APP depletion alters selective pre- and post-synaptic proteins

Isak Martinsson,Estibaliz Capetillo-Zarate,Mathilde Faideau,Katarina Willén,Noemi Esteras,Susanne Frykman,Lars O Tjernberg,Gunnar K Gouras

doi:10.1016/j.mcn.2019.02.003

Abstract

The normal role of Alzheimer's disease (AD)-linked amyloid precursor protein (APP) in the brain remains incompletely understood. Previous studies have reported that lack of APP has detrimental effects on spines and electrophysiological parameters. APP has been described to be important in synaptic pruning during development. The effect of APP knockout on mature synapses is complicated by this role in development. We previously reported on differential changes in synaptic proteins and receptors in APP mutant AD transgenic compared to wild-type neurons, which revealed selective decreases in levels of pre- and post-synaptic proteins, including of surface glutamate receptors. In the present study, we undertook a similar analysis of synaptic composition but now in APP knockout compared to wild-type mouse neurons. Here we demonstrate alterations in levels of selective pre- and post-synaptic proteins and receptors in APP knockout compared to wild-type mouse primary neurons in culture and brains of mice in youth and adulthood. Remarkably, we demonstrate selective increases in levels of synaptic proteins, such as GluA1, in neurons with APP knockout and with RNAi knockdown, which tended to be opposite to the reductions seen in AD transgenic APP mutant compared to wild-type neurons. These data reinforce that APP is important for the normal composition of synapses.

Highlights

Despite nearly three decades since the formulation of the amyloid cascade hypothesis of Alzheimer's disease (AD) the normal role of the βamyloid precursor protein (APP) and its various metabolites remain poorly understood
We previously reported on differential alterations in synaptic composition in cultured neurons from Tg2576 AD transgenic compared to wild-type mice (Almeida et al, 2005), where decreased levels in GluA1 and post-synaptic density-95 (PSD-95) were noted in the AD transgenic neurons
Since there are some discrepancies in the literature, we characterized our cortical cultures of APP knockout (APP KO) and wild-type (WT) neurons at 12 and 19 days in vitro (DIV)

Summary

Introduction

Despite nearly three decades since the formulation of the amyloid cascade hypothesis of Alzheimer's disease (AD) the normal role of the βamyloid precursor protein (APP) and its various metabolites remain poorly understood. Spine density was described as decreased in APP KO compared to wild-type mouse brains (Lee et al, 2010; Tyan et al, 2012), and in vitro knockdown of APP was shown to reduce spine density in rat hippocampal neurons in culture (Lee et al, 2010). Contrary to these findings, it was reported that cultured hippocampal knockout neurons showed increased synaptophysin puncta and longer dendrites compared to their wild-type counterparts (Priller et al, 2006); a subsequent study by the same group further reported increased spine density in cortical layers 3 and 5 of 4–6 months old APP KO mouse brains (Bittner et al, 2009), a discrepancy reviewed by Jung & Herms (Jung and Herms, 2012)

Methods

Results

Conclusion