Cyclic stretch of brain microvascular endothelial cells and regulation of amyloid processing and expression: evidence for contribution of vascular pulsatility in Alzheimer's disease

Alzheimer's disease (AD) is characterised by amyloid-β (Aβ) plaques arising from amyloid precursor protein (APP) processed by β secretase-1 (BACE-1). Increasing evidence suggests a role of vascular factors in AD, namely hypertension, elevated pulse pressure and arterial stiffness, all associated with increased vascular pulsatility imposing mechanical stretch on the endothelium. This thesis addresses the role of cyclic stretch on human cerebral microvascular endothelial cells (HCMECs) in expression and processing of APP. It also investigates effect of high salt diet on APP processing in rat brains, given associations of high salt diet and cognitive impairment. In vitro studies involved cultured HCMECs subjected to 0%, 5%, 10% or 15% stretch (18 hours, 1 Hz) and analysis of protein and RNA expression, nitric oxide and Aβ levels. In vivo study included treatment of rats with high (8% NaCl, HS) or a low (0.26% NaCl, control) for 10-13 weeks and examination of brain tissue. Established for the first time was that APP expression and Aβ secretion are altered in response to HCMECs stretch, and that this response is differentially mediated in early and late passage HCMECs. In late passage HCMECs, APP and BACE-1 expression increased 2-3-fold with 10 and 15% stretch compared to 0%, with proportional increases in Aβ42/Aβ40 with % stretch (R2=0.21). In early passage HCMECs stretched at 15%, APP expression, BACE-1, Aβ42 levels were decreased 2-3-fold compared to late passage HCMECs. Glycosphingolipid inhibition prior to cyclic stretching at 15% increased APP expression and Aβ42 secretion 1-fold. In vivo findings provide preliminary evidence of altered APP processing in HS rats compared to controls parallel with increases in markers of arterial stiffness. Overall results suggest a role of arterial stiffness and vascular pulsatility strengthening the evidence of vascular contributions to AD. Future studies identifying associated molecular mechanisms will provide novel therapeutic targets for AD.