A Second Space Age Spanning Omics, Platforms, and Medicine Across Orbits.

The recent acceleration of commercial, private, and multi-national spaceflight has created an unprecedented level of activity in low Earth orbit (LEO), concomitant with the highest-ever number of crewed missions entering space and preparations for exploration-class (>1 year) missions. Such rapid advancement into space from many new companies, countries, and space-related entities has enabled a"Second Space Age." This new era is also poised to leverage, for the first time, modern tools and methods of molecular biology and precision medicine, thus enabling precision aerospace medicine for the crews. The applications of these biomedical technologies and algorithms are diverse, encompassing multi-omic, single-cell, and spatial biology tools to investigate human and microbial responses to spaceflight. Additionally, they extend to the development of new imaging techniques, real-time cognitive assessments, physiological monitoring, and personalized risk profiles tailored for astronauts. Furthermore, these technologies enable advancements in pharmacogenomics (PGx), as well as the identification of novel spaceflight biomarkers and the development of corresponding countermeasures. In this review, we highlight some of the recent biomedical research from the National Aeronautics and Space Administration (NASA), Japan Aerospace Exploration Agency (JAXA), European Space Agency (ESA), and other space agencies, and also detail the commercial spaceflight sector's (e.g. SpaceX, Blue Origin, Axiom, Sierra Space) entrance into aerospace medicine and space biology, the first aerospace medicine biobank, and the myriad upcoming missions that will utilize these tools to ensure a permanent human presence beyond LEO, venturing out to other planets and moons.

Cellular and Molecular Signaling Meet the Space Environment.

During space missions that travel beyond the cocoon of the Earth's magnetosphere, astronauts are subjected to the microgravity and radiation stressors of outer space [...].

The Impact of SRT2104 on Skeletal Muscle Mitochondrial Function, Redox Biology, and Loss of Muscle Mass in Hindlimb Unloaded Rats.

Mechanical unloading during microgravity causes skeletal muscle atrophy and impairs mitochondrial energetics. The elevated production of reactive oxygen species (ROS) by mitochondria and Nox2, coupled with impairment of stress protection (e.g., SIRT1, antioxidant enzymes), contribute to atrophy. We tested the hypothesis that the SIRT1 activator, SRT2104 would rescue unloading-induced mitochondrial dysfunction. Mitochondrial function in rat gastrocnemius and soleus muscles were evaluated under three conditions (10 days): ambulatory control (CON), hindlimb unloaded (HU), and hindlimb-unloaded-treated with SRT2104 (SIRT). Oxidative phosphorylation, electron transfer capacities, H2O2 production, and oxidative and antioxidant enzymes were quantified using high-resolution respirometry and colorimetry. In the gastrocnemius, (1) integrative (per mg tissue) proton LEAK was lesser in SIRT than in HU or CON; (2) intrinsic (relative to citrate synthase) maximal noncoupled electron transfer capacity (ECI+II) was lesser, while complex I-supported oxidative phosphorylation to ECI+II was greater in HU than CON; (3) the contribution of LEAK to ECI+II was greatest, but cytochrome c oxidase activity was lowest in HU. In both muscles, H2O2 production and concentration was greatest in SIRT, as was gastrocnemius superoxide dismutase activity. In the soleus, H2O2 concentration was greater in HU compared to CON. These results indicate that SRT2104 preserves mitochondrial function in unloaded skeletal muscle, suggesting its potential to support healthy muscle cells in microgravity by promoting necessary energy production in mitochondria.

Nox2 Inhibition Regulates Stress Response and Mitigates Skeletal Muscle Fiber Atrophy during Simulated Microgravity.

