Sensory Neurons Get Touchy When STOML3 Is Around
Stomatin-like protein 3 (STOML3) boosts neural and channel responses to mechanical stimuli, according to a paper published March 24 in Nature Communications. Led by Gary Lewin of the Max-Delbrück Center for Molecular Medicine, Berlin, Germany, the researchers developed a method to precisely poke mechanosensitive sensory neurons at localized points along the plasma membrane, and found responses to displacements as small as 10 nanometers (nm). This fine sensitivity vanished in neurons lacking the scaffolding protein STOML3, however, suggesting that the STOML3 protein tunes mechanosensitivity. Consistent with this, STOML3 enhanced activation of the mechanosensitive ion channels Piezo 1 and Piezo 2.
“The big surprise for us was that you really only had to nudge these channels ever so slightly to open them, like 10 to 15 nm. That’s about three times the thickness of the plasma membrane,” Lewin told PRF.
The findings offer up STOML3 as a contributor to the range of sensitivities found among different sensory neurons in normal conditions, as well as to the heightened sensitivities present in some pain conditions.
“One thing that we don’t understand well at all is how sensory neurons that innervate the same tissue have widely differing sensitivities to mechanical stimulation,” said Miriam Goodman of Stanford University, Palo Alto, California, US, who was not involved in the study. “This discovery suggests that STOML3 is part of the answer.”
Goodman studies the C. elegans version of STOML3, called MEC-2, which enhances activation of touch-sensitive ion channels. The new study reveals a similar interaction between STOML3 and the mammalian ion channels Piezo 1 and Piezo 2 (see PRF related news story). These behemoths span the membrane over 30 times and transduce force directly into ion channel openings.
“I think this study convincingly shows that STOML3 is modulating the mechanical sensitivity of Piezo 1 and 2,” wrote Bertrand Coste in an email to PRF. Coste is a research scientist at Aix-Marseille Université, in France, who discovered the Piezo proteins while working with Ardem Patapoutian at the Scripps Research Institute in La Jolla, US.
A sign of STOML3’s involvement in pain first came in 2007, when Lewin and colleagues made a STOML3 knockout mouse (Wetzel et al., 2007). The animals seemed protected from neuropathic pain, a situation in which signals through mechanoreceptors responsible for sensing innocuous touch stimuli produce pain by tapping into pain circuitry. About half the mechanoreceptors in these mice didn’t work properly, which may have prevented the pain signal from developing.
The new work finds that STOML3 boosted the sensitivity of mechanoreceptors responsible for processing fine touch, and to a lesser extent, of nociceptors that sense intense, potentially painful stimuli.
“So disrupting STOML3 function might be a way to treat both nociceptor sensitization as well as neuropathic pain,” Lewin said.
Pillars and probes
The results also suggest that sensory neurons can sense finer movements than previously appreciated. Mechanosensitive channels are located on sensory neuron endings, where the channels are embedded in an extracellular matrix of proteins. Mechanosensitivity is typically measured in vitro by poking, from above, the cell body of isolated neurons, which indirectly jiggles mechanosensitive channels on the neuron’s underside, where the channels contact the extracellular matrix in a dish.
First author Kate Poole and colleagues developed a new method that places isolated sensory neurons atop an array of small, flexible pillars coated with an extracellular matrix. Pushing individual pillars to the side with a probe produced localized movements of the cell membrane, presumably close to mechanosensitive channels. With this method, tiny movements in the nanometer range evoked currents, compared to the microns of displacement needed with the cell body indentation method.
“This new technique pushes our understanding of the sensitivity of these cells orders of magnitude lower than what we had thought previously,” Goodman said.
Despite this, the actual force felt at the membrane remains unclear. “In the future, systems that allow precise quantification of the force required to gate these mechano-channels need to be developed,” Coste wrote.
The researchers found that sensitivity varied across different cell types isolated from the dorsal root ganglia (DRG) of mice. Mechanoreceptors responsible for sensing fine touch needed only about 10 nm of movement to evoke a current, whereas nociceptors required at least 10 times that. This tidy alignment suggests that neural differences in sensitivity stem from differences in mechanosensitivity of individual cells.
Sensitivity with STOML3
Using the microstimulation system, the researchers went on to show that STOML3 modulated mechanosensitivity in a subset of DRG neurons. Loss of STOML3 compromised the sensitivity of type II mechanoreceptors responsible for fine touch, but not of their type I counterparts attuned to larger movements. Nociceptors from STOML3 knockouts showed a slight decrease in sensitivity in the 100-250 nm range compared to controls.
Intriguingly, STOML3 modulated the force-sensing Piezo proteins in simplified cell preparations. Overexpressing STOML3 in a neural-like cell line known to express Piezo channels increased a cell’s mechanosensitivity. Similarly, expressing Piezo 1 or 2 together with STOML3 in HEK cells brought sizeable increases in mechanically activated currents over that seen when expressing Piezo 1 or 2 alone. The researchers also found evidence that STOML3 physically interacted with Piezo 1 and 2.
But the study falls short of connecting this interaction with mechanoreceptor sensitivity. “Since not all DRG neurons express Piezo proteins, it’s still not clear whether or not STOML3 effects on mechanoreceptors work through Piezo-dependent channels,” Goodman said.
Further experiments indicated that the stomatin domain of STOML3 was required for the sensitivity of Piezo proteins. Lewin suggested that STOML3 associates with other STOML3 molecules to make scaffolds lining the inner surface of the cell membrane. This would result in a more pliable membrane and expedite Piezo channel activation.
Michele Solis is a science writer and former neuroscientist who lives in Seattle, Washington, US.
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