Hierarchical Phosphorylation of Recombinant Tau by the Paired-Helical Filament-Associated Protein Kinase Is Dependent on Cyclic AMP-Dependent Protein Kinase

Abstract : Immunoaffinity-purified paired helical filaments (PHFs) from Alzheimer's disease (AD) brain homogenates contain an associated protein kinase activity that is able to induce the phosphorylation of PHF proteins on addition of exogenous MgCl₂ and ATP. PHF kinase activity is shown to be present in immunoaffinity-purified PHFs from both sporadic and familial AD, Down's syndrome, and Pick's disease but not from normal brain homogenates. Although initial studies failed to show that the kinase was able to induce the phosphorylation of tau, additional studies presented in this article show that only cyclic AMP-dependent protein kinase-pretreated recombinant tau is a substrate for the PHF kinase activity. Deletional mutagenesis, phosphopeptide mapping, and site-directed mutagenesis have identified the PHF kinase phosphorylation sites as amino acids Thr³⁶¹ and Ser⁴¹² in htau40. In addition, the cyclic AMP-dependent protein kinase phosphorylation sites that direct the PHF kinase have been mapped to amino acids Ser³⁵⁶ and Ser⁴⁰⁹ in htau40. Additional data demonstrate that these hierarchical phosphorylations in the extreme C terminus of tau allow for the incorporation of recombinant tau into exogenously added AD-derived PHFs, providing evidence that certain unique phosphorylations of tau may play a role in the pathogenesis of neurofibrillary pathology in AD.     

The Excitotoxin Quinolinic Acid Induces Tau Phosphorylation in Human Neurons

Some of the tryptophan catabolites produced through the kynurenine pathway (KP), and more particularly the excitotoxin quinolinic acid (QA), are likely to play a role in the pathogenesis of Alzheimer''s disease (AD). We have previously shown that the KP is over activated in AD brain and that QA accumulates in amyloid plaques and within dystrophic neurons. We hypothesized that QA in pathophysiological concentrations affects tau phosphorylation. Using immunohistochemistry, we found that QA is co-localized with hyperphosphorylated tau (HPT) within cortical neurons in AD brain. We then investigated in vitro the effects of QA at various pathophysiological concentrations on tau phosphorylation in primary cultures of human neurons. Using western blot, we found that QA treatment increased the phosphorylation of tau at serine 199/202, threonine 231 and serine 396/404 in a dose dependent manner. Increased accumulation of phosphorylated tau was also confirmed by immunocytochemistry. This increase in tau phosphorylation was paralleled by a substantial decrease in the total protein phosphatase activity. A substantial decrease in PP2A expression and modest decrease in PP1 expression were observed in neuronal cultures treated with QA. These data clearly demonstrate that QA can induce tau phosphorylation at residues present in the PHF in the AD brain. To induce tau phosphorylation, QA appears to act through NMDA receptor activation similar to other agonists, glutamate and NMDA. The QA effect was abrogated by the NMDA receptor antagonist memantine. Using PCR arrays, we found that QA significantly induces 10 genes in human neurons all known to be associated with AD pathology. Of these 10 genes, 6 belong to pathways involved in tau phosphorylation and 4 of them in neuroprotection. Altogether these results indicate a likely role of QA in the AD pathology through promotion of tau phosphorylation. Understanding the mechanism of the neurotoxic effects of QA is essential in developing novel therapeutic strategies for AD.     

Phosphorylation of microtubule-associated protein tau by Ca2+/calmodulin-dependent protein kinase II in its tubulin binding sites

The paired helical filaments (PHF) found in Alzheimer's disease (AD) brain are composed mainly of the hyperphosphorylated form of microtubule-associated protein tau (PHF-tau). It is well known that tau is a good in vitro substrate for Ca(2+)/calmodulin-dependent protein kinase II (CaM kinase II). To establish the phosphorylation sites, the longest human tau (hTau40) was bacterially expressed and phosphorylated by CaM kinase II, followed by digestion with lysyl endoprotease. The digests were subjected to liquid chromatography/mass spectrometry. We found that 5 of 22 identified peptides were phosphorylated. From the tandem mass spectrometry, two phosphorylation sites (serines 262 and 356) were identified in the tubulin binding sites. When tau was phosphorylated by CaM kinase II, the binding of tau to taxol-stabilized microtubules was remarkably impaired. As both serines 262 and 356 are reportedly phosphorylated in PHF-tau, CaM kinase II may be involved in hyperphosphorylation of tau in AD brain.     

