Therapeutic Potential of Targeting Regulated Intramembrane Proteolysis Mechanisms of Voltage-Gated Ion Channel Subunits and Cell Adhesion Molecules.

Several integral membrane proteins undergo regulated intramembrane proteolysis (RIP), a tightly controlled process through which cells transmit information across and between intracellular compartments. RIP generates biologically active peptides by a series of proteolytic cleavage events carried out by two primary groups of enzymes: sheddases and intramembrane-cleaving proteases (iCLiPs). Following RIP, fragments of both pore-forming and non-pore-forming ion channel subunits, as well as immunoglobulin super family (IgSF) members, have been shown to translocate to the nucleus to function in transcriptional regulation. As an example, the voltage-gated sodium channel ß1 subunit, which is also an IgSF-cell adhesion molecule (CAM), is a substrate for RIP. ß1 RIP results in generation of a soluble intracellular domain, which can regulate gene expression in the nucleus. In this review, we discuss the proposed RIP mechanisms of voltage-gated sodium, potassium, and calcium channel subunits as well as the roles of their generated proteolytic products in the nucleus. We also discuss other RIP substrates that are cleaved by similar sheddases and iCLiPs, such as IgSF macromolecules, including CAMs, whose proteolytically generated fragments function in the nucleus. Importantly, dysfunctional RIP mechanisms are linked to human disease. Thus, we will also review how understanding RIP events and subsequent signaling processes involving ion channel subunits and IgSF proteins may lead to the discovery of novel therapeutic targets. SIGNIFICANCE STATEMENT: Several ion channel subunits and immunoglobulin superfamily molecules have been identified as substrates of regulated intramembrane proteolysis (RIP). This signal transduction mechanism, which generates polypeptide fragments that translocate to the nucleus, is an important regulator of gene transcription. RIP may impact diseases of excitability, including epilepsy, cardiac arrhythmia, and sudden death syndromes. A thorough understanding of the role of RIP in gene regulation is critical as it may reveal novel therapeutic strategies for the treatment of previously intractable diseases.

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.

Voltage-Gated Sodium Channel ß Subunits and Their Related Diseases.

Voltage-gated sodium channels are protein complexes comprised of one pore forming α subunit and two, non-pore forming, ß subunits. The voltage-gated sodium channel ß subunits were originally identified to function as auxiliary subunits, which modulate the gating, kinetics, and localization of the ion channel pore. Since that time, the five ß subunits have been shown to play crucial roles as multifunctional signaling molecules involved in cell adhesion, cell migration, neuronal pathfinding, fasciculation, and neurite outgrowth. Here, we provide an overview of the evidence implicating the ß subunits in their conducting and non-conducting roles. Mutations in the ß subunit genes (SCN1B-SCN4B) have been linked to a variety of diseases. These include cancer, epilepsy, cardiac arrhythmias, sudden infant death syndrome/sudden unexpected death in epilepsy, neuropathic pain, and multiple neurodegenerative disorders. ß subunits thus provide novel therapeutic targets for future drug discovery.

Neonatal Scn1b-null mice have sinoatrial node dysfunction, altered atrial structure, and atrial fibrillation.

Loss-of-function (LOF) variants in SCN1B, encoding the voltage-gated sodium channel ß1/ß1B subunits, are linked to neurological and cardiovascular diseases. Scn1b-null mice have spontaneous seizures and ventricular arrhythmias and die by approximately 21 days after birth. ß1/ß1B Subunits play critical roles in regulating the excitability of ventricular cardiomyocytes and maintaining ventricular rhythmicity. However, whether they also regulate atrial excitability is unknown. We used neonatal Scn1b-null mice to model the effects of SCN1B LOF on atrial physiology in pediatric patients. Scn1b deletion resulted in altered expression of genes associated with atrial dysfunction. Scn1b-null hearts had a significant accumulation of atrial collagen, increased susceptibility to pacing induced atrial fibrillation (AF), sinoatrial node (SAN) dysfunction, and increased numbers of cholinergic neurons in ganglia that innervate the SAN. Atropine reduced the incidence of AF in null animals. Action potential duration was prolonged in null atrial myocytes, with increased late sodium current density and reduced L-type calcium current density. Scn1b LOF results in altered atrial structure and AF, demonstrating the critical role played by Scn1b in atrial physiology during early postnatal mouse development. Our results suggest that SCN1B LOF variants may significantly impact the developing pediatric heart.

