【作者】 刘丽；

【导师】 张海林；

【作者基本信息】 河北医科大学 ， 药理学， 2008， 硕士

【摘要】 G蛋白偶联受体（G protein-coupled-receptors,GPCRs）是目前发现的最大的细胞膜表面受体超家族,具有重要的生理病理学和药理学意义。其独特的结构特征和在信号转导中的重要作用决定了其可以作为很好的药物靶点。GPCRs特异性的激动剂及阻断剂具有良好的药物开发前景。事实上目前市场上超过一半的药物均是以GPCRs为靶点的。不同的GPCR通过和相应的G蛋白（G protein）偶联可以对细胞外多种刺激信号作出反应,在细胞内产生神经传递、味觉、嗅觉、视觉及细胞的新陈代谢、分化、增殖、分泌等一系列生理效应。G蛋白种类较多,但所有类型G蛋白都由3个不同的亚单位即α、β、γ所组成。α亚单位具有特异的GTP结合位点,有GTP酶活性。根据G蛋白的α亚单位的结构,可将其分为4个亚家族:G_s,G_i/o,G_q和G₁₂。M电流最早于1980年由Brown和Admas在牛蛙颈上交感神经节中发现,是一种慢激活、非失活的电压依赖型外向钾离子电流,因其可被激活后的毒蕈碱受体（M受体）所抑制而得名。M电流的抑制将导致神经系统兴奋性升高。M通道的功能失调与良性家族性新生儿惊厥症（BFNCs）、阿尔茨海默氏病（Alzheimer）、癫痫等疾病密切相关。M通道的分子基础是KCNQ钾离子通道,目前已经发现了5种KCNQ亚型:KCNQ1～5,其中KCNQ2-5,尤其是KCNQ2/Q3参与构成M电流。在对M电流调节机制的研究中已经发现一些G_q蛋白偶联受体激活后能引起M电流的抑制。由G_q蛋白偶联受体介导的磷脂酶C（PLC）信号转导通路被认为是其抑制M电流的主要途径。目前已证明,G_αq蛋白激活后首先激活PLC_β,进而水解胞膜PIP₂,PIP₂水解生成两种第二信使IP₃与DAG,IP₃可引发细胞内质网钙库释放,而DAG则进一步激活PKC。PIP₂水解、PKC及细胞内钙离子均可能在GPCRs调节M电流过程中发挥了作用。然而,对于是否所有G_q蛋白偶联受体激活对KCNQ/M电流的调节作用是否利用同样的机制还不甚明了。目的:观察HEK293细胞中外源性表达的组胺（H1）、血管紧张素Ⅱ（AT1）、嘌呤（P2Y1和P2Y2）受体对共表达的KCNQ2/3通道电流的抑制作用,以细胞膜PIP₂及细胞内钙释放为主要观察点,研究抑制作用的信号通路机制,并为以后的实验打下基础。方法:用全细胞膜片钳记录电流变化:用激光共聚焦显微镜（LSCM）观测细胞膜PIP₂敏感荧光探针（PLC -PH-GFP）在细胞膜与细胞浆间的转位情况,进而了解PIP₂的水解情况;用LSCM及双波长比例法钙成像技术测定细胞内钙离子浓度变化。结果:（1）用全细胞膜片钳方法记录到的HEK293细胞表达的外源性的KCNQ2/3电流可以被共表达的H1、AT1、P2Y1和P2Y2受体激活后所抑制,H1、AT1、P2Y1和P2Y2激活对KCNQ2/3电流的抑制率分别为87.7%±6.9%（n=7）,55.0%±4.9%（n=4）,57.7%±5.1%（n=6）和62.0%±3.1%（n=6）。（2）LSCM观察HEK293细胞膜PIP₂水解情况:以上四种受体激动后均可明显见到荧光探针（PLC -PH-GFP）从胞膜处向胞浆处的转位,洗掉受体激动药物后,荧光探针又重新由胞浆转位到胞膜（复位）,呈现一种可逆性变化。在PI4激酶抑制剂wortmannin持续存在的情况下,荧光探针由胞浆到胞膜的复位被抑制,说明荧光探针的复位是PI4激酶介导的PIP₂的再合成的结果。用PLC阻断剂U73122预先孵育细胞后,受体激活导致的由胞膜向胞浆的荧光转位不再发生,说明受体激活导致的荧光探针转位是PLC介导的PIP₂水解的结果。（3）用LSCM及双波长比例法钙成像技术测定细胞内钙离子浓度变化:以上四种受体激动后无论是否有无外钙存在,均可引起细胞内钙升高,这种作用可被U73122阻断。（4）组胺H1受体激动后抑制KCNQ2/3电流机制的研究:用全细胞膜片钳方法记录HEK293细胞表达的KCNQ2/3电流,观察激活表达的H1受体对KCNQ2/3电流抑制作用,并观察PLC阻断剂U-73122、PI4激酶阻断剂wortmannin和钙库耗竭剂thapsigargin对上述作用的影响,从而推断PIP₂及细胞内钙在H1受体激活抑制KCNQ2/3电流中的作用。PLC阻断剂U73122可使电流抑制率由74.5±0.6%（n=5）显著下降到7.1±0.6%（n=5,p＜0.01）,基本取消了H1受体激活对电流的抑制作用,说明H1受体是通过激活PLC对KCNQ2/3电流产生抑制的;P14激酶抑制剂wortmannin使电流被抑制后的恢复率由88.4±6.2%（n=7）显著下降到10.5±3.4%（n=8,p＜0.01）,说明PIP₂的再合成是组胺通过H1受体抑制KCNQ2/3电流后电流恢复的必要条件;内钙库耗竭剂thapsigargin应用前后电流抑制率分别为90.0±1.6%（n=6）,88.6±3.9%（n=6,p＞0.05）,无显著性差别,说明细胞内钙的释放没有参与组胺H1受体抑制KCNQ2/3电流的过程。结论:结果表明在HEK293细胞中,H1、AT1、P2Y1和P2Y2受体均可抑制KCNQ2/3 M电流,上述受体激动后通过激活PLC,可以水解细胞膜上的PIP₂,且均可引起细胞内钙升高。H1受体及其他GPCRs激活后可能主要是通过激动PLC产生的细胞膜PIP₂水解,而不是细胞内钙来抑制KCNQ2/3电流的。更多还原

【Abstract】 The G protein-coupled-receptors（GPCRs）are the largest superfamily of cell surface receptors,and have important pathophysiological and pharmacological significance.GPCRs play important roles in cellular signaling networks involving such processes as neurotransmission,taste,smell,vision,cellular metabolism,differentiation,growth and secretion.Different GPCRs respond to a wide variety of different external stimulants and initiate