Brain malformations, epilepsy, and infantile spasms

Quantitative Three-Dimensional Reconstructions of Excitatory Synaptic Boutons in the "Barrel Field" of the Adult "Reeler" Mouse Somatosensory Neocortex: A Comparative Fine-Scale Electron Microscopic Analysis with the Wild Type Mouse

Synapses are key structural determinants for information processing and computations in the normal and pathologically altered brain. Here, the quantitative morphology of excitatory synaptic boutons in the "reeler" mutant, a model system for various neurological disorders, was investigated and compared with wild-type (WT) mice using high-resolution, fine-scale electron microscopy (EM) and quantitative three-dimensional (3D) models of synaptic boutons. Beside their overall geometry, the shape and size of presynaptic active zones (PreAZs) and postsynaptic densities (PSDs) forming the active zones and the three pools of synaptic vesicles (SVs), namely the readily releasable pool (RRP), the recycling pool (RP), and the resting pool, were quantified. Although the reeler mouse neocortex is severely disturbed, no significant differences were found in most of the structural parameters investigated: the size of boutons (~3 μm2), size of the PreAZs and PSDs (~0.17 μm2), total number of SVs, and SVs within a perimeter (p) of 10 nm and p20 nm RRP; the p60 nm, p100 nm, and p60-p200 nm RP; and the resting pool, except the synaptic cleft width. Taken together, the synaptic organization and structural composition of synaptic boutons in the reeler neocortex remain comparably "normal" and may thus contribute to a "correct" wiring of neurons within the reeler cortical network.

Treatment of infantile spasms: why do we know so little?

INTRODUCTION

Infantile spasm (IS) is an epileptic syndrome with typical onset within the first 2 years of life. This condition might be caused by several etiologies. IS is associated with pathological neuronal networks; however, definite hypotheses on neurobiological processes are awaited.

AREAS COVERED

Changes in NMDA and GABA_B receptors and increase of Ca²⁺ conductance are some of the possible pathophysiological mechanisms. Animal models can help, but most have only some features of IS. Outcome is strongly affected by etiology and the timing of treatment, which relies still on ACTH, oral steroids, and vigabatrin. No significant differences in terms of efficacy have been documented, though a combination of ACTH and vigabatrin seems to be associated with better long-term outcomes. Despite the increasing knowledge about the etiology and pathophysiology of IS, in the last years, no new treatment approaches have been recognized to be able to modify the neurobiological process underlying IS. Precision medicine has far to come in IS.

EXPERT OPINION

Recently, no new therapeutic options for IS have emerged, probably due to the lack of reliable animal models and to the extreme variability in etiologies. Consequently, the outlook for patients and families is poor and early recognition and intervention remain research priorities.

Clinical Phenotypes Associated to Engrailed 2 Gene Alterations in a Series of Neuropediatric Patients

The engrailed homeobox protein (EN) plays an important role in the regionalization of the neural tube. EN distribution regulates the cerebellum and midbrain morphogenesis, as well as retinotectal synaptogenesis. In humans, the EN1 and EN2 genes code for the EN family of transcription factors. Genetic alterations in the expression of EN2 have been related to different neurologic conditions and more particularly to autism spectrum disorders (ASD). We aimed to study and compare the phenotypes of three series of patients: (1) patients with encephalic structural anomalies (ESA) and abnormalities in the genomic (DNA) and/or transcriptomic (RNAm) of EN2 (EN2-g), (2) ESA patients having other gene mutations (OG-g), and (3) ESA patients free of these mutations (NM-g).

Subjects and Methods: We have performed a descriptive study on 109 patients who suffer from mental retardation (MR), cerebral palsy (CP), epilepsy (EP), and behavioral disorders (BD), showing also ESA in their encephalic MRI. We studied genomic DNA and transcriptional analysis (cDNA) on EN2 gene (EN2), and in other genes (OG): LIS1, PTAFR, PAFAH1B2, PAFAH1B3, FGF8, PAX2, D17S379, D17S1866, and SMG6 (D17S5), as a routine genetic diagnosis in ESA patients.

Results: From 109 patients, fifteen meet the exclusion criteria. From the remaining 94 patients, 12 (12.8%) showed mutations in EN2 (EN2-g), 20 showed mutations in other studied genes (OG-g), and 62 did not showed any mutation (NM-g). All EN2-g patients, suffered from MR, nine EP, seven BD and four CP. The proportions of these phenotypes in EN2-g did not differ from those in the OG-g, but it was significantly higher when comparing EN2-g with NM-g (MR: p = 0.013; EP: p = 0.001; BD: p = 0.0001; CP: p = 0.07, ns). Groups EN2-g and OG-g showed a 100 and a 70% of comorbidity, respectively, being significantly (p = 0.04) greater than NM-group (62.9%).

