Role of tau in the polymerization of peptides from beta-amyloid precursor protein

Lack of tau proteins rescues neuronal cell death and decreases amyloidogenic processing of APP in APP/PS1 mice

Lack of tau expression has been reported to protect against excitotoxicity and to prevent memory deficits in mice expressing mutant amyloid precursor protein (APP) identified in familial Alzheimer disease. In APP mice, mutant presenilin 1 (PS1) enhances generation of Aβ42 and inhibits cell survival pathways. It is unknown whether the deficient phenotype induced by concomitant expression of mutant PS1 is rescued by absence of tau. In this study, we have analyzed the effect of tau deletion in mice expressing mutant APP and PS1. Although APP/PS1/tau(+/+) mice had a reduced survival, developed spatial memory deficits at 6 months and motor impairments at 12 months, these deficits were rescued in APP/PS1/tau(-/-) mice. Neuronal loss and synaptic loss in APP/PS1/tau(+/+) mice were rescued in the APP/PS1/tau(-/-) mice. The amyloid plaque burden was decreased by roughly 50% in the cortex and the spinal cord of the APP/PS1/tau(-/-) mice. The levels of soluble and insoluble Aβ40 and Aβ42, and the Aβ42/Aβ40 ratio were reduced in APP/PS1/tau(-/-) mice. Levels of phosphorylated APP, of β-C-terminal fragments (CTFs), and of β-secretase 1 (BACE1) were also reduced, suggesting that β-secretase cleavage of APP was reduced in APP/PS1/tau(-/-) mice. Our results indicate that tau deletion had a protective effect against amyloid induced toxicity even in the presence of mutant PS1 and reduced the production of Aβ.

Differential post-translational modifications of transthyretin in Alzheimer's disease: A study of the cerebral spinal fluid

Transthyretin (TTR) is a 55 kDa homotetrameric protein. TTR in the cerebral spinal fluid (CSF) is primarily synthesized by the choroid plexus. TTR can bind to the beta-amyloid peptide and a number of familial amyloidosis diseases (familial amyloid polyneuropathy) have been associated with its allele variants. In a transgenic mice model overexpression of TTR was positively correlated with a neuroprotective effect from the pathogenic APPsw mutation. TTR has a free reactive sulphydryl moiety located on the Cys(10) residue which has been implicated to undergo a variety of oxidation reactions. To examine the neuroprotective role of TTR, we investigated the conjugated forms of TTR with cysteine (Cys) and cysteinglycine (CsyGly) in the CSF of 39 probable Alzheimer's disease (AD)-affected subjects and in a cohort of subjects without cognitive impairment (27 subjects). Linear MALDI-TOF MS experiments were employed to obtain high-resolution protein profiling of TTR isoforms. Nano-LC-TANDEM MS combined with reflectron MALDI-TOF-MS was used to unequivocally assign the investigated TTR-conjugate signals. Our results indicate a differential distribution of TTR-Cys and TTR-CysGly adducts. Both oxidized forms of TTR are significantly less abundant in the AD group (p = 0.0001). The investigated population (66 subjects) was then diagnosed using the ratio of conjugated TTR to free TTR in each subject. A sensitivity >90% and a specificity >70% were derived from a receiver operating characteristic curve when the overall cohort is analysed by the TTR-Cys signals. This manuscript is the first report describing the presence of differential post-translational oxidations of TTR in the CSF of AD patients.

Proapoptotic effects of tau cleavage product generated by caspase-3

Using an in vitro translation assay to screen a human brain cDNA library, we isolated the microtubule-associated protein Tau and determined it to be a caspase-3 substrate whose C-terminal cleavage occurred during neuronal apoptosis. DeltaTau, the 50-kDa cleavage product, was detected by Western blot in apoptotic cortical cells probed with anti-PHF-1 and anti-Tau-5 antibodies, but not anti-T-46 antibody which recognizes the C-terminus. Overexpression of DeltaTau in SK-N-BE2(C) cells significantly increased the incidence of cell death. Staurosporine-induced Tau cleavage was blocked by 20 microM z-Asp-Glu-Val-Asp-chloromethylketone, a caspase-3 inhibitor, and in vitro, Tau was selectively cleaved by caspase-3 or calpain, a calcium-activated protease, but not by caspases-1, -8, or -9. (D421E)-Tau, a mutant in which Asp421 was replaced with a Glu, was resistant to cleavage by caspase-3 and tended to suppress staurosporine-induced cell death more efficiently than did wild-type Tau in both transient and stable expression systems. Finally, the incidence of DeltaTau-induced cell death was augmented by expression of Abeta precursor protein (APP) or Swedish APP mutant. Taken together, these results suggest that the caspase-3 cleavage product of Tau may contribute to the progression of neuronal cell death in Alzheimer's disease.

