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Members of the LIM homeodomain (LIM-HD) family of proteins are double zinc-finger containing transcription factors with important functions in pattern formation and cell lineage determination. The LIM-HD family member Lhx2 is required for normal eye, liver, and central nervous system formation. Lhx2(-/-) mice lack eyes, and experiments in Xenopus predict that Lhx2 forms a regulatory network with other eye field transcription factors to specify the eye field during eye formation. Here, we describe the structure and developmental expression pattern of the Xenopus laevis homologue, XLhx2. We show that XLhx2 shares significant amino acid sequence identity with other vertebrate Lhx2 proteins and Drosophila apterous (ap). The expression patterns of XLhx2 in the early neural plate and during eye development are consistent with a role in eye field specification and retinal differentiation. Despite highly similar expression patterns in the mouse and Xenopus central nervous system, divergent expression patterns were also observed. Phylogenetic analysis confirmed the identity of the isolated cDNA as a Xenopus ortholog of Lhx2. Therefore, in spite of structural similarities, the mouse and Xenopus Lhx2 expression patterns differ, suggesting potential functional differences in these species.
Figure 2. Expression patterns of XLhx2 during eye formation in whole-mount (A,C,D) and sectioned (B,E-I) Xenopus embryos. A,B: XLhx2 is expressed in the eye field of the stage 12.5 embryo. Arrow shows expression is limited to the neuroectoderm. C,D: Anterior views show how the XLhx2 expression pattern resolves from a single eye field (stage 15) into the two eyeprimordia (stage 18). E: Evaginating optic vesicles and ventralforebrain stain throughout for XLhx2. Arrow and arrowhead point to presumptive retinal pigment epithelium (PRPE), optic stalk (OS), and presumptive neural retina (PNR), respectively. F: Dashed line outlines PRPE and PRN while arrow points to developing lens placode, where XLhx2 expression was undetectable. G: Neural retina (NR) expresses XLhx2, but lens vesicle (LV) does not. Arrows point to the peripheral retina where Lhx2 levels appear elevated. H: At stage 35/36, XLhx2 expression is reduced in the (a) presumptive outer nuclear and (b) ganglion cell layers. I: At stage 41, XLhx2 expression is limited to a subset of cells in the inner nuclear layer (INL) and ciliary marginal zone (CMZ, arrow and outlined in red). In G-I, the black dashed line denotes the border between neural retina and lens (Le). Dorsal is top in all panels. J-L: Relative expression patterns of the retinal transcription factors Lhx2, Pax6, Six3, Rx1, and Optx2 in the stage 41 Xenopus eye. J: Lhx2 expression is shown in light blue. K: Pax6, Six3, and Pax6 + Six3 expression is illustrated in blue, red, and violet, respectively. L: Rx1, Optx2, and Rx1 + Optx2 expression is illustrated in pink, yellow, and orange, respectively. At stage 41, the retina forms three nuclear layers. The outer nuclear layer (ONL - Rx1+) consists of rod and cone photoreceptors. The inner nuclear layer (INL) is subdivided into inner and outer regions. The outer INL (Rx1+; Optx2+; Six3+) contains mostly horizontal and bipolar cells while the inner INL (Lhx2+; Pax6+; Optx2+) contains mostly amacrine, Müller, and a few bipolar cells. The retinal ganglion cell layer (GCL) expresses Pax6, Optx2, and Six3. See the text for additional details of the expression patterns at these and earlier developmental stages.
Figure 3. Divergent Lhx2 expression patterns among vertebrates. A: Arrows point to XLhx2 expression in the (a) pineal gland, (b) eyes, and (c) olfactory placodes. B,C: Lateral views of stage 35/36 embryos. XLhx2 expression (B) is undetectable in and around the liverprimordium (arrow) in spite of strong expression in the brain and eye. (Although not the focus of this study, readers interested in a comparative analysis of the Xenopus and mouse brain expression patterns of Lhx2 are directed to the work of Bachy et al. ([2001],[2002)] and Moreno et al. ([2004],[2005]). Shown are sections of stage 33/34 (D,E) and stage 41 (F,G) embryos; dorsal is the top. D,F: XLhx2 mRNA is undetected in and around the developing liver. In contrast, intense XHex expression is observed in the stage 33/34 liver diverticulum (E, arrow) and stage 41 liver bud (G, arrow). Images of stage 33/34 and 41 embryos were modified from Nieuwkoop and Faber ([1994]). St, stomach.
lhx2 (LIM homeobox 2 ) gene expression in Xenopus laevis embryo, assayed via in situ hybridization, NF stage 25, head region only, anterior view, dorsal up.
lhx2 ( LIM homeobox 2) gene expression in sectioned eye of Xenopus laevis embryo, at NF stage 41, lens epithelium/anterior right, dorsal up.
lhx2 (LIM homeobox 2) gene expression in Xenopus laevis embryo, assayed via in situ hybridization, NF stage 12.5, (A) anterior view, dorsal up, and (B) in section, anterior up.
lhx2 ( LIM homeobox 2) gene expression in Xenopus laevis embryo, assayed via in situ hybridization, NF stage 15, anterior view, dorsal up.
lhx2 (LIM homeobox 2 ) gene expression in Xenopus laevis embryo, assayed via in situ hybridization, NF stage 18, anterior view, dorsal up.
