Itch is a sensation resulting in a desire to scratch one’s skin to help remove irritants or parasites. Itch can be triggered by a variety of internal and external cues, such as pro-inflammatory agents (e.g., histamine) or the woolen fibers in a sweater. The molecular and cellular mechanisms of chemical itch have been elucidated over the past decade. In contrast, the mechanisms underlying mechanical itch are relatively unknown.
Now, new research led by Rose Hill from The Scripps Research Institute, California, US, shows the PIEZO1 ion channel is selectively expressed by itch-specific sensory neurons and plays a necessary role in their activation after mechanical stimulation.
“I think [mechanical itch] has been ignored because most of what we are accustomed to is an allergic type of itch where you’re encountering poison ivy. Personally, I think of mechanical itch feeling like something crawling on your skin and you’ve got to get it off, but that’s hard to study,” explained Kyle Baumbauer, University of Kansas, US, who was not involved in the current research.
“[However], this comprehensive paper used multiple techniques and approaches to get at this question and identify a transducer responsible for the mechanical property of itch in the somatic sensory system. A lot of times, when we are chasing a molecule down, it doesn’t necessarily translate nicely to a behavioral outcome. Here, we have something that we didn’t know much about, but [now know] it’s got an interesting role that might be clinically relevant.”
The research and accompanying editorial were published in Nature on June 22, 2022.
A brief history of PIEZO
The PIEZO ion channels – PIEZO1 and PIEZO2 – were discovered in the laboratory of senior author Ardem Patapoutian in 2010. Discovering these channels was a significant step forward in our understanding of how we detect touch – so much so that Patapoutian, together with David Julius, was jointly awarded the 2021 Nobel Prize in Physiology or Medicine (see the PRF story).
After the discovery of the PIEZO genes, early research used single-cell RNA sequencing to locate low but observable levels of PIEZO1 transcript expression in mouse dorsal root ganglion (DRG) neurons.
After joining the Patapoutian lab as a postdoc, Hill reassessed some of the initial PIEZO expression findings using the more novel and sensitive single-molecule fluorescence in-situ hybridization (FISH) method. Hill noticed an interesting pattern, where 92% of natriuretic polypeptide b neurons (Nppb) in the mouse DRG – which elicit scratching responses in mice when activated – expressed PIEZO1.
PIEZO1 expression was also observed in a subset of MAS-related G protein-coupled receptor D (Mrgprd) neurons, providing further evidence of a role for the ion channel in itch.
“The Mrgprd population of neurons is mechanically sensitive; they are a group of C-fibers that have a really well-known role in heat and mechanical pain,” Baumbauer highlighted. “To find PIEZO1 in there suggests that it could be coding mechanical itch as well.”
Examining Nppb neurons from a human DRG revealed similar levels of PIEZO1 expression to those observed in the murine model. Observing PIEZO1 expression on itch neurons piqued Hill’s interest, as the mechanisms underlying mechanical itch were, at that stage, poorly understood. These observations provided a tangible lead to explore further.
Simply being there isn’t enough
Hill next sought to determine whether the PIEZO1 that was expressed in somatosensory neurons was functional by using calcium imaging on cultured mouse DRG neurons.
“They made very clever use of an exogenous compound called Yoda1, which the Patapoutian lab had identified several years earlier as a PIEZO1 agonist after screening over three million compounds,” said Jorg Grandl, Duke University, US, who was also not involved in the current research.
Yoda1 application resulted in activation of the somatosensory neurons, as did known chemical itch compounds β-alanine and histamine. This implied that itch neurons express functional PIEZO1 channels.
To explore how PIEZO1 mediates itch behaviors, Hill and colleagues needed a mouse model where the ion channel had been knocked out. This was easier said than done, due to the role of PIEZO1 in regulating the development of vasculature in lymphatic vessels.
“A complete PIEZO1 knockout will result in embryonic lethality, so we couldn’t do the ‘normal’ thing of making a mouse where the channel is missing in a broad array of tissues. We had to hope that the sensory neurons were mediating the effects of PIEZO1 and took a risk [by] going through a specific Cre driver first,” Hill recalled.
The selected neuronal Cre driver was a success, and deleted PIEZO1 from peripheral neurons in the mouse model. The knockout mice displayed a profound decrease in mechanically evoked scratching compared to their littermates with normal PIEZO1 expression.
To increase PIEZO1 activity, Hill and colleagues next used a gain-of-function mouse model. These animals displayed increased mechanically evoked scratching behaviors in response to von Frey stimulation when compared to controls. Injecting Yoda1, the PIEZO1 agonist, into the gain-of-function mice resulted in scratching behaviors but did not lead to inflammation or pain sensitization.
Finally, Hill and colleagues sought to explore the relevance of PIEZO1-dependent mechanical itch in a chronic state. The team used an established model of chronic itch, which mimics specific aspects of human atopic dermatitis, in which calcipotriol (a vitamin D analogue) is applied daily to the neck and ears to induce erythema, itch-evoked scratching, and itch hypersensitivity.
PIEZO1 knockout mice that were exposed to the chronic itch model displayed significantly reduced mechanical hypersensitivity and spent less time scratching the experimentally induced wound.
“This experiment was really important because it is one of the first that looks at a specific molecular mechanism – an ion channel-based mechanism – underlying mechanical itch hypersensitivity in a chronic model,” Hill concluded. “It suggests there are different mechanisms underlying the spontaneous feeling of itch and the itch you might feel from wearing a wool sweater in a chronic itch setting.”
Answering one question and raising many more
Although Hill’s work is an important step in enhancing our understanding of the mechanisms underlying mechanical itch, it also opens several avenues for future research. Baumbauer’s mind immediately goes to his work focusing on afferent neurons and nociceptor function.
“One area of future investigation I’d love to see is what this specific population of neurons looks like, and which specific fiber types are involved. Are they in the A-delta range? Are they C-fibers?”
Hill hopes to continue on with work performed by the Patapoutian lab in recent years looking at the association between mutations in the PIEZO1 gene and various human health conditions.
“We are hoping to work with clinicians and patient populations to explore whether the [PIEZO1] gain-of-function mutation is associated with chronic itch disorders, which we might expect given some of our results using the gain-of-function mouse model in the current study,” Hill explained.
Grandl is excited about the translational potential of new treatments for chronic itch, which are sorely needed.
“Although PIEZO1 isn’t an intuitive pharmacological target, as it is expressed in so many different tissues, you could apply an antagonist locally and circumvent unspecific or unwanted side effects. I think this is exciting, as chronic itch is a debilitating condition with relatively limited treatments.”
Lincoln Tracy is a researcher and freelance writer from Melbourne, Australia. You can follow him on Twitter – @lincolntracy.
References
Hill RZ, Loud MC, Dubin AE, Peet B, Patapoutian A. PIEZO1 transduces mechanical itch in mice. Nature. 2022;607(7917):104-110. doi:10.1038/s41586-022-04860-5. PMID: 35732741
Follansbee T, Dong X. Touch-evoked itch pinned on Piezo1 ion-channel protein. Nature. 2022;607(7917):36-37. doi:10.1038/d41586-022-01571-9. PMID: 35732710