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 (Aβ) 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 τ deficient mice

Conflicting evidence supports a role for τ 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 τ in axonogenesis, we created τ deficient mice by disrupting the TAU gene. The engineered mice do not express the τ protein, appear physically normal and are able to reproduce. In contrast to a previously reported τ knockout mouse, embryonic hippocampal cultures from τ 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 τ knockout neurons. By mating human TAU-gene transgenic and τ knockout mice, we reconstituted τ-deficient neurons with human τ proteins and restored a normal pattern of axonal growth and neuronal maturation. The ability of human τ proteins to rescue τ-deficient mouse neurons confirms that τ 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.

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.

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.

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.

Polymerization of microtubule-associated protein tau under near-physiological conditions

Neurofibrillary tangles, which form in certain degenerating neurons in the brains of patients with Alzheimer's disease, are amassed from filaments having a straight or paired helical morphology. Solubilization of these filaments reveals that they are composed of the microtubule-associated protein tau. It has not previously been shown, however, that tau will assemble to form filaments of similar morphology under conditions representative of the intracellular environment. We have succeeded in forming such filaments using tau purified from porcine or rat microtubules. The filaments are relatively straight with narrowing at irregular intervals, and are about 10 nm wide, a morphology similar to that of straight filaments seen in Alzheimer's disease neurofibrillary tangles. At tau concentrations of 1-10 microM, in vitro assembly occurs at physiological pH, ionic strength, temperature, and reducing potential, and each one of these factors modulates the reaction. Assembly is judged to be only slowly reversible by the exponential rather than normal distribution of filament lengths, and by the limited disassembly observed under conditions which inhibit polymerization. Tau purified directly from whole brain tissue rather than from microtubules does not polymerize under conditions described in this report.

Presence of tau in isolated nuclei from human brain

Previous studies have demonstrated that the microtubule-associated protein (MAP) tau is present in the axonal and somatodendritic compartment of neurons. In cultured primate cell lines, tau has been found localized to the NOR regions of the acrocentric chromosomes in mitotic cells and the dense fibrillar regions of nucleoli in interphase cells. We report here the presence of nuclear tau in nuclei isolated from fresh, frozen human frontal cortex. Using several monoclonal antibodies against tau, Tau-1, Tau 46.1, and 5E2, we have established by both indirect immunofluorescence and Western blotting that tau is an integral component of nuclei isolated from Alzheimer's disease (AD) and pathologically normal control brains. Brain nuclear tau, like nuclear tau in primate cells, is insoluble in SDS and must first be extracted with formic acid prior to analysis by Western blot. Immunoblot analysis of isolated brain nuclei displays the characteristic ladder of tau proteins and demonstrates that all isoforms of tau are present. It is unclear whether levels of nuclear tau can be correlated to pathologic events in AD, but its insoluble nature along with reports of intranuclear PHFs warrant further studies of nuclear tau as a molecular candidate in the genesis of AD.

Localization of specific epitopes on human microtubule-associated protein 2

Microtubule-associated protein 2 (MAP-2) is an abundant neuronal cytoskeletal protein that binds to tubulin and stabilizes microtubules. Using fusion protein constructs we have defined the epitopes of 10 monoclonal antibodies (mAbs) to discrete regions of human MAP-2. Proteins were expressed in pATH vectors. After electrophoresis, immunoblotting was performed. By western blot analysis five of the mAbs (AP-14, AP-20, AP-21, AP-23, and AP-25) share epitopes with only the high molecular weight isoforms (MAP-2a, MAP-2b); two of the mAbs (AP-18 and tau 46) recognize MAP-2a, MAP-2b, and MAP-2c. Although AP-18 immunoreactivity was detected within heat-stable protein homogenates isolated from a human neuroblastoma cell line MSN, fusion protein constructs encompassing human MAP-2 were negative, suggesting that the AP-18 epitope is phosphorylated. Furthermore, AP-18 immunoreactivity was lost after alkaline phosphatase treatment of heat-stable protein preparations from MSN cells. Four of the mAbs (322, 636, 635, and 39) recognize epitopes located within amino acids 169-219 of human MAP-2. AP-21 maps to a region between amino acids 553 and 645. AP-23 maps between amino acids 645 and 993, whereas AP-20, AP-14, and AP-25 map between amino acids 995 and 1332. Expression of the region of MAP-2 between amino acids 1787 and 1824 was positive to tau 46.

