The carbon monoxide prodrug oCOm-21 increases Ca2+ sensitivity of the cardiac myofilament

Patients undergoing cardiopulmonary bypass procedures require inotropic support to improve hemodynamic function and cardiac output. Current inotropes such as dobutamine, can promote arrhythmias, prompting a demand for improved inotropes with little effect on intracellular Ca2+ flux. Low-dose carbon monoxide (CO) induces inotropic effects in perfused hearts. Using the CO-releasing pro-drug, oCOm-21, we investigated if this inotropic effect results from an increase in myofilament Ca2+ sensitivity. Male Sprague Dawley rat left ventricular cardiomyocytes were permeabilized, and myofilament force was measured as a function of -log [Ca2+ ] (pCa) in the range of 9.0-4.5 under five conditions: vehicle, oCOm-21, the oCOm-21 control BP-21, and levosimendan, (9 cells/group). Ca2+ sensitivity was assessed by the Ca2+ concentration at which 50% of maximal force is produced (pCa50 ). oCOm-21, but not BP-21 significantly increased pCa50 compared to vehicle, respectively (pCa50 5.52 vs. 5.47 vs. 5.44; p < 0.05). No change in myofilament phosphorylation was seen after oCOm-21 treatment. Pretreatment of cardiomyocytes with the heme scavenger hemopexin, abolished the Ca2+ sensitizing effect of oCOm-21. These results support the hypothesis that oCOm-21-derived CO increases myofilament Ca2+ sensitivity through a heme-dependent mechanism but not by phosphorylation. Further analyses will confirm if this Ca2+ sensitizing effect occurs in an intact heart.

Geroprotector drugs and exercise: friends or foes on healthy longevity?

Physical activity and several pharmacological approaches individually combat age-associated conditions and extend healthy longevity in model systems. It is tantalizing to extrapolate that combining geroprotector drugs with exercise could extend healthy longevity beyond any individual treatment. However, the current dogma suggests that taking leading geroprotector drugs on the same day as exercise may limit several health benefits. Here, we review leading candidate geroprotector drugs and their interactions with exercise and highlight salient gaps in knowledge that need to be addressed to identify if geroprotector drugs can have a harmonious relationship with exercise.

Mass Spectrometry-Based Multiomics Identifies Metabolic Signatures of Sarcopenia in Rhesus Monkey Skeletal Muscle

Sarcopenia is a progressive disorder characterized by age-related loss of skeletal muscle mass and function. Although significant progress has been made over the years to identify the molecular determinants of sarcopenia, the precise mechanisms underlying the age-related loss of contractile function remains unclear. Advances in "omics" technologies, including mass spectrometry-based proteomic and metabolomic analyses, offer great opportunities to better understand sarcopenia. Herein, we performed mass spectrometry-based analyses of the vastus lateralis from young, middle-aged, and older rhesus monkeys to identify molecular signatures of sarcopenia. In our proteomic analysis, we identified proteins that change with age, including those involved in adenosine triphosphate and adenosine monophosphate metabolism as well as fatty acid beta oxidation. In our untargeted metabolomic analysis, we identified metabolites that changed with age largely related to energy metabolism including fatty acid beta oxidation. Pathway analysis of age-responsive proteins and metabolites revealed changes in muscle structure and contraction as well as lipid, carbohydrate, and purine metabolism. Together, this study discovers new metabolic signatures and offers new insights into the molecular mechanisms underlying sarcopenia for the evaluation and monitoring of a therapeutic treatment of sarcopenia.

Mass spectrometry-based multi-omics identifies metabolic signatures of sarcopenia in rhesus monkey skeletal muscle

Sarcopenia is a progressive disorder characterized by age-related loss of skeletal muscle mass and function. Although significant progress has been made over the years to identify the molecular determinants of sarcopenia, the precise mechanisms underlying the age-related loss of contractile function remains unclear. Advances in omics technologies, including mass spectrometry-based proteomic and metabolomic analyses, offer great opportunities to better understand sarcopenia. Herein, we performed mass spectrometry-based analyses of the vastus lateralis from young, middle-aged, and older rhesus monkeys to identify molecular signatures of sarcopenia. In our proteomic analysis, we identified numerous proteins that change with age, including those involved in adenosine triphosphate and adenosine monophosphate metabolism as well as fatty acid beta oxidation. In our untargeted metabolomic analysis, we identified multiple metabolites that changed with age largely related to energy metabolism including fatty acid beta oxidation. Pathway analysis of age-responsive proteins and metabolites revealed changes in muscle structure and contraction as well as lipid, carbohydrate, and purine metabolism. Together, this study discovers new metabolic signatures and offer new insights into the molecular mechanism underlying sarcopenia for the evaluation and monitoring of therapeutic treatment of sarcopenia.

High sensitivity top-down proteomics captures single muscle cell heterogeneity in large proteoforms

Single-cell proteomics has emerged as a powerful method to characterize cellular phenotypic heterogeneity and the cell-specific functional networks underlying biological processes. However, significant challenges remain in single-cell proteomics for the analysis of proteoforms arising from genetic mutations, alternative splicing, and post-translational modifications. Herein, we have developed a highly sensitive functionally integrated top-down proteomics method for the comprehensive analysis of proteoforms from single cells. We applied this method to single muscle fibers (SMFs) to resolve their heterogeneous functional and proteomic properties at the single-cell level. Notably, we have detected single-cell heterogeneity in large proteoforms (>200 kDa) from the SMFs. Using SMFs obtained from three functionally distinct muscles, we found fiber-to-fiber heterogeneity among the sarcomeric proteoforms which can be related to the functional heterogeneity. Importantly, we detected multiple isoforms of myosin heavy chain (~223 kDa), a motor protein that drives muscle contraction, with high reproducibility to enable the classification of individual fiber types. This study reveals single muscle cell heterogeneity in large proteoforms and establishes a direct relationship between sarcomeric proteoforms and muscle fiber types, highlighting the potential of top-down proteomics for uncovering the molecular underpinnings of cell-to-cell variation in complex systems.

