Ephrin-B1 regulates the adult diastolic function through a late postnatal maturation of cardiomyocyte surface crests.

The rod-shaped adult cardiomyocyte (CM) harbors a unique architecture of its lateral surface with periodic crests, relying on the presence of subsarcolemmal mitochondria (SSM) with unknown role. Here, we investigated the development and functional role of CM crests during the postnatal period. We found in rodents that CM crest maturation occurs late between postnatal day 20 (P20) and P60 through both SSM biogenesis, swelling and crest-crest lateral interactions between adjacent CM, promoting tissue compaction. At the functional level, we showed that the P20-P60 period is dedicated to the improvement of relaxation. Interestingly, crest maturation specifically contributes to an atypical CM hypertrophy of its short axis, without myofibril addition, but relying on CM lateral stretching. Mechanistically, using constitutive and conditional CM-specific knock-out mice, we identified ephrin-B1, a lateral membrane stabilizer, as a molecular determinant of P20-P60 crest maturation, governing both the CM lateral stretch and the diastolic function, thus highly suggesting a link between crest maturity and diastole. Remarkably, while young adult CM-specific Efnb1 KO mice essentially exhibit an impairment of the ventricular diastole with preserved ejection fraction and exercise intolerance, they progressively switch toward systolic heart failure with 100% KO mice dying after 13 months, indicative of a critical role of CM-ephrin-B1 in the adult heart function. This study highlights the molecular determinants and the biological implication of a new late P20-P60 postnatal developmental stage of the heart in rodents during which, in part, ephrin-B1 specifically regulates the maturation of the CM surface crests and of the diastolic function.

Kinin B1 receptor: a potential therapeutic target in sepsis-induced vascular hyperpermeability.

BACKGROUND: In sepsis, the endothelial barrier becomes incompetent, with the leaking of plasma into interstitial tissues. VE-cadherin, an adherens junction protein, is the gatekeeper of endothelial cohesion. Kinins, released during sepsis, induce vascular leakage and vasodilation. They act via two G-protein coupled receptors: B1 (B1R) and B2 (B2R). B1R is inducible in the presence of pro-inflammatory cytokines, endotoxins or after tissue injury. It acts at a later stage of sepsis and elicits a sustained inflammatory response. The aim of our study was to investigate the relationships between B1R and VE-cadherin destabilization in vivo in a later phase of sepsis. METHODS: Experimental, prospective study in a university research laboratory. We used a polymicrobial model of septic shock by cecal ligation and puncture in C57BL6 male mice or C57BL6 male mice that received a specific B1R antagonist (R-954). We studied the influence of B1R on sepsis-induced vascular permeability 30 h after surgery for several organs, and VE-cadherin expression in the lung and kidneys by injecting R-954 just before surgery. The 96-h survival was determined in mice without treatment or in animals receiving R-954 as a "prophylactic" regimen (a subcutaneous injection of 200 µg/kg, prior to CLP and 24 h after CLP), or as a "curative" regimen (injection of 100 µg/kg at H6, H24 and H48 post-surgery). RESULTS: B1R inactivation helps to maintain MAP above 65 mmHg but induces different permeability profiles depending on whether or not organ perfusion is autoregulated. In our model, VE-cadherin was destabilized in vivo during septic shock. At a late stage of sepsis, the B1R blockade reduced the VE-cadherin disruption by limiting eNOS activation. The survival rate for mice that received R-954 after sepsis induction was higher than in animals that received an antagonist as a prophylactic treatment. CONCLUSIONS: B1R antagonizing reduced mortality in our model of murine septic shock by limiting the vascular permeability induced by VE-cadherin destabilization through maintenance of the macrohemodynamics, consequently limiting organ dysfunctions.

Structural evidence for a new elaborate 3D-organization of the cardiomyocyte lateral membrane in adult mammalian cardiac tissues.

