The Sialic Acid Component of the β1 Subunit Modulates Voltage-gated Sodium Channel Function

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 two classes of membrane-spanning subunit-an α-subunit (pore forming) and two 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 four β-subunits, β1-β4, encoded by four genes, SCN1B-SCN4B, each of which are 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. Whilst 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 on the two 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 anti-arrhythmic therapeutics.

Epilepsy and sudden unexpected death in epilepsy in a mouse model of human SCN1B-linked developmental and epileptic encephalopathy

Voltage-gated sodium channel β1 subunits are essential proteins that regulate excitability. They modulate sodium and potassium currents, function as cell adhesion molecules and regulate gene transcription following regulated intramembrane proteolysis. Biallelic pathogenic variants in SCN1B, encoding β1, are linked to developmental and epileptic encephalopathy 52, with clinical features overlapping Dravet syndrome. A recessive variant, SCN1B-c.265C>T, predicting SCN1B-p.R89C, was homozygous in two children of a non-consanguineous family. One child was diagnosed with Dravet syndrome, while the other had a milder phenotype. We identified an unrelated biallelic SCN1B-c.265C>T patient with a clinically more severe phenotype than Dravet syndrome. We used CRISPR/Cas9 to knock-in SCN1B-p.R89C to the mouse Scn1b locus (Scn1bR89/C89). We then rederived the line on the C57BL/6J background to allow comparisons between Scn1bR89/R89 and Scn1bC89/C89 littermates with Scn1b+/+ and Scn1b-/- mice, which are congenic on C57BL/6J, to determine whether the SCN1B-c.265C>T variant results in loss-of-function. Scn1bC89/C89 mice have normal body weights and ∼20% premature mortality, compared with severely reduced body weight and 100% mortality in Scn1b-/- mice. β1-p.R89C polypeptides are expressed in brain at comparable levels to wild type. In heterologous cells, β1-p.R89C localizes to the plasma membrane and undergoes regulated intramembrane proteolysis similar to wild type. Heterologous expression of β1-p.R89C results in sodium channel α subunit subtype specific effects on sodium current. mRNA abundance of Scn2a, Scn3a, Scn5a and Scn1b was increased in Scn1bC89/C89 somatosensory cortex, with no changes in Scn1a. In contrast, Scn1b-/- mouse somatosensory cortex is haploinsufficient for Scn1a, suggesting an additive mechanism for the severity of the null model via disrupted regulation of another Dravet syndrome gene. Scn1bC89/C89 mice are more susceptible to hyperthermia-induced seizures at post-natal Day 15 compared with Scn1bR89/R89 littermates. EEG recordings detected epileptic discharges in young adult Scn1bC89/C89 mice that coincided with convulsive seizures and myoclonic jerks. We compared seizure frequency and duration in a subset of adult Scn1bC89/C89 mice that had been exposed to hyperthermia at post-natal Day 15 versus a subset that were not hyperthermia exposed. No differences in spontaneous seizures were detected between groups. For both groups, the spontaneous seizure pattern was diurnal, occurring with higher frequency during the dark cycle. This work suggests that the SCN1B-c.265C>T variant does not result in complete loss-of-function. Scn1bC89/C89 mice more accurately model SCN1B-linked variants with incomplete loss-of-function compared with Scn1b-/- mice, which model complete loss-of-function, and thus add to our understanding of disease mechanisms as well as our ability to develop new therapeutic strategies.

Cardiac sodium channel complexes and arrhythmia: structural and functional roles of the β1 and β3 subunits

In cardiac myocytes, the voltage-gated sodium channel NaV 1.5 opens in response to membrane depolarisation and initiates the action potential. The NaV 1.5 channel is typically associated with regulatory β-subunits that modify gating and trafficking behaviour. These β-subunits contain a single extracellular immunoglobulin (Ig) domain, a single transmembrane α-helix and an intracellular region. Here we focus on the role of the β1 and β3 subunits in regulating NaV 1.5. We catalogue β1 and β3 domain specific mutations that have been associated with inherited cardiac arrhythmia, including Brugada syndrome, long QT syndrome, atrial fibrillation and sudden death. We discuss how new structural insights into these proteins raises new questions about physiological function.

Glycans and Carbohydrate-Binding/Transforming Proteins in Axon Physiology

The mature nervous system relies on the polarized morphology of neurons for a directed flow of information. These highly polarized cells use their somatodendritic domain to receive and integrate input signals while the axon is responsible for the propagation and transmission of the output signal. However, the axon must perform different functions throughout development before being fully functional for the transmission of information in the form of electrical signals. During the development of the nervous system, axons perform environmental sensing functions, which allow them to navigate through other regions until a final target is reached. Some axons must also establish a regulated contact with other cells before reaching maturity, such as with myelinating glial cells in the case of myelinated axons. Mature axons must then acquire the structural and functional characteristics that allow them to perform their role as part of the information processing and transmitting unit that is the neuron. Finally, in the event of an injury to the nervous system, damaged axons must try to reacquire some of their immature characteristics in a regeneration attempt, which is mostly successful in the PNS but fails in the CNS. Throughout all these steps, glycans perform functions of the outermost importance. Glycans expressed by the axon, as well as by their surrounding environment and contacting cells, encode key information, which is fine-tuned by glycan modifying enzymes and decoded by glycan binding proteins so that the development, guidance, myelination, and electrical transmission functions can be reliably performed. In this chapter, we will provide illustrative examples of how glycans and their binding/transforming proteins code and decode instructive information necessary for fundamental processes in axon physiology.

The Synthesis of UDP-N-acetylglucosamine 2-epimerase/N-acetylmannosamine Kinase (GNE), α-dystroglycan, and β-galactoside α-2,3-sialyltransferase 6 (ST3Gal6) By Skeletal Muscle Cell As a Response To Infection with Trichinella Spiralis

The Nurse cell of the parasitic nematode Trichinella spiralis is a unique structure established after genetic, morphological and functional modification of a small portion of invaded skeletal muscle fiber. Even if the newly developed cytoplasm of the Nurse cell is no longer contractile, this structure remains well integrated within the surrounding healthy tissue. Our previous reports suggested that this process is accompanied by an increased local biosynthesis of sialylated glycoproteins. In this work we examined the expressions of three proteins, functionally associated with the process of sialylation. The enzyme UDP-N-acetylglucosamine 2-epimerase/N-acetylmannosamine kinase (GNE) is a key initiator of the sialic acid biosynthetic pathway. The α-dystroglycan was the only identified sialylated glycoprotein in skeletal muscles by now, bearing sialyl-α-2,3-Gal-β-1,4-Gl-cNAc-β-1,2-Man-α-1-O-Ser/Thr glycan. The third protein of interest for this study was the enzyme β-galactoside α-2,3-sialyltransferase 6 (ST3Gal6), which transfers sialic acid preferably onto Gal-β-1,4-GlcNAc as an acceptor, and thus it was considered as a suitable candidate for the sialylation of the α-dystroglycan. The expressions of the three proteins were analyzed by real time-PCR and immunohistochemistry on modified methacarn fixed paraffin tissue sections of mouse skeletal muscle samples collected at days 0, 14 and 35 post infection. According to our findings, the up-regulation of GNE was a characteristic of the early and the late stage of the Nurse cell development. Additional features of this process were the elevated expressions of α-dystroglycan and the enzyme ST3Gal6. We provided strong evidence that an increased local synthesis of sialic acids is a trait of the Nurse cell of T. spiralis, and at least in part due to an overexpression of α-dystroglycan. In addition, circumstantially we suggest that the enzyme ST3Gal6 is engaged in the process of sialylation of the major oligosaccharide component of α-dystroglycan.