Differential distribution of tau proteins in developing cat cerebellum

The differential distribution and phosphorylation of tau proteins in cat cerebellum was studied with two well characterized antibodies, TAU-1 and TAU-2. TAU-1 detects tau proteins in axons, and the epitope in perikarya and dendrites is masked by phosphorylation. TAU-2 detects a phosphorylation-independent epitope on tau proteins. The molecular composition of tau proteins in the range of 45 kD to 64 kD at birth changed after the first postnatal month to a set of several adult variants of higher molecular weights in the range of 59 kD to 95 kD. The appearance of tau proteins in subsets of axons corresponds to the axonal maturation of cerebellar local-circuit neurons in granular and molecular layers and confirms previous studies. Tau proteins were also identified in synapses by immunofluorescent double-staining with synapsin I, located in the pinceau around the Purkinje cells, and in glomeruli. Dephosphorylation of juvenile cerebellar tissue by alkaline phosphatase indicated indirectly the presence of differentially phosphorylated tau forms mainly in juvenile ages. Additional TAU-1 immunoreactivity was unmasked in numerous perikarya and dendrites of stellate cells, and in cell bodies of granule cells. Purkinje cell bodies were stained transiently at juvenile ages. During postnatal development, the intensity of the phosphate-dependent staining decreased, suggesting that phosphorylation of tau proteins in perikarya and dendrites may be essential for early steps in neuronal morphogenesis during cat cerebellum development.

Expression of the cytoskeletal-associated protein filamin in adult rat organs

Filamin is a well-characterized actin-associated protein first isolated from chicken smooth muscle. Subsequently, this polypeptide and its nonmuscle homolog actin-binding protein have been shown to be expressed in avian muscle tissue, mammalian smooth muscle, mammalian macrophages and other blood cell types, as well as several cultured cell lines. In this report, the occurrence of this polypeptide in adult mammalian organs has been investigated. Immunoblot analysis using three anti-filamin monoclonal antibodies showed that this protein was largely detected in adult rat organs that possess a substantial smooth muscle component. Furthermore, the limited expression of filamin in smooth muscle tissue was corroborated by immunohistochemical analysis. In contrast to avian systems, filamin was never found in detectable quantities in either mammalian cardiac or skeletal muscle. Quantitative immunoblot analysis demonstrated that filamin amounts roughly correlated with the abundance of the smooth muscle component of a given organ, comprising as much as 16.5% of the total SDS-extractable protein in bovine aorta. Work in avian systems and cells in culture has suggested that filamin is a rather ubiquitous cytoskeletal element. By contrast, this work demonstrates that filamin is highly restricted in its expression in mammalian organ systems, in situ.

A novel tau transcript in cultured human neuroblastoma cells expressing nuclear tau

We previously reported the presence of the microtubule-associated protein, tau in the nuclei of primate cells in culture. The present study confirms the existence of nuclear tau in two human neuroblastoma cells lines by indirect immunofluorescence and Western blot using mAbs to tau. Northern blot analysis of poly A+ mRNA detects a novel 2-kb tau transcript coexpressed with the 6-kb message in cultured human cells and human frontal cortex. PCR and cDNA sequencing demonstrate that the 2-kb message contains the entire tau coding region. Furthermore, actinomycin D transcription inhibition experiments indicate that the 2-kb message is not derived from the 6-kb message, but instead arises from the original tau transcript. One of the human neuroblastoma cell lines examined contains both nuclear and cytoplasmic tau as assayed by both Western blot and indirect immunofluorescence. Northern blot analysis of this cell line indicates that copious amounts of the 2-kb message are present while little of the 6-kb transcript is obvious. Immunofluorescence analysis of this cell line demonstrates that the cytoplasmic tau is not localized to microtubules. Together, these results indicate that the 2-kb tau message in humans may specify tau for non-microtubule functions in both the cytoplasm and the nucleus. We hypothesize that this is accomplished via a message targeting mechanism mediated by the untranslated regions of the tau messages.

