Expression of nebulette during early cardiac development

Downregulation of NEBL promotes migration and invasion of clear cell renal cell carcinoma by inducing epithelial-mesenchymal transition

As a member of the nebulin protein family and a structural protein of cytoskeleton, NEBL plays an important role in cardiac diseases. Recently, literature have reported the involvement of NEBL in the occurrence and development of various cancers except clear cell renal cell carcinoma (ccRCC). In this study, we found that mRNA and protein of NEBL are downregulated remarkably in ccRCC tissues based on both the TCGA database and clinical samples we collected. The areas under curve values of NEBL analyzed based on the TCGA database, qRT-PCR and IHC results were 0.9376, 0.9733 and 0.9807, respectively. The lower mRNA level of NEBL was associated with worse outcomes in ccRCC patients. When overexpressing NEBL in ccRCC cell lines, the proliferation, migration and invasion of ccRCC cells were suppressed significantly, suggesting a tumor suppressor role of NEBL. In addition, we identified that NEBL is closely related to epithelial-mesenchymal transition (EMT), thereby reducing the motility of ccRCC cells. Furthermore, the lower expression of NEBL was correlated with ccRCC patients with distant organ metastasis. In summary, we firstly described the aberrant expression of NEBL and revealed its tumor suppressor role in ccRCC. Our data support that NEBL could serve as a valuable diagnostic and prognostic biomarker in ccRCC, as well as a promising therapeutic target.

Structural and signaling proteins in the Z-disk and their role in cardiomyopathies

The sarcomere is the smallest functional unit of muscle contraction. It is delineated by a protein-rich structure known as the Z-disk, alternating with M-bands. The Z-disk anchors the actin-rich thin filaments and plays a crucial role in maintaining the mechanical stability of the cardiac muscle. A multitude of proteins interact with each other at the Z-disk and they regulate the mechanical properties of the thin filaments. Over the past 2 decades, the role of the Z-disk in cardiac muscle contraction has been assessed widely, however, the impact of genetic variants in Z-disk proteins has still not been fully elucidated. This review discusses the various Z-disk proteins (alpha-actinin, filamin C, titin, muscle LIM protein, telethonin, myopalladin, nebulette, and nexilin) and Z-disk-associated proteins (desmin, and obscurin) and their role in cardiac structural stability and intracellular signaling. This review further explores how genetic variants of Z-disk proteins are linked to inherited cardiac conditions termed cardiomyopathies.

The Mechanisms of Thin Filament Assembly and Length Regulation in Muscles

The actin containing tropomyosin and troponin decorated thin filaments form one of the crucial components of the contractile apparatus in muscles. The thin filaments are organized into densely packed lattices interdigitated with myosin-based thick filaments. The crossbridge interactions between these myofilaments drive muscle contraction, and the degree of myofilament overlap is a key factor of contractile force determination. As such, the optimal length of the thin filaments is critical for efficient activity, therefore, this parameter is precisely controlled according to the workload of a given muscle. Thin filament length is thought to be regulated by two major, but only partially understood mechanisms: it is set by (i) factors that mediate the assembly of filaments from monomers and catalyze their elongation, and (ii) by factors that specify their length and uniformity. Mutations affecting these factors can alter the length of thin filaments, and in human cases, many of them are linked to debilitating diseases such as nemaline myopathy and dilated cardiomyopathy.

Deficiency in nebulin repeats of sarcomeric nebulette is detrimental for cardiomyocyte tolerance to exercise and biomechanical stress

The actin-binding sarcomeric nebulette (NEBL) protein provides efficient contractile flexibility via interaction with desmin intermediate filaments. NEBL gene mutations affecting the nebulin repeat (NR) domain are known to induce cardiomyopathy. The study aimed to explore the roles of NEBL in exercise and biomechanical stress response. We ablated exon3 encoding the first NR of Nebl and created global Nebl^ex3-/ex3- knockout mice. Cardiac function, structure, and transcriptome were assessed before and after a 4-wk treadmill regimen. A Nebl-based exercise signaling network was constructed using systems genetics methods. H9C2 and neonatal rat cardiomyocytes (NRCs) expressing wild-type or mutant NEBL underwent cyclic mechanical strain. Nebl^ex3-/ex3- mice demonstrated diastolic dysfunction with preserved systolic function at 6 mo of age. After treadmill running, 4-mo-old Nebl^ex3-/ex3- mice developed concentric cardiac hypertrophy and left ventricular dilation compared with running Nebl^+/+ and sedentary Nebl^ex3-/ex3- mice. Disturbance of sarcomeric Z-disks and thin filaments architecture and disruption of intercalated disks and mitochondria were found in exercised Nebl^ex3-/ex3- mice. A Nebl-based exercise signaling network included Csrp3, Des, Fbox32, Jup, Myh6, and Myh7. Disturbed expression of TM1, DES, JUP, β-catenin, MLP, α-actinin2, and vinculin proteins was demonstrated. In H9C2 cells, NEBL was recruited into focal adhesions at 24-h poststrain and redistributed along with F-actin at 72-h poststrain, suggesting time-dependent redistribution of NEBL in response to strain. NEBL mutations cause desmin disorganization in NRCs upon stretch. We conclude that Nebl's NR ablation causes disturbed sarcomere, Z-disks, and desmin organization, and prevents NEBL redistribution to focal adhesions in cardiomyocytes, weakening cardiac tolerance to biomechanical stress.NEW & NOTEWORTHY We demonstrate that ablation of first nebulin-repeats of sarcomeric nebulette (Nebl) causes diastolic dysfunction in Nebl^ex3-/ex3- mice. Exercise-induced development of diastolic dysfunction, cardiac hypertrophy and ventricular dilation in knockouts. This was associated with sarcomere disturbance, intercalated disks disruption, and mitochondrial distortion upon stress and altered expression of genes involved in Nebl-based stress network. We demonstrate that G202R and A592 mutations alter actin and desmin expression causing disorganization of desmin filaments upon cyclic strain.

