New insights on receptor-dependent and monoamine oxidase-dependent effects of serotonin in the heart

Monoamine Oxidase A Contributes to Serotonin-But Not Norepinephrine-Dependent Damage of Rat Ventricular Myocytes

Serotonin effects on cardiac hypertrophy, senescence, and failure are dependent either on activation of specific receptors or serotonin uptake and serotonin degradation by monoamine oxidases (MAOs). Receptor-dependent effects are specific for serotonin, but MAO-dependent effects are nonspecific as MAOs also metabolize other substrates such as catecholamines. Our study evaluates the role of MAO-A in serotonin- and norepinephrine-dependent cell damage. Experiments were performed in vivo to study the regulation of MAOA and MAOB expression and in vitro on isolated cultured adult rat ventricular cardiomyocytes (cultured for 24 h) to study the function of MAO-A. MAOA but not MAOB expression increased in maladaptive hypertrophic stages. Serotonin and norepinephrine induced morphologic cell damage (loss of rod-shaped cell structure). However, MAO-A inhibition suppressed serotonin-dependent but not norepinephrine-dependent damages. Serotonin but not norepinephrine caused a reduction in cell shortening in nondamaged cells. Serotonin induced mitochondria-dependent oxidative stress. In vivo, MAOA was induced during aging and hypertension but the expression of the corresponding serotonin uptake receptor (SLC6A4) was reduced and enzymes that reduce either oxidative stress (CAT) or accumulation of 5-hydroxyindolacetaldehyde (ALDH2) were induced. In summary, the data show that MAO-A potentially affects cardiomyocytes' function but that serotonin is not necessarily the native substrate.

Importance of Mitochondria in Cardiac Pathologies: Focus on Uncoupling Proteins and Monoamine Oxidases

On the one hand, reactive oxygen species (ROS) are involved in the onset and progression of a wide array of diseases. On the other hand, these are a part of signaling pathways related to cell metabolism, growth and survival. While ROS are produced at various cellular sites, in cardiomyocytes the largest amount of ROS is generated by mitochondria. Apart from the electron transport chain and various other proteins, uncoupling protein (UCP) and monoamine oxidases (MAO) have been proposed to modify mitochondrial ROS formation. Here, we review the recent information on UCP and MAO in cardiac injuries induced by ischemia-reperfusion (I/R) as well as protection from I/R and heart failure secondary to I/R injury or pressure overload. The current data in the literature suggest that I/R will preferentially upregulate UCP2 in cardiac tissue but not UCP3. Studies addressing the consequences of such induction are currently inconclusive because the precise function of UCP2 in cardiac tissue is not well understood, and tissue- and species-specific aspects complicate the situation. In general, UCP2 may reduce oxidative stress by mild uncoupling and both UCP2 and UCP3 affect substrate utilization in cardiac tissue, thereby modifying post-ischemic remodeling. MAOs are important for the physiological regulation of substrate concentrations. Upon increased expression and or activity of MAOs, however, the increased production of ROS and reactive aldehydes contribute to cardiac alterations such as hypertrophy, inflammation, irreversible cardiomyocyte injury, and failure.

Cardiac Roles of Serotonin (5-HT) and 5-HT-Receptors in Health and Disease

Serotonin acts solely via 5-HT₄-receptors to control human cardiac contractile function. The effects of serotonin via 5-HT₄-receptors lead to positive inotropic and chronotropic effects, as well as arrhythmias, in the human heart. In addition, 5-HT₄-receptors may play a role in sepsis, ischaemia, and reperfusion. These presumptive effects of 5-HT₄-receptors are the focus of the present review. We also discuss the formation and inactivation of serotonin in the body, namely, in the heart. We identify cardiovascular diseases where serotonin might play a causative or additional role. We address the mechanisms which 5-HT₄-receptors can use for cardiac signal transduction and their possible roles in cardiac diseases. We define areas where further research in this regard should be directed in the future, and identify animal models that might be generated to this end. Finally, we discuss in what regard 5-HT₄-receptor agonists or antagonists might be useful drugs that could enter clinical practice. Serotonin has been the target of many studies for decades; thus, we found it timely to summarise our current knowledge here.

