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New migratory targets in the postnatal brain for neuroblasts generated in the sub-ventricular zone: molecular and functional characterization

Final Activity Report Summary - NEURONAL MIGRATION (New migratory targets in the postnatal brain for neuroblasts generated in the subventricular zone: molecular and functional characterization)

Mammalian neurogenesis occurs primarily during embryonic development. Throughout postnatal development and adulthood, neurogenesis persists in only two regions, the subventricular zone (SVZ) of the lateral ventricles and the subgranular zone of the hippocampus. New neurons generated in the SVZ migrate to the olfactory bulb (OB) through the rostral migratory stream (RMS) where they functionally integrate into pre-existing neuronal networks.

In Prof. Monyer’s lab a transgenic mouse expressing the in vivo marker enhanced green fluorescent protein (EGFP) under the control of the 5-HT3 receptor promoter was generated. In these mice, immature neuroblasts arising from the SVZ were labelled, allowing for the visualisation of the migration patterns followed by newborn interneurons. In a first manuscript, published in 2008, we used BrdU injections and retroviral labelling to show that, during the first month after birth, numerous neuroblasts born in the SVZ exited the RMS and migrated not only to the OB but also to the cortex, where they developed into mature GABAergic interneurons. In a second manuscript, which was under preparation by the time of the project completion, we performed patch-clamp recordings from cortical postnatally-born interneurons at different stages of maturation. We found that the majority of these cells developed into small axonless interneurons, a distinct new subtype of cortical interneurons endowed with unique physiological and morphological properties. These cells got functionally integrated into pre-established neocortical networks.

In the light of the role played by postnatal neurogenesis in the olfactory bulb and the dentate gyrus, postnatally-born cortical neurons might contribute to information processing in the cortex. As a parallel project within this research line, we aimed to identify novel factors involved in neuroblast migration. Starting from an messenger ribonucleic acid (mRNA) profile of migrating neuroblasts that was performed in the lab, we selected a group of candidate genes based on their expression levels. With the results from this screen we decided to pursue in vivo experiments for the selected genes. We used retroviral vectors to over-express or knock down the expression levels of the specific sequences in living animals, along with time-lapse imaging. We found that a highly expressed sequence in the RMS that encoded for the gene known as diazepam binding inhibitor (DBI) affected neuroblast migration. The mechanism of action of this novel role for DBI was currently under investigation by the time of elaboration of this final report.