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

Tau inclusions in retinal ganglion cells of human P301S tau transgenic mice: effects on axonal viability

Tau inclusions play a key role in the pathogenesis of tauopathies. Altered tau levels have been detected in retina and optic nerve of patients with glaucoma, suggesting the possibility of shared pathogenic mechanisms with tauopathies. Here we report that hyperphosphorylated transgenic tau accumulates in the nerve fibre layer and, from 2 months of age, aggregates into filamentous inclusions in retinal ganglion cells of human P301S tau transgenic mice. Axonopathy and accumulation of hyperphosphorylated tau in the nerve fibre layer preceded inclusion formation. Hyperphosphorylated tau and tau inclusions were also detected in cultured retinal explants from 5-month-old transgenic mice. Axonal outgrowth was similar in transgenic and wild-type retinal explants under basal conditions. However, when exposed to growth-promoting stimuli, axon elongation was enhanced in explants from wild-type but not transgenic mice, indicating that the presence of abnormal tau can impair stimulated axonal outgrowth. These findings suggest that the retina is a good model system for investigating tau-driven neurodegeneration and for assessing potential pharmacological modifiers for tauopathies.

The role of tau in neurodegeneration

Since the identification of tau as the main component of neurofibrillary tangles in Alzheimer's disease and related tauopathies, and the discovery that mutations in the tau gene cause frontotemporal dementia, much effort has been directed towards determining how the aggregation of tau into fibrillar inclusions causes neuronal death. As evidence emerges that tau-mediated neuronal death can occur even in the absence of tangle formation, a growing number of studies are focusing on understanding how abnormalities in tau (e.g. aberrant phosphorylation, glycosylation or truncation) confer toxicity. Though data obtained from experimental models of tauopathies strongly support the involvement of pathologically modified tau and tau aggregates in neurodegeneration, the exact neurotoxic species remain unclear, as do the mechanism(s) by which they cause neuronal death. Nonetheless, it is believed that tau-mediated neurodegeneration is likely to result from a combination of toxic gains of function as well as from the loss of normal tau function. To truly appreciate the detrimental consequences of aberrant tau function, a better understanding of all functions carried out by tau, including but not limited to the role of tau in microtubule assembly and stabilization, is required. This review will summarize what is currently known regarding the involvement of tau in the initiation and development of neurodegeneration in tauopathies, and will also highlight some of the remaining questions in need of further investigation.

[Morphologic and molecular neuropathology of Alzheimer's disease]

Alzheimer disease lesions include the abnormal accumulation of two proteins normally present in neurons: tau protein and Abeta peptide. Tau protein aggregates into fibrils in the cell body of neurons (neurofibrillary tangles), in dendrites (neuropil threads) and in degenerating axons that constitute the corona of the senile plaque. Tau pathology progresses in the brain areas in a stereotyped manner and in parallel with the clinical symptoms. Abeta extracellular deposits may be diffuse or focal. The Abeta focal deposit constitutes the core of the senile plaque. Progression of the Abeta lesions, which initially affect the isocortex, then the hippocampus, basal ganglia, various brainstem nuclei and cerebellum, is not directly correlated with symptoms. Mutations involving the genes implicated in Abeta peptide metabolism are responsible for familial Alzheimer disease. Mutations of the tau gene are not associated with Alzheimer disease but with frontotemporal dementia. The link between altered Abeta peptide metabolism and tau pathology has not been fully elucidated. Animal models mimic several aspects of the disease and have contributed to a better understanding of the mechanisms of the lesions.

Knock-out and transgenic mouse models of tauopathies

Tauopathies, characterized by the dysfunction and aggregation of the microtubule-associated protein tau (MAPT), represent some of the most devastating neurodegenerative disorders afflicting the elderly, including Alzheimer's disease and progressive supranuclear palsy. Here we review the range of Mapt knock-out and MAPT transgenic mouse models which have proven successful at providing insights into the molecular mechanisms of neurodegenerative disease. In this overview we highlight several themes, including the insights such models provide into the cellular and molecular mechanisms of tauopathy, the direct relationship between neuropathology and behaviour, and the use of mouse models to help provide a platform for testing novel therapies. Mouse models have helped clarify the relationship between pathological forms of tau, cell death, and the emergence of disease, as well as the interaction between tau and other disease-associated molecules, such as the A beta peptide. Finally, we discuss potential future MAPT genomic DNA models to investigate the importance of alternative splicing of the MAPT locus and its role in sporadic tauopathies.

An update on the toxicity of Abeta in Alzheimer's disease

Alzheimer's disease is characterized histopathologically by deposition of insoluble forms of the peptide Abeta and the protein tau in brain. Abeta is the principal component of amyloid plaques and tau of neurofibrillary tangles. Familial cases of AD are associated with causal mutations in the gene encoding the amyloid precursor protein, APP, from which the amyloidogenic Abeta peptide is derived, and this supports a role for Abeta in disease. Abeta can promote tau pathology and at the same time its toxicity is also tau-dependent. Abeta can adopt different conformations including soluble oligomers and insoluble fibrillar species present in plaques. We discuss which of these conformations exert toxicity, highlight molecular pathways involved and discuss what has been learned by applying functional genomics.

Hyperphosphorylation of microtubule-associated protein tau: a promising therapeutic target for Alzheimer disease

Alzheimer disease (AD) is the most common cause of dementia in adults. The current therapy for AD has only moderate efficacy in controlling symptoms, and it does not cure the disease. Recent studies have suggested that abnormal hyperphosphorylation of tau in the brain plays a vital role in the molecular pathogenesis of AD and in neurodegeneration. This article reviews the current advances in understanding of tau protein, regulation of tau phosphorylation, and the role of its abnormal hyperphosphorylation in neurofibrillary degeneration. Furthermore, several therapeutic strategies for treating AD on the basis of the important role of tau hyperphosphorylation in the pathogenesis of the disease are described. These strategies include (1) inhibition of glycogen synthase kinase-3beta (GSK-3beta), cyclin-dependent kinase 5 (cdk5), and other tau kinases; (2) restoration of PP2A activity; and (3) targeting tau O-GlcNAcylation. Development of drugs on the basis of these strategies is likely to lead to disease-modifying therapies for AD.