Dendritic Excitability and Synaptic Plasticity In Vitro and In Vivo

Much of our understanding of dendritic and synaptic physiology comes from in vitro experimentation, where the afforded mechanical stability and convenience of applying drugs allowed patch-clamping based recording techniques to investigate ion channel distributions, their gating kinetics, and to uncover dendritic integrative and synaptic plasticity rules. However, with current efforts to study these questions in vivo, there is a great need to translate existing knowledge between in vitro and in vivo experimental conditions. In this review, we identify discrepancies between in vitro and in vivo ionic composition of extracellular media and discuss how changes in ionic composition alter dendritic excitability and plasticity induction. Here, we argue that under physiological in vivo ionic conditions, dendrites are expected to be more excitable and the threshold for synaptic plasticity induction to be lowered. Consequently, the plasticity rules described in vitro vary significantly from those implemented in vivo.

Cell Surface Engineering Enables Surfaceome Profiling

Cell surface proteins (CSPs) are vital molecular mediators for cells and their extracellular environment. Thus, understanding which CSPs are displayed on cells, especially in different cell states, remains an important endeavor in cell biology. Here, we describe the integration of cell surface engineering with radical-mediated protein biotinylation to profile CSPs. This method relies on the prefunctionalization of cells with cholesterol lipid groups, followed by sortase-catalyzed conjugation with an APEX2 ascorbate peroxidase enzyme. In the presence of biotin-phenol and H₂O₂, APEX2 catalyzes the formation of highly reactive biotinyl radicals that covalently tag electron-rich residues within CSPs for subsequent streptavidin-based enrichment and analysis by quantitative mass spectrometry. While APEX2 is traditionally used to capture proximity-based interactomes, we envisioned using it in a "baitless" manner on cell surfaces to capture CSPs. We evaluate this strategy in light of another CSP labeling method that relies on the presence of cell surface sialic acid. Using the APEX2 strategy, we describe the CSPs found in three mammalian cell lines and compare CSPs in adherent versus three-dimensional pancreatic adenocarcinoma cells.

The AMIGO1 adhesion protein activates Kv2.1 voltage sensors

Kv2 voltage-gated potassium channels are modulated by amphoterin-induced gene and open reading frame (AMIGO) neuronal adhesion proteins. Here, we identify steps in the conductance activation pathway of Kv2.1 channels that are modulated by AMIGO1 using voltage-clamp recordings and spectroscopy of heterologously expressed Kv2.1 and AMIGO1 in mammalian cell lines. AMIGO1 speeds early voltage-sensor movements and shifts the gating charge-voltage relationship to more negative voltages. The gating charge-voltage relationship indicates that AMIGO1 exerts a larger energetic effect on voltage-sensor movement than is apparent from the midpoint of the conductance-voltage relationship. When voltage sensors are detained at rest by voltage-sensor toxins, AMIGO1 has a greater impact on the conductance-voltage relationship. Fluorescence measurements from voltage-sensor toxins bound to Kv2.1 indicate that with AMIGO1, the voltage sensors enter their earliest resting conformation, yet this conformation is less stable upon voltage stimulation. We conclude that AMIGO1 modulates the Kv2.1 conductance activation pathway by destabilizing the earliest resting state of the voltage sensors.

iPSCs and DRGs: stepping stones to new pain therapies

There is a pressing need for more effective nonaddictive treatment options for pain. Pain signals are transmitted from the periphery into the spinal cord via dorsal root ganglion (DRG) neurons, whose excitability is driven by voltage-gated sodium (NaV) channels. Three NaV channels (NaV1.7, NaV1.8, and NaV1.9), preferentially expressed in DRG neurons, play important roles in pain signaling in humans. Blockade of these channels may provide a novel approach to the treatment of pain, but clinical translation of preclinical results has been challenging, in part due to differences between rodent and human DRG neurons. Human DRG neurons and iPSC-derived sensory neurons (iPSC-SNs) provide new preclinical platforms that may facilitate the development of novel pain therapeutics.

Absence of ST3Gal2 and ST3Gal4 sialyltransferase expressions in the nurse cell of Trichinella spiralis

This study was aimed to describe some glycosylation changes in the Nurse cell of Trichinella spiralis in mouse skeletal muscle. Tissue specimens were subjected to lectin histochemistry with Maackia amurensis lectin (MAL), Peanut agglutinin (PNA) and neuraminidase desialylation in order to verify and analyse the structure of α-2,3-sialylated glycoproteins, discovered within the affected sarcoplasm. The expressions of two sialyltransferases were examined by immunohistochemistry. It was found out that the occupied portion of skeletal muscle cell responded with synthesis of presumable sialyl-T-antigen and α-2,3-sialyllactosamine structure, that remained accumulated during the time course of Nurse cell development. The enzymes β-galactoside-α-2,3-sialyltransferases 2 and 4, which could be responsible for the sialylation of each of these structures, were however not present in the invaded muscle portions, although their expressions in the healthy surrounding tissue remained persistent. Our results contribute to the progressive understanding about the amazing abilities of Trichinella spiralis to manipulate the genetic programme of its host.

Molecular Pathology of Sodium Channel Beta-Subunit Variants

The voltage-gated Na⁺ channel regulates the initiation and propagation of the action potential in excitable cells. The major cardiac isoform Na_V1.5, encoded by SCN5A, comprises a monomer with four homologous repeats (I-IV) that each contain a voltage sensing domain (VSD) and pore domain. In native myocytes, Na_V1.5 forms a macromolecular complex with Na_Vβ subunits and other regulatory proteins within the myocyte membrane to maintain normal cardiac function. Disturbance of the Na_V complex may manifest as deadly cardiac arrhythmias. Although SCN5A has long been identified as a gene associated with familial atrial fibrillation (AF) and Brugada Syndrome (BrS), other genetic contributors remain poorly understood. Emerging evidence suggests that mutations in the non-covalently interacting Na_Vβ1 and Na_Vβ3 are linked to both AF and BrS. Here, we investigated the molecular pathologies of 8 variants in Na_Vβ1 and Na_Vβ3. Our results reveal that Na_Vβ1 and Na_Vβ3 variants contribute to AF and BrS disease phenotypes by modulating both Na_V1.5 expression and gating properties. Most AF-linked variants in the Na_Vβ1 subunit do not alter the gating kinetics of the sodium channel, but rather modify the channel expression. In contrast, AF-related Na_Vβ3 variants directly affect channel gating, altering voltage-dependent activation and the time course of recovery from inactivation via the modulation of VSD activation.

Overview of sialylation status in human nervous and skeletal muscle tissues during aging

Sialic acids (Sias) are a large and heterogeneous family of electronegatively charged nine-carbon monosaccharides containing a carboxylic acid and are mostly found as terminal residues in glycans of glycoproteins and glycolipids such as gangliosides. They are linked to galactose or N-acetylgalactosamine via α2,3 or α2,6 linkage, or to other Sias via α2,8 or more rarely α2,9 linkage, resulting in mono, oligo and polymeric forms. Given their characteristics, Sias play a crucial role in a multitude of human tissue biological processes in physiological and pathological conditions, ranging from development and growth to adult life until aging. Here, we review the sialylation status in human adult life focusing on the nervous and skeletal muscle tissues, which both display significant structural and functional changes during aging, strongly impacting on the whole human body and, therefore, on the quality of life. In particular, this review highlights the fundamental roles played by different types of glycoconjugates Sias in several cellular biological processes in the nervous and skeletal muscle tissues during adult life, also discussing how changes in Sia content during aging may contribute to the physiological decline of physical and nervous functions and to the development of age-related degenerative pathologies. Based on our current knowledge, further in-depth investigations could help to develop novel prophylactic strategies and therapeutic approaches that, by maintaining and/or restoring the correct sialylation status in the nervous and skeletal muscle tissues, could contribute to aging slowing and the prevention of age-related pathologies.

