Anti-PD-1 treatment protects against seizure by suppressing sodium channel function

AIMS

Although programmed cell death protein 1 (PD-1) typically serves as a target for immunotherapies, a few recent studies have found that PD-1 is expressed in the nervous system and that neuronal PD-1 might play a crucial role in regulating neuronal excitability. However, whether brain-localized PD-1 is involved in seizures and epileptogenesis is still unknown and worthy of in-depth exploration.

METHODS

The existence of PD-1 in human neurons was confirmed by immunohistochemistry, and PD-1 expression levels were measured by real-time quantitative PCR (RT-qPCR) and western blotting. Chemoconvulsants, pentylenetetrazol (PTZ) and cyclothiazide (CTZ), were applied for the establishment of in vivo (rodents) and in vitro (primary hippocampal neurons) models of seizure, respectively. SHR-1210 (a PD-1 monoclonal antibody) and sodium stibogluconate (SSG, a validated inhibitor of SH2-containing protein tyrosine phosphatase-1 [SHP-1]) were administrated to investigate the impact of PD-1 pathway blockade on epileptic behaviors of rodents and epileptiform discharges of neurons. A miRNA strategy was applied to determine the impact of PD-1 knockdown on neuronal excitability. The electrical activities and sodium channel function of neurons were determined by whole-cell patch-clamp recordings. The interaction between PD-1 and α-6 subunit of human voltage-gated sodium channel (Nav1.6) was validated by performing co-immunostaining and co-immunoprecipitation (co-IP) experiments.

RESULTS

Our results reveal that PD-1 protein and mRNA levels were upregulated in lesion cores compared with perifocal tissues of surgically resected specimens from patients with intractable epilepsy. Furthermore, we show that anti-PD-1 treatment has anti-seizure effects both in vivo and in vitro. Then, we reveal that PD-1 blockade can alter the electrophysiological properties of sodium channels. Moreover, we reveal that PD-1 acts together with downstream SHP-1 to regulate sodium channel function and hence neuronal excitability. Further investigation suggests that there is a direct interaction between neuronal PD-1 and Nav1.6.

CONCLUSION

Our study reveals that neuronal PD-1 plays an important role in epilepsy and that anti-PD-1 treatment protects against seizures by suppressing sodium channel function, identifying anti-PD-1 treatment as a novel therapeutic strategy for epilepsy.

Increased expression of Nav1.6 of reactive astrocytes in the globus pallidus is closely associated with motor deficits in a model of Parkinson's disease

Parkinson's disease (PD) is a common neurodegenerative disease in elderly people, which is characterized by motor disabilities in PD patients. Nav1.6 is the most abundant subtype of voltage-gated sodium channels (VGSCs) in the brain of adult mammals and rodents. Here we investigated the role of Nav1.6 in the external globus pallidus (GP) involved in the pathogenesis of motor deficits in unilateral 6-OHDA(6-hydroxydopamine)lesioned rats. The results show that Nav1.6 is dramatically increased in reactive astrocytes of the ipsilateral GP in the middle stage, but not different from the control rats in the later stage of the pathological process in 6-OHDA lesioned rats. Furthermore, the down-regulation of Nav1.6 expression in the ipsilateral GP can significantly improve motor deficits in 6-OHDA lesioned rats in the middle stage of the pathological process. The electrophysiological experiments show that the down-regulation of Nav1.6 expression in the ipsilateral GP significantly decreases the abnormal high synchronization between the ipsilateral M1 (the primary motor cortex) and GP in 6-OHDA lesioned rats. Ca2+ imaging reveals that the down-regulation of Nav1.6 expression reduces the intracellular concentration of Ca2+ ([Ca2+ ]i) in primary cultured astrocytes. These findings suggest that the increased Nav1.6 expression of reactive astrocytes in the GP play an important role in the pathogenesis of motor dysfunction in the middle stage in 6-OHDA lesioned rats, which may participate in astrocyte-neuron communication by regulating [Ca2+ ]i of astrocytes, thereby contributing to the formation of abnormal electrical signals of the basal ganglia (BG) in 6-OHDA lesioned rats.

Persistent Nav1.1 and Nav1.6 currents drive spinal locomotor functions through nonlinear dynamics

Persistent sodium current (INaP) in the spinal locomotor network promotes two distinct nonlinear firing patterns: a self-sustained spiking triggered by a brief excitation in bistable motoneurons and bursting oscillations in interneurons of the central pattern generator (CPG). Here, we identify the NaV channels responsible for INaP and their role in motor behaviors. We report the axonal Nav1.6 as the main molecular player for INaP in lumbar motoneurons. The inhibition of Nav1.6, but not of Nav1.1, in motoneurons impairs INaP, bistability, postural tone, and locomotor performance. In interneurons of the rhythmogenic CPG region, both Nav1.6 and Nav1.1 equally mediate INaP. Inhibition of both channels is required to abolish oscillatory bursting activities and the locomotor rhythm. Overall, Nav1.6 plays a significant role both in posture and locomotion by governing INaP-dependent bistability in motoneurons and working in tandem with Nav1.1 to provide INaP-dependent rhythmogenic properties of the CPG.

Role of the voltage‑gated sodium channel Nav1.6 in glioma and candidate drugs screening

Gliomas remain a clinical challenge, common and fatal. Treatment of glioblastoma remains elusive, and researchers have focused on discovering new mechanisms and drugs. It has been well established that the expression of voltage‑gated sodium channels (VGSCs) is abnormally increased in numerous malignancies and, in general, is rarely expressed in the corresponding normal tissues. This suggests that ion channel activity appears to be associated with malignant progression of tumors. VGSCs remain largely unknown as to how their activity leads to an increase in cancer cell activity or invasiveness. Certain sodium ion channel subtypes (for instance, Nav1.5 and Nav1.7) are associated with metastasis and invasion in cancers including breast and colorectal cancers. A previous study by the authors explored the expression of certain ion channels in glioma, but there are few studies related to Nav1.6. The current study aimed to elucidate the expression and role of Nav1.6 in glioma and to screen potential drugs for the treatment of glioma by virtual screening and drug sensitivity analysis. Nav1.6 relative expression of mRNA and protein was determined by reverse transcription‑quantitative PCR and western blot analysis. Cell proliferation was determined by Cell Counting Kit‑8 assay. Cell migration was assessed by cellular wound healing assay. Cell invasion and apoptosis were detected by Transwell cell invasion assay and flow cytometry. Last but not least, FDA‑approved drugs were screened using virtual screening, molecular docking and NCI‑60 drug sensitivity analyses based on the expression and structure of Nav1.6. In glioma cells, Nav1.6 was significantly upregulated and expressed mostly in the cytoplasm and cell membrane; its expression was positively correlated with pathological grade. A172 and U251 cells exhibited reduced proliferation, migration and invasion when Nav1.6 expression was knocked down, and apoptosis was increased. TNF‑α (100 pg/ml) acting on glioma cells was found to upregulate the expression level of Nav1.6, and TNF‑α was involved in the process of Nav1.6 promoting malignant progression of glioma. Finally, certain FDA‑approved drugs were identified by virtual screening and drug sensitivity analysis. In conclusion, the present study demonstrated the expression and role of Nav1.6 in glioma and identified several FDA‑approved drugs that are highly correlated with Nav1.6 and could be candidate drugs for patients with glioma.

