The Molecular Mechanisms of Mutations in Actin and Myosin that Cause Inherited Myopathy

A Glu209Lys substitution in DRG1/TaACT7, which disturbs F-actin organization, reduces plant height and grain length in bread wheat

Plant height and grain size are two important agronomic traits that are closely related to crop yield. Numerous dwarf and grain-shape mutants have been studied to identify genes that can be used to increase crop yield and improve breeding programs. In this study, we characterized a dominant mutant, dwarf and round grain 1 (drg1-D), in bread wheat (Triticum aestivum L.). drg1-D plants exhibit multiple phenotypic changes, including dwarfism, round grains, and insensitivity to brassinosteroids (BR). Cell structure observation in drg1-D mutant plants showed that the reduced organ size is due to irregular cell shape. Using map-based cloning and verification in transgenic plants, we found that a Glu209Lys substitution in the DRG1 protein is responsible for the irregular cell size and arrangement in the drg1-D mutant. DRG1/TaACT7 encodes an actin family protein that is essential for polymerization stability and microfilament (MF) formation. In addition, the BR response and vesicular transport were altered by the abnormal actin cytoskeleton in drg1-D mutant plants. Our study demonstrates that DRG1/TaACT7 plays an important role in wheat cell shape determination by modulating actin organization and intracellular material transport, which could in the longer term provide tools to better understand the polymerization of actin and its assembly into filaments and arrays.

Conformation of actin subunits at the barbed and pointed ends of F-actin with and without capping proteins

Advances in cryo-electron microscopy have made possible the determination of structures of the barbed and pointed ends of F-actin, both in the absence and the presence of capping proteins that block subunit exchange. The conformation of the two exposed protomers at the barbed end resembles the "flat" conformation of protomers in the middle of F-actin. The barbed end changes little upon binding of CapZ, which in turn undergoes a major conformational change. At the pointed end, however, protomers have the "twisted" conformation characteristic of G-actin, whereas tropomodulin binding forces a flat conformation upon the second subunit. The structures provide a mechanistic understanding for the asymmetric addition/dissociation of actin subunits at the ends of F-actin and open the way to future studies of other regulators of filament end dynamics.

Modeling Duchenne Muscular Dystrophy Cardiomyopathy with Patients' Induced Pluripotent Stem-Cell-Derived Cardiomyocytes

Duchenne muscular dystrophy (DMD) is an X-linked progressive muscle degenerative disease caused by mutations in the dystrophin gene, resulting in death by the end of the third decade of life at the latest. A key aspect of the DMD clinical phenotype is dilated cardiomyopathy, affecting virtually all patients by the end of the second decade of life. Furthermore, despite respiratory complications still being the leading cause of death, with advancements in medical care in recent years, cardiac involvement has become an increasing cause of mortality. Over the years, extensive research has been conducted using different DMD animal models, including the mdx mouse. While these models present certain important similarities to human DMD patients, they also have some differences which pose a challenge to researchers. The development of somatic cell reprograming technology has enabled generation of human induced pluripotent stem cells (hiPSCs) which can be differentiated into different cell types. This technology provides a potentially endless pool of human cells for research. Furthermore, hiPSCs can be generated from patients, thus providing patient-specific cells and enabling research tailored to different mutations. DMD cardiac involvement has been shown in animal models to include changes in gene expression of different proteins, abnormal cellular Ca²⁺ handling, and other aberrations. To gain a better understanding of the disease mechanisms, it is imperative to validate these findings in human cells. Furthermore, with the recent advancements in gene-editing technology, hiPSCs provide a valuable platform for research and development of new therapies including the possibility of regenerative medicine. In this article, we review the DMD cardiac-related research performed so far using human hiPSCs-derived cardiomyocytes (hiPSC-CMs) carrying DMD mutations.