Insufficient stress response and elevated oxidative stress can contribute to skeletal muscle atrophy during mechanical unloading (e.g., spaceflight and bedrest). Perturbations in heat shock proteins (e.g., HSP70), antioxidant enzymes, and sarcolemmal neuronal nitric oxidase synthase (nNOS) have been linked to unloading-induced atrophy. We recently discovered that the sarcolemmal NADPH oxidase-2 complex (Nox2) is elevated during unloading, downstream of angiotensin II receptor 1, and concomitant with atrophy. Here, we hypothesized that peptidyl inhibition of Nox2 would attenuate disruption of HSP70, MnSOD, and sarcolemmal nNOS during unloading, and thus muscle fiber atrophy. F344 rats were divided into control (CON), hindlimb unloaded (HU), and hindlimb unloaded +7.5 mg/kg/day gp91ds-tat (HUG) groups. Unloading-induced elevation of the Nox2 subunit p67phox-positive staining was mitigated by gp91ds-tat. HSP70 protein abundance was significantly lower in HU muscles, but not HUG. MnSOD decreased with unloading; however, MnSOD was not rescued by gp91ds-tat. In contrast, Nox2 inhibition protected against unloading suppression of the antioxidant transcription factor Nrf2. nNOS bioactivity was reduced by HU, an effect abrogated by Nox2 inhibition. Unloading-induced soleus fiber atrophy was significantly attenuated by gp91ds-tat. These data establish a causal role for Nox2 in unloading-induced muscle atrophy, linked to preservation of HSP70, Nrf2, and sarcolemmal nNOS.

Muscular apoptosis but not oxidative stress increases with old age in a long-lived diver, the Weddell seal.

Seals experience repeated bouts of ischemia-reperfusion while diving, potentially exposing their tissues to increased oxidant generation and thus oxidative damage and accelerated aging. We contrasted markers of oxidative damage with antioxidant profiles across age and sex for propulsive (longissismus dorsi) and maneuvering (pectoralis) muscles of Weddell seals to determine whether previously observed morphological senescence is associated with oxidative stress. In longissismus dorsi, old (age 17-26 years) seals exhibited a nearly 2-fold increase in apoptosis over young (age 9-16 years) seals. There was no evidence of age-associated changes in lipid peroxidation or enzymatic antioxidant profiles. In pectoralis, 4-hydroxynonenal-Lys (4-HNE-Lys) levels increased 1.5-fold in old versus young seals, but lipid hydroperoxide levels and apoptotic index did not vary with age. Glutathione peroxidase activity was 1.5-fold higher in pectoralis of old versus young animals, but no other antioxidants changed with age in this muscle. With respect to sex, no differences in lipid hydroperoxides or apoptosis were observed in either muscle. Males had higher HSP70 expression (1.4-fold) and glutathione peroxidase activity (1.3-fold) than females in longissismus dorsi, although glutathione reductase activity was 1.4-fold higher in females. No antioxidants varied with sex in pectoralis. These results show that apoptosis is not associated with oxidative stress in aged Weddell seal muscles. Additionally, the data suggest that adult seals utilize sex-specific antioxidant strategies in longissismus dorsi but not pectoralis to protect skeletal muscles from oxidative damage.

Effect of Eukarion-134 on Akt-mTOR signalling in the rat soleus during 7 days of mechanical unloading.