Cryo-EM structure of human voltage-gated sodium channel Na(v)1.6

Voltage-gated sodium channel Na_v1.6 plays a crucial role in neuronal firing in the central nervous system (CNS). Aberrant function of Na_v1.6 may lead to epilepsy and other neurological disorders. Specific inhibitors of Na_v1.6 thus have therapeutic potentials. Here we present the cryo-EM structure of human Na_v1.6 in the presence of auxiliary subunits β1 and fibroblast growth factor homologous factor 2B (FHF2B) at an overall resolution of 3.1 Å. The overall structure represents an inactivated state with closed pore domain (PD) and all "up" voltage-sensing domains. A conserved carbohydrate-aromatic interaction involving Trp302 and Asn326, together with the β1 subunit, stabilizes the extracellular loop in repeat I. Apart from regular lipids that are resolved in the EM map, an unprecedented Y-shaped density that belongs to an unidentified molecule binds to the PD, revealing a potential site for developing Na_v1.6-specific blockers. Structural mapping of disease-related Na_v1.6 mutations provides insights into their pathogenic mechanism.

CaMKII Inhibition Attenuates Distinct Gain-of-Function Effects Produced by Mutant Nav1.6 Channels and Reduces Neuronal Excitability

Aberrant Nav1.6 activity can induce hyperexcitability associated with epilepsy. Gain-of-function mutations in the SCN8A gene encoding Nav1.6 are linked to epilepsy development; however, the molecular mechanisms mediating these changes are remarkably heterogeneous and may involve post-translational regulation of Nav1.6. Because calcium/calmodulin-dependent protein kinase II (CaMKII) is a powerful modulator of Nav1.6 channels, we investigated whether CaMKII modulates disease-linked Nav1.6 mutants. Whole-cell voltage clamp recordings in ND7/23 cells show that CaMKII inhibition of the epilepsy-related mutation R850Q largely recapitulates the effects previously observed for WT Nav1.6. We also characterized a rare missense variant, R639C, located within a regulatory hotspot for CaMKII modulation of Nav1.6. Prediction software algorithms and electrophysiological recordings revealed gain-of-function effects for R639C mutant channel activity, including increased sodium currents and hyperpolarized activation compared to WT Nav1.6. Importantly, the R639C mutation ablates CaMKII phosphorylation at a key regulatory site, T642, and, in contrast to WT and R850Q channels, displays a distinct response to CaMKII inhibition. Computational simulations demonstrate that modeled neurons harboring the R639C or R850Q mutations are hyperexcitable, and simulating the effects of CaMKII inhibition on Nav1.6 activity in modeled neurons differentially reduced hyperexcitability. Acute CaMKII inhibition may represent a promising mechanism to attenuate gain-of-function effects produced by Nav1.6 mutations.

Nav1.6 promotes the progression of human follicular thyroid carcinoma cells via JAK-STAT signaling pathway

Follicular thyroid carcinoma (FTC) is one of the most common malignant tumors of the endocrine system. Recent studies have shown that voltage-gated sodium channels (VGSCs) affect the proliferation, migration, and invasion of tumor cells. However, the expression and functions of VGSCs, and the molecular pathways activated by VGSCs in FTC cells remain unclear. Our studies revealed that the expression of Nav1.6, encoded by SCN8A, was the predominantly upregulated subtype of VGSCs in FTC tissues. Knockdown of Nav1.6 significantly inhibited the proliferation, epithelial-mesenchymal transition and invasiveness of FTC cells. Using gene set enrichment analysis and Kyoto Encyclopedia of Genes and Genomics, SCN8A was predicted to be related to the JAK-STAT signaling pathway. Hence, we targeted the JAK-STAT pathway and demonstrated that Nav1.6 enhanced FTC cell proliferation, epithelial-mesenchymal transition, and invasion by phosphorylating JAK2 to activate STAT3. Furthermore, downregulating the expression of Nav1.6 improve the susceptibility of FTC cells to ubenimex in vitro. These results suggest Nav1.6 accelerates FTC progression through JAK/STAT signaling and may be a potential target for FTC therapy.

Molecular Determinants of Mechanical Itch Sensitization in Chronic Itch

Chronic itch is associated with sensitization of the somatosensory nervous system. Recent studies have identified the neural circuits transmitting acute itch; however, the mechanisms by which itch transforms into a pathological state remain largely unknown. We have previously shown that Aβ low-threshold mechanoreceptors, together with spinal urocortin 3-positive (Ucn3+) excitatory interneurons and neuropeptide Y-positive (NPY+) inhibitory interneurons, form a microcircuit that transmits and gates acute mechanical itch. Here, using whole-cell patch-clamp recordings, we observed increased excitability in spinal Ucn3+ neurons under chronic itch conditions. In contrast to Ucn3+ neurons, the excitability of spinal NPY+ neurons was largely reduced under chronic itch conditions. To explore the molecular mechanisms underlying sensitization of this microcircuit, we examined the mRNA expression levels of voltage-gated ion channels in recorded spinal Ucn3+ and NPY+ neurons by single-cell quantitative real-time PCR (qRT-PCR). We found that the expression levels of Nav1.6 and Cav2.3 channels were increased in spinal Ucn3+ neurons in chronic itch mice, while the expression level of SK3 channels was decreased. By contrast, the expression levels of Nav1.6 and BK channels were decreased in spinal NPY+ neurons in chronic itch mice. To determine the contribution of different ion channels in chronic itch sensitization, we then used a Markov Chain Monte Carlo method to parameterize a large number of biophysically distinct multicompartment models of Ucn3+ and NPY+ neurons. These models included explicit representations of the ion channels that we found to be up- or down-regulated under chronic itch conditions. Our models demonstrated that changes in Nav1.6 conductance are predominantly responsible for the changes in excitability of both Ucn3+ and NPY+ neurons during chronic itch pathogenesis. Furthermore, when simulating microcircuits of our Ucn3+ and NPY+ models, we found that reduced Nav1.6 conductance in NPY+ models played a major role in opening the itch gate under chronic itch conditions. However, changing SK, BK, or R-type calcium channel conductance had negligible effects on the sensitization of this circuit. Therefore, our results suggest that Nav1.6 channels may play an essential role in mechanical itch sensitization. The findings presented here may open a new avenue for developing pharmaceutical strategies to treat chronic itch.

Astrocyte reactivity in a mouse model of SCN8A epileptic encephalopathy

OBJECTIVE

SCN8A epileptic encephalopathy is caused predominantly by de novo gain-of-function mutations in the voltage-gated sodium channel Nav 1.6. The disorder is characterized by early onset of seizures and developmental delay. Most patients with SCN8A epileptic encephalopathy are refractory to current anti-seizure medications. Previous studies determining the mechanisms of this disease have focused on neuronal dysfunction as Nav 1.6 is expressed by neurons and plays a critical role in controlling neuronal excitability. However, glial dysfunction has been implicated in epilepsy and alterations in glial physiology could contribute to the pathology of SCN8A encephalopathy. In the current study, we examined alterations in astrocyte and microglia physiology in the development of seizures in a mouse model of SCN8A epileptic encephalopathy.

METHODS

Using immunohistochemistry, we assessed microglia and astrocyte reactivity before and after the onset of spontaneous seizures. Expression of glutamine synthetase and Nav 1.6, and Kir 4.1 channel currents were assessed in astrocytes in wild-type (WT) mice and mice carrying the N1768D SCN8A mutation (D/+).

