Biomedical subjects

E Touma

Publications and source records attributed to E Touma.

9 recordsLinked to original sources

PubMed2000

Competitive antagonism of recombinant P2X(2/3) receptors by 2', 3'-O-(2,4,6-trinitrophenyl) adenosine 5'-triphosphate (TNP-ATP).

TNP-ATP has become widely recognized as a potent and selective P2X receptor antagonist, and is currently being used to discriminate between subtypes of P2X receptors in a variety of tissues. We have investigated the ability of TNP-ATP to inhibit alpha,beta-methylene ATP (alpha,beta-meATP)-evoked responses in 1321N1 human astrocytoma cells expressing recombinant rat or human P2X(2/3) receptors. Pharmacological responses were measured using electrophysiological and calcium imaging techniques. TNP-ATP was a potent inhibitor of P2X(2/3) receptors, blocking both rat and human receptors with IC(50) values of 3 to 6 nM. In competition studies, 10 to 1000 microM alpha,beta-meATP was able to overcome TNP-ATP inhibition. Schild analysis revealed that TNP-ATP was a competitive antagonist with pA(2) values of -8.7 and -8.2. Inhibition of P2X(2/3) receptors by TNP-ATP was rapid in onset, reversible, and did not display use dependence. Although the onset kinetics of inhibition were concentration-dependent, the TNP-ATP off-kinetics were concentration-independent and relatively slow. Full recovery from TNP-ATP inhibition did not occur until >/=5 s after removal of the antagonist. Because of the slow off-kinetics of TNP-ATP, full competition with alpha,beta-meATP for receptor occupancy could be seen only after both ligands had reached a steady-state condition. It is proposed that the slowly desensitizing P2X(2/3) receptor allowed this competitive interaction to be observed over time, whereas the rapid desensitization of other P2X receptors (P2X(3)) may mask the detection of competitive inhibition by TNP-ATP.

Adenosine Triphosphate↗

PubMed1999

Pharmacological characterization of recombinant human and rat P2X receptor subtypes.

ATP functions as a fast neurotransmitter through the specific activation of a family of ligand-gated ion channels termed P2X receptors. In this report, six distinct recombinant P2X receptor subtypes were pharmacologically characterized in a heterologous expression system devoid of endogenous P2 receptor activity. cDNAs encoding four human P2X receptor subtypes (hP2X1, hP2X3, hP2X4, and hP2X7), and two rat P2X receptor subtypes (rP2X2 and rP2X3), were stably expressed in 1321N1 human astrocytoma cells. Furthermore, the rP2X2 and rP2X3 receptor subtypes were co-expressed in these same cells to form heteromultimeric receptors. Pharmacological profiles were determined for each receptor subtype, based on the activity of putative P2 ligands to stimulate Ca2+ influx. The observed potency and kinetics of each response was receptor subtype-specific and correlated with their respective electrophysiological properties. Each receptor subtype exhibited a distinct pharmacological profile, based on its respective sensitivity to nucleotide analogs, diadenosine polyphosphates and putative P2 receptor antagonists. Alphabeta-methylene ATP (alphabeta-meATP), a putative P2X receptor-selective agonist, was found to exhibit potent agonist activity only at the hP2X1, hP2X3 and rP2X3 receptor subtypes. Benzoylbenzoic ATP (BzATP, 2' and 3' mixed isomers), which has been reported to act as a P2X7 receptor-selective agonist, was least active at the rat and human P2X7 receptors, but was a potent (nM) agonist at hP2X1, rP2X3 and hP2X3 receptors. These data comprise a systematic examination of the functional pharmacology of P2X receptor activation.

Adenosine Triphosphate↗

PubMed1999

Gain of function mutation of the alpha7 nicotinic receptor: distinct pharmacology of the human alpha7V274T variant.

