Editor’s note: The 17th IASP World Congress on Pain took place September 12-16, 2018, in Boston, US. At the Congress, 12 early-career pain researchers took part in the PRF Correspondents program, a science communications training experience that provides participants with knowledge and skills needed to communicate science effectively to a wide range of pain researchers and to patients and the wider public. As part of this program, the Correspondents conducted interviews with plenary speakers.
Here, plenary speaker Makoto Tominaga, MD, PhD, sat down to chat with PRF Correspondent Tayler Sheahan, a postdoctoral fellow at the University of Pittsburgh, US. Tominaga is a professor in the Division of Cellular Signaling at the Okazaki Institute for Integrative Bioscience, National Institute for Physiological Sciences, Japan. He and his research group aim to understand the significance of temperature in living organisms, with a focus on TRP channels. In this interview, Tominaga discusses his path to TRP channel research, what he’s working on now, and much more. Below is an edited transcript of the conversation.
You first trained to be a physician. What inspired you to pursue a career in scientific research?
I worked as a cardiologist for four or five years. In Japan, many clinicians also get a PhD. In my case, I knocked on the door of basic science and eventually gave up my clinical career in 1995.
I then got an assistant professor position at the National Institute for Physiological Sciences in Japan, but I wanted to do basic research in the United States. I joined David Julius’ lab at UCSF and was involved in a big project involving the cloning and functional characterization of the capsaicin receptor, TRPV1. Since then, I have been working on TRP channels—for more than 20 years!
What led you to explore the role of TRP channels in contexts beyond pain?
It is now well known that TRPV1 is a thermosensitive TRP channel activated by temperatures of more than 43 degrees Celsius, and this is the temperature that causes pain in the body. My plenary lecture at the World Congress focuses on TRP channels and nociception, but I'm broadly interested in temperature sensation [see attached PDF article below]. I'm leading a huge research group supported by the Japanese government that focuses on what temperature is, how our body is regulated by temperature, and how temperature is involved in our physical function.
What are your main research projects right now?
There are 11 thermosensitive TRP channels, and my lab is working on all of them! Some of these channels are expressed in sensory neurons that are involved in nociception. But our bodies are exposed to temperature all the time. For example, consider the oral cavity. Sometimes you drink very cold water, and sometimes you drink hot tea; the oral cavity is exposed to a battery of temperature changes. As I mentioned, we are interested in the significance of temperature throughout the body, not just in nociceptors. We are studying TRP channels in the skin, which are activated by changes in body temperature. We are also studying TRP channels in immune cells because an increase in body temperature is important for enhancing the immune system.
Another main focus is the evolution of TRP channels. All of the animals on Earth have survived by adapting to ambient temperature changes during the evolutionary process, so they should have changed their temperature detection system as well. We have cloned TRP channels from many different species—from insects to mammals—and are comparing their temperature sensitivities and amino acid sequences.
What is the most surprising finding you’ve had about the functional evolution of TRP channels?
There are two kinds of animals: homoeothermic and poikilothermic animals. Poikilothermic animals cannot maintain their body temperature, which is affected by ambient temperatures. And yet upon cloning a lot of TRP channels from different species, surprisingly, their temperature thresholds are pretty much all similar. Evolutionarily, I’m not sure what this means.
How does the long functional evolutionary history of TRPA1 as a heat sensor bear on the possibility of TRPA1 acting as a cold sensor in mammals?
All the TRPA1 channels that we have cloned from insects to birds so far are heat sensors. This year, a very interesting paper came out in Nature by the Voets group. In a TRPM3, TRPV1, TRPA1 triple knockout mouse, heat-evoked nociceptive behavior was completely gone. This may suggest that TRPA1 is activated by heat, even in the mouse!
Our idea is the same as that of David Julius' group—that TRPA1 is not involved in sensing cold. But there is one human disease named familial episodic pain syndrome in which TRPA1 is mutated. Those patients feel more pain in a cold environment, which indicates that human TRPA1 is somehow related to cold.
What is the largest unanswered question about TRP channels?
Some TRP channels, including TRPV1, TRPA1, TRPM3, and TRPM8, are directly gated by temperature, but nobody knows how. David Julius’ and Yifan Cheng’s groups reported the TRPA1 structure at the atomic level a few years ago in Nature using cryo-electron microscopy. It was quite fascinating. But this technique provides just a snapshot of the channel; we can't see any dynamic changes of the actual protein with cryo-electron microscopy. This means that nobody in the world knows how temperature activation opens the channel!
We are taking many approaches to study temperature-dependent activation of these ion channels. First, we are doing a very simple lipid bilayer experiment where we test the temperature threshold for the channels under different lipid conditions, that is, oxidized or nonoxidized conditions. Second, we are cloning thermo-gated TRP channels from frogs that live in different temperature conditions and have somewhat different activation thresholds. Interestingly, there are very few species differences in the amino acid sequences. We are carefully looking at these differences because they might be linked to the amino acid motif involved in temperature-dependent activation of the ion channels. We are also using atomic force microscopy to study single-channel opening of TRP channels in response to heat.
Do you have a favorite TRP channel?
I love TRPA1. In the evolutionary process, TRPV1 emerges at the level of the fish. But then we realized that TRPA1 is present even in insects—in mosquitoes! We are cloning TRPA1 from four different mosquito species from Africa and from Japan. The temperature threshold for the heat activation is very different depending on the place they live. That's quite interesting.
Is there a piece of advice that someone gave you that really influenced your career?
When I gave up clinical work for basic science, a professor in cardiovascular medicine at Kyoto University in Japan said to me, “Don't come back.” There are superstars in the world who can do both clinical work and basic science, but at that time I realized I'm not that kind of person, which was important. I chose basic science, and I continued to work harder and harder.
Who has been your most influential mentor throughout your career?
Definitely David Julius—I’ve learned a lot from him. His team is small but publishes many high-profile papers. While I was in his lab, we had very intense discussions every day about the data we had collected. I’m quite happy to say that I am an alumnus of David Julius’ lab.
Do you have any advice for young scientists who are just starting to launch their careers?
Our institute is located in the small city of Okazaki, but it is a scientific center of Japan because we have many domestic and international collaborations. We are well funded and have lots of equipment, and young scientists oftendon't want to leave to go to the United States or European countries. But while I was at UCSF, I was able to see science from a different perspective. I also saw my country from the outside. That kind of experience is quite important.
What are your hobbies outside of the lab?
Working outside in the garden with my wife. The retirement age in Japan is 65, so I will definitely finish with my science at 65. Then I will have some more time to spend with my family, and I can do some clinical work. I haven’t done clinical work in more than 25 years, so I cannot catch up with the advances, but maybe I can see the elderly people in the local town—that's my dream.
Additional Reading
Understanding the significance of temperature in living organisms (see attached PDF article below from http://www.impact.pub)
Saito S, Nakatsuka K, Takahashi K, Fukuta N, Imagawa T, Ohta T, Tominaga M
J Biol Chem. 2012;287(36):30743-54.
A TRP channel trio mediates acute noxious heat sensing.
Vandewauw I, De Clercq K, Mulier M, Held K, Pinto S, Van Ranst N, Segal A, Voet T, Vennekens R, Zimmermann K, Vriens J, Voets T
Nature. 2018 Mar 29; 555(7698):662-6.
Structure of the TRPA1 ion channel suggests regulatory mechanisms.
Paulsen CE, Armache JP, Gao Y, Cheng Y, Julius D
Nature. 2015 Apr 23; 520(7548):511-7.