a wide spectrum of intracellular responses by activating a number of different GTP binding proteins（G proteins）.GPCRs are practical and potential drug targets due to the important physiological roles they play,and due to the nature of signaling transduction pathway they initiate.Thus agonists or antagonists of GPCRs are being used as,or have potential to become new drugs.In fact,GPCRs are targets for more than half of the current therapeutic agents on the market now.There are many kinds of G proteins but all are composed ofα、β、γsubunits.Theαsubunit has the GTPase activity and can hydrolyze bound GTP.According to the structure ofαsubunit,G proteins can be divided into four sub-families:G_s, G_i/o,G_q and G₁₂.M current was first discovered in bullfrog sympathetic neurons by Brown and Admas in 1980.It is a slowly activating,noninactivating, voltage-dependent potassium current.It is named M current because of its suppression by muscarinic receptor activation. Blocking of M current will result in hyperexcitability of the neuronal system.The dysfunction of M channel is closely associated with diseases like benign neonatal familial convulsions（BFNCs）, Alzheimer,epilepsy,etc.The molecular basis of M current is KCNQ potassium channels.So far five members of KCNQ（KCNQ1～5） have been found.Among them,KCNQ2-5,specially KCNQ2/Q3 heteromultimers are believed to form neuronal M currents.Ample evidence demonstrated that activation of some G_q protein-coupled-receptors will result in KCNQ/M current inhibition.The PLC signaling pathway is believed to mediate M current inhibition during G_q protein-coupled-receptor activation.PLC_β,when stimulated by G_αqproteins,hydrolyze phosphatidylinositol 4,5-bisphosphate （PIP₂）,yielding two intracellular second messengers IP₃ and DAG.IP₃ then trigger calcium release from endoplasmic reticulum,which in turn are involved in varieties of cell functions. DAG activates PKC.Membrane PIP₂ hydrolysis,PKC and intracellular calcium have all been demonstrated to be involved in G_q protein-coupled-receptor-mediated M current regulations.However, it is not clear if all G_q protein-coupled-receptors modulate KCNQ/M currents through same mechanism.Objective:To study the effect of G_q protein-coupled-receptor on KCNQ2/3 channel currents expressed in HEK293 cells.GPCRs observed in the study includes:histamine（HI）,angiotensinⅡ（AT1）, purine（P2Y1,P2Y2）receptors.The study will focus on two key signaling molecules:membrane PIP₂ and intracellular calcium,to study their role in KCNQ/M current inhibition induced by GPCR activation.This study will lay a foundation for future investigation.Methods:Whole cell patch clamp technique will be used to record KCNQ currents.Laser scanning confocal microscopy（LSCM） will be used to monitor translocation