This is the second in a series of Forum interviews with PRF’s eight new science advisors for 2014-2015.
Ru-Rong Ji, PhD, is distinguished professor and chief of pain research at Duke University Medical Center in Durham, North Carolina, US. His research focuses on how glial cells and neuroinflammation are involved in the pathogenesis of neuropathic and inflammatory pain. Ji is well known for his contributions to understanding the roles of MAP kinase signaling in chronic pain, and recently he and his colleagues demonstrated analgesic effects of the inflammation-resolving lipid mediators resolvins and neuroprotectins. Lately, he has been working on a novel approach to targeting sodium channels to relieve pain and studying the role of microRNAs in pain signaling. Ji spoke with Neil Andrews, PRF executive editor, by telephone to discuss the evolution of his research interests, his outlook for the future of pain research, and his advice for young investigators. Below is an edited transcript of their conversation.
How did you become interested in studying pain—and what path did your early career research take?
I was always fascinated with neuroplasticity that occurs following activity or injury. As a postdoctoral fellow at Beijing Medical University in China in the early 1990s with Ji-Sheng Han, I was inspired by pioneering work from Clifford Woolf on neuroplasticity in the spinal cord. I was also intrigued by a paper published in Nature by Stephen Hunt in 1987 and a paper published in the Journal of Neuroscience by Allan Basbaum in 1990. These papers reported that noxious stimulation could cause a very dramatic and rapid induction of the immediate early gene c-Fos in rat dorsal horn neurons. I thought it was amazing that this change in gene expression could be observed within an hour after stimulation and that it could serve as a marker for nociceptive transmission. I would go on to study c-Fos expression in the spinal cord in response to noxious stimuli in my own research early in my career.
I also spent two years at the Karolinska Institute in Stockholm with Tomas Hökfelt to study neuroplastic changes in gene expression in dorsal root ganglion and spinal cord neurons after nerve injury and inflammation. I then moved to the United States for a postdoctoral position with Fabio Rupp in the neuroscience department at Johns Hopkins University School of Medicine, Baltimore, US, where I tried to identify additional markers of nociceptive activity in the spinal cord. I identified a signaling molecule that caused phosphorylation of CREB, a transcription factor that is very important for neuroplasticity, in the spinal cord. In this particular case, noxious hindpaw stimulation with formalin caused phosphorylation of CREB within just five minutes in rat spinal cord neurons. I was very excited about this line of research, as CREB phosphorylation is probably one of the earliest markers of nociceptive activity.
After that I moved to Harvard Medical School and Massachusetts General Hospital to work with Clifford, whose work on neuroplasticity, as I mentioned, always fascinated me; I started in Clifford’s lab as a senior postdoc and immediately became an instructor at Harvard Medical School. While in Clifford’s lab, I focused on intracellular signal transduction mechanisms involving phosphorylation of the MAP kinase family member ERK. We found that brief noxious stimulation, such as with capsaicin injection, activated ERK in rat spinal cord nociceptive neurons and contributed to pain hypersensitivity. Clifford was very supportive of my work and encouraged me to apply for my first NIH [National Institutes of Health, US] grant, which enabled me to start my independent research.
What are some of the projects your lab is focusing on now?
We are working to learn more about the pain resolution mechanism. Most studies that people do now look at how pain is initiated, and we have learned a lot about how pain mediators can trigger pain. But we don't know much about how pain is resolved. In fact, I think that the problem of chronic pain is not a problem of initiation, but rather of resolution—if acute pain cannot be resolved, then chronic pain is the result.
I work in the same department as a colleague, Charles Serhan, who discovered resolvins and protectins; these are endogenous lipid mediators with potent anti-inflammatory and pro-resolution actions that have been demonstrated in the immune system. We thought that these mediators should also have an important role in pain regulation, so we started a collaboration and have published a series of papers showing that resolvins and protectins derived from fish oil have potent anti-inflammatory actions. They also act as potent and novel analgesics that can modulate glial activation as well as the activity of TRP [transient receptor potential] channels. We are continuing to study resolvins in mouse models of pain because they are a class of mediators that we think could represent a general mechanism for the resolution of pain. To increase translational potential, we are also testing these compounds in human dorsal root ganglion neurons.
