Study of the mechanical properties of myomesin proteins using dynamic force spectroscopy

Cardiomyocyte maturation and its reversal during cardiac regeneration

Cardiovascular disease is a leading cause of death worldwide. Due to the limited proliferative and regenerative capacity of adult cardiomyocytes, the lost myocardium is not replenished efficiently and is replaced by a fibrotic scar, which eventually leads to heart failure. Current therapies to cure or delay the progression of heart failure are limited; hence, there is a pressing need for regenerative approaches to support the failing heart. Cardiomyocytes undergo a series of transcriptional, structural, and metabolic changes after birth (collectively termed maturation), which is critical for their contractile function but limits the regenerative capacity of the heart. In regenerative organisms, cardiomyocytes revert from their terminally differentiated state into a less mature state (ie, dedifferentiation) to allow for proliferation and regeneration to occur. Importantly, stimulating adult cardiomyocyte dedifferentiation has been shown to promote morphological and functional improvement after myocardial infarction, further highlighting the importance of cardiomyocyte dedifferentiation in heart regeneration. Here, we review several hallmarks of cardiomyocyte maturation, and summarize how their reversal facilitates cardiomyocyte proliferation and heart regeneration. A detailed understanding of how cardiomyocyte dedifferentiation is regulated will provide insights into therapeutic options to promote cardiomyocyte de-maturation and proliferation, and ultimately heart regeneration in mammals.

The role of the M-band myomesin proteins in muscle integrity and cardiac disease

Transversal structural elements in cross-striated muscles, such as the M-band or the Z-disc, anchor and mechanically stabilize the contractile apparatus and its minimal unit—the sarcomere. The ability of proteins to target and interact with these structural sarcomeric elements is an inevitable necessity for the correct assembly and functionality of the myofibrillar apparatus. Specifically, the M-band is a well-recognized mechanical and signaling hub dealing with active forces during contraction, while impairment of its function leads to disease and death. Research on the M-band architecture is focusing on the assembly and interactions of the three major filamentous proteins in the region, mainly the three myomesin proteins including their embryonic heart (EH) isoform, titin and obscurin. These proteins form the basic filamentous network of the M-band, interacting with each other as also with additional proteins in the region that are involved in signaling, energetic or mechanosensitive processes. While myomesin-1, titin and obscurin are found in every muscle, the expression levels of myomesin-2 (also known as M-protein) and myomesin-3 are tissue specific: myomesin-2 is mainly expressed in the cardiac and fast skeletal muscles, while myomesin-3 is mainly expressed in intermediate muscles and specific regions of the cardiac muscle. Furthermore, EH-myomesin apart from its role during embryonic stages, is present in adults with specific cardiac diseases. The current work in structural, molecular, and cellular biology as well as in animal models, provides important details about the assembly of myomesin-1, obscurin and titin, the information however about the myomesin-2 and -3, such as their interactions, localization and structural details remain very limited. Remarkably, an increasing number of reports is linking all three myomesin proteins and particularly myomesin-2 to serious cardiovascular diseases suggesting that this protein family could be more important than originally thought. In this review we will focus on the myomesin protein family, the myomesin interactions and structural differences between isoforms and we will provide the most recent evidence why the structurally and biophysically unexplored myomesin-2 and myomesin-3 are emerging as hot targets for understanding muscle function and disease.

Myomesin is part of an integrity pathway that responds to sarcomere damage and disease

The structure and function of the sarcomere of striated muscle is well studied but the steps of sarcomere assembly and maintenance remain under-characterized. With the aid of chaperones and factors of the protein quality control system, muscle proteins can be folded and assembled into the contractile apparatus of the sarcomere. When sarcomere assembly is incomplete or the sarcomere becomes damaged, suites of chaperones and maintenance factors respond to repair the sarcomere. Here we show evidence of the importance of the M-line proteins, specifically myomesin, in the monitoring of sarcomere assembly and integrity in previously characterized zebrafish muscle mutants. We show that myomesin is one of the last proteins to be incorporated into the assembling sarcomere, and that in skeletal muscle, its incorporation requires connections with both titin and myosin. In diseased zebrafish sarcomeres, myomesin1a shows an early increase of gene expression, hours before chaperones respond to damaged muscle. We found that myomesin expression is also more specific to sarcomere damage than muscle creatine kinase, and our results and others support the use of myomesin assays as an early, specific, method of detecting muscle damage.

