Restoring force development by titin/connectin and assessment of Ig domain unfolding

Nair Preetha, Wu Yiming, Michiel Helmes, Fukuda Norio, Labeit Siegfried, Henk Granzier

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21 Scopus citations

Abstract

Titin/connectin is the main determinant of physiological levels of passive muscle force. This force is generated by the extensible I-band region of the molecule, which is composed of serially-linked immunoglobulin (Ig)-like domains and several unique sequence elements. Here we address the role of titin/connectin in sarcomeres shortened to below the slack length (length attained by an un-activated cell in absence of external forces). Such shortened cells develop so-called restoring forces that re-extend the cells upon relaxation. The experiments that we present are based on a high throughput method with a rapid solution switching system which allows unattached single cardiac myocytes to be activated (resulting in shortening below the slack length) and then to be rapidly relaxed while their maximal re-lengthening velocity is measured at the sarcomere level (dSL/dt max), with high-resolution imaging techniques. Experiments were carried out on myocytes that express different isoforms of titin/connectin. We measured the relation between dSL/dt max and the minimal SL during contraction (SL min) and determined the slope of this relation as a measure of 'restoring stiffness.' We found that the restoring stiffness correlates with the isoform expression profile with myocytes that express high levels of the stiff isoform (N2B) having the highest restoring stiffness. These results support the notion that titin/connectin is a bi-directional spring that develops passive force when stretched above the slack length and restoring force when shortened to below this length. We also discuss in detail the mechanisms that underlie titin/connectin's restoring force development and focus on whether or not unfolding of Ig domains plays a role.

Original languageEnglish (US)
Pages (from-to)307-317
Number of pages11
JournalJournal of Muscle Research and Cell Motility
Volume26
Issue number6-8
DOIs
StatePublished - Dec 1 2005

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ASJC Scopus subject areas

  • Biochemistry
  • Physiology
  • Cell Biology

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