaluating the latest and the current knowledge in regard to the mechanisms underlying molecular and cellular alteration, hyperexcitability is a consequence of an imbalanced state wherein enhance excitatory glutamatergic and reduced inhibitory GABAergic signaling is considered to be accountable for seizures associated damage. However, neurodegeneration contributing to epileptogenesis has become increasingly appreciated. The components at the helm of neurodegenerative alterations during epileptogenesis include GABAergic neuronal and receptor changes, neuroinflammation, alteration in axonal transport, oxidative stress, excitotoxicity, and other cellular as well as functional changes. Targeting neurodegeneration with vitamin E as an antioxidant, anti-inflammatory and neuroprotective may prove to be one of the therapeutic approaches useful in managing epilepsy. In this review, we discuss and converse about the seizure-induced episodes as a link for the development of neurodegenerative and pathological consequences of epilepsy. We also put forth a summary of the potential intervention with vitamin E therapy in the management of epilepsy.

Resection of tuber centers only for seizure control in tuberous sclerosis complex

Our previous studies suggest the tuber center is the seizure focus in tuberous sclerosis complex (TSC). We report findings from 5 epilepsy surgeries in 4 children with TSC and focal motor seizures from single tubers in primary sensorimotor cortex in which resection was limited to the cortex in the tuber center. Intraoperative electrocorticography showed epileptiform activity in the tuber center, with or without propagation to the tuber rim and surrounding perituberal cortex. Histopathology showed an abundance of dysmorphic neurons in the tuber center compared to the rim in four paired specimens, dysmorphic neurons being the reported epileptogenic cell line in TSC. Associated focal motor seizures were eliminated in all children (mean follow up 6.3 years) without postoperative deficits. Tuber center resections are a potential alternative to complete tuberectomy in patients with epileptogenic tubers in eloquent cortex and potentially also in children with a high tuber load and multifocal seizures.

Increased expression of fibroblast growth factor 13 in cortical lesions of the focal cortical dysplasia

Focal cortical dysplasias (FCDs) are well recognized as important causes of medically intractable epilepsy in both children and adults. To explore the potential role of fibroblast growth factor 13 (FGF13) in intractable epilepsy caused by FCDs, we examined the expression of FGF13 in cortical lesions from 23 patients with FCD type Ia (FCDIa), 24 patients with FCD type IIa (FCDIIa), and 12 patients with FCD type IIb (FCDIIb), and we compared the results with the FGF13 expression levels in control cortex (CTX) brain tissues from 12 nonepileptic normal subjects. Both the mRNA levels and protein levels of FGF13 were significantly higher in the cortical lesions from patients with FCD than in the control cortices. The immunohistochemical results showed that strong FGF13 immunoreactivity was observed in misshapen cells, including neuronal microcolumns, hypertrophic neurons, dysmorphic neurons, and most balloon cells. Moreover, double-label immunofluorescence analyses confirmed that FGF13 was mainly localized in neurons and nearly absent in glia-like cells. Taken together, our results suggest that the overexpression of FGF13 in FCDs and the cell-specific distribution patterns of FGF13 in misshapen neurons in FCDs could potentially contribute to intractable epilepsy caused by FCDs.

BRAFV600E expression in neural progenitors results in a hyperexcitable phenotype in neocortical pyramidal neurons