Divergent phosphorylation pattern of tau in P301L tau transgenic mice

Aggregates of hyperphosphorylated tau are prominent in brains of patients with Alzheimer's disease or frontotemporal dementia (FTD). They have been reproduced in animal models following the identification of tau mutations in familial cases of FTD. This includes our previously generated transgenic model, pR5, which expresses FTD (P301L) mutant tau in neurons. The mice are characterized by tau aggregation including tangle (NFT) formation, memory impairment and mitochondrial dysfunction. In 8-month-old mice, S422 phosphorylation of tau is linked to NFT formation, however, a detailed analysis of tau solubility, phosphorylation and aggregation has not been done nor have the mice been monitored until a high age. Here, we undertook an analysis by immunohistochemistry, Gallyas impregnation and Western blotting of brains from 3 month- up to 20 month-old mice. NFTs first appeared at 6 months in the amygdala, followed by the CA1 region of the hippocampus. As the mice get older, the solubility of tau is decreased as determined by sequential extractions. Histological analysis revealed increased phosphorylation at the AT180, AT270 and 12E8 epitopes with ageing. The numbers of AT8-positive neurons increased from 3 to 6 months old. However, whereas S422 appeared only late and concomitantly with NFT formation, the only neurons left with AT8-reactivity at 20 months were those that had undergone NFT formation. As hyperphosphorylated tau continued to accumulate, the lack of AT8-reactivity suggests regulatory mechanisms in specifically dephosphorylating the AT8 epitope in the remaining neurons. Thus, differential regulation of phosphorylation is important for NFT formation in neurodegenerative diseases with tau pathology.

Mechanisms of neuronal death in disease: defining the models and the players

Dysregulation of life and death at the cellular level leads to a variety of diseases. In the nervous system, aberrant neuronal death is an outstanding feature of neurodegenerative diseases. Since the discovery of the caspase family of proteases, much effort has been made to determine how caspases function in disease, including neurodegenerative diseases. Although many papers have been published examining caspases in neuronal death and disease, the pathways have not been fully clarified. In the present review, we examine the potential players in the death pathways, the current tools for examining these players and the models for studying neurological disease. Alzheimer's disease, the most common neurodegenerative disorder, and cerebral ischaemia, the most common cause of neurological death, are used to illustrate our current understanding of death signalling in neurodegenerative diseases. A better understanding of the neuronal death pathways would provide targets for the development of therapeutic interventions for these diseases.

Parkinsonism and impaired axonal transport in a mouse model of frontotemporal dementia

Frontotemporal dementia (FTD) is characterized by cognitive and behavioral changes and, in a significant subset of patients, Parkinsonism. Histopathologically, FTD frequently presents with tau-containing lesions, which in familial cases result from mutations in the MAPT gene encoding tau. Here we present a novel transgenic mouse strain (K3) that expresses human tau carrying the FTD mutation K369I. K3 mice develop a progressive histopathology that is reminiscent of that in human FTD with the K369I mutation. In addition, K3 mice show early-onset memory impairment and amyotrophy in the absence of overt neurodegeneration. Different from our previously generated tau transgenic strains, the K3 mice express the transgene in the substantia nigra (SN) and show an early-onset motor phenotype that reproduces Parkinsonism with tremor, bradykinesia, abnormal gait, and postural instability. Interestingly, motor performance of young, but not old, K3 mice improves upon L-dopa treatment, which bears similarities to Parkinsonism in FTD. The early-onset symptoms in the K3 mice are mechanistically related to selectively impaired anterograde axonal transport of distinct cargos, which precedes the loss of dopaminergic SN neurons that occurs in aged mice. The impaired axonal transport in SN neurons affects, among others, vesicles containing the dopamine-synthesizing enzyme tyrosine hydroxylase. Distinct modes of transport are also impaired in sciatic nerves, which may explain amyotrophy. Together, the K3 mice are a unique model of FTD-associated Parkinsonism, with pathomechanistic implications for the human pathologic process.

Detailed immunohistochemical characterization of temporal and spatial progression of Alzheimer's disease-related pathologies in male triple-transgenic mice

Background

Several transgenic animal models genetically predisposed to develop Alzheimer's disease (AD)-like pathology have been engineered to facilitate the study of disease pathophysiology and the vetting of potential disease-modifying therapeutics. The triple transgenic mouse model of AD (3xTg-AD) harbors three AD-related genetic loci: human PS1^M146V, human APP^swe, and human tau^P301L. These mice develop both amyloid plaques and neurofibrillary tangle-like pathology in a progressive and age-dependent manner, while these pathological hallmarks are predominantly restricted to the hippocampus, amygdala, and the cerebral cortex the main foci of AD neuropathology in humans. This model represents, at present, one of the most advanced preclinical tools available and is being employed ever increasingly in the study of mechanisms underlying AD, yet a detailed regional and temporal assessment of the subtleties of disease-related pathologies has not been reported.

Methods and results

In this study, we immunohistochemically documented the evolution of AD-related transgene expression, amyloid deposition, tau phosphorylation, astrogliosis, and microglial activation throughout the hippocampus, entorhinal cortex, primary motor cortex, and amygdala over a 26-month period in male 3xTg-AD mice. Intracellular amyloid-beta accumulation is detectable the earliest of AD-related pathologies, followed temporally by phospho-tau, extracellular amyloid-beta, and finally paired helical filament pathology. Pathology appears to be most severe in medial and caudal hippocampus. While astrocytic staining remains relatively constant at all ages and regions assessed, microglial activation appears to progressively increase temporally, especially within the hippocampal formation.

Conclusion

These data fulfill an unmet need in the ever-widening community of investigators studying 3xTg-AD mice and provide a foundation upon which to design future experiments that seek to examine stage-specific disease mechanisms and/or novel therapeutic interventions for AD.

Ethanol enhances tau accumulation in neuroblastoma cells that inducibly express tau

Chronic alcohol consumption causes pathological changes in the brain and neuronal loss. Ethanol toxicity may partially result from the perturbation of microtubule-associated proteins, like tau. Tau dysfunction is well known for its involvement in certain neurodegenerative diseases, such as Alzheimer's disease. In the present study, the effect of ethanol on tau was examined using differentiated human neuroblastoma cells that inducibly express the 4R0N isoform of tau via a tetracycline-off expression system. During tau induction, ethanol exposure (1.25-5mg/ml) dose-dependently increased tau protein levels and reduced cell viability. The increase in cell death likely resulted from tau accumulation since increased levels of tau were sufficient to reduce cell viability and ethanol was toxic to cells expressing tau but not to non-induced controls. Tau accumulation did not result from greater tetracycline-off induction since ethanol increased neither tau mRNA expression nor the expression of the tetracycline-controlled transactivator. Additionally, ethanol increased endogenous tau protein levels in neuroblastoma cells lacking the tetracycline-off induction system for tau. Ethanol delayed tau clearance suggesting ethanol impedes its degradation. Though ethanol inhibited neither cathepsin B, cathepsin D, nor chymotrypsin-like activity, it did significantly reduce calpain I expression and activity. Calpain I knockdown by shRNA increased tau levels indicating that calpain participates in tau degradation in this model. Moreover, the activation of calpain, by the calcium ionophore A23187, partially reversed the accumulation of tau resulting from ethanol exposure. Impaired calpain-mediated degradation may thus contribute to the increased accumulation of tau caused by ethanol.

