Tbr2-expressing intermediate progenitor cells in the adult mouse hippocampus are unipotent neuronal precursors with limited amplification capacity under homeostasis

Neurogenesis persists in two locations of the adult mammalian brain, the subventricular zone of the lateral ventricles and the subgranular zone of the dentate gyrus in the hippocampus. In the adult subgranular zone, radial gliallike cells (RGLs) are multipotent stem cells that can give rise to both astrocytes and neurons. In the process of generating neurons, RGLs divide asymmetrically to give rise to one RGL and one intermediate progenitor cell (IPC). IPCs are considered to be a population of transit amplifying cells that proliferate and eventually give rise to mature granule neurons. The properties of individual IPCs at the clonal level are not well understood. Furthermore, it is not clear whether IPCs can generate astrocytes or revert back to RGLs, besides generating neurons. Here we developed a genetic marking strategy for clonal analysis and lineage-tracing of individual Tbr2-expressing IPCs in the adult hippocampus in vivo using Tbr2-CreER^T2 mice. Using this technique we identified Tbr2-CreER^T2 labeled IPCs as unipotent neuronal precursors that do not generate astrocytes or RGLs under homeostasis. Additionally, we showed that these labeled IPCs rapidly generate immature neurons in a synchronous manner and do not undergo a significant amount of amplification under homeostasis, in animals subjected to an enriched environment/running, or in animals with different age. In summary, our study suggests that Tbr2-expressing IPCs in the adult mouse hippocampus are unipotent precursors and rapidly give rise to immature neurons without major amplification.