What insights into age-related changes in skeletal muscle are provided by animal models?

Tongue exercise and ageing effects on morphological and biochemical properties of the posterior digastric and temporalis muscles in a Fischer 344 Brown Norway rat model

OBJECTIVE

This study sought to examin effects of age and tongue exercise on the posterior digastric (opener) and the temporalis (closer). We hypothesized 1) age would result in differing morphological (cross sectional area) and biochemical (myosin heavy chain isoform) components of these muscles; 2) tongue exercise would result in coactivation of these muscles inducing a decrease in age-related differences between age groups.

DESIGN

Young adult (9 months) and old (32 months) Fischer 344 Brown Norway rats were randomized into a tongue exercise or control group. Post-training, posterior digastric and temporalis muscles were harvested and analyzed using: 1) Sodium Dodecyl Sulfate-Polyacrylamide Gel Electrophoresis (SDS-PAGE) to assess percent myosin heavy chain (MyHC) content; 2) Immunohistochemical staining to determine cross sectional area (CSA).

RESULTS

A larger proportion of slowly contracting MyHC isoforms in the posterior digastric and temporalis muscles were found in old. No significant main effects for age or exercise in fiber size were found in posterior digastric muscle. An interaction between age and exercise for temporalis cross sectional area indicated the old exercise group had smaller average cross sectional area than all other groups. CONCLUSIONS FINDINGS: suggest that: 1) Increasing age induces biochemical changes in muscles of the jaw, specifically showing an increase the proportion of slower contracting MyHC isoforms; 2) Increasing age and tongue exercise induce a reduction in muscle fiber cross sectional area in the temporalis muscle only. However, continued study of these cranial muscle systems is warranted to better understand these changes that occur with age and exercise.

Vocal training mitigates age-related changes within the vocal mechanism in old rats

Aging affects voice production and is associated with reduced communicative ability and quality of life. Voice therapy is a critical component of treatment, but its effects on neuromuscular mechanisms are unknown. The ultrasonic vocalizations (USVs) of rats can be used to test the effects of aging and voice use on the laryngeal neuromuscular system. This study tested the hypothesis that age-related changes in the USVs of rats and laryngeal neuromuscular junctions can be reversed through vocal exercise. Young and old rats were trained for 8 weeks to increase their USVs and were compared with a no intervention group pre- and post-treatment. USV acoustics and aspects of neuromuscular junction (NMJ) morphology were measured in the thyroarytenoid muscle. Vocal training reduced or eliminated some age differences found in both USVs and NMJs. We conclude that vocal exercise may assist in mitigating age-related changes in voice characteristics and underlying neuromuscular adaptations.

Videofluorographic assessment of deglutitive behaviors in a rat model of aging and Parkinson disease

Dysphagia is commonly associated with aging and Parkinson disease and can have a significant impact on a person's quality of life. In some cases, dysphagia may be life-threatening. Animal models may be used to study underlying mechanisms of dysphagia, but paradigms that allow adequate imaging of the swallow in combination with measurement of physiological variables have not been forthcoming. To begin development of methods that allow this, we used videofluorography to record the deglutition behaviors of 22 Fisher 344/Brown Norway rats in young adult (9 months old), old (32 months old), and parkinsonian (unilateral lesion to the medial forebrain bundle) groups. We hypothesized that the old and parkinsonian rats would manifest deficits in deglutition behaviors analogous to those found in human clinical populations. Our results supported our hypothesis in that the old group demonstrated reductions in bolus transport speeds and mastication rate while the parkinsonian rats showed impairments in oral processing. Interpretation of these results should consider the particular animal model, lesion type, and videofluorographic protocol used in this work. Future studies will link swallow imaging data of this kind with physiological and anatomical data in a manner not possible with human participants.

