sGC(alpha)1(beta)1 attenuates cardiac dysfunction and mortality in murine inflammatory shock models

Arterial Hypertension: Novel Pharmacological Targets and Future Perspectives

Arterial hypertension (HTN) is one of the major global contributors to cardiovascular diseases and premature mortality, particularly due to its impact on vital organs and the coexistence of various comorbidities such as chronic renal disease, diabetes, cerebrovascular diseases, and obesity. Regardless of the accessibility of several well-established pharmacological treatments, the percentage of patients achieving adequate blood pressure (BP) control is still significantly lower than recommended levels. Therefore, the pharmacological and non-pharmacological management of HTN is currently the major focus of healthcare systems. Various strategies are being applied, such as the development of new pharmacological agents that target different underlying physiopathological mechanisms or associated comorbidities. Additionally, a novel group of interventional techniques has emerged in recent years, specifically for situations when blood pressure is not properly controlled despite the use of multiple antihypertensives in maximum doses or when patients are unable to tolerate or desire not to receive antihypertensive medications. Nonetheless, reducing the focus on antihypertensive medication development by the pharmaceutical industry and increasing recognition of ineffective HTN control due to poor drug adherence demands ongoing research into alternative approaches to treatment. The aim of this review is to summarize the potential novel pharmacological targets for the treatment of arterial hypertension as well as the future perspectives of the treatment strategy.

DNA-PKcs Phosphorylates Cofilin2 to Induce Endothelial Dysfunction and Microcirculatory Disorder in Endotoxemic Cardiomyopathy

The presence of endotoxemia is strongly linked to the development of endothelial dysfunction and disruption of myocardial microvascular reactivity. These factors play a crucial role in the progression of endotoxemic cardiomyopathy. Sepsis-related multiorgan damage involves the participation of the catalytic subunit of DNA-dependent protein kinase (DNA-PKcs). However, whether DNA-PKcs contributes to endothelial dysfunction and myocardial microvascular dysfunction during endotoxemia remains unclear. Hence, we conducted experiments in mice subjected to lipopolysaccharide (LPS)-induced endotoxemic cardiomyopathy, as well as assays in primary mouse cardiac microvascular endothelial cells. Results showed that endothelial-cell-specific DNA-PKcs ablation markedly attenuated DNA damage, sustained microvessel perfusion, improved endothelial barrier function, inhibited capillary inflammation, restored endothelium-dependent vasodilation, and improved heart function under endotoxemic conditions. Furthermore, we show that upon LPS stress, DNA-PKcs recognizes a TQ motif in cofilin2 and consequently induces its phosphorylation at Thr25. Phosphorylated cofilin2 shows increased affinity for F-actin and promotes F-actin depolymerization, resulting into disruption of the endothelial barrier integrity, microvascular inflammation, and defective eNOS-dependent vasodilation. Accordingly, cofilin2-knockin mice expressing a phospho-defective (T25A) cofilin2 mutant protein showed improved endothelial integrity and myocardial microvascular function upon induction of endotoxemic cardiomyopathy. These findings highlight a novel mechanism whereby DNA-PKcs mediates cofilin2Thr25 phosphorylation and subsequent F-actin depolymerization to contribute to endotoxemia-related cardiac microvascular dysfunction.

MECHANISMS OF CARDIAC DYSFUNCTION IN SEPSIS

ABSTRACT

Studies in animal models of sepsis have elucidated an intricate network of signaling pathways that lead to the dysregulation of myocardial Ca 2+ handling and subsequently to a decrease in cardiac contractile force, in a sex- and model-dependent manner. After challenge with a lethal dose of LPS, male animals show a decrease in cellular Ca 2+ transients (ΔCa i ), with intact myofilament function, whereas female animals show myofilament dysfunction, with intact ΔCa i . Male mice challenged with a low, nonlethal dose of LPS also develop myofilament desensitization, with intact ΔCa i . In the cecal ligation and puncture (CLP) model, the causative mechanisms seem similar to those in the LPS model in male mice and are unknown in female subjects. ΔCa i decrease in male mice is primarily due to redox-dependent inhibition of sarco/endoplasmic reticulum Ca 2+ ATP-ase (SERCA). Reactive oxygen species (ROS) are overproduced by dysregulated mitochondria and the enzymes NADPH/NADH oxidase, cyclooxygenase, and xanthine oxidase. In addition to inhibiting SERCA, ROS amplify cardiomyocyte cytokine production and mitochondrial dysfunction, making the process self-propagating. In contrast, female animals may exhibit a natural redox resilience. Myofilament dysfunction is due to hyperphosphorylation of troponin I, troponin T cleavage by caspase-3, and overproduction of cGMP by NO-activated soluble guanylate cyclase. Depleted, dysfunctional, or uncoupled mitochondria likely synthesize less ATP in both sexes, but the role of energy deficit is not clear. NO produced by NO synthase (NOS)-3 and mitochondrial NOSs, protein kinases and phosphatases, the processes of autophagy and sarco/endoplasmic reticulum stress, and β-adrenergic insensitivity may also play currently uncertain roles.

CARDIOMYOCYTE REPROGRAMMING IN ANIMAL MODELS OF SEPTIC SHOCK

ABSTRACT

Cardiomyocyte reprogramming plays a pivotal role in sepsis-induced cardiomyopathy through the induction or overexpression of several factors and enzymes, ultimately leading to the characteristic decrease in cardiac contractility. The initial trigger is the binding of LPS to TLR-2, -3, -4, and -9 and of proinflammatory cytokines, such as TNF, IL-1, and IL-6, to their respective receptors. This induces the nuclear translocation of nuclear factors, such as NF-κB, via activation of MyD88, TRIF, IRAK, and MAPKs. Among the latter, ROS- and estrogen-dependent p38 and ERK 1/2 are proinflammatory, whereas JNK may play antagonistic, anti-inflammatory roles. Nuclear factors induce the synthesis of cytokines, which can amplify the inflammatory signal in a paracrine fashion, and of several effector enzymes, such as NOS-2, NOX-1, and others, which are ultimately responsible for the degradation of cardiomyocyte contractility. In parallel, the downregulation of enzymes involved in oxidative phosphorylation causes metabolic reprogramming, followed by a decrease in ATP production and the release of fragmented mitochondrial DNA, which may augment the process in a positive feedback loop. Other mediators, such as NO, ROS, the enzymes PI3K and Akt, and adrenergic stimulation may play regulatory roles, but not all signaling pathways that mediate cardiac dysfunction of sepsis do that by regulating reprogramming. Transcription may be globally modulated by miRs, which exert protective or amplifying effects. For all these mechanisms, differentiating between modulation of cardiomyocyte reprogramming versus systemic inflammation has been an ongoing but worthwhile experimental challenge.

Inhaled nitric oxide improves post-cardiac arrest outcomes via guanylate cyclase-1 in bone marrow-derived cells

RATIONALE

Nitric oxide (NO) exerts its biological effects primarily via activation of guanylate cyclase (GC) and production of cyclic guanosine monophosphate. Inhaled NO improves outcomes after cardiac arrest and cardiopulmonary resuscitation (CPR). However, mechanisms of the protective effects of breathing NO after cardiac arrest are incompletely understood.

OBJECTIVE

To elucidate the mechanisms of beneficial effects of inhaled NO on outcomes after cardiac arrest.

