Maintenance of mammalian oocyte quality and fertility during ageing via Sirtuin activity and NAD+ repletion

Abstract

The age-related decline in female fertility is one the earliest and most profound phenotype in natural human ageing, with mounting body of evidence pointing to the quality of the female egg, termed ‘oocyte’, as the primary determinant. With unrelenting trend in women delaying childbearing, the growing incidence of age-associated infertility is rapidly becoming a global health burden. Current Assisted Reproductive Technology (ART) options have been unable to overcome the underlying issue of poor oocyte quality. To this end, the following thesis aimed to: examine the problem of poor oocyte quality and infertility through an ageing perspective, explore key molecular factors underlying this pathology, and propose novel approaches to address this important health issue. Declining level of the spindle assembly checkpoint (SAC) protein budding uninhibited by benzimidazole-related 1 (BubR1) has been implicated in spindle morphology defects and aneuploidy in aged mouse oocytes. Accumulation of reactive oxygen species (ROS) is another precipitating factor in declining oocyte quality during ageing. Considering recently observed roles of SIRT2, a member of the sirtuin family, in stabilisation of BubR1 and regeneration of the antioxidant glutathione in somatic cells, we sought to examine the impact of genetic overexpression of SIRT2 on oocyte quality in a naturally-aged murine model. Assessment of various markers of oocyte quality revealed that increased SIRT2 activity resulted in superior oocyte quality, which translated to improved fertility when tested in a mating trial. SIRT2 is highly dependent on the availability of nicotinamide adenine dinucleotide (NAD+). To test the importance of sustaining NAD+ on oocyte quality, we utilised two distinct NAD-raising approaches, pharmacologically through administration of its precursor nicotinamide mononucleotide (NMN) and genetically via overexpression of the enzyme NMN adenylyltransferase (NMNAT). As with SIRT2 overexpression, pharmacological and genetic restoration of NAD+ resulted in amelioration of poor oocyte quality in aged mice. This thesis provides evidence that SIRT2 is crucial in preserving oocyte quality during biological ageing, and demonstrates that restoration of NAD+ levels could reverse age-associated defects in mammalian oocytes. These findings offer novel approaches which could dramatically alter our ability to address the fundamentally elusive problem of ageing-associated infertility.