Transcription factor EBF1 mitigates neuropathic pain by rescuing Kv1.2 expression in primary sensory neurons

Nerve injury-induced alternations of gene expression in primary sensory neurons of the dorsal root ganglion (DRG) are molecular basis of neuropathic pain genesis. Transcription factors regulate gene expression. In this study, we examined whether early B cell factor 1 (EBF1), a transcription factor, in the DRG, participated in neuropathic pain caused by chronic constriction injury (CCI) of the sciatic nerve. EBF1 was distributed exclusively in the neuronal nucleus and coexpressed with cytoplasmic/membrane Kv1.2 in individual DRG neurons. The expression of Ebf1 mRNA and protein was time-dependently downregulated in the ipsilateral lumbar (L) 3/4 DRGs after unilateral CCI. Rescuing this downregulation through microinjection of the adeno-associated virus 5 expressing full-length Ebf1 mRNA into the ipsilateral L3/4 DRGs reversed the CCI-induced decrease of DRG Kv1.2 expression and alleviated the development and maintenance of mechanical, heat and cold hypersensitivities. Conversely, mimicking the downregulation of DRG EBF1 through microinjection of AAV5-expressing Ebf1 shRNA into unilateral L3/4 DRGs produced a reduction of Kv1.2 expression in the ipsilateral L3/4 DRGs, spontaneous pain, and the enhanced responses to mechanical, heat and cold stimuli in naive mice. Mechanistically, EBF1 not only bound to the Kcna2 gene (encoding Kv1.2) promoter but also directly activated its activity. CCI decreased the EBF1 binding to the Kcna2 promoter in the ipsilateral L3/4 DRGs. Our findings suggest that DRG EBF1 downregulation contributes to neuropathic pain likely by losing its binding to Kcna2 promoter and subsequently silencing Kv1.2 expression in primary sensory neurons. Exogenous EBF1 administration may mitigate neuropathic pain by rescuing DRG Kv1.2 expression.

An epilepsy-associated KV1.2 charge-transfer-center mutation impairs KV1.2 and KV1.4 trafficking

We report on a heterozygous KCNA2 variant in a child with epilepsy. KCNA2 encodes KV1.2 subunits, which form homotetrameric potassium channels and participate in heterotetrameric channel complexes with other KV1-family subunits, regulating neuronal excitability. The mutation causes substitution F233S at the KV1.2 charge transfer center of the voltage-sensing domain. Immunocytochemical trafficking assays showed that KV1.2(F233S) subunits are trafficking deficient and reduce the surface expression of wild-type KV1.2 and KV1.4: a dominant-negative phenotype extending beyond KCNA2, likely profoundly perturbing electrical signaling. Yet some KV1.2(F233S) trafficking was rescued by wild-type KV1.2 and KV1.4 subunits, likely in permissible heterotetrameric stoichiometries: electrophysiological studies utilizing applied transcriptomics and concatemer constructs support that up to one or two KV1.2(F233S) subunits can participate in trafficking-capable heterotetramers with wild-type KV1.2 or KV1.4, respectively, and that both early and late events along the biosynthesis and secretion pathway impair trafficking. These studies suggested that F233S causes a depolarizing shift of ∼48 mV on KV1.2 voltage dependence. Optical tracking of the KV1.2(F233S) voltage-sensing domain (rescued by wild-type KV1.2 or KV1.4) revealed that it operates with modestly perturbed voltage dependence and retains pore coupling, evidenced by off-charge immobilization. The equivalent mutation in the Shaker K+ channel (F290S) was reported to modestly affect trafficking and strongly affect function: an ∼80-mV depolarizing shift, disrupted voltage sensor activation and pore coupling. Our work exposes the multigenic, molecular etiology of a variant associated with epilepsy and reveals that charge-transfer-center disruption has different effects in KV1.2 and Shaker, the archetypes for potassium channel structure and function.

Multifaceted Regulation of Potassium-Ion Channels by Graphene Quantum Dots

Graphene quantum dots (GQDs) are emerging as a versatile nanomaterial with numerous proposed biomedical applications. Despite the explosion in potential applications, the molecular interactions between GQDs and complex biomolecular systems, including potassium-ion (K+) channels, remain largely unknown. Here, we use molecular dynamics (MD) simulations and electrophysiology to study the interactions between GQDs and three representative K+ channels, which participate in a variety of physiological processes and are closely related to many disease states. Using MD simulations, we observed that GQDs adopt distinct contact poses with each of the three structurally distinct K+ channels. Our electrophysiological characterization of the effects of GQDs on channel currents revealed that GQDs interact with the extracellular voltage-sensing domain (VSD) of a Kv1.2 channel, augmenting current by left-shifting the voltage dependence of channel activation. In contrast, GQDs form a "lid" cluster over the extracellular mouth of inward rectifier Kir3.2, blocking the channel pore and decreasing the current in a concentration-dependent manner. Meanwhile, GQDs accumulate on the extracellular "cap domain" of K2P2 channels and have no apparent impact on the K+ flux through the channel. These results reveal a surprising multifaceted regulation of K+ channels by GQDs, which might help de novo design of nanomaterial-based channel probe openers/inhibitors that can be used to further discern channel function.

The sigma-1 receptor behaves as an atypical auxiliary subunit to modulate the functional characteristics of Kv1.2 channels expressed in HEK293 cells

Expression of Kv1.2 within Kv1.x potassium channel complexes is critical in maintaining appropriate neuronal excitability and determining the threshold for action potential firing. This is attributed to the interaction of Kv1.2 with a hitherto unidentified protein that confers bimodal channel activation gating, allowing neurons to adapt to repetitive trains of stimulation and protecting against hyperexcitability. One potential protein candidate is the sigma-1 receptor (Sig-1R), which regulates other members of the Kv1.x channel family; however, the biophysical nature of the interaction between Sig-1R and Kv1.2 has not been elucidated. We hypothesized that Sig-1R may regulate Kv1.2 and may further act as the unidentified modulator of Kv1.2 activation. In transiently transfected HEK293 cells, we found that ligand activation of the Sig-1R modulates Kv1.2 current amplitude. More importantly, Sig-1R interacts with Kv1.2 in baseline conditions to influence bimodal activation gating. These effects are abolished in the presence of the auxiliary subunit Kvβ2 and when the Sig-1R mutation underlying ALS16 (Sig-1R-E102Q), is expressed. These data suggest that Kvβ2 occludes the interaction of Sig-1R with Kv1.2, and that E102 may be a residue critical for Sig-1R modulation of Kv1.2. The results of this investigation describe an important new role for Sig-1R in the regulation of neuronal excitability and introduce a novel mechanism of pathophysiology in Sig-1R dysfunction.

TET1 Overexpression Mitigates Neuropathic Pain Through Rescuing the Expression of μ-Opioid Receptor and Kv1.2 in the Primary Sensory Neurons

Peripheral nerve injury downregulates the expression of the μ-opioid receptor (MOR) and voltage-gated potassium channel subunit Kv1.2 by increasing their DNA methylation in the dorsal root ganglion (DRG). Ten-eleven translocation methylcytosine dioxygenase 1 (TET1) causes DNA demethylation. Given that DRG MOR and Kv1.2 downregulation contribute to neuropathic pain genesis, this study investigated the effect of DRG TET1 overexpression on neuropathic pain. Overexpression of TET1 in the DRG through microinjection of herpes simplex virus expressing full-length TET1 mRNA into the injured rat DRG significantly alleviated the fifth lumbar spinal nerve ligation (SNL)-induced pain hypersensitivities during the development and maintenance periods, without altering acute pain or locomotor function. This microinjection also restored morphine analgesia and attenuated morphine analgesic tolerance development after SNL. Mechanistically, TET1 microinjection rescued the expression of MOR and Kv1.2 by reducing the level of 5-methylcytosine and increasing the level of 5-hydroxymethylcytosine in the promoter and 5' untranslated regions of the Oprml1 gene (encoding MOR) and in the promoter region of the Kcna2 gene (encoding Kv1.2) in the DRG ipsilateral to SNL. These findings suggest that DRG TET1 overexpression mitigated neuropathic pain likely through rescue of MOR and Kv1.2 expression in the ipsilateral DRG. Virus-mediated DRG delivery of TET1 may open a new avenue for neuropathic pain management.

