The sensation of light touch in mammals is mediated by Piezo2 ion channel-containing Merkel cells, which are concentrated in touch-sensitive areas of the skin. Now, a new study proposes a role for these mechanoreceptors in generating touch-induced itch.
Researchers led by Hongzhen Hu, Washington University, St Louis, US, show that a loss of Merkel cells is associated with aging and dry skin, resulting in alloknesis, a condition where innocuous mechanical stimuli provoke excessive itch. Activation of these cells via chemogenetics attenuated alloknesis in a dry skin model. Meanwhile, selective ablation of these cells and their associated Piezo2 resulted in increased alloknesis, suggesting a molecular mechanism by which the Merkel cells exert their effect.
The findings were published online May 4 in Science with an accompanying perspective from Amanda Lewis and Jörg Grandl, Duke University, Durham, US.
“I’m very excited about this paper,” Grandl told PRF. “It’s a very significant contribution, and I would predict that a lot of future studies will emerge from this research. What I particularly liked is that the research team investigated their hypothesis in a very methodological and strategic manner, by using five distinct models to alter Merkel cell activity and examine how this affects mechanical touch and itch.”
The young and the old
Excessive itch is the most common skin disorder in the elderly and is closely linked with aging and dry skin. Alloknesis is a specific form of itch evoked by light touch of the skin and is distinct from pruritus, which is usually associated with an underlying skin disease. Both forms of itch are evolutionarily conserved to promote scratching in response to chemicals such as histamine as well as in response to light tactile stimuli such as insects or parasites on the skin.
First author Jing Feng and colleagues first examined the scratching response to light touch in young (two months old) and aged (more than 24 months old) mice. Mechanical stimulation of the nape of the neck with von Frey hairs, at a range of forces, evoked substantially more scratching in aged compared to young mice. Interestingly, scratching in response to the classical itch-promoting substances histamine and chloroquine did not differ between the young and aged mice, suggesting that the pathways underlying chemical itch are unaffected by aging.
Previous studies investigating the neural circuitry of itch reported a subpopulation of neuropeptide Y-positive (NPY+) spinal inhibitory interneurons that are innervated by low-threshold mechanoreceptors (LTMRs) on hairy skin, and that these cells act as a gate for the transmission of itch after mechanical stimulation (Bourane et al., 2015, and see PRF related news story). Equipped with this knowledge, the team next analyzed the firing properties of LTMRs in young and aged mice via ex vivo skin-nerve recordings. Type 1 slowly adapting A-beta afferents (SAI) from skin-nerve preparations from young mice displayed the typical sustained robust firing in response to mechanical stimulation. In contrast, SAI from aged mice displayed truncated static firing (intermediately adapting responses), indicating the dysfunction of these afferents in aged skin.
Surprisingly, examination of the firing properties of SAI from glabrous (non-hairy) skin revealed the same scenario, which is in contrast to previous findings that NPY+ interneurons only receive mechanosensory input from hairy skin. This suggests the existence of a previously unrecognized subset of NPY-negative (NPY-) inhibitory interneurons that likely receive input from SAI afferents and act to gate mechanical itch.
“If NPY+ interneurons are only innervated by LTMRs from hairy skin as reports suggest, then there must be some NPY- interneurons that are innervated by LTMRs from glabrous skin,” said Hu. “Most likely these type 1 SAI A-beta fibers can send inputs to both NPY+ and NPY- interneurons in the spinal cord.”
Moving upstream
Turning next to the acetone-ether-water (AEW) model of dry skin itch, the researchers observed reduced numbers of Merkel cells in AEW-treated mice compared to water-treated controls, with accompanying reduced firing rates of SAI afferents. Seeking behavioral correlates for these observations, Hu and colleagues then assessed alloknesis in their model and found that the animals showed increased scratching in response to light stimulation with von Frey filaments. Interestingly, and consistent with the group’s observations in young versus aged mice, the response to histamine or chloroquine was similar between Merkel cell-deficient and naïve mice, nor were mechanical or thermal pain responses different between the two groups. This indicated a selective Merkel cell-dependent pathway mediating mechanical but not chemical itch.
“We believe that we have discovered Merkel cells to be the cutaneous sensors of these type 1 SAI afferents, driving inhibitory interneurons in the spinal cord to suppress mechanical itch,” said Hu. “In aged or dry skin mice, you lose the upstream signaling when you lose the Merkel cells and subsequently the afferent drive into the spinal inhibitory interneurons, therefore reducing inhibition there.”
“What we call dermatological or conventional itch is caused by pruritogens binding receptors primarily on C-fibers that when activated send itch signals to the dorsal horn,” said Anne Louise Oaklander, Massachusetts General Hospital and Harvard Medical School, Boston, US, who studies neuropathic itch but was not involved in the study. “But we know that mechanoreceptors can also initiate itch, not because they are itch neurons or receptors but because of the pattern of how the signals are transmitted to the CNS, which this paper does a nice job of describing. Much of neuropathic itch may involve this same pattern.”
Activate to alleviate
Because depleting Merkel cells exacerbated alloknesis, the researchers reasoned that activating these cells would offer itch relief. So, pursuing a chemogenetic strategy, they generated mice to express designer receptors exclusively activated by designer drugs (DREADDs) specifically in Merkel cells. Activating Merkel cells using the designer drug clozapine-N-oxide suppressed alloknesis in AEW-treated mice and increased the firing frequency of type 1 SAI afferents.
The group next turned to the molecular mechanism by which Merkel cells acted to suppress itch, setting their sights on Piezo2. The Piezo1 and 2 ion channels were the first mammalian mechanosensitive channels to be identified. Although at the time it was thought that Piezo2 might mediate pain as well as touch, subsequent studies showed that the channel was indispensable for touch but not pain (see PRF related news stories here and here).
Piezo2 is expressed in Merkel cells but whether or not it is involved in the conversion of touch to itch remained unclear. To address this issue, Hu and colleagues generated mice that specifically lacked Piezo2 in Merkel cells. These animals showed significantly increased scratching in response to light touch, even in the absence of AEW-induced dry skin. The study thus highlights important roles for both Merkel cells and Piezo2 channels in touch-induced itch, but what is the most important factor underlying this phenomenon?
“From my point of view, it’s the Merkel cells that appear to be the critical regulator here, as even if Piezo2 expression [on other cells] remains constant and you deplete Merkel cells you still lose the crucial input into the spinal cord, which results in alloknesis,” said Hu.
Grandl tends to agree. “Piezo2 channels are also expressed in a subset of sensory neurons that are in direct contact with Merkel cells and while both certainly contribute together in sensing light touch, this study indicates that it’s the Merkel cells that appear to be most crucial for the itch aspect,” he explained.
Dara Bree is a postdoctoral fellow at Beth Israel Deaconess Medical Center and Harvard Medical School, Boston, US.
Image courtesy of Feng et al., 2018
