(May 18th, 2017) Smell is one of our most important senses. But how does it develop? And why are some people more sensitive to certain odours? A UK research team looked into the noses of mice to find out.
Inside your nose, thousands of olfactory receptors are sensitive to chemicals in the air. They can, for instance, sense geosmin (wet soil after rain), rose-oxide (roses) or the repulsive mercaptoethanol (sadly present in most PAGE loading buffers). The combinatorial power of a thousand different olfactory receptors stands in sharp contrast to the three visual receptors in your eyes and the dozens of taste receptors in your tongue. With all those potential olfactory receptors to choose from, what determines, which one gets a place inside your nose? An international research team, led by the Wellcome Trust Sanger Institute in the UK, just published what they think is going on inside your nose, or technically, inside mice’s noses.
The olfactory sensory neurons of the main olfactory epithelium are like moss, covering the cavern of your nostrils. All of them have the genetic information for producing odour receptors on their membranes; however, it would be baffling to find a single cell expressing all the receptors all the time. I bet, if such a cell existed, it would probably work as coherently as a single PhD student finishing a dissertation, publishing a killer paper and keeping a fiancé at the same time. The safe path to smelling is to have around ten million olfactory sensory neurons, each one expressing predominantly one single olfactory receptor out of a thousand options.
Seeking to take an olfactory inventory, Darren Logan's team and his collaborators quantified all those receptors in the noses of individual mice. This was possible by RNA sequencing of the mice's dissected olfactory epithelium. Although this method doesn’t tell you directly whether the receptors exist on the cell membrane, it gives you an accurate estimation of the receptors that are actively transcribed and very likely used by the nose.
The detection of the different olfactory receptor-RNA’s showed that individuals of the same strain possess similar olfactory sensory neurons. This was expected but not the next result. Logan and colleagues found dozens of olfactory sensory neuron subtypes that apparently were determined by the odorants the mice had encountered during their lives. Even genetically identical mice had a different odour receptor repertoire, if they had grown up being exposed to different smells. Thus, the smells in the air you breathe can ultimately determine the types of olfactory receptors inside your nose. By contrast, when the mice were exposed to the same odorants during development, the variations in the olfactory receptors among individuals could be explained mainly by genetics. The authors found that sequences of non-coding DNA (cis-acting elements) in proximity to the receptors’ genes caused the variation when the environment was constant.
"We have shown that each individual has a very different combination of possible olfactory neurons, driven by genetics. In this study we also show that, with experience of different smells, these combinations of neurons change, so both genetics and environment interplay to give every individual a unique sense of smell," Logan explains in a press release.
If this amazing neuronal plasticity, induced by experience, is found in the neurons of our nose, what kind of plasticity can we expect from the neurons in our brains?