Newswise – Johns Hopkins Medicine researchers say they have evidence to potentially overturn a prevailing belief in an important type of signaling inside cells. The main idea is that a single protein receptor molecule, a kind of flag on the cell surface, stimulates the activity of up to hundreds of downstream protein molecules to produce a signal.
Their new findings from the study of genetically modified mice show that nasal cell receptors activate, on average, far fewer of these special proteins, usually one at most, to start a cascade of chemical reactions that reach in the odor-detecting parts of the mammalian brain. And most of the time, they say, the signage doesn’t happen at all.
The researchers’ findings were published Aug. 1 in the Proceedings of the National Academy of Sciences.
The signaling pathway, called G protein-coupled receptor (GPCR) signaling, is ubiquitous throughout the body and is an important focus for drug development to treat many diseases, from high blood pressure to pain and illness Parkinson’s The pathway also mediates several physiological processes, including vision, smell, mood regulation, inflammation, and the immune system.
“This signaling pathway is found in cells all over the body, and they serve all kinds of functions,” said King-Wai Yau, Ph.D., professor of neuroscience and ophthalmology at Johns Hopkins University School of Medicine.
The main idea in the 1980s was that a GPCR molecule on the cell surface, when stimulated, would activate hundreds of guanine nucleotide binding proteins, called G proteins. The activation of a high level of these G proteins is called high amplification. This, in turn, would set off a chemical chain reaction.
This idea began with research on light-sensitive cells called rod photoreceptors in the retina. Their visual pigment, called rhodopsin, is a GPCR that absorbs light particles called photons. Other researchers reported finding that when one rhodopsin molecule absorbs a photon, it activates up to 500 G proteins. The signal eventually reaches the brain, causing vision.
“In fact, over the next 30 years, scientists extrapolated, or generalized, this idea of high amplification to other GPCR signaling pathways involving G proteins,” Yau said.
In the current research on smell, however, the Johns Hopkins team found that signal amplification is actually very low, so low that the probability of an odorant receptor activating just one G protein would be perhaps only d ‘1 in 10,000. As such, the level of activation “is very weak,” Yau said.
For the experiments, Yau’s team, including first author Rong-Chang Li, Ph.D., genetically tagged mouse nasal olfactory cells in a laboratory dish with fluorescence. They then stimulated one of these cells with an odorant in solution for exactly 30 milliseconds.
In this way, they were able to estimate how many times the odor molecules intercepted the odor receptor during stimulation. Finally, they calculated how many collisions were needed to activate a G protein molecule. To calculate the probabilities, the team examined 20 cells and performed about 45 trials for each cell.
The findings suggest that when the odorant and the receptor interacted, 99.99% of the time the odor did not trigger the chemical chain reaction that sends a signal to the brain.
“The result is very different from the rod view,” Yau said.
In the future, the researchers will focus on whether the low probability of activating G proteins applies to other types of odors and their associated receptors. They also plan to study other types of receptors to confirm the team’s findings.
The researchers speculate that light receptors activate more G proteins than odor receptors because light receptors are so sensitive to light, to the point that they can absorb and signal a single photon of light. In the future, the researchers will test whether the low probability of signaling they found for a particular odorant receptor applies to other odorant receptors.
Other researchers who contributed to this study include Chih-Chun Lin and Xiaozhi Ren, formerly of the Johns Hopkins University School of Medicine; Laurie L. Molday and Robert Molday of the University of British Columbia; and Alexander Fleischmann of Brown University.
This work was supported by the National Institutes of Health (grant R01 DC14941) and the Canadian Institutes of Health Research.
This press release was researched and written by Science Writing Fellow Lauren Hines.