Morphological alterations of mouse skeletal muscles during early ageing are muscle specific

A Novel Research Paradigm for Sarcopenia of Limb Muscles: Lessons From the Perpetually Working Diaphragm's Anti-Aging Mechanisms

BACKGROUND

Skeletal muscle function and mass continuously decrease during aging. Most studies target limb muscles owing to their direct impact on mobility and falls risk. The diaphragm (DIA), also a type of skeletal muscle with different phenotype, has received less attention. Comparative research of the DIA and limb muscles can reveal their distinct aging characteristics. Critically, the potential endogenous anti-aging mechanisms of DIA that may provide new insights into the mechanisms of sarcopenia in limb muscles remain scarce.

METHODS

Treadmill and grip tests assessed limb muscle function, while a lung function system evaluated respiratory function in both adult (6-month-old) and old (22-month-old) mice. Histological assessments evaluated muscle mass in both the DIA and tibialis anterior (TA). Transcriptome sequencing identified differentially expressed genes (DEGs) between the DIA and TA with aging. Adeno-associated virus (AAV)-encoding short hairpin (sh) RNA targeting gene was injected into adult mice's TA muscles to knockdown target gene level in TA, and AAV-gene was injected into old mice's TA to overexpress target gene level.

RESULTS

Old mice displayed significantly reduced running distance (p = 0.0026), maximal speed (p = 0.0019), time to exhaustion (p = 0.0033) and grip strength (p = 0.0055) compared with adult mice, alongside TA's weight loss, decreased myofibre cross-sectional area (CSA) and autophagy deficiency. However, lung function indicators (respiratory rate, tidal volume, minute ventilation volume, forced vital capacity and ratio of forced expiratory volume in 100 or 200 ms to forced vital capacity), as well as DIA weight and morphology remained stable in old mice. Transcriptional analysis revealed 61 DEGs, with significant upregulation or downregulation observed in TA, but without changes in DIA during aging. Smox (spermine oxidase) is one of the DEGs, responsible for catalysing the conversion of spermine to spermidine. It was reported that in muscle atrophy models such as limb immobilisation, fasting and denervation, Smox's levels are positively correlated with muscle mass and function. Additionally, an increase in Smox also promotes mitochondrial biogenesis. In our study, AAV-shSmox adult mice decreased running distance, speed and time, myofibre CSA alongside mitochondrial function, compared with controls. In contrast, old mice with Smox overexpression showed enhanced mitochondrial function.

CONCLUSIONS

In conclusion, this study reveals aging diversities of TA and DIA, explores the sarcopenia of limb muscles based on the anti-aging properties of DIA, which offers a novel perspective on limb sarcopenia. Our findings suggest Smox as a potential target for developing strategies to mitigate sarcopenia progression.

Physical activity reverses the aging induced decline in angiogenic potential in the fast locomotory muscles of mice

Fast locomotory muscles, which are responsible for generating the highest power outputs, are more vulnerable to aging than slow muscles. In this study, we aimed to evaluate the impact of middle age and voluntary physical activity on capillarization and angiogenic potential in fast locomotory muscles. Middle-aged (M-group) and young (Y-group) wild-type FVB female mice were randomly assigned to either the sedentary or trained group undergoing 8-week spontaneous wheel running (8-sWR). Capillary density (assessed via immunohistochemical capillary staining and Western immunoblotting) of the fast locomotory muscles in the M-group (15-months old) was not significantly different compared to the Y-group (4-months old). Nevertheless, the expression of key pro-angiogenic genes in the fast muscle of the M-group was lower than that in the fast muscle of Y-group. 8-sWR had no impact on muscle capillarization; however, it increased fast muscle Vegfa expression in both the M and Y groups. We concluded that although fast muscle capillarization is still preserved in middle age, nevertheless the angiogenic potential (at least at the level of gene expression) is significantly reduced at this stage of aging. Moderate-intensity voluntary physical activity had no effect on capillary density, but it increased the angiogenic potential of the fast muscle.

