Mutation detection in the med and medJ alleles of the sodium channel Scn8a. Unusual splicing due to a minor class AT-AC intron

The contribution of NaV1.6 to the efficacy of voltage-gated sodium channel inhibitors in wild type and NaV1.6 gain-of-function (GOF) mouse seizure control

BACKGROUND AND PURPOSE

Inhibitors of voltage-gated sodium channels (NaVs) are important anti-epileptic drugs, but the contribution of specific channel isoforms is unknown since available inhibitors are non-selective. We aimed to create novel, isoform selective inhibitors of Nav channels as a means of informing the development of improved antiseizure drugs.

EXPERIMENTAL APPROACH

We created a series of compounds with diverse selectivity profiles enabling block of NaV1.6 alone or together with NaV1.2. These novel NaV inhibitors were evaluated for their ability to inhibit electrically evoked seizures in mice with a heterozygous gain-of-function mutation (N1768D/+) in Scn8a (encoding NaV1.6) and in wild-type mice.

KEY RESULTS

Pharmacologic inhibition of NaV1.6 in Scn8aN1768D/+ mice prevented seizures evoked by a 6-Hz shock. Inhibitors were also effective in a direct current maximal electroshock seizure assay in wild-type mice. NaV1.6 inhibition correlated with efficacy in both models, even without inhibition of other CNS NaV isoforms.

CONCLUSIONS AND IMPLICATIONS

Our data suggest NaV1.6 inhibition is a driver of efficacy for NaV inhibitor anti-seizure medicines. Sparing the NaV1.1 channels of inhibitory interneurons did not compromise efficacy. Selective NaV1.6 inhibitors may provide targeted therapies for human Scn8a developmental and epileptic encephalopathies and improved treatments for idiopathic epilepsies.

Attentional Deficits and Absence Epilepsy: A Tale of 2 Interneuronopathies

Prefrontal PV Interneurons Facilitate Attention and Are Linked to Attentional Dysfunction in a Mouse Model of Absence Epilepsy Ferguson B, Glick C, Huguenard JR. Elife . 2023;12:e78349. doi:10.7554/eLife.78349 Absence seizures are characterized by brief periods of unconsciousness accompanied by lapses in motor function that can occur hundreds of times throughout the day. Outside of these frequent moments of unconsciousness, approximately a third of people living with the disorder experience treatment-resistant attention impairments. Convergent evidence suggests prefrontal cortex (PFC) dysfunction may underlie attention impairments in affected patients. To examine this, we use a combination of slice physiology, fiber photometry, electrocorticography (ECoG), optogenetics, and behavior in the Scn8a+/-mouse model of absence epilepsy. Attention function was measured using a novel visual attention task where a light cue that varied in duration predicted the location of a food reward. In Scn8a+/-mice, we find altered parvalbumin interneuron (PVIN) output in the medial PFC (mPFC) in vitro and PVIN hypoactivity along with reductions in gamma power during cue presentation in vivo. This was associated with poorer attention performance in Scn8a+/-mice that could be rescued by gamma-frequency optogenetic stimulation of PVINs. This highlights cue-related PVIN activity as an important mechanism for attention and suggests PVINs may represent a therapeutic target for cognitive comorbidities in absence epilepsy.

A CMOS-based highly scalable flexible neural electrode interface

Perception, thoughts, and actions are encoded by the coordinated activity of large neuronal populations spread over large areas. However, existing electrophysiological devices are limited by their scalability in capturing this cortex-wide activity. Here, we developed an electrode connector based on an ultra-conformable thin-film electrode array that self-assembles onto silicon microelectrode arrays enabling multithousand channel counts at a millimeter scale. The interconnects are formed using microfabricated electrode pads suspended by thin support arms, termed Flex2Chip. Capillary-assisted assembly drives the pads to deform toward the chip surface, and van der Waals forces maintain this deformation, establishing Ohmic contact. Flex2Chip arrays successfully measured extracellular action potentials ex vivo and resolved micrometer scale seizure propagation trajectories in epileptic mice. We find that seizure dynamics in absence epilepsy in the Scn8a+/- model do not have constant propagation trajectories.

Targeting Ion Channels and Purkinje Neuron Intrinsic Membrane Excitability as a Therapeutic Strategy for Cerebellar Ataxia

In degenerative neurological disorders such as Parkinson's disease, a convergence of widely varying insults results in a loss of dopaminergic neurons and, thus, the motor symptoms of the disease. Dopamine replacement therapy with agents such as levodopa is a mainstay of therapy. Cerebellar ataxias, a heterogeneous group of currently untreatable conditions, have not been identified to have a shared physiology that is a target of therapy. In this review, we propose that perturbations in cerebellar Purkinje neuron intrinsic membrane excitability, a result of ion channel dysregulation, is a common pathophysiologic mechanism that drives motor impairment and vulnerability to degeneration in cerebellar ataxias of widely differing genetic etiologies. We further propose that treatments aimed at restoring Purkinje neuron intrinsic membrane excitability have the potential to be a shared therapy in cerebellar ataxia akin to levodopa for Parkinson's disease.

Prefrontal PV interneurons facilitate attention and are linked to attentional dysfunction in a mouse model of absence epilepsy

Absence seizures are characterized by brief periods of unconsciousness accompanied by lapses in motor function that can occur hundreds of times throughout the day. Outside of these frequent moments of unconsciousness, approximately a third of people living with the disorder experience treatment-resistant attention impairments. Convergent evidence suggests prefrontal cortex (PFC) dysfunction may underlie attention impairments in affected patients. To examine this, we use a combination of slice physiology, fiber photometry, electrocorticography (ECoG), optogenetics, and behavior in the Scn8a+/-mouse model of absence epilepsy. Attention function was measured using a novel visual attention task where a light cue that varied in duration predicted the location of a food reward. In Scn8a+/-mice, we find altered parvalbumin interneuron (PVIN) output in the medial PFC (mPFC) in vitro and PVIN hypoactivity along with reductions in gamma power during cue presentation in vivo. This was associated with poorer attention performance in Scn8a+/-mice that could be rescued by gamma-frequency optogenetic stimulation of PVINs. This highlights cue-related PVIN activity as an important mechanism for attention and suggests PVINs may represent a therapeutic target for cognitive comorbidities in absence epilepsy.

Sodium channelopathies of skeletal muscle and brain

Voltage-gated sodium channels initiate action potentials in nerve, skeletal muscle, and other electrically excitable cells. Mutations in them cause a wide range of diseases. These channelopathy mutations affect every aspect of sodium channel function, including voltage sensing, voltage-dependent activation, ion conductance, fast and slow inactivation, and both biosynthesis and assembly. Mutations that cause different forms of periodic paralysis in skeletal muscle were discovered first and have provided a template for understanding structure, function, and pathophysiology at the molecular level. More recent work has revealed multiple sodium channelopathies in the brain. Here we review the well-characterized genetics and pathophysiology of the periodic paralyses of skeletal muscle and then use this information as a foundation for advancing our understanding of mutations in the structurally homologous α-subunits of brain sodium channels that cause epilepsy, migraine, autism, and related comorbidities. We include studies based on molecular and structural biology, cell biology and physiology, pharmacology, and mouse genetics. Our review reveals unexpected connections among these different types of sodium channelopathies.

