Alzheimer's disease is characterized by the accumulation of beta amyloid pe
ptides in plaques and vessel walls and by the intraneuronal accumulation of
paired helical filaments composed of hyperphosphorylated tau. In this revi
ew, we concentrate on the biology of amyloid precursor protein, and on the
central role of amyloid in the pathogenesis of Alzheimer's disease, Amyloid
precursor protein (APP) is part of a super-family of transmembrane and sec
reted proteins. It appears to have a number of roles, including regulation
of haemostasis and mediation of neuroprotection. APP also has potentially i
mportant metal and heparin-binding properties, and the current challenge is
to synthesize all these varied activities into a coherent view of its func
tion. Cleavage of amyloid precursor protein by beta- and gamma-secretases r
esults in the generation of the A beta (beta A4) peptide, whereas alpha-sec
retase cleaves within the AP sequence and prevents formation from APP. Rece
nt findings indicate that the site of gamma-secretase cleavage is critical
to the development of amyloid deposits; A beta(1-42) is much more amyloidog
enic than A beta(1-40). A beta(1-42) formation is favoured by mutations in
the two presenilin genes (PS1 and PS2), and by the commonest amyloid precur
sor protein mutations. Transgenic mouse models of Alzheimer's disease incor
porating various mutations in the presenilin gene now exist, and have shown
amyloid accumulation and cognitive impairment. Neurofibrillary tangles hav
e not been reproduced in these models, however. While aggregated A beta is
neurotoxic, perhaps via an oxidative mechanism, the relationship between su
ch toxicity and neurofibrillary tangle formation remains a subject of ongoi
ng research.