[Role of zonisamid in treating epilepsy, Parkinson disorders and other neurological diseases]

On the basis of six randomized controlled trials, zonisamide (ZNS) can be prescribed as add-on treatment in focal adulthood epilepsy in USA and Europe. In Japan, it can be prescribed as first-line monotherapy drug - independent of age. ZNS may also be effective in idiopathic generalized epilepsy and some difficult-to-treat epilepsies including West, Lennox-Gastaut, or Dravet syndromes. The most frequent side effects of ZNS are related to central nervous system occurring in 19%. Kidney stones and oligohidrosis are ZNS-specific side effects. Loss of appetite and weight are usually "beneficial" effects. ZNS is not recommended in pregnancy ZNS can be taken once daily, which may be beneficial in non-compliance. The pathomechanism of ZNS is different from other antiepileptic drugs. ZNS has an effect on the voltage-gated Na+ - and T-type Ca2+ channels as well as on the dopaminerg, glutamaterg, cholinerg, and GABAerg systems. The multiple way of action may be the reason why ZNS seems to be a broad-spectrum drug and beneficial in various neurological disorders. ZNS reduces production of free radicals according to in vitro and in vivo studies. Animal experiments suggest that ZNS may be a neuroprotective agent. Based on an adequate randomized controlled trial, ZNS is effective in adjuctive treatment of Parkinson disorder. A peculiar benefit of the ZNS is that parallel to its positive effect on motor impairment it also reduces severity of dyskinesias. ZNS may be effective in bipolar disorder, obesity, eating disorders, and migraine prophylaxis.

A Nav1.7 channel mutation associated with hereditary erythromelalgia contributes to neuronal hyperexcitability and displays reduced lidocaine sensitivity

Mutations in the TTX-sensitive voltage-gated sodium channel subtype Nav1.7 have been implicated in the painful inherited neuropathy, hereditary erythromelalgia. Hereditary erythromelalgia can be difficult to treat and, although sodium channels are targeted by local anaesthetics such as lidocaine (lignocaine), some patients do not respond to treatment with local anaesthetics. This study examined electrophysiological differences in Nav1.7 caused by a hereditary erythromelalgia mutation (N395K) that lies within the local anaesthetic binding site of the channel. The N395K mutation produced a hyperpolarized voltage dependence of activation, slower kinetics of deactivation, and impaired steady-state slow inactivation. Computer simulations indicate that the shift in activation is the major determinant of the hyperexcitability induced by erythromelalgia mutations in sensory neurons, but that changes in slow inactivation can modulate the overall impact on excitability. This study also investigated lidocaine inhibition of the Nav1.7-N395K channel. We show that the N395K mutation attenuates the inhibitory effects of lidocaine on both resting and inactivated Nav1.7. The IC50 for lidocaine was estimated at 500 microM for inactivated wild-type Nav1.7 and 2.8 mM for inactivated Nav1.7-N395K. The N395K mutation also significantly reduced use-dependent inhibition of lidocaine on Nav1.7 current. In contrast, a different hereditary erythromelalgia mutation (F216S), not located in the local anaesthetic binding site, had no effect on lidocaine inhibition of Nav1.7 current. Our observation of reduced lidocaine inhibition on Nav1.7-N395K shows that the residue N395 is critical for lidocaine binding to Nav1.7 and suggests that the response of individuals with hereditary erythromelalgia to lidocaine treatment may be determined, at least in part, by their specific genotype.

Isoform-specific Effects of the β2 Subunit on Voltage-gated Sodium Channel Gating