AIMS: This study explored the lateral crest structures of adult cardiomyocytes (CMs) within healthy and diseased cardiac tissue. METHODS AND RESULTS: Using high-resolution electron and atomic force microscopy, we performed an exhaustive quantitative analysis of the three-dimensional (3D) structure of the CM lateral surface in different cardiac compartments from various mammalian species (mouse, rat, cow, and human) and determined the technical pitfalls that limit its observation. Although crests were observed in nearly all CMs from all heart compartments in all species, we showed that their heights, dictated by the subsarcolemmal mitochondria number, substantially differ between compartments from one species to another and tightly correlate with the sarcomere length. Differences in crest heights also exist between species; for example, the similar cardiac compartments in cows and humans exhibit higher crests than rodents. Unexpectedly, we found that lateral surface crests establish tight junctional contacts with crests from neighbouring CMs. Consistently, super-resolution SIM or STED-based immunofluorescence imaging of the cardiac tissue revealed intermittent claudin-5-claudin-5 interactions in trans via their extracellular part and crossing the basement membrane. Finally, we found a loss of crest structures and crest-crest contacts in diseased human CMs and in an experimental mouse model of left ventricle barometric overload. CONCLUSION: Overall, these results provide the first evidence for the existence of differential CM surface crests in the cardiac tissue as well as the existence of CM-CM direct physical contacts at their lateral face through crest-crest interactions. We propose a model in which this specific 3D organization of the CM lateral membrane ensures the myofibril/myofiber alignment and the overall cardiac tissue cohesion. A potential role in the control of sarcomere relaxation and of diastolic ventricular dysfunction is also discussed. Whether the loss of CM surface crests constitutes an initial and common event leading to the CM degeneration and the setting of heart failure will need further investigation.

G protein stoichiometry dictates biased agonism through distinct receptor-G protein partitioning.

Biased agonism at G protein coupled receptors emerges as an opportunity for development of drugs with enhanced benefit/risk balance making biased ligand identification a priority. However, ligand biased signature, classically inferred from ligand activity across multiple pathways, displays high variability in recombinant systems. Functional assays usually necessity receptor/effector overexpression that should be controlled among assays to allow comparison but this calibration currently fails. Herein, we demonstrate that Gα expression level dictates the biased profiling of agonists and, to a lesser extent of ß-blockers, in a Gα isoform- and receptor-specific way, depending on specific G protein activity in different membrane territories. These results have major therapeutic implications since they suggest that the ligand bias phenotype is not necessarily maintained in pathological cell background characterized by fluctuations in G protein expression. Thus, we recommend implementation of G protein stoichiometry as a new parameter in biased ligand screening programs.

Cardioprotective Angiotensin-(1-7) Peptide Acts as a Natural-Biased Ligand at the Angiotensin II Type 1 Receptor.

Hyperactivity of the renin-angiotensin-aldosterone system through the angiotensin II (Ang II)/Ang II type 1 receptor (AT1-R) axis constitutes a hallmark of hypertension. Recent findings indicate that only a subset of AT1-R signaling pathways is cardiodeleterious, and their selective inhibition by biased ligands promotes therapeutic benefit. To date, only synthetic biased ligands have been described, and whether natural renin-angiotensin-aldosterone system peptides exhibit functional selectivity at AT1-R remains unknown. In this study, we systematically determined efficacy and potency of Ang II, Ang III, Ang IV, and Ang-(1-7) in AT1-R-expressing HEK293T cells on the activation of cardiodeleterious G-proteins and cardioprotective ß-arrestin2. Ang III and Ang IV fully activate similar G-proteins than Ang II, the prototypical AT1-R agonist, despite weaker potency of Ang IV. Interestingly, Ang-(1-7) that binds AT1-R fails to promote G-protein activation but behaves as a competitive antagonist for Ang II/Gi and Ang II/Gq pathways. Conversely, all renin-angiotensin-aldosterone system peptides act as agonists on the AT1-R/ß-arrestin2 axis but display biased activities relative to Ang II as indicated by their differences in potency and AT1-R/ß-arrestin2 intracellular routing. Importantly, we reveal Ang-(1-7) a known Mas receptor-specific ligand, as an AT1-R-biased agonist, selectively promoting ß-arrestin activation while blocking the detrimental Ang II/AT1-R/Gq axis. This original pharmacological profile of Ang-(1-7) at AT1-R, similar to that of synthetic AT1-R-biased agonists, could, in part, contribute to its cardiovascular benefits. Accordingly, in vivo, Ang-(1-7) counteracts the phenylephrine-induced aorta contraction, which was blunted in AT1-R knockout mice. Collectively, these data suggest that Ang-(1-7) natural-biased agonism at AT1-R could fine-tune the physiology of the renin-angiotensin-aldosterone system.

