Endogenous gamma-secretase: Activity, substrate selectivity, and subunit composition

gamma-Secretase is a multi-subunit membrane bound aspartyl protease which plays a pivotal role in the pathogenesis of Alzheimer disease (AD). The study of rare forms of familial AD (FAD) mutations suggest that aberrant gamma-secretase processing of the amyloid precursor protein (APP) leads to the production and aggregation of toxic Abeta peptides resulting in neuronal cell death and AD. In addition to APP, gamma-secretase is known to process additional substrates which have limited sequence homology. However, the mechanism by which gamma-secretase achieves substrate regulation and whether or not gamma-secretase complexes located in discrete tissues and cell populations exhibit unique activity is unclear.;We have developed a small molecule affinity probe methodology to selectively study the relationship between subunit composition and activity of catalytically active endogenous gamma-secretase. Using this approach we characterized endogenous gamma-secretase from diverse cellular sources and in normal aging brain. gamma-Secretase derived from hematopoetic origins is distinct from classically studied epithelial derived gamma-secretase; hematopoetic gamma-secretase exhibits lower in vitro activity and has an atypical subunit stoichiometry. Significantly, we demonstrated that hematopoetic gamma-secretase has a greatly altered PS1-NTF:PS1-CTF ratio in active complexes and a subset of these complexes can be pharmacologically distinguished. gamma-Secretase isolated from mouse brain exhibits age and gender dependent regulation. Male mice exhibit reduced APP and Notch1 processing in aged brain whereas female mice exhibit increased APP but decreased Notch1 processing. Significantly, both male and female mice exhibit increased Abeta42:Abeta40 ratios in aged brain suggesting a possible role for altered gamma-secretase cleavage specificity in the development of sporadic AD. Furthermore, the activity difference between male and female APP processing in aged brain may partially explain the gender bias seen in the development of sporadic AD in the human population.;Our novel findings reveal that discrete gamma-secretase complexes exist and their formation is independently regulated by factors such as age, gender, and cell lineage. Significantly, these complexes exhibit unique activity profiles, substrate selectivity, and subunit stoichiometry thereby providing a plausible explanation of how a seemingly homogenous enzyme is able to process a multitude of diverse substrates and affect many physiological processes.