One of the antigenic determinants of paired helical filaments is related to tau protein

Paired helical filaments (PHF) are unusual neuronal fibers which accumulate progressively in the brain in Alzheimer's disease (AD). The insolubility of PHF in various kinds of solvents enabled us to obtain highly purified PHF, but prevented the application of conventional analytical methods to identify their components. Here we report that antibodies against purified PHF recognize tau protein, a brain-specific microtubule-associated protein, suggesting that a portion of PHF is tau protein.     

p38 kinase is activated in the Alzheimer's disease brain

The p38 mitogen-activated protein kinase is a stress-activated enzyme responsible for transducing inflammatory signals and initiating apoptosis. In the Alzheimer's disease (AD) brain, increased levels of phosphorylated (active) p38 were detected relative to age-matched normal brain. Intense phospho-p38 immunoreactivity was associated with neuritic plaques, neuropil threads, and neurofibrillary tangle-bearing neurons. The antibody against phosphorylated p38 recognized many of the same structures as an antibody against aberrantly phosphorylated, paired helical filament (PHF) tau, although PHF-positive tau did not cross-react with the phospho-p38 antibody. These findings suggest a neuroinflammatory mechanism in the AD brain, in which aberrant protein phosphorylation affects signal transduction elements, including the p38 kinase cascade, as well as cytoskeletal components.     

Passive Immunization with Phospho-Tau Antibodies Reduces Tau Pathology and Functional Deficits in Two Distinct Mouse Tauopathy Models

In Alzheimer’s disease (AD), an extensive accumulation of extracellular amyloid plaques and intraneuronal tau tangles, along with neuronal loss, is evident in distinct brain regions. Staging of tau pathology by postmortem analysis of AD subjects suggests a sequence of initiation and subsequent spread of neurofibrillary tau tangles along defined brain anatomical pathways. Further, the severity of cognitive deficits correlates with the degree and extent of tau pathology. In this study, we demonstrate that phospho-tau (p-tau) antibodies, PHF6 and PHF13, can prevent the induction of tau pathology in primary neuron cultures. The impact of passive immunotherapy on the formation and spread of tau pathology, as well as functional deficits, was subsequently evaluated with these antibodies in two distinct transgenic mouse tauopathy models. The rTg4510 transgenic mouse is characterized by inducible over-expression of P301L mutant tau, and exhibits robust age-dependent brain tau pathology. Systemic treatment with PHF6 and PHF13 from 3 to 6 months of age led to a significant decline in brain and CSF p-tau levels. In a second model, injection of preformed tau fibrils (PFFs) comprised of recombinant tau protein encompassing the microtubule-repeat domains into the cortex and hippocampus of young P301S mutant tau over-expressing mice (PS19) led to robust tau pathology on the ipsilateral side with evidence of spread to distant sites, including the contralateral hippocampus and bilateral entorhinal cortex 4 weeks post-injection. Systemic treatment with PHF13 led to a significant decline in the spread of tau pathology in this model. The reduction in tau species after p-tau antibody treatment was associated with an improvement in novel-object recognition memory test in both models. These studies provide evidence supporting the use of tau immunotherapy as a potential treatment option for AD and other tauopathies.     

