Vascular protection by tetrahydrobiopterin: progress and therapeutic prospects

The Alzheimer's Disease Brain, Its Microvasculature, and NADPH Oxidase

The deterioration of the brain's microvasculature, particularly in the hippocampus, appears to be a very early event in the development of Alzheimer's disease (AD), preceding even the deposition of amyloid-β. A damaged microvasculature reduces the supply of oxygen and glucose to this region and limits the production of energy, ATP. The damage may be a function of the rise with age in the expression and activity of NADPH oxidase (NOX) in these microvessels. This rise renders these vessels vulnerable to the effects of oxidative stress and inflammation. The rise in NOX activity with age is even more marked in the AD brain where an inverse correlation has been demonstrated between NOX activity and cognitive ability. Apocynin, a putative NOX inhibitor, has been shown to block the damaging effects of NOX activation. Apocynin acts as a strong scavenger of H2O2, and as a weak scavenger of superoxide. Like apocynin, sodium oxybate (SO) has also been shown to block the toxic effects of NOX activation. The application of SO generates NADPH and ATP. SO inhibits oxidative stress and maintains normal cerebral ATP levels under hypoxic conditions. Moreover, it acts epigenetically to attenuate the expression of NOX. SO may delay the onset and slow the progress of AD by suppling energy and maintaining an antioxidative environment in the brain throughout the night. The slow wave activity produced by SO may also activate the glymphatic system and promote the clearance of amyloid-β from the brain.

L-Arginine-eNOS-NO Functional System in Brain Damage and Cognitive Impairments in Cerebral Small Vessel Disease

Cerebral small vessel disease (CSVD) is a significant cause of cognitive impairment (CI), disability, and mortality. The insufficient effectiveness of antihypertensive therapy in curbing the disease justifies the search for potential targets for modifying therapy and indicators supporting its use. Using a laser-assisted optical rotational cell analyzer (LORRCA, Mechatronics, The Netherlands), the rheological properties and deformability of erythrocytes before and after incubation with 10 μmol/L of L-arginine, the nitric oxide (NO) donor, blood-brain barrier (BBB) permeability assessed by dynamic contrast-enhanced MRI, clinical, and MRI signs were studied in 73 patients with CSVD (48 women, mean age 60.1 ± 6.5 years). The control group consisted of 19 volunteers (14 women (73.7%), mean age 56.9 ± 6.4 years). The erythrocyte disaggregation rate (y-dis) after incubation with L-arginine showed better performance than other rheological characteristics in differentiating patients with reduced NO bioavailability/NO deficiency by its threshold values. Patients with y-dis > 113 s-1 had more severe CI, arterial hypertension, white matter lesions, and increased BBB permeability in grey matter and normal-appearing white matter (NAWM). A test to assess changes in the erythrocyte disaggregation rate after incubation with L-arginine can be used to identify patients with impaired NO bioavailability. L-arginine may be part of a therapeutic strategy for CSVD with CI.

[Nitric oxide availability in cerebral microangiopathy]

OBJECTIVE

To develop a test of individual nitric oxide (NO) availability based on changes in erythrocyte rheological properties after incubation with a NO donor and to evaluate the role of these disorders in brain damage and development of cognitive impairment (CI) in cerebral small vessel disease (cSVD).

MATERIAL AND METHODS

In 73 cSVD patients (48 (65.8%) women, mean age 60.1±6.5), the rheological properties of erythrocytes before and after incubation with 10 μmol/L L-arginine-NO donor were evaluated using a laser-optical rotating cell analyzer, and the blood-brain barrier (BBB) permeability by MRI-T1 dynamic contrast.

RESULTS

Among the studied parameters of erythrocyte rheological properties, the best characteristic by ROC analysis was the rate of erythrocyte disaggregation (y-dis) after incubation with L-arginine (area under the curve 0.733 (0.609-0.856), sensitivity 67%, specificity 79%). Patients with a y-dis threshold >113 sec-1 had more severe CI, arterial hypertension, white matter lesions, and increased BBB permeability in gray matter and normal-appearing white matter.

CONCLUSION

The prolonged rate of erythrocyte disaggregation in cSVD patients after incubation with L-arginine indicates the risk for disease progression due to decreased NO bioavailability/disruption of the functional L-arginine-eNOS-NO system. This test can be used to assess individual NO bioavailability and potentially identify indications for modifying therapy with NO donors such as L-arginine. Clinical trials are needed to standardize and evaluate the efficacy of NO donor therapy in patients with cSVD and CI.

Design, Synthesis, and Investigation of Novel Nitric Oxide (NO)-Releasing Aromatic Aldehyde as Drug Candidates for the Treatment of Sickle Cell Disease

Sickle cell disease (SCD) is caused by a single-point mutation, and the ensuing deoxygenation-induced polymerization of sickle hemoglobin (HbS), and reduction in bioavailability of vascular nitric oxide (NO), contribute to the pathogenesis of the disease. In a proof-of-concept study, we successfully incorporated nitrate ester groups onto two previously studied potent antisickling aromatic aldehydes, TD7 and VZHE039, to form TD7-NO and VZHE039-NO hybrids, respectively. These compounds are stable in buffer but demonstrated the expected release of NO in whole blood in vitro and in mice. The more promising VZHE039-NO retained the functional and antisickling activities of the parent VZHE039 molecule. Moreover, VZHE039-NO, unlike VZHE039, significantly attenuated RBC adhesion to laminin, suggesting this compound has potential in vivo RBC anti-adhesion properties relevant to vaso-occlusive events. Crystallographic studies show that, as with VZHE039, VZHE039-NO also binds to liganded Hb to make similar protein interactions. The knowledge gained during these investigations provides a unique opportunity to generate a superior candidate drug in SCD with enhanced benefits.

