Large-scale, multicenter study of cerebrospinal fluid tau protein phosphorylated at serine 199 for the antemortem diagnosis of Alzheimer's disease.

We surveyed a total of 570 cerebrospinal fluid (CSF) samples from a variety of diseases, including Alzheimer's disease (AD; n = 236), non-AD-demented and nondemented diseases (n = 239), and normal controls (n = 95) to quantitate levels of tau protein phosphorylated at serine 199 (CSF/phospho-tau199) by a recently established sandwich ELISA. The CSF/phospho-tau199 levels in the AD group were significantly elevated compared to those in all the other non-AD groups. Receiver operating characteristics curves showed that the diagnostic sensitivity and specificity for the AD group vs all the other non-AD groups using the CSF/phospho-tau199 were 85.2% and 85.0%, respectively. Furthermore, there was a significant positive correlation between CSF/phospho-tau199 and CSF/total-tau levels in the AD group. Elevated CSF/phospho-tau199 in the AD group was noted irrespective of age, gender, dementia severity, and number of apolipoprotein E4 alleles. Thus, we suggest that CSF/phospho-tau199 may be a novel and logical biomarker in supporting antemortem diagnosis of AD.

CSF phosphorylated tau protein and mild cognitive impairment: a prospective study.

Cerebrospinal fluid (CSF) tau protein phosphorylated at both Thr231 and Ser235 sites (CSF/phospho-tau(231-235)) and total tau (CSF/total-tau) were quantified by sandwich ELISA in 20 patients with mild cognitive impairment (MCI) who eventually developed AD on follow-up as well as seven memory complainers with no objective memory loss. 13/20 (65%) of the MCI patients had high CSF/total-tau and detectable levels of CSF/phospho-tau(231-235), whereas these markers were low and under a detectable level in all of the memory complainers. Although either a total-tau, phospho-tau measurement or a combination of these can help in predicting if MCI will develop AD, our results suggest that the pathogenic steps of AD may be at the stage that finally leads to an accumulation of abnormally phosphorylated tau and neuron death, at least in some brain areas, when MCI patients present with the earliest detectable clinical symptoms of dementia.

Phosphorylated tau in human cerebrospinal fluid is a diagnostic marker for Alzheimer's disease.

Microtubule-associated protein tau in cerebrospinal fluid (CSF) has been proposed as a diagnostic marker for Alzheimer's disease (AD), but there is overlap between AD patients and non-AD controls. To improve the diagnostic accuracy, we measured phosphorylated tau in CSF, because phosphorylated tau accumulates as pathological paired helical filaments in neurons of the AD brain. Immunoblot showed that CSF contained a 32 kDa N-terminal fragment of tau that was partially phosphorylated on Ser199, Thr231 and Ser235. A sandwich enzyme immunoassay revealed that phosphorylated CSF-tau levels were significantly higher in AD patients than those in non-AD controls. Discrimination between the two groups was clearer in phosphorylated CSF-tau than in total CSF-tau. The data indicate that elevated phosphorylated CSF-tau level is a more specific diagnostic marker for AD.

Phosphatidic acid-dependent phosphorylation of a 29-kDa protein by protein kinase Calpha in bovine brain cytosol.