METHODS

Adult male C57BL/6J wild-type (WT) mice, GC-1 knockout mice, and chimeric WT mice with WT or GC-1 knockout bone marrow were subjected to 8 min of potassium-induced cardiac arrest to determine the role of GC-1 in bone marrow-derived cells. Mice breathed air or 40 parts per million NO for 23 h starting at 1 h after CPR.

RESULTS

Breathing NO after CPR prevented hypercoagulability, cerebral microvascular occlusion, an increase in circulating polymorphonuclear neutrophils and neutrophil-to-lymphocyte ratio, and right ventricular dysfunction in WT mice, but not in GC-1 knockout mice, after cardiac arrest. The lack of GC-1 in bone marrow-derived cells diminished the beneficial effects of NO breathing after CPR.

CONCLUSIONS

GC-dependent signaling in bone marrow-derived cells is essential for the beneficial effects of inhaled NO after cardiac arrest and CPR.

NO-Sensitive Guanylate Cyclase Isoforms NO-GC1 and NO-GC2 Contribute to Noise-Induced Inner Hair Cell Synaptopathy

Nitric oxide (NO) activates the NO-sensitive soluble guanylate cyclase (NO-GC, sGC) and triggers intracellular signaling pathways involving cGMP. For survival of cochlear hair cells and preservation of hearing, NO-mediated cascades have both protective and detrimental potential. Here we examine the cochlear function of mice lacking one of the two NO-sensitive guanylate cyclase isoforms [NO-GC1 knockout (KO) or NO-GC2 KO]. The deletion of NO-GC1 or NO-GC2 did not influence electromechanical outer hair cell (OHC) properties, as measured by distortion product otoacoustic emissions, neither before nor after noise exposure, nor were click- or noise-burst-evoked auditory brainstem response thresholds different from controls. Yet inner hair cell (IHC) ribbons and auditory nerve responses showed significantly less deterioration in NO-GC1 KO and NO-GC2 KO mice after noise exposure. Consistent with a selective role of NO-GC in IHCs, NO-GC β1 mRNA was found in isolated IHCs but not in OHCs. Using transgenic mice expressing the fluorescence resonance energy transfer-based cGMP biosensor cGi500, NO-induced elevation of cGMP was detected in real-time in IHCs but not in OHCs. Pharmacologic long-term treatment with a NO-GC stimulator altered auditory nerve responses but did not affect OHC function and hearing thresholds. Interestingly, NO-GC stimulation exacerbated the loss of auditory nerve response in aged animals but attenuated the loss in younger animals. We propose NO-GC2 and, to some degree, NO-GC1 as targets for early pharmacologic prevention of auditory fiber loss (synaptopathy). Both isoforms provide selective benefits for hearing function by maintaining the functional integrity of auditory nerve fibers in early life rather than at old age.

Up-regulation of Intracellular Calcium Handling Underlies the Recovery of Endotoxemic Cardiomyopathy in Mice

BACKGROUND

In surviving patients, sepsis-induced cardiomyopathy is spontaneously reversible. In the absence of any experimental data, it is generally thought that cardiac recovery in sepsis simply follows the remission of systemic inflammation. Here the authors aimed to identify the myocardial mechanisms underlying cardiac recovery in endotoxemic mice.

METHODS

Male C57BL/6 mice were challenged with lipopolysaccharide (7 μg/g, intraperitoneally) and followed for 12 days. The authors assessed survival, cardiac function by echocardiography, sarcomere shortening, and calcium transients (with fura-2-acetoxymethyl ester) in electrically paced cardiomyocytes (5 Hz, 37°C) and myocardial protein expression by immunoblotting.

RESULTS

Left ventricular ejection fraction, cardiomyocyte sarcomere shortening, and calcium transients were depressed 12 h after lipopolysaccharide challenge, started to recover by 24 h (day 1), and were back to baseline at day 3. The recovery of calcium transients at day 3 was associated with the up-regulation of the sarcoplasmic reticulum calcium pump to 139 ± 19% (mean ± SD) of baseline and phospholamban down-regulation to 35 ± 20% of baseline. At day 6, calcium transients were increased to 123 ± 31% of baseline, associated with increased sarcoplasmic reticulum calcium load (to 126 ± 32% of baseline, as measured with caffeine) and inhibition of sodium/calcium exchange (to 48 ± 12% of baseline).

CONCLUSIONS

In mice surviving lipopolysaccharide challenge, the natural recovery of cardiac contractility was associated with the up-regulation of cardiomyocyte calcium handling above baseline levels, indicating the presence of an active myocardial recovery process, which included sarcoplasmic reticulum calcium pump activation, the down-regulation of phospholamban, and sodium/calcium exchange inhibition.

Distinct Myocardial Mechanisms Underlie Cardiac Dysfunction in Endotoxemic Male and Female Mice

In male mice, sepsis-induced cardiomyopathy develops as a result of dysregulation of myocardial calcium (Ca) handling, leading to depressed cellular Ca transients (ΔCai). ΔCai depression is partially due to inhibition of sarcoplasmic reticulum Ca ATP-ase (SERCA) via oxidative modifications, which are partially opposed by cGMP generated by the enzyme soluble guanylyl cyclase (sGC). Whether similar mechanisms underlie sepsis-induced cardiomyopathy in female mice is unknown.Male and female C57Bl/6J mice (WT), and mice deficient in the sGC α1 subunit activity (sGCα1), were challenged with lipopolysaccharide (LPS, ip). LPS induced mouse death and cardiomyopathy (manifested as the depression of left ventricular ejection fraction by echocardiography) to a similar degree in WT male, WT female, and sGCα1 male mice, but significantly less in sGCα1 female mice. We measured sarcomere shortening and ΔCai in isolated, externally paced cardiomyocytes, at 37°C. LPS depressed sarcomere shortening in both WT male and female mice. Consistent with previous findings, in male mice, LPS induced a decrease in ΔCai (to 30 ± 2% of baseline) and SERCA inhibition (manifested as the prolongation of the time constant of Ca decay, τCa, to 150 ± 5% of baseline). In contrast, in female mice, the depression of sarcomere shortening induced by LPS occurred in the absence of any change in ΔCai, or SERCA activity. This suggested that, in female mice, the causative mechanism lies downstream of the Ca transients, such as a decrease in myofilament sensitivity for Ca. The depression of sarcomere shortening shortening after LPS was less severe in female sGCα1 mice than in WT female mice, indicating that cGMP partially mediates cardiomyocyte dysfunction.These results suggest, therefore, that LPS-induced cardiomyopathy develops through distinct sex-specific myocardial mechanisms. While in males LPS induces sGC-independent decrease in ΔCai, in female mice LPS acts downstream of ΔCai, possibly via sGC-dependent myofilament dysfunction.

