Sodium channel ß1 subunits are post-translationally modified by tyrosine phosphorylation, S-palmitoylation, and regulated intramembrane proteolysis.

Voltage-gated sodium channel (VGSC) ß1 subunits are multifunctional proteins that modulate the biophysical properties and cell-surface localization of VGSC α subunits and participate in cell-cell and cell-matrix adhesion, all with important implications for intracellular signal transduction, cell migration, and differentiation. Human loss-of-function variants in SCN1B, the gene encoding the VGSC ß1 subunits, are linked to severe diseases with high risk for sudden death, including epileptic encephalopathy and cardiac arrhythmia. We showed previously that ß1 subunits are post-translationally modified by tyrosine phosphorylation. We also showed that ß1 subunits undergo regulated intramembrane proteolysis via the activity of ß-secretase 1 and γ-secretase, resulting in the generation of a soluble intracellular domain, ß1-ICD, which modulates transcription. Here, we report that ß1 subunits are phosphorylated by FYN kinase. Moreover, we show that ß1 subunits are S-palmitoylated. Substitution of a single residue in ß1, Cys-162, to alanine prevented palmitoylation, reduced the level of ß1 polypeptides at the plasma membrane, and reduced the extent of ß1-regulated intramembrane proteolysis, suggesting that the plasma membrane is the site of ß1 proteolytic processing. Treatment with the clathrin-mediated endocytosis inhibitor, Dyngo-4a, re-stored the plasma membrane association of ß1-p.C162A to WT levels. Despite these observations, palmitoylation-null ß1-p.C162A modulated sodium current and sorted to detergent-resistant membrane fractions normally. This is the first demonstration of S-palmitoylation of a VGSC ß subunit, establishing precedence for this post-translational modification as a regulatory mechanism in this protein family.

Drugging the undruggable: gabapentin, pregabalin and the calcium channel α2δ subunit.

In the post-genomic era, the idea of using the sequence of a protein to determine its potential role as a drug target has gained currency. The goal of this approach to drug discovery is to use the sequence of a protein that is known to bind a specific ligand or drug, along with the known structure of the ligand binding site, to predict other similar proteins that are also "druggable". Gabapentin (Neurontin) and pregabalin (Lyrica) are drugs currently in the clinic that were developed based on the hypothesis that generating non-hydrolyzable analogs of GABA would lead to the development of antiepileptic agents. While these compounds are indeed good anticonvulsants, their activity is surprisingly not due to activity in the GABAergic system. By purifying the protein to which gabapentin bound, and determining its identity as the α2δ1 subunit of voltage gated calcium channels, it was possible to make progress in developing new compounds with similar activities to gabapentin, including pregabalin. The recognition of the α2δ1 subunit as the receptor for these drugs also meant that related proteins, such as α2δ3, may be interesting targets for novel pain therapeutics.

Anxiolytic-like activity of pregabalin in the Vogel conflict test in α2δ-1 (R217A) and α2δ-2 (R279A) mouse mutants.

The α(2)δ auxiliary subunits (α(2)δ-1 and α(2)δ-2) of voltage-sensitive calcium channels are thought to be the site of action of pregabalin (Lyrica), a drug that has been shown to be anxiolytic in clinical trials for generalized anxiety disorder. Pregabalin and the chemically related drug gabapentin have similar binding and pharmacology profiles, demonstrating high-affinity, in vitro binding to both α(2)δ-1 and α(2)δ-2 subunits. Two independent point mutant mouse strains were generated in which either the α(2)δ-1 subunit (arginine-to-alanine mutation at amino acid 217; R217A) or the α(2)δ-2 subunit (arginine-to-alanine mutation at amino acid 279; R279A) were rendered insensitive to gabapentin or pregabalin binding. These strains were used to characterize the activity of pregabalin in the Vogel conflict test, a measure of anxiolytic-like activity. Pregabalin showed robust anticonflict activity in wild-type littermates from each strain at a dose of 10 mg/kg but was inactive in the α(2)δ-1 (R217A) mutants up to a dose of 320 mg/kg. In contrast, pregabalin was active in the α(2)δ-2 (R279A) point mutants at 10 and 32 mg/kg. The positive control phenobarbital was active in mice carrying either mutation. These data suggest that the anxiolytic-like effects of pregabalin are mediated by binding of the drug to the α(2)δ-1 subunit.

Sodium channel ß1 subunits participate in regulated intramembrane proteolysis-excitation coupling.

