Striated muscle cytoarchitecture: an intricate web of form and function

Styxl2 regulates de novo sarcomere assembly by binding to non-muscle myosin IIs and promoting their degradation

Styxl2, a poorly characterized pseudophosphatase, was identified as a transcriptional target of the Jak1-Stat1 pathway during myoblast differentiation in culture. Styxl2 is specifically expressed in vertebrate striated muscles. By gene knockdown in zebrafish or genetic knockout in mice, we found that Styxl2 plays an essential role in maintaining sarcomere integrity in developing muscles. To further reveal the functions of Styxl2 in adult muscles, we generated two inducible knockout mouse models: one with Styxl2 being deleted in mature myofibers to assess its role in sarcomere maintenance, and the other in adult muscle satellite cells (MuSCs) to assess its role in de novo sarcomere assembly. We find that Styxl2 is not required for sarcomere maintenance but functions in de novo sarcomere assembly during injury-induced muscle regeneration. Mechanistically, Styxl2 interacts with non-muscle myosin IIs, enhances their ubiquitination, and targets them for autophagy-dependent degradation. Without Styxl2, the degradation of non-muscle myosin IIs is delayed, which leads to defective sarcomere assembly and force generation. Thus, Styxl2 promotes de novo sarcomere assembly by interacting with non-muscle myosin IIs and facilitating their autophagic degradation.

Cocaine, amphetamine, or titin: Unraveling the genetic underpinnings of dilated cardiomyopathy

Key Clinical Message

An interesting case that shows the importance of identifying a pathogenic TTN gene mutation through genetic assessment in unexplained cardiomyopathy, especially with family history. This case highlights the need for genetic counseling and testing for at-risk relatives, and advocates for personalized management considering both genetic and lifestyle factors.

Abstract

This case report examines a 33-year-old Hispanic male with bipolar disorder, schizophrenia, and a history of substance use, presenting with acute respiratory failure and cardiac arrest. The patient's nonischemic dilated cardiomyopathy (DCM) highlights the critical role of genetic factors, particularly titin gene (TTN) mutations, in cardiomyopathy pathogenesis. Through genetic analysis, we explore the intersection of lifestyle factors and genetic predisposition in DCM, underscoring the importance of comprehensive genetic testing for accurate diagnosis and targeted therapy. This case contributes to the evolving understanding of DCM etiology, emphasizing the necessity of considering both environmental and genetic factors in clinical assessment and management.

Exploring TTN variants as genetic insights into cardiomyopathy pathogenesis and potential emerging clues to molecular mechanisms in cardiomyopathies

The giant protein titin (TTN) is a sarcomeric protein that forms the myofibrillar backbone for the components of the contractile machinery which plays a crucial role in muscle disorders and cardiomyopathies. Diagnosing TTN pathogenic variants has important implications for patient management and genetic counseling. Genetic testing for TTN variants can help identify individuals at risk for developing cardiomyopathies, allowing for early intervention and personalized treatment strategies. Furthermore, identifying TTN variants can inform prognosis and guide therapeutic decisions. Deciphering the intricate genotype-phenotype correlations between TTN variants and their pathologic traits in cardiomyopathies is imperative for gene-based diagnosis, risk assessment, and personalized clinical management. With the increasing use of next-generation sequencing (NGS), a high number of variants in the TTN gene have been detected in patients with cardiomyopathies. However, not all TTN variants detected in cardiomyopathy cohorts can be assumed to be disease-causing. The interpretation of TTN variants remains challenging due to high background population variation. This narrative review aimed to comprehensively summarize current evidence on TTN variants identified in published cardiomyopathy studies and determine which specific variants are likely pathogenic contributors to cardiomyopathy development.

Desmin and its molecular chaperone, the αB-crystallin: How post-translational modifications modulate their functions in heart and skeletal muscles?

Maintenance of the highly organized striated muscle tissue requires a cell-wide dynamic network through protein-protein interactions providing an effective mechanochemical integrator of morphology and function. Through a continuous and complex trans-cytoplasmic network, desmin intermediate filaments ensure this essential role in heart and in skeletal muscle. Besides their role in the maintenance of cell shape and architecture (permitting contractile activity efficiency and conferring resistance towards mechanical stress), desmin intermediate filaments are also key actors of cell and tissue homeostasis. Desmin participates to several cellular processes such as differentiation, apoptosis, intracellular signalisation, mechanotransduction, vesicle trafficking, organelle biogenesis and/or positioning, calcium homeostasis, protein homeostasis, cell adhesion, metabolism and gene expression. Desmin intermediate filaments assembly requires αB-crystallin, a small heat shock protein. Over its chaperone activity, αB-crystallin is involved in several cellular functions such as cell integrity, cytoskeleton stabilization, apoptosis, autophagy, differentiation, mitochondria function or aggresome formation. Importantly, both proteins are known to be strongly associated to the aetiology of several cardiac and skeletal muscles pathologies related to desmin filaments disorganization and a strong disturbance of desmin interactome. Note that these key proteins of cytoskeleton architecture are extensively modified by post-translational modifications that could affect their functional properties. Therefore, we reviewed in the herein paper the impact of post-translational modifications on the modulation of cellular functions of desmin and its molecular chaperone, the αB-crystallin.

Cooperative & competitive binding of anti-myosin tail antibodies revealed by super-resolution microscopy

The MF30 monoclonal antibody, which binds to the myosin subfragment-2 (S2), was found to increase the extent of myofibril shortening. Yet, previous observations found no effect of this antibody on actin sliding over myosin during in vitro motility assays with purified proteins in which myosin binding protein C (MyBPC) was absent. MF30 is hypothesized to enhance the availability of myosin heads (subfragment-1 or S1) to bind actin by destabilizing the myosin S2 coiled-coil and sterically blocking S2 from binding S1. The mechanism of action likely includes MF30's substantial size, thereby inhibiting S1 heads and MyBPC from binding S2. Hypothetically, MF30 should enhance the ON state of myosin, thereby increasing muscle contraction. Our findings indicate that MF30 binds preferentially to the unfolded heavy chains of S2, displaying positive cooperativity. However, the dose-response curve of MF30's enhancement of myofibril shortening did not suggest complex interactions with S2. Single, double, and triple-stained myofibrils with increasing amounts of antibodies against myosin rods indicate a possible competition with MyBPC. Additional assays revealed decreased fluorescence intensity at the C-zone (central zone in the sarcomere, where MyBPC is located), where MyBPC may inhibit MF30 binding. Another monoclonal antibody named MF20, which binds to the light meromyosin (LMM) without affecting myofibril contraction, showed less reduction in fluorescence intensity at the C-zone in expansion microscopy than MF30. Expansion microscopy images of myofibrils labeled with MF20 revealed labeling of the A-band (anisotropic band) and a slight reduction in the labeling at the C-zone. The staining pattern obtained from the expansion microscopy image was consistent with images from photolocalization microscopy which required the synthesis of unique photoactivatable quantum dots, and Zeiss Airyscan imaging as well as alternative expansion microscopy digestion methods. Consistent with the hypothesis that MF30 competes with MyBPC binding to S2, cardiac tissue from MyBPC knockout mice was stained more intensely, especially in the C-zone, by MF30 compared to the wild type.

The Relationship between Changes in MYBPC3 Single-Nucleotide Polymorphism-Associated Metabolites and Elite Athletes' Adaptive Cardiac Function

Athletic performance is a multifactorial trait influenced by a complex interaction of environmental and genetic factors. Over the last decades, understanding and improving elite athletes' endurance and performance has become a real challenge for scientists. Significant tools include but are not limited to the development of molecular methods for talent identification, personalized exercise training, dietary requirements, prevention of exercise-related diseases, as well as the recognition of the structure and function of the genome in elite athletes. Investigating the genetic markers and phenotypes has become critical for elite endurance surveillance. The identification of genetic variants contributing to a predisposition for excellence in certain types of athletic activities has been difficult despite the relatively high genetic inheritance of athlete status. Metabolomics can potentially represent a useful approach for gaining a thorough understanding of various physiological states and for clarifying disorders caused by strength-endurance physical exercise. Based on a previous GWAS study, this manuscript aims to discuss the association of specific single-nucleotide polymorphisms (SNPs) located in the MYBPC3 gene encoding for cardiac MyBP-C protein with endurance athlete status. MYBPC3 is linked to elite athlete heart remodeling during or after exercise, but it could also be linked to the phenotype of cardiac hypertrophy (HCM). To make the distinction between both phenotypes, specific metabolites that are influenced by variants in the MYBPC3 gene are analyzed in relation to elite athletic performance and HCM. These include theophylline, ursodeoxycholate, quinate, and decanoyl-carnitine. According to the analysis of effect size, theophylline, quinate, and decanoyl carnitine increase with endurance while decreasing with cardiovascular disease, whereas ursodeoxycholate increases with cardiovascular disease. In conclusion, and based on our metabolomics data, the specific effects on athletic performance for each MYBPC3 SNP-associated metabolite are discussed.

