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Lyashchenko AK, Redd KJ, Goldstein PA, Tibbs GR. cAMP control of HCN2 channel Mg2+ block reveals loose coupling between the cyclic nucleotide-gating ring and the pore. PLoS One 2014; 9:e101236. [PMID: 24983358 PMCID: PMC4077740 DOI: 10.1371/journal.pone.0101236] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/14/2014] [Accepted: 06/04/2014] [Indexed: 12/24/2022] Open
Abstract
Hyperpolarization-activated cyclic nucleotide-regulated HCN channels underlie the Na+-K+ permeable IH pacemaker current. As with other voltage-gated members of the 6-transmembrane KV channel superfamily, opening of HCN channels involves dilation of a helical bundle formed by the intracellular ends of S6 albeit this is promoted by inward, not outward, displacement of S4. Direct agonist binding to a ring of cyclic nucleotide-binding sites, one of which lies immediately distal to each S6 helix, imparts cAMP sensitivity to HCN channel opening. At depolarized potentials, HCN channels are further modulated by intracellular Mg2+ which blocks the open channel pore and blunts the inhibitory effect of outward K+ flux. Here, we show that cAMP binding to the gating ring enhances not only channel opening but also the kinetics of Mg2+ block. A combination of experimental and simulation studies demonstrates that agonist acceleration of block is mediated via acceleration of the blocking reaction itself rather than as a secondary consequence of the cAMP enhancement of channel opening. These results suggest that the activation status of the gating ring and the open state of the pore are not coupled in an obligate manner (as required by the often invoked Monod-Wyman-Changeux allosteric model) but couple more loosely (as envisioned in a modular model of protein activation). Importantly, the emergence of second messenger sensitivity of open channel rectification suggests that loose coupling may have an unexpected consequence: it may endow these erstwhile “slow” channels with an ability to exert voltage and ligand-modulated control over cellular excitability on the fastest of physiologically relevant time scales.
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Affiliation(s)
- Alex K. Lyashchenko
- Department of Anesthesiology, Columbia University, New York, New York, United States of America
| | - Kacy J. Redd
- Department of Neuroscience, Columbia University, New York, New York, United States of America
| | - Peter A. Goldstein
- Department of Anesthesiology, Weill Cornell Medical College, New York, New York, United States of America
| | - Gareth R. Tibbs
- Department of Anesthesiology, Columbia University, New York, New York, United States of America
- Department of Anesthesiology, Weill Cornell Medical College, New York, New York, United States of America
- * E-mail:
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Li RA. Gene- and cell-based bio-artificial pacemaker: what basic and translational lessons have we learned? Gene Ther 2012; 19:588-95. [PMID: 22673497 DOI: 10.1038/gt.2012.33] [Citation(s) in RCA: 16] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/09/2022]
Abstract
Normal rhythms originate in the sino-atrial node, a specialized cardiac tissue consisting of only a few thousands of nodal pacemaker cells. Malfunction of pacemaker cells due to diseases or aging leads to rhythm generation disorders (for example, bradycardias and sick-sinus syndrome (SSS)), which often necessitate the implantation of electronic pacemakers. Although effective, electronic devices are associated with such shortcomings as limited battery life, permanent implantation of leads, lead dislodging, the lack of autonomic responses and so on. Here, various gene- and cell-based approaches, with a particular emphasis placed on the use of pluripotent stem cells and the hyperpolarization-activated cyclic nucleotide-gated-encoded pacemaker gene family, that have been pursued in the past decade to reconstruct bio-artificial pacemakers as alternatives will be discussed in relation to the basic biological insights and translational regenerative potential.
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Affiliation(s)
- R A Li
- Center of Cardiovascular Research, Mount Sinai School of Medicine, New York, NY 10029, USA.
