Maurotoxin: a potent inhibitor of intermediate conductance Ca2+-activated potassium channels

Of Seven New K+ Channel Inhibitor Peptides of Centruroides bonito, α-KTx 2.24 Has a Picomolar Affinity for Kv1.2

Seven new peptides denominated CboK1 to CboK7 were isolated from the venom of the Mexican scorpion Centruroides bonito and their primary structures were determined. The molecular weights ranged between 3760.4 Da and 4357.9 Da, containing 32 to 39 amino acid residues with three putative disulfide bridges. The comparison of amino acid sequences with known potassium scorpion toxins (KTx) and phylogenetic analysis revealed that CboK1 (α-KTx 10.5) and CboK2 (α-KTx 10.6) belong to the α-KTx 10.x subfamily, whereas CboK3 (α-KTx 2.22), CboK4 (α-KTx 2.23), CboK6 (α-KTx 2.21), and CboK7 (α-KTx 2.24) bear > 95% amino acid similarity with members of the α-KTx 2.x subfamily, and CboK5 is identical to Ce3 toxin (α-KTx 2.10). Electrophysiological assays demonstrated that except CboK1, all six other peptides blocked the Kv1.2 channel with Kd values in the picomolar range (24-763 pM) and inhibited the Kv1.3 channel with comparatively less potency (Kd values between 20-171 nM). CboK3 and CboK4 inhibited less than 10% and CboK7 inhibited about 42% of Kv1.1 currents at 100 nM concentration. Among all, CboK7 showed out-standing affinity for Kv1.2 (Kd = 24 pM), as well as high selectivity over Kv1.3 (850-fold) and Kv1.1 (~6000-fold). These characteristics of CboK7 may provide a framework for developing tools to treat Kv1.2-related channelopathies.

Characterization and Chemical Synthesis of Cm39 (α-KTx 4.8): A Scorpion Toxin That Inhibits Voltage-Gated K+ Channel KV1.2 and Small- and Intermediate-Conductance Ca2+-Activated K+ Channels KCa2.2 and KCa3.1

A novel peptide, Cm39, was identified in the venom of the scorpion Centruroides margaritatus. Its primary structure was determined. It consists of 37 amino acid residues with a MW of 3980.2 Da. The full chemical synthesis and proper folding of Cm39 was obtained. Based on amino acid sequence alignment with different K+ channel inhibitor scorpion toxin (KTx) families and phylogenetic analysis, Cm39 belongs to the α-KTx 4 family and was registered with the systematic number of α-KTx 4.8. Synthetic Cm39 inhibits the voltage-gated K+ channel hKV1.2 with high affinity (Kd = 65 nM). The conductance-voltage relationship of KV1.2 was not altered in the presence of Cm39, and the analysis of the toxin binding kinetics was consistent with a bimolecular interaction between the peptide and the channel; therefore, the pore blocking mechanism is proposed for the toxin-channel interaction. Cm39 also inhibits the Ca2+-activated KCa2.2 and KCa3.1 channels, with Kd = 502 nM, and Kd = 58 nM, respectively. However, the peptide does not inhibit hKV1.1, hKV1.3, hKV1.4, hKV1.5, hKV1.6, hKV11.1, mKCa1.1 K+ channels or the hNaV1.5 and hNaV1.4 Na+ channels at 1 μM concentrations. Understanding the unusual selectivity profile of Cm39 motivates further experiments to reveal novel interactions with the vestibule of toxin-sensitive channels.

Pharmacological targeting of the mitochondrial calcium-dependent potassium channel KCa3.1 triggers cell death and reduces tumor growth and metastasis in vivo

Ion channels are non-conventional, druggable oncological targets. The intermediate-conductance calcium-dependent potassium channel (K_Ca3.1) is highly expressed in the plasma membrane and in the inner mitochondrial membrane (mitoK_Ca3.1) of various cancer cell lines. The role mitoK_Ca3.1 plays in cancer cells is still undefined. Here we report the synthesis and characterization of two mitochondria-targeted novel derivatives of a high-affinity K_Ca3.1 antagonist, TRAM-34, which retain the ability to block channel activity. The effects of these drugs were tested in melanoma, pancreatic ductal adenocarcinoma and breast cancer lines, as well as in vivo in two orthotopic models. We show that the mitochondria-targeted TRAM-34 derivatives induce release of mitochondrial reactive oxygen species, rapid depolarization of the mitochondrial membrane, fragmentation of the mitochondrial network. They trigger cancer cell death with an EC₅₀ in the µM range, depending on channel expression. In contrast, inhibition of the plasma membrane K_Ca3.1 by membrane-impermeant Maurotoxin is without effect, indicating a specific role of mitoK_Ca3.1 in determining cell fate. At sub-lethal concentrations, pharmacological targeting of mitoK_Ca3.1 significantly reduced cancer cell migration by enhancing production of mitochondrial reactive oxygen species and nuclear factor-κB (NF-κB) activation, and by downregulating expression of Bcl-2 Nineteen kD-Interacting Protein (BNIP-3) and of Rho GTPase CDC-42. This signaling cascade finally leads to cytoskeletal reorganization and impaired migration. Overexpression of BNIP-3 or pharmacological modulation of NF-κB and CDC-42 prevented the migration-reducing effect of mitoTRAM-34. In orthotopic models of melanoma and pancreatic ductal adenocarcinoma, the tumors at sacrifice were 60% smaller in treated versus untreated animals. Metastasis of melanoma cells to lymph nodes was also drastically reduced. No signs of toxicity were observed. In summary, our results identify mitochondrial K_Ca3.1 as an unexpected player in cancer cell migration and show that its pharmacological targeting is efficient against both tumor growth and metastatic spread in vivo.

Recombinant Expression in Pichia pastoris System of Three Potent Kv1.3 Channel Blockers: Vm24, Anuroctoxin, and Ts6

The Kv1.3 channel has become a therapeutic target for the treatment of various diseases. Several Kv1.3 channel blockers have been characterized from scorpion venom; however, extensive studies require amounts of toxin that cannot be readily obtained directly from venoms. The Pichia pastoris expression system provides a cost-effective approach to overcoming the limitations of chemical synthesis and E. coli recombinant expression. In this work, we developed an efficient system for the production of three potent Kv1.3 channel blockers from different scorpion venoms: Vm24, AnTx, and Ts6. Using the Pichia system, these toxins could be obtained in sufficient quantities (Vm24 1.6 mg/L, AnTx 46 mg/L, and Ts6 7.5 mg/L) to characterize their biological activity. A comparison was made between the activity of tagged and untagged recombinant peptides. Tagged Vm24 and untagged AnTx are nearly equivalent to native toxins in blocking Kv1.3 (Kd = 4.4 pM and Kd = 0.72 nM, respectively), whereas untagged Ts6 exhibits a 53-fold increase in Kd (Kd = 29.1 nM) as compared to the native peptide. The approach described here provides a method that can be optimized for toxin production to develop more selective and effective Kv1.3 blockers with therapeutic potential.

Regulatory role of KCa3.1 in immune cell function and its emerging association with rheumatoid arthritis

Rheumatoid arthritis (RA) is a common autoimmune disease characterized by chronic inflammation. Immune dysfunction is an essential mechanism in the pathogenesis of RA and directly linked to synovial inflammation and cartilage/bone destruction. Intermediate conductance Ca²⁺-activated K⁺ channel (KCa3.1) is considered a significant regulator of proliferation, differentiation, and migration of immune cells by mediating Ca²⁺ signal transduction. Earlier studies have demonstrated abnormal activation of KCa3.1 in the peripheral blood and articular synovium of RA patients. Moreover, knockout of KCa3.1 reduced the severity of synovial inflammation and cartilage damage to a significant extent in a mouse collagen antibody-induced arthritis (CAIA) model. Accumulating evidence implicates KCa3.1 as a potential therapeutic target for RA. Here, we provide an overview of the KCa3.1 channel and its pharmacological properties, discuss the significance of KCa3.1 in immune cells and feasibility as a drug target for modulating the immune balance, and highlight its emerging role in pathological progression of RA.

