Muscle mitochondrial health : ageing, physical activity and molecular mechanisms

Muscle mass and strength are reported to decline with age. Due to the vital role the muscle in daily life activities, increased loss of muscle mass and strength is associated with functional decline, decreased quality of life and increased hospitalisation rates. Therefore, maintaining skeletal muscle mass and strength with age is a key component in healthy ageing. One of the postulated causes of the decline in mass and strength is the decline in the amount and the quality of the muscle mitochondria, collectively referred to as mitochondrial capacity. Nevertheless, the exact contribution of the mitochondria is still under debate. Therefore, this thesis aims to obtain a better understanding of the role of skeletal muscle mitochondria during ageing. Understanding the role of the contributors to skeletal muscle ageing on the physiologic and molecular level could focus intervention strategies to ultimately to sustain muscle mass and strength with age.Current methods to assess mitochondrial capacity in humans are either invasive, such as the sampling of muscle tissue via a muscle biopsy, or less accessible, due to expensive and specialised equipment, such as 31P-MRS. In recent years, the assessment of mitochondrial capacity using near-infrared spectroscopy (NIRS) has offered relief to these limitations. Yet, additional effort is needed to extend the use of NIRS to also study the effect of age on mitochondrial capacity. For example, previous studies compared groups of subjects with large expected differences in mitochondrial capacity, and therefore it was unknown if NIRS is able to detect relatively smaller differences in mitochondrial capacity, such as might be expected between a young and older population. In chapter 2 we demonstrated that NIRS was able to detect differences in mitochondrial capacity in the gastrocnemius muscle in a homogenous population of high- and low-fitness males, with a smaller expected difference in mitochondrial capacity than was previously assessed. In chapter 3 we additionally showed that NIRS is able to detect differences in mitochondrial capacity the gastrocnemius muscle in a population of a high- and low-fitness females. Furthermore, we show that NIRS correlates with other measures of oxidative capacity, underlining the physiological relevance of NIRS assessment of mitochondrial capacity. This demonstrates that NIRS could be a valuable tool to study muscle mitochondrial capacity in an ageing populationIn chapter 4 we used NIRS to assess the effect of age on mitochondrial capacity in a population of older (65-71 years) and young (19-25 years) males. Due to the interaction between physical activity, mitochondrial capacity and age, the two age groups were selected based on self-reported, similar physical activity, which was verified using a 5-day accelerometry measurement. We showed that NIRS was able to detect differences in mitochondrial capacity between the two age groups in the gastrocnemius and vastus lateralis, but not tibialis anterior. This showed that not all muscle groups display similar mitochondrial ageing and, because we observed these effects despite similar physical activity, the lower mitochondrial capacity is likely a direct effect of ageing and cannot be completely prevented by physical activity. Nevertheless, a higher mitochondrial capacity was correlated with spending more time in moderate-to-vigorous physical activity, suggesting that physical activity might ameliorate part of the age-related decline in mitochondrial capacity. In chapter 5 we used transcriptome sequencing to identify molecular mechanisms of ageing in vastus lateralis muscle biopsies in the aforementioned population. The significant regulated processes in older compared to young muscle included: cell-adhesion, the matrisome, innervation and inflammation, which were largely upregulated, and oxidative metabolism, which was downregulated. In accordance with the transcriptome results, the protein expression of some mitochondrial respiratory complexes was lower in older compared to young muscle. Moreover, the expression of these complexes in the older group was correlated with in vivo mitochondrial capacity in the vastus lateralis. This showed that the observed lower mitochondrial capacity could be explained by a lower expression of mitochondrial complex proteins and further substantiated the use of NIRS to measure mitochondrial capacity in vivo.In chapter 6 and 7 we explored the role of protein propionylation as regulatory factor of mitochondrial and muscle function in aging. Post-translational protein modifications are an important regulatory mechanism for protein functionality and offers the cell a rapid and reversible mechanism to respond to changes in the environment. Protein propionylation might be an important post-translational modification in muscle physiology and ageing. In chapter 6 we turned to a pathophysiological human model in which levels of propionyl-CoA were elevated and protein propionylation was increased. We showed that fibroblasts from patients in this pathological state have impaired mitochondrial function compared to healthy donor cells, possibly due to aberrant propionylation of proteins involved in mitochondrial respiration. Furthermore, increasing propionylation by exposure to pathophysiological concentrations of propionate induced impaired mitochondrial function in cultured fibroblasts and liver cells. Yet, this effect was not observed in cultured muscle cells, possibly due to differences in metabolic handling of propionyl-CoA. Despite an absence of the effect of propionate exposure and increased propionylation on mitochondrial function in muscle cells, in chapter 7 we showed that exposure to propionate impairs skeletal muscle differentiation. Concomitant with this observation, we observed an increase in histone protein propionylation and acetylation. The increase in propionylation and acetylation occurred on regions of the genome that regulate muscle differentiation, possibly revealing an additional mechanism by which propionyl-CoA and propionylation can influence muscle cellular fates.In chapter 8 the conclusions are presented and the main findings of this thesis are further discussed. For example, we further discuss the way forward for NIRS assessment of mitochondrial capacity. Additionally, we discuss the limitations of the study design used in chapter 4 and 5. Lastly, the plausible role of protein propionylation in skeletal muscle ageing is discussed.In conclusion, this thesis aimed to obtain a better understanding of the role of mitochondria in skeletal muscle ageing. We obtained better understanding on the non-invasive assessment of skeletal muscle mitochondrial capacity using NIRS and newly applied this method to study the effect of age on mitochondrial capacity in locomotor muscles. Furthermore, we obtained better understanding on the molecular mechanisms of ageing and identified that the age-related decline in mitochondrial capacity in skeletal muscle occurs despite similar physical activity, although we demonstrated this effect is muscle dependent. Lastly, we explored the effect of protein propionylation on skeletal muscle cells. Although we did not find direct effects of increased propionylation on mitochondrial function in muscle, we observed that increased propionylation was associated with impaired muscle differentiation and propionylation could therefore play a role in muscle physiology and ageing.