Somatic mutations have emerged as the likely cause of focal epilepsies associated with developmental malformations and epilepsy-associated glioneuronal tumors (GNT). Somatic BRAFV600E mutations in particular have been detected in the majority of low-grade neuroepithelial tumors (LNETS) and in neurons in focal cortical dysplasias adjacent to epilepsy-associated tumors. Furthermore, conditional expression of an activating BRAF mutation in neocortex causes seizures in mice. In this study we characterized the cellular electrophysiology of layer 2/3 neocortical pyramidal neurons induced to express BRAFV600E from neural progenitor stages. In utero electroporation of a piggyBac transposase plasmid system was used to introduce transgenes expressing BRAF wild type (BRAFwt), BRAFV600E, and/or enhanced green fluorescent protein (eGFP) and monomeric red fluorescent protein (mRFP) into radial glia progenitors in mouse embryonic cortex. Whole cell patch-clamp recordings of pyramidal neurons in slices prepared from both juvenile and adult mice showed that BRAFV600E resulted in neurons with a distinct hyperexcitable phenotype characterized by depolarized resting membrane potentials, increased input resistances, lowered action potential (AP) thresholds, and increased AP firing frequencies. Some of the BRAFV600E-expressing neurons normally destined for upper cortical layers by their birthdate were stalled in their migration and occupied lower cortical layers. BRAFV600E-expressing neurons also displayed increased hyperpolarization-induced inward currents (Ih) and decreased sustained potassium currents. Neurons adjacent to BRAFV600E transgene-expressing neurons, and neurons with TSC1 genetically deleted by CRISPR or those induced to carry PIK3CAE545K transgenes, did not show an excitability phenotype similar to that of BRAFV600E-expressing neurons. Together, these results indicate that BRAFV600E leads to a distinct hyperexcitable neuronal phenotype.NEW & NOTEWORTHY This study is the first to report the cell autonomous effects of BRAFV600E mutations on the intrinsic neuronal excitability. We show that BRAFV600E alters multiple electrophysiological parameters in neocortical neurons. Similar excitability changes did not occur in cells neighboring BRAFV600E-expressing neurons, after overexpression of wild-type BRAF transgenes, or after introduction of mutations affecting the mammalian target of rapamycin (mTOR) or the catalytic subunit of phosphoinositide 3-kinase (PIK3CA). We conclude that BRAFV600E causes a distinct, cell autonomous, highly excitable neuronal phenotype when introduced somatically into neocortical neuronal progenitors.

Paroxysmal Discharges in Tissue Slices From Pediatric Epilepsy Surgery Patients: Critical Role of GABAB Receptors in the Generation of Ictal Activity

In the present study, we characterized the effects of bath application of the proconvulsant drug 4-aminopyridine (4-AP) alone or in combination with GABA_A and/or GABA_B receptor antagonists, in cortical dysplasia (CD type I and CD type IIa/b), tuberous sclerosis complex (TSC), and non-CD cortical tissue samples from pediatric epilepsy surgery patients. Whole-cell patch clamp recordings in current and voltage clamp modes were obtained from cortical pyramidal neurons (CPNs), interneurons, and balloon/giant cells. In pyramidal neurons, bath application of 4-AP produced an increase in spontaneous synaptic activity as well as rhythmic membrane oscillations. In current clamp mode, these oscillations were generally depolarizing or biphasic and were accompanied by increased membrane conductance. In interneurons, membrane oscillations were consistently depolarizing and accompanied by bursts of action potentials. In a subset of balloon/giant cells from CD type IIb and TSC cases, respectively, 4-AP induced very low-amplitude, slow membrane oscillations that echoed the rhythmic oscillations from pyramidal neurons and interneurons. Bicuculline reduced the amplitude of membrane oscillations induced by 4-AP, indicating that they were mediated principally by GABA_A receptors. 4-AP alone or in combination with bicuculline increased cortical excitability but did not induce seizure-like discharges. Ictal activity was observed in pyramidal neurons and interneurons from CD and TSC cases only when phaclofen, a GABA_B receptor antagonist, was added to the 4-AP and bicuculline solution. These results emphasize the critical and permissive role of GABA_B receptors in the transition to an ictal state in pediatric CD tissue and highlight the importance of these receptors as a potential therapeutic target in pediatric epilepsy.