Ubiquitination and abnormal phosphorylation of paired helical filaments in Alzheimer's disease

The most characteristic cellular change in Alzheimer's disease is the accumulation of aberrant filaments, the paired helical filaments (PHF), in the affected neurons. There is growing evidence from a number of laboratories that dementia correlates better with the accumulation of PHF than of the extracellular amyloid, the second major lesion of Alzheimer's disease. PHF are both morphologically and biochemically unlike any of the normal neurofibrils. The major polypeptides in isolated PHF are microtubule-associated protein tau. Tau in PHF is phosphorylated differently from tau in microtubules. This abnormal phosphorylation of tau in PHF occurs at several sites. The accumulation of abnormally phosphorylated tau in the affected neurons in Alzheimer's disease brain precedes both the formation and the ubiquitination of the neurofibrillary tangles. In Alzheimer's disease brain, tubulin is assembly competent, but the in vitro assembly of microtubules is not observed. In vitro, the phosphate groups in PHF are less accessible than those of tau to alkaline phosphatase. The in vitro dephosphorylated PHF polypeptides stimulate microtubule assembly from bovine tubulin. It is hypothesized that a defect in the protein phosphorylation/dephosphorylation system is one of the earliest events in the cytoskeletal pathology in Alzheimer's disease. Production of nonfunctional tau by its phosphorylation and its polymerization into PHF most probably contributes to a microtubule assembly defect, and consequently, to a compromise in both axoplasmic flow and neuronal function. Index Entries: Alzheimer's disease; mechanisms of neuronal degeneration; neurofibrillary changes; paired helical filaments: biochemistry; microtubule-associated protein tau; abnormal phosphorylation; ubiquitination; microtubule assembly; axoplasmic flow; protein phosphorylation/dephosphorylation.     

蛋白磷酸酯酶对Alzheimer神经原纤维缠结的松解作用

神经原纤维缠结是Ａｌｚｈｅｉｍｅｒ患者的特征性脑病理损伤,其形成机制至今不明．根据神经原纤维缠结的基本组分是异常磷酸化ｔａｕ蛋白的聚集形式双螺旋丝（ｐａｉｒｅｄｈｅｌｉｃａｌｆｉｌａｍｅｎｔｓ,ＰＨＦ）的研究结果,推测蛋白磷酸酯酶与蛋白激酶的失衡可能与ＰＨＦ的形成有关．将蛋白磷酸酯酶ＰＰ－２Ａ和ＰＰ－２Ｂ与ＰＨＦ一起在３７℃保温３０ｍｉｎ可使ＰＨＦ缠结结构松解,成为单个ＰＨＦ原纤维,延长去磷酸化反应时间至３ｈ可使ＰＨＦ结构进一步松解,释放一些游离ＰＨＦ原纤维片段．放免印迹定量分析结果表明：ＰＰ－２Ａ处理的ＰＨＦ样品比对照者释放游离ｔａｕ蛋白的量增加２５％．此外,ＰＰ－２Ａ和ＰＰ－２Ｂ去磷酸化的ＰＨＦ对脑中钙激活的中性蛋白水解酶的抗性降低．这些研究资料从结构上显示了Ａｌｚｈｅｉｍｅｒ病脑病理损伤的可逆性,为Ａｌｚｈｅｉｍｅｒ病治疗的可能性提供了实验依据     

Two-Dimensional Characterization of Paired Helical Filament-Tau from Alzheimer's Disease: Demonstration of an Additional 74-kDa Component and Age-Related Biochemical Modifications

Abstract: PHF-tau proteins are the major components of the paired helical filament (PHF) from Alzheimer's disease (AD) neurofibrillary lesions. They differ both qualitatively and quantitatively in their degree of phosphorylation when compared with native tau proteins. However, little is known about the extent and heterogeneity of phosphorylated sites or the isoform composition and the isoelectric variants of PHF-tau. Therefore, we have characterized PHF-tau proteins from cortical brain tissue homogenates of 13 AD patients using two-dimensional gel electrophoresis. Whatever the topographical origin of brain tissue homogenates, PHF-tau proteins shared the same two-dimensional gel electrophoresis profile made of a tau triplet of 55, 64, and 69 kDa. A 74-kDa hyperphosphorylated tau component was detected particularly in the youngest and most severely affected AD patients. This additional component of hyperphosphorylated tau was shown to correspond to the longest brain tau isoform. Furthermore, the isoelectric points of PHF-tau from older AD patients were significantly more basic, indicating a lower degree of phosphorylation. These results show that the severity of neurofibrillary degeneration of AD is modulated by age.     