RESULTS

Astrocytes in spontaneously seizing D/+ mice become reactive and increase expression of glial fibrillary acidic protein (GFAP), a marker of astrocyte reactivity. These same astrocytes exhibited reduced barium-sensitive Kir 4.1 currents compared to age-matched WT mice and decreased expression of glutamine synthetase. These alterations were only observed in spontaneously seizing mice and not before the onset of seizures. In contrast, microglial morphology remained unchanged before and after the onset of seizures.

SIGNIFICANCE

Astrocytes, but not microglia, become reactive only after the onset of spontaneous seizures in a mouse model of SCN8A encephalopathy. Reactive astrocytes have reduced Kir 4.1-mediated currents, which would impair their ability to buffer potassium. Reduced expression of glutamine synthetase would modulate the availability of neurotransmitters to excitatory and inhibitory neurons. These deficits in potassium and glutamate handling by astrocytes could exacerbate seizures in SCN8A epileptic encephalopathy. Targeting astrocytes may provide a new therapeutic approach to seizure suppression.

Soluble tumor necrosis factor-alpha-induced hyperexcitability contributes to retinal ganglion cell apoptosis by enhancing Nav1.6 in experimental glaucoma

BACKGROUND

Neuroinflammation plays an important role in the pathogenesis of glaucoma. Tumor necrosis factor-alpha (TNF-α) is a major pro-inflammatory cytokine released from activated retinal glial cells in glaucoma. Here, we investigated how TNF-α induces retinal ganglion cell (RGC) hyperexcitability and injury.

METHODS

Whole-cell patch-clamp techniques were performed to explore changes in spontaneous firing and evoked action potentials, and Na⁺ currents in RGCs. Both intravitreal injection of TNF-α and chronic ocular hypertension (COH) models were used. Western blotting, immunofluorescence, quantitative real-time polymerase chain reaction (q-PCR), and terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL) techniques were employed to investigate the molecular mechanisms of TNF-α effects on RGCs.

RESULTS

Intravitreal injection of soluble TNF-α significantly increased the spontaneous firing frequencies of RGCs in retinal slices. When the synaptic transmissions were blocked, more than 90% of RGCs still showed spontaneous firing; both the percentage of cells and firing frequency were higher than the controls. Furthermore, the frequency of evoked action potentials was also higher than the controls. Co-injection of the TNF-α receptor 1 (TNFR1) inhibitor R7050 eliminated the TNF-α-induced effects, suggesting that TNF-α may directly act on RGCs to induce cell hyperexcitability through activating TNFR1. In RGCs acutely isolated from TNF-α-injected retinas, Na⁺ current densities were upregulated. Perfusing TNF-α in RGCs of normal rats mimicked this effect, and the activation curve of Na⁺ currents shifted toward hyperpolarization direction, which was mediated through p38 MAPK and STAT3 signaling pathways. Further analysis revealed that TNF-α selectively upregulated Nav1.6 subtype of Na⁺ currents in RGCs. Similar to observations in retinas of rats with COH, intravitreal injection of TNF-α upregulated the expression of Nav1.6 proteins in both total cell and membrane components, which was reversed by the NF-κB inhibitor BAY 11-7082. Inhibition of TNFR1 blocked TNF-α-induced RGC apoptosis.

CONCLUSIONS

TNF-α/TNFR1 signaling induces RGC hyperexcitability by selectively upregulating Nav1.6 Na⁺ channels, thus contributing to RGC apoptosis in glaucoma.

Toxin Ct1a, from venom of Centruroides tecomanus, modifies the spontaneous firing frequency of neurons in the suprachiasmatic nucleus

The peptide, denominated Ct1a, is a β-toxin of 66 amino acids, isolated from venom of the scorpion, Centruroides tecomanus, collected in Colima, Mexico. This toxin was purified using size exclusion, cationic exchange, and reverse phase chromatography. It is the most abundant toxin, representing 1.7% of the soluble venom. Its molecular mass of 7588.9 Da was determined by mass spectrometry. The amino acid sequence was determined by Edman degradation and confirmed by transcriptomic analysis. Since neurons of the suprachiasmatic nucleus (SCN) maintain a spontaneous firing rate (SFR), we evaluated the physiological effects of toxin Ct1a on these neurons. The SFR exhibited a bimodal concentration-dependent response: 100 nM of Ct1a increased the SFR by 223%, whereas 500 nM and 1000 nM reduced it to 42% and 7%, respectively. Control experiments, consisting of recordings of the SFR during a time similar to that used in Ct1a testing, showed stability throughout the trials. Experiments carried out with denatured Ct1a toxin (500 nM) caused no variation in SFR recordings. Action potentials of SCN neurons, before and after Ct1a (100 nM) showed changes in the time constants of depolarization and repolarization phases, amplitude, and half-time. Finally, recordings of hNav1.6 sodium currents indicated that Ct1a shifts the channel activation to a more negative potential and reduces the amplitude of the peak current. These results all demonstrate that toxin Ct1a affects the SFR of SCN neurons by acting upon sodium channels of sub-type 1.6, implicating them in regulation of the SFR of SCN neurons.

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.

Syndecan-3 contributes to the regulation of the microenvironment at the node of Ranvier following end-to‑side neurorrhaphy: sodium image analysis

Syndecan-3 (SDC3) and Syndecan-4 (SDC4) are distributed throughout the nervous system (NS) and are favourable factors in motor neuron development. They are also essential for regulation of neurite outgrowth in the CNS. However, their roles in the reconstruction of the nodes of Ranvier after peripheral nerve injury (PNI) are still unclear. Present study used an in vivo model of end-to-side neurorrhaphy (ESN) for 1-3 months. The recovery of neuromuscular function was evaluated by grooming test. Expression and co-localization of SDC3, SDC4, and Nav1.6 channel (Nav1.6) at regenerating axons were detected by proximity ligation assay and confocal microscopy after ESN. Time-of-flight secondary ion mass spectrometry was used for imaging ions distribution on tissue. Our data showed that the re-clustering of sodium and Nav1.6 at nodal regions of the regenerating nerve corresponded to the distribution of SDC3 after ESN. Furthermore, the re-establishment of sodium and Nav1.6 correlated with the recovery of muscle power 3 months after ESN. This study suggested syndecans may involve in stabilizing Nav1.6 and further modulate the distribution of sodium at nodal regions after remyelination. The efficiency of sodium re-clustering was improved by the assistance of anionic syndecan, resulting in a better functional repair of PNI.

Three-dimensional neuron-astrocyte construction on matrigel enhances establishment of functional voltage-gated sodium channels

This study aimed to investigate and compare cell growth manners and functional differences of primary cortical neurons cultured on either poly-d-lysine (PDL) and or Matrigel, to delineate the role of extracellular matrix on providing resemblance to in vivo cellular interactions in nervous tissue. Primary cortical neurons, obtained from embryonic day 15 mice pups, seeded either on PDL- or Matrigel-coated culture ware were investigated by DIC/bright field and fluorescence/confocal microscopy for their morphology, 2D and 3D structure, and distribution patterns. Patch clamp, western blot, and RT-PCR studies were performed to investigate neuronal firing thresholds and sodium channel subtypes Nav1.2 and Nav1.6 expression. Cortical neurons cultured on PDL coating possessed a 2D structure composed of a few numbers of branched and tortuous neurites that contacted with each other in one to one manner, however, neurons on Matrigel coating showed a more complicated dimensional network that depicted tight, linear axonal bundles forming a 3D interacted neuron-astrocyte construction. This difference in growth patterns also showed a significant alteration in neuronal firing threshold which was recorded between 80 < Iinj > 120 pA on PDL and 2 < Iinj > 160 pA on Matrigel. Neurons grown up on Matrigel showed increased levels of sodium channel protein expression of Nav1.2 and Nav1.6 compared to neurons on PDL. These results have demonstrated that a 3D interacted neuron-astrocyte construction on Matrigel enhances the development of Nav1.2 and Nav1.6 in vitro and decreases neuronal firing threshold by 40 times compared to conventional PDL, resembling in vivo neuronal networks and hence would be a better in vitro model of adult neurons.