Binding of the general anesthetic sevoflurane to ion channels

The direct-site hypothesis assumes general anesthetics bind ion channels to impact protein equilibrium and function, inducing anesthesia. Despite advancements in the field, a first principle all-atom demonstration of this structure-function premise is still missing. We focus on the clinically used sevoflurane interaction to anesthetic-sensitive Kv1.2 mammalian channel to resolve if sevoflurane binds protein’s well-characterized open and closed structures in a conformation-dependent manner to shift channel equilibrium. We employ an innovative approach relying on extensive docking calculations and free-energy perturbation of all potential binding sites revealed by the latter, and find sevoflurane binds open and closed structures at multiple sites under complex saturation and concentration effects. Results point to a non-trivial interplay of site and conformation-dependent modes of action involving distinct binding sites that increase channel open-probability at diluted ligand concentrations. Given the challenge in exploring more complex processes potentially impacting channel-anesthetic interaction, the result is revealing as it demonstrates the process of multiple anesthetic binding events alone may account for open-probability shifts recorded in measurements.

General anesthetics are central to modern medicine, yet their microscopic mechanism of action is still unknown. Here, we demonstrate that a clinically used anesthetic, sevoflurane, binds the mammalian voltage-gated potassium channel Kv1.2 effecting a shift in its open probability, even at low concentrations. The results, supported by recent experimental measurements, are promising as they demonstrate that the molecular process of direct binding of anesthetic to ion channels play a relevant role in anesthesia.

Pharmacological characterization of human beta-defensins 3 and 4 on potassium channels: Evidence of diversity in beta-defensin-potassium channel interactions

Recent reports have identified defensins as a new type of potassium channel inhibitors; differential binding mechanisms of human β-defensins hBD1 and hBD2 point to complex interactions between human β-defensins and potassium channels. We investigated the inhibitory effects of human defensins hBD3 and hBD4 on potassium channels. The data indicate that hBD3 is a voltage-gated channel subfamily A member 3 (Kv1.3) inhibitor with an IC₅₀ value of 187.6 ± 25.7 nM; 1 μM hBD4 inhibited 34.0 ± 0.2% of Kv1.3 channel currents. Moreover, 1 μM hBD3 inhibited 50.6 ± 3.6% of Kv1.2 channel currents and had smaller effects on Kv1.1, SKCa3, and IKCa channel currents; these effects differed from the Kv1.3 channel-specific inhibitors hBD1 and hBD2. Similar to the pharmacological profiles of hBD1 and hBD2, hBD4 had lower inhibitory effects on Kv1.1, Kv1.2, SKCa3, and IKCa channels. Subsequent mutagenesis and channel activation experiments confirmed that hBD3 binds in a manner similar to that of hBD1, interacting with the outer pore region of the Kv1.3 channel without affecting Kv1.3 channel activation. Thus, the data indicate that the human β-defensin family is a novel group of potassium channel inhibitors with diverse types of human β-defensin-potassium channel interactions.

Molecular Dynamics Simulation Reveals Specific Interaction Sites between Scorpion Toxins and Kv1.2 Channel: Implications for Design of Highly Selective Drugs

The K_v1.2 channel plays an important role in the maintenance of resting membrane potential and the regulation of the cellular excitability of neurons, whose silencing or mutations can elicit neuropathic pain or neurological diseases (e.g., epilepsy and ataxia). Scorpion venom contains a variety of peptide toxins targeting the pore region of this channel. Despite a large amount of structural and functional data currently available, their detailed interaction modes are poorly understood. In this work, we choose four K_v1.2-targeted scorpion toxins (Margatoxin, Agitoxin-2, OsK-1, and Mesomartoxin) to construct their complexes with K_v1.2 based on the experimental structure of ChTx-K_v1.2. Molecular dynamics simulation of these complexes lead to the identification of hydrophobic patches, hydrogen-bonds, and salt bridges as three essential forces mediating the interactions between this channel and the toxins, in which four K_v1.2-specific interacting amino acids (D353, Q358, V381, and T383) are identified for the first time. This discovery might help design highly selective K_v1.2-channel inhibitors by altering amino acids of these toxins binding to the four channel residues. Finally, our results provide new evidence in favor of an induced fit model between scorpion toxins and K⁺ channel interactions.

Extracellular redox sensitivity of Kv1.2 potassium channels

Kv1.2 is a prominent potassium channel subtype in the nervous system and serves as an important structural template for investigation of ion channel function. However, Kv1.2 voltage-dependence exhibits dramatic cell-to-cell variability due to a gating mode shift that is regulated by an unknown mechanism. We report that this variable behavior is regulated by the extracellular redox environment. Exposure to reducing agents promotes a shift in gating properties towards an 'inhibited' gating mode that resists opening, and causes channels to exhibit pronounced use-dependent activation during trains of repetitive depolarizations. This sensitivity to extracellular redox potential is absent in other Kv1 channels, but is apparent in heteromeric channels containing Kv1.2 subunits, and overlaps with the reported physiological range of extracellular redox couples. Mutagenesis of candidate cysteine residues fails to abolish redox sensitivity. Therefore, we suggest that an extrinsic, redox-sensitive binding partner imparts these properties.

Ion Concentration- and Voltage-Dependent Push and Pull Mechanisms of Potassium Channel Ion Conduction

The mechanism of ion conduction by potassium channels is one of the central issues in physiology. In particular, it is still unclear how the ion concentration and the membrane voltage drive ion conduction. We have investigated the dynamics of the ion conduction processes in the Kv1.2 pore domain, by molecular dynamics (MD) simulations with several different voltages and ion concentrations. By focusing on the detailed ion movements through the pore including selectivity filter (SF) and cavity, we found two major conduction mechanisms, called the III-IV-III and III-II-III mechanisms, and the balance between the ion concentration and the voltage determines the mechanism preference. In the III-IV-III mechanism, the outermost ion in the pore is pushed out by a new ion coming from the intracellular fluid, and four-ion states were transiently observed. In the III-II-III mechanism, the outermost ion is pulled out first, without pushing by incoming ions. Increases in the ion concentration and voltage accelerated ion conductions, but their mechanisms were different. The increase in the ion concentrations facilitated the III-IV-III conductions, while the higher voltages increased the III-II-III conductions, indicating that the pore domain of potassium channels permeates ions by using two different driving forces: a push by intracellular ions and a pull by voltage.

Short-Term Sleep Disturbance-Induced Stress Does not Affect Basal Pain Perception, but Does Delay Postsurgical Pain Recovery

Chronic sleep disturbance-induced stress is known to increase basal pain sensitivity. However, most surgical patients frequently report short-term sleep disturbance/deprivation during the pre- and postoperation periods and have normal pain perception presurgery. Whether this short-term sleep disturbance affects postsurgical pain is elusive. Here, we report that pre- or postexposure to rapid eye movement sleep disturbance (REMSD) for 6 hours daily for 3 consecutive days did not alter basal responses to mechanical, heat, and cold stimuli, but did delay recovery in incision-induced reductions in paw withdrawal threshold to mechanical stimulation and paw withdrawal latencies to heat and cold stimuli on the ipsilateral side of male or female rats. This short-term REMSD led to stress shown by an increase in swim immobility time, a decrease in sucrose consumption, and an increase in the level of corticosterone in serum. Blocking this stress via intrathecal RU38486 or bilateral adrenalectomy abolished REMSD-caused delay in recovery of incision-induced reductions in behavioral responses to mechanical, heat, and cold stimuli. Moreover, this short-term REMSD produced significant reductions in the levels of mu opioid receptor and kappa opioid receptor, but not Kv1.2, in the ipsilateral L4/5 spinal cord and dorsal root ganglia on day 9 after incision (but not after sham surgery).

PERSPECTIVE

Our findings show that short-term sleep disturbance either pre- or postsurgery does not alter basal pain perception, but does exacerbate postsurgical pain hypersensitivity. The latter may be related to the reductions of mu and kappa opioid receptors in the spinal cord and dorsal root ganglia caused by REMSD plus incision. Prevention of short-term sleep disturbance may help recovery from postsurgical pain in patients.

