Immunocytochemistry of formalin-fixed human brain tissues: microwave irradiation of free-floating sections

Formalin fixation, the chemical process in which formaldehyde binds to cells and tissues, is widely used to preserve human brain specimens from autolytic decomposition. Ultrastructure of cellular and mitochondrial membranes is markedly altered by vesiculation, but this does not interfere with diagnostic evaluation of neurohistology by light microscopy. Serious difficulties are encountered, however, when immunocytochemical staining is attempted. Antigens that are immunoreactive in unfixed frozen sections and protein extracts appear to be concealed or destroyed in formalin-fixed tissues. In dilute aqueous solution, formaldehyde is in equilibrium with methylene glycol and its polymeric hydrates, the balance by far in favor of methylene glyco. Carbonylic formaldehyde is a reactive electrophilic species well known for crosslinking functional groups in tissue proteins, nucleic acids, and polysaccharides. Some of its methylene crosslinks are readily hydrolyzed. Others are stable and irreversible. During immunostaining reactions, intra- and inter-molecular links between macromolecules limit antibody permeation of tissue sections, alter protein secondary structure, and reduce accessibility of antigenic determinants . Accordingly, immunoreactivity is diminished for many antigens. Tissues are rapidly penetrated by methylene glycol, but formaldehyde binding to cellular constituents is relatively slow, increasing progressively until equilibrium is reached. In addition, prolonged storage in formalin may result in acidification of human brain specimens. Low pH favors dissociation of methylene glycol into formaldehyde, further reducing both classical staining and antigen detectability. Various procedures have been devised to counter the antigen masking effects of formaldehyde. Examples include pretreatment of tissue sections with proteases, formic acid, or ultrasound. Recently, heating of mounted sections in ionic salt solution by microwave energy was found to restore many antigens. Theory and practice of microwave antigen retrieval are covered extensively in the handbook Microwave Cookbook for Microscopists. A concise overview of microwave methods in the neurosciences has been published, and clinical applications have been reviewed. In this context, it should be noted that fresh tissues may be stabilized for immunocytochemistry by reversible, non-chemical binding processes such as cryosectioning after microwave treatment and freeze-drying. Thus, it may be possible to enhance immunostaining for some antigens by microwave irradiation of unfixed as well as fixed specimens. Parameters to be optimized for microwave retrieval of specific antigens include temperature, irradiation time, tissue buffer composition, salt concentration, and pH. Temperature, irradiation time, and pH are key variables. With this in mind, an optimal method was developed for retrieval of a wide variety of antigens in human brain tissues. Typical microwave protocols employ elevated temperatures that may reach 100 degrees C, where denaturation causes irreversible uncoiling and disruption of protein secondary and tertiary structures. Under these conditions, stable covalent bonds securing methylene crosslinks between polypeptides remain intact, but more reactive links formed by Schiff bases may be hydrolyzed. Resultant conformational changes presumably expose buried loops of continuous amino acids and protruding regions, increasing accessibility of their epitopes. Protein denaturation seems to be a reasonable explanation for the effects of microwaves on antigen retrieval. This idea is supported by the observation that denaturing solutions such as 6 M urea increase immunoreactivity of some antigens. Still, the molecular basis of these effects remains unresolved, in part due to the complex chemistry of formaldehyde reactions with tissue constituents. Indeed, some methylene bridges between similar groups such as NH2 and NH may be hydrolyzed by washing fixed tissues in distilled wa

Immunocytochemistry of tau phosphoserine 413 and tau protein kinase I in Alzheimer pathology

One unique phosphorylation site consistently found in paired helical filament tau, serine 413, is modified by tau protein kinase I/glycogen synthase kinase-3 beta but no other known tau kinase. Here we present immunocytochemistry from Alzheimer's disease brains showing that focal subpopulations of hippocampal CA1 pyramidal neurons and neuritic plaques are strongly reactive for tau protein kinase I/glycogen synthase kinase-3 beta and tau phosphoserine 413 in early stages of pathology. Colocalization of these epitopes suggests that tau protein kinase I/glycogen synthase kinase-3 beta abnormally phosphorylates tau and is in a position to disrupt neuronal metabolism in anatomical areas vulnerable to Alzheimer's disease.

