Tropomodulin capping of actin filaments in striated muscle development and physiology

Abstract

Efficient striated muscle contraction requires precise assembly and regulation of diverse actin filament systems, most notably the sarcomeric thin filaments of the contractile apparatus. By capping the pointed ends of actin filaments, tropomodulins (Tmods) regulate actin filament assembly, lengths, and stability. Here, we explore the current understanding of the expression patterns, localizations, and functions of Tmods in both cardiac and skeletal muscle. We first describe the mechanisms by which Tmods regulate myofibril assembly and thin filament lengths, as well as the roles of closely related Tmod family variants, the leiomodins (Lmods), in these processes. We also discuss emerging functions for Tmods in the sarcoplasmic reticulum. This paper provides abundant evidence that Tmods are key structural regulators of striated muscle cytoarchitecture and physiology.

Figure 1

Schematic of the molecular sequence and domain organization of Tmods and Lmods found in vertebrate striated muscle. Tmods contain an N-terminal TM-Cap domain and a C-terminal LRR-Cap domain, whose α-helices possess distinct combinations of TM-binding and actin-regulatory activities, as shown. The α-helices are numbered sequentially within each domain, and the NES and NLS of Tmods are highlighted. Lmods also contain a C-terminal extension with a polyproline region, two predicted α-helices, a basic region, and a WH2 domain.

Figure 2

Immunofluorescence localization of Tmods in skeletal muscle. Panels depict longitudinal cryosections of mouse tibialis anterior muscle immunostained for (a) Tmod1, (b) Tmod3, or (c) Tmod4, phalloidin-stained for F-actin, and imaged by confocal microscopy, prepared as in [31, 63]. Note the predominant localization of Tmod1 and Tmod4 at the pointed ends of the phalloidin-stained thin filaments. Tmod1 and Tmod4 also exhibit Z-line-flanking localizations, corresponding to an SR- or T-tubule-associated compartment. By contrast, the M-line and Z-line-flanking localization of Tmod3 is a signature of the SR [63]. Z, Z-line; M, M-line; P, thin filament pointed ends. Bars, 1 μm.

Figure 3

Model diagram of Tmods in the sarcomeres and SR of skeletal muscle. Thin filaments, consisting of α-actin subunits, are capped at their pointed ends by a combination of Tmod1 and Tmod4 in skeletal muscle, whereas Tmod3 is associated with cytoplasmic (nonmuscle) γ-actin filaments in the SR. The exact stoichiometries and distributions of Tmod1 versus Tmod4 on the thin filament pointed ends are unknown. Tmod-capped thin filaments extend past the distal N-terminus of nebulin into the middle of the sarcomere. By contrast, in cardiac muscle (not shown here), thin filament pointed ends are capped solely by Tmod1 and do not contain nebulin. Filaments composed of γ-actin, nonmuscle TMs, and Tmod3 are anchored to the SR membrane via sAnk1.5 and laterally connected by a spectrin network. Tmod3/nonmuscle TM/γ-actin filaments are connected to myofibrils via Tmod3 binding to sAnk1.5, which links to obscurin, which, in turn, is linked to myomesin and titin at the M-line. To date, Tmod3's localization with respect to the cardiac SR remains undetermined. Note that for clarity, the thick filaments are not drawn to scale with respect to the thin filaments.