Neuronal Tau Pathology Research Articles

Tau Nitration Occurs at Tyrosine 29 in the Fibrillar Lesions of Alzheimer's Disease and Other Tauopathies

The neurodegenerative tauopathies are a clinically diverse group of diseases typified by the pathological self-assembly of the microtubule-associated tau protein. Although tau nitration is believed to influence the pathogenesis of these diseases, the precise residues modified, and the resulting effects on tau function, remain enigmatic. Previously, we demonstrated that nitration at residue Tyr29 markedly inhibits the ability of tau to self-associate and stabilize the microtubule lattice (Reynolds et al., 2005b, 2006). Here, we report the first monoclonal antibody to detect nitration in a protein-specific and site-selective manner. This reagent, termed Tau-nY29, recognizes tau only when nitrated at residue Tyr29. It does not cross-react with wild-type tau, tau mutants singly nitrated at Tyr18, Tyr197, and Tyr394, or other proteins known to be nitrated in neurodegenerative diseases. By Western blot analysis, Tau-nY29 detects soluble tau and paired helical filament tau from severely affected Alzheimer's brain but fails to recognize tau from normal aged brain. This observation suggests that nitration at Tyr29 is a disease-related event that may alter the intrinsic ability of tau to self-polymerize. In Alzheimer's brain, Tau-nY29 labels the fibrillar triad of tau lesions, including neurofibrillary tangles, neuritic plaques, and, to a lesser extent, neuropil threads. Intriguingly, although Tau-nY29 stains both the neuronal and glial tau pathology of Pick disease, it detects only the neuronal pathology in corticobasal degeneration and progressive supranuclear palsy without labeling the predominant glial pathology. Collectively, our findings provide the first direct evidence that site-specific tau nitration is linked to the progression of the neurodegenerative tauopathies.

Impaired Glutamate Transport in a Mouse Model of Tau Pathology in Astrocytes

Filamentous tau inclusions in neurons and glia are neuropathological hallmarks of tauopathies. The discovery of microtubule-associated protein tau gene mutations that are pathogenic for a heterogenous group of neurodegenerative disorders, called frontotemporal dementia and parkinsonism linked to chromosome-17 (FTDP-17), directly implicate tau abnormalities in the onset/progression of disease. Although the role of tau pathology in neurons in disease pathogenesis is well accepted, the contribution of glial pathology is essentially unknown. We recently generated a transgenic (Tg) mouse model of tau pathology in astrocytes by expressing the human tau protein under the control of the glial fibrillary acidic protein (GFAP) promoter. Both wild-type and FTDP-17 mutant GFAP/tau Tg animals manifest an age-dependent accumulation of tau inclusions in astrocytes that resembles the pathology observed in human tauopathies. We further demonstrate that both strains of Tg mice manifest compromised motor function that correlates with altered expression of the glial glutamate-aspartate transporter and occurs before the development of tau pathology. Subsequently, the Tg mice manifest additional deficits in neuromuscular strength that correlates with reduced expression of glutamate transporter-1 (GLT-1) and occurs concurrent with tau inclusion pathology. Reduced GLT-1 expression was associated with a progressive decrease in sodium-dependent glutamate transport capacity. Reductions in GLT-1 expression were also observed in corticobasal degeneration, a tauopathy with prominent pathology in astrocytes. Less robust changes were observed in Alzheimer's disease in which neuronal tau pathology predominates. Thus, these Tg mice recapitulate features of astrocytic pathology observed in tauopathies and implicate a role for altered astrocyte function in the pathogenesis of these disorders.