Monoamine oxidase A-dependent ROS formation modulates human cardiomyocyte differentiation through AKT and WNT activation

During embryonic development, cardiomyocytes undergo differentiation and maturation, processes that are tightly regulated by tissue-specific signaling cascades. Although redox signaling pathways involved in cardiomyogenesis are established, the exact sources responsible for reactive oxygen species (ROS) formation remain elusive. The present study investigates whether ROS produced by the mitochondrial flavoenzyme monoamine oxidase A (MAO-A) play a role in cardiomyocyte differentiation from human induced pluripotent stem cells (hiPSCs). Wild type (WT) and MAO-A knock out (KO) hiPSCs were generated by CRISPR/Cas9 genome editing and subjected to cardiomyocyte differentiation. Mitochondrial ROS levels were lower in MAO-A KO compared to the WT cells throughout the differentiation process. MAO-A KO hiPSC-derived cardiomyocytes (hiPSC-CMs) displayed sarcomere disarray, reduced α- to β-myosin heavy chain ratio, GATA4 upregulation and lower macroautophagy levels. Functionally, genetic ablation of MAO-A negatively affected intracellular Ca2+ homeostasis in hiPSC-CMs. Mechanistically, MAO-A generated ROS contributed to the activation of AKT signaling that was considerably attenuated in KO cells. In addition, MAO-A ablation caused a reduction in WNT pathway gene expression consistent with its reported stimulation by ROS. As a result of WNT downregulation, expression of MESP1 and NKX2.5 was significantly decreased in MAO-A KO cells. Finally, MAO-A re-expression during differentiation rescued expression levels of cardiac transcription factors, contractile structure, and intracellular Ca2+ homeostasis. Taken together, these results suggest that MAO-A mediated ROS generation is necessary for the activation of AKT and WNT signaling pathways during cardiac lineage commitment and for the differentiation of fully functional human cardiomyocytes.

Monoamine oxidase is a source of cardiac oxidative stress in obese rats: the beneficial role of metformin

Diet-induced metabolic diseases, such as obesity, metabolic syndrome, and type 2 diabetes (T2DM) are the global threatening epidemics that share cardiovascular oxidative stress as common denominator. Monoamine oxidase (MAO) has recently emerged as a constant source of reactive oxygen species (ROS) in DM. Metformin, the first-line drug in T2DM, elicits cardiovascular protection via pleiotropic effects. The present study was aimed to assess the contribution of MAO to the early cardiac oxidative stress in a rat model of high-calorie junk food (HCJF) diet-induced obesity and prediabetes and whether metformin can alleviate it. After 6 months of HCJF, rats developed obesity and hyperglycemia. Hearts were isolated and used for the evaluation of MAO expression and ROS production. Experiments were performed in the presence vs absence of metformin (10 µM) and MAO-A and B inhibitors (clorgyline and selegiline, 10 µM), respectively. Both MAO isoforms were overexpressed and led to increased ROS generation in cardiac samples harvested from the obese animals. Acute treatment with metformin and MAO inhibitors was able to mitigate oxidative stress. More important, metformin downregulated MAO expression in the diseased samples. In conclusion, MAO contributes to oxidative stress in experimental obesity and can be targeted with metformin.

Preserved Left Ventricular Function despite Myocardial Fibrosis and Myopathy in the Dystrophin-Deficient D2.B10-Dmdmdx/J Mouse

Duchenne muscular dystrophy involves an absence of dystrophin, a cytoskeletal protein which supports cell structural integrity and scaffolding for signalling molecules in myocytes. Affected individuals experience progressive muscle degeneration that leads to irreversible loss of ambulation and respiratory diaphragm function. Although clinical management has greatly advanced, heart failure due to myocardial cell loss and fibrosis remains the major cause of death. We examined cardiac morphology and function in D2.B10-Dmd^mdx/J (D2-mdx) mice, a relatively new mouse model of muscular dystrophy, which we compared to their wild-type background DBA/2J mice (DBA/2). We also tested whether drug treatment with a specific blocker of mitochondrial permeability transition pore opening (Debio-025), or ACE inhibition (Perindopril), had any effect on dystrophy-related cardiomyopathy. D2-mdx mice were treated for six weeks with Vehicle control, Debio-025 (20 mg/kg/day), Perindopril (2 mg/kg/day), or a combination (n = 8/group). At 18 weeks, compared to DBA/2, D2-mdx hearts displayed greater ventricular collagen, lower cell density, greater cell diameter, and greater protein expression levels of IL-6, TLR4, BAX/Bcl2, caspase-3, PGC-1α, and notably monoamine oxidases A and B. Remarkably, these adaptations in D2-mdx mice were associated with preserved resting left ventricular function similar to DBA/2 mice. Compared to vehicle, although Perindopril partly attenuated the increase in heart weight and collagen at 18 weeks, the drug treatments had no marked impact on dystrophic cardiomyopathy.

Antioxidant Activity of Tetranitrosyl Iron Complex with Thiosulfate Ligands and Its Effect on Catalytic Activity of Mitochondrial Enzymes In vitro

The antioxidant and antiradical properties of the tetra nitrosyl iron complex with thiosulfate ligands (TNIC) were studied in vitro in mouse brain homogenates. It was found for the first time that TNIC is an effective antioxidant. The effect of TNIC on the catalytic activity of mitochondrial enzymes cytochrome c oxidase and monoamine oxidase A was studied. It was shown for the first time that TNIC is an inhibitor of the catalytic activity of cytochrome c oxidase and monoamine oxidase A in animal brain mitochondria in vitro.

Membranotropic and Antiradical Properties of 2-Nitroxysuccinate 3-Hydroxy-6-Methyl-2-Ethylpyridine

We studied membranotropic properties of NO donor 2-nitroxysuccinate 3-hydroxy-6-methyl-2-ethylpyridine and its structural analog succinate 3-hydroxy-6-methyl-2-ethylpyridine (Mexidol). It was shown that the compounds under study are incorporated into modeled membranes and form long-living complexes with pyrene in the region of fatty acid tails of phospholipids. Luminol-amplified chemiluminescence analysis showed that both compounds exhibited antiradical activity and in a concentration of 0.1 mM reduced chemiluminescence intensity by more than 70%. 2-Nitroxysuccinate 3-hydroxy-6-methyl-2-ethylpyridine inhibited catalytic activity of monoamine oxidase A more efficiently than its structural analogue Mexidol.

MAOA variants differ in oscillatory EEG & ECG activities in response to aggression-inducing stimuli

Among the genetic variations in the monoamine oxidase A (MAOA) gene, upstream variable number tandem repeats (uVNTRs) of the promoter have been associated with individual differences in human physiology and aggressive behaviour. However, the evidence for a molecular or neural link between MAOA uVNTRs and aggression remains ambiguous. Additionally, the use of inconsistent promoter constructs in previous studies has added to the confusion. Therefore, it is necessary to demonstrate the genetic function of MAOA uVNTR and its effects on multiple aspects of aggression. Here, we identified three MAOA alleles in Koreans: the predominant 3.5R and 4.5R alleles, as well as the rare 2.5R allele. There was a minor difference in transcriptional efficiency between the 3.5R and 4.5R alleles, with the greatest value for the 2.5R allele, in contrast to existing research. Psychological indices of aggression did not differ among MAOA genotypes. However, our electroencephalogram and electrocardiogram results obtained under aggression-related stimulation revealed oscillatory changes as novel phenotypes that vary with the MAOA genotype. In particular, we observed prominent changes in frontal γ power and heart rate in 4.5R carriers of men. Our findings provide genetic insights into MAOA function and offer a neurobiological basis for various socio-emotional mechanisms in healthy individuals.

The neuroprotective agent Rasagiline mesylate attenuates cardiac remodeling after experimental myocardial infarction

AIM

Rasagiline mesylate (N-propargyl-1 (R)-aminoindan) (RG) is a selective, potent irreversible inhibitor of monoamine oxidase-B with cardioprotective and anti-apoptotic properties. We investigated whether it could be cardioprotective in a rat model undergoing experimental myocardial infarction (MI) by permanent ligation of the left anterior descending coronary artery.

METHODS AND RESULTS

RG was administered, intraperitoneally, for 28 days (2 mg/kg) starting 24 h after MI induction. Echocardiography analysis revealed a significant reduction in left ventricular end-systolic and diastolic dimensions and preserved fractional shortening in RG-treated compared with normal saline group at 28 days post-MI (31.6 ± 2.3 vs. 19.6 ± 1.8, P < 0.0001), respectively. Treatment with RG prevented tissue fibrosis as indicated by interstitial collagen estimation by immunofluorescence staining and hydroxyproline content and attenuated the number of apoptotic myocytes in the border zone (65%) as indicated by terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL) assay. Caspase 3 relative protein levels were significantly decreased in the non-infarcted myocardium. Markedly decreased malondialdehyde levels in the border zone indicate a reduction in tissue oxidative stress.

CONCLUSIONS

Our study demonstrates a positive effect of RG in the post-MI period with a significant attenuation in cardiac remodelling.

Comparative Analysis of the Neurochemical Profile and MAO Inhibition Properties of N-(Furan-2-ylmethyl)-N-methylprop-2-yn-1-amine

The regulation of brain monoamine levels is paramount for cognitive functions, and the monoamine oxidase (MAO A and B) enzymes play a central role in these processes. The aim of this study was to evaluate whether the procognitive properties exerted by propargylamine N-(furan-2-ylmethyl)-N-methylprop-2-yn-1-amine (F2MPA) are related to changes in monoamine content via MAO inhibition. In vivo microdialysis and ex vivo amine metabolite measurement demonstrated region-specific alterations in monoamine metabolism that differ from both of the classic MAO A and MAO B inhibitors, clorgyline and l-deprenyl, respectively. Although all the inhibitors (1 and 4 mg/kg) increased cortical serotonin tissue content, only F2MPA increased the levels of cortical noradrenaline. In the striatum, clorgyline (1 mg/kg), but not F2MPA (1 mg/kg), reduced extracellular levels of dopamine metabolites at rest or stimulated by the intrastriatal application of the MAO substrate 3-methoxytyramine. In vitro, F2MPA exhibited a low affinity toward MAO B and MAO A. Nonetheless, it modified the B form of MAO, forming a flavin adduct structurally similar to that with deprenyl. F2MPA was rapidly metabolized in the presence of rat but not human microsomes, producing a hydroxylated derivative. In conclusion, the effect of F2MPA on cognition may arise from monoaminergic changes in the cortex, but the role of MAO in this process is likely to be negligible, consistent with the poor affinity of F2MPA for MAO.

Monoamine Oxidase Is Overactivated in Left and Right Ventricles from Ischemic Hearts: An Intriguing Therapeutic Target

Growing evidence indicates that reactive oxygen species (ROS) may play a key role in human heart failure (HF). Monoamine oxidase (MAO) is emerging as a major ROS source in several cardiomyopathies. However, little is known about MAO activity in human failing heart and its relationship with redox imbalance. Therefore, we measured MAO activity in the left (LV) and in the right (RV) ventricle of human nonfailing (NF) and in end-stage ischemic (IHD) and nonischemic failing hearts. We found that both MAO isoforms (MAO-A/B) significantly increased in terms of activity and expression levels only in IHD ventricles. Catalase and aldehyde dehydrogenase-2 activities (ALDH-2), both implicated in MAO-catalyzed catecholamine catabolism, were significantly elevated in the failing LV, whereas, in the RV, statistical significance was observed only for ALDH-2. Oxidative stress markers levels were significantly increased only in the failing RV. Actin oxidation was significantly elevated in both failing ventricles and related to MAO-A activity and to functional parameters. These data suggest a close association between MAO-A-dependent ROS generation, actin oxidation, and ventricular dysfunction. This latter finding points to a possible pathogenic role of MAO-A in human myocardial failure supporting the idea that MAO-A could be a new therapeutic target in HF.

