Implications for Combination Therapy of Selective Monoamine Reuptake Inhibitors on Dopamine Transporters.

Monoamine transporters, including dopamine, norepinephrine, and serotonin transporters (DAT, NET, and SERT, respectively), are important therapeutic targets due to their essential roles in the brain. To overcome the slow action of selective monoamine reuptake inhibitors, dual- or triple-acting inhibitors have been developed. Here, to examine whether combination treatments of selective reuptake inhibitors have synergistic effects, the pharmacological properties of DAT, NET, and SERT were investigated using the selective inhibitors of each transporter, which are vanoxerine, nisoxetine, and fluoxetine, respectively. Potencies were determined via fluorescence-based substrate uptake assays in the absence and presence of other inhibitors to test the multi-drug effects on individual transporters, resulting in antagonistic effects on DAT. In detail, fluoxetine resulted in a 1.6-fold increased IC50 value of vanoxerine for DAT, and nisoxetine produced a more drastic increase in the IC50 value by six folds. Furthermore, the effects of different inhibitors, specifically monovalent ions, were tested on DAT inhibition by vanoxerine. Interestingly, these ions also reduced vanoxerine potency in a similar manner. The homology models of DAT suggested a potential secondary inhibitor binding site that affects inhibition in an allosteric manner. These findings imply that the use of combination therapy with monoamine reuptake inhibitors should be approached cautiously, as antagonistic effects may occur.

A selective class of inhibitors for the CLC-Ka chloride ion channel.

CLC proteins are a ubiquitously expressed family of chloride-selective ion channels and transporters. A dearth of pharmacological tools for modulating CLC gating and ion conduction limits investigations aimed at understanding CLC structure/function and physiology. Herein, we describe the design, synthesis, and evaluation of a collection of N-arylated benzimidazole derivatives (BIMs), one of which (BIM1) shows unparalleled (>20-fold) selectivity for CLC-Ka over CLC-Kb, the two most closely related human CLC homologs. Computational docking to a CLC-Ka homology model has identified a BIM1 binding site on the extracellular face of the protein near the chloride permeation pathway in a region previously identified as a binding site for other less selective inhibitors. Results from site-directed mutagenesis experiments are consistent with predictions of this docking model. The residue at position 68 is 1 of only ∼20 extracellular residues that differ between CLC-Ka and CLC-Kb. Mutation of this residue in CLC-Ka and CLC-Kb (N68D and D68N, respectively) reverses the preference of BIM1 for CLC-Ka over CLC-Kb, thus showing the critical role of residue 68 in establishing BIM1 selectivity. Molecular docking studies together with results from structure-activity relationship studies with 19 BIM derivatives give insight into the increased selectivity of BIM1 compared with other inhibitors and identify strategies for further developing this class of compounds.

Double Blockade of Glioma Cell Proliferation and Migration by Temozolomide Conjugated with NPPB, a Chloride Channel Blocker.

Glioblastoma is the most common and aggressive primary malignant brain tumor. Temozolomide (TMZ), a chemotherapeutic agent combined with radiation therapy, is used as a standard treatment. The infiltrative nature of glioblastoma, however, interrupts effective treatment with TMZ and increases the tendency to relapse. Voltage-gated chloride channels have been identified as crucial regulators of glioma cell migration and invasion by mediating cell shape and volume change. Accordingly, chloride current inhibition by 5-nitro-2-(3-phenylpropylamino)-benzoate (NPPB), a chloride channel blocker, suppresses cell movement by diminishing the osmotic cell volume regulation. In this study, we developed a novel compound, TMZ conjugated with NPPB (TMZ-NPPB), as a potential anticancer drug. TMZ-NPPB blocked chloride currents in U373MG, a severely invasive human glioma cell line, and suppressed migration and invasion of U373MG cells. Moreover, TMZ-NPPB exhibited DNA modification activity similar to that of TMZ, and surprisingly showed remarkably enhanced cytotoxicity relative to TMZ by inducing apoptotic cell death via DNA damage. These findings indicate that TMZ-NPPB has a dual function in blocking both proliferation and migration of human glioma cells, thereby suggesting its potential to overcome challenges in current glioblastoma therapy.

Cooperative gating between ion channels.

Cooperative gating between ion channels, i.e. the gating of one channel directly coupled to the gating of neighboring channels, has been observed in diverse channel types at the single-channel level. Positively coupled gating could enhance channel-mediated signaling while negative coupling may effectively reduce channel gating noise. Indeed, the physiological significance of cooperative channel gating in signal transduction has been recognized in several in vivo studies. Moreover, coupled gating of ion channels was reported to be associated with some human disease states. In this review, physiological roles for channel cooperativity and channel clustering observed in vitro and in vivo are introduced, and stimulation-induced channel clustering and direct channel cross linking are suggested as the physical mechanisms of channel assembly. Along with physical clustering, several molecular mechanisms proposed as the molecular basis for functional coupling of neighboring channels are covered: permeant ions as a channel coupling mediator, concerted channel activation through the membrane, and allosteric mechanisms. Also, single-channel analysis methods for cooperative gating such as the binomial analysis, the variance analysis, the conditional dwell time density analysis, and the maximum likelihood fitting analysis are reviewed and discussed.