Expression of the Small Heat-Shock Protein αB-Crystallin in Tauopathies with Glial Pathology

Intracellular accumulations of filamentous material composed of tau proteins are defining features of sporadic and familial neurodegenerative disorders termed "tauopathies." In Alzheimer's disease, the most common tauopathy, tau pathology is predominantly localized within neurons; however, robust glial pathology occurs in other tauopathies. Although the pathogenesis of tauopathies remains primarily unknown, molecular chaperones such as heat-shock proteins (HSPs) are implicated in these tau disorders as well as other neurodegenerative diseases characterized by the accumulation of insoluble protein aggregates such as alpha-synuclein in Parkinson's disease and polyglutamine in Huntington's disease. We analyzed a variety of tauopathies with antibodies to a panel of HSPs to determine their role in the pathogenesis of these disorders. Although HSPs are not found in neuronal tau inclusions, we demonstrate increased expression of the small HSP alphaB-crystallin in glial inclusions of both sporadic and familial tauopathies. alphaB-crystallin was observed in a subset of astrocytic and oligodendrocytic tau inclusions as well as the neuropil thread pathology in cellular processes, but the co-expression of alphaB-crystallin with tau inclusions was relatively specific to tauopathies with extensive glial pathology. Thus, increased alphaB-crystallin expression in glial tau inclusions may represent a response by glia to the accumulation of misfolded or aggregated tau protein that is linked to the pathogenesis of the glial pathology and distinct from mechanisms underlying neuronal tau pathology in neurodegenerative disease.

Nitration of Tau Protein Is Linked to Neurodegeneration in Tauopathies

Oxidative and nitrative injury is implicated in the pathogenesis of Alzheimer's disease (AD) and Down syndrome (DS), but no direct evidence links this type of injury to the formation of neurofibrillary tau lesions. To address this, we generated a monoclonal antibody (mAb), n847, which recognizes nitrated tau and alpha-synuclein. n847 detected nitrated tau in the insoluble fraction of AD, corticobasal degeneration (CBD), and Pick's disease (PiD) brains by Western blots. Immunohistochemistry (IHC) showed that n847 labeled neurons in the hippocampus and neocortex of AD and DS brains. Double-label immunofluorescence with n847 and an anti-tau antibody revealed partial co-localization of tau and n847 positive tangles, while n847 immunofluorescence and Thioflavin-S double-staining showed that a subset of n847-labeled neurons were Thioflavin-S-positive. In addition, immuno-electron microscopy revealed that tau-positive filaments in tangle-bearing neurons were also labeled by n847 and IHC of other tauopathies showed that some of glial and neuronal tau pathologies in CBD, progressive supranuclear palsy, PiD, and frontotemporal dementia with parkinsonism linked to chromosome 17 also were n847-positive. Finally, nitrated and Thioflavin-S-positive tau aggregates were generated in a oligodendrocytic cell line after treatment with peroxynitrite. Taken together, these findings imply that nitrative injury is directly linked to the formation of filamentous tau inclusions.

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.

Perisomatic granules (non-plaque dystrophic dendrites) of hippocampal CA1 neurons in Alzheimer's disease and Pick's disease: a lesion distinct from granulovacuolar degeneration.

A number of pathological changes have been reported in relation to CA1 pyramidal cells in Alzheimer's disease (AD), among them hyperphosphorylation of tau protein followed by the formation of filamentous tau lesions, granulovacuolar degeneration (GVD), Hirano bodies and spindle-shaped dilatations of distal apical dendrites. Juxtacellular clusters of glutamate receptor (GluR)-positive granules around pyramidal cells of the CA1 sector have been recently reported under the term "non-plaque dystrophic dendrites". We independently found that CA1 pyramidal cells in AD patients are regularly surrounded by ubiquitin-positive granules measuring 1-4 microns in diameter, which we have termed perisomatic granules (PSG). Using confocal microscopy, ubiquitin- and GluR-reactive granules were found to largely coincide and to correspond to the same structure. By immunoelectron microscopy PSG were found to consist of GluR1-2-reactive enlarged synaptic boutons containing tubulo-filamentous or floccular material. PSG were found to be consistently associated with pyramidal (principal) cells but not with interneurons of the CA1 sector. Dual-labeling experiments have shown that PSG are preferentially associated with tau-immunoreactive "pretangle" neurons but not with cells containing filamentous tau inclusions or with tau-negative nerve cell bodies. The number of PSG was found to increase with the severity of AD changes with almost no PSG found in Braak stages I and II and few in stage III. Furthermore, PSG were not AD specific, as shown by their presence around CA1 pyramidal cells in Pick's disease. The reasons for GluR reactivity and ubiquitin complex formation in enlarged perisomatic boutons are unclear. Marked changes in GluR subunits have been observed in association with even moderate AD pathology in hippocampal pyramidal cells in AD and our findings suggest a pathogenic link between PSG and early tau pathology in CA1 neurons. PSG might represent residual and abnormally clustered GluR subunits in degenerating perisomatic neurites. Our work confirms and extend previous study on perisomatic "non-plaque dystrophic dendrites" in AD and establish PSG as a pathological entity distinct from GVD. In addition PSG should be acknowledged among main histological changes associated with hippocampal neurons in AD and Pick's disease.