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.

The voltage-gated sodium channel inhibitor, 4,9-anhydrotetrodotoxin, blocks human Nav1.1 in addition to Nav1.6.

Voltage-gated sodium channels (VGSCs) are responsible for the initiation and propagation of action potentials in neurons. The human genome includes ten human VGSC α-subunit genes, SCN(X)A, encoding Nav1.1-1.9 plus Nax. To understand the unique role that each VGSC plays in normal and pathophysiological function in neural networks, compounds with high affinity and selectivity for specific VGSC subtypes are required. Toward that goal, a structural analog of the VGSC pore blocker tetrodotoxin, 4,9-anhydrotetrodotoxin (4,9-ah-TTX), has been reported to be more selective in blocking Na+ current mediated by Nav1.6 than other TTX-sensitive VGSCs, including Nav1.2, Nav1.3, Nav1.4, and Nav1.7. While SCN1A, encoding Nav1.1, has been implicated in several neurological diseases, the effects of 4,9-ah-TTX on Nav1.1-mediated Na+ current have not been tested. Here, we compared the binding of 4,9-ah-TTX for human and mouse brain preparations, and the effects of 4,9-ah-TTX on human Nav1.1-, Nav1.3- and Nav1.6-mediated Na+ currents using the whole-cell patch clamp technique in heterologous cells. We show that, while 4,9-ah-TTX administration results in significant blockade of Nav1.6-mediated Na+ current in the nanomolar range, it also has significant effects on Nav1.1-mediated Na+ current. Thus, 4,9-ah-TTX is not a useful tool in identifying Nav1.6-specific effects in human brain networks.

Excitatory and inhibitory neuron defects in a mouse model of Scn1b-linked EIEE52.

OBJECTIVE: Human variants in voltage-gated sodium channel (VGSC) α and ß subunit genes are linked to developmental and epileptic encephalopathies (DEEs). Inherited, biallelic, loss-of-function variants in SCN1B, encoding the ß1/ß1B subunits, are linked to early infantile DEE (EIEE52). De novo, monoallelic variants in SCN1A (Nav1.1), SCN2A (Nav1.2), SCN3A (Nav1.3), and SCN8A (Nav1.6) are also linked to DEEs. While these VGSC-linked DEEs have similar presentations, they have diverse mechanisms of altered neuronal excitability. Mouse models have suggested that Scn2a-, Scn3a-, and Scn8a-linked DEE variants are, in general, gain of function, resulting in increased persistent or resurgent sodium current (INa ) and pyramidal neuron hyperexcitability. In contrast, Scn1a-linked DEE variants, in general, are loss-of-function, resulting in decreased INa and hypoexcitability of fast-spiking interneurons. VGSC ß1 subunits associate with Nav1.1, Nav1.2, Nav1.3, and Nav1.6 and are expressed throughout the brain, raising the possibility that insults to both pyramidal and interneuron excitability may drive EIEE52 pathophysiology. METHODS: We investigated excitability defects in pyramidal and parvalbumin-positive (PV +) interneurons in the Scn1b-/- model of EIEE52. We also used Scn1bFL/FL mice to delete Scn1b in specific neuronal populations. RESULTS: Scn1b-/- cortical PV + interneurons were hypoexcitable, with reduced INa density. Scn1b-/- cortical pyramidal neurons had population-specific changes in excitability and impaired INa density. Scn1b deletion in PV + neurons resulted in 100% lethality, whereas deletion in Emx1 + or Camk2a + neurons did not affect survival. INTERPRETATION: This work suggests that SCN1B-linked DEE variants impact both excitatory and inhibitory neurons, leading to the increased severity of EIEE52 relative to other DEEs.

Delayed maturation of GABAergic signaling in the Scn1a and Scn1b mouse models of Dravet Syndrome.

