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* Departments of Bioengineering and Orthopaedics, Biomedical Sciences Graduate Group, University of California and Veterans Administration Medical Centers, San Diego, California USA; Department of Cell Biology, Baylor College of Medicine, Houston, Texas USA; National Institutes of Health, Bethesda, Maryland USA; and Center for Basic Research, Foundation for Biomedical Research, Academy of Athens, Greece and Department of Biology, University of Patras, Greece
Correspondence: Address reprint requests to Richard L. Lieber, PhD, Dept. of Orthopaedics (9151), VA Medical Center and UC San Diego, 3350 La Jolla Village Dr., San Diego, CA 92161. Tel.: 858-552-8585 ext. 7016; Fax: 858-552-4381; E-mail: rlieber{at}ucsd.edu.
Mechanical interactions between desmin and Z-disks, costameres, and nuclei were measured during passive deformation of single muscle cells. Image processing and continuum kinematics were used to quantify the structural connectivity among these structures. Analysis of both wild-type and desmin-null fibers revealed that the costamere protein talin colocalized with the Z-disk protein 
-actinin, even at very high strains and stresses. These data indicate that desmin is not essential for mechanical coupling of the costamere complex and the sarcomere lattice. Within the sarcomere lattice, significant differences in myofibrillar connectivity were revealed between passively deformed wild-type and desmin-null fibers. Connectivity in wild-type fibers was significantly greater compared to desmin-null fibers, demonstrating a significant functional connection between myofibrils that requires desmin. Passive mechanical analysis revealed that desmin may be partially responsible for regulating fiber volume, and consequently, fiber mechanical properties. Kinematic analysis of -actinin strain fields revealed that knockout fibers transmitted less shear strain compared to wild-type fibers and experienced a slight increase in fiber volume. Finally, linkage of desmin intermediate filaments to muscle nuclei was strongly suggested based on extensive loss of nuclei positioning in the absence of desmin during passive fiber loading.
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