Coupling of Adjacent Tropomyosins Enhances Cross-Bridge-Mediated Cooperative Activation in a Markov Model of the Cardiac Thin Filament

Serine 19 phosphorylation of the myosin regulatory light chain (MRLC) appears to be the primary determinant of smooth muscle force development. The relationship between MRLC phosphorylation and force is nonlinear, showing that phosphorylation is not a simple switch regulating the number of cycling cross bridges. We reexamined the MRLC phosphorylation-force relationship in slow, tonic swine carotid media; fast, phasic rabbit urinary bladder detrusor; and very fast, tonic rat anococcygeus. We found a sigmoidal dependence of force on MRLC phosphorylation in all three tissues with a threshold for force development of ∼0.15 mol Pi/mol MRLC. This behavior suggests that force is regulated in a highly cooperative manner. We then determined whether a model that employs both the latch-bridge hypothesis and cooperative activation could reproduce the relationship between Ser19-MRLC phosphorylation and force without the need for a second regulatory system. We based this model on skeletal muscle in which attached cross bridges cooperatively activate thin filaments to facilitate cross-bridge attachment. We found that such a model describes both the steady-state and time-course relationship between Ser19-MRLC phosphorylation and force. The model required both cooperative activation and latch-bridge formation to predict force. The best fit of the model occurred when binding of a cross bridge cooperatively activated seven myosin binding sites on the thin filament. This result suggests cooperative mechanisms analogous to skeletal muscle that will require testing.

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Cardiac Thin Filament-Mediated Calcium Sensitization Modulates Cross-Bridge Kinetics

Biophysical Journal ◽

10.1016/j.bpj.2017.11.1777 ◽

2018 ◽

Vol 114 (3) ◽

pp. 315a-316a

Author(s):

Maicon Landim-Vieira ◽

David Gonzalez-Martinez ◽

Jamie R. Johnston ◽

Weikang Ma ◽

Olga Antipova ◽

...

Keyword(s):

Thin Filament ◽

Calcium Sensitization ◽

Cross Bridge

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Kinetics of a Ca(2+)-sensitive cross-bridge state transition in skeletal muscle fibers. Effects due to variations in thin filament activation by extraction of troponin C.

The Journal of General Physiology ◽

10.1085/jgp.98.2.233 ◽

1991 ◽

Vol 98 (2) ◽

pp. 233-248 ◽

Cited By ~ 32

Author(s):

J M Metzger ◽

R L Moss

Keyword(s):

Steady State ◽

Direct Effect ◽

Thin Filament ◽

Near Neighbor ◽

Troponin C ◽

Isometric Tension ◽

Cross Bridge ◽

Filament Activation ◽

Kinetics Of ◽

Partial Extraction

The rate constant of tension redevelopment (ktr; 1986. Proc. Natl. Acad. Sci. USA. 83:3542-3546) was determined at various levels of thin filament activation in skinned single fibers from mammalian fast twitch muscles. Activation was altered by (a) varying the concentration of free Ca2+ in the activating solution, or (b) extracting various amounts of troponin C (TnC) from whole troponin complexes while keeping the concentration of Ca2+ constant. TnC was extracted by bathing the fiber in a solution containing 5 mM EDTA, 10 mM HEPES, and 0.5 mM trifluoperazine dihydrochloride. Partial extraction of TnC resulted in a decrease in the Ca2+ sensitivity of isometric tension, presumably due to disruption of near-neighbor molecular cooperativity between functional groups (i.e., seven actin monomers plus associated troponin and tropomyosin) within the thin filament. Altering the level of thin filament activation by partial extraction of TnC while keeping Ca2+ concentration constant tested whether the Ca2+ sensitivity of ktr results from a direct effect of Ca2+ on cross-bridge state transitions or, alternatively, an indirect effect of Ca2+ on these transitions due to varying extents of thin filament activation. Results showed that the ktr-pCa relation was unaffected by partial extraction of TnC, while steady-state isometric tension exhibited the expected reduction in Ca2+ sensitivity. This finding provides evidence for a direct effect of Ca2+ on an apparent rate constant that limits the formation of force-bearing cross-bridge states in muscle fibers. Further, the kinetics of this transition are unaffected by disruption of near-neighbor thin filament cooperativity subsequent to extraction of TnC. Finally, the results support the idea that the steepness of the steady-state isometric tension-calcium relationship is at least in part due to mechanisms involving molecular cooperativity among thin filament regulatory proteins.

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The Myosin Duty Ratio Tunes the Calcium Sensitivity and Cooperative Activation of the Thin Filament

Biochemistry ◽

10.1021/bi400262h ◽

2013 ◽

Vol 52 (37) ◽

pp. 6437-6444 ◽

Cited By ~ 7

Author(s):

Milad Webb ◽

Del R. Jackson ◽

Travis J. Stewart ◽

Samuel P. Dugan ◽

Michael S. Carter ◽

...

Keyword(s):

Thin Filament ◽

Calcium Sensitivity ◽

Duty Ratio ◽

Cooperative Activation

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Ca2+ Regulation of Rabbit Skeletal Muscle Thin Filament Sliding: Role of Cross-Bridge Number

Biophysical Journal ◽

10.1016/s0006-3495(03)74607-4 ◽

2003 ◽

Vol 85 (3) ◽

pp. 1775-1786 ◽

Cited By ~ 27

Author(s):

Bo Liang ◽

Ying Chen ◽

Chien-Kao Wang ◽

Zhaoxiong Luo ◽

Michael Regnier ◽

...

