P-type ATPases as drug targets: tools for medicine and science

Plasma membrane calcium ATPases and cancer

The plasma membrane calcium ATPases (PMCAs) are vital regulators of basal Ca(2+) and shape the nature of intracellular free Ca(2+) transients after cellular stimuli and are thus regulators of a plethora of cellular processes. Studies spanning many years have identified that at least some cancers are associated with a remodeling of PMCA isoform expression. This alteration in Ca(2+) efflux capacity may have a variety of consequences including reduced sensitivity to apoptosis and increases in the responsiveness of cancer cells to proliferative stimuli. In this review we provide an overview of studies focused on PMCAs in the context of cancer. We discuss how the remodeling of PMCA expression could provide a survival and/or growth advantage to cancer cells, as well as the potential of pharmacological agents that target specific PMCA isoforms to be novel therapies for the treatment of cancer.

Structural insights into the high affinity binding of cardiotonic steroids to the Na+,K+-ATPase

The Na+,K+-ATPase belongs to the P-ATPase family, whose characteristic property is the formation of a phosphorylated intermediate. The enzyme is also a defined target for cardiotonic steroids which inhibit its functional activity and initiate intracellular signaling. Here we describe the 4.6 Å resolution crystal structure of the pig kidney Na+,K+-ATPase in its phosphorylated form stabilized by high affinity binding of the cardiotonic steroid ouabain. The steroid binds to a site formed at transmembrane segments αM1-αM6, plugging the ion pathway from the extracellular side. This structure differs from the previously reported low affinity complex with potassium. Most importantly, the A domain has rotated in response to phosphorylation and αM1-2 move towards the ouabain molecule, providing for high affinity interactions and closing the ion pathway from the extracellular side. The observed re-arrangements of the Na+,K+-ATPase stabilized by cardiotonic steroids may affect protein-protein interactions within the intracellular signal transduction networks.

Spiroindolones, a potent compound class for the treatment of malaria

Recent reports of increased tolerance to artemisinin derivatives--the most recently adopted class of antimalarials--have prompted a need for new treatments. The spirotetrahydro-beta-carbolines, or spiroindolones, are potent drugs that kill the blood stages of Plasmodium falciparum and Plasmodium vivax clinical isolates at low nanomolar concentration. Spiroindolones rapidly inhibit protein synthesis in P. falciparum, an effect that is ablated in parasites bearing nonsynonymous mutations in the gene encoding the P-type cation-transporter ATPase4 (PfATP4). The optimized spiroindolone NITD609 shows pharmacokinetic properties compatible with once-daily oral dosing and has single-dose efficacy in a rodent malaria model.

Proton translocating ATPase mediated fungicidal activity of eugenol and thymol

Eugenol (1) and thymol (2) exhibit excellent fungicidal activity against pathogenic yeasts, including isolates resistant to azoles. The rapid irreversible action of compound-1 and compound 2 on fungal cells suggested a membrane-located target for their action. We investigated their effect on H(+)-ATPase mediated H(+)-pumping by various Candida species. Both compounds inhibit H(+)-ATPase activity at their respective MIC values--500 and 100 μg/ml. Glucose stimulated H(+)-extrusion was also inhibited significantly by compound 1 and compound 2. Inhibition of H(+)-ATPase leads to intracellular acidification and cell death. Inhibition of cell growth and H(+)-efflux by test compounds suggests that their antifungal properties are related to their inhibitory effects on H(+)-ATPase.

High-throughput expression and purification of membrane proteins

High-throughput (HT) methodologies have had a tremendous impact on structural biology of soluble proteins. High-resolution structure determination relies on the ability of the macromolecule to form ordered crystals that diffract X-rays. While crystallization remains somewhat empirical, for a given protein, success is proportional to the number of conditions screened and to the number of variants trialed. HT techniques have greatly increased the number of targets that can be trialed and the rate at which these can be produced. In terms of number of structures solved, membrane proteins appear to be lagging many years behind their soluble counterparts. Likewise, HT methodologies for production and characterization of these hydrophobic macromolecules are only now emerging. Presented here is an HT platform designed exclusively for membrane proteins that has processed over 5000 targets.

