Human heart mitochondria do not produce physiologically relevant quantities of nitric oxide

Endogenous nitric oxide formation in cardiac myocytes does not control respiration during β-adrenergic stimulation

KEY POINTS

In the heart, endothelial nitric oxide (NO) controls oxygen consumption in the working heart through paracrine mechanisms. While cardiac myocytes contain several isoforms of NO synthases, it is unclear whether these can control respiration in an intracrine fashion. A long-standing controversy is whether a NOS exists within mitochondria. By combining fluorescence technologies with electrical field stimulation or the patch-clamp technique in beating cardiac myocytes, we identified a neuronal NO synthase (nNOS) as the most relevant source of intracellular NO during β-adrenergic stimulation, while no evidence for a mitochondria-located NOS was obtained. The amounts of NO produced by non-mitochondrial nNOS were insufficient to regulate respiration during β-adrenergic stimulation, arguing against intracrine control of respiration by NO within cardiac myocytes.

ABSTRACT

Endothelial nitric oxide (NO) controls cardiac oxygen (O₂ ) consumption in a paracrine way by slowing respiration at the mitochondrial electron transport chain. While NO synthases (NOSs) are also expressed in cardiac myocytes, it is unclear whether they control respiration in an intracrine way. Furthermore, the existence of a mitochondrial NOS is controversial. Here, by combining fluorescence imaging with electrical field stimulation, the patch-clamp method and knock-out technology, we determined the sources and consequences of intracellular NO formation during workload transitions in isolated murine and guinea pig cardiac myocytes and mitochondria. Using 4-amino-5-methylamino-2',7'-difluorofluorescein diacetate (DAF) as a fluorescent NO-sensor that locates to the cytosol and mitochondria, we observed that NO increased by ∼12% within 3 min of β-adrenergic stimulation in beating cardiac myocytes. This NO stems from neuronal NOS (nNOS), but not endothelial (eNOS). After patch clamp-mediated dialysis of cytosolic DAF, the remaining NO signals (mostly mitochondrial) were blocked by nNOS deletion, but not by inhibiting the mitochondrial Ca²⁺ uniporter with Ru360. While in isolated mitochondria exogenous NO inhibited respiration and reduced the NAD(P)H redox state, pyridine nucleotide redox states were unaffected by pharmacological or genetic disruption of endogenous nNOS or eNOS during workload transitions in cardiac myoctyes. We conclude that under physiological conditions, nNOS is the most relevant source for NO in cardiac myocytes, but this nNOS is not located in mitochondria and does not control respiration. Therefore, cardiac O₂ consumption is controlled by endothelial NO in a paracrine, but not intracrine, fashion.

Interaction between connexin 43 and nitric oxide synthase in mice heart mitochondria

Connexin 43 (Cx43), which is highly expressed in the heart and especially in cardiomyocytes, interferes with the expression of nitric oxide synthase (NOS) isoforms. Conversely, Cx43 gene expression is down-regulated by nitric oxide derived from the inducible NOS. Thus, a complex interplay between Cx43 and NOS expression appears to exist. As cardiac mitochondria are supposed to contain a NOS, we now investigated the expression of NOS isoforms and the nitric oxide production rate in isolated mitochondria of wild-type and Cx43-deficient (Cx43(Cre-ER(T)/fl) ) mice hearts. Mitochondria were isolated from hearts using differential centrifugation and purified via Percoll gradient ultracentrifugation. Isolated mitochondria were stained with an antibody against the mitochondrial marker protein adenine-nucleotide-translocator (ANT) in combination with either a neuronal NOS (nNOS) or an inducible NOS (iNOS) antibody and analysed using confocal laser scanning microscopy. The nitric oxide formation was quantified in purified mitochondria using the oxyhaemoglobin assay. Co-localization of predominantly nNOS (nNOS: 93 ± 4.1%; iNOS: 24.6 ± 7.5%) with ANT was detected in isolated mitochondria of wild-type mice. In contrast, iNOS expression was increased in Cx43(Cre-ER(T)/fl) mitochondria (iNOS: 90.7 ± 3.2%; nNOS: 53.8 ± 17.5%). The mitochondrial nitric oxide formation was reduced in Cx43(Cre-ER(T)/fl) mitochondria (0.14 ± 0.02 nmol/min./mg protein) in comparison to wild-type mitochondria (0.24 ± 0.02 nmol/min./mg). These are the first data demonstrating, that a reduced mitochondrial Cx43 content is associated with a switch of the mitochondrial NOS isoform and the respective mitochondrial rate of nitric oxide formation.

