The lens‐regenerating competence in the outer cornea and epidermis of larval Xenopus laevis is related to pax6 expression

Abstract

After lentectomy, larval Xenopus laevis can regenerate a new lens by transdifferentiation of the outer cornea and pericorneal epidermis (lentogenic area). This process is promoted by retinal factor(s) accumulated into the vitreous chamber. To understand the molecular basis of the lens-regenerating competence (i.e. the capacity to respond to the retinal factor forming a new lens) in the outer cornea and epidermis, we analysed the expression of otx2 , pax6 , sox3, pitx3, prox1, β B1-cry (genes all involved in lens development) by Real-time RT-PCR in the cornea and epidermis fragments dissected from donor larvae. The same fragments were also implanted into the vitreous chamber of host larvae to ascertain their lens-regenerating competence using specific anti-lens antibodies. The results demonstrate that there is a tight correlation between lens-regenerating competence and pax6 expression. In fact, (1) pax6 is the only one of the aforesaid genes to be expressed in the lentogenic area; (2) pax6 expression is absent in head epidermis outside the lentogenic area and in flank epidermis, both incapable of transdifferentiating into lens after implantation into the vitreous chamber; (3) in larvae that have undergone eye transplantation under the head or flank epidermis, pax6 re-expression was observed only in the head epidermis covering the transplanted eye. This is consistent with the fact that only the head epidermis reacquires the lens-regenerating competence after eye transplantation, forming a lens following implantation into the vitreous chamber; and (4) in larvae that have undergone removal of the eye, the epidermis covering the orbit maintained pax6 expression. This is consistent with the fact that after the eye enucleation the lentogenic area maintains the lens-regenerating competence, giving rise to a lens after implantation into the vitreous chamber. Moreover, we observed that misexpression of pax6 is sufficient to promote the acquisition of the lens-regenerating competence in flank epidermis. In fact, flank epidermis fragments dissected from pax6 RNA injected embryos could form lenses when implanted into the vitreous chamber. The data indicate for the first time that pax6 is a pivotal factor of lens-regenerating competence in the outer cornea and epidermis of larval X. laevis .

Expression of pluripotency factors in larval epithelia of the frog Xenopus: Evidence for the presence of cornea epithelial stem cells

“…Differential expression was also detected for growth differentiation factor 3 ( gdf3 ), where transcripts were present in regenerating corneas but absent from control corneas and flank ectoderm. Although it is not a pluripotency factor, it was previously reported that pax6 is only expressed in cornea tissue and not in flank ectoderm (Gargioli et al, 2008; Henry et al, 2002), which was verified here (Table 1). Together, these expression profiles define the samples as being comprised of three distinct tissues.…”

Retinoic acid regulation by CYP26 in vertebrate lens regeneration

“…However, the nature of the cornea that makes it competent to respond to those retinal signals is poorly understood. pax6 expression is one factor that has been associated with lens regeneration competence (Gargioli et al, 2008). We presently provide evidence that RA signaling must be attenuated within the cornea epithelium, which could in part be responsible for endowing this tissue with lens-forming competence.…”

Transdifferentiation: a cell and molecular reprogramming process

“…It has been shown that Pax6 gene and its homologue could form ectopic eyes by targeted expression in Drosophila and Xenopus (Halder et al 1995). Other studies have indicated the critical role of Pax6 transcription factor in transdifferentiation of the retinal pigmented epithelia into the neural retina in chicken (Azuma et al 2005) and also in lens-regenerating of larval Xenopus laevis (Gargioli et al 2008).…”