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Chan YC, Wang K, Au KW, Au KW, Lau CP, Tse HF, Li RA. Probing the bradycardic drug binding receptor of HCN-encoded pacemaker channels. Pflugers Arch 2010; 459:25-38. [PMID: 19756722 PMCID: PMC2765624 DOI: 10.1007/s00424-009-0719-2] [Citation(s) in RCA: 9] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/03/2009] [Revised: 07/23/2009] [Accepted: 08/21/2009] [Indexed: 11/26/2022]
Abstract
If (or Ih), encoded by the hyperpolarization-activated, cyclic nucleotide-gated (HCN1–4) channel gene family, contributes significantly to cardiac pacing. Bradycardic agents such as ZD7288 that target HCN channels have been developed, but the molecular configuration of their receptor is poorly defined. Here, we probed the drug receptor by systematically introducing alanine scanning substitutions into the selectivity filter (C347A, I348A, G349A, Y350A, G351A in the P-loop), outer (P355A, V356A, S357A, M358A in the P-S6 linker), and inner (M377A, F378A, V379A in S6) pore vestibules of HCN1 channels. When heterologously expressed in human embryonic kidney 293 cells for patch-clamp recordings, I348A, G349A, Y350A, G351A, P355A, and V356A did not produce measurable currents. The half-blocking concentration (IC50) of wild type (WT) for ZD7288 was 25.8 ± 9.7 μM. While the IC50 of M358A was identical to WT, those of C347A, S357A, F378A, and V379A markedly increased to 137.6 ± 56.4, 113.3 ± 34.1, 587.1 ± 167.5, and 1726.3 ± 673.4 μM, respectively (p < 0.05). Despite the proximity of the S6 residues studied, M377A was hypersensitive (IC50 = 5.1 ± 0.7 μM; p < 0.05) implicating site specificity. To explore the energetic interactions among the S6 residues, double and triple substitutions (M377A/F378A, M377A/V379A, F378A/V379A, and M377A/F378A/V379A) were generated for thermodynamic cycle analysis. Specific interactions with coupling energies (ΔΔG) >1 kT for M377–F378 and F378–V379 but not M377–V379 were identified. Based on these new data and others, we proposed a refined drug-blocking model that may lead to improved antiarrhythmics and bioartificial pacemaker designs.
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Affiliation(s)
- Yau-Chi Chan
- Division of Cardiology, Department of Medicine, Queen Mary Hospital, Li Ka Shing Faculty of Medicine, University of Hong Kong, Pokfulam, Hong Kong
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Chan YC, Siu CW, Lau YM, Lau CP, Li RA, Tse HF. Synergistic effects of inward rectifier (I) and pacemaker (I) currents on the induction of bioengineered cardiac automaticity. J Cardiovasc Electrophysiol 2009; 20:1048-54. [PMID: 19460073 DOI: 10.1111/j.1540-8167.2009.01475.x] [Citation(s) in RCA: 29] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Abstract
INTRODUCTION Normal heart rhythms originate in the sinoatrial node. HCN-encoded funny current (I(f)) and the Kir2-encoded inward rectifier (I(K1)) counteract each other by respectively oscillating and stabilizing the negative resting membrane potential, and controlling action potential firing. Therefore, I(K1) suppression and I(f) overexpression have been independently exploited to convert cardiomyocytes (CMs) into AP-firing bioartificial pacemakers. Although the 2 strategies have been largely assumed synergistic, their complementarity has not been investigated. METHODS AND RESULTS We explored the interrelationships of automaticity, I(f) and I(K1) by transducing single left ventricular (LV) CMs isolated from guinea pig hearts with the recombinant adenoviruses Ad-CMV-GFP-IRES-HCN1-AAA and/or Ad-CGI-Kir2.1 to mediate their current densities via a whole-cell patch clamp technique at 37 degrees C. Results showed that Ad-CGI-HCN1-AAA but not Ad-CGI-Kir2.1 transduction induced automaticity (181.1 +/- 13.1 bpm). Interestingly, Ad-CGI-HCN1-AAA/Ad-CGI-Kir2.1 cotransduction significantly promoted the induced firing frequency (320.0 +/- 15.8 bpm; P < 0.05). Correlation analysis revealed that the firing frequency, phase-4 slope and APD(90) of AP-firing LV CMs were correlated with I(f) (R(2) > 0.7) only when -2 >I(K1) >-4 pA/pF but not with I(K1) over the entire I(f) ranges examined (0.02 < R(2) < 0.4). Unlike I(f), I(K1) displayed correlation with neither the phase-4 slope (R(2)= 0.02) nor phase-4 length (R(2)= 0.04) when -2 > I(f) > -4 pA/pF. As anticipated, however, APD(90) was correlated with I(K1) (R(2)= 0.4). CONCLUSION We conclude that an optimal level of I(K1) maintains a voltage range for I(f) to operate most effectively during a dynamic cardiac cycle.
