Drosophila models have proven invaluable for the study of skeletal and cardiac muscle diseases. Although permeabilized indirect flight muscle (IFM) and jump muscle fibers have provided mechanical insights, these preparations are not suitable for characterizing muscle activation and relaxation kinetics because calcium diffusion is rate limiting across the diameter of permeabilized fibers. In contrast, myofibril preparations, with diameters of approximately 1 μm, allow for rapid calcium exchange and are ideal for measuring physiologically relevant activation and relaxation rates. Previous attempts to develop an IFM myofibril preparation have failed to produce myofibrils that generate measurable active force, likely due to IFM’s inherently low force output. To address this, we developed a myofibril preparation from Drosophila jump muscles, which produce much higher forces than IFM. By applying brief, low-amplitude sonication to permeabilized jump muscles, we successfully isolated myofibrils that produced a net active specific force of 19.8 ± 10.5 mN/mm 2 , the first active force measurements from an insect muscle, with an activation rate of 8.2 ± 4.0 s −1 . Jump myofibrils exhibited the same characteristic bi-phasic relaxation observed in vertebrate myofibrils, consisting of an initial slow, linear phase lasting 82 ± 9 ms, followed by a fast, exponential decay phase with a rate of −19.7 ± 9.6 s −1 . We also characterized myofibrils from jump muscles expressing a transgenic larval myosin isoform, EMB, which we previously reported possesses slower actin-binding and detachment kinetics than the native jump muscle isoform. EMB expression caused a 1.6-fold increase in active specific force, a 38% decrease in activation rate, but did not change relaxation parameters. These findings highlight the utility of transgenic Drosophila jump muscle myofibrils, expressing customizable sarcomeric proteins, to dissect their contributions to muscle activation and relaxation.
Fenwick et al. (Sun,) studied this question.