NEW FINDINGS: What is the central question of this study? Translocation of nNOSµ initiates catabolic signalling via FoxO3a and skeletal muscle atrophy during mechanical unloading. Recent evidence suggests that unloading-induced muscle atrophy and FoxO3a activation are redox sensitive. Will a mimetic of superoxide dismutase and catalase (i.e. Eukarion-134) also mitigate suppression of the Akt-mTOR pathway? What is the main finding and its importance? Eukarion-134 rescued Akt-mTOR signalling and sarcolemmal nNOSµ, which were linked to protection against the unloading phenotype, muscle fibre atrophy and partial fibre-type shift from slow to fast twitch. The loss of nNOSµ from the sarcolemma appears crucial to Akt phosphorylation and is redox sensitive, although the mechanisms remain unresolved. ABSTRACT: Mechanical unloading stimulates rapid changes in skeletal muscle morphology, characterized by atrophy of muscle fibre cross-sectional area and a partial fibre-type shift from slow to fast twitch. Recent studies revealed that oxidative stress contributes to activation of forkhead box O3a (FoxO3a), proteolytic signalling and unloading-induced muscle atrophy via translocation of the µ-splice variant of neuronal nitric oxide synthase (nNOSµ) and activation of FoxO3a. There is limited understanding of the role of reactive oxygen species in the Akt-mammalian target of rapamycin (mTOR) pathway signalling during unloading. We hypothesized that Eukarion-134 (EUK-134), a mimetic of the antioxidant enzymes superoxide dismutase and catalase, would protect Akt-mTOR signalling in the unloaded rat soleus. Male Fischer 344 rats were separated into the following three study groups: ambulatory control (n = 11); 7 days of hindlimb unloading + saline injections (HU, n = 11); or 7 days of HU + EUK-134; (HU + EUK-134, n = 9). EUK-134 mitigated unloading-induced dephosphorylation of Akt, as well as FoxO3a, in the soleus. Phosphorylation of mTOR in the EUK-treated HU rats was not different from that in control animals. However, EUK-134 did not significantly rescue p70S6K phosphorylation. EUK-134 attenuated translocation of nNOSµ from the membrane to the cytosol, reduced nitration of tyrosine residues and suppressed upregulation of caveolin-3 and dysferlin. EUK-134 ameliorated HU-induced remodelling, atrophy of muscle fibres and the 12% increase in type II myosin heavy chain-positive fibres. Attenuation of the unloaded muscle phenotype was associated with decreased reactive oxygen species, as assessed by ethidium-positive nuclei. We conclude that oxidative stress affects Akt-mTOR signalling in unloaded skeletal muscle. Direct linkage of abrogation of nNOSµ translocation with Akt-mTOR signalling during unloading is the subject of future investigation.

Mitochondria in the middle: exercise preconditioning protection of striated muscle.

Cellular and physiological adaptations to an atmosphere which became enriched in molecular oxygen spurred the development of a layered system of stress protection, including antioxidant and stress response proteins. At physiological levels reactive oxygen and nitrogen species regulate cell signalling as well as intracellular and intercellular communication. Exercise and physical activity confer a variety of stressors on skeletal muscle and the cardiovascular system: mechanical, metabolic, oxidative. Transient increases of stressors during acute bouts of exercise or exercise training stimulate enhancement of cellular stress protection against future insults of oxidative, metabolic and mechanical stressors that could induce injury or disease. This phenomenon has been termed both hormesis and exercise preconditioning (EPC). EPC stimulates transcription factors such as Nrf-1 and heat shock factor-1 and up-regulates gene expression of a cadre of cytosolic (e.g. glutathione peroxidase and heat shock proteins) and mitochondrial adaptive or stress proteins (e.g. manganese superoxide dismutase, mitochondrial KATP channels and peroxisome proliferator activated receptor γ coactivator-1 (PGC-1)). Stress response and antioxidant enzyme inducibility with exercise lead to protection against striated muscle damage, oxidative stress and injury. EPC may indeed provide significant clinical protection against ischaemia-reperfusion injury, Type II diabetes and ageing. New molecular mechanisms of protection, such as δ-opioid receptor regulation and mitophagy, reinforce the notion that mitochondrial adaptations (e.g. heat shock proteins, antioxidant enzymes and sirtuin-1/PGC-1 signalling) are central to the protective effects of exercise preconditioning.

EUK-134 ameliorates nNOSµ translocation and skeletal muscle fiber atrophy during short-term mechanical unloading.

