Segmental exchanges define 4-aminopyridine binding and the inner mouth of K+ pores

Electrophysiological characterization of human KCNT1 channel modulators and the therapeutic potential of hydroquinine and tipepidine in KCNT1 mutation-associated epilepsy mouse model

Patients suffering epilepsy caused by the gain-of-function mutants of the hKCNT1 potassium channels are drug refractory. In this study, we cloned a novel human KCNT1B channel isoform using the brain cDNA library and conducted patch-clamp and molecular docking analyses to characterize the pharmacological properties of the hKCNT1B channel using thirteen drugs. Among cinchona alkaloids, we found that hydroquinine exerted the strongest blocking effect on the hKCNT1B channel, especially the F313L mutant. In addition, we confirmed the antitussive drug tipepidine was also a potent inhibitor of the hKCNT1B channel. Subsequently, we proved that these two drugs produced an excellent therapeutic effect on the epileptic model of KCNT1 Y777H mutant male mice; thus, both could be ready-to-use anti-epileptic drugs. On the other hand, we demonstrated that the activation of the KCNT1 channel by loxapine and clozapine was through interacting with pore domain residues to reverse the run-down of the KCNT1 channel. Taken together, our results provide new insights into the mechanism of the modulators in regulating the KCNT1 channel activity as well as important candidates for clinical tests in the treatment of KCNT1 mutant-associated epilepsy.

Structural Basis for Voltage Gating and Dalfampridine Binding in the Shaker Potassium Channel

The generation and propagation of action potentials in neurons depend on the coordinated activation of voltage-dependent sodium and potassium channels. Potassium channels of the Shaker family regulate neuronal excitability through voltage-dependent opening and closing of their ion conduction pore. This family of channels is an important therapeutic target, particularly in multiple sclerosis where the inhibitor dalfampridine (4-aminopyridine) is used to improve nerve conduction. The molecular details of how the voltage sensor domain drives opening of the pore domain has been limited by the lack of closed-state structures, also impairing the search for novel drugs. Using AlphaFold2-based conformational sampling methods we identify a structural model for the closed Shaker potassium channel where movement of the voltage sensor drives the opening trough interactions between the S4-S5 linker and S6 helix. We show experimentally that breakage of a backbone hydrogen bond is a critical part of the activation pathway. Through docking we identify a hydrophobic cavity formed by the pore domain helices that binds dalfampridine in the closed state. Our results demonstrate how voltage sensor movement drives pore opening and provide a structural framework for developing new therapeutic agents targeting the closed state. We anticipate this work will enable structure-based drug design efforts focused on state-dependent modulation of voltage-gated ion channels for the treatment of neurological disorders.

Chemical and biophysical characterization of novel potassium channel blocker 3-fluoro-5-methylpyridin-4-amine

4-aminopyridine (4AP) is a potassium (K+) channel blocker used clinically to improve walking in people with multiple sclerosis (MS). 4AP binds to exposed K+ channels in demyelinated axons, reducing the leakage of intracellular K+ and enhancing impulse conduction. Multiple derivatives of 4AP capable of blocking K+ channels have been reported including three radiolabeled with positron emitting isotopes for imaging demyelinated lesions using positron emission tomography (PET). However, there remains a demand for novel molecules with suitable physicochemical properties and binding affinity that can potentially be radiolabeled and used as PET radiotracers. In this study, we introduce 3-fluoro-5-methylpyridin-4-amine (5Me3F4AP) as a novel trisubstituted K+ channel blocker with potential application in PET. 5Me3F4AP has comparable potency to 4AP and the PET tracer 3-fluoro-4-aminopyridine (3F4AP). Compared to 3F4AP, 5Me3F4AP exhibits comparable basicity (pKa = 7.46 ± 0.01 vs. 7.37 ± 0.07, P-value = 0.08), greater lipophilicity (logD = 0.664 ± 0.005 vs. 0.414 ± 0.002, P-value < 0.0001) and higher permeability to an artificial brain membrane (Pe = 88.1 ± 18.3 vs. 31.1 ± 2.9 nm/s, P-value = 0.03). 5Me3F4AP is also more stable towards oxidation in vitro by the cytochrome P450 enzyme CYP2E1 (IC50 = 36.2 ± 2.5 vs. 15.4 ± 5.1, P-value = 0.0003); the enzyme responsible for the metabolism of 4AP and 3F4AP. Taken together, 5Me3F4AP has promising properties as a candidate for PET imaging warranting additional investigation.

Inactivation of the Kv2.1 channel through electromechanical coupling

The Kv2.1 voltage-activated potassium (Kv) channel is a prominent delayed-rectifier Kv channel in the mammalian central nervous system, where its mechanisms of activation and inactivation are critical for regulating intrinsic neuronal excitability1,2. Here we present structures of the Kv2.1 channel in a lipid environment using cryo-electron microscopy to provide a framework for exploring its functional mechanisms and how mutations causing epileptic encephalopathies3-7 alter channel activity. By studying a series of disease-causing mutations, we identified one that illuminates a hydrophobic coupling nexus near the internal end of the pore that is critical for inactivation. Both functional and structural studies reveal that inactivation in Kv2.1 results from dynamic alterations in electromechanical coupling to reposition pore-lining S6 helices and close the internal pore. Consideration of these findings along with available structures for other Kv channels, as well as voltage-activated sodium and calcium channels, suggests that related mechanisms of inactivation are conserved in voltage-activated cation channels and likely to be engaged by widely used therapeutics to achieve state-dependent regulation of channel activity.

Chemical and biophysical characterization of novel potassium channel blocker 3-fluoro-5-methylpyridin-4-amine

4-aminopyridine (4AP) is a potassium (K+) channel blocker used clinically to improve walking in people with multiple sclerosis (MS). 4AP binds to exposed K+ channels in demyelinated axons, reducing the leakage of intracellular K+ and enhancing impulse conduction. Multiple derivatives of 4AP capable of blocking K+ channels have been reported including three radiolabeled with positron emitting isotopes for imaging demyelinated lesions using positron emission tomography (PET). Here, we describe 3-fluoro-5-methylpyridin-4-amine (5Me3F4AP), a novel K+ channel blocker with potential application in PET. 5Me3F4AP has comparable potency to 4AP and the PET tracer 3-fluoro-4-aminopyridine (3F4AP). Compared to 3F4AP, 5Me3F4AP is more lipophilic (logD = 0.664 ± 0.005 vs. 0.414 ± 0.002) and slightly more basic (pKa = 7.46 ± 0.01 vs. 7.37 ± 0.07). In addition, 5Me3F4AP appears to be more permeable to an artificial brain membrane and more stable towards oxidation by the cytochrome P450 enzyme family 2 subfamily E member 1 (CYP2E1), responsible for the metabolism of 4AP and 3F4AP. Taken together, 5Me3F4AP has promising properties for PET imaging warranting additional investigation.

Combining mKate2-Kv1.3 Channel and Atto488-Hongotoxin for the Studies of Peptide Pore Blockers on Living Eukaryotic Cells

The voltage-gated potassium Kv1.3 channel is an essential component of vital cellular processes which is also involved in the pathogenesis of some autoimmune, neuroinflammatory and oncological diseases. Pore blockers of the Kv1.3 channel are considered as potential drugs and are used to study Kv1 channels' structure and functions. Screening and study of the blockers require the assessment of their ability to bind the channel. Expanding the variety of methods used for this, we report on the development of the fluorescent competitive binding assay for measuring affinities of pore blockers to Kv1.3 at the membrane of mammalian cells. The assay constituents are hongotoxin 1 conjugated with Atto488, fluorescent mKate2-tagged Kv1.3 channel, which was designed to improve membrane expression of the channel in mammalian cells, confocal microscopy, and a special protocol of image processing. The assay is implemented in the "mix and measure", format and allows the screening of Kv1.3 blockers, such as peptide toxins, that bind to the extracellular vestibule of the K⁺-conducting pore, and analyzing their affinity.