Molecular characterization of mutant Arabidopsis plants with reduced plasma membrane proton pump activity

Arabidopsis mutants containing gene disruptions in AHA1 and AHA2, the two most highly expressed isoforms of the Arabidopsis plasma membrane H(+)-ATPase family, have been isolated and characterized. Plants containing homozygous loss-of-function mutations in either gene grew normally under laboratory conditions. Transcriptome and mass spectrometric measurements demonstrate that lack of lethality in the single gene mutations is not associated with compensation by increases in RNA or protein levels. Selected reaction monitoring using synthetic heavy isotope-labeled C-terminal tryptic peptides as spiked standards with a triple quadrupole mass spectrometer revealed increased levels of phosphorylation of a regulatory threonine residue in both isoforms in the mutants. Using an extracellular pH assay as a measure of in vivo ATPase activity in roots, less proton secreting activity was found in the aha2 mutant. Among 100 different growth conditions, those that decrease the membrane potential (high external potassium) or pH gradient (high external pH) caused a reduction in growth of the aha2 mutant compared with wild type. Despite the normal appearance of single mutants under ideal laboratory growth conditions, embryos containing homozygous double mutations are lethal, demonstrating that, as expected, this protein is absolutely essential for plant cell function. In conclusion, our results demonstrate that the two genes together perform an essential function and that the effects of their single mutations are mostly masked by overlapping patterns of expression and redundant function as well as by compensation at the post-translational level.

Ivermectin is a nonselective inhibitor of mammalian P-type ATPases

Ivermectin is a large spectrum antiparasitic drug that is very safe at the doses actually used. However, as it is being studied for new applications that would require higher doses, we should pay attention to its effects at high concentrations. As micromolar concentrations of ivermectin have been reported to inhibit the sarco-endoplasmic reticulum Ca(2+)-ATPase (SERCA), we decided to investigate its putative inhibitory effect on other two important P-type ATPases, namely the Na(+) , K(+)-ATPase and H(+)/K(+)-ATPase. We first extended the data on SERCA, using preparations from rat enriched in SERCA1a (extensor digitorum longus) and 1b (heart) isoforms. Secondly, we tested the effect of ivermectin in two preparations of rat Na(+), K(+)-ATPase in order to appreciate its putative selectivity towards the alpha(1) isoform (kidney) and the alpha(2)/alpha(3) isoforms (brain), and in an H(+)/K(+)-ATPase preparation from rat stomach. Ivermectin inhibited all these ATPases with similar IC(50) values (6-17 microM). With respect to the inhibition of the Na(+), K(+)-ATPase, ivermectin acts by a mechanism different from the classical cardiac glycosides, based on selectivity towards the isoforms, sensibility to the antagonistic effect of K(+) and to ionic conditions favoring different conformations of the enzyme. We conclude that ivermectin is a nonselective inhibitor of three important mammalian P-type ATPases, which is indicative of putative important adverse effects if this drug were used at high doses. As a consequence, we propose that novel analogs of ivermectin should be developed and tested both for their parasitic activity and in vitro effects on P-type ATPases.

Changes in electrostatic surface potential of Na+/K+-ATPase cytoplasmic headpiece induced by cytoplasmic ligand(s) binding

A set of single-tryptophan mutants of the Na(+)/K(+)-ATPase isolated, large cytoplasmic loop connecting transmembrane helices M4 and M5 (C45) was prepared to monitor effects of the natural cytoplasmic ligands (i.e., Mg(2+) and/or ATP) binding. We introduced a novel method for the monitoring of the changes in the electrostatic surface potential (ESP) induced by ligand binding, using the quenching of the intrinsic tryptophan fluorescence by acrylamide or iodide. This approach opens a new way to understanding the interactions within the proteins. Our experiments revealed that the C45 conformation in the presence of the ATP (without magnesium) substantially differed from the conformation in the presence of Mg(2+) or MgATP or in the absence of any ligand not only in the sense of geometry but also in the sense of the ESP. Notably, the set of ESP-sensitive residues was different from the set of geometry-sensitive residues. Moreover, our data indicate that the effect of the ligand binding is not restricted only to the close environment of the binding site and that the information is in fact transmitted also to the distal parts of the molecule. This property could be important for the communication between the cytoplasmic headpiece and the cation binding sites located within the transmembrane domain.