Isoform-specific effects of sialic acid on voltage-dependent Na+ channel gating: functional sialic acids are localized to the S5-S6 loop of domain I

Enzyme-Digested Edible Bird’s Nest (EBND) Prevents UV and arid Environment-Induced Cellular Oxidative Stress, Cell Death and DNA Damage in Human Skin Keratinocytes and Three-Dimensional Epithelium Equivalents

The aim of this study is to investigate the repressive effects of enzyme-digested edible bird’s nest (EBND) on the combination of arid environment and UV-induced intracellular oxidative stress, cell death, DNA double-strand breaks (DSBs) and inflammatory responses in human HaCaT keratinocytes and three-dimensional (3D) epithelium equivalents. An oxygen radical antioxidant capacity assay showed that EBND exhibited excellent peroxyl radical scavenging activity and significantly increased cellular antioxidant capacity in HaCaT cells. When EBND was administered to HaCaT cells and 3D epitheliums, it exhibited significant preventive effects on air-drying and UVA (Dry-UVA)-induced cell death and apoptosis. Dry-UVA markedly induced intracellular reactive oxygen species (ROS) generation in HaCaT cells and 3D epitheliums as quantified by CellROX® Green/Orange reagents. Once HaCaT cells and 3D epitheliums were pretreated with EBND, Dry-UVA-induced intracellular ROS were significantly reduced. The results from anti-γ-H2A.X antibody-based immunostaining showed that EBND significantly inhibited Dry-UVA-induced DSBs in HaCaT keratinocytes. Compared with sialic acid, EBND showed significantly better protection for both keratinocytes and 3D epitheliums against Dry-UVA-induced injuries. ELISA showed that EBND significantly suppressed UVB-induced IL-6 and TNF-α secretion. In conclusion, EBND could decrease arid environments and UV-induced harmful effects and inflammatory responses in human keratinocytes and 3D epithelium equivalents partially through its antioxidant capacity.

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.

Genomic and Non-Genomic Regulatory Mechanisms of the Cardiac Sodium Channel in Cardiac Arrhythmias

Nav1.5 is the predominant cardiac sodium channel subtype, encoded by the SCN5A gene, which is involved in the initiation and conduction of action potentials throughout the heart. Along its biosynthesis process, Nav1.5 undergoes strict genomic and non-genomic regulatory and quality control steps that allow only newly synthesized channels to reach their final membrane destination and carry out their electrophysiological role. These regulatory pathways are ensured by distinct interacting proteins that accompany the nascent Nav1.5 protein along with different subcellular organelles. Defects on a large number of these pathways have a tremendous impact on Nav1.5 functionality and are thus intimately linked to cardiac arrhythmias. In the present review, we provide current state-of-the-art information on the molecular events that regulate SCN5A/Nav1.5 and the cardiac channelopathies associated with defects in these pathways.

Neuraminidase-1 promotes heart failure after ischemia/reperfusion injury by affecting cardiomyocytes and invading monocytes/macrophages

Neuraminidase (NEU)1 forms a multienzyme complex with beta-galactosidase (β-GAL) and protective-protein/cathepsin (PPC) A, which cleaves sialic-acids from cell surface glycoconjugates. We investigated the role of NEU1 in the myocardium after ischemia/reperfusion (I/R). Three days after inducing I/R, left ventricles (LV) of male mice (3 months-old) displayed upregulated neuraminidase activity and increased NEU1, β-GAL and PPCA expression. Mice hypomorphic for neu1 (hNEU1) had less neuraminidase activity, fewer pro-inflammatory (Lin⁻CD11b⁺F4/80⁺Ly-6C^high), and more anti-inflammatory macrophages (Lin⁻CD11b⁺F4/80⁺Ly-6C^low) 3 days after I/R, and less LV dysfunction 14 days after I/R. WT mice transplanted with hNEU1-bone marrow (BM) and hNEU1 mice with WT-BM showed significantly better LV function 14 days after I/R compared with WT mice with WT-BM. Mice with a cardiomyocyte-specific NEU1 overexpression displayed no difference in inflammation 3 days after I/R, but showed increased cardiomyocyte hypertrophy, reduced expression and mislocalization of Connexin-43 in gap junctions, and LV dysfunction despite a similar infarct scar size to WT mice 14 days after I/R. The upregulation of NEU1 after I/R contributes to heart failure by promoting inflammation in invading monocytes/macrophages, enhancing cardiomyocyte hypertrophy, and impairing gap junction function, suggesting that systemic NEU1 inhibition may reduce heart failure after I/R.

N-Glycosylation of TREK-1/hK2P2.1 Two-Pore-Domain Potassium (K2P) Channels

Mechanosensitive hTREK-1 two-pore-domain potassium (hK_2P2.1) channels give rise to background currents that control cellular excitability. Recently, TREK-1 currents have been linked to the regulation of cardiac rhythm as well as to hypertrophy and fibrosis. Even though the pharmacological and biophysical characteristics of hTREK-1 channels have been widely studied, relatively little is known about their posttranslational modifications. This study aimed to evaluate whether hTREK-1 channels are N-glycosylated and whether glycosylation may affect channel functionality. Following pharmacological inhibition of N-glycosylation, enzymatic digestion or mutagenesis, immunoblots of Xenopus laevis oocytes and HEK-293T cell lysates were used to assess electrophoretic mobility. Two-electrode voltage clamp measurements were employed to study channel function. TREK-1 channel subunits undergo N-glycosylation at asparagine residues 110 and 134. The presence of sugar moieties at these two sites increases channel function. Detection of glycosylation-deficient mutant channels in surface fractions and recordings of macroscopic potassium currents mediated by these subunits demonstrated that nonglycosylated hTREK-1 channel subunits are able to reach the cell surface in general but with seemingly reduced efficiency compared to glycosylated subunits. These findings extend our understanding of the regulation of hTREK-1 currents by posttranslational modifications and provide novel insights into how altered ion channel glycosylation may promote arrhythmogenesis.

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.

A mutation of SCN1B associated with GEFS+ causes functional and maturation defects of the voltage-dependent sodium channel

Voltage-dependent sodium channels are responsible of the rising phase of the action potential in excitable cells. These integral membrane proteins are composed of a pore-forming α-subunit, and one or more auxiliary β subunits. Mutation p.Asp25Asn (D25N; c.73G > A) of the β1 subunit, coded by the gene SCN1B, has been reported in a patient with generalized epilepsy with febrile seizure plus type 1 (GEFS+). In human embryonic kidney 293 (HEK) cells, the heterologous coexpression of D25N-β1 subunit with Nav1.2, Nav1.4, and Nav1.5 α subunits, representative of brain, skeletal muscle, and heart voltage gated sodium channels, determines a reduced sodium channel functional expression and a negative shift of the activation and inactivation steady state curves. The D25N mutation of the β1 subunit causes a maturation (glycosylation) defect of the protein, leading to a reduced targeting to the plasma membrane. Also the β1-dependent gating properties of the sodium channels are abolished by the mutation, suggesting that D25N is no more able to interact with the α subunit. Our work underscores the role played by the β1 subunit, highlighting how a defective interaction between the sodium channel constituents could lead to a disabling pathological condition, and opens the possibility to design a mutation-specific GEFS+ treatment based on protein maturation.

