R-Spondin2 is a secreted activator of Wnt/beta-catenin signaling and is required for Xenopus myogenesis.
We have carried out a small pool expression screen for modulators of the Wnt/beta-catenin pathway and identified Xenopus R-spondin2 (Rspo2) as a secreted activator of this cascade. Rspo2 is coexpressed with and positively regulated by Wnt signals and synergizes with Wnts to activate beta-catenin. Analyses of functional interaction with components of the Wnt/beta-catenin pathway suggest that Rspo2 functions extracellularly at the level of receptor ligand interaction. In addition to activating the Wnt/beta-catenin pathway, Rspo2 overexpression blocks Activin, Nodal, and BMP4 signaling in Xenopus, raising the possibility that it may negatively regulate the TGF-beta pathway. Antisense Morpholino experiments in Xenopus embryos and RNAi experiments in HeLa cells reveal that Rspo2 is required for Wnt/beta-catenin signaling. In Xenopus embryos depleted of Rspo2, the muscle markers myoD and myf5 fail to be activated and later muscle development is impaired. Thus, Rspo2 functions in a positive feedback loop to stimulate the Wnt/beta-catenin cascade.
PubMed ID: 15469841
Article link: Dev Cell.
Genes referenced: act3 actl6a bmp4 cnot2 dvl2 en2 fgf8 hist1h4d mcf2 myc myf5 myod1 nodal nodal1 not otx2 rax rspo1 rspo2 rspo3 shh sox3 t tgfb1 ventx2.2 wnt1 wnt3a wnt8a
Morpholinos referenced: rspo2 MO1
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|Figure 2. Expression Analysis of Xenopus and Mouse R-spondins(A) Rspo2 expression during Xenopus development at the indicated embryonic stages analyzed by RT-PCR. Histone H4 was used for normalization. −RT, minus reverse transcription control.(B–H) Xenopus whole-mount in situ hybridizations of the indicated genes.(B) Stage 11 embryo, dorso-vegetal view; dbl, dorsal blastoporal lip.(C) Stage 12 embryo, dorsal view with anterior up. An anterior neural expression domain is indicated by arrowhead.(D) Stage 15 embryo, dorsal view with anterior up.(E) Stage 14 embryo, dorsal view with anterior up.(F and G) Tailbud stage embryos; ba, branchial arches; cm, cranial musculature; di, diencephalon; dnt, dorsal neural tube; mhb, midbrain-hindbrain boundary; ov, otic vesicle; pn, pronephros; pdm, proctodeum; s, somites; tb, tailbud mesoderm. Inset in (F) shows a transverse section at the level indicated by arrowhead, showing expression in dorsal neural tube and in the dorsal- and ventral-most parts of the somites.(H) Dissected Xenopus tadpole brain (lateral view) showing expression in diencephalons (di) and zona limitans intrathalamica (zli), where sonic hedgehog is expressed (inset). dt, vt, dorsal and ventral thalamus, respectively; sc, spinal cord; tel, telencephalon.(I–M) Mouse whole-mount in situ hybridizations of the indicated genes.(I) Limb buds of day 12.5 mouse embryos. AER, apical ectodermal ridge.(J) Day 7.5 mouse embryo showing Rspo3 expression in the primitive streak.(K–M) Day 9.5 mouse embryos. di, diencephalon; dnt, dorsal neural tube; met, metencephalon; tel, telencephalon.|
|Figure 3. Regulation of Xenopus R-spondins by Wnt Signaling(A) Comparison of XRspo2, XWnt8, and XWnt3A expression pattern in early neurula embryos by whole-mount in situ hybridization. Dorsal view, anterior up.(B) Top: Diagram of experiment. 4-cell stage embryos were injected with 50 pg pCS-ppl (preprolactin), pCS-XWnt8, or pCS-β-catenin into each blastomere, and DMZs were dissected, cultured until stage 11 equivalent, and analyzed by RT-PCR. Bottom: RT-PCR analysis of the indicated genes. −RT, minus reverse transcription control.(C–E) 4-cell stage embryos were injected with 50 pg pCS-Wnt8, pCS-Wnt3A, or pCS-ppl into one blastomere and fixed at stage 11 for in situ hybridization with XRspo2.|
|rspo3 (R-spondin) gene expression in Xenopus laevis embryo, assayed via in situ hybridization, NF stage 28, lateral view, anterior left, dorsal up.|
|rspo2 (R-spondin 2) gene expression in Xenopus laevis embryo, assayed via in situ hybridization, NF stage 11, vegetal view, dorsal up.|
|rspo2 (R-spondin2) gene expression in Xenopus laevis embryo, assayed via in situ hybridization, NF stage 12, dorsal view, anterior up.|
|rspo2 (R-spondin 2) gene expression in Xenopus laevis embryo, assayed via in situ hybridization, NF stage 15, dorsal view, anterior up.|
|rspo2 (R-spondin 2) gene expression in Xenopus laevis embryo, assayed via in situ hybridization, NF stage 28, lateral view, anterior left, dorsal up. Inset: transverse section, mid-trunk region.|
|rspo2 (R-spondin 2) gene expression in Xenopus laevis embryo, dissected brain, assayed via in situ hybridization, NF stage 50, lateral view, dorsal up, anterior left.|
|rspo3 (R-spondin 3 ) gene expression in Xenopus tropicalis embryo, assayed via in situ hybridization, NF stage 14, dorsal view, anterior up.|
