Sodium channel mRNA in the B104 neuroblastoma cell line

Fibroblast growth factor homologous factor 2 (FGF-13) associates with Nav1.7 in DRG neurons and alters its current properties in an isoform-dependent manner

Fibroblast Growth Factor Homologous Factors (FHF) constitute a subfamily of FGF proteins with four prototypes (FHF1-4; also known as FGF11-14). FHF proteins have been shown to bind directly to the membrane-proximal segment of the C-terminus in voltage-gated sodium channels (Nav), and regulate current density, availability, and frequency-dependent inhibition of sodium currents. Members of the FHF2 subfamily, FHF2A and FHF2B, differ in the length and sequence of their N-termini, and, importantly, differentially regulate Nav1.6 gating properties. Using immunohistochemistry, we show that FHF2 isoforms are expressed in adult dorsal root ganglion (DRG) neurons where they co-localize with Nav1.6 and Nav1.7. FHF2A and FHF2B show differential localization in neuronal compartments in DRG neurons, and levels of expression of FHF2 factors are down-regulated following sciatic nerve axotomy. Because Nav1.7 in nociceptors plays a critical role in pain, we reasoned that its interaction with FHF2 isoforms might regulate its current properties. Using whole-cell patch clamp in heterologous expression systems, we show that the expression of FHF2A in HEK293 cell line stably expressing Nav1.7 channels causes no change in activation, whereas FHF2B depolarizes activation. Both FHF2 isoforms depolarize fast-inactivation. Additionally, FHF2A causes an accumulation of inactivated channels at all frequencies tested due to a slowing of recovery from inactivation, whereas FHF2B has little effect on these properties of Nav1.7. Measurements of the Nav1.7 current in DRG neurons in which FHF2 levels are knocked down confirmed the effects of FHF2A on repriming, and FHF2B on activation, however FHF2A and B did not have an effect on fast inactivation. Our data demonstrates that FHF2 does indeed regulate the current properties of Nav1.7 and does so in an isoform and cell-specific manner.

Robust selection of cancer survival signatures from high-throughput genomic data using two-fold subsampling

Identifying relevant signatures for clinical patient outcome is a fundamental task in high-throughput studies. Signatures, composed of features such as mRNAs, miRNAs, SNPs or other molecular variables, are often non-overlapping, even though they have been identified from similar experiments considering samples with the same type of disease. The lack of a consensus is mostly due to the fact that sample sizes are far smaller than the numbers of candidate features to be considered, and therefore signature selection suffers from large variation. We propose a robust signature selection method that enhances the selection stability of penalized regression algorithms for predicting survival risk. Our method is based on an aggregation of multiple, possibly unstable, signatures obtained with the preconditioned lasso algorithm applied to random (internal) subsamples of a given cohort data, where the aggregated signature is shrunken by a simple thresholding strategy. The resulting method, RS-PL, is conceptually simple and easy to apply, relying on parameters automatically tuned by cross validation. Robust signature selection using RS-PL operates within an (external) subsampling framework to estimate the selection probabilities of features in multiple trials of RS-PL. These probabilities are used for identifying reliable features to be included in a signature. Our method was evaluated on microarray data sets from neuroblastoma, lung adenocarcinoma, and breast cancer patients, extracting robust and relevant signatures for predicting survival risk. Signatures obtained by our method achieved high prediction performance and robustness, consistently over the three data sets. Genes with high selection probability in our robust signatures have been reported as cancer-relevant. The ordering of predictor coefficients associated with signatures was well-preserved across multiple trials of RS-PL, demonstrating the capability of our method for identifying a transferable consensus signature. The software is available as an R package rsig at CRAN (http://cran.r-project.org).

