Myopathic mutations affect differently the inactivation of the two gating modes of sodium channels

Three groups of mutations of the alpha subunit of the rat skeletal muscle sodium channel (rSkM1), homologous to mutations linked to human muscle hereditary diseases, have been studied by heterologous expression in frog oocytes: S798F, G1299E, G1299V, and G1299A, linked with potassium-aggravated myotonia (PAM); T1306M, R1441C and R1441P, linked with paramyotonia congenita (PC); T698M and M1353V, linked with the hyperkalemic periodic paralysis (HyPP). Wild-type rSkM1 channels (WT) show two gating modes, M1 and M2, which differ mainly in the process of inactivation. The naturally most representative mode M1 is tenfold faster and develops at approximately 30 mV less depolarized potentials. A common feature of myopathy-linked mutants is an increase in the mode M2 probability, P(M2), but phenotype-specific alterations of voltage-dependence and kinetics of inactivation of both modes are also observed. The coexpression of the sodium channel beta1 subunit, which has been studied for WT and for the five best expressing mutants, generally caused a threefold reduction of P(M2) without changing the properties of the individual modes. This indicates that the mutations do not affect the alpha - beta1 interaction and that the phenotypic changes in P(M2) observed for the enhanced mode M2 behavior of the sole alpha subunits, although largely depressed in the native tissue, are likely to be the most important functional modification that causes the muscle hyperexcitability observed in all patients carrying the myotonic mutations. The interpretation of the more phenotype-specific changes revealed by our study is not obvious, but it may offer clues for understanding the different clinical manifestations of the diseases associated with the various mutations.

KCNE1-like gene is deleted in AMME contiguous gene syndrome: identification and characterization of the human and mouse homologs

We describe the identification and characterization of a new gene deleted in the AMME contiguous gene syndrome. This gene is predominantly expressed in heart, skeletal muscle, spinal cord, and brain. Screening of placenta and NT2 cDNA libraries enabled us to obtain the 1.5-kb full-length transcript, which shows a 426-bp open reading frame. Since the resulting 142-amino-acid peptide has a single putative transmembrane domain and a weak but suggestive homology with KCNE1 (minK), a protein associated with the KCNQ1 potassium channel (KVLQT1), we named this new gene KCNE1-like (KCNE1L). To obtain greater insight into this new member of an apparently distinct protein family, we have identified and characterized the homologous mouse gene (Kcne1l), which encodes a peptide of 143 amino acids with 91% homology and 80% identity. The expression pattern of mouse Kcne1l in the developing embryo revealed strong signal in ganglia, in the migrating neural crest cells of cranial nerves, in the somites, and in the myoepicardial layer of the heart. The specific distribution in adult tissues, the putative channel function, and the expression pp6tern in the developing mouse embryo suggest that KCNE1L could be involved in the development of the cardiac abnormalities as well as of some neurological signs observed in patients with AMME contiguous gene syndrome.

Tonic and phasic tetrodotoxin block of sodium channels with point mutations in the outer pore region

Tonic and use-dependent block by tetrodotoxin (TTX) has been studied in cRNA-injected Xenopus oocytes expressing mutants W386Y, E945Q, D1426K, and D1717Q, of the outer-pore region of the rat brain IIA alpha-subunit of sodium channels. The various phenotypes are tonically half-blocked at TTX concentrations, IC50(t), that span a range of more than three orders of magnitude, from 4 nM in mutant D1426K to 11 microM in mutant D1717Q. When stimulated with repetitive depolarizing pulses at saturating frequencies, all channels showed a monoexponential increase in their TTX-binding affinity with time constants that span an equally wide range of values ([TTX] approximately IC50(t), from approximately 60 s for D1426K to approximately 30 ms for D1717Q) and are in most phenotypes roughly inversely proportional to IC50(t). In contrast, all phenotypes show the same approximately threefold increase in their TTX affinity under stimulation. The invariance of the free-energy difference between tonic and phasic configurations of the toxin-receptor complex, together with the extreme variability of phasic block kinetics, is fully consistent with the trapped-ion mechanism of use dependence suggested by and developed by. Using this model, we estimated for each phenotype both the second-order association rate constant, kon, and the first-order dissociation rate constant, koff, for TTX binding. Except for mutant E945Q, all phenotypes have roughly the same value of kon approximately 2 microM-1 s-1 and owe their large differences in IC50(t) to different koff values. However, a 60-fold reduction in kon is the main determinant of the low TTX sensitivity of mutant E945Q. This suggests that the carboxyl group of E945 occupies a much more external position in the pore vestibule than that of the homologous residue D1717.

