Hallmarks of ageing in human skeletal muscle and implications for understanding the pathophysiology of sarcopenia in women and men.

Ageing is a complex biological process associated with increased morbidity and mortality. Nine classic, interdependent hallmarks of ageing have been proposed involving genetic and biochemical pathways that collectively influence ageing trajectories and susceptibility to pathology in humans. Ageing skeletal muscle undergoes profound morphological and physiological changes associated with loss of strength, mass, and function, a condition known as sarcopenia. The aetiology of sarcopenia is complex and whilst research in this area is growing rapidly, there is a relative paucity of human studies, particularly in older women. Here, we evaluate how the nine classic hallmarks of ageing: genomic instability, telomere attrition, epigenetic alterations, loss of proteostasis, deregulated nutrient sensing, mitochondrial dysfunction, cellular senescence, stem cell exhaustion, and altered intercellular communication contribute to skeletal muscle ageing and the pathophysiology of sarcopenia. We also highlight five novel hallmarks of particular significance to skeletal muscle ageing: inflammation, neural dysfunction, extracellular matrix dysfunction, reduced vascular perfusion, and ionic dyshomeostasis, and discuss how the classic and novel hallmarks are interconnected. Their clinical relevance and translational potential are also considered.

Aged neutrophils accumulate in lymphoid tissues from healthy elderly mice and infiltrate T- and B-cell zones.

The average age of the human population is rising, leading to an increasing burden of age-related diseases, including increased susceptibility to infection. However, immune function can decrease with age which could impact on processes that require a functional immune system. Aging is also characterized by chronic low-grade inflammation which could further impact immune cell function. While changes to neutrophils in blood during aging have been described, little is known in aging lymphoid organs. This study used female C57BL/6J mice comparing bone marrow (BM), spleen and lymph nodes from young mice aged 2-3 months (equivalent to 18 human years) with healthy elderly mice aged 22-24 months (equivalent to 60-70 human years). Neutrophil proportions increased in BM and secondary lymphoid organs of elderly mice relative to their younger counterparts and presented an atypical phenotype. Interestingly, neutrophils from elderly spleen and lymph nodes were long lived (with decreased apoptosis via Annexin V staining and increased proportion of BrdUneg mature cells) with splenic neutrophils also demonstrating a hypersegmented morphology. Furthermore, splenic neutrophils of elderly mice expressed a mixed phenotype with increased expression of activation markers, CD11b and ICAM-1, increased proinflammatory TNFα, yet increased anti-inflammatory transforming growth factor-beta. Elderly splenic architecture was compromised, as the marginal zone (required for clearing infections) was contracted. Moreover, neutrophils from elderly but not young mice accumulated in lymph node and splenic T- and B-cell zones. Overall, the expansion of functionally compromised neutrophils could contribute to increased susceptibility to infection observed in the elderly.

Age-related loss of VGLUT1 excitatory, but not VGAT inhibitory, immunoreactive terminals on motor neurons in spinal cords of old sarcopenic male mice.

Age-related changes in ventral lumbar spinal cord (L3-L5) were compared in young [3 month, (M)] and old (27 M) C57BL/6J male mice. The aged mice had previously been shown to exhibit sarcopenia and changes to peripheral nerve morphology. The putative connectivity of ß-III tubulin positive α-motor neurons was compared in immunostained transverse sections using excitatory and inhibitory terminal markers vesicular glutamate transporter-1 (VGLUT1) and vesicular GABA transporter (VGAT). Glial fibrillary acidic protein (GFAP) and ionized calcium binding adaptor molecule 1 (Iba1) immunostaining was used to monitor changes in astrocyte and microglial phenotype respectively. For a given motor neuron, the neuronal perimeter was outlined and terminals immunoreactive for VGLUT1 or VGAT in close apposition to the soma were identified. By 27 M, the percentage coverage and total number of VGLUT1 immunoreactive terminals immediately adjacent to the soma of α-motor neurons was significantly decreased compared with young mice. However, percentage coverage of immunoreactive VGAT inhibitory terminals did not change significantly with age. The gray matter of 27 M spinal cords showed increased astrocytic and microglial activity. The loss of VGLUT1 terminals on α-motor neurons, terminals known to be derived from proprioceptive muscle afferents, may further impair sensorimotor control of hind limb skeletal muscle function in old mice.

