Mutations in the pore region of ROMK enhance Ba2+ block

1996 ◽  
Vol 271 (6) ◽  
pp. C1949-C1956 ◽  
Author(s):  
H. Zhou ◽  
S. Chepilko ◽  
W. Schutt ◽  
H. Choe ◽  
L. G. Palmer ◽  
...  
Keyword(s):  
Single Channel ◽  
Ion Selectivity ◽  
Inward Rectification ◽  
Xenopus Laevis Oocytes ◽  
Channel Measurements ◽  
Pore Region ◽  
Selective Channel ◽  
Increased Sensitivity ◽  
Membrane Spanning ◽  
Zero Voltage

The sequence of the hydrophobic “P” (pore) region of a K(+)-selective channel from the kidney (ROMK2) was altered to match that of the closely related inward rectifier (IRK1) channel by changing two amino acids, leucine (L) 117 and valine (V) 121, to isoleucine (I) and threonine (T), respectively. The mutant channel expressed in Xenopus laevis oocytes had an apparent inhibition constant at zero voltage [Ki(0)] in the presence of Ba2+ of 0.07 +/- 0.01 mM, which was more than 50 times lower than the Ki(0) of the wild-type channel (4.7 +/- 1.0 mM). The increased sensitivity to Ba2+ was accounted for by the point mutation V121T. Single-channel measurements indicated that the increased affinity involved an increase in the on-rate for Ba2+ block and a decrease in the off-rate. Block by Ca+ was also enhanced. The single-channel conductance of the L1171/ V121T mutant was increased by 50%, whereas the degree of inward rectification, ion selectivity, and apparent affinity for K+ were essentially unchanged. When the neutral asparagine residue within the second putative membrane-spanning domain of the ROMK channel was substituted with aspartic acid, the corresponding amino acid in IRK1, the degree of inward rectification was enhanced but Ba2+ block and single-channel inward conductance were unaffected. Thus the site of Ba2+ binding appears to be distinct from the locus of internal Mg2+ block and from at least one of the sites that determines K+ conjuctivity.

scholarly journals A Point Mutation in the Pore Region Alters Gating, Ca2+Blockage, and Permeation of Olfactory Cyclic Nucleotide–Gated Channels

2000 ◽  
Vol 116 (3) ◽  
pp. 311-326 ◽  
Author(s):  
Paola Gavazzo ◽  
Cristiana Picco ◽  
Elisabeth Eismann ◽  
U. Benjamin Kaupp ◽  
Anna Menini
Keyword(s):  
Cyclic Nucleotide ◽  
Single Channel ◽  
Channel Gating ◽  
Cation Binding ◽  
Xenopus Laevis Oocytes ◽  
Wild Type ◽  
Pore Region ◽  
Α Subunit ◽  
Cyclic Nucleotide Gated Channels ◽  
Cation Binding Site

Upon stimulation by odorants, Ca2+ and Na+ enter the cilia of olfactory sensory neurons through channels directly gated by cAMP. Cyclic nucleotide–gated channels have been found in a variety of cells and extensively investigated in the past few years. Glutamate residues at position 363 of the α subunit of the bovine retinal rod channel have previously been shown to constitute a cation-binding site important for blockage by external divalent cations and to control single-channel properties. It has therefore been assumed, but not proven, that glutamate residues at the corresponding position of the other cyclic nucleotide–gated channels play a similar role. We studied the corresponding glutamate (E340) of the α subunit of the bovine olfactory channel to determine its role in channel gating and in permeation and blockage by Ca2+ and Mg2+. E340 was mutated into either an aspartate, glycine, glutamine, or asparagine residue and properties of mutant channels expressed in Xenopus laevis oocytes were measured in excised patches. By single-channel recordings, we demonstrated that the open probabilities in the presence of cGMP or cAMP were decreased by the mutations, with a larger decrease observed on gating by cAMP. Moreover, we observed that the mutant E340N presented two conductance levels. We found that both external Ca2+ and Mg2+ powerfully blocked the current in wild-type and E340D mutants, whereas their blockage efficacy was drastically reduced when the glutamate charge was neutralized. The inward current carried by external Ca2+ relative to Na+ was larger in the E340G mutant compared with wild-type channels. In conclusion, we have confirmed that the residue at position E340 of the bovine olfactory CNG channel is in the pore region, controls permeation and blockage by external Ca2+ and Mg2+, and affects channel gating by cAMP more than by cGMP.