Conclusion: Our series reflects a significant effect of EN2 gene alterations in neurodevelopmental abnormalities associated to ESA. Conversely, although these EN2 related anomalies might represent a predisposition to develop brain diseases, our results did not support direct relationship between EN2 mutations and specific clinical phenotypes.

Association between brain structural anomalies, electroencephalogram and history of seizures in Mucopolysaccharidosis type II (Hunter syndrome)

Mucopolysaccharidosis type II or Hunter syndrome (MPS II) is a genetic disease that can course with intellectual impairment and central nervous system (CNS) alterations. To date, no report has documented electroencephalogram (EEG) measures associated with CNS alterations, detected by imaging studies, and the history of seizures in patients with MPS II. Therefore, we decided to search this association. We included 9 patients with MPS II and performed imaging studies of the brain to detect the presence of cortico-subcortical atrophy, enlarged subarachnoid space and supratentorial ventricular size. Additionally, we performed EEG studies in sleep and awake conditions and a complete clinical description. Five out of the nine patients presented history of seizures and all except one patient (88.9%) presented some CNS structural alteration in the imaging studies, being the most frequent the cortico-subcortical atrophy (77.8%). The EEG results showed low amplitude in all patients and low voltage in sleep condition in eight patients with interhemispheric asymmetry in six patients during awake and sleep conditions. Although the five patients with history of seizures did not present a distinctive EEG anomaly, four of them presented some structural alteration in the imaging studies. In conclusion, most patients presented structural alterations in the CNS; likewise, all of them presented EEG anomalies mainly during sleep conditions. However, a clear association between EEG, CNS and the history of seizures was not established.

Populations of radial glial cells respond differently to reelin and neuregulin1 in a ferret model of cortical dysplasia

Radial glial cells play an essential role during corticogenesis through their function as neural precursors and guides of neuronal migration. Both reelin and neuregulin1 (NRG1) maintain the radial glial scaffold; they also induce expression of Brain Lipid Binding Protein (BLBP), a well known marker of radial glia. Although radial glia in normal ferrets express both vimentin and BLBP, this coexpression diverges at P3; vimentin is expressed in the radial glial processes, while BLBP appears in cells detached from the ventricular zone. Our lab developed a model of cortical dysplasia in the ferret, resulting in impaired migration of neurons into the cortical plate and disordered radial glia. This occurs after exposure to the antimitotic methylazoxymethanol (MAM) on the 24th day of development (E24). Ferrets treated with MAM on E24 result in an overall decrease of BLBP expression; radial glia that continue to express BLBP, however, show only mild disruption compared with the strongly disrupted vimentin expressing radial glia. When E24 MAM-treated organotypic slices are exposed to reelin or NRG1, the severely disrupted vimentin+ radial glial processes are repaired but the slightly disordered BLBP+ processes are not. The realignment of vimentin+ processes was linked with an increase of their BLBP expression. BLBP expressing radial glia are distinguished by being both less affected by MAM treatment and by attempts at repair. We further investigated the effects induced by reelin and found that signaling was mediated via VLDLR/Dab1/Pi3K activation while NRG1 signaling was mediated via erbB3/erbB4/Pi3K. We then tested whether radial glial repair correlated with improved neuronal migration. Repairing the radial glial scaffold is not sufficient to restore neuronal migration; although reelin improves migration of neurons toward the cortical plate signaling through ApoER2/Dab1/PI3K activation, NRG1 does not.

Levetiracetam decreases the seizure activity and blood-brain barrier permeability in pentylenetetrazole-kindled rats with cortical dysplasia

This study investigates the effects of levetiracetam (LEV) on the functional and structural properties of blood-brain barrier (BBB) in pentylenetetrazole (PTZ)-kindled rats with cortical dysplasia (CD). Pregnant rats were exposed to 145 cGy of gamma-irradiation on embryonic day 17. In offsprings, kindling was induced by giving subconvulsive doses of PTZ three times per week for 45 days. While all kindled rats with CD died during epileptic seizures evoked by the administration of a convulsive dose of PTZ in 15 to 25 min, one week LEV (80 mg/kg) pretreatment decreased the mortality to 38% in the same setting. LEV caused a remarkable decrease (p<0.01) in extravasation of sodium fluorescein dye into the brain tissue of kindled animals with CD treated with convulsive dose of PTZ. Occludin immunoreactivity and expression remained essentially unchanged in all groups. Immunoreactivity for glial fibrillary acidic protein (GFAP) was observed to be slightly increased by acute convulsive challenge in kindled rats with CD while LEV pretreatment led to GFAP immunoreactivity comparable to that of controls. An increased c-fos immunoreactivity in kindled rats with CD exposed to convulsive PTZ challenge was also observed with LEV pretreatment. Tight junctions were ultrastructurally intact, whereas LEV decreased the increased pinocytotic activity in brain endothelium of kindled rats with CD treated with convulsive dose of PTZ. The present study showed that LEV decreased the increased BBB permeability considerably by diminishing vesicular transport in epileptic seizures induced by convulsive PTZ challenge in kindled animals with CD.

