The principal pathological features of Alzheimer's disease (AD) are extracellular amyloid plaques and intracellular neurofibrillary tangles, the latter composed of the microtubule-binding protein tau assembled into paired helical and straight filaments. Recent studies suggest that these pathological entities may be functionally linked, although the mechanisms by which amyloid deposition promotes pathological tau filament assembly are poorly understood. Here, we report that tau is proteolyzed by multiple caspases at a highly conserved aspartate residue (Asp 421 ) in its C terminus in vitro and in neurons treated with amyloid-␤ (A␤) (1-42) peptide. Tau is rapidly cleaved at Asp 421 in A␤-treated neurons (within 2 h), and its proteolysis appears to precede the nuclear events of apoptosis. We also demonstrate that caspase cleavage of tau generates a truncated protein that lacks its C-terminal 20 amino acids and assembles more rapidly and more extensively into tau filaments in vitro than wild-type tau. Using a monoclonal antibody that specifically recognizes tau truncated at Asp 421 , we show that tau is proteolytically cleaved at this site in the fibrillar pathologies of AD brain. Taken together, our results suggest a novel mechanism linking amyloid deposition and neurofibrillary tangles in AD: A␤ peptides promote pathological tau filament assembly in neurons by triggering caspase cleavage of tau and generating a proteolytic product with enhanced polymerization kinetics. A lzheimer's disease (AD) is a progressive neurodegenerative disorder characterized by accelerated neuronal cell death leading to dementia (1). Its hallmark pathologic features are extracellular amyloid plaques and intraneuronal fibrillar structures, the latter including neurofibrillary tangles (NFTs), neuropil threads, and dystrophic neurites invading amyloid plaques (2). Amyloid plaques are formed by the extracellular deposition of proteolytic fragments of the amyloid precursor protein (APP) termed amyloid-␤ (A␤) (1, 3), whereas the fibrillar pathologies are composed of the microtubule-associated protein tau assembled into polymeric filaments (paired helical and straight filaments) (2). The pathogenic role of amyloid deposition in AD is underscored by the evidence that each of the disease-causing mutations in familial AD results in enhanced production of amyloidogenic A␤ peptides; these peptides are sufficient to induce apoptosis in cultured neurons (1, 3). Furthermore, the recent observation that tau mutations cause hereditary frontotemporal dementia and parkinsonism linked to chromosome 17 (FTDP-17), a class of diseases characterized by NFT-like deposition of polymeric tau and dementia without amyloid plaques, emphasizes the critical role that tau plays in neurodegenerative events (4-6). Although amyloid plaques and NFTs have been largely regarded as independent neuropathologic entities, recent work suggests they may be functionally linked: mutation of APP that results in amyloid deposition or direct intracranial injection of A␤ peptide increase...

Alzheimer's disease (AD) is the most common type of dementia. In connection with the global trend of prolonging human life and the increasing number of elderly in the population, the AD becomes one of the most serious health and socioeconomic problems of the present. Tau protein promotes assembly and stabilizes microtubules, which contributes to the proper function of neuron. Alterations in the amount or the structure of tau protein can affect its role as a stabilizer of microtubules as well as some of the processes in which it is implicated. The molecular mechanisms governing tau aggregation are mainly represented by several posttranslational modifications that alter its structure and conformational state. Hence, abnormal phosphorylation and truncation of tau protein have gained attention as key mechanisms that become tau protein in a pathological entity. Evidences about the clinicopathological significance of phosphorylated and truncated tau have been documented during the progression of AD as well as their capacity to exert cytotoxicity when expressed in cell and animal models. This paper describes the normal structure and function of tau protein and its major alterations during its pathological aggregation in AD.

Neurofibrillary tangles (NFT) are comprised of the microtubule-associated protein tau, in the form of filamentous aggregates. In addition to the well-known changes in phosphorylation state, tau undergoes multiple truncations and shifts in conformation as it transforms from an unfolded monomer to the structured polymer characteristic of NFT. Truncations at both the amino- and carboxy-termini directly influence the conformation into which the molecule folds, and hence the ability of tau to polymerize into fibrils. Certain of these truncations may be due to cleavage by caspases as part of the apoptotic cascade. In this review, we discuss evidence that strongly suggests that these truncations occur in an orderly pattern and directly influence the ability of tau to polymerize into filaments.

No abstract

The conformation-dependent antibodies Tau-66 and Alz-50 recognize discontinuous epitopes on the tau molecule (residues 155-244 & 305-314 and 5-15 & 312-322, respectively), thereby defining two distinct conformations. In double-and triple-label immunofluorescence experiments we discovered that specific populations of neurofibrillary tangles display either the Alz-50 or the Tau-66 conformation, but not both. In combination with other antibodies to several domains of the molecule we demonstrate that the conformation recognized by Alz-50 seems to be an early event in the formation of neurofibrillary tangles. This conformation is characterized by the presence of predominantly intact N-and C-termini. By contrast, the Tau-66 conformation is likely a later event in tangle development, being favored in structures containing truncations of both the N-and C-termini. We propose a sequence of events that occurs during the formation and evolution of neurofibrillary tangles based on the initial conformation adopted by tau. In this scheme, the Tau-66 conformation in neurofibrillary tangles may arise from amino and carboxy truncation of tau after it has assumed the Alz-50 conformation. These results indicate that tau structure within the NFT is dynamic in that tau can undergo a "refolding" event following N-and C-terminal truncation.