Resistance wheel exercise from mid-life has minimal effect on sciatic nerves from old mice in which sarcopenia was prevented.

The ability of resistance exercise, initiated from mid-life, to prevent age-related changes in old sciatic nerves, was investigated in male and female C57BL/6J mice. Aging is associated with cellular changes in old sciatic nerves and also loss of skeletal muscle mass and function (sarcopenia). Mature adult mice aged 15 months (M) were subjected to increasing voluntary resistance wheel exercise (RWE) over a period of 8 M until 23 M of age. This prevented sarcopenia in the old 23 M aged male and female mice. Nerves of control sedentary (SED) males at 3, 15 and 23 M of age, showed a decrease in the myelinated axon numbers at 15 and 23 M, a decreased g-ratio and a significantly increased proportion of myelinated nerves containing electron-dense aggregates at 23 M. Myelinated axon and nerve diameter, and axonal area, were increased at 15 M compared with 3 and 23 M. Exercise increased myelinated nerve profiles containing aggregates at 23 M. S100 protein, detected with immunoblotting was increased in sciatic nerves of 23 M old SED females, but not males, compared with 15 M, with no effect of exercise. Other neuronal proteins showed no significant alterations with age, gender or exercise. Overall the RWE had no cellular impact on the aging nerves, apart from an increased number of old nerves containing aggregates. Thus the relationship between cellular changes in aging nerves, and their sustained capacity for stimulation of old skeletal muscles to help maintain healthy muscle mass in response to exercise remains unclear.

The long and short of non-coding RNAs during post-natal growth and differentiation of skeletal muscles: Focus on lncRNA and miRNAs.

Post-natal growth of skeletal muscle is a dynamic process involving proliferation and fusion of myoblasts with elongating myofibres (hyperplasia of myonuclei) until 3 weeks post-natally in mice, with ongoing differentiation and further increases in myofibre size mostly by hypertrophy until about 12 weeks of age. The expression of mRNAs that control these events are well described, but little is known about the in vivo roles of non-coding RNAs (ncRNAs), including both microRNAs (miRNAs) and the lesser-studied long non-coding RNAs (lncRNAs). We analysed expression patterns for a broad range of lncRNAs (including Neat1, Malat1, Sra, Meg3, LncMyoD and linc-MD1), miRNAs and mRNAs in muscles of normal male C57Bl/6J mice at 2 days and 2, 4, 6 and 12 weeks after birth. These post-natal patterns were compared with expression of these RNAs during classic C2C12 myogenesis and differentiation in tissue culture. This overview of RNAs during post-natal skeletal muscle growth provides a novel focus on ncRNAs during this often overlooked growth period, with many potential applications to normal muscle growth in humans and livestock, and to childhood muscle disorders.

Lipid accumulation in dysferlin-deficient muscles.

