We found an important function for canonical Wnt/β-catenin signaling in balancing self-renewal of NSCs and neuronal differentiation in adult dentate gyrus. Our results (as summarized in Supplementary Fig. 15
) indicate that the Sox2 and TCF/LEF regulatory element in the Neurod1
promoter is critical for the transition from Sox2-mediated repression to Wnt/β-catenin-mediated activation, that the clear, dose-dependent activation of the Neurod1
promoter by the Wnt3a ligand is dependent on the Sox/LEF-binding site, that deletion of β-catenin leads to substantial loss of NeuroD1-positive cells, whereas the stem cell compartment remains intact in vivo
, and that Wnt/β-catenin–mediated neuronal differentiation is dependent on NeuroD1, at least in vitro
. Taken together, these findings indicate that the decreased adult neurogenesis observed in the Sox2–cre-gfp; Ctnnb1loxP/loxP
mice is probably a result of a failure of neuronal lineage commitment from Sox2-positive NSCs and of survival of Sox2- and NeuroD1-positive progenitor cells/neuroblasts.
During embryonic development, neurogenins function as pro-neural proteins that activate transcription of Neurod1
through E-protein binding sites in its promoter28–30
. However, the transcriptional/epigenetic mechanism that regulates NeuroD1 expression in the adult neurogenic niche is not clear. In the SGZ, hippocampal astrocyte–derived factors, such as Wnt proteins, signal to NSCs to promote adult neurogenesis. Among several Wnt proteins (Wnt3a, Wnt2a, Wnt5a, Wnt7a and Wnt8b)34–36
, Wnt3a has a dominant role in CNS development, as the deletion of Wnt3a (Wnt3a−/−
mice) results in the absence of dentate gyrus formation35
. Furthermore, it has been reported that β-catenin is involved in the dendritic development of newborn neurons37
. Both canonical4–8
Wnt/β-catenin signaling may contribute to the step-wise progression of adult hippocampal neurogenesis by removing Sox2 repression and turning on NeuroD1. Similar to a loss of Wnt/β-catenin signaling, NeuroD1 deficiency during hippocampal development leads to a complete loss of dentate gyrus formation in mice10,11
. In adult stages, when the hippocampal formation is fully developed, conditional deletion of β-catenin and NeuroD1 (ref. 15
) lead to a similar phenotype in dentate gyrus, that is, a decreased number of neuronal progenitors/newborn neurons, suggesting that the regulation of canonical Wnt/β-catenin signaling and Neurod1
gene expression are tightly linked and the functions of both β-catenin and NeuroD1 are indispensable for adult neurogenesis and for the survival of neuronal progenitors. Recently, it was shown that Wnt-mediated adult hippocampal neurogenesis contributed to learning and memory in rats38
. Our data suggest that Wnt signaling activation might be a major environmental factor that is relayed to the NSC genome for neuronal lineage commitment to modulate behavior.
Our finding that the Wnt-mediated regulatory mechanism is required for the activation of NeuroD1 can be broadly extended to the regulation of LINE-1. Because retro-element sequences are scattered throughout the genome and contain Sox/LEF DNA regulatory elements, one possibility is that Sox/LEF-binding sites act as bidirectional promoters and cause nearby neuronal gene loci to become de-silenced and activated during adult neurogenesis. Thus, to explore the possibility that LINE-1 sequences containing Sox2 and TCF/LEF sites might confer cell type–specific regulation as described for NeuroD1, we searched for LINE-1 sequences proximal to the transcriptional start sites of known protein-coding genes in human, mouse and rat genomes. We were able to identify 79, 84 and 25 such LINE-1 elements within −6,000 and +1,000 base pairs of the target human, mouse and rat genes, respectively (Supplementary Tables 1–3
). These bioinformatics analyses suggest that there is a global regulatory mechanism for controlling the activation/repression of neuronal gene expression that uses embedded retrotransposition sequences in the genome as a putative master regulatory pathway during adult neurogenesis. However, the causal relationship between LINE-1 sequences and transcriptional activation of nearby Sox/LEF-driven neuronal genes awaits future validation. Recently, it has been shown that environment is a robust stimulator of adult neurogenesis39
, possibly through the activation of the Sox/LEF regulatory elements described here. Consistent with these data, we recently observed that voluntary exercise increased LINE-1 retrotransposition in dentate gyrus40
. Our data provides a window into the molecular mechanism behind experience-dependent LINE-1 retrotransposition that may affect neuronal plasticity.