Axonopathy and amyotrophy in mice transgenic for human four-repeat tau protein

The emerging utility of animal models of chronic neurodegenerative diseases

The two most common neurodegenerative diseases are Alzheimer's disease (AD) and Parkinson's disease (PD). The symptoms are caused by the initially selective degeneration of neuronal subpopulations involved in memory (AD) or movement control (PD). The cause of both diseases is unknown, but ageing is an inevitable risk factor. The identification of disease-associated genes was a breakthrough for the understanding of molecular mechanisms of neurodegeneration and has provided the basis for the establishment of cell culture and animal model systems, instrumental for target validation and drug screening. Familial AD is caused by mutations in the beta-amyloid precursor protein (betaAPP) and in the gene products responsible for its proteolytic processing, namely the presenilins. Transgenic mice expressing these mutant genes develop characteristic AD plaques in an age-dependent manner. A reduction of plaque burden and amelioration of cognitive decline in these animals was recently achieved by vaccination with amyloid beta-protein fibrils. The other hallmark lesion of AD, the neurofibrillary tangle, has been modelled recently in transgenic mice expressing mutant tau protein linked to frontotemporal dementia. PD is characterised by intraneuronal cytoplasmic deposits (Lewy bodies) of the PD-associated gene product alpha-synuclein. Transgenic expression of alpha-synuclein recreated hallmark features of PD in mice and fruit flies, establishing alpha-synuclein as PD-causing drug target. Moreover, environmental risk factors such as the pesticide rotenone have been used successfully to generate rodent models of PD. Lesion models of PD are being exploited for the development of experimental gene therapy and transplantation approaches.

Tau phosphorylation in neuronal cell function and dysfunction

Tau is a group of neuronal microtubule-associated proteins that are formed by alternative mRNA splicing and accumulate in neurofibrillary tangles in Alzheimer's disease (AD) brain. Tau plays a key role in regulating microtubule dynamics, axonal transport and neurite outgrowth, and all these functions of tau are modulated by site-specific phosphorylation. There is significant evidence that a disruption of normal phosphorylation events results in tau dysfunction in neurodegenerative diseases, such as AD, and is a contributing factor to the pathogenic processes. Indeed, the abnormal tau phosphorylation that occurs in neurodegenerative conditions not only results in a toxic loss of function (e.g. decreased microtubule binding) but probably also a toxic gain of function (e.g. increased tau-tau interactions). Although tau is phosphorylated in vitro by numerous protein kinases, how many of these actually phosphorylate tau in vivo is unclear. Identification of the protein kinases that phosphorylate tau in vivo in both physiological and pathological processes could provide potential therapeutic targets for the treatment of AD and other neurodegenerative diseases in which there is tau pathology.

Neuroprotective strategies in MS: Lessons from C57BL/WldS mice

Valuable clues about how axons degenerate in MS can be gained from axon pathology in other disorders and experimental models. We discuss the similarities in mechanism and morphology of axon pathology in diverse circumstances revealed using mutant mice. The slow Wallerian degeneration mutation, Wld(S), delays three types of axon degeneration previously considered distinct: Wallerian degeneration of injured axons, 'dying-back' of axons in peripheral nervous system disease, and axonal spheroid pathology in gracile axonal dystrophy (gad) mice. Therefore, axon degeneration mechanisms are more uniform than previously thought and, in gad at least, axonal swelling is either related to or a consequence of Wallerian degeneration. Both axonal swelling and the accumulation of amyloid precursor protein through impaired axonal transport are common to MS, gad, and many other CNS disorders, indicating a degree of shared mechanism. YFP-H transgenic mice express YFP in a representative subset of neurons enabling unprecedented imaging of axon morphology and pathology over considerable longitudinal distances. Using this method, we have observed unbroken axons with multiple constrictions and dilatations in VEGF(delta/delta) mice, a model of amyotrophic lateral sclerosis (ALS). Similar morphologies have been described in MS, stroke, and other disorders, again suggesting a uniformity of axon degeneration mechanisms.

Characterization of a double (amyloid precursor protein-tau) transgenic: tau phosphorylation and aggregation

A double transgenic mouse expressing the amyloid precursor protein, bearing the Swedish mutations, and expressing tau protein containing three of the mutations present in frontotemporal dementia linked to chromosome 17 (FTDP-17), has been characterized. In the double transgenic mouse an increase in tau phosphorylation at serine S262 and S422 was observed compared with that found in simple transgenic mice. The phosphorylation at S262 was also found, in a much lower level, in the single transgenic mouse expressing amyloid precursor protein (APP), and it was absent in that overexpressing tau variant. Additionally, in the double transgenic mouse a slight increase in the amount of sarkosyl insoluble tau polymers was observed in comparison with that found in single transgenic tau mouse. Also, wider tau filaments were found in the double transgenic mouse compared with those found in the single transgenic mouse. Our results suggest that beta-amyloid peptide could facilitate the phosphorylation of tau at a site not directed by proline, such as serine 262, and that modification could facilitate tau aberrant aggregation. Also, they suggest that different types of tau filamentous polymers can occur in different mouse models for tauopathies, like those used for Alzheimer's disease or FTDP-17.

Inability of tau to properly regulate neuronal microtubule dynamics: a loss-of-function mechanism by which tau might mediate neuronal cell death

Interest in the microtubule-associated protein tau stems from its critical roles in neural development and maintenance, as well as its role in Alzheimer's, FTDP-17 and related neurodegenerative diseases. Under normal circumstances, tau performs its functions by binding to microtubules and powerfully regulating their stability and growing and shortening dynamics. On the other hand, genetic analyses have established a clear cause-and-effect relationship between tau dysfunction/mis-regulation and neuronal cell death and dementia in FTDP-17, but the molecular basis of tau's destructive action(s) remains poorly understood. One attractive model suggests that the intracellular accumulation of abnormal tau aggregates causes cell death, i.e., a gain-of-toxic function model. Here, we describe the evidence and arguments for an alternative loss-of-function model in which tau-mediated neuronal cell death is caused by the inability of affected cells to properly regulate their microtubule dynamic due to mis-regulation by tau. In support of this model, our recent data demonstrate that missense FTDP-17 mutations that alter amino acid residues near tau's microtubule binding region strikingly modify the ability of tau to modulate microtubule dynamics. Additional recent data from our labs support the notion that the same dysfunction occurs in the FTDP-17 regulatory mutations that alter tau RNA splicing patterns. Our model posits that the dynamics of microtubules in neuronal cells must be tightly regulated to enable them to carry out their diverse functions, and that microtubules that are either over-stabilized or under-stabilized, that is, outside an acceptable window of dynamic activity, lead to neurodegeneration. An especially attractive aspect of this model is that it readily accommodates both the structural and regulatory classes of FTDP-17 mutations.

