RaxL regulates chick ganglion cell development

RaxL is a paired-like homeobox gene involved in vertebrate eye morphogenesis. We examined RaxL protein expression patterns during chick retinal development in combination with ganglion cell markers including the RA4 antigen, cBrn-3, Islet-1 and neuronal type III beta-tubulin. Double-immunostaining demonstrated that downregulation of RaxL protein correlates with upregulation of ganglion cell markers in the ganglion cell layer (GCL). To explore this correlation in vivo, we performed gain- and loss-of-function experiments by electroporating retroviral vectors encoding wild-type and dominant-negative-RaxL into the optic vesicles of stage 10 chick embryos. Infection with virus expressing RaxL led to a 35% decrease in Islet-1-positive ganglion cells at E5.0 and a complete loss of ganglion cells at E15, with no effect on displaced amacrine cells in the GCL. When dominant-negative RaxL was expressed, the total number of cells in the GCL increased by approximately 40% at E5.0 but was reduced to 40% at E15, due to ectopic apoptosis in the GCL from E9 to E15. These results suggest that RaxL gives an inhibitory effect on ganglion cell development and that the loss of RaxL expression is required for maintenance of ganglion cells.