Identification of phosphorylation sites for adenosine 3',5'-cyclic phosphate dependent protein kinase on the voltage-sensitive sodium channel from Electrophorus electricus

Sclerotiorin, a novel inhibitor of lipoxygenase from Penicillium frequentans

Foods rich with unsaturated fatty acids are prone to enzymatic and nonenzymatic lipid peroxidation; lipoxygenase, a metalloenzyme and a free radical former, oxidizes polyunsaturated fatty acids and is one of the key enzymes in lipid oxidation. Here, we report sclerotiorin, purified from the fermented broth of Penicillium frequentans, as a potent reversible, uncompetitive inhibitor against soybean lipoxygenase-1 (LOX-1) with a half-maximal value (IC50) of 4.2 microM. The inhibitor also showed an antioxidant property by scavenging free radical with an ED50 of 0.12 microM; in addition, nonenzymatic lipid peroxidation was inhibited with a PD50 value of 64 microM and did not show metal chelation. The observations made in this study suggest that sclerotiorin possibly inhibits LOX in two ways: one, by interacting with the enzyme-substrate complex, and two, as an antioxidant by quenching or trapping the free radical intermediates formed in the enzyme reaction. Sclerotiorin compares well with other known natural and synthetic lipoxygenase inhibitors.

Ca2+/calmodulin-dependent protein kinase II is an essential mediator in the coordinated regulation of electrocyte Ca2+-ATPase by calmodulin and protein kinase A

The aim of this study was to investigate (a) whether Ca2+/calmodulin-dependent protein kinase II (CaM kinase II) participates in the regulation of plasma membrane Ca2+-ATPase and (b) its possible cross-talk with other kinase-mediated modulatory pathways of the pump. Using isolated innervated membranes of the electrocytes from Electrophorus electricus L., we found that stimulation of endogenous protein kinase A (PKA) strongly phosphorylated membrane-bound CaM kinase II with simultaneous substantial activation of the Ca2+ pump (approximately 2-fold). The addition of cAMP (5-50 pM), forskolin (10 nM), or cholera toxin (10 or 100 nM) stimulated both CaM kinase II phosphorylation and Ca2+-ATPase activity, whereas these activation processes were cancelled by an inhibitor of the PKA alpha-catalytic subunit. When CaM kinase II was blocked by its specific inhibitor KN-93, the Ca2+-ATPase activity decreased to the levels measured in the absence of calmodulin; the unusually high Ca2+ affinity dropped 2-fold; and the PKA-mediated stimulation of Ca2+-ATPase was no longer seen. Hydroxylamine-resistant phosphorylation of the Ca2+-ATPase strongly increased when the PKA pathway was activated, and this phosphorylation was suppressed by inhibition of CaM kinase II. We conclude that CaM kinase II is an intermediate in a complex regulatory network of the electrocyte Ca2+ pump, which also involves calmodulin and PKA.

Serotonin modulates the electric waveform of the gymnotiform electric fish Brachyhypopomus pinnicaudatus

The gymnotiform electric fish Brachyhypopomus pinnicaudatus communicates with a sexually dimorphic electric waveform, the electric organ discharge (EOD). Males display pronounced circadian rhythms in the amplitude and duration of their EODs. Changes in the social environment influence the magnitudes of these circadian rhythms and also produce more transient responses in the EOD waveforms. Here we show that injections of serotonin produce quick, transient, dose-dependent enhancements of the male EOD characters similar to those induced by encounters with another male. The response to serotonin administered peripherally begins 5-10 min post injection and lasts approximately 3 h. The magnitude of the response to serotonin is tightly associated with the magnitude of the day-to-night swing of the circadian rhythm prior to injection. Taken together these findings suggest that the male's social environment influences his response to serotonin by altering the function of some part of the downstream chain between the serotonin receptors and the ion channels involved in control of the EOD waveform. Although chronic activation of serotonin circuitry is widely known to elicit subordinate behavior, we find that 5-HT initially increases a dominance signal in these fish. These findings are consistent with the emerging view that serotonin facilitates different adaptive responses to acute and chronic social challenge and stress.

