Subtype-dependence of NMDA receptor channel open probability.
NMDA receptor-mediated calcium transients play a critical role in synaptogenesis, synaptic plasticity, and excitotoxicity. NMDA receptors are heteromeric complexes of NR1A combined with NR2A, NR2B, NR2C, and/or NR2D subunits. The NR2 subunits determine a variety of electrophysiological and pharmacological properties of the NMDA receptor complex. In this report, we provide evidence for the first time that there is also a significant difference in peak channel open probability (P(o)) between NMDA receptors composed of NR1A/NR2A and those of NR1A/NR2B subunits. First, whole-cell patch-clamp recordings from human embryonic kidney (HEK) 293 cells expressing NMDA receptors revealed that NR1A/NR2A-mediated peak current densities are approximately four times larger than those of NR1A/NR2B. We show that this fourfold difference is unlikely caused by differences in receptor surface expression, since these levels were similar for the two subtypes by Western blot analysis. To determine whether P(o) contributed to the difference in peak current densities, we used two different open channel antagonists, MK-801 and 9-aminoacridine, in a variety of experimental paradigms. Our results indicate that peak P(o) is significantly higher (twofold to fivefold) for NR1A/NR2A than NR1A/NR2B, with estimated values of approximately 0.35 and 0.07, respectively. These results suggest that a change in the relative expression levels of NR2A and NR2B can regulate peak amplitude of NMDA receptor-mediated excitatory postsynaptic potentials and therefore may play a role in mechanisms underlying synaptic plasticity. (+info)
Cytotoxicity and mutagenicity of frameshift-inducing agent ICR191 in mismatch repair-deficient colon cancer cells.
BACKGROUND: Deficiency of DNA mismatch repair is a common feature of cancers exhibiting instability of microsatellite DNA sequences. Cancers with microsatellite instability are recognizable by their high rate of spontaneous frameshift mutations within microsatellite sequences, their resistance to killing by cytotoxic agents, and their localization to specific tissues, e.g., the proximal colon and stomach. We hypothesized that the mismatch repair deficiency of these cancers would make them vulnerable to environmental or chemical frameshift-inducing agents. This study was undertaken to test whether exogenous frameshift-inducing agents selectively induce mutations in mismatch repair-deficient cells of mutagen-exposed tissues like the colon and whether cytotoxic doses of these agents would preferentially kill those cells. METHODS: Cytotoxicity of the acridine mutagen 6-chloro-9-[3-(2-chloroethylamino)propylamino]-2-methoxy-acridine (ICR191), a DNA frameshift inducer, was determined in the mismatch repair-deficient human colon carcinoma cell line HCT116 versus the repair-reconstituted derivative HCT116+C3. Vulnerability to the mutagenic effects of ICR191 was determined by transfection of HCT116 or HCT116+C3 cells with a frameshift reporter vector, followed by treatment of the cells with ICR191. Alternatively, the reporter vector was reacted ex vivo with ICR191, and the derivatized vector was then transfected into HCT116 or HCT116+C3 cells. RESULTS: ICR191 proved to be fivefold to 10-fold more potent in inducing mutations in mismatch repair-deficient HCT116 cells than in mismatch repair-proficient HCT116+C3 cells. Moreover, at cytotoxic doses of ICR191, repair-deficient HCT116 cells proved to be fivefold more vulnerable to killing than did HCT116+C3 cells. CONCLUSIONS: Frameshift-inducing mutagens can selectively induce mutations in mismatch repair-deficient cells versus mismatch repair-proficient cells. Environmental exposures may, therefore, favor development of cancers with microsatellite instability in tissues like the gut. Frameshift-inducing agents can, however, also preferentially kill mismatch repair-deficient cancer cells and, thus, may be promising as model therapeutic compounds. (+info)
Acridinecarboxamide topoisomerase poisons: structural and kinetic studies of the DNA complexes of 5-substituted 9-amino-(N-(2-dimethylamino)ethyl)acridine-4-carboxamides.
