Immunocytochemical studies of Alzheimer neuronal perikarya with intermediate filament antisera

Blood-brain barrier leakage in Alzheimer's disease: From discovery to clinical relevance

Alzheimer's disease (AD) is the most common form of dementia. AD brain pathology starts decades before the onset of clinical symptoms. One early pathological hallmark is blood-brain barrier dysfunction characterized by barrier leakage and associated with cognitive decline. In this review, we summarize the existing literature on the extent and clinical relevance of barrier leakage in AD. First, we focus on AD animal models and their susceptibility to barrier leakage based on age and genetic background. Second, we re-examine barrier dysfunction in clinical and postmortem studies, summarize changes that lead to barrier leakage in patients and highlight the clinical relevance of barrier leakage in AD. Third, we summarize signaling mechanisms that link barrier leakage to neurodegeneration and cognitive decline in AD. Finally, we discuss clinical relevance and potential therapeutic strategies and provide future perspectives on investigating barrier leakage in AD. Identifying mechanistic steps underlying barrier leakage has the potential to unravel new targets that can be used to develop novel therapeutic strategies to repair barrier leakage and slow cognitive decline in AD and AD-related dementias.

The neurobiology of Alzheimer's disease

The defining histological characteristics of Alzheimer's disease (AD) are neurofibrillary tangles and neuritic plaques, although neither is pathognomonic for this disorder. The distribution of AD histopathology suggests selective neuronal vulnerability, with specific cell populations affected within discrete regions of the cerebral hemispheres and within certain subcortical and brain-stem nuclear areas. At the ultrastructural level, tangles and plaque neurites contain paired helical filaments whose composition is unknown but may include altered cytoskeletal elements. Amyloid, deposited in plaque cores and often focally present within the cerebral vasculature, contains a polypeptide ("beta-protein," or "beta-amyloid") encoded by a chromosome 21 gene. At least in occasional families, AD has been linked to a separate chromosome 21 locus, but different underlying genetic factors may operate in other cases. Inorganic substances, including aluminum and silicon, are reported to co-localize within tangle-bearing neurons and plaque cores. Specific environmental agents have not been confirmed to be pathogenetically important, however, but may eventually prove to exert a permissive, facilitatory, or even causative role in many AD patients.

Immunodetection of protein composition in cerebral amyloid extracts in Alzheimer's disease: enrichment of retinol-binding protein

The protein profile of amyloid-enriched extracts from Alzheimer brain tissue was studied by enzyme immunoassay using antisera to 37 different antigens including neurofilament protein, the proteins of the prealbumin complex, immunoglobulins, light chains, and amyloid A and P components. The results demonstrate a significant enrichment of retinol-binding protein in Alzheimer brain extracts, but not in control extracts from non-Alzheimer brains or amyloid extracts from cases of amyloid A (secondary) or lambda light chain (primary) amyloidosis. The retinol-binding protein was enriched in the extracts from all four Alzheimer brains. In contrast, no enrichment of prealbumin, immunoglobulins or amyloid P component was observed. Two of the 4 different Alzheimer amyloid extracts showed significant accumulation of neurofilament protein. In addition, slightly increased amounts of free light chains were observed. The results suggest that retinol-binding protein may be involved in the pathogenesis of the amyloid deposits in Alzheimer's disease.

Recognition of tau epitopes by anti-neurofilament antibodies that bind to Alzheimer neurofibrillary tangles

