In Profile: Models of Ribosome Biogenesis Defects and Regulation of Protein Synthesis

Abstract

Ribosomes are the mediators of protein synthesis in the cell and therefore crucial to proper cell function. In addition, ribosomes are highly abundant, with ribosomal RNA making up 80% of the RNA in the cell. A large amount of resources go into maintaining this pool of ribosomes, so ribosome biogenesis and translation are highly controlled mechanisms. In this thesis we explore both ribosome biogenesis defects and physiological control of ribosome numbers and translation efficiency. In the first part of this thesis, we examined the role the nucleostemin gene family plays in ribosome biogenesis. We provide evidence that gnl2 and nucleostemin (ns), but not gnl3l, are required for ribosome biogenesis. All three mutants stabilize p53, which in ns is believed to occur through ribosomal stress, but in gnl3l occurs through another, as of yet unidentified mechanism. We argue that the overexpression of neural progenitor genes that we observe in the gnl2 and ns mutants is most likely a secondary effect of their ribosome biogenesis defects, since other ribosome biogenesis mutants display similar phenotypes. Inactivation of the insulin/insulin growth factor signalling (IIS) pathway in C elegans results in a remarkable increase in lifespan. In the second part of this thesis, we identified a strong downregulation of protein metabolism in a IIS mutant that was previously overlooked, using an unbiased mass spectrometry approach. This links the pathways of insulin/insulin growth factor signalling and dietary restriction mediated longevity more closely than was previously appreciated. In addition to a reduction in global translation, we also observed altered translation efficiency for several subsets of mRNAs. We identified a number of ribosome associated ncRNAs and provided evidence that one of these, tts-1, inhibits polysome formation and extends lifespan. In the last part of this thesis, unrelated to ribosomes, we investigated the interaction between the Von Hippel-Lindau (VHL) and the p53 tumour suppressors in a zebrafish model of vhl loss. In contrast to earlier in vitro studies, we observed increased stabilization of p53 in the absence of vhl. We identified a novel interaction between VHL and PDCD5 and showed that loss of VHL results in nuclear translocation of PDCD5 and subsequent degradation of MDM2, leading to increased p53 stabilization and target gene transcription. However, vhl is required for caspase activation and apoptosis to occur in response to DNA damage. These findings possibly explain why VHL and p53 are rarely found both mutated in RCC, since loss of p53 would not confer any additional growth advantage to VHL negative cells.