Accumulation of filamentous tau in the cerebral cortex of human tau R406W transgenic mice

Missense mutations of the tau gene cause autosomal dominant frontotemporal dementia and parkinsonism linked to chromosome 17 (FTDP-17), an illness characterized by progressive personality changes, dementia, and parkinsonism. There is prominent frontotemporal lobe atrophy of the brain accompanied by abundant tau accumulation with neurofibrillary tangles and neuronal cell loss. Using a hamster prion protein gene expression vector, we generated several independent lines of transgenic (Tg) mice expressing the longest form of the human four-repeat tau with the R406W mutation associated with FTDP-17. The TgTauR406W 21807 line showed tau accumulation beginning in the hippocampus and amygdala at 6 months of age, which subsequently spread to the cortices and subcortical areas. The accumulated tau was phosphorylated, ubiquitinated, conformationally changed, argyrophilic, and sarcosyl-insoluble. Activation of GSK-3beta and astrocytic induction of mouse tau were observed. Astrogliosis and microgliosis correlated with prominent tau accumulation. Electron microscopic examination revealed the presence of straight filaments. Behavioral tests showed motor disturbances and progressive acquired memory loss between 10 to 12 months of age. These findings suggested that TgTauR406W mice would be a useful model in the study of frontotemporal dementia and other tauopathies such as Alzheimer's disease (AD).

Evaluation of the mouse TgTP6.3 tauGFP transgene as a lineage marker in chimeras

The mouse TgTP6.3 transgene, encoding a tauGFP fusion protein, is becoming widely used but has yet to be fully characterized and evaluated as suitable lineage marker. The aim of the present study was to investigate the phenotype of TgTP6.3(+/+) homozygotes and TgTP6.3(+/-) hemizygotes, characterize the expression of the TgTP6.3 transgene in different tissues and critically evaluate its use as a lineage marker. TgTP6.3(+/+) homozygotes died between embryonic day 14.5 and weaning, whereas TgTP6.3(+/-) hemizygotes were mostly viable and fertile but smaller than non-transgenic siblings. TgTP6.3 expression began in the late two-cell stage, persisted in most fetal and adult tissues and was uniformly expressed in many (but not all) tissues. TgTP6.3(+/-) cells were readily identified in many chimeric tissues and their contribution appeared to be quantitatively and spatially normal. Overall, tauGFP expression in hemizygous TgTP6.3(+/-) cells fulfils the main criteria of a good lineage marker for many tissues. It provides a useful lineage marker, which should be particularly suitable for axons, blood vessels and pre-implantation embryos.

Tau alteration and neuronal degeneration in tauopathies: mechanisms and models

Tau becomes characteristically altered both functionally and structurally in several neurodegenerative diseases now collectively called tauopathies. Although increasing evidence supports that alterations of tau may directly cause neuronal degeneration and cell death, the mechanisms, which render tau to become a toxic agent are still unclear. In addition, it is obscure, whether neurodegeneration in tauopathies occurs via a common mechanism or specific differences exist. The aim of this review is to provide an overview about the different experimental models that currently exist, how they are used to determine the role of tau during degeneration and what has been learnt from them concerning the mechanistic role of tau in the disease process. The review begins with a discussion about similarities and differences in tau alteration in paradigmatic tauopathies such as frontotemporal dementia and parkinsonism linked to chromosome 17 (FTDP-17) and Alzheimer's disease (AD). The second part concentrates on major experimental models that have been used to address the mechanistic role of tau during degeneration. This will include a discussion of cell-free assays, culture models using cell lines or dissociated neurons, and animal models. How these models aid to understand (i) alterations in the function of tau as a microtubule-associated protein (MAP), (ii) direct cytotoxicity of altered tau protein, and (iii) the potential role of tau aggregation in neurodegenerative processes will be the central theme of this part. The review ends with concluding remarks about a general mechanistic model of the role of tau alteration and neuronal degeneration in tauopathies and future perspectives.

Mutations causing neurodegenerative tauopathies

Tau is the major component of the intracellular filamentous deposits that define a number of neurodegenerative diseases. They include the largely sporadic Alzheimer's disease (AD), progressive supranuclear palsy, corticobasal degeneration, Pick's disease and argyrophilic grain disease, as well as the inherited frontotemporal dementia and parkinsonism linked to chromosome 17 (FTDP-17). For a long time, it was unclear whether the dysfunction of tau protein follows disease or whether disease follows tau dysfunction. This was resolved when mutations in Tau were found to cause FTDP-17. Currently, 32 different mutations have been identified in over 100 families. About half of the known mutations have their primary effect at the protein level. They reduce the ability of tau protein to interact with microtubules and increase its propensity to assemble into abnormal filaments. The other mutations have their primary effect at the RNA level and perturb the normal ratio of three-repeat to four-repeat tau isoforms. Where studied, this resulted in a relative overproduction of tau protein with four microtubule-binding domains in the brain. Individual Tau mutations give rise to diseases that resemble progressive supranuclear palsy, corticobasal degeneration or Pick's disease. Moreover, the H1 haplotype of Tau has been identified as a significant risk factor for progressive supranuclear palsy and corticobasal degeneration. At a practical level, the new work is leading to the production of experimental animal models that reproduce the essential molecular and cellular features of the human tauopathies, including the formation of abundant filaments made of hyperphosphorylated tau protein and nerve cell degeneration.

C-terminal ECFP fusion impairs synaptotagmin 1 function: crowding out synaptotagmin 1

To allow the monitoring of synaptotagmin 1 trafficking in vivo, we generated transgenic mice expressing a synaptotagmin 1-enhanced cyan fluorescent protein (ECFP) fusion protein under control of the Thy1 promoter. Transgenic synaptotagmin 1-ECFP is expressed throughout the brain where it localizes to synapses and marks synapses in vivo. However, when we crossed transgenic synaptotagmin 1-ECFP mice with synaptotagmin 1 knock-out mice, we detected no rescue of survival or function. Furthermore, viral overexpression of synaptotagmin 1-ECFP in synaptotagmin 1-deficient neurons failed to restore normal Ca2+-triggered release, whereas overexpression of wild type synaptotagmin 1 did so efficiently. To determine whether synaptotagmin 1-ECFP is non-functional because the ECFP-fusion interferes with its biochemical activities, we measured Ca2+-independent binding of synaptotagmin 1-ECFP to SNARE complexes, and Ca2+-dependent binding of synaptotagmin 1-ECFP to phospholipids and to itself. Although the apparent Ca2+ affinity of synaptotagmin 1-ECFP was decreased compared with wild type synaptotagmin 1, we observed no major changes in Ca2+-dependent or -independent activities, indicating that the non-functionality of the synaptotagmin 1-ECFP fusion protein was not because of inactivation of its biochemical properties. These data suggest that synaptotagmin 1-ECFP is suitable for monitoring synaptic vesicle traffic in vivo because the synaptotagmin 1-ECFP marks synaptic vesicles without participating in exocytosis. In addition, the data demonstrate that synaptotagmin 1 function requires a free C terminus, possibly because of spatial constraints at the release sites.