Na(+) channel regulation by calmodulin kinase II in rat cerebellar granule cells

The effects of specific CaM kinase II inhibitors were investigated on Na(+) channels from rat cerebellar granule cells. A maximal effect of KN-62 was observed at 20 microM and consisted of an 80% reduction of the peak Na(+) current after only a 10-min application. A hyperpolarizing shift of 8 mV in the steady-state inactivation was also observed. KN-04 (20 microM), an inactive analog, had no detectable effect. KN-62 was however inactive on Na(+) currents recorded from Chinese hamster ovary cells expressing the type II A alpha subunit. We have also analyzed the inhibitory effects of CaM kinase II 296-311 and CaM kinase II 281-309 peptides. Both peptides (75 microM) induced a maximum peak Na(+) current reduction within 30 min. Under similar conditions, a truncated peptide CaM kinase II 284-302 was ineffective. These results demonstrate that CaM kinase II acts as a modulator of Na(+) channel activity in cerebellar granule cells.

Electrophorus electricus as a model system for the study of membrane excitability

The stunning sensations produced by electric fish, particularly the electric eel, Electrophorus electricus, have fascinated scientists for centuries. Within the last 50 years, however, electric cells of Electrophorus have provided a unique model system that is both specialized and appropriate for the study of excitable cell membrane electrophysiology and biochemistry. Electric tissue generates whole animal electrical discharges by means of membrane potentials that are remarkably similar to those of mammalian neurons, myocytes and secretory cells. Electrocytes express ion channels, ATPases and signal transduction proteins common to these other excitable cells. Action potentials of electrocytes represent the specialized end function of electric tissue whereas other excitable cells use membrane potential changes to trigger sophisticated cellular processes, such as myofilament cross-bridging for contraction, or exocytosis for secretion. Because electric tissue lacks these functions and the proteins associated with them, it provides a highly specialized membrane model system. This review examines the basic mechanisms involved in the generation of the electrical discharge of the electric eel and the membrane proteins involved. The valuable contributions that electric tissue continues to make toward the understanding of excitable cell physiology and biochemistry are summarized, particularly those studies using electrocytes as a model system for the study of the regulation of membrane excitability by second messengers and signal transduction pathways.

Protein kinase A activation increases sodium current magnitude in the electric organ of Sternopygus

The inactivation kinetics of the Na+ current of the weakly electric fish Sternopygus are modified by treatment with androgens. To determine whether phosphorylation could play a role in this effect, we examined whether activation of protein kinase A by 8 bromo cyclic AMP (8 Br cAMP) altered voltage-dependent properties of the current. Using a two-electrode voltage-clamp procedure, we found no effect of 8 Br cAMP on inactivation kinetics or other voltage-dependent properties of the Na+ current of the electrocytes. However, treatment with 8 Br cAMP did produce a dose-dependent increase in the Na+ current compared with saline controls: 17.6% at 100 microM, 42.4% at 1 mM, and 43.1% at 5 mM. This effect was blocked by 30 microM H89, a PKA inhibitor, indicating that the observed effect was attributable to 8 Br cAMP activation of PKA. We conclude that androgen-induced changes in Na+ current inactivation are not mediated by PKA and suggest that PKA-mediated increases in Na+ current underlie increases in the amplitude of the electric organ discharge observed in social interactions or with changes in water conductance.

