Yes / Conference abstract

Large conductance voltage and Ca2+ activated K+ (BK) channels control electrical excitability in many cell types. BK channels have the largest conductance (~250 pS) of all K+ selective channels. To explore whether a large entrance to the inner cavity of BK channels is required for their large conductance, I examined if changing the size of the entrance alters the single-channel current amplitude. Previous studies suggest that residues E321/E324 in BK channels are located at the entrance to the inner cavity. To test if positions 321/324 are accessible to intracellular ions, I compared single-channel outward current before and after attaching thiol reagents at E321C/E324C. Attachment of MBB and MTSET altered single-channel currents, indicating that positions 321/324 are accessible to the conduction pathway. Decreasing the size of the entrance to the inner cavity by substituting residues with larger side chains, such as tyrosine and tryptophan, at positions 321/324 decreased the conductance, whereas increasing the size of the entrance had little effect on conductance. Increasing [K+]i from 0.15 to 2.5 M negated differences in single-channel outward current associated with side chain volume. Substitutions had less effect on inward currents. Plots of conductance vs. substituted side chain volume could be approximated with a simple model for the conduction pathway described by two resistors in series, R1 and R2. R2 is a variable resistor, with the resistance proportional to the inverse of the volume of the entrance to the inner cavity not occupied by the side chains. R1 is a fixed resistor arising from the other parts of the conduction pathway including the selectivity filter. Fitting the experimental observations indicated that R1+R2 ~5.4 GΩ for glycine substitution, with an R1/R2 ratio of ~17, and an effective radius and length of the entrance to the inner cavity of ~9.0 and 5.4 Å, respectively. The volume of K+ and water were not taken into account. Taken together, the above observations suggest that a large entrance to the inner cavity is needed for the large conductance of BK channels, as my study shows that the entrance is large and that decreasing the entrance size decreases the currents.

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Regulator of K+ conductance (RCK) domains control the activity of a variety of K+ channels and transporters, including the prokaryotic TrkA/H K+ transport complex and the eukaryotic BK channel, through binding of cytoplasmic ligands such as ATP, H+, and Ca2+. Thus RCK domains transduce ligand binding to gate transmembrane K+ flux in response to signaling events and cellular metabolism, in organisms ranging from bacteria to humans. In this work, I utilize the prokaryotic RCK domain containing K+ channel, MthK as a model system to provide insight toward the structural basis of ion channel gating by RCK domains. In MthK, binding of Ca2+ to an octameric ring of RCK domains (the gating ring) which is tethered to the pore of the channel, leads to a series of conformational changes that facilitates channel opening and K+ conduction. Using electrophysiology and X-ray crystallography, I identify the presence of additional Ca2+ binding sites in the MthK RCK domain, showing that each RCK domain contributes to three different regulatory Ca2+ binding sites, two of which are located at the interfaces between adjacent RCK domains. The additional Ca2+ binding sites, resulting in a stoichiometry of 24 Ca2+ ions per channel, is consistent with the steep relation between [Ca2+] and MthK channel activity. Comparison of Ca2+ bound and unliganded RCK domains suggests a physical mechanism for Ca2+-dependent conformational changes that underlie gating in this class of channels. To gain insight toward mechanisms of RCK domain activation, I crystallized and solved the structure of the RCK domain of MthK bound with Ba2+. The Ba2+-bound RCK domain was assembled as an octomeric gating ring, as observed in structures of the full-length MthK channel, and shows Ba2+ bound at several positions, one of which overlaps with a known Ca2+ binding site. Functionally, I determined that Ba2+ could activate reconstituted MthK channels as observed in electrophysiological recordings. These results suggest a working hypothesis for a sequence of ligand-dependent conformational changes that may underlie RCK domain activation and channel gating. In an effort to more accurately describe the Ca2+-dependent gating process in MthK, I crystallized and solved structures of mutant and wild-type RCK domains, and found that distinct Ca2+ activation sites near the N- and C-termini of the RCK domain (termed C1 and C3, respectively) are allosterically coupled to one another, to affect tuning of Ca2+ affinity and Ca2+-dependent channel activation. These results define a structural mechanism of allosteric modulation in a ligand-gated K+ channel, and provide a framework for understanding similar mechanisms in related RCK-containing channels and transporters. / Biochemistry

京都大学 / 0048 / 新制・課程博士 / 博士(薬科学) / 甲第20304号 / 薬科博第73号 / 新制||薬科||8(附属図書館) / 京都大学大学院薬学研究科薬科学専攻 / (主査)教授 竹島 浩, 教授 中山 和久, 教授 金子 周司 / 学位規則第4条第1項該当 / Doctor of Pharmaceutical Sciences / Kyoto University / DFAM

