Processing of beta-amyloid precursor protein by cathepsin D

Area of Science:

• Biochemistry
• Neuroscience
• Proteomics

Background:

• Beta-amyloid (betaA4) formation from beta-amyloid precursor protein (betaAPP) involves secretase enzymes.
• Beta- and gamma-secretases, crucial for betaA4 production, remain unidentified.
• Cathepsin D is a candidate enzyme for gamma-secretase activity.

Purpose of the Study:

• To investigate the role of cathepsin D in betaAPP processing.
• To determine if cathepsin D functions as gamma-secretase.
• To identify cathepsin D cleavage sites on betaAPP substrates.

Main Methods:

• In vitro digestion of synthetic and recombinant betaAPP substrates with human cathepsin D.
• Liquid chromatography/mass spectrometry for precise cleavage site identification.
• Analysis of cathepsin D activity on native and denatured betaAPP substrates.

Main Results:

• Cathepsin D cleaved a synthetic peptide at Ala42-Thr43.
• Recombinant betaAPP substrates were cleaved at Leu49-Val50, Asp68-Ala69, and Phe93-Phe94.
• Cleavage at putative gamma-secretase sites (Val40-Ile41, Ala42-Thr43) was not observed.
• Denaturation of substrate revealed additional cleavage sites near the betaA4 amino terminus.
• Cathepsin D's cleavage pattern is influenced by betaAPP substrate structure.

Conclusions:

• Cathepsin D is unlikely to be the gamma-secretase responsible for generating amyloidogenic betaA4.
• Cathepsin D efficiently cleaves betaAPP at non-amyloidogenic sites.
• Cathepsin D may play a role in the general catabolism of betaAPP.