scholarly journals Single-channel Ca2+ imaging implicates Aβ1–42 amyloid pores in Alzheimer’s disease pathology

2011 ◽  
Vol 195 (3) ◽  
pp. 515-524 ◽  
Author(s):  
Angelo Demuro ◽  
Martin Smith ◽  
Ian Parker
Keyword(s):  
Alzheimer’S Disease ◽  
Alzheimer's Disease ◽  
Ion Channels ◽  
Single Channel ◽  
Membrane Integrity ◽  
Cytosolic Calcium ◽  
Xenopus Laevis Oocytes ◽  
Permeable Membrane ◽  
Membrane Pores ◽  
Pathological Mechanism

Oligomeric forms of Aβ peptides are implicated in Alzheimer’s disease (AD) and disrupt membrane integrity, leading to cytosolic calcium (Ca2+) elevation. Proposed mechanisms by which Aβ mediates its effects include lipid destabilization, activation of native membrane channels, and aggregation of Aβ into Ca2+-permeable pores. We distinguished between these using total internal reflection fluorescence (TIRF) microscopy to image Ca2+ influx in Xenopus laevis oocytes. Aβ1–42 oligomers evoked single-channel Ca2+ fluorescence transients (SCCaFTs), which resembled those from classical ion channels but which were not attributable to endogenous oocyte channels. SCCaFTs displayed widely variable open probabilities (Po) and stepwise transitions among multiple amplitude levels reminiscent of subconductance levels of ion channels. The proportion of high Po, large amplitude SCCaFTs grew with time, suggesting that continued oligomer aggregation results in the formation of highly toxic pores. We conclude that formation of intrinsic Ca2+-permeable membrane pores is a major pathological mechanism in AD and introduce TIRF imaging for massively parallel single-channel studies of the incorporation, assembly, and properties of amyloidogenic oligomers.

Electric field tomography: setup for single-channel measurements

2007 ◽  
Vol 28 (7) ◽  
pp. S279-S289 ◽  
Author(s):  
A V Korjenevsky ◽  
T S Tuykin
Keyword(s):  
Electric Field ◽  
Single Channel ◽  
Channel Measurements

Characterization of a newly found stretch-activated KCa,ATP channel in cultured chick ventricular myocytes

1999 ◽  
Vol 276 (6) ◽  
pp. H1827-H1838 ◽  
Author(s):  
Takashi Kawakubo ◽  
Keiji Naruse ◽  
Tatsuaki Matsubara ◽  
Nigishi Hotta ◽  
Masahiro Sokabe
Keyword(s):  
Single Channel ◽  
Ventricular Myocytes ◽  
Heart Cells ◽  
Patch Clamp Technique ◽  
Selective Channel ◽  
New Type ◽  
Inside Out ◽  
Ion Selectivities ◽  
Channel Type

With the use of the patch-clamp technique, five kinds of stretch-activated (SA) ion channels were identified on the basis of their single-channel conductances and ion selectivities in cultured chick ventricular myocytes. Because a high-conductance K+-selective channel predominated among these channels, we concentrated on characterizing its properties mostly using excised inside-out patches. With 145 mM KCl solution in the pipette and the bath, the channel had a conductance of 199.8 ± 8.2 pS ( n = 22). The ion selectivities among K+, Na+, Ca2+, and Cl− as estimated from their permeability ratios were P Na/ P K= 0.03, P Ca/ P K= 0.025, and P Cl/ P K= 0.026. The probability of the channel being open (Po) increased with the Ca2+concentration in the bath ([Ca2+]b; dissociation constant K d = 0.51 μM at +30 mV) and membrane potential (voltage at half-maximal Po= 39.4 mV at 0.35 μM [Ca2+]b). The channel was blocked by gadolinium, tetraethylammonium, and charybdotoxin from the extracellular surface and, consequently, was identified as a Ca2+-activated K+(KCa) channel type. The channel was also reversibly activated by ATP applied to the intracellular surface ( K d = 0.74 mM at 0.10 μM [Ca2+]bat +30 mV). From these data taken together, we concluded that the channel is a new type of KCachannel that could be designated as an “SA KCa,ATP channel.” To our knowledge, this is the first report of KCa channel in heart cells.