Region-Specific Cell Membrane N-Glycome of Functional Mouse Brain Areas Revealed by nanoLC-MS Analysis

N-glycosylation is a ubiquitous posttranslational modification that affects protein structure and function, including those of the central nervous system. N-glycans attached to cell membrane proteins play crucial roles in all aspects of biology, including embryogenesis, development, cell-cell recognition and adhesion, and cell signaling and communication. Although brain function and behavior are known to be regulated by the N-glycosylation state of numerous cell surface glycoproteins, our current understanding of brain glycosylation is limited, and glycan variations associated with functional brain regions remain largely unknown. In this work, we used a well-established cell surface glycomic nanoLC-Chip-Q-TOF platform developed in our laboratory to characterize the N-glycome of membrane fractions enriched in cell surface glycoproteins obtained from specific functional brain areas. We report the cell membrane N-glycome of two major developmental divisions of mice brain with specific and distinctive functions, namely the forebrain and hindbrain. Region-specific glycan maps were obtained with ∼120 N-glycan compositions in each region, revealing significant differences in "brain-type" glycans involving high mannose, bisecting, and core and antenna fucosylated species. Additionally, the cell membrane N-glycome of three functional regions of the forebrain and hindbrain, the cerebral cortex, hippocampus, and cerebellum, was characterized. In total, 125 N-glycan compositions were identified, and their region-specific expression profiles were characterized. Over 70 N-glycans contributed to the differentiation of the cerebral cortex, hippocampus, and cerebellum N-glycome, including bisecting and branched glycans with varying degrees of core and antenna fucosylation and sialylation. This study presents a comprehensive spatial distribution of the cell-membrane enriched N-glycomes associated with five discrete anatomical and functional brain areas, providing evidence for the presence of a previously unknown brain glyco-architecture. The region-specific molecular glyco fingerprints identified here will enable a better understanding of the critical biological roles that N-glycans play in the specialized functional brain areas in health and disease.

A novel gain-of-function sodium channel β2 subunit mutation in idiopathic small fiber neuropathy

Small fiber neuropathy (SFN) is a common condition affecting thinly myelinated Aδ and unmyelinated C fibers, often resulting in excruciating pain and dysautonomia. SFN has been associated with several conditions, but a significant number of cases have no discernible cause. Recent genetic studies have identified potentially pathogenic gain-of-function mutations in several the pore-forming voltage-gated sodium channel α subunits (Na_Vs) in a subset of patients with SFN, but the auxiliary sodium channel β subunits have been less implicated in the development of the disease. β subunits modulate Na_V trafficking and gating, and several mutations have been linked to epilepsy and cardiac dysfunction. Recently, we provided the first evidence for the contribution of a mutation in the β2-subunit to pain in human painful diabetic neuropathy. Here, we provide the first evidence for the involvement of a sodium channel β subunit mutation in the pathogenesis of SFN with no other known causes. We show, through current-clamp analysis, that the newly-identified Y69H variant of the β2 subunit induces neuronal hyperexcitability in dorsal root ganglion neurons, lowering the threshold for action potential firing and allowing for increased repetitive action potential spiking. Underlying the hyperexcitability induced by the β2-Y69H variant, we demonstrate an upregulation in tetrodotoxin-sensitive, but not tetrodotoxin-resistant sodium currents. This provides the first evidence for the involvement of β2 subunits in SFN and strengthens the link between sodium channel β subunits and the development of neuropathic pain in humans.

Tissue specific expression of sialic acid metabolic pathway: role in GNE myopathy

GNE myopathy is an adult-onset degenerative muscle disease that leads to extreme disability in patients. Biallelic mutations in the rate-limiting enzyme UDP-N-acetylglucosamine-2-epimerase/N-acetylmannosamine-kinase (GNE) of sialic acid (SA) biosynthetic pathway, was shown to be the cause of this disease. Other genetic disorders with muscle pathology where defects in glycosylation are known. It is yet not clear why a defect in SA biosynthesis and glycosylation affect muscle cells selectively even though they are ubiquitously present in all tissues. Here we have comprehensively examined the complete SA metabolic pathway involving biosynthesis, sialylation, salvage, and catabolism. To understand the reason for tissue-specific phenotype caused by mutations in genes of this pathway, we analysed the expression of different SA pathway genes in various tissues, during the muscle tissue development and in muscle tissues from GNE myopathy patients (p.Met743Thr) using publicly available databases. We have also analysed gene co-expression networks with GNE in different tissues as well as gene interactions that are unique to muscle tissues only. The results do show a few muscle specific interactions involving ANLN, MYO16 and PRAMEF25 that could be involved in specific phenotype. Overall, our results suggest that SA biosynthetic and catabolic genes are expressed at a very low level in skeletal muscles that also display a unique gene interaction network.

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.

Wheat germ agglutinin-conjugated fluorescent pH sensors for visualizing proton fluxes

Small-molecule fluorescent wheat germ agglutinin (WGA) conjugates are routinely used to demarcate mammalian plasma membranes, because they bind to the cell’s glycocalyx. Here, we describe the derivatization of WGA with a pH-sensitive rhodamine fluorophore (pHRho; pKa = 7) to detect proton channel fluxes and extracellular proton accumulation and depletion from primary cells. We found that WGA-pHRho labeling was uniform and did not appreciably alter the voltage gating of glycosylated ion channels, and the extracellular changes in pH correlated with proton channel activity. Using single-plane illumination techniques, WGA-pHRho was used to detect spatiotemporal differences in proton accumulation and depletion over the extracellular surface of cardiomyocytes, astrocytes, and neurons. Because WGA can be derivatized with any small-molecule fluorescent ion sensor, WGA conjugates should prove useful to visualize most electrogenic and nonelectrogenic events on the extracellular side of the plasma membrane.

Brugada Syndrome Caused by Sodium Channel Dysfunction Associated with a SCN1B Variant A197V

OBJECTIVE

We aimed to identify and characterize a SCN1B variant, A197V, associated with Brugada Syndrome (BrS).

METHODS

Whole-exome sequencing was employed to explore the potential causative genes in 8 unrelated clinically diagnosed BrS patients. A197V variant was only detected in exon 4 of SCN1B in a 46 year old patient, who was admitted due to syncope. Wild type (WT) and mutant (A197V) genes were co-expressed with SCN5A in human embryonic kidney cells (HEK293 cells) and studied using whole-cell patch clamp and immunodetection techniques.

RESULTS

Coexpression of 5A/WT + 1B/A197V resulted in a marked decrease in current density compared to 5A/WT + 1B/WT. The activation velocity was decelerated by A197V mutation. No significant changes were observed in recovery from inactivation parameters. Cell surface protein analyses confirmed that Na_v1.5 channel membrane distribution was affected by A197V mutation.

CONCLUSIONS

The current study is the first to report the functional analysis of SCN1B/ A197V, serving as a substrate responsible for BrS.

