Immunohistoehemistry of Alzheimer' s Disease
Introduction
Immunohistochemistry
has played a central role in the developing of our understanding of AD. This
short article will review potential pitfalls and findings relating to amyloid
and paired helical filament pathology.
Pitfalls
The relative
insolubility of the abnormal proteins in AD 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 Westerns 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 demonstrated in some areas.
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.
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.