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Heme Detoxification and Signaling in Blood-Feeding Organisms
Summary: The long-term goal of Pedro Oliveira's research is to understand how blood-feeding organisms have adapted to the ingestion of large amounts of heme. Specifically, he plans to characterize the mechanisms responsible for heme detoxification and for maintenance of the redox balance in the midgut of blood-feeding animals. He also studies the effects of heme on the cell signaling and gene expression of cells of the midgut of blood-feeding arthropods.
The most prevalent infectious diseases in developing countries are transmitted by arthropod vectors that feed on human blood. In all cases, such hematophagous organisms consume large amounts of blood. Thus, large quantities of free heme are produced during hemoglobin degradation. Heme, the prosthetic group of hemoglobin, is a potentially toxic molecule, capable of increasing the formation of free radicals and promoting oxidative damage. Heme is also a regulatory molecule that acts by modulating protein function and controlling the expression of stress-related genes. The hypothesis that drives our research is that adaptation to the ingestion of large amounts of heme is a major trend in the evolution of hematophagy. In recent years, we have been studying how various blood-sucking organisms cope with the high heme concentrations generated inside their gut as a consequence of their "vampire" way of life.
Invertebrate animals that feed on the blood of vertebrates do not have a common hematophagous ancestor. Present-day mosquitoes have a nonhematophagous dipteran ancestor, whereas triatomine bugs (vectors of Chagas' disease) and ticks probably arose independently from ancestors that preyed on other insects. Thus, the general nature of our working hypothesis led us to a comparative approach, using different organisms as study models: triatomine bugs, mosquitoes, ticks, and worms.
We have described a wide range of biological processes that contribute to attenuating the harmful effects of heme, including antioxidant enzymes and radical scavengers, heme-binding proteins, and the formation of heme aggregates. Our findings provide a description of several mechanisms that allow these animals to ingest large amounts of heme.
We found some protective mechanisms, also present in nonhematophagous animals, such as ubiquitous antioxidant enzymes, that play a role in ameliorating heme toxicity. The adaptation to hematophagy may involve a simple strategy—for example, high specific activities of an enzyme such as catalase in the midgut.
In other situations, novel adaptive mechanisms have evolved. One interesting example is the detoxification of heme. All organisms (from bacteria to humans) degrade heme by using the enzyme heme oxygenase (HO), an NADPH-dependent enzyme that uses O2 to break the heme porphyrin ring and generate biliverdin, CO, and iron. We recently found that the triatomine insect Rhodnius prolixus breaks down heme by using a unique and complex pathway. In this insect, heme first reacts with two glutathione molecules; only after that step can the oxidative cleavage of the modified heme occur, resulting in the release of iron and CO and the formation of biliverdin covalently attached to two cysteine residues. The mosquito Aedes aegypti makes use of a heme oxygenase similar to that of most other organisms, but, after biliverdin is produced, it is modified by the addition of two glutamine residues. This is an example of convergent evolution, whereby natural selection has recruited different molecular mechanisms, resulting, in both cases, in the formation of a polar derivative of biliverdin, which may facilitate excretion.
Another example is the formation of heme aggregates. We have shown that most of the heme these animals ingest is detoxified by aggregation pathways. It appears that, once digestive proteases cleave the hemoglobin polypeptide chain and heme is released from its heme pocket, the priority is to accelerate the insolubilization of heme to prevent its binding to cellular membranes. This task appears to be achieved by a different molecular pathway in each case. In Rhodnius prolixus and in the worm Schistosoma mansoni, heme is converted to hemozoin, a heme aggregate that is also found in Plasmodium, the malaria parasite. The mechanism of hemozoin formation is still unknown. In Plasmodium, it takes place inside the digestive vacuole. In Rhodnius, it involves the association of heme with extracellular phospholipid membranes that are found in the midgut lumen and are secreted by the midgut cells. In Schistosoma, hemozoin is formed inside lipid droplets that are found in the luminal gut content. In Aedes, no hemozoin is formed—instead, heme is trapped by a set of heme-binding proteins found in the extracellular mucous layer secreted by gut epithelial cells. Hemozoin formation does not occur in ticks either, where hemoglobin digestion is intracellular and heme is packed into organelles dedicated to this function—organelles that we have named hemosomes. The common trend for all four species is to trap heme in insoluble aggregates that are confined in space. Again, evolution has converged to physiologically equivalent processes but to distinct molecular mechanisms.
In addition to causing oxidative damage, heme acts as a signaling molecule. We have observed modulation of protein phosphorylation by heme in the blood-sucking Rhodnius and also in human neutrophils. In the latter case, heme showed a pro-inflammatory action, enhancing innate immunity mechanisms. Other researchers have shown that heme modifies gene expression in mammalian cells by promoting specific transcription factors. How these mechanisms operate in hematophagous invertebrates will also reveal important aspects about the adaptation of these animals to hematophagy.
In the hope of discovering novel molecules and metabolic pathways as well as additional functions for already known pathways and regulatory mechanisms, we continue to hunt for biological diversity generated by evolution in blood-feeding organisms. This may help us understand an important aspect of the biology of these organisms and how their ancestors adapted to hematophagy. The human organism also has a high concentration of heme in blood, and conditions such as hemorrhage and hemolytic disease expose our cells to high heme concentrations. Given that it has been shown to modulate an immune response, heme degradation has attracted much attention in the last few years, but we still lack basic information about some fundamental questions concerning the cell biology of heme. We do not yet know which routes are used to transport heme inside cells or the expression of which genes heme modulates. Information from blood-feeding organisms may shed some light on these basic biological problems.
Last updated October 2008
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