ASICs are the receptor for a proton synaptic messenger between Merkel cells and an afferent nerve

Transduction is the transformation of one form of energy into another, like a spoken voice transformed into radio waves by a cell phone.

Ten years ago, three research teams published a major breakthrough: the first description of the molecular mechanism of transduction in Merkel’s fingertip sensory cells, capable of transducing a gentle touch, a mechanical force, into electric current. This discovery earned a 2021 Nobel Prize in Physiology or Medicine for one of these team leaders, Ardem Patapoutian, Ph.D.

Yet a mystery remained.

How does the tactile signal emitted by these sensory Merkel cells in the fingertips propagate across the empty synapse space, from the Merkel cells to the end of a nerve cell? Jianguo Gu, Ph.D., and colleagues at the University of Alabama at Birmingham have now answered this mystery in a study published in the journal Neuron.

Gu reports that protons are the signals that cross the synapse gap from a Merkel cell to an Aβ afferent nerve, and his UAB team identified the receptors in the nerve that sense the protons as ASICs, or l-sensing ion channels. ‘acid. The protons trigger the ASIC to open its transmembrane channel, allowing a flood of sodium, or Na, ions⁺, in the nerve. This, in turn, can trigger action potential impulses that quickly propagate down the afferent nerve toward the brain.

“Protons have never been reported before as primary transmitters in any synapse, and their role as primary transmitters for touch transmission in Merkel cell-neurite complexes is quite unique,” Gu said. His discovery adds protons to the list of well-known neurotransmitters, such as glutamate, GABA, acetylcholine, dopamine or serotonin.

Small oval Merkel cells are essential for tactile discrimination, the ability to differentiate between the physical properties of objects, such as texture, shape, and edges. As the Nobel Prize organization noted in 2021, “The sense of touch, initiated by the detection of mechanical force, allows us to recognize the texture, size and shape of objects, as well as tactile and vibrational sensitivity . for example, allows us to recognize the softness of a pillow, the gentle caress of the skin or the feeling of a breeze.

While the fingertips of primates have the largest number of Merkel cells, making them the most sensitive part of the human body, in non-primate mammals the most sensitive touch receptors are the whiskers. Thus, the mustache follicle, with its numerous Merkel cells and exquisite sensitivity, is biologically analogous to the tip of the human finger. To study the transmission of tactile signals, Gu’s team studied the hair follicles of rodent whiskers.

In the whisker follicle, the tip of each afferent nerve divides into approximately 28 smaller neurites, thin strands protruding from the end of the nerve. Each neurite extends to form a synapse with a Merkel cell located at the front of the hair follicle.

To measure the reception of a tactile signal from a Merkel cell, the UAB researchers made patch-clamp recordings at a heminode, an accessible location for a patch-clamp on the neurite between the Merkel cell synapse and the afferent nerve tip, and at the first node of Ranvier, an accessible site on the afferent nerve just below the nerve tip.

It was a difficult feat, Gu said. “Patch-clamp recordings have never been performed before on Aβ afferent axons near Merkel neurite synapses due to technical difficulties because most parts of Aβ afferent axons are heavily myelinated and inaccessible to recording electrodes patch-clamp.”

The patch clamp uses an extremely thin glass pipette to attach to the membrane of a neuron. After punching a hole in the membrane, the patch clamp is able to measure electrical currents or membrane potential in the entire cell.

In the study, Merkel cells were evoked by mechanical stimulation or depolarizing currents, while patch clamps measured responses at the heminode neurite and the first node of Ranvier in the afferent nerve.

Several lines of evidence have shown that ASICs are the receptors on the neurite for tactile discrimination signals from Merkel cells, according to Gu and colleagues.

First, electrophysiology showed that the receptor appeared to be a Na⁺-selective channel, a class of ion channels that would include ASICs. Second, the researchers found that pharmacological and ASIC toxin blockers prevented Merkel cell signals from being received at the afferent nerve and genetically suppressed one of the ASIC subunits, making it less effective, thereby reducing reception of signals in the afferent nerve. Third, immune fluorescence imaging showed that ASICs are expressed at Aβ afferent terminals that innervate Merkel cell-neurite complexes.

Fourth, focal application of protons to neurites of Merkel cell-neurite complexes directly evoked excitatory currents in the afferent nerve. Fifth, neutralizing endogenous protons using alkalizing agents or preventing the release of protons from Merkel cell vacuoles blocked the transmission of the touch signal from Merkel cells to the afferent nerve.

A final piece of evidence came from animals in a behavioral test of tactile recognition.

When a rat or mouse is placed in an open field box containing two identical smooth objects placed in diagonal corners, they spend equal amounts of time using their whiskers to palpate and recognize each object. If one of the smooth objects is then replaced by an object of identical shape but with a rough surface, when the rodents are returned to the open field box, they spend significantly more time palpating the new object with a rough surface than the smooth object.

The UAB researchers found that giving rats two different pharmacological ASIC blockers eliminated this preference for the novel rough-surfaced object, suggesting that the animals treated with the ASIC blockers were unable to distinguish the rough object as a as a new tactile stimulus.

To further support the idea that ASIC channels are involved in tactile whisker discrimination, the researchers tested mice with deletion of one of the ASIC subunits. In tactile discrimination experiments, unlike controls, mice with this deletion of the ASIC subunit showed no preference for palpating the novel object. As in the rat experiment, this suggests that the ASIC mutant mice could not distinguish the rough-surfaced object as a novel tactile stimulus.

“Initially, ASICs were thought to be mechanical transducers for mammals,” Gu said. “However, many studies over the past decades have failed to establish that ASICs are mechanical transducers in mammals. In the present study, we found that instead of serving as mechanical transducers, ASICs provide Rapid excitatory synaptic transmission at Merkel cell-neurite complexes to encode tactile signals and enable tactile discrimination.

Gu, one of three team leaders who first described the molecular mechanism of transduction in fingertip sensory cells capable of transducing gentle touch into an electric current 10 years ago, is Edward A. Ernst, MD, chair and director of pain research in the UAB Department of Anesthesiology and Perioperative Medicine at the Marnix E. Heersink School of Medicine.

Co-authors with Gu in the Neuron study, “ASICs mediate rapid excitatory synaptic transmission for tactile discrimination,” are Akihiro Yamada, Jennifer Ling and Ayaka I. Yamada, UAB Department of Anesthesiology and Perioperative Medicine; and Hidemasa Furue, Hyogo Medical University, Nishinomiya, Japan.