Serotonin (5-HT) neurotransmission in the brain relies on a widespread axon terminal network originating from the hindbrain raphe nuclei. These projections are topographically organized such that the dorsal (DR), and median raphe (MnR) nuclei have different brain targets. However, the guidance molecules involved in this selective targeting in development are unknown. Here, we show the implication of ephrinA5 signaling in this process. We find that the EphA5 gene is selectively expressed in a subset of 5-HT neurons during embryonic and postnatal development. Highest co-expression of EphA5 and of the 5-HT marker Tph2 is found in the DR with lower co-expression in the MnR, and hardly any colocalization in the caudal raphe in the medulla. Accordingly, ephrinA induced a dose-dependent collapse response of 5-HT growth cones cultured from rostral but not caudal raphe. Ectopic expression of ephrinA3, after in utero electroporation in the amygdala and piriform cortex, repelled 5-HT raphe fiber ingrowth. Conversely, misplaced DR 5-HT axons were found in ephrin A5 KO mice in brain regions that are normally only targeted by MnR 5-HT axons. This causes an overall increase in the density of 5-HT innervation in the ventromedial hypothalamus, the suprachiasmatic nucleus and the olfactory bulb. All these brain areas have high expression of ephrinAs at the time of 5-HT fiber ingrowth. Present results show for the first time the role of a guidance molecule for the region-specific targeting of raphe neurons. This has important implications to understand how functional parsing of central 5-HT neurons is established during development.
Significance Statement Present results demonstrate a new role of ephrinA signaling for the selective targeting of 5-HT raphe nuclei. The tyrosine kinase EphA5 is differentially expressed across 5-HT neurons from the different raphe nuclei in mice, correlating with a different repulsive action of ephrinA on the growth of 5-HT axons. EphrinA5 loss of function causes a mis-targeting of dorsal raphe 5-HT axons, whereas over-expression of ephrinA inhibits the ingrowth of 5-HT raphe axons in the amygdala and piriform cortex that are main targets of the dorsal raphe 5-HT neurons. Thus Eph-ephrin signaling acts as a repulsive signal to differentially target 5-HT axons originating from different raphe nuclei.
The authors declare no competing financial interests.
Dr Teng Teng was financed by the China Scholarsip Council; Research was supported by the Fondation de la Recherche Médicale and the Investissements d'Avenir program ( ANR-11-0004-02), the Inserm, and the Université Pierre et Marie Curie. The team is part of the Ecole des Neurosciences de Paris training network and of the Bio-Psy Labex.