Comparison of K+ Channel Families

K⁺ channels enable potassium to flow across the membrane with great selectivity. There are four K⁺ channel families: voltage-gated K (K_v), calcium-activated (K_Ca), inwardly rectifying K (K_ir), and two-pore domain potassium (K_2P) channels. All four K⁺ channels are formed by subunits assembling into a classic tetrameric (4x1P = 4P for the K_v, K_Ca, and K_ir channels) or tetramer-like (2x2P = 4P for the K_2P channels) architecture. These subunits can either be the same (homomers) or different (heteromers), conferring great diversity to these channels. They share a highly conserved selectivity filter within the pore but show different gating mechanisms adapted for their function. K⁺ channels play essential roles in controlling neuronal excitability by shaping action potentials, influencing the resting membrane potential, and responding to diverse physicochemical stimuli, such as a voltage change (K_v), intracellular calcium oscillations (K_Ca), cellular mediators (K_ir), or temperature (K_2P).

Calcium-Activated K+ Channels (KCa) and Therapeutic Implications

Potassium channels are the most diverse and ubiquitous family of ion channels found in cells. The Ca²⁺ and voltage gated members form a subfamily that play a variety of roles in both excitable and non-excitable cells and are further classified on the basis of their single channel conductance to form the small conductance (SK), intermediate conductance (IK) and big conductance (BK) K⁺ channels.In this chapter, we will focus on the mechanisms underlying the gating of BK channels, whose function is modified in different tissues by different splice variants as well as the expanding array of regulatory accessory subunits including β, γ and LINGO subunits. We will examine how BK channels are modified by these regulatory subunits and describe how the channel gating is altered by voltage and Ca²⁺ whilst setting this in context with the recently published structures of the BK channel. Finally, we will discuss how BK and other calcium-activated channels are modulated by novel ion channel modulators and describe some of the challenges associated with trying to develop compounds with sufficient efficacy, potency and selectivity to be of therapeutic benefit.

Voltage-Gated K+/Na+ Channels and Scorpion Venom Toxins in Cancer

Ion channels have recently been recognized as novel therapeutic targets in cancer research since they are overexpressed in different histological tissues, and their activity is linked to proliferation, tumor progression, angiogenesis, metastasis, and apoptosis. Voltage gated-potassium channels (VGKC) are involved in cell proliferation, cancer progression, cell cycle transition, and apoptosis. Moreover, voltage-dependent sodium channels (VGSC) contribute to decreases in extracellular pH, which, in turn, promotes cancer cell migration and invasion. Furthermore, VGSC and VGKC modulate voltage-sensitive Ca2+ channel activity by controlling the membrane potential and regulating Ca2+ influx, which functions as a second messenger in processes related to proliferation, invasion, migration, and metastasis. The subgroup of these types of channels that have shown a high oncogenic potential have become known as "oncochannels", and the evidence has highlighted them as key potential therapeutic targets. Scorpion venoms contain a high proportion of peptide toxins that act by modulating voltage-gated Na+/K+ channel activity. Increasing scientific data have pointed out that scorpion venoms and their toxins can affect the activity of oncochannels, thus showing their potential for anticancer therapy. In this review, we provide an update of the most relevant voltage-gated Na+\K+ ion channels as cellular targets and discuss the possibility of using scorpion venom and toxins for anticancer therapy.

Pharmacology of Small- and Intermediate-Conductance Calcium-Activated Potassium Channels

The three small-conductance calcium-activated potassium (K_Ca2) channels and the related intermediate-conductance K_Ca3.1 channel are voltage-independent K⁺ channels that mediate calcium-induced membrane hyperpolarization. When intracellular calcium increases in the channel vicinity, it calcifies the flexible N lobe of the channel-bound calmodulin, which then swings over to the S4-S5 linker and opens the channel. K_Ca2 and K_Ca3.1 channels are highly druggable and offer multiple binding sites for venom peptides and small-molecule blockers as well as for positive- and negative-gating modulators. In this review, we briefly summarize the physiological role of K_Ca channels and then discuss the pharmacophores and the mechanism of action of the most commonly used peptidic and small-molecule K_Ca2 and K_Ca3.1 modulators. Finally, we describe the progress that has been made in advancing K_Ca3.1 blockers and K_Ca2.2 negative- and positive-gating modulators toward the clinic for neurological and cardiovascular diseases and discuss the remaining challenges.

Diverse Structural Features of Potassium Channels Characterized by Scorpion Toxins as Molecular Probes

Scorpion toxins are well-known as the largest potassium channel peptide blocker family. They have been successfully proven to be valuable molecular probes for structural research on diverse potassium channels. The potassium channel pore region, including the turret and filter regions, is the binding interface for scorpion toxins, and structural features from different potassium channels have been identified using different scorpion toxins. According to the spatial orientation of channel turrets with differential sequence lengths and identities, conformational changes and molecular surface properties, the potassium channel turrets can be divided into the following three states: open state with less hindering effects on toxin binding, half-open state or half-closed state with certain effects on toxin binding, and closed state with remarkable effects on toxin binding. In this review, we summarized the diverse structural features of potassium channels explored using scorpion toxin tools and discuss future work in the field of scorpion toxin-potassium channel interactions.

Venom-Derived Peptide Modulators of Cation-Selective Channels: Friend, Foe or Frenemy

Ion channels play a key role in our body to regulate homeostasis and conduct electrical signals. With the help of advances in structural biology, as well as the discovery of numerous channel modulators derived from animal toxins, we are moving toward a better understanding of the function and mode of action of ion channels. Their ubiquitous tissue distribution and the physiological relevancies of their opening and closing suggest that cation channels are particularly attractive drug targets, and years of research has revealed a variety of natural toxins that bind to these channels and alter their function. In this review, we provide an introductory overview of the major cation ion channels: potassium channels, sodium channels and calcium channels, describe their venom-derived peptide modulators, and how these peptides provide great research and therapeutic value to both basic and translational medical research.

KCa3.1 Channel Modulators as Potential Therapeutic Compounds for Glioblastoma

Background

The intermediate-conductance Ca²⁺-activated K⁺ channel KCa3.1 is widely expressed in cells of the immune system such as T- and B-lymphocytes, mast cells, macrophages and microglia, but also found in dedifferentiated vascular smooth muscle cells, fibroblasts and many cancer cells including pancreatic, prostate, leukemia and glioblastoma. In all these cell types K_Ca3.1 plays an important role in cellular activation, migration and proliferation by regulating membrane potential and Ca²⁺ signaling.

Methods and Results

KCa3.1 therefore constitutes an attractive therapeutic target for diseases involving excessive proliferation or activation of one more of these cell types and researchers both in academia and in the pharmaceutical industry have developed several potent and selective small molecule inhibitors of K_Ca3.1. This article will briefly review the available compounds (TRAM-34, senicapoc, NS6180), their binding sites and mechanisms of action, and then discuss the potential usefulness of these compounds for the treatment of brain tumors based on their brain penetration and their efficacy in reducing microglia activation in animal models of ischemic stroke and Alzheimer’s disease.

Conclusion

Senicapoc, which has previously been in Phase III clinical trials, would be available for repurposing, and could be used to quickly translate findings made with other K_Ca3.1 blocking tool compounds into clinical trials.

Calcium-dependent potassium channels control proliferation of cardiac progenitor cells and bone marrow-derived mesenchymal stem cells

KEY POINTS

Ex vivo proliferated c-Kit⁺ endogenous cardiac progenitor cells (eCPCs) obtained from mouse and human cardiac tissues have been reported to express a wide range of functional ion channels. In contrast to previous reports in cultured c-Kit⁺ eCPCs, we found that ion currents were minimal in freshly isolated cells. However, inclusion of free Ca²⁺ intracellularly revealed a prominent inwardly rectifying current identified as the intermediate conductance Ca²⁺ -activated K⁺ current (KCa3.1) Electrical function of both c-Kit⁺ eCPCs and bone marrow-derived mesenchymal stem cells is critically governed by KCa3.1 calcium-dependent potassium channels. Ca²⁺ -induced increases in KCa3.1 conductance are necessary to optimize membrane potential during Ca²⁺ entry. Membrane hyperpolarization due to KCa3.1 activation maintains the driving force for Ca²⁺ entry that activates stem cell proliferation. Cardiac disease downregulates KCa3.1 channels in resident cardiac progenitor cells. Alterations in KCa3.1 may have pathophysiological and therapeutic significance in regenerative medicine.