Androgen-sensitive hypertension associated with soluble guanylate cyclase-α1 deficiency is mediated by 20-HETE

Dysregulated nitric oxide (NO) signaling contributes to the pathogenesis of hypertension, a prevalent and often sex-specific risk factor for cardiovascular disease. We previously reported that mice deficient in the α1-subunit of the NO receptor soluble guanylate cyclase (sGCα1 (-/-) mice) display sex- and strain-specific hypertension: male but not female sGCα1 (-/-) mice are hypertensive on an 129S6 (S6) but not a C57BL6/J (B6) background. We aimed to uncover the genetic and molecular basis of the observed sex- and strain-specific blood pressure phenotype. Via linkage analysis, we identified a suggestive quantitative trait locus associated with elevated blood pressure in male sGCα1 (-/-)S6 mice. This locus encompasses Cyp4a12a, encoding the predominant murine synthase of the vasoconstrictor 20-hydroxy-5,8,11,14-eicosatetraenoic acid (20-HETE). Renal expression of Cyp4a12a in mice was associated with genetic background, sex, and testosterone levels. In addition, 20-HETE levels were higher in renal preglomerular microvessels of male sGCα1 (-/-)S6 than of male sGCα1 (-/-)B6 mice. Furthermore, treating male sGCα1 (-/-)S6 mice with the 20-HETE antagonist 20-hydroxyeicosa-6(Z),15(Z)-dienoic acid (20-HEDE) lowered blood pressure. Finally, 20-HEDE rescued the genetic background- and testosterone-dependent impairment of acetylcholine-induced relaxation in renal interlobar arteries associated with sGCα1 deficiency. Elevated Cyp4a12a expression and 20-HETE levels render mice susceptible to hypertension and vascular dysfunction in a setting of sGCα1 deficiency. Our data identify Cyp4a12a as a candidate sex-specific blood pressure-modifying gene in the context of deficient NO-sGC signaling.

Bone morphogenetic protein receptor type II deficiency and increased inflammatory cytokine production. A gateway to pulmonary arterial hypertension

RATIONALE

Mutations in bone morphogenetic protein receptor type II (BMPR-II) underlie most cases of heritable pulmonary arterial hypertension (PAH). However, disease penetrance is only 20-30%, suggesting a requirement for additional triggers. Inflammation is emerging as a key disease-related factor in PAH, but to date there is no clear mechanism linking BMPR-II deficiency and inflammation.

OBJECTIVES

To establish a direct link between BMPR-II deficiency, a consequentially heightened inflammatory response, and development of PAH.

METHODS

We used pulmonary artery smooth muscle cells from Bmpr2(+/-) mice and patients with BMPR2 mutations and compared them with wild-type controls. For the in vivo model, we used mice heterozygous for a null allele in Bmpr2 (Bmpr2(+/-)) and wild-type littermates.

MEASUREMENTS AND MAIN RESULTS

Acute exposure to LPS increased lung and circulating IL-6 and KC (IL-8 analog) levels in Bmpr2(+/-) mice to a greater extent than in wild-type controls. Similarly, pulmonary artery smooth muscle cells from Bmpr2(+/-) mice and patients with BMPR2 mutations produced higher levels of IL-6 and KC/IL-8 after lipopolysaccharide stimulation compared with controls. BMPR-II deficiency in mouse and human pulmonary artery smooth muscle cells was associated with increased phospho-STAT3 and loss of extracellular superoxide dismutase. Chronic lipopolysaccharide administration caused pulmonary hypertension in Bmpr2(+/-) mice but not in wild-type littermates. Coadministration of tempol, a superoxide dismutase mimetic, ameliorated the exaggerated inflammatory response and prevented development of PAH.

CONCLUSIONS

This study demonstrates that BMPR-II deficiency promotes an exaggerated inflammatory response in vitro and in vivo, which can instigate development of pulmonary hypertension.

Cardiovascular and pharmacological implications of haem-deficient NO-unresponsive soluble guanylate cyclase knock-in mice

Oxidative stress, a central mediator of cardiovascular disease, results in loss of the prosthetic haem group of soluble guanylate cyclase (sGC), preventing its activation by nitric oxide (NO). Here we introduce Apo-sGC mice expressing haem-free sGC. Apo-sGC mice are viable and develop hypertension. The haemodynamic effects of NO are abolished, but those of the sGC activator cinaciguat are enhanced in apo-sGC mice, suggesting that the effects of NO on smooth muscle relaxation, blood pressure regulation and inhibition of platelet aggregation require sGC activation by NO. Tumour necrosis factor (TNF)-induced hypotension and mortality are preserved in apo-sGC mice, indicating that pathways other than sGC signalling mediate the cardiovascular collapse in shock. Apo-sGC mice allow for differentiation between sGC-dependent and -independent NO effects and between haem-dependent and -independent sGC effects. Apo-sGC mice represent a unique experimental platform to study the in vivo consequences of sGC oxidation and the therapeutic potential of sGC activators.

Haem-free, NO-insensitive soluble guanylate cyclase (apo-sGC) generated during oxidative stress contributes to cardiovascular pathology. By generating and characterizing apo-sGC knock-in mice, Thoonen et al. provide a scientific ground for the therapeutic concept of sGC activators, and dissect the relevance of the NO-sGC axis.

Nitrate/Nitrite as Critical Mediators to Limit Oxidative Injury and Inflammation

SIGNIFICANCE

Nitric oxide (NO) is a critical signaling molecule marked by complex chemistry and varied biological responses depending on the context of the redox environment. In the setting of inflammation, NO can not only contribute to tissue injury and be causative of oxidative damage but can also signal as an adaptive molecule to limit inflammatory signaling in multiple cell types and tissues.

RECENT ADVANCES

An advance in our understanding of NO biology was the recognition of the nitrate-nitrite-NO axis, whereby nitrate (predominantly from dietary sources) could be converted to nitrite and nitrite could be reduced to NO.

CRITICAL ISSUES

Intriguingly, the recognition of multiple enzymes that serve as nitrite reductases in the setting of hypoxia or ischemia established the concept of nitrite as a circulating endocrine reservoir of NO, with the selective release of NO at sites that were primed for this reaction. This review highlights the anti-inflammatory roles of nitrite in numerous clinical conditions, including ischemia/reperfusion, transplant, cardiac arrest, and vascular injury, and in gastrointestinal inflammation.

FUTURE DIRECTIONS

These preclinical and clinical investigations set up further clinical trials and studies that elucidate the endogenous role this pathway plays in protection against inflammatory signaling.

Shedding of the tumor necrosis factor (TNF) receptor from the surface of hepatocytes during sepsis limits inflammation through cGMP signaling

Proteolytic cleavage of the tumor necrosis factor (TNF) receptor (TNFR) from the cell surface contributes to anti-inflammatory responses and may be beneficial in reducing the excessive inflammation associated with multiple organ failure and mortality during sepsis. Using a clinically relevant mouse model of polymicrobial abdominal sepsis, we found that the production of inducible nitric oxide synthase (iNOS) in hepatocytes led to the cyclic guanosine monophosphate (cGMP)-dependent activation of the protease TACE (TNF-converting enzyme) and the shedding of TNFR. Furthermore, treating mice with a cGMP analog after the induction of sepsis increased TNFR shedding and decreased systemic inflammation. Similarly, increasing the abundance of cGMP with a clinically approved phosphodiesterase 5 inhibitor (sildenafil) also decreased markers of systemic inflammation, protected against organ injury, and increased circulating amounts of TNFR1 in mice with sepsis. We further confirmed that a similar iNOS-cGMP-TACE pathway was required for TNFR1 shedding by human hepatocytes in response to the bacterial product lipopolysaccharide. Our data suggest that increasing the bioavailability of cGMP might be beneficial in ameliorating the inflammation associated with sepsis.

