Soluble EphA3 stimulates axon growth whereas axonal EphA4 decreases axon growth of retinal ganglion cells

OLIVIERI, VANESA; ORTALLI, ANA L; DI NAPOLI, JENNIFER; PASQUALE, ELENA B; CARRI, NESTOR G; SCICOLONE, GABRIEL

Buenos Aires, Argentina

Congreso; 4th International Meeting of the Latin American Society of Developmental Biology.; 2008

Latin American Society of Developmental Biology.

SOLUBLE EPH3 STIMULATES AXON GROWTH WHEREAS AXONAL EPHA4 DECREASES AXON GROWTH OF RETINAL GANGLION CELLS.   Vanesa Olivieri1, Ana L Ortalli1, Jennifer Di Napoli1, Elena B Pasquale3, Néstor G Carri2, Gabriel Scicolone1. 1. Inst Cell Biol. and Neurosci. Med School, UBA, Bs As, Argentina. 2. Molecular Biology, IMBICE (CIC,CCT-La Plata,CONICET), Argentina. 3. The Burnham Inst, La Jolla, USA.     The retinotectal system is useful to study topographic map formation because nasal retinal ganglion cells (RGC) connect to the caudal tectum and temporal ones connect to the rostral tectum. Eph receptor tyrosine kinases and their ligands, the ephrins, are expressed in complementary gradients in both the retina and the tectum and contribute to guide retinotectal projections. Both Ephs and ephrins function as receptors and ligands producing bidirectional signaling. Ephrin-As located in the caudal tectum repel temporal axons by activating the axonal EphA3. Ephrin-As expressed in nasal RGC activate EphA4 converting nasal RGC less sensitive to tectal ephrin-As. However, there is controversial evidence about the signal that stimulates axon growth to the caudal tectum and which is the receptor of this cue. As all optic fibers invade the tectum from the rostral to the caudal region we postulated that: a) EphA3 expressed in the rostral tectum stimulates the axon growth of RGCs. As it was previously demonstrated that axonal ephrin-As activate axonal EphA4, we postulated that EphA4 activation decreases axon growth and that tectal EphA3 increases axon growth by reducing EphA4 activation throughtout compiting with axonal EphA4 for axonal ephrinAs binding. Our objectives were : 1) to demonstrate that soluble EphA3 stimulates axon growth of RGC in vitro and 2) to demonstrate that activation of axonal EphA4 decreases axon growth of RGCs. Methods: We used retinal explants from 7 days-old chicken embryos to investigate: 1) the expression pattern of ephrin-As and EphA4 in growth cones in vitro by immunocytochemistry; and 2) the effect of soluble EphA3 on axon growth of RGCs and 3) the effect of inhibiting axonal EphA4 activaty by using an inhibitory peptide (KYL) (Molecular and Cel Neurosci. 24:1000, 2003). Results: EphA4 is expressed homogeneously in nasal and temporal growth cones. Ephrin-As are highly expressed in nasal and moderately in temporal growth cones. It was previously demonstrated that axonal ephrin-As activate axonal EphA4. Axons of RGC growth longer when they are stimulated with soluble EphA3. This effect is higher in nasal than in temporal RGCs. Control temporal RGCs grow statistically significant longer axons than nasal ones. Inhibition of axonal EphA4 produces a significant increase in axon growth of nasal RGC. Conclusions: The fact that EphA3 stimulates axon growth of RGCs in vitro suggests that tectal EphA3 could be the force that stimulates axon growth to the caudal tectum. The fact that inhibition of axonal EphA4 increases axon growth independently of tectal ephrinAs suggests that activation of axonal EphA4 by axonal ephrinAs decreases axon growth in vitro. This supports the idea that tectal EphA3 could stimulate axon growth throughtout binding with axonal ephrinAs and indirectly inhibiting EphA4 activity.   This work was supported by grants from CONICET and UBA