Titin domains with reduced core hydrophobicity cause dilated cardiomyopathy

The underlying genetic defect in most cases of dilated cardiomyopathy (DCM), a common inherited heart disease, remains unknown. Intriguingly, many patients carry single missense variants of uncertain pathogenicity targeting the giant protein titin, a fundamental sarcomere component. To explore the deleterious potential of these variants, we first solved the wild-type and mutant crystal structures of I21, the titin domain targeted by pathogenic variant p.C3575S. Although both structures are remarkably similar, the reduced hydrophobicity of deeply buried position 3575 strongly destabilizes the mutant domain, a scenario supported by molecular dynamics simulations and by biochemical assays that show no disulfide involving C3575. Prompted by these observations, we have found that thousands of similar hydrophobicity-reducing variants associate specifically with DCM. Hence, our results imply that titin domain destabilization causes DCM, a conceptual framework that not only informs pathogenicity assessment of gene variants but also points to therapeutic strategies counterbalancing protein destabilization.

Structure of the native myosin filament in the relaxed cardiac sarcomere

The thick filament is a key component of sarcomeres, the basic units of striated muscle1. Alterations in thick filament proteins are associated with familial hypertrophic cardiomyopathy and other heart and muscle diseases2. Despite the central importance of the thick filament, its molecular organization remains unclear. Here we present the molecular architecture of native cardiac sarcomeres in the relaxed state, determined by cryo-electron tomography. Our reconstruction of the thick filament reveals the three-dimensional organization of myosin, titin and myosin-binding protein C (MyBP-C). The arrangement of myosin molecules is dependent on their position along the filament, suggesting specialized capacities in terms of strain susceptibility and force generation. Three pairs of titin-α and titin-β chains run axially along the filament, intertwining with myosin tails and probably orchestrating the length-dependent activation of the sarcomere. Notably, whereas the three titin-α chains run along the entire length of the thick filament, titin-β chains do not. The structure also demonstrates that MyBP-C bridges thin and thick filaments, with its carboxy-terminal region binding to the myosin tails and directly stabilizing the OFF state of the myosin heads in an unforeseen manner. These results provide a foundation for future research investigating muscle disorders involving sarcomeric components.

Cryo-EM structure of the human cardiac myosin filament

Pumping of the heart is powered by filaments of the motor protein myosin that pull on actin filaments to generate cardiac contraction. In addition to myosin, the filaments contain cardiac myosin-binding protein C (cMyBP-C), which modulates contractility in response to physiological stimuli, and titin, which functions as a scaffold for filament assembly1. Myosin, cMyBP-C and titin are all subject to mutation, which can lead to heart failure. Despite the central importance of cardiac myosin filaments to life, their molecular structure has remained a mystery for 60 years2. Here we solve the structure of the main (cMyBP-C-containing) region of the human cardiac filament using cryo-electron microscopy. The reconstruction reveals the architecture of titin and cMyBP-C and shows how myosin's motor domains (heads) form three different types of motif (providing functional flexibility), which interact with each other and with titin and cMyBP-C to dictate filament architecture and function. The packing of myosin tails in the filament backbone is also resolved. The structure suggests how cMyBP-C helps to generate the cardiac super-relaxed state3; how titin and cMyBP-C may contribute to length-dependent activation4; and how mutations in myosin and cMyBP-C might disturb interactions, causing disease5,6. The reconstruction resolves past uncertainties and integrates previous data on cardiac muscle structure and function. It provides a new paradigm for interpreting structural, physiological and clinical observations, and for the design of potential therapeutic drugs.

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.

Basal oxidation of conserved cysteines modulates cardiac titin stiffness and dynamics

Titin, as the main protein responsible for the passive stiffness of the sarcomere, plays a key role in diastolic function and is a determinant factor in the etiology of heart disease. Titin stiffness depends on unfolding and folding transitions of immunoglobulin-like (Ig) domains of the I-band, and recent studies have shown that oxidative modifications of cryptic cysteines belonging to these Ig domains modulate their mechanical properties in vitro. However, the relevance of this mode of titin mechanical modulation in vivo remains largely unknown. Here, we describe the high evolutionary conservation of titin mechanical cysteines and show that they are remarkably oxidized in murine cardiac tissue. Mass spectrometry analyses indicate a similar landscape of basal oxidation in murine and human myocardium. Monte Carlo simulations illustrate how disulfides and S-thiolations on these cysteines increase the dynamics of the protein at physiological forces, while enabling load- and isoform-dependent regulation of titin stiffness. Our results demonstrate the role of conserved cysteines in the modulation of titin mechanical properties in vivo and point to potential redox-based pathomechanisms in heart disease.

