Glypican 4 modulates FGF signalling and regulates dorsoventral forebrain patterning inXenopusembryos

Abstract

Heparan sulphate proteoglycans such as glypicans are essential modulators of intercellular communication during embryogenesis. In Xenopus laevis embryos, the temporal and spatial distribution of Glypican 4 (Gpc4) transcripts during gastrulation and neurulation suggests functions in early development of the central nervous system. We have functionally analysed the role of Xenopus Gpc4 by using antisense morpholino oligonucleotides and show that Gpc4 is part of the signalling network that patterns the forebrain. Depletion of GPC4 protein results in a pleiotropic phenotype affecting both primary axis formation and early patterning of the anterior central nervous system. Molecular analysis shows that posterior axis elongation during gastrulation is affected in GPC4-depleted embryos, whereas head and neural induction are apparently normal. During neurulation, loss of GPC4 disrupts expression of dorsal forebrain genes, such as Emx2, whereas genes marking the ventral forebrain and posterior central nervous system continue to be expressed. This loss of GPC4 activity also causes apoptosis of forebrain progenitors during neural tube closure. Biochemical studies establish that GPC4 binds FGF2 and modulates FGF signal transduction. Inhibition of FGF signal transduction, by adding the chemical SU5402 to embryos from neural plate stages onwards,phenocopies the loss of gene expression and apoptosis in the forebrain. We propose that GPC4 regulates dorsoventral forebrain patterning by positive modulation of FGF signalling.

Signalling Mechanisms Underlying Congenital Malformation: The Gatekeepers, Glypicans

“…We have also shown that loss-of Gpc4 function in Xenopus embryos impairs forebrain patterning and cell survival from early neural plate stages onwards, and that these early developmental defects result in brains affected by microcephaly at later stages ( Fig. 2; Galli et al, 2003). Xgly4 physically interacts with Wnt11, and might enhance function in the Wnt/PCP pathway during gastrulation (Ohkawara et al, 2003).…”

“…In addition to Wnt11, we have demonstrated that Xgly4 also binds Fgf2. Inhibition of Fgf signalling results in dorsal forebrain phenotypes similar to those of Xgly4 depleted embryos, indicating that establishment and patterning of the dorsal forebrain territory may require modulation of Fgf signalling by Xgly4 (Galli et al, 2003). Fig.…”

“…We and others have shown that Gpcs are among the most abundant HSPGs in the developing nervous system and are expressed in embryonic and adult neural stem cells (Bandtlow and Zimmermann, 2000;Hagihara et al, 2000;Luxardi et al, 2007). Our embryological manipulations in Xenopus embryos have begun to provide insight into their role in brain size as abrogation of Gpc4 activity in Xenopus embryos disrupts forebrain patterning and cell survival, and causes microcephaly (see also above and Ficure 2; Galli et al, 2003). Therefore, our findings raise the possibility that some of the congenital microcephalies may arise as a consequence of disrupting Glpc-4 gene function during brain development.…”

Exploration of the extracellular space by a large-scale secretion screen in the early Xenopus embryo

“…We also identified Xenopus Glypican-4 (Xgly-4), which belongs to the family of heparan sulphate proteoglycans linked to the cell surface via a glycosylphosphatidylinositol (GPI) anchor; Xgly-4 acts as a co-receptor for Wnt and FGF signals and in this way affects convergence-extension movements during gastrulation and dorsoventral patterning of the forebrain, respectively (Ohkawara et al, 2003;Galli et al, 2003). Lunatic Fringe (lFng) was isolated twice.…”

Two promoters with distinct activities in different tissues drive the expression of heparanase in Xenopus

“…Furthermore, they bind to and control the biological function of signaling molecules, such as growth factors and morphogens (Allen and Rapraeger, 2003;Johnson et al, 2004;Kreuger et al, 2006). These interactions with signaling molecules underlie in part the importance of HSPGs in development (Lin et al, 1999;Galli et al, 2003;Bornemann et al, 2004).…”