The open gate of the K(V)1.2 channel: quantum calculations show the key role of hydration

The open gate of the Kv1.2 voltage-gated potassium channel can just hold a hydrated K(+) ion. Quantum calculations starting from the x-ray coordinates of the channel confirm this, showing little change from the x-ray coordinates for the protein. Water molecules not in the x-ray coordinates, and the ion itself, are placed by the calculation. The water molecules, including their orientation and hydrogen bonding, with and without an ion, are critical for the path of the ion, from the solution to the gate. A sequence of steps is postulated in which the potential experienced by the ion in the pore is influenced by the position of the ion. The gate structure, with and without the ion, has been optimized. The charges on the atoms and bond lengths have been calculated using natural bond orbital calculations, giving K(+) ~0.77 charges, rather than 1.0. The PVPV hinge sequence has been mutated in silico to PVVV (P407V in the 2A79 numbering). The water structure around the ion becomes discontinuous, separated into two sections, above and below the ion. PVPV conservation closely relates to maintaining the water structure. Finally, these results have implications concerning gating.

Disruption of myelin leads to ectopic expression of K(V)1.1 channels with abnormal conductivity of optic nerve axons in a cuprizone-induced model of demyelination

The molecular determinants of abnormal propagation of action potentials along axons and ectopic conductance in demyelinating diseases of the central nervous system, like multiple sclerosis (MS), are poorly defined. Widespread interruption of myelin occurs in several mouse models of demyelination, rendering them useful for research. Herein, considerable myelin loss is shown in the optic nerves of cuprizone-treated demyelinating mice. Immuno-fluorescence confocal analysis of the expression and distribution of voltage-activated K⁺ channels (K(V)1.1 and 1.2 α subunits) revealed their spread from typical juxta-paranodal (JXP) sites to nodes in demyelinated axons, albeit with a disproportionate increase in the level of K(V)1.1 subunit. Functionally, in contrast to monophasic compound action potentials (CAPs) recorded in controls, responses derived from optic nerves of cuprizone-treated mice displayed initial synchronous waveform followed by a dispersed component. Partial restoration of CAPs by broad spectrum (4-aminopyridine) or K(V)1.1-subunit selective (dendrotoxin K) blockers of K⁺ currents suggest enhanced K(V)1.1-mediated conductance in the demyelinated optic nerve. Biophysical profiling of K⁺ currents mediated by recombinant channels comprised of different K(V)1.1 and 1.2 stoichiometries revealed that the enrichment of K(V)1 channels K(V)1.1 subunit endows a decrease in the voltage threshold and accelerates the activation kinetics. Together with the morphometric data, these findings provide important clues to a molecular basis for temporal dispersion of CAPs and reduced excitability of demyelinated optic nerves, which could be of potential relevance to the patho-physiology of MS and related disorders.

Ion concentration-dependent ion conduction mechanism of a voltage-sensitive potassium channel

Voltage-sensitive potassium ion channels are essential for life, but the molecular basis of their ion conduction is not well understood. In particular, the impact of ion concentration on ion conduction has not been fully studied. We performed several micro-second molecular dynamics simulations of the pore domain of the Kv1.2 potassium channel in KCl solution at four different ion concentrations, and scrutinized each of the conduction events, based on graphical representations of the simulation trajectories. As a result, we observed that the conduction mechanism switched with different ion concentrations: at high ion concentrations, potassium conduction occurred by Hodgkin and Keynes' knock-on mechanism, where the association of an incoming ion with the channel is tightly coupled with the dissociation of an outgoing ion, in a one-step manner. On the other hand, at low ion concentrations, ions mainly permeated by a two-step association/dissociation mechanism, in which the association and dissociation of ions were not coupled, and occurred in two distinct steps. We also found that this switch was triggered by the facilitated association of an ion from the intracellular side within the channel pore and by the delayed dissociation of the outermost ion, as the ion concentration increased.

Dynamic interaction between sigma-1 receptor and Kv1.2 shapes neuronal and behavioral responses to cocaine

The sigma-1 receptor (Sig-1R), an endoplasmic reticulum (ER) chaperone protein, is an interorganelle signaling modulator that potentially plays a role in drug-seeking behaviors. However, the brain site of action and underlying cellular mechanisms remain unidentified. We found that cocaine exposure triggers a Sig-1R-dependent upregulation of D-type K(+) current in the nucleus accumbens (NAc) that results in neuronal hypoactivity and thereby enhances behavioral cocaine response. Combining ex vivo and in vitro studies, we demonstrated that this neuroadaptation is caused by a persistent protein-protein association between Sig-1Rs and Kv1.2 channels, a phenomenon that is associated to a redistribution of both proteins from intracellular compartments to the plasma membrane. In conclusion, the dynamic Sig-1R-Kv1.2 complex represents a mechanism that shapes neuronal and behavioral response to cocaine. Functional consequences of Sig-1R binding to K(+) channels may have implications for other chronic diseases where maladaptive intrinsic plasticity and Sig-1Rs are engaged.

The sodium-potassium pump controls the intrinsic firing of the cerebellar Purkinje neuron

In vitro, cerebellar Purkinje cells can intrinsically fire action potentials in a repeating trimodal or bimodal pattern. The trimodal pattern consists of tonic spiking, bursting, and quiescence. The bimodal pattern consists of tonic spiking and quiescence. It is unclear how these firing patterns are generated and what determines which firing pattern is selected. We have constructed a realistic biophysical Purkinje cell model that can replicate these patterns. In this model, Na(+)/K(+) pump activity sets the Purkinje cell's operating mode. From rat cerebellar slices we present Purkinje whole cell recordings in the presence of ouabain, which irreversibly blocks the Na(+)/K(+) pump. The model can replicate these recordings. We propose that Na(+)/K(+) pump activity controls the intrinsic firing mode of cerbellar Purkinje cells.

A theoretical model for calculating voltage sensitivity of ion channels and the application on Kv1.2 potassium channel

Voltage sensing confers conversion of a change in membrane potential to signaling activities underlying the physiological processes. For an ion channel, voltage sensitivity is usually experimentally measured by fitting electrophysiological data to Boltzmann distributions. In our study, a two-state model of the ion channel and equilibrium statistical mechanics principle were used to test the hypothesis of empirically calculating the overall voltage sensitivity of an ion channel on the basis of its closed and open conformations, and determine the contribution of individual residues to the voltage sensing. We examined the theoretical paradigm by performing experimental measurements with Kv1.2 channel and a series of mutants. The correlation between the calculated values and the experimental values is at respective level, R(2) = 0.73. Our report therefore provides in silico prediction of key conformations and has identified additional residues critical for voltage sensing.

Identification of selective inhibitors of the potassium channel Kv1.1-1.2((3)) by high-throughput virtual screening and automated patch clamp

Two voltage-dependent potassium channels, Kv1.1 (KCNA1) and Kv1.2 (KCNA2), are found to co-localize at the juxtaparanodal region of axons throughout the nervous system and are known to co-assemble in heteromultimeric channels, most likely in the form of the concatemer Kv1.1-1.2((3)) . Loss of the myelin sheath, as is observed in multiple sclerosis, uncovers the juxtaparanodal region of nodes of Ranvier in myelinated axons leading to potassium conductance, resulting in loss of nerve conduction. The selective blocking of these Kv channels is therefore a promising approach to restore nerve conduction and function. In the present study, we searched for novel inhibitors of Kv1.1-1.2((3)) by combining a virtual screening protocol and electrophysiological measurements on a concatemer Kv1.1-1.2((3)) stably expressed in Chinese hamster ovary K1 (CHO-K1) cells. The combined use of four popular virtual screening approaches (eHiTS, FlexX, Glide, and Autodock-Vina) led to the identification of several compounds as potential inhibitors of the Kv1.1-1.2((3)) channel. From 89 electrophysiologically evaluated compounds, 14 novel compounds were found to inhibit the current carried by Kv1.1-1.2((3)) channels by more than 80 % at 10 μM. Accordingly, the IC(50) values calculated from concentration-response curve titrations ranged from 0.6 to 6 μM. Two of these compounds exhibited at least 30-fold higher potency in inhibition of Kv1.1-1.2((3)) than they showed in inhibition of a set of cardiac ion channels (hERG, Nav1.5, and Cav1.2), resulting in a profile of selectivity and cardiac safety. The results presented herein provide a promising basis for the development of novel selective ion channel inhibitors, with a dramatically lower demand in terms of experimental time, effort, and cost than a sole high-throughput screening approach of large compound libraries.

