The mechanism of γ-Secretase dysfunction in familial Alzheimer diseaseMutations in presenilin (PSEN) and amyloid precursor protein (APP) cause dominant early-onset Alzheimer's disease (AD), but the mechanism involved is debated. Here, such mutations are shown to alter γ-secretase activity, leading to changes in Aβ peptide cleavage patterns.

Mutations in human presenilin (PS) genes cause aggressive forms of familial Alzheimer's disease. Presenilins are polytopic proteins that harbour the catalytic site of the c-secretase complex and cleave many type I transmembrane proteins including b-amyloid precursor protein (APP), Notch and syndecan 3. Contradictory results have been published concerning whether PS mutations cause 'abnormal' gain or (partial) loss of function of c-secretase. To avoid the possibility that wild-type PS confounds the interpretation of the results, we used presenilin-deficient cells to analyse the effects of different clinical mutations on APP, Notch, syndecan 3 and N-cadherin substrate processing, and on c-secretase complex formation. A loss in APP and Notch substrate processing at e and S3 cleavage sites was observed with all presenilin mutants, whereas APP processing at the c site was affected in variable ways. PS1-D9 and PS1-L166P mutations caused a reduction in b-amyloid peptide (Ab) 40 production whereas PS1-G384A mutant significantly increased Ab 42 . Interestingly PS2, a close homologue of PS1, appeared to be a less efficient producer of Ab than PS1. Finally, subtle differences in c-secretase complex assembly were observed. Overall, our results indicate that the different mutations in PS affect c-secretase structure or function in multiple ways.

Human lumbar CSF patterns of Ab peptides were analysed by urea-based b-amyloid sodium dodecyl sulphate polyacrylamide gel electrophoresis with western immunoblot (Ab-SDS-PAGE/immunoblot). A highly conserved pattern of carboxyterminally truncated Ab1-37/38/39 was found in addition to Ab1-40 and Ab1-42. Remarkably, Ab1-38 was present at a higher concentration than Ab1-42, being the second prominent Ab peptide species in CSF. Patients with Alzheimer's disease (AD, n ¼ 12) and patients with chronic inflammatory CNS disease (CID, n ¼ 10) were differentiated by unique CSF Ab peptide patterns from patients with other neuropsychiatric diseases (OND, n ¼ 37). This became evident only when we investigated the amount of Ab peptides relative to their total Ab peptide concentration (Ab1-x%, fractional Ab peptide pattern), which may reflect diseasespecific c-secretase activities. Remarkably, patients with AD and CID shared elevated Ab1-38% values, whereas otherwise the patterns were distinct, allowing separation of AD from CID or OND patients without overlap. The presence of one or two ApoE e4 alleles resulted in an overall reduction of CSF Ab peptides, which was pronounced for Ab1-42. The severity of dementia was significantly correlated to the fractional Ab peptide pattern but not to the absolute Ab peptide concentrations. Keywords: Alzheimer's disease (AD), b-amyloid protein precursor/metabolism, biological markers, cerebrospinal fluid, 2D-PAGE, western immunoblot.

The γ-secretase complex plays a role in Alzheimer’s disease (AD) and cancer progression. The development of clinical useful inhibitors, however, is complicated by the role of the γ-secretase complex in regulated intramembrane proteolysis of Notch and other essential proteins. Different γ-secretase complexes containing different Presenilin or Aph1 protein subunits are present in various tissues. Here we show that these complexes have heterogeneous biochemical and physiological properties. Specific inactivation of the Aph1B γ-secretase in a murine Alzheimer’s disease model led to improvements of Alzheimer’s disease-relevant phenotypic features without any Notch-related side effects. The Aph1B complex contributes to total γ-secretase activity in the human brain, thus specific targeting of Aph1B-containing γ-secretase complexes may be helpful in generating less toxic therapies for Alzheimer’s disease.