Characterization of a murine gene homologous to the bovine CaCC chloride channel

The bovine CaCC protein is a putative Ca2+-dependent Cl- channel of airway epithelial cells. Therefore, CaCC proteins could contribute to transepithelial Cl- transport and accordingly modify the phenotype of cystic fibrosis (CF) patients. We have identified a murine EST containing a full-length cDNA coding for a 902-amino-acid protein highly homologous to bovine CaCC. The murine gene (mCaCC) maps to chromosome 3 at the H2-H3 band and is expressed, as indicated by Northern blot analysis, in mouse skin and kidney but not in brain, heart, lung or testis. RT-PCR indicates a low expression in tracheal epithelial cells. Heterologous expression of mCaCC in Xenopus oocytes elicits membrane currents that are anion-selective and inhibited by DIDS and by niflumic acid, a blocker of the endogenous chloride current in oocytes. The identification of genes belonging to the CaCC family will help to evaluate their role as ion channels or channel regulators and their actual contribution to epithelial chloride transport.

Fast- and slow-gating modes of the sodium channel are altered by a paramyotonia congenita-linked mutation

We have studied the expression in frog oocytes of the alpha subunit of the rat skeletal muscle sodium channel mutation T1306M, homologous to the mutation T1313M of the human isoform that causes the muscular hereditary disease paramyotonia congenita. Wild-type (WT) channels show a bimodal behavior, with two gating modes characterized by inactivation time constants that differ at least by one order of magnitude and with voltage dependencies shifted by +27 mV in the slow mode (M2) relative to the fast (M1) mode. In the myopathy-linked mutant the propensity of the channel for the mode M2 is increased fourfold and the kinetics and voltage dependence of inactivation in both modes are altered. In mode M1, the onset of inactivation is faster and the recovery from inactivation is slower whereas both processes are slowed in mode M2. The half-inactivation potential of both modes is shifted by the mutation to positive potentials. Coexpression of beta subunit causes a threefold reduction of the M2 propensity of both WT and T1306M channels, with small changes in the voltage dependency and kinetic properties of inactivation. All the changes are consistent with the hyperexcitability of the muscle fibers observed in patients affected by potassium-aggrevated myotonia (PAM).

Calcium dependent shifts of Na+ channel activation correlated with the state dependence of calcium-binding to the pore

Calcium ions block the open configuration and antagonise the tonic binding of TTX to the closed state of sodium channels in very different ranges of extracellular concentration, [Ca]o. We measured the open-state block in channels expressed in Xenopus oocytes by alpha-subunits from rat brain (rBIIa) or adult rat skeletal muscle (rSkM1). Recordings of instantaneous tail-currents from cell-attached macro patches show that the binding of Ca2+ to the blocking site has a dissociation constant of about 20 mM at 0 mV and senses about 30% of the membrane potential drop, whereas the concentration of half-inhibition of TTX-binding is less than 1 mM and voltage-insensitive. Assuming that both effects involve a single binding site, a simple model predicts that the state-dependency of the dissociation constant entails positive shifts of activation and faster kinetics of deactivation at increasing [Ca]o. The shifts of activation measured for rBIIA and rSkM1 channels are comparable in size to those predicted by the model, which accounts also for the observed larger shifts of the rBIIA-mutant K226Q as a consequence of its reduced voltage-sensitivity. Shifts attributable to surface-charge screening effects seem smaller in the oocyte than in native cell-membranes. The experimental [Ca]o-dependence of deactivation kinetics is also consistent with the model and with the idea that Ca(2+)-binding changes to the same extent, but in opposite directions, the activation free-energies of both opening and closing transitions.

