Neuronal cytoskeletal abnormalities in human cerebral cortical dysplasia

Neurofilament heavy phosphorylated epitopes as biomarkers in ageing and neurodegenerative disease

From the day we are born, the nervous system is subject to insult, disease and degeneration. Aberrant phosphorylation states in neurofilaments, the major intermediate filaments of the neuronal cytoskeleton, accompany and mediate many pathological processes in degenerative disease. Neuronal damage, degeneration and death can release these internal components to the extracellular space and eventually the cerebrospinal fluid and blood. Sophisticated assay techniques are increasingly able to detect their presence and phosphorylation states at very low levels, increasing their utility as biomarkers and providing insights and differential diagnosis for the earliest stages of disease. Although a variety of studies focus on single or small clusters of neurofilament phosphorylated epitopes, this review offers a wider perspective of the phosphorylation landscape of the neurofilament heavy subunit, a major intermediate filament component in both ageing and disease.

Surgery for amygdala enlargement with mesial temporal lobe epilepsy: pathological findings and seizure outcome

OBJECTIVE

Amygdala enlargement (AE) has been suggested to be a subtype of mesial temporal lobe epilepsy (MTLE). However, most reports related to AE have referred to imaging studies, and there have been few reports regarding surgical and pathological findings. The present study was performed to clarify the surgical outcomes and pathology of AE.

METHODS

Eighty patients with drug-resistant MTLE were treated surgically at the Tokyo Metropolitan Neurological Hospital between April 2010 and July 2013. Of these patients, 11 were diagnosed as AE based on presurgical MRI. Nine patients with AE underwent selective amygdalohippocampectomy, while the remaining two patients underwent selective amygdalotomy with hippocampal transection. Intraoperative EEG was routinely performed. The histopathology of the resected amygdala tissue was evaluated and compared with the amygdala tissue of patients with hippocampal sclerosis.

RESULTS

Pathological findings indicated that 10 of 11 specimens had closely clustering hypertrophic neurons with vacuolisation of the background matrix. Slight gliosis was seen in nine specimens, while the remaining two showed no gliotic changes. Intraoperative EEG showed abnormal sharp waves that seemed to originate not from the amygdala but from the hippocampus in all cases. Ten patients became seizure-free during the postoperative follow-up period.

CONCLUSIONS

Histopathologically, clustering hypertrophic neurons and vacuolation with slight gliosis or without gliosis were considered to be pathological characteristics of AE. Amygdalohippocampectomy or hippocampal transection with amygdalotomy is effective for seizure control in patients with AE.

Cytoskeletal proteins in cortical development and disease: actin associated proteins in periventricular heterotopia

The actin cytoskeleton regulates many important cellular processes in the brain, including cell division and proliferation, migration, and cytokinesis and differentiation. These developmental processes can be regulated through actin dependent vesicle and organelle movement, cell signaling, and the establishment and maintenance of cell junctions and cell shape. Many of these processes are mediated by extensive and intimate interactions of actin with cellular membranes and proteins. Disruption in the actin cytoskeleton in the brain gives rise to periventricular heterotopia (PH), a malformation of cortical development, characterized by abnormal neurons clustered deep in the brain along the lateral ventricles. This disorder can give rise to seizures, dyslexia and psychiatric disturbances. Anatomically, PH is characterized by a smaller brain (impaired proliferation), heterotopia (impaired initial migration) and disruption along the neuroependymal lining (impaired cell-cell adhesion). Genes causal for PH have also been implicated in actin-dependent processes. The current review provides mechanistic insight into actin cytoskeletal regulation of cortical development in the context of this malformation of cortical development.

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.

Emerging concepts in Alzheimer's disease

Alzheimer's disease/senile dementia of the Alzheimer type (AD/SDAT) is the most common neuropathologic substrate of dementia. It is characterized by synapse loss (predominantly within neocortex) as well as deposition of certain distinctive lesions (the result of protein misfolding) throughout the brain. The latter include senile plaques, composed mainly of an amyloid (Aβ) core and a neuritic component; neurofibrillary tangles, composed predominantly of hyperphosphorylated tau; and cerebral amyloid angiopathy, a microangiopathy affecting both cerebral cortical capillaries and arterioles and resulting from Aβ deposition within their walls or (in the case of capillaries) immediately adjacent brain parenchyma. In this article, I discuss the hypothesized role these lesions play in causing cerebral dysfunction, as well as CSF and neuroimaging biomarkers (for dementia) that are especially relevant as immunotherapeutic approaches are being developed to remove Aβ from the brain parenchyma. In addition, I address the role of neuropathology in characterizing the sequelae of new AD/SDAT therapies and helping to validate CSF and neuroimaging biomarkers of disease. Comorbidity of AD/SDAT and various types of cerebrovascular disease is a major theme in dementia research, especially as cognitive impairment develops in the oldest old, who are especially vulnerable to ischemic and hemorrhagic brain lesions.

Surgical pathology of epilepsy-associated non-neoplastic cerebral lesions: a brief introduction with special reference to hippocampal sclerosis and focal cortical dysplasia

Among epilepsy-associated non-neoplastic lesions, mesial temporal lobe epilepsy with hippocampal sclerosis (mTLE-HS) and malformation of cortical development (MCD), including focal cortical dysplasia (FCD), are the two most frequent causes of drug-resistant focal epilepsies, constituting about 50% of all surgical pathology of epilepsy. Several distinct histological patterns have been historically recognized in both HS and FCD, and several studies have tried to perform clinicopathological correlations. However, results have been controversial, particularly in terms of post-surgical seizure outcome. Recently, the International League Against Epilepsy constituted a Task Forces of Neuropathology and FCD within the Commission on Diagnostic Methods, to establish an international consensus of histological classification of HS and FCD, respectively, based on agreement with the recognition of the importance of defining a histopathological classification system that reliably has some clinicopathological correlation. Such consensus classifications are likely to facilitate future clinicopathological studies. Meanwhile, we reviewed the neuropathology of 41 surgical cases of mTLE, and confirmed three type/patterns of HS along with no HS, based on the qualitative evaluation of the distribution and severity of neuronal loss and gliosis within hippocampal formation, that is, HS type 1 (61%) equivalent to "classical" Ammon's horn sclerosis, HS type 2 (2%) representing CA1 sclerosis, HS type 3 (17%) equivalent to end folium sclerosis, and no HS (19%). Furthermore, we performed a neuropathological comparative study on mTLE-HS and dementia-associated HS (d-HS) in the elderly, and confirmed that neuropathological features differ between mTLE-HS and d-HS in the distribution of hippocampal neuronal loss and gliosis, morphology of reactive astrocytes and their protein expression, and presence of concomitant neurodegenerative changes, particularly Alzheimer type and TDP-43 pathologies. These differences may account, at least in part, for the difference in pathogenesis and epileptogenicity of HS in mTLE and senile dementia. However, the etiology and pathogenesis of most epileptogenic lesions are yet to be elucidated.

