Heat-Activated Receptor Could Unlock New Pain Therapies

Now, investigators at the Jacobs School of Medicine and Biomedical Sciences have uncovered how heat causes a critical receptor protein within cells to unfold and relay pain. This newfound activation mechanism could open up new therapeutic targets for treating pain and contribute to development of needed alternatives to opioids.

This research is described in a Proceedings of the National Academies of Sciences (PNAS) paper published Jan. 7. 

Within cell membranes, ion channel receptors typically activate in response to electrical, chemical, or other specific stimuli, explains Feng Qin, PhD, professor of physiology and biophysics and senior author of the PNAS paper. These proteins act like gateways, opening and closing to permit the passage of ions critical to cell communication.

Researchers have long investigated the ion channels known as TRP (transient receptor potential) channels. They’ve focused particularly on TRPV1, a receptor found at the endings of peripheral nerves in the skin that’s integral to detecting temperature and pain and also responds to capsaicin, the component that makes chili peppers spicy.

How these receptors detect temperature has not been well understood, however. As opposed to a specific chemical or molecular stimulus triggering a designated sensor, the investigators’ research suggests that heat itself causes the receptors to become unstable and partially unfold, causing activation.

Proteins typically maintain structural stability to function correctly, yet the partial unfolding appears to be essential for activation — an unusual finding for ion channels, says Dinesh Indurthi, PhD, a research scientist in Qin’s lab. “This study actually shows that these TRPV channels in fact partially unfold as a part of their activation mechanism,” says Indurthi. “That is a new paradigm.”

While the team’s previous research sought to conceptualize much of this work, the current paper details molecular evidence behind these findings, says Qin, who also directs the Biophysics Graduate Program.

When exposed to heat, the receptor protein typically transitions uniformly from a closed to open activated state. So when the research team altered the protein’s temperature-sensing abilities, it still opened but in an uncoordinated, erratic manner. This observation suggests that its opening relies on how heat affects its overall structure rather than activation of a particular sensor. 

Further, the receptors were observed unfolding within the temperature range in which they’re supposed to detect. That, Qin notes, is unusual because a receptor would typically need to remain stable in order to detect and relay signals.

To carry out these investigations, the researchers have used technology capable of very quickly generating heat — jumping from room temperature to 60 or 70 degrees Celsius in a mere half millisecond — to mimic the receptors’ speedy functioning. Think of how quickly your hand flies back from a hot plate, which requires not only sensing the heat, but relaying the message to your brain, sending another message back to your hand, and, finally, retracting. 

The researchers also used a technique called differential scanning calorimetry, or DSC, which measures heat flow across materials as the temperature changes, to evaluate the receptors’ thermal transitions.

Understanding these proteins’ thermal activation mechanisms could be essential for developing new pain therapies, Qin says.

“These receptors are a very important therapeutic target for pain,” Qin says. “So whether we can utilize these receptors is dependent on whether we can preserve the temperature sensitivity, and, on the other hand, inhibit the sensitivity to other noxious chemical stimuli.”

To alleviate pain, previous potential therapies have blocked the TRPV1 receptor entirely. But because the receptor is believed to contribute to regulating body temperature, blocking it can cause hypothermia and impair temperature sensation, among other physiological disruptions, Qin notes.

“Understanding these temperature-sensing domains and how they work will help design better therapeutics that avoid unwanted noxious signaling,” Indurthi says. "If we identify temperature-sensitive domains and determine how they are involved in protein unfolding, we could also leverage this knowledge to potentially engineer temperature-responsive proteins for applications such as biosensors.”

To do so, the researchers say they’ll need to further investigate the TRPV1 domains and overall structure. Using techniques like cryogenic electron microscopy, or cryo-EM, could help pinpoint where exactly the protein unfolding takes place, presenting possible molecular targets.    

Qin says that “the theory behind this work could be applicable to many thermally sensitive biological processes,” indicating that these findings shed light on fundamental biology and could be potentially relevant to many other research avenues besides treating pain.

This work was supported by a National Institutes of Health grant. Additional paper authors include first author Andrew Njagi Mugo, PhD, of the Department of Physiology and Biophysics, and Ryan Chou of Duke University.