Plasticity of TTX-sensitive sodium channels PN1 and brain III in injured human nerves

Sensory neurones co-express voltage-gated sodium channels that mediate TTX-sensitive (TTX-S) and TTX-resistant (TTX-R) currents, which may contribute to chronic pain after nerve injury. We previously demonstrated that TTX-R channels were decreased acutely in human sensory cell bodies after central axotomy, but accumulated in nerve terminals after peripheral axotomy. We have now studied the TTX-S channels PN1 and Brain III, using specific antibodies for immunohistochemistry, in dorsal root ganglia (DRG) from 10 patients with traumatic central axotomy, nerves from 16 patients with peripheral axotomy, and controls. PN1 showed temporal changes similar to the TTX-R channels in sensory cell bodies of injured DRG. In contrast, Brain III was found only in injured nerves (not control nerves, or control/central axotomy DRG). PNI and Brain III are distinct targets for novel analgesics.

Non-coordinate regulation of endogenous epithelial sodium channel (ENaC) subunit expression at the apical membrane of A6 cells in response to various transporting conditions

In many epithelial tissues in the body (e.g. kidney distal nephron, colon, airways) the rate of Na(+) reabsorption is governed by the activity of the epithelial Na(+) channel (ENaC). ENaC activity in turn is regulated by a number of factors including hormones, physiological conditions, and other ion channels. To begin to understand the mechanisms by which ENaC is regulated, we have examined the trafficking and turnover of ENaC subunits in A6 cells, a polarized, hormonally responsive Xenopus kidney cell line. As previously observed by others, the half-life of newly synthesized ENaC subunits was universally short ( approximately 2 h). However, the half-lives of alpha- and gamma-ENaC subunits that reached the apical cell surface were considerably longer (t(12) > 24 h), whereas intriguingly, the half-life of cell surface beta-ENaC was only approximately 6 h. We then examined the effects of various modulators of sodium transport on cell surface levels of individual ENaC subunits. Up-regulation of ENaC-mediated sodium conductance by overnight treatment with aldosterone or by short term incubation with vasopressin dramatically increased cell surface levels of beta-ENaC without affecting alpha- or gamma-ENaC levels. Conversely, treatment with brefeldin A selectively decreased the amount of beta-ENaC at the apical membrane. Short term treatment with aldosterone or insulin had no effect on cell surface amounts of any subunits. Subcellular fractionation revealed a selective loss of beta-ENaC from early endosomal pools in response to vasopressin. Our data suggest the possibility that trafficking and turnover of individual ENaC subunits at the apical membrane of A6 cells is non-coordinately regulated. The selective trafficking of beta-ENaC may provide a mechanism for regulating sodium conductance in response to physiological stimuli.

An amiloride-sensitive and voltage-dependent Na+ channel in an HLA-DR-restricted human T cell clone

We investigated changes in voltage-gated Na+ currents and effects of extracellular Na+ on proliferation in HLA-DR-restricted human CD4+ alphabeta T cells after stimulation with a non-self antigenic peptide, M12p54-68. In the absence of antigenic peptide, neither single (n = 80) nor APC-contacted (n = 71) T cells showed voltage-gated inward currents recording with whole-cell patch-clamp techniques, even with Ca2+ and Na+ ions present in the perfusion solution. However, with the same recording conditions, 31% (26 of 84) of APC-contacted T cells stimulated with the antigenic peptide showed voltage-dependent inward currents that were elicited from -60 mV. The inward currents were not inhibited in extracellular Ca2+-free conditions or in the presence of 1 mM NiCl2. However, they were completely inhibited in extracellular Na+-free conditions, which were made by replacing Na+ with iso-osmotic N-methyl-d -glucamine or choline. The Na+ currents were insensitive to tetrodotoxin, a classical blocker of Na+ channels, but were dose-dependently inhibited by amiloride, a potassium-sparing pyrazine diuretic. Furthermore, the Ag-specific proliferative response of T cells was completely inhibited in Na+-free Tyrode's solution and was suppressed by amiloride in a dose-dependent manner. Our findings suggest that activation of amiloride-sensitive and voltage-gated Na+ channels would be an important step to allow an adequate influx of Na+ and maintain a sustained high Ca2+ level during T cell activation.