Inactivation of BACE1 increases expression of endothelial nitric oxide synthase in cerebrovascular endothelium

Cerebrovascular effects of β-site amyloid precursor protein-cleaving enzyme 1 (BACE1) inactivation have not been systematically studied. In the present study we employed cultured human brain microvascular endothelial cells (BMECs), BACE1-knockout (BACE1^-/-) mice and conditional (tamoxifen-induced) endothelium-specific BACE1-knockout (eBACE1^-/-) mice to determine effect of BACE1 inhibition on expression and function of endothelial nitric oxide synthase (eNOS). Deletion of BACE1 caused upregulation of eNOS and glypican-1 (GPC1) in human BMECs treated with BACE1-siRNA, and cerebral microvessels of male BACE1^-/- mice and male eBACE1^-/- mice. In addition, BACE1siRNA treatment increased NO production in human BMECs. These effects appeared to be independent of amyloid β-peptide production. Furthermore, adenoviral-mediated overexpression of BACE1 in human BMECs down-regulated GPC1 and eNOS. Treatment of human BMECs with GPC1siRNA suppressed mRNA and protein levels of eNOS. In basilar arteries of male eBACE1^-/- mice, endothelium-dependent relaxations to acetylcholine and endothelium-independent relaxations to NO donor, DEA-NONOate, were not affected, consistent with unchanged expression of eNOS and phosphorylation of eNOS at Ser1177 in large cerebral arteries. In aggregate, our findings suggest that under physiological conditions, inactivation of endothelial BACE1 increases expression of eNOS in cerebral microvessels but not in large brain arteries. This effect appears to be mediated by increased GPC1 expression.

Cumulative effect of simvastatin, L-arginine, and tetrahydrobiopterin on cerebral blood flow and cognitive function in Alzheimer's disease

BACKGROUND AND OBJECTIVES

Vascular disease is a known risk factor for Alzheimer's disease (AD). Endothelial dysfunction has been linked to reduced cerebral blood flow. Endothelial nitric oxide synthase pathway (eNOS) upregulation is known to support endothelial health. This single-center, proof-of-concept study tested whether the use of three medications known to augment the eNOS pathway activity improves cognition and cerebral blood flow (CBF).

METHODS

Subjects with mild AD or mild cognitive impairment (MCI) were sequentially treated with the HMG-CoA reductase synthesis inhibitor simvastatin (weeks 0-16), L-arginine (weeks 4-16), and tetrahydrobiopterin (weeks 8-16). The primary outcome of interest was the change in CBF as measured by MRI from baseline to week 16. Secondary outcomes included standard assessments of cognition.

RESULTS

A total of 11 subjects were deemed eligible and enrolled. One subject withdrew from the study after enrollment, leaving 10 subjects for data analysis. There was a significant increase in CBF from baseline to week 8 by ~13% in the limbic and ~15% in the cerebral cortex. Secondary outcomes indicated a modest but significant increase in the MMSE from baseline (24.2±3.2) to week 16 (26.0±2.7). Exploratory analysis indicated that subjects with cognitive improvement (reduction of the ADAS-cog 13) had a significant increase in their respective limbic and cortical CBF.

CONCLUSIONS

Treatment of mild AD/MCI subjects with medications shown to augment the eNOS pathway was well tolerated and associated with modestly increased cerebral blood flow and cognitive improvement.

TRIAL REGISTRATION

This study is registered in https://www.

CLINICALTRIALS

gov ; registration identifier: NCT01439555; date of registration submitted to registry: 09/23/2011; date of first subject enrollment: 11/2011.

Does sympathetic vasoconstriction contribute to metabolism: Perfusion matching in exercising skeletal muscle?

The process of matching skeletal muscle blood flow to metabolism is complex and multi-factorial. In response to exercise, increases in cardiac output, perfusion pressure and local vasodilation facilitate an intensity-dependent increase in muscle blood flow. Concomitantly, sympathetic nerve activity directed to both exercising and non-active muscles increases as a function of exercise intensity. Several studies have reported the presence of tonic sympathetic vasoconstriction in the vasculature of exercising muscle at the onset of exercise that persists through prolonged exercise bouts, though it is blunted in an exercise-intensity dependent manner (functional sympatholysis). The collective evidence has resulted in the current dogma that vasoactive molecules released from skeletal muscle, the vascular endothelium, and possibly red blood cells produce local vasodilation, while sympathetic vasoconstriction restrains vasodilation to direct blood flow to the most metabolically active muscles/fibers. Vascular smooth muscle is assumed to integrate a host of vasoactive signals resulting in a precise matching of muscle blood flow to metabolism. Unfortunately, a critical review of the available literature reveals that published studies have largely focused on bulk blood flow and existing experimental approaches with limited ability to reveal the matching of perfusion with metabolism, particularly between and within muscles. This paper will review our current understanding of the regulation of sympathetic vasoconstriction in contracting skeletal muscle and highlight areas where further investigation is necessary.

Aging increases circulating BH2 without modifying BH4 levels and impairs peripheral vascular function in healthy adults

Little is known about the mechanisms of aging on vascular beds and its relationship with tetra and di-hydrobiopterin (BH4 and BH2) levels. This observational clinical study analyzed the impact of aging on plasma and platelet biopterins, cutaneous blood flow (CBF), and coronary flow reserve (CFR) in healthy adults. The study enrolled healthy adults in 3 age groups: 18-30, 50-59, and 60-70 years (n = 25/group). Biopterins were assessed by LC-MS/MS using newly defined pre-analytical conditions limiting BH4 oxidation and improving long-term stability. CBF was measured by Laser Speckle Contrast Imaging coupled with acetylcholine-iontophoresis and CFR by adenosine stress cardiac magnetic resonance. In healthy adults, aging (60-70 years vs 18-30 years) significantly increased platelet BH2 (+75%, P = 0.033) and BH2 + BH4 (+31%, P = 0.033), and to a lesser extent plasma BH2 (+29%, P = 0.009) without affecting BH4 and BH4/BH2. Simultaneously, CBF was decreased (-23%, P = 0.004) but not CFR, CBF being inversely correlated with platelet BH2 (r = -0.42, P = 0.001) and BH2 + BH4 (r = -0.41, P = 0.002). The proportion of adults with abnormal platelet BH2 increased with age (+28% in 60-70y). These abnormal BH2 levels were significantly associated with reduced CBF and CFR (-16%, P = 0.03 and -26%, P = 0.02). In conclusion, our study showed that age-related peripheral endothelial dysfunction was associated with an increase in circulating BH2 without decreasing BH4, the effect being more marked in platelets, the most relevant blood compartment to assess biopterin bioavailability. Peripheral but not coronary vascular function is progressively impaired with aging in healthy adults. All these findings support biopterins as therapeutic targets to improve vascular function.

