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International audience; Transient receptor potential cation channel‐2 (TRPP2) is a nonspecific Ca2+‐dependent cation channel with versatile functions including control of extracellular calcium entry at the plasma membrane, release of intracellular calcium ([Ca2+]i) from internal stores of endoplasmic reticulum, and calcium‐dependent mechanosensation in the primary cilium. In early Xenopus embryos, TRPP2 is expressed in cilia of the gastrocoel roof plate (GRP) involved in the establishment of left‐right asymmetry, and in nonciliated kidney field (KF) cells, where it plays a central role in early specification of nephron tubule cells dependent on [Ca2+]i signaling. Identification of proteins binding to TRPP2 in embryo cells can provide interesting clues about the mechanisms involved in its regulation during these various processes. Using mass spectrometry, we have therefore characterized proteins from late gastrula/early neurula stage embryos coimmunoprecipitating with TRPP2. Binding of three of these proteins, golgin A2, protein kinase‐D1, and disheveled‐2 has been confirmed by immunoblotting analysis of TRPP2‐coprecipitated proteins. Expression analysis of the genes, respectively, encoding these proteins, golga2, prkd1, and dvl2 indicates that they are likely to play a role in these two regions. Golga2 and prkd1 are expressed at later stage in the developing pronephric tubule where golgin A2 and protein kinase‐D1 might also interact with TRPP2. Colocalization experiments using exogenously expressed fluorescent versions of TRPP2 and dvl2 in GRP and KF reveal that these two proteins are generally not coexpressed, and only colocalized in discrete region of cells. This was observed in KF cells, but does not appear to occur in the apical ciliated region of GRP cells.

The POU (Pit-Oct-Unc) genes encode a large transcription factor family comprising 6 classes (pou1f to pou6f ) involved in many developmental processes, such as cell commitment and differentiation. The pou3f class contains four members (pou3f1, pou3f2, pou3f3, pou3f4) characterized by expression in ectodermal tissue derivatives, such as nervous system and otic vesicle, during mammalian development. In order to obtain insights into the potential conservation of this class of transcription factors in vertebrates, we carried out a phylogenetic analysis and a comprehensive comparative study of pou3f expression in the frog Xenopus laevis. All vertebrates examined possessed members of the four pou3f subfamilies, excepting the zebrafish, which lacked a pou3f4 gene. Whole mount in situ hybridization and real-time quantitative polymerase chain reaction (RT-qPCR) analyses revealed that Xenopus pou3f genes were expressed in the forming neural tube and their expression was maintained in the brain, mostly in the dorsal part, at tailbud stages. The pou3f2, pou3f3, and pou3f4 genes were also expressed in the developing otic vesicle, and pou3f1 in some cells of the epidermis. Besides ectodermal derivatives, pou3f3 and pou3f4 were expressed in the developing kidney. Their expression started at the early tailbud stage in the pronephric anlage and partly overlapped. In the mature pronephric tubule, pou3f3 was restricted to the intermediate tubule, while pou3f4 was also expressed in the distal and connecting tubule. Together, our results highlight a significant conservation of pou3f gene expression in vertebrates and indicate that they may have distinct but also redundant functions during neural and renal development.

International audience; Vibrio harveyi is a marine bacterial pathogen responsible for episodic abalone epidemics associated with massive mortalities in France, Japan, and Australia. The aim of this study was the understanding of a possible role of the p38 MAPK in abalone haemocyte responses towards this bacterium. First, the pathogenicity of different V. harveyi strains was compared in both immersion and injection trials, and clear differences were detected. The three strains, ORM4, 04/092, and 05/053, all isolated from moribund abalone, induced up to 80% mortalities in immersion or injection challenges (LD(50) (ORM4) = 2.5 x 10(2) CFU animal(-1)). The two strains, LMG 4044T and LMG 7890 were non-pathogenic towards abalone in immersion trials, and needed very high numbers for killing by intramuscular injections (LD(50) = 8.9 x 10(4) and 1.6 x 10(5) CFU animal(-1), respectively). To start unraveling the mechanism explaining these differences, the p38-MAPK, a keyplayer in antimicrobial immune response, was studied. The non-pathogenic strain, LMG 7890 can be eliminated by abalone haemocytes and induces haemocyte phagocytosis and high ROS production. With different concentrations of a p38-specific inhibitor, SB203580, p38 implication was shown. This inhibitor reduced phagocytosis and ROS induction leading to LMG 7890 proliferation. In the case of the pathogenic ORM4 which can not be eliminated by abalone haemocytes, no phagocytosis and ROS production was induced, and a retarded p38 activation was observed. Taken together, our results suggest that p38 MAPK modulation may be one of the ways of virulent V. harveyi to attack its host and escape abalone immune response.

