Scientists Identify a Protein Channel that Mediates the Body's Ability to Feel Frigid Temperatures
A group of researchers from The Scripps Research Institute (TSRI) and the Genomics Institute of the Novartis Research Foundation (GNF) have identified and isolated a novel protein that mediates the body's ability to sense cold through the skin.
In an article that will appear in this week's issue of the journal Cell, the group describes the "ion channel" protein, called ANKTM1, which is the first noxious (painful) cold receptor identified, and may be an important basic target for pain-modulating drugs.
Despite the fact that researchers at several other laboratories had previously identified receptors that sense hot temperatures, warm temperatures, and cool temperatures, the protein that detects cold temperatures had been conspicuously absent. "This was one of the remaining puzzles," says TSRI Assistant Professor of Cell Biology Ardem Patapoutian, who led the effort with TSRI Research Associate Gina Story.
The cold receptor protein ANKTM1 was overlooked, note Patapoutian and Story, because it is distantly related to the hot, warm, and cool receptors. As such, ANKTM1 has very low sequence homology, or DNA similarity, with these other proteins.
But when they studied it in the laboratory, Patatpoutian and Story found that even though ANKTM1 did not "look" like a temperature receptor, it sure acted like one. "We found that if we applied very cold stimuli, the channel would open in response," says Story.
Hot, Cold, and Everything In Between
Humans and other vertebrate animals use specialized sensory neurons to detect temperature, pressure, and other physical stimuli on the skin. These neurons are located in the spinal column and are connected to the skin and organs through long extensions known as axons.
On the surface of these axons are the protein channel molecules, like ANKTM1 and its cousins the hot, warm, and cool receptors, which span the axon's membrane, connecting the inside with the outside. These receptors act like "molecular thermometers" by opening and closing according to the temperature. At a particular temperature, the receptors open. This allows an influx of calcium ions into the axon, and this electrical signal is relayed through the neuron to the brain.
The existence of specialized hot- and cold-neurons had been known for years, but the molecules that actually sense the temperatures and signal back to the neuron through the axon were a complete mystery. That changed in 1997 when a group cloned the first sensory molecule, a type of transient receptor potential (TRP) channel called TRPV1. TRPV1 opens when it senses hot temperaturesabove 42° C (108° F). That discovery opened the floodgates for identifying temperature-detecting proteins. Within a few years, several laboratories had identified additional temperature-detecting proteins.
Last year, Patapoutian and his TSRI and GNF colleagues identified and cloned a protein called TRPM8, which is the first-known signaling molecule that helps the body sense cool temperatures. The channel becomes activated below 25° C (77° F). Similarly, the group also identified a type of TRP channel called "TRPV3" that makes skin cells able to sense warm temperatures. It is activated around 33° C (92° F).
How Low Can You Go?
In their current study, Patapoutian and Story demonstrate that the channel ANKTM1 is inactive at room temperature and higher, and only becomes active at "noxious" cold temperatures. Below 15° C (59° F), the channel opens and allows an influx of positively charged ions into the axon, an electrical signal which is then communicated to the brain.
Biochemically, ANKTM1 is a bit of a puzzle because proteins are normally more active at higher temperatures. Even more bizarre is the fact that these cold-sensing ANKTM1 proteins are coexpressed with their cousins, the hot-sensing TRPV1 proteins on the same neurons. This means that the same neuron may be responsible for detecting hot and cold temperatures.
Scientists had long assumed that different neurons would detect different stimuli and be responsible for communicating those separately to the brain. But if the same neurons detect hot and cold, how does the brain tell the two stimuli apart? The answer, while unclear, may explain an old psychologist's observation that humans cannot tell the difference between a hot needlepoint and a cold needlepoint on their hand.
Significantly, ANKTM1's neuronal neighbor TRPV1 is involved in inflammation and in communicating pain to the brain, and several compounds that block TRPV1's action are currently under investigation for chronic pain indications. Since ANKTM1 is expressed in the same neurons, it, too, may be a target for pain therapeutics.
"This protein may be an important therapeutic target," says Patapoutian, "because, like these other TRP channels, it may be involved in inflammation and pain-mediation."
The research article "ANKTM1, a TRP-like Channel Expressed in Nociceptive Neurons, Is Activated by Cold Temperatures" is authored by Gina M. Story, Andrea M. Peier, Alison J. Reeve, Samer R. Eid, Johannes Mosbacher, Todd R. Hricik, Taryn J. Earley, Anne C. Hergarden, David A. Andersson, Sun Wook Hwang, Peter McIntyre, Tim Jegla, Stuart Bevan, and Ardem Patapoutian and appears in the March 21, 2003 issue of Cell.
The research was funded by the National Institutes of Health and by a grant to TSRI from Novartis.
Some neurons like it hot (and cold)