Diversity of expression of the sensory neuron-specific TTX-resistant voltage-gated sodium ion channels SNS and SNS2

The differential distribution of two tetrodotoxin resistant (TTXr) voltage-gated sodium channels SNS (PN3) and SNS2 (NaN) in rat primary sensory neurons has been investigated. Both channels are sensory neuron specific with SNS2 restricted entirely to those small dorsal root ganglion (DRG) cells with unmyelinated axons (C-fibers). SNS, in contrast, is expressed both in small C-fiber DRG cells and in 10% of cells with myelinated axons (A-fibers). All SNS expressing A-fiber cells are Trk-A positive and many express the vanilloid-like receptor VRL1. About half of C-fiber DRG neurons express either SNS or SNS2, and in most, the channels are colocalized. SNS and SNS2 are found both in NGF-responsive and GDNF-responsive C-fibers and many of these cells also express the capsaicin receptor VR1. A very small proportion of small DRG cells express either only SNS or only SNS2. At least four different classes of A- and C-fiber DRG neurons exist, therefore, with respect to expression of these sodium channels.

Epithelial Na+ channels and stomatin are expressed in rat trigeminal mechanosensory neurons

Caenorhabditis elegans MEC-4 and MEC-10 are subunits of the degenerin/epithelial Na+ channel (DEG/ENaC) ion channel superfamily thought to be associated with MEC-2 (a stomatin-like protein) in a mechanotransducing molecular complex in specialized touch sensory neurons. A key question is whether analogous molecular complexes in higher organisms transduce mechanical signals. To address this question, we selected mechanoreceptors of the rat vibrissal follicle-sinus complex in the mystacial pad and the trigeminal ganglia for an immunocytochemical and molecular biological study. RT-PCR of poly(A+) mRNA of rat trigeminal ganglia indicated that alpha-, beta-, and gamma-ENaC and stomatin mRNA are expressed in rat trigeminal ganglia. Using immunocytochemistry, we found that alpha-, beta-, and gamma-ENaC subunits and stomatin are localized in the perikarya of the trigeminal neurons and in a minor fraction of their termination site in the vibrissal follicle-sinus complex, where longitudinal lanceolate endings are immunopositive. We conclude that alpha-, beta-, and gamma-ENaC subunits as well as the candidate interacting protein stomatin are coexpressed in a mammalian mechanoreceptor, a location consistent with a possible role in mechanotransduction.

SNS/PN3 and SNS2/NaN sodium channel-like immunoreactivity in human adult and neonate injured sensory nerves

Two tetrodotoxin-resistant voltage-gated sodium channels, SNS/PN3 and SNS2/NaN, have been described recently in small-diameter sensory neurones of the rat, and play a key role in neuropathic pain. Using region-specific antibodies raised against different peptide sequences of their alpha subunits, we show by Western blot evidence for the presence of these channels in human nerves and sensory ganglia. The expected fully mature 260 kDa component of SNS/PN3 was noted in all injured nerve tissues obtained from adults; however, for SNS2/NaN, smaller bands were found, most likely arising from protein degradation. There was increased intensity of the SNS/PN3 260 kDa band in nerves proximal to the site of injury, whereas it was decreased distally, suggesting accumulation at sites of injury; all adult patients had a positive Tinel's sign at the site of nerve injury, indicating mechanical hypersensitivity. Injured nerves from human neonates showed similar results for both channels, but neonate neuromas lacked the SNS2/NaN 180 kDa molecular form, which was strongly present in adult neuromas. The distribution of SNS/PN3 and SNS2/NaN sodium channels in injured human nerves indicates that they represent targets for novel analgesics, and could account for some differences in the development of neuropathic pain in infants.

