When Cells Age: A New Contributor to Chronic Pain – and Potentially to Headache, Too

An animal study, supported by human data, shows that cellular senescence is associated with neuropathic and facial pain. The findings suggest that a similar process could also underlie headache conditions.

Graying hair, the appearance of wrinkles, moving a bit slower – these are just a few of the visible signs of getting older. But aging also happens on a much smaller scale, at the level of the cell. It’s known as cellular senescence, a process where cells stop multiplying but don’t die. This can cause problems like inflammation as well as damage to healthy cells nearby.

Cellular senescence is associated with many conditions, such as cancer, cardiovascular disease, and dementia. But while a smattering of previous research had connected cellular senescence to chronic pain, whether it actually plays any important role in the pain system, and particularly in nociceptors – the sensory neurons responsible for the detection of harmful stimuli, the first step on the way to pain sensation – remained a mostly unexplored question.

But now, researchers led by Saida Hadjab, from the Karolinska Institutet, Stockholm, Sweden, have identified a role for cellular senescence of nociceptors as a key player in chronic pain. To do so, they combined their own novel dataset with existing datasets, all of which contain information about single cells from the dorsal root ganglion (DRG) or the trigeminal ganglion (TG); these are cells that have been studied in animal pain models. The team found that senescence drove pain in models of nerve injury, but also discovered evidence for senescence in tissue samples from people with chronic pain conditions or painful diabetic neuropathy. Further, treatment with drugs that target senescence alleviated pain in nerve-injured animals.

“The findings are very convincing. They did a terrific job of bringing together all kinds of different datasets; it’s really amazing how many they brought together,” said Ted Price, a pain researcher at the University of Texas at Dallas, US, in an interview. Price was not involved with the new study.

But the headache field should also take note of the findings, which showed a role for senescence in a model of facial pain, specifically, compression of the trigeminal root entry zone of the TG nerve. That’s relevant to conditions like cluster headache and trigeminal neuralgia, where nerve compression by blood vessels plays a role, the authors wrote in their paper. What’s more, the discoveries have relevance to migraine, too.

“The TG data in this paper is not specifically related to headache, but these cell types are similar to those that are hyperexcitable in migraine and other headache disorders. So I believe that improved understanding of how trigeminal neurons respond in facial pain models could reveal overlapping mechanisms to those that occur in certain headache syndromes,” wrote William Renthal, a headache and pain researcher at Brigham and Women’s Hospital and Harvard Medical School, Boston, US, in an email to Migraine Science Collaborative. Renthal also did not take part in the new research.

The study appeared in Nature Communications on October 4, 2024.

Mapping the (cellular) world
One impetus for the new study, Hadjab told MSC in an email, was to understand whether different kinds of chronic pain have a shared pathophysiology.

“[One of my primary motivations was] to investigate whether a common molecular mechanism exists for the development and persistence of pain across various chronic pain conditions. Understanding this could potentially lead to more universally effective treatments,” she said.

To identify a mechanism, the investigators pursued an increasingly popular approach to understanding the pain system. Pain researchers, and those interested in headache as well, have been working hard to create “atlases” of the cells that contribute to pain in animal models, and also to pain in people. In addition to creating a picture of the different cell types that contribute to pain conditions, the atlases also include information about the genes the cells express.

This type of work became possible with methods that enable identification of the RNA sequences of single cells, and of single nuclei. All of that laborious effort has led to atlases of cells of the DRG, where the cell bodies of nociceptive neurons reside, as well as TG cells (see MSC related news article).

For the current study, the researchers integrated those published datasets with data of their own to create iPain, which comprises two different atlases: one for the DRG (iPainDRG), and one for the TG (iPainTG). The iPainDRG integrates roughly 192,000 cells into the atlas, while iPainTG captures information from almost 88,000 cells. With this unified approach, the group had a wealth of information about how DRG and TG cells respond in various animal pain models, such as models of nerve injury pain and inflammatory pain.

The study’s primary focus was iPainDRG, considering that it contains information about nociceptors from more pain models compared to iPainTG. It also has more data on the entire time course of how nociceptors respond in the pain models, from the time pain first develops in an animal in response to a painful experimental stimulus, all the way up until pain becomes chronic (here, that was pain lasting for 28 days). But the group would also look to their iPainTG atlas to study the profile of TG cells in a model of facial pain.

Moving from one microstate to another
Using the iPainDRG atlas, the authors were first able to identify “microstates” of the DRG nociceptors – that is, the characteristics of these neurons based on where the cells were in the pathway of pain progression over 28 days. So, neurons at baseline, before nerve injury, had certain characteristics, whereas at other timepoints – right after injury, at the establishment of pain, when pain recovers, or when pain instead becomes lasting – the neurons had their own unique features.

Once they showed how the neurons progressed through different microstates in various chronic pain models, the team looked for the underlying mechanisms, in particular, for the genes responsible for driving the transition between microstates. One of those driver genes was Sox11, which encodes a transcription factor that regulates gene expression (the turning on or off of genes).

