Growing nerve fibers get some guidance.

In the developing human embryo, a mere wisp of tissue called the neural plate gives rise to a nervous system consisting of billions of interconnected nerve cells. To establish the trillions of connections among them, these neurons extend long fibers, or axons, that snake their way through the embryo.

Scientists seeking to understand the chemical cues that guide these fibers are now focusing on a protein called Slit. In both invertebrates and vertebrates, Slit or its relatives repel the growth of certain axons. Yet the molecules have a friendly side, too. They can encourage axons to sprout secondary branches, another feat crucial to the development of the nervous system.

In four papers in the March 19 CELL and one in the April NEURON, scientists describe the proteins' contrasting roles. In fruit flies, for example, certain axons that try to cross the midline, which divides the insect's developing nervous system from top to bottom, come to a dead halt and retract. It's like hitting an "electric fence," says David L. Van Vactor of Harvard Medical School in Boston.

The shocking element in that fence turns out to be Slit. "Slit is expressed right down the midline. It's really potent at setting a boundary between the two sides of the nervous system," says Corey S. Goodman of the Howard Hughes Medical Institute (HHMI) at the University of California, Berkeley, who is a coauthor of three of the papers.

The story of Slit extends into higher organisms. Marc Tessier-Lavigne, an HHMI investigator at the University of California, San Francisco, and his colleagues report that mammals have at least three versions of Slit, all of which can repel axons at the midline or elsewhere.

For example, groups led by Yi Rao of Washington University School of Medicine in St. Louis and Alain Chedotal of Salpetriere Hospital in Paris show that in chicks and rodents, some of these Slit proteins repel growing axons that came from brain regions such as the hippocampus and olfactory bulb.

Goodman and his colleagues also have evidence that the protein can repel migrating muscle-precursor cells from certain areas of the fruit fly embryo. These areas have cells that secrete Slit into their surroundings, where it sticks to other proteins or slowly diffuses to extend the repellent zone.

The role of Slit proteins in axon branching may be as important as their ability to repel those fibers. Most nerve cells ultimately connect with multiple targets, in the brain or elsewhere, that are separated by considerable distances. An axon may grow steadily toward one main target, but secondary axons, or collaterals, can shoot off from the axon's main trunk. Some collateral axons don't appear until a day after a parent axon has passed by a secondary target.

"They just pop out of the axon," notes Dennis O'Leary of the Salk Institute for Biological Studies in La Jolla, Calif.

O'Leary and his colleagues have for years studied how vertebrate axons from the brain's cortex travel down to the spinal cord, forming branches all along the way. They've shown that the targets of collateral axons secrete soluble factors that penetrate the parent axon and induce it to branch at that site.

Slit proteins emerged as candidates for such a factor when Kuan Hong Wang, one of Tessier-Lavigne's colleagues, purified a molecule that seemed to provoke collateral branching in test tube studies. After determining the protein's amino acid sequence, he realized it was a fragment of one of the molecules that his laboratory mates had identified as axon repellents.

"There was absolutely no reason to believe there was any relationship between the two [projects]. We were totally surprised," says Tessier-Lavigne.

He and O'Leary stress that the mechanism behind the Slit fragment's ability to enhance collateral branching remains unclear. It may stimulate the budding of collateral axons, notes Tessier-Lavigne, or may merely stabilize fledgling axons induced to branch by other, unknown factors.

A fuller understanding of Slit proteins, he adds, might one day allow physicians to employ them in repairing the nervous system, such as in cases of spinal cord injuries.