Lens and retina formation require expression of Pitx3 in Xenopus pre‐lens ectoderm

Abstract

Pitx3 is expressed in tissues fated to contribute to eye development, namely, neurula stage ectoderm and pre-chordal mesoderm, then presumptive lens ectoderm, placode, and finally lens. Pitx3 overexpression alters lens, optic cup, optic nerve, and diencephalon development. Many of the induced anomalies are attributable to midline deficits; however, as assessed by molecular markers, ectopic Pitx3 appears to temporarily enlarge the lens field. These changes are usually insufficient to generate either ectopic lenses to enlarge the eye that eventually differentiates. Conversely, use of a repressor chimera or of antisense morpholinos alters early expression of marker genes, and later inhibits lens development, thereby abrogating retinal induction. Reciprocal grafting experiments using wild-type and morpholino-treated tissues demonstrate that Pitx3 expression in the presumptive lens ectoderm is required for lens formation. Contradictory to recent assertions that retina can form in the absence of a lens, the expression of Pitx3 in the presumptive lens ectoderm is critical for retina development.

Microarray‐based identification of Pitx3 targets during Xenopus embryogenesis

“…Morpholino specificity has been previously published and reported to selectively reduce Pitx3 transcript and protein levels, with the control-morpholino having none of these effects (Khosrowshahian et al, 2005). This specificity has subsequently been confirmed using a second Pitx3 morpholino and missense control (Smoczer et al, In Press).…”

“…Visual comparisons of gene expression patterns between right-side injected control-morpholino (Cmo) or Pitx3-morpholino (Pmo) embryos and their untreated contralateral control. A-E: Pitx3 expression patterns are presented for comparison (adapted from Khosrowshahian et al, 2005;Smoczer et al, In Press). A: demonstrates faint but detectable signal throughout the ectoderm and in agreement with reverse transcriptase-polymerase chain reaction (RT-PCR) results.…”

“…Zebrafish pitx3 morphants also exhibit small eyes with lens degeneration, along with misshapen heads, a bent dorsal axis, and reduced jaws and fins (Shi et al, 2005). Disruption of Pitx3 in Xenopus laevis impedes development of lens and retina, and recent evidence suggests an additional role in dorsal axis segmentation and in laterality (Khosrowshahian et al, 2005;Shi et al, 2005;Smoczer et al, In Press). In zebrafish, Pitx3 expresses in the hypoblast of gastrulating embryos (Dutta et al, 2005), and the transcript is detectable by reverse transcriptase-polymerase chain reaction (RT-PCR) in pregastrula Xenopus (Khosrowshahian et al, 2005).…”

“…Disruption of Pitx3 in Xenopus laevis impedes development of lens and retina, and recent evidence suggests an additional role in dorsal axis segmentation and in laterality (Khosrowshahian et al, 2005;Shi et al, 2005;Smoczer et al, In Press). In zebrafish, Pitx3 expresses in the hypoblast of gastrulating embryos (Dutta et al, 2005), and the transcript is detectable by reverse transcriptase-polymerase chain reaction (RT-PCR) in pregastrula Xenopus (Khosrowshahian et al, 2005). These two studies suggest an earlier involvement for the gene in dorso-anterior patterning than is generally understood.…”

“…Although the preponderance of literature regarding the gene relates to lens and mDA neurons, Pitx3 also expresses broadly throughout gastrulation, and later in somites, and lateral plate mesoderm (Pommereit et al, 2001;Khosrowshahian et al, 2005;Smoczer et al, In Press). In zebrafish, pitx3 expresses in the demarcation of the mesendodermderived polster (Dutta et al, 2005).…”