MicrobiologyBytes

Wolbachia pipientis is an intracellular maternally-inherited bacterial symbiont of invertebrates that is very common in insects, including a number of mosquito species. It can manipulate host reproduction in several ways, including cytoplasmic incompatibility, whereby certain crosses are rendered effectively sterile. Females that are uninfected produce infertile eggs when they mate with males that carry Wolbachia, while there is a “rescue” effect in Wolbachia-infected embryos such that infected females can reproduce successfully with any males. Therefore uninfected females suffer a frequency-dependent reproductive disadvantage. Wolbachia is able to rapidly invade populations using this powerful mechanism

Malaria is one of the world’s most devastating diseases, particularly in Africa, and new control strategies are desperately needed. Here we show that the presence of Wolbachia bacteria inhibits the development of a malaria parasite in the most important Anopheles mosquito species of Africa. In addition it shows that the presence of Wolbachia results in the switching on of immune genes that are known to affect development of many species of malaria parasite. When added to the lifespan-shortening effects of this particular strain of Wolbachia, and the general ability of Wolbachia to spread through insect populations, this study provides a stimulus for the development of Wolbachia-based malaria control methods. It also provides new insights into the wide range of effects of Wolbachia in insects.

Wolbachia Stimulates Immune Gene Expression and Inhibits Plasmodium Development in Anopheles gambiae. (2010) PLoS Pathog 6(10): e1001143. doi:10.1371/journal.ppat.1001143
The over-replicating wMelPop strain of the endosymbiont Wolbachia pipientis has recently been shown to be capable of inducing immune upregulation and inhibition of pathogen transmission in Aedes aegypti mosquitoes. In order to examine whether comparable effects would be seen in the malaria vector Anopheles gambiae, transient somatic infections of wMelPop were created by intrathoracic inoculation. Upregulation of six selected immune genes was observed compared to controls, at least two of which (LRIM1 and TEP1) influence the development of malaria parasites. A stably infected An. gambiae cell line also showed increased expression of malaria-related immune genes. Highly significant reductions in Plasmodium infection intensity were observed in the wMelPop-infected cohort, and using gene knockdown, evidence for the role of TEP1 in this phenotype was obtained. Comparing the levels of upregulation in somatic and stably inherited wMelPop infections in Ae. aegypti revealed that levels of upregulation were lower in the somatic infections than in the stably transinfected line; inhibition of development of Brugia filarial nematodes was nevertheless observed in the somatic wMelPop infected females. Thus we consider that the effects observed in An. gambiae are also likely to be more pronounced if stably inherited wMelPop transinfections can be created, and that somatic infections of Wolbachia provide a useful model for examining effects on pathogen development or dissemination. The data are discussed with respect to the comparative effects on malaria vectorial capacity of life shortening and direct inhibition of Plasmodium development that can be produced by Wolbachia.

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• Wolbachia
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Plasmodium falciparum is a blood parasite that lives for the most part inside red cells. It is responsible for the death of 1-2 million people through malaria every year. The mechanisms by which the parasite invades red cells are complex and not completely understood. For many years it has been known that proteins called glycophorins are used by the parasite to gain entry into the red cell. However, the existence of another protein that allows entry independent of glycophorins has been suspected for nearly as long. The identity of the alternative protein has been a difficult mystery to solve.

Researchers have now identified an alternative protein used by P. falciparum to invade red blood cells. The results may aid the development of a future vaccine for malaria. The researchers identified complement receptor 1 as the protein that enables P. falciparum to invade red blood cells. Proteins called glycophorins are used by the parasite to gain entry into the red cell. However, because infection can take place without glycophorins, researchers suspected that another protein is also involved. Complement receptor 1 (CR1), also known to help protect red cells from attack by the immune system, has been suspected of having other roles in the development of malaria complications. The team was able to demonstrate that this protein is important in the invasion of red cells by using several laboratory strains of malaria as well as strains obtained from Kenya.

P. falciparum may use the CR1 protein instead of glycophorins if the parasite encounters a variant that lacks the glycophorin receptor; if the immune system mounts a response against parasite proteins involved in the dominant pathway due to a previous infection; or if the host were treated with a vaccine that blocks the glycophorin pathway. The recognition of the additional role of complement receptor 1 in red cell invasion will allow the definitive identification of malaria proteins that interact with it and that could be used in a future vaccine cocktail to block red cell invasion.

