ABSTRACT
In Alzheimer's disease (AD), rod-like cofilin aggregates (cofilin-actin rods) and thread-like inclusions containing phosphorylated microtubule-associated protein (pMAP) tau form in the brain (neuropil threads), and the extent of their presence correlates with cognitive decline and disease progression. The assembly mechanism of these respective pathological lesions and the relationship between them is poorly understood, yet vital to understanding the causes of sporadic AD. We demonstrate that, during mitochondrial inhibition, activated actin-depolymerizing factor (ADF)/cofilin assemble into rods along processes of cultured primary neurons that recruit pMAP/tau and mimic neuropil threads. Fluorescence resonance energy transfer analysis revealed colocalization of cofilin-GFP (green fluorescent protein) and pMAP in rods, suggesting their close proximity within a cytoskeletal inclusion complex. The relationship between pMAP and cofilin-actin rods was further investigated using actin-modifying drugs and small interfering RNA knockdown of ADF/cofilin in primary neurons. The results suggest that activation of ADF/cofilin and generation of cofilin-actin rods is required for the subsequent recruitment of pMAP into the inclusions. Additionally, we were able to induce the formation of pMAP-positive ADF/cofilin rods by exposing cells to exogenous amyloid-beta (Abeta) peptides. These results reveal a common pathway for pMAP and cofilin accumulation in neuronal processes. The requirement of activated ADF/cofilin for the sequestration of pMAP suggests that neuropil thread structures in the AD brain may be initiated by elevated cofilin activation and F-actin bundling that can be caused by oxidative stress, mitochondrial dysfunction, or Abeta peptides, all suspected initiators of synaptic loss and neurodegeneration in AD.
Subject(s)
Actin Depolymerizing Factors/metabolism , Actins/metabolism , Neurites/metabolism , Neurons/pathology , tau Proteins/metabolism , Actin Depolymerizing Factors/genetics , Adenosine Triphosphate/pharmacology , Alzheimer Disease/pathology , Amino Acid Motifs/physiology , Amyloid beta-Peptides/pharmacology , Animals , Animals, Newborn , Antimycin A/analogs & derivatives , Antimycin A/pharmacology , Brain/pathology , Bridged Bicyclo Compounds, Heterocyclic/pharmacology , Carbonyl Cyanide m-Chlorophenyl Hydrazone/pharmacology , Cells, Cultured , Chick Embryo/cytology , Depsipeptides/pharmacology , Enzyme Inhibitors/pharmacology , Fluorescence Resonance Energy Transfer/methods , Green Fluorescent Proteins/genetics , Humans , Hydrogen Peroxide/pharmacology , Ionophores/pharmacology , Neurites/drug effects , Neurons/cytology , Neurons/drug effects , Organ Culture Techniques , Oxidants/pharmacology , Peptide Fragments/pharmacology , Phosphorylation/physiology , RNA, Small Interfering/genetics , RNA, Small Interfering/metabolism , Rats , Serine/metabolism , Thiazolidines/pharmacology , Transfection/methods , p21-Activated Kinases/genetics , p21-Activated Kinases/metabolismABSTRACT
Oxidative damage is associated with Alzheimer's disease and mild cognitive impairment, but its relationship to the development of neuropathological lesions involving accumulation of amyloid-beta (Abeta) peptides and hyperphosphorylated tau protein remains poorly understood. We show that inducing oxidative stress in primary chick brain neurons by exposure to sublethal doses of H(2)O(2 )increases levels of total secreted endogenous Abeta by 2.4-fold after 20 h. This occurs in the absence of changes to intracellular amyloid precursor protein or tau protein levels, while heat-shock protein 90 is elevated 2.5-fold. These results are consistent with the hypothesis that aging-associated oxidative stress contributes to increasing Abeta generation and up-regulation of molecular chaperones in Alzheimer's disease.
