Stabilization of the cyclin-dependent kinase 5 activator,p35, by paclitaxel decreases beta-amyloid toxicity in cortical neurons

One hallmark of Alzheimer's disease (AD) is the formation of neurofibrillary tangles, aggregated paired helical filaments composed of hyperphosphorylated tau. Amyloid-beta (Abeta) induces tau hyperphosphorylation, decreases microtubule (MT) stability and induces neuronal death. MT stabilizing agents have been proposed as potential therapeutics that may minimize Abeta toxicity and here we report that paclitaxel (taxol) prevents cell death induced by Abeta peptides, inhibits Abeta-induced activation of cyclin-dependent kinase 5 (cdk5) and decreases tau hyperphosphorylation. Taxol did not inhibit cdk5 directly but significantly blocked Abeta-induced calpain activation and decreased formation of the cdk5 activator, p25, from p35. Taxol specifically inhibited the Abeta-induced activation of the cytosolic cdk5-p25 complex, but not the membrane-associated cdk5-p35 complex. MT-stabilization was necessary for neuroprotection and inhibition of cdk5 but was not sufficient to prevent cell death induced by overexpression of p25. As taxol is not permeable to the blood-brain barrier, we assessed the potential of taxanes to attenuate Abeta toxicity in adult animals using a succinylated taxol analog (TX67) permeable to the blood-brain barrier. TX67, but not taxol, attenuated the magnitude of both basal and Abeta-induced cdk5 activation in acutely dissociated cortical cultures prepared from drug treated adult mice. These results suggest that MT-stabilizing agents may provide a therapeutic approach to decrease Abeta toxicity and neurofibrillary pathology in AD and other tauopathies.     

Species-specific and conserved epitopes on mouse and human E-selectin important for leukocyte adhesion

The cdk5 and its activator p35 constitute one of the main tau-phosphorylating systems in neuronal cells. Under normal conditions for neurons, its activity is required for modulating tau involvement in neuronal polarity and in development of the mammalian central nervous system. Recently, we reported that the treatment of rat hippocampal cells in culture with fibrillary beta-amyloid (Abeta) results in deregulation of the protein kinase cdk5. The neurotoxic effects of Abeta fibrils were prevented by inhibition of cdk5 activity by butyrolactone I or by using antisense oligonucleotides that control the expression of this kinase. Here, we show that the Abeta-promoted increase of cdk5 activity is associated with changes in tau phosphorylation patterns and in the intraneuronal distribution of tau. In addition to hippocampal cells, deregulation of cdk5 was observed in other cell types. However, butyrolactone I prevented Abeta-induced cell death only in neuronal cells in which cdk5 activation was sensitive to Abeta fibrils. This lost of cdk5 regulation in hippocampal cells exposed to Abeta fibrils appears to be associated with an increase in the cdk5-p35 complex stability. Complex stabilization was sensitive to phosphorylation of cdk5. However, no changes in cdk5 and p35 mRNAs were observed, suggesting that the main effects on cdk5 occur at the posttranslational level. These studies indicate that cdk5 phosphorylation and the formation of an abnormally active cdk5-p35 complex are directly involved in the molecular paths leading to the neurodegenerative process of rat hippocampal neurons triggered by Abeta fibrils.     

Beta-amyloid treatment of two complementary P301L tau-expressing Alzheimer's disease models reveals similar deregulated cellular processes

Alzheimer's disease (AD) is characterized by Abeta peptide-containing plaques and tau-containing neurofibrillary tangles (NFTs). Both pathologies have been combined by crossing Abeta plaque-forming APP mutant mice with NFT-forming P301L tau mutant mice or by stereotaxically injecting beta-amyloid peptide 1-42 (Abeta42) into brains of P301L tau mutant mice. In cell culture, Abeta42 induces filamentous tau aggregates. To understand which processes are disrupted by Abeta42 in the presence of tau aggregates, we applied comparative proteomics to Abeta42-treated P301L tau-expressing neuroblastoma cells and the amygdala of P301L tau transgenic mice stereotaxically injected with Abeta42. Remarkably, a significant fraction of proteins altered in both systems belonged to the same functional categories, i.e. stress response and metabolism. We also identified model-specific effects of Abeta42 treatment such as differences in cell signaling proteins in the cellular model and of cytoskeletal and synapse associated proteins in the amygdala. By Western blotting (WB) and immunohistochemistry (IHC), we were able to show that 72% of the tested candidates were altered in human AD brain with a major emphasis on stress-related unfolded protein responsive candidates. These data highlight these processes as potentially important initiators in the Abeta42-mediated pathogenic cascade in AD and further support the role of unfolded proteins in the course of AD.     

