Search

Older adults are vulnerable to glucocorticoid‐induced muscle atrophy and weakness, with sex potentially influencing their susceptibility to those effects. Aerobic exercise can reduce glucocorticoid‐induced muscle atrophy in young rodents. However, it is unknown whether aerobic exercise can prevent glucocorticoid myopathy in aged muscle. The objectives of this study were to define the extent to which sex influences the development of glucocorticoid myopathy in aged muscle, and to determine the extent to which aerobic exercise training protects against myopathy development. Twenty‐four‐month‐old female (n = 30) and male (n = 33) mice were randomized to either sedentary or aerobic exercise groups. Within their respective groups, mice were randomized to either daily treatment with dexamethasone (DEX) or saline. Upon completing treatments, the contractile properties of the triceps surae complex were assessed in situ. DEX marginally lowered muscle mass and soluble protein content in both sexes, which was attenuated by aerobic exercise only in females. DEX increased sub‐tetanic force and rate of force development only in females, which was not influenced by aerobic exercise. Muscle fatigue was higher in both sexes following DEX, but aerobic exercise prevented fatigue induction only in females. The sex‐specific differences to muscle function in response to DEX treatment coincided with sex‐specific changes to the content of proteins related to calcium handling, mitochondrial quality control, reactive oxygen species production, and glucocorticoid receptor in muscle. These findings define several important sexually dimorphic changes to aged skeletal muscle physiology in response to glucocorticoid treatment and define the capacity of short‐term aerobic exercise to protect against those changes. Key points: There are sexually dimorphic effects of glucocorticoids on aged skeletal muscle physiology.Glucocorticoid‐induced changes to aged muscle contractile properties coincide with sex‐specific differences in the content of calcium handling proteins.Aerobic exercise prevents glucocorticoid‐induced fatigue only in aged females and coincides with differences in the content of mitochondrial quality control proteins and glucocorticoid receptors. [ABSTRACT FROM AUTHOR]

The C-terminal region of cardiac troponin I (cTnI) is known to be important in cardiac function, as removal of the last 17 C-terminal residues of human cTnI has been associated with myocardial stunning. To investigate the C-terminal region of cTnI, three C-terminal deletion mutations in human cTnI were generated: Δ1 (deletion of residue 210), Δ3 (deletion of residues 208-210), and Δ5 (deletion of residues 206-210). Mammalian two-hybrid studies showed that the interactions between cTnI mutants and cardiac troponin C (cTnC) or cardiac troponin T (cTnT) were impaired in Δ3 and Δ5 mutants when compared to wild-type cTnI. Troponin complexes containing 2-[4'-(iodoacetamido) anilino] naphthalene-6-sulfonic acid (IAANS) labeled cTnC showed that the troponin complex containing cTnI Δ5 had a small increase in Ca(2+) affinity (P

This article discusses the role of the ryanodine receptor (RyR) in skeletal muscle and its connection to myosin, a protein involved in muscle contraction. Genetic variants of the RYR1 gene can affect the release of calcium from the sarcoplasmic reticulum, leading to altered muscle contraction and potentially causing congenital myopathies. The study examines the post-translational modifications of myosin in patients with RYR1-related myopathies and suggests that these modifications may contribute to the pathophysiology of the disease. The authors propose that changes in the intracellular environment, including altered calcium handling, can have secondary effects on myosin structure and function, impacting muscle contractility. [Extracted from the article]

Missense variant Ile79Asn in human cardiac troponin T (cTnT-I79N) has been associated with hypertrophic cardiomyopathy and sudden cardiac arrest in juveniles. cTnT-I79N is located in the cTnT N-terminal (TnT1) loop region and is known for its pathological and prognostic relevance. A recent structural study revealed that I79 is part of a hydrophobic interface between the TnT1 loop and actin, which stabilizes the relaxed (OFF) state of the cardiac thin filament. Given the importance of understanding the role of TnT1 loop region in Ca2+ regulation of the cardiac thin filament along with the underlying mechanisms of cTnT-I79N-linked pathogenesis, we investigated the effects of cTnT-I79N on cardiac myofilament function. Transgenic I79N (Tg-I79N) muscle bundles displayed increased myofilament Ca2+ sensitivity, smaller myofilament lattice spacing, and slower crossbridge kinetics. These findings can be attributed to destabilization of the cardiac thin filament's relaxed state resulting in an increased number of crossbridges during Ca2+ activation. Additionally, in the low Ca2+-relaxed state (pCa8), we showed that more myosin heads are in the disordered-relaxed state (DRX) that are more likely to interact with actin in cTnT-I79N muscle bundles. Dysregulation of the myosin super-relaxed state (SRX) and the SRX/DRX equilibrium in cTnT-I79N muscle bundles likely result in increased mobility of myosin heads at pCa8, enhanced actomyosin interactions as evidenced by increased active force at low Ca2+, and increased sinusoidal stiffness. These findings point to a mechanism whereby cTnT-I79N weakens the interaction of the TnT1 loop with the actin filament, which in turn destabilizes the relaxed state of the cardiac thin filament. [ABSTRACT FROM AUTHOR]

