Differential asparagine-linked glycosylation of voltage-gated K+ channels in mammalian brain and in transfected cells

Transcriptome-wide association study reveals two genes that influence mismatch negativity

Mismatch negativity (MMN) is a differential electrophysiological response measuring cortical adaptability to unpredictable stimuli. MMN is consistently attenuated in patients with psychosis. However, the genetics of MMN are uncharted, limiting the validation of MMN as a psychosis endophenotype. Here, we perform a transcriptome-wide association study of 728 individuals, which reveals 2 genes (FAM89A and ENGASE) whose expression in cortical tissues is associated with MMN. Enrichment analyses of neurodevelopmental expression signatures show that genes associated with MMN tend to be overexpressed in the frontal cortex during prenatal development but are significantly downregulated in adulthood. Endophenotype ranking value calculations comparing MMN and three other candidate psychosis endophenotypes (lateral ventricular volume and two auditory-verbal learning measures) find MMN to be considerably superior. These results yield promising insights into sensory processing in the cortex and endorse the notion of MMN as a psychosis endophenotype.

Reduced hybrid/complex N-glycosylation disrupts cardiac electrical signaling and calcium handling in a model of dilated cardiomyopathy

Dilated cardiomyopathy (DCM) is the third most common cause of heart failure, with ~70% of DCM cases considered idiopathic. We showed recently, through genetic ablation of the MGAT1 gene, which encodes an essential glycosyltransferase (GlcNAcT1), that prevention of cardiomyocyte hybrid/complex N-glycosylation was sufficient to cause DCM that led to heart failure and early death. Our findings are consistent with increasing evidence suggesting a link between aberrant glycosylation and heart diseases of acquired and congenital etiologies. However, the mechanisms by which changes in glycosylation contribute to disease onset and progression remain largely unknown. Activity and gating of voltage-gated Na⁺ and K⁺ channels (Na_v and K_v respectively) play pivotal roles in the initiation, shaping and conduction of cardiomyocyte action potentials (APs) and aberrant channel activity was shown to contribute to cardiac disease. We and others showed that glycosylation can impact Na_v and K_v function; therefore, here, we investigated the effects of reduced cardiomyocyte hybrid/complex N-glycosylation on channel activity to investigate whether chronic aberrant channel function can contribute to DCM. Ventricular cardiomyocytes from MGAT1 deficient (MGAT1KO) mice display prolonged APs and pacing-induced aberrant early re-activation that can be attributed to, at least in part, a significant reduction in K_v expression and activity that worsens over time suggesting heart disease-related remodeling. MGAT1KO Na_v demonstrate no change in expression or maximal conductance but show depolarizing shifts in voltage-dependent gating. Together, the changes in MGAT1KO Na_v and K_v function likely contribute to observed anomalous electrocardiograms and Ca²⁺ handling. These findings provide insight into mechanisms by which altered glycosylation contributes to DCM through changes in Na_v and K_v activity that impact conduction, Ca²⁺ handling and contraction. The MGAT1KO can also serve as a useful model to study the effects of aberrant electrical signaling on cardiac function and the remodeling events that can occur with heart disease progression.

N-glycosylation-dependent regulation of hK2P17.1 currents

Two pore-domain potassium (K_2P) channels mediate potassium background currents that stabilize the resting membrane potential and facilitate action potential repolarization. In the human heart, hK_2P17.1 channels are predominantly expressed in the atria and Purkinje cells. Reduced atrial hK_2P17.1 protein levels were described in patients with atrial fibrillation or heart failure. Genetic alterations in hK_2P17.1 were associated with cardiac conduction disorders. Little is known about posttranslational modifications of hK_2P17.1. Here, we characterized glycosylation of hK_2P17.1 and investigated how glycosylation alters its surface expression and activity. Wild-type hK_2P17.1 channels and channels lacking specific glycosylation sites were expressed in Xenopus laevis oocytes, HEK-293T cells, and HeLa cells. N-glycosylation was disrupted using N-glycosidase F and tunicamycin. hK_2P17.1 expression and activity were assessed using immunoblot analysis and a two-electrode voltage clamp technique. Channel subunits of hK_2P17.1 harbor two functional N-glycosylation sites at positions N65 and N94. In hemi-glycosylated hK_2P17.1 channels, functionality and membrane trafficking remain preserved. Disruption of both N-glycosylation sites results in loss of hK_2P17.1 currents, presumably caused by impaired surface expression. This study confirms diglycosylation of hK_2P17.1 channel subunits and its pivotal role in cell-surface targeting. Our findings underline the functional relevance of N-glycosylation in biogenesis and membrane trafficking of ion channels.

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.

Trafficking of Kv2.1 Channels to the Axon Initial Segment by a Novel Nonconventional Secretory Pathway

Kv2.1 is a major delayed-rectifier voltage-gated potassium channel widely expressed in neurons of the CNS. Kv2.1 localizes in high-density cell-surface clusters in the soma and proximal dendrites as well as in the axon initial segment (AIS). Given the crucial roles of both of these compartments in integrating signal input and then generating output, this localization of Kv2.1 is ideal for regulating the overall excitability of neurons. Here we used fluorescence recovery after photobleaching imaging, mutagenesis, and pharmacological interventions to investigate the molecular mechanisms that control the localization of Kv2.1 in these two different membrane compartments in cultured rat hippocampal neurons of mixed sex. Our data uncover a unique ability of Kv2.1 channels to use two molecularly distinct trafficking pathways to accomplish this. Somatodendritic Kv2.1 channels are targeted by the conventional secretory pathway, whereas axonal Kv2.1 channels are targeted by a nonconventional trafficking pathway independent of the Golgi apparatus. We further identified a new AIS trafficking motif in the C-terminus of Kv2.1, and show that putative phosphorylation sites in this region are critical for the restricted and clustered localization in the AIS. These results indicate that neurons can regulate the expression and clustering of Kv2.1 in different membrane domains independently by using two distinct localization mechanisms, which would allow neurons to precisely control local membrane excitability.SIGNIFICANCE STATEMENT Our study uncovered a novel mechanism that targets the Kv2.1 voltage-gated potassium channel to two distinct trafficking pathways and two distinct subcellular destinations: the somatodendritic plasma membrane and that of the axon initial segment. We also identified a distinct motif, including putative phosphorylation sites, that is important for the AIS localization. This raises the possibility that the destination of a channel protein can be dynamically regulated via changes in post-translational modification, which would impact the excitability of specific membrane compartments.

Eag1 K+ Channel: Endogenous Regulation and Functions in Nervous System

Ether-à-go-go1 (Eag1, Kv10.1, KCNH1) K⁺ channel is a member of the voltage-gated K⁺ channel family mainly distributed in the central nervous system and cancer cells. Like other types of voltage-gated K⁺ channels, the EAG1 channels are regulated by a variety of endogenous signals including reactive oxygen species, rendering the EAG1 to be in the redox-regulated ion channel family. The role of EAG1 channels in tumor development and its therapeutic significance have been well established. Meanwhile, the importance of hEAG1 channels in the nervous system is now increasingly appreciated. The present review will focus on the recent progress on the channel regulation by endogenous signals and the potential functions of EAG1 channels in normal neuronal signaling as well as neurological diseases.