HDAC6 modulates myofibril stiffness and diastolic function of the heart

Passive stiffness of the heart is determined largely by extracellular matrix and titin, which functions as a molecular spring within sarcomeres. Titin stiffening is associated with the development of diastolic dysfunction (DD), while augmented titin compliance appears to impair systolic performance in dilated cardiomyopathy. We found that myofibril stiffness was elevated in mice lacking histone deacetylase 6 (HDAC6). Cultured adult murine ventricular myocytes treated with a selective HDAC6 inhibitor also exhibited increased myofibril stiffness. Conversely, HDAC6 overexpression in cardiomyocytes led to decreased myofibril stiffness, as did ex vivo treatment of mouse, rat, and human myofibrils with recombinant HDAC6. Modulation of myofibril stiffness by HDAC6 was dependent on 282 amino acids encompassing a portion of the PEVK element of titin. HDAC6 colocalized with Z-disks, and proteomics analysis suggested that HDAC6 functions as a sarcomeric protein deacetylase. Finally, increased myofibril stiffness in HDAC6-deficient mice was associated with exacerbated DD in response to hypertension or aging. These findings define a role for a deacetylase in the control of myofibril function and myocardial passive stiffness, suggest that reversible acetylation alters titin compliance, and reveal the potential of targeting HDAC6 to manipulate the elastic properties of the heart to treat cardiac diseases.

Microbial production of megadalton titin yields fibers with advantageous mechanical properties

Manmade high-performance polymers are typically non-biodegradable and derived from petroleum feedstock through energy intensive processes involving toxic solvents and byproducts. While engineered microbes have been used for renewable production of many small molecules, direct microbial synthesis of high-performance polymeric materials remains a major challenge. Here we engineer microbial production of megadalton muscle titin polymers yielding high-performance fibers that not only recapture highly desirable properties of natural titin (i.e., high damping capacity and mechanical recovery) but also exhibit high strength, toughness, and damping energy - outperforming many synthetic and natural polymers. Structural analyses and molecular modeling suggest these properties derive from unique inter-chain crystallization of folded immunoglobulin-like domains that resists inter-chain slippage while permitting intra-chain unfolding. These fibers have potential applications in areas from biomedicine to textiles, and the developed approach, coupled with the structure-function insights, promises to accelerate further innovation in microbial production of high-performance materials.

The GSK-3β-FBXL21 Axis Contributes to Circadian TCAP Degradation and Skeletal Muscle Function

FBXL21 is a clock-controlled E3 ligase modulating circadian periodicity via subcellular-specific CRYPTOCHROME degradation. How FBXL21 regulates tissue-specific circadian physiology and what mechanism operates upstream is poorly understood. Here we report the sarcomere component TCAP as a cytoplasmic substrate of FBXL21. FBXL21 interacts with TCAP in a circadian manner antiphasic to TCAP accumulation in skeletal muscle, and circadian TCAP oscillation is disrupted in Psttm mice with an Fbxl21 hypomorph mutation. GSK-3β phosphorylates FBXL21 and TCAP to activate FBXL21-mediated, phosphodegron-dependent TCAP degradation. GSK-3β inhibition or knockdown diminishes FBXL21-Cul1 complex formation and delays FBXL21-mediated TCAP degradation. Finally, Psttm mice show significant skeletal muscle defects, including impaired fiber size, exercise tolerance, grip strength, and response to glucocorticoid-induced atrophy, in conjunction with cardiac dysfunction. These data highlight a circadian regulatory pathway where a GSK-3β-FBXL21 functional axis controls TCAP degradation via SCF complex formation and regulates skeletal muscle function.

Wirianto et al. find that the circadian E3 ligase FBXL21 drives rhythmic degradation of the sarcomeric protein TCAP in skeletal muscle. GSK-3β co-phosphorylates FBXL21 and TCAP and promotes SCF complex formation and phosphodegron-dependent TCAP turnover. Psttm mice, expressing a hypomorphic Fbxl21 mutant, show dysregulated TCAP degradation and impaired muscle function.

The nano-scale viscoelasticity using atomic force microscopy in liquid environment

We measured viscoelasticity of two nanoscale systems, single protein molecules and molecular layers of water confined between solid walls. In order to quantify the viscoelastic response of these nanoscale systems in liquid environment, the measurements are performed using two types of atomic force microscopes (AFMs), which employ different detection schemes to measure the cantilever response. We used a deflection detection scheme, available in commercial AFMs, that measures cantilever bending and a fibre-interferometer based detection which measures cantilever displacement. The hydrodynamics of the cantilever is modelled using Euler-Bernoulli equation with appropriate boundary conditions which accommodate both detection schemes. In a direct contradiction with many reports in the literature, the dissipation coefficient of a single octomer of titin I27₈ is found to be immeasurably low. The upper bound on the dissipation coefficient is 5 × 10^-7 kg s^-1, which is much lower than the reported values. The entropic stiffness of single unfolded domains of protein measured using both methods is in the range of 10 mN m^-1. We show that in a conventional deflection detection measurement, the phase of the bending signal can be a primary source of artefacts in the dissipation estimates. It is recognized that the measurement of cantilever displacement, which has negligibly small phase lag due to hydrodynamics of the cantilever at low excitation frequencies, is better suited for ensuring artefact-free measurement of viscoelasticity compared to the measurement of the cantilever bending. Further, it was possible to measure dissipation in molecular layers of water confined between the tip and the substrate using fibre interferometer based AFM with similar experimental parameters. It confirms that the dissipation coefficient of a single I27₈ is below the detection limit of AFM. The results shed light on the discrepancy observed in the measured diffusional dynamics of protein collapse measured using Force spectroscopic techniques and single-molecule optical techniques.