High-resolution multi-z confocal microscopy with a diffractive optical element

There has been recent interest in the development of fluorescence microscopes that provide high-speed volumetric imaging for life-science applications. For example, multi-z confocal microscopy enables simultaneous optically-sectioned imaging at multiple depths over relatively large fields of view. However, to date, multi-z microscopy has been hampered by limited spatial resolution owing to its initial design. Here we present a variant of multi-z microscopy that recovers the full spatial resolution of a conventional confocal microscope while retaining the simplicity and ease of use of our initial design. By introducing a diffractive optical element in the illumination path of our microscope, we engineer the excitation beam into multiple tightly focused spots that are conjugated to axially distributed confocal pinholes. We discuss the performance of this multi-z microscope in terms of resolution and detectability and demonstrate its versatility by performing in-vivo imaging of beating cardiomyocytes in engineered heart tissues and neuronal activity in c. elegans and zebrafish brains.

Cardiac myofibrillogenesis is spatiotemporally modulated by the molecular chaperone UNC45B

Sarcomeres are fundamental to cardiac muscle contraction. Their impairment can elicit cardiomyopathies, leading causes of death worldwide. However, the molecular mechanism underlying sarcomere assembly remains obscure. We used human embryonic stem cell (hESC)-derived cardiomyocytes (CMs) to reveal stepwise spatiotemporal regulation of core cardiac myofibrillogenesis-associated proteins. We found that the molecular chaperone UNC45B is highly co-expressed with KINDLIN2 (KIND2), a marker of protocostameres, and later its distribution overlaps with that of muscle myosin MYH6. UNC45B-knockout CMs display essentially no contractility. Our phenotypic analyses further reveal that (1) binding of Z line anchor protein ACTN2 to protocostameres is perturbed because of impaired protocostamere formation, resulting in ACTN2 accumulation; (2) F-ACTIN polymerization is suppressed; and (3) MYH6 becomes degraded, so it cannot replace non-muscle myosin MYH10. Our mechanistic study demonstrates that UNC45B mediates protocostamere formation by regulating KIND2 expression. Thus, we show that UNC45B modulates cardiac myofibrillogenesis by interacting spatiotemporally with various proteins.

Phosphoproteomics Profiling Defines a Target Landscape of the Basophilic Protein Kinases AKT, S6K, and RSK in Skeletal Myotubes

Phosphorylation-dependent signal transduction plays an important role in regulating the functions and fate of skeletal muscle cells. Central players in the phospho-signaling network are the protein kinases AKT, S6K, and RSK as part of the PI3K-AKT-mTOR-S6K and RAF-MEK-ERK-RSK pathways. However, despite their functional importance, knowledge about their specific targets is incomplete because these kinases share the same basophilic substrate motif RxRxxp[ST]. To address this, we performed a multifaceted quantitative phosphoproteomics study of skeletal myotubes following kinase inhibition. Our data corroborate a cross talk between AKT and RAF, a negative feedback loop of RSK on ERK, and a putative connection between RSK and PI3K signaling. Altogether, we report a kinase target landscape containing 49 so far unknown target sites. AKT, S6K, and RSK phosphorylate numerous proteins involved in muscle development, integrity, and functions, and signaling converges on factors that are central for the skeletal muscle cytoskeleton. Whereas AKT controls insulin signaling and impinges on GTPase signaling, nuclear signaling is characteristic for RSK. Our data further support a role of RSK in glucose metabolism. Shared targets have functions in RNA maturation, stability, and translation, which suggests that these basophilic kinases establish an intricate signaling network to orchestrate and regulate processes involved in translation.

Postmenopausal osteoporosis: a bioinformatics-integrated experimental study the pathogenesis

Postmenopausal osteoporosis (PMOP) is a chronic bone metabolic disease, which often causes fractures and various complications, it causes a great social and economic burden, and it is urgent to use modern research techniques to elucidate the pathogenesis of PMOP. At the same time, because of the complex physiological and pathological interaction mechanism between osteoporosis and sarcopenia, the correlation research has become a hot topic. Ovary removal is a commonly used experimental method to study the endocrine system of female animals, and it is also the best animal model to study PMOP. In this study, the preparation of the ovariectomized rat was confirmed through the detection of vaginal smear, the level of bone formation markers, and the analysis of bone tissue morphology. Transcriptome sequencing was used to analyze the molecular mechanism of PMOP in ovariectomized rats, qRT-PCR was used to verify the key targets. Results of Micro-CT and scanning electron microscopy (SEM) showed that the trabecular structure was disorganized and the symptoms of osteoporosis appeared, this indicating that the ovariectomized rats model was successfully prepared. Transcriptional sequencing results of femur tissue showed that 452 differentially expressed genes (DEGs) were screened. Bioinformatics analysis results showed that the osteoporosis caused by ovariectomized rats was mainly related to muscle contraction, calcium signaling pathway, etc. Results of qRT-PCR were consistent with transcriptome analysis. These results reveal the pathogenesis of PMOP in ovariectomized rats and also offer a possibility for elucidating the relevance of action between PMOP and sarcopenia.

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 Quality and Bacterial Community Changes in Freshwater Crawfish Stored at 4 °C in Vacuum Packaging

Crawfish can be easily spoiled due to their rich nutrition and high water content, which is difficult to preserve. In this study, the dominant spoilage organisms in crawfish which were stored at 4 °C in vacuum packaging were identified by high-throughput sequencing technology; after sequencing the full-length 16S rRNA gene, the changes in the bacterial community structure, diversity and quality (texture, flavor, etc.) were analyzed. Our results reflected that the specific spoilage organisms (SSOs) of crawfish were Aeromonas sobria, Shewanella putrefaciens, Trichococcus pasteurii and Enterococcus aquimarinus, since their abundances significantly increased after being stored for 12 days at 4 °C under vacuum conditions. At the same time, the abundance and diversity of the microbial community decreased with storage time, which was related to the rapid growth of the dominant spoilage organisms and the inhibition of other kinds of microorganisms at the end of the spoilage stage. Function prediction results showed that the gene which contributed to metabolism influenced the spoilage process. Moreover, the decline in texture of crawfish was negatively correlated to the richness of SSOs; this may be because SSOs can produce alkaline proteases to degrade the myofibrillar protein. On the contrary, the unpleasant flavor of crawfish, resulting from volatile flavor compounds such as S-containing compounds and APEOs, etc., is negatively correlated to the richness of SSOs, due to the metabolism of SSOs by secondary metabolites such as terpenoids, polyketides and lips, which can lead to decarboxylation, deamination and enzymatic oxidation. These results are very important to achieve the purpose of targeted inhibition of crawfish spoilage at 4 °C in vacuum packaging.

A comprehensive review of computational and image analysis techniques for quantitative evaluation of striated muscle tissue architecture

Unbiased evaluation of morphology is crucial to understanding development, mechanics, and pathology of striated muscle tissues. Indeed, the ability of striated muscles to contract and the strength of their contraction is dependent on their tissue-, cellular-, and cytoskeletal-level organization. Accordingly, the study of striated muscles often requires imaging and assessing aspects of their architecture at multiple different spatial scales. While an expert may be able to qualitatively appraise tissues, it is imperative to have robust, repeatable tools to quantify striated myocyte morphology and behavior that can be used to compare across different labs and experiments. There has been a recent effort to define the criteria used by experts to evaluate striated myocyte architecture. In this review, we will describe metrics that have been developed to summarize distinct aspects of striated muscle architecture in multiple different tissues, imaged with various modalities. Additionally, we will provide an overview of metrics and image processing software that needs to be developed. Importantly to any lab working on striated muscle platforms, characterization of striated myocyte morphology using the image processing pipelines discussed in this review can be used to quantitatively evaluate striated muscle tissues and contribute to a robust understanding of the development and mechanics of striated muscles.

