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Gene "Knockouts" Reveal Critical Links in Synapse Formation

For release: Thursday, May 16, 1996

New studies reveal exciting clues to the mystery of how synapses form between nerve and muscle cells. The findings shed new light on human development and may help reveal how molecular interactions are altered in muscular dystrophy.

Studies of mice with genes "knocked out" or inactivated reveal that two proteins, called agrin and muscle-specific receptor kinase (MuSK), are essential for motor nerves to form synapses with muscle. Mice missing either protein die shortly after birth due to profound defects in their neuromuscular synapses which leave them unable to move or breathe. The similar defects in the two mutants, with other findings from these studies, suggest that MuSK serves directly as a receptor for agrin.

"This new research is an important step forward in understanding how signaling mechanisms in the brain develop. These findings will be important for understanding both developmental disorders and regeneration after disease or injury," says Dr. Zach W. Hall, director of the National Institute of Neurological Disorders and Stroke (NINDS).

Synapses at the neuromuscular junction, where motor nerves signal to muscle, are crucial for normal movement and can serve as models for synapses in the brain and spinal cord. Researchers have identified many proteins found at neuromuscular synapses, but they are just beginning to describe how these molecules work. The severe defects seen in mice lacking either agrin or MuSK show that these molecules are critical links in development of motor nerve connections. The new studies are reported in the May 17 issue of Cell and were funded in part by NINDS.*,**,***

"For animals to have meaningful behavior, synapses need to be wired up properly," explains Dr. Joshua R. Sanes, an NINDS grantee at Washington University Medical School in St. Louis, MO, who led the effort to develop the agrin-deficient mice. Studies have shown that the nerve wiring process involves a complex molecular "conversation;" between nerve axons and receptors on muscle cells. This back-and-forth signaling guides the axons and receptors to move toward each other and form a synapse. For synapses to function efficiently, the receptors for nerve signaling molecules, or neurotransmitters, must cluster directly under the axon terminal.

Agrin is one of several proteins proposed as the signal axons use to induce receptor clustering in muscle cells. The new findings are the first to confirm in living animals that agrin is needed for normal clustering of acetylcholine (ACh) receptors on muscle cells. ACh is the main neurotransmitter used in motor signaling. The number of ACh receptor clusters was greatly reduced in agrin-deficient mice, although some very limited receptor clustering did occur, suggesting that a secondary signaling molecule may be at work. The agrin-deficient mice also showed unexpected defects in pre-synaptic axon terminals. They had fewer axons leaving the main nerve trunk than normal mice. The existing axons grew parallel to the muscle fibers for abnormally long distances, as if searching in vain for appropriate targets.

MuSK, a type of signaling molecule known as a receptor tyrosine kinase, is structurally similar to many growth factor receptors. MuSK, identified last year by scientists led by Dr. George D. Yancopoulos at Regeneron Pharmaceuticals of Tarrytown, NY, is concentrated at neuromuscular synapses. New research, led by Yancopoulos and involving an important collaboration with NINDS grantee Dr. Steven J. Burden of the Skirball Institute at New York University Medical Center, shows that MuSK is necessary for muscle cells to respond to agrin. Mice without MuSK receptors have defects very similar to those seen in agrin-deficient mice. While their muscles and skeleton appeared normal, the mice did not develop neuromuscular synapses and died soon after birth since they were unable to move or breathe.

"Our work shows that MuSK is critically important -- in its absence, agrin is unable to signal to muscle," says Yancopoulos. "MuSK is the first example of a receptor tyrosine kinase essential for synapse formation. Furthermore, our studies demonstrate that MuSK is a required receptor component for agrin, and that it binds agrin directly, although to do so requires an accessory component expressed only in muscle."

Previous evidence suggested that agrin might act by binding to dystroglycan, which is one of several molecules associated with dystrophin, the protein altered in Duchenne muscular dystrophy. Dystrophin-associated proteins interact to link structures inside muscle cells with the extracellular matrix that surrounds them. The current work suggests that dystroglycan is not the signaling receptor for agrin, but might serve as a link between MuSK and other associated proteins, expanding the receptor-associated complex.

While MuSK is restricted to the skeletal muscles, retina, and spleen, agrin is found throughout the nervous system. Agrin in the brain and spinal cord may act through a receptor similar to MuSK, or it may function completely differently than it does in motor nerves, Sanes says. He and his colleagues will soon begin studies of how agrin works in the brain. This work may lead to new insights about brain wiring abnormalities, which are thought to underlie psychiatric diseases such as schizophrenia.

Scientists must now identify the molecule or molecules needed for MuSK to respond to agrin and learn how MuSK activation leads to receptor clustering, Burden says. Answering these questions will help solve the mystery of how synapses form and lead to a better understanding of early development, muscular dystrophy, and diseases of abnormal nerve wiring.

The NINDS, one of the National Institutes of Health in Bethesda, Maryland, is the nation?s leading supporter of research on the brain and nervous system and a lead agency for the Congressionally designated Decade of the Brain.

*Gautam, M.; Noakes, P.G.; Moscoso, L., Rupp, F.; Scheller, R.H.; Merlie, J.P.; Sanes, J.R. "Defective Neuromuscular Synaptogenesis in Agrin-Deficient Mutant Mice." Cell, Vol. 85, No. 4, May 17, 1996 (pp. 525-536).

**DeChiara, T.M.; Bowen, D.C.; Valenzuela, D.M.; Simmons, M.V.; Poueymirou, W.T.; Thomas, S.; Kinetz, E.; Compton, D.L.; Rojas, E.; Park, J.S.; Smith, C.; DiStefano, P.S.; Glass, D.J.; Burden, S.J.; Yancopoulos, G.D. "The Receptor Tyrosine Kinase, MuSK, is Required for Neuromuscular Junction Formation in Vivo." Cell, Vol. 85, No. 4, May 17, 1996 (pp. 501-512).

***Glass, D.J.; Bowen, D.C.; Stitt, T.N.; Radziejewski, C.; Bruno, J.; Ryan, T.E.; Gies, D.R.; Shah, S.; Burden, S.J.; DiStefano, P.S.; Valenzuela, D.M.; DeChiara, T.M.; Yancopoulos, G.D. "Agrin Acts Via a MuSK Receptor Complex." Cell, Vol. 85, No. 4, May 17, 1996 (pp. 513-524).

Originally prepared by Natalie Larsen, NINDS Office of Communications and Public Liaison.

Last Modified August 7, 2009