An acute eccentric exercise increases circulating myomesin 3 fragments

Discovery of blood biomarkers to evaluate exercise-induced muscle damage have attracted many researchers and coaches. This study aimed to determine changes in circulating myomesin 3 fragments as a novel biomarker for exercise-induced muscle damage. Nine healthy males performed 10 sets of 40 repetitions of one-leg calf-raise exercise by the load corresponding to the half of their body weight. Muscle symptoms were evaluated by a visual analog scale (VAS). Blood samples were collected before and 2, 4, 24, 48, 72, and 96 h post-exercise. Plasma myomesin 3 fragments levels were significantly increased at 96 h after the eccentric exercise. The myomesin 3 fragments levels were correlated with other biomarkers of muscle damage and the muscle symptoms. These results suggest that the circulating myomesin 3 fragments levels are potential biomarkers reflecting eccentric exercise-induced muscle damage.

The Axial Alignment of Titin on the Muscle Thick Filament Supports Its Role as a Molecular Ruler

The giant protein titin is expressed in vertebrate striated muscle where it spans half a sarcomere from the Z-disc to the M-band and is essential for muscle organisation, activity and health. The C-terminal portion of titin is closely associated with the thick, myosin-containing filament and exhibits a complex pattern of immunoglobulin and fibronectin domains. This pattern reflects features of the filament organisation suggesting that it acts as a molecular ruler and template, but the exact axial disposition of the molecule has not been determined. Here, we present data that allow us to precisely locate titin domains axially along the thick filament from its tip to the edge of the bare zone. We find that the domains are regularly distributed along the filament at 4-nm intervals and we can determine the domains that associate with features of the filament, such as the 11 stripes of accessory proteins. We confirm that the nine stripes ascribed to myosin binding protein-C are not related to the titin sequence previously assumed; rather, they relate to positions approximately 18 domains further towards the C terminus along titin. This disposition also allows a subgroup of titin domains comprising two or three fibronectin domains to associate with each of the 49 levels of myosin heads in each half filament. The results strongly support the role of titin as a blueprint for the thick filament and the arrangement of the myosin motor domains.

A HaloTag-TEV genetic cassette for mechanical phenotyping of proteins from tissues

Single-molecule methods using recombinant proteins have generated transformative hypotheses on how mechanical forces are generated and sensed in biological tissues. However, testing these mechanical hypotheses on proteins in their natural environment remains inaccesible to conventional tools. To address this limitation, here we demonstrate a mouse model carrying a HaloTag-TEV insertion in the protein titin, the main determinant of myocyte stiffness. Using our system, we specifically sever titin by digestion with TEV protease, and find that the response of muscle fibers to length changes requires mechanical transduction through titin's intact polypeptide chain. In addition, HaloTag-based covalent tethering enables examination of titin dynamics under force using magnetic tweezers. At pulling forces < 10 pN, titin domains are recruited to the unfolded state, and produce 41.5 zJ mechanical work during refolding. Insertion of the HaloTag-TEV cassette in mechanical proteins opens opportunities to explore the molecular basis of cellular force generation, mechanosensing and mechanotransduction.

Switchable reinforced streptavidin

The complex of the small molecule biotin and the homotetrameric protein streptavidin is key to a broad range of biotechnological applications. Therefore, the behavior of this extraordinarily high-affinity interaction under mechanical force is intensively studied by single-molecule force spectroscopy. Recently, steered molecular dynamics simulations have identified a low force pathway for the dissociation of biotin from streptavidin, which involves partial unfolding of the N-terminal β-sheet structure of monovalent streptavidin's functional subunit. Based on these results, we now introduced two mutations (T18C,A33C) in the functional subunit of monovalent streptavidin to establish a switchable connection (disulfide bridge) between the first two β-strands to prevent this unfolding. In atomic force microscopy-based single-molecule force spectroscopy experiments, we observed unbinding forces of about 350 pN (at a force-loading rate of 10 nN s^-1) for pulling a single biotin out of an N-terminally anchored monovalent streptavidin binding pocket - about 1.5-fold higher compared with what has been reported for N-terminal force loading of native monovalent streptavidin. Upon addition of a reducing agent, the unbinding forces dropped back to 200 pN, as the disulfide bridge was destroyed. Switching from reducing to oxidizing buffer conditions, the inverse effect was observed. Our work illustrates how the mechanics of a receptor-ligand system can be tuned by engineering the receptor protein far off the ligand-binding pocket.

Protein unfolding by SDS: the microscopic mechanisms and the properties of the SDS-protein assembly

The effects of detergent sodium dodecyl sulfate (SDS) on protein structure and dynamics are fundamental to the most common laboratory technique used to separate proteins and determine their molecular weights: polyacrylamide gel electrophoresis. However, the mechanism by which SDS induces protein unfolding and the microstructure of protein-SDS complexes remain largely unknown. Here, we report a detailed account of SDS-induced unfolding of two proteins-I27 domain of titin and β-amylase-obtained through all-atom molecular dynamics simulations. Both proteins were found to spontaneously unfold in the presence of SDS at boiling water temperature on the time scale of several microseconds. The protein unfolding was found to occur via two distinct mechanisms in which specific interactions of individual SDS molecules disrupt the protein's secondary structure. In the final state of the unfolding process, the proteins are found to wrap around SDS micelles in a fluid necklace-and-beads configuration, where the number and location of bound micelles changes dynamically. The global conformation of the protein was found to correlate with the number of SDS micelles bound to it, whereas the number of SDS molecules directly bound to the protein was found to define the relaxation time scale of the unfolded protein. Our microscopic characterization of SDS-protein interactions sets the stage for future refinement of SDS-enabled protein characterization methods, including protein fingerprinting and sequencing using a solid-state nanopore.