Treadmill running increases the calcium sensitivity of myofilaments in diabetic rats

The cardiovascular benefits of regular exercise are unequivocal, yet patients with type 2 diabetes respond poorly to exercise due to a reduced cardiac reserve. The contractile response of diabetic cardiomyocytes to beta-adrenergic stimulation is attenuated, which may result in altered myofilament calcium sensitivity and post-translational modifications of cardiac troponin I (cTnI). Treadmill running increases myofilament calcium sensitivity in non‑diabetic rats, and thus we hypothesized that endurance training would increase calcium sensitivity of diabetic cardiomyocytes and alter site-specific phosphorylation of cTnI. Calcium sensitivity, or pCa₅₀, was measured in Zucker Diabetic Fatty (ZDF) non-diabetic (nDM) and diabetic (DM) rat hearts after 8 weeks of either a sedentary (SED) or progressive treadmill running (TR) intervention. Skinned cardiomyocytes were connected to a capacitance-gauge transducer and a torque motor to measure force as a function of pCa (‑log[Ca²⁺]). Specific phospho-sites on cTnI and O‑GlcNAcylation were quantified by immunoblot and total protein phosphorylation by fluorescent gel staining (ProQ Diamond). The novel finding in this study was that training increased pCa₅₀ in both DM and nDM cardiomyocytes (p = 0.009). Phosphorylation of cTnI amino acid residues Ser23/24, a crucial protein kinase A site, and Threonine (Thr)144, was lower in DM hearts, but there was no effect of training on site-specific phosphorylation. Additionally, total phosphorylation and O-GlcNAcylation levels were not different between SED and TR groups. These findings suggest that regular exercise may benefit the diabetic heart by specifically targeting myofilament contractile function.

Adiponectin receptor agonist AdipoRon improves skeletal muscle function in aged mice

The loss of skeletal muscle function with age, known as sarcopenia, significantly reduces independence and quality of life and can have significant metabolic consequences. Although exercise is effective in treating sarcopenia it is not always a viable option clinically, and currently, there are no pharmacological therapeutic interventions for sarcopenia. Here, we show that chronic treatment with pan-adiponectin receptor agonist AdipoRon improved muscle function in male mice by a mechanism linked to skeletal muscle metabolism and tissue remodeling. In aged mice, 6 weeks of AdipoRon treatment improved skeletal muscle functional measures in vivo and ex vivo. Improvements were linked to changes in fiber type, including an enrichment of oxidative fibers, and an increase in mitochondrial activity. In young mice, 6 weeks of AdipoRon treatment improved contractile force and activated the energy-sensing kinase AMPK and the mitochondrial regulator PGC-1a (peroxisome proliferator-activated receptor gamma coactivator one alpha). In cultured cells, the AdipoRon induced stimulation of AMPK and PGC-1a was associated with increased mitochondrial membrane potential, reorganization of mitochondrial architecture, increased respiration, and increased ATP production. Furthermore, the ability of AdipoRon to stimulate AMPK and PGC1a was conserved in nonhuman primate cultured cells. These data show that AdipoRon is an effective agent for the prevention of sarcopenia in mice and indicate that its effects translate to primates, suggesting it may also be a suitable therapeutic for sarcopenia in clinical application.

Increased myofilament calcium sensitivity is associated with decreased cardiac troponin I phosphorylation in the diabetic rat heart

NEW FINDINGS

What is the central question of this study? In Zucker Diabetic Fatty rats, does cardiomyocyte myofilament function change through the time course of diabetes and what are the mechanisms behind alterations in calcium sensitivity? What is the main finding and its importance? Zucker Diabetic Fatty rats had increased myofilament calcium sensitivity and reduced phosphorylation at cardiac troponin I without differential O-GlcNAcylation.

ABSTRACT

The diabetic heart has impaired systolic and diastolic function independent of other comorbidities. The availability of calcium is altered, but does not fully explain the cardiac dysfunction seen in the diabetic heart. Thus, we explored if myofilament calcium regulation of contraction is altered while also categorizing the levels of phosphorylation and O-GlcNAcylation in the myofilaments. Calcium sensitivity (pCa₅₀ ) was measured in Zucker Diabetic Fatty (ZDF) rat hearts at the initial stage of diabetes (12 weeks old) and after 8 weeks of uncontrolled hyperglycaemia (20 weeks old) and in non-diabetic (nDM) littermates. Skinned cardiomyocytes were connected to a capacitance-gauge transducer and a torque motor to measure force as a function of pCa (-log[Ca²⁺ ]). Fluorescent gel stain (ProQ Diamond) was used to measure total protein phosphorylation. Specific phospho-sites on cardiac troponin I (cTnI) and total cTnI O-GlcNAcylation were quantified using immunoblot. pCa₅₀ was greater in both 12- and 20-week-old diabetic (DM) rats compared to nDM littermates (P = 0.0001). Total cTnI and cTnI serine 23/24 phosphorylation were lower in DM rats (P = 0.003 and P = 0.01, respectively), but cTnI O-GlcNAc protein expression was not different. pCa₅₀ is greater in DM rats and corresponds with an overall reduction in cTnI phosphorylation. These findings indicate that myofilament calcium sensitivity is increased and cTnI phosphorylation is reduced in ZDF DM rats and suggests an important role for cTnI phosphorylation in the DM heart.

Sex differences in skeletal muscle alterations in a model of colorectal cancer

Cancer cachexia is the loss of lean muscle mass with or without loss of fat mass that is often highlighted by a progressive loss of skeletal muscle mass and function. The mechanisms behind the cachexia-related loss of skeletal muscle are poorly understood, including cachexia-related muscle functional impairments. Existing models have revealed some potential mechanisms, but appear limited to how the cancer develops and the type of tumors that form. We studied the C57BL6/J (B6) ApcMin/+ Tg::Fabp1-Cre TG::PIK3ca* (CANCER) mouse. In this model, mice develop highly aggressive intestinal cancers. We tested whether CANCER mice develop cancer cachexia, if muscle function is altered and if sex differences are present. Both female and male mice, B6 (CONTROL) and CANCER mice, were analyzed to determine body weight, hindlimb muscle mass, protein concentration, specific force, and fatigability. Female CANCER mice had reduced body weight and hindlimb muscle mass compared with female CONTROL mice, but lacked changes in protein concentration and specific force. Male CANCER mice had reduced protein concentration and reduced specific force, but lacked altered body weight and muscle mass. There were no changes in fatigability in either group. Our study demonstrates that CANCER mice present an early stage of cachexia, have reduced specific force in male CANCER mice and develop a sex-dependent cachexia phenotype. However, CANCER mice lack certain aspects of the syndrome seen in the human scenario and, therefore, using the CANCER mice as a preclinical model does not seem sufficient in order to maximize the translation of preclinical findings to humans.

