Commentary and Perspective
A five-course meal symposium on “The Future of Muscle is Now”
2022 Volume 19 Article ID: e190029
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2022 Volume 19 Article ID: e190029
Muscles are the source of mechanical force. Muscles enable us to move our arms and legs, speak, pump blood, and digest food. Muscle mechanics has been an important subject in biophysics. Accordingly, it is now possible to explain how mechanical force is produced and assembled at all levels of the hierarchy of the muscle contractile system, that is, from a single protein molecule at the smallest scale, to an assembly of the molecules (sarcomere; a highly ordered bipolar structure mainly composed of actin filaments that are protein polymers of actin monomers, and their counterpart myosin filaments that are of myosin motor proteins), to a myofibril (assembly of sarcomeres connected in series) and muscle cell, and finally, to a tissue. Then, are there no intriguing questions that can be asked regarding biophysics?
We have organized a symposium titled “The Future of Muscle is Now” at the 60th Annual Meeting of the Biophysical Society of Japan, held in September 2022 (Figure 1). In the symposium, we intend to demonstrate that the previously mentioned tragic perspective may be incorrect.
Overview of the symposium. Five presentations and their speakers are highlighted in the hierarchy of the muscle contractile system.
The current article introduces a five-course meal as a metaphor for the five presentations in the symposium. The meal begins with two senior researchers, Shin’ichi Ishiwata and Hiroyuki Iwamoto, who provide an overview of the current state of muscle research in biophysics to commence the symposium. They have pioneered single-molecule and synchrotron radiation X-ray studies in the muscle research, respectively, and have been actively contributing to the field until now. After appetizers and soup, Shuya Ishii introduces their recipes for activating muscle thin filaments in the absence of Ca2+. Subsequently, Michiya Matsusaki serves our main course, a three-dimensional (3D)-Bioprinted Wagyu Steak, which is a meat of the future. In addition, one of the authors of the symposium and of this manuscript, Madoka Suzuki, describes their recent discovery that heat can be an agonist of Ca2+ release channels, to close the meal for the day. Further details of the individual courses are provided below.
Shin’ichi discusses “About the present and future of research on muscle contraction/regulation mechanism.” The presentation begins with an overview of single-molecule studies that have successfully elucidated structural changes that motor proteins undergo during their chemo-mechanical energy conversion processes. Despite these significant advances, our understanding of muscle myosin remains limited. He emphasizes the importance of understanding the muscle at higher levels in the hierarchy of the muscle contractile system. As an illustration, he highlights protein assemblies, such as the lattice pattern, composed of muscle thick (myosin) and thin (actin) filaments; the lattice spring of Z-discs, where the thin filaments are attached in the sarcomere [1]; and the dynamic and heterogeneous motions of sarcomere structures examined in vitro and in vivo [2,3].
The second presentation is by Hiroyuki, titled “Rosy future of muscle research illuminated by bright synchrotron radiation X-rays.” He uncovered the static structures of proteins and protein assemblies in muscles as well as their changes using synchrotron radiation X-rays at SPring-8, a large synchrotron radiation facility located in Hyogo, Japan. In his presentation, he highlights two recently developed computational tools for muscle structure research. One of the tools is based on coherent diffractive imaging (CDI), one of the standard methods used in restoring 3D structures from X-ray diffraction patterns [4]. CDI is effective with isolated, highly contrasted objects, such as metal nanoparticles, that are completely illuminated by the X-ray beam. For fiber objects with low contrast that are predominantly composed of light atoms, such as carbons, alternative methods are required. Recently, he proposed such a method using the Patterson function and demonstrated its successful application in reconstructing actin and myosin filaments [5].
Calcium signaling is an important regulator of both cardiac and skeletal muscle contractions. During Ca2+ transients, striated muscles begin to contract by the activation of thin filaments in response to Ca2+ binding to troponin. The in vitro motility assay is an effective method for reconstituting the Ca2+ activation of thin filaments using purified actin, the regulatory proteins (tropomyosin and troponin), and myosin. The research presented by Shuya in his talk titled “Microscopic heat pulses induce activation of striated muscle thin filaments” combined the conventional assay with optical microheating to reveal thermo-sensing in muscle contractile systems. Rapid microheating to body temperature (37°C) initiated the sliding of reconstituted cardiac thin filaments even in the absence of Ca2+ [6]. This result suggests that cardiac thin filaments are partially activated at body temperature during relaxation [7]. Furthermore, a comparison of the sliding of reconstituted cardiac and skeletal thin filaments on both types of myosins revealed distinct thermo-sensing mechanisms in cardiac and skeletal muscles.
The reconstitution of biological systems is a way to understand the contribution of individual components in the system of interest. As Shuya demonstrates in his presentation, there are established methods to regenerate muscle thin filaments. However, the fabrication of cellular assemblies that retain the properties of in vivo tissue remains difficult [8]. Now, Michiya has a method of reconstituting artificial muscle tissue or meat, as he demonstrates in his presentation “3D-Bioprinted Wagyu Steak: Meat of the future?” What is this artificial meat? This meat is composed of muscles, fats, and blood vessels [9]. How is it manufactured? Similar to 3D-printing technology that creates 3D-structured objects by layer-by-layer printing of polymer inks, he has developed a 3D-printer that uses cells as “ink” to fabricate muscle, fat, and vascular cell fibers. Then, the three types of cell fibers are assembled with the desired location, ratio, and amount to construct the commercial beef, Japanese “Wagyu.” Is the meat of tomorrow tasty? We will discover the answer in the near future.
The final course of the symposium, titled “Thermal runaway in muscles studied using a local heat pulse method,” is presented by Madoka, one of the authors of this manuscript. Ca2+ homeostasis regulation is fundamental to all cellular functions, including muscle contractions. The process requires the chemical energy of ATP, of which a portion is lost as heat that diffuses through our cells and eventually raises our body temperature. Therefore, an unregulated Ca2+ homeostasis can lead to uncontrollable thermogenesis, such as in the disease of malignant hyperthermia studied here. The authors, including Toshiko Yamazawa, and the other author of this manuscript Kotaro Oyama discovered positive feedback from the heating process to the Ca2+ release through Ca2+ release channels, the type 1 ryanodine receptors (RyR1), using an optical method to heat cells locally under the optical microscope [10]. The novel response of RyR1 to heat was termed “heat-induced Ca2+ release (HICR).” HICR was also confirmed in skeletal muscle cells derived from disease-model mice established by Toshiko and colleagues recently [11]. HICR probably acts as an enhancer to accelerate thermogenesis in cells during malignant hyperthermia. In addition, that study suggests the potential role of HICR in normal Ca2+ homeostasis, that is, the role of heat as a mediator of intracellular Ca2+ signaling.
In conclusion, the symposium highlights several emerging fields in muscle research. The new studies are performed at the intersection of biophysical techniques, computational methodology, and material chemistry. In vivo examinations of protein assemblies shed light on previously unanswered questions in muscle physiology. On the basis of recent experimental findings at the protein and cellular levels, possible contributions of muscle thermogenesis to force generation and Ca2+ homeostasis are also discussed.
The authors thank all the invited speakers of this symposium. This work and symposium are partly supported by the Ministry of Education, Culture, Sports, Science and Technology (MEXT) “Grant-in-Aid for Transformative Research Areas (B),” “Trans-scale thermal signaling in muscle” (JP22H05052).