Analysis of N-glycans of pathological tau: possible occurrence of aberrant processing of tau in Alzheimer's disease

In a previous study [Wang et al. (1996) Nat. Med. 2, 871-875], Wang et al. found (i) that abnormally hyperphosphorylated tau (AD P-tau) isolated from Alzheimer's disease (AD) brain as paired helical filaments (PHF)-tau and as cytosolic AD P-tau but not tau from normal brain were stained by lectins, and (ii) that on in vitro deglycosylation the PHF untwisted into sheets of thin straight filaments, suggesting that tau only in AD brains is glycosylated. To elucidate the primary structure of N-glycans, we comparatively analyzed the N-glycan structures obtained from PHF-tau and AD P-tau. More than half of N-glycans found in PHF-tau and AD P-tau were different. High mannose-type sugar chains and truncated N-glycans were found in both taus in addition to a small amount of sialylated bi- and triantennary sugar chains. More truncated glycans were richer in PHF-tau than AD P-tau. This enrichment of more truncated glycans in PHF might be involved in promoting the assembly and or stabilizing the pathological fibrils in AD.     

Alzheimer's Disease Neurofibrillary Tangles Contain Mitosis-Specific Phosphoepitopes

Abstract: Paired helical filaments (PHFs) are the major components of neurofibrillary lesions present in Alzheimer's disease (AD). PHFs are composed of the microtubule-associated protein (MAP) τ, which is abnormally phosphorylated in AD. Normal fetal τ is also phosphorylated and shares certain phosphoepitopes with PHF-τ. The abnormal phosphorylation of PHF-τ is considered to be involved in the formation of PHFs and subsequent degeneration of AD neurons. We have previously shown that other neuronal MAPs, such as MAP1B, contain mitosis-specific phosphoepitopes. In addition to mitotic cells, these epitopes are also expressed in fetal brain and PC12 cells during differentiation and neurite outgrowth. One hypothesis regarding the etiology of AD involves the reactivation of a fetal-like state and mitotic conditions in selected neurons. To determine if similar mitosis-associated phosphoepitopes appeared in AD, sections of hippocampal tissue were stained for immunoreactivity with antibodies recognizing both τ and mitotic phosphoepitopes. Both the MPM2 mitotic phosphoepitope antibody and the AT8 PHF-τ antibody stained neurofibrillary lesions and colocalized to pyramidal neurons in AD samples. In addition, PHFs isolated from an AD brain reacted with both antibodies. The MPM2 antibody specifically reacted with τ in the isolated PHF fraction but not normal adult τ. In addition, MPM2 failed to react with normal fetal or adult τ obtained from rat brains. The MPM2 antibody also recognized human MAP1B; however, MAP1B was not present in the PHF fraction. Our results indicate that MPM2 recognized a phosphoepitope present on PHF-τ. Because normal fetal or adult rat brain τ did not express the MPM2 epitope, it is likely that this phosphoepitope is specific for the disease state.     

Developmental regulation of tau phosphorylation, tau kinases, and tau phosphatases

Tau is a neuronal microtubule-associated protein. Its hyperphosphorylation plays a critical role in Alzheimer disease (AD). Expression and phosphorylation of tau are regulated developmentally, but its dynamic regulation and the responsible kinases or phosphatases remain elusive. Here, we studied the developmental regulation of tau in rats during development from embryonic day 15 through the age of 24 months. We found that tau expression increased sharply during the embryonic stage and then became relatively stable, whereas tau phosphorylation was much higher in developing brain than in mature brain. However, the extent of tau phosphorylation at seven of the 14 sites studied was much less in developing brain than in AD brain. Tau phosphorylation during development matched the period of active neurite outgrowth in general. Tau phosphorylation at various sites had different topographic distributions. Several tau kinases appeared to regulate tau phosphorylation collectively at overlapping sites, and the decrease of overall tau phosphorylation in adult brain might be due to the higher levels of tau phosphatases in mature brain. These studies provide new insight into the developmental regulation of site-specific tau phosphorylation and identify the likely sites required for the abnormal hyperphosphorylation of tau in AD.     

