Touch Activates Mechanosensitive Ion Channels in Merkel Cells In Vitro

Abstract

Merkel cell-neurite complexes are gentle touch receptors that mediate slowly adapting type I (SAI) responses. Since their description in 1875, Merkel cells have been proposed to be mechanosensory cells that transduce mechanical stimuli into electrical signals that activate somatosensory neurons. Consistent with this model, conditional knockout mice that lack Merkel cells show the loss of touch-evoked SAI responses. Moreover, in vitro studies on cultured Merkel cells report calcium elevation in Merkel cells in response to swelling or membrane stretch. Previous studies support the contribution of Merkel cells to touch sensation, however, the central question of whether Merkel cells are intrinsically touch sensitive is unanswered. To tackle this problem, we performed live-cell imaging and electrophysiological recordings from mouse Merkel cells. Merkel cells were dissociated from the epidermal skin of transgenic Atoh1/nGFP mice, whose Merkel cells selectively express green fluorescent protein. GFP-positive cells were purified using fluorescence-activated cell sorting and cultured for 1-5 days. Individual Merkel cells were stimulated by families of displacements (≦0.3-μm steps) with a glass probe driven by a piezoelectric actuator. Touch-evoked responses were monitored with either ratiometric calcium imaging or tight-seal, whole-cell recordings. Merkel cells displayed reversible calcium responses to focal displacements applied to somata. Moreover, electrophysiological recordings demonstrated mechanically activated inward currents at a negative holding potential. Peak inward currents ranged from 107.3-431.3 pA. The 10-90% operating range of these mechanically activated currents was 1.7±0.1 μm (N=10 cells, mean±SE). Like mechanosensitive currents in hair cells and somatosensory neurons, Merkel-cell currents adapted exponentially to sustained stimuli. Quantitative PCR indicated that Merkel cells expressed both Piezo1 and Piezo2 genes. Together, these data demonstrate that Merkel cells are intrinsically mechanosensitive in the absence of other skin cells or somatosensory neurons.