A combined experimental approach to investigate the underlying mechanisms of history-dependent phenomena in skeletal muscle : the Drosophila jump muscle and single fiber tissue engineering

Abstract

Alternatively, muscle fibers engineered from mammalian skeletal muscle progenitor cells may provide an additional experimental platform to investigate history-dependent phenomena. Muscle fibers produced in vitro would allow for genetic modification capable in mammalian culture systems without the need for animal use or complicated dissections. Additionally, the engineering of functional muscle tissue in vitro may provide a successful strategy to address the clinical demand for muscle replacement. The loss of skeletal muscle function due to genetic or acquired conditions, such as traumatic injury, disease, or surgical excisions, causes a physiological deficit that remains without an effective clinical treatment. Current attempts to engineer muscle in vitro has been limited to two-dimensions or relied on scaffold-based approaches and have yet to achieve the dynamic mechanical responses of physiological muscle. To this end, we have developed a self-assembling, scaffold-free approach in conjunction with an electromechanical bioreactor to facilitate the engineering of three-dimensional functional muscle fibers. In addition to providing a platform to study muscle mechanics, including history-dependent behaviors, muscle fibers grown in vitro will offer a unique model to study muscle development, specifically the cues important for differentiating slow and fast muscle fibers.

Although observed and well characterized over the past seventy years, the underlying mechanisms of these phenomena remain unknown. Current limitations on the efficacy of proteomic manipulation in mammalian tissue make investigating the underlying mechanisms of these history-dependent phenomena difficult. The Drosophila muscle system is an excellent model to study the influence of contractile sarcomere protein isoforms on work and power production in functional muscle. We have utilized the Drosophila jump muscle, which is biomechanically similar to skeletal muscle, to not only investigate the presence of FD and FE, but also the influence of myosin kinetics, through complete isoform exchange, on the underlying mechanisms of these phenomena. This investigation demonstrates the presence of FD and FE in the jump muscle, how the mechanisms of these phenomena are different, and myosin's potential involvement in the underlying mechanisms.

Muscle is one of the most structurally and functionally complex tissues of the human body. Existing in multiple forms, muscle is responsible for critical bodily functions such as mobility, digestion, and blood transport. Mechanically, muscle possesses a passive viscoelasticity typical of soft musculoskeletal tissues, but also actively produces contractile force. At the core of functional muscle, the interaction of myosin and actin is predominantly responsible for the force generating properties. The relationships of force generation with length, velocity, and stimulation (neural excitation leading to sarcoplasmic reticulum release of calcium) have been well characterized. However, these relationships cannot explain the dynamic mechanical behavior, especially following active stretch or shortening. Two history-dependent phenomena of skeletal muscle, Force Depression (FD) and Force Enhancement (FE) are observed after active shortening and lengthening, respectively. Specifically, FD is a decrease in the steady-state muscle force than expected for the given length after shortening. On the other hand, FE is a muscle force higher than expected for a given length after active lengthening.