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S C Cannon

Publications and source records attributed to S C Cannon.

At least 37 records · Page 2Linked to original sources
PubMed1997

From mutation to myotonia in sodium channel disorders.

Hyperkalemic periodic paralysis, paramyotonia congenita, and the potassium-aggravated myotonias are all caused by point mutations in the alpha-subunit of a sodium channel expressed selectively in skeletal muscle. This review updates the growing list of genotype-phenotype correlations for these mutations and summarizes the alterations in channel function they produce. A toxin-based in vitro model demonstrates that subtle defects in sodium channel inactivation are sufficient to cause myotonia and computer modeling suggests that specific types of inactivation defect may predispose to paralysis or myotonia.

Amino Acid Sequence↗
PubMed1996

Ion-channel defects and aberrant excitability in myotonia and periodic paralysis.

The myotonias and periodic paralyses are a diverse group of skeletal muscle disorders that share a common pathophysiological mechanism: all are caused by derangements in the electrical excitability of the sarcolemma. Mutations within coding regions of ion-channel genes have been identified recently as the underlying molecular defects in these heritable disorders. Chloride-channel mutations cause a reduction in the resting conductance, which enhances excitability and gives rise to myotonia. By contrast, missense mutations in the L-type Ca2+ channel reduce the electrical excitability of the fiber and cause a form of periodic paralysis. Mutations of the sodium channel impair inactivation of the channel, which, depending on the type and severity of the functional defect, results in either paralysis or myotonia.

Calcium Channels↗
PubMed1996

Sequence of the voltage-gated sodium channel beta1-subunit in wild-type and in quivering mice.

SCN1B, the human gene encoding the beta1-subunit of the voltage-gated sodium channel has previously been cloned and mapped to Chr 19q13.1. The sequence of the homologous mouse gene, Scn1b, has now been determined from cDNA. The mouse gene is highly conserved, encoding a predicted protein with 99%, 98% and 96% amino acid identity to the rat, rabbit, and human homologs, respectively. DNA sequence conservation is also striking in the 3' untranslated region which shows 67% and 98% to human and rat, respectively. Unlike the human and rat homologs, high expression of mRNA from the mouse gene is confined to adult skeletal muscle and brain, and is not observed in heart. As Scnlb maps to Chr 7, in close genetic proximity to the quivering gene (qv), the coding region of Scnlb was also cloned from a qvJ/qvJ homozygous mouse and assessed as a candidate for the site of this genetic defect. Comparison of qv and wild-type cDNAs showed no changes in the predicted amino acid sequence that could cause the qv phenotype. However, three silent polymorphisms in the DNA coding region indicate that Scn1b is close to qv, and is within a region of genetic identity with DBA/2J, the inbred background on which the qvJ allele arose.

Amino Acid Sequence↗
PubMed1996

Inactivation defects caused by myotonia-associated mutations in the sodium channel III-IV linker.

Missense mutations in the skeletal muscle Na+ channel alpha subunit occur in several heritable forms of myotonia and periodic paralysis. Distinct phenotypes arise from mutations at two sites within the III-IV cytoplasmic loop: myotonia without weakness due to substitutions at glycine 1306, and myotonia plus weakness caused by a mutation at threonine 1313. Heterologous expression in HEK cells showed that substitutions at either site disrupted inactivation, as reflected by slower inactivation rates, shifts in steady-state inactivation, and larger persistent Na+ currents. For T1313M, however, the changes were an order of magnitude larger than any of three substitutions at G1306, and recovery from inactivation was hastened as well. Model simulations demonstrate that these functional difference have distinct phenotypic consequences. In particular, a large persistent Na+ current predisposes to paralysis due to depolarization-induced block of action potential generation.

Animals↗
PubMed1996

Sodium channel defects in myotonia and periodic paralysis.

Myotonias and periodic paralyses constitute a diverse group of inherited disorders of muscle in which the primary defect is an alteration in the electrical excitability of the muscle fiber. The ion channel defects underlying these excitability derangements have recently been elucidated at the molecular and functional levels. This review focuses on sodium channel mutations that disrupt inactivation and thereby cause both the enhanced excitability of myotonia (muscle stiffness due to repetitive discharges) and the inexcitability resulting from depolarization during attacks of paralysis.

