Sarah M. Greising

Hormone Therapy and Skeletal Muscle Strength: A Meta-Analysis

BACKGROUND Our objective was to perform a systematic review and meta-analysis of the research literature that compared muscle strength in postmenopausal women who were and were not on estrogen-based hormone therapy (HT). METHODS Twenty-three relevant studies were found. Effect sizes (ESs) were calculated as the standardized mean difference, and meta-analyses were completed using a random effects model. RESULTS HT was found to result in a small beneficial effect on muscle strength in postmenopausal women (overall ES = 0.23; p = .003) that equated to an approximately 5% greater strength for women on HT. Among the 23 studies, various muscle groups were assessed for strength, and those that benefitted the most were the thumb adductors (ES = 1.14; p < .001). Ten studies that compared muscle strength in rodents that were and were not estradiol deficient were also analyzed. The ES for absolute strength was moderate but not statistically significant (ES = 0.44; p = .12), whereas estradiol had a large effect on strength normalized to muscle size (ES = 0.66; p = .03). CONCLUSION Overall, estrogen-based treatments were found to beneficially affect strength.

Estrogen Regulates Estrogen Receptors and Antioxidant Gene Expression in Mouse Skeletal Muscle

Background Estrogens are associated with the loss of skeletal muscle strength in women with age. Ovarian hormone removal by ovariectomy in mice leads to a loss of muscle strength, which is reversed with 17β-estradiol replacement. Aging is also associated with an increase in antioxidant stress, and estrogens can improve antioxidant status via their interaction with estrogen receptors (ER) to regulate antioxidant gene expression. The purpose of this study was to determine if ER and antioxidant gene expression in skeletal muscle are responsive to changes in circulating estradiol, and if ERs regulate antioxidant gene expression in this tissue. Methodology/Principal Findings Adult C57BL/6 mice underwent ovariectomies or sham surgeries to remove circulating estrogens. These mice were implanted with placebo or 17β-estradiol pellets acutely or chronically. A separate experiment examined mice that received weekly injections of Faslodex to chronically block ERs. Skeletal muscles were analyzed for expression of ER genes and proteins and antioxidant genes. ERα was the most abundant, followed by Gper and ERβ in both soleus and EDL muscles. The loss of estrogens through ovariectomy induced ERα gene and protein expression in the soleus, EDL, and TA muscles at both the acute and chronic time points. Gpx3 mRNA was also induced both acutely and chronically in all 3 muscles in mice receiving 17β-estradiol. When ERs were blocked using Faslodex, Gpx3 mRNA was downregulated in the soleus muscle, but not the EDL and TA muscles. Conclusions/Significance These data suggest that Gpx3 and ERα gene expression are sensitive to circulating estrogens in skeletal muscle. ERs may regulate Gpx3 gene expression in the soleus muscle, but skeletal muscle regulation of Gpx3 via ERs is dependent upon muscle type. Further work is needed to determine the indirect effects of estrogen and ERα on Gpx3 expression in skeletal muscle, and their importance in the aging process.

Mechanisms behind estrogen's beneficial effect on muscle strength in females.

Muscle weakness ensues when serum testosterone declines with age in men. Testosterones female counterpart, estrogen, also has been implicated in age-related strength loss, but these results are less conclusive. Our working hypothesis is that estrogens do benefit muscle strength, and that the underlying mechanism involves estrogen receptors to improve muscle quality more so than quantity.

Diaphragm muscle sarcopenia in aging mice.