Neuroprotective effect of TNFalpha against the beta-amyloid neurotoxicity mediated by CDK5 kinase

The tumor necrosis factor alpha (TNFalpha) plays a dual role in producing either neurodegeneration or neuroprotection in the central nervous system. Despite that TNFalpha was initially described as a cell death inductor, neuroprotective effects against cell death induced by several neurotoxic insults have been reported. Tau hyperphosphorylation and neuronal death found in Alzheimer disease is mediated by deregulation of the cdk5/p35 complex induced by Abeta treatments. Since TNFalpha affects cdk5 activity, we investigated its possible protective role against the Abeta-induced neurodegeneration, as mediated by cdk5. TNFalpha pretreatments significantly reduced the hippocampal neuronal cell death induced by the effects of Abeta(42) peptide. In addition, this pretreatment reduced the increase in the activity of cdk5 induced by Abeta(42) in primary neurons. Next, we investigated the Alzheimer type phosphorylation of tau protein induced by Abeta(42). We observed that the pretreatment of neurons with TNFalpha reduces tau hyperphosphorylation. Taken together, these results define a novel neuroprotective effect of TNFalpha in preventing neuronal cell death and cdk5-dependent tau hyperphosphorylation. This phenomenon, taken together with other previous findings, suggests that the inflammatory response due to Abeta peptide plays a key role in the development of Alzheimer etiopathogenesis.     

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.     

Inhibition of protein phosphatase 2A overrides tau protein kinase I/glycogen synthase kinase 3 beta and cyclin-dependent kinase 5 inhibition and results in tau hyperphosphorylation in the hippocampus of starved mouse.

Hyperphosphorylated tau is the major component of paired helical filaments in neurofibrillary tangles found in Alzheimer's disease (AD) brain. Starvation of adult mice induces tau hyperphosphorylation at many paired helical filaments sites and with a similar regional selectivity as those in AD, suggesting that a common mechanism may be mobilized. Here we investigated the mechanism of starvation-induced tau hyperphosphorylation in terms of tau kinases and Ser/Thr protein phosphatases (PP), and the results were compared with those reported in AD brain. During starvation, tau hyperphosphorylation at specific epitopes was accompanied by decreases in tau protein kinase I/glycogen synthase kinase 3 beta (TPKI/GSK3 beta), cyclin-dependent kinase 5 (cdk5), and PP2A activities toward tau. These results demonstrate that the activation of TPKI/GSK3 beta and cdk5 is not necessary to obtain hyperphosphorylated tau in vivo, and indicate that inhibition of PP2A is likely the dominant factor in inducing tau hyperphosphorylation in the starved mouse, overriding the inhibition of key tau kinases such as TPKI/GSK3 beta and cdk5. Furthermore, these data give strong support to the hypothesis that PP2A is important for the regulation of tau phosphorylation in the adult brain, and provide in vivo evidence in support of a central role of PP2A in tau hyperphosphorylation in AD.     

Inhibition of tau phosphorylating protein kinase cdk5 prevents beta-amyloid-induced neuronal death.