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Affiliation(s)
- Yau-Chi Chan
- Cardiology Division, Department of Medicine, University of Hong Kong, Hong Kong
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P-loop residues critical for selectivity in K channels fail to confer selectivity to rabbit HCN4 channels. PLoS One 2009; 4:e7712. [PMID: 19890386 PMCID: PMC2766643 DOI: 10.1371/journal.pone.0007712] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/07/2009] [Accepted: 10/09/2009] [Indexed: 11/19/2022] Open
Abstract
HCN channels are thought to be structurally similar to Kv channels, but show much lower selectivity for K+. The ∼3.3 Å selectivity filter of K+ channels is formed by the pore-lining sequence XT(V/I)GYG, with X usually T, and is held stable by key residues in the P-loop. Differences in the P-loop sequence of HCN channels (eg. the pore-lining sequence L478C479IGYG) suggest these residues could account for differences in selectivity between these channel families. Despite being expressed, L478T/C479T HCN4 channels did not produce current. Since threonine in the second position is highly conserved in K+ channels, we also studied C479T channels. Based on permeability ratios (PX/PK), C479T HCN4 channels (K+(1)>Rb+(0.85)>Cs+(0.59)>Li+(0.50)≥Na+(0.49)) were less selective than WT rabbit HCN4 (K+(1)>Rb+(0.48)>Cs+(0.31)≥Na+(0.29)>Li+(0.03)), indicating that the TIGYG sequence is insufficient to confer K+ selectivity to HCN channels. C479T HCN4 channels had an increased permeability to large organic cations than WT HCN4 channels, as well as increased unitary K+ conductance, and altered channel gating. Collectively, these results suggest that HCN4 channels have larger pores than K+ channels and replacement of the cysteine at position 479 with threonine further increases pore size. Furthermore, selected mutations in other regions linked previously to pore stability in K+ channels (ie. S475D, S475E and F471W/K472W) were also unable to confer K+ selectivity to C479T HCN4 channels. Our findings establish the presence of the TIGYG pore-lining sequence does not confer K+ selectivity to rabbit HCN4 channels, and suggests that differences in selectivity of HCN4 versus K+ channels originate from differences outside the P-loop region.
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State-dependent accessibility of the P-S6 linker of pacemaker (HCN) channels supports a dynamic pore-to-gate coupling model. J Membr Biol 2009; 230:35-47. [PMID: 19609824 PMCID: PMC2718208 DOI: 10.1007/s00232-009-9184-2] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/03/2009] [Accepted: 06/11/2009] [Indexed: 11/08/2022]
Abstract
The hyperpolarization-activated cyclic nucleotide-modulated channel gene family (HCN1-4) encodes the membrane depolarizing current that underlies pacemaking. Although the topology of HCN resembles Kv channels, much less is known about their structure-function correlation. Previously, we identified several pore residues in the S5-P linker and P-loop that are externally accessible and/or influence HCN gating, and proposed an evolutionarily conserved pore-to-gate mechanism. Here we sought dynamic evidence by assessing the functional consequences of Cys-scanning substitutions in the unexplored P-S6 linker (residues 352–359), the HCN1-R background (that is, resistant to sulfhydryl-reactive agents). None of A352C, Q353C, A354C, P355C, V356C, S357C, M358C, or S359C produced functional currents; the loss-of-function of Q353C, A354C, S357C, and M358C could be rescued by the reducing agent dithiothreitol. Q353C, A354C, and S357C, but not M358C and HCN1-R, were sensitive to Cd2+ blockade (IC50 = 3–12 μM vs. >1 mM). External application of the positively charged covalent sulfhydryl modifier MTSET irreversibly reduced I−140mV of Q353C and A354C to 27.9 ± 3.4% and 58.2 ± 13.1% of the control, respectively, and caused significant steady-state activation shifts (∆V1/2 = –21.1 ± 1.6 for Q353C and −10.0 ± 2.9 mV for A354C). Interestingly, MTSET reactivity was also state dependent. MTSET, however, affected neither S357C nor M358C, indicating site specificity. Collectively, we have identified novel P-S6 residues whose extracellular accessibility was sterically and state dependent and have provided the first functional evidence consistent with a dynamic HCN pore-to-gate model.
