Structural and functional evaluation of branched myofibers lacking intermediate filaments.

Intermediate filaments (IFs), composed of desmin and keratins, link myofibrils to each other and to the sarcolemma in skeletal muscle. Fast-twitch muscle of mice lacking the IF proteins, desmin and keratin 19 (K19), showed reduced specific force and increased susceptibility to injury in earlier studies. Here we tested the hypothesis that the number of malformed myofibers in mice lacking desmin (Des^-/-), keratin 19 (K19^-/-), or both IF proteins (double knockout, DKO) is increased and is coincident with altered excitation-contraction (EC) coupling Ca²⁺ kinetics, as reported for mdx mice. We quantified the number of branched myofibers, characterized their organization with confocal and electron microscopy (EM), and compared the Ca²⁺ kinetics of EC coupling in flexor digitorum brevis myofibers from adult Des^-/-, K19^-/-, or DKO mice and compared them to agematched wild type (WT) and mdx myofibers. Consistent with our previous findings, 9.9% of mdx myofibers had visible malformations. Des^-/- myofibers had more malformations (4.7%) than K19^-/- (0.9%) or DKO (1.3%) myofibers. Confocal and EM imaging revealed no obvious changes in sarcomere misalignment at the branch points, and the neuromuscular junctions in the mutant mice, while more variably located, were limited to one per myofiber. Global, electrically evoked Ca²⁺ signals showed a decrease in the rate of Ca²⁺ uptake (decay rate) into the sarcoplasmic reticulum after Ca²⁺ release, with the most profound effect in branched DKO myofibers (44% increase in uptake relative to WT). Although branched DKO myofibers showed significantly faster rates of Ca²⁺ clearance, the milder branching phenotype observed in DKO muscle suggests that the absence of K19 corrects the defect created by the absence of desmin alone. Thus, there are complex roles for desmin-based and K19-based IFs in skeletal muscle, with the null and DKO mutations having different effects on Ca²⁺ reuptake and myofiber branching. [ABSTRACT FROM AUTHOR]