Kiyoshi Egawa

Dominant-Negative Mutations in α-II Spectrin Cause West Syndrome with Severe Cerebral Hypomyelination, Spastic Quadriplegia, and Developmental Delay

A de novo 9q33.3-q34.11 microdeletion involving STXBP1 has been found in one of four individuals (group A) with early-onset West syndrome, severe hypomyelination, poor visual attention, and developmental delay. Although haploinsufficiency of STXBP1 was involved in early infantile epileptic encephalopathy in a previous different cohort study (group B), no mutations of STXBP1 were found in two of the remaining three subjects of group A (one was unavailable). We assumed that another gene within the deletion might contribute to the phenotype of group A. SPTAN1 encoding alpha-II spectrin, which is essential for proper myelination in zebrafish, turned out to be deleted. In two subjects, an in-frame 3 bp deletion and a 6 bp duplication in SPTAN1 were found at the initial nucleation site of the alpha/beta spectrin heterodimer. SPTAN1 was further screened in six unrelated individuals with WS and hypomyelination, but no mutations were found. Recombinant mutant (mut) and wild-type (WT) alpha-II spectrin could assemble heterodimers with beta-II spectrin, but alpha-II (mut)/beta-II spectrin heterodimers were thermolabile compared with the alpha-II (WT)/beta-II heterodimers. Transient expression in mouse cortical neurons revealed aggregation of alpha-II (mut)/beta-II and alpha-II (mut)/beta-III spectrin heterodimers, which was also observed in lymphoblastoid cells from two subjects with in-frame mutations. Clustering of ankyrinG and voltage-gated sodium channels at axon initial segment (AIS) was disturbed in relation to the aggregates, together with an elevated action potential threshold. These findings suggest that pathological aggregation of alpha/beta spectrin heterodimers and abnormal AIS integrity resulting from SPTAN1 mutations were involved in pathogenesis of infantile epilepsy.

Decreased Tonic Inhibition in Cerebellar Granule Cells Causes Motor Dysfunction in a Mouse Model of Angelman Syndrome

Ube3a deficiency causes a decrease in cerebellar tonic inhibition by preventing degradation of GABA transporter 1 in a mouse model of Angelman syndrome. Elucidating the Mechanism of Ataxia in Angelman Syndrome Angelman syndrome is a neurodevelopmental disorder caused by loss of the ubiquitin E3 ligase Ube3a. A variety of symptoms, including severe developmental delay, speech impairment, epilepsy, and movement and balance problems, are associated with Angelman syndrome. In a new study, Egawa et al. investigate electrophysiological features of cerebellar dysfunction in a mouse model of Angelman syndrome. The authors report that extrasynaptic γ-aminobutyric acid type A (GABAA) receptor–mediated tonic inhibition decreases in cerebellar granule cells from the Ube3a-deficient Angelman mice. They then show that, normally, Ube3a binds to GABA transporter 1, a key modulator of extrasynaptic GABA, and controls its degradation in the mouse cerebellum. The Ube3a-deficient mice, however, showed a surplus of GABA transporter 1, resulting in decreased GABA concentrations in the extrasynaptic space and thus decreased tonic inhibition of cerebellar granule cells. Pharmacological compensation of decreased tonic inhibition by administering low doses of THIP, a selective agonist of extrasynaptic GABAA receptors, alleviated impaired motor function in the Ube3a-deficient mice. Thus, attenuated GABA transporter 1 degradation and a consequent decrease in tonic inhibition of cerebellar granule cells play a key role in the cerebellar dysfunction characteristic of Angelman syndrome. Pharmacologically increasing tonic inhibition with THIP may be a useful strategy for alleviating movement and balance problems in patients with this disease. Angelman syndrome is a neurodevelopmental disorder caused by loss of function of the UBE3A gene encoding a ubiquitin E3 ligase. Motor dysfunction is a characteristic feature of Angelman syndrome, but neither the mechanisms of action nor effective therapeutic strategies have yet been elucidated. We report that tonic inhibition is specifically decreased in cerebellar granule cells of Ube3a-deficient mice, a model of Angelman syndrome. As a mechanism underlying this decrease in tonic inhibition, we show that Ube3a controls degradation of γ-aminobutyric acid (GABA) transporter 1 (GAT1) and that deficiency of Ube3a induces a surplus of GAT1 that results in a decrease in GABA concentrations in the extrasynaptic space. Administering low doses of 4,5,6,7-tetrahydroisothiazolo-[5,4-c]pyridin-3-ol (THIP), a selective extrasynaptic GABAA receptor agonist, improves the abnormal firing properties of a population of Purkinje cells in cerebellar brain slices and reduces cerebellar ataxia in Ube3a-deficient mice in vivo. These results suggest that pharmacologically increasing tonic inhibition may be a useful strategy for alleviating motor dysfunction in Angelman syndrome.

