Age-related loss of skeletal muscle function; impairment of gene expression

Mechano Growth Factor (MGF) is derived from the insulin-like growth factor (IGF-I) but its sequence differs from the systemic IGF-I produced by the liver. MGF is expressed by mechanically overloaded muscle and is involved in tissue repair and adaptation. It is expressed as a pulse following muscle damage and involved in the activation of muscle satellite (stem) cells. These donate nuclei to the muscle fibers that are required for repair and for the hypertrophy processes which may have similar regulatory mechanisms. Muscles in the elderly are unable to upregulate MGF in response to exercise. This is also true in certain diseases and this helps to explain muscle loss in those conditions. There is evidence that MGF is a local tissue repair factor as well as a growth factor and that it has an important role in damage limitation and inducing repair in other post-mitotic tissues. As there is no cell replacement in these tissues there has to be an effective local cellular repair mechanism. With advancing years this seems to become deficient and there is an increased chance that the damaged cells will undergo cell death leading to progressive loss of tissue function.     

Age-related impairment of mesenchymal progenitor cell function

In most mesenchymal tissues a subcompartment of multipotent progenitor cells is responsible for the maintenance and repair of the tissue following trauma. With increasing age, the ability of tissues to repair themselves is diminished, which may be due to reduced functional capacity of the progenitor cells. The purpose of this study was to investigate the effect of aging on rat mesenchymal progenitor cells. Mesenchymal progenitor cells were isolated from Wistar rats aged 3, 7, 12 and 56 weeks. Viability, capacity for differentiation and cellular aging were examined. Cells from the oldest group accumulated raised levels of oxidized proteins and lipids and showed decreased levels of antioxidative enzyme activity. This was reflected in decreased fibroblast colony-forming unit (CFU-f) numbers, increased levels of apoptosis and reduced proliferation and potential for differentiation. These data suggest that the reduced ability to maintain mesenchymal tissue homeostasis in aged mammals is not purely due to a decline in progenitor cells numbers but also to a loss of progenitor functionality due to the accumulation of oxidative damage, which may in turn be a causative factor in a number of age-related pathologies such as arthritis, tendinosis and osteoporosis.     

Sirt1 increases skeletal muscle precursor cell proliferation

It is important to understand the mechanisms that control muscle precursor cell (MPC) proliferation for the development of countermeasures to offset the deleterious effects of the aging-related loss of skeletal muscle mass (and myonuclei) and the impaired ability of old muscle to regrow and regenerate. Over-expression of the NAD+-dependent histone deacetylase Sirt1 increased MPC proliferation and cell cycle progression as evidenced by increased 5-bromo-2'-deoxyuridine (BrdU) incorporation, an increase in cell number, proliferating cell nuclear antigen expression, and the phosphorylation of retinoblastoma protein. Associated with the Sirt1-mediated increase in MPC cycle progression were the bidirectional decreases and increases in the expression of the cyclin-dependent kinase inhibitors p21(Waf/Cip1) and p27(Kip1), respectively. Based upon our recent observation that lowering oxygen (O2) in culture from ambient (20%) to estimated physiological levels (5%) increased MPC proliferation, we next measured Sirt1 protein at 5% and 20% O2. Interestingly, in addition to increased proliferation in MPCs cultured at 5% O2, Sirt1 expression increased, compared to 20% O2. Using O2 levels as a platform to modulate basal Sirt1 protein, activation of Sirt1 activity with resveratrol in 20% O2 increased MPC proliferation while inhibition of Sirt1 with nicotinamide in 5% O2 lowered proliferation. For the first time, Sirt1 has been shown to increase MPC proliferation. These findings could have clinical significance since MPC proliferation has important implications in regulating skeletal muscle growth, maintenance, and repair, and the aging-related loss of skeletal muscle mass.     

Age-related changes in T cell function.

A comparison was made of the abilities of carrier (BGG)-primed T cell populations from young (4-month old), middle-aged (14- and 19-month old) and old (31- and 34-month old) mice to collaborate with hapten (DNP)-primed B cells from young mice in a cell-transfer system. The plaque-forming cell responses to 2,4-dinitrophenol (DNP) were measured by a modification of the Jerne plaque assay. The DNP-specific antibody-forming cell responses of old T cell/young B cell combinations were significantly lower than those of young T cell/young B cell combinations, both in the number of T cells needed for peak response and in the size of that response. These data indicate that the primed T cell populations of old mice are deficient by a factor of 6 in their ability to initiate B cell proliferation and differentiation into antibody-forming cells.     