Interaction between dihydropyridines and phospholipid bilayers: a molecular dynamics simulation

Interaction of the calcium-channel antagonist dihydropyridines (DHPs), lacidipine and nifedipine, with a phospholipid bilayer was studied using 600 ps molecular dynamic simulations. We have constructed a double layer membrane model composed of 42 dimirystoyl-phosphatidylcholine molecules. The DHP molecules locate at about 7 A from the centre of the membrane, inducing an asymmetry in the bilayer. While lacidipine did not induce significant local perturbations as judged by the gauche-trans isomerisation rate, nifedipine significantly decreased this rate, probably by producing a local rigidity of the membrane in the vicinity of the DHP.

Inactivation defects produced by a myopathic II-S6 mutation of the muscle sodium channel

We have studied the expression in frog oocytes of the alpha-subunit of the rat skeletal muscle sodium channel mutation S798F, homologous to the mutation S804F of the human isoform, that causes potassium aggravated myotony (PAM), a muscular hereditary disease in humans. Wild type channels show a bimodal inactivation, with two gating modes that inactivate with time constants that differ at least by one order of magnitude and a steady steady-state voltage dependence of the slow mode shifted by +27 mV relative to that of the fast mode. In the myopathy-linked mutant the propensity of the channel to gate in the slow mode is significantly increased and there are alterations in the inactivation properties of both modes. The half inactivation potential of the fast mode is shifted negatively, and the inactivation kinetics of both modes are slower, with an apparent shift in their voltage dependence. The changes on the inactivation properties of the mutant channel may be related with the muscle fibre hyperexcitability observed patients affected by PAM.

Simulation of action potential damping produced by anticonvulsant drugs

Anticonvulsant drugs reduce the ability of central neurones to sustain high-frequency repetitive firing of action potentials. In recent years, it has been demonstrated that this effect is primarily due to a Na+ conductance reduction. We have simulated the electrical behaviour of a neurone, including Ca2+ and various K+ conductances. Although a reduction of Na+ conductance produces a progressive reduction until a complete suppression of the action potential bust, a smaller reduction of this conductance is necessary to produce the same effect when the delayed-rectifier K+ conductance and the Ca2+ conductance are concomitantly reduced . The results indicate that the drugs action on conductances other than Na+ is important for determining their anticonvulsant effect on neurones at therapeutic concentrations.

Use dependence of tetrodotoxin block of sodium channels: a revival of the trapped-ion mechanism

The use-dependent block of sodium channels by tetrodotoxin (TTX) has been studied in cRNA-injected Xenopus oocytes expressing the alpha-subunit of rat brain IIA channels. The kinetics of stimulus-induced extra block are consistent with an underlying relaxation process involving only three states. Cumulative extra block induced by repetitive stimulations increases with hyperpolarization, with TTX concentration, and with extracellular Ca2+ concentration. We have developed a theoretical model based on the suggestion by Salgado et al. that TTX blocks the extracellular mouth of the ion pore less tightly when the latter has its external side occupied by a cation, and that channel opening favors a tighter binding by allowing the escape of the trapped ion. The model provides an excellent fit of the data, which are consistent with Ca2+ being more efficient than Na+ in weakening TTX binding and with bound Ca2+ stabilizing the closed state of the channel, as suggested by Armstrong and Cota. Reports arguing against the trapped-ion mechanism are critically discussed.

