Nitric oxide donors protect from myocardial ischemia-reperfusion injury, but the underlying mechanisms have been unclear. Edward T Chouchani et al. uncover the molecular target of such donors, a cysteine residue in a subunit of complex I of the mitochondrial respiratory chain, and suggest that this cysteine residue has a general role in regulating complex I activity and modulating ischemia-reperfusion injury. Oxidative damage from elevated production of reactive oxygen species (ROS) contributes to ischemia-reperfusion injury in myocardial infarction and stroke. The mechanism by which the increase in ROS occurs is not known, and it is unclear how this increase can be prevented. A wide variety of nitric oxide donors and S-nitrosating agents protect the ischemic myocardium from infarction, but the responsible mechanisms are unclear1,2,3,4,5,6. Here we used a mitochondria-selective S-nitrosating agent, MitoSNO, to determine how mitochondrial S-nitrosation at the reperfusion phase of myocardial infarction is cardioprotective in vivo in mice. We found that protection is due to the S-nitrosation of mitochondrial complex I, which is the entry point for electrons from NADH into the respiratory chain. Reversible S-nitrosation of complex I slows the reactivation of mitochondria during the crucial first minutes of the reperfusion of ischemic tissue, thereby decreasing ROS production, oxidative damage and tissue necrosis. Inhibition of complex I is afforded by the selective S-nitrosation of Cys39 on the ND3 subunit, which becomes susceptible to modification only after ischemia. Our results identify rapid complex I reactivation as a central pathological feature of ischemia-reperfusion injury and show that preventing this reactivation by modification of a cysteine switch is a robust cardioprotective mechanism and hence a rational therapeutic strategy.

Background—: Although vascular smooth muscle cell (VSMC) proliferation is implicated in atherogenesis, VSMCs in advanced plaques and cultured from plaques show evidence of VSMC senescence and DNA damage. In particular, plaque VSMCs show shortening of telomeres, which can directly induce senescence. Senescence can have multiple effects on plaque development and morphology; however, the consequences of VSMC senescence or the mechanisms underlying VSMC senescence in atherosclerosis are mostly unknown. Methods and Results—: We examined the expression of proteins that protect telomeres in VSMCs derived from human plaques and normal vessels. Plaque VSMCs showed reduced expression and telomere binding of telomeric repeat-binding factor-2 (TRF2), associated with increased DNA damage. TRF2 expression was regulated by p53-dependent degradation of the TRF2 protein. To examine the functional consequences of loss of TRF2, we expressed TRF2 or a TRF2 functional mutant (T188A) as either gain- or loss-of-function studies in vitro and in apolipoprotein E ^–/– mice. TRF2 overexpression bypassed senescence, reduced DNA damage, and accelerated DNA repair, whereas TRF2 ^188A showed opposite effects. Transgenic mice expressing VSMC-specific TRF2 ^T188A showed increased atherosclerosis and necrotic core formation in vivo, whereas VSMC-specific TRF2 increased the relative fibrous cap and decreased necrotic core areas. TRF2 protected against atherosclerosis independent of secretion of senescence-associated cytokines. Conclusions—: We conclude that plaque VSMC senescence in atherosclerosis is associated with loss of TRF2. VSMC senes cence promotes both atherosclerosis and features of plaque vulnerability, identifying prevention of senescence as a potential target for intervention.

Transplantation of bone marrow cells as well as circulating endothelial progenitor cells (EPC) enhances neovascularization after ischemia. The chemokine receptor CXCR4 is essential for migration and homing of hematopoietic stem cells. Therefore, we investigated the role of CXCR4 and its downstream signaling cascade for the angiogenic capacity of cultured human EPC. Ex vivo, differentiated EPC derived from peripheral blood abundantly expressed CXCR4. Incubation of EPC from healthy volunteers with neutralizing antibodies against CXCR4 profoundly inhibited vascular endothelial growth factor– and stromal-derived factor-1–induced migration as well as EPC-induced angiogenesis in an ex vivo assay. Preincubation of transplanted EPC with CXCR4 antibody reduced EPC incorporation and impaired blood-flow recovery in ischemic hindlimbs of nude mice (57±4% of normal perfusion versus untreated EPC: 80±11%, P <0.001). Bone marrow mononuclear cells (BM-MNC) or EPC of heterozygous CXCR4 ^+/− mice displayed reduced CXCR4 expression and disclosed impaired in vivo capacity to enhance recovery of ischemic blood flow in nude mice (blood flow 27±11% versus 66±25% using wild-type cells, P <0.01). Importantly, impaired blood flow in ischemic CXCR4 ^+/− mice was rescued by injection of wild-type BM-MNC. Next, we investigated the role of CXCR4 for functional capacities of EPC from patients with coronary artery disease (CAD). Surface expression of CXCR4 was similar in EPC from patients with CAD compared with healthy controls. However, basal Janus kinase (JAK)-2 phosphorylation was significantly reduced and less responsive to stromal-derived factor-1 in EPC from patients with CAD compared with healthy volunteers, indicating that CXCR4-mediated JAK-2 signaling is dysregulated in EPC from patients with CAD. The CXCR4 receptor signaling profoundly modulates the angiogenic activity and homing capacity of cultured human EPC. Disturbance of CXCR4 signaling, as demonstrated by reduced JAK-2 phosphorylation, may contribute to functional impairment of EPC from patients with CAD. Stimulating CXCR4 signaling might improve functional properties of EPC and may rescue impaired neovascularization capacity of EPC derived from patients with CAD.

