Human pyroglutamyl peptidases and their involvement in Alzheimer’s disease

Abstract

The N-terminally pyroglutamyl-modified β-amyloid (Aβ) peptides are found in abundance in the pathological Alzheimer disease (AD) brain deposits. Such modification not only increases the hydrophobic properties of a given molecule, but also plays a protecting role against proteolytic degradation. This project involved the study of the human type I and type II pyroglutamyl peptidases and their involvement in Aβ processing in AD. Human PcpI has been successfully overexpressed in Escherichia coli strain and purified to homogeneity. The protein displayed significant instability in vitro. To overcome this problem a number of methods were employed such as screening for an optimal protein expression system and buffer composition, site-directed mutagenesis and chemical modification of selected surface residues. This resulted in the selection of the HEPPS buffer system as providing the most stabilising conditions for human PcpI. Improvement in the protein stability enabled initial crystallisation experiments and the identification of favourable conditions for crystal production. Further optimization of this process is needed in order to obtain good quality crystals which are required for structural study. The study on human PcpII involved an extensive screening for optimal expression conditions in bacterial, baculovirus/insect and mammalian systems. The truncated PcpII isoform PcpII/S62-H1024, which lacks the N-terminal transmembrane domain, was successfully expressed and secreted from the HEK 293T cell line using three different pOPIN-based constructs. Moreover, homology modelling of human PcpII catalytic domain was performed, which helped to gain an insight into the three-dimensional structure of the protein and its mode of substrate binding. Lastly, immunohistochemical staining of the human AD brain tissue sections was performed to compare the level and distribution of PcpI and PcpII enzymes between diseased and control cases. The results confirmed that the neurodegenerative conditions lead to the increased synthesis of both enzymes in the cortical AD tissues. Additionally PcpI was shown to be able to participate in the degradation of pGlu-modified Aβ peptides.