Complement Receptor 1 Is a Sialic Acid-Independent Erythrocyte Receptor of Plasmodium falciparum. 2010 PLoS Pathog 6(6): e1000968. doi:10.1371/journal.ppat.1000968
Plasmodium falciparum is a highly lethal malaria parasite of humans. A major portion of its life cycle is dedicated to invading and multiplying inside erythrocytes. The molecular mechanisms of erythrocyte invasion are incompletely understood. P. falciparum depends heavily on sialic acid present on glycophorins to invade erythrocytes. However, a significant proportion of laboratory and field isolates are also able to invade erythrocytes in a sialic acid-independent manner. The identity of the erythrocyte sialic acid-independent receptor has been a mystery for decades. We report here that the complement receptor 1 (CR1) is a sialic acid-independent receptor for the invasion of erythrocytes by P. falciparum. We show that soluble CR1 (sCR1) as well as polyclonal and monoclonal antibodies against CR1 inhibit sialic acid-independent invasion in a variety of laboratory strains and wild isolates, and that merozoites interact directly with CR1 on the erythrocyte surface and with sCR1- coated microspheres. Also, the invasion of neuraminidase-treated erythrocytes correlates with the level of CR1 expression. Finally, both sialic acid-independent and dependent strains invade CR1 transgenic mouse erythrocytes preferentially over wild-type erythrocytes but invasion by the latter is more sensitive to neuraminidase. These results suggest that both sialic acid-dependent and independent strains interact with CR1 in the normal red cell during the invasion process. However, only sialic acid-independent strains can do so without the presence of glycophorin sialic acid. Our results close a longstanding and important gap in the understanding of the mechanism of erythrocyte invasion by P. falciparum that will eventually make possible the development of an effective blood stage vaccine.

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• New ways to beat malaria
• Researchers characterize potential protein targets for malaria vaccine
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Malaria caused by Plasmodium falciparum is a disease that is responsible for 880,000 deaths per year worldwide. Vaccine development has proved difficult and resistance has emerged for most antimalarial drugs. To discover new antimalarial chemotypes, researchers have used a chemical genetic approach to assay 309,474 chemicals. Many chemicals in the library of compounds tested showed potent in vitro activity against drug-resistant P. falciparum strains. A reverse chemical genetic study identified 19 new inhibitors of 4 validated drug targets and 15 novel binders among 61 malarial proteins. Phylochemogenetic profiling in several organisms revealed similarities between Toxoplasma gondii and mammalian cell lines and dissimilarities between P. falciparum and related protozoans. One exemplar compound displayed efficacy in a mouse model. These findings provide the scientific community with new starting points for malaria drug discovery.

Chemical genetics of Plasmodium falciparum. Nature 465, 311–315 (2010) doi:10.1038/nature09099

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• Drug discovery: Priming the antimalarial pipeline
• Nature Collections – Malaria
• New ways to beat malaria

Malaria and alcohol consumption both represent major public health problems. Alcohol consumption is rising in developing countries and, as efforts to manage malaria are expanded, understanding the links between malaria and alcohol consumption becomes crucial. Our aim was to ascertain the effect of beer consumption on human attractiveness to malaria mosquitoes in semi field conditions in Burkina Faso. We used a Y tube-olfactometer designed to take advantage of the whole body odour (breath and skin emanations) as a stimulus to gauge human attractiveness to Anopheles gambiae (the primary African malaria vector) before and after volunteers consumed either beer (n = 25 volunteers and a total of 2500 mosquitoes tested) or water (n = 18 volunteers and a total of 1800 mosquitoes). Water consumption had no effect on human attractiveness to An. gambiae mosquitoes, but beer consumption increased volunteer attractiveness. Body odours of volunteers who consumed beer increased mosquito activation (proportion of mosquitoes engaging in take-off and up-wind flight) and orientation (proportion of mosquitoes flying towards volunteers’ odours). The level of exhaled carbon dioxide and body temperature had no effect on human attractiveness to mosquitoes. Despite individual volunteer variation, beer consumption consistently increased attractiveness to mosquitoes. These results suggest that beer consumption is a risk factor for malaria and needs to be integrated into public health policies for the design of control measures.

Beer Consumption Increases Human Attractiveness to Malaria Mosquitoes. PLoS ONE 5(3): e9546. doi:10.1371/journal.pone.0009546

The malaria parasite life cycle constitutes one of the most complicated and fascinating life cycles of any organism and thus poses intriguing areas of study for cell biology, molecular biology, and immunology alike. Malaria as a disease is devastating developing countries, especially those in sub-Saharan Africa, causing approximately one million deaths each year, which are mainly attributable to a single parasite species, Plasmodium falciparum. The intricacy of malaria parasite biology has vexed vaccinologists and immunologists for nearly a century and is a major impediment to the development of a fully protective vaccine. A major part of the complexity associated with the malaria parasite life cycle is due to the parasite’s ability to change its cellular and molecular makeup, which is controlled by a genome with more than 5000 recognized genes, and develop in intracellular and extracellular niches in the mammalian host and the mosquito vector.

