Two mouse models of Alzheimer's disease accumulate amyloid at different rates and have distinct Aß oligomer profiles unaltered by ablation of cellular prion protein.

Oligomers formed from monomers of the amyloid ß-protein (Aß) are thought to be central to the pathogenesis of Alzheimer's disease (AD). Unsurprisingly for a complex disease, current mouse models of AD fail to fully mimic the clinical disease in humans. Moreover, results obtained in a given mouse model are not always reproduced in a different model. Cellular prion protein (PrPC) is now an established receptor for Aß oligomers. However, studies of the Aß-PrPC interaction in different mouse models have yielded contradictory results. Here we performed a longitudinal study assessing a range of biochemical and histological features in the commonly used J20 and APP-PS1 mouse models. Our analysis demonstrated that PrPC ablation had no effect on amyloid accumulation or oligomer production. However, we found that APP-PS1 mice had higher levels of oligomers, that these could bind to recombinant PrPC, and were recognised by the OC antibody which distinguishes parallel, in register fibrils. On the other hand, J20 mice had a lower level of Aß oligomers, which did not interact with PrPC when tested in vitro and were OC-negative. These results suggest the two mouse models produce diverse Aß assemblies that could interact with different targets, highlighting the necessity to characterise the conformation of the Aß oligomers concomitantly with the toxic cascade elicited by them. Our results provide an explanation for the apparent contradictory results found in APP-PS1 mice and the J20 mouse line in regards to Aß toxicity mediated by PrPC.

Transmission of amyloid-ß protein pathology from cadaveric pituitary growth hormone.

We previously reported1 the presence of amyloid-ß protein (Aß) deposits in individuals with Creutzfeldt-Jakob disease (CJD) who had been treated during childhood with human cadaveric pituitary-derived growth hormone (c-hGH) contaminated with prions. The marked deposition of parenchymal and vascular Aß in these relatively young individuals with treatment-induced (iatrogenic) CJD (iCJD), in contrast to other prion-disease patients and population controls, allied with the ability of Alzheimer's disease brain homogenates to seed Aß deposition in laboratory animals, led us to argue that the implicated c-hGH batches might have been contaminated with Aß seeds as well as with prions. However, this was necessarily an association, and not an experimental, study in humans and causality could not be concluded. Given the public health importance of our hypothesis, we proceeded to identify and biochemically analyse archived vials of c-hGH. Here we show that certain c-hGH batches to which patients with iCJD and Aß pathology were exposed have substantial levels of Aß40, Aß42 and tau proteins, and that this material can seed the formation of Aß plaques and cerebral Aß-amyloid angiopathy in intracerebrally inoculated mice expressing a mutant, humanized amyloid precursor protein. These results confirm the presence of Aß seeds in archived c-hGH vials and are consistent with the hypothesized iatrogenic human transmission of Aß pathology. This experimental confirmation has implications for both the prevention and the treatment of Alzheimer's disease, and should prompt a review of the risk of iatrogenic transmission of Aß seeds by medical and surgical procedures long recognized to pose a risk of accidental prion transmission2,3.

Prion Protein as a Toxic Acceptor of Amyloid-ß Oligomers.

The initial report that cellular prion protein (PrPC) mediates toxicity of amyloid-ß species linked to Alzheimer's disease was initially treated with scepticism, but growing evidence supports this claim. That there is a high-affinity interaction is now clear, and its molecular basis is being unraveled, while recent studies have identified possible downstream toxic mechanisms. Determination of the clinical significance of such interactions between PrPC and disease-associated amyloid-ß species will require experimental medicine studies in humans. Trials of compounds that inhibit PrP-dependent amyloid-ß toxicity are commencing in humans, and although it is clear that only a fraction of Alzheimer's disease toxicity could be governed by PrPC, a partial, but still therapeutically useful, role in human disease may soon be testable.

Post-Translational Incorporation of L-Phenylalanine into the C-Terminus of α-Tubulin as a Possible Cause of Neuronal Dysfunction.