Mitochondrial ion channels/transporters as sensors and regulators of cellular redox signaling

SIGNIFICANCE

Mitochondrial ion channels/transporters and the electron transport chain (ETC) serve as key sensors and regulators for cellular redox signaling, the production of reactive oxygen species (ROS) and nitrogen species (RNS) in mitochondria, and balancing cell survival and death. Although the functional and pharmacological characteristics of mitochondrial ion transport mechanisms have been extensively studied for several decades, the majority of the molecular identities that are responsible for these channels/transporters have remained a mystery until very recently.

RECENT ADVANCES

Recent breakthrough studies uncovered the molecular identities of the diverse array of major mitochondrial ion channels/transporters, including the mitochondrial Ca2+ uniporter pore, mitochondrial permeability transition pore, and mitochondrial ATP-sensitive K+ channel. This new information enables us to form detailed molecular and functional characterizations of mitochondrial ion channels/transporters and their roles in mitochondrial redox signaling.

CRITICAL ISSUES

Redox-mediated post-translational modifications of mitochondrial ion channels/transporters and ETC serve as key mechanisms for the spatiotemporal control of mitochondrial ROS/RNS generation.

FUTURE DIRECTIONS

Identification of detailed molecular mechanisms for redox-mediated regulation of mitochondrial ion channels will enable us to find novel therapeutic targets for many diseases that are associated with cellular redox signaling and mitochondrial ion channels/transporters.

A novel quantitative approach for eliminating sample-to-sample variation using a hue saturation value analysis program

OBJECTIVES

As computing technology and image analysis techniques have advanced, the practice of histology has grown from a purely qualitative method to one that is highly quantified. Current image analysis software is imprecise and prone to wide variation due to common artifacts and histological limitations. In order to minimize the impact of these artifacts, a more robust method for quantitative image analysis is required.

METHODS AND RESULTS

Here we present a novel image analysis software, based on the hue saturation value color space, to be applied to a wide variety of histological stains and tissue types. By using hue, saturation, and value variables instead of the more common red, green, and blue variables, our software offers some distinct advantages over other commercially available programs. We tested the program by analyzing several common histological stains, performed on tissue sections that ranged from 4 µm to 10 µm in thickness, using both a red green blue color space and a hue saturation value color space.

CONCLUSION

We demonstrated that our new software is a simple method for quantitative analysis of histological sections, which is highly robust to variations in section thickness, sectioning artifacts, and stain quality, eliminating sample-to-sample variation.

Strategic localization of heart mitochondrial NOS: a review of the evidence

Heart mitochondria play a central role in cell energy provision and in signaling. Nitric oxide (NO) is a free radical with primary regulatory functions in the heart and involved in a broad array of key processes in cardiac metabolism. Specific NO synthase (NOS) isoforms are confined to distinct locations in cardiomyocytes. The present article reviews the chemical reactions through which NO interacts with biomolecules and exerts some of its crucial roles. Specifically, the article discusses the reactions of NO with mitochondrial targets and the subcellular localization of NOS within the myocardium and analyzes the available data about heart mitochondrial NOS activity and identity. The article also describes the regulation of heart mtNOS by the distinctive mitochondrial environment by showing the effects of Ca(2+), O(2), l-arginine, mitochondrial transmembrane potential, and the metabolic states on heart mitochondrial NO production. The article depicts the effects of NO on heart function and highlights the relevance of NO production within mitochondria. Finally, the evidence on the functional implications of heart mitochondrial NOS is delineated with emphasis on chronic hypoxia and ischemia-reperfusion studies.

