BioWorld International Correspondent

LONDON - Scientists now have a molecular marker for the neural networks in the spinal cord that coordinate repetitive movements such as walking - and allows able-bodied people to run downstairs while thinking about something completely different.

The work, carried out in Sweden by molecular biologist Klas Kullander of Gothenburg University; AstraZeneca; neurophysiologist Ole Kiehn, of the department of neuroscience of the Karolinska Institute in Stockholm; and their colleagues showed that neurons bearing a particular tyrosine kinase receptor form part of the neural network in the spinal cord known as the central pattern generator.

This network issues instructions to the muscles of the limbs to carry out the complex movements needed for walking. It can be activated by electrical stimulation even in patients with spinal cord injury. Although scientists have described parts of the organization of the central pattern generator of other species, such as fish and frogs, the organization of the mammalian central pattern generator has until now remained elusive.

Kiehn told BioWorld International, "We are a very long way from having a cure for patients with spinal cord injury, but if we are ever going to come up with a treatment for these patients, we need to understand the machinery of the spinal cord and how the central pattern generator is organized and determined. The information from our study may allow us to find new ways of identifying specific drugs that can activate this network."

Kullander and Kiehn found that the receptor EphA4 was present on the neurons of the central pattern generator in mice. The molecule, and its ligand, called ephrinB3, is found in the developing mammalian brain. Both are known to play a role in guiding axons to their correct destinations during development.

An account of the study appears in the March 21, 2003, issue of Science in a paper titled "Role of EphA4 and EphrinB3 in Local Neuronal Circuits That Control Walking."

"This study is important because we have found the first molecular marker for the neurons in the central pattern generator in the mammalian spinal cord, and this has allowed us to define a group of neurons that we are sure is part of the central pattern generator," Kiehn said.

Kullander's group, and others, had already found that mice, in which the gene encoding the receptor EphA4 was knocked out, and those lacking the gene encoding its ligand, ephrinB3, could not walk normally. Instead of moving their hind legs alternately, these animals moved them synchronously, hopping rather like kangaroos, instead of walking.

To find out what was going on inside the spinal cord to account for this strange type of movement, Kullander enlisted the help of Kiehn, who has worked for more than 15 years on understanding the central pattern generator of the spinal cord in mammals.

Experiments carried out in Kiehn's laboratory, using isolated spinal cords from the knockout mice, showed that the hopping effect was not the result of signals from the brain, but was due to a reconfiguration of the neural network in the spinal cord.

Further studies of mice that lacked EphA4, which made it possible to trace the paths of individual neurons, showed that, unlike in wild-type mice, many axons crossed the midline of the spinal cord. Using immunohistochemistry, the authors showed that the cells that cross abnormally to the other side of the spinal cord are those that normally contain the EphA4 receptor. Those cells normally meet the ephrinB3 ligand in the midline. If the ligand is removed from the midline or the receptor is removed from the cells, then the cells cross over to the other side.

Writing in Science, the authors said, "We conclude that ephrinB3-induced EphA4 signaling repels some components of the mammalian spinal central pattern generator and restricts their axonal projections to one side of the spinal cord. We postulate that in the normal central pattern generator, ipsilateral neurons that express the EphA4 receptor are predominantly excitatory."

Kiehn, Kullander and colleagues are now carrying out intracellular recordings of the electrical activity of the EphA4-positive neurons, in order to try to determine in detail their precise function during locomotion. They are also generating new knockout mice lacking other genes that may encode proteins that are important in the central pattern generator.