Voltage-gated sodium channels (Nav) are complex glycoproteins comprised of an alpha subunit and often one to several beta subunits. We have shown that sialic acid residues linked to Nav alpha and beta1 subunits alter channel gating. To determine whether beta2-linked sialic acids similarly impact Nav gating, we co-expressed beta2 with Nav1.5 or Nav1.2 in Pro5 (complete sialylation) and in Lec2 (essentially no sialylation) cells. Beta2 sialic acids caused a significant hyperpolarizing shift in Nav1.5 voltage-dependent gating, thus describing for the first time an effect of beta2 on Nav1.5 gating. In contrast, beta2 caused a sialic acid-independent depolarizing shift in Nav1.2 gating. A deglycosylated mutant, beta(2-DeltaN), had no effect on Nav1.5 gating, indicating further the impact of beta2 N-linked sialic acids on Nav1.5 gating. Conversely, beta(2-DeltaN) modulated Nav1.2 gating virtually identically to beta2, confirming that beta2 N-linked sugars have no impact on Nav1.2 gating. Thus, beta2 modulates Nav gating through multiple mechanisms possibly determined by the associated alpha subunit. Beta1 and beta2 were expressed together with Nav1.5 or Nav1.2 in Pro5 and Lec2 cells. Together beta1 and beta2 produced a significantly larger sialic acid-dependent hyperpolarizing shift in Nav1.5 gating. Under fully sialylating conditions, the Nav1.2.beta1.beta2 complex behaved like Nav1.2 alone. When sialylation was reduced, only the sialic acid-independent depolarizing effects of beta2 on Nav1.2 gating were apparent. Thus, the varied effects of beta1 and beta2 on Nav1.5 and Nav1.2 gating are apparently synergistic and highlight the complex manner, through subunit- and sugar-dependent mechanisms, by which Nav activity is modulated.

Sodium channel beta2 subunits regulate tetrodotoxin-sensitive sodium channels in small dorsal root ganglion neurons and modulate the response to pain

Voltage-gated sodium channel (Na(v)1) beta2 subunits modulate channel gating, assembly, and cell-surface expression in CNS neurons in vitro and in vivo. beta2 expression increases in sensory neurons after nerve injury, and development of mechanical allodynia in the spared nerve injury model is attenuated in beta2-null mice. Thus, we hypothesized that beta2 modulates electrical excitability in dorsal root ganglion (DRG) neurons in vivo. We compared sodium currents (I(Na)) in small DRG neurons from beta2+/+ and beta2-/- mice to determine the effects of beta2 on tetrodotoxin-sensitive (TTX-S) and tetrodotoxin-resistant (TTX-R) Na(v)1 in vivo. Small-fast DRG neurons acutely isolated from beta2-/- mice showed significant decreases in TTX-S I(Na) compared with beta2+/+ neurons. This decrease included a 51% reduction in maximal sodium conductance with no detectable changes in the voltage dependence of activation or inactivation. TTX-S, but not TTX-R, I(Na) activation and inactivation kinetics in these cells were slower in beta2(-/-) mice compared with controls. The selective regulation of TTX-S I(Na) was supported by reductions in transcript and protein levels of TTX-S Na(v)1s, particularly Na(v)1.7. Low-threshold mechanical sensitivity was preserved in beta2-/- mice, but they were more sensitive to noxious thermal stimuli than wild type whereas their response during the late phase of the formalin test was attenuated. Our results suggest that beta2 modulates TTX-S Na(v)1 mRNA and protein expression resulting in increased TTX-S I(Na) and increases the rates of TTX-S Na(v)1 activation and inactivation in small-fast DRG neurons in vivo. TTX-R I(Na) were not significantly modulated by beta2.

Upregulation of the voltage-gated sodium channel beta2 subunit in neuropathic pain models: characterization of expression in injured and non-injured primary sensory neurons

The development of abnormal primary sensory neuron excitability and neuropathic pain symptoms after peripheral nerve injury is associated with altered expression of voltage-gated sodium channels (VGSCs) and a modification of sodium currents. To investigate whether the beta2 subunit of VGSCs participates in the generation of neuropathic pain, we used the spared nerve injury (SNI) model in rats to examine beta2 subunit expression in selectively injured (tibial and common peroneal nerves) and uninjured (sural nerve) afferents. Three days after SNI, immunohistochemistry and Western blot analysis reveal an increase in the beta2 subunit in both the cell body and peripheral axons of injured neurons. The increase persists for >4 weeks, although beta2 subunit mRNA measured by real-time reverse transcription-PCR and in situ hybridization remains unchanged. Although injured neurons show the most marked upregulation,beta2 subunit expression is also increased in neighboring non-injured neurons and a similar pattern of changes appears in the spinal nerve ligation model of neuropathic pain. That increased beta2 subunit expression in sensory neurons after nerve injury is functionally significant, as demonstrated by our finding that the development of mechanical allodynia-like behavior in the SNI model is attenuated in beta2 subunit null mutant mice. Through its role in regulating the density of mature VGSC complexes in the plasma membrane and modulating channel gating, the beta2 subunit may play a key role in the development of ectopic activity in injured and non-injured sensory afferents and, thereby, neuropathic pain.