SCN8A encephalopathy: Mechanisms and models

De novo mutations of the neuronal sodium channel SCN8A have been identified in approximately 2% of individuals with epileptic encephalopathy. These missense mutations alter the biophysical properties of sodium channel Nav1.6 in ways that lead to neuronal hyperexcitability. We generated two mouse models carrying patient mutations N1768D and R1872W to examine the effects on neuronal function in vivo. The conditional R1872W mutation is activated by expression of CRE recombinase, permitting characterization of the effects of the mutation on different classes of neurons and at different points in postnatal development. Preclinical drug testing in these mouse models provides support for several new therapies for this devastating disorder. In contrast with the gain-of-function mutations in epilepsy, mutations of SCN8A that result in partial or complete loss of function are associated with intellectual disability and other disorders.

Mutations in the Scn8a DIIS4 voltage sensor reveal new distinctions among hypomorphic and null Nav 1.6 sodium channels

Mutations in the voltage-gated sodium channel gene SCN8A cause a broad range of human diseases, including epilepsy, intellectual disability, and ataxia. Here we describe three mouse lines on the C57BL/6J background with novel, overlapping mutations in the Scn8a DIIS4 voltage sensor: an in-frame 9 bp deletion (Δ9), an in-frame 3 bp insertion (∇3) and a 35 bp deletion that results in a frameshift and the generation of a null allele (Δ35). Scn8a ^Δ9/+ and Scn8a ^∇3/+ heterozygous mutants display subtle motor deficits, reduced acoustic startle response, and are resistant to induced seizures, suggesting that these mutations reduce activity of the Scn8a channel protein, Na_v 1.6. Heterozygous Scn8a ^Δ35/+ mutants show no alterations in motor function or acoustic startle response, but are resistant to induced seizures. Homozygous mutants from each line exhibit premature lethality and severe motor impairments, ranging from uncoordinated gait with tremor (Δ9 and ∇3) to loss of hindlimb control (Δ35). Scn8a ^Δ9/Δ9 and Scn8a ^∇3/∇3 homozygous mutants also exhibit impaired nerve conduction velocity, while normal nerve conduction was observed in Scn8a ^Δ35/Δ35 homozygous mice. Our results suggest that hypomorphic mutations that reduce Na_v 1.6 activity will likely result in different clinical phenotypes compared to null alleles. These three mouse lines represent a valuable opportunity to examine the phenotypic impacts of hypomorphic and null Scn8a mutations without the confound of strain-specific differences.

Mouse germ line mutations due to retrotransposon insertions

Transposable element (TE) insertions are responsible for a significant fraction of spontaneous germ line mutations reported in inbred mouse strains. This major contribution of TEs to the mutational landscape in mouse contrasts with the situation in human, where their relative contribution as germ line insertional mutagens is much lower. In this focussed review, we provide comprehensive lists of TE-induced mouse mutations, discuss the different TE types involved in these insertional mutations and elaborate on particularly interesting cases. We also discuss differences and similarities between the mutational role of TEs in mice and humans.

Retrotransposon-induced mosaicism in the neural genome

Over the past decade, major discoveries in retrotransposon biology have depicted the neural genome as a dynamic structure during life. In particular, the retrotransposon LINE-1 (L1) has been shown to be transcribed and mobilized in the brain. Retrotransposition in the developing brain, as well as during adult neurogenesis, provides a milieu in which neural diversity can arise. Dysregulation of retrotransposon activity may also contribute to neurological disease. Here, we review recent reports of retrotransposon activity in the brain, and discuss the temporal nature of retrotransposition and its regulation in neural cells in response to stimuli. We also put forward hypotheses regarding the significance of retrotransposons for brain development and neurological function, and consider the potential implications of this phenomenon for neuropsychiatric and neurodegenerative conditions.

Normal audiogram but poor sensitivity to brief sounds in mice with compromised voltage-gated sodium channels (Scn8amedJ)

The Scn8a^medJ mutation of the gene for sodium channels at the nodes of Ranvier slows nerve conduction, resulting in motor abnormalities. This mutation is also associated with loss of spontaneous bursting activity in the dorsal cochlear nucleus. However initial tests of auditory sensitivity in mice homozygous for this mutation, using standard 400-ms tones, demonstrated normal hearing sensitivity. Further testing, reported here, revealed a severely compromised sensitivity to short-duration tones of 10 and 2 ms durations. Such a deficit might be expected to interfere with auditory functions that depend on rapid processing of auditory signals.

The anti-parkinsonian drug zonisamide reduces neuroinflammation: Role of microglial Nav 1.6

Parkinson's disease (PD), the second most common age-related progressive neurodegenerative disorder, is characterized by dopamine depletion and the loss of dopaminergic (DA) neurons with accompanying neuroinflammation. Zonisamide is an-anti-convulsant drug that has recently been shown to improve clinical symptoms of PD through its inhibition of monoamine oxidase B (MAO-B). However, zonisamide has additional targets, including voltage-gated sodium channels (Na_v), which may contribute to its reported neuroprotective role in preclinical models of PD. Here, we report that Na_v1.6 is highly expressed in microglia of post-mortem PD brain and of mice treated with the parkinsonism-inducing neurotoxin MPTP. Administration of zonisamide (20 mg/kg, i.p. every 4 h × 3) following a single injection of MPTP (12.5 mg/kg, s.c.) reduced microglial Na_v 1.6 and microglial activation in the striatum, as indicated by Iba-1 staining and mRNA expression of F4/80. MPTP increased the levels of the pro-inflammatory cytokine TNF-α and gp91^phox, and this was significantly reduced by zonisamide. Together, these findings suggest that zonisamide may reduce neuroinflammation through the down-regulation of microglial Na_v 1.6. Thus, in addition to its effects on parkinsonian symptoms through inhibition of MAO-B, zonisamide may have disease modifying potential through the inhibition of Na_v 1.6 and neuroinflammation.

L1 retrotransposition is a common feature of mammalian hepatocarcinogenesis

The retrotransposon Long Interspersed Element 1 (LINE-1 or L1) is a continuing source of germline and somatic mutagenesis in mammals. Deregulated L1 activity is a hallmark of cancer, and L1 mutagenesis has been described in numerous human malignancies. We previously employed retrotransposon capture sequencing (RC-seq) to analyze hepatocellular carcinoma (HCC) samples from patients infected with hepatitis B or hepatitis C virus and identified L1 variants responsible for activating oncogenic pathways. Here, we have applied RC-seq and whole-genome sequencing (WGS) to an Abcb4 (Mdr2)-/- mouse model of hepatic carcinogenesis and demonstrated for the first time that L1 mobilization occurs in murine tumors. In 12 HCC nodules obtained from 10 animals, we validated four somatic L1 insertions by PCR and capillary sequencing, including TF subfamily elements, and one GF subfamily example. One of the TF insertions carried a 3' transduction, allowing us to identify its donor L1 and to demonstrate that this full-length TF element retained retrotransposition capacity in cultured cancer cells. Using RC-seq, we also identified eight tumor-specific L1 insertions from 25 HCC patients with a history of alcohol abuse. Finally, we used RC-seq and WGS to identify three tumor-specific L1 insertions among 10 intra-hepatic cholangiocarcinoma (ICC) patients, including one insertion traced to a donor L1 on Chromosome 22 known to be highly active in other cancers. This study reveals L1 mobilization as a common feature of hepatocarcinogenesis in mammals, demonstrating that the phenomenon is not restricted to human viral HCC etiologies and is encountered in murine liver tumors.

