ABSTRACT
Amyloid beta (Abeta) is a metabolic product of amyloid-beta precursor protein (APP). Deposition of Abeta in the brain and neuronal degeneration are characteristic hallmarks of Alzheimer's disease (AD). Abeta induces neuronal degeneration, but the mechanism of neurotoxicity remains elusive. Here we show that overexpression of APP renders hippocampal neurons vulnerable to Abeta toxicity. Deletion of the extracellular Abeta sequence of APP prevents binding of APP to Abeta, and abolishes toxicity. Abeta 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 Abeta. Overexpression of PTX-insensitive Galpha-o subunit, but not Galpha-i subunit, of G protein restores Abeta 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 Abeta-species promotes toxicity in hippocampal neurons by a mechanism that involves APP-mediated Go protein activation, revealing an Abeta-receptor-like function of APP directly implicated in neuronal degeneration in AD.
Subject(s)
Alzheimer Disease/metabolism , Amyloid beta-Peptides/metabolism , Amyloid beta-Protein Precursor/metabolism , Brain/metabolism , GTP-Binding Protein alpha Subunits, Gi-Go/metabolism , Alzheimer Disease/genetics , Alzheimer Disease/physiopathology , Amyloid beta-Peptides/toxicity , Animals , Binding Sites/genetics , Brain/physiopathology , Cells, Cultured , GTP-Binding Protein alpha Subunits, Gi-Go/genetics , Hippocampus/metabolism , Hippocampus/physiopathology , Humans , Nerve Degeneration/genetics , Nerve Degeneration/metabolism , Nerve Degeneration/physiopathology , Neurons/drug effects , Neurons/metabolism , Neurons/pathology , Pertussis Toxin/pharmacology , Protein Binding/genetics , Protein Structure, Tertiary/physiology , RatsABSTRACT
Deposition of fibrillar amyloid beta (fAbeta) plays a critical role in Alzheimer's disease (AD). We have shown recently that fAbeta-induced dystrophy requires the activation of focal adhesion proteins and the formation of aberrant focal adhesion structures, suggesting the activation of a mechanism of maladaptative plasticity in AD. Focal adhesions are actin-based structures that provide a structural link between the extracellular matrix and the cytoskeleton. To gain additional insight in the molecular mechanism of neuronal degeneration in AD, here we explored the involvement of LIM kinase 1 (LIMK1), actin-depolymerizing factor (ADF), and cofilin in Abeta-induced dystrophy. ADF/cofilin are actin-binding proteins that play a central role in actin filament dynamics, and LIMK1 is the kinase that phosphorylates and thereby inhibits ADF/cofilin. Our data indicate that treatment of hippocampal neurons with fAbeta increases the level of Ser3-phosphorylated ADF/cofilin and Thr508-phosphorylated LIMK1 (P-LIMK1), accompanied by a dramatic remodeling of actin filaments, neuritic dystrophy, and neuronal cell death. A synthetic peptide, S3 peptide, which acts as a specific competitor for ADF/cofilin phosphorylation by LIMK1, inhibited fAbeta-induced ADF/cofilin phosphorylation, preventing actin filament remodeling and neuronal degeneration, indicating the involvement of LIMK1 in Abeta-induced neuronal degeneration in vitro. Immunofluorescence analysis of AD brain showed a significant increase in the number of P-LIMK1-positive neurons in areas affected with AD pathology. P-LIMK1-positive neurons also showed early signs of AD pathology, such as intracellular Abeta and pretangle phosphorylated tau. Thus, LIMK1 activation may play a key role in AD pathology.