BACE1 regulates voltage-gated sodium channels and neuronal activity

BACE1 activity is significantly increased in the brains of Alzheimer's disease patients, potentially contributing to neurodegeneration. The voltage-gated sodium channel (Na(v)1) beta2-subunit (beta2), a type I membrane protein that covalently binds to Na(v)1 alpha-subunits, is a substrate for BACE1 and gamma-secretase. Here, we find that BACE1-gamma-secretase cleavages release the intracellular domain of beta2, which increases mRNA and protein levels of the pore-forming Na(v)1.1 alpha-subunit in neuroblastoma cells. Similarly, endogenous beta2 processing and Na(v)1.1 protein levels are elevated in brains of BACE1-transgenic mice and Alzheimer's disease patients with high BACE1 levels. However, Na(v)1.1 is retained inside the cells and cell surface expression of the Na(v)1 alpha-subunits and sodium current densities are markedly reduced in both neuroblastoma cells and adult hippocampal neurons from BACE1-transgenic mice. BACE1, by cleaving beta2, thus regulates Na(v)1 alpha-subunit levels and controls cell-surface sodium current densities. BACE1 inhibitors may normalize membrane excitability in Alzheimer's disease patients with elevated BACE1 activity.

Critical interval of somal calcium transient after neurite transection determines B 104 cell survival

Nerve cells may survive or die after axonal or dendritic transection. After neurite transection near (<50 mum) the cell body of Fura-2-loaded B 104 neuroblastoma (rat brain-derived) cells, the somal calcium concentration (SCC) undergoes a three-phase transient change: a rapid (0-0.15-min post-transection [PT]) rise phase, followed by an early (0.15--1.5-min PT) rapid decay phase, and succeeded by a late (1.5-60-min PT) slower decay phase that restores SCC to preinjury levels. The SCC in a critical interval (1.5-12.5 min PT) of the third transient phase correlates with cell fate, i.e., most transected cells that exclude dye (restore a barrier) and die have a significantly higher (P<0.005) SCC in this critical interval than do transected cells that exclude dye and survive at 24-hr PT. Loading BAPTA (chelation of somal Ca(2+)) before, but not after, the critical interval increases the percentage of cells that survive compared to that of cells transected without BAPTA loading. Furthermore, most transected cells that die despite successful barrier restoration exhibit characteristics consistent with apoptosis initiated during the critical interval of the SCC, including caspase activation and plasmalemmal phosphatidylserine translocation. These data suggest that decreased cell survival for injuries near the soma is due to Ca(2+)-initiated apoptosis during the critical interval of the third phase of the SCC transient. (c) 2005 Wiley-Liss, Inc.

Presenilin/gamma-secretase-mediated cleavage of the voltage-gated sodium channel beta2-subunit regulates cell adhesion and migration

The voltage-gated sodium channel beta2-subunit (beta2) is a member of the IgCAM superfamily and serves as both an adhesion molecule and an auxiliary subunit of the voltage-gated sodium channel. Here we found that beta2 undergoes ectodomain shedding followed by presenilin (PS)-dependent gamma-secretase-mediated cleavage. 12-O-Tetradecanoylphorbol-13-acetate treatment or expression of an alpha-secretase enzyme, ADAM10, resulted in ectodomain cleavage of beta2 in Chinese hamster ovary cells. Subsequent cleavage of the remaining 15-kDa C-terminal fragment (beta2-CTF) was independently inhibited by three specific gamma-secretase inhibitors, expression of the dominant negative form of PS1, and in PS1/PS2 knock-out cells. gamma-Secretase inhibitor treatment also increased endogenous beta2-CTF levels in neuroblastoma cells and mouse primary neuronal cultures. In a cell-free gamma-secretase assay, we detected gamma-secretase activity-dependent generation of a 12 kDa beta2 intracellular domain (ICD), which was loosely associated with the membrane fraction. To assess the functional role of beta2 processing by gamma-secretase, we tested whether N-[N-(3,5-difluorophenylacetyl-l-alanyl)]-S-phenylglycine t-butylester (DAPT), a specific gamma-secretase inhibitor, would alter beta2-mediated cell adhesion and migration. We found that DAPT inhibited cell-cell aggregation and migration in a wound healing assay carried out with Chinese hamster ovary cells expressing beta2. DAPT also reduced migration of neuroblastoma cells in a modified Boyden chamber assay. Since DAPT treatment resulted in increased beta2-CTF levels, we also tested whether beta2-CTFs or beta2-ICDs would directly affect cell migration by overexpressing recombinant proteins. Interestingly, elevated levels of beta2-CTFs, but not ICDs, also blocked cell migration by 81 to 93%. Together, our findings show for the first time that beta2 is a PS/gamma-secretase substrate and gamma-secretase mediated cleavage of beta2-CTF is required for cell-cell adhesion and migration of beta2-expressing cells.

