Engineered specific and high-affinity inhibitor for a subtype of inward-rectifier K+ channels

Neuromedin B excites central lateral amygdala neurons and reduces cardiovascular output and fear-potentiated startle

Neuromedin B (NMB) and gastrin-releasing peptide (GRP) are the two mammalian analogs in the bombesin peptide family that exert a variety of actions including emotional processing, appetitive behaviors, cognition, and tumor growth. The bombesin-like peptides interact with three receptors: the NMB-preferring bombesin 1 (BB1) receptors, the GRP-preferring bombesin 2 (BB2) receptors and the orphan bombesin 3 (BB3) receptors. Whereas, injection of bombesin into the central amygdala reduces satiety and modulates blood pressure, the underlying cellular and molecular mechanisms have not been determined. As administration of bombesin induces the expression of Fos in the lateral nucleus of the central amygdala (CeL) which expresses BB1 receptors, we probed the effects of NMB on CeL neurons using in vitro and in vivo approaches. We showed that activation of the BB1 receptors increased action potential firing frequency recorded from CeL neurons via inhibition of the inwardly rectifying K+ (Kir) channels. Activities of phospholipase Cβ and protein kinase C were required, whereas intracellular Ca2+ release was unnecessary for BB1 receptor-elicited potentiation of neuronal excitability. Application of NMB directly into the CeA reduced blood pressure and heart rate and significantly reduced fear-potentiated startle. We may provide a cellular and molecular mechanism whereby bombesin-like peptides modulate anxiety and fear responses in the amygdala.

Explanation of Structure and Function of kv1.3 Potent Blocker From Mesobuthus eupeus Venom Gland: A New Promise in Drug Development

Background: Scorpions and other venomous animals are sought with great concern because venom is a source of novel peptides with exciting features. Some toxins of scorpion venom are effectors of potassium channels. Previous studies strongly support the importance of potassium channel toxins for use as pharmacological tools or potential drugs. Objectives: Here, a three-dimensional (3-D) structure and function of a potent acidic blocker of the human voltage-gated potassium ion channel, Kv1.3, previously identified in the scorpion Mesobuthus eupeus venom gland, were interpreted. Methods: The 3-D structure of meuK2-2 was generated using homology modeling. The interaction of meuK2-2 with the Kv1.3 channel was evaluated using a computational protocol employing peptide-protein docking experiments, pose clustering, and 100 ns molecular dynamic simulations to make the 3-D models of the meuK2-2/Kv1.3 complex trustworthy. Results: A CSα/β (cysteine-stabilized α-helical and β-sheet) fold was found for the 3-D structure of meuK2-2. In a different mechanism from what was identified so far, meuK2-2 binds to both turret and pore loop of Kv1.3 through two key residues (Ala28 and Ser11) and H-bonds. The binding of meuK2-2 induces some conformational changes to Kv1.3. Eventually, the side chain of a positively charged amino acid (His9) occupies the channel's pore. All together blocks the ion permeation pathway. Conclusions: MeuK2-2 could block Kv1.3 by a new mechanism. So, it could be a unique target for further investigations to develop a pharmacological tool and potential drug.

Roles of PLCβ, PIP2 , and GIRK channels in arginine vasopressin-elicited excitation of CA1 pyramidal neurons

