Decrease of EphA4 forward signaling allows nasal retinal ganglion cells axons to invade the rostral tectum and forme terminal zones in caudal tectum during retinotectal mapping.

LUCIANO FIORE; MARA MEDORI; CINDY OLMOS CARREÑO; MELINA RAPACIOLI; VIVIANA SANCHEZ; NÉSTOR GABRIEL CARRI; GABRIEL SCICOLONE

Rio de Janeiro

Congreso; IBRO 2015, 9th World Congress; 2015

IBRO

We demonstrated that tectal EphA3 stimulates axon growth of nasal retinal ganglion cells (RGC) toward the caudal tectum preventing them from branching in the rostral tectum. Here, we postulated that activation of axonal EphA4 and its downstream signaling (ephexin1 phosphorylation and RhoA activation) decreases axon growth whereas tectal EphA3 increases axon growth by reducing EphA4, ephexin1 and RhoA activation and increasing cdc42 activation throughout competing with axonal EphA4 for axonal ephrin-As binding. In order to investigate this hypothesis, we used retinal explants treated with EphA3, PIPLC (sheds ephrin-As) or KYL (EphA4 inhibitor), and we electroporated retinas in vivo/in vitro with EphA4, KiEphA4 (dominant negative) or GFP. We showed that: a) Nasal RGC axons present higher levels of ephrin-As, colocalization of ephrin-A2/EphA4, EphA4P and ephexin1P than temporal RGC axons. b) Axonal response to EphA3 is associated to ephrin-A, EphA4-P, ephexin1P and RhoA active expression. c) The EphA3, ephrin-A shedding both decrease the degree of EphA4-P, ephexin1P, RhoA activation and increase cdc42 activation. d) Removal of axonal ephrin-As and inhibition of ephrin-As-mediated EphA4 signaling recapitulate the effects of EphA3 on RGC axon growth and branching. e) In vitro overexpression of EphA4 produces neurons with shorter axons whereas neurons expressing KiEphA4 have longer axons than the control. In vivo overexpression of EphA4 produces nasal RGC axons with terminal zones closer to the rostral tectum than the control. Nasal RCGs expressing KiEphA4 form terminal zones closer to the caudal tectum. These results support the idea of a new molecular mechanism whereby tectal EphA3 increases axon growth toward the caudal tectum and collaborate to inhibit axon branching in the rostral tectum by decreasing ephrin-As-mediated EphA4 forward signaling. Supported by UBA and CONICET.