On conduction in a bacterial sodium channel

Voltage-gated Na⁺-channels are transmembrane proteins that are responsible for the fast depolarizing phase of the action potential in nerve and muscular cells. Selective permeability of Na⁺ over Ca²⁺ or K⁺ ions is essential for the biological function of Na⁺-channels. After the emergence of the first high-resolution structure of a Na⁺-channel, an anionic coordination site was proposed to confer Na⁺ selectivity through partial dehydration of Na⁺ via its direct interaction with conserved glutamate side chains. By combining molecular dynamics simulations and free-energy calculations, a low-energy permeation pathway for Na⁺ ion translocation through the selectivity filter of the recently determined crystal structure of a prokaryotic sodium channel from Arcobacter butzleri is characterised. The picture that emerges is that of a pore preferentially occupied by two ions, which can switch between different configurations by crossing low free-energy barriers. In contrast to K⁺-channels, the movements of the ions appear to be weakly coupled in Na⁺-channels. When the free-energy maps for Na⁺ and K⁺ ions are compared, a selective site is characterised in the narrowest region of the filter, where a hydrated Na⁺ ion, and not a hydrated K⁺ ion, is energetically stable.

Ion channels are integral membrane proteins that control the passive diffusion of ions down their electrochemical gradient. According to the most permeating ion species, ion channels are classified in three categories: K⁺-channels, Na⁺-channels, and Ca²⁺-channels. The atomic structure of a K⁺-channel was the first to be solved experimentally more than 10 years ago. This structure inspired numerous computational studies, which revealed the mechanisms of conduction and selectivity in K⁺-channels. Recently, the first atomic structure of a Na⁺ selective channel has been solved. Here, molecular dynamics simulations and free-energy calculations are described and a possible mechanism for Na⁺ conduction is identified. In contrast to what it is observed in K⁺-channels, ion movements through Na⁺-channels appeared highly uncorrelated.

More gating charges are needed to open a Shaker K+ channel than are needed to open an rBIIA Na+ channel

This study presents what is, to our knowledge, a novel technique by means of which the ratio of the single gating charges of voltage-gated rat brain IIA (rBIIA) sodium and Shaker potassium ion channels was estimated. In the experiment, multiple tandems of enhanced green fluorescent protein were constructed and inserted into the C-terminals of Na(+) and K(+) ion channels. cRNA of Na(+) and K(+) ion channels was injected and expressed in Xenopus laevis oocytes. The two electrode voltage-clamp technique allowed us to determine the total gating charge of sodium and potassium ion channels, while a relative measure of the amount of expressed channels could be established on the basis of the quantification of the fluorescence intensity of membrane-bound channels marked by enhanced green fluorescent proteins. As a result, gating charge and fluorescence intensity were found to be positively correlated. A relative comparison of the single gating charges of voltage-gated sodium and potassium ion channels could thus be established: the ratio of the single gating charges of the Shaker potassium channel and the rBIIA sodium channel was found to be 2.5 +/- 0.4. Assuming the single channel gating charge of the Shaker K(+) channel to be approximately 13 elementary charges (well supported by other studies), this leads to approximately six elementary charges for the rBIIA sodium channel, which includes a fraction of gating charge that is missed during inactivation.

Adaptive Brownian dynamics for shape estimation of sodium ion channels

Ion channels are protein macromolecules that form biological nanotubes across the membranes of living cells. Given many possible geometrical shapes of an ion channel, we propose a computational scheme of selecting the model that best replicates experimental observations, using adaptive Brownian dynamics simulations together with discrete optimization algorithms. Brownian dynamics simulations emulate the propagation of individual ions through the sodium channel nanotube at a femto time second time scale and Angstrom unit (10(-10) meter) spatial scale.

Changes in ultrastructure and endogenous ionic channels activity during culture of HEK 293 cell line

Human embryonic kidney (HEK) 293 cells were characterised as an expression system for voltage-activated cationic channels. Current density for cationic channels intrinsically expressed in HEK 293 cells as well as cell ultrastructure was described after 7-11, 29-30 and 49-63 days of cell culture. Slowly activating outward potassium current with the current density varying between +10 and +26 pA/pF was observed in 72% to 95% of investigated cells. Rapidly inactivating outward potassium current with the current density varying between +7 and +10 pA/pF was present in 38% to 48% of all cells. 30% of cells exhibited voltage-activated calcium channel with the current density less than -1 pA/pF. Tetrodotoxin-sensitive sodium current with amplitudes between -1.4 and -2.2 pA/pF was initially present in 5% of cells, nevertheless, after 49-63 days of cell culture this proportion increased to 35%. Ultrastructure of HEK 293 cell surface, but not of cell's interior changed during cell culture. The longer the time after thawing the more microvilli and protrusions appear on the cell surface. Irregular cell contours hinder the cells to appose and only small patches of membranes form attachments. Staining of cells with a polycationic dye ruthenium red initially increased and decreased again following prolonged period of time in culture indicating regression of negatively charged layers of the cell surface coat. We suggest that the optimal time window for patch clamp experiment is between days 7 and 63 of cell culture due to alterations of cell surface.

Electron cryomicroscopic visualization of PomA/B stator units of the sodium-driven flagellar motor in liposomes

A motor protein complex of the bacterial flagellum, PomA/B from Vibrio alginolyticus, was reconstituted into liposomes and visualized by electron cryomicroscopy. PomA/B is a sodium channel, composed of two membrane proteins, PomA and PomB, and converts ion flux to the rotation of the flagellar motor. Escherichia coli and Salmonella have a homolog called MotA/B, which utilizes proton instead of sodium ion. PomB and MotB have a peptidoglycan-binding motif in their C-terminal region, and therefore PomA/B and MotA/B are regarded as the stator. Energy filtering electron cryomicroscopy enhanced the image contrast of the proteins reconstituted into liposomes and showed that two extramembrane domains with clearly different sizes stick out of the lipid bilayers on opposite sides. Image analysis combined with gold labeling and deletion of the peptidoglycan-binding motif revealed that the longer one, approximately 70 A long, is likely to correspond to the periplasmic domain, and the other, about half size, to the cytoplasmic domain.

BetaIVSigma1 spectrin stabilizes the nodes of Ranvier and axon initial segments

Saltatory electric conduction requires clustered voltage-gated sodium channels (VGSCs) at axon initial segments (AIS) and nodes of Ranvier (NR). A dense membrane undercoat is present at these sites, which is thought to be key for the focal accumulation of channels. Here, we prove that βIVΣ1 spectrin, the only βIV spectrin with an actin-binding domain, is an essential component of this coat. Specifically, βIVΣ1 coexists with βIVΣ6 at both AIS and NR, being the predominant spectrin at AIS. Removal of βIVΣ1 alone causes the disappearance of the nodal coat, an increased diameter of the NR, and the presence of dilations filled with organelles. Moreover, in myelinated cochlear afferent fibers, VGSC and ankyrin G clusters appear fragmented. These ultrastructural changes can explain the motor and auditory neuropathies present in βIVΣ1 −/− mice and point to the βIVΣ1 spectrin isoform as a master-stabilizing factor of AIS/NR membranes.

