Measurements of half-life of rat cardiac myosin heavy chain with leucyl-tRNA used as precursor pool

Circadian regulation of cardiac muscle function and protein degradation

The circadian clock plays a fundamental role in physiology. In particular, the heart is a target organ where the clock orchestrates various aspects of cardiac function. At the molecular level, the clock machinery governs daily rhythms of gene expression. Such circadian regulation is in tune with the dynamic nature of heart structure and function, and provides the foundation for chronotherapeutic applications in cardiovascular diseases. In comparison, a regulatory role of the clock in cardiac protein degradation is poorly documented. Sarcomere is the structural and functional unit responsible for cardiac muscle contraction, and sarcomere components are closely regulated by protein folding and proteolysis. Emerging evidence supports a role of the circadian clock in governing sarcomere integrity and function. Particularly, recent studies uncovered a circadian regulation of a core sarcomere component TCAP. It is possible that circadian regulation of the cardiac muscle protein turnover is a key regulatory mechanism underlying cardiac remodeling in response to physiological and environmental stimuli. While the detailed regulatory mechanisms and the molecular links to cardiac (patho)physiology remain to be further studied, therapeutic strategies targeting circadian control in the heart may markedly enhance intervention outcomes against cardiovascular disease.

Insulin and Insulin-Like Growth Factor 1 Signaling Preserves Sarcomere Integrity in the Adult Heart

Insulin and insulin-like growth factor 1 (IGF1) signaling is transduced by insulin receptor substrate 1 (IRS1) and IRS2. To elucidate physiological and redundant roles of insulin and IGF1 signaling in adult hearts, we generated mice with inducible cardiomyocyte-specific deletion of insulin and IGF1 receptors or IRS1 and IRS2. Both models developed dilated cardiomyopathy, and most mice died by 8 weeks post-gene deletion. Heart failure was characterized by cardiomyocyte loss and disarray, increased proapoptotic signaling, and increased autophagy. Suppression of autophagy by activating mTOR signaling did not prevent heart failure. Transcriptional profiling revealed reduced serum response factor (SRF) transcriptional activity and decreased mRNA levels of genes encoding sarcomere and gap junction proteins as early as 3 days post-gene deletion, in concert with ultrastructural evidence of sarcomere disruption and intercalated discs within 1 week after gene deletion. These data confirm conserved roles for constitutive insulin and IGF1 signaling in suppressing autophagic and apoptotic signaling in the adult heart. The present study also identifies an unexpected role for insulin and IGF1 signaling in regulating an SRF-mediated transcriptional program, which maintains expression of genes encoding proteins that support sarcomere integrity in the adult heart, reduction of which results in rapid development of heart failure.

Cardiac Myosin Filaments are Maintained by Stochastic Protein Replacement

Myosin and myosin-binding protein C are exquisitely organized into giant filamentous macromolecular complexes within cardiac muscle sarcomeres, yet these proteins must be continually replaced to maintain contractile fidelity. The overall hypothesis that myosin filament structure is dynamic and allows for the stochastic replacement of individual components was tested in vivo, using a combination of mass spectrometry- and fluorescence-based proteomic techniques. Adult mice were fed a diet that marked all newly synthesized proteins with a stable isotope-labeled amino acid. The abundance of unlabeled and labeled proteins was quantified by high-resolution mass spectrometry over an 8-week period. The rates of change in the abundance of these proteins were well described by analytical models in which protein synthesis defined stoichiometry and protein degradation was governed by the stochastic selection of individual molecules. To test whether the whole myosin filaments or the individual components were selected for replacement, cardiac muscle was chemically skinned to remove the cellular membrane and myosin filaments were solubilized with ionic solutions. The composition of the filamentous and soluble fractions was quantified by mass spectrometry, and filament depolymerization was visualized by real-time fluorescence microscopy. Myosin molecules were preferentially extracted from ends of the filaments in the presence of the ionic solutions, and there was only a slight bias in the abundance of unlabeled molecules toward the innermost region on the myosin filaments. These data demonstrate for the first time that the newly synthesized myosin and myosin-binding protein C molecules are randomly mixed into preexisting thick filaments in vivo and the rate of mixing may not be equivalent along the length of the thick filament. These data collectively support a new model of cardiac myosin filament structure, with the filaments being dynamic macromolecular assemblies that allow for replacement of their components, rather than rigid bodies.

Under construction: The dynamic assembly, maintenance, and degradation of the cardiac sarcomere

The sarcomere is the basic contractile unit of striated muscle and is a highly ordered protein complex with the actin and myosin filaments at its core. Assembling the sarcomere constituents into this organized structure in development, and with muscle growth as new sarcomeres are built, is a complex process coordinated by numerous factors. Once assembled, the sarcomere requires constant maintenance as its continuous contraction is accompanied by elevated mechanical, thermal, and oxidative stress, which predispose proteins to misfolding and toxic aggregation. To prevent protein misfolding and maintain sarcomere integrity, the sarcomere is monitored by an assortment of protein quality control (PQC) mechanisms. The need for effective PQC is heightened in cardiomyocytes which are terminally differentiated and must survive for many years while preserving optimal mechanical output. To prevent toxic protein aggregation, molecular chaperones stabilize denatured sarcomere proteins and promote their refolding. However, when old and misfolded proteins cannot be salvaged by chaperones, they must be recycled via degradation pathways: the calpain and ubiquitin-proteasome systems, which operate under basal conditions, and the stress-responsive autophagy-lysosome pathway. Mutations to and deficiency of the molecular chaperones and associated factors charged with sarcomere maintenance commonly lead to sarcomere structural disarray and the progression of heart disease, highlighting the necessity of effective sarcomere PQC for maintaining cardiac function. This review focuses on the dynamic regulation of assembly and turnover at the sarcomere with an emphasis on the chaperones involved in these processes and describes the alterations to chaperones - through mutations and deficient expression - implicated in disease progression to heart failure.

The biochemical and metabolic profiles of dairy cows with mycotoxins-contaminated diets

BACKGROUND

Previous studies on the effects of mycotoxins have solely focused on their biochemical profiles or products in dairy ruminants. Changes in metabolism that occur after exposure to mycotoxins, as well as biochemical changes, have not been explored.

METHODS

We measured the biochemical and metabolic changes in dairy cows after exposure to mycotoxins using biochemical analyses and nuclear magnetic resonance. Twenty-four dairy cows were randomly assigned to three different treatment groups. Control cows received diets with 2 kg uncontaminated cottonseed. Cows in the 50% replacement group received the same diet as the control group, but with 1 kg of uncontaminated cottonseed and 1 kg of cottonseed contaminated with mycotoxins. Cows in the 100% replacement group received the same diet as the control, but with 2 kg contaminated cottonseed.

RESULTS

The results showed that serum γ-glutamyl transpeptidase and total antioxidant capacities were significantly affected by cottonseed contaminated with mycotoxins. There were also significant differences in isovalerate and NH3-N levels, and significant differences in the eight plasma metabolites among the three groups. These metabolites are mainly involved in amino acid metabolism pathways. Therefore, the results suggest that amino acid metabolism pathways may be affected by mycotoxins exposure.

Cardiac regeneration and remodelling of the cardiomyocyte cytoarchitecture

Adult mammals are unable to regenerate their hearts after cardiac injury, largely due to the incapacity of cardiomyocytes (CMs) to undergo cell division. However, mammalian embryonic and fetal CMs, similar to CMs from fish and amphibians during their entire life, exhibit robust replicative activity, which stops abruptly after birth and never significantly resumes. Converging evidence indicates that formation of the highly ordered and stable cytoarchitecture of mammalian mature CMs is coupled with loss of their proliferative potential. Here, we review the available information on the role of the cardiac cytoskeleton and sarcomere in the regulation of CM proliferation. The actin cytoskeleton, the intercalated disc, the microtubular network and the dystrophin-glycoprotein complex each sense mechanical cues from the surrounding environment. Furthermore, they participate in the regulation of CM proliferation by impinging on the yes-associated protein/transcriptional co-activator with PDZ-binding motif, β-catenin and myocardin-related transcription factor transcriptional co-activators. Mastering the molecular mechanisms regulating CM proliferation would permit the development of innovative strategies to stimulate cardiac regeneration in adult individuals, a hitherto unachieved yet fundamental therapeutic goal.