Tau pathology in neurons and glial cells of aged baboons.

The formation of cytoskeletal alterations associated with abnormally phosphorylated microtubule-associated tau protein is emerging as a common pathomechanism in several neurodegenerative diseases of the human brain. These disorders include Alzheimer’s disease (AD), the most common and most extensively studied disorder associated with abnormal tau protein’. Tau pathology is also seen in other “tauopathies” including corticobasal degeneration, Pick’s disease, progressive supranuclear palsy, and argyrophilic grain disease2,3,4. These diseases are characterized by taupositive inclusions deposited not only in nerve cells but also in glial cells3,5Finally, tau pathology develops in some families afflicted with hereditary frontotemporal dementia and parkinsonism linked to chromosome 176,7. Interestingly, FTDP-17 recently has been linked to a number of pathogenetic mutations of the tau gene8. At this point, the mechanisms of tau pathology in neurons and glial cells are not fully understood Animal models replicating characteristics of human tauopathies could provide information not obtainable from studies of the human brain. In an earlier report, we demonstrated the existence of neuronal and glial tau pathology in two out of four aged baboons9. Subsequently, we extended our sample to a total of 50

Filamentous tau pathology in nerve cells, astrocytes, and oligodendrocytes of aged baboons.

Intracellular filamentous inclusions containing abnormally phosphorylated tau protein are hallmarks of several human neurodegenerative disorders. This study reveals tau-positive cytoskeletal abnormalities in neurons and glial cells of aged baboons. The brains of four baboons (Papio hamadryas, 20-30 yr of age) were examined using the Gallyas silver technique for neurofibrillary changes and phosphorylation-dependent anti-tau antibodies (AT8, AT100, AT270, PHF-1, TG-3). Conspicuous changes were noted in two animals, 26 and 30 yr of age. In both animals, a combination of neuronal and glial cytoskeletal pathology was seen preferentially affecting limbic brain areas, including the hippocampal formation. In the 30-yr-old animal, numerous tau-positive inclusions were seen in the granule cells of the fascia dentata. These cells even exhibited an accumulation of argyrophilic neurofibrillary tangles. The glial changes affected both astrocytes and oligodendrocytes. Tau-positive astrocytes were seen in perivascular, subpial, and subependymal locations. Tau-positive oligodendrocytes preferentially occurred in limbic fiber tracts including the entorhinal perforant path. Ultrastructurally, tau-positive straight filaments (10-14 nm) in both neurons and glial cells were revealed by anti-tau immunoelectron microscopy. This study thus indicates the potential usefulness of aged baboons for experimental investigation of neuronal and glial filamentous tau pathology. This nonhuman primate species may provide valuable information pertinent to the broad spectrum of human tauopathies.

