A model for studying Alzheimer's Aβ42-induced toxicity in Drosophila melanogaster

Abstract

Alzheimer's disease is a neurological disorder resulting in the degeneration and death of brain neurons controlling memory, cognition and behavior. Although overproduction of Aβ peptides is widely considered a causative event in the disease, the mechanisms by which Aβ peptides cause neurodegeneration and the processes of Aβ clearance and degradation remain unclear. To address these issues, we have expressed the Aβ peptides in Drosophila melanogaster. We show that overexpression of Aβ₄₂ peptides in the nervous system results in phenotypes associated with neuronal degeneration in a dose- and age-dependent manner. We further show that a mutation in a Drosophila neprilysin gene suppresses the Aβ₄₂ phenotypes by lowering the levels of the Aβ₄₂ peptide, supporting the role of neprilysin in the catabolism of Aβ peptides in vivo. We propose that our Drosophila model is suitable for the study and elucidation of Aβ metabolism and toxicity at the genetic level.

Introduction

Alzheimer's disease (AD), the most prevalent form of senile dementia in humans, is diagnosed by the presence of neuritic plaques, composed mainly of Aβ peptides, and neurofibrillary tangles, composed of tau protein. Disease manifestation is age-dependent, with the incidence of Alzheimer's in the general population rising from 6% in those over 65 years to 30% in those over 85 years (Tariot and Winblat, 2001).

Aβ peptides are produced by proteolytic cleavage of the transmembrane receptor amyloid precursor protein (APP) at the β and γ sites. Familial mutations (FAD) in APP result in increased production of Aβ₄₂ peptide Citron et al., 1995, Suzuki et al., 1994, the amyloidogenic form of the two Aβ species, Aβ₄₀ and Aβ₄₂. Aβ₄₂ forms protofibrils and fibrils much more readily than Aβ₄₀ Harper et al., 1997, Jarrett et al., 1993 and is the predominant form of the peptide found in plaques (Tamaoka et al., 1995). The membrane-tethered aspartyl protease BACE Sinha et al., 1999, Vassar et al., 1999, Yan et al., 1999 cleaves APP at the β site, and the presenilins, PS1 and PS2, participate in APP cleavage at the γ site (Annaert et al., 1999), along with the genes nicastrin (nct), Aph-1 and Pen-2 (for review, see De Strooper, 2003).

Drosophila melanogaster carries homologs of AD-related genes, including APP (Rosen et al., 1989), presenilin (Boulianne et al., 1997), and tau (Heidary and Fortini, 2001). Recently, Drosophila has been used to study human neurodegenerative diseases, such as polyglutamine repeat diseases Fernandez-Funez et al., 2000, Jackson et al., 1998, Kazemi-Esfarjani and Benzer, 2000, Steffan et al., 2001, Warrick et al., 1999, Parkinson's disease Auluck et al., 2002, Feany and Bender, 2000, and tauopathy Jackson et al., 2002, Wittmann et al., 2001.

Here, we describe overexpression of the Aβ₄₂ peptide in Drosophila nervous system tissues, resulting in structural as well as behavioral phenotypes. The severity of Aβ₄₂ effects is dose- and age-dependent, resembling AD symptoms. In addition, in a genetic screen designed to identify modifiers of Aβ₄₂-induced phenotypes, we isolated a mutation in a Drosophila neprilysin homolog that causes amelioration of Aβ₄₂ phenotypes by reducing the steady-state levels of Aβ.

The pathways that affect degradation and clearance of Aβ are likely to influence incidence and progression of the disease (reviewed in Chen and Fernandez, 2001, Selkoe, 2001). Proteins fulfilling such roles include apolipoprotein E, alpha 2-macroglobulin, the LDL receptor-related protein, TFG-β1 (reviewed in Hyman et al., 2000, Poirier, 2000, Wyss-Coray et al., 2001) as well as insulin-degrading enzyme, plasmin, insulysin, and neprilysin Iwata et al., 2001, Mukherjee et al., 2000, Tucker et al., 2000, Vekrellis et al., 2000, but more remain to be discovered. The Aβ₄₂ Drosophila model presented here is a powerful tool for the dissection of mechanisms of Aβ toxicity and may lead to better understanding of the disease and potentially novel AD therapies.