FGF-8Is Associated with Anteroposterior Patterning and Limb Regeneration inXenopus

Abstract

FGF-8 has attracted attention particularly because of its importance for limb development in the chick and mouse, although it also has a number of earlier expression domains in these species. We have now cloned an FGF-8 homologue from Xenopus in which it is easier to do functional studies on early development. There is no maternal expression, while zygotic expression is highest in the gastrula and neurula stages. XFGF-8 is expressed as a ring around the blastopore and subsequently in the tail bud. There are several domains in the head including the hatching gland, the branchial clefts, and the midbrain-hindbrain border. At later stages there is a prominent band of expression in the limb bud epidermis. Although there is no morphological apical ridge, this band of expression suggests that the Xenopus limb bud contains a cryptic region with a similar ability to stimulate mesenchymal outgrowth. The mesoderm-inducing activity of XFGF-8 is somewhat lower than that of other FGFs, while the posteriorizing activity is similar. These differences are probably due to the different receptor specificity. The posterior expression and high posteriorizing activity suggest that XFGF-8 contributes to the patterning of the anterior-posterior axis by FGF family members during gastrulation. In contrast to the amniotes, Xenopus limb buds can regenerate following damage. We show that regeneration is correlated with the reexpression of XFGF-8 in the distal epidermis, suggesting that this ability is critical for successful limb regeneration.

Mechanisms of urodele limb regeneration

“…Nevertheless, Shh is expressed in a patch of posterior mesenchyme (Imokawa & Yoshizato, 1997; Torok, Gardiner, Izpisua Belmonte, & Bryant, 1999), while Fgf8 is expressed in the anterior mesenchyme and the basal cells of the AEC (Han et al., 2001). Fgf8 is associated with anterior patterning of the blastema during limb regeneration in Xenopus (Christen & Slack, 1997). Transfection of shh into anterior blastema tissue of the axolotl limb by vaccinia virus results in the regeneration of supernumerary digits, but not more proximal structures (Roy, Gardiner, & Bryant, 2000).…”

Beyond early development: Xenopus as an emerging model for the study of regenerative mechanisms

“…3GЈ). The basal epithelial cells change shape and become columnar (Pearl et al, 2008) and start expressing AEC markers like fgf-8 (Christen and Slack, 1997). With maturation, regeneration ability becomes more restricted, resulting in regenerates that are imperfect, with missing structures.…”

“…Third, in chick, the signals for outgrowth of the limb originate from a specialized structure at the distal end of the limb bud, the apical ectodermal ridge (AER), which extends along the dorsoventral boundary from anterior to posterior (Saunders, 1948). Amphibians do not have an AER as found in chick but some anurans, including Xenopus, have an analogous structure called the apical epithelial cap (AEC) that acts to support outgrowth of the limb bud (Christen and Slack, 1997).…”

“…There is a clear difference in the initial mechanism of dorsal-ventral patterning although later restricted expression of lmx1 in the dorsal mesoderm appears to be the same across tetrapod species (Christen and Slack, 1998;Matsuda et al, 2001). Genes involved in defining the proximal-distal (P-D) axis such as fgf-10 (Yokoyama et al, 2000) and fgf-8 are expressed in the distal mesenchyme and in the apical region of the limb bud, respectively (Christen and Slack, 1997). However the distal mesenchymal expression of the Bmp inhibitor Gremlin in mouse limbs, which regulates the growth of the limb by means of inhibitory effects on the AER specific Fgf-8 (Verheyden and Sun, 2008), is not seen in Xenopus (Pearl et al, 2008).…”

The molecular basis of amphibian limb regeneration: integrating the old with the new

“…Exogenous FGFs presumably substitute for the normal function of the apical epidermis in the production of FGFs, an ability that is lost coincident with the loss of regenerative ability. 33,34 Experimental treatment of amputated Xenopus limb buds with FGF10, which is normally expressed in the mesenchyme, induces a number of genes, including fgf8, in the epidermis, and partially rescues regeneration of late stage limbs that normally would not regenerate. 32 A comparable result is observed in amputated chick limb buds that are supplied with either FGF or a grafted apical epidermis (see Reference 31).…”