How Do You Feel?

by Pat McCaffrey on 10/8/2014

The 15th World Congress on Pain, running from now until October 11 in Buenos Aires, Argentina, kicked off with some exciting new data on touch-responsive channels in sensory neurons. Mechanosensation, the response of cells and organisms to pushing or stretching or stroking, remains one of the most mysterious of all the sensory modes. Understanding the channels that mediate mechanical pain could offer new targets for analgesics aimed at quieting peripheral neurons.

The first mammalian mechanosensitive channels, Piezo1 and 2, were discovered just a few years ago by Ardem Patapoutian and colleagues at the Scripps Research Institute, La Jolla, California, US (Coste et al., 2010). They subsequently showed that the fruit fly form of Piezo could respond to noxious mechanical probing (see PRF related news story on Kim et al., 2012). In the opening plenary presentation of the meeting, Patapoutian showed new data on the function of Piezo2 in the mammalian sensory nervous system. Based on experiments with conditional knockout mice, the channel appears indispensable for sensing touch, but not pain.

That leaves open the question of what channel or channels are responsible for sensing painful pressure. The answer may be revealed before too long, as Patapoutian’s lab is working on a clever high throughput assay for additional mechanosensors.

“He has found the touch transducer. He has not found the pain transducer… but he will likely find it soon, if he has not already,” said audience member Jeff Mogil, McGill University, Montreal, after the talk.

Patapoutian has a long-standing interest in channels that give peripheral sensory neurons their feelings. His lab discovered the TRPM8 cold sensor and, along with David Julius’s lab, the TRPA1 channel, which senses noxious chemicals and is a target for experimental new treatments for pain and cough.  Channels that open to force were known in lower organisms but the identity of the mammalian force-operated channel was unknown until Patapoutian’s group discovered Piezo1 and 2.  

In his new work, Patapoutian found that sensory neurons of the dorsal root ganglion express Piezo2, but not Piezo1. Piezo2 knockouts die soon after birth, so Patapoutian used a tissue-specific inducible knockout strategy to delete the channel in adult mice and only from DRG neurons. The mice showed the expected loss of most if not all Piezo2 protein in the DRG, and loss of the characteristic mechanically-activated current in DRG neurons. Electrophysiological recordings from ex vivo skin-nerve explants (done in collaboration with Gary Lewin, Max Delbrück Center for Molecular Medicine, Berlin, Germany) showed a loss of mechanical stimulation in the low threshold mechanotransducing Abeta sensory fibers (the fibers that respond to light touch), but not nociceptive C or Adelta fibers.

In several behavioral assays, the mice showed a drastic reduction in response to touch stimuli, including gentle poking with von Frey filaments, swiping with a cotton swab, and having a piece of adhesive tape placed on their back. In contrast, the detection of noxious force from von Frey filaments was completely normal in the knockouts, and they developed mechanical hypersensitivity in inflammatory and neuropathic pain models identically to the wild type mice.  

The results indicate that Piezo2 is a major transducer of mechanical touch sensation in mice. That fits with the distribution of the channel in skin, which Patapoutian found mainly in around low threshold mechanotransducing structures including Meissner corpuscles and Merkel cells using a GFP reporter construct. There was no detectable signal in free nerve endings typical of nociceptive fibers.

What about the pain channel? Finding Piezo took 11 months of poking and recording currents from individual cells. Piezo was the 72^nd gene that then-postdoc Bertrand Coste screened. Now, the lab has devised a high-throughput assay to detect currents evoked by shear stress in cells in 384-well plates. The new assay can do 3000 genes in 3 days, Patapoutian said. Chances are it will not take long to find a candidate transducer for the high-threshold mechanoreceptors.

Patapoutian’s impressive work so far earned him a Howard Hughes Investigator award earlier this year (see his investigator bio here). His excellent plenary was a fitting kickoff to the pain meeting, and to PRF’s coverage. Watch this space for more blogging from Buenos Aires. If you are at the meeting, chime in with your comments, too.