CSF plasmablasts differentiate MS from other neurologic disorders

Multiparametric flow cytometry (FC) of CSF allows one to easily estimate the percentage of lymphocyte subpopulations in CSF. We hypothesized that an increased ratio of B-lineage cells in CSF of MS patients, as assessed with FC, could be useful for diagnostics. We analyzed CSF of 137 patients (70 MS, 24 infectious/autoimmune neurologic disorders (INDs), and 43 non-infectious/autoimmune neurologic disorders (NINDs)), and showed that CSF plasmablasts of >0.1% had a sensitivity of 40% for MS and specificity of 92% when comparing MS and IND, while plasmablasts of >0.25% had sensitivity of 36%, and 100% specificity.

Diagnostic criteria of chronic inflammatory demyelinating polyneuropathy in diabetes mellitus

OBJECTIVE

The possibility of co-association between diabetes mellitus (DM) and chronic inflammatory demyelinating polyneuropathy (CIDP) has long been a focus of interest as well as of clinical significance. As CIDP is a potentially treatable condition, it is diagnosis in the context of DM is of great importance. However, diagnostic criteria to identify CIDP in patients with diabetes are not available. We propose a diagnostic tool that should help clinicians to decide what is the probability that a patient with diabetes might have CIDP.

METHODS

We list several clinical, electrophysiological, and laboratory parameters that, when combined, have the power of discriminating an immune-mediated neuropathy in patients with DM. By summing the points assigned to each of these parameters, we define four levels of probability for a patient with diabetes to have CIDP. To analyze the validity of the diagnostic toll, we applied it in three different patient populations: (i) Patients with diabetes with peripheral neuropathy, (ii) Patients with CIDP without DM, and (iii) Patients with diabetes with CIDP.

RESULTS

The scores of patients with diabetes without CIDP ranged from -7 to 2, while those of patients with DM-CIDP ranged from 2 to 20. The scores of non-diabetic patients with CIDP were similar to those of patients with DM-CIDP and ranged from 6 to 16. The mean score of patients with DM-CIDP was 9.083, while the score of patients with CIDP was 11.16 and that of patients with diabetic polyneuropathy was -3.59.

CONCLUSIONS

These results show that this diagnostic tool is able to identify patients with diabetes with overlapping CIDP.

The voltage-dependent potassium channel subunit Kv2.1 regulates insulin secretion from rodent and human islets independently of its electrical function

AIMS/HYPOTHESIS

It is thought that the voltage-dependent potassium channel subunit Kv2.1 (Kv2.1) regulates insulin secretion by controlling beta cell electrical excitability. However, this role of Kv2.1 in human insulin secretion has been questioned. Interestingly, Kv2.1 can also regulate exocytosis through direct interaction of its C-terminus with the soluble NSF attachment receptor (SNARE) protein, syntaxin 1A. We hypothesised that this interaction mediates insulin secretion independently of Kv2.1 electrical function.

METHODS

Wild-type Kv2.1 or mutants lacking electrical function and syntaxin 1A binding were studied in rodent and human beta cells, and in INS-1 cells. Small intracellular fragments of the channel were used to disrupt native Kv2.1-syntaxin 1A complexes. Single-cell exocytosis and ion channel currents were monitored by patch-clamp electrophysiology. Interaction between Kv2.1, syntaxin 1A and other SNARE proteins was probed by immunoprecipitation. Whole-islet Ca(2+)-responses were monitored by ratiometric Fura red fluorescence and insulin secretion was measured.

RESULTS

Upregulation of Kv2.1 directly augmented beta cell exocytosis. This happened independently of channel electrical function, but was dependent on the Kv2.1 C-terminal syntaxin 1A-binding domain. Intracellular fragments of the Kv2.1 C-terminus disrupted native Kv2.1-syntaxin 1A interaction and impaired glucose-stimulated insulin secretion. This was not due to altered ion channel activity or impaired Ca(2+)-responses to glucose, but to reduced SNARE complex formation and Ca(2+)-dependent exocytosis.

CONCLUSIONS/INTERPRETATION

Direct interaction between syntaxin 1A and the Kv2.1 C-terminus is required for efficient insulin exocytosis and glucose-stimulated insulin secretion. This demonstrates that native Kv2.1-syntaxin 1A interaction plays a key role in human insulin secretion, which is separate from the channel's electrical function.

Direct interaction of a brain voltage-gated K+ channel with syntaxin 1A: functional impact on channel gating

Presynaptic voltage-gated K(+) (Kv) channels play a physiological role in the regulation of transmitter release by virtue of their ability to shape presynaptic action potentials. However, the possibility of a direct interaction of these channels with the exocytotic apparatus has never been examined. We report the existence of a physical interaction in brain synaptosomes between Kvalpha1.1 and Kvbeta subunits with syntaxin 1A, occurring, at least partially, within the context of a macromolecular complex containing syntaxin, synaptotagmin, and SNAP-25. The interaction was altered after stimulation of neurotransmitter release. The interaction with syntaxin was further characterized in Xenopus oocytes by both overexpression and antisense knock-down of syntaxin. Direct physical interaction of syntaxin with the channel protein resulted in an increase in the extent of fast inactivation of the Kv1.1/Kvbeta1.1 channel. Syntaxin also affected the channel amplitude in a biphasic manner, depending on its concentration. At low syntaxin concentrations there was a significant increase in amplitudes, with no detectable change in cell-surface channel expression. At higher concentrations, however, the amplitudes decreased, probably because of a concomitant decrease in cell-surface channel expression, consistent with the role of syntaxin in regulation of vesicle trafficking. The observed physical and functional interactions between syntaxin 1A and a Kv channel may play a role in synaptic efficacy and neuronal excitability.

