Structural and functional evaluation of branched myofibers lacking intermediate filaments

Area of Science:

• Muscle Physiology
• Cell Biology
• Biochemistry

Background:

• Intermediate filaments (IFs), including desmin and keratin 19 (K19), are essential for skeletal muscle structure and function.
• Previous studies indicated that fast-twitch muscle lacking desmin and K19 exhibits reduced force and increased injury susceptibility.
• Alterations in excitation-contraction (EC) coupling calcium (Ca2+) kinetics are observed in muscular dystrophy models like mdx mice.

Purpose of the Study:

• To investigate the impact of desmin and K19 deficiency on myofiber malformations and EC coupling Ca2+ kinetics in skeletal muscle.
• To test the hypothesis that IF protein absence increases malformed myofibers and alters Ca2+ handling, similar to mdx mice.

Main Methods:

• Quantified branched myofibers and characterized their organization using confocal and electron microscopy (EM).
• Compared EC coupling Ca2+ kinetics in flexor digitorum brevis myofibers from desmin(-/-), K19(-/-), and double knockout (DKO) mice.
• Utilized age-matched wild type (WT) and mdx mice as controls.

Main Results:

• Desmin-deficient (Des(-/-)) myofibers showed significantly more malformations (4.7%) compared to K19(-/-) (0.9%) and DKO (1.3%) mice.
• Global Ca2+ signals revealed decreased Ca2+ uptake into the sarcoplasmic reticulum, most notably in branched DKO myofibers (44% increase in uptake relative to WT).
• Despite faster Ca2+ clearance in branched DKO myofibers, the reduced branching phenotype in DKO suggests K19 absence corrects desmin deficiency-related defects.

Conclusions:

• Desmin plays a more critical role in myofiber integrity than K19, as its absence leads to greater malformations.
• Both desmin and K19 IFs have complex roles in skeletal muscle, influencing Ca2+ reuptake and myofiber branching.
• The absence of K19 appears to mitigate the negative effects of desmin deficiency on myofiber structure and Ca2+ handling.