Human neuromuscular aging: Sex differences revealed at the myocellular level.

Age-related muscle loss (sarcopenia) is a major clinical problem affecting both men and women - accompanied by muscle weakness, dysfunction, disability, and impaired quality of life. Current definitions of sarcopenia do not fully encompass the age-related changes in skeletal muscle. We therefore examined the influence of aging and sex on elements of skeletal muscle health using a thorough histopathological analysis of myocellular aging and assessments of neuromuscular performance. Two-hundred and twenty-one untrained males and females were separated into four age cohorts [mean age 25 y (n = 47), 37 y (n = 79), 61 y (n = 51), and 72 y (n = 44)]. Total (-12%), leg (-17%), and arm (-21%) lean mass were lower in both 61 y and 72 y than in 25 y or 37 y (P < 0.05). Knee extensor strength (-34%) and power (-43%) were lower (P < 0.05) in the older two groups, and explosive sit-to-stand power was lower by 37 y (P < 0.05). At the histological/myocellular level, type IIx atrophy was noted by 37 y and type IIa atrophy by 61 y (P < 0.05). These effects were driven by females, noted by substantial and progressive type IIa and IIx atrophy across age. Aged female muscle displayed greater within-type myofiber size heterogeneity and marked type I myofiber grouping (~5-fold greater) compared to males. These findings suggest the predominant mechanisms leading to whole muscle atrophy differ between aging males and females: myofiber atrophy in females vs. myofiber loss in males. Future studies will be important to better understand the mechanisms underlying sex differences in myocellular aging and optimize exercise prescriptions and adjunctive treatments to mitigate or reverse age-related changes.

Heightened muscle inflammation susceptibility may impair regenerative capacity in aging humans.

The regenerative response of skeletal muscle to mechanically induced damage is impaired with age. Previous work in our laboratory suggests this may result from higher proinflammatory signaling in aging muscle at rest and/or a greater inflammatory response to damage. We, therefore, assessed skeletal muscle proinflammatory signaling at rest and 24 h after unaccustomed, loaded knee extension contractions that induced modest muscle damage (72% increase in serum creatine kinase) in a cohort of 87 adults across three age groups (AGE40, AGE61, and AGE76). Vastus lateralis muscle gene expression and protein cell signaling of the IL-6 and TNF-α pathways were determined by quantitative PCR and immunoblot analysis. For in vitro studies, cell signaling and fusion capacities were compared among primary myoblasts from young (AGE28) and old (AGE64) donors treated with TNF-α. Muscle expression was higher (1.5- to 2.1-fold) in AGE76 and AGE61 relative to AGE40 for several genes involved in IL-6, TNF-α, and TNF-like weak inducer of apoptosis signaling. Indexes of activation for the proinflammatory transcription factors signal transducer and activator of transcription-3 and NF-κB were highest in AGE76. Resistance loading reduced gene expression of IL-6 receptor, muscle RING finger 1, and atrogin-1, and increased TNF-like weak inducer of apoptosis receptor expression. Donor myoblasts from AGE64 showed impaired differentiation and fusion in standard media and greater NF-κB activation in response to TNF-α treatment (compared with AGE28). We show for the first time that human aging is associated with muscle inflammation susceptibility (i.e., higher basal state of proinflammatory signaling) that is present in both tissue and isolated myogenic cells and likely contributes to the impaired regenerative capacity of skeletal muscle in the older population.

Modulation of the dystrophin-associated protein complex in response to resistance training in young and older men.

The dystrophin-associated protein complex (DAPC) is a scaffold of proteins linking the intracellular cytoskeleton with the extracellular matrix that is integral to structural stability and integrity, signaling and mechanotransduction, and force transmission. We hypothesized that the expression of DAPC component proteins would be altered by resistance loading during progressive resistance training (PRT)-mediated myofiber hypertrophy, and we investigated whether aging influenced these changes. Seventeen young (27 yr) and 13 older (65 yr) men completed 16 wk of PRT with muscle biopsies at baseline (T1), 24 h after bout 1 (T2), and 24 h after the final bout at week 16 (T3). Myofiber hypertrophy in the young (type I 31%, P < 0.005; type II 40%, P < 0.001) far exceeded hypertrophy in the old (type II only, 19.5%, P < 0.05). PRT altered protein expression for caveolin-3 (decreased 24% by T3, P < 0.01), alpha(1)-syntrophin (increased 16% by T3, P < 0.05), alpha-dystrobrevin (fell 23% from T2 to T3, P < 0.01), and dystrophin [rose acutely (30% by T2, P < 0.05) and returned to baseline by T3]. The phosphorylation state of membrane neuronal nitric oxide synthase (Ser(1417)) decreased 70% (P < 0.005) by T3, particularly in the old (81%), whereas p38 MAPK phosphorylation increased twofold by T3 in the old (P < 0.01). We conclude that component proteins of the DAPC are modulated by PRT, which may serve to improve both structural and signaling functions during load-mediated myofiber hypertrophy. The blunted hypertrophic adaptation seen in old vs. young men may have resulted from overstress, as suggested by marked p38 MAPK activation in old men only.

Efficacy of myonuclear addition may explain differential myofiber growth among resistance-trained young and older men and women.

