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Title: Study of novel protein-protein interactions modulating PERP-mediated apoptosis
Author: McDonnell, S. J.
ISNI:       0000 0004 7658 126X
Awarding Body: University of Liverpool
Current Institution: University of Liverpool
Date of Award: 2018
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Multicellular organisms have evolved complex responses to cellular stress which are protective over the initiation of disease. In response to adverse environmental or cellular changes, cells upregulate pathways to repair any damage and restore homeostasis. If cell repair is not possible, cells are genetically programmed to undergo controlled cell death, known as apoptosis. This provides the ultimate protection against the proliferation of damaged cells which would contribute to disease development and progression. Conditions such as hypoxia and nutrient deprivation lead to robust cellular physiological responses by: (i) limiting the capacity of the endoplasmic reticulum (ER) to synthesise proteins, inducing an ER stress response which halts translation to aid cellular repair; (ii) inducing the degradation of cellular content by the autophagy pathway to provide nutrients and energy for survival. Importantly, both ER stress and autophagy pathways promote apoptosis after sustained, irreparable damage. Tumour cells hijack these protective responses to enable their hyperproliferatation in a 3D tumour mass by upregulating pro-survival ER stress and autophagy pathways and downregulating pro-death apoptosis pathways. The plasma membrane (PM) protein PERP induces apoptosis in many cell types. PERP is downregulated in tumours of multiple tissues and this often correlates which increased aggressiveness, metastasis risk and resistance to apoptosis-inducing chemo- and radiotherapies. The aim of this study was to characterise the pathways governing the regulation of PERP at the PM and its protein-protein interactions which mediate apoptosis induction. Here, we showed that PERP is synthesised in the ER and is trafficked to the PM in large post-Golgi carriers. PERP is distributed to protein islands across the PM of healthy cells and is present in cell-cell and cell-matrix adhesive membrane regions. We found that PERP interacts with ER proteins SERCA2b and ORP8L, which regulate calcium and lipid signalling respectively, at ER-PM points of contact. We therefore studied the role of PERP in the response to ER stress and found that PERP is upregulated in a transcription-independent manner and accumulates across the PM due to a reduction in its uptake and degradation. In line with previous studies, ER stress induced an upregulation in autophagy. We found that, PERP is degraded by a selective lysosomal pathway and sustained autophagy mediated an increase in PERP which correlated with apoptosis. Together, this indicated that PERP accumulates at the PM in response to ER stress due to an increase in its protein stability as a result of sustained induction of autophagy. Significantly, we showed that PERP and SERCA2b increasingly interact during ER stress and SERCA2b-mediated apoptosis at ER-PM junctions, which suggested that PERP regulates the generation of toxic ER calcium levels to induce apoptosis. The findings showed that PERP mediates apoptosis at inter-organelle points of contact via the regulation of calcium and lipid signals and provides an important cross-talk between the ER stress, autophagy and apoptosis pathways. These findings highlighted the protective functions of PERP against tumour formation and metastasis and identified novel avenues for therapeutic intervention.
Supervisor: Paraoan, Luminita ; Prior, Ian Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral