Over the past few years translational regulation has been shown to be critical, particularly in cases of cellular stress where it can elicit a rapid, efficient response at the protein level. It was, therefore, hypothesised that changes in mRNA translation might play an important role in compound-induced hepatotoxicity and that identifying such changes would provide a more complete understanding of the molecular mechanisms of such toxicity. The translational profiling technique frequently used for in vitro samples was modified slightly to enable its use with in vivo samples and, it revealed that, in cases of acute, compound-induced hepatotoxicity more mRNAs showed a change at the translational level than at the transcriptional level. This suggested that the liver was acting rapidly at the translational level to alter protein activity following toxic insult. One mRNA that exhibited translational repression was selected for further investigation. Following compound-induced hepatotoxicity, Dio3 demonstrated no change at the mRNA level, but a significant reduction in protein, thus, it was hypothesised that the translational repression of Dio3 was the predominant factor regulating the reduction in protein. Furthermore, it was proposed that due to the involvement of Dio3 in maintaining thyroid hormone levels, this was an important feedback mechanism by which the liver, upon early signs of damage, was acting rapidly to maintain its own energy equilibrium, avoiding global disruption of the hypothalamic-pituitary-thyroid axis. miRNAs have been identified as translational regulators; however, there was no evidence that the translational repression of Dio3 was miRNA-mediated. Global hepatic miRNA expression analysis did however, reveal that miRNAs appeared to mediate large networks important in the cellular response to compound-induced liver injury. Furthermore, by correlating miRNA and translational profiling data sets from the same models, it was possible to identify specific functionally-relevant miRNA-mRNA interactions. miRNA expression profiling was extended to the plasma of rats treated with one of the hepatotoxic compounds. A subset of miRNAs were differentially expressed following treatment and these appeared to mediate pathways involved in hepatic fibrosis and stellate cell activation, suggesting that they might function as predictive biomarkers following compound-induced hepatotoxicity. The work presented in this thesis has expanded the existing data available for a number of models of compound-induced hepatotoxicity to encompass global changes in both mRNA translation and miRNA expression. It has shown that mRNA translation, regardless of how it is regulated, plays an important role in the dynamic response of the liver to cellular stress induced by hepatotoxic compounds.

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Box C/D small nucleolar ribonucleic protein complexes (snoRNPs) either direct 2’-O-methylation of the pre-ribosomal RNA (pre-rRNA) or function as chaperones in pre-rRNA processing. It has been demonstrated that box C/D snoRNP biogenesis is a highly intricate and co-ordinated process that is mediated by a large, dynamic complex, known as the pre-snoRNP. This complex contains the box C/D snoRNA and a number of common core proteins, which become stably associated with the snoRNA, as well as factors linked to assembly, 3’ RNA processing and transport. Interestingly, the product of the gene linked to the neurodegenerative disorder spinal muscular atrophy, the survival of motor neurons protein (SMN), interacts with the box C/D snoRNP common core protein fibrillarin and is vital for the cellular localisation of both fibrillarin and box C/D snoRNA. The SMN protein operates with Gemins2-8 and UNRIP in what is termed the SMN complex, which has known functions in the assembly of the spliceosomal small nuclear RNPs (snRNPs). As the SMN protein interacts with fibrillarin and is also required for the cellular localisation of both fibrillarin and box C/D snoRNA this study set out to investigate the association of fibrillarin with the box C/D snoRNPs and the role of the SMN complex in this process. In this study it was revealed that the C terminal domain of fibrillarin is essential for cellular localisation and interactions with the box C/D snoRNP assembly factors, which suggests that this domain may mediate fibrillarin association with the box C/D snoRNPs. Also in this study the SMN protein was shown to interact with the box C/D snoRNP assembly factors NUFIP and BCD1, which are stable components of the pre-snoRNP. Analysis of the interaction of SMN with the pre-snoRNP, however, indicates that if SMN does associate with the pre-snoRNP then it is only a transient interaction. Further analysis revealed that as well as the SMN protein, Gemin2, 5, 6 and 7 are essential for the localisation of box C/D snoRNA and that Gemin5 is also required for the accumulation of box C/D snoRNA. This study strengthens the case that the SMN complex is involved in box C/D snoRNP biogenesis, with the data suggesting that it functions as a transport factor rather than in the association of fibrillarin with the box C/D snoRNPs. Analysis of the snRNP transport factor, snurportin1, revealed that it also interacts with numerous components of the pre-snoRNP; however, does not interact with SMN.