Amyloid-β Protofibrils in Alzheimer´s Disease Focus on Antibodies, Inflammation and Astrocytes

Abstract: Soluble amyloid-beta (Aβ) aggregates, including Aβ protofibrils, play a central role in Alzheimer’s disease (AD) and constitute a potential diagnostic biomarker and a therapeutic target. Aβ protofibrils promote synapse dysfunction and neurodegeneration, but the mechanisms behind these effects remain unclear. The aim of this thesis was to increase the knowledge of Aβ protofibrils in AD pathology.When measuring low abundant antigens, such as soluble Aβ aggregates, in plasma and CSF by immunoassays, there is a possibility of interference by heterophilic antibodies (HA). In paper I, we show that HA generate false positive signals, by cross-binding the assay antibodies, when plasma and CSF from AD patients and healthy controls were analyzed for soluble Aβ aggregates, using sandwich ELISAs.Natural anti-Aβ antibodies exist in AD patients and healthy individuals. Circulating Aβ and anti-Aβ antibodies may form immune complexes, masking epitopes on the anti-Aβ antibody, which makes the anti-Aβ antibody concentration difficult to measure. In paper II, the ELISpot technique enabled us to successfully measure B cell production of anti-Aβ antibodies. Our results show that anti-Aβ protofibril antibody production is present in both AD patients and healthy individuals, but is significantly higher in AD patients, indicating that the immune system attempt to eliminate the toxic Aβ species.Insufficient lysosomal degradation is proposed to cause sporadic AD. In paper III, we used a co-culture system of astrocytes, neurons and oligodendrocytes, to clarify the role of astrocytes in Aβ protofibril clearance. Astrocytes are the most prominent glial cell type in the brain, but their role in AD remains elusive. We found that astrocytes effectively engulf, but inefficiently degrade Aβprotofibrils. This result in a high intracellular load of toxic, partly N-terminally truncated Aβ and lysosomal dysfunction. Moreover, we found that secretion of microvesicles, containing N-terminally truncated Aβ, induce neuronal apoptosis. In paper IV, we show that treatment with the protofibril selective antibody mAb158 lead to enhanced Aβ clearance and thereby prevent Aβ neurotoxicity.Taken together, this thesis contributes with important knowledge on the role of Aβ protofibrils in AD pathogenesis and technical aspects that should be considered when measuring Aβ in human tissues.