Reduced mechanical loading during bedrest, spaceflight, and casting, causes rapid morphological changes in skeletal muscle: fiber atrophy and reduction of slow-twitch fibers. An emerging signaling event in response to unloading is the translocation of neuronal nitric oxide synthase (nNOSµ) from the sarcolemma to the cytosol. We used EUK-134, a cell-permeable mimetic of superoxide dismutase and catalase, to test the role of redox signaling in nNOSµ translocation and muscle fiber atrophy as a result of short-term (54 h) hindlimb unloading. Fischer-344 rats were divided into ambulatory control, hindlimb-unloaded (HU), and hindlimb-unloaded + EUK-134 (HU-EUK) groups. EUK-134 mitigated the unloading-induced phenotype, including muscle fiber atrophy and muscle fiber-type shift from slow to fast. nNOSµ immunolocalization at the sarcolemma of the soleus was reduced with HU, while nNOSµ protein content in the cytosol increased with unloading. Translocation of nNOS from the sarcolemma to cytosol was virtually abolished by EUK-134. EUK-134 also mitigated dephosphorylation at Thr-32 of FoxO3a during HU. Hindlimb unloading elevated oxidative stress (4-hydroxynonenal) and increased sarcolemmal localization of Nox2 subunits gp91phox (Nox2) and p47phox, effects normalized by EUK-134. Thus, our findings are consistent with the hypothesis that oxidative stress triggers nNOSµ translocation from the sarcolemma and FoxO3a dephosphorylation as an early event during mechanical unloading. Thus, redox signaling may serve as a biological switch for nNOS to initiate morphological changes in skeletal muscle fibers.

Bioreactor development for skeletal muscle hypertrophy and atrophy by manipulating uniaxial cyclic strain: proof of concept.

Skeletal muscles overcome terrestrial, gravitational loading by producing tensile forces that produce movement through joint rotation. Conversely, the microgravity of spaceflight reduces tensile loads in working skeletal muscles, causing an adaptive muscle atrophy. Unfortunately, the design of stable, physiological bioreactors to model skeletal muscle tensile loading during spaceflight experiments remains challenging. Here, we tested a bioreactor that uses initiation and cessation of cyclic, tensile strain to induce hypertrophy and atrophy, respectively, in murine lineage (C2C12) skeletal muscle myotubes. Uniaxial cyclic stretch of myotubes was conducted using a StrexCell® (STB-1400) stepper motor system (0.75 Hz, 12% strain, 60 min day^-1). Myotube groups were assigned as follows: (a) quiescent over 2- or (b) 5-day (no stretch), (c) experienced 2-days (2dHY) or (d) 5-days (5dHY) of cyclic stretch, or (e) 2-days of cyclic stretch followed by a 3-day cessation of stretch (3dAT). Using ß-sarcoglycan as a sarcolemmal marker, mean myotube diameter increased significantly following 2dAT (51%) and 5dAT (94%) vs. matched controls. The hypertrophic, anabolic markers talin and Akt phosphorylation (Thr308) were elevated with 2dHY but not in 3dAT myotubes. Inflammatory, catabolic markers IL-1ß, IL6, and NF-kappaB p65 subunit were significantly higher in the 3dAT group vs. all other groups. The ratio of phosphorylated FoxO3a/total FoxO3a was significantly lower in 3dAT than in the 2dHY group, consistent with elevated catabolic signaling during unloading. In summary, we demonstrated proof-of-concept for a spaceflight research bioreactor, using uniaxial cyclic stretch to produce myotube hypertrophy with increased tensile loading, and myotube atrophy with subsequent cessation of stretch.

Contribution of oxidative stress to pathology in diaphragm and limb muscles with Duchenne muscular dystrophy.

Duchenne muscular dystrophy (DMD) is a degenerative skeletal muscle disease that makes walking and breathing difficult. DMD is caused by an X-linked (Xp21) mutation in the dystrophin gene. Dystrophin is a scaffolding protein located in the sarcolemmal cytoskeleton, important in maintaining structural integrity and regulating muscle cell (muscle fiber) growth and repair. Dystrophin deficiency in mouse models (e.g., mdx mouse) destabilizes the interface between muscle fibers and the extracellular matrix, resulting in profound damage, inflammation, and weakness in diaphragm and limb muscles. While the link between dystrophin deficiency with inflammation and pathology is multi-factorial, elevated oxidative stress has been proposed as a central mediator. Unfortunately, the use of non-specific antioxidant scavengers in mouse and human studies has led to inconsistent results, obscuring our understanding of the importance of redox signaling in pathology of muscular dystrophy. However, recent studies with more mechanistic approaches in mdx mice suggest that NAD(P)H oxidase and nuclear factor-kappaB are important in amplifying dystrophin-deficient muscle pathology. Therefore, more targeted antioxidant therapeutics may ameliorate damage and weakness in human population, thus promoting better muscle function and quality of life. This review will focus upon the pathobiology of dystrophin deficiency in diaphragm and limb muscle primarily in mouse models, with a rationale for development of targeted therapeutic antioxidants in DMD patients.

