Muscle mitochondrial bioenergetic capacities are associated with multimorbidity burden in older adults: the Study of Muscle, Mobility and Aging (SOMMA)

BACKGROUND

The geroscience hypothesis posits that aging biological processes contribute to many age-related deficits, including the accumulation of multiple chronic diseases. Though only one facet of mitochondrial function, declines in muscle mitochondrial bioenergetic capacities may contribute to this increased susceptibility to multimorbidity.

METHODS

The Study of Muscle, Mobility and Aging (SOMMA) assessed ex vivo muscle mitochondrial energetics in 764 older adults (mean age =76.4, 56.5% women, 85.9% non-Hispanic white) by high-resolution respirometry of permeabilized muscle fibers. We estimated the proportional odds ratio (POR [95%CI]) for the likelihood of greater multimorbidity (four levels: 0 conditions, N=332; 1 condition, N=299; 2 conditions, N=98; or 3+ conditions, N=35) from an index of 11 conditions, per SD decrement in muscle mitochondrial energetic parameters. Distribution of conditions allowed for testing the associations of maximal muscle energetics with some individual conditions.

RESULTS

Lower oxidative phosphorylation supported by fatty acids and/or complex-I and -II linked carbohydrates (e.g., Max OXPHOSCI+CII) was associated with a greater multimorbidity index score (POR=1.32[1.13,1.54]) and separately with diabetes mellitus (OR=1.62[1.26,2.09]), depressive symptoms (OR=1.45[1.04,2.00]) and possibly chronic kidney disease (OR=1.57[0.98,2.52]) but not significantly with other conditions (e.g., cardiac arrhythmia, chronic obstructive pulmonary disease).

CONCLUSIONS

Lower muscle mitochondrial bioenergetic capacities was associated with a worse composite multimorbidity index score. Our results suggest that decrements in muscle mitochondrial energetics may contribute to a greater global burden of disease and is more strongly related to some conditions than others.

Associations of Skeletal Muscle Mass, Muscle Fat Infiltration, Mitochondrial Energetics, and Cardiorespiratory Fitness With Liver Fat Among Older Adults

BACKGROUND

Muscle mass loss may be associated with liver fat accumulation, yet scientific consensus is lacking and evidence in older adults is scant. It is unclear which muscle characteristics might contribute to this association in older adults.

METHODS

We associated comprehensive muscle-related phenotypes including muscle mass normalized to body weight (D3-creatine dilution), muscle fat infiltration (magnetic resonance imaging), carbohydrate-supported muscle mitochondrial maximal oxidative phosphorylation (respirometry), and cardiorespiratory fitness (VO2 peak) with liver fat among older adults. Linear regression models adjusted for age, gender, technician (respirometry only), daily minutes of moderate-to-vigorous physical activity, and prediabetes/diabetes status tested main effects and interactions of each independent variable with waist circumference (high: women-≥88 cm, men-≥102 cm) and gender.

RESULTS

Among older adults aged 75 (interquartile range: 73, 79 years; 59.8% women), muscle mass and liver fat were not associated overall (N = 362) but were positively associated among participants with a high waist circumference (β: 25.2; 95% confidence intervals [95% CI]: 11.7, 40.4; p = .0002; N = 160). Muscle fat infiltration and liver fat were positively associated (β: 15.2; 95% CI: 6.8, 24.3; p = .0003; N = 378). Carbohydrate-supported maximum oxidative phosphorylation (before adjustment) and VO2 peak (after adjustment; β: -12.9; 95% CI: -20.3, -4.8; p = .003; N = 361) were inversely associated with liver fat; adjustment attenuated the estimate for maximum oxidative phosphorylation although the point estimate remained negative (β: -4.0; 95% CI: -11.6, 4.2; p = .32; N = 321).

CONCLUSIONS

Skeletal muscle-related characteristics are metabolically relevant factors linked to liver fat in older adults. Future research should confirm our results to determine whether trials targeting mechanisms common to liver and muscle fat accumulation are warranted.

Associations Between D3Cr Muscle Mass and Magnetic Resonance Thigh Muscle Volume With Strength, Power, Physical Performance, Fitness, and Limitations in Older Adults in the SOMMA Study

BACKGROUND

How magnetic resonance (MR) derived thigh muscle volume and deuterated creatine dilution derived muscle mass (D3Cr muscle mass) differentially relate to strength, fitness, and other functions in older adults-and whether associations vary by sex-is not known.

METHODS

Men (N = 345) and women (N = 482) aged ≥70 years from the Study of Muscle, Mobility, and Aging completed leg extension strength (1-repetition max) and cardiopulmonary exercise testing to assess fitness (VO2peak). Correlations and adjusted regression models stratified by sex were used to assess the association between muscle size measures, study outcomes, and sex interactions.

RESULTS

D3Cr muscle mass and MR thigh muscle volume were correlated (men: r = 0.62, women: r = 0.51, p < .001). Each standard deviation (SD) decrement in D3Cr muscle mass was associated with lower 1-repetition max strength (-14 kg men, -4 kg women, p < .001 for both; p-interaction = .003) and lower VO2peak (-79 mL/min men, -30 mL/min women, p < .001 for both, p-interaction: .016). Each SD decrement in MR thigh muscle volume was also associated with lower strength (-32 kg men, -20 kg women, p < .001 for both; p-interaction = .139) and lower VO2peak (-217 mL/min men, -111 mL/min women, p < .001 for both, p-interaction = .010). There were associations, though less consistent, between muscle size or mass with physical performance and function; associations varied by sex.

CONCLUSIONS

Less muscle-measured by either D3Cr muscle mass or MR thigh muscle volume-was associated with lower strength and fitness. Varied associations by sex and assessment method suggest consideration be given to which measurement to use in future studies.

Energetics and clinical factors for the time required to walk 400 m: The Study of Muscle, Mobility and Aging (SOMMA)

BACKGROUND

Walking slows with aging often leading to mobility disability. Mitochondrial energetics has been found to be associated with gait speed over short distances. Additionally, walking is a complex activity but few clinical factors that may be associated with walk time have been studied.

METHODS

We examined 879 participants ≥70 years and measured the time to walk 400 m. We tested the hypothesis that decreased mitochondrial energetics by respirometry in muscle biopsies and magnetic resonance spectroscopy in the thigh and is associated with longer time to walk 400 m. We also used cardiopulmonary exercise testing to assess the energetic costs of walking: maximum oxygen consumption (VO2peak) and energy cost-capacity (the ratio of VO2, at a slow speed to VO2peak). In addition, we tested the hypothesis that selected clinical factors would also be associated with 400-m walk time.

RESULTS

Lower Max OXPHOS was associated with longer walk time, and the association was explained by the energetic costs of walking, leg power, and weight. Additionally, a multivariate model revealed that longer walk time was also significantly associated with lower VO2peak, greater cost-capacity ratio, weaker leg power, heavier weight, hip and knee stiffness, peripheral neuropathy, greater perceived exertion while walking slowly, greater physical fatigability, less moderate-to-vigorous exercise, less sedentary time, and anemia. Significant associations between age, sex, muscle mass, and peripheral artery disease with 400-m walk time were explained by other clinical and physiologic factors.

CONCLUSIONS

Lower mitochondrial energetics is associated with needing more time to walk 400 m. This supports the value of developing interventions to improve mitochondrial energetics. Additionally, doing more moderate-to-vigorous exercise, increasing leg power, reducing weight, treating hip and knee stiffness, and screening for and treating anemia may reduce the time required to walk 400 m and reduce the risk of mobility disability.

The Association of Skeletal Muscle Energetics With Recurrent Falls in Older Adults Within the Study of Muscle, Mobility and Aging

BACKGROUND

Falls in the older population are a major public health concern. While many physiological and environmental factors have been associated with fall risk, muscle mitochondrial energetics has not yet been investigated.

METHODS

In this analysis, 835 Study of Muscle, Mobility and Aging (SOMMA) participants aged 70-94 were surveyed for number of falls (total), recurrent falls (2+), and fall-related injuries over the past 12 months at baseline and again after 1 year. Skeletal muscle energetics were assessed at baseline in vivo using 31P Magnetic Resonance Spectroscopy for the maximal rate of adenosine triphosphate recovery (ATPmax) after an acute bout of exercise, and ex vivo by High-Resolution Respirometry for the maximal rate of complex I and II supported oxygen consumption (MaxOXPHOS) in permeabilized muscle fibers from the vastus lateralis.

RESULTS

At least 1 fall was reported in 28.7% of SOMMA participants in the first year of the study, with 12% of older adults reporting recurrent falls (2+). Individuals who experienced recurrent falls had a slower 400-m walk gait speed (1.0 ± 0.2 vs 1.1 ± 0.2, p < .001), reported fewer alcoholic drinks per week in the past year (2.4 ± 4.3 vs 2.8 ± 4.4, p = .054), and took a significantly greater number of medication in the 30 days before their baseline visit (5.6 ± 4.4 vs 4.2 ± 3.4, p < .05). A history of falls was reported in 63% of individuals who experienced recurrent falls in the first year of the study compared to 22.8% who experienced 1 or fewer falls. MaxOXPHOS was significantly lower in those who reported recurrent falls (p = .008) compared to those with 1 or fewer falls, but there was no significant difference in ATPmax (p = .369). Neither muscle energetics measure was significantly associated with total number of falls or injurious falls, but recurrent falls were significantly higher with lower MaxOXPHOS (risk ratio = 1.33, 95% confidence interval = 1.02-1.73, p = .033). However, covariates accounted for the increased risk.

CONCLUSIONS

Mitochondrial energetics were largely unrelated to fall risk in older adults when accounting for variables, suggesting that the complex etiology of falls may not be related to a single "hallmark of aging" biological pathway.