Animals↗
PubMed1995

Modulation of Na+ channel inactivation by the beta 1 subunit: a deletion analysis.

Na+ currents recorded from Xenopus oocytes expressing the Na+ channel alpha subunit alone inactivate with two exponential components. The slow component predominates in monomeric channels, while co-expression with the beta 1 subunit favors the fast component. Macropatch recordings show that the relative rates of these components are much greater than previously estimated from two-electrode measurements (approximately 30-fold vs approximately 5-fold). A re-assessment of steady-state inactivation, h infinity (V), shows that there is no depolarized shift of the slow component, provided a sufficiently long prepulse duration and repetition interval are used to achieve steady-state entry and recovery from inactivation, respectively. Deletion mutagenesis of the beta 1 subunit was used to define which regions of the subunit are required to modulate inactivation kinetics. The carboxy tail, comprising the entire predicted intracellular domain, can be deleted without a loss of activity; whereas small deletions in the extracellular amino domain or the signal peptide totally disrupt function.

Amino Acid Sequence↗
PubMed1995

Sodium channel inactivation is impaired in equine hyperkalemic periodic paralysis.

1. Equine hyperkalemic periodic paralysis (E-HPP) is a dominantly inherited disorder of muscle that causes recurrent episodes of stiffness (myotonia) and weakness in association with elevated serum K+. Affected horses carry a mutant allele of the skeletal muscle isoform of the Na channel alpha-subunit. To understand how this mutation may cause the disease phenotype, the functional defect in Na channel behavior was defined physiologically by recording unitary currents from cell-attached patches on normal and affected equine myotubes. 2. The presence of the mutation was confirmed in our cell line by restriction digest of polymerase chain reaction (PCR)-amplified genomic DNA. Myotubes from the affected horse were heterozygous for the point mutation that codes for a Phe to Leu substitution in S3 of domain IV. This assay provides a rapid technique to screen for the mutation in horses at risk. 3. The primary physiological defect in mutant Na channels was an impairment of inactivation. This defect was manifest as bursts of persistent activity during which the channel closed and reopened throughout a maintained depolarization. Disrupted inactivation slowed the decay of the ensemble-averaged current and produced an eightfold increase in the steady-state open probability measured at the end of a 40-ms pulse. This point mutation identifies a new region of the alpha subunit that is important for rapid inactivation of the channel. 4. The persistent Na current was produced by a distinct mode of gating. Failure of a mutant channel to inactivate was infrequent and occurred in groups of consecutive trials.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1993

GAP-43 augments G protein-coupled receptor transduction in Xenopus laevis oocytes.

The neuronal protein GAP-43 is thought to play a role in determining growth-cone motility, perhaps as an intracellular regulator of signal transduction, but its molecular mechanism of action has remained unclear. We find that GAP-43, when microinjected into Xenopus laevis oocytes, increases the oocyte response to G protein-coupled receptor agonists by 10- to 100-fold. Higher levels of GAP-43 cause a transient current flow, even without receptor stimulation. The GAP-43-induced current, like receptor-stimulated currents, is mediated by a calcium-activated chloride channel and can be desensitized by injection of inositol 1,4,5-trisphosphate. This suggests that neuronal GAP-43 may serve as an intracellular signal to greatly enhance the sensitivity of G protein-coupled receptor transduction.

Acetylcholine↗
PubMed1993

Modification of the Na+ current conducted by the rat skeletal muscle alpha subunit by coexpression with a human brain beta subunit.

Sodium channels are multimeric structures composed of alpha and beta subunits. Oocytes injected with RNA encoding only the alpha subunit express voltage-gated Na+ currents. The kinetics of inactivation, however, are abnormal. Co-injection of rat brain alpha and beta subunits modifies inactivation of INa such that it closely resembles endogenous currents. Here we show that a beta subunit derived from human brain directs the same functional modification of INa expressed by a rat skeletal muscle alpha subunit. This implies that functional domains for the interaction of alpha and beta subunits are highly conserved across both tissues and species.

