Overview

Can we stop the pain? It may be the oldest question in medicine, and it remains one of the most important. But with chronic pain afflicting billions of people worldwide, and few effective treatments besides highly addictive opioids, researchers are still searching for better answers.

On May 3-4, the New York Academy of Sciences, in collaboration with Science Translational Medicine, convened the Advances in Pain conference. Across the meeting’s two keynote presentations, nine sessions of talks, and concluding panel discussion, leading experts in many branches of pain research discussed the field’s biggest challenges and latest developments.

Highlights

• Specific ion channels on neurons, such as Nav1.7, are critical components of pain sensing and potential drug targets for new analgesics.
• Several novel laboratory models are revealing new details of nociception, or pain sensing.
• Large databases of genetic and clinical records are helping researchers link specific genes with common pain conditions.
• Neuroimaging and sleep studies may offer objective ways to measure the severity of chronic pain.
• New mechanistic data are pointing researchers toward novel strategies for analgesic drug development.
• A subset of gut epithelial cells is critical for sensing visceral pain.
• The immune system links tightly to pain sensation, through multiple mechanisms scientists are now beginning to uncover.
• Data mining reveals subsets of neurons with distinct responses to nerve injury, including chronic pain.
• Understanding sex and ethnic differences in pain perception requires new strategies in experimental design and data analysis.
• Besides neurons, Schwann cells can also carry pain signals.
• Novel drug discovery platforms and trial designs can accelerate the development of new analgesics.

Part 1

Speakers

Tuning into the pain channel

A life free of pain may sound ideal, but as David Bennett explained in the meeting’s opening keynote presentation, individuals with defects in pain sensing often suffer tremendous difficulties. Describing one 26-year-old man with such a condition, Bennett explained that “he had pretty much fractured every long bone in his body, he is stunted because he’s destroyed all the growth plates … and had severe burns and mouth injuries.” The patient’s sister, who had the same condition, died of undiagnosed sepsis.

Genetic analysis revealed that the patient had a rare set of loss-of-function mutations in the gene for Nav1.7, a sodium ion channel expressed in nociceptors, or pain sensing neurons. Using a sophisticated cell culture system that mimics pain signaling through nociceptors, Bennett and his colleagues have characterized Nav1.7 in detail, and determined that it acts early in the pain signaling process, amplifying the electrical signal in the nociceptors to ensure that it’s relayed to the central nervous system.

Patients with gain-of-function mutations that make Nav1.7 overactive have the opposite problem: incurable chronic pain. Bennett’s team studied the Nav1.7 mutations in these patients, and discovered that the degree of the biochemical defect in a patient’s channel proteins correlates directly with the time of onset of their pain condition.

Based on his findings in patients with these rare, extreme pain disorders, Bennett hypothesized that Nav1.7 could also drive more common conditions. As rates of diabetes skyrocket globally, millions of people are developing diabetic neuropathy, which causes chronic pain only in a subset of patients. In an effort to determine what distinguishes painful from pain-free diabetic neuropathy, Bennett’s team looked at Nav1.7 gene sequences for patients with the condition.

“The rare variants in Nav1.7 seemed to cluster much more in the painful versus the painless diabetic neuropathy groups, so this is now acting as a risk factor, in the sense that these people did not experience [chronic] pain prior to developing diabetes,” Bennett says.

Some variants of Nav1.7 apparently predispose people to develop chronic pain, but the condition doesn’t manifest itself until a second event, such as diabetes, triggers it. A closer look at clinical testing results in these patients revealed that those with the rare variants were also more sensitive to certain stimuli, such as burning pain and pressure pain.

Nav1.7 isn’t the only ion channel involved in pain, though. The researchers have also identified strong associations between pain disorders and mutations in the related channel proteins Nav1.8 and Nav1.9, highlighting the diversity of channelopathies that can derail pain sensing. Indeed, an analysis of data from the UK Biobank, which has whole genome sequences and medical records for 100,000 Britons, revealed that voltage-gated sodium channels were the largest group of variants associated with neuropathic pain.

Based on his findings, Bennett advocates using both clinical testing data and gene sequencing to stratify patients according to which treatments are most likely to work for them. In particular, sodium channel blocking drugs appear to work much better in patients with variant channels predisposing them to pain.

Where does it hurt?