Dravet syndrome (DS) is a catastrophic developmental and epileptic encephalopathy characterized by severe, pharmacoresistant seizures and the highest risk of Sudden Unexpected Death in Epilepsy (SUDEP) of all epilepsy syndromes. Here, we investigated the time course of maturation of neuronal GABAergic signaling in the Scn1b-/- and Scn1a+/- mouse models of DS. We found that GABAergic signaling remains immature in both DS models, with a depolarized reversal potential for GABAA-evoked currents compared to wildtype in the third postnatal week. Treatment of Scn1b-/- mice with bumetanide resulted in a delay in SUDEP onset compared to controls in a subset of mice, without prevention of seizure activity or amelioration of failure to thrive. We propose that delayed maturation of GABAergic signaling may contribute to epileptogenesis in SCN1B- and SCN1A-linked DS. Thus, targeting the polarity of GABAergic signaling in brain may be an effective therapeutic strategy to reduce SUDEP risk in DS.

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.

Structure-Based Analysis of Boronic Acids as Inhibitors of Acinetobacter-Derived Cephalosporinase-7, a Unique Class C ß-Lactamase.

Acinetobacter baumannii is a multidrug resistant pathogen that infects more than 12 000 patients each year in the US. Much of the resistance to ß-lactam antibiotics in Acinetobacter spp. is mediated by class C ß-lactamases known as Acinetobacter-derived cephalosporinases (ADCs). ADCs are unaffected by clinically used ß-lactam-based ß-lactamase inhibitors. In this study, five boronic acid transition state analog inhibitors (BATSIs) were evaluated for inhibition of the class C cephalosporinase ADC-7. Our goal was to explore the properties of BATSIs designed to probe the R1 binding site. Ki values ranged from low micromolar to subnanomolar, and circular dichroism (CD) demonstrated that each inhibitor stabilizes the ß-lactamase-inhibitor complexes. Additionally, X-ray crystal structures of ADC-7 in complex with five inhibitors were determined (resolutions from 1.80 to 2.09 Å). In the ADC-7/CR192 complex, the BATSI with the lowest Ki (0.45 nM) and greatest Δ Tm (+9 °C), a trifluoromethyl substituent, interacts with Arg340. Arg340 is unique to ADCs and may play an important role in the inhibition of ADC-7. The ADC-7/BATSI complexes determined in this study shed light into the unique recognition sites in ADC enzymes and also offer insight into further structure-based optimization of these inhibitors.

Inhibition of Acinetobacter-Derived Cephalosporinase: Exploring the Carboxylate Recognition Site Using Novel ß-Lactamase Inhibitors.

Boronic acids are attracting a lot of attention as ß-lactamase inhibitors, and in particular, compound S02030 ( Ki = 44 nM) proved to be a good lead compound against ADC-7 ( Acinetobacter-derived cephalosporinase), one of the most significant resistance determinants in A. baumannii. The atomic structure of the ADC-7/S02030 complex highlighted the importance of critical structural determinants for recognition of the boronic acids. Herein, to elucidate the role in recognition of the R2-carboxylate, which mimics the C3/C4 found in ß-lactams, we designed, synthesized, and characterized six derivatives of S02030 (3a). Out of the six compounds, the best inhibitors proved to be those with an explicit negative charge (compounds 3a-c, 3h, and 3j, Ki = 44-115 nM), which is in contrast to the derivatives where the negative charge is omitted, such as the amide derivative 3d ( Ki = 224 nM) and the hydroxyamide derivative 3e ( Ki = 155 nM). To develop a structural characterization of inhibitor binding in the active site, the X-ray crystal structures of ADC-7 in a complex with compounds 3c, SM23, and EC04 were determined. All three compounds share the same structural features as in S02030 but only differ in the carboxy-R2 side chain, thereby providing the opportunity of exploring the distinct binding mode of the negatively charged R2 side chain. This cephalosporinase demonstrates a high degree of versatility in recognition, employing different residues to directly interact with the carboxylate, thus suggesting the existence of a "carboxylate binding region" rather than a binding site in ADC enzymes. Furthermore, this class of compounds was tested against resistant clinical strains of A. baumannii and are effective at inhibiting bacterial growth in conjunction with a ß-lactam antibiotic.