Sapropterin (BH4) Aggravates Autoimmune Encephalomyelitis in Mice

Depletion of the enzyme cofactor, tetrahydrobiopterin (BH4), in T-cells was shown to prevent their proliferation upon receptor stimulation in models of allergic inflammation in mice, suggesting that BH4 drives autoimmunity. Hence, the clinically available BH4 drug (sapropterin) might increase the risk of autoimmune diseases. The present study assessed the implications for multiple sclerosis (MS) as an exemplary CNS autoimmune disease. Plasma levels of biopterin were persistently low in MS patients and tended to be lower with high Expanded Disability Status Scale (EDSS). Instead, the bypass product, neopterin, was increased. The deregulation suggested that BH4 replenishment might further drive the immune response or beneficially restore the BH4 balances. To answer this question, mice were treated with sapropterin in immunization-evoked autoimmune encephalomyelitis (EAE), a model of multiple sclerosis. Sapropterin-treated mice had higher EAE disease scores associated with higher numbers of T-cells infiltrating the spinal cord, but normal T-cell subpopulations in spleen and blood. Mechanistically, sapropterin treatment was associated with increased plasma levels of long-chain ceramides and low levels of the poly-unsaturated fatty acid, linolenic acid (FA18:3). These lipid changes are known to contribute to disruptions of the blood-brain barrier in EAE mice. Indeed, RNA data analyses revealed upregulations of genes involved in ceramide synthesis in brain endothelial cells of EAE mice (LASS6/CERS6, LASS3/CERS3, UGCG, ELOVL6, and ELOVL4). The results support the view that BH4 fortifies autoimmune CNS disease, mechanistically involving lipid deregulations that are known to contribute to the EAE pathology.

New Drugs and Interventional Strategies for the Management of Hypertension

Essential hypertension is an important cause of cardiovascular morbidity and mortality worldwide with significant clinical and economic implications. The field of antihypertensive treatment already numbers numerous agents and classes of drugs. However, patients are still developing uncontrolled hypertension. Hence there is a continuous need for novel agents with good tolerability. Advances in this field are focusing both on pharmacotherapy, with the developments in traditional and non-traditional targets, as well as interventional techniques such as renal denervation and baroreflex activation therapy. It is likely that future strategies may involve a tailored approach to the individual patient, with genetic modulation playing a key role.

Neuronal Nitric Oxide Synthase (nNOS) in Neutrophils: An Insight

NO (nitric oxide) is an important regulator of neutrophil functions and has a key role in diverse pathophysiological conditions. NO production by nitric oxide synthases (NOS) is under tight control at transcriptional, translational, and post-translational levels including interactions with heterologous proteins owing to its potent chemical reactivity and high diffusibility; this limits toxicity to other cellular components and promotes signaling specificity. The protein-protein interactions govern the activity and spatial distribution of NOS isoform to regulatory proteins and to their intended targets. In comparison with the vast literature available for endothelial, macrophages, and neuronal cells, demonstrating neuronal NOS (nNOS) interaction with other proteins through the PDZ domain, neutrophil nNOS, however, remains unexplored. Neutrophil's key role in both physiological and pathological conditions necessitates the need for further studies in delineating the NOS mediated NO modulations in signaling pathways operational in them. nNOS has been linked to depression, schizophrenia, and Parkinson's disease, suggesting the importance of exploring nNOS/NO-mediated neutrophil physiology in relation to such neuronal disorders. The review thus presents the scenario of neutrophil nNOS from the genetics to the functional level, including protein-protein interactions governing its intracellular sequestration in diverse cell types, besides speculating possible regulation in neutrophils and also addressing their clinical implications.

Nitric oxide and sickle cell disease-Is there a painful connection?

Sickle cell disease is the most common hemoglobinopathy and affects millions worldwide. The disease is associated with severe organ dysfunction, acute and chronic pain, and significantly decreased life expectancy. The large body of work demonstrating that hemolysis results in rapid consumption of the endogenous vasodilator nitric oxide, decreased nitric oxide production, and promotion of vaso-occlusion provides the basis for the hypothesis that nitric oxide bioavailability is reduced in sickle cell disease and that this deficit plays a role in sickle cell disease pain. Despite initial promising results, large clinical trials using strategies to increase nitric oxide bioavailability in sickle cell disease patients yielded no significant change in duration or frequency of acute pain crises. Further, recent investigations showed that sickle cell disease patients and mouse models have elevated baseline levels of blood nitrite, a reservoir for nitric oxide formation and a product of nitric oxide metabolism, regardless of pain phenotype. These conflicting results challenge the hypotheses that nitric oxide bioavailability is decreased and that it plays a significant role in the pathogenesis in sickle cell disease acute pain crises. Conversely, a large body of work demonstrates that nitric oxide, as a neurotransmitter, has a complex role in pain neurobiology, contributes to the development of central sensitization, and can mediate hyperalgesia in inflammatory and neuropathic pain. These results support an alternative hypothesis: one proposing that altered nitric oxide signaling may contribute to the development of neuropathic and/or inflammatory pain in sickle cell disease through its role as a neurotransmitter.

Comparative Metabolomics Study Revealed Difference in Central Carbon Metabolism between Sika Deer and Red Deer Antler

The antler regeneration has been well studied for the past two decades and adopted in the regenerative medicine model for studying on developmental biology. Despite our growing knowledge of functional molecules regulating antler regeneration, we still do not know whether antler from different deer species possess the exact same mechanism or not. Our previous comparative study between sika deer and red deer suggests that the metabolic pathways between them are profoundly different based on protein level. Therefore, the metabolomic technology is used to identify and quantify the metabolites in antler samples, providing interesting insights into differential metabolite profile of antlers between sika deer and red deer. The distinct metabolic characteristics of sika deer compared to red deer provide an opportunity to explain why the red deer antler with a larger size. The enrichment analysis of differential metabolites showed that three pathways including glycine and serine metabolism, methionine metabolism, and pterine biosynthesis had a significant difference between two antler groups.