Identification of functional voltage-gated Na(+) channels in cultured human pulmonary artery smooth muscle cells

Electrical excitability, which plays an important role in excitation-contraction coupling in the pulmonary vasculature, is regulated by transmembrane ion flux in pulmonary artery smooth muscle cells (PASMC). This study aimed to characterize the electrophysiological properties and molecular identities of voltage-gated Na(+) channels in cultured human PASMC. We recorded tetrodotoxin (TTX) sensitive and rapidly inactivating Na(+) currents with properties similar to those described in cardiac myocytes. Using RT-PCR, we detected transcripts of seven Na(+) channel alpha genes (SCN2A, 3A, 4A, 7A, 8A, 9A, and 11A), and two beta subunit genes (SCN1B and 2B). Our results demonstrate that human PASMC express TTX-sensitive voltage-gated Na(+) channels. Their physiological functions remain unresolved, although our data suggest that Na(+) channel activity does not directly influence membrane potential, intracellular Ca(2+) release, or proliferation in normal human PASMC. Whether their expression and/or activity are heightened in the pathological state is discussed.

Presenilin/gamma-secretase-mediated cleavage of the voltage-gated sodium channel beta2-subunit regulates cell adhesion and migration

The voltage-gated sodium channel beta2-subunit (beta2) is a member of the IgCAM superfamily and serves as both an adhesion molecule and an auxiliary subunit of the voltage-gated sodium channel. Here we found that beta2 undergoes ectodomain shedding followed by presenilin (PS)-dependent gamma-secretase-mediated cleavage. 12-O-Tetradecanoylphorbol-13-acetate treatment or expression of an alpha-secretase enzyme, ADAM10, resulted in ectodomain cleavage of beta2 in Chinese hamster ovary cells. Subsequent cleavage of the remaining 15-kDa C-terminal fragment (beta2-CTF) was independently inhibited by three specific gamma-secretase inhibitors, expression of the dominant negative form of PS1, and in PS1/PS2 knock-out cells. gamma-Secretase inhibitor treatment also increased endogenous beta2-CTF levels in neuroblastoma cells and mouse primary neuronal cultures. In a cell-free gamma-secretase assay, we detected gamma-secretase activity-dependent generation of a 12 kDa beta2 intracellular domain (ICD), which was loosely associated with the membrane fraction. To assess the functional role of beta2 processing by gamma-secretase, we tested whether N-[N-(3,5-difluorophenylacetyl-l-alanyl)]-S-phenylglycine t-butylester (DAPT), a specific gamma-secretase inhibitor, would alter beta2-mediated cell adhesion and migration. We found that DAPT inhibited cell-cell aggregation and migration in a wound healing assay carried out with Chinese hamster ovary cells expressing beta2. DAPT also reduced migration of neuroblastoma cells in a modified Boyden chamber assay. Since DAPT treatment resulted in increased beta2-CTF levels, we also tested whether beta2-CTFs or beta2-ICDs would directly affect cell migration by overexpressing recombinant proteins. Interestingly, elevated levels of beta2-CTFs, but not ICDs, also blocked cell migration by 81 to 93%. Together, our findings show for the first time that beta2 is a PS/gamma-secretase substrate and gamma-secretase mediated cleavage of beta2-CTF is required for cell-cell adhesion and migration of beta2-expressing cells.

