What Is the Shape of “Cool”, and How Could It Help Chronic Pain and Migraine Sufferers?

It’s that time of year when even the thought of leaving the house and being greeted by the dreary, cold weather literally sends chills up one’s spine. The US is currently facing a record-breaking cold snap in which temperatures have plummeted as low as -40°F (-40°C) and the death toll only continues to rise.

Thermoception, the ability of an organism to perceive temperature, comes naturally to most mammals. We quickly learn that approaching a naked flame feels hot and uncomfortable, whilst prolonged exposure to snow or ice eventually induces a burning sensation. Cool sensations are also detected through compounds such as menthol that can be consumed or applied topically.

Many decades of scientific study have led to the knowledge that transient receptor potential (TRP) channels are implicated in thermoception. Interestingly, such channels are also known to be associated with migraine and chronic pain conditions. However, the precise physiological mechanisms behind their activation and function remain vague. This is the research focus of Professor Seok-Yong Lee’s lab at the Duke University School of Medicine.

Previously, a group of scientists led by Lee characterized the architecture of TRPM8, a well-conserved cold-sensing protein. This discovery provided insight into the protein’s function but raised further questions.

The team’s latest publication, in the journal Science, describes their work in deducing the structure TRPM8 adopts when it is bound to menthol and a synthetic cooling agent icilin. A thorough understanding of TRPM8’s structure provides the opportunity to modulate its activation for clinical purposes. “Accumulating genetics and clinical studies have shown that TRPM8 is a target to treat chronic pain and migraine. Therefore, modulating the channel activity by small molecules holds promise to help relieve migraines and chronic pains” says Lee. 

To study the structure of TRPM8, Lee and colleagues used a technique novel to their laboratory known as cyro-electron microscopy (cryo-EM). “Recent advances of cryo-EM have enhanced the field of ion channel structural biology remarkably, because it requires smaller quantity of protein materials and does not involve protein crystallization for structure determination” Lee adds. The technique involves purifying the protein from cells and flash freezing it before bouncing electrons off the sample embedded in ice. The outcome was a composition of images encompassing millions of protein particles in various orientations, and a high-resolution three-dimensional structure for TRPM8.

The team’s triumphs were not achieved easily, however, “TRPM8 is an intrinsically unstable protein, which makes structural studies very difficult.” Consequently, they utilized TRPM8 channels from a small bird known as the collared flycatcher, which has channels similar to human TRPM8. 

They generated and isolated the protein in the lab before mixing it with either menthol or icilin and PIP2 (phosphatidylinositol 4,5-bisphosphate), a lipid molecule that sends signals within cells and has been previously shown to be necessary for TRPM8 to properly sense both menthol and icilin.

The team localized menthol and icilin binding to TRPM8 at a point adjacent to the binding site for PIP2. This suggests that PIP2 and the cooling agents likely cooperate to control structural changes in TRPM8, allowing the cold-sensing signal to travel to the brain.

"We figured out menthol binding to TRPM8, and we have some clue about how this channel opens, but we still don't fully understand how this menthol binding is coupled to the channel opening," commented Lee, adding that his group plan to focus on this connection in the future.

Whilst TRPM8 activation can lead to the relief of pain, in conditions such as cold allodynia, the ability to turn the channel off may hold the potential to reduce pain and alleviate symptoms. “So far most synthetic compounds modulating the channel activity bind to the menthol binding site. By visualizing the interactions of both natural and synthetic cooling agents with the TRPM8, our structure can provide a guidance to develop drugs targeting TRPM8 with better efficacy and specificity”. 

Reference: Ying Yin, Son C. Le, Allen L. Hsu, Mario J. Borgnia, Huanghe Yang, Seok-Yong Lee. 2019. Structural Basis of Cooling Agent and Lipid Sensing by the Cold-Activated TRPM8 Channel. Science. DOI:10.1126/science.aav9334.