Short-term calorie restriction enhances skeletal muscle stem cell function

Calorie restriction (CR) extends life span and ameliorates age-related pathologies in most species studied, yet the mechanisms underlying these effects remain unclear. Using mouse skeletal muscle as a model, we show that CR acts in part by enhancing the function of tissue-specific stem cells. Even short-term CR significantly enhanced stem cell availability and activity in the muscle of young and old animals, in concert with an increase in mitochondrial abundance and induction of conserved metabolic and longevity regulators. Moreover, CR enhanced endogenous muscle repair and CR initiated in either donor or recipient animals improved the contribution of donor cells to regenerating muscle after transplant. These studies indicate that metabolic factors play a critical role in regulating stem cell function and that this regulation can influence the efficacy of recovery from injury and the engraftment of transplanted cells.     

Optimization of human skeletal muscle precursor cell culture and myofiber formation in vitro

Muscle bioengineering is proposed as a treatment option for various conditions requiring restoration of muscle function. In order to allow for rapid clinical translation culture conditions have to be optimized for human application. The optimal isolation and culture technique should be able to support cell growth and differentiation using defined media only. Therefore, we have evaluated alternative culture conditions to determine the optimal growth condition for the engineering of human skeletal muscle. In this research, we present protocols for consistent isolation and growth of human muscle precursor cells (MPCs). MPCs were grown from human biopsies and expanded in culture using defined media and collagen coated dishes only. The best results were achieved using a one-step pre-plating and by supplementing the growth medium with insulin, dexamethasone, human basic fibroblast growth factor (hFGF) and human epithelial growth factor (hEGF). Detailed cell characterization using fluorescence-activated cell-sorting analysis and morphological analysis at different passages were performed. Further, the applicability of these cells for tissue engineering purposes was assessed by measuring expansion potential, formation of myofibers and fused myotubes. We have established a culture technique for human MPCs that allows for reliable cell growth and expansion using collagen coated dishes and defined media only. Cell characterization demonstrated a muscle phenotype and the ability to form myofibers in vitro.     

Impaired autoreactive T cell-induced T cell-T cell interaction in aged mice

Self-Ia-reactive (autoreactive) L3T4+ T cell clones have been shown earlier to stimulate the proliferation of syngeneic naive L3T4+ T cells and initiate a T cell-T cell (T-T) interaction leading to the generation of immunoregulatory circuits. Since aging has been shown to be associated with a decline of the immune responsiveness, age-related alterations in the T-T interaction was investigated in the present study. Using several I-Ed-specific autoreactive T cell clones isolated from 2- to 3-month-old (young) DBA/2 mice as stimulators, it was observed that L3T4+ T cells from 22- to 24-month-old (aged) DBA/2 mice, failed to demonstrate a significant response to the autoreactive T cells. In contrast, L3T4+ T cells from young mice responded strongly to the autoreactive T cell clones. The deficient T-T cell interaction in aged mice correlated with an impaired syngeneic mixed lymphocyte reaction in these mice, thereby suggesting that aging induces a defect both in the autoreactive T cells and in T cells which react with the autoreactive T cells. When exogenous recombinant interleukin 2 (rIL-2), recombinant interleukin 4 (rIL-4), or a combination of these was added to the interaction, it was observed that rIL-4 but not rIL-2 enhanced the T-T interaction in young mice. However, rIL-4 or a combination of rIL-2 and rIL-4 failed to correct the defective T-T interaction in aged mice. Since the T cell network is believed to play an important role in the maintenance of normal immune system homeostasis, the present study suggests that age-related alterations in T and B cell functions and increased susceptibility to autoimmune diseases with age may result from a defect in the T cell network regulation.     

Different sensitivity of rabbit heart and skeletal muscle to endotoxin-induced impairment of mitochondrial function.