scholarly journals Dynamic Interaction of Norfloxacin to Magnesium Monitored by Bacterial Outer Membrane Nanopores

2020 ◽  
Author(s):  
Jiajun Wang ◽  
Jigneshkumar Dahyabhai Prajapati ◽  
Ulrich Kleinekathöfer ◽  
Mathias Winterhalter
Keyword(s):  
Outer Membrane ◽  
Lipid Bilayers ◽  
Brownian Dynamics ◽  
Single Channel ◽  
Ion Selectivity ◽  
Computational Modelling ◽  
Free Energy Calculations ◽  
Divalent Ions ◽  
Dynamics Simulations ◽  
Brownian Dynamics Simulations

The effect of divalent ions on the permeability of norfloxacin across the major outer membrane channels from <i>E. coli</i> (OmpF, OmpC) and <i>E. aerogenes</i> (Omp35, Omp36) has been investigated at the single channel level. To understand the rate limiting steps in permeation, we reconstituted single porin into planar lipid bilayers and analyzed the ion current fluctuations caused in the presence of norfloxacin. To obtain an atomistic view, we complemented the experiments with millisecond-long free energy calculations based on temperature-accelerated Brownian dynamics simulations to identify the most probable permeation pathways of the antibiotics through the respective pore. Both, experimental analysis and computational modelling, suggest that norfloxacin is able to permeate through the larger porins, i.e., OmpF, OmpC, and Omp35, whereas it only binds to the slightly narrower porin Omp36. Moreover, divalent ions can bind to negatively charged residues inside the porin, reversing the ion selectivity of the pore. In addition, the divalent ions can chelate with the fluoroquinolones and alter their physicochemical properties. The results suggest that the conjugation must break with either one of them when the antibiotics molecules bypass the lumen of the porin, with the conjugation to the antibiotic being more stable than that to the pore. In general, the permeation or binding process of fluoroquinolone in porins occurs irrespective of the presence of divalent ions, but the presences of divalent ions can vary the kinetics significantly.

scholarly journals Identification of Amino Acid Residues in the α, β, and γ Subunits of the Epithelial Sodium Channel (ENaC) Involved in Amiloride Block and Ion Permeation

1997 ◽  
Vol 109 (1) ◽  
pp. 15-26 ◽  
Author(s):  
Laurent Schild ◽  
Estelle Schneeberger ◽  
Ivan Gautschi ◽  
Dmitri Firsov
Keyword(s):  
Amino Acid ◽  
Ion Selectivity ◽  
Site Directed Mutagenesis ◽  
Ion Permeation ◽  
Amino Acid Residues ◽  
Α Subunit ◽  
Na Ions ◽  
Membrane Spanning ◽  
A Site ◽  
Epithelial Sodium Channel Enac

The amiloride-sensitive epithelial Nachannel (ENaC) is a heteromultimeric channel made of three αβγ subunits. The structures involved in the ion permeation pathway have only been partially identified, and the respective contributions of each subunit in the formation of the conduction pore has not yet been established. Using a site-directed mutagenesis approach, we have identified in a short segment preceding the second membrane-spanning domain (the pre-M2 segment) amino acid residues involved in ion permeation and critical for channel block by amiloride. Cys substitutions of Gly residues in β and γ subunits at position βG525 and γG537 increased the apparent inhibitory constant (Ki) for amiloride by &gt;1,000-fold and decreased channel unitary current without affecting ion selectivity. The corresponding mutation S583 to C in the α subunit increased amiloride Ki by 20-fold, without changing channel conducting properties. Coexpression of these mutated αβγ subunits resulted in a nonconducting channel expressed at the cell surface. Finally, these Cys substitutions increased channel affinity for block by externalZn2+ ions, in particular the αS583C mutant showing a Ki for Zn2+of 29 μM. Mutations of residues αW582L or βG522D also increased amiloride Ki, the later mutation generating a Ca2+blocking site located 15% within the membrane electric field. These experiments provide strong evidence that αβγ ENaCs are pore-forming subunits involved in ion permeation through the channel. The pre-M2 segment of αβγ subunits may form a pore loop structure at the extracellular face of the channel, where amiloride binds within the channel lumen. We propose that amiloride interacts with Na+ions at an external Na+binding site preventing ion permeation through the channel pore.

