Titin as a modular spring: emerging mechanisms for elasticity control by titin in cardiac physiology and pathophysiology

The contribution of N-terminal truncated cMyBPC to in vivo cardiac function

Cardiac myosin binding protein C (cMyBPC) is an 11-domain sarcomeric protein (C0-C10) integral to cardiac muscle regulation. In vitro studies have demonstrated potential functional roles for regions beyond the N-terminus. However, the in vivo contributions of these domains are mostly unknown. Therefore, we examined the in vivo consequences of expression of N-terminal truncated cMyBPC (C3C10). Neonatal cMyBPC-/- mice were injected with AAV9-full length (FL), C3C10 cMyBPC, or saline, and echocardiography was performed 6 wk after injection. We then isolated skinned myocardium from virus-treated hearts and performed mechanical experiments. Our results show that expression of C3C10 cMyBPC in cMyBPC-/- mice resulted in a 28% increase in systolic ejection fraction compared to saline-injected cMyBPC-/- mice and a 25% decrease in left ventricle mass-to-body weight ratio. However, unlike expression of FL cMyBPC, there was no prolongation of ejection time compared to saline-injected mice. In vitro mechanical experiments demonstrated that functional improvements in cMyBPC-/- mice expressing C3C10 were primarily due to a 35% reduction in the rate of cross-bridge recruitment at submaximal Ca2+ concentrations when compared to hearts from saline-injected cMyBPC-/- mice. However, unlike the expression of FL cMyBPC, there was no change in the rate of cross-bridge detachment when compared to saline-injected mice. Our data demonstrate that regions of cMyBPC beyond the N-terminus are important for in vivo cardiac function, and have divergent effects on cross-bridge behavior. Elucidating the molecular mechanisms of cMyBPC region-specific function could allow for development of targeted approaches to manipulate specific aspects of cardiac contractile function.

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.

Cardiac Sarcomere Signaling in Health and Disease

The cardiac sarcomere is a triumph of biological evolution wherein myriad contractile and regulatory proteins assemble into a quasi-crystalline lattice to serve as the central point upon which cardiac muscle contraction occurs. This review focuses on the many signaling components and mechanisms of regulation that impact cardiac sarcomere function. We highlight the roles of the thick and thin filament, both as necessary structural and regulatory building blocks of the sarcomere as well as targets of functionally impactful modifications. Currently, a new focus emerging in the field is inter-myofilament signaling, and we discuss here the important mediators of this mechanism, including myosin-binding protein C and titin. As the understanding of sarcomere signaling advances, so do the methods with which it is studied. This is reviewed here through discussion of recent live muscle systems in which the sarcomere can be studied under intact, physiologically relevant conditions.

Interaction of chloramphenicol with titin I27 probed using single-molecule force spectroscopy

Titin is a giant elastic protein which is responsible for passive muscle stiffness when muscle sarcomeres are stretched. Chloramphenicol, besides being a broad-spectrum antibiotic, also acts as a muscle relaxant. Therefore, it is important to study the interaction between titin I27 and chloramphenicol. We investigated the interaction of chloramphenicol with octamer of titin I27 using single-molecule force spectroscopy and fluorescence spectroscopy. The fluorescence data indicated that binding of chloramphenicol with I27 results in fluorescence quenching. Furthermore, it is observed that chloramphenicol binds to I27 at a particular concentration ([Formula: see text] 40 μM). Single-molecule force spectroscopy shows that, in the presence of 40 μM chloramphenicol concentration, the I27 monomers become mechanically stable, resulting in an increment of the unfolding force. The stability was further confirmed by chemical denaturation experiments on monomers of I27, which corroborate the evidence for enhanced mechanical stability at 40 μM drug concentration. The free energy of stabilization for I27 (wild type) was found to be 1.95 ± 0.93 kcal/mole and I27 with 40 μM drug was 3.25 ± 0.63 kcal/mole. The results show a direct effect of the broad-spectrum antibiotic chloramphenicol on the passive elasticity of muscle protein titin. The I27 is stabilized both mechanically and chemically by chloramphenicol.

