内质网–线粒体交互在心血管疾病中的作用研究进展

线粒体和内质网的稳态在维持心血管正常功能中发挥重要作用,线粒体或内质网的结构功能异常参与了众多心血管疾病的发生发展。近年来研究发现线粒体与内质网存在物理和功能的交互,其交互作用调控线粒体、内质网功能,进而影响心肌细胞和平滑肌细胞的线粒体动力学平衡、钙转运及磷脂合成和转运。内质网–线粒体交互异常被认为是冠心病、心力衰竭、肺动脉高压和动脉粥样硬化等心血管疾病的关键机制。因此,理解内质网-线粒体交互机制可为预防和改善心血管疾病提供崭新靶点。     

血管内皮细胞线粒体动力学相关功能与心血管疾病关系的研究进展

细胞线粒体动力学相关功能是指线粒体通过不断地融合与分裂、线粒体自噬及线粒体-内质网结构偶联来维持细胞正常生理功能的过程。其异常与神经退行性病变、肿瘤、视神经萎缩及糖尿病等疾病的发生发展关系密切。近年来,血管内皮细胞（vascular endothelial cell,VEC）线粒体相关功能在心血管疾病中的研究受到广泛关注,研究发现VEC线粒体相关功能异常在心肌缺血/再灌注（I/R）损伤、冠状动脉粥样硬化、肺动脉高压及扩张型心肌病等疾病的发生发展中发挥重要作用。本文就VEC线粒体动力学相关功能及与心血管疾病的关系进行简要阐述。     

去除Mfn2基因PKA磷酸化位点对血管平滑肌细胞增殖的影响

目的:研究大鼠线粒体融合素基因2(Mfn2)在去除蛋白激酶A(PKA)磷酸化位点后对大鼠血管平滑肌细胞(VSMC)增殖的影响及其相关的信号通路.方法:构建携带去除PKA磷酸化位点的Mfn2重组腺病毒[Adv-Mfn2-PKA(△)]和携带Mfn2的重组腺病毒(Adv-Mfn2)并感染VSMC.Western blot分析法Mfn2-PKA(△)和Mfn2蛋白的表达;激光共聚焦显微镜观察其亚细胞定位;荧光显微镜观察细胞形态变化;四甲基偶氮唑盐(MTT)法比较其对细胞增殖的影响;Western blot法分析磷酸化ERK1/2(p-ERK1/2)蛋白表达变化.结果:激光共聚焦显微镜示Mfn2-PKA(△)与Mfn2蛋白都主要分布于线粒体外膜;荧光显微镜示Mfn2组细胞数较少,而Mfn2-PKA(△)组与对照组相似;MTT示,Mfn2-PKA(△)抑制VSMC增殖作用较Mfn2显著减弱(P＜0.01),与对照组无显著差异;Western blot结果显示,Mfn2-PKA(△)较Mfn2组p-ERK1/2表达显著升高(P＜0.01),与对照组无明显差异.结论:Mfn2-PKA(△)与Mfn2蛋白一样都定位于线粒体外膜,但抑制VSMC增殖的作用消失,对ERK1/2信号通路无抑制作用.表明PKA磷酸化位点是调控Mfn2抗VSMC增殖的重要功能位点.     

线粒体动力学异常与相关心血管疾病

线粒体是心肌能量代谢的主要场所,其通过分裂和融合的动态平衡维持正常的形态和功能.线粒体分裂和融合的动态转换称为线粒体动力学,受线粒体融合和分裂相关蛋白等多种蛋白调控.线粒体动力失衡可引起心脏结构和功能的紊乱,参与扩张型心肌病、缺血再灌注损伤、脓毒性心肌病、糖尿病心肌病和动脉粥样硬化等心血管疾病的发生和发展.维持线粒体动...     

