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Catherine E Clarke

Publications and source records attributed to Catherine E Clarke.

7 recordsLinked to original sources
PubMed2006

Zinc and copper: pharmacological probes and endogenous modulators of neuronal excitability.

As well as being key structural components of many proteins, increasing evidence suggests that zinc and copper ions function as signaling molecules in the nervous system and are released from the synaptic terminals of certain neurons. In this review, we consider the actions of these two ions on proteins that regulate neuronal excitability. In addition to the established actions of zinc, and to a lesser degree copper, on excitatory and inhibitory ligand-gated ion channels, we show that both ions have a number of actions on selected members of the voltage-gated-like ion channel superfamily. For example, zinc is a much more effective blocker of one subtype of tetrodotoxin (TTX)-insensitive sodium (Na+) channel (NaV1.5) than other Na+ channels, whereas a certain T-type calcium (Ca2+) channel subunit (CaV3.2) is particularly sensitive to zinc. For potassium (K+) channels, zinc can have profound effects on the gating of certain KV channels whereas zinc and copper have distinct actions on closely related members of the 2 pore domain potassium channel (K2P) channel family. In addition to direct actions on these proteins, zinc is able to permeate a number of membrane proteins such as (S)-alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA)/kainate receptors, Ca2+ channels and some transient receptor potential (trp) channels. There are a number of important physiological and pathophysiological consequences of these many actions of zinc and copper on membrane proteins, in terms of regulation of neuronal excitability and neurotoxicity. Furthermore, the concentration of free zinc and copper either in the synaptic cleft or neuronal cytoplasm may contribute to the etiology of certain disease states such as Alzheimer's disease (AD) and epilepsy.

Alzheimer Disease↗
PubMed2004

Selective block of the human 2-P domain potassium channel, TASK-3, and the native leak potassium current, IKSO, by zinc.

Background potassium channels control the resting membrane potential of neurones and regulate their excitability. Two-pore-domain potassium (2-PK) channels have been shown to underlie a number of such neuronal background currents. Currents through human TASK-1, TASK-2 and TASK-3 channels expressed in Xenopus oocytes were inhibited by extracellular acidification. For TASK-3, mutation of histidine 98 to aspartate or alanine considerably reduced this effect of pH. Zinc was found to be a selective blocker of TASK-3 with virtually no effect on TASK-1 or TASK-2. Zinc had an IC(50) of 19.8 microM for TASK-3, at +80 mV, with little voltage dependence associated with this inhibition. TASK-3 H98A had a much reduced sensitivity to zinc suggesting this site is important for zinc block. Surprisingly, TASK-1 also has histidine in position 98 but is insensitive to zinc block. TASK-3 and TASK-1 differ at position 70 with glutamate for TASK-3 and lysine for TASK-1. TASK-3 E70K also had a much reduced sensitivity to zinc while the corresponding reverse mutation in TASK-1, K70E, induced zinc sensitivity. A TASK-3-TASK-1 concatamer channel was comparatively zinc insensitive. For TASK-3, it is concluded that positions E70 and H98 are both critical for zinc block. The native cerebellar granule neurone (CGN) leak current, IK(SO), is sensitive to block by zinc, with current reduced to 0.58 of control values in the presence of 100 microM zinc. This suggests that TASK-3 channels underlie a major component of IK(SO). It has recently been suggested that zinc is released from inhibitory synapses onto CGNs. Therefore it is possible that inhibition of IK(SO) in cerebellar granule cells by synaptically released zinc may have important physiological consequences.

Animals↗
PubMed2004

Molecular basis of slow activation of the human ether-a-go-go related gene potassium channel.

The human ether-á-go-go related gene (HERG) encodes the pore forming alpha-subunit of the rapid delayed rectifier K(+) channel which is central to the repolarization phase of the cardiac action potential. HERG K(+) channels have unusual kinetics characterized by slow activation and deactivation, yet rapid inactivation. The fourth transmembrane domain (S4) of HERG, like other voltage-gated K(+) channels, contains multiple positive charges and is the voltage sensor for activation. In this study, we mutated each of the positively charged residues in this region to glutamine (Q), expressed the mutant and wild-type (WT) channels in Xenopus laevis oocytes and studied them using two-electrode voltage clamp methods. K525Q channels activated at more hyperpolarized potentials than WT, whereas all the other mutant channels activated at more depolarized potentials. All mutants except for R531Q also had a reduction in apparent gating charge associated with activation. Mutation of K525 to cysteine (C) resulted in a less dramatic phenotype than K525Q. The addition of the positively charged MTSET to K525C altered the phenotype to one more similar to K525Q than to WT. Therefore it is not charge per se, but the specific lysine side chain at position 525, that is crucial for stabilizing the closed state. When rates of activation and deactivation for WT and mutant channels were compared at equivalent total (chemical + electrostatic) driving forces, K525Q and R528Q accelerated activation but had no effect on deactivation, R531Q slowed activation and deactivation, R534Q accelerated activation but slowed deactivation and R537Q accelerated deactivation but had no effect on activation. The main conclusions we can draw from these data are that in WT channels K525 stabilizes the closed state, R531 stabilizes the open state and R534 participates in interactions that stabilize pre-open closed states.

Amino Acid Sequence↗
PubMed2003

Structure of the HERG K+ channel S5P extracellular linker: role of an amphipathic alpha-helix in C-type inactivation.

The HERG K+ channel has very unusual kinetic behavior that includes slow activation but rapid inactivation. These features are critical for normal cardiac repolarization as well as in preventing lethal ventricular arrhythmias. Mutagenesis studies have shown that the extracellular peptide linker joining the fifth transmembrane domain to the pore helix is critical for rapid inactivation of the HERG K+ channel. This peptide linker is also considerably longer in HERG K+ channels, 40 amino acids, than in most other voltage-gated K+ channels. In this study we show that a synthetic 42-residue peptide corresponding to this linker region of the HERG K+ channel does not have defined structural elements in aqueous solution; however, it displays two well defined helical regions when in the presence of SDS micelles. The helices correspond to Trp585-Ile593 and Gly604-Tyr611 of the channel. The Trp585-Ile593 helix has distinct hydrophilic and hydrophobic surfaces. The Gly604-Tyr611 helix corresponds to an N-terminal extension of the pore helix. Electrophysiological studies of HERG currents following application of exogenous S5P peptides show that the amphipathic helix in the S5P linker interacts with the pore region of the channel in a voltage-dependent manner.

Amino Acid Sequence↗
PubMed2003

What are the roles of the many different types of potassium channel expressed in cerebellar granule cells?

Potassium (K) channels have a key role in the regulation of neuronal excitability. Over a hundred different subunits encoding distinct K channel subtypes have been identified so far. A major challenge is to relate these many different channel subunits to the functional K currents observed in native neurons. In this review, we have concentrated on cerebellar granule neurons (CGNs). We have considered each of the three principal super families of K channels in turn, namely, the six transmembrane domain, voltage-gated super family, the two transmembrane domain, inward-rectifier super family and the four transmembrane domain, leak channel super family. For each super family, we have identified the subunits that are expressed in CGNs and related the properties of these expressed channel subunits to the functional currents seen in electrophysiological recordings from these neurons. In some cases, there are strong molecular candidates for proteins underlying observed currents. In other cases the correlation is less clear. We show that at least 26 potassium channel alpha subunits are moderately or strongly expressed in CGNs. Nevertheless, a good empirical model of CGN function has been obtained with just six distinct K conductances. The transient KA current in CGNs, seems due to expression of Kv4.2 channels or Kv4.2/4.3 heteromers, while the KCa current is due to expression of large-conductance slo channels. The G-protein activated KIR current is probably due to heteromeric expression of KIR3.1 and KIR3.2. Perhaps KIR2.2 subunits underlie the KIR current when it is constitutively active. The leak conductance can be attributed to TASK-1 and or TASK-3 channels. With less certainty, the IK-slow current may be due to expression of one or more members of the KCNQ or EAG family. Lastly, the delayed-rectifier Kv current has as many as six different potential contributors from the extensive Kv family of alpha subunits. Since many of these subunits are highly regulated by neurotransmitters, physiological regulators and, often, auxiliary subunits, the resulting electrical properties of CGNs may be highly dynamic and subject to constant fine-tuning.

Animals↗
PubMed2000

Cloning and functional expression of a human orthologue of rat vanilloid receptor-1.

Capsaicin, resiniferatoxin, protons or heat have been shown to activate an ion channel, termed the rat vanilloid receptor-1 (rVR1), originally isolated by expression cloning for a capsaicin sensitive phenotype. Here we describe the cloning of a human vanilloid receptor-1 (hVR1) cDNA containing a 2517 bp open reading frame that encodes a protein with 92% homology to the rat vanilloid receptor-1. Oocytes or mammalian cells expressing this cDNA respond to capsaicin, pH and temperature by generating inward membrane currents. Mammalian cells transfected with human VR1 respond to capsaicin with an increase in intracellular calcium. The human VR1 has a chromosomal location of 17p13 and is expressed in human dorsal root ganglia and also at low levels throughout a wide range of CNS and peripheral tissues. Together the sequence homology, similar expression profile and functional properties confirm that the cloned cDNA represents the human orthologue of rat VR1.

Amino Acid Sequence↗