Mutant myocilin impacts sarcomere ultrastructure in mouse gastrocnemius muscle

Myocilin (MYOC) is the gene with mutations most common in glaucoma. In the eye, MYOC is in trabecular meshwork, ciliary body, and retina. Other tissues with high MYOC transcript levels are skeletal muscle and heart. To date, the function of wild-type MYOC remains unknown and how mutant MYOC causes high intraocular pressure and glaucoma is ambiguous. By investigating mutant MYOC in a non-ocular tissue we hoped to obtain novel insight into mutant MYOC pathology. For this study, we utilized a transgenic mouse expressing human mutant MYOC Y437H protein and we examined its skeletal (gastrocnemius) muscle phenotype. Electron micrographs showed that sarcomeres in the skeletal muscle of mutant CMV-MYOC-Y437H mice had multiple M-bands. Western blots of soluble muscle lysates from transgenics indicated a decrease in two M-band proteins, myomesin 1 (MYOM1) and muscle creatine kinase (CKM). Immunoprecipitation identified CKM as a MYOC binding partner. Our results suggest that binding of mutant MYOC to CKM is changing sarcomere ultrastructure and this may adversely impact muscle function. We speculate that a person carrying the mutant MYOC mutation will likely have a glaucoma phenotype and may also have undiagnosed muscle ailments or vice versa, both of which will have to be monitored and treated.

Molecular basis of the mechanical hierarchy in myomesin dimers for sarcomere integrity

Myomesin is one of the most important structural molecules constructing the M-band in the force-generating unit of striated muscle, and a critical structural maintainer of the sarcomere. Using molecular dynamics simulations, we here dissect the mechanical properties of the structurally known building blocks of myomesin, namely α-helices, immunoglobulin (Ig) domains, and the dimer interface at myomesin's 13th Ig domain, covering the mechanically important C-terminal part of the molecule. We find the interdomain α-helices to be stabilized by the hydrophobic interface formed between the N-terminal half of these helices and adjacent Ig domains, and, interestingly, to show a rapid unfolding and refolding equilibrium especially under low axial forces up to ∼ 15 pN. These results support and yield atomic details for the notion of recent atomic-force microscopy experiments, namely, that the unique helices inserted between Ig domains in myomesin function as elastomers and force buffers. Our results also explain how the C-terminal dimer of two myomesin molecules is mechanically outperforming the helices and Ig domains in myomesin and elsewhere, explaining former experimental findings. This study provides a fresh view onto how myomesin integrates elastic helices, rigid immunoglobulin domains, and an extraordinarily resistant dimer into a molecular structure, to feature a mechanical hierarchy that represents a firm and yet extensible molecular anchor to guard the stability of the sarcomere.

Loss of muscleblind-like 1 results in cardiac pathology and persistence of embryonic splice isoforms

Cardiac dysfunction is a prominent cause of mortality in myotonic dystrophy I (DM1), a disease where expanded CUG repeats bind and disable the muscleblind-like family of splice regulators. Deletion of muscleblind-like 1 (Mbnl1^ΔE2/ΔE2) in 129 sv mice results in QRS, QTc widening, bundle block and STc narrowing at 2–4 months of age. With time, cardiac function deteriorates further and at 6 months, decreased R wave amplitudes, sinus node dysfunction, cardiac hypertrophy, interstitial fibrosis, multi-focal myocardial fiber death and calcification manifest. Sudden death, where no end point illness is overt, is observed at a median age of 6.5 and 4.8 months in ~67% and ~86% of male and female Mbnl1^ΔE2/ΔE2 mice, respectively. Mbnl1 depletion results in the persistence of embryonic splice isoforms in a network of cardiac RNAs, some of which have been previously implicated in DM1, regulating sodium and calcium currents, Scn5a, Junctin, Junctate, Atp2a1, Atp11a, Cacna1s, Ryr2, intra and inter cellular transport, Clta, Stx2, Tjp1, cell survival, Capn3, Sirt2, Csda, sarcomere and cytoskeleton organization and function, Trim55, Mapt, Pdlim3, Pdlim5, Sorbs1, Sorbs2, Fhod1, Spag9 and structural components of the sarcomere, Myom1, Tnnt2, Zasp. Thus this study supports a key role for Mbnl1 loss in the initiation of DM1 cardiac disease.

