Structure of the amphioxus nebulin gene and evolution of the nebulin family genes

The emergence of nebulin repeats and evolution of lasp family proteins

The LIM and SH3 domain protein (lasp) family, the smallest proteins in the nebulin superfamily, consists of vertebrate lasp-1 expressed in various non-muscle tissues, vertebrate lasp-2 expressed in the brain and cardiac muscle, and invertebrate lasp whose functions have been analyzed in Ascidiacea and Insecta. Gene evolution of the lasp family proteins was investigated by multiple alignments, comparison of gene structure, and synteny analyses in eukaryotes in which mRNA expression was confirmed. All invertebrates analyzed in this study belonging to the clade Filasterea, with the exception of Placozoa, have at least one lasp gene. The minimal actin-binding region (LIM domain and first nebulin repeat) and SH3 domain detected in vertebrate lasp-2 were found to be conserved among the lasp family proteins, and we showed that nematode lasp has actin-binding activity. The linker sequences vary among invertebrate lasp proteins, implying that the lasp family proteins have universal and diverse functions. Gene structures and syntenic analyses suggest that a gene fragment encoding two nebulin repeats and a linker emerged in Filasterea or Holozoa, and the first lasp gene was generated following combination of three gene fragments encoding the LIM domain, two nebulin repeats with a linker, and the SH3 domain. This article is protected by copyright. All rights reserved.

A great legacy of muscle research and education from Dr. Sumiko Kimura

Dr. Sumiko Kimura, a former professor of biology at Chiba University, was known as a distinguished biochemist who contributed considerably to our knowledge about the cytoskeleton of muscle cells, especially through her work on connectin (also called titin) and actin regulatory proteins. Sadly, she suddenly passed away in Tokyo on November 1, 2018 at the age of 71. She succumbed to multiple organ failure caused by a bacterial infection following a third operation on her heart. Dr. Kimura had been continuing her research into connectin right up until several months before her decease.

Titin and Nebulin in Thick and Thin Filament Length Regulation

In this review we discuss the history and the current state of ideas related to the mechanism of size regulation of the thick (myosin) and thin (actin) filaments in vertebrate striated muscles. Various hypotheses have been considered during of more than half century of research, recently mostly involving titin and nebulin acting as templates or 'molecular rulers', terminating exact assembly. These two giant, single-polypeptide, filamentous proteins are bound in situ along the thick and thin filaments, respectively, with an almost perfect match in the respective lengths and structural periodicities. However, evidence still questions the possibility that the proteins function as templates, or scaffolds, on which the thin and thick filaments could be assembled. In addition, the progress in muscle research during the last decades highlighted a number of other factors that could potentially be involved in the mechanism of length regulation: molecular chaperones that may guide folding and assembly of actin and myosin; capping proteins that can influence the rates of assembly-disassembly of the myofilaments; Ca²⁺ transients that can activate or deactivate protein interactions, etc. The entire mechanism of sarcomere assembly appears complex and highly dynamic. This mechanism is also capable of producing filaments of about the correct size without titin and nebulin. What then is the role of these proteins? Evidence points to titin and nebulin stabilizing structures of the respective filaments. This stabilizing effect, based on linear proteins of a fixed size, implies that titin and nebulin are indeed molecular rulers of the filaments. Although the proteins may not function as templates in the assembly of the filaments, they measure and stabilize exactly the same size of the functionally important for the muscles segments in each of the respective filaments.

Independent evolution of striated muscles in cnidarians and bilaterians

Striated muscles are present in bilaterian animals (for example, vertebrates, insects and annelids) and some non-bilaterian eumetazoans (that is, cnidarians and ctenophores). The considerable ultrastructural similarity of striated muscles between these animal groups is thought to reflect a common evolutionary origin. Here we show that a muscle protein core set, including a type II myosin heavy chain (MyHC) motor protein characteristic of striated muscles in vertebrates, was already present in unicellular organisms before the origin of multicellular animals. Furthermore, 'striated muscle' and 'non-muscle' myhc orthologues are expressed differentially in two sponges, compatible with a functional diversification before the origin of true muscles and the subsequent use of striated muscle MyHC in fast-contracting smooth and striated muscle. Cnidarians and ctenophores possess striated muscle myhc orthologues but lack crucial components of bilaterian striated muscles, such as genes that code for titin and the troponin complex, suggesting the convergent evolution of striated muscles. Consistently, jellyfish orthologues of a shared set of bilaterian Z-disc proteins are not associated with striated muscles, but are instead expressed elsewhere or ubiquitously. The independent evolution of eumetazoan striated muscles through the addition of new proteins to a pre-existing, ancestral contractile apparatus may serve as a model for the evolution of complex animal cell types.

Nebulin, a major player in muscle health and disease

Nebulin is a giant 600- to 900-kDa filamentous protein that is an integral component of the skeletal muscle thin filament. Its functions have remained largely nebulous because of its large size and the difficulty in extracting nebulin in a native state from muscle. Recent improvements in the field, especially the development of knockout mouse models deficient in nebulin (NEB-KO mice), indicate now that nebulin performs a surprisingly wide range of functions. In addition to a major role in thin-filament length specification, nebulin also functions in the regulation of muscle contraction, as indicated by the findings that muscle fibers deficient in nebulin have a higher tension cost, and develop less force due to reduced myofilament calcium sensitivity and altered crossbridge cycling kinetics. In addition, the function of nebulin extends to a role in calcium homeostasis. These novel functions indicate that nebulin might have evolved in vertebrate skeletal muscles to develop high levels of muscle force efficiently. Finally, the NEB-KO mouse models also highlight the role of nebulin in the assembly and alignment of the Z disks. Notably, rapid progress in understanding the roles of nebulin in vivo provides clinically important insights into how nebulin deficiency in patients with nemaline myopathy contributes to debilitating muscle weakness.

The Nebulin family: an actin support group

Nebulin, a giant, actin-binding protein, is the largest member of a family of proteins (including N-RAP, nebulette, lasp-1 and lasp-2) that are assembled in a variety of cytoskeletal structures, and expressed in different tissues. For decades, nebulin has been thought to act as a molecular ruler, specifying the precise length of actin filaments in skeletal muscle. However, emerging evidence suggests that nebulin should not be viewed as a ruler but as an actin filament stabilizer required for length maintenance. Nebulin has also been implicated recently in an array of regulatory functions independent of its role in actin filament length regulation. In this review, we discuss the current evolutionary, biochemical, and functional data for the nebulin family of proteins - a family whose members, both large and small, function as cytoskeletal scaffolds and stabilizers.