XATH5 PARTICIPATES IN A NETWORK OF BHLH GENES IN THE DEVELOPING XENOPUS-RETINA

We examined the function of basic-helix-loop-helix (bHLH) transcriptio n factors during retinal neurogenesis. We identified Xath5, a Xenopus bHLH gene related to Drosophila atonal, which is expressed in the deve loping Xenopus retina. Targeted expression of Xath5 in retinal progeni tor cells biased the differentiation of these cells toward a ganglion cell fate, suggesting that Xath5 can regulate the differentiation of r etinal neurons. We examined the relationship between the three bHLH ge nes Xash3, NeuroD, and Xath5 during retinal neurogenesis and found tha t Xash3 is expressed in early retinoblasts, followed by coexpression o f Xath5 and NeuroD in differentiating cells. We provide evidence that the expression of Xash3, NeuroD, and Xath5 is coupled and propose that these bHLH genes regulate successive stages of neuronal differentiati on in the developing retina.