Yes / X-ray crystallography has provided tremendous insight into the different structural states of membrane proteins and, in particular, of ion channels. However, the molecular forces that determine the thermodynamic stability of a particular state are poorly understood. Here we analyze the different X-ray structures of an inwardly rectifying potassium channel (Kir1.1) in relation to functional data we obtained for over 190 mutants in Kir1.1. This mutagenic perturbation analysis uncovered an extensive, state-dependent network of physically interacting residues that stabilizes the pre-open and open states of the channel, but fragments upon channel closure. We demonstrate that this gating network is an important structural determinant of the thermodynamic stability of these different gating states and determines the impact of individual mutations on channel function. These results have important implications for our understanding of not only K+ channel gating but also the more general nature of conformational transitions that occur in other allosteric proteins. / Wellcome Trust

No description available.

<p>During the last years, drugs that prolong the repolarisation phase of the myocardial action potential, due to inhibition of the rapid component of the delayed-rectifying potassium channel (I_Kr) have been in focus. In addition to arrhythmogenic potential, selective Ikr-blockers have also been shown to be embryotoxic and teratogenic in animal studies. The aim of this thesis was to investigate a theory that these developmental toxic results from pharmacologically induced episodes of embryonic cardiac arrhythmias leading to hypoxia related damage in the embryo. Almokalant (ALM) was used as a model compound for selective Ikr-blockers. ALM induced embryonic cardiac arrhythmia, and in similarity with results obtained by maternal hypoxia, ALM induced embryonic death and growth retardation in both rats, and mice. </p><p>The theory of a hypoxia-related mechanism was strengthened by the results that ALM induce phase specific external and visceral defects (e.g. cleft lip/palate, distal digital, cardiovascular, and urogenital defects), and that the skeletal defects (not shown before) showed a clear trend; the later the treatment the more caudal was the site of the defect, which is in accordance with results from maternal hypoxia induced by e.g. lowering of the O₂ content in the air. The spin trapping agent PBN decreased almokalant induced malformations, suggesting that the defects mainly are caused by reoxygenation damage after episodes of severe embryonic dysrhythmia, rather than "pure hypoxia".</p><p>Sotalol was tested in a third species, the rabbit who expresses functional I_Kr channels both in the embryo and in the adult, where it induced developmental toxicity, and indicating that the embryo is more sensitive than the adult towards arrhythmia caused by I_Kr-blockers. </p>

During the last years, drugs that prolong the repolarisation phase of the myocardial action potential, due to inhibition of the rapid component of the delayed-rectifying potassium channel (IKr) have been in focus. In addition to arrhythmogenic potential, selective Ikr-blockers have also been shown to be embryotoxic and teratogenic in animal studies. The aim of this thesis was to investigate a theory that these developmental toxic results from pharmacologically induced episodes of embryonic cardiac arrhythmias leading to hypoxia related damage in the embryo. Almokalant (ALM) was used as a model compound for selective Ikr-blockers. ALM induced embryonic cardiac arrhythmia, and in similarity with results obtained by maternal hypoxia, ALM induced embryonic death and growth retardation in both rats, and mice. The theory of a hypoxia-related mechanism was strengthened by the results that ALM induce phase specific external and visceral defects (e.g. cleft lip/palate, distal digital, cardiovascular, and urogenital defects), and that the skeletal defects (not shown before) showed a clear trend; the later the treatment the more caudal was the site of the defect, which is in accordance with results from maternal hypoxia induced by e.g. lowering of the O2 content in the air. The spin trapping agent PBN decreased almokalant induced malformations, suggesting that the defects mainly are caused by reoxygenation damage after episodes of severe embryonic dysrhythmia, rather than "pure hypoxia". Sotalol was tested in a third species, the rabbit who expresses functional IKr channels both in the embryo and in the adult, where it induced developmental toxicity, and indicating that the embryo is more sensitive than the adult towards arrhythmia caused by IKr-blockers.

