Double Act: How a Single Molecule Can Attract and Repel Growing Brain Connections

How can you find the same thing both attractive and repulsive? For growing neurons, the answer is in how they engage with it. The findings, published online in Neuron, stem from the 3D structure of Netrin-1 bound to one of the sensor molecules - receptors - the cell uses to detect it.

The work, by scientists at the European Molecular Biology Laboratory (EMBL) in Hamburg, Germany, the Dana-Farber Cancer Institute affiliated to Harvard Medical School in Boston, the USA, and Peking University in Beijing, China, could also have implications for cancer treatment.

“Although this is a challenging area for drug design, we found a mode of interaction that could be exploited to make cells respond to Netrin in a specific way, for instance to control proliferation or trigger programmed cell death,” says Rob Meijers, who led the work at EMBL.

Our brain’s ‘wiring’ is a set of protrusions that run from one neuron to another, like stretched-out arms. As connections between neurons are established - in the developing brain and throughout life - each of these wires, or axons, grows out from a neuron and extends through the brain until it reaches its destination: the neuron it is connecting to. To choose its path, a growing axon senses and reacts to different molecules it encounters along the way. One of these molecules, Netrin-1, posed an interesting puzzle: an axon can be both attracted to and repelled from this cue. The axon’s behaviour is determined by two types of receptor on its tip: DCC drives attraction, while UNC5 in combination with DCC drives repulsion.

When the scientists determined the 3D structure of Netrin-1 bound to DCC, they found the answer to this conundrum. The structure showed that Netrin-1 binds not to one, but to two DCC molecules. But most surprisingly, it binds those two molecules in different ways.

“Normally a receptor and a signal are like lock-and-key, they have evolved to bind each other and are highly specific – and that’s what we see in one Netrin site,” says Meijers. “But the second is a very unusual binding site, which is not specific for DCC.”

Most of the second binding site does not connect directly to a receptor. Instead, it requires small molecules that act as middle-men. These intermediary molecules seem to have a preference for UNC5, so if the axon has both UNC5 and DCC receptors, Netrin-1 will bind to one copy of UNC5 via those molecules and one copy of DCC at the DCC-specific site. This triggers a cascade of events inside the cell that ultimately drives the axon away from the source of Netrin-1, Yan Zhang’s lab at Peking University found. The researchers surmise that, if an axon has only DCC receptors, each Netrin-1 molecule binds two DCC molecules, which results in the axon being attracted to Netrin-1.

“So by controlling whether or not UNC5 is present on its tip, an axon can switch from moving towards Netrin to moving away from it, weaving through the brain to establish the right connection,” says Jia-Huai Wang, who heads labs at Dana-Farber Cancer Institute and Peking University, and co-initiated the research.

Knowing how neurons switch from being attracted to Netrin to being repelled opens the door to devise ways of activating that switch in other cells that respond to Netrin cues, too. For instance, many cancer cells produce Netrin to attract growing blood vessels that bring them nourishment and allow the tumour to grow, so switching off that attraction could starve the tumour, or at least prevent it from growing. On the other hand, when cancers metastasize they often stop being responsive to Netrin. In fact, the DCC receptor was first identified as a marker for an aggressive form of colon cancer, and DCC stands for ‘deleted in colorectal cancer’. Since colorectal cancer cells have no DCC, they are ‘immune’ to the programmed cell death that would normally follow once they move away from the lining of the gut and no longer have access to Netrin. As a result, these tumour cells continue to move into the bloodstream, and metastasize to other tissues.

Meijers and colleagues are now investigating how other receptors bind to Netrin-1, and exactly how the intermediary molecules ‘choose’ their preferred receptor. The answers could one day enable researchers to steer a cell’s response to Netrin, ultimately changing its fate.