Carbonyl-related posttranslational modification of neurofilament protein in the neurofibrillary pathology of Alzheimer's disease

We present the first evidence for carbonyl-related posttranslational modifications of neurofilaments in the neurofibrillary pathology of Alzheimer's disease (AD). Two distinct monoclonal antibodies that consistently labeled neurofibrillary tangles (NFTs), neuropil threads, and granulovacuolar degeneration in sections of AD tissue also labeled the neurofilaments within axons of the white matter following modification by reducing sugars, glutaraldehyde, formaldehyde, or malondialdehyde. The epitope recognized by these two antibodies shows a strict dependency for carbonyl modification of the neurofilament heavy subunit. The in vivo occurrence of this neurofilament modification in the neurofibrillary pathology of AD suggests that carbonyl modification is associated with a generalized cytoskeletal abnormality that may be critical in the pathogenesis of neurofibrillary pathology. Furthermore, the data presented here support the idea that extensive posttranslational modifications, including oxidative stress-type mechanisms, through the formation of cross-links, might account for the biochemical properties of NFTs and their resistance to degradation in vivo.

Extracellular neurofibrillary tangles reflect neuronal loss and provide further evidence of extensive protein cross-linking in Alzheimer disease

In this report we quantitatively assess the numbers of intracellular and extracellular neurofibrillary tangles (NFT) in the brains of a series of individuals with Alzheimer's disease and of controls and correlate these with neuronal loss. Our data indicate that in some cases, NFT are not removed from the brain throughout the disease process. This finding, together with our previous demonstration of carbonyl-related modifications in NFT, provides additional evidence that the protein constituents of NFT are resistant to proteolytic removal, possibly as a result of extensive cross-links. Additionally, correlation between the number of NFT and neuronal loss indicates that there are at least two distinct mechanisms responsible for neuronal death in Alzheimer's disease that are directly and indirectly related to the presence of neurofibrillary pathology.

Tau protein directly interacts with the amyloid beta-protein precursor: implications for Alzheimer's disease

The simultaneous presence of intracellular neurofibrillary tangles (NFT) and extracellular senile plaques in Alzheimer's disease (AD) suggests that the two lesions could be synergistically interrelated. However, although the main protein components of NFT and senile plaques, tau (tau) and amyloid beta-protein, respectively, are well characterized, the molecular mechanisms responsible for their deposition in lesions are unknown. We demonstrate, using four independent techniques, that tau directly interacts with a conformation-dependent domain of the amyloid beta-protein precursor (beta PP) encompassing residues beta PP714-723. The putative tau-binding domain includes beta PP717 mutation sites that are associated with familial forms of AD. Our findings strongly suggest that NFT and senile plaques, often thought of as independent structures, may play a role in each other's formation during the pathogenesis of AD.

Immunocytochemical evidence that the beta-protein precursor is an integral component of neurofibrillary tangles of Alzheimer's disease

Amyloid beta (A beta) immunoreactivity has been demonstrated in all extracellular neurofibrillary tangles (E-NFT) and most intraneuronal neurofibrillary tangles (I-NFT). We undertook this immunocytochemical study to understand the relationship between A beta immunoreactivity localized in NFT and beta-protein precursor (beta PP). We found epitopes of amino-, mid-, and carboxyl-terminal domains of beta PP in I-NFT and the majority of E-NFT. NFT retained beta PP after ionic detergent extraction, demonstrating that beta PP is an integral component of NFT. Finding beta PP in regions of A beta immunoreactivity raises the possibility that beta PP or its fragments associate with amyloid, and that the stability of A beta is responsible for its dominance in amyloid deposits.

Filaments of Lewy bodies contain insoluble cytoskeletal elements

The Lewy body is an intraneuronal inclusion body that is one of the histologic hallmarks of Parkinson's disease, a degenerative disease of the brain. Ultrastructural analysis has shown that the Lewy body is composed of straight 7-20 nm filaments and amorphous elements. Previous light microscopic, immunocytochemical studies have suggested the presence of neurofilament, microtubule, ubiquitin, and paired helical filament-related epitopes in Lewy bodies. Yet the biochemical composition of the Lewy body remains incompletely elucidated. The ultrastructural and immunocytochemical similarities and differences between the Lewy body and the neurofibrillary tangle of Alzheimer's disease raise questions as to their relation to each other and possible shared mechanisms of formation. In this study the authors examine whether ultrastructural immunocytochemical analysis of Lewy bodies confirms the light microscopic data, whether the structures and epitopes of Lewy bodies share with Alzheimer's disease neurofibrillary tangles the property of insolubility in sodium dodecyl sulfate, and speculate about the subunit composition of Lewy body filaments.

