Your nose is a genuinely wondrous sense. It can differentiate between one trillion different scents! This ability is remarkable, and it requires over 10 million specialized nerve cells, or neurons, in the nose, and more than 400 dedicated genes. How these genes and neurons work together to distinguish a scent has remained a mystery. Until now.
A new study is discovering a genomic mechanism through which a finite number of genes can distinguish a near-infinite number of scents. The new Columbia study suggests that the genome rearranges itself in three-dimensional space and coordinates the regulation of the genes in each neuron. The result is the biological diversity needed to detect the scents we all experience.
The sense of smell, which is technically known as olfaction, is complex. In short order, our noses must identify a scent, determine how strong the scent is, scan our memories to determine if the scent is familiar, and then determine if it is pleasing or toxic.
Specialized nerve cells, known as olfactory receptor neurons, make this process possible. Each neuron contains the full set of 400 dedicated olfactory receptor genes. However, only one of these genes is active in each neuron. The active gene is randomly chosen and is different from neuron to neuron. Deciphering how each olfactory receptor neuron activates only one of these genes and how it results in a finely tuned sense of smell has been a challenge for years.
The popular theory is that genes on different chromosomes rarely interact with each other. Using genomic sequencing known as situ Hi-C, chromosomes interact more frequently than thought. Hi-C is innovative in that it allows researchers to 3D map the entire genome inside a living cell. It is this that gives investigators a snapshot of the genome at any point in time.
The snapshots taken by the investigators show clusters of olfactory receptor genes located on different chromosomes, moving toward each other before choosing an olfactory receptor gene. Once the genes huddled, another element known as enhancers clustered in separate 3D compartments. These enhancers regulate the activity of genes. The enhancers create hotspots of activity that regulates the selected olfactory receptor genes. The protein Ldb 1 is also involved in the process. The protein holds the clusters together, allowing them to switch on a particular olfactory receptor gene and then, as a team, interpret the scent.
Although the focus of this study is olfaction, the research team believes that their findings can have implications in other areas of biology where chromosome interaction involvement is present. The interactions among chromosomes that result in shifts of the genome might play a role in causing cancer. The changes seen in the olfactory receptor may shape the activity of other cells. The research team hopes to continue their research with the hope that further study may benefit other areas of biology in the fight against diseases.