Single-cell quantification of Bax activation and mathematical modelling suggest pore formation on minimal mitochondrial Bax accumulation.

Mitochondrial outer membrane permeabilisation (MOMP) during apoptosis is triggered by the activation and oligomerisation of Bax and Bak, but a quantification of these processes in individual cells has not yet been performed. Single-cell imaging of Bax translocation and oligomerisation in Bax-deficient DU-145 cells expressing CFP-Bax and YFP-Bax revealed that both processes started only minutes before or concomitantly with MOMP, with the majority of Bax translocation and oligomerisation occurring downstream of MOMP. Quantification of YFP-Bax concentrations at mitochondria revealed an increase of only 1.8 + or - 1.5% at MOMP onset. This was increased to 11.2 + or - 3.6% in bak-silenced cells. These data suggested that Bax activation exceeded by far the quantities required for MOMP induction, and that minimal Bax or Bak activation may be sufficient to trigger rapid pore formation. In a cellular automaton modelling approach that incorporated the quantities and movement probabilities of Bax and its inhibitors, activators and enablers in the mitochondrial membrane, we could re-model rapid pore formation kinetics at submaximal Bax activation.