Search

The accumulation of tau is a hallmark of several neurodegenerative diseases and is associated with neuronal hypoactivity and presynaptic dysfunction. Oral administration of the adenosine A1 receptor antagonist rolofylline (KW-3902) has previously been shown to reverse spatial memory deficits and to normalize the basic synaptic transmission in a mouse line expressing full-length pro-aggregant tau (TauΔK) at low levels, with late onset of disease. However, the efficacy of treatment remained to be explored for cases of more aggressive tauopathy. Using a combination of behavioral assays, imaging with several PET-tracers, and analysis of brain tissue, we compared the curative reversal of tau pathology by blocking adenosine A1 receptors in three mouse models expressing different types and levels of tau and tau mutants. We show through positron emission tomography using the tracer [18F]CPFPX (a selective A1 receptor ligand) that intravenous injection of rolofylline effectively blocks A1 receptors in the brain. Moreover, when administered to TauΔK mice, rolofylline can reverse tau pathology and synaptic decay. The beneficial effects are also observed in a line with more aggressive tau pathology, expressing the amyloidogenic repeat domain of tau (TauRDΔK) with higher aggregation propensity. Both models develop a progressive tau pathology with missorting, phosphorylation, accumulation of tau, loss of synapses, and cognitive decline. TauRDΔK causes pronounced neurofibrillary tangle assembly concomitant with neuronal death, whereas TauΔK accumulates only to tau pretangles without overt neuronal loss. A third model tested, the rTg4510 line, has a high expression of mutant TauP301L and hence a very aggressive phenotype starting at ~3 months of age. This line failed to reverse pathology upon rolofylline treatment, consistent with a higher accumulation of tau-specific PET tracers and inflammation. In conclusion, blocking adenosine A1 receptors by rolofylline can reverse pathology if the pathological potential of tau remains below a threshold value that depends on concentration and aggregation propensity. [ABSTRACT FROM AUTHOR]

Accumulation of amyloidogenic proteins such as Tau is a hallmark of neurodegenerative diseases including Alzheimer disease and fronto-temporal dementias. To link Tau pathology to cognitive impairments and defects in synaptic plasticity, we created four inducible Tau transgenic mouse models with expression of pro- and anti-aggregant variants of either full-length human Tau (hTau40/ΔK280 and hTau40/ΔK280/PP) or the truncated Tau repeat domain (TauRD/ΔK280 and TauRD/ΔK280/PP). Here we review the histopathological features caused by pro-aggregant Tau, and correlate them with behavioral deficits and impairments in synaptic transmission. Both pro-aggregant Tau variants cause Alzheimer-like features, including synapse loss, mis-localization of Tau into the somatodendritic compartment, conformational changes and hyperphosphorylation. However, there is a clear difference in the extent of Tau aggregation and neurotoxicity. While pro-aggregant full-length hTau40/ΔK280 leads to a 'pre-tangle' pathology, the repeat domain TauRD/ΔK280 causes massive formation of neurofibrillary tangles and neuronal loss in the hippocampus. However, both Tau variants cause co-aggregation of human and mouse Tau and similar functional impairments. Thus, earlier Tau pathological stages and not necessarily neurofibrillary tangles are critical for the development of cognitive malfunctions. Most importantly, memory and synapses recover after switching off expression of pro-aggregant Tau. The rescue of functional impairments correlates with the rescue of most Tau pathological changes and most strikingly the recovery of synapses. This implies that tauopathies as such are reversible, provided that amyloidogenic Tau is removed. Therefore, our Tau transgenic mice may serve as model systems for in vivo validation of therapeutic strategies and drug candidates with regard to cognition and synaptic function. [ABSTRACT FROM AUTHOR]

