After mossy fiber elimination (P25), the number of BrdU-positive cells in DG-A::TeTxLC-tau-lacZ mice was significantly decreased (Figure 4E). Therefore, TeTxLC-expressing inactive DG neurons are eventually eliminated after axon retraction, which explains diminished lacZ signals from the whole hippocampus at P25 and P30 in DG::TeTxLC-tau-lacZ mice (Figures 3F and 3G). The result that TeTxLC-expressing DG axons were eliminated

between P15 and P25 in DG-A::TeTxLC-tau-lacZ mice, in which almost all mature DG neurons are inactivated (Figures 3G, 3H, and 4B), implies that either (1) DG axons are refined in CA3 by mechanisms other than activity-dependent competition, or (2) axons of mature DGCs compete with an additional neuronal population. To distinguish Bcl-2 inhibitor between these two possibilities, we globally suppressed neural activity by administering TTX into the hippocampus of DG-A::TeTxLC-tau-lacZ mice and examined DG axon elimination. We applied TTX by implanting TTX-containing

Elvax (Echegoyen et al., 2007) on the hippocampus at P15 (Figure S3A) and prepared horizontal sections at P23 (8 days total of TTX application). TTX applications significantly inhibited the elimination of inactive DG axons in DG-A::TeTxLC-tau-lacZ mice (Figure 5B), GSK-3 activity as quantified in Figure 5C (see Figure S3B for methods). Further quantitative analysis revealed that relative to P15 brains, the staining intensities at P23 were 94% in DG-A::tau-lacZ (no TeTxLC) mice, 27% in PBS-treated DG-A::TeTxLC-tau-lacZ mice, and 70% in TTX-treated DG-A::TeTxLC-tau-lacZ mice (Figure S3C). These results indicate that TTX effectively inhibited the elimination of TeTxLC-expressing DG axons in DG-A::TeTxLC-tau-lacZ mice. Therefore, the elimination of TeTxLC-expressing axons in DG-A::TeTxLC-tau-lacZ

mice, in which the vast majority of mature DGCs express the transgene, is largely the outcome much of activity-dependent competition. To identify axons that compete with TeTxLC-expressing axons in DG-A::TeTxLC-tau-lacZ mice, we characterized tTA-expressing neurons in the DG-A line. In the subgranular zone (SGZ) of the DG, neurons are continuously generated throughout life (Gage, 2000, Lie et al., 2004 and Ming and Song, 2005). In DG-A mice, all tTA-expressing neurons were NeuN-positive mature neurons (Figure 3B) and not Ki67-positive dividing neural progenitors in the SGZ (Kee et al., 2002) (Figure 6A). In addition, almost all doublecortin (DCX)-positive immature neurons located adjacent to the SGZ (Kempermann et al., 2003) or calretinin-positive young DGCs (Brandt et al., 2003, Kempermann et al., 2004, Ming and Song, 2005 and Li et al., 2009) failed to express tTA (Figures 6B and 6C; 88.8% ± 0.12% of calretinin-positive DGCs do not express tTA). These results raise the possibility that TeTxLC-expressing axons in DG-A::TeTxLC-tau-lacZ mice, which are of mature DGCs, might be competing with axons of young, DCX/calretinin-positive DG neurons during refinement.