Accumulation of long-lasting inactivation in rat brain K(+)-channels

We studied the phenomenon of cumulative inactivation in the voltage-dependent K+ channels of the Shaker-related subfamily Kv1 cloned from rat brain and expressed in Xenopus oocytes. In Kv1.4, repetitive stimulations at intervals shorter than 20 s produce cumulative inactivation even for brief stimuli that elicit K+ currents which do not show any significant decline during the depolarising pulse. These effects are absent or greatly reduced in the clones Kv1.1, Kv1.3, Kv1.5 and Kv1.6, and in the deletion mutant Kv1.4-delta-110, characterised by lack of "fast" (N-type) inactivation. We find that the inactivation caused by a single pulse increases after the pulse while the channels deactivate, and subsides with two time constants, indicating the existence of (at least) two inactivated states: IS, with a slow recovery kinetics and IF, with faster kinetics. In the simplest kinetic scheme accounting for our observations, IF is coupled sequentially to the open state O, while IS can be reached at a fast rate both from IF and from a pre-open, activated state, A, that is in fast equilibrium with O. The accumulation of long-lasting inactivation during the repolarisation is favoured by the prolongation of the lifetime of activated states due to the presence of IF. This explains the smaller accumulation effect observed in channels lacking fast inactivation. The physiological implications of these findings suggest how different channels of the Kv1 subfamily can affect differently the firing behaviour of neurones.

Primary peritoneal serous papillary carcinoma: a study of 25 cases and comparison with stage III-IV ovarian papillary serous carcinoma

The clinical characteristics and treatment outcome of patients with primary peritoneal serous papillary carcinoma (PPSC) (n = 22) was compared with stage III-IV papillary serous ovarian carcinoma (PSOC) patients (n = 63). There were no statistically significant differences between the PPSC and PSOC patients with regard to the mean age, menopausal status, parity, ascites fluid volume, proportion of stage IV disease, and the rate of optimal debulking achieved. The median disease-free interval was 15 and 18 months; the median survival was 21 and 26 months; and the 5-year survival was 18 and 24% for the PPSC and PSOC groups, respectively. The median survival time for patients with a residual tumor > or = 2 cm was 20.5 and 24 months, and for residual tumor > or = 2 cm was 46 and 41 months, in PPSC and PSOC patients, respectively. Survival was thus better, in both groups, when residual disease at the end of the operation was < 2 cm, though this was statistically significant only for PSOC (P < 0.02). We conclude that patients with PPSC should be treated as other stage II-IV PSOC patients. Combining optimal debulking with a platinum-based chemotherapy may offer the patient the most effective treatment.

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.

Prediction of polypeptide secondary structures analysing the oscillation of the hydropathy profile

The hydropathy profile of a polypeptide can reflect periodicity tightly linked, in particular, with alpha-helix and beta-strand conformations. We have developed an algorithm to detect such periodicity to predict the secondary structure of proteins. The method uses profiles constructed with the weighted average of hydropathy along the primary structure of the polypeptide and does the analysis of the variation of the profile for looking for periodic oscillations, using a variation detecting algorithm, without the definition of an empirical threshold. The independence of the method from known structures makes it particularly reliable for analysing membrane proteins.

Properties of the Kv1.1 rat brain potassium channels expressed in mammalian cells: temperature effects

We studied the temperature dependence of the currents mediated by potassium channels Kv1.1 cloned from rat brain expressed by permanent transfection in the human embryonic kidney cell line 292er. Voltage-gated outward currents, with a reversal potential near to the Nernst potential for potassium, were elicited by depolarising pulses. The currents activate fast and do not show inactivation during 50-ms test pulses. Increasing temperature decreased the amplitude of the currents, slowed the kinetics of activation and deactivation and shifted the activation curve to more negative potentials. We conclude that in Kv1.1 channels the closed states are associated with more ordered structure of the channel protein than the open state. The voltage-dependence and the kinetic properties are similar to those expressed in frog oocytes injected with cRNA coding the same Kv1.1.