Localization of the type VI voltage-gated sodium channel protein in human CNS

The cellular distribution of the type VI human voltage-gated sodium channel (Type VI) was examined in selected human brain regions. Antibodies designed to be specific to rat and human Type VI were raised against a synthetic peptide from the predicted NH2-terminal of the protein, and used for an immunohistochemical investigation. Immunoblot experiments showed that purified antibodies specifically detected the presence of Type VI in transfected cells and human brain membrane preparations. Immunohistochemistry on perfusion fixed human tissue revealed a predominantly somato-dendritic distribution of Type VI in major output neurons of the cerebellum, cerebral cortex and hippocampus. The observed localisation of this channel may reflect an important role in the integration of synaptic input in the human CNS.

Monoclonal antibodies against the Androctonus australis hector scorpion neurotoxin I: characterisation and use for venom neutralisation

A series of monoclonal antibodies (mAbs) specific for the alpha-neurotoxin I (Aah I) from the venom of the dangerous Androctonus australis hector scorpion were obtained using carrier protein-coupled toxin. Competitive RIA, receptor assays and mouse toxicity tests were performed to characterise mAbs in terms of affinity and neutralisation. Cross-reactivity studies and two-site ELISA results allowed some classification of mAbs into three groups. One mAb, 9C2, was particularly interesting since it recognised the parent toxin I with a K(D) of 0.15 nM and was also reactive with toxins of the same immunological group. Its ability to neutralise the toxic effect of the parent toxin and the venom fraction has been investigated. This anti-Aah I mAb 9C2, associated with anti-Aah II mAb 4C1, provides a valuable tool to neutralise the toxicity of the venom.

Immunochemical analysis of the sodium channel in rodent and human eye

The presence of the amiloride-sensitive sodium channel (ASSC) in ocular tissues was studied with the aid of a polyclonal antiserum raised against the 14 amino acid peptide QGLGKGDKREEQGL. This sequence corresponds to the region 44-58 of the alpha subunit of the channel, termed ENaC, cloned from rat colon. The antibody titers, measured by the ELISA technique, rose to 1∶2560 4 weeks after immunization, and this bleed was used in all subsequent experiments. Immunoblotting with the polyclonal anti-alphaENaC serum, revealed a major band of 82-86 kDa in extracts prepared from whole bovine or rat retina; a minor component of 92 kDa in the extract from bovine ciliary body may represent a glycosylated species. Immunohistochemistry, using the alphaENaC-specific antiserum, revealed strong fluorescence in specific areas of the rat and human eye. Pronounced labelling was observed in the epithelial cell layer of the retina, the lens, as well as both the pigmented and the nonpigmented epithelium of the ciliary body and the iris. All of the cell layers (epithelium, endothelium and fibroblasts) in the cornea, the blood vessels in the iris, and iris epithelium, were also strongly immunopositive. The somatic body of the photoreceptor cells (cones and rods) in the inner and outer segments could be traced to forming a synapse in both the internal and external portions of the internal nuclear layer. The bipolar cells and ganglia in the neuronal compartment also exhibited occasional immunofluorescence. The method of fixation and the source of the tissue were important parameters for the immunochemical localization of the ENaC. The resolution was very poor when rat eye was fixed in Bouin's solution but this method was satisfactory for human tissues. For rat eye, optimum resolution was obtained with AMeX fixation. This widespread distribution of the ENaC generally colocalizes with the previously observed immunopositivity for the mineralocorticoid receptor such that steroid hormone-mediated ion regulation would appear to add a new parameter to the functional expression of ocular tissues.