CaMKII enhances voltage-gated sodium channel Nav1.6 activity and neuronal excitability

Nav1.6 is the primary voltage-gated sodium channel isoform expressed in mature axon initial segments and nodes, making it critical for initiation and propagation of neuronal impulses. Thus, Nav1.6 modulation and dysfunction may have profound effects on input-output properties of neurons in normal and pathological conditions. Phosphorylation is a powerful and reversible mechanism regulating ion channel function. Because Nav1.6 and the multifunctional Ca²⁺/CaM-dependent protein kinase II (CaMKII) are independently linked to excitability disorders, we sought to investigate modulation of Nav1.6 function by CaMKII signaling. We show that inhibition of CaMKII, a Ser/Thr protein kinase associated with excitability, synaptic plasticity, and excitability disorders, with the CaMKII-specific peptide inhibitor CN21 reduces transient and persistent currents in Nav1.6-expressing Purkinje neurons by 87%. Using whole-cell voltage clamp of Nav1.6, we show that CaMKII inhibition in ND7/23 and HEK293 cells significantly reduces transient and persistent currents by 72% and produces a 5.8-mV depolarizing shift in the voltage dependence of activation. Immobilized peptide arrays and nanoflow LC-electrospray ionization/MS of Nav1.6 reveal potential sites of CaMKII phosphorylation, specifically Ser-561 and Ser-641/Thr-642 within the first intracellular loop of the channel. Using site-directed mutagenesis to test multiple potential sites of phosphorylation, we show that Ala substitutions of Ser-561 and Ser-641/Thr-642 recapitulate the depolarizing shift in activation and reduction in current density. Computational simulations to model effects of CaMKII inhibition on Nav1.6 function demonstrate dramatic reductions in spontaneous and evoked action potentials in a Purkinje cell model, suggesting that CaMKII modulation of Nav1.6 may be a powerful mechanism to regulate neuronal excitability.

Aberrant expression of Nav1.6 in the cochlear nucleus correlates with salicylate-induced tinnitus in rats

Hyperactivity in cochlear nucleus (CN) is one of the major neural correlates for tinnitus induction, yet the molecular factors that participate in the neuronal hyperexcitability remain unclear. The present study showed that acute and chronic administrations of salicylate were both capable of inducing reversible tinnitus in rats. The number of GAD 65/67-immunoreactive neurons in the AVCN and DCN was decreased, while the number of VGLUT 1/2-immunoreactive neurons in the AVCN and DCN was increased when rats were experiencing tinnitus, providing evidence for excitatory-inhibitory imbalance in CN is correlated with tinnitus. Interestingly, the expression level of Nav1.6, an important subtype of voltage-gated sodium channels was significantly increased in the DCN and AVCN of rats experiencing tinnitus, the up-regulation of Nav1.6 was returned to normal level following the disappearance of tinnitus. Double-labeling experiments revealed that Nav1.6 expression was down-regulated in the GAD 65/67-positive neurons in the DCN and AVCN of rats experiencing tinnitus. Notably, the percentage of co-localization of Nav1.6 and NeuN-labeling fusiform neurons was markedly increased in the DCN during tinnitus. These findings uncover the tinnitus-associated alteration in Nav1.6, a potential key contributor that can lead to hyperexcitability in CN and contribute to salicylate-induced tinnitus.

S-Palmitoylation of the sodium channel Nav1.6 regulates its activity and neuronal excitability

S-Palmitoylation is a reversible post-translational lipid modification that dynamically regulates protein functions. Voltage-gated sodium channels are subjected to S-palmitoylation and exhibit altered functions in different S-palmitoylation states. Our aim was to investigate whether and how S-palmitoylation regulates Nav1.6 channel function and to identify S-palmitoylation sites that can potentially be pharmacologically targeted. Acyl-biotin exchange assay showed that Nav1.6 is modified by S-palmitoylation in the mouse brain and in a Nav1.6 stable HEK 293 cell line. Using whole-cell voltage clamp, we discovered that enhancing S-palmitoylation with palmitic acid increases Nav1.6 current, whereas blocking S-palmitoylation with 2-bromopalmitate reduces Nav1.6 current and shifts the steady-state inactivation in the hyperpolarizing direction. Three S-palmitoylation sites (Cys¹¹⁶⁹, Cys¹¹⁷⁰, and Cys¹⁹⁷⁸) were identified. These sites differentially modulate distinct Nav1.6 properties. Interestingly, Cys¹⁹⁷⁸ is exclusive to Nav1.6 among all Nav isoforms and is evolutionally conserved in Nav1.6 among most species. Cys¹⁹⁷⁸ S-palmitoylation regulates current amplitude uniquely in Nav1.6. Furthermore, we showed that eliminating S-palmitoylation at specific sites alters Nav1.6-mediated excitability in dorsal root ganglion neurons. Therefore, our study reveals S-palmitoylation as a potential isoform-specific mechanism to modulate Nav activity and neuronal excitability in physiological and diseased conditions.

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.

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.

The MAP1B Binding Domain of Nav1.6 Is Required for Stable Expression at the Axon Initial Segment

Na_v1.6 (SCN8A) is a major voltage-gated sodium channel in the mammalian CNS, and is highly concentrated at the axon initial segment (AIS). As previously demonstrated, the microtubule associated protein MAP1B binds the cytoplasmic N terminus of Na_v1.6, and this interaction is disrupted by the mutation p.VAVP(77-80)AAAA. We now demonstrate that this mutation results in WT expression levels on the somatic surface but reduced surface expression at the AIS of cultured rat embryonic hippocampal neurons from both sexes. The mutation of the MAP1B binding domain did not impair vesicular trafficking and preferential delivery of Na_v1.6 to the AIS; nor was the diffusion of AIS inserted channels altered relative to WT. However, the reduced AIS surface expression of the MAP1B mutant was restored to WT levels by inhibiting endocytosis with Dynasore, indicating that compartment-specific endocytosis was responsible for the lack of AIS accumulation. Interestingly, the lack of AIS targeting resulted in an elevated percentage of persistent current, suggesting that this late current originates predominantly in the soma. No differences in the voltage dependence of activation or inactivation were detected in the MAP1B binding mutant relative to WT channel. We hypothesize that MAP1B binding to the WT Na_v1.6 masks an endocytic motif, thus allowing long-term stability on the AIS surface. This work identifies a critical and important new role for MAP1B in the regulation of neuronal excitability and adds to our understanding of AIS maintenance and plasticity, in addition to identifying new target residues for pathogenic mutations of SCN8A SIGNIFICANCE STATEMENT Na_v1.6 is a major voltage-gated sodium channel in human brain, where it regulates neuronal activity due to its localization at the axon initial segment (AIS). Na_v1.6 mutations cause epilepsy, intellectual disability, and movement disorders. In the present work, we show that loss of interaction with MAP1B within the Na_v1.6 N terminus reduces the steady-state abundance of Na_v1.6 at the AIS. The effect is due to increased Na_v1.6 endocytosis at this neuronal compartment rather than a failure of forward trafficking to the AIS. This work confirms a new biological role of MAP1B in the regulation of sodium channel localization and will contribute to future analysis of patient mutations in the cytoplasmic N terminus of Na_v1.6.

