EphA3 expressed in the chicken tectum stimulates nasal retinal ganglion cell axon growth and is required for retinotectal topographic map formation.
BACKGROUND
Retinotopic projection onto the tectum/colliculus constitutes the most studied model of topographic mapping and Eph receptors and their ligands, the ephrins, are the best characterized molecular system involved in this process. Ephrin-As, expressed in an increasing rostro-caudal gradient in the tectum/colliculus, repel temporal retinal ganglion cell (RGC) axons from the caudal tectum and inhibit their branching posterior to their termination zones. However, there are conflicting data regarding the nature of the second force that guides nasal axons to invade and branch only in the caudal tectum/colliculus. The predominant model postulates that this second force is produced by a decreasing rostro-caudal gradient of EphA7 which repels nasal optic fibers and prevents their branching in the rostral tectum/colliculus. However, as optic fibers invade the tectum/colliculus growing throughout this gradient, this model cannot explain how the axons grow throughout this repellent molecule.


RESULTS
By using chicken retinal cultures we showed that EphA3 ectodomain stimulates nasal RGC axon growth in a concentration dependent way. Moreover, we showed that nasal axons choose growing on EphA3-expressing cells and that EphA3 diminishes the density of interstitial filopodia in nasal RGC axons. Accordingly, in vivo EphA3 ectodomain misexpression directs nasal optic fibers toward the caudal tectum preventing their branching in the rostral tectum.


CONCLUSIONS
We demonstrated in vitro and in vivo that EphA3 ectodomain (which is expressed in a decreasing rostro-caudal gradient in the tectum) is necessary for topographic mapping by stimulating the nasal axon growth toward the caudal tectum and inhibiting their branching in the rostral tectum. Furthermore, the ability of EphA3 of stimulating axon growth allows understanding how optic fibers invade the tectum growing throughout this molecular gradient. Therefore, opposing tectal gradients of repellent ephrin-As and of axon growth stimulating EphA3 complement each other to map optic fibers along the rostro-caudal tectal axis.
Laboratory of Developmental Neurobiology, Institute of Cell Biology and Neurosciences Prof. E. De Robertis (UBA-CONICET), School of Medicine, University of Buenos Aires, Buenos Aires, Argentina.
Ana Laura
AL
Ortalli
Luciano
L
Fiore
Jennifer
J
Di Napoli
Melina
M
Rapacioli
Marcelo
M
Salierno
Roberto
R
Etchenique
Vladimir
V
Flores
Viviana
V
Sanchez
Néstor Gabriel
NG
Carri
Gabriel
G
Scicolone
D000818:Animals; D001369:Axons / Q000378:metabolism* / Q000502:physiology; D015153:Blotting, Western; D002478:Cells, Cultured; D002642:Chick Embryo; D002645:Chickens; D049452:Green Fluorescent Proteins / Q000235:genetics / Q000378:metabolism; D057809:HEK293 Cells; D006801:Humans; D007150:Immunohistochemistry; D018613:Microscopy, Confocal; D010766:Phosphorylation; D036121:Receptor, EphA3 / Q000096:biosynthesis* / Q000235:genetics / Q000378:metabolism; D012160:Retina / Q000196:embryology / Q000254:growth & development / Q000378:metabolism; D012165:Retinal Ganglion Cells / Q000378:metabolism*; D013477:Superior Colliculi / Q000196:embryology / Q000254:growth & development / Q000378:metabolism; D003336:Tectum Mesencephali / Q000196:embryology / Q000254:growth & development / Q000378:metabolism*; D013997:Time Factors; D059008:Time-Lapse Imaging; D046509:Tissue Culture Techniques; D014443:Tyrosine / Q000378:metabolism; D014795:Visual Pathways
D049452:Green Fluorescent Proteins; D014443:Tyrosine; D036121:Receptor, EphA3