Modulation of amyloid precursor protein cleavage by gamma-secretase activating protein through phase separation

Aberrant cleavage of amyloid precursor protein (APP) by gamma-secretase is closely associated with Alzheimer's disease (AD). gamma-secretase activating protein (GSAP) specifically promotes gamma-secretase-mediated cleavage of APP. However, the underlying mechanism remains enigmatic. Here, we demonstrate that the 16-kDa C-terminal fragment of GSAP (GSAP-16K) undergoes phase separation in vitro and forms puncta-like condensates in cells. GSAP-16K exerts dual modulation on gamma-secretase cleavage; GSAP-16K in dilute phase increases APP-C-terminal 99-residue fragment (C99) cleavage toward preferred production of p-amyloid peptide 42 (A beta 42), but GSAP-16K condensates reduce APP-C99 cleavage through substrate sequestration. Notably, the A beta 42/A beta 40 ratio is markedly elevated with increasing concentrations of GSAP-16K. GSAP-16K stably associates with APP-C99 through specific sequence elements. These findings mechanistically explain GSAP-mediated modulation of gamma-secretase activity that may have ramifications on the development of potential therapeutics.