Neuropil threads of Alzheimer's disease show a marked alteration of the normal cytoskeleton

Abnormal neurites, neuropil threads, are a widespread and characteristic lesion of Alzheimer's disease likely to play a major role in the cognitive impairment of this disease. Contrary to normal neurites, neuropil threads contain straight and paired helical filaments that contain the microtubule-associated protein tau and ubiquitin. It is not known whether these abnormal filaments are added to or replace the normal cytoskeleton. In this study, we examined the fine structure of neuropil threads and carried out a morphometric analysis of the neurofilaments and abnormal filaments contained in the neuropil threads by using an antiserum to tau and colloidal gold immuno-electron microscopy. Almost 70% of the neuropil threads contained straight or paired helical filaments with no neurofilaments. The total number of filaments in each neuropil thread remained essentially unchanged either when straight or paired helical filaments were present alone or when they coexisted either together or with neurofilaments. When the three types of filaments were expressed as a proportion of the total, a linear inverse correlation was found between neurofilaments and straight filaments as well as between straight and paired helical filaments. Approximately 10% of the neuropil threads were found to be myelinated axons. It is concluded that straight filaments are likely to replace neurofilaments, that they in turn might be replaced by paired helical filaments, and that this process occurs in axons as well as dendrites.

Neuronal and microglial involvement in beta-amyloid protein deposition in Alzheimer's disease

This study was undertaken to localize amyloid precursor protein (APP) and to determine how APP might be released and proteolyzed to yield the beta-amyloid protein deposits found in senile plaques in the brains of Alzheimer's disease patients. We found that antibodies to recombinantly expressed APP labeled many normal neurons and neurites. In addition, dystrophic neurites in different types of senile plaques and degenerating neurons in the temporal cortex and hippocampus of Alzheimer's disease patients were immunostained. We also detected small clusters of dystrophic APP immunoreactive neurites that were not associated with beta-amyloid protein deposits. Microglia was involved in different types of senile plaques and often were associated closely with APP immunoreactive neurites and neurons. The greatest concurrence of APP immunoreactivity and reactive microglia was seen in the subiculum and area CA1, regions with a high density of congophilic plaques and subject to intense Alzheimer's pathology. Our findings suggest that neuronally derived APP is the source for senile plaque beta-amyloid protein, while microglia may act as processing cells.

Immunochemical demonstration of tropomyosin in the neurofibrillary pathology of Alzheimer's disease

The focus of research on the neurofibrillary pathology (NFP) of Alzheimer disease has been not only to determine the component forming the paired helical filaments but also to determine whether they result from abnormal processes affecting a single protein. Therefore, although these studies have lead to controversy concerning the respective contribution of components of microtubules and neurofilaments, there has been essentially no consideration of whether other cytoskeletal systems might also be involved and of what are the common features for the incorporated components. Particularly relevant to this issue is our finding that several antisera raised to either skeletal or smooth muscle tropomyosin, a microfilament component, intensely recognize NFP. These antibodies continue to recognize NFP after affinity purification to tropomyosin or paired helical filament fractions. We show that the antibodies do not recognize NFP due to cross-reactivity with the previously identified NFP components related to neurofilaments and microtubules, tau, and MAP2, or neurofilament proteins because the antisera did not recognize these proteins on immunoblots or were not adsorbable by the proteins. Ultrastructural analysis of the immunoreaction showed that tropomyosin-related epitopes were clustered rather than uniformly distributed along paired helical and straight filaments. Although the distribution suggests that tropomyosin is an NFP-associated protein, its retention by paired helical and straight filaments after detergent extraction indicates that it is an integral component strongly and specifically associated with the filaments characteristic of NFP. These findings indicate that NFP involves the three primary neuronal cytoskeletal filament systems, microtubules, neurofilaments, as well as microfilaments, and therefore that NFP probably results from the reorganization of these normal filaments that interact to comprise the cytomatrix and may continue this interaction under the pathologic condition of Alzheimer's disease to generate novel, abnormal polymers.

