Human actin mutations associated with hypertrophic and dilated cardiomyopathies demonstrate distinct thin filament...

Area of Science:

• Molecular Biology
• Biophysics
• Cardiovascular Research

Background:

• Cardiomyopathies are a group of diseases affecting the heart muscle.
• Specific mutations in cardiac actin, such as E99K and R312H, are linked to hypertrophic and dilated cardiomyopathy, respectively.
• Understanding the molecular basis of these mutations is crucial for developing targeted therapies.

Purpose of the Study:

• To investigate the molecular mechanics and thin filament regulation differences between E99K and R312H actin mutations.
• To determine how these mutations affect cardiac muscle contractility at a molecular level.

Main Methods:

• Expression of human cardiac actin mutations (E99K, R312H) using a Baculovirus/insect cell system.
• In vitro motility assays to measure thin filament velocity under varying calcium and ATP conditions.
• Single molecule laser trap assays to analyze myosin-actin interactions, including step size and binding duration.

Main Results:

• Both E99K and R312H mutations significantly reduced thin filament velocity at maximal calcium compared to wild type (WT).
• R312H exhibited increased calcium sensitivity (pCa50) at submaximal calcium levels.
• E99K mutation impaired the frequency of myosin strong-binding events and reduced cooperativity in the presence of troponin and tropomyosin.
• Neither mutation significantly altered myosin's unitary step size or strong binding duration.

Conclusions:

• The E99K mutation inhibits thin filament activation by myosin strong-binding, contributing to hypertrophic cardiomyopathy.
• The R312H mutation enhances calcium activation, explaining its association with dilated cardiomyopathy.
• Divergent molecular mechanical properties of actin mutations underlie distinct cardiomyopathy phenotypes.