Neural stem cells in the adult olfactory bulb core generate mature neurons in vivo.

Although previous studies suggest that neural stem cells (NSCs) exist in the adult olfactory bulb (OB), their location, identity and capacity to generate mature neurons in vivo has been little explored. Here, we injected EGFP-expressing retroviral particles into the OB core of adult mice to label dividing cells and to track the differentiation/maturation of any neurons they might generate. EGFP labelled cells initially expressed adult NSC markers on days 1-3 post injection (dpi), including Nestin, GLAST, Sox2, Prominin-1 and GFAP. EGFP+ -doublecortin (DCX) cells with a migratory morphology were also detected and their abundance increased over a 7-day period. Furthermore, EGFP-labeled cells progressively became NeuN+ neurons, they acquired neuronal morphologies and they became immunoreactive for OB neuron subtype markers, the most abundant representing calretinin expressing interneurons. OB-NSCs also generated glial cells, suggesting they could be multipotent in vivo. Significantly, the newly-generated neurons established and received synaptic contacts, and they expressed presynaptic proteins and the transcription factor pCREB. By contrast, when the retroviral particles were injected into the subventricular zone (SVZ), nearly all (98%) EGFP+ -cells were post-mitotic when they reached the OB core, implying that the vast majority of proliferating cells present in the OB are not derived from the SVZ. Furthermore, we detected slowly dividing label-retaining cells (LRCs) in this region that could correspond to the population of resident NSCs. This is the first time NSCs located in the adult OB core have been shown to generate neurons that incorporate into OB circuits in vivo. © AlphaMed Press 2021 SIGNIFICANCE STATEMENT: The adult olfactory bulb (OB) receives neurons originating from the subventricular zone (SVZ) and from the elbow of the rostral migratory stream (RMS) but it remains unclear whether local neural stem cells (NSCs) contribute to neurogenesis in the OB. Our results show for the first time that NSCs in the adult OB core predominantly give rise to calretinin-expressing interneurons. Notably, not only do the newly generated neurons establish synapses but they also express synaptic proteins and pCREB in vivo, indicating they become incorporated into the OB's synaptic circuits.