Mitochondrial oxidative metabolism and uncoupling proteins in the failing heart

Despite significant progress in cardiovascular medicine, myocardial ischemia and infarction, progressing eventually to the final end point heart failure (HF), remain the leading cause of morbidity and mortality in the USA. HF is a complex syndrome that results from any structural or functional impairment in ventricular filling or blood ejection. Ultimately, the heart's inability to supply the body's tissues with enough blood may lead to death. Mechanistically, the hallmarks of the failing heart include abnormal energy metabolism, increased production of reactive oxygen species (ROS) and defects in excitation-contraction coupling. HF is a highly dynamic pathological process, and observed alterations in cardiac metabolism and function depend on the disease progression. In the early stages, cardiac remodeling characterized by normal or slightly increased fatty acid (FA) oxidation plays a compensatory, cardioprotective role. However, upon progression of HF, FA oxidation and mitochondrial oxidative activity are decreased, resulting in a significant drop in cardiac ATP levels. In HF, as a compensatory response to decreased oxidative metabolism, glucose uptake and glycolysis are upregulated, but this upregulation is not sufficient to compensate for a drop in ATP production. Elevated mitochondrial ROS generation and ROS-mediated damage, when they overwhelm the cellular antioxidant defense system, induce heart injury and contribute to the progression of HF. Mitochondrial uncoupling proteins (UCPs), which promote proton leak across the inner mitochondrial membrane, have emerged as essential regulators of mitochondrial membrane potential, respiratory activity and ROS generation. Although the physiological role of UCP2 and UCP3, expressed in the heart, has not been clearly established, increasing evidence suggests that these proteins by promoting mild uncoupling could reduce mitochondrial ROS generation and cardiomyocyte apoptosis and ameliorate thereby myocardial function. Further investigation on the alterations in cardiac UCP activity and regulation will advance our understanding of their physiological roles in the healthy and diseased heart and also may facilitate the development of novel and more efficient therapies.

Role of serotonin 5-HT2A receptors in the development of cardiac hypertrophy in response to aortic constriction in mice

Serotonin, in addition to its fundamental role as a neurotransmitter, plays a critical role in the cardiovascular system, where it is thought to be involved in the development of cardiac hypertrophy and failure. Indeed, we recently found that mice with deletion of monoamine oxidase A had enhanced levels of blood and cardiac 5-HT, which contributed to exacerbation of hypertrophy in a model of experimental pressure overload. 5-HT2A receptors are expressed in the heart and mediate a hypertrophic response to 5-HT in cardiac cells. However, their role in cardiac remodeling in vivo and the signaling pathways associated are not well understood. In the present study, we evaluated the effect of a selective 5-HT2A receptor antagonist, M100907, on the development of cardiac hypertrophy induced by transverse aortic constriction (TAC). Cardiac 5-HT2A receptor expression was transiently increased after TAC, and was recapitulated in cardiomyocytes, as observed with 5-HT2A in situ labeling by immunohistochemistry. Selective blockade of 5-HT2A receptors prevented the development of cardiac hypertrophy, as measured by echocardiography, cardiomyocyte area and heart weight-to-body weight ratio. Interestingly, activation of calmodulin kinase (CamKII), which is a core mechanism in cardiac hypertrophy, was reduced in cardiac samples from M100907-treated TAC mice compared to vehicle-treated mice. In addition, phosphorylation of histone deacetylase 4 (HDAC4), a downstream partner of CamKII was significantly diminished in M100907-treated TAC mice. Thus, our results show that selective blockade of 5-HT2A receptors has beneficial effect in the development of cardiac hypertrophy through inhibition of the CamKII/HDAC4 pathway.

Monoamine oxidases in development

Monoamine oxidases (MAOs) are flavoproteins of the outer mitochondrial membrane that catalyze the oxidative deamination of biogenic and xenobiotic amines. In mammals there are two isoforms (MAO-A and MAO-B) that can be distinguished on the basis of their substrate specificity and their sensitivity towards specific inhibitors. Both isoforms are expressed in most tissues, but their expression in the central nervous system and their ability to metabolize monoaminergic neurotransmitters have focused MAO research on the functionality of the mature brain. MAO activities have been related to neurodegenerative diseases as well as to neurological and psychiatric disorders. More recently evidence has been accumulating indicating that MAO isoforms are expressed not only in adult mammals, but also before birth, and that defective MAO expression induces developmental abnormalities in particular of the brain. This review is aimed at summarizing and critically evaluating the new findings on the developmental functions of MAO isoforms during embryogenesis.

From depression to neurodegeneration and heart failure: re-examining the potential of MAO inhibitors

Initially introduced in the 1950s for treating depression, monoamine oxidase (MAO) inhibitors were gradually abandoned, mainly owing to their potential for drug-drug and drug-food interactions, the most widely known being with tyramine-containing food (the 'cheese' effect). Since then, more selective MAO-A or MAO-B inhibitors have been developed with substantially reduced risks, and have been approved for the treatment of depression and Parkinson's disease, respectively. Recent research suggests that some of these drugs also have neuroprotective properties, while preclinical evidence expands the spectrum of potential indications to heart failure, renal diseases and multiple sclerosis. In this article, the authors review the relevance of MAO isoforms to disease, and they also outline current research and development efforts in this class of drugs, including newer multipotent compounds.