Purkinje Cell Signaling Deficits in Animal Models of Ataxia

Purkinje cell (PC) dysfunction or degeneration is the most frequent finding in animal models with ataxic symptoms. Mutations affecting intrinsic membrane properties can lead to ataxia by altering the firing rate of PCs or their firing pattern. However, the relationship between specific firing alterations and motor symptoms is not yet clear, and in some cases PC dysfunction precedes the onset of ataxic signs. Moreover, a great variety of ionic and synaptic mechanisms can affect PC signaling, resulting in different features of motor dysfunction. Mutations affecting Na⁺ channels (Na_V1.1, Na_V1.6, Na_Vβ4, Fgf14 or Rer1) reduce the firing rate of PCs, mainly via an impairment of the Na⁺ resurgent current. Mutations that reduce Kv3 currents limit the firing rate frequency range. Mutations of Kv1 channels act mainly on inhibitory interneurons, generating excessive GABAergic signaling onto PCs, resulting in episodic ataxia. Kv4.3 mutations are responsible for a complex syndrome with several neurologic dysfunctions including ataxia. Mutations of either Cav or BK channels have similar consequences, consisting in a disruption of the firing pattern of PCs, with loss of precision, leading to ataxia. Another category of pathogenic mechanisms of ataxia regards alterations of synaptic signals arriving at the PC. At the parallel fiber (PF)-PC synapse, mutations of glutamate delta-2 (GluD2) or its ligand Crbl1 are responsible for the loss of synaptic contacts, abolishment of long-term depression (LTD) and motor deficits. At the same synapse, a correct function of metabotropic glutamate receptor 1 (mGlu1) receptors is necessary to avoid ataxia. Failure of climbing fiber (CF) maturation and establishment of PC mono-innervation occurs in a great number of mutant mice, including mGlu1 and its transduction pathway, GluD2, semaphorins and their receptors. All these models have in common the alteration of PC output signals, due to a variety of mechanisms affecting incoming synaptic signals or the way they are processed by the repertoire of ionic channels responsible for intrinsic membrane properties. Although the PC is a final common pathway of ataxia, the link between specific firing alterations and neurologic symptoms has not yet been systematically studied and the alterations of the cerebellar contribution to motor signals are still unknown.

Minor spliceosome and disease

The U12-dependent (minor) spliceosome excises a rare group of introns that are characterized by a highly conserved 5' splice site and branch point sequence. Several new congenital or somatic diseases have recently been associated with mutations in components of the minor spliceosome. A common theme in these diseases is the detection of elevated levels of transcripts containing U12-type introns, of which a subset is associated with other splicing defects. Here we review the present understanding of minor spliceosome diseases, particularly those associated with the specific components of the minor spliceosome. We also present a model for interpreting the molecular-level consequences of the different diseases.

Heritable L1 retrotransposition in the mouse primordial germline and early embryo

LINE-1 (L1) retrotransposons are a noted source of genetic diversity and disease in mammals. To expand its genomic footprint, L1 must mobilize in cells that will contribute their genetic material to subsequent generations. Heritable L1 insertions may therefore arise in germ cells and in pluripotent embryonic cells, prior to germline specification, yet the frequency and predominant developmental timing of such events remain unclear. Here, we applied mouse retrotransposon capture sequencing (mRC-seq) and whole-genome sequencing (WGS) to pedigrees of C57BL/6J animals, and uncovered an L1 insertion rate of ≥1 event per eight births. We traced heritable L1 insertions to pluripotent embryonic cells and, strikingly, to early primordial germ cells (PGCs). New L1 insertions bore structural hallmarks of target-site primed reverse transcription (TPRT) and mobilized efficiently in a cultured cell retrotransposition assay. Together, our results highlight the rate and evolutionary impact of heritable L1 retrotransposition and reveal retrotransposition-mediated genomic diversification as a fundamental property of pluripotent embryonic cells in vivo.

Regulation of Thalamic and Cortical Network Synchrony by Scn8a

Voltage-gated sodium channel (VGSC) mutations cause severe epilepsies marked by intermittent, pathological hypersynchronous brain states. Here we present two mechanisms that help to explain how mutations in one VGSC gene, Scn8a, contribute to two distinct seizure phenotypes: (1) hypoexcitation of cortical circuits leading to convulsive seizure resistance, and (2) hyperexcitation of thalamocortical circuits leading to non-convulsive absence epilepsy. We found that loss of Scn8a leads to altered RT cell intrinsic excitability and a failure in recurrent RT synaptic inhibition. We propose that these deficits cooperate to enhance thalamocortical network synchrony and generate pathological oscillations. To our knowledge, this finding is the first clear demonstration of a pathological state tied to disruption of the RT-RT synapse. Our observation that loss of a single gene in the thalamus of an adult wild-type animal is sufficient to cause spike-wave discharges is striking and represents an example of absence epilepsy of thalamic origin.

Neuronal hyperexcitability in a mouse model of SCN8A epileptic encephalopathy

Patients with early infantile epileptic encephalopathy (EIEE) experience severe seizures and cognitive impairment and are at increased risk for sudden unexpected death in epilepsy (SUDEP). EIEE13 [Online Mendelian Inheritance in Man (OMIM) # 614558] is caused by de novo missense mutations in the voltage-gated sodium channel gene SCN8A Here, we investigated the neuronal phenotype of a mouse model expressing the gain-of-function SCN8A patient mutation, p.Asn1768Asp (Na_v1.6-N1768D). Our results revealed regional and neuronal subtype specificity in the effects of the N1768D mutation. Acutely dissociated hippocampal neurons from Scn8a^N1768D/+ mice showed increases in persistent sodium current (I_Na) density in CA1 pyramidal but not bipolar neurons. In CA3, I_Na,P was increased in both bipolar and pyramidal neurons. Measurement of action potential (AP) firing in Scn8a^N1768D/+ pyramidal neurons in brain slices revealed early afterdepolarization (EAD)-like AP waveforms in CA1 but not in CA3 hippocampal or layer II/III neocortical neurons. The maximum spike frequency evoked by depolarizing current injections in Scn8a^N1768D/+ CA1, but not CA3 or neocortical, pyramidal cells was significantly reduced compared with WT. Spontaneous firing was observed in subsets of neurons in CA1 and CA3, but not in the neocortex. The EAD-like waveforms of Scn8a^N1768D/+ CA1 hippocampal neurons were blocked by tetrodotoxin, riluzole, and SN-6, implicating elevated persistent I_Na and reverse mode Na/Ca exchange in the mechanism of hyperexcitability. Our results demonstrate that Scn8a plays a vital role in neuronal excitability and provide insight into the mechanism and future treatment of epileptogenesis in EIEE13.

The neurogenetics of alternative splicing

Alternative precursor-mRNA splicing is a key mechanism for regulating gene expression in mammals and is controlled by specialized RNA-binding proteins. The misregulation of splicing is implicated in multiple neurological disorders. We describe recent mouse genetic studies of alternative splicing that reveal its critical role in both neuronal development and the function of mature neurons. We discuss the challenges in understanding the extensive genetic programmes controlled by proteins that regulate splicing, both during development and in the adult brain.