New insights into a spectrum of developmental malformations related to mTOR dysregulations: challenges and perspectives

In recent years the role of the mammalian target of rapamycin (mTOR) pathway has emerged as crucial for normal cortical development. Therefore, it is not surprising that aberrant activation of mTOR is associated with developmental malformations and epileptogenesis. A broad spectrum of malformations of cortical development, such as focal cortical dysplasia (FCD) and tuberous sclerosis complex (TSC), have been linked to either germline or somatic mutations in mTOR pathway-related genes, commonly summarised under the umbrella term 'mTORopathies'. However, there are still a number of unanswered questions regarding the involvement of mTOR in the pathophysiology of these abnormalities. Therefore, a monogenetic disease, such as TSC, can be more easily applied as a model to study the mechanisms of epileptogenesis and identify potential new targets of therapy. Developmental neuropathology and genetics demonstrate that FCD IIb and hemimegalencephaly are the same diseases. Constitutive activation of mTOR signalling represents a shared pathogenic mechanism in a group of developmental malformations that have histopathological and clinical features in common, such as epilepsy, autism and other comorbidities. We seek to understand the effect of mTOR dysregulation in a developing cortex with the propensity to generate seizures as well as the aftermath of the surrounding environment, including the white matter.

Inflammatory aspects of epileptogenesis: contribution of molecular inflammatory mechanisms

OBJECTIVE

Epilepsy is a chronic neurological disease characterised with seizures. The aetiology of the most generalised epilepsies cannot be explicitly determined and the seizures are pronounced to be genetically determined by disturbances of receptors in central nervous system. Besides, neurotransmitter distributions or other metabolic problems are supposed to involve in epileptogenesis. Lack of adequate data about pharmacological agents that have antiepileptogenic effects point to need of research on this field. Thus, in this review, inflammatory aspects of epileptogenesis has been focussed via considering several concepts like role of immune system, blood-brain barrier and antibody involvement in epileptogenesis.

METHODS

We conducted an evidence-based review of the literatures in order to evaluate the possible participation of inflammatory processes to epileptogenesis and also, promising agents which are effective to these processes. We searched PubMed database up to November 2015 with no date restrictions.

RESULTS

In the present review, 163 appropriate articles were included. Obtained data suggests that inflammatory processes participate to epileptogenesis in several ways like affecting fibroblast growth factor-2 and tropomyosin receptor kinase B signalling pathways, detrimental proinflammatory pathways [such as the interleukin-1 beta (IL-1β)-interleukin-1 receptor type 1 (IL-1R1) system], mammalian target of rapamycin pathway, microglial activities, release of glial inflammatory proteins (such as macrophage inflammatory protein, interleukin 6, C-C motif ligand 2 and IL-1β), adhesion molecules that are suggested to function in signalling pathways between neurons and microglia and also linkage between these molecules and proinflammatory cytokines.

CONCLUSION

The literature research indicated that inflammation is a part of epileptogenesis. For this reason, further studies are necessary for assessing agents that will be effective in clinical use for therapeutic treatment of epileptogenesis.

Molecular biomarkers in drug-resistant epilepsy: Facts & possibilities

Despite great advances in our understanding of the process of epileptogenesis we are yet to develop reliable biomarkers that have the potential to accurately localize the epileptogenic zone (EZ), and to resolve the issue of heterogeneity in epilepsy surgery outcome. Inability to precisely localize the epileptogenic foci is one of the reason why more than 30% of these DRE patients are not benefited. Molecular and cellular biomarkers in combination with imaging and electrical investigations will provide a more specific platform for defining epileptogenic zone. Potential molecular biomarkers of epileptogenesis including markers of inflammation, synaptic alterations and neurodegeneration may also have the potential for localizing EZ. At molecular level components derived from epileptogenic tissues, such as metabolites, proteins, mRNAs and miRNAs that are significantly altered can serve as biomarkers and can be clubbed with existing techniques to preoperatively localize the EZ. Neurosurgeons across the world face problems while defining the margins of the epileptogenic tissues to be resected during surgery. In this review we discuss molecular biomarkers reported so far in the context of epileptogenesis and some of the unexplored markers which may have the potential to localize EZ during surgery. We also discuss "Intelligent knife" technique that couples electrosurgery and mass spectrometry allowing near-real-time characterization of human tissue and may prove to be instrumental in defining the margins of the epileptogenic zone during surgery.

