The best sugar in town for malaria transmission

Abstract

Anopheles mosquitoes feed on plant nectars as their main source of sugar. Wang et al. show that Asaia bacteria proliferate in the midgut of mosquitoes that feed on glucose or trehalose. Asaia increases the lumenal pH by down-regulating mosquito vacuolar ATPase expression, therefore increasing Plasmodium gametogenesis and vector competence.

Mosquitoes feed on plant nectars for the sugar that is their major source of energy and which is also used for the synthesis of amino acids, nucleotides, and storage molecules like triglycerides, trehalose, and glycogen. It has been shown that plant nectar defines the composition of the microbiome in Anopheles mosquitoes [1], and that the composition of the microbiome in the mosquito midgut affects the infectivity of Plasmodium to mosquitoes [2]. However, a direct link on how the composition of nectar sugar could affect vectorial capacity through modulation of the microbiome remains unexplored. By integrating metabolomics, RNAseq, and reverse genetic analysis, Wang et al. [3] investigated the role of plant nectar in influencing the infectivity of Plasmodium parasites to Anopheles stephensi mosquitoes. Metabolomic analysis of Plasmodium berghei-infected and noninfected mosquitoes showed a significant reduction in trehalose, glucose, succinate, and citrate, and an increase in pyruvate and acetate, suggesting an enhanced glycolytic activity induced by infection. Mosquitoes maintained on either a trehalose or glucose diet for 5 days prior to P. berghei infection had significantly higher oocyst numbers than mosquitoes kept on a sucrose diet. Interestingly, the number of Plasmodium falciparum oocysts increased in mosquitoes fed on trehalose but not on glucose or sucrose. The authors speculate that this could be because of the artificial nature of P. falciparum in vitro culturing which does not necessarily reflect physiological sub-strate concentrations [4]. Taken together, these results show that the mosquito’s sugar feed influences Plasmodium infectivity.

Interestingly, antibiotic treatment of mosquitoes prior to infection prevented the parasite-induced decrease in these carbohydrates, suggesting that the microbiota controls the metabolic changes observed upon P. berghei infection. P. berghei oocyst numbers were also reduced in antibiotic-treated mosquitoes, confirming previous reports. Pyrosequencing data revealed that mosquitoes fed on a glucose diet had a higher proportion of the bacterium Asaia bogorensis compared with sucrose-fed mosquitoes, showing that a glucose diet alters the midgut microbiome, favoring A. bogorensis proliferation. A. bogorensis is a symbiont commonly present in the midgut of Anopheles mosquitoes. To gauge the effect of A. bogorensis on parasite infectivity, mosquitoes were recolonized with A. bogorensis after antibiotic treatment and then maintained on either sucrose or glucose. Recolonization with A. bogorensis increased the number of Plasmodium oocysts only in glucose-fed mosquitoes. These experiments show that A. bogorensis proliferation in mosquitoes is facilitated by glucose and that the increase in A. bogorensis enhances Plasmodium infectivity. Of interest, the ingestion of a blood meal containing antibiotics increased both Asaia abundance after 24 h and oocyst formation in Anopheles gambiae mosquitoes maintained on fructose [5]. These findings support the positive correlation between Asaia levels and parasite infectivity reported by Wang et al. [3]. Future work should ascertain the effect of sugar in the microbiome composition of various mosquito species.

RNAseq analysis revealed the downregulation of genes encoding the vacuolar ATPase (V-ATPase) in the midguts of mosquitoes fed on glucose as compared to sucrose. The authors showed that glucose or trehalose feeding increased lumenal pH, which correlated with decreased expression of V-ATPase (Figure 1). RNAi silencing of V-ATPase gene expression in sucrose-fed mosquitoes increased the midgut pH and P. berghei infectivity. Since V-ATPase decreases the pH of the midgut by pumping protons toward the lumenal side [6], these findings imply that glucose metabolized by Asaia might increase the midgut pH. In addition, these results support the findings showing that an increase in midgut pH is one of the main signals inducing Plasmodium gamete activation and male exflagellation [7]. A basic midgut pH is also required for activation of proteases required for blood digestion and for detoxification of ferrous iron [8]. Therefore, the impact that different sugar sources might have in these physiological processes deserves further investigation. Since Plasmodium has its own V-ATPase and pH regulatory pathway [9], it will also be interesting to examine if such a pathway is also involved in regulating glucose/Asaia-mediated pH changes.

Figure 1. The mosquito’s sugar diet shapes vector competence during Plasmodium infection.

Upper panel. Glucose and trehalose promote Plasmodium berghei infectivity. Mosquitoes feeding on a diet rich in glucose or trehalose sustain the proliferation of Asaia bogorensis in the midgut. Wang et al. [3] propose that glucose or trehalose catabolized by A. bogorensis triggers a downregulation of mosquito V-ATPase by a yet unknown mechanism. Downregulation of the V-ATPase proton transporter elevates midgut lumenal pH which, in turn, promotes gametogenesis by stimulating exflagellation. These interactions increase the number of oocysts in the midgut, and therefore, vector competence. Lower panel. Mosquitoes feeding on sucrose reduce the abundance of A. bogorensis, thus lowering sugar metabolism in the mosquito’s lumen. Consequently, this triggers the upregulation of the mosquito’s V-ATPase and the acidification of lumenal pH, therefore hindering Plasmodium gametogenesis and vector competence.

Finally, Wang et al. [3] examined the effect of different plant nectars on Plasmodium infectivity. By feeding mosquitoes on combinations of sugars mimicking the nectar composition of plants commonly present in malaria-endemic regions, they found that artificial nectars containing higher concentrations of glucose positively correlate with the Asaia abundance in the midgut and the parasite oocyst numbers (Figure 1). Their data suggest that the nectar of certain plants in malaria-endemic areas can promote Asaia colonization and Plasmodium infectivity to mosquitoes, which could enhance malaria transmission in neighborhoods with a high density of these types of plants. Plants with nectars that do not promote parasite infectivity could be grown in these neighborhoods, although this approach has some limitations as mosquitoes are selective and attracted to some plants [10]. In the field, the composition of nectar and sap is complex, and it includes amino acids, vitamins, minerals, and powerful antioxidant molecules that could influence the mosquito microbiome population and parasite infection. Further studies will be important to establish the link between natural plant feeding and vector competence, and to identify new molecules that influence microbiota composition and mosquito physiology. In addition, the mechanism by which Plasmodium reduces mosquito sugars, shown in the metabolomic experiments, remains to be elucidated.

This study establishes for the first time a causal relationship in which the mosquito sugar diet alters the midgut microbiota composition, which then regulates the expression of V-ATPases. A downregulation of V-ATPases by Asaia proliferation on glucose-rich diet induces a reduction in midgut pH and an increase in parasite infectivity. The findings are timely and fill an important knowledge gap about the molecular interaction among plant nectars, gut microbiota, and the Plasmodium parasite.

Declaration of interests

The authors declare no competing interests.