Sodium channels and neurological disease: insights from Scn8a mutations in the mouse

Differential encoding of mammalian proprioception by voltage-gated sodium channels

Animals requiring purposeful movement for survival are endowed with mechanoreceptors, called proprioceptors, that provide essential sensory feedback from muscles and joints to spinal cord circuits, which modulates motor output. Despite the essential nature of proprioceptive signaling in daily life, the mechanisms governing proprioceptor activity are poorly understood. Here, we identified nonredundant roles for two voltage-gated sodium channels (NaVs), NaV1.1 and NaV1.6, in mammalian proprioception. Deletion of NaV1.6 in somatosensory neurons (NaV1.6cKO mice) causes severe motor deficits accompanied by loss of proprioceptive transmission, which contrasts with our previous findings using similar mouse models to target NaV1.1 (NaV1.1cKO). In NaV1.6cKO animals, we observed impairments in proprioceptor end-organ structure and a marked reduction in skeletal muscle myofiber size that were absent in NaV1.1cKO mice. We attribute the differential contributions of NaV1.1 and NaV1.6 to distinct cellular localization patterns. Collectively, we provide evidence that NaVs uniquely shape neural signaling within a somatosensory modality.

Allele-Specific Editing of a Dominant SCN8A Epilepsy Variant Protects against Seizures and Lethality in a Murine Model

OBJECTIVE

Developmental and epileptic encephalopathies (DEEs) can result from dominant, gain of function variants of neuronal ion channels. More than 450 de novo missense variants of the sodium channel gene SCN8A have been identified in individuals with DEE.

METHODS

We studied a mouse model carrying the patient Scn8a variant p.Asn1768Asp. An AAV-PHP.eB virus carrying an allele-specific single guide RNA (sgRNA) was administered by intracerebroventricular injection. Cas9 was provided by an inherited transgene.

RESULTS

Allele-specific disruption of the reading frame of the pathogenic transcript generated out-of-frame indels in 1/4 to 1/3 of transcripts throughout the brain. This editing efficiency was sufficient to rescue lethality and seizures. Neuronal hyperexcitability was reduced in cells expressing the virus.

INTERPRETATION

The data demonstrate efficient allele-specific editing of a dominant missense variant and support the feasibility of allele-specific therapy for DEE epilepsy. ANN NEUROL 2024;96:958-969.

Differential encoding of mammalian proprioception by voltage-gated sodium channels

Animals that require purposeful movement for survival are endowed with mechanosensory neurons called proprioceptors that provide essential sensory feedback from muscles and joints to spinal cord circuits, which modulates motor output. Despite the essential nature of proprioceptive signaling in daily life, the mechanisms governing proprioceptor activity are poorly understood. Here, we have identified distinct and nonredundant roles for two voltage-gated sodium channels (NaVs), NaV1.1 and NaV1.6, in mammalian proprioception. Deletion of NaV1.6 in somatosensory neurons (NaV1.6cKO mice) causes severe motor deficits accompanied by complete loss of proprioceptive transmission, which contrasts with our previous findings using similar mouse models to target NaV1.1 (NaV1.1cKO). In NaV1.6cKO animals, loss of proprioceptive feedback caused non-cell-autonomous impairments in proprioceptor end-organs and skeletal muscle that were absent in NaV1.1cKO mice. We attribute the differential contribution of NaV1.1 and NaV1.6 in proprioceptor function to distinct cellular localization patterns. Collectively, these data provide the first evidence that NaV subtypes uniquely shape neurotransmission within a somatosensory modality.

spatialHeatmap: visualizing spatial bulk and single-cell assays in anatomical images

Visualizing spatial assay data in anatomical images is vital for understanding biological processes in cell, tissue, and organ organizations. Technologies requiring this functionality include traditional one-at-a-time assays, and bulk and single-cell omics experiments, including RNA-seq and proteomics. The spatialHeatmap software provides a series of powerful new methods for these needs, and allows users to work with adequately formatted anatomical images from public collections or custom images. It colors the spatial features (e.g. tissues) annotated in the images according to the measured or predicted abundance levels of biomolecules (e.g. mRNAs) using a color key. This core functionality of the package is called a spatial heatmap plot. Single-cell data can be co-visualized in composite plots that combine spatial heatmaps with embedding plots of high-dimensional data. The resulting spatial context information is essential for gaining insights into the tissue-level organization of single-cell data, or vice versa. Additional core functionalities include the automated identification of biomolecules with spatially selective abundance patterns and clusters of biomolecules sharing similar abundance profiles. To appeal to both non-expert and computational users, spatialHeatmap provides a graphical and a command-line interface, respectively. It is distributed as a free, open-source Bioconductor package (https://bioconductor.org/packages/spatialHeatmap) that users can install on personal computers, shared servers, or cloud systems.

Role of NaV1.6-mediated persistent sodium current and bursting-pacemaker properties in breathing rhythm generation

Inspiration is the inexorable active phase of breathing. The brainstem pre-Bötzinger complex (preBötC) gives rise to inspiratory neural rhythm, but its underlying cellular and ionic bases remain unclear. The long-standing "pacemaker hypothesis" posits that the persistent Na+ current (INaP) that gives rise to bursting-pacemaker properties in preBötC interneurons is essential for rhythmogenesis. We tested the pacemaker hypothesis by conditionally knocking out and knocking down the Scn8a (Nav1.6 [voltage-gated sodium channel 1.6]) gene in core rhythmogenic preBötC neurons. Deleting Scn8a substantially decreases the INaP and abolishes bursting-pacemaker activity, which slows inspiratory rhythm in vitro and negatively impacts the postnatal development of ventilation. Diminishing Scn8a via genetic interference has no impact on breathing in adult mice. We argue that the Scn8a-mediated INaP is not obligatory but that it influences the development and rhythmic function of the preBötC. The ubiquity of the INaP in respiratory brainstem interneurons could underlie breathing-related behaviors such as neonatal phonation or rhythmogenesis in different physiological conditions.

Autonomic dysfunction in epilepsy mouse models with implications for SUDEP research

Epilepsy has a high prevalence and can severely impair quality of life and increase the risk of premature death. Sudden unexpected death in epilepsy (SUDEP) is the leading cause of death in drug-resistant epilepsy and most often results from respiratory and cardiac impairments due to brainstem dysfunction. Epileptic activity can spread widely, influencing neuronal activity in regions outside the epileptic network. The brainstem controls cardiorespiratory activity and arousal and reciprocally connects to cortical, diencephalic, and spinal cord areas. Epileptic activity can propagate trans-synaptically or via spreading depression (SD) to alter brainstem functions and cause cardiorespiratory dysfunction. The mechanisms by which seizures propagate to or otherwise impair brainstem function and trigger the cascading effects that cause SUDEP are poorly understood. We review insights from mouse models combined with new techniques to understand the pathophysiology of epilepsy and SUDEP. These techniques include in vivo, ex vivo, invasive and non-invasive methods in anesthetized and awake mice. Optogenetics combined with electrophysiological and optical manipulation and recording methods offer unique opportunities to study neuronal mechanisms under normal conditions, during and after non-fatal seizures, and in SUDEP. These combined approaches can advance our understanding of brainstem pathophysiology associated with seizures and SUDEP and may suggest strategies to prevent SUDEP.

Targeting Circuit Abnormalities in Neurodegenerative Disease

Despite significant improvement in our ability to diagnose both common and rare neurodegenerative diseases and understand their underlying biologic mechanisms, there remains a disproportionate lack of effective treatments, reflecting the complexity of these disorders. Successfully advancing novel treatments for neurodegenerative disorders will require reconsideration of traditional approaches, which to date have focused largely on specific disease proteins or cells of origin. This article proposes reframing these diseases as conditions of dysfunctional circuitry as a complement to ongoing efforts. Specifically reviewed is how aberrant spiking is a common downstream mechanism in numerous neurodegenerative diseases, often driven by dysfunction in specific ion channels. Surgical modification of this electrical activity via deep brain stimulation is already an approved modality for many of these disorders. Therefore, restoring proper electrical activity by targeting these channels pharmacologically represents a viable strategy for intervention, not only for symptomatic management but also as a potential disease-modifying therapy. Such an approach is likely to be a promising route to treating these devastating disorders, either as monotherapy or in conjunction with current drugs. SIGNIFICANCE STATEMENT: Despite extensive research and improved understanding of the biology driving neurodegenerative disease, there has not been a concomitant increase in approved therapies. Accordingly, it is time to shift our perspective and recognize these diseases also as disorders of circuitry to further yield novel drug targets and new interventions. An approach focused on treating dysfunctional circuitry has the potential to reduce or reverse patient symptoms and potentially modify disease course.

Structural Pharmacology of Voltage-Gated Sodium Channels

Voltage-gated sodium (Na_V) channels initiate and propagate action potentials in excitable tissues to mediate key physiological processes including heart contraction and nervous system function. Accordingly, Na_V channels are major targets for drugs, toxins and disease-causing mutations. Recent breakthroughs in cryo-electron microscopy have led to the visualization of human Na_V1.1, Na_V1.2, Na_V1.4, Na_V1.5 and Na_V1.7 channel subtypes at high-resolution. These landmark studies have greatly advanced our structural understanding of channel architecture, ion selectivity, voltage-sensing, electromechanical coupling, fast inactivation, and the molecular basis underlying Na_V channelopathies. Na_V channel structures have also been increasingly determined in complex with toxin and small molecule modulators that target either the pore module or voltage sensor domains. These structural studies have provided new insights into the mechanisms of pharmacological action and opportunities for subtype-selective Na_V channel drug design. This review will highlight the structural pharmacology of human Na_V channels as well as the potential use of engineered and chimeric channels in future drug discovery efforts.

Sodium channelopathies in neurodevelopmental disorders

The voltage-gated sodium channel α-subunit genes comprise a highly conserved gene family. Mutations of three of these genes, SCN1A, SCN2A and SCN8A, are responsible for a significant burden of neurological disease. Recent progress in identification and functional characterization of patient variants is generating new insights and novel approaches to therapy for these devastating disorders. Here we review the basic elements of sodium channel function that are used to characterize patient variants. We summarize a large body of work using global and conditional mouse mutants to characterize the in vivo roles of these channels. We provide an overview of the neurological disorders associated with mutations of the human genes and examples of the effects of patient mutations on channel function. Finally, we highlight therapeutic interventions that are emerging from new insights into mechanisms of sodium channelopathies.

Trafficking mechanisms underlying Nav channel subcellular localization in neurons

Voltage gated sodium channels (Nav) play a crucial role in action potential initiation and propagation. Although the discovery of Nav channels dates back more than 65 years, and great advances in understanding their localization, biophysical properties, and links to disease have been made, there are still many questions to be answered regarding the cellular and molecular mechanisms involved in Nav channel trafficking, localization and regulation. This review summarizes the different trafficking mechanisms underlying the polarized Nav channel localization in neurons, with an emphasis on the axon initial segment (AIS), as well as discussing the latest advances regarding how neurons regulate their excitability by modifying AIS length and location. The importance of Nav channel localization is emphasized by the relationship between mutations, impaired trafficking and disease. While this review focuses on Nav1.6, other Nav isoforms are also discussed.

Mapping of the FGF14:Nav1.6 complex interface reveals FLPK as a functionally active peptide modulating excitability

The voltage-gated sodium (Nav) channel complex is comprised of pore-forming α subunits (Nav1.1-1.9) and accessory regulatory proteins such as the intracellular fibroblast growth factor 14 (FGF14). The cytosolic Nav1.6 C-terminal tail binds directly to FGF14 and this interaction modifies Nav1.6-mediated currents with effects on intrinsic excitability in the brain. Previous studies have identified the FGF14V160 residue within the FGF14 core domain as a hotspot for the FGF14:Nav1.6 complex formation. Here, we used three short amino acid peptides around FGF14V160 to probe for the FGF14 interaction with the Nav1.6 C-terminal tail and to evaluate the activity of the peptide on Nav1.6-mediated currents. In silico docking predicts FLPK to bind to FGF14V160 with the expectation of interfering with the FGF14:Nav1.6 complex formation, a phenotype that was confirmed by the split-luciferase assay (LCA) and surface plasmon resonance (SPR), respectively. Whole-cell patch-clamp electrophysiology studies demonstrate that FLPK is able to prevent previously reported FGF14-dependent phenotypes of Nav1.6 currents, but that its activity requires the FGF14 N-terminal tail, a domain that has been shown to contribute to Nav1.6 inactivation independently from the FGF14 core domain. In medium spiny neurons in the nucleus accumbens, where both FGF14 and Nav1.6 are abundantly expressed, FLPK significantly increased firing frequency by a mechanism consistent with the ability of the tetrapeptide to interfere with Nav1.6 inactivation and potentiate persistent Na+ currents. Taken together, these results indicate that FLPK might serve as a probe for characterizing molecular determinants of neuronal excitability and a peptide scaffold to develop allosteric modulators of Nav channels.

