Download files
Abstract
As we age and across various types of disease, levels of the molecule NAD+ (nicoti- namide adenine dinucleotide) decline. Derived from different sources including vitamin B3, NAD+ is a coenzyme that plays a key role in the regulation of redox biology and cellular metabolism. It also serves as an essential substrate for the seven-member class of NAD+ consuming enzymes called Sirtuins (Sirt1-7). Sir- tuins and NAD+ have been shown to regulate ageing and various physiological parameters in mammals. Pharmacologically elevating NAD+ levels and Sirtuin activity positively regulates numerous metabolic and physiological parameters in mammals, and reduces the deleterious effects of high fat feeding and age-related deterioration of health, as well as supressing tumour formation in various instances. Recently, the mitochondrial Sirtuin Sirt5, has been characterised in depth and has been identified to regulate multiple new protein modifications which impact mitochondrial and cytosolic processes through the enzymatic activity of reversible lysine acylation. Several knock-out studies revealed a loss of function phenotype when Sirt5 is removed from cells. In this thesis, it was hypothesised that increasing the expression Sirt5 may exhibit a phenotype that is beneficial for health. Over- expressing Sirt5 in mice resulted in alterations to protein acylation status in two different dietary conditions, however Sirt5 TG mice displayed no overt phenotype. This may be due to the limiting availability of NAD+, the substrate essential for Sirtuin activity. To overcome this issue, mice that overexpress the mitochondrial enzyme responsible for synthesising NAD+ (Nicotinamide mononucleotide andeylyl transferase, NMNAT3) were next investigated, and demonstrated a metabolically advantageous phenotype marked by improvements to mitochondrial metabolism. Potential implications of NAD+ in a model of hepatocellular carcinoma are also investigated to address the role of NAD+ in cancer. Here, NMNAT3 transgenic mice were administered a single dose of the carcinogen diethylnitrosamine, which results in the spontaneous formation of hepatocellular carcinoma. Despite the improvements to cellular and mitochondrial metabolism, overexpression has no impact on the development of hepatocellular carcinoma. These results suggest improving mitochondrial NAD+ biosynthesis is sufficient to improve the metabolic health in the face of elevated lipid supply and may serve as an interesting target for improving metabolic function in aged mammals.