A different type of analysis then provided information about how nociceptors interact with different cells, such as immune cells and glial cells (a type of non-neuronal cell that supports the function of neurons), during the different microstates. The group also looked at pairs of ligands and the receptors to which they bind that are responsible for those cell-to-cell interactions.

The key finding was that for one group of ligand-receptor pairs, the p53 biological pathway appeared to play a prominent role. Since the p53 gene is an important factor in cellular senescence, the possibility now emerged that senescence could play a role in chronic pain.

Building the case for senescence
To prove this, the researchers first performed analyses showing that, after nerve injury, nociceptive cells took on features of senescent cells. That finding led to the hypothesis that pain persists because senescent cells that can’t revert to the state they were in before nerve injury stick around to cause problems.

It turned out that DRG nociceptors with senescent features were present in virtually all of the different nerve injury pain models captured by the iPainDRG atlas. Further, measures of cell senescence were higher in the pain and lasting pain microstates, compared to the recovery microstate.

What about nociceptors from the TG? To answer that question, the team looked to the rat pain model of compression of the trigeminal root entry zone of the TG nerve. Here, too, they found that TG cells exhibited features of senescent cells. Since senescence was a feature of cells in this model, senescence could also be a player in migraine and other headache conditions, though that is a question for future research.

“For headache researchers, the key takeaway is that the senescence mechanism observed in the DRG was also found in the TG. This suggests that the findings could be relevant not only to trigeminal neuralgia but [also] to headache disorders linked to the TG,” wrote Hadjab.

The group next turned to human samples of DRG obtained from patients diagnosed with chronic pain or painful diabetic neuropathy. Complementing the findings from the earlier analyses, a measure of senescence was elevated in DRG tissue from those individuals.

“What makes this discovery [of the role of senescence] particularly surprising and significant is its consistency across different pain models and its presence in both animal and human samples. This universality suggests that cellular senescence could be a fundamental mechanism underlying various pain disorders,” according to Hadjab.

Can you alleviate chronic pain by treating senescence?
Finally, to illustrate potential clinical applications of their work, the researchers used senolytics, which are drugs that target senescent cells. These agents have already been used in preclinical studies and clinical trials, so it made sense to test whether they could alleviate pain – and they did: Nerve-injured mice that received senolytics showed less mechanical pain four weeks after nerve injury, compared to baseline or compared to a control drug.

The senolytics appeared safe, associated with selective removal of just a small number of nociceptive neurons, and had no adverse systemic effects. Importantly, the drugs affected only mechanical hypersensitivity, and not other sensory modalities.

Would targeting the removal of senescent cells be a feasible treatment in people? Price told MSC that while the study certainly provided a proof of principle of that approach in animals, he wasn’t so sure that it would be an ideal therapy for humans.

“I kind of don’t think that, for a nervous system problem, just getting rid of the senescent neurons is something that a lot of people are going to say sounds like a good thing to do in a patient; it doesn’t sound like the best approach to me. It sounds like a perfectly reasonable approach for proof of principle, [but] for actually developing a therapeutic, I’m not so sure. I would prefer to have the cells get out of being in a senescent state and go back to how they were before,” said Price.

“A turning point”
While the clinical feasibility of senolytics for pain may be uncertain, one thing seems clear: The new work represents a big payoff from a methodological perspective, where the researchers made use of the atlases created by other investigators as well as their own to create something more comprehensive that can answer questions of interest to researchers. Price called this a “turning point” for the field.

“This is a beautiful example of how these datasets being generated and made publicly available enables really creative and insightful work. I love the paper from that perspective,” Price said.

An important avenue for future research is to bulk up the TG atlases. One current weakness in those atlases is that they don’t contain as much human data as the DRG atlases do. That’s not a surprise, considering that it is more difficult to obtain TG tissue from people compared to DRG tissue during the organ donor process. But Price said that is changing, with many investigators now interested in and working on this issue.

Learning more about the mechanisms of senescence is also a top priority. “I think an important next step is to study how the senescent cell state develops,” Renthal said. “What are the signals that initiate and maintain senescent states in chronic pain? I think we also need to learn more specifically about how senescence leads to sensitized nociception, as it will give us more opportunities to therapeutically target these pathways in the safest way possible for patients with chronic pain.”

As for Hadjab, she and her colleagues continue to push forward in this area of research.

“Efforts [are underway] to explore the detailed mechanisms that lead to cellular senescence in chronic pain and ways to revert it, to investigate the potential of senolytic treatments in various pain conditions, and possibly to translate these findings into human clinical trials,” she told MSC.

Neil Andrews is a science journalist and executive editor of the Migraine Science Collaborative.

Reference
The single-cell transcriptomic atlas iPain identifies senescence of nociceptors as a therapeutical target for chronic pain treatment.
Techameena et al.
Nat Commun. 2024 Oct 4;15(1):8585.

Image credit: 123RF Stock Photo.