Regulation and developmental roles of amyloid precursor protein family members: using zebrafish as a model system

Abstract

The amyloid precursor protein (APP) family, consisting of APP and its homologues APLP1 and APLP2, plays important roles during development. Although APP has been extensively studied due to its involvement in Alzheimer’s disease (AD), the regulatory mechanisms governing APP family expression and the physiological functions of APLP proteins during development remain incompletely understood. In paper I, we first demonstrated that mutations introducing premature termination codons (PTCs) in the zebrafish appb gene induce transcriptional adaptation (TA), resulting in upregulation of appa and aplp2, independently of Appb protein levels. We also showed that TA is activated through nonsense-mediated mRNA decay (NMD) and highlighted the role of upf1 and upf2 in fine-tuning App family members. In paper II, we generated appa and appb transgenic zebrafish models using CRISPR/Cas9-mediated targeted knock-in strategy carrying, a NanoLuciferase (NLuc) reporter upstream of the start codon to enable sensitive in vivo quantification of App expression. We then validated the NLuc model by treating larvae with drugs known to reduce App protein levels in other species, including cycloheximide, phenserine, and buntanetap, and observed similar responses in zebrafish. Finally, in paper III, we used aplp1 and aplp2 knock-in and knockout models to investigate the expression pattern and functional roles of these two genes during development. Using endogenous fluorescent reporters and loss-of-function mutants, we found out that Aplp1 and Aplp2 together perform essential and distinct developmental functions during vertebrate sensory organ and lateral line formation. In conclusion, this thesis provides insights into App family regulation and the roles of Aplp1 and Aplp2 on the lateral line formation during development, in addition to introducing a zebrafish-based model suitable for high-throughput screening of App modulators.