Tetrodotoxin-resistant voltage-gated sodium channel Nav 1.8 constitutively interacts with ankyrin G

Detrimental proarrhythmogenic interaction of Ca2+/calmodulin-dependent protein kinase II and NaV1.8 in heart failure

An interplay between Ca²⁺/calmodulin-dependent protein kinase IIδc (CaMKIIδc) and late Na⁺ current (I_NaL) is known to induce arrhythmias in the failing heart. Here, we elucidate the role of the sodium channel isoform Na_V1.8 for CaMKIIδc-dependent proarrhythmia. In a CRISPR-Cas9-generated human iPSC-cardiomyocyte homozygous knock-out of Na_V1.8, we demonstrate that Na_V1.8 contributes to I_NaL formation. In addition, we reveal a direct interaction between Na_V1.8 and CaMKIIδc in cardiomyocytes isolated from patients with heart failure (HF). Using specific blockers of Na_V1.8 and CaMKIIδc, we show that Na_V1.8-driven I_NaL is CaMKIIδc-dependent and that Na_V1.8-inhibtion reduces diastolic SR-Ca²⁺ leak in human failing cardiomyocytes. Moreover, increased mortality of CaMKIIδc-overexpressing HF mice is reduced when a Na_V1.8 knock-out is introduced. Cellular and in vivo experiments reveal reduced ventricular arrhythmias without changes in HF progression. Our work therefore identifies a proarrhythmic CaMKIIδc downstream target which may constitute a prognostic and antiarrhythmic strategy.

The Role of Voltage-Gated Sodium Channels in Pain Signaling

Acute pain signaling has a key protective role and is highly evolutionarily conserved. Chronic pain, however, is maladaptive, occurring as a consequence of injury and disease, and is associated with sensitization of the somatosensory nervous system. Primary sensory neurons are involved in both of these processes, and the recent advances in understanding sensory transduction and human genetics are the focus of this review. Voltage-gated sodium channels (VGSCs) are important determinants of sensory neuron excitability: they are essential for the initial transduction of sensory stimuli, the electrogenesis of the action potential, and neurotransmitter release from sensory neuron terminals. Nav1.1, Nav1.6, Nav1.7, Nav1.8, and Nav1.9 are all expressed by adult sensory neurons. The biophysical characteristics of these channels, as well as their unique expression patterns within subtypes of sensory neurons, define their functional role in pain signaling. Changes in the expression of VGSCs, as well as posttranslational modifications, contribute to the sensitization of sensory neurons in chronic pain states. Furthermore, gene variants in Nav1.7, Nav1.8, and Nav1.9 have now been linked to human Mendelian pain disorders and more recently to common pain disorders such as small-fiber neuropathy. Chronic pain affects one in five of the general population. Given the poor efficacy of current analgesics, the selective expression of particular VGSCs in sensory neurons makes these attractive targets for drug discovery. The increasing availability of gene sequencing, combined with structural modeling and electrophysiological analysis of gene variants, also provides the opportunity to better target existing therapies in a personalized manner.

Nonmuscle myosin II isoforms interact with sodium channel alpha subunits

Sodium channels play pivotal roles in health and diseases due to their ability to control cellular excitability. The pore-forming α-subunits (sodium channel alpha subunits) of the voltage-sensitive channels (i.e., Na_v1.1–1.9) and the nonvoltage-dependent channel (i.e., Na_x) share a common structural motif and selectivity for sodium ions. We hypothesized that the actin-based nonmuscle myosin II motor proteins, nonmuscle myosin heavy chain-IIA/myh9, and nonmuscle myosin heavy chain-IIB/myh10 might interact with sodium channel alpha subunits to play an important role in their transport, trafficking, and/or function. Immunochemical and electrophysiological assays were conducted using rodent nervous (brain and dorsal root ganglia) tissues and ND7/23 cells coexpressing Na_v subunits and recombinant myosins. Immunoprecipitation of myh9 and myh10 from rodent brain tissues led to the coimmunoprecipitation of Na_x, Na_v1.2, and Na_v1.3 subunits, but not Na_v1.1 and Na_v1.6 subunits, expressed there. Similarly, immunoprecipitation of myh9 and myh10 from rodent dorsal root ganglia tissues led to the coimmunoprecipitation of Na_v1.7 and Na_v1.8 subunits, but not Na_v1.9 subunits, expressed there. The functional implication of one of these interactions was assessed by coexpressing myh10 along with Na_v1.8 subunits in ND7/23 cells. Myh10 overexpression led to three-fold increase (P < 0.01) in the current density of Na_v1.8 channels expressed in ND7/23 cells. Myh10 coexpression also hyperpolarized voltage-dependent activation and steady-state fast inactivation of Na_v1.8 channels. In addition, coexpression of myh10 reduced (P < 0.01) the offset of fast inactivation and the amplitude of the ramp currents of Na_v1.8 channels. These results indicate that nonmuscle myosin heavy chain-IIs interact with sodium channel alpha subunits subunits in an isoform-dependent manner and influence their functional properties.

Ankyrin G Membrane Partners Drive the Establishment and Maintenance of the Axon Initial Segment

The axon initial segment (AIS) is a highly specialized neuronal compartment that plays a key role in neuronal development and excitability. It concentrates multiple membrane proteins such as ion channels and cell adhesion molecules (CAMs) that are recruited to the AIS by the scaffold protein ankyrin G (ankG). The crucial function of ankG in the anchoring of AIS membrane components is well established, but a reciprocal role of membrane partners in ankG targeting and stabilization remained elusive. In rat cultured hippocampal neurons and cortical organotypic slices, we found that shRNA-mediated knockdown of ankG membrane partners (voltage-gated sodium channels (Nav) or neurofascin-186) led to a decrease of ankG concentration and perturbed the AIS formation and maintenance. These effects were rescued by expressing a recombinant AIS-targeted Nav or by a minimal construct containing the ankyrin-binding domain of Nav1.2 and a membrane anchor (mABD). Moreover, overexpressing mABD in mature neurons led to ankG mislocalization. Altogether, these results demonstrate that a tight and precocious association of ankG with its membrane partners is a key step for the establishment and maintenance of the AIS.