Induction of inflammatory mediators and microglial activation in mice transgenic for mutant human P301S tau protein

Mice transgenic for human P301S tau protein exhibit many characteristics of the human tauopathies, including the formation of abundant filaments made of hyperphosphorylated tau protein and neurodegeneration leading to nerve cell loss. At 5 months of age, the pathological changes are most marked in brainstem and spinal cord. Here we show that these changes are accompanied by marked neuroinflammation. Many tau-positive nerve cells in brainstem and spinal cord were strongly immunoreactive for interleukin-1beta and cyclooxygenase-2, indicating induction and overproduction of proinflammatory cytokines and enzymes. In parallel, numerous activated microglial cells were present throughout brain and spinal cord of transgenic mice, where they concentrated around tau-positive nerve cells. These findings suggest that inflammation may play a significant role in the events leading to neurodegeneration in the tauopathies and that anti-inflammatory compounds may have therapeutic potential.

Myotonic dystrophy type 1 is associated with nuclear foci of mutant RNA, sequestration of muscleblind proteins and deregulated alternative splicing in neurons

Myotonic dystrophy type 1 (DM1) is caused by expansion of a CTG repeat in the DMPK gene. In skeletal muscles, DM1 may involve a novel, RNA-dominant disease mechanism in which transcripts from the mutant DMPK allele accumulate in the nucleus and compromise the regulation of alternative splicing. Here we show evidence for a similar disease mechanism in brain. Examination of post-mortem DM1 tissue by fluorescence in situ hybridization indicates that the mutant DMPK mRNA, with its expanded CUG repeat in the 3'-untranslated region, is widely expressed in cortical and subcortical neurons. The mutant transcripts accumulate in discrete foci within neuronal nuclei. Proteins in the muscleblind family are recruited into the RNA foci and depleted elsewhere in the nucleoplasm. In parallel, a subset of neuronal pre-mRNAs show abnormal regulation of alternative splicing. These observations suggest that CNS impairment in DM1 may result from a deleterious gain-of-function by mutant DMPK mRNA.

Amyloid‐induced neurofibrillary tangle formation in Alzheimer's disease: insight from transgenic mouse and tissue‐culture models

Of all forms of dementia, Alzheimer's disease is the most prevalent. It is histopathologically characterized by beta-amyloid-containing plaques, tau-containing neurofibrillary tangles, reduced synaptic density and neuronal loss in selected brain areas. For the rare familial forms of Alzheimer's disease, pathogenic mutations have been identified in both the gene encoding the precursor of the Abeta peptide, APP, itself and in the presenilin genes which encode part of the APP-protease complex. For the more frequent sporadic forms of Alzheimer's disease, the pathogenic trigger has not been unambiguously identified. Whether Abeta is again the main cause remains to be heavily discussed. In a related disorder termed frontotemporal dementia, which is characterized by tangles in the absence of beta-amyloid deposition, mutations have been identified in tau which also lead to neurodegeneration and dementia. For Alzheimer's disease the existence of familial forms lead to the proposition of the amyloid cascade hypothesis, which claims that beta-amyloid causes or enhances the tangle pathology. In this review, we describe tau transgenic mouse models in which aspects of the tau-associated pathology, including tangle formation, has been achieved. Moreover, tau transgenic mouse and tissue-culture models were used to test the amyloid cascade hypothesis. In addition, we discuss alternative hypotheses to explain the sporadic forms. The animal and tissue-culture models will provide insight into the underlying biochemical mechanisms of tau aggregation and nerve cell degeneration. These mechanisms may be partially shared between sporadic Alzheimer's disease, the familial forms and frontotemporal dementia. Eventually, Alzheimer's disease may be redefined based on biochemical events rather than phenotype.

Retarded axonal transport of R406W mutant tau in transgenic mice with a neurodegenerative tauopathy

Intracellular accumulations of filamentous tau inclusions are neuropathological hallmarks of neurodegenerative diseases known as tauopathies. The discovery of multiple pathogenic tau gene mutations in many kindreds with familial frontotemporal dementia with parkinsonism linked to chromosome 17 (FTDP-17) unequivocally confirmed the central role of tau abnormalities in the etiology of neurodegenerative disorders. To examine the effects of tau gene mutations and the role of tau abnormalities in neurodegenerative tauopathies, transgenic (Tg) mice were engineered to express the longest human tau isoform (T40) with or without the R406W mutation (RW and hWT Tg mice, respectively) that is pathogenic for FTDP-17 in several kindreds. RW but not hWT tau Tg mice developed an age-dependent accumulation of insoluble filamentous tau aggregates in neuronal perikarya of the cerebral cortex, hippocampus, cerebellum, and spinal cord. Significantly, CNS axons in RW mice contained reduced levels of tau when compared with hWT mice, and this was linked to retarded axonal transport and increased accumulation of an insoluble pool of RW but not hWT tau. Furthermore, RW but not hWT mice demonstrated neurodegeneration and a reduced lifespan. These data indicate that the R406W mutation causes reduced binding of this mutant tau to microtubules, resulting in slower axonal transport. This altered tau function caused by the RW mutation leads to increased accumulation and reduced solubility of RW tau in an age-dependent manner, culminating in the formation of filamentous intraneuronal tau aggregates similar to that observed in tauopathy patients.

Effects of alpha-tocopherol on an animal model of tauopathies

We have reported that transgenic (Tg) mice overexpressing human tau protein develop filamentous tau aggregates in the CNS. We overexpressed the smallest human tau isoform (T44) in the mouse CNS to model tauopathies. These tau Tg mice acquire age-dependent CNS pathologies, including insoluble, hyperphosphorylated tau and argyrophilic intraneuronal inclusions formed by tau-immunoreactive filaments. Therefore, these Tg mice are a model that can be exploited for drug discovery in studies that target amelioration of tau-induced neurodegeneration as well as for elucidating mechanisms of tau pathology in various neurodegenerative tauopathies. Oxidative stress has been implicated in the pathogenesis of various neurodegenerative diseases, including tauopathies, and many epidemiological, clinical, and basic studies have suggested the neuroprotective effects of vitamin E in neurodegenerative diseases. To elucidate the role of oxidative damage in the pathological mechanisms of these Tg mice, we fed them alpha-tocopherol, the major component of antioxidant vitamin E. Supplementation of alpha-tocopherol suppressed and/or delayed the development of tau pathology, which correlated with improvement in the health and attenuation of motor weakness in the Tg mice. These results suggest that oxidative damage is involved in the pathological mechanisms of the tau Tg mice and that treatment with antioxidative agents like alpha-tocopherol may prevent neurodegenerative tauopathies.

