Agonist specific L-type Ca(2+)-current stimulation in ventricular myocytes by a novel steroid-like compound

In cardiac muscle the amplitude of Ca(2+) transients can be increased by enhancing Ca(2+) influx. Among the processes leading to increased Ca(2+) influx, agonists of the L-type Ca(2+)-channel can play an important role. Known pharmacological Ca(2+)-channel agonists act on different binding sites on the channel protein, which may lead not only to enhanced peak currents, but also to distinct changes in other biophysical characteristics of the current. In this study, membrane currents were recorded with the patch-clamp technique in the whole-cell configuration in guinea pig isolated ventricular myocytes in combination with confocal fluorescence Ca(2+) imaging techniques and a variety of pharmacological tools. Testing a new positive inotropic steroid-like compound, we found that it increased the L-type Ca(2+)-current by 2.5-fold by shifting the voltage-dependence of activation by 20.2 mV towards negative potentials. The dose-response relationship revealed two vastly different affinities (EC(50(high-affinity))=4.5+/-1.7 nM, EC(50(low-affinity))=8.0+/-1.1 microM) exhibiting differential pharmacological interactions with three classes of Ca(2+)-current antagonists, suggesting more than one binding site on the channel protein. Therefore, we identified and characterized a novel positive inotropic compound (F90927) as a member of a new class of Ca(2+)-channel agonists exhibiting unique features, which set it apart from other presently known L-type Ca(2+)-channel agonists.

A novel steroid-like compound F90927 exerting positive-inotropic effects in cardiac muscle

Here we report a novel steroid-like compound F90363, exhibiting positive inotropy in vivo and in vitro in various cardiac muscle preparations. F90363 is a racemic mixture composed of the stereoisomers (-)-F90926 and (+)-F90927. Only F90927 exerted positive inotropy, while F90926 induced a weak negative inotropy, but only at concentrations 10(3) times higher than F90927 and most likely resulting from an unspecific interaction. The rapid time course of the action of F90927 suggested a direct interaction with a cellular target rather than a genomic alteration. We could identify the L-type Ca2+ current I(Ca(L)) as a main target of F90927, while excluding other components of cardiac Ca2+ signalling as potential contributors. In addition, several other signaling pathways known to lead to positive inotropy (e.g. alpha- and beta-adrenergic stimulation, cAMP pathways) could be excluded as targets of F90927. However, vessel contraction and stiffening of the cardiac muscle at high doses (>30 microM, 0.36 mg kg(-1), respectively) prevent the use of F90927 as a candidate for drug development. Since the compound may still find valuable applications in research, the aim of the present study was to identify the cellular target and the mechanism of inotropy of F90927.

Oxidation of 3-hydroxyanthranilic acid to the phenoxazinone cinnabarinic acid by peroxyl radicals and by compound I of peroxidases or catalase

Since 3-hydroxyanthranilic acid (3HAA), an oxidation product of tryptophan metabolism, is a powerful radical scavenger [Christen, S., Peterhans, E., ; Stocker, R. (1990) Proc. Natl. Acad. Sci. U.S.A. 87, 2506], its reaction with peroxyl radicals was investigated further. Exposure to aqueous peroxyl radicals generated at constant rate under air from the thermolabile radical initiator 2,2'-azobis[2-amid-inopropane] hydrochloride (AAPH) resulted in rapid consumption of 3HAA with initial accumulation of its cyclic dimer, cinnabarinic acid (CA). The initial rate of formation of the phenoxazinone CA accounted for approximately 75% of the initial rate of oxidation of 3HAA, taking into account that 2 mol of 3HAA are required to form 1 mol of CA. Consumption of 3HAA under anaerobic conditions (where alkyl radicals are produced from AAPH) was considerably slower and did not result in detectable formation of CA. Addition of superoxide dismutase enhanced autoxidation of 3HAA as well as the initial rates of peroxyl radical-induced oxidation of 3HAA and formation of CA by approximately 40-50%, whereas inclusion of xanthine/xanthine oxidase decreased the rate of oxidation of 3HAA by approximately 50% and inhibited formation of CA almost completely, suggesting that superoxide anion radical (O2.-) was formed and reacted with reaction intermediate(s) to curtail formation of CA. Formation of CA was also observed when 3HAA was added to performed compound I of horseradish peroxidase (HRPO) or catalytic amounts of either HRPO, myeloperoxidase, or bovine liver catalase together with glucose/glucose oxidase.(ABSTRACT TRUNCATED AT 250 WORDS)

