脊髓缺血再灌注时血脊髓屏障损伤机制及远端缺血预处理对其作用的研究进展

2016-10-19 04:32余奇劲黄锦秀胡霁
中国医药导报 2016年9期

余奇劲 黄锦秀 胡霁

[摘要] 血-脊髓屏障(BSCB)是血液循环和脊髓组织之间的生理和代谢物质扩散的屏障。BSCB的破坏是脊髓缺血再灌注损伤(SCIRI)的重要病理改变之一,在SCIRI的发生发展中起关键作用。药物、缺血预处理等可通过保护BSCB完整性减轻SCIRI。远端缺血预处理(RIPC)可对SCIRI产生保护作用,但其内在机制仍有待阐明。本文就BSCB的形态结构与功能、SCIRI后BSCB损伤的机制及RIPC对SCIRI的保护作用作一综述。

[关键词] 脊髓缺血;再灌注损伤;缺血预处理;血-脊髓屏障

[中图分类号] R651.2 [文献标识码] A [文章编号] 1673-7210(2016)03(c)-0068-05

[Abstract] The blood-spinal cord barrier (BSCB) is the physiological and metabolic substance diffusion barrier between blood circulation and spinal cord tissues. The disruption of the BSCB is one of the important pathological changes in the course of spinal cord ischemia reperfusion injury (SCIRI), which plays a pivotal role in the development and progression of SCIRI. Some interventions, such as drugs and ischemic preconditioning, can prevent the SCIRI by keeping BSCB intact. Remote ischemic preconditioning (RIPC) can prevent the SCIRI, but the internal mechanism remains to be elucidated. This paper reviews the morphological structure and function of the BSCB, the injury mechanism of BSCB resulting from SCIRI, and the effect of RIPC on it.

[Key words] Spinal cord ischemia; Reperfusion injury; Ischemic preconditioning; Blood-spinal cord barrier

胸腹主动脉瘤修复术可导致脊髓缺血再灌注损伤(spinal cord ischemic reperfusion injury,SCIRI),据报道其发生率为1%~32%,而脊髓损伤后一个灾难性的、不可预知的并发症是截瘫[1-2]。SCIRI的发生机制主要包括氧自由基介导的脂质过氧化作用、细胞内钙超载、白细胞活化、炎性反应及细胞凋亡等。其中,血-脊髓屏障(blood-spinal cord barrier,BSCB)的破坏是SCIRI的一个重要病理改变,它可加剧脊髓水肿,增加白细胞浸润,放大炎性反应及氧化应激,因而在SCIRI的演变及神经元的进一步损害中起重要作用[3-5]。BSCB是血液循环和脊髓组织之间的生理和代谢物质扩散的屏障,严格地调控着脊髓微环境的稳态,因此早期修复BSCB对于防治脊髓损伤具有十分重要的意义。

远端缺血预处理(remote ischemic preconditioning,RIPC)是指对非靶组织或器官进行短暂的几个循环的缺血再灌注后可对随后远隔组织或器官长时间持续性的缺血产生保护作用。已有研究表明,RIPC可对SCIRI产生保护作用[6-8],但其内在机制仍不十分清楚。本文就BSCB的形态结构与功能、SCIRI后BSCB损伤的机制及远端缺血预处理对SCIRI的保护作用作一综述。

1 BSCB的形态结构与功能

1.1 BSCB的形态结构

与血-脑屏障(blood-brain barrier,BBB)相似,BSCB的基本结构包括毛细血管内皮细胞及其间的紧密连接(tight junctions,TJ)、基膜、周细胞和星形胶质细胞终足[9]。BSCB的毛细血管内皮细胞与外周血循环的内皮细胞不同,其细胞膜无窗孔,胞浆中有含量十分丰富的线粒体,缺乏胞饮小泡,胞饮活性十分微弱[10]。内皮细胞间的紧密连接结构由一些特定跨膜蛋白组成,包括claudins(比如claudin-1、claudin-3、claudin-5)、occludin以及连接黏附分子(junction adherence molecular,JAM)。这些跨膜蛋白通过锚定于其上的胞质附着蛋白(如ZO-1、ZO-2、ZO-3)而与胞浆中的细胞骨架蛋白相互作用[11]。基膜環绕毛细血管内皮细胞及周细胞,其主要组成成分包括胶原蛋白、弹性蛋白、纤粘连蛋白、层粘连蛋白以及蛋白多糖等[12]。星形胶质细胞是中枢神经系统主要的胶质细胞,它发出足突包绕神经元突起及血管。