TNF-α/STAT3 pathway epigenetically upregulates Nav1.6 expression in DRG and contributes to neuropathic pain induced by L5-VRT

Background

Studies showed that upregulation of Nav1.6 increased the neuronal excitability and participated in neuropathic pain in the dorsal root ganglion (DRG). However, the molecular mechanisms underlying Nav1.6 upregulation were not reported yet.

Methods

The paw withdrawal threshold was measured in the rodents following lumbar 5 ventral root transection (L5-VRT). Then qPCR, western blotting, immunoprecipitation, immunohistochemistry, and chromatin immunoprecipitation assays were performed to explore the molecular mechanisms in vivo and in vitro.

Results

We found that the levels of Nav1.6 and phosphorylated STAT3 were significantly increased in DRG neurons following L5-VRT, and TNF-α incubation also upregulated the Nav1.6 expression in cultured DRG neurons. Furthermore, immunoprecipitation and chromatin immunoprecipitation assays demonstrated that L5-VRT increased the binding of STAT3 to the Scn8a (encoding Nav1.6) promoter and the interaction between STAT3 and p300, which contributed to the enhanced transcription of Scn8a by increasing histone H4 acetylation in Scn8a promoter in DRG. Importantly, intraperitoneal injection of the TNF-α inhibitor thalidomide reduced the phosphorylation of STAT3 and decreased the recruitment of STAT3 and histone H4 hyperacetylation in the Scn8a promoter, thus subsequently attenuating Nav1.6 upregulation in DRG neurons and mechanical allodynia induced by L5-VRT.

Conclusion

These results suggested a new mechanism for Nav1.6 upregulation involving TNF-α/STAT3 pathway activation and subsequent STAT3-mediated histone H4 hyperacetylation in the Scn8a promoter region in DRG, which contributed to L5-VRT-induced neuropathic pain.

Upregulation of Nav1.6 expression in the rostral ventrolateral medulla of stress-induced hypertensive rats

The rostral ventrolateral medulla (RVLM) plays a key role in mediating the development of stress-induced hypertension (SIH) by excitation and/or inhibition of sympathetic preganglionic neurons. The voltage-gated sodium channel Nav1.6 has been found to contribute to neuronal hyperexcitability. To examine the expression of Nav1.6 in the RVLM during SIH, a rat model was established by administering electric foot-shocks and noises. We found that Nav1.6 protein expression in the RVLM of SIH rats was higher than that of control rats, peaking at the tenth day of stress. Furthermore, we observed changes in blood pressure correlating with days of stress, with systolic blood pressure (SBP) found to reach a similarly timed peak at the tenth day of stress. Percentages of cells exhibiting colocalization of Nav1.6 with NeuN, a molecular marker of neurons, indicated a strong correlation between upregulation of Nav1.6 expression in NeuN-positive cells and SBP. The level of RSNA was significantly increased after 10 days of stress induction than control group. Compared with the SIHR, knockdown of Nav1.6 in RVLM of the SIHR decreased the level of SBP, heart rate (HR) and renal sympathetic nerve activity (RSNA). These results suggest that upregulated Nav1.6 expression within neurons in the RVLM of SIH rats may contribute to overactivation of the sympathetic system in response to SIH development.

Partial loss-of-function of sodium channel SCN8A in familial isolated myoclonus

Variants in the neuronal sodium channel gene SCN8A have been implicated in several neurological disorders. Early infantile epileptic encephalopathy type 13 results from de novo gain‐of‐function mutations that alter the biophysical properties of the channel. Complete loss‐of‐function variants of SCN8A have been identified in cases of isolated intellectual disability. We now report a novel heterozygous SCN8A variant, p.Pro1719Arg, in a small pedigree with five family members affected with autosomal dominant upper limb isolated myoclonus without seizures or cognitive impairment. Functional analysis of the p.Pro1719Arg variant in transfected neuron‐derived cells demonstrated greatly reduced Na_v1.6 channel activity without altered gating properties. Hypomorphic alleles of Scn8a in the mouse are known to result in similar movement disorders. This study expands the phenotypic and functional spectrum of SCN8A variants to include inherited nonepileptic isolated myoclonus. SCN8A can be considered as a candidate gene for isolated movement disorders without seizures.

Environmental Enrichment and Social Isolation Mediate Neuroplasticity of Medium Spiny Neurons through the GSK3 Pathway

Resilience and vulnerability to neuropsychiatric disorders are linked to molecular changes underlying excitability that are still poorly understood. Here, we identify glycogen-synthase kinase 3β (GSK3β) and voltage-gated Na+ channel Nav1.6 as regulators of neuroplasticity induced by environmentally enriched (EC) or isolated (IC) conditions-models for resilience and vulnerability. Transcriptomic studies in the nucleus accumbens from EC and IC rats predicted low levels of GSK3β and SCN8A mRNA as a protective phenotype associated with reduced excitability in medium spiny neurons (MSNs). In vivo genetic manipulations demonstrate that GSK3β and Nav1.6 are molecular determinants of MSN excitability and that silencing of GSK3β prevents maladaptive plasticity of IC MSNs. In vitro studies reveal direct interaction of GSK3β with Nav1.6 and phosphorylation at Nav1.6T1936 by GSK3β. A GSK3β-Nav1.6T1936 competing peptide reduces MSNs excitability in IC, but not EC rats. These results identify GSK3β regulation of Nav1.6 as a biosignature of MSNs maladaptive plasticity.

Selective Modulation of Axonal Sodium Channel Subtypes by 5-HT1A Receptor in Cortical Pyramidal Neuron

Serotonergic innervation of the prefrontal cortex (PFC) modulates neuronal activity and PFC functions. However, the cellular mechanism for serotonergic modulation of neuronal excitability remains unclear. We performed patch-clamp recording at the axon of layer-5 pyramidal neurons in rodent PFC slices. We found surprisingly that the activation of 5-HT1A receptors selectively inhibits Na+ currents obtained at the axon initial segment (AIS) but not those at the axon trunk. In addition, Na+ channel subtype NaV1.2 but not NaV1.6 at the AIS is selectively modulated by 5-HT1A receptors. Further experiments revealed that the inhibitory effect is attributable to a depolarizing shift of the activation curve and a facilitation of slow inactivation of AIS Na+ currents. Consistently, dual somatic and axonal recording and simulation results demonstrate that the activation of 5-HT1A receptors could decrease the success rate of action potential (AP) backpropagation toward the somatodendritic compartments, enhancing the segregation of axonal and dendritic activities. Together, our results reveal a selective modulation of NaV1.2 distributed at the proximal AIS region and AP backpropagation by 5-HT1A receptors, suggesting a potential mechanism for serotonergic regulation of functional polarization in the dendro-axonal axis, synaptic plasticity and PFC functions.