Functional innervation in tissue engineered models for in vitro study and testing purposes

The biotechnology industry is rapidly expanding and the emerging field of tissue engineering is projected to have a high impact in the near future. Recently the field of cellular, drug, and prosthetic delivery has melded with the field of tissue engineering to make simulated tissues. In addition to their roles as tissue substitutes for transplantation, these simulated tissues may provide more accurate models and environments for toxicology testing and the study of peripheral nerves. The current study demonstrates the importance of innervation, in general, for the function of engineered tissues. We observe that the presence of nerves in a tissue engineered (TE) human cornea model enhances the growth of the epithelium and the formation of its protective mucin layer. Innervation also confers protection to the epithelium from chemical insult, as determined by the level of post-treatment epithelial cell death. We demonstrate differential responses of the nerves to chemical stimuli by changes in intracellular sodium as measured by 2-photon microscopy. The 2-photon imaging techniques also allow for the visualization and study of the fine sensory axon fibers within the 3-dimensional tissue. This work demonstrates a role for innervation in the protective quality and function of the engineered tissue, and the potential to use the nerves themselves as indicators of the severity of an insult. These results are important to consider for the development of any optimized TE models for in vitro study and testing purposes.

Topology representing network enables highly accurate classification of protein images taken by cryo electron-microscope without masking

In single-particle analysis, a three-dimensional (3-D) structure of a protein is constructed using electron microscopy (EM). As these images are very noisy in general, the primary process of this 3-D reconstruction is the classification of images according to their Euler angles, the images in each classified group then being averaged to reduce the noise level. In our newly developed strategy of classification, we introduce a topology representing network (TRN) method. It is a modified method of a growing neural gas network (GNG). In this system, a network structure is automatically determined in response to the images input through a growing process. After learning without a masking procedure, the GNG creates clear averages of the inputs as unit coordinates in multi-dimensional space, which are then utilized for classification. In the process, connections are automatically created between highly related units and their positions are shifted where the inputs are distributed in multi-dimensional space. Consequently, several separated groups of connected units are formed. Although the interrelationship of units in this space are not easily understood, we succeeded in solving this problem by converting the unit positions into two-dimensional (2-D) space, and by further optimizing the unit positions with the simulated annealing (SA) method. In the optimized 2-D map, visualization of the connections of units provided rich information about clustering. As demonstrated here, this method is clearly superior to both the multi-variate statistical analysis (MSA) and the self-organizing map (SOM) as a classification method and provides a first reliable classification method which can be used without masking for very noisy images.

FRET between cardiac Na+ channel subunits measured with a confocal microscope and a streak camera

When and where proteins associate is a central question in many biomolecular studies. Förster resonance energy transfer (FRET) measurements can be used to address this question when the interacting proteins are labeled with appropriate donor and acceptor fluorophores. We describe an improved method to determine FRET efficiency that uses a mode-locked laser, a confocal microscope and a streak camera. We applied this method to study the association of alpha and beta(1) subunits of the human cardiac sodium channel. The subunits were tagged with the cyan and yellow variants of the green fluorescent protein (GFP) and expressed in human embryonic kidney (HEK293) cells. Pronounced FRET between the channel subunits in the endoplasmic reticulum (ER) suggested that the subunits associate before they reach the plasma membrane. The described method allows simultaneous measurement of donor and acceptor fluorescence decays and provides an intrinsically validated estimate of FRET efficiency.

Automatic particle pickup method using a neural network has high accuracy by applying an initial weight derived from eigenimages: a new reference free method for single-particle analysis

The single-particle analysis is a structure-determining method for electron microscope (EM) images which does not require crystal. In this method, the projections are picked up and averaged by the images of similar Euler angles to improve the signal to noise ratio, and then create a 3-D reconstruction. The selection of a large number of particles from the cryo-EM micrographs is a pre-requisite for obtaining a high resolution. To pickup a low-contrast cryo-EM protein image, we have recently found that a three-layer pyramidal-type neural network is successful in detecting such a faint image, which had been difficult to detect by other methods. The connection weights between the input and hidden layers, which work as a matching filter, have revealed that they reflect characters of the particle projections in the training data. The images stored in terms of the connection weights were complex, more similar to the eigenimages which are created by the principal component analysis of the learning images rather than to the averages of the particle projections. When we set the initial learning weights according to the eigenimages in advance, the learning period was able to be shortened to less than half the time of the NN whose initial weights had been set randomly. Further, the pickup accuracy increased from 90 to 98%, and a combination of the matching filters were found to work as an integrated matching filter there. The integrated filters were amazingly similar to averaged projections and can be used directly as references for further two-dimensional averaging. Therefore, this research also presents a brand-new reference-free method for single-particle analysis.

Sodium imaging of climbing fiber innervation fields in developing mouse Purkinje cells

Maturation of specific neuronal connections in the mature nervous system includes elimination of redundant synapses formed earlier during development. In the cerebellum of adult animals, each Purkinje cell (PC) is innervated by a single climbing fiber (CF). In early postnatal development each PC is innervated by multiple CFs and elimination of synapses formed by supernumerary CFs occurs until monoinnervation is established at around postnatal day 20 (P20) in mice. It is not clear whether multiple CFs, or only a single CF, translocate from the cell body of immature PCs to the developing dendrite and, in case several CFs translocate, whether they share or segregate their innervation fields. To localize CF innervation fields, we imaged changes in postsynaptic sodium concentration resulting from CF-mediated postsynaptic currents. We found that more than one CF translocates from an innervation field on the cell body of the PC to the developing dendrite and that these CFs share rather than segregate their innervation fields. We concluded that both the soma and the proximal dendrite of the PC are territories of competition for the developing CFs and that the overlapping of their termination fields may be the prerequisite for a local process of elimination of all but one CF, as previously demonstrated in the developing neuromuscular junction.

Development of nodes of Ranvier

The architecture and function of the nodes of Ranvier depend on several specialized cell contacts between the axon and myelinating glial cells. These sites contain highly organized multimolecular complexes of ion channels and cell adhesion molecules, closely connected with the cytoskeleton. Recent findings are beginning to reveal how this organization is achieved during the development of myelinated nerves. The role of membrane proteins involved in axoglial interactions and of associated cytoplasmic molecules is being elucidated, while studies of mutant mice have underlined the importance of glial cells and the specific role of axonal proteins in the organization of axonal domains.

Sodium channel Na(v)1.6 is expressed along nonmyelinated axons and it contributes to conduction

Nodes of Ranvier in myelinated fibers exhibit a complex architecture in which specific molecules organize in distinct nodal, paranodal and juxtaparanodal domains to support saltatory conduction. The clustering of sodium channel Na(v)1.6 within the nodal membrane has led to its identification as the major nodal sodium channel in myelinated axons. In contrast, much less is known about the molecular architecture of nonmyelinated fibers. In the present study, Na(v)1.6 is shown to be a significant component of nonmyelinated PNS axons. In DRG C-fibers, Na(v)1.6 is distributed continuously from terminal receptor fields in the skin to the dorsal root entry zone in the spinal cord. Na(v)1.6 is also present in the nerve endings of corneal C-fibers. Analysis of compound action potential recordings from wildtype and med mice, which lack Na(v)1.6, indicates that Na(v)1.6 plays a functional role in nonmyelinated fibers where it contributes to action potential conduction. These observations indicate that Na(v)1.6 functions not only in saltatory conduction in myelinated axons but also in continuous conduction in nonmyelinated axons.