Generation of a monoclonal antibody to the carboxy-terminal domain of tau by immunization with the amino-terminal domain of the amyloid precursor protein

A fusion protein between beta-galactosidase and the amino-terminal domain of amyloid precursor protein (APP) was used as an immunogen for the production of monoclonal antibodies. One of these antibodies, the 5D12 monoclonal antibody, labeled the neurofibrillary tangles (NFT) by immunohistochemistry, as well as isolated paired helical filaments (PHF) in electron microscopy. In immunoassay, the ascitic fluid produced by the 5D12 clone was demonstrated to contain a high titer of antibodies to heat-stable microtubule associated proteins (MAPs). By immunoblotting, the proteins recognized in heat-stable MAPs were found to correspond to tau proteins. The 5D12 antibody recognized normal tau isolated from rat and human brain homogenates, and PHF-tau isolated from the brain of patients with Alzheimer's disease (AD). By immunoblotting, the 5D12 antibody also recognized the full-length recombinant tau protein but not the fusion protein used as an immunogen. The immunoreactivity of the 5D12 antibody with tau was completely abolished when the half-carboxy domain of tau, containing the tubulin-binding repeats, was removed. This study demonstrates that the use of the amino-terminal domain of APP as an immunogen led to the generation of a monoclonal antibody to the half-carboxy domain of tau.

Beta-amyloid precursor protein is modified with O-linked N-acetylglucosamine

The beta-amyloid precursor protein (APP) has been implicated in the etiology of Alzheimer's disease (Kang et al.: Nature 325:733-736, 1987; Selkoe: Science 248:1058-1060, 1990; Selkoe: In Cowan et al. (eds): "Annual Review of Neuroscience." Palo Alto, CA: Annual Reviews, Inc., pp 489-519, 1994) and numerous studies have shown that beta-amyloid is involved in amyloid plaque formation (Rumble et al.: N Engl J Med 320:1446-1452, 1989; Sisodia et al.: Science 248: 492-495, 1990). Evidence is presented that APP is modified with N-acetylglucosamine linked to cytoplasmic serine or threonine residues (O-GlcNAc). This is the first report of a plasma membrane protein modified with this carbohydrate. It has been postulated that this modification, which is ubiquitous in all organisms studied to date except bacteria (Haltiwanger et al.: Biochem Soc Trans 20:264-269, 1992; Dong et al.: J Biol Chem 268:16679-16687, 1993; Elliot et al.: J Neurosci 13:2424-2429, 1993; Kelly et al.: J Biol Chem 268:10416-10424, 1993), may function as an alternative to phosphorylation (Dong et al., 1993) and is involved in the multimerization of proteins (Haltiwanger et al., 1992; Dong et al., 1993). O-GlcNAc occurs at "PEST" sequences (Rogers et al.: Science 234:364-368, 1986) and it has been suggested that this modification within such a sequence leads to increased proteolytic stability of the molecule (Dong et al., 1993).(ABSTRACT TRUNCATED AT 250 WORDS)

Examination of phosphorylated tau protein as a PHF-precursor at early stage Alzheimer's disease

Hyperphosphorylated tau protein which can be isolated on the basis of insolubility in 1% sarkosyl (A68-tau fraction) is thought to represent a precursor pool for PHF assembly, associated histologically with neuritic pathology, which feeds into a more resistant tangle-associated PHF pool via cross-linking and proteolysis. We examined these predictions at the earliest detectable stages of neurofibrillary pathology. We report that there is no evidence that neuritic pathology represents an early pathologic stage, no evidence of an association between neuritic pathology and phosphorylated tau, no evidence of selective accumulation of phosphorylated tau at early stages of pathology, and no evidence for a precursor/product relationship between phosphorylated tau and PHFs during progression of pathology. We conclude that altered phosphorylation is a secondary process affecting 5% of PHFs and does not explain PHF assembly in Alzheimer's disease.