The key target of this study was the tau protein kinase II system (TPK II) involving the catalytic subunit cdk5 and the regulatory component p35. TPK II is one of the tau phosphorylating systems in neuronal cells, thus regulating its functions in the cytoskeletal dynamics and the extension of neuronal processes. This research led to demonstration that the treatment of rat hippocampal cells in culture with fibrillary beta-amyloid (Abeta) results in a significant increase of the cdk5 enzymatic activity. Interestingly, the data also showed that the neurotoxic effect of 1-20 microM Abeta on primary cultures markedly diminished with co-incubation of hippocampal cells with the amyloid fibers plus the cdk5 inhibitor butyrolactone I. This inhibitor protected brain cells against Abeta-induced cell death in a concentration dependent fashion. Moreover, death was also prevented by a cdk5 antisense probe, but not by an oligonucleotide with a random sequence. The cdk5 antisense also reduced neuronal expression of cdk5 compared with the random oligonucleotide. The studies indicate that cdk5 plays a major role in the molecular path leading to the neurodegenerative process triggered by the amyloid fibers in primary cultures of rat hippocampal neurons. These findings are of interest in the context of the pathogenesis of Alzheimer's disease.     

Regulation of Phosphorylation of tau by Protein Kinases in Rat Brain

Microtubule associated protein tau is abnormally hyperphosphorylated in Alzheimer disease (AD) brain. To investigate the role of protein kinases involved in this lesion, metabolically active slices made from brains of adult rats were treated with or without various specific kinase activators in oxygenated artificial cerebrospinal fluid. The basal kinase activities of protein kinase-A (PKA), CaM Kinase II and GSK-3 were stimulated more than two-fold by isoproterenol, bradykinin and wortmannin, respectively. We found that cdk5 activity was co-stimulated with PKA by isoproterenol. Sequential activation of PKA (+cdk5), CaM Kinase II and GSK-3 produced hyperphosphorylation of tau at Ser-198/Ser-199/Ser-202, Ser-214, Thr-231/Ser-235, Ser-262, Ser-396/Ser-404 and Ser-422 sites. Like AD P-tau, the P-tau from brain slices bound to normal tau and its binding to tubulin was inhibited. These studies suggest that PKA, cdk5, CaM Kinase II and GSK-3 are involved in the regulation of phosphorylation of tau and that AD-type phosphorylation of tau is probably a product of the synergistic action of two or more of these kinases.     

Reduction of soluble Abeta and tau, but not soluble Abeta alone, ameliorates cognitive decline in transgenic mice with plaques and tangles

Increasing evidence points to soluble assemblies of aggregating proteins as a major mediator of neuronal and synaptic dysfunction. In Alzheimer disease (AD), soluble amyloid-beta (Abeta) appears to be a key factor in inducing synaptic and cognitive abnormalities. Here we report the novel finding that soluble tau also plays a role in the cognitive decline in the presence of concomitant Abeta pathology. We describe improved cognitive function following a reduction in both soluble Abeta and tau levels after active or passive immunization in advanced aged 3xTg-AD mice that contain both amyloid plaques and neurofibrillary tangles (NFTs). Notably, reducing soluble Abeta alone did not improve the cognitive phenotype in mice with plaques and NFTs. Our results show that Abeta immunotherapy reduces soluble tau and ameliorates behavioral deficit in old transgenic mice.     

Characterization of the in vitro phosphorylation of human tau by tau protein kinase II (cdk5/p20) using mass spectrometry

Hyperphosphorylated tau is an integral part of the neurofibrillary tangles that form within neuronal cell bodies, and tau protein kinase II is reported to play a role in the pathogenesis of Alzheimer's disease. Recently, we reported that tau protein kinase II (cdk5/p20)-phosphorylated human tau inhibits microtubule assembly, and tau protein kinase II (cdk5/p20) phosphorylation of microtubule-associated tau results in dissociation of phosphorylated tau from the microtubules and tubulin depolymerization. In the studies reported here, a combination of mass spectrometric techniques was used to study the phosphorylation of human recombinant tau by recombinant tau protein kinase II (cdk5/p20) in vitro. The extent of phosphorylation was determined by measuring the molecular mass of phosphorylated tau using mass spectrometry. Reaction of human recombinant tau with tau protein kinase II (cdk5/p20) resulted in the formation of two major species containing either five or six phosphate groups. The specific amino acid residues phosphorylated were determined by analyzing tryptic peptides by tandem mass spectrometry via either MALDI/TOF post-source decay or by electrospray tandem mass spectrometry. Based on these experiments, we conclude that tau protein kinase II (cdk5/p20) can phosphorylate human tau at Thr(181), Thr(205), Thr(212), Thr(217), Ser(396) and Ser(404).     

