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To stay or to go: Uncovering how neurons know where to end up

May 2008--For Guo-Li Ming, it’s all about the attraction. For Alex Kolodkin, it’s more about the repulsion. But rather than bifurcate across the halls of Hopkins, the two neuroscientists study what could be considered the yin and yang of nervous system development. For both researchers, the question is the same: With a hundred billon nerve cells each making thousands of connections during the formation of the central nervous system, how is it possible that the wiring ends up working at all?

It’s sort of like a highway, says Ming. “Some neurons grow up to a meter long in humans and make thousands of connections with other cells.” But where is the map that gets them there, and what’s the enticing factor that makes them follow the directions to their destination? “Guidepost cells and target cells—sort of relay stations along the way to a final destination,” says Ming. The relay stations send guidance signals—GO and TURN—that cajole the neuron to get where it needs to be.

To uncover clues to the cell’s journey, Ming used a micropipette to introduce a netrin protein gradient near a frog nerve cell in vitro. (Netrin is named after the Sanskrit word “netr,” meaning one who guides.) The slides show that the axon growth cone—with the morphology of a wild-haired Muppet—initially is determined to make a left, then senses something undeniably delectable and banks right, heading straight for the cue. With the obvious proclivity for the peptide established, Ming’s team turned their attention to what exactly makes the cell change direction in response to the netrin. What they found is a change in the calcium concentration inside the growth cone—a function that takes place through the same TRP channels that also regulate taste and heat sensing.

The next step for Ming’s lab is trying to understand how the calcium channel is regulated to respond to guidance cue stimulus. “Knowing how these guidance systems work will be critical in dealing with spinal cord injuries,” she says. “The big problem in humans is that the ability to regenerate neurons in the brain and spinal cord is very low, so we are trying to see if we can manipulate the signal to help promote regeneration.”

Of course, when you’re a growing nerve cell, knowing the right connection to make is contingent on knowing where to land and also not going in the wrong direction. Enter Kolodkin and his work on fruit fly nervous system development and the yin side of the equation. “In neural development there are as many inhibitory cues as attractive cues,” says Kolodkin, who argues that the STOP and YIELD signs formed by repellents that include the semaphorin and slit proteins are just as important as the green-light netrins. Take the case of a neuron that’s preprogrammed to head toward the spinal cord and come out the other side. While there’s strong attraction to the spinal cord stimulated by netrin, there must also be an “off” switch somewhere in order to allow the growth cone to migrate to the other side of the spinal cord rather than just stopping where the level of netrin is highest. Like Ming, Kolodkin and his team are looking to uncover how such proteins direct the neuron’s growth cone, in this case, telling it to back off or move on along.

To get a look at circuit development in vivo and uncover more about axonal guidance systems, Ming turned to the mouse hippocampus, where adult neural stem cells give birth to new neurons. “These neurons can send out axons and dendrites, and they eventually form connections and integrate into the existing, older circuitry,” she says. Ming found that within four weeks, the adult-born stem cells have integrated entirely into the preexisting system, and she also found that the newer cells have a greater degree of plasticity than the older cells. She says that it’s essential to know what controls the signals that establish these connections, since being able to guide the growth cones is key for developing a variety of therapies.

While guidance cues are critical for proper neural development, knocking out a single cue isn’t necessarily fatal, says Ming. “Even if the cues for a particular neuron are removed, the animal still develops. In adult animals, there is almost never a failure. There must be a very robust regulatory factor to make sure neurons go to the right place.” Kolodkin agrees but adds a caveat: “Yes, the animal does develop, and sometimes they’re born looking quite normal.” But, collaborating with neuroscientist David Ginty, Kolodkin has found that some mice missing guidance cues undergo epileptic-type seizures, and these mice likely have other behavioral disorders yet to be identified—deficits that may appear in humans whose neural development is similarly compromised. Given that many diseases of the nervous system, including schizophrenia, involve problems in nervous system development, we can expect that defining the yin and yang of attracting and repelling neurons will guide our own quest for understanding and treating mental illness. 

--Victoria Bruce

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