CHICAGO, Sept. 2 (Xinhua) -- Researchers at the University of Michigan (UM) have identified a receptor protein that can detect when winter is coming, revealing the first known cold-sensing protein to respond to extreme cold.

"Clearly, nerves in the skin can sense cold. But no one has been able to pinpoint exactly how they sense it," said Shawn Xu, a faculty member at the UM Life Sciences Institute and senior author of the study. "Now, I think we have an answer."

Capitalizing on the simplicity of Caenorhabditis elegans (C. elegans), the researchers looked across thousands of random genetic variations to determine which affected the worms' responses to cold.

They found that worms missing the glutamate receptor gene glr-3 no longer responded when temperatures dipped below 18 degrees Celsius.

This gene is responsible for making the GLR-3 receptor protein. Without this protein, the worms became insensitive to cold temperatures, indicating that the protein is required for the worms to sense cold.

What's more, the glr-3 gene is evolutionarily conserved across species, including humans. And it turns out the vertebrate versions of the gene can also function as a cold-sensing receptor.

When the researchers added the mammalian version of the gene to mutant worms lacking glr-3, they found that it rescued the worms' cold sensitivity.

They also added the worm, zebrafish, mouse and human versions of the genes to cold-insensitive mammalian cells. With all versions of the gene, the cells became sensitive to cold temperatures.

The mouse version of the gene, GluK2 abbreviated from glutamate ionotropic receptor kainate type subunit 2, is well known for its role in transmitting chemical signals within the brain.

The researchers discovered, however, that this gene is also active in a group of mouse sensory neurons that detect environmental stimuli, such as temperature, through sensory endings in the animals' skin.

Reducing the expression of GluK2 in mouse sensory neurons suppressed their ability to sense cold, but not cool, temperatures. The findings provide additional evidence that the GluK2 protein serves as a cold receptor in mammals.

"For all these years, attention has been focused on this gene's function in the brain. Now, we've found that it has a role in the peripheral sensory system, as well," said Xu.

"It's really exciting. This was one of the few remaining sensory receptors that had not yet been identified in nature," Xu said.

The study was published in the latest issue of the journal Cell.