Hypoxia induces a functionally significant and translationally efficient neuronal NO synthase mRNA variant

Evaluation of the therapeutic utility of phosphodiesterase 5A inhibition in the mdx mouse model of duchenne muscular dystrophy

Duchenne muscular dystrophy (DMD) is a devastating and ultimately fatal disease characterized by progressive muscle wasting and weakness. DMD is caused by the absence of a functional dystrophin protein, which in turn leads to reduced expression and mislocalization of dystrophin-associated proteins including neuronal nitric oxide (NO) synthase mu (nNOSμ). Disruption of nNOSμ signaling results in muscle fatigue and unopposed sympathetic vasoconstriction during exercise, thereby increasing contraction-induced damage in dystrophin-deficient muscles. The loss of normal nNOSμ signaling during exercise is central to the vascular dysfunction proposed over 40 years ago to be an important pathogenic mechanism in DMD. Recent preclinical studies focused on circumventing defective nNOSμ signaling in dystrophic skeletal and cardiac muscle by inhibiting phosphodiesterase 5A (PDE5A) have shown promising results. This review addresses nNOS signaling in normal and dystrophin-deficient muscles and the potential of PDE5A inhibition as a therapeutic approach for the treatment of cardiovascular deficits in DMD.

Regulation of the Pulmonary Circulation in the Fetus and Newborn

During the development of the pulmonary vasculature in the fetus, many structural and functional changes occur to prepare the lung for the transition to air breathing. The development of the pulmonary circulation is genetically controlled by an array of mitogenic factors in a temporo-spatial order. With advancing gestation, pulmonary vessels acquire increased vasoreactivity. The fetal pulmonary vasculature is exposed to a low oxygen tension environment that promotes high intrinsic myogenic tone and high vasocontractility. At birth, a dramatic reduction in pulmonary arterial pressure and resistance occurs with an increase in oxygen tension and blood flow. The striking hemodynamic differences in the pulmonary circulation of the fetus and newborn are regulated by various factors and vasoactive agents. Among them, nitric oxide, endothelin-1, and prostaglandin I2 are mainly derived from endothelial cells and exert their effects via cGMP, cAMP, and Rho kinase signaling pathways. Alterations in these signaling pathways may lead to vascular remodeling, high vasocontractility, and persistent pulmonary hypertension of the newborn.

Role of intermittent hypoxia in the treatment of bronchial asthma and chronic obstructive pulmonary disease

PURPOSE OF REVIEW

The purpose of this review is to describe the impact that exposure to intermittent hypoxic training (IHT) could have on bronchial asthma and chronic obstructive pulmonary disease (COPD). This is of particular interest, as an increasing number of patients suffer from severe symptoms of bronchial asthma and COPD and desire more effective and efficient treatment options with fewer side effects.

RECENT FINDINGS

Exposure to IHT has been shown to raise baroreflex sensitivity to normal levels and to selectively increase hypercapnic ventilatory response, total exercise time, total haemoglobin mass, and lung diffusion capacity for carbon monoxide in COPD patients. However, evidence proving that IHT leads to health benefit effects in bronchial asthma patients has not been produced by recent literature.

SUMMARY

Recent research outlines the value of IHT as a therapeutic strategy for the treatment of COPD patients, leading to more efficient ventilation. Additionally, IHT might represent an attractive method to complement the known beneficial effects of exercise training and to rebalance early autonomic dysfunction in COPD patients. Future research examining the potential risks and benefits of IHT could pave the way for the development of new therapeutic approaches for patients suffering from bronchial asthma and COPD.

Delayed gastric emptying and disruption of the interstitial cells of Cajal network after gastric ischaemia and reperfusion

BACKGROUND

Gastrointestinal tract is one of the most susceptible organ systems to ischaemia. Not only mucosal injury but also alterations of the intestinal motility and loss of interstitial cells of Cajal (ICC) have been reported in response to ischaemia and reperfusion (I/R). However, there are few reports on the changes in the gastric motility after gastric I/R. The present study was designed to investigate the alterations in gastric emptying, the ICC and enteric nerves that regulate smooth muscle function in response to gastric I/R.

METHODS

Seven-week-old male Wistar rats were exposed to gastric I/R, and the gastric emptying rates at 12 and 48 h after I/R were evaluated by the phenol red method. Expressions of gene product of c-kit receptor tyrosine kinase (c-Kit), a marker of ICC, and of neuronal proteins were also examined.

KEY RESULTS

Gastric emptying was transiently delayed at 12 h after I/R, but returned to normal by 48 h. Expression of c-Kit protein as assessed by Western blotting and immunofluorescent staining of the smooth muscle layer, as well as expression of the mRNA of stem cell factor, the ligand for c-Kit, were reduced at both 12 and 48 h after I/R. The expression of neuronal nitric oxide synthase (nNOS) protein as assessed by Western blotting and immunofluorescent staining was also decreased at 12 h after I/R, but was restored to normal by 48 h.

CONCLUSIONS & INFERENCES

Gastric I/R evokes transient gastroparesis with delayed gastric emptying, associated with disruption of the ICC network and nNOS-positive neurons.

Interplay between calcium and reactive oxygen/nitrogen species: an essential paradigm for vascular smooth muscle signaling

Signaling cascades initiated or regulated by calcium (Ca(2+)), reactive oxygen (ROS), and nitrogen (RNS) species are essential to diverse physiological and pathological processes in vascular smooth muscle. Stimuli-induced changes in intracellular Ca(2+) regulate the activity of primary ROS and RNS, producing enzymes including NADPH oxidases (Nox) and nitric oxide synthases (NOS). At the same time, alteration in intracellular ROS and RNS production reciprocates through redox-based post-translational modifications altering Ca(2+) signaling networks. These may include Ca(2+) pumps such as sarcoplasmic endoplasmic reticulum Ca(2+)-ATPase (SERCA), voltage-gated channels, transient receptor potential canonical (TRPC), melastatin2 (TRPM2), and ankyrin1 (TRPA1) channels, store operated Ca(2+) channels such as Orai1/stromal interaction molecule 1 (STIM1), and Ca(2+) effectors such as Ca(2+)/calmodulin-dependent protein kinase II (CaMKII). In this review, we summarize and highlight current experimental evidence supporting the idea that cross-talk between Ca(2+) and ROS/RNS may represent a well-integrated signaling network in vascular smooth muscle.

Nitric oxide synthases in the pathogenesis of cardiovascular disease: lessons from genetically modified mice

Nitric oxide (NO) is produced in almost all tissues and organs, exerting a variety of biological actions under both physiological and pathological conditions. NO is synthesized by three distinct NO synthase (NOS) isoforms (neuronal, inducible, and endothelial NOS), all of which are expressed in the human cardiovascular system. Although the regulatory roles of NOSs in cardiovascular diseases have been described in pharmacological studies with selective and non-selective NOS inhibitors, the specificity of the NOS inhibitors continues to be an issue of debate. To overcome this issue, genetically engineered animals have been used. All types of NOS gene-deficient animals, including singly, doubly, and triply NOS-deficient mice, and various types of NOS gene-transgenic (TG) animals, including conditional and non-conditional TG mice bearing endothelium-specific or cardiomyocyte-specific overexpression of each NOS gene, have thus been developed. The roles of individual NOS isoforms as well as the entire NOS system in the cardiovascular system have been extensively investigated in those mice, providing pivotal insights into an understanding of the pathophysiology of NOSs in human cardiovascular diseases. Based on studies with the murine NOS genetic models, this review briefly summarizes the latest knowledge of NOSs and cardiovascular diseases.