Pathophysiological mechanisms of focal cortical dysplasia: a critical review of human tissue studies and animal models

Cortical dysplasia (CD, also known as malformations of cortical development) are the pathological substrates in a large percentage of patients with pharmacoresistant epilepsy who may be amenable to surgical treatment. Therefore, research on the mechanisms of dysplastic lesion formation and epileptogenicity is of paramount importance for the prevention, detection, and treatment of CD-induced epilepsy. The purpose of this review is to discuss and critically evaluate the current state and results of human tissue experimentation (focusing on reported results of studies done on neocortical dysplastic tissue resected from patients with pharmacoresistant epilepsy), and to discuss some of the concerns related to research that uses surgically resected epileptic human tissue. The use of better animal models of CD as a tool toward the better understanding of the mechanisms of pathogenesis, epileptogenesis, and epileptogenicity of dysplastic lesions will be reviewed from the perspective of their usefulness in a model of translational research that should ultimately result in better diagnostic and therapeutic techniques of CD.

Alteration of interneuron migration in a ferret model of cortical dysplasia

During cerebral cortical development, gamma-aminobutyric acidergic (GABAergic) interneurons arise from a different site than projection neurons. GABAergic cells are generated in the subpallial ganglionic eminence (GE), while excitatory projection neurons arise from the neocortical ventricular zone. Our laboratory studies a model of cortical dysplasia that displays specific disruption of GABAergic mechanisms and an alteration in the overall balance of excitation in the neocortex. To produce this model, the birth of neurons on a specific gestational day in ferrets (embryonic day 33 [E33]) is interrupted by injection of the antimitotic methylazoxymethanol (MAM). We hypothesized that migration of interneurons might be disrupted in this cortical dysplasia paradigm. We observed that although interneurons migrate into the neocortex in both normal and dysplastic cortex, the migrating cells become disoriented over time after E33 MAM treatment. Coculture experiments using normal GE and MAM-treated cortex (and vice versa) demonstrate that cues dictating proper orientation of migrating interneurons arise from the cortex and are not intrinsic to the migrating cells. As a consequence, interneurons in mature brains of MAM-treated animals are abnormally distributed. We report that GABA(A) receptor activation is crucial to the proper positioning of interneurons migrating into the cortex from the GE in normal and MAM-treated animals.

Determinants of drug brain uptake in a rat model of seizure-associated malformations of cortical development

We examined the blood-brain barrier (BBB) function in methylazoxymethanol acetate (MAM)-treated rats, a model of human developmental brain malformations. We found aberrant vessels morphology and serum albumin leakage in the heterotopic (malformed) hippocampus; these changes were associated with a significant increase in endothelial P-glycoprotein (P-gp) expression. Seizures exacerbated BBB leakage and greatly augmented P-gp expression in vessels and additionally in perivascular/parenchymal astrocytes. The effects of seizures were observed to a much larger extent in malformed than in normal brain tissue. The intrinsic changes in BBB function in MAM-exposed rats were associated with increased blood-to-brain penetration of ondansetron, a P-gp substrate. However, a marked reduction in drug brain levels was provoked by seizures, and this effect was reversed by selective blockade of P-gp activity with tariquidar. Changes in BBB function may critically contribute to determine the brain uptake and distribution of P-gp substrates in epileptic tissue associated with developmental malformations.

LIS1 RNA interference blocks neural stem cell division, morphogenesis, and motility at multiple stages

Mutations in the human LIS1 gene cause the smooth brain disease classical lissencephaly. To understand the underlying mechanisms, we conducted in situ live cell imaging analysis of LIS1 function throughout the entire radial migration pathway. In utero electroporation of LIS1 small interference RNA and short hairpin dominant negative LIS1 and dynactin cDNAs caused a dramatic accumulation of multipolar progenitor cells within the subventricular zone of embryonic rat brains. This effect resulted from a complete failure in progression from the multipolar to the migratory bipolar state, as revealed by time-lapse analysis of brain slices. Surprisingly, interkinetic nuclear oscillations in the radial glial progenitors were also abolished, as were cell divisions at the ventricular surface. Those few bipolar cells that reached the intermediate zone also exhibited a complete block in somal translocation, although, remarkably, process extension persisted. Finally, axonal growth also ceased. These results identify multiple distinct and novel roles for LIS1 in nucleokinesis and process dynamics and suggest that nuclear position controls neural progenitor cell division.