Sarcomere maturation: function acquisition, molecular mechanism, and interplay with other organelles

During postnatal cardiac development, cardiomyocytes mature and turn into adult ones. Hence, all cellular properties, including morphology, structure, physiology and metabolism, are changed. One of the most important aspects is the contractile apparatus, of which the minimum unit is known as a sarcomere. Sarcomere maturation is evident by enhanced sarcomere alignment, ultrastructural organization and myofibrillar isoform switching. Any maturation process failure may result in cardiomyopathy. Sarcomere function is intricately related to other organelles, and the growing evidence suggests reciprocal regulation of sarcomere and mitochondria on their maturation. Herein, we summarize the molecular mechanism that regulates sarcomere maturation and the interplay between sarcomere and other organelles in cardiomyocyte maturation.

This article is part of the theme issue ‘The cardiomyocyte: new revelations on the interplay between architecture and function in growth, health, and disease’.

Exercise-Induced Muscle Damage and Protein Intake: A Bibliometric and Visual Analysis

Numerous studies have covered exercise-induced muscle damage (EIMD) topics, ranging from nutritional strategies to recovery methods, but few attempts have adequately explored and analyzed large volumes of scientific output. The purpose of this study was to assess the scientific output and research activity regarding EIMD and protein intake by conducting a bibliometric and visual analysis. Relevant publications from 1975-2022 were retrieved from the Web of Science Core Collection database. Quantitative and qualitative variables were collected, including the number of publications and citations, H-indexes, journals of citation reports, co-authorship, co-citation, and the co-occurrence of keywords. There were 351 total publications, with the number of annual publications steadily increasing. The United States has the highest total number of publications (26.21% of total publications, centrality 0.44). Institutional cooperation is mostly geographically limited, with few transnational cooperation links. EIMD and protein intake research is concentrated in high-quality journals in the disciplines of Sport Science, Physiology, Nutrition, and Biochemistry & Molecular Biology. The top ten journals in the number of publications are mostly high-quality printed journals, and the top ten journals in centrality have an average impact factor of 13.845. The findings of the co-citation clusters and major keyword co-occurrence reveal that the most discussed research topics are "exercise mode", "nutritional strategies", "beneficial outcomes", and "proposed mechanisms". Finally, we identified the following research frontiers and research directions: developing a comprehensive understanding of new exercise or training models, nutritional strategies, and recovery techniques to alleviate EIMD symptoms and accelerate recovery; applying the concept of hormesis in EIMD to induce muscle hypertrophy; and investigating the underlying mechanisms of muscle fiber and membrane damage.

Insight Into the Metabolic Adaptations of Electrically Pulse-Stimulated Human Myotubes Using Global Analysis of the Transcriptome and Proteome

Electrical pulse stimulation (EPS) has proven to be a useful tool to interrogate cell-specific responses to muscle contraction. In the present study, we aimed to uncover networks of signaling pathways and regulatory molecules responsible for the metabolic effects of exercise in human skeletal muscle cells exposed to chronic EPS. Differentiated myotubes from young male subjects were exposed to EPS protocol 1 (i.e. 2 ms, 10 V, and 0.1 Hz for 24 h), whereas myotubes from middle-aged women and men were exposed to protocol 2 (i.e. 2 ms, 30 V, and 1 Hz for 48 h). Fuel handling as well as the transcriptome, cellular proteome, and secreted proteins of EPS-treated myotubes from young male subjects were analyzed using a combination of high-throughput RNA sequencing, high-resolution liquid chromatography-tandem mass spectrometry, oxidation assay, and immunoblotting. The data showed that oxidative metabolism was enhanced in EPS-exposed myotubes from young male subjects. Moreover, a total of 81 differentially regulated proteins and 952 differentially expressed genes (DEGs) were observed in these cells after EPS protocol 1. We also found 61 overlapping genes while comparing the DEGs to mRNA expression in myotubes from the middle-aged group exposed to protocol 2, assessed by microarray. Gene ontology (GO) analysis indicated that significantly regulated proteins and genes were enriched in biological processes related to glycolytic pathways, positive regulation of fatty acid oxidation, and oxidative phosphorylation, as well as muscle contraction, autophagy/mitophagy, and oxidative stress. Additionally, proteomic identification of secreted proteins revealed extracellular levels of 137 proteins were changed in myotubes from young male subjects exposed to EPS protocol 1. Selected putative myokines were measured using ELISA or multiplex assay to validate the results. Collectively, our data provides new insight into the transcriptome, proteome and secreted proteins alterations following in vitro exercise and is a valuable resource for understanding the molecular mechanisms and regulatory molecules mediating the beneficial metabolic effects of exercise.

Fibroblast fate determination during cardiac reprogramming by remodeling of actin filaments

Fibroblasts can be reprogrammed into induced cardiomyocyte-like cells (iCMs) by forced expression of cardiogenic transcription factors. However, it remains unknown how fibroblasts adopt a cardiomyocyte (CM) fate during their spontaneous ongoing transdifferentiation toward myofibroblasts (MFs). By tracing fibroblast lineages following cardiac reprogramming in vitro, we found that most mature iCMs are derived directly from fibroblasts without transition through the MF state. This direct conversion is attributable to mutually exclusive induction of cardiac sarcomeres and MF cytoskeletal structures in the cytoplasm of fibroblasts during reprogramming. For direct fate switch from fibroblasts to iCMs, significant remodeling of actin isoforms occurs in fibroblasts, including induction of α-cardiac actin and decrease of the actin isoforms predominant in MFs. Accordingly, genetic or pharmacological ablation of MF-enriched actin isoforms significantly enhances cardiac reprogramming. Our results demonstrate that remodeling of actin isoforms is required for fibroblast to CM fate conversion by cardiac reprogramming.

Mutual dependence between tropomodulin and tropomyosin in the regulation of sarcomeric actin assembly in Caenorhabditis elegans striated muscle

Tropomodulin and tropomyosin are important components of sarcomeric thin filaments in striated muscles. Tropomyosin decorates the side of actin filaments and enhances tropomodulin capping at the pointed ends of the filaments. Their functional relationship has been extensively characterized in vitro, but in vivo and cellular studies in mammals are often complicated by the presence of functionally redundant isoforms. Here, we used the nematode Caenorhabditis elegans, which has a relatively simple composition of tropomodulin and tropomyosin genes, and demonstrated that tropomodulin (unc-94) and tropomyosin (lev-11) are mutually dependent on each other in their sarcomere localization and regulation of sarcomeric actin assembly. Mutation of tropomodulin caused sarcomere disorganization with formation of actin aggregates. However, the actin aggregation was suppressed when tropomyosin was depleted in the tropomodulin mutant. Tropomyosin was mislocalized to the actin aggregates in the tropomodulin mutants, while sarcomere localization of tropomodulin was lost when tropomyosin was depleted. These results indicate that tropomodulin and tropomyosin are interdependent in the regulation of organized sarcomeric assembly of actin filaments in vivo.

Bioengineering approaches to treat the failing heart: from cell biology to 3D printing

Successfully engineering a functional, human, myocardial pump would represent a therapeutic alternative for the millions of patients with end-stage heart disease and provide an alternative to animal-based preclinical models. Although the field of cardiac tissue engineering has made tremendous advances, major challenges remain, which, if properly resolved, might allow the clinical implementation of engineered, functional, complex 3D structures in the future. In this Review, we provide an overview of state-of-the-art studies, challenges that have not yet been overcome and perspectives on cardiac tissue engineering. We begin with the most clinically relevant cell sources used in this field and discuss the use of topological, biophysical and metabolic stimuli to obtain mature phenotypes of cardiomyocytes, particularly in relation to organized cytoskeletal and contractile intracellular structures. We then move from the cellular level to engineering planar cardiac patches and discuss the need for proper vascularization and the main strategies for obtaining it. Finally, we provide an overview of several different approaches for the engineering of volumetric organs and organ parts - from whole-heart decellularization and recellularization to advanced 3D printing technologies.