We are also following up our research on a mononclonal antibody to [the voltage-gated sodium channel] Nav1.7. In our earlier work, Seok-Yong Lee, who is in the biochemistry department here at Duke University, developed a selective monoclonal antibody to Nav1.7. We collaborated to demonstrate that the antibody is effective at blocking sodium currents and action potentials in native neurons, and we also observed that the antibody could ameliorate pain and itch in mouse models [see PRF related news story]. Now we are trying to develop additional monoclonal antibodies to Nav1.7 and also potentially to other ion channels that are important for pain.
What are your thoughts about Nav1.7 as a potential therapeutic target—and why take an antibody approach?
I think that Nav1.7 remains one of the most attractive targets to develop new pain therapeutics. Enthusiasm to target Nav1.7 is based on human genetic studies showing that loss-of-function mutations result in insensitivity to pain, while gain-of-function mutations increase sensitivity to pain. Also, there is a very strong electrophysiological basis for Nav1.7 as a pain target because this ion channel is important for the generation of action potentials in nociceptors and the conduction of pain signals, as shown by elegant work from John Wood and Steve Waxman. For these reasons, Nav1.7 is at the very top of the list of drug targets for many companies.
But while Nav1.7 is a very appealing target, the problem is that it is extremely difficult to design and develop selective small molecule inhibitors because of the homology among the different sodium channel subunits; the homology between subunits from the Nav1.1 channel all the way to the Nav1.9 channel is so close, such that small molecules and toxins are unable to have very selective actions. Using a monoclonal antibody is a much more selective approach.
What has the response been to the anti-Nav1.7 antibody work?
We’ve seen an overwhelmingly positive response from large pharmaceutical companies; everyone knows that Nav1.7 is an important target, and the companies have put so much effort into it. One of the reasons for the recent enthusiasm is that companies view biologics such as antibodies as a priority—as first-line drugs for which the companies can see a fast track to development. Other examples include anti-NGF [nerve growth factor] antibodies for pain, and anti-CGRP [calcitonin gene-related peptide] and anti-CGRP receptor antibodies for migraine, which have all been tested in clinical trials. There is a very high priority for large pharmaceutical companies to develop new antibody treatments.
It seems promising that industry is interested in the anti-Nav1.7 antibody, since some companies have pulled out of the pain arena or are devoting less effort to developing pain therapeutics. Are you optimistic that more companies will come back to the pain field?
I am cautiously optimistic. The companies know that chronic pain is an important area, and while companies can leave quickly, they can also come back quickly. If an anti-Nav1.7 antibody can become a therapeutic antibody, or if an anti-NGF or anti-CGRP antibody succeeds, then I think the companies will come back. If there's a way in, they will return.
You have also been studying the role of microRNAs in pain. Why is that an area that is beginning to catch on?
There is a lot of interest in microRNAs because of the potential to develop new drugs for pain treatment. MicroRNAs were well known to work intracellularly to regulate gene expression, but we recently found that secreted microRNAs acting extracellularly could activate the immune receptor Toll-like receptor 7 in nociceptive sensory neurons [see PRF related news story]. We further found that this activation of Toll-like receptor 7 was coupled to an ion channel, TRPA1, which can activate neurons.
This was a very surprising finding to us that could lead to a potential therapeutic for pain treatment. We are continuing this line of research, and we're also thinking that some of the microRNAs may serve as neuromodulators that may be secreted from the synapse to modulate synaptic transmission.
You have also worked to understand the contribution of glia to pain. Why did you become interested in that area of research?