Transgenic animal models of Alzheimer's disease and related disorders: histopathology, behavior and therapy

Alzheimer's disease (AD) is a devastating neurodegenerative disease that affects more than 15 million people worldwide. Within the next generation, these numbers will more than double. To assist in the elucidation of pathogenic mechanisms of AD and related disorders, such as frontotemporal dementia (FTDP-17), genetically modified mice, flies, fish and worms were developed, which reproduce aspects of the human histopathology, such as beta-amyloid-containing plaques and tau-containing neurofibrillary tangles (NFT). In mice, the tau pathology caused selective behavioral impairment, depending on the distribution of the tau aggregates in the brain. Beta-amyloid induced an increase in the numbers of NFT, whereas the opposite was not observed in mice. In beta-amyloid-producing transgenic mice, memory impairment was associated with increased levels of beta-amyloid. Active and passive beta-amyloid-directed immunization caused the removal of beta-amyloid plaques and restored memory functions. These findings have since been translated to human therapy. This review aims to discuss the suitability and limitations of the various animal models and their contribution to an understanding of the pathophysiology of AD and related disorders.

Stable-tau overexpression in human neuroblastoma cells: an open door for explaining neuronal death in tauopathies

Many neurodegenerative disorders referred to as "tauopathies" are characterized by the accumulation and aggregation of Tau proteins into filaments. In these pathologies, Tau proteins are hyperphosphorylated and also abnormally phosphorylated. Moreover, they differ from each other by the preferential aggregation of isoforms exhibiting either three microtubule-binding repeats (3R) or four repeats (4R) Tau. To investigate the effects of an intracellular accumulation of Tau, we stably transfected neuroblastoma cell line SY5Y with either 3R or 4R Tau. Our data showed that an increase in intracellular Tau expression has led to their hyperphosphorylation. Conversely, an abnormal Tau phosphorylation and/or aggregation were never observed. Furthermore, SY5Y cells transfected with 4R Tau showed an increased susceptibility to cell death. Finally, in apoptotic conditions, Tau proteins were degraded at their carboxy terminus by caspase, leading to an apparent decrease in Tau phosphorylation in this region. Because truncated Tau generated during apoptosis are not commonly found in Tau aggregates, apoptotic processes may not be of interest in neurofibrillary degeneration.

GSK-3beta inhibition reverses axonal transport defects and behavioural phenotypes in Drosophila

The tauopathies are a group of disorders characterised by aggregation of the microtubule-associated protein tau and include Alzheimer's disease (AD) and the fronto-temporal dementias (FTD). We have used Drosophila to analyse how tau abnormalities cause neurodegeneration. By selectively co-expressing wild-type human tau (0N3R isoform) and a GFP vesicle marker in motorneurons, we examined the consequences of tau overexpression on axonal transport in vivo. The results show that overexpression of tau disrupts axonal transport causing vesicle aggregation and this is associated with loss of locomotor function. All these effects occur without neuron death. Co-expression of constitutively active glycogen-synthase kinase-3beta (GSK-3beta) enhances and two GSK-3beta inhibitors, lithium and AR-A014418, reverse both the axon transport and locomotor phenotypes, suggesting that the pathological effects of tau are phosphorylation dependent. These data show that tau abnormalities significantly disrupt neuronal function, in a phosphorylation-dependent manner, before the classical pathological hallmarks are evident and also suggest that the inhibition of GSK-3beta might have potential therapeutic benefits in tauopathies.

Role of tau protein in both physiological and pathological conditions

The morphology of a neuron is determined by its cytoskeletal scaffolding. Thus proteins that associate with the principal cytoskeletal components such as the microtubules have a strong influence on both the morphology and physiology of neurons. Tau is a microtubule-associated protein that stabilizes neuronal microtubules under normal physiological conditions. However, in certain pathological situations, tau protein may undergo modifications, mainly through phosphorylation, that can result in the generation of aberrant aggregates that are toxic to neurons. This process occurs in a number of neurological disorders collectively known as tauopathies, the most commonly recognized of which is Alzheimer's disease. The purpose of this review is to define the role of tau protein under normal physiological conditions and to highlight the role of the protein in different tauopathies.

Transgenic mice expressing the nucleoprotein of Borna disease virus in either neurons or astrocytes: decreased susceptibility to homotypic infection and disease

The nucleoprotein (N) of Borna disease virus (BDV) is the major target of the disease-inducing antiviral CD8 T-cell response in the central nervous system of mice. We established two transgenic mouse lines which express BDV-N in either neurons (Neuro-N) or astrocytes (Astro-N). Despite strong transgene expression, neurological disease or gross behavioral abnormalities were not observed in these animals. When Neuro-N mice were infected as adults, replication of BDV was severely impaired and was restricted to brain areas with a low density of transgene-expressing cells. Notably, the virus failed to replicate in the transgene-expressing granular and pyramidal neurons of the hippocampus (which are usually the preferred host cells of BDV). When Neuro-N mice were infected within the first 5 days of life, replication of BDV was not suppressed in most neurons, presumably because the onset of transgene expression in the brain occurred after these cells became infected with BDV. Astro-N mice remained susceptible to BDV infection, but they were resistant to BDV-induced neurological disorder. Unlike their nontransgenic littermates, Neuro-N mice with persistent BDV infection did not develop neurological disease after immunization with a vaccinia virus vector expressing BDV-N. In contrast to the situation in wild-type mice, this treatment also failed to induce N-specific CD8 T cells in the spleens of both transgenic mouse lines. Thus, while resistance to BDV infection in N-expressing neurons appeared to result from untimely expression of a viral nucleocapsid component, the resistance to BDV-induced neuropathology probably resulted from immunological tolerance.

Characterisation of cytoskeletal abnormalities in mice transgenic for wild-type human tau and familial Alzheimer's disease mutants of APP and presenilin-1

To study the role of Abeta amyloid deposits in the generation of cytoskeletal lesions, we have generated a transgenic mouse line coexpressing in the same neurons a wild-type human tau isoform (0N3R), a mutant form of APP (751SL) and a mutant form of PS1 (M146L). These mice developed early cerebral extracellular deposits of Abeta, starting at 2.5 months. A somatodendritic neuronal accumulation of transgenic tau protein was observed in tau only and in tau/PS1/APP transgenic mice, including in neurons adjacent to Abeta deposits. The phosphorylation status of this somatodendritic tau was similar in the two transgenic lines. The Abeta deposits were surrounded by a neuritic reaction composed of axonal dystrophic processes, immunoreactive for many phosphotau epitopes and for the human tau transgenic protein. Ultrastructural observation showed in these dystrophic neurites a disorganisation of the microtubule and the neurofilament network but animals that were observed up to 18 months of age did not develop neurofibrillary tangles. These results indicate that overexpression of mutant PS1, mutant APP and of wild-type human tau were not sufficient per se to drive the formation of neurofibrillary tangles in a transgenic model. The Abeta deposits, however, were associated to marked changes in cytoskeletal organisation and in tau phosphorylation in adjacent dystrophic neurites.

Genetic disruptions of O/E2 and O/E3 genes reveal involvement in olfactory receptor neuron projection

The mammalian Olf1/EBF (O/E) family of repeated helix-loop-helix (rHLH)transcription factors has been implicated in olfactory system gene regulation,nervous system development and B-cell differentiation. Ebf(O/E1) mutant animals showed defects in B-cell lineage and brain regions where it is the only O/E family member expressed, but the olfactory epithelium appeared unaffected and olfactory marker expression was grossly normal in these animals. In order to further study the mammalian O/E proteins,we disrupted O/E2 and O/E3 genes in mouse and placed tau-lacZ and tau-GFP reporter genes under the control of the respective endogenous O/E promoters. Mice mutant for each of these genes display reduced viability and other gene-specific phenotypes. Interestingly, both O/E2 and O/E3 knockout mice as well as O/E2/O/E3 double heterozygous animals share a common phenotype:olfactory neurons (ORN) fail to project to dorsal olfactory bulb. We suggest that a decreased dose of O/E protein may alter expression of O/E target genes and underlie the ORN projection defect.

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.

Co-localization of glycogen synthase kinase-3 with neurofibrillary tangles and granulovacuolar degeneration in transgenic mice

Transgenic mice expressing human tau with P301L missense mutation (JNPL3) develop progressive amyotrophy, neurofibrillary degeneration, and neuronal loss. Mating of JNPL3 with transgenic mice expressing mutant amyloid precursor protein (Tg2576) leads to bigenic (TAPP) mice with enhanced neurofibrillary pathology. TAPP and JNPL3 mice were studied with immunocytochemistry and immunoblotting with antibodies to glycogen synthase kinase-3 (GKS3) to determine whether the development of tauopathy is associated with activation or increased expression of GSK3, and when the observed changes occur with respect to neurofibrillary tangle (NFT) formation. Accumulation of GSK3alpha/beta phosphorylated at Y279/216 was observed in neurons containing NFTs and granulovacuolar degeneration (GVD), but not in normal neurons or neurons with pretangles. More GSK3 immunoreactive NFTs were detected in TAPP than JNPL3 mice, especially in the amygdala. These differences were notable only in old animals. There was no significant difference between animals with and without NFTs in the level of total, inactive, or Y216-phosphorylated (pY216)GSK3beta. No apparent GSK3 accumulation was detected in neurons in Tg2576 mice. There was also no significant difference in the distribution of GSK3 in lysates fractionated based on their solubility in various reagents, including the sarkosyl-insoluble fraction. The results suggest that the pY216 GSK3beta accumulates in NFT and GVD due to redistribution rather than increased expression or activation, and that pre-existence of tau abnormalities is required for APP/Abeta to exert their effects on tau pathology in TAPP mice.

Biochemical and ultrastructural study of neurofibrillary tangles in amyotrophic lateral sclerosis/parkinsonism-dementia complex in the Kii peninsula of Japan

Amyotrophic lateral sclerosis/parkinsonism-dementia complex of the Kii peninsula (Kii ALS/PDC) is a neurodegenerative disorder endemic to natives in the southern coast area of the Kii peninsula of Japan. The disorder closely resembles Guamanian ALS/PDC clinically and neuropathologically. The characteristic neuropathological finding is abundant neurofibrillary tangles (NFTs) without amyloid deposition. To elucidate the biochemical properties of hyperphosphorylated tau protein, the major component of the NFTs, we examined Kii ALS/PDC brains by immunoblotting and immunohistochemical analysis using well-characterized anti-tau antibodies specific to phosphorylation-dependent or -independent epitopes. Hyperphosphorylated tau in Kii ALS/PDC had phosphorylated epitopes common to tau of paired helical filaments (PHFs) in Alzheimer disease (AD): immunoblot showed triplet bands composed of 6 tau isoforms. Ultrastructurally, NFTs revealed a twisted filamentous shape similar to PHF of AD. The biochemical properties of its phosphorylated tau protein and the ultrastructural characteristics of the NFTs of Kii ALS/PDC are very similar, if not identical, to PHF tau in AD, although they are different taupopathies.

Genetic epidemiology of amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis (ALS) is a late onset, rapidly progressive and ultimately fatal neurological disorder, caused by the loss of motor neurons in the brain and spinal cord. Familial aggregation of ALS, with an age-dependent but high penetrance, is a major risk factor for ALS. Familial ALS (FALS) is clinically and genetically heterogeneous. Three genes and linkage to four additional gene loci have been identified so far and may either predominantly lead to ALS (ALSI-ALS6) or cause multisystem neurodegeneration with ALS as an occasional symptom (tauopathies, ALS-dementia complex). This review presents a tentative classification of the "major" ALS genes and ALS "susceptibility" genes, that may act as susceptibility factors for neurodegeneration in interaction with other genetic or environmental risk factors. Considering that mutations in ALS genes explain approximately 10% of familial as well as sporadic ALS, and most remaining cases of the discase are thought to result form the interaction of several genes and environmental factors, ALS is a paradigm for multifactorial discases.

Tau and axonopathy in neurodegenerative disorders

The microtubule (MT)-associated protein (MAP) tau in neurons has been implicated as a significant factor in the axonal growth, development of neuronal polarity, and the maintenance of MT dynamics. Tau is localized to the axon, and is known to promote MT assembly and to stabilize axonal MTs. These functions of tau are primarily regulated by the activities of protein kinases and phosphatases. In Alzheimer's disease and other neurodegenerative disorders, abundant filamentous tau inclusions are found to be major neuropathological characteristics of these diseases. Both somato-dendritic and axonal tau lesions appear to be closely associated with axonal disruption. Furthermore, recent discoveries of pathogenic mutations on the tau gene suggest that abnormalities of tau alone are causative of neurodegeneration. Finally, analyses of transgenic mice that express human tau proteins have enabled in vivo quantitative assessments of axonal functions and have provided information about mechanistic relationships between pathological alteration of tau and axonal degeneration.