Living in a box or call of the wild? Revisiting lifetime inactivity and sarcopenia

SIGNIFICANCE

The accepted effects of aging in mammalian skeletal muscle are progressive atrophy and weakening, or sarcopenia. Canonical hallmarks of aging in skeletal muscle include a reduction in muscle fiber cross-sectional area, a loss in muscle fibers through apoptosis and denervation, and infiltration of connective tissue or fibrosis. Emerging thought suggests that pro-inflammatory signaling and oxidative stress may contribute to sarcopenia.

CRITICAL ISSUES

Unfortunately, most of the mammalian models used to examine and understand sarcopenia are confounded by the pervasive influence of prolonged physical inactivity. Further, the potential for underlying metabolic disorder and chronic disease (e.g., type II diabetes and cardiovascular disease) may accelerate skeletal muscle wasting. Because physical inactivity may share elevated pro-inflammatory (tumor necrosis factor-alpha and inducible nitric oxide synthase) and insufficient stress response (insulin-like growth factor-1 [IGF-1], heat-shock protein 25 [HSP25], NAD-dependent deacetylase sirtuin-3 [SIRT-3], and peroxisome proliferator-activated receptor-gamma coactivator 1[PGC-1α]) signaling with aging and chronic disease, it is critical to distinguish true aging from chronic inactivity or underlying disease. Conversely, the efficacy of exercise and caloric restrictive interventions against sarcopenia in aging populations appears highly effective when (a) conducted across the lifespan, or (b) at higher intensities when commenced in middle age or later.

RECENT ADVANCES

While the prospective mechanisms by which exercise or daily activity provide have not been elucidated, upregulation of HSPs, PGC-1α, and IGF-1 may ameliorate inflammatory signaling, apoptosis, and sarcopenia. Limited data indicate that the aging phenotype exhibited by mammals living in their natural habitat (Weddell seal and shrews) express limited apoptosis and fiber atrophy, whereas significant collagen accumulation remains. In addition, aging shrews displayed a remarkable ability to upregulate antioxidant enzymes (copper, zinc isoform of superoxide dismutase, manganese isoform of superoxide dismutase, catalase, and glutathione peroxidase).

FUTURE DIRECTIONS

It is possible that in healthy populations requiring daily activity to thrive, fibrosis and weakness, more than atrophy, may be the predominant phenotype of muscle aging until senescence. Elucidating the molecular mechanisms by which lifetime inactivity contributes to sarcopenia and chronic disease will be critical in managing the quality of life and health costs associated with our aging population.

Reduced somatosensory activations in swallowing with age

Understanding the neural functional organization of swallowing in healthy elders is essential in diagnosing and treating older adults with swallowing difficulties. The primary aims of this investigation were to identify the neural activation sites of different components of deglutition in healthy elders using functional Magnetic Resonance Imaging (fMRI) and to investigate age differences in the neural control of swallowing. Ten young (age range 19-25 years of age) and nine older (age range 66-77 years of age) right-handed healthy individuals were scanned in a 3-Tesla MRI scanner. Subjects were visually cued for both a "Swallow" task and for component/control tasks ("Prepare to swallow," "Tap your tongue," and "Clear your throat"). Behavioral interleaved gradient (BIG) methodology was used to address movement related artifacts. Between-group comparisons revealed statistically stronger activations in the primary somatosensory cortex of young adults during the motor tasks examined. Both groups showed activations in the major motor areas involved in the initiation and execution of movement; however, areas involved in sensory processing, sensorimotor integration and/or motor coordination and control, showed reduced or limited activity in the elderly. Potential implications of these findings for clinical practice are discussed.

Characterization of fiber types in different muscles of the hindlimb in female weanling and adult Wistar rats

We analyzed lesser diameter and distribution of fiber types in different skeletal muscles from female Wistar rats using a histoenzymology Myofibrillar Adenosine Tri-phosphatase (mATPase) method. Fragments from muscles were frozen and processed by mATPase in different pH. Adult and weanling rat soleus muscles presented a predominance of type I fibers and larger fiber diameters. In the plantar muscle in adult rats, the type IIB fibers demonstrated greater lesser diameter while in the weanling animals, types I and IIB fibers were larger. The plantar muscle of animals of both ages was composed predominantly of the type IID fibers. The type IID fibers were observed in similar amounts in the lateral gastrocnemius and the medial gastrocnemius muscles. Type IIB fibers showed predominance and presented higher size in comparison with other types in the EDL muscle. The present study shows that data on fiber type distribution and fiber lesser diameter obtained in adult animals cannot always be applied to weanling animals of the same species. Using the mATPase, despite the difficult handling, is an important tool to determine the different characteristics of the specific fibers in the skeletal muscle tissue.

