Regulation of morphogen signalling during neural patterning in the Xenopus embryo

Morphogens such as Hedghog, Wnt, FGF, and retinoic acid are important signals whose concentrations need to be tightly regulated in the vertebrate embryo to ensure body axis development and formation of the central nervous system. We first show that the intracellular cytoplasmic protein XSufu acts as a dual regulator of Hedgehog (Hh) and Wnt signals during neural induction and patterning in the Xenopus embryo. We further reveal an essential role of XSufu in the crosstalk of the two pathways, in which β-catenin activates Hh signalling upon overexpression of Gli1, and Gli inhibits Wnt signalling upon overexpression of β-catenin. A biphasic model for the role of XSufu in anteroposterior patterning of the neural plate is presented suggesting that XSufu suppresses anterior Gli and posterior β-catenin transcription factors in a dose-dependent manner.

Then we introduce the secreted serine protease xHtrA1 as feedforward stimulator of long-range FGF signalling. Fibroblast growth factor (FGF) signals activate transcription of xHtrA1, and xHtrA1 stimulates FGF4 and FGF8 gene activities, allowing positive feedback regulation. We also show that xHtrA1 triggers proteolytic cleavage of xBiglycan, xSyndecan-4, and xGlypican-4, suggesting a model, in which xHtrA1 through cleaving proteoglycans releases FGF/proteoglycan complexes that act as long-range messages during anteroposterior patterning, mesoderm induction, and neuronal differentiation.

Third, we present Xenopus retinol dehydrogenase-10 (XRDH10) as a critical enzyme for embryonic vitamin A metabolism and retinoic acid (RA) synthesis in the developing embryo. We show that XRDH10, which oxidizes vitamin A to retinal, is transcriptionally inhibited by RA, suggesting negative feedback regulation at the first step of RA biosynthesis. XRDH10 cooperates with XRALDH2, which further oxidizes retinal to bioactive RA, in Spemann’s organizer during dorsoventral patterning of the embryo. We also show that the nested gene expression and cooperate action of XRDH10 and XRALDH2 form a biosynthetic enzyme code that establishes RA gradients along the anteroposterior neuraxis.