In the orchestra of the worm Caenorhabditis elegans, which has 302 neurons, the protein NHR-49 was long cast as a conductor who runs nothing but the fat warehouse. But a new experiment shows that this conductor can walk onstage without a fat score in hand.
Researchers took worms with the nhr-49 gene completely switched off and placed them in a Petri dish where one side smelled of diacetyl, an odor that for a worm means "food over there." Healthy worms crawled toward the aroma with confidence, while the mutants lost their bearings. Yet the mutants responded normally to other attractive odors picked up by different neurons. So the protein is not needed for smell in general, only for one specific line of communication.
To pin down where the protein does its work, the scientists switched it on in different tissues: neurons, muscles, intestine, skin. Only neurons and muscles restored normal behavior. What is more, mutants showed less of a glowing reporter protein in their AWA neurons, a marker that reveals the activity of the odr-10 gene, the very one that encodes the diacetyl odor receptor. It looks as if NHR-49 directly props up the neuron's "antenna."
The same protein also turned out to be needed for getting used to touch. Worms were poked 30 times in a row at intervals of 10 seconds, and healthy worms backed up less and less with each poke. Mutants reacted to the first poke just as briskly, but by the end of the series their behavior had barely changed, as if they never learned that the poke was harmless.
So a metabolic "accountant" turns out to be a director of sensory scenes as well. It used to seem that NHR-49 influenced behavior only through fat metabolism, like a conductor who runs the orchestra by changing the lighting in the hall. Now it is clear that it can walk up to a particular musician and tune the instrument directly. For medicine, this is a hint: proteins that mind metabolism may also fine-tune the nerves, and glitches in that tuning could lie behind diseases where metabolism and behavior are tangled together.
Despite its tiny nervous system of 302 neurons, C. elegans distinguishes odors using specialized sensory neurons: AWA handles attractive aromas, AWC handles other attractants, and ASH handles repulsion. NHR-49 turned out to be a "conductor" only for AWA-dependent responses.
Not published on arXiv. Here are its page, the PDF and the formalization: you can check it yourself, not only read our account.