In-Silico Modeling of the Functional Role of Reduced Sialylation in Sodium and Potassium Channel Gating of Mouse Ventricular Myocytes

Cardiac ion channels are highly glycosylated membrane proteins with up to 30% of the protein's mass containing glycans. Heart diseases often accompany individuals with congenital disorders of glycosylation (CDG). However, cardiac dysfunction among CDG patients is not yet fully understood. There is an urgent need to study how aberrant glycosylation impacts cardiac electrical signaling. Our previous works reported that congenitally reduced sialylation achieved through deletion of the sialyltransferase gene, ST3Gal4, leads to altered gating of voltage-gated Na⁺ and K⁺ channels ( and , respectively). However, linking the impact of reduced sialylation on ion channel gating to the action potential (AP) is difficult without performing computer experiments. Also, decomposing the sum of K⁺ currents is difficult because of complex structures and components of channels (e.g., , and ). In this study, we developed in-silico models to describe the functional role of reduced sialylation in both and gating and the AP using in vitro experimental data. Modeling results showed that reduced sialylation changes gating as follows: 1) The steady-state activation voltages of isoforms are shifted to a more depolarized potential. 2) Aberrant K⁺ currents ( and ) contribute to a prolonged AP duration, and altered Na⁺ current ( ) contributes to a shortened AP refractory period. This study contributes to a better understanding of the functional role of reduced sialylation in cardiac dysfunction that shows strong potential to provide new pharmaceutical targets for the treatment of CDG-related heart diseases.

Contribution of KV1.5 Channel to Hydrogen Peroxide-Induced Human Arteriolar Dilation and Its Modulation by Coronary Artery Disease

RATIONALE

Hydrogen peroxide (H₂O₂) regulates vascular tone in the human microcirculation under physiological and pathophysiological conditions. It dilates arterioles by activating large-conductance Ca²⁺-activated K⁺ channels in subjects with coronary artery disease (CAD), but its mechanisms of action in subjects without CAD (non-CAD) when compared with those with CAD remain unknown.

OBJECTIVE

We hypothesize that H₂O₂-elicited dilation involves different K⁺ channels in non-CAD versus CAD, resulting in an altered capacity for vasodilation during disease.

METHODS AND RESULTS

H₂O₂ induced endothelium-independent vasodilation in non-CAD adipose arterioles, which was reduced by paxilline, a large-conductance Ca²⁺-activated K⁺ channel blocker, and by 4-aminopyridine, a voltage-gated K⁺ (K_V) channel blocker. Assays of mRNA transcripts, protein expression, and subcellular localization revealed that K_V1.5 is the major K_V1 channel expressed in vascular smooth muscle cells and is abundantly localized on the plasma membrane. The selective K_V1.5 blocker diphenylphosphine oxide-1 and the K_V1.3/1.5 blocker 5-(4-phenylbutoxy)psoralen reduced H₂O₂-elicited dilation to a similar extent as 4-aminopyridine, but the selective K_V1.3 blocker phenoxyalkoxypsoralen-1 was without effect. In arterioles from CAD subjects, H₂O₂-induced dilation was significantly reduced, and this dilation was inhibited by paxilline but not by 4-aminopyridine, diphenylphosphine oxide-1, or 5-(4-phenylbutoxy)psoralen. K_V1.5 cell membrane localization and diphenylphosphine oxide-1-sensitive K⁺ currents were markedly reduced in isolated vascular smooth muscle cells from CAD arterioles, although mRNA or total cellular protein expression was largely unchanged.

CONCLUSIONS

In human arterioles, H₂O₂-induced dilation is impaired in CAD, which is associated with a transition from a combined large-conductance Ca²⁺-activated K⁺- and K_V (K_V1.5)-mediated vasodilation toward a large-conductance Ca²⁺-activated K⁺-predominant mechanism of dilation. Loss of K_V1.5 vasomotor function may play an important role in microvascular dysfunction in CAD or other vascular diseases.

N-linked glycosylation of Kv1.2 voltage-gated potassium channel facilitates cell surface expression and enhances the stability of internalized channels

KEY POINTS

Kv1.2 and related voltage-gated potassium channels have a highly conserved N-linked glycosylation site in the first extracellular loop, with complex glycosylation in COS-7 cells similar to endogenous Kv1.2 glycosylation in hippocampal neurons. COS-7 cells expressing Kv1.2 show a crucial role of this N-linked glycosylation in the forward trafficking of Kv1.2 to the cell membrane. Although both wild-type and non-glycosylated mutant Kv1.2 channels that have reached the cell membrane are internalized at a comparable rate, mutant channels are degraded at a faster rate. Treatment of wild-type Kv1.2 channels on the cell surface with glycosidase to remove sialic acids also results in the faster degradation of internalized channels. Glycosylation of Kv1.2 is important with respect to facilitating trafficking to the cell membrane and enhancing the stability of channels that have reached the cell membrane.

ABSTRACT

Studies in cultured hippocampal neurons and the COS-7 cell line demonstrate important roles for N-linked glycosylation of Kv1.2 channels in forward trafficking and protein degradation. Kv1.2 channels can contain complex N-linked glycans, which facilitate cell surface expression of the channels. Additionally, the protein stability of cell surface-expressed Kv1.2 channels is affected by glycosylation via differences in the degradation of internalized channels. The present study reveals the importance of N-linked complex glycosylation in boosting Kv1.2 channel density. Notably, sialic acids at the terminal sugar branches play an important role in dampening the degradation of Kv1.2 internalized from the cell membrane to promote its stability.

Complex N-Glycans Influence the Spatial Arrangement of Voltage Gated Potassium Channels in Membranes of Neuronal-Derived Cells

The intrinsic electrical properties of a neuron depend on expression of voltage gated potassium (Kv) channel isoforms, as well as their distribution and density in the plasma membrane. Recently, we showed that N-glycosylation site occupancy of Kv3.1b modulated its placement in the cell body and neurites of a neuronal-derived cell line, B35 neuroblastoma cells. To extrapolate this mechanism to other N-glycosylated Kv channels, we evaluated the impact of N-glycosylation occupancy of Kv3.1a and Kv1.1 channels. Western blots revealed that wild type Kv3.1a and Kv1.1 α-subunits had complex and oligomannose N-glycans, respectively, and that abolishment of the N-glycosylation site(s) generated Kv proteins without N-glycans. Total internal reflection fluorescence microscopy images revealed that N-glycans of Kv3.1a contributed to its placement in the cell membrane while N-glycans had no effect on the distribution of Kv1.1. Based on particle analysis of EGFP-Kv proteins in the adhered membrane, glycosylated forms of Kv3.1a, Kv1.1, and Kv3.1b had differences in the number, size or density of Kv protein clusters in the cell membrane of neurites and cell body of B35 cells. Differences were also observed between the unglycosylated forms of the Kv proteins. Cell dissociation assays revealed that cell-cell adhesion was increased by the presence of complex N-glycans of Kv3.1a, like Kv3.1b, whereas cell adhesion was similar in the oligomannose and unglycosylated Kv1.1 subunit containing B35 cells. Our findings provide direct evidence that N-glycans of Kv3.1 splice variants contribute to the placement of these glycoproteins in the plasma membrane of neuronal-derived cells while those of Kv1.1 were absent. Further when the cell membrane distribution of the Kv channel was modified by N-glycans then the cell-cell adhesion properties were altered. Our study demonstrates that N-glycosylation of Kv3.1a, like Kv3.1b, provides a mechanism for the distribution of these proteins to the cell body and outgrowths and thereby can generate different voltage-dependent conductances in these membranes.