Paranodal myelin retraction in relapsing experimental autoimmune encephalomyelitis visualized by coherent anti-Stokes Raman scattering microscopy

How demyelination is initiated is a standing question for pathology of multiple sclerosis. By label-free coherent anti-Stokes Raman scattering (CARS) imaging of myelin lipids, we investigate myelin integrity in the lumbar spinal cord tissue isolated from naïve SJL mice, and from mice at the onset, peak acute, and remission stages of relapsing experimental autoimmune encephalomyelitis (EAE). Progressive demyelinating disease is initially characterized by the retraction of paranodal myelin both at the onset of disease and at the borders of acute demyelinating lesions. Myelin retraction is confirmed by elongated distribution of neurofascin proteins visualized by immunofluorescence. The disruption of paranodal myelin subsequently exposes Kv1.2 channels at the juxtaparanodes and lead to the displacement of Kv1.2 channels to the paranodal and nodal domains. Paranodal myelin is partially restored during disease remission, indicating spontaneous myelin regeneration. These findings suggest that paranodal domain injury precedes formation of internodal demyelinating lesions in relapsing EAE. Our results also demonstrate that CARS microscopy is an effective readout of myelin disease burden.

Comparative study of the energetics of ion permeation in Kv1.2 and KcsA potassium channels

Biological ion channels rely on a multi-ion transport mechanism for fast yet selective permeation of ions. The crystal structure of the KcsA potassium channel provided the first microscopic picture of this process. A similar mechanism is assumed to operate in all potassium channels, but the validity of this assumption has not been well investigated. Here, we examine the energetics of ion permeation in Shaker Kv1.2 and KcsA channels, which exemplify the six-transmembrane voltage-gated and two-transmembrane inward-rectifier channels. We study the feasibility of binding a third ion to the filter and the concerted motion of ions in the channel by constructing the potential of mean force for K(+) ions in various configurations. For both channels, we find that a pair of K(+) ions can move almost freely within the filter, but a relatively large free-energy barrier hinders the K(+) ion from stepping outside the filter. We discuss the effect of the CMAP dihedral energy correction that was recently incorporated into the CHARMM force field on ion permeation dynamics.

Contribution of Kv1.2 voltage-gated potassium channel to D2 autoreceptor regulation of axonal dopamine overflow

Impairments in axonal dopamine release are associated with neurological disorders such as schizophrenia and attention deficit hyperactivity disorder and pathophysiological conditions promoting drug abuse and obesity. The D2 dopamine autoreceptor (D2-AR) exerts tight regulatory control of axonal dopamine (DA) release through a mechanism suggested to involve K(+) channels. To evaluate the contribution of Kv1 voltage-gated potassium channels of the Shaker gene family to the regulation of axonal DA release by the D2-AR, the present study employed expression analyses, real time measurements of striatal DA overflow, K(+) current measurements and immunoprecipitation assays. Kv1.1, -1.2, -1.3, and -1.6 mRNA and protein were detected in midbrain DA neurons purified by fluorescence-activated cell sorting and in primary DA neuron cultures. In addition, Kv1.1, -1.2, and -1.6 were localized to DA axonal processes in the dorsal striatum. By means of fast scan cyclic voltammetry in striatal slice preparations, we found that the inhibition of stimulation-evoked DA overflow by a D2 agonist was attenuated by Kv1.1, -1.2, and -1.6 toxin blockers. A particular role for the Kv1.2 subunit in the process whereby axonal D2-AR inhibits DA overflow was established with the use of a selective Kv1.2 blocker and Kv1.2 knock-out mice. Moreover, we demonstrate the ability of D2-AR activation to increase Kv1.2 currents in co-transfected cells and its reliance on Gβγ subunit signaling along with the physical coupling of D2-AR and Kv1.2-containing channels in striatal tissue. These findings underline the contribution of Kv1.2 in the regulation of nigrostriatal DA release by the D2-AR and thereby offer a novel mechanism by which DA release is regulated.

Dynamics of Kv1 channel transport in axons

Concerted actions of various ion channels that are precisely targeted along axons are crucial for action potential initiation and propagation, and neurotransmitter release. However, the dynamics of channel protein transport in axons remain unknown. Here, using time-lapse imaging, we found fluorescently tagged Kv1.2 voltage-gated K(+) channels (YFP-Kv1.2) moved bi-directionally in discrete puncta along hippocampal axons. Expressing Kvbeta2, a Kv1 accessory subunit, markedly increased the velocity, the travel distance, and the percentage of moving time of these puncta in both anterograde and retrograde directions. Suppressing the Kvbeta2-associated protein, plus-end binding protein EB1 or kinesin II/KIF3A, by siRNA, significantly decreased the velocity of YFP-Kv1.2 moving puncta in both directions. Kvbeta2 mutants with disrupted either Kv1.2-Kvbeta2 binding or Kvbeta2-EB1 binding failed to increase the velocity of YFP-Kv1.2 puncta, confirming a central role of Kvbeta2. Furthermore, fluorescently tagged Kv1.2 and Kvbeta2 co-moved along axons. Surprisingly, when co-moving with Kv1.2 and Kvbeta2, EB1 appeared to travel markedly faster than its plus-end tracking. Finally, using fission yeast S. pombe expressing YFP-fusion proteins as reference standards to calibrate our microscope, we estimated the numbers of YFP-Kv1.2 tetramers in axonal puncta. Taken together, our results suggest that proper amounts of Kv1 channels and their associated proteins are required for efficient transport of Kv1 channel proteins along axons.

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.

Molecular basis of inhibitory peptide maurotoxin recognizing Kv1.2 channel explored by ZDOCK and molecular dynamic simulations

Inhibitory peptide-channel interactions have been utilized to characterize both channels and peptides; however, the fundamental basis for these interactions remains elusive. Here, combined computation methods were employed to study the specific binding of maurotoxin (MTX) peptide to Kv1.2 channel. In the first stage, numerous predicted complexes were generated by docking an ensemble of all 35 NMR conformations of MTX to Kv1.2 channel with ZDOCK program. Then the resulted complexes were clustered and classified into four main binding modes, based on experimental information and interaction energy analysis after the energy minimization and molecular dynamics (MD) simulations. By examining the stability of the plausible candidates through unrestrained MD simulations and calculation of the binding free energies, a final reasonable MTX-Kv1.2 complex was identified, with an overall high degree of correlation between the calculation and experiment on mutational effects. In the obtained complex structure model, MTX mainly used its beta-sheet domains to associate the channel mouth instead of the well-recognized functionally important S5P linkers of Kv1.2 channel. Structure analysis characterized that the most essential Tyr(32) residue of MTX was surrounded by a "pocket" formed by many nonpolar and polar residues of Kv1.2 channel, and revealed a pore-blocking Lys(23) and an important Lys(7) stabilized by strong electrostatic interactions with Asp(379) of Kv1.2. Furthermore, a stepwise structural arrangement for both ligand and receptor was found to accompany the tighter interaction of MTX into the target channel. The starting conformation of MTX, the side-chain conformation of the most important residue Tyr(32), and proper introduction of flexibility for candidate complexes were demonstrated to be considerably important factors for obtaining the final reasonable complex structure model. All these findings should not only be helpful for identifying more plausible K(+) channel-inhibitory peptide complex structures, but also provide intrinsically valuable structural biology information to interpret binding affinities, specificities, and diversity of K(+) channel-nature toxin interactions.

