Titin and Troponin: Central Players in the Frank-Starling Mechanism of the Heart

The basis of the Frank-Starling mechanism of the heart is the intrinsic ability of cardiac muscle to produce greater active force in response to stretch, a phenomenon known as length-dependent activation. A feedback mechanism transmitted from cross-bridge formation to troponin C to enhance Ca2+ binding has long been proposed to account for length-dependent activation. However, recent advances in muscle physiology research technologies have enabled the identification of other factors involved in length-dependent activation. The striated muscle sarcomere contains a third filament system composed of the giant elastic protein titin, which is responsible for most passive stiffness in the physiological sarcomere length range. Recent studies have revealed a significant coupling of active and passive forces in cardiac muscle, where titin-based passive force promotes cross-bridge recruitment, resulting in greater active force production in response to stretch. More currently, the focus has been placed on the troponin-based “on-off” switching of the thin filament state in the regulation of length-dependent activation. In this review, we discuss how myocardial length-dependent activation is coordinately regulated by sarcomere proteins

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

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

Sarcomere length-dependent Ca2+ activation in skinned rabbit psoas muscle fibers: coordinated regulation of thin filament cooperative activation and passive force

In skeletal muscle, active force production varies as a function of sarcomere length (SL). It has been considered that this SL dependence results simply from a change in the overlap length between the thick and thin filaments. The purpose of this study was to provide a systematic understanding of the SL-dependent increase in Ca2+ sensitivity in skeletal muscle, by investigating how thin filament “on–off” switching and passive force are involved in the regulation. Rabbit psoas muscles were skinned, and active force measurements were taken at various Ca2+ concentrations with single fibers, in the short (2.0 and 2.4 μm) and long (2.4 and 2.8 μm) SL ranges. Despite the same magnitude of SL elongation, the SL-dependent increase in Ca2+ sensitivity was more pronounced in the long SL range. MgADP (3 mM) increased the rate of rise of active force and attenuated SL-dependent Ca2+ activation in both SL ranges. Conversely, inorganic phosphate (Pi, 20 mM) decreased the rate of rise of active force and enhanced SL-dependent Ca2+ activation in both SL ranges. Our analyses revealed that, in the absence and presence of MgADP or Pi, the magnitude of SL-dependent Ca2+ activation was (1) inversely correlated with the rate of rise of active force, and (2) in proportion to passive force. These findings suggest that the SL dependence of active force in skeletal muscle is regulated via thin filament “on–off” switching and titin (connectin)-based interfilament lattice spacing modulation in a coordinated fashion, in addition to the regulation via the filament overlap

Post-induction MRD by FCM and GATA1-PCR are significant prognostic factors for myeloid leukemia of Down syndrome.

Myeloid leukemia of Down syndrome (ML-DS) is associated with good response to chemotherapy, resulting in favorable outcomes. However, no universal prognostic factors have been identified to date. To clarify a subgroup with high risk of relapse, the role of minimal residual disease (MRD) was explored in the AML-D11 trial by the Japanese Pediatric Leukemia/Lymphoma Study Group. MRD was prospectively evaluated at after induction therapy and at the end of all chemotherapy, using flow cytometry (FCM-MRD) and GATA1-targeted deep sequencing (GATA1-MRD). A total of 78 patients were eligible and 76 patients were stratified to the standard risk (SR) group by morphology. In SR patients, FCM-MRD and GATA1-MRD after induction were positive in 5/65 and 7/59 patients, respectively. Three-year event-free survival (EFS) and overall survival (OS) rates were 93.3% and 95.0% in the FCM-MRD-negative population, and 60.0% and 80.0% in the positive population. Three-year EFS and OS rates were both 96.2% in the GATA1-MRD-negative population, and 57.1% and 71.4% in the positive population. Adjusted hazard ratios for associations of FCM-MRD or GATA1-MRD with EFS were 10.98 (p = 0.01) and 27.68 (p < 0.01), respectively. Detection of MRD by either FCM or GATA1 after initial induction therapy represents a significant prognostic factor for predicting ML-DS relapse