Inward rectifier potassium channels in plants differ from their animal counterparts in response to voltage and channel modulators

We have investigated the electrophysiological basis of potassium inward rectification of the KAT1 gene product from Arabidopsis thaliana expressed in Xenopus oocytes and of functionally related K+ channels in the plasma membrane of guard and root cells from Vicia faba and Zea mays. The whole-cell currents passed by these channels activate, following steps to membrane potentials more negative than -100 mV, with half activation times of tens of milliseconds. This voltage dependence was unaffected by the removal of cytoplasmic magnesium. Consequently, unlike inward rectifier channels of animals, inward rectification of plant potassium channels is an intrinsic property of the channel protein itself. We also found that the activation kinetics of KAT1 were modulated by external pH. Decreasing the pH in the range 8.5 to 4.5 hastened activation and shifted the steady state activation curve by 19 mV per pH unit. This indicates that the activity of these K+ channels and the activity of the plasma membrane H(+)-ATPase may not only be coordinated by membrane potential but also by pH. The instantaneous current-voltage relationship, on the other hand, did not depend on pH, indicating that H+ do not block the channel. In addition to sensitivity towards protons, the channels showed a high affinity voltage dependent block in the presence of cesium, but were less sensitive to barium. Recordings from membrane patches of KAT1 injected oocytes in symmetric, Mg(2+)-free, 100 mM-K+, solutions allowed measurements of the current-voltage relation of single open KAT1 channels with a unitary conductance of 5 pS. We conclude that the inward rectification of the currents mediated by the KAT1 gene product, or the related endogenous channels of plant cells, results from voltage-modulated structural changes within the channel proteins. The voltage-sensing or the gating-structures appear to interact with a titratable acidic residue exposed to the extracellular medium.

Activation and deactivation properties of rat brain K+ channels of the Shaker-related subfamily

We studied the activation properties of members of the Shaker-related subfamily of voltage-gated K+ channels cloned from rat brain and expressed in Xenopus oocytes. We find that Kv1.1, Kv1.4, Kv1.5, and Kv1.6 have similar activation and deactivation kinetics. The k+ currents produced by step depolarisations increase with a sigmoidal time course that can be described by a delay and by the derivative of the current at the inflection point. The delay tends to zero and the logarithmic derivative seems to approach a finite value at large positive voltages, but these asymptotic values are not yet reached at +80 mV. Deactivation of the currents upon stepping to negative membrane potentials below -60 mV is fairly well described by a single exponential. The decrease of the deactivation time constant at increasingly negative voltages tends to become less steep, indicating that this parameter also has a finite limiting value, which is not yet reached, however, at -160 mV. The various clones studied have very similar voltage dependencies of activation with half-activation voltages ranging between -50 and -11 mV and maximum steepness yielding and e-fold change for voltage increments between 3.8 and 7.0 mV. The shallower activation curve of Kv1.4 is likely to be due to coupling with the fast inactivation process present in this clone.

Proline mutations on the S4 segment of rat brain sodium channel II

We have studied S4-proline mutants of the rat brain sodium channel II. In mutant A224P one proline was added on the S4 segment of repeat I, and in mutant P1313V a proline was removed from the segment S4 of the repeat II. In both mutants, the activation curve was shifted to more positive potentials, without changing the steepness of the voltage dependence. The time course of inactivation, consisting of two exponential components, was similar in the wild type and in mutant A224P. Differently, the decay of the current in mutant P1313V had only one component, with a time constant similar to that of the fast component of wild type channels. This change in kinetics was accompanied in mutant P1313V by a change in the voltage dependence of the apparent steady-state inactivation. We conclude that the addition or deletion of prolines in segment S4 does not affect significantly the activation of sodium channels, but alters their mode of inactivation.

On the structure of mitochondrial porins and its homologies with bacterial porins

By use of computer modelling, we have predicted a model of 16 transmembrane beta-strands for mitochondrial porins structure from human, Saccharomyces cerevisiae, Neurospora crassa and Dictyostelium discoideum. The proposed model takes into account biochemical and immunological data reported in the literature, as well as electrophysiological results obtained with yeast mitochondrial porins with mutations at selected amino acids. The predicted structure is very similar to that of some bacterial porins, as apparent from the homology of their hydropathic profiles.

Properties of the transient potassium currents in cerebellar granule cells

Macroscopic potassium currents were studied in cell-attached and inside-out patches from rat cerebellar granule cells. They were related with transient IA type potassium channels. Currents activated rapidly at potentials higher than -40 mV and did not inactivate completely. The magnitude of the current diminished when the membrane patches were excised. No differences in the activation and inactivation properties were found between patches in the integral cells and cell free membrane patches. A biophysical description of the currents is presented.