The Impact of Ageing on Diaphragm Function and Maximal Inspiratory Pressure: A Cross-Sectional Ultrasound Study

Background/Objectives: The effects of ageing on the diaphragm are unclear. This study examined the association between ageing and diaphragm thickness, thickening fraction (TF), and diaphragm excursion (DE) as assessed by ultrasonography after adjusting for other factors. The relationship between these parameters and maximal inspiratory pressure (MIP) was also investigated. Methods: From 2022 to 2024, ambulatory and communicative adult volunteers and outpatients were recruited from four Japanese medical institutions. Each participant's background factors (including height, weight, and underlying diseases) and pulmonary function test results were assessed. Diaphragm thickness, TF, and DE were evaluated using ultrasonography. Results: The study involved 230 individuals with a mean age of 55.5 years (older adults (65 years and over), n = 117; non-older adults, n = 113). In older adults, the diaphragm was thicker (2.1 vs. 1.7 mm, p < 0.001), and TF was lower (88.7% vs. 116.0%, p < 0.001), with no significant difference in DE. Multivariate linear regression analysis adjusted for sex, height, body mass index, and underlying diseases showed positive associations between age and diaphragm thickness (p = 0.001), but not with TF or DE. MIP was positively associated with DE (p < 0.001) but not with thickness or TF. Age was negatively associated with MIP, regardless of diaphragm thickness, TF, and DE (all p < 0.001). Conclusions: As the diaphragm thickens with age, neither thickness nor TF is associated with MIP; only DE is related to MIP. Additionally, ageing is negatively associated with MIP, independent of diaphragm thickness, TF, and DE. Diaphragm function should be assessed using DE rather than thickness or TF.

Relationship Between Diaphragm Function and Sarcopenia Assessed by Ultrasound: A Cross-Sectional Study

Background/Objectives: The diaphragm is important for respiration, but the effects of age-related muscle loss and sarcopenia on diaphragm function are unclear. We evaluated the associations of sarcopenia and skeletal muscle mass (SMM) with diaphragm function. Methods: This study was conducted at three Japanese hospitals from May 2023 to September 2024. The participants underwent bioelectrical impedance for SMM assessment, as well as pulmonary function tests. Diaphragm ultrasound was used to measure the thickness at functional residual capacity (FRC), thickening fraction (TF), and diaphragm excursion (DE) during deep breathing (DB), and their associations with sarcopenia and low skeletal muscle index (SMI) were analyzed. Results: Overall, 148 patients (mean age 78.1 years; sarcopenia, n = 35; non-sarcopenia, n = 103) were included. No statistically significant differences in thickness(FRC), TF and DE were observed between the sarcopenia group and the non-sarcopenia group. The low SMI group had significantly lower thickness (difference -0.22, 95% CI; -0.41, -0.29) and DE (difference -9.2, 95%CI; -14.0, -4.49) than the normal SMI group. Multivariable linear regression analyses adjusted for age, sex, and stature revealed no association between thickness (FRC) and sarcopenia (p = 0.98), but thickness (FRC) was negatively associated with low SMI (p = 0.034). DE during DB was negatively associated with sarcopenia (p = 0.024) and low SMI (p = 0.001). TF showed no associations. Conclusions: DE during DB was reduced in patients with sarcopenia and low SMI, and thickness (FRC) was reduced in those with low SMI without sarcopenia.

Perfusion Staining Methods for Visualization of Intact Microvascular Networks in Whole Mount Skeletal Muscle Preparations

INTRODUCTION

Visualization of the intact microvascular network in skeletal muscle requires labeling the entire network in whole mount preparations where muscle fibre length can be set to near optimal but the tools to do this are not clear.

METHODS

We intravascularly injected CD-1 mice with different fluorescently labelled lectins (fluorescent isolectin GS-IB4 [ISO], wheat germ agglutinin [WGA], lycopersicon esculentum [LYCO]) or FITC-labelled gel. Soleus, extensor digitorum longus, diaphragm, gluteus maximus and cremaster muscles were excised, pinned at optimal sarcomere length and viewed using fluorescence microscopy.

RESULTS

WGA and LYCO were effective at labeling the entire vascular network with WGA labeling capillaries more brightly. ISO labelled the arteriolar vasculature and early segments of the capillaries but not the full length of the capillaries or the venular network. FITC-labelled gel was effective at labelling the microvascular network but not all small vessels were consistently labelled. The pattern of staining for each labelling method was similar across all muscle fibre-types tested.

CONCLUSIONS

WGA was optimal for perfusion labeling and visualization of the intact microvascular network in whole mount skeletal muscle preparations and can be used in combination with ISO to distinguish the arteriolar and venous sides of the network.