Supramolecular and Liquid Crystalline Contributions to the Assembly of Myofibril

We compare steps observed during the fibrillogenesis of myofibrils with the sequence of steps predictable by a recent analysis of the structurization and functioning of striated muscles. The predicted assembly steps are based solely on fundamental equilibrium processes, particularly supramolecular interactions and liquid crystalline alignment of the rigid thick and thin filaments hosted within the sarcomer. Satisfactory agreement is obtained between several of the observed and the predicted fibrillogenesis steps. In several cases, however, the actual steps appear to be more complex than expected, evidencing the occurrence of transport and kinetic pathways that may assist the attainment of the equilibrium structure. The memory of the order of a precursor mesophase is imprinted during the remodeling of the surfaces at which the two sets of filaments are anchored. The relevance of the present analysis to the functioning of the myofibril is considered.

Structural diversity in the atomic resolution 3D fingerprint of the titin M-band segment

In striated muscles, molecular filaments are largely composed of long protein chains with extensive arrays of identically folded domains, referred to as “beads-on-a-string”. It remains a largely unresolved question how these domains have developed a unique molecular profile such that each carries out a distinct function without false-positive readout. This study focuses on the M-band segment of the sarcomeric protein titin, which comprises ten identically folded immunoglobulin domains. Comparative analysis of high-resolution structures of six of these domains ‒ M1, M3, M4, M5, M7, and M10 ‒ reveals considerable structural diversity within three distinct loops and a non-conserved pattern of exposed cysteines. Our data allow to structurally interpreting distinct pathological readouts that result from titinopathy-associated variants. Our findings support general principles that could be used to identify individual structural/functional profiles of hundreds of identically folded protein domains within the sarcomere and other densely crowded cellular environments.

Current Understanding of Residual Force Enhancement: Cross-Bridge Component and Non-Cross-Bridge Component

Muscle contraction is initiated by the interaction between actin and myosin filaments. The sliding of actin filaments relative to myosin filaments is produced by cross-bridge cycling, which is governed by the theoretical framework of the cross-bridge theory. The cross-bridge theory explains well a number of mechanical responses, such as isometric and concentric contractions. However, some experimental observations cannot be explained with the cross-bridge theory; for example, the increased isometric force after eccentric contractions. The steady-state, isometric force after an eccentric contraction is greater than that attained in a purely isometric contraction at the same muscle length and same activation level. This well-acknowledged and universally observed property is referred to as residual force enhancement (rFE). Since rFE cannot be explained by the cross-bridge theory, alternative mechanisms for explaining this force response have been proposed. In this review, we introduce the basic concepts of sarcomere length non-uniformity and titin elasticity, which are the primary candidates that have been used for explaining rFE, and discuss unresolved problems regarding these mechanisms, and how to proceed with future experiments in this exciting area of research.

Biomimetic Pulsating Vesicles with Both pH-Tunable Membrane Permeability and Light-Triggered Disassembly-Re-assembly Behaviors Prepared by Supra-Amphiphilic Helices

The reversible unfolding-refolding transition is considerably important for natural elastomeric proteins (e.g., titin) to fulfill their biological functions. It is of great importance to develop synthetic versions by borrowing their unique stretchable design principles. Herein, we present a novel pulsating vesicle by means of the aqueous self-assembly of supra-amphiphilic helices. Interestingly, this vesicle simultaneously features dynamic swelling and shrinkage movements in response to external proton triggers. Titin-like unfolding-refolding transformation of artificial helices was proved to play a crucial role in this pulsatile motion. Moreover, the vesicular membrane of this vesicle has exhibited tunable permeability during reversible expansion and contraction circulation. Meanwhile, light can also be used as a driving force to further regulate the disassembly-reassembly transformation of the pulsating vesicle. In addition, the drug delivery system was also employed as an investigating model to estimate the permeability variation and disassembly-reassembly behaviors of the pulsating vesicles, which displayed unique dual quick- and sustained-release behaviors toward anti-cancer agents. It is anticipated that this work opens an avenue for fabricating novel stretchable biomimetics by using the exclusive unfolding-refolding nature of artificial foldamers.

Challenge data set for macromolecular multi-microcrystallography

A synthetic data set demonstrating a particularly challenging case of indexing ambiguity in the context of radiation damage was generated. This set shall serve as a standard benchmark and reference point for the ongoing development of new methods and new approaches to robust structure solution when single-crystal methods are insufficient. Of the 100 short wedges of data, only the first 36 are currently necessary to solve the structure by `cheating', or using the correct reference structure as a guide. The total wall-clock time and number of crystals required to solve the structure without cheating is proposed as a metric for the efficacy and efficiency of a given multi-crystal automation pipeline.