Dual agonist occupancy of AT1-R-α2C-AR heterodimers results in atypical Gs-PKA signaling.

Hypersecretion of norepinephrine (NE) and angiotensin II (AngII) is a hallmark of major prevalent cardiovascular diseases that contribute to cardiac pathophysiology and morbidity. Herein, we explore whether heterodimerization of presynaptic AngII AT1 receptor (AT1-R) and NE α2C-adrenergic receptor (α2C-AR) could underlie their functional cross-talk to control NE secretion. Multiple bioluminescence resonance energy transfer and protein complementation assays allowed us to accurately probe the structures and functions of the α2C-AR-AT1-R dimer promoted by ligand binding to individual protomers. We found that dual agonist occupancy resulted in a conformation of the heterodimer different from that induced by active individual protomers and triggered atypical Gs-cAMP-PKA signaling. This specific pharmacological signaling unit was identified in vivo to promote not only NE hypersecretion in sympathetic neurons but also sympathetic hyperactivity in mice. Thus, we uncovered a new process by which GPCR heterodimerization creates an original functional pharmacological entity and that could constitute a promising new target in cardiovascular therapeutics.

Cardiac fibroblasts regulate sympathetic nerve sprouting and neurocardiac synapse stability.

Sympathetic nervous system (SNS) plays a key role in cardiac homeostasis and its deregulations always associate with bad clinical outcomes. To date, little is known about molecular mechanisms regulating cardiac sympathetic innervation. The aim of the study was to determine the role of fibroblasts in heart sympathetic innervation. RT-qPCR and western-blots analysis performed in cardiomyocytes and fibroblasts isolated from healthy adult rat hearts revealed that Pro-Nerve growth factor (NGF) and pro-differentiating mature NGF were the most abundant neurotrophins expressed in cardiac fibroblasts while barely detectable in cardiomyocytes. When cultured with cardiac fibroblasts or fibroblast-conditioned medium, PC12 cells differentiated into/sympathetic-like neurons expressing axonal marker Tau-1 at neurites in contact with cardiomyocytes. This was prevented by anti-NGF blocking antibodies suggesting a paracrine action of NGF secreted by fibroblasts. When co-cultured with cardiomyocytes to mimic neurocardiac synapse, differentiated PC12 cells exhibited enhanced norepinephrine secretion as quantified by HPLC compared to PC12 cultured alone while co-culture with fibroblasts had no effect. However, when supplemented to PC12-cardiomyocytes co-culture, fibroblasts allowed long-term survival of the neurocardiac synapse. Activated fibroblasts (myofibroblasts) isolated from myocardial infarction rat hearts exhibited significantly higher mature NGF expression than normal fibroblasts and also promoted PC12 cells differentiation. Within the ischemic area lacking cardiomyocytes and neurocardiac synapses, tyrosine hydroxylase immunoreactivity was increased and associated with local anarchical and immature sympathetic hyperinnervation but tissue norepinephrine content was similar to that of normal cardiac tissue, suggesting depressed sympathetic function. Collectively, these findings demonstrate for the first time that fibroblasts are essential for the setting of cardiac sympathetic innervation and neurocardiac synapse stability. They also suggest that neurocardiac synapse functionality relies on a triptych with tight interaction between sympathetic nerve endings, cardiomyocytes and fibroblasts. Deregulations of this triptych may be involved in pathophysiology of cardiac diseases.

Role of endothelial AADC in cardiac synthesis of serotonin and nitrates accumulation.