Top-down Mass Spectrometry of Sarcomeric Protein Post-translational Modifications from Non-human Primate Skeletal Muscle

Sarcomeric proteins, including myofilament and Z-disk proteins, play critical roles in regulating muscle contractile properties. A variety of isoforms and post-translational modifications (PTMs) of sarcomeric proteins have been shown to be associated with modulation of muscle functions and the occurrence of muscle diseases. Non-human primates (NHPs) are excellent research models for sarcopenia, a disease associated with alterations in sarcomeric proteins, due to their marked similarities to humans. However, the sarcomeric proteins in NHP skeletal muscle have not been well characterized. To gain a deeper understanding of sarcomeric proteins in NHP skeletal muscle, we employed top-down mass spectrometry (MS) to conduct a comprehensive analysis on isoforms and PTMs of sarcomeric proteins in rhesus macaque skeletal muscle. We identified 23 protein isoforms with 46 proteoforms of sarcomeric proteins, including 6 isoforms with 18 proteoforms from fast skeletal troponin T. Particularly, for the first time, a novel PDZ/LIM domain protein isoform, PDLIM7, was characterized with a newly identified protein sequence. Moreover, we also identified multiple PTMs on these proteins, including deamidation, methylation, acetylation, tri-methylation, phosphorylation, and S-glutathionylation. Most PTM sites were localized, including Asn13 deamidation on MLC-2S; His73 methylation on αactin; N-terminal acetylation on most identified proteins; N-terminal tri-methylation on MLC-1S, MLC-1F, MLC-2S, and MLC-2F; Ser14 phosphorylation on MLC-2S; and Ser15 and Ser16 phosphorylation on MLC-2F. In summary, a comprehensive characterization of sarcomeric proteins including multiple isoforms and PTMs in NHP skeletal muscle was achieved by analyzing intact proteins in the top-down MS approach.

Top-Down Proteomics Reveals Myofilament Proteoform Heterogeneity among Various Rat Skeletal Muscle Tissues

Heterogeneity in skeletal muscle contraction time, peak power output, and resistance to fatigue, among others, is necessary to accommodate the wide range of functional demands imposed on the body. Underlying this functional heterogeneity are a myriad of differences in the myofilament protein isoform expression and post-translational modifications; yet, characterizing this heterogeneity remains challenging. Herein, we have utilized top-down liquid chromatography (LC)-mass spectrometry (MS)-based proteomics to characterize myofilament proteoform heterogeneity in seven rat skeletal muscle tissues including vastus lateralis, vastus medialis, vastus intermedius, rectus femoris, soleus, gastrocnemius, and plantaris. Top-down proteomics revealed that myofilament proteoforms varied greatly across the seven different rat skeletal muscle tissues. Subsequently, we quantified and characterized myofilament proteoforms using online LC-MS. We have comprehensively characterized the fast and slow skeletal troponin I isoforms, which demonstrates the ability of top-down MS to decipher isoforms with high sequence homology. Taken together, we have shown that top-down proteomics can be used as a robust and high-throughput method to characterize the molecular heterogeneity of myofilament proteoforms from various skeletal muscle tissues.

Simultaneous Quantification of Protein Expression and Modifications by Top-down Targeted Proteomics: A Case of the Sarcomeric Subproteome

Determining changes in protein expression and post-translational modifications (PTMs) is crucial for elucidating cellular signal transduction and disease mechanisms. Conventional antibody-based approaches have inherent problems such as the limited availability of high-quality antibodies and batch-to-batch variation. Top-down mass spectrometry (MS)-based proteomics has emerged as the most powerful method for characterization and quantification of protein modifications. Nevertheless, robust methods to simultaneously determine changes in protein expression and PTMs remain lacking. Herein, we have developed a straightforward and robust top-down liquid chromatography (LC)/MS-based targeted proteomics platform for simultaneous quantification of protein expression and PTMs with high throughput and high reproducibility. We employed this method to analyze the sarcomeric subproteome from various muscle types of different species, which successfully revealed skeletal muscle heterogeneity and cardiac developmental changes in sarcomeric protein isoform expression and PTMs. As demonstrated, this targeted top-down proteomics platform offers an excellent 'antibody-independent' alternative for the accurate quantification of sarcomeric protein expression and PTMs concurrently in complex mixtures, which is generally applicable to different species and various tissue types.

Sex-Specific Skeletal Muscle Fatigability and Decreased Mitochondrial Oxidative Capacity in Adult Rats Exposed to Postnatal Hyperoxia

Premature birth affects more than 10% of live births, and is characterized by relative hyperoxia exposure in an immature host. Long-term consequences of preterm birth include decreased aerobic capacity, decreased muscular strength and endurance, and increased prevalence of metabolic diseases such as type 2 diabetes mellitus. Postnatal hyperoxia exposure in rodents is a well-established model of chronic lung disease of prematurity, and also recapitulates the pulmonary vascular, cardiovascular, and renal phenotype of premature birth. The objective of this study was to evaluate whether postnatal hyperoxia exposure in rats could recapitulate the skeletal and metabolic phenotype of premature birth, and to characterize the subcellular metabolic changes associated with postnatal hyperoxia exposure, with a secondary aim to evaluate sex differences in this model. Compared to control rats, male rats exposed to 14 days of postnatal hyperoxia then aged to 1 year demonstrated higher skeletal muscle fatigability, lower muscle mitochondrial oxidative capacity, more mitochondrial damage, and higher glycolytic enzyme expression. These differences were not present in female rats with the same postnatal hyperoxia exposure. This study demonstrates detrimental mitochondrial and muscular outcomes in the adult male rat exposed to postnatal hyperoxia. Given that young adults born premature also demonstrate skeletal muscle dysfunction, future studies are merited to determine whether this dysfunction as well as reduced aerobic capacity is due to reduced mitochondrial oxidative capacity and metabolic dysfunction.

Novel Sarcopenia-related Alterations in Sarcomeric Protein Post-translational Modifications (PTMs) in Skeletal Muscles Identified by Top-down Proteomics

Sarcopenia, the age-related loss of skeletal muscle mass and strength, is a significant cause of morbidity in the elderly and is a major burden on health care systems. Unfortunately, the underlying molecular mechanisms in sarcopenia remain poorly understood. Herein, we utilized top-down proteomics to elucidate sarcopenia-related changes in the fast- and slow-twitch skeletal muscles of aging rats with a focus on the sarcomeric proteome, which includes both myofilament and Z-disc proteins-the proteins that constitute the contractile apparatuses. Top-down quantitative proteomics identified significant changes in the post-translational modifications (PTMs) of critical myofilament proteins in the fast-twitch skeletal muscles of aging rats, in accordance with the vulnerability of fast-twitch muscles to sarcopenia. Surprisingly, age-related alterations in the phosphorylation of Cypher isoforms, proteins that localize to the Z-discs in striated muscles, were also noted in the fast-twitch skeletal muscle of aging rats. This represents the first report of changes in the phosphorylation of Z-disc proteins in skeletal muscle during aging. In addition, increased glutathionylation of slow skeletal troponin I, a novel modification that may help protect against oxidative damage, was observed in slow-twitch skeletal muscles. Furthermore, we have identified and characterized novel muscle type-specific proteoforms of myofilament proteins and Z-disc proteins, including a novel isoform of the Z-disc protein Enigma. The finding that the phosphorylation of Z-disc proteins is altered in response to aging in the fast-twitch skeletal muscles of aging rats opens new avenues for the investigation of the role of Z-discs in age-related muscle dysfunction.

Mitochondrial and Metabolic Gene Expression in the Aged Rat Heart

Aging is associated with a decline in cardiac function. Exercise intervention has been suggested as a way to improve this decrement. Age-related decline in cardiac function is associated with decreases in fatty acid oxidation, mitochondrial function, and AMP-activated protein kinase (AMPK) activity. The molecular mechanisms involved with age-related changes in mitochondrial function and substrate metabolism are poorly understood. We determined gene expression differences in hearts of Young (6 mo), Old (33 mo), and old exercise trained (Old + EXE) (34 mo) FBN rats, using Qiagen PCR arrays for Glucose, Fatty acid, and Mitochondrial metabolism. Old rats demonstrated decreased (p < 0.05) expression for key genes in fatty acid oxidation, mitochondrial function, and AMPK signaling. There were no differences in the expression of genes involved in glucose metabolism with age. These gene expression changes occurred prior to altered protein translation as we found no differences in the protein content of peroxisome proliferator activated receptor gamma, coactivators 1 alpha (PGC-1α), peroxisome proliferator activated receptor alpha (PPARα), and AMPKα₂ between young and old hearts. Four months of exercise training did not attenuate the decline in the gene expression in aged hearts. Despite this lack of change in gene expression, exercise-trained rats demonstrated increased exercise capacity compared to their sedentary counterparts. Taken together, our results show that differential expression of genes associated with fatty acid metabolism, AMPK signaling and mitochondrial function decrease in the aging heart which may play a role in age-related declines in fatty acid oxidation, AMPK activity, and mitochondrial function in the heart.

Top-Down Targeted Proteomics Reveals Decrease in Myosin Regulatory Light-Chain Phosphorylation That Contributes to Sarcopenic Muscle Dysfunction

Sarcopenia, the loss of skeletal muscle mass and function with advancing age, is a significant cause of disability and loss of independence in the elderly and thus, represents a formidable challenge for the aging population. Nevertheless, the molecular mechanism(s) underlying sarcopenia-associated muscle dysfunction remain poorly understood. In this study, we employed an integrated approach combining top-down targeted proteomics with mechanical measurements to dissect the molecular mechanism(s) in age-related muscle dysfunction. Top-down targeted proteomic analysis uncovered a progressive age-related decline in the phosphorylation of myosin regulatory light chain (RLC), a critical protein involved in the modulation of muscle contractility, in the skeletal muscle of aging rats. Top-down tandem mass spectrometry analysis identified a previously unreported bis-phosphorylated proteoform of fast skeletal RLC and localized the sites of decreasing phosphorylation to Ser14/15. Of these sites, Ser14 phosphorylation represents a previously unidentified site of phosphorylation in RLC from fast-twitch skeletal muscle. Subsequent mechanical analysis of single fast-twitch fibers isolated from the muscles of rats of different ages revealed that the observed decline in RLC phosphorylation can account for age-related decreases in the contractile properties of sarcopenic fast-twitch muscles. These results strongly support a role for decreasing RLC phosphorylation in sarcopenia-associated muscle dysfunction and suggest that therapeutic modulation of RLC phosphorylation may represent a new avenue for the treatment of sarcopenia.

Moderate intensity, but not high intensity, treadmill exercise training alters power output properties in myocardium from aged rats

Aging is characterized by a progressive decline in cardiac function, but endurance exercise training has been shown to retard a number of deleterious effects of aging. However, underlying mechanisms by which exercise training improves age-related decrements in myocardial contractile function are not well understood. The purpose of this study was to determine the effects of exercise training on power output properties in permeablized (skinned) myocytes of old rats. Thirty-month-old rats were divided into sedentary control (C) and groups undergoing 11 weeks of treadmill exercise training at moderate intensity (MI) and at high intensity (HI). Peak power output normalized to maximal force was significantly increased in MI but not in HI compared to C with significant increases in atrial myosin light chain 1 in ventricle. These results suggest that MI exercise training is beneficial as a significant increase was seen in the ability of the myocardium to do work, but this effect was not seen with HI training.

