α-Actinin/titin interaction: A dynamic and mechanically stable cluster of bonds in the muscle Z-disk

Titin: roles in cardiac function and diseases

The giant protein titin is an essential component of muscle sarcomeres. A single titin molecule spans half a sarcomere and mediates diverse functions along its length by virtue of its unique domains. The A-band of titin functions as a molecular blueprint that defines the length of the thick filaments, the I-band constitutes a molecular spring that determines cell-based passive stiffness, and various domains, including the Z-disk, I-band, and M-line, serve as scaffolds for stretch-sensing signaling pathways that mediate mechanotransduction. This review aims to discuss recent insights into titin's functional roles and their relationship to cardiac function. The role of titin in heart diseases, such as dilated cardiomyopathy and heart failure with preserved ejection fraction, as well as its potential as a therapeutic target, is also discussed.

Rare ACTN2 Frameshift Variants Resulting in Protein Extension Cause Distal Myopathy and Hypertrophic Cardiomyopathy through Protein Aggregation

Distal myopathies are a group of rare, inherited muscular disorders characterized by progressive loss of muscle fibers that begins in the distal parts of arms and legs. Recently, variants in a new disease gene, ACTN2 , have been shown to cause distal myopathy. ACTN2 , a gene previously only associated with cardiomyopathies, encodes alpha-actinin-2, a protein expressed in both cardiac and skeletal sarcomeres. The primary function of alpha-actinin-2 is to link actin and titin to the sarcomere Z-disk. New ACTN2 variants are continuously discovered, however, the clinical significance of many variants remains unknown. Thus, lack of clear genotype-phenotype correlations in ACTN2 -related diseases, actininopathies, persists.

Objective

The objective of the study is to characterize the pathomechanisms underlying actininopathies.

Methods

Functional characterization in C2C12 cell models of several ACTN2 variants is conducted, including frameshift and missense variants associated with dominant actininopathies. We assess the genotype-phenotype correlations of actininopathies using clinical data from several patients carrying these variants.

Results

The results show that the missense variants associated with a recessive form of actininopathy do not cause detectable alpha-actinin-2 aggregates in the cell model. Conversely, dominant frameshift variants causing a protein extension do produce alpha-actinin-2 aggregates.

Interpretation

The results suggest that alpha-actinin-2 aggregation is the disease mechanism underlying some dominant actininopathies, and thus we recommend that protein-extending frameshift variants in ACTN2 should be classified as pathogenic. However, this mechanism is likely elicited by only a limited number of variants. Alternative functional characterization methods should be explored to further investigate other molecular mechanisms underlying actininopathies.

TTN truncation variants produce sarcomere-integrating proteins of uncertain functional significance

Titin (TTN) is one of the largest and most complex proteins expressed in humans, and truncation variants are the most prevalent genetic lesion identified in individuals with dilated cardiomyopathy (DCM) or other disorders of impaired cardiac contractility. Two reports in this issue of the JCI shed light on a potential mechanism involving truncated TTN sarcomere integration and the potential for disruption of sarcomere structural integrity. Kellermayer, Tordai, and colleagues confirmed the presence of truncated TTN protein in human DCM samples. McAfee and authors developed a patient-specific TTN antibody to study truncated TTN subcellular localization and to explore its functional consequences. A "poison peptide" mechanism emerges that inspires alternative therapeutic approaches while opening new lines for inquiry, such as the role of haploinsufficiency of full-length TTN protein, mechanisms explaining sarcomere dysfunction, and explanations for variable penetrance.

Magnetic tweezers in cell mechanics

The chapter provides an overview of the applications of magnetic tweezers in living cells. It discusses the advantages and disadvantages of magnetic tweezers technology with a focus on individual magnetic tweezers configurations, such as electromagnetic tweezers. Solutions to the disadvantages identified are also outlined. The specific role of magnetic tweezers in the field of mechanobiology, such as mechanosensitivity, mechano-allostery and mechanotransduction are also emphasized. The specific usage of magnetic tweezers in mechanically probing cells via specific cell surface receptors, such as mechanosensitive channels is discussed and why mechanical probing has revealed the opening and closing of the channels. Finally, the future direction of magnetic tweezers is presented.

Disruption of Z-Disc Function Promotes Mechanical Dysfunction in Human Myocardium: Evidence for a Dual Myofilament Modulatory Role by Alpha-Actinin 2

The ACTN2 gene encodes α-actinin 2, located in the Z-disc of the sarcomeres in striated muscle. In this study, we sought to investigate the effects of an ACTN2 missense variant of unknown significance (p.A868T) on cardiac muscle structure and function. Left ventricular free wall samples were obtained at the time of cardiac transplantation from a heart failure patient with the ACTN2 A868T heterozygous variant. This variant is in the EF 3-4 domain known to interact with titin and α-actinin. At the ultrastructural level, ACTN2 A868T cardiac samples presented small structural changes in cardiomyocytes when compared to healthy donor samples. However, contractile mechanics of permeabilized ACTN2 A868T variant cardiac tissue displayed higher myofilament Ca2+ sensitivity of isometric force, reduced sinusoidal stiffness, and faster rates of tension redevelopment at all Ca2+ levels. Small-angle X-ray diffraction indicated increased separation between thick and thin filaments, possibly contributing to changes in muscle kinetics. Molecular dynamics simulations indicated that while the mutation does not significantly impact the structure of α-actinin on its own, it likely alters the conformation associated with titin binding. Our results can be explained by two Z-disc mediated communication pathways: one pathway that involves α-actinin's interaction with actin, affecting thin filament regulation, and the other pathway that involves α-actinin's interaction with titin, affecting thick filament activation. This work establishes the role of α-actinin 2 in modulating cross-bridge kinetics and force development in the human myocardium as well as how it can be involved in the development of cardiac disease.