An automatic particle pickup method using a neural network applicable to low-contrast electron micrographs

Three-dimensional reconstruction from electron micrographs requires the selection of many single-particle projection images; more than 10 000 are generally required to obtain 5- to 10-A structural resolution. Consequently, various automatic detection algorithms have been developed and successfully applied to large symmetric protein complexes. This paper presents a new automated particle recognition and pickup procedure based on the three-layer neural network that has a large application range than other automated procedures. Its use for both faint and noisy electron micrographs is demonstrated. The method requires only 200 selected particles as learning data and is able to detect images of proteins as small as 200 kDa.

The voltage-sensitive sodium channel is a bell-shaped molecule with several cavities

Voltage-sensitive membrane channels, the sodium channel, the potassium channel and the calcium channel operate together to amplify, transmit and generate electric pulses in higher forms of life. Sodium and calcium channels are involved in cell excitation, neuronal transmission, muscle contraction and many functions that relate directly to human diseases. Sodium channels--glycosylated proteins with a relative molecular mass of about 300,000 (ref. 5)--are responsible for signal transduction and amplification, and are chief targets of anaesthetic drugs and neurotoxins. Here we present the three-dimensional structure of the voltage-sensitive sodium channel from the eel Electrophorus electricus. The 19 A structure was determined by helium-cooled cryo-electron microscopy and single-particle image analysis of the solubilized sodium channel. The channel has a bell-shaped outer surface of 135 A in height and 100 A in side length at the square-shaped bottom, and a spherical top with a diameter of 65 A. Several inner cavities are connected to four small holes and eight orifices close to the extracellular and cytoplasmic membrane surfaces. Homologous voltage-sensitive calcium and tetrameric potassium channels, which regulate secretory processes and the membrane potential, may possess a related structure.

Three-dimensional reconstruction of single particle electron microscopy: the voltage sensitive sodium channel structure

Single particle analysis in electron microscopy allows direct observation of the reconstructed three-dimensional structures of protein molecules. This method enables a more comprehensive study of membrane proteins which have been problematic in structural studies using X-ray crystallography. These membrane proteins include the voltage-sensitive ion channel proteins, which play an important rule in neural activities, and have great medical significance. The method described is supported by the development of cryo-electron microscopy and the angular reconstitution method. This review summarizes certain principles governing single particle analysis employing angular reconstitution. This method was applied to our study of the voltage-sensitive sodium channel, and the results are discussed. With improvements in resolutions and statistical analyses, the single particle technique is considered to be advantageous in studies of the structural changes and molecular interactions of protein molecules.

mu-conotoxin GIIIA interactions with the voltage-gated Na(+) channel predict a clockwise arrangement of the domains

Voltage-gated Na(+) channels underlie the electrical activity of most excitable cells, and these channels are the targets of many antiarrhythmic, anticonvulsant, and local anesthetic drugs. The channel pore is formed by a single polypeptide chain, containing four different, but homologous domains that are thought to arrange themselves circumferentially to form the ion permeation pathway. Although several structural models have been proposed, there has been no agreement concerning whether the four domains are arranged in a clockwise or a counterclockwise pattern around the pore, which is a fundamental question about the tertiary structure of the channel. We have probed the local architecture of the rat adult skeletal muscle Na(+) channel (mu1) outer vestibule and selectivity filter using mu-conotoxin GIIIA (mu-CTX), a neurotoxin of known structure that binds in this region. Interactions between the pore-forming loops from three different domains and four toxin residues were distinguished by mutant cycle analysis. Three of these residues, Gln-14, Hydroxyproline-17 (Hyp-17), and Lys-16 are arranged approximately at right angles to each other in a plane above the critical Arg-13 that binds directly in the ion permeation pathway. Interaction points were identified between Hyp-17 and channel residue Met-1240 of domain III and between Lys-16 and Glu-403 of domain I and Asp-1532 of domain IV. These interactions were estimated to contribute -1.0+/-0.1, -0.9+/-0.3, and -1.4+/-0.1 kcal/mol of coupling energy to the native toxin-channel complex, respectively. mu-CTX residues Gln-14 and Arg-1, both on the same side of the toxin molecule, interacted with Thr-759 of domain II. Three analytical approaches to the pattern of interactions predict that the channel domains most probably are arranged in a clockwise configuration around the pore as viewed from the extracellular surface.

Subthreshold voltage noise due to channel fluctuations in active neuronal membranes

Voltage-gated ion channels in neuronal membranes fluctuate randomly between different conformational states due to thermal agitation. Fluctuations between conducting and nonconducting states give rise to noisy membrane currents and subthreshold voltage fluctuations and may contribute to variability in spike timing. Here we study subthreshold voltage fluctuations due to active voltage-gated Na+ and K+ channels as predicted by two commonly used kinetic schemes: the Mainen et al. (1995) (MJHS) kinetic scheme, which has been used to model dendritic channels in cortical neurons, and the classical Hodgkin-Huxley (1952) (HH) kinetic scheme for the squid giant axon. We compute the magnitudes, amplitude distributions, and power spectral densities of the voltage noise in isopotential membrane patches predicted by these kinetic schemes. For both schemes, noise magnitudes increase rapidly with depolarization from rest. Noise is larger for smaller patch areas but is smaller for increased model temperatures. We contrast the results from Monte Carlo simulations of the stochastic nonlinear kinetic schemes with analytical, closed-form expressions derived using passive and quasi-active linear approximations to the kinetic schemes. For all subthreshold voltage ranges, the quasi-active linearized approximation is accurate within 8% and may thus be used in large-scale simulations of realistic neuronal geometries.

Voltage-gated Na+ channel activity and connexin expression in Cx43-deficient cardiac myocytes

INTRODUCTION

Dynamic interplay between active and passive electrical properties of cardiac myocytes is based on interrelationships between various channels responsible for depolarizing and repolarizing ionic currents and intercellular conductances. Mice with targeted disruption of the connexin43 (Cx43) gene have hearts completely devoid of Cx43, the principal gap junctional protein expressed in mammalian hearts.

METHODS AND RESULTS

To determine whether cardiac myocytes that develop in an abnormal environment of reduced intercellular coupling have altered active membrane properties, we studied whole cell action potentials, Na+ channel currents, and Na+ channel expression and distribution via immunoblotting and confocal immunofluorescence in neonatal ventricular myocytes isolated from Cx43 wild-type, heterozygous, and homozygous null hearts. Action potential morphology, peak Na+ current, activation and inactivation kinetics, and Na+ channel protein expression and distribution were not different among myocytes isolated from wild-type, heterozygous, or null hearts. Active membrane properties and Na+ channel activity were completely normal in Cx43-deficient myocytes isolated from hearts that have been shown to exhibit markedly reduced Cx43 expression, gap junction number, and epicardial conduction delay.

CONCLUSION

Despite a genetic inability to produce Cx43 and a developmental history that culminates in marked gross cardiac morphologic abnormalities, premature death, and myocardial inexcitability ex vivo, cardiac Na+ channel distribution and function appear to be normal in Cx43 null hearts. Although intimate structural and functional interrelationships have been described between ion channels and gap junction channels, expression and function of Na+ channels is not affected by the absence of Cx43.