Post-transcriptional alterations in the expression of cardiac Na+ channel subunits in chronic heart failure

Clinical and experimental evidence has recently accumulated about the importance of alterations of Na(+) channel (NaCh) function and slow myocardial conduction for arrhythmias in infarcted and failing hearts (i.e., heart failure, HF). The present study evaluated the molecular mechanisms of local alterations in the expression of NaCh subunits which underlie Na(+) current (I(Na)) density decrease in HF. HF was induced in five dogs by sequential coronary microembolization and developed approximately 3 months after the last embolization (left ventricle (LV), ejection fraction = 27 +/- 7%). Five normal dogs served as a control group. Ventricular cardiomyocytes were isolated enzymatically from LV mid-myocardium and I(Na) was measured by whole-cell patch-clamp. The mRNA encoding the cardiac-specific NaCh alpha-subunit Na(v)1.5, and one of its auxiliary subunits beta 1 (NaCh beta 1), were analyzed by competitive reverse transcription-polymerase chain reaction. Protein levels of Na(v)1.5, NaCh beta 1 and NaCh beta 2 were evaluated by western blotting. The maximum density of I(Na)/C(m) was decreased in HF (n = 5) compared to control hearts (33.2 +/- 4.4 vs. 50.0 +/- 4.9 pA/pF, mean +/- S.E.M., n = 5, P < 0.05). The steady-state inactivation and activation of I(Na) remained unchanged in HF compared to control hearts. The levels of mRNA encoding Na(v)1.5, and NaCh beta 1 were unaltered in FH. However, Na(v)1.5 protein expression was reduced about 30% in HF, while NaCh beta 1 and NaCh beta 2 protein were unchanged. We conclude that experimental HF in dogs results in post-transcriptional changes in cardiac NaCh alpha-subunit expression.

[Molecular genetics of epilepsy]

Recent identifications of genes responsible for epilepsies are now contributing to diagnosis and treatment. Mutations of voltage-gated sodium channel genes SCN1A and SCN2A have been reported in epilepsies with a variety of phenotypes including generalized epilepsy with febrile seizures plus (GEFS +), severe myoclonic epilepsy in infancy (SMEI), intractable childhood epilepsy with generalized tonic-clonic seizures (ICEGTC), and benign familial neonatal-infantile seizures (BFNIS). We also identified a sporadic nonsense mutation of SCN2A in a patient with intractable epilepsy with severe mental decline. Lafora's disease (LD) is a fatal autosomal recessive epilepsy characterized by stimuli sensitive myoclonus, grand mal seizures, and progressive intellectual and neurological deterioration. The EPM2A gene has been reported to be responsible for LD. We found multiple disease mutations of EPM2A in LD patients, and also identified a subclass of LD who shows an early onset cognitive defect and correlated with EPM2A exon 1 mutations. We reported that the laforin protein encoded by the EPM2A gene has a dual-specificity phosphatase activity, associates with polyribosome, and interacts with the HIRIP5 protein with NifU-like domain. We recently generated and reported the EPM2A KO mice those develop neurodegeneration and other features similar to those of LD patients.

T594M mutation of the epithelial sodium channel beta-subunit gene in pre-eclampsia and eclampsia in Black South African women

The possible role of the beta-subunit of the epithelial sodium channel T594M polymorphism in hypertensive disorders of pregnancy has not been examined. This study compared Black South African women with pre-eclampsia (n= 204), early onset pre-eclampsia (n= 67), eclampsia (n= 120) and gestational hypertension (n= 78) with 338 women from the same ethnic group who had full-term normotensive pregnancies, for the presence of the T594M polymorphism. The variant allele was detected in 1.7% to 3.8% of the various patient groups and in 3.6% of the control group reflecting no significant difference. These results suggest that the T594M polymorphism in the sodium channel beta-subunit is not associated with the pathogenesis of pre-eclampsia or gestational hypertension.

Molecular basis of Mendelian idiopathic epilepsies

A genetic aetiology is estimated to be present in about 40% of patients with epilepsy. Significant progress has been made in understanding the molecular genetic basis of Mendelian epilepsies. Fourteen genes have been identified which underlie a group of rare, autosomal dominant Mendelian idiopathic epilepsies. All but two of these genes encode subunits of ion-channels, revealing that idiopathic Mendelian human epilepsies are predominantly channelopathies. The two non-ion-channel genes, LGl1 causing autosomal dominant lateral temporal lobe epilepsy and MASS1 causing febrile and afebrile seizures, both contain a novel repeat motif variously called the epilepsy-associated repeat (EAR) and epitempin (EPTP) repeat. This motif defines a subfamily of genes, some of which have also been implicated in epilepsy in mice and humans. Progress in dissecting the more common 'complex' genetic epilepsies remains slow, but ion channels represent the most biologically plausible candidates. Characterization of common population sequence variants for the entire cohort of ion channel genes and the development of high-throughput techniques should enable rapid advances in the understanding of the common idiopathic familial epilepsies.