Skeletal Muscle Energetics Explain the Sex Disparity in Mobility Impairment in the Study of Muscle, Mobility and Aging

The age-related decline in muscle mitochondrial energetics contributes to the loss of mobility in older adults. Women experience a higher prevalence of mobility impairment compared to men, but it is unknown whether sex-specific differences in muscle energetics underlie this disparity. In the Study of Muscle, Mobility and Aging (SOMMA), muscle energetics were characterized using in vivo phosphorus-31 magnetic resonance spectroscopy and high-resolution respirometry of vastus lateralis biopsies in 773 participants (56.4% women, age 70-94 years). A Short Physical Performance Battery (SPPB) score ≤8 was used to define lower-extremity mobility impairment. Muscle mitochondrial energetics were lower in women compared to men (eg, Maximal Complex I&II OXPHOS: Women = 55.06 ± 15.95; Men = 65.80 ± 19.74; p < .001) and in individuals with mobility impairment compared to those without (eg, Maximal Complex I&II OXPHOS in women: SPPB ≥ 9 = 56.59 ± 16.22; SPPB ≤ 8 = 47.37 ± 11.85; p < .001). Muscle energetics were negatively associated with age only in men (eg, Maximal ETS capacity: R = -0.15, p = .02; age/sex interaction, p = .04), resulting in muscle energetics measures that were significantly lower in women than men in the 70-79 age group but not the 80+ age group. Similarly, the odds of mobility impairment were greater in women than men only in the 70-79 age group (70-79 age group, odds ratio [OR]age-adjusted = 1.78, 95% confidence interval [CI] = 1.03, 3.08, p = .038; 80+ age group, ORage-adjusted = 1.05, 95% CI = 0.52, 2.15, p = .89). Accounting for muscle energetics attenuated up to 75% of the greater odds of mobility impairment in women. Women had lower muscle mitochondrial energetics compared to men, which largely explain their greater odds of lower-extremity mobility impairment.

Lower muscle mitochondrial energetics is associated with greater phenotypic frailty in older women and men: the Study of Muscle, Mobility and Aging

BACKGROUND

Phenotypic frailty syndrome identifies older adults at greater risk for adverse health outcomes. Despite the critical role of mitochondria in maintaining cellular function, including energy production, the associations between muscle mitochondrial energetics and frailty have not been widely explored in a large, well-phenotyped, older population.

METHODS

The Study of Muscle, Mobility and Aging (SOMMA) assessed muscle energetics in older adults (N = 879, mean age = 76.3 years, 59.2% women). 31Phosporous magnetic resonance spectroscopy measured maximal production of adenosine triphosphate (ATPmax) in vivo, while ex vivo high-resolution respirometry of permeabilized muscle fibers from the vastus lateralis measured maximal oxygen consumption supported by fatty acids and complex I- and II-linked carbohydrates (e.g., Max OXPHOSCI+CII). Five frailty criteria, shrinking, weakness, exhaustion, slowness, and low activity, were used to classify participants as robust (0, N = 397), intermediate (1-2, N = 410), or frail (≥ 3, N = 66). We estimated the proportional odds ratio (POR) for greater frailty, adjusted for multiple potential confounders.

RESULTS

One-SD decrements of most respirometry measures (e.g., Max OXPHOSCI+CII, adjusted POR = 1.5, 95%CI [1.2,1.8], p = 0.0001) were significantly associated with greater frailty classification. The associations of ATPmax with frailty were weaker than those between Max OXPHOSCI+CII and frailty. Muscle energetics was most strongly associated with slowness and low physical activity components.

CONCLUSIONS

Our data suggest that deficits in muscle mitochondrial energetics may be a biological driver of frailty in older adults. On the other hand, we did observe differential relationships between measures of muscle mitochondrial energetics and the individual components of frailty.

Role of Cardiorespiratory Fitness and Mitochondrial Oxidative Capacity in Reduced Walk Speed of Older Adults with Diabetes

Cardiorespiratory fitness and mitochondrial oxidative capacity are associated with reduced walking speed in older adults. The impact of cardiorespiratory fitness and mitochondrial oxidative capacity on walking speed in older adults with diabetes has not been clearly defined. We examined differences in cardiorespiratory fitness and skeletal muscle mitochondrial oxidative capacity between older adults with and without diabetes as well as determine their relative contribution to slower walking speed in older adults with diabetes. Participants with diabetes (n=159) had lower cardiorespiratory fitness and mitochondrial respiration in permeabilized fiber bundles when compared to those without diabetes (n=717), following adjustments for covariates including BMI, chronic comorbid health conditions, and physical activity. 4-m and 400-m walking speeds were slower in those with diabetes. Mitochondrial oxidative capacity alone or combined with cardiorespiratory fitness mediated ∼20-70% of the difference in walk speed between older adults with and without diabetes. Additional adjustments with BMI and co-morbidities further explained the group differences in walk speed. Cardiorespiratory fitness and skeletal muscle mitochondrial oxidative capacity contribute to slower walking speeds in older adults with diabetes.

Chronic exercise improves hepatic acylcarnitine handling

Exercise mediates tissue metabolic function through direct and indirect adaptations to acylcarnitine (AC) metabolism, but the exact mechanisms are unclear. We found that circulating medium-chain acylcarnitines (AC) (C12-C16) are lower in active/endurance trained human subjects compared to sedentary controls, and this is correlated with elevated cardiorespiratory fitness and reduced adiposity. In mice, exercise reduced serum AC and increased liver AC, and this was accompanied by a marked increase in expression of genes involved in hepatic AC metabolism and mitochondrial β-oxidation. Primary hepatocytes from high-fat fed, exercise trained mice had increased basal respiration compared to hepatocytes from high-fat fed sedentary mice, which may be attributed to increased Ca2+ cycling and lipid uptake into mitochondria. The addition of specific medium- and long-chain AC to sedentary hepatocytes increased mitochondrial respiration, mirroring the exercise phenotype. These data indicate that AC redistribution is an exercise-induced mechanism to improve hepatic function and metabolism.

Childhood adverse life events and skeletal muscle mitochondrial function

Social stress experienced in childhood is associated with adverse health later in life. Mitochondrial function has been implicated as a mechanism for how stressful life events "get under the skin" to influence physical well-being. Using data from the Study of Muscle, Mobility, and Aging (n = 879, 59% women), linear models examined whether adverse childhood events (i.e., physical abuse) were associated with two measures of skeletal muscle mitochondrial energetics in older adults: (i) maximal adenosine triphosphate production (ATPmax) and (ii) maximal state 3 respiration (Max OXPHOS). Forty-five percent of the sample reported experiencing one or more adverse childhood events. After adjustment, each additional event was associated with -0.08 SD (95% confidence interval = -0.13, -0.02) lower ATPmax. No association was observed with Max OXPHOS. Adverse childhood events are associated with lower ATP production in later life. Findings indicate that mitochondrial function may be a mechanism for understanding how early social stress influences health in later life.

The cellular bases of mobility from the Study of Muscle, Mobility and Aging (SOMMA)

Findings from the Study of Muscle, Mobility and Aging (SOMMA) in this issue of Aging Cell show that several biological pathways in skeletal muscle cells play an important role in determining mobility in older adults. These are based on assays in skeletal muscle biopsies obtained from participants, aged 70 years and older in SOMMA tested for association with assessments related to mobility, including muscle mass, strength, power, cardiopulmonary fitness, and 400 m walking speed. The papers show that, using mass spectrometry, oxidative modifications of proteins essential to myocellular function are associated with poorer mobility. Using RNA-seq to quantify gene expression, lower levels of expression of antioxidant enzymes located in mitochondria, autophagy, patterns of expression of genes involved in autophagy, and higher levels of RNA transcripts that increase with denervation were associated with poorer performance on tests of mobility. These results extend previous research from the Baltimore Longitudinal Study of Aging and recent studies from SOMMA showing the importance of mitochondrial energetics in mobility. Together, these findings are painting a picture of how fundamental cellular processes influence the loss of mobility with aging. They may also be a window on aging in other cells, tissues, and systems. The data collected in SOMMA are publicly available and SOMMA welcomes collaborations with scientists who are interested in research about human aging.

Usual-paced 400 m long distance corridor walk estimates cardiorespiratory fitness among older adults: The Study of Muscle, Mobility and Aging

BACKGROUND

Cardiopulmonary exercise testing (CPET), the gold-standard method to quantify cardiorespiratory fitness (CRF), is not always feasible due to cost, access, and burden. The usual-paced 400 m long distance corridor walk (LDCW), a measure of mobility among older adults, may provide an alternate method to assess CRF. The purpose of this study was to develop and validate an estimating equation to estimate VO2 peak from average 400 m walking speed (WS) among participants in the Study of Muscle, Mobility and Aging (SOMMA).

METHODS

At baseline, women (58%) and men age 70 years and older enrolled in SOMMA (N = 820, 76.2 ± 4.9 years, 86% Non-Hispanic White) completed a 400 m LDCW (400 m WS = 400 m/completion time in seconds) and symptom-limited maximal CPET (Modified Balke Protocol). VO2 peak (mL/kg/min) was considered the highest 30-second average oxygen consumption during CPET. Other covariates included: age, sex, race, physical activity (7-day wrist-worn accelerometer), physical function (Short Physical Performance Battery, range 0-12), perceived physical fatigability (Pittsburgh Fatigability Scale, range 0-50), and Borg Rating of Perceived Exertion (RPE, range 6-20) at completion of the 400 m LDCW. Stepwise linear regression was used. Internal validation was completed using data-splitting method (70%; 30%).

RESULTS

Mean VO2 peak was 20.2 ± 4.8 mL/kg/min and mean 400 m WS was 1.06 ± 0.2 m/s. Each 0.05 m/s increment in 400 m WS was associated with a 0.40 mL/kg/min higher VO2 peak after covariate adjustment. An estimating equation including 400 m WS, age, sex, race, and RPE was developed. Internal validation showed low overall bias (-0.26) and strong correlation (r = 0.71) between predicted and measured VO2 peak values. Bland-Altman plot and regression analyses indicated predicted VO2 peak was an acceptable alternative, despite mean underestimation of 4.53 mL/kg/min among the highly fit.

CONCLUSIONS

Usual-paced 400 m LDCW strongly correlates with direct measures of CRF during CPET in older adults with lower fitness and can be used to test both fitness and function.

Age is Associated with Dampened Circadian Patterns of Rest and Activity: The Study of Muscle, Mobility and Aging (SOMMA)

BACKGROUND

The effects of aging on circadian patterns of behavior are insufficiently described. To address this, we characterized age-specific features of rest-activity rhythms (RAR) in community dwelling older adults both overall, and in relation, to sociodemographic characteristics.

