MicroRNAs influence crucial decisions in the patterning of the early vertebrate embryo, reports a study online this week in Nature. The study is the first example of microRNAs regulating a fundamental signaling cascade, the authors say.
"It's an important finding," said Deepak Srivastava of the University of California, San Francisco, because it demonstrates that "microRNAs can play such a fundamental role in embryonic patterning." Srivastava was not involved in the study.
According to senior author Stefano Piccolo of the University of Padua in Italy, the finding also provides "a solution to a long-standing issue in vertebrate embryology."
This issue revolves around how the basic body plan is set up in a developing embryo. When a vertebrate egg is fertilized, the site of sperm entry establishes the future dorsal/ventral axis of the embryo by driving â-catenin signaling on the dorsal side. The â-catenin signals are eventually translated into a gradient of another signaling molecule called Nodal. High Nodal expression on the dorsal side of the embryo induces a set of cells called the Spemann organizer, which establishes major parts of the animal's body pattern. But researchers have not known how the â-catenin pathway and the Nodal pathway communicate with each other.
Researchers led by Graziano Martello, also of the University of Padua, first discovered that the total collection of Xenopus laevis microRNAs -- small RNAs that inhibit gene expression by binding to messenger RNA -- inhibits Nodal activity when injected into embryos.
They used computational tools to look for potential microRNA binding sites in members of the Nodal signaling pathway. They then identified two microRNAs -- miR-15 and miR-16 - that reduced Nodal signaling via a key receptor, and looked to if the pair's inhibition of Nodal signaling caused problems in the Spemann organizer. They discovered that injecting miR-15 downregulated organizer genes and led to problems in neural tissue patterning and other developmental events that the organizer controls. Inactivating miR-15 and miR-16, on the other hand, caused an expansion of organizer tissue.
Last, the researchers looked for a connection between miR-15 and miR-16 and the â-catenin signaling cascade. They found that overexpression of â-catenin inhibits expression of mature miR-15 and miR-16, while knocking down â-catenin increases expression and activity of the two miRNAs.
Overall, Piccolo told The Scientist, the results show that miR-15 and miR-16 provide the missing link between â-catenin and Nodal signaling. The â-catenin signaling induced by sperm entry inhibits miR-15 and miR-16 expression on the dorsal side of the embryo. Because miR-15 and miR-16 suppress Nodal signaling, their inhibition allows Nodal signaling to proceed on the dorsal side, while it is suppressed on the ventral side.
"This is an interesting novel regulatory mechanism," said Malcolm Whitman of Harvard Medical School in Boston, Mass., who was not involved in the work. However, it's not yet clear that miR-15 and miR-16 are actually upstream of Nodal in the signaling cascade, he said. Nodal upregulates its own signaling in a postive-feedback loop, which means that "there isn't a good way to distinguish the primary event" that increases Nodal signaling on the dorsal side of the embryo, Whitman told The Scientist. Although miR-15 and miR-16 clearly help differentiate the dorsal side from the ventral, "there are a lot of reinforcing mechanisms," he said.
Melissa Lee Phillips
Correction: A previous version of this article misspelled the name of Malcolm Whitman. The misspelling has now been corrected. The Scientist regrets the error.
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