Remarkable Homeostasis of Protein Sialylation in Skeletal Muscles of Hibernating Daurian Ground Squirrels (Spermophilus dauricus)

As the most common post-translational protein modification, glycosylation is intimately linked to muscle atrophy. This study aimed to investigate the performance of protein glycosylation in the soleus muscle (SOL) in Daurian ground squirrels (Spermophilus dauricus) and to determine the potential role of protein glycosylation in the mechanism underlying disuse muscle atrophy prevention. The results showed that (1) seven glycan structures comprising sialic acid α2-3 galactose (SAα2-3Gal) were altered during hibernation; (2) alterations in the SAα2-3Gal structure during hibernation were based on changes in the expression levels of beta-galactoside alpha-2 and 3-sialyltransferases; and (3) α2-3–linked sialylated modifications of heat shock cognate 70 and pyruvate kinase and expression of 14-3-3 epsilon protein were oscillatorily changed during hibernation. Our findings indicate that the skeletal muscles of hibernating Daurian ground squirrels maintain protein sialylation homeostasis by restoring sialylation modification during periodic interbout arousal, which might protect the skeletal muscles against disuse atrophy.

N-Glycosylation of the voltage-gated sodium channel β2 subunit is required for efficient trafficking of NaV1.5/β2 to the plasma membrane

The voltage-gated sodium channel is critical for cardiomyocyte function and consists of a protein complex comprising a pore-forming α subunit and two associated β subunits. It has been shown previously that the associated β2 subunits promote cell surface expression of the α subunit. The major α isoform in the adult human heart is Na_V1.5, and germline mutations in the Na_V1.5-encoding gene, sodium voltage-gated channel α subunit 5 (SCN5A), often cause inherited arrhythmias. Here, we investigated the mechanisms that regulate β2 trafficking and how they may determine proper Na_V1.5 cell surface localization. Using heterologous expression in polarized Madin-Darby canine kidney cells, we show that β2 is N-glycosylated in vivo and in vitro at residues 42, 66, and 74, becoming sialylated only at Asn-42. We found that fully nonglycosylated β2 was mostly retained in the endoplasmic reticulum, indicating that N-linked glycosylation is required for efficient β2 trafficking to the apical plasma membrane. The nonglycosylated variant reached the cell surface by bypassing the Golgi compartment at a rate of only approximately one-third of that of WT β2. YFP-tagged, nonglycosylated β2 displayed mobility kinetics in the plane of the membrane similar to that of WT β2. However, it was defective in promoting surface localization of Na_V1.5. Interestingly, β2 with a single intact glycosylation site was as effective as the WT in promoting Na_V1.5 surface localization. In conclusion, our results indicate that N-linked glycosylation of β2 is required for surface localization of Na_V1.5, a property that is often defective in inherited cardiac arrhythmias.

Regular alteration of protein glycosylation in skeletal muscles of hibernating Daurian ground squirrels (Spermophilus dauricus)

Glycosylation is one of the most common post-translational protein modifications and is closely associated with muscle atrophy. This study aims to investigate the changes in glycan profiles in the fast-twitch extensor digitorum longus (EDL) muscles of Daurian ground squirrels (Spermophilus dauricus) during hibernation as well as the correlation between protein glycosylation and muscle atrophy prevention in hibernating animals. The results showed that there was no significant change in the muscle-to-body mass ratio, muscle fiber cross-sectional area (CSA), fiber distribution and ultrastructures in the EDL muscles of ground squirrels during hibernation. Alterations of six glycans comprising sialic acid α2-3 galactose (Sia2-3Gal) and Fucα1-2Galβ1-4Glc(NAc) in the EDL muscles were observed. In addition, the observed downregulation of sialyltransferase (ST3Gals) mRNA levels and upregulation of fucosyltransferase (FUT1 and FUT2) mRNA levels during hibernation and the subsequent restoration to normal levels during periodic interbout arousal were consistent with the changes in sialic acid and fucose modifications. Our results indicate that changes in ST3Gals and FUTs in the EDL muscles of Daurian ground squirrels during hibernation can alter sialylation and fucosylation of muscle glycoproteins, which may protect the skeletal muscles of hibernating Daurian ground squirrels from disuse atrophy.

A recurrent missense variant in SLC9A7 causes nonsyndromic X-linked intellectual disability with alteration of Golgi acidification and aberrant glycosylation

We report two unrelated families with multigenerational nonsyndromic intellectual disability (ID) segregating with a recurrent de novo missense variant (c.1543C>T:p.Leu515Phe) in the alkali cation/proton exchanger gene SLC9A7 (also commonly referred to as NHE7). SLC9A7 is located on human X chromosome at Xp11.3 and has not yet been associated with a human phenotype. The gene is widely transcribed, but especially abundant in brain, skeletal muscle and various secretory tissues. Within cells, SLC9A7 resides in the Golgi apparatus, with prominent enrichment in the trans-Golgi network (TGN) and post-Golgi vesicles. In transfected Chinese hamster ovary AP-1 cells, the Leu515Phe mutant protein was correctly targeted to the TGN/post-Golgi vesicles, but its N-linked oligosaccharide maturation as well as that of a co-transfected secretory membrane glycoprotein, vesicular stomatitis virus G (VSVG) glycoprotein, was reduced compared to cells co-expressing SLC9A7 wild-type and VSVG. This correlated with alkalinization of the TGN/post-Golgi compartments, suggestive of a gain-of-function. Membrane trafficking of glycosylation-deficient Leu515Phe and co-transfected VSVG to the cell surface, however, was relatively unaffected. Mass spectrometry analysis of patient sera also revealed an abnormal N-glycosylation profile for transferrin, a clinical diagnostic marker for congenital disorders of glycosylation. These data implicate a crucial role for SLC9A7 in the regulation of TGN/post-Golgi pH homeostasis and glycosylation of exported cargo, which may underlie the cellular pathophysiology and neurodevelopmental deficits associated with this particular nonsyndromic form of X-linked ID.

Asparagine-linked glycosylation modifies voltage-dependent gating properties of CaV3.1-T-type Ca2+ channel

T-type channels are low-voltage-activated channels that play a role in the cardiovascular system particularly for pacemaker activity. Glycosylation is one of the most prevalent post-translational modifications in protein. Among various glycosylation types, the most common one is asparagine-linked (N-linked) glycosylation. The aim of this study was to elucidate the roles of N-linked glycosylation for the gating properties of the CaV3.1-T-type Ca2+ channel. N-linked glycosylation synthesis inhibitor tunicamycin causes a reduction of CaV3.1-T-type Ca2+ channel current (CaV3.1-ICa.T) when applied for 12 h or longer. Tunicamycin (24 h) significantly shifted the activation curve to the depolarization potentials, whereas the steady-state inactivation curve was unaffected. Use-dependent inactivation of CaV3.1-ICa.T was accelerated, and recovery from inactivation was prolonged by tunicamycin (24 h). CaV3.1-ICa.T was insensitive to a glycosidase PNGase F when the channels were expressed on the plasma membrane. These findings suggest that N-glycosylation contributes not only to the cell surface expression of the CaV3.1-T-type Ca2+ channel but to the regulation of the gating properties of the channel when the channel proteins were processed during the folding and trafficking steps in the cell.

Accumulation of α-2,6-sialyoglycoproteins in the Muscle Sarcoplasm Due to Trichinella Sp. Invasion

The sialylation of the glycoproteins in skeletal muscle tissue is not well investigated, even though the essential role of the sialic acids for the proper muscular function has been proven by many researchers. The invasion of the parasitic nematode Trichinella spiralis in the muscles with subsequent formation of Nurse cell-parasite complex initiates increased accumulation of sialylated glycoproteins within the affected area of the muscle fiber. The aim of this study is to describe some details of the α-2,6-sialylation in invaded muscle cells. Asynchronous invasion with infectious T. spiralis larvae was experimentally induced in mice. The areas of the occupied sarcoplasm were reactive towards α-2,6-sialic acid specific Sambucus nigra agglutinin during the whole process of transformation to a Nurse cell.The cytoplasm of the developing Nurse cell reacted with Helix pomatia agglutinin, Arachis hypogea agglutinin and Vicia villosa lectin-B4 after neuraminidase pretreatment.Up-regulation of the enzyme ST6GalNAc1 and down-regulation of the enzyme ST6GalNAc3 were detected throughout the course of this study. The results from our study assumed accumulation of sialyl-Tn-Ag, 6`-sialyl lactosamine, SiA-α-2,6-Gal-β-1,3-GalNAc-α-Ser/Thr and Gal-β-1,3-GalNAc(SiA-α-2,6-)-α-1-Ser/Thr oligosaccharide structures into the occupied sarcoplasm. Further investigations in this domain will develop the understanding about the amazing adaptive capabilities of skeletal muscle tissue.

