Fragmentation of the Golgi apparatus induced by the overexpression of wild-type and mutant human tau forms in neurons

Tau is a microtubule-associated protein enriched in the axonal compartment. In several neurodegenerative diseases including Alzheimer's disease, hyperphosphorylated tau accumulates in the somatodendritic compartment, self-aggregates, and forms neurofibrillary tangles. A fragmentation of the neuronal Golgi apparatus (GA) was also observed in Alzheimer's disease. In the present study, we examined the effect of overexpressing human tau on the organization of the neuronal GA in rat hippocampal cultures and in JNPL3 mice expressing tau mutant P301L. GA fragmentation was noted in a significantly higher percentage of hippocampal neurons overexpressing wild-type human tau than in control neurons over-expressing green fluorescent protein (GFP) alone. Most importantly, in neurons overexpressing mutant forms of human tau (P301L, V337M, or R406W), the percentage of neurons with a fragmented GA was 10% higher than that of neurons overexpressing wild-type human tau. In JNPL3 mice, a significantly higher percentage of motor neurons presented a fragmented GA compared to control mice. Interestingly, fragmentation of the GA was more frequent in neurons containing an accumulation and aggregation of hyperphosphorylated tau in the cell body than in neurons without these features. In both primary hippocampal neurons and JNPL3 mice, the tau-induced GA fragmentation was not caused by apoptosis. The pre-sent results implicate tau in GA fragmentation and show that this event occurs before the formation of neurofibrillary tangles.     

GSPE interferes with tau aggregation in vivo: implication for treating tauopathy

Tauopathies are characterized by progressive neurodegeneration caused by intracellular accumulation of hyperphosphorylated tau protein aggregates in the brain. The present study was designed to test whether a grape seed polyphenolic extract (GSPE) previously shown to inhibit tau protein aggregation in vitro could benefit tau-mediated neuropathology and behavior deficits in JNPL3 transgenic mice expressing a human tau protein containing the P301L mutation. Nine-month-old JNPL3 mice were treated with GSPE delivered through their drinking water for 6 months. We found that GSPE treatment significantly reduced the number of motor neurons immunoreactive for hyperphosphorylated and conformationally-modified tau in the ventral horns of the spinal cord identified using AT100, PHF-1, AT8, and Alz50 tau antibodies. This coincided with a drastically reduced level of hyperphosphorylated and sarcosyl-insoluble tau in spinal cord fractions. Furthermore, the reduction of tau pathology was accompanied by an improvement in the motor function assessed by a wire hang test. Collectively, our results suggest that GSPE can interfere with tau-mediated neurodegenerative mechanisms and ameliorate neurodegenerative phenotype in an animal model of tauopathy. Our studies support further evaluation of GSPE for preventing and/or treating of tauopathies in humans.     

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.     

Ultrastructural neuronal pathology in transgenic mice expressing mutant (P301L) human tau

Transgenic mice expressing mutant (P301L) human tau develop neurofibrillary tangles, amyotrophy and progressive motor disturbance. We present ultrastructural features of neuronal degeneration in this model that suggests involvement of both neurofibrillary and autophagic processes in neurodegeneration. Neurons undergoing neurofibrillary degeneration contain tau-immunoreactive, 15-20 nm-wide straight or wavy filaments with no periodic twists. Tau filaments were found in two types of affected neurons. One type resembled neurons with neurofibrillary tangles (NFT) that were filled with numerous filaments that displaced sparse cytoplasmic organelles to the periphery. Microtubules were almost completely absent. The nucleus remained centrally located, but showed lobulations due to deep infoldings. The other type resembled ballooned neurons seen in some human tauopathies. The nucleus was peripherally placed, but normal appearing. The cytoplasmic organelles were dispersed throughout the swollen perikarya, the Golgi complex was fragmented and duplicated, while mitochondria and other organelles appeared normal. Tau filaments similar to those in NFT were sparse and not tightly packed. Microtubules were also sparse. Many autophagic vacuoles were present in these cells. Heterogeneous appearing axonal swellings resembling spheroids in human tauopathies were present in gray and white matter. Unlike normal appearing axons, axonal spheroids were filled with tau-immunoreactive filaments and autophagic vacuoles, in addition to normal appearing neurofilaments and microtubules. These P301L transgenic mice exhibit many features common to human tauopathies, making them a valuable model to study the pathogenesis of these uncommon disorders.     