Antiidiotypic antibody recognizes an amiloride binding domain within the alpha subunit of the epithelial Na+ channel

We previously raised an antibody (RA6.3) by an antiidiotypic approach which was designed to be directed against an amiloride binding domain on the epithelial Na+ channel (ENaC). This antibody mimicked amiloride in that it inhibited transepithelial Na+ transport across A6 cell monolayers. RA6.3 recognized a 72-kDa polypeptide in A6 epithelia treated with tunicamycin, consistent with the size of nonglycosylated Xenopus laevis alphaENaC. RA6.3 specifically recognized an amiloride binding domain within the alpha-subunit of mouse and bovine ENaC. The deduced amino acid sequence of RA6.3 was used to generate a three-dimensional model structure of the antibody. The combining site of RA6.3 was epitope mapped using a novel computer-based strategy. Organic residues that potentially interact with the RA6.3 combining site were identified by data base screening using the program LUDI. Selected residues docked to the antibody in a manner corresponding to the ordered linear array of amino acid residues within an amiloride binding domain on the alpha-subunit of ENaC. A synthetic peptide spanning this domain inhibited the binding of RA6.3 to alphaENaC. This analysis provided a novel approach to develop models of antibody-antigen interaction as well as a molecular perspective of RA6.3 binding to an amiloride binding domain within alphaENaC.

Carboxylmethylation of the beta subunit of xENaC regulates channel activity

The action of aldosterone to increase apical membrane permeability in responsive epithelia is thought to be due to activation of sodium channels. Aldosterone stimulates methylation of a 95-kDa protein in apical membrane of A6 cells, and we have previously shown that methylation of a 95-kDa protein in the immunopurified Na+ channel complex increases open probability of these channels in planar lipid bilayers. We report here that aldosterone stimulates carboxylmethylation of the beta subunit of xENaC in A6 cells. In vitro translated beta subunit, but not alpha or gamma, serves as a substrate for carboxylmethylation. Carboxylmethylation of ENaC reconstituted in planar lipid bilayers leads to an increase in open probability only when beta subunit is present. When the channel complex is immunoprecipitated from A6 cells and analyzed by Western blot with antibodies to the three subunits of xENaC, all three subunits are recognized as constituents of the complex. The results suggest that Na+ channel activity in A6 cells is regulated, in part, by carboxylmethylation of the beta subunit of xENaC.

Definition of epitopes for monoclonal antibodies developed against purified sodium channel protein: implications for channel structure

To test sodium channel structural models, we defined the epitopes for nineteen independently cloned monoclonal antibodies previously generated against purified, detergent-solubilized, adult rat skeletal muscle sodium channel protein using channel proteolysis, synthetic peptides, and fusion proteins. All identified epitopes were continuous and unique to the skeletal muscle subtype alpha-subunit. Of the nineteen independent clones, seventeen had epitopes located either in the origin of the amino-terminus or in the interdomain 2-3 region while only two antibodies had epitopes located in the mid-portion of the interdomain 1-2 region. No immunogenic regions were identified on the alpha-subunit's extracellular regions, interdomain 3-4 segment, or carboxyl-terminus or on channel beta-subunits. While immune tolerance may explain the lack of immunogenicity of extracellular regions, the lack of immunogenicity of most of the channel's cytoplasmic mass may be due to segment inaccessibility from organization of these regions as globular domains, to insertion of parts of these regions into the membrane phase, or to interaction with other protein elements. The definition of monoclonal antibody epitopes allows us to reinterpret previously reported monoclonal antibody competition studies, providing independent support for our model of sodium channel cytoplasmic domain structure. In addition, these data suggest additional testable hypotheses concerning the interactions of the sodium channel amino- and carboxyl-termini with each other as well as with other protein elements.

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.