Loss-of-function (LOF) variants in SCN1B, encoding voltage-gated sodium channel ß1 subunits, are linked to human diseases with high risk of sudden death, including developmental and epileptic encephalopathy and cardiac arrhythmia. ß1 Subunits modulate the cell-surface localization, gating, and kinetics of sodium channel pore-forming α subunits. They also participate in cell-cell and cell-matrix adhesion, resulting in intracellular signal transduction, promotion of cell migration, calcium handling, and regulation of cell morphology. Here, we investigated regulated intramembrane proteolysis (RIP) of ß1 by BACE1 and γ-secretase and show that ß1 subunits are substrates for sequential RIP by BACE1 and γ-secretase, resulting in the generation of a soluble intracellular domain (ICD) that is translocated to the nucleus. Using RNA sequencing, we identified a subset of genes that are downregulated by ß1-ICD overexpression in heterologous cells but upregulated in Scn1b-null cardiac tissue, which lacks ß1-ICD signaling, suggesting that the ß1-ICD may normally function as a molecular brake on gene transcription in vivo. We propose that human disease variants resulting in SCN1B LOF cause transcriptional dysregulation that contributes to altered excitability. Moreover, these results provide important insights into the mechanism of SCN1B-linked channelopathies, adding RIP-excitation coupling to the multifunctionality of sodium channel ß1 subunits.

SCN1B-linked early infantile developmental and epileptic encephalopathy.

OBJECTIVE: Patients with Early Infantile Epileptic Encephalopathy (EIEE) 52 have inherited, homozygous variants in the gene SCN1B, encoding the voltage-gated sodium channel (VGSC) ß1 and ß1B non-pore-forming subunits. METHODS: Here, we describe the detailed electroclinical features of a biallelic SCN1B patient with a previously unreported variant, p.Arg85Cys. RESULTS: The female proband showed hypotonia from birth, multifocal myoclonus at 2.5 months, then focal seizures and myoclonic status epilepticus (SE) at 3 months, triggered by fever. Auditory brainstem response (ABR) showed bilateral hearing loss. Epilepsy was refractory and the patient had virtually no development. Administration of fenfluramine resulted in a significant reduction in seizure frequency and resolution of SE episodes that persisted after a 2-year follow-up. The patient phenotype is more compatible with early infantile developmental and epileptic encephalopathy (DEE) than with typical Dravet syndrome (DS), as previously diagnosed for other patients with homozygous SCN1B variants. Biochemical and electrophysiological analyses of the SCN1B variant expressed in heterologous cells showed cell surface expression of the mutant ß1 subunit, similar to wild-type (WT), but with loss of normal ß1-mediated modification of human Nav 1.1-generated sodium current, suggesting that SCN1B-p.Arg85Cys is a loss-of-function (LOF) variant. INTERPRETATION: Importantly, a review of the literature in light of our results suggests that the term, early infantile developmental and epileptic encephalopathy, is more appropriate than either EIEE or DS to describe biallelic SCN1B patients.

Channelopathy as a SUDEP Biomarker in Dravet Syndrome Patient-Derived Cardiac Myocytes.

Dravet syndrome (DS) is a severe developmental and epileptic encephalopathy with a high incidence of sudden unexpected death in epilepsy (SUDEP). Most DS patients carry de novo variants in SCN1A, resulting in Nav1.1 haploinsufficiency. Because SCN1A is expressed in heart and in brain, we proposed that cardiac arrhythmia contributes to SUDEP in DS. We generated DS patient and control induced pluripotent stem cell-derived cardiac myocytes (iPSC-CMs). We observed increased sodium current (INa) and spontaneous contraction rates in DS patient iPSC-CMs versus controls. For the subject with the largest increase in INa, cardiac abnormalities were revealed upon clinical evaluation. Generation of a CRISPR gene-edited heterozygous SCN1A deletion in control iPSCs increased INa density in iPSC-CMs similar to that seen in patient cells. Thus, the high risk of SUDEP in DS may result from a predisposition to cardiac arrhythmias in addition to seizures, reflecting expression of SCN1A in heart and brain.

Calcium channel alpha2-delta type 1 subunit is the major binding protein for pregabalin in neocortex, hippocampus, amygdala, and spinal cord: an ex vivo autoradiographic study in alpha2-delta type 1 genetically modified mice.

