New Insights into the Structural Roles of Nebulin in Skeletal Muscle

One important feature of muscle structure and function that has remained relatively obscure is the mechanism that regulates thin filament length. Filament length is an important aspect of muscle function as force production is proportional to the amount of overlap between thick and thin filaments. Recent advances, due in part to the generation of nebulin KO models, reveal that nebulin plays an important role in the regulation of thin filament length. Another structural feature of skeletal muscle that is not well understood is the mechanism involved in maintaining the regular lateral alignment of adjacent sarcomeres, that is, myofibrillar connectivity. Recent studies indicate that nebulin is part of a protein complex that mechanically links adjacent myofibrils. Thus, novel structural roles of nebulin in skeletal muscle involve the regulation of thin filament length and maintaining myofibrillar connectivity. When these functions of nebulin are absent, muscle weakness ensues, as is the case in patients with nemaline myopathy with mutations in nebulin. Here we review these new insights in the role of nebulin in skeletal muscle structure.

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Essential Role of Septin 7 in Skeletal Muscle Structure and Function

Biophysical Journal ◽

10.1016/j.bpj.2019.11.1500 ◽

2020 ◽

Vol 118 (3) ◽

pp. 258a

Author(s):

Laszlo Csernoch ◽

Mónika Gönczi ◽

Zsolt Ráduly ◽

László Szabó ◽

Nóra Dobrosi ◽

...

Keyword(s):

Skeletal Muscle ◽

Essential Role ◽

Structure And Function ◽

Muscle Structure ◽

And Function

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

The Journal of Cell Biology ◽

10.1083/jcb.201401094 ◽

2014 ◽

Vol 206 (4) ◽

pp. 559-572 ◽

Cited By ~ 27

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.

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Reduced thin filament length in nebulin-knockout skeletal muscle alters isometric contractile properties

AJP Cell Physiology ◽

10.1152/ajpcell.00503.2008 ◽

2009 ◽

Vol 296 (5) ◽

pp. C1123-C1132 ◽

Cited By ~ 43

Author(s):

David S. Gokhin ◽

Marie-Louise Bang ◽

Jianlin Zhang ◽

Ju Chen ◽

Richard L. Lieber

Keyword(s):

Skeletal Muscle ◽

Thin Filament ◽

Expression Patterns ◽

Nemaline Myopathy ◽

Myosin Heavy Chain Isoforms ◽

Filament Length ◽

Force Transmission ◽

Thin Filaments ◽

Tension Properties ◽

Tension Curves

Nebulin (NEB) is a large, rod-like protein believed to dictate actin thin filament length in skeletal muscle. NEB gene defects are associated with congenital nemaline myopathy. The functional role of NEB was investigated in gastrocnemius muscles from neonatal wild-type (WT) and NEB knockout (NEB-KO) mice, whose thin filaments have uniformly shorter lengths compared with WT mice. Isometric stress production in NEB-KO skeletal muscle was reduced by 27% compared with WT skeletal muscle on postnatal day 1 and by 92% on postnatal day 7, consistent with functionally severe myopathy. NEB-KO muscle was also more susceptible to a decline in stress production during a bout of 10 cyclic isometric tetani. Length-tension properties in NEB-KO muscle were altered in a manner consistent with reduced thin filament length, with length-tension curves from NEB-KO muscle demonstrating a 7.4% narrower functional range and an optimal length reduced by 0.13 muscle lengths. Expression patterns of myosin heavy chain isoforms and total myosin content did not account for the functional differences between WT and NEB-KO muscle. These data indicate that NEB is essential for active stress production, maintenance of functional integrity during cyclic activation, and length-tension properties consistent with a role in specifying normal thin filament length. Continued analysis of NEB's functional properties will strengthen the understanding of force transmission and thin filament length regulation in skeletal muscle and may provide insights into the molecular processes that give rise to nemaline myopathy.

