CNS Amyloid- , Soluble APP-  and -  Kinetics during BACE Inhibition

Abstract

BACE, a ␤-secretase, is an attractive potential disease-modifying therapeutic strategy for Alzheimer's disease (AD) as it results directly in the decrease of amyloid precursor protein (APP) processing through the ␤-secretase pathway and a lowering of CNS amyloid-␤ (A␤) levels. The interaction of the ␤-secretase and ␣-secretase pathway-mediated processing of APP in the rhesus monkey (nonhuman primate; NHP) CNS is not understood. We hypothesized that CNS inhibition of BACE would result in decreased newly generated A␤ and soluble APP␤ (sAPP␤), with increased newly generated sAPP␣.A stable isotope labeling kinetics experiment in NHPs was performed with a 13 C 6 -leucine infusion protocol to evaluate effects of BACE inhibition on CNS APP processing by measuring the kinetics of sAPP␣, sAPP␤, and A␤ in CSF. Each NHP received a low, medium, or high dose of MBI-5 (BACE inhibitor) or vehicle in a four-way crossover design. CSF sAPP␣, sAPP␤, and A␤ were measured by ELISA and newly incorporated label following immunoprecipitation and liquid chromatography-mass spectrometry. Concentrations, kinetics, and amount of newly generated APP fragments were calculated. sAPP␤ and sAPP␣ kinetics were similar, but both significantly slower than A␤. BACE inhibition resulted in decreased labeled sAPP␤ and A␤ in CSF, without observable changes in labeled CSF sAPP␣. ELISA concentrations of sAPP␤ and A␤ both decreased and sAPP␣ increased. sAPP␣ increased by ELISA, with no difference by labeled sAPP␣ kinetics indicating increases in product may be due to APP shunting from the ␤-secretase to the ␣-secretase pathway. These results provide a quantitative understanding of pharmacodynamic effects of BACE inhibition on NHP CNS, which can inform about target development.

Cellular Functions of the Amyloid Precursor Protein from Development to Dementia

“…Cutting of APP by BACE1 in endosomes releases sAPPb in the endosomal lumen, which would most likely target sAPPb for degradation in the lysosome. However, secreted sAPPb can be found in cerebrospinal fluid (CSF) (Dobrowolska et al, 2014) and can be secreted by neurons indicating that sAPPb might be additionally generated at the PM or sorted to the secretory trans-Golgi network (TGN). Alternatively, sAPPb could be exocytosed from lysosomes or sorted into recycling endosomes (Figure 3).…”

Systems Pharmacology Analysis of the Amyloid Cascade after -Secretase Inhibition Enables the Identification of an A 42 Oligomer Pool

“…To our knowledge, there are no quantitative data available from the literature on the ratio of APP moving down the a-secretase pathway and BACE1 pathway. Dobrowolska et al (2014b) compared sAPPa and sAPPb levels in human CSF from the lumbar region from cognitively normal and AD participants. They identified an sAPPb/sAPPa ratio of 0.59 6 0.4 (n 5 15) in cognitively normal healthy controls.…”

A Greek Tragedy: The Growing Complexity of Alzheimer Amyloid Precursor Protein Proteolysis

“…In monkeys, BACE1 inhibition decreased sAPP␤ and A␤ in cerebrospinal fluid (CSF) and increased sAPP␣ as measured by ELISA, but there was no change in labeled sAPP␣ kinetics, suggesting that BACE1 inhibition may not boost ␣-secretase processing of APP to the same degree (11). In human induced pluripotent stem cell-derived neurons, inhibition of BACE1 reduced sAPP␤ and A␤ and reciprocally increased sAPP␣ (12), whereas inhibition of ␥-secretase resulted in an increase in sAPP␣ and a decrease in sAPP␤ (9).…”