One molecule makes nerve cells grow longer. Another one makes them grow branches. These new experimental manipulations have taken Drexel researchers a step closer to understanding how nerve cells are repaired at their farthest reaches after injury.

“If you injure a peripheral nerve, it will spontaneously regenerate, but it goes very slowly. We’re trying to speed that up,” says Jeffery Twiss, a professor in the the Biology Department in Drexel’s College of Arts and Sciences.

Christopher Donnelly, now a postdoctoral fellow at Johns Hopkins University, led the study as part of his dissertation work in Twiss’ lab while at the University of Delaware.

Donnelly and Twiss knew from their previous research that two of the messenger RNA molecules involved in directing repair in injured axons compete against each other at an essential step in repairing damage. The mRNA molecules that “win” the competition get to make their particular repair-protein product.

So, experimentally, they rigged the competition between those molecules to see what would happen: Could one molecule make a difference in helping axons grow longer, faster?

The technical process of these experiments was complex, but the answers were clear.

They saw more branches in the axons when they added more mRNA used to make the repair protein beta-actin, while taking away the mRNA for the protein GAP-43. But they got the desired longer, less-branched axons from the opposite experiment, adding mRNA coding for GAP-43 and taking away that for beta-actin.

This was a promising result for developing potential therapies, Donnelly says. When nerves repair themselves after injury, there is currently no way to control their pattern of regrowth. But, “if you can induce longer growth quicker, rather than branching growth, you can help reach the target of faster recovery from an injury.”

Yet other modifications, selectively withholding the mRNA molecules, resulted in shorter axons, or in fewer axon branches—and the researchers found they could experimentally and selectively restore branching or lengthening growth in those deprived cells, too. Consistent with the first experiments’ results, adding more beta-actin mRNA again restored axon branching, and adding more GAP-43 mRNA again restored axon length.

But a key point in these experiments is that all of the changes only happened when they added mRNA molecules that functioned as the proteins’ “local recipe” used especially for making these proteins in the nerves’ axons. The “standard recipe” of mRNA, that directs cells to make those same proteins in the cell body, didn’t have these effects on the axons’ growth.

This experimental technique to manipulate axon growth was not previously tested, the authors noted. They found most of these results using adult rat neurons in vitro, but also confirmed the principles in vivo with the developing spinal cord of chicken embryos in collaboration with Dr. Gianluca Gallo of Temple University.

“This sets some of the groundwork needed to consider using these mechanisms for improving regeneration in the future,” Twiss says.