Hippocampal plasticity during the progression of Alzheimer's disease.

Neuroplasticity involves molecular and structural changes in central nervous system (CNS) throughout life. The concept of neural organization allows for remodeling as a compensatory mechanism to the early pathobiology of Alzheimer's disease (AD) in an attempt to maintain brain function and cognition during the onset of dementia. The hippocampus, a crucial component of the medial temporal lobe memory circuit, is affected early in AD and displays synaptic and intraneuronal molecular remodeling against a pathological background of extracellular amyloid-beta (Aß) deposition and intracellular neurofibrillary tangle (NFT) formation in the early stages of AD. Here we discuss human clinical pathological findings supporting the concept that the hippocampus is capable of neural plasticity during mild cognitive impairment (MCI), a prodromal stage of AD and early stage AD.

Conformational changes and cleavage of tau in Pick bodies parallel the early processing of tau found in Alzheimer pathology.

Neuronal protein inclusions are a common feature in Alzheimer disease (AD) and Pick disease. Even though the inclusions are morphologically different, flame-shape structure for AD vs. spherical structure for Pick disease, both have filaments mainly composed of tau protein. In AD, a well-defined pattern of conformational changes and truncation has been described. In this study, we used laser scanning confocal microscopy to characterize and compare the processing of tau protein during Pick disease with that found in AD. We found that tau protein of Pick disease preserves most of the relevant epitopes found in AD, the conformational foldings labelled by Alz-50 and Tau-66, the cleavage sites D(421) and E(391), as well as many phosphorylated sites, such as Ser(199/202), Thr(205) and Ser(396/404). We found a strong pattern of association between phosphorylation and cleavage at site D(421), as well as the phosphorylation and the conformational Alz-50 epitope. When we used late AD markers such as the conformational Tau-66 epitope and MN423 (cleavage at site E(391)) in Pick bodies (PBs), the overlap was significantly less. Furthermore, following morphological quantification, we found significantly higher numbers of phosphorylated tau in PBs. Overall, our findings suggest that phosphorylation is an early event, likely preceding the cleavage of tau at D(421). Despite this consistency with AD, we found a major distinction, namely that PBs lack beta-sheet conformation. We propose a scheme of early tau processing in these structures, similar to neurofibrillary tangles of AD.

Tau epitope display in progressive supranuclear palsy and corticobasal degeneration.

Filamentous aggregates of the protein tau are a prominent feature of Alzheimer's disease (AD), progressive supranuclear palsy (PSP) and corticobasal degeneration (CBD). However, the extent to which the molecular structure of the tau in these aggregates is similar or differs between these diseases is unclear. We approached this question by examining these disorders with a panel of antibodies that represent different structural, conformational, and cleavage-specific tau epitopes. Although each of these antibodies reveals AD pathology, they resolved into three classes with respect to PSP and CBD: AD2 and Tau-46.1 stained the most tau pathology in all cases; Tau-1, 2, 5, and 12 exhibited variable reactivity; and Tau-66 and MN423 did not reveal any tau pathology. In addition, hippocampal neurofibrillary tangles in these cases showed a predominantly PSP/CBD-like, rather than AD-like, staining pattern. These results indicate that the patterns of the tau epitopes represented by this panel that reside in the pathological aggregates of PSP and CBD are similar to each other but distinct from that of AD.

Inhibition of tau polymerization by its carboxy-terminal caspase cleavage fragment.

Abnormal aggregation of the microtubule-associated protein, tau, occurs in many neurodegenerative diseases, making it important to understand the mechanisms of tau polymerization. Previous work has indicated that the C-terminal region of tau inhibits polymerization in vitro, and a growing body of evidence implicates caspase cleavage of tau at Asp 421 in the C-terminus as an important inducer of tau polymerization in Alzheimer's disease. In the present study, we provide evidence that the C-terminal peptide fragment produced by caspase cleavage inhibits tau polymerization, suggesting that caspase cleavage of tau enhances its polymerization by removing the inhibitory control element. Moreover, we provide evidence that the peptide assumes an alpha-helical configuration and inhibits tau assembly by interacting with residues 321-375 in the microtubule binding repeat region. These findings indicate that formation of the fibrillar pathologies during the course of Alzheimer's disease may be driven or sustained by apoptotic events leading to caspase activation.