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Dysregulation of NRAP degradation by KLHL41 contributes to pathophysiology in nemaline myopathy

Human Molecular Genetics ◽

10.1093/hmg/ddz078 ◽

2019 ◽

Vol 28 (15) ◽

pp. 2549-2560 ◽

Cited By ~ 8

Author(s):

Caroline Jirka ◽

Jasmine H Pak ◽

Claire A Grosgogeat ◽

Michael Mario Marchetii ◽

Vandana A Gupta

Keyword(s):

Skeletal Muscle ◽

Thin Filament ◽

Muscle Development ◽

Nemaline Myopathy ◽

Therapeutic Interventions ◽

Transgenic Zebrafish ◽

Anchoring Protein ◽

And Function

Abstract Nemaline myopathy (NM) is the most common form of congenital myopathy that results in hypotonia and muscle weakness. This disease is clinically and genetically heterogeneous, but three recently discovered genes in NM encode for members of the Kelch family of proteins. Kelch proteins act as substrate-specific adaptors for Cullin 3 (CUL3) E3 ubiquitin ligase to regulate protein turnover through the ubiquitin-proteasome machinery. Defects in thin filament formation and/or stability are key molecular processes that underlie the disease pathology in NM; however, the role of Kelch proteins in these processes in normal and diseases conditions remains elusive. Here, we describe a role of NM causing Kelch protein, KLHL41, in premyofibil-myofibil transition during skeletal muscle development through a regulation of the thin filament chaperone, nebulin-related anchoring protein (NRAP). KLHL41 binds to the thin filament chaperone NRAP and promotes ubiquitination and subsequent degradation of NRAP, a process that is critical for the formation of mature myofibrils. KLHL41 deficiency results in abnormal accumulation of NRAP in muscle cells. NRAP overexpression in transgenic zebrafish resulted in a severe myopathic phenotype and absence of mature myofibrils demonstrating a role in disease pathology. Reducing Nrap levels in KLHL41 deficient zebrafish rescues the structural and function defects associated with disease pathology. We conclude that defects in KLHL41-mediated ubiquitination of sarcomeric proteins contribute to structural and functional deficits in skeletal muscle. These findings further our understanding of how the sarcomere assembly is regulated by disease-causing factors in vivo, which will be imperative for developing mechanism-based specific therapeutic interventions.

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Dysregulation of NRAP degradation by KLHL41 contributes to pathophysiology in Nemaline Myopathy

10.1101/487454 ◽

2018 ◽

Cited By ~ 1

Author(s):

Caroline Jirka ◽

Jasmine H Pak ◽

Claire A Grosgogeat ◽

Michael M Marchetti ◽

Vandana A Gupta

Keyword(s):

Skeletal Muscle ◽

Thin Filament ◽

Muscle Development ◽

Nemaline Myopathy ◽

Therapeutic Interventions ◽

Transgenic Zebrafish ◽

Sarcomeric Protein ◽

And Function

Nemaline myopathy (NM) is the most common form of congenital myopathy that results in hypotonia and muscle weakness. This disease is clinically and genetically heterogeneous, but three recently discovered genes in NM encode for members of the Kelch family of proteins. Kelch proteins act as substrate-specific-adapters for CUL3 E3 ubiquitin ligase to regulate protein turn-over through the ubiquitin-proteasome machinery. Defects in thin filament formation and/or stability are key molecular processes that underlie the disease pathology in NM, however, the role of Kelch proteins in these processes in normal and diseases conditions remains elusive in vivo. Here, we describe a role of NM causing Kelch protein, KLHL41, in premyofibil-myofibil transition during skeletal muscle development through a regulation of the thin filament chaperone, NRAP. KLHL41 binds to the thin filament chaperone NRAP and promotes ubiquitination and subsequent degradation of NRAP, a process that is critical for the formation of mature myofibrils. KLHL41 deficiency results in abnormal accumulation of NRAP in muscle cells. NRAP overexpression in transgenic zebrafish resulted in a severe myopathic phenotype and absence of mature myofibrils demonstrating a role in disease pathology. Reducing Nrap levels in KLHL41 deficient zebrafish rescues the structural and function defects associated with disease pathology. We conclude that defects in KLHL41-mediated ubiquitination of sarcomeric protein contribute to structural and functional deficits in skeletal muscle. These findings further our understanding of how the sarcomere assembly is regulated by disease causing factors in vivo, which will be imperative for developing mechanism-based specific therapeutic interventions.

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