Sarcopenia, defined as muscle weakness and fiber atrophy, of respiratory muscles such as the diaphragm (DIAm) has not been well characterized. The DIAm is the main inspiratory muscle and knowledge of DIAm sarcopenia is important for establishing the effects of aging on respiratory function. We hypothesized that aging is associated with a loss of DIAm force and reduced fiber cross-sectional area (CSA), and that these changes vary across fiber types. DIAm sarcopenia was assessed in young (5 month; n = 11) and old (23 month; n = 12) wild-type mice reflecting ~100 and 75% survival, respectively. In addition, DIAm sarcopenia was evaluated in BubR1(H/H) mice (n = 4) that display accelerated aging (~60% survival at 5 months) as a result of expression of a hypomorphic allele (H) of the mitotic checkpoint protein BubR1. Maximum specific force (normalized for CSA) of the DIAm was 34% less in old mice and 57% lower in BubR1(H/H) mice compared to young mice. Mean CSA of type IIx and/or IIb DIAm fibers was 27% smaller in old wild-type mice and 47% smaller in BubR1(H/H) mice compared to young mice. Mean CSA of type I or IIa fibers was not different between groups. Collectively these results demonstrate sarcopenia of the DIAm in aging wild-type mice and in BubR1(H/H) mice displaying accelerated aging. Sarcopenia may limit the ability of the DIAm to accomplish expulsive, non-ventilatory behaviors essential for airway clearance. As a result, these changes in the DIAm may contribute to respiratory complications with aging.

Prolonged C2 spinal hemisection-induced inactivity reduces diaphragm muscle specific force with modest, selective atrophy of type IIx and/or IIb fibers

The diaphragm muscle (DIAm) is critically responsible for sustaining ventilation. Previously we showed in a commonly used model of spinal cord injury, unilateral spinal cord hemisection at C(2) (SH), that there are minimal changes to muscle fiber cross-sectional area (CSA) and fiber type distribution following 14 days of SH-induced ipsilateral DIAm inactivity. In the present study, effects of long-term SH-induced inactivity on DIAm fiber size and force were examined. We hypothesized that prolonged inactivity would not result in substantial DIAm atrophy or force loss. Adult rats were randomized to control or SH groups (n = 34 total). Chronic bilateral DIAm electromyographic (EMG) activity was monitored during resting breathing. Minimal levels of spontaneous recovery of ipsilateral DIAm EMG activity were evident in 42% of SH rats (<25% of preinjury root mean square amplitude). Following 42 days of SH, DIAm specific force was reduced 39%. There was no difference in CSA for type I or IIa DIAm fibers in SH rats compared with age, weight-matched controls (classification based on myosin heavy chain isoform expression). Type IIx and/or IIb DIAm fibers displayed a modest 20% reduction in CSA (P < 0.05). Overall, there were no differences in the distribution of fiber types or the contribution of each fiber type to the total DIAm CSA. These data indicate that reduced specific force following prolonged inactivity of the DIAm is associated with modest, fiber type selective adaptations in muscle fiber size and fiber type distribution.

Estradiol's beneficial effect on murine muscle function is independent of muscle activity

Estradiol (E₂) deficiency decreases muscle strength and wheel running in female mice. It is not known if the muscle weakness results directly from the loss of E₂ or indirectly from mice becoming relatively inactive with presumably diminished muscle activity. The first aim of this study was to determine if cage activities of ovariectomized mice with and without E₂ treatment differ. Ovariectomized mice were 19-46% less active than E₂-replaced mice in terms of ambulation, jumping, and time spent being active (P ≤ 0.033). After E₂-deficient mice were found to have low cage activities, the second aim was to determine if E₂ is beneficial to muscle contractility, independent of physical activities by the mouse or its hindlimb muscles. Adult, female mice were ovariectomized or sham-operated and randomized to receive E₂ or placebo and then subjected to conditions that should maintain physical and muscle activity at a constant low level. After 2 wk of hindlimb suspension or unilateral tibial nerve transection, muscle contractile function was assessed. Soleus muscles of hindlimb-suspended ovariectomized mice generated 31% lower normalized (relative to muscle contractile protein content) maximal isometric force than suspended mice with intact ovaries (P ≤ 0.049). Irrespective of whether the soleus muscle was innervated, muscles from ovariectomized mice generated ∼20% lower absolute and normalized maximal isometric forces, as well as power, than E₂-replaced mice (P ≤ 0.004). In conclusion, E₂ affects muscle force generation, even when muscle activity is equalized.

