The dentate gyrus of the hippocampus is one of the few regions of the mammalian brain where new neurons are generated throughout adulthood. This adult neurogenesis has been proposed as a novel mechanism that mediates spatial memory. However, data showing a causal relationship between neurogenesis and spatial memory are controversial. Here, we developed an inducible transgenic strategy allowing specific ablation of adult-born hippocampal neurons. This resulted in an impairment of spatial relational memory, which supports a capacity for flexible, inferential memory expression. In contrast, less complex forms of spatial knowledge were unaltered. These findings demonstrate that adult-born neurons are necessary for complex forms of hippocampus-mediated learning.

Mitochondria are key players of apoptosis and can irreversibly commit the cell to death by releasing cytochrome c (Cyt.c) to the cytosol, where caspases 9 and 3 subsequently get activated. Under conditions of oxidative stress, opening of the mitochondrial permeability transition pore (PTP) represents an early trigger and is crucial in causing Cyt.c release. To account for the latter, current models propose that PTP gating would result, as is the case in vitro , in the rupture of the outer mitochondrial membrane caused by mitochondrial matrix swelling. Using live cell imaging and recombinant fluorescent probes based on the green fluorescent protein (GFP) and its mutants, we report that directed repetitive gating of the PTP triggers a delayed Cyt.c efflux, which is not associated with mitochondrial swelling. Instead, subcellular imaging shows that PTP opening signals the redistribution of the cytosolic protein Bax to the mitochondria, where it secondarily forms clusters that appear to be a prerequisite for Cyt.c release. Fluorescence resonance energy transfer imaging further reveals that Bax clustering coincides with the formation of Bax multimers. We conclude that the PTP is not itself a component of the Cyt.c release machinery, but that it acts indirectly by signaling Bax translocation and multimerization.