Mechanical properties of titin isoforms

Area of Science:

• Muscle Physiology
• Biophysics
• Molecular Biology

Background:

• Titin is a giant protein crucial for sarcomere structure and function.
• Differential splicing creates titin isoforms with varying lengths in different muscles.
• Titin contributes significantly to passive and restoring forces in cardiac and skeletal muscles.

Purpose of the Study:

• To investigate the mechanical properties and molecular basis of titin's extensibility in different muscle types.
• To model titin's behavior using its known molecular properties and test it against experimental data.
• To compare titin isoforms in cardiac myocytes and elucidate their role in passive tension.

Main Methods:

• Mechanical testing of skeletal muscle fibers and cardiac myocytes.
• Immunoelectron microscopy (IEM) to visualize titin structure and localization.
• Development and application of a mechanical model for titin's extensible region.

Main Results:

• A model of titin's extensible region as serially linked WLCs adequately predicted behavior in skeletal muscle but overestimated force at long SLs, suggesting Ig domain unfolding.
• Cardiac myocytes expressing N2B titin (mouse) showed a steeper passive tension-sarcomere length relation than those expressing N2BA titin (cow).
• IEM revealed the N2B sequence as an additional extensibility source in cardiac titin, and the PEVK segment was significantly longer in N2BA (200 nm) than N2B (60 nm).

Conclusions:

• Ig domain unfolding may limit titin's force at long sarcomere lengths in skeletal muscle.
• The N2B sequence contributes to cardiac titin extensibility.
• Longer PEVK segments in N2BA titin isoforms lead to lower passive tensions in cardiac muscle.