线粒体动力学、线粒体自噬和心血管疾病

线粒体是一种动态的细胞器,通过响应各种代谢和环境的信号, 分裂和融合改变其形态和结构,从而维持细胞的正常功能。它们短暂而快速的形态变化对于细胞周期、免疫、凋亡和线粒体自噬的质量控制等许多复杂的细胞过程至关重要。线粒体自噬与线粒体质量控制密切相关,通过将受损的功能障碍的线粒体转运到溶酶体进行降解,促进心肌细胞受损线粒体的更新,并有效地抑制功能障碍线粒体的积累。由于心脏作为一个复杂而高耗能的器官,心肌细胞严重地依赖线粒体氧化代谢过程作为其能量和营养供应的来源。许多研究表明,线粒体融合、分裂和线粒体自噬的诸多影响和调控功能的因子都与各种心血管疾病有关,维持线粒体的功能和其完整性对正常心肌细胞的运行是至关重要的。在这篇的综述中,我们将重点概述一下线粒体的融合、分裂和线粒体自噬的诸多调控因子与心血管疾病的最新研究进展。     

线粒体内质网结构偶联的蛋白组成和功能

线粒体内质网结构偶联(MAM)是线粒体与内质网二者之间形成的一个动态膜偶联结构,并且MAM可以参与这两个细胞器之间信息交流。研究证实MAM参与调控钙信号、脂质平衡、线粒体动态变化、线粒体自噬和内质网应激反应等。MAM与心血管疾病、神经退行性疾病和代谢性疾病等密切相关。本文综述了MAM的蛋白组成、功能以及与疾病的关系。     

线粒体相关内质网膜:心血管疾病治疗的新靶点

线粒体相关内质网膜是指内质网和线粒体之间高度动态的紧密连接部分,参与维持内质网和线粒体的正常功能,与细胞脂质代谢、钙稳态、线粒体动力学、自噬和凋亡、内质网应激和炎症等密切相关。研究显示线粒体相关内质网膜功能异常或者数量和结构改变参与心血管疾病的发生发展。本文总结了线粒体相关内质网膜的功能,阐述了其在心血管疾病中的作用及可能机制,为线粒体相关内质网膜成为心血管疾病治疗的新靶点提供理论参考。     

敲低NLRX1可上调线粒体融合蛋白并减轻缺氧心肌细胞凋亡

目的 探究模式识别受体NOD样受体家族成员X1(NLRX1)对心肌细胞缺氧损伤的影响及其分子机制。方法 分离培养SD大鼠乳鼠原代心肌细胞，转染NLRX1-siRNA 48 h后，给予不同时间的氧糖剥夺模拟心肌缺血。采用CCK-8法检测细胞活力，流式细胞术检测细胞凋亡，Western blot检测NLRX1蛋白和线粒体融合相关蛋白（Opa1、Mfn1、Mfn2）表达水平，激光扫描共聚焦显微镜观察线粒体形态。结果 NLRX1在心肌细胞中高表达，心肌细胞氧糖剥夺后NLRX1总蛋白水平不变，胞浆NLRX1上调（P<0.01）。与对照组相比，siRNA沉默NLRX1可上调线粒体融合蛋白Mfn2和Opa1的表达水平，促进缺氧心肌细胞线粒体融合，减轻缺氧诱导的心肌细胞凋亡（P<0.05）。结论 免疫调节分子NLRX1可能具有调控线粒体融合蛋白的作用，阻抑NLRX1可上调融合分子Mfn2, Opa1蛋白表达，减轻缺氧心肌损伤。     

AKAP1调控线粒体稳态及其在心血管疾病中的研究进展

线粒体是细胞的能量工厂,同时也是细胞内信号传导的枢纽,可调节细胞增殖、分化和存活。A型激酶锚定蛋白（A-kinase anchoring protein,AKAP）1是一种支架蛋白,因其可与蛋白激酶（protein kinase,PK） A结合而得名。近年研究发现AKAP1可将多种信号蛋白以及mRNA招募至线粒体外膜,调节线粒体功能及一系列相关生理病理过程。研究报道AKAP1可参与调控心肌肥厚、心肌细胞凋亡、血管舒张等活动,提示AKAP1与心血管疾病密切相关。本文将对AKAP1及其信号复合物调控线粒体功能的分子机制进行综述,并阐述AKAP1在心血管疾病中的研究进展。     