Nav1.6 promotes inflammation and neuronal degeneration in a mouse model of multiple sclerosis

BACKGROUND

In multiple sclerosis (MS) and in the experimental autoimmune encephalomyelitis (EAE) model of MS, the Nav1.6 voltage-gated sodium (Nav) channel isoform has been implicated as a primary contributor to axonal degeneration. Following demyelination Nav1.6, which is normally co-localized with the Na⁺/Ca²⁺ exchanger (NCX) at the nodes of Ranvier, associates with β-APP, a marker of neural injury. The persistent influx of sodium through Nav1.6 is believed to reverse the function of NCX, resulting in an increased influx of damaging Ca²⁺ ions. However, direct evidence for the role of Nav1.6 in axonal degeneration is lacking.

METHODS

In mice floxed for Scn8a, the gene that encodes the α subunit of Nav1.6, subjected to EAE we examined the effect of eliminating Nav1.6 from retinal ganglion cells (RGC) in one eye using an AAV vector harboring Cre and GFP, while using the contralateral either injected with AAV vector harboring GFP alone or non-targeted eye as control.

RESULTS

In retinas, the expression of Rbpms, a marker for retinal ganglion cells, was found to be inversely correlated to the expression of Scn8a. Furthermore, the gene expression of the pro-inflammatory cytokines Il6 (IL-6) and Ifng (IFN-γ), and of the reactive gliosis marker Gfap (GFAP) were found to be reduced in targeted retinas. Optic nerves from targeted eyes were shown to have reduced macrophage infiltration and improved axonal health.

CONCLUSION

Taken together, our results are consistent with Nav1.6 promoting inflammation and contributing to axonal degeneration following demyelination.

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.

Discovery of a Novel Nav1.7 Inhibitor From Cyriopagopus albostriatus Venom With Potent Analgesic Efficacy

Spider venoms contain a vast array of bioactive peptides targeting ion channels. A large number of peptides have high potency and selectivity toward sodium channels. Na_v1.7 contributes to action potential generation and propagation and participates in pain signaling pathway. In this study, we describe the identification of μ-TRTX-Ca2a (Ca2a), a novel 35-residue peptide from the venom of Vietnam spider Cyriopagopus albostriatus (C. albostriatus) that potently inhibits Na_v1.7 (IC₅₀ = 98.1 ± 3.3 nM) with high selectivity against skeletal muscle isoform Na_v1.4 (IC₅₀ > 10 μM) and cardiac muscle isoform Na_v1.5 (IC₅₀ > 10 μM). Ca2a did not significantly alter the voltage-dependent activation or fast inactivation of Na_v1.7, but it hyperpolarized the slow inactivation. Site-directed mutagenesis analysis indicated that Ca2a bound with Na_v1.7 at the extracellular S3–S4 linker of domain II. Meanwhile, Ca2a dose-dependently attenuated pain behaviors in rodent models of formalin-induced paw licking, hot plate test, and acetic acid-induced writhing. This study indicates that Ca2a is a potential lead molecule for drug development of novel analgesics.

The pharmacology of voltage-gated sodium channel activators

Toxins and venom components that target voltage-gated sodium (Na_V) channels have evolved numerous times due to the importance of this class of ion channels in the normal physiological function of peripheral and central neurons as well as cardiac and skeletal muscle. Na_V channel activators in particular have been isolated from the venom of spiders, wasps, snakes, scorpions, cone snails and sea anemone and are also produced by plants, bacteria and algae. These compounds have provided key insight into the molecular structure, function and pathophysiological roles of Na_V channels and are important tools due to their at times exquisite subtype-selectivity. We review the pharmacology of Na_V channel activators with particular emphasis on mammalian isoforms and discuss putative applications for these compounds. This article is part of the Special Issue entitled 'Venom-derived Peptides as Pharmacological Tools.'

Encoding of High Frequencies Improves with Maturation of Action Potential Generation in Cultured Neocortical Neurons

The ability of neocortical neurons to detect and encode rapid changes at their inputs is crucial for basic neuronal computations, such as coincidence detection, precise synchronization of activity and spike-timing dependent plasticity. Indeed, populations of cortical neurons can respond to subtle changes of the input very fast, on a millisecond time scale. Theoretical studies and model simulations linked the encoding abilities of neuronal populations to the fast onset dynamics of action potentials (APs). Experimental results support this idea, however mechanisms of fast onset of APs in cortical neurons remain elusive. Studies in neuronal cultures, that are allowing for accurate control over conditions of growth and microenvironment during the development of neurons and provide better access to the spike initiation zone, may help to shed light on mechanisms of AP generation and encoding. Here we characterize properties of AP encoding in neocortical neurons grown for 11-25 days in culture. We show that encoding of high frequencies improves upon culture maturation, which is accompanied by the development of passive electrophysiological properties and AP generation. The onset of APs becomes faster with culture maturation. Statistical analysis using correlations and linear model approaches identified the onset dynamics of APs as a major predictor of age-dependent changes of encoding. Encoding of high frequencies strongly correlated also with the input resistance of neurons. Finally, we show that maturation of encoding properties of neurons in cultures is similar to the maturation of encoding in neurons studied in slices. These results show that maturation of AP generators and encoding is, to a large extent, determined genetically and takes place even without normal micro-environment and activity of the whole brain in vivo. This establishes neuronal cultures as a valid experimental model for studying mechanisms of AP generation and encoding, and their maturation.

De novo and inherited SCN8A epilepsy mutations detected by gene panel analysis

OBJECTIVES

To determine the incidence of pathogenic SCN8A variants in a cohort of epilepsy patients referred for clinical genetic testing. We also investigated the contribution of SCN8A to autism spectrum disorder, intellectual disability, and neuromuscular disorders in individuals referred for clinical genetic testing at the same testing laboratory.

METHODS

Sequence data from 275 epilepsy panels screened by Emory Genetics Laboratory were reviewed for variants in SCN8A. Two additional cases with variants in SCN8A were ascertained from other testing laboratories. Parental samples were tested for variant segregation and clinical histories were examined. SCN8A variants detected from gene panel analyses for autism spectrum disorder, intellectual disability, and neuromuscular disorders were also examined.

RESULTS

Five variants in SCN8A were identified in five individuals with epilepsy. Three variants were de novo, one was inherited from an affected parent, and one was inherited from an unaffected parent. Four of the individuals have epilepsy and developmental delay/intellectual disability. The remaining individual has a milder epilepsy presentation without cognitive impairment. We also identified an amino acid substitution at an evolutionarily conserved SCN8A residue in a patient who was screened on the autism spectrum disorder panel. Additionally, we examined the distribution of pathogenic SCN8A variants across the Nav1.6 channel and identified four distinct clusters of variants. These clusters are primarily located in regions of the channel that are important for the kinetics of channel inactivation.