The design and discovery of T-type calcium channel inhibitors for the treatment of central nervous system disorders.

INTRODUCTION: Neuronal T-type calcium channels (T-type channels) are expressed throughout the central nervous system (CNS), regulating neuronal excitability. T-type channels in the CNS are involved in various neurophysiological and pathophysiological states, and thus have become a promising therapeutic target. AREAS COVERED: This article discusses T-type channel-related CNS disorders such as epilepsy, neuropathic pain, insomnia and tremor disorders including Parkinson's disease. In addition, the article reviews T-type channel inhibitors showing efficacy in animal models in such CNS disorders, with a focus on classical T-type channel inhibitors with limited specificity for T-type channels as well as novel inhibitors with high specificity. Furthermore, the article also presents and discusses the next generation of T-type channel inhibitor discovery, virtual as well as screening and high-throughput screening techniques. EXPERT OPINION: Although T-type channel-related CNS disorders seem to be diverse, it converges into the one major cause - abnormal hyperactivity of neurons. T-type channel-specific inhibitors could thus be commonly applied for the treatment of such CNS disorders regardless of the complexity of individual disorder. Structural information of inhibitor-binding sites would facilitate the discovery of the next generation of T-type channel-specific inhibitors by stimulating structure-based rational drug designs.

A designed inhibitor of a CLC antiporter blocks function through a unique binding mode.

The lack of small-molecule inhibitors for anion-selective transporters and channels has impeded our understanding of the complex mechanisms that underlie ion passage. The ubiquitous CLC "Chloride Channel" family represents a unique target for biophysical and biochemical studies because its distinctive protein fold supports both passive chloride channels and secondary-active chloride-proton transporters. Here, we describe the synthesis and characterization of a specific small-molecule inhibitor directed against a CLC antiporter (ClC-ec1). This compound, 4,4'-octanamidostilbene-2,2'-disulfonate (OADS), inhibits ClC-ec1 with low micromolar affinity and has no specific effect on a CLC channel (ClC-1). Inhibition of ClC-ec1 occurs by binding to two distinct intracellular sites. The location of these sites and the lipid dependence of inhibition suggest potential mechanisms of action. This compound will empower research to elucidate differences between antiporter and channel mechanisms and to develop treatments for CLC-mediated disorders.

ATP-sensitive potassium channels exhibit variance in the number of open channels below the limit predicted for identical and independent gating.

In small cells containing small numbers of ion channels, noise due to stochastic channel opening and closing can introduce a substantial level of variability into the cell's membrane potential. Negatively cooperative interactions that couple a channel's gating conformational change to the conformation of its neighbor(s) provide a potential mechanism for mitigating this variability, but such interactions have not previously been directly observed. Here we show that heterologously expressed ATP-sensitive potassium channels generate noise (i.e., variance in the number of open channels) below the level possible for identical and independent channels. Kinetic analysis with single-molecule resolution supports the interpretation that interchannel negative cooperativity (specifically, the presence of an open channel making a closed channel less likely to open) contributes to the decrease in noise. Functional coupling between channels may be important in modulating stochastic fluctuations in cellular signaling pathways.

Pharmacological studies of Cav3.1 T-type calcium channels using automated patch-clamp techniques.

T-type calcium channels are involved in a variety of physiological and pathophysiological processes, and thus could be therapeutic targets. However, there is no T-type channel selective blocker for use in clinical practice, demanding a need for the development of novel drugs where a higher-throughput screening system is required. Here we present pharmacological studies on Ca(v)3.1 T-type channels using automated patch-clamp. The IC(50) values obtained from automated patch-clamp and conventional one showed a good correlation (correlation coefficient of 0.82), suggesting that the automated patch-clamp is an efficient and reliable method for ranking the drug potencies for T-type channels.

hERG channel blockade by externally applied quaternary ammonium derivatives.

The human ether-à-go-go related gene potassium channel is a key player in cardiac rhythm regulation, thus being an important subject for a cardiac toxicity test. Ever since human ether-à-go-go related gene channel inhibition-related cardiac arrest was proven to be fatal, numerous numbers of data on human ether-à-go-go related gene channel inhibition have been piled up. However, there has been no quantitative study on human ether-à-go-go related gene channel inhibition by quaternary ammonium derivatives, well-known potassium channel blockers. Here, we present human ether-à-go-go related gene channel blockade by externally applied quaternary ammonium derivatives using automated whole-cell patch-clamp recordings as well as ab initio quantum calculations. The inhibitory constants and the relative binding energies for human ether-à-go-go related gene channel inhibition were obtained from quaternary ammoniums with systematically varied head and tail groups, indicating that more hydrophobic quaternary ammoniums have higher affinity blockade while cation-π interactions or size effects are not a deterministic factor for human ether-à-go-go related gene channel inhibition by quaternary ammoniums. Further studies on the effect of quaternary ammoniums on human ether-à-go-go related gene channel inactivation implied that hydrophobic quaternary ammoniums either with a longer tail group or with a bigger head group than tetraethylammonium permeate the cell membrane to easily access the high-affinity internal binding site in human ether-à-go-go related gene channel and exert stronger blockade. These results may be informative for the rational drug design to avoid cardiac toxicity.