α-Tubulin C-terminus undergoes post-translational, cyclic tyrosination/detyrosination, and L-Phenylalanine (Phe) can be incorporated in place of tyrosine. Using cultured mouse brain-derived cells and an antibody specific to Phe-tubulin, we showed that: (i) Phe incorporation into tubulin is reversible; (ii) such incorporation is not due to de novo synthesis; (iii) the proportion of modified tubulin is significant; (iv) Phe incorporation reduces cell proliferation without affecting cell viability; (v) the rate of neurite retraction declines as level of C-terminal Phe incorporation increases; (vi) this inhibitory effect of Phe on neurite retraction is blocked by the co-presence of tyrosine; (vii) microtubule dynamics is reduced when Phe-tubulin level in cells is high as a result of exogenous Phe addition and returns to normal values when Phe is removed; moreover, microtubule dynamics is also reduced when Phe-tubulin is expressed (plasmid transfection). It is known that Phe levels are greatly elevated in blood of phenylketonuria (PKU) patients. The molecular mechanism underlying the brain dysfunction characteristic of PKU is unknown. Beyond the differences between human and mouse cells, it is conceivable the possibility that Phe incorporation into tubulin is the first event (or among the initial events) in the molecular pathways leading to brain dysfunctions that characterize PKU.

Reversal of Synapse Degeneration by Restoring Wnt Signaling in the Adult Hippocampus.

Synapse degeneration occurs early in neurodegenerative diseases and correlates strongly with cognitive decline in Alzheimer's disease (AD). The molecular mechanisms that trigger synapse vulnerability and those that promote synapse regeneration after substantial synaptic failure remain poorly understood. Increasing evidence suggests a link between a deficiency in Wnt signaling and AD. The secreted Wnt antagonist Dickkopf-1 (Dkk1), which is elevated in AD, contributes to amyloid-ß-mediated synaptic failure. However, the impact of Dkk1 at the circuit level and the mechanism by which synapses disassemble have not yet been explored. Using a transgenic mouse model that inducibly expresses Dkk1 in the hippocampus, we demonstrate that Dkk1 triggers synapse loss, impairs long-term potentiation, enhances long-term depression, and induces learning and memory deficits. We decipher the mechanism involved in synapse loss induced by Dkk1 as it can be prevented by combined inhibition of the Gsk3 and RhoA-Rock pathways. Notably, after loss of synaptic connectivity, reactivation of the Wnt pathway by cessation of Dkk1 expression completely restores synapse number, synaptic plasticity, and long-term memory. These findings demonstrate the remarkable capacity of adult neurons to regenerate functional circuits and highlight Wnt signaling as a targetable pathway for neuronal circuit recovery after synapse degeneration.

mGlu5 receptors and cellular prion protein mediate amyloid-ß-facilitated synaptic long-term depression in vivo.

NMDA-type glutamate receptors (NMDARs) are currently regarded as paramount in the potent and selective disruption of synaptic plasticity by Alzheimer's disease amyloid ß-protein (Aß). Non-NMDAR mechanisms remain relatively unexplored. Here we describe how Aß facilitates NMDAR-independent long-term depression of synaptic transmission in the hippocampus in vivo. Synthetic Aß and Aß in soluble extracts of Alzheimer's disease brain usurp endogenous acetylcholine muscarinic receptor-dependent long-term depression, to enable long-term depression that required metabotropic glutamate-5 receptors (mGlu5Rs). We also find that mGlu5Rs are essential for Aß-mediated inhibition of NMDAR-dependent long-term potentiation in vivo. Blocking Aß binding to cellular prion protein with antibodies prevents the facilitation of long-term depression. Our findings uncover an overarching role for Aß-PrP(C)-mGlu5R interplay in mediating both LTD facilitation and LTP inhibition, encompassing NMDAR-mediated processes that were previously considered primary.

Dysfunction of Wnt signaling and synaptic disassembly in neurodegenerative diseases.

The molecular mechanisms that regulate synapse formation have been well documented. However, little is known about the factors that modulate synaptic stability. Synapse loss is an early and invariant feature of neurodegenerative diseases including Alzheimer's (AD) and Parkinson's disease. Notably, in AD the extent of synapse loss correlates with the severity of the disease. Hence, understanding the molecular mechanisms that underlie synaptic maintenance is crucial to reveal potential targets that will allow the development of therapies to protect synapses. Wnts play a central role in the formation and function of neuronal circuits. Moreover, Wnt signaling components are expressed in the adult brain suggesting their role in synaptic maintenance in the adult. Indeed, blockade of Wnts with the Wnt antagonist Dickkopf-1 (Dkk1) causes synapse disassembly in mature hippocampal cells. Dkk1 is elevated in brain biopsies from AD patients and animal models. Consistent with these findings, Amyloid-ß (Aß) oligomers induce the rapid expression of Dkk1. Importantly, Dkk1 neutralizing antibodies protect synapses against Aß toxicity, indicating that Dkk1 is required for Aß-mediated synapse loss. In this review, we discuss the role of Wnt signaling in synapse maintenance in the adult brain, particularly in relation to synaptic loss in neurodegenerative diseases.