Eukaryotic elongation factor 2 kinase inhibitor, A484954 lowered blood pressure in spontaneously hypertensive rats via inducing vasorelaxation

Eukaryotic elongation factor 2 (eEF2) kinase (eEF2K) suppresses protein translation. We previously reported eEF2K expression was upregulated in mesenteric arteries (MA) from spontaneously hypertensive rats (SHR). We have recently revealed A484954, an eEF2K inhibitor, acutely suppressed vasopressor agonists-induced increase of blood pressure (BP) in normal Wistar rats. In this study, we examined the acute effects of A484954 on BP in SHR and explored underlying mechanisms. BP was measured by a carotid cannulation method in SHR. Isometric contraction in MA from SHR was measured. Endothelial nitric oxide synthase (eNOS) dimerization was measured by low-temperature sodium dodecyl sulfate-polyacrylamide gel electrophoresis and Western blotting. A484954 lowered BP in 15-week-old SHR. A484954 induced relaxation in MA from both 4- and 7-9-week-old SHR. In MA from 4-week-old SHR, A484954-induced relaxation was inhibited almost completely by a NOS inhibitor, N^G-nitro-l-arginine methyl ester (l-NAME) and significantly by a β blocker, propranolol. In MA from 7-9-week-old SHR, on the other hand, A484954-induced relaxation was inhibited partly either by l-NAME, indomethacin, a cyclooxygenase inhibitor, or l-NAME + indomethacin. A484954 promoted the dimerization of eNOS in human endothelial cells. In summary, we have revealed A484954 lowers BP in SHR perhaps through the vasorelaxation via the production of endothelium-derived relaxing factors.

Combined l-citrulline and tetrahydrobiopterin therapy improves NO signaling and ameliorates chronic hypoxia-induced pulmonary hypertension in newborn pigs

Newborn pigs with chronic hypoxia-induced pulmonary hypertension (PH) have evidence of endothelial nitric oxide synthase (eNOS) uncoupling. In this model, we showed that therapies that promote eNOS coupling, either tetrahydrobiopterin (BH4), a NOS cofactor, or l-citrulline, a NO-l-arginine precursor, inhibit PH. We wanted to determine whether cotreatment with l-citrulline and a BH4 compound, sapropterin dihydrochloride, improves NO signaling and chronic hypoxia-induced PH more markedly than either alone. Normoxic (control) and hypoxic piglets were studied. Some hypoxic piglets received sole treatment with l-citrulline or BH4, or were cotreated with l-citrulline and BH4, from day 3 through day 10 of hypoxia. Catheters were placed for hemodynamic measurements, and pulmonary arteries were dissected to assess eNOS dimer-to-monomer ratios and NO production. In untreated hypoxic piglets, pulmonary vascular resistance (PVR) was higher and NO production and eNOS dimer-to-monomer ratios were lower than in normoxic piglets. Compared with the untreated hypoxic group, PVR was lower in hypoxic piglets cotreated with l-citrulline and BH4 and in those treated with l-citrulline alone but not for those treated solely with BH4. NO production and eNOS dimer-to-monomer ratios were greater for all three treated hypoxic groups compared with the untreated group. Notably, greater improvements in PVR, eNOS dimer-to-monomer ratios, and NO production were found in hypoxic piglets cotreated with l-citrulline and BH4 than in piglets treated with either alone. Cotreatment with l-citrulline and BH4 more effectively improves NO signaling and inhibits chronic hypoxia-induced PH than either treatment alone. Combination therapies may offer enhanced therapeutic capacity for challenging clinical conditions, such as chronic neonatal PH.

Therapeutic strategies for sickle cell disease: towards a multi-agent approach

For over 100 years, clinicians and scientists have been unravelling the consequences of the A to T substitution in the β-globin gene that produces haemoglobin S, which leads to the systemic manifestations of sickle cell disease (SCD), including vaso-occlusion, anaemia, haemolysis, organ injury and pain. However, despite growing understanding of the mechanisms of haemoglobin S polymerization and its effects on red blood cells, only two therapies for SCD - hydroxyurea and L-glutamine - are approved by the US Food and Drug Administration. Moreover, these treatment options do not fully address the manifestations of SCD, which arise from a complex network of interdependent pathophysiological processes. In this article, we review efforts to develop new drugs targeting these processes, including agents that reactivate fetal haemoglobin, anti-sickling agents, anti-adhesion agents, modulators of ischaemia-reperfusion and oxidative stress, agents that counteract free haemoglobin and haem, anti-inflammatory agents, anti-thrombotic agents and anti-platelet agents. We also discuss gene therapy, which holds promise of a cure, although its widespread application is currently limited by technical challenges and the expense of treatment. We thus propose that developing systems-oriented multi-agent strategies on the basis of SCD pathophysiology is needed to improve the quality of life and survival of people with SCD.

Mineralocorticoid receptor: A hidden culprit for hemodialysis vascular access dysfunction

Hemodialysis vascular access dysfunction is a common and intractable problem in clinical practice with no definitive therapy yet available. As a key mediator of vascular and cardiac maladaptive remodeling, mineralocorticoid receptor (MR) plays a pivotal role in vascular fibrosis and intimal hyperplasia (IH) and is potentiated locally in hemodialysis vascular access following diverse injuries, like barotrauma, cannulation and shear stress. MR-related genomic and non-genomic pathways are responsible for triggering vascular smooth muscle cell activation, proliferation, migration and extracellular matrix overproduction. In endothelial cells, MR signaling diminishes nitric oxide production and its bioavailability, but amplifies reactive oxygen species, leading to an inflammatory state. Moreover, MR favors macrophage polarization towards a pro-inflammatory phenotype. In clinical settings like post-angioplasty or stenting restenosis, the beneficial effect of MR antagonists on vascular fibrosis and IH has been validated. In aggregate, therapeutic targeting of MR may provide a new avenue to prevent hemodialysis vascular access dysfunction.