Radiochemical Synthesis and Evaluation of 3-[11C]Methyl-4-aminopyridine in Rodents and Nonhuman Primates for Imaging Potassium Channels in the CNS

Demyelination, the loss of the insulating sheath of neurons, causes failed or slowed neuronal conduction and contributes to the neurological symptoms in multiple sclerosis, traumatic brain and spinal cord injuries, stroke, and dementia. In demyelinated neurons, the axonal potassium channels K_v1.1 and K_v1.2, generally under the myelin sheath, become exposed and upregulated. Therefore, imaging these channels using positron emission tomography can provide valuable information for disease diagnosis and monitoring. Here, we describe a novel tracer for K_v1 channels, [¹¹C]3-methyl-4-aminopyridine ([¹¹C]3Me4AP). [¹¹C]3Me4AP was efficiently synthesized via Pd(0)-Cu(I) comediated Stille cross-coupling of a stannyl precursor containing a free amino group. Evaluation of its imaging properties in rats and nonhuman primates showed that [¹¹C]3Me4AP has a moderate brain permeability and slow kinetics. Additional evaluation in monkeys showed that the tracer is metabolically stable and that a one-tissue compartment model can accurately model the regional brain time-activity curves. Compared to the related tracers [¹⁸F]3-fluoro-4-aminopyridine ([¹⁸F]3F4AP) and [¹¹C]3-methoxy-4-aminopyridine ([¹¹C]3MeO4AP), [¹¹C]3Me4AP shows lower initial brain uptake, which indicates reduced permeability to the blood-brain barrier and slower kinetics, suggesting higher binding affinity consistent with in vitro studies. While the slow kinetics and strong binding affinity resulted in a tracer with less favorable properties for imaging the brain than its predecessors, these properties may make 3Me4AP useful as a therapeutic.

The nonconducting W434F mutant adopts upon membrane depolarization an inactivated-like state that differs from wild-type Shaker-IR potassium channels

Voltage-gated K⁺ (Kv) channels mediate the flow of K⁺ across the cell membrane by regulating the conductive state of their activation gate (AG). Several Kv channels display slow C-type inactivation, a process whereby their selectivity filter (SF) becomes less or nonconductive. It has been proposed that, in the fast inactivation-removed Shaker-IR channel, the W434F mutation epitomizes the C-type inactivated state because it functionally accelerates this process. By introducing another pore mutation that prevents AG closure, P475D, we found a way to record ionic currents of the Shaker-IR-W434F-P475D mutant at hyperpolarized membrane potentials as the W434F-mutant SF recovers from its inactivated state. This W434F conductive state lost its high K⁺ over Na⁺ selectivity, and even NMDG⁺ can permeate, features not observed in a wild-type SF. This indicates that, at least during recovery from inactivation, the W434F-mutant SF transitions to a widened and noncationic specific conformation.

Molecular basis for functional connectivity between the voltage sensor and the selectivity filter gate in Shaker K+ channels

In Shaker K⁺ channels, the S4-S5 linker couples the voltage sensor (VSD) and pore domain (PD). Another coupling mechanism is revealed using two W434F-containing channels: L361R:W434F and L366H:W434F. In L361R:W434F, W434F affects the L361R VSD seen as a shallower charge-voltage (Q-V) curve that crosses the conductance-voltage (G-V) curve. In L366H:W434F, L366H relieves the W434F effect converting a non-conductive channel in a conductive one. We report a chain of residues connecting the VSD (S4) to the selectivity filter (SF) in the PD of an adjacent subunit as the molecular basis for voltage sensor selectivity filter gate (VS-SF) coupling. Single alanine substitutions in this region (L409A, S411A, S412A, or F433A) are enough to disrupt the VS-SF coupling, shown by the absence of Q-V and G-V crossing in L361R:W434F mutant and by the lack of ionic conduction in the L366H:W434F mutant. This residue chain defines a new coupling between the VSD and the PD in voltage-gated channels.

Na+ and K+ channels: history and structure

In this perspective, we discuss the physiological roles of Na and K channels, emphasizing the importance of the K channel for cellular homeostasis in animal cells and of Na and K channels for cellular signaling. We consider the structural basis of Na and K channel gating in light of recent structural and electrophysiological findings.

Structure-activity relationship studies of four novel 4-aminopyridine K+ channel blockers

4-Aminopyridine (4AP) is a specific blocker of voltage-gated potassium channels (KV1 family) clinically approved for the symptomatic treatment of patients with multiple sclerosis (MS). It has recently been shown that [18F]3F4AP, a radiofluorinated analog of 4AP, also binds to KV1 channels and can be used as a PET tracer for the detection of demyelinated lesions in rodent models of MS. Here, we investigate four novel 4AP derivatives containing methyl (-CH3), methoxy (-OCH3) as well as trifluoromethyl (-CF3) in the 2 and 3 position as potential candidates for PET imaging and/or therapy. We characterized the physicochemical properties of these compounds (basicity and lipophilicity) and analyzed their ability to block Shaker K+ channel under different voltage and pH conditions. Our results demonstrate that three of the four derivatives are able to block voltage-gated potassium channels. Specifically, 3-methyl-4-aminopyridine (3Me4AP) was found to be approximately 7-fold more potent than 4AP and 3F4AP; 3-methoxy- and 3-trifluoromethyl-4-aminopyridine (3MeO4AP and 3CF34AP) were found to be about 3- to 4-fold less potent than 4AP; and 2-trifluoromethyl-4-AP (2CF34AP) was found to be about 60-fold less active. These results suggest that these novel derivatives are potential candidates for therapy and imaging.

KV channels and the regulation of vascular smooth muscle tone

VSMCs in resistance arteries and arterioles express a diverse array of K_V channels with members of the K_V 1, K_V 2 and K_V 7 families being particularly important. Members of the K_V channel family: (i) are highly expressed in VSMCs; (ii) are active at the resting membrane potential of VSMCs in vivo (-45 to -30 mV); (iii) contribute to the negative feedback regulation of VSMC membrane potential and myogenic tone; (iv) are activated by cAMP-related vasodilators, hydrogen sulfide and hydrogen peroxide; (v) are inhibited by increases in intracellular Ca²⁺ and vasoconstrictors that signal through G_q -coupled receptors; (vi) are involved in the proliferative phenotype of VSMCs; and (vii) are modulated by diseases such as hypertension, obesity, the metabolic syndrome and diabetes. Thus, K_V channels participate in every aspect of the regulation of VSMC function in both health and disease.

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.

