The interaction of neutrophils and amyloid-ß in Alzheimer’s disease

Abstract

Alzheimer’s disease (AD) is a progressive neurodegenerative disorder that affects memory and cognition. A key molecular pathological hallmark of AD is amyloid-ß deposition. Amyloid-ß is a small aggregation-prone peptide that forms the senile plaques characteristic of AD. Although amyloid-ß has long been a focus of AD research, clinical trials targeting the aggregation and deposition of the peptide have been largely unsuccessful. The shift away from amyloid-ß focused research and advancements in genetic profiling have enabled the discovery that many of the genes affected in sporadic AD are related to immunity. Neuroinflammation is now accepted as another of the key pathological hallmarks of AD; however, the exact mechanisms of how it contributes to disease onset are still undetermined.

Recent literature has shown that peripheral innate immune cells, such as neutrophils, contribute to AD pathology. Neutrophils invade the central nervous system in AD and undergo an immune response called NETosis, where they form networks of extracellular fibres, composed of DNA decorated with granule proteins. The formation of these neutrophil extracellular traps (NETs) has been previously shown to occur around amyloid-ß plaques. The precise interaction between neutrophils and the amyloid-ß deposits is unknown, but NETs may modify the immunological and biochemical properties of the plaques.

Thioflavin T fluorescent assays and electron microscopy were used to characterise and validate four different multimerization states of amyloid-ß42. These represent the different species found in the AD brain. Neutrophils were isolated from human blood and NETosis was induced using phorbol myristate acetate (PMA). Using a SYTOX™ green assay to measure NETosis, it was observed that amyloid-ß42 does not induce NET formation in neutrophils, irrespective of aggregation state or the presence of complement factors in serum or anti-amyloid-ß antibodies. Using immunocytochemistry and fluorescent microscopy, the binding of amyloid-ß42 to NETs was observed in an in vitro system. When micrococcal nuclease was used to degrade the DNA structure of NETs, a reduction was found in the binding of heavily aggregated amyloid-ß42 species to NETs. This reduction indicated that the binding between heavily aggregated amyloid-ß42 and NETs is likely mediated by the NET-DNA matrix. Microglia are the resident immune cells of the brain and were seen to interact with both NETs and all species of amyloid-ß42. This interaction could indicate that NET-plaque complexes are involved in initiating an immune response and contribute to the neuroinflammation seen in the AD brain.

Binding with amyloid-ß plaques could enable the persistence of NETs in the AD brain. Furthermore, NETs changing the plaque structural or chemical properties could potentially contribute to the chronic inflammation seen in AD patients. Therapeutics focused on inhibiting or depleting neutrophil entry into the central nervous system, or preventing the formation of NETs, could be beneficial in the treatment of AD.