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.

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.

Oxidation of KCNB1 K(+) channels in central nervous system and beyond

KCNB1, a voltage-gated potassium (K(+)) channel that conducts a major delayed rectifier current in the brain, pancreas and cardiovascular system is a key player in apoptotic programs associated with oxidative stress. As a result, this protein represents a bona fide drug target for limiting the toxic effects of oxygen radicals. Until recently the consensus view was that reactive oxygen species trigger a pro-apoptotic surge in KCNB1 current via phosphorylation and SNARE-dependent incorporation of KCNB1 channels into the plasma membrane. However, new evidence shows that KCNB1 can be modified by oxidants and that oxidized KCNB1 channels can directly activate pro-apoptotic signaling pathways. Hence, a more articulated picture of the pro-apoptotic role of KCNB1 is emerging in which the protein induces cell's death through distinct molecular mechanisms and activation of multiple pathways. In this review article we discuss the diverse functional, toxic and protective roles that KCNB1 channels play in the major organs where they are expressed.

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.

Benefits and pitfalls of secondary antibodies: why choosing the right secondary is of primary importance

Simultaneous labeling of multiple targets in a single sample, or multiplexing, is a powerful approach to directly compare the amount, localization and/or molecular properties of different targets in the same sample. Here we highlight the robust reliability of the simultaneous use of multiple mouse monoclonal antibodies (mAbs) of different immunoglobulin G (IgG) subclasses in a wide variety of multiplexing applications employing anti-mouse IgG subclass-specific secondary antibodies (2°Abs). We also describe the unexpected finding that IgG subclass-specific 2°Abs are superior to general anti-mouse IgG 2°Abs in every tested application in which mouse mAbs were used. This was due to a detection bias of general anti-mouse IgG-specific 2°Abs against mAbs of the most common mouse IgG subclass, IgG1, and to a lesser extent IgG2b mAbs. Thus, when using any of numerous mouse mAbs available through commercial and non-profit sources, for cleaner and more robust results each mAb should be detected with its respective IgG subclass-specific 2°Ab and not a general anti-mouse IgG-specific 2°Ab.

jShaw1, a low-threshold, fast-activating K(v)3 from the hydrozoan jellyfish Polyorchis penicillatus

Voltage-gated potassium (K(v)) channels work in concert with other ion channels to determine the frequency and duration of action potentials in excitable cells. Little is known about K(v)3 channels from invertebrates, but those that have been characterized generally display slow kinetics. Here, we report the cloning and characterization of jShaw1, the first K(v)3 isolated from a cnidarian, the jellyfish Polyorchis penicillatus, in comparison with mouse K(v)3.1 and K(v)3.2. Using a two-electrode voltage clamp on Xenopus laevis oocytes expressing the channels, we compared steady-state and kinetic properties of macroscopic currents. jShaw1 is fast activating, and opens at potentials approximately 40 mV more hyperpolarized than the mouse K(v)3 channels. There is an inverse relationship between the number of positive charges on the voltage sensor and the half-activation voltage of the channel, contrary to what would be expected with the simplest model of voltage sensitivity. jShaw1 has kinetic characteristics that are substantially different from the mammalian K(v)3 channels, including a much lower sensitivity of early activation rates to incremental voltage changes, and a much faster voltage-dependent transition in the last stages of opening. jShaw1 opening kinetics were affected little by pre-depolarization voltage, in contrast to both mouse channels. Similar to the mouse channels, jShaw1 was half-blocked by 0.7 mmol l(-1) tetraethyl ammonium and 5 mmol l(-1) 4-aminopyridine. Comparison of sequence and functional properties of jShaw1 with the mouse and other reported K(v)3 channels helps to illuminate the general relationship between amino acid sequence and electrophysiological activity in this channel family.

Importance of glycosylation on function of a potassium channel in neuroblastoma cells

The Kv3.1 glycoprotein, a voltage-gated potassium channel, is expressed throughout the central nervous system. The role of N-glycans attached to the Kv3.1 glycoprotein on conducting and non-conducting functions of the Kv3.1 channel are quite limiting. Glycosylated (wild type), partially glycosylated (N220Q and N229Q), and unglycosylated (N220Q/N229Q) Kv3.1 proteins were expressed and characterized in a cultured neuronal-derived cell model, B35 neuroblastoma cells. Western blots, whole cell current recordings, and wound healing assays were employed to provide evidence that the conducting and non-conducting properties of the Kv3.1 channel were modified by N-glycans of the Kv3.1 glycoprotein. Electrophoretic migration of the various Kv3.1 proteins treated with PNGase F and neuraminidase verified that the glycosylation sites were occupied and that the N-glycans could be sialylated, respectively. The unglycosylated channel favored a different whole cell current pattern than the glycoform. Further the outward ionic currents of the unglycosylated channel had slower activation and deactivation rates than those of the glycosylated Kv3.1 channel. These kinetic parameters of the partially glycosylated Kv3.1 channels were also slowed. B35 cells expressing glycosylated Kv3.1 protein migrated faster than those expressing partially glycosylated and much faster than those expressing the unglycosylated Kv3.1 protein. These results have demonstrated that N-glycans of the Kv3.1 glycoprotein enhance outward ionic current kinetics, and neuronal migration. It is speculated that physiological changes which lead to a reduction in N-glycan attachment to proteins will alter the functions of the Kv3.1 channel.

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.

Analysis and functional implications of phosphorylation of neuronal voltage-gated potassium channels

Phosphorylation is the most common and abundant post-translational modification to eukaryotic proteins, regulating a plethora of dynamic cellular processes. Here, we review and discuss recent advances in our knowledge of the breadth and importance of reversible phosphorylation in regulating the expression, localization and function of mammalian neuronal voltage-gated potassium (Kv) channels, key regulators of neuronal function. We highlight the role of modern mass spectrometric techniques and phosphospecific antibodies in revealing the extent and nature of phosphorylation at specific sites in Kv channels. We also emphasize the role of reversible phosphorylation in dynamically regulating diverse aspects of Kv channel biology. Finally, we discuss as important future directions the determination of the mechanistic basis for how altering phosphorylation state affects Kv channel expression, localization and function, the nature of macromolecular signaling complexes containing Kv channels and enzymes regulating their phosphorylation state, and the specific role of Kv channel phosphorylation in regulating neuronal function during physiological and pathophysiological events.

Tetramerization domain mutations in KCNA5 affect channel kinetics and cause abnormal trafficking patterns

The activity of voltage-gated K(+) (K(V)) channels plays an important role in regulating pulmonary artery smooth muscle cell (PASMC) contraction, proliferation, and apoptosis. The highly conserved NH(2)-terminal tetramerization domain (T1) of K(V) channels is important for proper channel assembly, association with regulatory K(V) beta-subunits, and localization of the channel to the plasma membrane. We recently reported two nonsynonymous mutations (G182R and E211D) in the KCNA5 gene of patients with idiopathic pulmonary arterial hypertension, which localize to the T1 domain of KCNA5. To study the electrophysiological properties and expression patterns of the mutants compared with the wild-type (WT) channel in vitro, we transfected HEK-293 cells with WT KCNA5, G182R, E211D, or the double mutant G182R/E211D channel. The mutants form functional channels; however, whole cell current kinetic differences between WT and mutant channels exist. Steady-state inactivation curves of the G182R and G182R/E211D channels reveal accelerated inactivation; the mutant channels inactivated at more hyperpolarized potentials compared with the WT channel. Channel protein expression was also decreased by the mutations. Compared with the WT channel, which was present in its mature glycosylated form, the mutant channels are present in greater proportion in their immature form in HEK-293 cells. Furthermore, G182R protein level is greatly reduced in COS-1 cells compared with WT. Immunostaining data support the hypothesis that, while WT protein localizes to the plasma membrane, mutant protein is mainly retained in intracellular packets. Overall, these data support a role for the T1 domain in channel kinetics as well as in KCNA5 channel subcellular localization.

Amino acids in the pore region of Kv1 potassium channels dictate cell-surface protein levels: a possible trafficking code in the Kv1 subfamily

Kv1.1 and Kv1.4 potassium channels have different pore region determinants that were found to affect their cell-surface levels positively and negatively [Zhu, Watanabe, Gomez and Thornhill (2001) J. Biol. Chem. 276, 39419-39427; Zhu, Watanabe, Gomez and Thornhill (2003) J. Biol. Chem. 278, 25558-25567; Zhu, Watanabe, Gomez and Thornhill (2003) Biochem. J. 375, 761-768]. In the present study, we focused on the deep pore region of Kv1 members to test whether a cell-surface trafficking code was dictated by two amino acids. Kv1 channels with a threonine/lysine amino acid pair in a non-contiguous pore region promoted high surface levels, whereas a serine/tyrosine amino acid pair inhibited high surface expression by inducing a high level of partial endoplasmic reticulum retention. Our work suggests that a possible positive trafficking amino acid pair coding here for the Kv1 subfamily is Thr/Lys>Thr/Val>Thr/Tyr>Thr/Arg approximately Thr/His>Ser/Val>Ser/Tyr>Ser/Lys. The Kv1 trafficking code was not transferable to a Kv2 family member and thus it appears that it only governs surface levels in the context of its Kv1 native pore loop region and/or its S5 and S6 regions. All members of a given Kv2, Kv3 or Kv4 potassium channel subfamily have identical amino acids at similar positions in their deep pore regions (Thr/Tyr or Thr/Val), which suggests that any difference in surface levels among members is not dictated by these amino acids. Thus a major determinant for cell-surface trafficking of Kv1 potassium channels is an amino acid pair in their deep pore regions, whereas the cell-surface levels of a given Kv2, Kv3 or Kv4 subfamily member are probably not affected by these amino acids.

Novel Kv3 glycoforms differentially expressed in adult mammalian brain contain sialylated N-glycans

The N-glycan pool of mammalian brain contains remarkably high levels of sialylated N-glycans. This study provides the first evidence that voltage-gated K+ channels Kv3.1, Kv3.3, and Kv3.4, possess distinct sialylated N-glycan structures throughout the central nervous system of the adult rat. Electrophoretic migration patterns of Kv3.1, Kv3.3, and Kv3.4 glycoproteins from spinal cord, hypothalamus, thalamus, cerebral cortex, hippocampus, and cerebellum membranes digested with glycosidases were used to identify the various glycoforms. Differences in the migration of Kv3 proteins were attributed to the desialylated N-glycans. Expression levels of the Kv3 proteins were highest in cerebellum, whereas those of Kv3.1 and Kv3.3 were much lower in the other 5 regions. The lowest level of Kv3.1 was expressed in the hypothalamus, whereas the lowest levels of Kv3.3 were expressed in both thalamus and hypothalamus. The other regions expressed intermediate levels of Kv3.3, with spinal cord expressing the highest. The expression level of Kv3.4 in the hippocampus was slightly lower than that in cerebellum, and was closely followed by the other 4 regions, with spinal cord expressing the lowest level. We suggest that novel Kv3 glycoforms may endow differences in channel function and expression among regions throughout the central nervous system.

Atypical sialylated N-glycan structures are attached to neuronal voltage-gated potassium channels

Mammalian brains contain relatively high amounts of common and uncommon sialylated N-glycan structures. Sialic acid linkages were identified for voltage-gated potassium channels, Kv3.1, 3.3, 3.4, 1.1, 1.2 and 1.4, by evaluating their electrophoretic migration patterns in adult rat brain membranes digested with various glycosidases. Additionally, their electrophoretic migration patterns were compared with those of NCAM (neural cell adhesion molecule), transferrin and the Kv3.1 protein heterologously expressed in B35 neuroblastoma cells. Metabolic labelling of the carbohydrates combined with glycosidase digestion reactions were utilized to show that the N-glycan of recombinant Kv3.1 protein was capped with an oligo/poly-sialyl unit. All three brain Kv3 glycoproteins, like NCAM, were terminated with α2,3-linked sialyl residues, as well as atypical α2,8-linked sialyl residues. Additionally, at least one of their antennae was terminated with an oligo/poly-sialyl unit, similar to recombinant Kv3.1 and NCAM. In contrast, brain Kv1 glycoproteins consisted of sialyl residues with α2,8-linkage, as well as sialyl residues linked to internal carbohydrate residues of the carbohydrate chains of the N-glycans. This type of linkage was also supported for Kv3 glycoproteins. To date, such a sialyl linkage has only been identified in gangliosides, not N-linked glycoproteins. We conclude that all six Kv channels (voltage-gated K+ channels) contribute to the α2,8-linked sialylated N-glycan pool in mammalian brain and furthermore that their N-glycan structures contain branched sialyl residues. Identification of these novel and unique sialylated N-glycan structures implicate a connection between potassium channel activity and atypical sialylated N-glycans in modulating and fine-tuning the excitable properties of neurons in the nervous system.

Syntaxin modulates Kv1.1 through dual action on channel surface expression and conductance

The Kv1.1 channel that is expressed throughout the central and peripheral nervous system is known to interact with syntaxin 1A, a member of the exocytosis machinery protein complex. This interaction was previously shown to increase the macroscopic currents of the presynaptic Kv1.1 channel when coexpressed in Xenopus oocytes, while it decreased the unitary channel conductance and open probability. This apparent discrepancy has been resolved in this work, using electrophysiological, biochemical, and immunohistochemical analyses in oocytes by overexpression and antisense knockdown of syntaxin. Here, we demonstrate that syntaxin plays a dual role in the modulation of Kv1.1 function: enhancement of the channel's surface expression along with attenuation of single channel ion flux. These findings broaden the scope of channels and transporters that are dually modulated by syntaxin. Although the dual functioning of syntaxin in modulation of Kv1.1 channel activity may seem antagonistic, the combination of the two mechanisms may provide a useful means for fine-tuning axonal excitability and synaptic efficacy.

Identification of an evolutionarily conserved extracellular threonine residue critical for surface expression and its potential coupling of adjacent voltage-sensing and gating domains in voltage-gated potassium channels

The dynamic expression of voltage-gated potassium channels (Kvs) at the cell surface is a fundamental factor controlling membrane excitability. In exploring possible mechanisms controlling Kv surface expression, we identified a region in the extracellular linker between the first and second of the six (S1-S6) transmembrane-spanning domains of the Kv1.4 channel, which we hypothesized to be critical for its biogenesis. Using immunofluorescence microscopy, flow cytometry, patch clamp electrophysiology, and mutagenesis, we identified a single threonine residue at position 330 within the Kv1.4 S1-S2 linker that is absolutely required for cell surface expression. Mutation of Thr-330 to an alanine, aspartate, or lysine prevented surface expression. However, surface expression occurred upon co-expression of mutant and wild type Kv1.4 subunits or mutation of Thr-330 to a serine. Mutation of the corresponding residue (Thr-211) in Kv3.1 to alanine also caused intracellular retention, suggesting that the conserved threonine plays a generalized role in surface expression. In support of this idea, sequence comparisons showed conservation of the critical threonine in all Kv families and in organisms across the evolutionary spectrum. Based upon the Kv1.2 crystal structure, further mutagenesis, and the partial restoration of surface expression in an electrostatic T330K bridging mutant, we suggest that Thr-330 hydrogen bonds to equally conserved outer pore residues, which may include a glutamate at position 502 that is also critical for surface expression. We propose that Thr-330 serves to interlock the voltage-sensing and gating domains of adjacent monomers, thereby yielding a structure competent for the surface expression of functional tetramers.

Activity-dependent heteromerization of the hyperpolarization-activated, cyclic-nucleotide gated (HCN) channels: role of N-linked glycosylation

Formation of heteromeric complexes of ion channels via co-assembly of different subunit isoforms provides an important mechanism for enhanced channel diversity. We have previously demonstrated co-association of the hyperpolarization activated cyclic-nucleotide gated (HCN1/HCN2) channel isoforms that was regulated by network (seizure) activity in developing hippocampus. However, the mechanisms that underlie this augmented expression of heteromeric complexes have remained unknown. Glycosylation of the HCN channels has been implicated in the stabilization and membrane expression of heteromeric HCN1/HCN2 constructs in heterologous systems. Therefore, we used in vivo and in vitro systems to test the hypothesis that activity modifies HCN1/HCN2 heteromerization in neurons by modulating the glycosylation state of the channel molecules. Seizure-like activity (SA) increased HCN1/HCN2 heteromerization in hippocampus in vivo as well as in hippocampal organotypic slice cultures. This activity increased the abundance of glycosylated HCN1 but not HCN2-channel molecules. In addition, glycosylated HCN1 channels were preferentially co-immunoprecipitated with the HCN2 isoforms. Provoking SA in vitro in the presence of the N-linked glycosylation blocker tunicamycin abrogated the activity-dependent increase of HCN1/HCN2 heteromerization. Thus, hippocampal HCN1 molecules have a significantly higher probability of being glycosylated after SA, and this might promote a stable heteromerization with HCN2.

Voltage gated K+ channel expression in arteries of Wistar-Kyoto and spontaneously hypertensive rats

BACKGROUND

We have previously demonstrated differences in the gene expression of voltage-gated K v1.X channel alpha-subunits in arteries from Wistar-Kyoto rats (WKYs) and spontaneously hypertensive rats (SHRs). The purpose of this study was to test the hypothesis that these differences are also present at the protein level.

METHODS

Proteins were isolated from the aorta, mesenteric (MAs) and tail arteries (TAs) of 12- to 15-week-old male WKY and SHR, and analyzed by immunoblotting. K(v) currents were recorded from MA myocytes by patch clamp methods.

RESULTS

Expression of Kv1.2, Kv1.5, and Kv2.1 was higher in MAs but was not different in aortas of SHRs as compared to WKYs. In the TA, expression of Kv1.2 and Kv1.5 was higher while that of Kv2.1 was lower in SHR compared to WKY. In the MA, the larger expression of an 80 kDa species of Kv1.2 in SHRs was associated with a lower expression of a 60 kDa species. Kv2.1 gene expression was larger in MAs from SHRs but not different in TAs. K(v) currents associated with Kv1.X and Kv2.1 channels were both larger in MA myocytes from SHRs but less than expected based upon differences in K(v) alpha-subunit protein expression.

CONCLUSIONS

For the MA, K(v) protein expression and current components between WKYs and SHRs were qualitatively consistent, but differences in gene and protein expression were not closely correlated. The higher expression of K(v) subunits in small mesenteric arteries (SMAs) of SHR would tend to maintain normal myogenic activity and vasoconstrictor reserve, and could be viewed as a form of homeostatic remodeling.

Trafficking-dependent phosphorylation of Kv1.2 regulates voltage-gated potassium channel cell surface expression

Kv1.2 alpha-subunits are components of low-threshold, rapidly activating voltage-gated potassium (Kv) channels in mammalian neurons. Expression and localization of Kv channels is regulated by trafficking signals encoded in their primary structure. Kv1.2 is unique in lacking strong trafficking signals and in exhibiting dramatic cell-specific differences in trafficking, which is suggestive of conditional trafficking signals. Here we show that a cluster of cytoplasmic C-terminal phosphorylation sites regulates Kv1.2 trafficking. Using tandem MS to analyze Kv1.2 purified from rat, human, and mouse brain, we identified in each sample in vivo phosphoserine (pS) phosphorylation sites at pS434, pS440, and pS441, as well as doubly phosphorylated pS440/pS441. We also found these sites, as well as pS449, on recombinant Kv1.2 expressed in heterologous cells. We found that phosphorylation at pS440/pS441 is present only on the post-endoplasmic reticulum (ER)/cell surface pool of Kv1.2 and is not detectable on newly synthesized and ER-localized Kv1.2, on which we did observe pS449 phosphorylation. Elimination of PS440/PS441 phosphorylation by mutation reduces cell-surface expression efficiency and functional expression of homomeric Kv1.2 channels. Interestingly, mutation of S449 reduces phosphorylation at pS440/pS441 and also decreases Kv1.2 cell-surface expression efficiency and functional expression. These mutations also suppress trafficking of Kv1.2/Kv1.4 heteromeric channels, suggesting that incorporation of Kv1.2 into heteromeric complexes confers conditional phosphorylation-dependent trafficking to diverse Kv channel complexes. These data support Kv1.2 phosphorylation at these clustered C-terminal sites as playing an important role in regulating trafficking of Kv1.2-containing Kv channels.

Regulation of Kv1 channel trafficking by the mamba snake neurotoxin dendrotoxin K

Modulation of voltage-gated potassium (Kv) channel surface expression can profoundly affect neuronal excitability. Some, but not all, mammalian Shaker or Kv1 alpha subunits contain a dominant endoplasmic reticulum (ER) retention signal in their pore region, preventing surface expression of Kv1.1 homotetrameric channels and of heteromeric Kv1 channels containing more than one Kv1.1 subunit. The critical amino acid residues within this ER pore-region retention signal are also critical for high-affinity binding of snake dendrotoxins (DTX). This suggests that ER retention may be mediated by an ER protein with a domain structurally similar to that of DTX. One facet of such a model is that expression of soluble DTX in the ER lumen should compete for binding to the retention protein and allow for surface expression of retained Kv1.1. Here, we show that luminal DTX expression dramatically increased both the level of cell surface Kv1.1 immunofluorescence staining and the proportion of Kv1.1 with processed N-linked oligosaccharides. Electrophysiological analyses showed that luminal DTX expression led to significant increases in Kv1.1 currents. Together, these data showed that luminal DTX expression increases surface expression of functional Kv1.1 homotetrameric channels and support a model whereby a DTX-like ER protein regulates abundance of cell surface Kv1 channels.

