Deletion of Drosophila muscle LIM protein decreases flight muscle stiffness and power generation

Abstract

Deletion of Drosophila muscle LIM protein decreases flight muscle stiffness and power generation. Am J Physiol Cell Physiol 301: C373-C382, 2011. First published May 11, 2011; doi:10.1152/ajpcell.00206.2010 can be found at the Z-disk of sarcomeres where it is hypothesized to be involved in sensing muscle stretch. Loss of murine MLP results in dilated cardiomyopathy, and mutations in human MLP lead to cardiac hypertrophy, indicating a critical role for MLP in maintaining normal cardiac function. Loss of MLP in Drosophila (mlp84B) also leads to muscle dysfunction, providing a model system to examine MLP's mechanism of action. Mlp84B-null flies that survive to adulthood are not able to fly or beat their wings. Transgenic expression of the mlp84B gene in the Mlp84B-null background rescues flight ability and restores wing beating ability. Mechanical analysis of skinned flight muscle fibers showed a 30% decrease in oscillatory power production and a slight increase in the frequency at which maximum power is generated for fibers lacking Mlp84B compared with rescued fibers. Mlp84B-null muscle fibers displayed a 25% decrease in passive, active, and rigor stiffness compared with rescued fibers, but no significant decrease in isometric tension generation was observed. Muscle ultrastructure of Mlp84B-null muscle fibers is grossly normal; however, the null fibers have a slight decrease, 11%, in thick filament number per unit cross-sectional area. Our data indicate that MLP contributes to muscle stiffness and is necessary for maximum work and power generation. muscle mechanics; wing beat frequency STRIATED MUSCLE IS A HIGHLY organized tissue that relies on regulated protein-protein interactions to perform its contractile functions. While actin-myosin interactions between the thick and thin filaments generate contractile force, the Z-disk serves as a conduit for this force to be transmitted through neighboring sarcomeres to muscle insertion points or tendons, and to the muscle cell membrane. Therefore, proper assembly, maintenance, and stiffness of the Z-disk are critical for efficient force transmission and power output by striated muscle. Proteins that cross-link actin at the Z-disk are a major contributor to Z-disk stiffness (28). While the major actin cross-linking protein in the Z-disk is ␣-actinin, genetic and biochemical studies indicate that other Z-disk-associated proteins also have important roles in maintaining Z-disk structure and function (17,50,55).Muscle LIM protein (MLP) is a LIM domain protein found at the Z-disk that associates with several other Z-disk proteins including ␣-actinin (27) and Titin via Tcap/telethonin (22). Mice lacking MLP develop dilated cardiomyopathy (3), and mutations in human MLP are linked to familial hypertrophic cardiomyopathy (2,5,13,22,35,38,49), indicating its importance for maintaining normal cardiac function. Characterization of the MLP-null mice indicate that while striated muscle development proceeds normally, the MLP-deficient hearts exhibit several defects including a widening...

Characterization of an MLP Homologue from Haemaphysalis longicornis (Acari: Ixodidae) Ticks

“…In partially fed adult ticks, HLMLP expression in the integument was significantly higher than that observed in the other tested tissues ( Figure 6B). This result is in accordance with the tissue specificity of MLP gene expression in Drosophila, in which gene expression is spatially restricted to somatic muscles [28]. HLMLP mRNA was generally upregulated in the four developmental stages ( Figure 6A), which was in accordance with the positive regulatory role of HLMLP in muscle development [7].…”

Muscle LIM Protein: Master regulator of cardiac and skeletal muscle functions

“…No report on zebrafish being utilized as animal models for MLP has been described so far but studies using Drosophila have resulted in highly concordant data to mouse. Genetic ablation of Mlp84B, the Drosophila homolog of MLP, was associated with pupal lethality and impaired muscle function (Clark et al, 2007), with significantly reduced muscle stiffness and decreased power generation (Clark et al, 2011). The cardiac-specific ablation of Mlp84B resulted in decreased lifespan, impaired diastolic function and disturbances in cardiac rhythm.…”

Drosophila melanogaster muscle LIM protein and alpha‐actinin function together to stabilize muscle cytoarchitecture: A potential role for Mlp84B in actin‐crosslinking

“…Weakened actin crosslinks at the Z-line and muscle-membrane attachments would account for the following: 1) increased compliancy and decreased muscle stiffness exhibited by Mlp84B mutant flight muscle (Clark et al 2011), 2) decreased force propagation and decreased power in the Mlp84B null muscle (Clark et al 2011), 3) increased compliancy and prolonged diastole in the Mlp84B mutant hearts (Mery et al 2008), and 4) lack of robust muscle contraction necessary for pupariation (Clark et al 2007). In summary, all the observed phenotypes in the Mlp84B null flies can be explained by weakened actin crosslinks, suggesting this is the primary cellular defect caused by loss of Mlp84B.…”

“…In a collaborative study to specifically investigate cardiac defects, we have shown that both Mlp84B -null flies as well as animals with a heart specific reduction of Mlp84B exhibit a prolonged diastole (Mery et al 2008), a phenotype similar to that seen in young MLP knock-out mice before the onset of cardiac hypertrophy (Lorenzen-Schmidt et al 2005). Mechanical studies of Mlp84B -null flight muscle indicate that loss of Mlp84B results in decreased muscle stiffness and power generation (Clark et al 2011). While these physiological observations describe the muscle defects present in the Mlp84B -null flies, they have not yet defined the underlying molecular mechanism by which Mlp84B maintains muscle integrity and function.…”