Molecular controls over cortical interneuron subtype migration

Abstract

The migration of GABAergic inhibitory interneurons plays a critical role in the assembly of cortical circuits, and alterations in this process are associated to psychiatric-relevant phenotypes. In mice about 70% of cortical interneurons are generated in the medial ganglionic eminences (MGE) of the subpallium, whereas about 30% originate from the caudal ganglionic eminences (CGE) and to a smaller extent in the preoptic area (PoA). The molecular mechanisms regulating the migration of MGE-derived interneurons have been the focus of many studies in the field, whereas little is know on molecular pathways controlling CGE-derived interneuron migration. In this work, I aimed to investigate the molecular controls over the migration of cortical interneurons (cINs) preferentially derived from the CGE. Using a microarray screen performed on CGE-derived INs, I found that the ionotropic serotonin receptor 3A (5-HT3AR) is transiently up-regulated in cINs from the CGE as they invade the cortical plate. Fate-mapping experiments revealed that the 5-HT3AR is specifically expressed in CGE but not MGE-derived INs. To assess the role of the 5-HT3AR in the migration of CGE INs, we used a combination of methods, including time-lapse imaging, calcium recordings and genetic manipulations. Functional investigations including calcium imaging and migration assays indicated that CGE INs increased their responsiveness to 5-HT3AR activation specifically during CP invasion. Genetic loss-of-function experiments combined to time-lapse imaging and in vivo grafts revealed a cell-autonomous requirement for the 5-HT3AR in regulating the migration of INs into the cortical plate. Altered migration into the CP due to 5-HT3AR deletion led to the persistent laminar mispositioning of a specific subtype of reelin-expressing interneurons. Interestingly this phenotype could be observed in a mouse model of serotonin depletion, the Tph2-KO mice. Finally, to identify 5-HT3AR-dependent down-stream genes regulating migration of cINs, I performed a microarray screen on CGE-derived cINs from Htr3a-KO mice. Using this strategy, I identified the guidance receptor PlexinA4 as a candidate gene that failed to normally up-regulate in Htr3a-KO cINs specifically during the phase of CP invasion. Functional in vivo studies provided evidence that PlexinA4 regulates the positioning of cINs. Taken together, these results reveal a mechanism whereby the time-specific upregulation of the 5-HT3AR is required for the migration and positioning of cINs into the developing cortex, and suggest that PlexinA4 could be a functional downstream target of the 5-HT3AR. Given the implication of interneuron dysfunction and early-life serotonin dysregulation in vulnerability to neuropsychiatric disorders, the 5-HT3AR represents an interesting and novel cell-type specific developmental target of early-life serotonin.