YAP and TAZ limit cytoskeletal and focal adhesion maturation to enable persistent cell motility

The importance of transcription during cell motility is controversial. Mason et al. show that YAP/TAZ-mediated transcription is required to limit cytoskeletal tension generation and permit persistent cell motility. This pathway is defined as a negative feedback loop whereby Rho-ROCK-myosin activate YAP and TAZ, which limit myosin activation through NUAK2 expression.

Cell migration initiates by traction generation through reciprocal actomyosin tension and focal adhesion reinforcement, but continued motility requires adaptive cytoskeletal remodeling and adhesion release. Here, we asked whether de novo gene expression contributes to this cytoskeletal feedback. We found that global inhibition of transcription or translation does not impair initial cell polarization or migration initiation, but causes eventual migratory arrest through excessive cytoskeletal tension and over-maturation of focal adhesions, tethering cells to their matrix. The transcriptional coactivators YAP and TAZ mediate this feedback response, modulating cell mechanics by limiting cytoskeletal and focal adhesion maturation to enable persistent cell motility and 3D vasculogenesis. Motile arrest after YAP/TAZ ablation was partially rescued by depletion of the YAP/TAZ-dependent myosin phosphatase regulator, NUAK2, or by inhibition of Rho-ROCK-myosin II. Together, these data establish a transcriptional feedback axis necessary to maintain a responsive cytoskeletal equilibrium and persistent migration.

Altered myocyte contractility and calcium homeostasis in alpha-myosin heavy chain point mutations linked to familial dilated cardiomyopathy

Mutations in the human cardiac motor protein beta-myosin heavy chain (βMHC) have been long recognized as a cause of familial hypertrophic cardiomyopathy. Recently, mutations (P830L and A1004S) in the less abundant but faster isoform alpha-myosin heavy chain (αMHC) have been linked to dilated cardiomyopathy (DCM). In this study, we sought to determine the cellular contractile phenotype associated with these point mutations. Ventricular myocytes were isolated from 2 month male Sprague Dawley rats. Cells were cultured in M199 media and infected with recombinant adenovirus containing the P830L or the A1004S mutant human αMHC at a MOI of 500 for 18 h. Uninfected cells (UI), human βMHC (MOI 500, 18 h), and human αMHC (MOI 500, 18 h) were used as controls. Cells were loaded with fura-2 (1 μM, 15 min) after 48 h. Sarcomere shortening and calcium transients were recorded in CO₂ buffered M199 media (36°±1 C) with and without 10 nM isoproterenol (Iso). The A1004S mutation resulted in decreased peak sarcomere shortening while P830L demonstrated near normal shortening kinetics at baseline. In the presence of Iso, the A1004S sarcomere shortening was identical to the βMHC shortening while the P830L was identical to the αMHC control. All experimental groups had identical calcium transients. Despite a shared association with DCM, the P830L and A1004S αMHC mutations alter myocyte contractility in completely different ways while at the same preserving peak intracellular calcium.

Engineering Circular Gliding of Actin Filaments Along Myosin-Patterned DNA Nanotube Rings To Study Long-Term Actin-Myosin Behaviors

Nature has evolved molecular motors that are critical in cellular processes occurring over broad time scales, ranging from seconds to years. Despite the importance of the long-term behavior of molecular machines, topics such as enzymatic lifetime are underexplored due to the lack of a suitable approach for monitoring motor activity over long time periods. Here, we developed an "O"-shaped Myosin Empowered Gliding Assay (OMEGA) that utilizes engineered micron-scale DNA nanotube rings with precise arrangements of myosin VI to trap gliding actin filaments. This circular gliding assay platform allows the same individual actin filament to glide over the same myosin ensemble (50-1000 motors per ring) multiple times. First, we systematically characterized the formation of DNA nanotubes rings with 4, 6, 8, and 10 helix circumferences. Individual actin filaments glide along the nanotube rings with high processivity for up to 12.8 revolutions or 11 min in run time. We then show actin gliding speed is robust to variation in motor number and independent of ring curvature within our sample space (ring diameter of 0.5-4 μm). As a model application of OMEGA, we then analyze motor-based mechanical influence on "stop-and-go" gliding behavior of actin filaments, revealing that the stop-to-go transition probability is dependent on motor flexibility. Our circular gliding assay may provide a closed-loop platform for monitoring long-term behavior of broad classes of molecular motors and enable characterization of motor robustness and long time scale nanomechanical processes.

Proteomics of muscle chronological ageing in post-menopausal women

Background

Muscle ageing contributes to both loss of functional autonomy and increased morbidity. Muscle atrophy accelerates after 50 years of age, but the mechanisms involved are complex and likely result from the alteration of a variety of interrelated functions. In order to better understand the molecular mechanisms underlying muscle chronological ageing in human, we have undertaken a top-down differential proteomic approach to identify novel biomarkers after the fifth decade of age.

Results

Muscle samples were compared between adult (56 years) and old (78 years) post-menopausal women. In addition to total muscle extracts, low-ionic strength extracts were investigated to remove high abundance myofibrillar proteins and improve the detection of low abundance proteins. Two-dimensional gel electrophoreses with overlapping IPGs were used to improve the separation of muscle proteins. Overall, 1919 protein spots were matched between all individuals, 95 were differentially expressed and identified by mass spectrometry, and they corresponded to 67 different proteins. Our results suggested important modifications in cytosolic, mitochondrial and lipid energy metabolism, which may relate to dysfunctions in old muscle force generation. A fraction of the differentially expressed proteins were linked to the sarcomere and cytoskeleton (myosin light-chains, troponin T, ankyrin repeat domain-containing protein-2, vinculin, four and a half LIM domain protein-3), which may account for alterations in contractile properties. In line with muscle contraction, we also identified proteins related to calcium signal transduction (calsequestrin-1, sarcalumenin, myozenin-1, annexins). Muscle ageing was further characterized by the differential regulation of several proteins implicated in cytoprotection (catalase, peroxiredoxins), ion homeostasis (carbonic anhydrases, selenium-binding protein 1) and detoxification (aldo-keto reductases, aldehyde dehydrogenases). Notably, many of the differentially expressed proteins were central for proteostasis, including heat shock proteins and proteins involved in proteolysis (valosin-containing protein, proteasome subunit beta type-4, mitochondrial elongation factor-Tu).

Conclusions

This study describes the most extensive proteomic analysis of muscle ageing in humans, and identified 34 new potential biomarkers. None of them were previously recognized as differentially expressed in old muscles, and each may represent a novel starting point to elucidate the mechanisms of muscle chronological ageing in humans.

Overexpression of smooth muscle myosin heavy chain leads to activation of the unfolded protein response and autophagic turnover of thick filament-associated proteins in vascular smooth muscle cells

Duplications spanning nine genes at the genomic locus 16p13.1 predispose individuals to acute aortic dissections. The most likely candidate gene in this region leading to the predisposition for dissection is MYH11, which encodes smooth muscle myosin heavy chain (SM-MHC). The effects of increased expression of MYH11 on smooth muscle cell (SMC) phenotypes were explored using mouse aortic SMCs with transgenic overexpression of one isoform of SM-MHC. We found that these cells show increased expression of Myh11 and myosin filament-associated contractile genes at the message level when compared with control SMCs, but not at the protein level due to increased protein degradation. Increased expression of Myh11 resulted in endoplasmic reticulum (ER) stress in SMCs, which led to a paradoxical decrease of protein levels through increased autophagic degradation. An additional consequence of ER stress in SMCs was increased intracellular calcium ion concentration, resulting in increased contractile signaling and contraction. The increased signals for contraction further promote transcription of contractile genes, leading to a feedback loop of metabolic abnormalities in these SMCs. We suggest that overexpression of MYH11 can lead to increased ER stress and autophagy, findings that may be globally implicated in disease processes associated with genomic duplications.

Label-free quantitative protein profiling of vastus lateralis muscle during human aging

Sarcopenia corresponds to the loss of muscle mass occurring during aging, and is associated with a loss of muscle functionality. Proteomic links the muscle functional changes with protein expression pattern. To better understand the mechanisms involved in muscle aging, we performed a proteomic analysis of Vastus lateralis muscle in mature and older women. For this, a shotgun proteomic method was applied to identify soluble proteins in muscle, using a combination of high performance liquid chromatography and mass spectrometry. A label-free protein profiling was then conducted to quantify proteins and compare profiles from mature and older women. This analysis showed that 35 of the 366 identified proteins were linked to aging in muscle. Most of the proteins were under-represented in older compared with mature women. We built a functional interaction network linking the proteins differentially expressed between mature and older women. The results revealed that the main differences between mature and older women were defined by proteins involved in energy metabolism and proteins from the myofilament and cytoskeleton. This is the first time that label-free quantitative proteomics has been applied to study of aging mechanisms in human skeletal muscle. This approach highlights new elements for elucidating the alterations observed during aging and may lead to novel sarcopenia biomarkers.

