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Thuma JB, Hooper SL. Choline and NMDG directly reduce outward currents: reduced outward current when these substances replace Na + is alone not evidence of Na +-activated K + currents. J Neurophysiol 2018; 120:3217-3233. [PMID: 30354793 DOI: 10.1152/jn.00871.2017] [Citation(s) in RCA: 7] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/19/2022] Open
Abstract
Choline chloride is often, and N-methyl-d-glucamine (NMDG) sometimes, used to replace sodium chloride in studies of sodium-activated potassium channels. Given the high concentrations used in sodium replacement protocols, it is essential to test that it is not the replacement substances themselves, as opposed to the lack of sodium, that cause any observed effects. We therefore compared, in lobster stomatogastric neurons and leech Retzius cells, the effects of applying salines in which choline chloride replaced sodium chloride, and in which choline hydroxide or sucrose was added to normal saline. We also tested, in stomatogastric neurons, the effect of adding NMDG to normal saline. These protocols allowed us to measure the direct effects (i.e., effects not due to changes in sodium concentration or saline osmolarity or ionic strength) of choline on stomatogastric and leech currents, and of NMDG on stomatogastric currents. Choline directly reduced transient and sustained depolarization-activated outward currents in both species, and NMDG directly reduced transient depolarization-activated outward currents in stomatogastric neurons. Experiments with lower choline concentrations showed that adding as little as 150 mM (stomatogastric) or 5 mM (leech) choline reduced at least some depolarization-activated outward currents. Reductions in outward current with choline chloride or NMDG replacement alone are thus not evidence of sodium-activated potassium currents. NEW & NOTEWORTHY We show that choline or N-methyl-d-glucamine (NMDG) directly (i.e., not due to changes in extracellular sodium) decrease outward currents. Prior work studying sodium-activated potassium channels in which sodium was replaced with choline or NMDG without an addition control may therefore be artifactual.
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Affiliation(s)
- Jeffrey B Thuma
- Department of Biological Sciences, Irvine Hall, Ohio University , Athens, Ohio
| | - Scott L Hooper
- Department of Biological Sciences, Irvine Hall, Ohio University , Athens, Ohio
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Koos B, Christmann J, Plettenberg S, Käding D, Becker J, Keteku M, Klein C, Imtiaz S, Janning P, Bastiaens PIH, Wehner F. Hypertonicity-induced cation channels in HepG2 cells: architecture and role in proliferation vs. apoptosis. J Physiol 2018; 596:1227-1241. [PMID: 29369356 DOI: 10.1113/jp275827] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/10/2018] [Accepted: 01/18/2018] [Indexed: 12/25/2022] Open
Abstract
KEY POINTS Na+ conducting hypertonicity-induced cation channels (HICCs) are key players in the volume restoration of osmotically shrunken cells and, under isotonic conditions, considered as mediators of proliferation - thereby opposing apoptosis. In an siRNA screen of ion channels and transporters in HepG2 cells, with the regulatory volume increase (RVI) as read-out, δENaC, TRPM2 and TRPM5 were identified as HICCs. Subsequently, all permutations of these channels were tested in RVI and patch-clamp recordings and, at first sight, HICCs were found to operate in an independent mode. However, there was synergy in the siRNA perturbations of HICC currents. Accordingly, proximity ligation assays showed that δENaC was located in proximity to TRPM2 and TRPM5 suggesting a physical interaction. Furthermore, δENaC, TRPM2 and TRPM5 were identified as mediators of HepG2 proliferation - their silencing enhanced apoptosis. Our study defines the architecture of HICCs in human hepatocytes as well as their molecular functions. ABSTRACT Hypertonicity-induced cation channels (HICCs) are a substantial element in the regulatory volume increase (RVI) of osmotically shrunken cells. Under isotonic conditions, they are key effectors in the volume gain preceding proliferation; HICC repression, in turn, significantly increases apoptosis rates. Despite these fundamental roles of HICCs in cell physiology, very little is known concerning the actual molecular architecture of these channels. Here, an siRNA screening of putative ion channels and transporters was performed, in HepG2 cells, with the velocity of RVI as the read-out; in this first run, δENaC, TRPM2 and TRPM5 could be identified as HICCs. In the second run, all permutations of these channels were tested in RVI and patch-clamp recordings, with special emphasis on the non-additivity and additivity of siRNAs - which would indicate molecular interactions or independent ways of channel functioning. At first sight, the HICCs in HepG2 cells appeared to operate rather independently. However, a proximity ligation assay revealed that δENaC was located in proximity to both TRPM2 and TRPM5. Furthermore, a clear synergy of HICC current knock-downs (KDs) was observed. δENaC, TRPM2 and TRPM5 were defined as mediators of HepG2 cell proliferation and their silencing increased the rates of apoptosis. This study provides a molecular characterization of the HICCs in human hepatocytes and of their role in RVI, cell proliferation and apoptosis.