Abnormally phosphorylated tau is associated with neuronal and axonal loss in experimental autoimmune encephalomyelitis and multiple sclerosis

The pathological correlate of clinical disability and progression in multiple sclerosis is neuronal and axonal loss; however, the underlying mechanisms are unknown. Abnormal phosphorylation of tau is a common feature of some neurodegenerative disorders, such as Alzheimer's disease. We investigated the presence of tau hyperphosphorylation and its relationship with neuronal and axonal loss in chronic experimental autoimmune encephalomyelitis (CEAE) and in brain samples from patients with secondary progressive multiple sclerosis. We report the novel finding of abnormal tau phosphorylation in CEAE. We further show that accumulation of insoluble tau is associated with both neuronal and axonal loss that correlates with progression from relapsing-remitting to chronic stages of EAE. Significantly, analysis of secondary progressive multiple sclerosis brain tissue also revealed abnormally phosphorylated tau and the formation of insoluble tau. Together, these observations provide the first evidence implicating abnormal tau in the neurodegenerative phase of tissue injury in experimental and human demyelinating disease.

Deficits in working memory and motor performance in the APP/PS1ki mouse model for Alzheimer's disease

The APP/PS1ki mouse model for Alzheimer's disease (AD) exhibits robust brain and spinal cord axonal degeneration and hippocampal CA1 neuron loss starting at 6 months of age. It expresses human mutant APP751 with the Swedish and London mutations together with two FAD-linked knocked-in mutations (PS1 M233T and PS1 L235P) in the murine PS1 gene. The present report covers a phenotypical analysis of this model using either behavioral tests for working memory and motor performance, as well as an analysis of weight development and body shape. At the age of 6 months, a dramatic, age-dependent change in all of these properties and characteristics was observed, accompanied by a significantly reduced ability to perform working memory and motor tasks. The APP/PS1ki mice were smaller and showed development of a thoracolumbar kyphosis, together with an incremental loss of body weight. While 2-month-old APP/PS1ki mice were inconspicuous in all of these tasks and properties, there is a massive age-related impairment in all tested behavioral paradigms. We have previously reported robust axonal degeneration in brain and spinal cord, as well as abundant hippocampal CA1 neuron loss starting at 6 months of age in the APP/PS1ki mouse model, which coincides with the onset of motor and memory deficits described in the present report.

Microtubule-associated protein tau in development, degeneration and protection of neurons

As a principal neuronal microtubule-associated protein, tau has been recognized to play major roles in promoting microtubule assembly and stabilizing the microtubules and to maintain the normal morphology of the neurons. Recent studies suggest that tau, upon alternative mRNA splicing and multiple posttranslational modifications, may participate in the regulations of intracellular signal transduction, development and viability of the neurons. Furthermore, tau gene mutations, aberrant mRNA splicing and abnormal posttranslational modifications, such as hyperphosphorylation, have also been found in a number of neurodegenerative disorders, collectively known as tauopathies. Therefore, changes in expression of the tau gene, alternative splicing of its mRNA and its posttranslational modification can modulate the normal architecture and functions of neurons as well as in a situation of tauopathies, such as Alzheimer's disease. The primary aim of this review is to summarize the latest developments and perspectives in our understanding about the roles of tau, especially hyperphosphorylation, in the development, degeneration and protection of neurons.

Motor impairment in Alzheimer's disease and transgenic Alzheimer's disease mouse models

In this commentary, we accent the accumulating evidence for motor impairment as a common feature of early Alzheimer's disease (AD) pathology. In addition, we summarize the state of knowledge on this phenotype in experimental mouse models, expressing AD-associated genes like tau or amyloid precursor protein.

Neurotrophic factors in Alzheimer's disease: role of axonal transport

Neurotrophic factors (NTF) are small, versatile proteins that maintain survival and function to specific neuronal populations. In general, the axonal transport of NTF is important as not all of them are synthesized at the site of its action. Nerve growth factor (NGF), for instance, is produced in the neocortex and the hippocampus and then retrogradely transported to the cholinergic neurons of the basal forebrain. Neurodegenerative dementias like Alzheimer’s disease (AD) are linked to deficits in axonal transport. Furthermore, they are also associated with imbalanced distribution and dysregulation of NTF. In particular, brain-derived neurotrophic factor (BDNF) plays a crucial role in cognition, learning and memory formation by modulating synaptic plasticity and is, therefore, a critical molecule in dementia and neurodegenerative diseases. Here, we review the changes of NTF expression and distribution (NGF, BDNF, neurotrophin-3, neurotrophin-4/5 and fibroblast growth factor-2) and their receptors [tropomyosin-related kinase (Trk)A, TrkB, TrkC and p75^NTR] in AD and AD models. In addition, we focus on the interaction with neuropathological hallmarks Tau/neurofibrillary tangle and amyloid-β (Abeta)/amyloid plaque pathology and their influence on axonal transport processes in order to unify AD-specific cholinergic degeneration and Tau and Abeta misfolding through NTF pathophysiology.

Lentivirus Tau (P301S) expression in adult amyloid precursor protein (APP)-transgenic mice leads to tangle formation

AIMS

In this study, we aimed to investigate the interaction between amyloid- and Tau-associated pathologies to gain further insights into the development of Alzheimer's disease. We examined the formation of neurofibrillary tangles (NFT) in adult mouse brain without the prior overexpression of Tau at embryonic or early post-natal stages to dissociate any developmental mechanisms.

METHODS

Lentivirus technology was used to examine the effects of overexpressing mutant Tau-P301S in the adult mouse brains of both wild-type and amyloid precursor protein (APP)-transgenic mice.

RESULTS

We find that injection of lentivirus Tau-P301S into the hippocampus of adult wild-type mice increases levels of hyperphosphorylated Tau, as early as 3 months post injection. However, no NFT are found even after 13 months of Tau expression. In contrast, the overexpression of Tau-P301S in adult APP-transgenic animals results in the formation of Gallyas-stained NFT.

CONCLUSIONS

Our current findings are the first to show that overexpression of Tau-P301S in adult mice overexpressing APP, but not wild-type mice, leads to enhanced Tau-related pathology.