Actin-Binding Proteins in Cardiac Hypertrophy

The heart reacts to a large number of pathological stimuli through cardiac hypertrophy, which finally can lead to heart failure. However, the molecular mechanisms of cardiac hypertrophy remain elusive. Actin participates in the formation of highly differentiated myofibrils under the regulation of actin-binding proteins (ABPs), which provides a structural basis for the contractile function and morphological change in cardiomyocytes. Previous studies have shown that the functional abnormality of ABPs can contribute to cardiac hypertrophy. Here, we review the function of various actin-binding proteins associated with the development of cardiac hypertrophy, which provides more references for the prevention and treatment of cardiomyopathy.

Taking flight: an educational primer for use with "A novel mechanism for activation of myosin regulatory light chain by protein kinase C-delta in Drosophila"

Muscles are required for animal movement, feeding, heartbeat, and reproduction. Disruption of muscle function can lead to mobility impairments and diseases like muscular dystrophy and cardiac myopathy; therefore, research in this area has significant implications for public health. Recent work by Vaziri and colleagues has taken genetic, cell biological, and biochemical approaches to identify Protein kinase C-d (Pkcδ) as a novel regulator of the essential myosin light chain 2 (MLC2) by phosphorylation. The authors determine which residues of MLC2 are modified by Pkcδ and show that phosphorylation by Pkcδ is required for proper sarcomere assembly and function. This study underscores the importance of Drosophila melanogaster as a model system for muscle function and highlights how protein phosphorylation is a vital part of post-translational gene regulation.

Critical Evaluation of Current Hypotheses for the Pathogenesis of Hypertrophic Cardiomyopathy

Hereditary hypertrophic cardiomyopathy (HCM), due to mutations in sarcomere proteins, occurs in more than 1/500 individuals and is the leading cause of sudden cardiac death in young people. The clinical course exhibits appreciable variability. However, typically, heart morphology and function are normal at birth, with pathological remodeling developing over years to decades, leading to a phenotype characterized by asymmetric ventricular hypertrophy, scattered fibrosis and myofibrillar/cellular disarray with ultimate mechanical heart failure and/or severe arrhythmias. The identity of the primary mutation-induced changes in sarcomere function and how they trigger debilitating remodeling are poorly understood. Support for the importance of mutation-induced hypercontractility, e.g., increased calcium sensitivity and/or increased power output, has been strengthened in recent years. However, other ideas that mutation-induced hypocontractility or non-uniformities with contractile instabilities, instead, constitute primary triggers cannot yet be discarded. Here, we review evidence for and criticism against the mentioned hypotheses. In this process, we find support for previous ideas that inefficient energy usage and a blunted Frank-Starling mechanism have central roles in pathogenesis, although presumably representing effects secondary to the primary mutation-induced changes. While first trying to reconcile apparently diverging evidence for the different hypotheses in one unified model, we also identify key remaining questions and suggest how experimental systems that are built around isolated primarily expressed proteins could be useful.

Comparison of Gene Expression Between Resistant and Susceptible Families Against VPAHPND and Identification of Biomarkers Used for Resistance Evaluation in Litopenaeus vannamei

Acute hepatopancreatic necrosis disease (AHPND) has caused a heavy loss to shrimp aquaculture since its outbreak. Vibrio parahaemolyticus (VP_AHPND) is regarded as one of the main pathogens that caused AHPND in the Pacific white shrimp Litopenaeus vannamei. In order to learn more about the mechanism of resistance to AHPND, the resistant and susceptible shrimp families were obtained through genetic breeding, and comparative transcriptome approach was used to analyze the gene expression patterns between resistant and susceptible families. A total of 95 families were subjected to VP_AHPND challenge test, and significant variations in the resistance of these families were observed. Three pairs of resistant and susceptible families were selected for transcriptome sequencing. A total of 489 differentially expressed genes (DEGs) that presented in at least two pairwise comparisons were screened, including 196 DEGs highly expressed in the susceptible families and 293 DEGs in the resistant families. Among these DEGs, 16 genes demonstrated significant difference in all three pairwise comparisons. Gene set enrichment analysis (GSEA) of all 27,331 expressed genes indicated that some energy metabolism processes were enriched in the resistant families, while signal transduction and immune system were enriched in the susceptible families. A total of 32 DEGs were further confirmed in the offspring of the detected families, among which 19 genes were successfully verified. The identified genes in this study will be useful for clarifying the genetic mechanism of shrimp resistance against Vibrio and will further provide molecular markers for evaluating the disease resistance of shrimp in the breeding program.

