Calcium-activated potassium channels

Calcium-activated potassium channels are fundamental regulators of neuronal excitability, participating in interspike interval and spike-frequency adaptation. For large-conductance calcium-activated potassium (BK) channels, recent experiments have illuminated the fundamental biophysical mechanisms of gating, demonstrating that BK channels are voltage gated and calcium modulated. Structurally, BK channels have been shown to possess an extracellular amino-terminal domain, different from other potassium channels. Domains and residues involved in calcium-gating, and perhaps calcium binding itself, have been identified. For small- and intermediate-conductance calcium-activated potassium channels, SK and IK channels, clones have only recently become available, and they show that SK channels are a distinct subfamily of potassium channels. The biophysical properties of SK channels demonstrate that kinetic differences between apamin-sensitive and apamin-insensitive slow afterhyperpolarizations are not attributable to intrinsic gating differences between the two subtypes. Interestingly, SK and IK channels may prove effective drug targets for diseases such as myotonic muscular dystrophy and sickle cell anemia.

Acute activation of Maxi-K channels (hSlo) by estradiol binding to the beta subunit

Maxi-K channels consist of a pore-forming alpha subunit and a regulatory beta subunit, which confers the channel with a higher Ca(2+) sensitivity. Estradiol bound to the beta subunit and activated the Maxi-K channel (hSlo) only when both alpha and beta subunits were present. This activation was independent of the generation of intracellular signals and could be triggered by estradiol conjugated to a membrane-impenetrable carrier protein. This study documents the direct interaction of a hormone with a voltage-gated channel subunit and provides the molecular mechanism for the modulation of vascular smooth muscle Maxi-K channels by estrogens.

Kinetics of Ca2+-activated K+ channels from rabbit muscle incorporated into planar bilayers. Evidence for a Ca2+ and Ba2+ blockade

The interaction of Ca2+ and Ba2+ with a Ca2+-activated K+ channel from rabbit skeletal muscle membranes is studied in planar lipid bilayers. At [Ca2+] greater than or equal to 100 microM in the cis side (the side to which the vesicles are added) and at positive voltages, the channel kinetics consisted of bursts of activity interrupted by long periods of quiescence. We found that the reciprocal of the mean burst time increases linearly with [Ca2+], whereas the mean time for the quiescent (closed) periods is independent of [Ca2+]. The number of quiescent periods is reduced by increasing [K+]. Micromolar amounts of cis Ba2+ do not activate the channel, but induce similar "slow" closings. Also, in this case, the mean burst time is inversely proportional to the [Ba2+] and the mean closed time is independent of [Ba2+]. Raising [K+] either symmetrically or only in the trans side relieved the Ba2+ effect. trans Ba2+ also induces changes in the slow kinetics, but in millimolar amounts. These results suggest that the quiescent periods correspond to a channel blocked by a Ba ion. The voltage dependence of the cis blockade indicates that the Ba2+ binding site is past the middle of the membrane field. The similarities in the slow kinetics induced by Ca2+ and Ba2+ suggest that Ca2+ blocks the channel by binding to the same site. However, binding of Ca2+ to the site is 10(5)-fold weaker.

Reconstitution in planar lipid bilayers of a Ca2+-dependent K+ channel from transverse tubule membranes isolated from rabbit skeletal muscle

Addition of membrane vesicles prepared from transverse tubule (T-tubule) membranes of rabbit skeletal muscle to the aqueous phase of a planar lipid bilayer induces a stepwise increase in conductance. This conductance is both voltage and Ca2+ dependent. At 1 mM Ca2+, the steady-state conductance is maximal at voltages higher than +20 mV and decreases for more negative voltages. (Voltages refer to the side to which the vesicles are added, cis) Decreasing the Ca2+ concentration reversibly shifts the conductance-voltage curve toward the right along the voltage axis. Furthermore, Ca2+ can activate the conductance only if added to the cis compartment. Neither Mg2+, Ba2+, nor Cd2+ can activate the conductance induced by T-tubule vesicles. Addition of 5 mM tetraethylammonium ion to the trans, but not the cis, side abolishes the T-tubule-induced conductance. The Ca2+-dependent conductance appears as a consequence of ionic channel formation. Single-channel activity appears in bursts followed by periods of time in which the channel remains "silent". The conductance of the open channel averages 226 pS in 0.1 M KC1 and is voltage and Ca2+ independent. However, the fraction of time that the channel remains in the open state is voltage and Ca2+ dependent in a manner that parallels the voltage and Ca2+ dependence of the multichannel membrane. The channel is 6.6 times more permeable to K+ than to Na+ and is impermeable to C1-.