去除蛋白激酶A磷酸化位点的线粒体融合素2基因对血管平滑肌细胞凋亡的影响

目的 研究大鼠线粒体融合素2基因在去除蛋白激酶A磷酸化位点后对大鼠血管平滑肌细胞凋亡的影响及其相关的信号通路.方法 利用携带去除蛋白激酶A磷酸化位点的线粒体融合素2基因重组腺病毒(AdvMfn2-PKA(△))和携带线粒体融合素2基因的重组腺病毒(Adv-Mfn2),感染培养的大鼠血管平滑肌细胞.免疫印迹分析相关蛋白的表达;激光共聚焦显微镜观察其亚细胞定位;细胞凋亡酶联免疫吸附法比较其对细胞凋亡的影响;免疫印迹法分析磷酸化蛋白激酶B蛋白表达变化.结果 外源基因转染后可表达特异性蛋白产物;激光共聚焦显微镜显示去除蛋白激酶A磷酸化位点后的线粒体融合素2基因表达产物主要分布于线粒体外膜;酶联免疫吸附法检测显示去除蛋白激酶A磷酸化位点后,线粒体融合素2基因诱导大鼠血管平滑肌细胞凋亡作用显著减弱(P＜0.01),与空白对照组差异无显著性;免疫印迹检测表明Mfn2-PKA(△)组磷酸化蛋白激酶B表达较Mfn2组显著升高(P＜0.01),与空白对照组差异无显著性.结论 去除蛋白激酶A磷酸化位点的线粒体融合素2基因表达产物仍主要分布于线粒体外膜,但其诱导血管平滑肌细胞凋亡的作用丧失,对蛋白激酶B信号通路也无抑制作用.这表明蛋白激酶A磷酸化位点是调控线粒体融合素2基因诱导血管平滑肌细胞凋亡的重要功能位点.     

Mitofusin 2-containing mitochondrial-reticular microdomains direct rapid cardiomyocyte bioenergetic responses via interorganelle Ca2+ crosstalk

Rationale: Mitochondrial Ca(2+) uptake is essential for the bioenergetic feedback response through stimulation of Krebs cycle dehydrogenases. Close association of mitochondria to the sarcoplasmic reticulum (SR) may explain efficient mitochondrial Ca(2+) uptake despite low Ca(2+) affinity of the mitochondrial Ca(2+) uniporter. However, the existence of such mitochondrial Ca(2+) microdomains and their functional role are presently unresolved. Mitofusin (Mfn) 1 and 2 mediate mitochondrial outer membrane fusion, whereas Mfn2 but not Mfn1 tethers endoplasmic reticulum to mitochondria in noncardiac cells. Objective: To elucidate roles for Mfn1 and 2 in SR-mitochondrial tethering, Ca(2+) signaling, and bioenergetic regulation in cardiac myocytes. Methods and Results: Fruit fly heart tubes deficient of the Drosophila Mfn ortholog MARF had increased contraction-associated and caffeine-sensitive Ca(2+) release, suggesting a role for Mfn in SR Ca(2+) handling. Whereas cardiac-specific Mfn1 ablation had no effects on murine heart function or Ca(2+) cycling, Mfn2 deficiency decreased cardiomyocyte SR-mitochondrial contact length by 30% and reduced the content of SR-associated proteins in mitochondria-associated membranes. This was associated with decreased mitochondrial Ca(2+) uptake (despite unchanged mitochondrial membrane potential) but increased steady-state and caffeine-induced SR Ca(2+) release. Accordingly, Ca(2+)-induced stimulation of Krebs cycle dehydrogenases during β-adrenergic stimulation was hampered in Mfn2-KO but not Mfn1-KO myocytes, evidenced by oxidation of the redox states of NAD(P)H/NAD(P)(+) and FADH(2)/FAD. Conclusions: Physical tethering of SR and mitochondria via Mfn2 is essential for normal interorganelle Ca(2+) signaling in the myocardium, consistent with a requirement for SR-mitochondrial Ca(2+) signaling through microdomains in the cardiomyocyte bioenergetic feedback response to physiological stress.     