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Natarajan S, Jakobsson E. Functional equivalency inferred from "authoritative sources" in networks of homologous proteins. PLoS One 2009; 4:e5898. [PMID: 19521530 PMCID: PMC2690840 DOI: 10.1371/journal.pone.0005898] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/09/2008] [Accepted: 04/29/2009] [Indexed: 11/18/2022] Open
Abstract
A one-on-one mapping of protein functionality across different species is a critical component of comparative analysis. This paper presents a heuristic algorithm for discovering the Most Likely Functional Counterparts (MoLFunCs) of a protein, based on simple concepts from network theory. A key feature of our algorithm is utilization of the user's knowledge to assign high confidence to selected functional identification. We show use of the algorithm to retrieve functional equivalents for 7 membrane proteins, from an exploration of almost 40 genomes form multiple online resources. We verify the functional equivalency of our dataset through a series of tests that include sequence, structure and function comparisons. Comparison is made to the OMA methodology, which also identifies one-on-one mapping between proteins from different species. Based on that comparison, we believe that incorporation of user's knowledge as a key aspect of the technique adds value to purely statistical formal methods.
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Affiliation(s)
- Shreedhar Natarajan
- Biophysics and Computational Biology, University of Illinois, Urbana-Champaign, Illinois, United States of America
| | - Eric Jakobsson
- Biophysics and Computational Biology, University of Illinois, Urbana-Champaign, Illinois, United States of America
- National Center for Supercomputing Applications, University of Illinois, Urbana-Champaign, Illinois, United States of America
- Department of Molecular and Integrative Physiology, University of Illinois, Urbana-Champaign, Illinois, United States of America
- * E-mail:
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Lieu DK, Chan YC, Lau CP, Tse HF, Siu CW, Li RA. Overexpression of HCN-encoded pacemaker current silences bioartificial pacemakers. Heart Rhythm 2008; 5:1310-7. [PMID: 18693074 DOI: 10.1016/j.hrthm.2008.05.010] [Citation(s) in RCA: 29] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 12/12/2007] [Accepted: 05/10/2008] [Indexed: 11/17/2022]
Abstract
BACKGROUND Current strategies of engineering bioartificial pacemakers from otherwise silent yet excitable adult atrial and ventricular cardiomyocytes primarily rely on either maximizing the hyperpolarization-activated I(f) or on minimizing its presumptive opponent, the inwardly rectifying potassium current I(K1). OBJECTIVE The purpose of this study was to determine quantitatively the relative current densities of I(f) and I(K1) necessary to induce automaticity in adult atrial cardiomyocytes. METHODS Automaticity of adult guinea pig atrial cardiomyocytes was induced by adenovirus (Ad)-mediated overexpression of the gating-engineered HCN1 construct HCN1-DeltaDeltaDelta with the S3-S4 linker residues EVY235-7 deleted to favor channel opening. RESULTS Whereas control atrial cardiomyocytes remained electrically quiescent and had no I(f), 18% of Ad-CMV-GFP-IRES-HCN1-DeltaDeltaDelta (Ad-CGI-HCN1-DeltaDeltaDelta)-transduced cells demonstrated automaticity (240 +/- 14 bpm) with gradual phase 4 depolarization (143 +/- 28 mV/s), a depolarized maximal diastolic potential (-45.3 +/- 2.2 mV), and substantial I(f) at -140 mV (I(f,-140 mV) = -9.32 +/- 1.84 pA/pF). In the remaining quiescent Ad-CGI-HCN1-DeltaDeltaDelta-transduced atrial cardiomyocytes, two distinct immediate phenotypes were observed: (1) 13% had a hyperpolarized resting membrane potential (-56.7 +/- 1.3 mV) with I(f,-140 mV) of -4.85 +/- 0.97 pA/pF; and (2) the remaining 69% displayed a depolarized resting membrane potential (-27.6 +/- 1.3 mV) with I(f,-140 mV) of -23.0 +/- 3.71 pA/pF. Upon electrical stimulation, both quiescent groups elicited a single action potential with incomplete phase 4 depolarization that was never seen in controls. Further electrophysiologic analysis indicates that an intricate balance of I(K1) and I(f) is necessary for induction of atrial automaticity. CONCLUSION Optimized pacing induction and modulation can be better achieved by engineering the I(f)/I(K1) ratio rather than the individual currents.