Arginine vasopressin (AVP) is a hormone exerting vasoconstrictive and antidiuretic action in the periphery and serves as a neuromodulator in the brain. Although the hippocampus receives vasopressinergic innervation and AVP has been shown to facilitate the excitability of CA1 pyramidal neurons, the involved ionic and signaling mechanisms have not been determined. Here we found that AVP excited CA1 pyramidal neurons by activation of V_1a receptors. Functions of G proteins and phospholipase Cβ (PLCβ) were required for AVP-elicited excitation of CA1 pyramidal neurons, whereas intracellular Ca²⁺ release and protein kinase C were unnecessary. PLCβ-mediated depletion of phosphatidylinositol 4,5-bisphosphate (PIP₂ ) was required for AVP-elicited excitation of CA1 pyramidal neurons. AVP augmented the input resistance and increased the time constants of CA1 pyramidal neurons. AVP induced an inward current in K⁺ -containing intracellular solution, whereas no inward currents were observed with Cs⁺ -containing intracellular solution. AVP-sensitive currents showed inward rectification with a reversal potential close to the K⁺ reversal potential, suggesting the involvement of inwardly rectifying K⁺ channels. AVP-induced currents were sensitive to the micromolar concentration of Ba²⁺ and tertiapin-Q, whereas application of ML 133, a selective Kir2 channel blocker had no effects, suggesting that AVP excited CA1 pyramidal neurons by depressing G protein-gated inwardly rectifying K⁺ channels. Activation of V_1a receptors in the CA1 region facilitated glutamatergic transmission onto subicular pyramidal neurons, suggesting that AVP modulates network activity in the brain. Our results may provide one of the cellular and molecular mechanisms to explain the in vivo physiological functions of AVP.

Antidepressive effect of an inward rectifier K+ channel blocker peptide, tertiapin-RQ

Renal outer medullary K⁺ channel, ROMK (Kir1.1, kcnj1) is expressed in the kidney and brain, but its role in the central nervous system remains unknown. Recent studies suggested an involvement of the ROMK channel in mental diseases. Tertiapin (TPN) is a European honey bee venom peptide and is reported to selectively block the ROMK channel. Here, we have chemically synthesized a series of mutated TPN peptides, including TPN-I8R and -M13Q (TPN-RQ), reported previously, and examined their blocking activity on the ROMK channel. Among 71 peptides tested, TPN-RQ was found to block the ROMK channel most effectively. Whole-cell patch-clamp recordings showed the essential roles of two disulfide bonds and the circular structure for the blockade activity. To examine the central role, we injected TPN-RQ intracerebroventricularly and examined the effects on depression- and anxiety-like behaviors in mice. TPN-RQ showed an antidepressive effect in tail-suspension and forced swim tests. The injection of TPN-RQ also enhanced the anxiety-like behavior in the elevated plus-maze and light/dark box tests and impaired spontaneous motor activities in balance beam and wheel running tests. Administration of TPM-RQ suppressed the anti-c-Fos immunoreactivity in the lateral septum, without affecting immunoreactivity in antidepressant-related nuclei, e.g. the dorsal raphe nucleus and locus coeruleus. TPN-RQ may exert its antidepressive effects through a different mechanism from current drugs.

Oxytocin receptors excite lateral nucleus of central amygdala by phospholipase Cβ- and protein kinase C-dependent depression of inwardly rectifying K+ channels

KEY POINTS

Activation of oxytocin receptors (OXTRs) facilitates neuronal excitability in rat lateral nucleus of central amygdala (CeL). OXTR-induced excitation is mediated by inhibition of inwardly rectifying K⁺ (Kir) channels. Phospholipase Cβ is necessary for OXTR-mediated excitation of CeL neurons and depression of Kir channels. OXTR-elicited depression of Kir channels and excitation of CeL neurons require the function of Ca²⁺ -dependent protein kinase C.

ABSTRACT

Oxytocin (OXT) is a nonapeptide that exerts anxiolytic effects in the brain. The amygdala is an important structure involved in the modulation of fear and anxiety. A high density of OXT receptors (OXTRs) has been detected in the capsular (CeC) and lateral (CeL) nucleus of the central amygdala (CeA). Previous studies have demonstrated that activation of OXTRs induces remarkable increases in neuronal excitability in the CeL/C. However, the signalling and ionic mechanisms underlying OXTR-induced facilitation of neuronal excitability have not been determined. We found that activation of OXTRs in the CeL increased action potential firing frequency recorded from neurons in this region via inhibition of the inwardly rectifying K⁺ channels. The functions of phospholipase Cβ and protein kinase C were required for OXTR-induced augmentation of neuronal excitability. Our results provide a cellular and molecular mechanism whereby activation of OXTRs exerts anxiolytic effects.

