Do cardiac actin mutations lead to altered actomyosin interactions?

Area of Science:

• Cardiovascular Biology
• Molecular Cardiology
• Biophysics

Background:

• Hypertrophic cardiomyopathy (HCM) and dilated cardiomyopathy (DCM) are hypothesized to result from altered cardiac muscle contractility.
• The cardiac actin gene (ACTC) is implicated in these conditions, but the precise molecular mechanisms remain under investigation.
• Understanding how ACTC mutations affect the fundamental interaction between actin and myosin is crucial for elucidating disease pathogenesis.

Purpose of the Study:

• To investigate the impact of eight cardiac actin (ACTC) mutations, found in HCM and DCM patients, on the actomyosin interaction.
• To determine if specific ACTC mutations alter actin-activated myosin S1 ATPase rates, in vitro motility, and duty ratios.
• To correlate observed molecular changes with potential mechanisms underlying HCM and DCM development.

Main Methods:

• Biochemical assays measuring actin-activated myosin S1 ATPase rates for wild-type (WT) and mutant ACTC proteins.
• In vitro motility assays were performed across varying ATP concentrations to assess kinetic parameters (KM, Vmax).
• Calculation of the duty ratio, representing the proportion of time myosin spends attached to actin during the cross-bridge cycle, using a defined myosin step size.

Main Results:

• The R312H ACTC mutation showed a decreased actin-activated myosin S1 ATPase rate compared to WT.
• The E99K ACTC mutation exhibited a significantly higher ATPase rate and a doubled duty ratio compared to WT.
• While E99K demonstrated altered kinetics (unchanged KM, decreased Vmax), other tested ACTC mutants did not show significant changes in basic actomyosin interactions.

Conclusions:

• Mutations in the cardiac actin gene (ACTC) can lead to cardiomyopathies through diverse molecular mechanisms.
• The E99K mutation's altered actomyosin kinetics, including increased ATP usage and tension, may contribute to HCM.
• For several other ACTC mutants, disease development likely involves molecular mechanisms independent of direct alterations in the fundamental actin-myosin interaction.