Cross-Linking of Cell Surface Amyloid Precursor Protein Leads to Increased  -Amyloid Peptide Production in Hippocampal Neurons: Implications for Alzheimer's Disease

Abstract

The accumulation of the β-amyloid peptide (Aβ) in Alzheimer’s disease (AD) is thought to play a causative role in triggering synaptic dysfunction in neurons leading to their eventual demise through apoptosis. Aβ is produced and secreted upon sequential cleavage of the amyloid precursor protein (APP) by β- and γ-secretases. However, while Aβ levels have been shown to be increased in AD patients’ brains, little is known about how the cleavage of APP and the subsequent generation of Aβ is influenced, or if the cleavage process changes over time. It has been proposed that Aβ can bind APP and promote amyloidogenic processing of APP, further enhancing Aβ production. This idea has remained controversial due to a lack of a clear mechanism and complicated by the promiscuous nature of Aβ binding. To work around this problem, we used an antibody-mediated approach to bind and cross-link cell surface APP in cultured rat primary hippocampal neurons. Here we show that cross-linking of APP is sufficient to raise the levels of Aβ in viable neurons with a concomitant increase in the levels of the β-secretase BACE1. This appears to occur as a result of a sorting defect, due to the caspase-3-mediated inactivation of a key sorting adaptor protein, namely GGA3, which prevents the lysosomal degradation of BACE1. Taken together, our data suggest the occurrence of a positive pathogenic feedback loop involving Aβ and APP in affected neurons possibly allowing Aβ to spread to nearby healthy neurons.

Amyloid beta1-42 peptide regulates microtubule stability independently of tau

“…Treatment of primary cortical neurons with Ab results in FAK activation (Williamson et al, 2002), and aberrant activation of focal adhesion proteins mediates Ab-induced neuronal dystrophy (Grace et al, 2002;Williamson et al, 2002). In addition, APP, a potential receptor for Ab (Lorenzo et al, 2000), is found at focal contacts (Sabo et al, 2001); it colocalizes with b1 integrins in neuronal cells (Yamazaki et al, 1997), and antibody-mediated clustering of APP induces significant loss of dendritic spines (Lefort et al, 2012) and neuronal injury through FAK activation (Xu et al, 2009). We found that clustering of APP was sufficient to induce Glu MTs and both APP and caspase-2 expression were required for the formation of stable Glu MTs by Ab, suggesting that induction of MT stability by Ab is initiated by the same APP/ caspase-2/RhoA pathway that leads to neurotoxicity by Ab in neurons (Lefort et al, 2012;Pozueta et al, 2013;Troy et al, 2000).…”

“…In addition, APP, a potential receptor for Ab (Lorenzo et al, 2000), is found at focal contacts (Sabo et al, 2001); it colocalizes with b1 integrins in neuronal cells (Yamazaki et al, 1997), and antibody-mediated clustering of APP induces significant loss of dendritic spines (Lefort et al, 2012) and neuronal injury through FAK activation (Xu et al, 2009). We found that clustering of APP was sufficient to induce Glu MTs and both APP and caspase-2 expression were required for the formation of stable Glu MTs by Ab, suggesting that induction of MT stability by Ab is initiated by the same APP/ caspase-2/RhoA pathway that leads to neurotoxicity by Ab in neurons (Lefort et al, 2012;Pozueta et al, 2013;Troy et al, 2000). Caspases have been already implicated in the induction of apoptosis, synaptic pruning and inhibition of LTP by Ab, and caspase activity is modulated by exposures to Ab (D'Amelio et al, 2011;Hyman, 2011;Jo et al, 2011;Klaiman et al, 2008;Nikolaev et al, 2009;Pozueta et al, 2013;Troy et al, 2000).…”

“…4A-C). We tested whether APP signaling was sufficient to induce Glu MTs and stimulated APP dimerization by treating serum-starved cells with a cross-linking APP antibody (22C11) (Lefort et al, 2012) (Fig. 4D,E).…”

“…Alterations in actin dynamics is generally considered the mechanism by which Rho contributes to the collapse of dendritic spines and impairs synaptic transmission, all early events occurring in neurons exposed to Ab (Lambert et al, 1998;Lefort et al, 2012;Lesné et al, 2006;Nimmrich and Ebert, 2009;Shankar et al, 2007). However, it is not known whether neurotoxicity might also derive from changes in MT dynamics and/or MT post-translational modifications induced by Rho activation through the action on its effector mDia1 (Goulimari et al, 2005;Palazzo et al, 2001a).…”

“…In neurons, APP and caspase-2 can regulate RhoA activation, and cells lacking either APP or caspase-2 are completely immune to Ab synaptotoxicity (Pozueta et al, 2013;Troy et al, 2000). In addition, dimerization of cellsurface APP by cross-linking cell-surface APP with a divalent antibody is sufficient to mimic the toxic effects of Ab in neurons (Lefort et al, 2012;Rohn et al, 2000). Interestingly, activation of RhoA in neuronal cells by lysophosphatidic acid (LPA) is also regulated by caspase-2 activity, suggesting that Ab and LPA share pathways (Pozueta et al, 2013).…”