Functional analysis of a gene-edited mouse model to gain insights into the disease mechanisms of a titin missense variant

Titin truncating variants are a well-established cause of cardiomyopathy; however, the role of titin missense variants is less well understood. Here we describe the generation of a mouse model to investigate the underlying disease mechanism of a previously reported titin A178D missense variant identified in a family with non-compaction and dilated cardiomyopathy. Heterozygous and homozygous mice carrying the titin A178D missense variant were characterised in vivo by echocardiography. Heterozygous mice had no detectable phenotype at any time point investigated (up to 1 year). By contrast, homozygous mice developed dilated cardiomyopathy from 3 months. Chronic adrenergic stimulation aggravated the phenotype. Targeted transcript profiling revealed induction of the foetal gene programme and hypertrophic signalling pathways in homozygous mice, and these were confirmed at the protein level. Unsupervised proteomics identified downregulation of telethonin and four-and-a-half LIM domain 2, as well as the upregulation of heat shock proteins and myeloid leukaemia factor 1. Loss of telethonin from the cardiac Z-disc was accompanied by proteasomal degradation; however, unfolded telethonin accumulated in the cytoplasm, leading to a proteo-toxic response in the mice.We show that the titin A178D missense variant is pathogenic in homozygous mice, resulting in cardiomyopathy. We also provide evidence of the disease mechanism: because the titin A178D variant abolishes binding of telethonin, this leads to its abnormal cytoplasmic accumulation. Subsequent degradation of telethonin by the proteasome results in proteasomal overload, and activation of a proteo-toxic response. The latter appears to be a driving factor for the cardiomyopathy observed in the mouse model.

Training Induced Changes to Skeletal Muscle Passive Properties Are Evident in Both Single Fibers and Fiber Bundles in the Rat Hindlimb

Introduction: The passive mechanical behavior of skeletal muscle represents both important and generally underappreciated biomechanical properties with little attention paid to their trainability. These experiments were designed to gain insight into the trainability of muscle passive mechanical properties in both single fibers and fiber bundles.

Methods: Rats were trained in two groups: 4 weeks of either uphill (UH) or downhill (DH) treadmill running; with a third group as sedentary control. After sacrifice, the soleus (SOL), extensor digitorum longus (EDL), and vastus intermedius (VI) were harvested. One hundred seventy-nine bundles and 185 fibers were tested and analyzed using a cumulative stretch-relaxation protocol to determine the passive stress and elastic modulus. Titin isoform expression was analyzed using sodium dodecyl sulfate vertical agarose gel electrophoresis (SDS-VAGE).

Results: Single fibers: passive modulus and stress were greater for the EDL at sarcomere lengths (SLs) ≥ 3.7 μm (modulus) and 4.0 μm (stress) with DH training compared to UH training and lesser for the SOL (SLs ≥ 3.3 μm) with DH training compared with control; there was no effect of UH training. Vastus intermedius was not affected by either training protocol. Fiber bundles: passive modulus and stress were greater for the EDL at SLs ≥ 2.5 μm (modulus) and 3.3 μm (stress) in the DH training group as compared with control, while no affects were observed in either the SOL or VI for either training group. No effects on titin isoform size were detected with training.

Conclusion: This study demonstrated that a trainability of passive muscle properties at both the single fiber and fiber bundle levels was not accompanied by any detectable changes to titin isoform size.