ABSTRACT

Endogenous c-Kit⁺ cardiac progenitor cells (eCPCs) and bone marrow (BM)-derived mesenchymal stem cells (MSCs) are being developed for cardiac regenerative therapy, but a better understanding of their physiology is needed. Here, we addressed the unknown functional role of ion channels in freshly isolated eCPCs and expanded BM-MSCs using patch-clamp, microfluorometry and confocal microscopy. Isolated c-Kit⁺ eCPCs were purified from dog hearts by immunomagnetic selection. Ion currents were barely detectable in freshly isolated c-Kit⁺ eCPCs with buffering of intracellular calcium (Ca²⁺_i ). Under conditions allowing free intracellular Ca²⁺ , freshly isolated c-Kit⁺ eCPCs and ex vivo proliferated BM-MSCs showed prominent voltage-independent conductances that were sensitive to intermediate-conductance K⁺ -channel (KCa3.1 current, I_KCa3.1 ) blockers and corresponding gene (KCNN4)-expression knockdown. Depletion of Ca²⁺_i induced membrane-potential (V_mem ) depolarization, while store-operated Ca²⁺ entry (SOCE) hyperpolarized V_mem in both cell types. The hyperpolarizing SOCE effect was substantially reduced by I_KCa3.1 or SOCE blockade (TRAM-34, 2-APB), and I_KCa3.1 blockade (TRAM-34) or KCNN4-knockdown decreased the Ca²⁺ entry resulting from SOCE. I_KCa3.1 suppression reduced c-Kit⁺ eCPC and BM-MSC proliferation, while significantly altering the profile of cyclin expression. I_KCa3.1 was reduced in c-Kit⁺ eCPCs isolated from dogs with congestive heart failure (CHF), along with corresponding KCNN4 mRNA. Under perforated-patch conditions to maintain physiological [Ca²⁺ ]_i , c-Kit⁺ eCPCs from CHF dogs had less negative resting membrane potentials (-58 ± 7 mV) versus c-Kit⁺ eCPCs from control dogs (-73 ± 3 mV, P < 0.05), along with slower proliferation. Our study suggests that Ca²⁺ -induced increases in I_KCa3.1 are necessary to optimize membrane potential during the Ca²⁺ entry that activates progenitor cell proliferation, and that alterations in KCa3.1 may have pathophysiological and therapeutic significance in regenerative medicine.

Structure, Gating and Basic Functions of the Ca2+-activated K Channel of Intermediate Conductance

BACKGROUND

The KCa3.1 channel is the intermediate-conductance member of the Ca2+- activated K channel superfamily. It is widely expressed in excitable and non-excitable cells, where it plays a major role in a number of cell functions. This paper aims at illustrating the main structural, biophysical and modulatory properties of the KCa3.1 channel, and providing an account of experimental data on its role in volume regulation and Ca2+ signals.

METHODS

Research and online content related to the structure, structure/function relationship, and physiological role of the KCa3.1 channel are reviewed.

RESULTS

Expressed in excitable and non-excitable cells, the KCa3.1 channel is voltage independent, its opening being exclusively gated by the binding of intracellular Ca2+ to calmodulin, a Ca2+- binding protein constitutively associated with the C-terminus of each KCa3.1 channel α subunit. The KCa3.1 channel activates upon high affinity Ca2+ binding, and in highly coordinated fashion giving steep Hill functions and relatively low EC50 values (100-350 nM). This high Ca2+ sensitivity is physiologically modulated by closely associated kinases and phosphatases. The KCa3.1 channel is normally activated by global Ca2+ signals as resulting from Ca2+ released from intracellular stores, or by the refilling influx through store operated Ca2+ channels, but cases of strict functional coupling with Ca2+-selective channels are also found. KCa3.1 channels are highly expressed in many types of cells, where they play major roles in cell migration and death. The control of these complex cellular processes is achieved by KCa3.1 channel regulation of the driving force for Ca2+ entry from the extracellular medium, and by mediating the K+ efflux required for cell volume control.

CONCLUSION

Much work remains to be done to fully understand the structure/function relationship of the KCa3.1 channels. Hopefully, this effort will provide the basis for a beneficial modulation of channel activity under pathological conditions.

Arthropod toxins acting on neuronal potassium channels

Arthropod venoms are a rich mixture of biologically active compounds exerting different physiological actions across diverse phyla and affecting multiple organ systems including the central nervous system. Venom compounds can inhibit or activate ion channels, receptors and transporters with high specificity and affinity providing essential insights into ion channel function. In this review, we focus on arthropod toxins (scorpions, spiders, bees and centipedes) acting on neuronal potassium channels. A brief description of the K⁺ channels classification and structure is included and a compendium of neuronal K⁺ channels and the arthropod toxins that modify them have been listed. This article is part of the Special Issue entitled 'Venom-derived Peptides as Pharmacological Tools.'

Structural Determinants for the Selectivity of the Positive KCa3.1 Gating Modulator 5-Methylnaphtho[2,1-d]oxazol-2-amine (SKA-121)

Intermediate-conductance (K_Ca3.1) and small-conductance (K_Ca2) calcium-activated K⁺ channels are gated by calcium binding to calmodulin (CaM) molecules associated with the calmodulin-binding domain (CaM-BD) of these channels. The existing K_Ca activators, such as naphtho[1,2-d]thiazol-2-ylamine (SKA-31), 6,7-dichloro-1H-indole-2,3-dione 3-oxime (NS309), and 1-ethylbenzimidazolin-2-one (EBIO), activate both channel types with similar potencies. In a previous chemistry effort, we optimized the benzothiazole pharmacophore of SKA-31 toward K_Ca3.1 selectivity and identified 5-methylnaphtho[2,1-d]oxazol-2-amine (SKA-121), which exhibits 40-fold selectivity for K_Ca3.1 over K_Ca2.3. To understand why introduction of a single CH₃ group in five-position of the benzothiazole/oxazole system could achieve such a gain in selectivity for K_Ca3.1 over K_Ca2.3, we first localized the binding site of the benzothiazoles/oxazoles to the CaM-BD/CaM interface and then used computational modeling software to generate models of the K_Ca3.1 and K_Ca2.3 CaM-BD/CaM complexes with SKA-121. Based on a combination of mutagenesis and structural modeling, we suggest that all benzothiazole/oxazole-type K_Ca activators bind relatively "deep" in the CaM-BD/CaM interface and hydrogen bond with E54 on CaM. In K_Ca3.1, SKA-121 forms an additional hydrogen bond network with R362. In contrast, NS309 sits more "forward" and directly hydrogen bonds with R362 in K_Ca3.1. Mutating R362 to serine, the corresponding residue in K_Ca2.3 reduces the potency of SKA-121 by 7-fold, suggesting that R362 is responsible for the generally greater potency of K_Ca activators on K_Ca3.1. The increase in SKA-121's K_Ca3.1 selectivity compared with its parent, SKA-31, seems to be due to better overall shape complementarity and hydrophobic interactions with S372 and M368 on K_Ca3.1 and M72 on CaM at the K_Ca3.1-CaM-BD/CaM interface.

