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When you press a fingertip against a desk, your skin changes shape for a moment, and your brain hears about it. Before Ardem Patapoutian's work, nobody knew which molecule in vertebrates turned that squash into an electrical signal; mechanical sensors had been found in bacteria, but not the ones behind our sense of touch (Nobel Assembly press release).
Patapoutian shared the 2021 Nobel Prize in Physiology or Medicine with David Julius "for their discoveries of receptors for temperature and touch", and each received half the prize (Nobel Prize facts page). He was born in 1967 in Beirut, Lebanon, moved from war-torn Beirut to Los Angeles in his youth, earned a PhD from Caltech in 1996 and has worked at Scripps Research in La Jolla, California, since 2000 (Nobel Assembly press release).
What did Ardem Patapoutian discover?
Piezo1 and Piezo2 are ion channels that are opened directly by pressure on a cell membrane (Nobel Assembly press release). Both are cation channels, meaning they let positively charged ions through when they open (Coste et al., Science 2010).
The name comes from the Greek word for pressure. Sensory neurons carry high levels of Piezo2, and later work showed it is essential for the sense of touch (Nobel Assembly press release).
How did Patapoutian find the Piezo genes?
The search was patient elimination. The Nobel Assembly describes it in four moves (Nobel Assembly press release):
Find a cell line that gives off a measurable electric signal when a single cell is poked with a micropipette.
Assume the touch sensor is an ion channel, and list 72 candidate genes that could encode it.
Silence the genes one at a time and poke the cells again.
Look for the one gene whose silencing makes the cells stop responding.
That gene was Piezo1, and its similarity to another gene led the team to Piezo2. The 2010 paper reports that the team used RNA interference in a mouse cell line, and that the two proteins were needed for the pressure-triggered current in those cells and in cultured sensory neurons (Coste et al., Science 2010).
What does Piezo2 do in the human body?
Mice lacking Piezo2 in their adult sensory neurons and Merkel cells showed an almost complete loss of light-touch sensation, without changes in their other body senses that the study tested, according to a 2014 study from Patapoutian's lab (Ranade et al., Nature 2014). The same study found that touch and pain can be separated, which suggested that a different channel, unknown at the time, handles painful pressure.
A 2015 study in mice showed that Piezo2 is the principal channel for proprioception, your sense of where your limbs are without looking (Woo et al., Nature Neuroscience 2015).
Evidence in people followed. A 2016 study described two patients with inactivating variants in the PIEZO2 gene: both had a selective loss of discriminative touch and sharply reduced proprioception, which got markedly worse when they could not see, yet they could still walk, talk and write (Chesler et al., NEJM 2016).
Piezo channels work beyond the skin too: they help regulate blood pressure, breathing and urinary bladder control (Nobel Assembly press release). For the temperature half of the prize, read how David Julius found the TRPV1 chilli heat receptor.
How do you explain Piezo channels in an exam answer?
When a question asks how touch becomes a nerve signal, this five-step chain keeps your answer complete:
Stimulus: pressure deforms the membrane of a sensory nerve ending in the skin.
Receptor: Piezo2 channels in that membrane open.
Ions: positive ions flow in, so the inside of the cell becomes less negative. If the resting potential is shaky, revise Julius Bernstein's membrane theory of the resting potential first.
Signal: a large enough change starts a nerve impulse that travels towards the spinal cord and brain.
Evidence: mice without Piezo2 in their sensory neurons and Merkel cells almost lose light touch (Ranade et al., 2014).
Step 5 turns a description into an explanation. Test yourself with a parent or friend: they read one step, you say the next.
For a stretch question, compare your skin with how a phone touchscreen senses your finger.
If the chain breaks at step 3 or 4, the gap is usually earlier biology. The Fermi Concept Library has a lesson for every concept in biology, each with its place in the map, so you can trace ion channels back to membrane potential. Fermi's AI tutor checks understanding as it goes, so you can explain the five steps in your own words and find the one you tend to skip.
Next time you tap a desk, run the chain in your head: push, Piezo2, ions, impulse, evidence.
