The atypic antipsychotic clozapine inhibits multiple cardiac ion channels

Clozapine is an atypical neuroleptic used to manage treatment-resistant schizophrenia which is known to inhibit cardiac hERG/K_V11.1 potassium channels, a pharmacological property associated with increased risk of potentially fatal Torsades de Pointes (TdP) and sudden cardiac death (SCD). Yet, the long-standing clinical practice of clozapine does not show a consistent association with increased incidence of TdP, although SCD is considerably higher among schizophrenic patients than in the general population. Here, we have established the inhibitory profile of clozapine at the seven cardiac ion currents proposed by the ongoing comprehensive in vitro pro-arrhythmia (CiPA) initiative to better predict new drug cardio-safety risk. We found that clozapine inhibited all CiPA currents tested with the following rank order of potency: K_V11.1 > Na_V1.5 _{(late current)} ≈ Ca_V1.2 ≈ Na_V1.5 _{(peak current)} ≈ K_V7.1 > K_V4.3 > K_ir2.1 _{(outward current)}. Half-maximal inhibitory concentrations (IC₅₀) at the repolarizing K_V11.1 and K_V7.1 channels, and at the depolarizing Ca_V1.2 and Na_V1.5 channels fell within a narrow half-log 3-10 µM concentration range, suggesting that mutual compensation could explain the satisfactory arrhythmogenic cardio-safety profile of clozapine. Although the IC₅₀ values determined herein using an automated patch-clamp (APC) technique are at the higher end of clozapine plasmatic concentrations at target therapeutic doses, this effective antipsychotic appears prone to distribute preferentially into the cardiac tissue, which supports the clinical relevance of our in vitro pharmacological findings.

In vitro ion channel profile and ex vivo cardiac electrophysiology properties of the R(-) and S(+) enantiomers of hydroxychloroquine

Hydroxychloroquine (HCQ) is a derivative of the antimalaria drug chloroquine primarily prescribed for autoimmune diseases. Recent attempts to repurpose HCQ in the treatment of corona virus disease 2019 has raised concerns because of its propensity to prolong the QT-segment on the electrocardiogram, an effect associated with increased pro-arrhythmic risk. Since chirality can affect drug pharmacological properties, we have evaluated the functional effects of the R(-) and S(+) enantiomers of HCQ on six ion channels contributing to the cardiac action potential and on electrophysiological parameters of isolated Purkinje fibers. We found that R(-)HCQ and S(+)HCQ block human K_ir2.1 and hERG potassium channels in the 1 μM-100 μM range with a 2-4 fold enantiomeric separation. Na_V1.5 sodium currents and Ca_V1.2 calcium currents, as well as K_V4.3 and K_V7.1 potassium currents remained unaffected at up to 90 μM. In rabbit Purkinje fibers, R(-)HCQ prominently depolarized the membrane resting potential, inducing autogenic activity at 10 μM and 30 μM, while S(+)HCQ primarily increased the action potential duration, inducing occasional early afterdepolarization at these concentrations. These data suggest that both enantiomers of HCQ can alter cardiac tissue electrophysiology at concentrations above their plasmatic levels at therapeutic doses, and that chirality does not substantially influence their arrhythmogenic potential in vitro.

Fluorescent- and tagged-protoxin II peptides: potent markers of the Nav 1.7 channel pain target

BACKGROUND AND PURPOSE

Protoxin II (ProTx II) is a high affinity gating modifier that is thought to selectively block the Na_v 1.7 voltage-dependent Na⁺ channel, a major therapeutic target for the control of pain. We aimed at producing ProTx II analogues entitled with novel functionalities for cell distribution studies and biochemical characterization of its Na_v channel targets.

EXPERIMENTAL APPROACH

We took advantage of the high affinity properties of the peptide, combined to its slow off rate, to design a number of new tagged analogues useful for imaging and biochemistry purposes. We used high-throughput automated patch-clamp to identify the analogues best matching the native properties of ProTx II and validated them on various Na_v -expressing cells in pull-down and cell distribution studies.

KEY RESULTS

Two of the produced ProTx II analogues, Biot-ProTx II and ATTO488-ProTx II, best emulate the pharmacological properties of unlabelled ProTx II, whereas other analogues remain high affinity blockers of Na_v 1.7. The biotinylated version of ProTx II efficiently works for the pull-down of several Na_v isoforms tested in a concentration-dependent manner, whereas the fluorescent ATTO488-ProTx II specifically labels the Na_v 1.7 channel over other Na_v isoforms tested in various experimental conditions.