CoordenaÃÃo de AperfeiÃoamento de Pessoal de NÃvel Superior / DisfunÃÃo erÃtil (DE) à definida como uma inabilidade para conseguir e manter uma ereÃÃo para satisfaÃÃo sexual. A fentolamina um antagonista a-adrenÃrgico tem sido usado para tratar DE desde 1994, principalmente em combinaÃÃo com outros agentes vasoativos. Mais recentemente uma formulaÃÃo oral mesilato de fentolamina foi desenvolvida para a doenÃa. A droga relaxa corpo cavernoso peniano inibindo a-adrenÃrgicos receptores. Desde o artigo de Traish (1998) se tem especulado que a fentolamina pode relaxar o mÃsculo liso peniano por um mecanismo nÃo adrenÃrgico. O objetivo deste estudo à compreender melhor a farmacocinÃtica da fentolamina (estudo in vivo) usando os dados de um teste de bioequivalÃncia, and investigar sua farmacodinÃmica no corpo cavernoso humano com o propÃsito de dirimir as dÃvidas sobre seu mecanismo nÃo adrenÃrgico neste sÃtio (estudo in vitro). MÃtodos (estudo in vivo): Trinta e seis voluntÃrios sÃos, masculinos, (idade mÃdia 21,5 anos) foram admitidos no estudo cujo desenho consistia de um ensaio duplo cruzado randomizado, com uma Ãnica dose, comparando (regitinaÃ) a uma formulaÃÃo padrÃo de fentolamina (VasomaxÃ). Estudo in vitro: Um total de 64 tiras isoladas de corpo cavernoso humano obtido de 16 doadores cadÃveres masculinos (16 â40 anos de idade) foram investigados. A atividade farmacolÃgica do relaxamento mediado pela fentolamina de fragmentos prÃ-contraÃdos de tecido erÃtil peniano foi estudada em banho de tecidos (meio nÃo adrenÃrgico/nÃo prostanÃide). Resultados (estudo in vivo): a razÃo da mÃdia geomÃtrica de Cmax da formulaÃÃo de Regitinaà 40 mg foi 108.29% (90% CI = 98.58 â 118.96) da formulaÃÃo de Vasomax 40 mg. A razÃo da mÃdia geomÃtrica da [AUC(0-720 min)] da formulaÃÃo de Regitinaà 40 mg foi 102.33 (90% CI = 97.21 â 19= 07.72) da formulaÃÃo de Vasomaxà 40 mg. A mÃdia dos parÃmetros farmacocinÃticos da fentolamina foram Cmax 15,4 ng/mL, Tmax 50 min e t1/2 3 h. (Estudo in vitro): A fentolamina causou relaxamento dependente da concentraÃÃo em tiras de corpo cavernoso humano prÃ-contraÃdas com o agonista a-adrenÃrgico fenilefrina bem como com os agentes nÃo adrenÃrgicos serotonina (10-4 M), prostaglandina F2a (10-4 M) e KCl (60 mM), com a melhor eficÃcia contra a fenilefrina (100% de relaxamento na concentraÃÃo de 10-3 M - IC50 = 1,5 x10-5 M). A Tetrodotoxina (TTX â 10-6 M) (bloqueador de canal de Na+) e atropina (10-5 M) (inibidor do receptor muscarÃnico) nÃo alterou o relaxamento da fentolamina no mÃsculo liso peniano (54,6  4,6% x 48,9 x 6,4%) (52,7  6,5% x 58,6  5,6%) (p > 0,05). O relaxamento da fentolamina nas tiras de corpo cavernoso humano prÃ-contraÃdos com KCl (40 mM) foi significantemente atenuado por NG-nitro-L-arginine L-NAME (10-4 M) ( inibidor da NO sintetase) (59,7  5,8% x 27,8  7,1%) (p < 0,05) e 1H-[1,2,4] Oxadiazole [4,3-a]quinoxalin-1-one ODQ (10-4 M) (inibidor da guanilato ciclase) (62,7  5,1% x 26,8  3,9%) (p < 0,05). O papel dos bloqueadores dos canais de K foram investigados. A glibenclamida (10-4 M) um inibidor do canal de potÃssio ativado por ATP (inibidor do KATP) causou uma inibiÃÃo quase completa (90%) do relaxamento da fentolamina em tiras de corpo cavernoso humano prÃ-contraÃdas com KCl (40 mM) (56,7  6,3% x 11,3  2,3%) (P < 0,05). InvestigaÃÃo com glibenclamida + L-NAME fez o mesmo efeito (54,6  5,6% x 5,7  1,4%) (p < 0,05). Os bloqueadores do canal de potÃssio dependente de CA++ (Kca) Charybdotoxina e apamina nÃo modificaram o relaxamento da fentolamina (54,6  4,6% v 59,3  5,2%) ConclusÃo: Os parÃmetros farmacocinÃticos mÃdios da fentolamina foram similares aos reportados pela literatura cientÃfica. As duas formulaÃÃes da fentolamina foram consideradas semelhantes. Os resultados dos estudos farmacolÃgicos da fentolamina sugerem que ela relaxa o corpo cavernoso de humanos