11β-hydroxysteroid dehydrogenase type 2 polymorphisms and activity in a Chilean essential hypertensive and normotensive cohort

BACKGROUND

11β-hydroxysteroid dehydrogenase type 2 enzyme (11β-HSD2) inactivates cortisol (F) to cortisone (E); its impairment is associated with hypertension. We reported that 15.7% of the Chilean essential hypertensives possessed a high F/E ratio suggesting a partial deficit in 11β-HSD2 activity. It has been reported that the G534A(Glu178/Glu) polymorphism in the HSD11B2 gene is associated with hypertension. Investigate the frequency of the G534A polymorphism and its correlation with the glucocorticoid profile in Chilean essential hypertensive and normotensive subjects.

METHODS

Essential hypertensive outpatients (n = 232) and normotensive subjects (n = 74) were recruited. A change in the AluI restriction enzyme digest pattern, caused by the presence of the G534A polymorphism, was utilized to screen DNA isolated from leukocytes within the cohort before confirmation by sequencing. Plasma renin activity (PRA), serum aldosterone, F, and E were measured by radioimmunoassay. Urinary tetrahydrocortisol (THF), 5α-tetrahydrocortisol (5α-THF), and tetrahydrocortisone (THE) were measured by gas chromatography-mass spectrometry.

RESULTS

G534A polymorphism frequency was similar between hypertensive patients (19 of 232; 8.2%) and normotensive subjects (7 of 74; 9.5%). When categorized by presence or absence of the G534A polymorphism, no significant differences in the serum F/E ratio or other measured biochemical variables were detected. Despite a previous report that the G534A polymorphism is associated with a neighboring C468A (Thr156/Thr) polymorphism, analysis within our cohort showed that only one patient in each group presented with this double polymorphism.

CONCLUSIONS

We report the frequency of the G534A polymorphism in the Spanish-Amerindian population. No correlation was detected between this polymorphism and the presence of hypertension and biochemical parameters in this Chilean cohort.

Epidemiologic, molecular, and functional evidence suggest A572D-SCN5A should not be considered an independent LQT3-susceptibility mutation

BACKGROUND

Considering that approximately 2% of Caucasian controls host rare, nonsynonymous variants in the SCN5A-encoded cardiac sodium channel, caution must be exercised when interpreting SCN5A genetic test results for long QT syndrome (LQTS).

OBJECTIVE

The purpose of this study was to determine if A572D-SCN5A is a pathogenic mutation, a possible functional modifier, or background "genetic noise."

METHODS

The frequency of A572D was compared between 3,741 LQTS referral cases (mostly Caucasian) and 1,437 Caucasian controls. A572D-SCN5A was engineered into SCN5A using the most commonly spliced transcript (Q1077del, hH1c clone) in the setting of either H558 or R558 for heterologous expression/patch clamp studies in HEK293 cells.

RESULTS

A572D-SCN5A was detected in 17 (0.45%) of 3,741 cases compared with 7 (0.49%) of 1,437 controls (P = .82). Among the 17 A572D-positive LQTS referrals, 10 (59%) hosted definite LQTS-causing mutations elsewhere (5 KCNQ1, 3 KCNH2, 2 SCN5A). Functional studies showed no gating kinetic or current density differences compared with wild-type channels in the context of H558 but showed moderate dysfunction when expressed in H558R-SCN5A, with which it is invariably associated.

CONCLUSION

There is sufficient evidence to conclude that A572D-SCN5A is not an independent, LQT3-causative mutation. A572D is present in approximately 0.5% of both cases and controls and has a wild-type phenotype when expressed in HEK293 cells. However, in the context of H558R-SCN5A, persistent late sodium current emerges, indicating that A572D/H558R could be a proarrhythmic factor akin to S1103Y. These findings underscore the scrutiny necessary to distinguish truly pathogenic mutations from functional polymorphisms and otherwise innocuous, rare genetic variants in SCN5A. These results also question how much cellular dysfunction for a mutation is required in vitro to support pathogenicity.