Direct interaction and functional coupling between voltage-gated CaV1.3 Ca2+ channel and GABAB receptor subunit 2.

Although CaV1.2 and CaV1.3 are two subtypes of L-type Ca2+ channels expressed in the CNS, functions of CaV1.3 have not been well elucidated compared to CaV1.2. Here, we found that CaV1.3-NT associates with GABABR2-CT using yeast two-hybrid, GST pull-down and co-immunoprecipitation assays. We also demonstrated co-localization of CaV1.3 and GABABR2 in HEK293 cells and cultured hippocampal neurons. Whole-cell patch-clamp and Ca2+-imaging experiments revealed that activation of GABABR increases CaV1.3 currents and intracellular Ca2+ via CaV1.3, but not CaV1.2. These results show a physical and functional interaction between CaV1.3 and GABABR, suggesting the potential pivotal roles of CaV1.3 in the CNS.

Inhibition of recombinant Ca(v)3.1 (alpha(1G)) T-type calcium channels by the antipsychotic drug clozapine.

Low voltage-activated T-type calcium channels are involved in the regulation of the neuronal excitability, and could be subject to many antipsychotic drugs. The effects of clozapine, an atypical antipsychotic drug, on recombinant Ca(v)3.1 T-type calcium channels heterologously expressed in human embryonic kidney 293 cells were examined using whole-cell patch-clamp recordings. At a standard holding potential of -100 mV, clozapine inhibited Ca(v)3.1 currents with an IC(50) value of 23.7+/-1.3 microM in a use-dependent manner. However, 10 microM clozapine inhibited more than 50% of the Ca(v)3.1 currents in recordings at a more physiologically relevant holding potential of -75 mV. Clozapine caused a significant hyperpolarizing shift in the steady-state inactivation curve of the Ca(v)3.1 channels, which is presumably the main mechanism accounting for the inhibition of the Ca(v)3.1 currents. In addition, clozapine slowed Ca(v)3.1 deactivation and inactivation kinetics but not activation kinetics. Clozapine-induced changes in deactivation and inactivation rates of the Ca(v)3.1 channel gating would likely facilitate calcium influx via Ca(v)3.1 T-type calcium channels. Thus, clozapine may exert its therapeutic and/or side effects by altering cell's excitability and firing properties through actions on T-type calcium channels.

Testing for violations of microscopic reversibility in ATP-sensitive potassium channel gating.

In pancreatic beta cells, insulin secretion is tightly controlled by the cells' metabolic state via the ATP-sensitive potassium (KATP) channel. ATP is a key mediator in this signaling process, where its role as an inhibitor of KATP channels has been extensively studied. Since the channel contains an ATPase as an accessory subunit, the possibility that ATP hydrolysis mediates KATP channel opening has also been proposed. However, a rigorous test of coupling between ATP hydrolysis and channel gating has not previously been performed. In the present work, we examine whether KATP channel gating obeys detailed balance in order to determine whether ATP hydrolysis is strongly coupled to the gating of the KATP channel. Single-channel records were obtained from inside-out patches of transiently transfected HEK-293 cells. Channel activity in membrane patches with exactly one channel shows no violations of microscopic reversibility. Although KATP channel gating shows long closed times on the time scale where ATP hydrolysis takes place, the time symmetry of channel gating indicates that it is not tightly coupled to ATP hydrolysis. This lack of coupling suggests that channel gating operates close to equilibrium; although detailed balance is not expected to hold for ATP hydrolysis, it still does so in channel gating. On the basis of these results, the function of the ATPase active site in channel gating may be to sense nucleotides by differential binding of ATP and ADP, rather than to drive a thermodynamically unfavorable conformational change.

Control of peptide product sizes by the energy-dependent protease ClpAP.

Processive proteases can unfold proteins and cleave them into fragments of a characteristic size. The detailed mechanism by which product sizes are controlled is still in question. One possible mechanism for the control of product sizes would be translocation of unfolded polypeptides to the protease active sites in units of defined length. We have investigated the mechanism by which ClpAP, an energy-dependent protease from Escherichia coli, controls the sizes of its peptide products. We show that ClpAP generates peptide products with a distribution of sizes that has a pronounced peak at a peptide length of 6-8 amino acid residues. This product size distribution, which is similar to that observed previously for the proteasome, is robust to perturbations that interfere with translocation or proteolysis. To explain these results, we propose a mechanism in which translocation alternates with proteolysis, allowing peptides of more or less uniform length to be cleaved processively from a translocating substrate. To estimate the rate and energy efficiency of ClpAP-catalyzed measurements of product sizes, we apply information theory to quantify how precisely the product sizes are controlled. This analysis may also prove to be useful in characterizing the mechanisms of other proteases and nucleases, such as the proteasome and Dicer, which control the sizes of their products.