Muscle drives locomotion and other important body functions, such as pumping blood and moving air during respiration, in all animal species. Muscle has undergone tremendous diversification during the course of evolution. As a result, muscle has become highly specialized among different species to serve different types of locomotion. In general, muscles contract very rapidly among small animals, where speed is critically important to avoid predators, and slowly among large animals, where economy is of great importance. Several molecular and cellular features are recognized as contributing to the speed versus economy spectrum. The investigator and others in his laboratory recently discovered a very systematic structural variation in a major muscle protein that is associated with species body mass. The main objective of this project is to test whether this variation contributes to the speed versus economy spectrum among mammals with very different body masses. Contraction will be studied in single muscle cells from different species. Tests will be conducted to determine if the structural variation in this protein contributes to muscles being fast in some species and more economical in other species. The results will have important evolutionary implications for better understanding how muscles have become specialized in different organisms and in organisms of different sizes. This project will provide excellent opportunities for students to be trained at both the molecular and cellular levels and to integrate laboratory findings with well known differences in locomotion characteristics between species. The project results will be made available to the general public (e.g., presentations at local schools, hosting laboratory visits by students) in an understandable manner to instill a greater appreciation for the variety of molecular and cellular mechanisms that animals employ to serve an extremely diverse range of muscle functions.
Myosin is a very large and complex protein that is of fundamental importance for muscle function. It is abundantly expressed in all types of muscle of all animals. Every muscle cell contains millions of myosin molecules. Each myosin molecule is comprised of six subunits - two very large heavy chains and two pairs of much smaller light chains. All of the subunits are expressed in muscles as families of isoforms (that is, as variants of one another). The presence of specific isoforms of myosin heavy chains and lights chains in a muscle cell determines how strong and/or how fast that cell contracts and how much power that cell can generate. Other proteins in muscle cells are also highly variable in their expression. The expression of all of these different proteins in muscle cells and the extraordinary spectrum of contractile properties of muscle cells in species that fly, swim, run or climb provides an opportunity to learn, at the molecular level, how animals are specialized to perform a range of physical activities. Work from this project has contributed to understanding which myosin subunit isoforms are expressed in different types of muscles among many vertebrate species. It is well known that the heavy chains of myosin are of pivotal importance in regulating muscle contractile properties. The primary objectives of this project were to further explore the patterns of myosin heavy and light chain isoforms in different muscle cell types across a range of vertebrate species and the roles of these proteins in regulating contraction. The collective results from the project indicate that the expression of several muscle proteins is much more complex than previously understood. This is especially true in craniofacial muscles, which include extraocular muscles that drive eye rotations, jaw-closing muscles that drive chewing, and laryngeal muscles that open and close the airway and that serve vocalizations, including speaking and singing. One unexpected finding from this project was the expression of a myosin light chain isoform in jaw-closing muscles of carnivores. These muscles express an evolutionarily ancient isoform of myosin heavy chain that is not expressed elsewhere in the body. This myosin allows these cells to generate very high force. It had been thought, for almost 35 years, that one of the light chains associated with this heavy chain is also unique and also evolutionarily ancient. Instead, a discovery from this project is that this light chain does not exist anywhere, apparently in any animal, and that the light chain that is expressed in these muscles is the same as the light chain that is normally expressed in skeletal muscles at the embryonic stage of development and in the atria of the heart in adults. This was a major discovery because of the importance of myosin light chains in regulating muscle contraction and it corrected a notion that had been around for more than three decades. This discovery has also raised several fascinating new questions, such as how, during the course of evolution, did this embryonic/atrial light chain become associated with the ancient heavy chain? Results from this project also show that the expression of proteins that are involved with how calcium ions turn on muscle contraction is also much more complex that previously understood. The variation in these proteins provides another mechanism for muscle cells to be specialized for specific types of physical activity, such as locomotion, chewing and eye rotations. The results of the studies in this project provide a more complete understanding of biological diversity on the planet and of the value in preserving this diversity. Understanding how different types of muscle cells are equipped with different proteins allows for the design of devices to perform specific types of work. Knowing how humans differ from other animals allows for a better appreciation of how humans have evolved relative to other species, and, by comparison with other species, we better understand our physical performance strengths and limitations. Finally, a more complete understanding of how complex patterns of protein expression in muscle serve different types of physical activities in different animals allows for a better understanding and eventual treatment of muscle diseases in humans.