Alpha1-syntrophin mutations identified in sudden infant death syndrome cause an increase in late cardiac sodium current

BACKGROUND

Sudden infant death syndrome (SIDS) is a leading cause of death during the first 6 months after birth. About 5% to 10% of SIDS may stem from cardiac channelopathies such as long-QT syndrome. We recently implicated mutations in alpha1-syntrophin (SNTA1) as a novel cause of long-QT syndrome, whereby mutant SNTA1 released inhibition of associated neuronal nitric oxide synthase by the plasma membrane Ca-ATPase PMCA4b, causing increased peak and late sodium current (I(Na)) via S-nitrosylation of the cardiac sodium channel. This study determined the prevalence and functional properties of SIDS-associated SNTA1 mutations.

METHODS AND RESULTS

Using polymerase chain reaction, denaturing high-performance liquid chromatography, and DNA sequencing of SNTA1's open reading frame, 6 rare (absent in 800 reference alleles) missense mutations (G54R, P56S, T262P, S287R, T372M, and G460S) were identified in 8 (approximately 3%) of 292 SIDS cases. These mutations were engineered using polymerase chain reaction-based overlap extension and were coexpressed heterologously with SCN5A, neuronal nitric oxide synthase, and PMCA4b in HEK293 cells. I(Na) was recorded using the whole-cell method. A significant 1.4- to 1.5-fold increase in peak I(Na) and 2.3- to 2.7-fold increase in late I(Na) compared with controls was evident for S287R-, T372M-, and G460S-SNTA1 and was reversed by a neuronal nitric oxide synthase inhibitor. These 3 mutations also caused a significant depolarizing shift in channel inactivation, thereby increasing the overlap of the activation and inactivation curves to increase window current.

CONCLUSIONS

Abnormal biophysical phenotypes implicate mutations in SNTA1 as a novel pathogenic mechanism for the subset of channelopathic SIDS. Functional studies are essential to distinguish pathogenic perturbations in channel interacting proteins such as alpha1-syntrophin from similarly rare but innocuous ones.

SCN4B-encoded sodium channel beta4 subunit in congenital long-QT syndrome

BACKGROUND

Congenital long-QT syndrome (LQTS) is potentially lethal secondary to malignant ventricular arrhythmias and is caused predominantly by mutations in genes that encode cardiac ion channels. Nearly 25% of patients remain without a genetic diagnosis, and genes that encode cardiac channel regulatory proteins represent attractive candidates. Voltage-gated sodium channels have a pore-forming alpha-subunit associated with 1 or more auxiliary beta-subunits. Four different beta-subunits have been described. All are detectable in cardiac tissue, but none have yet been linked to any heritable arrhythmia syndrome.

METHODS AND RESULTS

We present a case of a 21-month-old Mexican-mestizo female with intermittent 2:1 atrioventricular block and a corrected QT interval of 712 ms. Comprehensive open reading frame/splice mutational analysis of the 9 established LQTS-susceptibility genes proved negative, and complete mutational analysis of the 4 Na(vbeta)-subunits revealed a L179F (C535T) missense mutation in SCN4B that cosegregated properly throughout a 3-generation pedigree and was absent in 800 reference alleles. After this discovery, SCN4B was analyzed in 262 genotype-negative LQTS patients (96% white), but no further mutations were found. L179F was engineered by site-directed mutagenesis and heterologously expressed in HEK293 cells that contained the stably expressed SCN5A-encoded sodium channel alpha-subunit (hNa(V)1.5). Compared with the wild-type, L179F-beta4 caused an 8-fold (compared with SCN5A alone) and 3-fold (compared with SCN5A + WT-beta4) increase in late sodium current consistent with the molecular/electrophysiological phenotype previously shown for LQTS-associated mutations.

CONCLUSIONS

We provide the seminal report of SCN4B-encoded Na(vbeta)4 as a novel LQT3-susceptibility gene.