Pathological glial tau accumulations in neurodegenerative disease: review and case report.

Abnormal deposits of tau protein accumulate in glia in many neurodegenerative diseases. This suggests that in some instances the disease process may target glial tau, with neuronal degeneration a secondary consequence of this process. In this report, we summarize the pattern of glial tau pathology in various neurodegenerative disorders and add original findings from a case of sporadic frontotemporal dementia that exhibits astrocytic tau pathology. The neurodegenerative diseases span the spectrum of relative neuronal and glial tau involvement, from disorders affecting only neuronal tau to those in which abnormal tau deposits are found only in glia. From this, we conclude that glial tau can be a primary target of the disease process, and that this can lead to neuronal degeneration.

Tau-66: evidence for a novel tau conformation in Alzheimer's disease.

We have characterized a novel monoclonal antibody, Tau-66, raised against recombinant human tau. Immunohistochemistry using Tau-66 reveals a somatic-neuronal stain in the superior temporal gyrus (STG) that is more intense in Alzheimer's disease (AD) brain than in normal brain. In hippocampus, Tau-66 yields a pattern similar to STG, except that neurofibrillary lesions are preferentially stained if present. In mild AD cases, Tau-66 stains plaques lacking obvious dystrophic neurites (termed herein 'diffuse reticulated plaques') in STG and the hippocampus. Enzyme-linked immunosorbent assay (ELISA) analysis reveals that Tau-66 is specific for tau, as there is no cross-reactivity with MAP2, tubulin, Abeta(1-40), or Abeta(1-42), although Tau-66 fails to react with tau or any other polypeptide on western blots. The epitope of Tau-66, as assessed by ELISA testing of tau deletion mutants, appears discontinuous, requiring residues 155-244 and 305-314. Tau-66 reactivity exhibits buffer and temperature sensitivity in an ELISA format and is readily abolished by SDS treatment. Taken together these lines of evidence indicate that the Tau-66 epitope is conformation-dependent, perhaps involving a close interaction of the proline-rich and the third microtubule-binding regions. This is the first indication that tau can undergo this novel folding event and that this conformation of tau is involved in AD pathology.

Structural analysis of Pick's disease-derived and in vitro-assembled tau filaments.

Pick's and Alzheimer's diseases are distinct neurodegenerative disorders both characterized in part by the presence of intracellular filamentous tau protein inclusions. The tight bundles of paired helical filaments (PHFs) of tau protein found in Alzheimer's disease (AD) differ from the tau filaments of Pick's disease in their morphology, distribution, and pathological structure as identified by silver impregnation. The filaments of Pick's disease are loosely arranged in pathognomonic spherical inclusions found in ballooned neurons, whereas the tau pathology of AD is classically described as a triad of neuropil threads, neurofibrillary tangles, and dystrophic neurites surrounding and invading plaques. In this study we used the high-resolution technique of scanning transmission electron microscopy to characterize and compare the filaments found in Pick's disease with those found in AD. In addition, we determined the mass/nm length and density of arachidonic acid-induced in vitro-assembled filaments. Three morphologically distinct populations of Pick's filaments were identified but each was indistinguishable from AD-PHFs in mass/nm length and density. Filaments assembled in vitro from single isoforms were similar in mass/nm length, but less dense than AD-PHFs and Pick's disease filaments. Finally, we provide clear structural evidence that a PHF, whether found in disease or assembled in vitro, is composed of two distinct intertwined filaments.