Pronuclear injection for the production of transgenic mice

Transgenic mice have been instrumental in dissecting the role of various neuronal proteins under both physiological and pathological conditions. Pronuclear injection is the most widely used protocol for the generation of transgenic mice. Here, we describe all steps involved from DNA purification to the set up of a mouse colony including vasectomy, injection of the DNA into a donor zygote, transfer of injected zygotes into recipient foster mice, screening of offspring and establishment of transgenic mouse lines. We discuss the use of neuron-specific promoters to express proteins with a role in Alzheimer disease. Transgenic expression of a truncated form of the microtubule-associated protein tau (delta tau) is used as an example for the anticipated results.

Tauopathy models and human neuropathology: similarities and differences

Much of our current understanding of the pathogenic mechanisms in human neurodegenerative disorders has been derived from animal studies. As such, transgenic mouse models have significantly contributed to the development of novel pathogenic concepts underlying human tauopathies, a group of diseases comprising various forms of neurodegenerative disorders including Alzheimer's disease, corticobasal degeneration, argyrophilic grain disease, progressive supranuclear palsy, and Pick's disease as well as hereditary fronto-temporal dementia with parkinsonism linked to chromosome 17. Here, we will review in vivo models of human tauopathies with particular preference to transgenic mouse models. Strengths and limitations of these models in recapitulating the complex pathogenesis of tauopathies will be discussed.

Alzheimer disease models and human neuropathology: similarities and differences

Animal models aim to replicate the symptoms, the lesions or the cause(s) of Alzheimer disease. Numerous mouse transgenic lines have now succeeded in partially reproducing its lesions: the extracellular deposits of Abeta peptide and the intracellular accumulation of tau protein. Mutated human APP transgenes result in the deposition of Abeta peptide, similar but not identical to the Abeta peptide of human senile plaque. Amyloid angiopathy is common. Besides the deposition of Abeta, axon dystrophy and alteration of dendrites have been observed. All of the mutations cause an increase in Abeta 42 levels, except for the Arctic mutation, which alters the Abeta sequence itself. Overexpressing wild-type APP alone (as in the murine models of human trisomy 21) causes no Abeta deposition in most mouse lines. Doubly (APP x mutated PS1) transgenic mice develop the lesions earlier. Transgenic mice in which BACE1 has been knocked out or overexpressed have been produced, as well as lines with altered expression of neprilysin, the main degrading enzyme of Abeta. The APP transgenic mice have raised new questions concerning the mechanisms of neuronal loss, the accumulation of Abeta in the cell body of the neurons, inflammation and gliosis, and the dendritic alterations. They have allowed some insight to be gained into the kinetics of the changes. The connection between the symptoms, the lesions and the increase in Abeta oligomers has been found to be difficult to unravel. Neurofibrillary tangles are only found in mouse lines that overexpress mutated tau or human tau on a murine tau -/- background. A triply transgenic model (mutated APP, PS1 and tau) recapitulates the alterations seen in AD but its physiological relevance may be discussed. A number of modulators of Abeta or of tau accumulation have been tested. A transgenic model may be analyzed at three levels at least (symptoms, lesions, cause of the disease), and a reading key is proposed to summarize this analysis.

Tau-mediated neurodegeneration in Alzheimer's disease and related disorders

Advances in our understanding of the mechanisms of tau-mediated neurodegeneration in Alzheimer's disease (AD) and related tauopathies, which are characterized by prominent CNS accumulations of fibrillar tau inclusions, are rapidly moving this previously underexplored disease pathway to centre stage for disease-modifying drug discovery efforts. However, controversies abound concerning whether or not the deleterious effects of tau pathologies result from toxic gains-of-function by pathological tau or from critical losses of normal tau function in the disease state. This Review summarizes the most recent advances in our knowledge of the mechanisms of tau-mediated neurodegeneration to forge an integrated concept of those tau-linked disease processes that drive the onset and progression of AD and related tauopathies.

Early axonopathy preceding neurofibrillary tangles in mutant tau transgenic mice

Neurodegenerative diseases characterized by brain and spinal cord involvement often show widespread accumulations of tau aggregates. We have generated a transgenic mouse line (Tg30tau) expressing in the forebrain and the spinal cord a human tau protein bearing two pathogenic mutations (P301S and G272V). These mice developed age-dependent brain and hippocampal atrophy, central and peripheral axonopathy, progressive motor impairment with neurogenic muscle atrophy, and neurofibrillary tangles and had decreased survival. Axonal spheroids and axonal atrophy developed early before neurofibrillary tangles. Neurofibrillary inclusions developed in neurons at 3 months and were of two types, suggestive of a selective vulnerability of neurons to form different types of fibrillary aggregates. A first type of tau-positive neurofibrillary tangles, more abundant in the forebrain, were composed of ribbon-like 19-nm-wide filaments and twisted paired helical filaments. A second type of tau and neurofilament-positive neurofibrillary tangles, more abundant in the spinal cord and the brainstem, were composed of 10-nm-wide neurofilaments and straight 19-nm filaments. Unbiased stereological analysis indicated that total number of pyramidal neurons and density of neurons in the lumbar spinal cord were not reduced up to 12 months in Tg30tau mice. This Tg30tau model thus provides evidence that axonopathy precedes tangle formation and that both lesions can be dissociated from overt neuronal loss in selected brain areas but not from neuronal dysfunction.

The beta-propensity of Tau determines aggregation and synaptic loss in inducible mouse models of tauopathy

Neurofibrillary lesions are characteristic for a group of human diseases, named tauopathies, which are characterized by prominent intracellular accumulations of abnormal filaments formed by the microtubule-associated protein Tau. The tauopathies are accompanied by abnormal changes in Tau protein, including pathological conformation, somatodendritic mislocalization, hyperphosphorylation, and aggregation, whose interdependence is not well understood. To address these issues we have created transgenic mouse lines in which different variants of full-length Tau are expressed in a regulatable fashion, allowing one to switch the expression on and off at defined time points. The Tau variants differ by small mutations in the hexapeptide motifs that control the ability of Tau to adopt a beta-structure conformation and hence to aggregate. The "pro-aggregation" mutant DeltaK280, derived from one of the mutations observed in frontotemporal dementias, aggregates avidly in vitro, whereas the "anti-aggregation" mutant DeltaK280/PP cannot aggregate because of two beta-breaking prolines. In the transgenic mice, the pro-aggregation Tau induces a pathological conformation and pre-tangle aggregation, even at low expression levels, the anti-aggregation mutant does not. This illustrates that abnormal aggregation is primarily controlled by the molecular structure of Tau in vitro and in the organism. Both variants of Tau become mislocalized and hyperphosphorylated independently of aggregation, suggesting that localization and phosphorylation are mainly a consequence of increased concentration. These pathological changes are reversible when the expression of Tau is switched off. The pro-aggregation Tau causes a strong reduction in spine synapses.

