Millions of Americans (roughly 100 million) and more than a billion people around the world live with chronic pain. This not only affects quality of life but is of growing concern with the ongoing opioid epidemic. Although there is an urgent need for improved pain management strategies, there is little known about the mechanisms that mediate increased sensitivity to pain in injury and inflammation.
A research group led by Sharona E. Gordon, Ph.D., professor of physiology & biophysics at the University of Washington, has uncovered a novel way in which the inflammatory response to pain is regulated. The work was published in eLife on December 18th in a paper titled, “Reciprocal 1 regulation among TRPV1 channels and phosphoinositide 3-kinase in response to nerve growth factor.”
Inflammatory hyperalgesia, the increased sensitivity to pain (caused by thermal, chemical, and mechanical stimuli) can be divided into two phases, acute and chronic. As part of this condition, locally released inflammatory mediators, including growth factors, bradykinin, prostaglandins, ATP, and tissue acidification, directly stimulate and sensitize nociceptive fibers of primary sensory neurons. Nociceptive pain (pain arising from the stimulation of nerve cells) is the most common type of pain people experience. It develops when the nociceptive nerve fibers are triggered by inflammation, chemicals, or physical events.
Although it is well understood that the inflammatory mediator nerve growth factor (NGF) sensitizes pain-receptor neurons through increased trafficking of Transient Receptor Potential Vanilloid Subtype 1 (TRPV1) channels to the plasma membrane, the mechanism by which this occurs remains mysterious.
First-author Anastasiia Stratiievska, a graduate student in Dr. Gordon’s group, adds that although “it has been known for over a decade that […] sensitization requires the activation of an enzyme called phosphoinositide 3-kinase, or PI3K, the mechanism by which this occurs was still unknown, and we wanted to gather more insights into the process.”
The authors wrote that, “NGF activates phosphoinositide 3-kinase (PI3K), the enzyme that generates PI(3,4)P2 and PIP3, and PI3K activity is required for sensitization. One tantalizing hint came from the finding that the N-terminal region of TRPV1 interacts directly with PI3K.”
To study this, Stratiievska and her team used an imaging technique called 2-color total internal reflection fluorescence microscopy to study TRPV1-expressing cells. Their analysis revealed that TRPV1 potentiates NGF-induced PI3K activity. Although TRPV1 is a large, multi-domain protein embedded in the cell surface, a small fragment of the protein called the N-terminal Ankyrin repeats domain (ARD) was enough to cause this increase in activity.
“Because the ARD is structurally conserved among TRPV channels, we tested whether other channels besides TRPV1 could increase NGF-induced PI3K activity,” said Dr. Gordon. “We saw that this was indeed also true for TRPV2 and TRPV4.”
“Together, our findings reveal a previously unknown reciprocal regulation among multiple TRPV channels and PI3K,” Stratiievska concluded. “The next steps will be to identify ARD mutations that prevent this regulation. It would also be interesting to determine the exact role that reciprocal regulation plays in sensitization to painful stimuli within model organisms, as this could help with the development of more effective painkillers further down the line.”