For a series of antitumor-active 5-substituted 9-aminoacridine-4-carboxamide topoisomerase II poisons, we have used X-ray crystallography and stopped-flow spectrophotometry to explore relationships between DNA binding kinetics, biological activity, and the structures of their DNA complexes. The structure of 5-F-9-amino-[N-(2-dimethylamino)ethyl]-acridine-4-carboxamide bound to d(CGTACG)(2) has been solved to a resolution of 1.55 A in space group P6(4). A drug molecule intercalates between each of the CpG dinucleotide steps, its protonated dimethylamino group partially occupying positions close to the N7 and O6 atoms of guanine G2 in the major groove. A water molecule forms bridging hydrogen bonds between the 4-carboxamide NH and the phosphate group of the same guanine. Intercalation unwinds steps 1 and 2 by 12 degrees and 8 degrees, respectively compared with B-DNA, whereas the central TpA step is overwound by 10 degrees. Nonphenyl 5-substituents, on average, decrease mean DNA dissociation rates by a factor of three, regardless of their steric, hydrophobic, H-bonding, or electronic properties. Cytotoxicity is enhanced on average 4-fold and binding affinities rise by 3-fold, thus there is an apparent association between kinetics, affinity, and cytotoxicity. Taken together, the structural and kinetic studies imply that the main origin of this association is enhanced stacking interactions between the 5-substituent and cytosine in the CpG binding site. Ligand-dependent perturbations in base pair twist angles and their consequent effects on base pair-base pair stacking interactions may also contribute to the stability of the intercalated complex. 5-Phenyl substituents modify dissociation rates without affecting affinities, and variations in their biological activity are not correlated with DNA binding properties, which suggests that they interact directly with the topoisomerase protein. (+info)
Simultaneous measurement of deltapH and electron transport in chloroplast thylakoids by 9-aminoacridine fluorescence.
Electron transport and the electrochemical proton gradient across the thylakoid membrane are two fundamental parameters of photosynthesis. A combination of the electron acceptor, ferricyanide and the DeltapH indicator, 9-aminoacridine, was used to measure simultaneously electron transport rates and DeltapH solely by changes in the fluorescence of 9-aminoacridine. This method yields values for the rate of electron transport that are comparable with those obtained by established methods. Using this method a relationship between the rate of electron transport and DeltapH at various uncoupler concentrations or light intensities was obtained. In addition, the method was used to study the effect of reducing the disulfide bridge in the gamma-subunit of the chloroplast ATP synthase on the relation of electron transport to DeltapH. When the ATP synthase is reduced and alkylated, the threshold DeltapH at which the ATP synthase becomes leaky to protons is lower compared with the oxidized enzyme. Proton flow through the enzyme at a lower DeltapH may be a key step in initiation of ATP synthesis in the reduced enzyme and may be the way by which reduction of the disulfide bridge in the gamma-subunit enables high rates of ATP synthesis at low DeltapH values. (+info)
Anacardic acid-mediated changes in membrane potential and pH gradient across liposomal membranes.