[Animal models of neurodegenerative diseases]

Numerous evidences indicate that the phenotype of a neurodegenerative disease and its pathogenetic mechanism are only loosely linked. The phenotype is directly related to the topography of the lesions and is reproduced whatever the mechanism as soon as the same neurons are destroyed or deficient: the symptoms of Parkinson disease are mimicked by any destruction of the neurons of the substantia nigra, caused for instance by the toxin MPTP. This does not mean that idiopathic Parkinson disease is due to MPTP. In the same way, mouse lines such as Reeler, Weaver and Staggerer in which ataxia occurs spontaneously does not help to understand human ataxias: now that mutations responsible for these phenotypes have been identified, it appears that one is responsible for lissencephaly (mutation of the reelin gene) and the other two have no equivalent in man. Therapeutic attempts, however, rely on the understanding of the pathogenetic mechanisms. Introducing a mutated human transgene in the genome of an animal has, in many instances, significantly improved this understanding. Transgenic mice have proven useful in reproducing lesions seen in neurodegenerative disease such as the plaques of Alzheimer disease (in the APP mouse which has integrated the mutated gene of the amyloid protein precursor), the tau glial and neuronal accumulation (seen in cases of frontotemporal dementias due to tau mutation), the nuclear inclusions caused by CAG triplet expansion (seen in the mutation of Huntington disease and autosomal dominant spinocerebellar ataxias). These recent advances have fostered numerous therapeutic attempts. Transgenesis in drosophila and in the worm Caenorhabditis elegans have opened new possibilities in the screening of protein partners, modifier genes, and potential therapeutic molecules. However, it is also becoming clear that introducing a human mutated gene in an animal does not necessarily trigger pathogenetic cascades identical to those seen in the human disease. Human diseases have to be studied in parallel with their animal models to ensure that the model mimic at least a few original mechanisms, on which new therapeutics may be tested.

A Caenorhabditis elegans model of tau hyperphosphorylation: induction of developmental defects by transgenic overexpression of Alzheimer's disease-like modified tau

The microtubule-associated tau proteins become functionally and structurally altered in Alzheimer's disease (AD). To analyze tau modification and its role in a non-vertebrate animal model, we produced transgenic Caenorhabditis elegans strains with a panneuronal expression of human tau and a pseudohyperphosphorylated (PHP) tau construct that mimics AD-relevant tau modification. We show that human tau in C. elegans becomes highly phosphorylated and exhibits conformational changes similar to PHP tau and human PHF tau. Both, wt tau and PHP tau induced a progressive age-dependent development of a phenotype of uncoordinated locomotion (unc) in the absence of neuronal degeneration. However, only PHP tau induced a defective pattern of motor neuron development as indicated by the presence of gaps in the dorsal cord, commissures on the wrong side and local broadening of axons. The data indicate that C. elegans is capable of highly phosphorylating human tau to an AD-like state whereas only stable disease-like tau modification induce developmental defects suggesting a specific interference of pathologic tau with intracellular mechanisms of axonal outgrowth and pathfinding.

Monitoring synaptic function at the neuromuscular junction of a mouse expressing synaptopHluorin

We monitored presynaptic exocytosis and vesicle recycling at neuromuscular junctions of transgenic mice expressing synaptopHluorin (spH), using simultaneous optical and electrophysiological recordings. Synaptic transmission was indistinguishable from that in wild-type controls. Fluorescence rose during and decayed monotonically after stimulus trains to the nerve, with amplitudes and decay times increasing with the amount of stimulation. The relatively large size of synaptic terminals allowed us to examine the spatial profile of fluorescence changes. We identified hot spots of exocytosis, which were stable with repeated trains. Photobleach experiments showed that spH freshly exposed by nerve stimulation was not preferentially retrieved by compensatory endocytosis; instead, most retrieved spH preexisted in the surface membrane. Finally, we compared fluorescence and electrical [summed end-plate potentials (EPPs)] estimates of exocytosis, which diverged during repeated trains, as fluorescence exceeded summed EPPs, although the average amplitude of miniature EPPs was unchanged. This might reflect exocytosis of spH-containing, acetylcholine-free ("empty") vesicles or other organelles during intense stimulation.

Tau overexpression in transgenic mice induces glycogen synthase kinase 3β and β-catenin phosphorylation

The abnormal phosphorylations of tau, GSK3beta, and beta-catenin have been shown to perform a crucial function in the neuropathology of Alzheimer's disease (AD). The primary objective of the current study was to determine the manner in which overexpressed htau23 interacts and regulates the behavior and phosphorylation characteristics of tau, GSK3beta, and beta-catenin. In order to accomplish this, transgenic mice expressing neuron-specific enolase (NSE)-controlled human wild-type tau (NSE/htau23) were created. Transgenic mice evidenced the following: (i) tendency toward memory impairments at later stages, (ii) dramatic overexpression of the tau transgene, coupled with increased tau phosphorylation and paired helical filaments (PHFs), (iii) high levels of GSK3beta phosphorylation with advanced age, resulting in increases in the phosphorylations of tau and beta-catenin, (iv) an inhibitory effect of lithium on the phosphorylations of tau, GSK3beta, and beta-catenin, but not in the non-transgenic littermate group. Therefore, the overexpression of NSE/htau23 in the brains of transgenic mice induces abnormal phosphorylations of tau, GSK3beta, and beta-catenin, which are ultimately linked to neuronal degeneration in cases of AD. These transgenic mice are expected to prove useful for the development of new drugs for the treatment of AD.

Study of tauopathies by comparing Drosophila and human tau in Drosophila

The microtubule-binding protein tau has been investigated for its contribution to various neurodegenerative disorders. However, the findings from transgenic studies, using the same tau transgene, vary widely among different laboratories. Here, we have investigated the potential mechanisms underlying tauopathies by comparing Drosophila (d-tau) and human (h-tau) tau in a Drosophila model. Overexpression of a single copy of either tau isoform in the retina results in a similar rough eye phenotype. However, co-expression of Par-1 with d-tau leads to lethality, whereas co-expression of Par-1 with h-tau has little effect on the rough eye phenotype. We have found analogous results by comparing larval proteomes. Through genetic screening and proteomic analysis, we have identified some important potential modifiers and tau-associated proteins. These results suggest that the two tau genes differ significantly. This comparison between species-specific isoforms may help to clarify whether the homologous tau genes are conserved.