Experimental and theoretical studies of mechanical unfolding of different proteins

Mechanical properties of proteins are important for a wide range of biological processes including cell adhesion, muscle contraction, and protein translocation across biological membranes. It is necessary to reveal how proteins achieve their required mechanical stability under natural conditions in order to understand the biological processes and also to use the knowledge for constructing new biomaterials for medical and industrial purposes. In this connection, it is important to know how a protein will behave in response to various impacts. Theoretical and experimental works on mechanical unfolding of globular proteins will be considered in detail in this review.

Making muscle elastic: the structural basis of myomesin stretching

The muscle M-band protein myomesin comprises a 36-nm long filament made of repetitive immunoglobulin–helix modules that can stretch to 2.5-fold this length, demonstrating substantial molecular elasticity.

Skeletal and cardiac muscles are remarkable biological machines that support and move our bodies and power the rhythmic work of our lungs and hearts. As well as producing active contractile force, muscles are also passively elastic, which is essential to their performance. The origins of both active contractile and passive elastic forces can be traced to the individual proteins that make up the highly ordered structure of muscle. In this Primer, we describe the organization of sarcomeres—the structural units that produce contraction—and the nature of the proteins that make muscle elastic. In particular, we focus on an elastic protein called myomesin, whose novel modular architecture helps explain elasticity.

Mechanical unfolding of cardiac myosin binding protein-C by atomic force microscopy

Cardiac myosin-binding protein-C (cMyBP-C) is a thick-filament-associated protein that performs regulatory and structural roles within cardiac sarcomeres. It is a member of the immunoglobulin (Ig) superfamily of proteins consisting of eight Ig- and three fibronectin (FNIII)-like domains, along with a unique regulatory sequence referred to as the M-domain, whose structure is unknown. Domains near the C-terminus of cMyBP-C bind tightly to myosin and mediate the association of cMyBP-C with thick (myosin-containing) filaments, whereas N-terminal domains, including the regulatory M-domain, bind reversibly to myosin S2 and/or actin. The ability of MyBP-C to bind to both myosin and actin raises the possibility that cMyBP-C cross-links myosin molecules within the thick filament and/or cross-links myosin and thin (actin-containing) filaments together. In either scenario, cMyBP-C could be under mechanical strain. However, the physical properties of cMyBP-C and its behavior under load are completely unknown. Here, we investigated the mechanical properties of recombinant baculovirus-expressed cMyBP-C using atomic force microscopy to assess the stability of individual cMyBP-C molecules in response to stretch. Force-extension curves showed the presence of long extensible segment(s) that became stretched before the unfolding of individual Ig and FNIII domains, which were evident as sawtooth peaks in force spectra. The forces required to unfold the Ig/FNIII domains at a stretch rate of 500 nm/s increased monotonically from ∼30 to ∼150 pN, suggesting a mechanical hierarchy among the different Ig/FNIII domains. Additional experiments using smaller recombinant proteins showed that the regulatory M-domain lacks significant secondary or tertiary structure and is likely an intrinsically disordered region of cMyBP-C. Together, these data indicate that cMyBP-C exhibits complex mechanical behavior under load and contains multiple domains with distinct mechanical properties.

Fast-folding alpha-helices as reversible strain absorbers in the muscle protein myomesin

The highly oriented filamentous protein network of muscle constantly experiences significant mechanical load during muscle operation. The dimeric protein myomesin has been identified as an important M-band component supporting the mechanical integrity of the entire sarcomere. Recent structural studies have revealed a long α-helical linker between the C-terminal immunoglobulin (Ig) domains My12 and My13 of myomesin. In this paper, we have used single-molecule force spectroscopy in combination with molecular dynamics simulations to characterize the mechanics of the myomesin dimer comprising immunoglobulin domains My12-My13. We find that at forces of approximately 30 pN the α-helical linker reversibly elongates allowing the molecule to extend by more than the folded extension of a full domain. High-resolution measurements directly reveal the equilibrium folding/unfolding kinetics of the individual helix. We show that α-helix unfolding mechanically protects the molecule homodimerization from dissociation at physiologically relevant forces. As fast and reversible molecular springs the myomesin α-helical linkers are an essential component for the structural integrity of the M band.