CONCLUSIONS AND IMPLICATIONS

The properties of these ProTx II analogues as tools for Na_v channel purification and cell distribution studies pave the way for a better understanding of ProTx II channel receptors in pain and their pathophysiological implications in sensory neuronal processing. The new fluorescent ProTx II should also be useful in the design of new drug screening strategies.

Cannabidiol inhibits multiple cardiac ion channels and shortens ventricular action potential duration in vitro

Cannabidiol (CBD) is a non-psychoactive component of Cannabis which has recently received regulatory consideration for the treatment of intractable forms of epilepsy such as the Dravet and the Lennox-Gastaut syndromes. The mechanisms of the antiepileptic effects of CBD are unclear, but several pre-clinical studies suggest the involvement of ion channels. Therefore, we have evaluated the effects of CBD on seven major cardiac currents shaping the human ventricular action potential and on Purkinje fibers isolated from rabbit hearts to assess the in vitro cardiac safety profile of CBD. We found that CBD inhibits with comparable micromolar potencies the peak and late components of the Na_V1.5 sodium current, the Ca_V1.2 mediated L-type calcium current, as well as all the repolarizing potassium currents examined except K_ir2.1. The most sensitive channels were K_V7.1 and the least sensitive were K_V11.1 (hERG), which underly the slow (I_Ks) and rapid (I_Kr) components, respectively, of the cardiac delayed-rectifier current. In the Purkinje fibers, CBD decreased the action potential (AP) duration more potently at half-maximal than at near complete repolarization, and slightly decreased the AP amplitude and its maximal upstroke velocity. CBD had no significant effects on the membrane resting potential except at the highest concentration tested under fast pacing rate. These data show that CBD impacts cardiac electrophysiology and suggest that caution should be exercised when prescribing CBD to carriers of cardiac channelopathies or in conjunction with other drugs known to affect heart rhythm or contractility.

Lysophosphatidic Acid Receptor Agonism: Discovery of Potent Nonlipid Benzofuran Ethanolamine Structures

Lysophosphatidic acid (LPA) is the natural ligand for two phylogenetically distinct families of receptors (LPA_1-3, LPA_4-6) whose pathways control a variety of physiologic and pathophysiological responses. Identifying the benefit of balanced activation/repression of LPA receptors has always been a challenge because of the high lability of LPA and the limited availability of selective and/or stable agonists. In this study, we document the discovery of small benzofuran ethanolamine derivatives (called CpX and CpY) behaving as LPA_1-3 agonists. Initially found as rabbit urethra contracting agents, their elusive receptors were identified from [³⁵S]GTPγS-binding and β-arrestin2 recruitment investigations and then confirmed by [³H]CpX binding studies (urethra, hLPA_1-2 membranes). Both compounds induced a calcium response in hLPA_1-3 cells within a range of 0.4-1.5-log lower potency as compared with LPA. The contractions of rabbit urethra strips induced by these compounds perfectly matched binding affinities with values reaching the two-digit nanomolar level. The antagonist, KI16425, dose-dependently antagonized CpX-induced contractions in agreement with its affinity profile (LPA₁≥LPA₃>>LPA₂). The most potent agonist, CpY, doubled intraurethral pressure in anesthetized female rats at 3 µg/kg i.v. Alternatively, CpX was shown to inhibit human preadipocyte differentiation, a process totally reversed by KI16425. Together with original molecular docking data, these findings clearly established these molecules as potent agonists of LPA_1-3 and consolidated the pivotal role of LPA₁ in urethra/prostate contraction as well as in fat cell development. The discovery of these unique and less labile LPA_1-3 agonists would offer new avenues to investigate the roles of LPA receptors. SIGNIFICANCE STATEMENT: We report the identification of benzofuran ethanolamine derivatives behaving as potent selective nonlipid LPA_1-3 agonists and shown to alter urethra muscle contraction or preadipocyte differentiation. Unique at this level of potency, selectivity, and especially stability, compared with lysophosphatidic acid, they represent more appropriate tools for investigating the physiological roles of lysophosphatidic acid receptors and starting point for optimization of drug candidates for therapeutic applications.