Nucleus Mechanosensing in Cardiomyocytes

Cardiac muscle contraction is distinct from the contraction of other muscle types. The heart continuously undergoes contraction-relaxation cycles throughout an animal's lifespan. It must respond to constantly varying physical and energetic burdens over the short term on a beat-to-beat basis and relies on different mechanisms over the long term. Muscle contractility is based on actin and myosin interactions that are regulated by cytoplasmic calcium ions. Genetic variants of sarcomeric proteins can lead to the pathophysiological development of cardiac dysfunction. The sarcomere is physically connected to other cytoskeletal components. Actin filaments, microtubules and desmin proteins are responsible for these interactions. Therefore, mechanical as well as biochemical signals from sarcomeric contractions are transmitted to and sensed by other parts of the cardiomyocyte, particularly the nucleus which can respond to these stimuli. Proteins anchored to the nuclear envelope display a broad response which remodels the structure of the nucleus. In this review, we examine the central aspects of mechanotransduction in the cardiomyocyte where the transmission of mechanical signals to the nucleus can result in changes in gene expression and nucleus morphology. The correlation of nucleus sensing and dysfunction of sarcomeric proteins may assist the understanding of a wide range of functional responses in the progress of cardiomyopathic diseases.

Molecular basis of interactions between CaMKII and α-actinin-2 that underlie dendritic spine enlargement

Ca2+/calmodulin-dependent protein kinase II (CaMKII) is essential for long-term potentiation (LTP) of excitatory synapses that is linked to learning and memory. In this study, we focused on understanding how interactions between CaMKIIα and the actin-crosslinking protein α-actinin-2 underlie long-lasting changes in dendritic spine architecture. We found that association of the two proteins was unexpectedly elevated within 2 minutes of NMDA receptor stimulation that triggers structural LTP in primary hippocampal neurons. Furthermore, disruption of interactions between the two proteins prevented the accumulation of enlarged mushroom-type dendritic spines following NMDA receptor activation. α-Actinin-2 binds to the regulatory segment of CaMKII. Calorimetry experiments, and a crystal structure of α-actinin-2 EF hands 3 and 4 in complex with the CaMKII regulatory segment, indicate that the regulatory segment of autoinhibited CaMKII is not fully accessible to α-actinin-2. Pull-down experiments show that occupation of the CaMKII substrate-binding groove by GluN2B markedly increases α-actinin-2 access to the CaMKII regulatory segment. Furthermore, in situ labelling experiments are consistent with the notion that recruitment of CaMKII to NMDA receptors contributes to elevated interactions between the kinase and α-actinin-2 during structural LTP. Overall, our study provides new mechanistic insight into the molecular basis of structural LTP and reveals an added layer of sophistication to the function of CaMKII.

Stretching the story of titin and muscle function

The discovery of the giant protein titin, also known as connectin, dates almost half a century back. In this review, I recapitulate major advances in the discovery of the titin filaments and the recognition of their properties and function until today. I briefly discuss how our understanding of the layout and interactions of titin in muscle sarcomeres has evolved and review key facts about the titin sequence at the gene (TTN) and protein levels. I also touch upon properties of titin important for the stability of the contractile units and the assembly and maintenance of sarcomeric proteins. The greater part of my discussion centers around the mechanical function of titin in skeletal muscle. I cover milestones of research on titin's role in stretch-dependent passive tension development, recollect the reasons behind the enormous elastic diversity of titin, and provide an update on the molecular mechanisms of titin elasticity, details of which are emerging even now. I reflect on current knowledge of how muscle fibers behave mechanically if titin stiffness is removed and how titin stiffness can be dynamically regulated, such as by posttranslational modifications or calcium binding. Finally, I highlight novel and exciting, but still controversially discussed, insight into the role titin plays in active tension development, such as length-dependent activation and contraction from longer muscle lengths.

The role of single protein elasticity in mechanobiology

In addition to biochemical signals and genetic considerations, mechanical forces are rapidly emerging as a master regulator of human physiology. Yet the molecular mechanisms that regulate force-induced functionalities across a wide range of scales, encompassing the cell, tissue or organ levels, are comparatively not so well understood. With the advent, development and refining of single molecule nanomechanical techniques, enabling to exquisitely probe the conformational dynamics of individual proteins under the effect of a calibrated force, we have begun to acquire a comprehensive knowledge on the rich plethora of physicochemical principles that regulate the elasticity of single proteins. Here we review the major advances underpinning our current understanding of how the elasticity of single proteins regulates mechanosensing and mechanotransduction. We discuss the present limitations and future challenges of such a prolific and burgeoning field.

Mutation update for the ACTN2 gene

ACTN2 encodes alpha-actinin-2, a protein expressed in human cardiac and skeletal muscle. The protein, located in the sarcomere Z-disk, functions as a link between the anti-parallel actin filaments. This important structural protein also binds N-terminal titins, and thus contributes to sarcomere stability. Previously, ACTN2 mutations have been solely associated with cardiomyopathy, without skeletal muscle disease. Recently, however, ACTN2 mutations have been associated with novel congenital and distal myopathy. Previously reported variants are in varying locations across the gene, but the potential clustering effect of pathogenic locations is not clearly understood. Further, the genotype-phenotype correlations of these variants remain unclear. Here we review the previously reported ACTN2-related molecular and clinical findings and present an additional variant, c.1840-2A>T, that further expands the mutation and phenotypic spectrum. Our results show a growing body of clinical, genetic, and functional evidence, which underlines the central role of ACTN2 in the muscle tissue and myopathy. However, limited segregation and functional data are available to support the pathogenicity of most previously reported missense variants and clear-cut genotype-phenotype correlations are currently only demonstrated for some ACTN2-related myopathies.