Bidirectional activity-dependent regulation of neuronal ion channel phosphorylation

Activity-dependent dephosphorylation of neuronal Kv2.1 channels yields hyperpolarizing shifts in their voltage-dependent activation and homoeostatic suppression of neuronal excitability. We recently identified 16 phosphorylation sites that modulate Kv2.1 function. Here, we show that in mammalian neurons, compared with other regulated sites, such as serine (S)563, phosphorylation at S603 is supersensitive to calcineurin-mediated dephosphorylation in response to kainate-induced seizures in vivo, and brief glutamate stimulation of cultured hippocampal neurons. In vitro calcineurin digestion shows that supersensitivity of S603 dephosphorylation is an inherent property of Kv2.1. Conversely, suppression of neuronal activity by anesthetic in vivo causes hyperphosphorylation at S603 but not S563. Distinct regulation of individual phosphorylation sites allows for graded and bidirectional homeostatic regulation of Kv2.1 function. S603 phosphorylation represents a sensitive bidirectional biosensor of neuronal activity.

Regulation of TRP channels by N-linked glycosylation

A subset of TRP channel proteins undergoes regulatory N-linked glycosylation. A glycosylation site in the first extracellular loop of TRPV5 is enzymatically cleaved by a secreted glucuronidase, indirectly regulating channel function. Members of the TRPC family share a similar site, although details about a regulatory role are lacking. A second conserved TRP channel glycosylation site is found immediately adjacent to the channel pore-forming loop; both TRPV1 and TRPV4--and perhaps other TRPV family members--are influenced by glycosylation at this site. N-linked glycosylation, and the dynamic regulation of this process, substantially impacts function and targeting of TRP channels.

Immunolocalization of the Ca2+-activated K+ channel Slo1 in axons and nerve terminals of mammalian brain and cultured neurons

Ca(2+)-activated voltage-dependent K(+) channels (Slo1, KCa1.1, Maxi-K, or BK channel) play a crucial role in controlling neuronal signaling by coupling channel activity to both membrane depolarization and intracellular Ca(2+) signaling. In mammalian brain, immunolabeling experiments have shown staining for Slo1 channels predominantly localized to axons and presynaptic terminals of neurons. We have developed anti-Slo1 mouse monoclonal antibodies that have been extensively characterized for specificity of staining against recombinant Slo1 in heterologous cells, and native Slo1 in mammalian brain, and definitively by the lack of detectable immunoreactivity against brain samples from Slo1 knockout mice. Here we provide precise immunolocalization of Slo1 in rat brain with one of these monoclonal antibodies and show that Slo1 is accumulated in axons and synaptic terminal zones associated with glutamatergic synapses in hippocampus and GABAergic synapses in cerebellum. By using cultured hippocampal pyramidal neurons as a model system, we show that heterologously expressed Slo1 is initially targeted to the axonal surface membrane, and with further development in culture, become localized in presynaptic terminals. These studies provide new insights into the polarized localization of Slo1 channels in mammalian central neurons and provide further evidence for a key role in regulating neurotransmitter release in glutamatergic and GABAergic terminals.

Glycosylation of the osmoresponsive transient receptor potential channel TRPV4 on Asn-651 influences membrane trafficking

We identified a consensus N-linked glycosylation motif within the pore-forming loop between the fifth and sixth transmembrane segments of the osmoresponsive transient receptor potential (TRP) channel TRPV4. Mutation of this residue from Asn to Gln (i.e., TRPV4(N651Q)) resulted in loss of a slower migrating band on anti-TRPV4 immunoblots and a marked reduction in lectin-precipitable TRPV4 immunoreactivity. HEK293 cells transiently transfected with the mutant TRPV4(N651Q) exhibited increased calcium entry in response to hypotonic stress relative to wild-type TRPV4 transfectants. This increase in hypotonicity responsiveness was associated with an increase in plasma membrane targeting of TRPV4(N651Q) relative to wild-type TRPV4 in both HEK293 and COS-7 cells but had no effect on overall channel abundance in whole cell lysates. Residue N651 of TRPV4 is immediately adjacent to the pore-forming loop. Although glycosylation in this vicinity has not been reported for a TRP channel, the structurally related hexahelical hyperpolarization-activated cyclic nucleotide-gated channel, HCN2, and the voltage-gated potassium channel, human ether-a-go-go-related (HERG), share a nearly identically situated and experimentally confirmed N-linked glycosylation site which promotes rather than limits channel insertion into the plasma membrane. These data point to a potentially conserved structural and functional feature influencing membrane trafficking across diverse members of the voltage-gated-like ion channel superfamily.

A truncated Kv1.1 protein in the brain of the megencephaly mouse: expression and interaction

Background

The megencephaly mouse, mceph/mceph, is epileptic and displays a dramatically increased brain volume and neuronal count. The responsible mutation was recently revealed to be an eleven base pair deletion, leading to a frame shift, in the gene encoding the potassium channel Kv1.1. The predicted MCEPH protein is truncated at amino acid 230 out of 495. Truncated proteins are usually not expressed since nonsense mRNAs are most often degraded. However, high Kv1.1 mRNA levels in mceph/mceph brain indicated that it escaped this control mechanism. Therefore, we hypothesized that the truncated Kv1.1 would be expressed and dysregulate other Kv1 subunits in the mceph/mceph mice.

Results

We found that the MCEPH protein is expressed in the brain of mceph/mceph mice. MCEPH was found to lack mature (Golgi) glycosylation, but to be core glycosylated and trapped in the endoplasmic reticulum (ER). Interactions between MCEPH and other Kv1 subunits were studied in cell culture, Xenopus oocytes and the brain. MCEPH can form tetramers with Kv1.1 in cell culture and has a dominant negative effect on Kv1.2 and Kv1.3 currents in oocytes. However, it does not retain Kv1.2 in the ER of neurons.

Conclusion

The megencephaly mice express a truncated Kv1.1 in the brain, and constitute a unique tool to study Kv1.1 trafficking relevant for understanding epilepsy, ataxia and pathologic brain overgrowth.

Glycosylation of Eag1 (Kv10.1) potassium channels: intracellular trafficking and functional consequences

N-Linked glycosylation is a common post-translational modification of membrane proteins. Here we report that mature Eag1 potassium channels carry sugar moieties linked to asparagines at positions 388 and 406. Asn-388 seems to undergo only core glycosylation, but complex sugars are bound to Asn-406. Correct complex glycosylation is required for proper trafficking of Eag1 to the plasma membrane but is also crucial for the correct function of channels already inserted in the membrane.

Kv2.1: a voltage-gated k+ channel critical to dynamic control of neuronal excitability

Neurons use a variety of mechanisms to dynamically control their own signaling capabilities. Regulation of voltage-dependent K+ channel localization and function has long been recognized as a major mechanism to achieve dynamic regulation of intrinsic neuronal excitability in a number of mammalian and non-mammalian neurons. Our recent evidence, together with compelling data from other laboratories, suggests that in mammalian neurons the Kv2.1 channel may play an especially prominent role in determining intrinsic neuronal excitability. Kv2.1 is widely expressed in brain and composes the majority of delayed rectifier K+ current in pyramidal neurons in cortex and hippocampus, and is also widely expressed in interneurons. Dynamic modulation of Kv2.1 localization and function by a mechanism involving activity-dependent Kv2.1 dephosphorylation dramatically impacts intrinsic excitability of neurons. Here we review previous studies of Kv2.1 localization and function in neurons, and summarize recent work regarding dynamic regulation of these characteristics. We also discuss possible roles of the Kv2.1 channel in neuronal and network excitability.

