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(Circulation Research. 2008;103:974.)
© 2008 American Heart Association, Inc.

Cellular Biology

Acceleration of Crossbridge Kinetics by Protein Kinase A Phosphorylation of Cardiac Myosin Binding Protein C Modulates Cardiac Function

Carl W. Tong, Julian E. Stelzer, Marion L. Greaser, Patricia A. Powers, Richard L. Moss

From the Department of Physiology, University of Wisconsin School of Medicine and Public Health, Madison.

Correspondence to Richard L. Moss, PhD, Professor and Chair, Department of Physiology, University of Wisconsin School of Medicine and Public Health, Room 123 SMI, 1300 University Ave, Madison, WI 53706. E-mail rlmoss{at}physiology.wisc.edu

Normal cardiac function requires dynamic modulation of contraction. β1-Adrenergic–induced protein kinase (PK)A phosphorylation of cardiac myosin binding protein (cMyBP)-C may regulate crossbridge kinetics to modulate contraction. We tested this idea with mechanical measurements and echocardiography in a mouse model lacking 3 PKA sites on cMyBP-C, ie, cMyBP-C(t3SA). We developed the model by transgenic expression of mutant cMyBP-C with Ser-to-Ala mutations on the cMyBP-C knockout background. Western blots, immunofluorescence, and in vitro phosphorylation combined to show that non–PKA-phosphorylatable cMyBP-C expressed at 74% compared to normal wild-type (WT) and was correctly positioned in the sarcomeres. Similar expression of WT cMyBP-C at 72% served as control, ie, cMyBP-C(tWT). Skinned myocardium responded to stretch with an immediate increase in force, followed by a transient relaxation of force and finally a delayed development of force, ie, stretch activation. The rate constants of relaxation, k_rel (s-1), and delayed force development, k_df (s-1), in the stretch activation response are indicators of crossbridge cycling kinetics. cMyBP-C(t3SA) myocardium had baseline k_rel and k_df similar to WT myocardium, but, unlike WT, k_rel and k_df were not accelerated by PKA treatment. Reduced dobutamine augmentation of systolic function in cMyBP-C(t3SA) hearts during echocardiography corroborated the stretch activation findings. Furthermore, cMyBP-C(t3SA) hearts exhibited basal echocardiographic findings of systolic dysfunction, diastolic dysfunction, and hypertrophy. Conversely, cMyBP-C(tWT) hearts performed similar to WT. Thus, PKA phosphorylation of cMyBP-C accelerates crossbridge kinetics and loss of this regulation leads to cardiac dysfunction.

Key Words: cMyBP-C • phosphorylation • contraction kinetics

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