Mitochondrial dynamics and cell death in heart failure

The highly regulated processes of mitochondrial fusion (joining), fission (division) and trafficking, collectively called mitochondrial dynamics, determine cell-type specific morphology, intracellular distribution and activity of these critical organelles. Mitochondria are critical for cardiac function, while their structural and functional abnormalities contribute to several common cardiovascular diseases, including heart failure (HF). The tightly balanced mitochondrial fusion and fission determine number, morphology and activity of these multifunctional organelles. Although the intracellular architecture of mature cardiomyocytes greatly restricts mitochondrial dynamics, this process occurs in the adult human heart. Fusion and fission modulate multiple mitochondrial functions, ranging from energy and reactive oxygen species production to Ca²⁺ homeostasis and cell death, allowing the heart to respond properly to body demands. Tightly controlled balance between fusion and fission is of utmost importance in the high energy-demanding cardiomyocytes. A shift toward fission leads to mitochondrial fragmentation, while a shift toward fusion results in the formation of enlarged mitochondria and in the fusion of damaged mitochondria with healthy organelles. Mfn1, Mfn2 and OPA1 constitute the core machinery promoting mitochondrial fusion, whereas Drp1, Fis1, Mff and MiD49/51 are the core components of fission machinery. Growing evidence suggests that fusion/fission factors in adult cardiomyocytes play essential noncanonical roles in cardiac development, Ca²⁺ signaling, mitochondrial quality control and cell death. Impairment of this complex circuit causes cardiomyocyte dysfunction and death contributing to heart injury culminating in HF. Pharmacological targeting of components of this intricate network may be a novel therapeutic modality for HF treatment.     

Dissociation of mitochondrial from sarcoplasmic reticular stress in Drosophila cardiomyopathy induced by molecularly distinct mitochondrial fusion defects

Mitochondrial dynamism (fusion and fission) is responsible for remodeling interconnected mitochondrial networks in some cell types. Adult cardiac myocytes lack mitochondrial networks, and their mitochondria are inherently “fragmented”. Mitochondrial fusion/fission is so infrequent in cardiomyocytes as to not be observable under normal conditions, suggesting that mitochondrial dynamism may be dispensable in this cell type. However, we previously observed that cardiomyocyte-specific genetic suppression of mitochondrial fusion factors optic atrophy 1 (Opa1) and mitofusin/MARF evokes cardiomyopathy in Drosophila hearts. We posited that fusion-mediated remodeling of mitochondria may be critical for cardiac homeostasis, although never directly observed. Alternately, we considered that inner membrane Opa1 and outer membrane mitofusin/MARF might have other as-yet poorly described roles that affect mitochondrial and cardiac function. Here we compared heart tube function in three models of mitochondrial fragmentation in Drosophila cardiomyocytes: Drp1 expression, Opa1 RNAi, and mitofusin MARF RNA1. Mitochondrial fragmentation evoked by enhanced Drp1-mediated fission did not adversely impact heart tube function. In contrast, RNAi-mediated suppression of either Opa1 or mitofusin/MARF induced cardiac dysfunction associated with mitochondrial depolarization and ROS production. Inhibiting ROS by overexpressing superoxide dismutase (SOD) or suppressing ROMO1 prevented mitochondrial and heart tube dysfunction provoked by Opa1 RNAi, but not by mitofusin/MARF RNAi. In contrast, enhancing the ability of endoplasmic/sarcoplasmic reticulum to handle stress by expressing Xbp1 rescued the cardiomyopathy of mitofusin/MARF insufficiency without improving that caused by Opa1 deficiency. We conclude that decreased mitochondrial size is not inherently detrimental to cardiomyocytes. Rather, preservation of mitochondrial function by Opa1 located on the inner mitochondrial membrane, and prevention of ER stress by mitofusin/MARF located on the outer mitochondrial membrane, are central functions of these “mitochondrial fusion proteins”.     

Mitochondria at the interface between danger signaling and metabolism: role of unfolded protein responses in chronic inflammation