Chronic temporal lobe epilepsy is associated with enhanced Alzheimer-like neuropathology in 3×Tg-AD mice

The comorbidity between epilepsy and Alzheimer's disease (AD) is a topic of growing interest. Senile plaques and tauopathy are found in epileptic human temporal lobe structures, and individuals with AD have an increased incidence of spontaneous seizures. However, why and how epilepsy is associated with enhanced AD-like pathology remains unknown. We have recently shown β-secretase-1 (BACE1) elevation associated with aberrant limbic axonal sprouting in epileptic CD1 mice. Here we sought to explore whether BACE1 upregulation affected the development of Alzheimer-type neuropathology in mice expressing mutant human APP, presenilin and tau proteins, the triple transgenic model of AD (3×Tg-AD). 3×Tg-AD mice were treated with pilocarpine or saline (i.p.) at 6-8 months of age. Immunoreactivity (IR) for BACE1, β-amyloid (Aβ) and phosphorylated tau (p-tau) was subsequently examined at 9, 11 or 14 months of age. Recurrent convulsive seizures, as well as mossy fiber sprouting and neuronal death in the hippocampus and limbic cortex, were observed in all epileptic mice. Neuritic plaques composed of BACE1-labeled swollen/sprouting axons and extracellular AβIR were seen in the hippocampal formation, amygdala and piriform cortices of 9 month-old epileptic, but not control, 3×Tg-AD mice. Densities of plaque-associated BACE1 and AβIR were elevated in epileptic versus control mice at 11 and 14 months of age. p-Tau IR was increased in dentate granule cells and mossy fibers in epileptic mice relative to controls at all time points examined. Thus, pilocarpine-induced chronic epilepsy was associated with accelerated and enhanced neuritic plaque formation and altered intraneuronal p-tau expression in temporal lobe structures in 3×Tg-AD mice, with these pathologies occurring in regions showing neuronal death and axonal dystrophy.

Pathologic lesions in neurodegenerative diseases

This chapter will discuss two of the most widely used approaches to assessing brain structure: neuroimaging and neuropathology. Whereas neuropathologic approaches to studying the central nervous system have been utilized for many decades and have provided insights into morphologic correlates of dementia for over 100 years, accurate structural imaging techniques "blossomed" with the development and refinement of computerized tomographic scanning and magnetic resonance imaging (MRI), beginning in the late 1970s. As Alzheimer disease progresses over time, there is progressive atrophy of the hippocampus and neocortex--this can be quantified and regional accentuation of the atrophy can be evaluated using quantitative MRI scanning. Furthermore, ligands for amyloid proteins have recently been developed--these can be used in positron emission tomography studies to localize amyloid proteins, and (in theory) study the dynamics of their deposition (and clearance) within the brain over time. Neuropathologic studies of the brain, using highly specific antibodies, can demonstrate synapse loss and the deposition of proteins important in AD progression--specifically ABeta and phosphor-tau. Finally, neuropathologic assessment of (autopsy) brain specimens can provide important correlation with sophisticated neuroimaging techniques.

Altered firing rates and patterns in interneurons in experimental cortical dysplasia

Cortical dysplasia (CD) is associated with severe epilepsy in humans, and the in utero irradiation of fetal rats provides a model of this disorder. These animals show a selective loss of inhibitory interneurons, and the surviving interneurons have a reduced excitatory synaptic drive. The current study was undertaken to see how alterations in synaptic input would affect spontaneous firing of interneurons in dysplastic cortex. We recorded spontaneous action potentials and excitatory and inhibitory postsynaptic currents (EPSCs and IPSCs, respectively) from somatostatin (SST)-, parvalbumin (PV)-, and calretinin (CR)-immunoreactive (ir) interneurons. We found that SST- and PV-ir interneurons fired less frequently and with less regularity than controls. This corresponded to a relative imbalance in the ratio of EPSCs to IPSCs that favored inhibition. In contrast, CR-ir interneurons from CD showed no differences from controls in spontaneous firing or ratio of EPSCs to IPSCs. Additional studies demonstrated that synaptic input had a powerful effect on spontaneous firing in all interneurons. These findings demonstrate that a relative reduction in excitatory drive results in less active SST- and PV-ir interneurons in irradiated rats. This would further impair cortical inhibition in these animals and may be an important mechanism of epileptogenesis.

Focal brain malformations: seizures, signaling, sequencing

Focal malformations of cortical development are highly associated with intractable epilepsy in children and adults. Most patients with focal cortical malformations and epilepsy will require epilepsy surgery. Recent studies have provided new insights into the developmental pathogenesis of cortical malformations specifically relating to alterations in cell signaling though the mammalian target of rapamycin (mTOR) pathway. Focal cortical dysplasias, hemimegalencephaly, and tubers in tuberous sclerosis complex all exhibit evidence for hyperactive mTOR signaling, suggesting that these disorders form a spectrum of malformations or "TORopathies" characterized by disorganized cortical lamination, cytomegaly, and intractable seizures. Alterations in mTOR activity in focal brain malformations provide a potential pathogenic pathway to investigate for gene mutations and to exploit for animal models. Most importantly, however, if select focal cortical malformations result from enhanced mTOR signaling, new therapeutic antiepileptic compounds, such as rapamycin, can be designed and tested that specifically target mTOR signaling.

Pathological Tau Tangles Localize to Focal Cortical Dysplasia in Older Patients

PURPOSE

Reactivation of neurodevelopmental processes may contribute to neurodegeneration. For example, the proteins cyclin dependent kinase 5 (cdk5) and glycogen synthase kinase 3 beta (GSK3beta), which are essential to normal cortical development, can hyperphosphorylate tau and might contribute to the pathogenesis of Alzheimer's disease. Focal cortical dysplasia (FCD) is an important neurodevelopmental cause of refractory human epilepsy within which dysplastic neurons exhibit increased immunoreactivity for cdk5 and GSK3beta as well as neurofilamentous accumulations. We therefore hypothesized that the developmentally abnormal cortex of FCD might be more susceptible to tau-mediated neurodegeneration than adjacent histologically normal cortex.

MATERIALS AND METHODS

We examined a series of 15 cases of FCD, spanning a wide age range, for beta-amyloid, pathologically phosphorylated tau and neurofibrillary tangles using silver staining, immunohistochemistry for tau, AT8, RD3, RD4 and two-dimensional cell counting.

RESULTS

Beta-amyloid plaques, aberrantly phosphorylated tau and neurofibrillary tangles are only found in older patients. The hyperphosphorylated tau tangles are confined to dysplastic neurons. Immunoreactivity for 3- and 4-repeat tau was again only detected within regions of FCD in older patients. With increasing age, the dysplastic cortex became hypocellular and a higher proportion of dysplastic neurons exhibited pathological tau phosphorylation.

CONCLUSIONS

In older patients, FCD appears more susceptible to formation of pathologically phosphorylated tau neurofibrillary tangles than adjacent histologically normal cortex. Our results suggest a novel convergence of pathological neurodevelopment with pathological age-related neurodegeneration.

Neocortical hyperexcitability in a human case of tuberous sclerosis complex and mice lacking neuronal expression of TSC1

OBJECTIVE

To identify brain regions, cell types, or both that generate abnormal electrical discharge in tuberous sclerosis complex (TSC). Here we examined excitatory and inhibitory synaptic currents in human tissue samples obtained from a TSC patient with no discernible cortical tubers and acute neocortical brain slices from a mouse featuring synapsin-driven conditional deletion of a TSC1 gene. These studies were designed to assess whether TSC gene inactivation alters excitability.

