Scientists use genetic engineering to work out how odors are encoded

Yale scientists, working with the fruit fly as a model, have discovered how odors are encoded by the olfactory system into the complex messages that are sent to the brain.

The study, published in the June 25 issue of Cell, provides new insight into how animals sense and distinguish odors, a process that is essential to identifying food, mates and predators.

Graduate student Elissa Hallem and her advisor John Carlson, professor in the Department of Molecular, Cellular and Developmental Biology at Yale University, systematically tested the odor receptor proteins in the fruit fly antenna and recorded which odors they detect.

The noses of humans and the antennae of insects contain many odor receptor proteins, but it was previously not known how the entire collection of receptors act together to encode olfactory information.

Each fruit fly antenna has 32 odorant receptors, and Hallem and Carlson used a mutant fruit fly to determine their individual odor sensitivities. The antenna of their mutant fruit fly has an "empty" nerve cell, or neuron, that has lost its original odor receptor and does not respond to any odors.

Using genetic engineering, Hallem and Carlson created a series of mutant flies, each with a different fruit fly odor receptor in the previously empty neuron. They then tested the engineered neuron in each fly for the odor sensitivity of the receptor.

They found that some receptors responded strongly to many of the tested odors, while others responded strongly to only one or none. Some odors activated many receptors, and some odors activated only one. Some receptors are able to respond in different ways to different odors – activated by some odors and inhibited by others.

"We were able to create a map of which odor receptor is expressed in which type of neuron," said Hallem. According to Carlson, this receptor-to-neuron map is the first map of its kind of the olfactory system.

"We hope that this map in the fruit fly will serve as a model for the olfactory systems of insects such as mosquitoes that transmit disease as well as for more complex organisms, including humans," said Carlson.

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