Structural Insights into the Atomistic Mechanisms of Action of Small Molecule Inhibitors Targeting the KCa3.1 Channel Pore

The intermediate-conductance Ca2+-activated K+ channel (KCa3.1) constitutes an attractive pharmacological target for immunosuppression, fibroproliferative disorders, atherosclerosis, and stroke. However, there currently is no available crystal structure of this medically relevant channel that could be used for structure-assisted drug design. Using the Rosetta molecular modeling suite we generated a molecular model of the KCa3.1 pore and tested the model by first confirming previously mapped binding sites and visualizing the mechanism of TRAM-34 (1-[(2-chlorophenyl)diphenylmethyl]-1H-pyrazole), senicapoc (2,2-bis-(4-fluorophenyl)-2-phenylacetamide), and NS6180 (4-[[3-(trifluoromethyl)phenyl]methyl]-2H-1,4-benzothiazin-3(4H)-one) inhibition at the atomistic level. All three compounds block ion conduction directly by fully or partially occupying the site that would normally be occupied by K+ before it enters the selectivity filter. We then challenged the model to predict the receptor sites and mechanisms of action of the dihydropyridine nifedipine and an isosteric 4-phenyl-pyran. Rosetta predicted receptor sites for nifedipine in the fenestration region and for the 4-phenyl-pyran in the pore lumen, which could both be confirmed by site-directed mutagenesis and electrophysiology. While nifedipine is thus not a pore blocker and might be stabilizing the channel in a nonconducting conformation or interfere with gating, the 4-phenyl-pyran was found to be a classical pore blocker that directly inhibits ion conduction similar to the triarylmethanes TRAM-34 and senicapoc. The Rosetta KCa3.1 pore model explains the mechanism of action of several KCa3.1 blockers at the molecular level and could be used for structure-assisted drug design.

DNA methyltransferase DNMT3a contributes to neuropathic pain by repressing Kcna2 in primary afferent neurons

Nerve injury induces changes in gene transcription in dorsal root ganglion (DRG) neurons, which may contribute to nerve injury-induced neuropathic pain. DNA methylation represses gene expression. Here, we report that peripheral nerve injury increases expression of the DNA methyltransferase DNMT3a in the injured DRG neurons via the activation of the transcription factor octamer transcription factor 1. Blocking this increase prevents nerve injury-induced methylation of the voltage-dependent potassium (Kv) channel subunit Kcna2 promoter region and rescues Kcna2 expression in the injured DRG and attenuates neuropathic pain. Conversely, in the absence of nerve injury, mimicking this increase reduces the Kcna2 promoter activity, diminishes Kcna2 expression, decreases Kv current, increases excitability in DRG neurons and leads to spinal cord central sensitization and neuropathic pain symptoms. These findings suggest that DNMT3a may contribute to neuropathic pain by repressing Kcna2 expression in the DRG.

Potassium Channel-blockers as Therapeutic Agents to Interfere with Bone Resorption of Periodontal Disease

Inflammatory lesions of periodontal disease contain all the cellular components, including abundant activated/memory T- and B-cells, necessary to control immunological interactive networks and to accelerate bone resorption by RANKL-dependent and -independent mechanisms. Blockade of RANKL function has been shown to ameliorate periodontal bone resorption and other osteopenic disorders without affecting inflammation. Development of therapies aimed at decreasing the expression of RANKL and pro-inflammatory cytokines by T-cells constitutes a promising strategy to ameliorate not only bone resorption, but also inflammation. Several reports have demonstrated that the potassium channels Kv1.3 and IKCa1, through the use of selective blockers, play important roles in T-cell-mediated events, including T-cell proliferation and the production of pro-inflammatory cytokines. More recently, a potassium channel-blocker for Kv1.3 has been shown to down-regulate bone resorption by decreasing the ratio of RANKL-to-OPG expression by memory-activated T-cells. In this article, we first summarize the mechanisms by which chronically activated/memory T-cells, in concert with B-cells and macrophages, trigger inflammatory bone resorption. Then, we describe the main structural and functional characteristics of potassium channels Kv1.3 and IKCa1 in some of the cells implicated in periodontal disease progression. Finally, this review elucidates some recent advances in the use of potassium channel-blockers of Kv1.3 and IKCa1 to ameliorate the clinical signs or side-effects of several immunological disorders and to decrease inflammatory bone resorption in periodontal disease. ABBREVIATIONS: AICD, activation-induced cell death; APC, antigen-presenting cells; B(K), large conductance; CRAC, calcium release-activated calcium channels; DC, dendritic cell; GAPDH, glyceraldehyde-3-phosphate dehydrogenase; IFN-γ, interferon-γ; IP3, inositol (1,4,5)-triphosphate; (K)ir, inward rectifier; JNK, c-Jun N-terminal kinase; I(K), intermediate conductance; LPS, lipopolysaccharide; L, ligand; MCSF, macrophage colony-stimulating factor; MHC, major histocompatibility complex; NFAT, nuclear factor of activated T-cells; RANK, receptor activator of nuclear factor-κB; TCM, central memory T-cells; TEM, effector memory T-cells; TNF, tumor necrosis factor; TRAIL, TNF-related apoptosis-inducing ligand; OPG, osteoprotegerin; Omp29, 29-kDa outer membrane protein; PKC, protein kinase C; PLC, phospholipase C; RT-PCR, reverse-transcriptase polymerase chain-reaction; S(K), small conductance; TCR, T-cell receptor; and (K)v, voltage-gated.

G9a participates in nerve injury-induced Kcna2 downregulation in primary sensory neurons

Nerve injury-induced downregulation of voltage-gated potassium channel subunit Kcna2 in the dorsal root ganglion (DRG) is critical for DRG neuronal excitability and neuropathic pain genesis. However, how nerve injury causes this downregulation is still elusive. Euchromatic histone-lysine N-methyltransferase 2, also known as G9a, methylates histone H3 on lysine residue 9 to predominantly produce a dynamic histone dimethylation, resulting in condensed chromatin and gene transcriptional repression. We showed here that blocking nerve injury-induced increase in G9a rescued Kcna2 mRNA and protein expression in the axotomized DRG and attenuated the development of nerve injury-induced pain hypersensitivity. Mimicking this increase decreased Kcna2 mRNA and protein expression, reduced Kv current, and increased excitability in the DRG neurons and led to spinal cord central sensitization and neuropathic pain-like symptoms. G9a mRNA is co-localized with Kcna2 mRNA in the DRG neurons. These findings indicate that G9a contributes to neuropathic pain development through epigenetic silencing of Kcna2 in the axotomized DRG.

Scorpion toxin peptide action at the ion channel subunit level

This review categorizes functionally validated actions of defined scorpion toxin (SCTX) neuropeptides across ion channel subclasses, highlighting key trends in this rapidly evolving field. Scorpion envenomation is a common event in many tropical and subtropical countries, with neuropharmacological actions, particularly autonomic nervous system modulation, causing significant mortality. The primary active agents within scorpion venoms are a diverse group of small neuropeptides that elicit specific potent actions across a wide range of ion channel classes. The identification and functional characterisation of these SCTX peptides has tremendous potential for development of novel pharmaceuticals that advance knowledge of ion channels and establish lead compounds for treatment of excitable tissue disorders. This review delineates the unique specificities of 320 individual SCTX peptides that collectively act on 41 ion channel subclasses. Thus the SCTX research field has significant translational implications for pathophysiology spanning neurotransmission, neurohumoral signalling, sensori-motor systems and excitation-contraction coupling. This article is part of the Special Issue entitled 'Venom-derived Peptides as Pharmacological Tools.'

Small-Conductance Ca2+-Activated Potassium Channels Negatively Regulate Aldosterone Secretion in Human Adrenocortical Cells

Aldosterone, which plays a key role in maintaining water and electrolyte balance, is produced by zona glomerulosa cells of the adrenal cortex. Autonomous overproduction of aldosterone from zona glomerulosa cells causes primary hyperaldosteronism. Recent clinical studies have highlighted the pathological role of the KCNJ5 potassium channel in primary hyperaldosteronism. Our objective was to determine whether small-conductance Ca(2+)-activated potassium (SK) channels may also regulate aldosterone secretion in human adrenocortical cells. We found that apamin, the prototypic inhibitor of SK channels, decreased membrane voltage, raised intracellular Ca(2+) and dose dependently increased aldosterone secretion from human adrenocortical H295R cells. By contrast, 1-Ethyl-2-benzimidazolinone, an agonist of SK channels, antagonized apamin's action and decreased aldosterone secretion. Commensurate with an increase in aldosterone production, apamin increased mRNA expression of steroidogenic acute regulatory protein and aldosterone synthase that control the early and late rate-limiting steps in aldosterone biosynthesis, respectively. In addition, apamin increased angiotensin II-stimulated aldosterone secretion, whereas 1-Ethyl-2-benzimidazolinone suppressed both angiotensin II- and high K(+)-stimulated production of aldosterone in H295R cells. These findings were supported by apamin-modulation of basal and angiotensin II-stimulated aldosterone secretion from acutely prepared slices of human adrenals. We conclude that SK channel activity negatively regulates aldosterone secretion in human adrenocortical cells. Genetic association studies are necessary to determine whether mutations in SK channel subtype 2 genes may also drive aldosterone excess in primary hyperaldosteronism.