Mechanistic analysis of actin-binding compounds that affect the kinetics of cardiac myosin-actin interaction

Actin-myosin mediated contractile forces are crucial for many cellular functions, including cell motility, cytokinesis, and muscle contraction. We determined the effects of ten actin-binding compounds on the interaction of cardiac myosin subfragment 1 (S1) with pyrene-labeled F-actin (PFA). These compounds, previously identified from a small-molecule high-throughput screen (HTS), perturb the structural dynamics of actin and the steady-state actin-activated myosin ATPase activity. However, the mechanisms underpinning these perturbations remain unclear. Here we further characterize them by measuring their effects on PFA fluorescence, which is decreased specifically by the strong binding of myosin to actin. We measured these effects under equilibrium and steady-state conditions, and under transient conditions, in stopped-flow experiments following addition of ATP to S1-bound PFA. We observed that these compounds affect early steps of the myosin ATPase cycle to different extents. They increased the association equilibrium constant K₁ for the formation of the strongly bound collision complex, indicating increased ATP affinity for actin-bound myosin, and decreased the rate constant k₊₂ for subsequent isomerization to the weakly bound ternary complex, thus slowing the strong-to-weak transition that actin-myosin interaction undergoes early in the ATPase cycle. The compounds' effects on actin structure allosterically inhibit the kinetics of the actin-myosin interaction in ways that may be desirable for treatment of hypercontractile forms of cardiomyopathy. This work helps to elucidate the mechanisms of action for these compounds, several of which are currently used therapeutically, and sets the stage for future HTS campaigns that aim to discover new drugs for treatment of heart failure.

Prolonged Exposure to Microgravity Reduces Cardiac Contractility and Initiates Remodeling in Drosophila

Understanding the effects of microgravity on human organs is crucial to exploration of low-earth orbit, the moon, and beyond. Drosophila can be sent to space in large numbers to examine the effects of microgravity on heart structure and function, which is fundamentally conserved from flies to humans. Flies reared in microgravity exhibit cardiac constriction with myofibrillar remodeling and diminished output. RNA sequencing (RNA-seq) in isolated hearts revealed reduced expression of sarcomeric/extracellular matrix (ECM) genes and dramatically increased proteasomal gene expression, consistent with the observed compromised, smaller hearts and suggesting abnormal proteostasis. This was examined further on a second flight in which we found dramatically elevated proteasome aggregates co-localizing with increased amyloid and polyQ deposits. Remarkably, in long-QT causing sei/hERG mutants, proteasomal gene expression at 1g, although less than the wild-type expression, was nevertheless increased in microgravity. Therefore, cardiac remodeling and proteostatic stress may be a fundamental response of heart muscle to microgravity.

Laing Myopathy: Report of 4 New Families With Novel MYH7 Mutations, Double Mutations, and Severe Phenotype

Laing distal myopathy (LDM) is an autosomal dominant disorder caused by mutations in the slow skeletal muscle fiber myosin heavy chain (MYH7) gene on chromosome 14q11.2. The classic LDM phenotype-including early-onset, initial involvement of foot dorsiflexors and great toe extensors, followed by weakness of neck flexors and finger extensors-is well documented. Since the original report by Laing et al in 1995, the spectrum of MYH7-related myopathies has expanded to include congenital myopathies, late-onset myopathies, myosin storage myopathy, and scapuloperoneal myopathies. Most patients with LDM harbor mutations in the midrod domain of the MYH7 gene, but rare cases document disease-associated mutations in the globular head region. In this report, we add to the medical literature by describing the clinicopathological findings in 8 affected family members from 4 new LDM families-including 2 with novel MYH7 mutations (Y162D and A1438P), one with dual mutations (V39M and K1617del), and one family (E1508del) with severe early-onset weakness associated with contractures, respiratory insufficiency, and dilated cardiomyopathy. Our families highlight the ever-expanding clinical spectrum and genetic variation of the skeletal myopathies related to MYH7 gene mutations.

