The dual roles of apoptosis-inducing factor (AIF) in neuronal life and death

Abstract

Development of drugs to rescue neuronal cell death during acute neuronal injury such as stroke is challenging because of the multiple neuronal death pathways. Apart from the caspase pathway, neurons can also die in the absence of caspases. One of the key factors in the caspase independent pathway is the mitochondrial protein Apoptosis-inducing factor (AIF) which is released and translocated to the nucleus during cell death. In this thesis, I have first demonstrated that AIF is a critical factor in both DNA damage induced Bax dependent neuronal cell death and Bax independent excitotoxic cell death using the Hq mice that have reduced AIF expression. These results are the first to directly show the role of AIF in acute neuronal injury using both in vivo and in vitro models. Recent studies have also revealed an unexpected role of AIF in the mitochondria to maintain survival. It is therefore possible that after cell death induction, the release and subsequent loss of AIF from the mitochondria alone, but not its pro-apoptotic activity, is enough to kill the cell. To resolve this controversy, I have designed an AIF mutant construct that is mitochondrially anchored and cannot be released during cell death. The expression of this construct cannot protect cells from dying in the presence of the wildtype AIF that is able to translocate to the nucleus. This indicates that even when AIF is preserved in the mitochondria, the apoptotic activity of AIF can still kill the cell. This is the first definitive evidence that AIF is indeed pro-apoptotic during cell death, in addition to its survival role in healthy cells. AIF has suggested to be a reactive oxygen species (ROS) scavenger or to stabilize mitochondrial complex I, however, the exact role is still unclear. Using the forebrain specific AIF knockout animals, I have showed that rather than scavenging ROS, AIF may instead regulate mitochondrial structure and function through the mitochondrial fusion protein Opal. Taken together, this thesis illustrates the mechanisms of AIF in neuronal cell life and death, which are essential for the design of AIF as a drug target for acute neuronal injury such as stroke.