IKCa channels are a critical determinant of the slow AHP in CA1 pyramidal neurons

Control over the frequency and pattern of neuronal spike discharge depends on Ca2+-gated K+ channels that reduce cell excitability by hyperpolarizing the membrane potential. The Ca2+-dependent slow afterhyperpolarization (sAHP) is one of the most prominent inhibitory responses in the brain, with sAHP amplitude linked to a host of circuit and behavioral functions, yet the channel that underlies the sAHP has defied identification for decades. Here, we show that intermediate-conductance Ca2+-dependent K+ (IKCa) channels underlie the sAHP generated by trains of synaptic input or postsynaptic stimuli in CA1 hippocampal pyramidal cells. These findings are significant in providing a molecular identity for the sAHP of central neurons that will identify pharmacological tools capable of potentially modifying the several behavioral or disease states associated with the sAHP.

Dorsal root ganglion myeloid zinc finger protein 1 contributes to neuropathic pain after peripheral nerve trauma

Peripheral nerve injury-induced changes in gene transcription and translation in primary sensory neurons of the dorsal root ganglion (DRG) are considered to contribute to neuropathic pain genesis. Transcription factors control gene expression. Peripheral nerve injury increases the expression of myeloid zinc finger protein 1 (MZF1), a transcription factor, and promotes its binding to the voltage-gated potassium 1.2 (Kv1.2) antisense (AS) RNA gene in the injured DRG. However, whether DRG MZF1 participates in neuropathic pain is still unknown. Here, we report that blocking the nerve injury-induced increase of DRG MZF1 through microinjection of MZF1 siRNA into the injured DRG attenuated the initiation and maintenance of mechanical, cold, and thermal pain hypersensitivities in rats with chronic constriction injury (CCI) of the sciatic nerve, without affecting locomotor functions and basal responses to acute mechanical, heat, and cold stimuli. Mimicking the nerve injury-induced increase of DRG MZF1 through microinjection of recombinant adeno-associated virus 5 expressing full-length MZF1 into the DRG produced significant mechanical, cold, and thermal pain hypersensitivities in naive rats. Mechanistically, MZF1 participated in CCI-induced reductions in Kv1.2 mRNA and protein and total Kv current and the CCI-induced increase in neuronal excitability through MZF1-triggered Kv1.2 AS RNA expression in the injured DRG neurons. MZF1 is likely an endogenous trigger of neuropathic pain and might serve as a potential target for preventing and treating this disorder.

Role of the potassium channel KCa3.1 in diabetic nephropathy

There is an urgent need to identify novel interventions for mitigating the progression of diabetic nephropathy. Diabetic nephropathy is characterized by progressive renal fibrosis, in which tubulointerstitial fibrosis has been shown to be the final common pathway of all forms of chronic progressive renal disease, including diabetic nephropathy. Therefore targeting the possible mechanisms that drive this process may provide novel therapeutics which allow the prevention and potentially retardation of the functional decline in diabetic nephropathy. Recently, the Ca2+-activated K+ channel KCa3.1 (KCa3.1) has been suggested as a potential therapeutic target for nephropathy, based on its ability to regulate Ca2+ entry into cells and modulate Ca2+-signalling processes. In the present review, we focus on the physiological role of KCa3.1 in those cells involved in the tubulointerstitial fibrosis, including proximal tubular cells, fibroblasts, inflammatory cells (T-cells and macrophages) and endothelial cells. Collectively these studies support further investigation into KCa3.1 as a therapeutic target in diabetic nephropathy.

KCa and Ca(2+) channels: the complex thought

Potassium channels belong to the largest and the most diverse super-families of ion channels. Among them, Ca(2+)-activated K(+) channels (KCa) comprise many members. Based on their single channel conductance they are divided into three subfamilies: big conductance (BKCa), intermediate conductance (IKCa) and small conductance (SKCa; SK1, SK2 and SK3). Ca(2+) channels are divided into two main families, voltage gated/voltage dependent Ca(2+) channels and non-voltage gated/voltage independent Ca(2+) channels. Based on their electrophysiological and pharmacological properties and on the tissue where there are expressed, voltage gated Ca(2+) channels (Cav) are divided into 5 families: T-type, L-type, N-type, P/Q-type and R-type Ca(2+). Non-voltage gated Ca(2+) channels comprise the TRP (TRPC, TRPV, TRPM, TRPA, TRPP, TRPML and TRPN) and Orai (Orai1 to Orai3) families and their partners STIM (STIM1 to STIM2). A depolarization is needed to activate voltage-gated Ca(2+) channels while non-voltage gated Ca(2+) channels are activated by Ca(2+) depletion of the endoplasmic reticulum stores (SOCs) or by receptors (ROCs). These two Ca(2+) channel families also control constitutive Ca(2+) entries. For reducing the energy consumption and for the fine regulation of Ca(2+), KCa and Ca(2+) channels appear associated as complexes in excitable and non-excitable cells. Interestingly, there is now evidence that KCa-Ca(2+) channel complexes are also found in cancer cells and contribute to cancer-associated functions such as cell proliferation, cell migration and the capacity to develop metastases. This article is part of a Special Issue entitled: Calcium signaling in health and disease. Guest Editors: Geert Bultynck, Jacques Haiech, Claus W. Heizmann, Joachim Krebs, and Marc Moreau.

Impaired neuropathic pain and preserved acute pain in rats overexpressing voltage-gated potassium channel subunit Kv1.2 in primary afferent neurons

Voltage-gated potassium (Kv) channels are critical in controlling neuronal excitability and are involved in the induction of neuropathic pain. Therefore, Kv channels might be potential targets for prevention and/or treatment of this disorder. We reported here that a majority of dorsal root ganglion (DRG) neurons were positive for Kv channel alpha subunit Kv1.2. Most of them were large and medium, although there was a variety of sizes. Peripheral nerve injury caused by lumbar (L)₅ spinal nerve ligation (SNL) produced a time-dependent reduction in the number of Kv1.2-positive neurons in the ipsilateral L₅ DRG, but not in the contralateral L₅ DRG. Such reduction was also observed in the ipsilateral L₅ DRG on day 7 after sciatic nerve axotomy. Rescuing nerve injury-induced reduction of Kv1.2 in the injured L₅ DRG attenuated the development and maintenance of SNL-induced pain hypersensitivity without affecting acute pain and locomotor function. This effect might be attributed to the prevention of SNL-induced upregulation of endogenous Kv1.2 antisense RNA, in addition to the increase in Kv1.2 protein expression, in the injured DRG. Our findings suggest that Kv1.2 may be a novel potential target for preventing and/or treating neuropathic pain.

BcsTx3 is a founder of a novel sea anemone toxin family of potassium channel blocker

Sea anemone venoms have become a rich source of peptide toxins which are invaluable tools for studying the structure and functions of ion channels. In this work, BcsTx3, a toxin found in the venom of a Bunodosoma caissarum (population captured at the Saint Peter and Saint Paul Archipelago, Brazil) was purified and biochemically and pharmacologically characterized. The pharmacological effects were studied on 12 different subtypes of voltage-gated potassium channels (K(V)1.1-K(V)1.6; K(V)2.1; K(V)3.1; K(V)4.2; K(V)4.3; hERG and Shaker IR) and three cloned voltage-gated sodium channel isoforms (Na(V)1.2, Na(V)1.4 and BgNa(V)1.1) expressed in Xenopus laevis oocytes. BcsTx3 shows a high affinity for Drosophila Shaker IR channels over rKv1.2, hKv1.3 and rKv1.6, and is not active on NaV channels. Biochemical characterization reveals that BcsTx3 is a 50 amino acid peptide crosslinked by four disulfide bridges, and sequence comparison allowed BcsTx3 to be classified as a novel type of sea anemone toxin acting on K(V) channels. Moreover, putative toxins homologous to BcsTx3 from two additional actiniarian species suggest an ancient origin of this newly discovered toxin family.