Myosins and Disease

Myosins constitute a superfamily of actin-based molecular motor proteins that mediates a variety of cellular activities including muscle contraction, cell migration, intracellular transport, the formation of membrane projections, cell adhesion, and cell signaling. The 12 myosin classes that are expressed in humans share sequence similarities especially in the N-terminal motor domain; however, their enzymatic activities, regulation, ability to dimerize, binding partners, and cellular functions differ. It is becoming increasingly apparent that defects in myosins are associated with diseases including cardiomyopathies, colitis, glomerulosclerosis, neurological defects, cancer, blindness, and deafness. Here, we review the current state of knowledge regarding myosins and disease.

Actin-binding compounds, previously discovered by FRET-based high-throughput screening, differentially affect skeletal and cardiac muscle

Actin's interactions with myosin and other actin-binding proteins are essential for cellular viability in numerous cell types, including muscle. In a previous high-throughput time-resolved FRET (TR-FRET) screen, we identified a class of compounds that bind to actin and affect actomyosin structure and function. For clinical utility, it is highly desirable to identify compounds that affect skeletal and cardiac muscle differently. Because actin is more highly conserved than myosin and most other muscle proteins, most such efforts have not targeted actin. Nevertheless, in the current study, we tested the specificity of the previously discovered actin-binding compounds for effects on skeletal and cardiac α-actins as well as on skeletal and cardiac myofibrils. We found that a majority of these compounds affected the transition of monomeric G-actin to filamentous F-actin, and that several of these effects were different for skeletal and cardiac actin isoforms. We also found that several of these compounds affected ATPase activity differently in skeletal and cardiac myofibrils. We conclude that these structural and biochemical assays can be used to identify actin-binding compounds that differentially affect skeletal and cardiac muscles. The results of this study set the stage for screening of large chemical libraries for discovery of novel compounds that act therapeutically and specifically on cardiac or skeletal muscle.

Actin Mutations and Their Role in Disease

Actin is a widely expressed protein found in almost all eukaryotic cells. In humans, there are six different genes, which encode specific actin isoforms. Disease-causing mutations have been described for each of these, most of which are missense. Analysis of the position of the resulting mutated residues in the protein reveals mutational hotspots. Many of these occur in regions important for actin polymerization. We briefly discuss the challenges in characterizing the effects of these actin mutations, with a focus on cardiac actin mutations.

G65V Substitution in Actin Disturbs Polymerization Leading to Inhibited Cell Elongation in Cotton

The importance of the actin cytoskeleton for proper cell development has been well established in a variety of organisms. Actin protein sequences are highly conserved, and each amino acid residue may be essential for its function. In this study, we report the isolation and characterization of GhLi ₁ from an upland cotton mutant Ligon lintless-1 (Li₁), which harbors the G65V substitution in its encoded actin protein. Li₁ mutants exhibit pleiotropic malformed phenotypes, including dwarf plants, distorted organs, and extremely shortened fibers. Cytological analysis showed that the actin cytoskeleton was disorganized and the abundance of F-actin was decreased in the Li₁ cells. Vesicles were aggregated into patches, and excessive cellulose synthase complexes were inserted into the plasma membrane during the secondary cell wall biosynthesis stage, which dramatically affected the morphology of the Li₁ cells. Molecular model prediction suggested that the G65V substitution may affect the three-bodied G-actin interaction during F-actin assembly. Biochemical assays demonstrated that the recombinant GhLi₁ protein disturbs actin dynamics by inhibiting the nucleation and elongation processes. Therefore, our findings demonstrate that the G65V substitution in actin had dominant-negative effects on cell elongation, by disturbing actin polymerization and actin cytoskeleton-based biological processes such as intracellular transportation.