Are human and mouse satellite cells really the same?

Satellite cells are quiescent cells located under the basal lamina of skeletal muscle fibers that contribute to muscle growth, maintenance, repair, and regeneration. Mouse satellite cells have been shown to be muscle stem cells that are able to regenerate muscle fibers and self-renew. As human skeletal muscle is also able to regenerate following injury, we assume that the human satellite cell is, like its murine equivalent, a muscle stem cell. In this review, we compare human and mouse satellite cells and highlight their similarities and differences. We discuss gaps in our knowledge of human satellite cells, compared with that of mouse satellite cells, and suggest ways in which we may advance studies on human satellite cells, particularly by finding new markers and attempting to re-create the human satellite cell niche in vitro.

Senescent swallowing: impact, strategies, and interventions

The risk for disordered oropharyngeal swallowing (dysphagia) increases with age. Loss of swallowing function can have devastating health implications, including dehydration, malnutrition, pneumonia, and reduced quality of life. Age-related changes increase risk for dysphagia. First, natural, healthy aging takes its toll on head and neck anatomy and physiologic and neural mechanisms underpinning swallowing function. This progression of change contributes to alterations in the swallowing in healthy older adults and is termed presbyphagia, naturally diminishing functional reserve. Second, disease prevalence increases with age, and dysphagia is a comorbidity of many age-related diseases and/or their treatments. Sensory changes, medication, sarcopenia, and age-related diseases are discussed herein. Recent findings that health complications are associated with dysphagia are presented. Nutrient requirements, fluid intake, and nutrition assessment for older adults are reviewed relative to dysphagia. Dysphagia screening and the pros and cons of tube feeding as a solution are discussed. Optimal intervention strategies for elders with dysphagia ranging from compensatory interventions to more rigorous exercise approaches are presented. Compelling evidence of improved functional swallowing and eating outcomes resulting from active rehabilitation focusing on increasing strength of head and neck musculature is provided. In summary, although oropharyngeal dysphagia may be life threatening, so are some of the traditional alternatives, particularly for frail, elderly patients. Although the state of the evidence calls for more research, this review indicates that the behavioral, dietary, and environmental modifications emerging in this past decade are compassionate, promising, and, in many cases, preferred alternatives to the always present option of tube feeding.

Effects of 4 weeks recombinant human growth hormone administration on insulin resistance of skeletal muscle in rats

PURPOSE

Effect of recombinant human growth hormone (rhGH) administration on lipid storage, and its subsequent effect on insulin sensitivity have not yet been adequately examined. Thus, we investigated the effects of rhGH treatment on muscle triglyceride (TG) and ceramide content, and insulin sensitivity after 4 weeks of rhGH administration in rats.

MATERIALS AND METHODS

Fourteen rats were randomly assigned to two groups: rhGH injection group (GH, n = 7) and saline injection group (CON, n = 7). GH received rhGH by subcutaneous injections (130 microg.kg(-1).day(-1), 6 days.week(-1)) for 4 weeks, while CON received saline injections that were equivalent in volume to GH group. Intramuscular TG and ceramide content and hepatic TG content were measured. To determine insulin sensitivity, oral glucose tolerance test (OGTT) and muscle incubation for glucose transport rate were performed in rats, and used as indicators of insulin sensitivity. We also examined plasma lipid profiles.