Neurotransmitters and Sodium Channelopathies; Possible Link?

Investigators from the University of British Columbia, Great Ormond Street Hospital for Children, and the National Hospital reported their findings on neurotransmitter deficiencies in two patients with mutations in voltage-gated sodium genes (SCN2A and SCN8A) discovered by whole exome sequencing.

Local knockdown of Nav1.6 relieves pain behaviors induced by BmK I

Voltage-gated sodium channels (VGSCs) in peripheral nociceptive sensory neurons are critical to transmit pain signals. BmK I purified from the venom of scorpion Buthus martensi Karsch (BmK) has been demonstrated to be the primary contributor of envenomation-associated pain. However, the role of distinct VGSCs such as Nav1.6 in the induction and maintenance of pain behaviors induced by BmK I was ambiguous. Herein, using molecular and behavioral approaches we investigated the mRNA and protein expression profiles of Nav1.6 in rat DRG after intraplantar injection of BmK I and tested the pain behaviors after knockdown of Nav1.6 in BmK I-treated rats. It was shown that during induction and maintenance of pain responses induced by BmK I, the expression of Nav1.6 in DRG was found to be significantly increased at both mRNA and protein levels. The percentage of co-localization of Nav1.6 and Isolectin B4, a molecular marker of small diameter non-peptidergic DRG neurons, was increased at the maintenance phase of pain responses. Furthermore, spontaneous pain and mechanical allodynia, but not thermal hyperalgesia induced by BmK I, were significantly alleviated after knockdown of Nav1.6. These data strongly suggest that Nav1.6 plays an indispensable role in the peripheral pain hypersensitivity induced by BmK I.

mTOR-mediated Na+/Ca2+ exchange affects cell proliferation and metastasis of melanoma cells

Melanoma is a common malignant tumor, which is associated with high mortality rate. The multiple-drug resistance of tumor cells often results in failure of chemotherapy. The aim of our study is to investigate the expression of Nav 1.6 in human melanoma cells and human epidermal melanocytes. Additionally, the effect of Na+channels on Ca+ current and mTOR activity in melanoma cells were also analyzed. The protein expression levels of Nav1.6 in human melanocyte PIG1, WM266 and WM115 cells were investigated by western blot. After treatment of Na⁺ channel inhibitor Tetroadotoxin (TTX) or mTOR inhibitor rapamycin (RAPA), the electrophysiological activity (Na+ current and Ca2+ current) in WM266 and WM115 cells was detected by patch clamp technique. The expression of mTORC1 phosphorylates S6 kinase (p-S6), cell invasion and migration, cell proliferation and cell apoptosis were also performed. Results shown that Nav 1.6 was overexpressed in WM266 and WM115 cells, and the inhibition of Na⁺ channel by TTX reduced Na⁺ current. Both TTX and RAPA suppressed Ca²⁺ current and the expression of p-S6, thus inducing Na⁺ channel which activates the mTOR-Ca2+ signaling pathway. Both TTX and RAPA suppressed cell invasion, migration and proliferation, and promoted cell apoptosis of WM266 cells. Thus, the Nav1.6 sodium channel promotes cell proliferation and invasion through mTOR-mediated Na+/Ca2+ exchange in melanoma. The observations will provide a new perspective for understanding the malignant biological behavior of melanoma cells, and potentially provide a new drug target.

Alterations of Caspr2 and Nav1.6 on myelinated axon damage in a rat model of chronic cerebral hypoperfusion

Myelinated axons require the correct localization of key proteins that are essential for nerve conduction and cognitive function. Little is known regarding the altered expression of contactin-associated protein 2 (Caspr2) at the juxtaparanodal regions and Nav1.6 at the node of Ranvier in response to chronic cerebral hypoperfusion (CCH). The aim of the present study was to examine the alterations in the key protein of myelinated axons and the potential mechanisms that may follow CCH. We established a rat model of CCH by controllable partial narrowing of bilateral common carotid arteries. Then, we detected cerebral blood flow (CBF) after surgery. We also evaluated motor-evoked potentials (MEPs), assessed the Morris water maze test, analyzed Caspr2 expression through immunohistochemistry and Nav1.6 protein expression through western blot analysis at 2, 4 and 12 weeks. The results revealed that the mean CBF value was significantly decreased to 33.90±5.48%. The MEP latencies and the escaping latencies were significantly prolonged. There was also an elongation of the first time passing of the hidden platform with a reduction of crossing platform times in spatial probing. Furthermore, the Caspr2 immunoreactivity demonstrated that the Caspr2 level was significantly downregulated with abnormal locations in the corpus callosum. The western blot analysis of Nav1.6 protein revealed that the level was reduced significantly over time. The results demonstrate that CCH leads to central conductive function loss, cognitive function damage and alterations in the key protein of myelinated axons, which may provide a molecular basis and key link for white matter damage.

Transient upregulation of Nav1.6 expression in the genu of corpus callosum following middle cerebral artery occlusion in the rats

Focal ischemic stroke can lead to brain damage and cause human disability and death. Increased excitatory transmission and reduced neuronal inhibition are important pathological alterations in the cerebral ischemia, which can induce abnormal brain excitability. Nav1.6 is a key determinant of neuronal excitability in the nervous system. Here we investigate the expression of Nav1.6 at protein and mRNA levels in the rats subjected to middle cerebral artery occlusion (MCAO). Nav1.6 expression at mRNA levels in the ischemic and contralateral hemispheres of MCAO rats were persistently decreased at 6h, 12h and 24h after reperfusion compared to the sham-operated rats. However, a prominent, dynamic increase of Nav1.6 immunoreactivity in reactive astrocytes was observed in the genu of corpus callosum (GCC) of MCAO rats in the acute phase, reaching the peak at 6h after reperfusion, rapidly dropping at 12h and 24h after reperfusion. Furthermore, the upregulation of Nav1.6 expression was strongly correlated with the severity of reactive astrogliosis. Collectively, these findings suggest that this upregulated astrocytic sodium channel expression in the GCC of MCAO rats may contribute to the functional roles of reactive astrocytes in response to brain ischemia.

A gain-of-function mutation in Nav1.6 in a case of trigeminal neuralgia

Idiopathic trigeminal neuralgia (TN) is a debilitating pain disorder characterized by episodic unilateral facial pain along the territory of branches of the trigeminal nerve. Human painful disorders, but not TN, have been linked to gain-of-function mutations in peripheral voltage-gated sodium channels (Na_V1.7, Na_V1.8 and Na_V1.9). Gain-of-function mutations in Na_V1.6, which is expressed in myelinated and unmyelinated CNS and peripheral nervous system neurons and supports neuronal high-frequency firing, have been linked to epilepsy but not to pain. Here, we describe an individual who presented with evoked and spontaneous paroxysmal unilateral facial pain, and carried a diagnosis of TN. Magnetic resonance imaging showed unilateral neurovascular compression, consistent with pain in areas innervated by the second branch of the trigeminal nerve. Genetic analysis as part of a phase 2 clinical study in patients with TN conducted by Convergence Pharmaceuticals Ltd revealed a previously undescribed de novo missense mutation in Na_V1.6 (c.A406G; p.Met136Val). Whole-cell voltage-clamp recordings show that the Met136Val mutation significantly increases peak current density (1.5-fold) and resurgent current (1.6-fold) without altering gating properties. Current-clamp studies in trigeminal ganglion (TRG) neurons showed that Met136Val increased the fraction of high-firing neurons, lowered the current threshold and increased the frequency of evoked action potentials in response to graded stimuli. Our results demonstrate a novel Na_V1.6 mutation in TN, and show that this mutation potentiates transient and resurgent sodium currents and leads to increased excitability in TRG neurons. We suggest that this gain-of-function Na_V1.6 mutation may exacerbate the pathophysiology of vascular compression and contribute to TN.