Infantile hypophosphatasia due to a new compound heterozygous TNSALP mutation - functional evidence for a hydrophobic side-chain?

BACKGROUND: Infantile hypophosphatasia (IH) is an inherited disorder characterized by defective bone mineralization and a deficiency of alkaline phosphatase activity. OBJECTIVE/DESIGN: The aim of the study was to evaluate a new compound heterozygous TNSALP mutation for its residual enzyme activity and localization of the comprised amino acid residues in a 3D-modeling. PATIENT: We report on a 4-week old girl with craniotabes, severe defects of ossification, and failure to thrive. Typical clinical features as low serum alkaline phosphatase, high serum calcium concentration, increased urinary calcium excretion, and nephrocalcinosis were observed. Vitamin D was withdrawn and the patient was started on calcitonin and hydrochlorothiazide. Nonetheless, the girl died at the age of 5 months from respiratory failure. RESULTS: Sequence analysis of the patient's TNSALP gene revealed two heterozygous mutations [c.653T>C (I201T), c.1171C>T (R374C)]. Transfection studies of the unique I201T variant in COS-7 cells yielded a mutant TNSALP protein with only a residual enzyme activity (3.7%) compared with wild-type, whereas the R374C variant was previously shown to reduce normal activity to 10.3%. 3D-modeling of the mutated enzyme showed that I201T resides in a region that does not belong to any known functional site. CONCLUSION: We note that I201, which has been conserved during evolution, is buried in a hydrophobic pocket and, therefore, the I>T-change should affect its functional properties. Residue R374C is located in the interface between monomers and it has been previously suggested that this mutation affects dimerization. These findings explain the patient's clinical picture and severe course.

Aromatase deficiency in a female who is compound heterozygote for two new point mutations in the P450arom gene: impact of estrogens on hypergonadotropic hypogonadism, multicystic ovaries, and bone densitometry in childhood

We report on a female who is compound heterozygote for two new point mutations in the CYP19 gene. The allele inherited from her mother presented a base pair deletion (C) occurring at P408 (CCC, exon 9), causing a frameshift that results in a nonsense codon 111 bp (37 aa) further down in the CYP19 gene. The allele inherited from her father showed a point mutation from G-->A at the splicing point (canonical GT to mutational AT) between exon and intron 3. This mutation ignores the splice site and a stop codon 3 bp downstream occurs. Aromatase deficiency was already suspected because of the marked virilization occurring prepartum in the mother, and the diagnosis was confirmed shortly after birth. Extremely low levels of serum estrogens were found in contrast to high levels of androgens. Ultrasonographic follow-up studies revealed persistently enlarged ovaries (19.5-22 mL) during early childhood (2 to 4 yr) which contained numerous large cysts up to 4.8 x 3.7 cm and normal-appearing large tertiary follicles already at the age of 2 yr. In addition, both basal and GnRH-induced FSH levels remained consistently strikingly elevated. Low-dose estradiol (E2) (0.4 mg/day) given for 50 days at the age of 3 6/12 yr resulted in normalization of serum gonadotropin levels, regression of ovarian size, and increase of whole body and lumbar spine (L1-L4) bone mineral density. The FSH concentration and ovarian size returned to pretreatment levels shortly (150 days) after cessation of E2 therapy. Therefore, we recommend that affected females be treated with low-dose E2 in amounts sufficient to result in physiological prepubertal E2 concentrations using an ultrasensitive estrogen assay. However, E2 replacement needs to be adjusted throughout childhood and puberty to ensure normal skeletal maturation and adequate adolescent growth spurt, normal accretion of bone mineral density, and, at the appropriate age, female secondary sex maturation.