Effect of phospholipid surface charge on the conductance and gating of a Ca2+-activated K+ channel in planar lipid bilayers

A Ca-activated, K-selective channel from plasma membrane of rat skeletal muscle was studied in artificial lipid bilayers formed from either phosphatidylethanolamine (PE) or phosphatidylserine (PS). In PE, the single-channel conductance exhibited a complex dependence on symmetrical K+ concentration that could not be described by simple Michaelis-Menten saturation. At low K+ concentrations the channel conductance was higher in PS membranes, but approached the same conductance observed in PE above 0.4 m KCl. At the same Ca2+ concentration and voltage, the probability of channel opening was significantly greater in PS than PE. The differences in the conduction and gating, observed in the two lipids, can be explained by the negative surface charge of PS compared to the neutral PE membrane. Model calculations of the expected concentrations of K+ and Ca2+ at various distances from a PS membrane surface, using Gouy-Chapman-Stern theory, suggest that the K+-conduction and Ca2+-activation sites sense a similar fraction of the surface potential, equivalent to the local electrostatic potential at a distance of 9 A from the surface.

Structure, organization and characterization of the gene cluster involved in the production of microcin E492, a channel-forming bacteriocin

Microcin E492 is a low-molecular-weight, channel-forming bacteriocin produced and excreted by Klebsiella pneumoniae RYC492. A 13 kb chromosomal DNA fragment from K. pneumoniae RYC492 was sequenced, and it was demonstrated by random Tn5 mutagenesis that most of this segment, which has at least 10 cistrons, is needed for the production of active microcin and its immunity protein. Genes mceG and mceH correspond to an ABC exporter and its accessory protein, respectively, and they are closely related to the colicin V ABC export system. The microcin E492 system also requires the product of gene mceF as an additional factor for export. Despite the fact that this bacteriocin lacks post-translational modifications, genes mceC, mceI and mceJ are needed for the production of active microcin. Genes mceC and mceI are homologous to a glycosyl transferase and acyltransferase, respectively, whereas mceJ has no known homologue. Mutants in these three genes secrete an inactive form of microcin, able to form ion channels in a phospholipidic bilayer, indicating that the mutation of these microcin genes does not alter the process of membrane insertion. On the other hand, microcin isolated from mutants in genes mceC and mceJ has a lethal effect when incubated with spheroplasts of sensitive cells, indicating that the microcin defects in these mutants are likely to alter receptor recognition at the outer membrane. A model for synthesis and export is proposed as well as a novel maturation pathway that would involve conformational changes to explain the production of active microcin E492.

Microcin E492 forms ion channels in phospholipid bilayer membrane

Microcin E492, a polypeptide antibiotic, has been shown to have an M(r) of 6,000 by urea-SDS-polyacrylamide gel electrophoresis of the fluorescently labelled compound. It is known that the bactericidal action of microcin involves a loss of the transmembrane potential. In this study we show that microcin forms cation-selective channels in planar phospholipid bilayers. The channels have two main conductance states the current-voltage curves of which rectify. The reversal potentials measured under biionic conditions indicate a permeability sequence of NH4+ > K+ = Rb+ = Cs+ > Na+ = Na+ = Li+ > Tris+. The results suggest that membrane potential dissipation induced by microcin is a consequence of the formation of pores in the bacterial membrane.

Light-dependent channels from excised patches of Limulus ventral photoreceptors are opened by cGMP

The identity of the second messenger that directly activates the light-dependent conductance in invertebrate photoreceptors remains unclear; the available evidence provides some support for cGMP and Ca2+. To resolve this issue we have applied these second messengers to membrane patches excised from the light-sensitive lobe of Limulus ventral photoreceptors. Our results show that these patches contain channels that can be opened by cGMP, but not by Ca2+. These cGMP-activated channels closely resemble the channels activated by light in cell-attached patches. This evidence suggests that cGMP is the messenger that opens the light-dependent channel in invertebrate photoreceptors.