Enhanced translation of heme oxygenase-2 preserves human endothelial cell viability during hypoxia

Heme oxygenases (HOs) -1 and -2 catalyze the breakdown of heme to release carbon monoxide, biliverdin, and ferrous iron, which may preserve cell function during oxidative stress. HO-1 levels decrease in endothelial cells exposed to hypoxia, whereas the effect of hypoxia on HO-2 expression is unknown. The current study was carried out to determine if hypoxia alters HO-2 protein levels in human endothelial cells and whether this enzyme plays a role in preserving their viability during hypoxic stress. Human umbilical vein endothelial cells (HUVECs), human aortic endothelial cells (HAECs), and human blood outgrowth endothelial cells were exposed to 21% or 1% O(2) for 48 or 16 h in the presence or absence of tumor necrosis factor-alpha (10 ng/ml) or H(2)O(2) (100 microm). In all three endothelial cell types HO-1 mRNA and protein levels were decreased following hypoxic incubation, whereas HO-2 protein levels were unaltered. In HUVECs HO-2 levels were maintained during hypoxia despite a 57% reduction in steady-state HO-2 mRNA level and a 43% reduction in total protein synthesis. Polysome profiling revealed increased HO-2 transcript association with polysomes during hypoxia consistent with enhanced translation of these transcripts. Importantly, inhibition of HO-2 expression by small interference RNA increased oxidative stress, exacerbated mitochondrial membrane depolarization, and enhanced caspase activation and apoptotic cell death in cells incubated under hypoxic but not normoxic conditions. These data indicate that HO-2 is important in maintaining endothelial viability and may preserve local regulation of vascular tone, thrombosis, and inflammatory responses during reductions in systemic oxygen delivery.

Hypoxic repression of endothelial nitric-oxide synthase transcription is coupled with eviction of promoter histones

Hypoxia elicits endothelial dysfunction, in part, through reduced expression of endothelial nitric-oxide synthase (eNOS). Here we present evidence that hypoxia causes a rapid decrease in the transcription of the eNOS/NOS3 gene, accompanied by decreased acetylation and lysine 4 (histone H3) methylation of eNOS proximal promoter histones. Surprisingly, we demonstrate that histones are rapidly evicted from the eNOS proximal promoter during hypoxia. We also demonstrate endothelium-specific H2A.Z incorporation at the eNOS promoter and find that H2A.Z is also evicted by hypoxic stimulation. After longer durations of hypoxia, histones are reincorporated at the eNOS promoter, but these histones lack substantial histone acetylation. Additionally, we identify a key role for the chromatin remodeler, BRG1, in re-establishing eNOS expression following reoxygenation of hypoxic cells. We posit that post-translational histone modifications are required to maintain constitutive eNOS transcriptional activity and that histone eviction rapidly resets histone marks and is a proximal event in the hypoxic repression of eNOS. Although nucleosome eviction has been reported in models of transcriptional activation, the observation that eviction can also accompany transcriptional repression in hypoxic mammalian cells argues that eviction may be broadly relevant to both positive and negative changes in transcription.

Vascular dysfunction in retinopathy-an emerging role for arginase

Retinal neovascularization is a leading cause of visual disability. Retinal diseases involving neovascularization all follow the same progression, beginning with vascular inflammatory reactions and injury of the vascular endothelium and ending with neovascularization, fibrosis and retinal detachment. Understanding the mechanisms underlying this process is critical for its prevention and treatment. Research using retinopathy models has revealed that the NOX2 NADPH oxidase has a key role in inducing production of reactive oxygen species and angiogenic cytokines and causing vascular inflammatory reactions and neovascularization. This prospective review addresses the potential role of the urea/ornithine pathway enzyme arginase in this process. Studies of peripheral vessels isolated from diabetic animals have shown that increased arginase activity causes vascular endothelial cell dysfunction by decreasing availability of l-arginine to endothelial cell nitric oxide synthase which decreases nitric oxide bioavailability and increases oxidative stress. Increasing arginase activity also increases formation of polyamines and proline, which can induce cell growth and fibrosis. Studies in models of retinopathy show that increases in oxidative stress and signs of vascular inflammation are correlated with increases in arginase activity and arginase 1 expression and that decreasing arginase expression or inhibiting its activity blocks these effects. Furthermore, the induction of arginase during retinopathy is blocked by knocking out NOX2 or inhibiting NADPH oxidase activity. These observations suggest that NADPH oxidase-induced activation of the arginase pathway has a key role in causing retinal vascular dysfunction during retinopathy. Limiting the actions of arginase could provide a new strategy for treating this potentially blinding condition.

Retinoic acid-induced nNOS expression depends on a novel PI3K/Akt/DAX1 pathway in human TGW-nu-I neuroblastoma cells

Neuronal nitric oxide synthase (nNOS)-derived nitric oxide (NO) acts as a neurotransmitter and intracellular signaling molecule in the central and peripheral nervous system. NO regulates multiple processes like neuronal development, plasticity, and differentiation and is a mediator of neurotoxicity. The nNOS gene is highly complex with 12 alternative first exons, exon 1a-1l, transcribed from distinct promoters, leading to nNOS variants with different 5'-untranslated regions. Transcriptional control of the nNOS gene is not understood in detail. To investigate regulation of nNOS gene expression by retinoic acid (RA), we used the human neuroblastoma cell line TGW-nu-I as a model system. We show that RA induces nNOS transcription in a protein synthesis-dependent fashion. We identify the phosphatidylinositol 3-kinase (PI3K)/Akt signaling pathway and the atypical orphan nuclear receptor DAX1 (NR0B1) as critical mediators involved in RA-induced nNOS gene transcription. RA treatment increases DAX1 expression via PI3K/Akt signaling. Upregulation of DAX1 expression in turn induces nNOS transcription in response to RA. These results identify nNOS as a target gene of a novel RA/PI3K/Akt/DAX1-dependent pathway in human neuroblastoma cells and stress the functional importance of the transcriptional regulator DAX1 for nNOS gene expression in response to RA treatment.

Nitric oxide synthases and cardiovascular diseases: insights from genetically modified mice

Nitric oxide (NO) is produced in almost all tissues and organs, exerting a variety of biological actions under both physiological and pathological conditions. NO is synthesized by 3 distinct NO synthase (NOS) isoforms (neuronal, inducible, and endothelial NOS), all of which are expressed in the human cardiovascular system. The regulatory roles of NOSs in cardiovascular diseases have been described in pharmacological studies with selective and non-selective NOS inhibitors. However, the specificity of the NOS inhibitors continues to be an issue of debate. To overcome this issue, genetically engineered animals have been used. All types of NOS gene-deficient (knockout: KO) animals, including singly, doubly, and triply NOS-KO mice, and various types of NOS gene-transgenic (TG) animals, including conditional and non-conditional TG mice bearing endothelium-specific or cardiomyocyte-specific overexpression of each NOS gene, have thus far been developed. The roles of individual NOS isoforms, as well as the entire NOS system, in the cardiovascular system have been extensively investigated in those mice, and the results provide pivotal insights into the pathophysiology of NOSs in human cardiovascular diseases. Based on studies with murine NOS genetic models, this review summarizes the latest knowledge of NOSs and cardiovascular diseases.