Number of subunits comprising the epithelial sodium channel

The human epithelial sodium channel (hENaC) is a hetero-oligomeric complex composed of three subunits, alpha, beta, and gamma. Understanding the structure and function of this channel and its abnormal behavior in disease requires knowledge of the number of subunits that comprise the channel complex. We used freeze-fracture electron microscopy and electrophysiological methods to evaluate the number of subunits in the ENaC complex expressed in Xenopus laevis oocytes. In oocytes expressing wild-type hENaC (alpha, beta, and gamma subunits), clusters of particles appeared in the protoplasmic face of the plasma membrane. The total number of particles in the clusters was consistent with the whole-cell amiloride-sensitive current measured in the same cells. The size frequency histogram for the particles in the clusters suggested the presence of an integral membrane protein complex composed of 17 +/- 2 transmembrane alpha-helices. Because each ENaC subunit has two putative transmembrane helices, these data suggest that in the oocyte plasma membrane, the ENaC complex is composed of eight or nine subunits. At high magnification, individual ENaC particles exhibited a near-square geometry. Functional studies using wild-type alphabeta-hENaC coexpressed with gamma-hENaC mutants, which rendered the functional channel differentially sensitive to methanethiosulfonate reagents and cadmium, suggested that the functional channel complex contains more than one gamma subunit. These data suggest that functional ENaC consists of eight or nine subunits of which a minimum of two are gamma subunits.

Insulin increases sodium (Na+) channel density in A6 epithelia: implications for expression of hypertension

Essential or primary hypertension is a multifactorial disease that is expressed as a result of complex interactions between genes and environmental influences. Several mutations in many different proteins are associated with expression of hypertension, including abnormalities in the epithelial sodium channel (ENaC) found in absorptive organs (i.e., distal colon, distal tubule of the nephron). Some of these mutations result in structural and/or functional alterations in ENaC-mediated Na+ entry in epithelia responsible for fluid and electrolyte balance and are associated with expression of hypertension. Studies support the notion that there is a link between ENaC and hypertension of both the monogenic (single gene mutation) and primary or essential type (a multifactorial disease). Alterations of other aspects of the environment of absorptive cells (e.g., hyperinsulinemia, hyperaldosteronemia, high plasma cortisol, high plasma Na+) have also been shown to elicit hyperabsorption of Na+ via ENaC and therefore could contribute significantly to expression of hypertension in people with intermediate phenotypes. This article describes an initial study in which the effects of an environmental factor, extracellular levels of insulin, on ENaC were examined in a normal kidney cell model. Electrophysiologic techniques revealed that ENaC density rapidly increased in response to addition of insulin to the basolateral bath. This autoregulatory recruitment of Na+ total channel density masked a slight decrease in open channel probability. Insulin's effect on ENaC function and implications on fluid and electrolyte balance and expression of primary hypertension is discussed.

Feedback inhibition of rat amiloride-sensitive epithelial sodium channels expressed in Xenopus laevis oocytes

1. Regulation of the amiloride-sensitive epithelial sodium channel (ENaC) is essential for the control of body sodium homeostasis. The downregulation of the activity of this Na+ channel that occurs when the intracellular Na+ concentration ([Na+]i) is increased is known as feedback inhibition. Although intracellular Na+ is the trigger for this phenomenon, its cellular and molecular mediators are unknown. 2. We used the 'cut-open oocyte' technique to control the composition of the intracellular milieu of Xenopus oocytes expressing rat ENaCs to enable us to test several factors potentially involved in feedback inhibition. 3. The effects of perfusion of the intracellular space were demonstrated by an electromicrographic study and the time course of the intracellular solution exchange was established by observing the effect of intracellular pH: a decrease from pH 7.4 to 6.5 reduced the amiloride-sensitive current by about 40 % within 2 min. 4. Feedback inhibition was observed in non-perfused oocytes when Na+ entry induced a large increase in [Na+]i. Intracellular perfusion prevented feedback regulation even though the [Na+]i was allowed to increase to values above 50 mM. 5. No effects on the amiloride-sensitive current were observed after changes in the concentration of Na+ (from 1 to 50 mM), Ca2+ (from 10 to 1000 nM) or ATP (from nominally free to 1 or 5 mM) in the intracellular perfusate. 6. We conclude that feedback inhibition requires intracellular factors that can be removed by intracellular perfusion. Although a rise in [Na+]i may be the trigger for the feedback inhibition of the ENaC, this effect is not mediated by a direct effect of Na+, Ca2+ or ATP on the ENaC protein.

The sodium channel has four domains surrounding a central pore

The voltage-gated sodium channel generates the action potential. This 300-kDa protein has four homologous regions, which are also homologous to the voltage-sensitive tetrameric potassium channel. We isolated sodium channels from Electrophorus electricus electroplax by detergent solubilization and immunoaffinity chromatography and studied their structure by electron microscopy of negatively stained specimens. Different projections were aligned, classified, and averaged. In side view, the channel protein exhibits the shape of a truncated cone, 14 nm in height. One end has a diameter of 12 nm and is asymmetric, while the other is more symmetric and has a diameter of 7-10 nm. In top views, the sodium channel appears to consist of four domains of different size and to have a stain-filled pore in the center.

Effects of tapering geometry and inhomogeneous ion channel distribution in a neuron model

Recent experiments have produced direct evidence on the existence of various dendritic voltage-gated ion channels, indicating that these neuronal components are not just a passive medium for the propagation of synaptic excitation but a putative source of neuronal excitability that is reflected in the activity patterns occurring on the soma. In order to study possible changes in neuronal excitability when the distribution of dendritic voltage-activated channels is non-uniform, and the dendritic geometry is not necessarily cylindric, we have developed a neuron model that incorporates two voltage-activated currents [I(Na) and I(K)], and in which space-dependent distributions of the system parameters can be treated in a mathematically simple and efficient way. Simulation results with the model showed that both linearly and exponentially tapering geometries led to marked anisotropy of the propagation of excitation, favouring the soma-to-dendrite direction. Exponentially decaying densities of dendritic voltage-activated channels, with appropriate choice of the parameters, induced bistable behaviour between the normal resting state and an intrinsic, sustained oscillation with cylindric as well as linear and exponential tapering dendritic geometry. Bistability could not be evoked when the model was reduced to a space-independent one (point-like soma). These results suggest that both tapering dendritic geometry and inhomogeneous distribution of ion channels may crucially affect the propagation and integration of synaptic potentials, and that changes in dendritic channel densities might underlie pathological electrophysiological activities.

Solvation, water permeation, and ionic selectivity of a putative model for the pore region of the voltage-gated sodium channel

This paper describes a molecular dynamics and molecular mechanics study of the solvation and selectivity of the narrow pore and vestibule region of a model-built structure for the voltage-gated sodium channel. The particular structure used was one proposed by Guy and Durell. However, many of the features we saw would likely be shared with other possible models for this channel, such as the one proposed by Lipkind and Fozzard. It was found that the water mobility was reduced in the channel and the water orientations were significantly ordered by the channel environment. Water mobility depended on protein mobility; in a computer experiment in which the protein was artificially frozen, channel water at 300 degrees K was immobilized. Water motions were defined in significant part by a series of discrete moves from one pattern of hydrogen bonding with particular amino acids to another. However, there are so many different hydrogen bonding patterns that a description of the motion in terms of transitions among a small number of discrete states is not appropriate. In the model whose solvation we explored, several charged residues seem to play a particularly significant role in determining solvation and water motions. Based on energy minimization studies, the structure clearly shows selectivity for univalent cations over anions.