We have previously shown that anacardic acid has an uncoupling effect on oxidative phosphorylation in rat liver mitochondria using succinate as a substrate (Life Sci. 66 (2000) 229-234). In the present study, for clarification of the physicochemical characteristics of anacardic acid, we used a cyanine dye (DiS-C3(5)) and 9-aminoacridine (9-AA) to determine changes of membrane potential (DeltaPsi) and pH difference (DeltapH), respectively, in a liposome suspension in response to the addition of anacardic acid to the suspension. The anacardic acid quenched DiS-C3(5) fluorescence at concentrations higher than 300 nM, with the degree of quenching being dependent on the log concentration of the acid. Furthermore, the K(+) diffusion potential generated by the addition of valinomycin to the suspension decreased for each increase in anacardic acid concentration used over 300 nM, but the sum of the anacardic acid- and valinomycin-mediated quenching was additively increasing. This indicates that the anacardic acid-mediated quenching was not due simply to increments in the K(+) permeability of the membrane. Addition of anacardic acid in the micromolar range to the liposomes with DeltaPsi formed by valinomycin-K(+) did not significantly alter 9-AA fluorescence, but unexpectedly dissipated DeltaPsi. The DeltaPsi preformed by valinomycin-K(+) decreased gradually following the addition of increasing concentrations of anacardic acid. The DeltaPsi dissipation rate was dependent on the pre-existing magnitude of DeltaPsi, and was correlated with the logarithmic concentration of anacardic acid. Furthermore, the initial rate of DeltapH dissipation increased with logarithmic increases in anacardic acid concentration. These results provide the evidence for a unique function of anacardic acid, dissimilar to carbonylcyanide p-trifluoromethoxyphenylhydrazone or valinomycin, in that anacardic acid behaves as both an electrogenic (negative) charge carrier driven by DeltaPsi, and a 'proton carrier' that dissipates the transmembrane proton gradient formed. (+info)
Molecular rearrangements of the extracellular vestibule in NMDAR channels during gating.
Many N-methyl-D-aspartate receptor (NMDAR) channel blockers that have therapeutic potential can be trapped in the closed state. Using a combination of the substituted cysteine accessibility method and open channel blockers, we found that the M3 segment forms the core of the extracellular vestibule, including a deep site for trapping blockers. The M3 segment, as well as more superficial parts of the extracellular vestibule, undergo extensive remodeling during channel closure, but do not define the activation gate, which is located deeper in the pore. Rather, the pore walls lining the extracellular vestibule constrict during channel closure. This movement is essential for coupling ligand binding to activation gate opening and accounts for the different mechanisms of open channel block, including trapping. (+info)
Staggering of subunits in NMDAR channels.
Functional N-methyl-D-aspartate receptors (NMDARs) are heteromultimers formed by NR1 and NR2 subunits. The M3 segment, as contributed by NR1, forms the core of the extracellular vestibule, including binding sites for channel blockers, and represents a critical molecular link between ligand binding and channel opening. Taking advantage of the substituted cysteine accessibility method along with channel block and multivalent coordination, we studied the contribution of the M3 segment in NR2C to the extracellular vestibule. We find that the M3 segment in NR2C, like that in NR1, contributes to the core of the extracellular vestibule. However, the M3 segments from the two subunits are staggered relative to each other in the vertical axis of the channel. Compared to NR1, homologous positions in NR2C, including those in the highly conserved SYTANLAAF motif, are located about four amino acids more externally. The staggering of subunits may represent a key structural feature underlying the distinct functional properties of NMDARs. (+info)
9-Aminoacridine: an efficient reagent to improve human and plant chromosome banding patterns and to standardize chromosome image analysis.
BACKGROUND: Successful automated chromosome analysis requires the development of new techniques to increase and standardize chromosome length and improve banding patterns. METHODS: Human and plant cells were pretreated with the DNA intercalator 9-aminoacridine (9-AMA), and chromosomes were stained with GTG and aceto-orcein banding techniques and investigated by an image analysis system. RESULTS: The human optimal chromosome spreads with the 850 G-band resolution level, suitable for image analysis, were obtained by 9-AMA pretreatment for 1 h at a final concentration of 0.5-1 microg/ml, as compared with 600-700 bands after ethidium bromide treatment and about 400 bands without pretreatment. The best results for plant chromosomes were obtained after pretreatment with 1-2 microg/ml of 9-AMA for 12-24 h. The chromosomes elongated approximately 1.5-fold, and the resolution of chromosome banding patterns increased, reaching approximately 140 bands per haploid set in the case of camomile. CONCLUSIONS: 9-AMA is an efficient reagent for the standardization and increasing the resolution of chromosome banding patterns in human and plant chromosomes. It is extremely important for chromosome investigation in small plants. (+info)