Interaction of the Presenilins with the Amyloid Precursor Protein (APP)
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The genes encoding presenilin-1 (PS1 ) and presenilin-2 (PS2 ) were identified as the genes that harbour mutations that cause more than 60% of early onset familial Alzheimer’s disease cases (FAD) (1 -3 ). So far, more than 40 missense mutations have been described for presenilin-1 and two have been found in the gene coding for presenilin-2 (reviewed in refs. 4 and 5 ). Carriers of mutated presenilin genes develop in their brain neuropathological changes characteristic of Alzheimer’s disease including the deposition of amyloid Aβ peptide. The latter is released from its cognate amyloid precursor protein (APP) by a two-step proteolytic conversion: first, proteolysis of APP by β-secretase, which releases the N-terminus of Aβ, and second, conversion of the remaining fragment by γ-secretase, which cleaves within the predicted transmembrane region of APP. This releases the C-terminus of Aβ, which may end either at position 40 or, to a lesser extent, at position 42 (reviewed in ref. 6 ). The latter species, Aβ1-42 , is more prone to aggregation and deposition than Aβ1-40 and is produced at higher levels in the brains and primary fibroblasts of FAD patients carrying PS missense mutations (7 ). The same result was obtained when cultured cells transfected with mutated PS1 orPS2 , or transgenic mice harboring missense PS1 were analyzed for the production of Aβ1-42 : in every case increased amounts of the longer Aβ1-42 species were observed (8 -10 ). The mechanisms by which mutations in the PS genes affect the proteolytic processing of APP by γ-secretase have not been resolved in detail. There are two possibilities by which the normal processing of APP may be disturbed: either mutations in the presenilins affect APP metabolism in an indirect way by modulation of proteases or interaction with proteins involved in APP intracellular routing, or presenilins may modulate APP processing directly through physical interactions with APP. Such a direct interaction between presenilins and APP was first demonstrated by us for PS2 (11 ). Later on, formation of stable complexes with APP was reported not only for PS2 but also for PS1 (12 ,13 ,13a ).