Extending the translational potential of targeting NO/cGMP-regulated pathways in the CVS

The discovery of NO as both an endogenous signalling molecule and as a mediator of the cardiovascular effects of organic nitrates was acknowledged in 1998 by the Nobel Prize in Physiology/Medicine. The characterization of its downstream signalling, mediated through stimulation of soluble GC (sGC) and cGMP generation, initiated significant translational interest, but until recently this was almost exclusively embodied by the use of PDE5 inhibitors in erectile dysfunction. Since then, research progress in two areas has contributed to an impressive expansion of the therapeutic targeting of the NO-sGC-cGMP axis: first, an increased understanding of the molecular events operating within this complex pathway and second, a better insight into its dys-regulation and uncoupling in human disease. Already-approved PDE5 inhibitors and novel, first-in-class molecules, which up-regulate the activity of sGC independently of NO and/or of the enzyme's haem prosthetic group, are undergoing clinical evaluation to treat pulmonary hypertension and myocardial failure. These molecules, as well as combinations or second-generation compounds, are also being assessed in additional experimental disease models and in patients in a wide spectrum of novel indications, such as endotoxic shock, diabetic cardiomyopathy and Becker's muscular dystrophy. There is well-founded optimism that the modulation of the NO-sGC-cGMP pathway will sustain the development of an increasing number of successful clinical candidates for years to come.

New insights into the role of soluble guanylate cyclase in blood pressure regulation

PURPOSE OF REVIEW

Nitric oxide (NO)-soluble guanylate cyclase (sGC)-dependent signaling mechanisms have a profound effect on the regulation of blood pressure (BP). In this review, we will discuss recent findings in the field that support the importance of sGC in the development of hypertension.

RECENT FINDINGS

The importance of sGC in BP regulation was highlighted by studies using genetically modified animal models, chemical stimulators/activators and inhibitors of the NO/sGC signaling pathway, and genetic association studies in humans. Many studies further support the role of NO/sGC in vasodilation and vascular dysfunction, which is underscored by the early clinical success of synthetic sGC stimulators for the treatment of pulmonary hypertension. Recent work has uncovered more details about the structural basis of sGC activation, enabling the development of more potent and efficient modulators of sGC activity. Finally, the mechanisms involved in the modulation of sGC by signaling gases other than NO, as well as the influence of redox signaling on sGC, have been the subject of several interesting studies.

SUMMARY

sGC is fast becoming an interesting therapeutic target for the treatment of vascular dysfunction and hypertension, with novel sGC stimulating/activating compounds as promising clinical treatment options.

Analyzing left ventricular function in mice with Doppler echocardiography

Mice are widely used in heart failure research. Accurate evaluation of cardiac structure and function is key to modern cardiovascular research. Doppler echocardiography is a simple, reproducible, and non-invasive method, which allows a longitudinal study of these small animals. Besides common parameters such as left ventricular chamber size, mass, and function, new emerging echo tools are of great interest for small animal imaging. In this review, we describe the technical issues linked to murine cardiovascular anatomy and physiology and the most current echo parameters that can be used.

Transgenic mice for real-time visualization of cGMP in intact adult cardiomyocytes

RATIONALE

3',5'-Cyclic guanosine monophosphate (cGMP) is an important second messenger that regulates cardiac contractility and protects the heart from hypertrophy. However, because of the lack of real-time imaging techniques, specific subcellular mechanisms and spatiotemporal dynamics of cGMP in adult cardiomyocytes are not well understood.

OBJECTIVE

Our aim was to generate and characterize a novel cGMP sensor model to measure cGMP with nanomolar sensitivity in adult cardiomyocytes.

METHODS AND RESULTS

We generated transgenic mice with cardiomyocyte-specific expression of the highly sensitive cytosolic Förster resonance energy transfer-based cGMP biosensor red cGES-DE5 and performed the first Förster resonance energy transfer measurements of cGMP in intact adult mouse ventricular myocytes. We found very low (≈10 nmol/L) basal cytosolic cGMP levels, which can be markedly increased by natriuretic peptides (C-type natriuretic peptide >> atrial natriuretic peptide) and, to a much smaller extent, by the direct stimulation of soluble guanylyl cyclase. Constitutive activity of this cyclase contributes to basal cGMP production, which is balanced by the activity of clinically established phosphodiesterase (PDE) families. The PDE3 inhibitor, cilostamide, showed especially strong cGMP responses. In a mild model of cardiac hypertrophy after transverse aortic constriction, PDE3 effects were not affected, whereas the contribution of PDE5 was increased. In addition, after natriuretic peptide stimulation, PDE3 was also involved in cGMP/cAMP crosstalk.

CONCLUSIONS

The new sensor model allows visualization of real-time cGMP dynamics and pharmacology in intact adult cardiomyocytes. Förster resonance energy transfer imaging suggests the importance of well-established and potentially novel PDE-dependent mechanisms that regulate cGMP under physiological and pathophysiological conditions.

The role of soluble guanylyl cyclase in chronic obstructive pulmonary disease

RATIONALE

Soluble guanylyl cyclase (sGC), a cyclic guanosine 5'-monophosphate-generating enzyme, regulates smooth muscle tone and exerts antiinflammatory effects in animal models of asthma and acute lung injury. In chronic obstructive pulmonary disease (COPD), primarily caused by cigarette smoke (CS), lung inflammation persists and smooth muscle tone remains elevated, despite ample amounts of nitric oxide that could activate sGC.

OBJECTIVES

To determine the expression and function of sGC in patients with COPD and in a murine model of COPD.

METHODS

Expression of sGCα1, α2, and β1 subunits was examined in lungs of never-smokers, smokers without airflow limitation, and patients with COPD; and in C57BL/6 mice after 3 days, 4 weeks, and 24 weeks of CS exposure. The functional role of sGC was investigated in vivo by measuring bronchial responsiveness to serotonin in mice using genetic and pharmacologic approaches.

MEASUREMENTS AND MAIN RESULTS

Pulmonary expression of sGC, both at mRNA and protein level, was decreased in smokers without airflow limitation and in patients with COPD, and correlated with disease severity (FEV1%). In mice, exposure to CS reduced sGC, cyclic guanosine 5'-monophosphate levels, and protein kinase G activity. sGCα1(-/-) mice exposed to CS exhibited bronchial hyperresponsiveness to serotonin. Activation of sGC by BAY 58-2667 restored the sGC signaling and attenuated bronchial hyperresponsiveness in CS-exposed mice.

CONCLUSIONS

Down-regulation of sGC because of CS exposure might contribute to airflow limitation in COPD.

Deletion of the murine cytochrome P450 Cyp2j locus by fused BAC-mediated recombination identifies a role for Cyp2j in the pulmonary vascular response to hypoxia

Epoxyeicosatrienoic acids (EETs) confer vasoactive and cardioprotective functions. Genetic analysis of the contributions of these short-lived mediators to pathophysiology has been confounded to date by the allelic expansion in rodents of the portion of the genome syntenic to human CYP2J2, a gene encoding one of the principle cytochrome P450 epoxygenases responsible for the formation of EETs in humans. Mice have eight potentially functional genes that could direct the synthesis of epoxygenases with properties similar to those of CYP2J2. As an initial step towards understanding the role of the murine Cyp2j locus, we have created mice bearing a 626-kb deletion spanning the entire region syntenic to CYP2J2, using a combination of homologous and site-directed recombination strategies. A mouse strain in which the locus deletion was complemented by transgenic delivery of BAC sequences encoding human CYP2J2 was also created. Systemic and pulmonary hemodynamic measurements did not differ in wild-type, null, and complemented mice at baseline. However, hypoxic pulmonary vasoconstriction (HPV) during left mainstem bronchus occlusion was impaired and associated with reduced systemic oxygenation in null mice, but not in null mice bearing the human transgene. Administration of an epoxygenase inhibitor to wild-type mice also impaired HPV. These findings demonstrate that Cyp2j gene products regulate the pulmonary vascular response to hypoxia.