Inflammatory bowel diseases (IBDs), like many other chronic diseases, feature multiple cellular stress responses including endoplasmic reticulum (ER) unfolded protein response (UPR). Maintaining protein homeostasis is indispensable for cell survival and, consequently, distinct signaling pathways have evolved to transmit organelle stress. While the ER UPR, aiming to restore ER homeostasis after challenges to ER function, has been extensively studied in the context of chronic diseases, only recently the related mitochondrial UPR (mtUPR), induced by disturbances of mitochondrial proteostasis, has drawn some attention. ER and mitochondria are in close contact and interact physically and functionally. Accumulating data have placed mitochondria at the center of diverse cellular functions and suggest mitochondria as integrators of signaling pathways such as autophagy and inflammation. Consequently, it is likely that mitochondrial stress and ER stress cannot be regarded separately and that mitochondrial stress, as well as ER stress, participates in the pathology of IBD. Protein homeostasis is particularly sensitive toward infections, oxidative stress, and energy deficiency. Thus, environmental disturbances impacting organelle function lead to the concerted activation of distinct UPRs. The metabolic status might therefore serve as an innate mechanism to sense the epithelial environment, including luminal-derived and host-derived factors. This review highlights mtUPR and its interrelation with ER UPR, focuses on recent studies identifying mitochondria as integrators of cellular danger signaling, and, furthermore, illustrates the importance ER UPR and mitochondrial dysfunction in IBD.     

Bax regulates primary necrosis through mitochondrial dynamics

The defining event in apoptosis is mitochondrial outer membrane permeabilization (MOMP), allowing apoptogen release. In contrast, the triggering event in primary necrosis is early opening of the inner membrane mitochondrial permeability transition pore (mPTP), precipitating mitochondrial dysfunction and cessation of ATP synthesis. Bcl-2 proteins Bax and Bak are the principal activators of MOMP and apoptosis. Unexpectedly, we find that deletion of Bax and Bak dramatically reduces necrotic injury during myocardial infarction in vivo. Triple knockout mice lacking Bax/Bak and cyclophilin D, a key regulator of necrosis, fail to show further reduction in infarct size over those deficient in Bax/Bak. Absence of Bax/Bak renders cells resistant to mPTP opening and necrosis, effects confirmed in isolated mitochondria. Reconstitution of these cells or mitochondria with wild-type Bax, or an oligomerization-deficient mutant that cannot support MOMP and apoptosis, restores mPTP opening and necrosis, implicating distinct mechanisms for Bax-regulated necrosis and apoptosis. Both forms of Bax restore mitochondrial fusion in Bax/Bak-null cells, which otherwise exhibit fragmented mitochondria. Cells lacking mitofusin 2 (Mfn2), which exhibit similar fusion defects, are protected to the same extent as Bax/Bak-null cells. Conversely, restoration of fused mitochondria through inhibition of fission potentiates mPTP opening in the absence of Bax/Bak or Mfn2, indicating that the fused state itself is critical. These data demonstrate that Bax-driven fusion lowers the threshold for mPTP opening and necrosis. Thus, Bax and Bak play wider roles in cell death than previously appreciated and may be optimal therapeutic targets for diseases that involve both forms of cell death.     

Biology of endoplasmic reticulum stress in the heart

The endoplasmic reticulum (ER) is a multifunctional intracellular organelle supporting many processes required by virtually every mammalian cell, including cardiomyocytes. It performs diverse functions, including protein synthesis, translocation across the membrane, integration into the membrane, folding, posttranslational modification including N-linked glycosylation, and synthesis of phospholipids and steroids on the cytoplasmic side of the ER membrane, and regulation of Ca(2+) homeostasis. Perturbation of ER-associated functions results in ER stress via the activation of complex cytoplasmic and nuclear signaling pathways, collectively termed the unfolded protein response (UPR) (also known as misfolded protein response), leading to upregulation of expression of ER resident chaperones, inhibition of protein synthesis and activation of protein degradation. The UPR has been associated with numerous human pathologies, and it may play an important role in the pathophysiology of the heart. ER stress responses, ER Ca(2+) buffering, and protein and lipid turnover impact many cardiac functions, including energy metabolism, cardiogenesis, ischemic/reperfusion, cardiomyopathies, and heart failure. ER proteins and ER stress-associated pathways may play a role in the development of novel UPR-targeted therapies for cardiovascular diseases.     

