BackgroundIntracytoplasmic inclusions composed of filamentous tau proteins are defining characteristics of neurodegenerative tauopathies, but it remains unclear why different tau isoforms accumulate in different diseases and how they induce abnormal filamentous structures and pathologies. Two tau isoform-specific antibodies, RD3 and RD4, are widely used for immunohistochemical and biochemical studies of tau species in diseased brains.ResultsHere, we show that extensive irreversible post-translational deamidation takes place at asparagine residue 279 (N279) in the RD4 epitope of tau in Alzheimer’s disease (AD), but not corticobasal degeneration (CBD) or progressive supranuclear palsy (PSP), and this modification abrogates the immunoreactivity to RD4. An antiserum raised against deamidated RD4 peptide specifically recognized 4R tau isoforms, regardless of deamidation, and strongly stained tau in AD brain. We also found that mutant tau with N279D substitution showed reduced ability to bind to microtubules and to promote microtubule assembly.ConclusionThe biochemical and structural differences of tau in AD from that in 4R tauopathies found in this study may therefore have implications for prion-like propagation of tau.

Alzheimer’s disease (AD) is characterized by extracellular amyloid β (Aβ) deposition and intracellular tau aggregation. Many studies have indicated some association between these processes, but it remains unknown how the two pathologies are linked. In this study, we investigated whether expression of amyloid precursor protein (APP) influences extracellular seed-dependent intracellular tau accumulation in cultured cells. Treatment of tau-expressing SH-SY5Y cells with Aβ fibrils did not induce intracellular tau aggregation. On the other hand, in cells expressing both tau and APP, treatment with tau fibrils or Sarkosyl-insoluble tau from AD brains induced intracellular tau aggregation. The seed-dependent intracellular tau aggregation was not induced by expression of APP lacking the extracellular domain. The amount of phosphorylated tau aggregates in cultured cells was dose dependently elevated in response to increased levels of APP on the cell membrane. Our results indicate that the extracellular region of APP is involved in uptake of tau fibrils into cells, raising the possibility that APP, but not Aβ, influences cell-to-cell spreading of tau pathologies in AD by serving as a receptor of abnormal tau aggregates.Electronic supplementary materialThe online version of this article (doi:10.1007/s00401-015-1415-2) contains supplementary material, which is available to authorized users.

Treatment of PC12 cells with fungus-derived alkaloid neoechinulin A for more than 12 h renders the cells resistant to subsequent superoxide (O 2 )/nitric oxide (NO) insults derived from 3-morpholinosydnonimine (SIN-1). However, the underlying mechanism(s) remains largely unclear. To elucidate the mechanism(s), we assessed the specificity of the cytoprotection afforded by neoechinulin A treatment using other cytocidal stressors and also clarified the resulting cellular alterations, focusing on the antioxidant and metabolic enzymes systems. Neoechinulin A treatment for more than 12 h endowed PC12 cells with significant resistance to transient NO toxicity, but not persistent NO toxicity, bolus H 2 O 2 toxicity, or oxidative insult from the redox cycling quinone menadione. Cellular antioxidant system profiling revealed no substantial potentiation of the activity of any antioxidant enzyme in lysate from the neoechinulin A-treated cells excluding glutathione (GSH) content, which was significantly decreased (>50%), resulting in a proportional compromise in the thiol-reducing activity of the intact cells. In addition, no differences were observed in the activity for any nicotinamide adenine dinucleotide (phosphate) reduced form (NAD(P)H)-generating enzyme, steady-state NAD(P)H/nicotinamide adenine dinucleotide (phosphate) oxidized form (NAD(P) ) ratios, or the levels of total NAD(P)H. Nevertheless, the neoechinulin A-treated intact cells exhibited increased NAD(P)H redox turnover when driven by extracellular tetrazolium. The structurally inactive analog preechinulin failed to protect cells against NO toxicity or induce these alterations, suggesting their link with the cytoprotective mechanism. These results suggest that neoechinulin A, despite disabling the GSH defense system, confers cytoprotection against nitrosative stresses by elevating the cellular reserve capacity for NAD(P)H generation, which could offset crippling of energy-supplying systems due to nitrosative stress.

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