Abeta, tau and ApoE4 in Alzheimer's disease: the axonal connection

Mutations in amyloid precursor protein (APP), tau and apolipoprotein E4 (ApoE4) lead to Alzheimer's disease (AD) or related pathologies. Pathogenesis and interactions between these pathways have been studied in mouse models. Here, we highlight the fact that axons are important sites of cellular pathology in each pathway and propose that pathway convergence at the molecular level might occur in axons. Recent developments suggest that axonal transport of APP influences beta-amyloid deposition and that tau regulates axonal transport. ApoE4 influences both axonal tau phosphorylation and amyloid-induced neurite pathology. Thus, a better understanding of axonal events in AD might help connect the pathogenic mechanisms of beta-amyloid, ApoE4 and tau, indicating the most important steps for therapeutic targeting.

Ribonuclear foci at the neuromuscular junction in myotonic dystrophy type 1

In myotonic dystrophy type 1 (DM1) the muscle fibers express RNA containing an expanded CUG repeat (CUG(exp)). The CUG(exp) RNA is retained in the nucleus, forming ribonuclear foci. Splicing factors in the muscleblind (MBNL) family are sequestered in ribonuclear foci, resulting in abnormal regulation of alternative splicing. In extrajunctional nuclei, these effects on splicing regulation lead to reduced chloride conductance and altered insulin receptor signaling. Here we show that CUG(exp) RNA is also expressed in subsynaptic nuclei of muscle fibers and in motor neurons in DM1, causing sequestration of MBNL1 protein in both locations. In a transgenic mouse model, expression of CUG(exp) RNA at high levels in extrajunctional nuclei replicates many features of DM1, but the toxic RNA is poorly expressed in subsynaptic nuclei and the mice fail to develop denervation-like features of DM1 myopathology. Our findings indicate that subsynaptic nuclei and motor neurons are at risk for DM1-induced spliceopathy, which may affect function or stability of the neuromuscular junction.

Activation of PAR-1 kinase and stimulation of tau phosphorylation by diverse signals require the tumor suppressor protein LKB1

Aberrant phosphorylation of tau is associated with a number of neurodegenerative diseases, including Alzheimer's disease (AD). The molecular mechanisms by which tau phosphorylation is regulated under normal and disease conditions are not well understood. Microtubule affinity regulating kinase (MARK) and PAR-1 have been identified as physiological tau kinases, and aberrant phosphorylation of MARK/PAR-1 target sites in tau has been observed in AD patients and animal models. Here we show that phosphorylation of PAR-1 by the tumor suppressor protein LKB1 is required for PAR-1 activation, which in turn promotes tau phosphorylation in Drosophila. Diverse stress stimuli, such as high osmolarity and overexpression of the human beta-amyloid precursor protein, can promote PAR-1 activation and tau phosphorylation in an LKB1-dependent manner. These results reveal a new function for the tumor suppressor protein LKB1 in a signaling cascade through which the phosphorylation and function of tau is regulated by diverse signals under physiological and pathological conditions.

Tau aggregation in Alzheimer's disease: what role for phosphorylation?

The crucial role of the neuronal Tau protein in microtubule stabilization and axonal transport suggests that too little or too much Tau might lead to neuronal dysfunction. The presence of a hyper phosphorylated but non aggregated molecule as a toxic species that might sequester normal Tau is discussed. We present recent in vitro results that might allow us to dissect the role of individual phosphorylation sites on its structure and function. We also discuss in this review the role of phosphorylation for the aggregation of the neuronal Tau protein, and compare it to the aggregation induced by external poly anions.

A decade of tau transgenic animal models and beyond

The first tau transgenic mouse model was established more than a decade ago. Since then, much has been learned about the role of tau in Alzheimer's disease and related disorders. Animal models, both in vertebrates and invertebrates, were significantly improved and refined as a result of the identification of pathogenic mutations in Tau in human cases of frontotemporal dementia. They have been instrumental for dissecting the cross-talk between tau and the second hallmark lesion of Alzheimer's disease, the Abeta peptide-containing amyloid plaque. We discuss how the tau models have been used to unravel the pathophysiology of Alzheimer's disease, to search for disease modifiers and to develop novel treatment strategies. While tau has received less attention than Abeta, it is rapidly acquiring a more prominent position and the emerging view is one of a synergistic action of Abeta and tau in Alzheimer's disease. Moreover, the existence of a number of neurodegenerative diseases with tau pathology in the absence of extracellular deposits underscores the relevance of research on tau.

Animal models of tauopathies

Intracellular fibrillar amyloid lesions comprised of tau proteins are pathological hallmarks in diverse neurodegenerative disorders. As models of these tauopathies, transgenic mice overexpressing tau with or without mutations discovered in familial tauopathies were generated. Findings in these tau transgenic mice support the notion that impairments of tau proteins are causally related to tauopathies, while studies on crossbred mice have indicated initiation and promotion of tau-positive neuropathologies by crosstalk among several pathogenic molecules. Enhancement of tau pathology by amyloid beta (Abeta) deposition provided some of the most compelling evidence for such a cross-talk, and molecular processes linking abnormalities of Abeta and tan have been suggested to involve activation of calcium-dependent protease, calpain, based on analyses of amyloid precursor protein transgenic mice crossbred with other genetically engineered mice with altered calpain activity. It also should be noted that mice transgenic for both tau and alpha-synuclein exhibit facilitated polymerization of these molecules into pathological filaments. Roles of fibrillar tau deposits in nervous system injuries can be mechanistically pursued by longitudinal monitoring of brain amyloidosis and neuroglial degeneration in the time course of antiamyloid intervention. The possibility of in vivo detection of tau-positive amyloid lesions has been demonstrated by intravenous administration of potential tracers into tau transgenic mice and subsequent brain imaging. Moreover, visualization of glial responses in living brains may allow sensitive detection of degenerative changes in the central nervous system.

Plaques, tangles, and memory loss in mouse models of neurodegeneration

Within the past decade, our understanding of the pathogenic mechanisms in Alzheimer's disease (AD) has dramatically advanced because of the development of transgenic mouse models that recapitulate the key pathological and behavioral phenotypes of the disease. These mouse models have allowed investigators to test detailed questions about how pathology develops and to evaluate potential therapeutic approaches that could slow down the development of this disease. In this review, we discuss the status of transgenic mouse models and review the complex relationship between pathology and behavior in the development of neuropathological syndromes in AD.

