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Investigating the Pathogenesis of MYH7 Mutation Gly823Glu in Familial Hypertrophic Cardiomyopathy using a Mouse Model
Published on: August 8, 2022
Altered crossbridge kinetics in the alphaMHC403/+ mouse model of familial hypertrophic cardiomyopathy
E Blanchard1, C Seidman, J G Seidman
1Department of Molecular Physiology and Biophysics, University of Vermont Medical School, Burlington, VT, USA.
Insights
The Arg403Gln (R403Q) mutation in alpha-myosin heavy chain increases diastolic stiffness and submaximal tension in familial hypertrophic cardiomyopathy (FHC) mouse models. This mutation may reduce myosin-actin binding, impacting crossbridge function and leading to diverse FHC symptoms.
Area of Science:
- Cardiovascular Biology
- Muscle Physiology
- Molecular Cardiology
Background:
- Familial hypertrophic cardiomyopathy (FHC) is a genetic heart disease often caused by mutations in myosin heavy chain genes.
- The Arg403Gln (R403Q) mutation in cardiac beta-myosin heavy chain is linked to severe FHC in humans.
- Understanding the mechanical consequences of FHC-associated mutations is crucial for elucidating disease mechanisms.
Purpose of the Study:
- To investigate the mechanical properties of cardiac muscle with the R403Q mutation in alpha-myosin heavy chain (alphaMHC403/+) using a mouse model.
- To determine how this mutation affects muscle stiffness, tension generation, and crossbridge dynamics.
- To explore the molecular basis for the observed mechanical changes and their relation to FHC.
Main Methods:
- Utilized small-amplitude (0.25%) length-perturbation analysis on skinned left ventricular papillary muscle strips from alphaMHC403/+ mice and wild-type littermates.
- Assessed dynamic stiffness at resting conditions (pCa 8) and isometric tension at submaximal (pCa 6) and maximal (pCa 5) calcium activation.
- Measured characteristic frequencies of crossbridge steps (b and c) and oscillatory power output under varying calcium concentrations.
Main Results:
- alphaMHC403/+ muscle strips exhibited approximately 40% greater dynamic stiffness at rest compared to wild-type.
- At submaximal calcium activation (pCa 6), isometric tension was threefold higher in mutant strips, while maximal tension (pCa 5) was marginally lower.
- Reduced characteristic frequencies (b and c) and significantly lower oscillatory power were observed in mutant strips at maximal activation.
Conclusions:
- The R403Q mutation in alphaMHC leads to increased passive diastolic stiffness and altered force-generating properties of cardiac muscle.
- Results support the hypothesis that the mutation reduces myosin's strong binding affinity to actin, leading to preforce state accumulation.
- These calcium-dependent mechanical alterations, coupled with myofibrillar disarray, likely contribute to the heterogeneous clinical presentation of FHC.
Abstract:
A mutation in the cardiac beta-myosin heavy chain, Arg403Gln (R403Q), causes a severe form of familial hypertrophic cardiomyopathy (FHC) in humans. We used small-amplitude (0.25%) length-perturbation analysis to examine the mechanical properties of skinned left ventricular papillary muscle strips from mouse hearts bearing the R403Q mutation in the alpha-myosin heavy chain (alphaMHC403/+). Myofibrillar disarray with variable penetrance occurred in the left ventricular free wall of the alphaMHC403/+ hearts. In resting strips (pCa 8), dynamic stiffness was approximately 40% greater than in wild-type strips, consistent with elevated diastolic stiffness reported for murine hearts with FHC. At pCa 6 (submaximal activation), strip isometric tension was approximately 3 times higher than for wild-type strips, whereas at pCa 5 (maximal activation), tension was marginally lower. At submaximal calcium activation the characteristic frequencies of the work-producing (b) and work-absorbing (c) steps of the crossbridge were less in alphaMHC403/+ strips than in wild-type strips (b=11+/-1 versus 15+/-1 Hz; c= 58+/-3 versus 66+/-3 Hz; 27 degrees C). At maximal calcium activation, strip oscillatory power was reduced (0. 53+/-0.25 versus 1.03+/-0.18 mW/mm3; 27 degrees C), which is partly attributable to the reduced frequency b, at which crossbridge work is maximum. The results are consistent with the hypothesis that the R403Q mutation reduces the strong binding affinity of myosin for actin. Myosin heads may accumulate in a preforce state that promotes cooperative activation of the thin filament at submaximal calcium but blunts maximal tension and oscillatory power output at maximal calcium. The calcium-dependent effect of the mutation (whether facilitating or debilitating), together with a variable degree of fibrosis and myofibrillar disorder, may contribute to the diversity of clinical symptoms observed in murine FHC.
