Frontier: Plasticity: more than meets the eye

by Sandra A. Swanson

September, 2008

The brain’s capacity for rewiring early in life is well-established, but questions remain about what underlies that critical period. Now, researchers have identified a protein that triggers plasticity in visual systems. The study appears in the Aug. 8 issue of the journal Cell.

Parvalbumin (PV) cells, located in the visual cortex, help the brain rewire based on visual input. With colleagues in France and Japan, Takao Hensch of Children’s Hospital Boston studied the visual systems of postnatal mice, and the experiments revealed two major surprises related to the molecular triggers that determine when PV cells mature.

First, researchers determined that PV cell maturation was orchestrated by Otx2, a protein essential for fetal head formation but previously thought to disappear at birth, Hensch says.

The second surprise: Otx2 is synthesized not by PV cells but by the retina, from which it is sent to the cortex. Says Hensch: “Suddenly we find that it’s not the brain specifying the timing of the critical period, but in fact the eye is telling the brain when to become plastic.”

Hensch and his colleagues raised mice in the dark and determined that Otx2’s migration from eye to brain was activity-dependent, requiring an external stimulus to start the process. In another experiment, the researchers injected Otx2 directly into the visual cortex of mice raised in darkness – and their PV cells matured.

“It’s not the production of the protein that’s activity-dependent, but some step of getting it from the eye to the brain,” says Hensch. Further research will focus on that step.

The researchers speculate that other sensory systems may also have a molecular messenger that triggers plasticity. Hensch observes that if plasticity’s timing can be controlled, it could affect a range of needs, such as learning a language or recovering from a stroke.