Serotonin (5-HT) regulates different cardiac functions by acting directly on cardiomyocytes, fibroblasts and endothelial cells. Today, it is widely accepted that activated platelets represent a major source of 5-HT. In contrast, a supposed production of 5-HT in the heart is still controversial. To address this issue, we investigated the expression and localization of 5-HT synthesizing enzyme tryptophan hydroxylase (TPH) and L-aromatic amino acid decarboxylase (AADC) in the heart. We also evaluated their involvement in cardiac production of 5-HT. TPH1 was weakly expressed in mouse and rat heart and appeared restricted to mast cells. Degranulation of mast cells by compound 48/80 did not modify 5-HT cardiac content in mice. Western blots and immunolabelling experiments showed an abundant expression of AADC in the mouse and rat heart and its co-localization with endothelial cells. Incubation of cardiac homogenate with the AADC substrate (5-hydroxy-L-tryptophan) 5-HTP or intraperitoneal injection of 5-HTP in mice significantly increased cardiac 5-HT. These effects were prevented by the AADC inhibitor benserazide. Finally, 5-HTP administration in mice increased phosphorylation of aortic nitric oxide synthase 3 at Ser (1177) as well as accumulation of nitrates in cardiac tissue. This suggests that the increase in 5-HT production by AADC leads to activation of endothelial and cardiac nitric oxide pathway. These data show that endothelial AADC plays an important role in cardiac synthesis of 5-HT and possibly in 5-HT-dependent regulation of nitric oxide generation.

Deciphering biased-agonism complexity reveals a new active AT1 receptor entity.

Functional selectivity of G protein-coupled receptor (GPCR) ligands toward different downstream signals has recently emerged as a general hallmark of this receptor class. However, pleiotropic and crosstalk signaling of GPCRs makes functional selectivity difficult to decode. To look from the initial active receptor point of view, we developed new, highly sensitive and direct bioluminescence resonance energy transfer-based G protein activation probes specific for all G protein isoforms, and we used them to evaluate the G protein-coupling activity of [(1)Sar(4)Ile(8)Ile]-angiotensin II (SII), previously described as an angiotensin II type 1 (AT(1)) receptor-biased agonist that is G protein independent but ß-arrestin selective. By multiplexing assays sensing sequential signaling events, from receptor conformations to downstream signaling, we decoded SII as an agonist stabilizing a G protein-dependent AT(1A) receptor signaling module different from that of the physiological agonist angiotensin II, both in recombinant and primary cells. Thus, a biased agonist does not necessarily select effects from the physiological agonist but may instead stabilize and create a new distinct active pharmacological receptor entity.

Ephrin-B1 is a novel specific component of the lateral membrane of the cardiomyocyte and is essential for the stability of cardiac tissue architecture cohesion.

RATIONALE: Cardiac tissue cohesion relying on highly ordered cardiomyocytes (CM) interactions is critical because most cardiomyopathies are associated with tissue remodeling and architecture alterations. OBJECTIVE: Eph/ephrin system constitutes a ubiquitous system coordinating cellular communications which recently emerged as a major regulator in adult organs. We examined if eph/ephrin could participate in cardiac tissue cyto-organization. METHODS AND RESULTS: We reported the expression of cardiac ephrin-B1 in both endothelial cells and for the first time in CMs where ephrin-B1 localized specifically at the lateral membrane. Ephrin-B1 knock-out (KO) mice progressively developed cardiac tissue disorganization with loss of adult CM rod-shape and sarcomeric and intercalated disk structural disorganization confirmed in CM-specific ephrin-B1 KO mice. CMs lateral membrane exhibited abnormal structure by electron microscopy and notably increased stiffness by atomic force microscopy. In wild-type CMs, ephrin-B1 interacted with claudin-5/ZO-1 complex at the lateral membrane, whereas the complex disappeared in KO/CM-specific ephrin-B1 KO mice. Ephrin-B1 deficiency resulted in decreased mRNA expression of CM basement membrane components and disorganized fibrillar collagen matrix, independently of classical integrin/dystroglycan system. KO/CM-specific ephrin-B1 KO mice exhibited increased left ventricle diameter and delayed atrioventricular conduction. Under pressure overload stress, KO mice were prone to death and exhibited striking tissue disorganization. Finally, failing CMs displayed downregulated ephrin-B1/claudin-5 gene expression linearly related to the ejection fraction. CONCLUSIONS: Ephrin-B1 is necessary for cardiac tissue architecture cohesion by stabilizing the adult CM morphology through regulation of its lateral membrane. Because decreased ephrin-B1 is associated with molecular/functional cardiac defects, it could represent a new actor in the transition toward heart failure.