Nonspecific block of voltage-gated potassium channels has greater effect on distal schaffer collaterals than proximal schaffer collaterals during periods of high activity

Previous studies established different responses between proximal and distal portions of Schaffer collateral axons during high‐frequency and burst stimulation, with distal axons demonstrating biphasic changes in excitability (hyperexcitability followed by depression), but proximal axons showing only monophasic depression. Voltage‐dependent potassium (K_V) channels are important determinants of axonal excitability, and block of K_V channels can promote axon hyperexcitability. We therefore hypothesized that block of K_V channels should lead to biphasic response changes in proximal Schaffer collaterals, like those seen in distal Schaffer collaterals. To test this hypothesis, we made extracellular recordings of distal Schaffer collateral responses in stratum radiatum of hippocampal area CA1 and proximal Schaffer collateral responses in stratum pyramidale of area CA3 during high‐frequency stimulation (HFS) at 100 Hz and burst stimulation at 200 msec intervals (5 Hz or theta frequency). We then applied a nonselective K_V channel blocker, tetraethlylammonium (TEA, 10 mmol/L) or 4‐aminopyridine (4‐AP, 100 μmol/L), and assessed effects on Schaffer collateral responses. Surprisingly, block of K_V channels had little or no effect on proximal Schaffer collateral responses during high‐frequency or burst stimulation. In contrast, K_V channel blockade caused more rapid depression of distal Schaffer collateral responses during both high‐frequency and burst stimulation. These findings indicate that K_V channels are important for maintaining distal, but not proximal, Schaffer collateral excitability during period of sustained high activity. Differential sensitivity of distal versus proximal Schaffer collaterals to K_V channel block may reflect differences in channel density, diversity, or subcellular localization.

Smooth Muscle Ion Channels and Regulation of Vascular Tone in Resistance Arteries and Arterioles

Vascular tone of resistance arteries and arterioles determines peripheral vascular resistance, contributing to the regulation of blood pressure and blood flow to, and within the body's tissues and organs. Ion channels in the plasma membrane and endoplasmic reticulum of vascular smooth muscle cells (SMCs) in these blood vessels importantly contribute to the regulation of intracellular Ca2+ concentration, the primary determinant of SMC contractile activity and vascular tone. Ion channels provide the main source of activator Ca2+ that determines vascular tone, and strongly contribute to setting and regulating membrane potential, which, in turn, regulates the open-state-probability of voltage gated Ca2+ channels (VGCCs), the primary source of Ca2+ in resistance artery and arteriolar SMCs. Ion channel function is also modulated by vasoconstrictors and vasodilators, contributing to all aspects of the regulation of vascular tone. This review will focus on the physiology of VGCCs, voltage-gated K+ (KV) channels, large-conductance Ca2+-activated K+ (BKCa) channels, strong-inward-rectifier K+ (KIR) channels, ATP-sensitive K+ (KATP) channels, ryanodine receptors (RyRs), inositol 1,4,5-trisphosphate receptors (IP3Rs), and a variety of transient receptor potential (TRP) channels that contribute to pressure-induced myogenic tone in resistance arteries and arterioles, the modulation of the function of these ion channels by vasoconstrictors and vasodilators, their role in the functional regulation of tissue blood flow and their dysfunction in diseases such as hypertension, obesity, and diabetes. © 2017 American Physiological Society. Compr Physiol 7:485-581, 2017.

Fluorescent protein-scorpion toxin chimera is a convenient molecular tool for studies of potassium channels

Ion channels play a central role in a host of physiological and pathological processes and are the second largest target for existing drugs. There is an increasing need for reliable tools to detect and visualize particular ion channels, but existing solutions suffer from a number of limitations such as high price, poor specificity, and complicated protocols. As an alternative, we produced recombinant chimeric constructs (FP-Tx) consisting of fluorescent proteins (FP) fused with potassium channel toxins from scorpion venom (Tx). In particular, we used two FP, eGFP and TagRFP, and two Tx, OSK1 and AgTx2, to create eGFP-OSK1 and RFP-AgTx2. We show that these chimeras largely retain the high affinity of natural toxins and display selectivity to particular ion channel subtypes. FP-Tx are displaced by other potassium channel blockers and can be used as an imaging tool in ion channel ligand screening setups. We believe FP-Tx chimeras represent a new efficient molecular tool for neurobiology.

Complexes of Peptide Blockers with Kv1.6 Pore Domain: Molecular Modeling and Studies with KcsA-Kv1.6 Channel

Potassium voltage-gated Kv1.6 channel, which is distributed primarily in neurons of central and peripheral nervous systems, is of significant physiological importance. To date, several high-affinity Kv1.6-channel blockers are known, but the lack of selective ones among them hampers the studies of tissue localization and functioning of Kv1.6 channels. Here we present an approach to advanced understanding of interactions of peptide toxin blockers with a Kv1.6 pore. It combines molecular modeling studies and an application of a new bioengineering system based on a KcsA-Kv1.6 hybrid channel for the quantitative fluorescent analysis of blocker-channel interactions. Using this system we demonstrate that peptide toxins agitoxin 2, kaliotoxin1 and OSK1 have similar high affinity to the extracellular vestibule of the K⁺-conducting pore of Kv1.6, hetlaxin is a low-affinity ligand, whereas margatoxin and scyllatoxin do not bind to Kv1.6 pore. Binding of toxins to Kv1.6 pore has considerable inverse dependence on the ionic strength. Model structures of KcsA-Kv1.6 and Kv1.6 complexes with agitoxin 2, kaliotoxin 1 and OSK1 were obtained using homology modeling and molecular dynamics simulation. Interaction interfaces, which are formed by 15-19 toxin residues and 10 channel residues, are described and compared. Specific sites of Kv1.6 pore recognition are identified for targeting of peptide blockers. Analysis of interactions between agitoxin 2 derivatives with point mutations (S7K, S11G, L19S, R31G) and KcsA-Kv1.6 confirms reliability of the calculated complex structure.

Kv3.1 uses a timely resurgent K(+) current to secure action potential repolarization

High-frequency action potential (AP) transmission is essential for rapid information processing in the central nervous system. Voltage-dependent K_v3 channels play an important role in this process thanks to their high activation threshold and fast closure kinetics, which reduce the neuron's refractory period. However, premature K_v3 channel closure leads to incomplete membrane repolarization, preventing sustainable AP propagation. Here, we demonstrate that K_v3.1b channels solve this problem by producing resurgent K⁺ currents during repolarization, thus ensuring enough repolarizing power to terminate each AP. Unlike previously described resurgent Na⁺ and K⁺ currents, K_v3.1b's resurgent current does not originate from recovery of channel block or inactivation but results from a unique combination of steep voltage-dependent gating kinetics and ultra-fast voltage-sensor relaxation. These distinct properties are readily transferrable onto an orthologue K_v channel by transplanting the voltage-sensor's S3–S4 loop, providing molecular insights into the mechanism by which K_v3 channels contribute to high-frequency AP transmission.

K_v3 potassium channels have an important role in the repolarization of action potentials in fast-spiking neurons. Here, the authors use electrophysiology and modelling to report on an interesting mechanism that might explain their gating behaviour.

Modulation of Closed-State Inactivation in Kv2.1/Kv6.4 Heterotetramers as Mechanism for 4-AP Induced Potentiation

The voltage-gated K+ (Kv) channel subunits Kv2.1 and Kv2.2 are expressed in almost every tissue. The diversity of Kv2 current is increased by interacting with the electrically silent Kv (KvS) subunits Kv5-Kv6 and Kv8-Kv9, into functional heterotetrameric Kv2/KvS channels. These Kv2/KvS channels possess unique biophysical properties and display a more tissue-specific expression pattern, making them more desirable pharmacological and therapeutic targets. However, little is known about the pharmacological properties of these heterotetrameric complexes. We demonstrate that Kv5.1, Kv8.1 and Kv9.3 currents were inhibited differently by the channel blocker 4-aminopyridine (4-AP) compared to Kv2.1 homotetramers. In contrast, Kv6.4 currents were potentiated by 4-AP while displaying moderately increased affinities for the channel pore blockers quinidine and flecainide. We found that the 4-AP induced potentiation of Kv6.4 currents was caused by modulation of the Kv6.4-mediated closed-state inactivation: suppression by 4-AP of the Kv2.1/Kv6.4 closed-state inactivation recovered a population of Kv2.1/Kv6.4 channels that was inactivated at resting conditions, i.e. at a holding potential of -80 mV. This modulation also resulted in a slower initiation and faster recovery from closed-state inactivation. Using chimeric substitutions between Kv6.4 and Kv9.3 subunits, we demonstrated that the lower half of the S6 domain (S6c) plays a crucial role in the 4-AP induced potentiation. These results demonstrate that KvS subunits modify the pharmacological response of Kv2 subunits when assembled in heterotetramers and illustrate the potential of KvS subunits to provide unique pharmacological properties to the heterotetramers, as is the case for 4-AP on Kv2.1/Kv6.4 channels.