Eukaryotic Voltage-Gated Sodium Channels: On Their Origins, Asymmetries, Losses, Diversification and Adaptations

The appearance of voltage-gated, sodium-selective channels with rapid gating kinetics was a limiting factor in the evolution of nervous systems. Two rounds of domain duplications generated a common 24 transmembrane segment (4 × 6 TM) template that is shared amongst voltage-gated sodium (Na_v1 and Na_v2) and calcium channels (Ca_v1, Ca_v2, and Ca_v3) and leak channel (NALCN) plus homologs from yeast, different single-cell protists (heterokont and unikont) and algae (green and brown). A shared architecture in 4 × 6 TM channels include an asymmetrical arrangement of extended extracellular L5/L6 turrets containing a 4-0-2-2 pattern of cysteines, glycosylated residues, a universally short III-IV cytoplasmic linker and often a recognizable, C-terminal PDZ binding motif. Six intron splice junctions are conserved in the first domain, including a rare U12-type of the minor spliceosome provides support for a shared heritage for sodium and calcium channels, and a separate lineage for NALCN. The asymmetrically arranged pores of 4x6 TM channels allows for a changeable ion selectivity by means of a single lysine residue change in the high field strength site of the ion selectivity filter in Domains II or III. Multicellularity and the appearance of systems was an impetus for Na_v1 channels to adapt to sodium ion selectivity and fast ion gating. A non-selective, and slowly gating Na_v2 channel homolog in single cell eukaryotes, predate the diversification of Na_v1 channels from a basal homolog in a common ancestor to extant cnidarians to the nine vertebrate Na_v1.x channel genes plus Nax. A close kinship between Na_v2 and Na_v1 homologs is evident in the sharing of most (twenty) intron splice junctions. Different metazoan groups have lost their Na_v1 channel genes altogether, while vertebrates rapidly expanded their gene numbers. The expansion in vertebrate Na_v1 channel genes fills unique functional niches and generates overlapping properties contributing to redundancies. Specific nervous system adaptations include cytoplasmic linkers with phosphorylation sites and tethered elements to protein assemblies in First Initial Segments and nodes of Ranvier. Analogous accessory beta subunit appeared alongside Na_v1 channels within different animal sub-phyla. Na_v1 channels contribute to pace-making as persistent or resurgent currents, the former which is widespread across animals, while the latter is a likely vertebrate adaptation.

The role of the gap junction perinexus in cardiac conduction: Potential as a novel anti-arrhythmic drug target

Cardiovascular disease remains the single largest cause of natural death in the United States, with a significant cause of mortality associated with cardiac arrhythmias. Presently, options for treating and preventing myocardial electrical dysfunction, including sudden cardiac death, are limited. Recent studies have indicated that conduction of electrical activation in the heart may have an ephaptic component, wherein intercellular coupling occurs via electrochemical signaling across narrow extracellular clefts between cardiomyocytes. The perinexus is a 100-200 nm-wide stretch of closely apposed membrane directly adjacent to connexin 43 gap junctions. Electron and super-resolution microscopy studies, as well as biochemical analyses, have provided evidence that perinexal nanodomains may be candidate structures for facilitating ephaptic coupling. This work has included characterization of the perinexus as a region of close inter-membrane contact between cardiomyocytes (<30 nm) containing dense clusters of voltage-gated sodium channels. Here, we review what is known about perinexal structure and function and the potential that the perinexus may have novel and pivotal roles in disorders of cardiac conduction. Of particular interest is the prospect that cell adhesion mediated by the cardiac sodium channel β subunit (Scn1b) may be a novel anti-arrhythmic target.

Co-expression of β Subunits with the Voltage-Gated Sodium Channel NaV1.7: the Importance of Subunit Association and Phosphorylation and Their Effects on Channel Pharmacology and Biophysics

The voltage-gated sodium ion channel Na_V1.7 is crucial in pain signaling. We examined how auxiliary β2 and β3 subunits and the phosphorylation state of the channel influence its biophysical properties and pharmacology. The human Na_V1.7α subunit was co-expressed with either β2 or β3 subunits in HEK-293 cells. The β2 subunits and the Na_V1.7α, however, were barely associated as evidenced by immunoprecipitation. Therefore, the β2 subunits did not change the biophysical properties of the channel. In contrast, β3 subunit was clearly associated with Na_V1.7α. This subunit had a significant degree of glycosylation, and only the fully glycosylated β3 subunit was associated with the Na_V1.7α. Electrophysiological characterisation revealed that the β3 subunit had small but consistent effects: a right-hand shift of the steady-state inactivation and faster recovery from inactivation. Furthermore, the β3 subunit reduced the susceptibility of Na_V1.7α to several sodium channel blockers. In addition, we assessed the functional effect of Na_V1.7α phosphorylation. Inhibition of kinase activity increased channel inactivation, while the blocking phosphatases produced the opposite effect. In conclusion, co-expression of β subunits with Na_V1.7α, to better mimic the native channel properties, may be ineffective in cases when subunits are not associated, as shown in our experiments with β2. The β3 subunit significantly influences the function of Na_V1.7α and, together with the phosphorylation of the channel, regulates its biophysical and pharmacological properties. These are important findings to take into account when considering the role of Na_V1.7 channel in pain signaling.

Voltage-Gated Sodium Channel β1/β1B Subunits Regulate Cardiac Physiology and Pathophysiology

Cardiac myocyte contraction is initiated by a set of intricately orchestrated electrical impulses, collectively known as action potentials (APs). Voltage-gated sodium channels (NaVs) are responsible for the upstroke and propagation of APs in excitable cells, including cardiomyocytes. NaVs consist of a single, pore-forming α subunit and two different β subunits. The β subunits are multifunctional cell adhesion molecules and channel modulators that have cell type and subcellular domain specific functional effects. Variants in SCN1B, the gene encoding the Nav-β1 and -β1B subunits, are linked to atrial and ventricular arrhythmias, e.g., Brugada syndrome, as well as to the early infantile epileptic encephalopathy Dravet syndrome, all of which put patients at risk for sudden death. Evidence over the past two decades has demonstrated that Nav-β1/β1B subunits play critical roles in cardiac myocyte physiology, in which they regulate tetrodotoxin-resistant and -sensitive sodium currents, potassium currents, and calcium handling, and that Nav-β1/β1B subunit dysfunction generates substrates for arrhythmias. This review will highlight the role of Nav-β1/β1B subunits in cardiac physiology and pathophysiology.