Neuropathologic differentiation of progressive supranuclear palsy and corticobasal degeneration

Progressive supranuclear palsy (PSP) and corticobasal degeneration (CBD) are usually sporadic multisystem degenerations associated with filamentous tau inclusions in neurons and glia. As such they can be considered sporadic tauopathies in contrast to familial tauopathies linked to mutations in the tau gene. Mutations have not been found in the tau gene in either PSP or CBD. The clinical syndromes and neuroimaging of typical cases of PSP and CBD are distinct; however, atypical cases are described that have overlapping clinical and pathologic features. Both PSP and CBD have similar biochemical alterations in the tau protein, with the abnormal tau protein containing predominantly four-repeat tau. While there is overlap in the pathology in PSP and CBD, there are sufficient differences to continue the present day trend to consider these separate disorders. Several important pathologic features differentiate PSP from CBD. Ballooned neurons are frequent and nearly a sine qua non for CBD, but they are found in PSP at a frequency similar to that of other neurodegenerative diseases, such as Alzheimer’s disease. Astrocytic lesions are different, with tufted astrocytes found in motor cortex and striatum in PSP and astrocytic plaques in focal atrophic cortices in CBD. The most characteristic neuronal tau pathology in CBD is wispy, fine filamentous inclusions within neuronal cell bodies, while affected neurons in PSP have compact, dense filamentous aggregates characteristic of globose neurofibrillary tangles. Thread-like processes in gray and white matter are much more numerous and widespread in CBD than in PSP. The brunt of the pathology in CBD is in the cerebrum, while the basal ganglia, diencephalon and brainstem are the targets of PSP. Further clinicopathologic studies will refine our understanding of these disorders and open the possibility that common etiologic factors may be identified for these unusual sporadic tauopathies.

Neuropathologic differentiation of progressive supranuclear palsy and corticobasal degeneration.

Progressive supranuclear palsy (PSP) and corticobasal degeneration (CBD) are usually sporadic multi-system degenerations associated with filamentous tau inclusions in neurons and glia. As such they can be considered sporadic tauopathies in contrast to familial tauopathies linked to mutations in the tau gene. Mutations have not been found in the tau gene in either PSP or CBD. The clinical syndromes and neuroimaging of typical cases of PSP and CBD are distinct; however, atypical cases are described that have overlapping clinical and pathologic features. Both PSP and CBD have similar biochemical alterations in the tau protein, with the abnormal tau protein containing predominantly four-repeat tau. While there is overlap in the pathology in PSP and CBD, there are sufficient differences to continue the present day trend to consider these separate disorders. Several important pathologic features differentiate PSP from CBD. Ballooned neurons are frequent and nearly a sine qua non for CBD, but they are found in PSP at a frequency similar to that of other neurodegenerative diseases, such as Alzheimer's disease. Astrocytic lesions are different, with tufted astrocytes found in motor cortex and striatum in PSP and astrocytic plaques in focal atrophic cortices in CBD. The most characteristic neuronal tau pathology in CBD is wispy, fine filamentous inclusions within neuronal cell bodies, while affected neurons in PSP have compact, dense filamentous aggregates characteristic of globose neurofibrillary tangles. Thread-like processes in gray and white matter are much more numerous and widespread in CBD than in PSP. The brunt of the pathology in CBD is in the cerebrum, while the basal ganglia, diencephalon and brainstem are the targets of PSP. Further clinicopathologic studies will refine our understanding of these disorders and open the possibility that common etiologic factors may be identified for these unusual sporadic tauopathies.

Tau gene mutation in familial progressive subcortical gliosis.

Familial forms of frontotemporal dementias are associated with mutations in the tau gene. A kindred affected by progressive subcortical gliosis (PSG), a rare form of presenile dementia, has genetic linkage to chromosome 17q21-22. This kindred (PSG-1) is included in the 'frontotemporal dementias and Parkinsonism linked to chromosome 17' group along with kindreds affected by apparently different forms of atypical dementias. Some of these kindreds have mutations in the tau gene. We report here that PSG-1 has a tau mutation at position +16 of the intron after exon 10. The mutation destabilizes a predicted stem-loop structure and leads to an over-representation of the soluble four-repeat tau isoforms, which assemble into wide, twisted, ribbon-like filaments and ultimately result in abundant neuronal and glial tau pathology. The mutations associated with PSG and other atypical dementias can be subdivided into three groups according to their tau gene locations and effects on tau. The existence of tau mutations with distinct pathogenetic mechanisms may explain the phenotypic heterogeneity of atypical dementias that previously led to their classification into separate disease entities.