Identification and Characterization of the RLIP/RALBP1 Interacting Protein Xreps1 in Xenopus laevis Early Development

Background: The FGF/Ras/Ral/RLIP pathway is required for the gastrulation process during the early development of vertebrates. The Ral Interacting Protein (RLIP also known as RalBP1) interacts with GTP-bound Ral proteins. RLIP/RalBP1 is a modular protein capable of participating in many cellular functions. Methodology/Principal Findings: To investigate the role of RLIP in early development, a two-hybrid screening using a library of maternal cDNAs of the amphibian Xenopus laevis was performed. Xreps1 was isolated as a partner of RLIP/RalBP1 and its function was studied. The mutual interacting domains of Xreps1 and Xenopus RLIP (XRLIP) were identified. Xreps1 expressed in vivo, or synthesized in vitro, interacts with in vitro expressed XRLIP. Interestingly, targeting of Xreps1 or the Xreps1-binding domain of XRLIP (XRLIP(469–636)) to the plasma membrane through their fusion to the CAAX sequence induces a hyperpigmentation phenotype of the embryo. This hyperpigmented phenotype induced by XRLIP(469–636)-CAAX can be rescued by co-expression of a deletion mutant of Xreps1 restricted to the RLIP-binding domain (Xreps1(RLIP-BD)) but not by co-expression of a cDNA coding for a longer form of Xreps1. Conclusion/Significance: We demonstrate here that RLIP/RalBP1, an effector of Ral involved in receptor-mediated endocytosis and in the regulation of actin dynamics during embryonic development, also interacts with Reps1. Although these two proteins are present early during embryonic development, they are active only at the end of gastrulation. Ou

Extracorporeal Membrane Oxygenation for Severe Acute Respiratory Distress Syndrome associated with COVID-19: An Emulated Target Trial Analysis.

RATIONALE: Whether COVID patients may benefit from extracorporeal membrane oxygenation (ECMO) compared with conventional invasive mechanical ventilation (IMV) remains unknown. OBJECTIVES: To estimate the effect of ECMO on 90-Day mortality vs IMV only Methods: Among 4,244 critically ill adult patients with COVID-19 included in a multicenter cohort study, we emulated a target trial comparing the treatment strategies of initiating ECMO vs. no ECMO within 7 days of IMV in patients with severe acute respiratory distress syndrome (PaO2/FiO2 <80 or PaCO2 ≥60 mmHg). We controlled for confounding using a multivariable Cox model based on predefined variables. MAIN RESULTS: 1,235 patients met the full eligibility criteria for the emulated trial, among whom 164 patients initiated ECMO. The ECMO strategy had a higher survival probability at Day-7 from the onset of eligibility criteria (87% vs 83%, risk difference: 4%, 95% CI 0;9%) which decreased during follow-up (survival at Day-90: 63% vs 65%, risk difference: -2%, 95% CI -10;5%). However, ECMO was associated with higher survival when performed in high-volume ECMO centers or in regions where a specific ECMO network organization was set up to handle high demand, and when initiated within the first 4 days of MV and in profoundly hypoxemic patients. CONCLUSIONS: In an emulated trial based on a nationwide COVID-19 cohort, we found differential survival over time of an ECMO compared with a no-ECMO strategy. However, ECMO was consistently associated with better outcomes when performed in high-volume centers and in regions with ECMO capacities specifically organized to handle high demand. This article is open access and distributed under the terms of the Creative Commons Attribution Non-Commercial No Derivatives License 4.0 (http://creativecommons.org/licenses/by-nc-nd/4.0/)

Interaction analysis of XRLIP with Xreps1 by the two-hybrid technique.

<p>(A) Schematic representation of the XRLIP and Xreps1(253) deletion mutants used here. For XRLIP, a distinctive box represents each interacting domain. For Xreps1(253), the grey box corresponds to the EH homology domain and the black box to the coiled-coil motif. (B) Two-hybrid assay between Xreps1(253), Xreps1(RLIP-BD) or Xreps1(ΔRLIP-BD) and human RLIP (RLIP76), <i>Xenopus</i> RLIP (XRLIP), or Lamin or human Ras (Hras) as controls. (C) Two-hybrid assay between Xreps1(253), Xreps1(RLIP-BD) or Xreps1(ΔRLIP-BD) with different deletion mutants of XRLIP or Lamin as control.</p

Hypothetical model of the interaction of Reps with RLIP during early development.

<p>(A) During cleavage stage, and up to MBT, Ral, RLIP and Reps are present in embryo; however no interactions are detected between them. (B) As of MBT, Ral interacts with RLIP and recruits it at the plasma membrane. It is thought that through Cdc42 it destabilizes the actin cytoskeleton and through μ2 and consequently AP2 allows endocytosis via clathrin. At this stage of development RLIP interacts also with Reps1, but (C) this interaction could be exclusive of ectodermic cells and could use another partner, such as filaggrin.</p

Xreps1 protein interacts with XRLIP-CAAX at the plasma membrane.