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’.

The insect perspective on Z-disc structure and biology

Myofibrils are the intracellular structures formed by actin and myosin filaments. They are paracrystalline contractile cables with unusually well-defined dimensions. The sliding of actin past myosin filaments powers contractions, and the entire system is held in place by a structure called the Z-disc, which anchors the actin filaments. Myosin filaments, in turn, are anchored to another structure called the M-line. Most of the complex architecture of myofibrils can be reduced to studying the Z-disc, and recently, important advances regarding the arrangement and function of Z-discs in insects have been published. On a very small scale, we have detailed protein structure information. At the medium scale, we have cryo-electron microscopy maps, super-resolution microscopy and protein-protein interaction networks, while at the functional scale, phenotypic data are available from precise genetic manipulations. All these data aim to answer how the Z-disc works and how it is assembled. Here, we summarize recent data from insects and explore how it fits into our view of the Z-disc, myofibrils and, ultimately, muscles.

Contributions of Titin and Collagen to Passive Stress in Muscles from mdm Mice with a Small Deletion in Titin’s Molecular Spring

Muscular dystrophy with myositis (mdm) is a naturally occurring mutation in the mouse Ttn gene that results in higher passive stress in muscle fibers and intact muscles compared to wild-type (WT). The goal of this study was to test whether alternative splicing of titin exons occurs in mdm muscles, which contain a small deletion in the N2A-PEVK regions of titin, and to test whether splicing changes are associated with an increase in titin-based passive tension. Although higher levels of collagen have been reported previously in mdm muscles, here we demonstrate alternative splicing of titin in mdm skeletal muscle fibers. We identified Z-band, PEVK, and C-terminus Mex5 exons as splicing hotspots in mdm titin using RNA sequencing data and further reported upregulation in ECM-associated genes. We also treated skinned mdm soleus fiber bundles with trypsin, trypsin + KCl, and trypsin + KCL + KI to degrade titin. The results showed that passive stress dropped significantly more after trypsin treatment in mdm fibers (11 ± 1.6 mN/mm²) than in WT fibers (4.8 ± 1 mN/mm²; p = 0.0004). The finding that treatment with trypsin reduces titin-based passive tension more in mdm than in WT fibers supports the hypothesis that exon splicing leads to the expression of a stiffer and shorter titin isoform in mdm fibers. After titin extraction by trypsin + KCl + KI, mdm fibers (6.7 ± 1.27 mN/mm²) had significantly higher collagen-based passive stress remaining than WT fibers (2.6 ± 1.3 mN/mm²; p = 0.0014). We conclude that both titin and collagen contribute to higher passive tension of mdm muscles.

Engineered meatballs via scalable skeletal muscle cell expansion and modular micro-tissue assembly using porous gelatin micro-carriers

The emerging field of cultured meat faces several technical hurdles, including the scale-up production of quality muscle and adipose progenitor cells, and the differentiation and bioengineering of these cellular materials into large, meat-like tissue. Here, we present edible, 3D porous gelatin micro-carriers (PoGelat-MCs), as efficient cell expansion scaffolds, as well as modular tissue-engineering building blocks for lab-grown meat. PoGelat-MC culture in spinner flasks, not only facilitated the scalable expansion of porcine skeletal muscle satellite cells and murine myoblasts, but also triggered their spontaneous myogenesis, in the absence of myogenic reagents. Using 3D-printed mold and transglutaminase, we bio-assembled pork muscle micro-tissues into centimeter-scale meatballs, which exhibited similar mechanical property and higher protein content compared to conventional ground pork meatballs. PoGelat-MCs also supported the expansion and differentiation of 3T3L1 murine pre-adipocytes into mature adipose micro-tissues, which could be used as modular assembly unit for engineered fat-containing meat products. Together, our results highlight PoGelat-MCs, in combination with dynamic bioreactors, as a scalable culture system to produce large quantity of highly-viable muscle and fat micro-tissues, which could be further bio-assembled into ground meat analogues.

Active unfolding of the glucocorticoid receptor by the Hsp70/Hsp40 chaperone system in single-molecule mechanical experiments

One of the key unresolved questions in the field of molecular chaperones is how they can actively unfold proteins. In this study, we discovered that the Hsp70/Hsp40 chaperone system completely unfolds a native soluble substrate protein, the ligand-binding domain of the glucocorticoid receptor, in a concerted action. Our high-resolution optical tweezers data show in real time how the substrate is attacked by the chaperone machinery. As soon as the hormone has left the binding pocket, up to five Hsp70/Hsp40 complexes bind and unfold the protein in a stepwise manner. This finding constitutes direct evidence that the chaperone machinery can bind to the folded core of the receptor, thus providing a mechanism for Hsp70-induced protein unfolding.

The glucocorticoid receptor (GR) is an important transcription factor and drug target linked to a variety of biological functions and diseases. It is one of the most stringent physiological clients of the Hsp90/Hsp70/Hsp40 chaperone system. In this study, we used single-molecule force spectroscopy by optical tweezers to observe the interaction of the GR’s ligand-binding domain (GR-LBD) with the Hsp70/Hsp40 chaperone system (Hsp70/40). We show in real time that Hsp70/40 can unfold the complete GR-LBD in a stepwise manner. Each unfolding step involves binding of an Hsp70 to the GR-LBD and subsequent adenosine triphosphate (ATP) hydrolysis, stimulated by Hsp40. The kinetics of chaperone-mediated unfolding depend on chaperone concentrations as well as the presence of the nucleotide exchange factor BAG1. We find that Hsp70/40 can stabilize new unfolding intermediates, showing that Hsp70/40 can directly interact with the folded core of the protein when working as an unfoldase. Our results support an unfolding mechanism where Hsp70 can directly bind to folded protein structures and unfold them upon ATP hydrolysis. These results provide important insights into the regulation of GR by Hsp70/40.