RESULTS

After 4 weeks of rhGH treatment, the GH group had higher muscle and liver TG contents than the CON (p < 0.05). Ceramide content in GH was significantly greater than that in CON (p < 0.05). GH also had higher plasma levels of FFA (p < 0.05), glucose and insulin responses during OGTT (p < 0.05), and lower glucose transport rates in submaximal insulin concentration (p < 0.05) as compared with CON. Results indicate that rhGH treatment is associated with insulin resistance in rats.

CONCLUSION

rhGH treatment elevated muscle TG and ceramide content, and hepatic TG content. Thus, elevation of these compounded by rhGH treatment could contribute to the development of insulin resistance in rats.

Satellite-cell pool size does matter: defining the myogenic potency of aging skeletal muscle

The deteriorating in vivo environment is thought to play a major role in reduced stem cell function with age. The capacity of stem cells to support tissue maintenance depends not only on their response to cues from the surrounding niche, but also on their abundance. Here, we investigate satellite cell (myogenic stem cell) pool size and its potential to participate in muscle maintenance through old age. The numbers and performance of mouse satellite cells have been analyzed using molecular markers that exclusively characterize quiescent satellite cells and their progeny as they transit through proliferation, differentiation and generation of reserve cells. The study establishes that abundance of resident satellite cells declines with age in myofibers from both fast- and slow-twitch muscles. Nevertheless, the inherent myogenic potential of satellite cells does not diminish with age. Furthermore, the aging satellite cell niche retains the capacity to support effective myogenesis upon enrichment of the mitogenic milieu with FGF. Altogether, satellite cell abundance, but not myogenic potential, deteriorates with age. This study suggests that the population of satellite cells that participate in myofiber maintenance during routine muscle utilization is not fully replenished throughout life.

Contractile dysfunction and altered metabolic profile of the aging rat thyroarytenoid muscle

The larynx and its muscles are important for ventilation, coughing, sneezing, swallowing, Valsalva's maneuver, and phonation. Because of their functional demands, the intrinsic laryngeal muscles have a unique phenotype: very small and fast fibers with high mitochondrial content. How aging affects their function is largely unknown. In this study, we tested the hypothesis that an intrinsic laryngeal muscle (thyroarytenoid muscle, a vocal fold adductor) would become weaker, slower, and fatigable with age. Muscles from Fischer 344 x Brown Norway F1 hybrid rats (6, 18, and 30 mo of age) were used for in vitro contractile function and histology. Thyroarytenoid muscles generated significantly lower twitch and tetanic forces at 30 mo vs. 6 and 18 mo. Maximal shortening velocity decreased by 20% at 30 mo (vs. 6 mo), and velocity of unloaded shortening was slower at 18 and 30 mo by 19 and 27% vs. 6 mo. There was no histochemical evidence of altered myosin ATPase activity at 18 or 30 mo of age. Fatigue resistance was significantly decreased at 18 and 30 mo. We also found abundant mitochondrial clusters and ragged red fibers in the muscles of 30-mo-old rats, and there was an age-related increase in glycogen-positive fibers. We conclude that rat thyroarytenoid muscles become weaker, slower, and more fatigable with age. These functional changes are not due to alterations in myosin ATPase activity, but a switch in the expression of myosin isoforms remains a possibility. Finally, the alterations in mitochondrial and glycogen content indicate a shift in the metabolic characteristics of these muscles with age.

Adult and developmental myosin heavy chain isoforms in soleus muscle of aging Fischer Brown Norway rat

