Nav1.1 localizes to axons of parvalbumin-positive inhibitory interneurons: a circuit basis for epileptic seizures in mice carrying an Scn1a gene mutation

Antiepileptic Drug Discovery and Development: What Have We Learned and Where Are We Going?

Current marketed antiepileptic drugs (AEDs) consist of a variety of structural classes with different mechanisms of action. These agents typically have non-overlapping efficacy and side-effect profiles presenting multiple treatment options for the patient population. However, approximately 30% of seizure sufferers fail to respond to current therapies often because poorly tolerated side-effects limit adequate dosing. The scope of this review is to summarize selected advances in 2nd and 3rd generation AEDs as well as compounds in development with novel mechanisms of action.

Scn1a missense mutation impairs GABAA receptor-mediated synaptic transmission in the rat hippocampus

Mutations of the Na(v)1.1 channel subunit SCN1A have been implicated in the pathogenesis of human febrile seizures (FS). We have recently developed hyperthermia-induced seizure-susceptible (Hiss) rat, a novel rat model of FS, which carries a missense mutation (N1417H) in Scn1a[1]. Here, we conducted electrophysiological studies to clarify the influences of the Scn1a mutation on the hippocampal synaptic transmission, specifically focusing on the GABAergic system. Hippocampal slices were prepared from Hiss or F344 (control) rats and maintained in artificial cerebrospinal fluid saturated with 95% O(2) and 5% CO(2)in vitro. Single neuron activity was recorded from CA1 pyramidal neurons and their responses to the test (unconditioned) or paired pulse (PP) stimulation of the Schaffer collateral/commissural fibers were evaluated. Hiss rats were first tested for pentylenetetrazole-induced seizures and confirmed to show high seizure susceptibility to the blockade of GAGA(A) receptors. The Scn1a mutation in Hiss rats did not directly affect spike generation (i.e., number of evoked spikes and firing threshold) of the CA1 pyramidal neurons elicited by the Schaffer collateral/commissural stimulation. However, GABA(A) receptor-mediated inhibition of pyramidal neurons by the PP stimulation was significantly disrupted in Hiss rats, yielding a significant increase in the number of PP-induced firings at PP intervals of 32-256ms. The present study shows that the Scn1a missense mutation preferentially impairs GABA(A) receptor-mediated synaptic transmission without directly altering the excitability of the pyramidal neurons in the hippocampus, which may be linked to the pathogenesis of FS.

A missense mutation in a highly conserved alternate exon of dynamin-1 causes epilepsy in fitful mice

Dynamin-1 (Dnm1) encodes a large multimeric GTPase necessary for activity-dependent membrane recycling in neurons, including synaptic vesicle endocytosis. Mice heterozygous for a novel spontaneous Dnm1 mutation—fitful—experience recurrent seizures, and homozygotes have more debilitating, often lethal seizures in addition to severe ataxia and neurosensory deficits. Fitful is a missense mutation in an exon that defines the DNM1a isoform, leaving intact the alternatively spliced exon that encodes DNM1b. The expression of the corresponding alternate transcripts is developmentally regulated, with DNM1b expression highest during early neuronal development and DNM1a expression increasing postnatally with synaptic maturation. Mutant DNM1a does not efficiently self-assemble into higher order complexes known to be necessary for proper dynamin function, and it also interferes with endocytic recycling in cell culture. In mice, the mutation results in defective synaptic transmission characterized by a slower recovery from depression after trains of stimulation. The DNM1a and DNM1b isoform pair is highly conserved in vertebrate evolution, whereas invertebrates have only one isoform. We speculate that the emergence of more specialized forms of DNM1 may be important in organisms with complex neuronal function.

Epilepsy, a group of chronic disorders characterized by recurrent seizures, results from abnormal, synchronized neuronal activity in the brain. The mouse represents a powerful system to study novel mutations that model neurological disease, including epilepsy. Here we describe a new mouse mutation (“fitful”) in the gene encoding dynamin-1. Fitful mice have recurrent seizures and other neurological defects, including impaired hearing. Dynamin-1 is very well studied, but has yet to be linked to neurological disease. Dynamin-1 is a large multimeric enzyme that functions in membrane fission, primarily of vesicles after they release neurotransmitter at neuronal synapses. Fitful occurs in the region of dynamin-1 that is important for self-assembly of single dynamin subunits into the multimers required for enzymatic function. We show that fitful interferes with dynamin-1 self-assembly and with endocytosis. Moreover, the mutation resides in one of two alternate forms of dynamin-1 and affects what may be a necessary shift during brain development, with the expression of the mutated form being higher after maturation in fitful mice. This particular genetic specialization is unique to vertebrate dynamin. We speculate that specialized forms of dynamin-1 are important for modifying the self-assembly process to meet the demands complex brain activity in higher organisms.

Electroclinical characterization of epileptic seizures in leucine-rich, glioma-inactivated 1-deficient mice

Mutations of the LGI1 (leucine-rich, glioma-inactivated 1) gene underlie autosomal dominant lateral temporal lobe epilepsy, a focal idiopathic inherited epilepsy syndrome. The LGI1 gene encodes a protein secreted by neurons, one of the only non-ion channel genes implicated in idiopathic familial epilepsy. While mutations probably result in a loss of function, the role of LGI1 in the pathophysiology of epilepsy remains unclear. Here we generated a germline knockout mouse for LGI1 and examined spontaneous seizure characteristics, changes in threshold for induced seizures and hippocampal pathology. Frequent spontaneous seizures emerged in homozygous LGI1^−/− mice during the second postnatal week. Properties of these spontaneous events were examined in a simultaneous video and intracranial electroencephalographic recording. Their mean duration was 120 ± 12 s, and behavioural correlates consisted of an initial immobility, automatisms, sometimes followed by wild running and tonic and/or clonic movements. Electroencephalographic monitoring indicated that seizures originated earlier in the hippocampus than in the cortex. LGI1^−/− mice did not survive beyond postnatal day 20, probably due to seizures and failure to feed. While no major developmental abnormalities were observed, after recurrent seizures we detected neuronal loss, mossy fibre sprouting, astrocyte reactivity and granule cell dispersion in the hippocampus of LGI1^−/− mice. In contrast, heterozygous LGI1^+/− littermates displayed no spontaneous behavioural epileptic seizures, but auditory stimuli induced seizures at a lower threshold, reflecting the human pathology of sound-triggered seizures in some patients. We conclude that LGI1^+/− and LGI1^−/− mice may provide useful models for lateral temporal lobe epilepsy, and more generally idiopathic focal epilepsy.

The axon initial segment and the maintenance of neuronal polarity

Ion channel clustering at the axon initial segment (AIS) and nodes of Ranvier has been suggested to be a key evolutionary innovation that enabled the development of the complex vertebrate nervous system. This innovation epitomizes a signature feature of neurons, namely polarity. The mechanisms that establish neuronal polarity, channel clustering and axon-dendrite identity during development are becoming clearer. However, much less is known about how polarity is maintained throughout life. Here, I review the role of the AIS in the development and maintenance of neuronal polarity and discuss how disrupted polarity may be a common component of many diseases and injuries that affect the nervous system.