Phosphorylation of brain sodium channels in the I--II linker modulates channel function in Xenopus oocytes

Voltage-gated sodium channels, which initiate action potentials in mammalian brain neurons, are modulated functionally by cAMP-dependent protein kinase A (PKA), resulting in reduced sodium current amplitude. Comparing brain and muscle sodium channels, we show that only the brain channel is modulated by PKA. The brain sodium channel I-II linker is both necessary and sufficient for PKA modulation, as shown by exchanging the I-II linker regions of the two channels. PKA consensus sites in the brain channel I-II linker were eliminated by deletion and site-specific mutagenesis. The mutant channels demonstrated decreased levels of phosphorylation when metabolically labeled in oocytes with [gamma-32P]-ATP, and they did not respond with a reduction in current magnitude after PKA induction. Modulation of the brain channel by PKA phosphorylation was mimicked by adding fixed negative charges at the PKA consensus sites, suggesting that the decrease in current was a direct result of the negative charge at one or more of the PKA sites in the I-II linker.

Structure and function of the beta 2 subunit of brain sodium channels, a transmembrane glycoprotein with a CAM motif

Voltage-gated sodium channels in brain neurons are complexes of a pore-forming alpha subunit with smaller beta 1 and beta 2 subunits. cDNA cloning and sequencing showed that the beta 2 subunit is a 186 residue glycoprotein with an extracellular NH2-terminal domain containing an immunoglobulin-like fold with similarity to the neural cell adhesion molecule (CAM) contactin, a single transmembrane segment, and a small intracellular domain. Coexpression of beta 2 with alpha subunits in Xenopus oocytes increases functional expression, modulates gating, and causes up to a 4-fold increase in the capacitance of the oocyte, which results from an increase in the surface area of the plasma membrane microvilli. beta 2 subunits are unique among the auxiliary subunits of ion channels in combining channel modulation with a CAM motif and the ability to expand the cell membrane surface area. They may be important regulators of sodium channel expression and localization in neurons.

Kinetic characterization of rat brain type IIA sodium channel alpha-subunit stably expressed in a somatic cell line

1. The rat brain type IIA Na+ channel alpha-subunit was stably expressed in Chinese hamster ovary (CHO) cells. Current through the expressed Na+ channels was studied using the whole-cell configuration of the patch clamp technique. The transient Na+ current was sensitive to TTX and showed a bell-shaped peak current vs. membrane potential relation. 2. Na+ current inactivation was better described by the sum of two exponentials in the potential range -30 to + 40 mV, with a dominating fast component and a small slower component. 3. The steady-state inactivation, h infinity, was related to potential by a Boltzmann distribution, underlying three states of the inactivation gate. 4. Recovery of the channels from inactivation at different potentials in the range -70 to -120 mV were characterized by an initial delay which decreased with hyperpolarization. The time course was well fitted by the sum of two exponentials. In this case the slower exponential was the major component, and both time constants decreased with hyperpolarization. 5. For a working description of the Na+ channel inactivation in this preparation, with a minimal deviation from the Hodgkin-Huxley model, a three-state scheme of the form O<-->I1<-->I2 was proposed, replacing the original two-state scheme of the Hodgkin-Huxley model, and the rate constants are reported. 6. The instantaneous current-voltage relationship showed marked deviation from linearity and was satisfactorily fitted by the constant-field equation. 7. The time course of activation was described by an m chi model. However, the best-fitted value of chi varied with the membrane potential and had a mean value of 2. 8. Effective gating charge was determined to be 4.7e from the slope of the activation plot, plotted on a logarithmic scale. 9. The rate constants of activation, alpha m and beta m, were determined. Their functional dependence on the membrane potential was investigated.

Probing sodium channel cytoplasmic domain structure. Evidence for the interaction of the rSkM1 amino and carboxyl termini

Epitopes for monoclonal antibodies directed against the purified adult rat skeletal muscle sodium channel (rSkM1) were localized using channel proteolysis and fusion proteins. The interactions between these and other monoclonal antibodies with site-specific polyclonal antibodies were used to investigate the spatial relationships among rSkM1 cytoplasmic segments. Competition. between antibodies for binding was performed using a solution-phase assay in which solubilized channel protein retains many of the biophysical characteristics of the rSkM1 protein in vivo. Our results support a model in which: 1) the amino terminus assumes a rigid structure having a fixed orientation with respect to other intracellular segments; 2) the interdomain 2-3 region is centrally located on the cytoplasmic surface of the channel, extends farther into the cytoplasm, and has an intermediate degree of flexibility; 3) the beginning of the amino terminus and end of the carboxyl terminus specifically interact with each other; and 4) domains 1 and 4 are adjacent. The sequences responsible for the interaction of the amino and carboxyl termini were identified by demonstrating the specific binding of a synthetic peptide encompassing the first 30 residues of the rSkM1 amino terminus to a fusion protein containing the rSkM1 carboxyl terminus.

Evidence for voltage-dependent S4 movement in sodium channels

The mutation R1448C substitutes a cysteine for the outermost arginine in the fourth transmembrane segment (S4) of domain 4 in skeletal muscle sodium channels. We tested the accessibility of this cysteine residue to hydrophilic methanethiosulfonate reagents applied to the extracellular surface of cells expressing these mutant channels. The reagents irreversibly increase the rate of inactivation of R1448C, but not wild-type, channels. Cysteine modification is voltage dependent, as if depolarization extends this residue into the extracellular space. The rate of cysteine modification increases with depolarization and has the voltage dependence and kinetics expected for the movement of a voltage sensor controlling channel gating.

Synthetic peptides corresponding to the four P regions of Electrophorus electricus Na+ channel: interaction with and organization in model phospholipid membranes

The hydropathy plot of the alpha subunit of the voltage-gated Na+ channel reveals four homologous repeats, each of which is homologous to Shaker type K+ channel monomer and contains six putative transmembrane segments and a hydrophobic segment within the loop connecting transmembrane segments S5 and S6. Current models predict that the four homologous segments [designated H5 or P regions (PR)] from the S5-S6 loop of each repeat lie in the aqueous pore. Peptides corresponding to the P regions of the four domains of the Electrophorus electricus (eel) Na+ channel (25-36 aa long, designated as PR-I, PR-II, PR-III, and PR-IV) and a 23-mer preceding PR-II (designated pre-PR-II) were synthesized and fluorescently labeled. The segments were then structurally and functionally characterized for their interaction with phospholipid membranes. Although the sequences of the four P regions are significantly different, they all bind to zwitterionic phospholipid membranes with similar partition coefficients (approximately 10(4) M-1). The pre-PR-II does not bind membranes at all. Resonance energy transfer measurements, between donor/acceptor-labeled pairs of peptides, revealed that besides the PR-I/PR-III pair, all other pairs form heteroaggregates but do not coassemble with unrelated membrane-bound peptide. Circular dichroism (CD) spectroscopy revealed that PR-I, PR-II, and PR-III adopt similar partial alpha-helical structures (approximately 30%) in 40% trifluoroethanol and in solutions of 1% sodium dodecylsulfate (SDS). The PR-IV (36 aa) adopts approximately 18% alpha-helical structure, and pre-PR-II gives a low CD signal. These findings are in line with proposed models in which the P regions are packed in close proximity in the lumen of the hydrophobic core of the channel. Furthermore, the finding that the PRs adopt similar partial alpha-helical structures in two different hydrophobic environments might suggest that partial alpha-helical structures also exist in the native channel as proposed by recent models. The results are discussed in terms of proposals that various regions of membrane proteins participate in driving folding or oligomerization of the parent molecules.

