How Nerves Work
Sensory neurons come in many “flavors.” Different types respond to different stimuli – kisses, cuts, tickles, burns, baths, bug bites, buzzing cell phones, and on and on – and their specialties can be remarkably nuanced. “There are touch neurons, for example, that sense a hair moving and others that sense a hair being pulled,” explains Chesler of the NIH. Some neurons sense only one stimulus, such as an itch or a pinch, while others sense multiple stimuli, such as both the coolness and the caress of a fall breeze. One class of sensory neurons, known as polymodal nociceptors, responds to just about anything that hurts. “You can burn your hand, or somebody slaps you – doesn’t matter,” Chesler says. “They’ll respond no matter what it is, as long as it’s nasty.”
Receptors enable sensory neurons to detect physical stimuli by converting them into electrical signals. These receptor molecules act as gates in the tips of axons, or nerve fibers. When a receptor is activated – such as by heat, pressure, or a chemical reaction – its gate opens. This allows charged particles called ions to flow into a neuron, creating an electrical current that sets off a signaling cascade.
Illustration: Chris Philpot
The Promise of Better Painkillers
The holy grail of pain medicine is a drug that can alleviate agony without creating trouble elsewhere in the body. The problem with opioids, for instance, is that they act indiscriminately. Opioids not only numb nerves but also activate the brain’s reward centers (which causes a sense of euphoria that can lead to addiction) and suppress respiration (which is why an overdose can be fatal). “The pill doesn’t know where to go,” explains Allan Basbaum, PhD, a UCSF neuroscientist who specializes in pain physiology.
So when Julius’s lab discovered the capsaicin receptor, TRPV1, Basbaum says, the pain field was ecstatic. TRPV1 does more than trigger sensations of heat. It also plays a role in pain caused by inflammation, a hallmark of chronic conditions like arthritis, asthma, and inflammatory bowel disease. Most importantly, the receptor is found primarily in peripheral sensory neurons – those outside the central nervous system. Many pain scientists therefore believed that TRPV1 would lead them to an analgesic drug with few, if any, dangerous side effects.
Finding that drug, however, has been harder than they had thought. “Many groups tried to go after it and failed,” Basbaum says. Some candidates that made it to clinical trials flopped at relieving pain as well as animal models had predicted; others caused slight fevers or increased the risk of burn injuries because they prevented patients from feeling anything hot. But experts haven’t lost hope. Several promising compounds are still in various stages of testing, Basbaum says, and recent advances in molecular imaging may enable scientists to find ways of targeting TRPV1 that allay pain without interfering with important functions, like regulating body temperature and sensing heat. (See “How the Chili Pepper Sparked a Breakthrough in Molecular Imaging.”)
Other receptors in peripheral neurons besides TRPV1 could also make good targets for new pain drugs. Blocking the cold receptor TRPM8, for example, could bring relief to cancer patients taking the drug oxaliplatin, which causes hypersensitivity to cold. Another receptor, TRPA1, is especially intriguing. It responds to all sorts of piquant and painful things – garlic, wasabi, wildfire smoke, animal venoms, tear gas, and the byproducts of chemotherapy drugs, among other substances – and has been implicated in a variety of pain syndromes, including diabetic neuropathy, sickle cell disease, and a rare genetic disorder called familial episodic pain syndrome. Biochemist Candice Paulsen, PhD, an alum of Julius’s lab who now studies TRPA1 in her lab at Yale, calls it “a gatekeeper to the development of chronic pain.” Like TRPV1, the receptor reacts to inflammation but, unlike TRPV1, is not involved in regulating body heat. “The hope is that TRPA1 may be a better drug target because it doesn’t have the temperature side effects,” Paulsen says.
Hidden Worlds of Hurt
Surprisingly, neurons are not the only cells in your body involved in pain. Do your teeth twinge when you bite into ice cream? A recent study suggests that tooth cells called odontoblasts could be the culprit. These cells express the cold receptor TRPC5, the activation of which may make teeth extra sensitive to frosty food or drink, particularly when they are inflamed.
Enterochromaffin cells, cells in the lining of the digestive tract, are another suspected pain accomplice. Known as EC cells, they express the receptor TRPA1, which responds to mustard, radishes, and other spicy fare, explains James Bayrer, MD, PhD, a pediatric gastroenterologist at UCSF. Working with Julius’s lab and the lab of Holly Ingraham, PhD, UCSF’s Herzstein Professor of Molecular Physiology (and Julius’s wife), he has found that EC cells communicate with neurons to amplify sensory signals from the gut. When they’re activated, they can make normally benign sensations – such as a stretch of your intestine as food passes through it – feel painful.
Bayrer suspects this signal amplification is behind abdominal pain disorders like irritable bowel syndrome, which afflicts up to 30% of people and lacks a silver-bullet treatment. “If we could use what we’re learning to find a drug that could turn the volume knob down on the pain,” he says, “that would be fantastic.”
