Sodium channel β1 subunits are post-translationally modified by tyrosine phosphorylation, S-palmitoylation, and regulated intramembrane proteolysis

Development and characterization of the mode-of-action of inhibitory and agonist peptides targeting the voltage-gated sodium channel SCN1B beta-subunit

Cardiac arrhythmia treatment is a clinical challenge necessitating safer and more effective therapies. Recent studies have highlighted the role of the perinexus, an intercalated disc nanodomain enriched in voltage-gated sodium channels including both Nav1.5 and β1 subunits, adjacent to gap junctions. These findings offer insights into action potential conduction in the heart. A 19-amino acid SCN1B (β1/β1B) mimetic peptide, βadp1, disrupts VGSC beta subunit-mediated adhesion in cardiac perinexii, inducing arrhythmogenic changes. We aimed to explore βadp1's mechanism and develop novel SCN1B mimetic peptides affecting β1-mediated adhesion. Using patch clamp assays in neonatal rat cardiomyocytes and electric cell substrate impedance sensing (ECIS) in β1-expressing cells, we observed βadp1 maintained inhibitory effects for up to 5 h. A shorter peptide (LQLEED) based on the carboxyl-terminus of βadp1 mimicked this inhibitory effect, while dimeric peptides containing repeated LQLEED sequences paradoxically promoted intercellular adhesion over longer time courses. Moreover, we found a link between these peptides and β1-regulated intramembrane proteolysis (RIP) - a signaling pathway effecting gene transcription including that of VGSC subunits. βadp1 increased RIP continuously over 48 h, while dimeric agonists acutely boosted RIP for up to 6 h. In the presence of DAPT, an RIP inhibitor, βadp1's effects on ECIS-measured intercellular adhesion was reduced, suggesting a relationship between RIP and the peptide's inhibitory action. In conclusion, novel SCN1B (β1/β1B) mimetic peptides are reported with the potential to modulate intercellular VGSC β1-mediated adhesion, potentially through β1 RIP. These findings suggest a path towards the development of anti-arrhythmic drugs targeting the perinexus.

New focus on cardiac voltage-gated sodium channel β1 and β1B: Novel targets for treating and understanding arrhythmias?

Voltage-gated sodium channels (VGSCs) are transmembrane protein complexes that are vital to the generation and propagation of action potentials in nerve and muscle fibers. The canonical VGSC is generally conceived as a heterotrimeric complex formed by 2 classes of membrane-spanning subunit: an α-subunit (pore forming) and 2 β-subunits (non-pore forming). NaV1.5 is the main sodium channel α-subunit of mammalian ventricle, with lower amounts of other α-subunits, including NaV1.6, being present. There are 4 β-subunits (β1-β4) encoded by 4 genes (SCN1B-SCN4B), each of which is expressed in cardiac tissues. Recent studies suggest that in addition to assignments in channel gating and trafficking, products of Scn1b may have novel roles in conduction of action potential in the heart and intracellular signaling. This includes evidence that the β-subunit extracellular amino-terminal domain facilitates adhesive interactions in intercalated discs and that its carboxyl-terminal region is a substrate for a regulated intramembrane proteolysis (RIP) signaling pathway, with a carboxyl-terminal peptide generated by β1 RIP trafficked to the nucleus and altering transcription of various genes, including NaV1.5. In addition to β1, the Scn1b gene encodes for an alternative splice variant, β1B, which contains an identical extracellular adhesion domain to β1 but has a unique carboxyl-terminus. Although β1B is generally understood to be a secreted variant, evidence indicates that when co-expressed with NaV1.5, it is maintained at the cell membrane, suggesting potential unique roles for this understudied protein. In this review, we focus on what is known of the 2 β-subunit variants encoded by Scn1b in heart, with particular focus on recent findings and the questions raised by this new information. We also explore data that indicate β1 and β1B may be attractive targets for novel antiarrhythmic therapeutics.

Cleavage efficiency of the intramembrane protease γ-secretase is reduced by the palmitoylation of a substrate's transmembrane domain

The intramembrane protease γ-secretase has broad physiological functions, but also contributes to Notch-dependent tumors and Alzheimer's disease. While γ-secretase cleaves numerous membrane proteins, only few nonsubstrates are known. Thus, a fundamental open question is how γ-secretase distinguishes substrates from nonsubstrates and whether sequence-based features or post-translational modifications of membrane proteins contribute to substrate recognition. Using mass spectrometry-based proteomics, we identified several type I membrane proteins with short ectodomains that were inefficiently or not cleaved by γ-secretase, including 'pituitary tumor-transforming gene 1-interacting protein' (PTTG1IP). To analyze the mechanism preventing cleavage of these putative nonsubstrates, we used the validated substrate FN14 as a backbone and replaced its transmembrane domain (TMD), where γ-cleavage occurs, with the one of nonsubstrates. Surprisingly, some nonsubstrate TMDs were efficiently cleaved in the FN14 backbone, demonstrating that a cleavable TMD is necessary, but not sufficient for cleavage by γ-secretase. Cleavage efficiencies varied by up to 200-fold. Other TMDs, including that of PTTG1IP, were still barely cleaved within the FN14 backbone. Pharmacological and mutational experiments revealed that the PTTG1IP TMD is palmitoylated, which prevented cleavage by γ-secretase. We conclude that the TMD sequence of a membrane protein and its palmitoylation can be key factors determining substrate recognition and cleavage efficiency by γ-secretase.

