Loss of Smyhc1 or Hsp90α1 Function Results in Different Effects on Myofibril Organization in Skeletal Muscles of Zebrafish Embryos

Background: Myofibrillogenesis requires the correct folding and assembly of sarcomeric proteins into highly organized sarcomeres. Heat shock protein 90 alpha 1 (Hsp90 alpha 1) has been implicated as a myosin chaperone that plays a key role in myofibrillogenesis. Knockdown or mutation of hsp90 alpha 1 resulted in complete disorganization of thick and thin filaments and M-and Z-line structures. It is not clear whether the disorganization of these sarcomeric structures is due to a direct effect from loss of Hsp90 alpha 1 function or indirectly through the disorganization of myosin thick filaments. Methodology/Principal Findings: In this study, we carried out a loss-of-function analysis of myosin thick filaments via gene-specific knockdown or using a myosin ATPase inhibitor BTS (N-benzyl-p-toluene sulphonamide) in zebrafish embryos. We demonstrated that knockdown of myosin heavy chain 1 (myhc1) resulted in sarcomeric defects in the thick and thin filaments and defective alignment of Z-lines. Similarly, treating zebrafish embryos with BTS disrupted thick and thin filament organization, with little effect on the M-and Z-lines. In contrast, loss of Hsp90 alpha 1 function completely disrupted all sarcomeric structures including both thick and thin filaments as well as the M-and Z-lines. Conclusion/Significance: Together, these studies indicate that the hsp90 alpha 1 mutant phenotype is not simply due to disruption of myosin folding and assembly, suggesting that Hsp90 alpha 1 may play a role in the assembly and organization of other sarcomeric structures.