The switch from client holding to folding in the Hsp70/Hsp90 chaperone machineries is regulated by a direct interplay between co-chaperones

Folding of stringent clients requires transfer from Hsp70 to Hsp90. The co-chaperone Hop physically connects the chaperone machineries. Here, we define its role from the remodeling of Hsp70/40-client complexes to the mechanism of client transfer and the conformational switching from stalled to active client-processing states of Hsp90. We show that Hsp70 together with Hsp40 completely unfold a stringent client, the glucocorticoid receptor ligand-binding domain (GR-LBD) in large assemblies. Hop remodels these for efficient transfer onto Hsp90. As p23 enters, Hsp70 leaves the complex via switching between binding sites in Hop. Current concepts assume that to proceed to client folding, Hop dissociates and the co-chaperone p23 stabilizes the Hsp90 closed state. In contrast, we show that p23 functionally interacts with Hop, relieves the stalling Hsp90-Hop interaction, and closes Hsp90. This reaction allows folding of the client and is thus the key regulatory step for the progression of the chaperone cycle.

Nano-Precision Tweezers for Mechanosensitive Proteins and Beyond

Mechanical forces play pivotal roles in regulating cell shape, function, and fate. Key players that govern the mechanobiological interplay are the mechanosensitive proteins found on cell membranes and in cytoskeleton. Their unique nanomechanics can be interrogated using single-molecule tweezers, which can apply controlled forces to the proteins and simultaneously measure the ensuing structural changes. Breakthroughs in high-resolution tweezers have enabled the routine monitoring of nanometer-scale, millisecond dynamics as a function of force. Undoubtedly, the advancement of structural biology will be further fueled by integrating static atomic-resolution structures and their dynamic changes and interactions observed with the force application techniques. In this minireview, we will introduce the general principles of single-molecule tweezers and their recent applications to the studies of force-bearing proteins, including the synaptic proteins that need to be categorized as mechanosensitive in a broad sense. We anticipate that the impact of nano-precision approaches in mechanobiology research will continue to grow in the future.

Isoform-specific functions of synaptopodin-2 variants in cytoskeleton stabilization and autophagy regulation in muscle under mechanical stress

Protein homeostasis (proteostasis) in multicellular organisms depends on the maintenance of force-bearing and force-generating cellular structures. Within myofibrillar Z-discs of striated muscle, isoforms of synaptopodin-2 (SYNPO2/myopodin) act as adapter proteins that are engaged in proteostasis of the actin-crosslinking protein filamin C (FLNc) under mechanical stress. SYNPO2 directly binds F-actin, FLNc and α-actinin and thus contributes to the architectural features of the actin cytoskeleton. By its association with autophagy mediating proteins, i.e. BAG3 and VPS18, SYNPO2 is also engaged in protein quality control and helps to target mechanical unfolded and damaged FLNc for degradation. Here we show that deficiency of all SYNPO2-isoforms in myotubes leads to decreased myofibrillar stability and deregulated autophagy under mechanical stress. In addition, isoform-specific proteostasis functions were revealed. The PDZ-domain containing variant SYNPO2b and the shorter, PDZ-less isoform SYNPO2e both localize to Z-discs. Yet, SYNPO2e is less stably associated with the Z-disc than SYNPO2b, and is dynamically transferred into FLNc-containing myofibrillar lesions under mechanical stress. SYNPO2e also recruits BAG3 into these lesions via interaction with the WW domain of BAG3. Our data provide evidence for a role of myofibrillar lesions as a transient quality control compartment essential to prevent and repair contraction-induced myofibril damage in muscle and indicate an important coordinating activity for SYNPO2 therein.

Myocardial fibrosis reversion via rhACE2-electrospun fibrous patch for ventricular remodeling prevention

Myocardial fibrosis and ventricular remodeling were the key pathology factors causing undesirable consequence after myocardial infarction. However, an efficient therapeutic method remains unclear, partly due to difficulty in continuously preventing neurohormonal overactivation and potential disadvantages of cell therapy for clinical practice. In this study, a rhACE2-electrospun fibrous patch with sustained releasing of rhACE2 to shape an induction transformation niche in situ was introduced, through micro-sol electrospinning technologies. A durable releasing pattern of rhACE2 encapsulated in hyaluronic acid (HA)—poly(L-lactic acid) (PLLA) core-shell structure was observed. By multiple in vitro studies, the rhACE2 patch demonstrated effectiveness in reducing cardiomyocytes apoptosis under hypoxia stress and inhibiting cardiac fibroblasts proliferation, which gave evidence for its in vivo efficacy. For striking mice myocardial infarction experiments, a successful prevention of adverse ventricular remodeling has been demonstrated, reflecting by improved ejection fraction, normal ventricle structure and less fibrosis. The rhACE2 patch niche showed clear superiority in long term function and structure preservation after ischemia compared with intramyocardial injection. Thus, the micro-sol electrospun rhACE2 fibrous patch niche was proved to be efficient, cost-effective and easy-to-use in preventing ventricular adverse remodeling.