Variability of Potassium Channel Blockers in Mesobuthus eupeus Scorpion Venom with Focus on Kv1.1: AN INTEGRATED TRANSCRIPTOMIC AND PROTEOMIC STUDY

The lesser Asian scorpion Mesobuthus eupeus (Buthidae) is one of the most widely spread and dispersed species of the Mesobuthus genus, and its venom is actively studied. Nevertheless, a considerable amount of active compounds is still under-investigated due to the high complexity of this venom. Here, we report a comprehensive analysis of putative potassium channel toxins (KTxs) from the cDNA library of M. eupeus venom glands, and we compare the deduced KTx structures with peptides purified from the venom. For the transcriptome analysis, we used conventional tools as well as a search for structural motifs characteristic of scorpion venom components in the form of regular expressions. We found 59 candidate KTxs distributed in 30 subfamilies and presenting the cysteine-stabilized α/β and inhibitor cystine knot types of fold. M. eupeus venom was then separated to individual components by multistage chromatography. A facile fluorescent system based on the expression of the KcsA-Kv1.1 hybrid channels in Escherichia coli and utilization of a labeled scorpion toxin was elaborated and applied to follow Kv1.1 pore binding activity during venom separation. As a result, eight high affinity Kv1.1 channel blockers were identified, including five novel peptides, which extend the panel of potential pharmacologically important Kv1 ligands. Activity of the new peptides against rat Kv1.1 channel was confirmed (IC50 in the range of 1-780 nm) by the two-electrode voltage clamp technique using a standard Xenopus oocyte system. Our integrated approach is of general utility and efficiency to mine natural venoms for KTxs.

Mutations in the S6 gate isolate a late step in the activation pathway and reduce 4-AP sensitivity in shaker K(v) channel

Kv channels detect changes in the membrane potential via their voltage-sensing domains (VSDs) that control the status of the S6 bundle crossing (BC) gate. The movement of the VSDs results in a transfer of the S4 gating charges across the cell membrane but only the last 10-20% of the total gating charge movement is associated with BC gate opening, which involves cooperative transition(s) in the subunits. Substituting the proline residue P475 in the S6 of the Shaker channel by a glycine or alanine causes a considerable shift in the voltage-dependence of the cooperative transition(s) of BC gate opening, effectively isolating the late gating charge component from the other gating charge that originates from earlier VSD movements. Interestingly, both mutations also abolished Shaker's sensitivity to 4-aminopyridine, which is a pharmacological tool to isolate the late gating charge component. The alanine substitution (that would promote a α-helical configuration compared to proline) resulted in the largest separation of both gating charge components; therefore, BC gate flexibility appears to be important for enabling the late cooperative step of channel opening.

Comparative normal/failing rat myocardium cell membrane chromatographic analysis system for screening specific components that counteract doxorubicin-induced heart failure from Acontium carmichaeli

Cell membrane chromatography (CMC) derived from pathological tissues is ideal for screening specific components acting on specific diseases from complex medicines owing to the maximum simulation of in vivo drug-receptor interactions. However, there are no pathological tissue-derived CMC models that have ever been developed, as well as no visualized affinity comparison of potential active components between normal and pathological CMC columns. In this study, a novel comparative normal/failing rat myocardium CMC analysis system based on online column selection and comprehensive two-dimensional (2D) chromatography/monolithic column/time-of-flight mass spectrometry was developed for parallel comparison of the chromatographic behaviors on both normal and pathological CMC columns, as well as rapid screening of the specific therapeutic agents that counteract doxorubicin (DOX)-induced heart failure from Acontium carmichaeli (Fuzi). In total, 16 potential active alkaloid components with similar structures in Fuzi were retained on both normal and failing myocardium CMC models. Most of them had obvious decreases of affinities on failing myocardium CMC compared with normal CMC model except for four components, talatizamine (TALA), 14-acetyl-TALA, hetisine, and 14-benzoylneoline. One compound TALA with the highest affinity was isolated for further in vitro pharmacodynamic validation and target identification to validate the screen results. Voltage-dependent K⁺ channel was confirmed as a binding target of TALA and 14-acetyl-TALA with high affinities. The online high throughput comparative CMC analysis method is suitable for screening specific active components from herbal medicines by increasing the specificity of screened results and can also be applied to other biological chromatography models.

Ion channels as drug targets in central nervous system disorders

Ion channel targeted drugs have always been related with either the central nervous system (CNS), the peripheral nervous system, or the cardiovascular system. Within the CNS, basic indications of drugs are: sleep disorders, anxiety, epilepsy, pain, etc. However, traditional channel blockers have multiple adverse events, mainly due to low specificity of mechanism of action. Lately, novel ion channel subtypes have been discovered, which gives premises to drug discovery process led towards specific channel subtypes. An example is Na(+) channels, whose subtypes 1.3 and 1.7-1.9 are responsible for pain, and 1.1 and 1.2 - for epilepsy. Moreover, new drug candidates have been recognized. This review is focusing on ion channels subtypes, which play a significant role in current drug discovery and development process. The knowledge on channel subtypes has developed rapidly, giving new nomenclatures of ion channels. For example, Ca(2+)s channels are not any more divided to T, L, N, P/Q, and R, but they are described as Ca(v)1.1-Ca(v)3.3, with even newer nomenclature α1A-α1I and α1S. Moreover, new channels such as P2X1-P2X7, as well as TRPA1-TRPV1 have been discovered, giving premises for new types of analgesic drugs.

4-Aminopyridine ameliorates mobility but not disease course in an animal model of multiple sclerosis

Neuropathological changes following demyelination in multiple sclerosis (MS) lead to a reorganization of axolemmal channels that causes conduction changes including conduction failure. Pharmacological modulation of voltage-sensitive potassium channels (K(V)) has been found to improve conduction in experimentally induced demyelination and produces symptomatic improvement in MS patients. Here we used an animal model of autoimmune inflammatory neurodegeneration, namely experimental autoimmune encephalomyelitis (EAE), to test the influence of the K(V)-inhibitor 4-aminopyridine (4-AP) on various disease and immune parameters as well as mobility in MOG₃₅₋₅₅ immunized C57Bl/6 mice. We challenged the hypothesis that 4-AP exerts relevant immunomodulatory or neuroprotective properties. Neither prophylactic nor therapeutic treatment with 4-AP altered disease incidence or disease course of EAE. Histopathological signs of demyelination and neuronal damage as well as MRI imaging of brain volume changes were unaltered. While application of 4-AP significantly reduced the standing outward current of stimulated CD4(+) T cells compared to controls, it failed to impact intracellular calcium concentrations in these cells. Compatibly, KV channel inhibition neither influenced CD4(+) T cell effector functions (proliferation, IL17 or IFNγ production). Importantly however, despite equal disease severity scores 4-AP treated animals showed improved mobility as assessed by 2 independent methods, 1) foot print and 2) rotarod analysis (0.332 ± 0.03, n=7 versus 0.399 ± 0.08, n=14, p<0.001, respectively). Our data suggest that 4-AP while having no apparent immunomodulatory or direct neuroprotective effects, significantly ameliorates conduction abnormalities thereby improving gait and coordination. Improvement of mobility in this experimental model supports trial data and clinical experience with 4-AP in the symptomatic treatment of MS.

