The hedgehog pathway is a modulator of retina regeneration

Abstract

The embryonic chick has the ability to regenerate its retina after it has been completely removed. Here, we provide a detailed characterization of retina regeneration in the embryonic chick at the cellular level. Retina regeneration can occur in two distinct manners. The first is via transdifferentiation, which is induced by members of the Fibroblast growth factor (Fgf) family. The second type of retinal regeneration occurs from the anterior margin of the eye, near the ciliary body (CB) and ciliary marginal zone (CMZ). We show that regeneration from the CB/CMZ is the result of proliferating stem/progenitor cells. This type of regeneration is also stimulated by Fgf2, but we show that it can be activated by Sonic hedgehog(Shh) overexpression when no ectopic Fgf2 is present. Shh-stimulated activation of CB/CMZ regeneration is inhibited by the Fgf receptor (Fgfr)antagonist, PD173074. This indicates that Shh-induced regeneration acts through the Fgf signaling pathway. In addition, we show that the hedgehog (Hh)pathway plays a role in maintenance of the retina pigmented epithelium (RPE),as ectopic Shh expression inhibits transdifferentiation and Hh inhibition increases the transdifferentiation domain. Ectopic Shh expression in the regenerating retina also results in a decrease in the number of ganglion cells present and an increase in apoptosis mostly in the presumptive ganglion cell layer (GCL). However, Hh inhibition increases the number of ganglion cells but does not have an effect on cell death. Taken together, our results suggest that the hedgehog pathway is an important modulator of retina regeneration.

Fibroblast growth factor–hedgehog interdependence during retina regeneration

“…In addition, as already reported, Shh stimulated regeneration was significantly inhibited by the FGFR antagonist PD173074 (compare Fig. 1E with F and H; Spence et al, 2004). It is well documented that one of the pathways through which FGF2 works is the FGFR/MEK/Erk pathway in different cellular contexts (Galy et al, 2002;Rios-Muñ oz et al, 2005).…”

“…1; Spence et al, 2004), we wanted to determine whether Shh is able to activate the MAPK pathway. After placing our explants in culture medium, we added increasing concentrations of FGF2 or Shh and assayed for pErk.…”

“…The embryonic chick retina is able to regenerate during a window of early development Coulombre, 1965, 1970;Hollenberg, 1989, 1991;Spence et al, 2004) from two distinct sources (reviewed in Del Rio- Tsonis and Tsonis, 2003;Haynes and Del Rio-Tsonis, 2004;Vergara et al, 2005). One source of regeneration is by means of the process of transdifferentiation of the retina pigmented epithelium (RPE).…”

“…One source of regeneration is by means of the process of transdifferentiation of the retina pigmented epithelium (RPE). This type of regeneration can occur until approximately embryonic day (E) 4.5 when an ectopic source of fibroblast growth factor (FGF) is present Hollenberg, 1989, 1991;Spence et al, 2004). The other source of retina regeneration, which is the current focus of this work, consists of a pool of stem/progenitor cells located in the ciliary region of the chick eye.…”

“…We recently reported that both FGF2 and Shh are able to independently induce retina regeneration from the CB/CMZ in E4 chick eyes . Thus, both FGF and Shh are important in the regulation of stem/progenitor cells in the CB/CMZ of the embryonic and postnatal chick (Fischer et al, 2002;Fischer and Reh, 2003;Spence et al, 2004;Moshiri et al, , 2005.…”