The Jentsch lab identifies a protein long thought to be crucial to smell… and finds that it isn’t
Open a biology textbook to the chapter on the senses and you’ll find a story about the way nerve cells transmit information about odors to the brain. It usually goes like this: “smelly” molecules enter the nose and dock onto proteins on the surfaces of neurons. As a result, ion channels in cell membranes open and allow a passage of charged particles (ions), changing the voltage over the outer cell membrane. This change generates electrical impulses that travel to the next cell and on to the brain. The ion channel that opens in response to odor molecules lets positively charged sodium and calcium ions flow into the cell. The entry of calcium has been thought to trigger the opening of another channel by which negatively charged chloride ions exit the cell, but researchers haven’t been able to identify the channel protein. Thomas Jentsch’s lab at the FMP and MDC has just found it, and in the process they encountered a surprise: animals can smell just fine without the channel. The new study appears in the June issue of the journal Nature Neuroscience.
“Most researchers have seen the exit of chloride ions as important, probably even crucial, in mammals’ perception of smells,” Thomas says. “Presumably its function has been to amplify odor signals. Measurements based on cells from rodents have suggested that the release of chloride leads to a five-to-ten-time increase in the strength of electrochemical signals.”
One reason for the focus on chloride channels has surely been the fact that in freshwater animals, this ion is the major player in exciting neurons that transmit information about odors. The cells of mammals accumulate unusually high amounts of it during “quiet” phases and apparently release it when they are activated. Scientists have identified the molecule that draws the ions into cells: a co-transporter called Nkcc1. But until now, the channel that permits chloride to leave has remained a mystery.
And questions remained about the impact of chloride on smell, particularly after 2008, when another lab developed a line of mice that lacked Nkcc1. Without the protein, animal neurons accumulated much less chloride and thus had little that could be released. The researchers confirmed that this caused a drop in the strength of electrochemical signaling to the brain. However, the animals responded normally to odors. The results might have cast doubts on the “amplifier” function attributed to chloride, but most researchers interpreted it differently: in parallel to Nkcc1, cells might have another mechanism to accumulate chloride. In that case, the ions could still be released to augment the animals’ sense of smell.
Thomas’ lab has been systematically investigating ion channels, with a particular focus on chloride channels. Such molecules have been linked to a range of serious diseases; inheriting a defective version of one of them or losing its functions often causes a disruption of the nervous system. As a part of the group’s efforts, PhD student Gwendolyn Billig produced a line of mice lacking a protein called Ano2. This molecule belongs to a family of chloride channels that open in response to rising concentrations of another ion – calcium.
Several pieces of evidence showed that the researchers had found the elusive chloride channel. Björn Schröder, now a junior group leader at the MDC, had already proven that Ano2 transported chloride in response to calcium – so it was the right type of molecule. Now the scientists labeled it with an antibody that made it visible under the microscope. It appeared to be the only calcium-activated chloride channel in neurons of the main olfactory epithelium, which is the point of arrival for odor molecules. Finally, Gwendolyn and her colleagues demonstrated that in the mouse line which lacked Ano2, these sensory cells no longer generated chloride currents in response to high levels of calcium.
They now had a way to test the importance of chloride in odor detection. They carried out precise measurements of electrochemical stimulation in tissue from the main olfactory epithelium of animals without Ano2. They discovered that the strength of signals was reduced by 40 percent at most – much lower than previous estimates. And the animals could still detect odors and differentiate between them. In fact, no difference was found between the smell sensitivity of normal mice and those lacking the channel protein.
As a result of the study, Thomas says, researchers will have to rethink the role of chloride in odor reception. “These ions do amplify a signal, but much more modestly than people have believed. That boost doesn’t seem to be necessary for animals to achieve near-normal sensitivity to smells, at least under normal conditions. Interestingly, a few humans who suffer from a condition called von Willebrand disease also lack the Ano2 gene. There haven’t been any reports about deficits in their ability to smell.”
It’s possible, he admits, that the channel plays a role in the response to some odors – that remains to be seen. But it may also simply be a vestige of evolution, predating the rise of mammals. It may have been preserved because its signal-amplifying functions give animals a slight evolutionary edge. In any case, Thomas says, it’s satisfying to have found the elusive chloride channel. In understanding the molecular mechanics of smell, scientists can now stop chasing a Fata Morgana.
– Russ Hodge