The voltage-gated K+ channel Kv2.1 has been intimately linked with neuronal apoptosis. After ischemic, oxidative, or inflammatory insults, Kv2.1 mediates a pronounced, delayed enhancement of K+ efflux, generating an optimal intracellular environment for caspase and nuclease activity, key components of programmed cell death. This apoptosis-enabling mechanism is initiated via Zn2+-dependent dual phosphorylation of Kv2.1, increasing the interaction between the channel's intracellular C-terminus domain and the SNARE(soluble N-ethylmaleimide-sensitive factor activating protein receptor) protein syntaxin 1A. Subsequently, an upregulation of de novo channel insertion into the plasma membrane leads to the critical enhancement of K+ efflux in damaged neurons. Here, we investigated whether a strategy designed to interfere with the cell death-facilitating properties of Kv2.1, specifically its interaction with syntaxin 1A, could lead to neuroprotection following ischemic injury in vivo. The minimal syntaxin 1A-binding sequence of Kv2.1 C terminus (C1aB) was first identified via a far-Western peptide screen and used to create a protherapeutic product by conjugating C1aB to a cell-penetrating domain. The resulting peptide (TAT-C1aB) suppressed enhanced whole-cell K+ currents produced by a mutated form of Kv2.1 mimicking apoptosis in a mammalian expression system, and protected cortical neurons from slow excitotoxic injury in vitro, without influencing NMDA-induced intracellular calcium responses. Importantly, intraperitoneal administration of TAT-C1aB in mice following transient middle cerebral artery occlusion significantly reduced ischemic stroke damage and improved neurological outcome. These results provide strong evidence that targeting the proapoptotic function of Kv2.1 is an effective and highly promising neuroprotective strategy.

In previous studies, some channels, called the “sleeper channels,” were reported to contribute to the shaping of the action potential (AP) only under non-physiological conditions. These channels have been hypothesized to play a role in providing a protective mechanism to prevent damage from neuronal hyperexcitation. Here we applied two-electrode current clamp at the primary branch point (1°BP) and the presynaptic terminal simultaneously on crayfish axons. Cadmium had minimal effects on AP shaping, suggesting the absence of calcium-activated potassium channels. Application of 1 mM TEA had minimal impact on AP waveform. In the presence of 4-Aminopyridine (4-AP), the same tetraethylammonium (TEA) concentration significantly prolonged AP duration, resembling the behaviors of sleeper channels. The kinetics of the TEA-sensitive channel (Kv(TEA)) is similar to the Kv2 family of mammalian K+ channels. TEA depolarized the potential after an AP and increased the AP duration in a dose-dependent manner, indicating that these channels contributed to maintaining AP waveform majorly during the hyperpolarization. The terminals were more sensitive to the blockers, suggesting a probability of regulation on neurotransmitter release. However, the TEA-sensitive channels at the crayfish axon had a higher affinity to TEA than the Kv2 channels. Pharmacological profiles, spatial distinction and function of the Kv(TEA) in the crayfish axon require further study.

Inwardly rectifying potassium (Kir) channels are critically important for regulating resting membrane potential in excitable cells, a job underscored by the severe pathophysiology associated with channel dysfunction. While all Kir channels require the activating lipid PIP2, many of these channels have diverse modulatory factors that couple to PIP2-dependent gating. Channels in the Kir3 (GIRK) family, in particular, have several co-activating elements, including G-protein betagamma subunits, ethanol, and sodium. During stimulation of Gq-coupled receptors, downstream activation of Protein Kinase C can phosphorylate and inhibit Kir3 channels, yet the mechanism of inhibition and phosphorylation sites are incompletely understood. We took a combined experimental and computational approach using neuronal Kir3.2 to investigate how phosphorylation at a putative PKC site identified in Kir3.1/3.4 could lead to channel inhibition. Kir3.2 inhibition was found to depend on the phosphorylation state of Ser-196, although mutagenesis data suggest it functions as an allosteric regulator of PKC inhibition. MD simulations identified a molecular switch whereby phosphorylation of Ser-196 recruits a critical gating residue, Arg-201, away from the sodium coordination site Asp-228. Neutralization of Ser-196 or Arg-201 resulted in less active channels which exhibited increased sensitivity to PKC inhibition. Additionally the interplay of PIP2 and PKC inhibition was examined in depth using homomeric Kir3.2, revealing that increases in channel-PIP2 interactions limit sensitivity to PKC inhibition, whereas low levels of PIP2 increase PKC sensitivity. Neutralization of Ser-196 uncoupled PKC inhibition from this PIP2 dependence. These studies suggest a model whereby PKC inhibition can occur along PIP2-dependent and PIP2-independent pathways, depending on the phosphorylation state of Ser-196.