Truncations of the titin Z-disc predispose to a heart failure with preserved ejection phenotype in the context of pressure overload

Titin (TTN) Truncating variants (TTNtv) in the A-band of TTN predispose the mouse heart to systolic dysfunction when subjected to pressure-loading. However, the effects of TTNtv of the Z-disc are largely unexplored. A rat model of pressure-loaded heart is developed by trans-aortic constriction (TAC). Rats with TTNtv of the Z-disc were randomly assigned to TAC (Z-TAC) or sham-surgery (Z-Sham) and wildtype (WT) littermates served as controls (WT-TAC or WT-Sham). Left ventricular (LV) function was assessed by echocardiography. Pressure volume (PV) loops, histology and molecular profiling were performed eight months after surgery. Pressure-load by TAC increased LV mass in all cases when compared with Sham animals. Notably, systolic function was preserved in TAC animals throughout the study period, which was confirmed by terminal PV loops. Diastolic function was impaired in Z-disc TTNtv rats at baseline as compared to WT and became impaired further after TAC (dp/dtmin, mmHg/s): Z-TAC = -3435±763, WT-TAC = -6497±1299 (p<0.01). Z-TAC animals had greater cardiac fibrosis, with elevated collagen content and decreased vascular density as compared to WT-TAC animals associated with enhanced apoptosis of myocyte and non-myocyte populations. In the context of pressure overload, Z-disc TTNtv is associated with cardiac fibrosis, diastolic dysfunction, and capillary rarefaction in the absence of overt systolic dysfunction.

Multidomain proteins under force

Advancements in single-molecule force spectroscopy techniques such as atomic force microscopy and magnetic tweezers allow investigation of how domain folding under force can play a physiological role. Combining these techniques with protein engineering and HaloTag covalent attachment, we investigate similarities and differences between four model proteins: I10 and I91-two immunoglobulin-like domains from the muscle protein titin, and two α + β fold proteins-ubiquitin and protein L. These proteins show a different mechanical response and have unique extensions under force. Remarkably, when normalized to their contour length, the size of the unfolding and refolding steps as a function of force reduces to a single master curve. This curve can be described using standard models of polymer elasticity, explaining the entropic nature of the measured steps. We further validate our measurements with a simple energy landscape model, which combines protein folding with polymer physics and accounts for the complex nature of tandem domains under force. This model can become a useful tool to help in deciphering the complexity of multidomain proteins operating under force.

The insertion sequence of the N2A region of titin exists in an extended structure with helical characteristics

The giant sarcomere protein titin is the third filament in muscle and is integral to maintaining sarcomere integrity as well as contributing to both active and passive tension. Titin is a multi-domain protein that contains regions of repeated structural elements. The N2A region sits at the boundary between the proximal Ig region of titin that is extended under low force and the PEVK region that is extended under high force. Multiple binding interactions have been associated with the N2A region and it has been proposed that this region acts as a mechanical stretch sensor. The focus of this work is a 117 amino acid portion of the N2A region (N2A-IS), which resides between the proximal Ig domains and the PEVK region. Our work has shown that the N2A-IS region is predicted to contain helical structure in the center while both termini are predicted to be disordered. Recombinantly expressed N2A-IS protein contains 13% α-helical structure, as measured via circular dichroism. Additional α-helical structure can be induced with 2,2,2-trifluoroethanol, suggesting that there is transient helical structure that might be stabilized in the context of the entire N2A region. The N2A-IS region does not exhibit any cooperativity in either thermal or chemical denaturation studies while size exclusion chromatography and Fluorescence Resonance Energy Transfer demonstrates that the N2A-IS region has an extended structure. Combined, these results lead to a model of the N2A-IS region having a helical core with extended N- and C-termini.

Combination of Whole Genome Sequencing, Linkage, and Functional Studies Implicates a Missense Mutation in Titin as a Cause of Autosomal Dominant Cardiomyopathy With Features of Left Ventricular Noncompaction

Supplemental Digital Content is available in the text.

Background—

High throughput next-generation sequencing techniques have made whole genome sequencing accessible in clinical practice; however, the abundance of variation in the human genomes makes the identification of a disease-causing mutation on a background of benign rare variants challenging.

Methods and Results—

Here we combine whole genome sequencing with linkage analysis in a 3-generation family affected by cardiomyopathy with features of autosomal dominant left ventricular noncompaction cardiomyopathy. A missense mutation in the giant protein titin is the only plausible disease-causing variant that segregates with disease among the 7 surviving affected individuals, with interrogation of the entire genome excluding other potential causes. This A178D missense mutation, affecting a conserved residue in the second immunoglobulin-like domain of titin, was introduced in a bacterially expressed recombinant protein fragment and biophysically characterized in comparison to its wild-type counterpart. Multiple experiments, including size exclusion chromatography, small-angle x ray scattering, and circular dichroism spectroscopy suggest partial unfolding and domain destabilization in the presence of the mutation. Moreover, binding experiments in mammalian cells show that the mutation markedly impairs binding to the titin ligand telethonin.

Conclusions—

Here we present genetic and functional evidence implicating the novel A178D missense mutation in titin as the cause of a highly penetrant familial cardiomyopathy with features of left ventricular noncompaction. This expands the spectrum of titin’s roles in cardiomyopathies. It furthermore highlights that rare titin missense variants, currently often ignored or left uninterpreted, should be considered to be relevant for cardiomyopathies and can be identified by the approach presented here.