Autophagy in toxicology: self-consumption in times of stress and plenty

Autophagy is a critical cellular process orchestrating the lysosomal degradation of cellular components in order to maintain cellular homeostasis and respond to cellular stress. A growing research effort over the last decade has proven autophagy to be essential for constitutive protein and organelle turnover, for embryonic/neonatal survival and for cell survival during conditions of environmental stress. Emphasizing its biological importance, dysfunctional autophagy contributes to a diverse set of human diseases. Cellular stress induced by xenobiotic exposure typifies environmental stress, and can result in the induction of autophagy as a cytoprotective mechanism. An increasing number of xenobiotics are notable for their ability to modulate the induction or the rate of autophagy. The role of autophagy in normal cellular homeostasis, the intricate relationship between cellular stress and the induction of autophagy, and the identification of specific xenobiotics capable of modulating autophagy, point to the importance of the autophagic process in toxicology. This review will summarize the importance of autophagy and its role in cellular response to stress, including examples in which consideration of autophagy has contributed to a more complete understanding of toxicant-perturbed systems.

Tail muscle parvalbumin content is decreased in chronic sacral spinal cord injured rats with spasticity

In rats, chronic sacral spinal isolation eliminates both descending and afferent inputs to motoneurons supplying the segmental tail muscles, eliminating daily tail muscle EMG activity. In contrast, chronic sacral spinal cord transection preserves afferent inputs, causing tail muscle spasticity that generates quantitatively normal daily EMG. Compared with normal rats, rats with spinal isolation and transection/spasticity provide a chronic model of progressive neuromuscular injury. Using normal, spinal isolated and spastic rats, we characterized the activity dependence of calcium-handling protein expression for parvalbumin, fast sarco(endo)plasmic reticulum Ca(2+)-ATPase (SERCA1) and slow SERCA2. As these proteins may influence fatigue resistance, we also assayed the activities of oxidative (citrate synthase; CS) and glycolytic enzymes (glyceraldehyde phosphate dehydrogenase; GAPDH). We hypothesized that, compared with normal rats, chronic isolation would cause decreased parvalbumin, SERCA1 and SERCA2 expression and CS and GAPDH activities. We further hypothesized that chronic spasticity would promote recovery of parvalbumin, SERCA1 and SERCA2 expression and of CS and GAPDH activities. Parvalbumin, SERCA1 and SERCA2 were quantified with Western blotting. Citrate synthase and GAPDH activities were quantified photometrically. Compared with normal rats, spinal isolation caused large decreases in parvalbumin (95%), SERCA1 (70%) and SERCA2 (68%). Compared with spinal isolation, spasticity promoted parvalbumin recovery (ninefold increase) and a SERCA2-to-SERCA1 transformation (84% increase in the ratio of SERCA1 to SERCA2). Compared with normal values, CS and GAPDH activities decreased in isolated and spastic muscles. In conclusion, with complete paralysis due to spinal isolation, parvalbumin expression is nearly eliminated, but with muscle spasticity after spinal cord transection, parvalbumin expression partly recovers. Additionally, spasticity after transection causes a slow-to-fast SERCA isoform transformation that may be compensatory for decreased parvalbumin content.

Correction of radiolabel pulse-chase data by a mathematical model: application to mitochondrial turnover studies

Metabolic labelling pulse-chase experiments are important means to study molecular turnover rates. However, the inherent problem associated with the method is precursor re-utilization, which can cause a significant overestimation of the actual rates of molecular degradation. In published studies on mitochondrial degradation, this problem has led to widely differing results. Practically, the extra information required to correct these errors is not easy to obtain. Using an example of a mitochondrial protein degradation study with NaH(14)CO(3) as the precursor label, we explain the limitations of the method and our approaches to mathematical correction. A dynamic model, including error, used the full power of the data and resulted in sensitive and specific distributed parameter estimates, helping to reduce numbers of experimental animals. This example has important implications not only for similar pulse-chase experiments, but also in a more general context where comparable types of data are generated.

Gene transfer, expression, and sarcomeric incorporation of a headless myosin molecule in cardiac myocytes: evidence for a reserve in myofilament motor function

The purpose of this study was to implement a living myocyte in vitro model system to test whether a motor domain-deleted headless myosin construct could be incorporated into the sarcomere and affect contractility. To this end we used gene transfer to express a "headless" myosin heavy chain (headless-MHC) in complement with the native full-length myosin motors in the cardiac sarcomere. An NH2-terminal Flag epitope was used for unique detection of the motor domain-deleted headless-MHC. Total MHC content (i.e., headless-MHC+endogenous MHC) remained constant, while expression of the headless-MHC in transduced myocytes increased from 24 to 72 h after gene transfer until values leveled off at 96 h after gene transfer, at which time the headless-MHC comprised ∼20% of total MHC. Moreover, immunofluorescence labeling and confocal imaging confirmed expression and demonstrated incorporation of the headless-MHC in the A band of the cardiac sarcomere. Functional measurements in intact myocytes showed that headless-MHC modestly reduced amplitude of dynamic twitch contractions compared with controls (P<0.05). In chemically permeabilized myocytes, maximum steady-state isometric force and the tension-pCa relationship were unaltered by the headless-MHC. These data suggest that headless-MHC can express to 20% of total myosin and incorporate into the sarcomere yet have modest to no effects on dynamic and steady-state contractile function. This would indicate a degree of functional tolerance in the sarcomere for nonfunctional myosin molecules.

AFos inhibits phenylephrine-mediated contractile dysfunction by altering phospholamban phosphorylation

Using neonatal rat ventricular myocytes, we previously reported that the expression of a dominant negative form of the c-Fos proto-oncogene (AFos) inhibited activator protein 1 activity and blocked the induction of the pathological gene profile stimulated by phenylephrine (PE) while leaving growth unaffected. We now extend these observations to the adult rat ventricular myocyte (ARVM) to understand the relationship between gene expression, growth, and function. Ventricular myocytes were isolated from adult rats and infected with adenovirus expressing beta-galactosidase (control) or AFos. The cells were subsequently treated with PE, and protein synthesis, gene program, calcium transients, and contractility were evaluated. As seen with the neonatal rat ventricular myocytes, in control cells PE stimulated an increase in protein synthesis, induced the pathological gene profile, and exhibited both depressed contractility and calcium transients. Although ARVMs expressing AFos still had PE-induced growth, pathological gene expression as well as contractility and calcium handling abnormalities were inhibited. To determine a possible mechanism of the preserved myocyte function in AFos-expressing cells, we examined phospholamban (PLB) and sarco(endo)plasmic reticulum calcium-ATPase proteins. Although there was no change in total PLB or sarco(endo)plasmic reticulum calcium-ATPase expression in response to PE treatment, PE decreased the phosphorylation of PLB at serine-16, an observation that was prevented in AFos-expressing cells. In conclusion, although PE-induced growth was unaffected in AFos-expressing ARVMs, the expression of the pathological gene profile was inhibited and both contractile function and calcium cycling were preserved. The inhibition of functional deterioration was, in part, due to the preservation of PLB phosphorylation.

Carbonylation of myosin heavy chains in rat heart during diabetes

Cardiac inotropy progressively declines during diabetes mellitus. To date, the molecular mechanisms underlying this defect remain incompletely characterized. This study tests the hypothesis that ventricular myosin heavy chains (MHC) undergo carbonylation by reactive carbonyl species (RCS) during diabetes and these modifications contribute to the inotropic decline. Male Sprague-Dawley rats were injected with streptozotocin (STZ). Fourteen days later the animals were divided into two groups: one group was treated with the RCS blocker aminoguanidine for 6 weeks, while the other group received no treatment. After 8 weeks of diabetes, cardiac ejection fraction, fractional shortening, left ventricular pressure development (+dP/dt) and myocyte shortening were decreased by 9%, 16%, 34% and 18%, respectively. Ca(2+)- and Mg(2+)-actomyosin ATPase activities and peak actomyosin syneresis were also reduced by 35%, 28%, and 72%. MHC-alpha to MHC-beta ratio was 12:88. Mass spectrometry and Western blots revealed the presence of carbonyl adducts on MHC-alpha and MHC-beta. Aminoguanidine treatment did not alter MHC composition, but it blunted formation of carbonyl adducts and decreases in actomyosin Ca(2+)-sensitive ATPase activity, syneresis, myocyte shortening, cardiac ejection fraction, fractional shortening and +dP/dt induced by diabetes. From these new data it can be concluded that in addition to isozyme switching, modification of MHC by RCS also contributes to the inotropic decline seen during diabetes.

