Epidermal Merkel cells are mechanosensory cells that tune mammalian touch receptors.
Touch submodalities, such as flutter and pressure, are mediated by somatosensory afferents whose terminal specializations extract tactile features and encode them as action potential trains with unique activity patterns. Whether non-neuronal cells tune touch receptors through active or passive mechanisms is debated. Terminal specializations are thought to function as passive mechanical filters analogous to the cochlea's basilar membrane, which deconstructs complex sounds into tones that are transduced by mechanosensory hair cells. The model that cutaneous specializations are merely passive has been recently challenged because epidermal cells express sensory ion channels and neurotransmitters; however, direct evidence that epidermal cells excite tactile afferents is lacking. Epidermal Merkel cells display features of sensory receptor cells and make 'synapse-like' contacts with slowly adapting type I (SAI) afferents. These complexes, which encode spatial features such as edges and texture, localize to skin regions with high tactile acuity, including whisker follicles, fingertips and touch domes. Here we show that Merkel cells actively participate in touch reception in mice. Merkel cells display fast, touch-evoked mechanotransduction currents. Optogenetic approaches in intact skin show that Merkel cells are both necessary and sufficient for sustained action-potential firing in tactile afferents. Recordings from touch-dome afferents lacking Merkel cells demonstrate that Merkel cells confer high-frequency responses to dynamic stimuli and enable sustained firing. These data are the first, to our knowledge, to directly demonstrate a functional, excitatory connection between epidermal cells and sensory neurons. Together, these findings indicate that Merkel cells actively tune mechanosensory responses to facilitate high spatio-temporal acuity. Moreover, our results indicate a division of labour in the Merkel cell-neurite complex: Merkel cells signal static stimuli, such as pressure, whereas sensory afferents transduce dynamic stimuli, such as moving gratings. Thus, the Merkel cell-neurite complex is an unique sensory structure composed of two different receptor cell types specialized for distinct elements of discriminative touch.
1] Department of Dermatology, Columbia University, New York, New York 10032, USA [2].
Srdjan
S
Maksimovic
1] Department of Dermatology, Columbia University, New York, New York 10032, USA [2] Graduate School of System Design and Management, Keio University, Yokohama 223-8526, Japan [3].
Masashi
M
Nakatani
1] Department of Dermatology, Columbia University, New York, New York 10032, USA [2].
Yoshichika
Y
Baba
1] Department of Dermatology, Columbia University, New York, New York 10032, USA [2] Department of Neuroscience, Baylor College of Medicine, Houston, Texas 77006, USA.
Aislyn M
AM
Nelson
Department of Dermatology, Columbia University, New York, New York 10032, USA.
Kara L
KL
Marshall
Department of Neuroscience, Baylor College of Medicine, Houston, Texas 77006, USA.
Scott A
SA
Wellnitz
Department of Neuroscience, Baylor College of Medicine, Houston, Texas 77006, USA.
Pervez
P
Firozi
Howard Hughes Medical Institute, Molecular and Cellular Neuroscience, The Scripps Research Institute, La Jolla California 92037, USA.
Seung-Hyun
SH
Woo
Howard Hughes Medical Institute, Molecular and Cellular Neuroscience, The Scripps Research Institute, La Jolla California 92037, USA.
Sanjeev
S
Ranade
Howard Hughes Medical Institute, Molecular and Cellular Neuroscience, The Scripps Research Institute, La Jolla California 92037, USA.
Ardem
A
Patapoutian
1] Department of Dermatology, Columbia University, New York, New York 10032, USA [2] Department of Physiology & Cellular Biophysics, Columbia University, New York, New York 10032, USA [3] Program in Neurobiology & Behavior, Columbia University, New York, New York 10032, USA.
Ellen A
EA
Lumpkin
D000200:Action Potentials; D000344:Afferent Pathways; D000818:Animals; D051792:Basic Helix-Loop-Helix Proteins / Q000378:metabolism; D004553:Electric Conductivity; D000078404:Epidermal Cells; D004817:Epidermis / Q000294:innervation*; D005260:Female; D007473:Ion Channels / Q000378:metabolism; D008297:Male; D040542:Mechanotransduction, Cellular; D018862:Merkel Cells / Q000378:metabolism*; D051379:Mice; D008954:Models, Biological; D016501:Neurites / Q000378:metabolism; D009475:Neurons, Afferent / Q000378:metabolism; D062308:Optogenetics; D011312:Pressure; D014110:Touch / Q000502:physiology*
D051792:Basic Helix-Loop-Helix Proteins; D007473:Ion Channels; C500943:Atoh1 protein, mouse; C552921:Piezo2 protein, mouse