Axon initial segment dysfunction in a mouse model of genetic epilepsy with febrile seizures plus

Febrile seizures are a common childhood seizure disorder and a defining feature of genetic epilepsy with febrile seizures plus (GEFS+), a syndrome frequently associated with Na+ channel mutations. Here, we describe the creation of a knockin mouse heterozygous for the C121W mutation of the beta1 Na+ channel accessory subunit seen in patients with GEFS+. Heterozygous mice with increased core temperature displayed behavioral arrest and were more susceptible to thermal challenge than wild-type mice. Wild-type beta1 was most concentrated in the membrane of axon initial segments (AIS) of pyramidal neurons, while the beta1(C121W) mutant subunit was excluded from AIS membranes. In addition, AIS function, an indicator of neuronal excitability, was substantially enhanced in hippocampal pyramidal neurons of the heterozygous mouse specifically at higher temperatures. Computational modeling predicted that this enhanced excitability was caused by hyperpolarized voltage activation of AIS Na+ channels. This heat-sensitive increased neuronal excitability presumably contributed to the heightened thermal seizure susceptibility and epileptiform discharges seen in patients and mice with beta1(C121W) subunits. We therefore conclude that Na+ channel beta1 subunits modulate AIS excitability and that epilepsy can arise if this modulation is impaired.

Optogenetic localization and genetic perturbation of saccade-generating neurons in zebrafish

The optokinetic response (OKR) to a visual stimulus moving at constant velocity consists of a series of two alternating components, a slow phase, during which the eyes follow the stimulus, and a quick phase, which resets the eyes to begin a new response cycle. The quick phases of the OKR resemble the saccades observed during free viewing. It is unclear to what extent the premotor circuitry underlying these two types of jerky, conjugate eye movements is conserved among vertebrates. Zebrafish (Danio rerio) larvae, broadly expressing halorhodopsin (NpHR) or channelrhodopsin-2 (ChR2) in most neurons, were used to map the location of neurons involved in this behavior. By blocking activity in localized groups of NpHR-expressing neurons with an optic fiber positioned above the head of the fish and by systematically varying the site of photostimulation, we discovered that activity in a small hindbrain area in rhombomere 5 was necessary for saccades to occur. Unilateral block of activity at this site affected behavior in a direction-specific manner. Inhibition of the right side suppressed rightward saccades of both eyes, while leaving leftward saccades unaffected, and vice versa. Photostimulation of this area in ChR2-transgenic fish was sufficient to trigger saccades that were precisely locked to the light pulses. These extra saccades could be induced both during free viewing and during the OKR, and were distinct in their kinetics from eye movements elicited by stimulating the abducens motor neurons. Zebrafish double indemnity (didy) mutants were identified in a chemical mutagenesis screen based on a defect in sustaining saccades during OKR. Positional cloning, molecular analysis, and electrophysiology revealed that the didy mutation disrupts the voltage-gated sodium channel Scn1lab (Nav1.lb). ChR2 photostimulation of the putative hindbrain saccade generator was able to fully reconstitute saccades in the didy mutant. Our studies demonstrate that an optogenetic approach is useful for targeted loss-of-function and gain-of-function manipulations of neural circuitry underlying eye movements in zebrafish and that the saccade-generating circuit in this species shares many of its properties with that in mammals.

Molecular identity of dendritic voltage-gated sodium channels

Active invasion of the dendritic tree by action potentials (APs) generated in the axon is essential for associative synaptic plasticity and neuronal ensemble formation. In cortical pyramidal cells (PCs), this AP back-propagation is supported by dendritic voltage-gated Na+ (Nav) channels, whose molecular identity is unknown. Using a highly sensitive electron microscopic immunogold technique, we revealed the presence of the Nav1.6 subunit in hippocampal CA1 PC proximal and distal dendrites. Here, the subunit density is lower by a factor of 35 to 80 than that found in axon initial segments. A gradual decrease in Nav1.6 density along the proximodistal axis of the dendritic tree was also detected without any labeling in dendritic spines. Our results reveal the characteristic subcellular distribution of the Nav1.6 subunit, identifying this molecule as a key substrate enabling dendritic excitability.

A missense mutation of the gene encoding voltage-dependent sodium channel (Nav1.1) confers susceptibility to febrile seizures in rats

Although febrile seizures (FSs) are the most common convulsive syndrome in infants and childhood, the etiology of FSs has remained unclarified. Several missense mutations of the Na(v)1.1 channel (SCN1A), which alter channel properties, have been reported in a familial syndrome of GEFS+ (generalized epilepsy with febrile seizures plus). Here, we generated Scn1a-targeted rats carrying a missense mutation (N1417H) in the third pore region of the sodium channel by gene-driven ENU (N-ethyl-N-nitrosourea) mutagenesis. Despite their normal appearance under ordinary circumstances, Scn1a mutant rats exhibited remarkably high susceptibility to hyperthermia-induced seizures, which involve generalized clonic and/or tonic-clonic convulsions with paroxysmal epileptiform discharges. Whole-cell patch-clamp recordings from HEK cells expressing N1417H mutant channels and from hippocampal GABAergic interneurons of N1417H mutant rats revealed a significant shift of the inactivation curve in the hyperpolarizing direction. In addition, clamp recordings clearly showed the reduction in action potential amplitude in the hippocampal interneurons of these rats. These findings suggest that a missense mutation (N1417H) of the Na(v)1.1 channel confers susceptibility to FS and the impaired biophysical properties of inhibitory GABAergic neurons underlie one of the mechanisms of FS.

Voltage-gated sodium channels as therapeutic targets in epilepsy and other neurological disorders

Voltage-gated sodium channels (VGSCs) are key mediators of intrinsic neuronal and muscle excitability. Abnormal VGSC activity is central to the pathophysiology of epileptic seizures, and many of the most widely used antiepileptic drugs, including phenytoin, carbamazepine, and lamotrigine, are inhibitors of VGSC function. These antiepileptic drugs might also be efficacious in the treatment of other nervous system disorders, such as migraine, multiple sclerosis, neurodegenerative diseases, and neuropathic pain. In this Review, we summarise the structure and function of VGSCs and their involvement in the pathophysiology of several neurological disorders. We also describe the biophysical and molecular bases for the mechanisms of action of antiepileptic VGSC blockers and discuss the efficacy of these drugs in the treatment of epileptic and non-epileptic disorders. Overall, clinical and experimental data indicate that these drugs are efficacious for a range of diseases, and that the development of drugs with enhanced selectivity for specific VGSC isoforms might be an effective and novel approach for the treatment of several neurological diseases.

