Ossipova O et al. (2020), Pinhead signaling regulates mesoderm heterogene...

XB-ART-57304

Development 2020 Sep 11;14717:. doi: 10.1242/dev.188094.

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Pinhead signaling regulates mesoderm heterogeneity via the FGF receptor-dependent pathway.

Ossipova O , Itoh K , Radu A , Ezan J , Sokol SY .

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Among the three embryonic germ layers, the mesoderm plays a central role in the establishment of the vertebrate body plan. The mesoderm is specified by secreted signaling proteins from the FGF, Nodal, BMP and Wnt families. No new classes of extracellular mesoderm-inducing factors have been identified in more than two decades. Here, we show that the pinhead (pnhd) gene encodes a secreted protein that is essential for the activation of a subset of mesodermal markers in the Xenopus embryo. RNA sequencing revealed that many transcriptional targets of Pnhd are shared with those of the FGF pathway. Pnhd activity was accompanied by Erk phosphorylation and required FGF and Nodal but not Wnt signaling. We propose that during gastrulation Pnhd acts in the marginal zone to contribute to mesoderm heterogeneity via an FGF receptor-dependent positive feedback mechanism.

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Species referenced: Xenopus laevis
Genes referenced: cdx4 dkk1 esr-5 fgf2 fgfr1 fgfr4 hoxd1 mapk1 msgn1 nodal pnhd smad1 smad2 tbxt wnt8a
GO keywords: fibroblast growth factor receptor signaling pathway [+]
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???displayArticle.gses??? GSE143795: Xenbase, NCBI
Phenotypes: Xla Wt + Xla.pnhd-FLAG (Fig.1.D) [+]