Infantile tauopathies: Hemimegalencephaly; tuberous sclerosis complex; focal cortical dysplasia 2; ganglioglioma

Tau is a normal microtubule-associated protein; mutations to phosphorylated or acetylated forms are neurotoxic. In many dementias of adult life tauopathies cause neuronal degeneration. Four developmental disorders of the fetal and infant brain are presented, each of which exhibits up-regulation of tau. Microtubules are cytoskeletal structures that provide the strands of mitotic spindles and specify cellular polarity, growth, lineage, differentiation, migration and axonal transport of molecules. Phosphorylated tau is abnormal in immature as in mature neurons. Several malformations are demonstrated in which upregulated tau may be important in pathogenesis. All produce highly epileptogenic cortical foci. The prototype infantile tauopathy is (1) hemimegalencephaly (HME); normal tau is degraded by a mutant AKT3 or AKT1 gene as the aetiology of focal somatic mosaicism in the periventricular neuroepithelium. HME may be isolated or associated with neurocutaneous syndromes, particularly epidermal naevus syndromes, also due to somatic mutations. Other tauopathies of early life include: (2) tuberous sclerosis complex; (3) focal cortical dysplasia type 2b (FCD2b); and (4) ganglioglioma, a tumor with dysplastic neurons and neoplastic glial cells. Pathological tau in these infantile cases alters cellular growth and architecture, synaptic function and tissue organization, but does not cause neuronal loss. All infantile tauopathies are defined neuropathologically as a tetrad of (1) dysmorphic and megalocytic neurons; (2) activation of the mTOR signaling pathway; (3) post-zygotic somatic mosaicism; and (4) upregulation of phosphorylated tau. HME and FCD2b may be the same disorder with different timing of the somatic mutation in the mitotic cycles of the neuroepithelium. HME and FCD2b may be the same disorder with different timing of the somatic mutation in the mitotic cycles of the neuroepithelium. Tauopathies must be considered in infantile neurological disease and no longer restricted to adult dementias. The mTOR inhibitor everolimus, already demonstrated to be effective in TSC, also may be a potential treatment in other infantile tauopathies.

Basic mechanisms of epileptogenesis in pediatric cortical dysplasia

Cortical dysplasia (CD) is a neurodevelopmental disorder due to aberrant cell proliferation and differentiation. Advances in neuroimaging have proven effective in early identification of the more severe lesions and timely surgical removal to treat epilepsy. However, the exact mechanisms of epileptogenesis are not well understood. This review examines possible mechanisms based on anatomical and electrophysiological studies. CD can be classified as CD type I consisting of architectural abnormalities, CD type II with the presence of dysmorphic cytomegalic neurons and balloon cells, and CD type III which occurs in association with other pathologies. Use of freshly resected brain tissue has allowed a better understanding of basic mechanisms of epileptogenesis and has delineated the role of abnormal cells and synaptic activity. In CD type II, it was demonstrated that balloon cells do not initiate epileptic activity, whereas dysmorphic cytomegalic and immature neurons play an important role in generation and propagation of epileptic discharges. An unexpected finding in pediatric CD was that GABA synaptic activity is not reduced, and in fact, it may facilitate the occurrence of epileptic activity. This could be because neuronal circuits display morphological and functional signs of dysmaturity. In consequence, drugs that increase GABA function may prove ineffective in pediatric CD. In contrast, drugs that counteract depolarizing actions of GABA or drugs that inhibit the mammalian target of rapamycin (mTOR) pathway could be more effective.

Expression of TRPV1 in cortical lesions from patients with tuberous sclerosis complex and focal cortical dysplasia type IIb