Skeletal muscle stem (satellite) cells supporting growth/regeneration are thought to be activated and incorporated into growing myofibers by both endocrine and locally expressed autocrine/paracrine growth factors, the latter being load sensitive. We recently found that myofiber hypertrophy with resistance training is superior in young men (YM) vs. young women and older adults (Kosek DJ, Kim JS, Petrella JK, Cross JM, and Bamman MM. J Appl Physiol 101: 531-544, 2006). We hypothesized that the advanced myofiber hypertrophy in YM is facilitated by myonuclear addition in response to a milieu promoting stem cell activation. Twenty-six young (27.0 +/- 1 yr, 50% women) and 26 older (63.7 +/- 1 yr, 50% women) adults completed 16 wk of knee extensor resistance training. Vastus lateralis biopsies were obtained at baseline, 24 h after one bout, and after 16 wk. Muscle stem cells were identified immunohistochemically with anti-neural cell adhesion molecule (NCAM+). Muscle transcript levels of IGF-I and mechanogrowth factor (MGF) were determined by RT-PCR. Serum IGF-I, IGF-binding protein (IGFBP)-3, IGFBP-1, total and free testosterone, sex hormone-binding globulin (SHBG), and androstenedione were assessed by radioimmunoassay. Myofiber hypertrophy was twofold greater in YM vs. others, and only YM increased NCAM+ cells per 100 myofibers (49%) and myonuclei per fiber (19%) (P < 0.05). IGF-IEa mRNA was higher in young and increased acutely (29%) with summation by 16 wk (96%) (P < 0.05). MGF mRNA increased only in young after one bout (81%) and by 16 wk (85%) (P < 0.001). Circulating IGF-I was twofold higher in young, whereas IGFBP-1 was lowest in YM (P < 0.05). Among men, free testosterone was 59% higher in YM (P < 0.01). Myonuclear addition was most effectively accomplished in YM, which likely drove the superior growth.

Efficacy of 3 days/wk resistance training on myofiber hypertrophy and myogenic mechanisms in young vs. older adults.

Resistance training (RT) has shown the most promise in reducing/reversing effects of sarcopenia, although the optimum regime specific for older adults remains unclear. We hypothesized myofiber hypertrophy resulting from frequent (3 days/wk, 16 wk) RT would be impaired in older (O; 60-75 yr; 12 women, 13 men), sarcopenic adults compared with young (Y; 20-35 yr; 11 women, 13 men) due to slowed repair/regeneration processes. Myofiber-type distribution and cross-sectional area (CSA) were determined at 0 and 16 wk. Transcript and protein levels of myogenic regulatory factors (MRFs) were assessed as markers of regeneration at 0 and 24 h postexercise, and after 16 wk. Only Y increased type I CSA 18% (P < 0.001). O showed smaller type IIa (-16%) and type IIx (-24%) myofibers before training (P < 0.05), with differences most notable in women. Both age groups increased type IIa (O, 16%; Y, 25%) and mean type II (O, 23%; Y, 32%) size (P < 0.05). Growth was generally most favorable in young men. Percent change scores on fiber size revealed an age x gender interaction for type I fibers (P < 0.05) as growth among Y (25%) exceeded that of O (4%) men. Myogenin and myogenic differentiation factor D (MyoD) mRNAs increased (P < 0.05) in Y and O, whereas myogenic factor (myf)-5 mRNA increased in Y only (P < 0.05). Myf-6 protein increased (P < 0.05) in both Y and O. The results generally support our hypothesis as 3 days/wk training led to more robust hypertrophy in Y vs. O, particularly among men. However, this differential hypertrophy adaptation was not explained by age variation in MRF expression.

Resting and load-induced levels of myogenic gene transcripts differ between older adults with demonstrable sarcopenia and young men and women.

Regenerative capacity appears to be impaired in sarcopenic muscle. As local growth factors and myogenic regulatory factors (MRFs) modulate repair/regeneration responses after overload, we hypothesized that resistance loading (RL)-induced expression of MRFs and muscle IGF-I-related genes would be blunted in older (O) males (M) and females (F) with demonstrable sarcopenia vs. young (Y) adults. Y (20-35 yr, 10 YF, 10 YM) and O (60-75 yr, 9 OF, 9 OM) underwent vastus lateralis biopsy before and 24 h after knee extensor RL. Sarcopenia was assessed by cross-sectional area of type I, IIa, and IIx myofibers. Transcript levels were assessed by relative RT-PCR and analyzed by age x gender x load repeated-measures ANOVA. O were sarcopenic based on type II atrophy with smaller type IIa (P < 0.05) and IIx (P < 0.001) myofibers. Within-gender cross-sectional area differences were more marked in F (OF < YF: IIa 21%, IIx 42%). Load effects (P < 0.05) were seen for four of seven mRNAs as IGF-IEa (34%), myogenin (53%), and MyoD (20%) increased, and myf-6 declined 10%. Increased IGF-IEa was driven by O (48%) and/or M (43%). An age x gender x load interaction was found for MyoD (P < 0.05). An age x load interaction for type 1 IGF receptor (P < 0.05) was driven by a small increase in O (16%, P < 0.05). A gender x load interaction (P < 0.05) was noted for IGF binding protein-4. Age effects (P < 0.05) resulted from higher MyoD (54%), myf-5 (21%), and IGF binding protein-4 (17%) in O and were primarily localized to F at baseline (OF > YF; MyoD 94%, myf-5 47%, P < 0.05). We conclude that RL acutely increases mRNA expression of IGF-IEa and myogenin, which may promote growth/regeneration in both Y and O. Higher resting levels of MRFs in OF vs. YF suggest elevated basal regenerative activity in sarcopenic muscle of OF.