Regeneration of the newt retina: order of appearance of photoreceptors and ganglion cells

The adult newt regenerates a functional retina following removal or destruction of the original retina. We studied the order of appearance of cell types in the regenerating retina by using immunohistochemical techniques. An antibody that recognizes the alpha subunit (260 kDa) of voltage-dependent Na+ channels was found to label a 255-kDa band in Western blots of crude membrane fractions from the normal retina. Cryosections of normal retina revealed intense Na+ channel immunoreactivity in somata and axons of ganglion cells, weaker immunoreactivity in somata of amacrine cells, and no immunoreactivity in the inner plexiform layer. In the same sections, immunoreactivity to a monoclonal antibody (RB-1) specific to newt cones was intense in the photoreceptor layer. In regenerating retinas, double staining with the Na+ channel antibody as a possible marker of ganglion cells and RB-1 antibody first revealed immunoreactive cells at the intermediate stage (three to five cells thick), which does not exhibit segregated synaptic layers. Na+ channel-immunoreactive ganglion cells appeared before the RB-1-immunoreactive photoreceptors. Because ganglion cells also appear before photoreceptor cells in normal development, common mechanisms may control both the generation and the regeneration of the newt retina.

Analysis of local structure in the D2/S1-S2 region of the rat skeletal muscle type 1 sodium channel using insertional mutagenesis

A reporter epitope was inserted at 11 positions in a region encompassing proposed transmembrane segments S1 and S2 in the second repeat domain (D2) of the rat skeletal muscle type 1 sodium channel. All mutations produced full-length membrane-associated protein following transfection into cultured cells, although the level of expression varied with insertion position. Characterization of cognate cRNAs for each mutation in Xenopus oocytes by two-electrode voltage clamp defined a permissive region between the proposed transmembrane regions in which these large insertions did not interfere with channel function. Two of the mutations, in which the point of insertion was within the proposed S1-S2 loop, demonstrated extracellular membrane labeling when studied either by antibody binding in oocytes or by confocal analysis following transfection into primary muscle cells. Our results define the likely boundaries of an extramembrane region linking the S1 and S2 transmembrane segments in D2 and confirm the extracellular location of this S1-S2 loop predicted by current models of channel tertiary structure.

Morphogenesis of the node of Ranvier: co-clusters of ankyrin and ankyrin-binding integral proteins define early developmental intermediates

AnkyrinG 480/270 kDa and three ankyrin-binding integral membrane proteins (neurofascin, NrCAM, and the voltage-dependent sodium channel) colocalize within a specialized domain of the spectrin-actin network found at axonal segments of nodes of Ranvier in myelinated axons. Before myelination in embryonic nerves, ankyrinG 480/270 kDa and the related ankyrin isoform ankyrinB 440 kDa are co-expressed along with NrCAM in an abundant, continuous distribution along the length of axons. This study has resolved intermediate stages in the developmental transition from a continuous distribution of ankyrinG 480/270 kDa in all axons to a highly polarized localization at the node of Ranvier in the developing rat sciatic nerve. The first detected event is formation of clusters containing the cell adhesion molecules neurofascin and NrCAM at sites independent of myelin-associated glycoprotein (MAG)-staining Schwann cell processes. Subsequent steps involve recruitment of ankyrinG 480/270 kDa and the voltage-dependent sodium channel to cluster sites containing cell adhesion molecules, and elaboration of MAG-staining Schwann cell processes adjacent to these cluster sites. Formation of the mature node of Ranvier results from the fusion of asynchronously formed pairs of clusters associated with MAG-positive Schwann cells flanking the site of presumed node formation. Studies with the hypomyelinating mutant mouse trembler demonstrate that the elaboration of compact myelin is not required for the formation of these clustered nodal intermediates. Clustering of neurofascin and NrCAM precedes redistribution of ankyrinG 480/270 kDa and the voltage-dependent sodium channel, suggesting that the adhesion molecules define the initial site for subsequent assembly of ankyrin and the voltage-dependent sodium channel.