The secreted Wnt antagonist Dickkopf-1 is required for amyloid ß-mediated synaptic loss.

Extensive evidence supports a central role for amyloid-ß (Aß) in the pathogenesis of Alzheimer's disease (AD). Synaptic loss mediated by Aß in early stages of the disease might contribute to cognitive impairments. However, little is known about the mechanism by which Aß induces the loss of synapses. The expression of the Wnt antagonist Dickkopf-1 (Dkk1) is increased in brains of AD patients and in AD transgenic mouse models, suggesting that dysfunction of Wnt signaling could contribute to AD pathology. Here we report that acute exposure to Aß oligomers induces Dkk1 expression together with the loss of synaptic sites. Importantly, Dkk1-neutralizing antibodies suppress Aß-induced synapse loss in mouse brain slices. In mature rat hippocampal neurons, Dkk1 decreases the number of synapses without affecting cell viability. Ultrastructural analyses revealed that Wnt blockade decreases the size of presynaptic and postsynaptic terminals. Time-lapse recordings of RFP-labeled stable synaptic sites demonstrate that Dkk1 induces the dispersal of synaptic components. These findings identify Dkk1 as a potential therapeutic target for the treatment of AD.

Phytoremediation of 2,4-dichlorophenol using wild type and transgenic tobacco plants.

INTRODUCTION: Transgenic plant strategies based on peroxidase expression or overexpression would be useful for phenolic compound removal since these enzymes play an important role in phenolic polymerizing reactions. MATERIAL AND METHODS: Thus, double transgenic (DT) plants for basic peroxidases were obtained and characterized in order to compare the tolerance and efficiency for 2,4-dichlorophenol (2,4-DCP) removal with WT and simple transgenic plants expressing TPX1 or TPX2 gene. Several DT plants showed the expression of both transgenes and proteins, as well as increased peroxidase activity. RESULTS: DT lines showed higher tolerance to 2,4-DCP at early stage of development since their germination index was higher than that of WT seedlings exposed to 25 mg/L of the pollutant. High 2,4-DCP removal efficiencies were found for WT tobacco plants. TPX1 transgenic plants and DT (line d) reached slightly higher removal efficiencies for 10 mg/L of 2,4-DCP than WT plants, while DT plants (line A) showed the highest removal efficiencies (98%). These plants showed an increase of 21% and 14% in 2,4-DCP removal efficiency for solutions containing 10 and 25 mg/L 2,4-DCP, respectively, compared with WT plants. In addition, an almost complete toxicity reduction of postremoval solutions using WT and DT plants was obtained through AMPHITOX test, which indicates that the 2,4-DCP degradation products would be similar for both plants. CONCLUSION: These results are relevant in the field of phytoremediation application and, moreover, they highlight the safety of using DT tobacco plants because nontoxic products were formed after an efficient 2,4-DCP removal.

Lack of stabilized microtubules as a result of the absence of major maps in CAD cells does not preclude neurite formation.

In many laboratories, the requirement of microtubule-associated proteins (MAPs) and the stabilization of microtubules for the elongation of neurites has been intensively investigated, with controversial results being obtained. We have observed that the neurite microtubules of Cath.a-differentiated (CAD) cells, a mouse brain derived cell, are highly dynamic structures, and so we analyzed several aspects of the cytoskeleton to investigate the molecular causes of this phenomenon. Microtubules and microfilaments were present in proportions similar to those found in brain tissue and were distributed similarly to those in normal neurons in culture. Neurofilaments were also present. Analysis of tubulin isospecies originating from post-translational modifications revealed an increased amount of tyrosinated tubulin, a diminished amount of the detyrosinated form and a lack of the Delta2 form. This tyrosination pattern is in agreement with highly dynamic microtubules. Using western blot analyses with specific antibodies, we found that CAD cells do not express several MAPs such as MAP1b, MAP2, Tau, doublecortin, and stable-tubule-only-peptide. The presence of the genes corresponding to these MAPs was verified. The absence of the corresponding mRNAs confirmed the lack of expression of these proteins. The exception was Tau, whose mRNA was present. Among the several MAPs investigated, LIS1 was the only one to be expressed in CAD cells. In addition, we determined that neurites of CAD cells form and elongate at the same rate as processes in a primary culture of hippocampal neurons. Treatment with nocodazol precluded the formation of neurites, and induced the retraction of previously formed neurites. We conclude that the formation and elongation of neurites, at least in CAD cells, are dependent on microtubule integrity but not on their stabilization or the presence of MAPs.