Properties of St I and St II, two isotoxins isolated from Stichodactyla helianthus: a comparison

Sticholysins I and II are two highly hemolytic polypeptides purified from the Caribbean Sea anemone Stichodactyla helianthus. Their high sequence homology (93%) indicates that they correspond to isoforms of the same hemolysin. The spectroscopic measurements show a close similarity in the secondary structure content, conformation and stability of both toxins. Exposure of the toxins to high pHs (>11), a free radical source (AAPH), urea or temperature produce permanent changes in the toxin that lead to a significant loss of HA. It is significant to note that this loss of hemolytic activity occurs when other indicators, probably with the only exception of near-UV CD spectra, barely detect changes in the protein structure. This emphasizes the sensitivity of the protein function to changes in the macromolecule conformation. The most noticeable difference between both toxins is the considerably higher activity of St II, both measured in terms of erythrocyte internal K(+) exit or hemolysis; which is related to enthalpic factors. This difference is not due to an incomplete association of St I to the membrane. We consider then that the different pore forming capacity of both toxins in erythrocytes can be explained in terms of the difference in charge of the N-terminal fragment, than can considerably reduce the St I insertion rate in the membrane probably due to the negatively charged outer leaflet of the red blood cell, without a significant reduction of its capacity to bind to the cell membrane. This electrostatic effect, together with a slightly more relaxed structure in St II, could explain the higher pore forming capacity of St II in the red blood cell membrane.

Increase of apamin receptors in skeletal muscle induced by colchicine: possible role in myotonia

We have shown an increase of apamin receptors in rat skeletal muscle membranes following the application of colchicine to the sciatic nerve. 125I-apamin binding to partially purified membrane fractions was observed since day 4, reached a maximum around days 6-15, and was negligible at day 35 after the application of colchicine. Control muscles (nerves treated with buffer solution) showed low binding values (11 fmol/mg protein). Maximal 125I-apamin binding values to partially purified muscle membranes of colchicine-treated rats (42 fmol/mg protein) were lower than those obtained in denervated muscle (95 fmol/mg protein). The affinity binding constant values were 37 (colchicine) and 95 pM (denervation). No signs of muscle denervation were observed on histological examination of the nerve submitted to colchicine treatment nor in the muscles innervated by it. Muscle tension developed by indirect stimulation was the same as in controls. We here show also that partially purified membranes of normal untreated muscles have measurable amounts of 125I-apamin binding (13 fmol/mg protein), similar to those obtained in control muscles. Electromyographic recordings of the muscles after colchicine treatment of the nerve showed abnormal repetitive electrical discharges, similar to myotonic discharges, that were present with a similar temporal course as the increase in apamin receptors. The myotonic-like discharges were suppressed by the topical application of apamin to the muscle, whereas the toxin had no effect on anthracene-9-carbolytic acid-induced myotonia. Our results suggest that a neurotrophic factor that travels by axonal flow is involved in the regulation of the expression of apamin receptors in skeletal muscle membranes.(ABSTRACT TRUNCATED AT 250 WORDS)

Synthesis and content of polyamines in bloodstream Trypanosma brucei

The sensitive dansyl procedure was used to detect putrescine and spermidine, but not spermine and cadaverine, in pleomorphic Trypanosoma brucei. The polyamines were synthesized in vitro from [3H]ornithine, [14C]arginine and [14C]methionine. Proline, agmatine, and citrulline, but not glutamine, glutamic or pyroglutamic acids, stimulated spermidine formation from [4C]methionine. Putrescine and sperimidine synthesis occurred rapidly from ornithine: putrescine synthesis peaked in 0.5 h, spermidine in 1 h. Trypanosoma brucei assimilated exogenous 14C-labeled putrescine, spermidine, and spermine; spermidine and spermine were taken up 5 times as rapidly as putrescine. Polyamine syntheses may therefore be a practical target for novel trypanocies.

Clinical and neuropathological study of six patients with spastic paraparesis associated with HTLV-I: an axomyelinic degeneration of the central nervous system

Between 1990 to 1994, 6 TSP/HAM patients, 3 women and 3 men with an average age of 57.1 years (39 to 76 years old), who died in the Salvador Hospital were submitted to postmortem examination. The mean time of paraparesis was 7 years (3 to 17 years), and 2 patients had pseudobulbar signs. Three cases had macroscopic atrophy of the spinal cord. Histologically, all cases had lesions in the pyramidal tracts and 4 cases showed somatotopic lesions of the Goll's tracts which followed a "dying back" ascendant and descendant distribution, respectively. In 2 cases, both of which had intellectual impairment, demyelination of the subcortical and parathalamic areas was observed without U fiber involvement. Abnormal vessels with gross thickening of the adventitia, many of them with lymphocytic cuffs, were seen everywhere, especially in the spinal cord, brain stem, midbrain and meninges, but no relation between these findings and the parenchymal lesions was observed. Also, in the cases with posterior column involvement, neuronal changes and proliferation of satellite cells in the dorsal ganglia were found. All cases showed histological sialoadenitis and none had inflammatory muscle changes. We conclude that the lesions affected the neuraxis in a systemic axial fashion as in degenerative diseases, and did not seem to be secondary to vascular or inflammatory abnormalities.