Sodium channel distribution in a spider mechanosensory organ

A site-directed antibody was used immunocytochemically to measure the distribution of sodium channels in the tissues of a spider mechanoreceptor organ. The VS-3 slit sense organ contains 7-8 pairs of bipolar sensory neurons; these neurons are representative of a wide range of arthropod mechanoreceptors. Sensory transduction is thought to occur at the tips of the dendrites and to cause action potentials that are regeneratively conducted to the cell bodies, although it has not been possible to confirm this by direct intracellular recordings from the dendrites. Wholemount preparations were labelled by immunofluorescence and thin sections were immunogold labelled, using an antibody to the highly conserved SP19 sequence of the voltage-activated sodium channel. Labelling for sodium channels was found in the neurons and in their surrounding glial cells. Both cytoplasm and membranes were labelled, but immunogold particles were clearly aligned along cell membranes, indicating that the majority of labelling represented membrane-bound sodium channels. Channel density in the dendrites was similar to the axons and higher than in the cell bodies, supporting the idea of active conduction in the sensory dendrites. Labelling in glial cell membranes was indistinguishable from the neighboring neurons, suggesting a significant role for sodium channels in the functions of these supporting cells.

Immunocytochemical localization of amiloride-sensitive sodium channels in the lower intestine of the hen

We have used polyclonal antibodies generated against purified bovine renal amiloride-sensitive Na+ channels to localize amiloride-sensitive Na+ channels within the lower intestine (colon and coprodeum) of the hen. These antibodies cross-reacted with two polypeptides exhibiting M(r)'s of 235 and 150 kDa on immunoblots of detergent-solubilized apical membrane fractions from both the colon and coprodeum. The apparent molecular masses of theses polypeptides are in agreement with the M(r)'s of 2 of the subunits of the renal high amiloride-affinity Na+ channel, namely the alpha and the beta (= amiloride binding) subunits. The cellular distribution of Na+ channels was determined by immunoperoxidase and indirect immunofluorescence cytochemical techniques. The apical (luminal) membrane and cytoplasm of villar principal cells in both colon and coprodeum exhibited immunoreactivity, whereas goblet cells were negative. Both principal and goblet cells of the crypts were also negative. We conclude that the amiloride-sensitive Na+ channels are localized to the principal cells of the intestinal villi and that these cells are responsible for intestinal Na+ absorption.

Sodium-dependent regulation of epithelial sodium channel densities in frog skin; a role for the cytoskeleton

1. A weak electroneutral sodium channel blocker 6-chloro-3,5-diamino-pyrazine-2-carboxamide was used to perform noise analysis on isolated epithelium from Rana fuscigula to determine the cellular mechanism underlying autoregulation of Na+ channel densities in response to a reduction in the mucosal Na+ concentration. 2. The inherent transport rates of these tissues were generally lower than in other frog skins. The macroscopic sodium current, INa, averaged 10.71 microA/cm2 and was mainly determined by the number of open channels (N(o)) which averaged 21.6 million/cm2. The calculated mean channel open probability (beta') was 0.38, and corresponded very closely to values previously determined by patch clamp. 3. Reducing the mucosal Na+ from 110 to 10 mM caused large increases in the open channel density, which stabilized the Na+ transport rate. N(o) increased from a mean value of 26.6 to 64.3 million/cm2 within 2 min. 4. Autoregulatory changes were induced primarily by increasing beta' by about 60% and to a lesser extent by an increase in NT, the total number of open and closed channels. 5. We also examined the role of the cytoskeleton in the regulation of Na+ channel densities. Colchicine treatment, which disrupted microtubules, had no apparent effect on the ability of the tissues to autoregulate their Na+ channel densities. 6. The integrity of the microfilaments were essential for autoregulatory changes in N(o). After we had disrupted the microfilaments with cytochalasin B, we observed a marked reduction in the ability of the tissues to increase N(o). 7. The mean N(o) did not increase in response to a drop in mucosal Na+ despite the fact that beta' increased by 69%. We, therefore, assumed that cytochalasin B did not affect Na+ channels already present in the membrane but interfered with recruitment of new channels. Significantly, we did not observe any increase in NT. 8. In kidney and other tight epithelia, microfilaments are responsible for regulating the delivery of newly synthesized membrane proteins. We believe that our results with cytochalasin-treated tissues support the theory that autoregulatory changes in N(o) are also regulated by the recruitment of channels from a cytoplasmic pool.

Ca(2+)-blockable, poorly selective cation channels in the apical membrane of amphibian epithelia. UO2(2+) reveals two channel types

This study deals with the effect of mucosal UO2(2+) on the Ca(2+)-blockable, poorly selective cation channels in the apical membrane of frog skin and toad urinary bladder. Our data show that UO2(2+) inhibits the Na+ currents through the amiloride-insensitive cation pathway and confirm a previously described stimulatory effect on the amiloride-blockade Na+ transport. Noise analysis of the Ca(2+)-blockable current demonstrates that the divalent also depresses the low-frequency Lorentzian (fc = 11.7 Hz) in the power density spectrum (PDS) and reveals the presence of high-frequency relaxation noise (fc = 58.5 Hz). The action of UO2(2+) is not reversed upon washout and is not accompanied by noise, typically induced by reversible blockers. The divalent merely depresses the plateau of the low-frequency Lorentzian, demonstrating a decrease in the number of conductive cation channels. Similarly, with mucosal K+ and Rb+, UO2(2+) also unmasks the high-frequency Lorentzian by depressing the noise from the slowly fluctuating cation channels (type S). In all experiments with mucosal Cs+, the PDS contains high-frequency relaxation noise (fc = 75.1 Hz in Rana temporaria, and 65.4 Hz in Rana ridibunda). An effect of UO2(2+) on the Cs+ currents and Lorentzian plateaus could not be demonstrated, suggesting that this monovalent cation does not pass through type S channels. Experiments with the urinary bladder revealed only a UO2(2+)-insensitive pathway permeable for Na+, K+, Rb+, and Cs+. We submit that in frog skin two cation-selective channels occur, distinguished by their spontaneous gating kinetics, their sensitivity to UO2(2+), and their permeability for Cs+. In toad urinary bladder, only one kind of cation-selective channel is observed, which resembles the UO2(2+)-insensitive channel in frog skin, with fast open-closed kinetics (type F).

Ca(2+)-blockable, poorly selective cation channels in the apical membrane of amphibian epithelia. Tetracaine blocks the UO2(2+)-insensitive pathway

We examined the effect of the local anesthetic tetracaine on the Ca(2+)-blockable, poorly selective cation channels in the isolated skin of Rana temporaria and the urinary bladder of Bufo marinus using noise analysis and microelectrode impalements. Experiments with frog skin demonstrated that mucosal concentrations of the compound up to 100 microM did not affect the Na+ current through type S channels (slowly fluctuating, UO2(2+)-blockable channels) and the associated noise. On the other hand, 20 microM mucosal tetracaine already suffices to inhibit approximately 50% of the current carried by Cs+ and Na+ through channel type F (fast fluctuating, UO2(2+)-insensitive channel) and So of the associated Lorentzian component. With 100 microM of the inhibitor the current and So values were reduced by at least 70-80%. The time course of the response to serosal tetracaine was markedly slower and the effects on the current and So were smaller. Possible effects on the basolateral K+ conductance were excluded on the basis of the lack of response of transepithelial K+ movements to 100 microM tetracaine. UO2(2+) and tetracaine together blocked the poorly selective cation pathways almost completely. Moreover, both agents retain their inhibitory effect in the presence of the other. In toad urinary bladder, the Ca(2+)-blockable channel is also tetracaine blockable. The concentration required for half-maximal inhibition is approximately 100 microM in SO4(2-) and approximately 20 microM in Cl-. The data with tetracaine complement those obtained with UO2(2+) and support the idea that the Ca(2+)-blockable current proceeds through two distinct classes of cation channels. Using tetracaine and UO2(2+) as channel-specific compounds, we demonstrated with microelectrode measurements that both channel types are located in the granulosum cells.