Effect of aging on power output properties in rat skinned cardiac myocytes

Aging is generally associated with a decline in several indices of cardiac function. The cellular mechanisms for this decline are not completely understood. The ability of the myocardium to perform external work (power output) is a critical aspect of ventricular function. The purpose of this study was to determine the effect of aging on loaded shortening and power output properties. We measured force-velocity properties in permeabilized (skinned) myocytes from the hearts of 9-, 24-, and 33-month-old male Fisher 344 × Brown Norway F1 hybrid rats (F344BN) during loaded contractions using a force-clamp technique. Power output was calculated by multiplying force and shortening velocity values. We found that peak power output normalized to maximal force was significantly decreased by 18% and 31% in myocytes from 24- and 33-month-old group, respectively, compared with 9-month group (p < .05). These results suggest that aging is associated with a significant decrease in the ability of the myocardium to do work.

Morphometry, ultrastructure, myosin isoforms, and metabolic capacities of the “mini muscles” favoured by selection for high activity in house mice

Prolonged selective breeding of mice (Mus musculus) for high levels of voluntary wheel running has favoured an unusual phenotype ("mini muscles"), apparently caused by a single Mendelian recessive allele, in which most hind-limb muscles are markedly reduced in mass, but have increased mass-specific activities of mitochondrial enzymes. We examined whether these changes reflect changes in fibre size, number or ultrastructure in normal and "mini-muscle" mice within the two (of four) selectively bred lines (lab designations L3 and L6) that exhibit the phenotype at generations 26 and 27. In both lines, the gastrocnemius and plantaris muscles are smaller in mass (by >50% and 20%, respectively) in affected individuals. The mass-specific activities of mitochondrial enzymes in the gastrocnemius and plantaris muscles were increased in the mini phenotype in both lines, with stronger effects in the gastrocnemius muscle. In the gastrocnemius, the % myosin heavy chain (MHC) IIb was reduced by 50% in L3 and by 30% in L6, whereas the % MHC IIa and I were higher, particularly in L3. Fibre number in the plantaris muscle did not significantly differ between mini and normal muscles, although muscle mass was a significant positive correlate of fibre number. Small fibres were more abundant in mini than normal muscles in L3. Mitochondrial volume density was significantly higher in mini than normal muscle fibres in L3, but not in L6. Microscopy revealed a surprising attribute of the mini muscles: an abundance of small, minimally differentiated, myofibril-containing cells positioned in a disorderly fashion, particularly in the surface layer. We hypothesise that these unusual cells may be satellite cells or type IIb fibres that did not complete their differentiation. Together, these observations suggest that mice with the mini phenotype have reduced numbers of type IIb fibres in many of their hind-limb muscles, leading to a decrease in mass and an increase in mass-specific aerobic capacity in muscles that typically have a high proportion of type IIb fibres. Moreover, the several statistically significant interactions between muscle phenotype and line indicate that the effect of the underlying allele is altered by genetic background.

Adaptation of cardiac myocyte contractile properties to exercise training

Recent work suggests that chronic exercise induces alterations in the contractile properties of cardiac myocytes. These alterations include increased sensitivity to activation by Ca, changes in the force-length relationship, and increased power output. A recently observed shift in expression of myosin light chain 1 subunit isoforms induced by training may provide a molecular mechanism for these contractile alterations.

Regional differences in effects of exercise training on contractile and biochemical properties of rat cardiac myocytes

Myocardial function is enhanced by endurance exercise training, but the cellular mechanisms underlying this improved function remain unclear. A number of studies have shown that the characteristics of cardiac myocytes vary across the width of the ventricular wall. We have previously shown that endurance exercise training alters the Ca2+ sensitivity of tension as well as contractile protein isoform expression in rat cardiac myocytes. We tested the hypothesis that these effects of training are not uniform across the ventricular wall but are more pronounced in the subendocardial (Endo) region of the myocardium. Female Sprague-Dawley rats were divided into sedentary control (C) and exercise trained (T) groups. T rats underwent 11 wk of progressive treadmill exercise. Myocytes were isolated from the Endo region of the myocardium and from the subepicardial (Epi) region of both T and C hearts. We found an increase in the Ca2+ sensitivity of tension in T cells compared with C cells, but this difference was larger in the Endo cells than in the Epi cells. In addition, we found a training-induced increase in atrial myosin light chain 1 (aMLC1) expression that was larger in the Endo compared with Epi samples. We conclude that effects of exercise training on myocyte contractile and biochemical properties are greater in myocytes from the Endo region of the myocardium than those from the Epi region. In addition, these results provide evidence that the increase in aMLC1 expression may be responsible for some of the training-induced increase in myocyte Ca2+ sensitivity of tension.

Altered single cell force-velocity and power properties in exercise-trained rat myocardium

Myocardial function is enhanced by endurance exercise training, but the cellular mechanisms underlying this improved function remain unclear. The ability of the myocardium to perform external work is a critical aspect of ventricular function, but previous studies of myocardial adaptation to exercise training have been limited to measurements of isometric tension or unloaded shortening velocity, conditions in which work output is zero. We measured force-velocity properties in single permeabilized myocyte preparations to determine the effect of exercise training on loaded shortening and power output. Female Sprague-Dawley rats were divided into sedentary control (C) and exercise trained (T) groups. T rats underwent 11 wk of progressive treadmill exercise. Myocytes were isolated from T and C hearts, chemically skinned, and attached to a force transducer. Shortening velocity was determined during loaded contractions at 15 degrees C by using a force-clamp technique. Power output was calculated by multiplying force times velocity values. We found that unloaded shortening velocity was not significantly different in T vs. C myocytes (T = 1.43 muscle lengths/s, n = 46 myocytes; C = 1.12 muscle lengths/s, n = 43 myocytes). Training increased the velocity of loaded shortening and increased peak power output (peak power = 0.16 P/P(o) x muscle length/s for T myocytes; peak power = 0.10 P/P(o) x muscle length/s for C myocytes, where P/P(o) is relative tension). We found no effect of training on myosin heavy chain isoform content. These results suggest that training alters power output properties of single cardiac myocytes and that this adaptation may improve the work capacity of the myocardium.