Axon initial segment dysfunction in epilepsy

The axon initial segment (AIS) contains the site of action potential initiation and plays a major role in neuronal excitability. AIS function relies on high concentrations of different ion channels and complex regulatory mechanisms that orchestrate molecular microarchitecture. We review recent evidence that a large number of ion channels associated with epilepsy are enriched at the AIS, making it a 'hotspot' for epileptogenesis. Furthermore, we present novel data on the clustering of GABRgamma2 receptors in the AIS of cortical and hippocampal neurons in a knock in mouse model of a human genetic epilepsy. This article highlights the molecular coincidence of epilepsy mutations at the AIS and reviews pathogenic mechanisms converging at the AIS.

Dysfunction of the Scn8a voltage-gated sodium channel alters sleep architecture, reduces diurnal corticosterone levels, and enhances spatial memory

Voltage-gated sodium channels (VGSCs) are responsible for the initiation and propagation of transient depolarizing currents and play a critical role in the electrical signaling between neurons. A null mutation in the VGSC gene SCN8A, which encodes the transmembrane protein Na(v)1.6, was identified previously in a human family. Heterozygous mutation carriers displayed a range of phenotypes, including ataxia, cognitive deficits, and emotional instability. A possible role for SCN8A was also proposed in studies examining the genetic basis of attempted suicide and bipolar disorder. In addition, mice with a Scn8a loss-of-function mutation (Scn8a(med-Tg/+)) show altered anxiety and depression-like phenotypes. Because psychiatric abnormalities are often associated with altered sleep and hormonal patterns, we evaluated heterozygous Scn8a(med-jo/+) mutants for alterations in sleep-wake architecture, diurnal corticosterone levels, and behavior. Compared with their wild-type littermates, Scn8a(med-jo/+) mutants experience more non-rapid eye movement (non-REM) sleep, a chronic impairment of REM sleep generation and quantity, and a lowered and flattened diurnal rhythm of corticosterone levels. No robust differences were observed between mutants and wild-type littermates in locomotor activity or in behavioral paradigms that evaluate anxiety or depression-like phenotypes; however, Scn8a(med-jo/+) mutants did show enhanced spatial memory. This study extends the spectrum of phenotypes associated with mutations in Scn8a and suggests a novel role for altered sodium channel function in human sleep disorders.

Sodium channel gene family: epilepsy mutations, gene interactions and modifier effects

The human sodium channel family includes seven neuronal channels that are essential for the initiation and propagation of action potentials in the CNS and PNS. In view of their critical role in neuronal firing and their strong sequence conservation during evolution, it is not surprising that mutations in the sodium channel genes are responsible for a growing spectrum of channelopathies. Nearly 700 mutations of the SCN1A gene have been identified in patients with Dravet's syndrome (severe myoclonic epilepsy of infancy), making this the most commonly mutated gene in human epilepsy. A small number of mutations have been found in SCN2A, SCN3A and SCN9A, and studies in the mouse suggest that SCN8A may also contribute to seizure disorders. Interactions between genetic variants of SCN2A and KCNQ2 in the mouse and variants of SCN1A and SCN9A in patients provide models of potential genetic modifier effects in the more common human polygenic epilepsies. New methods for generating induced pluripotent stem cells and neurons from patients will facilitate functional analysis of amino acid substitutions in channel proteins. Whole genome sequencing and exome sequencing in patients with epilepsy will soon make it possible to detect multiple variants and their interactions in the genomes of patients with seizure disorders.

Electrogenic tuning of the axon initial segment

Action potentials (APs) provide the primary means of rapid information transfer in the nervous system. Where exactly these signals are initiated in neurons has been a basic question in neurobiology and the subject of extensive study. Converging lines of evidence indicate that APs are initiated in a discrete and highly specialized portion of the axon-the axon initial segment (AIS). The authors review key aspects of the organization and function of the AIS and focus on recent work that has provided important insights into its electrical signaling properties. In addition to its main role in AP initiation, the new findings suggest that the AIS is also a site of complex AP modulation by specific types of ion channels localized to this axonal domain.

NaV1.1 channels and epilepsy

Voltage-gated sodium channels initiate action potentials in brain neurons, and sodium channel blockers are used in therapy of epilepsy. Mutations in sodium channels are responsible for genetic epilepsy syndromes with a wide range of severity, and the NaV1.1 channel encoded by the SCN1A gene is the most frequent target of mutations. Complete loss-of-function mutations in NaV1.1 cause severe myoclonic epilepsy of infancy (SMEI or Dravet's Syndrome), which includes severe, intractable epilepsy and comorbidities of ataxia and cognitive impairment. Mice with loss-of-function mutations in NaV1.1 channels have severely impaired sodium currents and action potential firing in hippocampal GABAergic inhibitory neurons without detectable effect on the excitatory pyramidal neurons, which would cause hyperexcitability and contribute to seizures in SMEI. Similarly, the sodium currents and action potential firing are also impaired in the GABAergic Purkinje neurons of the cerebellum, which is likely to contribute to ataxia. The imbalance between excitatory and inhibitory transmission in these mice can be partially corrected by compensatory loss-of-function mutations of NaV1.6 channels, and thermally induced seizures in these mice can be prevented by drug combinations that enhance GABAergic neurotransmission. Generalized epilepsy with febrile seizures plus (GEFS+) is caused by missense mutations in NaV1.1 channels, which have variable biophysical effects on sodium channels expressed in non-neuronal cells, but may primarily cause loss of function when expressed in mice. Familial febrile seizures is caused by mild loss-of-function mutations in NaV1.1 channels; mutations in these channels are implicated in febrile seizures associated with vaccination; and impaired alternative splicing of the mRNA encoding these channels may also predispose some children to febrile seizures. We propose a unified loss-of-function hypothesis for the spectrum of epilepsy syndromes caused by genetic changes in NaV1.1 channels, in which mild impairment predisposes to febrile seizures, intermediate impairment leads to GEFS+ epilepsy, and severe or complete loss of function leads to the intractable seizures and comorbidities of SMEI.

Altered function of the SCN1A voltage-gated sodium channel leads to gamma-aminobutyric acid-ergic (GABAergic) interneuron abnormalities

Voltage-gated sodium channels are required for the initiation and propagation of action potentials. Mutations in the neuronal voltage-gated sodium channel SCN1A are associated with a growing number of disorders including generalized epilepsy with febrile seizures plus (GEFS+),(7) severe myoclonic epilepsy of infancy, and familial hemiplegic migraine. To gain insight into the effect of SCN1A mutations on neuronal excitability, we introduced the human GEFS+ mutation SCN1A-R1648H into the orthologous mouse gene. Scn1a(RH/RH) mice homozygous for the R1648H mutation exhibit spontaneous generalized seizures and premature death between P16 and P26, whereas Scn1a(RH/+) heterozygous mice exhibit infrequent spontaneous generalized seizures, reduced threshold and accelerated propagation of febrile seizures, and decreased threshold to flurothyl-induced seizures. Inhibitory cortical interneurons from P5-P15 Scn1a(RH/+) and Scn1a(RH/RH) mice demonstrated slower recovery from inactivation, greater use-dependent inactivation, and reduced action potential firing compared with wild-type cells. Excitatory cortical pyramidal neurons were mostly unaffected. These results suggest that this SCN1A mutation predominantly impairs sodium channel activity in interneurons, leading to decreased inhibition. Decreased inhibition may be a common mechanism underlying clinically distinct SCN1A-derived disorders.