Compound heterozygous mutations affect protein folding and function in patients with congenital sucrase-isomaltase deficiency

BACKGROUND & AIMS: Congenital sucrase-isomaltase (SI) deficiency is an autosomal-recessive intestinal disorder characterized by a drastic reduction or absence of sucrase and isomaltase activities. Previous studies have indicated that single mutations underlie individual phenotypes of the disease. We investigated whether compound heterozygous mutations, observed in some patients, have a role in disease pathogenesis. METHODS: We introduced mutations into the SI complementary DNA that resulted in the amino acid substitutions V577G and G1073D (heterozygous mutations found in one group of patients) or C1229Y and F1745C (heterozygous mutations found in another group). The mutant genes were expressed transiently, alone or in combination, in COS cells and the effects were assessed at the protein, structural, and subcellular levels. RESULTS: The mutants SI-V577G, SI-G1073D, and SI-F1745C were misfolded and could not exit the endoplasmic reticulum, whereas SI-C1229Y was transported only to the Golgi apparatus. Co-expression of mutants found on each SI allele in patients did not alter the protein's biosynthetic features or improve its enzymatic activity. Importantly, the mutations C1229Y and F1745C, which lie in the sucrase domains of SI, prevented its targeting to the cell's apical membrane but did not affect protein folding or isomaltase activity. CONCLUSIONS: Compound heterozygosity is a novel pathogenic mechanism of congenital SI deficiency. The effects of mutations in the sucrase domain of SIC1229Y and SIF1745C indicate the importance of a direct interaction between isomaltase and sucrose and the role of sucrose as an intermolecular chaperone in the intracellular transport of SI.

Unique mixed phenotype and unexpected functional effect revealed by novel compound heterozygosity mutations involving SCN5A.

BACKGROUND Functional characterization of mutations involving the SCN5A-encoded cardiac sodium channel has established the pathogenic mechanisms for type 3 long QT syndrome and type 1 Brugada syndrome and has provided key insights into the physiological importance of essential structure-function domains. OBJECTIVE This study sought to present the clinical and biophysical phenotypes discerned from compound heterozygosity mutations in SCN5A on different alleles in a toddler diagnosed with QT prolongation and fever-induced ventricular arrhythmias. METHODS A 22-month-old boy presented emergently with fever and refractory ventricular tachycardia. Despite restoration of sinus rhythm, the infant sustained profound neurological injury and died. Using polymerase chain reaction, denaturing high-performance liquid chromatography, and direct DNA sequencing, comprehensive open-reading frame/splice mutational analysis of the 12 known long QT syndrome susceptibility genes was performed. RESULTS The infant had 2 SCN5A mutations: a maternally inherited N-terminal frame shift/deletion (R34fs/60) and a paternally inherited missense mutation, R1195H. The mutations were engineered by site-directed mutagenesis and heterologously expressed transiently in HEK293 cells. As expected, the frame-shifted and prematurely truncated peptide, SCN5A-R34fs/60, showed no current. SCN5A-R1195H had normal peak and late current but abnormal voltage-dependent gating parameters. Surprisingly, co-expression of SCN5A-R34fs/60 with SCN5A-R1195H elicited a significant increase in late sodium current, whereas co-expression of SCN5A-WT with SCN5A-R34fs/60 did not. CONCLUSIONS A severe clinical phenotype characterized by fever-induced monomorphic ventricular tachycardia and QT interval prolongation emerged in a toddler with compound heterozygosity involving SCN5A: R34fs/60, and R1195H. Unexpectedly, the 94-amino-acid fusion peptide derived from the R34fs/60 mutation accentuated the late sodium current of R1195H-containing Na(V)1.5 channels in vitro.