IGF-I alleviates diabetes-induced RhoA activation, eNOS uncoupling, and myocardial dysfunction

IGF-I rescues diabetic heart defects and oxidative stress, although the underlying mechanism of action remains poorly understood. This study was designed to delineate the beneficial effects of IGF-I with a focus on RhoA, Akt, and eNOS coupling. Echocardiography was performed in normal or diabetic Friend Virus-B type (FVB) and IGF-I transgenic mice. Cardiomyocyte contractile properties were evaluated using peak shortening (PS), time-to-90% relengthening (TR90), and intracellular Ca2+ rise and decay. Diabetes reduced fraction shortening, PS, and intracellular Ca2+; it increased chamber size, prolonged TR90, and intracellular Ca2+ decay. Levels of RhoA mRNA, active RhoA, and O2(-) were elevated, whereas nitric oxide (NO) levels were reduced in diabetes. Diabetes-induced O2(-) accumulation was ablated by the NO synthase (NOS) inhibitor nitro-L-arginine methyl ester (L-NAME), indicating endothelial NOS (eNOS) uncoupling, all of which except heart size were negated by IGF-I. The IGF-I-elicited beneficial effects were mimicked by the Rho kinase inhibitor Y27632 and BH4. Diabetes depressed expression of Kv1.2 and dihydrofolate reductase (DHFR), increased beta-myosin heavy-chain expression, stimulated p38 MAPK, and reduced levels of total Akt and phosphorylated Akt/eNOS, all of which with the exception of myosin heavy chain were attenuated by IGF-I. In addition, Y27632 and the eNOS coupler folate abrogated glucose toxicity-induced PS decline, TR90 prolongation, while it increased O2(-) and decreased NO and Kv1.2 levels. The DHFR inhibitor methotrexate impaired myocyte function, NO/O2(-) balance, and rescued Y27632-induced cardiac protection. These results revealed that IGF-I benefits diabetic hearts via Rho inhibition and antagonism of diabetes-induced decrease in pAkt, eNOS uncoupling, and K+ channel expression.

[Optimization of method for detecting the activation of potassium channels in rat cardiac muscle cells]

To detect the phosphorylation of potassium channel protein (Kv1.2 and Kv1.5) in rat cardiac muscle cells accurately, we applied the combined method of immunoprecipitation and Western blot in this study. Compared with using Western blot alone, the combination of immunoprecipitation and Western blot displayed high sensitivity to detect the activation of potassium channel proteins. Because of its simplicity, quickness and reproducibility, we find that this method was promising for detecting the phosphorylation of Kv1.2 and Kv1.5 proteins or other potassium channel proteins in rat cardiac muscle cells.

Glucose homeostasis, insulin secretion, and islet phospholipids in mice that overexpress iPLA2beta in pancreatic beta-cells and in iPLA2beta-null mice

Studies with genetically modified insulinoma cells suggest that group VIA phospholipase A(2) (iPLA(2)beta) participates in amplifying glucose-induced insulin secretion. INS-1 insulinoma cells that overexpress iPLA(2)beta, for example, exhibit amplified insulin-secretory responses to glucose and cAMP-elevating agents. To determine whether similar effects occur in whole animals, we prepared transgenic (TG) mice in which the rat insulin 1 promoter (RIP) drives iPLA(2)beta overexpression, and two characterized TG mouse lines exhibit similar phenotypes. Their pancreatic islet iPLA(2)beta expression is increased severalfold, as reflected by quantitative PCR of iPLA(2)beta mRNA, immunoblotting of iPLA(2)beta protein, and iPLA(2)beta enzymatic activity. Immunofluorescence microscopic studies of pancreatic sections confirm iPLA(2)beta overexpression in RIP-iPLA(2)beta-TG islet beta-cells without obviously perturbed islet morphology. Male RIP-iPLA(2)beta-TG mice exhibit lower blood glucose and higher plasma insulin concentrations than wild-type (WT) mice when fasting and develop lower blood glucose levels in glucose tolerance tests, but WT and TG blood glucose levels do not differ in insulin tolerance tests. Islets from male RIP-iPLA(2)beta-TG mice exhibit greater amplification of glucose-induced insulin secretion by a cAMP-elevating agent than WT islets. In contrast, islets from male iPLA(2)beta-null mice exhibit blunted insulin secretion, and those mice have impaired glucose tolerance. Arachidonate incorporation into and the phospholipid composition of RIP-iPLA(2)beta-TG islets are normal, but they exhibit reduced Kv2.1 delayed rectifier current and prolonged glucose-induced action potentials and elevations of cytosolic Ca(2+) concentration that suggest a molecular mechanism for the physiological role of iPLA(2)beta to amplify insulin secretion.

Kv1 potassium channel C-terminus constant HRETE region: arginine substitution affects surface protein level and conductance level of subfamily members differentially

We have shown previously that truncating all of the variable cytoplasmic C-terminus of Kv1.1 potassium channels to G421stop had only a small inhibitory effect on their cell surface conductance density levels and cell surface protein levels. Here we investigated the role of a highly conserved cytoplasmic C-terminal charged region of five amino acids (HRETE) of the S6 transmembrane domain in the protein and conductance expression of Kv1.1, Kv1.2, and Kv1.4 channels. For Kv1.1 we found that E420stop, T419stop, and E418stop showed cell surface conductance densities and cell surface protein levels similar to full length control, whereas R417stop and H416stop exhibited essentially no conductance but their surface protein levels were similar to full length control. A bulky non-negatively charged hydrophilic amino acid at position 417 appeared to be critical for wild type gating of Kv1.1 because R417K and R417Q rescued conductance levels whereas R417A or R417E did not. The R417A mutation in the full length Kv1.1 also exhibited surface protein levels similar to control but it did not exhibit significant conductance. In contrast, mutation of the equivalent arginine to alanine in full length Kv1.2 and Kv1.4 appeared to have little or no effect on channel conductance but rather decreased cell surface protein levels by inducing partial high ER retention. These findings are consistent with the notion that the arginine amino acid in the HRETE region plays a different role in affecting conductance levels or cell surface protein levels of very closely related Kv1 potassium channels.

Molecular basis of dysfunctional Kv channels in small coronary artery smooth muscle cells of streptozotocin-induced diabetic rats

We have previously shown that diabetes impaired cAMP-mediated endothelium independent vasodilation of rat small coronary arteries. Inhibition of Kv channel activity plays an important role in the decrease of cAMP mediated vasodilation. The present study investigated the effect of streptozotocin (STZ)-induced diabetes on mRNA and protein expressions of Kv1.2 and Kv1.5 channels in vascular smooth muscle cells of rat small coronary artery using RT-PCR, Western blot and immunohistochemistry methods. STZ-induced diabetes obviously impaired mRNA expression of Kv1.2 and Kv1.5 channel. The mRNA levels of Kv1.2 channel were 0.65 +/- 0.08 and 1.02 +/- 0.17 in STZ rats and control rats, respectively (n = 7, P < 0.05). Whereas the levels of Kv1.5 channel were 0.58 +/- 0.05 and 0.94 +/- 0.13 in STZ rats and control rats, respectively (n = 7, P < 0.05). Western blotting analysis showed that protein expression of Kv1.2 channel was decreased significantly but not Kv1.5 channel. Protein expressions of Kv1.2 channel were 0.49 +/- 0.04 and 0.70 +/- 0.06 in STZ rats and control rats, respectively (n = 5, P < 0.05), but those of Kv1.5 channel were 0.61 +/- 0.12 and 0.59 +/- 0.14 in STZ rats and control rats, respectively (n = 5, P > 0.05). Immunohistochemistry identification indicated that immunological reaction of Kv1.2 channel protein was attenuated, but Kv1.5 channel protein was not altered. Positive staining intensity normalized by gray values of Kv1.2 channel were 173 +/- 13 and 131 +/- 11 in STZ rats and control rats, respectively (n = 5, P < 0.05), but those of Kv1.5 channel were 139 +/- 16 and 141 +/- 12 in STZ rats and control rats, respectively (n = 5, P > 0.05). These results suggested that impairment of cAMP-mediated endothelium independent vasodilation of rat small coronary artery by STZ-induced diabetes was resulted from decrease of mRNA and protein expressions of Kv channels, and which eventually leads to a reduced current from Kv channels.