Expression and distribution of voltage-sensitive sodium channels in pyrethroid-susceptible and pyrethroid-resistant Musca domestica

Knockdown resistance (kdr) to pyrethroid insecticides has been found in numerous insect species. kdr causes nerve insensitivity by altering the primary target of these insecticides, the voltage-sensitive sodium channel. In Musca domestica, cloning and sequencing of susceptible, kdr, and super-kdr alleles of the sodium channel gene (Msc) homologous to the Drosophila melanogaster para channel gene has revealed point mutations. The conservation of the nature and of the position of these mutations strongly suggests a role in the kdr mechanism. To determine if these mutations are associated with modifications of channel expression in adult flies, we investigated the localization of the Msc transcripts, and the size and the tissue distribution of the channel protein in pyrethroid-susceptible and super-kdr strains. Msc channels were mainly found in the cortical regions of the central nervous system with additional labeling in some neuronal processes and in the eyes. No qualitative or quantitative difference was observed between the strains. In immunoblotting experiments, anti-Msc antibodies bound to only one polypeptide of 260 kDa in adult brain. No differences were found in antibody staining between susceptible and pyrethroid-resistant strains. These results were correlated with those on Drosophila melanogaster, for which two sodium channel genes have been identified.

The binding site on cochlear stereocilia for antisera raised against renal Na+ channels is blocked by amiloride and dihydrostreptomycin

The mechanoelectrical transduction channels on hair cells have been suggested to be operated by tip links that are stretched when the hair bundle is deflected in the direction of the tallest row of stereocilia. Localising these channels is therefore an important test of this hypothesis. The transduction channels are known to be amiloride-sensitive and immunogold labelling with antibodies raised against the amiloride-sensitive epithelial Na+ channel from kidney (alpha NaCh), has suggested that sites with similar characteristics are located in the region where the tips of the shorter stereocilia appear to come into contact with the sides of the adjacent taller stereocilia rather than being associated directly with the tip links. Now, further immunocytochemical experiments have been performed to determine if amiloride and dihydrostreptomycin, both of which can block transduction, can affect this labelling. Immunofluorescent labelling of the stereocilia is obtained when surface preparations of the organ of Corti are fixed and incubated with alpha NaCh followed by an appropriate secondary antibody. This labelling is abolished by trypsinization prior to fixation but retained if the tissue is pretreated with amiloride and then trypsinized in its presence. Because amiloride is known to protect amiloride-binding sites from degradation by trypsin, these results suggest that alpha NaCh is revealing amiloride-binding sites on the stereocilia. Similarly, immunofluorescent labelling of the stereocilia is abolished if cochlear tissue is pretreated with dihydrostreptomycin (DHS) and fixed in its presence prior to incubation with alpha NaCh. Quantitative analysis of colloidal gold labelling using transmission electron microscopy shows that DHS treatment produces a significant reduction in the number of gold particles on stereocilia, especially in the region of contact between them. These results suggest that anti-Na+ recognises a site with characteristics similar to the mechanoelectrical transduction channels.

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.

Antibodies against GM1 ganglioside affect K+ and Na+ currents in isolated rat myelinated nerve fibers

High titers of anti-GM1 ganglioside antibodies (anti-GM1 antibodies) may be implicated in lower motor neuron disease. We studied the pathogenic role of anti-GM1 antibody using the petroleum jelly-gap voltage clamp technique on isolated single myelinated rat nerve fibers. Anti-GM1 antisera were obtained from rabbits immunized with GM1 ganglioside. Extracellularly applied anti-GM1 antisera without complement activity increased both the rate of rise and the amplitude of the K+ current elicited by step depolarization, with little effect on Na+ current. In the presence of active complement, however, anti-GM1 antibodies decreased the Na+ current, and caused a progressive increase of nonspecific leakage current. Neither complement alone nor complement-supplemented antisera from which anti-GM1 antibodies were depleted by affinity chromatography had any effect on ionic current. These observations indicate that anti-GM1 antibodies themselves can uncover K+ channels in the paranodal region, while anti-GM1 antibodies bound to the nodal membrane in the presence of complement may form antibody-complement complexes that block Na+ channels and disrupt the membrane at the node of Ranvier.