The involvement of mitochondrial dysfunction in septic disturbances of tissues is controversial. The aim of this study was to investigate the effects of endotoxin-induced sepsis on the function of heart and skeletal muscle mitochondria. Rabbits were made septic by subcutaneous injection of endotoxin (lipopolysaccharide, LPS) from Escherichia coli at concentrations of 100 or 150 microg LPS.kg(-1) 24 h prior to the experiments. Mitochondrial respiration was measured in saponin-skinned muscle fibers and compared with photometrically detected activities of respiratory chain enzymes as well as with function of perfused hearts. In heart fibers a dosage of 100 microg LPS.kg(-1) caused a significant decrease of state 3-respiration for the substrates pyruvate (-38%), octanoyl-carnitine (-38%) and succinate (-30%) with correspondingly decreased respiratory control indexes (RCI). In addition, endotoxin caused a decreased temporal stability of the rate of state 3-respiration. At least in part these changes can be attributed to a reduced activity of complex I + III (-50%) of the respiratory chain. State 4-respiration rates were not significantly altered. The lowered state 3-respiration in heart mitochondria seems to contribute to the impairment of heart muscle function as detected by an increase of coronary vascular resistance (CVR) in endotoxin-treated hearts. Functional properties of mitochondria from M. Vastus lasteralis were not affected by 100 microg LPS.kg(-1) but a higher dosage of 150 microg LPS.kg(-1) caused decreased RCI for the substrates pyruvate (-29%) and octanoyl-carnitine (-32%). Also the activity of complex I + III was not significantly affected at lower dose of endotoxin but decreased (-42%) after treatment with 150 microg LPS.kg(-1). Results demonstrate the involvement of impaired mitochondria in the pathophysiology of septic organ failure and a tissue specificity of endotoxaemia.     

Age-related mitochondrial genotypic and phenotypic alterations in human skeletal muscle

To have a clearer picture of how mitochondrial damages are associated to aging, a comprehensive study of phenotypic and genotypic alterations was carried out, analyzing with histochemical and molecular biology techniques the same skeletal muscle specimens of a large number of healthy subjects from 13 to 92 years old. Histochemical data showed that ragged red fibers (RRF) appear at about 40 years of age and are mostly cytochrome c oxidase (COX)-positive, whereas they are almost all COX-negative thereafter. Molecular analyses showed that the 4977 bp deletion of mitochondrial DNA (mtDNA(4977)) and the 7436 bp deletion of mtDNA (mtDNA(7436)) are already present in individuals younger than 40 years of age, but their occurrence does not change with age. After 40 years of age the number of mtDNA deleted species, as revealed by Long Extension PCR (LX-PCR), increases, the 10422 bp deletion of mtDNA (mtDNA(10422)) appears, although with a very low frequency of occurrence, and mtDNA content is more than doubled. Furthermore, mtDNA(4977) level directly correlates with that of COX-negative fibers in the same analyzed subjects. These data clearly show that, after 40 years of age, the phenotypic and genotypic mitochondrial alterations here studied appear in human skeletal muscle and that they are closely related.     

骨骼肌细胞的凋亡

细胞凋亡是受基因调控的细胞死亡方式,与机体组织的萎缩和退化关系密切,近年来有人将骨骼肌的萎缩和退化与细胞凋亡联系起来,认为细胞凋亡可能在骨骼肌萎缩和退化中发挥重要作用,本文综述了近年来对骨骼肌肌肉细胞和未分化肌细胞的凋亡及其基因调控的研究,以期进一步阐明骨骼肌萎缩和退化的发生机制,为临床上寻找延缓骨骼肌萎缩的治疗方法提供一些思路。     

骨骼肌的内分泌功能

长期以来,骨骼肌被认为是一种效应器官,接受神经和体液的调节。近年大量实验研究资料发现骨骼肌也具有分泌活性物质的功能,能表达、合成和分泌多种生物信号分子,包括调节肽、细胞因子和生长因子等,也是一种重要的内分泌器官。骨骼肌分泌的活性物质以旁分泌和／或自分泌方式调节骨骼肌的生长、代谢和运动功能;甚至以血液循环内分泌的方式调节机体远隔器官组织的功能。骨骼肌生成和分泌的活性物质在运动系统疾病和某些全身性疾病的发病中具有重要的作用。本文将对骨骼肌分泌的主要活性物质及其生理和病理生理学意义进行综述。     