Imaging plasma membrane proteins in large membrane patches of Xenopus oocytes

We describe the preparation of a Xenopus oocyte plasma membrane patch attached to a cover-slip with its intracellular face exposed to the bath solution. The proteins attached to the plasma membrane were visualized by confocal microscopy after fluorescence labelling. Since cortical microfilament elements were detected in these plasma membrane preparations we termed the patches plasma membrane-cortex patches. The way these patches are formed and the low concentration of proteins needed for cytochemical detection make the membrane-cortex patches similar to electrophysiological membrane patches and therefore allow the cytochemical study of ion channels to be correlated with electrophysiological experiments. Furthermore, the described patch is similar to manually isolated plasma membranes used for biochemical analysis by sodium dodecylsulphate-polyacrylamide gel electrophoresis (SDS-PAGE). Cytochemical analysis of membrane-cortex patches also enables the detection of the two-dimensional pattern of organization of membrane proteins (clustered or non-clustered forms). In addition, patch preparations enable cytochemical study of the relative localization of membrane proteins. The methodology enables integration of electrophysiological, biochemical and cytochemical studies of ion channels, giving a comprehensive perspective on ion channel function.

Modal behavior of the Kv1.1 channel conferred by the Kvbeta1.1 subunit and its regulation by dephosphorylation of Kv1.1

Modulation of fast-inactivating voltage-gated K+ channels can produce plastic changes in neuronal signaling. Previously, we showed that the voltage-dependent K+ channel composed of brain Kv1.1 and Kvbeta1.1 subunits (alpha(beta) channel) gives rise to a current that has a fast-inactivating and a sustained component; the proportion of the fast-inactivating component could be decreased by dephosphorylation of a basally phosphorylated Ser-446 on the alpha subunit. To account for our results we suggested a model that assumes a bimodal gating of the alpha(beta) channel. In this study, using single-channel analysis, we confirm this model. Two modes of gating were identified: (1) an inactivating mode characterized by low open probability and single openings early in the voltage step, and (2) a non-inactivating gating mode with bursts of openings. These two modes were non-randomly distributed, with spontaneous shifts between them. Each mode is characterized by a different set of open time constants (tau) and mean open times (t(0)). The non-inactivating mode is similar to the gating mode of a homomultimeric alpha channel. The phosphorylation-deficient alphaS446Abeta channel has the same two gating modes. Furthermore, alkaline phosphatase promoted the transition to the non-inactivating mode. This is the first report of modal behavior of a fast-inactivating K+ channel; furthermore, it substantiates the notion that direct phosphorylation is one mechanism that regulates the equilibrium between the two modes and thereby regulates the extent of macroscopic fast inactivation of a brain K+ channel.

A scorpion alpha-like toxin that is active on insects and mammals reveals an unexpected specificity and distribution of sodium channel subtypes in rat brain neurons

Several scorpion toxins have been shown to exert their neurotoxic effects by a direct interaction with voltage-dependent sodium channels. Both classical scorpion alpha-toxins such as Lqh II from Leiurus quiquestratus hebraeus and alpha-like toxins as toxin III from the same scorpion (Lqh III) competitively interact for binding on receptor site 3 of insect sodium channels. Conversely, Lqh III, which is highly toxic in mammalian brain, reveals no specific binding to sodium channels of rat brain synaptosomes and displaces the binding of Lqh II only at high concentration. The contrast between the low-affinity interaction and the high toxicity of Lqh III indicates that Lqh III binding sites distinct from those present in synaptosomes must exist in the brain. In agreement, electrophysiological experiments performed on acute rat hippocampal slices revealed that Lqh III strongly affects the inactivation of voltage-gated sodium channels recorded either in current or voltage clamp, whereas Lqh II had weak, or no, effects. In contrast, Lqh III had no effect on cultured embryonic chick central neurons and on sodium channels from rat brain IIA and beta1 subunits reconstituted in Xenopus oocytes, whereas sea anemone toxin ATXII and Lqh II were very active. These data indicate that the alpha-like toxin Lqh III displays a surprising subtype specificity, reveals the presence of a new, distinct sodium channel insensitive to Lqh II, and highlights the differences in distribution of channel expression in the CNS. This toxin may constitute a valuable tool for the investigation of mammalian brain function.

Fast inactivation of a brain K+ channel composed of Kv1.1 and Kvbeta1.1 subunits modulated by G protein beta gamma subunits

Modulation of A-type voltage-gated K+ channels can produce plastic changes in neuronal signaling. It was shown that the delayed-rectifier Kv1.1 channel can be converted to A-type upon association with Kvbeta1.1 subunits; the conversion is only partial and is modulated by phosphorylation and microfilaments. Here we show that, in Xenopus oocytes, expression of Gbeta1gamma2 subunits concomitantly with the channel (composed of Kv1.1 and Kvbeta1.1 subunits), but not after the channel's expression in the plasma membrane, increases the extent of conversion to A-type. Conversely, scavenging endogenous Gbetagamma by co-expression of the C-terminal fragment of the beta-adrenergic receptor kinase reduces the extent of conversion to A-type. The effect of Gbetagamma co-expression is occluded by treatment with dihydrocytochalasin B, a microfilament-disrupting agent shown previously by us to enhance the extent of conversion to A-type, and by overexpression of Kvbeta1.1. Gbeta1gamma2 subunits interact directly with GST fusion fragments of Kv1.1 and Kvbeta1.1. Co-expression of Gbeta1gamma2 causes co-immunoprecipitation with Kv1.1 of more Kvbeta1.1 subunits. Thus, we suggest that Gbeta1gamma2 directly affects the interaction between Kv1.1 and Kvbeta1.1 during channel assembly which, in turn, disrupts the ability of the channel to interact with microfilaments, resulting in an increased extent of A-type conversion.

