Swelling-Activated Ca(2+) Channels Trigger Ca(2+) Signals in Merkel Cells

Merkel cell-neurite complexes are highly sensitive touch receptors comprising epidermal Merkel cells and sensory afferents. Based on morphological and molecular studies, Merkel cells are proposed to be mechanosensory cells that signal afferents via neurotransmission; however, functional studies testing this hypothesis in intact skin have produced conflicting results. To test this model in a simplified system, we asked whether purified Merkel cells are directly activated by mechanical stimulation. Cell shape was manipulated with anisotonic solution changes and responses were monitored by Ca(2+) imaging with fura-2. We found that hypotonic-induced cell swelling, but not hypertonic solutions, triggered cytoplasmic Ca(2+) transients. Several lines of evidence indicate that these signals arise from swelling-activated Ca(2+)-permeable ion channels. First, transients were reversibly abolished by chelating extracellular Ca(2+), demonstrating a requirement for Ca(2+) influx across the plasma membrane. Second, Ca(2+) transients were initially observed near the plasma membrane in cytoplasmic processes. Third, voltage-activated Ca(2+) channel (VACC) antagonists reduced transients by half, suggesting that swelling-activated channels depolarize plasma membranes to activate VACCs. Finally, emptying internal Ca(2+) stores attenuated transients by 80%, suggesting Ca(2+) release from stores augments swelling-activated Ca(2+) signals. To identify candidate mechanotransduction channels, we used RT-PCR to amplify ion-channel transcripts whose pharmacological profiles matched those of hypotonic-evoked Ca(2+) signals in Merkel cells. We found 11 amplicons, including PKD1, PKD2, and TRPC1, channels previously implicated in mechanotransduction in other cells. Collectively, these results directly demonstrate that Merkel cells are activated by hypotonic-evoked swelling, identify cellular signaling mechanisms that mediate these responses, and support the hypothesis that Merkel cells contribute to touch reception in the Merkel cell-neurite complex.