Pathophysiological power of improper tonic GABAA conductances in mature and immature models

High-affinity extrasynaptic gamma-aminobutyric acid A (GABAA) receptors are tonically activated by low and consistent levels of ambient GABA, mediating chronic inhibition against neuronal excitability (tonic inhibition) and the modulation of neural development. Synaptic (phasic) inhibition is spatially and temporally precise compared with tonic inhibition, which provides blunt yet strong integral inhibitory force by shunting electrical signaling. Although effects of acute modification of tonic inhibition are known, its pathophysiological significance remains unclear because homeostatic regulation of neuronal excitability can compensate for long-term deficit of extrasynaptic GABAA receptor activation. Nevertheless, tonic inhibition is of great interest for its pathophysiological involvement in central nervous system (CNS) diseases and thus as a therapeutic target. Together with the development of experimental models for various pathological states, recent evidence demonstrates such pathological involvements of tonic inhibition in neuronal dysfunction. This review focuses on the recent progress of tonic activation of GABAA conductance on the development and pathology of the CNS. Findings indicate that neuronal function in various brain regions are exacerbated with a gain or loss of function of tonic inhibition by GABA spillover. Disturbance of tonic GABAA conductance mediated by non-synaptic ambient GABA may result in brain mal-development. Therefore, various pathological states (epilepsy, motor dysfunctions, psychiatric disorders, and neurodevelopmental disorders) may be partly attributable to abnormal tonic GABAA conductances. Thus, the tone of tonic conductance and level of ambient GABA may be precisely tuned to maintain the regular function and development of the CNS. Therefore, receptor expression and factors for regulating the ambient GABA concentration are highlighted to gain a deeper understanding of pathology and therapeutic strategy for CNS diseases.

Cl− homeodynamics in gap junction‐coupled astrocytic networks on activation of GABAergic synapses

•  Astrocytes encapsulate GABAergic synapses and express GABAA receptors and GABA transporters. They are tightly coupled by gap junctions, and are referred to as the gap junction‐coupled astrocytic network. •  With higher [Cl−]i, GABA application can mediate bidirectional Cl− fluxes in astrocytes, Cl− efflux via GABAA receptors, and Cl− influx along with GABA uptake via GABA transporters. •  We focused on the Cl− dynamics of the astrocytic network under GABAergic synapse transmission. Spillover of GABA predominantly induced Cl− efflux via GABAA receptors, presumably because they are localized more closely to the synaptic cleft. •  GABAA receptor‐mediated currents were propagated via gap junctions within the astrocytic network. These results indicate that Cl− efflux from astrocytes mediated by GABAergic transmission is homeostatically maintained within gap junction‐coupled astrocytic networks. •  Blockage of gap junctional coupling by octanol promoted the collapse of the driving force for neuronal inhibitory transmission during intense activation of GABAergic synapses. Thus, the astrocytic network may play a role in maintaining GABAergic transmission by regulating [Cl−]o.

Dynamic Statistical Parametric Mapping for Analyzing the Magnetoencephalographic Epileptiform Activity in Patients With Epilepsy

Our current purpose is to evaluate the applicability of dynamic statistical parametric mapping, a novel method for localizing epileptiform activity recorded with magnetoencephalography in patients with epilepsy. We report four pediatric patients with focal epilepsies. Magnetoencephalographic data were collected with a 306-channel whole-head helmet-shaped sensor array. We calculated equivalent current dipoles and dynamic statistical parametric mapping movies of the interictal epileptiform discharges that were based in the minimum-L2 norm estimate, minimizing the square sum of the dipole element amplitudes. The dynamic statistical parametric mapping analysis of interictal epileptiform discharges can demonstrate the rapid change and propagation of interical epileptiform discharges. According to these findings, specific epileptogenic lesion—focal cortical dysplasia could be found and patients could be operated on successfully. The presurgical analysis of interictal epileptiform discharges using dynamic statistical parametric mapping seems to be promising in patients with a possible underlying focal cortical dysplasia and might help to guide the placement of invasive electrodes. (J Child Neurol 2005;20:363—369).

Aberrant somatosensory-evoked responses imply GABAergic dysfunction in Angelman syndrome.