Activation of phospholipase D (PLD) is involved in receptor-mediated signal transduction responses. Signaling from PLD to a downstream molecule(s) appears to be mediated by the PLD product phosphatidic acid (PA). A target molecule(s) of PA, however, has not yet been identified. The present study sought to define such a target molecule(s) of PA. In bovine brain cytosol, proteins with apparent molecular weights of 29,000 (p29) and 32,000 (p32) were prominently phosphorylated in the presence of PA, but not in its absence, indicating that there is a PA-regulated protein kinase (PARK) in bovine brain that phosphorylates p29 and p32. One of these substrates, p29, was purified to near homogeneity. Its partial amino acid sequence was determined and found to be identical to that of a known brain-specific 25-kDa protein (p25). The purified p29 was also readily recognized by and immunoprecipitated with an anti-p25 antibody. These results suggest that p29 is very similar to or identical with p25. Using the purified p29 as a substrate, PARK was purified to near homogeneity. The purified PARK had an apparent molecular weight of 80,000, was strongly recognized by an antiprotein kinase C (PKC)alpha antibody, and was activated by phosphatidylserine (PS) as well as PA. The PA- and PS-stimulated PARK activity was extremely augmented by the presence of 1 microM free Ca2+. In the presence of 1 mM EGTA, phorbol 12-myristate 13-acetate activated PARK synergistically with PA or PS. Similar results were obtained with the purified recombinant PKCalpha. From these results, it is suggested that the PARK activity purified might be attributed to PKCalpha. In p25-depleted bovine brain cytosol, which was prepared by treatment of bovine brain cytosol with the anti-p25 antibody, PA-dependent phosphorylation of p29, but not p32, was almost completely eliminated. When PKCalpha in bovine brain cytosol was depleted by its precipitation with the anti-PKCalpha antibody, neither p29 nor p32 in this PKCalpha-depleted cytosol was phosphorylated in the presence of PA. These results indicate that in bovine brain cytosol PA activates PKCalpha, which, in turn, phosphorylates p29, which may be identical with p25.

Characterization of tau phosphorylation in glycogen synthase kinase-3beta and cyclin dependent kinase-5 activator (p23) transfected cells.

One of the histopathological markers in Alzheimer's disease is the accumulation of hyperphosphorylated tau in neurons called neurofibrillary tangles (NFT) composing paired helical filaments (PHF). Combined tau protein kinase II (TPK II), which consists of CDK5 and its activator (p23), and glycogen synthase kinase-3beta (GSK-3beta) phosphorylate tau to the PHF-form in vitro. To investigate tau phosphorylation by these kinases in intact cells, the phosphorylation sites were examined in detail using well-characterized phosphorylation-dependent anti-tau antibodies after overexpressing the kinases in COS-7 cells with a human tau isoform. The overexpression of tau in COS-7 cells showed extensive phosphorylation at Ser-202 and Ser-404. The p23 overexpression induced a mobility shift of tau, but most of the phosphorylation sites overlapped the endogenous phosphorylation sites. GSK-3beta transfection showed the phosphorylation at Ser-199, Thr-231, Ser-396, and Ser-413. Triplicated transfection resulted in phosphorylation of tau at 8 observed sites (Ser-199, Ser-202, Thr-205, Thr-231, Ser-235, Ser-396, Ser-404, and Ser-413).

Possible role of tau protein kinases in pathogenesis of Alzheimer's disease.

Tau protein kinases (TPK) I and II were isolated as candidate enzymes responsible for the hyperphosphorylation observed in PHF-tau. Four phosphorylation sites of tau were identified for each kinase, accounting for most, but not all, of the major phosphorylation sites of PHF-tau. Immunostaining with anti-TPKI antibody indicated that this kinase is up-regulated in AD brain. Such up-regulation of TPKI and phosphorylatioin of tau were reproduced by treating cultured hippocampal cells with amyloid beta (Abeta) protein. In addition, we found that TPKI can phosphorylate and inactivate pyruvate dehydrogenase (PDH), which is expected to result in depletion of acetyl-CoA, a key substrate of acetyl choline synthesis. Indeed, when septum cells were treated with Abeta, the level of acetyl choline decreased dramatically.

Immunocytochemistry of formalin-fixed human brain tissues: microwave irradiation of free-floating sections.