Dysferlin is a membrane associated protein involved in vesicle trafficking and fusion. Defects in dysferlin result in limb-girdle muscular dystrophy type 2B and Miyoshi myopathy in humans and myopathy in A/J(dys-/-) and BLAJ mice, but the pathomechanism of the myopathy is not understood. Oil Red O staining showed many lipid droplets within the psoas and quadriceps muscles of dysferlin-deficient A/J(dys-/-) mice aged 8 and 12 months, and lipid droplets were also conspicuous within human myofibers from patients with dysferlinopathy (but not other myopathies). Electron microscopy of 8-month-old A/J(dys-/-) psoas muscles confirmed lipid droplets within myofibers and showed disturbed architecture of myofibers. In addition, the presence of many adipocytes was confirmed, and a possible role for dysferlin in adipocytes is suggested. Increased expression of mRNA for a gene involved in early lipogenesis, CCAAT/enhancer binding protein-δ, in 3-month-old A/J(dys-/-) quadriceps (before marked histopathology is evident), indicates early induction of lipogenesis/adipogenesis within dysferlin-deficient muscles. Similar results were seen for dysferlin-deficient BLAJ mice. These novel observations of conspicuous intermyofibrillar lipid and progressive adipocyte replacement in dysferlin-deficient muscles present a new focus for investigating the mechanisms that result in the progressive decline of muscle function in dysferlinopathies.

Protein thiol oxidation does not change in skeletal muscles of aging female mice.

Oxidative stress caused by reactive oxygen species is proposed to cause age related muscle wasting (sarcopenia). Reversible oxidation of protein thiols by reactive oxygen species can affect protein function, so we evaluated whether muscle wasting in normal aging was associated with a pervasive increase in reversible oxidation of protein thiols or with an increase in irreversible oxidative damage to macromolecules. In gastrocnemius muscles of C57BL/6J female mice aged 3, 15, 24, 27, and 29 months there was no age related increase in protein thiol oxidation. In contrast, there was a significant correlation (R (2) = 0.698) between increasing protein carbonylation, a measure of irreversible oxidative damage to proteins, and loss of mass of gastrocnemius muscles in aging female mice. In addition, there was an age-related increase in lipofuscin content, an aggregate of oxidised proteins and lipids, in quadriceps limb muscles in aging female mice. However, there was no evidence of an age-related increase in malondialdehyde or F2-isoprostanes levels, which are measures of oxidative damage to lipids, in gastrocnemius muscles. In summary, this study does not support the hypothesis that a pervasive increase in protein thiol oxidation is a contributing factor to sarcopenia. Instead, the data are consistent with an aging theory which proposes that molecular damage to macromolecules leads to the structural and functional disorders associated with aging.

Electrical stimulation of biofidelic engineered muscle enhances myotube size, force, fatigue resistance, and induces a fast-to-slow-phenotype shift.

Therapeutic development for skeletal muscle diseases is challenged by a lack of ex vivo models that recapitulate human muscle physiology. Here, we engineered 3D human skeletal muscle tissue in the Biowire II platform that could be maintained and electrically stimulated long-term. Increasing differentiation time enhanced myotube formation, modulated myogenic gene expression, and increased twitch and tetanic forces. When we mimicked exercise training by applying chronic electrical stimulation, the "exercised" skeletal muscle tissues showed increased myotube size and a contractility profile, fatigue resistance, and gene expression changes comparable to in vivo models of exercise training. Additionally, tissues also responded with expected physiological changes to known pharmacological treatment. To our knowledge, this is the first evidence of a human engineered 3D skeletal muscle tissue that recapitulates in vivo models of exercise. By recapitulating key features of human skeletal muscle, we demonstrated that the Biowire II platform may be used by the pharmaceutical industry as a model for identifying and optimizing therapeutic drug candidates that modulate skeletal muscle function.

Short-term feed deprivation rapidly induces the protein degradation pathway in skeletal muscles of young mice.