The meeting’s first regular session focused on efforts to dissect the central pain circuits in the nervous system. For Sarah Ross, the dissection is literal: she carefully removes a piece of a mouse spinal cord, along with the sensory nerves connected to a patch of skin from the animal’s hind paw, keeping all of the neuronal connections intact. Using luminescent probes, her team can then watch the activation of specific neurons in response to stimuli.

“We can see some neurons respond to heat, other neurons will respond to cool, other neurons will respond to mechanical stimuli,” said Ross.

Many neurons also respond to multiple stimuli, and mapping these responses reveals that distinct classes of neurons function as amplifiers, tuners, and integrators of pain signals.

Jing Wang studies what happens to pain signals in the cerebral cortex of the brain. Using optogenetics, which allows him to stimulate specific neurons in the brains of mice with light, he has identified subsets of neurons in the anterior cingulate cortex and prefrontal cortex that respond to pain.

In mice with experimentally induced chronic pain, low-intensity stimulation of the prefrontal cortex restores normal pain control. Wang’s lab is now studying ways to achieve similar responses with less invasive methods, including the drug ketamine and brain-machine interfaces.

“The cortex processes and regulates pain, but its normal endogenous function can be impaired by chronic pain, and [restoring cortical regulation] has the potential to transform pain treatment,” said Wang.

Part 2

Speakers

The pains of the father

Aarno Palotie began the meeting’s session on the genetics of pain by discussing his results from large-scale studies on migraine. With the exception of some rare, strictly inherited forms of the condition, these sporadic, debilitating headaches usually stem from variations in numerous common genes. To identify those genes, Palotie and a large team of collaborators scrutinized genetic and medical data from hundreds of thousands of migraine sufferers.

The effort revealed over 100 gene loci linked to migraine, mostly in regulatory regions associated with genes expressed in cardiovascular tissue and the central nervous system. Tracking those variants in another large data set revealed a cumulative effect.

“We can see that those with a high polygenic risk score, meaning a high load of common variants, they seem to have an earlier onset of migraine,” said Palotie.

Using data from the 500,000 participants in the UK Biobank, Luda Diatchenko and her colleagues have performed a similar analysis to identify genetic variants linked to chronic pain. The investigators subdivided chronic pain patients based on the type of pain they experienced, such as back pain, hip pain, knee pain, and multi-site pain.

Analyzing gene sequences for these sub-groups showed that multi-site pain had the highest correlation with specific gene variants. The gene most strongly linked to multi-site pain encodes a receptor protein involved in guiding nerve axons in development.

“This is one example of how [genome-wide association studies] can show us a new mechanism which contributes to human chronic pain conditions,” said Diatchenko.

On a scale of one to ten

The meeting’s third session focused on one of the biggest challenges in studying pain: measuring it. Clinical studies attempt to quantify pain severity with patient questionnaires, while animal experiments rely on behavioral responses, but both methods are notoriously unreliable.

Ilene Tracey hopes to solve that problem with neuroimaging, linking specific patterns of neuronal activation to painful stimuli.

“We’ve got now quite a good array of tools that are reasonably well developed and robust, that allow you to look at … ways that patients will experience their pain,” said Tracey.

By combining functional magnetic resonance imaging with electroencephalography, video analysis, and other sensing methods, this approach could allow researchers to quantify patient responses to pain treatment more reliably than current, fundamentally qualitative methods. Using machine learning, Tracey’s team can now measure pain and also distinguish different categories of it, such as physical versus emotional pain.

Sleep disturbances might also provide a pain gauge.

“The vast majority of patients with chronic pain suffer from poor sleep quality,” said Alban Latremoliere, who has been studying this connection as a potential pain biomarker.

By tracking electroencephalography and other measurements in sleeping mice, he and his colleagues have found that nerve injury, which causes chronic neuropathic pain, also changes the animals’ sleep architecture. Compared to uninjured animals, those with injured nerves suffer multiple brief interruptions in the non-REM phase of their sleep. When the injury heals, the normal sleep architecture returns; Latremoliere now hopes to use these patterns to quantify neuropathic pain severity and treatment efficacy in humans.

Part 3

Speakers

The hurt blocker

As Greg Scherrer pointed out in the meeting’s fourth session, the real problem with pain isn’t that it exists, but that it’s unpleasant.

“If we were to understand how our brain collects this information from sensory neurons and the spinal cord to make pain unpleasant … maybe we’ll discover new ways to treat pain,” said Scherrer.

