The impact of aging on mitochondrial function and biogenesis pathways in skeletal muscle of sedentary high- and low-functioning elderly individuals

Abstract

Age-related loss of muscle mass and strength (sarcopenia) leads to a decline in physical function and frailty in the elderly. Among the many proposed underlying causes of sarcopenia, mitochondrial dysfunction is inherent in a variety of aged tissues. The intent of this study was to examine the effect of aging on key groups of regulatory proteins involved in mitochondrial biogenesis and how this relates to physical performance in two groups of sedentary elderly participants, classified as high- and low-functioning based on the Short Physical Performance Battery test. Muscle mass was decreased by 38% and 30% in low-functioning elderly (LFE) participants when compared to young and high-functioning elderly participants, respectively, and positively correlated to physical performance. Mitochondrial respiration in permeabilized muscle fibers was reduced (41%) in the LFE group when compared to the young, and this was associated with a 30% decline in cytochrome c oxidase activity. Levels of key metabolic regulators, SIRT3 and PGC-1α, were significantly reduced (50%) in both groups of elderly participants when compared to young. Similarly, the fusion protein OPA1 was lower in muscle from elderly subjects; however, no changes were detected in Mfn2, Drp1 or Fis1 among the groups. In contrast, protein import machinery components Tom22 and cHsp70 were increased in the LFE group when compared to the young. This study suggests that aging in skeletal muscle is associated with impaired mitochondrial function and altered biogenesis pathways and that this may contribute to muscle atrophy and the decline in muscle performance observed in the elderly population.

Fig. 1

Mitochondrial bioenergetics in muscle from young and functionally-distinct elderly individuals. (A) Respiration in permeabilized muscle fibers from young (Y), high-functioning elderly (HFE) and low-functioning elderly (LFE) subjects was measured with pyruvate/malate (5 mM/2 mM) in basal respiratory state 4 or ADP-stimulated (2 mM) respiratory state 3 and expressed as pmol/s/mg of dry weight. Values are means ± SE; n = 5–9 per group.*P < 0.05 vs. young; †P < 0.05 vs. HFE (B) Cytochrome c oxidase (COX) activity, expressed as unit per gram of tissue. Values are means ± SE; n = 5–9 per group.*P < 0.05 vs. young.

Fig. 2

Effect of age on cellular signaling molecules. Representative Western blots of phosphorylated 5′ AMP-activated protein kinase (P-AMPKα), total AMPK (AMPK), phosphorylated p38 mitogen-activated protein kinase (p-p38) and total p38 (p38). Summary of experiments of AMPK and p38 activation are shown below as the phosphorylated form of the kinase over the total content and expressed as a fold of young. Values are means ± SE; n = 11–12 per group.

Fig. 3

Age-related changes in key metabolic and transcriptional regulators. (A) Western blot analysis of SIRT3 = sirtuin 3; RIP140 (nrip1) = nuclear receptor-interacting protein 140; PGC-1α =peroxisome proliferator activated receptor gamma (PPARγ) Coactivator 1-alpha; NRF-1 = nuclear respiratory factor 1; Tfam = mitochondrial transcription factor A; COX I and IV = cytochrome c oxidase subunit I and IV. A summary of repeated experiments (left) along with the corresponding protein blot (right). All values were normalized to actin and expressed as a fold of young. Values are means ± SE; n = 11–12 per group. *P < 0.05 vs. young. (B) Relationship between COX activity and PGC-1α in young and elderly individuals. n = 7–9 per group. r = Pearson correlation coefficient.

Fig. 4

Association between PGC-1α and physical function in elderly subjects. (A) Gait speed in HFE and LFE individuals with each point representing an individual. n = 11–12 per group. *P < 0.05 vs. HFE. (B) Relationship between gait speed and PGC-1α protein in HFE and LFE subjects. n = 9–12 per group. r = Pearson correlation coefficient.

Fig. 5

Mitochondrial fusion and fission proteins in young and elderly subjects. Western blots of fusion Mfn2 (mitofusin 2) and Opa1 (optic Atrophy Type I), and fission proteins Drp1 (dynamin-related protein 1) and Fis1 (fission 1). Two bands were detected for Opa1 representing the long and short isoforms. Both bands were quantified for total Opa1 protein. Summary of repeated experiments is shown below. All values were normalized to actin and expressed as a fold of young. Values are means ± SE; n = 11–12 per group. *P < 0.05 vs. young.

Fig. 6

Alterations in mitochondrial protein import machinery (PIM) components. Western blot analysis of PIM with a graphical representation depicted below. Tim23 = translocase of the inner membrane protein 23; Tom22 = translocase of the outer membrane protein 22; cHsp70 = cytosolic heat shock protein 70; mtHsp70 = mitochondrial heat shock protein 70; Hsp60 = heat shock protein 60. All values were normalized to actin and expressed as a fold of young. Values are means ± SE; n = 11–12 per group. *P < 0.05 vs. young; †P < 0.05 vs. HFE.