Amyloid-β precursor protein mediates neuronal toxicity of amyloid β through Go protein activation

Abstract

Amyloid beta (Aβ) is a metabolic product of amyloid-β precursor protein (APP). Deposition of Aβ in the brain and neuronal degeneration are characteristic hallmarks of Alzheimer's disease (AD). Aβ induces neuronal degeneration, but the mechanism of neurotoxicity remains elusive. Here we show that overexpression of APP renders hippocampal neurons vulnerable to Aβ toxicity. Deletion of the extracellular Aβ sequence of APP prevents binding of APP to Aβ, and abolishes toxicity. Aβ toxicity is also abrogated by deletion of the cytoplasmic domain of APP, or by deletions comprising the Go protein-binding sequence of APP. Treatment with Pertussis toxin (PTX) abrogates APP-dependent toxicity of Aβ. Overexpression of PTX-insensitive Gα-o subunit, but not Gα-i subunit, of G protein restores Aβ toxicity in the presence of PTX, and this requires the integrity of APP-binding site for Go protein. Altogether, these experiments indicate that interaction of APP with toxic Aβ-species promotes toxicity in hippocampal neurons by a mechanism that involves APP-mediated Go protein activation, revealing an Aβ-receptor-like function of APP directly implicated in neuronal degeneration in AD.

Introduction

Alzheimer's disease (AD) is the most frequent cause of memory loss and dementia in the elderly human population. The abundance of senile plaques and neuronal degeneration are characteristic features of AD histopathology. Senile plaques are complex lesions mainly composed of aggregated amyloid-β (Aβ) protein, and typically surrounded by dystrophic neurites. Aggregation confers toxicity to Aβ (Pike et al., 1991, Busciglio et al., 1992), suggesting its direct involvement in AD neurodegeneration. The mechanism of Aβ toxicity is not completely understood. Recent evidence suggests that different Aβ-aggregated species, such as Aβ-fibrils or Aβ-oligomers induce toxicity through distinct mechanisms (Deshpande et al., 2006). Toxicity of Aβ fibrils (fAβ), the most conspicuous species of Aβ in AD brain, appears to require the interaction of the fibrils with cell-surface receptors, resulting in altered modulation of signal transduction pathways. Consistent with this possibility it was shown that fAβ abnormally modulates the activity of focal adhesion proteins (Grace and Busciglio, 2003), actin-binding proteins (Heredia et al., 2006, Mendoza-Naranjo et al., 2007) and tau (Busciglio et al., 1995), indicating that fAβ may alter signaling-cascades resulting in neuronal degeneration around senile plaques.

Aβ is a metabolic product of the amyloid-β precursor protein (APP), and familial forms of AD can arise from mutations in APP or the Presenilins (Selkoe, 2001). These mutations alter APP-processing and the generation of Aβ predisposing to Aβ aggregation, which implicates an altered APP processing in the pathogenesis of familial AD (Hardy and Selkoe, 2002). More recently, it was found that increased gene dosage of wild-type APP is sufficient to cause early onset AD (Rovelet-Lecrux et al., 2006), indicating that a slight elevation in APP-expression predisposes to AD-neurodegeneration. The protein structure and cellular functions of APP are consistent with its potential role as a cell-surface receptor implicated in cell adhesion, although a natural ligand for APP remains to be established (for a review see Zheng and Koo, 2006). Importantly, APP binds fAβ and modulates their toxicity (Lorenzo et al., 2000, Van Nostrand et al., 2002), suggesting that fAβ may be a pathological ligand for APP. Moreover, deposition of fAβ promotes cell-surface accumulation of APP (Heredia et al., 2004), in a way that resembles clustering of cell-adhesion receptors upon binding to their extracellular ligands. However, it is unclear whether APP may directly mediate signaling events required for fAβ toxicity. In this work, we show that interaction of fAβ with the ectodomain of APP promotes APP-dependent activation of heterotrimeric Go protein, resulting in degeneration of hippocampal neurons. Taken together these results reveal that, in addition of being the source of Aβ, APP might function as a receptor for fAβ that may be directly implicated in neuronal degeneration in AD.