Genomic and Non-Genomic Regulatory Mechanisms of the Cardiac Sodium Channel in Cardiac Arrhythmias

Nav1.5 is the predominant cardiac sodium channel subtype, encoded by the SCN5A gene, which is involved in the initiation and conduction of action potentials throughout the heart. Along its biosynthesis process, Nav1.5 undergoes strict genomic and non-genomic regulatory and quality control steps that allow only newly synthesized channels to reach their final membrane destination and carry out their electrophysiological role. These regulatory pathways are ensured by distinct interacting proteins that accompany the nascent Nav1.5 protein along with different subcellular organelles. Defects on a large number of these pathways have a tremendous impact on Nav1.5 functionality and are thus intimately linked to cardiac arrhythmias. In the present review, we provide current state-of-the-art information on the molecular events that regulate SCN5A/Nav1.5 and the cardiac channelopathies associated with defects in these pathways.

A computational approach to quantitatively define sarcomere dimensions and arrangement in skeletal muscle

BACKGROUND AND OBJECTIVE

The skeletal muscle is composed of integrated tissues mainly composed of myofibers i.e., long, cylindrical syncytia, whose cytoplasm is mostly occupied by parallel myofibrils. In section, each myofibril is organized in serially end-to-end arranged sarcomeres connected by Z lines. In muscle disorders, these structural and functional units can undergo structural alterations in terms of Z-line and sarcomere lengths, as well as lateral alignment of Z-line among adjacent myofibrils. In this view, objectifying alterations of the myofibril and sarcomere architecture would provide a solid foundation for qualitative observations. In this work, specific quantitative parameters characterizing the sarcomere and myofibril arrangement were defined using a computerized analysis of ultrastructural images.

METHODS

computerized analysis was carried out on transmission electron microscopy pictures of the murine vastus lateralis muscle. Samples from both euploid (control) and trisomic (showing myofiber alterations) Ts65Dn mice were used. Two routines were written in MATLAB to measure specific structural parameters on sarcomeres and myofibrils. The output included the Z-line, M-line, and sarcomere lengths, the Aspect Ratio (AsR) and Curviness (Cur) sarcomere shape parameters, myofibril axis (α angle), and the H parameter (evaluation of sequence of Z-lines of adjacent myofibrils).

RESULTS

Both routines worked well in control (euploid) skeletal muscle yielding consistent quantitative data of sarcomere and myofibril structural organization. In comparison with euploid, trisomic muscle showed statistically significant lower Z-line length, similar M-line length, and statistically significant lower sarcomere length. Both AsR and Cur were statistically significantly lower in trisomic muscle, suggesting the sarcomere is barrel-shaped in the latter. The angle (α) distribution showed that the sarcomere axes are almost parallel in euploid muscle, while a large variability occurs in trisomic tissue. The mean value of H was significantly higher in trisomic versus euploid muscle indicating that Z-lines are not perfectly aligned in trisomic muscle.

CONCLUSIONS

Our procedure allowed us to accurately extract and quantify sarcomere and myofibril parameters from the high-resolution electron micrographs thereby yielding an effective tool to quantitatively define trisomy-associated muscle alterations. These results pave the way to future objective quantification of skeletal muscle changes in pathological conditions.

SHORT ABSTRACT

The skeletal muscle is composed of integrated tissues mainly composed of myofibers i.e., long, cylindrical syncytia, whose cytoplasm is mostly occupied by parallel myofibrils organized in serially end-to-end arranged sarcomeres. Several pieces of evidence have highlighted that in muscle disorders and diseases the sarcomere structure may be altered. Therefore, objectifying alterations of the myofibril and sarcomere architecture would provide a solid foundation for qualitative observations. A computerized analysis was carried out on transmission electron microscopy images of euploid (control) and trisomic (showing myofiber alterations) skeletal muscle. Two routines were written in MATLAB to measure nine sarcomere and myofibril structural parameters. Our computational method confirmed and expanded on previous qualitative ultrastructural findings defining several trisomy-associated skeletal muscle alterations. The proposed procedure is a potentially useful tool to quantitatively define skeletal muscle changes in pathological conditions involving the sarcomere.

PRC1 Stabilizes Cardiac Contraction by Regulating Cardiac Sarcomere Assembly and Cardiac Conduction System Construction

Cardiac development is a complex process that is strictly controlled by various factors, including PcG protein complexes. Several studies have reported the critical role of PRC2 in cardiogenesis. However, little is known about the regulation mechanism of PRC1 in embryonic heart development. To gain more insight into the mechanistic role of PRC1 in cardiogenesis, we generated a PRC1 loss-of-function zebrafish line by using the CRISPR/Cas9 system targeting rnf2, a gene encoding the core subunit shared by all PRC1 subfamilies. Our results revealed that Rnf2 is not involved in cardiomyocyte differentiation and heart tube formation, but that it is crucial to maintaining regular cardiac contraction. Further analysis suggested that Rnf2 loss-of-function disrupted cardiac sarcomere assembly through the ectopic activation of non-cardiac sarcomere genes in the developing heart. Meanwhile, Rnf2 deficiency disrupts the construction of the atrioventricular canal and the sinoatrial node by modulating the expression of bmp4 and other atrioventricular canal marker genes, leading to an impaired cardiac conduction system. The disorganized cardiac sarcomere and defective cardiac conduction system together contribute to defective cardiac contraction. Our results emphasize the critical role of PRC1 in the cardiac development.

Sarc-Graph: Automated segmentation, tracking, and analysis of sarcomeres in hiPSC-derived cardiomyocytes

A better fundamental understanding of human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) has the potential to advance applications ranging from drug discovery to cardiac repair. Automated quantitative analysis of beating hiPSC-CMs is an important and fast developing component of the hiPSC-CM research pipeline. Here we introduce “Sarc-Graph,” a computational framework to segment, track, and analyze sarcomeres in fluorescently tagged hiPSC-CMs. Our framework includes functions to segment z-discs and sarcomeres, track z-discs and sarcomeres in beating cells, and perform automated spatiotemporal analysis and data visualization. In addition to reporting good performance for sarcomere segmentation and tracking with little to no parameter tuning and a short runtime, we introduce two novel analysis approaches. First, we construct spatial graphs where z-discs correspond to nodes and sarcomeres correspond to edges. This makes measuring the network distance between each sarcomere (i.e., the number of connecting sarcomeres separating each sarcomere pair) straightforward. Second, we treat tracked and segmented components as fiducial markers and use them to compute the approximate deformation gradient of the entire tracked population. This represents a new quantitative descriptor of hiPSC-CM function. We showcase and validate our approach with both synthetic and experimental movies of beating hiPSC-CMs. By publishing Sarc-Graph, we aim to make automated quantitative analysis of hiPSC-CM behavior more accessible to the broader research community.

Heart disease is the leading cause of death worldwide. Because of this, many researchers are studying heart cells in the lab and trying to create artificial heart tissue. Recently, there has been a growing focus on human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs). These are cells that are safely sampled from living humans, for example from the blood or skin, that are then transformed into human heart muscle cells. One active research goal is to use these cells to repair the damaged heart. Another active research goal is to test new drugs on these cells before testing them in animals and humans. However, one major challenge is that hiPSC-CMs often have an irregular internal structure that is difficult to analyze. At present, their behavior is far from fully understood. To address this, we have created software to automatically analyze movies of beating hiPSC-CMs. With our software, it is possible to quantify properties such as the amount and direction of beating cell contraction, and the variation in behavior across different parts of each cell. These tools will enable further quantitative analysis of hiPSC-CMs. With these tools, it will be easier to understand, control, and optimize artificial heart tissue created with hiPSC-CMs, and quantify the effects of drugs on hiPSC-CM behavior.