Deglycosylation of Shaker KV channels affects voltage sensing and the open-closed transition

Voltage-gated ion channels are subject to posttranslational modification, including glycosylation. Lopez-Rodriguez and Holmgren show that, in Shaker K_V channels, deglycosylation influences voltage sensing and open–closed transitions but not binding of ligands to the protein.

Most membrane proteins are subject to posttranslational glycosylation, which influences protein function, folding, solubility, stability, and trafficking. This modification has been proposed to protect proteins from proteolysis and modify protein–protein interactions. Voltage-activated ion channels are heavily glycosylated, which can result in up to 30% of the mature molecular mass being contributed by glycans. Normally, the functional consequences of glycosylation are assessed by comparing the function of fully glycosylated proteins with those in which glycosylation sites have been mutated or by expressing proteins in model cells lacking glycosylation enzymes. Here, we study the functional consequences of deglycosylation by PNGase F within the same population of voltage-activated potassium (K_V) channels. We find that removal of sugar moieties has a small, but direct, influence on the voltage-sensing properties and final opening–closing transition of Shaker K_V channels. Yet, we observe that the interactions of various ligands with different domains of the protein are not affected by deglycosylation. These results imply that the sugar mass attached to the voltage sensor neither represents a cargo for the dynamics of this domain nor imposes obstacles to the access of interacting molecules.

Biphasic voltage-dependent inactivation of human NaV 1.3, 1.6 and 1.7 Na+ channels expressed in rodent insulin-secreting cells

KEY POINTS

Na+ current inactivation is biphasic in insulin-secreting cells, proceeding with two voltage dependences that are half-maximal at ∼-100 mV and -60 mV. Inactivation of voltage-gated Na+ (NaV ) channels occurs at ∼30 mV more negative voltages in insulin-secreting Ins1 and primary β-cells than in HEK, CHO or glucagon-secreting αTC1-6 cells. The difference in inactivation between Ins1 and non-β-cells persists in the inside-out patch configuration, discounting an involvement of a diffusible factor. In Ins1 cells and primary β-cells, but not in HEK cells, inactivation of a single NaV subtype is biphasic and follows two voltage dependences separated by 30-40 mV. We propose that NaV channels adopt different inactivation behaviours depending on the local membrane environment.

ABSTRACT

Pancreatic β-cells are equipped with voltage-gated Na+ channels that undergo biphasic voltage-dependent steady-state inactivation. A small Na+ current component (10-15%) inactivates over physiological membrane potentials and contributes to action potential firing. However, the major Na+ channel component is completely inactivated at -90 to -80 mV and is therefore inactive in the β-cell. It has been proposed that the biphasic inactivation reflects the contribution of different NaV α-subunits. We tested this possibility by expression of TTX-resistant variants of the NaV subunits found in β-cells (NaV 1.3, NaV 1.6 and NaV 1.7) in insulin-secreting Ins1 cells and in non-β-cells (including HEK and CHO cells). We found that all NaV subunits inactivated at 20-30 mV more negative membrane potentials in Ins1 cells than in HEK or CHO cells. The more negative inactivation in Ins1 cells does not involve a diffusible intracellular factor because the difference between Ins1 and CHO persisted after excision of the membrane. NaV 1.7 inactivated at 15--20 mV more negative membrane potentials than NaV 1.3 and NaV 1.6 in Ins1 cells but this small difference is insufficient to solely explain the biphasic inactivation in Ins1 cells. In Ins1 cells, but never in the other cell types, widely different components of NaV inactivation (separated by 30 mV) were also observed following expression of a single type of NaV α-subunit. The more positive component exhibited a voltage dependence of inactivation similar to that found in HEK and CHO cells. We propose that biphasic NaV inactivation in insulin-secreting cells reflects insertion of channels in membrane domains that differ with regard to lipid and/or membrane protein composition.

Mutation E87Q of the β1-subunit impairs the maturation of the cardiac voltage-dependent sodium channel

Voltage-dependent sodium channels are responsible of the rising phase of the action potential in excitable cells. These membrane integral proteins are composed by a pore-forming α-subunit, and one or more auxiliary β subunits. Mutation E87Q of the β1 subunit is correlated with Brugada syndrome, a genetic disease characterised by ventricular fibrillation, right precordial ST segment elevation on ECG and sudden cardiac death. Heterologous expression of E87Q-β1 subunit in CHO cells determines a reduced sodium channel functional expression. The effect the E87Q mutation of the β1 subunit on sodium currents and α protein expression is correlated with a reduced availability of the mature form of the α subunit in the plasma membrane. This finding offers a new target for the treatment of the Brugada syndrome, based on protein maturation management. This work highlights the role played by the β1 subunit in the maturation and expression of the entire sodium channel complex and underlines how the defective interaction between the sodium channel constituents could lead to a disabling pathological condition.

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.

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.

Sialic acid accelerates the electrophoretic velocity of injured dorsal root ganglion neurons

Peripheral nerve injury has been shown to result in ectopic spontaneous discharges on soma and injured sites of sensory neurons, thereby inducing neuropathic pain. With the increase of membrane proteins on soma and injured site neurons, the negatively charged sialic acids bind to the external domains of membrane proteins, resulting in an increase of this charge. We therefore speculate that the electrophoretic velocity of injured neurons may be faster than non-injured neurons. The present study established rat models of neuropathic pain via chronic constriction injury. Results of the cell electrophoresis test revealed that the electrophoretic velocity of injured neuronal cells was faster than that of non-injured (control) cells. We then treated cells with divalent cations of Ca²⁺ and organic compounds with positive charges, polylysine to counteract the negatively charged sialic acids, or neuraminidase to specifically remove sialic acids from the membrane surface of injured neurons. All three treatments significantly reduced the electrophoretic velocity of injured neuronal cells. These findings suggest that enhanced sialic acids on injured neurons may accelerate the electrophoretic velocity of injured neurons.

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.