Formalin fixation, the chemical process in which formaldehyde binds to cells and tissues, is widely used to preserve human brain specimens from autolytic decomposition. Ultrastructure of cellular and mitochondrial membranes is markedly altered by vesiculation, but this does not interfere with diagnostic evaluation of neurohistology by light microscopy. Serious difficulties are encountered, however, when immunocytochemical staining is attempted. Antigens that are immunoreactive in unfixed frozen sections and protein extracts appear to be concealed or destroyed in formalin-fixed tissues. In dilute aqueous solution, formaldehyde is in equilibrium with methylene glycol and its polymeric hydrates, the balance by far in favor of methylene glyco. Carbonylic formaldehyde is a reactive electrophilic species well known for crosslinking functional groups in tissue proteins, nucleic acids, and polysaccharides. Some of its methylene crosslinks are readily hydrolyzed. Others are stable and irreversible. During immunostaining reactions, intra- and inter-molecular links between macromolecules limit antibody permeation of tissue sections, alter protein secondary structure, and reduce accessibility of antigenic determinants . Accordingly, immunoreactivity is diminished for many antigens. Tissues are rapidly penetrated by methylene glycol, but formaldehyde binding to cellular constituents is relatively slow, increasing progressively until equilibrium is reached. In addition, prolonged storage in formalin may result in acidification of human brain specimens. Low pH favors dissociation of methylene glycol into formaldehyde, further reducing both classical staining and antigen detectability. Various procedures have been devised to counter the antigen masking effects of formaldehyde. Examples include pretreatment of tissue sections with proteases, formic acid, or ultrasound. Recently, heating of mounted sections in ionic salt solution by microwave energy was found to restore many antigens. Theory and practice of microwave antigen retrieval are covered extensively in the handbook Microwave Cookbook for Microscopists. A concise overview of microwave methods in the neurosciences has been published, and clinical applications have been reviewed. In this context, it should be noted that fresh tissues may be stabilized for immunocytochemistry by reversible, non-chemical binding processes such as cryosectioning after microwave treatment and freeze-drying. Thus, it may be possible to enhance immunostaining for some antigens by microwave irradiation of unfixed as well as fixed specimens. Parameters to be optimized for microwave retrieval of specific antigens include temperature, irradiation time, tissue buffer composition, salt concentration, and pH. Temperature, irradiation time, and pH are key variables. With this in mind, an optimal method was developed for retrieval of a wide variety of antigens in human brain tissues. Typical microwave protocols employ elevated temperatures that may reach 100 degrees C, where denaturation causes irreversible uncoiling and disruption of protein secondary and tertiary structures. Under these conditions, stable covalent bonds securing methylene crosslinks between polypeptides remain intact, but more reactive links formed by Schiff bases may be hydrolyzed. Resultant conformational changes presumably expose buried loops of continuous amino acids and protruding regions, increasing accessibility of their epitopes. Protein denaturation seems to be a reasonable explanation for the effects of microwaves on antigen retrieval. This idea is supported by the observation that denaturing solutions such as 6 M urea increase immunoreactivity of some antigens. Still, the molecular basis of these effects remains unresolved, in part due to the complex chemistry of formaldehyde reactions with tissue constituents. Indeed, some methylene bridges between similar groups such as NH2 and NH may be hydrolyzed by washing fixed tissues in distilled wa

Cloning of cDNA and expression of the gene encoding rat presenilin-2.

We have cloned the rat homologue of the presenilin-2 (PS-2) cDNA. PS-2 is responsible for chromosome 1-linked familial Alzheimer's disease. Sequence analysis predicted that the rat PS-2 encodes a 448 amino acid (aa) protein, and there was a very high degree of amino acid identity between rat and human PS-2 (95%). All the mutated codons in PS-2 and PS-1 in chromosome 1- or 14-linked familial Alzheimer's disease patients were conserved in rat PS-2. The expression of PS-2 was weaker than that of PS-1. The alternatively spliced short form of PS-2 mRNA, which was detected in human tissues was not detected in various rat tissues. During brain development, the expression level of both PS-2 and PS-1 increased but decreased in the adult. No remarkable change was observed in neural differentiation of PC12 cells.

Physiology and pathology of tau protein kinases in relation to Alzheimer's disease.