tambÃm por mecanismo nÃo adrenÃrgico-nÃo colinÃrgico, ativando o canal de potÃssio KATP. / Erectile dysfunction (ED) is defined as the Inability to achieve or maintain an erection adequate for sexual satisfaction. Phentolamine an a-adrenergic antagonist has been used to treat ED since 1994, mostly in combination with other vasoactive agents. More recently oral formulation of Phentolamine mesylate were developed for the disease. The drug is thought to relax penile smooth muscle by a inhibition over a-receptors. Since the paper of Traish (1998) has been speculated that phentolamine may also relax penile smooth muscle by a non-adrenergic mechanism. The aim of this study is to understand the pharmacokinetics aspects of Phentolamine (in vivo study) using the data from a bioequivalence test, and to investigate its pharmacodynamics with the purpose to clear its non-adrenergic mechanism in human corpus cavernosum (in vitro study). Methods (In vivo study): Thirty six healthy male volunteers (mean age 21,5 years old) were enrolled in the study that consisted in a single dose, two-way randomized crossover design comparing one phentolamine formulation (regitinaÃ) to one standard phentolamine formulation (VasomaxÃ). In vitro study: A total of 64 isolated human corporeal tissue strips obtained from 16 male donor cadaver (16 â40 years old) were investigate. The pharmacologic activity of phentolamine-mediated relaxation, of pre-contracted erectile tissue strips of human corpus cavernosum were studied in organ bath chambers(non-adrenergic mean). Results: (in vivo study): Regitinaà 40 mg formulation Cmax geometric mean ratio was 108.29% (90% CI = 98.58 â 118.96 of Vasomax 40 mg formulation. Regitineà 40 mg formulation [AUC(0-720 min)] geometric mean ratio was 102.33 (90% CI = 97.21 â 19= 07.72) of Vasomaxà 40 mg formulation. The average phentolamine pharmacokinetics parameters were Cmax 15,4 ng/mL, Tmax 50 min and t1/2 3 h. (In vitro study): Phentolamine caused concentration dependent relaxation in human corpus cavernosum strips pre-contracted with the a-adrenergic agonist phenylephrine as well as with the non-adrenergic serotonin (10-4 M), prostaglandin F2a (10-4 M) and KCl (60 mM) agents, with the best efficacy against phenylephrine (100% of relaxation at 10-3 M - IC50 = 1,5 x10-5M). Tetrodotoxin (TTX â 10-6 M) (Na+ channel blocker) and atropine (10-5 M) (muscarinic receptor inhibitor) did not cause alterations in the phentolamine relaxation of the penile smooth muscle (54,6  4,6% x 48,9 x 6,4%) (52,7  6,5% x 58,6  5,6%) (p > 0,05). The relaxation of phentolamine of the human corpus cavernosum strips pre-contracted with KCl (40 mM) was significantly attenuated by NG-nitro-L-arginine L-NAME (10-4 M) (NO synthase inhibitor) (59,7  5,8% x 27,8  7,1%) (p < 0,05) and 1H-[1,2,4] Oxadiazole [4,3-a]quinoxalin-1-one ODQ (10-4 M) (inibidor da guanilato ciclase (62,7  5,1% x 26,8  3,9%) (p < 0,05). The role of the K channel blockers were investigated. Glibenclamide (10-4 M) an inhibitor of ATP-activated K+ -channels (KATP- inhibitor) caused a almost completely inhibition (90%) of the human corpus cavernosum strips phentolamine relaxation, pre-contracted with KCl (40 mM) (56,7  6,3% x 11,3  2,3%) (P < 0,05). Investigation with Glibenclamide + L-NAME did the same effect (54,6  5,6% x 5,7  1,4%) (p < 0,05). Charybdotoxin and apamin (blockers of CA++-activated K+ channels â Kca) did not alter the phentolamine relaxations (54,6  4,6% v 59,3  5,2%) Conclusion: The average phentolamine pharmacokinetics parameters were similar to the reported by scientific literature. The two drugs are bioequivalents for the rate and extent of absorption. The results from the Pharmacologic studies suggest that Phentolamine relaxes human corpus cavernosum by a nonadrenergic noncholinergic mechanism activating the ATP-activated K+ -channel (KATP).