Nonmuscle γcyto-actin is expressed at very low levels in skeletal muscle but uniquely localizes to costameres, the cytoskeletal networks that couple peripheral myofibrils to the sarcolemma. We generated and analyzed skeletal muscle-specific γcyto-actin knockout (Actg1-msKO) mice. Although muscle development proceeded normally, Actg1-msKO mice presented with overt muscle weakness accompanied by a progressive pattern of muscle fiber necrosis/regeneration. Functional deficits in whole-body tension and isometric twitch force were observed, consistent with defects in the connectivity between muscle fibers and/or myofibrils or at the myotendinous junctions. Surprisingly, γcyto-actin-deficient muscle did not demonstrate the fibrosis, inflammation, and membrane damage typical of several muscular dystrophies but rather presented with a novel progressive myopathy. Together, our data demonstrate an important role for minimally abundant but strategically localized γcyto-actin in adult skeletal muscle and describe a new mouse model to study the in vivo relevance of subcellular actin isoform sorting.
Bibliographical noteFunding Information:
The authors thank Michele Jaeger and Kurt Prins for technical assistance and Drs. James Lessard and Judith Melki for reagents and mice, respectively. This work was supported by an American Heart Association Pre-Doctoral Fellowship and grants from the Muscular Dystrophy Association, the National Institutes of Health (AR049899), and National Research Service Award T32 HL07936 from the University of Wisconsin-Madison Cardiovascular Research Center.