IL-5 blocks apoptosis and tau hyperphosphorylation induced by Aβ<Subscript>25–35</Subscript> peptide in PC12 cells

The primary features of Alzheimer’s disease (AD) are extracellular amyloid plaques consisting mainly of deposits of amyloid β (Aβ) peptides and intracellular neurofibrillary tangles (NFTs). Sets of evidence suggest that interleukin-5 (IL-5) is involved in the pathogenesis of AD. Herein, we investigated the protective role of IL-5 in PC12 cells, to provide new insights into understanding this disease. Western blot was employed to assess the protein levels of Bax and phospho-tau as well as phospho-JAK2; MTT assay was performed to decipher cell viability. Treatment of IL-5 decreased Aβ_25–35-induced tau phosphorylation and apoptosis, effects blunted by JAK2 inhibition. IL-5 prevents Aβ_25–35-evoked tau protein hyperphosphorylation and apoptosis through JAK2 signaling.     

Design, synthesis, and testing of an 6-O-linked series of benzimidazole based inhibitors of CDK5/p25

Alzheimer’s disease (AD) is a progressive neurodegenerative disease resulting in cognitive and behavioral impairment. The two classic pathological hallmarks of AD include extraneuronal deposition of amyloid ?? (A??) and intraneuronal formation of neurofibrillary tangles (NFTs). NFTs contain hyperphosphorylated tau. Tau is the major microtubule-associated protein in neurons and stabilizes microtubules (MTs). Cyclin dependent kinase 5 (CDK5), when activated by the regulatory binding protein p25, phosphorylates tau at a number of proline-directed serine/threonine residues, resulting in formation of phosphorylated tau as paired helical filaments (PHFs) then in subsequent deposition of PHFs as NFTs. Beginning with the structure of Roscovitine, a moderately selective CDK5 inhibitor, we sought to conduct structural modifications to increase inhibitory potency of CDK5 and increase selectivity over a similar enzyme, cyclin dependent kinase 2 (CDK2). The design, synthesis, and testing of a series of 1-isopropyl-4-aminobenzyl-6-ether-linked benzimidazoles is presented.     

Brain levels of CDK5 activator p25 are not increased in Alzheimer's or other neurodegenerative diseases with neurofibrillary tangles

Elevated levels of p25 and constitutive activation of CDK5 have been observed in AD brains. This has led to the hypothesis that increased p25 levels could promote neurofibrillary tangles (NFT) through CDK5-mediated hyperphosphorylation of tau, the principal component of NFTs. We examined p25 immunoreactivity in brains from sporadic and familial AD cases, as well as other neurologic diseases that exhibit NFT, such as Down's syndrome (DS), Pick's disease (Pick), corticobasal degeneration (CBD), progressive supranuclear palsy (PSP), frontotemporal dementia (FTD). Neither the p25 immunoreactivity nor the p25/p35 ratio was elevated in the AD brains or in the other tauopathies (n = 34) compared with controls (n = 11). Although Abeta peptides have been suggested to activate calpain-mediated cleavage of p35 to p25 in cultured neurons, p25 levels in brains of TgCRND8 mice, which express high levels of brain Abeta peptides, were similar to those of non-Tg littermates. Our data suggest that high Abeta levels in brain do not activate p35 proteolysis, and p25 is unlikely to be a causative agent for NFT formation in AD or other tauopathies.     

Formation of tau inclusions in knock-in mice with familial Alzheimer disease (FAD) mutation of presenilin 1 (PS1)