Solubility of cytoskeletal proteins in immunohistochemistry and the influence of fixation

For accurate and quantitative immunohistochemical localization of antigens it is crucial to know the solubility of tissue proteins and their degree of loss during processing. In this study we focused on the solubility of several cytoskeletal proteins in cat brain tissue at various ages and their loss during immunohistochemical procedures. We further examined whether fixation affected either solubility or immunocytochemical detectability of several cytoskeletal proteins. An assay was designed to measure the solubility of cytoskeletal proteins in cryostat sections. Quantity and quality of proteins lost or remaining in tissue were measured and analyzed by electrophoresis and immunoblots. Most microtubule proteins were found to be soluble in unfixed and alcohol fixed tissues. Furthermore, the microtubule proteins remaining in the tissue had a changed cellular distribution. In contrast, brain spectrin and all three neurofilament subunits were insoluble and remained in the tissue, allowing their immunocytochemical localization in alcohol-fixed tissue. Synapsin I, a protein associated with the spectrin cytoskeleton, was soluble, and aldehyde fixation is advised for its immunohistochemical localization. With aldehyde fixation, the immunoreactivity of some antibodies against neurofilament proteins was reduced in axons unveiling novel immunogenic sites in nuclei that may represent artifacts of fixation. In conclusion, protein solubility and the effects of fixation are influential factors in cytoskeletal immunohistochemistry, and should be considered before assessments for a quantitative distribution are made.

Actin-binding protein is a component of bovine erythrocytes

Actin-binding protein (ABP) is a well-characterized polypeptide capable of crosslinking filamentous actin. To date, this polypeptide has been shown to exist in a number of tissues and cultured cell lines. This report shows that by using a panel of three monoclonal antibodies for immunoblotting and immunofluorescence analysis, that ABP is present in bovine erythrocytes. Moreover, the data obtained suggest that this protein is a component of the erythrocyte membrane skeleton. Additionally, bovine erythrocyte ABP is shown to possess both an apparent molecular weight and an isoelectric point identical to that of bovine smooth muscle filamin, implying that these two polypeptides are identical.

Identification of the intermediate filament-associated protein gyronemin as filamin. Implications for a novel mechanism of cytoskeletal interaction

In a previous paper, a monoclonal antibody (designated M1.4) that recognized a 240 kDa polypeptide was characterized. This antibody stained the intermediate filaments in several cell lines, and biochemical characteristics of the 240 kDa polypeptide led us to conclude that it was a novel intermediate filament-associated protein, which we termed gyronemin. Here we report that gyronemin is expressed in adult rat organs that contain a substantial smooth muscle component. Taking advantage of this observation, this protein was purified from bovine uterine tissue and, by biochemical, immunological and amino acid sequence analysis, found to be homologous to the actin-associated protein filamin. Three novel monoclonal antibodies raised using purified bovine gyronemin as the immunogen show this protein to be associated with actin-containing stress fibers, although our original M1.4 antibody continued to be localized along vimentin filaments. Since two-dimensional electrophoretic analysis did not demonstrate a difference in either relative molecular mass or isoelectric point of this polypeptide when associated with either filamentous system, we conclude that filamin is a bifunctional protein capable of associating with both the intermediate filament and actin cytoskeletal systems.

Hydrofluoric acid-treated tau PHF proteins display the same biochemical properties as normal tau

Tau (tau) is a major constituent of paired helical filaments (PHF) found in Alzheimer's disease. The current study examines the possibility that the distinct properties of PHF-associated tau proteins (tau PHF) result from post-translational modifications of normal soluble tau (tau s). Following hydrofluoric acid (HF) treatment, tau PHF proteins are heat- and acid-stable, soluble in 2-(N-morpholino)ethanesulfonic acid buffers and display the same molecular weight, pI, and immunochemical properties as normal tau s. Alkaline phosphatase treatment of dissociated PHF results in similar, although less extensive, electrophoretic changes and a reduction in PHF-1 immunoreactivity. Therefore, phosphorylation of normal tau s appears to be responsible for the distinct properties of tau PHF. Although our results suggest that all of the normal tau isoforms are in PHF, the relative abundance of individual tau species differs in HF-treated PHF and tau s samples. Moreover, the loss of PHF following HF treatment suggests that post-translational modifications contribute to the structural stability of PHF.

