Critical role of a K+ channel in Plasmodium berghei transmission revealed by targeted gene disruption.

Regulated K(+) transport across the plasma membrane is of vital importance for the survival of most cells. Two K(+) channels have been identified in the Plasmodium falciparum genome; however, their functional significance during parasite life cycle in the vertebrate host and during transmission through the mosquito vector remains unknown. We hypothesize that these two K(+) channels mediate the transport of K(+) in the parasites, and thus are important for parasite survival. To test this hypothesis, we identified the orthologue of one of the P. falciparum K(+) channels, PfKch1, in the rodent malaria parasite P. berghei (PbKch1) and examined the biological role by performing a targeted disruption of the gene encoding PbKch1. The deduced amino acid sequence of the six transmembrane domains of PfKch1 and PbKch1 share 82% identity, and in particular the pore regions are completely identical. The PbKch1-null parasites were viable despite a marked reduction in the uptake of the K(+) congener (86)Rb(+), and mice infected with PbKch1-null parasites survived slightly longer than mice infected with WT parasites. However, the most striking feature of the phenotype was the virtually complete inhibition of the development of PbKch1-null parasites in Anopheles stephensi mosquitoes. In conclusion, these studies demonstrate that PbKch1 contributes to the transport of K(+) in P. berghei parasites and supports the growth of the parasites, in particular the development of oocysts in the mosquito midgut. K(+) channels therefore may constitute a potential antimalarial drug target.

[Transport proteins as drug targets in Plasmodium falciparum. New perspectives in the treatment of malaria]. / Transportproteiner som drug-targets hos Plasmodium falciparum. Nye perspektiver i behandlingen af malaria.

The malaria parasite, Plasmodium falciparum, infects and replicates in human erythrocytes. Through the use of substrate-specific transport proteins, P. falciparum takes up nutrients from the erythrocyte's cytoplasm. The sequencing and publishing of the P. falciparum genome have made it possible to identify, clone and characterise a number of these transport proteins from the parasite. Since the P. falciparum transport proteins differ from their human homologues, they may provide potential drug targets in the treatment of malaria. An example of a P. falciparum transport protein which seems promising as a drug target is the parasite's hexose transporter. Furthermore, the antimalarial drug artemisinin has been shown to interact specifically with the parasite's Ca2+ pump. A number of other transport proteins are also discussed as possible drug targets.

Molecular cloning of a K(+) channel from the malaria parasite Plasmodium falciparum.

In most living cells, K(+) channels are important for the generation of the membrane potential and for volume regulation. The parasite Plasmodium falciparum, which causes malignant malaria, must be able to deal with large variations in the ambient K(+) concentration: it is exposed to high concentrations of K(+) when inside the erythrocyte and low concentrations when in plasma. In the recently published genome of P. falciparum, we have identified a gene, pfkch1, encoding a potential K(+) channel, which to some extent resembles the big-conductance (BK) K(+) channel. We have cloned the approximately 6000 nucleotide (nt) fragment from cDNA, studied the pattern of expression of pfkch1 throughout the intraerythrocytic part of the parasite's life-cyclus, and characterized the channel on the basis of similarity to other K(+) channels from pro- and eukaryotic organisms. This P. falciparum K(+) channel could be a potential drug target.

Nizatidine and omeprazole enhance the effect of metronidazole on Helicobacter pylori in vitro.

Treatment failures are common in patients infected with metronidazole-resistant Helicobacter pylori in the gastric mucosa when triple therapy including metronidazole is used. In patients with treatment failure and metronidazole-resistant H. pylori, a higher eradication rate for H. pylori was found after secondary treatment with bismuth/ranitidine in combination with antibiotics including metronidazole, compared with the same antibiotics combined with a standard dose of omeprazole. This agrees with our previous finding that bismuth was able to reduce the susceptibility of H. pylori to metronidazole. In this study, we have found that nizatidine, an H(2)-receptor antagonist, is also able to reduce the susceptibility of H. pylori to metronidazole in vitro, despite having no direct inhibitory effect on the growth of H. pylori. This agrees with earlier findings that compounds having the ability to reverse antibiotic resistance do not necessarily have an antibiotic or chemotherapeutic effect in the sense of growth inhibition. Therefore, it was decided to investigate the effect of nizatidine and omeprazole on the oxidative respiratory chain, as it is known that metronidazole is able to inhibit the activity of fumarate reductase of H. pylori. This enzyme is a key enzyme in the alternative respiratory chain under anaerobic conditions. Nizatidine was, in these preliminary experiments, found to inhibit fumarate reductase in a dose-dependent way, like metronidazole, whereas omeprazole had almost no effect on fumarate reductase. No other significant effects on the enzymes of the respiratory chain were found. The synergistic effect of nizatidine on metronidazole resistant H. pylori strains could be explained by the effect on fumarate reductase, whereas the effect of omeprazole is different and could be an inhibition of a proton pump in H. pylori. Reversal of antimicrobial resistance with the help of different non-antibiotics seems to be possible by using quite different compounds, and is therefore to be explained by different molecular mechanisms.