SCN8A encephalopathy: Research progress and prospects

On April 21, 2015, the first SCN8A Encephalopathy Research Group convened in Washington, DC, to assess current research into clinical and pathogenic features of the disorder and prepare an agenda for future research collaborations. The group comprised clinical and basic scientists and representatives of patient advocacy groups. SCN8A encephalopathy is a rare disorder caused by de novo missense mutations of the sodium channel gene SCN8A, which encodes the neuronal sodium channel Nav 1.6. Since the initial description in 2012, approximately 140 affected individuals have been reported in publications or by SCN8A family groups. As a result, an understanding of the severe impact of SCN8A mutations is beginning to emerge. Defining a genetic epilepsy syndrome goes beyond identification of molecular etiology. Topics discussed at this meeting included (1) comparison between mutations of SCN8A and the SCN1A mutations in Dravet syndrome, (2) biophysical properties of the Nav 1.6 channel, (3) electrophysiologic effects of patient mutations on channel properties, (4) cell and animal models of SCN8A encephalopathy, (5) drug screening strategies, (6) the phenotypic spectrum of SCN8A encephalopathy, and (7) efforts to develop a bioregistry. A panel discussion of gaps in bioregistry, biobanking, and clinical outcomes data was followed by a planning session for improved integration of clinical and basic science research. Although SCN8A encephalopathy was identified only recently, there has been rapid progress in functional analysis and phenotypic classification. The focus is now shifting from identification of the underlying molecular cause to the development of strategies for drug screening and prioritized patient care.

Minor intron splicing is regulated by FUS and affected by ALS-associated FUS mutants

Fused in sarcoma (FUS) is a ubiquitously expressed RNA-binding protein proposed to function in various RNA metabolic pathways, including transcription regulation, pre-mRNA splicing, RNA transport and microRNA processing. Mutations in the FUS gene were identified in patients with amyotrophic lateral sclerosis (ALS), but the pathomechanisms by which these mutations cause ALS are not known. Here, we show that FUS interacts with the minor spliceosome constituent U11 snRNP, binds preferentially to minor introns and directly regulates their removal. Furthermore, a FUS knockout in neuroblastoma cells strongly disturbs the splicing of minor intron-containing mRNAs, among them mRNAs required for action potential transmission and for functional spinal motor units. Moreover, an ALS-associated FUS mutant that forms cytoplasmic aggregates inhibits splicing of minor introns by trapping U11 and U12 snRNAs in these aggregates. Collectively, our findings suggest a possible pathomechanism for ALS in which mutated FUS inhibits correct splicing of minor introns in mRNAs encoding proteins required for motor neuron survival.

Single amino acid deletion in transmembrane segment D4S6 of sodium channel Scn8a (Nav1.6) in a mouse mutant with a chronic movement disorder

Mutations of the neuronal sodium channel gene SCN8A are associated with lethal movement disorders in the mouse and with human epileptic encephalopathy. We describe a spontaneous mouse mutation, Scn8a(9J), that is associated with a chronic movement disorder with early onset tremor and adult onset dystonia. Scn8a(9J) homozygotes have a shortened lifespan, with only 50% of mutants surviving beyond 6 months of age. The 3 bp in-frame deletion removes 1 of the 3 adjacent isoleucine residues in transmembrane segment DIVS6 of Nav1.6 (p.Ile1750del). The altered helical orientation of the transmembrane segment displaces pore-lining amino acids with important roles in channel activation and inactivation. The predicted impact on channel activity was confirmed by analysis of cerebellar Purkinje neurons from mutant mice, which lack spontaneous and induced repetitive firing. In a heterologous expression system, the activity of the mutant channel was below the threshold for detection. Observations of decreased nerve conduction velocity and impaired behavior in an open field are also consistent with reduced activity of Nav1.6. The Nav1.6Δ1750 protein is only partially glycosylated. The abundance of mutant Nav1.6 is reduced at nodes of Ranvier and is not detectable at the axon initial segment. Despite a severe reduction in channel activity, the lifespan and motor function of Scn8a(9J/9J) mice are significantly better than null mutants lacking channel protein. The clinical phenotype of this severe hypomorphic mutant expands the spectrum of Scn8a disease to include a recessively inherited, chronic and progressive movement disorder.

The Influence of LINE-1 and SINE Retrotransposons on Mammalian Genomes

Transposable elements have had a profound impact on the structure and function of mammalian genomes. The retrotransposon Long INterspersed Element-1 (LINE-1 or L1), by virtue of its replicative mobilization mechanism, comprises ∼17% of the human genome. Although the vast majority of human LINE-1 sequences are inactive molecular fossils, an estimated 80-100 copies per individual retain the ability to mobilize by a process termed retrotransposition. Indeed, LINE-1 is the only active, autonomous retrotransposon in humans and its retrotransposition continues to generate both intra-individual and inter-individual genetic diversity. Here, we briefly review the types of transposable elements that reside in mammalian genomes. We will focus our discussion on LINE-1 retrotransposons and the non-autonomous Short INterspersed Elements (SINEs) that rely on the proteins encoded by LINE-1 for their mobilization. We review cases where LINE-1-mediated retrotransposition events have resulted in genetic disease and discuss how the characterization of these mutagenic insertions led to the identification of retrotransposition-competent LINE-1s in the human and mouse genomes. We then discuss how the integration of molecular genetic, biochemical, and modern genomic technologies have yielded insight into the mechanism of LINE-1 retrotransposition, the impact of LINE-1-mediated retrotransposition events on mammalian genomes, and the host cellular mechanisms that protect the genome from unabated LINE-1-mediated retrotransposition events. Throughout this review, we highlight unanswered questions in LINE-1 biology that provide exciting opportunities for future research. Clearly, much has been learned about LINE-1 and SINE biology since the publication of Mobile DNA II thirteen years ago. Future studies should continue to yield exciting discoveries about how these retrotransposons contribute to genetic diversity in mammalian genomes.

The importance of de novo mutations for pediatric neurological disease--It is not all in utero or birth trauma

The advent of next generation sequencing (NGS, which consists of massively parallel sequencing to perform TGS (total genome sequencing) or WES (whole exome sequencing)) has abundantly discovered many causative mutations in patients with pediatric neurological disease. A surprisingly high number of these are de novo mutations which have not been inherited from either parent. For epilepsy, autism spectrum disorders, and neuromotor disorders, including cerebral palsy, initial estimates put the frequency of causative de novo mutations at about 15% and about 10% of these are somatic. There are some shared mutated genes between these three classes of disease. Studies of copy number variation by comparative genomic hybridization (CGH) proceded the NGS approaches but they also detect de novo variation which is especially important for ASDs. There are interesting differences between the mutated genes detected by CGS and NGS. In summary, de novo mutations cause a very significant proportion of pediatric neurological disease.