Animals↗
PubMed1993

Functional expression of sodium channel mutations identified in families with periodic paralysis.

Two mutations in the sodium channel alpha subunit that have been implicated as the cause of periodic paralysis were studied by functional expression in a mammalian cell line. Both mutations disrupted inactivation without affecting the time course of the onset of the sodium current or the single-channel conductance. This is the same functional defect that was observed in myotubes cultured from affected patients and proves that these mutations are not benign polymorphisms. Unlike the currents in the myotubes, however, there was no consistent potassium dependence for the noninactivating component. These mutations also define new regions of the sodium channel alpha subunit that are involved in the process of inactivation.

Amino Acid Sequence↗
PubMed1993

Theoretical reconstruction of myotonia and paralysis caused by incomplete inactivation of sodium channels.

Muscle fibers from individuals with hyperkalemic periodic paralysis generate repetitive trains of action potentials (myotonia) or large depolarizations and block of spike production (paralysis) when the extracellular K+ is elevated. These pathologic features are thought to arise from mutations of the sodium channel alpha subunit which cause a partial loss of inactivation (steady-state Popen approximately 0.02, compared to < 0.001 in normal channels). We present a model that provides a possible mechanism for how this small persistent sodium current leads to repetitive firing, why the integrity of the T-tubule system is required to produce myotonia, and why paralysis will occur when a slightly larger proportion of channels fails to inactivate. The model consists of a two-compartment system to simulate the surface and T-tubule membranes. When the steady-state sodium channel open probability exceeds 0.0075, trains of repetitive discharges occur in response to constant current injection. At the end of the current injection, the membrane potential may either return to the normal resting value, continue to discharge repetitive spikes, or settle to a new depolarized equilibrium potential. This after-response depends on both the proportion of noninactivating sodium channels and the magnitude of the activity-driven K+ accumulation in the T-tubular space. A reduced form of model is presented in which a two-dimensional phase-plane analysis shows graphically how this diversity of after-responses arises as extracellular [K+] and the proportion of noninactivating sodium channels are varied.

Animals↗
PubMed1993

The cloning and expression of a sodium channel beta 1-subunit cDNA from human brain.

Electrical excitability of neurons and muscle cells is mediated largely through the actions of the voltage-gated sodium channel. Initiation and propagation of the action potential is a direct result of the precisely controlled inward flux of sodium through these channels. Much attention has been paid to the sodium channel alpha-subunit, the major, pore-forming component. However, alpha-subunits are associated with one or more smaller beta-subunits, which have been implicated in the critical fine tuning of the gating properties of the channel. To investigate the properties of the beta-subunit, we have isolated a cDNA encoding the human brain beta 1-subunit and assigned the corresponding gene to chromosome 19. We have also examined the effects of expressing the brain beta 1-subunit on the kinetics of a coexpressed muscle sodium channel alpha-subunit. Our results underscore the functional importance of the beta 1-subunit and imply a conserved mechanism for the interaction of the beta 1-subunit with different alpha-subunits.

Amino Acid Sequence↗
PubMed1993

Loss of Na+ channel inactivation by anemone toxin (ATX II) mimics the myotonic state in hyperkalaemic periodic paralysis.