Tuberous sclerosis complex (TSC) and focal cortical dysplasia type IIb (FCDIIb) are recognized as causes of intractable epilepsy. Transient receptor potential vanilloid receptor 1 (TRPV1), a member of the transient receptor potential family, is the capsaicin receptor and is known to be involved in peripheral nociception. Recent evidence suggested that TRPV1 may be a contributing factor in epileptogenicity. Here, we evaluated the expression of TRPV1 in the cortical lesions of TSC and FCDIIb relative to normal control cortex. TRPV1 was studied in epilepsy surgery cases with TSC (cortical tubers; n=12) and FCDIIb (n=12) using immunocytochemistry, confocal analysis, and Western blotting (WB). Immunohistochemical location of the TRPV1 was predominately detected in the abnormal cell types, such as dysmorphic neurons, balloon cells (BCs) and giant cells. Co-localization assays further revealed that cells expressing TRPV1 mainly had a neuronal lineage, apart from some BCs in FCDIIb, which obviously were of astrocytic lineage. The increased TRPV1 expression within the dysplastic cortex of TSC and FCDIIb was confirmed by WB. Interestingly, both immunohistochemical and WB data indicated that TRPV1 might have both cytoplasm and nuclear distribution, suggesting a potential nuclear role of TRPV1. The over-expression of TRPV1 in cortical lesions of TSC and FCDIIb suggested the possible involvement of TRPV1 in the intrinsic and increased epileptogenicity of malformations of cortical development associated epilepsy diseases and may represent a potential antiepileptogenic target. However, the current data are merely descriptive, and further electrophysiological investigation is needed in the future.

Evolving neurobiology of tuberous sclerosis complex

Over the past decade, there have been numerous advances in our understanding of the molecular pathogenesis of tuberous sclerosis complex (TSC). Following the identification of the TSC1 and TSC2 genes, a link to regulatory control of the mammalian target of rapamycin (mTOR) signaling pathway has paved the way for new therapeutic interventions, and now even approved therapies for TSC. Gene identification has permitted establishment of cell lines and conditional knockout mouse strains to assay how abnormalities in brain structure lead to enhanced excitability, seizures, cognitive disabilities, and other neuropsychological disorders in TSC. Furthermore, work in in vitro systems and analysis of rodent models and human tissue has allowed investigators to study how brain lesions form in TSC. Evolving questions over the next decade include understanding the high clinical variability of TSC, defining why there is a lack of clear genotype-phenotype correlations, and identifying biomarkers for prognosis and stratification. The study of TSC has in many ways reflected a paradigm "bench-to-bedside" success story that serves as a model of many other neurological disorders.

A circuitry and biochemical basis for tuberous sclerosis symptoms: from epilepsy to neurocognitive deficits

Tuberous sclerosis complex (TSC) is an autosomal dominant monogenetic disorder that is characterized by the formation of benign tumors in several organs as well as brain malformations and neuronal defects. TSC is caused by inactivating mutations in one of two genes, TSC1 and TSC2, resulting in increased activity of the mammalian Target of Rapamycin (mTOR). Here, we explore the cytoarchitectural and functional CNS aberrations that may account for the neurological presentations of TSC, notably seizures, hydrocephalus, and cognitive and psychological impairments. In particular, recent mouse models of brain lesions are presented with an emphasis on using electroporation to allow the generation of discrete lesions resulting from loss of heterozygosity during perinatal development. Cortical lesions are thought to contribute to epileptogenesis and worsening of cognitive defects. However, it has recently been suggested that being born with a mutant allele without loss of heterozygosity and associated cortical lesions is sufficient to generate cognitive and neuropsychiatric problems. We will thus discuss the function of mTOR hyperactivity on neuronal circuit formation and the potential consequences of being born heterozygous on neuronal function and the biochemistry of synaptic plasticity, the cellular substrate of learning and memory. Ultimately, a major goal of TSC research is to identify the cellular and molecular mechanisms downstream of mTOR underlying the neurological manifestations observed in TSC patients and identify novel therapeutic targets to prevent the formation of brain lesions and restore neuronal function.

Electrocorticographic evidence of perituberal cortex epileptogenicity in tuberous sclerosis complex

OBJECT

Tuberous sclerosis complex (TSC) is a multisystem autosomal dominant disorder resulting in hamartomas of several organs. Cortical tubers are the most prominent brain lesions in TSC. Treatment-resistant epilepsy often develops early in life in patients with TSC and is associated with severe intellectual and behavioral impairments. Seizures may remit following epilepsy surgery in selected cases, yet it remains unclear whether the tuber or the perituberal cortex is the source of seizure onset. In this study, the authors reviewed the onset of seizures in patients in whom depth electrodes had been placed within or adjacent to cortical tubers.