INTRODUCTION

Visualization of the intact microvascular network in skeletal muscle requires labeling the entire network in whole mount preparations where muscle fibre length can be set to near optimal but the tools to do this are not clear.

METHODS

We intravascularly injected CD-1 mice with different fluorescently labelled lectins (fluorescent isolectin GS-IB4 [ISO], wheat germ agglutinin [WGA], lycopersicon esculentum [LYCO]) or FITC-labelled gel. Soleus, extensor digitorum longus, diaphragm, gluteus maximus and cremaster muscles were excised, pinned at optimal sarcomere length and viewed using fluorescence microscopy.

RESULTS

WGA and LYCO were effective at labeling the entire vascular network with WGA labeling capillaries more brightly. ISO labelled the arteriolar vasculature and early segments of the capillaries but not the full length of the capillaries or the venular network. FITC-labelled gel was effective at labelling the microvascular network but not all small vessels were consistently labelled. The pattern of staining for each labelling method was similar across all muscle fibre-types tested.

CONCLUSIONS

WGA was optimal for perfusion labeling and visualization of the intact microvascular network in whole mount skeletal muscle preparations and can be used in combination with ISO to distinguish the arteriolar and venous sides of the network.

Breathing and Oxygen Carrying Capacity in Ts65Dn and Down Syndrome

Individuals with Down syndrome (Ds) are at increased risk of respiratory infection, aspiration pneumonia, and apnea. The Ts65Dn mouse is a commonly used model of Ds, but there have been no formal investigations of awake breathing and respiratory muscle function in these mice. We hypothesized that breathing would be impaired in Ts65Dn vs. wild-type (WT), and would be mediated by both neural and muscular inputs. Baseline minute ventilation was not different at 3, 6, or 12 mo of age. However, VT/Ti, a marker of the neural drive to breathe, was lower in Ts65Dn vs. WT and central apneas were more prevalent. The response to breathing hypoxia was not different, but the response to hypercapnia was attenuated, revealing a difference in carbon dioxide sensing, and/or motor output in Ts65Dn. Oxygen desaturations were present in room air, demonstrating that ventilation may not be sufficient to maintain adequate oxygen saturation in Ts65Dn. We observed no differences in arterial PO2 or PCO2, but Ts65Dn had lower hemoglobin and hematocrit. A retrospective medical record review of 52,346 Ds and 52,346 controls confirmed an elevated relative risk of anemia in Ds. We also performed eupneic in-vivo electromyography and in-vitro muscle function and histological fiber typing of the diaphragm, and found no difference between strains. Overall, conscious respiration is impaired in Ts65Dn, is mediated by neural mechanisms, and results in reduced hemoglobin saturation. Oxygen carrying capacity is reduced in Ts65Dn vs. WT, and we demonstrate that individuals with Ds are also at increased risk of anemia.

Slow or fast: Implications of myofibre type and associated differences for manifestation of neuromuscular disorders

Many neuromuscular disorders can have a differential impact on a specific myofibre type, forming the central premise of this review. The many different skeletal muscles in mammals contain a spectrum of slow- to fast-twitch myofibres with varying levels of protein isoforms that determine their distinctive contractile, metabolic, and other properties. The variations in functional properties across the range of classic 'slow' to 'fast' myofibres are outlined, combined with exemplars of the predominantly slow-twitch soleus and fast-twitch extensor digitorum longus muscles, species comparisons, and techniques used to study these properties. Other intrinsic and extrinsic differences are discussed in the context of slow and fast myofibres. These include inherent susceptibility to damage, myonecrosis, and regeneration, plus extrinsic nerves, extracellular matrix, and vasculature, examined in the context of growth, ageing, metabolic syndrome, and sexual dimorphism. These many differences emphasise the importance of carefully considering the influence of myofibre-type composition on manifestation of various neuromuscular disorders across the lifespan for both sexes. Equally, understanding the different responses of slow and fast myofibres due to intrinsic and extrinsic factors can provide deep insight into the precise molecular mechanisms that initiate and exacerbate various neuromuscular disorders. This focus on the influence of different myofibre types is of fundamental importance to enhance translation for clinical management and therapies for many skeletal muscle disorders.

