title: The titin A-band rod domain is dispensable for initial thick filament assembly in zebrafish.
abstract: The sarcomeres of skeletal and cardiac muscle are highly structured protein arrays, consisting of thick and thin filaments aligned precisely to one another and to their surrounding matrix. The contractile mechanisms of sarcomeres are generally well understood, but how the patterning of sarcomeres is initiated during early skeletal muscle and cardiac development remains uncertain. Two of the most widely accepted hypotheses for this process include the "molecular ruler" model, in which the massive protein titin defines the length of the sarcomere and provides a scaffold along which the myosin thick filament is assembled, and the "premyofibril" model, which proposes that thick filament formation does not require titin, but that a "premyofibril" consisting of non-muscle myosin, α-actinin and cytoskeletal actin is used as a template. Each model posits a different order of necessity of the various components, but these have been difficult to test in vivo. Zebrafish motility mutants with developmental defects in sarcomere patterning are useful for the elucidation of such mechanisms, and here we report the analysis of the herzschlag mutant, which shows deficits in both cardiac and skeletal muscle. The herzschlag mutant produces a truncated titin protein, lacking the C-terminal rod domain that is proposed to act as a thick filament scaffold, yet muscle patterning is still initiated, with grossly normal thick and thin filament assembly. Only after embryonic muscle contraction begins is breakdown of sarcomeric myosin patterning observed, consistent with the previously noted role of titin in maintaining the contractile integrity of mature sarcomeres. This conflicts with the "molecular ruler" model of early sarcomere patterning and supports a titin-independent model of thick filament organization during sarcomerogenesis. These findings are also consistent with the symptoms of human titin myopathies that exhibit a late onset, such as tibial muscular dystrophy.
authors.affiliation: Department of Biological Sciences, CW405, Biological Sciences Building, University of Alberta, Edmonton, Canada AB T6G 2E9.
authors.forename: J Layne
authors.initials: JL
authors.lastname: Myhre
authors.affiliation: Department of Biological Sciences, CW405, Biological Sciences Building, University of Alberta, Edmonton, Canada AB T6G 2E9.
authors.forename: Jordan A
authors.initials: JA
authors.lastname: Hills
authors.affiliation: Department of Biological Sciences, CW405, Biological Sciences Building, University of Alberta, Edmonton, Canada AB T6G 2E9.
authors.forename: Kendal
authors.initials: K
authors.lastname: Prill
authors.affiliation: Department of Biological Sciences, CW405, Biological Sciences Building, University of Alberta, Edmonton, Canada AB T6G 2E9.
authors.forename: Serene L
authors.initials: SL
authors.lastname: Wohlgemuth
authors.affiliation: Department of Biological Sciences, CW405, Biological Sciences Building, University of Alberta, Edmonton, Canada AB T6G 2E9. Electronic address: dave.pilgrim@ualberta.ca.
authors.forename: David B
authors.initials: DB
authors.lastname: Pilgrim
mesh_terms: D000818:Animals; D064211:Connectin / Q000235:genetics*; D006321:Heart / Q000196:embryology*; D009119:Muscle Contraction / Q000235:genetics; D024510:Muscle Development / Q000235:genetics*; D018482:Muscle, Skeletal / Q000196:embryology*; D009206:Myocardium; D016376:Oligonucleotides, Antisense / Q000235:genetics; D017434:Protein Structure, Tertiary; D012518:Sarcomeres / Q000235:genetics / Q000378:metabolism; D015027:Zebrafish / Q000196:embryology* / Q000235:genetics
chemical_list: D064211:Connectin; D016376:Oligonucleotides, Antisense
keywords: Muscle development; Myofibrillogenesis; Myogenesis; Sarcomere; Titin; Zebrafish