Indeed, a patient whose basolateral amygdala was removed to treat epilepsy could still sense painful stimuli, but didn’t label them as painful; the unpleasantness was gone. Examining mice with various alterations to the same brain region, Scherrer and his colleagues believe they have identified the amygdala cells responsible for connecting pain to unpleasantness. The investigators are now trying to identify receptors on those cells that would be good drug targets for new pain treatments.

Venetia Zachariou is also dissecting cellular signaling pathways to target in pain treatment, and her lab has uncovered several promising leads in recent years. When the COVID-19 pandemic derailed that work, though, the scientists quickly pivoted to apply their skills and techniques to study the new disease’s neuronal pathogenesis.

In a hamster model, they found that SARS-CoV-2, the virus that causes COVID-19, can acutely infect nerves in the dorsal root ganglia, which are also involved in pain sensing. Looking more closely at both the hamster model and a mouse model of SARS-CoV-2 infection, Zachariou has identified distinct changes in neurons’ gene expression patterns after virus infection, including a signature similar to that seen in models of neuropathic pain.

One of the most popular targets for researchers trying to develop new pain therapies is the sodium channel Nav1.7, a “pain amplifier” that several speakers at the meeting discussed. Rajesh Khanna is also interested in Nav1.7, but instead of targeting the protein directly, his team is trying to identify proteins that interact with it. That work led them to focus on collapsin response mediator protein 2 (Crmp2), which regulates Nav1.7 signaling.

Mice lacking Crmp2 are resistant to chronic pain, suggesting that drugs inhibiting its action would be good pain therapy candidates. After conducting extensive mechanistic studies, Khanna started a company to identify such inhibitors. So far, the company has optimized a lead compound that appears to stop chronic pain in animal models, without causing detectable side effects or tolerance.

You feel it in your gut

The meeting’s first day concluded with a keynote presentation by David Julius, who discussed his work on chronic visceral pain. This subtype of chronic pain, which can be caused by gut infection or non-infectious conditions such as inflammatory bowel disease, affects about 15% of the population. It’s three times more common in women than men, but nobody knows why.

“We’re interested in a particular aspect of visceral pain signaling, and that involves the interaction of sensory nerve fibers with the gut epithelium,” said Julius.

A subset of gut epithelial cells, called enterochromaffin cells, plays an outsize role in that interaction. Comprising only a fraction of a percentage of all gut epithelial cells, enterochromaffin cells make about 90% of the body’s serotonin, a potent neurotransmitter protein. They also fire electrical signals that could propagate to nearby neurons.

Julius wanted to analyze that process in live mice, but wasn’t happy with the standard mouse system for those types of experiments. That model involves putting irritants into a mouse’s gut to trigger a major inflammatory response, after which the animal remains hypersensitive to physical stimuli such as colon distention.

“Do we need to … put the mouse through all that, or can you have a model that’s simpler [and] does not require all the sequellae of an inflammatory episode?” asked Julius.

Instead, he and his colleagues first tried studying enterochromaffin cells in the context of cultured enteroids, pieces of intestinal epithelium that can mimic many aspects of gut biology in a petri dish. That system revealed that enterochromaffin cells respond to numerous compounds that fall into three general classes: ingested irritants, metabolites of common gut microbes, and endogenous regulatory hormones.

“So, we want to know how these cells integrate all this information, and what role this plays in maladaptive situations like [inflammatory bowel disease],” said Julius.

Based on those results, the researchers moved to a more complex system, an explanted piece of a mouse colon with its connecting nerves. Monitoring the electrical signals in the connected nerves reveals sensory signals from the explanted gut. In this setup, bathing the colon section with isovalerate, a bacterial metabolite that triggered a response from enterochromaffin cells in the enteroid experiment, makes it hypersensitive to subsequent physical or biochemical stimuli. This system also revealed different response patterns in guts from male and female mice.

Having demonstrated that isovalerate could induce gut hypersensitivity without the inflammatory response of harsher irritants, Julius’s team next tried looking at its effect in live mice. They used a small balloon in the colon, similar to an endoscope, as a stimulus, and monitored abdominal muscle contraction, a behavioral response to pain. Treating the mice with isovalerate increased the magnitude of subsequent pain responses potently in male mice, but less so in females, consistent with the explant results.

Subsequent experiments showed that enterochromaffin cells mediate these responses in live mice, apparently through both serotonin secretion and direct electrical signaling to neurons, and that these cells seem to respond differently in male and female mice.

Part 4

Speakers

Is antibody hurt?