CONCLUSIONS

Variants in SCN8A may be responsible for a spectrum of epilepsies as well as other neurodevelopmental disorders without seizures. The predominant pathogenic mechanism appears to involve disruption of channel inactivation, leading to gain-of-function effects.

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.

An oral NaV1.8 blocker improves motor function in mice completely deficient of myelin protein P0

Mice deficient of myelin protein P0 are established models of demyelinating Charcot-Marie-Tooth (CMT) disease. Dysmyelination in these mice is associated with an ectopic expression of the sensory neuron specific sodium channel isoform NaV1.8 on motor axons. We reported that in P0+/-, a model of CMT1B, the membrane dysfunction could be acutely improved by a novel oral NaV1.8 blocker referred to as Compound 31 (C31, Bioorg. Med. Chem. Lett. 2010, 20, 6812; AbbVie Inc.). The aim of this study was to investigate the extent to which C31 treatment could also improve the motor axon function in P0-/-, a CMT model with a much more severe neuropathy. We found that the progressive impairment of motor performance from 1 to 4 months of age in P0-/- could be acutely reversed by C31 treatment. The effect was associated with an improvement of the amplitude of the plantar CMAP evoked by tibial nerve stimulation. The corresponding motor nerve excitability studies by "threshold tracking" showed changes after C31 consistent with attenuation of a resting membrane depolarization. Our data suggest that the depolarizing motor conduction failure in P0-/- could be acutely improved by C31. This provides proof-of-concept that treatment with oral subtype-selective NaV1.8 blockers could be used to improve the motor function in severe forms of demyelinating CMT.

Progression of motor axon dysfunction and ectopic Nav1.8 expression in a mouse model of Charcot-Marie-Tooth disease 1B

Mice heterozygously deficient for the myelin protein P0 gene (P0+/-) develop a slowly progressing neuropathy modeling demyelinating Charcot-Marie-Tooth disease (CMT1B). The aim of the study was to investigate the long-term progression of motor dysfunction in P0+/- mice at 3, 7, 12 and 20months. By comparison with WT littermates, P0+/- showed a decreasing motor performance with age. This was associated with a progressive reduction in amplitude and increase in latency of the plantar compound muscle action potential (CMAP) evoked by stimulation of the tibial nerve at ankle. This progressive functional impairment was in contrast to the mild demyelinating neuropathy of the tibial nerve revealed by histology. "Threshold-tracking" studies showed impaired motor axon excitability in P0+/- from 3months. With time, there was a progressive reduction in threshold deviations during both depolarizing and hyperpolarizing threshold electrotonus associated with increasing resting I/V slope and increasing strength-duration time constant. These depolarizing features in excitability in P0+/- as well as the reduced CMAP amplitude were absent in P0+/- NaV1.8 knockouts, and could be acutely reversed by selective pharmacologic block of NaV1.8 in P0+/-. Mathematical modeling indicated an association of altered passive cable properties with a depolarizing shift in resting membrane potential and increase in the persistent Na(+) current in P0+/-. Our data suggest that ectopic NaV1.8 expression precipitates depolarizing conduction failure in CMT1B, and that motor axon dysfunction in demyelinating neuropathy is pharmacologically reversible.

Analgesic Effects of GpTx-1, PF-04856264 and CNV1014802 in a Mouse Model of NaV1.7-Mediated Pain

Loss-of-function mutations of Na(V)1.7 lead to congenital insensitivity to pain, a rare condition resulting in individuals who are otherwise normal except for the inability to sense pain, making pharmacological inhibition of Na(V)1.7 a promising therapeutic strategy for the treatment of pain. We characterized a novel mouse model of Na(V)1.7-mediated pain based on intraplantar injection of the scorpion toxin OD1, which is suitable for rapid in vivo profiling of Na(V)1.7 inhibitors. Intraplantar injection of OD1 caused spontaneous pain behaviors, which were reversed by co-injection with Na(V)1.7 inhibitors and significantly reduced in Na(V)1.7(-/-) mice. To validate the use of the model for profiling Na(V)1.7 inhibitors, we determined the Na(V) selectivity and tested the efficacy of the reported Na(V)1.7 inhibitors GpTx-1, PF-04856264 and CNV1014802 (raxatrigine). GpTx-1 selectively inhibited Na(V)1.7 and was effective when co-administered with OD1, but lacked efficacy when delivered systemically. PF-04856264 state-dependently and selectively inhibited Na(V)1.7 and significantly reduced OD1-induced spontaneous pain when delivered locally and systemically. CNV1014802 state-dependently, but non-selectively, inhibited Na(V) channels and was only effective in the OD1 model when delivered systemically. Our novel model of Na(V)1.7-mediated pain based on intraplantar injection of OD1 is thus suitable for the rapid in vivo characterization of the analgesic efficacy of Na(V)1.7 inhibitors.

SCN4A pore mutation pathogenetically contributes to autosomal dominant essential tremor and may increase susceptibility to epilepsy

Essential tremor (ET) is the most prevalent movement disorder, affecting millions of people in the USA. Although a positive family history is one of the most important risk factors for ET, the genetic causes of ET remain unknown. In an attempt to identify genetic causes for ET, we performed whole-exome sequencing analyses in a large Spanish family with ET, in which two patients also developed epilepsy. To further assess pathogenicity, site-directed mutagenesis, mouse and human brain expression analyses, and patch clamp techniques were performed. A disease-segregating mutation (p.Gly1537Ser) in the SCN4A gene was identified. Posterior functional analyses demonstrated that more rapid kinetics at near-threshold potentials altered ion selectivity and facilitated the conductance of both potassium and ammonium ions, which could contribute to tremor and increase susceptibility to epilepsy, respectively. In this report, for the first time, we associated the genetic variability of SCN4A with the development of essential tremor, which adds ET to the growing list of neurological channelopathies.