Prevalence and risk factors for gallstone disease

Gallstone disease is a main public health problem. The overall prevalence data range from 3.9% in the pre-echographic era to 13.7% when ultrasonography was used as a diagnostic tool. This study is aimed to determine the prevalence of gallstone disease in a medium income level population in Lima, as well as the relationship with some risk factors: age, sex, familiar history and obesity. A total of 534 adult men and women from a medium economic level underwent ultrasonographic examination of abdomen for detection of gallstone disease (July 2003). The echographic evaluation was performed by 10 general surgeons trained in ultrasonography. Likewise, 4 risk factors--age, gender, familial history, and obesity--were analyzed. Pearson chi2 test (2-sided) was used with a probability of <0.05 for statistical significance and logistic regression analyses for assessment of confounding factors. The prevalence founded was 15%. Eighty-one of 534 participants had lithiasis. Compared to the age group under 30, the odds ratio for the 31 to 50 years and >50 years of age group was 0.9 and 1.1, respectively. The female-male ratio was 1.07 and the odds ratio 0.8. The prevalence of gallstone disease in people reporting a first-degree relative with lithiasis was 21%, whereas in participants without such a condition, it was 13%. On the other hand, a familial history was present in 38% of the lithiasis group and in 25% of the nonlithiasis group. The odds ratio for familial history was 1.8 (P = 0.01, 95% confidence interval 1.1-2.9). The prevalence of the disease for body mass index <24, 25 to 29, and higher than 30 was 17%, 14% and 13%, respectively. Compared to the reference group (body mass index <24), the other 2 groups (body mass index 25-29 and >30) both had a similar odds ratio, 0.8. Logistic regression analyses showed an odds ratio of 1.9 for familiar history (95% confidence interval 1.1-3.2), whereas the odds ratio of the overweight (body mass index 25-29) and obese group (body mass index >30) when compared to the normal group, BMI <24, was 0.7 and 0.9, respectively. The prevalence data for gallstone disease remain slightly higher than those previously reported. Although the familiar history was the only characteristic with a statistically significant positive relationship with lithiasis, additional studies are needed because few biases could not be completely avoided and some confounding factors were not controlled.

Ryanodine receptor dysfunction in porcine stunned myocardium

We investigated the effects of myocardial stunning on the function of the two main Ca2+ transport proteins of the sarcoplasmic reticulum (SR), the Ca(2+)-adenosinetriphosphatase and the Ca(2+)-release channel or ryanodine receptor. Regional myocardial stunning was induced in open-chest pigs (n = 6) by a 10-min occlusion of the left anterior descending coronary artery (LAD) and 2 h reperfusion. SR vesicles isolated from the LAD-perfused region (stunned) and the normal left circumflex coronary artery (LC)-perfused region were used to assess the oxalate-supported 45Ca2+ uptake, [3H]ryanodine binding, and single-channel recordings of ryanodine-sensitive Ca(2+)-release channels in planar lipid bilayers. Myocardial stunning decreased LAD systolic wall thickening to 20% of preischemic values. The rate of SR 45Ca2+ uptake in the stunned LAD bed was reduced by 37% compared with that of the normal LC bed (P < 0.05). Stunning was also associated with a 38% reduction in the maximal density of high-affinity [3H]ryanodine binding sites (P < 0.05 vs. normal LC) but had no effect on the dissociation constant. The open probability of ryanodine-sensitive Ca(2+)-release channels determined by single channel recordings in planar lipid bilayers was 26 +/- 2% for control SR (n = 33 channels from 3 animals) and 14 +/- 2% for stunned SR (n = 21 channels; P < 0.05). This depressed activity of SR function observed in postischemic myocardium could be one of the mechanisms underlying myocardial stunning.