METHODS

We used visualized patch-clamp (human and mouse) and extracellular field (mouse) recordings. Additional mice were processed for immunohistochemistry or Western blot analysis.

RESULTS

Detailed anatomic studies in brain tissue sections from synapsin-TSC1 conditional knock-out mice failed to uncover gross anatomic defects, loss of lamination, or frank tuber formation. However, regions of abnormal and potentially activated neocortex were shown using antibodies to nonphosphorylated neurofilaments (SMI-311) and immediate early genes (c-Fos). Extracellular recordings from neocortical slices, examining synaptic activity in these regions, demonstrated clear differences in excitability between conditional knock-out and age-matched control mice. Whole-cell patch-clamp recordings demonstrated excitatory synaptic currents with strikingly long duration and epileptiform discharge patterns, similar to waveforms observed in our human tissue samples. These events were 1-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) receptor mediated and were most prominent in neocortex. Normal-appearing inhibitory postsynaptic currents (human) and intrinsic neuronal firing patterns (mouse) were also recorded.

INTERPRETATION

This combination of human and mouse tissue studies suggests, for the first time, that synaptic excitation is altered in a direction that favors seizure generation in TSC brain tissue regardless of cortical tubers.

Dystrophic neuritic processes in epileptic cortex

Cortical dysplasia is a frequent finding in cortical resections from children with refractory epilepsy. Diagnostic criteria and a classification scheme for cortical dysplasia has been proposed, though the relationship between specific cortical dysplasia features and their causal relationship with epilepsy is poorly understood. We reviewed 28 surgical resections from children and identified a common and easily recognized feature of cortical dysplasia: maloriented, misshapen and occasionally coarse neurofilament stained process forming a dystrophic neuritic background. The dystrophic neuritic background was associated with other features of cortical dysplasia in all 28 patients with cortical dysplasia, 26 with refractory epilepsy and 2 patients with other neurologic diagnoses. In seven children with refractory epilepsy due to other pathologic diagnosis such as vascular or glial lesions, the dystrophic neuritic background was only found in one patient with a ganglioglioma and other features suggestive of an associated cortical dysplasia. Our data indicate that a dystrophic neuritic background is a common and relatively specific neuropathologic finding in cortical dysplasia.

Neurodevelopmental disorders as a cause of seizures: neuropathologic, genetic, and mechanistic considerations

This review will consider patterns of developmental neuropathologic abnormalities-malformations of cortical development (MCD)--encountered in infants (often with infantile spasms), children, and adults with intractable epilepsy. Treatment of epilepsy associated with some MCD, such as focal cortical dysplasia and tubers of tuberous sclerosis, may include cortical resection performed to remove the "dysplastic" region of cortex. In extreme situations (eg, hemimegalencephaly), hemispherectomy may be carried out on selected patients. Neuropathologic (including immunohistochemical) findings within these lesions will be considered. Other conditions that cause intractable epilepsy and often mental retardation, yet are not necessarily amenable to surgical treatment (eg, lissencephaly, periventricular nodular heterotopia, double cortex syndrome) will be discussed. Over the past 10 years there has been an explosion of information on the genetics of MCD. The genes responsible for many MCD (eg, TSC1, TSC2, LIS-1, DCX, FLN1) have been cloned and permit important mechanistic studies to be carried out with the purpose of understanding how mutations within these genes result in abnormal cortical cytoarchitecture and anomalous neuroglial differentiation. Finally, novel techniques allowing for analysis of patterns of gene expression within single cells, including neurons, is likely to provide answers to the most vexing and important question about these lesions: Why are they epileptogenic?

Epilepsy-induced microarchitectural changes in the brain

Our understanding of the pathogenesis of the neuropathology of epilepsy has been challenged by a need to separate the "lesions" that cause epilepsy from the "lesions" that are produced by the epilepsy. Significant clinical, genetic, pathologic, and experimental studies of Ammon horn sclerosis (AHS) suggest that AHS is the result and cause of seizures. The data support the idea that seizures cause alterations in cell numbers, cell shape, and organization of neuronal circuitry, thus setting up an identifiable seizure-genic focus. As such, AHS represents a slowly progressive lesion and a search for the cause of the initiating seizure has led to the identification of ion channel mutations. In this report, the neuropathology of other conditions associated with intractable epilepsy is considered, suggesting that in them similar epilepsy-produced alterations in microarchitecture can be observed. The idea is important to define the optimum time for epilepsy surgery and the underlying etiology of these seizure-genic lesions.

Aberrant neuronal-glial differentiation in Taylor-type focal cortical dysplasia (type IIA/B)

Focal cortical dysplasia (FCD) type IIA/B (Taylor type) is a malformation of cortical development characterized by laminar disorganization and dysplastic neurons. FCD IIA and FCD IIB denote subtypes in which balloon cells are absent or present, respectively. The etiology of FCD IIA/B is unknown, but previous studies suggest that its pathogenesis may involve aberrant, mixed neuronal-glial differentiation. To investigate whether aberrant differentiation is a consistent phenotype in FCD IIA/B, we studied a panel of neuronal and glial marker antigens in a series of 15 FCD IIB cases, and 2 FCD IIA cases. Double-labeling immunofluorescence and confocal imaging revealed that different combinations of neuronal and glial antigens were co-expressed by individual cells in all cases of FCD IIA/B, but not in control cases of epilepsy due to other causes. Co-expression of neuronal and glial markers was most common in balloon cells, but was also observed in dysplastic neurons. The relative expression of neuronal and glial antigens varied over a broad range. Microtubule-associated protein 1B, an immature neuronal marker, was more frequently co-expressed with glial antigens than were mature neuronal markers, such as neuronal nuclear antigen. Our results indicate that aberrant neuronal-glial differentiation is a consistent and robust phenotype in FCD IIA/B, and support the hypothesis that developmental defects of neuronal and glial fate specification play an important role in its pathogenesis.

Activated remodeling and N-methyl-D-aspartate (NMDA) receptors in cortical dysplasia

Cortical dysplasia is now recognized as one of the major etiologies causing intractable epilepsy in childhood. Dysplastic cortex displays cortical dyslamination, which is often associated with dysmorphic large neurons and less frequently with balloon cells. The dysmorphic large neurons are commonly located in the subcortical white matter and cerebral cortex, with enlarged nuclei with a single prominent nucleolus and showing aberrant cytoskeletal changes. I have shown that dysmorphic large neurons have several immature types of cytoskeletal proteins, such as the low-molecular-weight form of microtubule-associated protein 2 (MAP2) and MAP1B, which are involved in the outgrowth and modeling of neuronal processes in the immature brain. I have also reported that dysmorphic large neurons also have enhanced gene expression of growth-associated protein GAP43, which is a phosphoprotein enriched at presynaptic nerve terminals and is thought to be involved in axonal outgrowth and plasticity in synaptic connections. Finally, I have shown that the N-methyl-D-aspartate acid (NMDA) receptor R1 gene is up-regulated in the dysmorphic large neurons and nearly normal-sized neurons located in the dysplastic cortex. This evidence suggests that growth of neuronal processes and activated excitatory synaptic remodeling exist in the epileptic conditions of cortical dysplasia.