Purification of the CIC-0 chloride channel from Torpedo california electroplax identification of a phosphorylation site for cAMP-dependent protein kinase

The voltage-gated chloride channel (CIC-0) from the electric organ of Torpedo californica was purified by immunoaffinity chromatography. A polyclonal antibody was shown to specifically recognize the CIC-0 channel (M(r) 85,000-90,000) in a Western blot of total membrane proteins. As monitored by immunoprecipitation, the formation of antibody-antigen complexes in solution strongly depends on the detergent composition. The highest yield of precipitated CIC-0 was obtained from an incubation mixture containing both an anionic detergent, cholate or lauryl sulfate, and the zwitterionic detergent CHAPS. In contrast, immuno-precipitation of CIC-0 was largely reduced when cholate was exchanged for the nonionic detergent Triton x-100, suggesting that the efficient formation of immuno-complexes is favored by negatively charged detergent. In initial immunopurification experiments, in addition to CIC-0 a major contaminating polypeptide of M(r) approximately 115,000 was copurified, which represents the SITS-binding protein [Jentsch et al. (1989) Biochem. J. 261, 155]. Purification of CIC-0 could be increased from about 35% up to 60% homogeneity when immunoaffinity chromatography was performed in the presence of N-acetylglucosamine. Therefore the highly glycosylated SITS-binding protein most likely interacts with the CIC-0 protein via its carbohydrate parts. The purified CIC-0 channel was found to be phosphorylated by PKA in vitro to a level of 0.35-0.4 mol of phosphate incorporated per mol of CIC-0. Proteolytic digestion with endoproteinase GluC and HPLC-separation revealed two major phosphopeptides, which could be identified by amino acid sequence analysis as different size fragments of the same consensus phosphorylation site. Comparison of the peptide sequences with the deduced protein sequence of CIC-0 [Jentsch et al. (1990) Nature 348, 510; O'Neill et al. (1991) Biochem. Biophys. Acta 1129, 131] indicates serine 600 as the phosphorylated residue. Therefore, our results provide strong evidence that CIC-0 is phosphorylated at this single site by PKA in vitro.

Diacylglycerol-induced activation of protein kinase C attenuates Na+ currents by enhancing inactivation from the closed state

The causes of attenuation of Na+ currents by diacylglycerol (DAG)-induced protein kinase C (PKC) activation in mouse neuroblastoma N1E-115 cells were investigated using the cell-attached patch, and the perforated-patch (nystatin based) whole-cell voltage-clamp techniques. Activation of PKC by DAG attenuated Na+ currents. Attenuation occurred in the absence of significant changes in the time-course of Na+ currents. However, the steady-state inactivation curve of these currents shifted to more negative voltages by approximately 20 mV. Here we demonstrate that the time-course of inactivation is accelerated by treatment with DAG-like substances in a voltage-dependent manner (time constant of inactivation decreased by 2- and 3.6-fold at -60, and -30 mV, respectively). In cell-attached patches, treatment with DAG compounds increased the percentage of current traces showing no single Na+ channel openings in response to depolarizing voltage-clamp pulses. Moreover, the average of current traces containing single Na+ channel openings was essentially the same in control conditions and after treatment with DAG compounds. Removal of Na+ channel inactivation by the alkaloid batrachotoxin prevented the attenuation of Na+ currents by PKC activation via DAGs. Taken together, these data strongly suggest that PKC-induced attenuation of Na+ currents is linked to an enhancement of Na+ channel inactivation. This attenuation is caused by an increase in the number of Na+ channels inactivating directly from the closed state(s). This inactivation pathway represents a simple and efficient physiological mechanism by which PKC activation might modulate the electrical activity of excitable cells.

Cloned Ca(2+)-dependent K+ channel modulated by a functionally associated protein kinase

Calcium-dependent potassium (KCa) channels carry ionic currents that regulate important cellular functions. Like some other ion channels, KCa channels are modulated by protein phosphorylation. The recent cloning of complementary DNAs encoding Slo KCa channels has enabled KCa channel modulation to be investigated. We report here that protein phosphorylation modulates the activity of Drosophila Slo KCa channels expressed in Xenopus oocytes. Application of ATP-gamma S to detached membrane patches increases Slo channel activity by shifting channel voltage sensitivity. This modulation is blocked by a specific inhibitor of cyclic AMP-dependent protein kinase (PKA). Mutation of a single serine residue in the channel protein also blocks modulation by ATP-gamma S, demonstrating that phosphorylation of the Slo channel protein itself modulates channel activity. The results also indicate that KCa channels in oocyte membrane patches can be modulated by an endogenous PKA-like protein kinase which remains functionally associated with the channels in the detached patch.

