| Summary: | The molecular pathology of Alzheimer’s disease (AD) remains unclear, though it is widely acknowledged that AD is triggered by the accumulation of a small peptide, Amyloid-β (Aβ) in the brain, typically 40 to 42 amino acids in length. Specifically, Aβ oligomers, can disrupt membrane homeostasis and are often identified as the primary cytotoxic entities in AD. Despite Aβ42 being less abundant than Aβ40, it has greater amyloid forming properties and cytotoxic. One hypothesis suggests that wild-type Aβ42 oligomers can form ion channel pores in the plasma membrane and lead to loss of cellular homeostasis. In this thesis, a snapshot of Aβ42 prefibrillar assemblies is studied using cryo-electron tomography and cryo-electron microscopy single particle analysis (Chapter 3). The images indicate that curvilinear protofibrils and smaller oligomers are a continuum of the same assembly with an identical 28 Å cross-section diameter. The dimensions of these structures suggest they laterally associate to form amyloid fibrils. The 3D reconstruction of annular assemblies, reveal a ring-shaped structure with a diameter of 65 Å and length 54 Å; capable of spanning the membrane. While the central internal channel is ca. 14 Å in diameter. The internal diameter of the channel closely matches predicted channel dimensions from ion-channel conductance measurements. Further patch-clamp conductance measurements are performed for prefibrillar assemblies of a Aβ mutant isoforms associated with familiar early-on-set AD (Chapter 4). The Arctic (E22G) mutant oligomers exhibited behaviour distinct from wild-type Aβ oligomers. Aβ40(WT) failed to form ion channels, while Aβ40(Arctic) showed a strong propensity to form amyloid channels across cellular plasma membranes. The inner pore size, determined from the observed conductance for Aβ40(Arctic), Aβ42(Arctic), and Aβ42(WT), was consistent with the cryo-EM 3D reconstruction of annular assemblies. Point mutations with Aβ peptide are heterozygous and as such both wild-type and Aβ familiar mutans are ...
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