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Affiliation(s)
- Björn Koos
- Max Planck Institute of Molecular Physiology, Department of Systemic Cell Biology, Otto-Hahn-Strasse 11, 44227, Dortmund, Germany
| | - Jens Christmann
- Max Planck Institute of Molecular Physiology, Department of Systemic Cell Biology, Otto-Hahn-Strasse 11, 44227, Dortmund, Germany
| | - Sandra Plettenberg
- Max Planck Institute of Molecular Physiology, Department of Systemic Cell Biology, Otto-Hahn-Strasse 11, 44227, Dortmund, Germany
| | - Domenic Käding
- Max Planck Institute of Molecular Physiology, Department of Systemic Cell Biology, Otto-Hahn-Strasse 11, 44227, Dortmund, Germany
| | - Julia Becker
- Max Planck Institute of Molecular Physiology, Department of Systemic Cell Biology, Otto-Hahn-Strasse 11, 44227, Dortmund, Germany
| | - Melody Keteku
- Max Planck Institute of Molecular Physiology, Department of Systemic Cell Biology, Otto-Hahn-Strasse 11, 44227, Dortmund, Germany
| | - Christian Klein
- Max Planck Institute of Molecular Physiology, Department of Systemic Cell Biology, Otto-Hahn-Strasse 11, 44227, Dortmund, Germany
| | - Sarah Imtiaz
- Max Planck Institute of Molecular Physiology, Department of Systemic Cell Biology, Otto-Hahn-Strasse 11, 44227, Dortmund, Germany
| | - Petra Janning
- Max Planck Institute of Molecular Physiology, Department of Systemic Cell Biology, Otto-Hahn-Strasse 11, 44227, Dortmund, Germany
| | - Philippe I H Bastiaens
- Max Planck Institute of Molecular Physiology, Department of Systemic Cell Biology, Otto-Hahn-Strasse 11, 44227, Dortmund, Germany
| | - Frank Wehner
- Max Planck Institute of Molecular Physiology, Department of Systemic Cell Biology, Otto-Hahn-Strasse 11, 44227, Dortmund, Germany
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Numata T, Sato K, Christmann J, Marx R, Mori Y, Okada Y, Wehner F. The ΔC splice-variant of TRPM2 is the hypertonicity-induced cation channel in HeLa cells, and the ecto-enzyme CD38 mediates its activation. J Physiol 2012; 590:1121-38. [PMID: 22219339 PMCID: PMC3381820 DOI: 10.1113/jphysiol.2011.220947] [Citation(s) in RCA: 25] [Impact Index Per Article: 2.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/30/2011] [Accepted: 12/31/2011] [Indexed: 12/13/2022] Open
Abstract
Hypertonicity-induced cation channels (HICCs) are key-players in proliferation and apoptosis but their molecular correlate remains obscure. Furthermore, the activation profile of HICCs is not well defined yet. We report here that, in HeLa cells, intracellular adenosine diphosphate ribose (ADPr) and cyclic ADPr (cADPr), as supposed activators of TRPM2, elicited cation currents that were virtually identical to the osmotic activation of HICCs. Silencing of the expression of TRPM2 and of the ecto-enzyme CD38 (as a likely source of ADPr and cADPr) inhibited HICC as well as nucleotide-induced currents and, in parallel, the hypertonic volume response of cells (the regulatory volume increase, RVI) was attenuated. Quantification of intracellular cADPr levels and the systematic application of extra- vs. intracellular nucleotides indicate that the outwardly directed gradient rather than the cellular activity of ADPr and cADPr triggers TRPM2 activation, probably along with a simultaneous biotransformation of nucleotides.Cloning of TRPM2 identified the ΔC-splice variant as the molecular correlate of the HICC, which could be strongly supported by a direct comparison of the respective Ca²⁺ selectivity. Finally, immunoprecipitation and high-resolution FRET/FLIM imaging revealed the interaction of TRPM2 and CD38 in the native as well as in a heterologous (HEK293T) expression system. We propose transport-related nucleotide export via CD38 as a novel mechanism of TRPM2/HICC activation. With the biotransformation of nucleotides running in parallel, continuous zero trans-conditions are achieved which will render the system infinitely sensitive.
