scholarly journals Disruption of cardiac thin filament assembly arising from a mutation in LMOD2: A novel mechanism of neonatal dilated cardiomyopathy

2019 ◽  
Vol 5 (9) ◽  
pp. eaax2066 ◽  
Author(s):  
Rebecca C. Ahrens-Nicklas ◽  
Christopher T. Pappas ◽  
Gerrie P. Farman ◽  
Rachel M. Mayfield ◽  
Tania M. Larrinaga ◽  
...  
Keyword(s):  
Dilated Cardiomyopathy ◽  
Cardiac Transplantation ◽  
Thin Filament ◽  
Force Production ◽  
Actin Binding ◽  
Isolated Cardiomyocytes ◽  
Thin Filaments ◽  
Familial Dilated Cardiomyopathy ◽  
Filament Assembly ◽  
Explanted Heart

Neonatal heart failure is a rare, poorly-understood presentation of familial dilated cardiomyopathy (DCM). Exome sequencing in a neonate with severe DCM revealed a homozygous nonsense variant in leiomodin 2 (LMOD2, p.Trp398*). Leiomodins (Lmods) are actin-binding proteins that regulate actin filament assembly. While disease-causing mutations in smooth (LMOD1) and skeletal (LMOD3) muscle isoforms have been described, the cardiac (LMOD2) isoform has not been previously associated with human disease. Like our patient, Lmod2-null mice have severe early-onset DCM and die before weaning. The infant’s explanted heart showed extraordinarily short thin filaments with isolated cardiomyocytes displaying a large reduction in maximum calcium-activated force production. The lack of extracardiac symptoms in Lmod2-null mice, and remarkable morphological and functional similarities between the patient and mouse model informed the decision to pursue cardiac transplantation in the patient. To our knowledge, this is the first report of aberrant cardiac thin filament assembly associated with human cardiomyopathy.

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

2015 ◽  
Vol 112 (44) ◽  
pp. 13573-13578 ◽  
Author(s):  
Christopher T. Pappas ◽  
Rachel M. Mayfield ◽  
Christine Henderson ◽  
Nima Jamilpour ◽  
Cathleen Cover ◽  
...  
Keyword(s):  
Dilated Cardiomyopathy ◽  
Thin Filament ◽  
Striated Muscle ◽  
Heart Function ◽  
Contractile Force ◽  
Actin Binding ◽  
Actin Assembly ◽  
Thin Filaments ◽  
Filament Assembly ◽  
Muscle Thin Filament

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.

scholarly journals Functional Analysis of a Unique Troponin C Mutation, GLY159ASP, that Causes Familial Dilated Cardiomyopathy, Studied in Explanted Heart Muscle

2009 ◽  
Vol 2 (5) ◽  
pp. 456-464 ◽  
Author(s):  
Emma C. Dyer ◽  
Adam M. Jacques ◽  
Anita C. Hoskins ◽  
Douglas G. Ward ◽  
Clare E. Gallon ◽  
...  
Keyword(s):  
Functional Analysis ◽  
Dilated Cardiomyopathy ◽  
Heart Muscle ◽  
Troponin C ◽  
Familial Dilated Cardiomyopathy ◽  
Explanted Heart

scholarly journals The nebulin repeat protein Lasp regulates I-band architecture and filament spacing in myofibrils

2014 ◽  
Vol 206 (4) ◽  
pp. 559-572 ◽  
Author(s):  
Isabelle Fernandes ◽  
Frieder Schöck
Keyword(s):  
Thin Filament ◽  
Muscle Protein ◽  
Nemaline Myopathy ◽  
Single Amino Acid ◽  
Actin Binding ◽  
Filament Length ◽  
Thin Filaments ◽  
Single Amino Acid Change ◽  
Filament Spacing

Mutations in nebulin, a giant muscle protein with 185 actin-binding nebulin repeats, are the major cause of nemaline myopathy in humans. Nebulin sets actin thin filament length in sarcomeres, potentially by stabilizing thin filaments in the I-band, where nebulin and thin filaments coalign. However, the precise role of nebulin in setting thin filament length and its other functions in regulating power output are unknown. Here, we show that Lasp, the only member of the nebulin family in Drosophila melanogaster, acts at two distinct sites in the sarcomere and controls thin filament length with just two nebulin repeats. We found that Lasp localizes to the Z-disc edges to control I-band architecture and also localizes at the A-band, where it interacts with both actin and myosin to set proper filament spacing. Furthermore, introducing a single amino acid change into the two nebulin repeats of Lasp demonstrated different roles for each domain and established Lasp as a suitable system for studying nebulin repeat function.

scholarly journals The number of Z-repeats and super-repeats in nebulin greatly varies across vertebrates and scales with animal size

2020 ◽  
Vol 153 (3) ◽  
Author(s):  
Jochen Gohlke ◽  
Paola Tonino ◽  
Johan Lindqvist ◽  
John E. Smith ◽  
Henk Granzier
Keyword(s):  
Binding Site ◽  
Thin Filament ◽  
Muscle Protein ◽  
Isometric Force ◽  
Force Production ◽  
Actin Binding ◽  
Skeletal Muscle Protein ◽  
Shortening Velocity ◽  
Large Animals ◽  
Simple Repeats

