How nose distinguishes rose scent from rotten eggs
A research on fruit fly has identified a braking mechanism in olfactory neurons that helps generate an amazing diversity of sensors in the nose.
The human nose has millions of olfactory neurons grouped into hundreds of different neuron types. Each of these neuron types expresses only one odorant receptor, and all neurons expressing the same odorant receptor plug into one region in the brain, an organization that allows for specific odors to be sensed.
For example, when you smell a rose, only those neurons that express a specific odor receptor that detects a chemical the rose emits get activated, which in turn activates a specific region in the brain.
Rotten eggs on the other hand, activate a different class of neurons that express a different (rotten egg) receptor and activate a different part of the brain. How the one-receptor-per-neuron pattern - critical for odor discrimination - is achieved in olfactory neurons is a mystery that has frustrated scientists for long.
Now a team of scientists, led by neurobiologists at the University of California, Riverside, has an explanation.
Focusing on the olfactory receptor for detecting carbon dioxide in Drosophila (fruit fly), the researchers identified a large multi-protein complex in olfactory neurons, called MMB/dREAM, that plays a major role in selecting the carbon dioxide receptors to be expressed in appropriate neurons.
According to the researchers, a molecular mechanism first blocks the expression of most olfactory receptor genes (~60) in the fly's antennae. This mechanism, which acts like a brake, relies on repressive histones -proteins that tightly wrap DNA around them. All insects and mammals are equipped with this mechanism, which keeps the large families of olfactory receptor genes repressed.
"How, then, do you release this brake so that only the carbon dioxide receptor is expressed in the carbon dioxide neuron while the remaining receptors are repressed?" said Anandasankar Ray, an assistant professor of entomology, whose lab conducted the research.
"Our lab, in collaboration with a lab at Stanford University, has found that the MMB/dREAM multi-protein complex can act on the genes of the carbon dioxide receptors and de-repress the braking mechanism - akin to taking the foot off the brake pedal. This allows these neurons to express the receptors and respond to carbon dioxide," he stated.
Next, the researchers will test whether the receptor-braking mechanism they identified in Drosophila is also involved in other organisms like mosquitoes. They also will examine the other receptors in Drosophila to explain what de-represses each one of them.
The researchers also found that the activity of the MMB/dREAM multi-protein complex in Drosophila can alter levels of the carbon dioxide receptor and modulate the level of response to carbon dioxide.
Study results appeared in the Genes and Development.