Identification of putative potassium channel homologues in pathogenic protozoa

K⁺ channels play a vital homeostatic role in cells and abnormal activity of these channels can dramatically alter cell function and survival, suggesting that they might be attractive drug targets in pathogenic organisms. Pathogenic protozoa lead to diseases such as malaria, leishmaniasis, trypanosomiasis and dysentery that are responsible for millions of deaths each year worldwide. The genomes of many protozoan parasites have recently been sequenced, allowing rational design of targeted therapies. We analyzed the genomes of pathogenic protozoa and show the existence within them of genes encoding putative homologues of K⁺ channels. These protozoan K⁺ channel homologues represent novel targets for anti-parasitic drugs. Differences in the sequences and diversity of human and parasite proteins may allow pathogen-specific targeting of these K⁺ channel homologues.

Intracellular recordings of subnucleus reticularis dorsalis neurones revealed novel electrophysiological properties and windup mechanisms

Aδ- and/or C-fibre nociceptive inputs drive subnucleus reticularis dorsalis (SRD) neurones projecting to a variety of regions including the spinal cord and the nucleus reticularis gigantocellularis (NRGc), but their electrophysiological properties are largely unknown. Here we intracellularly recorded the SRD neuronal responses to injection of polarising current pulses as well as to electrical stimulation of the cervical spinal posterior quadrant (PQ) and the NRGc. Three different classes of neurones with distinct electrophysiological properties were found: type I were characterised by the absence of a fast postspike hyperpolarisation, type II by the presence of a postspike hyperpolarisation followed by a depolarisation resembling low threshold calcium spikes (LTSs), and type III (lacking LTSs) had a fast postspike hyperpolarisation deinactivating A-like potassium channels leading to enlarged interspike intervals. All three classes generated depolarising sags to hyperpolarising current pulses and showed 3-4.5 Hz subthreshold oscillatory activity leading to windup when intracellularly injecting low-frequency repetitive depolarising pulses as well as in response to 0.5-2 Hz NRGc and PQ electrical stimulation. About half of the 132 sampled neurones responded antidromically to NRGc stimulation with more than 65% of the NRGc-antidromic cells, pertaining to all three types, also responding antidromically to PQ stimulation. NRGc stimulation induced exclusively excitatory first-synaptic-responses whilst PQ stimulation induced first-response excitation in most cases, but inhibitory postsynaptic potentials in a few type II and type III neurones not projecting to the spinal cord that also displayed cumulative inhibitory effects (inverse windup). The results show that SRD cells (i) can actively regulate different temporal firing patterns due to their intrinsic electrophysiological properties, (ii) generate windup upon gradual membrane depolarisation produced by low-frequency intracellular current injection and by C-fibre tonic input, both processes leading subthreshold oscillations to threshold, and (iii) collateralise to the NRGc and the spinal cord, potentially providing simultaneous regulation of ascending noxious information and motor reactions to pain.

Modulation of the frequency response of Shaker potassium channels by the quiver peptide suggesting a novel extracellular interaction mechanism

Recent studies have indicated that the Shaker potassium channel regulates sleep in Drosophila. The Drosophila quiver (qvr) gene encodes a novel potassium channel subunit that modulates the Shaker potassium channel. The Qvr peptide contains a signal sequence for extracellular localization and may regulate a unique feature of the Shaker I(A) current that confers special neuronal excitability patterns. Thus, studies of the Shaker channel properties in the qvr mutant background should provide an opportunity to uncover a new form of physiologic modulation of potassium channels. We have begun to investigate the impact of qvr protein on the Shaker channel properties and its implications in synaptic function in vivo. We studied synaptic transmission at the larval neuromuscular junction and characterized the transient potassium current I(A) in larval muscles. We identified two different functional states of I(A) in qvr larval muscles, as reflected by two distinct components, I(AF) and I(AS), differing in their kinetics of recovery from inactivation and sensitivity to a K(+) channel blocker. Correspondingly, qvr mutant larvae exhibit multiple synaptic discharges following individual nerve stimuli during repetitive activity.

Potassium channel blockers in multiple sclerosis: Neuronal Kv channels and effects of symptomatic treatment

Multiple sclerosis (MS) is an inflammatory disease of the central nervous system (CNS) characterized by demyelination, with a relative sparing of axons. In MS patients, many neurologic signs and symptoms have been attributed to the underlying conduction deficits. The idea that neurologic function might be improved if conduction could be restored in CNS demyelinated axons led to the testing of potassium (K(+)) channel blockers as a symptomatic treatment. To date, only 2 broad-spectrum K(+) channel blockers, 4-aminopyridine (4-AP) and 3,4-diaminopyridine (3,4-DAP), have been tested in MS patients. Although both 4-AP and 3,4-DAP produce clear neurologic benefits, their use has been limited by toxicity. Here we review the current status of basic science and clinical research related to the therapeutic targeting of voltage-gated K(+) channels (K(v)) in MS. By bringing together 3 distinct but interrelated disciplines, we aim to provide perspective on a vast body of work highlighting the lengthy and ongoing process entailed in translating fundamental K(v) channel knowledge into new clinical treatments for patients with MS and other demyelinating diseases. Covered are (1) K(v) channel nomenclature, structure, function, and pharmacology; (2) classic and current experimental morphology and neurophysiology studies of demyelination and conduction deficits; and (3) a comprehensive overview of clinical trials utilizing 4-AP and 3,4-DAP in MS patients.

Status of the intracellular gate in the activated-not-open state of shaker K+ channels

Voltage-dependent K⁺ channels like Shaker use an intracellular gate to control ion flow through the pore. When the membrane voltage becomes more positive, these channels traverse a series of closed conformations before the final opening transition. Does the intracellular gate undergo conformational changes before channel opening? To answer this question we introduced cysteines into the intracellular end of the pore and studied their chemical modification in conditions favoring each of three distinct states, the open state, the resting closed state, and the activated-not-open state (the closed state adjacent to the open state). We used two independent ways to isolate the channels in the activated-not-open state. First, we used mutations in S4 (ILT; Smith-Maxwell, C.J., J.L. Ledwell, and R.W. Aldrich. 1998. J. Gen. Physiol. 111:421–439; Ledwell, J.L., and R.W. Aldrich. 1999. J. Gen. Physiol. 113:389–414) that separate the final opening step from earlier charge-movement steps. Second, we used the open channel blocker 4-aminopyridine (4-AP), which has been proposed to promote closure of the intracellular gate and thus specifically to stabilize the activated-not-open state of the channels. Supporting this proposed mechanism, we found that 4-AP enters channels only after opening, remaining trapped in closed channels, and that in the open state it competes with tetraethylammonium for binding. Using these tools, we found that in the activated-not-open state, a cysteine located at a position considered to form part of the gate (Shaker 478) showed higher reactivity than in either the open or the resting closed states. Additionally, we have found that in this activated state the intracellular gate continued to prevent access to the pore by molecules as small as Cd²⁺ ions. Our results suggest that the intracellular opening to the pore undergoes some rearrangements in the transition from the resting closed state to the activated-not-open state, but throughout this process the intracellular gate remains an effective barrier to the movement of potassium ions through the pore.