Determinants of voltage-gated potassium channel surface expression and localization in Mammalian neurons

Neurons strictly regulate expression of a wide variety of voltage-dependent ion channels in their surface membranes to achieve precise yet dynamic control of intrinsic membrane excitability. Neurons also exhibit extreme morphological complexity that underlies diverse aspects of their function. Most ion channels are preferentially targeted to either the axonal or somatodendritic compartments, where they become further localized to discrete membrane subdomains. This restricted accumulation of ion channels enables local control of membrane signaling events in specific microdomains of a given compartment. Voltage-dependent K+ (Kv) channels act as potent modulators of diverse excitatory events such as action potentials, excitatory synaptic potentials, and Ca2+ influx. Kv channels exhibit diverse patterns of cellular expression, and distinct subtype-specific localization, in mammalian central neurons. Here we review the mechanisms regulating the abundance and distribution of Kv channels in mammalian neurons and discuss how dynamic regulation of these events impacts neuronal signaling.

Calnexin regulates mammalian Kv1 channel trafficking

Voltage-gated Kv1 channels are key factors regulating excitability in the mammalian central nervous system. Diverse posttranslational regulatory mechanisms operate to determine the density, subunit composition, and localization of Kv1 channel complexes in the neuronal plasma membrane. In this study, we investigated the role of the endoplasmic reticulum chaperone calnexin in the intracellular trafficking of Kv1 channels. We found that coexpressing calnexin with the Kv1.2alpha subunit in transfected mammalian COS-1 cells produced a dramatic dose-dependent increase in cell surface Kv1.2 channel complexes. In calnexin-transfected COS-1 cells, the proportion of Kv1.2 channels with mature N-linked oligosaccharide chains was comparable to that observed in neurons. In contrast, calnexin coexpression exerted no effects on trafficking of the intracellularly retained Kv1.1 or Kv1.6alpha subunits. We also found that calnexin and auxiliary Kvbeta2 subunit coexpression was epistatic, suggesting that they share a common pathway for promoting Kv1.2 channel surface expression. These results provide yet another component in the elaborate repertoire of determinants regulating the density of Kv1 channels in the plasma membrane.

Assembly, Maturation, and Turnover of KATP Channel Subunits

ATP-sensitive K(+), or K(ATP), channels are comprised of K(IR)6.x and sulfonylurea receptor (SUR) subunits that assemble as octamers, (K(IR)/SUR)(4). The assembly pathway is unknown. Pulse-labeling studies show that when K(IR)6.2 is expressed individually, its turnover is biphasic; approximately 60% is lost with t((1/2)) approximately 36 min. The remainder converts to a long-lived species (t((1/2)) approximately 26 h) with an estimated half-time of 1.2 h. Expressed alone, SUR1 has a long half-life, approximately 25.5 h. When K(IR)6.2 and SUR1 are co-expressed, they associate rapidly and the fast degradation of K(IR)6.2 is eliminated. Based on changes in the glycosylation state of SUR1, the half-time for the maturation of K(ATP) channels, including completion of assembly, transit to the Golgi, and glycosylation, is approximately 2.2 h. Estimation of the turnover rates of mature, fully glycosylated SUR1 associated with K(IR)6.2 and of K(IR)6.2 associated with Myc-tagged SUR1 gave similar values for the half-life of K(ATP) channels, a mean value of approximately 7.3 h. K(ATP) channel subunits in INS-1 beta-cells displayed qualitatively similar kinetics. The results imply the octameric channels are stable. Two mutations, K(IR)6.2 W91R and SUR1 DeltaF1388, identified in patients with the severe form of familial hyperinsulinism, profoundly alter the rate of K(IR)6.2 and SUR1 turnover, respectively. Both mutant subunits associate with their respective partners but dissociate freely and degrade rapidly. The data support models of channel formation in which K(IR)6.2-SUR1 heteromers assemble functional channels and are inconsistent with models where SUR1 can only assemble with K(IR)6.2 tetramers.

Trafficking of Kv1.4 potassium channels: interdependence of a pore region determinant and a cytoplasmic C-terminal VXXSL determinant in regulating cell-surface trafficking

Kv1.4 and Kv1.1 potassium channel homomers have been shown to exhibit different intracellular trafficking programmes and cell-surface expression levels in cell lines: a determinant in the pore region of Kv1.4 and Kv1.1 [Zhu, Watanabe, Gomez and Thornhill (2001) J. Biol. Chem. 276, 39419-39427] and a cytoplasmic C-terminal VXXSL determinant on Kv1.4 [Li, Takimoto and Levitan (2000) J. Biol. Chem. 275, 11597-11602] have been described, which affected trafficking and cell-surface expression levels. In the present study, we examined whether trafficking pore determinants influenced any cytoplasmic C-terminal trafficking determinant. We found that removal of VXXSL from a Kv1.4 chimaera that contained the pore of Kv1.1 did not affect cell-surface trafficking. Therefore removal of the C-terminal VXXSL of Kv1.4 inhibited protein surface levels only in the presence of the Kv1.4 pore. In contrast, truncating the cytoplasmic C-terminus of Kv1.1 or truncating a Kv1.1 chimaera with the pore of Kv1.4, had little effect on surface protein levels. Furthermore, the subregion of the Kv1.4 pore trafficking determinant that was required for the inhibitory effect of VXXSL removal was mapped to a threonine residue in the deep pore region. Therefore the Kv1.4 pore determinant affected the trafficking and cell-surface levels directed by the C-terminal VXXSL determinant. Different Kv1 trafficking programmes would affect cell-surface expression levels either positively or negatively and also cell signalling. Cells may use differential trafficking programmes of membrane proteins as a post-translational mechanism to regulate surface protein levels and cell function.

Allowed N-glycosylation sites on the Kv1.2 potassium channel S1-S2 linker: implications for linker secondary structure and the glycosylation effect on channel function

N-glycosylation is a post-translational modification that plays a role in the trafficking and/or function of some membrane proteins. We have shown previously that N-glycosylation affected the function of some Kv1 voltage-gated potassium (K+) channels [Watanabe, Wang, Sutachan, Zhu, Recio-Pinto and Thornhill (2003) J. Physiol. (Cambridge, U.K.) 550, 51-66]. Kv1 channel S1-S2 linkers vary in length but their N-glycosylation sites are at similar relative positions from the S1 or S2 membrane domains. In the present study, by a scanning mutagenesis approach, we determined the allowed N-glycosylation sites on the Kv1.2 S1-S2 linker, which has 39 amino acids, by engineering N-glycosylation sites and assaying for glycosylation, using their sensitivity to glycosidases. The middle section of the linker (54% of linker) was glycosylated at every position, whereas both end sections (46% of linker) near the S1 or S2 membrane domains were not. These findings suggested that the middle section of the S1-S2 linker was accessible to the endoplasmic reticulum glycotransferase at every position and was in the extracellular aqueous phase, and presumably in a flexible conformation. We speculate that the S1-S2 linker is mostly a coiled-loop structure and that the strict relative position of native glycosylation sites on these linkers may be involved in the mechanism underlying the functional effects of glycosylation on some Kv1 K+ channels. The S3-S4 linker, with 16 amino acids and no N-glycosylation site, was not glycosylated when an N-glycosylation site was added. However, an extended linker, with an added N-linked site, was glycosylated, which suggested that the native linker was not glycosylated due to its short length. Thus other ion channels or membrane proteins may also have a high glycosylation potential on a linker but yet have similarly positioned native N-glycosylation sites among isoforms. This may imply that the native position of the N-glycosylation site may be important if the carbohydrate tree plays a role in the folding, stability, trafficking and/or function of the protein.