1. Mutations that impair inactivation of the sodium channel in skeletal muscle have recently been postulated to cause several heritable forms of myotonia in man. A peptide toxin from Anemonia sulcata (ATX II) selectively disrupts the inactivation mechanism of sodium channels in a way that mimics these mutations. We applied ATX II to rat skeletal muscle to test the hypothesis that myotonia is inducible by altered sodium channel function. 2. Single-channel sodium currents were measured in blebs of surface membrane that arose from the mechanically disrupted fibres. ATX II impaired inactivation as demonstrated by persistent reopenings of sodium channels at strongly depolarized test potentials. A channel failed to inactivate, however, in only a small proportion of the depolarizing steps. With micromolar amounts of ATX II, the ensemble average open probability at the steady state was 0.01-0.02. 3. Ten micromolar ATX II slowed the relaxation of tension after a single twitch by an order of magnitude. Delayed relaxation is the in vitro analogue of the stiffness experienced by patients with myotonia. However, peak twitch force was not affected within the range of 0-10 microM ATX II. 4. Intracellular injection of a long-duration, constant current pulse elicited a train of action potentials in ATX II-treated fibres. After-depolarizations and repetitive firing often persisted beyond the duration of the stimulus. Trains of action potentials varied spontaneously in amplitude and firing frequency in a similar way to the electromyogram of a myotonic muscle. Both the after-depolarization and the post-stimulus firing were abolished by detubulating the fibres with glycerol. 5. We conclude that a loss of sodium channel inactivation alone, without changes in resting membrane conductance, is sufficient to produce the electrical and mechanical features of myotonia. Furthermore, in support of previous studies on myotonic muscle from patients, this model provides direct evidence that only a small proportion of sodium channels needs to function abnormally to cause myotonia.

Action Potentials↗
PubMed1991

A sodium channel defect in hyperkalemic periodic paralysis: potassium-induced failure of inactivation.

Hyperkalemic periodic analysis (HPP) is an autosomal dominant disorder characterized by episodic weakness lasting minutes to days in association with a mild elevation in serum K+. In vitro measurements of whole-cell currents in HPP muscle have demonstrated a persistent, tetrodotoxin-sensitive Na+ current, and we have recently shown by linkage analysis that the Na+ channel alpha subunit gene may contain the HPP mutation. In this study, we have made patch-clamp recordings from cultured HPP myotubes and found a defect in the normal voltage-dependent inactivation of Na+ channels. Moderate elevation of extracellular K+ favors an aberrant gating mode in a small fraction of the channels that is characterized by persistent reopenings and prolonged dwell times in the open state. The Na+ current, through noninactivating channels, may cause the skeletal muscle weakness in HPP by depolarizing the cell, thereby inactivating normal Na+ channels, which are then unable to generate an action potential. Thus the dominant expression of HPP is manifest by inactivation of the wild-type Na+ channel through the influence of the mutant gene product on membrane voltage.

Electrophysiology↗
PubMed1989

Anesthesia effects on the electrically evoked middle latency response in guinea pigs.

Recent data indicate that the electrically evoked middle latency response (EMLR) is useful for patient selection for cochlear implantation and may provide a test for determining safe levels of electrical stimulation in cochlear implant recipients. Some anesthetic agents have been reported to alter the auditory evoked middle latency response. The aim of our study was to examine the effects of ketamine and xylazine anesthesia on the EMLR in guinea pigs. A consistent, reproducible, and significant depression in the EMLR was observed after anesthesia. Response latencies were increased and the suprathreshold amplitudes were depressed initially, but later increased above preanesthetic values. Changes followed a predictable time course of depression and overshoot, which allows the investigator to compensate for these effects of anesthesia. No change in threshold was observed. The lack of threshold change and the predictable course of suprathreshold depression indicates that the EMLR may be useful to evaluate responsiveness of the auditory system to electrical stimulation in the anesthetized animal.

Anesthetics↗
PubMed1989

Evaluation of eighth nerve integrity by the electrically evoked middle latency response.

A reliable objective test for estimating the number and distribution of surviving eighth nerve fibers needs to be identified for selection of candidates for cochlear implantation. Kanamycin and ethacrynic acid administration in guinea pigs resulted in graded amounts of eighth nerve degeneration over time. The electrically-induced middle latency response (EMLR) was acutely recorded in these animals at specific post-drug times, followed by the immediate killing of the animals, histologic preparation, and spiral ganglion cell density determination. Significant progressive spiral ganglion cell loss was noted by 4 weeks that increased over time. While EMLR threshold remained stable over time, the slope of the EMLR input/output function decreased with increasing post-drug intervals in a manner directly correlated with reduction in spiral ganglion cell density.

Animals↗