Actinin BioID reveals sarcomere crosstalk with oxidative metabolism through interactions with IGF2BP2

Actinins are strain-sensing actin cross-linkers that are ubiquitously expressed and harbor mutations in human diseases. We utilize CRISPR, pluripotent stem cells, and BioID to study actinin interactomes in human cardiomyocytes. We identify 324 actinin proximity partners, including those that are dependent on sarcomere assembly. We confirm 19 known interactors and identify a network of RNA-binding proteins, including those with RNA localization functions. In vivo and biochemical interaction studies support that IGF2BP2 localizes electron transport chain transcripts to actinin neighborhoods through interactions between its K homology (KH) domain and actinin’s rod domain. We combine alanine scanning mutagenesis and metabolic assays to disrupt and functionally interrogate actinin-IGF2BP2 interactions, which reveal an essential role in metabolic responses to pathological sarcomere activation using a hypertrophic cardiomyopathy model. This study expands our functional knowledge of actinin, uncovers sarcomere interaction partners, and reveals sarcomere crosstalk with IGF2BP2 for metabolic adaptation relevant to human disease.

Ladha et al. combine BioID, human cardiomyocytes, and CRISPR-Cas9 to interrogate the actinin interactome. This reveals 324 actinin proximity partners, including RNA-binding proteins that bind transcripts encoding ETC functional components. Among these RNA-binding proteins, IGF2BP2 directly binds actinin, and actinin-IGF2BP2 interactions regulate ETC transcript localization and metabolic adaptation to sarcomere function.

Loss of sarcomeric proteins via upregulation of JAK/STAT signaling underlies interferon-γ-induced contractile deficit in engineered human myocardium

The level of circulating interferon-γ (IFNγ) is elevated in various clinical conditions including autoimmune and inflammatory diseases, sepsis, acute coronary syndrome, and viral infections. As these conditions are associated with high risk of myocardial dysfunction, we investigated the effects of IFNγ on 3D fibrin-based engineered human cardiac tissues ("cardiobundles"). Cardiobundles were fabricated from human pluripotent stem cell-derived cardiomyocytes, exposed to 0-20 ng/ml of IFNγ on culture days 7-14, and assessed for changes in tissue structure, viability, contractile force and calcium transient generation, action potential propagation, cytokine secretion, and expression of select genes and proteins. We found that application of IFNγ induced a dose-dependent reduction in contractile force generation, deterioration of sarcomeric organization, and cardiomyocyte disarray, without significantly altering cell viability, action potential propagation, or calcium transient amplitude. At molecular level, the IFNγ-induced structural and functional deficits could be attributed to altered balance of pro- and anti-inflammatory cytokines, upregulation of JAK/STAT signaling pathway (JAK1, JAK2, and STAT1), and reduced expression of myosin heavy chain, myosin light chain-2v, and sarcomeric α-actinin. Application of clinically used JAK/STAT inhibitors, tofacitinib and baricitinib, fully prevented IFNγ-induced cardiomyopathy, confirming the critical roles of this signaling pathway in inflammatory cardiac disease. Taken together, our in vitro studies in engineered myocardial tissues reveal direct adverse effects of pro-inflammatory cytokine IFNγ on human cardiomyocytes and establish the foundation for a potential use of cardiobundle platform in modeling of inflammatory myocardial disease and therapy. STATEMENT OF SIGNIFICANCE: Various inflammatory and autoimmune diseases including rheumatoid arthritis, sepsis, lupus erythematosus, Chagas disease, and others, as well as viral infections including H1N1 influenza and COVID-19 show increased systemic levels of a pro-inflammatory cytokine interferon-γ (IFNγ) and are associated with high risk of heart disease. Here we explored for the first time if chronically elevated levels of IFNγ can negatively affect structure and function of engineered human heart tissues in vitro. Our studies revealed IFNγ-induced deterioration of myofibrillar organization and contractile force production in human cardiomyocytes, attributed to decreased expression of multiple sarcomeric proteins and upregulation of JAK/STAT signaling pathway. FDA-approved JAK inhibitors fully blocked the adverse effects of IFNγ, suggesting a potentially effective strategy against human inflammatory cardiomyopathy.

Molecular Mechanisms of Muscle Tone Impairment under Conditions of Real and Simulated Space Flight

Kozlovskaya et al. [1] and Grigoriev et al. [2] showed that enormous loss of muscle stiffness (atonia) develops in humans under true (space flight) and simulated microgravity conditions as early as after the first days of exposure. This phenomenon is attributed to the inactivation of slow motor units and called reflectory atonia. However, a lot of evidence indicating that even isolated muscle or a single fiber possesses substantial stiffness was published at the end of the 20th century. This intrinsic stiffness is determined by the active component, i.e. the ability to form actin-myosin cross-bridges during muscle stretch and contraction, as well as by cytoskeletal and extracellular matrix proteins, capable of resisting muscle stretch. The main facts on intrinsic muscle stiffness under conditions of gravitational unloading are considered in this review. The data obtained in studies of humans under dry immersion and rodent hindlimb suspension is analyzed. The results and hypotheses regarding reduced probability of cross-bridge formation in an atrophying muscle due to increased interfilament spacing are described. The evidence of cytoskeletal protein (titin, nebulin, etc.) degradation during gravitational unloading is also discussed. The possible mechanisms underlying structural changes in skeletal muscle collagen and its role in reducing intrinsic muscle stiffness are presented. The molecular mechanisms of changes in intrinsic stiffness during space flight and simulated microgravity are reviewed.

