Faculty Opinions recommendation of Force-Dependent Recruitment from the Myosin Off State Contributes to Length-Dependent Activation.

At the subcellular level, the Frank-Starling law of the heart is described by an increase in calcium sensitivity and force with increased sarcomere length (SL). We examine how this relationship is affected by a dilated cardiomyopathy-associated mutation in tropomyosin (D230N, denoted Tm D230N ) by measuring contractility of intact and permeabilized cardiac muscle preparations at short (2.0 μm) and long (2.3 μm) SL. Transgenic mouse hearts containing the Tm D230N mutation have significantly dilated hearts and reduced cardiac output by ~6 months of age. Intact trabeculae were electrically stimulated and paced at 1 Hz with oxygenated solution (30°C) circulating through the experimental chamber, and permeabilized preparations were bathed in solutions (15°C) of progressively increased [Ca 2+ ] for measures of steady-state force. For intact muscle we found that the Tm D230N mutation results in significantly reduced twitch forces at SL 2.0 and 2.3 μm relative to wild-type (WT). Also, WT trabeculae displayed a significant increase in twitch force upon increase in SL (as expected) but Tm D230N trabeculae did not, demonstrating a loss of SL dependence of contraction. In permeabilized preparations, maximal activation (pCa 4.5) of both WT and Tm D230N preparations exhibited significant SL-dependent increases in force. However, at submaximal Ca 2+ (pCa 5.8), where the heart operates, WT preparations had significant increases in force with increasing length (comparing SL 2.0 to 2.3 μm), while this length-dependence of force augmentation in Tm D230N was absent. The increase in pCa 50 (pCa that produces half-maximal force) going from SL 2.0 to 2.3 μm was significantly less for Tm D230N preparations compared to WT, owing to a significantly smaller increase in pCa 50 at SL 2.3 μm (the pCa 50 at SL 2.0 μm was not significantly different between WT and Tm D230N ). These results suggest that the Tm D230N mutation limits an increase in the Ca 2+ sensitivity of contraction as the muscle lengthens by damping thin filament activation. To further examine length-dependent effects of the Tm D230N mutation, future experiments will test conditions that augment cross-bridge binding/inhibition, and other models of dilated cardiomyopathy that inhibit thin filament activation. Funding: HL111197

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Allosteric coupling between Mn2+ and dsDNA controls the catalytic efficiency and fidelity of cGAS

Nucleic Acids Research ◽

10.1093/nar/gkaa084 ◽

2020 ◽

Vol 48 (8) ◽

pp. 4435-4447 ◽

Cited By ~ 4

Author(s):

Richard M Hooy ◽

Guido Massaccesi ◽

Kimberly E Rousseau ◽

Michael A Chattergoon ◽

Jungsan Sohn

Keyword(s):

Immune Responses ◽

Catalytic Efficiency ◽

Genotoxic Stress ◽

Innate Immune ◽

Activation Mechanism ◽

Innate Immune Responses ◽

Allosteric Coupling ◽

Pathogen Invasion ◽

Length Dependent Activation ◽

Dsdna Binding

Abstract Cyclic-G/AMP (cGAMP) synthase (cGAS) triggers host innate immune responses against cytosolic double-stranded (ds)DNA arising from genotoxic stress and pathogen invasion. The canonical activation mechanism of cGAS entails dsDNA-binding and dimerization. Here, we report an unexpected activation mechanism of cGAS in which Mn2+ activates monomeric cGAS without dsDNA. Importantly, the Mn2+-mediated activation positively couples with dsDNA-dependent activation in a concerted manner. Moreover, the positive coupling between Mn2+ and dsDNA length-dependent activation requires the cognate ATP/GTP substrate pair, while negative-cooperativity suppresses Mn2+ utilization by either ATP or GTP alone. Additionally, while Mn2+ accelerates the overall catalytic activity, dsDNA length-dependent dimerization specifically accelerates the cyclization of cGAMP. Together, we demonstrate how the intrinsic allostery of cGAS efficiently yet precisely tunes its activity.

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Titin strain contributes to the Frank–Starling law of the heart by structural rearrangements of both thin- and thick-filament proteins

Proceedings of the National Academy of Sciences ◽

10.1073/pnas.1516732113 ◽

2016 ◽

Vol 113 (8) ◽

pp. 2306-2311 ◽

Cited By ~ 73

Author(s):

Younss Ait-Mou ◽

Karen Hsu ◽

Gerrie P. Farman ◽

Mohit Kumar ◽

Marion L. Greaser ◽

...

