Bad Targets the Permeability Transition Pore Independent of Bax or Bak to Switch between Ca²⁺-Dependent Cell Survival and Death

Calcium oscillations exert physiological control on mitochondrial energy metabolism and can also lead to mitochondrial membrane permeabilization and cell death. The outcome of the mitochondrial calcium signaling is altered by stress factors such as ceramide or staurosporine. However, the mechanism of this proapoptotic switch remains unclear. Using genetic, biochemical, pharmacological, and functional approaches, we here show that ceramide and staurosporine target PP2A and protein kinases A and C, respectively, in a mitochondria-associated signaling complex to induce dephosphorylation of the BH3-only protein Bad. Dephosphorylated Bad sensitizes the mitochondrial permeability transition pore (PTP) to Ca²⁺ through a Bcl-xL-sensitive and VDAC-mediated process. Furthermore, the Bad-induced sensitization of the PTP to Ca²⁺ does not require Bax or Bak. Thus, phospho-regulatory mechanisms converge on Bad to switch between the survival and apoptotic functions of mitochondrial calcium signaling by activating a mechanism whereby a BH3-only protein bypasses Bax/Bak and engages the PTP.