A fundamental role for KChIPs in determining the molecular properties and trafficking of Kv4.2 potassium channels

Kv4 potassium channels regulate action potentials in neurons and cardiac myocytes. Co-expression of EF hand-containing Ca2+-binding proteins termed KChIPs with pore-forming Kv4 alpha subunits causes changes in the gating and amplitude of Kv4 currents (An, W. F., Bowlby, M. R., Betty, M., Cao, J., Ling, H. P., Mendoza, G., Hinson, J. W., Mattsson, K. I., Strassle, B. W., Trimmer, J. S., and Rhodes, K. J. (2000) Nature 403, 553-556). Here we show that KChIPs profoundly affect the intracellular trafficking and molecular properties of Kv4.2 alpha subunits. Co-expression of KChIPs1-3 causes a dramatic redistribution of Kv4.2, releasing intrinsic endoplasmic reticulum retention and allowing for trafficking to the cell surface. KChIP co-expression also causes fundamental changes in Kv4.2 steady-state expression levels, phosphorylation, detergent solubility, and stability that reconstitute the molecular properties of Kv4.2 in native cells. Interestingly, the KChIP4a isoform, which exhibits unique effects on Kv4 channel gating, does not exert these effects on Kv4.2 and negatively influences the impact of other KChIPs. We provide evidence that these KChIP effects occur through the masking of an N-terminal Kv4.2 hydrophobic domain. These studies point to an essential role for KChIPs in determining both the biophysical and molecular characteristics of Kv4 channels and provide a molecular basis for the dramatic phenotype of KChIP knockout mice.

Voltage-gated K+ channels in rat small cerebral arteries: molecular identity of the functional channels

Voltage-gated potassium (KV) channels represent an important dilator influence in the cerebral circulation, but the composition of these tetrameric ion channels remains unclear. The goals of the present study were to evaluate the contribution of KV1 family channels to the resting membrane potential and diameter of small rat cerebral arteries, and to identify the alpha-subunit composition of these channels using patch-clamp, molecular and immunological techniques. Initial studies indicated that 1 micromol l(-1) correolide (COR), a specific antagonist of KV1 channels, depolarized vascular smooth muscle cells (VSMCs) in pressurized (60 mmHg) cerebral arteries from -55 +/- 1 mV to -34 +/- 1 mV, and reduced the resting diameter from 152 +/- 15 microm to 103 +/- 20 microm. In patch clamped VSMCs from these arteries, COR-sensitive KV1 current accounted for 65 % of total outward KV current and was observed at physiological membrane potentials. RT-PCR identified mRNA encoding each of the six classical KV1 alpha-subunits, KV1.1-1.6, in rat cerebral arteries. However, only the KV1.2 and 1.5 proteins were detected by Western blot. The expression of these proteins in VSMCs was confirmed by immunocytochemistry and co-immunoprecipitation of KV1.2 and 1.5 from VSMC membranes suggested KV1.2/1.5 channel assembly. Subsequently, the pharmacological and voltage-sensitive properties of KV1 current in VSMCs were found to be consistent with a predominant expression of KV1.2/1.5 heterotetrameric channels. The findings of this study suggest that KV1.2/1.5 heterotetramers are preferentially expressed in rat cerebral VSMCs, and that these channels contribute to the resting membrane potential and diameter of rat small cerebral arteries.

Voltage-dependent K(+) channels in pancreatic beta cells: role, regulation and potential as therapeutic targets

Insulin secretion from pancreatic islet beta cells is acutely regulated by a complex interplay of metabolic and electrogenic events. The electrogenic mechanism regulating insulin secretion from beta cells is commonly referred to as the ATP-sensitive K(+) (K(ATP)) channel dependent pathway. Briefly, an increase in ATP and, perhaps more importantly, a decrease in ADP stimulated by glucose metabolism depolarises the beta cell by closing K(ATP) channels. Membrane depolarisation results in the opening of voltage-dependent Ca(2+) channels, and influx of Ca(2+) is the main trigger for insulin secretion. Repolarisation of pancreatic beta cell action potential is mediated by the activation of voltage-dependent K(+) (Kv) channels. Various Kv channel homologues have been detected in insulin secreting cells, and recent studies have shown a role for specific Kv channels as modulators of insulin secretion. Here we review the evidence supporting a role for Kv channels in the regulation of insulin secretion and discuss the potential and the limitations for beta-cell Kv channels as therapeutic targets. Furthermore, we review recent investigations of mechanisms regulating Kv channels in beta cells, which suggest that Kv channels are active participants in the regulation of beta-cell electrical activity and insulin secretion.

Heteromeric Kv1 potassium channel expression: amino acid determinants involved in processing and trafficking to the cell surface

Kv1.4 and Kv1.1 potassium channels are expressed in brain as mature glycoproteins that are trans-Golgi glycosylated. When expressed in cell lines these homomers had very different trans-Golgi glycosylation efficiencies and cell surface expression levels with Kv1.4 > Kv1.1 for both parameters (Zhu, J., Watanabe, I., Gomez, B., and Thornhill, W. B. (2001) J. Biol. Chem. 276, 39419-39427). This previous study identified determinants in the outer pore region of Kv1.4 and Kv1.1 that positively and negatively, respectively, affected these events when expressed as homomers. Here we investigated which subunit exhibited positive or negative effects on these processes when expressed as heteromers. Kv1.4/Kv1.1 heteromers, by coexpression or expression as tandem-linked heteromers, were expressed on the cell surface at approximately 20-fold lower levels versus Kv1.4 homomers but they were trans-Golgi glycosylated. The lower Kv1.4/Kv1.1 expression level was not rescued by Kvbeta 2.1 subunits. Thus Kv1.1 inhibited high cell surface expression and partially retained the heteromer in the endoplasmic reticulum, whereas Kv1.4 stimulated trans-Golgi glycosylation. The subunit determinants and cellular events responsible for these differences were investigated. In a Kv1.4/Kv1.1 heteromer, the Kv1.1 pore was a major negative determinant, and it inhibited high cell surface expression because it induced high partial endoplasmic reticulum retention and it decreased protein stability. Other Kv1.1 regions also inhibited high surface expression of heteromers. The Kv1.1 C terminus induced partial Golgi retention and contributed to a decreased protein stability, whereas the Kv1.1 N terminus contributed to only a decreased protein stability. Thus a neuron may regulate its cell surface K+ channel protein levels by different Kv1 subfamily homomeric and heteromeric combinations that affect intracellular retention characteristics and protein stability.