- In Xenopus, the Noggin protein, accomplishes two major functions of the organizer: it
induces dorsal ectoderm to form neural tissues, and it dorsalizes mesoderm cells. Which one of the following observations is correct with respect to Noggin?
(1) If a plasmid clone expressing Noggin protein is microinjected into a lithium chloride treated
Xenopus gastrula, it should rescue the abnormalities induced by lithium chloride treatment.
(2) If a plasmid clone expressing Noggin protein is microinjected into UV-treated embryo which
does not give rise to neural tube, it will rescue the abnormality.
(3) RNA in situ hybridization of noggin cDNA on Xenopus embryo will show its presence in all
regions except the dorsal blastopore lip.
(4) Microinjection of noggin mRNA into the embryo region fated to make the ventral part will
promote its ventralization.
Introduction
Noggin is a crucial signaling protein secreted by the Spemann organizer in Xenopus embryos, performing major functions including neural induction and dorsal mesoderm specification. UV irradiation of Xenopus embryos disrupts cortical rotation and dorsal axis formation, resulting in ventralized embryos lacking neural structures. Microinjection of Noggin protein or its expressing plasmid into these UV-treated embryos rescues the formation of dorsal tissues such as the neural tube, demonstrating Noggin’s central role in embryonic patterning.
UV Treatment in Xenopus Embryos
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UV irradiation of early Xenopus embryos destroys molecular determinants responsible for dorsal specification by inhibiting cortical rotation, leading to ventralized embryos.
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These embryos lack dorsal structures including the Spemann organizer, notochord, and neural tube.
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The ventralization phenotype serves as a model to test dorsalizing factors like Noggin.
Noggin Function and BMP Antagonism
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Noggin is a secreted extracellular protein that binds to BMP molecules, preventing BMP signaling which normally promotes ventral and epidermal fates.
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By blocking BMP signals, Noggin induces ectoderm to adopt a neural fate and dorsalizes mesodermal cells.
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Noggin mRNA or protein microinjection can restore dorsal structures lost after UV ventralization treatment.
Rescue of UV-Ventralized Embryos by Noggin
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Microinjection of Noggin expressing plasmid into UV-treated embryos reinitiates dorsal specification and neural induction, rescuing neural tube formation.
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The rescued embryos develop dorsal structures demonstrating the sufficiency of Noggin to restore organizer activity disrupted by UV irradiation.
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This rescue validates Noggin’s fundamental role in establishing dorsal-ventral polarity and neural induction.
Incorrect Options Explanation
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Microinjection of Noggin plasmid into lithium chloride-treated embryos (option 1) will not rescue because LiCl leads to hyperdorsalization (excess β-catenin stabilization) rather than ventralization.
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Noggin is expressed primarily in the dorsal blastopore lip and not absent from it (option 3 is wrong).
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Microinjection of Noggin mRNA into the ventral region leads to dorsalization, not ventralization (option 4 is incorrect).
Summary Table: Evaluation of Statements
| Option | Description | Correctness | Explanation |
|---|---|---|---|
| (1) | Noggin plasmid rescues lithium chloride-treated embryo abnormalities | Incorrect | LiCl causes dorsalization, so rescue is not applicable here |
| (2) | Noggin plasmid rescues UV-treated embryo neural tube formation | Correct | Noggin rescues ventralized embryos by restoring dorsal structures |
| (3) | Noggin cDNA expression absent in dorsal blastopore lip | Incorrect | Noggin is expressed in the dorsal blastopore lip (organizer) |
| (4) | Noggin mRNA injection in ventral region promotes ventralization | Incorrect | Noggin induces dorsalization, opposing ventral fate |
Conclusion
Among the given options, microinjecting a plasmid clone expressing Noggin into a UV-treated Xenopus embryo (option 2) rescues the abnormality by promoting neural tube formation and dorsal organizer function. This underscores Noggin’s critical role as an organizer protein that induces dorsal and neural cell fates by antagonizing BMP signaling, enabling recovery from UV-induced ventralization.
FAQ
Q: What effect does UV treatment have on Xenopus embryos?
UV treatment ventralizes embryos by disrupting cortical rotation and dorsal determinants, preventing neural tube formation.
Q: How does Noggin rescue UV-treated embryos?
Noggin blocks BMP signaling, inducing dorsal mesoderm and neural tissue, restoring normal axis formation.
Q: Will Noggin rescue lithium chloride-treated embryos?
No, lithium chloride causes hyperdorsalization and head enlargement, so Noggin rescue is not relevant here.
This focused insight is valuable for students and researchers studying embryonic patterning and organizer signaling in amphibians and related developmental biology fields.
