Researchers find clues to nerve cell development

Similar to an insect’s antennae, filopodia are finger-like projections on the tips of developing nerve cells that extend out to detect environmental clues and help direct axons to their proper destinations. Until now, scientists didn’t know what kind of signals filopodia sent back to the cell tip, called a growth cone, or how they controlled movement.

UW Medical School researchers have found that the ends of filopodia generate tiny bursts of calcium that travel back to the growth cone to stimulate movement in the right direction.

“These bursts, which usually occur at the very ends of the filopodia, are extremely brief-about a 300-millisecond pulse–which may explain why they were undetected in earlier studies,” says lead scientist Timothy Gomez, an assistant professor of anatomy at the medical school. “Since many other types of cells have these finger-like projections, we believe that these brief calcium bursts may be a universal signaling mechanism for all motile, or moving, cells, including immune, epithelial and metastatic cells.”

The study, conducted by Gomez while he was a post-doctoral fellow in the laboratory of Nicholas Spitzer at University of California-San Diego, appears in the March 9 issue of Science.

In the developing nervous system, neurons in the brain, spinal cord and elsewhere in the body send out long axons, which in the mature nervous system transfer signals to a target cell. A growth cone is located at the tip of each embryonic axon, and later becomes a synapse, the communication connection between nerve cells.

Filopodia sprout from each growth cone and are in constant motion, in search of guidance cues deposited by supporting cells and secreted by target cells, which are often located a great distance from the neuron’s cell body. As the filopodia help steer the growth cones to their target cells, axons are formed along the way.

Gomez and his colleagues studied frog spinal nerve cells they kept alive in a culture dish. They loaded the neurons with a fluorescent calcium indicator that allowed them to observe calcium activity with a confocal microscope, taking eight pictures every second. Collecting images for brief periods of time, the researchers found that calcium bursts occurred repeatedly, beginning at the end of filopodia and traveling down to the growth cone in less than a second.

Calcium modulates many cell activities, and calcium spikes, or transients, similar to the one the Wisconsin researchers observed are not uncommon. But the calcium transients they have discovered in filopodia have novel functions not identified before.

To test the impact of the filopodial calcium bursts on growth cone movements, the researchers exposed them to eight different environments, or substrates, that stimulate embryonic nerve cell growth, some being more favorable than others. They found that the frequency of the bursts varied with the type of substrate as well as its concentration. They saw that the burst frequency correlated with the motile behavior of the filopodia and growth cone.

What’s more, when the researchers experimentally generated calcium bursts in the filopodia on just one side of the growth cone, they found that the signal was sufficient to orient nerve growth. “These findings suggest that local calcium signals play an important role in the development of the nervous system and may be a general mechanism for the regulation of cell motility,” said Gomez. “This new information offers unique possibilities for medical intervention in conditions involving motile cells, such as abnormal development, ineffective nerve regeneration and even tumor metastasis.”