Alzheimer's disease (AD) is characterized by neuronal cell death and two kinds of deposits, neurofibrillary tangles (NFT) and senile plaques. The main component of NFT is paired helical filaments (PHF), which mainly consist of hyperphosphorylated tau protein. Tau protein kinases I and II were found as candidate enzymes responsible for hyperphosphorylation of tau to induce the formation of PHF. Since prior phosphorylation of tau by TPKII strongly enhanced the action of TPKI, it was thought that TPKII was involved in the formation of PHF-tau in concert with TPKI. After cloning, TPKI was found to be identical with glycogen synthase kinase 3 beta (GSK3 beta), while TPKII consists of a novel 23 kDa protein activator and a catalytic subunit that is identical with cyclin-dependent kinase 5 (CDK5). The phosphorylation sites on tau by TPKI and TPKII could account for the most, but not all, of the major phosphorylation sites of fetal tau and PHF-tau. An antibody for a site specifically phosphorylated by TPKI (Ser413) could identify all three neurofibrillary lesions in the AD brain, and double staining for either TPKI or TPKII and NFT in the brain of Down's syndrome patients clearly demonstrated that TPKI and TPKII are both associated with NFT in vivo, suggesting that the level of TPKI or TPKII is elevated in AD brain by some mechanism. On the other hand, the levels of both TPKs change developmentally, being high in the neonatal period when the phosphorylation of fetal tau proceeds actively, suggesting that the TPKI/TPKII cooperative system has an important physiological role in the formation of neural networks. In AD brain, aberrant accumulation of amyloid-beta protein (A beta) occurs ahead of the accumulation of PHF in NFT. When a primary culture of embryonic rat hippocampus was treated with 20 microM A beta, induction of TPKI, extensive phosphorylation of tau and then programmed cell death were observed, indicating that TPKI induced by A beta phosphorylates tau, followed by disruption of axonal transportation and finally cell death. By using a yeast two hybrid system, TPKI was found to interact with pyruvate dehydrogenase (PDH), which is a key enzyme in the glycolytic pathway. PDH was phosphorylated in vitro by TPKI to reduce the activity converting pyruvate into acetyl-CoA, which is required for acetylcholine synthesis. In a primary culture of rat hippocampal cells treated with A beta, PDH was inactivated in inverse relation to the activation of TPKI, resulting in accumulation of pyruvate or lactate, energy failure induced by the disturbance of glucose metabolism, and a shortage of acetylcholine owing to deficiency of acetyl-CoA, all of which are characteristic of AD brain. In cholinergic neurons such as those of the septum, non-aggregated A beta, specifically A beta (1-42), not A beta (1-40), caused a shortage of acetylcholine by activation of TPKI and inactivation of PDH without cell death.

Nontoxic amyloid beta peptide 1-42 suppresses acetylcholine synthesis. Possible role in cholinergic dysfunction in Alzheimer's disease.

We show here that amyloid beta peptide1-42 (Abeta1-42) may play a key role in the pathogenesis of the cholinergic dysfunction seen in Alzheimer's disease (AD), in addition to its putative role in amyloid plaque formation. Abeta1-42 freshly solubilized in water (non-aged Abeta1-42), which was not neurotoxic without preaggregation, suppressed acetylcholine (ACh) synthesis in cholinergic neurons at very low concentrations (10-100 nM), although non-aged Abeta1-40 was ineffective. Non-aged Abeta1-42 impaired pyruvate dehydrogenase (PDH) activity by activating mitochondrial tau protein kinase I/glycogen synthase kinase-3beta, as we have already shown in hippocampal neurons (Hoshi, M., Takashima, A., Noguchi, K., Murayama, M., Sato, M., Kondo, S., Saitoh, Y., Ishiguro, K., Hoshino, T., and Imahori, K. (1996) Proc. Natl. Acad. Sci. U. S. A. 93, 2719-2723). Neither choline acetyltransferase activity nor choline metabolism was affected. Therefore, the major cause of reduced ACh synthesis was considered to be an inadequate supply of acetyl-CoA owing to PDH impairment. Soluble Abeta1-42 increases specifically in AD brain (Kuo, Y.-M., Emmerling, M. R., Vigo-Pelfrey, C., Kasunic, T. C., Kirkpatrick, J. B., Murdoch, G. H., Ball, M. J., and Roher, A. E. (1996) J. Biol. Chem. 271, 4077-4081). This increase in soluble Abeta1-42 may disturb cholinergic function, leading to the deterioration of memory and cognitive function that is characteristic of AD.

Immunocytochemistry of tau phosphoserine 413 and tau protein kinase I in Alzheimer pathology.