Effects of myosin heavy chain manipulation in experimental heart failure

The myosin heavy chain (MHC) isoforms, alpha- and beta-MHC, are expressed in developmental- and chamber-specific patterns. Healthy human ventricle contains approximately 2-10% alpha-MHC and these levels are reduced even further in the failing ventricle. While down-regulation of alpha-MHC in failing myocardium is considered compensatory, we previously demonstrated that persistent transgenic (TG) alpha-MHC expression in the cardiomyocytes is cardioprotective in rabbits with tachycardia-induced cardiomyopathy (TIC). We sought to determine if this benefit extends to other types of experimental heart failure and focused on two models relevant to human heart failure: myocardial infarction (MI) and left ventricular pressure overload. TG and nontransgenic rabbits underwent either coronary artery ligation at 8 months or aortic banding at 10 days of age. The effects of alpha-MHC expression were assessed at molecular, histological and organ levels. In the MI experiments, we unexpectedly found modest functional advantages to alpha-MHC expression. In contrast, despite subtle benefits in TG rabbits subjected to aortic banding, cardiac function was minimally affected. We conclude that the benefits of persistent alpha-MHC expression depend upon the mechanism of heart failure. Importantly, in none of the scenarios studied did we find any detrimental effects associated with persistent alpha-MHC expression. Thus manipulation of MHC composition may be beneficial in certain types of heart failure and does not appear to compromise heart function in others. Future considerations of myosin isoform manipulation as a therapeutic strategy should consider the underlying etiology of cardiac dysfunction.

Designing heart performance by gene transfer

The birth of molecular cardiology can be traced to the development and implementation of high-fidelity genetic approaches for manipulating the heart. Recombinant viral vector-based technology offers a highly effective approach to genetically engineer cardiac muscle in vitro and in vivo. This review highlights discoveries made in cardiac muscle physiology through the use of targeted viral-mediated genetic modification. Here the history of cardiac gene transfer technology and the strengths and limitations of viral and nonviral vectors for gene delivery are reviewed. A comprehensive account is given of the application of gene transfer technology for studying key cardiac muscle targets including Ca(2+) handling, the sarcomere, the cytoskeleton, and signaling molecules and their posttranslational modifications. The primary objective of this review is to provide a thorough analysis of gene transfer studies for understanding cardiac physiology in health and disease. By comparing results obtained from gene transfer with those obtained from transgenesis and biophysical and biochemical methodologies, this review provides a global view of cardiac structure-function with an eye towards future areas of research. The data presented here serve as a basis for discovery of new therapeutic targets for remediation of acquired and inherited cardiac diseases.

Build it up-Tear it down: protein quality control in the cardiac sarcomere

The assembly and maintenance of the cardiac sarcomere, which contains the basic contractile components of actin and myosin, are essential for cardiac function. While often described as a static structure, the sarcomere is actually dynamic and undergoes constant turnover, allowing it to adapt to physiological changes while still maintaining function. A host of new factors have been identified that play a role in the regulation of protein quality control in the sarcomere, including chaperones that mediate the assembly of sarcomere components and ubiquitin ligases that control their specific degradation. There is clear evidence of sarcomere disorganization in animal models lacking muscle-specific chaperone proteins, illustrating the importance of these molecules in sarcomere structure and function. Although ubiquitin ligases have been found within the sarcomere structure itself, the role of the ubiquitin proteasome system in cardiac sarcomere regulation, and the factors that control its activity, are only just now being elucidated. The number of ubiquitin ligases identified with specificity for sarcomere proteins, each with distinct target substrates, is growing, allowing for tight regulation of this system. In this review, we highlight the dynamic interplay between sarcomere-specific chaperones and ubiquitin-dependent degradation of sarcomere proteins that is necessary in order to maintain structure and function of the cardiac sarcomere.

Peroxynitrite inhibits myofibrillar protein function in an in vitro assay of motility

We determined the effects of peroxynitrite (ONOO-) on cardiac myosin, actin, and thin filaments in order to more clearly understand the impact of this reactive compound in ischemia/reperfusion injury and heart failure. Actin filaments, native thin filaments, and alpha-cardiac myosin from rat hearts were exposed to ONOO- in the presence of 2 mM bicarbonate. Filament velocities over myosin, calcium sensitivity, and relative force generated by myosin were assessed in an in vitro motility assay in the absence of reducing agents. ONOO- concentrations > or =10 microM significantly reduced the velocities of thin filaments or bare actin filaments over alpha-cardiac myosin when any of these proteins were exposed individually. These functional deficits were linearly related to the degree of tyrosine nitration, with myosin being the most sensitive. However, at 10 microM ONOO- the calcium sensitivity of thin filaments remained unchanged. Cotreatment of myosin and thin filaments, analogous to the in vivo situation, resulted in a significantly greater functional deficit. The load supported by myosin after ONOO- exposure was estimated using mixtures experiments to be increased threefold. These data suggest that nitration of myofibrillar proteins can contribute to cardiac contractile dysfunction in pathologic states in which ONOO- is liberated.

Calcium-independent negative inotropy by beta-myosin heavy chain gene transfer in cardiac myocytes

Increased relative expression of the slow molecular motor of the heart (beta-myosin heavy chain [MyHC]) is well known to occur in many rodent models of cardiovascular disease and in human heart failure. The direct effect of increased relative beta-MyHC expression on intact cardiac myocyte contractility, however, is unclear. To determine the direct effects of increased relative beta-MyHC expression on cardiac contractility, we used acute genetic engineering with a recombinant adenoviral vector (AdMYH7) to genetically titrate beta-MyHC protein expression in isolated rodent ventricular cardiac myocytes that predominantly expressed alpha-MyHC (fast molecular motor). AdMYH7-directed beta-MyHC protein expression and sarcomeric incorporation was observed as soon as 1 day after gene transfer. Effects of beta-MyHC expression on myocyte contractility were determined in electrically paced single myocytes (0.2 Hz, 37 degrees C) by measuring sarcomere shortening and intracellular calcium cycling. Gene transfer-based replacement of alpha-MyHC with beta-MyHC attenuated contractility in a dose-dependent manner, whereas calcium transients were unaffected. For example, when beta-MyHC expression accounted for approximately 18% of the total sarcomeric myosin, the amplitude of sarcomere-length shortening (nanometers, nm) was depressed by 42% (151.0+/-10.7 [control] versus 87.0+/-5.4 nm [AdMYH7 transduced]); and genetic titration of beta-MyHC, leading to 38% beta-MyHC content, attenuated shortening by 57% (138.9+/-13.0 versus 59.7+/-7.1 nm). Maximal isometric cross-bridge cycling rate was also slower in AdMYH7-transduced myocytes. Results indicate that small increases of beta-MyHC expression (18%) have Ca2+ transient-independent physiologically relevant effects to decrease intact cardiac myocyte function. We conclude that beta-MyHC is a negative inotrope among the cardiac myofilament proteins.

Effects of sex and estrogen on myosin COOH-terminal isoforms and contractility in rat aorta

We reported that estrogen treatment of ovariectomized rats increased uterine smooth muscle contractility and the ratio of the COOH-terminal myosin heavy chain isoform SM1 (204 kDa) and SM2 [200 kDa; Hewett TE, Martin AF, Paul RJ. J Physiol (Lond) 460: 351-364, 1993]. We extended this model to study sex and estrogen effects on vascular contractility. Experimental groups included 10- to 14-wk-old male (M), female (F), ovariectomized female (OF), and OF treated with estrogen (OF&E) for 7 days with a subcutaneous pellet delivery system, resulting in 17beta-estradiol of 85 (OF&E) vs. 5 (OF or M) pg/ml. The SM1-to-SM2 ratio increased from 1.8 to 2.6 in thoracic aorta, similar to uterine muscle. Isometric force was measured in 5-mm segments of intact and endothelium-denuded (-endo) aorta. With KCl, the maximum forces were in the order OF approximately M > OF&E, and ED50 OF&E > OF approximately M. Differences in ED50 with estrogen persisted after endothelial denudation. The decreased force in -endo OF aorta was not seen in OF&E, suggesting that estrogen altered an endothelium-dependent effect. No differences in maximum forces were noted with norepinephrine: ED50 OF > OF&E > M. Estrogen treatment, in contrast to KCl, increased sensitivity. Endothelial denudation increased sensitivity but reduced the differences between groups. With ACh relaxation, males were more sensitive than females, and estrogen had no effect. In the abdominal aorta, there were no changes in SM1/SM2 with 17beta-estradiol, and differences in contractility were blunted. In summary, estrogen treatment decreased responses to KCl but increased sensitivity to norepinephrine; male rats always demonstrated the highest contractility. An increase in the COOH-terminal myosin heavy chain isoform SM1-to-SM2 ratio with 17beta-estradiol treatment may underlie the changes observed in contractility.