Knockdown of sodium channel NaV1.6 blocks mechanical pain and abnormal bursting activity of afferent neurons in inflamed sensory ganglia

Inflammatory processes in the sensory ganglia contribute to many forms of chronic pain. We previously showed that local inflammation of the lumbar sensory ganglia rapidly leads to prolonged mechanical pain behaviors and high levels of spontaneous bursting activity in myelinated cells. Abnormal spontaneous activity of sensory neurons occurs early in many preclinical pain models and initiates many other pathological changes, but its molecular basis is not well understood. The sodium channel isoform NaV1.6 can underlie repetitive firing and excitatory persistent and resurgent currents. We used in vivo knockdown of this channel via local injection of siRNA to examine its role in chronic pain after local inflammation of the rat lumbar sensory ganglia. In normal dorsal root ganglion (DRG), quantitative polymerase chain reaction showed that cells capable of firing repetitively had significantly higher relative expression of NaV1.6. In inflamed DRG, spontaneously active bursting cells expressed high levels of NaV1.6 immunoreactivity. In vivo knockdown of NaV1.6 locally in the lumbar DRG at the time of DRG inflammation completely blocked development of pain behaviors and abnormal spontaneous activity, while having only minor effects on unmyelinated C cells. Current research on isoform-specific sodium channel blockers for chronic pain is largely focused on NaV1.8 because it is present primarily in unmyelinated C fiber nociceptors, or on NaV1.7 because lack of this channel causes congenital indifference to pain. However, the results suggest that NaV1.6 may be a useful therapeutic target for chronic pain and that some pain conditions may be mediated primarily by myelinated A fiber sensory neurons.

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.

The splicing regulator Rbfox2 is required for both cerebellar development and mature motor function

The Rbfox proteins (Rbfox1, Rbfox2, and Rbfox3) regulate the alternative splicing of many important neuronal transcripts and have been implicated in a variety of neurological disorders. However, their roles in brain development and function are not well understood, in part due to redundancy in their activities. Here we show that, unlike Rbfox1 deletion, the CNS-specific deletion of Rbfox2 disrupts cerebellar development. Genome-wide analysis of Rbfox2(-/-) brain RNA identifies numerous splicing changes altering proteins important both for brain development and mature neuronal function. To separate developmental defects from alterations in the physiology of mature cells, Rbfox1 and Rbfox2 were deleted from mature Purkinje cells, resulting in highly irregular firing. Notably, the Scn8a mRNA encoding the Na(v)1.6 sodium channel, a key mediator of Purkinje cell pacemaking, is improperly spliced in RbFox2 and Rbfox1 mutant brains, leading to highly reduced protein expression. Thus, Rbfox2 protein controls a post-transcriptional program required for proper brain development. Rbfox2 is subsequently required with Rbfox1 to maintain mature neuronal physiology, specifically Purkinje cell pacemaking, through their shared control of sodium channel transcript splicing.

Neuronal P/Q-type calcium channel dysfunction in inherited disorders of the CNS

The past two decades have witnessed the emergence of a new and expanding field of neurological diseases--the genetic ion channelopathies. These disorders arise from mutations in genes that encode ion channel subunits, and manifest as paroxysmal attacks involving the brain or spinal cord, and/or muscle. The voltage-gated P/Q-type calcium channel (P/Q channel) is highly expressed in the cerebellum, hippocampus and cortex of the mammalian brain. The P/Q channel has a fundamental role in mediating fast synaptic transmission at central and peripheral nerve terminals. Autosomal dominant mutations in the CACNA1A gene, which encodes voltage-gated P/Q-type calcium channel subunit α(1) (the principal pore-forming subunit of the P/Q channel) are associated with episodic and progressive forms of cerebellar ataxia, familial hemiplegic migraine, vertigo and epilepsy. This Review considers, from both a clinical and genetic perspective, the various neurological phenotypes arising from inherited P/Q channel dysfunction, with a focus on recent advances in the understanding of the pathogenetic mechanisms underlying these disorders.

Two Nedd4-binding motifs underlie modulation of sodium channel Nav1.6 by p38 MAPK

Sodium channel Na(v)1.6 is essential for neuronal excitability in central and peripheral nervous systems. Loss-of-function mutations in Na(v)1.6 underlie motor disorders, with homozygous-null mutations causing juvenile lethality. Phosphorylation of Na(v)1.6 by the stress-induced p38 MAPK at a Pro-Gly-Ser(553)-Pro motif in its intracellular loop L1 reduces Na(v)1.6 current density in a dorsal root ganglion-derived cell line, without changing its gating properties. Phosphorylated Pro-Gly-Ser(553)-Pro motif is a putative binding site to Nedd4 ubiquitin ligases, and we hypothesized that Nedd4-like ubiquitin ligases may contribute to channel ubiquitination and internalization. We report here that p38 activation in hippocampal neurons from wild-type mice, but not from Scn8a(medtg) mice that lack Na(v)1.6, reduces tetrodotoxin-S sodium currents, suggesting isoform-specific modulation of Na(v)1.6 by p38 in these neurons. Pharmacological block of endocytosis completely abolishes p38-mediated Na(v)1.6 current reduction, supporting our hypothesis that channel internalization underlies current reduction. We also report that the ubiquitin ligase Nedd4-2 interacts with Na(v)1.6 via a Pro-Ser-Tyr(1945) motif in the C terminus of the channel and reduces Na(v)1.6 current density, and we show that this regulation requires both the Pro-Gly-Ser-Pro motif in L1 and the Pro-Ser-Tyr motif in the C terminus. Similarly, both motifs are necessary for p38-mediated reduction of Na(v)1.6 current, whereas abrogating binding of the ubiquitin ligase Nedd4-2 to the Pro-Ser-Tyr motif results in stress-mediated increase in Na(v)1.6 current density. Thus, phosphorylation of the Pro-Gly-Ser-Pro motif within L1 of Na(v)1.6 is necessary for stress-induced current modulation, with positive or negative regulation depending upon the availability of the C-terminal Pro-Ser-Tyr motif to bind Nedd4-2.

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.

Multiple regulatory elements mediating neuronal-specific expression of zebrafish sodium channel gene, scn8aa

Zebrafish scn8aa sodium channels mediate the majority of sodium conductance, which is essential for the embryonic locomotor activities. Here, we investigated the transcriptional regulation of scn8aa in developing zebrafish embryos by constructing a GFP reporter driven by a 15-kb fragment of scn8aa gene designed as scn8aa:GFP. GFP expression patterns of scn8aa:GFP temporally and spatially recapitulated the expression of endogenous scn8aa mRNA during zebrafish embryonic development, with one exception in the inner nuclear layer of the retina. Three novel elements, along with an evolutionarily conserved element shared with mouse SCN8A, modulated neuronal-specific expression of scn8aa. The deletion of each positive element reduced the expression levels in neurons without inducing ectopic GFP expression in non-neuronal cells. Our results demonstrate that these four regulatory elements function cooperatively to enhance scn8aa expression in the zebrafish nervous system.

