Study on protective effects of antimalarial drugs at different stages of malaria

Abstract

Malaria is one of the world’s public health problems, because it is a medical emergency with a high risk of mortality. The infection of malaria parasite induces a great number of deaths annually in many tropical and subtropical regions of the world. However, malaria has developed resistance to antimalarial drugs, such as sulfadoxine/pyrimethamine, mefloquine, halofantrine, and quinine. The development of new drugs should be needed to overcome the problem of drug resistance. The malaria parasite is a multi-stage protozoan with a complex life cycle. The life cycle is composed of three stages; the mosquito stage, the liver stage, and the blood stage. In this study, the effects of antimalarial drugs that are no signs of resistnace are investigated at blood and liver stages. In addition, human and non-human malarial vector mosquitoes are investigated to compare the functional features of their pump organs using synchrotron X-ray microscopic computed tomography (SR-μCT). The parasite Plasmodium falciparum has the most dangerous form of malaria to human. It has exhibited a bunch of resistance to antimalarial drugs. The resurgence of malaria is partly attributed to wide distribution of drug-resistant strains. Mitochondria of P. falciparum enable it to adapt to the host morphologically and physiologically. Thus, the inhibition of mitochondria of P. falciparum has been suggested as a novel strategy of survival. The enzyme protein farnesyltransferase is known as an ideal drug target for P. falciparum. The target of farnesyltransferase includes members of the Ras superfamily, which are critical to cell-cycle progression. The effect of farnesyltransferase inhibitor (FTI) on the functions of mitochondria of P. falciparum is investigated experimentally. As a result, FTI R115777 suppresses the infection rate of malaria parasite under in vitro condition. It also reduces the copy number of mtDNA-encoded cytochrome c oxidase III. In addition, the mitochondrial membrane potential (ΔΨm) and the green fluorescence intensity of MitoTracker are decreased by FTI R115777. These results indicate that FTI R115777 has strong influence on the mitochondrial function of P. falciparum as a target of the mitochondria of malaria parasites. In the liver stage, genistein has been known to inhibit the development of sporozoite under in vivo condition. The reduction of sporozoite in the liver relieves the severity of the disease in subsequent blood stages. Thus, the effect of genistein is investigated in P. berghei-infected mice models. Genistein is found to inhibit malaria-induced splenomegaly. After treating genistein, the disrupted architectures of the liver and spleen exhibit protective effect, compared to the uninfected ones. The enzymatic activity and expression pattern of proteins are then investigated in the liver and spleen of malaria-infected models. In addition, single-photon emission computed tomography/computed tomography (SPECT/CT) technique are used to examine the functions of malaria-infected spleen. The trapping capacity of the malaria-infected spleen is increased in comparison with the naïve group. However, genistein leads to prevention of P. berghei-induced spleen enlargement by inhibiting accumulation of heat-damaged radiolabeled RBCs or malaria parasites. Therefore, a genistein-based antimalarial combination therapy may be helpful for preventing the spleen impairment due to malaria infection. Lastly, the functional features of the pump organs of human and non-human malarial vector mosquitoes are investigated using a synchrotron X-ray microscopic computed tomography (SR-μCT). The pump organ of a mosquito is composed of cibarial pump (CP) and pharyngeal pump (PP). The volumetric variations and feeding behaviors of the two feeding pump chambers in Anopheles sinensis and Aedes togoi are quantitatively compared. In addition, differences in the actin-based structures of the heads of An. sinensis and Ae. togoi are comparatively investigated. The expansion ratio of CP/PP for Ae. togoi is about 60% larger than that of An. sinensis. Near the pump chamber PP, F-actin is clearly more localized in Ae. togoi than in An. sinensis. The results obtained in this study would be helpful for understanding the antimalarial drug activities in the blood and liver stages and the functional features of malaria and non-malaria vector mosquitoes.