Hydrophilic loop 1 of Presenilin-1 and the APP GxxxG transmembrane motif regulate γ-secretase function in generating Alzheimer-causing Aβ peptides

gamma-Secretase is responsible for the proteolysis of amyloid precursor protein (APP) into amyloid-beta (A beta) peptides, which are centrally implicated in the pathogenesis of Alzheimer's disease (AD). The biochemical mechanism of how processing by gamma-secretase is regulated, especially as regards the interaction between enzyme and substrate, remains largely unknown. Here, mutagenesis reveals that the hydrophilic loop-1 (HL-1) of presenilin-1 (PS1) is critical for both gamma-secretase step-wise cleavages (processivity) and its allosteric modulation by heterocyclic gamma-modulatory compounds. Systematic mutagenesis of HL-1, including all of its familial AD mutations and additional engineered variants, and quantification of the resultant A beta products show that HL-1 is necessary for proper sequential gamma-secretase processivity. We identify Y106, L113, and Y115 in HL-1 as key targets for heterocyclic gamma-secretase modulators (GSMs) to stimulate processing of pathogenic A beta peptides. Further, we confirm that the GxxxG domain in the APP transmembrane region functions as a critical substrate motif for gamma-secretase processivity: a G29A substitution in APP-C99 mimics the beneficial effects of GSMs. Together, these findings provide a molecular basis for the structural regulation of gamma-processivity by enzyme and substrate, facilitating the rational design of new GSMs that lower AD-initiating amyloidogenic A beta peptides.

Automated summary

The hydrophilic loop-1 of presenilin-1 is crucial for the processivity of gamma-secretase and its modulation by heterocyclic gamma-secretase modulators. Mutations in key amino acids within HL-1 affect the production of pathogenic A beta peptides. The GxxxG motif in the APP transmembrane region is also essential for gamma-secretase processivity. Together, these findings provide insights for designing new GSMs to reduce the production of amyloidogenic A beta peptides in AD.