The Sick and the Weak: Neuropathies/Myopathies in the Critically Ill

Critical illness polyneuropathies (CIP) and myopathies (CIM) are common complications of critical illness. Several weakness syndromes are summarized under the term intensive care unit-acquired weakness (ICUAW). We propose a classification of different ICUAW forms (CIM, CIP, sepsis-induced, steroid-denervation myopathy) and pathophysiological mechanisms from clinical and animal model data. Triggers include sepsis, mechanical ventilation, muscle unloading, steroid treatment, or denervation. Some ICUAW forms require stringent diagnostic features; CIM is marked by membrane hypoexcitability, severe atrophy, preferential myosin loss, ultrastructural alterations, and inadequate autophagy activation while myopathies in pure sepsis do not reproduce marked myosin loss. Reduced membrane excitability results from depolarization and ion channel dysfunction. Mitochondrial dysfunction contributes to energy-dependent processes. Ubiquitin proteasome and calpain activation trigger muscle proteolysis and atrophy while protein synthesis is impaired. Myosin loss is more pronounced than actin loss in CIM. Protein quality control is altered by inadequate autophagy. Ca(2+) dysregulation is present through altered Ca(2+) homeostasis. We highlight clinical hallmarks, trigger factors, and potential mechanisms from human studies and animal models that allow separation of risk factors that may trigger distinct mechanisms contributing to weakness. During critical illness, altered inflammatory (cytokines) and metabolic pathways deteriorate muscle function. ICUAW prevention/treatment is limited, e.g., tight glycemic control, delaying nutrition, and early mobilization. Future challenges include identification of primary/secondary events during the time course of critical illness, the interplay between membrane excitability, bioenergetic failure and differential proteolysis, and finding new therapeutic targets by help of tailored animal models.

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.

Nav1.5 channels can reach the plasma membrane through distinct N-glycosylation states

BACKGROUND

Like many voltage-gated sodium channels, the cardiac isoform Nav1.5 is well known as a glycoprotein which necessarily undergoes N-glycosylation processing during its transit to the plasma membrane. In some cardiac disorders, especially the Brugada syndrome (BrS), mutations in Nav1.5 encoding gene lead to intracellular retention and consequently trafficking defect of these proteins. We used two BrS mutants as tools to clarify both Nav1.5 glycosylation states and associated secretory behaviors.

METHODS

Patch-clamp recordings and surface biotinylation assays of HEK293T cells expressing wild-type (WT) and/or mutant Nav1.5 proteins were performed to assess the impact of mutant co-expression on the membrane activity and localization of WT channels. Enzymatic deglycosylation assays and brefeldin A (BFA) treatments were also employed to further characterize recombinant and native Nav1.5 maturation.

RESULTS

The present data demonstrate that Nav1.5 channels mainly exist as two differentially glycosylated forms. We reveal that dominant negative effects induced by BrS mutants upon WT channel current result from the abnormal surface expression of the fully-glycosylated forms exclusively. Furthermore, we show that core-glycosylated channels can be found at the surface membrane of BFA-treated or untreated cells, but obviously without generating any sodium current.

CONCLUSIONS

Our findings provide evidence that native and recombinant Nav1.5 subunits are expressed as two distinct matured forms. Fully-glycosylated state of Nav1.5 seems to determine its functionality whereas core-glycosylated forms might be transported to the plasma membrane through an unconventional Golgi-independent secretory route.

GENERAL SIGNIFICANCE

This work highlights that N-linked glycosylation processing would be critical for Nav1.5 membrane trafficking and function.

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.

Ionic mechanisms of microsecond-scale spike timing in single cells

Electric fish image their environments and communicate by generating electric organ discharges through the simultaneous action potentials (APs) of electric organ cells (electrocytes) in the periphery. Steatogenys elegans generates a biphasic electrocyte discharge by the precisely regulated timing and waveform of APs generated from two excitable membranes present in each electrocyte. Current-clamp recordings of electrocyte APs reveal that the posterior membrane fires first, followed ∼30 μs later by an AP on the anterior membrane. This delay was maintained even as the onset of the first AP was advanced >5 ms by increasing stimulus intensity and across multiple spikes during bursts of APs elicited by prolonged stimulation. Simultaneous cell-attached loose-patch recordings of Na(+) currents on each membrane revealed that activation voltage for Na(+) channels on the posterior membrane was 10 mV hyperpolarized compared with Na(+) channels on the anterior membrane, with no differences in activation or inactivation kinetics. Computational simulations of electrocyte APs demonstrated that this difference in Na(+) current activation voltage was sufficient to maintain the proper firing order and the interspike delay. A similar difference in activation threshold has been reported for the Na(+) currents of the axon initial segment compared with somatic Na(+) channels of pyramidal neurons, suggesting convergent evolution of spike initiation and timing mechanisms across different systems of excitable cells.

The role of protein N-glycosylation in neural transmission

Recent studies have explored the function of N-linked glycosylation in the nervous system, demonstrating essential roles of carbohydrate structures in neural development. The function of N-glycans in neural physiology remains less understood; however, increasing evidence indicates that N-glycans can play specific modulatory roles controlling neural transmission and excitability of neural circuits. These roles are mediated via effects on synaptic proteins involved in neurotransmitter release, transporters that regulate nerotransmitter concentrations, neurotransmitter receptors, as well as via regulation of proteins that control excitability and response to milieu stimuli, such as voltage-gated ion channels and transient receptor potential channels, respectively. Sialylated N-glycan structures are among the most potent modulators of cell excitability, exerting prominent effects on voltage gated Na(+) and K(+) channels. This modulation appears to be underlain by complex molecular mechanisms involving electrostatic effects, as well as interaction modes based on more specific steric effects and interactions with lectins and other molecules. Data also indicate that particular features of N-glycans, such as their location on a protein and structural characteristics, can be specifically associated with the effect of glycosylation. These features and their functional implications can vary between different cell types, which highlight the importance of in vivo analyses of glycan functions. Experimental challenges are associated with the overwhelming complexity of the nervous system and glycosylation pathways in vertebrates, and thus model organisms like Drosophila should help elucidate evolutionarily conserved mechanisms underlying glycan functions. Recent studies supported this notion and shed light on functions of several glycosylation genes involved in the regulation of the nervous system.

N-glycosylation in regulation of the nervous system

Protein N-glycosylation can influence the nervous system in a variety of ways by affecting functions of glycoproteins involved in nervous system development and physiology. The importance of N-glycans for different aspects of neural development has been well documented. For example, some N-linked carbohydrate structures were found to play key roles in neural cell adhesion and axonal targeting during development. At the same time, the involvement of glycosylation in the regulation of neural physiology remains less understood. Recent studies have implicated N-glycosylation in the regulation of neural transmission, revealing novel roles of glycans in synaptic processes and the control of neural excitability. N-Glycans were found to markedly affect the function of several types of synaptic proteins involved in key steps of synaptic transmission, including neurotransmitter release, reception, and uptake. Glycosylation also regulates a number of channel proteins, such as TRP channels that control responses to environmental stimuli and voltage-gated ion channels, the principal determinants of neuronal excitability. Sialylated carbohydrate structures play a particularly prominent part in the modulation of voltage-gated ion channels. Sialic acids appear to affect channel functions via several mechanisms, including charge interactions, as well as other interactions that probably engage steric effects and interactions with other molecules. Experiments also indicated that some structural features of glycans can be particularly important for their function. Since glycan structures can vary significantly between different cell types and depend on the metabolic state of the cell, it is important to analyze glycan functions using in vivo approaches. While the complexity of the nervous system and intricacies of glycosylation pathways can create serious obstacles for in vivo experiments in vertebrates, recent studies have indicated that more simple and experimentally tractable model organisms like Drosophila should provide important advantages for elucidating evolutionarily conserved functions of N-glycosylation in the nervous system.