Analysis of protein kinase B (AKT) and S6 kinase1 (p70S6K) activity is widely used to assess the efficacy of interventions designed to increase or maintain skeletal muscle mass; these studies are often performed on feed-deprived mice. One problem associated with feed deprivation is that it promotes catabolism, and young or metabolically compromised mice may have less tolerance. The aim of our study was to determine the effect of various times of feed deprivation on the activity of AKT and p70S6K signaling and markers of protein catabolism in young, growing mice compared with adult mice. Young 23-d-old and adult 3-mo-old mice were feed deprived for 8, 10, and 12 h starting at 0700 h. In addition, adult mice were feed deprived for 24 h. AKT(Ser473) phosphorylation decreased by 50 and 76% from fed amounts by 10 and 12 h of feed deprivation, respectively, in young but not adult muscles. In adult muscles, feed deprivation for 24 h reduced AKT(Ser473) phosphorylation by 70%. Significant de-phosphorylation of p70S6K(Thr389) occurred in all feed-deprived young and adult mice. There was an increase in muscle RING-finger protein-1 (Murf1; 133-1245%) and muscle atrophy F-box protein or Atrogin-1 (Fbxo32; 210-2420%) mRNA in all young but not adult groups deprived of feed for 8-12 h, and there was a trend (P = 0.08) toward increased MURF1 associated with the contractile protein-enriched fraction isolated from young muscles of mice feed deprived for 12 h. This study demonstrates that skeletal muscles of young mice respond rapidly to feed deprivation by decreasing AKT activity and upregulating the protein degradation program.

New horizons in the pathogenesis, diagnosis and management of sarcopenia.

Sarcopenia is the age-related loss of skeletal muscle mass and function. It is now recognised as a major clinical problem for older people and research in the area is expanding exponentially. One of the most important recent developments has been convergence in the operational definition of sarcopenia combining measures of muscle mass and strength or physical performance. This has been accompanied by considerable progress in understanding of pathogenesis from animal models of sarcopenia. Well-described risk factors include age, gender and levels of physical activity and this knowledge is now being translated into effective management strategies including resistance exercise with recent interest in the additional role of nutritional intervention. Sarcopenia is currently a major focus for drug discovery and development although there remains debate about the best primary outcome measure for trials, and various promising avenues to date have proved unsatisfactory. The concept of 'new tricks for old drugs' is, however, promising, for example, there is some evidence that the angiotensin-converting enzyme inhibitors may improve physical performance. Future directions will include a deeper understanding of the molecular and cellular mechanisms of sarcopenia and the application of a lifecourse approach to understanding aetiology as well as to informing the optimal timing of interventions.

Age-related gene expression signatures from limb skeletal muscles and the diaphragm in mice and rats reveal common and species-specific changes.

BACKGROUND: As a result of aging, skeletal muscle undergoes atrophy and a decrease in function. This age-related skeletal muscle weakness is known as "sarcopenia". Sarcopenia is part of the frailty observed in humans. In order to discover treatments for sarcopenia, it is necessary to determine appropriate preclinical models and the genes and signaling pathways that change with age in these models. METHODS AND RESULTS: To understand the changes in gene expression that occur as a result of aging in skeletal muscles, we generated a multi-time-point gene expression signature throughout the lifespan of mice and rats, as these are the most commonly used species in preclinical research and intervention testing. Gastrocnemius, tibialis anterior, soleus, and diaphragm muscles from male and female C57Bl/6J mice and male Sprague Dawley rats were analyzed at ages 6, 12, 18, 21, 24, and 27 months, plus an additional 9-month group was used for rats. More age-related genes were identified in rat skeletal muscles compared with mice; this was consistent with the finding that rat muscles undergo more robust age-related decline in mass. In both species, pathways associated with innate immunity and inflammation linearly increased with age. Pathways linked with extracellular matrix remodeling were also universally downregulated. Interestingly, late downregulated pathways were exclusively found in the rat limb muscles and these were linked to metabolism and mitochondrial respiration; this was not seen in the mouse. CONCLUSIONS: This extensive, side-by-side transcriptomic profiling shows that the skeletal muscle in rats is impacted more by aging compared with mice, and the pattern of decline in the rat may be more representative of the human. The observed changes point to potential therapeutic interventions to avoid age-related decline in skeletal muscle function.

The Activin/FLRG Pathway Associates with Poor COVID-19 Outcomes in Hospitalized Patients.