Establishment of transgenic tobacco hairy roots expressing basic peroxidases and its application for phenol removal.

Transgenic hairy root (HR) systems constitute an interesting alternative to improve the efficiency of phytoremediation process. Since peroxidases (Px) have been associated with phenolic compounds removal, in the present work, transgenic tobacco HR, which expressed basic Px genes from tomato (tpx1 and tpx2), were established and assayed for phenol removal. Tobacco HR clones were obtained, including those transgenic for TPX1 or TPX2, those double transgenic (DT) for both Px and the corresponding controls. Based on growth index, the presence of rol C sequence, tpx1 and/or tpx2 genes and the coded proteins, as well as Px activity determinations, we selected 10 tobacco HR clones for phenol removal assays. The removal efficiencies were high for all the HR, although, some transgenic HR showed significantly higher removal efficiencies compared with controls. The results demonstrate that TPX1 is involved in phenol removal not only when it was overexpressed in tomato, but also when it was expressed in other plant, such as tobacco. The higher efficiency of TPX2 transgenic HR showed that this Px also participates in the process. The contribution of other mechanisms (adsorption, H2O2 independent enzymatic processes) could be considered depreciable, which establishes the great implication of Px in phenol removal.

Wnt regulates axon behavior through changes in microtubule growth directionality: a new role for adenomatous polyposis coli.

Axon guidance and target-derived signals control axonal behavior by regulating the cytoskeleton through poorly defined mechanisms. In particular, how these signaling molecules regulate the growth and directionality of microtubules is not well understood. Here we examine the effect of Wnts on growth cone remodeling, a process that precedes synapse formation. Time-lapse recordings reveal that Wnt3a rapidly inhibits growth cone translocation while inducing growth cone enlargement. These changes in axonal behavior are associated with changes in the organization of microtubules. Time-lapse imaging of EB3-GFP (green fluorescent protein)-labeled microtubule plus-ends demonstrates that Wnt3a regulates microtubule directionality, resulting in microtubule looping, growth cone pausing, and remodeling. Analyses of Dishevelled-1 (Dvl1) mutant neurons demonstrate that Dvl1 is required for Wnt-mediated microtubule reorganization and axon remodeling. Wnt signaling directly affects the microtubule cytoskeleton by unexpectedly inducing adenomatous polyposis coli (APC) loss from microtubule plus-ends. Consistently, short hairpin RNA knockdown of APC mimics Wnt3a function. Together, our findings define APC as a key Wnt signaling target in the regulation of microtubule growth direction.

Tubulin must be acetylated in order to form a complex with membrane Na(+),K (+)-ATPase and to inhibit its enzyme activity.

In cells of neural and non-neural origin, tubulin forms a complex with plasma membrane Na(+),K(+)-ATPase, resulting in inhibition of the enzyme activity. When cells are treated with 1 mM L-glutamate, the complex is dissociated and enzyme activity is restored. Now, we found that in CAD cells, ATPase is not activated by L-glutamate and tubulin/ATPase complex is not present in membranes. By investigating the causes for this characteristic, we found that tubulin must be acetylated in order to associate with ATPase and to inhibit its catalytic activity. In CAD cells, the acetylated tubulin isotype is absent. Treatment of CAD cells with deacetylase inhibitors (trichostatin A or tubacin) caused appearance of acetylated tubulin, formation of tubulin/ATPase complex, and reduction of membrane ATPase activity. In these treated cells, addition of 1 mM L-glutamate dissociated the complex and restored the enzyme activity. Cytosolic tubulin from trichostatin A-treated but not from non-treated cells inhibited ATPase activity. These findings indicate that the acetylated isotype of tubulin is required for interaction with membrane Na(+),K(+)-ATPase and consequent inhibition of enzyme activity.

Inhibitors of protein phosphatase 1 and 2A decrease the level of tubulin carboxypeptidase activity associated with microtubules.

