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]

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]

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]

Background. Obesity, osteoporosis, and sarcopenia may individually occur due to age-related gradual alterations in body composition. This study investigates the cooccurrence of these age-related diseases in female animals with low levels of ovarian hormone in the absence of complex multifactorial process of chronological aging. Methods. Thirty-six 5- and 10-month-old female rats were chosen to model pre- and postmenopausal women, respectively. Rats were divided into three treatment groups in each age category—sham, ovariectomized (ovx), and ovx + E2 (17β-estradiol, 10 μg/kg)—and were pair-fed. Volunteer wheel running activity, body composition, bone microstructure, serum C-telopeptides of type I collagen, bone specific alkaline phosphatase, E2, and gastrocnemius and soleus muscles were analyzed. Results. The cooccurrence of osteoporosis, sarcopenia, and obesity was observed in the older ovx rats associated with a significant (p<0.05) increased fat mass (30%), bone loss (9.6%), decreased normalized muscle mass-to-body-weight ratio (10.5%), and a significant decrease in physical activity (57%). The ratio of tibial bone mineral density to combined muscle mass was significantly decreased in both ovx age categories. Conclusion. Ovariectomized rat could be used as an experimental model to examine the effect of loss of ovarian hormones, while controlling for energy intake and expenditure, to conduct obesity and body composition translational research in females without the confounding effect of genetic background. [ABSTRACT FROM AUTHOR]

The digital flexors of horses must produce high force to support the body weight during running, and a need for these muscles to generate power is likely limited during locomotion over level ground. Measurements of power output from horse muscle fibers close to physiological temperatures, and when cyclic strain is imposed, will help to better understand the in vivo performance of the muscles as power absorbers and generators. Skinned fibers from the deep (DDF) and superficial (SDF) digital flexors, and the soleus (SOL) underwent sinusoidal oscillations in length over a range of frequencies (0.5-16 Hz) and strain amplitudes (0.01-0.06) under maximum activation ( pCa 5) at 30°C. Results were analyzed using both workloop and Nyquist plot analyses to determine the ability of the fibers to absorb or generate power and the frequency dependence of those abilities. Power absorption was dominant at most cycling frequencies and strain amplitudes in fibers from all three muscles. However, small amounts of power were generated (0.002-0.05 Wkg−1) at 0.01 strain by all three muscles at relatively slow cycling frequencies: DDF (4-7 Hz), SDF (4-5 Hz) and SOL (0.5-1 Hz). Nyquist analysis, reflecting the influence of cross-bridge kinetics on power generation, corroborated these results. The similar capacity for power generation by DDF and SDF versus lower for SOL, and the faster frequency at which this power was realized in DDF and SDF fibers, are largely explained by the fast myosin heavy chain isoform content in each muscle. Contractile function of DDF and SDF as power absorbers and generators, respectively, during locomotion may therefore be more dependent on their fiber architectural arrangement than on the physiological properties of their muscle fibers. [ABSTRACT FROM AUTHOR]

Sensing biological agents at the genomic level, while enhancing the response time for biodetection over commonly used, optics-based techniques such as nucleic acid microarrays or enzyme-linked immunosorbent assays (ELISAs), is an important criterion for new biosensors. Here, we describe the successful detection of a 35-base, single-strand nucleic acid target by Hall-based magnetic transduction as a mimic for pathogenic DNA target detection. The detection platform has low background, large signal amplification following target binding and can discriminate a single, 350nm superparamagnetic bead labeled with DNA. Detection of the target sequence was demonstrated at 364pM (<2 target DNA strands per bead) target DNA in the presence of 36 μM nontarget (noncomplementary) DNA (<10 ppm target DNA) using optical microscopy detection on a GaAs Hall mimic. The use of Hall magnetometers as magnetic transduction biosensors holds promise for multiplexing applications that can greatly improve point-of-care (POC) diagnostics and subsequent medical care. [ABSTRACT FROM AUTHOR]

Microfabricated thermoelectric controllers can be employed to investigate mechanisms underlying myosin-driven sliding of Ca2+- regulated actin and disease-associated mutations in myofilament proteins. Specifically, we examined actin filament sliding—with or without human cardiac troponin (Tn) and α-tropomyosin (Tm)—propelled by rabbit skeletal heavy meromyosin, when temperature was varied continuously over a wide range (∼20-63°C). At the upper end of this temperature range, reversible dysregulation of thin filaments occurred at pCa 9 and 5; actomyosin function was unaffected. Tn-Tm enhanced sliding speed at pCa 5 and increased a transition temperature (Tt) between a high activation energy (Ea) but low temperature regime and a low Ea but high temperature regime. This was modulated by factors that alter cross-bridge number and kinetics. Three familial hypertrophic cardiomyopathy (FHC) mutations, cTnI R145G, cTnI K206Q, and cTnT R278C, cause dysregulation at temperatures ∼5-8°C lower; the latter two increased speed at pCa 5 at all temperatures. [ABSTRACT FROM AUTHOR]

