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In the Central Nervous System (CNS) there are some niches of undifferentiated, neural progenitor/stem cells that produce active neurogenesis originating functionally integrated neurons. In the chicken eye, there is a neurogenic niche in the ciliary margin (CM) which has the ability to originate all the cell types of the neural retina. During retinal development, cells acquire positional values along the radial and tangential axes. These positional values are the necessary base for the formation of neural circuits. In this work, we have analyzed whether neural progenitor cells (NPCs) of CM have positional values regarding the radial and tangential axes, and if they have the potential to differentiate into retinal ganglion cells (RGCs) in vitro. Furthermore, we analyzed whether these RGCs preserve positional values along the tangential axis and respond to the Eph/ephrin axon guidance system. In order to answer these questions, we cultured NPCs obtained from the CM favoring the formation of neurospheres. Our results showed that the expanding neurospheres are polarized structures in which their cells have specific positional values along their radial axis, recapitulating the apical-basal polarity of the CM and the neuroepithelium. We also showed that NPCs obtained from CM possess positional values along the nasal-temporal retinal axis. When the neurospheres were submitted to differentiation conditions, we observed that NPCs can differentiate into RGCs. These RGCs present long axons that express different members of the Eph/ephrin system and they are competent to respond to this axon guidance cue system, recapitulating the axonal behavior during retinotectal neural map development. All these findings contribute to understand the cellular and molecular mechanisms involved in CNS development and regeneration., (Copyright © 2022 Elsevier Ltd. All rights reserved.)

The Eph and ephrins are involved in the genesis of topographic ordered connections at the visual system. Previously we demonstrated that tectal EphA3 stimulates axon growth of nasal retinal ganglion cells (RGCs) toward the caudal tectum preventing them from branching in the rostral tectum. Now we investigated whether tectal EphA3 plays this role by modulating the axonal EphA4 forward signaling or throughout axonal ephrin-As reverse signaling. For this purpose we used cultures of nasal retinal explants and dissociated retinal neurons from chicken embryos. We treated them with clustered EphA3-Fc, Fc (control), PI-PLC (sheds ephrin-As) or KYL (inhibits ephrin-As-mediated EphA4 activation). We achieved in vitro and in vivo electroporations of chicken embryo retinas with wild type EphA4, Ki-EphA4 (kinase inactive dominant negative EphA4) or EGFP in pMES expression vector. We performed immunocytochemistry, immunoprecipitation and Western blot against Eph/ephrin-As system. Our results showed that: 1) shedding of ephrin-As and the inhibition of ephrin-A-mediated EphA4 activity increase axon length and decrease axonal interstitial filopodia density of nasal RGCs; and 2) a dominant negative form of EphA4 increases axon growth in vitro and induces nasal RGC axons to grow passing throughout their target area in the caudal tectum meanwhile overexpression of EphA4 produces the opposite effects. All together, these results demonstrate that ephrin-A-mediated EphA4 forward signaling decreases the level of axon growth and increases the density of axonal interstitial filopodia of nasal RGCs. Besides, our results showed that: 3) EphA3 ectodomain increases axon growth and decreases the density of axonal interstitial filopodia and branching in vitro and in vivo and 4) EphA3 ectodomain diminishes the ephrin-A2/EphA4 colocalization, and the EphA4 and ephexin1 phosphorylation. All together, these results show that the EphA3 ectodomain produces the opposite effects than the EphA4 forward signaling, by decreasing this signaling pathway throughout competing with EphA4 for ephrin-As binding. Furthermore, it is proposed that tectal EphA3 participates in the establishment of retinotectal mapping throughout this mechanism and that EphAs can regulate axon growth and branching by modulating other EphA receptors forward signaling., (Copyright © 2018 Elsevier Ltd. All rights reserved.)