By John Rennie, Science Editor & Journalist
When a bite of spicy chili pepper seems to set your mouth aflame, the comparison is not just a figment of your imagination. Rather, the sensations of extreme spiciness and burning heat are intertwined in your nervous system at a molecular level: the same pain sensors scream out in response to both stimuli. Similarly, the refreshing coolness of menthol comes from your body’s impression that it is experiencing actual cold.
Those quirks of physiology embody important lessons about how evolution works. They could also be the key to developing more effective painkillers that would snuff out discomfort without causing the dangerous sleepiness of many current analgesics.
Credit for these surprising discoveries—and others—belongs to molecular physiologist David Julius, currently the chair of the physiology department at the School of Medicine of the University of California, San Francisco (UCSF). The string of remarkable finds emerging from his laboratory over the years has brought him professional acclaim and major awards including the first Perl-UNC Neuroscience Prize in 2000, the Run Run Shaw Prize in 2010, and more recently, the Dr. Paul Janssen Award for Biomedical Research in 2014.
Growing up as the middle son of three in Brighton Beach, a Russian-immigrant neighborhood of Brooklyn, N.Y., Julius had few thoughts of becoming a scientist. He spent one year as a student at the exclusive Stuyvesant High School in Manhattan, but then opted to transfer to the public high school closer to his home, having grown tired of incessant testing and grade competition at Stuyvesant. At Lincoln High, the tutelage of an entertaining physics teacher who showed his students the relevance of science to daily life made Julius reconsider his ambitions. He headed to the Massachusetts Institute of Technology in 1973 with plans to become a physician.
At M.I.T., however, the opportunities for hand-on research experience won him over. While a junior, Julius began bench work under Alexander Rich, a living legend in the study of biological macromolecules. “Alex’s lab was a creative, freewheeling, and messy place, dispelling any ideas I might have had about labs being stark and sterile environments reserved for quiet personalities and dispassionate experimentation,” Julius wrote of that experience in his Shaw Prize autobiography.
In 1977, Julius moved on to graduate studies in the biochemistry department of the University of California, Berkeley. His advisers were the noted biochemists Jeremy Thorner and Randy Schekman (who would win a Nobel Prize in 2013). His dual mentors gave him a rich appreciation of what the combined forces of molecular genetics and pharmacological biochemistry could do to probe cells’ inner workings. He set about studying yeast and the molecular mechanisms that regulate their production of peptide hormones.
Notwithstanding the enjoyable success Julius was having in studies of yeast molecular biology, he had also become intrigued about neurobiology in more complex organisms. He was especially intrigued about the possibility of using molecular genetics and pharmacological methods to decipher how neurons functioned. In 1984 he therefore pursued post-doctoral work at Columbia University with the neurobiologist Richard Axel (who, like Schekman, would also eventually be a Nobel laureate, in 2004).
“When I was there, everybody worked on something different,” Julius says. “The unifying theme was that everybody was trying to clone genes for key molecules” governing cell physiology. He set out to clone a serotonin receptor from rat brain, knowing that it would illuminate the role of serotonin in certain psychiatric conditions. Eventually, he succeeded in cloning the 5HT1c serotonin receptor.
As Julius’s focus of attention shifted to the nervous system, he became increasingly curious about why naturally occurring compounds in foods and other products could sometimes act on receptors in the brain and induce hallucinations or other changes in brain function. The conceptual gulf between receptor molecules in the brain and complicated brain phenomena seemed dauntingly vast. Still, he drew inspiration from eminent scientists like Solomon Snyder of Johns Hopkins Medical School, who had discovered how opioid compounds made by plants (like morphine) trigger receptors in the brain that respond to endogenous opioids.