Cross-kingdom auxiliary subunit modulation of a voltage-gated sodium channel

Voltage-gated, sodium ion-selective channels (NaV) generate electrical signals contributing to the upstroke of the action potential in animals. NaVs are also found in bacteria and are members of a larger family of tetrameric voltage-gated channels that includes CaVs, KVs, and NaVs. Prokaryotic NaVs likely emerged from a homotetrameric Ca2+-selective voltage-gated progenerator, and later developed Na+ selectivity independently. The NaV signaling complex in eukaryotes contains auxiliary proteins, termed beta (β) subunits, which are potent modulators of the expression profiles and voltage-gated properties of the NaV pore, but it is unknown whether they can functionally interact with prokaryotic NaV channels. Herein, we report that the eukaryotic NaVβ1-subunit isoform interacts with and enhances the surface expression as well as the voltage-dependent gating properties of the bacterial NaV, NaChBac in Xenopus oocytes. A phylogenetic analysis of the β-subunit gene family proteins confirms that these proteins appeared roughly 420 million years ago and that they have no clear homologues in bacterial phyla. However, a comparison between eukaryotic and bacterial NaV structures highlighted the presence of a conserved fold, which could support interactions with the β-subunit. Our electrophysiological, biochemical, structural, and bioinformatics results suggests that the prerequisites for β-subunit regulation are an evolutionarily stable and intrinsic property of some voltage-gated channels.

Implantation of hyaluronic acid hydrogel prevents the pain phenotype in a rat model of intervertebral disc injury

Painful intervertebral disc degeneration is mediated by inflammation that modulates glycosylation and induces hyperinnervation and sensory sensitization, which result in discogenic pain. Hyaluronic acid (HA) used as a therapeutic biomaterial can reduce inflammation and pain, but the effects of HA therapy on glycosylation and pain associated with disc degeneration have not been previously determined. We describe a novel rat model of pain induced by intervertebral disc injury, with validation of the pain phenotype by morphine treatment. Using this model, we assessed the efficacy of HA hydrogel for the alleviation of pain, demonstrating that it reduced nociceptive behavior, an effect associated with down-regulation of nociception markers and inhibition of hyperinnervation. Furthermore, HA hydrogel altered glycosylation and modulated key inflammatory and regulatory signaling pathways, resulting in attenuation of inflammation and regulation of matrix components. Our results suggest that HA hydrogel is a promising clinical candidate for the treatment of back pain caused by degenerated discs.

Voltage-gated sodium channel β subunits: The power outside the pore in brain development and disease

Voltage gated sodium channels (VGSCs) were first identified in terms of their role in the upstroke of the action potential. The underlying proteins were later identified as saxitoxin and scorpion toxin receptors consisting of α and β subunits. We now know that VGSCs are heterotrimeric complexes consisting of a single pore forming α subunit joined by two β subunits; a noncovalently linked β1 or β3 and a covalently linked β2 or β4 subunit. VGSC α subunits contain all the machinery necessary for channel cell surface expression, ion conduction, voltage sensing, gating, and inactivation, in one central, polytopic, transmembrane protein. VGSC β subunits are more than simple accessories to α subunits. In the more than two decades since the original cloning of β1, our knowledge of their roles in physiology and pathophysiology has expanded immensely. VGSC β subunits are multifunctional. They confer unique gating mechanisms, regulate cellular excitability, affect brain development, confer distinct channel pharmacology, and have functions that are independent of the α subunits. The vast array of functions of these proteins stems from their special station in the channelome: being the only known constituents that are cell adhesion and intra/extracellular signaling molecules in addition to being part of channel complexes. This functional trifecta and how it goes awry demonstrates the power outside the pore in ion channel signaling complexes, broadening the term channelopathy beyond defects in ion conduction. This article is part of the Special Issue entitled 'Channelopathies.'

Posttranslational Modification of Sodium Channels

Voltage-gated sodium channels (VGSCs) are critical determinants of excitability. The properties of VGSCs are thought to be tightly controlled. However, VGSCs are also subjected to extensive modifications. Multiple posttranslational modifications that covalently modify VGSCs in neurons and muscle have been identified. These include, but are not limited to, phosphorylation, ubiquitination, palmitoylation, nitrosylation, glycosylation, and SUMOylation. Posttranslational modifications of VGSCs can have profound impact on cellular excitability, contributing to normal and abnormal physiology. Despite four decades of research, the complexity of VGSC modulation is still being determined. While some modifications have similar effects on the various VGSC isoforms, others have isoform-specific interactions. In addition, while much has been learned about how individual modifications can impact VGSC function, there is still more to be learned about how different modifications can interact. Here we review what is known about VGSC posttranslational modifications with a focus on the breadth and complexity of the regulatory mechanisms that impact VGSC properties.

Voltage-Gated Sodium Channel β Subunits and Their Related Diseases

Voltage-gated sodium channels are protein complexes comprised of one pore forming α subunit and two, non-pore forming, β subunits. The voltage-gated sodium channel β 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 β 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 β subunits in their conducting and non-conducting roles. Mutations in the β 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. β subunits thus provide novel therapeutic targets for future drug discovery.

Aberrant sialylation causes dilated cardiomyopathy and stress-induced heart failure

Dilated cardiomyopathy (DCM), the third most common cause of heart failure, is often associated with arrhythmias and sudden cardiac death if not controlled. The majority of DCM is of unknown etiology. Protein sialylation is altered in human DCM, with responsible mechanisms not yet described. Here we sought to investigate the impact of clinically relevant changes in sialylation on cardiac function using a novel model for altered glycoprotein sialylation that leads to DCM and to chronic stress-induced heart failure (HF), deletion of the sialyltransferase, ST3Gal4. We previously reported that 12- to 20-week-old ST3Gal4 (-/-) mice showed aberrant cardiac voltage-gated ion channel sialylation and gating that contribute to a pro-arrhythmogenic phenotype. Here, echocardiography supported by histology revealed modest dilated and thinner-walled left ventricles without increased fibrosis in ST3Gal4 (-/-) mice starting at 1 year of age. Cardiac calcineurin expression in younger (16-20 weeks old) ST3Gal4 (-/-) hearts was significantly reduced compared to WT. Transverse aortic constriction (TAC) was used as a chronic stressor on the younger mice to determine whether the ability to compensate against a pathologic insult is compromised in the ST3Gal4 (-/-) heart, as suggested by previous reports describing the functional implications of reduced cardiac calcineurin levels. TAC'd ST3Gal4 (-/-) mice presented with significantly reduced systolic function and ventricular dilation that deteriorated into congestive HF within 6 weeks post-surgery, while constricted WT hearts remained well-adapted throughout (ejection fraction, ST3Gal4 (-/-) = 34 ± 5.2 %; WT = 53.8 ± 7.4 %; p < 0.05). Thus, a novel, sialo-dependent model for DCM/HF is described in which clinically relevant reduced sialylation results in increased arrhythmogenicity and reduced cardiac calcineurin levels that precede cardiomyopathy and TAC-induced HF, suggesting a causal link among aberrant sialylation, chronic arrhythmia, reduced calcineurin levels, DCM in the absence of a pathologic stimulus, and stress-induced HF.

Developmental and Regulatory Functions of Na(+) Channel Non-pore-forming β Subunits

Voltage-gated Na(+) channels (VGSCs) isolated from mammalian neurons are heterotrimeric complexes containing one pore-forming α subunit and two non-pore-forming β subunits. In excitable cells, VGSCs are responsible for the initiation of action potentials. VGSC β subunits are type I topology glycoproteins, containing an extracellular amino-terminal immunoglobulin (Ig) domain with homology to many neural cell adhesion molecules (CAMs), a single transmembrane segment, and an intracellular carboxyl-terminal domain. VGSC β subunits are encoded by a gene family that is distinct from the α subunits. While α subunits are expressed in prokaryotes, β subunit orthologs did not arise until after the emergence of vertebrates. β subunits regulate the cell surface expression, subcellular localization, and gating properties of their associated α subunits. In addition, like many other Ig-CAMs, β subunits are involved in cell migration, neurite outgrowth, and axon pathfinding and may function in these roles in the absence of associated α subunits. In sum, these multifunctional proteins are critical for both channel regulation and central nervous system development.

