Proteomic response to metabolic stress and cellular dysfunction in relation to Alzheimer's disease
Herrmann, Abigail Grace
Vascular risk factors inducing a state of chronic cerebral hypoperfusion and metabolic stress are thought to influence the onset and progression of Alzheimer’s disease (AD). To investigate the complex molecular changes underpinning cellular adaptation to metabolic stress, the first aim of this thesis was to define the proteomic response of the SH-SY5Y human neuroblastoma cell line after exposure to the metabolic challenge of oxygen glucose deprivation (OGD). 958 proteins across multiple subcellular compartments were detected and quantified by label-free liquid chromatography mass spectrometry (LC-MS). The levels of 130 proteins were significantly increased (P<0.01) after OGD and the levels of 63 proteins were significantly decreased (P<0.01) while expression of the majority of proteins (765) was not altered. Ingenuity Pathway Analysis identified novel protein-protein interactomes involved with mitochondrial energy production, protein folding, and protein degradation, indicative of coherent and integrated proteomic responses to the metabolic challenge. Approximately one third (61) of the differentially expressed proteins were associated with the endoplasmic reticulum and mitochondria. Electron microscopic analysis of these subcellular structures showed morphologic changes consistent with the identified proteomic alterations. Pertinent to AD research, amyloid binding alcohol dehydrogenase (ABAD) was found to be significantly increased in response to OGD. ABAD is emerging as a key player in mitochondrial dysfunction in AD, yet full understanding of the biochemical pathways in which this protein is involved remain elusive. Using immunoprecipitation coupled to LC-MS (IP-MS), the second aim of the thesis was to characterise the ABAD protein interactome in SH-SY5Y cells and its response to metabolic stress. 67 proteins were identified as potential ABAD interactors under control conditions, and 69 proteins were identified as potential ABAD interactors under OGD conditions. The Database for Annotation, Visualization and Integrated Discovery (DAVID) was used to determine the subcellular locations and biological functions of the ABAD interacting proteins in control and OGD conditions. DAVID identified the nuclei and mitochondria to contain the greatest number of changes in ABAD interacting proteins following OGD. “Glucose Metabolic Process” (GO:0006006) was the top functional cluster for ABAD interacting proteins in both control and OGD conditions. Independent immunoprecipitations, western blotting, immunohistochemistry and electron microscopy were used to validate specific protein interactions. OGD was found to initiate a novel interaction between ABAD and glucose-regulated protein 75 (GRP75), a finding confirmed in human AD tissue. GRP75 is a mitochondrial protein and marker of the mitochondrial associated membrane (MAM), a specialised region between the mitochondria and the ER. The MAM is known to be enriched with presenilin proteins, involved in the proteolytic cleavage of amyloid precursor protein (APP). These data were used to generate an “ABAD-GRP75-MAM hypothesis of mitochondrial dysfunction in AD”, which might provide a novel link between chronic metabolic stress, ABAD, mitochondrial dysfunction and the onset / progression of AD. The third aim of the thesis was to test this novel hypothesis. Western blotting revealed APP to be significantly decreased following OGD, concurrent with an increase in ABAD protein levels. Over-expression of ABAD protein in SH-SY5Y cells was used to test whether the increased levels of ABAD following OGD were the driving force behind APP down-regulation. ABAD over-expression in SH-SY5Y cells was found to have no detectable effect on APP. Conversely, electron microscopy revealed a dynamic response of the MAM to metabolic stress. This result, along with the interaction of ABAD with GRP75, and the enrichment of presenilins at the MAM, suggests that this specialised membrane region may have an important role to play in AD.