Sf. Lichtenthaler et al., MUTATIONS IN THE TRANSMEMBRANE DOMAIN OF APP ALTERING GAMMA-SECRETASESPECIFICITY, Biochemistry, 36(49), 1997, pp. 15396-15403
Alzheimer's disease (AD) beta-amyloid peptide (A beta and beta A4) is
derived from the amyloid precursor protein (APP) by the subsequent act
ion of the so-far unidentified beta- and gamma-secretases. gamma-secre
tase, which generates the C-terminus of A beta, cleaves within the tra
nsmembrane domain of APP, preferentially after A beta-residue 40 (A be
ta 40) but also after residue 42 (A beta 42). This A beta 42 represent
s the major subunit of the plaques in AD, Since the position of gamma-
secretase cleavage is crucial for understanding the pathogenic pathway
, we investigated the effect of different point mutations at Thr43 on
gamma-secretase specificity in SPA4CT (SPC99)-expressing COS7 cells. T
hese constructs only require gamma-cleavage for A beta release. We obs
erved that all Thr43 mutations altered the specificity of gamma-secret
ase. Small hydrophobic residues favored the generation of A beta 42, l
eading to an increase in the 42/40 ratio of A beta (1.6-2.8-fold), The
increase was even stronger (5.6-5.8-fold) when combined with the fami
lial mutation Val46Phe. Thus, these constructs might be highly valuabl
e for the generation of animal models for AD. Processing of full-lengt
h APP or SPA4CT yielded the same 42/40 ratio of A beta (4.7%). Both co
nstructs, bearing the familial AD mutation Val46Phe, led to a similar
increase in the 42/40 ratio (3.3- versus 3.6-fold). The p3 fragment, p
roduced by alpha- and gamma-secretase, showed 42/40 ratios similar to
A beta when derived from wild-type and mutant proteins, These results
suggest that the different A beta- and p3-species are generated by gam
ma-cleavage activities with a similar enzymatic mechanism.