When Julius moved on to UCSF to start his own lab in 1989, his first project was to clone another serotonin receptor of particular interest, one for a serotonin subtype called 5HT3. Julius and his group showed that neurons immediately fire when serotonin binds to 5HT3 receptor because the receptor, which is a gated ion channel, instantly opens. (In contrast, many serotonin receptors work more slowly and indirectly by releasing secondary messengers that in turn open ion channels.) That speed and efficiency made the 5HT3 receptor an appealing target for drug development.
pain was—and in many ways, still is—a great mystery."But Julius’s team also noticed that the 5HT3 receptor is expressed heavily in sensory neurons that are linked to the perception of pain (such as those in the dorsal root ganglia). That fact opened up an exciting new avenue for further research because pain was—and in many ways, still is—a great mystery.Neuroscientists had broken the phenomenon of pain into two components: nociception, the sensory process that detects noxious stimuli and signals to the central nervous system about them, and pain perception, which is the interpretive process that renders the sensory information into an unpleasant experience. Perception is complex because it integrates so many aspects of an individual’s condition and history (which is why anesthesiologists struggle to meaningfully quantify how much patients suffer).
Yet even nociception was murky in its details. At the time when Julius cloned the 5HT3 receptor, neuroscientists were divided about whether nociceptors even existed as a distinct class of sensory neurons: an opposing theory held that the brain and spinal cord recognized the signature of painful stimuli from some pattern or intensity in the neural signals for other touch sensations.
For decades, researchers had been studying pain in the laboratory with doses of capsaicin—the pungently spicy extract of chili peppers that gives them their kick. No one knew why capsaicin had this effect, however.
Julius hesitated to wade into such an unexplored area. Yet one day when he and his spouse, fellow UCSF professor Holly A. Ingraham, were at the supermarket, she found him staring at the spice aisle, lost in a reverie. She told him to take the plunge, and he did.
His lab’s initial efforts to find and clone the gene for the receptor that responded to capsaicin repeatedly failed because the gene was not active enough to be caught by conventional assays. The breakthrough came when Julius’s colleague Michael J. Caterina brought in new equipment that would allow them to map the influx of calcium ions into cells, a telltale sign of activity in neurons responsive to the capsaicin. Within about a month, they had cloned and isolated the gene for the capsaicin receptor, which they dubbed TRPV1. The TRPV1 receptor is a gated cation channel protein: like the 5HT3 receptor, it is a regulated pore in the cell membrane that can be triggered to let positive ions into a neuron.
The key question then was, what did this receptor do to signal pain in daily life when capsaicin was not around? Julius and his colleagues began throwing various other pain-inducing compounds at the receptor to little effect. The result was more dramatic, however, when they exposed the receptor to heat: at temperatures above 43°C (109°F), the receptor readily opened. This temperature also happens to be roughly the threshold for what the body normally considers an uncomfortable extreme of heat.
Julius, Caterina, and their colleagues published their discovery of TRPV1 in 1997 to huge acclaim. In the nearly two decades since that time, the laboratory has continued to study the behavior of the TRPV1 receptor, but also identified other receptors in the same molecular family that confirmed their status as painful temperature sensors. In 2002, Julius and Ardem Patapoutian of the Scripps Research Institute independently identified TRPM8, the receptor that conveys the cool sensation of menthol. Diana M. Bautista in Julius’s laboratory showed in 2007 that TRPM8 serves as a primary detector of environmental cold: it activates when temperatures drop below 26°C (79°F). In 2006, Sven-Eric Jordt in Julius’s group found that TRPA1 is the receptor that responds to the noxious irritation of wasabi, horseradish, and mustard, which differs from the spiciness of capsaicin. TRPA1 seems to help us sense irritating chemicals in the air, as well as compounds associated with inflammation.
The picture emerging from the work in Julius’s lab and elsewhere thus confirmed that nociceptors do exist as a distinct class of sensory neurons. Equally important was that it clarified how those pain receptors work. It even explained why the same receptors respond to stimuli as different as heat and chili extract.
When burning temperatures trigger the TRPV1 receptors, for example, the body immediately reacts to the signal by mounting an inflammatory response around the injured area. Some of the natural substances released in that response also interact with TRPV1 receptors and make them more responsive. Think of how your skin feels immediately after a sunburn: it becomes uncomfortably sensitive to even small amounts of heat.
On the other hand, the capacity of those nociceptors to respond is not inexhaustible. That could be why topical pain-relieving ointments containing capsaicin bring relief: their gentle, prolonged stimulation of the TRPV1 receptors may desensitize the neurons and reduce the local inflammatory response.
For drug developers, the fact that specific components of pain sensation can be teased apart at a molecular level is exciting. It should be possible to develop analgesics that can selectively subdue the pain of inflammation, for example, without the broad side effects of opioid painkillers. Several experimental painkillers aimed at TRPV1 receptors have gone into clinical trials, but an initial problem has been that they had the undesirable effect of causing fevers or diminishing the protective response to heat.
According to Julius, these effects may be an “on-target effect,” an incidental consequence of diminishing sensitivity to noxious heat. “Because those sensors are also involved in detecting environmental temperature, you thermoadapt,” he says. “That’s why people like to eat hot peppers in hot climates.” The hope, he says, is that future drugs targeting TRPV1 or TRPA1 may block the receptor’s modulation of sensations more selectively. Rather than blocking the receptor outright, he says, those drugs would subtly modulate its behavior, taking out the hypersensitivity to pain from inflammation but leaving the environmental sensitivity important for maintaining healthy body temperatures.
Pharmaceutical applications aside, however, for Julius, learning how sensory receptor molecules work in more detail is reward enough. His work also lets him appreciate the inventiveness of evolution in finding radically novel uses for established molecules.
Chili peppers evolved capsaicin, for example, because it served as a defensive exploitation of animals’ inflammatory pain mechanism. Hungry creatures that ate those plants paid a price in fiery discomfort. The capsaicin-triggered pain receptor is a vulnerability that nature has exploited on other occasions, too: in 2006 Julius’s lab discovered that it is also the key to why the venom in a tarantula’s bite is devastatingly painful.
“We’ve enjoyed looking at sensory systems from that perspective,” Julius says. One night when he and his family were watching a television documentary about rattlesnakes, he idly wondered whether the snakes’ ability to sense the body heat had anything to do with the receptor molecules he was studying. At that time, researchers still debated whether snakes’ infrared-sensitive pit organ might have evolved out of the visual system.
…finding the right balance between chasing exciting ideas that might transform fields and keeping a more sustained focus that yields detailed knowledge and defines your expertise."In 2010, Elena O. Gracheva, Julius, and other members of his lab published proof that the infrared detector in the pit organ was an adaptation of the wasabi-sensitive TRPA1 receptor—a noxious chemical sensor co-opted to work as a heat sensor. The following year, they showed that vampire bats find vulnerable blood vessels in their warm-blooded prey with the help of heat sensors around the nose derived from TRPV1.Julius thinks that sensory systems are a particularly fruitful place to look for molecular adaptations. Whereas the molecules that handle basic housekeeping chores for the nervous system tend to be similar among all animals, he says, “in sensory systems you see more rapid evolution of molecules as they respond to the salient environment of that organism.” Toxin biology is another great area for such evolution studies, he adds. “Animals have had millions of years to evolve toxins. Molecules get repurposed over and over again.”Reflecting on the diverse successes of his career, Julius notes that “attracting great people to your lab” and having reliable funding for curiosity-based research are always key. In his own studies, he credits finding the right balance between chasing exciting ideas that might transform fields and keeping a more sustained focus that yields detailed knowledge and defines your expertise. “There comes a point when you find the things you’re really interested in,” Julius says, “and then you do deep.”
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Bautista DM. (2015) Spicy science: David Julius and the discovery of temperature-sensitive TRP channels. Temperature. 2(2): 135-141. doi: 10.1080/23328940.2015.1047077
Caterina MJ et al. (1997) The capsaicin receptor: a heat-activated ion channel in the pain pathway. Nature 389:816-824. doi:10.1038/39807
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Grachevna EO et al. (2010) Molecular basis of infrared detection by snakes. Nature. 464:1006-1012. doi:10.1038/nature08943
Grachevna EO et al. (2011) Ganglion-specific splicing of TRPV1 underlies infrared sensation in vampire bats. Nature. 476:88-92. doi: 10.1038/nature10245
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