Over 30 years ago, researchers considered a pain-signaling protein called substance P the next big therapeutic target for pain treatment. But, clinical trials testing drugs that blocked the activity of substance P failed over and over again, so investigators lost hope in the strategy.
Now, researchers discover a new way that substance P sends pain signals in neurons, and show that the clinical trials may have gone awry because the pain field under-appreciated the full picture of how the protein works.
A multi-year project spanning three research teams at Monash University, Australia, reveals that substance P doesn’t just activate a neuron at the surface of the cell, but also from the inside. Since many previous drugs working against substance P couldn’t get inside the cell, this may explain why they didn’t relieve pain.
And, to overcome this hurdle, the researchers chemically redesign a drug previously developed to fight substance P so that it could now get inside the neuron. This new approach made the drug better able to quell pain, at least in mice.
“It’s fantastic work,” says William Schmidt, NorthStar Consulting, Davis, US, an expert on pain drug development who was not involved in the study. “It opens up a new way of thinking about pain by considering intracellular processes,” he says, referring to what happens inside a cell.
The work was published online May 31 in the journal Science Translational Medicine.
High hopes, then disappointment
Early experiments in animals suggested substance P was an important player in chronic pain. Substance P sits inside pain neurons throughout the body. When something painful occurs, those neurons release substance P into the spinal cord.
Once there, substance P attaches to another protein called neurokinin-1 receptor (NK1R), which sits on the cell surface of spinal neurons. In turn, those neurons are activated and send a pain signal from the spinal cord up to the brain.
From this knowledge appeared a simple solution: Preventing substance P from attaching to NK1R would alleviate pain.
Excited by this hypothesis, researchers developed a long list of drugs that could block the substance P-NK1R interaction. The drugs worked well enough in animals, but not in people—not a single one relieved pain. It was a mystery why—until now.
Recycling isn’t just for the environment
According to Nigel Bunnett, who led one of the teams in the new study, the previous view of how substance P and NK1R interacted was overly simplistic.
NK1R is a special type of protein called a GPCR. When GPCRs are activated, they undergo a process called endocytosis. Here, part of the membrane that surrounds the cell buds off from the cell surface, engulfing the GPCR into a spherical compartment called an endosome that travels into the cell. Eventually the GPCR can be sent back to the cell surface, where it becomes active again. Researchers think of endocytosis as a sort of recycling process that keeps cells functioning properly.
The conventional wisdom was that endocytosis “turns off” the protein, keeping it quiet. But recent studies have shown that GPCRs aren’t turned off once inside the endosome. Instead, they can stay on for extended periods.
Bunnett wondered if a similar process was occurring with NK1R. If so, previous pain drugs that blocked NK1R at the cell surface wouldn’t work, since many of them couldn’t get inside the cell. To be successful pain relievers, then, they would need to access the interior of the cell to stop the activity of NK1R within the endosome.
Testing the hypothesis
Bunnett and colleagues first showed that endocytosis of NK1R did indeed take place. They did so by using a technique in which cells are manipulated to become fluorescent. This allowed the team to follow NK1R on its travels from the cell surface and into endosomes.
They also found that NK1R remained turned on while inside the endosomes. Further, preventing NK1R from being engulfed into endosomes reduced pain in mice.
The team then redesigned a drug that quiets NK1R. Here, the drug was modified so that it would follow NK1R into endosomes. This would allow the drug to act as an “endocytosis stowaway” so that it could block NK1R’s continued activity inside the endosome.
Testing this revamped drug, they met with success: The drug reached the endosome, turned down the activity of NK1R, and more importantly reduced pain in mice. Critically, it provided stronger and longer-lasting pain relief than the original drug.
Interestingly, “The rationale behind these experiments was to understand the intricate function behind GPCR signaling in endosomes, not necessarily to find a new pain therapeutic. I’m very pleased, however, that this may have implications for treating patients,” said Bunnett.
To read about the research in more detail, see the related IASP Pain Research Forum news story here.
Nathan Fried is a postdoctoral fellow at the University of Pennsylvania, Philadelphia, US.