Domain structure and antiparallel dimers of microtubule-associated protein 2 (MAP2)

We have studied the microtubule-associated protein MAP2 from porcine brain and its subfragments by limited proteolysis, antibody labeling, and electron microscopy. Two major chymotryptic fragments start at lys 1528 and arg 1664, generating microtubule-binding fragments of Mr 36 kDa (303 residues, analogous to the "assembly domain" of Vallee, 1980) and 18 kDa (167 residues). These fragments can be labeled with the antibody 2-4 which recognizes the last internal repeat of MAP2 (Dingus et al., 1991). The epitope of another monoclonal antibody, AP18 (Binder et al., 1986), was mapped to the first 151 residues of MAP2. The interaction with AP18 is phosphorylation dependent; dephosphorylated MAP2 is not recognized. Intact MAP2 forms rod-like particles of 97 nm mean length, similar to Gottlieb and Murphy's (1985) observations. Both antibodies bind near an end of the rod, suggesting that the sequence and the structure are approximately colinear. There is a pronounced tendency for MAP2 to form dimers whose components are nearly in register but of opposite polarity. MAP2 can also fold in a hairpin-like fashion, generating 50-nm rods, and it can self-associate into oligomers and fibers. The 36-kDa microtubule-binding fragment also has a rod-like shape; its mean length is 49 nm, half of the intact molecule, even though the fragment contains only one-sixth of the mass. The antibody 2-4 decorates one end of the rod, similar to the intact protein. The fragment also forms antiparallel dimers, but its tendency for higher self-assembly forms is much lower than with intact MAP2.

Identification of nuclear tau isoforms in human neuroblastoma cells

The tau proteins have been reported only in association with microtubules and with ribosomes in situ, in the normal central nervous system. In addition, tau has been shown to be an integral component of paired helical filaments, the principal constituent of the neurofibrillary tangles found in brains of patients with Alzheimer disease and of most aged individuals with Down syndrome (trisomy 21). We report here the localization of the well-characterized Tau-1 monoclonal antibody to the nucleolar organizer regions of the acrocentric chromosomes and to their interphase counterpart, the fibrillar component of the nucleolus, in human neuroblastoma cells. Similar localization to the nucleolar organizer regions was also observed in other human cell lines and in one monkey kidney cell line but was not seen in non-primate species. Immunochemically, we further demonstrate the existence of the entire tau molecule in the isolated nuclei of neuroblastoma cells. Nuclear tau proteins, like the tau proteins of the paired helical filaments, cannot be extracted in standard SDS-containing electrophoresis sample buffer but require pretreatment with formic acid prior to immunoblot analysis. This work indicates that tau may function in processes not directly associated with microtubules and that highly insoluble complexes of tau may also play a role in normal cellular physiology.

Expression of microtubule-associated protein 2 by reactive astrocytes

After an injury to the central nervous system, a dramatic change in the astrocytes bordering the wound occurs. The most characteristic feature of this process, termed reactive gliosis, is the upregulation of the intermediate filament protein, glial fibrillary acidic protein. In the present study, we show that reactive astrocytes express high levels of microtubule-associated protein 2 (MAP-2), a protein normally found in the somatodendritic compartment of neurons. When sections of injured brain are double-stained with antibodies directed against MAP-2 and glial fibrillary protein, all of the reactive astrocytes are found to contain MAP-2. The high levels of this protein appear to represent a permanent change in reactive astrocytes. In parallel quantitative studies, an elevated level of MAP-2 in the injured brain is confirmed by an immunoblot analysis of injured and normal white matter. This report demonstrates the direct involvement of a microtubule protein in the process of reactive gliosis.

Alzheimer disease proteins (A68) share epitopes with tau but show distinct biochemical properties