Pregabalin is a synthetic amino acid compound effective in clinical trials for the treatment of post-herpetic neuralgia, diabetic peripheral neuropathy, generalized anxiety disorder and adjunctive therapy for partial seizures of epilepsy. However, the mechanisms by which pregabalin exerts its therapeutic effects are not yet completely understood. In vitro studies have shown that pregabalin binds with high affinity to the alpha(2)-delta (alpha(2)-delta) subunits (Type 1 and 2) of voltage-gated calcium channels. To assess whether alpha(2)-delta Type 1 is the major central nervous system (CNS) binding protein for pregabalin in vivo, a mutant mouse with an arginine-to-alanine mutation at amino acid 217 of the alpha(2)-delta Type 1 protein (R217A mutation) was generated. Previous site-directed mutagenesis studies revealed that the R217A mutation dramatically reduces alpha(2)-delta 1 binding to pregabalin in vitro. In this autoradiographic analysis of R217A mice, we show that the mutation to alpha(2)-delta Type 1 substantially reduces specific pregabalin binding in CNS regions that are known to preferentially express the alpha(2)-delta Type 1 protein, notably the neocortex, hippocampus, basolateral amygdala and spinal cord. In mutant mice, pregabalin binding was robust throughout regions where the alpha(2)-delta Type 2 subunit mRNA is abundant, such as cerebellum. These findings, in conjunction with prior in vitro binding data, provide evidence that the alpha(2)-delta Type 1 subunit of voltage-gated calcium channels is the major binding protein for pregabalin in CNS. Moreover, the distinct localization of alpha(2)-delta Type 1 and mutation-resistant binding (assumed to be alpha(2)-delta Type 2) in brain areas subserving different functions suggests that identification of subunit-specific ligands could further enhance pharmacologic specificity.

Anticonvulsant activity of pregabalin in the maximal electroshock-induced seizure assay in α2δ1 (R217A) and α2δ2 (R279A) mouse mutants.

Pregabalin has been shown to have anticonvulsant, analgesic, and anxiolytic activity in animal models. Pregabalin binds with high affinity to the α2δ1 and α2δ2 subunits of voltage-gated calcium channels. In order to better understand the relative contribution that binding to either the α2δ1 or α2δ2 subunits confers on the anticonvulsant activity of pregabalin, we characterized the anticonvulsant activity of pregabalin in different wild-type (WT) and mutant mouse strains. Two targeted mouse mutants have been made in which either the α2δ1 subunit was mutated (arginine-to-alanine mutation at amino acid 217; R217A) or the α2δ2 subunit was mutated (arginine-to-alanine mutation at amino acid 279; R279A). These mutations in α2δ1 or α2δ2 render the subunits relatively insensitive to pregabalin binding. The anticonvulsant activity of pregabalin was assessed in these different mouse lines using the maximal electroshock-induced seizure (MES) model. Pregabalin reduced the percentage of seizures and increased the latency to seizure in the MES model in two parental mouse strains used to construct the mutants. Pregabalin also reduced the percentage of seizures and increased latency to seizure similarly in the α2δ2 (R279A) and WT littermate control mice. In contrast, pregabalin's anticonvulsant efficacy was significantly reduced in α2δ1 (R217A) mutants compared with WT littermate control mice. Phenytoin showed anticonvulsant activity across all WT and mutant mice. These data show that the anticonvulsant activity of pregabalin in the MES model requires binding to the α2δ1 subunit.

Altered cardiac electrophysiology and SUDEP in a model of Dravet syndrome.

OBJECTIVE: Dravet syndrome is a severe form of intractable pediatric epilepsy with a high incidence of SUDEP: Sudden Unexpected Death in epilepsy. Cardiac arrhythmias are a proposed cause for some cases of SUDEP, yet the susceptibility and potential mechanism of arrhythmogenesis in Dravet syndrome remain unknown. The majority of Dravet syndrome patients have de novo mutations in SCN1A, resulting in haploinsufficiency. We propose that, in addition to neuronal hyperexcitability, SCN1A haploinsufficiency alters cardiac electrical function and produces arrhythmias, providing a potential mechanism for SUDEP. METHODS: Postnatal day 15-21 heterozygous SCN1A-R1407X knock-in mice, expressing a human Dravet syndrome mutation, were used to investigate a possible cardiac phenotype. A combination of single cell electrophysiology and in vivo electrocardiogram (ECG) recordings were performed. RESULTS: We observed a 2-fold increase in both transient and persistent Na(+) current density in isolated Dravet syndrome ventricular myocytes that resulted from increased activity of a tetrodotoxin-resistant Na(+) current, likely Nav1.5. Dravet syndrome myocytes exhibited increased excitability, action potential duration prolongation, and triggered activity. Continuous radiotelemetric ECG recordings showed QT prolongation, ventricular ectopic foci, idioventricular rhythms, beat-to-beat variability, ventricular fibrillation, and focal bradycardia. Spontaneous deaths were recorded in 2 DS mice, and a third became moribund and required euthanasia. INTERPRETATION: These data from single cell and whole animal experiments suggest that altered cardiac electrical function in Dravet syndrome may contribute to the susceptibility for arrhythmogenesis and SUDEP. These mechanistic insights may lead to critical risk assessment and intervention in human patients.