Nitric oxide signaling: classical, less classical, and nonclassical mechanisms

Although nitric oxide (NO) was identified more than 150 years ago and its effects were clinically tested in the form of nitroglycerine, it was not until the decades of 1970-1990 that it was described as a gaseous signal transducer. Since then, a canonical pathway linked to cyclic GMP (cGMP) as its quintessential effector has been established, but other modes of action have emerged and are now part of the common body of knowledge within the field. Classical (or canonical) signaling involves the selective activation of soluble guanylate cyclase, the generation of cGMP, and the activation of specific kinases (cGMP-dependent protein kinases) by this cyclic nucleotide. Nonclassical signaling alludes to the formation of NO-induced posttranslational modifications (PTMs), especially S-nitrosylation, S-glutathionylation, and tyrosine nitration. These PTMs are governed by specific biochemical mechanisms as well as by enzymatic systems. In addition, a less classical but equally important pathway is related to the interaction between NO and mitochondrial cytochrome c oxidase, which might have important implications for cell respiration and intermediary metabolism. Cross talk trespassing these necessarily artificial conceptual boundaries is progressively being identified and hence an integrated systems biology approach to the comprehension of NO function will probably emerge in the near future.

Functional evidence for nitric oxide production by skeletal-muscle mitochondria from lipopolysaccharide-treated mice

The possible existence of a mitochondrially localized nitric oxide (NO) synthase (mtNOS) is controversial. To clarify this, we studied the ability of intact mitochondria to generate NO and the effect of mitochondrial NO on respiration. Respiratory rates and oxygen kinetics (P(50) values) were determined by high-resolution respirometry in skeletal-muscle mitochondria from control mice and mice injected with Escherichia coli lipopolysaccharide (LPS). In the presence of the NOS substrate L-arginine, mitochondria from LPS-treated mice had lower respiration rates and higher P(50) values than control animals. These effects were prevented by the NOS inhibitor L-NMMA. Our results suggest that mitochondrially derived NO is generated by an LPS-inducible NOS protein other than iNOS and modulates oxygen consumption in mouse skeletal muscle.

The ups and downs of mitochondrial calcium signalling in the heart

Regulation of intramitochondrial free calcium ([Ca2+]m) is critical in both physiological and pathological functioning of the heart. The full extent and importance of the role of [Ca2+]m is becoming apparent as evidenced by the increasing interest and work in this area over the last two decades. However, controversies remain, such as the existence of beat-to-beat mitochondrial Ca2+ transients; the role of [Ca2+]m in modulating whole-cell Ca2+ signalling; whether or not an increase in [Ca2+]m is essential to couple ATP supply and demand; and the role of [Ca2+]m in cell death by both necrosis and apoptosis, especially in formation of the mitochondrial permeability transition pore. The role of [Ca2+]m in heart failure is an area that has also recently been highlighted. [Ca2+]m can now be measured reasonably specifically in intact cells and hearts thanks to developments in fluorescent indicators and targeted proteins and more sensitive imaging technology. This has revealed interactions of the mitochondrial Ca2+ transporters with those of the sarcolemma and sarcoplasmic reticulum, and has gone a long way to bringing the mitochondrial Ca2+ transporters to the forefront of cardiac research. Mitochondrial Ca2+ uptake occurs via the ruthenium red sensitive Ca2+ uniporter (mCU), and efflux via an Na+/Ca2+ exchanger (mNCX). The purification and cloning of the transporters, and development of more specific inhibitors, would produce a step-change in our understanding of the role of these apparently critical but still elusive proteins. In this article we will summarise the key physiological roles of [Ca2+]m in ATP production and cell Ca2+ signalling in both adult and neonatal hearts, as well as highlighting some of the controversies in these areas. We will also briefly discuss recent ideas on the interactions of nitric oxide with [Ca2+]m.

