Drosophila jump muscle myofibrils: A new tool for investigating activation and relaxation

Drosophila models have proven invaluable for studying skeletal and cardiac muscle diseases. While permeabilized indirect flight muscle (IFM) and jump muscle fibers from Drosophila yield insightful mechanical data, these preparations cannot resolve the kinetics of activation and relaxation because calcium diffusion into the fiber core is rate limited at this scale. In contrast, myofibrils have a diameter of only 1-3 μm, making them ideal for measuring physiologically relevant activation and relaxation rates. However, previous attempts using IFMs failed to produce myofibrils that generated measurable active force, likely due to the muscle's inherently low force output. Therefore, instead of using IFMs as the source, we developed a method to isolate myofibrils from the Drosophila jump muscle. By applying brief, low-amplitude sonication to permeabilized jump muscles, we isolated myofibrils that produced 19.8 ± 10.5 mN/mm² net active tension, the first active force measurements from an insect myofibril, with an activation rate of 8.2 ± 4.0 s^-1. Jump muscle myofibrils exhibited the typical biphasic relaxation seen in vertebrates: an initial slow, linear phase lasting 75.6 ± 21.1 ms, followed by a fast exponential decay with a rate constant of 19.7 ± 9.6 s^-1. We also characterized myofibrils from jump muscles transgenically expressing a Drosophila larval body wall myosin isoform (EMB), which we previously reported displays slower actin-binding and detachment kinetics. EMB expression caused a 1.6-fold increase in active tension and a 38% slower activation rate compared with controls but did not change relaxation parameters. These characterizations and comparisons highlight the scientific value of expressing customizable sarcomeric proteins in transgenic Drosophila jump muscle myofibrils to elucidate their contributions to muscle activation and relaxation.