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How this maze of neurons establishes itself seems mind-boggling. Yet scientists are beginning to discover the navigational signs that guide neurons and help them compete for survival. This knowledge not only sheds light on the fascinating problem of how brains establish their wiring but also reveals how faults can lead to problems. “Understanding normal wiring will help us understand miswiring,” says former HHMI investigator Marc Tessier-Lavigne, now executive vice president of research drug discovery at Genentech, Inc., in South San Francisco. “And how it might be possible to rewire the brain following injury or disease.”
Since the late 1800s, neuroscientists have used Golgi staining, named for its inventor, Italian Nobel laureate Camillo Golgi, to trace the paths of neurons. In fixed tissue, Golgi staining randomly colors a small number of neurons and reveals their complex structure: the bulbous cell body in the middle, a bushy canopy of dendrites on one side, and the long sinewy cable-like axon on the other. But Golgi staining has its limitations. For example, it cannot tint specific types of neurons, or neurons in which a particular gene is mutated.
To improve on the method, HHMI investigator Liqun Luo of Stanford University has devised a way to label individual neurons by using genetics. In his MARCM technique, short for “mosaic analysis with a repressible cell marker,” Luo engineers fruit flies so that a small number of neurons express a fluorescent protein and consequently light up under a microscope. Moreover, Luo can restrict this labeling to particular types of neurons, so he knows exactly what kind of cell he's observing.
Luo also uses MARCM to spur mutations, so he can manipulate genes and then probe how a neuron grows or makes connections in response. The approach offers several benefits. First, it allows researchers to identify mutated cells, because each mutated cell glows. Second, scientists can probe how a genetic change influences a single neuron among a group of normal neurons.
Luo has used the technique to understand how the brain wires up its smell sensors—olfactory neurons—each of which carries a single olfactory receptor on its surface. Because flies carry 50 different odor receptors, they have 50 different types of olfactory neurons, with each one tuned to grab onto a corresponding kind of odor molecule. In the brain, all the neurons that produce the same receptor stem from a single spot, called a glomerulus. So the fly brain has 50 glomeruli, one for each receptor type. At each glomerulus, axons of the olfactory neurons relay smell information to dendrites of another type of neuron—a projection neuron—which transmits the information throughout the brain.
Photo: João Canziani