Ankyrin-B is lipid-modified by S-palmitoylation to promote dendritic membrane scaffolding of voltage-gated sodium channel NaV1.2 in neurons

Neuronal ankyrin-B is an intracellular scaffolding protein that plays multiple roles in the axon. By contrast, relatively little is known about the function of ankyrin-B in dendrites, where ankyrin-B is also localized in mature neurons. Recently, we showed that ankyrin-B acts as a scaffold for the voltage-gated sodium channel, NaV1.2, in dendrites of neocortical pyramidal neurons. How ankyrin-B is itself targeted to the dendritic membrane is not well understood. Here, we report that ankyrin-B is lipid-modified by S-palmitoylation to promote dendritic localization of NaV1.2. We identify the palmitoyl acyl transferase zDHHC17 as a key mediator of ankyrin-B palmitoylation in heterologous cells and in neurons. Additionally, we find that zDHHC17 regulates ankyrin-B protein levels independently of its S-acylation function through a conserved binding mechanism between the ANK repeat domain of zDHHC17 and the zDHHC ankyrin-repeat binding motif of ankyrin-B. We subsequently identify five cysteines in the N-terminal ankyrin repeat domain of ankyrin-B that are necessary for ankyrin-B palmitoylation. Mutation of these five cysteines to alanines not only abolishes ankyrin-B palmitoylation, but also prevents ankyrin-B from scaffolding NaV1.2 at dendritic membranes of neurons due to ankyrin-B's inability to localize properly at dendrites. Thus, we show palmitoylation is critical for localization and function of ankyrin-B at dendrites. Strikingly, loss of ankyrin-B palmitoylation does not affect ankyrin-B-mediated axonal cargo transport of synaptic vesicle synaptotagmin-1 in neurons. This is the first demonstration of S-palmitoylation of ankyrin-B as an underlying mechanism required for ankyrin-B localization and function in scaffolding NaV1.2 at dendrites.

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 β1 subunit, which is also an IgSF-cell adhesion molecule (CAM), is a substrate for RIP. β1 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.

Palmitoylation of Voltage-Gated Ion Channels

Protein lipidation is one of the most common forms of posttranslational modification. This alteration couples different lipids, such as fatty acids, phospho- and glycolipids and sterols, to cellular proteins. Lipidation regulates different aspects of the protein’s physiology, including structure, stability and affinity for cellular membranes and protein–protein interactions. In this scenario, palmitoylation is the addition of long saturated fatty acid chains to amino acid residues of the proteins. The enzymes responsible for this modification are acyltransferases and thioesterases, which control the protein’s behavior by performing a series of acylation and deacylation cycles. These enzymes target a broad repertoire of substrates, including ion channels. Thus, protein palmitoylation exhibits a pleiotropic role by differential modulation of the trafficking, spatial organization and electrophysiological properties of ion channels. Considering voltage-gated ion channels (VGICs), dysregulation of lipidation of both the channels and the associated ancillary subunits correlates with the development of various diseases, such as cancer or mental disorders. Therefore, a major role for protein palmitoylation is currently emerging, affecting not only the dynamism and differential regulation of a moiety of cellular proteins but also linking to human health. Therefore, palmitoylation of VGIC, as well as related enzymes, constitutes a novel pharmacological tool for drug development to target related pathologies.

Subcellular dynamics and functional activity of the cleaved intracellular domain of the Na+ channel β1 subunit

The voltage-gated Na⁺ channel β1 subunit, encoded by SCN1B, regulates cell surface expression and gating of α subunits and participates in cell adhesion. β1 is cleaved by α/β and γ-secretases, releasing an extracellular domain and intracellular domain (ICD), respectively. Abnormal SCN1B expression/function is linked to pathologies including epilepsy, cardiac arrhythmia, and cancer. In this study, we sought to determine the effect of secretase cleavage on β1 function in breast cancer cells. Using a series of GFP-tagged β1 constructs, we show that β1-GFP is mainly retained intracellularly, particularly in the endoplasmic reticulum and endolysosomal pathway, and accumulates in the nucleus. Reduction in endosomal β1-GFP levels occurred following γ-secretase inhibition, implicating endosomes and/or the preceding plasma membrane as important sites for secretase processing. Using live-cell imaging, we also report β1ICD-GFP accumulation in the nucleus. Furthermore, β1-GFP and β1ICD-GFP both increased Na⁺ current, whereas β1STOP-GFP, which lacks the ICD, did not, thus highlighting that the β1-ICD is necessary and sufficient to increase Na⁺ current measured at the plasma membrane. Importantly, although the endogenous Na⁺ current expressed in MDA-MB-231 cells is tetrodotoxin (TTX)-resistant (carried by Na_v1.5), the Na⁺ current increased by β1-GFP or β1ICD-GFP was TTX-sensitive. Finally, we found β1-GFP increased mRNA levels of the TTX-sensitive α subunits SCN1A/Na_v1.1 and SCN9A/Na_v1.7. Taken together, this work suggests that the β1-ICD is a critical regulator of α subunit function in cancer cells. Our data further highlight that γ-secretase may play a key role in regulating β1 function in breast cancer.