The heart is an early target of anthrax lethal toxin in mice: a protective role for neuronal nitric oxide synthase (nNOS)

Anthrax lethal toxin (LT) induces vascular insufficiency in experimental animals through unknown mechanisms. In this study, we show that neuronal nitric oxide synthase (nNOS) deficiency in mice causes strikingly increased sensitivity to LT, while deficiencies in the two other NOS enzymes (iNOS and eNOS) have no effect on LT-mediated mortality. The increased sensitivity of nNOS-/- mice was independent of macrophage sensitivity to toxin, or cytokine responses, and could be replicated in nNOS-sufficient wild-type (WT) mice through pharmacological inhibition of the enzyme with 7-nitroindazole. Histopathological analyses showed that LT induced architectural changes in heart morphology of nNOS-/- mice, with rapid appearance of novel inter-fiber spaces but no associated apoptosis of cardiomyocytes. LT-treated WT mice had no histopathology observed at the light microscopy level. Electron microscopic analyses of LT-treated mice, however, revealed striking pathological changes in the hearts of both nNOS-/- and WT mice, varying only in severity and timing. Endothelial/capillary necrosis and degeneration, inter-myocyte edema, myofilament and mitochondrial degeneration, and altered sarcoplasmic reticulum cisternae were observed in both LT-treated WT and nNOS-/- mice. Furthermore, multiple biomarkers of cardiac injury (myoglobin, cardiac troponin-I, and heart fatty acid binding protein) were elevated in LT-treated mice very rapidly (by 6 h after LT injection) and reached concentrations rarely reported in mice. Cardiac protective nitrite therapy and allopurinol therapy did not have beneficial effects in LT-treated mice. Surprisingly, the potent nitric oxide scavenger, carboxy-PTIO, showed some protective effect against LT. Echocardiography on LT-treated mice indicated an average reduction in ejection fraction following LT treatment in both nNOS-/- and WT mice, indicative of decreased contractile function in the heart. We report the heart as an early target of LT in mice and discuss a protective role for nNOS against LT-mediated cardiac damage.

HIF-1 and ventilatory acclimatization to chronic hypoxia

Ventilatory acclimatization to hypoxia (VAH) is a time-dependent increase in ventilation and ventilatory O2-sensitivity that involves plasticity in carotid body chemoreceptors and CNS respiratory centers. Hypoxia inducible factor-1alpha (HIF-1alpha) controls the expression of several genes that increase physiological O2 supply. Studies using transgenic mice show HIF-1alpha expression in the carotid bodies and CNS with chronic sustained and intermittent hypoxia is important for VAH. Other O2-sensitive transcription factors such as HIF-2alpha may be important for VAH by reducing metabolic O2 demands also. Specific gene targets of HIF-1alpha shown to be involved in VAH include erythropoietin, endothelin-1, neuronal nitric oxide synthase and tyrosine hydroxylase. Other HIF-1alpha targets that may be involved in VAH include vascular endothelial growth factor, heme oxygenase 1 and cytoglobin. Interactions between these multiple pathways and feedback control of HIF-1alpha expression from some of the targets support a complex and powerful role for HIF-1alpha in neural plasticity of physiological control circuits with chronic hypoxia.

Expression of neuronal nitric oxide synthase splice variants in atherosclerotic plaques of apoE knockout mice

OBJECTIVE

We previously reported that deletion of brain type neuronal nitric oxide synthase-alpha (nNOS-alpha) accelerates atherosclerosis in apolipoproteinE (apoE) knockout (ko) mice. The regulation of nNOS expression is complex, involving the generation of mRNA splice variants. The current study investigates occurrence and distribution of nNOS variants in atherosclerotic lesions of apoE ko and apoE/nNOS-alpha double ko (dko) animals.

METHODS

Mice were fed a high fat diet for 20 weeks. Immunohistochemistry and western blot analysis were performed using antibodies detecting the carboxy terminal-, or amino terminal-residue of the nNOS protein. Confocal microscopy and in situ hybridization were used to identify the compartment of cellular expression.

RESULTS

In situ hybridization revealed the presence of nNOS-alpha and -gamma mRNA variants in apoE ko plaques, while only nNOS-gamma was detectable in apoE/nNOS dko plaques. Consistent with mRNA expression nNOS-alpha protein can be detected in the neointima of apoE ko, but not apoE/nNOS dko animals. In contrast, the carboxy terminal antibody stained the neointima and media in apoE ko vessels and showed residual nNOS immunoreactivity in apoE/nNOS dko lesions. Confocal microscopy showed predominant nNOS expression in vascular smooth muscle cells, while colocalization with macrophages was less pronounced.

CONCLUSIONS

Our study shows that nNOS-alpha and -gamma splice variants are expressed in atherosclerotic plaques of apoE ko mice. nNOS variants colocalized with markers for vascular smooth muscle cells and macrophages but not for endothelial cells. Since nNOS-alpha is atheroprotective, other nNOS splice variants which differ in enzyme kinetic and subcellular localization may also influence plaque formation.

Cerebral adaptations to chronic anemia in a model of erythropoietin-deficient mice exposed to hypoxia

Anemia and hypoxia in rats result in an increase in factors potentially involved in cerebral angiogenesis. Therefore, the aim of this study was to assess the effect of chronic anemia and/or chronic hypoxia on cerebral cellular responses and angiogenesis in wild-type and anemic transgenic mice. These studies were done in erythropoietin-deficient mice (Epo-TAgh) in normoxia and following acute (one day) and chronic (14 days, barometric pressure = 420 mmHg) hypoxia. In normoxia, Epo-TAgh mice showed an increase in transcript and protein levels of hypoxia-inducible factor 1α (HIF-1α), vascular endothelial growth factor (VEGF), erythropoietin receptors (EpoR), phospho-STAT-5/STAT-5 ratio, and neuronal neuronal nitric oxide synthase (nNOS) along with a higher cerebral capillary density. In wild-type (WT) mice, acute hypoxia increased all of the studied factors, while in chronic hypoxia, HIF-1α, EpoR, phospho-STAT-5/STAT-5 ratio, nNOS, and inducible NOS remained elevated, with an increase in capillary density. Surprisingly, in Epo-TAgh mice, chronic hypoxia did not further increase any factor except the nitric oxide metabolites, while HIF-1α, EpoR, and phospho-STAT-5/STAT-5 ratio were reduced. Normoxic Epo-TAgh mice developed cerebral angiogenesis through the HIF-1α/VEGF pathway. In acute hypoxia, WT mice up-regulated all of the studied factors, including cerebral NO. Polycythemia and angiogenesis occurred with acclimatization to chronic hypoxia only in WT mice. In Epo-TAgh, the decrease in HIF-1α, VEGF proteins, and phospho-STAT-5 ratio in chronic hypoxia suggest that neuroprotective and angiogenesis pathways are altered.

