Muscle-specific stress fibers give rise to sarcomeres in cardiomyocytes

• DOI: 10.7554/elife.42144
• Publication Date: 2018-12-12T13:01:35Z
• Authors: Aidan M Fenix; Abigail C Neininger; Nilay Taneja; Karren Hyde; Mike R Visetsouk; Ryan J Garde; Baohong Liu; Benjamin R Nixon; Annabelle E Manalo; Jason R Becker; Scott W Crawley; David M Bader; Matthew J Tyska; Qi Liu; Jennifer H Gutzman; Dylan T Burnette
• Keywords: Sarcomeres; 0301 basic medicine; QH301-705.5; Science; Muscle Fibers, Skeletal; Formins; cardiomyocytes; myosin; 7. Clean energy; sarcomeres; Cell Line; 03 medical and health sciences; Cell Line, Tumor; Stress Fibers; Humans; Myocytes, Cardiac; Biology (General); Microscopy, Confocal; Nonmuscle Myosin Type IIB; Myosin Heavy Chains; Molecular Motor Proteins; Q; Microfilament Proteins; R; Cell Biology; Actins; formins; Actin Cytoskeleton; 13. Climate action; Medicine; RNA Interference; actin; HeLa Cells

The sarcomere is the contractile unit within cardiomyocytes driving heart muscle contraction. We sought to test the mechanisms regulating actin and myosin filament assembly during sarcomere formation. Therefore, we developed an assay using human cardiomyocytes to monitor sarcomere assembly. We report a population of muscle stress fibers, similar to actin arcs in non-muscle cells, which are essential sarcomere precursors. We show sarcomeric actin filaments arise directly from muscle stress fibers. This requires formins (e.g., FHOD3), non-muscle myosin IIA and non-muscle myosin IIB. Furthermore, we show short cardiac myosin II filaments grow to form ~1.5 μm long filaments that then ‘stitch’ together to form the stack of filaments at the core of the sarcomere (i.e., the A-band). A-band assembly is dependent on the proper organization of actin filaments and, as such, is also dependent on FHOD3 and myosin IIB. We use this experimental paradigm to present evidence for a unifying model of sarcomere assembly.