Fiber type shifts in aging skeletal muscle have been studied with myofibrillar ATPase histochemistry and gel electrophoresis, but less commonly with immunohistochemistry. Immunohistochemical study of myosin heavy chains (MHCs) in single myofibers yields additional information about aged skeletal muscle. Furthermore, many studies of aging rodent skeletal muscle have been performed on fast-MHC-predominant muscle and in several different strains. The aim of this study was to evaluate immunohistochemically MHC characteristics in the slow-MHC-predominant soleus muscle in the Fischer Brown Norway F1 hybrid aging rat (FBN). Three age groups of FBN rats were studied: 12 months, 30 months, and 36 months. Soleus muscles were excised, quick-frozen, and stained immunohistochemically for slow, fast, developmental, and neonatal MHC isoforms. Cross-sections were evaluated for the number and cross-sectional areas of fibers expressing each isoform. Single myofibers in soleus muscles of the aged rats showed significantly greater amounts of coexpression of slow and fast MHC than did younger animals. This change began by 30 months of age, but did not reach statistical significance until 36 months of age. The soleus from 36-month-old rats also expressed greater amounts of developmental MHC than did the other groups. These developmental MHC-positive myofibers also coexpressed either slow or slow and fast MHC. The age-related increase in MHC coexpression of slow with fast isoforms may indicate a fiber type shift suggestive of denervation that outpaces reinnervation. The developmental MHC-positive fibers provide evidence of ongoing myofiber remodeling in the oldest rats in the midst of the fiber degeneration of aging.

Expression profiling identifies dysregulation of myosin heavy chains IIb and IIx during limb immobilization in the soleus muscles of old rats

Aged individuals suffer from multiple dysfunctions during skeletal muscle atrophy. The purpose of this study was to determine differential changes in gene expression in atrophied soleus muscle induced by hindlimb immobilization in young (3-4 months) and old (30-31 months) rats. The hypothesis was that differentially expressed mRNAs with age-atrophy interactions would reveal candidates that induce loss of function responses in aged animals. Each muscle was applied to an independent set of Affymetrix micoarrays, with 385 differentially expressed mRNAs with atrophy and 354 age-atrophy interactions detected by two-factor ANOVA (alpha of 0.05 with a Bonferroni adjustment). Functional trends were observed for 23 and 15 probe sets involved in electron transport and the extracellular matrix, respectively, decreasing more in the young than in the old. Other functional categories with atrophy in both ages included chaperones, glutathione-S-transferases, the tricarboxylic acid cycle, reductions in Z-line-associated proteins and increases in probe sets for protein degradation. Surprisingly, myosin heavy chain IIb and IIx mRNAs were suppressed in the atrophied soleus muscle of old rats as opposed to the large increases in the young animals (16- and 25-fold, respectively, with microarrays, and 61- and 68-fold, respectively, with real-time PCR). No significant changes were observed in myosin heavy chain IIb and IIx mRNA with micoarrays in the atrophied soleus muscles of old rats, but they were found to increase six- and fivefold, respectively, with real-time PCR. Therefore, deficiencies in pre-translational signals that normally upregulate myosin heavy chain IIb and IIx mRNAs during atrophy may exist in the soleus muscle of old animals.

Morphological characteristics of skeletal muscles in relation to gender

BACKGROUND AND AIMS

The aim of this study was to ascertain whether there are gender-related differences in the morphological characteristics of the soleus and tibialis anterior muscles in young adult and old Fischer 344/Brown Norway F1 rats.

METHODS

We tested 1) whether there was a gender-related difference between the fiber type composition of these muscles, and 2) whether the cross-sectional area of individual muscle fibers demonstrated gender-associated differences, fibers from males being larger than fibers from females.

RESULTS

Gender differences were not found in the fiber type composition of the soleus and tibialis anterior muscles, but were present in the single skeletal fiber cross-sectional area of the tibialis anterior muscle. The cross-sectional area of type I fibers in females was greater than that in males at both 12 (16%) and 30 (5%) months of age. In contrast, the cross-sectional area of type Ila fibers of 12-month-old males was larger than that of 12-month-old females. No significant differences between genders were found for the cross-sectional area of type Ilb fibers in either age group. In the soleus muscle, 30-month-old males had larger single fiber cross-sectional areas of both fiber types I and lIa. At 12 months of age, type I fibers from females were larger than those from males.

CONCLUSIONS

Our findings indicate that gender-related differences exist in the size of individual skeletal fibers from the soleus and tibialis anterior muscles and that they may influence metabolism and the adaptive response to rehabilitation programs.