Clinical-grade human umbilical cord-derived mesenchymal stem cells improved skeletal muscle dysfunction in age-associated sarcopenia mice

With the expansion of the aging population, age-associated sarcopenia (AAS) has become a severe clinical disease of the elderly and a key challenge for healthy aging. Regrettably, no approved therapies currently exist for treating AAS. In this study, clinical-grade human umbilical cord-derived mesenchymal stem cells (hUC-MSCs) were administrated to two classic mouse models (SAMP8 mice and D-galactose-induced aging mice), and their effects on skeletal muscle mass and function were investigated by behavioral tests, immunostaining, and western blotting. Core data results showed that hUC-MSCs significantly restored skeletal muscle strength and performance in both mouse models via mechanisms including raising the expression of crucial extracellular matrix proteins, activating satellite cells, enhancing autophagy, and impeding cellular aging. For the first time, the study comprehensively evaluates and demonstrates the preclinical efficacy of clinical-grade hUC-MSCs for AAS in two mouse models, which not only provides a novel model for AAS, but also highlights a promising strategy to improve and treat AAS and other age-associated muscle diseases. This study comprehensively evaluates the preclinical efficacy of clinical-grade hUC-MSCs in treating age-associated sarcopenia (AAS), and demonstrates that hUC-MSCs restore skeletal muscle strength and performance in two AAS mouse models via raising the expression of extracellular matrix proteins, activating satellite cells, enhancing autophagy, and impeding cellular aging, which highlights a promising strategy for AAS and other age-associated muscle diseases.

Muscle-Specific Cellular and Molecular Adaptations to Late-Life Voluntary Concurrent Exercise

Murine exercise models can provide information on factors that influence muscle adaptability with aging, but few translatable solutions exist. Progressive weighted wheel running (PoWeR) is a simple, voluntary, low-cost, high-volume endurance/resistance exercise approach for training young mice. In the current investigation, aged mice (22-mo-old) underwent a modified version of PoWeR for 8 wk. Muscle functional, cellular, biochemical, transcriptional, and myonuclear DNA methylation analyses provide an encompassing picture of how muscle from aged mice responds to high-volume combined training. Mice run 6-8 km/d, and relative to sedentary mice, PoWeR increases plantarflexor muscle strength. The oxidative soleus of aged mice responds to PoWeR similarly to young mice in every parameter measured in previous work; this includes muscle mass, glycolytic-to-oxidative fiber type transitioning, fiber size, satellite cell frequency, and myonuclear number. The oxidative/glycolytic plantaris adapts according to fiber type, but with modest overall changes in muscle mass. Capillarity increases markedly with PoWeR in both muscles, which may be permissive for adaptability in advanced age. Comparison to published PoWeR RNA-sequencing data in young mice identified conserved regulators of adaptability across age and muscles; this includes Aldh1l1 which associates with muscle vasculature. Agrn and Samd1 gene expression is upregulated after PoWeR simultaneous with a hypomethylated promoter CpG in myonuclear DNA, which could have implications for innervation and capillarization. A promoter CpG in Rbm10 is hypomethylated by late-life exercise in myonuclei, consistent with findings in muscle tissue. PoWeR and the data herein are a resource for uncovering cellular and molecular regulators of muscle adaptation with aging.

Evaluation of Diaphragm Thickness and Stiffness Using Ultrasound and Shear-Wave Elastography

ABSTRACT

The purposes of this study are to measure the thickness and stiffness of diaphragm in association with the respiratory cyclus and to assess the reproducibility of these measurements. Forty individuals who are volunteered for participating diaphragm evaluation were included in the study. Two radiologists with 14 and 15 years of experiences in abdominal ultrasound performed all examinations independently. Furthermore, 8 chronic obstructive pulmonary disease (COPD) patients were examined by only the first radiologist. Gray scale and shear-wave elastography imaging of only the right hemidiaphragm of all participants were performed. Thickness and stiffness of diaphragm were measured at the peak inspiration and end expiration phases. Intraclass correlation coefficients test was used to assess the interobserver agreement. The thickness and stiffness of diaphragm significantly increased with inspiration (P < 0.001). The mean ± SD stiffness of diaphragm in peak inspiration and end expiration phases was 51.84 ± 16.83 kPa and 38.49 ± 9.42 kPa, respectively, for the first radiologist and 49.61 ± 13.83 kPa and 37.52 ± 10.71 kPa, respectively, for the second radiologist. Intraclass correlation coefficient values for diaphragm stiffness were 0.667 and 0.736 in peak inspiration and end expiration phases, respectively. In COPD patients, there was no significant difference between stiffness measurements of respiratory phases. In conclusion, the current study revealed that diaphragm thickness and stiffness increase at inspiration, and these measurements are slightly more reproducible at the end of expiration. However, diaphragm stiffness changes between respiratory phases may not be valid for COPD patients, and this may be related to loss of force-generating capacity of diaphragm in COPD patients.

Age-related structural changes show that loss of fibers is not a significant contributor to muscle atrophy in old mice

Age-related loss of skeletal muscle mass is widely considered a consequence of both fiber atrophy and fiber death. Evidence for fiber death derives largely from an age-related reduction in fiber numbers in muscle cross-sections, however it is unclear how age-related alterations in muscle morphology affect accuracy of such counts. To explore this we performed an examination of muscle and tendon length, muscle mass and girth, and pennation angle, in addition to histological section fiber counts of parallel-fibered (sternomastoid), fusiform (biceps brachii), and pennate (tibialis anterior, extensor digitorum longus, soleus) muscles from 31 mice aged 6-32 months. Age-related decline in mass and girth occurred in soleus (p = 0.026; p = 0.040), tibialis anterior (p = 0.004; p = 0.039), and extensor digitorum longus (p = 0.040; p = 0.022) muscles, for which location of maximal girth also changed. Tendon length and pennation angle remained consistent across the lifespan in all except soleus which showed elongation of both proximal and distal tendons coupled with alterations in pennation angle. Age-related decreases in fiber number were observed in transversely sectioned soleus and extensor digitorum longus muscles however when age-related changes in morphology were accounted for via oblique sectioning the age-related decrease in fiber number was eliminated. Findings show loss of fibers is not a significant contributor to age-related muscle wasting in mice, and that age-related changes in connective tissue selectively impact muscle structure. Fiber shortening is a likely contributor to loss of mass and change in function in muscles of old mice.

A High-Fat Diet Aggravates the Age-Related Decline in Skeletal Muscle Structure and Function

The age-related decline in muscle function is aggravated by a high-fat diet (HFD)-induced increase in fat mass. The hypothesis is that an HFD leads to a faster accumulation of intramyocellular lipids (IMCL) and an earlier onset of muscle dysfunction in old than in young-adult individuals. The IMCL accumulation is attenuated in young-adult organisms by an elevated oxidative capacity. Methionine restriction enhances mitochondrial biogenesis and is promising to combat obesity across the ages.

Effects of Aging on Hemodynamic Kinetics in Different Intensities of Dynamic Exercise

Healthy aging hemodynamics is known to exhibit a time-dependent loss of function. We aimed at verifying whether older men would have a slowed cardiac output and stroke volume dynamics in response to the onset ("on") and on recovery ("off") of exercise in comparison to young men. Twenty healthy active men (10 young and 10 older) were recruited. Participants performed an incremental cardiopulmonary exercise testing on a cycle ergometer, and on another day, 3 constant workload tests in different intensities. Compared to younger, older men exhibited a slower cardiac output and stroke volume dynamics in both on and off transients for all exercise intensities (all P < 0.05). During higher intensities, both younger and older men had slower hemodynamic kinetics compared to lower intensities (all P < 0.05). There was strong negative relationship between the time constant of cardiac output on-kinetics during high-intensity with maximal exercise performance in both groups (r = -0.88, P < 0.01). We interpret these findings to mean that healthy older men have slowed hemodynamic kinetics compared to younger, but this difference becomes less evident in higher intensities of exercise.

Effects of melatonin on insulin signaling and inflammatory pathways of rats with apical periodontitis

AIM

To verify the effects of melatonin supplementation on insulin sensitivity, plasma concentrations of inflammatory cytokines, insulin signalling and inflammatory pathways in the soleus (SM) and extensor digitorum longus (EDL) muscles of rats with apical periodontitis (AP).

METHODOLOGY

Seventy-two Wistar rats were distributed into 4 groups: (a) control (C), (b) control supplemented with melatonin (M), (c) AP (AP), and (d) AP supplemented with melatonin (AP + M). AP was induced by pulp exposure of the maxillary and mandibular right first and second molars to the oral environment. After AP induction, oral supplementation with 5 mg kg^-1 melatonin (diluted in drinking water) for 60 days was initiated. At the end of the treatment, the following were analysed: (1) plasma concentrations of insulin and inflammatory cytokines (TNF-α, IL-6, IL-1β and IL-10) using ELISA kits; (2) glycaemia using enzymatic assay; (3) insulin resistance using homoeostasis model assessment of insulin resistance (HOMA-IR) index; and (4) phosphorylation status of pp185 tyrosine, Akt serine, IKKα/β, and JNK in SM and EDL using Western blot. Analysis of variance of two or three factors was performed, followed by the Bonferroni test. P values < 0.05 were considered statistically significant.