Voltage-Gated Na+ Channels: Not Just for Conduction

Voltage-gated sodium channels (VGSCs), composed of a pore-forming α subunit and up to two associated β subunits, are critical for the initiation of the action potential (AP) in excitable tissues. Building on the monumental discovery and description of sodium current in 1952, intrepid researchers described the voltage-dependent gating mechanism, selectivity of the channel, and general structure of the VGSC channel. Recently, crystal structures of bacterial VGSC α subunits have confirmed many of these studies and provided new insights into VGSC function. VGSC β subunits, first cloned in 1992, modulate sodium current but also have nonconducting roles as cell-adhesion molecules and function in neurite outgrowth and neuronal pathfinding. Mutations in VGSC α and β genes are associated with diseases caused by dysfunction of excitable tissues such as epilepsy. Because of the multigenic and drug-resistant nature of some of these diseases, induced pluripotent stem cells and other novel approaches are being used to screen for new drugs and further understand how mutations in VGSC genes contribute to pathophysiology.

Sodium channel β subunits: emerging targets in channelopathies

Voltage-gated sodium channels (VGSCs) are responsible for the initiation and propagation of action potentials in excitable cells. VGSCs in mammalian brain are heterotrimeric complexes of α and β subunits. Although β subunits were originally termed auxiliary, we now know that they are multifunctional signaling molecules that play roles in both excitable and nonexcitable cell types and with or without the pore-forming α subunit present. β subunits function in VGSC and potassium channel modulation, cell adhesion, and gene regulation, with particularly important roles in brain development. Mutations in the genes encoding β subunits are linked to a number of diseases, including epilepsy, sudden death syndromes like SUDEP and SIDS, and cardiac arrhythmia. Although VGSC β subunit-specific drugs have not yet been developed, this protein family is an emerging therapeutic target.

Post-translational modifications of voltage-gated sodium channels in chronic pain syndromes

In the peripheral sensory nervous system the neuronal expression of voltage-gated sodium channels (Navs) is very important for the transmission of nociceptive information since they give rise to the upstroke of the action potential (AP). Navs are composed of nine different isoforms with distinct biophysical properties. Studying the mutations associated with the increase or absence of pain sensitivity in humans, as well as other expression studies, have highlighted Nav1.7, Nav1.8, and Nav1.9 as being the most important contributors to the control of nociceptive neuronal electrogenesis. Modulating their expression and/or function can impact the shape of the AP and consequently modify nociceptive transmission, a process that is observed in persistent pain conditions. Post-translational modification (PTM) of Navs is a well-known process that modifies their expression and function. In chronic pain syndromes, the release of inflammatory molecules into the direct environment of dorsal root ganglia (DRG) sensory neurons leads to an abnormal activation of enzymes that induce Navs PTM. The addition of small molecules, i.e., peptides, phosphoryl groups, ubiquitin moieties and/or carbohydrates, can modify the function of Navs in two different ways: via direct physical interference with Nav gating, or via the control of Nav trafficking. Both mechanisms have a profound impact on neuronal excitability. In this review we will discuss the role of Protein Kinase A, B, and C, Mitogen Activated Protein Kinases and Ca++/Calmodulin-dependent Kinase II in peripheral chronic pain syndromes. We will also discuss more recent findings that the ubiquitination of Nav1.7 by Nedd4-2 and the effect of methylglyoxal on Nav1.8 are also implicated in the development of experimental neuropathic pain. We will address the potential roles of other PTMs in chronic pain and highlight the need for further investigation of PTMs of Navs in order to develop new pharmacological tools to alleviate pain.

Glycosylation of solute carriers: mechanisms and functional consequences

Solute carriers (SLCs) are one of the largest groups of multi-spanning membrane proteins in mammals and include ubiquitously expressed proteins as well as proteins with highly restricted tissue expression. A vast number of studies have addressed the function and organization of SLCs as well as their posttranslational regulation, but only relatively little is known about the role of SLC glycosylation. Glycosylation is one of the most abundant posttranslational modifications of animal proteins and through recent advances in our understanding of protein-glycan interactions, the functional roles of SLC glycosylation are slowly emerging. The purpose of this review is to provide a concise overview of the aspects of glycobiology most relevant to SLCs, to discuss the roles of glycosylation in the regulation and function of SLCs, and to outline the major open questions in this field, which can now be addressed given major technical advances in this and related fields of study in recent years.

Increased sialylation as a phenomenon in accommodation of the parasitic nematode Trichinella spiralis (Owen, 1835) in skeletal muscle fibres

The biology of sialic acids has been an object of interest in many models of acquired and inherited skeletal muscle pathology. The present study focuses on the sialylation changes in mouse skeletal muscle after invasion by the parasitic nematode Trichinella spiralis (Owen, 1835). Asynchronous infection with T. spiralis was induced in mice that were sacrificed at different time points of the muscle phase of the disease. The amounts of free sialic acid, sialylated glycoproteins and total sialyltransferase activity were quantified. Histochemistry with lectins specific for sialic acid was performed in order to localise distribution of sialylated glycoconjugates and to clarify the type of linkage of the sialic acid residues on the carbohydrate chains. Elevated intracellular accumulation of α-2,3- and α-2,6-sialylated glycoconjugates was found only within the affected sarcoplasm of muscle fibres invaded by the parasite. The levels of free and protein-bound sialic acid were increased and the total sialyltransferase activity was also elevated in the skeletal muscle tissue of animals with trichinellosis. We suggest that the biological significance of this phenomenon might be associated with securing integrity of the newly formed nurse cell within the surrounding healthy skeletal muscle tissue. The increased sialylation might inhibit the affected muscle cell contractility through decreased membrane ion gating, helping the parasite accommodation process.

A new look at sodium channel β subunits

Voltage-gated sodium (Nav) channels are intrinsic plasma membrane proteins that initiate the action potential in electrically excitable cells. They are a major focus of research in neurobiology, structural biology, membrane biology and pharmacology. Mutations in Nav channels are implicated in a wide variety of inherited pathologies, including cardiac conduction diseases, myotonic conditions, epilepsy and chronic pain syndromes. Drugs active against Nav channels are used as local anaesthetics, anti-arrhythmics, analgesics and anti-convulsants. The Nav channels are composed of a pore-forming α subunit and associated β subunits. The β subunits are members of the immunoglobulin (Ig) domain family of cell-adhesion molecules. They modulate multiple aspects of Nav channel behaviour and play critical roles in controlling neuronal excitability. The recently published atomic resolution structures of the human β3 and β4 subunit Ig domains open a new chapter in the study of these molecules. In particular, the discovery that β3 subunits form trimers suggests that Nav channel oligomerization may contribute to the functional properties of some β subunits.