In mice and humans, the CYP2J class of cytochrome P450 epoxygenases metabolizes arachidonic acid (AA) to epoxyeicosatrienoic acids (EETs), short-lived mediators with effects on both the pulmonary and systemic vasculature. Genetic dissection of CYP2J function to date has been complicated by allelic expansion in the rodent genome. In this study, the mouse chromosomal locus syntenic to human CYP2J2, containing eight presumed genes and two pseudogenes, was deleted via generation of a recombinant template created by homologous and site-specific recombination steps that joined two precursor bacterial artificial chromosomes (BACs). The Cyp2j null mice were subsequently complemented by transgenic delivery of BAC sequences encoding human CYP2J2. Hypoxic pulmonary vasoconstriction (HPV) and systemic oxygenation during regional alveolar hypoxia were unexpectedly found to be impaired in null mice, but not in null mice bearing the transgenic human allele, suggesting that Cyp2j products contribute to the pulmonary vascular response to hypoxia.

The soluble guanylate cyclase activator BAY 58-2667 protects against morbidity and mortality in endotoxic shock by recoupling organ systems

Sepsis and septic shock are associated with high mortality rates and the majority of sepsis patients die due to complications of multiple organ failure (MOF). The cyclic GMP (cGMP) producing enzyme soluble guanylate cyclase (sGC) is crucially involved in the regulation of (micro)vascular homeostasis, cardiac function and, consequently, organ function. However, it can become inactivated when exposed to reactive oxygen species (ROS). The resulting heme-free sGC can be reactivated by the heme- and nitric oxide (NO)-independent sGC activator BAY 58-2667 (Cinaciguat). We report that late (+8 h) post-treatment with BAY 58-2667 in a mouse model can protect against lethal endotoxic shock. Protection was associated with reduced hypothermia, circulating IL-6 levels, cardiomyocyte apoptosis, and mortality. In contrast to BAY 58-2667, the sGC stimulator BAY 41-2272 and the phosphodiesterase 5 inhibitor Sildenafil did not have any beneficial effect on survival, emphasizing the importance of the selectivity of BAY 58-2667 for diseased vessels and tissues. Hemodynamic parameters (blood pressure and heart rate) were decreased, and linear and nonlinear indices of blood pressure variability, reflective for (un)coupling of the communication between the autonomic nervous system and the heart, were improved after late protective treatment with BAY 58-2667. In conclusion, our results demonstrate the pivotal role of the NO/sGC axis in endotoxic shock. Stabilization of sGC function with BAY 58-2667 can prevent mortality when given in the correct treatment window, which probably depends on the dynamics of the heme-free sGC pool, in turn influenced by oxidative stress. We speculate that, considering the central role of sGC signaling in many pathways required for maintenance of (micro)circulatory homeostasis, BAY 58-2667 supports organ function by recoupling inter-organ communication pathways.

SERCA Cys674 sulphonylation and inhibition of L-type Ca2+ influx contribute to cardiac dysfunction in endotoxemic mice, independent of cGMP synthesis

The goal of this study was to identify the cellular mechanisms responsible for cardiac dysfunction in endotoxemic mice. We aimed to differentiate the roles of cGMP [produced by soluble guanylyl cyclase (sGC)] versus oxidative posttranslational modifications of Ca(2+) transporters. C57BL/6 mice [wild-type (WT) mice] were administered lipopolysaccharide (LPS; 25 μg/g ip) and euthanized 12 h later. Cardiomyocyte sarcomere shortening and Ca(2+) transients (ΔCai) were depressed in LPS-challenged mice versus baseline. The time constant of Ca(2+) decay (τCa) was prolonged, and sarcoplasmic reticulum Ca(2+) load (CaSR) was depressed in LPS-challenged mice (vs. baseline), indicating decreased activity of sarco(endo)plasmic Ca(2+)-ATPase (SERCA). L-type Ca(2+) channel current (ICa,L) was also decreased after LPS challenge, whereas Na(+)/Ca(2+) exchange activity, ryanodine receptors leak flux, or myofilament sensitivity for Ca(2+) were unchanged. All Ca(2+)-handling abnormalities induced by LPS (the decrease in sarcomere shortening, ΔCai, CaSR, ICa,L, and τCa prolongation) were more pronounced in mice deficient in the sGC main isoform (sGCα1(-/-) mice) versus WT mice. LPS did not alter the protein expression of SERCA and phospholamban in either genotype. After LPS, phospholamban phosphorylation at Ser(16) and Thr(17) was unchanged in WT mice and was increased in sGCα1(-/-) mice. LPS caused sulphonylation of SERCA Cys(674) (as measured immunohistochemically and supported by iodoacetamide labeling), which was greater in sGCα1(-/-) versus WT mice. Taken together, these results suggest that cardiac Ca(2+) dysregulation in endotoxemic mice is mediated by a decrease in L-type Ca(2+) channel function and oxidative posttranslational modifications of SERCA Cys(674), with the latter (at least) being opposed by sGC-released cGMP.

Soluble guanylate cyclase modulates alveolarization in the newborn lung

Nitric oxide (NO) regulates lung development through incompletely understood mechanisms. NO controls pulmonary vascular smooth muscle cell (SMC) differentiation largely through stimulating soluble guanylate cyclase (sGC) to produce cGMP and increase cGMP-mediated signaling. To examine the role of sGC in regulating pulmonary development, we tested whether decreased sGC activity reduces alveolarization in the normal and injured newborn lung. For these studies, mouse pups with gene-targeted sGC-α1 subunit truncation were used because we determined that they have decreased pulmonary sGC enzyme activity. sGC-α1 knockout (KO) mouse pups were observed to have decreased numbers of small airway structures and lung volume compared with wild-type (WT) mice although lung septation and body weights were not different. However, following mild lung injury caused by breathing 70% O2, the sGC-α1 KO mouse pups had pronounced inhibition of alveolarization, as evidenced by an increase in airway mean linear intercept, reduction in terminal airway units, and decrease in lung septation and alveolar openings, as well as reduced somatic growth. Because cGMP regulates SMC phenotype, we also tested whether decreased sGC activity reduces lung myofibroblast differentiation. Cellular markers revealed that vascular SMC differentiation decreased, whereas myofibroblast activation increased in the hyperoxic sGC-α1 KO pup lung. These results indicate that lung development, particularly during hyperoxic injury, is impaired in mouse pups with diminished sGC activity. These studies support the investigation of sGC-targeting agents as therapies directed at improving development in the newborn lung exposed to injury.

Pathophysiology of hypertension in the absence of nitric oxide/cyclic GMP signaling

The nitric oxide (NO)-cyclic guanosine monophosphate (cGMP) signaling system is a well-characterized modulator of cardiovascular function, in general, and blood pressure, in particular. The availability of mice mutant for key enzymes in the NO-cGMP signaling system facilitated the identification of interactions with other blood pressure modifying pathways (e.g. the renin-angiotensin-aldosterone system) and of gender-specific effects of impaired NO-cGMP signaling. In addition, recent genome-wide association studies identified blood pressure-modifying genetic variants in genes that modulate NO and cGMP levels. Together, these findings have advanced our understanding of how NO-cGMP signaling regulates blood pressure. In this review, we will summarize the results obtained in mice with disrupted NO-cGMP signaling and highlight the relevance of this pathway as a potential therapeutic target for the treatment of hypertension.