Reduced Nav1.6 Sodium Channel Activity in Mice Increases In Vivo Sensitivity to Volatile Anesthetics

Na_v1.6 is a major voltage-gated sodium channel in the central and peripheral nervous systems. Within neurons, the channel protein is concentrated at the axon initial segment and nodes of Ranvier, where it functions in initiation and propagation of action potentials. We examined the role of Na_v1.6 in general anesthesia using two mouse mutants with reduced activity of Na_v1.6, Scn8a^medJ/medJ and Scn8a^9J/9J. The mice were exposed to the general anesthetics isoflurane and sevoflurane in step-wise increments; the concentration required to produce loss of righting reflex, a surrogate for anesthetic-induced unconsciousness in rodents, was determined. Mice homozygous for these mutations exhibited increased sensitivity to both isoflurane and sevoflurane. The increased sensitivity was observed during induction of unconsciousness but not during the recovery phase, suggesting that the effect is not attributable to compromised systemic physiology. Electroencephalographic theta power during baseline waking was lower in mutants, suggesting decreased arousal and reduced neuronal excitability. This is the first report linking reduced activity of a specific voltage-gated sodium channel to increased sensitivity to general anesthetics in vivo.

Selective blocking effects of 4,9-anhydrotetrodotoxin, purified from a crude mixture of tetrodotoxin analogues, on NaV1.6 channels and its chemical aspects

Tetrodotoxin (TTX) is a potent neurotoxin found in a number of marine creatures including the pufferfish, where it is synthesized by bacteria and accumulated through the food chain. It is a potent and selective blocker of some types of voltage-gated Na⁺ channel (Na_V channel). 4,9-Anhydrotetrodotoxin (4,9-anhydroTTX) was purified from a crude mixture of TTX analogues (such as TTX, 4-epiTTX, 6-epiTTX, 11-oxoTTX and 11-deoxyTTX) by the use of liquid chromatography-fluorescence detection (LC-FLD) techniques. Recently, it has been reported that 4,9-anhydroTTX selectively blocks the activity of Na_V1.6 channels with a blocking efficacy 40–160 times higher than that for other TTX-sensitive Na_V1.x channel isoforms. However, little attention has been paid to the molecular properties of the α-subunit in Na_V1.6 channels and the characteristics of binding of 4,9-anhydroTTX. From a functional point of view, it is important to determine the relative expression of Na_V1.6 channels in a wide variety of tissues. The aim of this review is to discuss briefly current knowledge about the pharmacology of 4,9-anhydroTTX, and provide an analysis of the molecular structure of native Na_V1.6 channels. In addition, chemical aspects of 4,9-anhydroTTX are briefly covered.

Convulsive seizures and SUDEP in a mouse model of SCN8A epileptic encephalopathy

De novo mutations of the voltage-gated sodium channel gene SCN8A have recently been recognized as a cause of epileptic encephalopathy, which is characterized by refractory seizures with developmental delay and cognitive disability. We previously described the heterozygous SCN8A missense mutation p.Asn1768Asp in a child with epileptic encephalopathy that included seizures, ataxia, and sudden unexpected death in epilepsy (SUDEP). The mutation results in increased persistent sodium current and hyperactivity of transfected neurons. We have characterized a knock-in mouse model expressing this dominant gain-of-function mutation to investigate the pathology of the altered channel in vivo. The mutant channel protein is stable in vivo. Heterozygous Scn8a^N1768D/+ mice exhibit seizures and SUDEP, confirming the causality of the de novo mutation in the proband. Using video/EEG analysis, we detect ictal discharges that coincide with convulsive seizures and myoclonic jerks. Prior to seizure onset, heterozygous mutants are not defective in motor learning or fear conditioning, but do exhibit mild impairment of motor coordination and social discrimination. Homozygous mutant mice exhibit earlier seizure onset than heterozygotes and more rapid progression to death. Analysis of the intermediate phenotype of functionally hemizygous Scn8a^N1768D/− mice indicates that severity is increased by a double dose of mutant protein and reduced by the presence of wild-type protein. Scn8a^N1768D mutant mice provide a model of epileptic encephalopathy that will be valuable for studying the in vivo effects of hyperactive Na_v1.6 and the response to therapeutic interventions.

Role of the hippocampus in Nav1.6 (Scn8a) mediated seizure resistance

SCN1A mutations are the main cause of the epilepsy disorders Dravet syndrome (DS) and genetic epilepsy with febrile seizures plus (GEFS+). Mutations that reduce the activity of the mouse Scn8a gene, in contrast, are found to confer seizure resistance and extend the lifespan of mouse models of DS and GEFS+. To investigate the mechanism by which reduced Scn8a expression confers seizure resistance, we induced interictal-like burst discharges in hippocampal slices of heterozygous Scn8a null mice (Scn8a(med/+)) with elevated extracellular potassium. Scn8a(med/+) mutants exhibited reduced epileptiform burst discharge activity after P20, indicating an age-dependent increased threshold for induction of epileptiform discharges. Scn8a deficiency also reduced the occurrence of burst discharges in a GEFS+ mouse model (Scn1a(R1648H/+)). There was no detectable change in the expression levels of Scn1a (Nav1.1) or Scn2a (Nav1.2) in the hippocampus of adult Scn8a(med/+) mutants. To determine whether the increased seizure resistance associated with reduced Scn8a expression was due to alterations that occurred during development, we examined the effect of deleting Scn8a in adult mice. Global Cre-mediated deletion of a heterozygous floxed Scn8a allele in adult mice was found to increase thresholds to chemically and electrically induced seizures. Finally, knockdown of Scn8a gene expression in the adult hippocampus via lentiviral Cre injection resulted in a reduction in the number of EEG-confirmed seizures following the administration of picrotoxin. Our results identify the hippocampus as an important structure in the mediation of Scn8a-dependent seizure protection and suggest that selective targeting of Scn8a activity might be efficacious in patients with epilepsy.

Physiological and genetic analysis of multiple sodium channel variants in a model of genetic absence epilepsy

In excitatory neurons, SCN2A (NaV1.2) and SCN8A (NaV1.6) sodium channels are enriched at the axon initial segment. NaV1.6 is implicated in several mouse models of absence epilepsy, including a missense mutation identified in a chemical mutagenesis screen (Scn8a(V929F)). Here, we confirmed the prior suggestion that Scn8a(V929F) exhibits a striking genetic background-dependent difference in phenotypic severity, observing that spike-wave discharge (SWD) incidence and severity are significantly diminished when Scn8a(V929F) is fully placed onto the C57BL/6J strain compared with C3H. Examination of sequence differences in NaV subunits between these two inbred strains suggested NaV1.2(V752F) as a potential source of this modifier effect. Recognising that the spatial co-localisation of the NaV channels at the axon initial segment (AIS) provides a plausible mechanism for functional interaction, we tested this idea by undertaking biophysical characterisation of the variant NaV channels and by computer modelling. NaV1.2(V752F) functional analysis revealed an overall gain-of-function and for NaV1.6(V929F) revealed an overall loss-of-function. A biophysically realistic computer model was used to test the idea that interaction between these variant channels at the AIS contributes to the strain background effect. Surprisingly this modelling showed that neuronal excitability is dominated by the properties of NaV1.2(V752F) due to "functional silencing" of NaV1.6(V929F) suggesting that these variants do not directly interact. Consequent genetic mapping of the major strain modifier to Chr 7, and not Chr 2 where Scn2a maps, supported this biophysical prediction. While a NaV1.6(V929F) loss of function clearly underlies absence seizures in this mouse model, the strain background effect is apparently not due to an otherwise tempting Scn2a variant, highlighting the value of combining physiology and genetics to inform and direct each other when interrogating genetic complex traits such as absence epilepsy.

