Xath5 Participates in a Network of bHLH Genes in the Developing Xenopus Retina

Abstract

We examined the function of basic-helix-loop-helix (bHLH) transcription factors during retinal neurogenesis. We identified Xath5, a Xenopus bHLH gene related to Drosophila atonal, which is expressed in the developing Xenopus retina. Targeted expression of Xath5 in retinal progenitor cells biased the differentiation of these cells toward a ganglion cell fate, suggesting that Xath5 can regulate the differentiation of retinal neurons. We examined the relationship between the three bHLH genes Xash3, NeuroD, and Xath5 during retinal neurogenesis and found that Xash3 is expressed in early retinoblasts, followed by coexpression of 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 differentiation in the developing retina.

Xath5 regulates neurogenesis in the Xenopus olfactory placode

“…Xath5 misexpressing embryos had ectopic N-tubulin staining throughout the injected side (100%, n ϭ 27, Fig. 2G), indicating that Xath5 can promote neurogenesis, as previously described (Kanekar et al, 1997). N-tubulin staining was largely restricted to the developing nervous system on the uninjected side (Fig.…”

“…Paradoxically, we have previously shown that overexpression of XNeuroD causes an expansion of Xath5 expression in anterior of the embryo (Kanekar et al, 1997), raising the question of why Xath5 promotes expanded Xomp2 expression when overexpressed by RNA injection, but not when its expression is activated after XNeuroD overexpression. XNeuroD may be activating the expression of additional genes that may interfere with ORN differentiation, such as negative regulatory factors.…”

“…Previous analysis revealed that Xath5 expression is limited to the pineal gland, retina, and the developing olfactory placode in Xenopus (Kanekar et al, 1997). In addition, Xath5 overexpression in Xenopus embryos has demonstrated that this bHLH factor can promote ectopic neurogenesis at neural plate stages as well as bias retinal progenitors to a retinal ganglion cell fate (Kanekar et al, 1997); however, Xath5 function in the olfactory lineage had not been examined.…”

“…In addition, Xath5 overexpression in Xenopus embryos has demonstrated that this bHLH factor can promote ectopic neurogenesis at neural plate stages as well as bias retinal progenitors to a retinal ganglion cell fate (Kanekar et al, 1997); however, Xath5 function in the olfactory lineage had not been examined. To determine whether Xath5 influences ORN generation in the developing olfactory epithelium, we injected mRNA encoding the lineage tracer ␤-galactosidase (␤-gal) alone or together with mRNA for Xath5 into one cell of a two-cell stage embryo.…”

FIZ1 is expressed during photoreceptor maturation, and synergizes with NRL and CRX at rod-specific promoters in vitro

“…(Carl, et al, 2002, Mathers and Jamrich, 2000, Porter, et al, 1997 The bHLH (basic Helix Loop Helix) gene family of transcription factors also has pivotal roles in the unique patterning of transcription factors that guide retinal development (Mash1, Math3, Ngn2). (Akagi, et al, 2004, Brown, et al, 1998, Brown, et al, 2001, Hojo, et al, 2000, Kanekar, et al, 1997, Marquardt and Gruss, 2002, Moore, et al, 2002, Morrow, et al, 1999, Yan and Wang, 1998 Several transcription factors are clearly essential for photoreceptor development, and their mutations cause retinal degenerations: NRL, CRX, Otx2, Trβ2 (thyroid hormone receptor β2), and NR2E3. (Bessant, et al, 1999, DeAngelis, et al, 2002, Freund, et al, 1997, Haider, et al, 2000, Haider, et al, 2001, Martinez-Gimeno, et al, 2001, Ng, et al, 2001, Nishida, et al, 2003, Swain, et al, 1997 However, our knowledge of the interactions of these cell-specific proteins with each other and upstream signal transduction proteins remains sparse.…”