RESULTS

AP promoted insulin resistance, significantly increased (P < 0.05) plasma concentrations of pro-inflammatory cytokines (TNF-α, IL-6, and IL-1β), significantly decreased (P < 0.05) the concentration of anti-inflammatory cytokine IL-10, impaired insulin signalling in SM, and increased IKKα/β phosphorylation status in SM and EDL. Melatonin supplementation in rats with AP improved insulin sensitivity, significantly decreased (P < 0.05) TNF-α and IL-1β, significantly increased (P < 0.05) IL-10 plasma concentrations, and changed the insulin signalling in soleus muscle and IKKα/β phosphorylation status in SM and EDL muscles.

CONCLUSIONS

Melatonin is a potent adjuvant treatment for improving apical periodontitis-associated changes in insulin sensitivity, insulin signalling and inflammatory pathways. In addition, the negative impact of AP on general health was also demonstrated.

Age-related changes in isolated mouse skeletal muscle function are dependent on sex, muscle, and contractility mode

The present study aimed to simultaneously examine the age-related, muscle-specific, sex-specific, and contractile mode-specific changes in isolated mouse skeletal muscle function and morphology across multiple ages. Measurements of mammalian muscle morphology, isometric force and stress (force/cross-sectional area), absolute and normalized (power/muscle mass) work-loop power across a range of contractile velocities, fatigue resistance, and myosin heavy chain (MHC) isoform concentration were measured in 232 isolated mouse (CD-1) soleus, extensor digitorum longus (EDL), and diaphragm from male and female animals aged 3, 10, 30, 52, and 78 wk. Aging resulted in increased body mass and increased soleus and EDL muscle mass, with atrophy only present for female EDL by 78 wk despite no change in MHC isoform concentration. Absolute force and power output increased up to 52 wk and to a higher level for males. A 23–36% loss of isometric stress exceeded the 14–27% loss of power normalized to muscle mass between 10 wk and 52 wk, although the loss of normalized power between 52 and 78 wk continued without further changes in stress ( P > 0.23). Males had lower power normalized to muscle mass than females by 78 wk, with the greatest decline observed for male soleus. Aging did not cause a shift toward slower contractile characteristics, with reduced fatigue resistance observed in male EDL and female diaphragm. Our findings show that the loss of muscle quality precedes the loss of absolute performance as CD-1 mice age, with the greatest effect seen in male soleus, and in most instances without muscle atrophy or an alteration in MHC isoforms.

Regular endurance exercise of overloaded muscle of young and old male mice does not attenuate hypertrophy and improves fatigue resistance

It has been observed that there is an inverse relationship between fiber size and oxidative capacity due to oxygen, ADP, and ATP diffusion limitations. We aimed to see if regular endurance exercise alongside a hypertrophic stimulus would lead to compromised adaptations to both, particularly in older animals. Here we investigated the effects of combining overload with regular endurance exercise in young (12 months) and old (26 months) male mice. The plantaris muscles of these mice were overloaded through denervation of synergists to induce hypertrophy and the mice ran on a treadmill for 30 min per day for 6 weeks. The hypertrophic response to overload was not blunted by endurance exercise, and the increase in fatigue resistance with endurance exercise was not reduced by overload. Old mice demonstrated less hypertrophy than young mice, which was associated with impaired angiogenesis and a reduction in specific tension. The data of this study suggest that combining endurance exercise and overload induces the benefits of both types of exercise without compromising adaptations to either. Additionally, the attenuated hypertrophic response to overload in old animals may be due to a diminished capacity for capillary growth.

