Expression of Pax-6 in the developing olfactory system of Xenopus laevis.

Abstract

The expression of the transcription factor Pax-6 in the developing olfactory system of the African clawed frog Xenopus laevis was examined using the Xenopus Pax-6 homolog. Pax-6 cDNA clones were isolated from a Xenopus cDNA library with a zebrafish Pax-6 probe. Using a Xenopus Pax-6 sequence that spans the paired box and a majority of the paired-type homeobox, I performed whole mount in situ hybridization analysis on unfertilized eggs and on animals ranging from stages 1 through 66 (i.e., through metamorphosis). Pax-6 is expressed in Xenopus beginning at the late gastrula stage and continuing through metamorphosis. Expression is detected in the olfactory system, the developing eye and regions of the forebrain, hindbrain and spinal cord. The expression pattern of Pax-6 is consistent with a role for Pax-6 in influencing developmental signaling between the olfactory placode and the developing olfactory bulb. Pax-6 is detected in the frog olfactory system from the earliest indication of the olfactory system, the olfactory placode, at stage 23. Light expression of Pax-6 remains in the developing nose as the olfactory axons contact and synapse on the developing olfactory bulb, and throughout formation of the layers of the olfactory bulb. The olfactory bulb primordium expresses Pax-6 from its earliest detection and throughout its development. Therefore, it appears that Pax-6 may play a role in the development and maturation of the olfactory system, as well as the eye and several regions within the central nervous system of Xenopus. Six additional cDNA clones were sequenced and compared at the nucleotide level and predicted amino acid level. Comparisons between Xenopus Pax-6 and other known Pax-6 sequences reveal a striking degree of sequence similarity. The Xenopus Pax-6 sequence encodes two DNA binding regions: the paired domain and the paired-type homeodomain. Greater than 90% sequence identity exists among the paired domains, and there is 100% identity among the paired-type homeodomains. Unlike other Pax-6 sequences reported, some Xenopus Pax-6 cDNAs differ in their predicted carboxy termini. One type of Xenopus Pax-6 cDNA sequence encodes a predicted carboxy terminus nearly identical to Pax-6 sequences reported in other animals. The second type of Xenopus Pax-6 cDNA sequence encodes a predicted protein with an altered carboxy terminal end due to the absence of 151 bp in the 3$\sp\prime$ end and a consequent downstream change in the reading frame. Based on PCR analysis of RNA from animals at different stages, these two sequences each are expressed in a unique and developmentally significant manner.