Regulation of hippocampal progenitor cell survival, proliferation and dendritic development by BDNF

Abstract

Background: Environmental enrichment (EE) is known to enhance BDNF levels and neurogenesis in the adult hippocampus. To examine the role of BDNF in modulating EE-mediated adult hippocampal neurogenesis, we conditionally ablated BDNF expression in the hippocampus (cKO mice) and have assessed proliferation, survival, differentiation and dendritic development of hippocampal progenitors.

Results: We show that while the extent of cell proliferation and neuronal fate differentiation in the hippocampus of cKO mice is not different from wild-type (WT) littermates maintained in either standard or enriched conditions, reduced BDNF levels significantly impaired the survival of newborn cells in both housing conditions. In addition, while highly active enriched WT mice exhibited a robust increase in progenitor cell proliferation, highly active cKO mice showed a modest increase in cell proliferation compared to standard housed or underactive cKO mice.

Conclusions: There results argue that while BDNF plays a role in exercise-induced cell proliferation, other factors must contribute to this phenomenon. We also show that dendritic development was impaired in cKO mice maintained in standard housing conditions, and that EE rescued this phenotype.

Figure 1

cKO mice show reduced hippocampal BDNF levels. A, Quantification of BDNF protein levels (Mean ± SEM, pg/mg). 5-13 animals/group. *, P < 0.05; **, P < 0.01. B, Levels of hippocampal BDNF after sub-grouping the EE groups into low-activity and high-activity groups. *, P < 0.05; **, P < 0.01.

Figure 2

cKO mice are not obese, anxiety-prone nor hyperactive. cKO mice are not obese, anxiety-prone nor hyperactive. A, Body weights in WT and cKO mice. 7-13 animals/group. *, P < 0.05. B, Locomotor activity in WT and cKO mice. 7 animals/group. C-D, Total number of bites (C) and first bite latency (D) in the resident intruder paradigm. 7 animals/group.

Figure 3

Reduced BDNF levels affect cell survival and exercise-mediated cell proliferation. A-D, Photomicrographs of BrdU⁺ cells 1 day after BrdU injection in the DG of WT (A and B) and cKO (C and D). The mice were maintained in either standard (A and C) or enriched conditions (B and D). Green or yellow, BrdU⁺ cells; Red, NeuN⁺ cells. Scale bar: 100 μm. E, Quantification of BrdU⁺ cells (Mean ± SEM; 6-12 animals/group). *, P < 0.05; **, P < 0.01. F, Quantification of BrdU⁺ cells after sub-grouping the EE groups into low-activity and high-activity groups. *, P < 0.05. G-H, Photomicrographs of Ki67⁺ cells in the DG of WT (G) and cKO-T50 (H) mice in standard condition. Scale bar: 100 μm. I, Quantification of Ki67⁺ cells in cKO-T50 and control littermates (Mean ± SEM; 8 animals/group).

Figure 4

Reduced BDNF levels do not affect neuronal fate differentiation. A-L, Confocal images of BrdU⁺ cells in slices of WT or cKO mice. The slices were triply stained for the BrdU (A, E and I), NeuN (B, F and J), and GFAP (C, G and K). The merged images of the three labels (D, H and L) demonstrate cells with neuronal (A-D) or glial (E-H) properties or neither of them (I-L). Arrows indicate the position of BrdU⁺ cells. Scale bar: 20 μm. M, Percentage of BrdU⁺ cells colabeled with NeuN or GFAP, or neither of them. 6-9 animals/group.

Figure 5

Dendritic development is impaired in cKO mice in standard condition, but not in enriched condition. A-F, Photomicrographs of DCX⁺ cells in the DG of WT (A, B and C) and cKO (D, E and F). The mice were maintained in either standard conditions (A, B, D and E) or enriched conditions (C and F). Scale bar: 80 μm in A or 20 μm in B. G-H, Quantification of total dendritic length (G) and dendritic complexity (H) of DCX⁺ cells in WT and cKO mice (Mean ± SEM; 6 animals/group). *, P < 0.05.