Sigma receptors have been implicated in the regulation of the cardiovascular system. Some of the cardiovascular effects of sigma receptors may be through the modulation of autonomic neurons. Studies on the expression and cellular function of sigma receptors in autonomic neurons were conducted in neonatal rat intracardiac (ICG) and superior cervical ganglia (SCG). Individual neurons from SCG and ICG were shown to express transcripts encoding the sigma-1 receptor. The effects of sigma receptors activation on high-voltage-activated Ca2+ channels was studied in isolated neurons of these ganglia. Bath application of sigma receptor agonists depressed peak calcium channel currents in a dose-dependent manner and the rank order potency of haloperidol> ibogaine > (+)-pentazocine > DTG is consistent with the effects being mediated by a sigma-2 receptor. Sigma receptor antagonist, metaphit, blocked DTG-mediated inhibition of Ca2+ current. Sigma ligands also altered the biophysical properties of these channels. Activation of sigma receptors reversibly blocked delayed outwardly rectifying potassium channels, large conductance Ca2+-sensitive K+ channels, and the M-current with maximal inhibition >80%. The rank order potency of different sigma ligands suggests that the effect is mediated by sigma-1 receptor. While bath application of sigma ligands depolarized ICG neurons, the number of action potentials (AP) fired by the cells in response to depolarizing current pulses was decreased. Experiments on the signal transduction cascade mediated the inhibition of K+ and Ca2+ channels by sigma ligands showed that the signal transduction pathway does not involve a diffusible cytosolic second messenger or a G-protein. Sigma ligands also modulate voltage-gated Na+ channels (VGSC) in ICG neurons. Bath application of sigma ligands inhibited VGSC current with maximal inhibition >90% and altered the biophysical properties of VGSC. The latency of AP generation during depolarizing current ramp was increased by sigma ligands and this effect is through the inhibition of VGSC. These data suggest that activation of sigma receptors on autonomic neurons modulates voltage-gated Ca2+, K+ and Na+ channels and as a result, the generation of AP is inhibited in these neurons. Sigma receptors are likely altering the cell-to-cell signaling in autonomic ganglia and thus regulating cardiac function by the peripheral nervous system.

Neuronal, cardiac, and skeletal muscle electrical signaling is achieved through the highly regulated activity of several types of voltage-gated ion channels to produce an action potential (AP). Voltage-gated potassium (Kv) channels are responsible for repolarization of the AP. Kv channels are uniquely and heavily glycosylated proteins. Previous reports indicate glycosylation modulates gating of some Kv channel isoforms; often, terminal sialic acid residues alter Kv channel gating. Here, we questioned whether alterations in glycosylation impact Kv channel gating, thus altering APs and cardiac excitability. ST3Gal-IV, a sialyltransferase expressed at uniform levels throughout the heart, adds sialic acids to N- and O-glycans through alpha 2-3 linkages. Electrocardiograms (ECGs) suggest that cardiac conduction/rhythm are altered in ST3Gal-IV(-/-) animals, which show an increased incidence of arrhythmic beats. AP waveform parameters and two components of IK, the transient outward, Ito, and the slowly inactivating, IK,slow, were compared in neonatal control versus ST3Gal-IV(-/-) and glycosidase treated atrial and ventricular myocytes. Action potential durations (APDs) measured from ST3Gal-IV(-/-) and glycosidase treated atrial myocytes were lengthened significantly (~25-150%) compared to control; however, ventricular APDs were unaffected by changes in glycosylation. Consistently, atrial Ito and IK,slow activation were shifted to more depolarized potentials (by ~9-17 mV) in ST3Gal-IV(-/-) and glycosidase treated myocytes, while ventricular K+ currents were unaltered. Those channels responsible for producing Ito and IK,slow were examined under conditions of full and reduced glycosylation. Sialylation and N-glycosylation uniquely and differently impact gating of two mammalian Shaker family Kv channel isoforms, Kv1.4 and Kv1.5; Kv1.4 gating was unaffected by changes in channel glycosylation, while N-linked sialic acids, acting through electrostatic mechanisms, fully account for glycan effects on Kv1.5 gating. In addition, sialic acids modulate the gating of three Kv channel isoforms that are not N-glycosylated, Kv2.1, Kv4.2, and Kv4.3, through apparent electrostatic mechanisms. Click chemistry was utilized to confirm that these three isoforms are O-glycosylated and sialylated; thus, O-linked sialylation modulates gating of Kv2.1, Kv4.2, and Kv4.3. This study suggests that regulated or aberrant glycosylation alters the gating of channels producing IK in a chamber-specific manner, thus altering the rate of cardiac repolarization and potentially leading to arrhythmias.