Ebselen reduces nitration and restores voltage-gated potassium channel function in small coronary arteries of diabetic rats

Small coronary arteries (SCA) from diabetic rats exhibit enhanced peroxynitrite (ONOO(-)) formation and concurrent impairment of voltage-dependent potassium (K(v)) channel function. However, it is unclear whether ONOO(-) plays a causative role in this impairment. We hypothesized that functional loss of K(v) channels in coronary smooth muscle cells (SMC) in diabetes is due to ONOO(-) with subsequent tyrosine nitration of K(v) channel proteins. Diabetic rats and nondiabetic controls were treated with or without ebselen (Eb) for 4 wk. SCA were prepared for immunohistochemistry (IHC), immunoprecipitation (IP) followed by Western blot (WB), videomicroscopy, and patch-clamp analysis. IHC revealed excess ONOO(-) in SCA from diabetic rats. IP and WB revealed elevated nitration of the K(v)1.2 alpha-subunit and reduced K(v)1.2 protein expression in diabetic rats. Each of these changes was improved in Eb-treated rats. Protein nitration and K(v)1.5 expression were unchanged in SCA from diabetic rats. Forskolin, a direct cAMP activator that induces K(v)1 channel activity, dilated SCA from nondiabetic rats in a correolide (Cor; a selective K(v)1 channel blocker)-sensitive fashion. Cor did not alter the reduced dilation to forskolin in diabetic rats; however, Eb partially restored the Cor-sensitive component of dilation. Basal K(v) current density and response to forskolin were improved in smooth muscle cells from Eb-treated DM rats. We conclude that enhanced nitrosative stress in diabetes mellitus contributes to K(v)1 channel dysfunction in the coronary microcirculation. Eb may be beneficial for the therapeutic treatment of vascular complications in diabetes mellitus.

Functional interactions at the interface between voltage-sensing and pore domains in the Shaker K(v) channel

Voltage-activated potassium (K(v)) channels contain a central pore domain that is partially surrounded by four voltage-sensing domains. Recent X-ray structures suggest that the two domains lack extensive protein-protein contacts within presumed transmembrane regions, but whether this is the case for functional channels embedded in lipid membranes remains to be tested. We investigated domain interactions in the Shaker K(v) channel by systematically mutating the pore domain and assessing tolerance by examining channel maturation, S4 gating charge movement, and channel opening. When mapped onto the X-ray structure of the K(v)1.2 channel the large number of permissive mutations support the notion of relatively independent domains, consistent with crystallographic studies. Inspection of the maps also identifies portions of the interface where residues are sensitive to mutation, an external cluster where mutations hinder voltage sensor activation, and an internal cluster where domain interactions between S4 and S5 helices from adjacent subunits appear crucial for the concerted opening transition.

Dissecting the coupling between the voltage sensor and pore domains

The gating mechanism of K(v) channels is not known. In this issue of Neuron, Soler-Llavina et al. present fascinating results that support the concept of relatively independent voltage-sensing modules. However, they also find that its interactions with the pore domain are rather complex, with specific S4-S5 intersubunit contacts underlying the concerted transition leading to the channel opening.

Structure-based secondary structure-independent approach to design protein ligands: Application to the design of Kv1.2 potassium channel blockers

We have developed a structure-based approach to the design of protein ligands. This approach is based on the transfer of a functional binding motif of amino acids, often referred as to the "hot spot", on a host protein able to reproduce the functional topology of these residues. The scaffolds were identified by a systematic in silico search in the Protein Data Bank for proteins possessing a group of residues in a topology similar to that adopted by the functional motif in a reference ligand of known 3D structure. In contrast to previously reported studies, this search is independent of the particular secondary structure supporting the functional motif. To take into account the global properties of the host protein, two additional criteria were taken into account in the selection process: (1) Only those scaffolds sterically compatible with the positioning of the functional motif as observed in a reference complex model were retained. (2) Host proteins displaying electrostatic potentials, in the region of the transferred functional motif, similar to that of the reference ligand were selected. This approach was applied to the development of protein ligands of the Kv1.2 channel using BgK, a small protein isolated from the sea anemone Bunodosoma granulifera, as the reference ligand. Four proteins obtained by this approach were produced for experimental evaluation. The X-ray structure of one of these proteins was determined to check for similarity of the transferred functional motif with the structure it adopts in the reference ligand. Three of these protein ligands bind the Kv1.2 channel with inhibition constants of 0.5, 1.5, and 1.6 microM. Several mutants of these designed protein ligands gave binding results consistent with the presumed binding mode. These results show that protein ligands can be designed by transferring a binding motif on a protein host selected to reproduce the functional topology of this motif, irrespective to the secondary structure supporting the functional motif, if the host protein possesses steric and electrostatic properties compatible with the binding to the target. This result opens the way to the design of protein ligands by taking advantage of the considerable structural repertoire of the Protein Data Bank.

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.

Structure prediction for the down state of a potassium channel voltage sensor

Voltage-gated potassium (Kv) channels, essential for regulating potassium uptake and cell volume in plants and electrical excitability in animals, switch between conducting and non-conducting states as a result of conformational changes in the four voltage-sensing domains (VSDs) that surround the channel pore. This process, known as gating, is initiated by a cluster of positively charged residues on the fourth transmembrane segment (S4) of each VSD, which drives the VSD into a 'down state' at negative voltages and an 'up state' at more positive voltages. The crystal structure of Kv1.2 probably corresponds to the up state, but the local environment of S4 in the down state and its motion in voltage gating remains unresolved. Here we employed several conditional lethal/second-site suppressor yeast screens to determine the transmembrane packing of the VSD in the down state. This screen relies on the ability of KAT1, a eukaryotic Kv channel, to conduct potassium when its VSDs are in the down state, thereby rescuing potassium-transport-deficient yeast. Starting with KAT1 channels bearing conditional lethal mutations, we identified second-site suppressor mutations throughout the VSD that recover yeast growth. We then constructed a down state model of the channel using six pairs of interacting residues as structural constraints and verified this model by engineering suppressor mutations on the basis of spatial considerations. A comparison of this down state model with the up state Kv1.2 structure suggests that the VSDs undergo large rearrangements during gating, whereas the S4 segment remains positioned between the central pore and the remainder of the VSD in both states.

Inhibition by bis(7)-tacrine of native delayed rectifier and KV1.2 encoded potassium channels

Bis(7)-tacrine [bis(7)-tetrahydroaminacrine] acts as an AChE inhibitor and also exerts modulatory effects on many ligand-gated ion channels and voltage-gated Ca(2+) and K(+) channels. It has been reported previously that tacrine and some other AChE inhibitors suppressed I(K(A)) in central and peripheral neurons. The present study aimed to explore whether bis(7)-tacrine could modulate the function of native delayed rectifier potassium channels in DRG neurons and K(V)1.2 encoded potassium channels expressed in oocytes. We found that both delayed rectifier potassium currents (I(K(DR))) in rat DRG neurons and the currents recorded from oocytes expressing K(V)1.2 (I(K(K(V)1.2))) were suppressed by bis(7)-tacrine, the potency of which was two orders greater than that of tacrine. The IC(50) values for bis(7)-tacrine and tacrine inhibition of I(K(KD)) in DRG neurons were 0.72+/-0.05 and 58.3+/-3.7 microM, respectively; while the two agents inhibited I(K(K(V)1.2)) in oocytes with an IC(50) of 0.24+/-0.06 and 102.1+/-21.5 microM, respectively. The possible mechanism for bis(7)-tacrine inhibition of I(K(A)) and I(K(K(V)1.2)) was identified as the suppression of their activation, inactivation.

