Research Article: Rat Merkel Cells Are Mechanoreceptors and Osmoreceptors

Date Published: November 9, 2009

Publisher: Public Library of Science

Author(s): Nicholas Boulais, Jean-Pierre Pennec, Nicolas Lebonvallet, Ulysse Pereira, Nathalie Rougier, Germaine Dorange, Christophe Chesné, Laurent Misery, Christophe Egles.

Abstract: Merkel cells (MCs) associated with nerve terminals constitute MC-neurite complexes, which are involved in slowly-adapting type I mechanoreception. Although MCs are known to express voltage-gated Ca2+ channels and hypotonic-induced membrane deformation is known to lead to Ca2+ transients, whether MCs initiate mechanotransduction is currently unknown. To answer to this question, rat MCs were transfected with a reporter vector, which enabled their identification. Their properties were investigated through electrophysiological studies. Voltage-gated K+, Ca2+ and Ca2+-activated K+ (KCa) channels were identified, as previously described. Here, we also report the activation of Ca2+ channels by histamine and their inhibition by acetylcholine. As a major finding, we demonstrated that direct mechanical stimulations induced strong inward Ca2+ currents in MCs. Depolarizations were dependent on the strength and the length of the stimulation. Moreover, touch-evoked currents were inhibited by the stretch channel antagonist gadolinium. These data confirm the mechanotransduction capabilities of MCs. Furthermore, we found that activation of the osmoreceptor TRPV4 in FM1-43-labeled MCs provoked neurosecretory granule exocytosis. Since FM1-43 blocks mechanosensory channels, this suggests that hypo-osmolarity activates MCs in the absence of mechanotransduction. Thus, mechanotransduction and osmoreception are likely distinct pathways.

Partial Text: The sense of touch is not fully understood in mammals [1]. The slowly adapting type I mechanoreceptor (SAI) formed by the Merkel cell (MC)-neurite complex is critical for shape and texture discrimination [2]. SAI is concentrated at touch sensitive areas of the skin, such as fingertips, lips, touch domes and vibrissal outer root sheath in rodents (for review see [3], [4]). However, since previous work has produced conflicting results, it is still unclear whether MCs are able to initiate mechanotransduction by themselves [5], [6]. Mechanotransduction requires stimulation of mechanically sensitive proteins, the opening of ion channels and the subsequent activation of nerve terminals, which generate action potentials. For MCs, electrophysiological evidence has demonstrated the presence of L-type (Cav1.2), P/Q-type (Cav2.1) and N-type (Cav2.2) voltage-gated Ca2+ channels and the role of Ca2+-induced Ca2+ release (CICR) in the evocation of robust intracellular Ca2+ transients [7], [8], [9]. Consecutive synaptic transmission to somatosensory neurons was bolstered by tight connections with nerve terminals, which were observed by confocal imaging and ultrastructural studies [10], [11]. Furthermore, essential components of the synaptic machinery were detected [12], [13], [14]. However, direct mechanical stimulation previously failed to activate quinacrine-labeled MCs [7].

The mechanotransduction properties of MCs in the MC-neurite complex remain controversial [5], [6]. Recently, increasing evidence has demonstrated the importance of Ca2+ signaling to induce depolarization of MCs stimulated by hypo-osmolarity [9], [22], [23]. In this study, we described a process by which to transiently transfect rat MCs. By performing electrophysiological recordings, we report the presence of voltage–activated Ca2+, K+ and KCa channels in MCs. These findings confirm recently published data. Histamine and acetylcholine modulated the activation of Ca2+ channels in MCs. Importantly, we provide evidence that MCs act as mechanoreceptors because direct mechanical stimulation induced sustained strength-dependent depolarizations. The inhibition of these currents by Gd3+ confirmed the presence of cationic non-selective stretch activated channels on MCs. Finally, we demonstrated that hypo-osmolarity led to neurosecretory granule exocytosis, possibly through the activation of TRPV4 in FM1–43-labeled MCs. Taking into account that FM1–43 inhibits mechanotransducer channels [19], [20], our results suggest that MCs act as mechanoreceptors and osmoreceptors.



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