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Focal Ca2+ Transient Detection in Smooth Muscle
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Published on: June 29, 2009

A force-activated kinase in a catch smooth muscle.

Thomas M Butler1, Marion J Siegman

  • 1Department of Molecular Physiology and Biophysics, Jefferson Medical College, Thomas Jefferson University, Philadelphia, PA 19107, USA. thomas.butler@jefferson.edu

Journal of Muscle Research and Cell Motility
|February 3, 2011
PubMed
Summary

Stretching smooth muscle in the catch state activates a force-sensitive kinase, increasing peptide phosphorylation. This kinase activity is inhibited by ML-9 and ML-7, suggesting a role in muscle force regulation.

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Area of Science:

  • Muscle Physiology
  • Biochemistry
  • Molecular Biology

Background:

  • The anterior byssus retractor muscle (ABRM) of Mytilus edulis exhibits catch, a state of sustained force maintenance at low intracellular calcium levels.
  • In catch, twitchin is unphosphorylated and thought to mediate force through filament tethering, with force highly sensitive to length changes.
  • A force-dependent kinase activation has been postulated for titin and twitchin, but direct evidence was lacking.

Purpose of the Study:

  • To investigate whether a kinase is activated by mechanical stretch in the smooth muscle ABRM.
  • To determine if this stretch-dependent kinase activation is linked to the catch state and twitchin phosphorylation.
  • To identify potential kinases involved using specific inhibitors.

Main Methods:

  • Permeabilized ABRM from Mytilus edulis were utilized as an experimental system.
  • A synthetic peptide, a known substrate for molluscan twitchin kinase, was used to measure kinase activation via phosphorylation.
  • Muscle preparations were subjected to stretch in different states (catch and activated), and kinase activity was assessed in the presence of kinase inhibitors (ML-9, ML-7, Y-27632).

Main Results:

  • A 10% stretch of ABRM in the catch state caused a two-fold increase in peptide phosphorylation, indicating kinase activation.
  • This increased phosphorylation was due to kinase activation, not phosphatase inhibition, and was reduced when catch force was absent.
  • Stretch also increased peptide phosphorylation during activation at pCa 5, and force-sensitive kinase activity was inhibited by ML-9 and ML-7, but not Y-27632. Twitchin phosphorylation also increased with stretch.

Conclusions:

  • A kinase in ABRM is activated by mechanical stretch, particularly in the catch state.
  • This force-sensitive kinase activity is likely mediated by twitchin kinase, as suggested by inhibitor effects and increased twitchin phosphorylation.
  • The findings support a model where twitchin senses force output, leading to kinase activation and potentially autophosphorylation, contributing to muscle force regulation.