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MR signaling is instrumental in both insufficient outward remodeling and exuberant inward remodeling of AVF.

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The effects of MR in VSMC, endothelial cell, and macrophage act synergistically to promote IH and vascular fibrosis in AVF.

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Pharmacological targeting of MR represents a novel therapeutic strategy to prevent hemodialysis vascular access dysfunction.

Arginase: A Multifaceted Enzyme Important in Health and Disease

The arginase enzyme developed in early life forms and was maintained during evolution. As the last step in the urea cycle, arginase cleaves l-arginine to form urea and l-ornithine. The urea cycle provides protection against excess ammonia, while l-ornithine is needed for cell proliferation, collagen formation, and other physiological functions. In mammals, increases in arginase activity have been linked to dysfunction and pathologies of the cardiovascular system, kidney, and central nervous system and also to dysfunction of the immune system and cancer. Two important aspects of the excessive activity of arginase may be involved in diseases. First, overly active arginase can reduce the supply of l-arginine needed for the production of nitric oxide (NO) by NO synthase. Second, too much l-ornithine can lead to structural problems in the vasculature, neuronal toxicity, and abnormal growth of tumor cells. Seminal studies have demonstrated that increased formation of reactive oxygen species and key inflammatory mediators promote this pathological elevation of arginase activity. Here, we review the involvement of arginase in diseases affecting the cardiovascular, renal, and central nervous system and cancer and discuss the value of therapies targeting the elevated activity of arginase.

Obesity-induced vascular dysfunction and arterial stiffening requires endothelial cell arginase 1

AIMS

Elevation of arginase activity has been linked to vascular dysfunction in diabetes and hypertension by a mechanism involving decreased nitric oxide (NO) bioavailability due to L-arginine depletion. Excessive arginase activity also can drive L-arginine metabolism towards the production of ornithine, polyamines, and proline, promoting proliferation of vascular smooth muscle cells and collagen formation, leading to perivascular fibrosis. We hypothesized that there is a specific involvement of arginase 1 expression within the vascular endothelial cells in this pathology.

METHODS AND RESULTS

To test this proposition, we used models of type 2 diabetes and metabolic syndrome. Studies were performed using wild type (WT), endothelial-specific arginase 1 knockout (EC-A1-/-) and littermate controls(A1con) mice fed high fat-high sucrose (HFHS) or normal diet (ND) for 6 months and isolated vessels exposed to palmitate-high glucose (PA/HG) media. Some WT mice or isolated vessels were treated with an arginase inhibitor, ABH [2-(S)-amino-6-boronohexanoic acid. In WT mice, the HFHS diet promoted increases in body weight, fasting blood glucose, and post-prandial insulin levels along with arterial stiffening and fibrosis, elevated blood pressure, decreased plasma levels of L-arginine, and elevated L-ornithine. The HFHS diet or PA/HG treatment also induced increases in vascular arginase activity along with oxidative stress, reduced vascular NO levels, and impaired endothelial-dependent vasorelaxation. All of these effects except obesity and hypercholesterolemia were prevented or significantly reduced by endothelial-specific deletion of arginase 1 or ABH treatment.

CONCLUSION

Vascular dysfunctions in diet-induced obesity are prevented by deletion of arginase 1 in vascular endothelial cells or arginase inhibition. These findings indicate that upregulation of arginase 1 expression/activity in vascular endothelial cells has an integral role in diet-induced cardiovascular dysfunction and metabolic syndrome.

Computational analysis of interactions of oxidative stress and tetrahydrobiopterin reveals instability in eNOS coupling

In cardiovascular and neurovascular diseases, an increase in oxidative stress and endothelial dysfunction has been reported. There is a reduction in tetrahydrobiopterin (BH4), which is a cofactor for the endothelial nitric oxide synthase (eNOS), resulting in eNOS uncoupling. Studies of the enhancement of BH4 availability have reported mixed results for improvement in endothelial dysfunction. Our understanding of the complex interactions of eNOS uncoupling, oxidative stress and BH4 availability is not complete and a quantitative understanding of these interactions is required. In the present study, we developed a computational model for eNOS uncoupling that considers the temporal changes in biopterin ratio in the oxidative stress conditions. Using the model, we studied the effects of cellular oxidative stress (Qsupcell) representing the non-eNOS based oxidative stress sources and BH4 synthesis (QBH4) on eNOS NO production and biopterin ratio (BH4/total biopterins (TBP)). Model results showed that oxidative stress levels from 0.01 to 1nM·s-1 did not affect eNOS NO production and eNOS remained in coupled state. When the Qsupcell increased above 1nM·s-1, the eNOS coupling and NO production transitioned to an oscillatory state. Oxidative stress levels dynamically changed the biopterin ratio. When Qsupcell increased from 1 to 100nM·s-1, the endothelial cell NO production, TBP levels and biopterin ratio reduced significantly from 26.5 to 2nM·s-1, 3.75 to 0.002μM and 0.99 to 0.25, respectively. For an increase in BH4 synthesis, the improvement in NO production rate and BH4 levels were dependent on the extent of cellular oxidative stress. However, a 10-fold increase in QBH4 at higher oxidative stresses did not restore the NO-production rate and the biopterin ratio. Our mechanistic analysis reveals that a combination of enhancing tetrahydrobiopterin level with a reduction in cellular oxidative stress may result in significant improvement in endothelial dysfunction.