Mutations in the presenilin 1 (PS1) gene are responsible for the early onset of familial Alzheimer disease (FAD). Accumulating evidence shows that PS1 is involved in gamma-secretase activity and that FAD-associated mutations of PS1 commonly accelerate Abeta(1-42) production, which causes Alzheimer disease (AD). Recent studies suggest, however, that PS1 is involved not only in Abeta production but also in other processes that lead to neurodegeneration. To better understand the causes of neurodegeneration linked to the PS1 mutation, we analyzed the development of tau pathology, another key feature of AD, in PS1 knock-in mice. Hippocampal samples taken from FAD mutant (I213T) PS1 knock-in mice contained hyperphosphorylated tau that reacted with various phosphodependent tau antibodies and with Alz50, which recognizes the conformational change of PHF tau. Some neurons exhibited Congo red birefringence and Thioflavin T reactivity, both of which are histological criteria for neurofibrillary tangles (NFTs). Biochemical analysis of the samples revealed SDS-insoluble tau, which under electron microscopy examination, resembled tau fibrils. These results indicate that our mutant PS1 knock-in mice exhibited NFT-like tau pathology in the absence of Abeta deposition, suggesting that PS1 mutations contribute to the onset of AD not only by enhancing Abeta(1-42) production but by also accelerating the formation and accumulation of filamentous tau.     

Chronic copper exposure exacerbates both amyloid and tau pathology and selectively dysregulates cdk5 in a mouse model of AD

Excess copper exposure is thought to be linked to the development of Alzheimer's disease (AD) neuropathology. However, the mechanism by which copper affects the CNS remains unclear. To investigate the effect of chronic copper exposure on both beta-amyloid and tau pathologies, we treated young triple transgenic (3×Tg-AD) mice with 250 ppm copper-containing water for a period of 3 or 9 months. Copper exposure resulted in altered amyloid precursor protein processing; increased accumulation of the amyloid precursor protein and its proteolytic product, C99 fragment, along with increased generation of amyloid-beta peptides and oligomers. These changes were found to be mediated via up-regulation of BACE1 as significant increases in BACE1 levels and deposits were detected around plaques in mice following copper exposure. Furthermore, tau pathology within hippocampal neurons was exacerbated in copper-exposed 3×Tg-AD group. Increased tau phosphorylation was closely correlated with aberrant cdk5/p25 activation, suggesting a role for this kinase in the development of copper-induced tau pathology. Taken together, our data suggest that chronic copper exposure accelerates not only amyloid pathology but also tau pathology in a mouse model of AD.     

Potentiation of GSK-3-catalyzed Alzheimer-like phosphorylation of human tau by cdk5

Tau protein from Alzheimer disease (AD) brain is hyperphosphorylated by both proline-dependent protein kinases (PDPKs) and non-PDPKs. It is presently unclear how PDPKs and non-PDPKs interact in tau hyperphosphorylation. Previously we have shown that non-PDPKs can positively modulate the activity of a PDPK (GSK-3) in tau phosphorylation (Singh et al. (1995) FEBS Lett. 358, 267-272). In this study we have investigated whether (A) non-PDPKs can also modulate the activity of the PDPK, cdk5, (B) a PDPK can modulate the activities of another PDPK, as well as non-PDPKs. We found that, like GSK-3, the activity of cdk5 is stimulated if tau were first prephosphorylated by any of several non-PDPKs (A-kinase, C-kinase, CK-1, CaM-kinase II). Prephosphorylation of tau by cdk5 stimulated both the rate and extent of a subsequent phosphorylation catalyzed by GSK-3. Under these conditions thr 231 phosphorylation was especially enhanced (9-fold). No significant stimulation of phosphorylation was obser ved when the order of these kinases was reversed (i.e. GSK-3 followed by cdk5). By contrast, prephosphorylation of tau by cdk5 served to inhibit subsequent phosphorylation catalyzed by C-kinase and CK-1, but not by A-kinase or CaM-kinase II. Our results suggest that in tau hyperphosphorylation in AD brain, cdk5-catalyzed phosphorylation may serve to up-regulate the activity of GSK-3 and down-regulate the activities of C-kinase and CK-1. (Mol Cell Biochem 167: 99-105, 1997)     

Interleukin-6 induces Alzheimer-type phosphorylation of tau protein by deregulating the cdk5/p35 pathway