Age-dependent axonal degeneration in an Alzheimer mouse model

Some neurodegenerative diseases including Alzheimer's disease (AD) and amyotrophic lateral sclerosis (ALS) exhibit prominent defects in axonal transport. These defects can manifest as axonal swellings or spheroids, which correspond to axonal enlargements and aberrant accumulation of axonal cargoes, cytoskeletal proteins and lipids. Recently, a controversial scientific debate focussed on the issue whether Abeta serves as a trigger for aberrant axonal transport in the pathophysiology of AD. Prominent axonopathy has been shown to be induced by overexpression of proteins involved in several neurodegenerative diseases. Neurofilament, apolipoprotein E, Niemann-Pick protein and Tau transgenic mice with axonal trafficking deficits have been reported. Furthermore, motor deficits are frequently observed in patients with AD, which has been attributed to the typical tauopathy in post-mortem brain tissue. In the present report, we analyzed axonal neuropathology in the brain and spinal cord of a transgenic mouse model with abundant intraneuronal Abeta42 production and provide compelling evidence for axonal degeneration. The APP/PS1ki mice showed characteristic axonal swellings, spheroids, axonal demyelination and ovoids, which are myelin remnants of degenerated nerve fibers in an age-dependent manner. Abundant accumulation of intraneuronal N-modified Abeta, Thioflavin S-positive material and ubiquitin was found within the somatodendritic compartment of neurons. We conclude that the intraneuronal accumulation of Abeta-amyloid peptides is followed by axonal degeneration, and thus might be a causative factor for the axonal changes seen in AD.

Alzheimer's disease-like tau neuropathology leads to memory deficits and loss of functional synapses in a novel mutated tau transgenic mouse without any motor deficits

Tau transgenic mice are valuable models to investigate the role of tau protein in Alzheimer's disease and other tauopathies. However, motor dysfunction and dystonic posture interfering with behavioral testing are the most common undesirable effects of tau transgenic mice. Therefore, we have generated a novel mouse model (THY-Tau22) that expresses human 4-repeat tau mutated at sites G272V and P301S under a Thy1.2-promotor, displaying tau pathology in the absence of any motor dysfunction. THY-Tau22 shows hyperphosphorylation of tau on several Alzheimer's disease-relevant tau epitopes (AT8, AT100, AT180, AT270, 12E8, tau-pSer396, and AP422), neurofibrillary tangle-like inclusions (Gallyas and MC1-positive) with rare ghost tangles and PHF-like filaments, as well as mild astrogliosis. These mice also display deficits in hippocampal synaptic transmission and impaired behavior characterized by increased anxiety, delayed learning from 3 months, and reduced spatial memory at 10 months. There are no signs of motor deficits or changes in motor activity at any age investigated. This mouse model therefore displays the main features of tau pathology and several of the pathophysiological disturbances observed during neurofibrillary degeneration. This model will serve as an experimental tool in future studies to investigate mechanisms underlying cognitive deficits during pathogenic tau aggregation.

Progressive white matter pathology in the spinal cord of transgenic mice expressing mutant (P301L) human tau

Transgenic mice expressing mutant (P301L) tau develop paresis, neurofibrillary tangles and neuronal loss in spinal motor neurons beginning at 4 to 6 months of age. Astrocytes and oligodendrocytes acquire filamentous tau inclusions at later ages. Here we report pathology in the spinal white matter of these animals. Progressive white matter pathology, detected as early as 2 months of age, was most marked in lateral and anterior columns, with sparing of posterior columns until late in the disease. Early changes in Luxol fast blue/periodic acid Schiff (LFB/PAS) and toluidine blue stained sections were vacuolation of myelin followed by accumulation of myelin figures within previous axonal tubes and finally influx of PAS-positive macrophages. Myelin debris and vacuoles were found in macrophages. At the ultrastructural level, myelinated axons showed extensive vacuolation of myelin sheaths formed by splitting of myelin lamellae at the intra-period line, while axons were atrophic and contained densely packed neurofilaments. Other axons were lost completely, resulting in collapse and phagocytosis of myelin sheaths. Also present were spheroids derived from swollen axons with thin myelin sheaths containing neurofilaments, tau filaments and degenerating organelles. Many oligodendrocytes had membrane-bound cytoplasmic bodies composed of tightly stacked lamellae capped by dense material. The vacuolar myelopathy in this model to some extent resembles that reported in acquired immune deficiency syndrome and vitamin B12 deficiency. The progressive axonal pathology is most consistent with a dying-back process caused by abnormal accumulation of tau in upstream neurons, while vacuolar myelinopathy may be a secondary manifestation of neuroinflammation.

Do axonal defects in tau and amyloid precursor protein transgenic animals model axonopathy in Alzheimer's disease?

The subcellular localization of organelles, mRNAs and proteins is particularly challenging in neurons. Owing to their extended morphology, with axons in humans exceeding a meter in length, in addition to which they are not renewed but persist for the entire lifespan, it is no surprise that neurons are highly vulnerable to any perturbation of their sophisticated transport machinery. There is emerging evidence that impaired transport is not only causative for a range of motor disorders, but possibly also for Alzheimer's disease (AD) and related neurodegenerative disorders. Support for this hypothesis comes from transgenic animal models. Overexpression of human tau and amyloid precursor protein (APP) in mice and flies models the key hallmark histopathological characteristics of AD, such as somatodendritic accumulation of phosphorylated forms of tau and beta-amyloid (Abeta) peptide-containing amyloid plaques, as well as axonopathy. The latter has also been demonstrated in mutant mice with altered levels of Alzheimer-associated genes, such as presenilin (PS). In Abeta-producing APP transgenic mice, axonopathy was observed before the onset of plaque formation and tau hyperphosphorylation. In human AD brain, an axonopathy was revealed for early but not late Braak stages. The overall picture is that key players in AD, such as tau, APP and PS, perturb axonal transport early on in AD, causing impaired synaptic plasticity and reducing survival rates. It will be challenging to determine the molecular mechanisms of these different axonopathies, as this might assist in the development of new therapeutic strategies.