Tetrahydrobiopterin activates brown adipose tissue and regulates systemic energy metabolism

Brown adipose tissue (BAT) is a central organ that acts to increase energy expenditure; its regulatory factors could be clinically useful in the treatment of obesity. Tetrahydrobiopterin (BH4) is an essential cofactor of tyrosine hydroxylase and nitric oxide synthase (NOS). Although BH4 regulates the known regulatory factors of BAT, such as noradrenaline (NA) and NO, participation of BH4 in BAT function remains unclear. In the present study, we investigate the role of BH4 in the regulation of BAT. Hph-1 mice, a mouse model of BH4 deficiency, exhibit obesity, adiposity, glucose intolerance, insulin resistance, and impaired BAT function. Impaired BAT function was ameliorated together with systemic metabolic disturbances by BAT transplantation from BH4-sufficient mice (control mice) into BH4-deficient mice, strongly suggesting that BH4-induced BAT has a critical role in the regulation of systemic energy metabolism. Both NA derived from the sympathetic nerve and NO derived from endothelial NOS in the blood vessels participate in the regulation of BH4. In addition, a direct effect of BH4 in the stimulation of brown adipocytes via NO is implicated. Taken together, BH4 activates BAT and regulates systemic energy metabolism; this suggests an approach for metabolic disorders, such as obesity and diabetes.

The potential of stimulating nitric oxide formation in the treatment of hypertension

INTRODUCTION

Hypertension is a leading cause of morbidity and mortality worldwide. A major pathophysiological factor contributing to hypertension is reduced nitric oxide (NO) bioavailability. Strategies to address this pathophysiological mechanism could offer significant advantages. Areas covered: In this review we aimed at examining a variety of drugs (statins, beta-adrenergic receptor blockers, calcium channel blockers, angiotensin converting enzyme inhibitors, angiotensin II type-1 receptor blockers) used to treat hypertension and other cardiovascular diseases, particularly with respect to their potential of increasing NO bioavailability and activity in the cardiovascular system. There is now evidence supporting the notion that many cardiovascular drugs activate NO signaling or enhance NO bioavailability as a contributing mechanism to their beneficial cardiovascular effects. Moreover, other drugs may attenuate NO inactivation by superoxide and other reactive oxygen species by exerting antioxidant effects. More recently, the NO oxidation products nitrite and nitrate have been acknowledged as sources of NO after recycling back to NO. Activation of the nitrate-nitrite-NO pathway is an alternate pathway that may generate NO from both anions and exert antihypertensive effects. Expert opinion: In this review, we provide an overview of the possible mechanisms by which these drugs enhance NO bioavailability and help in the therapy of hypertension.

Impaired Endothelial Nitric Oxide Synthase Homodimer Formation Triggers Development of Transplant Vasculopathy - Insights from a Murine Aortic Transplantation Model

Transplant vasculopathy (TV) represents a major obstacle to long-term graft survival and correlates with severity of ischemia reperfusion injury (IRI). Donor administration of the nitric oxide synthases (NOS) co-factor tetrahydrobiopterin has been shown to prevent IRI. Herein, we analysed whether tetrahydrobiopterin is also involved in TV development. Using a fully allogeneic mismatched (BALB/c to C57BL/6) murine aortic transplantation model grafts subjected to long cold ischemia time developed severe TV with intimal hyperplasia (α-smooth muscle actin positive cells in the neointima) and endothelial activation (increased P-selectin expression). Donor pretreatment with tetrahydrobiopterin significantly minimised these changes resulting in only marginal TV development. Severe TV observed in the non-treated group was associated with increased protein oxidation and increased occurrence of endothelial NOS monomers in the aortic grafts already during graft procurement. Tetrahydrobiopterin supplementation of the donor prevented all these early oxidative changes in the graft. Non-treated allogeneic grafts without cold ischemia time and syngeneic grafts did not develop any TV. We identified early protein oxidation and impaired endothelial NOS homodimer formation as plausible mechanistic explanation for the crucial role of IRI in triggering TV in transplanted aortic grafts. Therefore, targeting endothelial NOS in the donor represents a promising strategy to minimise TV.

Tetrahydrobiopterin oral therapy recouples eNOS and ameliorates chronic hypoxia-induced pulmonary hypertension in newborn pigs

We previously showed that newborn piglets who develop pulmonary hypertension during exposure to chronic hypoxia have diminished pulmonary vascular nitric oxide (NO) production and evidence of endothelial NO synthase (eNOS) uncoupling (Fike CD, Dikalova A, Kaplowitz MR, Cunningham G, Summar M, Aschner JL. Am J Respir Cell Mol Biol 53: 255-264, 2015). Tetrahydrobiopterin (BH4) is a cofactor that promotes eNOS coupling. Current clinical strategies typically invoke initiating treatment after the diagnosis of pulmonary hypertension, rather than prophylactically. The major purpose of this study was to determine whether starting treatment with an oral BH4 compound, sapropterin dihydrochloride (sapropterin), after the onset of pulmonary hypertension would recouple eNOS in the pulmonary vasculature and ameliorate disease progression in chronically hypoxic piglets. Normoxic (control) and hypoxic piglets were studied. Some hypoxic piglets received oral sapropterin starting on day 3 of hypoxia and continued throughout an additional 7 days of hypoxic exposure. Catheters were placed for hemodynamic measurements, and pulmonary arteries were dissected to assess eNOS dimer-to-monomer ratios (a measure of eNOS coupling), NO production, and superoxide (O2·-) generation. Although higher than in normoxic controls, pulmonary vascular resistance was lower in sapropterin-treated hypoxic piglets than in untreated hypoxic piglets. Consistent with eNOS recoupling, eNOS dimer-to-monomer ratios and NO production were greater and O2·- generation was less in pulmonary arteries from sapropterin-treated than untreated hypoxic animals. When started after disease onset, oral sapropterin treatment inhibits chronic hypoxia-induced pulmonary hypertension at least in part by recoupling eNOS in the pulmonary vasculature of newborn piglets. Rescue treatment with sapropterin may be an effective strategy to inhibit further development of pulmonary hypertension in newborn infants suffering from chronic cardiopulmonary conditions associated with episodes of prolonged hypoxia.