Na+/Ca2+ exchanger in Drosophila: cloning, expression, and transport differences

cDNAs for the Na+/Ca2+ exchanger from Drosophila melanogaster (Dmel/Nck) have been cloned by homology screening using the human heart Na+/Ca2+ exchanger cDNA. The overall deduced protein structure for Dmel/Nck is similar to that of mammalian Na+/Ca2+ exchanger genes NCX1 and NCX2, having six hydrophobic regions in the amino terminus separated from six at the carboxy-terminal end by a large intracellular loop. Sequence comparison of the Drosophila exchanger cDNAs with NCX1 and NCX2 Na+/Ca2+ exchangers are approximately 46% identical at the deduced amino acid level. Consensus phosphorylation sites for both protein kinase C and protein kinase A are present on the intracellular loop region of the Dmel/Nck. Alternative splicing for the Dmel/Nck gene is suggested in the same intracellular loop region as demonstrated for NCX1. Functionally, the Drosophila Na+/ Ca2+ exchanger expressed in oocytes differs from expressed mammalian NCX1 with regard to Ca2+ transport in Ca2+/ Ca2+ exchange and the effect of monovalent-dependent Ca2+/ Ca2+ exchange. The Dmel/Nck gene maps to chromosome 3 (93A-B) using in situ hybridization to polytene chromosomes, the same position as the Na(+)-K(+)-ATPase, a related transporter. We conclude that, although extracellular Na+ concentration-dependent Ca2+ transport is subserved by both human and Drosophila Na+/Ca2+ exchangers, there are clear and important differences in the transporters, which should be useful in deducing how the Na+/Ca2+ exchanger protein function depends on its structure.

The immunophilin FK506-binding protein modulates Ca2+ release channel closure in rat heart

1. The nature of the signal that terminates the release of Ca2+ from the cardiac sarcoplasmic reticulum has remained elusive. This study was intended to examine whether FK506-binding protein (FKBP), which is tightly associated to the ryanodine receptor (RyR)/Ca2+ release channel, plays a role in the termination of Ca(2+)-induced Ca2+ release (CICR) in heart. 2. Confocal microscopy and the Ca2+ indicator fluo-3 were used to visualize the elementary release events, i.e. 'Ca2+ sparks' in rat ventricular myocytes under resting or voltage-clamped conditions. Additionally, electrophysiological single-channel recordings, at constant [Ca2+] or during [Ca2+] steps produced by photorelease of caged Ca2+, were obtained from rat cardiac RyRs incorporated in planar lipid bilayers. 3. Inhibition of FKBP by the immunosuppressants FK506 or rapamycin increased the duration of spontaneous or depolarization-evoked Ca2+ sparks 6- to 7-fold. In addition, Ca2+ sparks were seen with two-level amplitudes, corresponding to full and half normal spark amplitude. 4. FK506 potentiated and prolonged electrically stimulated [Ca2+]i transients and contractions, but did not affect the amplitude and kinetics of the L-type Ca2+ channel current. 5. In planar lipid bilayers, FK506 (15 microM) prolonged approximately 7-fold the mean open lifetime of reconstituted single RyRs, induced the appearance of long-lasting subconductance states, and markedly slowed the spontaneous decay of RyR activity elicited by fast and sustained Ca2+ stimuli. The time constant of the spontaneous decay of activity increased from 1.8 s in control to > or = 20 s in the presence of FK506. 6. We conclude that FKBP may afford an intrinsic mechanism to terminate RyR openings and it may thus exert a negative feedback on CICR in heart cells.

Alternative splicing of the Na(+)-Ca2+ exchanger gene, NCX1

We describe an analysis of the NCX1 gene and show that various tissues express different alternatively spliced forms of the gene. Alternative splicing has been confirmed by the genomic analysis of the Na(+)-Ca2+ exchanger gene. We also describe the Drosophila Na(+)-Ca2+ exchanger as having many of the same structural characteristics of the mammalian exchangers and this locus as possibly undergoing alternative splicing in the same region that has been described in the NCX1 gene. The general structure of the exchangers is similar to that of the alpha-subunit of the (Na(+)+ K+)-A Pase. Finally, sequence comparison of the various molecules demonstrates that structural characteristics of these molecules are more strongly conserved than the primary sequence of these products.