In the human alpha7 nicotinic receptor, valine-274 in the pore-lining transmembrane-2 region was mutated to threonine to produce the variant human alpha7V274T, which was evaluated electrophysiologically following expression in Xenopus laevis oocytes. Inward current rectification was strong in human alpha7V274T as in the human alpha7 wild type nicotinic receptor. However, human alpha7V274T was 100-fold more sensitive to the nicotinic receptor agonists acetylcholine, (-)-nicotine and 1,1-dimethyl-4-phenylpiperazinium. Choline also activated human alpha7V274T (EC50 = 12 microM) and was 82-fold more potent than at human alpha7 wild type nicotinic receptor. (-)-Cotinine, (2,4)-dimethoxybenzylidene anabaseine (GTS-21) and 2-methyl-3-(2-(S)-pyrrolidinylmethoxy)pyridine (ABT-089), weak partial agonists at human alpha7 wild type, were much stronger agonists at human alpha7V274T with EC50 values of 70 microM, 4 microM and 28 microM and fractional activation values of 93%, 96% and 40%, respectively. However, (-)-lobeline, a human alpha7 wild type nicotinic receptor antagonist, and dihydro-beta-erythroidine, which activates chick mutagenized alpha7 nicotinic receptors, had only weak agonist-like activity at human alpha7V274T (< or = 4% of the maximal acetylcholine response). Methyllycaconitine, mecamylamine, d-tubocurarine and dihydro-beta-erythroidine retained antagonist activity and, indeed, appeared to be at least as potent at human alpha7V274T as at human alpha7 wild type. These results support and extend the concept that human nicotinic receptor pharmacology can be profoundly altered by single amino acid changes in the pore-lining segment.

Acetylcholine↗

PubMed1999

Holocarboxylase synthetase deficiency: report of a case with onset in late infancy.

A case of holocarboxylase synthetase (HCS) deficiency of late-infantile onset is presented and compared with the common manifestations in previously reported patients. Our patient had her first episode at 20 months followed by recurrent episodes of metabolic acidosis with ketolactic acidosis responding dramatically to a short trial of biotin and thiamin. The main clinical findings were metabolic acidosis with alteration in consciousness and respiration, which are in accordance with findings in earlier reported patients with both neonatal-onset and infantile-onset forms of HCS deficiency. The diagnosis of HCS deficiency was made only at the age of 5.5 years during a metabolic work-up when organic acid analysis was performed. This revealed elevated urinary excretion of the characteristics metabolites, 3-hydroxypropionate, 3-hydroxyisovalerate and methylcitrate, suggesting multiple carboxylase deficiency (MCD). MCD was demonstrated in fibroblasts of our patient, but only when the cells were grown in a medium with a very low biotin concentration of 10(-10) mol/L. Kinetics studies of reactivation of deficient propionyl-CoA carboxylase activity with biotin in intact fibroblasts revealed a midly decreased reactivation rate and only a 3-5 times higher biotin requirement as compared with controls. These findings are in accordance with a mild form of HCS deficiency. This child responded to 10 mg/day of biotin with normal lymphocyte carboxylase activities and adequate school performance at 10 years of age.

Age of Onset↗

PubMed1999

Molecular and functional characterization of human P2X(2) receptors.

P2X receptors are a family of ATP-gated ion channels. Four cDNAs with a high degree of homology to the rat P2X(2) receptor were isolated from human pituitary and pancreas RNA. Genomic sequence indicated that these cDNAs represent alternatively spliced messages. Northern analysis revealed high levels of human P2X(2) (hP2X(2)) message in the pancreas, and splice variants could be detected in a variety of tissues. Two cDNAs encoded functional ion channels when expressed in Xenopus oocytes, a receptor structurally homologous to the prototype rat P2X(2) receptor (called hP2X(2a)) and a variant containing a deletion within its cytoplasmic C terminus (called hP2X(2b)). Pharmacologically, these functional human P2X(2) receptors were virtually indistinguishable, with the P2X receptor agonists ATP, 2-methylthio-ATP, 2' and 3'-O-(4-benzoylbenzoyl)-ATP, and ATP5'-O-(3-thiotriphosphate) being approximately equipotent (EC(50) = 1 microM) in eliciting extracellular Ca(2+) influx. The P2 receptor agonists alpha,beta-methylene ATP, adenosine, adenosine 5'-O-(2-thiodiphosphate), and UTP were inactive at concentrations up to 100 microM. Both hP2X(2a) and hP2X(2b) receptors were sensitive to the P2 receptor antagonist pyridoxal-5-phosphate-6-azophenyl-2', 4'-disulfonic acid (IC(50) = 3 microM). In contrast to the analogous rat P2X(2) and P2X(2b) receptors, the desensitization rates of the hP2X(2a) and hP2X(2b) receptors were equivalent. Both functional forms of the human P2X(2) receptors formed heteromeric channels with the human P2X(3) receptor. These data demonstrate that the gene structure and mRNA heterogeneity of the P2X(2) receptor subtype are evolutionarily conserved between rat and human, but also suggest that alternative splicing serves a function other than regulating the desensitization rate of the human receptor.

Adenosine Triphosphate↗

PubMed1999

P2X receptor-mediated ionic currents in dorsal root ganglion neurons.

