First molecule discovered that directs nerve cells to connect with each other

By Wallace Ravven

UCSF scientists have identified for the first time a molecule that directs neurons to form connections with each other during an animal’s early development - creating synapses essential to all behavior.

The molecule may be one of only a few “matchmaker” proteins that instruct one type of neuron to form a synapse with another type, essentially wiring up the body during embryological development, the researchers say. Such molecules have been sought for decades, but this is the first discovered in a living animal.

The matchmaker protein, known as SYG-1, was discovered in studies of egg-laying behavior in the roundworm C. elegans.  It is a member of a large family of proteins known as the immunoglobulin superfamily, and is closely related to proteins in fruit flies, mice and humans. The related molecules are always found where two different types of cells form a close connection, and SYG-1 probably receives a signal to form a synapse from the animal’s egg-laying tissue, the scientists report.

Discovery of matchmaker proteins in humans may help treat disorders such as chronic epilepsy and chronic pain in which synapse formation goes awry and neurons form the “wrong” connections, said Cornelia Bargmann, PhD, professor of anatomy and Howard Hughes Medical Institute investigator at UCSF. Knowing which proteins direct synapse formation may also help in treatment of peripheral nerve damage, which requires nerves to reconnect with precisely the right partner from among many in their immediate environment.

Scientists think that each neuron in a developing animal is destined to a particular behavioral task, and the synaptic connection allows it to achieve its destiny, connecting with the right partner from among many dozens of kinds of neurons it encounters.

“Like people, neurons can make good or bad choices about who they associate with,” Bargmann said. “Molecules like the protein we discovered are needed to make the best choice.”

With the synapse established, the “pre-synaptic” neuron is able to emit neurotransmitters which prompt the “post-synaptic” neuron into action - signaling a muscle to contract, for example, or passing the relay on to another neuron down the line.

Bargmann is senior author on a paper reporting the research in CELL. Lead author is Kang Shen, PhD, a post-doctoral scientist in her laboratory.

For almost a century, scientists have sought to identify what triggers synapse formation during development. A number of studies have suggested candidate molecules or structures, but never in live animals where the full array of choices is made.

To study synapse formation in living animals, Bargmann and Shen focused on egg-laying in C. elegans because the process is essentially driven by just two types of neurons connected through a synapse. In the adult, the pre-synaptic neuron, known as HSN, responds to cues from the brain and signals the post-synaptic neuron, VC, and post-synaptic egg-laying muscles to contract.

As in all synapse formation, packets of neurotransmitters, known as synaptic vesicles, appear in the pre-synaptic neuron where the synapse is forming.
Researchers had assumed that physical contact between these two neurons during early development would trigger them to connect, but by studying mutant worms lacking the post-synaptic neuron, Bargmann and Shen showed that the vesicles would cluster at the normal location for the synapse even when this normal target was missing.

They discovered that the key to synapse formation is the protein SYG-1, which their research showed concentrates in the pre-synaptic neuron just before the synaptic vesicles form. The vesicles appear beneath the area of SYG-1 expression, and the post-synaptic VC neuron meets the pre-synaptic neuron in the area beneath the vesicles.

SYG-1 is probably a receptor for a signal from the egg-laying tissue which acts as a “guidepost” during development, directing the two neurons to join near the egg-laying muscles they will eventually control, the scientists conclude.

“It’s surprising that the two partner cells in the synapse aren’t able to get together on their own,” Bargmann said. “They need help from the guidepost cells to tell them where to find each other.”  Bargmann and Shen have not defined the precise signal SYG-1 responds to, but it may be another immunoglobulin protein.

Although their study shows that the pre-synaptic neuron does not have to be in physical contact with its target, the neuron apparently does make physical contact with the guidepost cells that signal SYG-1.

“The pre-synaptic neuron uses SYG-1 to sniff out the location of the guidepost cell, and then it organizes the synapse around the guidepost,” Bargmann explains. “That ensures that it always makes the right connection in the right place.”

She expects that SYG-1 likely plays a similar matchmaking role in synapse formation in other organisms, perhaps including humans. A closely related protein has been detected during brain synapse development in mice. Other closely related immunoglobulin proteins have been studied in muscle, eye and brain development of fruit flies, as well as in the mouse brain and human kidney. As in their role in the immune system, these immunoglobulin superfamily proteins recognize other molecules with high precision.

The pre-synaptic HSN neuron not only affects egg-laying through its connection with the post-synaptic neuron, but also can form a synapse directly with egg-laying muscle cells. Bargmann and Shen found that SYG-1 directed the formation of the synapse between HSN and the muscle, just as it triggered synapse formation between the HSN and VC neurons.
Shen used a fluorescence technology to allow him to look through the skin of the live, transparent worm and see intense yellow spots where synaptic vesicles are clustered. Then, focusing on the egg-laying neurons, he looked for changes in the pattern of yellow spots in worms with various mutations.

Worms lacking the syg-1 gene, he
found, were unable to make the right synapse. Instead, they scattered synapses in many locations.

In early development, Bargmann says, neurons are “desperate” to connect up with a partner. “Neurons want to wire up,” she says. If conditions are not right, they may form a synapse with the wrong neuron.  In experiments in culture, a neuron will even synapse with itself.

The researchers conclude that highly specific molecules such as SYG-1 (the protein coded by the syg-1 gene) may represent the “first choice” for a correct synapse, but the neuron’s drive to form a synapse will eventually “override the absence of the correct partner.”

The research was supported by the Howard Hughes Medical Institute.