Apelin prevents cardiac fibroblast activation and collagen production through inhibition of sphingosine kinase 1.

AIMS: Activation of cardiac fibroblasts and their differentiation into myofibroblasts is a key event in the progression of cardiac fibrosis that leads to end-stage heart failure. Apelin, an adipocyte-derived factor, exhibits a number of cardioprotective properties; however, whether apelin is involved in cardiac fibroblast activation and myofibroblast formation remains unknown. The aim of this study was to determine the effects of apelin in activated cardiac fibroblasts, the potential related mechanisms and impact on cardiac fibrotic remodelling process. METHODS AND RESULTS: In vitro experiments were performed in mouse cardiac fibroblasts obtained from normal and pressure-overload hearts. Pretreatment of naive cardiac fibroblasts with apelin (1-100 nM) inhibited Transforming growth factor-ß (TGF-ß)-mediated expression of the myofibroblast marker α-smooth muscle actin (α-SMA) and collagen production. Furthermore, apelin decreased the spontaneous collagen production in cardiac fibroblasts isolated from hearts after aortic banding. Knockdown strategy and pharmacological inhibition revealed that prevention of collagen accumulation by apelin was mediated by a reduction in sphingosine kinase 1 (SphK1) activity. In vivo studies using the aortic banding model indicated that pretreatment with apelin attenuated the development of myocardial fibrotic remodelling and inhibited cardiac SphK1 activity and α-SMA expression. Moreover, administration of apelin 2 weeks after aortic banding prevented cardiac remodelling by inhibiting myocyte hypertrophy, cardiac fibrosis, and ventricular dysfunction. CONCLUSION: Our data provide the first evidence that apelin inhibits TGF-ß-stimulated activation of cardiac fibroblasts through a SphK1-dependent mechanism. We also demonstrated that the administration of apelin during the phase of reactive fibrosis prevents structural remodelling of the myocardium and ventricular dysfunction. These findings may have important implications for designing future therapies for myocardial performance during fibrotic remodelling, affecting the clinical management of patients with progressive heart failure.

Characterization of monoamine oxidases in mesenchymal stem cells: role in hydrogen peroxide generation and serotonin-dependent apoptosis.

Early death of grafted bone marrow mesenchymal stem cells (MSCs) represents a major limit to their use in cell therapy of solid organs. It is well known that oxidative stress plays a major role in cell death. We have recently shown that the serotonin-degrading enzyme monoamine oxidase A (MAO-A) generates large amount of hydrogen peroxide (H2O2) responsible for cell apoptosis. Hydrogen peroxide generation requires 5-HT internalization into the cell and its degradation by MAO-A. In the present study, we investigated whether MAO-A is expressed in MSCs and we defined its role in serotonin-dependent MSCs apoptosis. RT-PCR analysis and western blots showed that the serotonin transporter (SERT) and the 2 MAO isoenzymes, A and B, are expressed in MSCs. As shown by enzyme assays using [14C]serotonin or [14C]ß-phenylethylamine as selective MAO-A or MAO-B substrates, MAO-A is largely predominant in MSCs. Incubation of MSCs with the MAO substrate tyramine led to a time-dependent generation of H2O2 that was prevented by the MAO inhibitor pargyline. Finally, exposure of the cells to serotonin promoted an increase in MSCs apoptosis prevented by pargyline and the SERT inhibitor imipramine. The pro-apoptotic effect of serotonin was associated to a decrease in the expression of the anti-apoptotic factor Bcl-2. In conclusion, these results show for the first time that the 5-HT-degrading enzyme MAO-A is an important source of H2O2 in MSCs and plays a major role in 5-HT-dependent MSCs apoptosis.

Mesenchymal stem cells promote matrix metalloproteinase secretion by cardiac fibroblasts and reduce cardiac ventricular fibrosis after myocardial infarction.