The lysosomal system in neurons. Involvement at multiple stages of Alzheimer's disease pathogenesis

Disturbed lysosomal function may be implicated at several stages of Alzheimer's pathogenesis. Lysosomes and acid hydrolases accumulate in the majority of neocortical pyramidal neurons before typical degenerative changes can be detected, indicating that altered lysosome function is among the earliest markers of metabolic dysfunction in Alzheimer's disease. These early alterations could reflect accelerated membrane and protein turnover, defective lysosome or hydrolase function, abnormal lysosomal trafficking or any combination of these possibilities. Because APP is partly metabolized in lysosomes, early disturbances in lysosomal function could promote the production of abnormal and/or neurotoxic APP fragments within intact cells. Lysosomal abnormalities progressively worsen as neurons begin to degenerate. Based on existing literature on cell death, increased perturbation and instability of the lysosomal system may be expected to contribute to the atrophy and eventual lysis of the neuron. Finally, the release of hydrolase-filled lysosomes and lipofuscin aggregates from dying neurons accounts for the abundant deposition of enzymatically active acid hydrolases of all classes in the extracellular space--a phenomenon that may be unique to Alzheimer's disease. Acting on APP present in surrounding dystrophic neurites, cellular debris and astrocyte processes, dysregulated hydrolases may cleave APP in atypical sequential patterns, thereby generating self-aggregating protease-resistant APP fragments that can be only processed to beta-amyloid. Genetic mutations or posttranslational factors of APP should further enhance the generation of amyloidogenic fragments by a dysregulated lysosomal system. Given that very little, if any, beta-amyloid is detected intracellularly, yet extracellular beta-amyloid is very abundant, our data suggest that the final steps of APP processing and the generation of most beta-amyloid in the brain parenchyma occur extracellularly and may involve one or more lysosomal proteases.

Lysosomal proteinase antigens are prominently localized within senile plaques of Alzheimer's disease: evidence for a neuronal origin

To investigate the role of proteolysis in amyloid formation, we studied the localization of the proteolytic enzymes, cathepsin D and cathepsin B, in the prefrontal cerebral cortex and hippocampus of human postmortem brains from patients with Alzheimer's disease and from individuals free of neurological disease. In control and Alzheimer brains, cathepsin immunoreactivity within cells was localized to lysosome-related structures, which were particularly abundant in neuronal perikarya. In Alzheimer brain, cathepsin immunoreactivity was also heavily concentrated extracellularly within senile plaques. Cathepsin immunoreactivity associated with plaques was not confined to lysosomes and was distributed throughout the plaque. Isolated amyloid cores, however, were not immunostained. Cathepsin-laden perikarya of degenerating neurons were frequently seen within senile plaques and, in the more advanced stages of degeneration, cathepsin immunoreactivity was present throughout the cytoplasm. Other identified constituents of senile plaques appeared to be less significant sources of cathepsin immunoreactivity, including astrocytes, degenerating neurites, microglia and macrophages. These results demonstrate that lysosomal proteinases are major constituents of the senile plaque and that degenerating neuronal perikarya are a principal source of the cathepsin immunoreactivity. We propose that the unregulated action of extracellular cathepsins liberated from degenerating neurons may lead to abnormal processing of the amyloid precursor protein and to the formation of amyloid locally within senile plaques in Alzheimer's disease.

Aluminum salts induce the accumulation of neurofilaments in perikarya of NB2a/dl neuroblastoma

NB2a/dl neuroblastoma cells were exposed to aluminum chloride or aluminum lactate (0.1-1 mM) for 3 and 6 days. Additional cultures were exposed to aluminum salts as the cells were stimulated to elaborate axonal neurites by dibutyryl cyclic AMP. By phase-contrast microscopy, aluminum salts had no effect on the morphology of undifferentiated (NB2a(-] or differentiated (NB2a(+] cells, or on neuritic elaboration and maintenance. Silver straining by the Bielschowsky method, however, demonstrated argyrophilic accumulations in perikarya of many NB2a(-) and NB2a(+) cells treated with aluminum salts. At the ultrastructural level, whorls of intermediate filaments were the most prominent abnormalities in neuronal perikarya. Although phosphorylated high-molecular weight neurofilament subunits (NF-H) are normally detected by immunocytochemical analyses only within axonal neurites of NB2a/dl cells, aluminum salt treatment caused the detection of phosphorylated epitopes of NF-H within perikaryal of NB2a(-) and NB2a(+) cytoskeletons, suggesting that the argyrophilic filamentous accumulations are composed at least partly of phosphorylated NF-H.