Dose responses of three 4-aminopyridine derivatives on axonal conduction in spinal cord trauma

To explore novel treatments for enhancing conduction through traumatically injured spinal cord we have synthesized structurally distinct pyridine based compounds; N-(4-pyridyl) methyl carbamate, N-(4-pyridyl) ethyl carbamate, and N-(4-pyridyl) t-butyl carbamate. With the use of a double sucrose gap-recording chamber we perform a dose-response assay to examine the effects of these compounds on axonal conduction following an in vitro stretch injury. The tested compounds significantly enhanced axonal conduction to the stretch injured cord at 1 microM, a dose that coincides with the clinically relevant dose of potassium channel blocker 4-aminopyridine (4-AP). Methyl carbamate enhanced conduction maximally at 100 microM. This is also the most effective concentration of 4-AP in vitro. The other compounds ethyl carbamate and t-butyl carbamate enhanced conduction maximally at lower concentrations of 10 and 1 microM. At higher concentrations each of these compounds continued to increased CAP amplitude, however not significantly. Additionally, two of the compounds ethyl and t-butyl carbamate appear to have negative effects on CAP amplitude when administered at or beyond 100 microM. These compounds demonstrate the possibility that derivatives of 4-AP can retain the ability to increase axonal conduction in the injured spinal cord.

Molecular physiology and modulation of somatodendritic A-type potassium channels

The somatodendritic subthreshold A-type K+ current (ISA) in nerve cells is a critical component of the ensemble of voltage-gated ionic currents that determine somatodendritic signal integration. The underlying K+ channel belongs to the Shal subfamily of voltage-gated K+ channels. Most Shal channels across the animal kingdom share a high degree of structural conservation, operate in the subthreshold range of membrane potentials, and exhibit relatively fast inactivation and recovery from inactivation. Mammalian Shal K+ channels (Kv4) undergo preferential closed-state inactivation with features that are generally inconsistent with the classical mechanisms of inactivation typical of Shaker K+ channels. Here, we review (1) the physiological and genetic properties of ISA, 2 the molecular mechanisms of Kv4 inactivation and its remodeling by a family of soluble calcium-binding proteins (KChIPs) and a membrane-bound dipeptidase-like protein (DPPX), and (3) the modulation of Kv4 channels by protein phosphorylation.

Bicuculline free base blocks voltage-activated K+ currents in rat medial preoptic neurons

The effects of the well-known GABA(A)-receptor blocker bicuculline on voltage-gated K(+) currents were studied in neurons from the medial preoptic nucleus (MPN) of rat. Whole-cell currents were recorded using the perforated-patch technique. Voltage steps from -54 to +6 mV resulted in tetraethylammonium-sensitive K(+) currents of delayed rectifier type. The total K(+) current (at 300 ms), including Ca(2+)-dependent and Ca(2+)-independent components, was reversibly reduced (17 +/- 4%) by 100 microM bicuculline methiodide and (37 +/- 5%) by 100 microM bicuculline as free base. The Ca(2+)-independent fraction (77 +/- 2%) of K(+) current evoked by a voltage step was, however, reduced (54 +/- 6%) only by bicuculline free base, but was not affected by bicuculline methiodide. The half-saturating concentration of bicuculline free base for blocking this purely voltage-gated K(+) current was 113 microM, whereas for blocking a steady Ca(2+)-dependent K(+) current it was 36 microM. The bicuculline-sensitive voltage-gated K(+) current was composed of 4-AP-sensitive and 4-AP-resistant components with different kinetic properties. No component of the purely voltage-gated K(+) current was affected neither by 100 nM alpha-dendrotoxin nor by 100 nM I-dendrotoxin. The possible K(+)-channel subtypes mediating the bicuculline-sensitive current in MPN neurons are discussed.

Mechanism of Rectification in Inward-Rectifier K+ Channels

Inward rectifiers are a class of K+ channels that can conduct much larger inward currents at membrane voltages negative to the K+ equilibrium potential than outward currents at voltages positive to it, even when K+ concentrations on both sides of the membrane are made equal. This conduction property, called inward rectification, enables inward rectifiers to perform many important physiological tasks. Rectification is not an inherent property of the channel protein itself, but reflects strong voltage dependence of channel block by intracellular cations such as Mg2+ and polyamines. This voltage dependence results primarily from the movement of K+ ions across the transmembrane electric field along the pore, which is energetically coupled to the blocker binding and unbinding. This mutual displacement mechanism between several K+ ions and a blocker explains the signature feature of inward rectifier K+ channels, namely, that at a given concentration of intracellular K+, their macroscopic conductance depends on the difference between membrane voltage and the K+ equilibrium potential rather than on membrane voltage itself.

The use of 4-aminopyridine (fampridine) in demyelinating disorders

4-Aminopyridine (4-AP or fampridine) is a potassium channel-blocking agent that has been shown to restore conduction in focally demyelinated axons. A sustained-release matrix tablet form of 4-AP (fampridine-SR) is currently undergoing multicenter clinical trials in patients with multiple sclerosis or chronic spinal cord injury. This review describes the pharmacology and mechanisms of action of 4-AP, its pharmacokinetics in human subjects, and the outcomes of clinical trials employing either immediate-release or sustained-release formulations of the drug. The randomized clinical trials that have been completed to date indicate that K+ channel blockade may prove to be a useful strategy for ameliorating central conduction deficits due to demyelination. Diverse neurological gains have been reported for both motor and sensory domains. At the present time, however, the clinical trials have not provided sufficiently robust or definitive evidence of efficacy to gain regulatory approval for the symptomatic management of patients with either multiple sclerosis or spinal cord injury.

Local anesthetic block of Kv channels: role of the S6 helix and the S5-S6 linker for bupivacaine action

To gain insights in the molecular mechanisms of anesthesia, we analyzed the effects of bupivacaine on a series of voltage-gated K+ channels (Kv1.1, -1.2, -1.5, -2.1, -3.1, and -3.2) and various mutant channels derived from Kv2.1, using Xenopus laevis oocytes. Two phenomenologically different blocking effects were seen at room temperature: a time-dependent block of Kv1 and Kv3 channels (Kd between 110 and 240 microM), and a time-independent block on Kv2.1 (Kd = 220 microM). At 32 degrees C, however, Kv2.1 also showed a time-dependent block. Swapping the S6 helix between Kv1.2 and Kv2.1 introduced Kv1.2 features in Kv2.1. Critical residues were located in the N-terminal end of S6, positions 395 and 398. The triple substitution of residues 372, 373, and 374 in the S5-S6 linker decreased the bupivacaine affinity by 5-fold (Kd increased from 220 to 1170 microM). The results suggest that bupivacaine blocks Kv channels by an open-state-dependent mechanism and that Kv2.1 deviates from the other channels in allowing a partial closure of the channel with bupivacaine bound. The results also suggest that the binding site is located in the internal vestibule and that residues in the descending P-loop and the upper part of S6 are critical for the binding, most likely by allosteric mechanisms. A simple mechanistic scenario that explains the observations is presented. Thermodynamic considerations suggest that the interaction between bupivacaine and the channels is hydrophobic.

