BS53 The role of BACE1 in metabolic dysfunction and cerebrovascular disease

Beta secretase 1 (BACE1) is a well-established causative molecule in Alzheimer's Disease where it's cleavage of the amyloid precursor protein (APP) produces amyloid-beta peptides, which can accumulate into plaques. BACE1 has also been linked to metabolic dysfunction and cardiovascular disease (CVD), where its role is less defined. Significantly, numerous clinical trials have shown that inhibiting BACE1 is safe, offering the potential for drug-repurposing. The aim of this project is to define the role of BACE1 in the vascular dysfunction frequently observed in metabolic dysfunction and dementia. A combination of methods was used to define the role of BACE1 in vascular dysfunction, including; (1) datamining and bioinformatic identification of novel BACE1 substrates, including those differentially expressed in disease; (2) RNA-sequencing of hCMEC/d3 brain endothelial cells overexpressing BACE1; (3) a UK Biobank analysis of single nucleotide polymorphisms (SNP) of BACE1 associated with plasma markers of BACE1 activity, including amyloid-beta 40 and 42, physiological measures of vascular function and incidence of disease. Datamining and bioinformatics analysis identified 533 BACE1 regulated proteins, of which, 120 were predicted as substrates. Of this list, 26 BACE1 dependent proteins were differentially expressed in vascular dementia, including the L1 family of cell adhesion molecules. Comparison with genes differentially expressed in endothelial cells in Alzheimer's Disease, identified seven members of the protein tyrosine phosphatase receptor (PTPR) family as novel BACE1 substrates, including PTPRD and PTPRK. Experimental validation in BACE1 knockout primary isolated brain endothelial cells observed a significant 1.95 fold increase (p<0.05) in PTPRD expression when compared against wildtype cells. In hCMEC/D3 cells treated with a BACE1 inhibitor (M3), a 1.31 fold increase in expression of PTPRD was observed (p<0.05). RNA-sequencing analysis identified associations with 'cytokine-cytokine receptor interactions' (p<0.01), 'insulin resistance' (p<0.05), 'lipids and atherosclerosis' (p<0.05) and 'protein tyrosine/threonine activity' (p<0.05). It also identified MAF BZIP Transcription Factor F (MAFF) as a novel potential regulator of BACE1 activity, as well as associations with the inhibitor of DNA binding (ID) family. UK Biobank analysis of 18 BACE1 SNPs identified associations with plasma proteins and functional associations including; lipoproteins, leukocyte migration, negative regulation of gluconeogenesis and cellular response to insulin stimulus and diagnosis of diabetes (p<0.05). Of these, three as yet uncharacterised BACE1 SNPs were associated with significant differences in plasma amyloid beta 40 and/or 42 (p<0.05).and several validated and predicted substrates, including SEZ6L and L1CAM (p<0.05). This analysis shows BACE1 may be an important enzyme in the vascular dysfunction observed with metabolic dysfunction and in cerebrovascular contributions to dementia. It offers insights into novel substrates and functional associations of BACE1, offering a deeper understanding into the functionality of BACE1, for which the potential for drug-repurposing exists. Conflict of Interest N/A