Agonist-independent inactivation and agonist-induced desensitization of the G protein-activated K+ channel (GIRK) in Xenopus oocytes

The G-protein-activated K+ channels of the GIRK (Kir 3) family are activated by Gbetagamma subunits of heterotrimeric Gi/Go proteins. Atrial GIRK currents evoked by acetylcholine (ACh)1 via muscarinic m2 receptors (m2R) display prominent desensitization. We studied desensitization of basal and ACh-evoked whole-cell GIRK currents in Xenopus oocytes. In the absence of receptor and/or agonist, the basal GIRK activity showed inactivation which was prominent when the preparation was bathed in a low-Na+, high-K+ extracellular solution (96 mM [K+]out and 2 mM [Na+]out) but did not occur in a normal physiological solution. Ion substitution experiments showed that this basal, agonist-independent inactivation was caused by the decrease in [Na+]out rather than by the increased [K+]out. We hypothesize that it reflects a depletion of intracellular Na+. ACh-evoked GIRK currents desensitized faster than the basal ones. The agonist-induced desensitization was present when the preparation was bathed in all solutions tested, independently of [Na+]out. A protein kinase C (PKC) activator inhibited the GIRK currents both in high and low [Na+]out, apparently mimicking agonist-induced desensitization; however, a potent serine/threonine protein kinase blocker, staurosporine, blocked only a minor part of desensitization. We conclude that basal inactivation and agonist-induced desensitization are separate processes, the former caused by changes in Na+ concentrations, and the latter by unknown factor(s) with only a minor contribution of PKC.

Activation of a metabotropic glutamate receptor and protein kinase C reduce the extent of inactivation of the K+ channel Kv1.1/Kvbeta1.1 via dephosphorylation of Kv1.1

Various brain K+ channels, which may normally exist as complexes of alpha (pore-forming) and beta (auxiliary) subunits, were subjected to regulation by metabotropic glutamate receptors. Kv1.1/Kvbeta1.1 is a voltage-dependent K+ channel composed of alpha and beta proteins that are widely expressed in the brain. Expression of this channel in Xenopus oocytes resulted in a current that had fast inactivating and noninactivating components. Previously we showed that basal and protein kinase A-induced phosphorylation of the alpha subunit at Ser-446 decreases the fraction of the noninactivating component. In this study we investigated the effect of protein kinase C (PKC) on the channel. We showed that a PKC-activating phorbol ester (phorbol 12-myristate 13-acetate (PMA)) increased the noninactivating fraction via activation of a PKC subtype that was inhibited by staurosporine and bisindolylmaleimide but not by calphostin C. However, it was not a PKC-induced phosphorylation but rather a dephosphorylation that mediated the effect. PMA reduced the basal phosphorylation of Ser-446 significantly in plasma membrane channels and failed to affect the inactivation of channels having an alpha subunit that was mutated at Ser-446. Also, the activation of coexpressed mGluR1a known to activate phospholipase C mimicked the effect of PMA on the inactivation via induction of dephosphorylation at Ser-446. Thus, this study identified a potential neuronal pathway initiated by activation of metabotropic glutamate receptor 1a coupled to a signaling cascade that possibly utilized PKC to induce dephosphorylation and thereby to decrease the extent of inactivation of a K+ channel.

Inactivation of a voltage-dependent K+ channel by beta subunit. Modulation by a phosphorylation-dependent interaction between the distal C terminus of alpha subunit and cytoskeleton

Kv1.1/Kvbeta1.1 (alphabeta) K+ channel expressed in Xenopus oocytes was shown to have a fast inactivating current component. The fraction of this component (extent of inactivation) is increased by microfilament disruption induced by cytochalasins or by phosphorylation of the alpha subunit at Ser-446, which impairs the interaction of the channel with microfilaments. The relevant sites of interaction on the channel molecules have not been identified. Using a phosphorylation-deficient mutant of alpha, S446A, to ensure maximal basal interaction of the channel with the cytoskeleton, we show that one relevant site is the end of the C terminus of alpha. Truncation of the last six amino acids resulted in alphabeta channels with an extent of inactivation up to 2.5-fold larger and its further enhancement by cytochalasins being reduced 2-fold. The wild-type channels exhibited strong inactivation, which could not be markedly increased either by cytochalasins or by the C-terminal mutations, indicating that the interaction of the wild-type channels with microfilaments was minimal to begin with, presumably because of extensive basal phosphorylation. Since the C-terminal end of Kv1.1 was shown to participate in channel clustering via an interaction with members of the PSD-95 family of proteins, we propose that a similar interaction with an endogenous protein takes place, contributing to channel connection to the oocyte cytoskeleton. This is the first report to assign a modulatory role to such an interaction: together with the state of phosphorylation of the channel, it regulates the extent of inactivation conferred by the beta subunit.

Deletion of the N-terminus of a K+ channel brings about short-term modulation by cAMP and beta 1-adrenergic receptor activation

On deletion of the N-terminus of RCK1 K+ channel, acute modulation of the channel by cAMP-elevating treatments is revealed. This modulation is studied in Xenopus oocytes using two-electrode voltage-clamp, site-directed mutagenesis, and SDS-PAGE analyses. Treatments by Sp-8-Br-cAMPS, a membrane-permeant cAMP analog, and by isoproterenol, a beta 1-adrenergic receptor (beta 1R) agonist, both increased the current amplitudes with no effect on the voltage dependency of activation. The effect of isoproterenol was blocked by coexpression of either G alpha S or G alpha i3 proteins. The channel protein is phosphorylated on the Sp-8-Br-cAMPS treatment at Ser446; however, a phosphorylation-deficient variant in which this site has been altered is still modulated by Sp-8-Br-cAMPS and isoproterenol. Expression of the full-length channel with Kv beta 1.1 auxiliary subunit renders the channel at the same modulation as that of the truncated one. Taken together, the RCK1 channel can be acutely modulated by cAMP and beta 1R activation possibly through protein kinase A (PKA) activation, but not through direct channel phosphorylation; the involvement of the N-terminus in this modulation is discussed.

