In times of energy depletion, a cell will attempt to maintain metabolic homeostasis and viability by degrading and recycling organelles and intracellular components and proteins in a process referred to as autophagy. However, if the energy depletion persists, the cell will be overwhelmed by the autophagic process and will succumb to autophagic cell death. This form of cell death has been implicated in cardiac remodeling during heart failure and damage during ischemic injury. Two proteins that have been previously shown to play a role in virtually every form of regulated cell death, including autophagy, are Bax and Bak. These effectors are responsible for cytochrome-c release during apoptosis and effect mitochondrial permeability transition pore opening during regulated necrosis. Although the expression of either Bax or Bak is required for autophagic cell death to occur, the role of Bax/Bak in this type of cell death is poorly understood, although the lysosome appears to be centrally involved. Here we show that Bax/Bak DKO MEFs subjected to several days of serum starvation contain intact lysosomes compared to WT MEFs. Furthermore, the acidity of the lysosomes in starved DKO MEFs is preserved compared with starved WT MEFs. Bax and Bak are both found in isolated lysosomal preparations and Bax targeted to the lysosome can completely restore autophagic cell death in DKO MEFs. Finally, although Bax oligomerization is required for apoptosis, it is not necessary for autophagic cell death, as DKO MEFs expressing an oligomerization defective mutant of Bax are still susceptible to this form of death, as monomeric Bax can still increase membrane permeability. In conclusion our results suggest that lysosomal membrane permeability through Bax or Bak is required for autophagic cell death to occur and without Bax or Bak the lysosomes remain intact where they can function as an energy source during times of nutrient deprivation.
Decision letter: Autophagic cell death is dependent on lysosomal membrane permeability through Bax and Bak