Depletion of intracellular Ca2+ stores activates a Ca(2+)-selective channel in vascular endothelium

1994 ◽  
Vol 267 (4) ◽  
pp. C920-C925 ◽  
Author(s):  
L. Vaca ◽  
D. L. Kunze
Keyword(s):  
Vascular Endothelium ◽  
Reversal Potential ◽  
Inward Rectification ◽  
Ion Permeability ◽  
Channel Activity ◽  
Permeability Ratio ◽  
Extracellular Solution ◽  
Aortic Endothelial Cells ◽  
Bovine Aortic Endothelial Cells ◽  
Selective Channel

The present study was designed to identify the channel responsible for Ca2+ influx after depletion of intracellular Ca2+ stores. Different maneuvers that deplete intracellular Ca2+ stores activated a Ca(2+)-selective channel. Superfusion of single bovine aortic endothelial cells with 50 nmol/l bradykinin, 10 mumol/l ATP, or 10 mumol/l 2,5-di(tert-butyl)-1,4-benzohydroquinone produced activation of channels of the same amplitude in cell-attached patches. Channel activity declined within the first minute after patch excision. The channel showed strong inward rectification and a reversal potential of 0 mV in symmetrical sodium sulfate (Na2SO4) solution. Under these conditions, the conductance was 5 pS in the inward direction. Addition of 10 mmol/l Ca2+ to the extracellular solution shifted the reversal potential to +30 +/- 5 mV, and the conductance for inward current was 11 pS. The reversal potential was used to calculate an ion permeability ratio of Ca2+/Na+ > 10:1.

scholarly journals Spermine Block of the Strong Inward Rectifier Potassium Channel Kir2.1

2002 ◽  
Vol 120 (1) ◽  
pp. 53-66 ◽  
Author(s):  
Lai-Hua Xie ◽  
Scott A. John ◽  
James N. Weiss
Keyword(s):  
Single Channel ◽  
Quantitative Model ◽  
Inward Rectification ◽  
Channel Blockers ◽  
Surface Charges ◽  
Open Probability ◽  
Dependent Component ◽  
Voltage Dependent ◽  
Single Channel Currents ◽  
Inside Out

Inward rectification in strong inward rectifiers such as Kir2.1 is attributed to voltage-dependent block by intracellular polyamines and Mg2+. Block by the polyamine spermine has a complex voltage dependence with shallow and steep components and complex concentration dependence. To understand the mechanism, we measured macroscopic Kir2.1 currents in excised inside-out giant patches from Xenopus oocytes expressing Kir2.1, and single channel currents in the inside-out patches from COS7 cells transfected with Kir2.1. We found that as spermine concentration or voltage increased, the shallow voltage-dependent component of spermine block at more negative voltages was caused by progressive reduction in the single channel current amplitude, without a decrease in open probability. We attributed this effect to spermine screening negative surface charges involving E224 and E299 near the inner vestibule of the channel, thereby reducing K ion permeation rate. This idea was further supported by experiments in which increasing ionic strength also decreased Kir2.1 single channel amplitude, and by mutagenesis experiments showing that this component of spermine block decreased when E224 and E299, but not D172, were neutralized. The steep voltage-dependent component of block at more depolarized voltages was attributed to spermine migrating deeper into the pore and causing fast open channel block. A quantitative model incorporating both features showed excellent agreement with the steady-state and kinetic data. In addition, this model accounts for previously described substate behavior induced by a variety of Kir2.1 channel blockers.