From identification to functional characterization of cyriotoxin-1a, an antinociceptive toxin from the spider Cyriopagopus schioedtei

BACKGROUND AND PURPOSE

The Na_V 1.7 channel is highly expressed in dorsal root ganglia of the sensory nervous system and plays a central role in the pain signalling process. We investigated a library prepared from original venoms of 117 different animals to identify new selective inhibitors of this target.

EXPERIMENTAL APPROACH

We used high throughput screening of a large venom collection using automated patch-clamp experiments on human voltage-gated sodium channel subtypes and then in vitro and in vivo electrophysiological experiments to characterize the active peptides that have been purified, sequenced, and chemically synthesized. Analgesic effects were evaluated in vivo in mice models.

KEY RESULTS

We identified cyriotoxin-1a (CyrTx-1a), a novel peptide isolated from Cyriopagopus schioedtei spider venom, as a candidate for further characterization. This 33 amino acids toxin belongs to the inhibitor cystine knot structural family and inhibits hNa_V 1.1-1.3 and 1.6-1.7 channels in the low nanomolar range, compared to the micromolar range for hNa_V 1.4-1.5 and 1.8 channels. CyrTx-1a was 920 times more efficient at inhibiting tetrodotoxin (TTX)-sensitive than TTX-resistant sodium currents recorded from adult mouse dorsal root ganglia neurons and in vivo electrophysiological experiments showed that CyrTx-1a was approximately 170 times less efficient than huwentoxin-IV at altering mouse skeletal neuromuscular excitability properties. CyrTx-1a exhibited an analgesic effect in mice by increasing reaction time in the hot-plate assay.

CONCLUSIONS AND IMPLICATIONS

The pharmacological profile of CyrTx-1a paves the way for further molecular engineering aimed to optimize the potential antinociceptive properties of this peptide.

Electrophysiological and Pharmacological Characterization of Human Inwardly Rectifying Kir2.1 Channels on an Automated Patch-Clamp Platform

Inwardly rectifying I_K1 potassium currents of the heart control the resting membrane potential of ventricular cardiomyocytes during diastole and contribute to their repolarization after each action potential. Mutations in the gene encoding K_ir2.1 channels, which primarily conduct ventricular I_K1, are associated with inheritable forms of arrhythmias and sudden cardiac death. Therefore, potential iatrogenic inhibition of K_ir2.1-mediated I_K1 currents is a cardiosafety concern during new drug discovery and development. K_ir2.1 channels are part of the panel of cardiac ion channels currently considered for refined early compound risk assessment within the Comprehensive in vitro Proarrhythmia Assay initiative. In this study, we have validated a cell-based assay allowing functional quantification of K_ir2.1 inhibitors using whole-cell recordings of Chinese hamster ovary cells stably expressing human K_ir2.1 channels. We reproduced key electrophysiological and pharmacological features known for native I_K1, including current enhancement by external potassium and voltage- and concentration-dependent blockade by external barium. Furthermore, the K_ir inhibitors ML133, PA-6, and chloroquine, as well as the multichannel inhibitors chloroethylclonidine, chlorpromazine, SKF-96365, and the class III antiarrhythmic agent terikalant demonstrated slowly developing inhibitory activity in the low micromolar range. The robustness of this assay authorizes medium throughput screening for cardiosafety purposes and could help to enrich the currently limited K_ir2.1 pharmacology.

Corrigendum: The NaV1.7 Channel Subtype as an Antinociceptive Target for Spider Toxins in Adult Dorsal Root Ganglia Neurons

[This corrects the article DOI: 10.3389/fphar.2018.01000.].

The NaV1.7 Channel Subtype as an Antinociceptive Target for Spider Toxins in Adult Dorsal Root Ganglia Neurons

Although necessary for human survival, pain may sometimes become pathologic if long-lasting and associated with alterations in its signaling pathway. Opioid painkillers are officially used to treat moderate to severe, and even mild, pain. However, the consequent strong and not so rare complications that occur, including addiction and overdose, combined with pain management costs, remain an important societal and economic concern. In this context, animal venom toxins represent an original source of antinociceptive peptides that mainly target ion channels (such as ASICs as well as TRP, Ca_V, K_V and Na_V channels) involved in pain transmission. The present review aims to highlight the Na_V1.7 channel subtype as an antinociceptive target for spider toxins in adult dorsal root ganglia neurons. It will detail (i) the characteristics of these primary sensory neurons, the first ones in contact with pain stimulus and conveying the nociceptive message, (ii) the electrophysiological properties of the different Na_V channel subtypes expressed in these neurons, with a particular attention on the Na_V1.7 subtype, an antinociceptive target of choice that has been validated by human genetic evidence, and (iii) the features of spider venom toxins, shaped of inhibitory cysteine knot motif, that present high affinity for the Na_V1.7 subtype associated with evidenced analgesic efficacy in animal models.

