Characterisation of peptides from within the secreted amyloid precursor protein alpha as potential therapeutics in Alzheimer's disease

Abstract

Amyloid precursor protein (APP) is a transmembrane protein that is cleaved by specific proteases to produce a variety of peptides and larger protein products that contribute to the development, maintenance and plasticity of the brain. Mutations in the genes for APP and its processing enzymes result in changes to the available proportions of APP’s proteolytic products, and are associated with the onset of Alzheimer’s disease (AD). One of the protein products of APP, secreted APP-alpha (sAPPα), is neurotrophic and neuroprotective, while also facilitating hippocampal long-term potentiation (LTP). sAPPα production is downregulated in AD, and multiple AD therapies focusing on increasing endogenous expression or exogenous treatment with sAPPα have been the focus of several recent studies. At 612 amino acids, sAPPα is a relatively large protein with several post-translational modifications that make it challenging and costly to produce and deliver into the brain. As peptide-based therapies may offer a more viable therapeutic approach, the experiments in this thesis explored the effectiveness of small peptides derived from sAPPα in mimicking sAPPα’s effects in enhancing LTP and neurogenesis, key elements affected in the pathophysiology of AD. To address whether small peptide domains are sufficient to elicit some of sAPPα’s effects, a 16 amino acid peptide from the C-terminus region (16mer), and a tri-peptide (RER) from within sAPPα were both examined. The N terminal amino acid of each peptide was acetylated to enhance stability (indicated by the prefix ac-). The effects of the ac-16 mer and the ac-RER on neural progenitor cell (NPC) proliferation was measured by the size of neurospheres generated from peptide treated progenitor cells isolated from neonatal mouse tissue. Peptide treatment was administered at a range of concentrations and both peptides produced concentration-dependent increases in neurosphere sizes. The results of immunofluorescent analysis of differentiating NPCs identified a concentration-dependent increase in the proportion of neurons produced when treated with the peptides. The ability of the peptides to enhance the induction and persistence of LTP at Schaffer collateral/commissural synapses in area CA1 of rat hippocampal slices was then assessed using field potential recordings. Both the ac-16 mer and ac-RER produced concentration-dependent increases in both the induction and stabilisation of LTP. These effects were protein trafficking dependent, and protein synthesis dependent. Finally, the ac-16 mer was able to rescue impaired LTP in acute slices from transgenic APP/PS1 mice. The results of these studies indicate that small peptide sequences from sAPPα play key roles in the parent protein’s functionality, and may have potential as isolated peptide therapeutics for improving deficits in synaptic plasticity and neurotrophism associated with neurodegenerative diseases.