Like humans, mosquitoes are prone to picking up nasty infections, and like us, they also have a specialised immune system that fights off undesirable invaders. This defensive system, found in haemolymph, is propelled through the body by the heart. However, very little is known about how the mosquito heart controls haemolymph circulation and Julián Hillyer, an insect physiologist from Vanderbilt University, USA, wondered, ‘If haemocirculation is so important for how a mosquito fights infection, then how do we know so little about it?’ Intrigued, Hillyer decided to investigate what makes a mosquito’s heart tick and turned to CCAP, a neurohormone peptide, which he knew regulates cardiac function in a wide range of other arthropods, including crabs and fruit flies (p. 601).
Recruiting the help of his research assistant Tania Estévez-Lao and an undergraduate student, Dacia Boyce, Hillyer and his team carefully reared over 300 adult mosquitoes to begin their investigation into the role of the CCAP hormone in haemocirculation. Taking advantage of the mosquitoes’ see-through cuticle, the team was able to visualise and record the changes in the wave-like heart contractions following CCAP injection. They were also able to measure the haemolymph velocity after treating the mosquitoes with CCAP by injecting tiny fluorescent particles into the body and tracking their movement through the mosquitoes’ heart using a microscope.
These elegant experiments, whilst almost routine now, were far from trivial at first, remembers Hillyer. He recalls that it was very tricky to restrain the mosquitoes because they were prone to escaping. However, the hours spent delicately handling and recording the mosquitoes’ heart rate eventually paid off. After injection of CCAP into the mosquito’s body, the team saw an increase in heart rate of up to 28%, rising to over two beats per second. Exposure to the hormone also sped up haemolymph flow to a speedy 8.6 mm s–1, with a maximum increase in the velocity of up to 33%. To confirm that CCAP was responsible for these racing hearts, the team then reduced expression of CCAP and saw that as they lowered production of the neurohormone they also slowed down heart contraction rates.
Having established the role of CCAP in speeding up heartbeats, the team next wanted to determine exactly where CCAP was being produced. By dissecting the mosquitoes’ head, thorax and abdomen, they were able to determine that CCAP was mostly produced in the head. With the expertise of colleague Hans-Willi Honegger, they were further able to pinpoint expression of the hormone to specific neurons in the brain that project onto the upper part of the circulatory system.
This is the first time that the adult mosquito heart has been shown to be under partial neuronal control, remarks Hillyer. However, perhaps the most exciting aspect for Hillyer is the new opportunity that CCAP offers to manipulate heart rates, allowing him to investigate how changes in haemolymph flow can affect a mosquito’s ability to fight off infections and hopefully avoid passing them onto us the next time they snack on our blood.