Modulation of BmK AS, a scorpion neurotoxic polypeptide, on voltage-gated Na+ channels in B104 neuronal cell line

The modulation of BmK AS, a neurotoxic polypeptide from scorpion Buthus martensi Karsch, on the voltage-gated Na(+) channels has been investigated in the B104 neuroblastoma cell line by whole-cell patch clamping. It was found that the whole Na(+) currents and tetrodotoxin-resistant Na(+) current were depressed in a biphasic manner and both steady-state activation and inactivation curves were shifted toward the hyperpolarizing direction in the presence of BmK AS. The results suggested that BmK AS could inhibit both tetrodotoxin-sensitive and -resistant Na(+) currents via modulating the gating mechanism of the voltage-gated Na(+) channels in B104 cells.

Characterization and developmental changes of Na+ currents of petrosal neurons with projections to the carotid body

Carotid body chemoreceptors transduce a decrease in arterial oxygen tension into an increase in spiking activity on the sinus nerve, and this response increases with postnatal age over the first week or two of life. Previous work from our laboratory has suggested a major role of axonal Na(+) channels in the initiation of afferent spiking activity. Using RT-PCR of the petrosal ganglia we identified Na(+) channel TTX-S isoforms Na(v)1.1, Na(v)1.6, and Na(v)1.7 and the TTX-resistant (TTX-R) isoforms Na(v)1.8 and Na(v)1.9 at high levels. Electrophysiologic recordings (at 3 ages: 3 days, 9 days, 18-20 days) of neurons that project to the carotid body exhibited predominantly fast-inactivating sodium currents, with a bimodal recovery from inactivation at -80 mV (fast component approximately 8 ms; slow component approximately 90 ms). Developmental age had little effect with no change in peak current density (approximately 1.4 nA/pF) and was associated with a slight, but significant increase in the speed of recovery from inactivation at -140 and -120 mV but not at other potentials. Assuming that the same Na(+) channel complement is present at the nerve terminal as at the soma, the association of a sensory modality (chemoreception) with a relatively uniform Na(+) channel profile suggests that the rapid kinetics of TTX-S channels may be essential for some aspects of chemoreceptor function beyond mediating simple axonal conduction.

Na(v)1.5 underlies the 'third TTX-R sodium current' in rat small DRG neurons

In addition to slow-inactivating and persistent TTX-R Na(+) currents produced by Na(v)1.8 and Na(v)1.9 Na(+) channels, respectively, a third TTX-R Na(+) current with fast activation and inactivation can be recorded in 80% of small neurons of dorsal root ganglia (DRG) from E15 rats, but in only 3% of adult small DRG neurons. The half-time for activation, the time constant for inactivation, and the midpoints of activation and inactivation of the third TTX-R Na(+) currents are significantly different from those of Na(v)1.8 and Na(v)1.9 Na(+) currents. The estimated TTX K(i) (2.11+/-0.34 microM) of the third TTX-R Na(+) current is significantly lower than those of Na(v)1.8 and Na(v)1.9 Na(+) currents. The Cd(2+) sensitivity of third TTX-R Na(+) current is closer to cardiac Na(+) currents. A concentration of 1 mM Cd(2+) is required to completely block this current, which is significantly lower than the 5 mM required to block Na(v)1.8 and Na(v)1.9 currents. The third TTX-R Na(+) channel is not co-expressed with Na(v)1.8 and Na(v)1.9 Na(+) channels in DRG neurons of E18 rats, at a time when all three currents show comparable densities. The physiological and pharmacological profiles of the third TTX-R Na(+) current are similar to those of the cardiac Na(+) channel Na(v)1.5 and RT-PCR and restriction enzyme polymorphism analysis, show a parallel pattern of expression of Na(v)1.5 in DRG during development. Taken together, these results demonstrate that Na(v)1.5 is expressed in a developmentally regulated manner in DRG neurons and suggest that Na(v)1.5 Na(+) channel produces the third TTX-R current.