Voluntary running induces fiber type-specific angiogenesis in mouse skeletal muscle

Adult skeletal muscle undergoes adaptation in response to endurance exercise, including fast-to-slow fiber type transformation and enhanced angiogenesis. The purpose of this study was to determine the temporal and spatial changes in fiber type composition and capillary density in a mouse model of endurance training. Long-term voluntary running (4 wk) in C57BL/6 mice resulted in an approximately twofold increase in capillary density and capillary-to-fiber ratio in plantaris muscle as measured by indirect immunofluorescence with an antibody against the endothelial cell marker CD31 (466 ± 16 capillaries/mm2and 0.95 ± 0.04 capillaries/fiber in sedentary control mice vs. 909 ± 55 capillaries/mm2and 1.70 ± 0.04 capillaries/fiber in trained mice, respectively; P < 0.001). A significant increase in capillary-to-fiber ratio was present at day 7 with increased concentration of vascular endothelial growth factor (VEGF) in the muscle, before a significant increase in percentage of type IIa myofibers, suggesting that exercise-induced angiogenesis occurs first, followed by fiber type transformation. Further analysis with simultaneous staining of endothelial cells and isoforms of myosin heavy chains (MHCs) showed that the increase in capillary contact manifested transiently in type IIb + IId/x fibers at the time ( day 7) of significant increase in total capillary density. These findings suggest that endurance training induces angiogenesis in a subpopulation of type IIb + IId/x fibers before switching to type IIa fibers.

Impact of beta-myosin heavy chain isoform expression on cross-bridge cycling kinetics

Myosin heavy chain (MHC) isoforms alpha and beta have intrinsically different ATP hydrolysis activities (ATPase) and therefore cross-bridge cycling rates in solution. There is considerable evidence of altered MHC expression in rodent cardiac disease models; however, the effect of incremental beta-MHC expression over a wide range on the rate of high-strain, isometric cross-bridge cycling is yet to be ascertained. We treated male rats with 6-propyl-2-thiouracil (PTU; 0.8 g/l in drinking water) for short intervals (6, 11, 16, and 21 days) to generate cardiac MHC patterns in transition from predominantly alpha-MHC to predominantly beta-MHC. Steady-state calcium-dependent tension development and tension-dependent ATP consumption (tension cost; proportional to cross-bridge cycling) were measured in chemically permeabilized (skinned) right ventricular muscles at 20 degrees C. To assess dynamic cross-bridge cycling kinetics, the rate of force redevelopment (ktr) was determined after rapid release-restretch of fully activated muscles. MHC isoform content in each experimental muscle was measured by SDS-PAGE and densitometry. alpha-MHC content decreased significantly and progressively with length of PTU treatment [68 +/- 5%, 58 +/- 4%, 37 +/- 4%, and 27 +/- 6% for 6, 11, 16, and 21 days, respectively; P < 0.001 (ANOVA)]. Tension cost decreased, linearly, with decreased alpha-MHC content [6.7 +/- 0.4, 5.6 +/- 0.5, 4.0 +/- 0.4, and 3.9 +/- 0.3 ATPase/tension for 6, 11, 16, and 21 days, respectively; P < 0.001 (ANOVA)]. Likewise, ktr was significantly and progressively depressed with length of PTU treatment [11.1 +/- 0.6, 9.1 +/- 0.5, 8.2 +/- 0.7, and 6.2 +/- 0.3 s(-1) for 6, 11, 16, and 21 days, respectively; P < 0.05 (ANOVA)] Thus cross-bridge cycling, under high strain, for alpha-MHC is three times higher than for beta-MHC. Furthermore, under isometric conditions, alpha-MHC and beta-MHC cross bridges hydrolyze ATP independently of one another.

The intracellular free amino acid pool represents tracer precursor enrichment for calculation of protein synthesis in cultured fibroblasts and myocytes

We assessed the approach of using intracellular free amino acid enrichment as precursor enrichment for calculating the fractional synthetic rate of using the stable isotope tracer incorporation technique. We assumed that the true rate of protein synthesis was reflected by the rate of tracer incorporation over time divided by the plateau enrichment in protein. Isolated human fibroblasts and myocytes were cultured in medium supplemented with [(15)N]glycine, [(15)N]proline, and [d(5)]phenylalanine. Culture medium and cells were collected daily from d 1 to 5. A portion of cells harvested on d 5 was subcultured for an additional 3 passages to d 20. Protein enrichments in both cell types reached a plateau after 20 d of cell culture. In fibroblasts, the true protein synthesis rates were 0.74, 0.85, and 0.86%/h, using protein plateau enrichments of [(15)N]glycine, [(15)N]proline, and [d(5)]phenylalanine as precursor enrichments, respectively. When the corresponding intracellular free amino acid enrichments were used, protein synthesis rates were 0.76, 0.79, and 0.76%/h, respectively. Similarly, in myocytes, the true protein synthesis rates were 0.98 and 1.14%/h by protein plateau enrichments of [(15)N]glycine and [d(5)]phenylalanine, respectively. The synthesis rates were 0.94 and 1.01%/h by the corresponding intracellular enrichments, respectively. Extracellular amino acid enrichments resulted in underestimation of protein synthesis by a variable amount. We conclude that the intracellular free amino acid enrichment is an optimal surrogate for precursor enrichment to quantify protein synthesis.

Effects of different activity and inactivity paradigms on myosin heavy chain gene expression in striated muscle

The goal of this mini-review is to summarize findings concerning the role that different models of muscular activity and inactivity play in altering gene expression of the myosin heavy chain (MHC) family of motor proteins in mammalian cardiac and skeletal muscle. This was done in the context of examining parallel findings concerning the role that thyroid hormone (T(3), 3,5,3'-triiodothyronine) plays in MHC expression. Findings show that both cardiac and skeletal muscles of experimental animals are initially undifferentiated at birth and then undergo a marked level of growth and differentiation in attaining the adult MHC phenotype in a T(3)/activity level-dependent fashion. Cardiac MHC expression in small mammals is highly sensitive to thyroid deficiency, diabetes, energy deprivation, and hypertension; each of these interventions induces upregulation of the beta-MHC isoform, which functions to economize circulatory function in the face of altered energy demand. In skeletal muscle, hyperthyroidism, as well as interventions that unload or reduce the weight-bearing activity of the muscle, causes slow to fast MHC conversions. Fast to slow conversions, however, are seen under hypothyroidism or when the muscles either become chronically overloaded or subjected to intermittent loading as occurs during resistance training and endurance exercise. The regulation of MHC gene expression by T(3) or mechanical stimuli appears to be strongly regulated by transcriptional events, based on recent findings on transgenic models and animals transfected with promoter-reporter constructs. However, the mechanisms by which T(3) and mechanical stimuli exert their control on transcriptional processes appear to be different. Additional findings show that individual skeletal muscle fibers have the genetic machinery to express simultaneously all of the adult MHCs, e.g., slow type I and fast IIa, IIx, and IIb, in unique combinations under certain experimental conditions. This degree of heterogeneity among the individual fibers would ensure a large functional diversity in performing complex movement patterns. Future studies must now focus on 1) the signaling pathways and the underlying mechanisms governing the transcriptional/translational machinery that control this marked degree of plasticity and 2) the morphological organization and functional implications of the muscle fiber's capacity to express such a diversity of motor proteins.