Anemia and cerebral outcomes: many questions, fewer answers

A number of clinical studies have associated acute anemia with cerebral injury in perioperative patients. Evidence of such injury has been observed near the currently accepted transfusion threshold (hemoglobin [Hb] concentration, 7-8 g/dL), and well above the threshold for cerebral tissue hypoxia (Hb 3-4 g/dL). However, hypoxic and nonhypoxic mechanisms of anemia-induced cerebral injury have not been clearly elucidated. In addition, protective mechanisms which may minimize cerebral injury during acute anemia have not been well defined. Vasodilatory mechanisms, including nitric oxide (NO), may help to maintain cerebral oxygen delivery during anemia as all three NO synthase (NOS) isoforms (neuronal, endothelial, and inducible NOS) have been shown to be up-regulated in different experimental models of acute hemodilutional anemia. Recent experimental evidence has also demonstrated an increase in an important transcription factor, hypoxia inducible factor (HIF)-1alpha, in the cerebral cortex of anemic rodents at clinically relevant Hb concentrations (Hb 6-7 g/dL). This suggests that cerebral oxygen homeostasis may be in jeopardy during acute anemia. Under hypoxic conditions, cytoplasmic HIF-1alpha degradation is inhibited, thereby allowing it to accumulate, dimerize, and translocate into the nucleus to promote transcription of a number of hypoxic molecules. Many of these molecules, including erythropoietin, vascular endothelial growth factor, and inducible NOS have also been shown to be up-regulated in the anemic brain. In addition, HIF-1alpha transcription can be increased by nonhypoxic mediators including cytokines and vascular hormones. Furthermore, NOS-derived NO may also stabilize HIF-1alpha in the absence of tissue hypoxia. Thus, during anemia, HIF-1alpha has the potential to regulate cerebral cellular responses under both hypoxic and normoxic conditions. Experimental studies have demonstrated that HIF-1alpha may have either neuroprotective or neurotoxic capacity depending on the cell type in which it is up-regulated. In the current review, we characterize these cellular processes to promote a clearer understanding of anemia-induced cerebral injury and protection. Potential mechanisms of anemia-induced injury include cerebral emboli, tissue hypoxia, inflammation, reactive oxygen species generation, and excitotoxicity. Potential mechanisms of cerebral protection include NOS/NO-dependent optimization of cerebral oxygen delivery and cytoprotective mechanisms including HIF-1alpha, erythropoietin, and vascular endothelial growth factor. The overall balance of these activated cellular mechanisms may dictate whether or not their up-regulation leads to cytoprotection or cellular injury during anemia. A clearer understanding of these mechanisms may help us target therapies that will minimize anemia-induced cerebral injury in perioperative patients.

eNOS and stroke: prevention, treatment and recovery

It is common knowledge that ischemic stroke has major social and economic consequences. However, until now, translation of experimental studies into clinical reality has been sorely lacking. So far, most studies have focused on acute stroke outcome and early treatment paradigms affording neuroprotection. It is increasingly recognized that it will be necessary to harness the capacity of the brain for neuroregeneration to improve longer-term outcome. Endothelial nitric oxide synthase (eNOS) is emerging as a key target in molecular stroke research. eNOS ameliorates acute ischemic injury and promotes recovery following cerebral ischemia. This review summarizes the effects of eNOS on the regulation of cerebral blood flow, hemostasis, inflammation, angiogenesis as well as neurogenesis. The possible impact on stroke prevention as well as on strategies aimed at improving long-term stroke outcome are discussed.

Faulty old ideas about translational regulation paved the way for current confusion about how microRNAs function

Despite a recent surge of reports about how microRNAs might regulate translation, the question has not been answered. The proposed mechanisms contradict one another, and none is supported by strong evidence. This review explains some deficiencies in the experiments with microRNAs. Some of the problems are traceable to bad habits carried over from older studies of translational regulation, here illustrated by discussing two models involving mRNA binding proteins. One widely-accepted model, called into doubt by recent findings, is the maskin hypothesis for translational repression of cyclin B1 in Xenopus oocytes. The second dubious model postulates repression of translation of ceruloplasmin by mRNA binding proteins. A big fault in the latter case is reconstructing the imagined mechanism before looking carefully at the real thing--a criticism that applies also to studies with microRNAs. Experiments with microRNAs often employ internal ribosome entry sequences (IRESs) as tools, necessitating brief discussion of that topic. A sensitive new assay reveals that many putative IRESs promote expression of downstream cistrons via splicing rather than internal initiation of translation. Recent claims about the biological importance of IRES-binding proteins--including suggestions that these proteins might serve as targets for cancer therapy--are not supported by any meaningful evidence. The bottom line is that older studies of mRNA binding proteins and putative IRESs have created a confusing picture of translational regulation which is not helpful when trying to understand how microRNAs might work. The obvious biological importance of microRNAs makes it essential to understand how they do what they do. Fresh ways of thinking and looking are needed.

Endothelial nitric oxide synthase gene expression during murine embryogenesis: commencement of expression in the embryo occurs with the establishment of a unidirectional circulatory system

To elucidate the role of endothelial NO synthase (eNOS)-derived NO during mammalian embryogenesis, we assessed the expression of the eNOS gene during development. Using transgenic eNOS promoter/reporter mice (with beta-galactosidase and green fluorescent protein reporters), in situ cRNA hybridization, and immunohistochemistry to assess transcription, steady-state mRNA levels, and protein expression, respectively, we noted that eNOS expression in the developing cardiovascular system was highly restricted to endothelial cells of medium- and large-sized arteries and the endocardium. The onset of transcription of the native eNOS gene and reporters coincided with the establishment of robust, unidirectional blood flow at embryonic day 9.5, as assessed by Doppler ultrasound biomicroscopy. Interestingly, reporter transgene expression and native eNOS mRNA were also observed in discrete regions of the developing skeletal musculature and the apical ectodermal ridge of developing limbs, suggesting a role for eNOS-derived NO in limb development. In vitro studies of promoter/reporter constructs indicated that similar eNOS promoter regions operate in both embryonic skeletal muscle and vascular endothelial cells. In summary, transcriptional activity of the eNOS gene in the murine circulatory system occurred following the establishment of embryonic blood flow. Thus, the eNOS gene is a late-onset gene in endothelial ontogeny.

Neuronal nitric oxide synthase contributes to the regulation of hematopoiesis

Nitric oxide (NO) signaling is important for the regulation of hematopoiesis. However, the role of individual NO synthase (NOS) isoforms is unclear. Our results indicate that the neuronal NOS isoform (nNOS) regulates hematopoiesis in vitro and in vivo. nNOS is expressed in adult bone marrow and fetal liver and is enriched in stromal cells. There is a strong correlation between expression of nNOS in a panel of stromal cell lines established from bone marrow and fetal liver and the ability of these cell lines to support hematopoietic stem cells; furthermore, NO donor can further increase this ability. The number of colonies generated in vitro from the bone marrow and spleen of nNOS-null mutants is increased relative to wild-type or inducible- or endothelial NOS knockout mice. These results describe a new role for nNOS beyond its action in the brain and muscle and suggest a model where nNOS, expressed in stromal cells, produces NO which acts as a paracrine regulator of hematopoietic stem cells.