Phosphorylation of a K+ channel alpha subunit modulates the inactivation conferred by a beta subunit. Involvement of cytoskeleton

Voltage-gated K+ channels isolated from mammalian brain are composed of alpha and beta subunits. Interaction between coexpressed Kv1.1 (alpha) and Kvbeta1.1 (beta) subunits confers rapid inactivation on the delayed rectifier-type current that is observed when alpha subunits are expressed alone. Integrating electrophysiological and biochemical analyses, we show that the inactivation of the alphabeta current is not complete even when alpha is saturated with beta, and the alphabeta current has an inherent sustained component, indistinguishable from a pure alpha current. We further show that basal and protein kinase A-induced phosphorylations at Ser-446 of the alpha protein increase the extent, but not the rate, of inactivation of the alphabeta channel, without affecting the association between alpha and beta. In addition, the extent of inactivation is increased by agents that lead to microfilament depolymerization. The effects of phosphorylation and of microfilament depolymerization are not additive. Taken together, we suggest that phosphorylation, via a mechanism that involves the interaction of the alphabeta channel with microfilaments, enhances the extent of inactivation of the channel. Furthermore, phosphorylation at Ser-446 also increases current amplitudes of the alphabeta channel as was shown before for the alpha channel. Thus, phosphorylation enhances in concert inactivation and current amplitudes, thereby leading to a substantial increase in A-type activity.

A potential site of functional modulation by protein kinase A in the cardiac Ca2+ channel alpha 1C subunit

The well-characterized enhancement of the cardiac Ca2+ L-type current by protein kinase A (PKA) is not observed when the corresponding channel is expressed in Xenopus oocytes, possibly because it is fully phosphorylated in the basal state. However, the activity of the expressed channel is reduced by PKA inhibitors. Using this paradigm as an assay to search for PKA sites relevant to channel modulation, we have found that mutation of serine 1928 of the alpha 1C subunit to alanine abolishes the modulation of the expressed channel by PKA inhibitors. This effect was independent of the presence of the beta subunit. Phosphorylation of serine 1928 of alpha 1C may mediate the modulatory effect of PKA on the cardiac voltage-dependent ca2+ channel.

Modulation by protein kinase C activation of rat brain delayed-rectifier K+ channel expressed in Xenopus oocytes

The modulation by protein kinase C (PKC) of the RCK1 K+ channel was investigated in Xenopus oocytes by integration of two-electrode voltage clamp, site-directed mutagenesis and SDS-PAGE analysis techniques. Upon application of beta-phorbol 12-myristate 13-acetate (PMA) the current was inhibited by 50-90%. No changes in the voltage sensitivity of the channel, changes in membrane surface area or selective elimination of RCK1 protein from the plasma membrane could be detected. The inhibition was mimicked by 1-oleoyl-2-acetyl-rac-glycerol (OAG) but not by alphaPMA, and was blocked by staurosporine and calphostin C. Upon deletion of most of the N-terminus a preceding enhancement of about 40% of the current was prominent in response to PKC activation. Its physiological significance is discussed. The N-terminus deletion eliminated 50% of the inhibition. However, phosphorylation of none of the ten classical PKC phosphorylation sites on the channel molecule could account, by itself or in combination with others, for the inhibition. Thus, our results show that PKC activation can modulate the channel conductance in a bimodal fashion. The N-terminus is involved in the inhibition, however, not via its direct phosphorylation.

Regulation of RCK1 currents with a cAMP analog via enhanced protein synthesis and direct channel phosphorylation

We have recently shown that the rat brain Kv1.1 (RCK1) voltage-gated K+ channel is partially phosphorylated in its basal state in Xenopus oocytes and can be further phosphorylated upon treatment for a short time with a cAMP analog (Ivanina, T., Perts, T., Thornhill, W. B., Levin, G., Dascal, N., and Lotan, I. (1994) Biochemistry 33, 8786-8792). In this study, we show, by two-electrode voltage clamp analysis, that whereas treatments for a short time with various cAMP analogs do not affect the channel function, prolonged treatment with 8-bromoadenosine 3',5'-cyclic monophosphorothioate ((Sp)-8-Br-cAMPS), a membrane-permeant cAMP analog, enhances the current amplitude. It also enhances the current amplitude through a mutant channel that cannot be phosphorylated by protein kinase A activation. The enhancement is inhibited in the presence of (Rp)-8-Br-cAMPS, a membrane-permeant protein kinase A inhibitor. Concomitant SDS-polyacrylamide gel electrophoresis analysis reveals that this treatment not only brings about phosphorylation of the wild-type channel, but also increases the amounts of both wild-type and mutant channel proteins; the latter effect can be inhibited by cycloheximide, a protein synthesis inhibitor. In the presence of cycloheximide, the (Sp)-8-Br-cAMPS treatment enhances only the wild-type current amplitudes and induces accumulation of wild-type channels in the plasma membrane of the oocyte. In summary, prolonged treatment with (Sp)-8-Br-cAMPS regulates RCK1 function via two pathways, a pathway leading to enhanced channel synthesis and a pathway involving channel phosphorylation that directs channels to the plasma membrane.

Phosphorylation by protein kinase A of RCK1 K+ channels expressed in Xenopus oocytes

Phosphorylation-mediated regulation of voltage-gated K+ channels has been implicated in numerous electrophysiological studies; however, complementary biochemical studies have so far been hampered by the failure to isolate and characterize any K+ channel proteins of distinct molecular identity. We used the Xenopus oocyte expression system to study the biosynthesis and phosphorylation by protein kinase A (PKA) of rat brain RCK1 (Kv1.1) K+ channel protein. RCK1 protein was isolated by immunoprecipitation from oocytes injected with RCK1 cRNA and analyzed by SDS-polyacrylamide gel electrophoresis (SDS-PAGE). The channel protein was expressed in the form of several polypeptides. The 57-kDa polypeptide, usually the major constituent, resided both in the cytosol and in the plasma membrane. Its levels were correlated with RCK1 current amplitudes (IRCK1) and upon incubation of the cRNA-injected oocytes with tunicamycin, its molecular weight was decreased and at the same time IRCK1 was reduced. These results suggest that the membranal 57-kDa polypeptides represent functional channels that are N-glycosylated. Furthermore, a study of the phosphorylation of the RCK1 polypeptides revealed that the 57-kDa polypeptide was specifically targeted for phosphorylation by PKA. It could be phosphorylated in vitro by the catalytic subunit of PKA (PKA-CS). In its native state in intact oocytes, the 57-kDa polypeptide was partially phosphorylated and could be further phosphorylated in vivo by addition of a membrane-permeant cAMP analog. Site-directed mutagenesis demonstrated that phosphorylation of a single site on the C-terminus of the channel molecule fully accounts for these phosphorylations.(ABSTRACT TRUNCATED AT 250 WORDS)

Mechanism of modulation of single sodium channels from skeletal muscle by the beta 1-subunit from rat brain

We studied the molecular mechanism of the rat skeletal muscle alpha-subunit (alpha microI) gating kinetics modulation by the brain beta 1-subunit by heterologous expression of single sodium channels from alpha microI and beta 1 in Xenopus laevis oocytes. Coexpression of beta 1 reduced mean open time at -10 mV to approximately 21% when compared to channels expressed by alpha microI alone. Channels formed by alpha microI exerted multiple openings per depolarization, which occurred in bursts, in contrast to the channels formed by the alpha microI/beta 1 complex that opened in average only once per depolarizing voltage pulse. Macroscopic current decay (mcd), as evidenced by reconstructed open probability vs. time (po(t)), was greatly accelerated by beta 1, closely resembling mcd of sodium currents from native skeletal muscle. Generally po(t) was larger for channels expressed from the pure alpha microI subunit. From our single channel data we conclude that beta 1 accelerates the inactivation process of the sodium channel complex.

Modulation of the skeletal muscle sodium channel alpha-subunit by the beta 1-subunit

Co-expression of cloned sodium channel beta 1-subunit with the rat skeletal muscle-subunit (alpha microI) accelerated the macroscopic current decay, enhanced the current amplitude, shifted the steady state inactivation curve to more negative potentials and decreased the time required for complete recovery from inactivation. Sodium channels expressed from skeletal muscle mRNA showed a similar behaviour to that observed from alpha microI/beta 1, indicating that beta 1 restores 'physiological' behaviour. Northern blot analysis revealed that the Na+ channel beta 1-subunit is present in high abundance (about 0.1%) in rat heart, brain and skeletal muscle, and the hybridization with untranslated region of the 'brain' beta 1 cDNA to skeletal muscle and heart mRNA indicated that the different Na+ channel alpha-subunits in brain, skeletal muscle and heart may share a common beta 1-subunit.

Evidence for the existence of RNA of Ca(2+)-channel alpha 2/delta subunit in Xenopus oocytes

Ba(2+)-currents (IBa) through voltage-dependent Ca(2+)-channels were studied in Xenopus oocytes injected with RNA from several excitable tissues, using the two-electrode voltage-clamp technique. Previous studies have shown that the expression of cardiac Ca(2+)-channels can be suppressed by an hybrid-arrest procedure that includes co-injection of the tissue-derived RNA with an 'antisense' oligonucleotide complementary to a part of RNA coding for the Ca(2+)-channel alpha 1 subunit. In this study, this method was used to investigate the role of the alpha 2/delta subunit. Co-injection of RNA extracted from either rabbit heart, rat brain or rat skeletal muscle (SkM) with 'antisense' oligonucleotides complementary to the alpha 2/delta subunit RNA did not substantially affect the expression of IBa in the oocytes. Using the Northern blot hybridization method, it was shown that native oocytes contain large amounts of alpha 2/delta subunit RNA of Ca(2+)-channel. It is proposed that te oligonucleotide treatment fails to eliminate the alpha 2/delta RNA because of the vast excess of endogenous alpha 2/delta RNA. These results impose a limit on the use of the hybrid-arrest method.

Protein kinase A reduces voltage-dependent Na+ current in Xenopus oocytes

The voltage-dependent Na+ channel of the brain is a good substrate for phosphorylation by the cAMP-dependent protein kinase (protein kinase A, or PKA), but the physiological effects of PKA on Na+ channels are poorly documented. We studied modulation by PKA of voltage-dependent Na+ channels expressed in Xenopus oocytes injected with RNA coding for the alpha-subunit of the channel protein (rat brain type IIA and its variant VA200), using the two electrode voltage-clamp technique. Intracellularly injected cAMP or catalytic subunit of PKA, or extracellularly applied forskolin, inhibited the Na+ current by 20-30%. The effect of cAMP was attenuated by prior injection of PKA inhibitors. Injection of small doses of protein phosphatase 2A increased the Na+ current by 10%, whereas larger doses of protein phosphatase 1 and alkaline phosphatase were without effect. The inhibition by PKA showed little voltage dependence, being only slightly stronger at holding potentials at which the availability of the channels was reduced. The voltage dependence of activation and inactivation processes was not altered by cAMP. Similar effects were exerted by forskolin and cAMP on the Na+ channels expressed after the injection of heterologous (total) RNA from rat brain. Thus, PKA modulates the Na+ channel by a mechanism that does not involve major changes in the voltage dependency of the current and is exerted on the channel-forming alpha-subunit.