Recent studies showed that mesenchymal stem cells (MSCs) transplantation significantly decreased cardiac fibrosis; however, the mechanisms involved in these effects are still poorly understood. In this work, we investigated whether the antifibrotic properties of MSCs involve the regulation of matrix metalloproteinases (MMPs) and matrix metalloproteinase endogenous inhibitor (TIMP) production by cardiac fibroblasts. In vitro experiments showed that conditioned medium from MSCs decreased viability, alpha-smooth muscle actin expression, and collagen secretion of cardiac fibroblasts. These effects were concomitant with the stimulation of MMP-2/MMP-9 activities and membrane type 1 MMP expression. Experiments performed with fibroblasts from MMP2-knockout mice demonstrated that MMP-2 plays a preponderant role in preventing collagen accumulation upon incubation with conditioned medium from MSCs. We found that MSC-conditioned medium also decreased the expression of TIMP2 in cardiac fibroblasts. In vivo studies showed that intracardiac injection of MSCs in a rat model of postischemic heart failure induced a significant decrease in ventricular fibrosis. This effect was associated with the improvement of morphological and functional cardiac parameters. In conclusion, we showed that MSCs modulate the phenotype of cardiac fibroblasts and their ability to degrade extracellular matrix. These properties of MSCs open new perspectives for understanding the mechanisms of action of MSCs and anticipate their potential therapeutic or side effects.

Platelet derived serotonin drives the activation of rat cardiac fibroblasts by 5-HT2A receptors.

Platelet activation occurs in different acute and chronic heart diseases including myocardial infarction, obstructive hypertrophic cardiomyopathy and valve stenosis. Recent studies suggested that some factors secreted by activated platelets may participate in cardiac remodeling. In the present study, we investigated whether platelets and platelet-released serotonin (5-HT) are directly involved in the functional regulation of cardiac fibroblasts. Treatment of neonatal rat cardiac fibroblasts with platelet lysate, 5-HT and the 5-HT2A receptor agonist DOI increased the expression of alpha-SMA protein, a marker of fibroblast differentiation into myofibroblasts. Platelet lysate, 5-HT and DOI also induced a time-dependent stimulation of cardiac fibroblast migration that was inhibited by the 5-HT2A receptor antagonist ketanserin. Finally, incubation of cardiac fibroblasts with platelet lysate or 5-HT enhanced secretion of TGF-beta1 and expression of MMP-3 and MMP-13. As observed for fibroblast migration, these effects were prevented by ketanserin. These results demonstrated for the first time that factors released from platelet directly regulate cardiac fibroblasts by enhancing secretion of TGF-beta1 and MMPs and promoting their migration and differentiation. 5-HT released by platelets appears to be a major contributor of platelet effects which are mediated through 5-HT2A receptors.

Ex vivo pretreatment with melatonin improves survival, proangiogenic/mitogenic activity, and efficiency of mesenchymal stem cells injected into ischemic kidney.

Bone marrow mesenchymal stem cells (MSCs) have shown great potential in cell therapy of solid organs. Approaches to improving the ability of grafted MSCs to survive and secrete paracrine factors represent one of the challenges for the further development of this novel therapy. In the present study, we designed a strategy of ex vivo pretreatment with the pineal hormone melatonin to improve survival, paracrine activity, and efficiency of MSCs. Using a rat model of acute renal failure, we showed that melatonin pretreatment strongly increased survival of MSCs after intraparenchymal injection. This effect was concomitant with overstimulation of angiogenesis, proliferation of renal cells, and accelerated recovery of renal function. To gain insight into the mechanisms involved in the effects observed in vivo, melatonin was tested in vitro on cultured MSCs. Our results show that through stimulation of specific melatonin receptors, melatonin induced an overexpression of the antioxidant enzyme catalase and superoxide dismutase-1 and increased the resistance of MSCs to hydrogen peroxide-dependent apoptosis. Compared with untreated cells, MSCs incubated with melatonin displayed a higher expression of basic fibroblast growth factor and hepatocyte growth factor. In addition, conditioned culture media from melatonin-treated MSCs stimulated tube formation by endothelial progenitor cells and proliferation of proximal tubule cells in culture. In conclusion, our results show that melatonin behaves as a preconditioning agent increasing survival, paracrine activity, and efficiency of MSCs. The use of this molecule for pretreatment of stem cells may represent a novel and safe approach to improving the beneficial effects of cell therapy of solid organs.