Multiple effects of protein kinase C activators on Na+ currents in mouse neuroblastoma cells

The effects of externally applied different protein kinase C (PKC) activators on Na+ currents in mouse neuroblastoma cells were studied using the perforated-patch (nystatin-based) whole cell voltage clamp technique. Two diacylglycerol-like compounds, OAG (1-oleoyl-2-acetyl-sn-glycerol), and DOG (1-2-dioctanoyl-rac-glycerol) attenuated Na+ currents without affecting the time course of activation or inactivation. The reduction in Na+ current amplitude caused by OAG or DOG was dependent on membrane potential, being more intense at positive voltages. The steady-state activation curve was also unaffected by these substances. However, both OAG and DOG shifted the steady-state inactivation curve of Na+ currents to more hyperpolarized voltages. Surprisingly, phorbol esters did not affect Na+ currents. Cis-unsaturated fatty acids (linoleic, linolenic, and arachidonic) attenuated Na+ currents without modifying the steady-state activation. As with DOG and OAG, cis-unsaturated fatty acids also shifted the steady-state inactivation curve to more negative voltages. Interestingly, inward currents were more effectively attenuated by cis-fatty acids than outward currents. Oleic acid, also a cis-unsaturated fatty acid, enhanced Na+ currents. This enhancement was not accompanied by changes in kinetic or steady-state properties of currents. Enhancement of Na+ currents caused by oleate was voltage dependent, being stronger at negative voltages. The inhibitory or stimulatory effects caused by all PKC activators on Na+ currents were completely prevented by pretreating cells with PKC inhibitors (calphostin C, H7, staurosporine or polymyxin B). By themselves, PKC inhibitors did not affect membrane currents. Trans-unsaturated or saturated fatty acids, which do not activate PKC's, did not modify Na+ currents. Taken together, the experimental results suggest that PKC activation modulates the behavior of Na+ channels by at least three distinct mechanisms. Because qualitatively different results were obtained with different PKC activators, it is not clear how Na+ currents would respond to activation of PKC under physiological conditions.

Regulation of the eel electroplax Na channel and phosphorylation of residues on amino- and carboxyl-terminal domains by cAMP-dependent protein kinase

Previous studies have shown that the short-motif electroplax Na channel is phosphorylated in vitro by cyclic AMP-dependent protein kinase (PKA) at serines 6 or 7 and 1776 and threonine 17 (Emerick & Agnew, 1989). We here show that phosphatase treatment of solubilized, purified Na channels enhanced subsequent PKA labeling of four of five tryptic phosphopeptides, indicating that these sites are phosphorylated in vivo. Microsequencing and analysis of PTH-amino acid products revealed endogenous labeling of serines 6, 444, 1680, and 1776. Serines 1680 and 1776 lie in the carboxyl-terminal cytoplasmic domain, while serine 6 lies in the amino terminus and serine 444 is in the cytoplasmic loop between domains I and II. Endogenous phosphorylation of serine 6 establishes experimentally that the Na channel amino terminus is cytoplasmic. In electrophysiological experiments, brief exposure of inside-out membrane patches excised from Sachs-organ cells to MgATP and purified PKA catalytic subunit produced rapid, sustained reduction of Na current amplitude by approximately 80% and a hyperpolarizing shift in the conductance/voltage relation by 10-12 mV. The effect was absent in controls omitting either PKA or MgATP. Serines 6 and 1776 and threonine 17 are labeled rapidly and extensively in vitro, and only threonine 17 appears to be unphosphorylated in vivo. We suggest that phosphorylation of the amino and carboxyl domains, perhaps especially at threonine 17, underlies the demonstrated downregulation of the electroplax Na channel.

