VOLUME 12, NUMBER 20 • APRIL 20, 2001

Brivanlou Lab Discovers Promoter of Neural Tissue in Frogs

According to Greek mythology, a person’s destiny is decided at birth by a set of goddesses known as the Fates. While predestiny may be just a myth, in the case of living cells it is partly true: a cell’s fate is tightly sealed at a very young age. But if not in the hands of the Fates, then what controls a cell’s ultimate destiny?

Professor Ali Hemmati Brivanlou (who now goes by Ali H. Brivanlou) is answering this question by studying the very early development of frog embryos. He is interested in uncovering the molecular signals that establish cell fate.

"How do cells know to become liver, heart or bone?" asks Brivanlou, head of the Laboratory of Molecular Vertebrate Embryology.

Professor Ali H. Brivanlou (right) and Chenbei Chang, a postdoctoral associate, report the discovery of a protein that indirectly leads to the creation of neural cells in frog embryos.

In a recent paper in Nature, Brivanlou and Chenbei Chang, a postdoctoral associate, report the discovery of a protein that indirectly leads to the creation of neural cells in frog embryos. Specifically, they show that Twisted gastrulation (Tsg), a protein found in frogs, is an inhibitor of a very important molecule called bone morphogenic protein (BMP). BMP, which was first identified by Brivanlou and Paul A. Wilson, a former research associate in Brivanlou’s lab, at Rockefeller in 1995, tells early embryonic cells to become skin cells instead of neural cells.

Because BMP is itself an inhibitor of neural tissue formation, Tsg actually promotes neural cells by inhibiting the inhibitor. In other words, Tsg tells a primitive cell to become a brain cell instead of a skin cell. But it does this indirectly by preventing BMP from relaying its message to cells.

The work also shows that Tsg binds to both BMP and another protein called chordin, a well-known BMP inhibitor, to form a three-part complex. In addition, the researchers find that Tsg and chordin, when present together, inhibit BMP more efficiently.

"We have found that BMP signaling can be regulated in much more complex ways than we thought," says Chang, lead author of the paper.

The new research may come as a surprise to the scientific community: A report published in Nature last year by another group claimed that Tsg worked together with BMP to inhibit the growth of neural cells in frogs and fruit flies. Brivanlou’s results in frogs have been replicated in fruit flies and zebra fish by researchers at the University of California at Irvine and at the University of Wisconsin. All three groups reported their findings in the March 22 issue of Nature.

Brivanlou, in 1994, pioneered the "default model" of neural induction, which states that early embryonic cells will mature, by default, into neural tissue in the absence of any signals. But in the presence of BMP, cells will become skin cells; in order for a cell to become neural, BMP must be inhibited by what scientists sometimes call neural inducers. So far, five of these BMP inhibitors, also called antagonists, have been identified, including chordin, noggin and follistatin, the third of which was discovered by Brivanlou’s lab in 1994. Twisted gastrulation marks the sixth.

Both BMP and its inhibitors have several potential medical applications: BMPs could be used in plastic surgery for burn victims and wound healing, and the BMP inhibitors, or neural inducers, might lead to the replacement of damaged neural tissue in patients with stroke or neurodegenerative diseases, such as Alzheimer’s or Parkinson’s. In addition, BMP and its inhibitors are important regulators of bone, cartilage and joint formation in adults, and thus may be used in the treatment of bone fractures or diseases associated with skeletal defects, like osteoporosis.

"There are direct medical applications for any modifier of BMP," says Brivanlou.

Originally identified in the fruit fly Drosophila, Twisted gastrulation was named after mutant fruit flies lacking the gene, which appeared twisted while gastrulating. Gastrulation is a crucial stage of embryogenesis when the three germ layers begin to take shape and the primary body axes are constructed. It is at this time that cell fate is established.

In order to determine whether Tsg could act as an antagonist of BMP, the researchers employed a classical embryology experiment called cell dissociation. Originally performed more than a decade ago, these early experiments offered Brivanlou the first clue to his default theory of neural induction.

In the initial studies, early embryonic cells were separated from each other and grown in such a way that they received no contact from other cells. What resulted was unexpected: the dissociated cells became neural in the absence of any signals; previously it was thought that skin cells represented the default mode. Brivanlou later showed that BMP, when added to the dissociated cells, prevented them from becoming neural and instead led to the initiation of skin cells.

In this recent set of experiments, the researchers found that Tsg could "rescue" the neural fate of dissociated cells grown in the presence of BMP. In other words, Tsg prevented BMP from transforming neural cells into skin cells, thereby indirectly promoting the creation of neural cells.

The researchers also looked at the function of Tsg in developing tadpoles. They overexpressed the protein in frog embryos, then checked to see if their organs were working properly at the tadpole stage. Both blood and heart tissues——other organs known to require BMP signaling——exhibited a stunted development, confirming that Tsg blocks BMP activity in living cells.

Tadpoles overexpressing Tsg, however, did not show the telltale sign of a BMP inhibitor. When other BMP antagonists, like chordin and noggin, are injected into the part of a frog embryo that does not normally become a head, a second body axis will develop in the tadpole, along with a second head. Embryos injected with Tsg, on the other hand, matured into tadpoles with one smaller head. Brivanlou says that Twisted gastrulation is not as strong an inhibitor as the others and must have some other function yet to be described.

"The story is not complete," says Brivanlou. "We leave the door open."

Support for this research was funded by a grant from the National Institutes of Health.