Key factors in the discovery and development of new antiepileptic drugs

Since the early 1990s, many new antiepileptic drugs (AEDs) that offer appreciable advantages in terms of their favourable pharmacokinetics, improved tolerability and lower potential for drug-drug interactions have entered the market. However, despite the therapeutic arsenal of old and new AEDs, approximately 30% of patients with epilepsy still suffer from seizures. Thus, there remains a substantial need for the development of more efficacious AEDs for patients with refractory seizures. Here, we briefly review the emerging knowledge on the pathological basis of epilepsy and how it might best be used in the design of new therapeutics. We also discuss the current approach to AED discovery and highlight some of the unique features of newer models of pharmacoresistance and epileptogenesis that have emerged in recent years.

Advances on the genetics of mendelian idiopathic epilepsies

Genetic factors play an increasingly recognized role in idiopathic epilepsies. Since 1995, positional cloning strategies in multi-generational families with autosomal dominant transmission have revealed 11 genes (KCNQ2, KCNQ3, CHRNA4, CHRNA2, CHRNB2, SCN1B, SCN1A, SCN2A, GABRG2, GABRA1, and LGI1) and numerous loci for febrile seizures and epilepsies. To date, all genes with the exception of LGI1 (leucine-rich glioma inactivated 1), encode neuronal ion channel or neurotransmitter receptor subunits. Molecular approaches have revealed great genetic heterogeneity, with the vast majority of genes remaining to be identified. One of the major challenges is now to understand phenotype-genotype correlations. This review focuses on the current knowledge on the molecular basis of these rare Mendelian autosomal dominant forms of idiopathic epilepsies.

De novo mutations of voltage-gated sodium channel alphaII gene SCN2A in intractable epilepsies

BACKGROUND

Mutations of voltage-gated sodium channel alpha(II) gene, SCN2A, have been described in a wide spectrum of epilepsies. While inherited SCN2A mutations have been identified in multiple mild epilepsy cases, a de novo SCN2A-R102X mutation, which we previously reported in a patient with sporadic intractable childhood localization-related epilepsy, remains unique. To validate the involvement of de novo SCN2A mutations in the etiology of intractable epilepsies, we sought to identify additional instances.

METHODS

We performed mutational analyses on SCN2A in 116 patients with severe myoclonic epilepsy in infancy, infantile spasms, and other types of intractable childhood partial and generalized epilepsies and did whole-cell patch-clamp recordings on Na(v)1.2 channels containing identified mutations.

RESULTS

We discovered 2 additional de novo SCN2A mutations. One mutation, SCN2A-E1211K, was identified in a patient with sporadic infantile spasms. SCN2A-E1211K produced channels with altered electrophysiologic properties compatible with both augmented (an approximately 18-mV hyperpolarizing shift in the voltage dependence of activation) and reduced (an approximately 22-mV hyperpolarizing shift in the voltage dependence of steady-state inactivation and a slowed recovery from inactivation) channel activities. The other de novo mutation, SCN2A-I1473M, was identified in a patient with sporadic neonatal epileptic encephalopathy. SCN2A-I1473M caused an approximately 14-mV hyperpolarizing shift in the voltage dependence of activation.

CONCLUSIONS

The identified de novo mutations SCN2A-E1211K, -I1473M, and -R102X indicate that SCN2A is an etiologic candidate underlying a variety of intractable childhood epilepsies. The phenotypic variations among patients might be due to the different electrophysiologic properties of mutant channels.

Subtype-selective targeting of voltage-gated sodium channels

Voltage-gated sodium channels are key to the initiation and propagation of action potentials in electrically excitable cells. Molecular characterization has shown there to be nine functional members of the family, with a high degree of sequence homology between the channels. This homology translates into similar biophysical and pharmacological properties. Confidence in some of the channels as drug targets has been boosted by the discovery of human mutations in the genes encoding a number of them, which give rise to clinical conditions commensurate with the changes predicted from the altered channel biophysics. As a result, they have received much attention for their therapeutic potential. Sodium channels represent well-precedented drug targets as antidysrhythmics, anticonvulsants and local anaesthetics provide good clinical efficacy, driven through pharmacology at these channels. However, electrophysiological characterization of clinically useful compounds in recombinant expression systems shows them to be weak, with poor selectivity between channel types. This has led to the search for subtype-selective modulators, which offer the promise of treatments with improved clinical efficacy and better toleration. Despite developments in high-throughput electrophysiology platforms, this has proven very challenging. Structural biology is beginning to offer us a greater understanding of the three-dimensional structure of voltage-gated ion channels, bringing with it the opportunity to do real structure-based drug design in the future. This discipline is still in its infancy, but developments with the expression and purification of prokaryotic sodium channels offer the promise of structure-based drug design in the not too distant future.

A role of SCN9A in human epilepsies, as a cause of febrile seizures and as a potential modifier of Dravet syndrome

A follow-up study of a large Utah family with significant linkage to chromosome 2q24 led us to identify a new febrile seizure (FS) gene, SCN9A encoding Na(v)1.7. In 21 affected members, we uncovered a potential mutation in a highly conserved amino acid, p.N641Y, in the large cytoplasmic loop between transmembrane domains I and II that was absent from 586 ethnically matched population control chromosomes. To establish a functional role for this mutation in seizure susceptibility, we introduced the orthologous mutation into the murine Scn9a ortholog using targeted homologous recombination. Compared to wild-type mice, homozygous Scn9a(N641Y/N641Y) knockin mice exhibit significantly reduced thresholds to electrically induced clonic and tonic-clonic seizures, and increased corneal kindling acquisition rates. Together, these data strongly support the SCN9A p.N641Y mutation as disease-causing in this family. To confirm the role of SCN9A in FS, we analyzed a collection of 92 unrelated FS patients and identified additional highly conserved Na(v)1.7 missense variants in 5% of the patients. After one of these children with FS later developed Dravet syndrome (severe myoclonic epilepsy of infancy), we sequenced the SCN1A gene, a gene known to be associated with Dravet syndrome, and identified a heterozygous frameshift mutation. Subsequent analysis of 109 Dravet syndrome patients yielded nine Na(v)1.7 missense variants (8% of the patients), all in highly conserved amino acids. Six of these Dravet syndrome patients with SCN9A missense variants also harbored either missense or splice site SCN1A mutations and three had no SCN1A mutations. This study provides evidence for a role of SCN9A in human epilepsies, both as a cause of FS and as a partner with SCN1A mutations.

