In contrast to a long held belief, the adult mammalian brain is capable of generating new neurons. It is now established that adult neurogenesis occurs in the subventricular zone of the lateral ventricles and the subgranular zone of the hippocampal dentate gyrus. As detection methods improve, it is likely that other areas of the brain will also be considered neurogenic.1 Studies have shown that a variety of extrinsic factors including exercise, environmental enrichment, and injury stimulate neurogenesis in the adult brain.2 The current challenge is to determine the signaling pathways that govern this intriguing biological phenomenon.
Previous studies showed that astrocytes release Wnt-3 to stimulate adult neurogenesis in vitro and in vivo, but the underlying molecular mechanism and downstream genetic targets have yet to be identified.3 Two recent complementary reports focused on the involvement of NeuroD1, a proneural basic helix-loop-helix (bHLH) transcription factor.4, 5 During development, NeuroD1 is critical for the generation of granule neurons in the hippocampus.6, 7, 8 In addition, NeuroD1 overexpression was shown to induce neuronal differentiation in neural progenitor cells isolated from adult rat hippocampus.9
To test the hypothesis that Wnt-mediated adult neurogenesis is dependent on NeuroD1, Kuwabara et al. studied the Neurod1 promoter sequence.4 Using in silico analysis, the authors identified a T cell factor/lymphoid enhancer factor (TCF/LEF) regulatory element. TCF/LEF is the major downstream transcription factor that transduces canonical Wnt/beta-catenin signaling. Kuwabara and colleagues discovered that this DNA sequence also contains an overlapping binding site for the HMG-box transcription factor, SOX2. This was an intriguing finding because SOX2 is known to prevent neurogenesis during development and is thought to be essential for maintaining neural stem cell populations in neonatal brain.10, 11, 12
||SOX/LEF Elements Regulate NeuroD1 Expression and Neurogenesis in the Adult Hippocampus. Neural stem cells in the subgranular zone of the hippocampal dentate gyrus differentiate toward mature granule neurons following the release of Wnt-3a from astrocytes. Recent data suggest that differentiation is dependent on a SOX/LEF dual site within the Neurod1 promoter sequence. Following the release of Wnt-3a, a SOX2/HDAC1 repressor complex is replaced by a beta-catenin/TCF/LEF activation complex, which leads to NeuroD1 expression and neural differentiation. Binding of beta-catenin/TCF/LEF also promotes the expression of LINE-1, a retrotransposon thought to be important for neuronal diversification.
Using reporter constructs, Kuwabara et al. conducted experiments in adult rat hippocampal neural stem cells.4 Their results showed that administration of Wnt-3a upregulated Neurod1 promoter activity, and this effect was dependent on a functional TCF/LEF binding site. The data also suggested that SOX2 and the histone deacetylase HDAC1 repressor protein associate with the Neurod1 promoter in undifferentiated cells. Following incubation of the cells with Wnt-3a, the SOX2/HDAC1 repressor complex was replaced by a beta-catenin/TCF/LEF activation complex, which induced NeuroD1 expression and neuronal differentiation. Collectively, these in vitro findings support the presence of dual regulatory elements (SOX/LEF sites) in the Neurod1 promoter that bi-directionally regulate adult neurogenesis in the hippocampus. This hypothesis was supported by a series of immunohistochemical, small interfering RNA (siRNA), and chromatin immunoprecipitation (ChIP) experiments.
To investigate the functional relevance of these in vitro findings, a complementary study designed in vivo experiments using a conditional genetic disruption model. Knock out of the Neurod1 gene results in mice that develop neonatal diabetes and die perinatally. To address this technical challenge, Gao et al. generated Neurod1 conditional knock out mice (NeuroD1 cKO) in which NeuroD1 expression was specifically ablated in the subgranular zone of the dentate gyrus in the hippocampus.5 Immunohistochemical analysis revealed no change in the number of total neural stem cells in this neurogenic region following deletion of Neurod1, suggesting that NeuroD1 is not required for the formation of early stem or progenitor cells. However, there was a selective loss of newborn granule neurons in NeuroD1 cKO mice, further supporting the hypothesis that NeuroD1 is functionally required for the formation of late stage progenitors and their differentiation into hippocampal granule neurons.
Additional studies by Kuwabara et al. found dual SOX/LEF sites in the Long Interspersed Element-1 (LINE-1) retrotransposon.4 Originally described as "jumping genes", retrotransposons are sequences of DNA that can replicate and reinsert into the genome at different positions.13 Although their physiological significance remains unclear, these elements are known to upregulate and retrotranspose during neurogenesis.14 Collectively, these recent studies suggest that dual SOX/LEF sites may represent a molecular switch that couples neural generation and diversification. Their discovery may present a novel pharmacological target to promote the generation of new neurons in response to disease or injury.
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