Physiological and morphological characterization of dentate granule cells in the p35 knock-out mouse hippocampus: evidence for an epileptic circuit

There is a high correlation between pediatric epilepsies and neuronal migration disorders. What remains unclear is whether there are intrinsic features of the individual dysplastic cells that give rise to heightened seizure susceptibility, or whether these dysplastic cells contribute to seizure activity by establishing abnormal circuits that alter the balance of inhibition and excitation. Mice lacking a functional p35 gene provide an ideal model in which to address these questions, because these knock-out animals not only exhibit aberrant neuronal migration but also demonstrate spontaneous seizures. Extracellular field recordings from hippocampal slices, characterizing the input-output relationship in the dentate, revealed little difference between wild-type and knock-out mice under both normal and elevated extracellular potassium conditions. However, in the presence of the GABA(A) antagonist bicuculline, p35 knock-out slices, but not wild-type slices, exhibited prolonged depolarizations in response to stimulation of the perforant path. There were no significant differences in the intrinsic properties of dentate granule cells (i.e., input resistance, time constant, action potential generation) from wild-type versus knock-out mice. However, antidromic activation (mossy fiber stimulation) evoked an excitatory synaptic response in over 65% of granule cells from p35 knock-out slices that was never observed in wild-type slices. Ultrastructural analyses identified morphological substrates for this aberrant excitation: recurrent axon collaterals, abnormal basal dendrites, and mossy fiber terminals forming synapses onto the spines of neighboring granule cells. These studies suggest that granule cells in p35 knock-out mice contribute to seizure activity by forming an abnormal excitatory feedback circuit.

Epileptic-like convulsions associated with LIS-1 in the cytoskeletal control of neurotransmitter signaling in Caenorhabditis elegans

Cortical malformations are a collection of disorders affecting brain development. Mutations in the LIS1 gene lead to a disorganized and smooth cerebral cortex caused by failure in neuronal migration. Among the clinical consequences of lissencephaly are mental retardation and intractable epilepsy. It remains unclear whether the seizures result from aberrant neuronal placement, disruption of intrinsic properties of neurons, or both. The nematode Caenorhabditis elegans offers an opportunity to study such convulsions in a simple animal with a defined nervous system. Here we show that convulsions mimicking epilepsy can be induced by a mutation in a C. elegans lis-1 allele (pnm-1), in combination with a chemical antagonist of gamma-aminobutyric acid (GABA) neurotransmitter signaling. Identical convulsions were obtained using C. elegans mutants defective in GABA transmission, whereas none of these mutants or the antagonist alone caused convulsions, indicating a threshold was exceeded in response to this combination. Crosses between pnm-1 and fluorescent marker strains designed to exclusively illuminate either the processes of GABAergic neurons or synaptic vesicles surprisingly showed no deviations in neuronal architecture. Instead, presynaptic defects in GABAergic vesicle distribution were clearly evident and could be phenocopied by RNAi directed against cytoplasmic dynein, a known LIS1 interactor. Furthermore, mutations in UNC-104, a neuronal-specific kinesin, and SNB-1, a synaptic vesicle-associated protein termed synaptobrevin, exhibit similar convulsion phenotypes following chemical induction. Taken together, these studies establish C. elegans as a system to investigate subtle cytoskeletal mechanisms regulating intrinsic neuronal activity and suggest that it may be possible to dissociate the epileptic consequences of lissencephaly from the more phenotypically overt cortical defects associated with neuronal migration.

MAOA knockout mice are more susceptible to seizures but show reduced epileptogenesis

The role of elevated neuroactive amine exposure during embryonic and early postnatal development on seizure threshold and epileptogenesis was examined using both electrical and pentylenetetrazol (PTZ) kindling in monoamine oxidase A knockout (MAO(A) KO) mice and their wildtype, parental strain (C3H). In the first experiment permanent bilateral electrodes were implanted in the amygdala of both C3H and MAO(A) KO mice. The mice had their afterdischarge threshold determined and then seizures were kindled daily for a total of 20 days. We observed that the MAO(A) KO mice had lower afterdischarge thresholds and less severe seizures compared to the C3H mice. In the second experiment, seizures were elicited in experimentally naive mice using 50mg/kg of PTZ once daily for 7 days. We observed that the MAO(A) KO mice had shorter latencies to the onset of the first seizure, shorter total duration of seizures and fewer seizures per day. Overall the results of both experiments suggest that MAO(A) KO mice have an increased susceptibility to seizures, but are more resistant to epileptogenesis. We conclude that the high levels of neuroactive amines in the MAO(A) KO mice reorganize the brain to make the mice more susceptible to seizures but the remaining high levels of serotonin and norepinephrine likely inhibit epileptogenesis.