Alternative splicing of myomesin 1 gene is aberrantly regulated in myotonic dystrophy type 1

Myotonic dystrophy type 1 (DM1) is a multisystemic disease caused by a CTG repeat expansion in the 3'-UTR of dystrophia myotonica-protein kinase. Aberrant regulation of alternative splicing is a characteristic feature of DM. Dozens of genes have been found to be abnormally spliced; however, few reported splicing abnormalities explain the phenotypes of DM1 patients. Thus, we hypothesized that other, unknown abnormal splicing events exist. Here, by using exon array, we identified aberrant inclusion of myomesin 1 (MYOM1) exon 17a as a novel splicing abnormality in DM1 muscle. A cellular splicing assay with a MYOM1 minigene revealed that not only MBNL1-3 but also CELF1 and 2 decreased the inclusion of MYOM1 exon 17a in HEK293T cells. Expression of expanded CUG repeat impeded MBNL1 activity but did not affect CELF1 activity on the splicing of MYOM1 minigene. Our results suggest that the downregulation of MBNL proteins should lead to the abnormal splicing of MYOM1 exon 17a in DM1 muscle.

Calpain activity and muscle wasting in sepsis

Muscle wasting in sepsis reflects activation of multiple proteolytic mechanisms, including lyosomal and ubiquitin-proteasome-dependent protein breakdown. Recent studies suggest that activation of the calpain system also plays an important role in sepsis-induced muscle wasting. Perhaps the most important consequence of calpain activation in skeletal muscle during sepsis is disruption of the sarcomere, allowing for the release of myofilaments (including actin and myosin) that are subsequently ubiquitinated and degraded by the 26S proteasome. Other important consequences of calpain activation that may contribute to muscle wasting during sepsis include degradation of certain transcription factors and nuclear cofactors, activation of the 26S proteasome, and inhibition of Akt activity, allowing for downstream activation of Foxo transcription factors and GSK-3beta. The role of calpain activation in sepsis-induced muscle wasting suggests that the calpain system may be a therapeutic target in the prevention and treatment of muscle wasting during sepsis. Furthermore, because calpain activation may also be involved in muscle wasting caused by other conditions, including different muscular dystrophies and cancer, calpain inhibitors may be beneficial not only in the treatment of sepsis-induced muscle wasting but in other conditions causing muscle atrophy as well.

Molecular basis of the C-terminal tail-to-tail assembly of the sarcomeric filament protein myomesin

Sarcomeric filament proteins display extraordinary properties in terms of protein length and mechanical elasticity, requiring specific anchoring and assembly mechanisms. To establish the molecular basis of terminal filament assembly, we have selected the sarcomeric M-band protein myomesin as a prototypic filament model. The crystal structure of the myomesin C-terminus, comprising a tandem array of two immunoglobulin (Ig) domains My12 and My13, reveals a dimeric end-to-end filament of 14.3 nm length. Although the two domains share the same fold, an unexpected rearrangement of one beta-strand reveals how they are evolved into unrelated functions, terminal filament assembly (My13) and filament propagation (My12). The two domains are connected by a six-turn alpha-helix, of which two turns are void of any interactions with other protein parts. Thus, the overall structure of the assembled myomesin C-terminus resembles a three-body beads-on-the-string model with potentially elastic properties. We predict that the found My12-helix-My13 domain topology may provide a structural template for the filament architecture of the entire C-terminal Ig domain array My9-My13 of myomesin.

Regulation of skeletal muscle sarcomere integrity and postnatal muscle function by Mef2c

Myocyte enhancer factor 2 (MEF2) transcription factors cooperate with the MyoD family of basic helix-loop-helix (bHLH) transcription factors to drive skeletal muscle development during embryogenesis, but little is known about the potential functions of MEF2 factors in postnatal skeletal muscle. Here we show that skeletal muscle-specific deletion of Mef2c in mice results in disorganized myofibers and perinatal lethality. In contrast, neither Mef2a nor Mef2d is required for normal skeletal muscle development in vivo. Skeletal muscle deficient in Mef2c differentiates and forms normal myofibers during embryogenesis, but myofibers rapidly deteriorate after birth due to disorganized sarcomeres and a loss of integrity of the M line. Microarray analysis of Mef2c null muscles identified several muscle structural genes that depend on MEF2C, including those encoding the M-line-specific proteins myomesin and M protein. We show that MEF2C directly regulates myomesin gene transcription and that loss of Mef2c in skeletal muscle results in improper sarcomere organization. These results reveal a key role for Mef2c in maintenance of sarcomere integrity and postnatal maturation of skeletal muscle.

