title: Redox regulation of mitochondrial responses in denervated and ageing skeletal muscle
Creators: name: Scalabrin, Mattia
Publisher: name: University of Liverpool Repository
Year: 2020

description: Sarcopenia is a characteristic of ageing where a substantial reduction of muscle tissue, strength and function are seen (Baumann et al, 2016). Previous reports have suggested that an age-related loss of stability of the neuromuscular junction can result in muscle fibre denervation, which in turn impacts on the capacity to activate muscles as well as altering muscle fibre architecture, composition, cross sectional area and number. Hence, muscle fibre denervation may play a fundamental role in sarcopenia. Previous studies (Muller et al, 2007; Bhattacharya et al, 2009; Pollock et al, 2017) have highlighted a significant increase in mitochondrial peroxide release in denervated muscle fibres but the physiological role of this increase is still unclear. Therefore, the main aim of this study was to determine the sources of peroxide in denervated muscle fibres and the physiological effect that this peroxide release has on skeletal muscle. A specific focus was given to the adaptive responses induced by the increased peroxide release and its effects on the metabolism and mitochondria of denervated muscle fibres. The main findings of this thesis are: • In response to surgically induced denervation of the Tibialis Anterior and Extensor Digitorum Longus muscles, a significant increase in peroxide release from mitochondria occurred from enzymes not involved in the electron transport chain. During prolonged denervation, muscles underwent significant atrophy accompanied by increased content of cytosolic enzymes such as phospholipase A2 and NADPH Oxidase 2 which may be responsible for the initiation of proteolysis. • In the experimental model of denervation, there was a significant increased content of mitochondrial fission markers with no apparent changes in fusion together with a progressive accumulation of p62 indicating mitophagy impairment. There was also a reduction of mitochondria content at 21 days post-denervation with the activation of pro-apoptotic pathways involving the Bcl-2 family. • Nuclear magnetic resonance metabolomics showed that prolonged denervation was associated with metabolic changes in pathways related to insulin resistance, increase of protein breakdown and activation of purine metabolism. • Given the extensive cellular alterations observed with prolonged, surgically induced denervation, similar measurements were performed in skeletal muscle from old mice to identify whether these changes were also evident during ageing. From 18 months, mice exhibited an instability of the neuromuscular junction with progressive fragmentation and nerve sprouting. However, not all of the biochemical changes observed in skeletal muscle following surgically induced denervation were also seen in skeletal muscle during ageing. In conclusion, the results of the present work suggest that the increase in peroxide release in fully denervated muscle fibres triggers multiple responses resulting in impairment of mitophagy, insulin resistance, increased proteolysis and fibre atrophy.