How The Chili Pepper Sparked A Breakthrough In Molecular Imaging
About a decade after Julius made history by locating the capsaicin receptor, the molecule in human nerve cells that gives chilies their kick, his team had examined it in every way they could think of – except for one. They had cloned the gene that codes for it. They had bombarded it with all matter of stimuli to see what sets it off. They had even mutated it in mice and created animals that were immune to spice and some amount of heat and pain. “They had studied this protein six ways from Sunday,” says Yale’s Candice Paulsen. “But they had never seen it.”
For a molecular biologist, seeing the structure of a molecule is revelatory. Suddenly you grasp how the thing is put together – and how you might heal it or cripple it or use its parts for other purposes. Such scrutiny allows researchers to know a molecule intimately, pinpoint its vulnerabilities, and promptly find drugs that target it by testing up to billions of compounds virtually using computer simulations.
Until recently, the best way to image molecules was with X-ray crystallography; scientists coax a molecule to form a crystal, blast it with X-rays, and then reconstruct its structure from the radiation patterns. When Erhu Cao, PhD, joined Julius’s lab in 2007, he tried to use this technique with the capsaicin receptor, to no avail. “He toiled away for three, almost four years, trying to get crystals,” Julius says. “Many proteins are just very difficult to crystalize,” explains Cao, now a biochemist at the University of Utah. He needed a new tool.
Meanwhile, two floors above Julius’s lab, UCSF biophysicists David Agard, PhD, and Yifan Cheng, PhD, were working to improve what was then a relatively obscure technology called cryo-electron microscopy, or cryo-EM. Invented in the 1970s, cryo-EM uses beams of electrons fired at frozen molecules to photograph their structures. For decades, however, the resolution was laughably poor. Most photographs came out looking like blobs, Cheng says. “That’s why, at the time, people doing cryo-EM were called ‘blobologists.’”
The cryo-EM pioneers believed they could do better. Digital cameras had made the process more efficient, but because the first-generation cameras worked with light, the electrons had to be converted to photons, which blurred the images. Hence the blobs. So Agard and his colleagues, including scientists at Lawrence Berkeley National Laboratory and the Massachusetts Institute of Technology, set about building a new kind of camera that detects electrons directly, resulting in sharper pictures. Agard enlisted Cheng to test and refine the prototypes, and Julius’s lab provided the perfect subject: the capsaicin receptor.
By the end of 2013, the team had published the first cryo-EM portraits of the famed receptor. The resolution was so good that you could make out the placement of each atom. “That sent a shock wave through the field,” Cheng says, because it showed that medically important molecules that resisted crystallization, like the capsaicin receptor, could be imaged with cryo-EM. “Almost every structural biology lab switched to doing cryo-EM overnight,” he says.
Today, more than 90% of molecular structures are solved using cryo-EM – including viruses. “After the first SARS outbreak, in 2003, it took years to work out the virus’s structure using X-ray crystallography,” Cheng points out. “With SARS-CoV-2, it took weeks.” Detailed cryo-EM images have allowed experts to track the evolution of coronavirus variants and quickly develop vaccines and therapies. “All this was done thanks to cryo-EM,” Cheng says. “If we did not push for this technology – if the field did not progress in this way – our understanding of the virus and our capability to fight it would lag way behind.”
When Elena Gracheva, PhD, was a fellow in Julius’s lab, she went to Texas to dissect some rattlesnakes. In a building with no windows (“so the snakes didn’t escape”), she cut from each viper’s brain a bundle of nerves projecting to organs in its face called pits. Pits allow rattlers and other snakes, including boas and pythons, to “see” the warm bodies of their prey via infrared radiation. Gracheva carried the snakes’ neurons in a cooler back to UCSF, where she isolated the molecule responsible: TRPA1.
In humans and other mammals, TRPA1 plays a role primarily in sensations of pain and itch. (See “The Promise of Better Painkillers” and “Why We Scratch.”) But in snakes, the receptor evolved to detect radiant heat. “This is the beauty of evolution,” says Gracheva, now a physiologist and neuroscientist at Yale. “One molecule can take on many different functions.” She has since discovered other extraordinary adaptations, which, besides being just plain cool, could reveal ways of tweaking sensory molecules for therapeutic purposes.
Hibernating squirrels and hamsters, for example, have evolved TRPM8 receptors that are not sensitive to cold, as they are in humans, enabling these creatures to survive frozen winters quite comfortably. Similarly, a modification in the receptor TRPV1, which in humans responds to relatively modest heat, allows desert camels to withstand extremely hot climates. Another variant of TRPV1 endows vampire bats with an infrared sense like that of rattlesnakes. This sense is so acute that the blood-drinking bats can make out veins under the skin of their prey.