Sodium channel beta4, a new disulfide-linked auxiliary subunit with similarity to beta2

The principal alpha subunit of voltage-gated sodium channels is associated with auxiliary beta subunits that modify channel function and mediate protein-protein interactions. We have identified a new beta subunit termed beta4. Like the beta1-beta3 subunits, beta4 contains a cleaved signal sequence, an extracellular Ig-like fold, a transmembrane segment, and a short intracellular C-terminal tail. Using TaqMan reverse transcription-PCR analysis, in situ hybridization, and immunocytochemistry, we show that beta4 is widely distributed in neurons in the brain, spinal cord, and some sensory neurons.beta4 is most similar to the beta2 subunit (35% identity), and, like the beta2 subunit, the Ig-like fold of beta4 contains an unpaired cysteine that may interact with the alpha subunit. Under nonreducing conditions, beta4 has a molecular mass exceeding 250 kDa because of its covalent linkage to Nav1.2a, whereas on reduction, it migrates with a molecular mass of 38 kDa, similar to the mature glycosylated forms of the other beta subunits. Coexpression of beta4 with brain Nav1.2a and skeletal muscle Nav1.4 alpha subunits in tsA-201 cells resulted in a negative shift in the voltage dependence of channel activation, which overrode the opposite effects of beta1 and beta3 subunits when they were present. This novel, disulfide-linked beta subunit is likely to affect both protein-protein interactions and physiological function of multiple sodium channel alpha subunits.

Generalized epilepsy with febrile seizures plus: mutation of the sodium channel subunit SCN1B

Generalized epilepsy with febrile seizures plus (GEFS(+)) is an important childhood genetic epilepsy syndrome with heterogeneous phenotypes, including febrile seizures (FS) and generalized epilepsies of variable severity. Forty unrelated GEFS(+) and FS patients were screened for mutations in the sodium channel beta-subunits SCN1B and SCN2B, and the second GEFS(+) family with an SCN1B mutation is described here. The family had 19 affected individuals: 16 with typical GEFS(+) phenotypes and three with other epilepsy phenotypes. Site-specific mutation within SCN1B remains a rare cause of GEFS(+), and the authors found no evidence to implicate SCN2B in this syndrome.

The voltage-gated sodium channel beta2-subunit gene and idiopathic generalized epilepsy

Recent identification of ion channel gene mutations in Mendelian epilepsies suggests that genetically driven neuronal hyperexcitability plays an important role in epileptogenesis. In this study, we tested the hypothesis that genetic variation in the human SCN2B gene confers liability to common subtypes of idiopathic generalized epilepsies (IGE). A systematic search for mutations was performed in 92 IGE patients. We detected a novel single nucleotide polymorphism (SNP), however, allele frequencies did not differ between IGE patients and controls (chi2 = 0.19, df = 1, p = 0.744). Furthermore, a missense mutation in codon 209 (Asn209Pro) was identified in one patient, but was found to be absent in an affected sibling of the index patient. Thus, our results do not suggest a major role of the SCN2B gene in the etiology of common IGE subtypes.

Exclusion of the SCN2B gene as candidate for CMT4B

Charcot-Marie-Tooth disease type 4B (CMT4B) is a demyelinating autosomal recessive motor and sensory neuropathy characterised by focally folded myelin sheaths in the peripheral nerve. The CMT4B gene has been localised by homozygosity mapping and haplotype sharing in the 11q23 region. A cDNA encoding for the beta 2 subunit of the human brain sodium channel, SCN2B, has been recently assigned to the same chromosomal interval by FISH. The SCN2B gene has been considered a good candidate for CMT4B on the basis of protein homology, chromosomal localisation, and putative biological function of the coded product. In this paper, we report the genomic structure of the SCN2B gene consisting of 4 exons and 3 introns spanning a region of approximately 12 Kb. In addition, a search for mutations in patients affected with CMT4B as well as a refined physical localisation excludes SCN2B as the CMT4B gene.