Distinct functions of BACE1 and BACE2 in neuregulin processing and melanocyte migration

Abstract

mechanism known as the unfolded protein response (UPR). When unfolded proteins accumulate in the endoplasmic reticulum (ER), UPR aims to restore cellular homeostasis. However, if stress is prolonged, it induces cell death and, thus, neurodegeneration. We investigated the order of events and the impact of UPR activation in Alzheimer’s disease (AD) model systems. Specifically, we developed an in vitro model system of human neuroblastoma cells (SK-N-SH) that overexpress wild type (WT) amyloid-b precursor protein (APP) or familial AD (FAD)-associated mutants, Swedish (S) and Swedish/Indiana (S/I) forms.Methods:We ’artificially’ induced ER stress in differentiated cells using tunicamycin at eight time points and assessed their viability. Subsequently, Real time PCR (RT-PCR) was used to analyse the gene expression levels of eight UPR markers downstream of all three receptors (IRE1, PERK, and ATF6). This allowed us to examine which ER stress pathways are activated and when in the three model systems. Results: Our data demonstrate that all cells activate stress post tunicamycin treatment. However, only the WT cells recover. More specifically, WT cells show stress activation in terms of ATF4 at three hours. ATF4 is part of the PERK pathway and functions by promoting translational attenuation, which is essential for cell survival. In mutant cells this activation occurs at four hours which we hypothesise that is too late for the UPR mechanism to promote cell survival and the cells are possibly already fated to apoptosis. We also have evidence that the S mutation is more toxic than the S/I orWTAPP overexpression. S cells exhibit higher levels of stress as seen from the mRNA levels of their respective markers and appear to be more susceptible to apoptosis. Conclusions: Overall, there is strong evidence that UPR is involved in the progression of AD. The data support the hypothesis that an initial activation of the UPR in AD neurons might work as neuroprotective to restore homeostasis while its sustained activation could induce neurodegeneration. Future studies need to address the therapeutic opportunities of this pathway for the treatment of AD.