A subset of hospitalized COVID-19 patients, particularly the aged and those with comorbidities, develop the most severe form of the disease, characterized by acute respiratory disease syndrome (ARDS), coincident with experiencing a "cytokine storm." Here, we demonstrate that cytokines which activate the NF-κB pathway can induce activin A. Patients with elevated activin A, activin B, and FLRG at hospital admission were associated with the most severe outcomes of COVID-19, including the requirement for mechanical ventilation, and all-cause mortality. A prior study showed that activin A could decrease viral load, which indicated there might be a risk to giving COVID-19 patients an inhibitor of activin. To evaluate this, the role for activin A was examined in a hamster model of SARS-CoV-2 infection, via blockade of activin A signaling. The hamster model demonstrated that use of an anti-activin A antibody did not worsen the disease and there was no evidence for increase in lung viral load and pathology. The study indicates blockade of activin signaling may be beneficial in treating COVID-19 patients experiencing ARDS.

Partial Inhibition of mTORC1 in Aged Rats Counteracts the Decline in Muscle Mass and Reverses Molecular Signaling Associated with Sarcopenia.

There is a lack of pharmacological interventions available for sarcopenia, a progressive age-associated loss of muscle mass, leading to a decline in mobility and quality of life. We found mTORC1 (mammalian target of rapamycin complex 1), a well-established positive modulator of muscle mass, to be surprisingly hyperactivated in sarcopenic muscle. Furthermore, partial inhibition of the mTORC1 pathway counteracted sarcopenia, as determined by observing an increase in muscle mass and fiber type cross-sectional area in select muscle groups, again surprising because mTORC1 signaling has been shown to be required for skeletal muscle mass gains in some models of hypertrophy. Additionally, several genes related to senescence were downregulated and gene expression indicators of neuromuscular junction denervation were diminished using a low dose of a "rapalog" (a pharmacological agent related to rapamycin). Therefore, partial mTORC1 inhibition may delay the progression of sarcopenia by directly and indirectly modulating multiple age-associated pathways, implicating mTORC1 as a therapeutic target to treat sarcopenia.

Age-Related Gene Expression Signature in Rats Demonstrate Early, Late, and Linear Transcriptional Changes from Multiple Tissues.

To understand the changes in gene expression that occur as a result of age, which might create a permissive or causal environment for age-related diseases, we produce a multi-time point age-related gene expression signature (AGES) from liver, kidney, skeletal muscle, and hippocampus of rats, comparing 6-, 9-, 12-, 18-, 21-, 24-, and 27-month-old animals. We focus on genes that changed in one direction throughout the lifespan of the animal, either early in life (early logistic changes), at mid-age (mid-logistic), late in life (late-logistic), or linearly, throughout the lifespan of the animal. The pathways perturbed because of chronological age demonstrate organ-specific and more-global effects of aging and point to mechanisms that could potentially be counter-regulated pharmacologically to treat age-associated diseases. A small number of genes are regulated by aging in the same manner in every tissue, suggesting they may be more-universal markers of aging.

TORC1 inhibition enhances immune function and reduces infections in the elderly.

Inhibition of the mechanistic target of rapamycin (mTOR) protein kinase extends life span and ameliorates aging-related pathologies including declining immune function in model organisms. The objective of this phase 2a randomized, placebo-controlled clinical trial was to determine whether low-dose mTOR inhibitor therapy enhanced immune function and decreased infection rates in 264 elderly subjects given the study drugs for 6 weeks. A low-dose combination of a catalytic (BEZ235) plus an allosteric (RAD001) mTOR inhibitor that selectively inhibits target of rapamycin complex 1 (TORC1) downstream of mTOR was safe and was associated with a significant (P = 0.001) decrease in the rate of infections reported by elderly subjects for a year after study drug initiation. In addition, we observed an up-regulation of antiviral gene expression and an improvement in the response to influenza vaccination in this treatment group. Thus, selective TORC1 inhibition has the potential to improve immune function and reduce infections in the elderly.