METHODS

We examined cross-sectional associations between RAR and age, sex, race, education, multimorbidity burden, financial, work, martial, health, and smoking status using assessments of older adults with wrist-worn free-living actigraphy data (N=820, Age=76.4 yrs, 58.2% women) participating in the Study of Muscle, Mobility and Aging (SOMMA). RAR parameters were determined by mapping an extension to the traditional cosine curve to activity data. Functional principal component analysis determined variables accounting for variance.

RESULTS

Age was associated with several metrics of dampened RAR; women had stronger and more robust RAR vs. men (all P < 0.05). Total activity (56%) and time of activity (20%) accounted for most the RAR variance. Compared to the latest decile of acrophase, those in the earliest decile had higher average amplitude (P <0.001). Compared to the latest decile of acrophase, those in the earliest and midrange categories had more total activity (P=0.02). Being in a married-like relationship and a more stable financial situation were associated with stronger rhythms; higher education was associated with less rhythm strength (all P < 0.05).

CONCLUSIONS

Older age was associated with dampened circadian behavior; behaviors were sexually dimorphic. Some sociodemographic characteristics were associated with circadian behavior. We identified a behavioral phenotype characterized by early time-of-day of peak activity, high rhythmic amplitude, and more total activity.

Associations of accelerometry-measured and self-reported physical activity and sedentary behavior with skeletal muscle energetics: The Study of Muscle, Mobility and Aging (SOMMA)

BACKGROUND

Skeletal muscle energetics decline with age, and physical activity (PA) has been shown to offset these declines in older adults. Yet, many studies reporting these effects were based on self-reported PA or structured exercise interventions. Therefore, we examined the associations of accelerometry-measured and self-reported PA and sedentary behavior (SB) with skeletal muscle energetics and explored the extent to which PA and sedentary behavior would attenuate the associations of age with muscle energetics.

METHODS

As part of the Study of Muscle, Mobility and Aging, enrolled older adults (n = 879), 810 (age = 76 ± 5 years old, mean ± SD; 58% women) had maximal muscle oxidative capacity measured ex vivo via high-resolution respirometry of permeabilized myofibers (maximal oxidative phosphorylation (maxOXPHOS)) and in vivo by 31phosphorus magnetic resonance spectroscopy (maximal adenosine triphosphate (ATPmax)). Accelerometry-measured sedentary behavior, light activity, and moderate-to-vigorous PA (MVPA) were assessed using a wrist-worn ActiGraph GT9X over 7 days. Self-reported sedentary behavior, MVPA, and all PA were assessed with the Community Healthy Activities Model Program for Seniors (CHAMPS) questionnaire. Linear regression models with progressive covariate adjustments evaluated the associations of sedentary behavior and PA with muscle energetics, as well as the attenuation of the age/muscle energetics association by MVPA and sedentary behavior. As a sensitivity analysis, we also examined activPAL-measured daily step count and time spent in sedentary behavior and their associations with muscle energetics.

RESULTS

Every 30 min/day more of ActiGraph-measured MVPA was associated with 0.65 pmol/(s × mg) higher maxOXPHOS and 0.012 mM/s higher ATPmax after adjusting for age, site/technician, and sex (p < 0.05). Light activity was not associated with maxOXPHOS or ATPmax. Meanwhile, every 30 min/day spent in ActiGraph-measured sedentary behavior was associated with 0.39 pmol/s × mg lower maxOXPHOS and 0.006 mM/s lower ATPmax (p < 0.05). Only associations with ATPmax held after further adjusting for socioeconomic status, body mass index, lifestyle factors, and multimorbidity. CHAMPS MVPA and all PA yielded similar associations with maxOXPHOS and ATPmax (p < 0.05), but sedentary behavior did not. Higher activPAL step count was associated with higher maxOXHPOS and ATPmax (p < 0.05), but time spent in sedentary behavior was not. Additionally, age was significantly associated with muscle energetics for men only (p < 0.05); adjusting for time spent in ActiGraph-measured MVPA attenuated the age association with ATPmax by 58% in men.

CONCLUSION

More time spent in accelerometry-measured or self-reported daily PA, especially MVPA, was associated with higher skeletal muscle energetics. Interventions aimed specifically at increasing higher intensity activity might offer potential therapeutic interventions to slow age-related decline in muscle energetics. Our work also emphasizes the importance of taking PA into consideration when evaluating associations related to skeletal muscle energetics.

Signatures of cysteine oxidation on muscle structural and contractile proteins are associated with physical performance and muscle function in older adults: Study of Muscle, Mobility and Aging (SOMMA)

Oxidative stress is considered a contributor to declining muscle function and mobility during aging; however, the underlying molecular mechanisms remain poorly described. We hypothesized that greater levels of cysteine (Cys) oxidation on muscle proteins are associated with decreased measures of mobility. Herein, we applied a novel redox proteomics approach to measure reversible protein Cys oxidation in vastus lateralis muscle biopsies collected from 56 subjects in the Study of Muscle, Mobility and Aging (SOMMA), a community-based cohort study of individuals aged 70 years and older. We tested whether levels of Cys oxidation on key muscle proteins involved in muscle structure and contraction were associated with muscle function (leg power and strength), walking speed, and fitness (VO2 peak on cardiopulmonary exercise testing) using linear regression models adjusted for age, sex, and body weight. Higher oxidation levels of select nebulin Cys sites were associated with lower VO2 peak, while greater oxidation of myomesin-1, myomesin-2, and nebulin Cys sites was associated with slower walking speed. Higher oxidation of Cys sites in key proteins such as myomesin-2, alpha-actinin-2, and skeletal muscle alpha-actin were associated with lower leg power and strength. We also observed an unexpected correlation (R = 0.48) between a higher oxidation level of eight Cys sites in alpha-actinin-3 and stronger leg power. Despite this observation, the results generally support the hypothesis that Cys oxidation of muscle proteins impairs muscle power and strength, walking speed, and cardiopulmonary fitness with aging.

Vigor to Frailty As a Continuum-A New Approach in the Study of Muscle, Mobility, and Aging Cohort

BACKGROUND

Frailty can occur in older adults without disability or multimorbidity. Current methods focus on the most frail, but poorly discriminate among those "not frail."

METHODS

The Study of Muscle, Mobility, and Aging (SOMMA) included 879 adults aged 70 years and older without mobility disability. We operationalized frailty domains using: peak oxygen consumption (endurance), digit symbol substitution test (speed), leg power (strength), perceived fatigability, D3 creatine dilution (sarcopenia), and accelerometry (sedentary behavior) to construct a frailty score of 0-12 summing tertiles (0-2) of each component. We used linear or logistic regression with and without adjustment for confounders to examine associations with age, reported, and performance function.

RESULTS

The SOMMA frailty score distribution was broad and strongly associated with age (r = 0.33, p < .0001). Each point was associated with a 30%-50% higher odds of having reported difficulty with activities of daily living or mobility. After grouping the total score (0-3, 4-7, and 8-12) those in the highest group were 9-31 times more likely to have functional limitation, and at least 8 times more likely to have poorer function after full adjustment. Higher scores identified those less likely to report ease of walking or higher physical activity. Peak oxygen consumption, leg power, fatigability, and digit symbol score contributed most to these associations.

CONCLUSIONS

The SOMMA frailty score characterizes frailty as a continuum from frail to vigorous with assessments that are amenable to change. Associations with age and function suggest utility for distinguishing a wide range of vigor and vulnerability in relatively well-functioning older adults.

Age is Associated with Dampened Circadian Patterns of Rest and Activity: The Study of Muscle, Mobility and Aging (SOMMA)

Background

Aging is associated with declines in circadian functions. The effects of aging on circadian patterns of behavior are insufficiently described. We characterized age-specific features of rest-activity rhythms (RAR) in community dwelling older adults, both overall, and in relation, to sociodemographic characteristics.

Methods

We analyzed baseline assessments of older adults with wrist-worn free-living wrist-worn actigraphy data (N=820, Age=76.4 yrs, 58.2% women) participating in the Study of Muscle, Mobility and Aging (SOMMA). We applied an extension to the traditional cosine curve to map RAR to activity data, calculating the parameters: rhythmic strength (amplitude); robustness (pseudo-F statistic); and timing of peak activity (acrophase). We also used function principal component analysis to determine 4 components describing underlying patterns of activity accounting for RAR variance. Linear models were used to examine associations between RAR and sociodemographic variables.

Results

Age was associated with several metrics of dampened RAR; women had stronger and more robust RAR metrics vs. men (all P < 0.05). Total activity (56%) and time of activity (20%) accounted for most the RAR variance. Compared to the latest decile of acrophase, those in the earliest decile had higher average amplitude (P <0.001). Compared to the latest decile of acrophase, those is the earliest and midrange categories had more total activity (P=0.02). RAR was associated with some sociodemographic variables.

Conclusions

Older age was associated with dampened circadian behavior; and behaviors were sexually dimorphic. We identified a behavioral phenotype characterized by early time-of-day of peak activity, high rhythmic amplitude, and more total activity.

Associations between D3Cr muscle mass and MR thigh muscle volume with strength, power, physical performance, fitness, and limitations in older adults in the SOMMA study

Background

Different measures to assess muscle size - magnetic resonance (MR) derived thigh muscle volume and d3-creatine dilution derived muscle mass (D3Cr muscle mass) - may have similar associations with strength, power, physical performance, fitness, and functional limitations in older adults.

Methods

Men (N=345) and women (N=482) aged ≥70 years from the Study of Muscle, Mobility and Aging completed exams including leg extension strength (1-repetition max) and cardiopulmonary exercise testing to assess fitness (VO2peak). Correlations and adjusted regression models stratified by sex were used to assess the association between muscle size measures and study outcomes; we tested for sex interactions.