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	Fig. 1 Pnhd is a secreted protein that promotes posterior development. (A) Alignment of CK domains from Pnhd and other secreted proteins. Spacing is indicated by numbers of non-conserved amino acids between conserved cysteine residues. X.l., Xenopus laevis; S.p., Stegastes partitus; H.s., Homo sapiens. (B) Secretion of Pnhd by Xenopus gastrula cells. Four-cell embryos were injected with 0.5 ng of Flag-Pnhd RNA for each blastomere, cultured to the onset of gastrulation and dissociated to individual cells. Pnhd levels were compared in the media conditioned for 3 h and the corresponding cell lysates. (C) Pnhd is secreted by transfected HEK293Tcells. Deletion of the putative signal peptide in PnhdSP prevents secretion. (D) Head defects in embryos injected dorsally with 2 ng of pnhd RNA at the four-cell stage. Frequencies of embryos with head defects and their total number are indicated. The results are representative of more than five independent experiments. A, anterior; cg, cement gland; CM, conditioned medium; IB, immunoblot; IP, immunoprecipitation; P, posterior.
	Fig. 2. Pnhd transcript localization at different developmental stages. WISH was carried out with albino embryos using pnhd antisense and sense RNA probes. (A) Vegetal view of a stage 10 embryo (dorsal is up). Arrowhead points to vegetal endoderm. (B) Cross-section of a stage 10 embryo. (C) Control embryo, stage 10 (sense probe). (D) Vegetal view of a stage 11.5 embryo (dorsal is up). (E) Vegetal view of a stage 12 embryo (dorsal side is up). Arrows in A,D,E indicate mesodermal expression. (F) Dorsal view of a stage 14 embryo (anterior is up). Arrowhead marks the neuroectoderm. (G) Cross-section of the embryo shown in F. (H) Anterior view of a stage 19 embryo (dorsal is up). (I) Posterior view of a stage 19 embryo (dorsal is up). Arrow indicates staining in the tailbud. (J) Side view of a stage 25 embryo. (K) Head of a stage 25 embryo (anterior is left). Arrowhead points to the signal in the superficial ectoderm cells that are dorsal to the cement gland. (L) Cross-section corresponding to the midbrain level of embryo in J. (M) Dorsal view of a stage 25 embryo. (N) Stage 25, control sense probe. (O) Lateral view of a stage 25 embryo tailbud. Anterior is to the left in J,K,M-O. (P) Transverse section of a stage 25 embryo corresponding to J and O. Dashed lines mark the approximate level of corresponding sections (indicated by bold letters). Dorsoventral (D/V) and anteroposterior (A/P) embryonic axes are indicated. Abbreviations: bc, blastocoel; bv, brain ventricle; cg, cement gland; df, dorsal fin; dl, dorsal blastopore lip; en, endoderm; ev, eye vesicle; lm, lateral mesoderm; me, mesoderm; mhb, midbrain-hindbrain boundary; ne, neuroectoderm; nt, neural tube; ntb, neural tube border; no, notochord; ov, otic vesicle; s, somites; sm, somitic mesoderm; tb, tailbud. Scale bars: 50 µm (B,G,L); 25 µm (P).
	Fig. 3. Pnhd induces mesoderm in ectodermal explants. (A-E) Early embryos were injected with 1-2 ng of Pnhd RNA. Ectoderm explants were dissected at late blastula stages and cultured until stage 12 to examine morphology (B,C) and gene expression by RT-PCR (D,E). (B,C) Pnhd RNA induced animal cap elongation by stage 12. Frequencies of elongated explants and their total number are indicated. The results represent more than five independent experiments. (D) Induction of selected mesodermal markers by Flag-Pnhd RNA (1 ng). (E) Flag-Pnhd RNA has the same ability to induce tbxt as untagged Pnhd RNA in RT-qPCR, but this activity is lost in Flag-PnhdSP lacking the signal peptide (2 ng of each RNA). (F) Pnhd protein was purified from the supernatants of transfected HEK293T cells. To assess its mesoderm-inducing activity, ectoderm explants were dissected from stage 10 embryos and cultured in 0.6×MMR solution containing 1.5 µg/ml or 6.5 µg/ml of Pnhd. RT-qPCR was carried out for tbxt at stage 11 or stage 14. Data are mean±s.d. Significance was determined by an unpaired two-tailed Student's t-test. P<0.01, *P<0.001.
	Fig. 4. RNA sequencing defines Pnhd target genes. (A) Heatmap of gene expression in the Pnhd-expressing and control uninjected animal pole cells that were cultured until stages 11 and 12. The duplicate samples are highly similar. (B) Volcano plot shows top genes upregulated by Pnhd. (C) Differentially expressed genes that are induced by Pnhd RNA (1.5 ng) in animal caps. The list was derived from the top 100 genes induced by Pnhd in four independent RNA-seq experiments. (D) qRT-PCR validation of Pnhd targets in animal caps. Data are mean±s.d. Significance was determined by an unpaired two-tailed Student's t-test. P<0.01, P<0.001, ***P<0.0001.
