02 Dec 2002

We all know that plants grow toward a light source-but would you believe the same of neurons? Neuronal "phototropism" is exquisitely demonstrated by a report published online in last week's PNAS.

Researchers from the University of Texas, Austin, and the University of Leipzig, Germany, led by principle author Josef Käs, used a titanium sapphire laser to "guide" growing neurons in vitro. First author Allen Ehrlicher et al. focused the laser beam on a small section of the neuronal growth cone and observed the extending neurons making dramatic turns, changing their direction by up to 90 degrees toward the light. Statistically these effects were highly significant, the cones altering course toward the laser almost 80 percent of the time.

Exactly why neurons grow toward the light is unclear. What seems certain is that this is not a classic phototropic response as these cells are devoid of photoreceptors. The authors also rule out heat as a reason for the response because the lasers have very little power and can heat up the surrounding tissue by at most a few degrees. Instead, the authors suggest that the light influences the dynamics of actin polymerization and depolymerization, which is a key process in regulating the movement of the leading edge of the growth cone. The laser exerts very little force, however, insufficient to move large oligomeric structures like actin polymers. Ehrlicher et al. favor a model whereby the light beam, which creates a weak electrical gradient, attracts soluble globular proteins, such as actin monomers and fragments of actin filaments. This increases the likelihood that polymerization will take place near the laser beam, thus causing the growth cone to expand in that direction.—Tom Fagan