Structural requirements for interaction of sodium channel beta 1 subunits with ankyrin

Sodium channel beta subunits modulate channel kinetic properties and cell surface expression levels and function as cell adhesion molecules. beta 1 and beta 2 participate in homophilic cell adhesion resulting in ankyrin recruitment to cell contact sites. We hypothesized that a tyrosine residue in the cytoplasmic domain of beta 1 may be important for ankyrin recruitment and tested our hypothesis using beta 1 mutants replacing Tyr(181) with alanine (beta 1Y181A), phenylalanine (beta 1Y181F), or glutamate (beta 1Y181E), or a truncated construct deleting all residues beyond Tyr(181) (beta 1L182(STOP)). Ankyrin recruitment was observed in beta 1L182(STOP), showing that residues Ile(166)-Tyr(181) contain the major ankyrin recruiting activity of beta 1. Ankyrin recruitment was abolished in beta 1Y181E, suggesting that tyrosine phosphorylation of beta 1 may inhibit beta 1-ankyrin interactions. Ankyrin(G) and beta 1 associate in rat brain membranes and in transfected cells expressing beta 1 and ankyrin(G) in the absence of sodium channel alpha subunits. beta 1 subunits are recognized by anti-phosphotyrosine antibodies following treatment of these cell lines with fibroblast growth factor. beta 1 and ankryin(G) association is not detectable in cells following treatment with fibroblast growth factor. Ankyrin(G) and beta 1Y181E do not associate even in the absence of fibroblast growth factor treatment. beta 1 subunit-mediated cell adhesion and ankyrin recruitment may contribute to sodium channel placement at nodes of Ranvier. The phosphorylation state of beta 1Y181 may be a critical regulatory step in these developmental processes.

Expression profiles of voltage-gated Na(+) channel alpha-subunit genes in rat and human prostate cancer cell lines

BACKGROUND

Voltage-gated Na(+) channel (VGSC) activity has been implicated in prostate cancer (PC) metastasis. Although VGSCs can occur as multiple-subunit assemblies, the alpha-subunits (VGSCalphas) alone can encode functional channels. The VGSCalpha gene(s) responsible for the functional VGSCalpha expression in strongly metastatic PC cell lines is not known.

METHODS

Two reverse transcription-PCR (RT-PCR) methods, degenerate primer screening and a novel semi quantitative PCR (SQT-PCR) technique, were used. These methods enabled a detailed qualitative and quantitative investigation of VGSCalpha mRNA expression in rat (MAT-LyLu/AT-2) and human (PC-3/LNCaP) PC cells of markedly different metastatic potential.

RESULTS

Expression of eight different VGSCalpha genes (SCN1A-4A, SCN7A-9A, and SCN11A) was determined in the PC cell lines. Most were expressed as multiple splice variants. SQT-PCR results were consistent with a basal level of VGSCalpha mRNA expression occurring in weakly metastatic (AT-2/LNCaP) cells, and this being greatly elevated in cells of stronger metastatic potential (MAT-LyLu/PC-3), primarily due to the elevated expression of the SCN9A gene (also termed PN1/hNe-Na).

CONCLUSIONS

(1) Several VGSCalpha genes and their splice variants are expressed similarly in both rat and human PC cell lines. (2) Expression levels are much higher in the strongly metastatic (MAT-LyLu/PC-3) cells. (3) Levels of SCN9A mRNA specifically are predominant in MAT-LyLu and PC-3 cells; thus, SCN9A is highly likely to be the main source of the functional VGSC detected.

Fibroblast Growth Factor Homologous Factor 1B Binds to the C Terminus of the Tetrodotoxin-resistant Sodium Channel rNav1.9a (NaN)

In this study we demonstrate a direct interaction between a cytosolic fibroblast growth factor family member and a sodium channel. A yeast two-hybrid screen for proteins that associate with the cytoplasmic domains of the tetrodotoxin-resistant sodium channel rNa(v)1.9a (NaN) led to the identification of fibroblast growth factor homologous factor 1B (FHF1B), a member of the fibroblast growth factor family, as an interacting partner of rNa(v)1.9a. FHF1B selectively interacts with the C-terminal region but not the other four intracellular segments of rNa(v)1.9a. FHF1B binds directly to the C-terminal polypeptide of rNa(v)1.9a both in vitro and in mammalian cell lines. The N-terminal 5-77 amino acid residues of FHF1B are essential for binding to rNa(v)1.9a. FHF1B did not interact with C termini of two other sodium channels hNa(v)1.7a (hNaNE) and rNa(v)1.8a (SNS), which share 50% similarity to the C-terminal polypeptide of rNa(v)1.9a. FHF1B is the first growth factor found to bind specifically to a sodium channel. Although the functional significance of this interaction is not clear, FHF1B may affect the rNa(v)1.9a channel directly or by recruiting other proteins to the channel complex. Alternatively, it is possible that rNa(v)1.9a may help deliver this factor to the cell membrane, where it exerts its function.