Nebulin and titin modulate cross-bridge cycling and length-dependent calcium sensitivity

Various mutations in the structural proteins nebulin and titin that are present in human disease are known to affect the contractility of striated muscle. Loss of nebulin is associated with reduced actin filament length and impairment of myosin binding to actin, whereas titin is thought to regulate muscle passive elasticity and is likely involved in length-dependent activation. Here, we sought to assess the modulation of muscle function by these sarcomeric proteins by using the computational platform muscle simulation code (MUSICO) to quantitatively separate the effects of structural changes, kinetics of cross-bridge cycling, and calcium sensitivity of the thin filaments. The simulations show that variation in thin filament length cannot by itself account for experimental observations of the contractility in nebulin-deficient muscle, but instead must be accompanied by a decreased myosin binding rate. Additionally, to match the observed calcium sensitivity, the rate of TnI detachment from actin needed to be increased. Simulations for cardiac muscle provided quantitative estimates of the effects of different titin-based passive elasticities on muscle force and activation in response to changes in sarcomere length and interfilament lattice spacing. Predicted force-pCa relations showed a decrease in both active tension and sensitivity to calcium with a decrease in passive tension and sarcomere length. We conclude that this behavior is caused by partial redistribution of the muscle load between active muscle force and titin-dependent passive force, and also by redistribution of stretch along the thin filament, which together modulate the release of TnI from actin. These data help advance understanding of how nebulin and titin mutations affect muscle function.

The multiple roles of titin in muscle contraction and force production

Titin is a filamentous protein spanning the half-sarcomere, with spring-like properties in the I-band region. Various structural, signaling, and mechanical functions have been associated with titin, but not all of these are fully elucidated and accepted in the scientific community. Here, I discuss the primary mechanical functions of titin, including its accepted role in passive force production, stabilization of half-sarcomeres and sarcomeres, and its controversial contribution to residual force enhancement, passive force enhancement, energetics, and work production in shortening muscle. Finally, I provide evidence that titin is a molecular spring whose stiffness changes with muscle activation and actin-myosin-based force production, suggesting a novel model of force production that, aside from actin and myosin, includes titin as a "third contractile" filament. Using this three-filament model of sarcomeres, the stability of (half-) sarcomeres, passive force enhancement, residual force enhancement, and the decrease in metabolic energy during and following eccentric contractions can be explained readily.

From stem cells to cardiomyocytes: the role of forces in cardiac maturation, aging, and disease

Stem cell differentiation into a variety of lineages is known to involve signaling from the extracellular niche, including from the physical properties of that environment. What regulates stem cell responses to these cues is there ability to activate different mechanotransductive pathways. Here, we will review the structures and pathways that regulate stem cell commitment to a cardiomyocyte lineage, specifically examining proteins within muscle sarcomeres, costameres, and intercalated discs. Proteins within these structures stretch, inducing a change in their phosphorylated state or in their localization to initiate different signals. We will also put these changes in the context of stem cell differentiation into cardiomyocytes, their subsequent formation of the chambered heart, and explore negative signaling that occurs during disease.

New titin (connectin) isoforms and their functional role in striated muscles of mammals: facts and suppositions

This review summarizes results of our studies on titin isoform composition in vertebrate striated muscles under normal conditions, during hibernation, real and simulated microgravity, and under pathological conditions (stiff-person syndrome, post-apoplectic spasticity, dilated cardiomyopathy, cardiac hypertrophy). Experimental evidence for the existence in mammalian striated muscles of higher molecular weight isoforms of titin (NT-isoforms) in addition to the known N2A-, N2BA-, and N2B-titin isoforms was obtained. Comparative studies of changes in titin isoform composition and structure-functional properties of human and animal striated muscles during adaptive and pathological processes led to a conclusion about the key role of NT-isoforms of titin in maintenance of sarcomere structure and contractile function of these muscles.

N-terminal phosphorylation of cardiac troponin-I reduces length-dependent calcium sensitivity of contraction in cardiac muscle

Protein kinase A (PKA) phosphorylation of myofibrillar proteins constitutes an important pathway for β-adrenergic modulation of cardiac contractility. In myofilaments PKA targets troponin I (cTnI), myosin binding protein-C (cMyBP-C) and titin. We studied how this affects the sarcomere length (SL) dependence of force-pCa relations in demembranated cardiac muscle. To distinguish cTnI from cMyBP-C/titin phosphorylation effects on the force-pCa relationship, endogenous troponin (Tn) was exchanged in rat ventricular trabeculae with either wild-type (WT) Tn, non-phosphorylatable cTnI (S23/24A) Tn or phosphomimetic cTnI (S23/24D) Tn. PKA cannot phosphorylate either cTnI S23/24 variant, leaving cMyBP-C/titin as PKA targets. Force was measured at 2.3 and 2.0 μm SL. Decreasing SL reduced maximal force (F(max)) and Ca(2+) sensitivity of force (pCa(50)) similarly with WT and S23/24A trabeculae. PKA treatment of WT and S23/24A trabeculae reduced pCa(50) at 2.3 but not at 2.0 μm SL, thus eliminating the SL dependence of pCa(50). In contrast, S23/24D trabeculae reduced pCa(50) at both SL values, primarily at 2.3 μm, also eliminating SL dependence of pCa(50). Subsequent PKA treatment moderately reduced pCa(50) at both SLs. At each SL, F(max) was unaffected by either Tn exchange and/or PKA treatment. Low-angle X-ray diffraction was performed to determine whether pCa(50) shifts were associated with changes in myofilament spacing (d(1,0)) or thick-thin filament interaction. PKA increased d(1,0) slightly under all conditions. The ratios of the integrated intensities of the equatorial X-ray reflections (I(1,1)/I(1,0)) indicate that PKA treatment increased crossbridge proximity to thin filaments under all conditions. The results suggest that phosphorylation by PKA of either cTnI or cMyBP-C/titin independently reduces the pCa(50) preferentially at long SL, possibly through reduced availability of thin filament binding sites (cTnI) or altered crossbridge recruitment (cMyBP-C/titin). Preferential reduction of pCa(50) at long SL may not reduce cardiac output during periods of high metabolic demand because of increased intracellular Ca(2+) during β-adrenergic stimulation.

Fiber Types in Mammalian Skeletal Muscles

Mammalian skeletal muscle comprises different fiber types, whose identity is first established during embryonic development by intrinsic myogenic control mechanisms and is later modulated by neural and hormonal factors. The relative proportion of the different fiber types varies strikingly between species, and in humans shows significant variability between individuals. Myosin heavy chain isoforms, whose complete inventory and expression pattern are now available, provide a useful marker for fiber types, both for the four major forms present in trunk and limb muscles and the minor forms present in head and neck muscles. However, muscle fiber diversity involves all functional muscle cell compartments, including membrane excitation, excitation-contraction coupling, contractile machinery, cytoskeleton scaffold, and energy supply systems. Variations within each compartment are limited by the need of matching fiber type properties between different compartments. Nerve activity is a major control mechanism of the fiber type profile, and multiple signaling pathways are implicated in activity-dependent changes of muscle fibers. The characterization of these pathways is raising increasing interest in clinical medicine, given the potentially beneficial effects of muscle fiber type switching in the prevention and treatment of metabolic diseases.

Muscle giants: molecular scaffolds in sarcomerogenesis

Myofibrillogenesis in striated muscles is a highly complex process that depends on the coordinated assembly and integration of a large number of contractile, cytoskeletal, and signaling proteins into regular arrays, the sarcomeres. It is also associated with the stereotypical assembly of the sarcoplasmic reticulum and the transverse tubules around each sarcomere. Three giant, muscle-specific proteins, titin (3-4 MDa), nebulin (600-800 kDa), and obscurin (approximately 720-900 kDa), have been proposed to play important roles in the assembly and stabilization of sarcomeres. There is a large amount of data showing that each of these molecules interacts with several to many different protein ligands, regulating their activity and localizing them to particular sites within or surrounding sarcomeres. Consistent with this, mutations in each of these proteins have been linked to skeletal and cardiac myopathies or to muscular dystrophies. The evidence that any of them plays a role as a "molecular template," "molecular blueprint," or "molecular ruler" is less definitive, however. Here we review the structure and function of titin, nebulin, and obscurin, with the literature supporting a role for them as scaffolding molecules and the contradictory evidence regarding their roles as molecular guides in sarcomerogenesis.

Non-linear myofilament elasticity in frog intact muscle fibres

The aim of the present investigation was to elucidate the elastic properties of the myofilaments during tetanic activity in striated muscle. The study was carried out on intact single muscle fibres from the anterior tibialis muscle of Rana temporaria (2.0-2.5 degrees C). The instantaneous stiffness was measured as the change in force that occurred in response to a high-frequency (2-4 kHz) length oscillation while the fibre was released to shorten against a pre-set constant load that ranged between 40 and 70% of maximum tetanic force in different experiments. Measurements of fibre stiffness were carried out, at a given load, both at 2.20 microm sarcomere length (S(2.20)), i.e. at full overlap between the thick and thin filaments, and at 2.60 microm sarcomere length (S(2.60)). The fact that the load on the fibre was constant during the stiffness measurements at the two sarcomere lengths implies that the stiffness of elastic elements, acting in series with the myofilaments, was constant at the two sarcomere lengths. The fibre stiffness was consistently lower at the extended sarcomere length, the S(2.60)/S(2.20) ratio ranging from 0.83 to 0.97 at the different loads investigated. Based on the S(2.60)/S(2.20) ratio, the compliance of the free portions of the thick and thin filaments could be calculated. The myofilament stiffness was found to increase progressively as the load was raised from 40 to 70% of maximum tetanic force. At 2.20 microm sarcomere length and at 40% of maximum load on the fibre, the calculated myofilament stiffness was approximately 2.5 times the maximum cross-bridge stiffness.

In indirect flight muscles Drosophila projectin has a short PEVK domain, and its NH2-terminus is embedded at the Z-band

Insect indirect flight muscles (IFM) contain a third filament system made up of elastic connecting or C-filaments. The giant protein projectin is the main, if not the only, component of these structures. In this study we found that projectin is oriented within the IFM sarcomere with its NH2-terminus embedded in the Z-bands. We demonstrate that this protein has an elastic region that can be detected by the movement of specific epitopes following stretch. One possible elastic region is the PEVK-like domain located close to the NH2-terminus. The amino acid length of this region is short, and 52% of its residues are P, E, V or K. We propose a model in which projectin extends from the Z-band to the lateral borders of the A-band. The PEVK-like domain and a series of Ig domains spanning the intervening I-band may provide the elastic properties of projectin.

Xtn3 is a developmentally expressed cardiac and skeletal muscle-specific novex-3 titin isoform

Titin proteins play an essential role in maintaining muscle function and structure. Recent work has implicated the involvement of the novex-3 titin isoform in sarcomere restructuring and disease. Unlike avian and mammalian systems, Xenopus laevis myogenesis is characterized by a wave of primary myogenesis followed by apoptosis of the primary muscles and formation of new muscles by secondary myogenesis. We show here that the Xenopus laevis novex-3 titin isoform (Xtn3) is developmentally expressed throughout the somites, heart, and primary muscles of the developing embryo. Downregulation of Xtn3 expression at tadpole stages appears to coincide with the change in myofiber composition from solely embryonic "fast" fiber types to myofibers containing both "fast" and "slow" fiber types. We demonstrate that Xtn3 is expressed early in the presomitic mesoderm and remains expressed in the somites, ventral myoblasts, and developing jaw muscles through late tailbud stage. Furthermore, we show that Xtn3 is expressed in the cardiac primordia prior to linear heart tube formation and remains expressed in the heart until tadpole stage, at which point it is downregulated in the heart except in discrete patches of cardiac cells. Finally, we demonstrate that Xtn3 transcripts are detectable in adult heart and muscle tissues.

Caenorhabditis elegans kettin, a large immunoglobulin-like repeat protein, binds to filamentous actin and provides mechanical stability to the contractile apparatuses in body wall muscle

Kettin is a large actin-binding protein with immunoglobulin-like (Ig) repeats, which is associated with the thin filaments in arthropod muscles. Here, we report identification and functional characterization of kettin in the nematode Caenorhabditis elegans. We found that one of the monoclonal antibodies that were raised against C. elegans muscle proteins specifically reacts with kettin (Ce-kettin). We determined the entire cDNA sequence of Ce-kettin that encodes a protein of 472 kDa with 31 Ig repeats. Arthropod kettins are splice variants of much larger connectin/titin-related proteins. However, the gene for Ce-kettin is independent of other connectin/titin-related genes. Ce-kettin localizes to the thin filaments near the dense bodies in both striated and nonstriated muscles. The C-terminal four Ig repeats and the adjacent non-Ig region synergistically bind to actin filaments in vitro. RNA interference of Ce-kettin caused weak disorganization of the actin filaments in body wall muscle. This phenotype was suppressed by inhibiting muscle contraction by a myosin mutation, but it was enhanced by tetramisole-induced hypercontraction. Furthermore, Ce-kettin was involved in organizing the cytoplasmic portion of the dense bodies in cooperation with alpha-actinin. These results suggest that kettin is an important regulator of myofibrillar organization and provides mechanical stability to the myofibrils during contraction.

Smooth muscle alpha-actinin interaction with smitin

Actin-myosin II filament-based contractile structures in striated muscle, smooth muscle, and nonmuscle cells also contain the actin filament-crosslinking protein alpha-actinin. In striated muscle sarcomeres, interactions between the myosin-binding protein titin and alpha-actinin in the Z-line provide an important structural linkage. We previously discovered a titin-like protein, smitin, associated with the contractile apparatus of smooth muscle cells. Purified native smooth muscle alpha-actinin binds with nanomolar affinity to smitin in smitin-myosin coassemblies in vitro. Smooth muscle alpha-actinin also interacts with striated muscle titin. In contrast to striated muscle alpha-actinin interaction with titin and smitin, which is significantly enhanced by PIP2, smooth muscle alpha-actinin interacts with smitin and titin equally well in the presence and absence of PIP2. Using expressed regions of smooth muscle alpha-actinin, we have demonstrated smitin-binding sites in the smooth muscle alpha-actinin R2-R3 spectrin-like repeat rod domain and a C-terminal domain formed by cryptic EF-hand structures. These smitin-binding sites are highly homologous to the titin-binding sites of striated muscle alpha-actinin. Our results suggest that direct interaction between alpha-actinin and titin or titin-like proteins is a common feature of actin-myosin II contractile structures in striated muscle and smooth muscle cells and that the molecular bases for alpha-actinin interaction with these proteins are similar, although regulation of these interactions may differ according to tissue.

Titin isoform-dependent effect of calcium on passive myocardial tension

We studied the effects of Ca2+ on titin (connectin)-based passive tension in skinned myocardium expressing either predominantly N2B titin (rat right ventricle, RRV) or predominantly N2BA titin (bovine left atrium, BLA). Actomyosin-based tension was abolished to undetectably low levels by selectively removing the thin filaments with a Ca2+-insensitive gelsolin fragment (FX-45). Myocardium was stretched in the presence and absence of Ca2+, and passive tension was measured. Ca2+ significantly increased passive tension during and after stretch in the BLA. The increase was insensitive to the actomyosin inhibitor 2,3-butanedione 2-monoxime, supporting the conclusion that the effect is titin based. Passive tension did not respond to calcium in the RRV, indicating that passive tension developed by N2B titin is calcium insensitive. Western blot analysis and immunofluorescence studies indicated that N2BA titin expresses E-rich PEVK motifs, whereas they are absent from N2B titin, supporting earlier single molecule studies that reported that E-rich motifs are required for calcium sensitivity. We conclude that calcium affects passive myocardial tension in a titin isoform-dependent manner.