Glycobiology of ion transport in the nervous system

The nervous system is richly endowed with large transmembrane proteins that mediate ion transport, including gated ion channels as well as energy-consuming pumps and transporters. Transport proteins undergo N-linked glycosylation which can affect expression, location, stability, and function. The N-linked glycans of ion channels are large, contributing between 5 and 50 % of their molecular weight. Many contain a high density of negatively charged sialic acid residues which modulate voltage-dependent gating of ion channels. Changes in the size and chemical composition of glycans are responsible for developmental and cell-specific variability in the biophysical and functional properties of many ion channels. Glycolipids, principally gangliosides, exert considerable influence on some forms of ion transport, either through direct association with ion transport proteins or indirectly through association with proteins that activate transport through appropriate signaling. Examples of both pumps and ion channels have been revealed which depend on ganglioside regulation. While some of these processes are localized in the plasma membrane, ganglioside-regulated ion transport can also occur at various loci within the cell including the nucleus. This chapter will describe ion channel and ion pump structures with a focus on the functional effects of glycosylation on ion channel availability and function, and effects of alterations in glycosylation on nervous system function. It will also summarize highlights of the research on glycolipid/ganglioside-mediated regulation of ion transport.

Modulation of voltage-gated ion channels by sialylation

Control and modulation of electrical signaling is vital to normal physiology, particularly in neurons, cardiac myocytes, and skeletal muscle. The orchestrated activities of variable sets of ion channels and transporters, including voltage-gated ion channels (VGICs), are responsible for initiation, conduction, and termination of the action potential (AP) in excitable cells. Slight changes in VGIC activity can lead to severe pathologies including arrhythmias, epilepsies, and paralyses, while normal excitability depends on the precise tuning of the AP waveform. VGICs are heavily posttranslationally modified, with upward of 30% of the mature channel mass consisting of N- and O-glycans. These glycans are terminated typically by negatively charged sialic acid residues that modulate voltage-dependent channel gating directly. The data indicate that sialic acids alter VGIC activity in isoform-specific manners, dependent in part, on the number/location of channel sialic acids attached to the pore-forming alpha and/or auxiliary subunits that often act through saturating electrostatic mechanisms. Additionally, cell-specific regulation of sialylation can affect VGIC gating distinctly. Thus, channel sialylation is likely regulated through two mechanisms that together contribute to a dynamic spectrum of possible gating motifs: a subunit-specific mechanism and regulated (aberrant) changes in the ability of the cell to glycosylate. Recent studies showed that neuronal and cardiac excitability is modulated through regulated changes in voltage-gated Na(+) channel sialylation, suggesting that both mechanisms of differential VGIC sialylation contribute to electrical signaling in the brain and heart. Together, the data provide insight into an important and novel paradigm involved in the control and modulation of electrical signaling.

The role of Drosophila cytidine monophosphate-sialic acid synthetase in the nervous system

While sialylation plays important functions in the nervous system, the complexity of glycosylation pathways and limitations of genetic approaches preclude the efficient analysis of these functions in mammalian organisms. Drosophila has recently emerged as a promising model for studying neural sialylation. Drosophila sialyltransferase, DSiaT, was shown to be involved in the regulation of neural transmission. However, the sialylation pathway was not investigated in Drosophila beyond the DSiaT-mediated step. Here we focused on the function of Drosophila cytidine monophosphate-sialic acid synthetase (CSAS), the enzyme providing a sugar donor for DSiaT. Our results revealed that the expression of CSAS is tightly regulated and restricted to the CNS throughout development and in adult flies. We generated CSAS mutants and analyzed their phenotypes using behavioral and physiological approaches. Our experiments demonstrated that mutant phenotypes of CSAS are similar to those of DSiaT, including decreased longevity, temperature-induced paralysis, locomotor abnormalities, and defects of neural transmission at neuromuscular junctions. Genetic interactions between CSAS, DSiaT, and voltage-gated channel genes paralytic and seizure were consistent with the hypothesis that CSAS and DSiaT function within the same pathway regulating neural excitability. Intriguingly, these interactions also suggested that CSAS and DSiaT have some additional, independent functions. Moreover, unlike its mammalian counterparts that work in the nucleus, Drosophila CSAS was found to be a glycoprotein-bearing N-glycans and predominantly localized in vivo to the Golgi compartment. Our work provides the first systematic analysis of in vivo functions of a eukaryotic CSAS gene and sheds light on evolutionary relationships among metazoan CSAS proteins.

Expression of the sialyltransferase, ST3Gal4, impacts cardiac voltage-gated sodium channel activity, refractory period and ventricular conduction

The sequential glycosylation process typically ends with sialic acid residues added through trans-Golgi sialyltransferase activity. Individuals afflicted with congenital disorders of glycosylation often have reduced glycoprotein sialylation and present with multi-system symptoms including hypotonia, seizures, arrhythmia and cardiomyopathy. Cardiac voltage-gated Na(+) channel (Nav) activity can be influenced by sialic acids likely contributing to an external surface potential causing channels to gate at less depolarized voltages. Here, a possible pathophysiological role for reduced sialylation is investigated by questioning the impact of gene deletion of the uniformly expressed beta-galactoside alpha-2,3-sialyltransferase 4 (ST3Gal4) on cardiac Nav activity, cellular refractory period and ventricular conduction. Whole-cell patch-clamp experiments showed that ventricular Nav from ST3Gal4 deficient mice (ST3Gal4(-/-)) gated at more depolarized potentials, inactivated more slowly and recovered from fast inactivation more rapidly than WT controls. Current-clamp recordings indicated a 20% increase in time to action potential peak and a 30ms decrease in ST3Gal4(-/-) myocyte refractory period, concurrent with increased Nav recovery rate. Nav expression, distribution and maximal Na(+) current levels were unaffected by ST3Gal4 expression, indicating that reduced sialylation does not impact Nav surface expression and distribution. However, enzymatic desialylation suggested that ST3Gal4(-/-) ventricular Nav are less sialylated. Consistent with the shortened myocyte refractory period, epicardial conduction experiments using optical mapping techniques demonstrated a 27% reduction in minimum ventricular refractory period and increased susceptibility to arrhythmias in ST3Gal4(-/-) ventricles. Thus, deletion of a single sialyltransferase significantly impacts ventricular myocyte electrical signaling. These studies offer insight into diseases of glycosylation that are often associated with pathological changes in excitability and highlight the importance of glycosylation in cardiac physiology.

