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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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TRPV4 is not required for hypotonic-evoked [Ca2+] transients in Merkel cells.The paired histograms display peak osmotic responses (N = 37–43 cells).
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pone-0001750-g005: TRPV4 is not required for hypotonic-evoked [Ca2+] transients in Merkel cells.The paired histograms display peak osmotic responses (N = 37–43 cells).

Mentions: Hypotonic induced extracellular Ca2+ influx implies the presence of a swelling-activated ion channel in the plasma membrane but does not speak to its identity. The most obvious candidate is TRPV4 (GenBank accession number NM_022017), which is expressed in Merkel cells [32] and is activated by hypotonic solutions when expressed in human embryonic kidney (HEK) cells [33], [34]. To determine if TRPV4 is required for hypotonic activation, we analyzed hypotonic-induced Ca2+ transients in Merkel cells from TRPV4-deficient mice (Fig. 5). The magnitude and time course of Ca2+ transients in Merkel cells from TRPV4 -/- mice were indistinguishable from those of heterozygous littermate controls (Fig. 5) and from wild-type responses. Thus, TRPV4 is unlikely to mediate hypotonic induced Ca2+ influx in Merkel cells.


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

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

TRPV4 is not required for hypotonic-evoked [Ca2+] transients in Merkel cells.The paired histograms display peak osmotic responses (N = 37–43 cells).
© Copyright Policy
Related In: Results  -  Collection

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

pone-0001750-g005: TRPV4 is not required for hypotonic-evoked [Ca2+] transients in Merkel cells.The paired histograms display peak osmotic responses (N = 37–43 cells).
Mentions: Hypotonic induced extracellular Ca2+ influx implies the presence of a swelling-activated ion channel in the plasma membrane but does not speak to its identity. The most obvious candidate is TRPV4 (GenBank accession number NM_022017), which is expressed in Merkel cells [32] and is activated by hypotonic solutions when expressed in human embryonic kidney (HEK) cells [33], [34]. To determine if TRPV4 is required for hypotonic activation, we analyzed hypotonic-induced Ca2+ transients in Merkel cells from TRPV4-deficient mice (Fig. 5). The magnitude and time course of Ca2+ transients in Merkel cells from TRPV4 -/- mice were indistinguishable from those of heterozygous littermate controls (Fig. 5) and from wild-type responses. Thus, TRPV4 is unlikely to mediate hypotonic induced Ca2+ influx in Merkel cells.

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