Mitochondrial dysfunction in skeletal muscle of amyloid precursor protein-overexpressing mice

Inclusion body myositis, the most common muscle disorder in the elderly, is partly characterized by abnormal expression of amyloid precursor protein (APP) and intracellular accumulation of its proteolytic fragments collectively known as β-amyloid. The present study examined the effects of β-amyloid accumulation on mitochondrial structure and function of skeletal muscle from transgenic mice (MCK-βAPP) engineered to accumulate intramyofiber β-amyloid. Electron microscopic analysis revealed that a large fraction of myofibers from 2-3-month-old MCK-βAPP mice contained numerous, heterogeneous alterations in mitochondria, and other cellular organelles. [(1)H-decoupled](13)C NMR spectroscopy showed a substantial reduction in TCA cycle activity and indicated a switch from aerobic to anaerobic glucose metabolism in the MCK-βAPP muscle. Isolated muscle fibers from the MCK-βAPP mice also exhibited a reduction in cytoplasmic pH, an increased rate of ROS production, and a partially depolarized plasmalemma. Treatment of MCK-βAPP muscle cells with Ru360, a mitochondrial Ca(2+) uniporter antagonist, reversed alterations in the plasmalemmal membrane potential (V(m)) and pH. Consistent with altered redox state of the cells, treatment of MCK-βAPP muscle cells with glutathione reversed the effects of β-amyloid accumulation on Ca(2+) transient amplitudes. We conclude that structural and functional alterations in mitochondria precede the reported appearance of histopathological and clinical features in the MCK-βAPP mice and may represent key early events in the pathogenesis of inclusion body myositis.     

T cell-induced secretion of MHC class II-peptide complexes on B cell exosomes

Antigen-specific interactions between B cells and T cells are essential for the generation of an efficient immune response. Since this requires peptide–MHC class II complexes (pMHC-II) on the B cell to interact with TCR on antigen-specific T cells, we have examined the mechanisms regulating the persistence, loss, and secretion of specific pMHC-II complexes on activated B cells. Using a mAb that recognizes specific pMHC-II, we found that activated B cells degrade approximately 50% of pMHC-II every day and release 12% of these pMHC-II from the cell on small membrane vesicles termed exosomes. These exosomes directly stimulate primed, but not naïve, CD4 T cells. Interestingly, engagement of antigen-loaded B cells with specific CD4 T cells stimulates exosome release in a manner that can be mimicked by pMHC-II crosslinking. Biochemical studies revealed that the pMHC-II released on exosomes was previously expressed on the plasma membrane of the B cells, suggesting that regulated exosome release from activated B cells is a mechanism to allow pMHC-II to escape intracellular degradation and decorate secondary lymphoid organs with membrane-associated pMHC-II complexes.     

Age-related differences in polyfunctional T cell responses

Background

A reduced number of naïve T cells along with an accumulation of differentiated cell types in aging have been described but little is known about the polyfunctionality of the T cell responses. In this study we compared the individual and polyfunctional expression of IFN-γ, MIP-1α, TNF-α, perforin, and IL-2 by T cell subsets, including the newly described stem cell like memory T cells (T_SCM), in response to stimulation with superantigen staphylococcal enterotoxin B (SEB) in older (median age 80, n?=?23) versus younger (median age 27; n?=?23) adults.

Results

Older age was associated with a markedly lower frequency of CD8+ naïve T cells (11% vs. 47%; p?<?0.0001) and an expansion in memory T cell subsets including central memory (p?<?0.05), effector memory and effector T cells (p?<?0.001 for both). There was also a decline in CD4+ naïve T cells in older subjects (33% vs. 45%; p?=?0.02). There were no differences in frequencies or polyfunctional profiles of T_SCM between groups. CD8+ naïve cells in the older group had increased expression of all functional parameters measured compared to the younger subjects and exhibited greater polyfunctionality (p?=?0.04). CD4+ naïve T cells in the older group also showed greater polyfunctionality with a TNF-α and IL-2 predominance (p?=?0.005). CD8+ effector memory and effector T cells exhibited increased polyfunctionality in the older group compared with younger (p?=?0.01 and p?=?0.003).

