2. 西南医科大学心血管医学研究所, 泸州 646000
2. Institute of Cardiovasology, Southwest Medical University, Luzhou 646000, Sichuan, China
协调一致的肌浆网(sarcoplasmic reticulum,SR)钙释放对心肌细胞正常兴奋收缩偶联(excitation-contraction coupling,ECC)具有关键作用,理想情况下在心脏舒张期SR的钙释放通道应处于完全关闭状态[1-2]。然而,心肌细胞内会出现其他SR钙释放事件,称为钙泄漏(calcium leak),主要包括钙火花(calcium sparks)、钙波(calcium waves)和难以检测的比钙火花更小的钙释放事件等[3]。钙泄漏增多可能会导致很多疾病的发生,如心房颤动(atrial fibrillation, AF)[4-6]、心力衰竭(heart failure, HF)[7]。目前认为,心房肌细胞钙泄漏是AF发生和维持的重要因素之一,2型雷尼丁受体(ryanodine receptor 2,RyR2)是SR上最主要的钙释放通道,磷酸化[6, 8]、氧化修饰[6]、自身空间排列结构的改变[9]、自身突变[6]和线粒体功能[10]等的影响均会导致钙泄漏,而晚钠通道同钠钙交换蛋白(Na+-Ca2+ exchanger 1,NCX1)的功能偶联也可以诱发舒张期的钙泄漏,最终导致AF的发生和维持[11]。
1 钙泄漏的主要表现形式 1.1 钙火花钙火花是激光共聚焦显微镜下观测到的局部自发性SR钙释放事件,是最明显和广泛认可的舒张期钙泄漏[12]。钙火花是由某一簇RyR2通道随机开放导致的,通常局限于一个接头间隙(junctional cleft)内,相邻接头处(横向距离0.5~1.0 μm,纵向距离约2 μm)的局部钙离子浓度受钙扩散及SR摄钙的影响,难以继续激活钙释放,进而诱发钙波[13]。
1.2 钙波钙火花通常难以在细胞内传导扩散,然而在某些情况下,如细胞质Ca2+浓度或SR Ca2+浓度过高、RyR2通道过度敏感时,某一接头处的钙火花可以激活周边相邻接头出现钙火花样的钙释放,从而产生全细胞内的钙波[14]。钙波是指心肌细胞内Ca2+负荷增高、细胞内Ca2+在局部自发性释放增加并伴传导的现象,其特点是在激光共聚焦显微镜下可见心肌细胞内Ca2+在某个区域瞬时性增高,并快速在细胞内传播[14]。钙火花和钙波是心肌细胞钙泄漏的基本表现形式[1, 3, 15]。
1.3 非钙火花介导的钙泄漏钙泄漏既包括钙火花或爆发式的钙波,也包括无形的钙泄漏[16-17]。有学者通过定量分析心肌细胞SR钙泄漏的不同成分,发现一部分RyR2介导的钙泄漏形式并非钙火花,可被丁卡因、钌红以及高浓度Mg2+阻断,即共聚焦显微镜难以观测到结果,但RyR2通道开放仍然存在[18-19]。研究发现,该形式的钙泄漏通常是由单个RyR2通道开放导致,亦称为钙夸克(calcium quark);或是数个RyR2通道开放但不足以诱发出一个完整的钙火花[20]。
2 钙泄漏的主要影响因素 2.1 细胞质Ca2+浓度可被细胞质内Ca2+激活是RyR2通道最基本的特性之一。平面脂质膜系统是定量检测RyR2通道性质的重要手段。实验表明,RyR2的开放概率受细胞质Ca2+浓度的调节[21]。50 nmol/L的细胞质Ca2+可降低钙泄漏,且几乎检测不到钙火花的发生,提高浓度至50~250 nmol/L可增加钙泄漏,可见钙火花和非钙火花形式钙泄漏均增多;而继续增加至350 nmol/L可诱发钙波,导致SR内Ca2+快速排空;该调节机制并不依赖于钙离子/钙调蛋白依赖性激酶Ⅱ(Ca2+-calmodulin-dependent protein kinaseⅡ,CaMKⅡ),而是直接调节RyR2通道活性[21]。
影响细胞质Ca2+最终浓度的因素很多,包括细胞质Mg2+、ATP、pH以及SR腔内的Ca2+浓度等[22-23]。Ca2+浓度越高,越容易导致RyR2钙泄漏,使病情进一步恶化,该机制在临床HF和AF患者心肌标本中已经得到证实[4, 24]。
2.2 SR Ca2+浓度细胞质条件固定时,RyR2的开放概率对于SR腔内的Ca2+浓度十分敏感,高于生理浓度0.4~2.0 mmol/L的SR Ca2+浓度可以增加RyR2的开放概率。SR Ca2+浓度调节RyR2的开放概率的分子机制尚不明确。Györke等[25]发现,集钙蛋白(calsequestrin,CSQ)可以与junctin/triadin等直接结合形成复合体,经变构作用影响RyR2。心肌细胞SR肌浆网-内质网钙离子转运ATP酶(sacro-endoplasmic reticulum calcium transport ATPase,SERCA)在心肌细胞SR钙稳态的调节中发挥重要作用。研究发现SERCA2过表达小鼠模型中SR摄钙和钙瞬变增多,钙泄漏未见显著改变;而在过表达SERCA负性调节蛋白sarcolipin的小鼠中,SR摄钙及钙瞬变均减少,钙泄漏同样未见显著改变,可见选择性调节SERCA可以调节SR钙含量,且不影响钙泄漏[25]。