Knockdown of zebrafish Nav1.6 sodium channel impairs embryonic locomotor activities

Although multiple subtypes of sodium channels are expressed in most neurons, the specific contributions of the individual sodium channels remain to be studied. The role of zebrafish Na(v)1.6 sodium channels in the embryonic locomotor movements has been investigated by the antisense morpholino (MO) knockdown. MO1 and MO2 are targeted at the regions surrounding the translation start site of zebrafish Na(v)1.6 mRNA. MO3 is targeted at the RNA splicing donor site of exon 2. The correctly spliced Na(v)1.6 mRNA of MO3 morphants is 6% relative to that of the wild-type embryos. Na(v)1.6-targeted MO1, MO2 and MO3 attenuate the spontaneous contraction, tactile sensitivity, and swimming in comparison with a scrambled morpholino and mutated MO3 morpholino. No significant defect is observed in the development of slow muscles, the axonal projection of primary motoneurons, and neuromuscular junctions. The movement impairments caused by MO1, MO2, and MO3 suggest that the function of Na(v)1.6 sodium channels is essential on the normal early embryonic locomotor activities.

Sodium currents in mesencephalic trigeminal neurons from Nav1.6 null mice

Previous studies using pharmacological methods suggest that subthreshold sodium currents are critical for rhythmical burst generation in mesencephalic trigeminal neurons (Mes V). In this study, we characterized transient (I(NaT)), persistent (I(N)(aP)), and resurgent (I(res)) sodium currents in Na(v)1.6-null mice (med mouse, Na(v)1.6(-/-)) lacking expression of the sodium channel gene Scn8a. We found that peak transient, persistent, and resurgent sodium currents from med (Na(v)1.6(-/-)) mice were reduced by 18, 39, and 76% relative to their wild-type (Na(v)1.6(+/+)) littermates, respectively. Current clamp recordings indicated that, in response to sinusoidal constant amplitude current (ZAP function), all neurons exhibited membrane resonance. However, Mes V neurons from med mice had reduced peak amplitudes in the impedance-frequency relationship (resonant Q-value) and attenuated subthreshold oscillations despite the similar passive membrane properties compared with wild-type littermates. The spike frequency-current relationship exhibited reduced instantaneous discharge frequencies and spike block at low stimulus currents and seldom showed maintained spike discharge throughout the stimulus in the majority of med neurons compared with wild-type neurons. Importantly, med neurons never exhibited maintained stimulus-induced rhythmical burst discharge unlike those of wild-type littermates. The data showed that subthreshold sodium currents are critical determinants of Mes V electrogenesis and burst generation and suggest a role for resurgent sodium currents in control of spike discharge.

Physiological maturation of photoreceptors depends on the voltage-gated sodium channel NaV1.6 (Scn8a)

Voltage-gated sodium channels (VGSCs) ensure the saltatory propagation of action potentials along axons by acting as signal amplifiers at the nodes of Ranvier. In the retina, activity mediated by VGSCs is important for the refinement of the retinotectal map. Here, we conducted a full-field electroretinogram (ERG) study on mice null for the sodium channel NaV1.6. Interestingly, the light-activated hyperpolarization of photoreceptor cells (the a-wave) and the major "downstream" components of the ERG, the b-wave and the oscillatory potentials, are markedly reduced and delayed in these mice. The functional deficit was not associated with any morphological abnormality. We demonstrate that Scn8a is expressed in the ganglion and inner nuclear layers and at low levels in the outer nuclear layer beginning shortly before the observed ERG deficit. Together, our data reveal a previously unappreciated role for VGSCs in the physiological maturation of photoreceptors.

Inactivation of sodium channel Scn8A (Nav1.6) in purkinje neurons impairs learning in Morris Water Maze and delay but not trace eyeblink classical conditioning

To examine the isolated effects of altered currents in cerebellar Purkinje neurons, the authors used Scn8a-super(flox/flox), Purkinje cell protein-CRE (Pcp-CRE) mice in which Exon 1 of Scn8a is deleted only in Purkinje neurons. Twenty male Purkinje Scn8a knockout (PKJ Scn8a KO) mice and 20 male littermates were tested on the Morris water maze (MWM). Subsequently, half were tested in 500-ms delay and half were tested in 500-ms trace eyeblink conditioning. PKJ Scn8a KO mice were impaired in delay conditioning and MWM but not in trace conditioning. These results provide additional support for the necessary participation of cerebellar cortex in normal acquisition of delay eyeblink conditioning and MWM and raise questions about the role, if any, of cerebellar cortex in trace eyeblink conditioning.

Sodium channel mutations in epilepsy and other neurological disorders

Since the first mutations of the neuronal sodium channel SCN1A were identified 5 years ago, more than 150 mutations have been described in patients with epilepsy. Many are sporadic mutations and cause loss of function, which demonstrates haploinsufficiency of SCN1A. Mutations resulting in persistent sodium current are also common. Coding variants of SCN2A, SCN8A, and SCN9A have also been identified in patients with seizures, ataxia, and sensitivity to pain, respectively. The rapid pace of discoveries suggests that sodium channel mutations are significant factors in the etiology of neurological disease and may contribute to psychiatric disorders as well.

Theoretical investigation of the neuronal Na+ channel SCN1A: abnormal gating and epilepsy

Epilepsy is a paroxysmal neurological disorder resulting from abnormal cellular excitability and is a common cause of disability. Recently, some forms of idiopathic epilepsy have been causally related to genetic mutations in neuronal ion channels. To understand disease mechanisms, it is crucial to understand how a gene defect can disrupt channel gating, which in turn can affect complex cellular dynamic processes. We develop a theoretical Markovian model of the neuronal Na+ channel NaV1.1 to explore and explain gating mechanisms underlying cellular excitability and physiological and pathophysiological mechanisms of abnormal neuronal excitability in the context of epilepsy. Genetic epilepsy has been shown to result from both mutations that give rise to a gain of channel function and from those that reduce the Na+ current. These data may suggest that abnormal excitation can result from both hyperexcitability and hypoexcitability, the mechanisms of which are presumably distinct, and as yet elusive. Revelation of the molecular origins will allow for translation into targeted pharmacological interventions that must be developed to treat syndromes resulting from divergent mechanisms. This work represents a first step in developing a comprehensive theoretical model to investigate the molecular mechanisms underlying runaway excitation that cause epilepsy.

