David Ginty tells the graduate students and postdocs in his lab to be fearless about learning and using new approaches. “You have to do whatever it takes to answer outstanding questions,” he says. “That means trying a new technique or coming up with one if it doesn’t already exist.” Ginty has followed this strategy throughout his career—grabbing molecular biology tools to delve into the processes responsible for laying out the complex networks of cells and connections that make up the vertebrate nervous system.
Ginty started working in this research field as a postdoc in John Wagner’s laboratory at the Dana-Farber Cancer Institute in Boston, studying the mechanism of action of nerve growth factor (NGF). This protein was known to be critical to the survival of sympathetic neurons, a family of neurons originating from spinal ganglia—masses of nerve tissue near the spinal cord that contain the cell bodies of neurons—and reaching out to target organs such as blood vessels, the heart, and glands.
When Wagner announced he was moving to Cornell University in New York City, Ginty had to choose whether to follow or look for another position. “I agonized over the decision because my project was going well and I thought that if I switched labs I would lose momentum,” he says. But his wife had recently given birth to their first son and they thought Boston would be an easier place to raise a family. So, Ginty moved across the street to the laboratory of Michael Greenberg at Harvard Medical School.
That choice turned out to be a good one. In Greenberg’s lab, Ginty demonstrated that when NGF binds to its receptor on a nerve cell it “turns on” the activity of a molecule called CREB (cAMP response element binding). In the cell’s nucleus, CREB, a transcription factor, regulates the expression of a large cohort of genes that may control the growth, differentiation, and survival of neurons.
But to study how NGF regulates CREB, Ginty needed a way to easily measure whether CREB was phosphorylated—in other words, in its active form—or not. In a technical feat, Ginty developed the first antibody that specifically recognizes the phosphorylated form of CREB. “Once we had the antibody, my experiments became 1,000 times easier,” he says.
After establishing his own lab at the Johns Hopkins University School of Medicine, Ginty used his antibody in combination with a variety of technologies to ask how NGF, a signaling molecule that binds to a receptor located on the distal part of a nerve projection, or axon, could regulate CREB and other nuclear proteins located in the neuron’s cell body at the opposite end. Ginty’s lab discovered that when NGF binds its receptor, the complex is incorporated in a membrane-bound compartment called an endosome. Inside the endosome, NGF travels through the axon to the cell body, a meter-long journey for some human nerve cells, to activate molecular events that occur in the cell’s nucleus, including gene transcription.
Eventually Ginty’s quest to add more pieces to this puzzle brought him to studies in mice. In collaboration with HHMI investigator Alex Kolodkin, who had started his own lab at Hopkins at around the same time, Ginty began to study the function of another nerve growth signal called semaphorin. One of the first questions they asked was, “What is the receptor for this molecule?” Through a series of experiments they determined that a protein called neuropilin is the receptor for one type of semaphorin called Sema3A; another related protein called neuropilin-2 is a receptor for other semaphorins. They then engineered “knockout” mice lacking one or the other neuropilin genes, demonstrating that both molecules are required for semaphorin function in vivo. “That question is what first got us into mouse genetics,” says Ginty. “And I have not looked back since.”
For the past 10 years, Ginty has been taking advantage of sophisticated mouse genetic approaches to identify the key molecular events that underlie the growth and survival of neurons in the peripheral nervous system. One of his recent discoveries is that sympathetic neurons compete for their survival through a series of feedback mechanisms. As axons extend toward their targets, they are only modestly responsive to NGF and other survival signals produced by target tissues. But as they innervate their target, these neurons’ responses to NGF become amplified through processes that require the transcription of several genes, resulting in large differences among neurons. The neurons that are more responsive to NGF become stronger and “punish” their neighbors, by producing signals that harm them. As a result, the stronger neurons survive and the others perish. That is one way the organism ensures that the right number of nerve cells end up innervating the intended target.
“I love how powerful the molecular-genetic approach has been to understand how neurons grow, extend axons into target fields, mature, and survive,” says Ginty. “It has been fascinating on so many levels.” And as Ginty follows the paths of his neurons, he is constantly being taken to new research areas. “We have recently identified a subset of sensory neurons that send their axons to the skin to respond to touch,” he says. “There are so many questions we now want to ask.”
When Ginty is not chasing neurons he oversees the graduate program in neuroscience at Johns Hopkins, imparting his excitement about science to students. “I love working with students,” he says. “When I was a postdoc my heart was in doing experiments and in using my hands to answer questions. I thought I would have to keep doing that to enjoy science. But after I set up my own lab I realized that it is even more satisfying when a student or a postdoc in the lab has a breakthrough. There is nothing better in the world.”