BACE2 variant identified from HSCR patient causes AD-like phenotypes in hPSC-derived brain organoids

Juan Luo,Elly Sau-Wai Ngan,Ke Huang,Hailin Zou,Yibo Guo,Peng Li

doi:10.1038/s41420-022-00845-5

Abstract

β-site APP-cleaving enzyme 2 (BACE2) is a homolog of BACE1, which is considered as the most promising therapeutic target for Alzheimer’s disease (AD). However, the expression and functional role of BACE2 in central nervous system (CNS) remain obscured. Previously, we identified several BACE2 rare variants in Hirschsprung disease (HSCR) patients and proved that BACE2-mediated APP cleavage might represent a novel HSCR pathogenesis mechanism in enteric nervous system. Here, we validated that these HSCR-associated BACE2 variants were loss-of-function mutations. Using the human pluripotent stem cell (hPSC)-derived brain organoids (BOs), we further demonstrated that BACE2 was mainly expressed in the ventricular zone and cortical plate of BOs, and its expression level was gradually increased along with the BO maturation. Functionally, we found that the BOs carrying the BACE2 loss-of-function mutation (BACE2G446R) showed greater apoptosis and increased levels of Aβ oligomers compared to the control BOs, resembling with the AD-associated phenotypes. All these phenotypes could be rescued via the removal of APP protein in BACE2G446R BOs. Furthermore, rather than BACE2G446R, BACE2WT overexpression in BOs carrying the APP Swedish/Indiana mutations attenuated the AD-associated phenotypes, including Aβ accumulation and neuronal cell death. Taken together, our results unravel that BACE2 can protect the neuronal cell from apoptosis caused by Aβ accumulation, and the deficiency of BACE2-mediated APP cleavage may represent a common pathological mechanism for both HSCR and AD.

Highlights

AD is the most common neurodegenerative disease leading to dementia, and it is clinically manifested with progressive memory loss and cognitive dysfunction
In this study, we developed the hPSC-derived BOs and demonstrated the BOs with HSCR-associated BACE2 loss-of-function mutation exhibited AD-like phenotypes, including Aβ accumulation and neuronal cell death, which could be detected in BOs with familial APPSwe/Idl mutations, indicating that dysregulation of BACE2-mediated amyloid precursor protein (APP) cleavage represents a possible pathogenesis mechanism of AD (Fig. 5)
In our hPSC-derived BOs, we observed that BACE2 expression level was gradually increased following the maturation of organoids, and its expression was mainly detected in the ventricular zone and cortical plate

Summary

Introduction

AD is the most common neurodegenerative disease leading to dementia, and it is clinically manifested with progressive memory loss and cognitive dysfunction. Aβ peptide is generated from the sequential cleavage of amyloid precursor protein (APP) by β- and γ-secretases. The β-site cleavage of APP within the luminal domain releases a C-terminal fragment (C99), which can be further cleaved by γ-secretase to produce the N-terminal fragment Aβ [2, 3]. BACE1 was initially identified as a β-site APP-cleaving enzyme in vivo to generate Aβ, owing that genetic deletion of BACE1 in an AD mouse model has abolished the Aβ production [7, 8]. BACE2 could cleave the pro-proliferative plasma membrane protein TMEM27 in pancreas, and thereby impacting the β cell mass and functions [13, 14]. BACE2-mediated PMEL protein cleavage in pigment cells was essential for melanosome formation, and genetic deletion of BACE2 in mouse caused coat color defects [15]. Its impact on AD pathogenesis or therapy is still obscured

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