Altered sodium channel-protein associations in critical illness myopathy

Background

During the acute phase of critical illness myopathy (CIM) there is inexcitability of skeletal muscle. In a rat model of CIM, muscle inexcitability is due to inactivation of sodium channels. A major contributor to this sodium channel inactivation is a hyperpolarized shift in the voltage dependence of sodium channel inactivation. The goal of the current study was to find a biochemical correlate of the hyperpolarized shift in sodium channel inactivation.

Methods

The rat model of CIM was generated by cutting the sciatic nerve and subsequent injections of dexamethasone for 7 days. Skeletal muscle membranes were prepared from gastrocnemius muscles, and purification and biochemical analyses carried out. Immunoprecipitations were performed with a pan-sodium channel antibody, and the resulting complexes probed in Western blots with various antibodies.

Results

We carried out analyses of sodium channel glycosylation, phosphorylation, and association with other proteins. Although there was some loss of channel glycosylation in the disease, as assessed by size analysis of glycosylated and de-glycosylated protein in control and CIM samples, previous work by other investigators suggest that such loss would most likely shift channel inactivation gating in a depolarizing direction; thus such loss was viewed as compensatory rather than causative of the disease. A phosphorylation site at serine 487 was identified on the Na_V 1.4 sodium channel α subunit, but there was no clear evidence of altered phosphorylation in the disease. Co-immunoprecipitation experiments carried out with a pan-sodium channel antibody confirmed that the sodium channel was associated with proteins of the dystrophin associated protein complex (DAPC). This complex differed between control and CIM samples. Syntrophin, dystrophin, and plectin associated strongly with sodium channels in both control and disease conditions, while β-dystroglycan and neuronal nitric oxide synthase (nNOS) associated strongly with the sodium channel only in CIM. Recording of action potentials revealed that denervated muscle in mice lacking nNOS was more excitable than control denervated muscle.

Conclusion

Taken together, these data suggest that the conformation/protein association of the sodium channel complex differs in control and critical illness myopathy muscle membranes; and suggest that nitric oxide signaling plays a role in development of muscle inexcitability.

Mining the virgin land of neurotoxicology: a novel paradigm of neurotoxic peptides action on glycosylated voltage-gated sodium channels

Voltage-gated sodium channels (VGSCs) are important membrane protein carrying on the molecular basis for action potentials (AP) in neuronal firings. Even though the structure-function studies were the most pursued spots, the posttranslation modification processes, such as glycosylation, phosphorylation, and alternative splicing associating with channel functions captured less eyesights. The accumulative research suggested an interaction between the sialic acids chains and ion-permeable pores, giving rise to subtle but significant impacts on channel gating. Sodium channel-specific neurotoxic toxins, a family of long-chain polypeptides originated from venomous animals, are found to potentially share the binding sites adjacent to glycosylated region on VGSCs. Thus, an interaction between toxin and glycosylated VGSC might hopefully join the campaign to approach the role of glycosylation in modulating VGSCs-involved neuronal network activity. This paper will cover the state-of-the-art advances of researches on glycosylation-mediated VGSCs function and the possible underlying mechanisms of interactions between toxin and glycosylated VGSCs, which may therefore, fulfill the knowledge in identifying the pharmacological targets and therapeutic values of VGSCs.

Biology of cardiac sodium channel Nav1.5 expression

Na(v)1.5, the pore forming α-subunit of the voltage-dependent cardiac Na(+) channel, is an integral membrane protein involved in the initiation and conduction of action potentials. Mutations in the gene-encoding Na(v)1.5, SCN5A, have been associated with a variety of arrhythmic disorders, including long QT, Brugada, and sick sinus syndromes as well as progressive cardiac conduction defect and atrial standstill. Moreover, alterations in the Na(v)1.5 expression level and/or sodium current density have been frequently noticed in acquired cardiac disorders, such as heart failure. The molecular mechanisms underlying these alterations are poorly understood, but are considered essential for conception of arrhythmogenesis and the development of therapeutic strategies for prevention or treatment of arrhythmias. The unravelling of such mechanisms requires critical molecular insight into the biology of Na(v)1.5 expression and function. Therefore, the aim of this review is to provide an up-to-date account of molecular determinants of normal Na(v)1.5 expression and function. The parts of the Na(v)1.5 life cycle that are discussed include (i) regulatory aspects of the SCN5A gene and transcript structure, (ii) the nature, molecular determinants, and functional consequences of Na(v)1.5 post-translational modifications, and (iii) the role of Na(v)1.5 interacting proteins in cellular trafficking. The reviewed studies have provided valuable information on how the Na(v)1.5 expression level, localization, and biophysical properties are regulated, but also revealed that our understanding of the underlying mechanisms is still limited.

Defective polysialylation and sialylation induce opposite effects on gating of the skeletal Na+ channel NaV1.4 in Chinese hamster ovary cells

Polysialic acid (polySia) is a large, negatively charged homopolymer of 2,8-linked N-acetylneuraminic acid residues resulting from remodeling and extension of protein-bound sialic acid (Sia) residues and seems to have a key role in regulating neural cell development and function. The aim of this study was to explore and compare the effects of polySia and sialylation on gating of voltage-gated sodium channels. The skeletal muscle α-subunit NaV1.4 was transiently expressed in wild-type Chinese hamster ovary (CHO) cells or in mutant CHO cells with deficits in their capacity to produce sialylated or polysialylated membrane components. Expression in both mutant cell lines resulted in larger peak current amplitudes as compared to wild-type CHO cells. Loss of Sia and polySia also resulted in significant shifts of voltage-dependent activation and steady-state inactivation, however, in opposite directions. Furthermore, only the loss of Sia had a significant effect on recovery from fast inactivation. Our data demonstrate for the first time that gating of voltage-gated sodium channels seems to be differentially regulated by polySia and Sia.

The endo-lysosomal system as an NAADP-sensitive acidic Ca(2+) store: role for the two-pore channels

Accumulating evidence suggests that the endo-lysosomal system provides a substantial store of Ca(2+) that is tapped by the Ca(2+)-mobilizing messenger, NAADP. In this article, we review evidence that NAADP-mediated Ca(2+) release from this acidic Ca(2+) store proceeds through activation of the newly described two-pore channels (TPCs). We discuss recent advances in defining the sub-cellular targeting, topology and biophysics of TPCs. We also discuss physiological roles and the evolution of this ubiquitous ion channel family.