A functional null mutation of SCN1B in a patient with Dravet syndrome

Dravet syndrome (also called severe myoclonic epilepsy of infancy) is one of the most severe forms of childhood epilepsy. Most patients have heterozygous mutations in SCN1A, encoding voltage-gated sodium channel Na(v)1.1 alpha subunits. Sodium channels are modulated by beta1 subunits, encoded by SCN1B, a gene also linked to epilepsy. Here we report the first patient with Dravet syndrome associated with a recessive mutation in SCN1B (p.R125C). Biochemical characterization of p.R125C in a heterologous system demonstrated little to no cell surface expression despite normal total cellular expression. This occurred regardless of coexpression of Na(v)1.1 alpha subunits. Because the patient was homozygous for the mutation, these data suggest a functional SCN1B null phenotype. To understand the consequences of the lack of beta1 cell surface expression in vivo, hippocampal slice recordings were performed in Scn1b(-/-) versus Scn1b(+/+) mice. Scn1b(-/-) CA3 neurons fired evoked action potentials with a significantly higher peak voltage and significantly greater amplitude compared with wild type. However, in contrast to the Scn1a(+/-) model of Dravet syndrome, we found no measurable differences in sodium current density in acutely dissociated CA3 hippocampal neurons. Whereas Scn1b(-/-) mice seize spontaneously, the seizure susceptibility of Scn1b(+/-) mice was similar to wild type, suggesting that, like the parents of this patient, one functional SCN1B allele is sufficient for normal control of electrical excitability. We conclude that SCN1B p.R125C is an autosomal recessive cause of Dravet syndrome through functional gene inactivation.

Severe epilepsy syndromes of early childhood: the link between genetics and pathophysiology with a focus on SCN1A mutations

Advances in genetics have increased our understanding of the underlying pathophysiologic mechanisms that cause severe epilepsy syndromes of early childhood. Many of the mutations associated with these syndromes are located in genes coding for ion channels or their accessory subunits, giving rise to the concept of epilepsy ;;channelopathies.'' In particular, the SCN1A gene coding for the pore-forming a-subunit of the voltage-gated sodium channel Na(V)1.1 appears to be a common target for epilepsy syndrome-specific mutations. An SCN1A mutation can potentially result in either a gain or loss of sodium channel function. Epilepsies linked to SCN1A mutations range from a relatively benign syndrome called generalized epilepsy with febrile seizures plus to severe childhood epilepsies such as severe myoclonic epilepsy of infancy (Dravet syndrome). The availability of genetic tests for SCN1A mutations is expanding awareness of the spectrum of diseases mediated by this gene and is beginning to permit genotype- phenotype correlations. Eventually, such information might enable clinicians to select an appropriate therapeutic regimen for patients with specific epilepsy gene mutations.

Sodium and potassium channel dysfunctions in rare and common idiopathic epilepsy syndromes

Mutations in the SCN1A gene are found in up to 80% of individuals with severe myoclonic epilepsy of infancy (SMEI), and mutations in KCNQ2 and KCNQ3 were identified in benign familial neonatal convulsions (BFNC) as well as in single families with Rolandic epilepsy (RE) and idiopathic generalized epilepsies (IGE). This paper summarizes recent findings concerning sodium (SCN1A) and potassium channel (KCNQ2 and KCNQ3) dysfunctions in the pathogenesis of rare and common idiopathic epilepsies (IE). SMEI, severe idiopathic generalized epilepsy of infancy (SIGEI), and myoclonic-astatic epilepsy (MAE) are rare IE. Because of some semeiologic overlap, a comparative analysis of the SCN1A gene performed in 20 patients with MAE and in 18 with SIGEI. This revealed mutations in three subjects with SIGEI only. Since BFNC are over-represented in families with RE, a mutational analysis was performed in 58 families with RE with and without BNFC. This revealed functionally relevant mutations in two index cases with BNFC, and three missense mutations (one resulting in a significantly reduced potassium current amplitude) in three patients with RE, but without BNFC. One KCNQ3 missense variant was also detected in eight out of 455 IGE patients but not in 454 controls, and a silent KCNQ2-SNP was found over-represented in both epilepsy samples. These findings confirm that mutations in the SCN1A gene are mainly involved in the pathogenesis of SMEI, rarely in that of SIGEI, and are commonly not found in patients with MAE. They also demonstrate that sequence variations of the KCNQ2 and KCNQ3 genes may contribute to the etiology of common IE syndromes.

Molecular genetics of migraine

Migraine is an episodic neurovascular disorder that is clinically divided into two main subtypes that are based on the absence or presence of an aura: migraine without aura (MO) and migraine with aura (MA). Current molecular genetic insight into the pathophysiology of migraine predominantly comes from studies of a rare monogenic subtype of migraine with aura called familial hemiplegic migraine (FHM). Three FHM genes have been identified, which all encode ion transporters, suggesting that disturbances in ion and neurotransmitter balances in the brain are responsible for this migraine type, and possibly the common forms of migraine. Cellular and animal models expressing FHM mutations hint toward neuronal hyperexcitability as the likely underlying disease mechanism. Additional molecular insight into the pathophysiology of migraine may come from other monogenic syndromes (for instance cerebral autosomal dominant arteriopathy with subcortical infarcts and leukoencephalopathy, which is caused by NOTCH3 mutations), in which migraine is prominent. Investigating patients with common forms of migraine has had limited successes. Except for 5',10'-methylenetetrahydrolate reductase, an enzyme in folate metabolism, the large majority of reported genetic associations with candidate migraine genes have not been convincingly replicated. Genetic linkage studies using migraine subtypes as an end diagnosis did not yield gene variants thus far. Clinical heterogeneity in migraine diagnosis may have hampered the identification of such variants. Therefore, the recent introduction of more refined methods of phenotyping, such as latent-class analysis and trait component analysis, may be certainly helpful. Combining the new phenotyping methods with genome-wide association studies may be a successful strategy toward identification of migraine susceptibility genes. Likely the identification of reliable biomarkers for migraine diagnosing will make these efforts even more successful.

A BAC transgenic mouse model reveals neuron subtype-specific effects of a Generalized Epilepsy with Febrile Seizures Plus (GEFS+) mutation

Mutations in the voltage-gated sodium channel SCN1A are responsible for a number of seizure disorders including Generalized Epilepsy with Febrile Seizures Plus (GEFS+) and Severe Myoclonic Epilepsy of Infancy (SMEI). To determine the effects of SCN1A mutations on channel function in vivo, we generated a bacterial artificial chromosome (BAC) transgenic mouse model that expresses the human SCN1A GEFS+ mutation, R1648H. Mice with the R1648H mutation exhibit a more severe response to the proconvulsant kainic acid compared with mice expressing a control Scn1a transgene. Electrophysiological analysis of dissociated neurons from mice with the R1648H mutation reveal delayed recovery from inactivation and increased use-dependent inactivation only in inhibitory bipolar neurons, as well as a hyperpolarizing shift in the voltage dependence of inactivation only in excitatory pyramidal neurons. These results demonstrate that the effects of SCN1A mutations are cell type-dependent and that the R1648H mutation specifically leads to a reduction in interneuron excitability.