Inactivation and pharmacological properties of sqKv1A homotetramers in Xenopus oocytes cannot account for behavior of the squid "delayed rectifier" K(+) conductance

Considerable published evidence suggests that alpha-subunits of the cloned channel sqKv1A compose the "delayed rectifier" in the squid giant axon system, but discrepancies regarding inactivation properties of cloned versus native channels exist. In this paper we define the mechanism of inactivation for sqKv1A channels in Xenopus oocytes to investigate these and other discrepancies. Inactivation of sqKv1A in Xenopus oocytes was found to be unaffected by genetic truncation of the N-terminus, but highly sensitive to certain amino acid substitutions around the external mouth of the pore. External TEA and K(+) ions slowed inactivation of sqKv1A channels in oocytes, and chloramine T (Chl-T) accelerated inactivation. These features are all consistent with a C-type inactivation mechanism as defined for Shaker B channels. Treatment of native channels in giant fiber lobe neurons with TEA or high K(+) does not slow inactivation, nor does Chl-T accelerate it. Pharmacological differences between the two channel types were also found for 4-aminopyridine (4AP). SqKv1A's affinity for 4AP was poor at rest and increased after activation, whereas 4AP block occurred much more readily at rest with native channels than when they were activated. These results suggest that important structural differences between sqKv1A homotetramers and native squid channels are likely to exist around the external and internal mouths of the pore.

Determinants of 4-aminopyridine sensitivity in a human brain kv1.4 k(+) channel: phenylalanine substitutions in leucine heptad repeat region stabilize channel closed state

The biophysical and pharmacological effects of individual phenylalanine-for-leucine (Phe-for-Leu) substitutions in the leucine heptad repeat region located at the cytosolic surface of the channel pore, on whole-cell K(+) currents, were studied in cloned and mutated human brain Kvl.4 K(+) channels (hKvl.4) transiently transfected into HeLa cells. Although L2 and L5 are not considered part of the 4-aminopyridine (4-AP) binding site, unlike the L4 heptad leucine, Phe substitutions at L2 (L464) or L5 (L485) increase 4-AP sensitivity by 400-fold, as seen previously in the L4F mutant channel. Greater depolarizing shifts manifest in the voltage dependence of activation and inactivation in L2F (20 mV) and L5F (30 mV) than in L4F (10 mV) relative to hKv1.4. L1F (L457) and L3F (L471) increase 4-AP sensitivity by 8- and 150-fold, respectively, and produce depolarizing shifts in activation of approximately 5 mV without affecting inactivation. The apparent free energy differences of 4-AP binding in each mutant suggest enhanced drug-channel interactions (L2F > or = L4F > or = L5F > L3F > L1F). Deactivation kinetics are accelerated in L2F (11-fold), L5F (8-fold), L1F (5-fold), and L3F (2-fold), at -50 mV. All Phe-for-heptad-Leu substitutions produce gating changes suggesting variable stabilization of the channel closed state conformation, with L1F, L2F, and L5F exhibiting the strongest correlations between altered gating and increased 4-AP sensitivity. If 4-AP blocks the open channel by promoting closure of the activation gate (recent Armstrong-Loboda model), then changes in the leucine heptad repeat that stabilize the channel closed state may contribute to increased 4-AP sensitivity by amplifying the mechanism of 4-AP block.

Inhibition of slow Ca(2+)-activated K(+) current by 4-aminopyridine in rat hippocampal CA1 pyramidal neurones

1. The effect of 4-aminopyridine (4-AP) on the slow afterhyperpolarization (sAHP) seen after high frequency dendritic or somatic firing was investigated in rat hippocampal CA1 pyramidal neurones (PC). Intracellular recordings were obtained from the distal apical dendrites and somata and suprathreshold depolarizing current pulses were used to evoke a sAHP. The sAHP was blocked by low concentrations of carbacholine (Cch) but insensitive to high concentrations of apamin. 2. In the presence of extracellular 4-AP, the first dendritic sAHP evoked was reduced compared to a maximal sAHP evoked in the absence of 4-AP. The reduction was evident at submillimolar concentration and increased to about 80% with 4 mM 4-AP. 3. The stability of the 4-AP-induced block was affected by the type of anion used in the electrode solution. With K(+) acetate (KAc) or K(+) methylsulphate (KMeSO(4)) containing electrodes, the block was progressively removed during the initial 300 - 400 s of recordings. With KCl containing electrodes, the block remained stable and was 10% larger than that obtained with acetate. Detailed investigations showed that intracellular acetate promotes the removal of the 4-AP-induced block in an activity-dependent manner. 4. Intracellularly applied 4-AP also induced an acetate-sensitive block of the dendritic sAHP. 5. 4-AP also blocked the somatic sAHP and the stability of the block showed the same sensitivity towards anions as the dendritic sAHP. 6. Thus 4-AP appears to block the slow Ca(2+)-activated K(+) current underlying the sAHP in a complex manner which is sensitive to certain types of anions.

A model for 4-aminopyridine action on K channels: similarities to tetraethylammonium ion action

We present a model for the action of 4-aminopyridine (4AP) on K channels. The model is closely based on the gating current studies of the preceding paper and has been extended to account for ionic current data in the literature. We propose that 4AP, like tetraethylammonium ion and other quaternary ammonium ions, enters and leaves the channel only when the activation gate is open, a proposal that is strongly supported by the literature. Once in the open channel, 4AP's major action is to bias the activation gate toward the closed conformation by approximately the energy of a hydrogen bond. S4 segment movement, as reflected in gating currents, is almost normal for a 4AP-occupied channel; only the final opening transition is affected. The model is qualitatively the same as the one used for many years to explain the action of quaternary ammonium ions.

Functional studies of a chimeric protein containing portions of the Na(+)/glucose and Na(+)/myo-inositol cotransporters

We obtained cDNA chimeras between Na/glucose cotransporter (SGLT1) and the homologous Na(+)/myo-inositol cotransporter (SMIT) by creating random chimeras in plasmids. Of 12 chimeras, two were functional when expressed in Xenopus laevis oocytes but, upon sequencing, only one of them (C1) produced an actual chimeric protein. In C1, the first 69 amino acids of SGLT1 were replaced by the corresponding 50 amino acids of SMIT. C1 transports the same sugars as does SGLT1. C1's affinity for all sugar substrates was systematically increased by a factor of 3.3+/-0.4 but the V(max) was diminished by a factor of 15-40. In contrast, the cotransport affinity for Na(+) was unchanged. The surface expression of C1 was one seventh that of SGLT1, which explains part of the reduced V(max) and implies a significant reduction in turnover rate. N-terminal truncated constructs of SGLT1 cDNA showed that deleting amino acids 2-14 does not affect cotransporter activity, but that the pentapeptide T(14)RPVET(19) is important for normal levels of SGLT1 current. The main result of a kinetic analysis of the systematic increase in apparent affinity for sugars, together with the intact Na apparent affinity, suggests enhanced access to the sugar binding site in C1.

Local movement in the S2 region of the voltage-gated potassium channel hKv2.1 studied using cysteine mutagenesis

The positively charged S4 region of voltage-dependent potassium channels moves outward during depolarization, leading to channel opening, but possible movement of the negatively charged S2 region may be more complex. Here we have studied possible movement of the S2 region of the slowly activating human voltage-dependent potassium channel hKv2.1. For this, cysteine mutants in the S2 region were expressed in Xenopus oocytes by injection of cRNA. Whole-cell currents were measured using the two-electrode voltage-clamp technique, and the effect of the membrane-impermeable cysteine-binding reagent parachloromercuribenzenesulfonate (PCMBS) was studied. For mutant S223C (located just outside the membrane in the S2 region), PCMBS inhibited currents and caused faster deactivation of tail currents. The time course of reactivity of PCMBS on tail current amplitudes was faster at more negative holding potentials. There was no effect of PCMBS on potassium channel currents for mutants D225C, N226C, A230C, and V232C. These data suggest that residue S223 is exposed to the extracellular phase at normal resting potentials, making it accessible to PCMBS, but upon depolarization there is a conformational change, making it less accessible, possibly by a local rather than global movement of S2 residues into the membrane. Voltage-dependent movements of nearby residues could also explain the results.