Infections commonly cause pain, which researchers had long assumed was just a byproduct of the body’s inflammatory response. However, as Isaac Chiu explained in the meeting’s session on neuroimmune and autoimmune mechanisms in pain, infecting pathogens can also interact directly with nociceptors, or pain-sensing neurons. In one set of mouse experiments, for example, Chiu’s team found that nociceptors in the intestine can detect infection with Salmonella enterica, triggering a response that decreases the number of M cells, the specialized intestinal epithelial cells S. enterica preferentially infects.

“These neurons actually regulate cell numbers, [which] not only shuts down the number of gates for pathogen entry, it also helps a protective microbe … attach better on the surface of the epithelium,” said Chiu.

Camila Svensson discussed a pain condition that has baffled researchers and clinicians for decades: fibromyalgia. Characterized by pain hypersensitivity in soft tissues, sometimes coupled with neuropathic pain, the condition has long eluded efforts to uncover its etiology and underlying mechanisms.

After serendipitously discovering evidence for autoantibodies in fibromyalgia patients, Svensson has now developed human tissue and mouse models to characterize these antibodies in more detail. Transferring antibodies from fibromyalgia patients into mice causes pain hypersensitivity in the animals, and patients with higher levels of antibodies that react with human dorsal root ganglia cells have more severe disease.

“This suggests that there is an autoimmunity in subpopulations of fibromyalgia patients,” said Svensson, adding that besides suggesting a mechanism for the disease, autoantibody levels could help stratify patients in clinical trials.

The body’s own immune response is also a key contributor to chronic neuropathic pain, especially through neuroinflammation. Alexander Davies presented his work on another component of neuropathic pain: the cytotoxic cellular response.

Cytotoxic cells normally detect cancerous or virally-infected cells and target them for destruction, but they can also target injured neurons. Dissecting this response in an extensive series of experiments in mice, Davies and his colleagues have found that a specific receptor on cytotoxic cells allows them to target nociceptors after nerve injury, leading to degeneration of the damaged axons and resolution of pain hypersensitivity.

“So, our data suggest that intact sensory networks are a source of ongoing neuropathic hypersensitivity, and that by targeting those, we can help to resolve that,” said Davies.

Short, sharp shocks

Dana Orange gave the first of two short “data blitz” presentations, providing an overview of her group’s work on rheumatoid arthritis pain. Though inflammation of joints is a hallmark of this form of arthritis, Orange noticed an odd discrepancy.

“Patients who really don’t have a lot of inflammation were reporting a lot of pain,” she said.

Through a combination of human gene expression and mouse studies, she’s found that nerve development may play a bigger role than inflammation in driving rheumatoid arthritis pain.

Shrinivasan Raghuraman described his approach to characterizing chronic pain mechanisms, using a rat model. By collecting thousands of data points from individual rat neurons under different conditions, his lab has identified 19 different subsets of neurons with distinct responses to nerve injury. Raghuraman hopes that correlating the cells’ electrical responses with their gene transcription profiles will identify the underlying mechanisms driving chronic pain, and how different candidate drugs can influence it.

Sex and race

In the session on sex and ethnic differences in pain, Jeffrey Mogil began by pointing out a critical flaw in traditional pain research methods. Despite ample evidence that women experience more pain than men, “80 percent of preclinical studies use male rats or male mice only,” said Mogil.

That skew overlooks important differences in the biology of pain in males and females, though. In a mouse model of chronic neuropathic pain, for example, Mogil’s lab has linked chronic pain to premature shortening of chromosome ends, or telomeres – but only in male mice. Besides studying both sexes instead of just one, Mogil argued that researchers need to extend their animal studies to monitor chronic pain for longer time periods, to account for age-related phenomena such as telomere shortening.

Frank Porreca also looks at sex differences in pain, but focuses on the role of stress. Clinical data clearly show that stress exacerbates functional pain syndromes such as inflammatory bowel disease, migraine, and fibromyalgia, all of which are more prevalent in women than men.

To study such syndromes, Porreca’s team developed a mouse model in which they restrain the animals for a short time to induce stress, then treat them with a compound that causes headaches. These stress-primed mice develop allodynia, interpreting normally non-painful stimuli as painful, while controls that only got the headache-inducing compound didn’t.

While both male and female mice exhibited the same response, Porreca found that it operates through different biochemical mechanisms in the two sexes, underscoring the importance of studying both in preclinical research.

Unlike sex, race and ethnicity lack clear biological definitions.

