Dissecting caspase-2-mediated cell death: from intrinsic PIDDosome activation to chemical modulation

Caspase-2, a highly conserved member of the Caspase family, is considered an initiator Caspase that triggers Apoptosis in response to some cellular stresses. Previous studies suggest that an intracellular multi-protein complex PIDDosome, induced by genotoxic stress, serves as a platform for caspase-2 activation. However, due to caspase-2's inability to process effector caspases, the mechanism underlying caspase-2-mediated cell death upon PIDDosome activation remains unclear. Here we conducted an unbiased genome-wide genetic screen and identified that the Bcl2 family protein BID is required for PIDDosome-induced, caspase-2-mediated Apoptosis. PIDDosome-activated caspase-2 directly and functionally processes BID to signal the mitochondrial pathway for Apoptosis induction. Additionally, a designed chemical screen identified a compound, HUHS015, that specifically activates caspase-2-mediated Apoptosis. HUHS015-stimulated Apoptosis also requires BID but is independent of the PIDDosome. Through extensive structure-activity relationship efforts, we identified a derivative with a potency of ~ 60 nmol/L in activating caspase-2-mediated Apoptosis. The HUHS015-series of compounds act as efficient agonists that directly target the interdomain linker in caspase-2, representing a new mode of initiator Caspase activation. Human and mouse caspase-2 differ in two crucial residues in the linker, rendering a selectivity of the agonists for human caspase-2. The caspase-2 agonists are valuable tools to explore the physiological roles of caspase-2-mediated cell death and a base for developing small-molecule drugs for relevant diseases.