Special Issue: The Actin-Myosin Interaction in Muscle: Background and Overview

Muscular contraction is a fundamental phenomenon in all animals; without it life as we know it would be impossible. The basic mechanism in muscle, including heart muscle, involves the interaction of the protein filaments myosin and actin. Motility in all cells is also partly based on similar interactions of actin filaments with non-muscle myosins. Early studies of muscle contraction have informed later studies of these cellular actin-myosin systems. In muscles, projections on the myosin filaments, the so-called myosin heads or cross-bridges, interact with the nearby actin filaments and, in a mechanism powered by ATP-hydrolysis, they move the actin filaments past them in a kind of cyclic rowing action to produce the macroscopic muscular movements of which we are all aware. In this special issue the papers and reviews address different aspects of the actin-myosin interaction in muscle as studied by a plethora of complementary techniques. The present overview provides a brief and elementary introduction to muscle structure and function and the techniques used to study it. It goes on to give more detailed descriptions of what is known about muscle components and the cross-bridge cycle using structural biology techniques, particularly protein crystallography, electron microscopy and X-ray diffraction. It then has a quick look at muscle mechanics and it summarises what can be learnt about how muscle works based on the other studies covered in the different papers in the special issue. A picture emerges of the main molecular steps involved in the force-producing process; steps that are also likely to be seen in non-muscle myosin interactions with cellular actin filaments. Finally, the remarkable advances made in studying the effects of mutations in the contractile assembly in causing specific muscle diseases, particularly those in heart muscle, are outlined and discussed.

Intrinsic Disorder of the BAF Complex: Roles in Chromatin Remodeling and Disease Development

The two-meter-long DNA is compressed into chromatin in the nucleus of every cell, which serves as a significant barrier to transcription. Therefore, for processes such as replication and transcription to occur, the highly compacted chromatin must be relaxed, and the processes required for chromatin reorganization for the aim of replication or transcription are controlled by ATP-dependent nucleosome remodelers. One of the most highly studied remodelers of this kind is the BRG1- or BRM-associated factor complex (BAF complex, also known as SWItch/sucrose non-fermentable (SWI/SNF) complex), which is crucial for the regulation of gene expression and differentiation in eukaryotes. Chromatin remodeling complex BAF is characterized by a highly polymorphic structure, containing from four to 17 subunits encoded by 29 genes. The aim of this paper is to provide an overview of the role of BAF complex in chromatin remodeling and also to use literature mining and a set of computational and bioinformatics tools to analyze structural properties, intrinsic disorder predisposition, and functionalities of its subunits, along with the description of the relations of different BAF complex subunits to the pathogenesis of various human diseases.

Myosin motor domains carrying mutations implicated in early or late onset hypertrophic cardiomyopathy have similar properties

Hypertrophic cardiomyopathy (HCM) is a common genetic disorder characterized by left ventricular hypertrophy and cardiac hyper-contractility. Mutations in the β-cardiac myosin heavy chain gene (β-MyHC) are a major cause of HCM, but the specific mechanistic changes to myosin function that lead to this disease remain incompletely understood. Predicting the severity of any β-MyHC mutation is hindered by a lack of detailed examinations at the molecular level. Moreover, because HCM can take ≥20 years to develop, the severity of the mutations must be somewhat subtle. We hypothesized that mutations that result in early onset disease would have more severe changes in function than do later onset mutations. Here, we performed steady-state and transient kinetic analyses of myosins carrying one of seven missense mutations in the motor domain. Of these seven, four were previously identified in early onset cardiomyopathy screens. We used the parameters derived from these analyses to model the ATP-driven cross-bridge cycle. Contrary to our hypothesis, the results indicated no clear differences between early and late onset HCM mutations. Despite the lack of distinction between early and late onset HCM, the predicted occupancy of the force-holding actin·myosin·ADP complex at [Actin] = 3 K _app along with the closely related duty ratio (the fraction of myosin in strongly attached force-holding states), and the measured ATPases all changed in parallel (in both sign and degree of change) compared with wildtype (WT) values. Six of the seven HCM mutations were clearly distinct from a set of previously characterized DCM mutations.