Systems biology of skeletal muscle: fiber type as an organizing principle

Skeletal muscle force generation and contraction are fundamental to countless aspects of human life. The complexity of skeletal muscle physiology is simplified by fiber type classification where differences are observed from neuromuscular transmission to release of intracellular Ca2+ from the sarcoplasmic reticulum and the resulting recruitment and cycling of cross‐bridges. This review uses fiber type classification as an organizing and simplifying principle to explore the complex interactions between the major proteins involved in muscle force generation and contraction. WIREs Syst Biol Med 2012. doi: 10.1002/wsbm.1184

Estradiol treatment, physical activity, and muscle function in ovarian-senescent mice

Estradiol (E(2)) treatment in young adult, ovariectomized mice increases physical activity and reverses deleterious effects on skeletal muscle. Here we test the hypothesis that E(2) treatment improves muscle function and physical activity in aged, ovarian-senescent mice. Plasma E(2) levels and vaginal cytology confirmed ovarian senescence in 20-month-old C57BL/6 mice. Mice were then randomly divided into activity groups, having access to a running wheel or not, and further into those receiving E(2) or placebo. Placebo-treated mice wheel ran more than E(2)-treated mice (P=0.03), with no difference between treatment groups in cage activities such as time spent being active and ambulation distance (P≥0.55). Soleus muscles from aged mice that wheel ran adapted by getting larger and stronger, irrespective of E(2) status (P≤0.02). Soleus muscle fatigue resistance was greater in mice treated with E(2) (P=0.02), but maximal isometric tetanic force was not affected (P≥0.79). Because E(2) treatment did not improve physical activity or overall muscle function in the aged, ovarian-senescent mice as predicted, a second study was initiated to examine E(2) treatment of young adult mice prematurely ovarian senescent from exposure to the chemical, 4-vinylcyclohexene diepoxide (VCD). Four-month-old C57BL/6 female mice were dosed with oil (control) or VCD. Vaginal cytology confirmed ovarian senescence in all mice treated with VCD 63 days after the onset of dosing, and then a subset of the VCD mice received E(2) (VCD+E(2)). Wheel running distance did not differ among control, VCD, and VCD+E(2) mice (P≥0.34). Soleus muscle concentric, isometric, and eccentric in vitro forces were greater in VCD+E(2) than in VCD mice (P<0.04), indicating beneficial estrogenic effects on muscle function. In general, aged and young mice with senescent ovaries were less responsive to E(2) treatment, in terms of physical activities and muscle function, than what has previously been shown for young, ovariectomized mice. These results bring forth the possibility that some component of the residual, follicle-depleted ovarian tissue influences physical activity in mice or that aging diminishes the responsiveness of skeletal muscle and related tissues to E(2) treatment.

Challenges to acellular biological scaffold mediated skeletal muscle tissue regeneration.

Volumetric muscle loss (VML) injuries present a complex and heterogeneous clinical problem that results in a chronic loss of muscle tissue and strength. The primary limitation to muscle tissue regeneration after VML injury is the frank loss of all native muscle constituents in the defect, especially satellite cells and the basal lamina. Recent advancements in regenerative medicine have set forth encouraging and emerging translational and therapeutic options for these devastating injuries including the surgical implantation of acellular biological scaffolds. While these biomaterials can modulate the wound environment, the existing data do not support their capacity to promote appreciable muscle fiber regeneration that can contribute to skeletal muscle tissue functional improvements. An apparent restriction of endogenous satellite cell (i.e., pax7(+)) migration to acellular biological scaffolds likely underlies this deficiency. This work critically evaluates the role of an acellular biological scaffold in orchestrating skeletal muscle tissue regeneration, specifically when used as a regenerative medicine approach for VML injury.

Ageing and neurotrophic signalling effects on diaphragm neuromuscular function

Sarcopenia of the diaphragm muscle, i.e. loss of muscle force and size with increasing age, may contribute to respiratory impairment in old age but the exact mechanisms underlying this are currently unknown. Across the lifespan in mice there is worsening neuromuscular function of the diaphragm muscle, specifically reduced force and impaired neuromuscular transmission. We tested the hypothesis that age‐related changes to the diaphragm muscle depend on brain‐derived neurotrophic factor (BDNF), acting through its high affinity receptor. BDNF improves neuromuscular transmission in the diaphragm muscle into early old age, but not older ages. Inhibition of BDNF signalling impairs neuromuscular transmission only in young adult mice. Our results suggest that the loss of endogenous BDNF precedes reduced activity of the high affinity receptor tropomyosin‐related kinase receptor B in the ageing mouse diaphragm muscle.