Sometimes nerve is all you need. But nerves have needs, too, including the use of synapses -- tiny junctions that coordinate communication between nerves and the muscles they control.
Scientists long suspected that chemical cues released from youthful nerve cells, meandering through the developing embryo, helped point the way to the birthplace of synapses. But a new study led by Salk scientists reverses this dogma.
"We found that the formation of neuromuscular synapses is triggered by cues in muscle cells," said Kuo-Fen Lee, a Salk associate professor and senior author of the study, which appears in today's issue of Nature.
"This finding overturns essentially 30 years of thinking and has implications for attempts to reconstitute a functional nervous system in cases like spinal cord injury."
The investigators examined the interactions between nerve and muscle in growing mouse embryos. Specifically, they monitored an accepted indicator of synapse formation, the clustering of neurotransmitter receptors -- specialized molecules that can receive messages released by neurons -- on muscle cells.
"Most studies looked at 18-day old embryos," said Lee. "We were curious about earlier time points."
When the investigators examined 14-day old embryos they found that receptor clusters were not apposed by nerves -- that is, no nerves were in proximity to the clusters.
"That was the first clue that perhaps the initiation was nerve-independent," said Lee. "But we couldn't rule out possibilities such as messages from nerve cells traveling further than we'd thought they could."
The group then genetically manipulated mouse embryos to eliminate nerve cells from the equation. These mutants do not grow a phrenic nerve, which normally innervates the diaphragm muscle that is essential in controlling breathing. When embryonic diaphragm muscle was examined, receptor clustering in the muscle was normal.
"This is the first evidence that muscles can initiate synapse formation completely in the absence of nerve stimulation," said Lee.
The receptor clustering occurs along what is called the midband of the muscle, which is where contraction will take place in the fully formed muscle. Lee and his colleagues think it is possible that by beginning to form synapses along the midband, the muscle attracts growing nerve cells to the appropriate location to form connections.
"We know that at later time points, the nerve releases factors that nourish the synapse and help it to develop fully," said Lee. "But the initial placement may be determined by the muscle.
"Therefore," he added, "if you're working on nerve regeneration in cases like spinal cord repair, it may not be enough to grow new nerves. You may have to look toward the muscle for help in establishing the proper connections to restore full function."
Additional Salk co-authors include Weichun Lin, Bertha Dominguez and Samuel Pfaff. The study was done in collaboration with Robert Burgess and Joshua Sanes at Washington University Medical School in St. Louis, Mo. The study, titled "Distinct roles of nerve and muscle in postsynaptic differentiation of the neuromuscular synapse," was supported by the National Institutes of Health. Lee and Pfaff are Pew Scholars.
The Salk Institute for Biological Studies, located in La Jolla, Calif., is an independent nonprofit institution dedicated to fundamental discoveries in the life sciences, the improvement of human health and conditions, and the training of future generations of researchers.
The Institute was founded in 1960 by Jonas Salk, M.D., with a gift of land from the City of San Diego and the financial support of the March of Dimes Birth Defects Foundation. - By Suzanne Clancy
The Salk Institute for Biological Studies
[Contact: Suzanne Clancy, Warren Froelich]