Nemaline myopathies: a current view

Nemaline myopathies are a heterogenous group of congenital myopathies caused by de novo, dominantly or recessively inherited mutations in at least twelve genes. The genes encoding skeletal α-actin (ACTA1) and nebulin (NEB) are the commonest genetic cause. Most patients have congenital onset characterized by muscle weakness and hypotonia, but the spectrum of clinical phenotypes is broad, ranging from severe neonatal presentations to onset of a milder disorder in childhood. Most patients with adult onset have an autoimmune-related myopathy with a progressive course. The wide application of massively parallel sequencing methods is increasing the number of known causative genes and broadening the range of clinical phenotypes. Nemaline myopathies are identified by the presence of structures that are rod-like or ovoid in shape with electron microscopy, and with light microscopy stain red with the modified Gömöri trichrome technique. These rods or nemaline bodies are derived from Z lines (also known as Z discs or Z disks) and have a similar lattice structure and protein content. Their shape in patients with mutations in KLHL40 and LMOD3 is distinctive and can be useful for diagnosis. The number and distribution of nemaline bodies varies between fibres and different muscles but does not correlate with severity or prognosis. Additional pathological features such as caps, cores and fibre type disproportion are associated with the same genes as those known to cause the presence of rods. Animal models are advancing the understanding of the effects of various mutations in different genes and paving the way for the development of therapies, which at present only manage symptoms and are aimed at maintaining muscle strength, joint mobility, ambulation, respiration and independence in the activities of daily living.

Small molecule studies: the fourth wave of muscle research

The study of muscle and contractility is an unusual scientific endeavour since it has from the start been focussed on one problem-What makes muscle work?-and yet has needed a vast range of different approaches and techniques to study it. Its uniqueness lies in the fundamental fascination of a large scale molecular machine that converts chemical energy into mechanical energy at ambient temperature and with high efficiency that is also controlled by an exquisitely intricate yet utterly reliable regulatory system and is an essential component of animal life. The investigation of muscle is as innovative as any other field of research. As soon as one approach appears to be played out another comes along. It is instructive to consider this as a series of waves of novel and heightened activity starting in the 1950s. The thesis of this article is that we are approaching the fourth wave with the recent rise of interest in small molecules as research tools and possible therapies for muscle diseases.

K336I mutant actin alters the structure of neighbouring protomers in filaments and reduces affinity for actin-binding proteins

Mutation of the Lys-336 residue of actin to Ile (K336I) or Asp (K336E) causes congenital myopathy. To understand the effect of this mutation on the function of actin filaments and gain insight into the mechanism of disease onset, we prepared and biochemically characterised K336I mutant actin from Dictyostelium discoideum. Subtilisin cleavage assays revealed that the structure of the DNase-I binding loop (D-loop) of monomeric K336I actin, which would face the adjacent actin-protomer in filaments, differed from that of wild type (WT) actin. Although K336I actin underwent normal salt-dependent reversible polymerisation and formed apparently normal filaments, interactions of K336I filaments with alpha-actinin, myosin II, and cofilin were disrupted. Furthermore, co-filaments of K336I and WT actins also exhibited abnormal interactions with cofilin, implying that K336I actin altered the structure of the neighbouring WT actin protomers such that interaction between cofilin and the WT actin protomers was prevented. We speculate that disruption of the interactions between co-filaments and actin-binding proteins is the primary reason why the K336I mutation induces muscle disease in a dominant fashion.