	Fig. 5. Pnhd is required for mesoderm formation. (A-D) WISH validates changes in cdx4 expression in embryos with manipulated Pnhd levels in one half of each embryo. Red-gal was used as a β-galactosidase substrate (red) for lineage tracing of the injected area. Compare gene expression (dark staining) between the injected (red) and uninjected sides. (E) Quantification of changes in cdx4 RNA in Pnhd-depleted or overexpressing embryos. (F) RT-qPCR confirmation of the downregulation of cdx4, hoxd1, msgn1 and tbxt in stage 10.5 marginal zone explants depleted of pnhd. Data are mean±s.d. Significance was determined by an unpaired two-tailed Student's t-test. P<0.05, *P<0.01.
	Fig. 6. Pnhd response requires FGF but not Wnt signaling. (A-C) Embryos were injected in the animal pole region at the two-cell stage with 1-2 ng of Pnhd RNA, FGFR1-Fc, FGFR4-Fc or dnFGFR1 RNA (2 ng each), 1 ng of Wnt8 or 300 pg of Dkk1 RNA, as indicated. Ectoderm explants were dissected at stages 9-10 and cultured until stages 11-11.5 for gene expression analysis by RT-qPCR. (A) The induction of tbxt and cdx4 by Pnhd is blocked by the FGF inhibitor SU5402 (100 µm). Stimulation with bFGF was used as a positive control. (B) Gene target activation by Pnhd was prevented by DN-FGFR1 and secreted forms of FGFR1-Fc and FGFR4-Fc. (C) The Wnt inhibitor Dkk1 did not affect Pnhd signaling but effectively blocked Wnt8 responses. Data are mean±s.d. Significance was determined by an unpaired two-tailed Student's t-test. P<0.05, P<0.01, P<0.001, **P<0.0001.
	Fig. 7. Pnhd inhibits Akt, but activates Erk1. (A) Schematic of the experiments shown in B-D. Embryos were injected with RNAs encoding Flag-Pnhd (1.5 ng), p110CAAX (0.5 ng), or treated with FGF protein, as indicated. Ectoderm explants were dissected at stages 8, 9.5 or 10, and cultured until the desired stage for immunoblotting with indicated antibodies. (B) Comparison of Pnhd and FGF effects on blastula ectoderm. Pnhd inhibits Akt phosphorylation in ectoderm explants isolated at stage 8 and analyzed at stage 10. FGF has no effect on Akt, but activates Erk. (C) Pnhd inhibits Akt but induces Erk phosphorylation in ectoderm isolated at stage 9.5 and cultured until stage 10.25. This result has been obtained in at least ten experiments. The separator line serves to indicate that several irrelevant gel lanes have been omitted. (D) Pnhd-dependent stimulation of Erk is not affected by the Akt activator p110CAAX. (E) Erk1 phosphorylation in Pnhd-expressing embryos at stage 11. Embryos were injected with RNAs encoding Flag-Pnhd or Flag-PnhdSP (1.5 ng each). (F) Downregulation of Erk1 phosphorylation in lysates of stage 11 embryos injected with Pnhd MOsp. Embryos were injected with RNAs encoding dnFGFR1 (1.5 ng each) or 40 ng of Pnhd MOsp, as indicated. There are no detectable changes in β-catenin or phospho-Smad1. (G) Pnhd promotes Smad2 phosphorylation by Activin. Ectoderm explants were dissected from the injected embryos at stage 8 and cultured for 1 h with or without Activin. Immunoblot analysis with anti-pSmad2 antibodies is shown. Pnhd is detected by anti-Flag antibodies (arrowhead). Erk1 is a control for loading. WE, whole embryo controls in C,D,F.
	Fig. 8. Pnhd cooperates with endogenous-inducing signals to promote mesoderm formation during gastrulation. (A) Schematic of the experiments presented in B,C. Mesoderm-inducing signals are indicated by orange arrows, and the cells receiving them are in light brown. Each animal blastomere of two-cell embryos received 1 ng of Pnhd RNA. Pnhd-expressing or control ectoderm explants were isolated at stage 8 or stage 10, as indicated. When the control embryos reached stage 12.5, the explants were lysed for immunoblotting with antibodies specific for pErk1 and Erk (B) and for RT-qPCR analysis of cdx4 and tbxt transcripts (C). (B) Erk is synergistically activated by Pnhd and endogenous signals in ectoderm explants isolated at stage 10. (C) Cooperative activation of mesodermal gene targets by Pnhd and endogenous signals in ectoderm dissected at stage 10. (D,E) Pnhd is required for mesoderm formation in response to endogenous-inducing signals. (D) Schematic of the experiment shown in E. Model for Pnhd function (right). Pnhd is activated in the marginal zone by Nodal and FGF signaling and functions within the newly induced mesodermal layer. D, dorsal mesoderm; V, ventral mesoderm. The green color indicates presumptive endoderm in the vegetal pole, the gray color indicates presumptive ectoderm at the animal pole, and the future mesoderm is represented by brown shading. (E) Pnhd is required for mesoderm formation in animal-vegetal conjugates. Pnhd MOsp-injected or control animal pole explants were combined with vegetal explants at stage 8. After culture until stage 11, levels of cdx4 and hoxd1 transcripts were determined in the conjugates by RT-qPCR. Data are mean±s.d. Significance was determined by an unpaired two-tailed Student's t-test. **P<0.01.

References [+] :

Amaya, Expression of a dominant negative mutant of the FGF receptor disrupts mesoderm formation in Xenopus embryos. 1991, Pubmed, Xenbase