Results

D3Cr muscle mass and MR thigh muscle volume were correlated (men: r=0.62, women: r=0.51, p<.001). Lower D3Cr muscle mass and lower MR thigh muscle volume were associated with lower strength and lower VO2peak in both men and women; D3Cr muscle mass was more strongly associated with strength in men than in women (p-int<0.05). There were correlations, though less consistent, between muscle size or mass with physical performance and function. Associations between the muscle size measures and the study outcomes occasionally varied by sex, and associations of MR thigh muscle volume were, at times, slightly more strongly associated with the study outcomes than was D3Cr muscle mass.

Conclusions

Less muscle -measured by either D3Cr muscle mass or MR thigh muscle volume - was associated with lower strength and worse performance. Varied associations by sex and assessment method suggest consideration be given to which measurement to use in future studies.

Associations between regional adipose tissue distribution and skeletal muscle bioenergetics in older men and women

Objective

Examine the association of ectopic adipose tissue (AT) with skeletal muscle (SM) mitochondrial bioenergetics in older adults.

Methods

Cross-sectional data from 829 older adults ≥70 years was used. Total abdominal, subcutaneous, and visceral AT; and thigh muscle fat infiltration (MFI) was quantified by MRI. SM mitochondrial energetics were characterized using in vivo 31 P-MRS (ATP max ) and ex vivo high-resolution respirometry (maximal oxidative phosphorylation (OXPHOS)). ActivPal was used to measure PA (step count). Linear regression models adjusted for covariates were applied, with sequential adjustment for BMI and PA.

Results

Independent of BMI, total abdominal (standardized (Std.) β=-0.21; R 2 =0.09) and visceral AT (Std. β=-0.16; R 2 =0.09) were associated with ATP max ( p <0.01), but not after further adjustment for PA (p≥0.05). Visceral AT (Std. β=-0.16; R 2 =0.25) and thigh MFI (Std. β=-0.11; R 2 =0.24) were negatively associated with carbohydrate-supported maximal OXPHOS independent of BMI and PA ( p <0.05). Total abdominal AT (Std. β=-0.19; R 2 =0.24) and visceral AT (Std. β=-0.17; R 2 =0.24) were associated with fatty acid-supported maximal OXPHOS independent of BMI and PA (p<0.05).

Conclusions

Skeletal MFI and abdominal visceral, but not subcutaneous AT, are inversely associated with SM mitochondrial bioenergetics in older adults independent of BMI. Associations between ectopic AT and in vivo mitochondrial bioenergetics are attenuated by PA.

Energetics and Clinical Factors for the Time Required to Walk 400 Meters The Study of Muscle, Mobility and Aging (SOMMA)

Background

Walking slows with aging often leading to mobility disability. Mitochondrial energetics has been found to influence gait speed over short distances. Additionally, walking is a complex activity but few clinical factors that may influence walk time have been studied.

Methods

We examined 879 participants ≥70 years and measured the time to walk 400m. We tested the hypothesis that decreased mitochondrial energetics by respirometry in muscle biopsies and magnetic resonance spectroscopy in the thigh, is associated with longer time to walk 400m. We also used cardiopulmonary exercise testing to assess the energetic costs of walking: maximum oxygen consumption (VO 2 peak) and energy cost-capacity (the ratio of VO2, at a slow speed to VO 2 peak). In addition, we tested the hypothesis that selected clinical factors would also be associated with 400m walk time.

Results

Lower Max OXPHOS was associated with longer walk time and the association was explained by the energetics costs of walking, leg power and weight. Additionally, a multivariate model revealed that longer walk time was also significantly associated with lower VO 2 peak, greater cost-capacity ratio, weaker leg power, heavier weight, hip and knee stiffness, peripheral neuropathy, greater perceived exertion while walking slowly, greater physical fatigability, less moderate-to-vigorous exercise, less sedentary time and anemia. Significant associations between age, sex, muscle mass, and peripheral artery disease with 400m walk time were explained by other clinical and physiologic factors.

Conclusions

Lower mitochondrial energetics is associated with needing more time to walk 400m. This supports the value of developing interventions to improve mitochondrial energetics. Additionally, doing more moderate-to-vigorous exercise, increasing leg power, reducing weight, treating hip and knee stiffness, and screening for and treating anemia may reduce the time required to walk 400m and reduce the risk of mobility disability.

Skeletal muscle energetics explain the sex disparity in mobility impairment in the Study of Muscle, Mobility and Aging (SOMMA)

The age-related decline in muscle mitochondrial energetics contributes to the loss of mobility in older adults. Women experience a higher prevalence of mobility impairment compared to men, but it is unknown whether sex-specific differences in muscle energetics underlie this disparity. In the Study of Muscle, Mobility and Aging (SOMMA), muscle energetics were characterized using in vivo phosphorus-31 magnetic resonance spectroscopy and high-resolution respirometry of vastus lateralis biopsies in 773 participants (56.4% women, age 70-94 years). A Short Physical Performance Battery score ≤ 8 was used to define lower-extremity mobility impairment. Muscle mitochondrial energetics were lower in women compared to men (e.g. Maximal Complex I&II OXPHOS: Women=55.06 +/- 15.95; Men=65.80 +/- 19.74; p<0.001) and in individuals with mobility impairment compared to those without (e.g., Maximal Complex I&II OXPHOS in women: SPPB≥9=56.59 +/- 16.22; SPPB≤8=47.37 +/- 11.85; p<0.001). Muscle energetics were negatively associated with age only in men (e.g., Maximal ETS capacity: R=-0.15, p=0.02; age/sex interaction, p=0.04), resulting in muscle energetics measures that were significantly lower in women than men in the 70-79 age group but not the 80+ age group. Similarly, the odds of mobility impairment were greater in women than men only in the 70-79 age group (70-79 age group, OR age-adjusted =1.78, 95% CI=1.03, 3.08, p=0.038; 80+ age group, OR age-adjusted =1.05, 95% CI=0.52, 2.15, p=0.89). Accounting for muscle energetics attenuated up to 75% of the greater odds of mobility impairment in women. Women had lower muscle mitochondrial energetics compared to men, which largely explain their greater odds of lower-extremity mobility impairment.

The association of skeletal muscle energetics with recurrent falls in older adults within the Study of Muscle, Mobility and Aging (SOMMA)

Background

Falls in the older population are a major public health concern. While many physiological and environmental factors have been associated with fall risk, muscle mitochondrial energetics has not yet been investigated.

Methods

In this analysis, 835 Study of Muscle, Mobility and Aging (SOMMA) participants aged 70-94 were surveyed for recurrent falls (2+) after one year. Skeletal muscle energetics were assessed at baseline in vivo using 31 P Magnetic Resonance Spectroscopy (MRS) (ATPmax) and ex vivo by High Resolution Respirometry (HRR) of permeabilized muscle fibers from the vastus lateralis (MaxOXPHOS).

Results

SOMMA participants who reported recurrent falls (12%) had a slower 400m walk gait speed compared to those with 0-1 falls (1.0 +/-0.2 vs. 1.1 +/-0.2, p<.001) and took a greater number of medication in the 30 days before their baseline visit (5.6 +/-4.4 vs. 4.2 +/-3.4, p<0.05). MaxOXPHOS was significantly lower in those who reported recurrent falls (p=0.008) compared to those with one or fewer falls, but there was no significant difference in ATPmax (p=0.369). Neither muscle energetics measure was significantly associated with total number of falls or injurious falls, but recurrent falls were significantly higher with lower MaxOXPHOS (RR=1.33, 95% CI= 1.02-1.73, p=0.033). However, covariates accounted for the increased risk.

Conclusions

Ex vivo maximal muscle mitochondrial energetics were lower in older adults who experienced recurrent falls, but covariates accounted for its association with recurrent fall risk, suggesting this "hallmark of aging" may not be directly implicated in the complex etiology of falls.

Signatures of Cysteine Oxidation on Muscle Structural and Contractile Proteins Are Associated with Physical Performance and Muscle Function in Older Adults: Study of Muscle, Mobility and Aging (SOMMA)

Oxidative stress is considered a contributor to declining muscle function and mobility during aging; however, the underlying molecular mechanisms remain poorly described. We hypothesized that greater levels of cysteine (Cys) oxidation on muscle proteins are associated with decreased measures of mobility. Herein, we applied a novel redox proteomics approach to measure reversible protein Cys oxidation in vastus lateralis muscle biopsies collected from 56 subjects in the Study of Muscle, Mobility and Aging (SOMMA), a community-based cohort study of individuals aged 70 years and older. We tested whether levels of Cys oxidation on key muscle proteins involved in muscle structure and contraction were associated with muscle function (leg power and strength), walking speed, and fitness (VO2 peak on cardiopulmonary exercise testing) using linear regression models adjusted for age, sex, and body weight. Higher oxidation levels of select nebulin Cys sites were associated with lower VO2 peak, while greater oxidation of myomesin-1, myomesin-2, and nebulin Cys sites was associated with slower walking speed. Higher oxidation of Cys sites in key proteins such as myomesin-2, alpha-actinin-2, and skeletal muscle alpha-actin were associated with lower leg power and strength. We also observed an unexpected correlation (r = 0.48) between a higher oxidation level of 8 Cys sites in alpha-actinin-3 and stronger leg power. Despite this observation, the results generally support the hypothesis that Cys oxidation of muscle proteins impair muscle power and strength, walking speed, and cardiopulmonary fitness with aging.

Childhood adverse life events and skeletal muscle mitochondrial function

Social stress experienced in childhood is associated with adverse health later in life. Mitochondrial function has been implicated as a mechanism for how stressful life events "get under the skin" to influence physical wellbeing. Using data from the Study of Muscle, Mobility and Aging (n=879, 59% women), linear models examined whether adverse childhood events (i.e., physical abuse) were associated with two measures of skeletal muscle mitochondrial energetics in older adults: (1) maximal adenosine triphosphate production (ATP max ) and (2) maximal state 3 respiration (Max OXPHOS). Forty-five percent of the sample reported experiencing 1+ adverse childhood event. After adjustment, each additional event was associated with -0.07 SD (95% CI= - 0.12, -0.01) lower ATP max . No association was observed with Max OXPHOS. Adverse childhood events are associated with lower ATP production in later life. Findings indicate that mitochondrial function may be a mechanism in understanding how early social stress influences health in later life.