Mechanical unfolding of proteins: insights into biology, structure and folding

Since user-friendly atomic force microscopes came onto the market a few years ago, scientists have explored the response of many proteins to applied force. This field has now matured beyond the phenomenological with exciting recent developments, particularly with regards to research into biological questions. For example, detailed mechanistic studies have suggested how mechanically active proteins perform their functions. Also, in vitro forced unfolding has been compared with in vivo protein import and degradation. Additionally, investigations have been carried out that probe the relationship between protein structure and response to applied force, an area that has benefited significantly from synergy between experiments and simulations. Finally, recent technological developments offer exciting new avenues for experimental studies.

Charge-transfer complex study by chemical force spectroscopy: a dynamic force spectroscopic approach

Charge-transfer interaction, as a reversible and rapid phenomenon, was evidenced by force microscopy. Pull-off forces were measured between a tip grafted with a trinitrofluorenone derivative and a surface functionalized with an electron-rich aromatic anthracene compound in a dodecane environment. The effect of the sweep time on the measured interaction forces is described, together with an extensive study of a competitive influence of free aromatic molecules in dodecane diluted solutions. These forces depend on the nature of the competitor and its concentration as well as on the velocity of tip/sample separation.

Dynamic force spectroscopy of parallel individual Mucin1-antibody bonds

We used atomic force microscopy to measure the binding forces between Mucin1 (MUC1) peptide and a single-chain variable fragment (scFv) antibody selected from a scFv library screened against MUC1. This binding interaction is central to the design of molecules used for targeted delivery of radioimmunotherapeutic agents for prostate and breast cancer treatment. Our experiments separated the specific binding interaction from nonspecific interactions by tethering the antibody and MUC1 molecules to the atomic force microscope tip and sample surface with flexible polymer spacers. Rupture force magnitude and elastic characteristics of the spacers allowed identification of the rupture events corresponding to different numbers of interacting proteins. We used dynamic force spectroscopy to estimate the intermolecular potential widths and equivalent thermodynamic off rates for monovalent, bivalent, and trivalent interactions. Measured interaction potential parameters agree with the results of molecular docking simulation. Our results demonstrate that an increase of the interaction valency leads to a precipitous decline in the dissociation rate. Binding forces measured for monovalent and multivalent interactions match the predictions of a Markovian model for the strength of multiple uncorrelated bonds in a parallel configuration. Our approach is promising for comparison of the specific effects of molecular modifications as well as for determination of the best configuration of antibody-based multivalent targeting agents.

The nanomechanics of polycystin-1 extracellular region

Recent evidence suggests that polycystin-1 (PC1) acts as a mechanosensor, receiving signals from the primary cilia, neighboring cells, and extracellular matrix and transduces them into cellular responses that regulate proliferation, adhesion, and differentiation that are essential for the control of renal tubules and kidney morphogenesis. PC1 has an unusually long extracellular region ( approximately 3000 amino acids) with a multimodular structure. Proteins with a similar architecture have structural and mechanical roles. Based on the structural similarities between PC1 and other modular proteins that have elastic properties we hypothesized that PC1 functions mechanically by providing a flexible and elastic linkage between cells. Here we directly tested this hypothesis by analyzing the mechanical properties of the entire PC1 extracellular region by using single molecule force spectroscopy. We show that the PC1 extracellular region is highly extensible and that this extensibility is mainly caused by the unfolding of its Ig-like domains. Stretching the native PC1 extracellular region results in a sawtooth pattern with equally spaced force peaks that have a wide range of unfolding forces (50-200 pN). By combining single-molecule force spectroscopy and protein engineering techniques, we demonstrate that the sawtooth pattern in native PC1 extracellular region corresponds to the sequential unfolding of individual Ig-like domains. We found that Ig-like domains refold after mechanical unfolding. Hence, the PC1 extracellular region displays a dynamic extensibility whereby the resting length might be regulated through unfolding/refolding of its Ig-like domains. These force-driven reactions may be important for cell elasticity and the regulation of cell signaling events mediated by PC1.