Identification of Aethina tumida Kir Channels as Putative Targets of the Bee Venom Peptide Tertiapin Using Structure-Based Virtual Screening Methods

Venoms are comprised of diverse mixtures of proteins, peptides, and small molecules. Identifying individual venom components and their target(s) with mechanism of action is now attainable to understand comprehensively the effectiveness of venom cocktails and how they collectively function in the defense and predation of an organism. Here, structure-based computational methods were used with bioinformatics tools to screen and identify potential biological targets of tertiapin (TPN), a venom peptide from Apis mellifera (European honey bee). The small hive beetle (Aethina tumida (A. tumida)) is a natural predator of the honey bee colony and was found to possess multiple inwardly rectifying K⁺ (Kir) channel subunit genes from a genomic BLAST search analysis. Structure-based virtual screening of homology modelled A. tumida Kir (atKir) channels found TPN to interact with a docking profile and interface “footprint” equivalent to known TPN-sensitive mammalian Kir channels. The results support the hypothesis that atKir channels, and perhaps other insect Kir channels, are natural biological targets of TPN that help defend the bee colony from infestations by blocking K⁺ transport via atKir channels. From these in silico findings, this hypothesis can now be subsequently tested in vitro by validating atKir channel block as well as in vivo TPN toxicity towards A. tumida. This study highlights the utility and potential benefits of screening in virtual space for venom peptide interactions and their biological targets, which otherwise would not be feasible.

µ-Opioid receptors inhibit the exercise pressor reflex by closing N-type calcium channels but not by opening GIRK channels in rats

µ-Opioid G protein-coupled receptors (MOR) interact with ion channels to decrease neuronal excitability. In humans, intrathecal administration of the MOR agonist fentanyl inhibits the exercise pressor reflex, an effect that can be attributed to either the opening of inward rectifying potassium channels (GIRK) or the closing of N-type calcium channels. The purpose of this study was to determine if the highly selective MOR agonist [d-Ala², N-MePhe⁴,Gly-ol]-enkephalin (DAMGO) attenuates the exercise pressor reflex and which of these two channels are responsible for this effect. In decerebrate rats, we determined the effect of intrathecal injection of either tertiapin-LQ, which blocks the GIRK channel or ω-conotoxin-GVIA, which blocks the N-type calcium channel on the exercise pressor reflex, which was evoked by contracting the triceps surae muscles. Initially, we established that intrathecal injection of DAMGO inhibited the exercise pressor reflex relative to no intrathecal injection or intrathecal saline injection ( P < 0.001, n = 5). We then found that intrathecal injection of two doses of tertiapin-LQ (1 and 10 µg) had no effect on the exercise pressor reflex ( n = 6 and n = 7, respectively; P > 0.05). Importantly, neither dose of tertiapin-LQ prevented the DAMGO-induced inhibition of the exercise pressor reflex. Last, we found that intrathecal injection of ω-conotoxin-GVIA markedly attenuated the exercise pressor reflex ( P < 0.001, n = 7). The cardioaccelerator response to contraction did not appear to be effected in any of the experiments. We conclude that N-type voltage-gated calcium channel inhibition appears to be the mechanism by which MOR activation inhibits the exercise pressor reflex in decerebrate rats.

Animal toxins for channelopathy treatment

Ion channels are transmembrane proteins that allow passive flow of ions inside and/or outside of cells or cell organelles. Except mutations lead to nonfunctional protein production or abolished receptor entrance on the membrane surface an altered channel may have two principal conditions that can be corrected. The channel may conduct fewer ions through (loss-of-function mutations) or too many ions (gain-of-function mutations) compared to a normal channel. Toxins from animal venoms are specialised molecules that are generally oriented toward interactions with ion channels. This is a result of long coevolution between predators and their prey. On the molecular level, toxins activate or inhibit ion channels, so they are ideal molecules for restoring conductance in mutated channels. Another aspect of this long coevolution is that a broad variety of toxins have been fine tuned to recognize the channels of different species, keeping many amino acids substitution among sequences. Many peptide ligands with high selectivity to specific receptor subtypes have been isolated from animal venoms, some of which are absolutely non-toxic to humans and mammalians. It is expected that molecules that are selective to each known receptor can be found in animal venoms, but the pool of toxins currently does not override all receptors described as being involved in channelopathies. Modern investigating methods have enhanced the search process for selective ligands. One prominent method is a site-directed mutagenesis of existing toxins to change the selectivity or/and affinity to the selected receptor, which has shown positive results. This article is part of the Special Issue entitled 'Channelopathies.'