Immunochemical properties of ubiquitin conjugates in the paired helical filaments of Alzheimer disease

Immunocytochemical and peptide sequencing studies indicate that the regulatory protein ubiquitin (Ub) is incorporated into the paired helical filaments (PHF) of Alzheimer disease. In this study, we showed that some antibodies raised to PHF recognize epitopes of Ub. Analysis of the Ub sequences recognized by the antibodies raised to PHF, along with the known specificity of several monoclonal antibodies raised to artificial Ub conjugates, indicates the immunochemical representation of Ub residues 34-76 in PHF. The Ub epitopes recognized by antibodies raised to PHF are distinct from those recognized by antibodies raised to artificial Ub conjugates in two respects. First, antibodies that are raised to PHF and that recognize Ub react with PHF equally, whether denatured or not, whereas those raised to artificial Ub conjugates show greater reaction after denaturation. Second, mapping of the epitopes recognized by two monoclonal antibodies to PHF onto Ub indicates a distinction in the Ub residues recognized, compared with monoclonal antibodies raised to artificial Ub conjugates. The proximity of their epitopes to the site of conjugation, as well as their affinity for PHF polypeptides, suggests that the PHF antibodies that recognize Ub may be directed specifically to Ub epitopes defined by the protein conjugated to Ub.

Immunoaffinity demonstration that paired helical filaments of Alzheimer disease share epitopes with neurofilaments, MAP2 and tau

Identification of the neuronal components incorporated into the neurofibrillary tangles of Alzheimer disease has primarily been derived from immunocytochemical procedures. Previous antibody studies have been able to directly determine the shared epitopes of known neuronal proteins with neurofibrillary tangles (NFT) only when the appropriate monoclonal antibodies were available. In this study, we use an immuno-affinity purification protocol to directly determine the properties of the epitopes recognized by two antisera which recognize NFT. Characterization of the purified antibodies demonstrates that NFT share epitopes with the two heavier neurofilament subunits. NFH and NFM, as well as MAP2 and tau. Further, this method indicates that the epitopes shared with neurofilaments and tau are distinct from each other.

Ubiquitin is associated with abnormal cytoplasmic filaments characteristic of neurodegenerative diseases

Several degenerative diseases of the nervous system are characterized by the presence of neuronal inclusions. Most of these inclusions are made of abnormal filaments and share epitopes with cytoskeletal proteins. One of these inclusions, the neurofibrillary tangle of Alzheimer disease, has recently been shown to contain ubiquitin, a regulatory protein thought to play a role in the degradation of abnormal proteins. We carried out light and electron microscopic immunocytochemistry with several polyclonal and monoclonal antibodies to investigate the presence of ubiquitin in neuronal inclusions of neurodegenerative diseases. Ubiquitin was present not only in paired helical filaments that form the neurofibrillary tangle of Alzheimer disease, but also in the filamentous components of the inclusion characteristic of Parkinson disease, Pick disease, and progressive supranuclear palsy. In contrast, ubiquitin was not detected in other neuronal inclusions often found in aging and in Alzheimer disease, such as Hirano bodies and granulovacuolar degeneration. Reactivity with monoclonal antibodies suggests differences in the ubiquitin-acceptor proteins present in the inclusions studied. It is concluded that ubiquitin is selectively present in neuronal inclusions of degenerative diseases.

Immunocytochemical properties of Alzheimer straight filaments

In addition to paired helical filaments (PHF), the Alzheimer neurofibrillary tangle (NFT) contains straight filaments measuring 10-15 nm in width, as well as amorphous material. In previous immunoelectron microscopic studies, we have demonstrated that PHF and the amorphous material share antigens with the microtubule associated protein, tau, and the 200 kDa subunit of neurofilaments (NF). The present immunoelectron microscopic study focuses on the antigens present in the straight filaments contained in the NFT. Indirect and direct immunogold staining of Vibratome sections of Alzheimer hippocampus with a monoclonal antibody to NF200 kDa (1.1.1.) and polyclonal antibodies to NF, tau, and to PHF yielded intense and specific reaction with NFT. Within the same NFT both straight filaments and PHF were decorated by all the antibodies. Similarly decorated within NFT were filaments displaying a transitional form between straight filaments and PHF. Both PHF and straight filaments were also recognized by all the antibodies after extraction with an ionic detergent. These findings show that straight filaments contain most, if not all, of the antigens known to be present in PHF and share with PHF insolubility in ionic detergents. It is suggested that straight filaments result from alternative pathways of organization of the same components as form the PHF.