Smooth muscle NOS, colocalized with caveolin-1, modulates contraction in mouse small intestine

Neuronal nitric oxide synthase (nNOS) in myenteric neurons is activated during peristalsis to produce nitric oxide which relaxes intestinal smooth muscle. A putative nNOS is also found in the membrane of intestinal smooth muscle cells in mouse and dog. In this study we studied the possible functions of this nNOS expressed in mouse small intestinal smooth muscle colocalized with caveolin-1(Cav-1). Cav-1 knockout mice lacked nNOS in smooth muscle and provided control tissues. 60 mM KCl was used to increase intracellular [Ca²⁺] through L-type Ca²⁺ channel opening and stimulate smooth muscle NOS activity in intestinal tissue segments. An additional contractile response to LNNA (100 μM, NOS inhibitor) was observed in KCl-contracted tissues from control mice and was almost absent in tissues from Cav-1 knockout mice. Disruption of caveolae with 40 mM methyl-β cyclodextrin in tissues from control mice led to the loss of Cav-1 and nNOS immunoreactivity from smooth muscle as shown by immunohistochemistry and a reduction in the response of these tissues to N-ω-nitro-L-arginine (LNNA). Reconstitution of membrane cholesterol using water soluble cholesterol in the depleted segments restored the immunoreactivity and the response to LNNA added after KCl. Nicardipine (1 μM) blocked the responses to KCl and LNNA confirming the role of L-type Ca²⁺ channels. ODQ (1 μM, soluble guanylate cyclase inhibitor) had the same effect as inhibition of NOS following KCl. We conclude that the activation of nNOS, localized in smooth muscle caveolae, by calcium entering through L-type calcium channels triggers nitric oxide production which modulates muscle contraction by a cGMP-dependent mechanism.

Regulation of smooth muscle by inducible nitric oxide synthase and NADPH oxidase in vascular proliferative diseases

Inflammation plays a critical role in promoting smooth muscle migration and proliferation during vascular diseases such as postangioplasty restenosis and atherosclerosis. Another common feature of many vascular diseases is the contribution of reactive oxygen (ROS) and reactive nitrogen (RNS) species to vascular injury. Primary sources of ROS and RNS in smooth muscle are several isoforms of NADPH oxidase (Nox) and the cytokine-regulated inducible nitric oxide (NO) synthase (iNOS). One important example of the interaction between NO and ROS is the reaction of NO with superoxide to yield peroxynitrite, which may contribute to the pathogenesis of hypertension. In this review, we discuss the literature that supports an alternate possibility: Nox-derived ROS modulate NO bioavailability by altering the expression of iNOS. We highlight data showing coexpression of iNOS and Nox in vascular smooth muscle demonstrating the functional consequences of iNOS and Nox during vascular injury. We describe the relevant literature demonstrating that the mitogen-activated protein kinases are important modulators of proinflammatory cytokine-dependent expression of iNOS. A central hypothesis discussed is that ROS-dependent regulation of the serine/threonine kinase protein kinase Cdelta is essential to understanding how Nox may regulate signaling pathways leading to iNOS expression. Overall, the integration of nonphagocytic NADPH oxidase with cytokine signaling in general and in vascular smooth muscle in particular is poorly understood and merits further investigation.

Endothelin(A) (ET(A)) and ET(B) receptor-mediated regulation of nitric oxide synthase 1 (NOS1) and NOS3 isoforms in the renal inner medulla

AIM

Our laboratory and others have shown that endothelin (ET)-1 directly stimulates nitric oxide (NO) production in inner medullary collecting duct (IMCD) cells. The goal of this study was to determine which NO synthase (NOS) isoforms in IMCD are sensitive to ET-1, and the role of ET(A) and ET(B) receptor activation in vivo and in vitro.

METHODS

NOS enzymatic activity and NOS isoform protein expression were examined in cultured IMCD-3 cells and isolated renal inner medulla. ET(B) receptor-deficient homozygous rats (sl/sl) have elevated levels of circulating ET-1 and lack a functional ET(B) signalling pathway in kidneys, and furthermore provides a unique model to study ET(A) receptor signalling in the renal inner medulla in vivo.

RESULTS

Incubation of IMCD-3 cells with exogenous ET-1 (50 nm) resulted in ET(A)-dependent increased NOS1 protein expression in IMCD-3 cells with no effect on NOS2 or NOS3 expression. ET(B) receptor antagonism has no effect on NOS expression in IMCD-3 cells. Consistent with in vitro results, cytosolic NOS1 protein expression was significantly greater in the renal inner medulla of sl/sl rats compared with heterozygous (sl/+) controls, with no alteration in NOS3 expression. In contrast to protein expression data, NOS1- and NOS3-specific enzymatic activities decreased in the cytosolic fraction from the renal inner medulla of sl/sl compared with sl/+.

CONCLUSION

These results provide evidence that both ET(A) and ET(B) receptors regulate NOS isoform activity in the renal inner medulla and specifically support the hypothesis that ET(A) receptor activation increases NOS1 expression.

Oxygen regulation of arterial smooth muscle cell proliferation and survival

The purpose of this study was to determine if hypoxia elicits different proliferative and apoptotic responses in systemic arterial smooth muscle cells incubated under conditions that do or do not result in cellular ATP depletion and whether these effects are relevant to vascular remodeling in vivo. Gene expression profiling was used to identify potential regulatory pathways. In human aortic smooth muscle cells (HASMCs) incubated at 3% O(2), proliferation and progression through the G1/S interphase are enhanced. Incubation at 1% O(2) reduced proliferation, delayed G1/S transition, increased apoptotic cell death, and is associated with mitochondrial membrane depolarization and reduced cellular ATP levels. In aorta and mesenteric artery from rats exposed to hypoxia (10% O(2), 48 h), both proliferation and apoptosis are increased, as are medial nuclear density and smooth muscle cell content. Although nuclear levels of hypoxia-inducible factor 1-alpha (HIF-1alpha) are increased to a similar extent in HASMCs incubated at 1 and 3% O(2), expression of tumor protein p53, its transcriptional target p21, as well as their regulatory factors and downstream effectors, are differentially affected under these two conditions, suggesting that the bidirectional effects of hypoxia are mediated by this pathway. We conclude that hypoxia induces a state of enhanced cell turnover through increased rates of both smooth muscle cell proliferation and death. This confers the ability to remodel the vasculature in response to changing tissue metabolic needs while avoiding the accumulation of mutations that may lead to malignant transformation or the formation of abnormal vascular structures.

Neuronal nitric oxide synthase deficiency decreases survival in bacterial peritonitis and sepsis

OBJECTIVE

To investigate the role of neuronal nitric oxide synthase (NOS1) in murine polymicrobial peritonitis and sepsis.

DESIGN

Randomized experimental trial.

SETTING

Animal research facility.