METHODS

After obtaining institutional review board approval, the authors retrospectively reviewed data from 12 pediatric patients with multifocal TSC and treatment-resistant epilepsy who had undergone invasive intracranial electroencephalographic monitoring. Tubers were identified on postimplantation MRI, and all depth electrodes were located. Depth electrode contacts were classified visually as either tuber/perituberal cortex or nontuber/nonperituberal cortex. Board-certified clinical neurophysiologists reviewed the seizures to identify all electrodes involved in the ictal onset.

RESULTS

Among 309 recorded seizures, 104 unique ictal onset patterns were identified. Of the 11 patients with electrodes recording in a tuber, 9 had seizure onsets involving the tuber. Similarly, of the 9 patients with perituberal recording electrodes, 7 had perituberal ictal onsets. Overall, there was no difference in the percentage of contacts involved in seizure onset between the tuber and perituberal cortex. In a subset of 7 patients in whom at least 1 depth electrode contact was within the tuber and 1 was in the perituberal cortex, there was no difference between the percentage of tuber and perituberal onsets.

CONCLUSIONS

Findings demonstrated heterogeneity in the ictal onset patterns as well as involvement of the tuber and perituberal cortex within and between patients. Although the data are limited by the restricted region(s) sampled with intracranial electrodes, they do suggest that cortical hyperexcitability in TSC may derive from the tuber or surrounding cortex.

Are developmental dysplastic lesions epileptogenic?

Cortical dysplasia of various types, reflecting abnormalities of brain development, have been closely associated with epileptic activities. Yet, there remains considerable discussion about if/how these structural lesions give rise to seizure phenomenology. Animal models have been used to investigate the cause-effect relationships between aberrant cortical structure and epilepsy. In this article, we discuss three such models: (1) the Eker rat model of tuberous sclerosis, in which a gene mutation gives rise to cortical disorganization and cytologically abnormal cellular elements; (2) the p35 knockout mouse, in which the genetic dysfunction gives rise to compromised cortical organization and lamination, but in which the cellular elements appear normal; and (3) the methylazoxymethanol-exposed rat, in which time-specific chemical DNA disruption leads to abnormal patterns of cell formation and migration, resulting in heterotopic neuronal clusters. Integrating data from studies of these animal models with related clinical observations, we propose that the neuropathologic features of these cortical dysplastic lesions are insufficient to determine the seizure-initiating process. Rather, it is their interaction with a more subtly disrupted cortical "surround" that constitutes the circuitry underlying epileptiform activities as well as seizure propensity and ictogenesis.

Epilepsy and epileptic syndrome

Epilepsy is one of the most common neurological disorders. In most patients with epilepsy, seizures respond to available medications. However, a significant number of patients, especially in the setting of medically-intractable epilepsies, may experience different degrees of memory or cognitive impairment, behavioral abnormalities or psychiatric symptoms, which may limit their daily functioning. As a result, in many patients, epilepsy may resemble a neurodegenerative disease. Epileptic seizures and their potential impact on brain development, the progressive nature of epileptogenesis that may functionally alter brain regions involved in cognitive processing, neurodegenerative processes that relate to the underlying etiology, comorbid conditions or epigenetic factors, such as stress, medications, social factors, may all contribute to the progressive nature of epilepsy. Clinical and experimental studies have addressed the pathogenetic mechanisms underlying epileptogenesis and neurodegeneration.We will primarily focus on the findings derived from studies on one of the most common causes of focal onset epilepsy, the temporal lobe epilepsy, which indicate that both processes are progressive and utilize common or interacting pathways. In this chapter we will discuss some of these studies, the potential candidate targets for neuroprotective therapies as well as the attempts to identify early biomarkers of progression and epileptogenesis, so as to implement therapies with early-onset disease-modifying effects.

