Acidic pH Greatly Enhances Calbindin-D28K's Inhibitory Effect on Caspase-3: Implications for Alzheimer's Therapeutics.

In Alzheimer's disease (AD), the interaction between many AD-relevant proteins, including amyloid precursor protein (APP) and tau, with the cysteine protease caspase-3 (Cas3) enhances AD pathogenesis. Searching for effective Cas3 inhibitors, we turned to the sensor calbindin-D28K (D28K) which specifically inhibits Cas3. The specific D28K conformation that elicits Cas3 inhibition remains elusive. To shed more light on this D28K conformation, we investigated likely environmental regulators. Recently, evidence of pH imbalance in aging brains has been implicated in AD pathologies. Using pull-down assays, Cas3 inhibition assays, and molecular docking, we identified pH as a critical regulator for tuning the strength of D28K's inhibitory actions and subsequently identified Loop4 in Cas3 as the structural element targeted by D28K.