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Affiliation(s)
- Deborah K Lieu
- Stem Cell Program, University of California, Davis, California, USA
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Lyashchenko AK, Tibbs GR. Ion binding in the open HCN pacemaker channel pore: fast mechanisms to shape "slow" channels. ACTA ACUST UNITED AC 2008; 131:227-43. [PMID: 18270171 PMCID: PMC2248720 DOI: 10.1085/jgp.200709868] [Citation(s) in RCA: 14] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022]
Abstract
IH pacemaker channels carry a mixed monovalent cation current that, under physiological ion gradients, reverses at ∼−34 mV, reflecting a 4:1 selectivity for K over Na. However, IH channels display anomalous behavior with respect to permeant ions such that (a) open channels do not exhibit the outward rectification anticipated assuming independence; (b) gating and selectivity are sensitive to the identity and concentrations of externally presented permeant ions; (c) the channels' ability to carry an inward Na current requires the presence of external K even though K is a minor charge carrier at negative voltages. Here we show that open HCN channels (the hyperpolarization-activated, cyclic nucleotide sensitive pore forming subunits of IH) undergo a fast, voltage-dependent block by intracellular Mg in a manner that suggests the ion binds close to, or within, the selectivity filter. Eliminating internal divalent ion block reveals that (a) the K dependence of conduction is mediated via K occupancy of site(s) within the pore and that asymmetrical occupancy and/or coupling of these sites to flux further shapes ion flow, and (b) the kinetics of equilibration between K-vacant and K-occupied states of the pore (10–20 μs or faster) is close to the ion transit time when the pore is occupied by K alone (∼0.5–3 μs), a finding that indicates that either ion:ion repulsion involving Na is adequate to support flux (albeit at a rate below our detection threshold) and/or the pore undergoes rapid, permeant ion-sensitive equilibration between nonconducting and conducting configurations. Biophysically, further exploration of the Mg site and of interactions of Na and K within the pore will tell us much about the architecture and operation of this unusual pore. Physiologically, these results suggest ways in which “slow” pacemaker channels may contribute dynamically to the shaping of fast processes such as Na-K or Ca action potentials.
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Affiliation(s)
- Alex K Lyashchenko
- Department of Anesthesiology, Columbia University, New York, NY 10032, USA
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Au KW, Siu CW, Lau CP, Tse HF, Li RA. Structural and functional determinants in the S5-P region of HCN-encoded pacemaker channels revealed by cysteine-scanning substitutions. Am J Physiol Cell Physiol 2007; 294:C136-44. [PMID: 17989208 DOI: 10.1152/ajpcell.00340.2007] [Citation(s) in RCA: 9] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022]
Abstract
Hyperpolarization-activated cyclic nucleotide-modulated (HCN) channels are responsible for the membrane pacemaker current that underlies the spontaneous generation of bioelectrical rhythms. However, their structure-function relationship is poorly understood. Previously, we identified several pore residues that influence HCN gating properties and proposed a pore-to-gate mechanism. Here, we systematically introduced cysteine-scanning substitutions into the descending portion of the P loop (residues 339-345) of HCN1-R (where R is resistance to sulfhydryl-reactive agents) channels, in which all endogenous cysteines except C303 have been removed or replaced. F339C, K340C, A341C, M342C, S343C, and M345C did not produce functional currents. Interestingly, the loss of function phenotype of F339C could be rescued by the reducing agent dithiothreitol (DTT). H344C but not HCN1-R and DTT-treated F339C channels were sensitive to blockade by divalent Cd(2+) (current with 100 microM Cd(2+)/control current at -140 mV = 67.6 +/- 2.9%, 109.3 +/- 3.1%, and 103.8 +/- 1.7%, respectively). Externally applied methanethiosulfate ethylammonium, a covalent sulfhydryl-reactive compound, irreversibly modified H344C by reducing the current at -140 mV (to 43.7 +/- 6.5%), causing a hyperpolarizing steady-state activation shift (change in half-activation voltage: approximately 6 mV) and decelerated gating kinetics (by up to 3-fold). Based on these results, we conclude that pore residues 339-345 are important determinants of the structure-function properties of HCN channels and that the side chain of H344 is externally accessible.