1.2 BSCB各个组成元素的功能

毛细血管内皮细胞是BSCB结构中最重要的组成部分,它严格地控制血源性物质的跨细胞自由转运,细胞内丰富的线粒体可为选择性主动转运提供能量并维持钙离子稳态[13]。此外,内皮细胞还可表达抗氧化剂血红素氧合酶-1(heme oxygenase,HO-1)及脑源性神经营养因子(brain derived neurotrophic factor,BDNF),HO-1、BDNF表达含量的增加有助于神经功能的修复[14-15]。内皮细胞间的紧密连接严格限制细胞旁转运途径。周细胞是小的血管壁细胞,与内皮细胞有共同的基膜,并对内皮细胞的增殖、迁移、分化起重要的调节作用[9]。星形胶质细胞足突可表达水通道蛋白4(aquaporin 4,AQP-4),AQP-4在中枢神经系统水平衡中发挥重要的调控作用[16]。

2 SCIRI后BSCB损伤的机制

2.1 MMPs介导BSCB的破坏

MMPs是锌依赖性肽链内切酶,可降解和重塑包括基膜蛋白、紧密连接蛋白等的细胞外基质。正常情况下,MMPs以无活性的酶原形式分泌,在缺血再灌注损伤中,炎症细胞产生的大量活性氧物质以及促炎细胞因子[如肿瘤坏死因子α(TNF-α)、白细胞介素(IL)-1β]可强烈促进MMPs的表达和活化[17]。其中,MMP-9是研究最为广泛的酶。Fang等[18]和Li等[19]研究发现,MMP-9表达的上调可增加BSCB通透性,MMP-9还参与小胶质细胞的浸润、迁移,增加促炎细胞因子和趋化因子的产生,从而放大炎性反应,进一步加重BSCB的破坏和神经元凋亡。通过药物如右美托咪定、七氟烷预处理或者鞘内进行骨髓基质细胞移植均可减少SCIRI中MMP-9的表达,从而保护BSCB完整性,改善神经功能[18-20]。以上研究表明,通过一定的干预措施抑制MMP-9的表达将有助于稳定BSCB结构,减轻SCIRI。

2.2 炎性反应介导BSCB的破坏

炎症因子在BSCB损伤中发挥重要作用,它可使ZO-1从细胞骨架复合体中解离,可上调MMPs及TNF-α的表达水平,从而引起BSCB通透性增加,而BSCB的破坏又可进一步加剧炎性反应,从而使脊髓发生严重的不可逆的损害[4]。最近,TLRs(Toll-like receptors),尤其是TLR4,因其在SCIRI后炎症应答中的重要作用而引起广泛关注。TLR4是一组调控固有免疫应答的跨膜蛋白,在小胶质细胞膜上表达量最多,可特异性识别LPS配体。有研究表明,SCIRI可引起小胶质细胞早期大量持续的活化,活化的小胶质细胞膜表面表达TLR4增加,一旦TLR4与配体结合即可触发NF-κB从胞质转位到胞核,进而调控其靶基因IL-1β的表达。TLR4-小胶质细胞-NF-κB/IL-1β信号通路可形成正反馈加重炎性反应及BSCB损害[6]。此外,研究发现TLR4通过作用于其下游受体MyD88及TRIF激活NF-κB,TLR4/MyD88通路主要在早期炎症阶段起作用,而TLR4/TRIF通路主要在晚期炎症阶段起作用,并可被MyD88信号通路放大[5]。

炎性反应和BSCB的破坏均是SCIRI的重要病理生理机制,炎性反应可破坏BSCB结构,而BSCB通透性的增加又反过来增加白细胞浸润,放大炎症应答,从而形成恶性循环,进一步加重脊髓组织的损伤。

2.3 氧化应激介导BSCB的破坏

在正常生理条件下,机体的氧化系统和抗氧化系统处于动态平衡状态,但在缺血再灌注发生时,大量炎症细胞的浸润使得氧自由基大量产生,从而使这种动态平衡被打破。有研究提示,过多的超氧化物的产生可损害血脑屏障的内皮细胞[21]。超氧化物可与NO结合形成过氧亚硝基,过氧亚硝基可通过脂质过氧化、消耗内源性抗氧化酶及诱导线粒体衰竭而引起微血管的严重损伤[22]。研究提示,氧化应激可导致重要紧密连接蛋白如claudin-5、occludin、ZO-1和JAM-1的表达下调或重排[21-23],而NO、活性氧物质及过氧亚硝基均可激活MMP-9,加剧紧密连接蛋白和基膜蛋白的降解,从而进一步增加BBB通透性。因此,通过一定的干预措施减少SCIRI中的活性氧物质的产生,或者增强脊髓组织的抗氧化能力,有助于维持BSCB完整性。