Alz 50, a monoclonal antibody raised against Alzheimer brain homogenate, reacts with neurofibrillary tangles, microtubule-associated proteins tau, and Alzheimer brain proteins of molecular weight 70-60 kDa (A68). To study the relationship between A68 and normal human tau we compared the biochemical properties of these proteins and tested the reactivity of A68 with eight antibodies (Alz 50, Tau 60, Tau-2, Tau 14, Tau-1, Ab 636.7, NP14, Tau 46) that bind to various regions of tau molecule. On Western blots, all tau-reactive antibodies, except Tau-1, recognized A68. Pretreatment with alkaline phosphatase was required for the Tau-1 binding to A68. A68 consisted of three polypeptides of 68, 64, and 60 kDa, while tau contained 4-6 polypeptides of 50-65 kDa. A68 was less heterogenous than tau in the number of pI variants on two-dimensional gels. All A68 variants were more acidic (pI 5.5-6.5) than human tau (pI 6.5-8.5). Phosphatase treatment had only a minor effect on the pI and mobility of A68. Limited proteolysis of A68 with trypsin or chymotrypsin generated large fragments of 56-66 kDa (chymotrypsin) and 40-45 kDa (trypsin). While none of the fragments was recognized by Alz 50, the chymotryptic fragments were reactive with all the other tau antibodies, and the tryptic fragments were positive with five of the antibodies (Tau 14, Tau-1, Ab 636.7, NP14, and Tau 46). The peptide maps of A68 differed from that of tau in the number and the size of the peptide fragments. The differences in biochemical properties of these proteins and the sharing multiple epitopes suggest that A68 is a modified form of tau. The modification in part may be due to phosphorylation, although other changes rendering different isoelectrical properties and susceptibility to proteases need to be considered. The removal of the Alz 50 epitope by a cleavage of a 2-3 kDa fragment which does not contain the most C-terminal epitope (Tau 46) indicates that the Alz 50 epitope is located at the N-terminal periphery of the A68 molecule.

Identification and characterization of a novel mammalian intermediate filament-associated protein

A novel monoclonal antibody, designated M1.4, recognizes the high molecular weight microtubule-associated protein MAP1A (ca. Mr 380 kD) in both bovine and rat brain. In HeLa cells, however, M1.4 binds to a 240 kD polypeptide on immunoblots and co-localizes with both vimentin and cytokeratin filaments using double-label immunofluorescence microscopy. Immunoelectron microscopy indicates that the 240 kD polypeptide localizes along bundled intermediate filaments in a periodic manner. Two-dimensional electrophoretic analysis indicates that the 240 kD polypeptide has a basic pI of 7.7. When HeLa cell intermediate filaments are isolated using standard non-ionic detergent/high-salt conditions the 240 kD polypeptide does not sediment with the intermediate filaments, unlike the established intermediate filament-associated protein plectin. Immunoblot analysis with M1.4 shows the 240 kD polypeptide is expressed in a number of mammalian cell lines. Additionally, double-label immunofluorescence shows the 240 kD polypeptide to associate with vimentin filaments in African Green Monkey kidney (CV-1) and JC neuroblastoma cells. Due to its unique biochemical and biological characteristics, the 240 kD polypeptide is clearly a novel intermediate filament-associated protein for which we have proposed the designation gyronemin (Gr. gyros: around; nemin: filament).

Abnormal processing of multiple proteins in Alzheimer disease

Cerebrovascular amyloid is the main constituent of the perivascular and neuritic plaques typical of Alzheimer disease, whereas neurofilaments and microtubule-associated tau protein have been considered primary contributors to the formation of the characteristic Alzheimer tangles. Plaques and tangles and their constituents have at times been ascribed a role in pathogenesis of the disease. Normally, neurofilaments become phosphorylated only upon axonal entry. In many neurologic disorders, neurofilament phosphorylation, as detected by any of the available monoclonal antibodies (mAbs) to neurofilament phosphorylated epitopes is shifted from an axonal to a cell-body location. An exception is provided by Alzheimer disease, where tangles (which are neuronal cell-body-derived structures) exhibit only one phosphorylated epitope. However, the very presence of neurofilaments in tangles and plaques has been questioned because of a reported cross-reaction of mAbs to phosphorylated neurofilaments with tau protein. On reinvestigating this cross-reactivity we found that four of five mAbs to phosphorylated neurofilaments and four of five mAbs to nonphosphorylated neurofilaments failed to react with tau protein. A fifth mAb (07-5) to phosphorylated neurofilament cross-reacted with partially denatured tau protein at an affinity 1/1700th of that for denatured neurofilaments; nondenatured tau protein in tissue sections did not cross-react. A fifth mAb (02-40) to nonphosphorylated neurofilament also cross-reacted weakly. In Alzheimer disease normal-appearing axons were revealed with all the mAbs to phosphorylated neurofilaments, but tangles were revealed with only one of them (mAb 07-5). mAb to tau protein did not stain or did so indistinctly. Four of five mAbs to nonphosphorylated neurofilaments failed to reveal axons. Upon dephosphorylation of tissue, staining by mAbs to phosphorylated neurofilaments disappeared, and axons were revealed with the mAb to tau protein and all mAbs to the nonphosphorylated neurofilaments. Tangles became stained with tau mAb and one mAb to the nonphosphorylated neurofilaments (mAb 10-1). Quantitative evaluation of immunocytochemical staining intensities and immunoblot cross-reactivity showed that neurofilaments are, indeed, constituents of tangles--apparently exceeding the concentration of tau protein 17-fold. Contribution of both conformation and primary structure to IgG specificity may explain the lack of any cross-reaction of mAbs to neurofilaments with tau protein in intact tissue and the appearance of cross-reaction in immunoblots where conformation specificity may be largely lost. The present data extend earlier findings of abnormal processing of neurofilaments and tau protein in Alzheimer disease and, together with reported abnormal processing of cerebrovascular amyloid beta-protein, suggest that inhibition of the processing of multiple proteins is basic to the pathogenesis of Alzheimer disease, whereas formation of plaques and tangles could be merely the most striking histologic result.