Mammalian target of rapamycin (mTOR) activation in focal cortical dysplasia and related focal cortical malformations

Focal cortical dysplasia (FCD) and other localized malformations of cortical development represent common causes of intractable pediatric epilepsy. Insights into the cellular and molecular pathogenesis of focal cortical malformations may reveal information about associated mechanisms of epileptogenesis and suggest new therapies for seizures caused by these developmental lesions. In animal models and human studies of FCD and the related disease of Tuberous Sclerosis Complex (TSC), the mammalian target of rapamycin (mTOR) pathway has been implicated in mediating cellular and molecular changes leading to the formation of the cortical malformations and the expression of epilepsy. The use of mTOR inhibitors may represent a rational therapeutic strategy for treating or even preventing epilepsy due to FCD and TSC.

Enhanced GABAergic network and receptor function in pediatric cortical dysplasia Type IIB compared with Tuberous Sclerosis Complex

Tuberous Sclerosis Complex (TSC) and cortical dysplasia Type IIB (CDIIB) share histopathologic features that suggest similar epileptogenic mechanisms. This study compared the morphological and electrophysiological properties of cortical cells in tissue from pediatric TSC (n=20) and CDIIB (n=20) patients using whole-cell patch clamp recordings and biocytin staining. Cell types were normal-appearing and dysmorphic-cytomegalic pyramidal neurons, interneurons, and giant/balloon cells, including intermediate neuronal-glial cells. In the cortical mantle, giant/balloon cells occurred more frequently in TSC than in CDIIB cases, whereas cytomegalic pyramidal neurons were found more frequently in CDIIB. Cell morphology and membrane properties were similar in TSC and CDIIB cases. Except for giant/balloon and intermediate cells, all neuronal cell types fired action potentials and displayed spontaneous postsynaptic currents. However, the frequency of spontaneous glutamatergic postsynaptic currents in normal pyramidal neurons and interneurons was significantly lower in CDIIB compared with TSC cases and the GABAergic activity was higher in all neuronal cell types in CDIIB. Further, acutely dissociated pyramidal neurons displayed higher sensitivity to exogenous application of GABA in CDIIB compared with TSC cases. These results indicate that, in spite of similar histopathologic features and basic cell membrane properties, TSC and CDIIB display differences in the topography of abnormal cells, excitatory and inhibitory synaptic network properties, and GABA(A) receptor sensitivity. These differences support the notion that the mechanisms of epileptogenesis could differ in patients with TSC and CDIIB. Consequently, pharmacologic therapies should take these findings into consideration.

A pulse rapamycin therapy for infantile spasms and associated cognitive decline

Infantile spasms are seizures manifesting within a spectrum of epileptic encephalopathies of infancy that often lead to cognitive impairment. Their current therapies, including adrenocorticotropic hormone (ACTH), high dose steroids, or vigabatrin, are not always effective and may be associated with serious side effects. Overactivation of the TORC1 complex of the mTOR pathway is implicated in the pathogenesis of certain genetic and acquired disorders that are linked with infantile spasms, like tuberous sclerosis. Here, we tested the therapeutic potential of rapamycin, a TORC1 inhibitor, as a potential treatment for infantile spasms in the multiple-hit rat model of ACTH-refractory symptomatic infantile spasms, which is not linked to tuberous sclerosis. Rapamycin or vehicle was given after spasms appeared. Their effects on spasms, other seizures, performance in Barnes maze, and expression of the phosphorylated S6 ribosomal protein (pS6: a TORC1 target) in the cortex, using immunofluorescence, were compared. Rapamycin suppressed spasms dose-dependently and improved visuospatial learning, although it did not reduce the frequency of other emerging seizures. High-dose pulse rapamycin effected acute and sustained suppression of spasms and improved cognitive outcome, without significant side effects. Therapeutically effective rapamycin doses normalized the pS6 expression, which was increased in perilesional cortical regions of pups with spasms. These findings support that pathological overactivation of TORC1 may be implicated in the pathogenesis of infantile spasms, including those that are not linked to tuberous sclerosis. Furthermore, a high-dose, pulse rapamycin treatment is a promising, well tolerated and disease-modifying new therapy for infantile spasms, including those refractory to ACTH.