Inhibition of neuronal maturation in primary hippocampal neurons from tau deficient mice.

Conflicting evidence supports a role for tau as an essential neuronal cytoskeletal protein or as a redundant protein whose function can be fulfilled by other microtubule-associated proteins. To investigate the function of tau in axonogenesis, we created tau deficient mice by disrupting the TAU gene. The engineered mice do not express the tau protein, appear physically normal and are able to reproduce. In contrast to a previously reported tau knockout mouse, embryonic hippocampal cultures from tau deficient mice show a significant delay in maturation as measured by axonal and neuritic extensions. The classic technique of selectively enhancing axonal growth by growth on laminin substrates failed to restore normal neuronal maturation of tau knockout neurons. By mating human TAU-gene transgenic and tau knockout mice, we reconstituted tau-deficient neurons with human tau proteins and restored a normal pattern of axonal growth and neuronal maturation. The ability of human tau proteins to rescue tau-deficient mouse neurons confirms that tau expression affects the rate of neurite extension.

Oxidative regulation of fatty acid-induced tau polymerization.

Alzheimer's disease (AD) is characterized by the presence of amyloid-positive senile plaques and tau-positive neurofibrillary tangles. Aside from these two pathological hallmarks, a growing body of evidence indicates that the amount of oxidative alteration of vulnerable molecules such as proteins, DNA, and fatty acids is elevated in the brains of AD patients. It has been hypothesized that the elevated amounts of protein oxidation could lead directly to the formation of neurofibrillary tangles through a cysteine-dependent mechanism. We have tested this hypothesis in an in vitro system in which tau assembly is induced by fatty acids. Using sulfhydryl protective agents and site-directed mutagenesis, we found that cysteine-dependent oxidation of the tau molecule is not required for its polymerization and may even be inhibitory. However, by adjusting the oxidative environment of the polymerization reaction through the addition of a strong antioxidant or through the addition of an oxidizing system consisting of iron, adenosine diphosphate, and ascorbate, we found that oxidation does play a major role in our in vitro paradigm. The results indicated that fatty acid oxidation, the amount of which is found to be elevated in AD patients, can facilitate the polymerization of tau. However, "overoxidation" of the fatty acids can inhibit the process. Therefore, we postulate that specific fatty acid oxidative products could provide a direct link between oxidative stress mechanisms and the formation of neurofibrillary tangles in AD.

C-terminal inhibition of tau assembly in vitro and in Alzheimer's disease.

Alzheimer's disease (AD) is, in part, defined by the polymerization of tau into paired helical and straight filaments (PHF/SFs) which together comprise the fibrillar pathology in degenerating brain regions. Much of the tau in these filaments is modified by phosphorylation. Additionally, a subset also appears to be proteolytically truncated, resulting in the removal of its C terminus. Antibodies that recognize tau phosphorylated at S(396/404 )or truncated at E(391) do not stain control brains but do stain brain sections very early in the disease process. We modeled these phosphorylation and truncation events by creating pseudo-phosphorylation and deletion mutants derived from a full-length recombinant human tau protein isoform (ht40) that contains N-terminal exons 2 and 3 and all four microtubule-binding repeats. In vitro assembly experiments demonstrate that both modifications greatly enhance the rates of tau filament formation and that truncation increases the mass of polymer formed, as well. Removal of as few as 12 or as many as 121 amino acids from the C terminus of tau greatly increases the rate and extent of tau polymerization. However, deletion of an additional 7 amino acids, (314)DLSKVTS(320), from the third microtubule-binding repeat results in the loss of tau's ability to form filaments in vitro. These results suggest that only part of the microtubule-binding domain (repeats 1, 2 and a small portion of 3) is crucial for tau polymerization. Moreover, the C terminus of tau clearly inhibits the assembly process; this inhibition can be partially reversed by site-specific phosphorylation and completely removed by truncation events at various sites from S(320) to the end of the molecule.