Dravet syndrome or genetic (generalized) epilepsy with febrile seizures plus?

Dravet syndrome and genetic epilepsy with febrile seizures plus (GEFS+) can both arise due to mutations of SCN1A, the gene encoding the alpha 1 pore-forming subunit of the sodium channel. GEFS+ refers to a familial epilepsy syndrome where at least two family members have phenotypes that fit within the GEFS+ spectrum. The GEFS+ spectrum comprises a range of mild to severe phenotypes varying from classical febrile seizures to Dravet syndrome. Dravet syndrome is a severe infantile onset epilepsy syndrome with multiple seizure types, developmental slowing and poor outcome. More than 70% of patients with Dravet syndrome have mutations of SCN1A; these include both truncation and missense mutations. In contrast, only 10% of GEFS+ families have SCN1A mutations and these comprise missense mutations. GEFS+ has also been associated with mutations of genes encoding the sodium channel beta 1 subunit, SCN1B, and the GABA(A) receptor gamma 2 subunit, GABRG2. The phenotypic heterogeneity that is characteristic of GEFS+ families is likely to be due to modifier genes. Interpretation of the significance of a SCN1A missense mutation requires a thorough understanding of the phenotypes in the GEFS+ spectrum whereas a de novo truncation mutation is likely to be associated with a severe phenotype. Early recognition of Dravet syndrome is important as aggressive control of seizures may improve developmental outcome.

Molecular basis of severe myoclonic epilepsy in infancy

Severe myoclonic epilepsy (SMEI) or Dravet syndrome is caused by mutations of the SCN1A gene that encodes voltage-gated sodium channel alpha-1 subunit. Recently, we generated and characterized a knock-in (KI) mice with an SCN1A nonsense mutation that appeared in three independent SMEI patients. The SCN1A-KI mice well reproduced the SMEI disease phenotypes. Both homozygous and heterozygous knock-in mice developed epileptic seizures within the first postnatal month. In heterozygous knock-in mice, trains of evoked action potentials in inhibitory neurons exhibited pronounced spike amplitude decrement late in the burst but not in pyramidal neurons. We further showed that in wild-type mice the Nav1.1 protein is expressed dominantly in axons and moderately in somata of parbalbumin (PV) - positive inhibitory interneurons. Our immunohistochemical observations of the Nav1.1 are clearly distinct to the previous studies, and our findings has corrected the view of the Nav1.1 protein distribution. The data indicate that Nav1.1 plays critical roles in the spike output from PV interneurons and further, that the specifically altered function of these inhibitory circuits may contribute to epileptic seizures in the mice. These information should contribute to the understanding of molecular pathomechanism of SMEI and to develop its effective therapies.

The borderland of epilepsy: clinical and molecular features of phenomena that mimic epileptic seizures

Paroxysmal losses of consciousness and other episodic neurological symptoms have many causes. Distinguishing epileptic from non-epileptic disorders is fundamental to diagnosis, but even this basic dichotomy is often challenging and is certainly not new. In 1907, the British neurologist William Richard Gowers published his book The Border-land of Epilepsy in which he discussed paroxysmal conditions "in the border-land of epilepsy-near it, but not of it" and their clinical differentiation from epilepsy itself. Now, a century later, we revisit the epilepsy borderland, focusing on syncope, migraine, vertigo, parasomnias, and some rarer paroxysmal disorders. For each condition, we review the clinical distinction from epileptic seizures. We then integrate current understanding of the molecular pathophysiology of these disorders into this clinical framework. This analysis shows that, although the clinical manifestations of paroxysmal disorders are highly heterogeneous, striking similarities in molecular pathophysiology are seen among many epileptic and non-epileptic paroxysmal phenomena.

The functional organization and assembly of the axon initial segment

Action potential initiation, modulation, and duration in neurons depend on a variety of Na+ and K+ channels that are highly enriched at the axon initial segment (AIS). The AIS also has high densities of cell adhesion molecules (CAMs), modulatory proteins, and a unique extracellular matrix (ECM). In contrast to other functional domains of axons (e.g. the nodes of Ranvier and axon terminals) whose development depends on the interactions with different cells (e.g. myelinating glia and postsynaptic cells), the recruitment and retention of AIS proteins is intrinsically specified through axonal cytoskeletal and scaffolding proteins. We speculate that the AIS has previously unappreciated forms of plasticity that influence neuronal excitability, and that AIS plasticity is regulated by the developmental or activity-dependent modulation of scaffolding protein levels rather than directly altering ion channel expression.

Cell-type-dependent molecular composition of the axon initial segment

The exact site of initiation and shape of action potentials vary among different neuronal types. The reason for this variability is largely unknown, but the subunit composition, density and distribution of voltage-gated sodium (Nav) and potassium (Kv) channels within the axon initial segment (AIS) are likely to play a key role. Here, we asked how heterogeneous are the density and distribution of Nav and Kv channels within the AISs of a variety of excitatory and inhibitory neurons. Most of the studied cell types expressed a high density of Nav1.6, Kv1.1, and Kv1.2 subunits in their AIS, but the Nav1.1 subunit could only be detected in GABAergic interneurons. A proximo-distal gradient in the density of these subunits was observed within the AIS of certain nerve cells but not in others. For example, a gradual increase of the Nav1.6 subunit was observed in cortical layer 2/3 and hippocampal CA1 pyramidal cell (PC) AISs, whereas its density was rather uniform in layer 5 PC AISs. The Nav1.1 subunit was distributed evenly along the AIS of short-axon cells of the main olfactory bulb but was restricted to the proximal part of the AIS in cortical and cerebellar interneurons. Our results reveal a cell type-dependent expression of sodium and potassium channel subunits with varying densities along the proximo-distal axis of the AISs. This precise arrangement is likely to contribute to the diversity of firing properties observed among central neurons.

Variability in the subcellular distribution of ion channels increases neuronal diversity

The exact location of an ion channel on the axo-somato-dendritic surface of a nerve cell crucially affects its functional impact. Recent high-resolution immunolocalization experiments examining the distribution of GABA and glutamate receptors, voltage-gated potassium and sodium channels and hyperpolarization-activated mixed cation (HCN) channels clearly demonstrate the lack of simple rules concerning their subcellular distribution. For example, the density of HCN1 subunits in pyramidal cells increases 60-fold from soma to distal dendrites but is uniform over the somato-dendritic surface of olfactory bulb external tufted cells and is highest in the axon of cortical and cerebellar basket cells. Such findings highlight the necessity of determining the precise subcellular location and density of each ion channel in every cell type. Here, I suggest that variations in the subcellular distribution of ion channels are previously unrecognized means of increasing neuronal diversity and, thus, the computational power of the brain.

Does a SCN1A gene mutation confer earlier age of onset of febrile seizures in GEFS+?