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.

Identification of Amino Acid Residues in Fibroblast Growth Factor 14 (FGF14) Required for Structure-Function Interactions with Voltage-gated Sodium Channel Nav1.6

The voltage-gated Na(+) (Nav) channel provides the basis for electrical excitability in the brain. This channel is regulated by a number of accessory proteins including fibroblast growth factor 14 (FGF14), a member of the intracellular FGF family. In addition to forming homodimers, FGF14 binds directly to the Nav1.6 channel C-tail, regulating channel gating and expression, properties that are required for intrinsic excitability in neurons. Seeking amino acid residues with unique roles at the protein-protein interaction interface (PPI) of FGF14·Nav1.6, we engineered model-guided mutations of FGF14 and validated their impact on the FGF14·Nav1.6 complex and the FGF14:FGF14 dimer formation using a luciferase assay. Divergence was found in the β-9 sheet of FGF14 where an alanine (Ala) mutation of Val-160 impaired binding to Nav1.6 but had no effect on FGF14:FGF14 dimer formation. Additional analysis revealed also a key role of residues Lys-74/Ile-76 at the N-terminal of FGF14 in the FGF14·Nav1.6 complex and FGF14:FGF14 dimer formation. Using whole-cell patch clamp electrophysiology, we demonstrated that either the FGF14(V160A) or the FGF14(K74A/I76A) mutation was sufficient to abolish the FGF14-dependent regulation of peak transient Na(+) currents and the voltage-dependent activation and steady-state inactivation of Nav1.6; but only V160A with a concomitant alanine mutation at Tyr-158 could impede FGF14-dependent modulation of the channel fast inactivation. Intrinsic fluorescence spectroscopy of purified proteins confirmed a stronger binding reduction of FGF14(V160A) to the Nav1.6 C-tail compared with FGF14(K74A/I76A) Altogether these studies indicate that the β-9 sheet and the N terminus of FGF14 are well positioned targets for drug development of PPI-based allosteric modulators of Nav channels.

Improved Methods for Fluorescence Microscopy Detection of Macromolecules at the Axon Initial Segment

The axonal initial segment (AIS) is the subcellular compartment required for initiation of the action potential in neurons. Scaffolding and regulatory proteins at the AIS cluster with ion channels ensuring the integrity of electrical signaling. Interference with the configuration of this protein network can lead to profound effects on neuronal polarity, excitability, cell-to-cell connectivity and brain circuit plasticity. As such, the ability to visualize AIS components with precision provides an invaluable opportunity for parsing out key molecular determinants of neuronal function. Fluorescence-based immunolabeling is a sensitive method for morphological and molecular characterization of fine structures in neurons. Yet, even when combined with confocal microscopy, detection of AIS elements with immunofluorescence has been limited by the loss of antigenicity caused by fixative materials. This technical barrier has posed significant limitations in detecting AIS components alone or in combination with other markers. Here, we designed improved protocols targeted to confocal immunofluorescence detection of the AIS marker fibroblast growth factor 14 (FGF14) in combination with the cytoskeletal-associated protein Ankyrin-G, the scaffolding protein βIV-spectrin, voltage-gated Na⁺ (Nav) channels (especially the Nav1.6 isoform) and critical cell type-specific neuronal markers such as parvalbumin, calbindin, and NeuN in the mouse brain. Notably, we demonstrate that intracardiac perfusion of animals with a commercially available solution containing 1% formaldehyde and 0.5% methanol, followed by brief fixation with cold acetone is an optimal and sensitive protocol for FGF14 and other AIS marker detection that guarantees excellent tissue integrity. With variations in the procedure, we also significantly improved the detection of Nav1.6, a Nav isoform known for its fixative-sensitivity. Overall, this study provides an ensemble of immunohistochemical recipes that permit excellent staining of otherwise invisible molecules within well-preserved tissue architecture. While improving the specific investigation of AIS physiology and cell biology, our thorough study can also serve as a roadmap for optimizing immunodetection of other fixative-sensitive proteins expanding the repertoire of enabling methods for brain studies.

Myelin loss and axonal ion channel adaptations associated with gray matter neuronal hyperexcitability

Myelination and voltage-gated ion channel clustering at the nodes of Ranvier are essential for the rapid saltatory conduction of action potentials. Whether myelination influences the structural organization of the axon initial segment (AIS) and action potential initiation is poorly understood. Using the cuprizone mouse model, we combined electrophysiological recordings with immunofluorescence of the voltage-gated Nav1.6 and Kv7.3 subunits and anchoring proteins to analyze the functional and structural properties of single demyelinated neocortical L5 axons. Whole-cell recordings demonstrated that neurons with demyelinated axons were intrinsically more excitable, characterized by increased spontaneous suprathreshold depolarizations as well as antidromically propagating action potentials ectopically generated in distal parts of the axon. Immunofluorescence examination of demyelinated axons showed that βIV-spectrin, Nav1.6, and the Kv7.3 channels in nodes of Ranvier either dissolved or extended into the paranodal domains. In contrast, while the AIS in demyelinated axons started more closely to the soma, ankyrin G, βIV-spectrin, and the ion channel expression were maintained. Structure-function analysis and computational modeling, constrained by the AIS location and realistic dendritic and axonal morphologies, confirmed that a more proximal onset of the AIS slightly reduced the efficacy of action potential generation, suggesting a compensatory role. These results suggest that oligodendroglial myelination is not only important for maximizing conduction velocity, but also for limiting hyperexcitability of pyramidal neurons.

Revealing the Function and the Structural Model of Ts4: Insights into the "Non-Toxic" Toxin from Tityus serrulatus Venom

The toxin, previously described as a "non-toxic" toxin, was isolated from the scorpion venom of Tityus serrulatus (Ts), responsible for the most severe and the highest number of accidents in Brazil. In this study, the subtype specificity and selectivity of Ts4 was investigated using six mammalian Nav channels (Nav1.2→Nav1.6 and Nav1.8) and two insect Nav channels (DmNav1 and BgNav). The electrophysiological assays showed that Ts4 specifically inhibited the fast inactivation of Nav1.6 channels, the most abundant sodium channel expressed in the adult central nervous system, and can no longer be classified as a "non-toxic peptide". Based on the results, we could classify the Ts4 as a classical α-toxin. The Ts4 3D-structural model was built based on the solved X-ray Ts1 3D-structure, the major toxin from Ts venom with which it shares high sequence identity (65.57%). The Ts4 model revealed a flattened triangular shape constituted by three-stranded antiparallel β-sheet and one α-helix stabilized by four disulfide bonds. The absence of a Lys in the first amino acid residue of the N-terminal of Ts4 is probably the main responsible for its low toxicity. Other key amino acid residues important to the toxicity of α- and β-toxins are discussed here.