The ubiquitin ligase Ozz decreases the replacement rate of embryonic myosin in myofibrils

Myosin, the most abundant myofibrillar protein in skeletal muscle, functions as a motor protein in muscle contraction. Myosin polymerizes into the thick filaments in the sarcomere where approximately 50% of embryonic myosin (Myh3) are replaced within 3 h (Ojima K, Ichimura E, Yasukawa Y, Wakamatsu J, Nishimura T, Am J Physiol Cell Physiol 309: C669-C679, 2015). The sarcomere structure including the thick filament is maintained by a balance between protein biosynthesis and degradation. However, the involvement of a protein degradation system in the myosin replacement process remains unclear. Here, we show that the muscle-specific ubiquitin ligase Ozz regulates replacement rate of Myh3. To examine the direct effect of Ozz on myosin replacement, eGFP-Myh3 replacement rate was measured in myotubes overexpressing Ozz by fluorescence recovery after photobleaching. Ozz overexpression significantly decreased the replacement rate of eGFP-Myh3 in the myofibrils, whereas it had no effect on other myosin isoforms. It is likely that ectopic Ozz promoted myosin degradation through increment of ubiquitinated myosin, and decreased myosin supply for replacement, thereby reducing myosin replacement rate. Intriguingly, treatment with a proteasome inhibitor MG132 also decreased myosin replacement rate, although MG132 enhanced the accumulation of ubiquitinated myosin in the cytosol where replaceable myosin is pooled, suggesting that ubiquitinated myosin is not replaced by myosin in the myofibril. Collectively, our findings showed that Myh3 replacement rate was reduced in the presence of overexpressed Ozz probably through enhanced ubiquitination and degradation of Myh3 by Ozz.

Protective Effects of Sal B on Oxidative Stress-Induced Aging by Regulating the Keap1/Nrf2 Signaling Pathway in Zebrafish

The models of oxidative damage-induced aging were established by adding ethanol (C2H5OH), hydrogen peroxide (H2O2) and 6-hydroxydopamine (6-OHDA) to zebrafish embryos in this research. To find effective protective drugs/foods, Salvianolic acid B (Sal B) was added after the embryos were treated by these oxidative reagents. After being treated with ethanol, H2O2 and 6-OHDA, the morphological changes were obvious and the deformities included spinal curvature, heart bleeding, liver bleeding, yolk sac deformity and pericardial edema, and the expression of oxidative stress-related genes Nrf2b, sod1 and sod2 and aging-related genes myl2a and selenbp1 were significantly up-regulated compared to the control group. While after adding 0.05 μg/mL and 0.5 μg/mL Sal B to the ethanol-treated group, death rates and MDA levels decreased, the activity of antioxidant enzyme (SOD, CAT and GSH-Px) changed and Nrf2b, sod1, sod2, myl2a, selenbp1, p53 and p21 were down-regulated compared to the ethanol-treated group. The bioinformatics analysis also showed that oxidative stress-related factors were associated with a variety of cellular functions and physiological pathways. In conclusion, Sal B can protect against aging through regulating the Keap1/Nrf2 pathway as well as antioxidative genes and enzyme activity.

Investigating the correlation of muscle function tests and sarcomere organization in C. elegans

Background

Caenorhabditis elegans has been widely used as a model to study muscle structure and function. Its body wall muscle is functionally and structurally similar to vertebrate skeletal muscle with conserved molecular pathways contributing to sarcomere structure, and muscle function. However, a systematic investigation of the relationship between muscle force and sarcomere organization is lacking. Here, we investigate the contribution of various sarcomere proteins and membrane attachment components to muscle structure and function to introduce C. elegans as a model organism to study the genetic basis of muscle strength.

Methods

We employ two recently developed assays that involve exertion of muscle forces to investigate the correlation of muscle function to sarcomere organization. We utilized a microfluidic pillar-based platform called NemaFlex that quantifies the maximum exertable force and a burrowing assay that challenges the animals to move in three dimensions under a chemical stimulus. We selected 20 mutants with known defects in various substructures of sarcomeres and compared the physiological function of muscle proteins required for force generation and transmission. We also characterized the degree of sarcomere disorganization using immunostaining approaches.

Results

We find that mutants with genetic defects in thin filaments, thick filaments, and M-lines are generally weaker, and our assays are successful in detecting the functional changes in response to each sarcomere location tested. We find that the NemaFlex and burrowing assays are functionally distinct informing on different aspects of muscle physiology. Specifically, the burrowing assay has a larger bandwidth in phenotyping muscle mutants, because it could pick ten additional mutants impaired while exerting normal muscle force in NemaFlex. This enabled us to combine their readouts to develop an integrated muscle function score that was found to correlate with the score for muscle structure disorganization.

Conclusions

Our results highlight the suitability of NemaFlex and burrowing assays for evaluating muscle physiology of C. elegans. Using these approaches, we discuss the importance of the studied sarcomere proteins for muscle function and structure. The scoring methodology we have developed enhances the utility of  C. elegans as a genetic model to study muscle function.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13395-021-00275-4.

Integrated proteomic and transcriptomic profiling identifies aberrant gene and protein expression in the sarcomere, mitochondrial complex I, and the extracellular matrix in Warmblood horses with myofibrillar myopathy

Background

Myofibrillar myopathy in humans causes protein aggregation, degeneration, and weakness of skeletal muscle. In horses, myofibrillar myopathy is a late-onset disease of unknown origin characterized by poor performance, atrophy, myofibrillar disarray, and desmin aggregation in skeletal muscle. This study evaluated molecular and ultrastructural signatures of myofibrillar myopathy in Warmblood horses through gluteal muscle tandem-mass-tag quantitative proteomics (5 affected, 4 control), mRNA-sequencing (8 affected, 8 control), amalgamated gene ontology analyses, and immunofluorescent and electron microscopy.

Results

We identified 93/1533 proteins and 47/27,690 genes that were significantly differentially expressed. The top significantly differentially expressed protein CSRP3 and three other differentially expressed proteins, including, PDLIM3, SYNPO2, and SYNPOL2, are integrally involved in Z-disc signaling, gene transcription and subsequently sarcomere integrity. Through immunofluorescent staining, both desmin aggregates and CSRP3 were localized to type 2A fibers. The highest differentially expressed gene CHAC1, whose protein product degrades glutathione, is associated with oxidative stress and apoptosis. Amalgamated transcriptomic and proteomic gene ontology analyses identified 3 enriched cellular locations; the sarcomere (Z-disc & I-band), mitochondrial complex I and the extracellular matrix which corresponded to ultrastructural Z-disc disruption and mitochondrial cristae alterations found with electron microscopy.

Conclusions

A combined proteomic and transcriptomic analysis highlighted three enriched cellular locations that correspond with MFM ultrastructural pathology in Warmblood horses. Aberrant Z-disc mechano-signaling, impaired Z-disc stability, decreased mitochondrial complex I expression, and a pro-oxidative cellular environment are hypothesized to contribute to the development of myofibrillar myopathy in Warmblood horses. These molecular signatures may provide further insight into diagnostic biomarkers, treatments, and the underlying pathophysiology of MFM.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12864-021-07758-0.

Contribution of Early-Postmortem Proteome and Metabolome to Ultimate pH and Pork Quality

This study's objectives were to identify how subtle differences in ultimate pH relate to differences in pork quality and to understand how early-postmortem glycolysis contributes to variation in ultimate pH. The hypothesis was that elements in early-postmortem longissimus thoracis et lumborum proteome and metabolome could be used to predict quality defects associated with pH decline. Temperature and pH of the longissimus thoracis et lumborum were measured at 45 min, 24 h, and 14 d postmortem. Quality measurements were made after 14 d of aging. Groups were classified as normal pH (NpH; x̄ = 5 . 59 [5.53–5.67]; NpH, n = 10) and low pH (LpH; x̄ = 5 . 42 [5.38–5.45]; LpH, n = 10) at 14 d postmortem. Metabolites from 45 min postmortem were identified using GC-MS. Relative differences between proteins were quantified with two-dimensional difference in gel electrophoreses, and spots were identified with MALDI-MS. Western blot analyses were used to measure phosphofructokinase, peroxiredoxin-2, and reduced and non-reduced adenosine monophosphate deaminase-2 at 45 min and 14 d postmortem. Ultimate pH classification did not affect 45-min-postmortem pH (P = 0.64); 14-d pH was different between groups (P < 0.01). NpH had less purge loss (P < 0.01), was darker (P < 0.01), had lower star probe (P < 0.01), and had less intact day-7 desmin (P = 0.02). More pyruvate (P = 0.01) and less lactate (P = 0.09) was observed in NpH, along with more soluble lactate dehydrogenase (P = 0.03) and pyruvate kinase (P < 0.10). These observations indicate that differences in enzyme abundance or solubility may produce more pyruvate and less lactate. Fructose 6-phosphate was more abundant (P = 0.08) in the LpH group, indicating that phosphofructokinase may be involved in glycolytic differences. Furthermore, greater abundance of heat shock proteins, peroxiredoxin-2 (P = 0.02), and malate (P = 0.01) early postmortem all suggest differences in mitochondrial function and oxidative stability that contribute to quality differences. These results show that even subtle changes in ultimate pH can influence pork quality. The proteome and metabolome at 45 min postmortem are associated with variation in the extent of pH decline.