Keyword(s):

Thick Filament ◽

Structural Rearrangements ◽

X Ray Diffraction ◽

Thin Filaments ◽

Time Resolved ◽

X Ray ◽

Isolated Myocardium ◽

Splice Isoform ◽

Length Dependent Activation

The Frank–Starling mechanism of the heart is due, in part, to modulation of myofilament Ca2+ sensitivity by sarcomere length (SL) [length-dependent activation (LDA)]. The molecular mechanism(s) that underlie LDA are unknown. Recent evidence has implicated the giant protein titin in this cellular process, possibly by positioning the myosin head closer to actin. To clarify the role of titin strain in LDA, we isolated myocardium from either WT or homozygous mutant (HM) rats that express a giant splice isoform of titin, and subjected the muscles to stretch from 2.0 to 2.4 μm of SL. Upon stretch, HM compared with WT muscles displayed reduced passive force, twitch force, and myofilament LDA. Time-resolved small-angle X-ray diffraction measurements of WT twitching muscles during diastole revealed stretch-induced increases in the intensity of myosin (M2 and M6) and troponin (Tn3) reflections, as well as a reduction in cross-bridge radial spacing. Independent fluorescent probe analyses in relaxed permeabilized myocytes corroborated these findings. X-ray electron density reconstruction revealed increased mass/ordering in both thick and thin filaments. The SL-dependent changes in structure observed in WT myocardium were absent in HM myocardium. Overall, our results reveal a correlation between titin strain and the Frank–Starling mechanism. The molecular basis underlying this phenomenon appears not to involve interfilament spacing or movement of myosin toward actin but, rather, sarcomere stretch-induced simultaneous structural rearrangements within both thin and thick filaments that correlate with titin strain and myofilament LDA.

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Hypertrophic cardiomyopathy mutation in cardiac troponin T (R95H) attenuates length-dependent activation in guinea pig cardiac muscle fibers

AJP Heart and Circulatory Physiology ◽

10.1152/ajpheart.00369.2017 ◽

2017 ◽

Vol 313 (6) ◽

pp. H1180-H1189 ◽

Cited By ~ 6

Author(s):

Alexis V. Mickelson ◽

Murali Chandra

Keyword(s):

Hypertrophic Cardiomyopathy ◽

Guinea Pig ◽

Central Region ◽

Cardiac Troponin ◽

Sarcomere Length ◽

Troponin T ◽

Cardiac Troponin T ◽

Cross Bridge ◽

Cardiac Muscle Fibers ◽

Length Dependent Activation

The central region of cardiac troponin T (TnT) is important for modulating the dynamics of muscle length-mediated cross-bridge recruitment. Therefore, hypertrophic cardiomyopathy mutations in the central region may affect cross-bridge recruitment dynamics to alter myofilament Ca2+ sensitivity and length-dependent activation of cardiac myofilaments. Given the importance of the central region of TnT for cardiac contractile dynamics, we studied if hypertrophic cardiomyopathy-linked mutation (TnTR94H)-induced effects on contractile function would be differently modulated by sarcomere length (SL). Recombinant wild-type TnT (TnTWT) and the guinea pig analog of the human R94H mutation (TnTR95H) were reconstituted into detergent-skinned cardiac muscle fibers from guinea pigs. Steady-state and dynamic contractile measurements were made at short and long SLs (1.9 and 2.3 µm, respectively). Our results demonstrated that TnTR95H increased pCa50 (−log of free Ca2+ concentration) to a greater extent at short SL; TnTR95H increased pCa50 by 0.11 pCa units at short SL and 0.07 pCa units at long SL. The increase in pCa50 associated with an increase in SL from 1.9 to 2.3 µm (ΔpCa50) was attenuated nearly twofold in TnTR95H fibers; ΔpCa50 was 0.09 pCa units for TnTWT fibers but only 0.05 pCa units for TnTR95H fibers. The SL dependency of rate constants of cross-bridge distortion dynamics and tension redevelopment was also blunted by TnTR95H. Collectively, our observations on the SL dependency of pCa50 and rate constants of cross-bridge distortion dynamics and tension redevelopment suggest that mechanisms underlying the length-dependent activation cardiac myofilaments are attenuated by TnTR95H. NEW & NOTEWORTHY Mutant cardiac troponin T (TnTR95H) differently affects myofilament Ca2+ sensitivity at short and long sarcomere length, indicating that mechanisms underlying length-dependent activation are altered by TnTR95H. TnTR95H enhances myofilament Ca2+ sensitivity to a greater extent at short sarcomere length, thus attenuating the length-dependent increase in myofilament Ca2+ sensitivity.