Proteolysis of tau by calpain

The calpain-induced proteolysis of tau associated with twice-cycled microtubules or from a total brain heat-stable fraction was studied. Twice-cycled microtubule tau was rapidly hydrolyzed by calpain. In contrast, tau purified from the total brain heat-stable fraction was very resistant to degradation by calpain. These results clearly demonstrate that there are at least 2 populations of tau in the brain based on calpain-sensitivity, a calpain-sensitive form that is associated with microtubules and a calpain-resistant form that may represent another population of tau in the brain.

Estramustine binds a MAP-1-like protein to inhibit microtubule assembly in vitro and disrupt microtubule organization in DU 145 cells

The twofold purpose of the study was (a) to determine if a MAP-1-like protein was expressed in human prostatic DU 145 cells and (b) to demonstrate whether a novel antimicrotubule drug, estramustine, binds the MAP-1-like protein to disrupt microtubules. SDS-PAGE and Western blots showed that a 330-kD protein was associated with microtubules isolated in an assembly buffer containing 10 microM taxol and 10 mM adenylylimidodiphosphate. After purification to homogeneity on an A5m agarose column, the 330-kD protein was found to promote 6 S tubulin assembly. Turbidimetric (A350), SDS-PAGE, and electron microscopic studies revealed that micromolar estramustine inhibited assembly promoted by the 330-kD protein. Similarly, estramustine inhibited binding of the 330-kD protein to 6-S microtubules independently stimulated to assemble with taxol. Immunofluorescent studies with beta-tubulin antibody (27B) and MAP-1 antibody (MI-AI) revealed that 60 microM estramustine (a) caused disassembly of MAP-1 microtubules in DU 145 cells and (b) removed MAP-1 from the surfaces of microtubules stabilized with 0.1 microM taxol. Taken together the data suggested that estramustine binds to a 330-kD MAP-1-like protein to disrupt microtubules in tumor cells.

The sequential appearance of low- and high-molecular-weight forms of MAP2 in the developing cerebellum

Mammalian microtubule-associated protein 2 (MAP2) exists in high-molecular-weight (Mr approximately 280,000) and low-molecular-weight (Mr approximately 70,000) forms, with the latter protein being more abundant in embryonic brain homogenates than in preparations from mature brain (Riederer and Matus, 1985). In the current study, we have shown that avian MAP2 also exists as both high- (Mr approximately 260,000) and low-molecular-weight (Mr approximately 65,000) forms whose relative abundance changes during brain maturation, indicating a conserved function for these proteins during vertebrate neuronal morphogenesis. Using indirect immunohistochemistry, we have determined the cellular distribution of the high- and low-molecular-weight forms of MAP2 in the developing avian cerebellum. In the embryonic cerebellum, low-molecular-weight MAP2 is found in the external granular layer and in epithelial cells. High-molecular-weight MAP2 is found only in neurons that have commenced dendrogenesis, i.e., Purkinje cells and neurons within the internal granular layer. Thus, low-molecular-weight MAP2 is not only more abundant in embryonic nervous tissue than in the adult, but it also appears in glia and in differentiating neurons before the high-molecular-weight form. We have also shown that in the mature cerebellum high-molecular-weight MAP2 cannot be detected with monoclonal antibodies or polyclonal antisera in Purkinje cell dendrites. Polyclonal antisera against the regulatory subunit of the cAMP-dependent protein kinase, which is associated with MAP2 in the Purkinje cell dendrites of the rat, also fail to stain Purkinje cell dendrites in the mature quail cerebellum. This suggests that high-molecular-weight MAP2 may be necessary for the establishment of dendrites but is not necessary for the maintenance of dendritic form.