One unique phosphorylation site consistently found in paired helical filament tau, serine 413, is modified by tau protein kinase I/glycogen synthase kinase-3 beta but no other known tau kinase. Here we present immunocytochemistry from Alzheimer's disease brains showing that focal subpopulations of hippocampal CA1 pyramidal neurons and neuritic plaques are strongly reactive for tau protein kinase I/glycogen synthase kinase-3 beta and tau phosphoserine 413 in early stages of pathology. Colocalization of these epitopes suggests that tau protein kinase I/glycogen synthase kinase-3 beta abnormally phosphorylates tau and is in a position to disrupt neuronal metabolism in anatomical areas vulnerable to Alzheimer's disease.

Preferential labeling of Alzheimer neurofibrillary tangles with antisera for tau protein kinase (TPK) I/glycogen synthase kinase-3 beta and cyclin-dependent kinase 5, a component of TPK II.

Using immunohistochemistry, we examined the localization of four types of proline-directed kinases in the brains of control rats and in the brains of non-demented aged human subjects, subjects with Alzheimer's disease and those with Down's syndrome. The four kinases were: cyclin-dependent kinase (cdk) 5, a component of tau protein kinase (TPK) II; TPK I/glycogen synthase kinase (GSK)-3 beta; GSK-3 alpha; and mitogen-activated protein kinase (MAPK/ERK2). Each of these kinases has been reported to promote the hyperphosphorylation of tau protein in vitro. The kinases were located essentially in neurons, although the intensity and distribution of labeling varied. Antiserum for cdk5 showed the most preferential and consistent labeling of intraneuronal neurofibrillary tangles (NFT). Antiserum for TPK I/GSK-3 beta also labeled intraneuronal NFT. Double immunolabeling for TPK I/GSK-3 beta and tau 1 showed that TPK I/GSK-3 beta was closely associated with NFT. Antiserum for GSK-3 alpha labeled neurons weakly, and the intensity of labeling did not differ between neurons with and without NFT. Antiserum for MAPK labeled neurons in superficial cortical layers, but NFT appeared in both superficial and deep cortical layers. These findings suggest that cdk5 and TPK I/GSK-3 beta are the critically important kinases for the generation in vivo of hyperphosphorylated tau, the main component of the paired helical filaments in NFT.

Characterization of human presenilin 1 using N-terminal specific monoclonal antibodies: Evidence that Alzheimer mutations affect proteolytic processing.

The majority of cases of early-onset familial Alzheimer disease are caused by mutations in the recently identified presenilin 1 (PS1) gene, located on chromosome 14. PS1, a 467 amino acid protein, is predicted to be an integral membrane protein containing seven putative transmembrane domains and a large hydrophilic loop between the sixth and seventh membrane-spanning domain. We produced 7 monoclonal antibodies that react with 3 non-overlapping epitopes on the N-terminal hydrophilic tail of PS1. The monoclonal antibodies can detect the full-size PS1 at Mr 47000 and a more abundant Mr 28000 product in membrane extracts from human brain and human cell lines. PC12 cells transiently transfected with PS1 constructs containing two different Alzheimer mutations fail to generate the 28 kDa degradation product in contrast to PC12 cells transfected with wild-type PS1. Our results indicate that missense mutations in this form of familial Alzheimer disease may act via a mechanism of impaired proteolytic processing of PS1.

Regulation of mitochondrial pyruvate dehydrogenase activity by tau protein kinase I/glycogen synthase kinase 3beta in brain.

According to the amyloid hypothesis for the pathogenesis of Alzheimer disease, beta-amyloid peptide (betaA) directly affects neurons, leading to neurodegeneration and tau phosphorylation. In rat hippocampal culture, betaA exposure activates tau protein kinase I/glycogen synthase kinase 3beta (TPKI/GSK-3beta), which phosphorylates tau protein into Alzheimer disease-like forms, resulting in neuronal death. To elucidate the mechanism of betaA-induced neuronal death, we searched for substrates of TPKI/GSK-3beta in a two-hybrid system and identified pyruvate dehydrogenase (PDH), which converts pyruvate to acetyl-CoA in mitochondria. PDH was phosphorylated and inactivated by TPKI/GSK-3beta in vitro and also in betaA-treated hippocampal cultures, resulting in mitochondrial dysfunction, which would contribute to neuronal death. In cholinergic neurons, betaA impaired acetylcholine synthesis without affecting choline acetyltransferase activity, which suggests that PDH is inactivated by betaA-induced TPKI/GSK-3beta. Thus, TPKI/GSK-3beta regulates PDH and participates in energy metabolism and acetylcholine synthesis. These results suggest that TPKI/GSK-3beta plays a key role in the pathogenesis of Alzheimer disease.

