Cytoskeletal Tropomyosin Tm5NM1 Is Required for Normal Excitation–Contraction Coupling in Skeletal Muscle

Abstract

The functional diversity of the actin microfilaments relies in part on the actin binding protein tropomyosin (Tm). The muscle-specific Tms regulate actin-myosin interactions and hence contraction. However, there is less known about the roles of the numerous cytoskeletal isoforms. We have shown previously that a cytoskeletal Tm, Tm5NM1, defines a Z-line adjacent cytoskeleton in skeletal muscle. Recently, we identified a second cytoskeletal Tm in this region, Tm4. Here we show that Tm4 and Tm5NM1 define separate actin filaments; the former associated with the terminal sarcoplasmic reticulum (SR) and other tubulovesicular structures. In skeletal muscles of Tm5NM1 knockout (KO) mice, Tm4 localization was unchanged, demonstrating the specificity of the membrane association. Tm5NM1 KO muscles exhibit potentiation of T-system depolarization and decreased force rundown with repeated T-tubule depolarizations consistent with altered T-tubule function. These results indicate that a Tm5NM1-defined actin cytoskeleton is required for the normal excitation-contraction coupling in skeletal muscle.

A cytoskeletal tropomyosin can compromise the structural integrity of skeletal muscle

“…One cytoskeletal Tm, Tm5NM1, localizes to the sarcolemma and to a region adjacent to the Z-line consistent with T-tubule localization [Kee et al, 2004;Vlahovich et al, 2009]. In keeping with a role in T-tubule membrane function there is altered excitation-contraction coupling in myofibers from Tm5NM1 knock-out mice [Vlahovich et al, 2009].…”

“…Recently, we have identified in skeletal muscle a number of actin filaments defined by the cytoskeletal Tms [Kee et al, 2004;Vlahovich et al, 2008Vlahovich et al, , 2009. One cytoskeletal Tm, Tm5NM1, localizes to the sarcolemma and to a region adjacent to the Z-line consistent with T-tubule localization [Kee et al, 2004;Vlahovich et al, 2009].…”

Tropomyosin – master regulator of actin filament function in the cytoskeleton

“…Loss of Tpm3.1 from T-tubules results in increased T-tubule dysmorphology (Vlahovich et al, 2009). In addition, altered excitation-contractioncoupling has been observed which is consistent with an altered uptake of Ca 2+ through the triad, possibly because of changes in T-tubule function (Vlahovich et al, 2009). In addition, expression of the cytoskeletal Tpm1.7, but not of Tpm3.1, in skeletal muscle results in muscular dystrophy (Kee et al, 2009a).…”

Tropomyosin isoforms and reagents

“…92 Both the Tm3 and Tm5NM1 transgenics together with the complementary Tm5NM1 knockout mice have become valuable resources for the isolation of primary embryonic cells such as neuronal and fibroblast cells. Primary cortical neurons isolated from these mice have revealed the importance of Tm isoforms in the development and maintenance of neuronal morphogenesis, previously only inferred from subcellular localization studies.…”