Significance of Calcium Signaling in Flight of Transgenic Drosophila Expressing Channelrhodopsin in Asynchronous Flight Muscles

This abstract was presented at the NeuroDoWo Conference, University of Regensburg (2018). The study investigates calcium-mediated control of flight muscle mechanics in transgenic Drosophila using optogenetic activation. Locomotion of animals is required for localization of food and mates, escape from predators, defense of territory, and response to stress and is thus an integral component of most animal behaviors. The energetic cost of locomotion broadly outscores all other metabolic activities in animals and varies as animals keep on changing walking, swimming or flight speed. Locomotor muscles compensate these changes by varying their mechanical power output. This requires adjustments of neural activity in the animal’s brain, resulting in fine adjustments of intracellular calcium concentration in locomotor muscles. In Drosophila, asynchronous flight muscles (AFM) reside in a thoracic shell that simultaneously drives both wings during wing flapping. Calcium is the major cellular component that regulates the contraction dynamics in muscle cells by controlling the molecular mechanism with which contractile proteins bind to each other. To study calcium control in AFM, we employed the Gal4/UAS system and genetically modified muscle calcium control by genetic insertion of the calcium channel channelrhodopsin, expressed under the Act88F promotor. By application of laser light, the selected approach allows us to elicit artificial changes in intracellular calcium concentration in flying flies, without invasive, experimental interference with the animal. During level flight, AFM power output precisely matches the power requirements for wing flapping. If the animal provides calcium-depended mechanical power in excess to what is actually needed for wing flapping, the wing hinge must destroy waste power to avoid structural damages, which leads to a reduction in flight efficiency. Alternatively, the animal might allow changes wing kinematics that potentially leads to unwanted changes in body posture stability. To distinguish between these two strategies, we tested the consequences of laser-triggered calcium activation by flying transgene fruit flies in a virtual reality flight simulator under closed-loop feedback control. Drosophila is sensitive to motion in any part of its visual field. The visual cue was provided to keep the fly flying in the flight arena. Data show that during laser activation, muscle calcium changes stroke amplitude only weakly, while stroke frequency decreases. The latter result is quite unexpected because aerodynamic theory predicts that an increase in muscle calcium concentration increases muscle power that should lead in turn to an increase in wing flapping frequency and not vice versa. The finding might highlight a more sophisticated control mechanism for wing kinematics as previously assumed. Since Act88F expression is specific for AFM, a possible explanation for our results is that the increase of calcium influx causes an increase in muscle stiffness, hampering thorax resonance and thus stretch activation of the asynchronous flight muscle. In sum, our study highlights the need for precise calcium control in AFM of Drosophila for flight control as previously suggested by experiments using calcium imaging approaches.