Muscle fatigue is characterized by a reduction in force and shortening velocity due to intracellular acidosis. To determine whether a relationship does exist between pH and force, 31P-NMR experiments were performed at different temperatures on intact skeletal muscle from mouse. The results revealed inhibition of tetanic force in hypercapnia (25% carbon dioxide) at the temperature range of 15 to 25 degrees Centigrade. Temperature-dependent inhibition of force was greater at lower temperatures, suggesting that temperature rather than pH influences muscle force development.

Assay and chromatographic studies indicate that the direct inhibition of actomysin during muscle fatigue does not involve adenosine diphosphate (ADP), as confirmed by a measurement of physiological concentrations of MgADP on single skinned fibers' unloaded shortening velocity and isometric force during fatigue of rabbit fast- and slow-twitch muscles. Changes in creatine concentration using buffer solutions help vary the MgADP levels in the presence of phosphocreatine and creatine kinase and enable ADP and ATP temporal and spatial buffering.

The interaction between the reactive thiol SH1 region of myosin subfragment 1 and the N-terminal segment on actin was studied with modifications of SH1 and SH2 by N-ethylamaleimide in the presence of actin, actin complexed with the N-terminal antibody Fab(1-7) and actin with its N-terminal acidic residues modified by carbodiimide coupling of ethylenediamine. An indirect interaction between the SH1 site on S1 and the N-terminus actin was shown.

Abstract Background The efficacy of adult stem cells is known to be compromised as a function of age. This therefore raises questions about the effectiveness of autologous cell therapy in elderly patients. Results We demonstrated that the expression profile of stemness markers was altered in BM-MSCs derived from old rats. BM-MSCs from young rats (4 months) expressed Oct-4, Sox-2 and NANOG, but we failed to detect Sox-2 and NANOG in BM-MSCs from older animals (15 months). Chondrogenic, osteogenic and adipogenic potential is compromised in old BM-MSCs. Stimulation with a cocktail mixture of bone morphogenetic protein (BMP-2), fibroblast growth factor (FGF-2) and insulin-like growth factor (IGF-1) induced cardiomyogenesis in young BM-MSCs but not old BM-MSCs. Significant differences in the expression of gap junction protein connexin-43 were observed between young and old BM-MSCs. Young and old BM-MSCs fused with neonatal ventricular cardiomyocytes in co-culture and expressed key cardiac transcription factors and structural proteins. Cells from old animals expressed significantly lower levels of VEGF, IGF, EGF, and G-CSF. Significantly higher levels of DNA double strand break marker γ-H2AX and diminished levels of telomerase activity were observed in old BM-MSCs. Conclusion The results suggest age related differences in the differentiation capacity of BM-MSCs. These changes may affect the efficacy of BM-MSCs for use in stem cell therapy.

Ants use their mandibles for a variety of functions and behaviors. We investigated mandibular muscle structure and function from major workers of the Florida carpenter ant Camponotus floridanus: force-pCa relation and velocity of unloaded shortening of single, permeabilized fibres, primary sequences of troponin subunits (TnC, TnI and TnT) from a mandibular muscle cDNA library, and muscle fibre ultrastructure. From the mechanical measurements, we found Ca2+-sensitivity of isometric force was markedly shifted rightward compared with vertebrate striated muscle. From the troponin sequence results, we identified features that could explain the rightward shift of Ca2+-activation: the N-helix of TnC is effectively absent and three of the four EF-hands of TnC (sites I, II and III) do not adhere to canonical sequence rules for divalent cation binding; two alternatively spliced isoforms of TnI were identified with the alternatively spliced exon occurring in the region of the IT-arm α-helical coiled-coil, and the N-terminal extension of TnI may be involved in modulation of regulation, as in mammalian cardiac muscle; and TnT has a Glu-rich C-terminus. In addition, a structural homology model was built of C. floridanus troponin on the thin filament. From analysis of electron micrographs, we found thick filaments are almost as long as the 6.8 μm sarcomeres, have diameter of ~ 16 nm, and typical center-to-center spacing of ~ 46 nm. These results have implications for the mechanisms by which mandibular muscle fibres perform such a variety of functions, and how the structure of the troponin complex aids in these tasks. [ABSTRACT FROM AUTHOR]