Maintenance of type 2 glycolytic myofibers with age by Mib1-Actn3 axis

Age-associated muscle atrophy is a debilitating condition associated with loss of muscle mass and function with age that contributes to limitation of mobility and locomotion. However, the underlying mechanisms of how intrinsic muscle changes with age are largely unknown. Here we report that, with age, Mind bomb-1 (Mib1) plays important role in skeletal muscle maintenance via proteasomal degradation-dependent regulation of α-actinin 3 (Actn3). The disruption of Mib1 in myofibers (Mib1^ΔMF) results in alteration of type 2 glycolytic myofibers, muscle atrophy, impaired muscle function, and Actn3 accumulation. After chronic exercise, Mib1^ΔMF mice show muscle atrophy even at young age. However, when Actn3 level is downregulated, chronic exercise-induced muscle atrophy is ameliorated. Importantly, the Mib1 and Actn3 levels show clinical relevance in human skeletal muscles accompanied by decrease in skeletal muscle function with age. Together, these findings reveal the significance of the Mib1-Actn3 axis in skeletal muscle maintenance with age and suggest the therapeutic potential for the treatment or amelioration of age-related muscle atrophy.

The number of Z-repeats and super-repeats in nebulin greatly varies across vertebrates and scales with animal size

Nebulin is a skeletal muscle protein that associates with the sarcomeric thin filaments and has functions in regulating the length of the thin filament and the structure of the Z-disk. Here we investigated the nebulin gene in 53 species of birds, fish, amphibians, reptiles, and mammals. In all species, nebulin has a similar domain composition that mostly consists of ∼30-residue modules (or simple repeats), each containing an actin-binding site. All species have a large region where simple repeats are organized into seven-module super-repeats, each containing a tropomyosin binding site. The number of super-repeats shows high interspecies variation, ranging from 21 (zebrafish, hummingbird) to 31 (camel, chimpanzee), and, importantly, scales with body size. The higher number of super-repeats in large animals was shown to increase thin filament length, which is expected to increase the sarcomere length for optimal force production, increase the energy efficiency of isometric force production, and lower the shortening velocity of muscle. It has been known since the work of A.V. Hill in 1950 that as species increase in size, the shortening velocity of their muscle is reduced, and the present work shows that nebulin contributes to the mechanistic basis. Finally, we analyzed the differentially spliced simple repeats in nebulin's C terminus, whose inclusion correlates with the width of the Z-disk. The number of Z-repeats greatly varies (from 5 to 18) and correlates with the number of super-repeats. We propose that the resulting increase in the width of the Z-disk in large animals increases the number of contacts between nebulin and structural Z-disk proteins when the Z-disk is stressed for long durations.

Real-time visualization of titin dynamics reveals extensive reversible photobleaching in human induced pluripotent stem cell-derived cardiomyocytes

Fluorescence recovery after photobleaching (FRAP) has been useful in delineating cardiac myofilament biology, and innovations in fluorophore chemistry have expanded the array of microscopic assays used. However, one assumption in FRAP is the irreversible photobleaching of fluorescent proteins after laser excitation. Here we demonstrate reversible photobleaching regarding the photoconvertible fluorescent protein mEos3.2. We used CRISPR/Cas9 genome editing in human induced pluripotent stem cells (hiPSCs) to knock-in mEos3.2 into the COOH terminus of titin to visualize sarcomeric titin incorporation and turnover. Upon cardiac induction, the titin-mEos3.2 fusion protein is expressed and integrated in the sarcomeres of hiPSC-derived cardiomyocytes (CMs). STORM imaging shows M-band clustered regions of bound titin-mEos3.2 with few soluble titin-mEos3.2 molecules. FRAP revealed a baseline titin-mEos3.2 fluorescence recovery of 68% and half-life of ~1.2 h, suggesting a rapid exchange of sarcomeric titin with soluble titin. However, paraformaldehyde-fixed and permeabilized titin-mEos3.2 hiPSC-CMs surprisingly revealed a 55% fluorescence recovery. Whole cell FRAP analysis in paraformaldehyde-fixed, cycloheximide-treated, and untreated titin-mEos3.2 hiPSC-CMs displayed no significant differences in fluorescence recovery. FRAP in fixed HEK 293T expressing cytosolic mEos3.2 demonstrates a 58% fluorescence recovery. These data suggest that titin-mEos3.2 is subject to reversible photobleaching following FRAP. Using a mouse titin-eGFP model, we demonstrate that no reversible photobleaching occurs. Our results reveal that reversible photobleaching accounts for the majority of titin recovery in the titin-mEos3.2 hiPSC-CM model and should warrant as a caution in the extrapolation of reliable FRAP data from specific fluorescent proteins in long-term cell imaging.

A Supramolecular Polymerization Approach to the Growth of the Myofibril

Extended linear structures self-assemble by the multi-stage-open-association mechanism of supramolecular polymerization (MSOA). Application of the model requires the identification of a repeating unit, the main-chain supramolecular bond, and the binding constant. The strength of the bond and the degree of polymerization become extremely large when multiple sites for non-covalent interactions occur. These expectations had been previously verified in the case of the neuronal axon, for which the above parameters were assessed from its known molecular structure. The more complex case of the myofibril is analyzed here. The specific interactions that connect neighboring sarcomers have been a matter of debate. Recent work has focused on the bond between titin and α-actinin localized at the terminal Z-zones of each sarcomer. Elaboration of literature data suggests that titin-α-actinin interactions do bridge neighboring sarcomers, promoting the polymerization of myofibrils that attain macroscopic dimensions consistently with the MSOA predictions. The rationale for the complex structuration of single sarcomers is discussed.