On the presence of serotonin in mammalian cardiomyocytes

Pleiotropic effects of serotonin (5-HT) in the cardiovascular system are well documented. However, it remains to be elucidated, whether 5-HT is present in adult mammalian cardiomyocytes. To address this issue, we investigated the levels of 5-HT in blood, plasma, platelets, cardiac tissue, and cardiomyocytes from adult mice and for comparison in human right atrial tissue. Immunohistochemically, 5-HT was hardly found in mouse cardiac tissue, but small amounts could be detected in renal preparations, whereas adrenal preparations revealed a strong positive immunoreaction for 5-HT. Using a sensitive HPLC detection system, 5-HT was also detectable in the mouse heart and human atrium. Furthermore, we could identify 5-HT in isolated cardiomyocytes from adult mice. These findings were supported by detection of the activity of 5-HT-forming enzymes-tryptophan hydroxylase and aromatic L-amino acid decarboxylase-in isolated cardiomyocytes from adult mice and by inhibition of these enzymes with p-chlorophenylalanine and 3-hydroxybenzyl hydrazine. Addition of the first intermediate of 5-HT generation, that is 5-hydroxytryptophan, enhanced the 5-HT level and inhibition of monoamine oxidase by tranylcypromine further increased the level of 5-HT. Our findings reveal the presence and synthesis of 5-HT in cardiomyocytes of the mammalian heart implying that 5-HT may play an autocrine and/or paracrine role in the heart.

The cellular and molecular origin of reactive oxygen species generation during myocardial ischemia and reperfusion

Myocardial ischemia-reperfusion injury is an important cause of impaired heart function in the early postoperative period subsequent to cardiac surgery. Reactive oxygen species (ROS) generation increases during both ischemia and reperfusion and it plays a central role in the pathophysiology of intraoperative myocardial injury. Unfortunately, the cellular source of these ROS during ischemia and reperfusion is often poorly defined. Similarly, individual ROS members tend to be grouped together as free radicals with a uniform reactivity towards biomolecules and with deleterious effects collectively ascribed under the vague umbrella of oxidative stress. This review aims to clarify the identity, origin, and progression of ROS during myocardial ischemia and reperfusion. Additionally, this review aims to describe the biochemical reactions and cellular processes that are initiated by specific ROS that work in concert to ultimately yield the clinical manifestations of myocardial ischemia-reperfusion. Lastly, this review provides an overview of several key cardioprotective strategies that target myocardial ischemia-reperfusion injury from the perspective of ROS generation. This overview is illustrated with example clinical studies that have attempted to translate these strategies to reduce the severity of ischemia-reperfusion injury during coronary artery bypass grafting surgery.

Nitric oxide/reactive oxygen species generation and nitroso/redox imbalance in heart failure: from molecular mechanisms to therapeutic implications

Adaptation of the heart to intrinsic and external stress involves complex modifications at the molecular and cellular levels that lead to tissue remodeling, functional and metabolic alterations, and finally to failure depending upon the nature, intensity, and chronicity of the stress. Reactive oxygen species (ROS) have long been considered as merely harmful entities, but their role as second messengers has gradually emerged. At the same time, our comprehension of the multifaceted role of nitric oxide (NO) and the related reactive nitrogen species (RNS) has been upgraded. The tight interlay between ROS and RNS suggests that their imbalance may implicate the impairment in physiological NO/redox-based signaling that contributes to the failing of the cardiovascular system. This review initially provides basic concepts on the role of nitroso/oxidative stress in the pathophysiology of heart failure with a particular focus on sources of ROS/RNS, their downstream targets, and endogenous modulators. Then, the role of NO/redox regulation of cardiomyocyte function, including calcium homeostasis, electrogenesis, and insulin signaling pathways, is described. Finally, an overview of old and emerging therapeutic opportunities in heart failure is presented, focusing on modulation of NO/redox mechanisms and discussing benefits and limitations.