Membrane topology of NAADP-sensitive two-pore channels and their regulation by N-linked glycosylation

Two-pore channels (TPCs) localize to the endolysosomal system and have recently emerged as targets for the Ca(2+)-mobilizing messenger, nicotinic acid adenine dinucleotide phosphate (NAADP). However, their membrane topology is unknown. Using fluorescence protease protection assays, we show that human TPC1 and TPC2 possess cytosolic N and C termini and therefore an even number of transmembrane regions. Fluorophores placed at position 225 or 347 in TPC1, or 339 in TPC2 were also cytosolic, whereas a fluorophore at position 628 in TPC1 was luminal. These data together with sequence similarity to voltage-gated Ca(2+) and Na(+) channels, and unbiased in silico predictions are consistent with a topology in which two homologous domains are present, each comprising 6 transmembrane regions and a re-entrant pore loop. Immunocytochemical analysis of selectively permeabilized cells using antipeptide antibodies confirmed that the C-terminal tails of recombinant TPCs are cytosolic and that residues 240-254 of TPC2 prior to putative pore 1 are luminal. Both TPC1 and TPC2 are N-glycosylated with residues 599, 611, and 616 contributing to glycosylation of TPC1. This confirms the luminal position of these residues, which immediately precede the putative pore loop of the second domain. Mutation of all three glycosylation sites in TPC1 enhances NAADP-evoked cytosolic Ca(2+) signals. Our data establish essential features of the topology of two-pore channels.

Sialic acids attached to O-glycans modulate voltage-gated potassium channel gating

Neuronal, cardiac, and skeletal muscle action potentials are produced and conducted through the highly regulated activity of several types of voltage-gated ion channels. Voltage-gated potassium (K(v)) channels are responsible for action potential repolarization. Glycans can be attached to glycoproteins through N- and O-linkages. Previous reports described the impact of N-glycans on voltage-gated ion channel function. Here, we show that sialic acids attached through O-linkages modulate gating of K(v)2.1, K(v)4.2, and K(v)4.3. The conductance-voltage (G-V) relationships for each isoform were shifted uniquely by a depolarizing 8-16 mV under conditions of reduced sialylation. The data indicate that sialic acids modulate K(v) channel activation through apparent electrostatic mechanisms that promote channel activity. Voltage-dependent steady-state inactivation was unaffected by changes in sialylation. N-Linked sialic acids cannot be responsible for the G-V shifts because K(v)4.2 and K(v)4.3 cannot be N-glycosylated, and immunoblot analysis confirmed K(v)2.1 is not N-glycosylated. Glycosidase gel shift analysis suggested that K(v)2.1, K(v)4.2, and K(v)4.3 were O-glycosylated and sialylated. To confirm this, azide-modified sugar residues involved specifically in O-glycan and sialic acid biosynthesis were shown to incorporate into all three K(v) channel isoforms using Cu(I)-catalyzed cycloaddition chemistry. Together, the data indicate that sialic acids attached to O-glycans uniquely modulate gating of three K(v) channel isoforms that are not N-glycosylated. These data provide the first evidence that external O-glycans, with core structures distinct from N-glycans in type and number of sugar residues, can modulate K(v) channel function and thereby contribute to changes in electrical signaling that result from regulated ion channel expression and/or O-glycosylation.

Sialyltransferase regulates nervous system function in Drosophila

In vertebrates, sialylated glycans participate in a wide range of biological processes and affect the development and function of the nervous system. While the complexity of glycosylation and the functional redundancy among sialyltransferases provide obstacles for revealing biological roles of sialylation in mammals, Drosophila possesses a sole vertebrate-type sialyltransferase, Drosophila sialyltransferase (DSiaT), with significant homology to its mammalian counterparts, suggesting that Drosophila could be a suitable model to investigate the function of sialylation. To explore this possibility and investigate the role of sialylation in Drosophila, we inactivated DSiaT in vivo by gene targeting and analyzed phenotypes of DSiaT mutants using a combination of behavioral, immunolabeling, electrophysiological, and pharmacological approaches. Our experiments demonstrated that DSiaT expression is restricted to a subset of CNS neurons throughout development. We found that DSiaT mutations result in significantly decreased life span, locomotor abnormalities, temperature-sensitive paralysis, and defects of neuromuscular junctions. Our results indicate that DSiaT regulates neuronal excitability and affects the function of a voltage-gated sodium channel. Finally, we showed that sialyltransferase activity is required for DSiaT function in vivo, which suggests that DSiaT mutant phenotypes result from a defect in sialylation of N-glycans. This work provided the first evidence that sialylation has an important biological function in protostomes, while also revealing a novel, nervous system-specific function of alpha2,6-sialylation. Thus, our data shed light on one of the most ancient functions of sialic acids in metazoan organisms and suggest a possibility that this function is evolutionarily conserved between flies and mammals.

Hereditary Inclusion Body Myopathy (HIBM2)

Hereditary inclusion body myopathy type 2 (HIBM2) is a myopathy characterized by progressive muscle weakness with early adult onset. The disease is the result of a recessive mutation in the Glucosamine (UDP-N-acetyl)-2-epimerase/N-acetylmannosamine kinase gene (GNE), which results in reduced enzyme function and sialic acid levels. A majority of individuals with HIBM2 are from Iranian-Jewish or Japanese decent, but isolated cases have been identified world wide. This article reviews the diagnostic criteria for HIBM2. Current research with a highlight on the biology of the disease and the role of GNE in the sialic acid pathway are assessed. Finally, therapeutic investigations and animal models are discussed with a focus on future studies to better understand the pathology of Hereditary Inclusion Body Myopathy and move therapeutic agents towards clinical trials.

Triple N-glycosylation in the long S5-P loop regulates the activation and trafficking of the Kv12.2 potassium channel

Mammalian voltage-dependent potassium (Kv) channels regulate the excitability of nerve and muscle cells. Kv12.2 features the longest S5-P loop among all known mammalian Kv channels with the most N-linked glycosylation sites (three sites). Despite its unique structural features, Kv12.2 is not well characterized. Because glycosylation plays important roles in the folding, trafficking, and function of various Kv channels, we focused on the N-glycosylation of Kv12.2. We show that Kv12.2 is N-glycosylated in Chinese hamster ovary (CHO) cells and in cultured neurons as well as in the mouse brain. As an effect of N-glycosylation on the function of Kv12.2, we demonstrate that removal of sugar chains causes a depolarizing shift in the steady-state activation without a significant reduction in current amplitude. Unlike the previously reported shift for Shaker-type Kv channels, this shift does not appear to be due to negatively charged sialic acid residues in the sugar chains. We next examined the trafficking in CHO cells to address whether the unglycosylated Kv12.2 channels are utilized in vivo. Although double mutants, retaining only one glycosylation site, are trafficked to the surface of CHO cells irrespective of the position of the glycosylated site, unglycosylated channels are not trafficked to the cell surface. Furthermore, we could not detect unglycosylated channels in the mouse brain. Our data suggest that only glycosylated Kv12.2 channels show proper voltage dependence and are utilized in vivo.

Regulated and aberrant glycosylation modulate cardiac electrical signaling

Millions afflicted with Chagas disease and other disorders of aberrant glycosylation suffer symptoms consistent with altered electrical signaling such as arrhythmias, decreased neuronal conduction velocity, and hyporeflexia. Cardiac, neuronal, and muscle electrical signaling is controlled and modulated by changes in voltage-gated ion channel activity that occur through physiological and pathological processes such as development, epilepsy, and cardiomyopathy. Glycans attached to ion channels alter channel activity through isoform-specific mechanisms. Here we show that regulated and aberrant glycosylation modulate cardiac ion channel activity and electrical signaling through a cell-specific mechanism. Data show that nearly half of 239 glycosylation-associated genes (glycogenes) were significantly differentially expressed among neonatal and adult atrial and ventricular myocytes. The N-glycan structures produced among cardiomyocyte types were markedly variable. Thus, the cardiac glycome, defined as the complete set of glycan structures produced in the heart, is remodeled. One glycogene, ST8sia2, a polysialyltransferase, is expressed only in the neonatal atrium. Cardiomyocyte electrical signaling was compared in control and ST8sia2((-/-)) neonatal atrial and ventricular myocytes. Action potential waveforms and gating of less sialylated voltage-gated Na+ channels were altered consistently in ST8sia2((-/-)) atrial myocytes. ST8sia2 expression had no effect on ventricular myocyte excitability. Thus, the regulated (between atrium and ventricle) and aberrant (knockout in the neonatal atrium) expression of a single glycogene was sufficient to modulate cardiomyocyte excitability. A mechanism is described by which cardiac function is controlled and modulated through physiological and pathological processes that involve regulated and aberrant glycosylation.

Inactivation of sodium channels underlies reversible neuropathy during critical illness in rats

Neuropathy and myopathy can cause weakness during critical illness. To determine whether reduced excitability of peripheral nerves, rather than degeneration, is the mechanism underlying acute neuropathy in critically ill patients, we prospectively followed patients during the acute phase of critical illness and early recovery and assessed nerve conduction. During the period of early recovery from critical illness, patients recovered from neuropathy within days. This rapidly reversible neuropathy has not to our knowledge been previously described in critically ill patients and may be a novel type of neuropathy. In vivo intracellular recordings from dorsal root axons in septic rats revealed reduced action potential amplitude, demonstrating that reduced excitability of nerve was the mechanism underlying neuropathy. When action potentials were triggered by hyperpolarizing pulses, their amplitudes largely recovered, indicating that inactivation of sodium channels was an important contributor to reduced excitability. There was no depolarization of axon resting potential in septic rats, which ruled out a contribution of resting potential to the increased inactivation of sodium channels. Our data suggest that a hyperpolarized shift in the voltage dependence of sodium channel inactivation causes increased sodium inactivation and reduced excitability. Acquired sodium channelopathy may be the mechanism underlying acute neuropathy in critically ill patients.

Deglycosylation altered the gating properties of rNav1.3: glycosylation/deglycosylation homeostasis probably complicates the functional regulation of voltage-gated sodium channel

OBJECTIVE

To examine the effect of deglycosylation on gating properties of rNav1.3.

METHODS

rNav1.3 was expressed in Xenopus oocyte, with glycosylation inhibition by using tunicamycin. Two-electrode voltage clamp was employed to record the whole-cell sodium current and data were analyzed by Origin software. Those of glycosylated rNav1.3 were kept as control.

RESULTS

Compared with glycosylated ones, the steady-state activation curve of deglycosylated rNav1.3 was positively shifted by about 10 mV, while inactivation curve was negatively shifted by about 8 mV.

CONCLUSION

Glycosylation altered the gating properties of rNav1.3 and contributed to the functional diversity.

The Kv1.2 potassium channel: the position of an N-glycan on the extracellular linkers affects its protein expression and function

Voltage-gated potassium Kv1 channels have three extracellular linkers, the S1-S2, the S3-S4, and the S5-P. The S1-S2 is the only linker that has an N-glycan and it is at a conserved position on this linker on Kv1.1-Kv1.5 and Kv1.7 channels. We hypothesize that an N-glycan is found at only this position due to its effect on folding, trafficking, and/or function of these channels. To investigate this hypothesis, N-glycosylation sites were engineered at different positions on the extracellular linkers of Kv1.2 to determine the effects of N-glycans on channel surface protein expression and function. Our data suggest that for Kv1 channels, (1) placing an N-glycan at non-native positions on the S1-S2 linker decreased cell surface protein expression but the N-glycan still affected function similarly as if it were at its native position, (2) placing a non-native N-glycan on the S3-S4 linker significantly altered function, and (3) placing a non-native N-glycan on the S5-P linker disrupted both trafficking and function. We suggest that Kv1 channels have an N-glycan at a conserved position on only the S1-S2 linker to overcome the constraints for proper folding, trafficking, and function that appear to occur if the N-glycan is moved from this position.

Gating of the shaker potassium channel is modulated differentially by N-glycosylation and sialic acids

N-linked glycans, including sialic acids, are integral components of ion channel complexes. To determine if N-linked sugars can modulate a rapidly inactivating K+ channel, the glycosylated Drosophila melanogaster Shaker K+ channel (ShB) and the N-glycosylation-deficient mutant (ShNQ), were studied under conditions of full and reduced sialylation. Through an apparent electrostatic mechanism, full sialylation induced uniform and significant hyperpolarizing shifts in all measured voltage-dependent ShB gating parameters compared to those measured under conditions of reduced sialylation. Steady-state gating of ShNQ was unaffected by changes in sialylation and was nearly identical to that observed for ShB under conditions of reduced sialylation, indicating that N-linked sialic acids were wholly responsible for the observed effects of sialic acid on ShB gating. Interestingly, the rates of transition among channel states and the voltage-independent rates of activation and inactivation were significantly slower for ShNQ compared to ShB. Both effects were independent of sialylation, indicating that N-linked sugars other than sialic acids alter ShB gating kinetics but have little to no effect on the steady-state distribution of channels among states. The effect of sialic acids on channel gating, particularly inactivation gating, and the impact of other N-linked sugars on channel gating kinetics are unique to the ShB isoform. Thus, ShB gating is modulated by two complementary but distinct sugar-dependent mechanisms, (1) an N-linked sialic acid-dependent surface charge effect and (2) a sialic acid-independent effect that is consistent with N-linked sugars affecting the stability of ShB among its functional states.

Role of extracellular sialic acid in regulation of neuronal and network excitability in the rat hippocampus

The extracellular membrane surface contains a substantial amount of negatively charged sialic acid residues. Some of the sialic acids are located close to the pore of voltage-gated channel, substantially influencing their gating properties. However, the role of sialylation of the extracellular membrane in modulation of neuronal and network activity remains primarily unknown. The level of sialylation is controlled by neuraminidase (NEU), the key enzyme that cleaves sialic acids. Here we show that NEU treatment causes a large depolarizing shift of voltage-gated sodium channel activation/inactivation and action potential (AP) threshold without any change in the resting membrane potential of hippocampal CA3 pyramidal neurons. Cleavage of sialic acids by NEU also reduced sensitivity of sodium channel gating and AP threshold to extracellular calcium. At the network level, exogenous NEU exerted powerful anticonvulsive action both in vitro and in acute and chronic in vivo models of epilepsy. In contrast, a NEU blocker (N-acetyl-2,3-dehydro-2-deoxyneuraminic acid) dramatically reduced seizure threshold and aggravated hippocampal seizures. Thus, sialylation appears to be a powerful mechanism to control neuronal and network excitability. We propose that decreasing the amount of extracellular sialic acid residues can be a useful approach to reduce neuronal excitability and serve as a novel therapeutic approach in the treatment of seizures.