Filamentous tau in oligodendrocytes and astrocytes of transgenic mice expressing the human tau isoform with the P301L mutation

We recently reported a transgenic mouse line (JNPL3) that expresses mutant (P301L) tau and develops neurofibrillary tangles composed of filamentous tau aggregates. Here we show that these mice have abnormal tau filaments not only in neurons, but also in oligodendrocytes and astrocytes. Similar results were detected in another transgenic line (JNPL2+3+) that expresses the longest human tau isoform with the P301L mutation. The ultrastructure of the tau filaments and immunoreactivity with tau and ubiquitin antibodies were similar in glia and neurons. Given similarities of the lesions in the mice to human neuronal and glial inclusions, these transgenic mice appear to be a valuable model to study pathogenesis of the neurodegenerative tauopathies.

Transgenic zebrafish model of neurodegeneration

In Alzheimer's disease (AD), the microtubule-associated protein, tau, is compromised in its normal association with microtubules and forms into paired helical filaments (PHF) that are the hallmark cytoskeletal pathology of the disease. Several posttranslational modifications of tau including phosphorylation have been implicated in AD pathogenesis. In addition, and importantly, mutations in the genes encoding human tau have recently been implicated in a variety of hereditary dementias, collectively termed frontotemporal dementia with parkinsonism linked to chromosome 17 (FTDP-17). This has rekindled interest in the importance of tau in neurodegenerative diseases (cf. Vogel [1998] Science 280:1524-1525; Goedert et al. [1998] Neuron 21:955-958; D'Souza et al. [1999] PNAS 96:5598-5603). Despite significant progress in the field of tau biology and neurodegenerative diseases, several important issues remain unresolved. The early functional consequences of tau alterations in living neurons is incompletely understood, and it is not clear how tau in neurodegenerative diseases becomes redistributed from its normal concentration in neuronal axons to pathological inclusions in neuronal soma known as neurofibrillary tangles (NFT). One of the reasons for these gaps in knowledge is the relative paucity of model systems to study these processes. We have developed a transgenic model system to study the functional consequences and trafficking patterns in zebrafish neurons of human tau either mutated on sites associated with hereditary dementias or altered at select posttranslational modification sites. The overall guiding hypothesis is that the model allows dissection of a hierarchy of events relevant to potential mechanisms of neurodegenerative diseases related to critical early stages in development of disease. We showed that a FTDP-17 mutant form of human tau expressed in zebrafish neurons produced a cytoskeletal disruption that closely resembled the NFT in human disease. This model system will prove useful in the study of other mutant taus in vertebrate neurons in vivo, and the approaches developed here will have broad usefulness in the study of functional consequences and potential genetic analyses of introducing into living vertebrate neurons other molecules involved in the pathogenesis of neurodegenerative diseases.

Assembly of tau in transgenic animals expressing P301L tau: alteration of phosphorylation and solubility

Transgenic mice (JNPL3), which develop neurofibrillary degeneration and express four-repeat human tau with P301L missense mutation, were characterized biochemically to determine whether the development of aggregated tau from soluble tau involves an intermediate stage. Homogenates from mice of different ages were separated into buffer-soluble (S1), sarkosyl- and salt-extractable (S2) and sarkosyl-insoluble pellet (P3) fractions, and analyzed for human tau distribution, phosphorylation and filament formation. S1 and S2 fractions contained 50-60-kDa tau whereas the S2 fraction also had 64-kDa tau. The level of tau in the P3 fraction increased in an age-dependent manner and correlated positively with the soluble tau concentration. The P3 fraction from 2.5-6.5-month-old mice contained 64- and 50-60-kDa tau, whereas that from 8.5-month and older transgenic animals contained mostly 64-kDa and higher molecular weight tau. The S2 and P3 fractions contained comparable amounts of 64-kDa tau. The 64-kDa tau was predominantly human, and phosphorylated at multiple sites: Thr181, Ser202/Thr205, Thr212, Thr231, Ser262, Ser396/Ser404, Ser409 and Ser422. Most of these sites were phosphorylated to a lesser extent in S2 than in P3 fractions. Tau polymers were detected in P3 fractions from 3-month and older female JNPL3 mice, but not in non-transgenic controls. The results suggest that tau in S2 represents an intermediate from which insoluble tau is derived, and that phosphorylation may play a role in filament formation and/or stabilization.

Abundant tau filaments and nonapoptotic neurodegeneration in transgenic mice expressing human P301S tau protein

The identification of mutations in the Tau gene in frontotemporal dementia and parkinsonism linked to chromosome 17 (FTDP-17) has made it possible to express human tau protein with pathogenic mutations in transgenic animals. Here we report on the production and characterization of a line of mice transgenic for the 383 aa isoform of human tau with the P301S mutation. At 5-6 months of age, homozygous animals from this line developed a neurological phenotype dominated by a severe paraparesis. According to light microscopy, many nerve cells in brain and spinal cord were strongly immunoreactive for hyperphosphorylated tau. According to electron microscopy, abundant filaments made of hyperphosphorylated tau protein were present. The majority of filaments resembled the half-twisted ribbons described previously in cases of FTDP-17, with a minority of filaments resembling the paired helical filaments of Alzheimer's disease. Sarkosyl-insoluble tau from brains and spinal cords of transgenic mice ran as a hyperphosphorylated 64 kDa band, the same apparent molecular mass as that of the 383 aa tau isoform in the human tauopathies. Perchloric acid-soluble tau was also phosphorylated at many sites, with the notable exception of serine 214. In the spinal cord, neurodegeneration was present, as indicated by a 49% reduction in the number of motor neurons. No evidence for apoptosis was obtained, despite the extensive colocalization of hyperphosphorylated tau protein with activated MAP kinase family members. The latter may be involved in the hyperphosphorylation of tau.

Axon pathology in neurological disease: a neglected therapeutic target

In the C57BL/Wld(S) mouse, a dominant mutation dramatically delays Wallerian degeneration in injury and disease, possibly by influencing multi-ubiquitination. Studies on this mouse show that axons and synapses degenerate by active and regulated mechanisms that are akin to apoptosis. Axon loss contributes to neurological symptoms in disorders as diverse as multiple sclerosis, stroke, traumatic brain and spinal cord injury, peripheral neuropathies and chronic neurodegenerative diseases, but it has been largely neglected in neuroprotective strategies. Defects in axonal transport, myelination or oxygenation could trigger such mechanisms of active axon degeneration. Understanding how these diverse insults might initiate an axon-degeneration process could lead to new therapeutic interventions.

Mouse and fly models of neurodegeneration

One of the most surprising discoveries of the past decade (at least in the field of neurodegeneration) was that protein misfolding underlies several seemingly disparate neurological diseases. Animal models were crucial to this discovery. In this article, we will discuss the CAG repeat diseases, the tauopathies and Parkinson disease, highlighting how mouse and fly models have contributed to our understanding of pathogenesis. In each case, we will stress what has been learned about the role of protein clearance and the questions that remain about how misfolded proteins acquire their toxicity.