Modulation of cardiac Ca2+ channels in Xenopus oocytes by protein kinase C

L-Type calcium channel was expressed in Xenopus laevis oocytes injected with RNAs coding for different cardiac Ca2+ channel subunits, or with total heart RNA. The effects of activation of protein kinase C (PKC) by the phorbol ester PMA (4 beta-phorbol 12-myristate 13-acetate) were studied. Currents through channels composed of the main (alpha 1) subunit alone were initially increased and then decreased by PMA. A similar biphasic modulation was observed when the alpha 1 subunit was expressed in combination with alpha 2/delta, beta and/or gamma subunits, and when the channels were expressed following injection of total rat heart RNA. No effects on the voltage dependence of activation were observed. The effects of PMA were blocked by staurosporine, a protein kinase inhibitor. beta subunit moderate the enhancement caused by PMA. We conclude that both enhancement and inhibition of cardiac L-type Ca2+ currents by PKC are mediated via an effect on the alpha 1 subunit, while the beta subunit may play a mild modulatory role.

Level of expression controls modes of gating of a K+ channel

Several distinct subfamilies of K+ channel genes have been discovered by molecular cloning, however, in some cases the structural differences among them do not account for the diversity of K+ current types, ranging from transient A-type to slowly inactivating delayed rectifier-type, as members within each subfamily have been shown to code for K+ channels of different inactivation kinetics and pharmacological properties. We show that a single K+ channel cDNA of the Shaker subfamily (ShH4) can express in Xenopus oocytes not only a transient A-type K+ current but also, upon increased level of expression, slowly inactivating K+ currents with markedly reduced sensitivity to tetraethylammonium. In correlation with the macroscopic currents there are single-channel gating modes ranging from the fast-inactivation mode which underlies the transient A-type current, to slow-inactivation modes characterized by bursts of longer openings, and corresponding to the slowly inactivating macroscopic currents.

Calcium channel currents in Xenopus oocytes injected with rat skeletal muscle RNA

1. Ba2+ currents (IBa) through voltage-dependent Ca2+ channels were studied in Xenopus laevis oocytes injected with heterologous RNA extracted from skeletal muscle (SkM) of young rats, using the two-electrode voltage clamp technique. 2. With 40 or 50 mM-extracellular Ba2+, native oocytes of most frogs displayed IBa between -5 and -20 nA at 0 mV. However, in 'variant' native oocytes of four frogs, IBa exceeded -30 nA and reached up to -100 nA. In oocytes injected with SkM RNA, IBa of up to -250 nA was observed. 3. In SkM RNA-injected oocytes and 'variant' native oocytes, the decay of IBa displayed two kinetic components. The faster component was selectively blocked by 40-100 microM-Ni2+ and thus was termed the Ni(2+)-sensitive IBa. The slower component was Ni2+ resistant, being inhibited only 10-20% by 100-200 microM-Ni2+. The half-activation and the half-inactivation voltages of the Ni(2+)-sensitive IBa were more negative (by 14.5 and 28.7 mV, respectively) than those of the Ni(2+)-resistant IBa. 4. Neither Ni(2+)-sensitive nor Ni(2+)-resistant IBa in native or SkM RNA-injected oocytes were affected by dihydropyridine antagonists nifedipine and (+) PN 200-110 (1-10 microM), by the dihydropyridine agonist (-)Bay K 8644 (0.01-2 microM), or by verapamil below 50 microM. IBa was blocked by diltiazem (half-block at about 500 microM). Thus, the pharmacology of IBa in SkM RNA-injected and in native oocytes was not characteristic of the L-type Ca2+ channel abundant in the skeletal muscle. 5. Destruction of the RNA coding for the channel-forming alpha 1-subunit of the SkM L-type Ca2+ channel using a hybrid arrest method failed to selectively suppress the appearance of either Ni(2+)-sensitive or Ni(2+)-resistant IBa in SkM RNA-injected oocytes. 6. Our results suggest that the appearance of large voltage-dependent Ba2+ currents in SkM RNA-injected oocytes is not due to the expression of the alpha 1-subunit of the SkM L-type Ca2+ channel. The possibility that the expression of a channel-forming subunit of another Ca2+ channel type underlies one of these currents cannot be rejected. However, since the Ba2+ currents in SkM RNA-injected oocytes resemble those observed in native oocytes, we suggest that their appearance may be the result of an enhanced activity of the native Ca2+ channels, possibly due to the expression of the 'auxiliary' subunits of the SkM Ca2+ channel that form complexes with a native alpha 1-subunit.

Molecular mechanism of protein kinase C modulation of sodium channel alpha-subunits expressed in Xenopus oocytes

The mechanism of modulation of sodium channel alpha-subunits (Type IIA) by a protein kinase C (PKC) activator was studied on single channel level. It was found that: (i) time constants for channel activation were prolonged; (ii) inactivation remained virtually unchanged; (iii) peak sodium inward current was reduced as evidenced by calculation of average sodium currents; and (iv) time constants for current activation and decay were prolonged. (i), (iii) and (iv) were voltage dependent, being most prominent at threshold potentials. The data show that a voltage dependent action on the activation gate can account for the observed reduction of peak inward sodium current and prolongation of current decay in macroscopic experiments.

The roles of the subunits in the function of the calcium channel

Dihydropyridine-sensitive voltage-dependent L-type calcium channels are critical to excitation-secretion and excitation-contraction coupling. The channel molecule is a complex of the main, pore-forming subunit alpha 1 and four additional subunits: alpha 2, delta, beta, and gamma (alpha 2 and delta are encoded by a single messenger RNA). The alpha 1 subunit messenger RNA alone directs expression of functional calcium channels in Xenopus oocytes, and coexpression of the alpha 2/delta and beta subunits enhances the amplitude of the current. The alpha 2, delta, and gamma subunits also have pronounced effects on its macroscopic characteristics, such as kinetics, voltage dependence of activation and inactivation, and enhancement by a dihydropyridine agonist. In some cases, specific modulatory functions can be assigned to individual subunits, whereas in other cases the different subunits appear to act in concert to modulate the properties of the channel.