Computing Average Passive Forces in Sarcomeres in Length-Ramp Simulations

Passive forces in sarcomeres are mainly related to the giant protein titin. Titin’s extensible region consists of spring-like elements acting in series. In skeletal muscles these elements are the PEVK segment, two distinct immunoglobulin (Ig) domain regions (proximal and distal), and a N2A portion. While distal Ig domains are thought to form inextensible end filaments in intact sarcomeres, proximal Ig domains unfold in a force- and time-dependent manner. In length-ramp experiments of single titin strands, sequential unfolding of Ig domains leads to a typical saw-tooth pattern in force-elongation curves which can be simulated by Monte Carlo simulations. In sarcomeres, where more than a thousand titin strands are arranged in parallel, numerous Monte Carlo simulations are required to estimate the resultant force of all titin filaments based on the non-uniform titin elongations. To simplify calculations, the stochastic model of passive forces is often replaced by linear or non-linear deterministic and phenomenological functions. However, new theories of muscle contraction are based on the hypothesized binding of titin to the actin filament upon activation, and thereby on a prominent role of the structural properties of titin. Therefore, these theories necessitate a detailed analysis of titin forces in length-ramp experiments. In our study we present a simple and efficient alternative to Monte Carlo simulations. Based on a structural titin model, we calculate the exact probability distributions of unfolded Ig domains under length-ramp conditions needed for rigorous analysis of expected forces, distribution of unfolding forces, etc. Due to the generality of our model, the approach is applicable to a wide range of stochastic protein unfolding problems.

We provide a simple and stable algorithm to determine the exact solution of passive forces in a half sarcomere in length-ramp simulations. The approach is applicable to a wide range of stochastic models of protein unfolding.

Molecular investigations into the mechanics of a muscle anchoring complex

The titin-telethonin complex, essential for anchoring filaments in the Z-disk of the sarcomere, is composed of immunoglobulin domains. Surprisingly, atomic force microscopy experiments showed that it resists forces much higher than the typical immunoglobulin domain and that the force distribution is unusually broad. To investigate the origin of this behavior, we developed a multiscale simulation approach, combining minimalist and atomistic models (SOP-AT). By following the mechanical response of the complex on experimental timescales, we found that the mechanical stability of titin-telethonin is modulated primarily by the strength of contacts between telethonin and the two titin chains, and secondarily by the timescales of conformational excursions inside telethonin and the pulled titin domains. Importantly, the conformational transitions executed by telethonin in simulations support its proposed role in mechanosensing. Our SOP-AT computational approach thus provides a powerful tool for the exploration of the link between conformational diversity and the broadness of the mechanical response, which can be applied to other multidomain complexes.

Shared Genetic Predisposition in Peripartum and Dilated Cardiomyopathies

Background Peripartum cardiomyopathy shares some clinical features with idiopathic dilated cardiomyopathy, a disorder caused by mutations in more than 40 genes, including TTN, which encodes the sarcomere protein titin. Methods In 172 women with peripartum cardiomyopathy, we sequenced 43 genes with variants that have been associated with dilated cardiomyopathy. We compared the prevalence of different variant types (nonsense, frameshift, and splicing) in these women with the prevalence of such variants in persons with dilated cardiomyopathy and with population controls. Results We identified 26 distinct, rare truncating variants in eight genes among women with peripartum cardiomyopathy. The prevalence of truncating variants (26 in 172 [15%]) was significantly higher than that in a reference population of 60,706 persons (4.7%, P=1.3×10(-7)) but was similar to that in a cohort of patients with dilated cardiomyopathy (55 of 332 patients [17%], P=0.81). Two thirds of identified truncating variants were in TTN, as seen in 10% of the patients and in 1.4% of the reference population (P=2.7×10(-10)); almost all TTN variants were located in the titin A-band. Seven of the TTN truncating variants were previously reported in patients with idiopathic dilated cardiomyopathy. In a clinically well-characterized cohort of 83 women with peripartum cardiomyopathy, the presence of TTN truncating variants was significantly correlated with a lower ejection fraction at 1-year follow-up (P=0.005). Conclusions The distribution of truncating variants in a large series of women with peripartum cardiomyopathy was remarkably similar to that found in patients with idiopathic dilated cardiomyopathy. TTN truncating variants were the most prevalent genetic predisposition in each disorder.

Titin-Based Nanoparticle Tension Sensors Map High-Magnitude Integrin Forces within Focal Adhesions

Mechanical forces transmitted through integrin transmembrane receptors play important roles in a variety of cellular processes ranging from cell development to tumorigenesis. Despite the importance of mechanics in integrin function, the magnitude of integrin forces within adhesions remains unclear. Literature suggests a range from 1 to 50 pN, but the upper limit of integrin forces remains unknown. Herein we challenge integrins with the most mechanically stable molecular tension probe, which is comprised of the immunoglobulin 27th (I27) domain of cardiac titin flanked with a fluorophore and gold nanoparticle. Cell experiments show that integrin forces unfold the I27 domain, suggesting that integrin forces exceed ∼30-40 pN. The addition of a disulfide bridge within I27 "clamps" the probe and resists mechanical unfolding. Importantly, incubation with a reducing agent initiates SH exchange, thus unclamping I27 at a rate that is dependent on the applied force. By recording the rate of S-S reduction in clamped I27, we infer that integrins apply 110 ± 9 pN within focal adhesions of rat embryonic fibroblasts. The rates of S-S exchange are heterogeneous and integrin subtype-dependent. Nanoparticle titin tension sensors along with kinetic analysis of unfolding demonstrate that a subset of integrins apply tension many fold greater than previously reported.