Plasmodium sporozoites are the product of a complex developmental process in the mosquito vector and are destined to infect the mammalian liver. Attention has been drawn to the mosquito stages and pre-erythrocytic stages owing to recognition that these are bottlenecks in the parasite life cycle and that intervention at these stages can block transmission and prevent infection. Parasite progression in the Anopheles mosquito, sporozoite transmission to the mammalian host by mosquito bite, and subsequent infection of the liver are characterized by extensive migration of invasive stages, cell invasion, and developmental changes. Preparation for the liver phase in the mammalian host begins in the mosquito with an extensive reprogramming of the sporozoite to support efficient infection and survival. This review discusses what is known about the molecular and cellular basis of the developmental progression of parasites and their interactions with host tissues in the mosquito and during the early phase of mammalian infection.

Malaria parasite development in the mosquito and infection of the mammalian host. Ann Rev Microbiol. 2009 63: 195-221

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• The origin of malignant malaria
• Nature Collections – Malaria
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Anopheles gambiae mosquitoes are the principal vectors of human malaria, a disease with devastating consequences for public health and the economic development of disease-endemic countries. The creation of new tools to control vector populations is a focal point of intensive efforts to eradicate the burden of malaria. As mosquitoes generally copulate only once during their lives, interfering with the mating process is a promising avenue for research into vector control. Unfortunately, very little is known about the molecular or physiological basis of mating and insemination in malaria vectors. Of particular concern is our lack of knowledge about factors and pathways ensuring male reproductive success, such as those that result in sperm storage, oviposition, and the inhibition of remating in females. Improving our understanding of mating biology might not only inform currently proposed strategies for vector control, but could potentially allow the development of entirely novel tools for combating malaria.

Stopping male mosquitoes from sealing their sperm inside females with a ‘mating plug’ could prevent mosquitoes from reproducing, and offer a potential new way to combat malaria. The new study focuses on An. gambiae, the species of mosquito primarily responsible for the transmission of malaria in Africa. These mosquitoes mate only once in their lives, which means that disrupting the reproductive process offers a good way of dramatically reducing their populations. When these mosquitos mate, the male transfers sperm to the female and then afterwards transfers a coagulating mass of proteins and seminal fluids known as a mating plug. This plug is not found in any other species of mosquito and until now, very little was known about the role it plays in An. gambiae reproduction. The authors show that the mating plug is essential for ensuring that sperm is correctly retained in the female’s sperm storage organ, from where she can fertilise eggs over the course of her lifetime. Without the mating plug, sperm is not stored correctly, and fertilisation cannot occur.

The researchers analysed the composition of the protein-rich mating plug and discovered that it is formed when an enzyme called transglutaminase interacts with other proteins in the male mosquito’s seminal fluid. This interaction causes the seminal fluids to coagulate into a gelatinous solid mass. When the research team removed this enzyme in male mosquitoes in the lab, the fluids failed to coagulate and form the plug. Furthermore, when these males, lacking the key protein and therefore the plug, mated with females, reproduction was not successful. The male mating plug is not a simple barrier to insemination from rival males, as has been previously suggested. Instead, the plug plays an important role in allowing the female to successfully store sperm in the correct way inside her, and as such is vital for successful reproduction. If in the future we can develop an inhibitor that prevents the coagulating enzyme doing its job inside male An. gambiae mosquitoes in such a way that can be deployed easily in the field – for example in the form of a spray as it is done with insecticides – then we could effectively induce sterility in female mosquitoes in the wild. This could provide a new way of limiting the population of this species of mosquito, and could be one more weapon in the arsenal against malaria.

Transglutaminase-Mediated Semen Coagulation Controls Sperm Storage in the Malaria Mosquito. 2009 PLoS Biol 7(12): e1000272. doi:10.1371/journal.pbio.1000272
Insect seminal fluid proteins are powerful modulators of many aspects of female physiology and behaviour including longevity, egg production, sperm storage, and remating. The crucial role of these proteins in reproduction makes them promising targets for developing tools aimed at reducing the population sizes of vectors of disease. In the malaria mosquito Anopheles gambiae, seminal secretions produced by the male accessory glands (MAGs) are transferred to females in the form of a coagulated mass called the mating plug. The potential of seminal fluid proteins as tools for mosquito control demands that we improve our limited understanding of the composition and function of the plug. Here, we show that the plug is a key determinant of An. gambiae reproductive success. We uncover the composition of the plug and demonstrate it is formed through the cross-linking of seminal proteins mediated by a MAG-specific transglutaminase (TGase), a mechanism remarkably similar to mammalian semen coagulation. Interfering with TGase expression in males inhibits plug formation and transfer, and prevents females from storing sperm with obvious consequences for fertility. Moreover, we show that the MAG-specific TGase is restricted to the anopheline lineage, where it functions to promote sperm storage rather than as a mechanical barrier to re-insemination. Taken together, these data represent a major advance in our understanding of the factors shaping Anopheles reproductive biology.

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• New methods of biocontrol for malaria
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Short Disney film (The Winged Scourge, 1943):

via Twisted Bacteria, via Microbeworld