Protein triggers nerve connections.

Neuroscientists are slowly learning that neurons aren't the only stars in the brain. Instead, they're part of an ensemble performance involving other types of brain cells collectively known as glia (SN: 4/7/01, p. 222).

In particular, glia known as astrocytes make up about half the cells in the brain, although their exact role has remained murky. In 2001, a research group led by Ben Barres of Stanford University School of Medicine reported that astrocytes somehow enable nerve cells to form the specialized brain connections called synapses. Nerve cells grown without glial cells in the neighborhood develop far fewer synapses than normal, the scientists found.

Now, Barres and his colleagues have identified a molecule secreted by astrocytes that controls synapse formation. It's a relatively large protein called thrombospondin. When applied to nerve cells growing without glia in a dish, says Barres, "thrombospondin is sufficient to greatly increase the number of synapses." The protein, he adds, appears in the developing brain at just about the time when synapses first emerge.

Thrombospondin isn't the whole story. The synapses that it spurs to form look normal, but they don't function, notes Barres. At least one still-undiscovered signal from astrocytes turns on the newly formed synapses, he says.--J.T.