During the first stage of my research career, I thought that it was only neuronal activation that was important for neuroplasticity—many others thought that as well. But I became interested in glia when I moved my lab to Brigham and Women’s Hospital in Boston. It was a natural extension of my earlier work. I was studying MAP kinase activation and neuroplasticity in chronic conditions such as neuropathic pain, and, unexpectedly, we found that the p38 MAP kinase in the spinal cord was very dramatically activated in microglial cells in a rat neuropathic pain model.
At that time, it was very surprising to me that glia were involved, but our work and that of others, including Linda Watkins, Joyce DeLeo, Michael Salter, Yves De Koninck, Kazuhide Inoue, and many others, all helped to show an important role for glia in pain. Our work would further demonstrate that activation of intracellular MAP kinase signaling was critical for the production of glial mediators such as cytokines and chemokines. We next moved to studying neuron-glial interactions, and it was surprising that these cytokines and chemokines produced by microglia and astrocytes had extremely potent actions on neuroplasticity and synaptic transmission—in fact, their effects as modulators of synaptic transmission are actually more potent than those of classical neurotransmitters.
Now, we are focusing on how microglia produce cytokines such as TNF [tumor necrosis factor] to modulate spinal cord synaptic transmission. We are also interested in how astrocytes produce chemokines such as CXCL1 and CCL2 to modulate pain sensitivity. I have an NIH grant with Maiken Nedergaard to study how connexin-43, a hemi-channel and also a gap junction protein, regulates astrocytic release of chemokines and neuropathic pain [see PRF related news story].
Let’s look at the pain field more broadly now. Where do you think breakthroughs will emerge?
One area where I think there will be huge advancement is in the study of pain circuitry. This is because of new technologies like optogenetics and chemical genetics, which will allow us to delineate pain circuitry and especially the circuitry for different modalities of pain, such as mechanical pain versus thermal pain. We will be able to specifically dissect pain circuits starting in primary sensory neurons, then going to different populations of neurons in the spinal cord, and then to different populations of neurons in various brain regions. This is a very exciting line of research, and I expect in the next few years we will see big breakthroughs because of all the research tools that are available now.
I also think there will be breakthroughs in studying mechanosensation, because I think in the near future we will have specific activators and inhibitors for different populations of sensory fibers. For example, now we use capsaicin to selectively activate C-fibers, but we lack selective activators of A-fibers, such as Aβ fibers, that are very important for the maintenance of chronic pain. Nor do we have selective inhibitors of Aβ fibers. The development of selective activators and inhibitors will dramatically move the field forward, not only advancing our understanding of pain pathways, but also leading to novel therapeutics.
What are the challenges facing the pain field now?
The big challenge is that despite the progress in basic research that we have seen, we don't have new drugs. One of the issues is the relevance of our animal models. The animal models rely on reflex responses, but the question is whether this really represents pain sensation in people. This is why using measures of pain such as conditioned place preference, which allows for the assessment of spontaneous pain and affective aspects of pain, will be important; this is an approach that has been developed by Frank Porreca, Jeffrey Mogil, and Howard Fields. Using these measures will help us learn more about pain mechanisms and enable us to test novel analgesics.
Another very tough challenge in the field now is that the analgesics currently available only give transient relief and are unable to provide resolution or reversal of pain—I think this is the most difficult task for pain management. But, looking at work from Yves De Koninck, Jürgen Sandkühler, Min Zhou, and others, if pain is considered a memory and we can erase that memory, then this may be a way to alleviate pain—it’s a very interesting idea.
However, to erase a memory of pain is to have an effect on neuroplasticity, but if there is an injury and ongoing pathology, such as neuroinflammation, this may still drive and cause pain. So in addition to targeting neuroplasticity, we also need to control neuroinflammation and the progression of disease—that's what will lead to a final cure for chronic pain. This will be very difficult to accomplish, and I don’t see it happening in the near future, but it is our final goal, and I'm cautiously optimistic that we will eventually reach it. Because pain is a very complicated phenomenon, what we will need is combination therapy: likely a drug that can treat neuroplasticity and another drug to control pathology such as neuroinflammation.