Three-dimensional structure of the basketweave Z-band in midshipman fish sonic muscle

Striated muscle enables movement in all animals by the contraction of myriads of sarcomeres joined end to end by the Z-bands. The contraction is due to tension generated in each sarcomere between overlapping arrays of actin and myosin filaments. At the Z-band, actin filaments from adjoining sarcomeres overlap and are cross-linked in a regular pattern mainly by the protein α-actinin. The Z-band is dynamic, reflected by the 2 regular patterns seen in transverse section electron micrographs; the so-called small-square and basketweave forms. Although these forms are attributed, respectively, to relaxed and actively contracting muscles, the basketweave form occurs in certain relaxed muscles as in the muscle studied here. We used electron tomography and subtomogram averaging to derive the 3D structure of the Z-band in the swimbladder sonic muscle of type I male plainfin midshipman fish (Porichthys notatus), into which we docked the crystallographic structures of actin and α-actinin. The α-actinin links run diagonally between connected pairs of antiparallel actin filaments and are oriented at an angle of about 25° away from the actin filament axes. The slightly curved and flattened structure of the α-actinin rod has a distinct fit into the map. The Z-band model provides a detailed understanding of the role of α-actinin in transmitting tension between actin filaments in adjoining sarcomeres.

Proteins with calmodulin-like domains: structures and functional roles

The appearance of modular proteins is a widespread phenomenon during the evolution of proteins. The combinatorial arrangement of different functional and/or structural domains within a single polypeptide chain yields a wide variety of activities and regulatory properties to the modular proteins. In this review, we will discuss proteins, that in addition to their catalytic, transport, structure, localization or adaptor functions, also have segments resembling the helix-loop-helix EF-hand motifs found in Ca2+-binding proteins, such as calmodulin (CaM). These segments are denoted CaM-like domains (CaM-LDs) and play a regulatory role, making these CaM-like proteins sensitive to Ca2+ transients within the cell, and hence are able to transduce the Ca2+ signal leading to specific cellular responses. Importantly, this arrangement allows to this group of proteins direct regulation independent of other Ca2+-sensitive sensor/transducer proteins, such as CaM. In addition, this review also covers CaM-binding proteins, in which their CaM-binding site (CBS), in the absence of CaM, is proposed to interact with other segments of the same protein denoted CaM-like binding site (CLBS). CLBS are important regulatory motifs, acting either by keeping these CaM-binding proteins inactive in the absence of CaM, enhancing the stability of protein complexes and/or facilitating their dimerization via CBS/CLBS interaction. The existence of proteins containing CaM-LDs or CLBSs substantially adds to the enormous versatility and complexity of Ca2+/CaM signaling.

An Active, Ligand-Responsive Pulling Geometry Reports on Internal Signaling between Subdomains of the DnaK Nucleotide-Binding Domain in Single-Molecule Mechanical Experiments

Single-molecule mechanical experiments have proven to be ideal tools for probing the energetics and mechanics of large proteins and domains. In this paper, we investigate the nucleotide-dependent unfolding mechanics of the nucleotide-binding domain (NBD) of the Hsp70 chaperone DnaK. The NBD binds ADP or ATP in the binding cleft formed by lobe I and lobe II, which consists of two subdomains each. When force is applied to the termini of the NBD, the observed unfolding forces are independent of the nucleotide state. In contrast, when force is applied across the nucleotide-binding pocket, the unfolding forces report specifically on the nucleotide-phosphate state. In this active, ligand-responsive pulling geometry, we observed a bifurcation of the unfolding pathway; the pathway proceeds either through a cooperative "coupled pathway" or through a noncooperative "uncoupled pathway". The partitioning between individual unfolding pathways can be effectively tuned by mutation or by the nucleotide exchange factor GrpE, i.e., by the factors affecting the strength of the lobe I-lobe II interactions within the native NBD. These experiments provide important insight into the molecular origin of the internal signaling between the subdomains of the nucleotide-binding domain of Hsp70 proteins and how signals are efficiently transferred inside the protein molecule.

Titin in muscular dystrophy and cardiomyopathy: Urinary titin as a novel marker

Titin, encoded by the gene TTN, is the largest human protein, and plays central roles in sarcomeric structures and functions in skeletal and cardiac muscles. Mutations of TTN are causally related to specific types of muscular dystrophies and cardiomyopathies. A developed methodology of next generation sequencing has recently led to the identification of novel TTN mutations in such diseases. The clinical significance of titin is now emerging as a target for genetic strategies. Titin-related muscular dystrophies include tibial muscular dystrophy, limb-girdle muscular dystrophy, Emery-Dreifuss muscular dystrophy, hereditary myopathy with early respiratory failure, central core myopathy, centronuclear myopathies, and Salih myopathy. Truncation mutations of TTN have been identified as the most frequent genetic cause of dilated cardiomyopathy. In this review article, we highlight the role of titin and impact of TTN mutations in the pathogenesis of muscular dystrophies and cardiomyopathies. Recently, a novel sensitive sandwich enzyme-linked immunosorbent assay (ELISA) for the detection of the urinary titin N-terminal fragments (U-TN) has been established. We discuss the clinical significance of U-TN in the diagnosis of muscular dystrophies and differential diagnosis of cardiomyopathies, as well as risk stratification in dilated cardiomyopathy.

Actininopathy: A new muscular dystrophy caused by ACTN2 dominant mutations

OBJECTIVE

To clinically and pathologically characterize a cohort of patients presenting with a novel form of distal myopathy and to identify the genetic cause of this new muscular dystrophy.

METHODS

We studied 4 families (3 from Spain and 1 from Sweden) suffering from an autosomal dominant distal myopathy. Affected members showed adult onset asymmetric distal muscle weakness with initial involvement of ankle dorsiflexion later progressing also to proximal limb muscles.

RESULTS

In all 3 Spanish families, we identified a unique missense variant in the ACTN2 gene cosegregating with the disease. The affected members of the Swedish family carry a different ACTN2 missense variant.

