Parkinson’s disease (PD) is a progressive neurodegenerative disease characterised by the loss of dopaminergic (DA) neurons, causing deficits in motor function. At present, mitochondrial dysfunction and the endoplasmic reticulum (ER) unfolded protein response (UPR) are perceived to be molecular features of PD. The UPR is an evolutionarily conserved adaptive cellular response to unfolded or misfolded proteins in the ER. Three ER-resident proteins sense the UPR, one of which is the protein kinase RNA (PKR)-like ER kinase (PERK). PERK activation leads to translational repression via phosphorylation of eukaryotic translation initiation factor-2 α (eIF2α) as well as activation of activating transcription factor 4 (ATF4). This triggers a transcriptional response, initially promoting cell survival. Eventually, the sustained activation of the UPR leads to cell death. PERK has also been reported to modulate mitochondrial function. The overarching aim of this thesis was to investigate the relationship between PERK kinase and mitochondrial dysfunction in the pathogenesis of PD using the fruit fly Drosophila melanogaster as a model organism. To characterise the Drosophila PERK (dPerk) expressional landscape, I combined microarray and quantitative proteomics analysis from adult flies overexpressing dPerk. Using this approach, I identified tribbles (trbl) and Heat shock protein 22 (Hsp22) as two novel Drosophila ATF4 (dAtf4) regulated transcripts. Furthermore, I established that dPerk expression leads to translational repression of several mitochondrial proteins. The mitochondria-ER tether ATPase Family AAA Domain Containing 3A (ATAD3A) has recently been suggested to function as a negative regulator of PERK in mammalian models. I show that the ATAD3A fly orthologue Belphegor (Bor) does not act as an inhibitor of dPerk. Furthermore, I propose that this is due to the lack of the proline-rich motif in Drosophila, otherwise present in the protein structure of its mammalian orthologue. Mutations in PTEN-induced putative kinase 1 (PINK1), a mitophagy gene, cause neurodegeneration in some autosomal recessive forms of PD. In Drosophila, mutations in the pink1 gene cause mitochondrial dysfunction and the degeneration of DA neurons. Pink1 mutants also show overactivation of the dPerk/eIF2α/dAtf4 axis. Therefore, I used the pink1 PD Drosophila model to further probe the role of dPerk in the pathomechanism of PD. PD patients experience gastrointestinal issues that often precede the onset of motor symptoms, implicating the gut-brain axis in the pathogenesis of this disease. Likewise, in pink1 mutants, mitochondrial dysfunction leads to cell death and proliferation in the Drosophila midgut. Suppressing intestinal dysfunction is neuroprotective. I found that the intestinal expression of dPerk causes intestinal damage, and silencing dPerk in the pink1 intestine leads to a rescue of intestinal dysfunction and neurodegeneration. The ER stress marker Trbl also acts as an inhibitor of Akt, implicating this pseudokinase as a negative regulator of insulin signalling. The human Trbl orthologue tribbles pseudokinase 3 (TRIB3) is upregulated in insulin resistant as well as obese adults, and TRIB3 polymorphisms are associated with type 2 diabetes and insulin resistance. The fat body is a fly organ homologous to the mammalian liver and adipose tissue. It functions to synthesise and store triacylglycerol and regulate endocrine and immune signalling, implicating it in the regulation of complex behaviours such as sleep. My results show that the FB expression of Trbl leads to metabolic defects, systemic repression of insulin signalling and a reduction in night-time sleep. This thesis investigates inter-organelle as well as inter-organ signalling using Drosophila melanogaster as a model organism. My research shows that the ER UPR kinase dPerk is an important regulator of mitochondrial function and intestinal homeostasis, suggesting dPerk as a potential therapeutic target for PD. Furthermore, analysis of Trbl pseudokinase function in Drosophila melanogaster proposes Trbl as a molecular link between animal nutrient state and behaviour.