The transcription factors Pax-6 and X-dll3 influence visual and olfactory system development in Xenopus laevis

Abstract

Pax-6 and X-dll3 are homeobox-containing transcription factors that are expressed in developing anterior neural structures in Xenopus. Characterization of multiple Xenopus Pax-6 cDNAs reveals that they fall into four distinct classes. These classes are defined based on the presence or absence of a 42 base pair sequence in the paired box, and the presence or absence of a 151 base pair sequence located downstream of the homeobox. Transcripts containing the 151 base pair sequence encode a highly conserved form of Pax-6, and are referred to as Xenopus A variants. Transcripts lacking the 151 base pair sequence, producing a change in reading frame that encodes a novel carboxy terminus, are referred to as Xenopus B variants. Xenopus A and B transcripts are produced from an alternate splicing event that occurs in each of at least two Pax-6 genes existing in the Xenopus genome. Both A and B transcripts are expressed in brain and eye tissue. Antibodies generated against unique peptide sequences located in the carboxy-terminal domains of the proteins produced from Xenopus A and B transcripts distinguish expression patterns for the two resulting proteins, and indicate that they are expressed differentially in the developing retina and lens. Five phenotypes result from overexpression of these Pax-6 transcripts: ectopic lens crystallin expression, reduction of endogenous lens tissue, retinas with altered size and morphology, ectopic eye formation, and a reduction of the olfactory placode. Both Xenopus A and Xenopus B can elicit the reduction of both lens and olfactory placodes, and the expansion of retinal tissue, while only Xenopus A can elicit ectopic eye formation and ectopic lens crystallin expression. These studies indicate that overexpression of Pax-6 can produce differential effects on Xenopus eye development, and support a model whereby the concentration of Pax-6 proteins is a critical factor for the development of the Xenopus lens and retina. Overexpression of X-dll3 transcripts also produces morphological defects in both the developing eye and olfactory system. The developing eye appears reduced in size, and this reduction seems to be predominant in the anterior portion of the eye. The developing forebrain and olfactory placode, in contrast, appear expanded. These results support a model whereby overexpression of X-dll3 expands the presumptive olfactory fields, at the expense of the anterior portion of the presumptive eye field. The combined results from these experiments indicate that overexpression of Pax-6 and X-dll3 transcripts exert differential effects on visual and olfactory system development in Xenopus.