The Role of Calcium Affinity and C-I Interaction in 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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Analysis of hystereses in force length and force calcium relations

AJP Heart and Circulatory Physiology ◽

10.1152/ajpheart.00722.2003 ◽

2005 ◽

Vol 288 (1) ◽

pp. H389-H399 ◽

Cited By ~ 4

Author(s):

Yael Yaniv ◽

Raphael Sivan ◽

Amir Landesberg

Keyword(s):

Sarcomere Length ◽

Force Response ◽

Shortening Velocity ◽

Low Frequencies ◽

Strong State ◽

Length Relationship ◽

Calcium Affinity ◽

Insight Into ◽

Mechanical Feedback

Analysis of the hystereses in the force-length relationship at constant Ca2+ concentration and in the force-calcium relationship at constant sarcomere length (SL) provides insight into the mechanisms that control cross-bridge (XB) recruitment. The hystereses are related here to two mechanisms that regulate the number of strong XBs: the cooperativity, whereby the number of strong XBs determines calcium affinity, and the mechanical feedback, whereby the shortening velocity determines the duration for which the XBs are in the strong state. The study simulates the phenomena and defines the role of these feedbacks. The model that couples calcium kinetics with XB cycling was built on Simulink software (Matlab). Counterclockwise (CCW) hysteresis, wherein the force response lags behind the SL oscillations, at a constant calcium level, is obtained in the force-length plane when neglecting the mechanical feedback and accounting only for the cooperativity mechanism. Conversely, the force response precedes the SL oscillations, yielding a clockwise (CW) hysteresis when only the mechanical feedback is allowed to exist. In agreement with experimental observations, either CW or CCW hysteresis is obtained when both feedbacks coexist: CCW hystereses are obtained at low frequencies (<3 Hz), and the direction is reversed to CW at higher frequencies (>3 Hz). The cooperativity dominates at low frequencies and allows the muscle to adapt XB recruitment to slow changes in the loading conditions. The changeover frequency from CCW to CW hysteresis defines the velocity limit above which the muscle absorbs rather than generates energy. The hysteresis in the force-calcium relation is conveniently explained by the same cooperativity mechanism. We propose that a single cooperativity mechanism that depends on the number of strong XBs can explain the hystereses in the force-length as well as in the force-calcium relationships.

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Regulatory mechanism of length-dependent activation in skinned porcine ventricular muscle: role of thin filament cooperative activation in the Frank-Starling relation

The Journal of General Physiology ◽

10.1085/jgp.201010502 ◽

2010 ◽

Vol 136 (4) ◽

pp. 469-482 ◽

Cited By ~ 22

Author(s):

Takako Terui ◽

Yuta Shimamoto ◽

Mitsunori Yamane ◽

Fuyu Kobirumaki ◽

Iwao Ohtsuki ◽

...

Keyword(s):

Thin Filament ◽

Linear Regression Analysis ◽

Left Ventricular ◽

Active Force ◽

Force Measurements ◽

Systematic Understanding ◽

Cardiac Sarcomeres ◽

Length Dependent Activation ◽

Cooperative Activation

Cardiac sarcomeres produce greater active force in response to stretch, forming the basis of the Frank-Starling mechanism of the heart. The purpose of this study was to provide the systematic understanding of length-dependent activation by investigating experimentally and mathematically how the thin filament “on–off” switching mechanism is involved in its regulation. Porcine left ventricular muscles were skinned, and force measurements were performed at short (1.9 µm) and long (2.3 µm) sarcomere lengths. We found that 3 mM MgADP increased Ca2+ sensitivity of force and the rate of rise of active force, consistent with the increase in thin filament cooperative activation. MgADP attenuated length-dependent activation with and without thin filament reconstitution with the fast skeletal troponin complex (sTn). Conversely, 20 mM of inorganic phosphate (Pi) decreased Ca2+ sensitivity of force and the rate of rise of active force, consistent with the decrease in thin filament cooperative activation. Pi enhanced length-dependent activation with and without sTn reconstitution. Linear regression analysis revealed that the magnitude of length-dependent activation was inversely correlated with the rate of rise of active force. These results were quantitatively simulated by a model that incorporates the Ca2+-dependent on–off switching of the thin filament state and interfilament lattice spacing modulation. Our model analysis revealed that the cooperativity of the thin filament on–off switching, but not the Ca2+-binding ability, determines the magnitude of the Frank-Starling effect. These findings demonstrate that the Frank-Starling relation is strongly influenced by thin filament cooperative activation.