Venom-derived peptides inhibiting Kir channels: Past, present, and future

Inwardly rectifying K⁺ (Kir) channels play a significant role in vertebrate and invertebrate biology by regulating the movement of K⁺ ions involved in membrane transport and excitability. Yet unlike other ion channels including their ancestral K⁺-selective homologs, there are very few venom toxins known to target and inhibit Kir channels with the potency and selectivity found for the Ca²⁺-activated and voltage-gated K⁺ channel families. It is unclear whether this is simply due to a lack of discovery, or instead a consequence of the evolutionary processes that drive the development of venom components towards their targets based on a collective efficacy to 1) elicit pain for defensive purposes, 2) promote paralysis for prey capture, or 3) facilitate delivery of venom components into the circulation. The past two decades of venom screening has yielded three venom peptides with inhibitory activity towards mammalian Kir channels, including the discovery of tertiapin, a high-affinity pore blocker from the venom of the European honey bee Apis mellifera. Venomics and structure-based computational approaches represent exciting new frontiers for venom peptide development, where re-engineering peptide 'scaffolds' such as tertiapin may aid in the quest to expand the palette of potent and selective Kir channel blockers for future research and potentially new therapeutics. This article is part of the Special Issue entitled 'Venom-derived Peptides as Pharmacological Tools.'

A computational design approach for virtual screening of peptide interactions across K(+) channel families

Ion channels represent a large family of membrane proteins with many being well established targets in pharmacotherapy. The ‘druggability’ of heteromeric channels comprised of different subunits remains obscure, due largely to a lack of channel-specific probes necessary to delineate their therapeutic potential in vivo. Our initial studies reported here, investigated the family of inwardly rectifying potassium (Kir) channels given the availability of high resolution crystal structures for the eukaryotic constitutively active Kir2.2 channel. We describe a ‘limited’ homology modeling approach that can yield chimeric Kir channels having an outer vestibule structure representing nearly any known vertebrate or invertebrate channel. These computationally-derived channel structures were tested ""in silico for ‘docking’ to NMR structures of tertiapin (TPN), a 21 amino acid peptide found in bee venom. TPN is a highly selective and potent blocker for the epithelial rat Kir1.1 channel, but does not block human or zebrafish Kir1.1 channel isoforms. Our Kir1.1 channel-TPN docking experiments recapitulated published in vitro ""findings for TPN-sensitive and TPN-insensitive channels. Additionally, in silico site-directed mutagenesis identified ‘hot spots’ within the channel outer vestibule that mediate energetically favorable docking scores and correlate with sites previously identified with in vitro thermodynamic mutant-cycle analysis. These ‘proof-of-principle’ results establish a framework for virtual screening of re-engineered peptide toxins for interactions with computationally derived Kir channels that currently lack channel-specific blockers. When coupled with electrophysiological validation, this virtual screening approach may accelerate the drug discovery process, and can be readily applied to other ion channels families where high resolution structures are available.

Differential phospholipase C-dependent modulation of TASK and TREK two-pore domain K+ channels in rat thalamocortical relay neurons