“It’s important to keep in mind that race and ethnicity are not causal factors, but rather proxies for these many psychosocial and biopsychosocial factors, largely driven by systemic societal and environmental exposures,” said Roger Fillingim.

At the same time, the groups that suffer disproportionately from racial and ethnic health disparities are often the least-studied. That’s certainly the case in pain research and treatment. Indeed, experiments suggest that Black patients may experience more pain than white ones, but health data show they’re less likely to be treated for pain in hospitals and clinics.

Summarizing a large body of additional evidence for similar skews in various minoritized groups, Fillingim advocated more holistic approaches to pain research across and within sub-populations.

Part 5

Speakers

Get the sensation

The meeting’s penultimate session focused on how sensory signals such as pain propagate toward the central nervous system. Alexander Chesler started the session with a discussion of his work on peripheral sensory neurons.

To study these cells, Chesler and his colleagues initially developed an elegant system that allowed them to probe the responses of individual mouse cells in the trigenimal ganglion, a nerve cluster that receives sensory signals. That revealed a specific subset of neurons that responded only to a painful stimulus, while other subsets responded to gentle touches. By extending the system with gene expression profiling, and correlating responses in the mouse with those in a human patient who lacks a receptor critical for mechanical sensation, the scientists are now tracing pain-sensing pathways in unprecedented detail.

Neurons aren’t the only cells carrying pain signals, though, as Patrik Ernfors has discovered. In tracing sensory circuits, he and his colleagues discovered that Schwann cells, support cells closely associated with peripheral neurons, are also stem cells that form a sensory organ under the skin.

Using genetically modified mouse models that allowed them to selectively activate these Schwann cells, Ernfors and his colleagues discovered that both the Schwann cells and their associated neurons can initiate acute pain sensations. Further work revealed that the Schwann cells also appear to become sensitized during the development of arthritis.

“We believe that we have found the mechanosensory skin organ that is associated with [mechanical pain sensation],” said Ernfors, adding that these cells could contribute to allodynia in arthritis.

Something for the pain

Clifford Woolf began the meeting’s final session, on finding new ways to treat pain, with a summary of his team’s novel approach to drug discovery. Currently, most pharmaceutical companies focus on finding compounds that can target specific cellular molecules known to be involved in pain, then trying to develop them into drugs.

In 2010, Woolf advocated an alternative strategy, screening drugs to find those that inhibit stem cell-derived pain-sensing neurons, without worrying about their mechanisms of action.

“However, the question was how to execute on this,” he said.

After extensive effort, his team can now derive the correct neuron types from patients’ cells. Screening libraries of compounds against these cells has yielded several promising hits, which inhibit pain signaling in nociceptors without affecting other cell types.

Others hope to broaden the scope of target-based drug screening, which has focused on a large and diverse class of cell surface proteins called G-protein coupled receptors, or GPCRs.

“But … when we mapped the drugs onto the phylogeny of all the [GPCRs] in the genome, only a few targets actually came out as being targets of approved drugs,” said Bryan Roth, adding that “there are many, many other potential targets for treating pain and other serious conditions.”

To test those targets, Roth’s team developed an assay that allows them to test drugs against a library encompassing 90% of GPCRs encoded in the human genome. That has revealed several new targets, which the researchers are now testing with more specific screens, ultimately hoping to develop safer opioids.

Kelly Knopp began the meeting’s final talk with the grim statistics of chronic pain: affecting about one fourth of the global population, the direct and indirect costs of this condition add up to more than a trillion dollars.

“[Meanwhile,] the probability of technical success for pain [drugs] is worse than any other therapeutic area,” said Knopp.

To address that, she and her colleagues have focused on establishing standardized protocols for phase 2 proof-of-concept trials of pain treatments. Their approach uses sophisticated statistical techniques and uniform trial designs to enable testing of many more drug candidates, without exceeding available funding and medical trial capacity.

After the presentations, a panel of speakers from the meeting discussed several of the field’s biggest challenges. Chief among them are the immense burden of opioid addiction, and the difficulty of shifting real-world clinical treatment toward less addictive but possibly less effective therapies for chronic pain. Despite the difficulties, many researchers in the field remain optimistic.

As Ilene Tracey said in her presentation, “We’re often quite doom and gloom in the pain field, [but] we’ve actually got a lot of different tools at our disposal, [and] we should be more confident about where the field has got to and where it can go quite rapidly.”