Inflammation is a process that has been actively related with the onset of several neurodegenerative disorders including Alzheimer disease (AD). However, the precise implications of inflammatory response for neurodegeneration have not been elucidated. A current hypothesis considers that extracellular insults to neurons could trigger the production of inflammatory cytokines by astrocytes and microglia. These cytokines, namely, interleukin (IL)-1beta, TNFalpha, and IL-6, could affect the normal behavior of neuronal cells. In the present study, we describe the effect of the administration at physiologic doses of one of these cytokines, IL-6, to hippocampal neurons, on the protein kinase pathways as well as on the tau phosphorylation patterns. IL-6-treated neurons exhibited an increase in the amount of anomalously hyperphosphorylated tau protein in epitopes dependent on proline-directed protein kinases (PDPKs). On the basis of our data, the observed increase of tau epitopes of Alzheimer type is explained by an increase of intraneuronal levels of p35 activator and in the activity of the protein kinase cdk5 in response to this cytokine. Further confirmation of cdk5 involvement in this process was based on the findings that inhibition of the kinase activity with butyrolactone-I prevents the appearance of tau of Alzheimer type in IL-6-treated neurons. Additional studies suggest that an increase of cdk5 activity could be mediated by a known signaling cascade described for IL-6 function, namely, the MAPK-p38 signaling pathway. Stimulation of the IL-6 pathway appears to increase the tau epitopes of Alzheimer type, as demonstrated in studies with specific inhibitors. These results support the findings of a pathologic role for IL-6 in the neuroinflammatory response as related with the pathogenesis of neuronal degeneration.     

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.     

Cultured cell and transgenic mouse models for tau pathology linked to beta-amyloid

The two histopathological signatures of Alzheimer's disease (AD) are amyloid plaques and neurofibrillary tangles, prompting speculation that a causal relationship exists between the respective building blocks of these abnormal brain structures: the beta-amyloid peptides (Abeta) and the neuron-enriched microtubule-associated protein called tau. Transgenic mouse models have provided in vivo evidence for such connections, and cultured cell models have allowed tightly controlled, systematic manipulation of conditions that influence links between Abeta and tau. The emerging evidence supports the view that amyloid pathology lies upstream of tau pathology in a pathway whose details remain largely mysterious. In this communication, we review and discuss published work about the Abeta-tau connection. In addition, we present some of our own previously unpublished data on the effects of exogenous Abeta on primary brain cultures that contain both neurons and glial cells. We report here that continuous exposure to 5 microM non-fibrillar Abeta40 or Abeta42 kills primary brain cells by apoptosis within 2-3 weeks, Abeta42 is more toxic and selective for neurons than Abeta40, and Abeta42, but not Abeta40, induces a transient increase in neurons that are positive for the AD-like PHF1 epitope. These findings demonstrate the greater potency of Abeta42 than Abeta40 at inducing tau pathology and programmed cell death, and corroborate and extend reports that tau-containing cells are more sensitive to Abeta peptides than cells that lack or express low levels of tau.     

Glucocorticoids trigger Alzheimer disease-like pathobiochemistry in rat neuronal cells expressing human tau

Amyloid precursor protein (APP) mis-processing and aberrant tau hyperphosphorylation are causally related to the pathogenesis and neurodegenerative processes that characterize Alzheimer's disease (AD). Abnormal APP metabolism leads to the generation of neurotoxic amyloid beta (Abeta), whereas tau hyperphosphorylation culminates in cytoskeletal disturbances, neuronal dysfunction and death. Many AD patients hypersecrete glucocorticoids (GC) while neuronal structure, function and survival are adversely influenced by elevated GC levels. We report here that a rat neuronal cell line (PC12) engineered to express the human ortholog of the tau protein (PC12-htau) becomes more vulnerable to the toxic effects of either Abeta or GC treatment. Importantly, APP metabolism in GC-treated PC12-htau cells is selectively shifted towards increased production of the pro-amyloidogenic peptide C99. Further, GC treatment results in hyperphosphorylation of human tau at AD-relevant sites, through the cyclin-dependent kinase 5 (E.C. 2.7.11.26) and GSK3 (E.C. 2.7.11.22) protein kinases. Pulse-chase experiments revealed that GC treatment increased the stability of tau protein rather than its de novo synthesis. GC treatment also induced accumulation of transiently expressed EGFP-tau in the neuronal perikarya. Together with previous evidence showing that Abeta can activate cyclin-dependent kinase 5 and GSK3, these results uncover a potential mechanism through which GC may contribute to AD neuropathology.