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Affiliation(s)
- Ka-Wing Au
- Department of Medicine, Queen Mary Hospital, University of Hong Kong, Hong Kong
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Lyashchenko AK, Redd KJ, Yang J, Tibbs GR. Propofol inhibits HCN1 pacemaker channels by selective association with the closed states of the membrane embedded channel core. J Physiol 2007; 583:37-56. [PMID: 17569731 PMCID: PMC2277223 DOI: 10.1113/jphysiol.2007.136465] [Citation(s) in RCA: 26] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/08/2022] Open
Abstract
Activation of native IH pacemaker channels and channels formed on heterologous expression of some isoforms of their pore forming HCN (hyperpolarization-activated, cyclic nucleotide-regulated) subunits is inhibited by the intravenous general anaesthetic propofol (2,6-diisopropylphenol). Here, we show that inhibition of homomeric HCN1 channels is mediated through anaesthetic association with the membrane embedded channel core, a domain that is highly conserved between this isoform and the relatively insensitive HCN2 and 4 subunits. Decoupling of HCN channel gating from cAMP and internal protons reveals that changes in these second messengers are neither necessary nor sufficient to account for propofol's actions. Modelling of the equilibrium and kinetic behaviour of HCN1 channels in the absence and presence of anaesthetic reveals that (1) gating is best described by models wherein closed and open states communicate via a voltage-independent reaction with no significant equilibrium occupancy of a deactivated open state at non-permissive voltages, and (2) propofol modifies gating by preferentially associating with closed-resting and closed-activated states but a low affinity interaction with the activated open state shapes the effect of the drug under physiological conditions. Our findings illuminate the mechanism of HCN channel gating and provide a framework that will facilitate development of propofol derivates that have altered pharmacological properties and therapeutic potentials.
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Affiliation(s)
- Alex K Lyashchenko
- Department of Anesthesiology, Columbia University, and Eye Institute Research Annex, EI3-305, 160 Fort Washington Avenue, New York, NY 10032, USA
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Siu CW, Lieu DK, Li RA. HCN-encoded pacemaker channels: from physiology and biophysics to bioengineering. J Membr Biol 2007; 214:115-22. [PMID: 17558529 DOI: 10.1007/s00232-006-0881-9] [Citation(s) in RCA: 28] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/31/2006] [Revised: 11/24/2006] [Indexed: 12/15/2022]
Abstract
The depolarizing membrane ionic current I(h) (also known as I(f), "f" for funny), encoded by the hyperpolarization-activated cyclic-nucleotide-modulated (HCN1-4) channel gene family, was first discovered in the heart over 25 years ago. Later, I(h) was also found in neurons, retina, and taste buds. HCN channels structurally resemble voltage-gated K(+) (Kv) channels but the molecular features underlying their opposite gating behaviors (activation by hyperpolarization rather than depolarization) and non-selective permeation profiles (> or =25 times less selective for K(+) than Kv channels) remain largely unknown. Although I(h) has been functionally linked to biological processes from the autonomous beating of the heart to pain transmission, the underlying mechanistic actions remain largely inferential and, indeed, somewhat controversial due to the slow kinetics and negative operating voltage range relative to those of the bioelectrical events involved (e.g., cardiac pacing). This article reviews the current state of our knowledge in the structure-function properties of HCN channels in the context of their physiological functions and potential HCN-based therapies via bioengineering.
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Affiliation(s)
- C-W Siu
- Stem Cell Program, University of California, Davis, CA 95817, USA
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13
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Ouyang Q, Goeritz M, Harris-Warrick RM. Panulirus interruptus Ih-channel gene PIIH: modification of channel properties by alternative splicing and role in rhythmic activity. J Neurophysiol 2007; 97:3880-92. [PMID: 17409170 DOI: 10.1152/jn.00246.2007] [Citation(s) in RCA: 18] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022] Open
Abstract
We cloned 10 full-length variants of PIIH, the gene for I(h) from the spiny lobster, Panulirus interruptus, using reverse transcription-PCR (RT-PCR) and rapid amplification of cDNA ends (RACE). This gene shows a significant amount of alternative splicing in the S3-S4 and S4-S5 linkers, in the P-loop and the entire S6 transmembrane domain, in the cyclic nucleotide binding domain (CNBD), and near the 3' end of the gene. Functional expression of seven splice variants in Xenopus oocytes generated slowly activating hyperpolarization-activated inward currents, which were blocked by the I(h) channel blockers CsCl and ZD7288. The different splice variants had markedly varying activation kinetics and voltage dependence of activation. Bath application of 8-Br-cAMP shifted the V(1/2) to more positive potentials and accelerated the activation kinetics in an isoform-specific manner. Two variants containing a segment with an ER-retention motif in the S4-S5 loop did not produce currents in oocytes. Overexpression of one splice variant, PIIH AB(S)-I, in pyloric dilator (PD) neurons in the lobster stomatogastric ganglion produced an average threefold increase in I(h) without evoking a compensatory increase in I(A). The voltage for half-maximal activation of I(h) in PIIH AB(S)-I-expressing PDs was shifted in the depolarizing direction by 9 mV, whereas the slope factor decreased by 3.8 mV. Moreover, its activation kinetics were significantly faster than in control PDs. PIIH AB(S)-I overexpression enhanced PD neuron rhythmic firing in an amplitude-dependent manner above a minimal threshold two- to threefold increase in amplitude.
