Organelle-specific initiation of cell death pathways

Nuclear DNA damage and ligation of plasma-membrane death receptors have long been recognized as initial triggers of apoptosis that induce mitochondrial membrane permeabilization (MMP) and/or the direct activation of caspases. Accumulating evidence suggests that other organelles, including the endoplasmic reticulum (ER), lysosomes and the Golgi apparatus, are also major points of integration of pro-apoptotic signalling or damage sensing. Each organelle possesses sensors that detect specific alterations, locally activates signal transduction pathways and emits signals that ensure inter-organellar cross-talk. The ER senses local stress through chaperones, Ca2+-binding proteins and Ca2+ release channels, which might transmit ER Ca2+ responses to mitochondria. The ER also contains several Bcl-2-binding proteins, and Bcl-2 has been reported to exert part of its cytoprotective effect within the ER. Upon membrane destabilization, lysosomes release cathepsins that are endowed with the capacity of triggering MMP. The Golgi apparatus constitutes a privileged site for the generation of the pro-apoptotic mediator ganglioside GD3, facilitates local caspase-2 activation and might serve as a storage organelle for latent death receptors. Intriguingly, most organelle-specific death responses finally lead to either MMP or caspase activation, both of which might function as central integrators of the death pathway, thereby streamlining lysosome-, Golgi- or ER-elicited responses into a common pathway.