That a finger can distinguish the texture of satin from suede is an exquisite sensory discrimination. Largely relying on small sensory organs in the fingertips called Merkel discs containing Merkel Cells. Jianguo Gu, Ph.D., of the University of Alabama at Birmingham, has now unraveled how the sensory information is processed in the Merkel discs. As well as further conveyed to the ending of a sensory nerve, the start of its journey to the brain.
Such molecular understanding about the sensory information transmission between Merkel cells and nerve endings could be radical. And may lay the foundation to treat patients’ intense pain felt by a gentle touch of their inflamed skin. A pathological pain also known as tactile allodynia. This knowledge may also point to how diabetes patients lose their sense of touch. And this new knowledge may lead to preventive care.
“Cancer patients often have touch-induced pain after chemotherapy,” said Gu, the Edward A. Ernst, M.D., Endowed Professor in the UAB Department of Anesthesiology and Perioperative Medicine. “Touch-induced pain is also commonly seen in clinical conditions such as fibromyalgia. As well as traumatic injury and in inflammation from sunburn. Our new findings may have profound implications in these conditions.”
A Merkel disc consists of a Merkel cell and a closely associated nerve ending that branches from a single sensory nerve. Until recently, it was unclear how the physical pressure of a light touch gets transduced from a mechanical force to an electrical nerve signal in Merkel discs.
Merkel Cell Signals
In 2014, Gu’s research team overturned the common assumption that transduction from the mechanical force takes place at the endings of the sensory nerves in Merkel discs. Instead, as he reported in the journal Cell, that mechanical transduction at Merkel discs initiates primarily in the Merkel cells. His team further pinpointed that a new ion channel in the Merkel cells—called Piezo2—is the mechanical transducing molecule.
Now Gu and colleagues have discovered how the signal transduced by Piezo2 is passed from the Merkel cells to the nerve endings. Reporting in Proceedings of the National Academy of Sciences that the Piezo2 transducer triggers Merkel cells to release the neurotransmitter serotonin. This serotonin crosses the tiny gap to the nerve ending, where it activates 5-HT receptors and triggers nerve impulses.
Such gaps from one nerve cell to the next are called synapses, and they are conventional in neural communication. The newly discovered Merkel cell-nerve ending synapse is unique, Gu says. “Because it is the only example of a synapse formed between a non-neuronal cell and a nerve cell. And it is also the first synapse that is found underneath the skin.”
Other types of sensory nerves from the skin—which detect sensations. Sensations like heat, cold or pain have their first synapse where the sensory nerve meets the spinal column.
Elucidation of a Merkel disc serotonin synapse in the skin opens several areas for future investigation.
“The serotonergic transmission in the epidermis, probably like that in the central nervous system, can be regulated by factors affecting serotonin uptake and release,” Gu and colleagues write in their PNAS paper.
By Jeff Hansen, University of Alabama at Birmingham
10/12/2016
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