Mosquitoes are cosmopolitan pests and disease vectors. Current methods for the control of mosquito populations rely primarily on chemical or biological agents that exert their insecticidal function in the mosquito hemocoel (body cavity) or in the gut. When in the hemocoel, insecticidal agents are spread throughout the body by the natural flow of hemolymph (blood) and are exposed to mosquito immune and detoxifying factors. In addition, pathogens transmitted by mosquitoes undergo an obligate migration through the hemocoel where they must also overpower mosquito immune and detoxifying factors. While characterizing the structural mechanics of the mosquito heart, Hillyer and colleagues observed that systemic bacterial infection elicited immune cells to aggregate in areas surrounding the valves of the heart (periostial regions), leading them to hypothesize that the circulatory and immune systems work in concert to eliminate systemic infections. The goal of this research is to characterize the interaction between immune cells and the periostial regions of the mosquito heart in an effort to uncover fundamental aspects of the physiological interaction between the mosquito circulatory and immune systems. Objective 1 will employ imaging methodologies to structurally characterize periostial immune cell foci, determine the kinetics of periostial immune cell foci formation, and test whether heart contraction dynamics change in response to bacterial infection. Objective 2 will employ molecular and imaging methodologies to uncover hormones that regulate mosquito heart activity. Finally, objective 3 will test whether manipulating heart contraction dynamics alters the kinetics of periostial immune cell foci formation and the ability of mosquitoes to survive a systemic infection. Data arising from these experiments could aid in the development of novel pest and disease control strategies. In addition to its scientific impact, this project will result in the training of graduate, undergraduate and high school students in biological research.
This project investigated the biology of an immune response that occurs on the surface of the mosquito heart, and explored the hormonal control of mosquito heart contractions. The project was based on the observation that during the course of an infection, pathogens accumulate in distinct regions of the heart, and that these regions correlate with the locations of the heart valves, called ostia (these regions were named the periostial regions). At the beginning of the project, the investigators hypothesized that the accumulation of pathogens in the periostial regions of the heart was due to an active and inducible cellular immune response. Thus, among other things (see the publication list), the investigators sought to characterize the biology behind the aggregation of pathogens at this anatomical location. The investigators found that, even in the absence of infection, immune cells, called hemocytes, are present at the periostial regions of the heart. Within seconds of infection, the periostial hemocytes capture, phagocytose, and degrade pathogens that are flowing with the hemolymph (insect blood), with this capture happening immediately prior to these pathogens gaining entry into the heart. As this occurs, additional hemocytes migrate to the periostial regions where they bind the periostial hemocytes as well as the cardiac musculature, which leads to an amplification of the phagocytosis response. The process of periostial hemocyte aggregation occurs in a time and infection dose-dependent manner, and once it is initiated, the number of periostial hemocytes remains elevated for the lifetime of the mosquito. After identifying the cellular nature of pathogen aggregation on the surface of the heart, the investigators examined whether pathogens and hemocytes also aggregate in other areas of this insect, and found that this was not the case. Regardless of the status or time of infection, the areas of highest density of hemocytes and pathogens are the periostial regions of the heart. Altogether, the findings in this portion of the project identified a novel immune response in mosquitoes, with the investigators hypothesizing that the location of this response is advantageous for the insect, as it places immune cells in areas of high hemolymph flow. In light of this, the investigators turned their attention to identifying mosquito factors that may control the flow of hemolymph and the rhythmicity of heart contractions. They initially found that a neuropeptide called corazonin has little impact on mosquito heart physiology. They then found that a neuropeptide called CCAP as well as two FMRFamide-containing peptides modulate mosquito heart rhythmicity. Specifically, CCAP is cardioacceleratory whereas FMRFamide-containing peptides are cardioacceleratory when present at low levels but cardioinhibitory when present at high levels. Overall, this project described a novel immune response of mosquitoes that occurs on the surface of the heart, and identified endogenous factors that modulate mosquito heart physiology. This work was carried out in mosquitoes, an important insect group because of its role as a pest and as a pathogen transmitter. Most of the work was conducted in Anopheles gambiae, which is a major vector of malaria in Africa. Thus, data arising from these experiments could one day contribute to the development of novel pest and disease control strategies. In addition to the experimental findings, this project was also fruitful in providing mentorship to developing scientists, as a large proportion of the experiments were carried out by graduate students, undergraduate students, and high school students.
