Temporal and spatial expression of transcription factors FoxN4, Ptf1a, Prox1, Isl1 and Lim1 mRNA in the developing chick retina.

Transcription factors are pivotal in regulating cell fate and development. We analyzed five transcription factors - FoxN4, Ptf1a, Prox1, Isl1 and Lim1 - with putative functions in the formation of early-generated retinal interneurons. A full-length chicken FoxN4 cDNA was characterized and in situ as well as RT-PCR showed that FoxN4 expression commenced already in the stage 12-14 optic vesicles. Ptf1a, Prox1, Isl1 and Lim1 expression appeared later by stage 20-24, concomitant with the first post-mitotic ganglion-, amacrine- and horizontal cells. The FoxN4 and Ptf1a expression was transient with peak levels by stage 32-35. Expression disappeared as the retinal progenitor cells differentiated. Prox1, Isl1 and Lim1 expression remained in several differentiated cells including the horizontal cells. The order of expression supports a scheme where Ptf1a and Prox1 is downstream of FoxN4 and that FoxN4 and Ptf1a have transient roles during fate specification while Prox1, Isl1 and Lim1 have roles that are important for the generation of the neuronal subtypes.