Knockout of Lmod2 results in shorter thin filaments followed by dilated cardiomyopathy and juvenile lethality

Abstract

Leiomodin 2 (Lmod2) is an actin-binding protein that has been implicated in the regulation of striated muscle thin filament assembly; its physiological function has yet to be studied. We found that knockout of Lmod2 in mice results in abnormally short thin filaments in the heart. We also discovered that Lmod2 functions to elongate thin filaments by promoting actin assembly and dynamics at thin filament pointed ends. Lmod2-KO mice die as juveniles with hearts displaying contractile dysfunction and ventricular chamber enlargement consistent with dilated cardiomyopathy. Lmod2-null cardiomyocytes produce less contractile force than wild type when plated on micropillar arrays. Introduction of GFP-Lmod2 via adeno-associated viral transduction elongates thin filaments and rescues structural and functional defects observed in Lmod2-KO mice, extending their lifespan to adulthood. Thus, to our knowledge, Lmod2 is the first identified mammalian protein that functions to elongate actin filaments in the heart; it is essential for cardiac thin filaments to reach a mature length and is required for efficient contractile force and proper heart function during development.actin-thin filaments | cardiomyopathy | cytoskeletal dynamics S triated muscle cells contain arrays of protein filaments assembled into contractile units that are nearly crystalline in structure. Efficient contraction at the molecular level is predicated upon accurate overlap of actin-containing thin and myosin-containing thick filaments. Therefore, proper control of filament assembly is absolutely critical.In striated muscle it is currently thought that the thin-filament pointed end capping protein tropomodulin (Tmod) is the predominant regulator of thin filament length, with Tmod1 being the sole isoform expressed in cardiomyocytes (1). Extensive in vitro work has revealed that Tmod1 uses two actin-and two tropomyosin-binding sites to associate with the end of the thin filament and to prevent addition or loss of actin monomers, thereby controlling length of the thin filament (2-7). Tmod1 is essential for life; Tmod1-KO mice are embryonic lethal because of cardiac defects (8-11).Identification of additional but structurally different members of the Tmod family of proteins, the leiomodins (Lmods), raises the possibility that thin filament lengths are not regulated solely by Tmod at thin filament pointed ends (12). Although there are three Lmod genes (Lmod1-3), Lmod2 and 3 are expressed in striated muscle with Lmod2 being the predominant isoform in cardiac muscle and Lmod3 the predominant isoform in skeletal muscle (12-16). The Lmods share ∼40% sequence identity at the protein level with the Tmods but do not contain a recognizable second tropomyosin-binding domain and have an additional C-terminal extension that includes a proline-rich region and an actin-binding Wiskott-Aldrich syndrome protein homology 2 (WH2) domain (12, 17). Lmod2 has been proposed to be the long-sought muscle actin filament nucleator because it robustly nucleates actin filament formation in ...

Loss of LMOD1 impairs smooth muscle cytocontractility and causes megacystis microcolon intestinal hypoperistalsis syndrome in humans and mice

“…Recent gene knockout experiments have further illuminated the role of LMOD family members. For example, Lmod2 knockout mice exhibit shortened cardiac actin filaments and dilated cardiomyopathy although no known LMOD2 mutations have been uncovered in humans (26). On the other hand, human mutations in LMOD3 are linked to nemaline myopathy (20), a complex congenital skeletal muscle disease characterized by rod-like nemaline bodies that are thought to cause muscle weakness (27,28).…”

Co-expression networks reveal the tissue-specific regulation of transcription and splicing

“…In the tissue group including breastmammary and the two adipose tissues, the top tissue-specific TE-IR hub was TTC36, a gene highly expressed in breast cancer (Liu et al 2008). The second ranked hub gene for the tissue group including skeletal muscle and heart-left ventricle was LMOD2, which was observed to be abundantly expressed in both tissues and has been reported to regulate the thin filament length in muscles affecting cardiomyopathy in mice (Pappas et al 2015;Li et al 2016b).…”

Tropomodulins and Leiomodins: Actin Pointed End Caps and Nucleators in Muscles

“…In cells, Tmods regulate actin filament length and stability by capping pointed ends in cytoskeletal architectures as diverse as the spectrin-based membrane skeleton and the contractile sarcomeres of striated muscle myofibrils (reviewed in (8)(9)(10)). The function of Lmods have been studied primarily in cardiac and skeletal muscles, where they are important for the regulation of the length of thin filaments in sarcomeres and normal muscle contractility (11)(12)(13)(14)(15). Interest in this family of proteins has increased dramatically, owing to recent discoveries linking the expression of Lmod2 and Lmod3, respectively, to dilated cardiomyopathy (11,12) and nemaline myopathy (13,14,(16)(17)(18).…”

“…The function of Lmods have been studied primarily in cardiac and skeletal muscles, where they are important for the regulation of the length of thin filaments in sarcomeres and normal muscle contractility (11)(12)(13)(14)(15). Interest in this family of proteins has increased dramatically, owing to recent discoveries linking the expression of Lmod2 and Lmod3, respectively, to dilated cardiomyopathy (11,12) and nemaline myopathy (13,14,(16)(17)(18). Lmod1, on the other hand, has been linked to autoimmune disorders (19,20), and a recent study finds that its deficiency impairs smooth muscle cytocontractility and causes megacystis microcolon intestinal hypoperistalsis syndrome (MMIHS) in humans and mice (21).…”