Molecular basis of potassium channels in pancreatic duct epithelial cells

Potassium channels regulate excitability, epithelial ion transport, proliferation, and apoptosis. In pancreatic ducts, K⁺ channels hyperpolarize the membrane potential and provide the driving force for anion secretion. This review focuses on the molecular candidates of functional K⁺ channels in pancreatic duct cells, including KCNN4 (K_Ca3.1), KCNMA1 (K_Ca1.1), KCNQ1 (K_v7.1), KCNH2 (K_v11.1), KCNH5 (K_v10.2), KCNT1 (K_Ca4.1), KCNT2 (K_Ca4.2), and KCNK5 (K_2P5.1). We will give an overview of K⁺ channels with respect to their electrophysiological and pharmacological characteristics and regulation, which we know from other cell types, preferably in epithelia, and, where known, their identification and functions in pancreatic ducts and in adenocarcinoma cells. We conclude by pointing out some outstanding questions and future directions in pancreatic K⁺ channel research with respect to the physiology of secretion and pancreatic pathologies, including pancreatitis, cystic fibrosis, and cancer, in which the dysregulation or altered expression of K⁺ channels may be of importance.

OcyKTx2, a new K⁺-channel toxin characterized from the venom of the scorpion Opisthacanthus cayaporum

Opisthacanthus cayaporum belongs to the Liochelidae family, and the scorpions from this genus occur in southern Africa, Central America and South America and, therefore, can be considered a true Gondwana heritage. In this communication, the isolation, primary structure characterization, and K⁺-channel blocking activity of new peptide from this scorpion venom are reported. OcyKTx2 is a 34 amino acid long peptide with four disulfide bridges and molecular mass of 3807 Da. Electrophysiological assays conducted with pure OcyKTx2 showed that this toxin reversibly blocks Shaker B K⁺-channels with a Kd of 82 nM, and presents an even better affinity toward hKv1.3, blocking it with a Kd of ∼18 nM. OcyKTx2 shares high sequence identity with peptides belonging to subfamily 6 of α-KTxs that clustered very closely in the phylogenetic tree included here. Sequence comparison, chain length and number of disulfide bridges analysis classify OcyKTx2 into subfamily 6 of the α-KTx scorpion toxins (systematic name, α-KTx6.17).

A long noncoding RNA contributes to neuropathic pain by silencing Kcna2 in primary afferent neurons

Neuropathic pain is a refractory disease characterized by maladaptive changes in gene transcription and translation in the sensory pathway. Long noncoding RNAs (lncRNAs) are emerging as new players in gene regulation, but how lncRNAs operate in the development of neuropathic pain is unclear. Here we identify a conserved lncRNA, named Kcna2 antisense RNA, for a voltage-dependent potassium channel mRNA, Kcna2, in first-order sensory neurons of rat dorsal root ganglion (DRG). Peripheral nerve injury increased Kcna2 antisense RNA expression in injured DRG through activation of myeloid zinc finger protein 1, a transcription factor that binds to the Kcna2 antisense RNA gene promoter. Mimicking this increase downregulated Kcna2, reduced total voltage-gated potassium current, increased excitability in DRG neurons and produced neuropathic pain symptoms. Blocking this increase reversed nerve injury-induced downregulation of DRG Kcna2 and attenuated development and maintenance of neuropathic pain. These findings suggest endogenous Kcna2 antisense RNA as a therapeutic target for the treatment of neuropathic pain.

Two conserved arginine residues from the SK3 potassium channel outer vestibule control selectivity of recognition by scorpion toxins

Potassium channel functions are often deciphered by using selective and potent scorpion toxins. Among these toxins, only a limited subset is capable of selectively blocking small conductance Ca(2+)-activated K(+) (SK) channels. The structural bases of this selective SK channel recognition remain unclear. In this work, we demonstrate the key role of the electric charges of two conserved arginine residues (Arg-485 and Arg-489) from the SK3 channel outer vestibule in the selective recognition by the SK3-blocking BmP05 toxin. Indeed, individually substituting these residues with histidyl or lysyl (maintaining the positive electric charge partially or fully), although decreasing BmP05 affinity, still preserved the toxin sensitivity profile of the SK3 channel (as evidenced by the lack of recognition by many other types of potassium channel-sensitive charybdotoxin). In contrast, when Arg-485 or Arg-489 of the SK3 channel was mutated to an acidic (Glu) or alcoholic (Ser) amino acid residue, the channel lost its sensitivity to BmP05 and became susceptible to the "new" blocking activity by charybdotoxin. In addition to these SK3 channel basic residues important for sensitivity, two acidic residues, Asp-492 and Asp-518, also located in the SK3 channel outer vestibule, were identified as being critical for toxin affinity. Furthermore, molecular modeling data indicate the existence of a compact SK3 channel turret conformation (like a peptide screener), where the basic rings of Arg-485 and Arg-489 are stabilized by strong ionic interactions with Asp-492 and Asp-518. In conclusion, the unique properties of Arg-485 and Arg-489 (spatial orientations and molecular interactions) in the SK3 channel account for its toxin sensitivity profile.

Structural basis of the selective block of Kv1.2 by maurotoxin from computer simulations

The 34-residue polypeptide maurotoxin (MTx) isolated from scorpion venoms selectively inhibits the current of the voltage-gated potassium channel Kv1.2 by occluding the ion conduction pathway. Here using molecular dynamics simulation as a docking method, the binding modes of MTx to three closely related channels (Kv1.1, Kv1.2 and Kv1.3) are examined. We show that MTx forms more favorable electrostatic interactions with the outer vestibule of Kv1.2 compared to Kv1.1 and Kv1.3, consistent with the selectivity of MTx for Kv1.2 over Kv1.1 and Kv1.3 observed experimentally. One salt bridge in the bound complex of MTx-Kv1.2 forms and breaks in a simulation period of 20ns, suggesting the dynamic nature of toxin-channel interactions. The toxin selectivity likely arises from the differences in the shape of the channel outer vestibule, giving rise to distinct orientations of MTx on block. Potential of mean force calculations show that MTx blocks Kv1.1, Kv1.2 and Kv1.3 with an IC₅₀ value of 6 µM, 0.6nM and 18 µM, respectively.

Expression and Role of the Intermediate-Conductance Calcium-Activated Potassium Channel KCa3.1 in Glioblastoma

Glioblastomas are characterized by altered expression of several ion channels that have important consequences in cell functions associated with their aggressiveness, such as cell survival, proliferation, and migration. Data on the altered expression and function of the intermediate-conductance calcium-activated K (KCa3.1) channels in glioblastoma cells have only recently become available. This paper aims to (i) illustrate the main structural, biophysical, pharmacological, and modulatory properties of the KCa3.1 channel, (ii) provide a detailed account of data on the expression of this channel in glioblastoma cells, as compared to normal brain tissue, and (iii) critically discuss its major functional roles. Available data suggest that KCa3.1 channels (i) are highly expressed in glioblastoma cells but only scantly in the normal brain parenchima, (ii) play an important role in the control of glioblastoma cell migration. Altogether, these data suggest KCa3.1 channels as potential candidates for a targeted therapy against this tumor.

Structural and functional diversity of acidic scorpion potassium channel toxins

Background

Although the basic scorpion K⁺ channel toxins (KTxs) are well-known pharmacological tools and potential drug candidates, characterization the acidic KTxs still has the great significance for their potential selectivity towards different K⁺ channel subtypes. Unfortunately, research on the acidic KTxs has been ignored for several years and progressed slowly.