Are you encouraged by the current state of pain research?
There has been great progress in pain research, as can be seen by the increasing number of papers published in very high-impact journals like Cell, Neuron, Nature Neuroscience, and others, which now publish much more pain research than they used to. This can also be seen by all the papers that make it to PRF’s Papers of the Week—there are more and more papers each week of interest to pain researchers.
Do you have any advice for young investigators entering the pain field?
I encourage young investigators, as a first step, to gain a very solid understanding of molecular and cellular mechanisms of pain. Of course, translational research is important, but having a very solid background in basic science, and publishing basic science work in high-quality journals, is very important. If this doesn’t occur, then obviously there will be nothing to translate.
I also think that it isn’t necessary for young investigators to follow authority—they should have their own independent thinking and try to think outside the box. It is telling that people have come from other fields to solve problems in the pain field. For example, David Julius came from outside the pain field but made one of the most important findings in pain research by identifying and cloning TRPV1 by working together with leaders in the pain field such as Allan Basbaum and Jon Levine.
It’s also very important to have a supportive environment and mentors that give you the opportunity to follow your own thinking and to have your own ideas. I was very fortunate to have Clifford Woolf serve in that role for my development as a pain researcher.
Finally, young investigators need to be focused. As I mentioned, there are many papers published in the literature each week, and you do have to be knowledgeable, but you also must be focused and have confidence that what you are doing is enough.
Thank you for speaking to PRF.
Thank you for the opportunity.
PRF Related Content:
See "Related Content" box in the right column of this page.
Chen G, Park CK, Xie RG, Berta T, Nedergaard M, Ji RR. Connexin-43 induces chemokine release from spinal cord astrocytes to maintain late-phase neuropathic pain in mice. Brain. 2014 Aug;137(Pt 8):2193-209.
Ji RR, Xu ZZ, Gao YJ. Emerging targets in neuroinflammation-driven chronic pain. Nat Rev Drug Discov. 2014 Jul;13(7):533-48.
Lee JH, Park CK, Chen G, Han Q, Xie RG, Liu T, Ji RR, Lee SY. A monoclonal antibody that targets a NaV1.7 channel voltage sensor for pain and itch relief. Cell. 2014 Jun 5;157(6):1393-404.
Park CK, Xu ZZ, Berta T, Han Q, Chen G, Liu XJ, Ji RR. Extracellular microRNAs activate nociceptor neurons to elicit pain via TLR7 and TRPA1. Neuron. 2014 Apr 2; 82(1):47-54.
Park CK, Xu ZZ, Liu T, Lü N, Serhan CN, Ji RR. Resolvin D2 is a potent endogenous inhibitor for transient receptor potential subtype V1/A1, inflammatory pain, and spinal cord synaptic plasticity in mice: distinct roles of resolvin D1, D2, and E1. J Neurosci. 2011 Dec 14;31(50):18433-8.
Xu ZZ, Zhang L, Liu T, Park JY, Berta T, Yang R, Serhan CN, Ji RR. Resolvins RvE1 and RvD1 attenuate inflammatory pain via central and peripheral actions. Nat Med. 2010 May;16(5):592-7.
Jin SX, Zhuang ZY, Woolf CJ, Ji RR. p38 mitogen-activated protein kinase is activated after a spinal nerve ligation in spinal cord microglia and dorsal root ganglion neurons and contributes to the generation of neuropathic pain. J Neurosci. 2003 May 15;23(10):4017-22.
Ji RR, Baba H, Brenner GJ, Woolf CJ. Nociceptive-specific activation of ERK in spinal neurons contributes to pain hypersensitivity. Nat Neurosci. 1999 Dec;2(12):1114-9.
Ji RR, Rupp F. Phosphorylation of transcription factor CREB in rat spinal cord after formalin-induced hyperalgesia: relationship to c-fos induction. J Neurosci. 1997 Mar 1;17(5):1776-85.
Other Forum Interviews with PRF's 2014-2015 Science Advisors: