SciCombinator

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Concept: Ion channel

169

The TRPM8 channel is a principal cold transducer that is expressed on some primary afferents of the somatic and cranial sensory systems. However, it is uncertain whether TRPM8-expressing afferent neurons have the ability to convey innocuous and noxious cold stimuli with sensory discrimination between the two sub-modalities. Using rat dorsal root ganglion (DRG) neurons and the patch-clamp recording technique, we characterized membrane and action potential properties of TRPM8-expressing DRG neurons at 24°C and 10°C. TRPM8-expressing neurons could be classified into TTX-sensitive (TTXs/TRPM8) and TTX-resistant (TTXr/TRPM8) subtypes based on the sensitivity to tetrodotoxin (TTX) block of their action potentials. These two subtypes of cold-sensing cells displayed different membrane and action potential properties. Voltage-activated inward Na+ currents were highly susceptible to cooling temperature and abolished by ~95% at 10°C in TTXs/TRPM8 DRG neurons, but remained substantially large at 10°C in TTXr/TRPM8 cells. In both TTXs/TRPM8 and TTXr/TRPM8 cells, voltage-activated outward K+ currents were substantially inhibited at 10°C, and the cooling-sensitive outward currents resembled A-type K+ currents. TTXs/TRPM8 neurons and TTXr/TRPM8 neurons were shown to fire action potentials at innocuous and noxious cold temperatures respectively, demonstrating sensory discrimination between innocuous and noxious cold by the two subpopulations of cold-sensing DRG neurons. The effects of cooling temperatures on voltage-gated Na+ channels and A-type K+ currents are likely to be contributing factors to sensory discrimination of cold by TTXs/TRPM8 and TTXr/TRPM8 afferent neurons.

Concepts: Neuron, Action potential, Ion channel, Temperature, Membrane potential, Potassium channel, Cold, Tetrodotoxin

168

Lubeluzole, which acts on various targets in vitro, including voltage-gated sodium channels (NaChs), was initially proposed as neuroprotectant. Lubeluzole structure contains a benzothiazole moiety (R-like) related to riluzole and a phenoxy-propranol-amine moiety (A-core) recalling propranolol. Both riluzole and propranolol are efficient NaCh blockers. We studied in detail the effects of lubeluzole (racemic mixture and single isomers), aforementioned lubeluzole moieties, and riluzole on NaChs to increase our knowledge about drug-channel molecular interactions. Compounds were tested on hNav1.4 NaChs, and F1586C or Y1593C mutants functionally expressed in HEK293 cells, using patch-clamp. Lubeluzole blocked NaChs with a remarkable effectiveness. No stereoselectivity was found. Compared to mexiletine, dissociation constant for inactivated channels was ≈600 times lower (≈11 nM), conferring to lubeluzole a huge use-dependence of great therapeutic value. The F1586C mutation impaired use-dependent block only partially, suggesting that additional amino acids are critically involved in high-affinity binding. Lubeluzole moieties were modest NaCh blockers. Riluzole blocked NaChs efficiently but lacked use-dependence, similarly to R-like. F1586C fully abolished A-core use-dependence, suggesting that A-core binds to the local anesthetic receptor. Thus lubeluzole likely binds to the local anesthetic receptor through its phenoxy-propranol-amine moiety, with consequent use-dependent behavior. Nevertheless, compared to other known NaCh blockers, lubeluzole adds a third pharmacophoric point through its benzothiazole moiety, that greatly enhances high-affinity binding and use-dependent block. If sufficient isoform specificity can be attained, the huge use-dependent block may help in the development of new NaCh inhibitors to provide pharmacotherapy for membrane excitability disorders, such as myotonia, epilepsy, or chronic pain.

Concepts: Protein, Acid, Amine, Functional group, Ion channel, Local anesthetic, Racemic mixture, Adderall

164

Identifying the determinants of neuronal energy consumption and their relationship to information coding is critical to understanding neuronal function and evolution. Three of the main determinants are cell size, ion channel density, and stimulus statistics. Here we investigate their impact on neuronal energy consumption and information coding by comparing single-compartment spiking neuron models of different sizes with different densities of stochastic voltage-gated Na(+) and K(+) channels and different statistics of synaptic inputs. The largest compartments have the highest information rates but the lowest energy efficiency for a given voltage-gated ion channel density, and the highest signaling efficiency (bits spike(-1)) for a given firing rate. For a given cell size, our models revealed that the ion channel density that maximizes energy efficiency is lower than that maximizing information rate. Low rates of small synaptic inputs improve energy efficiency but the highest information rates occur with higher rates and larger inputs. These relationships produce a Law of Diminishing Returns that penalizes costly excess information coding capacity, promoting the reduction of cell size, channel density, and input stimuli to the minimum possible, suggesting that the trade-off between energy and information has influenced all aspects of neuronal anatomy and physiology.Journal of Cerebral Blood Flow & Metabolism advance online publication, 19 June 2013; doi:10.1038/jcbfm.2013.103.

Concepts: Neuron, Action potential, Ion channel, Voltage-gated ion channel, Membrane potential, Information theory, Potassium channel, Voltage-dependent calcium channel

147

The venom of predatory marine cone snails mainly contains a diverse array of unique bioactive peptides commonly referred to as conopeptides or conotoxins. These peptides have proven to be valuable pharmacological probes and potential drugs because of their high specificity and affinity to important ion channels, receptors and transporters of the nervous system. Most previous studies have focused specifically on the conopeptides from piscivorous and molluscivorous cone snails, but little attention has been devoted to the dominant vermivorous species.

Concepts: Central nervous system, Nervous system, Ion channel, Neuroscience, Conotoxin, Conus, Conidae, Radula

144

Previous research has demonstrated that ClC-3 is responsible for volume-regulated Cl(-)current (ICl.vol) in vascular smooth muscle cells (VSMCs). However, it is still not clear whether and how ClC-3 is transported to cell membranes, resulting in alteration ofICl.vol.Methods and Results:Volume-regulated chloride current (ICl.vol) was recorded by whole-cell patch clamp recording, and Western blotting and co-immunoprecipitation were performed to examine protein expression and protein-protein interaction. Live cell imaging was used to observe ClC-3 transporting. The results showed that an overexpression of endophilin A2 could increaseICl.vol, while endophilin A2 knockdown decreasedICl.vol. In addition, the SH3 domain of endophilin A2 mediated its interaction with ClC-3 and promotes ClC-3 transportation from the cytoplasm to cell membranes. The regulation of ClC-3 channel activity was also verified in basilar arterial smooth muscle cells (BASMCs) isolated from endophilin A2 transgenic mice. Moreover, endophilin A2 increase VSMCs proliferation induced by endothelin-1 or hypo-osmolarity.

Concepts: Protein, Gene expression, Molecular biology, Cell membrane, Cytoplasm, Ion channel, Smooth muscle, Vascular smooth muscle

88

Fibroblast-like synoviocytes (FLS) are a key cell-type involved in rheumatoid arthritis (RA) progression. We previously identified the KCa1.1 potassium channel (Maxi-K, BK, Slo 1, KCNMA1) as a regulator of FLS and that KCa1.1 inhibition reduces disease severity in RA animal models. However, systemic KCa1.1 block causes multiple side effects and in this study, we aimed to determine whether the KCa1.1 β1-3-specific venom peptide blocker iberiotoxin (IbTX) reduces disease severity in animal models of RA without inducing major side effects. We used immunohistochemistry to identify IbTX-sensitive KCa1.1 subunits in joints of rats with a model of RA. Patch clamp and functional assays were used to determine if IbTX can regulate FLS through targeting KCa1.1. We then tested the efficacy of IbTX in ameliorating disease in two rat models of RA. Finally, we determined if IbTX causes side-effects including incontinence or tremors in rats, compared to those treated with the small molecule KCa1.1 blocker paxilline. IbTX-sensitive subunits of KCa1.1 are expressed by FLS in joints of rats with experimental arthritis. IbTX inhibits KCa1.1 channels expressed by FLS from patients with RA and by FLS from rat models of RA and reduces FLS invasiveness. IbTX significantly reduces disease severity in two rat models of RA. Unlike paxilline, IbTX does not induce tremors or incontinence in rats. Overall, IbTX inhibits KCa1.1 channels on FLS and treats rat models of RA without inducing side effects associated with non-specific KCa1.1 blockade and could become the basis for the development of a new treatment for RA.

Concepts: Protein, Rheumatoid arthritis, Rheumatology, Ion channel, Osteoarthritis, Enzyme inhibitor, Arthritis, Scorpion

77

Proopiomelanocortin (POMC) neurons in the arcuate nucleus of the hypothalamus (ARC) respond to numerous hormonal and neural signals, resulting in changes in food intake. Here, we demonstrate that ARC POMC neurons express capsaicin-sensitive transient receptor potential vanilloid 1 receptor (TRPV1)-like receptors. To show expression of TRPV1-like receptors in ARC POMC neurons, we use single-cell reverse transcription-polymerase chain reaction (RT-PCR), immunohistochemistry, electrophysiology, TRPV1 knock-out (KO), and TRPV1-Cre knock-in mice. A small elevation of temperature in the physiological range is enough to depolarize ARC POMC neurons. This depolarization is blocked by the TRPV1 receptor antagonist and by Trpv1 gene knockdown. Capsaicin-induced activation reduces food intake that is abolished by a melanocortin receptor antagonist. To selectively stimulate TRPV1-like receptor-expressing ARC POMC neurons in the ARC, we generate an adeno-associated virus serotype 5 (AAV5) carrying a Cre-dependent channelrhodopsin-2 (ChR2)-enhanced yellow fluorescent protein (eYFP) expression cassette under the control of the two neuronal POMC enhancers (nPEs). Optogenetic stimulation of TRPV1-like receptor-expressing POMC neurons decreases food intake. Hypothalamic temperature is rapidly elevated and reaches to approximately 39 °C during treadmill running. This elevation is associated with a reduction in food intake. Knockdown of the Trpv1 gene exclusively in ARC POMC neurons blocks the feeding inhibition produced by increased hypothalamic temperature. Taken together, our findings identify a melanocortinergic circuit that links acute elevations in hypothalamic temperature with acute reductions in food intake.

Concepts: Nervous system, Protein, Hypothalamus, Signal transduction, Receptor, Ion channel, Ligand, Receptor antagonist

49

The ability of azobenzene trimethylammonium bromide (azoTAB) to sensitize cardiac tissue excitability to light was recently reported. The dark, thermally relaxed trans- isomer of azoTAB suppressed spontaneous activity and excitation propagation speed, whereas the cis- isomer had no detectable effect on the electrical properties of cardiomyocyte monolayers. As the membrane potential of cardiac cells is mainly controlled by activity of voltage-gated ion channels, this study examined whether the sensitization effect of azoTAB was exerted primarily via the modulation of voltage-gated ion channel activity. The effects of trans- and cis- isomers of azoTAB on voltage-dependent sodium (INav), calcium (ICav), and potassium (IKv) currents in isolated neonatal rat cardiomyocytes were investigated using the whole-cell patch-clamp technique. The experiments showed that azoTAB modulated ion currents, causing suppression of sodium (Na+) and calcium (Ca2+) currents and potentiation of net potassium (K+) currents. This finding confirms that azoTAB-effect on cardiac tissue excitability do indeed result from modulation of voltage-gated ion channels responsible for action potential.

Concepts: Neuron, Action potential, Ion channel, Electrophysiology, Voltage-gated ion channel, Voltage-gated potassium channel, Potassium channel, Patch clamp

46

The single-span membrane protein KCNE3 modulates a variety of voltage-gated ion channels in diverse biological contexts. In epithelial cells, KCNE3 regulates the function of the KCNQ1 potassium ion (K(+)) channel to enable K(+) recycling coupled to transepithelial chloride ion (Cl(-)) secretion, a physiologically critical cellular transport process in various organs and whose malfunction causes diseases, such as cystic fibrosis (CF), cholera, and pulmonary edema. Structural, computational, biochemical, and electrophysiological studies lead to an atomically explicit integrative structural model of the KCNE3-KCNQ1 complex that explains how KCNE3 induces the constitutive activation of KCNQ1 channel activity, a crucial component in K(+) recycling. Central to this mechanism are direct interactions of KCNE3 residues at both ends of its transmembrane domain with residues on the intra- and extracellular ends of the KCNQ1 voltage-sensing domain S4 helix. These interactions appear to stabilize the activated “up” state configuration of S4, a prerequisite for full opening of the KCNQ1 channel gate. In addition, the integrative structural model was used to guide electrophysiological studies that illuminate the molecular basis for how estrogen exacerbates CF lung disease in female patients, a phenomenon known as the “CF gender gap.”

Concepts: Protein, Cell membrane, Action potential, Ion channel, Voltage-gated ion channel, Cystic fibrosis, Ion, Voltage-gated potassium channel

43

Efference copies refer to internal duplicates of movement-producing neural signals. Their primary function is to predict, and often suppress, the sensory consequences of willed movements. Efference copies have been almost exclusively investigated in the context of overt movements. The current electrophysiological study employed a novel design to show that inner speech - the silent production of words in one’s mind - is also associated with an efference copy. Participants produced an inner phoneme at a precisely specified time, at which an audible phoneme was concurrently presented. The production of the inner phoneme resulted in electrophysiological suppression, but only if the content of the inner phoneme matched the content of the audible phoneme. These results demonstrate that inner speech - a purely mental action - is associated with an efference copy with detailed auditory properties. These findings suggest that inner speech may ultimately reflect a special type of overt speech.

Concepts: Neuron, Ion channel, Electrophysiology, Neuroscience, Novel, Neurophysiology, Suppression, Suppression of dissent