Volume 2, Issue 13
Signaling Brain Cells
UC Berkeley neurobiologist Lu Chen believes that one of the best ways to learn about the brain is to build one of its key components. She and her colleagues are exploring how synapses form between neurons to make the circuits of the nervous system. Their approach is to identify the fewest ingredients necessary to create a synapse, mix them together in a "test tube" of non-neuronal cells, and let biology do the rest.
In 2004, Lu Chen was awarded a prestigious Packard Fellowship for Science and Engineering from the David and Lucile Packard Foundation.
"Our goal is to understand the molecular and cellular mechanisms of synaptogenesis, the formation of synapses," says Chen, associate professor in the Department of Molecular & Cell Biology, as well as a member of the Helen Wills Neuroscience Institute.
The basic unit of the nervous system is a neuron. The brain contains 100 billion of these nerve cells that are connected by 100 trillion synapses, junctions where the signal from one neuron is transmitted to its neighbor. The first neuron releases a chemical neurotransmitter that's converted into an electrical signal for propagation to the second neuron. The structure that releases the neurotransmitter across the synapse is known as a presynaptic cell while the receiving cell is postsynaptic.
The signals and proteins involved in the formation of the postsynaptic structure remain a mystery though. Historically, scientists have attempted to suss out the essential proteins for synapse formation by studying "knock out mice," rodents genetically engineered so that suspect proteins are not produced. The problem with that approach, Chen says, is that there's often redundancy in the system. If one protein isn't available, another may pick up the slack.
"Sometimes you can't identify a protein's function just by removing it," she says.
The left image shows a non-neuronal cell expressing neronal proteins (tagged to fluoresce green) forming a synapse with neuronal axons (stained red). The right panel shows synaptic responses recorded from a non-neuronal cell that expresses four neuronal proteins and receives input from neurons in the same culture. (courtesy the researchers)
That's why Chen takes the opposite approach. She essentially starts with nothing and builds a synapse from the bottom up.
"Rather than study a complicated system containing both pre- and postsynaptic structures, we replace one of the structures with a non-neuronal cell," she explains. "Then we can selectively add specific neuronal proteins into these 'clean' test-tubes and see how far we can get."
A synapse may contain hundreds of proteins, but only a few categories of them are involved in the postsynaptic assembly, Chen says. These include receptors for the neurotransmitters, scaffolding proteins--molecular links to connect receptors, signaling molecules, and the cell's structural support — and adhesion molecules that glue the pre- and postsynaptic structures together. The "glues" also serve to recruit the other proteins that combine together to form the synapse. The researchers added the proteins to non-neuronal cells and cultured them with neurons that are still actively seeking to form synapses. Axons extending from each neuron recognized the proteins in the non-neuronal cells and began to instruct the formation of a postsynaptic structure.
"We've shown that you can reproduce a synaptic response in non-neuronal cells," Chen says. "We're also looking at adding other proteins that can boost the process, increase the response, and stabilize the structure."
An image demonstrating that beta-neurexin, a "glue" protein (red), induces the accumulation of postsynaptic proteins (green) in contacting neuronal cells while other adhesion molecules (blue) do not have the same effect. (courtesy the researchers)
In recent months, Chen and graduate student Christine Nam removed the presynaptic axon from the stew. The aim was to identify the minimal signal an axon must send to induce the formation of a postsynaptic component.
"We knew that once the axon recognizes the proteins of a postsynaptic partner, something magical happens and the postsynaptic site starts to accumulate the necessary proteins to result in a fully-functional synapse," she says.
After plenty of informed trial-and-error, Chen and Nam determined that a single glue molecule and a chemical neurotransmitter are enough to get the postsynaptic assembly started. They published their results in the Proceedings of the National Academy of Sciences.
"In the next five to ten years we hope to test our knowledge of synapse function by building a rudimentary model system that actually works," Chen says.