Distinctive Properties and Powerful Neuromodulation of Nav1.6 Sodium Channels Regulates Neuronal Excitability

Voltage-gated sodium channels (Navs) are critical determinants of cellular excitability. These ion channels exist as large heteromultimeric structures and their activity is tightly controlled. In neurons, the isoform Na_v1.6 is highly enriched at the axon initial segment and nodes, making it critical for the initiation and propagation of neuronal impulses. Changes in Na_v1.6 expression and function profoundly impact the input-output properties of neurons in normal and pathological conditions. While mutations in Na_v1.6 may cause channel dysfunction, aberrant changes may also be the result of complex modes of regulation, including various protein-protein interactions and post-translational modifications, which can alter membrane excitability and neuronal firing properties. Despite decades of research, the complexities of Na_v1.6 modulation in health and disease are still being determined. While some modulatory mechanisms have similar effects on other Nav isoforms, others are isoform-specific. Additionally, considerable progress has been made toward understanding how individual protein interactions and/or modifications affect Na_v1.6 function. However, there is still more to be learned about how these different modes of modulation interact. Here, we examine the role of Na_v1.6 in neuronal function and provide a thorough review of this channel’s complex regulatory mechanisms and how they may contribute to neuromodulation.

The voltage-gated sodium channel β2 subunit associates with lipid rafts by S-palmitoylation

The voltage-gated sodium channel is critical for cardiomyocyte function. It consists of a protein complex comprising a pore-forming α subunit and associated β subunits. In polarized Madin-Darby canine kidney cells, we show evidence by acyl-biotin exchange that β2 is S-acylated at Cys-182. Interestingly, we found that palmitoylation increases β2 association with detergent-resistant membranes. β2 localizes exclusively to the apical surface. However, depletion of plasma membrane cholesterol, or blocking intracellular cholesterol transport, caused mislocalization of β2, as well as of the non-palmitoylable C182S mutant, to the basolateral domain. Apical β2 did not undergo endocytosis and displayed limited diffusion within the plane of the membrane; such behavior suggests that, at least in part, it is cytoskeleton anchored. Upon acute cholesterol depletion, its mobility was greatly reduced, and a slight reduction was also measured as a result of lack of palmitoylation, supporting β2 association with cholesterol-rich lipid rafts. Indeed, lipid raft labeling confirmed a partial overlap with apical β2. Although β2 palmitoylation was not required to promote surface localization of the α subunit, our data suggest that it is likely implicated in lipid raft association and the polarized localization of β2.

Sodium channel β1 subunits participate in regulated intramembrane proteolysis-excitation coupling

Loss-of-function (LOF) variants in SCN1B, encoding voltage-gated sodium channel β1 subunits, are linked to human diseases with high risk of sudden death, including developmental and epileptic encephalopathy and cardiac arrhythmia. β1 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 β1 by BACE1 and γ-secretase and show that β1 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 β1-ICD overexpression in heterologous cells but upregulated in Scn1b-null cardiac tissue, which lacks β1-ICD signaling, suggesting that the β1-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 β1 subunits.

Cell-Adhesion Properties of β-Subunits in the Regulation of Cardiomyocyte Sodium Channels

Voltage-gated sodium (Nav) channels drive the rising phase of the action potential, essential for electrical signalling in nerves and muscles. The Nav channel α-subunit contains the ion-selective pore. In the cardiomyocyte, Nav1.5 is the main Nav channel α-subunit isoform, with a smaller expression of neuronal Nav channels. Four distinct regulatory β-subunits (β1–4) bind to the Nav channel α-subunits. Previous work has emphasised the β-subunits as direct Nav channel gating modulators. However, there is now increasing appreciation of additional roles played by these subunits. In this review, we focus on β-subunits as homophilic and heterophilic cell-adhesion molecules and the implications for cardiomyocyte function. Based on recent cryogenic electron microscopy (cryo-EM) data, we suggest that the β-subunits interact with Nav1.5 in a different way from their binding to other Nav channel isoforms. We believe this feature may facilitate trans-cell-adhesion between β1-associated Nav1.5 subunits on the intercalated disc and promote ephaptic conduction between cardiomyocytes.