Eleven anti-neurofilament (anti-NF) monoclonal antibodies were studied for their reactivity with heat-stable, microtubule-associated proteins and Alzheimer neurofibrillary tangles (ANT). On immunoblots of NF proteins, the antibodies recognized epitopes that were variably sensitive to Escherichia coli alkaline phosphatase. Eight of the antibodies showed reactivity with ANT and decreased binding to electroblotted NF after phosphatase treatment. The same eight antibodies reacted with tau proteins from bovine and rat brain, binding to tau proteins was also substantially reduced by phosphatase. Of the eight antibodies that bound to animal tau proteins, five also bound to tau proteins from normal human brain. All of the antibodies that bound to animal tau proteins stained ANT in frozen tissue sections. Brief treatment of tissue sections with trypsin in most cases enhanced antibody binding to ANT. All antibodies that lacked reactivity with tau proteins failed to bind ANT. Phosphatase treatment of Alzheimer tissue sections did not change the immunoreactivity of ANT and neurites in senile plaques with ANT-reactive, anti-NF antibodies, except for two antibodies that showed decreased binding to ANT. In contrast, axonal staining was decreased or eliminated by phosphatase treatment, similar to the response of electroblotted NF and tau proteins. These results suggest that staining of ANT by anti-NF antibodies may be due to cross-reaction of anti-NF with epitopes in tau proteins, the epitopes in axons, NF, and tau are sensitive to the effect of phosphatase, whereas the majority of those in ANT are not, and some of the epitopes in ANT that are shared with NF and tau proteins are not readily accessible to antibody binding.

Two monoclonal antibodies recognize Alzheimer's neurofibrillary tangles, neurofilament, and microtubule-associated proteins

Two monoclonal antibodies that recognize Alzheimer's neurofibrillary tangles (ANTs), AD10 and AB18, have been characterized by immunoblotting against human and calf spinal cord neurofilament (NF) and calf brain microtubule preparations. Both antibodies bind to the 200-kilodalton (kd) (NF-H) and 160-kd (NF-M) but not to the 68-kd (NF-L) NF triplet proteins. They also bind to high-molecular-weight microtubule-associated proteins (MAPs) and tau. AD10 immunostains MAP2 and MAP1 families, whereas AB18 stains mainly MAP1 bands. Preincubation of intact filament preparation or nitrocellulose strips containing electroblotted NF proteins with Escherichia coli alkaline phosphatase completely blocks AD10 binding and partially blocks binding of AB18. These results suggest that the determinants recognized by these antibodies are phosphorylated. Immunoblotting of peptide fragments generated by limited proteolysis of NF proteins with alpha-chymotrypsin and Staphylococcus aureus V8 protease shows that the localization of the antigenic determinants to AD10 and AB18 in NF-H is approximately 100 and 60 kd, respectively, away from the carboxy terminal, a region previously shown to form the NF projection side arm. In NF-M, the antigenic determinants to both antibodies are located also in the projection side arm, in a 60-kd polypeptide adjacent to the alpha-helical filament core. The results show that ANTs contain at least two phosphorylated antigenic sites that are present in NF and MAPs, a finding suggesting that ANTs may be composed of proteins or their fragments with epitopes shared by cytoskeletal proteins.

Alzheimer neurofibrillary tangles contain phosphorylated and hidden neurofilament epitopes

Three monoclonal antibodies to neurofilaments (RT97, BF10 and 147), two of which also recognised neurofibrillary tangles (RT97 and BF10), have all been shown to be specific for phosphorylated epitopes. Treatment of histological sections with alkaline phosphatase prior to immunostaining resulted in reduction of axonal neurofilament staining with all three whilst the neurofibrillary tangles staining with BF10 was unaffected. Antibody 147 was found to recognise weakly some neurofibrillary tangles following alkaline phosphatase treatment. The results presented confirm the presence of structurally abnormal but phosphorylated neurofilaments in neurofibrillary tangles.

Transglutaminase and the neuronal cytoskeleton in Alzheimer's disease

Transglutaminase [EC 2.3.2.13, (R)-glutaminyl-peptide:amine gamma-glutamyltransferase], an enzyme that catalyzes the introduction of glutamine-lysine cross-links into proteins, was purified. Neurofilament and microtubule proteins were substrates for this enzyme but the insoluble neurofibrillary tangles (NFT) isolated from Alzheimer's disease brain were not substrates. In vitro cross-linking of neurofilaments and microtubules by the enzyme did not produce paired helical filaments (PHF), which are the major ultrastructural component of NFT. These results make it unlikely that PHF are formed by the straightforward cross-linking of neurofilaments or microtubules.

Neurofibrillary tangles of Alzheimer disease share antigenic determinants with the axonal microtubule-associated protein tau (tau)

The relationship of the neurofibrillary tangle, found in Alzheimer disease and aged brains, to normal or abnormal cytoskeletal proteins remains elusive. Although immunohistochemical studies have yielded disparate results, most antigenic determinants localized to neurofibrillary tangles are cytoskeletal constituents normally present in neuronal perikarya or dendrites. We report light and electron microscopic immunolabeling of neurofibrillary tangles by a monoclonal antibody to the microtubule-associated protein tau (tau). Dephosphorylation of tissue slices not only increased the number of tau-positive tangles but also produced marked positive immunoreactivity of neuritic plaques. The localization of tau, an axonal protein, to neurofibrillary tangles in the perikaryon in particular suggests that abnormal synthesis, modification, or aggregation of tau may induce aberrant cytoskeletal--cell organelle interactions, subsequent interference with axonal flow, and resultant tangle formation.

Controlled induction of paired helical filaments of the Alzheimer type in cultured human neurons, by glutamate and aspartate

One characteristic histopathological feature of brains of patients with Senile Dementia of the Alzheimer Type (SDAT) is a neuronal change known as neurofibrillary degeneration. Ultrastructurally, this degeneration presents as aggregates of Paired Helical Filaments (PHFs). We have reported (De Boni and Crapper 1978) that brain affected with SDAT contains a factor which induces PHFs similar to those occurring in Alzheimer's disease, in human CNS neurons in vitro. We now report that PHFs may be induced in cultured, fetal human spinal cord lesions by factors other than extracts from brain affected by SDAT. Specifically, ultrastructural analyses have shown that PHFs closely similar to those found in SDAT are induced by the excitotoxic aminoacids, glutamate (Glu) and aspartate (Asp). PHFs were found with Glu and Asp in combination at 1.1 and 0.45 mM, respectively and by Glu alone at 2.2 mM, when added to the culture medium. Cultures grown and maintained in MEM Eagle medium without Glu and Asp were invariably composed of tightly packed neuropil with abundant synaptic contacts, were free of vacuolated somata or processes and did not exhibit the presence of PHFs. In contrast, cultures exposed to Glu and Asp invariably responded with the presence of vacuolated neuronal somata and degenerating neuronal processes containing intermediate filaments, frequently paired into PHFs. These induced PHFs are composed of intermediate filaments morphologically identical (diameter 10 +/- 0.9 nm, n = 55) to neurofilaments. While the induced PHFs exhibit a range of periodicities (50-540 nm) with an overall mean period of 140 +/- 68 nm (n = 245), approximately 4% are morphologically closely similar (mean period 76 +/- 10 nm, n = 10) to those in SDAT.

DNA binding protein profiles in Alzheimer's disease

Eleven frozen autopsy specimens from cerebral cortex were tested for DNA-binding protein profiles. Six were Alzheimer's disease (AD) brains, 1 was Parkinson's/senile dementia of the Alzheimer's type and 4 were age-matched control brains. Proteins were extracted in a guanidine thiocyanate-containing solvent and freed of all nucleic acids by density gradient sedimentation. The proteins were separated by sodium dodecyl sulfate gel electrophoresis and transferred to nitrocellulose by electroblotting under conditions which favor renaturation of proteins containing only one type of polypeptide. The nitrocellulose was treated with partially denatured radiolabeled DNA, washed and subjected to autoradiography. An Mr = 43 000 (43 K) DNA-binding protein was detected in 5 of the 6 AD brains and was found to be absent or at least greatly reduced in any of the other 6 brains. No other DNA-binding proteins were found which could be associated with AD brains. The nature of the 43 K protein has yet to be determined.

Amyloid-like (Congophilic) neurofibrillary tangles do not react with neurofilament antisera in Alzheimer's cerebral cortex

The immunohistological properties of Alzheimer's neurofibrillary tangles (NFT) were studied by immunofluorescence with neurofilament (NF) antisera and with antiserum raised to paired helical filaments (PHF) in NFT preparations, brain smears, and cryostat sections. NFT decorated by NF antisera were Congo red-negative. Conversely, PHF antisera stained Congo red-positive NFT but failed to decorate NF-positive NFT. It is concluded that NF do not cross react with typical NFT, i.e., NFT displaying amyloid-like birefringence, under the conditions reported in this study.