Microarray expression analysis of effects of exercise training: increase in atrial MLC-1 in rat ventricles

Previous studies have shown that endurance exercise training increases myocardial contractility. We have previously described training-induced alterations in myocardial contractile function at the cellular level, including an increase in the Ca(2+) sensitivity of tension. To determine the molecular mechanism(s) of these changes, oligonucleotide microarrays were used to analyze the gene expression profile in ventricles from endurance-trained rats. We used an 11-wk treadmill training protocol that we have previously shown results in increased contractility in cardiac myocytes. After the training, the hearts were removed and RNA was isolated from the ventricles of nine trained and nine control rats. With the use of an Affymetrix Rat Genome U34A Array, we detected altered expression of 27 genes. Several genes previously found to have increased expression in hypertrophied myocardium, such as atrial natriuretic factor and skeletal alpha-actin, were decreased with training in this study. From the standpoint of altered contractile performance, the most significant finding was an increase in the expression of atrial myosin light chain 1 (aMLC-1) in the trained ventricular tissue. We confirmed microarray results for aMLC-1 using RT-PCR and also confirmed a training-induced increase in aMLC-1 protein using two-dimensional gel electrophoresis. aMLC-1 content has been previously shown to be increased in human cardiac hypertrophy and has been associated with increased Ca(2+) sensitivity of tension and increased power output. These results suggest that increased expression of aMLC-1 in response to training may be responsible, at least in part, for previously observed training-induced enhancement of contractile function.

Exercise training alters length dependence of contractile properties in rat myocardium

Myocardial function is enhanced by endurance exercise training, but the cellular mechanisms underlying this improved function remain unclear. Exercise training increases the sensitivity of rat cardiac myocytes to activation by Ca(2+), and this Ca(2+) sensitivity has been shown to be highly dependent on sarcomere length. We tested the hypothesis that exercise training increases this length dependence in cardiac myocytes. Female Sprague-Dawley rats were divided into sedentary control (C) and exercise-trained (T) groups. The T rats underwent 11 wk of progressive treadmill exercise. Heart weight increased by 14% in T compared with C rats, and plantaris muscle citrate synthase activity showed a 39% increase with training. Steady-state tension was determined in permeabilized myocytes by using solutions of various Ca(2+) concentration (pCa), and tension-pCa curves were generated at two different sarcomere lengths for each myocyte (1.9 and 2.3 microm). We found an increased sarcomere length dependence of both maximal tension and pCa(50) (the Ca(2+) concentration giving 50% of maximal tension) in T compared with C myocytes. The DeltapCa(50) between the long and short sarcomere length was 0.084 +/- 0.023 (mean +/- SD) in myocytes from C hearts compared with 0.132 +/- 0.014 in myocytes from T hearts (n = 50 myocytes per group). The Deltamaximal tension was 5.11 +/- 1.42 kN/m(2) in C myocytes and 9.01 +/- 1.28 in T myocytes. We conclude that exercise training increases the length dependence of maximal and submaximal tension in cardiac myocytes, and this change may underlie, at least in part, training-induced enhancement of myocardial function.

Altered expression of skeletal muscle myosin isoforms in cancer cachexia

Cachexia is commonly seen in cancer and is characterized by severe muscle wasting, but little is known about the effect of cancer cachexia on expression of contractile protein isoforms such as myosin. Other causes of muscle atrophy shift expression of myosin isoforms toward increased fast (type II) isoform expression. We injected mice with murine C-26 adenocarcinoma cells, a tumor cell line that has been shown to cause muscle wasting. Mice were killed 21 days after tumor injection, and hindlimb muscles were removed. Myosin heavy chain (MHC) and myosin light chain (MLC) content was determined in muscle homogenates by SDS-PAGE. Body weight was significantly lower in tumor-bearing (T) mice. There was a significant decrease in muscle mass in all three muscles tested compared with control, with the largest decrease occurring in the soleus. Although no type IIb MHC was detected in the soleus samples from control mice, type IIb comprised 19% of the total MHC in T soleus. Type I MHC was significantly decreased in T vs. control soleus muscle. MHC isoform content was not significantly different from control in plantaris and gastrocnemius muscles. These data are the first to show a change in myosin isoform expression accompanying muscle atrophy during cancer cachexia.

Exercise training increases the Ca(2+) sensitivity of tension in rat cardiac myocytes

The heart is known to respond to a program of chronic exercise in ways that enhance cardiac function. However, the cellular mechanisms involved in training-induced improvements in the contractile function of the myocardium are not known. In this study we tested the hypothesis that increased contractility of the myocardium associated with exercise training is due, in part, to increases in the Ca(2+) sensitivity of steady-state tension. Female Sprague-Dawley rats were randomly divided into sedentary control (C) and exercise-trained (T) groups. The T rats underwent 11 wk of progressive treadmill exercise (1 h/day, 5 days/wk, 26 m/min, 20% grade). Evidence of training effect included a 5.9% increase in heart mass, increases in heart weight-to-body weight ratio, and a 60% increase in skeletal muscle citrate synthase activity in T rats compared with C rats. After the training program, cardiac myocytes were isolated from T and C hearts. Myocytes were chemically skinned (i.e., the sarcolemma was removed) and attached to a force transducer, and steady-state tension was determined in solutions of various Ca(2+) concentrations ([Ca(2+)]). Myocytes isolated from the hearts of T rats showed a significantly (P < 0.01) increased sensitivity of tension to [Ca(2+)]. The [Ca(2+)] giving 50% of maximal tension (pCa(50)) was 5.90 +/- 0.033 and 5.82 +/- 0.023 (SD) in T and C myocytes, respectively (n = 70 myocytes/group). This result suggests that exercise training affects the myofibrillar proteins, such that Ca(2+) sensitivity is increased, and that this may be the mechanism that underlies, at least in part, the effect of training to increase myocardial contractility.