Transglutaminase bonds in neurofibrillary tangles and paired helical filament tau early in Alzheimer's disease.

Transglutaminase-catalyzed epsilon(gamma-glutamyl)lysine cross-links exist in Alzheimer's disease (AD) paired helical filament (PHF) tau protein but not normal soluble tau. To test the hypothesis that these cross-links could play a role in the formation of neurofibrillary tangles (NFT), we used single- and double-label immunofluorescence confocal microscopy and immunoaffinity purification and immunoblotting to examine epsilon(gamma-glutamyl)lysine cross-links in AD and control brains. The number of neurons that are immunoreactive with an antibody directed at the epsilon-(gamma-glutamyl)lysine bond was significantly higher in AD cortex compared with age-matched controls and schizophrenics. PHF tau-directed antibodies AT8, MC-1 and PHF-1 co-localized with epsilon(gamma-glutamyl)lysine immunolabeling in AD NFT. Immunoaffinity purification and immunoblotting experiments demonstrated that PHF tau contains epsilon(gamma-glutamyl)lysine bonds in parietal and frontal cortex in AD. In control cases with NFT present in the entorhinal cortex and hippocampus, indicative of Braak and Braak stage II, epsilon(gamma-glutamyl)lysine bonds were present in PHF tau in parietal and frontal cortex, despite the lack of microscopically detectable NFT or senile plaques in these cortical regions. The presence of PHF tau with epsilon(gamma-glutamyl)lysine bonds in brain regions devoid of NFT in stage II (but regions, which would be expected to contain NFT in stage III) suggests that these bonds occur early in the formation of NFT.     

A potent mechanism-inspired O-GlcNAcase inhibitor that blocks phosphorylation of tau in vivo

Pathological hyperphosphorylation of the microtubule-associated protein tau is characteristic of Alzheimer's disease (AD) and the associated tauopathies. The reciprocal relationship between phosphorylation and O-GlcNAc modification of tau and reductions in O-GlcNAc levels on tau in AD brain offers motivation for the generation of potent and selective inhibitors that can effectively enhance O-GlcNAc in vertebrate brain. We describe the rational design and synthesis of such an inhibitor (thiamet-G, K(i) = 21 nM; 1) of human O-GlcNAcase. Thiamet-G decreased phosphorylation of tau in PC-12 cells at pathologically relevant sites including Thr231 and Ser396. Thiamet-G also efficiently reduced phosphorylation of tau at Thr231, Ser396 and Ser422 in both rat cortex and hippocampus, which reveals the rapid and dynamic relationship between O-GlcNAc and phosphorylation of tau in vivo. We anticipate that thiamet-G will find wide use in probing the functional role of O-GlcNAc in vertebrate brain, and it may also offer a route to blocking pathological hyperphosphorylation of tau in AD.     

beta-Amyloid induces paired helical filament-like tau filaments in tissue culture

Paired helical filaments (PHF) are the principal pathologic components of neurofibrillary tangles in Alzheimer's disease (AD). To reproduce the formation of PHF in tissue culture, we stably expressed human tau with and without pathogenic mutations in human SH-SY5Y cells and exposed them for 5 days to aggregated synthetic beta-amyloid peptide (A beta 42). This caused a decreased solubility of tau along with the generation of PHF-like tau-containing filaments. These were 20 nm wide and had periodicities of 130-140 nm in the presence of P301L mutant tau or 150-160 nm in the presence of wild-type tau. Mutagenesis of the phosphoepitope serine 422 of tau prevented both the A beta 42-mediated decrease in solubility and the generation of PHF-like filaments, suggesting a role of serine 422 or its phosphorylation in tau filament formation. Together, our data underscore a role of A beta 42 in the formation of PHF-like filaments. Our culture system will be useful to map phosphoepitopes of tau involved in PHF formation and to identify and characterize modifiers of the tau pathology. Further adaptation of the system may allow the screening and validation of compounds designed to prevent PHF formation.     