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.     

Characterization of a novel transgenic rat carrying human tau with mutation P301L

We have established a novel transgenic rat line carrying human microtubule-associated protein Tau-40 with mutation P301L. hTau-40/P301L transgenic male and female rats were followed up to 2 years of age. The hTau-40/P301L rats expressed human tau mRNA and protein in the limbic cortex and associated white matter, hippocampus and spinal cord. With increasing age, the staining density for phosphorylated tau increased in all these areas. Neither silver stains nor Fluoro-Jade staining indicated the presence of dying neurons, or axonal degeneration, and there was no evidence of increased gliosis or inflammation. However, some neurons did display dendritic abnormalities, and immunoblots revealed the presence of sarcosyl insoluble tau. A large test battery revealed no behavioral abnormalities in these rats, except a mild hyperactivity in the elevated plus maze. In conclusion, this transgenic tau rat may be a useful model for ‘pretangle’ pathology, although in this study conditions were not sufficient to induce significant neuronal loss or behavioral deficits.     

Analysis of tau phosphorylation and truncation in a mouse model of human tauopathy

Recent evidence has suggested that truncation of tau protein at the caspase cleavage site D421 precedes hyperphosphorylation and may be necessary for the assembly of tau into filaments in Alzheimer's disease and other tauopathies. Here we have investigated the time course of the appearance of phosphorylated and truncated tau in the brain and spinal cord of mice transgenic for mutant human P301S tau protein. This mouse line recapitulates the essential molecular and cellular features of the human tauopathies, including tau hyperphosphorylation, tau filament formation, and neurodegeneration. Soluble tau was strongly phosphorylated at 1 to 6 months of age. Low levels of phosphorylated, sarkosyl-insoluble tau were detected at 2 months, with a steady increase up to 6 months of age. Tau truncated at D421 was detected at low levels in Tris-soluble and detergent-soluble tau at 3 to 6 months of age. By immunoblotting, it was not detected in sarkosyl-insoluble tau. However, by immunoelectron microscopy, a small percentage of tau in filaments from brain and spinal cord of transgenic mice was truncated at D421. Similar findings were obtained using dispersed filaments from Alzheimer's disease and FTDP-17 brains. The late appearance and low abundance of tau ending at D421 indicate that it is unlikely that truncation at this site is necessary for the assembly of tau into filaments.     

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.     

Age-dependent axonal transport and locomotor changes and tau hypophosphorylation in a "P301L" tau knockin mouse

Tauopathies are characterized by hyperphosphorylation of the microtubule-associated protein tau and its accumulation into fibrillar aggregates. Toxic effects of aggregated tau and/or dysfunction of soluble tau could both contribute to neural defects in these neurodegenerative diseases. We have generated a novel knockin mouse model of an inherited tauopathy, frontotemporal dementia with parkinsonism linked to tau mutations on chromosome 17 (FTDP-17T). We incorporated a single mutation, homologous to the common FTDP-17T P301L mutation, directly into the endogenous mouse gene, mimicking the human disease situation. These mice express P301L-equivalent mutant tau at normal physiological levels from the knockin allele. Importantly, in contrast to existing transgenic mouse models that overexpress human P301L mutant tau, no overt tau pathology developed during the normal lifespan of the knockin mice. In fact, overall phosphorylation of tau was reduced, perhaps due to reduced microtubule binding. However, homozygous knockin mice did display intriguing age-dependent changes in axonal transport of mitochondria, and increased spontaneous locomotor activity in old age. These could represent early consequences of the tau dysfunction that eventually precipitates pathogenesis in humans.     

