Developmental Programming and Life Course Regulation of Skeletal Muscle Inflammation and Oxidative Function

Abstract

Skeletal muscle is continuously adapting in size and oxidative capacity to meet the physical and metabolic demands of the organism. This commences prior to birth with exposure to adverse nutrition in utero having a lifelong adverse impact on skeletal muscle health. After birth, through to old age, periods of inadequate diet and activity significantly impair muscle mass and oxidative capacity. There are many cellular mechanism that mediate these complex adverse events, however these appear to be clustered around intracellular inflammation and disordered regulation of mitochondrial function. The aim of this thesis was to undertake analysis of potential molecular mechanisms that are integral to the regulation of inflammation, oxidative function and oxidative stress in the development and maintenance of lifelong skeletal muscle function. Poor maternal health and diet during fetal development elicits permanent changes in skeletal muscle structure and function. Therefore, these studies commenced with the use of a rodent model to examine the intergenerational effects of a maternal high-fat (HF) diet on offspring skeletal muscle inflammation and oxidative function. First, to determine the effects of a maternal HF diet on offspring skeletal muscle inflammation, dams were fed a HF diet during pregnancy and lactation vs. standard chow diet, with or without maternal supplementation of conjugative linoleic acid (CLA). HF offspring displayed elevated intramuscular inflammatory responses, and increased expression of catabolic factors, which was partially reversed with the maternal supplementation of the antiinflammatory CLA. The consequential actions of inflammation also include alterations in mitochondrial function. Therefore, subsequent studies targeted analysis of the regulation of the mitochondrial genome, electron transport system (ETS) activity, free radical production and mitochondrial turnover. First, in response to the maternal HF diet, the mitochondrial transcription factors, NRF1 and mtTFA, were down-regulated in HF offspring. There was also a decrease in the expression of downstream target genes encoding the mitochondrial ETS respiratory complex subunits. These alterations in gene expression translated into a downregulation in protein abundance of the complex I subunit in HF offspring, paralleled by decreased maximal catalytic linked activity of complex I and III. To further characterise mitochondrial function, high-resolution respirometry was used to examine skeletal muscle respiration rates and metabolic flexibility in offspring born to dams fed a HF diet. Consistent with the gene and enzyme data, HF offspring displayed blunted mitochondrial oxidation, but only of carbohydrate based substrates. Conversely, oxidation of lipid-based substrates remained intact between groups. Taken together, these studies demonstrate that in utero exposure to a maternal HF diet has lifelong implications on intramuscular inflammation and oxidative function, contributing to an increased risk of developing metabolic disease in adulthood. A model of short-term limb immobilisation was used in a clinical study to quantify the impact of physical inactivity on mitochondrial function, with additional analysis of mitochondrial ROS production and mitochondrial turnover. Two weeks of limb immobilisation had no effect on mitochondrial respiration or H2O2 emissions; whereas returning to normal ambulation for 2 weeks and a further 2 weeks of resistance exercise training resulted in robust increases in H2O2 emission from mitochondria and NADPH oxidase. Whilst H2O2 was not increased with immobolisation, a key mitochondria associated apoptotic factor (AIFM2) was increased, suggesting an upregulation of mitophagy following muscle disuse. This was normalised with normal ambulation and resistance training. Similarly, the expression of fission and fusion factors (OPA1, Fis1, and MFN) are also increased with the resumption of normal, indicative of increased mitochondrial turnover. Collectively, the increase in H2O2 emissions combined with increased expression of fission and fusion factors following the resistance training period suggest that increases in ROS production is a feature of the adaptive responses, rather than being increased during a short immobilisation period that results in muscle loss. The completed sequence of studies highlight the complex regulation of skeletal muscle mass and oxidative function, both as a consequence of adverse nutrition during the in utero phase of life and subsequent physical inactivity in later adulthood. The significance of in utero programming for lifelong impact on oxidative function suggests greater emphasis on optimizing pregnancy health and the subsequent identification on the epigenetic mechanisms of action. How this may further translate to increased risks during later life remains to be determined. Notably, the robustness of the mitochondrial adaptations to resistance exercise in adulthood points to the importance of examining how this can be applied to improving metabolic function in those at risk of metabolic disease. These studies only continue to hint at the complexity of the mitochondria and its regulation in skeletal muscle.