Proteomics analysis as an approach to understand the formation of pale, soft, and exudative (PSE) pork

We investigated ten pale, soft, and exudative (PSE), and ten normal meat samples from pig carcasses. The meat quality at 0, 5, 12, and 24 h post-mortem and the key enzyme activities at 0 and 24 h post-mortem were determined. We selected three PSE and three normal samples for proteomics analysis at 0 h and 24 h post-mortem. No remarkable differences in pyruvate kinase (PK) and lactate dehydrogenase (LDH) activity were observed between samples at 0 h post-mortem; however, creatine kinase (CK) activity was significantly higher in PSE meat. Hexokinase (HK) activity in PSE samples was higher than that in normal samples at 24 h post-mortem. Bioinformatics analysis of the proteome showed that PSE was related to glycolysis, TCA cycle, oxidative phosphorylation, muscle tissue structure, signal transduction, and molecular chaperones. This research found that proteins such as troponin T slow skeletal muscle isoform X, GADPH, L-lactate dehydrogenase A chain, and gamma-enolase isoform X1 might be responsible for PSE.

Breakdown of Filamentous Myofibrils by the UPS-Step by Step

Protein degradation maintains cellular integrity by regulating virtually all biological processes, whereas impaired proteolysis perturbs protein quality control, and often leads to human disease. Two major proteolytic systems are responsible for protein breakdown in all cells: autophagy, which facilitates the loss of organelles, protein aggregates, and cell surface proteins; and the ubiquitin-proteasome system (UPS), which promotes degradation of mainly soluble proteins. Recent findings indicate that more complex protein structures, such as filamentous assemblies, which are not accessible to the catalytic core of the proteasome in vitro, can be efficiently degraded by this proteolytic machinery in systemic catabolic states in vivo. Mechanisms that loosen the filamentous structure seem to be activated first, hence increasing the accessibility of protein constituents to the UPS. In this review, we will discuss the mechanisms underlying the disassembly and loss of the intricate insoluble filamentous myofibrils, which are responsible for muscle contraction, and whose degradation by the UPS causes weakness and disability in aging and disease. Several lines of evidence indicate that myofibril breakdown occurs in a strictly ordered and controlled manner, and the function of AAA-ATPases is crucial for their disassembly and loss.

Life Cycle of the Cardiac Voltage-Gated Sodium Channel NaV1.5

Cardiac voltage-gated sodium channel Na_V1.5, encoded by SCN5A, is crucial for the upstroke of action potential and excitation of cardiomyocytes. Na_V1.5 undergoes complex processes before it reaches the target membrane microdomains and performs normal functions. A variety of protein partners are needed to achieve the balance between SCN5A transcription and mRNA decay, endoplasmic reticulum retention and export, Golgi apparatus retention and export, selective anchoring and degradation, activation, and inactivation of sodium currents. Subtle alterations can impair Na_V1.5 in terms of expression or function, eventually leading to Na_V1.5-associated diseases such as lethal arrhythmias and cardiomyopathy.

FHOD-1 is the only formin in Caenorhabditis elegans that promotes striated muscle growth and Z-line organization in a cell autonomous manner

The striated body wall muscles of Caenorhabditis elegans are a simple model for sarcomere assembly. Previously, we observed deletion mutants for two formin genes, fhod-1 and cyk-1, develop thin muscles with abnormal dense bodies (the sarcomere Z-line analogs). However, this work left in question whether these formins work in a muscle cell autonomous manner, particularly since cyk-1(∆) deletion has pleiotropic effects on development. Using a fast acting temperature-sensitive cyk-1(ts) mutant, we show here that neither postembryonic loss nor acute loss of CYK-1 during embryonic sarcomerogenesis cause lasting muscle defects. Furthermore, mosaic expression of CYK-1 in cyk-1(∆) mutants is unable to rescue muscle defects in a cell autonomous manner, suggesting muscle phenotypes caused by cyk-1(∆) are likely indirect. Conversely, mosaic expression of FHOD-1 in fhod-1(Δ) mutants promotes muscle cell growth and proper dense body organization in a muscle cell autonomous manner. As we observe no effect of loss of any other formin on muscle development, we conclude FHOD-1 is the only worm formin that directly promotes striated muscle development, and the effects on formin loss in C. elegans are surprisingly modest compared to other systems.

Cryo-electron tomography of cardiac myofibrils reveals a 3D lattice spring within the Z-discs

The Z-disc forms a boundary between sarcomeres, which constitute structural and functional units of striated muscle tissue. Actin filaments from adjacent sarcomeres are cross-bridged by α-actinin in the Z-disc, allowing transmission of tension across the myofibril. Despite decades of studies, the 3D structure of Z-disc has remained elusive due to the limited resolution of conventional electron microscopy. Here, we observed porcine cardiac myofibrils using cryo-electron tomography and reconstructed the 3D structures of the actin-actinin cross-bridging complexes within the Z-discs in relaxed and activated states. We found that the α-actinin dimers showed contraction-dependent swinging and sliding motions in response to a global twist in the F-actin lattice. Our observation suggests that the actin-actinin complex constitutes a molecular lattice spring, which maintains the integrity of the Z-disc during the muscle contraction cycle.

Deficiency and overexpression of Rtl1 in the mouse cause distinct muscle abnormalities related to Temple and Kagami-Ogata syndromes

Temple and Kagami-Ogata syndromes are genomic imprinting diseases caused by maternal and paternal duplication of human chromosome 14, respectively. They exhibit different postnatal muscle-related symptoms as well as prenatal placental problems. Using the mouse models for these syndromes, it has been demonstrated that retrotransposon gag like 1 [Rtl1, also known as paternally expressed 11 (Peg11)] located in the mouse orthologous imprinted region is responsible for the prenatal placental problems because it is an essential placental gene for maintenance of fetal capillary network during gestation. However, the causative imprinted gene for the postnatal muscle-related symptoms remains unknown. Here, we demonstrate that Rtl1 also plays an important role in fetal/neonatal skeletal muscle development: its deletion and overproduction in mice lead to neonatal lethality associated with severe but distinct skeletal muscle defects, similar to those of Temple and Kagami-Ogata syndromes, respectively. Thus, it is strongly suggested that RTL1 is the major gene responsible for the muscle defects in addition to the placental defects in these two genomic imprinting diseases. This is the first example of an LTR retrotransposon-derived gene specific to eutherians contributing to eutherian skeletal muscle development.

Summary: Rtl1 plays an important role in fetal and neonatal skeletal muscle development. Abnormal expression of human RTL1 is the major cause of the muscle symptoms observed in Temple and Kagami-Ogata syndromes.

Characterizing the actin-binding ability of Zasp52 and its contribution to myofibril assembly

In sarcomeres, α-actinin crosslinks thin filaments and anchors them at the Z-disc. Drosophila melanogaster Zasp52 also localizes at Z-discs and interacts with α-actinin via its extended PDZ domain, thereby contributing to myofibril assembly and maintenance, yet the detailed mechanism of Zasp52 function is unknown. Here we show a strong genetic interaction between actin and Zasp52 during indirect flight muscle assembly, indicating that this interaction plays a critical role during myofibril assembly. Our results suggest that Zasp52 contains an actin-binding site, which includes the extended PDZ domain and the ZM region. Zasp52 binds with micromolar affinity to monomeric actin. A co-sedimentation assay indicates that Zasp52 can also bind to F-actin. Finally, we use in vivo rescue assays of myofibril assembly to show that the α-actinin-binding domain of Zasp52 is not sufficient for a full rescue of Zasp52 mutants suggesting additional contributions of Zasp52 actin-binding to myofibril assembly.