Sodium Channel Trafficking

Voltage-gated sodium channels (VGSC) are critical determinants of cellular electrical activity through the control of initiation and propagation of action potential. To ensure this role, these proteins are not consistently delivered to the plasma membrane but undergo drastic quality controls throughout various adaptive processes such as biosynthesis, anterograde and retrograde trafficking, and membrane targeting. In pathological conditions, this quality control could lead to the retention of functional VGSC and is therefore the target of different pharmacological approaches. The present chapter gives an overview of the current understanding of the facets of VGSC life cycle in the context of both cardiac and neuronal cell types.

Ankyrin G Expression Regulates Apical Delivery of the Epithelial Sodium Channel (ENaC)

The epithelial sodium channel (ENaC) is the limiting entry point for Na⁺ reabsorption in the distal kidney nephron and is regulated by numerous hormones, including the mineralocorticoid hormone aldosterone. Previously we identified ankyrin G (AnkG), a cytoskeletal protein involved in vesicular transport, as a novel aldosterone-induced protein that can alter Na⁺ transport in mouse cortical collecting duct cells. However, the mechanisms underlying AnkG regulation of Na⁺ transport were unknown. Here we report that AnkG expression directly regulates Na⁺ transport by altering ENaC activity in the apical membrane. Increasing AnkG expression increased ENaC activity while depleting AnkG reduced ENaC-mediated Na⁺ transport. These changes were due to a change in ENaC directly rather than through alterations to the Na⁺ driving force created by Na⁺/K⁺-ATPase. Using a constitutively open mutant of ENaC, we demonstrate that the augmentation of Na⁺ transport is caused predominantly by increasing the number of ENaCs at the surface. To determine the mechanism of AnkG action on ENaC surface number, changes in rates of internalization, recycling, and membrane delivery were investigated. AnkG did not alter ENaC delivery to the membrane from biosynthetic pathways or removal by endocytosis. However, AnkG did alter ENaC insertion from constitutive recycling pathways. These findings provide a mechanism to account for the role of AnkG in the regulation of Na⁺ transport in the distal kidney nephron.

Infantile Pain Episodes Associated with Novel Nav1.9 Mutations in Familial Episodic Pain Syndrome in Japanese Families

Painful peripheral neuropathy has been correlated with various voltage-gated sodium channel mutations in sensory neurons. Recently Nav1.9, a voltage-gated sodium channel subtype, has been established as a genetic influence for certain peripheral pain syndromes. In this study, we performed a genetic study in six unrelated multigenerational Japanese families with episodic pain syndrome. Affected participants (n = 23) were characterized by infantile recurrent pain episodes with spontaneous mitigation around adolescence. This unique phenotype was inherited in an autosomal-dominant mode. Linkage analysis was performed for two families with 12 affected and nine unaffected members, and a single locus was identified on 3p22 (LOD score 4.32). Exome analysis (n = 14) was performed for affected and unaffected members in these two families and an additional family. Two missense variants were identified: R222H and R222S in SCN11A. Next, we generated a knock-in mouse model harboring one of the mutations (R222S). Behavioral tests (Hargreaves test and cold plate test) using R222S and wild-type C57BL/6 (WT) mice, young (8-9 weeks old; n = 10-12 for each group) and mature (36-38 weeks old; n = 5-6 for each group), showed that R222S mice were significantly (p < 0.05) more hypersensitive to hot and cold stimuli than WT mice. Electrophysiological studies using dorsal root ganglion neurons from 8-9-week-old mice showed no significant difference in resting membrane potential, but input impedance and firing frequency of evoked action potentials were significantly increased in R222S mice compared with WT mice. However, there was no significant difference among Nav1.9 (WT, R222S, and R222H)-overexpressing ND7/23 cell lines. These results suggest that our novel mutation is a gain-of-function mutation that causes infantile familial episodic pain. The mouse model developed here will be useful for drug screening for familial episodic pain syndrome associated with SCN11A mutations.

Trafficking regulates the subcellular distribution of voltage-gated sodium channels in primary sensory neurons

Voltage-gated sodium channels (Na_vs) comprise at least nine pore-forming α subunits. Of these, Na_v1.6, Na_v1.7, Na_v1.8 and Na_v1.9 are the most frequently studied in primary sensory neurons located in the dorsal root ganglion and are mainly localized to the cytoplasm. A large pool of intracellular Na_vs raises the possibility that changes in Na_v trafficking could alter channel function. The molecular mediators of Na_v trafficking mainly consist of signals within the Na_vs themselves, interacting proteins and extracellular factors. The surface expression of Na_vs is achieved by escape from the endoplasmic reticulum and proteasome degradation, forward trafficking and plasma membrane anchoring, and it is also regulated by channel phosphorylation and ubiquitination in primary sensory neurons. Axonal transport and localization of Na_vs in afferent fibers involves the motor protein KIF5B and scaffold proteins, including contactin and PDZ domain containing 2. Localization of Na_v1.6 to the nodes of Ranvier in myelinated fibers of primary sensory neurons requires node formation and the submembrane cytoskeletal protein complex. These findings inform our understanding of the molecular and cellular mechanisms underlying Na_v trafficking in primary sensory neurons.