Nociceptive neurons in the dorsal root ganglia (DRG) are activated by extracellular ATP, implicating P2X receptors as potential mediators of painful stimuli. However, the P2X receptor subtype(s) underlying this activity remain in question. Using electrophysiological techniques, the effects of P2X receptor agonists and antagonists were examined on acutely dissociated adult rat lumbar DRG neurons. Putative P2X-expressing nociceptors were identified by labeling neurons with the lectin IB4. These neurons could be grouped into three categories based on response kinetics to extracellularly applied ATP. Some DRG responses (slow DRG) were relatively slowly activating, nondesensitizing, and activated by the ATP analogue alpha,beta-meATP. These responses resembled those recorded from 1321N1 cells expressing recombinant heteromultimeric rat P2X2/3 receptors. Other responses (fast DRG) were rapidly activating and desensitized almost completely during agonist application. These responses had properties similar to those recorded from 1321N1 cells expressing recombinant rat P2X3 receptors. A third group (mixed DRG) activated and desensitized rapidly (P2X3-like), but also had a slow, nondesensitizing component that functionally prolonged the current. Like the fast component, the slow component was activated by both ATP and alpha, beta-meATP and was blocked by the P2X antagonist TNP-ATP. But unlike the fast component, the slow component could follow high-frequency activation by agonist, and its amplitude was potentiated under acidic conditions. These characteristics most closely resemble those of rat P2X2/3 receptors. These data suggest that there are at least two populations of P2X receptors present on adult DRG nociceptive neurons, P2X3 and P2X2/3. These receptors are expressed either separately or together on individual neurons and may play a role in the processing of nociceptive information from the periphery to the spinal cord.

Animals↗

PubMed1995

Stable expression and pharmacological properties of the human alpha 7 nicotinic acetylcholine receptor.

The alpha 7 neuronal nicotinic acetylcholine receptor subtype forms a Ca(2+)-permeable homooligomeric ion channel sensitive to alpha-bungarotoxin in Xenopus oocytes. In this study, we have stably and functionally expressed the human alpha 7 cDNA in a mammalian cell line, HEK-293 and examined its pharmacologic properties. [125I] alpha-Bungarotoxin bound to transfected cells with a Kd value of 0.7 nM and a Bmax value of 973 pmoL/mg protein. No specific binding was detected in untransfected cells. Specific binding could be displaced by unlabeled alpha-bungarotoxin (Ki = 0.5 nM) and an excellent correlation was observed between binding affinities of a series of nicotinic cholinergic ligands in transfected cells and those in the human neuroblastoma IMR-32 cell line. Additionally, cell surface expression of alpha 7 receptors was detected by fluorescein isothiocyanate-conjugated alpha-bungarotoxin in transfected cells. Whole cell currents sensitive to blockade by alpha-bungarotoxin, and with fast kinetics of activation and inactivation, were recorded from transfected cells upon rapid application of (-)-nicotine or acetylcholine with EC50 values of 49 microM and 155 microM respectively. We conclude that the human alpha 7 subunit when expressed alone can form functional ion channels and that the stably transfected HEK-293 cell line serves as a unique system for studying human alpha 7 nicotinic receptor function and regulation, and for examining ligand interactions.

Acetylcholine↗

PubMed1995

Cloning and transient expression of genes encoding the human alpha 4 and beta 2 neuronal nicotinic acetylcholine receptor (nAChR) subunits.

Partial cDNA clones generated by RT-PCR were used as probes to clone the cDNAs encoding the human alpha 4 and beta 2 neuronal nicotinic acetylcholine receptor (nAChR) subunits. The 2.1-kb alpha 4 cDNA shows 84 and 76% identity to the rat and chicken cDNA sequences, respectively. The deduced amino-acid sequence shares 89 and 84% similarity, respectively, with the corresponding rat and chicken proteins, with most of the divergence occurring in the cytoplasmic domain. The 1721-nucleotide beta 2 sequence was identical to the human beta 2 sequence previously reported. Transfection of the alpha 4 and beta 2 clones into HEK293 cells resulted in the formation of binding sites that display high affinity towards [3H] cytisine, a characteristic of the alpha 4 beta 2 subtype produced in vivo.

Amino Acid Sequence↗

PubMed1989

[Neonatal pneumococcus laryngitis].

Report of a case of laryngitis beginning within the first 10 hours of life. The epiglottis was red, swollen, and covered by a membrane. Culture of laryngeal swabs isolated Streptococcus pneumoniae. Complete recovery was obtained with antibiotic therapy.

Anti-Bacterial Agents↗