Molecular pathogenesis of focal cortical dysplasia and hemimegalencephaly

My laboratory recently demonstrated that there is selective expression of phosphoribosomal S6 protein in balloon cells in focal cortical dysplasia and hemimegalencephaly but no expression of the upstream kinase, phospho-p70S6 kinase. Two proteins activated by phospho-p70S6 kinase, phospho-STAT3 and phospho-4EBP1, were not detected in balloon cells. Using complementary DNA arrays in hemimegalencephaly specimens, we found increased expression of cyclin D1 and c-myc messenger ribonucleic acids (RNAs). Expression of cyclin D1 and c-myc genes is transcriptionally activated by beta-catenin. Western analysis demonstrated increased levels of nonphosphorylated beta-catenin in hemimegalencephalic cortex. Reduced levels of Ser33, Ser37, and Thr41 phospho-beta-catenin, sites known to be phosphorylated by glycogen synthase kinase 3 and to be essential for beta-catenin inactivation, were detected in hemimegalencephaly. Enhanced transcription of cyclin D1 and c-myc messenger RNAs, increased transcriptionally active beta-catenin, and decreased Ser33/Ser37/Thr41 phospho-beta-catenin suggest activation of the Wnt-1/beta-catenin cascade in hemimegalencephaly, which can lead to aberrant cell proliferation and hemispheric enlargement during brain development. Enhanced activation of phospho-S6 and beta-catenin suggests two converging cell pathways that can be pivotal in the pathogenesis of focal cortical dysplasia and hemimegalencephaly.

Insulin signaling pathways in cortical dysplasia and TSC-tubers: tissue microarray analysis

To evaluate the possible roles of the Akt/PKB-mTOR-p70S6K-S6 and cap-dependent translation (eIF4G) pathways in the pathogenesis of tuberous sclerosis complex (TSC)-associated cortical tubers and focal cortical dysplasia (FCD), we performed qualitative and semiquantitative immunohistochemical evaluation on surgically resected corticectomy specimens to detect phosphorylated molecules as activated downstream targets of the signaling pathways. A tissue microarray paraffin block was constructed from 63 archival specimens of surgically resected TSC tubers, FCDs with balloon cells, cortical dysplasia without balloon cells, and histologically normal-appearing neocortex obtained from cases with Rasmussen encephalitis, cystic-gliotic encephalopathy, and temporal lobe epilepsy. Abnormal neuroglial cells were positive for phospho-S6 and phospho-eIF4G with various staining intensities in FCDs and TSC tubers. Both proteins were much less abundantly expressed in normal-appearing neocortex. Phospho-mTOR expression was observed in neurons in all groups. The expression of phospho-S6 and phospho-eIF4G was associated with dysplastic lesions (p < 0.05), and the cytoplasmic phospho-p70S6K expression was most specific for and abundant in TSC tubers and much less prominent in other groups (p < 0.01). These results suggest that constitutive activation of cytoplasmic p70S6K plays a pivotal role in the pathogenesis of TSC tubers and that FCDs possess a distinct mechanism for activation of S6 and eIF4G.

mTOR cascade activation distinguishes tubers from focal cortical dysplasia

Balloon cells (BCs) in focal cortical dysplasia (FCD) and giant cells (GCs) in tubers of the tuberous sclerosis complex (TSC) share phenotypic similarities. TSC1 or TSC2 gene mutations in TSC lead to mTOR pathway activation and p70S6kinase (phospho-S6K) and ribosomal S6 (phospho-S6) protein phosphorylation. Phospho-S6K, phospho-S6, and phospho-S6K-activated proteins phospho-STAT3 and phospho-4EBP1 were detected immunohistochemically in GCs, whereas only phospho-S6 was observed in BCs. Expression of four candidate gene families (cell signaling, cell adhesion, growth factor/receptor, and transcription factor mRNAs) was assayed in single, microdissected phospho-S6-immunolabeled BCs and GCs as a strategy to define whether BCs and GCs exhibit differential transcriptional profiles. Among 60 genes, differential expression of 24 mRNAs distinguished BCs from GCs and only 4 genes showed similar expression profiles between BCs and GCs. Tuberin mRNA levels were reduced in GCs from TSC patients with TSC2 gene mutations but were unchanged in BCs. Phospho-S6K, -S6, -STAT3, and -4EBP1 expression in GCs reflects loss of hamartin-tuberin-mediated mTOR pathway inhibition. Phospho-S6 expression alone in BCs does not support mTOR cascade activation in FCD. Differential gene expression profiles in BCs and GCs supports the hypothesis that these cell types derive by distinct pathogenic mechanisms.

Long-term outcome after epilepsy surgery for focal cortical dysplasia

OBJECT

Reports of outcomes for surgical treatment of cortical dysplasia associated with epilepsy are conflicting due to the inclusion of patients with a wide range of malformations of cortical development. The authors report their experience and the long-term outcome for a subgroup of patients with the histopathological diagnosis of focal cortical dysplasia of Taylor.

METHODS

The records of 22 patients with focal cortical dysplasia of Taylor (15 with the balloon-cell type and seven with the nonballoon-cell type) were reviewed. There were 11 female and 11 male patients whose mean age was 26 +/- 17.6 years (mean +/- standard deviation [SD]) at surgery. The details of their epilepsy evaluation and resection were analyzed. Extent of resection was preoperatively planned using information obtained from long-term intracranial monitoring (15 patients) and/or more definitively determined by histopathologically proven clear margins during resection when feasible (12 patients). The mean duration of follow up was 6.3 +/- 5.1 years (mean +/- SD, range 0.5-15.6 years). Risk factors for epilepsy were trauma (seven patients) or meningoencephalitis (one patient); 14 patients (64%) had no obvious risk factors. The mean age at seizure onset was 9.2 years and the mean duration of their epilepsy was 16.1 +/- 9 years. In two patients there were no adverse findings on magnetic resonance (MR) imaging. In 15 patients (68%), the epileptogenic zone identified on long-term intracranial monitoring extended beyond the abnormality observed on MR images. Focal resection (lesion plus margins) was performed in 14 patients (64%), whereas eight (36%) underwent partial/tailored lobectomy. Two patients underwent multiple subpial transections in addition to partial lesionectomy because their lesions involved the sensorimotor cortex. In these two, functional MR imaging confirmed a normal functional anatomy despite the presence of the cortical dysplasia. Eleven (92%) of 12 patients who underwent resection guided by histopathologically proven clear margins and three (43%) of seven patients who underwent histopathologically proven subtotal resection have remained seizure free. Evidence of clear margins was significantly associated with an improved seizure outcome (p = 0.003). Postoperatively, expected deficits included nondisabling visual field defects, which occurred in three patients (14%), and transient sensorimotor deficits, which appeared in five (23%). Two patients had meningitis, which was successfully treated with antibiotics. Overall, 16 patients (73%) are either seizure free (13 patients), have rare nondisabling partial seizures (one patient), or had one seizure after their medication was changed (two patients). Thirteen patients (59%) have discontinued anticonvulsant medications or are being maintained on monotherapy. Of five patients (23%), two have had rare disabling seizures or significant reduction in their seizure frequency (three patients). One patient's seizures have remained the same.