Sodium channel SCN8A (Nav1.6): properties and de novo mutations in epileptic encephalopathy and intellectual disability

The sodium channel Nav1.6, encoded by the gene SCN8A, is one of the major voltage-gated channels in human brain. The sequences of sodium channels have been highly conserved during evolution, and minor changes in biophysical properties can have a major impact in vivo. Insight into the role of Nav1.6 has come from analysis of spontaneous and induced mutations of mouse Scn8a during the past 18 years. Only within the past year has the role of SCN8A in human disease become apparent from whole exome and genome sequences of patients with sporadic disease. Unique features of Nav1.6 include its contribution to persistent current, resurgent current, repetitive neuronal firing, and subcellular localization at the axon initial segment (AIS) and nodes of Ranvier. Loss of Nav1.6 activity results in reduced neuronal excitability, while gain-of-function mutations can increase neuronal excitability. Mouse Scn8a (med) mutants exhibit movement disorders including ataxia, tremor and dystonia. Thus far, more than ten human de novo mutations have been identified in patients with two types of disorders, epileptic encephalopathy and intellectual disability. We review these human mutations as well as the unique features of Nav1.6 that contribute to its role in determining neuronal excitability in vivo. A supplemental figure illustrating the positions of amino acid residues within the four domains and 24 transmembrane segments of Nav1.6 is provided to facilitate the location of novel mutations within the channel protein.

FGF14 localization and organization of the axon initial segment

The axon initial segment (AIS) is highly enriched in the structural proteins ankyrin G and βIV-spectrin, the pore-forming (α) subunits of voltage-gated sodium (Nav) channels, and functional Nav channels, and is critical for the initiation of action potentials. We previously reported that FGF14, a member of the intracellular FGF (iFGF) sub-family, is expressed in cerebellar Purkinje neurons and that the targeted inactivation of Fgf14 in mice (Fgf14(-/-)) results in markedly reduced Purkinje neuron excitability. Here, we demonstrate that FGF14 immunoreactivity is high in the AIS of Purkinje neurons and is distributed in a decreasing, proximal to distal, gradient. This pattern is evident early in the postnatal development of Purkinje neurons and is also observed in many other types of central neurons. In (Scn8a(med)) mice, which are deficient in expression of the Nav1.6 α subunit, FGF14 immunoreactivity is markedly increased and expanded in the Purkinje neuron AIS, in parallel with increased expression of the Nav1.1 (Scn1a) α subunit and expanded expression of βIV-spectrin. Although Nav1.1, FGF14, and βIV-spectrin are affected, ankyrin G immunoreactivity at the AIS of Scn8a(med) and wild type (WT) Purkinje neurons was not significantly different. In Fgf14(-/-) Purkinje neurons, βIV-spectrin and ankyrin G immunoreactivity at the AIS were also similar to WT Purkinje neurons, although both the Nav1.1 and Nav1.6 α subunits are modestly, but significantly (p<0.005), reduced within sub-domains of the AIS, changes that may contribute to the reduced excitability of Fgf14(-/-) Purkinje neurons.

CELF expression in epilepsy linked to sodium channels

Evaluation of: Sun W, Wagnon JL, Mahaffey CL et al. Aberrant sodium channel activity in the complex seizure disorder of Celf4 mutant mice. J. Physiol. 591(1), 241–255 (2013). The choreographed expression of ion channels is critical for normal brain activity, as evidenced by the range of epilepsy syndromes associated with ion channel genetic variation. As large-scale sequencing efforts, such as Epi4K and EuroEPINOMICS, systematically deconstruct epilepsy genomes, it is likely that further progress will be made in our understanding of how ion channel dysfunction results in epilepsy. By contrast, our knowledge of non-ion channel genes in epilepsy is far less advanced, a problem frequently compounded by the lack of understanding of the basic neurobiology of these genes. Sun et al. address this shortcoming by providing an elegant account of how a key RNA metabolism gene, Celf4, gives rise to seizures by the regulation of axonal ion channels. This has implications for epilepsy genomics, where a priori knowledge of the gene networks that participate in seizure genesis is critical for bioinformatics filtering used to identify diagnostic and prognostic variants. The convergence on an ion channel target affirms their central role in seizure genesis but, more importantly, raises the idea that drugs that target key regulators such as CELF4 could be effective in epilepsy.

Resurgent-like currents in mouse vas deferens myocytes are mediated by NaV1.6 voltage-gated sodium channels

Patch-clamp experiments were performed to investigate the molecular properties of resurgent-like currents in single smooth muscle cells dispersed from mouse vas deferens, utilizing both Na(V)1.6-null mice (Na(V)1.6(-/-)), lacking the expression of the Scn8a Na(+) channel gene, and their wild-type littermates (Na(V)1.6(+/+)). Na(V)1.6 immunoreactivity was clearly visible in dispersed smooth muscle cells obtained from Na(V)1.6(+/+), but not Na(V)1.6(-/-), vas deferens. Following a depolarization to +30 mV from a holding potential of -70 mV (to produce maximal inactivation of the Na(+) current), repolarization to voltages between -60 and +20 mV elicited a tetrodotoxin (TTX)-sensitive inward current in Na(V)1.6(+/+), but not Na(V)1.6(-/-), vas deferens myocytes. The resurgent-like current in Na(V)1.6(+/+) vas deferens myocytes peaked at approximately -20 mV in the current-voltage relationship. The peak amplitude of the resurgent-like current remained at a constant level when the membrane potential was repolarized to -20 mV following the application of depolarizing rectangular pulses to more positive potentials than +20 mV. 4,9-Anhydrotetrodotoxin (4,9-anhydroTTX), a selective Na(V)1.6 blocking toxin, purified from a crude mixture of TTX analogues by LC-FLD techniques, reversibly suppressed the resurgent-like currents. β-Pompilidotoxin, a voltage-gated Na(+) channel activator, evoked sustained resurgent-like currents in Na(V)1.6(+/+) but not Na(V)1.6(-/-) murine vas deferens myocytes. These results strongly indicate that, primarily, resurgent-like currents are generated as a result of Na(V)1.6 channel activity.

Anticancer drug oxaliplatin induces acute cooling-aggravated neuropathy via sodium channel subtype Na(V)1.6-resurgent and persistent current

Infusion of the chemotherapeutic agent oxaliplatin leads to an acute and a chronic form of peripheral neuropathy. Acute oxaliplatin neuropathy is characterized by sensory paresthesias and muscle cramps that are notably exacerbated by cooling. Painful dysesthesias are rarely reported for acute oxaliplatin neuropathy, whereas a common symptom of chronic oxaliplatin neuropathy is pain. Here we examine the role of the sodium channel isoform Na(V)1.6 in mediating the symptoms of acute oxaliplatin neuropathy. Compound and single-action potential recordings from human and mouse peripheral axons showed that cooling in the presence of oxaliplatin (30-100 μM; 90 min) induced bursts of action potentials in myelinated A, but not unmyelinated C-fibers. Whole-cell patch-clamp recordings from dissociated dorsal root ganglion (DRG) neurons revealed enhanced tetrodotoxin-sensitive resurgent and persistent current amplitudes in large, but not small, diameter DRG neurons when cooled (22 °C) in the presence of oxaliplatin. In DRG neurons and peripheral myelinated axons from Scn8a(med/med) mice, which lack functional Na(V)1.6, no effect of oxaliplatin and cooling was observed. Oxaliplatin significantly slows the rate of fast inactivation at negative potentials in heterologously expressed mNa(V)1.6r in ND7 cells, an effect consistent with prolonged Na(V) open times and increased resurgent and persistent current in native DRG neurons. This finding suggests that Na(V)1.6 plays a central role in mediating acute cooling-exacerbated symptoms following oxaliplatin, and that enhanced resurgent and persistent sodium currents may provide a general mechanistic basis for cold-aggravated symptoms of neuropathy.