A role for gamma-aminobutyric acid (GABA)ergic inhibition in cortical sensory processing is one of the principle concerns of brain research. Angelman syndrome (AS) is thought to be one of the few neurodevelopmental disorders with GABAergic-related genetic involvement. AS results from a functional deficit of the imprinted UBE3A gene, located at 15q11-q13, resulting mainly from a 4-Mb deletion that includes GABA(A) receptor subunit genes. These genes are believed to affect the GABAergic system and modulate the clinical severity of AS. To understand the underlying cortical dysfunction, we have investigated the primary somatosensory-evoked responses in AS patients. Subjects included eleven AS patients with a 15q11-q13 deletion (AS Del), two AS patients without a 15q11-q13 deletion, but with a UBE3A mutation (AS non-Del), six epilepsy patients (non-AS) and eleven normal control subjects. Somatosensory-evoked fields (SEFs) in response to median nerve stimulation were measured by magnetoencephalography. The N1m peak latency in AS Del patients was significantly longer (34.6+/-4.8 ms) than in non-AS patients (19.5+/-1.2 ms, P<0.001) or normal control subjects (18.4+/-1.8 ms, P<0.001). The next component, P1m, was prolonged and ambiguous and was only detected in patients taking clonazepam. In contrast, SEF waveforms of AS non-Del patients were similar to those of control individuals, rather than to AS Del patients. Thus, GABAergic dysfunction in AS Del patients is likely due to hemizygosity of GABA(A) receptor subunit genes, suggesting that GABAergic inhibition plays an important role in synchronous activity of human sensory systems.

[11C]Flumazenil Positron Emission Tomography Analyses of Brain Gamma-Aminobutyric Acid Type A Receptors in Angelman Syndrome

OBJECTIVE To evaluate the role of the gamma-aminobutyric acid type A (GABA(A)) receptor in Angelman syndrome (AS). STUDY DESIGN We performed [(11)C]flumazenil positron emission tomography (PET) and examined GABA(A) receptor expression in 7 patients with AS of various genotypes (5 with the deletion, 1 with an imprinting defect [ID], and 1 with a UBE3A mutation) and 4 normal control healthy volunteers. RESULTS Relative to the control subjects, the [(11)C]flumazenil binding potentials (BPs) were significantly higher in the cerebral cortex and cerebellum in the 5 patients with the deletion and in the 1 patient with a UBE3A mutation, and were less frequently or barely increased in adult patients with the deletion and in the patient with IDs. CONCLUSIONS Total GABA(A) receptor expression was increased in patients with AS with various genotypes. We suggest that a developmental dysregulation of the GABA(A) receptor subunits occurs in patients with AS.

Chloride Dysregulation, Seizures, and Cerebral Edema: A Relationship with Therapeutic Potential

Pharmacoresistant seizures and cytotoxic cerebral edema are serious complications of ischemic and traumatic brain injury. Intraneuronal Cl- concentration ([Cl-]i) regulation impacts on both cell volume homeostasis and Cl--permeable GABAA receptor-dependent membrane excitability. Understanding the pleiotropic molecular determinants of neuronal [Cl-]i - cytoplasmic impermeant anions, polyanionic extracellular matrix (ECM) glycoproteins, and plasmalemmal Cl- transporters - could help the identification of novel anticonvulsive and neuroprotective targets. The cation/Cl- cotransporters and ECM metalloproteinases may be particularly druggable targets for intervention. We establish here a paradigm that accounts for recent data regarding the complex regulatory mechanisms of neuronal [Cl-]i and how these mechanisms impact on neuronal volume and excitability. We propose approaches to modulate [Cl-]i that are relevant for two common clinical sequela of brain injury: edema and seizures.

Efficacy of perampanel for controlling seizures and improving neurological dysfunction in a patient with dentatorubral-pallidoluysian atrophy (DRPLA)

We administered perampanel (PER) to a bedridden 13-year-old male patient with dentatorubral-pallidoluysian atrophy (DRPLA). The DRPLA diagnosis was based on the presence of a CAG trinucleotide repeat in the ATN1 gene. The patient experienced continuous myoclonic seizures and weekly generalized tonic–clonic seizures (GTCs). PER stopped the patients myoclonic seizures and reduced the GTCs to fragmented clonic seizures. The patient recovered his intellectual abilities and began to walk again with assistance. We suggest that PER be considered as one of the key drugs used to treat patients with DRPLA.

Electroclinical features of epilepsy in patients with juvenile type dentatorubral-pallidoluysian atrophy

Purpose:  To clarify the electroclinical characteristics of epileptic seizures in patients with juvenile type dentatorubral‐pallidoluysian atrophy (DRPLA).