Full Length Bid is sufficient to induce apoptosis of cultured rat hippocampal neurons

Background: Bcl-2 homology domain (BH) 3-only proteins are pro-apoptotic proteins of the Bcl2 family that couple stress signals to the mitochondrial cell death pathways. The BH3- only protein Bid can be activated in response to death receptor activation via caspase 8-mediated cleavage into a truncated protein ( tBid), which subsequently translocates to mitochondria and induces the release of cytochrome- C. Using a single- cell imaging approach of Bid cleavage and translocation during apoptosis, we have recently demonstrated that, in contrast to death receptor- induced apoptosis, caspase- independent excitotoxic apoptosis involves a translocation of full length Bid ( FLBid) from the cytosol to mitochondria. We induced a delayed excitotoxic cell death in cultured rat hippocampal neurons by a 5- min exposure to the glutamate receptor agonist N- methyl- Daspartate ( NMDA; 300 mu M). Results: Western blot experiments confirmed a translocation of FL-Bid to the mitochondria during excitotoxic apoptosis that was associated with the release of cytochrome- C from mitochondria. These results were confirmed by immunofluorescence analysis of Bid translocation during excitotoxic cell death using an antibody raised against the amino acids 1 - 58 of mouse Bid that is not able to detect tBid. Finally, inducible overexpression of FL- Bid or a Bid mutant that can not be cleaved by caspase- 8 was sufficient to induce apoptosis in the hippocampal neuron cultures. Conclusion: Our data suggest that translocation of FL-Bid is sufficient for the activation of mitochondrial cell death pathways in response to glutamate receptor overactivation.