Statistical Metamodeling and Sequential Design of Computer Experiments to Model Glyco-Altered Gating of Sodium Channels in Cardiac Myocytes

Glycan structures account for up to 35% of the mass of cardiac sodium ( Nav ) channels. To question whether and how reduced sialylation affects Nav activity and cardiac electrical signaling, we conducted a series of in vitro experiments on ventricular apex myocytes under two different glycosylation conditions, reduced protein sialylation (ST3Gal4(-/-)) and full glycosylation (control). Although aberrant electrical signaling is observed in reduced sialylation, realizing a better understanding of mechanistic details of pathological variations in INa and AP is difficult without performing in silico studies. However, computer model of Nav channels and cardiac myocytes involves greater levels of complexity, e.g., high-dimensional parameter space, nonlinear and nonconvex equations. Traditional linear and nonlinear optimization methods have encountered many difficulties for model calibration. This paper presents a new statistical metamodeling approach for efficient computer experiments and optimization of Nav models. First, we utilize a fractional factorial design to identify control variables from the large set of model parameters, thereby reducing the dimensionality of parametric space. Further, we develop the Gaussian process model as a surrogate of expensive and time-consuming computer models and then identify the next best design point that yields the maximal probability of improvement. This process iterates until convergence, and the performance is evaluated and validated with real-world experimental data. Experimental results show the proposed algorithm achieves superior performance in modeling the kinetics of Nav channels under a variety of glycosylation conditions. As a result, in silico models provide a better understanding of glyco-altered mechanistic details in state transitions and distributions of Nav channels. Notably, ST3Gal4(-/-) myocytes are shown to have higher probabilities accumulated in intermediate inactivation during the repolarization and yield a shorter refractory period than WTs. The proposed statistical design of computer experiments is generally extensible to many other disciplines that involve large-scale and computationally expensive models.

On the multiple roles of the voltage gated sodium channel β1 subunit in genetic diseases

Voltage-gated sodium channels are intrinsic plasma membrane proteins that initiate the action potential in electrically excitable cells. They are composed of a pore-forming α-subunit and associated β-subunits. The β1-subunit was the first accessory subunit to be cloned. It can be important for controlling cell excitability and modulating multiple aspects of sodium channel physiology. Mutations of β1 are implicated in a wide variety of inherited pathologies, including epilepsy and cardiac conduction diseases. This review summarizes β1-subunit related channelopathies pointing out the current knowledge concerning their genetic background and their underlying molecular mechanisms.

Regulation of the cardiac Na+ channel NaV1.5 by post-translational modifications

The cardiac voltage-gated Na(+) channel, Na(V)1.5, is responsible for the upstroke of the action potential in cardiomyocytes and for efficient propagation of the electrical impulse in the myocardium. Even subtle alterations of Na(V)1.5 function, as caused by mutations in its gene SCN5A, may lead to many different arrhythmic phenotypes in carrier patients. In addition, acquired malfunctions of Na(V)1.5 that are secondary to cardiac disorders such as heart failure and cardiomyopathies, may also play significant roles in arrhythmogenesis. While it is clear that the regulation of Na(V)1.5 protein expression and function tightly depends on genetic mechanisms, recent studies have demonstrated that Na(V)1.5 is the target of various post-translational modifications that are pivotal not only in physiological conditions, but also in disease. In this review, we examine the recent literature demonstrating glycosylation, phosphorylation by Protein Kinases A and C, Ca(2+)/Calmodulin-dependent protein Kinase II, Phosphatidylinositol 3-Kinase, Serum- and Glucocorticoid-inducible Kinases, Fyn and Adenosine Monophosphate-activated Protein Kinase, methylation, acetylation, redox modifications, and ubiquitylation of Na(V)1.5. Modern and sensitive mass spectrometry approaches, applied directly to channel proteins that were purified from native cardiac tissues, have enabled the determination of the precise location of post-translational modification sites, thus providing essential information for understanding the mechanistic details of these regulations. The current challenge is first, to understand the roles of these modifications on the expression and the function of Na(V)1.5, and second, to further identify other chemical modifications. It is postulated that the diversity of phenotypes observed with Na(V)1.5-dependent disorders may partially arise from the complex post-translational modifications of channel protein components.

Reduced sialylation impacts ventricular repolarization by modulating specific K+ channel isoforms distinctly

Voltage-gated K(+) channels (Kv) are responsible for repolarizing excitable cells and can be heavily glycosylated. Cardiac Kv activity is indispensable where even minimal reductions in function can extend action potential duration, prolong QT intervals, and ultimately contribute to life-threatening arrhythmias. Diseases such as congenital disorders of glycosylation often cause significant cardiac phenotypes that can include arrhythmias. Here we investigated the impact of reduced sialylation on ventricular repolarization through gene deletion of the sialyltransferase ST3Gal4. ST3Gal4-deficient mice (ST3Gal4(-/-)) had prolonged QT intervals with a concomitant increase in ventricular action potential duration. Ventricular apex myocytes isolated from ST3Gal4(-/-) mice demonstrated depolarizing shifts in activation gating of the transient outward (Ito) and delayed rectifier (IKslow) components of K(+) current with no change in maximum current densities. Consistently, similar protein expression levels of the three Kv isoforms responsible for Ito and IKslow were measured for ST3Gal4(-/-) versus controls. However, novel non-enzymatic sialic acid labeling indicated a reduction in sialylation of ST3Gal4(-/-) ventricular Kv4.2 and Kv1.5, which contribute to Ito and IKslow, respectively. Thus, we describe here a novel form of regulating cardiac function through the activities of a specific glycogene product. Namely, reduced ST3Gal4 activity leads to a loss of isoform-specific Kv sialylation and function, thereby limiting Kv activity during the action potential and decreasing repolarization rate, which likely contributes to prolonged ventricular repolarization. These studies elucidate a novel role for individual glycogene products in contributing to a complex network of cardiac regulation under normal and pathologic conditions.

Sialic acids attached to N- and O-glycans within the Nav1.4 D1S5-S6 linker contribute to channel gating

BACKGROUND

Voltage-gated Na+ channels (Nav) are responsible for the initiation and conduction of neuronal and muscle action potentials. Nav gating can be altered by sialic acids attached to channel N-glycans, typically through isoform-specific electrostatic mechanisms.

METHODS

Using two sets of Chinese Hamster Ovary cell lines with varying abilities to glycosylate glycoproteins, we show for the first time that sialic acids attached to O-glycans and N-glycans within the Nav1.4 D1S5-S6 linker modulate Nav gating.

RESULTS

All measured steady-state and kinetic parameters were shifted to more depolarized potentials under conditions of essentially no sialylation. When sialylation of only N-glycans or of only O-glycans was prevented, the observed voltage-dependent parameter values were intermediate between those observed under full versus no sialylation. Immunoblot gel shift analyses support the biophysical data.

CONCLUSIONS

The data indicate that sialic acids attached to both N- and O-glycans residing within the Nav1.4 D1S5-S6 linker modulate channel gating through electrostatic mechanisms, with the relative contribution of sialic acids attached to N- versus O-glycans on channel gating being similar.

GENERAL SIGNIFICANCE

Protein N- and O-glycosylation can modulate ion channel gating simultaneously. These data also suggest that environmental, metabolic, and/or congenital changes in glycosylation that impact sugar substrate levels, could lead, potentially, to changes in Nav sialylation and gating that would modulate AP waveforms and conduction.

Physiologic and pathophysiologic consequences of altered sialylation and glycosylation on ion channel function

Voltage-gated ion channels are transmembrane proteins that regulate electrical excitability in cells and are essential components of the electrically active tissues of nerves, muscle and the heart. Potassium channels are one of the largest subfamilies of voltage sensitive channels and are among the most-studied of the voltage-gated ion channels. Voltage-gated channels can be glycosylated and changes in the glycosylation pattern can affect ion channel function, leading to neurological and neuromuscular disorders and congenital disorders of glycosylation (CDG). Alterations in glycosylation can also be acquired and appear to play a role in development and aging. Recent studies have focused on the impact of glycosylation and sialylation on ion channels, particularly for voltage-gated potassium and sodium channels. The terminal step of sialylation often affects channel activation and inactivation kinetics. The presence of sialic acids on O or N-glycans can alter the gating mechanism and cause conformational changes in the voltage-sensing domains due to sialic acid's negative charges. This manuscript will provide an overview of sialic acids, potassium and sodium channel function, and the impact of sialylation on channel activation and deactivation.