Modulation of a Shaker potassium A-channel by protein kinase C activation

Brain fast transient K+ channel (A channel) is known to be modulated by PKC activation. We studied, by two-electrode voltage clamp, the molecular mechanism of modulation by PKC activation of A-channels expressed in Xenopus oocytes from the Shaker H4 clone. The modulation is inhibitory affecting primarily the maximal conductance of the channels. A secondary effect is a small change in the voltage-dependence of activation and inactivation of the channel.

Activation of protein kinase C alters voltage dependence of a Na+ channel

Phorbol esters and purified protein kinase C (PKC) have been shown to down-modulate the voltage-dependent Na+ channels expressed in Xenopus oocytes injected with chick brain RNA. We used the two-electrode voltage-clamp technique to demonstrate that a Na+ channel expressed in oocytes injected with RNA coding for the alpha subunit of the channel alone (VA200, a variant of rat brain type IIA) is also inhibited by PKC activation. The inhibition of Na+ currents, expressed in oocytes injected with either alpha subunit RNA (rat) or total brain RNA (chick), is voltage-dependent, being stronger at negative potentials. It appears to result mainly from a shift in the activation curve to the right and possibly a decrease in the steepness of the voltage dependence of activation. There is little effect on the inactivation process and maximal Na+ conductance. Thus, PKC modulates the Na+ channel by a mechanism involving changes in voltage-dependent properties of its main, channel-forming alpha subunit.

Modulation of vertebrate brain Na+ and K+ channels by subtypes of protein kinase C

Effects of purified subtypes I, II and III of protein kinase C (PKC) on voltage-dependent transient K+ (A) and Na+ channels were studied in Xenopus oocytes injected with chick brain RNA. The experiments were performed in the constant presence of 10 nM beta-phorbol 12-myristate-13-acetate (PMA). Intracellular injection of subtype I (tau) reduced the A-current (IA), with no effect on Na+ current (INa). PKC subtype II (beta 1 + beta 2) and III (alpha) reduced both currents. PKC did not affect the response to kainate. Inactivated (heated) or unactivated (injected in the absence of PMA) enzyme and vehicle alone had no effect. Our results strongly suggest that INa and IA in vertebrate neurons are modulated by PKC; all PKC subtypes exert a similar effect on the A-channel while only subtypes II and III modulate the Na+ channel.

Evidence for the existence of a cardiac specific isoform of the alpha 1 subunit of the voltage dependent calcium channel

Biochemical, pharmacological and electrophysiological evidence implies the existence of tissue specific isoforms of the L-type VDCC. The alpha 1 and alpha 2 subunits of the skeletal muscle calcium channel have been previously cloned and their amino acid sequence deduced. Here we report the isolation and sequencing of a partial cDNA that encodes a heart specific isoform of the alpha 1 subunit. The amino acid sequence deduced from this part cDNA clone shows 64.7% similarity with the skeletal muscle alpha 1 subunit. Northern analysis reveals 2 hybridizing bands, 8.5 and 13 kb, in contrast to one 6.5 kb band in the skeletal muscle. Selective inhibition of mRNA expression in Xenopus oocytes by complementary oligodeoxy-nucleotides derived from the heart clone provides further evidence that the cDNA corresponds to an essential component of the VDCC. These data further support the existence of tissue-specific isoforms of the L-type VDCC.

Specific block of calcium channel expression by a fragment of dihydropyridine receptor cDNA

Although the structure of rabbit skeletal muscle dihydropyridine (DHP) receptor, deduced from cDNA sequence, indicates that this protein is the channel-forming subunit of voltage-dependent calcium channel (VDCC), no functional proof for this prediction has been presented. Two DNA oligonucleotides complementary to DHP-receptor RNA sequences coding for putative membrane-spanning regions of the DHP receptor specifically suppress the expression of the DHP-sensitive VDCC from rabbit and rat heart in Xenopus oocytes. However, these oligonucleotides do not suppress the expression of the DHP-insensitive VDCC and of voltage-dependent sodium and potassium channels. Thus, the gene for DHP receptor of rabbit skeletal muscle is closely related, or identical to, a gene expressed in heart that encodes a component of the DHP-sensitive VDCC. The DHP-sensitive and DHP-insensitive VDCCs are distinct molecular entities.

Blockade of ion channel expression in Xenopus oocytes with complementary DNA probes to Na+ and K+ channel mRNAs

Ionic currents were recorded from Xenopus oocytes injected with RNA isolated from chick or mouse brain. Three currents were studied: a rapid tetrodotoxin-sensitive Na+ current (Ina), an early outward K+ current sensitive to 4-aminopyridine (IA), and an inward current activated by the excitatory amino acid receptor agonist kainate. Oligonucleotides (60-80 bases long) complementary to rat brain Na+ channel sequences were prehybridized to chick brain RNA. These DNA sequences, upon injection into oocytes, specifically inhibited expression of INa relative to IA and the kainate-induced current in a dose-dependent manner. By contrast, prehybridization of oligonucleotides complementary to sequences either from the Drosophila Shaker locus (which codes for an early K+ current in Drosophila muscle) or from a homologous clone from mouse brain did not block the expression of the early outward K+ current induced in the oocytes by mRNA from chick or mouse brain. This method provides a convenient means for testing the functional role of cloned DNA species.

Selective elimination of mRNAs in vivo: complementary oligodeoxynucleotides promote RNA degradation by an RNase H-like activity

Oligodeoxynucleotides lead to translation arrest of complementary mRNAs in the wheat germ translation system by a degradation of the mRNA. In an attempt to develop an effective reverse genetic approach in vivo, we demonstrate that injection of short (15- to 30-nucleotide) oligonucleotides into Xenopus oocytes leads to complete degradation of both injected and endogenous mRNAs by means of an RNase H-like activity.