KEY POINTS

During the behavioural states of sleep and wakefulness thalamocortical relay neurons fire action potentials in high frequency bursts or tonic sequences, respectively. The modulation of specific K(+) channel types, termed TASK and TREK, allows these neurons to switch between the two modes of activity. In this study we show that the signalling lipids phosphatidylinositol 4,5-bisphosphate (PIP2) and diacylglycerol (DAG), which are components of their membrane environment, switch on and shut off TREK and TASK channels, respectively. These channel modulations contribute to a better understanding of the molecular basis of the effects of neurotransmitters such as ACh which are released by the brainstem arousal system. The present report introduces PIP2 and DAG as new elements of signal transduction in the thalamus. The activity of two-pore domain potassium channels (K2P ) regulates the excitability and firing modes of thalamocortical (TC) neurons. In particular, the inhibition of two-pore domain weakly inwardly rectifying K(+) channel (TWIK)-related acid-sensitive K(+) (TASK) channels and TWIK-related K(+) (TREK) channels, as a consequence of the stimulation of muscarinic ACh receptors (MAChRs) which are coupled to phosphoinositide-specific phospholipase C (PLCβ), induces a shift from burst to tonic firing. By using a whole cell patch-clamp approach, the contribution of the membrane-bound second messenger molecules phosphatidylinositol 4,5-bisphosphate (PIP2 ) and diacylglycerol (DAG) acting downstream of PLCβ was probed. The standing outward current (ISO ) was used to monitor the current through TASK and TREK channels in TC neurons. By exploiting different manoeuvres to change the intracellular PIP2 level in TC neurons, we here show that the scavenging of PIP2 (by neomycin) results in an increased muscarinic effect on ISO whereas increased availability of PIP2 (inclusion to the patch pipette; histone-based carrier) decreased muscarinic signalling. The degree of muscarinic inhibition specifically depends on phosphatidylinositol phosphate (PIP) and PIP2 but no other phospholipids (phosphatidic acid, phosphatidylserine). The use of specific blockers revealed that PIP2 is targeting TREK but not TASK channels. Furthermore, we demonstrate that the inhibition of TASK channels is induced by the application of the DAG analogue 1-oleoyl-2-acetyl-sn-glycerol (OAG). Under current clamp conditions the activation of MAChRs and PLCβ as well as the application of OAG resulted in membrane depolarization, while PIP2 application via histone carrier induced a hyperpolarization. These results demonstrate a differential role of PIP2 and DAG in K2P channel modulation in native neurons which allows a fine-tuned inhibition of TREK (via PIP2 depletion) and TASK (via DAG) channels following MAChR stimulation.

Inward rectifier potassium (Kir) current in dopaminergic periglomerular neurons of the mouse olfactory bulb

Dopaminergic (DA) periglomerular (PG) neurons are critically placed at the entry of the bulbar circuitry, directly in contact with both the terminals of olfactory sensory neurons and the apical dendrites of projection neurons; they are autorhythmic and are the target of numerous terminals releasing a variety of neurotransmitters. Despite the centrality of their position, suggesting a critical role in the sensory processing, their properties -and consequently their function- remain elusive. The current mediated by inward rectifier potassium (Kir) channels in DA-PG cells was recorded by adopting the perforated-patch configuration in thin slices; IKir could be distinguished from the hyperpolarization-activated current (I h ) by showing full activation in <10 ms, no inactivation, suppression by Ba(2+) in a typical voltage-dependent manner (IC50 208 μM) and reversal potential nearly coincident with EK. Ba(2+) (2 mM) induces a large depolarization of DA-PG cells, paralleled by an increase of the input resistance, leading to a block of the spontaneous activity, but the Kir current is not an essential component of the pacemaker machinery. The Kir current is negatively modulated by intracellular cAMP, as shown by a decrease of its amplitude induced by forskolin or 8Br-cAMP. We have also tested the neuromodulatory effects of the activation of several metabotropic receptors known to be present on these cells, showing that the current can be modulated by a multiplicity of pathways, whose activation in some case increases the amplitude of the current, as can be observed with agonists of D2, muscarinic, and GABAA receptors, whereas in other cases has the opposite effect, as it can be observed with agonists of α1 noradrenergic, 5-HT and histamine receptors. These characteristics of the Kir currents provide the basis for an unexpected plasticity of DA-PG cell function, making them potentially capable to reconfigure the bulbar network to allow a better flexibility.

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.

Novel molecular targets for atrial fibrillation therapy

Atrial fibrillation is the most common type of cardiac arrhythmia, and is responsible for substantial morbidity and mortality in the general population. Current treatments have moderate efficacy and considerable risks, especially of pro-arrhythmia, highlighting the need for new therapeutic strategies. In recent years, substantial efforts have been invested in developing novel treatments that target the underlying molecular determinants of atrial fibrillation, and several new compounds are under development. This Review focuses on the mechanistic rationale for the development of new anti-atrial fibrillation drugs, on the molecular and structural motifs that they target and on the results obtained so far in experimental and clinical studies.