Induction of tau pathology by intracerebral infusion of amyloid-beta -containing brain extract and by amyloid-beta deposition in APP x Tau transgenic mice

Alzheimer's disease presents morphologically with senile plaques, primarily made of extracellular amyloid-beta (A beta) deposits, and neurofibrillary lesions, which consist of intracellular aggregates of hyperphosphorylated tau protein. To study the in vivo induction of tau pathology, dilute brain extracts from aged A beta-depositing APP23 transgenic mice were intracerebrally infused in young B6/P301L tau transgenic mice. Six months after the infusion, tau pathology was induced in the injected hippocampus but also in brain regions well beyond the injection sites such as the entorhinal cortex and amygdala, areas with neuronal projection to the injection site. No or only modest tau induction was observed when brain extracts from aged nontransgenic control mice and aged tau-depositing B6/P301L transgenic mice were infused. To further study A beta-induced tau lesions B6/P301L tau transgenic mice were crossed with APP23 mice. Although A beta deposition in double-transgenic mice did not differ from single APP23 transgenic mice, double-transgenic mice revealed increased tau pathology compared to single B6/P301L tau transgenic mice predominately in areas with high A beta plaque load. The present results suggest that both extract-derived A beta species and deposited fibrillary A beta can induce the formation of tau neurofibrillary pathology. The observation that infused A beta can trigger the tau pathology in the absence of A beta deposits provides an explanation for the discrepancy between the neuroanatomical location of A beta deposits and the development and spreading of tau lesions in Alzheimer's disease brain.     

Accelerated human mutant tau aggregation by knocking out murine tau in a transgenic mouse model

Many models of human tauopathies have been generated in mice by expression of a human mutant tau with maintained expression of mouse endogenous tau. Because murine tau might interfere with the toxic effects of human mutant tau, we generated a model in which a pathogenic human tau protein is expressed in the absence of wild-type tau protein, with the aim of facilitating the study of the pathogenic role of the mutant tau and to reproduce more faithfully a human tauopathy. The Tg30 line is a tau transgenic mouse model overexpressing human 1N4R double-mutant tau (P301S and G272V) that develops Alzheimer's disease-like neurofibrillary tangles in an age-dependent manner. By crossing Tg30 mice with mice invalidated for their endogenous tau gene, we obtained Tg30xtau(-/-) mice that express only exogenous human double-mutant 1N4R tau. Although Tg30xtau(-/-) mice express less tau protein compared with Tg30, they exhibit signs of decreased survival, increased proportion of sarkosyl-insoluble tau in the brain and in the spinal cord, increased number of Gallyas-positive neurofibrillary tangles in the hippocampus, increased number of inclusions in the spinal cord, and a more severe motor phenotype. Deletion of murine tau accelerated tau aggregation during aging of this mutant tau transgenic model, suggesting that murine tau could interfere with the development of tau pathology in transgenic models of human tauopathies.     

Dendritic degeneration, neurovascular defects, and inflammation precede neuronal loss in a mouse model for tau-mediated neurodegeneration

Adeno-associated virus (AAV)-mediated expression of wild-type or mutant P301L protein tau produces massive degeneration of pyramidal neurons without protein tau aggregation. We probed this novel model for genetic and structural factors and early parameters of pyramidal neurodegeneration. In yellow fluorescent protein-expressing transgenic mice, intracerebral injection of AAV-tauP301L revealed early damage to apical dendrites of CA1 pyramidal neurons, whereas their somata remained normal. Ultrastructurally, more and enlarged autophagic vacuoles were contained in degenerating dendrites and manifested as dark, discontinuous, vacuolated processes surrounded by activated astrocytes. Dendritic spines were lost in AAV-tauP301L-injected yellow fluorescent protein-expressing transgenic mice, and ultrastructurally, spines appeared dark and degenerating. In CX3CR1(EGFP/EGFP)-deficient mice, microglia were recruited early to neurons expressing human tau. The inflammatory response was accompanied by extravasation of plasma immunoglobulins. α2-Macroglobulin, but neither albumin nor transferrin, became lodged in the brain parenchyma. Large proteins, but not Evans blue, entered the brain of mice injected with AAV-tauP301L. Ultrastructurally, brain capillaries were constricted and surrounded by swollen astrocytes with extensions that contacted degenerating dendrites and axons. Together, these data corroborate the hypothesis that neuroinflammation participates essentially in tau-mediated neurodegeneration, and the model recapitulates early dendritic defects reminiscent of "dendritic amputation" in Alzheimer's disease.     

