γ-Secretase Dependent Production of Intracellular Domains Is Reduced in Adult Compared to Embryonic Rat Brain Membranes

Background: gamma-Secretase is an intramembrane aspartyl protease whose cleavage of the amyloid precursor protein (APP) generates the amyloid beta-peptide (A beta) and the APP intracellular domain. A beta is widely believed to have a causative role in Alzheimer's disease pathogenesis, and therefore modulation of gamma-secretase activity has become a therapeutic goal. Besides APP, more than 50 substrates of gamma-secretase with different cellular functions during embryogenesis as well as adulthood have been revealed. Prior to gamma-secretase cleavage, substrates are ectodomain shedded, producing membrane bound C-terminal fragments (CTFs). Principal Findings: Here, we investigated gamma-secretase cleavage of five substrates; APP, Notch1, N-cadherin, ephrinB and p75 neurotrophin receptor (p75-NTR) in membranes isolated from embryonic, young or old adult rat brain by analyzing the release of the corresponding intracellular domains (ICDs) or A beta 40 by western blot analysis and ELISA respectively. The highest levels of all ICDs and A beta were produced by embryonic membranes. In adult rat brain only cleavage of APP and Notch1 could be detected and the A beta 40 and ICD production from these substrates was similar in young and old adult rat brain. The CTF levels of Notch1, N-cadherin, ephrinB and p75-NTR were also clearly decreased in the adult brain compared to embryonic brain, whereas the APP CTF levels were only slightly decreased. Conclusions: In summary our data suggests that gamma-secretase dependent ICD production is down-regulated in the adult brain compared to embryonic brain. In addition, the present approach may be useful for evaluating the specificity of gamma-secretase inhibitors.