Familial Alzheimer mutations stabilize synaptotoxic γ-secretase-substrate complexes.

Area of Science:

• Neuroscience
• Molecular Biology
• Biochemistry

Background:

• Familial Alzheimer's disease (FAD) is linked to mutations in amyloid precursor protein (APP) and presenilin, affecting amyloid-beta peptide (Aβ) production.
• The precise role of Aβ as the primary driver of FAD remains debated, prompting investigation into alternative pathogenic mechanisms.

Purpose of the Study:

• To investigate how FAD mutations impact the initial proteolytic steps in the γ-secretase-mediated processing of the APP substrate C99.
• To determine whether the accumulation of Aβ or the disruption of the enzymatic process itself is responsible for FAD pathogenesis.

Main Methods:

• Utilized cryoelectron microscopy to capture the transition state of γ-secretase bound to an APP C99 substrate mimetic.
• Employed molecular dynamics simulations and in silico modeling to analyze enzyme-substrate interactions.
• Conducted in cellulo fluorescence microscopy and neuronal expression studies in Caenorhabditis elegans to assess synaptic loss.

Main Results:

• FAD mutations were found to disrupt the initial proteolytic events in C99 processing by γ-secretase.
• Cryoelectron microscopy revealed a stabilized enzyme-substrate complex structure, further supported by molecular dynamics simulations.
• Neuronal expression of FAD-mutant C99 or presenilin-1 in C. elegans induced synaptic loss, which was also observed with mutations stabilizing the enzyme-substrate complex and blocking Aβ production.

Conclusions:

• The findings implicate the stalled process of γ-secretase cleavage, rather than the resulting Aβ products, as the key factor in FAD pathogenesis.
• Disruption of the enzymatic processing of APP substrates by FAD-mutant γ-secretase is a critical mechanism underlying synaptic dysfunction in Alzheimer's disease.