Phylogenetic conservation of brain microtubule-associated proteins MAP2 and tau

The major rat brain microtubule-associated proteins, MAP2 and tau, exhibit various properties that implicate them in the mechanisms underlying the growth of axons and dendrites during neuronal development. To determine if these properties represent fundamental morphogenetic mechanisms, we have examined the phylogenetic conservation of these proteins in Xenopus laevis, quail and rat with respect to their molecular form, cytological distribution and developmental expression. In all three species, the high-molecular weight form of MAP2 migrates as a pair of polypeptides (MAP2a and MAP2b); this doublet as well as the low-molecular weight form of MAP2 (MAP2c) and the tau proteins are markedly similar in size in the different classes of vertebrates. Immunohistochemical staining of the Xenopus and quail cerebellum showed that MAP2 is highly concentrated in dendrites whereas the tau proteins are predominantly confined to axons, exactly as they are in rat. The developmental regulation of these proteins in Xenopus and rat is also conserved. Between the larva and the adult (i.e. during metamorphosis) MAP2c undergoes a marked decrease while MAP2a undergoes a large increase. Thus, in both classes of vertebrates the timing of changes in MAP2 expression coincides with the maturation of neuronal morphology. Taken together, these conserved properties of MAP2 and tau in three phylogenetically divergent classes of vertebrates suggest that these proteins serve fundamental functions during neuronal morphogenesis.

Immunochemical and biochemical characterization of tau proteins in normal and Alzheimer's disease brains with Alz 50 and Tau-1

Microtubule-associated protein tau was characterized in 5 Alzheimer and 5 control brains using two monoclonal antibodies, Alz 50 and Tau-1. Quantitative analysis of immunoblots with the antibodies showed that both homogenate and supernatant fractions (12,000 x g) from Alzheimer brains contained 38-65% less tau immunoreactivity compared to normal brains. The reduction was found in all brain regions studied (frontal and temporal lobes and thalamus) and in both gray and white matter. In partially purified tau preparations, the yield of protein was lower in Alzheimer (by 35%) than in control brain. Incubation of brain proteins, transferred onto nitrocellulose paper, with alkaline phosphatase had either no effect or slightly increased the antibody binding to tau proteins from both brain tissues. Immunoblots of tau-enriched preparations subjected to two-dimensional gel electrophoresis showed no major changes in the staining pattern of tau isoforms in Alzheimer samples except for a weaker reactivity of the basic isovariants as compared to non-Alzheimer samples. The elution volume of tau from Alzheimer brain supernatant on a Sepharose CL-6B column was similar to that from non-Alzheimer brain and equal to that of aldolase (Mr = 158,000). Our data suggest that most of tau proteins from both types of brain have similar biochemical properties. The reduction in tau reactivity in Alzheimer tissue may be due to a reduction in neuronal cell population or incorporation of soluble tau into stable structures such as neurofibrillary tangles, since the tangles have been shown to react with anti-tau antibodies.

Neuronal microtubule-associated proteins in the embryonic avian spinal cord

We have used monoclonal antibodies to study the distribution of three developmentally regulated microtubule-associated proteins-MAP2, MAP5, and tau-during the morphogenesis of the thoracic spinal cord and peripheral nervous system in the quail. MAP5 is the only one of the three that is present in growing motor neuron processes in the day 3 embryo. The low-molecular weight form of MAP2, MAP2c, is found in motor neuron cell bodies at embryonic day 3. At later stages MAP2c appears in axons and in glia; it decreases in abundance between embryonic days 5 and 7. High-molecular weight MAP2 appears in motor neuron cell bodies and spinal cord gray matter at embryonic day 4, and is never encountered in axons. Tau is found in axons, but only at embryonic day 3.5, after they have commenced active extension. The molecular form and patterns of intracellular compartmentalization of each of the microtubule-associated proteins studied is conserved in mammalian and avian neurons. We conclude that MAP5 may be involved in the active growth of neuronal processes, whereas MAP2 and tau are not, and that high-molecular weight MAP2 and tau may stabilize dendritic and axonal processes, respectively.

