Retinoic acid (RA) is an important morphogen that regulates many biological processes, including the development of the central nervous system (CNS). Its synthesis from vitamin A (retinol) occurs in two steps, with the second reaction -catalyzed by retinal dehydrogenases (RALDHs) -long considered to be crucial for tissue-specific RA production in the embryo. We have recently identified the Xenopus homologue of retinol dehydrogenase 10 (XRDH10) that mediates the first step in RA synthesis from retinol to retinal. XRDH10 is specifically expressed in the dorsal blastopore lip and in other domains of the early embryo that partially overlap with XRALDH2 expression. We show that endogenous RA suppresses XRDH10 gene expression, suggesting negativefeedback regulation. In mRNA-injected Xenopus embryos, XRDH10 mimicked RA responses, influenced the gene expression of organizer markers, and synergized with XRALDH2 in posteriorizing the developing brain. Knockdown of XRDH10 and XRALDH2 by specific antisense morpholino oligonucleotides had the opposite effects on organizer gene expression, and caused a ventralized phenotype and anteriorization of the brain. These data indicate that the conversion of retinol into retinal is a developmentally controlled step involved in specification of the dorsoventral and anteroposterior body axes, as well as in pattern formation of the CNS. We suggest that the combinatorial gene expression and concerted action of XRDH10 and XRALDH2 constitute a 'biosynthetic enzyme code' for the establishment of a morphogen gradient in the embryo.

We found that the secreted serine protease xHtrA1, expressed in the early embryo and transcriptionally activated by FGF signals, promotes posterior development in mRNA-injected Xenopus embryos. xHtrA1 mRNA led to the induction of secondary tail-like structures, expansion of mesoderm, and formation of ectopic neurons in an FGF-dependent manner. An antisense morpholino oligonucleotide or a neutralizing antibody against xHtrA1 had the opposite effects. xHtrA1 activates FGF/ERK signaling and the transcription of FGF genes. We show that Xenopus Biglycan, Syndecan-4, and Glypican-4 are proteolytic targets of xHtrA1 and that heparan sulfate and dermatan sulfate trigger posteriorization, mesoderm induction, and neuronal differentiation via the FGF signaling pathway. The results are consistent with a mechanism by which xHtrA1, through cleaving proteoglycans, releases cell-surface-bound FGF ligands and stimulates long-range FGF signaling.

Hedgehog (Hh) and Wnt proteins are important signals implicated in several aspects of embryonic development, including the early development of the central nervous system. We found that Xenopus Suppressor-of-fused (XSufu) affects neural induction and patterning by regulating the Hh/Gli and Wnt/β-catenin pathways. Microinjection of XSufu mRNA induced expansion of the epidermis at the expense of neural plate tissue and caused enlargement of the eyes. An antisense morpholino oligonucleotide against XSufu had the opposite effect. Interestingly, both gain- and loss-of-function experiments resulted in a posterior shift of brain markers, suggesting a biphasic effect of XSufu on anteroposterior patterning. XSufu blocked early Wnt/β-catenin signaling, as indicated by the suppression of XWnt8-induced secondary axis formation in mRNA-injected embryos, and activation of Wnt target genes in XSufu-MO-injected ectodermal explants. We show that XSufu binds to XGli1 and Xβ-catenin. In Xenopus embryos and mouse embryonic fibroblasts, Gli1 inhibits Wnt signaling under overexpression of β-catenin, whereas β-catenin stimulates Hh signaling under overexpression of Gli1. Notably, endogenous Sufu is critically involved in this crosstalk. The results suggest that XSufu may act as a common regulator of Hh and Wnt signaling and contribute to intertwining the two pathways.

Down-regulation of the IL-17RA receptor by shRNA decreased IL-6 expression induced by IL-17A via p38 MAPK and ERK1/2 phosphorylation in HSCs. Suppression of IL-17RA expression may be a strategy to reduce the inflammatory response induced by IL-17A in the liver.

able. In situ hybridization on E14.5 and E16.5 forelimbs confirmed delayed chondrocyte differentiation since Indian hedgehog (Ihh) is not expressed in the ulna of Ulnaless or Hoxa11//d11/ mice. Fibroblast growth factor 3 (Fgfr3), which is expressed at low levels in distal cells and at high levels in columnar chondrocytes, shows low expression in the Ulnaless and Hoxa11//d11/ zeugopod resembling the expression level in distal chondrocytes. However, the Unique cartilage matrix-associated protein (UCMA), a specific marker for distal chondrocytes, is only expressed in the outermost cell layers in ulna and radius of both mutant mouse lines.These results indicate that chondrocyte differentiation is arrested at an early step in Ulnaless and Hoxa11/d11 double mutant mice, before the differentiation of distal and columnar chondrocytes takes place.In the developing central nervous system, the balanced activity of different SoxB family transcription factors plays an important role in the regulation of neuronal proliferation and differentiation. The SoxB1 gene Sox2 has been shown to inhibit neurogenesis, maintaining cells in a proliferative neuronal precursor state in the chicken neural tube. Conversely the SoxB2 family member Sox21 promotes neurogenesis by antagonizing Sox2 activity [Sandberg, 2005. Nat. Neurosci. 8 (8) 2005, 995-1001]. A similar interplay may regulate the embryonic production of hair cells in the sensory epithelia of the inner ear. In fact, the Sox2 gene is expressed in progenitor cells that give rise to sensory hair cells, and a previous study has reported expression of Sox21 in the chicken inner ear at HH stage 30 [E6-6.5; Uchikawa, 1999. Mech. Dev. 84 (1-2), 103-120]. Here we analysed in detail the expression pattern of Sox21 during the development of the chicken inner ear, and found that it is first detected in the prosensory domains at E5. Sox21 expression becomes gradually restricted to the hair cell layer as development of the inner ear progresses. We also obtained preliminary data showing that overexpression of Sox21 can down-regulate the expression of the progenitor cell marker Prox-1. These findings suggest that Sox21 could play a role in the specification of inner ear sensory cells. Fibroblast growth factors (FGFs) play an important role in development and homeostasis. We previously presented the xHtrA1 as positive regulator of FGF signaling in the extracellular space [Hou etal., 2007]. The data suggested that xHtrA1 through cleaving proteoglycans release cell-surface bound FGF ligands and stimulate long-range FGF signaling during establishment of the embryonic body plan [Gallagher, 2007]. If not tightly controlled, the proteolytic activity of xHtrA1 would lead to an unlimited amplification and propagation of FGF signals. We have isolated a full-length cDNA clone encoding a secreted serine protease inhibitor (xSPI) that may act as a negative regulator of xHtrA1/FGF signals. xSPI shows distinct expression in the early embryo and promotes anterior development in mRNA-injected Xenopus embryos. x...