Pain acts as an alert system that tells us to pause and pay attention to our bodies, and has long been recognized as one of evolution’s most reliable tools to detect the presence of harm as well as signal that something is wrong. Newly reported research in mice now suggests that pain may be more than just a mere alarm bell, and may itself represent a form of protection. The study, headed by researchers at Harvard Medical School (HMS), suggests that in mice, pain neurons in the gut regulate the presence of protective mucus under normal conditions and stimulate intestinal cells to release more mucus during states of inflammation.
The work details the steps of a complex signaling cascade, showing that pain neurons engage in direct crosstalk with mucus-containing gut cells, known as goblet cells. “It turns out that pain may protect us in more direct ways than its classic job to detect potential harm and dispatch signals to the brain, said research lead Isaac Chiu, PhD, associate professor of immunobiology at the Blavatnik Institute at HMS. “Our work shows how pain-mediating nerves in the gut talk to nearby epithelial cells that line the intestines. This means that the nervous system has a major role in the gut beyond just giving us an unpleasant sensation and that it’s a key player in gut barrier maintenance and a protective mechanism during inflammation.”
Chiu and colleagues reported their findings in Cell, in a paper titled, “Nociceptor neurons direct goblet cells via a CGRP-RAMP1 axis to drive mucus production and gut barrier protection,” in which they note that the results could also have implications for the way that pain is treated in patients with colitis.
The body’s intestines and airways are studded with goblet cells. Named for their cup-like appearance, these cells contain a gel-like mucus made of proteins and sugars, which acts as a protective coating, shielding the surface of organs from abrasion and damage. In the gut system, “Goblet cells are specialized epithelial cells that produce mucins and associated proteins to form the mucus layer, a loosening structural mucin gradient created by mucus-associated enzymes,” the team explained. This mucus acts to defend against invasion by pathogenic microbes, while maintaining commensal homeostasis. However, the investigators continued, “Mucus defects increase susceptibility to pathogens and are associated with the development of inflammatory bowel diseases.”
Goblet cells are triggered to release mucus by microbial and immune cues, and the newly reported research by Chiu and collaborators found that intestinal goblet cells also release protective mucus when triggered by direct interaction with pain-sensing neurons in the gut. In one set of experiments, the researchers observed that mice lacking pain neurons produced less protective mucus and experienced changes in their intestinal microbial composition, effectively an imbalance in beneficial and harmful microbes, or dysbiosis.
To clarify just how this protective crosstalk occurs, the researchers analyzed the behavior of goblet cells in the presence and in absence of pain neurons. They found that the surfaces of goblet cells contain a type of receptor, called RAMP1, that ensures the cells can respond to adjacent pain neurons, which are activated by dietary and microbial signals, as well as mechanical pressure, chemical irritation, or drastic changes in temperature.
The experiments further showed that these receptors connect with the neuropeptide calcitonin-gene-related peptide (CGRP), which is released by nearby pain neurons when the neurons are stimulated. These RAMP1 receptors, the researchers found, are also present in both human and mouse goblet cells, thus rendering them responsive to pain signals. “The receptor for CGRP is formed by receptor-activity-modifying protein 1 (Ramp1) and its co-receptor, calcitonin-receptor-like receptor (Calcrl),” the investigators further commented. However, they noted, “how nociceptors are involved in regulating the gut epithelial barrier at homeostasis and in inflammation is poorly defined.”
The team’s experiments have now shown that the presence of certain gut microbes activated the release of CGRP to maintain gut homeostasis. “This finding tells us that these nerves are triggered not only by acute inflammation but also at baseline,” Chiu said. “Just having regular gut microbes around appears to tickle the nerves and causes the goblet cells to release mucus.” This feedback loop, Chiu said, ensures that microbes signal to neurons, neurons regulate the mucus, and the mucus keeps gut microbes healthy.
In addition to microbial presence, dietary factors also played a role in activating pain receptors, the study showed. When mice were given capsaicin—the main ingredient in chili peppers known for its ability to trigger intense, acute pain—the animals’ pain neurons were swiftly activated, causing goblet cells to release abundant amounts of protective mucus. By contrast, mice lacking either pain neurons or goblet cell receptors for CGRP were more susceptible to colitis, a form of gut inflammation. This finding could explain why people with gut dysbiosis may be more prone to colitis. “Nociceptor ablation led to decreased mucus thickness and dysbiosis, while chemogenetic nociceptor activation or capsaicin treatment induced mucus growth,” they noted in their paper.
Then, when researchers gave pain-signaling CGRP to animals lacking pain neurons, the mice experienced rapid improvement in mucus production. The treatment protected mice against colitis even in the absence of pain neurons.
The findings indicate that CGRP is a key instigator of the signaling cascade that leads to the secretion of protective mucus. “Pain is a common symptom of chronic inflammatory conditions of the gut, such as colitis, but our study shows that acute pain plays a direct protective role as well,” said study first author Daping Yang, PhD, a postdoctoral researcher in the Chiu Lab. As the authors further noted, “In this study, we identify a role for nociceptors in driving intestinal mucus release through neuron-goblet cell communication. Nociceptor activation leads to release of CGRP, which acts on Ramp1 on intestinal goblet cells to induce rapid mucus secretion.”
The team’s experiments showed that mice lacking pain receptors also had worse damage from colitis when it occurred. Given that pain medications are often used to treat patients with colitis, it may be important to consider the possible detrimental consequences of blocking pain, the researchers suggested. “Pain is a major symptom of inflammatory bowel diseases. However, acute pain and its accompanying release of CGRP may protect the gut barrier. We confirmed that eliminating nociceptors in DSS-induced colitis leads to worsening of pathology … Given that pain treatments are often used to treat patients with colitis, it may be important to consider detrimental consequences of blocking pain.”
“In people with inflammation of the gut, one of the major symptoms is pain, so you might think that we’d want to treat and block the pain to alleviate suffering,” Chiu said. “But some part of this pain signal could be directly protective as a neural reflex, which raises important questions about how to carefully manage pain in a way that does not lead to other harms.”
Additionally, a class of common migraine medications that suppress the secretion of CGRP may damage gut barrier tissues by interfering with this protective pain signaling, the researchers said. “Anti-CGRP therapies are widely used to treat or prevent chronic migraine. While these therapies are currently lauded as having minimal side effects, our work reveals potential detrimental side effects in gut barrier tissues.”
Chiu asked, “Given that CGRP is a mediator of goblet cell function and mucus production, if we are chronically blocking this protective mechanism in people with migraine and if they are taking these medications long-term, what happens? … Are the drugs going to interfere with the mucosal lining and people’s microbiomes?”
Goblet cells have multiple other functions in the gut. They provide a passage for antigens—proteins found on viruses and bacteria that initiate a protective immune response by the body—and they produce antimicrobial chemicals that protect the gut from pathogens. “One question that arises from our current work is whether pain fibers also regulate these other functions of goblet cells,” Yang said.
Another line of inquiry, Yang added, would be to explore disruptions in the CGRP signaling pathway and determine whether malfunctions are at play in patients with a genetic predisposition to inflammatory bowel disease. While the authors noted the limitations of their study, they concluded, “Overall, our findings demonstrate a nociceptor neuron-goblet cell axis that critically orchestrates gut barrier maintenance at homeostasis and protection during intestinal inflammation.”