Associations of Objective and Self-Reported Physical Activity and Sedentary Behavior with Skeletal Muscle Energetics: The Study of Muscle, Mobility and Aging (SOMMA)

Background

Skeletal muscle energetics decline with age, and physical activity (PA) has been shown to counteract these declines in older adults. Yet, many studies were based on self-reported PA or structured exercise interventions. We examined the associations of objective daily PA and sedentary behavior (SB) with skeletal muscle energetics and also compared with self-reported PA and SB. We also explored the extent to which PA would attenuate the associations of age with muscle energetics.

Methods

Among the Study of Muscle, Mobility and Aging (SOMMA) enrolled older adults, 810 (mean age=76±5, 58% women) had maximal muscle oxidative capacity measured ex vivo via high-resolution respirometry of permeabilized myofibers (maxOXPHOS) and in vivo by 31 Phosphorus magnetic resonance spectroscopy (ATP max ). Objective PA was measured using the wrist-worn ActiGraph GT9X over 7-days to capture sedentary behavior (SB), light, and moderate-to-vigorous PA (MVPA). Self-reported SB, MVPA, and all exercise-related PA were assessed with The Community Healthy Activities Model Program for Seniors questionnaire. Linear regression models with progressive covariate adjustments evaluated the associations between SB, PA and muscle energetics, and the attenuation of the age / muscle energetic association by PA.

Results

Every 30 minutes more objective MVPA was associated with 0.65 pmol/s*mg higher maxOXPHOS and 0.012 mM/sec higher ATP max , after adjustment for age, site/technician and sex. More time spent in objective light+MVPA was significantly associated with higher ATP max , but not maxOXPHOS. In contrast, every 30 minutes spent in objective SB was associated with 0.43 pmol/s*mg lower maxOXPHOS and 0.004 mM/sec lower ATP max . Only associations with ATP max held after further adjusting for socioeconomic status, body mass index, lifestyle factors and multimorbidities. Self-reported MVPA and all exercise-related activities, but not SB, yielded similar associations with maxOXPHOS and ATP max . Lastly, age was only significantly associated with muscle energetics in men. Adjusting for objective time spent in MVPA attenuated the age association with ATP max by nearly 60% in men.

Conclusion

More time spent in daily PA, especially MVPA, were associated with higher muscle energetics. Interventions that increase higher intensity activity might offer potential therapeutic interventions to slow the age-related decline in muscle energetics. Our work also emphasizes the importance of taking PA into consideration when evaluating associations related to skeletal muscle energetics.

Usual-Paced 400m Long Distance Corridor Walk Estimates Cardiorespiratory Fitness among Older Adults: The Study of Muscle, Mobility and Aging (SOMMA)

Background

Cardiopulmonary exercise testing (CPET), the gold-standard method to quantify cardiorespiratory fitness (CRF), is not always feasible due to cost, access, and burden. The usual-paced 400m long-distance corridor walk (LDCW), a measure of mobility among older adults, may provide an alternate method to assess CRF among populations unable to complete maximal intensity testing. The purpose of this study was to develop and validate an estimating equation to estimate VO 2 peak from average 400m walking speed (WS) among participants in the Study of Muscle, Mobility and Aging (SOMMA).

Methods

At baseline, participants (N=820, 76.2±4.9 years, 58% Women, 86% Non-Hispanic White) completed a 400m LDCW (400m WS=400m/completion time in seconds) and symptom-limited maximal CPET (Modified Balke Protocol). VO 2 peak (mL/kg/min) was considered the highest 30-second average oxygen consumption during CPET. Other covariates included: age, sex, race, physical activity (7-day wrist-worn accelerometer), physical function (Short Physical Performance Battery, range 0-12), perceived physical fatigability (Pittsburgh Fatigability Scale, range 0-50), and Borg Rating of Perceived Exertion (RPE, range 6-20) at completion of the 400m LDCW. Stepwise linear regression was used. Internal validation was completed using data-splitting method (70%; 30%).

Results

Mean VO 2 peak was 20.2±4.8 mL/kg/min and mean 400m WS was 1.06±0.2 m/s. Each 0.05 m/s increment in 400m WS was associated with a 0.40 mL/kg/min higher VO 2 peak after adjustment for covariates. An estimating equation including 400m WS, age, sex, race, and RPE was developed. Internal validation showed low overall bias (-0.26) and strong correlation (r = 0.71) between predicted and measured VO 2 peak values. Bland-Altman plot and regression analyses indicated predicted VO 2 peak was an acceptable alternative, despite mean underestimation of 4.53 mL/kg/min among those with CPET VO 2 peak ≥25 mL/kg/min.

Conclusions

Usual-paced 400m LDCW strongly correlates with direct measures of cardiorespiratory fitness during CPET in older adults with lower fitness and can be used to test both fitness and function.

Expression of mitochondrial oxidative stress response genes in muscle is associated with mitochondrial respiration, physical performance, and muscle mass in the Study of Muscle, Mobility and Aging (SOMMA)

Gene expression in skeletal muscle of older individuals may reflect compensatory adaptations in response to oxidative damage that preserve tissue integrity and maintain function. Identifying associations between oxidative stress response gene expression patterns and mitochondrial function, physical performance, and muscle mass in older individuals would further our knowledge of mechanisms related to managing molecular damage that may be targeted to preserve physical resilience. To characterize expression patterns of genes responsible for the oxidative stress response, RNA was extracted and sequenced from skeletal muscle biopsies collected from 575 participants (≥70 years old) from the Study of Muscle, Mobility and Aging. Expression levels of twenty-one protein coding RNAs related to the oxidative stress response were analyzed in relation to six phenotypic measures, including: maximal mitochondrial respiration from muscle biopsies (Max OXPHOS), physical performance (VO2 peak, 400m walking speed, and leg strength), and muscle size (thigh muscle volume and whole-body D3Cr muscle mass). The mRNA level of the oxidative stress response genes most consistently associated across outcomes are preferentially expressed within the mitochondria. Higher expression of mRNAs that encode generally mitochondria located proteins SOD2, TRX2, PRX3, PRX5, and GRX2 were associated with higher levels of mitochondrial respiration and VO2 peak. In addition, greater SOD2, PRX3, and GRX2 expression was associated with higher physical performance and muscle size. Identifying specific mechanisms associated with high functioning across multiple performance and physical domains may lead to targeted antioxidant interventions with greater impacts on mobility and independence.

Role of Cardiorespiratory Fitness and Mitochondrial Energetics in Reduced Walk Speed of Older Adults with Diabetes in the Study of Muscle, Mobility and Aging (SOMMA)

Rationale

Cardiorespiratory fitness and mitochondrial energetics are associated with reduced walking speed in older adults. The impact of cardiorespiratory fitness and mitochondrial energetics on walking speed in older adults with diabetes has not been clearly defined.

Objective

To examine differences in cardiorespiratory fitness and skeletal muscle mitochondrial energetics between older adults with and without diabetes. We also assessed the contribution of cardiorespiratory fitness and skeletal muscle mitochondrial energetics to slower walking speed in older adults with diabetes.

Findings

Participants with diabetes had lower cardiorespiratory fitness and mitochondrial energetics when compared to those without diabetes, following adjustments for covariates including BMI, chronic comorbid health conditions, and physical activity. 4-m and 400-m walking speeds were slower in those with diabetes. Mitochondrial oxidative capacity alone or combined with cardiorespiratory fitness mediated ∼20-70% of the difference in walk speed between older adults with and without diabetes. Further adjustments of BMI and co-morbidities further explained the group differences in walk speed.

Conclusions

Skeletal muscle mitochondrial energetics and cardiorespiratory fitness contribute to slower walking speeds in older adults with diabetes. Cardiorespiratory fitness and mitochondrial energetics may be therapeutic targets to maintain or improve mobility in older adults with diabetes.

ARTICLE HIGHLIGHTS

Why did we undertake this study? To determine if mitochondrial energetics and cardiorespiratory fitness contribute to slower walking speed in older adults with diabetes. What is the specific question(s) we wanted to answer? Are mitochondrial energetics and cardiorespiratory fitness in older adults with diabetes lower than those without diabetes? How does mitochondrial energetics and cardiorespiratory fitness impact walking speed in older adults with diabetes? What did we find? Mitochondrial energetics and cardiorespiratory fitness were lower in older adults with diabetes compared to those without diabetes, and energetics, and cardiorespiratory fitness, contributed to slower walking speed in those with diabetes. What are the implications of our findings? Cardiorespiratory fitness and mitochondrial energetics may be key therapeutic targets to maintain or improve mobility in older adults with diabetes.

Higher Expression of Denervation-responsive Genes is Negatively Associated with Muscle Volume and Performance Traits in the Study of Muscle, Mobility and Aging (SOMMA)

With aging skeletal muscle fibers undergo repeating cycles of denervation and reinnervation. In approximately the 8 th decade of life reinnervation no longer keeps pace, resulting in the accumulation of persistently denervated muscle fibers that in turn cause an acceleration of muscle dysfunction. The significance of denervation in important clinical outcomes with aging is poorly studied. The Study of Muscle, Mobility and Aging (SOMMA) is a large cohort study with the primary objective to assess how aging muscle biology impacts clinically important traits. Using transcriptomics data from vastus lateralis muscle biopsies in 575 participants we have selected 49 denervation-responsive genes to provide insights to the burden of denervation in SOMMA, to test the hypothesis that greater expression of denervation-responsive genes negatively associates with SOMMA participant traits that included time to walk 400 meters, fitness (VO 2peak ), maximal mitochondrial respiration, muscle mass and volume, and leg muscle strength and power. Consistent with our hypothesis, increased transcript levels of: a calcium-dependent intercellular adhesion glycoprotein (CDH15), acetylcholine receptor subunits (Chrna1, Chrnd, Chrne), a glycoprotein promoting reinnervation (NCAM1), a transcription factor regulating aspects of muscle organization (RUNX1), and a sodium channel (SCN5A) were each negatively associated with at least 3 of these traits. VO 2peak and maximal respiration had the strongest negative associations with 15 and 19 denervation-responsive genes, respectively. In conclusion, the abundance of denervation-responsive gene transcripts is a significant determinant of muscle and mobility outcomes in aging humans, supporting the imperative to identify new treatment strategies to restore innervation in advanced age.