CONCLUSIONS

Focal cortical dysplasias are a distinct subgroup of malformations of cortical development and have a favorable outcome after resection. The epileptogenic zone often extends beyond the abnormality found on neuroimaging. Resection of the epileptogenic zone guided by histopathologically proven clear margins is associated with an improved seizure outcome.

Malformations of cortical development: molecular pathogenesis and experimental strategies

Malformations of cortical development (MCD) are developmental brain lesions characterized by abnormal formation of the cerebral cortex and a high clinical association with epilepsy in infants, children, and adults. Despite multiple anti-epileptic drugs (AEDs), treatment of epilepsy associated with MCD may require cortical resection performed to remove the cytoarchitecturally abnormal region of cortex. Single genes responsible for distinct MCD including lissencephaly, subcortical band heterotopia, and tuberous sclerosis, have been identified and permit important mechanistic insights into how gene mutations result in abnormal cortical cytoarchitecture. The pathogenesis of MCD such as focal cortical dysplasia, hemimegalencephaly, and polymicrogyria, remains unknown. A variety of new techniques including cDNA array analysis now allow for analysis of gene expression within MCD.

Novel MR contrasts to reveal more about the brain

Twenty percent of patients with refractory focal epilepsy have an undetermined etiologic basis for their epilepsy despite extensive investigation, including optimal MR imaging. Surgical treatment of this group is associated with a less favorable postoperative outcome. Even with improvements in imaging techniques, a proportion of these patients will remain "MR imaging-negative." It is likely, however, that some of the discrete macroscopic focal lesions that are currently occult will be identified by imaging techniques interrogating different microstructural characteristics. Furthermore, these methods may provide pathologic specificity when used in combination. The description and application of these techniques in epilepsy are the focus of this article.

Altered corticogenesis and neuronal morphology in irradiation-induced cortical dysplasia: a Golgi-Cox study

Cortical dysplasia has a strong clinical association with epilepsy and mental retardation, but the relationship between alterations in cortical structure and function in dysplasia-related disorders is poorly understood. The cerebral cortex of irradiated rats, an experimental model of cortical dysplasia, was studied using cresyl violet-stained sections and the Golgi-Cox method. The irradiated cortex is characterized by reductions in size, volume, and number of neurons and fibers reflecting the original lethal injury to neuronal precursors. Consequently, only neurons that survived this injury were able to continue their, albeit altered, development. The result is an altered corticogenesis characterized by neuronal, fiber circuitry, and microvascular alterations. Abnormal aggregates (nodules) of excitatory pyramidal neurons with altered dendritic profiles and functional territories are found between 200 and 400 microm from the pial surface. Their horizontal dendritic profiles and functional territories contrast with the vertical (columnar) dendritic profiles and functional territories of normal pyramidal neurons. This horizontal concentration of spiny dendrites and, hence, of excitatory synaptic contacts suggests a response to the presence of an abnormal horizontal plexus of afferent fibers terminals. Stellate neurons, some morphologically compatible with inhibitory basket cells, are also essential components of these nodules. Some neuronal nodules are characterized by a rich plexus of anastomotic capillaries that contrasts with the sparser vasculature of surrounding gray matter tissue. The presence of well-vascularized aggregates of altered pyramidal and inhibitory neurons suggests a high level of metabolic activity. Well-vascularized deep heterotopias are also found. We propose that the functional activity of well-vascularized neuronal nodules and heterotopias could play a role in the abnormal cortical function in this model.

Cortical dysplasia and epilepsy: animal models

Cortical dysplasia syndromes--those conditions of abnormal brain structure/organization that arise during aberrant brain development--frequently involve epileptic seizures. Neuropathological and neuroradiological analyses have provided descriptions and categorizations based on gross anatomical and cellular histological features (e.g., lissencephaly, heterotopia, giant cells), as well as on the developmental mechanisms likely to be involved in the abnormality (e.g., cell proliferation, migration). Recently, the genes responsible for several cortical dysplastic conditions have been identified and the underlying molecular processes investigated. However, it is still unclear how the various structural abnormalities associated with cortical dysplasia are related to (i.e., "cause") chronic seizures. To elucidate these relationships, a number of animal models of cortical dysplasia have been developed in rats and mice. Some models are based on laboratory manipulations that injure the brain (e.g., freeze, undercut, irradiation, teratogen exposure) of immature animals; others are based on spontaneous genetic mutations or on gene manipulations (knockouts/transgenics) that give rise to abnormal cortical structures. Such models of cortical dysplasia provide a means by which investigators can not only study the developmental mechanisms that give rise to these brain lesions, but also examine the cause-effect relationships between structural abnormalities and epileptogenesis.

Histopathology of brain tissue from patients with infantile spasms

This chapter reviews the patterns of neuropathologic abnormality encountered in cortical resections performed for the treatment of infantile spasms (ISS) in the broader context of the larger "universe" of neuropathology seen in the central nervous system of infants with this syndrome as well as older children with intractable epilepsy. Although destructive lesions (encephalomalacia), Sturge-Weber-Dimitri syndrome, and even neoplasms can cause ISS, its most common neuropathologic substrate is cortical dysplasia (CD), representing the result of aberrant neuronal migration to the neocortex from the germinal matrix. Ways to recognize CD morphologically and assess its severity are reviewed, as are special stains and immunohistochemical methods that may be useful in confirming the diagnosis. The similarity and relationship between (sporadic) CD and tubers of tuberous sclerosis are discussed. Future potentially fruitful research directions aimed at refining the clinicopathologic analysis of the role of CD in ISS and epilepsies of older children are briefly considered.

Neurotrophin receptor immunoreactivity in severe cerebral cortical dysplasia

PURPOSE

Cerebral cortical dysplasia (CD) is one of the important causes of intractable epilepsies and characterized histologically by disorganized cortical lamination and cytomegalic dysplastic neurons. Although it has been suggested that neurotrophins play an important role in differentiation, growth, and survival of developmental neurons, their pathogenetic role in CD has rarely been investigated.

METHODS

To know the pathogenetic role of various neurotrophins on dysplastic neurons, immunohistochemical staining was performed using antibodies against NGFRp75, trkA, trkB, and trkC in surgical specimens of 20 patients with CD.

RESULTS

TrkB and trkC were strongly expressed in dysplastic neurons of severe CD, and NGFRp75 was also expressed in some dysplastic neurons.

CONCLUSIONS

It is known that brain-derived neurotrophic factor (BDNF) and neurotrophin-3 (NT-3) contribute to the differentiation of neuronal precursor cells, dendritic and axonal arborization, synaptic plasticity, and cellular hyperexcitability, so increased expression of trkB and trkC may have a critical pathogenetic role in cytoskeletal abnormalities and epileptogenicity in dysplastic neurons of CD.

Abnormal expression of cdk5 in focal cortical dysplasia in humans

Focal cortical dysplasia (FCD) is an important cause of refractory epilepsy in humans. The origin of its pathognomonic abnormal cell types and the links between abnormal cell morphology and epileptogenicity remain unknown. The developmentally-regulated kinase cdk5 and its neuronal activator p35 are known to be central to a number of key components in neuronal development, cellular morphology, cytoskeletal function, synaptic plasticity and neurodegeneration. Here we examine eight cases of human FCD for expression of cdk5. We show abnormal cdk5 immunoreactivity and aggregation of protein suggesting alterations in cdk5 may also be involved in this important epileptogenic human pathology.