Combined effects of heavy ion exposure and simulated Lunar gravity on skeletal muscle.

BACKGROUND: The limitations to prolonged spaceflight include unloading-induced atrophy of the musculoskeletal system which may be enhanced by exposure to the space radiation environment. Previous results have concluded that partial gravity, comparable to the Lunar surface, may have detrimental effects on skeletal muscle. However, little is known if these outcomes are exacerbated by exposure to low-dose rate, high-energy radiation common to the space environment. Therefore, the present study sought to determine the impact of highly charge, high-energy (HZE) radiation on skeletal muscle when combined with partial weightbearing to simulate Lunar gravity. We hypothesized that partial unloading would compromise skeletal muscle and these effects would be exacerbated by radiation exposure. METHODS: For month old female BALB/cByJ mice were -assigned to one of 2 groups; either full weight bearing (Cage Controls, CC) or partial weight bearing equal to 1/6th bodyweight (G/6). Both groups were then divided to receive either a single whole body absorbed dose of 0.5 Gy of 300 MeV 28Si ions (RAD) or a sham treatment (SHAM). Radiation exposure experiments were performed at the NASA Space Radiation Laboratory (NSRL) located at Brookhaven National Laboratory on Day 0, followed by 21 d of CC or G/6 loading. Muscles of the hind limb were used to measure protein synthesis and other histological measures. RESULTS: Twenty-one days of Lunar gravity (G/6) resulted in lower soleus, plantaris, and gastrocnemius muscle mass. Radiation exposure did not further impact muscle mass. 28Si exposure in normal ambulatory animals (RAD+CC) did not impact gastrocnemius muscle mass when compared to SHAM+CC (p>0.05), but did affect the soleus, where mass was higher following radiation compared to SHAM (p<0.05). Mixed gastrocnemius muscle protein synthesis was lower in both unloading groups. Fiber type composition transitioned towards a faster isoform with partial unloading and was not further impacted by radiation. The combined effects of partial loading and radiation partially mitigated fiber cross-sectional area when compared to partial loading alone. Radiation and G/6 reduced the total number of myonuclei per fiber while leading to elevated BrdU content of skeletal muscle. Similarly, unloading and radiation resulted in higher collagen content of muscle when compared to controls, but the effects of combined exposure were not additive. CONCLUSIONS: The results of this study confirm that partial weightbearing causes muscle atrophy, in part due to reductions of muscle protein synthesis in the soleus and gastrocnemius as well as reduced peripheral nuclei per fiber. Additionally, we present novel data illustrating 28Si exposure reduced nuclei in muscle fibers despite higher satellite cell fusion, but did not exacerbate muscle atrophy, CSA changes, or collagen content. In conclusion, both partial loading and HZE radiation can negatively impact muscle morphology.

Exercise training reduces fibrosis and matrix metalloproteinase dysregulation in the aging rat heart.

Aging impairs function in the nonischemic heart and is associated with mechanical remodeling. This process includes accumulation of collagen (i.e., fibrosis) and dysregulation of active matrix metalloproteinases (MMPs). Exercise training (ET) improves cardiac function, but the pathways of protection remain poorly understood. Young (3 mo) and old (31 mo) FBNF1 rats were assigned into sedentary and exercise groups, with ET group rats training on a treadmill 45 min/d, 5 d/wk for 12 wk. Nonlinear optical microscopy (NLOM), histology, immunohistochemistry (IHC), and Western blot analyses were performed on the left ventricle and septum. NLOM, IHC, and histological imaging revealed that ET reduced age-associated elevation of collagen type I fibers. Active MMP-1, active MMP-2, and MMP-14 in the ECM fraction of the left ventricle were reduced by aging, an effect abrogated by ET. Tissue inhibitor of MMP (TIMP-1) was elevated with age but protected by ET. Transforming growth factor-ß (TGF-ß), upstream regulator of TIMP-1, increased with age but was attenuated by ET. Therefore, exercise training could protect the aging heart against dysregulation of MMPs and fibrosis by suppressing elevation of TIMP-1 and TGF-ß.