Short-term Low-Dose mTORC1 Inhibition in Aged Rats Counter-Regulates Age-Related Gene Changes and Blocks Age-Related Kidney Pathology.

Rapalogs, inhibitors of mTORC1 (mammalian target of rapamycin complex 1), increase life span and delay age-related phenotypes in many species. However, the molecular mechanisms have not been fully elucidated. We determined gene expression changes comparing 6- and 24-month-old rats in the kidney, liver, and skeletal muscle, and asked which of these changes were counter-regulated by a clinically-translatable (short-term and low-concentration) treatment, with a rapalog (RAD001). Surprisingly, RAD001 had a more pronounced effect on the kidney under this regimen in comparison to the liver or skeletal muscle. Histologic evaluation of kidneys revealed that the severity of chronic progressive nephropathy lesions was lower in kidneys from 24-month-old rats treated with RAD001 compared with vehicle. In addition to other gene expression changes, c-Myc, which has been shown to regulate aging, was induced by aging in the kidney and counter-regulated by RAD001. RAD001 caused a decrease in c-Myc protein, which could be rescued by a proteasome inhibitor. These findings point to settings for use of mTORC1 inhibitors to treat age-related disorders, and highlight c-Myc regulation as one of the potential mechanisms by which mTORC1 inhibition is perturbing age-related phenotypes.

IGF1 stimulates greater muscle hypertrophy in the absence of myostatin in male mice.

Insulin-like growth factors (IGFs) and myostatin have opposing roles in regulating the growth and size of skeletal muscle, with IGF1 stimulating, and myostatin inhibiting, growth. However, it remains unclear whether these proteins have mutually dependent, or independent, roles. To clarify this issue, we crossed myostatin null (Mstn-/-) mice with mice overexpressing Igf1 in skeletal muscle (Igf1+) to generate six genotypes of male mice; wild type (Mstn+/+ ), Mstn+/-, Mstn-/-, Mstn+/+:Igf1+, Mstn+/-:Igf1+ and Mstn-/-:Igf1+ Overexpression of Igf1 increased the mass of mixed fibre type muscles (e.g. Quadriceps femoris) by 19% over Mstn+/+ , 33% over Mstn+/- and 49% over Mstn-/- (P < 0.001). By contrast, the mass of the gonadal fat pad was correspondingly reduced with the removal of Mstn and addition of Igf1 Myostatin regulated the number, while IGF1 regulated the size of myofibres, and the deletion of Mstn and Igf1+ independently increased the proportion of fast type IIB myosin heavy chain isoforms in T. anterior (up to 10% each, P < 0.001). The abundance of AKT and rpS6 was increased in muscles of Mstn-/-mice, while phosphorylation of AKTS473 was increased in Igf1+mice (Mstn+/+:Igf1+, Mstn+/-:Igf1+ and Mstn-/-:Igf1+). Our results demonstrate that a greater than additive effect is observed on the growth of skeletal muscle and in the reduction of body fat when myostatin is absent and IGF1 is in excess. Finally, we show that myostatin and IGF1 regulate skeletal muscle size, myofibre type and gonadal fat through distinct mechanisms that involve increasing the total abundance and phosphorylation status of AKT and rpS6.

High mTORC1 signaling is maintained, while protein degradation pathways are perturbed in old murine skeletal muscles in the fasted state.