Accelerated tau pathology with synaptic and neuronal loss in a novel triple transgenic mouse model of Alzheimer's disease

There is pivotal evidence that tau pathology can be triggered by amyloid-β (Aβ) pathology in experimental systems. On the other side, studies on human brain specimen have elucidated that tau pathology may occur before amyloid pathology is present indicating that in principle tau pathology could also trigger Aβ aggregation. To address this question, we have crossed 5XFAD mice coexpressing human mutant APP695 with the Swedish, Florida, and London mutations and human mutant presenilin-1 (PS1) with the M146L and L286V mutations with the PS19 model overexpressing human mutant tau with the P301S mutation. The resulting triple transgenic model 5XFAD/PS19 has been characterized at 3 and 9 months of age. A dramatic aggravation of hyperphosphorylated tau pathology together with a dramatically increased inflammatory response and a loss of synapses and hippocampal CA1 neurons in aged 5XFAD/PS19 mice were observed. Extracellular amyloid deposits were unaltered. These data support the assumption of tau pathology being downstream of amyloid pathology, suggesting that both pathologies together trigger the severe neuron loss in the hippocampus in the 5XFAD/PS19 mouse model.     

Sensitive quantitative assays for tau and phospho-tau in transgenic mouse models

Transgenic mouse models have been an invaluable resource in elucidating the complex roles of β-amyloid and tau in Alzheimer's disease. Although many laboratories rely on qualitative or semiquantitative techniques when investigating tau pathology, we have developed 4 Low-Tau, Sandwich enzyme-linked immunosorbent assays (ELISAs) that quantitatively assess different epitopes of tau relevant to Alzheimer's disease: total tau, pSer-202, pThr-231, and pSer-396/404. In this study, after comparing our assays with commercially available ELISAs, we demonstrate our assay's high specificity and quantitative capabilities using brain homogenates from tau transgenic mice, htau, JNPL3, and tau knockout. All 4 ELISAs show excellent specificity for mouse and human tau, with no reactivity to tau knockout animals. An age-dependent increase of serum tau in both tau transgenic models was also seen. Taken together, these assays are valuable methods to quantify tau and phospho-tau levels in transgenic animals, by examining tau levels in brain and measuring tau as a potential serum biomarker.     

Cortical neuronal and glial pathology in TgTauP301L transgenic mice: neuronal degeneration, memory disturbance, and phenotypic variation

Recapitulation of tau pathologies in an animal model has been a long-standing goal in neurodegenerative disease research. We generated transgenic (TgTauP301L) mice expressing a frontotemporal dementia with parkinsonism linked to chromosome 17 (FTPD-17) mutation within the longest form of tau (2N, 4R). TgTauP301L mice developed florid pathology including neuronal pretangles, numerous Gallyas-Braak-positive neurofibrillary tangles, and glial fibrillary tangles in the frontotemporal areas of the cerebrum, in the brainstem, and to a lesser extent in the spinal cord. These features were accompanied by gliosis, neuronal loss, and cerebral atrophy. Accumulated tau was hyperphosphorylated, conformationally changed, ubiquitinated, and sarkosyl-insoluble, with electron microscopy demonstrating wavy filaments. Aged TgTauP301L mice exhibited impairment in hippocampally dependent and independent behavioral paradigms, with impairments closely related to the presence of tau pathologies and levels of insoluble tau protein. We conclude that TgTauP301L mice recreate the substantial phenotypic variation and spectrum of pathologies seen in FTDP-17 patients. Identification of genetic and/or environmental factors modifying the tau phenotype in these mice may shed light on factors modulating human tauopathies. These transgenic mice may aid therapeutic development for FTDP-17 and other diseases featuring accumulations of four-repeat tau, such as Alzheimer's disease, corticobasal degeneration, and progressive supranuclear palsy.     

Cooexpression of FTDP-17 tau and GSK-3beta in transgenic mice induce tau polymerization and neurodegeneration

Here we have tested whether tau modification either by point mutation or by hyperphosphorylation can exert maximal pathogenic effects or if, on the contrary, both types of tau modifications can act synergistically to induce neuropathology. For this, we have combined transgenic mice overexpressing the enzyme GSK-3beta (Tet/GSK-3beta mice), with transgenic mice expressing Tau with a triple FTDP-17 mutation which develop prefibrillar tau-aggregates (VLW mice). Tet/GSK-3beta/VLW transgenic mice show tau hyperphosphorylation in hippocampal neurons. This is accompanied by thioflavin-S staining, and formation of filaments similar in width to those found in tauophaties. Finally, the atrophy of the hippocampal dentate gyrus observed in Tet/GSK-3beta mice develops much faster in Tet/GSK-3beta/VLW mice. All these morphological and biochemical data demonstrate that there is a synergistic contribution of both types of tau modifications and that the potential of GSK-3 inhibitors for AD therapeutics also extends to tauopathies caused by point mutations in tau gene.     