The association of tubulin carboxypeptidase with microtubules may be involved in the determination of the tyrosination state of the microtubules, i.e. their proportion of tyrosinated vs. nontyrosinated tubulin. We investigated the role of protein phosphatases in the association of carboxypeptidase with microtubules in COS cells. Okadaic acid and other PP1/PP2A inhibitors, when added to culture medium before isolation of the cytoskeletal fraction, produced near depletion of the carboxypeptidase activity associated with microtubules. Isolation of the native assembled and nonassembled tubulin fractions from cells treated and not treated with okadaic acid, and subsequent in vitro assay of the carboxypeptidase activity, revealed that the enzyme was dissociated from microtubules by okadaic acid treatment and recovered in the soluble fraction. There was no effect by nor-okadaone (an inactive okadaic acid analogue) or inhibitors of PP2B and of tyrosine phosphatases which do not affect PP1/PP2A activity. When tested in an in vitro system, okadaic acid neither dissociated the enzyme from microtubules nor inactivated it. In living cells, prior stabilization of microtubules with taxol prevented the dissociation of carboxypeptidase by okadaic acid indicating that dynamic microtubules are needed for okadaic acid to exert its effect. On the other hand, stabilization of microtubules subsequent to okadaic acid treatment did not reverse the dissociating effect of okadaic acid. These results suggest that dephosphorylation (and presumably also phosphorylation) of the carboxypeptidase or an intermediate compound occurs while it is not associated with microtubules, and that the phosphate content determines whether or not the carboxypeptidase is able to associate with microtubules.

Post-translational incorporation of the antiproliferative agent azatyrosine into the C-terminus of alpha-tubulin.

Detyrosination/tyrosination of tubulin is a post-translational modification that occurs at the C-terminus of the alpha-subunit, giving rise to microtubules rich in either tyrosinated or detyrosinated tubulin which coexist in the cell. We hereby report that the tyrosine analogue, azatyrosine, can be incorporated into the C-terminus of alpha-tubulin instead of tyrosine. Azatyrosine is structurally identical to tyrosine except that a nitrogen atom replaces carbon-2 of the phenolic group. Azatyrosine competitively excluded incorporation of [14C]tyrosine into tubulin of soluble brain extract. A newly developed rabbit antibody specific to C-terminal azatyrosine was used to study incorporation of azatyrosine in cultured cells. When added to the culture medium (Ham's F12K), azatyrosine was incorporated into tubulin of glioma-derived C6 cells. This incorporation was reversible, i.e. after withdrawal of azatyrosine, tubulin lost azatyrosine and reincorporated tyrosine. Azatyrosinated tubulin self-assembled into microtubules to a similar degree as total tubulin both in vitro and in vivo. Studies by other groups have shown that treatment of certain types of cultured cancer cells with azatyrosine leads to reversion of phenotype to normal, and that administration of azatyrosine into animals harbouring human proto-oncogenic c-Ha- ras prevents tumour formation. These interesting observations led us to study this phenomenon in relation to tubulin status. Under conditions in which tubulin was mostly azatyrosinated, C6 cells remained viable but did not proliferate. After 7-10 days under these conditions, morphology changed from a fused, elongated shape to a rounded soma with thin processes. Incorporation of azatyrosine into the C-terminus of alpha-tubulin is proposed as one possible cause of reversion of the malignant phenotype.

Incorporation of 3-nitrotyrosine into the C-terminus of alpha-tubulin is reversible and not detrimental to dividing cells.

The C-terminus of the alpha-chain of tubulin is subject to reversible incorporation of tyrosine by tubulin tyrosine ligase and removal by tubulin carboxypeptidase. Thus, microtubules rich in either tyrosinated or detyrosinated tubulin can coexist in the cell. Substitution of the terminal tyrosine by 3-nitrotyrosine has been claimed to cause microtubule dysfunction and consequent injury of epithelial lung carcinoma A549 cells. Nitrotyrosine is formed in cells by nitration of tyrosine by nitric oxide-derived species. We studied properties of tubulin modified by in vitro nitrotyrosination at the C-terminus of the alpha-subunit, and the consequences for cell functioning. Nitrotyrosinated tubulin was a good substrate of tubulin carboxypeptidase, and showed a similar capability to assemble into microtubules in vitro to that of tyrosinated tubulin. Tubulin of C6 cells cultured in F12K medium in the presence of 500 micro m nitrotyrosine became fully nitrotyrosinated. This nitrotyrosination was shown to be reversible. No changes in morphology, proliferation, or viability were observed during cycles of nitrotyrosination, denitrotyrosination, and re-nitrotyrosination. Similar results were obtained with CHO, COS-7, HeLa, NIH-3T3, NIH-3T3(TTL-), and A549 cells. C6 and A549 cells were subjected to several passages during 45 days or more in the continuous presence of 500 micro m nitrotyrosine without noticeable alteration of morphology, viability, or proliferation. The microtubular networks visualized by immunofluorescence with antibodies to nitrotyrosinated and total tubulin were identical. Furthermore, nitrotyrosination of tubulin in COS cells did not alter the association of tubulin carboxypeptidase with microtubules. Our results demonstrate that substitution of C-terminal tyrosine by 3-nitrotyrosine has no detrimental effect on dividing cells.