This study investigated age-associated changes to protein synthesis and degradation pathways in the quadriceps muscles of male C57BL/6J mice at 5 ages, between 4 and 24 months (m). Sarcopenia was evident by 18m and was accompanied by hyper-phosphorylation of S6K1, indicating increased mTORC1 signaling. Proteasomal and autophagosomal degradation pathways were also impacted by aging. In the 1% NP40 insoluble protein fraction, the abundance of MuRF1 increased at 24m, while p62 increased at 15m, and remained elevated at older ages. In addition, we investigated how protein synthesis and degradation pathways are modulated by fasting in young (4m) and old (24m) muscles, and showed that old mice respond to fasting less robustly compared with young. Overnight fasting for 16h caused de-phosphorylation of AKT and molecules downstream of mTORC1 (S6K1, rpS6 and 4E-BP1) in young, but not old muscles. A longer time of fasting (24h) was required to reduce phosphorylation of these molecules in old mice. Induction of MuRF1 and Fbxo32 mRNA was also more robust in young compared with old muscles following fasting for 16h. In addition, a 16h fast reduced ULK1 phosphorylation at the mTORC1 specific site Ser757 only in young muscles. The striking accumulation of insoluble p62 protein in muscles of all old male mice (fed or fasted), suggests age-related dysregulation of autophagy and protein aggregation. These data provide an insight into the mechanisms of metabolic responses that affect protein homeostasis in old skeletal muscles, with applications to design of clinical interventions that target sarcopenia.

Voluntary resistance wheel exercise from mid-life prevents sarcopenia and increases markers of mitochondrial function and autophagy in muscles of old male and female C57BL/6J mice.

BACKGROUND: There is much interest in the capacity of resistance exercise to prevent the age-related loss of skeletal muscle mass and function, known as sarcopenia. This study investigates the molecular basis underlying the benefits of resistance exercise in aging C57BL/6J mice of both sexes. RESULTS: This study is the first to demonstrate that long-term (34 weeks) voluntary resistance wheel exercise (RWE) initiated at middle age, from 15 months, prevents sarcopenia in selected hindlimb muscles and causes hypertrophy in soleus, by 23 months of age in both male and female C57BL/6J mice. Compared with 23-month-old sedentary (SED) controls, RWE (0-6 g of resistance) increased intramuscular mitochondrial density and oxidative capacity (measured by citrate synthase and NADH-TR) and increased LC3II/I ratios (a marker of autophagy) in exercised mice of both sexes. RWE also reduced mRNA expression of Gadd45α (males only) and Runx1 (females only) but had no effect on other markers of denervation including Chrng, Chrnd, Musk, and Myog. RWE increased heart mass in all mice, with a more pronounced increase in females. Significant sex differences were also noted among SED mice, with Murf1 mRNA levels increasing in male, but decreasing in old female mice between 15 and 23 months. CONCLUSIONS: Overall, long-term RWE initiated from 15 month of age significantly improved some markers of the mitochondrial and autophagosomal pathways and prevented age-related muscle wasting.

A Neurogenic Perspective of Sarcopenia: Time Course Study of Sciatic Nerves From Aging Mice.

To elucidate the neural basis for age-related sarcopenia, we quantified morphologic and molecular changes within sciatic nerves of aging male and female C57BL/6J mice aged between 3 and 27 months using immunoblotting, immunohistochemistry, and electron microscopy. Protein analyses by immunoblotting of nerves of male mice aged 4, 15, 18, 22, and 24 months showed increased levels of heavy chain SMI-32-positive neurofilaments, vimentin, tau5, choline acetyltransferase (ChAT), and p62 by 18-22 months. Similar protein increases were seen in 26-month-old compared with 3-month-old female mice. Immunostaining of longitudinal sections of old (27-month-old) male sciatic nerves revealed intense staining for tau5 and p62 that was increased compared with that at 3 months, but there were decreased numbers of axon profiles stained for ChAT or isolectin B4 (motor and sensory axons, respectively). Ultrastructural analysis revealed electron-dense aggregates within axons in peripheral nerves of old male mice; the proportion of axons that contained aggregates more than doubled between 15 and 27 months. Overall, the observed age-related accumulation of many proteins from about 18 months of age onward suggests impaired mechanisms for axonal transport and protein turnover. These peripheral nerve changes may contribute to the morphological and functional muscle deficits associated with sarcopenia.