Histochemistry of Alzheimer’s Disease
Immunohistochemistry has played a central role in the developing of our understanding of Alzheimer’s Disease. This short article will review potential pitfalls and findings relating to amyloid and paired helical filament pathology.
The relative insolubility of the abnormal proteins in Alzheimer’s Disease has made immunohistochemistry a valuable tool for studying the biochemistry of this condition. However, care must be taken in interpretation. Plagues and tangles, as well as neuronal cytoplasm, are ‘sticky’ and non-specific binding is easy to produce. Demonstration of an epitope does not demonstrate the protein. Multiple antibodies to different parts of the molecule should be used. Fixation often has unpreditable effects. Conformational differences between sections and blots may render Western Blot of limited value in some circumstances. Validation by Western is now almost obligatory however. Site of immunoreactivcity is not necessarily site of production (in situ hybridization an important parallel study to perform).
Immunohistochemistry for BA4 protein
a) Deposit types
Numerous classifications have been presented. Simplest is into diffuse deposits, with little or no alteration of the general structure of the neuropil and plaques with amyloid deposition, with or without a distinct central core.
b) Deposit distribution
Deposits are not randomly distributed. Diffuse deposits are present in many more areas than are classical plaques. Some areas only ever show diffuse deposits (parvopyramidal layer of the presubiculum). Cored plaques are especially frequent in the calcarine cortex, CA4 subfield of the hippocampus and the amygdala.
c) Temporal sequence
From studies of Down’s syndrome, especially, it has become clear that diffuse deposits ‘mature’ into coarser deposits with increasing neuritic change. Such a sequence is not inevitable in all areas and it is not clear whether or not it is inevitable in all cases.
d) Relationship of deposits to other structures
Deposition of BA4 in blood vessels is important yet not fully understood. Earlier suggestions that plaques form in relation to blood vessels have not been borne out in studies of BA4 deposition. Positioning of glia seems to be secondary. PHF-bearing neurites occur more frequently in areas/cases where there is tangle formation. Dendritic association has been demonstrated in some areas.
Other plaque components
Although the impression is often given that BA4 is the only component of plaques many other proteins have been identified. (Serum amyloid P, complement components, alpha-antichymotrypsin, cholinesterase.) The significance of many of these observations is unclear. Immunohistochemistry is also of great value in demonstrating of cellular elements namely astrocytes and microglia.
Immunohistochemistry of tangles
a) Cytoskeletal components
Early observations were followed by similar studies demonstratng the presence of phosphorylated microtubule associated proteins, especially tau. Alz 50 immunoreactivity relates closely to that of abnormally phosphorylated tau and gives a much clearer background than other tau antibodies but is present in normal brain also. Similar immunoreactivity is reported in other PHF structures, namely neuropil threads and plaque neurites. Other MAPs have also been reported in tangles.
b) Other components
Tangles also stain for complement components, serum amyloid P, ubiquitin and rarely PGP 9.5. Sporadic reports describe immuno staining for parts of APP but a coherent picture has yet to emerge.
c) Extracellular tangles
Tangles in the extracellular space (presumably because their host cell has died), become immunoreactive for GFAP as astrocyte processes interleave between the bundles of PHF.They also adsorb proteins from the extracellular space including BA4 protein and bFGF.
A) Full Text Papers:
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