Basic Science and Pathogenesis.

Area of Science:

• Biochemistry
• Structural Biology
• Neuroscience

Background:

• Amyloid-beta (Ab) peptide deposition in the brain is a key feature of Alzheimer's disease.
• Gamma-secretase complexes (GSECs) generate Ab peptides through sequential proteolysis of amyloid precursor protein (APP).
• The length of Ab peptides, influenced by Presenilin (PSEN) and APH-1 subunits, is critical for Alzheimer's pathogenesis, yet GSEC's proteolytic mechanisms remain unclear.

Purpose of the Study:

• To elucidate the mechanistic details of Ab proteolysis by GSEC.
• To understand the role of the APH-1 subunit in modulating GSEC activity.
• To provide structural insights into GSEC-substrate interactions.

Main Methods:

• Reconstitution of human GSEC (PSEN1/APH-1B) into lipid nanodiscs.
• Cryo-electron microscopy (cryo-EM) to determine structures of apo GSEC and GSEC-Ab46 complex.
• Characterization of substrate interactions and enzyme rearrangements.

Main Results:

• A divergent loop in APH-1, along with PSEN1, mediates substrate-binding-induced rearrangements in the enzyme-substrate complex.
• The structure of the intermediate Ab46 substrate and its specific interactions with PSEN1 were characterized.
• Polar interactions, including a novel interaction with the PSEN1 loop1, were identified as stabilizing Ab during GSEC-mediated proteolysis.

Conclusions:

• These findings enhance the understanding of GSEC proteolytic mechanisms.
• The structural insights are crucial for developing targeted GSEC inhibitors and modulators.
• This research has implications for future therapies for Alzheimer's disease and cancer.