Up-regulated monoamine oxidase in the mouse uterus during the peri-implantation period

PURPOSE

Monoamine oxidases (MAO), functioning as the metabolism of neuroamines, have been reported to be required for endometrial receptivity recently. The aim of this study was to examine the expression patterns of the two subtypes, MAOA and MAOB, during the peri-implantation period and to investigate whether MAOA or MAOB is a useful marker for receptivity.

METHODS

A total of 30 uteri were collected from females of gestational day 0, 2, 4, 6, 8, and mRNA and protein expression of MAOA/B were examined by real-time PCR, Western blot analysis and Immunohistochemistry analysis, respectively.

RESULTS

We found that the mRNA of MAOA expressed in the uteri at all stages of peri-implantation and began to rise on day 4 with a continuous increase up to day 8 of pregnancy, consistent with the changes of MAOA protein expression. The summit of MAOB mRNA and protein level was observed on day 4. Immunohistochemistry analysis revealed that both of MAOA and MAOB were mainly localized in the glandular and luminal epithelium cells, as well as their intense staining signals observed in the trophoblast cells on day 6 and 8.

CONCLUSIONS

Both MAOA and MAOB were up-regulated in the uteri during the peri-implantation period, which could play a role in mouse embryo implantation and endometrial receptivity; the temporal and spatial marked increase of MAOs may be traced as a useful marker for mouse endometrial receptivity; the expression mode of mouse MAOs should pave a way for further study.

Monoamine oxidases (MAO) in the pathogenesis of heart failure and ischemia/reperfusion injury

Recent evidence highlights monoamine oxidases (MAO) as another prominent source of oxidative stress. MAO are a class of enzymes located in the outer mitochondrial membrane, deputed to the oxidative breakdown of key neurotransmitters such as norepinephrine, epinephrine and dopamine, and in the process generate H(2)O(2). All these monoamines are endowed with potent modulatory effects on myocardial function. Thus, when the heart is subjected to chronic neuro-hormonal and/or peripheral hemodynamic stress, the abundance of circulating/tissue monoamines can make MAO-derived H(2)O(2) production particularly prominent. This is the case of acute cardiac damage due to ischemia/reperfusion injury or, on a more chronic stand, of the transition from compensated hypertrophy to overt ventricular dilation/pump failure. Here, we will first briefly discuss mitochondrial status and contribution to acute and chronic cardiac disorders. We will illustrate possible mechanisms by which MAO activity affects cardiac biology and function, along with a discussion as to their role as a prominent source of reactive oxygen species. Finally, we will speculate on why MAO inhibition might have a therapeutic value for treating cardiac affections of ischemic and non-ischemic origin. This article is part of a Special Issue entitled: Mitochondria and Cardioprotection.

Monoamine oxidase A-mediated enhanced catabolism of norepinephrine contributes to adverse remodeling and pump failure in hearts with pressure overload

RATIONALE

Monoamine oxidases (MAOs) are mitochondrial enzymes that catabolize prohypertrophic neurotransmitters, such as norepinephrine and serotonin, generating hydrogen peroxide. Because excess reactive oxygen species and catecholamines are major contributors to the pathophysiology of congestive heart failure, MAOs could play an important role in this process.

OBJECTIVE

Here, we investigated the role of MAO-A in maladaptive hypertrophy and heart failure.

METHODS AND RESULTS

We report that MAO-A activity is triggered in isolated neonatal and adult myocytes on stimulation with norepinephrine, followed by increase in cell size, reactive oxygen species production, and signs of maladaptive hypertrophy. All of these in vitro changes occur, in part, independently from alpha- and beta-adrenergic receptor-operated signaling and are inhibited by the specific MAO-A inhibitor clorgyline. In mice with left ventricular dilation and pump failure attributable to pressure overload, norepinephrine catabolism by MAO-A is increased accompanied by exacerbated oxidative stress. MAO-A inhibition prevents these changes, and also reverses fetal gene reprogramming, metalloproteinase and caspase-3 activation, as well as myocardial apoptosis. The specific role of MAO-A was further tested in mice expressing a dominant-negative MAO-A (MAO-A(neo)), which were more protected against pressure overload than their wild-type littermates.

CONCLUSIONS

In addition to adrenergic receptor-dependent mechanisms, enhanced MAO-A activity coupled with increased intramyocardial norepinephrine availability results in augmented reactive oxygen species generation, contributing to maladaptive remodeling and left ventricular dysfunction in hearts subjected to chronic stress.