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Role of Thin Filament Cooperative Activation in Length-dependent Activation in Skinned Porcine Ventricular Muscle

Biophysical Journal ◽

10.1016/j.bpj.2008.12.1936 ◽

2009 ◽

Vol 96 (3) ◽

pp. 223a

Author(s):

Takako Terui ◽

Yuta Shimamoto ◽

Munguntsetseg Sodnomtseren ◽

Mitsuhiro Yamane ◽

Iwao Ohtsuki ◽

...

Keyword(s):

Thin Filament ◽

Ventricular Muscle ◽

Length Dependent Activation ◽

Cooperative Activation

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Role of Calcium in Protein Folding and Function of Tva, the Receptor of Subgroup A Avian Sarcoma and Leukosis Virus

Journal of Virology ◽

10.1128/jvi.75.5.2051-2058.2001 ◽

2001 ◽

Vol 75 (5) ◽

pp. 2051-2058 ◽

Cited By ~ 16

Author(s):

Qing-Yin Wang ◽

Klavs Dolmer ◽

Wen Huang ◽

Peter G. W. Gettins ◽

Lijun Rong

Keyword(s):

Protein Folding ◽

Calcium Binding ◽

Titration Calorimetry ◽

Viral Receptor ◽

Calcium Affinity ◽

Binding Residues ◽

And Function ◽

Avian Sarcoma

ABSTRACT Tva is the cellular receptor for subgroup A avian sarcoma and leukosis virus (ASLV-A). The viral receptor function of Tva is determined by a 40-residue cysteine-rich motif called the LDL-A module. In this study, we expressed and purified the wild-type (wt) Tva LDL-A module as well as several mutants and examined their in vitro folding properties. We found that, as for other LDL-A modules, correct folding and structure of the Tva LDL-A module is Ca2+ dependent. When calcium was present during in vitro protein folding, the wt module was eluted as a single peak by reverse-phase high-pressure liquid chromatography. Furthermore, two-dimensional nuclear magnetic resonance (NMR) spectroscopy gave well-dispersed spectra in the presence of calcium. In contrast, the same protein folded in vitro in the absence of calcium was eluted as multiple broad peaks and gave a poorly dispersed NMR spectrum in the presence of calcium. The calcium affinity (Kd ) of the Tva LDL-A module, determined by isothermal titration calorimetry, is approximately 40 μM. Characterization of several Tva mutants provided further evidence that calcium is important in protein folding and function of Tva. Mutations of the Ca2+-binding residues (D46A and E47A) completely abrogated the Ca2+-binding ability of Tva, and the proteins were not correctly folded. Interestingly, mutations of two non-calcium-binding residues (W48A and L34A) also exerted adverse effect on Ca2+-dependent folding, albeit to a much less extent. Our results provide new insights regarding the structure and function of Tva in ASLV-A entry.

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The allosteric activation of cGAS underpins its dynamic signaling landscape

eLife ◽

10.7554/elife.39984 ◽

2018 ◽

Vol 7 ◽

Cited By ~ 19

Author(s):

Richard M Hooy ◽

Jungsan Sohn

Keyword(s):

Gene Expression ◽

Signaling Pathway ◽

Substrate Binding ◽

Allosteric Activation ◽

Interferon Responses ◽

Length Dependent Activation ◽

Antiviral Gene ◽

Substrate Concentrations

Cyclic G/AMP synthase (cGAS) initiates type-1 interferon responses against cytosolic double-stranded (ds)DNA, which range from antiviral gene expression to apoptosis. The mechanism by which cGAS shapes this diverse signaling landscape remains poorly defined. We find that substrate-binding and dsDNA length-dependent binding are coupled to the intrinsic dimerization equilibrium of cGAS, with its N-terminal domain potentiating dimerization. Notably, increasing the dimeric fraction by raising cGAS and substrate concentrations diminishes duplex length-dependent activation, but does not negate the requirement for dsDNA. These results demonstrate that reaction context dictates the duplex length dependence, reconciling competing claims on the role of dsDNA length in cGAS activation. Overall, our study reveals how ligand-mediated allostery positions cGAS in standby, ready to tune its signaling pathway in a switch-like fashion.