Tau protein directly interacts with the amyloid beta-protein precursor: implications for Alzheimer's disease

The simultaneous presence of intracellular neurofibrillary tangles (NFT) and extracellular senile plaques in Alzheimer's disease (AD) suggests that the two lesions could be synergistically interrelated. However, although the main protein components of NFT and senile plaques, tau (tau) and amyloid beta-protein, respectively, are well characterized, the molecular mechanisms responsible for their deposition in lesions are unknown. We demonstrate, using four independent techniques, that tau directly interacts with a conformation-dependent domain of the amyloid beta-protein precursor (beta PP) encompassing residues beta PP714-723. The putative tau-binding domain includes beta PP717 mutation sites that are associated with familial forms of AD. Our findings strongly suggest that NFT and senile plaques, often thought of as independent structures, may play a role in each other's formation during the pathogenesis of AD.

Aggregation of tau protein by aluminum

Aluminum has been detected in Alzheimer neurofibrillary tangles, but the significance of its presence is unknown. The principal component of tangles is the paired helical filament (PHF), comprised of tau protein. We investigated whether aluminum could induce tau protein to form filaments or aggregate. When 10 microM bovine tau or non-phosphorylated recombinant human tau was combined with 400 microM or more aluminum, tau protein appeared to aggregate, observed as a dose-dependent decrease in electrophoretic mobility on SDS-PAGE. Tau appeared as a smear above the region of the expected tau bands and, at higher aluminum doses, failed to enter the gel. A tau fragment encompassing the microtubule binding domains did not show decreased mobility in the presence of aluminum, but did form aggregates that failed to electrophorese. However no fibrillar structures were observed in the aluminum-treated tau samples when observed by electron microscopy. The effect of aluminum on tau mobility was reversed by incubating with 1 mM deferoxamine. In contrast, the morphology of PHF fibrils was unaffected by deferoxamine treatment and the characteristic abnormal mobility of PHF-tau was not reduced by deferoxamine. This suggests that aluminum is not, by itself, a significant factor in maintaining the assembly of PHF-tau as fibrils or in its abnormal mobility on SDS gels. Aluminum treatment of 3T3 fibroblasts transfected with human tau resulted in toxicity, but did not change tau expression levels or induce tau aggregation. In conclusion, aluminum appears to induce isolated tau protein to aggregate in a phosphate-independent way, without the formation of fibrils.(ABSTRACT TRUNCATED AT 250 WORDS)

Comparative toxicity of amyloid beta-peptide in neuroblastoma cell lines: effects of albumin and physalaemin

1. Synthetic amyloid beta-peptide was toxic to NB41A3 neuroblastoma cells in serum-free culture as judged by decreasing cell numbers and release of the cytosolic enzyme, lactic dehydrogenase. 2. Without amyloid beta-peptide, bovine serum albumin increased the number of cells surviving in culture. 3. In the presence of amyloid beta-peptide, BSA appeared to potentiate the amyloid beta-peptide toxicity. 4. The toxic dose response for amyloid beta-peptide varied between different cell lines (NB41A3, NB2a and IMR32), in a range of 100-1000 nM amyloid beta-peptide. 5. Amyloid beta-peptide toxicity was inhibited by the concurrent treatment of the cells with the tachykinin physalaemin with an ED50 of 10(-6) M.

Phosphorylation of tau by proline-directed protein kinase (p34cdc2/p58cyclin A) decreases tau-induced microtubule assembly and antibody SMI33 reactivity

Tau protein was evaluated as a substrate for a proline-directed protein kinase (p34cdc2/p58cyclin A) which recognizes the phosphorylation site motif X-Ser/Thr-Pro-X. The shortest human tau isoform, expressed as a recombinant protein, was phosphorylated to a stoichiometry of 2 mol phosphate/mol tau. Phosphoamino acid analysis revealed phosphorylation of both serine and threonine residues. Phosphorylation of recombinant tau resulted in a decreased ability to induce microtubule assembly but had no effect on the final extent of microtubule formation or on the rate of cold-induced microtubule disassembly. Phosphorylation of tau by the proline-directed protein kinase completely blocked immunoreactivity with antibody SMI33. Phosphorylation did not create the epitopes for the phosphate-dependent antibodies SMI31 or SMI34. Antibody SMI33 recognizes neurofibrillary tangles after treatment with alkaline phosphatase, suggesting that the proline-directed protein kinase may phosphorylate tau at sites that are phosphorylated in Alzheimer's disease.