Familial hypertrophic cardiomyopathy (FHC) is a disease of cardiac sarcomeres. To identify molecular mechanisms underlying FHC pathology, functional and structural differences in three FHC-related mutations in recombinant a-Tm (V95A, D175N, and E180G) were characterized using both conventional and modified in vitro motility assays and circular dichroism spectroscopy. Mutant Tm's exhibited reduced a-helical structure and increased unordered structure. When thin filaments were fully occupied by regulatory proteins, little or no motion was detected at pCa 9, and maximum speed (pCa 5) was similar for all tropomyosins. Ca2+-responsiveness of filament sliding speed was increased either by increased pCa50 (V95A), reduced cooperativity n (D175N), or both (E180G). When temperature was increased, thin filaments with E180G exhibited dysregulation at temperatures ∼10°C lower, and much closer to body temperature, than WT. When HMM density was reduced, thin filaments with D175N required fewer motors to initiate sliding or achieve maximum sliding speed. [ABSTRACT FROM AUTHOR]

Thin-filament regulation of isometric force redevelopment ( ktr) was examined in rabbit psoas fibres by substituting native TnC with either cardiac TnC (cTnC), a site I-inactive skeletal TnC mutant (xsTnC), or mixtures of native purified skeletal TnC (sTnC) and a site I- and II-inactive skeletal TnC mutant (xxsTnC). Reconstituted maximal Ca2+-activated force (r Fmax) decreased as the fraction of sTnC in sTnC: xxsTnC mixtures was reduced, but maximal ktr was unaffected until r Fmax was <0.2 of pre-extracted Fmax. In contrast, reconstitution with cTnC or xsTnC reduced maximal ktr to 0.48 and 0.44 of control ( P < 0.01), respectively, with corresponding r Fmax of 0.68 ± 0.03 and 0.25 ± 0.02 Fmax. The ktr–pCa relation of fibres containing sTnC: xxsTnC mixtures (r Fmax > 0.2 Fmax) was little effected, though ktr was slightly elevated at low Ca2+ activation. The magnitude of the Ca2+-dependent increase in ktr was greatly reduced following cTnC or xsTnC reconstitution because ktr at low levels of Ca2+ was elevated and maximal ktr was reduced. Solution Ca2+ dissociation rates ( koff) from whole Tn complexes containing sTnC (26 ± 0.1 s−1), cTnC (38 ± 0.9 s−1) and xsTnC (50 ± 1.2 s−1) correlated with ktr at low Ca2+ levels and were inversely related to r Fmax. At low Ca2+ activation, ktr was similarly elevated in cTnC-reconstituted fibres with ATP or when cross-bridge cycling rate was increased with 2-deoxy-ATP. Our results and model simulations indicate little or no requirement for cooperative interactions between thin-filament regulatory units in modulating ktr at any [Ca2+] and suggest Ca2+ activation properties of individual troponin complexes may influence the apparent rate constant of cross-bridge detachment. [ABSTRACT FROM AUTHOR]

Myosin heavy chain (MHC) isoforms in vertebrate striated muscles are distinguished functionally by differences in chemomechanical kinetics. These kinetic differences may influence the cross-bridge-dependent co-operativity of thin filament Ca2+ activation. To determine whether Ca2+ sensitivity of unloaded thin filament sliding depends upon MHC isoform kinetics, we performed in vitro motility assays with rabbit skeletal heavy meromyosin (rsHMM) or porcine cardiac myosin (pcMyosin). Regulated thin filaments were reconstituted with recombinant human cardiac troponin (rhcTn) and α-tropomyosin (rhcTm) expressed in Escherichia coli. All three subunits of rhcTn were coexpressed as a functional complex using a novel construct with a glutathione S-transferase (GST) affinity tag at the N-terminus of human cardiac troponin T (hcTnT) and an intervening tobacco etch virus (TEV) protease site that allows purification of rhcTn without denaturation, and removal of the GST tag without proteolysis of rhcTn subunits. Use of this highly purified rhcTn in our motility studies resulted in a clear definition of the regulated motility profile for both fast and slow MHC isoforms. Maximum sliding speed (pCa 5) of regulated thin filaments was roughly fivefold faster with rsHMM compared with pcMyosin, although speed was increased by 1.6- to 1.9-fold for regulated over unregulated actin with both MHC isoforms. The Ca2+ sensitivity of regulated thin filament sliding speed was unaffected by MHC isoform. Our motility results suggest that the cellular changes in isoform expression that result in regulation of myosin kinetics can occur independently of changes that influence thin filament Ca2+ sensitivity. [ABSTRACT FROM AUTHOR]