Functional implications of mitofusin 2-mediated mitochondrial-SR tethering

Cardiomyocyte mitochondria have an intimate physical and functional relationship with sarcoplasmic reticulum (SR). Under normal conditions mitochondrial ATP is essential to power SR calcium cycling that drives phasic contraction/relaxation, and changes in SR calcium release are sensed by mitochondria and used to modulate oxidative phosphorylation according to metabolic need. When perturbed, mitochondrial-SR calcium crosstalk can evoke programmed cell death. Physical proximity and functional interplay between mitochondria and SR are maintained in part through tethering of these two organelles by the membrane protein mitofusin 2 (Mfn2). Here we review and discuss findings from our two laboratories that derive from genetic manipulation of Mfn2 and closely related Mfn1 in mouse hearts and other experimental systems. By comparing the findings of our two independent research efforts we arrive at several conclusions that appear to be strongly supported, and describe a few areas of incomplete understanding that will require further study. In so doing we hope to clarify some misconceptions regarding the many varied roles of Mfn2 as both physical trans-organelle tether and mitochondrial fusion protein. This article is part of a Special Issue entitled "Mitochondria: From Basic Mitochondrial Biology to Cardiovascular Disease."     

Mitochondrial-Shaping Proteins in Cardiac Health and Disease – the Long and the Short of It!

Mitochondrial health is critically dependent on the ability of mitochondria to undergo changes in mitochondrial morphology, a process which is regulated by mitochondrial shaping proteins. Mitochondria undergo fission to generate fragmented discrete organelles, a process which is mediated by the mitochondrial fission proteins (Drp1, hFIS1, Mff and MiD49/51), and is required for cell division, and to remove damaged mitochondria by mitophagy. Mitochondria undergo fusion to form elongated interconnected networks, a process which is orchestrated by the mitochondrial fusion proteins (Mfn1, Mfn2 and OPA1), and which enables the replenishment of damaged mitochondrial DNA. In the adult heart, mitochondria are relatively static, are constrained in their movement, and are characteristically arranged into 3 distinct subpopulations based on their locality and function (subsarcolemmal, myofibrillar, and perinuclear). Although the mitochondria are arranged differently, emerging data supports a role for the mitochondrial shaping proteins in cardiac health and disease. Interestingly, in the adult heart, it appears that the pleiotropic effects of the mitochondrial fusion proteins, Mfn2 (endoplasmic reticulum-tethering, mitophagy) and OPA1 (cristae remodeling, regulation of apoptosis, and energy production) may play more important roles than their pro-fusion effects. In this review article, we provide an overview of the mitochondrial fusion and fission proteins in the adult heart, and highlight their roles as novel therapeutic targets for treating cardiac disease.     

From the Cover: Feature Article: Dual autonomous mitochondrial cell death pathways are activated by Nix/BNip3L and induce cardiomyopathy

Dysregulation of programmed cell death due to abnormal expression of Bcl-2 proteins is implicated in cancer, neurodegenerative diseases, and heart failure. Among Bcl-2 family members, BNip proteins uniquely stimulate cell death with features of both apoptosis and necrosis. Localization of these factors to mitochondria and endoplasmic reticulum (ER) provides additional complexity. Previously, we observed regulation of intracellular calcium stores by reticular Nix. Here, we report effects of Nix targeting to mitochondria or ER on cell death pathways and heart failure progression. Nix-deficient fibroblasts expressing mitochondrial-directed or ER-directed Nix mutants exhibited similar cytochrome c release, caspase activation, annexin V and TUNEL labeling, and cell death. ER-Nix cells, but not mitochondrial-Nix cells, showed dissipation of mitochondrial inner membrane potential, Δψ_m, and were protected from cell death by cyclosporine A or ppif ablation, implicating the mitochondrial permeability transition pore (MPTP). ER-Nix cells were not protected from death by caspase inhibition or combined ablation of Bax and Bak. Combined inhibition of caspases and the MPTP fully protected against Nix-mediated cell death. To determine the role of the dual pathways in heart failure, mice conditionally overexpressing Nix or Nix mutants in hearts were created. Cardiomyocte death caused by mitochondrial- and ER-directed Nix was equivalent, but ppif ablation fully protected only ER-Nix. Thus, Nix stimulates dual autonomous death pathways, determined by its subcellular localization. Mitochondrial Nix activates Bax/Bak- and caspase-dependent apoptosis, whereas ER-Nix activates Bax/Bak-independent, MPTP-dependent necrosis. Complete protection against programmed cell death mediated by Nix and related factors can be achieved by simultaneous inhibition of both pathways.