FGF Signaling Is Required for Lens Regeneration in Xenopus laevis

Abstract

In species of the frog genus Xenopus, lens regeneration occurs through a process of transdifferentiation, in which cornea epithelial cells presumably undergo dedifferentiation and subsequently redifferentiate to form a new lens. Experimental studies have shown that the retina provides the key signal(s) required to trigger this process once the original lens is removed. A previous study showed that addition of an exogenous Fibroblast Growth Factor (i.e., FGF1 protein) could initiate transdifferentiation of cornea epithelial cells in culture. To determine the role of FGF signaling in X. laevis lens regeneration, we have examined the presence of specific FGFs and their receptors (FGFRs) during this process and evaluated the necessity of FGFR signaling. RT-PCR analyses reveal that a number of FGF family members are expressed in cornea epithelium and retinal tissues both before and during the process of lens regeneration. Of these, FGF1, FGF8, and FGF9 are expressed principally in retinal tissue and not in the cornea epithelium. Hence, these ligands could represent key signaling factors originating from the retina that trigger regeneration. The results of experiments using an in vitro eye culture system and an FGFR inhibitor (SU5402) suggest that FGFR signaling is required for lens regeneration in Xenopus.

Lens regeneration from the cornea requires suppression of Wnt/β-catenin signaling

“…Eyes were lentectomized and cultured ex vivo , in the presence of either 1-azakenpaullone or BIO for seven days. In the ex vivo system, lentectomized eyes are removed and the cornea epithelium is tucked inside of the eyecup to ensure close proximity to the neural retina (Fukui and Henry, 2011; Thomas and Henry, 2014). Positive cases of regeneration are histologically scored based on the presence of morphologically distinct lentoids that are positively stained by an anti-lens antibody.…”

“…Eyes were lentectomized and cultured in an ex vivo system (formerly referred to as in vitro culture), as described in Fukui and Henry (2011). In the ex vivo culture system, eyes are lentectomized and the cornea epithelium is tucked into the eyecup to ensure close proximity to the retina.…”

“…Culture media were changed daily. After 7 days of culture (sufficient time for a lentoid to regenerate, see Fukui and Henry, 2011; Thomas and Henry, 2014), eyes were fixed for 3 hours in 3.7% formaldehyde, embedded in Paraplast Plus (McCormick Scientific, Richmond, IL) and serially sectioned at 8 µm (as described by Humason, 1972). To identify regenerated lenses, immunohistochemistry was carried out on serial sections using a polycolonal anti-lens antibody (Henry and Grainger, 1987).…”

“…In recent years, several signaling pathways have been shown to be important for cornea-lens regeneration such as the Fibroblast Growth Factor (FGF, Bosco et al, 1997; Fukui and Henry, 2011), Retinoic acid (Thomas and Henry, 2014), and Bone Morphogenetic Protein (BMP) signaling pathways (Day and Beck, 2011). The involvement of multiple pathways suggests that this process is regulated by a complex signaling network, and it is possible that other pathways may also be involved.…”

“…While Wolffian lens regeneration and cornea-lens regeneration occur in different organisms and from different tissues, it appears that some signaling (e.g. FGF signaling) is somewhat conserved between these two systems (Fukui and Henry, 2011; Hayashi et al, 2004). It is possible that the role of Wnt/β-catenin signaling in cornea-lens regeneration may also be conserved.…”