Direct evidence for high affinity blockade of NaV1.6 channel subtype by huwentoxin-IV spider peptide, using multiscale functional approaches

The Chinese bird spider huwentoxin-IV (HwTx-IV) is well-known to be a highly potent blocker of Na_V1.7 subtype of voltage-gated sodium (Na_V) channels, a genetically validated analgesic target, and thus promising as a potential lead molecule for the development of novel pain therapeutics. In the present study, the interaction between HwTx-IV and Na_V1.6 channel subtype was investigated using multiscale (from in vivo to individual cell) functional approaches. HwTx-IV was approximatively 2 times more efficient than tetrodotoxin (TTX) to inhibit the compound muscle action potential recorded from the mouse skeletal neuromuscular system in vivo, and 30 times more effective to inhibit nerve-evoked than directly-elicited muscle contractile force of isolated mouse hemidiaphragms. These results strongly suggest that the inhibition of nerve-evoked skeletal muscle functioning, produced by HwTx-IV, resulted from a toxin-induced preferential blockade of Na_V1.6, compared to Na_V1.4, channel subtype. This was confirmed by whole-cell automated patch-clamp experiments performed on human embryonic kidney (HEK)-293 cells overexpressing hNa_V1.1-1.8 channel subtypes. HwTx-IV was also approximatively 850 times more efficient to inhibit TTX-sensitive than TTX-resistant sodium currents recorded from mouse dorsal root ganglia neurons. Finally, based on our data, we predict that blockade of the Na_V1.6 channel subtype was involved in the in vivo toxicity of HwTx-IV, although this toxicity was more than 2 times lower than that of TTX. In conclusion, our results provide detailed information regarding the effects of HwTx-IV and allow a better understanding of the side-effect mechanisms involved in vivo and of channel subtype interactions resulting from the toxin activity.

Automated Patch-Clamp Methods for the hERG Cardiac Potassium Channel

The human Ether-a-go-go Related Gene (hERG) product has been identified as a central ion channel underlying both familial forms of elongated QT interval on the electrocardiogram and drug-induced elongation of the same QT segment. Indeed, reduced function of this potassium channel involved in the repolarization of the cardiac action potential can produce a type of life-threatening cardiac ventricular arrhythmias called Torsades de Pointes (TdP). Therefore, hERG inhibitory activity of newly synthetized molecules is a relevant structure-activity metric for compound prioritization and optimization in medicinal chemistry phases of drug discovery. Electrophysiology remains the gold standard for the functional assessment of ion channel pharmacology. The recent years have witnessed automatization and parallelization of the manual patch-clamp technique, allowing higher throughput screening on recombinant hERG channels. However, the multi-well plate format of automatized patch-clamp does not allow visual detection of potential micro-precipitation of poorly soluble compounds. In this chapter we describe bench procedures for the culture and preparation of hERG-expressing CHO cells for recording on an automated patch-clamp workstation. We also show that the sensitivity of the assay can be improved by adding a surfactant to the extracellular medium.

Functional Studies of Sodium Channels: From Target to Compound Identification

Over the last six decades, voltage-gated sodium (Na_v ) channels have attracted a great deal of scientific and pharmaceutical interest, driving fundamental advances in both biology and technology. The structure and physiological function of these channels have been extensively studied; clinical and genetic data have uncovered their implication in diseases such as epilepsy, arrhythmias, and pain, bringing them into focus as current and future drug targets. While different techniques have been established to record the activity of Na_v channels, proper determination of their properties still presents serious challenges, depending upon the experimental conditions and the desired subtype of channel to be characterized. The aim of this unit is to review the characteristics of Na_v channels, their properties, the cells in which they can be studied, and the currently available techniques. Topics covered include the determination of Na_v -channel biophysical properties as well as the use of toxins to discriminate between subtypes using electrophysiological or optical methods. Perspectives on the development of high-throughput screening assays with their advantages and limitations are also discussed to allow a better understanding of the challenges encountered in voltage-gated sodium channel preclinical drug discovery. © 2016 by John Wiley & Sons, Inc.

