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Hypertrophic and dilated cardiomyopathy mutations differentially affect the molecular force generation of mouse -cardiac myosin in the laser trap assay

¹Department of Molecular Physiology and Biophysics, University of Vermont, Burlington, Vermont; ²Institute of Pharmacology and Toxicology, University of Würzburg, Würzburg, Germany; ³Department of Physiology and Biophysics, Boston University School of Medicine, Boston; ⁴Department of Genetics and Howard Hughes Medical Institute, Harvard Medical School, Boston; and ⁵Cardiovascular Division and ⁶Howard Hughes Medical Institute, Brigham and Women's Hospital, Boston, Massachusetts

Submitted 31 January 2007 ; accepted in final form 3 March 2007

Point mutations in cardiac myosin, the heart's molecular motor, produce distinct clinical phenotypes: hypertrophic (HCM) and dilated (DCM) cardiomyopathy. Do mutations alter myosin's molecular mechanics in a manner that is predictive of the clinical outcome? We have directly characterized the maximal force-generating capacity (F_max) of two HCM (R403Q, R453C) and two DCM (S532P, F764L) mutant myosins isolated from homozygous mouse models using a novel load-clamped laser trap assay. F_max was 50% (R403Q) and 80% (R453C) greater for the HCM mutants compared with the wild type, whereas F_max was severely depressed for one of the DCM mutants (65% S532P). Although F_max was normal for the F764L DCM mutant, its actin-activated ATPase activity and actin filament velocity (V_actin) in a motility assay were significantly reduced (Schmitt JP, Debold EP, Ahmad F, Armstrong A, Frederico A, Conner DA, Mende U, Lohse MJ, Warshaw D, Seidman CE, Seidman JG. Proc Natl Acad Sci USA 103: 14525–14530, 2006.). These F_max data combined with previous V_actin measurements suggest that HCM and DCM result from alterations to one or more of myosin's fundamental mechanical properties, with HCM-causing mutations leading to enhanced but DCM-causing mutations leading to depressed function. These mutation-specific changes in mechanical properties must initiate distinct signaling cascades that ultimately lead to the disparate phenotypic responses observed in HCM and DCM.

familial hypertrophic cardiomyopathy; in vitro motility; force-velocity relationship; heart failure

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