Assessing murine resistance artery function using pressure myography

Pressure myograph systems are exquisitely useful in the functional assessment of small arteries, pressurized to a suitable transmural pressure. The near physiological condition achieved in pressure myography permits in-depth characterization of intrinsic responses to pharmacological and physiological stimuli, which can be extrapolated to the in vivo behavior of the vascular bed. Pressure myograph has several advantages over conventional wire myographs. For example, smaller resistance vessels can be studied at tightly controlled and physiologically relevant intraluminal pressures. Here, we study the ability of 3(rd) order mesenteric arteries (3-4 mm long), preconstricted with phenylephrine, to vaso-relax in response to acetylcholine. Mesenteric arteries are mounted on two cannulas connected to a pressurized and sealed system that is maintained at constant pressure of 60 mmHg. The lumen and outer diameter of the vessel are continuously recorded using a video camera, allowing real time quantification of the vasoconstriction and vasorelaxation in response to phenylephrine and acetylcholine, respectively. To demonstrate the applicability of pressure myography to study the etiology of cardiovascular disease, we assessed endothelium-dependent vascular function in a murine model of systemic hypertension. Mice deficient in the α1 subunit of soluble guanylate cyclase (sGCα1(-/-)) are hypertensive when on a 129S6 (S6) background (sGCα1(-/-S6)) but not when on a C57BL/6 (B6) background (sGCα1(-/-B6)). Using pressure myography, we demonstrate that sGCα1-deficiency results in impaired endothelium-dependent vasorelaxation. The vascular dysfunction is more pronounced in sGCα1(-/-S6) than in sGCα1(-/-B6) mice, likely contributing to the higher blood pressure in sGCα1(-/-S6) than in sGCα1(-/-B6) mice. Pressure myography is a relatively simple, but sensitive and mechanistically useful technique that can be used to assess the effect of various stimuli on vascular contraction and relaxation, thereby augmenting our insight into the mechanisms underlying cardiovascular disease.

Genetic modification of hypertension by sGCα1

Hypertension is an important modifiable risk factor for coronary heart disease, congestive heart failure, stroke, end-stage renal disease, and peripheral vascular disease, but many of the molecular mechanisms and genetic factors underlying the development of the most common forms of human hypertension remain to be defined. Abundant evidence suggests that nitric oxide (NO) and one of its primary targets, the cyclic guanosine monophosphate (cGMP)-generating enzyme soluble guanylate cyclase (sGC), have a critical role in regulating blood pressure. The availability of murine models of hypertension and the revolution in human genetics research (e.g., genome-wide association studies [GWAS]), resulting in the identification of dozens of genetic loci that affect normal variation in blood pressure and susceptibility to hypertension, provide a unique opportunity to dissect the mechanisms by which NO-cGMP signaling regulates blood pressure and to gain important insights into the pathogenesis of hypertension. In this review, we will give an overview of the current knowledge relating to the role of sGC in the regulation of blood pressure, discussing data obtained from genetically modified mouse models as well as from human genetic studies.

Reduction of cardiomyocyte S-nitrosylation by S-nitrosoglutathione reductase protects against sepsis-induced myocardial depression

Myocardial depression is an important contributor to morbidity and mortality in septic patients. Nitric oxide (NO) plays an important role in the development of septic cardiomyopathy, but also has protective effects. Recent evidence has indicated that NO exerts many of its downstream effects on the cardiovascular system via protein S-nitrosylation, which is negatively regulated by S-nitrosoglutathione reductase (GSNOR), an enzyme promoting denitrosylation. We tested the hypothesis that reducing cardiomyocyte S-nitrosylation by increasing GSNOR activity can improve myocardial dysfunction during sepsis. Therefore, we generated mice with a cardiomyocyte-specific overexpression of GSNOR (GSNOR-CMTg mice) and subjected them to endotoxic shock. Measurements of cardiac function in vivo and ex vivo showed that GSNOR-CMTg mice had a significantly improved cardiac function after lipopolysaccharide challenge (LPS, 50 mg/kg) compared with wild-type (WT) mice. Cardiomyocytes isolated from septic GSNOR-CMTg mice showed a corresponding improvement in contractility compared with WT cells. However, systolic Ca(2+) release was similarly depressed in both genotypes after LPS, indicating that GSNOR-CMTg cardiomyocytes have increased Ca(2+) sensitivity during sepsis. Parameters of inflammation were equally increased in LPS-treated hearts of both genotypes, and no compensatory changes in NO synthase expression levels were found in GSNOR-overexpressing hearts before or after LPS challenge. GSNOR overexpression however significantly reduced total cardiac protein S-nitrosylation during sepsis. Taken together, our results indicate that increasing the denitrosylation capacity of cardiomyocytes protects against sepsis-induced myocardial depression. Our findings suggest that specifically reducing protein S-nitrosylation during sepsis improves cardiac function by increasing cardiac myofilament sensitivity to Ca(2+).

Critical role of parathyroid hormone (PTH) receptor-1 phosphorylation in regulating acute responses to PTH

Agonist-induced phosphorylation of the parathyroid hormone (PTH) receptor 1 (PTHR1) regulates receptor signaling in vitro, but the role of this phosphorylation in vivo is uncertain. We investigated this role by injecting "knock-in" mice expressing a phosphorylation-deficient (PD) PTHR1 with PTH ligands and assessing acute biologic responses. Following injection with PTH (1-34), or with a unique, long-acting PTH analog, PD mice, compared with WT mice, exhibited enhanced increases in cAMP levels in the blood, as well as enhanced cAMP production and gene expression responses in bone and kidney tissue. Surprisingly, however, the hallmark hypercalcemic and hypophosphatemic responses were markedly absent in the PD mice, such that paradoxical hypocalcemic and hyperphosphatemic responses were observed, quite strikingly with the long-acting PTH analog. Spot urine analyses revealed a marked defect in the capacity of the PD mice to excrete phosphate, as well as cAMP, into the urine in response to PTH injection. This defect in renal excretion was associated with a severe, PTH-induced impairment in glomerular filtration, as assessed by the rate of FITC-inulin clearance from the blood, which, in turn, was explainable by an overly exuberant systemic hypotensive response. The overall findings demonstrate the importance in vivo of PTH-induced phosphorylation of the PTHR1 in regulating acute ligand responses, and they serve to focus attention on mechanisms that underlie the acute calcemic response to PTH and factors, such as blood phosphate levels, that influence it.

Cellular basis of burn-induced cardiac dysfunction and prevention by mesenteric lymph duct ligation

BACKGROUND

Myocardial contractile depression develops 4 to 24 h after major burn injury. We have reported previously that in a rat burn injury model (≈40% of total body surface area burn), mesenteric lymph duct ligation (LDL) prior to burn prevented myocardial dysfunction. However, the underlying cellular and molecular mechanisms are not well understood.

MATERIALS AND METHODS

Left ventricular myocytes were isolated from sham burn (control), sham burn with LDL (sham + LDL), burn, and burn with LDL (burn + LDL) rats at 4 and 24 h after burn or sham burn. Electrophysiological techniques were used to study myocyte size, contractility and L-type Ca2+ channel current (ICa). Further studies examined changes in the messenger RNA expression levels of pore-forming subunit of the L-type Ca(2+) channel, α1C, and its auxiliary subunits, β1, β2, β3, and α2δ1, which modulate the abundance of the ICa in post-burn hearts.

RESULTS

Depressed myocyte contractility (≈20%) developed during 4 to 24 h post-burn compared with control, sham + LDL, or burn + LDL groups, a pattern of changes consistent with whole heart studies. There was no significant alteration in myocyte size. The ICa density was significantly decreased (≈30%) at 24 h post-burn, whereas the messenger RNA expression levels of Ca(2+) channel gene were not significantly altered at 4 and 24 h after burn injury.