Design and validation of a homogeneous time-resolved fluorescence cell-based assay targeting the ligand-gated ion channel 5-HT3A

Ligand-gated ion channels (LGICs) are considered as attractive protein targets in the search for new therapeutic agents. Nowadays, this strategy involves the capability to screen large chemical libraries. We present a new Tag-lite ligand binding assay targeting LGICs on living cells. This technology combines the use of suicide enzyme tags fused to channels of interest with homogeneous time-resolved fluorescence (HTRF) as the detection readout. Using the 5-HT3 receptor as system model, we showed that the pharmacology of the HALO-5HT3 receptor was identical to that of the native receptor. After validation of the assay by using 5-HT3 agonists and antagonists of reference, a pilot screen enabled us to identify azelastine, a well-known histamine H1 antagonist, as a potent 5-HT3 antagonist. This interesting result was confirmed with electrophysiological experiments. The method described here is easy to implement and could be applicable for other LGICs, opening new ways for the screening of chemical libraries.

Transforming TRP channel drug discovery using medium-throughput electrophysiological assays

Since the cloning of its first member in 1998, transient receptor potential (TRP) cation channels have become one of the most studied ion channel families in drug discovery. These channels, almost all calcium permeant, have been studied in many different (patho)-physiological and therapeutic areas as diverse as pain; neurodegenerative, cardiovascular, and inflammatory diseases; and cancer. At the same time, implementation of automated electrophysiology screening platforms has significantly increased the tractability of ion channels, mainly voltage gated, as drug targets. The work presented in this article shows the design and validation of TRP screening assays using the IonWorks Quattro platform (Molecular Devices, Sunnyvale, CA), allowing a significant increase in throughput to support drug discovery programs. This new player has a direct impact on resources and timelines by prioritizing potential candidates and reducing the number of molecules requiring final testing by manual patch-clamp, which is still today the gold standard technology for this challenging drug target class.

Development of an inducible NMDA receptor stable cell line with an intracellular Ca2+ reporter

Cytotoxicity associated with NMDA receptor activation has impeded the establishment of cell lines expressing recombinant subtypes of this ligand-gated ion channel class. To circumvent this toxicity, we describe in this report the use of a potent inducible promoter in the construction of a cell line stably expressing the NR1a/NR2A subtype of the NMDA receptor. Western blot analysis using subunit selective antibodies revealed that NR2A subunits were constitutively expressed in this cell line, whereas expression of NR1a subunits was tightly regulated by tetracycline. Upon tetracycline removal, electrophysiological recordings using the patch clamp technique indicated the expression of functional receptors with biophysical and pharmacological properties corresponding to those expected of the NR1a/NR2A subtype. In addition, we utilized this cell line with the recombinant membrane targeted Ca2+ reporter, aequorin, in a functional assay of NMDA receptor activation. An evaluation of the coupling efficiency of NMDA receptor activation and aequorin response, as well as the pharmacological profile of this assay, illustrates the suitability of this cell line and the Ca2+ reporter assay to functionally identify novel NMDA receptor antagonists.

Cloning and characterization of a novel human inwardly rectifying potassium channel predominantly expressed in small intestine

A new member of the two transmembrane domain potassium (K+) channel family was identified and isolated from a human brain cDNA library. The cDNA clone contains an open reading frame which encodes a 360 amino acid sequence with a characteristic P domain flanked by two hydrophobic regions representing the membrane spanning segments. The closest homologue of this gene product is the inwardly rectifying potassium channel subunit, Kir1.2 (identity approximately 42%). Northern blot analysis of human tissues with a selective cDNA probe for this new K+ subunit showed a single major transcript of 3.4 kb predominantly expressed at high levels in small intestine, with lower levels in stomach, kidney and brain. The main regions of expression in the central nervous system were medulla, hippocampus and corpus callosum. cRNA-injected oocytes and transiently transfected HEK293 cells expressed a K+ conductance which displays an inward rectification. This conductance is blocked by cesium and barium but is insensitive to tolbutamide and diazoxide even upon co-transfection of this novel subunit with the plasmid encoding the sulfonylurea receptor SUR1. Taken together, these results demonstrate that we have isolated and characterized a novel K+ channel subunit belonging to the inwardly rectifying K+ (Kir) channel family to which, upon homology classification, we have given the nomenclature Kir7.1.