In Vivo Generation of Post-infarct Human Cardiac Muscle by Laminin-Promoted Cardiovascular Progenitors

Regeneration of injured human heart muscle is limited and an unmet clinical need. There are no methods for the reproducible generation of clinical-quality stem cell-derived cardiovascular progenitors (CVPs). We identified laminin-221 (LN-221) as the most likely expressed cardiac laminin. We produced it as human recombinant protein and showed that LN-221 promotes differentiation of pluripotent human embryonic stem cells (hESCs) toward cardiomyocyte lineage and downregulates pluripotency and teratoma-associated genes. We developed a chemically defined, xeno-free laminin-based differentiation protocol to generate CVPs. We show high reproducibility of the differentiation protocol using time-course bulk RNA sequencing developed from different hESC lines. Single-cell RNA sequencing of CVPs derived from hESC lines supported reproducibility and identified three main progenitor subpopulations. These CVPs were transplanted into myocardial infarction mice, where heart function was measured by echocardiogram and human heart muscle bundle formation was identified histologically. This method may provide clinical-quality cells for use in regenerative cardiology.

Assembly and Maintenance of Sarcomere Thin Filaments and Associated Diseases

Sarcomere assembly and maintenance are essential physiological processes required for cardiac and skeletal muscle function and organism mobility. Over decades of research, components of the sarcomere and factors involved in the formation and maintenance of this contractile unit have been identified. Although we have a general understanding of sarcomere assembly and maintenance, much less is known about the development of the thin filaments and associated factors within the sarcomere. In the last decade, advancements in medical intervention and genome sequencing have uncovered patients with novel mutations in sarcomere thin filaments. Pairing this sequencing with reverse genetics and the ability to generate patient avatars in model organisms has begun to deepen our understanding of sarcomere thin filament development. In this review, we provide a summary of recent findings regarding sarcomere assembly, maintenance, and disease with respect to thin filaments, building on the previous knowledge in the field. We highlight debated and unknown areas within these processes to clearly define open research questions.

Overview of the Muscle Cytoskeleton

Cardiac and skeletal striated muscles are intricately designed machines responsible for muscle contraction. Coordination of the basic contractile unit, the sarcomere, and the complex cytoskeletal networks are critical for contractile activity. The sarcomere is comprised of precisely organized individual filament systems that include thin (actin), thick (myosin), titin, and nebulin. Connecting the sarcomere to other organelles (e.g., mitochondria and nucleus) and serving as the scaffold to maintain cellular integrity are the intermediate filaments. The costamere, on the other hand, tethers the sarcomere to the cell membrane. Unique structures like the intercalated disc in cardiac muscle and the myotendinous junction in skeletal muscle help synchronize and transmit force. Intense investigation has been done on many of the proteins that make up these cytoskeletal assemblies. Yet the details of their function and how they interconnect have just started to be elucidated. A vast number of human myopathies are contributed to mutations in muscle proteins; thus understanding their basic function provides a mechanistic understanding of muscle disorders. In this review, we highlight the components of striated muscle with respect to their interactions, signaling pathways, functions, and connections to disease. © 2017 American Physiological Society. Compr Physiol 7:891-944, 2017.

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.

Nebulette is a powerful cytolinker organizing desmin and actin in mouse hearts

Nebulette physically links desmin to sarcomeric actin in hearts. An intact desmin network is required for nebulette to function as major actin-binding protein in sarcomeres. This study provides biochemical evidence that the desmin–nebulette complex is involved in filament-forming desminopathy.