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Affiliation(s)
- Tomohiro Numata
- Department of Cell Physiology, National Institute of Physiological Sciences, Okazaki 444–8585, Japan
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Wang Z, Wong NC, Cheng Y, Kehl SJ, Fedida D. Control of voltage-gated K+ channel permeability to NMDG+ by a residue at the outer pore. ACTA ACUST UNITED AC 2010; 133:361-74. [PMID: 19332619 PMCID: PMC2699102 DOI: 10.1085/jgp.200810139] [Citation(s) in RCA: 15] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022]
Abstract
Crystal structures of potassium (K(+)) channels reveal that the selectivity filter, the narrow portion of the pore, is only approximately 3-A wide and buttressed from behind, so that its ability to expand is highly constrained, and the permeation of molecules larger than Rb(+) (2.96 A in diameter) is prevented. N-methyl-d-glucamine (NMDG(+)), an organic monovalent cation, is thought to be a blocker of Kv channels, as it is much larger (approximately 7.3 A in mean diameter) than K(+) (2.66 A in diameter). However, in the absence of K(+), significant NMDG(+) currents could be recorded from human embryonic kidney cells expressing Kv3.1 or Kv3.2b channels and Kv1.5 R487Y/V, but not wild-type channels. Inward currents were much larger than outward currents due to the presence of intracellular Mg(2+) (1 mM), which blocked the outward NMDG(+) current, resulting in a strong inward rectification. The NMDG(+) current was inhibited by extracellular 4-aminopyridine (5 mM) or tetraethylammonium (10 mM), and largely eliminated in Kv3.2b by an S6 mutation that prevents the channel from opening (P468W) and by a pore helix mutation in Kv1.5 R487Y (W472F) that inactivates the channel at rest. These data indicate that NMDG(+) passes through the open ion-conducting pore and suggest a very flexible nature of the selectivity filter itself. 0.3 or 1 mM K(+) added to the external NMDG(+) solution positively shifted the reversal potential by approximately 16 or 31 mV, respectively, giving a permeability ratio for K(+) over NMDG(+) (P(K)(+)/P(NMDG)(+)) of approximately 240. Reversal potential shifts in mixtures of K(+) and NMDG(+) are in accordance with P(K)(+)/P(NMDG)(+), indicating that the ions compete for permeation and suggesting that NMDG(+) passes through the open state. Comparison of the outer pore regions of Kv3 and Kv1.5 channels identified an Arg residue in Kv1.5 that is replaced by a Tyr in Kv3 channels. Substituting R with Y or V allowed Kv1.5 channels to conduct NMDG(+), suggesting a regulation by this outer pore residue of Kv channel flexibility and, as a result, permeability.
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Affiliation(s)
- Zhuren Wang
- Department of Anesthesiology, Pharmacology, and Therapeutics, University of British Columbia, Vancouver, British Columbia V6T 1Z3, Canada
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Hoffmann EK, Lambert IH, Pedersen SF. Physiology of cell volume regulation in vertebrates. Physiol Rev 2009; 89:193-277. [PMID: 19126758 DOI: 10.1152/physrev.00037.2007] [Citation(s) in RCA: 1014] [Impact Index Per Article: 67.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/25/2022] Open
Abstract
The ability to control cell volume is pivotal for cell function. Cell volume perturbation elicits a wide array of signaling events, leading to protective (e.g., cytoskeletal rearrangement) and adaptive (e.g., altered expression of osmolyte transporters and heat shock proteins) measures and, in most cases, activation of volume regulatory osmolyte transport. After acute swelling, cell volume is regulated by the process of regulatory volume decrease (RVD), which involves the activation of KCl cotransport and of channels mediating K(+), Cl(-), and taurine efflux. Conversely, after acute shrinkage, cell volume is regulated by the process of regulatory volume increase (RVI), which is mediated primarily by Na(+)/H(+) exchange, Na(+)-K(+)-2Cl(-) cotransport, and Na(+) channels. Here, we review in detail the current knowledge regarding the molecular identity of these transport pathways and their regulation by, e.g., membrane deformation, ionic strength, Ca(2+), protein kinases and phosphatases, cytoskeletal elements, GTP binding proteins, lipid mediators, and reactive oxygen species, upon changes in cell volume. We also discuss the nature of the upstream elements in volume sensing in vertebrate organisms. Importantly, cell volume impacts on a wide array of physiological processes, including transepithelial transport; cell migration, proliferation, and death; and changes in cell volume function as specific signals regulating these processes. A discussion of this issue concludes the review.