Sodium channel beta subunits: anything but auxiliary

Voltage-gated sodium channels are glycoprotein complexes responsible for initiation and propagation of action potentials in excitable cells such as central and peripheral neurons, cardiac and skeletal muscle myocytes, and neuroendocrine cells. Mammalian sodium channels are heterotrimers, composed of a central, pore-forming alpha subunit and two auxiliary beta subunits. The alpha subunits form a gene family with at least 10 members. Mutations in alpha subunit genes have been linked to paroxysmal disorders such as epilepsy, long QT syndrome, and hyperkalemic periodic paralysis in humans, and motor endplate disease and cerebellar ataxia in mice. Three genes encode sodium channel beta subunits with at least one alternative splice product. A mutation in the beta 1 subunit gene has been linked to generalized epilepsy with febrile seizures plus type 1 (GEFS + 1) in a human family with this disease. Sodium channel beta subunits are multifunctional. They modulate channel gating and regulate the level of channel expression at the plasma membrane. More recently, they have been shown to function as cell adhesion molecules in terms of interaction with extracellular matrix, regulation of cell migration, cellular aggregation, and interaction with the cytoskeleton. Structure-function studies have resulted in the preliminary assignment of functional domains in the beta 1 subunit. A sodium channel signaling complex is proposed that involves beta subunits as channel modulators as well as cell adhesion molecules, other cell adhesion molecules such as neurofascin and contactin, RPTP beta, and extracellular matrix molecules such as tenascin.

Brevetoxin derivatives that inhibit toxin activity

BACKGROUND

The brevetoxins are marine neurotoxins that interfere with the normal functions of the voltage-gated Na(+) channel. We have identified two brevetoxin derivatives that do not exhibit pharmacological properties typical of the brevetoxins and that function as brevetoxin antagonists.

RESULTS

PbTx-3 and benzoyl-PbTx-3 elicited Na(+) channel openings during steady-state depolarizations; however, two PbTx-3 derivatives retained their ability to bind to the receptor, but did not elicit Na(+) channel openings. alpha-Naphthoyl-PbTx-3 acted as a PbTx-3 antagonist but did not affect Na(+) channels that were not exposed to PbTx-3. beta-Naphthoyl-PbTx-3 reduced openings of Na(+) channels that were not exposed to PbTx-3.

CONCLUSIONS

Some modifications to the brevetoxin molecule do not alter either the binding properties or the activity of these toxins. Larger modifications to the K-ring sidechain do not interfere with binding but have profound effects on their pharmacological properties. This implies a critical function for the K-ring sidechain of the native toxin.

Coding sequence, genomic organization, and conserved chromosomal localization of the mouse gene Scn11a encoding the sodium channel NaN

Previous studies have shown that sodium channel alpha-subunit NaN is preferentially expressed in small-diameter sensory neurons of dorsal root ganglia and trigeminal ganglia. These neurons include high-threshold nociceptors that are involved in transduction of pain associated with tissue and nerve injury. In this study, we show that mouse NaN is a 1765-amino-acid peptide that is predicted to produce a current that is resistant to tetrodotoxin (TTX-R). Mouse and rat NaN are 80 and 89% identical at the nucleotide and amino acid levels, respectively. The Scn11a gene encoding this cDNA is organized into 24 exons. Unlike some alpha-subunits, Scn11a does not have an alternative exon 5 in domain I. Introns of the U2 and U12 spliceosome types are present at conserved positions relative to other members of this family. Scn11a is located on mouse chromosome 9, close to the two other TTX-R sodium channel genes, Scn5a and Scn10a. The human gene, SCN11A, was mapped to the conserved linkage group on chromosome 3p21-p24, close to human SCN5A and SCN10A. The colocalization of the three sodium channel genes supports a common lineage of the TTX-R sodium channels.