Keyword(s):

Skeletal Muscle ◽

Thin Filament ◽

Rabbit Skeletal Muscle ◽

Cross Bridge ◽

Muscle Thin Filament ◽

Bridge Number

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The Effect of HCM and RCM Causing Mutations of Troponin T on Force Generation and Cross-Bridge Kinetics in Thin-Filament Reconstituted Bovine Cardiac Muscle Fibers

Biophysical Journal ◽

10.1016/j.bpj.2011.11.853 ◽

2012 ◽

Vol 102 (3) ◽

pp. 156a

Author(s):

Fan Bai ◽

Hannah M. Caster ◽

Jose R. Pinto ◽

James D. Potter ◽

Masataka Kawai

Keyword(s):

Cardiac Muscle ◽

Thin Filament ◽

Troponin T ◽

Muscle Fibers ◽

Force Generation ◽

Cross Bridge ◽

Cardiac Muscle Fibers

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Myosin MgADP release rate decreases at longer sarcomere length to prolong myosin attachment time in skinned rat myocardium

AJP Heart and Circulatory Physiology ◽

10.1152/ajpheart.00555.2015 ◽

2015 ◽

Vol 309 (12) ◽

pp. H2087-H2097 ◽

Cited By ~ 9

Author(s):

Bertrand C. W. Tanner ◽

Jason J. Breithaupt ◽

Peter O. Awinda

Keyword(s):

Release Rate ◽

Thin Filament ◽

Molecular Mechanisms ◽

Sarcomere Length ◽

Cardiac Contractility ◽

Force Production ◽

Cross Bridge ◽

Thin Filament Proteins ◽

Filament Activation ◽

Ventricular Filling

Cardiac contractility increases as sarcomere length increases, suggesting that intrinsic molecular mechanisms underlie the Frank-Starling relationship to confer increased cardiac output with greater ventricular filling. The capacity of myosin to bind with actin and generate force in a muscle cell is Ca2+ regulated by thin-filament proteins and spatially regulated by sarcomere length as thick-to-thin filament overlap varies. One mechanism underlying greater cardiac contractility as sarcomere length increases could involve longer myosin attachment time ( t on) due to slowed myosin kinetics at longer sarcomere length. To test this idea, we used stochastic length-perturbation analysis in skinned rat papillary muscle strips to measure t on as [MgATP] varied (0.05–5 mM) at 1.9 and 2.2 μm sarcomere lengths. From this t on-MgATP relationship, we calculated cross-bridge MgADP release rate and MgATP binding rates. As MgATP increased, t on decreased for both sarcomere lengths, but t on was roughly 70% longer for 2.2 vs. 1.9 μm sarcomere length at maximally activated conditions. These t on differences were driven by a slower MgADP release rate at 2.2 μm sarcomere length (41 ± 3 vs. 74 ± 7 s−1), since MgATP binding rate was not different between the two sarcomere lengths. At submaximal activation levels near the pCa50 value of the tension-pCa relationship for each sarcomere length, length-dependent increases in t on were roughly 15% longer for 2.2 vs. 1.9 μm sarcomere length. These changes in cross-bridge kinetics could amplify cooperative cross-bridge contributions to force production and thin-filament activation at longer sarcomere length and suggest that length-dependent changes in myosin MgADP release rate may contribute to the Frank-Starling relationship in the heart.

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Three-dimensional image reconstruction of insect flight muscle. I. The rigor myac layer.

The Journal of Cell Biology ◽

10.1083/jcb.109.3.1085 ◽

1989 ◽

Vol 109 (3) ◽

pp. 1085-1102 ◽

Cited By ~ 22

Author(s):

K A Taylor ◽

M C Reedy ◽

L Córdova ◽

M K Reedy

Keyword(s):

Thin Filament ◽

Flight Muscle ◽

Insect Flight ◽

Three Dimensional ◽

Bridge Structure ◽

Insect Flight Muscle ◽

Single Filament ◽

Cross Bridge ◽

Cross Bridges

We have obtained detailed three-dimensional images of in situ cross-bridge structure in insect flight muscle by electron microscopy of multiple tilt views of single filament layers in ultrathin sections, supplemented with data from thick sections. In this report, we describe the images obtained of the myac layer, a 25-nm longitudinal section containing a single layer of alternating myosin and actin filaments. The reconstruction reveals averaged rigor cross-bridges that clearly separate into two classes constituting lead and rear chevrons within each 38.7-nm axial repeat. These two classes differ in tilt angle, size and shape, density, and slew. This new reconstruction confirms our earlier interpretation of the lead bridge as a two-headed cross-bridge and the rear bridge as a single-headed cross-bridge. The importance of complementing tilt series with additional projections outside the goniometer tilt range is demonstrated by comparison with our earlier myac layer reconstruction. Incorporation of this additional data reveals new details of rigor cross-bridge structure in situ which include clear delineation of (a) a triangular shape for the lead bridge, (b) a smaller size for the rear bridge, and (c) density continuity across the thin filament in the lead bridge. Within actin's regular 38.7-nm helical repeat, local twist variations in the thin filament that correlate with the two cross-bridge classes persist in this new reconstruction. These observations show that in situ rigor cross-bridges are not uniform, and suggest three different myosin head conformations in rigor.

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