“I love talking about the molecular structure [I discovered]”, said Ardem Patapoutian, who leads a lab at the Scripps Research Institute in San Diego, as he was presenting his work to an audience gathered at the Champalimaud Foundation (CF), in Lisbon, last week. “It's very easy for me to talk about it when I have PowerPoint slides, but what if I'm in a restaurant or in a bar and I want to tell someone about how it works? I decided to get a tattoo of the structure and I want to share it with you.”
He next takes his suit jacket off (“I'm not going to completely undress, you don't have to worry about it”, he jokes), rolls up his right arm’s shirt sleeve and shows the public a tattoo, centred on his elbow, which depicts the molecular structure in case. He coined it PIEZO (a name derived from the Greek word for pressure). PIEZO proteins are ion channels and Patapoutian discovered two of them. In other words, they are “holes” or “pores” in the cell membrane, which block or allow the entry of ions. PIEZO1 and PIEZO2 are present in many cells across the body and they are called mechanosensors because they respond to pressure changes – by stretching. They look somewhat like airplane propellers.
“I wanted to place [the PIEZO structure] in the exact situation where I could talk about the opening and closing of the pore. So I put it on my arm, with the elbow being the pore. This way, it's very easy for me to demonstrate how it works. I stretch my arm, the pore opens and the ions fall through”. And entering the cell, they trigger a process that transforms mechanical stimuli into chemical signals that cells – and neurons in particular – can understand. “I've yet to use it in a bar”, says Patapoutian about his tattoo. “But who knows?”
Patapoutian’s discovery of the PIEZO1 and PIEZO2 molecules won him the Nobel Prize in 2021. “He asked such an important question and went after it when it was really not obvious that he would actually be able to answer it”, said Carlos Ribeiro, principal investigator at the FC’s Behaviour and Metabolism lab (who was instrumental in bringing Patapoutian to Lisbon), while presenting Patapoutian to a full auditorium. “And he not only solved one of the big mysteries of biology, but his discovery opened up a completely new field of research into questions that we didn't even know actually depended on mechanosensation”.
What was the fundamental question Patapoutian asked? It had to do with our most elusive sense: touch. “How do we feel?”, he wanted to know. Or, more to the point, how is a mechanical stimulus – a poke on the shoulder, for instance – sensed by our body and transformed, inside our nerve endings and skin sensor cells, into an electrical signal that can then be sent to the brain for processing?
Touch is truly an exquisite sense: we can feel and distinguish myriad different sensations, from light caresses to painful pinches. And contrary to our other senses, which are localised (in the eyes, the ears, the tongue, the nose) and that we can suppress at will (just close your eyes and you can’t see), touch is distributed all around our body and cannot easily be “turned off”.
Touch was also, before Patapoutian’s research, the least understood sense. Until 2010, when he made his discovery, no mechanosensors in vertebrate animals were known. He and his team changed that.
“Many people were trying to find ion channels in neurons that responded to touch”, Patapoutian explained. But it was complicated because neurons don't divide, so it's hard to do genomic level manipulations on them.”
At his lab, in 2009, they adopted a completely different approach, spearheaded by postdoc Bertrand Kost, which did not involve genetically manipulating neurons. “Forget the neurons, we said”, Patapoutian recalled. “Let's find the mechanosensor from any cell line that is easy to culture and divide in a dish.” So they started searching, in cell cultures, for a cell line that fired electrical currents in response to pokes by a micropipette. After screening about 30 cell lines, they found one.
It would take them another year to find PIEZO1, which they did by silencing, in this cell line, one candidate gene after another (with a technique called RNA interference), and then testing whether that canceled the cells’ sensitivity to mechanical force. They had about 300 candidate genes in view, and they hit the jackpot with candidate 72, when they saw, in the knockout cell line, a massive reduction in the mechanically activated currents. Finally, they cloned the PIEZO1 gene and put it in cells that were not normally mechanosensitive to see whether that was sufficient to induce mechanosensitivity. It was.
This also led to the discovery of a “sister gene”, PIEZO2. Together, the proteins for which these genes code – the ion channels that sit in cell membranes, look like propellers, and stretch under mechanical stimulation – are responsible, not just for the sense of touch we’re most familiar with, which originates in the skin, but for two other equally vital aspects of the sense of touch: proprioception and interoception.
Proprioception is what allows us to know, at all times, how our body is positioned in space, even when we close our eyes. Human patients who have a deficient PIEZO2 gene not only lose their sense of touch, but also have trouble standing and walking with their eyes closed and coordinating their movements. As to interoception, it allows us, for instance, to feel our heart beating or knowing that our bladder is full.
PIEZO2 has been shown to play a key role in proprioception, and both the PIEZO1 and PIEZO2 ion channels are known today for regulating important physiological processes including blood pressure, breathing and urinary bladder control – all aspects of interoception.
Furthermore, research has also been ongoing on the role of PIEZO1 in many types of non-neuronal cells, which are also known to respond to mechanical forces. In particular, it has been shown, by Patapoutian’s team and others, that PIEZO1 plays important roles in the development of the cardiovascular system, bone formation, and in red blood cells.
Even more surprisingly, Patapoutian explained that his team recently showed that PIEZO genes are expressed at the tips of plant roots. “PIEZO genes are expressed precisely there”, he said. “And when we knock them out in Arabidopsis [a small flowering plant related to cabbage and mustard], we see clear differences in the plant’s ability to penetrate hard surfaces. In the PIEZO knockouts, the roots cannot penetrate hard soil, so they go around it. Plants use PIEZO ion channels to sense the hardness of the soil.”
One of the potential medical applications of his research could be finding treatments for chronic pain, and in particular for “tactile allodynia”, a condition in which even an innocuous touch is painful. “Many people suffering from neuropathic pain experience this all the time, and there are no good medications for it” said Patapoutian. “We showed that, in mice, tactile allodynia is completely dependent on PIEZO2.” And a simple experiment performed in people without functional PIEZO2 channels gave similar results, which means the results in animals could translate to humans.
However, he adds, there are many challenges to translating this into medicine. “One of them is that we can’t orally administer any compound that blocks PIEZO2 to sufferers because it would also knock out their sense of touch and proprioception”, Patapoutian explained. “But we are actually developing these kinds of compounds for localised, topical application [i.e., in the form of a cream], hoping that they might be useful in some indications.”
Patapoutian has a singular personal link to Portugal: the first funding he ever received – to study at the American University of Beirut after graduating from highschool – came from none other than… the Gulbenkian Foundation, when he was 17 years old. He proudly shows the application he filled at the time. “Socioeconomically, it was very hard for me to be able to go to college, and Gulbenkian’s and later grants, and other foundations and government agencies, really helped in my education”, he said.
Towards the end of his talk, he described himself like this: “An Armenian immigrant from Lebanon, who was initially funded by a Portuguese agency, studied in the US, won the Nobel, who now comes back to visit Lisbon, and is having a great time at it. And he added: “This is quite a full circle moment.”
Text by Ana Gerschenfeld, Helath&Science Writer of the Champalimaud Foundation.