Dissociation of acetylcholine- and cyclic GMP-induced currents in Xenopus oocytes

In Xenopus follicular oocytes, activation of muscarinic receptors evokes a slow potassium current (H-response); a similar current is evoked by intracellular injection of cyclic guanosine 3',5'-monophosphate, cGMP (Dascal et al. 1984). We have tested the hypothesis that cGMP may be the second messenger that mediates the opening of K channel by acetylcholine (ACh). ACh elevated the intracellular level of cGMP with a time course similar to that of the development of the muscarinic H-response; maximal increase in cGMP concentration above the control was about 0.2 pmole/oocyte. The amount of injected cGMP that produced a detectable K current ("threshold dose") varied between 0.5 and 3 pmole/oocyte. At low doses of cGMP, the slope of log dose-log response curve was about 2.5, suggesting involvement of a biochemical process with a positive cooperativity of at least 3. Higher doses of cGMP evoked, in addition to the outward current, an irregular, rapidly developing, long-lasting inward current, that never reached amplitudes comparable to those of ACh-evoked Cl currents. The K current elicited by cGMP was insensitive to elevation or depletion of external Ca. It was potentiated by isobutylmethylxanthine (IBMX). ACh strongly inhibited the cGMP-evoked K current when applied at the plateau of the latter. 4-Phorbol 12,13-dibutyrate (PDBu) (1 microM) rapidly and completely inhibited the cGMP response. It is concluded, that most of the results presented in this report contradict the hypothesis that cGMP is the intracellular mediator of ACh-induced changes in membrane conductance in the oocytes.

Adenosine-induced K+ current in Xenopus oocyte and the role of adenosine 3',5'-monophosphate

Voltage clamp technique was used in Xenopus laevis oocytes in order to study and compare membrane currents evoked by extracellularly applied adenosine (0.1-10 microM) and intracellularly injected cyclic AMP (0.15-10 microM). The adenosine response is a late long-lasting outward K+ current ("H" current), mediated by the Ra purine receptor subtype. The H current amplitude is directly proportional to (occupancy)3; the KD for adenosine is 3.34 microM. The H current is inhibited by the intracellular injection of protein kinase inhibitors, types II and III (5-450 ng/oocyte) and is usually potentiated by intracellular injection of theophylline (100-300 microM), though extracellular application of theophylline (1-100 microM) reversibly blocks the receptor. Occasionally, the H current is contaminated by a small Cl- current. The cyclic AMP current is also a long-lasting K+ outward current which is potentiated by extracellular theophylline (2 mM). Injection of cyclic AMP inhibits the membrane response to subsequent application of adenosine. The converse inhibition of a cyclic AMP response by an earlier adenosine response is also observed but at very high concentrations of adenosine (greater than 0.6 mM). It was shown by radioimmunoassay that extracellular adenosine increases the level of the intracellular cAMP within a few seconds by about 30%. Intracellular injection of a comparable amount of cAMP was shown to evoke a measurable K+ current. It is proposed that the adenosine-evoked K+ outward current is mediated by a rise in intracellular cAMP.

Adenosine-induced slow ionic currents in the Xenopus oocyte

Adenosine and its 5'-phosphorylated congeners evoke specific membrane-mediated responses in excitable tissues. Available data suggest that inhibition of the target cell occurs due to hyperpolarization, and in some preparations a compound effect of ATP (excitation and inhibition) has been found. However, the ionic mechanism of the purinergic-mediated response has not been studied by standard intracellular voltage-clamping techniques. Recently, we have discovered purinergic receptors in the Xenopus oocyte, a well defined giant cell amenable to rigorous electrophysiological and biochemical studies. We report here that in these cells, adenosine-induced slow membrane responses consisted of an early depolarizing (D) transient current carried by Cl ions, followed by a steady hyperpolarizing (H) current involving K+ ions. The relative potency sequence for the D current was ATP congruent to ADP greater than AMP congruent to adenosine; this order was reversed for the H current.

pH dependence of the acetylcholine receptor channel: a species variation

The effects of pH changes on the miniature endplate current (mepc) and on endplate current fluctuations (acetylcholine [ACh] noise) were examined at the neuromuscular junction in vitro in two species of frogs. In Rana pipiens the relationship between the decay time constant of the mepc (tau') and pH had a symmetrical bell shape; the value of tau' being largest at pH 7 and decreasing at more acid or more alkaline pH. In acid pH the mepc amplitude (A) decreased relative to its value at pH 7, and in alkaline pH A increased. In Rana ridibunda a narrower and asymmetric bell-shaped dependence of tau' on pH, having a maximum of pH 5.5, was found. The mepc amplitude was again reduced in acid pH but had a peak at pH 5.5. Also, its value at pH 9 was larger than at pH 7. These results were obtained with a number of different buffers and were not found to be sensitive to the nature of the buffer chosen. By performing ACh-noise analysis we found that in Rana pipiens at acid pH (5.5-5.0), the single channel conductance (gamma) and the single channel open time (tau) were significantly reduced relative to their value at pH 7. However, in Rana ridibunda at acid pH (5.4) gamma was unchanged and tau was markedly increased relative to their values at pH 7. The results can be explained quantitatively by electrostatic interaction between two fixed and titratable ionic groups and a mobile charge in the receptor molecule. The model fits the data for groups having pKs approximately 4.8 and approximately 9.8 for Rana pipiens and approximately 4.6 and approximately 6.3 for Rana ridibunda. The groups can be tentatively identified as amino acid residues; glutamic or aspartic and lysine or tyrosine for Rana pipiens; glutamic or aspartic and histidine for Rana ridibunda. The difference in the fitted values of the other model parameters for these two species can be attributed to differences in the spatial configuration of the charged groups.