Outpatient endometrial sampling with the Pipelle curette

This study compares outpatient endometrial sampling using the Pipelle endometrial sampling curette with conventional dilation and curettage (D&C) in patients with abnormal uterine bleeding. Endometrial sampling with the Pipelle curette was performed in 172 and D&C in 97 women. No complications were encountered with either of these procedures. One hundred and seventy (98.8%) of the Pipelle aspirations attempted were successfully completed. Sufficient material for histological assessment was obtained in 154 (90.6%) of the women who underwent Pipelle endometrial sampling and in only 66 (68%) of those who underwent D&C (p < 0.0001). In postmenopausal women, adequate specimens were obtained in 74 of 88 (84.1%) by Pipelle and in only 22 of 48 (45.8%) by D&C. In 45 cases the histologic diagnosis of the endometrium obtained by Pipelle sampling was compared with the one of endometrium obtained by D&C or hysterectomy performed shortly thereafter. The diagnosis was identical in 43 (95.5%) cases. Endometrial sampling with the Pipelle was well tolerated causing occasionally only slight discomfort.

Cytologic examination of ovarian cyst fluid for the distinction between benign and malignant tumors

OBJECTIVE

To assess the feasibility of using cytologic examination of ovarian cyst fluid for distinguishing between benign and malignant tumors.

METHODS

Cyst fluid was aspirated at surgery in 83 women with benign and 35 women with malignant (25 invasive and ten borderline) ovarian tumors and submitted for routine cytologic examination.

RESULTS

The specificity of the cytologic examination was 100%, but the sensitivity and negative predictive value were only 26 and 76%, respectively.

CONCLUSION

The low sensitivity and negative predictive value of cytologic cyst fluid examination preclude its use for the distinction between benign and malignant ovarian tumors.

Modulation of voltage-dependent calcium channels by glutamate in rat cerebellar granule cells in culture

Voltage-dependent calcium channels of cerebellar granule cells maintained in a Ca(2+)-free depolarising solution were recorded using the cell-attached configuration of the patch-clamp technique. An increase in the maximum open probability of calcium channels and a shift in their activation curve toward more hyperpolarizing potentials were found in the presence of glutamate, a natural, excitatory amino acid. Such an increase in the activity of calcium channels was not due to ionic fluxes activated by glutamate, and was probably produced by a second messenger pathway triggered by the binding of glutamate to its receptor.

Channels in mitochondrial membranes: knowns, unknowns, and prospects for the future

Rapid diffusion of hydrophilic molecules across the outer membrane of mitochondria has been related to the presence of a protein of 29 to 37 kDa, called voltage-dependent anion channel (VDAC), able to generate large aqueous pores when integrated in planar lipid bilayers. Functional properties of VDAC from different origins appear highly conserved in artificial membranes: at low transmembrane potentials, the channel is in a highly conducting state, but a raise of the potential (both positive and negative) reduces drastically the current and changes the ionic selectivity from slightly anionic to cationic. It has thus been suggested that VDAC is not a mere molecular sieve but that it may control mitochondrial physiology by restricting the access of metabolites of different valence in response to voltage and/or by interacting with a soluble protein of the intermembrane space. The latest application of the patch clamp and tip-dip techniques, however, has indicated both a different electric behavior of the outer membrane and that other proteins may play a role in the permeation of molecules. Biochemical studies, use of site-directed mutants, and electron microscopy of two-dimensional crystal arrays of VDAC have contributed to propose a monomeric beta barrel as the structural model of the channel. An important insight into the physiology of the inner membrane of mammalian mitochondria has come from the direct observation of the membrane with the patch clamp. A slightly anionic, voltage-dependent conductance of 107 pS and one of 9.7 pS, K(+)-selective and ATP-sensitive, are the best characterized at the single channel level. Under certain conditions, however, the inner membrane can also show unselective nS peak transitions, possibly arising from a cooperative assembly of multiple substrates.