International audience; Translational control was investigated in sea urchin eggs and embryos in response to the DNA-damaging agent methyl methanesulfonate (MMS). We have shown in this report that exposure of sea urchin embryos to MMS induces drastic effects on protein synthesis activity, and on translation factors level, integrity and post-translational modifications. In response to the treatment of embryos by the DNA-damaging agent MMS, protein synthesis is inhibited independently of the translation inhibitor 4E-BP and in correlation with phosphorylation of the translation factor eIF2alpha subunit. Furthermore, a low molecular weight form of translation initiation factor eIF4G is detected correlatively with MMS-induced apoptosis. We propose that modifications of translation factors play an important role in protein synthesis modulation that occurs during DNA-damage induced apoptosis.

Dystroglycan (Dg) is a transmembrane receptor for laminin that must be expressed at the right time and place in order to be involved in notochord morphogenesis. The function of Dg was examined in Xenopus laevis embryos by knockdown of Dg and overexpression and replacement of the endogenous Dg with a mutated form of the protein. This analysis revealed that Dg is required for correct laminin assembly, for cell polarization during mediolateral intercalation and for proper differentiation of vacuoles. Using mutations in the cytoplasmic domain, we identified two sites that are involved in cell polarization and are required for mediolateral cell intercalation, and a site that is required for vacuolation. Furthermore, using a proteomic analysis, the cytoskeletal non-muscle myosin IIA has been identified for the first time as a molecular link between the Dg-cytoplasmic domain and cortical actin. The data allowed us to identify the adhesome laminin-Dg-myosin IIA as being required to maintain the cortical actin cytoskeleton network during vacuolation, which is crucial to maintain the shape of notochordal cells.

International audience; Background information In Xenopus, the functional kidney of the tadpole, the pronephros, forms from the kidney field, which is specified at completion of gastrulation. Specification of the kidney field requires retinoic acid (RA) signalling during gastrulation, while fibroblast growth factor (FGF) signals inhibit should be inhibit this process. Results We have analysed the functional interactions taking place during gastrulation between RA and FGF signals in the lateral marginal zone (LMZ), that is in the environment of unspecified pronephric mesoderm precursors. Inhibition of FGF receptor (FGFR) signalling with SU5402 does not significantly affect expression of genes encoding RA metabolism enzymes and RA receptor-a in LMZ explants. Furthermore, SU5402 has no effect on the expression of hoxa1, a major RA target in the LMZ, showing that FGF is not antagonising RA in the LMZ. Disruption of RA signalling affects FGF ligand production to some extent, especially FGF8b, but the strongest effect is the down-regulation of the mitogen-activated protein kinase phosphatase-3 (MKP3)-encoding gene, mkp3. A strong up-regulation of mkp3 occurs in response to exogenous RA. This effect is reduced in a context of FGFR inhibition, suggesting that RA and FGF signals are co-operating upstream of mkp3. Mkp3 knockdown results in an inhibition of the kidney field markers pax8 and lhx1 and in a defective development of the pronephros. Conclusions FGF is not negatively influencing pronephric specification by antagonising RA signalling. Functional interactions between RA and FGF rather take place at the level of the transcriptional regulation of mkp3, indicating that RA may antagonise FGF signalling at the level of the extracellular signal-regulated kinase (Erk) pathway. A fine tuning of Erk signalling by MKP3 is important for the proper establishment of the kidney field.