Transgenic mouse model of tauopathies with glial pathology and nervous system degeneration

Frontotemporal dementias (FTDs), including corticobasal degeneration (CBD) and progressive supranuclear palsy (PSP), are neurodegenerative tauopathies characterized by widespread CNS neuronal and glial tau pathologies, but there are no tau transgenic (Tg) mice that model neurodegeneration with glia tau lesions. Thus, we generated Tg mice overexpressing human tau in neurons and glia. No neuronal tau aggregates were detected, but old mice developed Thioflavin S- and Gallyas-positive glial tau pathology resembling CBD astrocytic plaques. Tau-immunoreactive and Gallyas-positive oligodendroglial coiled bodies (similar to CBD and PSP), glial degeneration, and motor deficits were associated with age-dependent accumulations of insoluble hyperphosphorylated human tau and tau immunopositive filaments in degenerating glial cells. Thus, tau-positive glial lesions similar to human FTDs occur in these Tg mice, and these pathologies are linked to glial and axonal degeneration.

Amyotrophic lateral sclerosis/parkinsonism dementia complex: transgenic mice provide insights into mechanisms underlying a common tauopathy in an ethnic minority on Guam

Intracytoplasmic filamentous tau inclusions are neuropathological hallmarks of amyotrophic lateral sclerosis/parkinsonism-dementia complex (ALS/PDC) of Guam and the defining lesions of other neurodegenerative disorders known as tauopathies. Here we review current insights into the cell and molecular neuropathology of ALS/PDC, a common tauopathy in the Chamorro population on Guam. We also summarize recent advances in understanding this disorder through studies of transgenic (Tg) mouse models of this tauopathy. Briefly, overexpression of human tau isoforms in the central nervous system of Tg mice resulted in a neurodegenerative tauopathy with a phenotype similar to ALS/PDC. Specifically, argyrophilic, congophilic, and tau immunoreactive inclusions accumulated with age in cortical and brainstem neurons of these mice, but they were most abundant in spinal cord neurons, and the inclusions contained 10- to 20-nm tau-positive straight filaments. There also was extensive gliosis in spinal cord associated with axonal degeneration in the ventral roots, while remaining axons in spinal nerves showed a loss of microtubules and reduced fast axonal transport. With advancing age, these Tg mice showed increasing motor weakness, and this was accompanied by a progressive increase in the phosphorylation and insolubility of brain and spinal cord tau proteins. Thus, tau Tg mice recapitulate key phenotypic features of ALS/PDC neuropathology in an ethnic minority on Guam, and these animal models provide new opportunities to discover novel therapies for this and related tauopathies.

Axonopathy, tau abnormalities, and dyskinesia, but no neurofibrillary tangles in p25-transgenic mice

Neurofibrillary tangles, one of the pathologic hallmarks of Alzheimer's disease (AD), are composed of abnormally polymerized tau protein. The hyperphosphorylation of tau alters its normal cellular function and is thought to promote the formation of neurofibrillary tangles. Growing evidence suggests that cyclin-dependent kinase 5 (cdk5) plays a role in tau phosphorylation, but the function of the enzyme in tangle formation remains uncertain. In AD, cdk5 is constitutively activated by p25, a highly stable, 25kD protein thought to be increased in the AD brain. To test the hypothesis that p25/cdk5 interactions promote neurofibrillary pathology, we created transgenic mouse lines that overexpress the human p25 protein specifically in neurons. Mice with high transgenic p25 expression have augmented cdk5 activity and develop severe hindlimb semiparalysis and mild forelimb dyskinesia beginning at approximately 3 months of age. Immunohistochemical and ultrastructural analyses showed widespread axonal degeneration with focal accumulation of tau in various regions of the brain and, to a lesser extent, the spinal cord. However, there was no evidence of neurofibrillary tangles in neuronal somata or axons, nor were paired helical filaments evident ultrastructurally. These studies confirm that p25 overexpression can lead to tau abnormalities and axonal degeneration in vivo but do not support the hypothesis that p25-related induction of cdk5 is a primary event in the genesis of neurofibrillary tangles.

Activation of mitogen-activated protein kinase cascade and phosphorylation of cytoskeletal proteins after neurone-specific activation of p21ras. II. Cytoskeletal proteins and dendritic morphology

In the present study, we analysed changes in the expression, subcellular distribution and phosphorylation state of the microtubule-associated protein tau and other cytoskeletal proteins after neurone-specific activation of the mitogen-activated protein kinase (MAPK) in the CNS in vivo. We used transgenic mice with a neurone-specific expression of activated ras protein (p21H-ras(Val12), synapsin I promoter) that is associated with an augmented activity of the MAPK. Chronic activation of MAPK cascade influenced tau protein phosphorylation, localisation and dendritic morphology. While the amount of tau protein was elevated by 9%, phospho-epitopes detected by the monoclonal antibodies AT270, 12E8 and SMI34 were increased by about 21%, 40% and 59% respectively. Steady-state levels of tau mRNA were not affected. Thus, the increase in tau protein was most likely due to stabilisation of tau protein by augmented phosphorylation. While in wild-type animals tau protein was preferentially localised in axons, a prominent immunoreactivity was found in the somatodendritic compartment of transgenic mice. This subcellular translocation typically seen in pyramidal neurones was associated with an increase in the dendritic calibre by about 30% and is paralleled by an increase in tubulin of 19%. We were unable to obtain any morphological indication of neurodegenerative processes in these animals. We suggest that the moderate increase in tau protein and phosphorylation may be part of the neuroprotective mechanism. However, further studies on aged transgenic mice will be necessary to establish potential effects on neuronal viability.

FTDP-17 mutations in tau transgenic mice provoke lysosomal abnormalities and Tau filaments in forebrain

The tauopathies, which include Alzheimer's disease (AD) and frontotemporal dementias, are a group of neurodegenerative disorders characterized by filamentous Tau aggregates. That Tau dysfunction can cause neurodegeneration is indicated by pathogenic tau mutations in frontotemporal dementia and parkinsonism linked to chromosome 17 (FTDP-17). To investigate how Tau alterations provoke neurodegeneration we generated transgenic mice expressing human Tau with four tubulin-binding repeats (increased by FTDP-17 splice donor mutations) and three FTDP-17 missense mutations: G272V, P301L, and R406W. Ultrastructural analysis of mutant Tau-positive neurons revealed a pretangle appearance, with filaments of Tau and increased numbers of lysosomes displaying aberrant morphology similar to those found in AD. Lysosomal alterations were confirmed by activity analysis of the marker acid phosphatase, which was increased in both transgenic mice and transfected neuroblastoma cells. Our results show that Tau modifications can provoke lysosomal aberrations and suggest that this may be a cause of neurodegeneration in tauopathies.