Autophagy gene expression in skeletal muscle of older individuals is associated with physical performance, muscle volume and mitochondrial function in the Study of Muscle, Mobility and Aging (SOMMA)

Autophagy is an essential component of proteostasis and a key pathway in aging. Identifying associations between autophagy gene expression patterns in skeletal muscle and physical performance outcomes would further our knowledge of mechanisms related with proteostasis and healthy aging. Muscle biopsies were obtained from participants in the Study of Muscle, Mobility and Aging (SOMMA). For 575 participants, RNA was sequenced and expression of 281 genes related to autophagy regulation, mitophagy and mTOR/upstream pathways were determined. Associations between gene expression and outcomes including mitochondrial respiration in muscle fiber bundles (MAX OXPHOS), physical performance (VO2 peak, 400m walking speed, and leg power), and thigh muscle volume were determined using negative binomial regression models. For autophagy, key transcriptional regulators including TFE3 and NFKB-related genes (RELA, RELB, NFKB1) were negatively associated with outcomes. On the contrary, regulators of oxidative metabolism that also promote overall autophagy, mitophagy and pexophagy (PPARGC1A, PPARA, EPAS1) were positively associated with multiple outcomes. In line with this, several mitophagy, fusion and fission related genes (NIPSNAP2, DNM1L, OPA1) were also positively associated with outcomes. For mTOR pathway and related genes, expression of WDR59 and WDR24, both subunits of GATOR2 complex (an indirect inhibitor of mTORC1) and PRKAG3, which is a regulatory subunit of AMPK, were negatively correlated with multiple outcomes. Our study identifies autophagy and selective autophagy such as mitophagy gene expression patterns in human skeletal muscle related to physical performance, muscle volume and mitochondrial function in older persons which may lead to target identification to preserve mobility and independence.

The Study of Muscle, Mobility and Aging (SOMMA): A Unique Cohort Study About the Cellular Biology of Aging and Age-related Loss of Mobility

BACKGROUND

The Study of Muscle, Mobility and Aging (SOMMA) aims to understand the biological basis of many facets of human aging, with a focus on mobility decline, by creating a unique platform of data, tissues, and images.

METHODS

The multidisciplinary SOMMA team includes 2 clinical centers (University of Pittsburgh and Wake Forest University), a biorepository (Translational Research Institute at Advent Health), and the San Francisco Coordinating Center (California Pacific Medical Center Research Institute). Enrollees were age ≥70 years, able to walk ≥0.6 m/s (4 m); able to complete 400 m walk, free of life-threatening disease, and had no contraindications to magnetic resonance or tissue collection. Participants are followed with 6-month phone contacts and annual in-person exams. At baseline, SOMMA collected biospecimens (muscle and adipose tissue, blood, urine, fecal samples); a variety of questionnaires; physical and cognitive assessments; whole-body imaging (magnetic resonance and computed tomography); accelerometry; and cardiopulmonary exercise testing. Primary outcomes include change in walking speed, change in fitness, and objective mobility disability (able to walk 400 m in 15 minutes and change in 400 m speed). Incident events, including hospitalizations, cancer diagnoses, fractures, and mortality are collected and centrally adjudicated by study physicians.

RESULTS

SOMMA exceeded its goals by enrolling 879 participants, despite being slowed by the COVID-19 pandemic: 59.2% women; mean age 76.3 ± 5.0 years (range 70-94); mean walking speed 1.04 ± 0.20 m/s; 15.8% identify as other than Non-Hispanic White. Over 97% had data for key measurements.

CONCLUSIONS

SOMMA will provide the foundation for discoveries in the biology of human aging and mobility.

Associations of Skeletal Muscle Mass, Muscle Fat Infiltration, Mitochondrial Energetics, and Cardiorespiratory Fitness with Liver Fat Among Older Adults

Background

Muscle mass loss may be associated with liver fat accumulation, yet scientific consensus is lacking and evidence in older adults is scant. It is unclear which muscle characteristics might contribute to this association in older adults.

Methods

We associated comprehensive muscle-related phenotypes including muscle mass normalized to body weight (D 3 -creatine dilution), muscle fat infiltration (MRI), carbohydrate-supported muscle mitochondrial maximal oxidative phosphorylation (respirometry), and cardiorespiratory fitness (VO 2 peak) with liver fat among older adults. Linear regression models adjusted for age, gender, technician (respirometry only), daily minutes of moderate to vigorous physical activity, and prediabetes/diabetes status tested main effects and interactions of each independent variable with waist circumference (high: women-≥88 cm, men-≥102 cm) and gender.

Results

Among older adults aged 75 (IQR 73, 79 years; 59.8% women), muscle mass and liver fat were not associated overall but were positively associated among participants with a high waist circumference (β: 25.2; 95%CI 11.7, 40.4; p =.0002; N=362). Muscle fat infiltration and liver fat were positively associated (β: 15.2; 95%CI 6.8, 24.3; p =.0003; N=378). Carbohydrate-supported maximum oxidative phosphorylation and VO 2 peak (adjusted β: -12.9; 95%CI -20.3, -4.8; p =0.003; N=361) were inversely associated with liver fat; adjustment attenuated the estimate for maximum oxidative phosphorylation although the point estimate remained negative (β: -4.0; 95%CI -11.6, 4.2; p =0.32; N=321).

Conclusions

Skeletal muscle-related characteristics are metabolically relevant factors linked to liver fat in older adults. Future research should confirm our results to determine whether trials targeting mechanisms common to liver and muscle fat accumulation are warranted.

Associations between skeletal muscle energetics and accelerometry-based performance fatigability: Study of Muscle, Mobility and Aging

Performance fatigability is typically experienced as insufficient energy to complete daily physical tasks, particularly with advancing age, often progressing toward dependency. Thus, understanding the etiology of performance fatigability, especially cellular-level biological mechanisms, may help to delay the onset of mobility disability. We hypothesized that skeletal muscle energetics may be important contributors to performance fatigability. Participants in the Study of Muscle, Mobility and Aging completed a usual-paced 400-m walk wearing a wrist-worn ActiGraph GT9X to derive the Pittsburgh Performance Fatigability Index (PPFI, higher scores = more severe fatigability) that quantifies percent decline in individual cadence-versus-time trajectory from their maximal cadence. Complex I&II-supported maximal oxidative phosphorylation (max OXPHOS) and complex I&II-supported electron transfer system (max ETS) were quantified ex vivo using high-resolution respirometry in permeabilized fiber bundles from vastus lateralis muscle biopsies. Maximal adenosine triphosphate production (ATPmax ) was assessed in vivo by 31 P magnetic resonance spectroscopy. We conducted tobit regressions to examine associations of max OXPHOS, max ETS, and ATPmax with PPFI, adjusting for technician/site, demographic characteristics, and total activity count over 7-day free-living among older adults (N = 795, 70-94 years, 58% women) with complete PPFI scores and ≥1 energetics measure. Median PPFI score was 1.4% [25th-75th percentile: 0%-2.9%]. After full adjustment, each 1 standard deviation lower max OXPHOS, max ETS, and ATPmax were associated with 0.55 (95% CI: 0.26-0.84), 0.39 (95% CI: 0.09-0.70), and 0.54 (95% CI: 0.27-0.81) higher PPFI score, respectively. Our findings suggested that therapeutics targeting muscle energetics may potentially mitigate fatigability and lessen susceptibility to disability among older adults.

Human skeletal muscle tissue chip autonomous payload reveals changes in fiber type and metabolic gene expression due to spaceflight

Microphysiological systems provide the opportunity to model accelerated changes at the human tissue level in the extreme space environment. Spaceflight-induced muscle atrophy experienced by astronauts shares similar physiological changes to muscle wasting in older adults, known as sarcopenia. These shared attributes provide a rationale for investigating molecular changes in muscle cells exposed to spaceflight that may mimic the underlying pathophysiology of sarcopenia. We report the results from three-dimensional myobundles derived from muscle biopsies from young and older adults, integrated into an autonomous CubeLab™, and flown to the International Space Station (ISS) aboard SpaceX CRS-21 as part of the NIH/NASA funded Tissue Chips in Space program. Global transcriptomic RNA-Seq analyses comparing the myobundles in space and on the ground revealed downregulation of shared transcripts related to myoblast proliferation and muscle differentiation. The analyses also revealed downregulated differentially expressed gene pathways related to muscle metabolism unique to myobundles derived from the older cohort exposed to the space environment compared to ground controls. Gene classes related to inflammatory pathways were downregulated in flight samples cultured from the younger cohort compared to ground controls. Our muscle tissue chip platform provides an approach to studying the cell autonomous effects of spaceflight on muscle cell biology that may not be appreciated on the whole organ or organism level and sets the stage for continued data collection from muscle tissue chip experimentation in microgravity. We also report on the challenges and opportunities for conducting autonomous tissue-on-chip CubeLabTM payloads on the ISS.

Mitochondrial Energetics in Skeletal Muscle Are Associated With Leg Power and Cardiorespiratory Fitness in the Study of Muscle, Mobility and Aging

BACKGROUND

Mitochondrial energetics are an important property of aging muscle, as generation of energy is pivotal to the execution of muscle contraction. However, its association with functional outcomes, including leg power and cardiorespiratory fitness, is largely understudied.

METHODS

In the Study of Muscle, Mobility, and Aging, we collected vastus lateralis biopsies from older adults (n = 879, 70-94 years, 59.2% women). Maximal State 3 respiration (Max OXPHOS) was assessed in permeabilized fiber bundles by high-resolution respirometry. Capacity for maximal adenosine triphosphate production (ATPmax) was measured in vivo by 31P magnetic resonance spectroscopy. Leg extension power was measured with a Keiser press system, and VO2 peak was determined using a standardized cardiopulmonary exercise test. Gender-stratified multivariate linear regression models were adjusted for age, race, technician/site, adiposity, and physical activity with beta coefficients expressed per 1-SD increment in the independent variable.