Glycosylation and cell surface expression of Kv1.2 potassium channel are regulated by determinants in the pore region

Voltage-gated K(+) channels contain six membrane spanning segments and a pore-forming domain. We used site-directed mutation to examine the role of specific amino acids in the extracellular region of the pore in Kv1.2. When expressed in CHO cells, a K(+) current was not observed for mutants S356A, S360A, T383A and T384A. However, coexpression of the Kvbeta2 subunit and the S360A mutant resulted in a robust peak current. Immunocytochemistry for Kv1.2 showed staining throughout the cytoplasm in cells coexpressing the beta2 and S360A, whereas only the perinuclear region was stained in cells expressing the S360A mutant. Western blotting revealed that the major immunoreactive protein in wild-type- and mutant-expressing cells is 60-kDa, but 87-kDa bands were also detected in cells expressing wild-type Kv1.2 and cells coexpressing beta2and S360A. These results suggest that amino acids in the pore region help regulate ion permeability or cellular trafficking by affecting glycosylation of Kv1.2.

Voltage sensor conformations in the open and closed states in ROSETTA structural models of K(+) channels

Voltage-gated ion channels control generation and propagation of action potentials in excitable cells. Significant progress has been made in understanding structure and function of the voltage-gated ion channels, highlighted by the high-resolution open-state structure of the voltage-gated potassium channel, K(v)1.2. However, because the structure of the closed state is unknown, the gating mechanism remains controversial. We adapted the rosetta membrane method to model the structures of the K(v)1.2 and KvAP channels using homology, de novo, and domain assembly methods and selected the most plausible models using a limited number of experimental constraints. Our model of K(v)1.2 in the open state is very similar in overall topology to the x-ray structure of this channel. Modeling of KvAP in the open state suggests that orientation of the voltage-sensing domain relative to the pore-forming domain is considerably different from the orientation in the K(v)1.2 open state and that the magnitude of the vertical movement of S4 is significantly greater. Structural modeling of closed state of K(v)1.2 suggests gating movement that can be viewed as a sum of two previously suggested mechanisms: translation (2-4 A) plus rotation ( approximately 180 degrees ) of the S4 segment as proposed in the original "sliding helix" or "helical screw" models coupled with a rolling motion of the S1-S3 segments around S4, similar to recent "transporter" models of gating. We propose a unified mechanism of voltage-dependent gating for K(v)1.2 and KvAP in which this major conformational change moves the gating charge across the electric field in an analogous way for both channels.

Environment of the gating charges in the Kv1.2 Shaker potassium channel

Recently, the structure of the Shaker channel Kv1.2 has been determined at a 2.9-angstroms resolution. This opens new possibilities in deciphering the mechanism underlying the function of voltage-gated potassium (Kv) channels. Molecular dynamics simulations of the channel, embedded in a membrane environment show that the channel is in its open state and that the gating charges carried by S4 are exposed to the solvent. The hydrated environment of S4 favors a local collapse of the electrostatic potential, which generates high electric-field gradients around the arginine gating charges. Comparison to experiments suggests furthermore that activation of the channel requires mainly a lateral displacement of S4. Overall, the results agree with the transporter model devised for Kv channels from electrophysiology experiments, and provide a possible pathway for the mechanistic response to membrane depolarization.

Potassium channels: complete and undistorted

The recently determined structure of a mammalian voltage-gated potassium channel has important implications for our understanding of voltage-sensing and gating mechanisms in channels. It is also the first crystal structure of an overexpressed eukaryotic membrane protein.

Remyelination of dorsal column axons by endogenous Schwann cells restores the normal pattern of Nav1.6 and Kv1.2 at nodes of Ranvier

Demyelination of CNS axons occurs in a number of pathological conditions, including multiple sclerosis and contusion-type spinal cord injury. The demyelination can be repaired by remyelination in both humans and rodents, and even within the CNS remyelination can be achieved by endogenous and/or exogenous Schwann cells, the myelinating cells of the PNS. Remyelinated axons can often conduct impulses securely, but the organization of ion channels at long-term remyelinated nodes is not known. In the present study, the expression of voltage-gated sodium (Na(v)) and potassium (K(v)) channels along central axons remyelinated by endogenous Schwann cells has been studied in lesions induced more than 1 year previously by the intraspinal injection of ethidium bromide (EB). The expression of the channels at long-term nodes formed by Schwann cell remyelination has been compared with that present in nascent nodes formed in the adult at 18 and 23 days post-EB injection. Immunohistochemical studies revealed that long-term nodes formed by Schwann cell remyelination exhibit a clustering of Na(v)1.6 sodium channels within the nodal membrane, with the Shaker-type potassium channel K(v)1.2 segregated within the juxtaparanodal region, similar to the arrangement at normal mature CNS nodes. Na(v)1.2 was not detected at nodes formed by Schwann cells at any stage of their development. Moreover, Na(v)1.6, but not Na(v)1.2, was clustered at nascent nodes formed by remyelinating Schwann cells 18 and 23 days following EB injection. These observations show that endogenous Schwann cells can establish and maintain nodes of Ranvier on central axons for over one year, and that the nodes exhibit an apparently normal distribution of sodium and potassium channels, with Na(v)1.6 the predominant subtype of sodium channel present at such nodes at all stages of their development.

Functional Changes in Genetically Dysmyelinated Spinal Cord Axons ofShivererMice: Role of Juxtaparanodal Kv1 Family K+Channels

Axonal dysfunction after spinal cord injury (SCI) and other types of neurotrauma is associated with demyelination and exposure of juxtaparanodal K+channels. In this study, sucrose gap electrophysiology using selective and nonselective K+channel blockers, confocal immunohistochemistry, and Western blotting were used to study the role of Kv1.1 and Kv1.2 K+channel subunits in dysmyelination-induced spinal cord axonal dysfunction in s hiverer mice, which lack the gene encoding myelin basic protein (MBP) and exhibit incomplete myelin sheath formation on CNS axons. The s hiverer spinal cord axons exhibited smaller amplitude of compound action potentials (CAPs), reduced conduction velocity, reduced excitability, and greater degree of high-frequency conduction failure. The “fast” K+channel blocker 4-aminopyridine, the toxin DTX-I, which targets the Kv1.1 and Kv1.2, but not DTX- K, which has higher selectivity for Kv1.1, increased the amplitude and area of CAPs of shiverer mice spinal cord axons but had insignificant effects in wild-type mice. Confocal immunohistochemistry showed that, unlike wild-type mice, which have a precise juxtaparanodal localization of the Kv1.l and Kv1.2 K+channel subunits, shiverer mouse axons displayed a dispersed distribution of these subunits along the internodes. In contrast, the Kv1.l and Kv1.2 subunits, Na+channels remained highly localized to the nodal regions. Western blotting showed an increased expression of Kv 1.1 and 1.2 in the shiverer mouse spinal cord. These results provide evidence that the neurological deficits associated with myelin deficiency reflect the altered distribution and expression of the K+channel subunits Kv1.l and Kv1.2 along the internodes of spinal cord axons associated with the biophysical consequences caused by alterations in the myelin sheaths.

Voltage-gated ion channels in the axon initial segment of human cortical pyramidal cells and their relationship with chandelier cells

The axon initial segment (AIS) of pyramidal cells is a critical region for the generation of action potentials and for the control of pyramidal cell activity. Here we show that Na+ and K+ voltage-gated channels, together with other molecules involved in the localization of ion channels, are distributed asymmetrically in the AIS of pyramidal cells situated in the human temporal neocortex. There is a high density of Na+ channels distributed along the length of the AIS together with the associated proteins spectrin betaIV and ankyrin G. In contrast, Kv1.2 channels are associated with the adhesion molecule Caspr2, and they are mostly localized to the distal region of the AIS. In general, the distal region of the AIS is targeted by the GABAergic axon terminals of chandelier cells, whereas the proximal region is innervated, mostly by other types of GABAergic interneurons. We suggest that this molecular segregation and the consequent regional specialization of the GABAergic input to the AIS of pyramidal cells may have important functional implications for the control of pyramidal cell activity.