Neurofibrillary lesions of abnormal Tau are hallmarks of Alzheimer disease and frontotemporal dementias. Our regulatable (Tet-OFF) mouse models of tauopathy express variants of human full-length Tau in the forebrain (CaMKIIα promoter) either with mutation ΔK280 (pro-aggregant) or ΔK280/I277P/I308P (anti-aggregant). Co-expression of luciferase enables in vivo quantification of gene expression by bioluminescence imaging. Pro-aggregant mice develop synapse loss and Tau-pathology including missorting, phosphorylation and early pretangle formation, whereas anti-aggregant mice do not. We correlated hippocampal Tau pathology with learning/memory performance and synaptic plasticity. Pro-aggregant mice at 16 months of gene expression exhibited severe cognitive deficits in Morris water maze and in passive-avoidance paradigms, whereas anti-aggregant mice were comparable to controls. Cognitive impairment of pro-aggregant mice was accompanied by loss of hippocampal LTP in CA1 and CA3 areas and by a reduction of synaptic proteins and dendritic spines, although no neuronal loss was observed. Remarkably, memory and LTP recovered when pro-aggregant Tau was switched-OFF for ~4 months, Tau phosphorylation and missorting were reversed, and synapses recovered. Moreover, soluble and insoluble pro-aggregant hTau40 disappeared, while insoluble mouse Tau was still present. This study links early Tau pathology without neurofibrillary tangles and neuronal death to cognitive decline and synaptic dysfunction. It demonstrates that Tau-induced impairments are reversible after switching-OFF pro-aggregant Tau. Therefore, our mouse model may mimic an early phase of AD when the hippocampus does not yet suffer from irreversible cell death but cognitive deficits are already striking. It offers potential to evaluate drugs with regard to learning and memory performance. [ABSTRACT FROM AUTHOR]

This report describes the behavioral and electrophysiological analysis of regulatable transgenic mice expressing mutant repeat domains of human Tau (TauRD). Mice were generated to express TauRD in two forms, differing in their propensity forβ-structure and thus in their tendency for aggregation ("pro-aggregant" or "anti-aggregant") (Mocanu et al., 2008). Only pro-aggregant mice show pronounced changes typical for Tau pathology in Alzheimer's disease (aggregation, missorting, hyperphosphorylation, synaptic and neuronal loss), indicating that the β-propensity and hence the ability to aggregate is a key factor in the disease. We now tested the mice with regard to neuromotor parameters, behavior, learning and memory, and synaptic plasticity and correlated this with histological and biochemical parameters in different stages of switching TauRD on or off. The mice are normal in neuromotor tests. However, pro-aggregant TauRD mice are strongly impaired in memory and show pronounced loss of long-term potentiation (LTP), suggesting that Tau aggregation specifically perturbs these brain functions. Remarkably, when the expression of human pro-aggregant TauRDisswitchedonfor ~10 months and off for ~4 months, memory and LTP recover, whereas aggregates decrease moderately and change their composition from mixed human plus mouse Tau to mouse Tau only. Neuronalloss persists, but synapses are partially rescued. This argues that continuous presence of amyloidogenic pro-aggregant TauRD constitutes the main toxic insult for memory and LTP, rather than the aggregates as such. [ABSTRACT FROM AUTHOR]

Background: Aggregates of the tau protein are a hallmark of Alzheimer’s and several other neurodegenerative diseases. Various transgenic mouse models have been generated to study the aggregation process. Since wild-type tau is highly soluble and does not aggregate readily, most models make use of tau mutations that occur in human frontotemporal dementias and are more prone to aggregate. These mouse models show neurofibrillary tangles similar to those of Alzheimer’s disease. However, since the mice contain both endogenous wild-type mouse tau and exogenous human mutant tau, the relative contribution of these components to the aggregates has been a matter of debate. Objective:Using a new set of regulatable transgenic mouse models, we sought to determine whether mouse tau coaggregates with human tau when it is switched on. Furthermore, we asked what type of tau remains in the aggregates after switching off the expression of exogenous tau. Methods: We generated doxycycline-inducible transgenic mice expressing either full-length human tau or the tau repeat domain (tauRD). In addition, both types of human tau derivatives were expressed in a ‘proaggregant’ form (with the frontotemporal dementia with parkinsonism linked to chromosome 17 mutation ΔK280), or in an ‘antiaggregant’ form (with additional proline mutations to block β-structure and aggregation). Results:The proaggregant tauRD mice develop tangles rapidly after induction, the antiaggregant mice do not. Analysis by biochemistry and immunohistology reveals that the tangles contain both exogenous and endogenous mouse tau. After switching off the proaggregant tauRD, tangles persist for extended periods. However, they are composed entirely of mouse tau. Conclusions:Mouse tau and exogenous human tau can coaggregate in transgenic models of tauopathy. The aggregates are in dynamic equilibrium with their subunits, so that exogenous tau disappears when its expression is switched off. Once the seeds of aggregation are generated by the foreign tau species, they propagate in a ‘prion-like’ fashion within the cell even after the foreign tau has disappeared. Copyright © 2010 S. Karger AG, Basel [ABSTRACT FROM AUTHOR]