Casein kinase 1 delta is associated with pathological accumulation of tau in several neurodegenerative diseases.

The distribution of casein kinase 1 delta (Cki delta) was studied by immunohistochemistry and correlated with other pathological hallmarks in Alzheimer's disease (AD), Down syndrome (DS), progressive supranuclear palsy (PSP), parkinsonism dementia complex of Guam (PDC), Pick's disease (PiD), pallido-ponto-nigral degeneration (PPND), Parkinson's disease (PD), dementia with Lewy bodies (DLB), amyotrophic lateral sclerosis (ALS), and elderly controls. Cki delta was found to be associated generally with granulovacuolar bodies and tau-containing neurofibrillary tangles in AD, DS, PSP, PDC, PPND, and controls, and Pick bodies and ballooned neurons in PiD. It was not associated with tau-containing inclusions in astroglia and oligodendroglia in PPND, PSP, and PDC. It was also not associated with tau-negative Lewy bodies in PD and DLB, Hirano bodies in PDC, Marinesco bodies in PD, AD, and controls and "skein"-like inclusions in anterior motor neurons in ALS. The colocalization of the kinase Cki delta and its apparent substrate tau suggests a function for Cki delta in the abnormal processing of tau.

In vitro polymerization of tau protein monitored by laser light scattering: method and application to the study of FTDP-17 mutants.

Tau polymerization into the filaments that compose neurofibrillary tangles is seminal to the development of many neurodegenerative diseases. It is therefore important to understand the mechanisms involved in this process. However, a consensus method for monitoring tau polymerization in vitro has been lacking. Here we demonstrate that illuminating tau polymerization reactions with laser light and measuring the increased scattering at 90 degrees to the incident beam with a digital camera results in data that closely approximate the mass of tau polymer formation in vitro. The validity of the technique was demonstrated over a range of tau concentrations and through multiple angle scattering measurements. In addition, laser light scattering data closely correlated with quantitative electron microscopy measurements of the mass of tau filaments. Laser light scattering was then used to measure the efficiency with which the mutant tau proteins found in frontotemporal dementia and Parkinsonism linked to chromosome 17 (FTDP-17) form filamentous structures. Several of these mutant proteins display enhanced polymerization in the presence of arachidonic acid, suggesting a direct role for these mutations in tau the filament formation that characterizes FTDP-17.

Differential assembly of human tau isoforms in the presence of arachidonic acid.

Six tau isoforms arise from the alternative splicing of a single gene in humans. Insoluble, filamentous deposits of tau protein occur in a number of neurodegenerative diseases, and in some of these diseases, the deposition of polymers enriched in certain tau isoforms has been documented. Because of these findings, we have undertaken studies on the efficacy of fatty acid-induced polymerization of the individual tau isoforms found in the adult human CNS. The polymerization of each tau isoform in the presence of two concentrations of arachidonic acid indicated that isoforms lacking N-terminal exons e2 and e3 formed small, globular oligomers that did not go on to elongate into straight (SF) or paired helical (PHF) filaments under our buffer conditions. The polymerization of all isoforms containing e2 or e2 and e3 occurred readily at a high arachidonic acid concentration. Conversely, at a lower arachidonic acid concentration, only tau isoforms containing four microtubule binding repeats assembled well. Under all buffer conditions employed, filaments formed from three of the isoforms containing e2 and e3 resembled SFs in morphology but began to form PHF-like structures following extended incubation at 37 degrees C. These results indicate that polymerization of the intact tau molecule may be facilitated by e2 and e3. Moreover, tau isoforms containing three versus four microtubule binding repeats display different assembly properties depending on the solvent conditions employed.

Ligand-dependent tau filament formation: implications for Alzheimer's disease progression.