Genetic approaches to a better understanding of bipolar disorder.

Bipolar disorder is a disease which causes major disability. The disease has both a manic and depressive component. Current standard of care consists of atypical antipsychotics for the treatment of mania, antidepressants for the treatment of depression, and mood stabilizers for the maintenance of euthymia. The molecular mechanisms which cause the disease are not well understood. Genome wide association studies have provided a set of genes which are linked to the disease. These genes show linkage to physiological and neuroanatomical alterations which are also seen in bipolar disorder.

Methodology for rapid measures of glutamate release in rat brain slices using ceramic-based microelectrode arrays: basic characterization and drug pharmacology.

Excessive excitability or hyperexcitability of glutamate-containing neurons in the brain has been proposed as a possible explanation for anxiety, stress-induced disorders, epilepsy, and some neurodegenerative diseases. However, direct measurement of glutamate on a rapid time scale has proven to be difficult. Here we adapted enzyme-based microelectrode arrays (MEA) capable of detecting glutamate in vivo, to assess the effectiveness of hyperexcitability modulators on glutamate release in brain slices of the rat neocortex. Using glutamate oxidase coated ceramic MEAs coupled with constant voltage amperometry, we measured resting glutamate levels and synaptic overflow of glutamate after K(+) stimulation in brain slices. MEAs reproducibly detected glutamate on a second-by-second time scale in the brain slice preparation after depolarization with high K(+) to evoke glutamate release. This stimulus-evoked glutamate release was robust, reproducible, and calcium dependent. The K(+)-evoked glutamate release was modulated by ligands to the α(2)δ subunit of voltage sensitive calcium channels (PD-0332334 and PD-0200390). Meanwhile, agonists to Group II metabotropic glutamate (mGlu) receptors (LY379268 and LY354740), which are known to alter hyperexcitability of glutamate neurons, attenuated K(+)-evoked glutamate release but did not alter resting glutamate levels. This new MEA technology provides a means of directly measuring the chemical messengers involved in glutamate neurotransmission and thereby helping to reveal the role multiple glutamatergic system components have on glutamate signaling.

The alpha2-delta protein: an auxiliary subunit of voltage-dependent calcium channels as a recognized drug target.

Currently, there are two drugs on the market, gabapentin (Neurontin) and pregabalin (Lyrica), that are proposed to exert their therapeutic effect through binding to the alpha2-delta subunit of voltage-sensitive calcium channels. This activity was unexpected, as the alpha2-delta subunit had previously been considered not to be a pharmacological target. In this review, the role of the alpha2-delta subunits is discussed and the mechanism of action of the alpha2-delta ligands in vitro and in vivo is summarized. Finally, new insights into the mechanism of drugs that bind to this protein are discussed.

Identification of the alpha2-delta-1 subunit of voltage-dependent calcium channels as a molecular target for pain mediating the analgesic actions of pregabalin.

Neuropathic pain is a debilitating condition affecting millions of people around the world and is defined as pain that follows a lesion or dysfunction of the nervous system. This type of pain is difficult to treat, but the novel compounds pregabalin (Lyrica) and gabapentin (Neurontin) have proven clinical efficacy. Unlike traditional analgesics such as nonsteroidal antiinflammatory drugs or narcotics, these agents have no frank antiinflammatory actions and no effect on physiological pain. Although extensive preclinical studies have led to a number of suggestions, until recently their mechanism of action has not been clearly defined. Here, we describe studies on the analgesic effects of pregabalin in a mutant mouse containing a single-point mutation within the gene encoding a specific auxiliary subunit protein (alpha2-delta-1) of voltage-dependent calcium channels. The mice demonstrate normal pain phenotypes and typical responses to other analgesic drugs. We show that the mutation leads to a significant reduction in the binding affinity of pregabalin in the brain and spinal cord and the loss of its analgesic efficacy. These studies show conclusively that the analgesic actions of pregabalin are mediated through the alpha2-delta-1 subunit of voltage-gated calcium channels and establish this subunit as a therapeutic target for pain control.