Calcium-dependent block of ryanodine receptor channel of swine skeletal muscle by direct binding of calmodulin

The interaction of the Ca2+ binding protein calmodulin (CaM) with the ryanodine receptor of the sarcoplasmic reticulum (SR) of pig skeletal muscle was investigated by [3H]-ryanodine binding, planar bilayer recordings, and rapid filtration of 45Ca(2+)-loaded SR. Inhibition of [3H]-ryanodine binding by CAM was phosphorylation-independent, had an IC50 of approximately 0.1 microM and was optimal at 10 microM Ca(2+). CaM also inhibited [3H]-ryanodine binding to CHAPS-solubilized and purified ryanodine receptors, suggesting a direct CaM-ryanodine receptor interaction. In single channel recordings, CaM blocked Ca2+ release channels in a Ca(2+)-dependent manner by decreasing the number of open events per unit time without affecting the mean open time or unitary channel conductance. Rapid filtration of 45Ca2+ passively loaded into SR vesicles showed that CaM blocked Ca2+ release within milliseconds of exposure of SR to a Ca2+ release medium containing 10 microM CaM. In controls, an increase in extravesicular Ca2+ from 7 nM to 10 microM resulted in a release of 47 +/- 10% of the 45Ca2+ in 20 ms. CaM reduced the release to 23 +/- 12% in the same period. These results are compatible with a direct mechanism of Ca2+ release channel blockade by CaM and suggest that CaM could play a significant role in the inactivation of SR Ca2+ release during excitation-contraction coupling.

Activation of the Ca2+ release channel of skeletal muscle sarcoplasmic reticulum by palmitoyl carnitine

Studies of [3H]ryanodine binding, 45Ca2+ efflux, and single channel recordings in planar bilayers indicated that the fatty acid metabolite palmitoyl carnitine produced a direct stimulation of the Ca2+ release channel (ryanodine receptor) of rabbit and pig skeletal muscle junctional sarcoplasmic reticulum. At a concentration of 50 microM, palmitoyl carnitine (a) stimulated [3H]ryanodine binding 1.6-fold in a competitive manner at all pCa in the range 6 to 3; (b) released approximately 65% (30 nmol) of passively loaded 45Ca2+/mg protein; and (c) increased 7-fold the open probability of Ca2+ release channels incorporated into planar bilayers. Neither carnitine nor palmitic acid could reproduce the effect of palmitoyl carnitine on [3H]ryanodine binding, 45Ca2+ release, or channel open probability. 45Ca2+ release was induced by several long-chain acyl carnitines (C14, C16, C18) and acyl coenzyme A derivatives (C12, C14, C16), but not by the short-chain derivative C8 or by free saturated fatty acids of chain length C8 to C18, at room temperature or 36 degrees C. This newly identified interaction of esterified fatty acids and ryanodine receptors may represent a pathway by which metabolism of skeletal muscle could influence intracellular Ca2+ and may be responsible for the pathophysiology of disorders of beta-oxidation such as carnitine palmitoyl transferase II deficiency.

Fast release of 45Ca2+ induced by inositol 1,4,5-trisphosphate and Ca2+ in the sarcoplasmic reticulum of rabbit skeletal muscle: evidence for two types of Ca2+ release channels

The kinetics of Ca2+ release induced by the second messenger D-myoinositol 1,4,5 trisphosphate (IP3), by the hydrolysis-resistant analogue D-myoinositol 1,4,5 trisphosphorothioate (IPS3), and by micromolar Ca2+ were resolved on a millisecond time scale in the junctional sarcoplasmic reticulum (SR) of rabbit skeletal muscle. The total Ca2+ mobilized by IP3 and IPS3 varied with concentration and with time of exposure. Approximately 5% of the 45Ca2+ passively loaded into the SR was released by 2 microM IPS3 in 150 ms, 10% was released by 10 microM IPS3 in 100 ms, and 20% was released by 50 microM IPS3 in 20 ms. Released 45Ca2+ reached a limiting value of approximately 30% of the original load at a concentration of 10 microM IP3 or 25-50 microM IPS3. Ca(2+)-induced Ca2+ release (CICR) was studied by elevating the extravesicular Ca2+ while maintaining a constant 5-mM intravesicular 45Ca2+. An increase in extravesicular Ca2+ from 7 nM to 10 microM resulted in a release of 55 +/- 7% of the passively loaded 45Ca2+ in 150 ms. CICR was blocked by 5 mM Mg2+ or by 10 microM ruthenium red, but was not blocked by heparin at concentrations as high as 2.5 mg/ml. In contrast, the release produced by IPS3 was not affected by Mg2+ or ruthenium red but was totally inhibited by heparin at concentrations of 2.5 mg/ml or lower. The release produced by 10 microM Ca2+ plus 25 microM IPS3 was similar to that produced by 10 microM Ca2+ alone and suggested that IP3-sensitive channels were present in SR vesicles also containing ruthenium red-sensitive Ca2+ release channels. The junctional SR of rabbit skeletal muscle may thus have two types of intracellular Ca2+ releasing channels displaying fast activation kinetics, namely, IP3-sensitive and Ca(2+)-sensitive channels.