Muscle-specific RING finger 1 is a bona fide ubiquitin ligase that degrades cardiac troponin I

Muscle-specific RING finger protein 1 (MuRF1) is a sarcomere-associated protein that is restricted to cardiac and skeletal muscle. In skeletal muscle, MuRF1 is up-regulated by conditions that provoke atrophy, but its function in the heart is not known. The presence of a RING finger in MuRF1 raises the possibility that it is a component of the ubiquitin-proteasome system of protein degradation. We performed a yeast two-hybrid screen to search for interaction partners of MuRF1 in the heart that might be targets of its putative ubiquitin ligase activity. This screen identified troponin I as a MuRF1 partner protein. MuRF1 and troponin I were found to associate both in vitro and in vivo in cultured cardiomyocytes. MuRF1 reduced steady-state troponin I levels when coexpressed in COS-7 cells and increased degradation of endogenous troponin I protein in cardiomyocytes. The degradation of troponin I in cardiomyocytes was associated with the accumulation of ubiquitylated intermediates of troponin I and was proteasome-dependent. In vitro, MuRF1 functioned as a ubiquitin ligase to catalyze ubiquitylation of troponin I through a RING finger-dependent mechanism. In isolated cardiomyocytes, MuRF1 reduced indices of contractility. In cardiomyocytes, these processes may determine the balance between hypertrophic and antihypertrophic signals and the regulation of myocyte contractile responses in the setting of heart failure.

The role of unstructured extensions in the rotational diffusion properties of a globular protein: the example of the titin i27 module

The possibility of predicting the overall shape of a macromolecule in solution from its diffusional properties has gained increasing importance in the structural genomic era. Here we explore and quantify the influence that unstructured and flexible regions have on the motions of a globular protein, a situation that can occur from the presence of such regions in the natural sequence or from additional tags. I27, an immunoglobulin-like module from the muscle protein titin, whose structure and properties are well characterized, was selected for our studies. The backbone dynamics and the overall tumbling of three different constructs of I27 were investigated using (15)N NMR relaxation collected at two (15)N frequencies (60.8 and 81.1 MHz) and fluorescence depolarization spectroscopy after labeling of a reactive cysteine with an extrinsic fluorophore. Our data show that the presence of disordered tags clearly exerts a frictional drag that increases with the length of the tags, thus affecting the module tumbling in solution. We discuss the use and the limitations of current approaches to hydrodynamic calculations, especially when having to take into account local flexibility.

Maternal nutrient restriction alters gene expression in the ovine fetal heart

Adequate maternal nutrient supply is critical for normal fetal organogenesis. We previously demonstrated that a global 50% nutrient restriction during the first half of gestation causes compensatory growth of both the left and right ventricles of the fetal heart by day 78 of gestation. Thus, it was hypothesized that maternal nutrient restriction significantly altered gene expression in the fetal cardiac left ventricle (LV). Pregnant ewes were randomly grouped into control (100% national research council (NRC) requirements) or nutrient-restricted groups (50% NRC requirements) from day 28 to day 78 of gestation, at which time fetal LV were collected. Fetal LV mRNA was used to construct a suppression subtraction cDNA library from which 11 cDNA clones were found by differential dot blot hybridization and virtual Northern analysis to be up-regulated by maternal nutrient restriction: caveolin, stathmin, G-1 cyclin, alpha-actin, titin, cardiac ankyrin repeat protein (CARP), cardiac-specific RNA-helicase activated by MEF2C (CHAMP), endothelial and smooth muscle derived neuropilin (ESDN), prostatic binding protein, NADH dehydrogenase subunit 2, and an unknown protein. Six of these clones (cardiac alpha-actin, cyclin G1, stathmin, NADH dehydrogenase subunit 2, titin and prostatic binding protein) have been linked to cardiac hypertrophy in other species including humans. Of the remaining clones, caveolin, CARP and CHAMP have been shown to inhibit remodelling of hypertrophic tissue. Compensatory growth of fetal LV in response to maternal undernutrition is concluded to be associated with increased transcription of genes related to cardiac hypertrophy, compensatory growth or remodelling. Counter-regulatory gene transcription may be increased, in part, as a response to moderating the degree of cardiac remodelling. The short- and long-term consequences of these changes in fetal heart gene expression and induction of specific homeostatic mechanisms in response to maternal undernutrition remain to be determined.