<p>Representative confocal micrographs of animal caps dissected at the 2000-cell stage, showing the distribution of Myc-Xreps1(RLIP-BD)-CAAX expressed alone (A) or in the presence of XRLIP(379-Cter)-CAAX (G). Myc-Xreps1(RLIP-BD) mRNA (1 ng) and XRLIP(379-Cter)-CAAX (0.5 ng) were microinjected into the animal hemisphere of 2-cell stage embryos and Myc-Xreps1(RLIP-BD) was visualized with an FITC anti-Myc antibody. The animal caps were fixed and Myc-XRLIP(379-Cter)-CAAX and Myc-Xreps1(RLIP-BD) immuno-stained with an FITC anti-Myc antibody, (panels A, D and G stained in green), the F-actin with phalloidin-rhodamine, (panels B, E and H stained in red) and the merge of Myc-FITC with phalloidin-rhodamine are shown (panels C, F and I). Scale bars represent 50 µm.</p

Characterization of the Xreps1(253) domain critical for interaction with XRLIP, and interaction <i>in vivo</i> of Xreps1(253) and Xreps1(RLIP-BD) with XRLIP.

<p>(A, B) GST-XRLIP or GST alone as control were tested for their ability to pull down Xreps1(253) or Xreps1(RLIP-BD) synthesized as Myc-tagged proteins. The interaction of XRLIP with RalB G23V served as positive control. Xreps1(253) precipitated with 1 µg GST-XRLIP was analyzed. (A) The input or supernatant ranged from 1.25 µl of the 25 µl of reticulocyte extract), or (B) half an embryo from extracts of 10 embryos, were analyzed by SDS-PAGE. (C) Myc-Xreps1(253) or Myc-Xreps1(RLIP-BD) were synthesized in embryos and used to interact with endogenous XRLIP. Total lysate from 10 embryos were incubated with an anti-Myc agarose-conjugated antibody (SC-40, Santa Cruz Biotechnology) and centrifuged. The supernatant and immunoprecipitated proteins were resolved by SDS-PAGE and immunoblotted with anti-RLIP, or anti-Myc antibodies. With Myc-Xreps1(RLIP-BD), the RLIP signal was weakly detected just above an unspecific signal (<). (D) Two µg of GST-Xreps1(253), GST-Xreps1(RLIP-BD), or 1 µg of GST-XralA (G23V) preloaded using GTP-g-S, or GST alone were incubated with Myc-XRLIP (lysate from 10 embryos) and purified with glutathione-agarose beads. The precipitated proteins were resolved by SDS-PAGE, transferred onto a Hybond-P membrane (Amersham) and blotted with a monoclonal anti-Myc antibody (9E10). (E) The table recapitulates the results obtained on the <i>in vivo</i> and <i>in vitro</i> interaction of XRLIP with Xreps1.</p

Phenotypes induced by XRLIP(172–495)-CAAX, XRLIP(330-Cter)-CAAX or XRLIP(469-Cter)-CAAX are amplified by Xreps1(253) and are rescued by Xreps1(RLIP-BD) but not.

<p>(A) The injected embryos were photographed at the gastrula stage. Embryos at the 2-cell stage were injected (a, d, g) with 1 ng of respectively XRLIP(172–495)-CAAX, XRLIP(330-Cter)-CAAX or XRLIP(469-Cter)-CAAX mRNA alone, or co-injected (b, e, h) with 1 ng of Xreps1(253) mRNA, or co-injected (c, f, i) with 1 ng of Xreps1(RLIP-BD) mRNA. Only the hyperpigmented phenotype (a, d, g) disappears (c, f, i). In g and h the yellow arrows show the hyperpigmented cells. (B) Proteins expression, from embryos injected with XRLIP(469-Cter)-CAAX alone (j) or respectively co-injected with Xreps (k) and Xreps(RLIP-BD) (l), were analyzed by Western blot with a 9E10 anti-Myc antibody.</p

Reps1 is uniformly expressed in early development.

<p><i>In situ</i> hybridization of Reps (a and b), and Chordin (c and d) in gastrula (a and c) and neurula (b and d). Each embryo was hybridized with one antisens, and one sens probe. Embryos hybridized with the sens probe are marked with *.</p

Effect of Xreps1(RLIP-BD) and Xreps1(253) on embryo pigmentation during gastrulation.

<p>(A) Animal views of gastrulae. Control uninjected embryos (a). Embryos injected in animal pole of one blastomere at the 2-cell stage with 1 ng mRNA of Myc-Xreps1(RLIP-BD) (b), Myc-Xreps1(253) (c), Mreps1 (d), or Mreps1-CAAX (e) and enlargement of hyperpigmented cell area from an embryo injected with Mreps1-CAAX (f) Red arrows show hypepigmentation area. (B) Protein expression of Xreps1(RLIP-BD) and Xreps1(253) was analyzed by Western blot with a 9E10 anti-Myc antibody, control lanes a, Xreps1(RLIP-BD) b and Xreps1(253) c, and analysis of ³⁵S signal from reticulocyte extracts; control d, Xreps1(RLIP-BD) e and Xreps1(253) f.</p

Interacting domain of RLIP involved in endocytosis.

<p>RLIP through its N-terminal and C-terminal domains interacts with μ2, and POB1 or Reps1 proteins respectively. These proteins interact in turn with complexes involved in clathrin endocytosis. The RhoGAP sequence of RLIP controls the Cdc42 activity and the actin dynamic. The Ral binding domain binds with Ral, in response to activation tyosine kinase receptor through the activation of the Ras protein.</p