Adjustment of muscle mechanics model parameters to simulate dynamic contractions in older adults

The generation of muscle-actuated simulations that accurately represent the movement of old adults requires a model that accounts for changes in muscle properties that occur with aging. An objective of this study was to adjust the parameters of Hill-type musculo-tendon models to reflect nominal age-related changes in muscle mechanics that have been reported in the literature. A second objective was to determine whether using the parametric adjustments resulted in simulated dynamic ankle torque behavior similar to that seen in healthy old adults. The primary parameter adjustment involved decreasing maximum isometric muscle forces to account for the loss of muscle mass and specific strength with age. A review of the literature suggested the need for other modest adjustments that account for prolonged muscular deactivation, a reduction in maximum contraction velocity, greater passive muscle stiffness and increased normalized force capacity during lengthening contractions. With age-related changes incorporated, a musculo-tendon model was used to simulate isometric and isokinetic contractions of ankle plantarflexor and dorsiflexor muscles. The model predicted that ankle plantarflexion power output during 120 deg/s shortening contractions would be over 40% lower in old adults compared to healthy young adults. These power losses with age exceed the 30% loss in isometric strength assumed in the model but are comparable to 39-44% reductions in ankle power outputs measured in healthy old adults of approximately 70 years of age. Thus, accounting for age-related changes in muscle properties, other than decreased maximum isometric force, may be particularly important when simulating movements that require substantial power development.

A comparison of the effects of unloading in young adult and aged skeletal muscle

PURPOSE

The objective of this investigation was to determine whether morphological adaptations to unloading are different in young adult and aged skeletal muscle.

METHODS

Sixteen young adult (8-month) Fischer 344 rats were randomly assigned to either a control or hindlimb suspension (HS) group. Sixteen aged (22-month) rats were similarly assigned to either control or HS conditions. After 4 wk, animals were euthanized and soleus and EDL muscles were histochemically analyzed.

RESULTS

In controls, neither the soleus nor EDL displayed age-related differences in fiber size or composition. Unloading elicited fiber atrophy of the soleus in both age groups but to a greater extent (P < 0.05) in aged rats. Only in aged solei were HS-induced fiber type conversions (Type I --> II) detected. In the EDL, unloading caused atrophy only among the aged.

CONCLUSION

These data suggest that aged muscle experiences greater detriment as a result of unloading. This may have important consequences in the aged because they are more likely to be restricted to bed rest or limb immobilization due to falls and other afflictions.

The effect of dietary restriction on body composition in adult male and female rhesus macaques

Dietary restriction is the only intervention shown to increase maximal life span, and to retard the rate of aging in rodents. As part of a long-term randomized trial of the effects of a 20-30% dietary restriction (DR) on adult rhesus macaques, female (N = 30) and male (N = 16) monkeys were assessed at baseline and 6, 12 and 18 months, following randomization to control (C) or dietary restricted (R) groups, for body composition by dual-energy x-ray absorptiometry. At baseline, there were no significant differences between C and R groups in any body composition parameters measured. Males had significantly (p < 0.05) greater values at baseline than females for body weight (BW), body mass index (BMI), total body lean tissue mass (LTM), appendicular skeletal muscle mass (ASM), and total body bone mineral content (BMC). When analyzed longitudinally through 18 months of DR, C females had significantly increased BW, total body fat tissue mass (FTM), total body percent fat tissue mass (%FTM), LTM, ASM, BMC and abdominal fat tissue mass (AbFTM) relative to R animals. Male C animals had significantly increased BW, FTM, %FTM, BMC and AbFTM relative to R males. The primary effect of DR on body composition in these animals was on FTM.

Aging and protein metabolism

During aging, there are qualitative and quantitative modifications of proteins in various tissues. In muscle, myofibrillar and mitochondrial proteins are affected, resulting in a loss of strength and, to a lesser degree, endurance. Mechanisms of sarcopenia remain not well known and probably involve loss of motoneurons, muscle disuse and hormonal alterations. Partial prevention of muscle loss is possible by resistance training. In all tissues, and particularly in the brain, oxidative changes in proteins are likely to alter various functions of proteins.