Polarized distribution of ion channels within microdomains of the axon initial segment

Voltage-gated sodium (Na(v)) channels accumulate at the axon initial segment (IS), where their high density supports spike initiation. Maintenance of this high density of Na(v) channels involves a macromolecular complex that includes the cytoskeletal linker protein ankyrin-G, the only protein known to bind Na(v) channels and localize them at the IS. We found previously that Na(v)1.6 is the predominant Na(v) channel isoform at IS of adult rodent retinal ganglion cells. However, here we report that Na(v)1.6 immunostaining is consistently reduced or absent in short regions of the IS proximal to the soma, although both ankyrin-G and pan-Na(v) antibodies stain this region. We show that this proximal IS subregion is a unique axonal microdomain, containing an accumulation of Na(v)1.1 channels that are spatially segregated from the Na(v)1.6 channels of the distal IS. Additionally, we find that axonal K(v)1.2 potassium channels are present within the distal IS, but are also excluded from the Na(v)1.1-enriched proximal IS microdomain. Because ankyrin-G was prominent in both proximal and distal subcompartments of the IS, where it colocalized with either Na(v)1.1 or Na(v)1.6, respectively, mechanisms other than association with ankyrin-G must mediate differential targeting of Na(v) channel subtypes to achieve the spatial precision observed within the IS. This precise arrangement of ion channels within the axon initial segment is likely an important determinant of the firing properties of ganglion cells and other mammalian neurons.

Expression and biophysical properties of Kv1 channels in supragranular neocortical pyramidal neurones

Potassium channels are extremely diverse regulators of neuronal excitability. As part of an investigation into how this molecular diversity is utilized by neurones, we examined the expression and biophysical properties of native Kv1 channels in layer II/III pyramidal neurones from somatosensory and motor cortex. Single-cell RT-PCR, immunocytochemistry, and whole cell recordings with specific peptide toxins revealed that individual pyramidal cells express multiple Kv1 alpha-subunits. The most abundant subunit mRNAs were Kv1.1 > 1.2 > 1.4 > 1.3. All of these subunits were localized to somatodendritic as well as axonal cell compartments. These data suggest variability in the subunit complexion of Kv1 channels in these cells. The alpha-dendrotoxin (alpha-DTX)-sensitive current activated more rapidly and at more negative potentials than the alpha-DTX-insensitive current, was first observed at voltages near action potential threshold, and was relatively insensitive to holding potential. The alpha-DTX-sensitive current comprised about 10% of outward current at steady-state, in response to steps from -70 mV. From -50 mV, this percentage increased to approximately 20%. All cells expressed an alpha-DTX-sensitive current with slow inactivation kinetics. In some cells a transient component was also present. Deactivation kinetics were voltage dependent, such that deactivation was slow at potentials traversed by interspike intervals during repetitive firing. Because of its kinetics and voltage dependence, the alpha-DTX-sensitive current should be most important at physiological resting potentials and in response to brief stimuli. Kv1 channels should also be important at voltages near threshold and corresponding to interspike intervals.

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.

Voltage gating and anions, especially phosphate: a model system

The voltage sensor of voltage gated sodium and potassium channels consists of four sets of transmembrane segments, of which one, called S4, contains at least four arginines; these are presumed to each carry positive charges. The channel opening is usually attributed to the outward (i.e., toward the extracellular side of the membrane) motion of S4. The evidence for this motion is based on certain experiments that appear to show differential access to parts of S4 from the intracellular and extracellular sides of the membrane in the open and closed states. A newly available structure [S.B. Long, E.B. Campbell and R. MacKinnon, Crystal structure of a mammalian voltage-dependent Shaker family K(+) channel. Science 309 (2005) 897-903; S.B. Long, E.B. Campbell, R. MacKinnon, Voltage sensor of Kv1.2: structural basis of electromechanical coupling. Science 309 (2005) 903-908][1,2] has now been used to argue for a large scale motion, although, as a static structure, it is not conclusive. In this paper, we consider the effect of anions in the surrounding medium. Phosphate is present in the intracellular as well as the extracellular fluid, apparently at hundreds of micromolar concentration, or more. There is evidence in the literature suggesting that phosphate-arginine complexes are rather strong. In a recent calculation one of us [M.E. Green, A possible role for phosphate in complexing the arginines of S4 in voltage gated channels J. Theor. Biol. 233 (2005) 337-341][3] has shown that a model peptide with a 2:1 arg:phosphate complex should have a favorable geometry. Here, we present NMR evidence of the existence of phosphate complexes of a model peptide with two arginines separated by two hydrophobic residues, the same spacing as in S4 segments. The complexes (there are different complexes for HPO(4)(2-) and for H(2)PO(4)(-) [3]) form with concentrations of peptide in the range of hundreds of micromolar, making it significant in the biological context. NMR spectra provide changes in chemical shift as functions of both phosphate concentration and pH. The resulting curves show titration of the phosphate, with its standard pK. Possible implications for other anion-S4 interactions, including ion pairs rather than complexes, as with Cl(-), are also discussed.

Developmental regulation and adult maintenance of potassium channel proteins (Kv1.1 and Kv1.2) in the cochlear nucleus of the rat

The development and maintenance of the adult expression and distribution of Kv 1.1 and Kv 1.2, two voltage-dependent potassium channel subunits, were investigated in the anteroventral cochlear nucleus (AVCN) of the rat. Both Kv 1.1 and Kv 1.2 were found in AVCN neuronal cell bodies at birth, as detected by in situ hybridization and immunocytochemistry. However, Kv 1.1 and Kv 1.2 were not seen in axons until the end of the third postnatal week. From postnatal day 21 through adulthood, labeling for both potassium channels was in axonal processes, whereas the number of cell bodies labeled for Kv 1.1 decreased and there were no cell bodies labeled for Kv 1.2. Therefore, these two potassium channel proteins are targeted to their final subcellular destinations in axons well after hearing onset. Once the adult distribution pattern of Kv 1.1 and Kv 1.2 is attained, its maintenance does not depend on signals from auditory nerve synapses. Eliminating auditory nerve input to the cochlear nucleus by means of bilateral cochleotomy did not change Kv 1.1 or Kv 1.2 expression or distribution, as seen by in situ hybridization, immunocytochemistry and Western blot. Thus, normal excitatory synaptic input in adult animals is not a requirement to regulate the expression and cellular and subcellular distribution of these potassium channel proteins.

Role of SHP-1, Kv.1.2, and cGMP in nitric oxide-induced ERK1/2 MAP kinase dephosphorylation in rat vascular smooth muscle cells

OBJECTIVE

Nitric oxide (NO) elicits relaxation in vascular smooth muscle cells (VSMC) that is associated with guanylate cyclase (GC) and K(+) channel activation. In this study we determined the mechanisms that lead to ERK1/2 MAP kinase dephosphorylation in response to NO.

METHODS

VSMC were treated with the NO donor SNAP or sodium nitroprusside (SNP), and ERK1/2, Src homology (SH) 1 domain-containing protein tyrosine phosphatase (SHP-1), and Kv.1.2 phosphorylation were assessed by immunoprecipitation and Western blot analysis.

RESULTS

NO decreased basal ERK1/2 phosphorylation in a dose- and time-dependent manner. NO-induced ERK1/2 dephosphorylation was detected at 1 min and sustained for 30 min. Pre-treatment with the GC inhibitor ODQ or the protein tyrosine phosphatase inhibitor I prevented ERK1/2 dephosphorylation induced by SNAP. The inhibition of protein phosphatase 1A/2A had no effect on ERK1/2 dephosphorylation induced by SNAP. Treatment with cromakalim A, a nonspecific K(+) channel activator, also induced ERK1/2 dephosphorylation, while blockade of Kv.1.2 K(+) channels (AM92016 hydrochloride) prevented NO-induced ERK1/2 dephosphorylation. In addition, SNAP induced SHP-1 phosphorylation, and the Kv.1.2 dephosphorylation increase and SHP-1 phosphorylation was blocked by ODQ or AM92016. The basal interaction between ERK1/2 and SHP-1 was decreased in response to SNAP stimulation. SHP-1 also interacted with Kv.1.2 under basal conditions and participates in Kv.1.2 activation. Using the mouse mesenteric resistance artery, we found that ERK1/2 MAP kinase is involved in regulation of myogenic tone.

CONCLUSION

Thus, our study provides the first evidence that NO controls basal ERK1/2 phosphorylation by a signaling cascade that involves a dynamic signaling complex between cGMP, Kv.1.2 and SHP-1.