Chemical synthesis, structural and functional characterisation of noxiustoxin, a powerful blocker of lymphocyte voltage-dependent K+ channels

Two forms of the Centrudoides noxius scorpion noxiustoxin, containing an amidated and an acid C-terminus, were synthesized on a solid support by using Fmoc-chemistry and 2-(1H-benzotriazol-1-yl)-1,1,3,3-tetramethyluronium hexafluorophosphate (HBTU) coupling. Comparison of the two synthetic forms with the native toxin by tryptic mapping and CD spectroscopy shows that noxiustoxin possesses an amidated C-terminus and the same fold as all short scorpion toxins. Patch-clamp assays on B lymphocytes demonstrate that noxiustoxin inhibits the voltage-dependent K+ channels with 2 nM affinity, but does not affect the Ca(2+)-activated K+ channels. This toxin, because of its high affinity and specificity for voltage-gated K+ channel, may provide a powerful tool in the investigation of the role(s) of these channels in the T and B lymphocyte activation and proliferation.

A primary T-cell immunodeficiency associated with defective transmembrane calcium influx

We investigated a T-cell activation deficiency in a 3-month-old boy with protracted diarrhea, serious cytomegalovirus pneumonia, and a family history (in a brother) of cytomegalovirus infection and toxoplasmosis. In spite of detection of normal number of peripheral lymphocytes, T cells did not proliferate after activation by anti-CD3 and anti-CD2 antibodies, although proliferation induced by antigens was detectable. We sought to determine the origin of this defect as it potentially represented a valuable tool to analyze T-cell physiology. T-cell activation by anti-CD3 antibody or phytohemagglutinin (PHA) led to reduced interleukin-2 (IL-2) production and abnormal nuclear factor-activated T cell (NF-AT; a complex regulating the IL-2 gene transcription) binding activity to a specific oligonucleotide. T-cell proliferation was restored by IL-2. Early events of T-cell activation, such as anti-CD3 antibody-induced cellular protein tyrosine phosphorylation, p59fyn and p56lck kinase activities, and phosphoinositide turnover, were found to be normal. In contrast, anti-CD3 antibody-induced Ca2+ flux was grossly abnormal. Release from endoplasmic reticulum stores was detectable as tested in the presence of anti-CD3 antibody or thapsigargin after cell membrane depolarization in a K+ rich medium, whereas extracellular entry of Ca2+ was defective. The latter abnormality was not secondary to defective K+ channel function, which was found to be normal. A similar defect was found in other hematopoietic cell lineages and in fibroblasts as evaluated by both cytometry and digital video imaging experiments at a single-cell level. This primary T-cell immunodeficiency appears, thus, to be due to defective Ca2+ entry through the plasma membrane. The same abnormality did not alter B-cell proliferation, platelet function, and polymorphonuclear neutrophil (PMN) function. Elucidation of the mechanism underlying this defect would help to understand the physiology of Ca2+ mobilization in T cells.

The calcium current activated by T cell receptor and store depletion in human lymphocytes is absent in a primary immunodeficiency

Stimulation of antigen receptors of lymphocytes triggers a transitory release of Ca2+ from internal stores and the opening of a transmembrane Ca2+ conductive pathway. The latter underlies the sustained increase of intracellular free calcium concentration, and it seems to be a key event in the Ca(2+)-dependent biochemical cascade leading to T cell proliferation. Alternatively, pharmacological depletion of internal stores by itself activates Ca2+ influx. This has led to the hypothesis that antigen-triggered Ca2+ influx is secondary to Ca2+ release from internal stores. However, the precise relationship between antigen and Ca2+ release-activated Ca2+ currents remains unclear, particularly since neither of them has been electrophysiologically recorded in normal lymphocytes. Using the whole-cell and the perforated configurations of the patch clamp technique on peripheral blood lymphocytes, we found that a low amplitude Ca(2+)-selective current was triggered when intracellular stores were depleted by stimuli such as the intracellular perfusion of inositol triphosphate or thapsigargin and the extracellular perfusion of ionomycin. A similar current was elicited by the cross-linking of the T cell receptor-CD3 complex. This current displayed an inward rectification below 0 mV and was completely blocked by the divalent cation Cd2+. It was very selective for Ca2+ over Na+ and insensitive to changes in chloride concentration. The physiological relevance of this conductance was investigated with the analysis of abnormal Ca2+ signaling in lymphocytes from a patient suffering from a primary immunodeficiency associated with a defective T cell proliferation. Using fura-2 video imaging, an absence of Ca2+ influx was established in the patient's lymphocytes, whereas the Ca2+ release from internal stores was normal. This was the case whether cells were stimulated physiologically through their antigen receptors or with store depleting pharmacological agents. Most importantly, no Ca(2+)-selective current was elicited in these cells. Our data strongly suggest that the Ca2+ release-activated current underlies the sustained Ca2+ influx during antigenic stimulation and that it plays a key role in the immune function.