Testosterone relaxes abdominal aorta in male Sprague-Dawley rats by opening potassium (K(+)) channel and blockade of calcium (Ca(2+)) channel

AIM

To investigate the direct effect of testosterone and its precursor/derivative dehydroepiandrosterone (DHEA) on isolated rat abdominal aortic rings.

MATERIALS AND METHODS

3mm abdominal aortic rings that were obtained from 3 months old male Sprague-Dawley rats were suspended in organ baths containing Hepes buffered PSS bubbled with 100% oxygen. Relaxation response to testosterone and DHEA was studied in noradrenalin pre-contracted rings. The role of aromatase and androgen receptor was assessed by inhibition using aminogluthetemide and blockade using flutamide respectively. Relaxation responses of the rings to testosterone in the presence of l-NAME, indomethacin, barium chloride, apamin, charybdotoxin, iberiotoxin, and nifedipine were also determined.

RESULTS

Both aromatase inhibition and androgen receptor blockade did not block the relaxing effect of testosterone on rings from rat abdominal aorta. Also there was no significant difference between testosterone relaxation response in the presence or absence of l-NAME and indomethacin. However 3μM, BaCl(2) almost completely abolished the aortic ring relaxation response to testosterone while 1μM, nifedipine potentiated the vasorelaxing effect of testosterone.

CONCLUSION

Testosterone relaxes abdominal aorta directly via a non-genomic pathway which is independent of endothelial derived vasoactive substances, but involves activation of inward rectifying potassium channel (K(IR)) and blockade of l-type calcium channel.

Brown spider (Loxosceles genus) venom toxins: tools for biological purposes

Venomous animals use their venoms as tools for defense or predation. These venoms are complex mixtures, mainly enriched of proteic toxins or peptides with several, and different, biological activities. In general, spider venom is rich in biologically active molecules that are useful in experimental protocols for pharmacology, biochemistry, cell biology and immunology, as well as putative tools for biotechnology and industries. Spider venoms have recently garnered much attention from several research groups worldwide. Brown spider (Loxosceles genus) venom is enriched in low molecular mass proteins (5–40 kDa). Although their venom is produced in minute volumes (a few microliters), and contain only tens of micrograms of protein, the use of techniques based on molecular biology and proteomic analysis has afforded rational projects in the area and permitted the discovery and identification of a great number of novel toxins. The brown spider phospholipase-D family is undoubtedly the most investigated and characterized, although other important toxins, such as low molecular mass insecticidal peptides, metalloproteases and hyaluronidases have also been identified and featured in literature. The molecular pathways of the action of these toxins have been reported and brought new insights in the field of biotechnology. Herein, we shall see how recent reports describing discoveries in the area of brown spider venom have expanded biotechnological uses of molecules identified in these venoms, with special emphasis on the construction of a cDNA library for venom glands, transcriptome analysis, proteomic projects, recombinant expression of different proteic toxins, and finally structural descriptions based on crystallography of toxins.

Random mutagenesis screening indicates the absence of a separate H(+)-sensor in the pH-sensitive Kir channels

Several inwardly-rectifying (Kir) potassium channels (Kir1.1, Kir4.1 and Kir4.2) are characterised by their sensitivity to inhibition by intracellular H(+) within the physiological range. The mechanism by which these channels are regulated by intracellular pH has been the subject of intense scrutiny for over a decade, yet the molecular identity of the titratable pH-sensor remains elusive. In this study we have taken advantage of the acidic intracellular environment of S. cerevisiae and used a K(+) -auxotrophic strain to screen for mutants of Kir1.1 with impaired pH-sensitivity. In addition to the previously identified K80M mutation, this unbiased screening approach identified a novel mutation (S172T) in the second transmembrane domain (TM2) that also produces a marked reduction in pH-sensitivity through destabilization of the closed-state. However, despite this extensive mutagenic approach, no mutations could be identified which removed channel pH-sensitivity or which were likely to act as a separate H(+) -sensor unique to the pH-sensitive Kir channels. In order to explain these results we propose a model in which the pH-sensing mechanism is part of an intrinsic gating mechanism common to all Kir channels, not just the pH-sensitive Kir channels. In this model, mutations which disrupt this pH-sensor would result in an increase, not reduction, in pH-sensitivity. This has major implications for any future studies of Kir channel pH-sensitivity and explains why formal identification of these pH-sensing residues still represents a major challenge.