Vesicular monoamine transporter 1 mediates dopamine secretion in rat proximal tubular cells.

Renal dopamine, synthesized by proximal tubules, plays an important role in the regulation of renal sodium excretion. Although the renal dopaminergic system has been extensively investigated in both physiological and pathological situations, the mechanisms whereby dopamine is stored and secreted by proximal tubule cells remain obscure. In the present study we investigated whether vesicular monoamine transporters (VMAT)-1 and -2, which participate in amine storing and secretion, are expressed in rat renal proximal tubules, and we defined their involvement in dopamine secretion. By combining RT-PCR, Western blot, and immunocytochemistry we showed that VMAT-1 is the predominant isoform expressed in isolated proximal tubule cells. These results were confirmed by immunohistochemistry analysis of rat renal cortex showing that VMAT-1 was found in proximal tubules but not in glomeruli. Functional studies showed that, as previously reported for VMAT-dependent amine transporters, dopamine release by cultured proximal tubule cells was partially inhibited by disruption of intracellular H(+) gradient. In addition, dopamine secretion was prevented by the VMAT-1/VMAT-2 inhibitor reserpine but not by the VMAT-2 inhibitor tetrabenazine. Finally, we demonstrated that tubular VMAT-1 mRNA and protein expression were significantly upregulated during a high-sodium diet. In conclusion, our results show for the first time the expression of a VMAT in the renal proximal tubule and its involvement in regulation of dopamine secretion. These data represent the first step toward the comprehension of the role of this transporter in renal dopamine handling and its involvement in pathological situations.

Oxidative stress-dependent sphingosine kinase-1 inhibition mediates monoamine oxidase A-associated cardiac cell apoptosis.

The mitochondrial enzyme monoamine oxidase (MAO), its isoform MAO-A, plays a major role in reactive oxygen species-dependent cardiomyocyte apoptosis and postischemic cardiac damage. In the current study, we investigated whether sphingolipid metabolism can account for mediating MAO-A- and reactive oxygen species-dependent cardiomyocyte apoptosis. In H9c2 cardiomyoblasts, MAO-A-dependent reactive oxygen species generation led to mitochondria-mediated apoptosis, along with sphingosine kinase-1 (SphK1) inhibition. These phenomena were associated with generation of proapoptotic ceramide and decrease in prosurvival sphingosine 1-phosphate. These events were mimicked by inhibition of SphK1 with either pharmacological inhibitor or small interfering RNA, as well as by extracellular addition of C(2)-ceramide or H(2)O(2). In contrast, enforced expression of SphK1 protected H9c2 cells from serotonin- or H(2)O(2)-induced apoptosis. Analysis of cardiac tissues from wild-type mice subjected to ischemia/reperfusion revealed significant upregulation of ceramide and inhibition of SphK1. It is noteworthy that SphK1 inhibition, ceramide accumulation, and concomitantly infarct size and cardiomyocyte apoptosis were significantly decreased in MAO-A-deficient animals. In conclusion, we show for the first time that the upregulation of ceramide/sphingosine 1-phosphate ratio is a critical event in MAO-A-mediated cardiac cell apoptosis. In addition, we provide the first evidence linking generation of reactive oxygen species with SphK1 inhibition. Finally, we propose sphingolipid metabolites as key mediators of postischemic/reperfusion cardiac injury.

Oxidative stress by monoamine oxidase mediates receptor-independent cardiomyocyte apoptosis by serotonin and postischemic myocardial injury.