Defective transmembrane calcium influx demonstrated in a primary immunodeficiency by video-imaging

A rise of intracellular calcium concentration triggered by the engagement of various membrane receptors is a key event in the control of cell growth. This increase involves both a release of calcium from intracellular stores and the opening of a transmembrane calcium conductive pathway. Using video imaging to measure intracellular calcium concentration in individual fura-2-loaded cells, we detected a defect in calcium influx in lymphocytes and fibroblasts collected from patients affected by a rare and new form of primary immunodeficiency. In these cells, pharmacological agents such as thapsigargin or ionomycin, and the physiological activator bradykinin, only induced transient increases in cytoplasmic calcium level, due to the emptying of internal stores, while in control cells, this initial step is followed by an additional and sustained transmembrane calcium influx. The fact that calcium influx is absent in patient's fibroblasts indicates that the related deficiency, which is clinically associated with a lack of proliferation of T lymphocytes, also affects cells of the non-hematopoietic lineages. This study emphasizes the adequacy of single cell imaging for determining whether some forms of pathologies are associated with a disregulation of ionic fluxes, and for identifying them accurately.

Pattern of potassium channel expression in proliferating B lymphocytes depends upon the mode of activation

Ionic channel expression is highly regulated during mitogenesis. But it is not clear whether these regulations only follow intrinsic programs during the course of the cell cycle or if they also depend upon the external factors used to promote cell activation. B lymphocytes express two classes of potassium channels and can be stimulated to enter the cell cycle by distinct pathways. Thus, we have analyzed, with the patch-clamp technique, if the expression of channels varies when the cells are activated by different signals that lead to cell proliferation. We found that stimulation through Ag receptors increases the expression of calcium- and voltage-activated potassium channels, whereas a bacterial mitogen, LPS, only enhances the expression of the latter. Moreover, channel expression can still be modified in proliferating cells because stimulation of LPS-activated cells through Ag receptors induces rapid expression of calcium-activated channels. The use of inhibitors of mRNA synthesis revealed that this process depends upon gene transcription. Thus, differential induction of the expression of potassium channels is not only linked to the entry into the cell cycle but depends also on pathways of stimulation.

Pattern of potassium channel expression in proliferating B lymphocytes depends upon the mode of activation

Ionic channel expression is highly regulated during mitogenesis. But it is not clear whether these regulations only follow intrinsic programs during the course of the cell cycle or if they also depend upon the external factors used to promote cell activation. B lymphocytes express two classes of potassium channels and can be stimulated to enter the cell cycle by distinct pathways. Thus, we have analyzed, with the patch-clamp technique, if the expression of channels varies when the cells are activated by different signals that lead to cell proliferation. We found that stimulation through Ag receptors increases the expression of calcium- and voltage-activated potassium channels, whereas a bacterial mitogen, LPS, only enhances the expression of the latter. Moreover, channel expression can still be modified in proliferating cells because stimulation of LPS-activated cells through Ag receptors induces rapid expression of calcium-activated channels. The use of inhibitors of mRNA synthesis revealed that this process depends upon gene transcription. Thus, differential induction of the expression of potassium channels is not only linked to the entry into the cell cycle but depends also on pathways of stimulation.

Cross-linking of IgG receptors inhibits membrane immunoglobulin-stimulated calcium influx in B lymphocytes