The glycosylation state of Kv1.2 potassium channels affects trafficking, gating, and simulated action potentials

We presented evidence previously that decreasing the glycosylation state of the Kv1.1 potassium channel modified its gating by a combined surface potential and a cooperative subunit interaction mechanism and these effects modified simulated action potentials. Here we continued to test the hypothesis that glycosylation affects channel function in a predictable fashion by increasing and decreasing the glycosylation state of Kv1.2 channels. Compared with Kv1.2, increasing the glycosylation state shifted the V(1/2) negatively with a steeper G-V slope, increased activation kinetics with little change in deactivation kinetics or in their voltage-dependence, and decreased the apparent level of C-type inactivation. Decreasing the glycosylation state had essentially the opposite effects and shifted the V(1/2) positively with a shallower G-V slope, decreased activation kinetics (and voltage-dependence), decreased deactivation kinetics, and increased the apparent level of C-type inactivation. Single channel conductance was not affected by the different glycosylation states of Kv1.2 tested here. Hyperpolarized or depolarized shifts in V(1/2) from wild type were apparently due to an increased or decreased level of channel sialylation, respectively. Data and modeling suggested that the changes in activation properties were mostly predictable within and between channels and were consistent with a surface potential mechanism, but those on deactivation properties were not predictable and were more consistent with a conformational mechanism. Moreover the effect on the deactivation process appeared to be channel-type dependent as well as glycosylation-site dependent. The glycosylation state of Kv1.2 also affected action potentials in simulations. In addition, preventing N-glycosylation decreased cell surface Kv1.2 expression levels by approximately 40% primarily by increasing partial endoplasmic reticulum retention and this effect was completely rescued by Kv1.4 subunits, which are glycosylated, but not by cytoplasmic Kvbeta2.1 subunits. The nonglycosylated Kv1.2 protein had a similar protein half-life as the glycosylated protein and appeared to be folded properly. Thus altering the native Kv1.2 glycosylation state affected its trafficking, gating, and simulated action potentials. Differential glycosylation of ion channels could be used by excitable cells to modify cell signaling.

Recombinant human voltage-gated skeletal muscle sodium channels are pharmacologically functional in planar lipid bilayers

Human voltage-gated sodium ion channels are major sites of action for drugs and toxins that modulate cellular excitability, and are therefore key molecular targets for ion channel research, high throughput screening for new drugs, and toxin detection. Protein suitable for these applications must be produced in a functionally active form. We report the successful use of ion metal affinity chromatography (IMAC) to purify C-terminal polyhistidine tagged human skeletal muscle voltage-gated sodium (hSkM1-HT) channels from Sf9 insect cells; hSkM1 channels were pharmacologically functional when reconstituted into liposomes and incorporated into planar bilayer lipid membranes. hSkM1-HT single channel currents activated by veratridine had a conductance of 21 pS and those activated by brevetoxin, 16 pS. Channel activity was inhibited by tetrodotoxin and saxitoxin. This protein is suitable for the development of biosensor and high throughput screening technologies.

Glycosylation-induced depolarization facilitates subthreshold membrane oscillation in injured primary sensory neurons

Subthreshold membrane potential oscillations (SMPO) in the injured dorsal root ganglion (DRG) neurons are involved in the generation of spontaneous activity, which can directly evoke neuropathic pain. Nerve injury usually triggers the synthesis of large quantities of membrane protein in nerve injured DRG neurons. Membrane proteins are glycosylated by addition of sugars, especially negatively charged sialic acid residues, which may depolarize the resting membrane potential (Vm), open voltage-gated channels in injured neurons, and cause spontaneous activity. In the present study, we aimed to determine if increased negative charge on the cell surface, carried by the sialic acid residues, could contribute to the generation of SMPO in injured DRG neurons. Intracellular recording was performed in DRG neurons following chronic constrictive injury (CCI) of the sciatic nerve. Results indicated that both A- and C-type injured DRG neurons exhibited a higher incidence of SMPO and more depolarized Vm than those of the control neurons. Ca(2+), Mg(2+), Mn(2+), or poly-lysine, a positively charged organic compound, when topically applied to the DRG, not only reduced SMPO but also caused a rapid hyperpolarizing shift in Vm. Topical application of neuraminidase to selectively remove sialic acid residues on the extracellular membrane normalized the depolarized Vm and inhibited both spontaneous and evoked SMPO. However, application of Ca(2+), Mg(2+), Mn(2+) or neuraminidase had no effect on excitability and Vm in normal neurons. The results demonstrated that the increase in negatively charged sialic acid residues on the extracellular membrane of neuronal somata is a critical factor in the generation of SMPO and hyperexcitability in injured sensory neurons.

Isoform-specific Effects of the β2 Subunit on Voltage-gated Sodium Channel Gating

Voltage-gated sodium channels (Nav) are complex glycoproteins comprised of an alpha subunit and often one to several beta subunits. We have shown that sialic acid residues linked to Nav alpha and beta1 subunits alter channel gating. To determine whether beta2-linked sialic acids similarly impact Nav gating, we co-expressed beta2 with Nav1.5 or Nav1.2 in Pro5 (complete sialylation) and in Lec2 (essentially no sialylation) cells. Beta2 sialic acids caused a significant hyperpolarizing shift in Nav1.5 voltage-dependent gating, thus describing for the first time an effect of beta2 on Nav1.5 gating. In contrast, beta2 caused a sialic acid-independent depolarizing shift in Nav1.2 gating. A deglycosylated mutant, beta(2-DeltaN), had no effect on Nav1.5 gating, indicating further the impact of beta2 N-linked sialic acids on Nav1.5 gating. Conversely, beta(2-DeltaN) modulated Nav1.2 gating virtually identically to beta2, confirming that beta2 N-linked sugars have no impact on Nav1.2 gating. Thus, beta2 modulates Nav gating through multiple mechanisms possibly determined by the associated alpha subunit. Beta1 and beta2 were expressed together with Nav1.5 or Nav1.2 in Pro5 and Lec2 cells. Together beta1 and beta2 produced a significantly larger sialic acid-dependent hyperpolarizing shift in Nav1.5 gating. Under fully sialylating conditions, the Nav1.2.beta1.beta2 complex behaved like Nav1.2 alone. When sialylation was reduced, only the sialic acid-independent depolarizing effects of beta2 on Nav1.2 gating were apparent. Thus, the varied effects of beta1 and beta2 on Nav1.5 and Nav1.2 gating are apparently synergistic and highlight the complex manner, through subunit- and sugar-dependent mechanisms, by which Nav activity is modulated.