A new family of ion channels may answer a pressing question in pain biology: How do we detect painful touch? In two papers published February 19 online in Nature, Ardem Patapoutian at The Scripps Research Institute in La Jolla, California, US, and collaborators make the case that the newly discovered Piezo proteins—a widely conserved family of gigantic membrane-spanning polypeptides—function as mechanosensitive ion channels in neurons and other cells. And, they show that, at least in fruit flies, the proteins are required specifically for perceiving painful prodding.
The discoveries raise hopes that researchers are at last closing in on the long-sought force-activated channels that signal touch—and pain—in higher animals.
Based on the new findings, the mouse protein Piezo2 “is the best candidate we have for an actual mechanotransduction ion channel in nociceptors,” said Cheryl Stucky, Medical College of Wisconsin, Milwaukee, US, who studies ion channels in sensory neurons. Stucky, who was not involved in the Piezo work, told PRF that if, as in flies, Piezos prove to transduce noxious mechanical stimuli in mammals, “It really could open up the door for treatments for pain.”
Opening moves
The ability of cells to detect mechanical force is crucial to a wide variety of physiological functions, including hearing, blood pressure regulation, and the sensation of both pleasant and painful touch. It is thought that, in sensory nerve endings, mechanical stimuli open force-sensitive excitatory cation channels that depolarize the cell membrane, leading to neuron firing. But the identities of the crucial proteins that convert mechanical stimuli into electrical signals have been hard to pin down, especially in higher animals (for a review, see Delmas et al., 2011; also see PRF related news story).
In 2010, Patapoutian’s lab reported the discovery of two related proteins required for mechanically activated currents in mouse neurons (Coste et al., 2010). The group dubbed the proteins Piezo1 and 2—after the Greek píesi, meaning pressure. Piezos are enormous proteins containing 30 to 40 membrane-crossing domains, and have few similarities to known ion channels. Piezo2 is highly expressed in sensory neurons of the dorsal root ganglion (DRG), making it a candidate touch and pain detector. In support of that idea, in the 2010 study the researchers showed that the protein is required for some kinds of mechanically activated currents in isolated DRG neurons. But the work raised some questions. Could these curious monstrosities be ion channels? And what is their physiological function?
Channeling pain
To address the physiological role, Patapoutian and his group turned to fruit flies, working in collaboration with Boaz Cook, also at Scripps. As reported in one of the two new papers, first author Sung Eun Kim found that, in flies, Piezo is required for sensing noxious mechanical force. Deleting the single Drosophila melanogaster Piezo homolog dramatically reduced the number of larvae that twisted away from harsh pokes (for more on the stereotyped response of fly larvae to noxious stimuli, see PRF related news story). Depleting Piezo in a subset of nociceptive sensory neurons diminished the animals’ reaction to the jabs, indicating that the protein is required specifically in pain-sensing cells. Notably, Piezo-deficient larvae responded normally to gentle touches and high temperature, suggesting that the protein uniquely detects noxious force, and not other types of stimuli.
In an accompanying paper, first author Bertrand Coste and his colleagues, in collaboration with Mauricio Montal at the University of California, San Diego, also in La Jolla, show that the Piezo protein is a force-responsive ion channel. As previously reported for mouse Piezos, Drosophila Piezo expression was sufficient to render cells responsive to mechanical force: Poking or stretching human embryonic kidney (HEK) cells containing fly Piezo resulted in large inward ion currents that were not seen in HEK cells lacking Piezo. Further, purified mouse Piezo1 reconstituted in lipid bilayers conducted sodium and potassium ions, solidifying the case that the proteins are true ion channels. Piezo seems to work alone: Protein crosslinking analysis by co-first author Bailong Xiao revealed no accessory proteins that tightly associate with the channel. Strikingly, Xiao also found evidence that Piezo1 assembles as a tetramer, bringing the mass of the functional complex to 1.2 million Daltons, bigger than any other plasma membrane ion channel known.
The studies answer some questions about Piezos, but raise others. Where in the massive complex is the ion pore, and how does the protein sense force? Patapoutian wonders, “Does the big size have anything to do with it being mechanosensitive?” And perhaps most important, what is the function of Piezos in mammals? Patapoutian said his group is making knockout mice to answer that question.
As researchers forge ahead in characterizing Piezos, the evolutionary conservation of the proteins should be a huge help. Patapoutian told PRF that his group is finding that human Piezo1 and 2 have similar properties to their mouse homologs. Further, he noted, “This is the first time that we have a direct link to mechanotransduction in mammalian and fly proteins of the same family.” He said he wants to take advantage of the link to work out the pathways in which Piezo participates, a task “much more easily done in a genetically amenable system such as flies than in mammalian systems.”
Image: Piezo (red) is expressed in a subset of nociceptive sensory neurons in Drosophila larvae, and is required for the animals to react to harsh pokes. Image credit: Adapted by permission from Macmillan Publishers Ltd: Nature, advance online publication, 19 February 2012 (doi: 10.1038/nature10801).
