Alzheimer's-Causing Mutations Shift Aß Length by Destabilizing γ-Secretase-Aßn Interactions.

Alzheimer's disease (AD)-linked mutations in Presenilins (PSEN) and the amyloid precursor protein (APP) lead to production of longer amyloidogenic Aß peptides. The shift in Aß length is fundamental to the disease; however, the underlying mechanism remains elusive. Here, we show that substrate shortening progressively destabilizes the consecutive enzyme-substrate (E-S) complexes that characterize the sequential γ-secretase processing of APP. Remarkably, pathogenic PSEN or APP mutations further destabilize labile E-S complexes and thereby promote generation of longer Aß peptides. Similarly, destabilization of wild-type E-S complexes by temperature, compounds, or detergent promotes release of amyloidogenic Aß. In contrast, E-Aßn stabilizers increase γ-secretase processivity. Our work presents a unifying model for how PSEN or APP mutations enhance amyloidogenic Aß production, suggests that environmental factors may increase AD risk, and provides the theoretical basis for the development of γ-secretase/substrate stabilizing compounds for the prevention of AD.

Deletion of exons 9 and 10 of the Presenilin 1 gene in a patient with Early-onset Alzheimer Disease generates longer amyloid seeds.

Presenilin 1 (PSEN1) mutations are the main cause of autosomal dominant Early-onset Alzheimer Disease (EOAD). Among them, deletions of exon 9 have been reported to be associated with a phenotype of spastic paraparesis. Using exome data from a large sample of 522 EOAD cases and 584 controls to search for genomic copy-number variations (CNVs), we report here a novel partial, in-frame deletion of PSEN1, removing both exons 9 and 10. The patient presented with memory impairment associated with spastic paraparesis, both starting from the age of 56years. He presented a positive family history of EOAD. We performed functional analysis to elucidate the impact of this novel deletion on PSEN1 activity as part of the γ-secretase complex. The deletion does not affect the assembly of a mature protease complex but has an extreme impact on its global endopeptidase activity. The mutant carboxypeptidase-like activity is also strongly impaired and the deleterious mutant effect leads to an incomplete digestion of long Aß peptides and enhances the production of Aß43, which has been shown to be potently amyloidogenic and neurotoxic in vivo.

Familial Alzheimer's Disease Mutations in Presenilin Generate Amyloidogenic Aß Peptide Seeds.

Recently it was proposed that the familial Alzheimer's disease (FAD) causing presenilin (PSEN) mutations PSEN1-L435F and PSEN1-C410Y do not support the generation of Aß-peptides from the amyloid precursor protein (APP). This challenges the amyloid hypothesis and disagrees with previous work showing that PSEN1 FAD causing mutations generate invariably long Aß and seed amyloid. We contrast here the proteolytic activities of these mutant PSEN alleles with the complete loss-of-function PSEN1-D257A allele. We find residual carboxy- and endo-peptidase γ-secretase activities, similar to the formerly characterized PSEN1-R278I. We conclude that the PSEN1-L435F and -C410Y mutations are extreme examples of the previously proposed "dysfunction" of γ-secretase that characterizes FAD-associated PSEN. This Matters Arising paper is in response to Xia et al. (2015), published in Neuron. See also the response by Xia et al. (2016), published in this issue.

Qualitative changes in human γ-secretase underlie familial Alzheimer's disease.

Presenilin (PSEN) pathogenic mutations cause familial Alzheimer's disease (AD [FAD]) in an autosomal-dominant manner. The extent to which the healthy and diseased alleles influence each other to cause neurodegeneration remains unclear. In this study, we assessed γ-secretase activity in brain samples from 15 nondemented subjects, 22 FAD patients harboring nine different mutations in PSEN1, and 11 sporadic AD (SAD) patients. FAD and control brain samples had similar overall γ-secretase activity levels, and therefore, loss of overall (endopeptidase) γ-secretase function cannot be an essential part of the pathogenic mechanism. In contrast, impaired carboxypeptidase-like activity (γ-secretase dysfunction) is a constant feature in all FAD brains. Significantly, we demonstrate that pharmacological activation of the carboxypeptidase-like γ-secretase activity with γ-secretase modulators alleviates the mutant PSEN pathogenic effects. Most SAD cases display normal endo- and carboxypeptidase-like γ-secretase activities. However and interestingly, a few SAD patient samples display γ-secretase dysfunction, suggesting that γ-secretase may play a role in some SAD cases. In conclusion, our study highlights qualitative shifts in amyloid-ß (Aß) profiles as the common denominator in FAD and supports a model in which the healthy allele contributes with normal Aß products and the diseased allele generates longer aggregation-prone peptides that act as seeds inducing toxic amyloid conformations.

The dynamic conformational landscape of gamma-secretase.

The structure and function of the gamma-secretase proteases are of great interest because of their crucial roles in cellular and disease processes. We established a novel purification protocol for the gamma-secretase complex that involves a conformation- and complex-specific nanobody, yielding highly pure and active enzyme. Using single particle electron microscopy, we analyzed the gamma-secretase structure and its conformational variability. Under steady-state conditions, the complex adopts three major conformations, which differ in overall compactness and relative position of the nicastrin ectodomain. Occupancy of the active or substrate-binding sites by inhibitors differentially stabilizes subpopulations of particles with compact conformations, whereas a mutation linked to familial Alzheimer disease results in enrichment of extended-conformation complexes with increased flexibility. Our study presents the csecretase complex as a dynamic population of interconverting conformations, involving rearrangements at the nanometer scale and a high level of structural interdependence between subunits. The fact that protease inhibition or clinical mutations, which affect amyloid beta (Abeta) generation, enrich for particular subpopulations of conformers indicates the functional relevance of the observed dynamic changes, which are likely to be instrumental for highly allosteric behavior of the enzyme.