Adult Drosophila Muscle Reassembled Disrupted Myofibrils Within One Week; the Same Damage Accumulates During Normal Aging

Adult Drosophila Muscle Reassembled Disrupted Myofibrils Within One Week; the Same Damage Accumulates During Normal Aging

On September 15, biologists from Dalhousie University posted an unreviewed preprint on bioRxiv showing how the femur muscle of an adult Drosophila recovers after severe injury. Structure and locomotion returned nearly to baseline within one week, and the decisive factor was the protein filamin, whose ability to toggle between two conformations directly at the damage site proved essential.

A myofibril is a filament inside a muscle fiber whose shortening generates contractile force; it consists of repeating sarcomeres separated by Z-discs. The authors chose the femur muscle because, unlike the wing (which would require repeated dissection), the femur is visible through the transparent cuticle, allowing the same muscle in the same fly to be imaged over several days. Damage was induced by two methods: a light-sensitive or heat-sensitive ion channel was genetically expressed in the muscle beforehand, and the fly was then exposed to red light or heated to 37°C for one to two hours. Both approaches trigger sustained contraction in the muscle without surgery.

Immediately after stimulation the fly was nearly paralyzed, but over seven days walking speed returned to baseline regardless of the method of injury. A fluorescent Z-disc marker showed that 50–60% of the muscle area lost its structure completely right away, yet this fraction dropped to zero by day three. Electron microscopy confirmed the same picture: the spacing between Z-discs expanded nearly fivefold, from 191 to 897 nanometers, and returned to the baseline range of 187–191 over one week.

In vertebrates, large muscle tears are repaired by satellite stem cells, while focal myofibril damage within an intact fiber is healed by the fiber itself, without new cells: the related protein filamin C moves to the injury site. In insects this type of repair has barely been studied. In 2024 the same laboratory showed in Drosophila flight muscle that filamin switching protects the Z-disc from rupture during contraction, but did not test whether the muscle can reassemble a myofibril de novo after severe destruction. The present work answers that question.

The repair depends on filamin, a protein that sits on the Z-disc and crosslinks actin filaments of adjacent sarcomeres; its flexible domain opens and closes under mechanical load. The authors compared wild-type filamin with two locked variants, one closed and one open. The closed variant, like wild-type, left the Z-disc after injury, but the resulting damage was three times more severe and did not fully resolve even after one week. The open variant did not leave its position and barely impeded recovery. Repair therefore depends on the mobility of this conformational switch, and locking it in the closed state abolishes that mobility.

The same signature, filamin accumulation and its departure from the Z-disc, appeared in aging flies that had received no injury at all. In flies carrying normal filamin, mild damage was already visible by the second week of life; severe damage appeared by the third week and continued to worsen. In flies carrying two copies of the closed variant, protein aggregates appeared as early as the first week. A separate optical assay showed the same trend: the structural integrity of the muscle declined with age in all lines, most sharply in flies whose filamin switching was impaired.

The same filamin C in humans, when mutated, causes hereditary myofibrillar myopathies, a form of muscle weakness resulting from the accumulation of damaged sarcomeric structures. The authors state their conclusion as follows:

This suggests that, as in vertebrates, the ability of muscle to efficiently repair and reassemble itself declines with age.