Arterial KIC as marker of liver and muscle intracellular leucine pools in healthy and type 1 diabetic humans

In human protein turnover studies with isotopically labeled leucine (Leu) as a tracer, plasma ketoisocaproate (KIC) enrichment is extensively used as a surrogate measure of intracellular leucine enrichment. To test how accurately arterial ketoisocaproate (A-KIC) represents leucine isotopic enrichment in the hepatic (HV) and femoral veins (FV), which drain liver and muscle beds, we measured Leu and KIC enrichments in samples collected from HV, FV, and femoral artery (A) in 24 control and 6 type I diabetic subjects after a primed, continuous infusion of L-[1-(13)C,(15)N]-Leu. Studies were performed during insulin deprivation or insulin replacement in the diabetic group, whereas the effect of normal saline or three different doses of insulin infusion (0.25, 0.50, and 1 mU. kg(-1). min(-1)) were assessed in healthy controls. The ratios of baseline isotopic enrichments of A-KIC to HV Leu and FV Leu were 0.93 +/- 0.01 and 0.94 +/- 0.02, respectively, in normal subjects and 1.07 +/- 0.04 and 1.05 +/- 0.03, respectively, in diabetic subjects (P < 0.01, diabetic vs. normal subjects). Insulin did not change A-KIC-to-HV Leu ratios in either group, but the A-KIC-to-FV Leu ratio decreased during insulin infusion in normal subjects (P < 0.05). In conclusion, A-KIC represents a reliable surrogate measure of HV Leu enrichment at different levels of circulating insulin in humans. The present data support the use of A-KIC as a surrogate precursor pool for hepatic protein synthesis.

Aminoacyl-tRNA and tissue free amino acid pools are equilibrated after a flooding dose of phenylalanine

The flooding dose method, which is used to measure tissue protein synthesis, assumes equilibration of the isotopic labeling between the aminoacyl-tRNA pool and the tissue and blood free amino acid pools. However, this has not been verified for a phenylalanine tracer in an in vivo study. We determined the specific radioactivity of [(3)H]phenylalanine in the aminoacyl-tRNA and the tissue and blood free amino acid pools of skeletal muscle and liver 30 min after administration of a flooding dose of phenylalanine along with [(3)H]phenylalanine. Studies were performed in neonatal pigs in the fasted and refed states and during hyperinsulinemic-euglycemic-amino acid clamps. The results showed that, 30 min after the administration of a flooding dose of phenylalanine, there was equilibration of the specific radioactivity of phenylalanine among the blood, tissue, and tRNA precursor pools. Equilibration of the specific radioactivity of the three precursor pools for protein synthesis occurred in both skeletal muscle and liver. Neither feeding nor insulin status affected the aminoacyl-tRNA specific radioactivity relative to the tissue free amino acid specific radioactivity. The results support the assumption that the tissue free amino acid pool specific radioactivity is a valid measure of the precursor pool specific radioactivity and thus can be used to calculate protein synthesis rates in skeletal muscle and liver when a flooding dose of phenylalanine is administered.

Altered turnover of calcium regulatory proteins of the sarcoplasmic reticulum in aged skeletal muscle

We have measured the in vivo protein turnover for the major calcium regulatory proteins of the sarcoplasmic reticulum from the skeletal muscle of young adult (7 months) and aged (28 months) Fischer 344 rats. From the time course of the incorporation and decay of protein-associated radioactivity after a pulse injection of [14C]leucine and correcting for leucine reutilization, in young rats, the apparent half-lives for calsequestrin, the 53-kDa glycoprotein, and ryanodine receptor are 5.4 +/- 0.4, 6.3 +/- 1.3, and 8.3 +/- 1.3 days, respectively. A half-life of 14.5 +/- 2.5 days was estimated for the Ca-ATPase isolated from young muscle. Differences in protein turnover associated with aging were determined using sequential injection of two different isotopic labels ([14C]leucine and [3H]leucine) to provide an estimate of protein synthesis and degradation within the same animal. The Ca-ATPase and ryanodine receptor isolated from aged muscle exhibits 27 +/- 5% and 25 +/- 3% slower protein turnover, respectively, relative to that from young muscle. In contrast, the 53-kDa glycoprotein exhibits a 25 +/- 5% more rapid turnover in aged SR, while calsequestrin exhibits no age-dependent alteration in turnover. Statistical analysis comparing the sensitivity of various methods for discriminating different rates of protein turnover validates the approach used in this study and demonstrates that the use of two isotopic labels provides at least a 6-fold more sensitive means to detect age-related differences in protein turnover relative to other methods.

The effect of insulin-like growth factor-I (IGF-I) on protein turnover in the meat-type chicken (Gallus domesticus)

Insulin-like growth factor-I (IGF-I) effects on chicken growth and development are poorly understood. This study examined the effect of IGF-I on protein synthesis rates in various tissues in the male broiler chicken. At three weeks of age, osmotic minipumps were subcutaneously implanted in the scapular area. Chickens were infused with either chicken IGF-I (450 micrograms/kg BW/day) or saline. After treatment for 5 days, the chickens received a flooding dose of [3H]-phenylalanine, and were sacrificed 20 min later. Wing vein blood samples were taken at 0, 5, 10 and 20 min post-injection. The following tissues were removed and frozen for analysis: pectoralis muscle, gastrocnemius muscle, heart, liver, and small intestine. In vivo total protein synthesis measurements were made using the double-label technique. Contractile protein degradation was evaluated using intracellular free 3-methylhistidine concentrations in skeletal muscle. There were no significant differences in absolute or relative body growth rates over the treatment period. Skeletal muscle (pectoralis and gastrocnemius) weights were significantly decreased with IGF-I treatment, while heart weight was significantly increased. Plasma insulin levels were significantly lower in IGF-treated chickens compared to that in control birds. There was no effect of IGF-I on protein synthesis rates in any of the tissues examined. Intracellular free 3-methylhistidine concentrations were higher in both the gastrocnemius (17%) and pectoralis muscles (25%) of chickens treated with IGF-I. This data demonstrates that IGF-I may have an indirect effect to regulate muscle protein turnover rates.

Protein half-lives of calmodulin and the plasma membrane Ca-ATPase in rat brain

We report the half-lives for two proteins involved in the regulation of intracellular calcium in the brain: the plasma membrane Ca-ATPase and its regulatory protein, calmodulin. [14C]-labeled leucine was injected into seven month old adult Fischer 344 rats and the time-dependent appearance and loss of radioactivity was monitored in both the serum and proteins from the brains of rats sacrificed from 4 hours to 13 days after injection. Experimental data obtained for calmodulin and the plasma membrane Ca-ATPase are best described by theoretical curves accounting for leucine reutilization that assume apparent half-lives of 18 (+/-2) hours and 12 (+/-1) days, respectively.

Adenovirus E1A represses cardiac gene transcription and reactivates DNA synthesis in ventricular myocytes, via alternative pocket protein- and p300-binding domains

To examine the potential impact of disrupting "pocket" protein function on cardiac differentiation and growth, we introduced 12 S E1A genes into neonatal ventricular myocytes, by adenoviral gene transfer. In the absence of E1B, E1A was cytotoxic, with features typical of apoptosis. In the presence of E1B, E1A preferentially inhibited transcription of cardiac-restricted alpha-actin promoters, and reactivated DNA synthesis in cardiac myocytes, without cell death. Mutations that abrogate known activities of the amino terminus of E1A, versus conserved region 2, demonstrate that the "pocket" protein- and p300-binding domains each suffice, in the absence of the other, for transcriptional repression and re-entry into S phase.

Precursor pools of protein synthesis: a stable isotope study in a swine model

The accuracy of using other free pools in lieu of tRNA for calculation of tissue protein synthesis in liver (L), skeletal muscle (SM), and heart (H) was assessed in six adult miniature swine using L-[1-13C]leucine and L-[ring-2H5]phenylalanine as tracers. L leucyl-tRNA enrichment was higher than arterial plasma leucine and ketoisocaproate (KIC) enrichments, and L phenylalanyl-tRNA enrichment was higher than arterial phenylalanine enrichment (P < 0.05). No such differences were noted in SM and H. Leucyl- and phenylalanyl-tRNA enrichments in L were best predicted by the respective amino acid enrichments in tissue fluid [TF; Leu: slope (m) = 0.954 +/- 0.035; Phe: m = 1.011 +/- 0.032] using linear regression analysis to determine the accuracy of the prediction, whereas plasma phenylalanine reasonably predicted phenylalanyl-tRNA (artery: m = 0.821 +/- 0.032; vein: m = 0.947 +/- 0.135). In SM, plasma KIC (artery: m = 0.846 +/- 0.046; vein: m = 0.881 +/- 0.043) and TF leucine (m = 0.788 +/- 0.034) predicted leucyl-tRNA with high accuracy. In H tissue, TF (m = 0.991 +/- 0.044) was the best predictor of leucyl-tRNA enrichment, whereas arterial phenylalanine (m = 0.912 +/- 0.015) was the most reliable predictor of phenylalanyl-tRNA enrichment. The relationships between aminoacyl-tRNA and other free pools in the same species under the same study conditions differ in different tissues. Use of KIC in lieu of leucyl-tRNA for calculating muscle protein synthesis is supported by this study.

