Sonic Hedgehog Shapes The Brain

During development, the brain becomes organized into highly specialized groups of neurons, called brain nuclei, each expressing its own set of genes and participating in very specific neural functions. Little is known, however, about how brain nuclei of the appropriate size, shape and location are generated. The researchers show that this process can be coordinated by the secretion of a single molecule, Sonic Hedgehog, that operates as a 'positional signal'.

"A positional signal is a neat mechanism for creating patterns of different types of cells," said Cliff Ragsdale, Ph.D., assistant professor in neurobiology, pharmacology and physiology at the University of Chicago, and principal investigator in the study. "Target cells respond differently to a signaling molecule according to their distance from the source of the signal."

Ragsdale's group studies this process in the midbrain, a major division of the central nervous system, of embryo chicks. The midbrain was an attractive model for studying the formation of the organization of brain, or morphogenesis, because it is very simply organized during development into a set of cell columns, called arcs, each with distinct molecular identities.

The group found that Sonic Hedgehog is normally expressed along the midline during midbrain development, suggesting that Sonic Hedgehog might be the morphogen setting up the midbrain pattern.

This signal is called a morphogen because it is a "form-producing substance." Though the idea of morphogens has been around since 1952 when the British mathematician Alan Turing coined the term, it is only in the last ten years that true morphogens used in animal development have been molecularly identified.

To study the role of Sonic Hedgehog in this process, the group transferred the gene for Sonic Hedgehog into the midbrain of two-day-old embryo chicks. The brains were examined when the embryos were five days old, when all the midbrain arcs are present.

"We used an extremely efficient gene delivery system called electroporation," said Seema Agarwala, Ph.D., research associate in neurobiology, pharmacology and physiology at the University of Chicago, and lead author of the study. "This allowed us to control precisely the time and place where the additional Sonic Hedgehog was expressed."

"We put the molecule in the dorsal midbrain, an area that never sees Sonic Hedgehog," said Agarwala. "To our surprise, we got a complete duplication of the pattern we saw in ventral midbrain, a full set of arcs. The Sonic Hedgehog was enough to make dorsal midbrain tissue into ventral midbrain tissue, producing the entire pattern."

In the next set of experiments, the researchers showed that Sonic Hedgehog could control both shape and size of the pattern produced. When the researchers introduced a linear source of Sonic Hedgehog perpendicular to the normal source, they produced a series of stripes that wrapped around the brain like a barber pole. In a further set of experiments, the researchers showed that a spot source of sonic hedgehog turned the arcs into a pattern very much like the eyespot of a butterfly's wing.

"What we have found in brain development is actually a very simple principle", said Ragsdale. "It was known that morphogens like Sonic Hedgehog can induce different cell-types according to their concentration, but it was never clear how you got those different cell-types to form patterns of the correct size and shape. What we have shown is that the size and shape of the morphogen source can control the size and shape of the cell patterns produced."

"This single molecule has tremendous power to shape the brain," said Ragsdale.

The name Sonic Hedgehog originated with a mutant fruit fly embryo that had cuticle all over its body. Researchers found three different versions of the "hedgehog" gene but only two kinds of real hedgehogs, so they named the third gene after the cartoon character.

Timothy Sanders, Ph.D., of the University of Chicago, was an additional author of the study, which became available online March 9.

This work was supported by the National Institute of Neurological Disorders and Stroke of the National Institutes of Health.