No description available.

Lysophosphatidylcholine (LPC) is a by product of phospholipid metabolism, that under physiological conditions is maintained at a low level. However, through an enhanced degradation of phospholipids and/or a reduced catabolism, LPC accumulates in the plasma and fluids of patients with disorders underscored by inflammation - such as atherosclerosis, diabetes, ischaemia and epilepsy. Previous studies have demonstrated LPC to possess vasoactive properties, able to both induce and inhibit vasodilation. Furthermore, a variety of proteins are sensitive to LPC, including non-selective cation (NSC) channels and Ca2+-activated K+ (KCa) channels. These channels are intimately associated with the maintenance and regulation of vascular tone. The aim of this study was to elucidate the mechanisms underlying the vascular effect of LPC.Aortic segments were constricted with phenylephrine and exposed to cumulative concentrations of LPC, with an ensuing endothelium-dependent, concentration-dependent vasodilation. Inhibitors of nitric oxide synthase (NOS) and soluble guanylyl cyclase (sGC) abolished LPC-induced responses, implicating nitric oxide (NO) as the mediator. Two cation fluxes were implicated in the dilator activity of LPC - Ca2+ and K+. NSC channel antagonists and reduced extracellular Ca2+ concentration attenuated dilation and reduced the Ca2+ signal activated in isolated rat aortic endothelial cells (RAEC) by LPC, implicating endothelial Ca2+ influx in the response. In addition, LPC also evoked a robust hyperpolarisation of isolated RAEC membrane potential. The K+ channel antagonists TEA+, TRAM-34 and apamin, inhibitors of KCa channels, attenuated both the LPC-induced dilation and RAEC membrane hyperpolarisation, highlighting their potential role in mediating both these processes. HEK293 cells, which lack many of the channels and signalling pathways possessed by other cells, mimicked RAEC in their sensitivity to LPC, generating robust elevations of intracellular Ca2+ when exposed to this lysolipid. Likewise, membrane hyperpolarisations were also observed in HEK293 cells, however, these only occurred when cells expressed recombinant KCa channels. This suggests that KCa channel activation is dependent upon Ca2+ influx, not vice versa. Phospholipase C (PLC) inhibitor U73122, attenuated LPC-induced hyperpolarisation, raising the question as to the possible involvement of G-protein coupled receptors in the bioactivity of LPC. Alternately, LPC might initiate PLC activity, and subsequent NSC channel opening and Ca2+ influx via a perturbation of membrane integrity, like certain local anaesthetics. It is proposed that endothelial NSC-channel activation by LPC initiates endothelial cell signalling, with concomitant activation of Ca2+-sensitive proteins such as NOS, to bring about vasodilation, and KCa channels, which modulate membrane potential and in turn the driving force for Ca2+ entry.

GIRK2 est situé sur le chromosome 21, dont la trisomie cause le syndrome de Down (DS). Les proportionss des sous-populations de lymphocytes T sont altérées, le nombre de lymphocytes B circulants est diminué. Notre hypothèse est un défaut de contrôle de la domiciliation/recirculation des leucocytes par les cellules endothéliales (CE). Les CE formant la paroi des vaisseaux, assurent la néovascularisation, interagissent avec les cellules circulantes, initient l’adhésion donc, la réponse immune. Pour élucider l’influence de GIRK2 sur la fonction des CE, un modèle cellulaire in vitro a été mis au point. Des lignées de CE furent établies à partir de: moelle osseuse, thymus, ganglions lymphatiques périphériques, plaques de Peyer et cerveau de souris transgéniques dotées de copies additionnelles du gène et de souris contrôles. La biologie de l’endothélium fut abordée quant aux molécules d’adhésion, et processus d’adhésion et d’angiogenèse. Les CE issues des souris transgéniques expriment différents niveaux de CD29, CD34, leurs propriétés d’adhésion des lymphocytes ainsi que d’angiogenèse sont dramatiquement affectées. Le profil d’expression des gènes des CE de souris transgéniques montrent que parmi les molécules d’adhésion, chimiokines et récepteurs, VEGFs et récepteurs, plus d’un quart des ARNm est considérablement modifié par rapport aux contrôles. Nos résultats montrent clairement que le gène GIRK2 influence la function endothéliale des patients atteints de DS. / GIRK2 is located on chromosome 21, which trisomy is the cause of Down syndrome (DS). In DS, among other features, proportions of T lymphocytes subpopulations are altered and number of circulating B cells are decreased. We hypothesized that it is due to the disturbed control of homing/recirculation of lymphocytes by endothelial cells (ECs). ECs constitute the vessel wall, achieve the neovascularisation, interact with circulating cells, initiate the adhesion process thus, immunological response. To assess the GIRK2 gene influence on the function of ECs, an in vitro cellular model was established. ECs lines were established from bone marrow, thymus, peripheral lymph nodes, Peyer’s patches and brain from transgenic mice with additional copies of the gene and from normal control mice. Endothelium biology was investigated in the aspect of adhesion molecules as well as processes of adhesion and angiogenesis. ECs from transgenic mice have altered levels of CD29, CD34, their adhesive properties towards lymphoid cells are affected and their angiogenic properties are drastically different. cDNA microarray display for the gene expression pattern of ECs from transgenic mice showed that among adhesion molecules, chemokines, chemokine receptors, VEGFs and VEGFs receptors, more than one fourth of the mRNA was significantly modified compared to controls. Presented results give clear evidence that GIRK2 gene can influence the function of endothelial cells in DS patients.

La protéine humaine hERG (human ether-à-go-go related gene) s’associe en homo-tétramère pour former le canal potassique voltage-dépendant Kv11.1. C’est un acteur majeur de la repolarisation du potentiel d’action cardiaque par sa capacité à externaliser le potassium du cardiomyocyte. L’altération de sa fonction induit le syndrome du QT long à l’origine d’arythmies cardiaques et pouvant conduire à un arrêt du cœur. Ce syndrome parfois génétique provient le plus souvent d’une inhibition pharmacologique. De nombreux médicaments ont montré leur capacité à inhiber hERG en se fixant dans la lumière du canal. L’étude des interactions moléculaires entre hERG et médicaments intéresse les scientifiques depuis de nombreuses années. Très récemment, la première structure atomique de hERG à l’état ouvert par cryo-microscopie électronique a permis une avancée majeure dans la compréhension de l’agencement du pore du canal. De nombreuses questions restent malgré tout non résolues concernant les mécanismes de liaison des ligands. Plus encore, le développement d’approches biophysiques à partir de canal purifié permettraient de caractériser et d’anticiper des interactions avec les médicaments. Dans cette perspective, nous avons testé plusieurs stratégies pour obtenir le canal hERG purifié dans une forme stable, homogène et fonctionnelle. Notre étude est basée sur une construction simplifiée et chimérique du canal hERG, la version hERG(S1-coil). Chaque étape permettant la production et la purification d’une protéine membranaire a été optimisée en testant différentes techniques proposées par la littérature. Nous avons comparé les rendements d’expression du canal dans différents systèmes recombinants procaryotes ou eucaryotes. La quantité de protéine totale et le pourcentage de protéine fonctionnelle dans les membranes ont été étudiés. Dans un deuxième temps, le canal a été solubilisé puis purifié. Nous avons comparé les rendements de solubilisation et la stabilité protéique en fonction du type de détergent. En parallèle, nous avons mis au point des moyens techniques pour évaluer la fonction du canal au fur et à mesure du processus de production et purification. Le canal hERG(S1-coil) tétramérique et fonctionnel a finalement été identifié dans la fraction purifiée. Cependant, des optimisations sont encore à apporter pour conserver l’agencement tétramérique et empêcher l’agrégation au cours du temps avant de pouvoir envisager des études biophysiques et structurales. A terme, ces travaux pourraient profiter à la production et à la purification d’autres protéines membranaires oligomériques. / The human protein hERG (human ether-à-go-go related gene) assembles as homo-tetramer to form the voltage-gated potassium channel Kv11.1. This channel is involved in repolarization of the cardiac action potential by regulating the potassium release from cardiomyocytes. hERG malfunction was found to cause long QT syndrome, a disorder that predisposes affected patients to arrhythmias and sudden death. This can be due to congenital mutation in the hERG gene and, most frequently, it is caused by pharmacological agents. Several drugs are known to block the channel ion pathway, resulting in off-target inhibition of hERG. Consequently, understanding the molecular basis of drug binding to hERG has become a high priority. The recent determination of a near-atomic resolution structure of the opened channel, using cryo-electron microscopy, provides insights into how this channel work. But several questions are still unanswered to understand the mechanisms of hERG function and drug binding. Moreover, new biophysical protocols with the purified hERG channel would help scientists and industries to anticipate drug side effects. In this context, we investigated strategies to purify a stable, homogenous and functional hERG channel. Our study was based on a shorter and chimeric hERG channel, the hERG(S1-coil) version. We optimized each step from production to purification of membrane proteins by testing experimental protocols found in the literature. In this thesis project, we first compared production rates of the channel in several prokaryote and eukaryotes recombinant systems. Total protein produced and the percent of functional channel were investigated in membranes from each recombinant system. Then, the channel was extracted from membranes before purification. Solubilizing rates and channel stability were compared depending on detergents. In another hand, we also developed protocols to investigate the channel stability and function along production and purification. A tetrameric and functional channel was finally purified and identified by this strategy. More work however is still needed to improve channel homogeneity and stability before to be suitable for biophysical and structural studies. In the future, this work could also help investigations in production and purification of other oligomeric membrane proteins.

<p>Experimentally derived biomolecular structures were determined by Nuclear Magnetic Resonance (NMR). The properties of selected peptides and proteins in solution and in membrane mimicking micelles were observed by circular Dichroism (CD), mass spectrometry (MS), and other spectroscopic techniques.</p><p>The mDpl(1-30) peptide (30 residues) of the mouse Doppel protein was found to be positioned as an α-helix in a DHPC micelle. The same peptide can disrupt and cause leakage in small unilamellar vesicles.</p><p>Single D-amino acid isomers of Trp-cage (20 residues), the smallest peptide with a protein-like fold, were analyzed by CD spectroscopy and were found to have different secondary structures and melting temperatures. They were compared against MS measurements specially designed to reveal the secondary structure of proteins.</p><p>We studied a novel protein in E. coli of unknown structure that is encoded by the putative transcription factor ORF: ygiT (131 residues). This protein comprises a helix-turn-helix (HTH) domain in the C-terminus and contains two CxxC motives in the N-terminal domain, which binds Zn. This protein was named 2CxxC. We succeeded in overexpressing and purifying 2CxxC in E. coli with enough yield for a 13C, 15N uniformly labeled NMR sample. The chemical shift assignment was completed and the NMR structure was calculated in reducing, slightly acidic conditions (1mM DTT, pH 5.5). The determined HTH domain shows good similarity with structures predicted by a homology search, while the N-terminal domain has no other homologous structure in the Protein Data Bank (PDB).</p><p>The structure of the paddle region (27 residues) of the HsapBK(233-260) voltage and Ca+2 activated potassium channel, in DPC micelles, was determined by NMR. It shows a helix-turn-helix loop, which agrees well with the expected structure and could help to verify the proposed models of the voltage gating mechanism.</p><p>The C-repressor (dimer of 99 residues) of bacteriophage P2 was analyzed by NMR. We assigned the chemical shifts and NMR structure determination is under way.</p>