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Swelling-activated Ca2+ channels trigger Ca2+ signals in Merkel cells.

Haeberle H, Bryan LA, Vadakkan TJ, Dickinson ME, Lumpkin EA - PLoS ONE (2008)

Bottom Line: 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.Third, voltage-activated Ca2+ channel (VACC) antagonists reduced transients by half, suggesting that swelling-activated channels depolarize plasma membranes to activate VACCs.We found 11 amplicons, including PKD1, PKD2, and TRPC1, channels previously implicated in mechanotransduction in other cells.

View Article: PubMed Central - PubMed

Affiliation: Neuroscience Graduate Program, University of California San Francisco, San Francisco, California, United States of America.

ABSTRACT
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 Ca2+ imaging with fura-2. We found that hypotonic-induced cell swelling, but not hypertonic solutions, triggered cytoplasmic Ca2+ transients. Several lines of evidence indicate that these signals arise from swelling-activated Ca2+-permeable ion channels. First, transients were reversibly abolished by chelating extracellular Ca2+, demonstrating a requirement for Ca2+ influx across the plasma membrane. Second, Ca2+ transients were initially observed near the plasma membrane in cytoplasmic processes. Third, voltage-activated Ca2+ channel (VACC) antagonists reduced transients by half, suggesting that swelling-activated channels depolarize plasma membranes to activate VACCs. Finally, emptying internal Ca2+ stores attenuated transients by 80%, suggesting Ca2+ release from stores augments swelling-activated Ca2+ signals. To identify candidate mechanotransduction channels, we used RT-PCR to amplify ion-channel transcripts whose pharmacological profiles matched those of hypotonic-evoked Ca2+ 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.

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A model of hypotonic-induced Ca2+ transients in Merkel cells.Hypotonic solutions stimulate water influx across the cell membrane, causing cell swelling. The increased tension on the membrane activates Ca2+ permeable channels. These channels both contribute to a rise in Ca2+ influx and depolarize the membrane, opening VACCs. VACCs and Ca2+ release from internal stores amplify Ca2+ signals.
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pone-0001750-g007: A model of hypotonic-induced Ca2+ transients in Merkel cells.Hypotonic solutions stimulate water influx across the cell membrane, causing cell swelling. The increased tension on the membrane activates Ca2+ permeable channels. These channels both contribute to a rise in Ca2+ influx and depolarize the membrane, opening VACCs. VACCs and Ca2+ release from internal stores amplify Ca2+ signals.

Mentions: This study demonstrates that dissociated Merkel cells are directly activated by hypotonic-evoked cell swelling, and introduces a robust in vitro assay for analyzing the molecular mechanisms that underlie swelling-evoked signals. Based on our findings, we propose that swelling triggers Ca2+ entry through an as yet unknown cation channel; the resultant depolarization activates VACCs and together these two sources of Ca2+ influx activate Ca2+ release from internal stores (Fig. 7). We identified 11 TRP channels expressed in Merkel-cells, seven of which have pharmacological profiles matching the hypotonic-evoked Ca2+ responses we observed. Of these, PKD1 and PKD2 are promising candidates because they have been previously implicated in mechanotransduction in cilia of kidney cells [36]. Our data support a model in which skin indentation applies force to Merkel cells, whose mechanosensitive channels allow Ca2+ influx. This Ca2+ influx is augmented by VACCs and Ca2+ release from internal stores. These Ca2+ increases could trigger synaptic signaling to the underlying sensory afferent.


Swelling-activated Ca2+ channels trigger Ca2+ signals in Merkel cells.

Haeberle H, Bryan LA, Vadakkan TJ, Dickinson ME, Lumpkin EA - PLoS ONE (2008)

A model of hypotonic-induced Ca2+ transients in Merkel cells.Hypotonic solutions stimulate water influx across the cell membrane, causing cell swelling. The increased tension on the membrane activates Ca2+ permeable channels. These channels both contribute to a rise in Ca2+ influx and depolarize the membrane, opening VACCs. VACCs and Ca2+ release from internal stores amplify Ca2+ signals.
© Copyright Policy
Related In: Results  -  Collection

Show All Figures
getmorefigures.php?uid=PMC2365925&req=5

pone-0001750-g007: A model of hypotonic-induced Ca2+ transients in Merkel cells.Hypotonic solutions stimulate water influx across the cell membrane, causing cell swelling. The increased tension on the membrane activates Ca2+ permeable channels. These channels both contribute to a rise in Ca2+ influx and depolarize the membrane, opening VACCs. VACCs and Ca2+ release from internal stores amplify Ca2+ signals.
Mentions: This study demonstrates that dissociated Merkel cells are directly activated by hypotonic-evoked cell swelling, and introduces a robust in vitro assay for analyzing the molecular mechanisms that underlie swelling-evoked signals. Based on our findings, we propose that swelling triggers Ca2+ entry through an as yet unknown cation channel; the resultant depolarization activates VACCs and together these two sources of Ca2+ influx activate Ca2+ release from internal stores (Fig. 7). We identified 11 TRP channels expressed in Merkel-cells, seven of which have pharmacological profiles matching the hypotonic-evoked Ca2+ responses we observed. Of these, PKD1 and PKD2 are promising candidates because they have been previously implicated in mechanotransduction in cilia of kidney cells [36]. Our data support a model in which skin indentation applies force to Merkel cells, whose mechanosensitive channels allow Ca2+ influx. This Ca2+ influx is augmented by VACCs and Ca2+ release from internal stores. These Ca2+ increases could trigger synaptic signaling to the underlying sensory afferent.

Bottom Line: 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.Third, voltage-activated Ca2+ channel (VACC) antagonists reduced transients by half, suggesting that swelling-activated channels depolarize plasma membranes to activate VACCs.We found 11 amplicons, including PKD1, PKD2, and TRPC1, channels previously implicated in mechanotransduction in other cells.

View Article: PubMed Central - PubMed

Affiliation: Neuroscience Graduate Program, University of California San Francisco, San Francisco, California, United States of America.

ABSTRACT
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 Ca2+ imaging with fura-2. We found that hypotonic-induced cell swelling, but not hypertonic solutions, triggered cytoplasmic Ca2+ transients. Several lines of evidence indicate that these signals arise from swelling-activated Ca2+-permeable ion channels. First, transients were reversibly abolished by chelating extracellular Ca2+, demonstrating a requirement for Ca2+ influx across the plasma membrane. Second, Ca2+ transients were initially observed near the plasma membrane in cytoplasmic processes. Third, voltage-activated Ca2+ channel (VACC) antagonists reduced transients by half, suggesting that swelling-activated channels depolarize plasma membranes to activate VACCs. Finally, emptying internal Ca2+ stores attenuated transients by 80%, suggesting Ca2+ release from stores augments swelling-activated Ca2+ signals. To identify candidate mechanotransduction channels, we used RT-PCR to amplify ion-channel transcripts whose pharmacological profiles matched those of hypotonic-evoked Ca2+ 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.

Show MeSH
Related in: MedlinePlus