Role of satellite cells in altering myosin expression during avian skeletal muscle hypertrophy

This study examined whether satellite cells express an embryonic isoform of myosin upon fusion with hypertrophying muscle fibers. Anterior latissimus dorsi (ALD) muscle hypertrophy was induced in adult chickens by weighting one wing. One and 7 days of wing-weighting produced significant increases in ALD muscle wet weight and in the number of mature fibers expressing ventricular-like embryonic (V-EMB) myosin. V-EMB myosin expression could be an event during regeneration of fibers injured by overload or part of the hypertrophy process itself. Although there was an increase in both the number of damaged fibers and the number of mature fibers expressing embryonic myosin after wing-weighting, results from this study suggest that these two events were not necessarily related. The apparent health of fibers expressing V-EMB myosin and the lack of correlation between the numbers of damaged and V-EMB myosin positive fibers (r = 0.20) suggest that embryonic myosin expression in mature fibers was likely a feature of the hypertrophy process itself. The appearance of V-EMB myosin in mature fibers 1 day after wing-weighting suggests that the change in myosin expression did not involve satellite cells since 24 hr is too short a time to permit more than limited satellite cell fusion. The relationship between satellite cells and embryonic myosin expression was examined more closely by labeling dividing satellite cells and their progeny with 5-bromo-2-deoxyuridine, and then colocalizing labeled myofiber nuclei and embryonic myosin in consecutive transverse sections of hypertrophied ALD muscle. One week of wing-weighting resulted in marked increases in myofiber nuclear labeling index and myofiber nuclear density compared to contralateral control. V-EMB myosin was not expressed uniformly throughout individual fibers, but rather in discrete regions of varying length. Many V-EMB myosin positive regions had a higher labeled nuclear density than V-EMB myosin negative regions indicating that V-EMB myosin expression was associated with an accumulation of satellite cell progeny in a restricted area. However, it was also clear that satellite cell progeny were not the sole source of V-EMB myosin since labeled nuclei were completely absent from 41% of the V-EMB positive regions. Furthermore, the presence of new nuclei did not result in obligatory expression of embryonic myosin because many V-EMB negative regions had a high labeled nuclear density. Thus, recently incorporated nuclei arising by satellite cell division are implicated as one, but not the sole source of embryonic myosin in hypertrophying muscle.(ABSTRACT TRUNCATED AT 400 WORDS)

Insulin action on protein metabolism

On the basis of the preceding observations, the following sequence of events can be postulated during insulin deficiency or excess. The main feature of insulin deficiency is the disruption of protein balance in muscle that rapidly leads to emaciation and wasting. Muscle protein degradation is greatly enhanced while increased amino acid availability maintains protein synthesis. In splanchnic tissues, both degradation and synthesis are increased but with an altered pattern, so that the levels of some proteins are increased (e.g. proteins of the acute-phase response), while those of others are decreased (e.g. albumin). As a result, intracellular protein content in liver is maintained but secretion of plasma proteins is abnormal. In healthy subjects, an acute increase in insulin concentration, as occurs after a meal, leads to a rapid suppression of protein breakdown in the splanchnic area. If hyperinsulinaemia is not supported by an exogenous amino acid supply, as might occur during a protein-free meal or experimentally during euglycaemic hyperinsulinaemic clamping, the plasma as well as muscle free amino acid concentration drops, owing to reduced splanchnic release. With reduced amino acid availability, insulin is not anabolic in muscle. If amino acid concentrations are maintained at normal or high levels, e.g. following a mixed meal, a net protein deposition in muscle may occur, primarily because of a stimulation of synthesis and possibly owing to inhibition of breakdown.

Suppression of protein degradation in progressive cardiac hypertrophy of chronic aortic regurgitation

BACKGROUND

The heart adapts to the volume overload of aortic regurgitation with dilation and hypertrophy. The development of left ventricular hypertrophy at the protein level is a dynamic process resulting from an imbalance between cardiac protein synthesis and degradation. The objective of the present study was to determine in vivo the relative contributions of cardiac protein synthesis and degradation to the progressive hypertrophy that occurs in response to chronic aortic regurgitation and to compare these with responses earlier in the course of this stress.

METHODS AND RESULTS

Continuous intravenous infusions of [3H]-leucine were administered 3 days and 1 month after surgical induction of aortic regurgitation and sham operation in rabbits. Total cardiac protein and myosin heavy chain fractional synthesis rates were obtained by analysis of plasma and protein hydrolysate data using [14C]-dansyl chloride assays. Left ventricular growth rates were determined from serial echocardiographic and postmortem left ventricular weight and protein concentration measurements; protein degradation rates were determined by subtraction of growth rates from synthesis rates.

CONCLUSIONS

In comparison with sham-operated control rabbits, protein fractional synthesis rates were increased at 3 days but not at 1 month after induction of aortic regurgitation Progressive cardiac hypertrophy occurring at 1 month was caused by a decrease in protein fractional degradation rates. An increase in protein synthesis contributes only to the early phase of hypertrophy caused by acute aortic regurgitation, whereas progressive eccentric hypertrophy in chronic volume overload is due to suppression of protein degradation.

High-performance liquid chromatographic measurement of leucine and alpha-ketoisocaproate in whole blood: application to fetal protein metabolism

Evaluation of fetal protein metabolism requires measurement of a number of variables including umbilical blood flow, CO2 radioactivity, as well as plasma specific activities, whole blood concentration, and radioactivity of leucine and alpha-ketoisocaproate. This report details methods of analysis for whole blood concentration and radioactivity of leucine and alpha-ketoisocaproate using high-performance liquid chromatography that can be done on minimal blood volumes and are sufficiently accurate to detect the small arteriovenous differences important in measurements of fetal metabolism. Using these methods, the important components of fetal protein metabolism such as protein synthesis can be calculated with sufficient accuracy to detect differences as small as 10% provided appropriate experimental designs are used.

Protein and amino acid metabolism in cancer cachexia: investigative techniques and therapeutic interventions

Cancer cachexia is a complex syndrome characterized primarily by diminished nutrient intake and progressive tissue depletion that is manifest clinically as anorexia and host weight loss. The gradual loss of host protein stores is central to this process. This review outlines the techniques that have been used to evaluate human amino acid metabolism, their application in patients with cancer cachexia, and possible therapeutic interventions designed to overcome alterations in host protein and amino acid metabolism associated with malignant cachexia. The techniques of nitrogen balance and 3-methylhistidine excretion provide indirect estimates of overall nitrogen metabolism and skeletal muscle myofibrillar protein breakdown. Measurement of circulating amino acid concentrations, particularly when combined with assessment of arterial-venous differences and regional amino acid balance allows for investigation of interorgan amino acid metabolism. One of the most significant advances in in vivo amino acid metabolic research has been the development of labeled amino acid tracer studies to evaluate whole body and regional amino acid kinetics. The use of stable and unstable amino acid isotopes in these techniques is reviewed in detail. Virtually all of these techniques have now been employed in the evaluation of human cancer cachexia. The results of studies evaluating amino acid concentrations, regional amino acid balance, and 3-methylhistidine excretion are summarized. The use of regional and whole body kinetic studies in cancer cachexia are reviewed extensively. Most investigators have observed increased rates of whole body protein turnover, synthesis, and catabolism in both weight-stable and weight-losing cancer patients. Some studies have suggested a relationship between the extent of disease and the degree of aberration in amino acid kinetic parameters. Investigators have attempted to reverse some of these alterations by provision of substrate (nutritional support) or administration of specific pharmacologic or anabolic agents such as hydrazine sulfate, insulin, growth hormone, and beta-2 agonists. The role of total parenteral nutrition (TPN) in cancer and its effects on protein and amino acid kinetics and tumor growth are addressed. The possible benefits of specific amino acid nutritional formulations with increased branched chain amino acids, arginine, and glutamine are reviewed. Although many of these approaches appear promising, significant impact on clinically definable parameters remains to be demonstrated. A better understanding of the underlying protein catabolic mechanisms of cancer cachexia will likely lead to more effective therapies to reverse the protein calorie malnutrition associated with cancer cachexia.

