Our laboratory is focused on understanding the molecular biology of membrane transport in a disease-related context covering Alzheimer’s disease and congenital disorders of glycosylation type II.
Related to Alzheimer’s, APP cleavage by gamma-secretase leads to amyloid beta peptide production, one possible cause of Alzheimer’s symptoms. gamma-secretase is composed of four proteins – presenilin, nicastrin, PEN-2 and APH-1 – which must come together for cleavage activity. Starting from the idea that Alzheimer’s disease might be slowed by inhibiting g-
secretase, we have now identified an endogenous inhibitor that prevents gamma-secretase complex assembly and activity and thus might be targeted for therapy (Spasic et al., 2007). Although all four components are present in the ER, their assembly into functional gamma-secretase is somehow restricted. Assembly begins with the binding of nicastrin to APH-1. This binding is competed early in the secretion pathway by Rer1p, a membrane receptor that retrieves proteins from the Golgi back to the ER. Rer1p binds to nicastrin, thus interfering with nicastrin’s ability to bind APH-1. Decreasing the amount of Rer1p led to an increase in gamma-secretase activity. Exactly what triggers Rer1p to release nicastrin and allow it to bind to APH-1, and subsequently to the other gamma-secretase components, remains to be determined. Preventing this release might provide a means to reduce gamma-secretase activity and thus amyloid plaque formation.
Sporadic Alzheimer’s disease is also characterized by dysfunctions in the neuronal endo-lysosomal system. In this context, we hypothesize that the differential localization of distinct gamma-secretase complexes and substrates at the cell surface and endosomes is part of the mechanism defining specificity in intramembrane proteolysis. We are currently applying high-resolution confocal microscopy to resolve this microdomain association of APP and gamma-secretase components. In addition, we use superparamagnetic nanoparticles for neuronal endosomal targeting. Magnetic isolation followed by ‘organellar proteomics’ will generate proteome inventories of axonal and dendritic endosomal carriers and identify novel early biomarkers for endosomal dysfunction in Alzheimer’s disease and beyond. In a related line of research, we are screening a 10K small-compound library for their effects on inducing axonal and dendritic outgrowth/arborization using a medium-to-high throughput InCell1000 system.
In CDG-II, we identified mutations in sub-units of the COG complex, which are now being used to study the role of this complex in maintaining Golgi-integrity. In a collaborative effort, we also identified patients with mutations in a sub-unit of the V-ATPase linking pH-dependency to proper glycosylation.