Conclusions

These data suggest that aging does not have a negative effect on polyfunctionality and therefore this is likely not a major contributor to the immunesenescence described with aging.

     

Age-related resistance to skeletal muscle fatigue is preserved during ischemia.

During voluntary contractions, the skeletal muscle of healthy older adults often fatigues less than that of young adults, a result that has been explained by relatively greater reliance on muscle oxidative metabolism in the elderly. Our aim was to investigate whether this age-related fatigue resistance was eliminated when oxidative metabolism was minimized via ischemia induced by cuff (220 mmHg). We hypothesized that 1) older men (n = 12) would fatigue less than young men (n = 12) during free-flow (FF) contractions; 2) both groups would fatigue similarly during ischemia; and 3) reperfusion would reestablish the fatigue resistance of the old. Subjects performed 6 min of intermittent, maximal voluntary isometric contractions of the ankle dorsiflexors under FF and ischemia-reperfusion (IR) conditions. Ischemia was maintained for the first 3 min of contractions, followed by rapid cuff deflation and reperfusion for 3 additional minutes of contractions. Central activation, peripheral activation, and muscle contractile properties were measured at 3 and 6 min of contractions. Older men fatigued less than young men during FF (P 相似文献   

Laminin-211 in skeletal muscle function

A chain is no stronger than its weakest link is an old idiom that holds true for muscle biology. As the name implies, skeletal muscle’s main function is to move the bones. However, for a muscle to transmit force and withstand the stress that contractions give rise to, it relies on a chain of proteins attaching the cytoskeleton of the muscle fiber to the surrounding extracellular matrix. The importance of this attachment is illustrated by a large number of muscular dystrophies caused by interruption of the cytoskeletal-extracellular matrix interaction. One of the major components of the extracellular matrix is laminin, a heterotrimeric glycoprotein and a major constituent of the basement membrane. It has become increasingly apparent that laminins are involved in a multitude of biological functions, including cell adhesion, differentiation, proliferation, migration and survival. This review will focus on the importance of laminin-211 for normal skeletal muscle function.     

Age-related apoptotic responses to stretch-induced hypertrophy in quail slow-tonic skeletal muscle

In the present study, we examined the responses of apoptosis and apoptotic regulatory factors to muscle hypertrophy induced by stretch overload in quail slow-tonic muscles. The wings from one side of young and aged Japanese quails were loaded by attaching a tube weight corresponding to 12% of the bird's body weight for 7 or 21 days. Muscle from the contralateral side served as the intraanimal control. Relative to the intraanimal contralateral control side, the muscle wet weight increased by 96% in young birds, whereas the muscle weight gain in aged birds was not significant after 7 days of loading. After 21 days of loading, muscle weight significantly increased by 179% and 102% in young and aged birds, respectively. Heat shock protein (HSP)72 and HSP27 protein contents in the loaded sides were higher than on the control sides exclusively in young birds after 7 days of loading. Compared with the contralateral control muscle, the extent of apoptotic DNA fragmentation and the total cytosolic apoptosis-inducing factor protein content were reduced in all loaded muscles except for the 7-day-loaded muscles from the aged birds. Bax protein content was diminished in the loaded muscle relative to the control side from all groups, whereas Bcl-2 protein content was reduced in the young and aged muscles after 21 days of loading. The total cytosolic cytochrome c protein content was decreased and the X chromosome-linked inhibitor of apoptosis protein content was elevated in 7- and 21-day-loaded muscles relative to the intraanimal control muscle from young birds. Furthermore, after 7 days of loading the muscles of aged birds, H₂O₂ content and the total cytosolic protein content of second mitochondrial activator of caspases/direct inhibitor of apoptosis-binding protein with low isoelectric point were elevated compared with the intraanimal control side. These data suggest that stretch overload-induced muscle hypertrophy is associated with changes in apoptosis in slow-tonic skeletal muscle. Moreover, discrepant apoptotic responses to muscle overload in young and aged muscles may account in part for the age-related decline in the capability for muscle hypertrophy. aging; sarcopenia; Bcl-2; Bax; heat shock proteins; apoptosis-inducing factor