Spinal cord neurofibrillary pathology in Alzheimer disease and Guam Parkinsonism-dementia complex

We examined spinal cords of neurodegenerative disease patients and controls living on the Island of Guam and in the continental United States. These patients had pathologically confirmed parkinsonism dementia-complex (PDC) with or without amyotrophic lateral sclerosis (ALS), or Alzheimer disease (AD), respectively. Nearly all of the spinal cords examined from both groups of patients contained neurofibrillary tangles (NFT). The immunohistochemical profile of these NFTs indicates that they are composed of hyperphosphorylated tau protein like their counterparts in the brains of these patients. Western blot analysis confirmed this by revealing that sarcosyl insoluble tau in spinal cord extracts from patients with NFTs exhibited the presence of all 6 tau isoforms similar to that from AD and ALS/PDC cortical gray matter.

Pathophysiological aspects of frontotemporal dementia--emphasis on cytoskeleton proteins and autoimmunity

The aim of this study was to investigate two putative pathophysiological aspects of the common neurodegenerative disorder frontotemporal dementia (FTD). To this end, cerebrospinal fluid (CSF) levels of tau (total tau) and the light subtype of the neurofilament proteins (NFL) were studied in patients with FTD (n=16) and in age-matched controls (n=16). In addition, serum was analysed for IgG and IgM antibodies to the most common gangliosides and sulfatide in FTD patients (n=13) and in age-matched controls (n=20). The CSF-NFL levels were increased in FTD (1606+/-1151 pg/ml, mean+/-S.D.; P<0.001) compared with controls (308+/-203 pg/ml), whereas the CSF-tau levels were normal. In serum, autoantibody IgG-GA1 was significantly increased in FTD (P<0.05) compared with controls. No correlations were found between the effect parameters and demographic variables in any group. The results of this study suggest that cytoskeleton proteins other than tau are also involved in the pathophysiology of FTD and that autoimmunity may be part of the pathophysiological processes in FTD, as it is believed to be in Alzheimer's disease.

Reduced protein phosphatase 2A activity induces hyperphosphorylation and altered compartmentalization of tau in transgenic mice

Hyperphosphorylated isoforms of the microtubule-associated protein tau are the major components of neurofibrillary lesions in Alzheimer's disease (AD). Protein phosphatase (PP) 2A is a major phosphatase implicated in tau dephosphorylation in vitro. Dephosphorylation of tau can be blocked in vivo by okadaic acid, a potent inhibitor of PP2A. Moreover, activity of PP2A is reduced in AD brains. To elucidate the role of PP2A in tau phosphorylation and pathogenesis, we expressed a dominant negative mutant form of the catalytic subunit Calpha of PP2A, L199P, in mice by using a neuron-specific promoter. We obtained mice with high expression levels of Calpha L199P in cortical, hippocampal, and cerebellar neurons. PP2A activity in brain homogenates of transgenic mice was reduced to 66%. Endogenous tau protein was hyperphosphorylated at distinct sites including the AT8 epitope Ser-202/Thr-205, a major AD-associated tau phosphoepitope. AT8-positive tau aggregates accumulated in the soma and dendrites of cortical pyramidal cells and cerebellar Purkinje cells and co-localized with ubiquitin. Our data establish that PP2A plays a crucial role in tau phosphorylation. Our results also show that reduced PP2A activity is associated with altered compartmentalization and ubiquitination of tau, resembling a key pathological finding in AD.

Analysis of tauopathies with transgenic mice

Intraneuronal filamentous inclusions composed of the microtubule-associated protein tau are a feature of several neurodegenerative diseases (including Alzheimer's disease) known as tauopathies. A pivotal finding was the identification in 1998 of mutations in tau associated with frontotemporal dementia with parkinsonism linked to chromosome 17. This demonstrated that tau dysfunction is sufficient to cause neurodegeneration, and indicated that tau is likely to play a crucial role in the pathogenesis of other tauopathies. However, the mechanism by which tau filamentous lesions form and their role in neurodegeneration remains uncertain. Recent progress in the development of transgenic mouse models of human tauopathy is allowing these questions to be addressed.

Neurodegenerative tauopathies

The defining neuropathological characteristics of Alzheimer's disease are abundant filamentous tau lesions and deposits of fibrillar amyloid beta peptides. Prominent filamentous tau inclusions and brain degeneration in the absence of beta-amyloid deposits are also hallmarks of neurodegenerative tauopathies exemplified by sporadic corticobasal degeneration, progressive supranuclear palsy, and Pick's disease, as well as by hereditary frontotemporal dementia and parkinsonism linked to chromosome 17 (FTDP-17). Because multiple tau gene mutations are pathogenic for FTDP-17 and tau polymorphisms appear to be genetic risk factors for sporadic progressive supranuclear palsy and corticobasal degeneration, tau abnormalities are linked directly to the etiology and pathogenesis of neurodegenerative disease. Indeed, emerging data support the hypothesis that different tau gene mutations are pathogenic because they impair tau functions, promote tau fibrillization, or perturb tau gene splicing, thereby leading to formation of biochemically and structurally distinct aggregates of tau. Nonetheless, different members of the same kindred often exhibit diverse FTDP-17 syndromes, which suggests that additional genetic or epigenetic factors influence the phenotypic manifestations of neurodegenerative tauopathies. Although these and other hypothetical mechanisms of neurodegenerative tauopathies remain to be tested and validated, transgenic models are increasingly available for this purpose, and they will accelerate discovery of more effective therapies for neurodegenerative tauopathies and related disorders, including Alzheimer's disease.

Frontotemporal dementia and tauopathy

The presence of abundant neurofibrillary lesions made of hyperphosphorylated tau proteins is the characteristic neuropathology of a subset of neurodegenerative disorders classified as "tauopathies." The discovery of mutations in the tau gene in frontotemporal dementia and parkinsonism linked to chromosome 17 (FTDP-17) constitutes convincing evidence that tau proteins play a key role in the pathogenesis of neurodegenerative disorders. Moreover, it now is known that the most common form of sporadic frontotemporal dementia (FTD), which is characterized by frontotemporal neuron loss, gliosis, and microvacuolar change, also is a tauopathy caused by a loss of tau protein expression. Thus, these discoveries have begun to change the classification and the neuropathologic diagnosis of FTD and tauopathies, as well as current understanding of the disease mechanisms underlying them. Although transgenic mice expressing wild-type human tau or variants thereof with an FTDP-17 mutation result in tau pathologies and brain degeneration similar to that seen in human tauopathies, the precise mechanisms leading to the onset and progression of neurodegenerative disorders remain incompletely understood. Here, we review current understanding of human neurodegenerative tauopathies and prospects for translative recent insights about these into therapeutic interventions to prevent or ameliorate them.