Augmentation of recovery from inactivation by site-3 Na channel toxins. A single-channel and whole-cell study of persistent currents

Site-3 toxins isolated from several species of scorpion and sea anemone bind to voltage-gated Na channels and prolong the time course of INa by interfering with inactivation with little or no effect on activation, effects that have similarities to those produced by genetic diseases in skeletal muscle (myotonias and periodic paralysis) and heart (long QT syndrome). Some published reports have also reported the presence of a noninactivating persistent current in site-3 toxin-treated cells. We have used the high affinity site-3 toxin Anthopleurin B to study the kinetics of this current and to evaluate kinetic differences between cardiac (in RT4-B8 cells) and neuronal (in N1E-115 cells) Na channels. By reverse transcription-PCR from N1E-115 cell RNA multiple Na channel transcripts were detected; most often isolated were sequences homologous to rBrII, although at low frequency sequences homologous to rPN1 and rBrIII were also detected. Toxin treatment induced a voltage-dependent plateau current in both isoforms for which the relative amplitude (plateau current/peak current) approached a constant value with depolarization, although the magnitude was much greater for neuronal (17%) than cardiac (5%) INa. Cell-attached patch recordings revealed distinct quantitative differences in open times and burst durations between isoforms, but for both isoforms the plateau current comprised discrete bursts separated by quiescent periods, consistent with toxin induction of an increase in the rate of recovery from inactivation rather than a modal failure of inactivation. In accord with this hypothesis, toxin increased the rate of whole-cell recovery at all tested voltages. Moreover, experimental data support a model whereby recovery at negative voltages is augmented through closed states rather than through the open state. We conclude that site-3 toxins produce qualitatively similar effects in cardiac and neuronal channels and discuss implications for channel kinetics.

NaN, a novel voltage-gated Na channel, is expressed preferentially in peripheral sensory neurons and down-regulated after axotomy

Although physiological and pharmacological evidence suggests the presence of multiple tetrodotoxin-resistant (TTX-R) Na channels in neurons of peripheral nervous system ganglia, only one, SNS/PN3, has been identified in these cells to date. We have identified and sequenced a novel Na channel alpha-subunit (NaN), predicted to be TTX-R and voltage-gated, that is expressed preferentially in sensory neurons within dorsal root ganglia (DRG) and trigeminal ganglia. The predicted amino acid sequence of NaN can be aligned with the predicted structure of known Na channel alpha-subunits; all relevant landmark sequences, including positively charged S4 and pore-lining SS1-SS2 segments, and the inactivation tripeptide IFM, are present at predicted positions. However, NaN exhibits only 42-53% similarity to other mammalian Na channels, including SNS/PN3, indicating that it is a novel channel, and suggesting that it may represent a third subfamily of Na channels. NaN transcript levels are reduced significantly 7 days post axotomy in DRG neurons, consistent with previous findings of a reduction in TTX-R Na currents. The preferential expression of NaN in DRG and trigeminal ganglia and the reduction of NaN mRNA levels in DRG after axonal injury suggest that NaN, together with SNS/PN3, may produce TTX-R currents in peripheral sensory neurons and may influence the generation of electrical activity in these cells.

Differential expression of sodium channel genes in retinal ganglion cells

Action potential electrogenesis in the axons of retinal ganglion cells is supported by voltage-gated sodium channels, and a tetrodotoxin (TTX)-inhibitable sodium conductance participates in anoxic injury of these axons within the optic nerve. However, the subtypes of sodium channels expressed in retinal ganglion cells have not been identified. In this study, we used reverse transcription-polymerase chain reaction (RT-PCR) and restriction enzyme mapping, together with in situ hybridization, to examine the expression of transcripts for sodium channel alpha-subunits I, II, III, NaG, Na6, hNE/PN1 and SNS, and beta-subunits 1 and 2, in the retina of the adult rat. RT-PCR yielded high levels of amplification of I, II, III, Na6, beta1 and beta2 transcripts. In situ hybridization demonstrated the presence of all these mRNAs in the cell bodies of retinal ganglion cells. Retinal ganglion cells thus express multiple sodium channel mRNAs, suggesting that they deploy several different types of sodium channels.