Prevalence of Titin Truncating Variants in General Population

Background

Truncating titin (TTN) mutations, especially in A-band region, represent the most common cause of dilated cardiomyopathy (DCM). Clinical interpretation of these variants can be challenging, as these variants are also present in reference populations. We carried out systematic analyses of TTN truncating variants (TTNtv) in publicly available reference populations, including, for the first time, data from Exome Aggregation Consortium (ExAC). The goal was to establish more accurate estimate of prevalence of different TTNtv to allow better clinical interpretation of these findings.

Methods and Results

Using data from 1000 Genomes Project, Exome Sequencing Project (ESP) and ExAC, we estimated the prevalence of TTNtv in the population. In the three population datasets, 52–54% of TTNtv were not affecting all TTN transcripts. The frequency of truncations affecting all transcripts in ExAC was 0.36% (0.32% - 0.41%, 95% CI) and 0.19% (0.16% - 0.23%, 95% CI) for those affecting the A-band. In the A-band region, the prevalences of frameshift, nonsense and essential splice site variants were 0.057%, 0.090%, and 0.047% respectively. Cga/Tga (arginine/nonsense–R/*) transitional change at CpG mutation hotspots was the most frequent type of TTN nonsense mutation accounting for 91.3% (21/23) of arginine residue nonsense mutation (R/*) at TTN A-band region. Non-essential splice-site variants had significantly lower proportion of private variants and higher proportion of low-frequency variants compared to essential splice-site variants (P = 0.01; P = 5.1 X 10⁻⁴, respectively).

Conclusion

A-band TTNtv are more rare in the general population than previously reported. Based on this analysis, one in 500 carries a truncation in TTN A-band suggesting the penetrance of these potentially harmful variants is still poorly understood, and some of these variants do not manifest as autosomal dominant DCM. This calls for caution when interpreting TTNtv in individuals and families with no history of DCM. Considering the size of TTN, expertise in DNA library preparation, high coverage NGS strategies, validated bioinformatics approach, accurate variant assessment strategy, and confirmatory sequencing are prerequisites for reliable evaluation of TTN in clinical settings, and ideally with the inclusion of mRNA and/or protein level assessment for a definite diagnosis.

Titin: central player of hypertrophic signaling and sarcomeric protein quality control

The giant sarcomeric protein titin has multiple important functions in striated muscle cells. Due to its gigantic size, its central position in the sarcomere and its elastic I-band domains, titin is a scaffold protein that is important for sarcomere assembly, and serves as a molecular spring that defines myofilament distensibility. This review focuses on the emerging role of titin in mechanosensing and hypertrophic signaling, and further highlights recent evidence that links titin to sarcomeric protein turnover.

Protein denaturation at a single-molecule level: the effect of nonpolar environments and its implications on the unfolding mechanism by proteases

Most proteins are typically folded into predetermined three-dimensional structures in the aqueous cellular environment. However, proteins can be exposed to a nonpolar environment under certain conditions, such as inside the central cavity of chaperones and unfoldases during protein degradation. It remains unclear how folded proteins behave when moved from an aqueous solvent to a nonpolar one. Here, we employed single-molecule atomic force microscopy and molecular dynamics (MD) simulations to investigate the structural and mechanical variations of a polyprotein, I278, during the change from a polar to a nonpolar environment. We found that the polyprotein was unfolded into an unstructured polypeptide spontaneously when pulled into nonpolar solvents. This finding was corroborated by MD simulations where I27 was dragged from water into a nonpolar solvent, revealing details of the unfolding process at the water/nonpolar solvent interface. These results highlight the importance of water in maintaining folding stability, and provide insights into the response of folded proteins to local hydrophobic environments.

Mechanostability of cohesin-dockerin complexes in a structure-based model: anisotropy and lack of universality in the force profiles

We use a structure-based coarse grained model to elucidate stretching of three cohesin-dockerin complexes that are found in the cellulosome. The average strength of mechanostability is comparable to that of the I27 domain of titin, but the force profiles depend on the pulling direction and anisotropy effects can be substantial. Even though the force profiles for individual cohesins and dockerins are similar, those for their complexes are visibly distinct for any pulling direction.

¹H, ¹⁵N, and ¹³C backbone chemical shift assignment of titin domains A59-A60 and A60 alone

The giant protein titin is the third most abundant protein of vertebrate striated muscle. The titin molecule is >1 μm long and spans half the sarcomere, from the Z-disk to the M-line, and has important roles in sarcomere assembly, elasticity and intracellular signaling. In the A-band of the sarcomere titin is attached to the thick filaments and mainly consists immunoglobulin-like and fibronectin type III-like domains. These are mostly arranged in long-range patterns or 'super-repeats'. The large super-repeats each contain 11 domains and are repeated 11 times, thus forming nearly half the titin molecule. Through interactions with myosin and C-protein, they are involved in thick filament assembly. The importance of titin in muscle assembly is highlighted by the effect of mutations in the A-band portion, which are the commonest cause of dilated cardiomyopathy, affecting ~1 in 250 (Herman et al. in N Engl J Med 366:619-628, 2012). Here we report backbone (15)N, (13)C and (1)H chemical shift and (13)Cβ assignments for the A59-A60 domain tandem from the titin A59-A69 large super-repeat, completed using triple resonance NMR. Since, some regions of the backbone remained unassigned in A60 domain of the complete A59-A60 tandem, a construct containing a single A60 domain, A60sd, was also studied using the same methods. Considerably improved assignment coverage was achieved using A60sd due to its lower mass and improved molecular tumbling rate; these assignments also allowed the analysis of inter-domain interactions using chemical shift mapping against A59-A60.