The First Heartbeat-Origin of Cardiac Contractile Activity

The amniote embryonic heart starts as a crescent of mesoderm that transitions through a midline linear heart tube in the course of developing into the four chambered heart. It is unusual in having to contract rhythmically while still undergoing extensive morphogenetic remodeling. Advances in imaging have allowed us to determine when during development this contractile activity starts. In the mouse, focal regions of contractions can be detected as early as the cardiac crescent stage. Calcium transients, required to trigger contraction, can be detected even earlier, prior to contraction. In this review, we outline what is currently known about how this early contractile function is initiated and the impact early contractile function has on cardiac development.

Come Together: Protein Assemblies, Aggregates and the Sarcostat at the Heart of Cardiac Myocyte Homeostasis

Homeostasis in vertebrate systems is contingent on normal cardiac function. This, in turn, depends on intricate protein-based cellular machinery, both for contractile function, as well as, durability of cardiac myocytes. The cardiac small heat shock protein (csHsp) chaperone system, highlighted by αB-crystallin (CRYAB), a small heat shock protein (sHsp) that forms ∼3–5% of total cardiac mass, plays critical roles in maintaining proteostatic function via formation of self-assembled multimeric chaperones. In this work, we review these ancient proteins, from the evolutionarily preserved role of homologs in protists, fungi and invertebrate systems, as well as, the role of sHsps and chaperones in maintaining cardiac myocyte structure and function. We propose the concept of the “sarcostat” as a protein quality control mechanism in the sarcomere. The roles of the proteasomal and lysosomal proteostatic network, as well as, the roles of the aggresome, self-assembling protein complexes and protein aggregation are discussed in the context of cardiac myocyte homeostasis. Finally, we will review the potential for targeting the csHsp system as a novel therapeutic approach to prevent and treat cardiomyopathy and heart failure.

Investigation of the Sarcoplasmic Proteome Contribution to the Development of Pork Loin Tenderness

The study objectives were to determine the extent to which the sarcoplasmic proteome explains variations in aged pork loin star probe value. Pork loins (n=12) were categorized by differences in star probe at 21 d post mortem from a larger set of loins. Loins were categorized into low star probe (LSP) group (n=6; star probe<5.80 kg) and high star probe (HSP) group (n=6; star probe>7.00 kg) based on 21-d star probe value with inclusion criteria of marbling score (1.0–3.0) and 24-h pH (5.69–5.98). Quality traits were measured at 1-, 8-, 14-, and 21-d aging. Desmin and troponin-T degradation, peroxiredoxin-2 abundance, calpain-1 autolysis, and sarcomere length were determined. Two-dimensional difference gel electrophoresis and mass spectrometry were used to identify proteins that differed in abundance due to category. Star probe values were lower (P<0.01) in LSP at each day of aging compared with HSP. Greater pH values were observed (P<0.05)in LSP compared with HSP at each day of aging. Marbling score was greater (P<0.05) in LSP compared with HSP at each day of aging. Greater (P<0.05) desmin and troponin-T degradation was detected in LSP chops at 14- and 21-d aging and 8-, 14-, and 21-d aging, respectively. Greater (P<0.05) sarcomere length was determined in LSP compared with HSP at 1-,8-, and 21-d aging. Sarcoplasmic proteins from HSP chops had greater abundance (P<0.10) of metabolic and regulatory proteins, whereas the LSP chops had greater abundance (P<0.10) of stress response proteins. Star probe values were affected by pH, marbling score, protein degradation, sarcomere length, and sarcoplasmic proteome.

Striated myocyte structural integrity: Automated analysis of sarcomeric z-discs

As sarcomeres produce the force necessary for contraction, assessment of sarcomere order is paramount in evaluation of cardiac and skeletal myocytes. The uniaxial force produced by sarcomeres is ideally perpendicular to their z-lines, which couple parallel myofibrils and give cardiac and skeletal myocytes their distinct striated appearance. Accordingly, sarcomere structure is often evaluated by staining for z-line proteins such as α-actinin. However, due to limitations of current analysis methods, which require manual or semi-manual handling of images, the mechanism by which sarcomere and by extension z-line architecture can impact contraction and which characteristics of z-line architecture should be used to assess striated myocytes has not been fully explored. Challenges such as isolating z-lines from regions of off-target staining that occur along immature stress fibers and cell boundaries and choosing metrics to summarize overall z-line architecture have gone largely unaddressed in previous work. While an expert can qualitatively appraise tissues, these challenges leave researchers without robust, repeatable tools to assess z-line architecture across different labs and experiments. Additionally, the criteria used by experts to evaluate sarcomeric architecture have not been well-defined. We address these challenges by providing metrics that summarize different aspects of z-line architecture that correspond to expert tissue quality assessment and demonstrate their efficacy through an examination of engineered tissues and single cells. In doing so, we have elucidated a mechanism by which highly elongated cardiomyocytes become inefficient at producing force. Unlike previous manual or semi-manual methods, characterization of z-line architecture using the metrics discussed and implemented in this work can quantitatively evaluate engineered tissues and contribute to a robust understanding of the development and mechanics of striated muscles.

Type I collagen promotes the migration and myogenic differentiation of C2C12 myoblastsviathe release of interleukin-6 mediated by FAK/NF-κB p65 activation

Type I collagen has the potential to promote the migration and differentiation of C₂C₁₂myoblastviaIL-6 release that was mediated by FAK/NF-κB pathway.

Skeletal muscle: A review of molecular structure and function, in health and disease

Decades of research in skeletal muscle physiology have provided multiscale insights into the structural and functional complexity of this important anatomical tissue, designed to accomplish the task of generating contraction, force and movement. Skeletal muscle can be viewed as a biomechanical device with various interacting components including the autonomic nerves for impulse transmission, vasculature for efficient oxygenation, and embedded regulatory and metabolic machinery for maintaining cellular homeostasis. The "omics" revolution has propelled a new era in muscle research, allowing us to discern minute details of molecular cross-talk required for effective coordination between the myriad interacting components for efficient muscle function. The objective of this review is to provide a systems-level, comprehensive mapping the molecular mechanisms underlying skeletal muscle structure and function, in health and disease. We begin this review with a focus on molecular mechanisms underlying muscle tissue development (myogenesis), with an emphasis on satellite cells and muscle regeneration. We next review the molecular structure and mechanisms underlying the many structural components of the muscle: neuromuscular junction, sarcomere, cytoskeleton, extracellular matrix, and vasculature surrounding muscle. We highlight aberrant molecular mechanisms and their possible clinical or pathophysiological relevance. We particularly emphasize the impact of environmental stressors (inflammation and oxidative stress) in contributing to muscle pathophysiology including atrophy, hypertrophy, and fibrosis. This article is categorized under: Physiology > Mammalian Physiology in Health and Disease Developmental Biology > Developmental Processes in Health and Disease Models of Systems Properties and Processes > Cellular Models.

O-GlcNAcylation as a regulator of the functional and structural properties of the sarcomere in skeletal muscle: An update review

Although the O-GlcNAcylation process was discovered in 1984, its potential role in the physiology and physiopathology of skeletal muscle only emerged 20 years later. An increasing number of publications strongly support a key role of O-GlcNAcylation in the modulation of important cellular processes which are essential for skeletal muscle functions. Indeed, over a thousand of O-GlcNAcylated proteins have been identified within skeletal muscle since 2004, which belong to various classes of proteins, including sarcomeric proteins. In this review, we focused on these myofibrillar proteins, including contractile and structural proteins. Because of the modification of motor and regulatory proteins, the regulatory myosin light chain (MLC2) is related to several reports that support a key role of O-GlcNAcylation in the fine modulation of calcium activation parameters of skeletal muscle fibres, depending on muscle phenotype and muscle work. In addition, another key function of O-GlcNAcylation has recently emerged in the regulation of organization and reorganization of the sarcomere. Altogether, this data support a key role of O-GlcNAcylation in the homeostasis of sarcomeric cytoskeleton, known to be disturbed in many related muscle disorders.