CONCLUSIONS

These results suggest that the post-burn contractile phenotype in vivo was also present in isolated myocytes in vitro, but cellular remodeling was not a major factor. The results also suggest that changes in ICa regulation, but not from Ca(2+) channel gene modification, may be a key element involved in post-burn contractile depression and the beneficial effects of LDL.

Genetic modifiers of hypertension in soluble guanylate cyclase α1-deficient mice

Nitric oxide (NO) plays an essential role in regulating hypertension and blood flow by inducing relaxation of vascular smooth muscle. Male mice deficient in a NO receptor component, the α1 subunit of soluble guanylate cyclase (sGCα1), are prone to hypertension in some, but not all, mouse strains, suggesting that additional genetic factors contribute to the onset of hypertension. Using linkage analyses, we discovered a quantitative trait locus (QTL) on chromosome 1 that was linked to mean arterial pressure (MAP) in the context of sGCα1 deficiency. This region is syntenic with previously identified blood pressure-related QTLs in the human and rat genome and contains the genes coding for renin. Hypertension was associated with increased activity of the renin-angiotensin-aldosterone system (RAAS). Further, we found that RAAS inhibition normalized MAP and improved endothelium-dependent vasorelaxation in sGCα1-deficient mice. These data identify the RAAS as a blood pressure-modifying mechanism in a setting of impaired NO/cGMP signaling.

Nitric oxide synthase and cyclic GMP signaling in cardiac myocytes: from contractility to remodeling

Cyclic guanosine 3'5'monophosphate (cGMP) is the common downstream second messenger of natriuretic peptides and nitric oxide. In cardiac myocytes, the physiological effects of cGMP are exerted through the activation of protein kinase G (PKG) signaling, and the activation and/or inhibition of phosphodiesterases (PDEs), providing an integration point between cAMP and cGMP signals. Specificity of cGMP signals is achieved through compartmentalization of cGMP synthesis by guanylate cyclases, and cGMP hydrolysis by PDEs. Increasing evidence suggests that cGMP-dependent signaling pathways play an important role in inhibiting cardiac remodeling, through the inhibition Ca(2+) handling upstream of pathological Ca(2+)-dependent signaling pathways. Thus, enhancing cardiac myocyte cGMP signaling represents a promising therapeutic target for treatment of cardiovascular disease. This article is part of a Special Issue entitled "Local Signaling in Myocytes."

Inorganic nitrite therapy: historical perspective and future directions

Over the past several years, investigators studying nitric oxide (NO) biology and metabolism have come to learn that the one-electron oxidation product of NO, nitrite anion, serves as a unique player in modulating tissue NO bioavailability. Numerous studies have examined how this oxidized metabolite of NO can act as a salvage pathway for maintaining NO equivalents through multiple reduction mechanisms in permissive tissue environments. Moreover, it is now clear that nitrite anion production and distribution throughout the body can act in an endocrine manner to augment NO bioavailability, which is important for physiological and pathological processes. These discoveries have led to renewed hope and efforts for an effective NO-based therapeutic agent through the unique action of sodium nitrite as an NO prodrug. More recent studies also indicate that sodium nitrate may also increase plasma nitrite levels via the enterosalivary circulatory system resulting in nitrate reduction to nitrite by microorganisms found within the oral cavity. In this review, we discuss the importance of nitrite anion in several disease models along with an appraisal of sodium nitrite therapy in the clinic, potential caveats of such clinical uses, and future possibilities for nitrite-based therapies.

The alpha1 isoform of soluble guanylate cyclase regulates cardiac contractility but is not required for ischemic preconditioning

Nitric oxide (NO)-dependent soluble guanylate cyclase (sGC) activation is an important component of cardiac signal transduction pathways, including the cardioprotective signaling cascade induced by ischemic preconditioning (IPC). The sGCα subunit, which binds to the common sGCβ1 subunit, exists in two different isoforms, sGCα1 and sGCα2, but their relative physiological roles remain unknown. In the present study, we studied Langendorff-perfused isolated hearts of genetically engineered mice lacking functional sGCα1 (sGCα1KO mice), which is the predominant isoform in the heart. Our results show that the loss of sGCα1 has a positive inotropic and lusitropic effect on basal cardiac function, indicating an important role for sGCα1 in regulating basal myocardial contractility. Surprisingly, IPC led to a similar 35-40% reduction in infarct size and concomitant protein kinase Cε (PKCε) phosphorylation in both wild-type (WT) and sGCα1KO hearts subjected to 40 min of global ischemia and reperfusion. Inhibition of the activation of all sGC isoforms by 1H-[1,2,4]oxadiazolo[4,3-a]quinoxaline-1-one (ODQ, 10 μmol/L) completely abolished the protection by IPC in WT and sGCα1KO hearts. NO-stimulated cGMP production was severely attenuated in sGCα1KO hearts compared to WT hearts, indicating that the sGCα2 isoform only produces minute amounts of cGMP after NO stimulation. Taken together, our results indicate that although sGCα1 importantly regulates cardiac contractility, it is not required for cardioprotection by IPC. Instead, our results suggest that possibly only minimal sGC activity, which in sGCα1KO hearts is provided by the sGCα2 isoform, is sufficient to transduce the cardioprotective signal induced by IPC via phosphorylation of PKCε.

Soluble guanylate cyclase-α1 is required for the cardioprotective effects of inhaled nitric oxide

Reperfusion injury limits the benefits of revascularization in the treatment of myocardial infarction (MI). Breathing nitric oxide (NO) reduces cardiac ischemia-reperfusion injury in animal models; however, the signaling pathways by which inhaled NO confers cardioprotection remain uncertain. The objective of this study was to learn whether inhaled NO reduces cardiac ischemia-reperfusion injury by activating the cGMP-generating enzyme, soluble guanylate cyclase (sGC), and to investigate whether bone marrow (BM)-derived cells participate in the sGC-mediated cardioprotective effects of inhaled NO. Wild-type (WT) mice and mice deficient in the sGC α(1)-subunit (sGCα(1)(-/-) mice) were subjected to cardiac ischemia for 1 h, followed by 24 h of reperfusion. During ischemia and for the first 10 min of reperfusion, mice were ventilated with oxygen or with oxygen supplemented with NO (80 parts per million). The ratio of MI size to area at risk (MI/AAR) did not differ in WT and sGCα(1)(-/-) mice that did not breathe NO. Breathing NO decreased MI/AAR in WT mice (41%, P = 0.002) but not in sGCα(1)(-/-) mice (7%, P = not significant). BM transplantation was performed to restore WT BM-derived cells to sGCα(1)(-/-) mice. Breathing NO decreased MI/AAR in sGCα(1)(-/-) mice carrying WT BM (39%, P = 0.031). In conclusion, these results demonstrate that a global deficiency of sGCα(1) does not alter the degree of cardiac ischemia-reperfusion injury in mice. The cardioprotective effects of inhaled NO require the presence of sGCα(1). Moreover, our studies suggest that BM-derived cells are key mediators of the ability of NO to reduce cardiac ischemia-reperfusion injury.