Cortical dysgenesis and epilepsy

Cortical dysgenesis (CD) describes a wide spectrum of brain anomalies that involve abnormal development of the cerebral cortex. There is a strong association between CD and epilepsy, and it comprises a significant proportion of children and adults whose epilepsy cannot be controlled with medications. There has been intense effort to define the relationship between CD and epilepsy so that more effective therapies can be devised. These efforts have ranged from detailed study of people with CD and epilepsy from a clinical standpoint to single-cell analysis of mRNA expression and postsynaptic receptor function. Animal models have also been developed to mimic certain aspects of CD in a situation when quantitative, controlled, and interventional experiments can be performed that would not be possible in a clinical setting. This review will give an overview of human CD syndromes and their causes, when possible, and describe some specific abnormalities in dysplastic cortex that may underlie its epileptogenic potential. It will also review several animal models of CD that have been studied mechanistically from the standpoint of epileptogenesis. In conclusion, some general trends will be proposed based on human and animal studies to encapsulate our current understanding of CD and how it causes epilepsy.

A review of gene expression patterns in the malformed brain

Major advances in the identification of genes expressed in malformation-associated epileptic disorders have been made. Some of these changes reflect the complex gene interactions necessary for proper neurodevelopment, whereas others suggest specific synaptic aberrations that could result in a hyperexcitable, and ultimately, epileptic condition. Here we review reported changes in gene expression associated with a malformed brain, with particular emphasis on how these changes provide clues to seizure genesis.

Pathologic characteristics of the cortical dysplasias

The gross and microscopic features of cortical dysplasia (malformations caused by abnormalities of cortical development) are reviewed and illustrated in this article. The pathologic associations of neurocutaneous disorders, neoplasms, and hippocampal sclerosis with cortical dysplasia also are discussed.

Drug resistance in epilepsy: expression of drug resistance proteins in common causes of refractory epilepsy

Epilepsy is resistant to drug treatment in about one-third of cases, but the mechanisms underlying this drug resistance are not understood. In cancer, drug resistance has been studied extensively. Amongst the various resistance mechanisms, overexpression of drug resistance proteins, such as multi-drug resistance gene-1 P-glycoprotein (MDR1) and multidrug resistance-associated protein 1 (MRP1), has been shown to correlate with cellular resistance to anticancer drugs. Previous studies in human epilepsy have shown that MDR1 and MRP1 may also be overexpressed in brain tissue from patients with refractory epilepsy; expression has been shown in glia and neurones, which do not normally express these proteins. We examined expression of MDR1 and MRP1 in refractory epilepsy from three common causes, dysembryoplastic neuroepithelial tumours (DNTs; eight cases), focal cortical dysplasia (FCD; 14 cases) and hippocampal sclerosis (HS; eight cases). Expression was studied immunohistochemically in lesional tissue from therapeutic resections and compared with expression in histologically normal adjacent tissue. With the most sensitive antibodies, in all eight DNT cases, reactive astrocytes within tumour nodules expressed MDR1 and MRP1. In five of eight HS cases, reactive astrocytes within the gliotic hippocampus expressed MDR1 and MRP1. Of 14 cases of FCD, MDR1 and MRP1 expression was noted in reactive astrocytes in all cases. In five FCD cases, MRP1 expression was also noted in dysplastic neurones. In FCD and DNTs, accentuation of reactivity was noted around lesional vessels. Immunoreactivity was always more frequent and intense in lesional reactive astrocytes than in glial fibrillary acidic protein-positive reactive astrocytes in adjacent histologically normal tissue. MDR1 is able to transport some antiepileptic drugs (AEDs), and MRP1 may also do so. The overexpression of these drug resistance proteins in tissue from patients with refractory epilepsy suggests one possible mechanism for drug resistance in patients with these pathologies. We propose that overexpressed resistance proteins lower the interstitial concentration of AEDs in the vicinity of the epileptogenic pathology and thereby render the epilepsy caused by these pathologies resistant to treatment with AEDs.

Nestin expression in cortical dysplasia

OBJECT

It is recognized that cortical dysplasia (CD) is associated with an increased incidence of glioneuronal neoplasms. Among hypothetical considerations, there is the possibility that CD and other neuronal migration abnormalities harbor dysmature cells with the potential to give rise to glioneuronal neoplasms. Such cells, if present, would be reasonably expected to display immature features. The goal of the present study was to characterize the expression of nestin, a neuroepithelial precursor/stem cell antigen, in CD, along with other pathological and clinical features of this entity.

METHODS

Clinical and surgical features of 10 recent cases meeting the histological criteria for CD were reviewed. Expressions of nestin, MAP2, neurofilament, and glial fibrillary acidic protein (GFAP) were assessed using immunohistochemical analysis and confocal scanning laser microscopy. Immunoreactivity for both glial and neuronal antigens as well as nestin was found in a select group of cells within regions of CD. Immunohistochemical and confocal microscopic findings demonstrated that these cells with neuronal or ambiguous features are a mixed population, some of which are dysmature neurons (positive for nestin and MAP2), whereas others are astrocytic (positive for nestin and GFAP).

CONCLUSIONS

Further insight into the nature of nestin-positive neurons may shed light on the cause and pathogenesis of the associated glioneuronal tumors and the accompanying chronic seizures.

Cortical dysplasia: electroclinical, imaging, and neuropathologic study of 13 patients

PURPOSE

The aim of this study was to correlate the electroclinical and radiologic data with the neuropathologic findings and surgical outcome in epileptic patients with epilepsy and Taylor's focal cortical dysplasia (TFCD) and to characterize further the abnormal intermediate filaments expression in the balloon cell present in the peculiar dysplasia.

METHODS

We retrospectively selected 13 TFCD patients who underwent surgery for intractable epilepsy with the aim of removing the magnetic resonance (MR)-detectable lesion and/or the epileptogenic zone defined by stereoelectroencephalographic recordings. The surgical specimens were analyzed by means of routine neuropathologic and immunocytochemical studies. Antisera against different intermediate filaments also were used in serial adjacent sections to evaluate their coexpression in balloon cells.

RESULTS

Histopathologic abnormalities typical of TFCD were found not only within the MR-visible lesions but also in most of the epileptogenic zones with no MR signal alterations. Furthermore, the MR-visible lesions contained a high proportion of cells with an abnormal expression of intermediate filament proteins. After a long follow-up, 10 of the patients are now seizure free.

CONCLUSIONS

Our findings indicate that highly epileptogenic zones may correspond to tissue alterations not revealed by neuroimaging. Furthermore, the immunocytochemical data show that the dysplastic tissue detected by MR contained high concentrations of cells filled with abnormal intermediate filaments. The detected colocalization of neuronal and glial markers in balloon cells indicates a failure of cellular commitment during development.

Focal cortical dysplasia: comparison of MRI and FDG-PET

PURPOSE

The purpose of this work was to compare the use of MRI and [18F]fluorodeoxyglucose (FDG) positron emission tomography (PET) in the diagnosis of focal cortical dysplasia (FCD).