Charge-pairing mechanism of phosphorylation effect upon amyloid fibrillation of human tau core peptide

Phosphorylation of a fibrillogenic protein, human tau, is believed to play crucial roles in the pathogenesis of Alzheimer's disease. For elucidating molecular mechanisms of the phosphorylation effect on tau fibrillation, we synthesized a peptide, VQIVY 310K (PHF6) and its phosphorylated derivative (PHF6pY). PHF6 is a partial peptide surrounding a plausible in vivo phosphorylation site Tyr310 and forms amyloid-type fibrils similar to those generated by full-length tau. Fibrillation of PHF6 and PHF6pY were studied by spectroscopic and microscopic methods, and the critical concentration of the fibrillation was determined for comparing the fibril stability. The results showed that the phosphorylation strongly influenced the fibrillation propensity of PHF6 by changing its dependency on pH and ionic strength. On the basis of the observations, we suggested that charged sites on the phosphate group and its electrostatic pairing with the neighboring charged residues were physical origins of the phosphorylation effect. To verify this charge-pairing mechanism, we conducted experiments using a series of PHF6 derivatives with non-native charge distributions. The electrostatic interaction in an intermolecular mode was also demonstrated by the system composed of two different peptide species, which found that fibrillation of nonphosphorylated PHF6 was drastically enhanced when a trace amount of phosphorylated PHF6 molecules coexisted. A simulation analysis utilizing crystal coordinates of the PHF6 fibril was also performed for interpreting the experimental results in a molecular level. The present study using the model peptide system gave us a microscopically insightful view on the roles of tau phosphorylation in amyloid-related diseases.     

Degradation of tau protein by puromycin-sensitive aminopeptidase in vitro

Tau, a microtubule associated protein, aggregates into intracellular paired helical filaments (PHFs) by an unknown mechanism in Alzheimer's disease (AD) and other tauopathies. A contributing factor may be a failure to metabolize free cytosolic tau within the neuron. The buildup of tau may then drive the aggregation process through mass action. Therefore, proteases that normally degrade tau are of great interest. A recent genetic screen identified puromycin-sensitive aminopeptidase (PSA) as a potent modifier of tau-induced pathology and suggested PSA as a possible tau-degrading enzyme. Here we have extended these observations using human recombinant PSA purified from Escherichia coli. The enzymatic activity and characteristics of the purified PSA were verified using chromogenic substrates, metal ions, and several specific and nonspecific protease inhibitors, including puromycin. PSA was shown to digest recombinant human full-length tau in vitro, and this activity was hindered by puromycin. The mechanism of amino terminal degradation of tau was confirmed using a novel N-terminal cleavage-specific tau antibody (Tau-C6g, specific for cleavage between residues 13-14) and a C-terminal cleavage-specific tau antibody (Tau-C3). Additionally, PSA was able to digest soluble tau purified from normal human brain to a greater extent than either soluble or PHF tau purified from AD brain, indicating that post-translational modifications and/or polymerization of tau may affect its digestion by PSA. These results are consistent with observations that PSA modulates tau levels in vivo and suggest that this enzyme may be involved in tau degradation in human brain.     

