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Role of Host Factors in Establishing Malaria Infection and Disease
Summary: Maria Mota aims to determine the host molecules and mechanisms required for the malaria parasite Plasmodium to become established inside the liver cells of its host and the host factors involved in developing pathology. She uses a systematic RNA interference screen and rodent models of malaria disease in her research.
Malaria is caused by a protozoan parasite of the genus Plasmodium. After the bite of an infected female Anopheles mosquito, Plasmodium sporozoites enter the mammalian host and travel to the liver. Once there, they cross several hepatocytes before invading a final one, forming a parasitophorous vacuole. Each invading parasite then replicates asexually into several thousand merozoites that constitute a so-called exoerythrocytic form of Plasmodium. This asymptomatic stage of infection is followed by the release of merozoites into the bloodstream, where they invade erythrocytes, initiating the blood stage of infection, which is when the symptoms of malaria occur.
A potential approach to malaria control is to target mechanisms crucial for the development of Plasmodium or the pathology caused by the infection. This requires detailed knowledge of the complex interaction between host cell and Plasmodium. While hepatocyte-Plasmodium interactions during liver stages constitute an ideal target for intervention strategies, the blood stages of infection are the ultimate goal for therapeutic strategies against the disease. Our overall goal is to elucidate the role of host components in establishing a malaria infection (liver stage) as well as in developing pathology (blood stage).
We aim to exploit postgenome knowledge of the host to understand the requirements that govern Plasmodium development inside the hepatocyte. The marked preference of Plasmodium for hepatocytes over other cell types suggests that the host cell strongly influences the parasite's development and, ultimately, infection. By applying microarray analysis and a systematic RNA interference (RNAi) screen of host factors, we are determining a number of host molecules that affect normal parasite invasion and development. Given that the combined activities of kinases and phosphatases can tightly control numerous cellular processes, it seems logical to assume that they can modulate the cell's behavior after infection by an intracellular pathogen. Thus, we also used RNAi to selectively knock down the expression of 727 genes in the human genome, encoding all the known proteins with putative kinase activity, as well as several kinase-interacting proteins. We monitored the effect of gene knock down on Plasmodium infection of HuH-7 cells, a human hepatoma cell line, by high-throughput, high-content immunofluorescence microscopy. We are identifying a number of host kinases that play a potential role in Plasmodium sporozoite infection of hepatocytes and analyzing them in more detail.
Using rodent models of malaria, we also aim to elucidate the role of certain host factors in establishing pathology. Although the pathogenic processes leading to severe malaria are poorly understood, activation of microvascular endothelial cells seems to play a pivotal role. Plasmodium infection of red blood cells causes extensive hemolysis. About 40 percent of the hemoglobin in each infected red blood cell can be released and readily oxidized. This leads to the generation of free heme, a molecule that is cytotoxic to the parasite when generated within red blood cells and to the host when released into the circulation. Plasmodium has developed strategies to cope with free heme generated within red blood cells, polymerizing it into hemozoin. When exposed to free heme, host cells (human or rodent) upregulate the expression of heme oxygenase-1 (HO-1, encoded by Hmox1), a stress-responsive enzyme that catabolizes heme into iron (Fe), biliverdin, and carbon monoxide (CO).
In rodents, we have shown that HO-1 prevents the development of experimental cerebral malaria (ECM), a complex neurological syndrome with many similarities to cerebral malaria in humans, a major cause of death by malaria. BALB/c mice infected with Plasmodium berghei ANKA exhibited upregulated HO-1 expression and activity and did not develop ECM. Deletion of Hmox1 and inhibition of HO activity increased ECM incidence to 83 and 78 percent, respectively. HO-1 upregulation was lower in infected C57BL/6 than in BALB/c mice, and all infected C57BL/6 mice developed ECM. Pharmacological induction of HO-1 and exposure to CO, an end product of HO-1 activity, reduced ECM incidence in C57BL/6 mice to 10 and 0 percent, respectively. Whereas HO-1 and CO did not affect parasitemia, they prevented blood-brain barrier disruption, brain microvasculature congestion, and neuroinflammation, including CD8+ T cell sequestration in the brain. These effects were mediated via binding of CO to hemoglobin, preventing hemoglobin oxidation and the generation of free heme, a molecule that triggers ECM pathogenesis. This study reveals that the pathogenesis of cerebral malaria might be controlled through the expression of a so-called "protective gene" in the host. We have shown this for HO-1 in the context of ECM, but we cannot exclude the possibility that other "protective genes" might act in a similar manner to prevent the onset of cerebral malaria or other forms of severe acute malaria in humans. Although caution is called for when extrapolating from rodent models to human malaria infection, these findings, which provide insights into the pathogenesis of ECM, may lead to new therapeutic approaches to suppress the pathogenesis of human cerebral malaria based on the modulation of HO-1 expression or the administration of CO.
With increasing knowledge about the molecular mechanisms in the host underlying Plasmodium establishment and disease development, it may become possible to design drugs that do not interfere with normal host functions but do prevent infection.
Last updated April 2007
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