Association between total immunoglobulin E and antibody responses to naturally acquired Ascaris lumbricoides infection and polymorphisms of immune system-related LIG4, TNFSF13B and IRS2 genes

The 13q33-34 region harbours a susceptibility locus to Ascaris lumbricoides, although the underlying genes are unknown. Immunoglobulin (Ig)E and IgG confer protective immunity and here we sought to investigate in an endemic population whether LIG4, TNFSF13B and IRS2 genes influence IgE and IgG levels against Ascaris and the ABA-1 allergen as a putative resistance marker. Mite-allergic asthmatic patients were analysed for potential relationships between Ascaris predisposition and allergy. One thousand and sixty-four subjects from Cartagena, Colombia, were included. Single nucleotide polymorphisms (SNPs) were genotyped using TaqMan assays. Antibody levels were measured by enzyme-linked immunosorbent assay. Linear and logistic regressions were used to model effects of genotypes on antibody levels. The GG genotype of LIG4 (rs1805388) was associated with higher IgE levels to Ascaris compared with other genotypes. TNFSF13B (rs10508198) was associated positively with IgG levels against Ascaris extract and IgE levels against ABA-1. In asthmatics, IRS2 (rs2289046) was associated with high total IgE levels. Associations held up after correction by population stratification using a set of 52 ancestry markers, age, sex and disease status. There was no association with asthma or mite sensitization. In a tropical population, LIG4 and TNFSF13B polymorphisms are associated with specific IgE and IgG to Ascaris, supporting previous linkage studies implicating the 13q33 region. Our results suggest that genes protecting against parasite infections can be different to those predisposing to asthma and atopy.

Squamous cell carcinoma antigen (SCC Ag) in the diagnosis and prognosis of lung cancer

OBJECTIVE

We have studied the usefulness of squamous cell carcinoma antigen (SCC Ag) in diagnosis and prognosis of lung cancer (LC).

MATERIAL AND METHODS

We have measured the serum SCC Ag levels in 388 subjects: 69 healthy persons; 103 with nonmalignant lung diseases (NMLD); 24 with lung metastasis of extrapulmonary origin (LMEO); and 192 with LC (88, with squamous cell carcinoma [SCC] type). In 55 with SCC, we analyzed the survival time.

RESULTS

Serum SCC Ag was above 2.5 ng/ml in 1.4 percent of healthy persons; 2.9 percent of those with NMLD; 8.3 percent of those with LMEO; and 27.6 percent of those with LC. Such percentage was 47.7 percent in SCC. In this type, there were significant differences according to the extent of disease (61.6 percent in advanced stages, and 26.5 percent in localized stages, p = 0.002). In the other types, the sensitivity was substantially lower. The initial SCC Ag has prognostic significance (p = 0.02) in the univariate analysis, but it loses such significance in a multivariate model, including the stage.

CONCLUSIONS

Therefore, we do not recommend this marker in the clinical management of patients with LC, even it can be useful in the differential diagnosis if used in combination with other markers.

Role of PrP(C) Expression in Tau Protein Levels and Phosphorylation in Alzheimer's Disease Evolution

Alzheimer's disease (AD) is characterized by the presence of amyloid plaques mainly consisting of hydrophobic β-amyloid peptide (Aβ) aggregates and neurofibrillary tangles (NFTs) composed principally of hyperphosphorylated tau. Aβ oligomers have been described as the earliest effectors to negatively affect synaptic structure and plasticity in the affected brains, and cellular prion protein (PrP(C)) has been proposed as receptor for these oligomers. The most widely accepted theory holds that the toxic effects of Aβ are upstream of change in tau, a neuronal microtubule-associated protein that promotes the polymerization and stabilization of microtubules. However, tau is considered decisive for the progression of neurodegeneration, and, indeed, tau pathology correlates well with clinical symptoms such as dementia. Different pathways can lead to abnormal phosphorylation, and, as a consequence, tau aggregates into paired helical filaments (PHF) and later on into NFTs. Reported data suggest a regulatory tendency of PrP(C) expression in the development of AD, and a putative relationship between PrP(C) and tau processing is emerging. However, the role of tau/PrP(C) interaction in AD is poorly understood. In this study, we show increased susceptibility to Aβ-derived diffusible ligands (ADDLs) in neuronal primary cultures from PrP(C) knockout mice, compared to wild-type, which correlates with increased tau expression. Moreover, we found increased PrP(C) expression that paralleled with tau at early ages in an AD murine model and in early Braak stages of AD in affected individuals. Taken together, these results suggest a protective role for PrP(C) in AD by downregulating tau expression, and they point to this protein as being crucial in the molecular events that lead to neurodegeneration in AD.

