Apoptosis-inducing factor is a key factor in neuronal cell death propagated by BAX-dependent and BAX-independent mechanisms.

Mitochondria release proteins that propagate both caspase-dependent and caspase-independent cell death pathways. AIF (apoptosis-inducing factor) is an important caspase-independent death regulator in multiple neuronal injury pathways. Presently, there is considerable controversy as to whether AIF is neuroprotective or proapoptotic in neuronal injury, such as oxidative stress or excitotoxicity. To evaluate the role of AIF in BAX-dependent (DNA damage induced) and BAX-independent (excitotoxic) neuronal death, we used Harlequin (Hq) mice, which are hypomorphic for AIF. Neurons carrying double mutations for Hq/Apaf1-/- (apoptosis proteases-activating factor) are impaired in both caspase-dependent and AIF-mediated mitochondrial cell death pathways. These mutant cells exhibit extended neuroprotection against DNA damage, as well as glutamate-induced excitotoxicity. Specifically, AIF is involved in NMDA- and kainic acid- but not AMPA-induced excitotoxicity. In vivo excitotoxic studies using kainic acid-induced seizure showed that Hq mice had significantly less hippocampal damage than wild-type littermates. Our results demonstrate an important role for AIF in both BAX-dependent and BAX-independent mechanisms of neuronal injury.

Oligodendrocyte progenitor enrichment in the connexin32 null-mutant mouse.

Before the establishment of chemical synapses, neural progenitors are often coupled by connexin-mediated gap junctions providing a robust mechanism for cell-cell communication in developing brain. The present study was undertaken to determine whether alterations in junctional coupling also affect neural progenitor proliferation, survival, and differentiation in adult brain. We localized the connexin32 gap junction protein to a subset of NG2+ and platelet-derived growth factor alpha receptor+ early oligodendrocyte progenitors in the dentate gyrus of adult mice. In connexin32-deficient mice, we found an increase in the total number of proliferating nestin+ and NG2+ progenitors in the subgranular zone, hilus, and polymorphonuclear layer of the dentate gyrus in vivo and in the total number of nestin+ progenitors capable of clonogenic expansion in vitro. By bromodeoxyuridine labeling, lineage analysis, and terminal deoxynucleotidyl nick end labeling, we demonstrate that turnover of these cells is constitutively enhanced in the connexin32 knock-out dentate gyrus reflecting a dynamic defect in oligodendrogenesis in this population. Analyses of surviving bromodeoxyuridine-labeled cells at 1, 3, 7, and 28 d after injection demonstrate that this transient amplifying population fails to terminally differentiate and is deleted by an apoptotic-like mechanism within 3 d of labeling. These data provide empirical evidence to support the hypothesis that connexin expression influences adult progenitor number and specifically implicate connexin32-mediated signaling in the activation, survival, and differentiation of a subset of early oligodendrocyte progenitors in postnatal brain.