SCN1A is the most clinically relevant epilepsy gene and is associated with generalized epilepsy and febrile seizure plus (GEFS+) and Dravet syndrome. We postulated that earlier onset of febrile seizures in the febrile seizure (FS) and febrile seizure plus (FS+) phenotypes may occur in the presence of a SCN1A mutation. This was because of the age-related onset of Dravet syndrome, which typically begins in the first year of life. We found that patients with FS and FS+ with SCN1A mutations had earlier median onset of febrile seizures compared to the population median. Patients with GABRG2 mutations had a similar early onset in contrast to patients with SCN1B mutations where onset was later. This study is the first to demonstrate that a specific genetic abnormality directly influences the FS and FS+ phenotype in terms of age of onset.

Topiramate and zonisamide prevent paradoxical intoxication induced by carbamazepine and phenytoin

The mechanisms of paradoxical aggravation of epileptic seizures induced by selected antiepileptic drugs (AEDs) remain unclear. The present study addressed this issue by determining the seizure-threshold doses of carbamazepine (CBZ) and phenytoin (PHT), as well the dose-dependent effects of CBZ, PHT, and carbonic anhydrase-inhibiting AEDs, acetazolamide (AZM), topiramate (TPM), and zonisamide (ZNS), on neurotransmitter release in rat hippocampus. The dose-dependent effects of AEDs on hippocampal extracellular levels of glutamate (Glu), GABA, norepinephrine (NE), dopamine (DA), and serotonin (5-HT) were determined by microdialysis with high-speed and high-sensitive extreme liquid chromatography. Proconvulsive effects of AEDs were determined by telemetric-electrocorticography. Therapeutically relevant doses of AZM, CBZ, TPM, and ZNS increased hippocampal extracellular levels of GABA, NE, DA, and 5-HT, while PHT had no effect. Supratherapeutic doses of AZM, CBZ, PHT, TPM, and ZNS decreased extracellular levels of GABA, NE, DA, and 5-HT, without affecting Glu levels. Toxic doses of CBZ and PHT produced seizures (paradoxical intoxication), markedly increasing all transmitter levels, but TPM and ZNS even at toxic doses did not produce seizure. Co-administration experiments showed that therapeutically relevant doses of CBZ or PHT reduced the seizure-threshold doses of PHT or CBZ, respectively. In contrast, therapeutically relevant doses of AZM, TPM, and ZNS elevated the seizure-threshold doses of CBZ and PHT. These results suggested that blockade of high percentage of the population of voltage-dependent sodium channels by CBZ and PHT might be important in inducing paradoxical intoxication/reaction, and that inhibition of carbonic anhydrase inhibits this effect. TPM and ZNS are candidate first-choice agents in treatment of epilepsy when first-line AEDs are ineffective.

Temperature- and age-dependent seizures in a mouse model of severe myoclonic epilepsy in infancy

Heterozygous loss-of-function mutations in the alpha subunit of the type I voltage-gated sodium channel Na(V)1.1 cause severe myoclonic epilepsy in infancy (SMEI), an infantile-onset epileptic encephalopathy characterized by normal development followed by treatment-refractory febrile and afebrile seizures and psychomotor decline. Mice with SMEI (mSMEI), created by heterozygous deletion of Na(V)1.1 channels, develop seizures and ataxia. Here we investigated the temperature and age dependence of seizures and interictal epileptiform spike-and-wave activity in mSMEI. Combined video-EEG monitoring demonstrated that mSMEI had seizures induced by elevated body core temperature but wild-type mice were unaffected. In the 3 age groups tested, no postnatal day (P)17-18 mSMEI had temperature-induced seizures, but nearly all P20-22 and P30-46 mSMEI had myoclonic seizures followed by generalized seizures caused by elevated core body temperature. Spontaneous seizures were only observed in mice older than P32, suggesting that mSMEI become susceptible to temperature-induced seizures before spontaneous seizures. Interictal spike activity was seen at normal body temperature in most P30-46 mSMEI but not in P20-22 or P17-18 mSMEI, indicating that interictal epileptic activity correlates with seizure susceptibility. Most P20-22 mSMEI had interictal spike activity with elevated body temperature. Our results define a critical developmental transition for susceptibility to seizures in SMEI, demonstrate that body temperature elevation alone is sufficient to induce seizures, and reveal a close correspondence between human and mouse SMEI in the striking temperature and age dependence of seizure frequency and severity and in the temperature dependence and frequency of interictal epileptiform spike activity.

Genetic influences on ketogenic diet efficacy

An average of 15-20% of patients on the ketogenic diet (KD) experience a >50% reduction in seizure frequency; however, 10-40% discontinue the diet due to either a lack of response or adverse side effects. This variability in patient response raises the possibility that genetic factors may influence the efficacy of the KD. As a first step towards identifying these factors, we evaluated the ability of the KD to alter seizure thresholds in four commonly used inbred mouse strains: C57BL/6J, FVB/NJ, A/J, and DBA/2J. We observed strain-specific differences, indicating that genetic factors are likely to influence efficacy.

Efhc1 deficiency causes spontaneous myoclonus and increased seizure susceptibility

Mutations in EFHC1 gene have been previously reported in patients with epilepsies, including those with juvenile myoclonic epilepsy. Myoclonin1, also known as mRib72-1, is encoded by the mouse Efhc1 gene. Myoclonin1 is dominantly expressed in embryonic choroid plexus, post-natal ependymal cilia, tracheal cilia and sperm flagella. In this study, we generated viable Efhc1-deficient mice. Most of the mice were normal in outward appearance, and both sexes were found to be fertile. However, the ventricles of the brains were significantly enlarged in the null mutants, but not in the heterozygotes. Although the ciliary structure was found intact, the ciliary beating frequency was significantly reduced in null mutants. In adult stages, both the heterozygous and null mutants developed frequent spontaneous myoclonus. Furthermore, the threshold of seizures induced by pentylenetetrazol was significantly reduced in both heterozygous and null mutants. These observations seem to further suggest that decrease or loss of function of myoclonin1 may be the molecular basis for epilepsies caused by EFHC1 mutations.

Migraine and epilepsy: genetically linked?

Most molecular genetic knowledge in migraine so far comes from the study of a rare subtype, familial hemiplegic migraine (FHM). The three known FHM genes (CACNA1A, ATP1A2 and SCN1A) are ion transporter genes. Mutations in all three FHM genes can also be associated with epilepsy. Of the many epilepsy genes that have been discovered, an association with migraine has been reported only for SCN1A. There is probably a lack of systematic studies of migraine in epilepsy families. A genetically determined dysfunction of ion transporters seems to point, at least to certain extent, at a common underlying mechanism for both paroxysmal disorders. The effect of ion channel mutations on neuronal neurotransmitter release is probably of major importance. In this article, we will discuss the arguments for a genetic relationship between migraine and epilepsy. A possible genetic link could give insight into the pathophysiology of both syndromes, and offer possibilities to develop specific preventive treatment aimed at the underlying ion transporter dysfunction and its consequences.