Nebulin is a skeletal muscle protein that associates with the sarcomeric thin filaments and has functions in regulating the length of the thin filament and the structure of the Z-disk. Here we investigated the nebulin gene in 53 species of birds, fish, amphibians, reptiles, and mammals. In all species, nebulin has a similar domain composition that mostly consists of ∼30-residue modules (or simple repeats), each containing an actin-binding site. All species have a large region where simple repeats are organized into seven-module super-repeats, each containing a tropomyosin binding site. The number of super-repeats shows high interspecies variation, ranging from 21 (zebrafish, hummingbird) to 31 (camel, chimpanzee), and, importantly, scales with body size. The higher number of super-repeats in large animals was shown to increase thin filament length, which is expected to increase the sarcomere length for optimal force production, increase the energy efficiency of isometric force production, and lower the shortening velocity of muscle. It has been known since the work of A.V. Hill in 1950 that as species increase in size, the shortening velocity of their muscle is reduced, and the present work shows that nebulin contributes to the mechanistic basis. Finally, we analyzed the differentially spliced simple repeats in nebulin's C terminus, whose inclusion correlates with the width of the Z-disk. The number of Z-repeats greatly varies (from 5 to 18) and correlates with the number of super-repeats. We propose that the resulting increase in the width of the Z-disk in large animals increases the number of contacts between nebulin and structural Z-disk proteins when the Z-disk is stressed for long durations.

scholarly journals Thin filaments are not of uniform length in rat skeletal muscle.

1983 ◽  
Vol 96 (1) ◽  
pp. 100-103 ◽  
Author(s):  
L Traeger ◽  
M A Goldstein
Keyword(s):  
Skeletal Muscle ◽  
Thin Filament ◽  
Fast Muscle ◽  
Filament Length ◽  
Rat Skeletal Muscle ◽  
Adult Rats ◽  
Thin Filaments ◽  
Filament Assembly ◽  
Old Rats ◽  
Two Stages

The variation in thin filament length was investigated in slow and fast muscle from adult and neonatal rats. Soleus (slow) muscle from adult, 3-, 7-, and 9-d-old rats, and extensor digitorum longus (EDL; fast) muscle from adult rats were serially cross-sectioned. The number of thin filaments per 0.06 microns2 (TF#) was counted for individual myofibrils followed from the H zone of one sarcomere, through the I-Z-I region, to the H zone of an adjacent sarcomere TF# was pooled by distance from the Z band or AI junction. In both adult muscles, thin filament length varied from 0.18 to 1.20 microns, with approximately 25% of the thin filaments less than 0.7 microns in length. In 7- and 9-d soleus, thin filament length ranged from 0.18 to 1.08 microns; except for the longest (0.18 to 1.20 microns) filaments, the distribution of thin filament lengths was similar to that in adult muscle. In 3-d soleus, thin filament length was more uniform, with less than 5% of the filaments shorter than 0.7 microns. In all neonatal muscles, there were approximately 15% fewer thin filaments per unit area as compared to adult muscles. We conclude: (a) In rat skeletal muscle, thin filaments are not of uniform length, ranging in length from 0.18 to 1.20 microns. (b) There may be two stages of thin filament assembly in neonatal muscle: between 3 and 7 d when short thin filaments may be preferentially or synthesized or inserted near the Z-band, and between 9 d and adult when thin filaments of all lengths may be synthesized or inserted into the myofibril.

scholarly journals Mechanisms of thin filament assembly in embryonic chick cardiac myocytes: tropomodulin requires tropomyosin for assembly.

1995 ◽  
Vol 129 (3) ◽  
pp. 683-695 ◽  
Author(s):  
C C Gregorio ◽  
V M Fowler
Keyword(s):  
Actin Filaments ◽  
Thin Filament ◽  
Striated Muscle ◽  
Cell Model ◽  
Significant Proportion ◽  
Embryonic Chick ◽  
Permeabilized Cell ◽  
Thin Filaments ◽  
Filament Assembly ◽  
Cell Biol

Tropomodulin is a pointed end capping protein for tropomyosin-coated actin filaments that is hypothesized to play a role in regulating the precise lengths of striated muscle thin filaments (Fowler, V. M., M. A. Sussman, P. G. Miller, B. E. Flucher, and M. P. Daniels. 1993. J. Cell Biol. 120:411-420; Weber, A., C. C. Pennise, G. G. Babcock, and V. M. Fowler. 1994, J. Cell Biol. 127:1627-1635). To gain insight into the mechanisms of thin filament assembly and the role of tropomodulin therein, we have characterized the temporal appearance, biosynthesis and mechanisms of assembly of tropomodulin onto the pointed ends of thin filaments during the formation of striated myofibrils in primary embryonic chick cardiomyocyte cultures. Our results demonstrate that tropomodulin is not assembled coordinately with other thin filament proteins. Double immunofluorescence staining and ultrastructural immunolocalization demonstrate that tropomodulin is incorporated in its characteristic sarcomeric location at the pointed ends of the thin filaments after the thin filaments have become organized into periodic I bands. In fact, tropomodulin assembles later than all other well characterized myofibrillar proteins studied including: actin, tropomyosin, alpha-actinin, titin, myosin and C-protein. Nevertheless, at steady state, a significant proportion (approximately 39%) of tropomodulin is present in a soluble pool throughout myofibril assembly. Thus, the absence of tropomodulin in some striated myofibrils is not due to limiting quantities of the protein. In addition, kinetic data obtained from [35S]methionine pulse-chase experiments indicate that tropomodulin assembles more slowly into myofibrils than does tropomyosin. This observation, together with results obtained using a novel permeabilized cell model for thin filament assembly, indicate that tropomodulin assembly is dependent on the prior association of tropomyosin with actin filaments. We conclude that tropomodulin is a late marker for the assembly of striated myofibrils in cardiomyocytes; its assembly appears to be linked to their maturity. We propose that tropomodulin is involved in maintaining and stabilizing the final lengths of thin filaments after they are assembled.

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