Investigating nutrient excess and endothelial dysfunction
: a role for BACE1 and amyloid?

  • Bethany Mae Coull

    Student thesis: Doctoral ThesisDoctor of Philosophy

    Abstract

    Background: Endothelial dysfunction (ED) often occurs secondary to metabolic pathologies associated with nutrient excess, such as hyperglycaemia and dyslipidaemia. ED, characterised by reduced nitric oxide (NO) bioavailability and activity, and inflammation, is associated with cardiovascular diseases, stroke, and neurodegeneration.

    Nutrient excess increases amyloid beta peptides (Aβ), which are produced in varying lengths by beta-site APP cleaving enzyme 1 (BACE1). Increased circulating Aβ42 levels is reported to promote ED, by reducing NO bioavailability, suggesting a role for Aβ in the pathogenesis of ED, but the underlying mechanism is unclear. 

    One potential mechanism involves mitochondrial dysfunction, whereby mitochondria exhibit impaired bioenergetics, increased reactive oxygen species (ROS) production, and dysregulated mitochondrial dynamics. Functional mitochondria are key to endothelial cell health and impairments may lead to reduced ATP production and increased ROS generation, resulting in a decrease in NO bioavailability and activity.

    Furthermore, nutrient excess and Aβ peptides are reported to induce mitochondrial dysfunction.
    Therefore, the work described herein sought to determine whether the early pathogenesis of ED, promoted by nutrient excess, is in part due to raised BACE1 activity and circulating Aβ levels. Further, it aimed to investigate the potential mechanistic role of mitochondrial dysfunction in this process.

    Methods: Microvascular endothelial function was measured in vivo using laser Doppler imaging in several mouse models of nutrient excess and varying circulating Aβ levels. A 6-month old 10-week high-fat diet (HFD)-fed mouse (45% HFD) and an 8-week old db/db mouse were used as models of nutrient excess. A 10-12-week old, 10-week HFD-fed BACE1 knockout (BACE1KO) mouse was used to investigate the role of BACE1 and Aβ in the pathogenesis of ED. Two 10-12 week old, 10-week HFD-fed cohorts of mice, infused either centrally or peripherally with Aβ42 peptides (3.36mg/kg/day) for 4 weeks were investigated as models of raised circulating Aβ levels. Finally, the APP23 mouse, a genetically engineered model of increased Aβ, was infused peripherally with a BACE1 inhibitor (10mg/kg/day) for 4 weeks to investigate the effects of reducing Aβ Peptides on endothelial function. Following in vivo investigation, proteins involved in mitochondrial dynamics were analysed using Western blot in tissues of interest (vessel enriched brain fraction, aorta) as a measure of mitochondrial function.

    Results: Impaired endothelial function was observed in vivo in db/db mice, but not in the HFD-fed model. BACE1KO mice had improved endothelial function compared to wild-type and heterozygous littermates. Both cohorts infused with Aβ, as well as the APP23 mouse, exhibited reduced endothelial function, whilst treating the APP23 mouse with a BACE1 inhibitor and thereby reducing Aβ levels rescued the endothelial dysfunction phenotype. No significant findings were observed in relation to mitochondrial dynamics proteins in any of the models studied.

    Conclusion: The data presented herein suggest a role for increased BACE1 activity and circulating Aβ, due to nutrient excess, in the early pathogenesis of ED. However, it is unlikely that this process involves altered mitochondrial dynamics processes. These studies highlight the possibility of repurposing BACE1 inhibitors for the treatment of ED and subsequent vascular diseases.
    Date of Award2020
    Original languageEnglish
    SponsorsAlzheimer's Society
    SupervisorMichael Ashford (Supervisor) & Calum Sutherland (Supervisor)

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