TY - JOUR
T1 - Activation of skeletal muscle is controlled by a dual-filament mechano-sensing mechanism
AU - Brunello, Elisabetta
AU - Marcucci, Lorenzo
AU - Irving, Malcolm
AU - Fusi, Luca
N1 - Funding Information:
We are grateful to Marty Rajaratnam (King’s College London) for mechanical engineering support and Yin-Biao Sun (King’s College London) for providing the TnC probes. This work and the investigators were supported by the Wellcome Trust/Royal Society, Medical Research Council (Grant MR/R01700X/1 awarded to M.I.) and by the British Heart Foundation (BHF). E.B. was funded by a BHF Intermediate Basic Science Research Fellowship (FS/17/3/32604). L.F. was funded by a Sir Henry Dale Fellowship awarded by the Wellcome Trust and the Royal Society (210464/Z/18/Z). L.M. was funded by the European Union via Horizon 2020 Research and Innovation Programme under the Marie Sklodowska-Curie grant agreement no. 886232. We acknowledge the CINECA award under the ISCRA initiative for providing high-performance computing resources and support in the project HP10C847NF.
Funding Information:
ACKNOWLEDGMENTS. We are grateful to Marty Rajaratnam (King’s College London) for mechanical engineering support and Yin-Biao Sun (King’s College London) for providing the TnC probes. This work and the investigators were supported by the Wellcome Trust/Royal Society, Medical Research Council (Grant MR/R01700X/1 awarded to M.I.) and by the British Heart Foundation (BHF). E.B. was funded by a BHF Intermediate Basic Science Research Fellowship (FS/17/3/32604). L.F. was funded by a Sir Henry Dale Fellowship awarded by the Wellcome Trust and the Royal Society (210464/Z/18/Z). L.M. was funded by the European Union via Horizon 2020 Research and Innovation Programme under the Marie Sklodowska-Curie grant agreement no. 886232. We acknowledge the CINECA award under the ISCRA initiative for providing high-performance computing resources and support in the project HP10C847NF.
Publisher Copyright:
Copyright © 2023 the Author(s). Published by PNAS.
PY - 2023/5/30
Y1 - 2023/5/30
N2 - Contraction of skeletal muscle is triggered by a transient rise in intracellular calcium concentration leading to a structural change in the actin-containing thin filaments that allows binding of myosin motors from the thick filaments. Most myosin motors are unavailable for actin binding in resting muscle because they are folded back against the thick filament backbone. Release of the folded motors is triggered by thick filament stress, implying a positive feedback loop in the thick filaments. However, it was unclear how thin and thick filament activation mechanisms are coordinated, partly because most previous studies of the thin filament regulation were conducted at low temperatures where the thick filament mechanisms are inhibited. Here, we use probes on both troponin in the thin filaments and myosin in the thick filaments to monitor the activation states of both filaments in near-physiological conditions. We characterize those activation states both in the steady state, using conventional titrations with calcium buffers, and during activation on the physiological timescale, using calcium jumps produced by photolysis of caged calcium. The results reveal three activation states of the thin filament in the intact filament lattice of a muscle cell that are analogous to those proposed previously from studies on isolated proteins. We characterize the rates of the transitions between these states in relation to thick filament mechano-sensing and show how thin- and thick-filament-based mechanisms are coupled by two positive feedback loops that switch on both filaments to achieve rapid cooperative activation of skeletal muscle.
AB - Contraction of skeletal muscle is triggered by a transient rise in intracellular calcium concentration leading to a structural change in the actin-containing thin filaments that allows binding of myosin motors from the thick filaments. Most myosin motors are unavailable for actin binding in resting muscle because they are folded back against the thick filament backbone. Release of the folded motors is triggered by thick filament stress, implying a positive feedback loop in the thick filaments. However, it was unclear how thin and thick filament activation mechanisms are coordinated, partly because most previous studies of the thin filament regulation were conducted at low temperatures where the thick filament mechanisms are inhibited. Here, we use probes on both troponin in the thin filaments and myosin in the thick filaments to monitor the activation states of both filaments in near-physiological conditions. We characterize those activation states both in the steady state, using conventional titrations with calcium buffers, and during activation on the physiological timescale, using calcium jumps produced by photolysis of caged calcium. The results reveal three activation states of the thin filament in the intact filament lattice of a muscle cell that are analogous to those proposed previously from studies on isolated proteins. We characterize the rates of the transitions between these states in relation to thick filament mechano-sensing and show how thin- and thick-filament-based mechanisms are coupled by two positive feedback loops that switch on both filaments to achieve rapid cooperative activation of skeletal muscle.
UR - https://www.pnas.org/doi/10.1073/pnas.2302837120
UR - http://www.scopus.com/inward/record.url?scp=85159847966&partnerID=8YFLogxK
U2 - 10.1073/pnas.2302837120
DO - 10.1073/pnas.2302837120
M3 - Article
SN - 0027-8424
VL - 120
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
IS - 22
M1 - e2302837120
ER -