Leiomodin 3 and Tropomodulin 4 have overlapping functions during skeletal myofibrillogenesis

Abstract

Precise regulation of thin filament length is essential for optimal force generation during muscle contraction. The thin filament capping protein tropomodulin (Tmod) contributes to thin filament length uniformity by regulating elongation and depolymerization at thin filament ends. The leiomodins (Lmod1-3) are structurally related to Tmod1-4 and also localize to actin filament pointed ends, but in vitro biochemical studies indicate that Lmods act instead as robust nucleators. Here, we examined the roles of Tmod4 and Lmod3 during Xenopus skeletal myofibrillogenesis. Loss of Tmod4 or Lmod3 resulted in severe disruption of sarcomere assembly and impaired embryonic movement. Remarkably, when Tmod4-deficient embryos were supplemented with additional Lmod3, and Lmod3-deficient embryos were supplemented with additional Tmod4, sarcomere assembly was rescued and embryonic locomotion improved. These results demonstrate for the first time that appropriate levels of both Tmod4 and Lmod3 are required for embryonic myofibrillogenesis and, unexpectedly, both proteins can function redundantly during in vivo skeletal muscle thin filament assembly. Furthermore, these studies demonstrate the value of Xenopus for the analysis of contractile protein function during de novo myofibril assembly.

Tropomodulin1 directly controls thin filament length in both wild-type and tropomodulin4-deficient skeletal muscle

“…and Lmod3 mRNAs were detected in all striated muscles, in agreement with previous studies (Cenik et al, 2015;Chereau et al, 2008;Conley et al, 2001;Nworu et al, 2015;Tian et al, 2015;Yuen et al, 2014), but an unexpected muscle type dependence was observed, with Lmod2 mRNA predominating in slow-twitch soleus, diaphragm, and left ventricular muscles and Lmod3 mRNA more highly expressed than Lmod2 mRNA in fast-twitch TA muscle (Fig. 1C).…”

“…4C), as expected (Nworu et al, 2015;Yuen et al, 2014). The Lmod3 striations were ∼0.95 µm from the Z-line (Fig.…”

“…This is probably due to the compensatory upregulation of Tmod1 being of sufficient magnitude to repopulate nearly all the thin filament pointed ends available upon Tmod4 deletion, which occurs during embryonic skeletal muscle development and does not require postnatal muscle use. Such a compensation mechanism appears to be a mammalian innovation, as Tmod4 deficiency leads to severe myofibril disruption and nemaline myopathy-like phenotypes in zebrafish and Xenopus (Berger et al, 2014;Nworu et al, 2015). The dispensability of Tmod4 for mammalian skeletal muscle development and organization also agrees with the fact that no human TMOD4-based nemaline myopathies have been identified to date, despite the fact that numerous nemaline myopathy-causing mutations in other thin filament structural components and actin-associated proteins have been identified (Romero et al, 2013).…”

Severe myopathy in mice lacking the MEF2/SRF-dependent gene leiomodin-3

“…Analysis of the proteome of skeletal muscle from Klhl40-mutant mice revealed pronounced downregulation of leiomodin-3 (LMOD3), a putative actin-nucleating protein that had not been previously studied (6). Subsequently, frame-shift and nonsense mutations in LMOD3 were found to cause nemaline myopathy in humans, and inhibition of Lmod3 in zebrafish and Xenopus through morpholino-mediated knockdown was shown to cause myofiber disarray (10,11).…”

Nebulin, a multi-functional giant

“…The Lmods are structurally similar to the Tmods; however, they lack a second tropomyosin-binding domain and possess an extended C-terminal region containing an additional (third) actin-binding domain. Mutations in LMOD3 were recently discovered to underlie autosomal recessive nemaline myopathy, and Lmod3 is essential for myofibrillogenesis and/or organization of skeletal muscle thin filaments (Cenik et al, 2015;Tian et al, 2015;Nworu et al, 2015;Yuen et al, 2014; for review, see Sandaradura and North 2015). Furthermore, knockout of a different Lmod family member (Lmod2) reduces thin filament length in the heart and results in a rapid onset dilated cardiomyopathy (Pappas et al, 2015).…”