Ion channel clustering at the axon initial segment and node of Ranvier evolved sequentially in early chordates

In many mammalian neurons, dense clusters of ion channels at the axonal initial segment and nodes of Ranvier underlie action potential generation and rapid conduction. Axonal clustering of mammalian voltage-gated sodium and KCNQ (Kv7) potassium channels is based on linkage to the actin-spectrin cytoskeleton, which is mediated by the adaptor protein ankyrin-G. We identified key steps in the evolution of this axonal channel clustering. The anchor motif for sodium channel clustering evolved early in the chordate lineage before the divergence of the wormlike cephalochordate, amphioxus. Axons of the lamprey, a very primitive vertebrate, exhibited some invertebrate features (lack of myelin, use of giant diameter to hasten conduction), but possessed narrow initial segments bearing sodium channel clusters like in more recently evolved vertebrates. The KCNQ potassium channel anchor motif evolved after the divergence of lampreys from other vertebrates, in a common ancestor of shark and humans. Thus, clustering of voltage-gated sodium channels was a pivotal early innovation of the chordates. Sodium channel clusters at the axon initial segment serving the generation of action potentials evolved long before the node of Ranvier. KCNQ channels acquired anchors allowing their integration into pre-existing sodium channel complexes at about the same time that ancient vertebrates acquired myelin, saltatory conduction, and hinged jaws. The early chordate refinements in action potential mechanisms we have elucidated appear essential to the complex neural signaling, active behavior, and evolutionary success of vertebrates.

Inherited neuronal ion channelopathies: new windows on complex neurological diseases

Studies of genetic forms of epilepsy, chronic pain, and migraine caused by mutations in ion channels have given crucial insights into molecular mechanisms, pathogenesis, and therapeutic approaches to complex neurological disorders. Gain-of-function missense mutations in the brain type-I sodium channel Na(V)1.1 are a primary cause of generalized epilepsy with febrile seizures plus. Loss-of-function mutations in Na(V)1.1 channels cause severe myoclonic epilepsy of infancy, an intractable childhood epilepsy. Studies of a mouse model show that this disease is caused by selective loss of sodium current and excitability of GABAergic inhibitory interneurons, which leads to hyperexcitability, epilepsy, and ataxia. Mutations in the peripheral sodium channel Na(V)1.7 cause familial pain syndromes. Gain-of-function mutations cause erythromelalgia and paroxysmal extreme pain disorder as a result of hyperexcitability of sensory neurons, whereas loss-of-function mutations cause congenital indifference to pain because of attenuation of action potential firing. These experiments have defined correlations between genotype and phenotype in chronic pain diseases and focused attention on Na(V)1.7 as a therapeutic target. Familial hemiplegic migraine is caused by mutations in the calcium channel, Ca(V)2.1, which conducts P/Q-type calcium currents that initiate neurotransmitter release. These mutations increase activation at negative membrane potentials and increase evoked neurotransmitter release at cortical glutamatergic synapses. Studies of a mouse genetic model show that these gain-of-function effects lead to cortical spreading depression, aura, and potentially migraine. Overall, these experiments indicate that imbalance in the activity of excitatory and inhibitory neurons is an important underlying cause of these diseases.

AnkyrinG is required for maintenance of the axon initial segment and neuronal polarity

The axon initial segment (AIS) functions as both a physiological and physical bridge between somatodendritic and axonal domains. Given its unique molecular composition, location, and physiology, the AIS is thought to maintain neuronal polarity. To identify the molecular basis of this AIS property, we used adenovirus-mediated RNA interference to silence AIS protein expression in polarized neurons. Some AIS proteins are remarkably stable with half-lives of at least 2 wk. However, silencing the expression of the cytoskeletal scaffold ankyrinG (ankG) dismantles the AIS and causes axons to acquire the molecular characteristics of dendrites. Both cytoplasmic- and membrane-associated proteins, which are normally restricted to somatodendritic domains, redistribute into the former axon. Furthermore, spines and postsynaptic densities of excitatory synapses assemble on former axons. Our results demonstrate that the loss of ankG causes axons to acquire the molecular characteristics of dendrites; thus, ankG is required for the maintenance of neuronal polarity and molecular organization of the AIS.

Loss of inhibitory neuron AMPA receptors contributes to ataxia and epilepsy in stargazer mice

Stargazer mice are characterized by ataxia and seizures, which resemble the human disorder absence epilepsy. Stargazin, the protein mutated in stargazer mice, promotes the expression and function of neuronal AMPA receptors (AMPARs). However, it is unclear how decreased expression of excitatory AMPARs generates stargazer seizures, given that seizures often result from increased neuronal excitability. Additionally, although stargazer ataxia has been attributed to loss of AMPARs from cerebellar granule cells, other cerebellar neurons have not been examined. To examine the role of AMPAR dysfunction in these behavioral phenotypes, electrophysiological recordings were used to probe AMPAR regulation in relevant brain regions. We found that both cerebellar Purkinje cells and inhibitory thalamic reticular nucleus neurons have strongly reduced synaptic AMPAR function in stargazer mice. Together, our data suggest that impaired AMPAR regulation in multiple neuron populations may contribute to the behavioral phenotypes of absence seizures and ataxia seen in stargazer mice and imply that an understanding of human genetic disorders will require knowledge of both the genes that are mutated as well as their precise cellular expression pattern.

Role of Axonal NaV1.6 Sodium Channels in Action Potential Initiation of CA1 Pyramidal Neurons

In many neuron types, the axon initial segment (AIS) has the lowest threshold for action potential generation. Its active properties are determined by the targeted expression of specific voltage-gated channel subunits. We show that the Na+ channel NaV1.6 displays a striking aggregation at the AIS of cortical neurons. To assess the functional role of this subunit, we used Scn8a med mice that are deficient for NaV1.6 subunits but still display prominent Na+ channel aggregation at the AIS. In CA1 pyramidal cells from Scn8a med mice, we found a depolarizing shift in the voltage dependence of activation of the transient Na+ current ( INaT), indicating that NaV1.6 subunits activate at more negative voltages than other NaV subunits. Additionally, persistent and resurgent Na+ currents were significantly reduced. Current-clamp recordings revealed a significant elevation of spike threshold in Scn8a med mice as well as a shortening of the estimated delay between spike initiation at the AIS and its arrival at the soma. In combination with simulations using a realistic computer model of a CA1 pyramidal cell, our results imply that a hyperpolarized voltage dependence of activation of AIS NaV1.6 channels is important both in determining spike threshold and localizing spike initiation to the AIS. In addition to altered spike initiation, Scn8a med mice also showed a strongly reduced spike gain as expected with combined changes in persistent and resurgent currents and spike threshold. These results suggest that NaV1.6 subunits at the AIS contribute significantly to its role as spike trigger zone and shape repetitive discharge properties of CA1 neurons.