A New Chemotaxis Assay

A new chemotaxis assay reveals nerve cells' surprising sensitivity. A new method for studying the guidance (change in direction) of neurons amid a sea of protein molecules shows how sensitive this process is to the surrounding protein gradient.

Chemotaxis is the process by which living cells sniff out their local environment and act accordingly, which usually means moving or growing toward higher concentrations of beneficial molecules.

In the case of neurons removed from their natural setting and put down on a bed of collagen gel in a dish, growth will follow the increasing gradient of proteins in their vicinity, such as the nerve growth factor (NGF) protein. Neuronal growth, the way in which the long axon bodies of a nerve cells wire themselves into a network, is of great interest since this aids in knowing how brains form.

Now a team of scientists at Georgetown University has developed a new method for measuring the gradient of local proteins (which have been fluorescently tagged) and the axon's response. In this case the neural cells come originally from a rat's brain.

The Georgetown team of neuroscientists and physicists find that axon growth is sensitive to gradients so small (0.1%) that they correspond to about one additional molecule across the spatial extent of the axon's "growth cone," the sensing device at the tip of the growing axon.

This is a remarkable feat considering that, at any one instant, there are large statistical fluctuations in the 1000 or so NGF molecules in the vicinity of the growth cone. The researchers suggest that axons may thus be "nature's most-sensitive gradient detectors." (Rosoff et al., Nature Neuroscicence, June 2004; contact Jeffrey Urbach, urbach@physics.georgetown.edu, 202-687-6594; or Geoffrey Goodhill, geoff@georgetown.edu.)