Sodium channels and microglial function

Microglia are resident immune cells that provide continuous surveillance within the central nervous system (CNS) and respond to perturbations of brain and spinal cord parenchyma with an array of effector functions, including proliferation, migration, phagocytosis, secretions of multiple cytokines/chemokines and promotion of repair. To sense alterations within their environment, microglia express a large number of cell surface receptors, ion channels and adhesion molecules, which activate complex and dynamic signaling pathways. In the present chapter, we review studies that demonstrate that microglia in vivo and in vitro express specific voltage-gated sodium channel isoforms, and that blockade of sodium channel activity can attenuate several effector functions of microglia. These studies also provide strong evidence that Nav1.6 is the predominant sodium channel isoform expressed in microglia and that its activity contributes to the response of microglia to multiple activating signals.

Genetic modifiers of neurological disease

Genetic modifiers make an important contribution to neurological disease phenotypes. Significant progress has been made by studying genetic modifiers in model organisms. The ability to study complex genetic interactions in model systems contributes to our understanding of the genetic factors that influence neurological disease. This will lead to the development of novel therapeutic strategies and personalized treatment based on genetic risk.

Persistent Nav1.6 current at axon initial segments tunes spike timing of cerebellar granule cells

Cerebellar granule (CG) cells generate high-frequency action potentials that have been proposed to depend on the unique properties of their voltage-gated ion channels. To address the in vivo function of Nav1.6 channels in developing and mature CG cells, we combined the study of the developmental expression of Nav subunits with recording of acute cerebellar slices from young and adult granule-specific Scn8a KO mice. Nav1.2 accumulated rapidly at early-formed axon initial segments (AISs). In contrast, Nav1.6 was absent at early postnatal stages but accumulated at AISs of CG cells from P21 to P40. By P40-P65, both Nav1.6 and Nav1.2 co-localized at CG cell AISs. By comparing Na(+) currents in mature CG cells (P66-P74) from wild-type and CG-specific Scn8a KO mice, we found that transient and resurgent Na(+) currents were not modified in the absence of Nav1.6 whereas persistent Na(+) current was strongly reduced. Action potentials in conditional Scn8a KO CG cells showed no alteration in threshold and overshoot, but had a faster repolarization phase and larger post-spike hyperpolarization. In addition, although Scn8a KO CG cells kept their ability to fire action potentials at very high frequency, they displayed increased interspike-interval variability and firing irregularity in response to sustained depolarization. We conclude that Nav1.6 channels at axon initial segments contribute to persistent Na(+) current and ensure a high degree of temporal precision in repetitive firing of CG cells.

Characterization of NaV1.6-mediated Na+ currents in smooth muscle cells isolated from mouse vas deferens

Patch-clamp experiments were performed to investigate the behavior of voltage-activated inward currents in vas deferens myocytes from Na(V)1.6-null mice (Na(V)1.6(-/-)) lacking the expression of the Na(+) channel gene, Scn8a, and their wild-type littermates (Na(V)1.6(+/+)). Immunohistochemistry confirmed expression of Na(V)1.6 in the muscle of Na(V)1.6(+/+), but not Na(V)1.6(-/-), vas deferens. PCR analysis revealed that the only beta(1)-subunit gene expressed in Na(V)1.6(+/+) vas deferens was Scn1b. In Na(V)1.6(+/+) myocytes, the threshold for membrane currents evoked by 20 msec voltage ramps (-100 mV to 60 mV) was -38.5 +/- 4.6 mV and this was shifted to a more positive potential (-31.2 +/- 4.9 mV) by tetrodotoxin (TTX). In Na(V)1.6(-/-) myocytes, the threshold was -30.4 +/- 3.4 mV and there was no TTX-sensitive current. The Na(+) current (I(Na)) in Na(V)1.6(+/+) myocytes had a bell-shaped current-voltage relationship that peaked at approximately -10 mV. Increasing the duration of the voltage ramps beyond 20 msec reduced the peak amplitude of I(Na). I(Na) displayed both fast (tau approximately 10 msec) and slow (tau approximately 1 sec) recovery from inactivation, the magnitude of the slow component increasing with the duration of the conditioning pulse (5-40 msec). During repetitive activation (5-40 msec pulses), I(Na) declined at stimulation frequencies > 0.5 Hz and at 10 Hz <or= 50% of the current remained. These findings indicate that I(Na) is due solely to Na(V)1.6 in Na(V)1.6(+/+) myocytes. The gating properties of these channels suggest they play a major role in regulating smooth muscle excitability, particularly in response to rapid depolarizing stimuli.

Animal models of human cerebellar ataxias: a cornerstone for the therapies of the twenty-first century

Cerebellar ataxias represent a group of disabling neurological disorders. Our understanding of the pathogenesis of cerebellar ataxias is continuously expanding. A considerable number of laboratory animals with neurological mutations have been reported and numerous relevant animal models mimicking the phenotype of cerebellar ataxias are becoming available. These models greatly help dissecting the numerous mechanisms of cerebellar dysfunction, a major step for the assessment of therapeutics targeting a given deleterious pathway and for the screening of old or newly synthesized chemical compounds. Nevertheless, differences between animal models and human disorders should not be overlooked and difficulties in terms of characterization should not be occulted. The identification of the mutations of many hereditary ataxias, the development of valuable animal models, and the recent identifications of the molecular mechanisms underlying cerebellar disorders represent a combination of key factors for the development of anti-ataxic innovative therapies. It is anticipated that the twenty-first century will be the century of effective therapies in the field of cerebellar ataxias. The animal models are a cornerstone to reach this goal.

Actions of veratridine on tetrodotoxin-sensitive voltage-gated Na currents, Na1.6, in murine vas deferens myocytes

BACKGROUND AND PURPOSE

The effects of veratridine, an alkaloid found in Liliaceae plants, on tetrodotoxin (TTX)-sensitive voltage-gated Na(+) channels were investigated in mouse vas deferens.

EXPERIMENTAL APPROACH

Effects of veratridine on TTX-sensitive Na(+) currents (I(Na)) in vas deferens myocytes dispersed from BALB/c mice, homozygous mice with a null allele of Na(V)1.6 (Na(V)1.6(-/-)) and wild-type mice (Na(V)1.6(+/+)) were studied using patch-clamp techniques. Tension measurements were also performed to compare the effects of veratridine on phasic contractions in intact tissues.