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Titin Determines the Frank-Starling Relation in Early Diastole

The Journal of General Physiology ◽

10.1085/jgp.20028652 ◽

2003 ◽

Vol 121 (2) ◽

pp. 97-110 ◽

Cited By ~ 80

Author(s):

Michiel Helmes ◽

Chee Chew Lim ◽

Ronglih Liao ◽

Ajit Bharti ◽

Lei Cui ◽

...

Keyword(s):

Restoring Force ◽

Cell Width ◽

Vitro Assay ◽

Rat Cardiomyocytes ◽

Early Diastole ◽

Restoring Forces ◽

Slack Length ◽

Length Dependent Activation

Titin, a giant protein spanning half the sarcomere, is responsible for passive and restoring forces in cardiac myofilaments during sarcomere elongation and compression, respectively. In addition, titin has been implicated in the length-dependent activation that occurs in the stretched sarcomere, during the transition from diastole to systole. The purpose of this study was to investigate the role of titin in the length-dependent deactivation that occurs during early diastole, when the myocyte is shortened below slack length. We developed a novel in vitro assay to assess myocyte restoring force (RF) by measuring the velocity of recoil in Triton-permeabilized, unloaded rat cardiomyocytes after rigor-induced sarcomere length (SL) contractions. We compared rigor-induced SL shortening to that following calcium-induced (pCa) contractions. The RF–SL relationship was linearly correlated, and the SL-pCa curve displayed a characteristic sigmoidal curve. The role of titin was defined by treating myocytes with a low concentration of trypsin, which we show selectively degrades titin using mass spectroscopic analysis. Trypsin treatment reduced myocyte RF as shown by a decrease in the slope of the RF-SL relationship, and this was accompanied by a downward and leftward shift of the SL-pCa curve, indicative of sensitization of the myofilaments to calcium. In addition, trypsin digestion did not alter the relationship between SL and interfilament spacing (assessed by cell width) after calcium activation. These data suggest that as the sarcomere shortens below slack length, titin-based restoring forces act to desensitize the myofilaments. Furthermore, in contrast to length-dependent activation at long SLs, length-dependent deactivation does not depend on interfilament spacing. This study demonstrates for the first time the importance of titin-based restoring force in length-dependent deactivation during the early phase of diastole.

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Role of dactinomycin in the improved survival of children with Wilms' tumor

JAMA ◽

10.1001/jama.195.12.1005 ◽

1966 ◽

Vol 195 (12) ◽

pp. 1005-1009 ◽

Cited By ~ 18

Author(s):

D. J. Fernbach

Keyword(s):

Wilms Tumor

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Role of the allergist in diagnosis and management of patients with uveitis

JAMA ◽

10.1001/jama.195.3.167 ◽

1966 ◽

Vol 195 (3) ◽

pp. 167-172 ◽

Cited By ~ 2

Author(s):

T. E. Van Metre

Keyword(s):

Diagnosis And Management

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The power of norms to sway fused group members

Behavioral and Brain Sciences ◽

10.1017/s0140525x18001644 ◽

2018 ◽

Vol 41 ◽

Cited By ~ 1

Author(s):

Winnifred R. Louis ◽

Craig McGarty ◽

Emma F. Thomas ◽

Catherine E. Amiot ◽

Fathali M. Moghaddam

Keyword(s):

Evolutionary Anthropology ◽

Normative Systems ◽

Violent Extremism ◽

Identity Fusion ◽

Group Members

AbstractWhitehouse adapts insights from evolutionary anthropology to interpret extreme self-sacrifice through the concept of identity fusion. The model neglects the role of normative systems in shaping behaviors, especially in relation to violent extremism. In peaceful groups, increasing fusion will actually decrease extremism. Groups collectively appraise threats and opportunities, actively debate action options, and rarely choose violence toward self or others.

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