Characteristics and function of cardiac mitochondrial nitric oxide synthase

We used laser scanning confocal microscopy in combination with the nitric oxide (NO)-sensitive fluorescent dye DAF-2 and the reactive oxygen species (ROS)-sensitive dyes CM-H(2)DCF and MitoSOX Red to characterize NO and ROS production by mitochondrial NO synthase (mtNOS) in permeabilized cat ventricular myocytes. Stimulation of mitochondrial Ca(2+) uptake by exposure to different cytoplasmic Ca(2+) concentrations ([Ca(2+)](i) = 1, 2 and 5 microm) resulted in a dose-dependent increase of NO production by mitochondria when L-arginine, a substrate for mtNOS, was present. Collapsing the mitochondrial membrane potential with the protonophore FCCP or blocking the mitochondrial Ca(2+) uniporter with Ru360 as well as blocking the respiratory chain with rotenone or antimycin A in combination with oligomycin inhibited mitochondrial NO production. In the absence of L-arginine, mitochondrial NO production during stimulation of Ca(2+) uptake was significantly decreased, but accompanied by increase in mitochondrial ROS production. Inhibition of mitochondrial arginase to limit L-arginine availability resulted in 50% inhibition of Ca(2+)-induced ROS production. Both mitochondrial NO and ROS production were blocked by the nNOS inhibitor (4S)-N-(4-amino-5[aminoethyl]aminopentyl)-N'-nitroguanidine and the calmodulin antagonist W-7, while the eNOS inhibitor L-N(5)-(1-iminoethyl)ornithine (L-NIO) or iNOS inhibitor N-(3-aminomethyl)benzylacetamidine, 2HCl (1400W) had no effect. The superoxide dismutase mimetic and peroxynitrite scavenger MnTBAP abolished Ca(2+)-induced ROS generation and increased NO production threefold, suggesting that in the absence of MnTBAP either formation of superoxide radicals suppressed NO production or part of the formed NO was transformed quickly to peroxynitrite. In the absence of L-arginine, mitochondrial Ca(2+) uptake induced opening of the mitochondrial permeability transition pore (PTP), which was blocked by the PTP inhibitor cyclosporin A and MnTBAP, and reversed by L-arginine supplementation. In the presence of the mtNOS cofactor (6R)-5,6,7,8,-tetrahydrobiopterin (BH(4); 100 microm) mitochondrial ROS generation and PTP opening decreased while mitochondrial NO generation slightly increased. These data demonstrate that mitochondrial Ca(2+) uptake activates mtNOS and leads to NO-mediated protection against opening of the mitochondrial PTP, provided sufficient availability of l-arginine and BH(4). In conclusion, our data show the importance of L-arginine and BH(4) for cardioprotection via regulation of mitochondrial oxidative stress and modulation of PTP opening by mtNOS.

Mitochondrial nitric oxide synthase: current concepts and controversies

New discoveries in the last decade significantly altered our view on mitochondria. They are no longer viewed as energy-making slaves but rather individual cells-within-the-cell. In particular, it has been suggested that many important cellular mechanisms involving specific enzymes and ion channels, such as nitric oxide synthase (NOS), ATP-dependent K+ (KATP) channels, and poly-(APD-ribose) polymerase (PARP), have a distinct, mitochondrial variant. Unfortunately, exploring these parallel systems in mitochondria have technical limitations and inappropriate methods often led to inconsistent results. For example, the intriguing possibility that mitochondria are significant sources of nitric oxide (NO) via a unique mitochondrial NOS variant has attracted intense interest among research groups because of the potential for NO to affect functioning of the electron transport chain. Nonetheless, conclusive evidence concerning the existence of mitochondrial NO synthesis is yet to be presented. This review summarizes the experimental evidence gathered over the last decade in this field and highlights new areas of research that reveal surprising dimensions of NO production and metabolism by mitochondria.

Nitric oxide regulation of microvascular oxygen

The role of nitric oxide (NO) as a highly diffusible free radical gaseous vasodilator is intrinsically linked to the control of blood flow and oxygen (O(2)) delivery to tissue. NO also is involved in regulating mitochondrial O(2) metabolism, growth of new blood vessels, and blood oxygenation through control of respiratory ventilation. Hemoglobin and myoglobin may help to conserve NO for subsequent release of a NO-related vasoactive species under hypoxic conditions. NO has a major role in regulating microvascular O(2), and dysfunctional NO signaling is associated with the pathogenesis of metabolic and cardiovascular diseases.