Reconstitution of sodium channels in large liposomes formed by the addition of acidic phospholipids and freeze-thaw sonication

Phosphatidylcholine (PC) alone or with phosphatidylethanolamine (PE) are sufficient for the reconstitution of Na+ channels in planar lipid bilayers. However, when Na+ channels were first reconstituted into liposomes using the freeze-thaw-sonication method, addition of acidic phospholipids, such as phosphatidylserine (PS), to the neutral phospholipids was necessary to obtain a significant toxin-modulated 22Na uptake. To further investigate the acidic phospholipid effect on reconstitution into liposomes, Na+ channels purified from Electrophorus electricus electrocytes were reconstituted into liposomes of different composition by freeze-thaw sonication and the effect of batrachotoxin and tetrodotoxin on the 22Na flux was measured. The results revealed that, under our experimental conditions, the presence of an acidic phospholipid was also necessary to obtain a significant neurotoxin-modulated 22Na influx. Though neurotoxin-modulated 22Na fluxes have been reported in proteoliposomes made with purified Na+ channels and PC alone, the 22Na fluxes were smaller than those found using lipid mixtures containing acidic phospholipids. Electron microscopy of negatively stained proteoliposomes prepared with PC, PC/PS (1:1 molar ratio), and PS revealed that the acidic phospholipid increases the size of the reconstituted proteoliposomes. The increment in size caused by the acidic phospholipid, due to the associated increase in internal volume for 22Na uptake and in area for Na+ channel incorporation, appears to be responsible for the large neurotoxin-modulated 22Na fluxes observed.

Fast and slow twitch skeletal muscle fibres differ in their distribution of Na channels near the endplate

Sodium channel distributions were measured in fast and slow twitch rodent skeletal muscle fibres using the loose patch voltage clamp technique. Large differences were found between these fibre types with respect to Na channel density in the perijunctional region. Fast twitch fibres exhibited a large increase in Na channel density near the endplate, while slow twitch fibres did not.

Amiloride-sensitive sodium channel is linked to the cytoskeleton in renal epithelial cells

Amiloride-sensitive sodium channels are localized to the microvillar domain of apical membranes in sodium-transporting renal epithelial cells. To elucidate the elements that maintain sodium channel distribution at the apical membrane, we searched for specific proteins associating with the channel. Triton X-100 extraction of A6 epithelial cells reveals that sodium channels are associated with detergent-insoluble and assembled cytoskeleton. Indirect immunofluorescence and confocal microscopy show that sodium channels are segregated to the apical microvillar membrane and colocalize with ankyrin, fodrin, and actin. We document by immunoblot analysis that ankyrin and fodrin remain associated with sodium channels after isolation and purification from bovine renal papillae. 125I-labeled ankyrine can be precipitated by anti-sodium-channel antibodies only in the presence of purified bovine sodium-channel complex. Direct binding of 125I-labeled ankyrin shows ankyrin binds to the 150-kDa subunit of the channel. Fluorescence photobleach lateral-diffusion measurements indicate sodium channels are severely restricted in their lateral mobility. We conclude that ankyrin links the amiloride-sensitive sodium channel to the underlying cytoskeleton and this association may sequester sodium channels at apical microvilli and maintain their polarized distribution in renal epithelial cells.

Gating properties of cardiac Na+ channels in cell-free conditions

In patches from neonatal rat heart myocytes, elementary Na+ currents were recorded at near threshold potentials in order to compare cardiac Na+ channels kinetics in the cell-attached mode with those in the inside-out mode. The transition from cell-attached to cell-free recording conditions caused a small prolongation of the conductive state of about 20%. This appeared within 8 min after patch excision regardless of the anionic composition (in mmol/liter) at the cytoplasmic membrane surface: 20 C1- plus 120 aspartate, 140 C1-, or 140 F-. Prolonged exposure (up to 50 min) to cell-free conditions evoked no additional changes and, specifically, left the monoexponential open-time distribution unchanged. Increased burst activity only developed in the cytoplasmic presence of F-, indicating that it is this artificial anion which influences reopening, but not the isolation of the Na+ channels from their natural environment per se. The mean number of openings per sequence (increase by a factor of 1.23 +/- 0.04) and tau decay of reconstructed macroscopic INa (increase by a factor of 1.32 +/- 0.06) responded rather weakly to F-. Cooling from 19 to 9 degrees C accentuated this F- effect significantly and led, at -65 mV, to pronounced burst activity. Moreover, the combined influence of F- and cooling induced a second, long-lasting and sometimes dominating open state. It is concluded that isolated cardiac Na+ channels largely preserve their intrinsic kinetic properties when facing a cytoplasmic environment with a quasi-physiological anionic composition.

Distribution of sodium channels in chronically demyelinated spinal cord axons: immuno-ultrastructural localization and electrophysiological observations

The immuno-ultrastructural localization of voltage-sensitive sodium channels was demonstrated within a central demyelinating lesion induced in the rat spinal cord by ethidium bromide/irradiation using polyclonal antibody 7493. Antibody 7493 has previously been shown to immunostain intensely axon membrane at nodes of Ranvier, and also perinodal astrocyte processes. At 25-35 days post injection/irradiation, the central portion of the demyelinating lesion is populated with chronically demyelinated axons and there is an absence of glial processes. Sodium channel immunoreactivity was not observed on the chronically demyelinated axolemma within this central portion of the lesion. Within the peripheral portion of the lesion demyelinated axons were occasionally abutted by astrocyte and Schwann cell processes. At these focal sites of apposition, the axon membrane displayed intense sodium channel immunoreactivity, while the abutting astrocyte and Schwann cell processes did not exhibit immunostaining. Also in the periphery of the lesion, some axons become ensheathed and myelinated by oligodendrocytes and Schwann cells. The axon membrane of circumferentially ensheathed axons displayed antibody 7493 immunostaining, and this immunoreactivity persisted on the axolemma until the ensheathing cytoplasmic processes compacted into myelin. Internodal axon membrane beneath the myelin sheath did not display sodium channel immunoreactivity, though (putative) developing nodal axon membrane adjacent to terminal paranodal loops exhibited robust sodium channel staining. Electrophysiological recordings within the ethidium bromide/irradiation lesion demonstrated that at least some axons conducted action potentials within the lesion, while others exhibited conduction block. These results indicate that there is a reorganization of sodium channels within the axon membrane of chronically demyelinated central axons.