High titer antibody to mammalian neuronal sodium channels produces sustained channel block

Antibodies specific for neuronal sodium channels recognized the alpha and beta subunits of the voltage-gated sodium channel on immunoblots of crude rat and cat brain membranes and purified rat brain sodium channels. These antibodies did not recognize channels from rat cardiac or rabbit skeletal muscle. Antibody binding to blots of crude rat brain membranes was blocked by preabsorption of the antibody with purified rat brain sodium channels. Staining of the sodium channel alpha subunit on immunoblots of crude rat brain membranes was easily visualized at antibody dilutions of up to 1:150,000, which is at least 15-fold higher than that reported in previous studies. Addition of antibody produced in one chicken to the extracellular face of batrachotoxin-activated rat brain sodium channels in planar lipid bilayers produced a sustained block of the channel at either hyperpolarized (-65 to -45 mV) or depolarized potentials (+45 to +75 mV). This block was not produced when the antibody was added to the cytoplasmic face of the channel, or if preimmune antibody was added to the extracellular face of rat brain sodium channels.

Cell-specific expression of epithelial sodium channel alpha, beta, and gamma subunits in aldosterone-responsive epithelia from the rat: localization by in situ hybridization and immunocytochemistry

A highly selective, amiloride-sensitive, epithelial sodium channel from rat colon (rENaC), composed of three homologous subunits termed alpha, beta, and gamma rENaC, has been cloned by functional expression and was proposed to mediate electrogenic sodium reabsorption in aldosterone-responsive epithelia. To determine whether rENaC could account for sodium absorption in vivo, we studied the cellular localization of the sodium channel messenger RNA subunits by in situ hybridization and their cellular and subcellular distribution by immunocytochemistry in the kidney, colon, salivary, and sweat glands of the rat. In the kidney, we show that the three subunit mRNAs are specifically co-expressed in the renal distal convoluted tubules (DCT), connecting tubules (CNT), cortical collecting ducts (CCD), and outer medullary collecting ducts (OMCD), but not in the inner medullary collecting ducts (IMCD). We demonstrate co-localization of alpha, beta, and gamma subunit proteins in the apical membrane of a majority of cells of CCD and OMCD. Our data indicate that alpha, beta, and gamma subunit mRNAs and proteins are co-expressed in the distal nephron (excepting IMCD), a localization that correlates with the previously described physiological expression of amiloride-sensitive electrogenic sodium transport. Our data, however, suggest that another sodium transport protein mediates electrogenic amiloride-sensitive sodium reabsorption in IMCD. We also localized rENaC to the surface epithelial cells of the distal colon and to the secretory ducts of the salivary gland and sweat gland, providing further evidence consistent with the hypothesis that the highly selective, amiloride-sensitive sodium channel is physiologically expressed in aldosterone-responsive cells.

Conformational mapping of the cytosolic linker between domains III and IV of the cardiac Na+ channel protein and binding studies with a site-directed channel modifying antibody

By combining antibody binding studies with conformational mapping using synthetic peptides, the structure of the cytosolic linker between domains III and IV of the cardiac Na+ channel alpha-subunit was analyzed. Inside-out patch clamp experiments with isolated cardiac Na+ channels from neonatal rat cardiocytes confirmed that a polyclonal antibody against amino acids 1490-1507 of the cardiac Na+ channel recognizes the linker in situ since Na+ inactivation became significantly retarded. Epitope fine mapping with a series of overlapping peptides identified the sequence YYNAMKKLG (corresponding to amino acids 1496-1504 of the cardiac sodium channel alpha-subunit) as the binding locus of the site directed antibody, an interesting result with respect to structure-function relationships because the functionally important hydrophobic amino-acid cluster in position 1487-1489 is not included. Circular dichroism measurements of synthetic 20-mer peptides in hydrophilic and lipophilic environments provided indications for a notable alpha-helical content only for segment GGQDIFMTEEQKKYYNAMKK. This sequence corresponds to amino acids 1483-1502 in the linker and adopts a highly ordered pattern of charge distribution due to this helical conformation. Ordered structure and helix dipole moment represent physical properties which may be important in a refined model for explaining the function of the linker in terminating the open channel configuration.