Direct binding of verapamil to the ryanodine receptor channel of sarcoplasmic reticulum

Radioligand binding experiments and single channel recordings demonstrate that verapamil interacts with the ryanodine receptor Ca2+ release channel of the sarcoplasmic reticulum of rabbit skeletal muscle. In isolated triads, verapamil decreased binding of [3H]Ryanodine with an IC50 of approximately 8 microM at an optimal pH 8.5 and pCa 4.3. Nitrendipine and d-cis-diltiazem did not interfere with binding of [3H]Ryanodine to triads, suggesting that the action of verapamil does not involve the dihydropyridine receptor. Single channel recordings showed that verapamil blocked Ca2+ release channels by decreasing open probability, duration of open events, and number of events per unit time. A direct interaction of verapamil with the ryanodine receptor peptide was demonstrated after purification of the approximately 400 kDa receptor protein from Chaps-solubilized triads. The purified receptor displayed high affinity for [3H]Ryanodine with a Kd of approximately 5 nM and a Bmax of approximately 400 pmol/mg. Verapamil and D600 decreased [3H]Ryanodine binding noncompetitively by reducing the Bmax. Thus the presence of binding sites for phenylalkylamines in the Ca2+ release channel was confirmed. Verapamil blockade of Ca2+ release channels may explain some of the paralyzing effects of phenylalkylamines observed during excitation-contraction coupling of skeletal muscle.

Ca2+ release by inositol-trisphosphorothioate in isolated triads of rabbit skeletal muscle

The effectiveness of the nonmetabolizable second messenger analogue DL-myo-inositol 1,4,5-trisphosphorothioate (IPS3) described by Cooke, A. M., R. Gigg, and B. V. L. Potter, (1987b. Jour. Chem. Soc. Chem. Commun. 1525-1526.) was examined in triads purified from rabbit skeletal muscle. A Ca2+ electrode uptake-release assay was used to determine the size and sensitivity of the IPS3-releasable pool of Ca2+ in isolated triads. Uptake was initiated by 1 mM MgATP, pCa 5.8, pH 7.5 Release was initiated when the free Ca2+ had lowered to pCa approximately 7. We found that 5-25 microM myo-inositol 1,4,5-trisphosphate (IP3), and separately IPS3, consistently released 5-20% of the Ca2+ pool actively loaded into triads. Single channel recording was used to determine if ryanodine receptor Ca2+ release channels were affected by IPS3 at the same myoplasmic Ca2+ and IPS3 concentrations. Open probability of ryanodine receptor Ca2+ release channels was monitored in triads fused to bilayers over long periods (200 s) in the absence and following addition of 30 microM IPS3 to the same channel. At myoplasmic pCa approximately 7, IPS3 had no effect in the absence of MgATP (Po = 0.0094 +/- 0.001 in control and Po = 0.01 +/- 0.006 after IPS3) and slightly increased activity in the presence of 1 mM MgATP (Po = 0.024 +/- 0.03 in control and Po = 0.05 +/- 0.03 after IPS3). Equally small effects were observed at higher myoplasmic Ca2+. The onset of channel activation by IPS3 or IP3 was slow, on the time scale 20-60 s. We suggest that in isolated triads of rabbit skeletal muscle, IP3-induced release of stored Ca2+ is probably not mediated by the opening of Ca2+ release channels.