Precipitating factors and targeted therapies in combating the perils of sickle cell disease--- A special nutritional consideration

Nutritional research in sickle cell disease has been the focus in recent times owing to not only specific nutritional deficiencies, but also the improvements associated with less painful episodes. Though hydroxyurea remains the drug of choice, certain adverse health effects on long term supplementation makes room for researches of different compounds. Macro and micro nutrient deficiencies, along with vitamins, play an important role in not only meeting the calorific needs, but also reducing clinical complications and growth abnormalities. Symptoms of hyper protein metabolism, increased cell turnover, increased cardiac output, and appetite suppression due to enhanced cytokine production, might give us leads for better understanding of the mechanisms involved. Different nutritional approaches comprising of traditional herbal therapies, antioxidants, flavonoids, vitamins, minerals etc., reducing oxidative stress and blood aggregation, have been tried out to increase the health potential. Nutritional therapies may also serve complementary to the newer therapies using ozone, hematopoietic stem cell transplantation, antifungal medications, erythropoietin etc. Herein we try to present a holistic picture of the different patho-physiological mechanisms, and nutritional strategies adopted.

Downregulation of tetrahydrobiopterin inhibits tumor angiogenesis in BALB/c-nu mice with hepatocellular carcinoma

Hepatocellular carcinoma (HCC) is a highly vascular tumor, and treatment options for patients of advanced-stage are limited. Nitric oxide (NO), which is derived from endothelial nitric oxide synthase (eNOS), provides crucial signals for angiogenesis in the tumor microenvironment. Tetrahydrobiopterin (BH4) is an essential cofactor eNOS and represents a critical determinant of NO production. To examine whether treatment of 2,4-diamino-6-hydroxypyrimidine (DAHP) inhibits angiogenesis of HCC, BALB/c-nu mice were injected with HepG-2 cells with DAHP. Supplemental DAHP treatment decreased K-ras mRNA transcripts, inhibition of phosphorylation of eNOS and Akt, inhibition of guanosine triphosphate cyclohydrolase (GTPCH), and decreased significantly NO synthesis, and then inhibited angiogenesis, compared with the results observed in the saline group. Histopathology demonstrated angiogenesis and tumor formation were significantly inhibited in HCC. DAHP downregulates GTPCH protein expression, corresponding to decreased levels of BH4 and the contents of NO. In addition, DAHP downregulates eNOS and Akt protein expression, corresponding to decreased eNOS phosphorylation at Ser1177 and Akt phosphorylation, compared with the saline control. We suggest that DAHP, recognized as a specific competitive inhibitor of GTPCH, can decrease tumor BH4 and NO by the inhibition of the wild-type Ras-PI3K/Akt pathway, and then inhibiting angiogenesis, and may provide a novel and promising way to target BH4 synthetic pathways to inhibit angiogenesis and to control potential progression of HCC. Whether DAHP has a therapeutic potential will require more direct testing in humans.

Endothelial nitric oxide synthase: a potential therapeutic target for cerebrovascular diseases

Endothelial nitric oxide (NO) is a significant signaling molecule that regulates cerebral blood flow (CBF), playing a pivotal role in the prevention and treatment of cerebrovascular diseases. However, achieving the expected therapeutic efficacy is difficult using direct administration of NO donors. Therefore, endothelial nitric oxide synthase (eNOS) becomes a potential therapeutic target for cerebrovascular diseases. This review summarizes the current evidence supporting the importance of CBF to cerebrovascular function, and the roles of NO and eNOS in CBF regulation.

Genetic predisposition in patients with hypertension and normal ejection fraction to oxidative stress

The role of oxidative stress (OXS) due to myocardial nitric oxide synthase (NOS) uncoupling related to oxidative depletion of its cofactor tetrahydrobiopterin (BH4) emerged in the pathogenesis of heart failure with preserved ejection fraction. We determined the prevalence of six single nucleotide polymorphisms (SNPs) of genes encoding enzymes related to OXS, BH4 metabolism, and NOS function in ≥60-year-old 94 patients with hypertension and 18 age-matched controls with normal ejection fraction. Using echocardiography, 56/94 (60%) patients with hypertension had left ventricular (LV) diastolic dysfunction (HTDD+ group) and 38/94 (40%) patients had normal LV diastolic function (HTDD- group). Four SNPs (rs841, rs3783641, rs10483639, and rs807267) of guanosine triphosphate cyclohydrolase-1, the rate-limiting enzyme in BH4 synthesis, one (rs4880) of manganese superoxide dismutase, and one (rs1799983) of endothelial NOS genes were genotyped using real-time polymerase chain reaction method and Taqman probes. Protein carbonylation, BH4, and total biopterin levels were measured from plasma samples. No between-groups difference in minor allele frequency of SNPs was found. We calculated a genetic score indicating risk for OXS based on the minor allele frequencies of the SNPs. A high genetic risk for OXS was significantly associated with HTDD+ even after adjustment for confounding variables (odds ratio [95% confidence interval]:4.79 [1.12-20.54]; P = .035). In both patient groups protein carbonylation (P < .05 for both), plasma BH4 (P < .01 for both) and in the HTDD+ group total biopterin (P < .05) increased versus controls. In conclusion, in patients with hypertension and normal ejection fraction, a potential precursor of heart failure with preserved ejection fraction, a partly genetically determined increased OXS, seems to be associated with the presence of LV diastolic dysfunction.

Ascorbic acid improves brachial artery vasodilation during progressive handgrip exercise in the elderly through a nitric oxide-mediated mechanism

The proposed mechanistic link between the age-related attenuation in vascular function and free radicals is an attractive hypothesis; however, direct evidence of free radical attenuation and a concomitant improvement in vascular function in the elderly is lacking. Therefore, this study sought to test the hypothesis that ascorbic acid (AA), administered intra-arterially during progressive handgrip exercise, improves brachial artery (BA) vasodilation in a nitric oxide (NO)-dependent manner, by mitigating free radical production. BA vasodilation (Doppler ultrasound) and free radical outflow [electron paramagnetic resonance (EPR) spectroscopy] were measured in seven healthy older adults (69 ± 2 yr) during handgrip exercise at 3, 6, 9, and 12 kg (∼13-52% of maximal voluntary contraction) during the control condition and nitric oxide synthase (NOS) inhibition via N(G)-monomethyl-L-arginine (L-NMMA), AA, and coinfusion of l-NMMA + AA. Baseline BA diameter was not altered by any of the treatments, while L-NMMA and L-NMMA + AA diminished baseline BA blood flow and shear rate. AA improved BA dilation compared with control at 9 kg (control: 6.5 ± 2.2%, AA: 10.9 ± 2.5%, P = 0.01) and 12 kg (control: 9.5 ± 2.7%, AA: 15.9 ± 3.7%, P < 0.01). NOS inhibition blunted BA vasodilation compared with control and when combined with AA eliminated the AA-induced improvement in BA vasodilation. Free radical outflow increased with exercise intensity but, interestingly, was not attenuated by AA. Collectively, these results indicate that AA improves BA vasodilation in the elderly during handgrip exercise through an NO-dependent mechanism; however, this improvement appears not to be the direct consequence of attenuated free radical outflow from the forearm.

