Chronological Age Does not Influence Ex-vivo Mitochondrial Respiration and Quality Control in Skeletal Muscle

Abstract

Background: Considerable debate continues to surround the concept of mitochondrial dysfunction in aging muscle. We tested the overall hypothesis that age per se does not influence mitochondrial function and markers of mitochondria quality control, that is, expression of fusion, fission, and autophagy proteins. We also investigated the influence of cardiorespiratory fitness (VO2max) and adiposity (body mass index) on these associations.

Methods: Percutaneous biopsies of the vastus lateralis were obtained from sedentary young (n = 14, 24±3 years), middle-aged (n = 24, 41±9 years) and older adults (n = 20, 78±5 years). A physically active group of young adults (n = 10, 27±5 years) was studied as a control. Mitochondrial respiration was determined in saponin permeabilized fiber bundles. Fusion, fission and autophagy protein expression was determined by Western blot. Cardiorespiratory fitness was determined by a graded exercise test.

Results: Mitochondrial respiratory capacity and expression of fusion (OPA1 and MFN2) and fission (FIS1) proteins were not different among sedentary groups despite a wide age range (21 to 88 years). Mitochondrial respiratory capacity and fusion and fission proteins were, however, negatively associated with body mass index, and mitochondrial respiratory capacity was positively associated with cardiorespiratory fitness. The young active group had higher respiration, complex I and II respiratory control ratios, and expression of fusion and fission proteins. Finally, the expression of fusion, fission, and autophagy proteins were linked with mitochondrial respiration.

Conclusions: Mitochondrial respiration and markers of mitochondrial dynamics (fusion and fission) are not associated with chronological age per se, but rather are more strongly associated with body mass index and cardiorespiratory fitness.

Keywords: Aging; Body composition; Mitochondria; Muscle; Physical activity.

Figure 1.

Mitochondrial respiration in permeabilized myofibers from young active (YA), and sedentary young (YS), middle-aged (MAS), and older (OS) participants. (A) Mitochondrial respiration consisting of leak (L_I), complex I supported OXPHOS (P_I), complex I+II supported OXPHOS (P_I+II), maximal electron transfer system capacity (E_I+II), and ETS with substrates for complex II only (E_II). (B) Respiratory flux control ratios were determined as an estimation of leak and OXPHOS capacity within the ETS capacity. (C) Complex I (CI) and Complex II (CII) control factors. Data presented as Mean and SD; n = 9–20 per group. *p < .05 vs YS, MAS, and OS; **p < .05 vs OS.

Figure 2.

Association between mitochondrial respiration with body mass index (BMI) and cardiorespiratory fitness (VO₂max). Bivariate correlation between Leak (L_I) respiration with BMI (A) and VO₂max (B), complex I+II supported OXPHOS (P_I+II) respiration with BMI (C) and VO₂max (D), and maximal electron transfer capacity (E_I+II) with BMI (E) and (F) VO₂max. Black circles represent young active subjects and white circles represent sedentary subjects, n = 58–59.

Figure 3.

Expression of quality control proteins in skeletal muscle samples from active young (YA), and sedentary young (YS), middle-aged (MAS), and older (OS) participants. (A) Western blot analysis of mitochondrial fusion proteins. (B) Western blot analysis of mitochondrial fission proteins. (C) Western blot analysis of autophagy proteins. (D) Western blot analysis of mitochondrial content (OXPHOS, complexes I-V and Total). Values were normalized to α-tubulin and a loading control. Data are presented as Mean and SD for figures A, B, and C, and as Mean and SE for figure D. Results are expressed as arbitrary units (AU); Vertical and horizontal dividing lines were used in the Western blot images to present lanes from the same gel that were reorganized for presentation purpose; n = 5–17 per group. *p < .05 vs YS, MAS, and OS **p < .05 vs MAS and OS, ***p < .05 vs MAS, § p < .05 vs YS, §§ p < .05 vs YA and YS.

Figure 4.

Association between mitochondrial fusion and fission proteins with body mass index (BMI). Correlation of mitochondrial proteins (A) OPA1, (B) MFN2, (C) FIS1, and (D) DRP1 with BMI. Results are expressed as arbitrary units (AU). Black circles represent young active subjects and white circles represent sedentary subjects n = 34–41.