The discovery, reported this month in the journal Developmental Cell by researchers at theA major step in the development of the vertebrate embryo - the establishment of a back that morphs into a brain, spinal cord and muscles - turns out to be so important that the body uses at least three signals to make sure it happens properly.

Postdoctoral fellow Bill Smith and Harland found the first of these BMP antagonists, noggin, in 1992, confirming its activity by repeating Spemann and Mangold's experiment but injecting noggin instead of implanting an organizer from another embryo. They also discovered xnr3, and other researchers isolated three more factors - chordin, follistatin and cerberus. While Harland and his UC Berkeley colleagues found numerous roles for noggin at later stages of development, they were unable to prove that noggin was essential to brain and spinal cord development. Blocking any one of these five factors had only a minimal effect on the fate of the embryo.

Khokha and Harland decided to try blocking more than one, and chose to work in embryos of the frog Xenopus tropicalis, a close relative of the more common laboratory frog Xenopus laevis. X. tropicalis is easier to work with, because it, like humans, is diploid, that is, it has only two copies of each gene, instead of four, as in the tetraploid X. laevis. Lab specimens of X. tropicalis also have less genetic variation than outbred populations of X. laevis, and the genome of X. tropicalis has been sequenced. Harland is pioneering the use of X. tropicalis in genetic studies, creating numerous mutants that can be used to explore the role of various genes in development.

Using standard knock-out techniques, their team inactivated the function of these BMP antagonists in various combinations, finding the most dramatic effect by simultaneously knocking out noggin, follistatin, and chordin.

Harland noted that the signaling pathway they blocked is one of several developmental pathways proceeding at the same time in the embryo. While BMP antagonists allow the formation of the back and belly, another group of antagonists creates the head and tail, while a third sets up left and right.

"It's been interesting that what one thinks of embryologically as the dominant signaling center actually is a source of a cocktail of inhibitors," he said, "so inhibition is just as specific a signal in the embryo as is an activating signal."

Since they discovered noggin, Harland and his colleagues have shown that in later stages of development, this protein factor is critical in laying down cartilage to make joints, and even plays an important role after birth. Recently, he and Stanford University colleagues showed that noggin may be important in preventing the premature fusion of the bones in the skull, and thus may be critical to allowing the brain to grow larger after birth. All of these findings are from mice or amphibians, but the researchers say that the same is almost certain to be the case in humans.

"One of the things that's been nicely shown is that the organizer itself, while it was originally identified in newts, is conserved through all vertebrate evolution," Khokha said. "If you cut out a similar tissue and transplant it in a mouse, you also get the Siamese twin phenomenon. So, we expect it to also be true in humans."

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