Insertion of a SNS-specific tetrapeptide in S3-S4 linker of D4 accelerates recovery from inactivation of skeletal muscle voltage-gated Na channel mu1 in HEK293 cells

Na channel subunits alphaSNS (PN3) and alpha mu1(SkM1) produce slowly inactivating/TTX-resistant and rapidly inactivating/TTX-sensitive currents, respectively. AlphaSNS (PN3) current recovers from inactivation (reprimes) rapidly. Sequence alignment identified the tetrapeptide SLEN, in the S3-S4 linker of D4, as alphaSNS-specific. To determine whether SLEN endows Na channels with slow kinetics and/or rapid repriming, we analyzed the transient Na current produced by a chimera mu1SLEN in HEK293 cells. Neither kinetics nor voltage dependence of activation and inactivation was affected. However, repriming was twice as fast as in the wild type at -100 mV. This suggests that SLEN may contribute to the rapid repriming of TTX-resistant Na current.

TTX-sensitive and -resistant Na+ currents, and mRNA for the TTX-resistant rH1 channel, are expressed in B104 neuroblastoma cells

To examine the molecular basis for membrane excitability in a neuroblastoma cell line, we used whole cell patch-clamp methods and reverse transcription-polymerase chain reaction (RT-PCR) to study Na+ currents and channels in B104 cells. We distinguished Tetrodotoxin (TTX)-sensitive and -resistant Na+ currents and detected the mRNA for the cardiac rH1 channel in B104 cells. Na+ currents could be recorded in 65% of cells. In the absence of TTX, mean peak Na+ current density was 126 +/- 19 pA/pF, corresponding to a channel density of 2.7 +/- 0.4/micron 2 (mean +/- SE). Time-to-peak (t-peak), activation (tau m), and inactivation time constants (tau h) for Na+ currents in B104 cells were 1.0 +/- 0.04, 0.4 +/- 0.06, and 0.9 +/- 0.04 ms at -10 mV. The peak conductance-voltage relationship had a V 1/2 of -39.8 +/- 1.5 mV. V 1/2 for steady-state inactivation was -81.6 +/- 1.5 mV. TTX-sensitive and -resistant components of the Na current had half-maximal inhibitions (IC50), respectively, of 1.2 nM and, minimally, 575.5 nM. The TTX-sensitive and -resistant Na+ currents were kinetically distinct; time-to-peak, tau m, and tau h for TTX-sensitive currents were shorter than for TTX-resistant currents. Steady-state voltage dependence of the two currents was indistinguishable. The presence of TTX-sensitive and -resistant Na+ currents, which are pharmacologically and kinetically distinct, led us to search for mRNAs known to be associated with TTX-resistant channels, in addition to the alpha subunit mRNAs, which have previously been shown to be expressed in these cells. Using RT-PCR and restriction enzyme mapping, we were unable to detect alpha SNS, but detected mRNA for rH1, which is known to encode a TTX-resistant channel, in B104 cells. B104 neuroblastoma cells thus express TTX-sensitive and -resistant Na+ currents. These appear to be encoded by neuronal-type and cardiac Na+ channel mRNAs including the RH1 transcript. This cell line may be useful for studies on the rH1 channel, which is known to be mutated in the long-QT syndrome.

Action potential-like responses in B104 cells with low Na+ channel densities

Action potential generation and Na+ currents were studied in B104 neuroblastoma cells in vitro using the whole-cell patch-clamp method in voltage-clamp and current-clamp mode. Action potential-like responses were elicited in 38 of 42 cells, with a threshold close to -55 mV for depolarizing stimuli, and -56 mV for anode-break stimuli. Response amplitudes were larger when cells were held at more negative prepulse potentials, and were well fit by a Boltzmann distribution with a midpoint of approx. -75 mV, close to the V1/2 for Na+ current steady-state inactivation in these cells. Cells displaying action potential-like responses exhibited a peak Na+ current density of 133 +/- 0.14 pA/pF (range, 10.2-296.2 pA/pF) and a low gK:gNa ratio (0.0067 +/- 0.0023). Exposure to 0.1 mM Cd2+ did not block the generation of action potential-like responses in B104 cells, while 1 microM TTX abolished the responses. We conclude that low densities of Na+ channels (< 3/microns2, and < 1/micron2 in some cells) can support the generation of action potential-like responses in B104 cells if they are held at hyperpolarized levels to remove inactivation. The low leak and K+ conductance of these cells may contribute to their ability to generate action potential-like responses under these circumstances.