A nucleated assembly mechanism of Alzheimer paired helical filaments

Abstract

Alzheimer's disease is characterized by two types of fibrous aggregates in the affected brains, the amyloid fibers (consisting of the A␤-peptide, generating the amyloid plaques), and paired helical filaments (PHFs; made up of tau protein, forming the neurofibrillary tangles). Hence, tau protein, a highly soluble protein that normally stabilizes microtubules, becomes aggregated into insoluble fibers that obstruct the cytoplasm of neurons and cause a loss of microtubule stability. We have developed recently a rapid assay for monitoring PHF assembly and show here that PHFs arise from a nucleated assembly mechanism. The PHF nucleus comprises about 8-14 tau monomers. A prerequisite for nucleation is the dimerization of tau because tau dimers act as effective building blocks. PHF assembly can be seeded by preformed filaments (made either in vitro or isolated from Alzheimer brain tissue). These results suggest that dimerization and nucleation are the rate-limiting steps for PHF formation in vivo.Alzheimer's disease, the most common age-relate dementia, is characterized by two pathological protein deposits in the brain, the amyloid plaques, consisting largely of amyloid fibers assembled from the A␤-peptide [a derivative of the membrane protein APP (amyloid precursor protein); reviewed in ref. 1] and the neurofibrillary tangles (NFT), which are bundles of paired helical filaments (PHFs) whose main constituent is the microtubule-associated protein tau (for reviews, see refs. 2 and 3). The uncontrolled precipitation of these aggregates is believed to be largely responsible for the neuronal degeneration, and the disease has been classified into several stages on the basis of the spreading of neurofibrillary deposits (4). It is therefore important to understand the factors underlying the abnormal aggregation of A␤ and tau. Both form filaments of Ϸ10 nm in width; in the case of A␤ they are smooth, while most tau filaments from Alzheimer brains show a characteristic ''paired helical'' structure, resembling two strands wound around one another, with a crossover periodicity of Ϸ80 nm and width varying between 10 and 20 nm (for review, see ref.The membrane-derived A␤-peptide is partly hydrophobic so that its tendency to aggregate is intuitively understandable (6, 7). By contrast, the cytosolic tau has a very hydrophilic character and is highly soluble (8, 9). Thus, it shows hardly any tendency to aggregate in physiological buffer conditions, and the formation of aggregates is very slow (days or weeks; ref. 10). Soluble tau protein contains very little secondary structure (␣-helix or ␤-sheet content Ͻ 5%), and the same holds for Alzheimer PHFs, in spite of their long-range periodicity (11). It therefore is not obvious why tau should aggregate in a specific manner and which structural principle could be responsible for this. Progress in understanding the mechanism has been correspondingly slow. Several factors supporting assembly have emerged in recent years. (i) The repeat domain in the C-terminal half of tau forms PHFs mo...

Stepwise proteolysis liberates tau fragments that nucleate the Alzheimer-like aggregation of full-length tau in a neuronal cell model

“…5b, illustrating the roles of sequential cleavage and folding. It illustrates that tau aggregation follows a nucleation-elongation mechanism whose rate-limiting step is the formation of an oligomeric nucleus (11). As with other polymerization reactions of this kind it can be accelerated by preformed seeds (nuclei or filaments).…”

“…However, aggregation can be dramatically accelerated by polyanionic cofactors such as sulfated glycosaminoglycans, RNA, acidic peptides, or fatty acid micelles (7)(8)(9)(10). The aggregation is thought to follow a nucleation-elongation pathway (11), whereby an oligomeric nucleus is first formed by self-association of protein subunits, followed by addition of subunits to filament ends. However, several issues remain open in this scheme: (i) Although tau fragments aggregate spontaneously (12), full-length tau is difficult to fibrillize even at high concentration unless aided by polyanions.…”

Tau Protein Assembles into Isoform- and Disulfide-dependent Polymorphic Fibrils with Distinct Structural Properties

“…A Thiol-disulfide Status in Both Repeat Isoforms of Fulllength Tau Impacts Fibrillation Kinetics-From the 1990s, kinetic effects of disulfide formation on Tau fibrillation in vitro have been studied by using several truncated Tau peptides comprised of pseudo-repeats and also using several full-length Tau isoforms such as 0N3R (also called htau23) and 2N4R (also called htau40) (14,(31)(32)(33). Based upon those previous studies, formation of a disulfide-linked three-repeat Tau dimer has been shown to significantly accelerate fibrillation kinetics, whereas a disulfide-linked monomer of four-repeat Tau (often called a "compact monomer") has been regarded as a fibrillation-incompetent species.…”

Characterization of Prefibrillar Tau Oligomers in Vitro and in Alzheimer Disease

“…Past attempts to characterize the earliest stages of Tau multimer formation in vitro were contingent upon disulfide bridge formation (26,29). However, the formation of disulfide bridges can inhibit aggregation of Tau isoforms containing four microtubule binding repeats (MTBRs) (30 -32), and aggregates of the four repeat isoforms are associated with many of the neurodegenerative tauopathies (33).…”