International audience; Cell growth, proliferation, differentiation and survival are influenced by the availability of oxygen. The effect of hypoxia on embryonic cells and the underlying molecular mechanisms to maintain cellular viability are still poorly understood. In this study, we show that hypoxia during Xenopus embryogenesis rapidly leads to a significant developmental delay and to cell apoptosis after prolonged exposure. We provide strong evidence that hypoxia does not affect somitogenesis but affects the number of mitotic cells and muscle-specific protein accumulation in somites, without interfering with the expression of MyoD and MRF4 transcription factors. We also demonstrate that hypoxia reversibly decreases Akt phosphorylation and increases the total amount of the translational repressor 4E-BP, in combination with an increase of the 4E-BP associated with eIF4E. Interestingly, the inhibition of PI3-kinase or mTOR, with LY29002 or rapamycin, respectively, triggers the 4E-BP accumulation in Xenopus embryos. Finally, the overexpression of the non-phosphorylatable 4E-BP protein induces, similar to hypoxia, a decrease in mitotic cells and a decrease in muscle-specific protein accumulation in somites. Taken together, our studies suggest that 4E-BP plays a central role under hypoxia in promoting the cap-independent translation at the expense of cap-dependent translation and triggers specific defects in muscle development.

BACKGROUND: We had previously reported that the Suppression Subtractive Hybridization (SSH) approach was relevant for the isolation of new mammalian genes involved in oogenesis and early follicle development. Some of these transcripts might be potential new oocyte and granulosa cell markers. We have now characterized one of them, named TOPAZ1 for the Testis and Ovary-specific PAZ domain gene. PRINCIPAL FINDINGS: Sheep and mouse TOPAZ1 mRNA have 4,803 bp and 4,962 bp open reading frames (20 exons), respectively, and encode putative TOPAZ1 proteins containing 1,600 and 1653 amino acids. They possess PAZ and CCCH domains. In sheep, TOPAZ1 mRNA is preferentially expressed in females during fetal life with a peak during prophase I of meiosis, and in males during adulthood. In the mouse, Topaz1 is a germ cell-specific gene. TOPAZ1 protein is highly conserved in vertebrates and specifically expressed in mouse and sheep gonads. It is localized in the cytoplasm of germ cells from the sheep fetal ovary and mouse adult testis. CONCLUSIONS: We have identified a novel PAZ-domain protein that is abundantly expressed in the gonads during germ cell meiosis. The expression pattern of TOPAZ1, and its high degree of conservation, suggests that it may play an important role in germ cell development. Further characterization of TOPAZ1 may elucidate the mechanisms involved in gametogenesis, and particularly in the RNA silencing process in the germ line

International audience; Using sea urchin early embryos as a pertinent model, chromium(III) provoked cell cycle arrest and induced apoptosis. The molecular machinery of translation initiation was investigated. Chromium provoked a time- and dose-dependent increase in the level of 4E-BP protein, the natural regulator of the cap-dependent initiation factor 4E (eIF4E). The 4E-BP increase was the result of 4E-BP stabilization and appeared functional for physiological eIF4E binding, removal of eIF4E from the initiation factor eIF4G, and almost full inhibition of cap-dependent translation in vivo. The protein 4E-BP may be involved in the biological pathway of apoptosis associated with the activation of the DNA-damaged checkpoint of the cell cycle.

In Xenopus laevis embryos, kidney field specification is dependent on retinoic acid (RA) and coincides with a dramatic increase of Ca 2+ transients, but the role of Ca 2+ signaling in the kidney field is unknown. Here, we identify TRPP2, a member of the transient receptor potential (TRP) superfamily of channel proteins encoded by the pkd2 gene, as a central component of Ca 2+ signaling in the kidney field. TRPP2 is strongly expressed at the plasma membrane where it might regulate extracellular Ca 2+ entry. Knockdown of pkd2 in the kidney field results in the downregulation of pax8 , but not of other kidney field genes ( lhx1 , osr1 and osr2 ). We further show that inhibition of Ca 2+ signaling with an inducible Ca 2+ chelator also causes downregulation of pax8 , and that pkd2 knockdown results in a severe inhibition of Ca 2+ transients in kidney field explants. Finally, we show that disruption of RA results both in an inhibition of intracellular Ca 2+ signaling and of TRPP2 incorporation into the plasma membrane of kidney field cells. We propose that TRPP2-dependent Ca 2+ signaling is a key component of pax8 regulation in the kidney field downstream of RA-mediated non-transcriptional control of TRPP2.