The nociceptive and anti-nociceptive effects of bee venom injection and therapy: a double-edged sword

Bee venom injection as a therapy, like many other complementary and alternative medicine approaches, has been used for thousands of years to attempt to alleviate a range of diseases including arthritis. More recently, additional theraupeutic goals have been added to the list of diseases making this a critical time to evaluate the evidence for the beneficial and adverse effects of bee venom injection. Although reports of pain reduction (analgesic and antinociceptive) and anti-inflammatory effects of bee venom injection are accumulating in the literature, it is common knowledge that bee venom stings are painful and produce inflammation. In addition, a significant number of studies have been performed in the past decade highlighting that injection of bee venom and components of bee venom produce significant signs of pain or nociception, inflammation and many effects at multiple levels of immediate, acute and prolonged pain processes. This report reviews the extensive new data regarding the deleterious effects of bee venom injection in people and animals, our current understanding of the responsible underlying mechanisms and critical venom components, and provides a critical evaluation of reports of the beneficial effects of bee venom injection in people and animals and the proposed underlying mechanisms. Although further studies are required to make firm conclusions, therapeutic bee venom injection may be beneficial for some patients, but may also be harmful. This report highlights key patterns of results, critical shortcomings, and essential areas requiring further study.

Inwardly rectifying potassium channels: their structure, function, and physiological roles

Inwardly rectifying K(+) (Kir) channels allow K(+) to move more easily into rather than out of the cell. They have diverse physiological functions depending on their type and their location. There are seven Kir channel subfamilies that can be classified into four functional groups: classical Kir channels (Kir2.x) are constitutively active, G protein-gated Kir channels (Kir3.x) are regulated by G protein-coupled receptors, ATP-sensitive K(+) channels (Kir6.x) are tightly linked to cellular metabolism, and K(+) transport channels (Kir1.x, Kir4.x, Kir5.x, and Kir7.x). Inward rectification results from pore block by intracellular substances such as Mg(2+) and polyamines. Kir channel activity can be modulated by ions, phospholipids, and binding proteins. The basic building block of a Kir channel is made up of two transmembrane helices with cytoplasmic NH(2) and COOH termini and an extracellular loop which folds back to form the pore-lining ion selectivity filter. In vivo, functional Kir channels are composed of four such subunits which are either homo- or heterotetramers. Gene targeting and genetic analysis have linked Kir channel dysfunction to diverse pathologies. The crystal structure of different Kir channels is opening the way to understanding the structure-function relationships of this simple but diverse ion channel family.

A designer ligand specific for Kv1.3 channels from a scorpion neurotoxin-based library

Venomous animals immobilize prey using protein toxins that act on ion channels and other targets of biological importance. Broad use of toxins for biomedical research, diagnosis, and therapy has been limited by inadequate target discrimination, for example, among ion channel subtypes. Here, a synthetic toxin is produced by a new strategy to be specific for human Kv1.3 channels, critical regulators of immune T cells. A phage display library of 11,200 de novo proteins is designed using the alpha-KTx scaffold of 31 scorpion toxin sequences known or predicted to bind to potassium channels. Mokatoxin-1 (moka1) is isolated by affinity selection on purified target. Moka1 blocks Kv1.3 at nanomolar levels that do not inhibit Kv1.1, Kv1.2, or KCa1.1. As a result, moka1 suppresses CD3/28-induced cytokine secretion by T cells without cross-reactive gastrointestinal hyperactivity. The 3D structure of moka1 rationalizes its specificity and validates the engineering approach, revealing a unique interaction surface supported on an alpha-KTx scaffold. This scaffold-based/target-biased strategy overcomes many obstacles to production of selective toxins.