Three ENU-induced neurological mutations in the pore loop of sodium channel Scn8a (Na(v)1.6) and a genetically linked retinal mutation, rd13

The goal of The Jackson Laboratory Neuroscience Mutagenesis Facility is to generate mouse models of human neurological disease. We describe three new models obtained from a three-generation screen for recessive mutations. Homozygous mutant mice from lines nmf2 and nmf5 exhibit hind limb paralysis and juvenile lethality. Homozygous nmf58 mice exhibit a less severe movement disorder that includes sustained dystonic postures. The mutations were mapped to the distal region of mouse Chromosome (Chr) 15. Failure to complement a mutant allele of a positional candidate gene, Scn8a, demonstrated that the mutations are new alleles of Scn8a. Missense mutations of evolutionarily conserved residues of the sodium channel were identified in the three lines, with the predicted amino acid substitutions N1370T, I1392F, and L1404H. These residues are located within the pore loop of domain 3 of sodium channel Na(v)1.6. The lethal phenotypes suggest that the new alleles encode proteins with partial or complete loss of function. Several human disorders are caused by mutation in the pore loop of domain 3 of paralogous sodium channel genes. Line nmf5 contains a second, independent mutation in the rd13 locus that causes a reduction in cell number in the outer nuclear layer of the retina. rd13 was mapped to the distal 4 Mb of Chr 15. No coding or splice site mutations were detected in Pde1b, a candidate gene for rd13. The generation of three independent Scn8a mutations among 1100 tested G3 families demonstrates that the Scn8a locus is highly susceptible to ENU mutagenesis. The new alleles of Scn8a will be valuable for analysis of sodium channel physiology and disease.

Sodium Currents in Subthalamic Nucleus Neurons From Nav1.6-Null Mice

In some central neurons, including cerebellar Purkinje neurons and subthalamic nucleus (STN) neurons, TTX-sensitive sodium channels show unusual gating behavior whereby some channels open transiently during recovery from inactivation. This “resurgent” sodium current is effectively activated immediately after action potential-like waveforms. Earlier work using Purkinje neurons suggested that the great majority of resurgent current originates from Nav1.6 sodium channels. Here we used a mouse mutant lacking Nav1.6 to explore the contribution of these channels to resurgent, transient, and persistent components of TTX-sensitive sodium current in STN neurons. The resurgent current of STN neurons from Nav1.6−/− mice was reduced by 63% relative to wild-type littermates, a less dramatic reduction than that observed in Purkinje neurons recorded under identical conditions. The transient and persistent currents of Nav1.6−/− STN neurons were reduced by ∼40 and 55%, respectively. The resurgent current present in Nav1.6−/− null STN neurons was similar in voltage dependence to that in wild-type STN and Purkinje neurons, differing only in having somewhat slower decay kinetics. These results show that sodium channels other than Nav1.6 can make resurgent sodium current much like that from Nav1.6 channels.

High-resolution mapping of the sodium channel modifier Scnm1 on mouse chromosome 3 and identification of a 1.3-kb recombination hot spot

Variation between inbred strains of mice can be used to identify modifier genes affecting the susceptibility to inherited disease. The medJ allele of the sodium channel Scn8a contains a splice site mutation that results in sodium channel deficiency. The severity of the neurological disorder is determined by the modifier locus Scnm1. The wild-type allele of the modifier results in correct splicing of 10% of Scn8amedJ pre-mRNA and a dystonic phenotype. The susceptible allele of the modifier in strain C57BL/6J results in 5% correctly spliced transcripts and a lethal phenotype. A mapping cross with C3H using 26 new markers and 2304 affected F2 animals localized the modifier gene to a 950-kb interval on mouse chromosome 3. Fine mapping of recombination breakpoints revealed a recombination hot spot of 1.3 kb. The ratio of genetic to physical distance in the hot spot is 85 cM/Mb, two orders of magnitude higher than the mouse genome average of 0.5 cM/Mb. The role of the modifier in other disorders in human and mouse can be tested with linked markers described here.

Mutations in cardiac sodium channels: clinical implications

Voltage-gated sodium channels (VGSCs) are critical transmembrane proteins responsible for the rapid action potential upstroke in most excitable cells. Recently discovered mutations in VGSCs, which underlie idiopathic clinical disease, have emphasized the importance of these channels in tissues such as skeletal muscle, nervous system, and myocardium. Mutations in the gene encoding the cardiac sodium channel isoform (SCN5A) have been linked to at least three abnormal phenotypes: variant 3 of the Long QT syndrome (LQT-3); Brugada's syndrome (BrS); and isolated cardiac conduction disease (ICCD). Mutations in SCN5A manifest as one or more of these clinical phenotypes - the precise distinction between these diseases is increasingly subtle. Clinical management of LQT-3 and diagnosis of BrS with the local anesthetic flecainide has proven promising. Channels associated with LQT-3 (D1790G) and BrS (Y1795H) both show more sensitivity to flecainide than wild-type (WT) channels, while lidocaine sensitivity is unchanged. One plausible explanation for differential drug sensitivity is that mutant channels may allow more access to a receptor site compared with WT through altered protein allosteric changes during an action potential. The high affinity binding site for local anesthetic block has been identified in the pore region of the channel. This region is not water accessible during the closed state, thus requiring channel opening for charged drug (flecainide and mexiletine) access and block. Channel mutations which disrupt inactivation biophysics lead to increased drug binding by altering the time the binding site is accessible during an action potential. Neutral drugs (lidocaine) which are not dependent on channel opening for binding site access will not be sensitive to mutations that alter channel inactivation properties. Interestingly another LQT-3 mutant (Y1795C) shows no change in flecainide sensitivity, suggesting that although drug effects of SCN5A mutations cross disease boundaries, clinical management with flecainide will be beneficial to patients in a mutation-specific manner.

Floxed allele for conditional inactivation of the voltage-gated sodium channelScn8a (Nav1.6)

The sodium channel gene Scn8a encodes the channel NaV1.6, which is widely distributed in the central and peripheral nervous system. NaV1.6 is the major channel at the nodes of Ranvier in myelinated axons. Mutant alleles of mouse Scn8a result in neurological disorders including ataxia, tremor, paralysis, and dystonia. We generated a floxed allele of Scn8a by inserting loxP sites around the first coding exon. The initial targeted allele containing the neo-cassette was a severe hypomorph. In vivo deletion of the neo-cassette by Flp recombinase produced a floxed allele that generates normal expression of NaV1.6 protein. Ubiquitous deletion of the floxed exon by Cre recombinase in ZP3-Cre transgenic mice produced the Scn8a(del) allele. The null phenotype of Scn8a(del) homozygotes confirms the in vivo inactivation of Scn8a. Conditional inactivation of the floxed allele will make it possible to circumvent the lethality that results from complete loss of Scn8a in order to investigate the physiologic role of NaV1.6 in subpopulations of neurons.

