p62 : linking protein homeostasis and the DNA damage response during ageing

Abstract

The accumulation of DNA damage has long been thought to contribute to both cellular and organismal ageing. The catabolic degradation process autophagy has also been implicated in the ageing process, as an age-dependent functional decline has been reported in many organisms. Moreover, interventions known to delay ageing such as dietary restriction and rapamycin treatment have been shown to require active autophagy. Recent studies have suggested a role for autophagy in the DNA damage response as well as DNA damage repair; however the mechanisms are still poorly understood. In this thesis we set out to understand how autophagy can influence DNA repair. We will also investigate how cross-talk between these processes is relevant to the ageing process. Firstly, we show that cells lacking autophagy have an impaired DNA damage repair kinetic, as measured by the resolution of DNA damage foci (DDF). Importantly, these differences were shown to be dependent on the intracellular levels of autophagy adaptor protein p62. It was recently shown that p62 shuttles rapidly between cytoplasmic and nuclear compartments. However, the role of p62 in the nucleus is still relatively unknown. Mechanistically, we show that both the PB1 and UBA domain of p62 are required for its effect on DNA damage repair. Furthermore we show p62 is recruited to DDF in response to DNA damage induction. Next, we show p62 interacts with the cytoskeletal protein FLNA and DNA damage protein RAD15 within the nucleus facilitating their proteasomal degradation. Both FLNA and RAD51 have previously been suggested to influence DNA repair via the homologous recombination pathway. Cells lacking p62 have higher nuclear levels of RAD51 and FLNA. Importantly these high levels correspond with an increased formation and resolution of RAD51 foci following induction of DNA damage, suggestive of an increase in DNA repair via the homologous recombination pathway. Finally, we observed an increase in co-localisation of p62 with the marker of DDF H2A.X in mouse liver during ageing. Additionally, we found that life-long dietary restriction, an intervention known to extend lifespan in mice, prevented the age-dependent increase in frequencies of p62-H2A.X foci. We propose that p62 plays a novel and important role in DNA damage repair and hypothesise that declining autophagy or dysregulation of p62 can contribute to organismal ageing.