KEY RESULTS

In whole-cell configuration, veratridine had a concentration-dependent dual action on the peak amplitude of I(Na): I(Na) was enhanced by veratridine (1-10 microM), while higher concentrations (> or =30 microM) inhibited I(Na). Additionally, two membrane current components were evoked by veratridine, namely a sustained inward current during the duration of the depolarizing rectangular pulse and a tail current at the repolarization. Although veratridine caused little shift of the voltage dependence of the steady-state inactivation curve and the activation curve for I(Na), veratridine enhanced a non-inactivating component of I(Na). Veratridine caused no detectable contractions in vas deferens from Na(V)1.6(-/-) mice, although in tissues from Na(V)1.6(+/+) mice, veratridine (> or =3 microM) induced TTX-sensitive contractions. Similarly, no detectable inward currents were evoked by veratridine in Na(V)1.6(-/-) vas deferens myocytes, while veratridine elicited both the sustained and tail currents in cells taken from Na(V)1.6(+/+) mice.

CONCLUSIONS AND IMPLICATIONS

These results suggest that veratridine possesses a dual action on I(Na) and that the veratridine-induced activation of contraction is induced by the activation of Na(V)1.6 channels.

Cerebellum-related characteristics of Scn8a-mutant mice

Among ten sodium channel alpha-subunit genes mapped in human and mouse genomes, the SCN8A gene is primarily expressed in neurons and glia. Mice with two types of Scn8a null mutations--Scn8a ( med ) and Scn8a ( medTg )--live for only 21-24 days, but those with incomplete mutations-Scn8a ( medJ ) and Scn8a ( medJo )--and those with knockout of Scn8a only in cerebellar Purkinje cells live to adult age. We review here previous work on cerebellum and related regions of Scn8a mutant mice and include some newer immunohistochemical and microchemical results. The resurgent sodium current that underlies the repeated firing of Purkinje cells is reduced in Scn8a mutant and knockout mice. Purkinje cells of mutant mice have greatly reduced spontaneous activity, as do the analogous cartwheel cells of the dorsal cochlear nucleus. Up-regulation of GABA(A) receptors in regions to which Purkinje cells project may partially compensate for their decreased activity in the mutant mice. The somata of cerebellar Purkinje cells of Scn8a ( medJ ) and Scn8a ( medJo ) mice, as revealed by PEP-19 immunoreaction, are slightly smaller than normal, and their axons, especially in Scn8a ( medJo ) mice, sometimes show enlargements similar to those in other types of mutant mice. Density of GABA-like immunoreactivity is decreased in Purkinje somata and regions of termination in deep cerebellar and vestibular nuclei of Scn8a ( medJ ) mice, but measured GABA concentration is not significantly reduced in microdissected samples of these regions. The concentrations of taurine and glutamine are significantly increased in cerebellar-related regions of Scn8a ( medJ ) mice, possibly suggesting up-regulation of glial amino acid metabolism.

Actions of kurtoxin on tetrodotoxin-sensitive voltage-gated Na+ currents, NaV1.6, in murine vas deferens myocytes

Kurtoxin is described as a selective inhibitor of Ca(V)3.1. Using patch-clamp techniques, the modulatory effects of kurtoxin on tetrodotoxin-sensitive voltage-gated Na(+) currents (I(Na)) recorded from mouse vas deferens myocytes were investigated. Kurtoxin increased the peak amplitude of I(Na) between -40 and -30 mV, whilst inhibited the peak amplitude at more positive potentials than -10 mV, thereby demonstrating a dual action on the peak amplitude of I(Na). The time to reach the peak amplitude of I(Na) became significantly longer in the presence of kurtoxin in comparison with that of the controls. Kurtoxin also slowed the deactivation of I(Na) at more positive membrane potentials than -30 mV. Kurtoxin enhanced the total amount of electrical charge of I(Na) in a concentration-dependent manner. These results suggest that kurtoxin is a modulator of Na(V)1.6 in native freshly dispersed smooth muscle cells from mouse vas deferens as well as its action on Ca(V)3.1.

Heterozygous mutations of the voltage-gated sodium channel SCN8A are associated with spike-wave discharges and absence epilepsy in mice

In a chemical mutagenesis screen, we identified the novel Scn8a^8J allele of the gene encoding the neuronal voltage-gated sodium channel Na_v1.6. The missense mutation V929F in this allele alters an evolutionarily conserved residue in the pore loop of domain 2 of Na_v1.6. Electroencephalography (EEG) revealed well-defined spike-wave discharges (SWD), the hallmark of absence epilepsy, in Scn8a^8J heterozygotes and in heterozygotes for two classical Scn8a alleles, Scn8a^med (null) and Scn8a^med-jo (missense). Mouse strain background had a significant effect on SWD, with mutants on the C3HeB/FeJ strain showing a higher incidence than on C57BL/6J. The abnormal EEG patterns in heterozygous mutant mice and the influence of genetic background on SWD make SCN8A an attractive candidate gene for common human absence epilepsy, a genetically complex disorder.

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.

A targeted deleterious allele of the splicing factor SCNM1 in the mouse

The auxiliary spliceosomal protein SCNM1 contributes to recognition of nonconsensus splice donor sites. SCNM1 was first identified as a modifier of the severity of a sodium channelopathy in the mouse. The most severely affected strain, C57BL/6J, carries the variant allele SCNM1R187X, which is defective in splicing the mutated donor site in the Scn8a(medJ) transcript. To further probe the in vivo function of SCNM1, we constructed a floxed allele and generated a mouse with constitutive deletion of exons 3-5. The SCNM1Delta3-5 protein is produced and correctly localized to the nucleus, but is more functionally impaired than the C57BL/6J allele. Deficiency of SCNM1 did not significantly alter other brain transcripts. We characterized an ENU-induced allele of Scnm1 and evaluated the ability of wild-type SCNM1 to rescue lethal mutations of I-mfa and Brunol4. The phenotypes of the Scnm1Delta3-5 mutant confirm the role of this splice factor in processing the Scn8a(medJ) transcript and provide a new allele of greater severity for future studies.

Molecular and biophysical properties of voltage-gated Na+ channels in murine vas deferens

The biological and molecular properties of tetrodotoxin (TTX)-sensitive voltage-gated Na(+) currents (I(Na)) in murine vas deferens myocytes were investigated using patch-clamp techniques and molecular biological analyses. In whole-cell configuration, a fast, transient inward current was evoked in the presence of Cd(2+), and was abolished by TTX (K(d) = 11.2 nM), mibefradil (K(d) = 3.3 microM), and external replacement of Na(+) with monovalent cations (TEA(+), Tris(+), and NMDG(+)). The fast transient inward current was enhanced by veratridine, an activator of voltage-gated Na(+) channels, suggesting that the fast transient inward current was a TTX-sensitive I(Na). The values for half-maximal (V(half)) inactivation and activation of I(Na) were -46.3 mV and -26.0 mV, respectively. RT-PCR analysis revealed the expression of Scn1a, 2a, and 8a transcripts. The Scn8a transcript and the alpha-subunit protein of Na(V)1.6 were detected in smooth muscle layers. Using Na(V)1.6-null mice (Na(V)1.6(-/-)) lacking the expression of the Na(+) channel gene, Scn8a, I(Na) were not detected in dispersed smooth muscle cells from the vas deferens, while TTX-sensitive I(Na) were recorded in their wild-type (Na(V)1.6(+/+)) littermates. This study demonstrates that the molecular identity of the voltage-gated Na(+) channels responsible for the TTX-sensitive I(Na) in murine vas deferens myocytes is primarily Na(V)1.6.