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Evidence for increased myofibrillar mobility in desmin-null mouse skeletal muscle
1 Departments of Orthopaedics and Bioengineering, Veterans Affairs Medical Center and University of California at San Diego, San Diego, CA 92093, USA,
2 Institute of Biomedical Engineering, National Cheng Kung University, Tainan, Taiwan,
3 Department of Cell Biology, Baylor College of Medicine, Houston, TX 77030, USA and
4 Department of Hand Surgery, Sahlgrenska University Hospital, Göteborg, Sweden
*Author for correspondence (e-mail: rlieber{at}ucsd.edu)
Accepted 22 November 2001
Quantitative electron microscopy was used to characterize the longitudinal mobility of myofibrils during muscle extension to investigate the functional roles of skeletal muscle intermediate filaments. Extensor digitorum longus fifth toe muscles from wild-type (+/+) and desmin-null (des –/–) animals were passively stretched to varying lengths, and the horizontal displacement of adjacent Z-disks in neighboring myofibrils (
xmyo) and average sarcomere length (SL) were calculated. At short SL (<2.20 µm), wild-type and desmin-null xmyo were not significantly different, although there was a trend towards greater Z-disk misalignment in muscles from knockout animals (xmyo 0.34±0.04 µm versus 0.22±0.09 µm; P>0.2; means ± S.E.M.). However, at higher SL (>2.90 µm), muscles from knockout animals displayed a dramatically increased xmyo relative to wild-type muscles (0.49±0.10 µm versus 0.25±0.07 µm; P<0.05). The results, which establish a maximum extension of the desmin network surrounding the Z-disk, provide what we believe to be the first quantitative estimation of the functional limits of the desmin intermediate filament system in the presence of an intact myofibrillar lattice. The existence of a limit on the extension of desmin suggests a mechanism for the recruitment of desmin into a network of force transmission, whether as a longitudinal load bearer or as a component in a radial force-transmission system.
Key words: passive strain, electron microscopy, intermediate filament, force transmission, muscle, mouse.
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