P1-23 “Studying the roles of the OST subunits in Alzheimer's disease”

Abstract

In eukaryotic cells, N-glycosylation is typically the most common protein modification that occurs in the ER lumen. N-glycosylation is facilitated by a large heterologous complex called the oligosaccharyltransferase (OST) which allows the attachment of a high mannose oligosaccharide from a dolichol pyrophosphate donor en bloc onto a free asparagine residue of a newly synthesised nascent chain during the translocation in the ER lumen. Defects in the process of N-glycosylation are often associated with a variety of diseases such as congenital disorders of glycosylation and Non-syndromic mental retardation. The focus of this work was based on our previous findings that some of the subunits of the OST may have alternative functions and play a role in the processing pathway of amyloid precursor protein (APP). We use a combination of methodologies developed during the project including RNA interference, Western blotting, and a fluorescent tripeptide assay to tackle these questions. Recent investigations reveal that depletion of some of the OST subunits, can affect APP processing leading to a reduction in both C-terminal fragments and full length N-glycosylated APP. The site of cleavage is of clinical relevance since Aβ peptides (Aβ 40-42) derived from γ-secretase cleavage of APP are major components of the amyloid plaques that are a characteristic brain lesion of individuals with Alzheimer's disease. Globally, depletion of some of the OST subunits has no effect upon N-glycosylation. The primary goal of this project is to establish if depleting these subunits would have any implication for Alzheimer's disease. We show for the first time that overexpression of some of the OST subunits causes an increase in the active γ-secretase complex particularly the N-terminal fragment of PS1 that is generated by endoproteolysis. The results from this study will enable us to understand better how the mammalian OST machinery attaches N-linked glycans to proteins and to understand the alternative roles of the OST subunits in Neurological diseases such as Alzheimer's disease and NSMR.