Background— Transplantation of endothelial progenitor cells (EPCs) improves neovascularization after ischemia, but patients with coronary artery disease (CAD) or diabetes mellitus show a reduced number of EPCs and impaired functional activity. Therefore, we investigated the effects of risk factors, such as glucose and TNF-α, on the number of EPCs in vitro to elucidate the underlying mechanisms. Methods and Results— EPCs of patients or healthy subjects were isolated from peripheral blood. Incubation with glucose or TNF-α dose-dependently reduced the number of EPCs (79.9±1.3% and 74.3±8.1% of control; P <0.05, respectively). This reduction was not caused by apoptosis. TNF-α and glucose induced a dose- and time-dependent activation of the p38 MAP kinase, the downstream kinase mitogen- and stress-activated kinase 1, and the transcription factor cAMP-responsive element–binding protein (CREB), in EPCs. Moreover, EPCs from CAD patients had significantly higher basal p38-phosphorylation levels (1.83±0.2-fold increase; P <0.05) compared with healthy subjects. The inhibition of the p38-kinase by SB203580 or infection with a dominant negative p38 kinase adenovirus significantly increased basal number of EPCs (136.7±6.3% and 142.9±18% versus control, respectively). Likewise, ex vivo cultivation of EPCs from patients with CAD with SB203580 significantly increased the number of EPCs and partially reversed the impaired capacity for neovascularization of EPCs in vivo (relative blood flow: 0.40±0.03 versus 0.64±0.08, P <0.05). The increased numbers of EPCs by SB203580 were associated with an augmentation of EPC proliferation and a reduction of cells expressing the monocytic marker proteins CD14 and CD64, suggesting that p38 regulates proliferation and differentiation events. Conclusions— These results demonstrate that p38 MAP kinase plays a pivotal role in the signal transduction pathways regulating the number of EPCs ex vivo. SB203580 can prevent the negative effects of TNF-α and glucose on the number of EPCs and may be useful to improve the number of EPCs for potential cell therapy.

Objective—: Mitochondrial DNA (mtDNA) damage is present in murine and human atherosclerotic plaques. However, whether endogenous levels of mtDNA damage are sufficient to cause mitochondrial dysfunction and whether decreasing mtDNA damage and improving mitochondrial respiration affects plaque burden or composition are unclear. We examined mitochondrial respiration in human atherosclerotic plaques and whether augmenting mitochondrial respiration affects atherogenesis. Approach and Results—: Human atherosclerotic plaques showed marked mitochondrial dysfunction, manifested as reduced mtDNA copy number and oxygen consumption rate in fibrous cap and core regions. Vascular smooth muscle cells derived from plaques showed impaired mitochondrial respiration, reduced complex I expression, and increased mitophagy, which was induced by oxidized low-density lipoprotein. Apolipoprotein E–deficient (ApoE ^−/− ) mice showed decreased mtDNA integrity and mitochondrial respiration, associated with increased mitochondrial reactive oxygen species. To determine whether alleviating mtDNA damage and increasing mitochondrial respiration affects atherogenesis, we studied ApoE ^−/− mice overexpressing the mitochondrial helicase Twinkle (Tw ⁺ /ApoE ^−/− ). Tw ⁺ /ApoE ^−/− mice showed increased mtDNA integrity, copy number, respiratory complex abundance, and respiration. Tw ⁺ /ApoE ^−/− mice had decreased necrotic core and increased fibrous cap areas, and Tw ⁺ /ApoE ^−/− bone marrow transplantation also reduced core areas. Twinkle increased vascular smooth muscle cell mtDNA integrity and respiration. Twinkle also promoted vascular smooth muscle cell proliferation and protected both vascular smooth muscle cells and macrophages from oxidative stress–induced apoptosis. Conclusions—: Endogenous mtDNA damage in mouse and human atherosclerosis is associated with significantly reduced mitochondrial respiration. Reducing mtDNA damage and increasing mitochondrial respiration decrease necrotic core and increase fibrous cap areas independently of changes in reactive oxygen species and may be a promising therapeutic strategy in atherosclerosis.

Objective— Sphingosine-1-phosphate (S1P) is a bioactive lipid, which influences migration and proliferation of endothelial cells through activation of S1P receptors and has been shown to support SDF-1 induced migration and bone marrow homing of CD34 ⁺ progenitors. Methods and Results— Here, we show that incubation of patient-derived endothelial progenitor cells (EPCs) with S1P or its synthetic analog FTY720 improved blood flow recovery in ischemic hind limbs. Likewise, recovery of blood flow was dramatically reduced after induction of hindlimb ischemia in mice deficient for the S1P receptor 3 (S1P ₃ ). S1P ₃ ^−/− bone marrow–derived mononuclear cells (BMCs) failed to augment neovascularization after hind limb ischemia. Of note, treatment of BMCs derived from S1P ₃ ^−/− mice with S1P did not rescue blood flow recovery. Mechanistically, S1P and FTY720 induced phosphorylation of CXCR ₄ , activated the Src kinase, and stimulated phosphorylation of JAK2. The contribution of CXCR ₄ for S1P-mediated effects was further supported by the findings that S1P preincubation failed to stimulate invasion capacity and in vivo blood flow recovery of BMCs from CXCR ₄ ^+/− mice. The activation of CXCR ₄ was dependent on the Src kinase family as demonstrated by preincubation with the Src inhibitor PP2. The activation of the CXCR ₄ signaling by S1P is mediated via the S1P ₃ receptor, since S1P-induced Src phosphorylation was abrogated in EPC from S1P ₃ ^−/− mice. Conclusions— S1P agonists might serve as sensitizers of CXCR ₄ -mediated signaling and may be applied in clinical progenitor cell therapy to improve EPC or BMC function in patients with coronary artery disease.