Tropomyosin 4 defines novel filaments in skeletal muscle associated with muscle remodelling/regeneration in normal and diseased muscle

Abstract

The organisation of structural proteins in muscle into highly ordered sarcomeres occurs during development, regeneration and focal repair of skeletal muscle fibers. The involvement of cytoskeletal proteins in this process has been documented, with nonmuscle gamma-actin found to play a role in sarcomere assembly during muscle differentiation and also shown to be up-regulated in dystrophic muscles which undergo regeneration and repair [Lloyd et al.,2004; Hanft et al.,2006]. Here, we show that a cytoskeletal tropomyosin (Tm), Tm4, defines actin filaments in two novel compartments in muscle fibers: a Z-line associated cytoskeleton (Z-LAC), similar to a structure we have reported previously [Kee et al.,2004], and longitudinal filaments that are orientated parallel to the sarcomeric apparatus, present during myofiber growth and repair/regeneration. Tm4 is upregulated in paradigms of muscle repair including induced regeneration and focal repair and in muscle diseases with repair/regeneration features, muscular dystrophy and nemaline myopathy. Longitudinal Tm4-defined filaments also are present in diseased muscle. Transition of the Tm4-defined filaments from a longitudinal to a Z-LAC orientation is observed during the course of muscle regeneration. This Tm4-defined cytoskeleton is a marker of growth and repair/regeneration in response to injury, disease state and stress in skeletal muscle.

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

“…We demonstrated previously that a cytoskeletal Tm from the ␥-TM gene, Tm5NM1, localizes to the sarcolemma and to a region adjacent to the Z-line in skeletal muscle fibers . In addition, we have shown that Tm4, the single product of the ␦-TM gene, is also found in this region and in longitudinal filaments in regenerating or repairing muscle (Vlahovich et al, 2008). Both Tm5NM1 and Tm4 colocalize with a ␥-actin cytoskeleton at these sites, suggesting a role for these filament systems in stabilization of the muscle fibers and linking myofibrillar networks to the membrane systems.…”

“…Mouse muscles were fixed in 4% paraformaldehyde (PFA) (hindlimb muscles were stretched and held during fixation) and infused with 1.8 M sucrose/20% polyvinylpyrrolidone as described by Vlahovich et al (2008). Semithin (0.5-to 0.8-m) sections were cut at Ϫ60°C using an Ultracut UCT ultramicrotome (Leica, Wetzler, Germany) equipped with an EM FCS cryochamber (Leica).…”

Localization and function of Xinα in mouse skeletal muscle

“…3A, mXin␣ was coimmunoprecipitated from gastrocnemius muscle extracts using anti-tropomyosin (LC24), antifilamin, anti-vinculin, or anti-talin mAb but not anti-␤-catenin mAb or normal mouse serum control. mAb LC24 recognizes tropomyosin isoform 4 that is known to locate together with ␥-actin external to the sarcomere but adjacent to the Z-line and in the subsarcolemmal region including MTJ in skeletal muscle cells (28,58). The antibodies used in our experiments specifically precipitated their respective antigens from skeletal muscle extract except for anti-␤-catenin (Fig.…”

Actin stress fibers – assembly, dynamics and biological roles

“…Specifically, the Tm4 isoform is involved in recruitment of myosin II to stress fibers in U2OS cells (Tojkander et al, 2011). Interestingly, TPM4 might also contribute to the assembly of premyofibrils (Vlahovich et al, 2008) and to myosin-IIA-rich podosome cores of osteoclasts (McMichael et al, 2006), suggesting that the role of this TPM isoform in the formation of distinct myosin-based structures is conserved. TPMs also regulate actomyosin interactions, including the assembly of contractile ring in fission yeast Clayton et al, 2010).…”