SUBJECTS

B6129S NOS1+/+ and B6;129S4 NOS-/- mice.

INTERVENTIONS

NOS1+/+ and NOS1-/- animals underwent cecal ligation and puncture (CLP) or sham surgery and received the NOS1 inhibitor 7-nitroindazole (7-NI) or vehicle.

MEASUREMENTS AND MAIN RESULTS

After CLP, genetic deficiency and pharmacologic inhibition of NOS1 significantly increased risk of mortality [8.69 (3.27, 23.1), p<0.0001 and 1.71 (1.00, 2.92) p=0.05, hazard ratio of death (95% confidence interval) for NOS1-/- and 7-NI-treated NOS1+/+ respectively] compared with NOS1+/+ animals. In 7-NI-treated NOS1+/+ animals, there were increases (6 h) and then decreases (24 h), whereas in NOS-/- animals persistent increases in blood bacteria counts (p=0.04 for differing effects of 7-NI and NOS1-/-) were seen compared with NOS1(+/+) animals. After CLP, NOS1(-/-) had upregulation of inducible NOS and proinflammatory cytokines and greater increases in serum tumor necrosis factor-alpha and interleukin-6 levels compared with NOS1+/+ mice (all p<0.05). Following CLP, there were similar significant decreases in circulating leukocytes and lung lavage cells (p

CONCLUSIONS

Deficiency and inhibition of NOS1 increases mortality, possibly by increasing proinflammatory cytokine response and impairing bacterial clearance after CLP. These data suggest that NOS1 is important for survival, bacterial clearance, and regulation of cytokine response during infection and sepsis.

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Key Words

• sepsis
• peritonitis
• nos1
• nnos
• no
• rodents

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MESH Headings

• Animals
• Female
• Gene Expression
• Mice
• Nitric Oxide Synthase Type I/adverse effects
• Nitric Oxide Synthase Type I/analysis
• Nitric Oxide Synthase Type I/deficiency
• Nitric Oxide Synthase Type I/genetics
• Nitric Oxide Synthase Type I/metabolism
• Nitric Oxide Synthase Type I/pharmacology
• Peritonitis/mortality
• Peritonitis/physiopathology
• Random Allocation
• Risk Assessment
• Sepsis/mortality
• Sepsis/physiopathology
• Survival Analysis
• United States

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Grants

• Z01 CL008047 Intramural NIH HHS
• Z01 CL008047-06 Intramural NIH HHS
• Z01 CL009009 Intramural NIH HHS
• Z01 CL009009-01 Intramural NIH HHS

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Affiliation(s)

Xizhong Cui Critical Care Medicine Department, National Institutes of Health Clinical Center, National Institutes of Health, Bethesda, MD, USA
Virginia Besch
Alfia Khaibullina
Adrienne Hergen
Martha Quezado
Peter Eichacker
Zenaide M N Quezado

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Low-molecular-weight S-nitrosothiols and blood vessel injury

S-nitrosothiol signaling reactions are argued to play key modulatory roles in mediating the actions of NOS in health and disease. A report by Palmer et al. in this issue of the JCI provides new insight into the in vivo biology of S-nitrosothiols (see the related article beginning on page 2592). The authors examine the chronic effects of exogenous nitrosothiol therapy and demonstrate that the commonly used antioxidant N-acetylcysteine (NAC) induces pulmonary arterial hypertension in mice. Importantly, the authors argue that the vascular pathology they observe in the lungs of these animals is functionally and morphologically equivalent to that observed in chronic hypoxia. These findings raise the concern that chronic NAC therapy may induce similar vascular pathology in patients.

Time-course of changes to nitric oxide signaling pathways in form-deprivation myopia in guinea pigs

The aim of this study was to investigate the time-course change of nitric oxide synthase (NOS) activity and cyclic GMP (cGMP) concentration in the posterior retina, choroid and sclera after differing periods of form-deprivation in guinea pigs. Three groups of guinea pigs were subjected to monocular FD for 7, 14 or 21 days. NOS activity and cGMP concentrations in ocular tissues of FD eyes and control eyes were analyzed by radioimmunoassay. The presence of NOS isoforms was detected by immunohistochemistry. Guinea pigs presented with considerable myopia after 14 days of FD. Retinal NOS activity in the FD group was lower than in the control group after 7 days of FD and was higher than in the control group after 14 and 21 days of FD. The choroidal and scleral NOS activities in the FD groups were higher than in the control groups after 21 days. The cGMP concentrations in the FD groups were higher than in the control groups at 21 days of the retinal, choroidal, and scleral tissues. Furthermore, the retinal cGMP concentration in the FD group was also significantly elevated at 14 days relative to the control group. We detected expression of three NOS isoforms in guinea pig ocular tissues. Our main observations were a change in NOS activity and an up-regulation in cGMP concentrations in posterior ocular tissues during the development of myopia. The function of elevated NOS activity may be mediated by cGMP.

Hemodynamics and muscle sympathetic nerve activity after 8 h of sustained hypoxia in healthy humans

Hemodynamics, muscle sympathetic nerve activity (MSNA), and forearm blood flow were evaluated in 12 normal subjects before, during (1 and 7 h), and after ventilatory acclimatization to hypoxia achieved with 8 h of continuous poikilocapnic hypoxia. All results are means +/- SD. Subjects experienced mean oxygen saturation of 84.3 +/- 2.3% during exposure. The exposure resulted in hypoxic acclimatization as suggested by end-tidal CO(2) [44.7 +/- 2.7 (pre) vs. 39.5 +/- 2.2 mmHg (post), P < 0.001] and by ventilatory response to hypoxia [1.2 +/- 0.8 (pre) vs. 2.3 +/- 1.3 l x min(-1).1% fall in saturation(-1) (post), P < 0.05]. Subjects exhibited a significant increase in heart rate across the exposure that remained elevated even upon return to room air breathing compared with preexposure (67.3 +/- 15.9 vs. 59.8 +/- 12.1 beats/min, P < 0.008). Although arterial pressure exhibited a trend toward an increase across the exposure, this did not reach significance. MSNA initially increased from room air to poikilocapnic hypoxia (26.2 +/- 10.3 to 32.0 +/- 10.3 bursts/100 beats, not significant at 1 h of exposure); however, MSNA then decreased below the normoxic baseline despite continued poikilocapnic hypoxia (20.9 +/- 8.0 bursts/100 beats, 7 h Hx vs. 1 h Hx; P < 0.008 at 7 h). MSNA decreased further after subjects returned to room air (16.6 +/- 6.0 bursts/100 beats; P < 0.008 compared with baseline). Forearm conductance increased after exposure from 2.9 +/- 1.5 to 4.3 +/- 1.6 conductance units (P < 0.01). These findings indicate alterations of cardiovascular and respiratory control following 8 h of sustained hypoxia producing not only acclimatization but sympathoinhibition.