Differential localization of the high- and low-molecular weight variants of MAP2 in the developing retina

Microtubule-associated protein 2 (MAP2) occurs in developing mammalian neuronal tissue as both high (280 kDa)- and low (70 kDa)-molecular weight forms with temporally regulated expression. We have studied the developing avian retina with a monoclonal antibody that recognizes both the high- and low-molecular weight forms of MAP2 and a second monoclonal antibody that recognizes only high-molecular weight MAP2. The developmentally regulated, low-molecular weight protein, MAP2c, has a more widespread distribution in the embryonic avian retina than high-molecular weight MAP2. Our results suggest that MAP2c is the first form of MAP2 to appear in differentiated embryonic retinal neurons, and that the high-molecular weight isoforms of MAP2 appear only later when they may confer stability to neuronal processes.

The expression and distribution of the microtubule-associated proteins tau and microtubule-associated protein 2 in hippocampal neurons in the rat in situ and in cell culture

Using a monoclonal antibody against the microtubule-associated protein tau we compared the distribution and the biochemical maturation of this protein in hippocampal pyramidal neurons in the rat in tau and in culture. In tissue sections from mature animals tau was localized heterogeneously within neurons. It was concentrated in axons; dendrites and somata showed little or no staining. In hippocampal cultures ranging from 12 h to 4 weeks in vitro tau was present in neurons but not in glial cells, as it is in situ. Within cultured neurons, however, tau was not compartmentalized but was present throughout the dendrites, axons and somata. Immunoblotting experiments showed that the biochemical maturation of tau that occurs in situ also failed to occur in culture. The young form of tau persisted, and the adult forms did not develop. In contrast the biochemical maturation and the compartmentalization of microtubule-associated protein 2 occurred normally in hippocampal cultures. These results show that the biochemical maturation and the intraneuronal compartmentalization of these two microtubule-associated proteins are independently controlled. Despite the non-restricted distribution of tau in hippocampal neurons in culture, and despite the presence of only the immature isoform which has a lessened stimulatory effect on microtubule polymerization, axons and dendrites appear to grow normally and to exhibit appropriate functional properties.

Phosphorylation determines two distinct species of Tau in the central nervous system

The monoclonal antibody, Tau-1, which had previously been used to localize tau to the axonal compartment in brain has been reutilized for light and electron microscopic immunohistochemistry following phosphatase treatment of tissue. We report here that a significant quantity of tau in the central nervous system is phosphorylated in situ at or near the Tau-1 epitope, preventing the binding of the Tau-1 antibody. Upon removal of this/these phosphate group(s), however, Tau-1 was observed in the somatodendritic compartment of neurons as well as in axons. Furthermore, intense staining was also observed in astrocytes and in perineuronal glial cells. This immunoreactivity was present along the lengths of microtubules and on ribosomes (polysomes). Treatment of immunoblots of extracts of whole cerebral cortex with phosphatase confirmed the immunohistochemical results in that a 50-65% increase in Tau-1 binding to the tau region of the blot was noted. Moreover, a novel monoclonal antibody, Tau-2, was also used in these experiments. This antibody binds only to tau and localizes along microtubules in axons, somata, dendrites, and astrocytes and on ribosomes (polysomes) without phosphatase pretreatment.

Evidence that MAP-2 may be involved in pigment granule transport in squirrel fish erythrophores

We have demonstrated the presence of MAP-2 in squirrel fish erythrophores using SDS-PAGE, immunoblot, and immunoprecipitation techniques. The monoclonal antibodies used (AP-9, -13, -14) were raised against distinct antigenic sites on Chinese hamster brain MAP-2. Immunoprecipitation studies demonstrated that all three antibodies bind a 300 K protein found in crude cell extracts and in partially purified MAP fractions isolated from erythrophores of the squirrel fish Holocentrus rufus. Immunofluorescent studies confirmed that the 300 K protein was present in cultured erythrophores. Studies of cells induced to aggregate and disperse their pigment granules revealed that the 300 K protein comigrated with the pigment, suggesting that the 300 K protein may constitute part of the "alpha-cytomatrix" involved in pigment translocations.