What is the clinical evidence from this study?

Study design: Other. Population: Muscle contraction. Intervention: Recombinant N-terminal fragments of MyBP-C isoforms (ssC1C2, fsC1C2, C0C2) vs. Untreated control. Primary outcome: Thin filament sliding velocity and force production at varying Ca2+ levels.

February 2, 2018Open Access

Skeletal myosin binding protein-C isoforms regulate thin filament activity in a Ca2+-dependent manner

BLBrian L. LinMedical College of Wisconsin ALAmy LiThe University of Sydney JMJi Young MunKorea Brain Research Institute

Key Result

Cardiac and slow-skeletal MyBP-C isoforms activate thin filament sliding at low Ca2+, whereas fast-skeletal and cardiac MyBP-C reduce sliding velocities and increase force production at higher Ca2+.

Structured PICO

Population

Preclinical models: permeabilized rat papillary muscle fibers (n=7-9 animals), mouse ventricular myosin and native thin filaments, reconstituted thin filaments, and adult rat ventricular myocytes (ARVM).

Intervention

Recombinant N-terminal fragments of MyBP-C isoforms (ssC1C2, fsC1C2, C0C2 at 0.10-10 µM) and adenoviral-mediated overexpression of full-length MyBP-C isoforms.

Comparator

Vehicle control, low Ca2+ control filaments, or uninfected controls.

Outcome

Thin filament activation, sliding velocities, force development, Ca2+ sensitivity, and relaxation kinetics.surrogate

Cardiac MyBP-C regulates contraction over a full range of Ca2+ levels, whereas slow- and fast-skeletal isoforms regulate contraction at lower and higher Ca2+ ranges, respectively.

Limitations

Use of N-terminal fragments in isolation may exaggerate functional effects outside the C-zone.
Predominant myosin isoforms in human hearts differ from those of the mouse models used.
Focus on thin filament interactions does not preclude effects of MyBP-C on myosin.

Abstract

Abstract Muscle contraction, which is initiated by Ca 2+ , results in precise sliding of myosin-based thick and actin-based thin filament contractile proteins. The interactions between myosin and actin are finely tuned by three isoforms of myosin binding protein-C (MyBP-C): slow-skeletal, fast-skeletal, and cardiac (ssMyBP-C, fsMyBP-C and cMyBP-C, respectively), each with distinct N-terminal regulatory regions. The skeletal MyBP-C isoforms are conditionally coexpressed in cardiac muscle, but little is known about their function. Therefore, to characterize the functional differences and regulatory mechanisms among these three isoforms, we expressed recombinant N-terminal fragments and examined their effect on contractile properties in biophysical assays. Addition of the fragments to in vitro motility assays demonstrated that ssMyBP-C and cMyBP-C activate thin filament sliding at low Ca 2+ . Corresponding 3D electron microscopy reconstructions of native thin filaments suggest that graded shifts of tropomyosin on actin are responsible for this activation (cardiac > slow-skeletal > fast-skeletal). Conversely, at higher Ca 2+ , addition of fsMyBP-C and cMyBP-C fragments reduced sliding velocities in the in vitro motility assays and increased force production in cardiac muscle fibers. We conclude that due to the high frequency of Ca 2+ cycling in cardiac muscle, cardiac MyBP-C may play dual roles at both low and high Ca 2+ . However, skeletal MyBP-C isoforms may be tuned to meet the needs of specific skeletal muscles.

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