Principal Findings

Here, we describe the identification of nine new acidic KTxs by cDNA cloning and bioinformatic analyses. Seven of these toxins belong to three new α-KTx subfamilies (α-KTx28, α-KTx29, and α-KTx30), and two are new members of the known κ-KTx2 subfamily. ImKTx104 containing three disulfide bridges, the first member of the α-KTx28 subfamily, has a low sequence homology with other known KTxs, and its NMR structure suggests ImKTx104 adopts a modified cystine-stabilized α-helix-loop-β-sheet (CS-α/β) fold motif that has no apparent α-helixs and β-sheets, but still stabilized by three disulfide bridges. These newly described acidic KTxs exhibit differential pharmacological effects on potassium channels. Acidic scorpion toxin ImKTx104 was the first peptide inhibitor found to affect KCNQ1 channel, which is insensitive to the basic KTxs and is strongly associated with human cardiac abnormalities. ImKTx104 selectively inhibited KCNQ1 channel with a K_d of 11.69 µM, but was less effective against the basic KTxs-sensitive potassium channels. In addition to the ImKTx104 toxin, HeTx204 peptide, containing a cystine-stabilized α-helix-loop-helix (CS-α/α) fold scaffold motif, blocked both Kv1.3 and KCNQ1 channels. StKTx23 toxin, with a cystine-stabilized α-helix-loop-β-sheet (CS-α/β) fold motif, could inhibit Kv1.3 channel, but not the KCNQ1 channel.

Conclusions/Significance

These findings characterize the structural and functional diversity of acidic KTxs, and could accelerate the development and clinical use of acidic KTxs as pharmacological tools and potential drugs.

Developing a comparative docking protocol for the prediction of peptide selectivity profiles: investigation of potassium channel toxins

During the development of selective peptides against highly homologous targets, a reliable tool is sought that can predict information on both mechanisms of binding and relative affinities. These tools must first be tested on known profiles before application on novel therapeutic candidates. We therefore present a comparative docking protocol in HADDOCK using critical motifs, and use it to “predict” the various selectivity profiles of several major αKTX scorpion toxin families versus K_v1.1, K_v1.2 and K_v1.3. By correlating results across toxins of similar profiles, a comprehensive set of functional residues can be identified. Reasonable models of channel-toxin interactions can be then drawn that are consistent with known affinity and mutagenesis. Without biological information on the interaction, HADDOCK reproduces mechanisms underlying the universal binding of αKTX-2 toxins, and K_v1.3 selectivity of αKTX-3 toxins. The addition of constraints encouraging the critical lysine insertion confirms these findings, and gives analogous explanations for other families, including models of partial pore-block in αKTX-6. While qualitatively informative, the HADDOCK scoring function is not yet sufficient for accurate affinity-ranking. False minima in low-affinity complexes often resemble true binding in high-affinity complexes, despite steric/conformational penalties apparent from visual inspection. This contamination significantly complicates energetic analysis, although it is usually possible to obtain correct ranking via careful interpretation of binding-well characteristics and elimination of false positives. Aside from adaptations to the broader potassium channel family, we suggest that this strategy of comparative docking can be extended to other channels of interest with known structure, especially in cases where a critical motif exists to improve docking effectiveness.

Therapeutic potential of KCa3.1 blockers: recent advances and promising trends

The Ca(2+)-activated K(+) channel K(Ca)3.1 regulates membrane potential and calcium signaling in erythrocytes, activated T and B cells, macrophages, microglia, vascular endothelium, epithelia, and proliferating vascular smooth muscle cells and fibroblasts. K(Ca)3.1 has therefore been suggested as a potential therapeutic target for diseases such as sickle cell anemia, asthma, coronary restenosis after angioplasty, atherosclerosis, kidney fibrosis and autoimmunity, where activation and excessive proliferation of one or more of these cell types is involved in the pathology. This article will review the physiology and pharmacology of K(Ca)3.1 and critically examine the available preclinical and clinical data validating K(Ca)3.1 as a therapeutic target.

The Lymphocyte Potassium Channels Kv1.3 and KCa3.1 as Targets for Immunosuppression

The voltage-gated Kv1.3 and the calcium-activated KCa3.1 potassium channel modulate many calcium-dependent cellular processes in immune cells, including T-cell activation and proliferation, and have therefore been proposed as novel therapeutic targets for immunomodulation. Kv1.3 is highly expressed in CCR7(-) effector memory T cells and is emerging as a target for T-cell mediated diseases like multiple sclerosis, rheumatoid arthritis, type-1 diabetes mellitus, allergic contact dermatitis, and psoriasis. KCa3.1 in contrast is expressed in CCR7(+) naïve and central memory T cells, as well as in mast cells, macrophages, dedifferentiated vascular smooth muscle cells, fibroblasts, vascular endothelium, and airway epithelium. Given this expression pattern, KCa3.1 is a potential therapeutic target for conditions ranging from inflammatory bowel disease, multiple sclerosis, arthritis, and asthma to cardiovascular diseases like atherosclerosis and post-angioplasty restenosis. Results from animal studies have been supportive of the therapeutic potential of both Kv1.3 and KCa3.1 blockers and have also not shown any toxicities associated with pharmacological Kv1.3 and KCa3.1 blockade. To date, two compounds targeting Kv1.3 are in preclinical development but, so far, no Kv1.3 blocker has advanced into clinical trials. KCa3.1 blockers, on the other hand, have been evaluated in clinical trials for sickle cell anemia and exercise-induced asthma, but have so far not shown efficacy. However, the trial results support KCa3.1 as a safe therapeutic target, and will hopefully help enable clinical trials for other medical conditions that might benefit from KCa3.1 blockade.

Negative gating modulation by (R)-N-(benzimidazol-2-yl)-1,2,3,4-tetrahydro-1-naphthylamine (NS8593) depends on residues in the inner pore vestibule: pharmacological evidence of deep-pore gating of K(Ca)2 channels

Acting as a negative gating modulator, (R)-N-(benzimidazol-2-yl)-1,2,3,4-tetrahydro-1-naphthylamine (NS8593) shifts the apparent Ca(2+)-dependence of the small-conductance Ca(2+)-activated K(+) channels K(Ca)2.1-2.3 to higher Ca(2+) concentrations. Similar to the positive K(Ca) channel-gating modulators 1-ethyl-2-benzimidazolinone (1-EBIO) and cyclohexyl-[2-(3,5-dimethyl-pyrazol-1-yl)-6-methylpyrimidin-4-yl]-amine (CyPPA), the binding site for NS8593 has been assumed to be located in the C-terminal region, in which these channels interact with their Ca(2+) sensor calmodulin. However, by using a progressive chimeric approach, we were able to localize the site-of-action of NS8593 to the K(Ca)2 pore. For example, when we transferred the C terminus from the NS8593-insensitive intermediate-conductance K(Ca)3.1 channel to K(Ca)2.3, the chimeric channel remained as sensitive to NS8593 as wild-type K(Ca)2.3. In contrast, when we transferred the K(Ca)2.3 pore to K(Ca)3.1, the channel became sensitive to NS8593. Using site-directed mutagenesis, we subsequently identified two specific residues in the inner vestibule of K(Ca)2.3 (Ser507 and Ala532) that determined the effect of NS8593. Mutation of these residues to the corresponding residues in K(Ca)3.1 (Thr250 and Val275) made K(Ca)2.3 insensitive to NS8593, whereas introduction of serine and alanine into K(Ca)3.1 was sufficient to render this channel highly sensitive to NS8593. It is noteworthy that the same two residue positions have been found previously to mediate sensitivity of K(Ca)3.1 to clotrimazole and 1-[(2-chlorophenyl)diphenylmethyl]-1H-pyrazole (TRAM-34). The location of Ser507 in the pore-loop near the selectivity filter and Ala532 in an adjacent position in S6 are within the region predicted to contain the K(Ca)2 channel gate. Hence, we propose that NS8593-mediated gating modulation occurs via interaction with gating structures at a position deep within the inner pore vestibule.

Differential molecular information of maurotoxin peptide recognizing IK(Ca) and Kv1.2 channels explored by computational simulation

BACKGROUND

Scorpion toxins are invaluable tools for ion channel research and are potential drugs for human channelopathies. However, it is still an open task to determine the molecular basis underlying the diverse interactions between toxin peptides and ion channels. The inhibitory peptide Maurotoxin (MTX) recognized the distantly related IK(Ca) and Kv1.2 channel with approximately the same potency and using the same functional residues, their differential binding mechanism remain elusive. In this study, we applied computational methods to explore the differential binding modes of MTX to Kv1.2 and IK(Ca) channels, which would help to understand the diversity of channel-toxin interactions and accelerate the toxin-based drug design.