of PIP₂ sensitive florescence probe（PLC -PH-GFP）between the membrane and the cytosol thus to monitor membrane PIP₂ hydrolysis.LSCM and ratio-imaging microscopy technique will be used to monitor intracellular Ca²⁺ release.Results:（1）Activation of H1、AT1、P2Y1 and P2Y2 receptors all inhibited co-expressed KCNQ2/3 channel currents recorded using whole cell patch clamp in HEK293 cells.Activation of H1、AT1、P2Y1 and P2Y2 receptors inhibited KCNQ2/3 currents by 87.7%±6.9%（n=7）,55.0%±4.9%（n=4）,57.7%±5.1%（n=6）and 62.0%±3.1%（n=6）,respectively.（2）For membrane PIP₂ hydrolysis study,application of agonists for these four GPCRs caused a translocation of the fluorescent probe from the membrane to the cytosol which was reversible after washout of the agonists. Wortmannin,a blocker of PI4 kinase,blocked the recovery of the florescence from the cytosol to the membrane,indicating the recovery is the result of PI4 kinase-mediated PIP₂ resynthesis. U73122,a blocker of PLC,totally abolished fluorescent translocation from the membrane to the cytosol induced by activation of the GPCRs,indicating the translocation is a result of PLC-mediated membrane PIP₂ hydrolysis.（3）Intracellular calcium([Ca²⁺]_i) monitored using LSCM and ratio-imaging microscopy system demonstrated a rising of[Ca²⁺]_i in responding to activation of receptors expressed in HEK293 cells,with or without presence of extracellular calcium.[Ca²⁺]_i rising was completely abolished by treatment with U-73122.（4）Inhibition of KCNQ2/3 currents by activation of histamine H1 receptor in HEK293 cells is due to hydrolysis of membrane PIP₂ and is independent of[Ca²⁺]_i signals. U-73122,a PLC inhibitor,antagonized histamine-induced KCNQ2/3 current inhibition;the inhibition rate was reduced significantly from 88.4±6.2%（n=7）to 10.5±3.4%（n=8,p＜0.01）.Treatment with PI4 kinase inhibitor wortmannin,which blocks the synthesis of PIP₂, made histamine-induced KCNQ2/3 currents inhibition irreversible; the recovery rate was reduced significantly from 74.5±0.6%（n=5）to 7.1±0.6%（n=5,p＜0.01）.Histamine-induced inhibition of KCNQ2/3 currents was not affected by thapsigargin,which depletes internal Ca²⁺stores;application of histamine after thapisgargin treatment still produced a remarkable inhibition of 88.6±3.9%（n=6）,which is not significantly different from a 90.0±1.6%（n=6）inhibition in the absence of thapisgargin.Conclusion:The results demonstrate that in HEK 293 cells, activation of four types of GPCRs,H1、AT1、P2Y1 and P2Y2,all inhibited co-expressed KCNQ2/3 channel currents.Activation of these receptors leads to PIP₂ hydrolysis and intracellular calcium increase through stimulation of PLC.Activation of H1 and other GPCR.s in HEK293 cells inhibits KCNQ2/3 current possibly through PIP₂ hydrolysis rather than[Ca²⁺]_i signaling.更多还原