Α- and β-subunit composition of voltage-gated sodium channels investigated with μ-conotoxins and the recently discovered μO§-conotoxin GVIIJ

We investigated the identities of the isoforms of the α (NaV1)- and β (NaVβ)-subunits of voltage-gated sodium channels, including those responsible for action potentials in rodent sciatic nerves. To examine α-subunits, we used seven μ-conotoxins, which target site 1 of the channel. With the use of exogenously expressed channels, we show that two of the μ-conotoxins, μ-BuIIIB and μ-SxIIIA, are 50-fold more potent in blocking NaV1.6 from mouse than that from rat. Furthermore, we observed that μ-BuIIIB and μ-SxIIIA are potent blockers of large, myelinated A-fiber compound action potentials (A-CAPs) [but not small, unmyelinated C-fiber CAPs (C-CAPs)] in the sciatic nerve of the mouse (unlike A-CAPs of the rat, previously shown to be insensitive to these toxins). To investigate β-subunits, we used two synthetic derivatives of the recently discovered μO§-conotoxin GVIIJ that define site 8 of the channel, as previously characterized with cloned rat NaV1- and NaVβ-subunits expressed in Xenopus laevis oocytes, where it was shown that μO§-GVIIJ is a potent inhibitor of several NaV1-isoforms and that coexpression of NaVβ2 or -β4 (but not NaVβ1 or -β3) totally protects against block by μO§-GVIIJ. We report here the effects of μO§-GVIIJ on 1) sodium currents of mouse NaV1.6 coexpressed with various combinations of NaVβ-subunits in oocytes; 2) A- and C-CAPs of mouse and rat sciatic nerves; and 3) sodium currents of small and large neurons dissociated from rat dorsal root ganglia. Our overall results lead us to conclude that action potentials in A-fibers of the rodent sciatic nerve are mediated primarily by NaV1.6 associated with NaVβ2 or NaVβ4.

Characterization of a de novo SCN8A mutation in a patient with epileptic encephalopathy

OBJECTIVE

Recently, de novo SCN8A missense mutations have been identified as a rare dominant cause of epileptic encephalopathies (EIEE13). Functional studies on the first described case demonstrated gain-of-function effects of the mutation. We describe a novel de novo mutation of SCN8A in a patient with epileptic encephalopathy, and functional characterization of the mutant protein.

DESIGN

Whole exome sequencing was used to discover the variant. We generated a mutant cDNA, transfected HEK293 cells, and performed Western blotting to assess protein stability. To study channel functional properties, patch-clamp experiments were carried out in transfected neuronal ND7/23 cells.

RESULTS

The proband exhibited seizure onset at 6 months of age, diffuse brain atrophy, and more profound developmental impairment than the original case. The mutation p.Arg233Gly in the voltage sensing transmembrane segment D1S4 was present in the proband and absent in both parents. This mutation results in a temperature-sensitive reduction in protein expression as well as reduced sodium current amplitude and density and a relative increased response to a slow ramp stimulus, though this did not result in an absolute increased current at physiological temperatures.

CONCLUSION

The new de novo SCN8A mutation is clearly deleterious, resulting in an unstable protein with reduced channel activity. This differs from the gain-of-function attributes of the first SCN8A mutation in epileptic encephalopathy, pointing to heterogeneity of mechanisms. Since Nav1.6 is expressed in both excitatory and inhibitory neurons, a differential effect of a loss-of-function of Nav1.6 Arg223Gly on inhibitory interneurons may underlie the epilepsy phenotype in this patient.

Analgesic effects of clinically used compounds in novel mouse models of polyneuropathy induced by oxaliplatin and cisplatin

BACKGROUND

Peripheral neuropathy is the major dose-limiting side effect of cisplatin and oxaliplatin, and there are currently no effective treatments available. The aim of this study was to assess the pharmacological mechanisms underlying chemotherapy-induced neuropathy in novel animal models based on intraplantar administration of cisplatin and oxaliplatin and to systematically evaluate the analgesic efficacy of a range of therapeutics.

METHODS

Neuropathy was induced by a single intraplantar injection of cisplatin or oxaliplatin in C57BL/6J mice and assessed by quantification of mechanical and thermal allodynia. The pharmacological basis of cisplatin-induced neuropathy was characterized using a range of selective pharmacological inhibitors. The analgesic effects of phenytoin, amitriptyline, oxcarbazepine, mexiletine, topiramate, retigabine, gabapentin, fentanyl, and Ca(2+/)Mg(2+) were assessed 24 hours after induction of neuropathy.

RESULTS

Intraplantar administration of cisplatin led to the development of mechanical allodynia, mediated through Nav1.6-expressing sensory neurons. Unlike intraplantar injection of oxaliplatin, cold allodynia was not observed with cisplatin, consistent with clinical observations. Surprisingly, only fentanyl was effective at alleviating cisplatin-induced mechanical allodynia despite a lack of efficacy in oxaliplatin-induced cold allodynia. Conversely, lamotrigine, phenytoin, retigabine, and gabapentin were effective at reversing oxaliplatin-induced cold allodynia but had no effect on cisplatin-induced mechanical allodynia. Oxcarbazepine, amitriptyline, mexiletine, and topiramate lacked efficacy in both models of acute chemotherapy-induced neuropathy.

CONCLUSION

This study established a novel animal model of cisplatin-induced mechanical allodynia consistent with the A-fiber neuropathy seen clinically. Systematic assessment of a range of therapeutics identified several candidates that warrant further clinical investigation.

The Nav channel bench series: Plasmid preparation

Research involving recombinant voltage-gated sodium (Nav) channels has unique challenges. Multiple factors contribute, but undoubtedly at the top of the list is these channels' DNA instability. Once introduced into bacterial hosts, Nav channel plasmid DNA will almost invariably emerge mutagenized and unusable, unless special conditions are adopted. This is particularly true for Nav1.1 (gene name SCN1A), Nav1.2 (SCN2A), and Nav1.6 (SCN8A), but less so for Nav1.4 (SCN4A) and Nav1.5 (SCN5A) while other Nav channel isoforms such as Nav1.7 (SCN9A) lie in between. The following recommendations for Nav plasmid DNA amplification and preparation address this problem. Three points are essential:•Bacterial propagation using Stbl2 cells at or below 30 °C.•Bias toward slow-growing, small bacterial colonies.•Comprehensive sequencing of the entire Nav channel coding region.

Modeling human epilepsy by TALEN targeting of mouse sodium channel Scn8a

To evaluate the efficiency of TALEN technology for introducing mutations into the mouse genome we targeted Scn8a, a member of a multigene family with nine closely related paralogs. Our goal was to generate a model of early onset epileptic encephalopathy by introduction of the Scn8a missense mutation p.Asn1768Asp. We used a pair of TALENs that were highly active in transfected cells. The targeting template for homologous recombination contained a 4 kb genomic fragment. Microinjection of TALENs with the targeting construct into the pronucleus of 350 fertilized mouse eggs generated 67 live-born potential founders, of which 5 were heterozygous for the pathogenic mutation, a yield of 7% correctly targeted mice. Twenty-four mice carried one or two Scn8a indels, including 12 frameshift mutations and the novel amino acid deletion p.Asn1759del. Nine off-site mutations in the paralogs sodium channel genes Scn5a and Scn4a were identified. The data demonstrate the feasibility and efficiency of targeting members of multigene families using TALENs. The Scn8a(tm) (1768DMm) mouse model will be useful for investigation of the pathogenesis and therapy of early onset seizure disorders.

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.