Deficiency of nuclear receptor interaction protein leads to cardiomyopathy by disrupting sarcomere structure and mitochondrial respiration

BACKGROUND

Cardiomyopathy is a common and lethal complication in patients with limb-girdle muscular dystrophy (LGMD), one of the most prevalent forms of muscular dystrophy. The pathogenesis underlying LGMD-related cardiomyopathy remains unclear. NRIP (gene name DCAF6), a Ca²⁺-dependent calmodulin binding protein, was reduced in dystrophic muscles from LGMD patients. Mice lacking NRIP exhibit a myopathic phenotype resembling that in LGMD patients, making NRIP deficiency a potential culprit leading to cardiomyopathy. This study aimed to determine if NRIP deficiency leads to cardiomyopathy and to explore the underlying molecular mechanisms.

METHODS AND RESULTS

NRIP expression was reduced in both human and mouse failing hearts. Muscle-specific NRIP knockout (MCK-Cre::Dcaf6^flox/flox) mouse heart and isolated cardiomyocytes exhibited markedly reduced contractility. Transmission electron microscopy revealed abnormal sarcomere structures and mitochondrial morphology in MCK-Cre::Dcaf6^flox/flox hearts. Protein co-immunoprecipitation and confocal imaging revealed that NRIP interacts with α-actinin 2 (ACTN2) at the Z-disc. We found that NRIP facilitated ACTN2-mediated F-actin bundling, and that NRIP deficiency resulted in reduced binding between Z-disc proteins ACTN2 and Cap-Z. In addition, NRIP-deficiency led to increased mitochondrial ROS and impaired mitochondrial respiration/ATP production owing to elevated cellular NADH/NAD⁺ ratios. Treatment with mitochondria-directed antioxidant mitoTEMPO or NAD⁺ precursor nicotinic acid restored mitochondrial function and cardiac contractility in MCK-Cre::Dcaf6^flox/flox mice.

CONCLUSIONS

NRIP is essential to maintain sarcomere structure and mitochondrial/contractile function in cardiomyocytes. Our results revealed a novel role for NRIP deficiency in the pathogenesis of LGMD and heart failure. Targeting NRIP, therefore, could be a powerful new approach to treat myocardial dysfunction in LGMD and heart failure patients.

Whole-exome sequencing identifies a donor splice-site variant in SMPX that causes rare X-linked congenital deafness

Background

X‐linked deafness‐4 (DFNX4) caused by functional loss of SMPX is a nonsyndromic form of progressive hearing loss with post‐lingual onset. Herein, we describe a male neonate from an ethnic Han Chinese family who presented with congenital deafness.

Methods

The proband and the family members were subjected to comprehensively hearing screen. Genetic testing was carried out using whole‐exome sequencing (WES). The result was verified by Sanger sequencing. Functional characterization of the identified variant was completed by reverse transcription PCR (RT‐PCR), Sanger sequencing, and fluorogenic quantitative PCR (qPCR).

Results

The proband was diagnosed with progressive sensorineural hearing loss. The proband's mother showed normal hearing at present. The proband's maternal grandmother exhibited mild HL since the age of 50. Using whole‐exome sequencing (WES), we identified a donor splice‐site variant (NM_014332.2: c.132 + 1G>A) in the SMPX gene in the proband. The mother and maternal grandmother were both carriers, which suggested a X‐linked inheritance of the condition in the family. RT‐PCR and Sanger sequencing revealed that four alternative splice pairs within intron 3 have led to four aberrant RNAs transcripts, including two non‐canonical splice‐pairs (GC‐AG and CT‐AG). The variant generated a novel frameshift variant, creating a premature termination codon (PTC) upstream of a newly formed splice site (p.Met45Glyfs*16). SMPX mRNA expression assay showed that the PTC has caused degradation of mRNA via nonsense‐mediated mRNA decay (NMD).

Conclusion

This is the first study to report a SMPX (DFNX4) splicing variant in a Chinese family. These findings, especially congenital deafness, contributed to existing knowledge regarding the genotypic and phenotypic spectrum of SMPX‐associated hearing loss.

Influence of postmortem aging and post-aging freezing on pork loin quality attributes

The objectives were to determine 1) the interaction between aging duration and post-aging freezing on pork loin quality attributes and 2) the relationship between pork loin star probe (SP) and Warner-Bratzler shear force (WBS). Loins from 20 carcasses were collected 1 d postmortem. Chops (n = 9; 2.54-cm thick) were fabricated from each loin and vacuum packaged. Four chops from each loin were aged at 4°C for 1, 8, 14, and 21 d and immediately evaluated (Fresh). Four adjacent chops were aged (1, 8, 14, and 21 d), frozen for 14 d, and thawed for evaluation (Frozen). An additional chop was used for evaluation of sarcomere length, intact desmin, and troponin-T degradation. Purge, objective color, pH, subjective color and marbling score, cook loss, SP, and WBS were evaluated at each aging period. Desmin and troponin-T degradation, and sarcomere length were measured on fresh samples at each day of aging. Post-aging freezing had no significant impact on SP, WBS, pH, and subjective color or marbling score at any aging period. Fresh chop purge increased at each day of aging (P < 0.01). Post-aging freezing resulted in greater purge at 1, 8, and 14 d aging (P < 0.01). Fresh chop cook loss was greater than post-aging freezing chop cook loss at 14 and 21 d aging (P < 0.05). Across all aging periods and treatments, SP was correlated (r = 0.85; P < 0.01) with WBS. Fresh chop SP and WBS decreased from 1 to 8 d aging but was not different after 8 d aging. The abundance of intact desmin decreased (P < 0.01) between 1, 8, and 14 d aging. Troponin-T degradation increased (P < 0.01) with each aging period. Sarcomere length was not different across aging periods (P > 0.05). Aging, without freezing, for 14 or 21 d did not improve SP or WBS observed at 8 d, corresponding with changes in desmin degradation.

Alterations of redox dynamics and desmin post-translational modifications in skeletal muscle models of desminopathies

Desminopathies are a type of myofibrillar myopathy resulting from mutations in DES, encoding the intermediate filament protein desmin. They display heterogeneous phenotypes, suggesting environment influences. Patient muscle proteins show oxidative features linking oxidative stress, protein aggregation, and abnormal protein deposition. To improve understanding of redox balance in desminopathies, we further developed cellular models of four pathological mutants localized in 2B helical domain (the most important region for desmin polymerization) to explore desmin behavior upon oxidative stress. We show that the mutations desQ389P and desD399Y share common stress-induced aggregates, desR406W presents more scattered cytoplasmic aggregative pattern, and pretreatment with N-acetyl-l-cysteine (NAC), an antioxidant molecule, prevents all type of aggregation. Mutants desD399Y and desR406W had delayed oxidation kinetics following H₂O₂ stress prevented by NAC pretreatment. Further, we used AAV-injected mouse models to confirm in vivo effects of N-acetyl-l-cysteine. AAV-desD399Y-injected muscles displayed similar physio-pathological characteristics as observed in patients. However, after 2 months of NAC treatment, they did not have reduced aggregates. Finally, in both models, stress induced some post-translational modifications changing Isoelectric Point, such as potential hyperphosphorylations, and/or molecular weight of human desmin by proteolysis. However, each mutant presented its own pattern that seemed to be post-aggregative. In conclusion, our results indicate that individual desmin mutations have unique pathological molecular mechanisms partly linked to alteration of redox homeostasis. Integrating these mutant-specific behaviors will be important when considering future therapeutics.

Engineering hiPSC cardiomyocyte in vitro model systems for functional and structural assessment

The study of human cardiomyopathies and the development and testing of new therapies has long been limited by the availability of appropriate in vitro model systems. Cardiomyocytes are highly specialized cells whose internal structure and contractile function are sensitive to the local microenvironment and the combination of mechanical and biochemical cues they receive. The complementary technologies of human induced pluripotent stem cell (hiPSC) derived cardiomyocytes (CMs) and microphysiological systems (MPS) allow for precise control of the genetics and microenvironment of human cells in in vitro contexts. These combined systems also enable quantitative measurement of mechanical function and intracellular organization. This review describes relevant factors in the myocardium microenvironment that affect CM structure and mechanical function and demonstrates the application of several engineered microphysiological systems for studying development, disease, and drug discovery.