Mitochondrial dysfunction and release of pro-apoptotic factors such as cytochrome c or apoptosis-inducing factor (AIF) from mitochondria are key features of neuronal cell death. The precise mechanisms of how these proteins are released from mitochondria and their particular role in neuronal cell death signaling are however largely unknown. Here, we demonstrate by fluorescence video microscopy that 8-10 h after induction of glutamate toxicity, AIF rapidly translocates from mitochondria to the nucleus and induces nuclear fragmentation and cell death within only a few minutes. This markedly fast translocation of AIF to the nucleus is preceded by increasing translocation of the pro-apoptotic bcl-2 family member Bid (BH3-interacting domain death agonist) to mitochondria, perinuclear accumulation of Bid-loaded mitochondria, and loss of mitochondrial membrane integrity. A small molecule Bid inhibitor preserved mitochondrial membrane potential, prevented nuclear translocation of AIF, and abrogated glutamate-induced neuronal cell death, as shown by experiments using Bid small interfering RNA (siRNA). Cell death induced by truncated Bid was inhibited by AIF siRNA, indicating that caspase-independent AIF signaling is the main pathway through which Bid mediates cell death. This was further supported by experiments showing that although caspase-3 was activated, specific caspase-3 inhibition did not protect neuronal cells against glutamate toxicity. In conclusion, Bid-mediated mitochondrial release of AIF followed by rapid nuclear translocation is a major mechanism of glutamate-induced neuronal death. Progressive degeneration and death of neurons are the major features of several acute and chronic neurodegenerative diseases such as ischemic stroke, Alzheimer's disease, or Parkinson's disease. 1 The main mechanisms of neuronal cell death are, for example, disturbed calcium homeostasis, oxidative stress, breakdown of the mitochondrial membrane potential, and release of mitochondrial factors that initiate downstream apoptotic cell death programs.2 In particular, mitochondrial membrane permeabilization is considered as a critical step for the release of pro-apoptotic proteins such as cytochrome c, Smac/Diablo (second mitochondria-derived activator of caspase/direct IAP binding protein with low pI), HtrA2/Omi, apoptosis-inducing factor (AIF), or endonuclease G, which trigger caspase-dependent or caspase-independent mechanisms of DNA degradation and cell death. 3,4 An increasing number of recent studies provide evidence that AIF is a major factor for an alternative post-mitochondrial cell death pathway, for example, following hypoxia, 5,6 ischemia, 7,8 or excitotoxic lesions. 9,10 AIF is a 63 kDa flavoprotein located at the inner mitochondrial membrane that is released early after oxygen-glucose deprivation in vitro or cerebral ischemia in vivo.7 Using harlequin (Hq) mutant mice expressing low AIF levels and small interfering RNA (siRNA) approaches, we recently demonstrated a causal role of AIF in neuronal cell death in models of i...