Kinetics and mechanism of St I modification by peroxyl radicals

St I is a toxin present in the Caribbean Sea anemone Stichodactyla helianthus which is highly hemolytic in the nanomolar concentration range. Exposure of the toxin to free radicals produced in the pyrolysis of 2,2'-azobis(2-amidinopropane) hydrochloride leads to a progressive loss of hemolytic activity. This loss of hemolytic activity is accompanied by extensive modification of tryptophan residues. On the average, three tryptophan residues are modified by each inactivated toxin. The loss of hemolytic activity of St I takes place without significant changes in the protein structure, as evidenced by the similarity of the fluorescence and CD spectra of native and modified proteins. Also, the native and modified ensembles present a similar resistance to their denaturation by guanidinium chloride. The hemolytic behavior and the performance of the toxin at the single-channel level when incorporated to black lipid membranes suggest that the modified ensemble can be considered as composed of inactive toxins and active toxins whose behavior is similar to that of the native proteins. These results, together with the lack of induction time in the activity loss, suggest that the fall of hemolytic activity takes place by an all-or-nothing inactivation mechanism in which the molecules become inactive when a critical amino acid residue is modified.

Three-year effectiveness of intravenous pamidronate versus pamidronate plus slow-release sodium fluoride for postmenopausal osteoporosis

All currently available and approved therapies for osteoporosis inhibit bone resorption. But, despite their great value, antiresorptive agents are generally not associated with dramatic increases in bone mass. In light of these data, the aim of our prospective, placebo-controlled, randomized clinical trial, with a 3-year follow up, was to examine the effects of cyclic intravenous pamidronate and fluoride in combination, versus pamidronate alone, on bone mineral density (BMD) at vertebral and femoral levels, biochemical markers of bone turnover, IGF-1 serum levels, and safety and tolerability in 40 postmenopausal women with osteoporosis. During the treatment period, pamidronate alone reduced both markers of bone formation and bone resorption, resulting in an increase of BMD, after 3 years, of 7.07% at the lumbar level and of 6.76% at the femoral level. In the group treated with pamidronate and fluoride, markers of bone turnover had a different trend: after 3 years, there was a lower reduction of bone resorption and an increase of bone formation markers, with a concomitant increase in IGF-1 levels. This resulted, after 3 years of treatment, in a marked variation of BMD at the lumbar level (+12.74%) and a reduced, but still significant, increase at the femoral level (3.89%). Spine radiography and clinical evaluation did not reveal any vertebral fractures in either treatment group. In conclusion, the combined use of pamidronate and fluoride produced somewhat larger, continuous increases in BMD, at the lumbar level, than pamidronate alone.

A NOS1 gene polymorphism associated with asthma and specific immunoglobulin E response to mite allergens in a Colombian population

BACKGROUND

Nitric oxide (NO) is involved in asthma pathogenesis and is synthesized by three isoforms of NO synthase, one of them encoded by NOS1 gene. The CA-repeat and the C5266T SNP in NOS1 exon 29 have been associated with asthma and IgE levels. We thought to test the association of asthma and asthma-related phenotypes with the exon 29 CA-repeat and the C5266T SNP in a Colombian population sample.

METHODS

The CA-repeat and the C5266T SNP were genotyped in 167 asthmatics and 166 controls using PCR-based fragment length polymorphism and TaqMan assay. We also determined total and mite-specific IgE against Blomia tropicalis and Dermatophagoides pteronyssinus.

RESULTS

Three new CA-repeat alleles, 14, 23 and 24 repeats were detected. Allele comprising 16 repeats was associated with asthma (OR: 1.90 (CI 1.22-2.97, p(c) = 0.028) and low total (p(c) = 0.02) and specific IgE to B. tropicalis (p(c) < 0.0001) and D. pteronyssinus (p(c) < 0.0001). We found no association of the C5266T SNP and asthma or IgE levels.

CONCLUSION

NOS1 exon 29 CA-repeat may be a risk factor for asthma susceptibility and mite specific IgE response in a Colombian population.