Region-Specific Proteome Changes of the Intestinal Epithelium during Aging and Dietary Restriction

The small intestine is responsible for nutrient absorption and one of the most important interfaces between the environment and the body. During aging, changes of the epithelium lead to food malabsorption and reduced barrier function, thus increasing disease risk. The drivers of these alterations remain poorly understood. Here, we compare the proteomes of intestinal crypts from mice across different anatomical regions and ages. We find that aging alters epithelial immunity, metabolism, and cell proliferation and is accompanied by region-dependent skewing in the cellular composition of the epithelium. Of note, short-term dietary restriction followed by refeeding partially restores the epithelium by promoting stem cell differentiation toward the secretory lineage. We identify Hmgcs2 (3-hydroxy-3-methylglutaryl-coenzyme A [CoA] synthetase 2), the rate-limiting enzyme for ketogenesis, as a modulator of stem cell differentiation that responds to dietary changes, and we provide an atlas of region- and age-dependent proteome changes of the small intestine.

The impact of a high-fat diet in mice is dependent on duration and age, and differs between muscles

Prolonged high-fat diets (HFDs) can cause intramyocellular lipid (IMCL) accumulation that may negatively affect muscle function. We investigated the duration of a HFD required to instigate these changes, and whether the effects are muscle specific and aggravated in older age. Muscle morphology was determined in the soleus, extensor digitorum longus (EDL) and diaphragm muscles of female CD-1 mice from 5 groups: young fed a HFD for 8 weeks (YS-HFD, n=16), young fed a HFD for 16 weeks (YL-HFD, n=28) and young control (Y-Con, n=28). The young animals were 20 weeks old at the end of the experiment. Old (70 weeks) female CD-1 mice received either a normal diet (O-Con, n=30) or a HFD for 9 weeks (OS-HFD, n=30). Body mass, body mass index and intramyocellular lipid (IMCL) content increased in OS-HFD (P≤0.003). In the young mice, this increase was seen in YL-HFD and not YS-HFD (P≤0.006). The soleus and diaphragm fibre cross-sectional area (FCSA) in YL-HFD was larger than that in Y-Con (P≤0.004) while OS-HFD had a larger soleus FCSA compared with that of O-Con after only 9 weeks on a HFD (P<0.001). The FCSA of the EDL muscle did not differ significantly between groups. The oxidative capacity of fibres increased in young mice only, irrespective of HFD duration (P<0.001). High-fat diet-induced morphological changes occurred earlier in the old animals than in the young, and adaptations to HFD were muscle specific, with the EDL being least responsive.

Ageing of the Diaphragm Muscle

The diaphragm muscle is the most important contractile district used for breathing. Like other muscles in the human body, it is subject to ageing and sarcopenia. Sarcopenia can be classified as primary (or age-related) when there are no local or systemic pathologies that cause a functional and morphological detriment of skeletal musculature. Secondary sarcopenia occurs when there is a cause or more pathological causes (illness, malnutrition, immobility) related or unrelated to ageing. In the elderly population, transdiaphragmatic pressure (Pdi) decreases by 20-41%, with a decline in the overall strength of 30% (the strength of the expiratory muscles also decreases). The article discusses the adaptation of the diaphragm muscle to ageing and some other ailments and co-morbidities, such as back pain, emotional alterations, motor incoordination, and cognitive disorders, which are related to breathing.

Diaphragm muscle sarcopenia into very old age in mice

Sarcopenia is the age-related decline of skeletal muscle mass and function. Diaphragm muscle (DIAm) sarcopenia may contribute to respiratory complications, a common cause of morbidity and mortality in the elderly. From 6 to 24 months (mo) of age, representing ~100% and ~80% survival in C57BL/6 × 129 male and female mice, there is a significant reduction in DIAm force generation (~30%) and cross-sectional area (CSA) of type IIx and/or IIb muscle fibers (~30%), impacting the ability to perform high force, non-ventilatory behaviors. To date, there is little information available regarding DIAm sarcopenia in very old age groups. The present study examined DIAm sarcopenia in C57BL/6 × 129 male and female mice at 24, 27, and 30 mo, representing ~80%, ~60%, and ~30% survival, respectively. We hypothesized that survival into older ages will show no further worsening of DIAm sarcopenia and functional impairment in 30 mo mice compared to 24 or 27 mo C57BL/6 × 129 mice. Measurements included resting ventilation, transdiaphragmatic pressure (Pdi) generation across a range of motor behaviors, muscle fiber CSA, and proportion of type-identified DIAm fibers. Maximum Pdi and resting ventilation did not change into very old age (from 24 to 30 mo). Type IIx and/or IIb fiber CSA and proportions did not change into very old age. The results of the study support a critical threshold for the reduction in DIAm force and Pdi such that survival into very old age is not associated with evidence of progression of DIAm sarcopenia or impairment in ventilation.