The untranslated region of exon 2 of the human neuronal nitric oxide synthase (NOS1) gene exerts regulatory activity

Expressional dysregulation of the human neuronal nitric oxide synthase (NOS1) gene represents an important mechanism in the pathogenesis of certain neuronal disease states. The structure and regulation of the human NOS1 gene is highly complex based on cell type- and stimulus-dependent usage of multiple exon 1 variants. Here we demonstrate that the untranslated region of exon 2 exerts promoter and enhancer functions as well, facilitated in large part by cooperative interaction of two conserved adjacent CREB/AP-1 binding sites. In human neuronal A673 cells, NOS1 expression is stimulated by several compounds which act through these sites, but also stimulate the combined promoter region of exons 1f and 1g. While stimulation of NOS1 expression by dibutyryl-cAMP is mediated by protein kinase A (blocked by H-89), the antiepileptic drug valproic acid is likely to activate phosphatidylinositol-3 kinase (inhibited by LY 294002).

Nitric Oxide in the Pulmonary Vasculature

Homeostasis in the pulmonary vasculature is maintained by the actions of vasoactive compounds, including nitric oxide (NO). NO is critical for normal development of the pulmonary vasculature and continues to mediate normal vasoregulation in adulthood. Loss of NO bioavailability is one component of the endothelial dysfunction and vascular pathology found in pulmonary hypertension (PH). A broad research effort continues to expand our understanding of the control of NO production and NO signaling and has generated novel theories on the importance of pulmonary NO production in the control of the systemic vasculature. This understanding has led to exciting developments in our ability to treat PH, including inhaled NO and phosphodiesterase inhibitors, and to several promising directions for future therapies using nitric oxide-donor compounds, stimulators of soluble guanylate cyclase, progenitor cells expressing NO synthase (NOS), and NOS gene manipulation.

Multi-parameter in vivo cardiac magnetic resonance imaging demonstrates normal perfusion reserve despite severely attenuated beta-adrenergic functional response in neuronal nitric oxide synthase knockout mice

AIMS

The role of neuronal nitric oxide synthase (nNOS) in regulating contractile function remains controversial, and in regulating myocardial perfusion is uninvestigated. We used magnetic resonance imaging (MRI) to phenotype nNOS(-/-) and wild-type (WT) mice regarding left ventricular (LV) structure, baseline function, beta-adrenergic responsiveness, and perfusion reserve.

METHODS AND RESULTS

Cine MRI showed higher LV mass to end-diastolic volume ratio (2.3 +/- 0.2 mg/microL nNOS(-/-) vs. 1.7 +/- 0.1 mg/microL WT; P=0.032) and LV ejection fraction (64.9 +/- 2.1% nNOS(-/-) vs. 55.8 +/- 1.1% WT; P = 0.003) in nNOS(-/-). Myocardial tagging demonstrated similar baseline systolic circumferential strain (Ecc) in nNOS(-/-) and WT. With dobutamine, the normal change in Ecc was nearly absent in nNOS(-/-) (-0.5 +/- 0.3% nNOS(-/-) vs. -2.2 +/- 0.3% WT; P = 0.001), and the systolic strain rate (dEcc/dt) response to dobutamine seen in WT was reduced in nNOS(-/-) (-29 +/- 13%/s nNOS(-/-) vs. -106+/-16%/s WT; P = 0.001). Diastolic strain rate increased significantly with dobutamine only in WT. Arterial spin labelling showed that baseline perfusion and perfusion reserve with either dobutamine or an adenosine receptor agonist are normal in nNOS(-/-).

CONCLUSION

MRI provides non-invasive in vivo evidence that nNOS does not play a role in basal contractile function or myocardial perfusion, but is required for increasing cardiac inotropy and lusitropy upon beta-adrenergic stimulation.

Mechanisms of disease: L-arginine in coronary atherosclerosis--a clinical perspective

L-arginine is the substrate of endothelial nitric oxide synthase and the main precursor of nitric oxide in the vascular endothelium, thus its effects are mediated largely by increases in nitric oxide production. L-arginine has antioxidant and antiapoptotic properties, increases smooth muscle relaxation, inhibits the expression of adhesion molecules and chemotactic peptides, decreases endothelin-1 expression, and inhibits platelet aggregation. This amino acid also improves endothelial function in patients with coronary artery disease and dilates human epicardial atheromatous coronary arteries. Despite the positive results from small case-control studies, it is still unclear whether chronic administration of L-arginine has any effect on clinical outcome in patients with coronary artery disease. In addition, other indirect strategies, such as the inhibition of arginase, could prove more effective at improving intracellular L-arginine bioavailability than exogenous L-arginine administration. The potential clinical usefulness of L-arginine, therefore, needs further evaluation in large, prospective clinical trials. Here, we present a critique of the existing literature about the role of L-arginine in the prevention of atherosclerosis.

Hypoxia-inducible expression of a natural cis-antisense transcript inhibits endothelial nitric-oxide synthase

The destabilization of endothelial nitric-oxide synthase (eNOS) mRNA in hypoxic endothelial cells may be important in the etiology of vascular diseases, such as pulmonary hypertension. Recently, an overlapping antisense transcript to eNOS/NOS3 was implicated in the post-transcriptional regulation of eNOS. We demonstrate here that expression of sONE, also known as eNOS antisense (NOS3AS) or autophagy 9-like 2 (APG9L2), is robustly induced by hypoxia or functional deficiency of von Hippel-Lindau protein. sONE is also up-regulated in the aortas of hypoxic rats. In hypoxic endothelial cells, sONE expression negatively correlates with eNOS expression. Blocking the hypoxic induction of sONE by RNA interference attenuates the fall in both eNOS RNA and protein. We provide evidence that the induction of sONE primarily involves transcript stabilization rather than increased transcriptional activity and is von Hippel-Lindaubut not hypoxia-inducible factor 2alpha-dependent. We also demonstrate that sONE transcripts are enriched in the nucleus of normoxic cells and that hypoxia promotes an increase in the level of cytoplasmic and polyribosome-associated, sONE mRNA. The finding that eNOS expression can be regulated by an overlapping cis-antisense transcript in a stimulus-dependent fashion provides evidence that sense/antisense interactions may play a previously unappreciated role in vascular disease pathogenesis.

Chronic intermittent hypoxia modulates nNOS mRNA and protein expression in the rat hypothalamus

Exposure to chronic intermittent hypoxia (CIH) as observed in obstructive sleep apnea (OSA) elicits a sustained elevation of sympathetic activity and arterial blood pressure. Our overall hypothesis is that intermittent hypoxia might increase sympathetic activity, in part by altering neuronal nitric oxide synthase (nNOS) expression in the hypothalamus, where nitric oxide is sympathoinhibitory. In this study, we begin investigation of this hypothesis by testing the more specific hypothesis that the CIH alters nNOS expression in regions of the hypothalamus associated with cardiovascular regulation. To test the effect of CIH on NOS expression we subjected male Sprague-Dawley rats to cyclic intermittent hypoxia for 8h/day, for 35 days. Experimental rats showed an increase in systemic blood pressure. In situ hybridization and immunohistochemistry were performed on hypothalamic sections, respectively. The CIH rats displayed significantly lower levels of both nNOS mRNA and protein in the paraventricular hypothalamic nucleus (PVN) with different changes in the subareas of the PVN. There was a decreased level of nNOS mRNA and protein in the subfornical organ and the periventricular hypothalamic nucleus of the CIH rats, but no significant change in the supraoptic nucleus or the lateral hypothalamic area. This work suggests that examination of central regulation of sympathetic activity may help elucidate the mechanisms of hypertension after CIH.

