Loss of cell adhesion in Xenopus laevis embryos mediated by the cytoplasmic domain of XLerk, an erythropoietin-producing hepatocellular ligand

Abstract

The erythropoietin-producing hepatocellular (Eph) family of ligands and receptors has been implicated in the control of axon guidance and the segmental restriction of cells during embryonic development. In this report, we show that ectopic expression of XLerk, a Xenopus homologue of the murine Lerk-2 (ephrin-B1) transmembrane ligand, causes dissociation of Xenopus embryonic blastomeres by the mid-blastula transition. Moreover, a mutant that lacks the extracellular receptor binding domain can induce this phenotype. The carboxyl-terminal 19 amino acids of the cytoplasmic domain of XLerk are necessary but not sufficient to induce cellular dissociation. Basic fibroblast growth factor, but not activin, can rescue both the loss of cell adhesion and mesoderm induction in ectodermal explants expressing XLerk. Collectively, these results show that the cytoplasmic domain of XLerk has a signaling function that is important for cell adhesion, and fibroblast growth factor signaling modulates this function.

Figure 1

XLerk-induced blastomere dissociation in Xenopus embryos. Each blastomere of the 2-cell stage embryo was injected with 2.5 ng of either capped XLerk mRNA or XLerk^1–215 mRNA, and development proceeded to the mid-blastula stage.

Figure 2

Western blot analysis of embryos injected with RNA encoding normal and mutant forms of XLerk. 2-Cell embryos were injected with 2.5 ng of RNA encoding all XLerk forms, except in the cases of XLerk^{215–327,215–307,215–287}, where 0.5 ng of RNA were used. Extracts were prepared from stage 9 embryos, and three embryo equivalents were fractionated on 12% SDS-PAGE and then immunoblotted for the Flag epitope.

Figure 3

Localization of normal and mutant forms of XLerk by immunofluorescence. Embryos were injected with 2.5 ng of RNA encoding normal and mutant XLerk constructs. At stage 9, the embryos were fixed in Carnoy’s solution, embedded in parafin, and sectioned; then, immunofluorescence was performed using Flag M2 antibody and fluorescein-conjugated anti-mouse-Ig. These immunofluorescent images show that XLerk mutants retaining the transmembrane and cytoplasmic domains are localized to the membrane. (A) XLerk; (B) XLerk^215–327; (C) XLerk^1–308; (D) XLerk^215–308. Note that an intact blastocoel is revealed in embryos expressing a C-terminal deletion of the last 19 amino acids in C. (E) Diffuse staining of XLerk^250–327 that retains only the cytoplasmic domain. (F) Very low levels of diffuse staining of XLerk^1–215, which just retains the ectodomain. (G) Membrane localization of XLerk^{1–250/307–327} and also an intact blastocoel. (H) Flag peptide competition control showing the specificity of the staining.

Figure 4

Scanning electron microscopy of XLerk-expressing embryos during the early blastula stage of development. 2-Cell stage embryos were injected with 2.5 ng of capped XLerk, XLerk^1–308, or XLerk^1–288 mRNA in each blastomere. Embryos were then fixed at stage 8 in 4% formaldehyde/1.25% glutaraldehyde and prepared for SEM.

Figure 5

(A) Western analysis of embryos overexpressing XLerk and C-cadherin. Embryos were either left alone or injected with 4 ng of XLerk RNA, or coinjected with 1 ng of C-cadherin RNA. At stage 9, extracts were prepared, and three embryo equivalents were fractionated by SDS-PAGE. Samples were immunoblotted with C-cadherin antibodies and a C-terminal XLerk antibody (cadherin appears as a doublet, perhaps representing the precursor and product forms). (B) C-Cadherin and β-catenin expression and association in XLerk-injected embryos. Embryos were either left alone or injected with 5 ng of XLerk RNA. Stage 9 embryos were harvested, and extracts were prepared. Three embryo equivalents were fractionated by SDS PAGE, and five embryo equivalents were subjected to immunoprecipitation analysis using β-catenin antibodies (β-Cat IP) and then fractionated by SDS-PAGE. Samples were immunoblotted with antibodies to β-catenin, C-cadherin, or phosphotyrosine.

Figure 6

(A) Morphological analysis of animal caps demonstrating the rescue of XLerk-injected embryos by bFGF. Animal caps were dissected at stage 8.5 and cultured at 23°C in 67% Leibovitz’s L-15 medium, 7 mM Tris⋅HCl (pH 7.5), and 1 mg/ml gentamycin with or without 50 ng/ml Activin or 100 ng/ml bFGF. The animal caps were harvested at stage 26 as determined by uninjected embryos. These results are representative of four independent experiments. (B) Effect of XLerk on gene expression in the animal cap. Embryos were injected with either 5 ng of XLerk mRNA or β-galactosidase mRNA at the 2-cell stage. The animal caps were explanted at stage 8 and cultured in 50 ng/ml activin or 100 ng/ml bFGF until stage 11. Upon harvesting, RT-PCR analysis using primers specific for gsc, Xbra, and EF-1α was performed.