Passive stiffness in Drosophila indirect flight muscle reduced by disrupting paramyosin phosphorylation, but not by embryonic myosin S2 hinge substitution

Abstract

High passive stiffness is one of the characteristic properties of the asynchronous indirect flight muscle (IFM) found in many insects like Drosophila. To evaluate the effects of two thick filament protein domains on passive sarcomeric stiffness, and to investigate their correlation with IFM function, we used microfabricated cantilevers and a high resolution imaging system to study the passive IFM myofibril stiffness of two groups of transgenic Drosophila lines. One group (hinge-switch mutants) had a portion of the endogenous S2 hinge region replaced by an embryonic version; the other group (paramyosin mutants) had one or more putative phosphorylation sites near the N-terminus of paramyosin disabled. Both transgenic groups showed severely compromised flight ability. In this study, we found no difference (compared to the control) in passive elastic modulus in the hinge-switch group, but a 15% reduction in the paramyosin mutants. All results were corroborated by muscle fiber mechanics experiments performed on the same lines. The fact that myofibril elasticity is unaffected by hinge switching implies alternative S2 hinges do not critically affect passive sarcomere stiffness. In contrast, the mechanical defects observed upon disrupting paramyosin phosphorylation sites in Drosophila suggests that paramyosin phosphorylation is important for maintaining high passive stiffness in IFM myofibrils, probably by affecting paramyosin's interaction with other sarcomeric proteins.

FIGURE 1

Elastic (A) and viscous modulus (B) values for passive (pCa 8) IFM fibers across muscle oscillation frequencies for hinge-switch mutants (15b-47 and 15b-108) and their positive control (pwMhc2). Values are mean ± SE. No significant differences were found between the three lines. Temperature = 15°C.

FIGURE 2

Elastic (A) and viscous modulus (B) values for passive (pCa 8) IFM fibers across muscle oscillation frequencies for paramyosin mutants (pm^S18A and pm^SA4) and their positive control (pm). Values are mean ± SE. * indicates a span of frequencies over which there is a significant difference (p < 0.05) between the pm and the pm^S18A and pm^SA4 lines. Temperature = 15°C.