Phosphorylation of myosin regulatory light chain eliminates force-dependent changes in relaxation rates in skeletal muscle

The rate of relaxation from steady-state force in rabbit psoas fiber bundles was examined before and after phosphorylation of myosin regulatory light chain (RLC). Relaxation was initiated using diazo-2, a photolabile Ca2+ chelator that has low Ca2+ binding affinity (K(Ca) = 4.5 x 10(5) M(-1)) before photolysis and high affinity (K(Ca) = 1.3 x 10(7) M(-1)) after photolysis. Before phosphorylating RLC, the half-times for relaxation initiated from 0.27 +/- 0.02, 0.51 +/- 0.03, and 0.61 +/- 0.03 Po were 90 +/- 6, 140 +/- 6, and 182 +/- 9 ms, respectively. After phosphorylation of RLC, the half-times for relaxation from 0.36 +/- 0.03 Po, 0.59 +/- 0.03 Po, and 0.65 +/- 0.02 Po were 197 +/- 35 ms, 184 +/- 35 ms, and 179 +/- 22 ms. This slowing of relaxation rates from steady-state forces less than 0.50 Po was also observed when bundles of fibers were bathed with N-ethylmaleimide-modified myosin S-1, a strongly binding cross-bridge derivative of S1. These results suggest that phosphorylation of RLC slows relaxation, most likely by slowing the apparent rate of transition of cross-bridges from strongly bound (force-generating) to weakly bound (non-force-generating) states, and reduces or eliminates Ca2+ and cross-bridge activation-dependent changes in relaxation rates.

Altered kinetics of contraction in skeletal muscle fibers containing a mutant myosin regulatory light chain with reduced divalent cation binding

We examined the kinetic properties of rabbit skinned skeletal muscle fibers in which the endogenous myosin regulatory light chain (RLC) was partially replaced with a mutant RLC (D47A) containing a point mutation within the Ca2+/Mg2+ binding site that severely reduced its affinity for divalent cations. We found that when approximately 50% of the endogenous RLC was replaced by the mutant, maximum tension declined to approximately 60% of control and the rate constant of active tension redevelopment (ktr) after mechanical disruption of cross-bridges was reduced to approximately 70% of control. This reduction in ktr was not an indirect effect on kinetics due to a reduced number of strongly bound myosin heads, because when the strongly binding cross-bridge analog N-ethylmaleimide-modified myosin subfragment1 (NEM-S1) was added to the fibers, there was no effect upon maximum ktr. Fiber stiffness declined after D47A exchange in a manner indicative of a decrease in the number of strongly bound cross-bridges, suggesting that the force per cross-bridge was not significantly affected by the presence of D47A RLC. In contrast to the effects on ktr, the rate of tension relaxation in steadily activated fibers after flash photolysis of the Ca2+ chelator diazo-2 increased by nearly twofold after D47A exchange. We conclude that the incorporation of the nondivalent cation-binding mutant of myosin RLC decreases the proportion of cycling cross-bridges in a force-generating state by decreasing the rate of formation of force-generating bridges and increasing the rate of detachment. These results suggest that divalent cation binding to myosin RLC plays an important role in modulating the kinetics of cross-bridge attachment and detachment.

Myosin regulatory light chain modulates the Ca2+ dependence of the kinetics of tension development in skeletal muscle fibers

To determine the role of myosin regulatory light chain (RLC) in modulating contraction in skeletal muscle, we examined the rate of tension development in bundles of skinned skeletal muscle fibers as a function of the level of Ca(2+) activation after UV flash-induced release of Ca(2+) from the photosensitive Ca(2+) chelator DM-nitrophen. In control fiber bundles, the rate of tension development was highly dependent on the concentration of activator Ca(2+) after the flash. There was a greater than twofold increase in the rate of tension development when the post-flash [Ca(2+)] was increased from the lowest level tested (which produced a steady tension that was 42% of maximum tension) to the highest level (producing 97% of maximum tension). However, when 40-70% of endogenous myosin RLC was extracted from the fiber bundles, tension developed at the maximum rate, regardless of the post-flash concentration of Ca(2+). Thus, the Ca(2+) dependence of the rate of tension development was eliminated by partial extraction of myosin RLC, an effect that was partially reversed by recombination of RLC back into the fiber bundles. The elimination of the Ca(2+) dependence of the kinetics of tension development was specific to the extraction of RLC rather than an artifact of the co-extraction of both RLC and Troponin C, because the rate of tension development was still Ca(2+) dependent, even when nearly 50% of endogenous Troponin C was extracted from fiber bundles fully replete with RLC. Thus, myosin RLC appears to be a key component in modulating Ca(2+) sensitive cross-bridge transitions that limit the rate of force development after photorelease of Ca(2+) in skeletal muscle fibers.

Effects of a non-divalent cation binding mutant of myosin regulatory light chain on tension generation in skinned skeletal muscle fibers

Each myosin molecule contains two heavy chains and a total of four low-molecular weight light chain subunits, two "essential" and two "regulatory" light chains (RLCs). Although the roles of myosin light chains in vertebrate striated muscle are poorly understood at present, recent studies on the RLC have suggested that it has a modulatory role with respect to Ca2+ sensitivity of tension and the rate of tension development, effects that may be mediated by Ca2+ binding to the RLC. To examine possible roles of the RLC Ca2+/Mg2+ binding site in tension development by skeletal muscle, we replaced endogenous RLC in rabbit skinned psoas fibers with an avian mutant RLC (D47A) having much reduced affinity for divalent cations. After replacement of up to 80% of the endogenous RLC with D47A RLC, maximum tension (at pCa 4.5) was significantly reduced compared with preexchange tension, and the amount of decrease was directly related to the extent of D47A exchange. Fiber stiffness changed in proportion to tension, indicating that the decrease in tension was due to a decrease in the number of tension-generating cross-bridges. Decreases in both tension and stiffness were substantially, although incompletely, reversed after reexchange of native RLC for D47A. RLC exchange was also performed using a wild-type RLC. Although a small decrease in tension was observed after wild-type RLC exchange, the decrease was not proportional to the extent of RLC exchange and was not reversed by reexchange of the native RLC. D47A exchange also decreased the Ca2+ sensitivity of tension and reduced the apparent cooperativity of tension development. The results suggest that divalent cation binding to myosin RLC plays an important role in tension generation in skeletal muscle fibers.