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Affiliation(s)
- Else K Hoffmann
- Department of Biology, University of Copenhagen, Copenhagen, Denmark.
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Bondarava M, Li T, Endl E, Wehner F. alpha-ENaC is a functional element of the hypertonicity-induced cation channel in HepG2 cells and it mediates proliferation. Pflugers Arch 2009; 458:675-87. [PMID: 19241091 PMCID: PMC2704294 DOI: 10.1007/s00424-009-0649-z] [Citation(s) in RCA: 43] [Impact Index Per Article: 2.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/14/2009] [Revised: 02/04/2009] [Accepted: 02/08/2009] [Indexed: 11/25/2022]
Abstract
The molecular correlate of hypertonicity-induced cation channels (HICCs) and their role in proliferation vs. apoptosis is a matter of debate. We report in this paper that, in whole-cell patch-clamp recordings, hypertonic stress (340→450 mosM) reversibly increased the Na+ conductance of HepG2 cells from 0.8 to 5.8 nS. The effect was dose-dependently inhibited by flufenamate and amiloride, known blockers of HICCs, with some 50% efficiency at 300 μM. In parallel, both drugs decreased HepG2 cell proliferation [in 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assays and with automatic cell counting]. Small interfering RNA (siRNA) silencing of the α-subunit of the epithelial Na+ channel (ENaC) reduced hypertonicity-induced Na+ currents to 60%, whereas the rate of HepG2 cell proliferation was approximately half of that of the control. Moreover, α-ENaC siRNA inhibited the regulatory volume increase of HepG2 cells (measured with scanning acoustic microscopy) by 60%. In florescence-activated cell sorting measurements, silencing of α-ENaC led to a significant decrease in the G1 and an increase in the G2/M phase of the cell cycle, whereas the S phase was not changing. Finally (determined by a caspase 3/7 assay), HICC inhibition by flufenamate and silencing of α-ENaC increased the rate of apoptosis in HepG2 cells. It is concluded that α-ENaC is one functional element of the HICC in HepG2 cells and that the channel is an important mediator of cell proliferation; likewise, HICC blockage shifts the system from a proliferative into a rather apoptotic one. This is the first report of a role of α-ENaC in cell proliferation.
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Affiliation(s)
- Maryna Bondarava
- Department of Systemic Cell Biology, Max Planck Institute of Molecular Physiology, Otto-Hahn-Strasse 11, 44227 Dortmund, Germany
| | - Tongju Li
- Department of Systemic Cell Biology, Max Planck Institute of Molecular Physiology, Otto-Hahn-Strasse 11, 44227 Dortmund, Germany
- Institute of Physiological Chemistry, University of Essen, Essen, Germany
| | - Elmar Endl
- Institute of Molecular Physiology and Experimental Immunology, University of Bonn, Bonn, Germany
| | - Frank Wehner
- Department of Systemic Cell Biology, Max Planck Institute of Molecular Physiology, Otto-Hahn-Strasse 11, 44227 Dortmund, Germany
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Wehner F, Bondarava M, ter Veld F, Endl E, Nürnberger HR, Li T. Hypertonicity-induced cation channels. Acta Physiol (Oxf) 2006; 187:21-5. [PMID: 16734739 DOI: 10.1111/j.1748-1716.2006.01561.x] [Citation(s) in RCA: 31] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/26/2022]
Abstract
Whenever studied in a quantitative fashion, hypertonicity-induced cation channels (HICCs) are found to be the main mediators of regulatory volume increase. In most instances, these channels are either inhibited by amiloride (but insensitive to Gd3+ and flufenamate) or they are efficiently blocked by Gd3+ and flufenamate (but insensitive to amiloride). Of note, however, from two preparations so far a mixed type of pharmacology has also been reported. Whereas the ion selectivity of amiloride-sensitive HICCs has not been studied in much detail yet, amiloride-insensitive channels are either equally permeable to Na+, K+, Cs+ and Li+ but impermeable to N-methyl-D-glucamine (NMDG+) or they exhibit a permeability to Li+ and NMDG+ that amounts to some 50% when compared with that of Na+. Also in this respect, however, some peculiarities do exist. Concerning the actual molecular correlate, evidence was reported that HICCs may be related to the (amiloride-sensitive) epithelial Na+ channel and/or to transient receptor potential channels. Recent findings suggest that HICCs may contribute to cell proliferation, just as the K+ channels that are employed in regulatory volume decrease are mediators of the opposing process, i.e. apoptosis.
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Affiliation(s)
- F Wehner
- Max-Planck-Institut für molekulare Physiologie, Dortmund, Germany
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