Molecular cloning and expression of the rat homologue of presenilin-1.

The rat homologue of the presenilin-1 (PS-1) gene, which is responsible for early-onset familial Alzheimer's disease linked to chromosome 14, was cloned and sequenced. The predicted amino acid sequence showed quite high homology among rat, mouse, and human PS-1. Especially, the amino acid sequences of the putative transmembrane domains were highly conserved among the three species. The expression level of the PS-1 gene increased during brain development and the number of transcripts of the PS-1 gene changed during brain development. We found one transcript of the PS-1 gene in embryonic day 12 (E12)-E15 rat brain and two transcripts in E18-adult rat brain. Therefore, PS-1 may play a role in neurogenesis.

Exposure of rat hippocampal neurons to amyloid beta peptide (25-35) induces the inactivation of phosphatidyl inositol-3 kinase and the activation of tau protein kinase I/glycogen synthase kinase-3 beta.

Exposure of rat hippocampal neurons to the peptide amyloid beta (A beta) (25-35) as well as A beta (1-40) peptides enhances phosphorylation of tau to a paired helical filament (PHF)-state through activation of tau protein kinase I (TPK I)/glycogen synthase kinase-3 beta (GSK-3 beta) [Busciglio, J., Lorenzo, A., Yeh, J. and Yankner, B.A., Neuron, 14 (1995) 879-888; Takashima, A., Ishiguro, K., Noguchi, K., Michel, G., Hoshi, M., Sato, K., Takahashi, M., Hoshino, T., Uchida, T. and Imahori, K., Neurosci. Meeting Abstr., 671 (1995) 17]. In order to examine the effects of A beta treatment on intracellular signaling mechanism, we have investigated the role of phosphatidyl inositol-3 (PI-3) kinase in the phosphorylation of tau. A beta (25-35) exposure induced an inactivation of PI-3 kinase and an activation of TPK I/GSK-3 beta in rat hippocampal culture. Wortmannin, an inhibitor of PI-3 kinase, also activated TPK I/GSK-3 beta, leading to an enhancement of tau phosphorylation and neuronal death in hippocampal culture. These results suggest that A beta (25-35) inhibition of PI-3 kinase results in the activation of TPK I/GSK-3 beta, the phosphorylation of tau, and resultant neuronal death in rat hippocampal neurons.

Correlation among secondary structure, amyloid precursor protein accumulation, and neurotoxicity of amyloid beta(25-35) peptide as analyzed by single alanine substitution.

Structure-neurotoxicity relationships of amyloid beta (25-35) peptide were studied by replacing each amino acid with Ala. In contrast to the general tendency in hydrophobicity-toxicity relationships, replacement of Asn27 yielded a more hydrophobic but less toxic analog and that of Met35 gave a less hydrophobic but more toxic one. Sedimentation profiles and CD spectra indicated that peptide aggregation via intermolecular beta-sheet formation is essential for the neurotoxicity of amyloid beta (25-35) peptide. The correlation between neurotoxicity and amyloid precursor protein accumulation suggested that the latter is one of the pathways of the neuronal death caused by amyloid beta protein.

Analysis of phosphorylation of tau with antibodies specific for phosphorylation sites.

Previously, we determined sites of tau protein phosphorylation by tau protein kinase (TPK) I/glycogen synthase kinase 3 beta (GSK-3 beta) and TPKII/(cyclin-dependent kinase 5 (CDK5) + p23). We prepared antibodies specific for these sites of tau phosphorylated by TPKI and TPKII, using chemically synthesized phosphopeptides as antigens. Each antibody specifically reacts with each phosphorylation site. With these antibodies, it was confirmed that TPKI and TPKII are responsible for these phosphorylation sites, as reported previously, except that Ser404 is also weakly phosphorylated by TPKI alone. It was also observed that TPKII-phosphorylation enhances TPKI-phosphorylation. These results indicate that these antibodies are useful tools for investigation of the phosphorylation of tau by TPKI and TPKII.