In the hearts of patients bearing nebulette mutations, a severe general disorganization in cardiomyocytes of the extrasarcomeric desmin intermediate filament system is frequently observed. However, the molecular and functional relationship between the desmin cytoskeleton and nebulette-containing sarcomeres is still unclear. Here we report a high-affinity in vitro interaction between nebulette and desmin filaments. A major interaction site has been mapped to the desmin α-helical rod domain, indicating that the filament core is directly involved in the binding of nebulette. The disease-mutant desmin variants E245D and T453I exhibited increased binding affinity for nebulette, delayed filament assembly kinetics, and caused significant weakening of networks. In isolated chick cardiomyocytes and sections from canine heart, we revealed by ground-state depletion and confocal microscopies that module 5 of nebulette extends outward from Z-disk–associated desmin filaments toward the center of the sarcomere. Accordingly, in the myocardium of Des^−/− mice, elevated levels of cardiac actin correlated with alterations in the distribution of nebulette. Our data suggest that a well-organized desmin network is required to accommodate an optimal conformation of nebulette on sarcomeres to bind and recruit cardiac α-actin. Hence we propose that nebulette acts in synergy with nebulin to reinforce and temporally fine-tune striated muscle relaxation–contraction cycles.

Mutations in NEBL encoding the cardiac Z-disk protein nebulette are associated with various cardiomyopathies

INTRODUCTION

Transgenic mice overexpressing mutated NEBL, encoding the cardiac-specific Z-disk protein nebulette, develop severe cardiac phenotypes. Since cardiomyopathies are commonly familial and because mutations in a single gene may result in variable phenotypes, we tested the hypothesis that NEBL mutations are associated with cardiomyopathy.

MATERIAL AND METHODS

We analyzed 389 patients, including cohorts of patients with dilated cardiomyopathy (DCM), hypertrophic cardiomyopathy (HCM), and left ventricular non-compaction cardiomyopathy (LVNC). The 28 coding exons of the NEBL gene were sequenced. Further bioinformatic analysis was used to distinguish variants.

RESULTS

In total, we identified six very rare heterozygous missense mutations in NEBL in 7 different patients (frequency 1.8%) in highly conserved codons. The mutations were not detectable in 320 Caucasian sex-matched unrelated individuals without cardiomyopathy and 192 Caucasian sex-matched blood donors without heart disease. Known cardiomyopathy genes were excluded in these patients. The mutations p.H171R and p.I652L were found in 2 HCM patients. Further, p.Q581R and p.S747L were detected in 2 DCM patients, while the mutation p.A175T was identified independently in two unrelated patients with DCM. One LVNC patient carried the mutation p.P916L. All HCM and DCM related mutations were located in the nebulin-like repeats, domains responsible for actin binding. Interestingly, the mutation associated with LVNC was located in the C-terminal serine-rich linker region.

CONCLUSIONS

Our data suggest that NEBL mutations may cause various cardiomyopathies. We herein describe the first NEBL mutations in HCM and LVNC. Our findings underline the notion that the cardiomyopathies are true allelic diseases.

Identification of Xin-repeat proteins as novel ligands of the SH3 domains of nebulin and nebulette and analysis of their interaction during myofibril formation and remodeling

The striated muscle–specific actin-binding proteins Xin and Xirp2 are identified as novel ligands of the SH3 domains of the thin filament ruler nebulin and nebulette. The interaction is spatially restricted to structures associated with myofibril development or remodeling, indicating a role for these proteins in myofibril assembly and repair.

The Xin actin-binding repeat–containing proteins Xin and XIRP2 are exclusively expressed in striated muscle cells, where they are believed to play an important role in development. In adult muscle, both proteins are concentrated at attachment sites of myofibrils to the membrane. In contrast, during development they are localized to immature myofibrils together with their binding partner, filamin C, indicating an involvement of both proteins in myofibril assembly. We identify the SH3 domains of nebulin and nebulette as novel ligands of proline-rich regions of Xin and XIRP2. Precise binding motifs are mapped and shown to bind both SH3 domains with micromolar affinity. Cocrystallization of the nebulette SH3 domain with the interacting XIRP2 peptide PPPTLPKPKLPKH reveals selective interactions that conform to class II SH3 domain–binding peptides. Bimolecular fluorescence complementation experiments in cultured muscle cells indicate a temporally restricted interaction of Xin-repeat proteins with nebulin/nebulette during early stages of myofibril development that is lost upon further maturation. In mature myofibrils, this interaction is limited to longitudinally oriented structures associated with myofibril development and remodeling. These data provide new insights into the role of Xin actin-binding repeat–containing proteins (together with their interaction partners) in myofibril assembly and after muscle damage.