Nitric oxide and calcium signaling regulate myocardial tumor necrosis factor-α expression and cardiac function in sepsis

Myocardial tumor necrosis factor-alpha (TNF-alpha), a proinflammatory cytokine, is a critical inducer of myocardial dysfunction in sepsis. The purpose of this review is to summarize the mechanisms through which TNF-alpha production is regulated in cardiomyocytes in response to lipopolysaccharide (LPS), a key pathogen-associated molecular pattern (PAMP) in sepsis. These mechanisms include Nox2-containing NAD(P)H oxidase, phospholipase C (PLC)gamma1, and Ca2+ signaling pathways. Activation of these pathways increases TNF-alpha expression via activation of extracellular signal-regulated kinases 1 and 2 (ERK1/2) and p38 mitogen-activated protein kinase (MAPK). Conversely, activation of c-Jun NH2-terminal kinase 1 (JNK1) negatively regulates TNF-alpha production through inhibition of ERK1/2 and p38 MAPK activity. Interestingly, endothelial nitric oxide synthase (eNOS) promotes TNF-alpha expression by enhancing p38 MAPK activation, whereas neuronal NOS (nNOS) inhibits TNF-alpha production by reducing Ca2+-dependent ERK1/2 activity. Therefore, the JNK1 and nNOS inhibitory pathways represent a "brake" that limits myocardial TNF-alpha expression in sepsis. Further understanding of these signal transduction mechanisms may lead to novel pharmacological therapies in sepsis.

Endothelial nitric oxide synthase transgenic models of endothelial dysfunction

Endothelial production of nitric oxide is critical to the regulation of vascular responses, including vascular tone and regional blood flow, leukocyte-endothelial interactions, platelet adhesion and aggregation, and vascular smooth muscle cell proliferation. A relative deficiency in the amount of bioavailable vascular NO results in endothelial dysfunction, with conditions that are conducive to the development of atherosclerosis: thrombosis, inflammation, neointimal proliferation, and vasoconstriction. This review focuses on mouse models of endothelial dysfunction caused by direct genetic modification of the endothelial nitric oxide synthase (eNOS) gene. We first describe the cardiovascular phenotypes of eNOS knockout mice, which are a model of total eNOS gene deficiency and thus the ultimate model of endothelial dysfunction. We then describe S1177A and S1177D eNOS mutant mice as mouse models with altered eNOS phosphorylation and therefore varying degrees of endothelial dysfunction. These include transgenic mice that carry the eNOS S1177A and S1177D transgenes, as well as knockin mice in which the endogenous eNOS gene has been mutated to carry the S1177A and S1177D mutations. Together, eNOS knockout mice and eNOS S1177 mutant mice are useful tools to study the effects of total genetic deficiency of eNOS as well as varying degrees of endothelial dysfunction caused by eNOS S1177 phosphorylation.

Endotoxin impairs cardiac hemodynamics by affecting loading conditions but not by reducing cardiac inotropism

Acute myocardial dysfunction is a typical manifestation of septic shock. Experimentally, the administration of endotoxin [lipopolysacharride (LPS)] to laboratory animals is frequently used to study such dysfunction. However, a majority of studies used load-dependent indexes of cardiac function [including ejection fraction (EF) and maximal systolic pressure increment (dP/dt(max))], which do not directly explore cardiac inotropism. Therefore, we evaluated the direct effects of LPS on myocardial contractility, using left ventricular (LV) pressure-volume catheters in mice. Male BALB/c mice received an intraperitoneal injection of E. coli LPS (1, 5, 10, or 20 mg/kg). After 2, 6, or 20 h, cardiac function was analyzed in anesthetized, mechanically ventilated mice. All doses of LPS induced a significant drop in LV stroke volume and a trend toward reduced cardiac output after 6 h. Concomitantly, there was a significant decrease of LV preload (LV end-diastolic volume), with no apparent change in LV afterload (evaluated by effective arterial elastance and systemic vascular resistance). Load-dependent indexes of LV function were markedly reduced at 6 h, including EF, stroke work, and dP/dt(max). In contrast, there was no reduction of load-independent indexes of LV contractility, including end-systolic elastance (ejection phase measure of contractility) and the ratio dP/dt(max)/end-diastolic volume (isovolumic phase measure of contractility), the latter showing instead a significant increase after 6 h. All changes were transient, returning to baseline values after 20 h. Therefore, the alterations of cardiac function induced by LPS are entirely due to altered loading conditions, but not to reduced contractility, which may instead be slightly increased.

Soluble guanylate cyclase alpha1beta1 limits stroke size and attenuates neurological injury

BACKGROUND AND PURPOSE

Nitric oxide mediates endothelium-dependent vasodilation, modulates cerebral blood flow, and determines stroke outcome. Nitric oxide signals in part by stimulating soluble guanylate cyclase (sGC) to synthesize cGMP. To study the role of sGC in stroke injury, we compared the outcome of cerebral ischemia and reperfusion in mice deficient in the alpha(1) subunit of sGC (sGCalpha(1)(-/-)) with that in wild-type mice.

METHODS

Blood pressure, cerebrovascular anatomy, and vasoreactivity of pressurized carotid arteries were compared in both mouse genotypes. Cerebral blood flow was measured before and during middle cerebral artery occlusion and reperfusion. We then assessed neurological deficit and infarct volume after 1 hour of occlusion and 23 hours of reperfusion and after 24 hours of occlusion.

RESULTS

Blood pressure and cerebrovascular anatomy were similar between genotypes. We found that vasodilation of carotid arteries in response to acetylcholine or sodium nitroprusside was diminished in sGCalpha(1)(-/-) compared with wild-type mice. Cerebral blood flow deficits did not differ between the genotypes during occlusion, but during reperfusion, cerebral blood flow was 45% less in sGCalpha(1)(-/-) mice. Infarct volumes and neurological deficits were similar after 24 hours of occlusion in both genotypes. After 1 hour of ischemia and 23 hours of reperfusion, infarct volumes were 2-fold larger and neurological deficits were worse in sGCalpha(1)(-/-) than in the wild-type mice.

CONCLUSIONS

sGCalpha(1) deficiency impairs vascular reactivity to nitric oxide and is associated with incomplete reperfusion, larger infarct size, and worse neurological damage, suggesting that cGMP generated by sGCalpha(1)beta(1) is protective in ischemic stroke.

Nitrite protects against morbidity and mortality associated with TNF- or LPS-induced shock in a soluble guanylate cyclase-dependent manner

Nitrite (NO₂⁻), previously viewed as a physiologically inert metabolite and biomarker of the endogenous vasodilator NO, was recently identified as an important biological NO reservoir in vasculature and tissues, where it contributes to hypoxic signaling, vasodilation, and cytoprotection after ischemia–reperfusion injury. Reduction of nitrite to NO may occur enzymatically at low pH and oxygen tension by deoxyhemoglobin, deoxymyoglobin, xanthine oxidase, mitochondrial complexes, or NO synthase (NOS). We show that nitrite treatment, in sharp contrast with the worsening effect of NOS inhibition, significantly attenuates hypothermia, mitochondrial damage, oxidative stress and dysfunction, tissue infarction, and mortality in a mouse shock model induced by a lethal tumor necrosis factor challenge. Mechanistically, nitrite-dependent protection was not associated with inhibition of mitochondrial complex I activity, as previously demonstrated for ischemia–reperfusion, but was largely abolished in mice deficient for the soluble guanylate cyclase (sGC) α1 subunit, one of the principal intracellular NO receptors and signal transducers in the cardiovasculature. Nitrite could also provide protection against toxicity induced by Gram-negative lipopolysaccharide, although higher doses were required. In conclusion, we show that nitrite can protect against toxicity in shock via sGC-dependent signaling, which may include hypoxic vasodilation necessary to maintain microcirculation and organ function, and cardioprotection.