METHOD

Nineteen patients with surgically proven FCD were analyzed retrospectively. MRI was performed in all patients, and FDG-PET was performed in 17 patients. We compared the MR and FDG-PET findings of FCD according to the histologic findings that were classified into three grades.

RESULTS

Four cases were classified as Grade I, 4 cases as Grade II, and 11 cases as Grade III FCD. The lesions were detected on MRI in 9 (82%) of the 11 patients with Grade III FCD and in only 1 (13%) of the 8 patients with Grade I and II FCD. Cortical hypometabolism of the lesion was revealed on FDG-PET in 6 (86%) of the 7 patients with Grade I and II FCD and in 9 (90%) of the 10 patients with Grade III FCD. The extent of the cortical abnormality was larger on FDG-PET than on MRI in 11 (65%) of the 17 patients.

CONCLUSION

FDG-PET is more useful in delineating the cortical abnormality in patients with mild degrees of FCD. The extent of the lesion was larger or similar on FDG-PET compared with that of the MRI.

Electrocorticographic coherence patterns

The availability of implantable subdural electrode arrays has made systematic studies of electrocorticographic (ECoG) coherence possible. Studies of coherence patterns recorded directly from human cortex are reviewed along with the presentation of original human clinical data, which reveal reliable and characteristic patterns of coherence. A data-driven technique for discriminating between reliable and unreliable coherence and phase values is described and used to reveal the relationship between coherence and cortical anatomy, such as in the region of the central sulcus, where low phase coherence declines and high phase-shifted coherence increases. Analysis of coherence magnitude and phase makes it possible to determine which signals likely arise from the cortical surface, and which arise from the depths of a sulcus. Alterations in coherence patterns caused by tumors or epilepsy are described and may be used to identify normal and pathological functional relationships between distant cortical areas. Some electrophysiologic/pathologic correlations indicate at least two types of epileptic abnormality, implying a sequence in breakdown of epileptic tissue. The relationship between coherence patterns and behavior and cognition is introduced and compared to similar studies of single-unit binding in animals.

Mechanisms underlying epileptogenesis in cortical malformations

The presence of developmental cortical malformations is associated with epileptogenesis and other neurological disorders. In recent years, animal models specific to certain malformations have been developed to study the underlying epileptogenic mechanisms. Teratogens (chemical, thermal or radiation) applied during cortical neuroblast division and migration result in lissencephaly and focal cortical dysplasia. Animals with these malformations have a lowered seizure threshold as well as histopathologies typical of those found in human dysgenic brains. Alterations that may promote epileptogenesis have been identified in lissencephalic brains, such as increased numbers of bursting types of neurons, and abnormal connections between hippocampus, subcortical heterotopia, and neocortex. A distinct set of pathological properties is present in animal models of 4-layered microgyria, induced with cortical lesions made during late stages of cortical neuroblast migration. Hyperexcitability has been demonstrated in cortex adjacent to the microgyrus (paramicrogyral zone) in in vitro slice preparations. A number of observations suggest that cellular differentiation is delayed in microgyric brains. Other studies show increases in postsynaptic glutamate receptors and decreases in GABA(A) receptors in microgyric cortex. These alterations could promote epileptogenesis, depending on which cell types have the altered receptors. The microgyrus lacks thalamic afferents from sensory relay nuclei, that instead appear to project to the paramicrogyral region, thereby increasing excitatory connectivity within this epileptogenic zone. These studies have provided a necessary first step in understanding molecular and cellular mechanisms of epileptogenesis associated with cortical malformations.

Focal cortical dysplasia: a neuropathological and developmental perspective

Focal cortical dysplasia (FCD) is a rare, sporadic disorder which is a recognised cause of chronic epilepsy. It is proposed to result from disordered neuronal migration and differentiation and has characteristic histological features which include disturbed cortical lamination, large abnormal neurons and the presence of large balloon cells with glassy eosinophilic cytoplasm and pleomorphic eccentric nuclei. These latter express both glial and neuronal markers indicative of abnormal neuroglial differentiation. In this paper we review the current literature on the neuropathology of FCD and discuss potential mechanisms. We focus on growth factors, signalling pathways and candidate genes with known roles in Drosophila and vertebrate brain development that could be responsible for the developmental brain changes seen in FCD. At issue are the factors that influence cell fate and differentiation and which regulate neural migration. Some of the molecular pathways, such as those involving the Notch and the Wnt pathways have particularly important roles in neuroglial differentiation in vertebrates, and these are proposed as potential candidates.

Taylor's cortical dysplasia: a confocal and ultrastructural immunohistochemical study

In the present report we describe the neuropathological characteristics of tissue surgically resected from three patients affected by intractable epilepsy secondary to cortical dysplasia. Common features, suggestive of a focal cortical dysplasia of Taylor, were observed in all specimens. Immunocytochemical procedures were performed using neuronal and glial markers and the sections were observed at light traditional and confocal microscopes. This part of the investigation pointed out: 1. cortical laminar disruption; 2. very large neurons displaying a pyramidal or round shape; 3. ballooned cells; 4. decrease of calcium binding proteins immunoreactivity; 5. abnormal nets of parvalbumin- and glutamic acid decarboxylase-positive puncta around giant neurons but not around ballooned cells. Ultrastructural investigation on the same material provided evidence of a high concentration of neurofilaments in giant neurons and of glial intermediate filaments in ballooned cells. In addition, immunolabeled GABAergic terminals clustered around giant neurons were not found to establish synapses on their cell bodies. The present data, derived from a limited sample of patients but showing very consistent features, suggest that in Taylor's type of cortical dysplasia a disturbance of migratory events could be paralleled by a disruption of cell differentiation and maturation and by an impairment of synaptogenesis. This latter mechanism seemed to affect especially the inhibitory elements, and could account for the hyperexcitability of this tissue and thus for the high epileptogenicity of Taylor's dysplasia.

Characterization of nodular neuronal heterotopia in children

Neuronal heterotopia are seen in various pathologies and are associated with intractable epilepsy. We examined brain tissue from four children with subcortical or periventricular nodular heterotopia of different aetiologies: one with severe epilepsy following focal brain trauma at 17 weeks gestation, one with hemimegalencephaly and intractable epilepsy, one with focal cortical dysplasia and intractable epilepsy, and one dysmorphic term infant with associated hydrocephalus and polymicrogyria. The connectivity of nodules was investigated using histological and carbocyanine dye (DiI) tracing techniques. DiI crystal placement adjacent to heterotopic nodules revealed numerous DiI-labelled fibres within a 2-3 mm radius of the crystals. Although we observed labelled fibres closely surrounding nodules, the majority did not penetrate them. Placement of DiI crystals within nodules also identified a limited number of projections out of the nodules and in one case there was evidence for connectivity between adjacent nodules. The cellular and neurochemical composition of nodules was also examined using immunohistochemistry for calretinin and neuropeptide Y (NPY), which are normally expressed in GABAergic cortical interneurons. Within heterotopic nodules from all cases, numerous calretinin-positive neurons were identified, along with a few cell bodies and many processes positive for NPY. Calretinin-positive neurons within nodules were less morphologically complex than those in the cortex, which may reflect incomplete differentiation into an inhibitory neuronal phenotype. There were also abnormal clusters of calretinin-positive cells in the overlying cortical plate, indicating that the migratory defect which produces heterotopic nodules also affects development of the cortex itself. Thus, heterotopic nodules consisting of multiple neuronal cell types are associated with malformation in the overlying cortical plate, and have limited connectivity with other brain regions. This abnormal development of connectivity may affect neuronal maturation and consequently the balance of excitation and inhibition in neuronal circuits, leading to their epileptogenic potential.