Analysis of protease-sensitive regions in the skeletal muscle sodium channel in vitro and implications for channel tertiary structure

The tertiary structure of the rat skeletal muscle sodium channel was probed in vitro by determining regions of sensitivity to V-8 protease, trypsin, and chymotrypsin. Resultant channel fragments were identified with antibodies to defined sequences distributed along the primary structure. The temporal pattern of proteolysis was followed with channel protein in either detergent-phospholipid micelles or membrane fragments as well as with channel exposed to sodium dodecyl sulfate. Proteolysis in micelles and membranes occurred in discrete, reproducible steps that were similar in both systems. Although the size of intermediates varied slightly, their sequence of appearance was similar for all enzymes, suggesting that the observed pattern was determined by the relative accessibility of selected sites in the tertiary structure. No major change in channel organization appeared to occur after solubilization of membranes in nonionic detergents. Highly accessible sites in the native structure included the carboxyl terminus and the region linking the second and third internal repeat domains, while the amino terminus and the repeat domains themselves were relatively resistant to proteolysis unless the protein was denatured. Kinetically, interdomain II-III was the most readily cleaved; interdomains I-II and especially III-IV were less easily accessible. While domains I and IV appeared to remain intact throughout our experiments, limit fragments for epitopes associated with domains II and III suggest that cleavage eventually occurs at sites between the putative S5 and S6 helices in these domains.

Expression of sodium channel subtypes during development in rat skeletal muscle

This study contrasts the developmental patterns of expression of 2 subtypes of the voltage-dependent sodium channel in rat muscle that are differentiated by their immunoreactivity with monoclonal antibodies raised to the purified muscle sodium channel protein. One subtype is found in the transverse tubular (T) system of slow twitch fibers as well as the plasma membrane of fast and slow twitch fibers in the anterior tibial and soleus muscles. The second is present in the plasma membrane in all fibers of both muscles. The transverse tubular subtype exhibits 2 immunocytochemical staining patterns within muscle fibers, reticular and homogeneous, which may represent labeling of the developing T tubular system and of a cytoplasmic pool of alpha subunits of the sodium channel respectively. The reticular pattern eventually disappears in fast twitch fibers but persists into the adult stage in slow twitch fibers. The homogeneous pattern is also seen with antibodies to the plasma membrane subtype and disappears in early development as immunoreactivity to both subtypes gradually appears in the surface membrane. A reticular pattern is never seen with the plasma membrane subtype. The factors that modulate the expression of these subtypes is unknown.

Sodium channels in the cytoplasm of Schwann cells

Immunoblotting, ultrastructural immunocytochemistry, and tritiated saxitoxin ([3H]STX) binding experiments were used to study sodium channel localization in Schwann cells. Polyclonal antibody 7493, which is directed against purified sodium channels from rat brain, specifically recognizes a 260-kDa protein corresponding to the alpha subunit of the sodium channel in immunoblots of crude glycoproteins from rat sciatic nerve. Electron microscopic localization of sodium channel immunoreactivity within adult rat sciatic nerves reveals heavy staining of the axon membrane at the node of Ranvier, in contrast to the internodal axon membrane, which does not stain. Schwann cells including perinodal processes also exhibit antibody 7493 immunoreactivity, localized within both the cytoplasm and the plasmalemma of the Schwann cell. To examine further the possibility that sodium channels are localized within Schwann cell cytoplasm, [3H]STX binding was studied in cultured rabbit Schwann cells, both intact and after homogenization. Saturable binding of STX was significantly higher in homogenized Schwann cells (410 +/- 37 fmol/mg of protein) than in intact Schwann cells (214 +/- 21 fmol/mg of protein). Moreover, the equilibrium dissociation constant was higher for homogenized preparations (1.77 +/- 0.37 nM) than for intact Schwann cells (1.06 +/- 0.29 nM). These data suggest the presence of an intracellular pool of sodium channels or channel precursors in Schwann cells.

Modification of Na channel inactivation by alpha-chymotrypsin in single cardiac myocytes

The effects of alpha-chymotrypsin and trypsin on the macroscopic Na current in isolated guinea pig ventricular myocytes at 16 degrees C were investigated using the whole-cell voltage-clamp technique. Intracellular application of both enzymes reduced the extent of Na current inactivation during 20- to 50-ms depolarizing pulses. Elimination of fast inactivation by alpha-chymotrypsin was accompanied by a slowing of the rate of Na current decay through changes in both the time constants of current decay and the proportions of current undergoing a fast vs slow rate of decay. Treatment that reduced Na current decay to less than or equal to 10% within 20 ms was accompanied by a hyperpolarizing shift of the Na conductance/voltage relationship and an increase in the time-to-peak current that was most prominent for small depolarizations. Evidence for a significant slow inactivation process was obtained following removal of fast inactivation. The effect of trypsin (0.15-0.3 mg/ml) was less specific than alpha-chymotrypsin in that it also reduced Na conductance and increased leak current.

Beta 1 subunits of sodium channels. Studies with subunit-specific antibodies

The purified Na+ channel from rat brain consists of alpha (260 kDa), beta 1 (36 kDa), and beta 2 (33 kDa) subunits. Pure beta 1 subunits were prepared from purified rat brain Na+ channels which had been adsorbed to hydroxylapatite resin and used to prepare specific anti-beta 1 subunit antiserum. Antibodies purified from this antiserum by antigen affinity chromatography immunoprecipitate 125I-labeled, purified beta 1 subunits and purified Na+ channels (measured as high affinity [3H] saxitoxin binding sites) and recognize beta 1 subunits on immunoblots of solubilized rat brain membranes. The affinity-purified anti-beta 1 antibodies recognize beta 1 subunits in rat spinal cord, heart, skeletal muscle, and sciatic nerve, but do not detect immunoreactive beta 1 subunits in eel electroplax or eel brain. The developmental time course of expression of immunoreactive beta 1 subunits in rat forebrain was measured by immunoprecipitation followed by immunoblotting with affinity-purified anti-beta 1 antibodies. The amount of immunoreactive beta 1 subunits increased steadily to adult levels during the first 21 days of postnatal development.

Structural and developmental differences between three types of Na channels in dorsal root ganglion cells of newborn rats

The changes in Na current during development were studied in the dorsal root ganglion (DRG) cells using the whole-cell patch-clamp technique. Cells obtained from rats 1-3 and 5-8 days after birth were cultured and their Na currents were compared. On top of the two types of Na currents reported in these cells (fast-FA current and slow-S current) a new fast current was found (FN). The main characteristics of the three currents are: (i) The voltages of activation are -37, -36 and -23 mV for the FN, FA and S currents, respectively. (ii) The activation and inactivation kinetics of FN and FA currents are about five times faster than those of the S current. (iii) The voltages at which inactivation reaches 50% are -139, -75 and -23 mV for the FN, FA and S currents, respectively. The kinetics and voltage-dependent parameters of the three currents and their density do not change during the first eight days after birth. However, their relative frequency in the cells changes. In the 1-3 day-old rats the percent of cells with S, FA, and mixed S + FN currents is 22, 18, and 60% of the cells, respectively. In the 5-8 day-old, the percent of cells with S, FA, and FN + S is 10, 66 and 22%. The relative increase in the frequency of cells with FA current during development can contribute to the ease of action potential generation compared with cells with FN currents, which are almost completely inactivated under physiological conditions. The predominance of FA cells also results in a significant decrease in the relative frequency of cells with the high-threshold, slow current. Antibodies directed against a part of the S4 region of internal repeat I of the sodium channel (C1+, amino acids 210-223, eel channel numbering) were found to shift the voltage dependence of FA current inactivation (but not of FN or S currents) to more negative potentials. The effect was found only when the antibodies were applied externally. The results suggest that FN, FA and S types of Na currents are generated by channels, which are different in the topography of the C1+ region in the membrane.