最近有研究提示,一些药物或化合物可通过上调抗氧化剂HO-1的表达使紧密连接蛋白ZO-1、occludin的表达增加,减轻BBB/BSCB通透性,改善中枢屏障功能[24-25]。另外,远端肢体缺血后处理可通过激活Nrf2-ARE通路上调HO-1的表达发挥抗氧化作用,从而减轻脑缺血再灌注损伤[26]。HO-1通路对于稳定BBB/BSCB在氧化应激状态下的完整性是十分关键的。

2.4 AQP-4的作用

AQPs是一组提供水跨膜转运的水通道蛋白。其中,AQP-4在中枢神经系统中含量最为丰富,主要表达于包绕毛细血管的星形胶质细胞足突。在缺血再灌注损伤中,大脑海马CA1和皮层区域的AQP-4表达明显增加,AQP-4在缺血导致的脑水肿中起重要作用,而在AQP-4敲除的小鼠,脑缺血后细胞毒性脑水肿减轻且神经功能得到改善[27]。在远端缺血后处理对脑缺血再灌注损伤保护作用研究中,脑水肿的减轻伴随着AQP-4表达的下降[28]。同样的,AQP-4在脊髓组织的水平衡调控中也发挥重要作用。AQP-4的表达与脊髓水肿呈正相关[29]。一些药物预处理方法可下调AQP-4的表达从而减轻SCIRI后的脊髓水肿[30]。

3 RIPC对SCIRI的保护作用

RIPC是一种创新。目前,在临床上应用比较广泛的RIPC方法是使用血压袖带绑扎上肢,对上肢进行短暂的几个循环的缺血再灌注处理。大量临床证据也已表明RIPC可对多种器官手术如心脏冠脉搭桥手术、经皮冠脉介入术、选择性颈椎减压术、肾移植术、腹主动脉瘤术等产生器官保护作用[31]。

近几年,一些动物实验已表明RIPC可对SCIRI产生保护作用。Dong等[7]通过对兔双侧股动脉进行缺血预处理,發现可明显改善SCIRI后的神经功能并减轻组织损伤。最近,有学者发现,对猪左后肢进行短暂的缺血预处理可保护脊髓免受缺血损伤[7]。在临床研究中,RIPC对脊髓缺血的保护作用也得到了验证。Hu等[32]通过对要进行选择性颈椎减压术患者的右上肢进行缺血预处理,发现其有助于术后早期患者神经功能的恢复。

尽管很多研究已提示RIPC对脊髓缺血的保护作用,但其内在的保护机制仍不十分清楚。体液通路可能参与了RIPC的保护作用机制,之前的研究表明热休克蛋白、内源性大麻素、活性氧物质触发的抗氧化通路介导了RIPC对脊髓缺血的耐受[8-9,33]。近几年,BSCB在SCIRI中的作用引起了关注。Fang等[3]发现缺血预处理可减轻缺血段脊髓水肿程度,降低BSCB通透性。Ren等[34]研究发现远端缺血后处理可减轻鼠BBB通透性及缺血脑组织水肿程度,从而减少缺血再灌注损伤所致脑梗死面积。最近,Li等[35]发现远端缺血后处理可减轻脑缺血后的水肿程度及BBB通透性,并可下调星形胶质细胞AQP-4的表达,因而推测远端缺血后处理可通过下调AQP-4的表达改善神经学功能。

4 結语

SCIRI后BSCB的破坏主要是由MMPs、炎性反应、氧化应激及AQP-4共同作用导致的,但究竟哪种因素占主导、各因素发生的先后顺序以及它们之间的关联性仍然不是很清楚。研究已证实RIPC可对SCIRI起到保护作用,但其内在机制仍不清楚,并且RIPC的实施方法并不统一,究竟哪种方法更好还未见文献报道。此外,关于RIPC对SCIRI保护作用的临床证据仍不足,大样本、多中心的随机对照临床试验以及最佳的RIPC方式仍然有待于进一步开展和验证。

远端缺血后处理可通过减轻BBB通透性及脑水肿程度而对脑缺血再灌注损伤产生保护作用,BSCB与BBB同属于中枢神经系统屏障,RIPC是否可通过保护BSCB完整性、减轻脊髓水肿而对脊髓缺血产生耐受,以及内在的保护机制,仍然需要进一步证实和探究。

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(收稿日期:2015-12-05 本文編辑:张瑜杰)