By cross-linking membrane immunoglobulins (mIg), the antigenic stimulation of B lymphocytes induces an increase in intracellular free calcium levels ([Ca2+]i) because of a combination of release from intracellular stores and transmembrane influx. It has been suggested that both events are linked, as in a number of other cases of receptor-induced increase in [Ca2+]i. Conversely, in B lymphocytes, type II receptors for the Fc fragment of IgG (Fc gamma RII) inhibit mIg-mediated signaling. Thus, we have investigated at the level of single cells if these receptors could act on specific phases of mIg Ca2+ signaling. Lipopolysaccharide-activated murine B splenocytes and B lymphoma cells transfected with intact or truncated Fc gamma RII-cDNA were used to determine the domains of Fc gamma RII implicated in the inhibition of the Ca2+ signal. [Ca2+]i was measured in single fura-2-loaded cells by microfluorometry. The phases of release from intracellular stores and of transmembrane influx were discriminated by using manganese, which quenches fura-2, in the external medium as a tracer for bivalent cation entry. The role of membrane potential was studied by recording [Ca2+]i in cells voltage-clamped using the perforated patch-clamp method. Cross-linking of mIgM or mIgG with F(ab')2 fragments of anti-Ig antibodies induced a sustained rise in [Ca2+]i due to an extremely fast and transitory release of Ca2+ from intracellular stores and a long lasting transmembrane Ca2+ influx. The phase of influx, but not that of release, was inhibited by membrane depolarization. The increase in [Ca2+]i occurred after a delay inversely related to the dose of ligand. Co-cross-linking mIgs and Fc gamma RII with intact anti-Ig antibodies only triggered transitory release of Ca2+ from intracellular stores but no Ca2+ influx, even when the cell was voltage-clamped at negative membrane potentials. These transitory Ca2+ rises had similar amplitudes and delays to those induced by cross-linking mIgs alone. Thus, our data show that Fc gamma RII does not mediate an overall inhibition of mIg signaling but specifically affects transmembrane Ca2+ influx without affecting the release of Ca2+ from intracellular stores. Furthermore, this inhibition is not mediated by cell depolarization. Thus, Fc gamma RII represents a tool to dissociate physiologically the phases of release and transmembrane influx of Ca2+ triggered through antigen receptors.

Differential regulation of voltage- and calcium-activated potassium channels in human B lymphocytes

The expression and characteristics of K+ channels of human B lymphocytes were studied by using single and whole-cell patch-clamp recordings. They were gated by depolarization (voltage-gated potassium current, IKv, 11-20 pS) and by an increase in intracellular Ca2+ concentration (calcium-activated potassium current, IKCa, 26 pS), respectively. The level of expression of these channels was correlated with the activational status of the cell. Both conductances are blocked by tetraethylammonium, verapamil, and charybdotoxin, and are insensitive to apamin; 4-aminopyridine blocks IK, preferentially. We used a protein kinase C activator (PMA) or antibodies to membrane Ig (anti-mu) to activate resting splenocytes in culture. Although IKv was recorded in the majority of the resting lymphocytic population, less than 20% of the activated cells expressed this conductance. However, in this subset the magnitude of IKv was 20-fold larger than in resting cells. On the other hand, IKCa was detected in nearly one half of the resting cells, whereas all activated cells expressed this current. The magnitude of IKCa was, on average, 30 times larger in activated than in nonactivated cells. These results probably reflect that during the course of activation 1) the number of voltage-dependent K+ channels per cell decreases and increases in a small subset and 2) the number of Ca(2+)-dependent K+ channels per cell increases in all cells. We suggest that the expression of functional Ca(2+)- and voltage-activated K+ channels are under the control of different regulatory signals.

Differential regulation of voltage- and calcium-activated potassium channels in human B lymphocytes

The expression and characteristics of K+ channels of human B lymphocytes were studied by using single and whole-cell patch-clamp recordings. They were gated by depolarization (voltage-gated potassium current, IKv, 11-20 pS) and by an increase in intracellular Ca2+ concentration (calcium-activated potassium current, IKCa, 26 pS), respectively. The level of expression of these channels was correlated with the activational status of the cell. Both conductances are blocked by tetraethylammonium, verapamil, and charybdotoxin, and are insensitive to apamin; 4-aminopyridine blocks IK, preferentially. We used a protein kinase C activator (PMA) or antibodies to membrane Ig (anti-mu) to activate resting splenocytes in culture. Although IKv was recorded in the majority of the resting lymphocytic population, less than 20% of the activated cells expressed this conductance. However, in this subset the magnitude of IKv was 20-fold larger than in resting cells. On the other hand, IKCa was detected in nearly one half of the resting cells, whereas all activated cells expressed this current. The magnitude of IKCa was, on average, 30 times larger in activated than in nonactivated cells. These results probably reflect that during the course of activation 1) the number of voltage-dependent K+ channels per cell decreases and increases in a small subset and 2) the number of Ca(2+)-dependent K+ channels per cell increases in all cells. We suggest that the expression of functional Ca(2+)- and voltage-activated K+ channels are under the control of different regulatory signals.