The mechanism through which arachidonic acid induces the polymerization of tau protein into filaments under reducing conditions was characterized through a combination of fluorescence spectroscopy and electron microscopy. Results show that polymerization follows a ligand-mediated mechanism, where binding of arachidonic acid is an obligate step preceding tau-tau interaction. Homopolymerization begins with rapid (on the order of seconds) nucleation, followed by a slower elongation phase (on the order of hours). Although essentially all synthetic filaments have straight morphology at early time points, they interact with thioflavin-S and monoclonal antibody Alz50 much like authentic paired helical filaments, suggesting that the conformation of tau protein is similar in the two filament forms. Over a period of days, synthetic straight filaments gradually adopt paired helical morphology. These results define a novel pathway of tau filament formation under reducing conditions, where oxidation may contribute to final paired helical morphology, but is not a necessary prerequisite for efficient nucleation or elongation of tau filaments.

A new molecular link between the fibrillar and granulovacuolar lesions of Alzheimer's disease.

Alzheimer's Disease (AD) is a progressive neurodegenerative disorder involving select neurons of the hippocampus, neocortex, and other regions of the brain. Markers of end stage disease include fibrillar lesions, which accumulate hyperphosphorylated tau protein polymerized into filaments, and granulovacuolar lesions, which appear primarily within the hippocampus. The mechanism by which only select populations of neurons develop these lesions as well as the relationship between them is unknown. To address these questions, we have turned to AD tissue to search for enzymes specifically involved in tau hyperphosphorylation. Recently, we showed that the principal phosphotransferases associated with AD brain-derived tau filaments are members of the casein kinase-1 (CK1) family of protein kinases. Here we report the distribution of three CK1 isoforms (Ckialpha, Ckidelta, and Ckiepsilon) in AD and control brains using immunohistochemistry and Western analysis. In addition to colocalizing with elements of the fibrillar pathology, CK1 is found within the matrix of granulovacuolar degeneration bodies. Furthermore, levels of all CK1 isoforms are elevated in the CA1 region of AD hippocampus relative to controls, with one isoform, Ckidelta, being elevated >30-fold. We propose that overexpression of this protein kinase family plays a key role in the hyperphosphorylation of tau and in the formation of AD-related pathology.

Free fatty acids stimulate the polymerization of tau and amyloid beta peptides. In vitro evidence for a common effector of pathogenesis in Alzheimer's disease.

Alzheimer's disease is a degenerative disorder of the central nervous system, characterized by the concomitant deposition of extracellular filaments composed of beta-amyloid peptides and intracellular filaments composed of the microtubule-associated protein tau. We have discovered that free fatty acids (FFAs) stimulate the assembly of both amyloid and tau filaments in vitro. The minimal concentration of arachidonic acid observed to stimulate tau assembly ranged from 10 to 20 mumol/L, depending on the source of the purified tau. Tau preparations that do not exhibit spontaneous assembly were among those induced to polymerize by arachidonic acid. All long-chain FFAs tested enhanced assembly to some extent, although greater stimulation was usually associated with unsaturated forms. Utilizing fluorescence spectroscopy, unsaturated FFAs were also demonstrated to induce beta-amyloid assembly. The minimal concentration of oleic or linoleic acid observed to stimulate the assembly of amyloid was 40 mumol/L. The filamentous nature of these thioflavin-binding amyloid polymers was verified by electron microscopy. These data define a new set of tools for examining the polymerization of amyloid and tau proteins and suggest that cortical elevations of FFAs may constitute a unifying stimulatory event driving the formation of two of the obvious pathogenetic lesions in Alzheimer's disease.

Nucleolar localization of the microtubule-associated protein tau in neuroblastomas using sense and anti-sense transfection strategies.