Role of glycosylation in hyperphosphorylation of tau in Alzheimer's disease

In Alzheimer's disease (AD) brain, microtubule-associated protein tau is abnormally modified by hyperphosphorylation and glycosylation, and is aggregated as neurofibrillary tangles of paired helical filaments. To investigate the role of tau glycosylation in neurofibrillary pathology, we isolated various pools of tau protein from AD brain which represent different stages of tau pathology. We found that the non-hyperphosphorylated tau from AD brain but not normal brain tau was glycosylated. Monosaccharide composition analyses and specific lectin blots suggested that the tau in AD brain was glycosylated mainly through N-linkage. In vitro phosphorylation indicated that the glycosylated tau was a better substrate for cAMP-dependent protein kinase than the deglycosylated tau. These results suggest that the glycosylation of tau is an early abnormality that can facilitate the subsequent abnormal hyperphosphorylation of tau in AD brain.     

Isoaspartate formation and neurodegeneration in Alzheimer's disease

We reviewed here that protein isomerization is enhanced in amyloid-beta peptides (Abeta) and paired helical filaments (PHFs) purified from Alzheimer's disease (AD) brains. Biochemical analyses revealed that Abeta purified from senile plaques and vascular amyloid are isomerized at Asp-1 and Asp-7. A specific antibody recognizing isoAsp-23 of Abeta further suggested the isomerization of Abeta at Asp-23 in vascular amyloid as well as in the core of senile plaques. Biochemical analyses of purified PHFs also revealed that heterogeneous molecular weight tau contains L-isoaspartate at Asp-193, Asn-381, and Asp-387, indicating a modification, other than phosphorylation, that differentiates between normal tau and PHF tau. Since protein isomerization as L-isoaspartate causes structural changes and functional inactivation, or enhances the aggregation process, this modification is proposed as one of the progression factors in AD. Protein L-isoaspartyl methyltransferase (PIMT) is suggested to play a role in the repair of isomerized proteins containing L-isoaspartate. We show here that PIMT is upregulated in neurodegenerative neurons and colocalizes in neurofibrillary tangles (NFTs) in AD. Taken together with the enhanced protein isomerization in AD brains, it is implicated that the upregulated PIMT may associate with increased protein isomerization in AD. We also reviewed studies on PIMT-deficient mice that confirmed that PIMT plays a physiological role in the repair of isomerized proteins containing L-isoaspartate. The knockout study also suggested that the brain of PIMT-deficient mice manifested neurodegenerative changes concomitant with accumulation of L-isoaspartate. We discuss the pathological implications of protein isomerization in the neurodegeneration found in model mice and AD.     

Increased tau phosphorylation on mitogen-activated protein kinase consensus sites and cognitive decline in transgenic models for Alzheimer's disease and FTDP-17: evidence for distinct molecular processes underlying tau abnormalities

Abnormal tau phosphorylation occurs in several neurodegenerative disorders, including Alzheimer's disease (AD) and frontotemporal dementia with Parkinsonism linked to chromosome 17 (FTDP-17). Here, we compare mechanisms of tau phosphorylation in mouse models of FTDP-17 and AD. Mice expressing a mutated form of human tau associated with FTDP-17 (tau(V337M)) showed age-related increases in exogenous tau phosphorylation in the absence of increased activation status of a number of kinases known to phosphorylate tau in vitro. In a "combined" model, expressing both tau(V337M) and the familial amyloid precursor protein AD mutation APP(V717I) in a CT100 fragment, age-dependent tau phosphorylation occurred at the same sites and was significantly augmented compared to "single" tau(V337M) mice. These effects were concomitant with increased activation status of mitogen-activated protein kinase (MAPK) family members (extracellular regulated kinases 1 and 2, p38, and c-Jun NH(2)-terminal kinase) but not glycogen synthase kinase-3alphabeta or cyclin-dependent kinase 5. The increase in MAPK activation was a discrete effect of APP(V717I)-CT100 transgene expression as near identical changes were observed in single APP(V717I)-CT100 mice. Age-dependent deficits in memory were also associated with tau(V337M) and APP(V717I)-CT100 expression. The data reveal distinct routes to abnormal tau phosphorylation in models of AD and FTDP-17 and suggest that in AD, tau irregularities may be linked to processing of APP C-terminal fragments via specific effects on MAPK activation status.