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.     

Active glycogen synthase kinase-3 and tau pathology-related tyrosine phosphorylation in pR5 human tau transgenic mice

We studied underlying pathomechanisms in tauopathies using pR5 mice that express the P301L tau mutation found in familial forms of frontotemporal dementia. In a longitudinal study we investigated the functional status of glycogen synthase kinase-3 and correlated it with the appearance of distinct tau phospho-epitopes. Neurons displaying increases in activating phosphorylation of glycogen synthase kinase-3α/β at tyrosine 279/216 also showed an intense rather than moderate AT8 (phospho-Ser202/Thr205 tau) immunoreactivity, and immunoreactivity for AT100 (phospho-Ser212/Thr214 tau) and phosphorylated Ser422, phospho-epitopes associated with fibrillar tau pathology. These neurons were rare in 8.5-month-old, but numerous in 18.5- and 28-month-old pR5 mice. Two antibodies that detect phosphotyrosine residues more generally only stained these neurons. In contrast, we did not find increased phosphotyrosine in neuronal perikarya of mice with an amyloid-β plaque pathology. Our results suggest a link between increased tyrosine phosphorylation and tau aggregation. They also reveal for the mouse models studied, that tau- rather than an amyloid-β peptide-induced pathology is associated with increased neuronal tyrosine phosphorylation.     

Neurofibrillary tangle-related synaptic alterations of spinal motor neurons of P301L tau transgenic mice

We investigated axosomatic synapses of anterior horn cells of transgenic (TG) mice expressing mutant P301L human tau and non-transgenic (NTG) mice using electron microscopic methods to demonstrate the relationship between neurofibrillary tangles (NFTs) and synaptic alterations. Animals aged 3.5-8.5 months were used because at this age many motor neurons in TG mice have NFTs. We measured the perimeter of anterior horn cell perikarya, the number of boutons and total length of boutons in contact with the neuronal perikarya from the micrographs of NFT and non-NFT-bearing neurons. We also calculated the proportion of the perimeter covered by boutons, density of boutons and mean size of boutons. The density of synaptic boutons in contact with NFT-bearing neurons was significantly decreased compared to non-NFT-bearing neurons. These findings suggest that synaptic reduction occurs during neurofibrillary degeneration and is probably associated with NFT. In addition, synaptic boutons were detached from NFT-bearing neurons with the resulting space occupied by astrocytic processes, suggesting that astrocytes may be involved in the observed synaptic alterations.     

Age-dependent induction of congophilic neurofibrillary tau inclusions in tau transgenic mice

Intraneuronal filamentous tau inclusions such as neurofibrillary tangles (NFTs) are neuropathological hallmarks of Alzheimer's disease (AD) and related sporadic and familial tauopathies. NFTs identical to those found in AD brains have also been detected in the hippocampus and entorhinal cortex of cognitively normal individuals as they age. To recapitulate age-induced NFT formation in a mouse model, we examined 12- to 24-month-old transgenic (Tg) mice overexpressing the smallest human brain tau isoform. These Tg mice develop congophilic tau inclusions in several brain regions including the hippocampus, amygdala, and entorhinal cortex. NFT-like inclusions were first detected in Tg mice at 18 to 20 months of age and they were detected by histochemical dyes that bind specifically to crossed beta-pleated sheet structures (eg, Congo red, Thioflavin S). Moreover, ultrastructurally these lesions contained straight tau filaments comprised of both mouse and human tau proteins but not other cytoskeletal proteins (eg, neurofilaments, microtubules). Isolated tau filaments were also recovered from detergent-insoluble tau fractions and insoluble tau proteins accumulated in brain in an age-dependent manner. Thus, overexpression of the smallest human brain tau isoform resulted in late onset and age-dependent formation of congophilic tau inclusions with properties similar to those in the tangles of human tauopathies, thereby implicating aging in the pathogenesis of fibrous tau inclusions.