Activity-induced regulation of myosin isoform distribution: comparison of two contractile activity programs

This study examined the role of specific types of contractile activity in regulating myosin heavy chain (MHC) isoform expression in rodent soleus. A combination of hindlimb suspension (SN) and two programmed contractile training activity paradigms, either isometric contractile activity (ST-IM) or high-load slowly shortening isovelocity activity, were utilized. Both training paradigms increased muscle mass compared with SN alone. However, only ST-IM resulted in a partial prevention of the suspension-induced decrease in type I MHC. With the use of a fluorescently labeled antibody to type IIa MHC, the distribution of MHCs among fibers was examined immunohistochemically. In SN, the percentage of cells staining positive for type IIa MHC was increased but the staining intensity of the positively staining cells was unchanged compared with control cells. In the ST-IM soleus, the percentage of positively staining fibers was unchanged but the intensity of the positively staining cells was decreased compared with SN values. These results suggest that: 1) isometric contractile activity is more effective than isovelocity activity in preventing suspension-induced shifts in soleus MHC distribution and 2) changes associated with both suspension and training occur in only a small number of fibers, with the majority of fibers apparently unresponsive to these interventions.

Interaction of various mechanical activity models in regulation of myosin heavy chain isoform expression

The purpose of this study was to determine the effects of a novel combination of mechanical activity paradigms on the isomyosin distribution in rat hindlimb muscles. Thirty female Sprague-Dawley rats were divided into five experimental groups as follows: normal control, functional overload (OV) of the plantaris, OV in conjunction with hindlimb suspension (OV-S), and a combination of OV-S and either static standing weight-bearing activity (OV-SS) or high-incline treadmill exercise (OV-SE). OV of the plantaris resulted in significant hypertrophy and significant fast-to-slow isomyosin shifts. These changes were completely inhibited by the addition of hindlimb suspension (OV-S). Also, neither of the two weight-bearing regimes (OV-SS and OV-SE) was able to attenuate the suspension-induced atrophy. In the vastus intermedius and vastus lateralis, however, OV-SS was able to partially retard the atrophy associated with suspension. In both the plantaris and vastus intermedius, only OV-SS was able to partially reverse the slow-to-fast isomyosin transitions associated with suspension. These results suggest that the type of mechanical activity is important in determining adaptation to altered loading conditions, with OV-SS appearing more effective than OV-SE in reversing the effects of unweighting.

Control of myosin heavy chain expression: interaction of hypothyroidism and hindlimb suspension

The aim of this study was to contrast competing influences, hypothyroidism and hindlimb suspension, on myosin heavy chain (MHC) expression studied at the protein level and mRNA level. Female Sprague-Dawley rats were assigned to either normal control (NC), normal suspended (NS), or hypothyroid (thyroidectomized) control (TC) and suspended (TS) groups. NS and TS animals were suspended for 14 days following which myofibrils and total RNA were purified from the hindlimb muscles. In the soleus and vastus intermedius (VI), there was an increase in type I MHC and a decrease in type IIa MHC in both the TC and TS groups and a decrease in type I and increase in type IIa MHC in the NS group. At the mRNA level, similar shifts were observed with the exception that 1) the increased type IIa MHC seen in the soleus and VI of the NS animals was not accompanied by an increase in IIa mRNA and 2) type IIb mRNA was increased in the NS soleus without concomitant changes in IIb protein levels. These data suggest the following: 1) a hypothyroid state predominates over mechanical unweighting factors in the control of MHC distribution in slow muscles; and 2) translational or posttranslational factors may be important in the regulation of type IIa and IIb MHC expression during hindlimb suspension.

Contractile and biochemical properties of rat soleus and plantaris after hindlimb suspension

This study examined the relationship between contractile and isomyosin changes occurring in rat soleus (SOL) and plantaris (PLAN) muscles after 28 days of hindlimb suspension. SOL muscles from suspended animals exhibited a 45% decline in muscle weight compared with controls (P less than 0.05) accompanied by a 49% decrease in peak twitch tension (Pt) and a 59% reduction in peak tetanic tension (Po). Smaller reductions were observed in muscle weight, Pt, and Po (12, 43, and 24%, respectively) for the suspended PLAN. Maximal shortening velocity (Vmax) of the suspended SOL and the velocity of unloaded shortening were increased by 36 and 35%, respectively, but there was no suspension-induced change in PLAN Vmax. Suspension induced a 22% increase in SOL myosin adenosinetriphosphatase (ATPase) activity that was accompanied by a shift in the native myosin isoform distribution characterized by an increase in the relative amounts of intermediate and fast myosin. The more modest changes in the contractile function of suspended PLAN were accompanied by a small (7%) increase in myosin ATPase activity but no significant changes in myosin isoform distribution. The results of this study confirm that hindlimb suspension results in significant speeding of SOL contractile properties and suggest that the shift toward faster myosin isoforms with a higher myosin ATPase activity likely accounts for these mechanical changes.

Effects of endurance exercise on isomyosin patterns in fast- and slow-twitch skeletal muscles

Although endurance training has been shown to profoundly affect the oxidative capacity of skeletal muscle, little information is available concerning the impact of endurance training on skeletal muscle isomyosin expression across a variety of muscle fiber types. Therefore, a 10-wk running program (1 h/day, 5 days/wk, 20% grade, 1 mile/h) was conducted to ascertain the effects of endurance training on isomyosin expression in the soleus, vastus intermedius (VI), plantaris (PLAN), red and white medial gastrocnemius (RMG and WMG), and red and white vastus lateralis muscles (RVL and WVL). Evidences of training were noted by the presence of a resting and a submaximal exercise bradycardia, as well as an enhancement in peak O2 consumption in the trained rodents relative to the nontrained controls. No evidence for skeletal muscle hypertrophy was observed subsequent to training when muscle weight was normalized to body weight. Shifts in the isomyosin profile of the trained VI, RMG, RVL, and PLAN were seen relative to the nontrained controls. Specifically, training affected the slow myosin (SM) composition of the VI by decreasing the relative content of the SM2 isoform by 14% while increasing that of the SM1 isoform (P less than 0.05). In addition, training elicited various degrees of a fast to slower myosin transformation in the RMG, RVL, and PLAN. All three muscles showed a significant reduction in the fast myosin 2 isoform (P less than 0.05), with significant increases in intermediate myosin in the RVL and PLAN along with elevations in SM2 in the RMG and PLAN (P less than 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)