Thanks to new research by scientists at the Salk Institute for Biological Studies, we are closer to understanding how these complicated network connections are wired up during embryonic development. Salk researchers have discovered that the same chemicals (called neurotransmitters) that are responsible for nerve signals are also involved in the wiring of synapses, the network's crucial contact points between nerves, or between nerves and muscle cells.
The study, published in the May issue of the journal Neuron, showed that as the motor neurons grow from their home base in the spinal cord towards muscles throughout the body, they release two opposing chemical signals. These signals act to preserve synapses that link a motor neuron to its correct muscle cell. 'Spare' sites for potential synapses that fail to team up with a motor neuron are dismantled.
"Our study provides the first evidence in a living animal system that the neurotransmitters themselves are sculpturing the developing nervous system," said Kuo-Fen Lee, Associate Professor at the Salk, who heads the research team reporting its results in Neuron.
Using mice as a model for human biology, Lee and colleagues showed that each long, thin muscle cell in the developing embryo prepares for the arrival of its motor neurons by creating sites for many potential synapses along its length. However, three weeks after conception, all the sites have disappeared, except those that connected with a newly arrived motor neuron and formed a fully functioning synapse. The scientists wanted to know: how does the embryo 'weed out' the potential synapse sites that are not needed? The answer to this question is crucial because it might shine light on how the nervous system could make new connections in medical conditions such as spinal cord injury.
Lee, along with Salk colleagues Weichun Lin, Bertha Dominguez, Jiefei Yang, Prafulla Aryal, Eugene Brandon and Fred Gage, discovered that the creation of synapses is controlled by the nerves themselves. As they grow towards the muscle cells, the nerve cells release a powerful chemical messenger from their growing ends. Called acetylcholine, this neurotransmitter 'edits out' the potential synapse sites on the muscle cell not destined to connect to a nerve. In mature animals, acetylcholine is a key neurotransmitter responsible for transmitting signals between nerve cells and muscle.
Using a combination of genetic and pharmacological techniques to block the various components of the chemical pathways involved, the Salk researchers painstakingly showed that acetylcholine works in tandem with another chemical produced by nerve cells, called agrin. Where the end of the nerve touches the muscle cell, agrin is concentrated enough to overcome the 'editing' effect of the acetylcholine. Further away from the nerve end, the levels of agrin are not high enough to overcome the more powerful influence of acetylcholine, and the redundant synapse sites are dismantled.
"The result is an interesting mechanism whereby two opposing forces work together to create the crucial synaptic connections between motor neurons and muscle cells," said co-author Prafulla Aryal.
"Although we have suspected for 25 years that something like this was happening, until now no-one has been able to demonstrate it in a living system," said Lee. "It is likely that this process occurs all over the nervous system. If you're going to repair or regenerate nerves in, for example, spinal cord injury you need to know how to form synapses for the right connections to be made."
The Salk Institute for Biological Studies in La Jolla, California, is an independent nonprofit organization dedicated to fundamental discoveries in the life sciences, the improvement of human health and the training of future generations of researchers. Jonas Salk, M.D., whose polio vaccine all but eradicated the crippling disease poliomyelitis in 1955, opened the Institute in 1965 with a gift of land from the City of San Diego and the financial support of the March of Dimes.