Exacerbation of pathology by oxidative stress in respiratory and locomotor muscles with Duchenne muscular dystrophy.

Duchenne muscular dystrophy (DMD) is the most devastating type of muscular dystrophy, leading to progressive weakness of respiratory (e.g. diaphragm) and locomotor muscles (e.g. gastrocnemius). DMD is caused by X-linked defects in the gene that encodes for dystrophin, a key scaffolding protein of the dystroglycan complex (DCG) within the sarcolemmal cytoskeleton. As a result of a compromised dystroglycan complex, mechanical integrity is impaired and important signalling proteins (e.g. nNOS, caveolin-3) and pathways are disrupted. Disruption of the dystroglycan complex leads to high susceptibility to injury with repeated, eccentric contractions as well as inflammation, resulting in significant damage and necrosis. Chronic damage and repair cycling leads to fibrosis and weakness. While the link between inflammation with damage and weakness in the DMD diaphragm is unresolved, elevated oxidative stress may contribute to damage, weakness and possibly fibrosis. While utilization of non-specific antioxidant interventions has yielded inconsistent results, recent data suggest that NAD(P)H oxidase could play a pivotal role in elevating oxidative stress via integrated changes in caveolin-3 and stretch-activated channels (SACs). Oxidative stress may act as an amplifier, exacerbating disruption of the dystroglycan complex, upregulation of the inflammatory transcription factor NF-B, and thus functional impairment of force-generating capacity.

Nox2 signaling and muscle fiber remodeling are attenuated by losartan administration during skeletal muscle unloading.

Reduced mechanical loading results in atrophy of skeletal muscle fibers. Increased reactive oxygen species (ROS) are causal in sarcolemmal dislocation of nNOS and FoxO3a activation. The Nox2 isoform of NADPH oxidase and mitochondria release ROS during disuse in skeletal muscle. Activation of the angiotensin II type 1 receptor (AT1R) can elicit Nox2 complex formation. The AT1R blocker losartan was used to test the hypothesis that AT1R activation drives Nox2 assembly, nNOS dislocation, FoxO3a activation, and thus alterations in morphology in the unloaded rat soleus. Male Fischer 344 rats were divided into four groups: ambulatory control (CON), ambulatory + losartan (40 mg kg-1  day-1 ) (CONL), 7 days of tail-traction hindlimb unloading (HU), and HU + losartan (HUL). Losartan attenuated unloading-induced loss of muscle fiber cross-sectional area (CSA) and fiber-type shift. Losartan mitigated unloading-induced elevation of ROS levels and upregulation of Nox2. Furthermore, AT1R blockade abrogated nNOS dislocation away from the sarcolemma and elevation of nuclear FoxO3a. We conclude that AT1R blockade attenuates disuse remodeling by inhibiting Nox2, thereby lessening nNOS dislocation and activation of FoxO3a.

Effect of combined fish oil & Curcumin on murine skeletal muscle morphology and stress response proteins during mechanical unloading.