The physiological role of cardiac cytoskeleton and its alterations in heart failure

Cardiac muscle cells are equipped with specialized biochemical machineries for the rapid generation of force and movement central to the work generated by the heart. During each heart beat cardiac muscle cells perceive and experience changes in length and load, which reflect one of the fundamental principles of physiology known as the Frank-Starling law of the heart. Cardiac muscle cells are unique mechanical stretch sensors that allow the heart to increase cardiac output, and adjust it to new physiological and pathological situations. In the present review we discuss the mechano-sensory role of the cytoskeletal proteins with respect to their tight interaction with the sarcolemma and extracellular matrix. The role of contractile thick and thin filament proteins, the elastic protein titin, and their anchorage at the Z-disc and M-band, with associated proteins are reviewed in physiologic and pathologic conditions leading to heart failure. This article is part of a Special Issue entitled: Reciprocal influences between cell cytoskeleton and membrane channels, receptors and transporters. Guest Editor: Jean Claude Hervé

Nebulette mutations are associated with dilated cardiomyopathy and endocardial fibroelastosis

OBJECTIVES

Four variants (K60N, Q128R, G202R, and A592E) in the nebulette gene were identified in patients with dilated cardiomyopathy (DCM) and endocardial fibroelastosis. We sought to determine if these mutations are cardiomyopathy causing.

BACKGROUND

Nebulette aligns thin filaments and connects them with the myocardial Z-disk, playing a role in mechanosensation.

METHODS

We generated transgenic mice with cardiac-restricted overexpression of human wild-type or mutant nebulette. Chimera and transgenic mice were examined at 4, 6, and 12 months of age by echocardiography and cardiac magnetic resonance imaging. The hearts from embryos and adult mice were assessed by histopathologic, immunohistochemical, ultrastructural, and protein analyses. Rat H9C2 cardiomyoblasts with transient expression of nebulette underwent cyclic mechanical strain.

RESULTS

We identified lethal cardiac structural abnormalities in mutant embryonic hearts (K60N and Q128R). Founders of the mutant mouse lines developed DCM with severe heart failure. An irregular localization pattern for nebulette and impaired desmin expression were noted in the proband and chimeric Q128R mice. Mutant G202R and A592E mice exhibited left ventricular dilation and impaired function with specific changes in I-band and Z-disk proteins by 6 months of age. The mutations modulated distribution of nebulette in the sarcomere and Z-disk during stretch of H9C2 cells.

CONCLUSIONS

Nebulette is a new susceptibility gene for endocardial fibroelastosis and DCM. Different mutations in nebulette trigger specific mechanisms, converging to a common pathological cascade leading to endocardial fibroelastosis and DCM.

Nonmuscle myosin IIB, a sarcomeric component in the extraocular muscles

Extraocular muscles (EOMs) are categorized as skeletal muscles; however, emerging evidence indicates that their gene expression profile, metabolic characteristics and functional properties are significantly different from the prototypical members of this muscle class. Gene expression profiling of developing and adult EOM suggest that many myofilament and cytoskeletal proteins have unique expression patterns in EOMs, including the maintained expression of embryonic and fetal isoforms of myosin heavy chains (MyHC), the presence of a unique EOM specific MyHC and mixtures of both cardiac and skeletal muscle isoforms of thick and thin filament accessory proteins. We demonstrate that nonmuscle myosin IIB (nmMyH IIB) is a sarcomeric component in approximately 20% of the global layer fibers in adult rat EOMs. Comparisons of the myofibrillar distribution of nmMyHC IIB with sarcomeric MyHCs indicate that nmMyH IIB co-exists with slow MyHC isoforms. In longitudinal sections of adult rat EOM, nmMyHC IIB appears to be restricted to the A-bands. Although nmMyHC IIB has been previously identified as a component of skeletal and cardiac sarcomeres at the level of the Z-line, the novel distribution of this protein within the A band in EOMs is further evidence of both the EOMs complexity and unconventional phenotype.

Nebulette interacts with filamin C

The actin-binding proteins, nebulette, and nebulin, are comprised of a four-domain layout containing an acidic N-terminal region, a repeat domain, a serine-rich-linker region, and a Src homology-3 domain. Both proteins contain homologous N-terminal regions that are predicted to be in different environments within the sarcomere. The nebulin acidic N-terminal region is found at the distal ends of the thin filaments. Nebulette, however, is predicted to extend 150 nm from the center of the Z-line. To dissect out the functions of the N-terminal domain of nebulette, we have performed a yeast two-hybrid screen using nebulette residues 1-86 as bait. We have identified filamin-C, ZASP-1, and tropomyosin-1 as binding partners. Characterization of the nebulette-filamin interaction indicates that filamin-C predominantly interacts with the modules. These data suggest that filamin-C, a known component of striated muscle Z-lines, interacts with nebulette modules.