A study in mice by University of Michigan researchers has identified a protein known as GluK2 (glutamate ionotropic receptor kainate type subunit 2) that enables mammals to sense cold. The discovery helps to bridge a long-standing knowledge gap in the field of sensory biology, and could let scientists unravel how we sense and suffer from cold temperature in the winter, and potentially better understand why some patients experience cold differently under particular disease conditions. The results also point to GluK2 as a potential therapeutic target for cold pain.

“This discovery of GluK2 as a cold sensor in mammals opens new paths to better understand why humans experience painful reactions to cold, and even perhaps offers a potential therapeutic target for treating that pain in patients whose cold sensation is overstimulated,” said research lead Shawn Xu, PhD, a professor at the U-M Life Sciences Institute. Xu is senior author of the team’s published paper in Nature Neuroscience, titled “The kainite receptor GluK2 mediates cold sensing in mice,” in which they concluded “In the current study, by characterizing GluK2 KO mice, we provided multiple lines of evidence uncovering a key role of GluK2 in cold sensing. Our results demonstrate that GluK2 mediates cold sensing in mice, supporting GluK2 as a cold sensor.”

Temperature has a “profound impact on the life of all organisms on Earth,” the authors stated. “To survive and adapt to the ever-changing environment, animals and humans have evolved a specialized somatosensory system to detect temperature changes in the environment, thereby adjusting their behavior and physiology accordingly,” they continued.

However, while thermosensors detecting cool, warm and hot temperatures have all been extensively characterized, little is known about those sensing cold temperatures. “The field started uncovering these temperature sensors over 20 years ago, with the discovery of a heat-sensing protein called TRPV1,” said neuroscientist Xu. “Various studies have found the proteins that sense hot, warm, even cool temperatures—but we’ve been unable to confirm what senses temperatures below about 60 degrees Fahrenheit.”

The authors further explained, “Though several candidates, particularly those thermosensitive TRP channels, have been proposed as cold sensors, their roles in mediating cold sensing in somatosensory neurons in vivo have not been validated. As such, the molecular identities of cold sensors remain elusive, leaving a knowledge gap in our understanding of thermosensation.”

In a 2019 study, researchers in Xu’s lab discovered the first cold-sensing receptor protein in Caenorhabditis elegans, a species of millimeter-long worm that the lab studies as a model system for understanding sensory responses. The gene that encodes the C. elegans protein is evolutionarily conserved across many species, including mice and humans, so the finding provided a starting point for verifying GluK2 as the cold sensor in mammals.

The researchers, from the Life Sciences Institute and the U-M College of Literature, Science, and the Arts, tested their hypothesis in mice that were missing the GluK2 gene, and thus could not produce any GluK2 proteins. “… we characterized mice lacking the kainate-type glutamate receptor GluK2, a mammalian homolog of the Caenorhabditis elegans cold sensor GLR-3,” they wrote. Through a series of experiments to test the animals’ behavioral reactions to temperature and other mechanical stimuli, the investigators found that the mice responded normally to hot, warm, and cool temperatures, but showed no response to noxious cold. “While GluK2 knockout mice respond normally to heat and mechanical stimuli, they exhibit a specific deficit in sensing cold but not cool temperatures,” the scientists pointed out. They also confirmed that selective deletion of GluK2 from dorsal root ganglion (DRG) neurons led to similar cold-sensing phenotypes, “… suggesting that GluK2 functions in DRG neurons to mediate cold sensing.”

GluK2 is primarily found on neurons in the brain, where it receives chemical signals to facilitate communication between neurons. But it is also expressed in sensory neurons in the peripheral nervous system (outside the brain and spinal cord). “Our results identify an unexpected function of GluK2—a kainate receptor best known to transmit chemical synaptic signals in the central nervous system—in sensing cold temperatures in the periphery,” the team concluded.

“We now know that this protein serves a totally different function in the peripheral nervous system, processing temperature cues instead of chemical signals to sense cold,” said Bo Duan, PhD, U-M associate professor of molecular, cellular, and developmental biology and co-senior author of the study.

While GluK2 is best known for its role in the brain, Xu speculates that this temperature-sensing role may have been one of the protein’s original purposes. The GluK2 gene has relatives across the evolutionary tree, going all the way back to single-cell bacteria.
“A bacterium has no brain, so why would it evolve a way to receive chemical signals from other neurons?” said Xu, who is also a professor of molecular and integrative physiology at the U-M Medical School. “But it would have great need to sense its environment, and perhaps both temperature and chemicals. So I think temperature sensing may be an ancient function, at least for some of these glutamate receptors, that was eventually co-opted as organisms evolved more complex nervous systems.”

The authors further pointed out, “Unlike thermosensitive TRP channels, homologs of GluK2 exist in both vertebrate and invertebrate species, and, importantly, they all can function as cold sensors at least in vitro. We thus propose that GluK2 represents an ancient class of evolutionarily conserved thermosensor.”

In addition to filling a gap in the temperature-sensing puzzle, Xu believes the new finding could have implications for human health and well-being. Cancer patients receiving chemotherapy, for example, often experience painful reactions to cold. “This discovery of GluK2 as a cold sensor in mammals opens new paths to better understand why humans experience painful reactions to cold, and even perhaps offers a potential therapeutic target for treating that pain in patients whose cold sensation is overstimulated,” Xu said.

In their paper the authors further pointed out, “Exposure to cold, particularly chronic cold, causes tissue damage and evokes pain. Cold-induced pain represents a severe health condition. Currently, there are no effective cures for cold pain. Our study identifies GluK2 as a new drug target for developing therapeutics treating cold pain.”