Motor disturbances in mice with deficiency of the sodium channel gene Scn8a show features of human dystonia

The med(J) mouse with twisting movements related to deficiency of the sodium channel Scn8a has been proposed as a model of kinesiogenic dystonia. This prompted us to examine the phenotype of these mice in more detail. By cortical electroencephalographic (EEG) recordings, we could not detect any changes, demonstrating that the motor disturbances are not epileptic in nature, an important similarity to human dystonia. The significantly decreased body weight of med(J) mice was related to reduced food intake. Observations in the open field and by video recordings revealed that the mice exhibit sustained abnormal postures and movements of limbs, trunk and tail not only during locomotor activity but also at rest. With the exception of the head tremor, the other motor impairments were persistent rather than paroxysmal. When several neurological reflexes were tested, alterations were restricted to the posture and righting reflexes. Results of the wire hang test confirmed the greatly reduced muscle strength in the med(J) mouse. In agreement with different types of human dystonia, biperiden, haloperidol and diazepam moderately reduced the severity of motor disturbances in med(J) mice. In view of the sodium channel deficiency in med(J) mice, the beneficial effects of the sodium channel blocker phenytoin was an unexpected finding. By immunohistochemical examinations, the density of nigral dopaminergic neurons was found to be unaltered, substantiating the absence of pathomorphological abnormalities within the brain of med(J) mice shown by previous studies. With the exception of muscle weakness, many of the features of the med(J) mouse are similar to human idiopathic dystonia.

Polarized domains of myelinated axons

The entire length of myelinated axons is organized into a series of polarized domains that center around nodes of Ranvier. These domains, which are crucial for normal saltatory conduction, consist of distinct multiprotein complexes of cell adhesion molecules, ion channels, and scaffolding molecules; they also differ in their diameter, organelle content, and rates of axonal transport. Juxtacrine signals from myelinating glia direct their sequential assembly. The composition, mechanisms of assembly, and function of these molecular domains will be reviewed. I also discuss similarities of this domain organization to that of polarized epithelia and present emerging evidence that disorders of domain organization and function contribute to the axonopathies of myelin and other neurologic disorders.

Pharmacogenomics in the treatment of epilepsy

Genetic variability has recently been implicated in the development of familial epilepsy syndromes and in heterogeneous responses of epilepsy patients to drug treatment. Mutations in distinct proteins have been shown to underlie the development of epilepsy, increase propensity for drug resistance, and alter drug metabolism. Improved understanding of how individual genetic variability may alter the efficacy of pharmacological therapeutic interventions is an important and timely goal. The investigation of relationships between genotype and patient responses to drug treatment is termed pharmacogenomics.

SCNM1, a putative RNA splicing factor that modifies disease severity in mice

The severity of many inherited disorders is influenced by genetic background. We describe a modifier interaction in C57BL/6Jmice that converts a chronic movement disorder into a lethal neurological disease. The primary mutation (medJ) changes a splice donor site of the sodium channel gene Scn8a (Nav1.6). The modifier mutation is characteristic of strain C57BL/6Jand introduces a nonsense codon into sodium channel modifier 1 (SCNM1), a zinc finger protein and a putative splice factor. An internally deleted SCNM1 protein is also predicted as a result of exon skipping associated with disruption of a consensus exonic splicing enhancer. The effect of the modifier mutation is to reduce the abundance of correctly spliced sodium channel transcripts below the threshold for survival. Our finding that genetic variation in a putative RNA splicing factor influences disease susceptibility in mice raises the possibility that a similar mechanism modifies the severity of human inherited disorders.

TSRC1, a widely expressed gene containing seven thrombospondin type I repeats

The thrombospondin type 1 repeat domain is found in nearly 100 mammalian proteins with diverse biological functions that include cellular adhesion, angiogenesis, and patterning of the developing nervous system. We have characterized a novel thrombospondin type 1 repeat containing gene, TSRC1, encoding a predicted protein with seven thrombospondin repeats, six of which are clustered at the C-terminus. The 17 coding exons and two nontranslated exons of TSRC1 span 10 kb of genomic DNA. The human and mouse genes encode proteins of 1074 and 1036 amino acids, respectively, with 76% amino acid sequence identity. Thirty of the extra amino acids in the human protein are encoded by exon 6. Mouse Tsrc1 is expressed in all fetal and adult tissues tested. Three conserved noncoding sequence elements with potential regulatory function are located in intron 1. Mouse Tsrc1 was genetically mapped to chromosome 3 within the nonrecombinant region for the sodium channel modifier locus Scnm1. The sensitive and resistant alleles of Scnm1 did not differ in Tsrc1 protein sequence, transcript length, or transcript abundance. Human TSRC1 is located on chromosome 1q21 within an 11.7 Mb segment of conserved synteny. TSRC1 and the closely linked gene ADAM15 appear to be derived by a chromosomal inversion that interrupted an ancestral ADAMTS gene.

Sodium channels SCN1A, SCN2A and SCN3A in familial autism

Autism is a psychiatric disorder with estimated heritability of 90%. One-third of autistic individuals experience seizures. A susceptibility locus for autism was mapped near a cluster of voltage-gated sodium channel genes on chromosome 2. Mutations in two of these genes, SCN1A and SCN2A, result in the seizure disorder GEFS+. To evaluate these sodium channel genes as candidates for the autism susceptibility locus, we screened for variation in coding exons and splice sites in 117 multiplex autism families. A total of 27 kb of coding sequence and 3 kb of intron sequence were screened. Only six families carried variants with potential effects on sodium channel function. Five coding variants and one lariat branchpoint mutation were each observed in a single family, but were not present in controls. The variant R1902C in SCN2A is located in the calmodulin binding site and was found to reduce binding affinity for calcium-bound calmodulin. R542Q in SCN1A was observed in one autism family and had previously been identified in a patient with juvenile myoclonic epilepsy. The effect of the lariat branchpoint mutation was tested in cultured lymphoblasts. Additional population studies and functional tests will be required to evaluate pathogenicity of the coding and lariat site variants. SNP density was 1/kb in the genomic sequence screened. We report 38 sodium channel SNPs that will be useful in future association and linkage studies.

Genomic scale profiling of autoimmune inflammation in the central nervous system: the nervous response to inflammation

Using gene microarray technology, we found that inflammation in the central nervous system (CNS) not only induced the expression of many immune-related genes, but also significantly altered the gene expression profile of neural cells. Two unique groups of CNS genes were identified. The first group includes genes encoding ion channels, neural transmitters and growth factors. The second group includes genes that are important for nervous tissue regeneration. Additionally, a distinct pattern of gene expression was also identified in recovering animals. Thus, during autoimmune inflammation, the CNS actively responds to immune attacks by activating its own defense and repair genes.