Structure, function and expression of voltage-dependent sodium channels

Voltage-dependent sodium channels control the transient inward current responsible for the action potential in most excitable cells. Members of this multigene family have been cloned, sequenced, and functionally expressed from various tissues and species, and common features of their structure have clearly emerged. Site-directed mutagenesis coupled with in vitro expression has provided additional insight into the relationship between structure and function. Subtle differences between sodium channel isoforms are also important, and aspects of the regulation of sodium channel gene expression and the modulation of channel function are becoming topics of increasing importance. Finally, sodium channel mutations have been directly linked to human disease, yielding insight into both disease pathophysiology and normal channel function. After a brief discussion of previous work, this review will focus on recent advances in each of these areas.

A topographical study of the electroplax sodium channel with site-directed antibodies

Three peptides corresponding to residues (13-23C), (Y1419-1431), and (1809-1820) of the electric eel sodium channel have been synthesized and used to raise antisera in rabbits. All the antibodies produced specifically recognized the corresponding peptides in an ELISA assay. However, their avidities to the channel protein were different. Two antibodies, against the sequence (1809-1820) and (13-23C) which are directed to the C-terminus region and to the adjacent portion of N-terminus, respectively, recognized the 250 kDa channel protein in the immunoblot and an ELISA assay. Binding of the two antibodies to the sodium channel in oriented electroplax membrane vesicles was increased 4-5-fold after permeabilizing the vesicles with 0.01% saponin, implying cytoplasmic orientation for the two peptides. The cytoplasmic orientation of the N- and C-terminal regions were further confirmed by immunogold electron microscopy. By contrast, antibody raised against the sequence (Y1419-1431) which has been proposed to be the transmembrane segment S4 of internal repeat IV, did not react with the channel protein not only after the saponin treatment but also even under the immunoblot conditions following SDS-PAGE. The antibody could recognize fragments of the channel protein after digestion with lysyl endoproteinase, suggesting that the region may apparently form a strictly well-ordered conformation in the transmembrane part of the channel molecule.

Voltage-sensitive Na+ channels: motifs, modes and modulation

Much recent progress has been made in understanding the structural organization and functional properties of voltage-dependent Na+ channels, in particular in the areas of activation, ion conductance, and inactivation. At the same time, however, electrophysiological studies have revealed new, more complex functional properties in the form of at least two gating modes and the existence of as yet unidentified modulatory factors.

Patch recordings from the electrocytes of Electrophorus electricus. Na currents and PNa/PK variability

Sodium currents were recorded in cell-attached and inside-out patches from the innervated membrane of Electrophorus electrocytes. Electrocytes from Sachs and main electric organs were prepared as described by Pasquale et al. (1986. J. Membr. Biol. 93:195.). Maximal currents in the Sachs organ, measured with 1-2 microns diameter patch pipettes and at room temperature, were in the range of 20 to 300 pA (27 patches) and were obtained near +10 mV. This range of current corresponds to approximately 70 to 1,300 channels in a patch. Maximal current in main organ cells also occurred near +10 mV and were in the range of 100 to 400 pA. Delayed K current was observed in a few patches. The inactivation phase of the currents during maintained depolarizations appears to be a single-exponential relaxation. The time constant decreases from 1 ms near -55 mV to a minimum of 0.3 ms near 0 mV, and then gradually increases with stronger depolarization. The mean currents are half inactivated near -90 mV with an apparent voltage dependence of e-fold per 6 mV. No apparent differences were observed in the decay time course or steady-state inactivation of the currents in the same patch before and after excision. From ensemble fluctuation analysis the peak open probability was found to be approximately 0.5 at +25 mV and increased only gradually with larger depolarizations. The single channel conductances were approximately 20 pS with 200 mM Na outside and 200 mM K inside, and 40 pS in 400 mM solutions. Reversal potentials in the 200 Na parallel 200 K solutions ranged from +51 to +94 mV in multichannel patches, corresponding to selectivity ratios PNa/PK from 8 to 43. Large differences in reversal potentials were seen even among patches from the same cell. Several controls rule out obvious sources of error in the reversal potential measurements. It is concluded that there is heterogeneity in the selectivity properties of the Na channels.