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Affiliation(s)
- Qing Ouyang
- Dept. of Neurobiology and Behavior, Cornell University, W159 Seeley G. Mudd Hall, Ithaca, NY 14853. )
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Xue T, Siu CW, Lieu DK, Lau CP, Tse HF, Li RA. Mechanistic role of I(f) revealed by induction of ventricular automaticity by somatic gene transfer of gating-engineered pacemaker (HCN) channels. Circulation 2007; 115:1839-50. [PMID: 17389267 PMCID: PMC2698014 DOI: 10.1161/circulationaha.106.659391] [Citation(s) in RCA: 30] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 11/16/2022]
Abstract
BACKGROUND Although I(f), encoded by the hyperpolarization-activated cyclic-nucleotide-modulated (HCN) channel gene family, is known to be functionally important in pacing, its mechanistic action is largely inferential and indeed somewhat controversial. To dissect in detail the role of I(f), we investigated the functional consequences of overexpressing in adult guinea pig left ventricular cardiomyocytes (LVCMs) various HCN1 constructs that have been engineered to exhibit different gating properties. METHODS AND RESULTS We created the recombinant adenoviruses Ad-CMV-GFP-IRES (CGI), Ad-CGI-HCN1, Ad-CGI-HCN1-delta delta delta, and Ad-CGI-HCN1-Ins, which mediate ectopic expression of GFP alone, WT, EVY235-7delta delta delta, and Ins HCN1 channels, respectively; EVY235-7delta delta delta and Ins encode channels in which the S3-S4 linkers have been shortened and lengthened to favor and inhibit opening, respectively. Ad-CGI-HCN1, Ad-CGI-HCN1-delta delta delta, and Ad-CGI-HCN1-Ins, but not control Ad-CGI, transduction of LVCMs led to robust expression of I(f) with comparable densities when fully open (approximately = -22 pA/pF at -140 mV; P>0.05) but distinctive activation profiles (V(1/2) = -70.8+/-0.6, -60.4+/-0.7, and -87.7+/-0.7 mV; P<0.01, respectively). Whereas control (nontransduced or Ad-CGI-transduced) LVCMs were electrically quiescent, automaticity (206+/-16 bpm) was observed exclusively in 61% of Ad-HCN1-delta delta delta-transduced cells that displayed depolarized maximum diastolic potential (-60.6+/-0.5 versus -70.6+/-0.6 mV of resting membrane potential of control cells; P<0.01) and gradual phase 4 depolarization (306+/-32 mV/s) that were typical of genuine nodal cells. Furthermore, spontaneously firing Ad-HCN1-delta delta delta-transduced LVCMs responded positively to adrenergic stimulation (P<0.05) but exhibited neither overdrive excitation nor suppression. In contrast, the remaining 39% of Ad-HCN1-delta delta delta-transduced cells exhibited no spontaneous action potentials; however, a single ventricular action potential associated with a depolarized resting membrane potential and a unique, incomplete "phase 4-like" depolarization that did not lead to subsequent firing could be elicited on simulation. Such an intermediate phenotype, similarly observed in 100% of Ad-CGI-HCN- and Ad-CGI-HCN1-Ins-transduced LVCMs, could be readily reversed by ZD7288, hinting at a direct role of I(f). Correlation analysis revealed the specific biophysical parameters required for I(f) to function as an active membrane potential oscillator. CONCLUSIONS Our results not only contribute to a better understanding of cardiac pacing but also may advance current efforts that focus primarily on automaticity induction to the next level by enabling bioengineering of central and peripheral cells that make up the native sinoatrial node.
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Affiliation(s)
- Tian Xue
- Department of Cell Biology and Human Anatomy, University of California, Davis, USA
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