Bayesian orientation estimate and structure information from sparse single-molecule x-ray diffraction images

We developed a Bayesian method to extract macromolecular structure information from sparse single-molecule x-ray free-electron laser diffraction images. The method addresses two possible scenarios. First, using a "seed" structural model, the molecular orientation is determined for each of the provided diffraction images, which are then averaged in three-dimensional reciprocal space. Subsequently, the real space electron density is determined using a relaxed averaged alternating reflections algorithm. In the second approach, the probability that the "seed" model fits to the given set of diffraction images as a whole is determined and used to distinguish between proposed structures. We show that for a given x-ray intensity, unexpectedly, the achievable resolution increases with molecular mass such that structure determination should be more challenging for small molecules than for larger ones. For a sufficiently large number of recorded photons (>200) per diffraction image an M^{1/6} scaling is seen. Using synthetic diffraction data for a small glutathione molecule as a challenging test case, successful determination of electron density was demonstrated for 20000 diffraction patterns with random orientations and an average of 82 elastically scattered and recorded photons per image, also in the presence of up to 50% background noise. The second scenario is exemplified and assessed for three biomolecules of different sizes. In all cases, determining the probability of a structure given set of diffraction patterns allowed successful discrimination between different conformations of the test molecules. A structure model of the glutathione tripeptide was refined in a Monte Carlo simulation from a random starting conformation. Further, effective distinguishing between three differently arranged immunoglobulin domains of a titin molecule and also different states of a ribosome in a tRNA translocation process was demonstrated. These results show that the proposed method is robust and enables structure determination from sparse and noisy x-ray diffraction images of single molecules spanning a wide range of molecular masses.

Titin-mediated control of cardiac myofibrillar function

According to the Frank-Starling relationship, ventricular pressure or stroke volume increases with end-diastolic volume. This is regulated, in large part, by the sarcomere length (SL) dependent changes in cardiac myofibrillar force, loaded shortening, and power. Consistent with this, both cardiac myofibrillar force and absolute power fall at shorter SL. However, when Ca(2+) activated force levels are matched between short and long SL (by increasing the activator [Ca(2+)]), short SL actually yields faster loaded shortening and greater peak normalized power output (PNPO). A potential mechanism for faster loaded shortening at short SL is that, at short SL, titin becomes less taut, which increases the flexibility of the cross-bridges, a process that may be mediated by titin's interactions with thick filament proteins. We propose a more slackened titin yields greater myosin head radial and azimuthal mobility and these flexible cross-bridges are more likely to maintain thin filament activation, which would allow more force-generating cross-bridges to work against a fixed load resulting in faster loaded shortening. We tested this idea by measuring SL-dependence of power at matched forces in rat skinned cardiac myocytes containing either N2B titin or a longer, more compliant N2BA titin. We predicted that, in N2BA titin containing cardiac myocytes, power-load curves would not be shifted upward at short SL compared to long SL (when force is matched). Consistent with this, peak normalized power was actually less at short SL versus long SL (at matched force) in N2BA-containing myocytes (N2BA titin: ΔPNPO (Short SL peak power minus long SL peak power)=-0.057±0.049 (n=5) versus N2B titin: ΔPNPO=+0.012±0.012 (n=5). These findings support a model whereby SL per se controls mechanical properties of cross-bridges and this process is mediated by titin. This myofibrillar mechanism may help sustain ventricular power during periods of low preloads, and perhaps a breakdown of this mechanism is involved in impaired function of failing hearts.

[Comparative studies of amyloid properties of muscles proteins and brain Abeta-peptides and identification of approaches to destruction of their amyloids in vitro]

In this review our data on the comparative study of amyloid properties of titin family proteins and brain Abeta-peptides are represented. Approaches to the destruction of amyloid fibrils of muscle X-protein and brain Abeta(1-42)-peptides by various chemical compounds are also described.

Silk fibroin scaffolds enhance cell commitment of adult rat cardiac progenitor cells

The use of three-dimensional (3D) cultures may induce cardiac progenitor cells to synthesize their own extracellular matrix (ECM) and sarcomeric proteins to initiate cardiac differentiation. 3D cultures grown on synthetic scaffolds may favour the implantation and survival of stem cells for cell therapy when pharmacological therapies are not efficient in curing cardiovascular diseases and when organ transplantation remains the only treatment able to rescue the patient's life. Silk fibroin-based scaffolds may be used to increase cell affinity to biomaterials and may be chemically modified to improve cell adhesion. In the present study, porous, partially orientated and electrospun nanometric nets were used. Cardiac progenitor cells isolated from adult rats were seeded by capillarity in the 3D structures and cultured inside inserts for 21 days. Under this condition, the cells expressed a high level of sarcomeric and cardiac proteins and synthesized a great quantity of ECM. In particular, partially orientated scaffolds induced the synthesis of titin, which is a fundamental protein in sarcomere assembly.