Insects are among the most successful animals that have conquered air. As flying is energetically demanding, they have evolved ingenious ways to reduce the energetic costs of flight. Modern insects have developed indirect flight muscles with a remarkable property called ‘stretch activation’, where the contraction of the muscle is triggered by stretching. Since its discovery, scientists have looked for a mechanistic explanation for this phenomenon. In a recent paper published in PNAS a team of US scientists guided by Mike Reedy provides fascinating insights into the underlying mechanism by observing X-ray diffraction patterns of beating flight muscles.
The benefit of stretch activation is evident when we look at the anatomy of insect indirect flight muscles. They do not move the wings directly but change the shape of the elastic thorax, which translates into up and down movements of the wings. The muscles are organized in antagonistic pairs of longitudinal depressors and dorsoventral elevators. When the depressor contracts it stretches and activates the elevator, and vice versa, yielding freely oscillating muscle contractions and buzzing wings. But how can stretch activation be explained at the molecular level? In normal muscle fibers, muscular contractions, where actin and myosin filaments slide past each other, are triggered by increased levels of Ca2+. In resting muscles, the interaction between actin and myosin is blocked by tropomyosin but, when a contraction is initiated, Ca2+ binds to a protein called troponin, which induces a conformational change in tropomyosin so that it no longer prevents myosin from binding to actin. However, insect flight muscles are special as a Ca2+ stimulus is still necessary to prime contraction but it alone is not sufficient to unlock the tropomyosin bar. So, what is the switch that is necessary to open the gate?
To answer this question, Reedy and his colleagues studied the flight muscles of the giant water bug Lethocerus, which is a convenient model system because the isolated muscle fibers oscillate for several hours. Using electron microscopy, they first found novel cross-bridges between actin and myosin filaments involving troponin, which they named troponin bridges. Then, in a remarkable experiment, the team exposed the oscillating muscle to a very intense beam of X-rays, and recorded the muscle's diffraction patterns as they varied in time with the muscular contractions.
The results of this experiment were stunning: an X-ray movie showing the movements of single molecules within the oscillating muscle fibers. As the scientists were able to assign single reflections to myosin heads, actin filaments, troponin and tropomyosin, they could measure their relative positions at any single moment of the contraction cycle. Based on these data they proposed a model in which the troponin bridges mechanically assist in pulling tropomyosin away and exposing the myosin binding sites on actin when the fibers are stretched. This mechanical signal transduction appears to act together with Ca2+ binding to troponin, to unlock the tropomyosin gate but leave it shut until stretch finally pushes it open.
Reedy and his colleagues propose a model explaining stretch activation by a lock-out release mechanism involving Ca2+ and the newly discovered troponin bridges. Stretch activation also plays a role in the mammalian heart muscle, where it is believed to assist in blood ejection from the ventricles. The molecular mechanism of this process is not completely understood, but maybe future work will also discover troponin bridges in the mammalian heart.