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Force-length relations in cardiac muscle segments

AJP Heart and Circulatory Physiology ◽

10.1152/ajpheart.1983.244.5.h701 ◽

1983 ◽

Vol 244 (5) ◽

pp. H701-H707

Author(s):

L. L. Huntsman ◽

J. F. Rondinone ◽

D. A. Martyn

Keyword(s):

Cardiac Muscle ◽

Central Region ◽

Muscle Length ◽

The Other ◽

Segment Length ◽

Isometric Contractions ◽

Papillary Muscles ◽

Passive Force ◽

A New Technique ◽

Length Dependent Activation

Using a new technique that measures the length of a segment in the central region of isolated papillaries, we have determined the force-segment length relation for ferret papillary muscles at 27 degrees C. The muscles contracted under muscle length isometric (auxotonic) and segment isometric conditions in physiological solutions containing 9.0, 4.5, 2.25, and 1.125 mM Ca2+. Force-segment length relations obtained from auxotonic and segment isometric contractions were identical in a given Ca2+ concentration. Calcium variations, however, changed the position, shape, and segment length intercept of the force-segment length relation. Force, at a given segment length, increased with increasing Ca2+ up to 9.0 mM Ca2+. The force-segment length relation changed shape from linear, in 4.5 mM Ca2+, to concave in 1.125 mM Ca2+. The segment length intercept was found by extrapolation to be 68, 69, and 74% SLmax in 4.5, 2.25, and 1.125 mM Ca2+, respectively. Two passive force corrections were used to calculate the developed force-segment length relations. Assuming passive force to be related primarily to segment length yields curves that change shape with Ca2+ concentration, suggesting length-dependent activation. On the other hand, assuming passive force to be related to muscle length results in curves for different Ca2+ concentrations that are nearly vertically shifted versions of each other, suggesting the influence of internal loads.

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Length-Dependent Activation is Reduced in Myocardium from Patients with Non-ischemic Heart Failure

Biophysical Journal ◽

10.1016/j.bpj.2017.11.2997 ◽

2018 ◽

Vol 114 (3) ◽

pp. 548a-549a

Author(s):

Bertrand C.W. Tanner ◽

Peter O. Awinda ◽

Cheavar A. Blair ◽

Maya A. Guglin ◽

Kenneth S. Campbell

Keyword(s):

Heart Failure ◽

Ischemic Heart ◽

Ischemic Heart Failure ◽

Length Dependent Activation

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Length‐dependent activation in three striated muscle types of the rat

The Journal of Physiology ◽

10.1113/jphysiol.2002.024505 ◽

2002 ◽

Vol 544 (1) ◽

pp. 225-236 ◽

Cited By ~ 93

Author(s):

John P. Konhilas ◽

Thomas C. Irving ◽

Pieter P. Tombe

Keyword(s):

Striated Muscle ◽

Muscle Types ◽

Length Dependent Activation

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Troponin and Titin Coordinately Regulate Length-dependent Activation in Skinned Porcine Ventricular Muscle

The Journal of General Physiology ◽

10.1085/jgp.200709895 ◽

2008 ◽

Vol 131 (3) ◽

pp. 275-283 ◽

Cited By ~ 38

Author(s):

Takako Terui ◽

Munguntsetseg Sodnomtseren ◽

Douchi Matsuba ◽

Jun Udaka ◽

Shin'ichi Ishiwata ◽

...

Keyword(s):

Skeletal Muscle ◽

Thin Filament ◽

Left Ventricular ◽

Ventricular Muscle ◽

Passive Force ◽

Active Force ◽

Fast Skeletal Muscle ◽

Thin Filaments ◽

Cross Bridges ◽

Length Dependent Activation

We investigated the molecular mechanism by which troponin (Tn) regulates the Frank-Starling mechanism of the heart. Quasi-complete reconstitution of thin filaments with rabbit fast skeletal Tn (sTn) attenuated length-dependent activation in skinned porcine left ventricular muscle, to a magnitude similar to that observed in rabbit fast skeletal muscle. The rate of force redevelopment increased upon sTn reconstitution at submaximal levels, coupled with an increase in Ca2+ sensitivity of force, suggesting the acceleration of cross-bridge formation and, accordingly, a reduction in the fraction of resting cross-bridges that can potentially produce additional active force. An increase in titin-based passive force, induced by manipulating the prehistory of stretch, enhanced length-dependent activation, in both control and sTn-reconstituted muscles. Furthermore, reconstitution of rabbit fast skeletal muscle with porcine left ventricular Tn enhanced length-dependent activation, accompanied by a decrease in Ca2+ sensitivity of force. These findings demonstrate that Tn plays an important role in the Frank-Starling mechanism of the heart via on–off switching of the thin filament state, in concert with titin-based regulation.

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