RESULTS

A reasonably stable MTX-IK(Ca) complex was obtained by combining various computational methods and by in-depth comparison with the previous model of the MTX-Kv1.2 complex. Similarly, MTX adopted the β-sheet structure as the interacting surface for binding both channels, with Lys23 occluding the pore. In contrast, the other critical residues Lys27, Lys30, and Tyr32 of MTX adopted distinct interactions when associating with the IK(Ca) channel. In addition, the residues Gln229, Ala230, Ala233, and Thr234 on the IK(Ca) channel turret formed polar and non-polar interactions with MTX, whereas the turret of Kv1.2 was almost not involved in recognizing MTX. In all, the pairs of interacting residues on MTX and the IK(Ca) channel of the bound complex indicated that electrostatic and Van der Waal interactions contributed equally to the formation of a stable MTX-IK(Ca) complex, in contrast to the MTX-Kv1.2 binding that is dominantly mediated by electrostatic forces.

CONCLUSIONS

Despite sharing similar pharmacological profiles toward both IK(Ca) and Kv1.2 channels, MTX adopted totally diverging modes in the two association processes. All the molecular information unveiled here could not only offer a better understanding about the structural differences between the IK(Ca) and Kv1.2 channels, but also provide novel structural clues that will help in the designing of more selective molecular probes to discriminate between these two channels.

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.

Ca2+-activated K channels in parotid acinar cells: The functional basis for the hyperpolarized activation of BK channels

Fluid secretion relies on a close interplay between Ca(2+)-activated Cl and K channels. Salivary acinar cells contain both large conductance, BK, and intermediate conductance, IK1, K channels. Physiological fluid secretion occurs with only modest (<500 nM) increases in intracellular Ca(2+) levels but BK channels in many cell types and in heterologous expression systems require very high concentrations for significant activation. We report here our efforts to understand this apparent contradiction. We determined the Ca(2+) dependence of IK1 and BK channels in mouse parotid acinar cells. IK1 channels activated with an apparent Ca(2+) affinity of about 350 nM and a Hill coefficient near 3. Native parotid BK channels activated at similar Ca(2+) levels unlike the BK channels in other cell types. Since the parotid BK channel is encoded by an uncommon splice variant, we examined this clone in a heterologous expression system. In contrast to the native parotid channel, activation of this expressed "parSlo" channel required very high levels of Ca(2+). In order to understand the functional basis for the special properties of the native channels, we analyzed the parotid BK channel in the context of the Horrigan-Aldrich model of BK channel gating. We found that the shifted activation of parotid BK channels resulted from a hyperpolarizing shift of the voltage dependence of voltage sensor activation and channel opening and included a large change in the coupling of these two processes.

The functional network of ion channels in T lymphocytes

For more than 25 years, it has been widely appreciated that Ca2+ influx is essential to trigger T-lymphocyte activation. Patch clamp analysis, molecular identification, and functional studies using blockers and genetic manipulation have shown that a unique contingent of ion channels orchestrates the initiation, intensity, and duration of the Ca2+ signal. Five distinct types of ion channels--Kv1.3, KCa3.1, Orai1+ stromal interacting molecule 1 (STIM1) [Ca2+-release activating Ca2+ (CRAC) channel], TRPM7, and Cl(swell)--comprise a network that performs functions vital for ongoing cellular homeostasis and for T-cell activation, offering potential targets for immunomodulation. Most recently, the roles of STIM1 and Orai1 have been revealed in triggering and forming the CRAC channel following T-cell receptor engagement. Kv1.3, KCa3.1, STIM1, and Orai1 have been found to cluster at the immunological synapse following contact with an antigen-presenting cell; we discuss how channels at the synapse might function to modulate local signaling. Immuno-imaging approaches are beginning to shed light on ion channel function in vivo. Importantly, the expression pattern of Ca2+ and K+ channels and hence the functional network can adapt depending upon the state of differentiation and activation, and this allows for different stages of an immune response to be targeted specifically.

Neurotoxic and cytotoxic effects of venom from different populations of the Egyptian Scorpio maurus palmatus

Neurotoxic and cytotoxic effects of venoms from Scorpio maurus palmatus taken from different populations were assessed for geographic based variability in toxicity, and to evaluate their insecticidal potency. Scorpions were collected from four regions. Three locations were mutually isolated pockets in the arid area of Southern Sinai. The fourth sample was collected from a population inhabiting the semi-arid environment of Western Mediterranean Coastal Desert. The neurotoxic (paralytic) effect of the venom from each population was assayed by its ability to induce permanent disability in adult cockroaches within 3h. Venom was applied using microinjection techniques through an intersegmental membrane. Probit analysis was used to calculate the Paralytic Effective Dose (PED(50), ng/100mg). Levels of glutathione, lipid peroxidation, protein carbonyl content and nitric oxide, as well as the activities of superoxide dismutase, catalase and cholinesterase, were measured to assess the cytotoxicity of the venom. The results show that the injected venom from each population induced obvious spasticity, followed by flaccid paralysis. All the tested biochemical parameters, except glutathione content, revealed significant differences in toxicity in venom taken from the different scorpion populations. We conclude that (i) the venom of this scorpion has significant neurotoxic and cytotoxic effects on insect cells, (ii) its efficacy, as assessed by the PED(50) unit, exhibited variation across its geographic range, and (iii) components in the venom may have the potential for being developed into effective and environmentally friendly bioinsecticides.

The K+ channels K(Ca)3.1 and K(v)1.3 as novel targets for asthma therapy

Asthma affects 10% of the UK population and is an important cause of morbidity and mortality at all ages. Current treatments are either ineffective or carry unacceptable side effects for a number of patients; in consequence, development of new approaches to therapy are important. Ion channels are emerging as attractive therapeutic targets in a variety of non-excitable cells. Ion channels conducting K(+) modulate the activity of several structural and inflammatory cells which play important roles in the pathophysiology of asthma. Two channels of particular interest are the voltage-gated K(+) channel K(v)1.3 and the intermediate conductance Ca(2+)-activated K(+) channel K(Ca)3.1 (also known as IK(Ca)1 or SK4). K(v)1.3 is expressed in IFNgamma-producing T cells while K(Ca)3.1 is expressed in T cells, mast cells, macrophages, airway smooth muscle cells, fibroblasts and epithelial cells. Both channels play important roles in cell activation, migration, and proliferation through the regulation of membrane potential and calcium signalling. We hypothesize that K(Ca)3.1- and/or K(v)1.3-dependent cell processes are one of the common denominators in asthma pathophysiology. If true, these channels might serve as novel targets for the treatment of asthma. Emerging evidence lends support to this hypothesis. Further validation through the study of the role that these channels play in normal and asthmatic airway cell (patho)physiology and in vivo models will provide further justification for the assessment of small molecule blockers of K(v)1.3 and K(Ca)3.1 in the treatment of asthma.

Molecular and cellular basis of small--and intermediate-conductance, calcium-activated potassium channel function in the brain

Small conductance calcium-activated potassium (SK or K_Ca2) channels link intracellular calcium transients to membrane potential changes. SK channel subtypes present different pharmacology and distribution in the nervous system. The selective blocker apamin, SK enhancers and mice lacking specific SK channel subunits have revealed multifaceted functions of these channels in neurons, glia and cerebral blood vessels. SK channels regulate neuronal firing by contributing to the afterhyperpolarization following action potentials and mediating I_AHP, and partake in a calcium-mediated feedback loop with NMDA receptors, controlling the threshold for induction of hippocampal long-term potentiation. The function of distinct SK channel subtypes in different neurons often results from their specific coupling to different calcium sources. The prominent role of SK channels in the modulation of excitability and synaptic function of limbic, dopaminergic and cerebellar neurons hints at their possible involvement in neuronal dysfunction, either as part of the causal mechanism or as potential therapeutic targets.