INTERPRETATION

ACTN2 encodes for alpha actinin2, which is highly expressed in the sarcomeric Z-disk with a major structural and functional role. Actininopathy is thus a new genetically determined distal myopathy. ANN NEUROL 2019;85:899-906.

Protein arginylation of cytoskeletal proteins in the muscle: modifications modifying function

The cytoskeleton drives many essential processes in normal physiology, and its impairments underlie many diseases, including skeletal myopathies, cancer, and heart failure, that broadly affect developed countries worldwide. Cytoskeleton regulation is a field of investigation of rapidly emerging global importance and a new venue for the development of potential therapies. This review overviews our present understanding of the posttranslational regulation of the muscle cytoskeleton through arginylation, a tRNA-dependent addition of arginine to proteins mediated by arginyltransferase 1. We focus largely on arginylation-dependent regulation of striated muscles, shown to play critical roles in facilitating muscle integrity, contractility, regulation, and strength.

Unforgettable force – crosstalk and memory of mechanosensitive structures

The ability of cells to sense and respond to mechanical stimuli is crucial for many developmental and homeostatic processes, while mechanical dysfunction of cells has been associated with numerous pathologies including muscular dystrophies, cardiovascular defects and epithelial disorders. Yet, how cells detect and process mechanical information is still largely unclear. In this review, we outline major mechanisms underlying cellular mechanotransduction and we summarize the current understanding of how cells integrate information from distinct mechanosensitive structures to mediate complex mechanoresponses. We also discuss the concept of mechanical memory and describe how cells store information on previous mechanical events for different periods of time.

Single-molecule force spectroscopy reveals folding steps associated with hormone binding and activation of the glucocorticoid receptor

The glucocorticoid receptor (GR) is a prominent nuclear receptor linked to a variety of diseases and an important drug target. Binding of hormone to its ligand binding domain (GR-LBD) is the key activation step to induce signaling. This process is tightly regulated by the molecular chaperones Hsp70 and Hsp90 in vivo. Despite its importance, little is known about GR-LBD folding, the ligand binding pathway, or the requirement for chaperone regulation. In this study, we have used single-molecule force spectroscopy by optical tweezers to unravel the dynamics of the complete pathway of folding and hormone binding of GR-LBD. We identified a "lid" structure whose opening and closing is tightly coupled to hormone binding. This lid is located at the N terminus without direct contacts to the hormone. Under mechanical load, apo-GR-LBD folds stably and readily without the need of chaperones with a folding free energy of [Formula: see text] The folding pathway is largely independent of the presence of hormone. Hormone binds only in the last step and lid closure adds an additional [Formula: see text] of free energy, drastically increasing the affinity. However, mechanical double-jump experiments reveal that, at zero force, GR-LBD folding is severely hampered by misfolding, slowing it to less than 1·s^-1 From the force dependence of the folding rates, we conclude that the misfolding occurs late in the folding pathway. These features are important cornerstones for understanding GR activation and its tight regulation by chaperones.

Early Deсline in Rat Soleus Passive Tension with Hindlimb Unloading: Inactivation of Cross-bridges or Activation of Calpains?

The study was aimed at testing the hypotheses about the role of cross-bridges and calpains in reduction of rat soleus passive tension under conditions of hindlimb unloading. For this purpose, we used an inhibitor of μ-calpain PD 150606 as well as a blocker of actomyosin interaction (blebbistatin). It was found for the first time that a decrease in passive tension of rat soleus after 3-day hindlimb unloading is associated with the activity of μ-calpain and does not depend on the processes of cross-bridges formation.

Nucleotide-Dependent Dimer Association and Dissociation of the Chaperone Hsp90

Hsp90 is an essential molecular chaperone, which has to be in a dimeric form for its correct function. While the affinity of the dimer has previously been measured, little is known about how it associates and dissociates and the factors that influence this. We perform an in-depth single molecule characterization of the C-terminal association and dissociation of Hsp90. We find more than one dissociation rate, indicating that the dimer has a stable and an unstable state. Furthermore, we find that the stability of the C-terminal association is dependent on the presence of ATP, despite the C-terminal dimerization interface being distal to the catalytic site.

A folding nucleus and minimal ATP binding domain of Hsp70 identified by single-molecule force spectroscopy

The folding pathways of large proteins are complex, with many of them requiring the aid of chaperones and others folding spontaneously. Along the folding pathways, partially folded intermediates are frequently populated; their role in the driving of the folding process is unclear. The structures of these intermediates are generally not amenable to high-resolution structural techniques because of their transient nature. Here we employed single-molecule force measurements to scrutinize the hierarchy of intermediate structures along the folding pathway of the nucleotide binding domain (NBD) of Escherichia coli Hsp70 DnaK. DnaK-NBD is a member of the sugar kinase superfamily that includes Hsp70s and the cytoskeletal protein actin. Using optical tweezers, a stable nucleotide-binding competent en route folding intermediate comprising lobe II residues (183-383) was identified as a critical checkpoint for productive folding. We obtained a structural snapshot of this folding intermediate that shows native-like conformation. To assess the fundamental role of folded lobe II for efficient folding, we turned our attention to yeast mitochondrial NBD, which does not fold without a dedicated chaperone. After replacing the yeast lobe II residues with stable E. coli lobe II, the obtained chimeric protein showed native-like ATPase activity and robust folding into the native state, even in the absence of chaperone. In summary, lobe II is a stable nucleotide-binding competent folding nucleus that is the key to time-efficient folding and possibly resembles a common ancestor domain. Our findings provide a conceptual framework for the folding pathways of other members of this protein superfamily.