|Figure 4. Rspo2 Inhibits TGF-β Signaling in Xenopus. (A) 4-cell stage embryos were injected animally with 100 pg Rspo2, 100 pg Xwnt8, or 200 pg β-catenin mRNA in each blastomere. At stage 8, animal caps were dissected, cultured until stage 18 equivalent, and analyzed for expression of the indicated marker genes. −RT, minus reverse transcription control. (B) 2-cell stage embryos were injected with 100 pg Rspo2 or preprolactin (ppl) mRNA in each blastomere. Note ectopic cement glands (arrowhead) and shortened body axis in Rspo2-injected embryo. (C-J) Whole-mount in situ hybridizations of the indicated genes. 8-cell stage embryos were injected with 100 pg of Rspo2 or ppl mRNAs as indicated into one animal blastomere. LacZ mRNA was coinjected as lineage tracer in all panels except (C), (G), and (H). (C-H) Stage 15 neurulae in anterior view. (I and J) Stage 11 gastrulae in vegetal view, dorsal up. (K-N) 4-cell stage embryos were injected animally with indicated RNAs. At stage 8, animal caps were dissected, cultured until stage 10 equivalent, and analyzed for expression of Xvent2 (K) or Xbra (L-N). −RT, minus reverse transcription control. Embryos injected with 100 pg preprolactin (ppl) were used as control. Amounts of mRNAs used: 100 pg Rspo2, 50 pg or 250 pg of BMP-4, 50 pg of activin, 25 or 100 pg of Wnt8, 200 pg of β-catenin, 50 or 100 pg of Xnr1, 2 or 20 pg of FGF8. (O) BMP responsive luciferase reporter assay in 293T cells. Concentrations of plasmids were: BRE4-luc, 10 ng, BMP4, 10 ng, Wnt1, 5 ng, Rspo2, 5 ng.|
|Figure 5. Rspo2 Promotes Myogenesis in Xenopus Embryos (A) Diagram of the experiments. 4-cell stage embryos were injected with 50 pg plasmid DNA constructs in all blastomeres, and the indicated fragments were explanted at stage 10.5, cultured, and processed for whole-mount in situ hybridization (C) or RT-PCR (D). (B) Stage 40 equivalent VMZ or LMZ explants. Note tail-like structures in VMZs from Rspo2-injected embryos. (C) In situ hybridization of stage 25 VMZs for muscle actin. (D) RT-PCR analysis for the indicated genes in stage 11 equivalent DMZ and stage 25 equivalent VMZ explants. Xdd1, dominant-negative Xenopus dishevelled, Co, preprolactin. −RT, minus reverse transcription control.|
|Figure 6. Xenopus Rspo2 Is Required for Muscle Formation (A) Rspo2Mo specifically inhibits translation of its cognate DNA. Top: Diagram of the experiment. 2-cell stage Xenopus embryos were injected with 100 pg Myc-tagged Rspo2 mRNA in the animal region, and at 8-cell stage, the same embryos were then injected with 5 ng of Rspo2Mo or CoMo in all animal blastomeres, harvested at stage 11, and processed for Western blot analysis of Myc-tagged Rspo2 and α-tubulin. (B) Depletion of Rspo2 protein causes muscle defects and downregulation of myogeneic markers. 4-cell stage embryos were injected equatorially into one blastomere with 5 ng control morpholino oligonucleotides (CoMo) or Rspo2Mo as indicated together with 50 pg ppl mRNA or 50 pg LacZ RNA as lineage tracer and analyzed at tailbud or gastrula stage by in situ hybridization for the indicated genes. In (a)-(f), double in situ hybridization for gene of interest (dark blue) and for ppl (red) was used. (a-d) Stage 25 embryos. (a and b) Myotomes, visualized by muscle actin expression, show malformations on Rspo2Mo-injected side (b). (c and d) Transversal section at the trunk level showing reduced muscle volume in (d). (e-h) myf5 and myoD expression (dark blue) is downregulated in the Rspo2Mo-injected region (red in [e], [f] or light blue in [g], [h]). (i-l) Xbra and Xnot2 expression (dark blue) is not affected in the region of Rspo2Mo injections (light blue). (C) Rspo2Mo act specifically. Rescue of myf5 reduction by coinjected Rspo2 mRNA. 4-cell stage embryos were injected in one blastomere with 5 ng of control morpholino (CoMo), Rspo2Mo, 50 pg ppl mRNA, or 50 pg Rspo2 mRNA containing mismatches to Rspo2Mo. Expression of myf5 was analyzed by in situ hybridization at gastrula stage. The percentage of embryos with strongly affected (A), moderately reduced (B), and normal (C) myf5 expression as displayed in the representative embryos is indicated. Standard deviation was calculated from three independent experiments.(D) Rspo2Mo blocks Wnt signaling upstream of dishevelled during muscle formation. 4-cell stage embryos were radially injected with 5 ng of Rspo2Mo or CoMo, 50 pg pCS-XWnt8, pCS-dishevelled (Xdsh), pCS-dominant-negative GSK-3β (dnGSK), or pCS-β-catenin. At stage 10.5 DMZs (a) or VMZs (b) or LMZs (c and d) were explanted and cultured until stage 11 (a, c) or 25 (b, d) for RT-PCR analysis of the genes indicated. −RT: minus reverse transcription control.|
|Figure 7. R-spondins Are Required for Wnt Signaling in HeLa Cells (A) RT-PCR analysis showing differential expression of human Rspo1-4 in HeLa and 293T cell lines. Actin was used for normalization. (B) Specificity of siRNAs. 293T cells were cotransfected with pSUPER-Rspo2 or 3 (siRNA), FLAG-tagged Rspo3, and GFP (transfection control). Expression of Rspo3 and GFP were analyzed by Western blot. (C and D) R-spondins are required for Wnt/β-catenin signaling in HeLa cells. Wnt luciferase reporter assay in HeLa cells cotransfected with the indicated constructs. Wnt3A was added as conditioned medium. RLU, relative light units.|