2.3 RyR2大分子复合体功能异常RyR是一个体积较大的大分子复合体,包括4个RyR2单体以及他克莫司结合蛋白12/12.6(FK506 binding protein-12/12.6,FKBP 12.6)[26]、钙调蛋白(calmodulin,CaM)、蛋白激酶A(protein kinase A,PKA)[11]、CaMKⅡ[11]、磷脂酶1、磷脂酶2A[27]、亲联蛋白2(junctophilin-2,JPH2)[28]等分子,这些分子均可影响RyR2介导的钙泄漏。CaM、FKBP12.6和JPH2等蛋白与RyR2结合可抑制其通道开放,其中CaM可使钙火花发生率降低约70%[29],而开关蛋白FKBP12.6降低了18%的钙火花发生率[30],JPH2基因突变或其蛋白表达降低均可增加钙火花的发生率[28, 31]。研究发现,心肌肥大患者心肌中CaMKⅡ和PKA均可导致RyR2蛋白磷酸化,但与正常对照患者差异无统计学意义,抑制这些蛋白激酶可减少钙泄漏;相较于心肌肥大患者,HF患者心肌细胞钙泄漏增加近2倍,CaMKⅡ导致的RyR2磷酸化也增加,但PKA的磷酸化作用未见显著改变[30]。受制于疾病种类和发展阶段的复杂性,目前调节RyR2功能活性的具体激酶作用尚未有定论。此外,RyR2通道自身点突变是导致心律失常疾病,如儿茶酚胺引起的多形性室性心动过速(catecholamine-induced polymorphic ventricular tachycardia,CPVT)的重要机制之一[32]。研究发现目前已知最严重的CPVT相关点突变K4750Q可通过降低细胞质Ca2+浓度激活RyR2通道阈值、抑制细胞质Ca2+/Mg2+介导的RyR2通道失活以及降低SR Ca2+浓度激活RyR2通道阈值等机制诱发钙泄漏[32]。
3 钙泄漏与AF心房肌细胞钙稳态异常是AF发生的重要机制之一,SR钙通道RyR2稳定性下降导致的钙泄漏是钙稳态异常的重要原因。心肌细胞SR RyR2是最主要的钙释放通道,在心肌的舒张期应保持理想的关闭状态,其对细胞内钙稳态具有重要作用[33]。舒张期RyR2异常开放会导致钙泄漏增多,产生钙火花以及钙波等自发性钙释放事件,从而导致细胞质Ca2+浓度异常升高,影响心肌细胞的钙稳态[33]。在这种情况下,Ca2+通过细胞膜上的NCX1外排,与细胞外的Na+以1:3的比例进行交换,进而产生异常的净内向阳离子移动即瞬时内向电流(transient inward current,Iti),导致心肌细胞延迟后除极(delayed after depolarization,DAD)。一旦达到心肌细胞兴奋的阈值,即可诱发自发性动作电位和心肌局部的异位电触发活动,而局部电活动可进展为折返环(reentrant circuit)维持AF[33]。
前期动物模型及患者来源标本的研究提示,RyR2通道功能异常和SR自发钙泄漏事件发生率增高等致心律失常性的异常电触发活动是AF发生的可能机制。研究发现,阵发性AF患者的发病机制包括SERCA2a活性增强导致的SR钙含量增多和RyR2蛋白表达水平及开放率增加[33],如miR-106b-25缺乏可导致RyR2蛋白表达水平的增加,最终增加AF的风险[5]。此外,JPH2是新发现的位于心肌横管与SR膜之间的膜偶联蛋白,研究发现JPH2-E169K突变的肥厚性心肌病患者由于RyR2与JPH2的结合减少、RyR2释放钙火花和钙泄漏增多,可出现阵发性AF的临床表现[30]。
目前研究认为,慢性AF或长程持续性AF患者的发病机制可能与PKA和CaMKⅡ分别作用于RyR2的S2808和S2814位点导致RyR2超磷酸化[34],从而增大了RyR2通道的开放概率有关。磷酸酶活性的调节异常,如磷酸酶抑制剂1(phosphatase inhibitor 1,PPI-1)的功能异常也可能导致心律失常患者的RyR2超磷酸化[35]。此外,线粒体氧化应激会导致RyR2通道被氧化,钙泄漏增多,进而导致心律失常的发生,而针对线粒体的抗氧化剂可减轻这一现象[36]。Li等[4]研究发现,钙泄漏在AF中的作用不仅限于诱发心房肌细胞异常电活动,亦可经由钙调磷酸酶/活化T淋巴细胞核因子等钙依赖结构重构机制,参与AF异常基质的生成,如心房扩大、传导障碍、心肌肥厚等,进而导致AF的进展。近期研究发现,晚钠通道可以通过激活CaMKⅡ和PKA参与心房肌细胞的钙稳态异常,抑制晚钠电流可减少患者心房肌细胞钙泄漏[11]。之后研究证实CaMKⅡ主要参与经RyR2的钙泄漏,而PKA参与增强心房肌细胞的SR摄钙,可能是通过磷酸化受磷蛋白(phospholamban,PLN)“去抑制”PLN/SERCA2a通路发挥作用,提示CaMKⅡ及PKA的共同激活参与了钙稳态异常的形成与维持,是AF发生的重要机制之一。
4 小结大量临床和动物实验研究表明,RyR2活性异常和钙泄漏增加可导致DAD及异常电触发活动,但由于AF临床发展阶段和AF动物模型的不同,心房肌细胞钙泄漏的具体机制各有不同。因此,进一步研究单个RyR2通道的特性、RyR2通道的翻译后调节机制、细胞水平DAD的检测,以及不同AF发展阶段患者心肌标本的生物学改变,对于阐明AF发生、发展的分子机制非常重要。而研发针对RyR2通道功能异常的药物有望从钙稳态异常的临床治疗思路出发,抑制钙泄漏诱发的电活动异常及其导致的折返基质等心房结构重构,临床应用前景良好。
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