Blood Meal-Derived Heme Decreases ROS Levels in the Midgut of Aedes aegypti and Allows Proliferation of Intestinal Microbiota

The presence of bacteria in the midgut of mosquitoes antagonizes infectious agents, such as Dengue and Plasmodium, acting as a negative factor in the vectorial competence of the mosquito. Therefore, knowledge of the molecular mechanisms involved in the control of midgut microbiota could help in the development of new tools to reduce transmission. We hypothesized that toxic reactive oxygen species (ROS) generated by epithelial cells control bacterial growth in the midgut of Aedes aegypti, the vector of Yellow fever and Dengue viruses. We show that ROS are continuously present in the midgut of sugar-fed (SF) mosquitoes and a blood-meal immediately decreased ROS through a mechanism involving heme-mediated activation of PKC. This event occurred in parallel with an expansion of gut bacteria. Treatment of sugar-fed mosquitoes with increased concentrations of heme led to a dose dependent decrease in ROS levels and a consequent increase in midgut endogenous bacteria. In addition, gene silencing of dual oxidase (Duox) reduced ROS levels and also increased gut flora. Using a model of bacterial oral infection in the gut, we show that the absence of ROS resulted in decreased mosquito resistance to infection, increased midgut epithelial damage, transcriptional modulation of immune-related genes and mortality. As heme is a pro-oxidant molecule released in large amounts upon hemoglobin degradation, oxidative killing of bacteria in the gut would represent a burden to the insect, thereby creating an extra oxidative challenge to the mosquito. We propose that a controlled decrease in ROS levels in the midgut of Aedes aegypti is an adaptation to compensate for the ingestion of heme.
Author Summary Mosquitoes are vectors of human pathogens, such as Dengue virus and Malaria parasites, which profoundly affect health worldwide, killing millions of people annually. Recent studies have demonstrated that the presence of bacteria in the gut of mosquitoes is able to antagonize the establishment of pathogens. Therefore, mechanisms with the potential to regulate bacterial growth in the digestive tract of mosquitoes may hamper disease transmission. Here, we show that reactive oxygen species (ROS) are present in the gut epithelia of sugar-fed mosquitoes and are drastically reduced after blood feeding through a mechanism that involves activation of protein kinase C by heme. ROS levels are inversely correlated with the presence of bacteria in the midgut and therefore we investigated if ROS are involved in fighting bacterial infections in the gut. We discovered that mosquitoes producing low levels of ROS challenged with an oral bacterial infection exhibited increased mortality due to intense bacterial proliferation and epithelial cell damage. We propose that reduction in ROS levels after blood feeding is an adaptation to compensate for the ingestion of blood, a pro-oxidant meal. This finding has consequences for the understanding of disease transmission due to the ability of ROS to modulate the gut bacterial levels.