Towards the three-dimensional structure of voltage-gated potassium channels

Electrical excitability is a fundamental property of the neuromuscular systems of metazoans. The varied response of neurons to electrical excitation is largely accounted for by a diverse set of voltage-gated potassium (KV) channels in the excitable membrane. The complete structure of a KV channel is not yet available. However, recent structural biological experiments have begun to provide new insight into how specific KV channels are formed and regulated, and how they function and interact with other proteins. In particular, the selectivity of KV channels for K+ and suggestions as to how these structural elements might assemble into a functional KV channel are discussed.

Delayed rectifier currents in rat globus pallidus neurons are attributable to Kv2.1 and Kv3.1/3.2 K(+) channels

The symptoms of Parkinson disease are thought to result in part from increased burst activity in globus pallidus neurons. To gain a better understanding of the factors governing this activity, we studied delayed rectifier K(+) conductances in acutely isolated rat globus pallidus (GP) neurons, using whole-cell voltage-clamp and single-cell RT-PCR techniques. From a holding potential of -40 mV, depolarizing voltage steps in identified GP neurons evoked slowly inactivating K(+) currents. Analysis of the tail currents revealed rapidly and slowly deactivating currents of similar amplitude. The fast component of the current deactivated with a time constant of 11. 1 +/- 0.8 msec at -40 mV and was blocked by micromolar concentrations of 4-AP and TEA (K(D) approximately 140 microM). The slow component of the current deactivated with a time constant of 89 +/- 10 microseconds at -40 mV and was less sensitive to TEA (K(D) = 0.8 mM) and 4-AP (K(D) approximately 6 mM). Organic antagonists of Kv1 family channels had little or no effect on somatic currents. These properties are consistent with the hypothesis that the rapidly deactivating current is attributable to Kv3.1/3.2 channels and the slowly deactivating current to Kv2.1-containing channels. Semiquantitative single-cell RT-PCR analysis of Kv3 and Kv2 family mRNAs supported this conclusion. An alteration in the balance of these two channel types could underlie the emergence of burst firing after dopamine-depleting lesions.

Activation and inactivation of the voltage-gated sodium channel: role of segment S5 revealed by a novel hyperkalaemic periodic paralysis mutation

Hyperkalaemic periodic paralysis, paramyotonia congenita, and potassium-aggravated myotonia are three autosomal dominant skeletal muscle disorders linked to the SCN4A gene encoding the alpha-subunit of the human voltage-sensitive sodium channel. To date, approximately 20 point mutations causing these disorders have been described. We have identified a new point mutation, in the SCN4A gene, in a family with a hyperkalaemic periodic paralysis phenotype. This mutation predicts an isoleucine-to-phenylalanine substitution at position 1495 located in the transmembrane segment S5 in the fourth homologous domain of the human alpha-subunit sodium channel. Introduction of the I1495F mutation into the wild-type channels disrupted the macroscopic current inactivation decay and shifted both steady-state activation and inactivation to the hyperpolarizing direction. The recovery from fast inactivation was slowed, and there was no effect on channel deactivation. Additionally, a significant enhancement of slow inactivation was observed in the I1495F mutation. In contrast, the T704M mutation, a hyperkalaemic periodic paralysis mutation located in the cytoplasmic interface of the S5 segment of the second domain, also shifted activation in the hyperpolarizing direction but had little effect on fast inactivation and dramatically impaired slow inactivation. These results, showing that the I1495F and T704M hyperkalaemic periodic paralysis mutations both have profound effects on channel activation and fast-slow inactivation, suggest that the S5 segment maybe in a location where fast and slow inactivation converge.

Inactivation gating and 4-AP sensitivity in human brain Kv1.4 potassium channel

Voltage-gated K(+) channels vary in sensitivity to block by 4-aminopyridine (4-AP) over a 1000-fold range. Most K(+) channel phenotypes with leucine at the fourth position (L4) in the leucine heptad repeat region, spanning the S4-S5 linker, exhibit low 4-AP sensitivity, while channels with phenylalanine exhibit high sensitivity. Mutational analysis on delayed rectifier type K(+) channels demonstrate increased 4-AP sensitivity upon mutation of the L4 heptad leucine to phenylalanine. This mutation can also influence inactivation gating, which is known to compete with 4-AP in rapidly inactivating A-type K(+) channels. Here, in a rapidly inactivating human brain Kv1.4 channel, we demonstrate a 400-fold increase in 4-AP sensitivity following substitution of L4 with phenylalanine. Accompanying this mutation is a slowing of inactivation, an acceleration of deactivation, and depolarizing shifts in the voltage dependence of activation and steady-state inactivation. To test the relative role of fast inactivation in modulating 4-AP block, N-terminal deletions of the fast inactivation gate were carried out in both channels. These deletions produced no change in 4-AP sensitivity in the mutant channel and approximately a six-fold increase in the wild type channel. These results support the view that changes at L4 which increase 4-AP sensitivity are largely due to 4-AP binding and may, in part, arise from alterations in channel conformation. Primarily, this study demonstrates that the fast inactivation gate is not a critical determinant of 4-AP sensitivity in Kv1.4 channels.

Auxiliary Hyperkinetic beta subunit of K+ channels: regulation of firing properties and K+ currents in Drosophila neurons

Auxiliary Hyperkinetic beta subunit of K+ channels: regulation of firing properties and K+ currents in Drosophila neurons. Molecular analysis and heterologous expression have shown that K+ channel beta subunits regulate the properties of the pore-forming alpha subunits, although how they influence neuronal K+ currents and excitability remains to be explored. We studied cultured Drosophila "giant" neurons derived from mutants of the Hyperkinetic (Hk) gene, which codes for a K+ channel beta subunit. Whole cell patch-clamp recording revealed broadened action potentials and, more strikingly, persistent rhythmic spontaneous activities in a portion of mutant neurons. Voltage-clamp analysis demonstrated extensive alterations in the kinetics and voltage dependence of K+ current activation and inactivation, especially at subthreshold membrane potentials, suggesting a role in regulating the quiescent state of neurons that are capable of tonic firing. Altered sensitivity of Hk currents to classical K+ channel blockers (4-aminopyridine, alpha-dendrotoxin, and TEA) indicated that Hk mutations modify interactions between voltage-activated K+ channels and these pharmacological probes, apparently by changing both the intra- and extracellular regions of the channel pore. Correlation of voltage- and current-clamp data from the same cells indicated that Hk mutations affect not only the persistently active neurons, but also other neuronal categories. Shaker (Sh) mutations, which alter K+ channel alpha subunits, increased neuronal excitability but did not cause the robust spontaneous activity characteristic of some Hk neurons. Significantly, Hk Sh double mutants were indistinguishable from Sh single mutants, implying that the rhythmic Hk firing pattern is conferred by intact Shalpha subunits in a distinct neuronal subpopulation. Our results suggest that alterations in beta subunit regulation, rather than elimination or addition of alpha subunits, may cause striking modifications in the excitability state of neurons, which may be important for complex neuronal function and plasticity.