Every heartbeat relies on cyclical interactions between myosin thick and actin thin filaments orchestrated by rising and falling Ca2+ levels. Thin filaments are comprised of two actin strands, each harboring equally separated troponin complexes, which bind Ca2+ to move tropomyosin cables away from the myosin binding sites and, thus, activate systolic contraction. Recently, structures of thin filaments obtained at low (pCa ~9) or high (pCa ~3) Ca2+ levels revealed the transition between the Ca2+-free and Ca2+-bound states. However, in working cardiac muscle, Ca2+ levels fluctuate at intermediate values between pCa ~6 and pCa ~7. The structure of the thin filament at physiological Ca2+ levels is unknown. We used cryoelectron microscopy and statistical analysis to reveal the structure of the cardiac thin filament at systolic pCa = 5.8. We show that the two strands of the thin filament consist of a mixture of regulatory units, which are composed of Ca2+-free, Ca2+-bound, or mixed (e.g., Ca2+ free on one side and Ca2+ bound on the other side) troponin complexes. We traced troponin complex conformations along and across individual thin filaments to directly determine the structural composition of the cardiac native thin filament at systolic Ca2+ levels. We demonstrate that the two thin filament strands are activated stochastically with short-range cooperativity evident only on one of the two strands. Our findings suggest a mechanism by which cardiac muscle is regulated by narrow range Ca2+ fluctuations. [ABSTRACT FROM AUTHOR]

Heart disease remains the number one killer of women in the US. Nonetheless, studies in women and female animal models continue to be underrepresented in cardiac research. Hypertrophic cardiomyopathy (HCM), the most commonly inherited cardiac disorder, has been tied to sarcomeric protein variants in both sexes. Among the susceptible genes, TNNC1—encoding cardiac troponin C (cTnC)—causes a substantial HCM phenotype in mice. Mice bearing an HCM‐associated cTnC‐A8V point mutation exhibited a significant decrease in stroke volume and left ventricular diameter and volume. Importantly, isovolumetric contraction time was significantly higher for female HCM mice. We utilized a transcriptomic approach to investigate the basis underlying the sexual dimorphism observed in the cardiac physiology of adult male and female HCM mice. RNA sequencing revealed several altered canonical pathways within the HCM mice versus WT groups including an increase in eukaryotic initiation factor 2 signaling, integrin‐linked kinase signaling, actin nucleation by actin‐related protein‐Wiskott‐Aldrich syndrome family protein complex, regulation of actin‐based motility by Rho kinase, vitamin D receptor/retinoid X receptor activation, and glutathione redox reaction pathways. In contrast, valine degradation, tricarboxylic acid cycle II, methionine degradation, and inositol phosphate compound pathways were notably down‐regulated in HCM mice. These down‐regulated pathways may be reduced in response to altered energetics in the hypertrophied hearts and may represent conservation of energy as the heart is compensating to meet increased contractile demands. HCM male versus female mice followed similar trends of the canonical pathways altered between HCM and WT. In addition, seven of the differentially expressed genes in both WT and HCM male versus female comparisons swapped directions in fold‐change between the sexes. These findings suggest a sexually‐dimorphic HCM phenotype due to a sarcomeric mutation and pinpoint several key targetable pathways and genes that may provide the means to alleviate the more severe decline in female cardiac function. [ABSTRACT FROM AUTHOR]

Delta 14 mutation of troponin T, associated with familial hypertrophic cardiomyapathy, results in an increase in ATPase rate like that seen with wild-type troponin in the presence of NEM-S1is shown. An analysis of the effect of the delta 14 mutation on the equilibrium binding of S1-ADP to actin was consistent with stabilization of an active state of actin.

Chase, P. Bryant, Ying Chen, Kristi L. Kulin, and Thomas L. Daniel. Viscosity and solute dependence of F-actin translocation by rabbit skeletal heavy meromyosin. Am J Physiol Cell Physiol 278: C1088-C1098, 2000.--We tested the hypothesis that solvent viscosity affects translocation of rhodamine phalloidin-labeled F-actin by rabbit skeletal heavy meromyosin (HMM). When viscosity was increased using either glycerol, fructose, sucrose, or dextran (1.5, 6.0, or 15-20 kDa mol mass), there was little or no effect on the fraction of moving filaments, whereas sliding speed decreased in inverse proportion to viscosity. The results could be explained neither by an effect of osmotic pressure at high solute concentrations nor by altered solvent drag on the actin filament. Elevated viscosity inhibited HMM ATPase activity in solution, but only at much higher viscosities than were needed to reduce sliding speed. Polyethylene glycols (300, 1,000, or 3,000 mol wt) also inhibited speed via elevated viscosity but secondarily inhibited by enhancing electrostatic interactions. These results demonstrate that a diffusion-controlled process intrinsic to cross-bridge cycling can be limiting to actomyosin function. in vitro motility assay; protein dynamics of biological motors; diffusion; sugars; polyethylene glycols