Skeletal muscle is a highly adaptable tissue capable of remodeling when dynamic stress is altered, including changes in mechanical loading and stretch. When muscle is subjected to an unloaded state (e.g., bedrest, immobilization, spaceflight) the resulting loss of muscle cross sectional area (CSA) impairs force production. In addition, muscle fiber-type shifts from slow to fast-twitch fibers. Unloading also results in a downregulation of heat shock proteins (e.g., HSP70) and anabolic signaling, which further exacerbate these morphological changes. Our lab recently showed reactive oxygen species (ROS) are causal in unloading-induced alterations in Akt and FoxO3a phosphorylation, muscle fiber atrophy, and fiber-type shift. Nutritional supplements such as fish oil and curcumin enhance anabolic signaling, glutathione levels, and heat shock proteins. We hypothesized that fish oil, rich in omega-3-fatty acids, combined with the polyphenol curcumin would enhance stress protective proteins and anabolic signaling in the rat soleus muscle, concomitant with synergistic protection of morphology. C57BL/6 mice were assigned to 3 groups (n = 6/group): ambulatory controls (CON), hindlimb unloading (HU), and hindlimb unloading with 5% fish oil, 1% curcumin in diet (FOC). FOC treatments began 10 days prior to HU and tissues were harvested following 7 days of HU. FOC mitigated the unloading induced decrease in CSA. FOC also enhanced abundance of HSP70 and anabolic signaling (Akt phosphorylation, p70S6K phosphorylation), while reducing Nox2, a source of oxidative stress. Therefore, we concluded that the combination of fish oil and curcumin prevents skeletal muscle atrophy due to a boost of heat shock proteins and anabolic signaling in an unloaded state.

Modulation of age-induced apoptotic signaling and cellular remodeling by exercise and calorie restriction in skeletal muscle.

Aging is inevitably associated with a progressive loss of muscle mass and strength, a condition also known as sarcopenia of aging. Although the precise mechanisms underlying this syndrome have not been completely elucidated, recent studies point toward several key cellular mechanisms that could contribute to age-associated muscle loss. Among these, mitochondrial dysfunction and deregulation of apoptotic signaling have emerged as critical players in the onset and progression of sarcopenia. Interestingly, calorie restriction, a well-known antiaging intervention, and, more recently, exercise training have been shown to beneficially affect both mitochondrial function and apoptotic signaling in skeletal muscle from young and old animals. Preliminary observations also indicate that even a small (8%) reduction in food intake may still provide protective effects against sarcopenia and cellular remodeling in aging skeletal muscle, with the advantage of being more applicable to human subjects than the traditional 30-40% restriction regimen. The most recent evidence on the relevance of skeletal muscle apoptosis to sarcopenia, as well as its modulation by calorie restriction and exercise, is reviewed.

Lifelong exercise and mild (8%) caloric restriction attenuate age-induced alterations in plantaris muscle morphology, oxidative stress and IGF-1 in the Fischer-344 rat.

Muscle atrophy is a highly prevalent condition among older adults, and results from reduced muscle mass and fiber cross-sectional area. Resistive exercise training and moderate (30-40%) caloric restriction may reduce the rate of sarcopenia in animal models. We tested the hypothesis that lifelong, voluntary exercise combined with mild (8%) caloric restriction would attenuate the reduction of muscle fiber cross-sectional area in the rat plantaris. Fischer-344 rats were divided into: young adults (6 mo) fed ad libitum (YAL); 24 mo old fed ad libitum (OAL); 24 mo old on 8% caloric restriction (OCR); lifelong wheel running with 8% CR (OExCR). Plantaris fiber cross-sectional area was significantly lower in OAL than YAL (-27%), but protected in OCR and OExCR, while mass/body mass ratio was preserved in OExCR only. Furthermore, 8% CR and lifelong wheel running attenuated the age-induced increases in extramyocyte space and connective tissue. Citrate synthase activity decreased with age, but was not significantly protected in OCR and OExCR. Total hydroperoxides were higher in OAL than YAL, but were not elevated in OExCR, with out a change in MnSOD. IGF-1 levels were lower in OAL (-57%) than YAL, but partially protected in the OExCR group (+51%).

Exercise training attenuates age-induced elevation in Bax/Bcl-2 ratio, apoptosis, and remodeling in the rat heart.

Aging is characterized by loss of myocytes, remodeling, and impaired contractile function in the heart. The rate of programmed cell death, or "apoptosis," in the left ventricle increases with age, and contributes to a 30% reduction in myocytes. Aging may preferentially target the Bcl-2 pathway of apoptosis in the heart. Exercise can protect cardiac function of the aging heart, although the mechanisms are poorly understood. We tested the hypothesis that 12 wk of exercise training would attenuate age-induced increases in remodeling, apoptosis, and Bax/Bcl-2 ratio in rat left ventricle. We found that exercise training provided significant protection against loss of cardiac myocytes, reduction in number of myonuclei, reactive hypertrophy of remaining myocytes, and increased connective tissue in left ventricle of the aging rat heart. Exercise training significantly attenuated age-induced increases of apoptosis in the left ventricle, as indicated by lower DNA fragmentation, TUNEL-positive staining, and caspase-3 cleavage, when compared with left ventricles from the age-matched sedentary group. Further, exercise training in the aging reduced caspase-9 levels and Bax/Bcl-2 ratio by lowering Bax protein expression while increasing Bcl-2 levels. These are the first data to demonstrate protective effects of endurance exercise training against elevated apoptosis and remodeling in the aging heart.

Exercise training attenuates age-induced changes in apoptotic signaling in rat skeletal muscle.

The aging process in skeletal muscle is characterized by a loss of myocytes and reduction in cross-sectional area of the remaining myocytes, particularly in Type II (fast-twitch) muscle fibers. In multinucleated skeletal muscle, apoptosis may contribute to both fiber atrophy and loss of muscle fibers. Recent evidence suggests that the mitochondrial Bcl-2 family pathway may be a target of aging. Here the authors demonstrated that aging increased DNA fragmentation, cleaved caspase-3, and pro-apoptotic Bax in rat skeletal muscle. Twelve weeks of treadmill exercise training increased anti-apoptotic Bcl-2, while markedly reducing DNA fragmentation, and cleaved caspase-3, Bax, and Bax/Bcl-2 ratio in the white gastrocnemius and soleus muscles of old rats. Upstream anti-apoptotic NF-kappaB activity decreased in aging skeletal muscle, and increased with exercise training. Regulation of NF-kappaB activity with aging and exercise was not related to changes in NF-kappaB subunit protein levels. Instead, changes in post-translational activation of NF-kappaB occurred as a function of altered phosphorylation of IkappaB. These results indicate that treadmill exercise training attenuates fiber atrophy and pro-apoptotic signaling in aging skeletal muscle.

Age-related alterations in the sarcolemmal environment are attenuated by lifelong caloric restriction and voluntary exercise.

Age-related loss of skeletal muscle mass and function, referred to as sarcopenia, is mitigated by lifelong calorie restriction as well as exercise. In aged skeletal muscle fibers there is compromised integrity of the cell membrane that may contribute to sarcopenia. The purpose of this study was to determine if lifelong mild (8%) caloric restriction (CR) and lifelong CR+voluntary wheel running (WR) could ameliorate disruption of membrane scaffolding and signaling proteins during the aging process, thus maintaining a favorable, healthy membrane environment in plantaris muscle fibers. Fischer-344 rats were divided into four groups: 24-month old adults fed ad libitum (OAL); 24-month old on 8% caloric restriction (OCR); 24month old 8% caloric restriction+wheel running (OCRWR); and 6-month old sedentary adults fed ad libitum (YAL) were used to determine age-related changes. Aging resulted in discontinuous membrane expression of dystrophin glycoprotein complex (DGC) proteins: dystrophin and α-syntrophin. Older muscle also displayed decreased content of neuronal nitric oxide synthase (nNOS), a key DGC signaling protein. In contrast, OCR and OCRWR provided significant protection against age-related DGC disruption. In conjunction with the age-related decline in membrane DGC patency, key membrane repair proteins (MG53, dysferlin, annexin A6, and annexin A2) were significantly increased in the OAL plantaris. However, lifelong CR and CRWR interventions were effective at maintaining membrane repair proteins near YAL levels of. OAL fibers also displayed reduced protein content of NADPH oxidase isoform 2 (Nox2) subunits (p67phox and p47phox), consistent with a perturbed sarcolemmal environment. Loss of Nox2 subunits was prevented by lifelong CR and CRWR. Our results are therefore consistent with the hypothesis that lifelong CR and WR are effective countermeasures against age-related alterations in the myofiber membrane environment.