Tuberous sclerosis-related gene expression in normal and dysplastic brain

Cortical dysplasia (CD) broadly defines a complex cerebral malformative lesion associated clinically with intractable, pharmacoresistant epilepsy (including infantile spasms), especially in infants and children. In CD, the spectrum of structural brain abnormalities includes (at a minimum) neuronal dyslamination and (in severe cases) neuronal cytomegaly with cytoskeletal alterations and the presence of gemistocyte-like 'balloon cells'. In some CD variants, the neuropathological features are essentially indistinguishable from those of a tuber of tuberous sclerosis (TSC). Two genes associated with the autosomal dominant, multi-system disorder TSC have recently been cloned: TSC2 (on chromosome 16p13.3) encodes the protein tuberin and TSC1 (on 9q34) encodes hamartin. Tuberin has been immunolocalized to neurons and possibly astrocytes in normal brain and CD/TSC tubers, and is widely expressed in normal viscera; loss of heterozygosity and tissue culture studies suggest it functions as a growth suppressor. The TSC1 gene has been cloned within the last year and hamartin as yet has no well-defined cellular function, though its protein product may also function as a growth suppressor. This article focuses on the cellular pathogenesis of CD and TSC brain lesions and how the two may be biologically related. Studies of how TSC1 and TSC2 function in normal and dysplastic cerebral neocortex may provide a paradigm for understanding the neurobiology of other genes that determine epilepsy-associated cerebral malformations (e.g. lissencephaly, double cortex).

Immunocytochemical investigation on dysplastic human tissue from epileptic patients

In this report we describe three patients with developmental cortical abnormalities (generally referred as cortical dysplasia), revealed by MRI and operated on for intractable epilepsy. Tissue, removed for strictly therapeutic reasons, was defined as the epileptogenic area by electroclinical data and stereo EEG (SEEG) recordings. Tissue samples were processed initially for histology, and selected sections were further processed for immunocytochemical investigation in order to determine whether the region of cortical dysplasia was co-extensive with the epileptogenic area. In two patients with nodular heterotopia, disorganized aggregates of neurons (as revealed by neuronal cytoskeletal markers) were found within the nodules. Both pyramidal and local circuit neurons were present in the nodules, but no reactive gliosis was present. When nodules reached the cortex, the cortical layers were disrupted. In the patient with localized cortical dysplasia, a complete disorganization of the cortical lamination was found, and numerous neurons were also present in the white matter. Disoriented pyramidal neurons weakly labelled with cytoskeletal neuronal markers were also present but no cytomegalic cells were found. One of the patients with nodular heterotopia underwent only partial resection of both the 'epileptogenic area' and of the lesion; this patient still presents with seizures. The other patient with nodular heterotopia is seizure-free after a complete lesionectomy and excision of the epileptogenic area. The third patient, with focal cortical dysplasia, had two surgeries; she became seizure-free only after the excision of the epileptogenic area detected by SEEG recording. The present data suggest that the dysplastic areas identified by MRI should not be considered as the only place of origin of the ictal discharges. From the neuropathological point of view, the focal cortical dysplasia can be considered as a pure form of migrational disorder. However, the presence of large aggregates of neurons interspersed within the white matter, in the subcortical nodular heterotopia, suggests that a defect of neuronal migration could be associated with an exuberant production of neuroblasts and/or a disruption of mechanisms for naturally occurring cell death.

Cellular and molecular basis of cerebral dysgenesis

Maldevelopment of the cerebral cortex, cortical dysgenesis (CD), may be associated with epilepsy, mental retardation (MR), and focal or widespread neurologic deficits. The histologic hallmark of CD is disrupted cytoarchitecture, including disorganized lamination, malpositioned neurons with respect to their normal radial orientation, abnormal dendritic arborization, and heterotopic neurons within the white matter. Seizures in these patients are particularly difficult to control with conventional anti-epileptic drugs (AEDs) and may require epilepsy surgery to remove these abnormal foci. Focal CD has been reported in up to 30% of epilepsy surgery specimens and are believed to provide the central pathologic substrate responsible for seizures in these patients. How and why CD results in epileptiform activity is unknown. Advances in understanding the pathogenesis of some types of CD have occurred recently with the cloning genes responsible for a few types of X-linked and autosomal CD. This review will outline the major subtypes of CD, the pathologic findings, and the molecular etiologies for a variety of CD. We will also address recent experimental advances in studying the pathogenesis of CD.

Neuronal migration disorders and epilepsy: a morphological analysis of three surgically treated patients

Despite the increasing number of patients affected by neuronal migration disorders (NMDs) recently diagnosed in vivo by means of magnetic resonance imaging (MRI), few detailed data on the correlation between the neuroradiological and the anatomical features in the single NMD case are available. The present paper reports a combined cytoarchitectural and immunocytochemical analysis, by means of antisera recognizing specific neuronal and glial markers, of three MRI diagnosed NMD patients surgically treated for the relief of intractable seizures. The first case was a giant subcortical nodular heterotopia of morphologically normal neurons lacking any type of cortical lamination. The second case was a layered polymicrogyria with an abnormal amount of ectopic neurons in the underlying white matter. The third case was a focal cortical dysplasia characterized by a dramatic disruption of the normal cortical layering associated with marked cytological abnormalities. The present data demonstrate that the macroscopical and microscopical brain abnormalities can be markedly different in different NMD subtypes, and suggest that different anatomical substrates can underlie the intrinsic hyperexcitability of these brain malformations. The relevance of further prospective clinico-morphological studies for a better understanding of the mechanisms determining the development of these brain malformations is underlined.

Enhanced expression of microtubule-associated protein 2 in large neurons of cortical dysplasia

To evaluate neuronal cytoarchitectural changes in cortical dysplasia, we examined microtubule-associated protein 2 (MAP2) expression in surgically resected specimens obtained from 20 patients (age range, 3 months to 10 years) treated for intractable epilepsy. Large neurons were investigated in the specimens from all patients and showed significantly strong immunoreactivity with antibodies against MAP2 in the perikaryon and proximal portion of their processes. In situ hybridization with MAP2 antitense riboprobe showed increased hybridization signal intensities in the large neurons, which correlated with the pattern of immunoreactivity for MAP2. We conclude that MAP2 is strongly expressed in the large neurons in cortical dysplasia. The results of preliminary immunoblotting in 1 patient with focal cortical dysplasia showed that the low-molecular-weight form of MAP2 (MAP2c) was strongly expressed in the dysplastic cortex, suggesting that MAP2c may be a major component contributing to the increased expression of MAP2 in the large neurons of cortical dysplasia. Since it has been suggested that MAP2 plays a crucial role in the branching and remodeling of neuronal processes, increased expression of MAP2 may reflect activated plasticity of the large neurons in cortical dysplasia.