The Tau-1 monoclonal antibody was localized to the nucleolus of interphase cells and the nucleolar organizing regions (NORs) of acrocentric chromosomes in cultured human cells. Putative nucleolar and NOR tau was found in CG neuroblastoma cells which contain nucleolar tau and little cytoplasmic tau. To further establish the presence of tau in the nucleolus of these cells, sense and anti-sense transfection strategies were used. CG neuroblastoma cells were transfected with tau sense cDNA and immunostained with Tau-1. Cytoplasmic Tau-1 staining was greatly increased in CG cells which contain very little endogenous cytoplasmic tau. Nucleolar Tau-1 staining was also increased in certain CG cells indicating an increase in nucleolar tau in a subset of transfected cells. CG cells were also transfected with tau anti-sense cDNA which abolished Tau-1 staining in the nucleolus. These results contribute to a growing body of evidence defining tau as a multifunctional protein found in both the cytoplasm and nucleoli of primate cells.

The structural basis of monoclonal antibody Alz50's selectivity for Alzheimer's disease pathology.

The epitope on tau protein recognized by the monoclonal antibody Alz50 was defined through internal deletion mutagenesis and quantified by affinity measurements. The epitope is discontinuous and requires both a previously identified N-terminal segment and the microtubule binding region for efficient binding of Alz50. The interaction between these regions is consistent with an intramolecular reaction mechanism, suggesting that Alz50 binding depends on the conformation of individual tau monomers. The results suggest that tau adopts a distinct conformation when polymerized into filaments and that this conformation is recognized selectively by Alz50.

Tau as a nucleolar protein in human nonneural cells in vitro and in vivo.

The Tau-1 monoclonal antibody was localized to the nucleolus of interphase cells and the nucleolar organizing regions (NORs) of acrocentric chromosomes in cultured human cells. Putative nucleolar and NOR tau was found in HeLa cells and lymphoblasts as well as in nontransformed fibroblasts and lymphocytes. To confirm the presence of tau in the nuclei of these nonneural cells, immunoblotting analysis was performed on isolated nuclei from lymphoblasts. Several tau bands were noted on the blot of the nuclear extract suggesting the presence of multiple tau isoforms. Tau-1 immunostaining demonstrated variable staining intensities between individual acrocentric chromosomes in all cells tested. In cultured peripheral lymphocytes, these staining patterns were the same from one chromosome spread to the next within an individual. This consistency of Tau-1 staining and its variability among NORs was reminiscent of staining patterns obtained using the silver-NOR procedure. Comparisons of Tau-1 immunostaining with silver staining of chromosome spreads from human lymphocytes demonstrated that Tau-1 did not immunostain all of the NORs that were silver stained. The intensity of Tau-1 fluorescence in nucleoli was further shown to be increased in phytohemagglutinin-stimulated lymphocytes, indicating an upregulation of nuclear tau when cells reentered the cell cycle. These results contribute to a growing body of evidence defining tau as a multifunctional protein that may be involved in ribosomal biogenesis and/or rRNA transcription in the nucleus of all cells as well as microtubule-stabilizing functions in the neuronal cytoplasm.

Functional implications for the microtubule-associated protein tau: localization in oligodendrocytes.

We present evidence that the microtubule-associated protein tau is present in oligodendrocytes (OLGs), the central nervous system cells that make myelin. By showing that tau is distributed in a pattern similar to that of myelin basic protein, our results suggest a possible involvement of tau in some aspect of myelination. Tau protein has been identified in OLGs in situ and in vitro. In interfascicular OLGs, tau localization, revealed by monoclonal antibody Tau-5, was confined to the cell somata. However, in cultured ovine OLGs with an exuberant network of processes, tau was detected in cell somata, cellular processes, and membrane expansions at the tips of these processes. Moreover, in such cultures, tau appeared localized adjacent to or coincident with myelin basic protein in membrane expansions along and at the ends of the cellular processes. The presence of tau mRNA was documented using fluorescence in situ hybridization. The distribution of the tau mRNA was similar to that of the tau protein. Western blot analysis of cultured OLGs showed the presence of many tau isoforms. Together, these results demonstrate that tau is a genuine oligodendrocyte protein and pave the way for determining its functional role in these cells.