Harry M. Vars Research Award. A new model to determine in vivo the relationship between amino acid transmembrane transport and protein kinetics in muscle

The bidirectional transmembrane transport rates of leucine (Leu), valine (Val), phenylalanine (Phe), lysine (Lys), and alanine (Ala) were measured in vivo in the hindlimb muscle of five dogs and related to the rates of protein synthesis and degradation. The compartmental model was based on the systemic continuous infusion of stable isotopic tracers of the amino acids, and the measurement of the enrichment and concentration in the arterial and femoral vein plasma and the intracellular free water in muscle (obtained by biopsy). The transport rate from plasma to tissue (in micromoles per minute) was: Leu, 18.1 +/- 1.8; Val, 26.9 +/- 3.5; Phe, 10.5 +/- 1.6 Lys; 12.2 +/- 1.8; and Ala, 10.7 +/- 3.4. The transport rate from tissue to plasma (in micromoles per minute) was: Leu, 25.5 +/- 2.5; Val, 32.4 +/- 2.8; Phe, 17.0 +/- 2.8; Lys, 24.9 +/- 3.4; Ala, 34.4 +/- 9.0. When the transmembrane transport rate was normalized per unit of amino acid concentration in the source pool, we found that the transport of Leu, Val, and Phe was significantly faster (p less than .05) than the transport of Lys and Ala. The calculated rates of incorporation into hindlimb muscle protein of Phe and Lys (in micromoles per minute) were 4.2 +/- 1.3 and 19.4 +/- 5.3, respectively, and the rates of intracellular appearance from breakdown were 10.7 +/- 1.9 and 32.1 +/- 6.6, respectively. We concluded, therefore, that (1) the transmembrane amino acid transport rate can be measured in vivo in muscle with a relatively noninvasive technique, (2) in the dog hindlimb the equilibration between tissue and plasma free amino acid pool is different for each amino acid depending on the kinetics of the transmembrane transport systems, and (3) the transport rates of amino acids and their rate of appearance from protein breakdown are roughly comparable, suggesting that variations in transport rates could play a role in controlling the rate of protein synthesis.

Stimulation of protein synthesis in human tumours by parenteral nutrition: evidence for modulation of tumour growth

Eighteen patients with localized colorectal carcinoma were randomized to receive intravenous nutrition or to be fasted during the 24 h before surgery. Protein synthesis, an index of tumour growth, was then measured by the incorporation of [13C]leucine into tumour protein immediately before surgery. The mean (s.e.m.) rate of tumour protein synthesis in patients receiving nutrition (42.7(3.5) per cent per day) was 89 per cent higher than the rate in the fasted group (22.6(1.9) per cent per day) (P = 0.002). As tumours consist of a variety of different cell types, in vitro rates of protein synthesis were measured in malignant cells isolated from colorectal tumours and cultured with autologous serum obtained from the patient in either the fasted or the fed state. There was a mean increase of 81 per cent in protein synthesis when fed rather than fasted serum was used (P less than 0.02), indicating that the malignant cells themselves respond to nutrient supply. This increase in tumour protein synthesis provides the first evidence in vivo that the exogenous supply of nutrients can modulate the rate of growth of a human tumour.

Measurement of hepatic protein synthesis in unrestrained mice-evaluation of the 'flooding technique'

Controversy exists regarding the validity of various techniques for estimating rates of protein synthesis in vivo. In the present report, we have compared estimates of hepatic protein synthesis in normal mice with a pulse labelling of [1-14C]leucine and calculated hepatic protein synthetic rates in a conventional two-pool model and in a five-pool compartment analysis. Results obtained with pulse labelling were also compared to those obtained in animals receiving a flooding dose of 1.5 mumol L-phenylalanine and 0.4 microCi [U-14C]phenylalanine per gram of body weight or 1.0 mumol L-leucine and 0.4 microCi [l-14C]leucine per gram of body weight. Estimates of protein synthesis were calculated with plasma free amino acid, liver acid-soluble fraction and acylated tRNA specific radioactivities as being representative of the precursor pool for protein synthesis. Rates of hepatic protein synthesis obtained with pulse labelling and either leu-tRNA or acid-soluble fractions of liver leucine as the precursor for protein synthesis gave similar results (37 +/- 5 vs 42 +/- 5% per day) in a two-pool model, but disagreed in a five-pool model (37 +/- 5 vs 6 +/- 2% per day). Estimates based on plasma enrichment in leucine were only one fifth of values obtained with tRNA in labelling experiments. When the plasma pool with tracer amino acids was used to indicate the precursor labelling of protein synthesis, values obtained with the flooding dose of either phenylalanine or leucine agreed with those obtained with pulse labelling and enrichment in tRNA (30 +/- 3 nmol min-1 vs 28 +/- 4 nmol min-1); with however no agreement when the enrichment in the liver mixed tissue pool was used (76 +/- 5 nmol min-1). Complete equilibration of the amino acid pools did not occur despite flooding. Therefore, the flooding technique may only represent an approximate method to measure protein synthesis in vivo, although it gives absolute values that agree well with results from labelling techniques based on tRNA enrichment provided the plasma pool is used as the precursor enrichment.

Regulation of myocardial isomyosin V1 in uraemic rats

Cardiac hypertrophy of various aetiologies is consistently associated with increased expression of V3 isomyosin. Uraemia is associated with cardiac hypertrophy. In the present study, we examined regulation of isomyosin in uraemic rats, using gel electrophoresis. Cardiac hypertrophy in uraemic animals was associated with a relative increase in V1 isomyosin. An increased proportion of V1 isomyosin was demonstrable 3 days after subtotal nephrectomy (NX 63.0 +/- 8.8%; control 43.6 +/- 7.2%; P less than 0.01) and persisted during uraemia of 80 days duration. Elevation of V1 isomyosin, relative to pair-fed controls, was observed in uraemic animals of various age. The proportion of V1 isomyosin changed in the same direction as controls when several manouevers were used which changed the isomyosin pattern, but the difference between uraemic animals and controls persisted. We studied the effect of carbohydrate loading or deprivation, starvation or administration of energetically inadequate diets, castration or administration of androgens and sodium depletion. With each of the above interventions, a difference between subtotally nephrectomized animals and sham-operated pair-fed control animals was statistically significant (P less than 0.05). Elevation of V1 isomyosin persisted during combined alpha and beta blockade and was still found when blood pressure was normalized by ACE inhibition using Ramipril. It is concluded that cardiac hypertrophy of uraemia differs from all other forms of cardiac hypertrophy by the occurrence of increased proportion of V1 isomyosin. The proportion of V1 isomyosin responds adequately to regulatory signals but is set at an abnormally high level.

In vivo measurement of myocardial protein turnover using an indicator dilution technique

We applied a nondestructive tracer technique, previously developed for measuring skeletal muscle protein turnover, to the measurement of myocardial protein turnover in vivo. During a continuous infusion of L-[ring-2,6-3H]phenylalanine to anesthetized, overnight-fasted dogs, we measured the uptake of radiolabeled phenylalanine from plasma and the release of unlabeled phenylalanine from myocardial proteolysis using arterial and coronary sinus catheterization and analytic methods previously applied to skeletal muscle. Using these measurements, together with a model of myocardial protein synthesis that assumes rapid equilibration of tracer specific activity between myocardial phenylalanyl-tRNA and circulating phenylalanine, we estimated the rates of heart protein synthesis and degradation. The rate of heart protein synthesis was also estimated directly from the incorporation of labeled phenylalanine into tissue protein. The use of [3H]phenylalanine was compared with L-[1-14C]leucine in the measurement of heart protein turnover in dogs given simultaneous infusion of both tracers. Leucine uptake and release by the myocardium exceeded that of phenylalanine by 3.1 +/- 0.4- and 1.7 +/- 0.3-fold, respectively, consistent with leucine's 2.4-fold greater abundance in heart protein and its metabolism via other pathways. Phenylalanine is the preferred tracer for use with this method because of its limited metabolic fate in muscle. One theoretical limitation to the method, slow equilibration of circulating labeled phenylalanine with myocardial phenylalanyl-tRNA, was resolved by comparison of these specific activities after a 30-minute infusion of labeled phenylalanine in the rat. A second, empirical limitation involves precision in the measurement of the small decrements in phenylalanine specific activity that occur with each pass of blood through the coronary circulation. This was addressed by improving the precision of both the measurements of phenylalanine concentration and phenylalanine specific activity using high-performance liquid chromatography. We conclude that the in vivo measurement of phenylalanine tracer exchange across the myocardium permits the nondestructive estimation of heart protein turnover in the intact animal.