RESULTS

Max OXPHOS was associated with leg power for both women (β = 0.12 Watts/kg, p < .001) and men (β = 0.11 Watts/kg, p < .050). ATPmax was associated with leg power for men (β = 0.09 Watts/kg, p < .05) but was not significant for women (β = 0.03 Watts/kg, p = .11). Max OXPHOS and ATPmax were associated with VO2 peak in women and men (Max OXPHOS, β women = 1.03 mL/kg/min, β men = 1.32 mL/kg/min; ATPmax β women = 0.87 mL/kg/min, β men = 1.50 mL/kg/min; all p < .001).

CONCLUSIONS

Higher muscle mitochondrial energetics measures were associated with both better cardiorespiratory fitness and greater leg power in older adults. Muscle mitochondrial energetics explained a greater degree of variance in VO2 peak compared to leg power.

Lower mitochondrial respiration does not lead to decreased fat oxidation in young African American women without obesity

OBJECTIVE

The prevalence of type 2 diabetes in African American women (AAW) is nearly twice that of White women. Lower insulin sensitivity and decreased mitochondrial function may be contributing factors. The purpose of this study was to compare fat oxidation in AAW and White women.

METHODS

Participants were 22 AAW and 22 White women, matched for age (18.7-38.3 years) and BMI (< 28 kg/m2). Participants completed two submaximal (50% VO2max) exercise tests with indirect calorimetry and stable isotope tracers to assess total, plasma, and intramyocellular triglyceride fat oxidation.

RESULTS

The respiratory quotient during the exercise test was nearly identical in AAW and White women (0.813 ± 0.008 vs. 0.810 ± 0.008, p = 0.83). Although absolute total and plasma fat oxidation was lower in AAW, adjusting for the lower workload in AAW eliminated these racial differences. There was no racial difference in plasma and intramyocellular triglyceride source of fat for oxidation. No racial differences were observed in rates of ex vivo fat oxidation. Exercise efficiency was lower in AAW when adjusted to leg fat free mass.

CONCLUSIONS

The data suggest that fat oxidation is not lower in AAW compared with White women, but additional studies are needed across exercise intensity, body weight, and age to confirm these results.

Source of nicotinamide governs its metabolic fate in cultured cells, mice, and humans

Metabolic routing of nicotinamide (NAM) to NAD⁺ or 1-methylnicotinamide (MeNAM) has impacts on human health and aging. NAM is imported by cells or liberated from NAD⁺. The fate of ²H₄-NAM in cultured cells, mice, and humans was determined by stable isotope tracing. ²H₄-NAM is an NAD⁺ precursor via the salvage pathway in cultured A549 cells and human PBMCs and in A549 cell xenografts and PBMCs from ²H₄-NAM-dosed mice and humans, respectively. ²H₄-NAM is a MeNAM precursor in A549 cell cultures and xenografts, but not isolated PBMCs. NAM released from NAD⁺ is a poor MeNAM precursor. Additional A549 cell tracer studies yielded further mechanistic insight. NAMPT activators promote NAD⁺ synthesis and consumption. Surprisingly, NAM liberated from NAD⁺ in NAMPT activator-treated A549 cells is also routed toward MeNAM production. Metabolic fate mapping of the dual NAM sources across the translational spectrum (cells, mice, humans) illuminates a key regulatory node governing NAD⁺ and MeNAM synthesis.

Exercise and ageing impact the kynurenine/tryptophan pathway and acylcarnitine metabolite pools in skeletal muscle of older adults

Exercise-induced perturbation of skeletal muscle metabolites is a probable mediator of long-term health benefits in older adults. Although specific metabolites have been identified to be impacted by age, physical activity and exercise, the depth of coverage of the muscle metabolome is still limited. Here, we investigated resting and exercise-induced metabolite distribution in muscle from well-phenotyped older adults who were active or sedentary, and a group of active young adults. Percutaneous biopsies of the vastus lateralis were obtained before, immediately after and 3 h following a bout of endurance cycling. Metabolite profile in muscle biopsies was determined by tandem mass spectrometry. Mitochondrial energetics in permeabilized fibre bundles was assessed by high resolution respirometry and fibre type proportion was assessed by immunohistology. We found that metabolites of the kynurenine/tryptophan pathway were impacted by age and activity. Specifically, kynurenine was elevated in muscle from older adults, whereas downstream metabolites of kynurenine (kynurenic acid and NAD⁺ ) were elevated in muscle from active adults and associated with cardiorespiratory fitness and muscle oxidative capacity. Acylcarnitines, a potential marker of impaired metabolic health, were elevated in muscle from physically active participants. Surprisingly, despite baseline group difference, acute exercise-induced alterations in whole-body substrate utilization, as well as muscle acylcarnitines and ketone bodies, were remarkably similar between groups. Our data identified novel muscle metabolite signatures that associate with the healthy ageing phenotype provoked by physical activity and reveal that the metabolic responsiveness of muscle to acute endurance exercise is retained [NB]:AUTHOR: Please ensure that the appropriate material has been provide for Table S2, as well as for Figures S1 to S7, as also cited in the text with age regardless of activity levels. KEY POINTS: Kynurenine/tryptophan pathway metabolites were impacted by age and physical activity in human muscle, with kynurenine elevated in older muscle, whereas downstream products kynurenic acid and NAD⁺ were elevated in exercise-trained muscle regardless of age. Acylcarnitines, a marker of impaired metabolic health when heightened in circulation, were elevated in exercise-trained muscle of young and older adults, suggesting that muscle act as a metabolic sink to reduce the circulating acylcarnitines observed with unhealthy ageing. Despite the phenotypic differences, the exercise-induced response of various muscle metabolite pools, including acylcarnitine and ketone bodies, was similar amongst the groups, suggesting that older adults can achieve the metabolic benefits of exercise seen in young counterparts.

The TAS1R2 sweet taste receptor regulates skeletal muscle mass and fitness

Muscle fitness and mass deteriorate under the conditions of obesity and aging for reasons yet to be fully elucidated. Herein, we describe a novel pathway linking peripheral nutrient sensing and skeletal muscle function through the sweet taste receptor TAS1R2 and the involvement of ERK2-PARP1-NAD signaling axis. Muscle-specific deletion of TAS1R2 (mKO) in mice produced elevated NAD levels due to suppressed PARP1 activity, improved mitochondrial function, increased muscle mass and strength, and prolonged running endurance. Deletion of TAS1R2 in obese or aged mice also ameliorated the decline in muscle mass and fitness arising from these conditions. Remarkably, partial loss-of-function of TAS1R2 (rs35874116) in older, obese humans recapitulated the healthier muscle phenotype displayed by mKO mice in response to exercise training. Our findings show that inhibition of the TAS1R2 signaling in skeletal muscle is a promising therapeutic approach to preserve muscle mass and function.

The mitochondrial multi-omic response to exercise training across tissues

Mitochondria are adaptable organelles with diverse cellular functions critical to whole-body metabolic homeostasis. While chronic endurance exercise training is known to alter mitochondrial activity, these adaptations have not yet been systematically characterized. Here, the Molecular Transducers of Physical Activity Consortium (MoTrPAC) mapped the longitudinal, multi-omic changes in mitochondrial analytes across 19 tissues in male and female rats endurance trained for 1, 2, 4 or 8 weeks. Training elicited substantial changes in the adrenal gland, brown adipose, colon, heart and skeletal muscle, while we detected mild responses in the brain, lung, small intestine and testes. The colon response was characterized by non-linear dynamics that resulted in upregulation of mitochondrial function that was more prominent in females. Brown adipose and adrenal tissues were characterized by substantial downregulation of mitochondrial pathways. Training induced a previously unrecognized robust upregulation of mitochondrial protein abundance and acetylation in the liver, and a concomitant shift in lipid metabolism. The striated muscles demonstrated a highly coordinated response to increase oxidative capacity, with the majority of changes occurring in protein abundance and post-translational modifications. We identified exercise upregulated networks that are downregulated in human type 2 diabetes and liver cirrhosis. In both cases HSD17B10, a central dehydrogenase in multiple metabolic pathways and mitochondrial tRNA maturation, was the main hub. In summary, we provide a multi-omic, cross-tissue atlas of the mitochondrial response to training and identify candidates for prevention of disease-associated mitochondrial dysfunction.

Microphysiological system for studying contractile differences in young, active, and old, sedentary adult derived skeletal muscle cells

Microphysiological systems (MPS), also referred to as tissue chips, incorporating 3D skeletal myobundles are a novel approach for physiological and pharmacological studies to uncover new medical treatments for sarcopenia. We characterize a MPS in which engineered skeletal muscle myobundles derived from donor-specific satellite cells that model aged phenotypes are encapsulated in a perfused tissue chip platform containing platinum electrodes. Our myobundles were derived from CD56+ myogenic cells obtained via percutaneous biopsy of the vastus lateralis from adults phenotyped by age and physical activity. Following 17 days differentiation including 5 days of a 3 V, 2 Hz electrical stimulation regime, the myobundles exhibited fused myotube alignment and upregulation of myogenic, myofiber assembly, signaling and contractile genes as demonstrated by gene array profiling and localization of key components of the sarcomere. Our results demonstrate that myobundles derived from the young, active (YA) group showed high intensity immunofluorescent staining of α-actinin proteins and responded to electrical stimuli with a ~1 μm displacement magnitude compared with non-stimulated myobundles. Myobundles derived from older sedentary group (OS) did not display a synchronous contraction response. Hypertrophic potential is increased in YA-derived myobundles in response to stimulation as shown by upregulation of insulin growth factor (IGF-1), α-actinin (ACTN3, ACTA1) and fast twitch troponin protein (TNNI2) compared with OS-derived myobundles. Our MPS mimics disease states of muscle decline and thus provides an aged system and experimental platform to investigate electrical stimulation mimicking exercise regimes and may be adapted to long duration studies of compound efficacy and toxicity for therapeutic evaluation against sarcopenia.

Weight Loss and Exercise Differentially Affect Insulin Sensitivity, Body Composition, Cardiorespiratory Fitness, and Muscle Strength in Older Adults With Obesity: A Randomized Controlled Trial

BACKGROUND

Aging-related disease risk is exacerbated by obesity and physical inactivity. It is unclear how weight loss and increased activity improve risk in older adults. We aimed to determine the effects of diet-induced weight loss with and without exercise on insulin sensitivity, VO2peak, body composition, and physical function in older obese adults.

