TY - JOUR
T1 - Dual Filament Regulation of Relaxation in Mammalian Fast Skeletal Muscle
AU - Hill, Cameron
AU - Kalakoutis, Michaeljohn
AU - Arcidiacono, Alice
AU - Paradine Cullup, Flair
AU - Wang, Yanhong
AU - Fukutani, Atsuki
AU - Narayanan, Theyencheri
AU - Brunello, Elisabetta
AU - Fusi, Luca
AU - Irving, Malcolm
PY - 2025/1/24
Y1 - 2025/1/24
N2 - Muscle contraction is driven by myosin motors from the thick filaments pulling on the actin-containing thin filaments of the sarcomere, and it is regulated by structural changes in both filaments. Thin filaments are activated by an increase in intracellular calcium concentration [Ca2+]i and by myosin binding to actin. Thick filaments are activated by direct sensing of the filament load. However, these mechanisms cannot explain muscle relaxation when [Ca2+]i decreases at high load and myosin motors are attached to actin. There is, therefore, a fundamental gap in our understanding of muscle relaxation, despite its importance for muscle function in vivo, for example, for rapid eye movements or, on slower timescales, for the efficient control of posture. Here, we used time-resolved X-ray diffraction to determine how muscle thin and thick filaments switch OFF in extensor digitorum longus muscles of the mouse in response to decreases in either [Ca2+]i or muscle load and to describe the distribution of muscle sarcomere lengths during relaxation. We show that reducing load at high [Ca2+]i is more effective in switching OFF both the thick and thin filaments than reducing [Ca2+]i at high load in normal relaxation. In the latter case, the thick filaments initially remain fully ON, although the number of myosin motors bound to actin decreases and the force per attached motor increases. That initial slow phase of relaxation is abruptly terminated by yielding of one population of sarcomeres, triggering a redistribution of sarcomere lengths that leads to the rapid completion of mechanical relaxation.
AB - Muscle contraction is driven by myosin motors from the thick filaments pulling on the actin-containing thin filaments of the sarcomere, and it is regulated by structural changes in both filaments. Thin filaments are activated by an increase in intracellular calcium concentration [Ca2+]i and by myosin binding to actin. Thick filaments are activated by direct sensing of the filament load. However, these mechanisms cannot explain muscle relaxation when [Ca2+]i decreases at high load and myosin motors are attached to actin. There is, therefore, a fundamental gap in our understanding of muscle relaxation, despite its importance for muscle function in vivo, for example, for rapid eye movements or, on slower timescales, for the efficient control of posture. Here, we used time-resolved X-ray diffraction to determine how muscle thin and thick filaments switch OFF in extensor digitorum longus muscles of the mouse in response to decreases in either [Ca2+]i or muscle load and to describe the distribution of muscle sarcomere lengths during relaxation. We show that reducing load at high [Ca2+]i is more effective in switching OFF both the thick and thin filaments than reducing [Ca2+]i at high load in normal relaxation. In the latter case, the thick filaments initially remain fully ON, although the number of myosin motors bound to actin decreases and the force per attached motor increases. That initial slow phase of relaxation is abruptly terminated by yielding of one population of sarcomeres, triggering a redistribution of sarcomere lengths that leads to the rapid completion of mechanical relaxation.
KW - Mammalian skeletal muscle
KW - small-angle X-ray diffraction
KW - actin
KW - myosin
KW - tropomyosin
M3 - Article
SN - 1091-6490
JO - Proceedings of the National Academy of Sciences of the United States of America
JF - Proceedings of the National Academy of Sciences of the United States of America
ER -