scholarly journals The TRPV3 channel of the bovine rumen: localization and functional characterization of a protein relevant for ruminal ammonia transport

2020 ◽  
Vol 472 (6) ◽  
pp. 693-710 ◽  
Author(s):  
Franziska Liebe ◽  
Hendrik Liebe ◽  
Sabine Kaessmeyer ◽  
Gerhard Sponder ◽  
Friederike Stumpff
Keyword(s):  
Single Channel ◽  
Functional Characterization ◽  
Nitrogen Efficiency ◽  
Channel Measurements ◽  
Hek 293 ◽  
Bovine Rumen ◽  
Ruminal Ph ◽  
293 Cells ◽  
Ammonia Transport ◽  
Ruminal Epithelium

Abstract Large quantities of ammonia (NH3 or NH4+) are absorbed from the gut, associated with encephalitis in hepatic disease, poor protein efficiency in livestock, and emissions of nitrogenous climate gasses. Identifying the transport mechanisms appears urgent. Recent functional and mRNA data suggest that absorption of ammonia from the forestomach of cattle may involve TRPV3 channels. The purpose of the present study was to sequence the bovine homologue of TRPV3 (bTRPV3), localize the protein in ruminal tissue, and confirm transport of NH4+. After sequencing, bTRPV3 was overexpressed in HEK-293 cells and Xenopus oocytes. An antibody was selected via epitope screening and used to detect the protein in immunoblots of overexpressing cells and bovine rumen, revealing a signal of the predicted ~ 90 kDa. In rumen only, an additional ~ 60 kDa band appeared, which may represent a previously described bTRPV3 splice variant of equal length. Immunohistochemistry revealed staining from the ruminal stratum basale to stratum granulosum. Measurements with pH-sensitive microelectrodes showed that NH4+ acidifies Xenopus oocytes, with overexpression of bTRPV3 enhancing permeability to NH4+. Single-channel measurements revealed that Xenopus oocytes endogenously expressed small cation channels in addition to fourfold-larger channels only observed after expression of bTRPV3. Both endogenous and bTRPV3 channels conducted NH4+, Na+, and K+. We conclude that bTRPV3 is expressed by the ruminal epithelium on the protein level. In conjunction with data from previous studies, a role in the transport of Na+, Ca2+, and NH4+ emerges. Consequences for calcium homeostasis, ruminal pH, and nitrogen efficiency in cattle are discussed.

CFTR in K562 human leukemic cells

2003 ◽  
Vol 285 (2) ◽  
pp. C480-C488 ◽  
Author(s):  
Yanina A. Assef ◽  
Alicia E. Damiano ◽  
Elsa Zotta ◽  
Cristina Ibarra ◽  
Basilio A. Kotsias
Keyword(s):  
Single Channel ◽  
Functional Characterization ◽  
Leukemic Cells ◽  
Cftr Gene ◽  
Human Leukemia ◽  
Selectivity Ratio ◽  
Open Probability ◽  
Selective Channel ◽  
Human Leukemic Cells

In this study, the expression and functional characterization of CFTR (cystic fibrosis transmembrane regulator) was determined in K562 chronic human leukemia cells. Expression of the CFTR gene product was determined by RT-PCR and confirmed by immunohistochemistry and Western blot analysis. Functional characterization of CFTR Cl- channel activity was conducted with patch-clamp techniques. Forskolin, an adenylyl cyclase activator, induced an anion-selective channel with a linear current-voltage relationship and a single-channel conductance of 11 pS. This cAMP-activated channel had a Pgluconate/PCl or PF/PCl perm-selectivity ratio of 0.35 and 0.30, respectively, and was inhibited by the CFTR blocker glibenclamide and the anti-CFTR antibody MAb 13-1, when added to the cytoplasmatic side of the patch. Glibenclamide decreased the open probability increasing the frequency of open-to-closed transitions. Addition of 200 μM DIDS caused an irreversible block of the channels when added to the cytosolic side of inside-out patches. These and other observations indicate a widespread distribution of CFTR gene expression and suggest that this channel protein may function in most human cells to help maintain cellular homeostasis.

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