METHODS

Physically inactive older (68.6 ± 4.5 years) obese (body mass index 37.4 ± 4.9 kg/m2) adults were randomized to health education control (HEC; n = 25); diet-induced weight loss (WL; n = 31); or weight loss and exercise (WLEX; n = 28) for 6 months. Insulin sensitivity was measured by hyperinsulinemic-euglycemic clamp, body composition by dual-energy X-ray absorptiometry and MRI, strength by isokinetic dynamometry, and VO2peak by graded exercise test.

RESULTS

WLEX improved (p < .05) peripheral insulin sensitivity (+75 ± 103%) versus HEC (+12 ± 67%); WL (+36 ± 47%) versus HEC did not reach statistical significance. WLEX increased VO2peak (+7 ± 12%) versus WL (-2 ± 24%) and prevented reductions in strength and lean mass induced by WL (p < .05). WLEX decreased abdominal adipose tissue (-16 ± 9%) versus HEC (-3 ± 8%) and intermuscular adipose tissue (-15 ± 13%) versus both HEC (+9 ± 15%) and WL (+2 ± 11%; p < .01).

CONCLUSIONS

Exercise with weight loss improved insulin sensitivity and VO2peak, decreased ectopic fat, and preserved lean mass and strength. Weight loss alone decreased lean mass and strength. Older adults intending to lose weight should perform regular exercise to promote cardiometabolic and functional benefits, which may not occur with calorie restriction-induced weight loss alone.

State of Knowledge on Molecular Adaptations to Exercise in Humans: Historical Perspectives and Future Directions

For centuries, regular exercise has been acknowledged as a potent stimulus to promote, maintain, and restore healthy functioning of nearly every physiological system of the human body. With advancing understanding of the complexity of human physiology, continually evolving methodological possibilities, and an increasingly dire public health situation, the study of exercise as a preventative or therapeutic treatment has never been more interdisciplinary, or more impactful. During the early stages of the NIH Common Fund Molecular Transducers of Physical Activity Consortium (MoTrPAC) Initiative, the field is well-positioned to build substantially upon the existing understanding of the mechanisms underlying benefits associated with exercise. Thus, we present a comprehensive body of the knowledge detailing the current literature basis surrounding the molecular adaptations to exercise in humans to provide a view of the state of the field at this critical juncture, as well as a resource for scientists bringing external expertise to the field of exercise physiology. In reviewing current literature related to molecular and cellular processes underlying exercise-induced benefits and adaptations, we also draw attention to existing knowledge gaps warranting continued research effort. © 2021 American Physiological Society. Compr Physiol 12:3193-3279, 2022.

Skeletal muscle transcriptome response to a bout of endurance exercise in physically active and sedentary older adults

Age-related declines in cardiorespiratory fitness and physical function are mitigated by regular endurance exercise in older adults. This may be due, in part, to changes in the transcriptional program of skeletal muscle following repeated bouts of exercise. However, the impact of chronic exercise training on the transcriptional response to an acute bout of endurance exercise has not been clearly determined. Here, we characterized baseline differences in muscle transcriptome and exercise-induced response in older adults who were active/endurance trained or sedentary. RNA-sequencing was performed on vastus lateralis biopsy specimens obtained before, immediately after, and 3 h following a bout of endurance exercise (40 min of cycling at 60%-70% of heart rate reserve). Using a recently developed bioinformatics approach, we found that transcript signatures related to type I myofibers, mitochondria, and endothelial cells were higher in active/endurance-trained adults and were associated with key phenotypic features including V̇o_2peak, ATP_max, and muscle fiber proportion. Immune cell signatures were elevated in the sedentary group and linked to visceral and intermuscular adipose tissue mass. Following acute exercise, we observed distinct temporal transcriptional signatures that were largely similar among groups. Enrichment analysis revealed catabolic processes were uniquely enriched in the sedentary group at the 3-h postexercise timepoint. In summary, this study revealed key transcriptional signatures that distinguished active and sedentary adults, which were associated with difference in oxidative capacity and depot-specific adiposity. The acute response signatures were consistent with beneficial effects of endurance exercise to improve muscle health in older adults irrespective of exercise history and adiposity.NEW & NOTEWORTHY Muscle transcript signatures associated with oxidative capacity and immune cells underlie important phenotypic and clinical characteristics of older adults who are endurance trained or sedentary. Despite divergent phenotypes, the temporal transcriptional signatures in response to an acute bout of endurance exercise were largely similar among groups. These data provide new insight into the transcriptional programs of aging muscle and the beneficial effects of endurance exercise to promote healthy aging in older adults.

Skeletal Muscle Energetics and Mitochondrial Function Are Impaired Following 10 Days of Bed Rest in Older Adults

Background

Older adults exposed to periods of inactivity during hospitalization, illness, or injury lose muscle mass and strength. This, in turn, predisposes poor recovery of physical function upon reambulation and represents a significant health risk for older adults. Bed rest (BR) results in altered skeletal muscle fuel metabolism and loss of oxidative capacity that have recently been linked to the muscle atrophy program. Our primary objective was to explore the effects of BR on mitochondrial energetics in muscle from older adults. A secondary objective was to examine the effect of β-hydroxy-β-methylbuturate (HMB) supplementation on mitochondrial energetics.

Methods

We studied 20 older adults before and after a 10-day BR intervention, who consumed a complete oral nutritional supplement (ONS) with HMB (3.0 g/d HMB, n = 11) or without HMB (CON, n = 9). Percutaneous biopsies of the vastus lateralis were obtained to determine mitochondrial respiration and H₂O₂ emission in permeabilized muscle fibers along with markers of content. RNA sequencing and lipidomics analyses were also conducted.

Results

We found a significant up-regulation of collagen synthesis and down-regulation of ribosome, oxidative metabolism and mitochondrial gene transcripts following BR in the CON group. Alterations to these gene transcripts were significantly blunted in the HMB group. Mitochondrial respiration and markers of content were both reduced and H₂O₂ emission was elevated in both groups following BR.

Conclusions

In summary, 10 days of BR in older adults causes a significant deterioration in mitochondrial energetics, while transcriptomic profiling revealed that some of these negative effects may be attenuated by an ONS containing HMB.

A Novel Endocrine Role for the BAT-Released Lipokine 12,13-diHOME to Mediate Cardiac Function

BACKGROUND

Brown adipose tissue (BAT) is an important tissue for thermogenesis, making it a potential target to decrease the risks of obesity, type 2 diabetes, and cardiovascular disease, and recent studies have also identified BAT as an endocrine organ. Although BAT has been implicated to be protective in cardiovascular disease, to this point there are no studies that identify a direct role for BAT to mediate cardiac function.

METHODS

To determine the role of BAT on cardiac function, we utilized a model of BAT transplantation. We then performed lipidomics and identified an increase in the lipokine 12,13-dihydroxy-9Z-octadecenoic acid (12,13-diHOME). We utilized a mouse model with sustained overexpression of 12,13-diHOME and investigated the role of 12,13-diHOME in a nitric oxide synthase type 1 deficient (NOS1^-/-) mouse and in isolated cardiomyocytes to determine effects on function and respiration. We also investigated 12,13-diHOME in a cohort of human patients with heart disease.

RESULTS

Here, we determined that transplantation of BAT (+BAT) improves cardiac function via the release of the lipokine 12,13-diHOME. Sustained overexpression of 12,13-diHOME using tissue nanotransfection negated the deleterious effects of a high-fat diet on cardiac function and remodeling, and acute injection of 12,13-diHOME increased cardiac hemodynamics via direct effects on the cardiomyocyte. Furthermore, incubation of cardiomyocytes with 12,13-diHOME increased mitochondrial respiration. The effects of 12,13-diHOME were absent in NOS1^-/- mice and cardiomyocytes. We also provide the first evidence that 12,13-diHOME is decreased in human patients with heart disease.

CONCLUSIONS

Our results identify an endocrine role for BAT to enhance cardiac function that is mediated by regulation of calcium cycling via 12,13-diHOME and NOS1.

Sedentary and Trained Older Men Have Distinct Circulating Exosomal microRNA Profiles at Baseline and in Response to Acute Exercise

Exercise has multi-systemic benefits and attenuates the physiological impairments associated with aging. Emerging evidence suggests that circulating exosomes mediate some of the beneficial effects of exercise via the transfer of microRNAs between tissues. However, the impact of regular exercise and acute exercise on circulating exosomal microRNAs (exomiRs) in older populations remains unknown. In the present study, we analyzed circulating exomiR expression in endurance-trained elderly men (n = 5) and age-matched sedentary males (n = 5) at baseline (Pre), immediately after a forty minute bout of aerobic exercise on a cycle ergometer (Post), and three hours after this acute exercise (3hPost). Following the isolation and enrichment of exosomes from plasma, exosome-enriched preparations were characterized and exomiR levels were determined by sequencing. The effect of regular exercise on circulating exomiRs was assessed by comparing the baseline expression levels in the trained and sedentary groups. The effect of acute exercise was determined by comparing baseline and post-training expression levels in each group. Regular exercise resulted in significantly increased baseline expression of three exomiRs (miR-486-5p, miR-215-5p, miR-941) and decreased expression of one exomiR (miR-151b). Acute exercise altered circulating exomiR expression in both groups. However, exomiRs regulated by acute exercise in the trained group (7 miRNAs at Post and 8 at 3hPost) were distinct from those in the sedentary group (9 at Post and 4 at 3hPost). Pathway analysis prediction and reported target validation experiments revealed that the majority of exercise-regulated exomiRs are targeting genes that are related to IGF-1 signaling, a pathway involved in exercise-induced muscle and cardiac hypertrophy. The immediately post-acute exercise exomiR signature in the trained group correlates with activation of IGF-1 signaling, whereas in the sedentary group it is associated with inhibition of IGF-1 signaling. While further validation is needed, including measurements of IGF-1/IGF-1 signaling in blood or skeletal muscle, our results suggest that training status may counteract age-related anabolic resistance by modulating circulating exomiR profiles both at baseline and in response to acute exercise.