BACKGROUND: Serotonin (5-hydroxytryptamine [5-HT]), released by activated platelets during cardiac ischemia, is metabolized by the mitochondrial enzyme monoamine oxidase A (MAO-A). Because hydrogen peroxide is one of the byproducts of 5-HT degradation by MAO-A, we investigated the potential role of reactive oxygen species generated by MAOs in 5-HT-dependent cardiomyocyte death and post-ischemia-reperfusion cardiac damage. METHODS AND RESULTS: Treatment of isolated adult rat cardiomyocytes with 5-HT induced intracellular oxidative stress and cell apoptosis. The apoptotic cascade triggered by 5-HT involves release of cytochrome c, upregulation of proapoptotic Bax protein, and downregulation of antiapoptotic Bcl-2 protein. These effects were prevented by inhibition of amine transporter or MAO, antioxidants, or iron chelation. In contrast, cardiomyocyte apoptosis was only slightly affected by the 5-HT(2B) receptor antagonist SB 206553. In vivo, inhibition of MAO-A largely reduced myocardial ultrastructural damage induced by 30 minutes of ischemia followed by 60 minutes of reperfusion in the rat heart. Cardioprotective effects of MAO inhibitors were associated with the prevention of postischemic oxidative stress, neutrophil accumulation, and mitochondrial-dependent cell death and were not reverted by SB 206553. Administration of MAO-A inhibitors during ischemia was still effective in preventing cardiac damage. CONCLUSIONS: Our results supply the first direct evidence that oxidative stress induced by MAO is responsible for receptor-independent apoptotic effects of 5-HT in cardiomyocytes and postischemic myocardial injury. These findings provide new insight into the mechanisms of 5-HT action in the heart and may constitute the basis for novel therapies.

Involvement of peripheral benzodiazepine receptor in the oxidative stress, death-signaling pathways, and renal injury induced by ischemia-reperfusion.

The peripheral benzodiazepine receptor (PBR) is a critical component of the mitochondrial permeability transition pore, which is involved in the regulation of cell death. In the present study we investigated the role of PBR in the regulation of signaling pathways leading to apoptotic and necrotic damage and renal dysfunction in a rat model of ischemia-reperfusion. Renal ischemia-reperfusion led to extended tubular apoptosis and necrosis that were associated with peroxidative damage, high levels of proapoptotic Bax expression, and low levels of antiapoptotic Bcl-2 expression, cleavage of death substrate, poly(ADP-ribose) polymerase (PARP), and activation of a key effector of apoptosis, caspase-3. Rat pretreatment with a novel PBR antagonist, SSR180575, significantly decreased postreperfusion oxidative stress and tubular apoptosis and necrosis. This effect was associated with inhibition of caspase-3 activation and PARP cleavage, upregulation of Bcl-2, and downregulation of Bax. Furthermore, inhibition of PBR accelerated the recovery of normal renal function, as assessed by measurement of levels of plasma creatinine and blood urea nitrogen. These findings reveal a role for PBR as a modulator of necrotic and apoptotic cell death induced by ischemia-reperfusion and suggest that regulation of PBR may provide new therapeutic implications for the prevention of acute renal failure.

Prevention of apoptotic and necrotic cell death, caspase-3 activation, and renal dysfunction by melatonin after ischemia/reperfusion.

The pineal hormone melatonin has been reported to protect tissue from oxidative damage. This study was designed to determine whether melatonin could prevent cell events leading to tissue injury and renal dysfunction after ischemia/reperfusion (I/R). Using an in vivo rat model of I/R, we show a significant increase in kidney malondialdehyde concentrations, reflecting lipid peroxidation, and cell apoptosis measured by TUNEL staining. This apoptotic cell death was associated with an increase in the activity of the proapoptotic factor caspase-3, determined by fluorometric protease activity assay. Histomorphological analysis of ischemic kidneys revealed that the most extensive tubular necrosis occurred at 24 and 48 h after reperfusion, which correlated with peak elevations in blood urea nitrogen and creatinine. Rat pretreatment with melatonin prevented lipid peroxidation, cell apoptosis, and necrosis and blocked caspase-3 activity. The prevention of tissue injury was associated with the improvement of renal function as shown by the decrease in blood urea nitrogen and creatinine concentrations. The demonstration that melatonin prevents postreperfusion apoptotic and necrotic cell death and improves renal function suggests that melatonin may represent a novel therapeutic approach for prevention of I/R injury.