Axonopathy in an APP/PS1 transgenic mouse model of Alzheimer's disease

While axonopathy is a prominent feature in a variety of neurodegenerative diseases, it has been largely neglected in Alzheimer's disease (AD), despite the observation of frequent motoric deficits in AD patients. In the present report we used transgenic mice overexpressing human mutant beta-amyloid precursor protein (APP(751SL)) and presenilin-1 (PS1(M146L)) that exhibit elevated intraneuronal Abeta42 levels. We observed abundant age-dependent axonopathy in the spinal cord: axons immunopositive for ubiquitin in the dorsal column; axonal swellings (spheroids) which accumulated APP, neurofilament, and ubiquitin; as well as myelin ovoid structures, which serve as markers for nerve fiber degeneration in both white and gray matter. Both descending and ascending axonal tracts in white matter were affected. Neuritic plaques also developed in an age-dependent manner starting in the cervical region. Furthermore, early intraneuronal Abeta was detected in some but not all motor neurons before plaque formation. In the present APP/PS1 transgenic mouse model we could show for the first time that elevated intracellular Abeta levels lead to an axonopathy characterized by the formation of axonal spheroids and myelin ovoids. The same pathological alterations are known from AD patients or transgenic models overexpressing Tau or ApoE, however, these disturbances in axonal transport occur in the absence of any signs of concomitant Tau pathology. This strengthens the prevailing amyloid hypothesis as a primary trigger of AD-typical pathological alterations.

Altered phosphorylation of cytoskeletal proteins in mutant protein phosphatase 2A transgenic mice

Protein phosphatase 2A (PP2A) is a family of heterotrimeric enzymes with diverse functions under physiologic and pathologic conditions such as Alzheimer's disease. All PP2A holoenzymes have in common a catalytic subunit C and a structural scaffolding subunit A. These core subunits assemble with various regulatory B subunits to form heterotrimers with distinct functions in the cell. Substrate specificity of PP2A in vitro is determined by regulatory subunits with leucine 309 of the catalytic subunit C playing a crucial role in the recruitment of regulatory subunits into the complex. Here we expressed a mutant form of Calpha, L309A, in brain and Harderian (lacrimal) gland of transgenic mice. We found an altered recruitment of regulatory subunits into the complex, demonstrating a role for the carboxyterminal leucine of Calpha in regulating holoenzyme assembly in vivo. This was associated with an increased phosphorylation of tau in brain and an impaired dephosphorylation of vimentin demonstrating that both cytoskeletal proteins are in vivo substrates of distinct PP2A holoenzyme complexes.

Axonal degeneration induced by targeted expression of mutant human tau in oligodendrocytes of transgenic mice that model glial tauopathies

Abundant filamentous tau inclusions in oligodendrocytes (OLGs) are hallmarks of neurodegenerative tauopathies, including sporadic corticobasal degeneration and hereditary frontotemporal dementia with parkinsonism linked to chromosome 17 (FTDP-17). However, mechanisms of neurodegeneration in these tauopathies are unclear in part because of the lack of animal models for experimental analysis. We address this by generating transgenic (Tg) mice expressing human tau exclusively in OLGs using the 2',3'-cyclic nucleotide 3'-phosphodiesterase promoter. Filamentous OLG tau inclusions developed in these Tg mice as a result of human tau expression in OLGs, especially those expressing the FTDP-17 human P301L mutant tau. Notably, structural disruption of myelin and axons preceded the emergence of thioflavin-S positive tau inclusions in OLGs, but impairments in axonal transport occurred even earlier, whereas motor deficits developed subsequently, especially in Tg mice with the highest tau expression levels. These data suggest that the accumulation of tau in OLG cause neurodegeneration, and we infer they do so by disrupting axonal transport. We suggest that similar defects may also occur in sporadic and hereditary human tauopathies with OLG tau pathologies.

Progressive neurodegeneration in C. elegans model of tauopathy

Discovery of various mutations in the tau gene among frontotemporal dementia and parkinsonism linked to chromosome 17 (FTDP-17) families suggests gain-of-toxic function of wild-type or mutant tau as the mechanism for extensive neuronal loss. We thus generated transgenic nematode (Caenorhabditis elegans) expressing wild-type or mutant (P301L and R406W) tau in the touch (mechanosensory) neurons. Whereas the worm expressing wild-type tau showed a small decrease in the touch response across the lifespan, the worm expressing mutant tau displayed a large and progressive decrease. When the touch neurons lost their function, neuritic abnormalities were found prominent, and microtubular loss became remarkable in the later stage. A substantial fraction of degenerating neurons developed tau accumulation in the cell body and neuronal processes. This neuronal dysfunction is not related to the apoptotic process because little recovery from touch abnormality was observed in the ced-3 or ced-4-deficient background. Expression of GSK3 brought about slight deterioration in the touch response, while expression of HSP70 led to some improvement.

Transgenic models of Alzheimer's disease: learning from animals

As the scope of the problem of Alzheimer's disease (AD) grows due to an aging population, research into the devastating condition has taken on added urgency. Rare inherited forms of AD provide insight into the molecular pathways leading to degeneration and have made possible the development of transgenic animal models. Several of these models are based on the overexpression of amyloid precursor protein (APP), presenilins, or tau to cause production and accumulation of amyloid-beta into plaques or hyperphosphorylated tau into neurofibrillary tangles. Producing these characteristic neuropathological lesions in animals causes progressive neurodegeneration and in some cases similar behavioral disruptions to those seen in AD patients. Knockout models of proteins involved in AD have also been generated to explore the native functions of these genes and examine whether pathogenesis is due to loss of function or toxic gain of function in these systems. Although none of the transgenic lines models the human condition exactly, the ability to study similar pathological processes in living animals have provided numerous insights into disease mechanisms and opportunities to test therapeutic agents. This chapter reviews animal models of AD and their contributions to developing therapeutic approaches for AD.

Dysregulation of protein phosphorylation/dephosphorylation in Alzheimer's disease: a therapeutic target

Studies during the last two decades have provided new insights into the molecular mechanism of Alzheimer's disease (AD). One of the milestone findings in AD research was the demonstration that neurofibrillary degeneration characterized by tau pathology is central to the pathogenesis of AD and other tauopathies and that abnormal hyperphosphorylation of tau is pivotal to neurofibrillary degeneration. This article reviews the recent research advances in tau pathology and the underlying dysregulation of the protein phosphorylation/dephosphorylation system. An updated model of the mechanism of neurofibrillary degeneration is also presented, and a promising therapeutic target to treat AD by correcting dysregulation of protein phosphorylation/dephosphorylation is discussed.

Tau alternative splicing and frontotemporal dementia

A number of neurodegenerative diseases are characterized by the presence of abundant deposits containing Tau protein. Expression of the human tau gene is under complex regulation. Mutations in the tau gene have been identified in patients with frontotemporal lobe dementia. These mutations affect either biochemical/biophysical properties or the delicate balance of different splicing isoforms. In this review, we summarize recent advances in our understanding of genetics and molecular pathogenesis of tauopathies with the focus on frontotemporal lobe dementia. We review published studies on tau pre-mRNA splicing regulation. Understanding molecular mechanisms of tauopathies may help in developing effective therapies for neurodegenerative tauopathies and related disorders, including Alzheimer disease.