Antenatal betamethasone administration has a dual effect on adult sheep vascular reactivity

The effect of antenatal steroids on blood pressure in humans remains an unresolved question. Here we report the effects of prenatal exposure to clinically relevant doses of betamethasone on endothelial and/or vascular smooth muscle function. Pregnant sheep were randomly treated with betamethasone (0.17 mg/kg) or vehicle at 80 and 81 d of gestation. We studied arterial segments (4th-5th generation) of the right brachial artery obtained at 1-2 y of age under general anesthesia. We demonstrate that in brachial arteries of steroid exposed offspring: KCl induced contraction is increased after endothelium removal or incubation with inhibitors of nitric oxide synthase or cyclooxygenase; acetylcholine-induced relaxation is increased; sensitivity to endothelin-1 (ET-1) is increased and this effect is decreased by the ETB antagonist BQ-788. These data suggest that, in sheep treated with clinically relevant doses of betamethasone at a gestational stage when human fetuses are routinely exposed to glucocorticoids, there is a dual effect of betamethasone on the adult sheep brachial artery, i.e. endothelial dysfunction with an impairment of endothelin-1 ETB receptor-induced release of nitric oxide and an increased contribution of the ETB receptor in smooth muscle to the contractile effects of ET-1.

Induction of matrix metalloproteinase-2 enhances systemic arterial contraction after hypoxia

This study was carried out to determine the role of increased vascular matrix metalloproteinase-2 (MMP-2) expression in the changes in systemic arterial contraction after prolonged hypoxia. Rats and mice were exposed to hypoxia (10% and 8% O(2), respectively) or normoxia (21% O(2)) for 16 h, 48 h, or 7 days. Aortae and mesenteric arteries were either mounted in organ bath myographs or frozen in liquid nitrogen. MMP-2 inhibition with cyclic CTTHWGFTLC (CTT) reduced contraction to phenylephrine (PE) in aortae and mesenteric arteries from rats exposed to hypoxia for 7 days but not in vessels from normoxic rats. Similarly, CTT reduced contraction to Big endothelin-1 (Big ET-1) in aortae from rats exposed to hypoxia for 7 days. Responses to PE were reduced in hypoxic MMP-2(-/-) mice compared with MMP-2(+/+) mice. Increased contraction to Big ET-1 after hypoxia was observed in MMP-2(+/+) mice but not in MMP-2(-/-) mice. Rat aortic MMP-2 and membrane type 1 (MT1)-MMP protein levels and MMP activity were increased after 7 days of hypoxia. Rat aortic MMP-2 and MT1-MMP mRNA levels were increased in the deep medial vascular smooth muscle. We conclude that hypoxic induction of MMP-2 expression potentiates contraction in systemic conduit and resistance arteries. This may preserve the capacity to regulate the systemic circulation in the transition between the alterations in vascular tone and structural remodeling that occurs during prolonged hypoxic epochs.

Increased expression of HIF-1alpha, nNOS, and VEGF in the cerebral cortex of anemic rats

This study tested the hypothesis that specific hypoxic molecules, including hypoxia-inducible factor-1alpha (HIF-1alpha), neuronal nitric oxide synthase (nNOS), and vascular endothelial growth factor (VEGF), are upregulated within the cerebral cortex of acutely anemic rats. Isoflurane-anesthetized rats underwent acute hemodilution by exchanging 50% of their blood volume with pentastarch. Following hemodilution, mean arterial pressure and arterial Pa(O(2)) values did not differ between control and anemic rats while the hemoglobin concentration decreased to 57 +/- 2 g/l. In anemic rats, cerebral cortical HIF-1alpha protein levels were increased, relative to controls (1.7 +/- 0.5-fold, P < 0.05). This increase was associated with an increase in mRNA levels for VEGF, erythropoietin, CXCR4, iNOS, and nNOS (P < 0.05 for all), but not endothelial NOS. Cerebral cortical nNOS and VEGF protein levels were increased in anemic rats, relative to controls (2.0 +/- 0.2- and 1.5 +/- 0.4-fold, respectively, P < 0.05 for both). Immunohistochemistry demonstrated increased HIF-1alpha and VEGF staining in perivascular regions of the anemic cerebral cortex and an increase in the number of nNOS-positive cerebral cortical cells (3.2 +/- 1.0-fold, P < 0.001). The nNOS-positive cells costained with the neuronal marker, Neu-N, but not with the astrocytic marker glial fibrillary acidic protein (GFAP). These nNOS-positive neurons frequently sent axonal projections toward cerebral blood vessels. Conversely, VEGF immunostaining colocalized with both neuronal (NeuN) and astrocytic markers (GFAP). In conclusion, acute normotensive, normoxemic hemodilution increased the levels of HIF-1alpha protein and mRNA for HIF-1-responsive molecules. nNOS and VEGF protein levels were also increased within the cerebral cortex of anemic rats at clinically relevant hemoglobin concentrations.

Nitric oxide regulation of mitochondrial oxygen consumption I: cellular physiology

Mitochondrial biochemistry is complex, expanding from oxygen consumption, oxidative phosphorylation, lipid catabolism, heme biosynthesis, to apoptosis, calcium homeostasis, and production of reactive oxygen species, including nitric oxide (NO). The latter molecule is produced by a mitochondrial NO synthase (mtNOS). The rates of consumption and production determine the steady-state concentration of NO at subcellular levels, leading to regulation of mitochondrial events. Temporospatial processes tightly regulate production of NO in mitochondria to maximize target effects and minimize deleterious reactions. Temporal regulatory mechanisms of mtNOS include activation by calcium signaling and transcriptional/translational regulations. Calcium-activated mtNOS inhibits mitochondrial respiration, resulting in a decrease of the oxygen consumption. This negative regulation antagonizes the effects of calcium on calcium-dependent dehydrogenases in the citric acid cycle, preventing the formation of anoxic foci. Temporal regulation of NO production by intracellular calcium signaling is a complex process, considering the heterogeneous intracellular calcium response and distribution. NO production in mitochondria is spatially regulated by mechanisms that determine subcellular localization of mtNOS, likely acylation and protein-protein interactions, in addition to transcriptional regulation as neuronal NOS. Because NO rapidly decays in mitochondria, subcellular localization of mtNOS is crucial for NO to function as a signal molecule. These temporospatial processes are biologically important to allow NO to act as an effective signal molecule to regulate mitochondrial events such as oxygen consumption and reactive oxygen species production.

New insights into nNOS regulation of vascular homeostasis

An important physiological response to changes in local or systemic oxygenation is the modulation of vascular tone, which is mediated in part by changes in the activities of the 3 NO synthase (NOS) isoforms. In arterial smooth muscle cells, acute hypoxia induces increased vascular tone, which is attenuated if hypoxia persists. In this issue of the JCI, Ward et al. demonstrate that changes in O2 concentration have effects on neuronal NOS enzymatic activity and gene expression that contribute to vascular homeostasis under conditions of acute and chronic hypoxia (see the related article beginning on page 3128).