Characterization of cerebral microvasculature in transgenic mice with endothelium targeted over-expression of GTP-cyclohydrolase I

Tetrahydrobiopterin (BH4) is a critical determinant of nitric oxide (NO) production by nitric oxide synthase (NOS) in the vascular endothelium and its biosynthesis is regulated by the enzymatic activity of GTP-cyclohydrolase I (GTPCH I). The present study was designed to determine the effects of endothelium-targeted overexpression of GTPCH I (eGCH-Tg) on murine cerebral vascular function. Endothelium targeted over-expression of GTPCH I was associated with a significant increase in levels of BH4, as well as its oxidized product, 7,8-dihydrobiopterin (7,8-BH2) in cerebral microvessels. Importantly, ratio of BH4 to 7,8-BH2, indicative of BH4 available for eNOS activation, was significantly increased in eGCH-Tg mice. However, expression of endothelial NOS, levels of nitrate/nitrite--indicative of NO production--remained unchanged between cerebral microvessels of wild-type and eGCH-Tg mice. Furthermore, increased BH4 biosynthesis neither affected production of superoxide anion nor expression of antioxidant proteins. Moreover, endothelium-specific GTPCH I overexpression did not alter intracellular levels of cGMP, reflective of NO signaling in cerebral microvessels. The obtained results suggest that, despite a significant increase in BH4 bioavailability, generation of endothelial NO in cerebral microvessels remained unchanged in eGCH-Tg mice. We conclude that under physiological conditions the levels of BH4 in the cerebral microvessels are optimal for activation of endothelial NOS and NO/cGMP signaling.

Treatment with tetrahydrobiopterin overcomes brain death-associated injury in a murine model of pancreas transplantation

Brain death (BD) has been associated with an immunological priming of donor organs and is thought to exacerbate ischemia reperfusion injury (IRI). Recently, we showed that the essential nitric oxide synthase co-factor tetrahydrobiopterin (BH4) abrogates IRI following experimental pancreas transplantation. We therefore studied the effects of BD in a murine model of syngeneic pancreas transplantation and tested the therapeutic potential of BH4 treatment. Compared with sham-operated controls, donor BD resulted in intragraft inflammation reflected by induced IL-1ß, IL-6, VCAM-1, and P-selectin mRNA expression levels and impaired microcirculation after reperfusion (p < 0.05), whereas pretreatment of the BD donor with BH4 significantly improved microcirculation after reperfusion (p < 0.05). Moreover, BD had a devastating impact on cell viability, whereas BH4-treated grafts showed a significantly higher percentage of viable cells (p < 0.001). Early parenchymal damage in pancreatic grafts was significantly more pronounced in organs from BD donors than from sham or non-BD donors (p < 0.05), but BH4 pretreatment significantly ameliorated necrotic lesions in BD organs (p < 0.05). Pretreatment of the BD donor with BH4 resulted in significant recipient survival (p < 0.05). Our data provide novel insights into the impact of BD on pancreatic isografts, further demonstrating the potential of donor pretreatment strategies including BH4 for preventing BD-associated injury after transplantation.

Uncoupling of endothelial nitric oxide synthase in cerebral vasculature of Tg2576 mice

In this study, we tested the hypothesis that reduced bioavailability of tetrahydrobiopterin (BH4) is a major mechanism responsible for pathogenesis of endothelial dysfunction in cerebral microvessels of transgenic mice expressing the Swedish double mutation of human amyloid precursor protein (APP) (Tg2576 mice). Endothelial nitric oxide synthase (eNOS) protein expression was significantly increased in cerebral vasculature of Tg2576 mice. In contrast, bioavailability of BH4 was significantly reduced (p < 0.05). Moreover, superoxide anion production was increased in cerebral microvessels of Tg2576 mice (p < 0.05). Incubation with NOS inhibitor, Nω-nitro-L-arginine methyl ester, decreased superoxide anion indicating that uncoupled eNOS is most likely the source of superoxide anion. Increasing BH4 bioavailability either exogenously by BH4 supplementation or endogenously by treatment with the selective peroxisome proliferator-activated receptor--delta activator GW501516 (2 mg/kg/day, 14 days) attenuated eNOS uncoupling and decreased superoxide anion production in cerebral microvessels of Tg2576 mice (p < 0.05). Treatment with GW501516 restored the biological activity of endothelial nitric oxide in cerebral microvessels of Tg2576 mice, as indicated by the increased nitrite/nitrate content and 3,5-cyclic guanosine monophosphate levels (p < 0.05). Our studies indicate that sub-optimal BH4 bioavailability in cerebral vasculature is an important contributor to oxidant stress and endothelial dysfunction in Tg2576 mouse model of Alzheimer's disease. Existing evidence suggests that Aβ peptides-induced up-regulation of expression and activity of NADPH oxidase causes increased production of superoxide anion (.O2(-)). .O2(-) can also be converted to hydrogen peroxide (H2O2) by enzymatic activity of superoxide dismutase (SOD) or spontaneous dismutation. Elevation of .O2(-) and H2O2 might cause oxidation of tetrahydrobiopterin (BH4) to dihydrobiopterin (BH2) and subsequent uncoupling of endothelial nitric oxide synthase (eNOS) (a) thus reducing levels of nitric oxide (NO) and 3',5'-cyclic guanosine monophosphate (cGMP). Supplementation of BH4 or activation of PPARδ prevents detrimental effects of eNOS uncoupling by restoring bioavailability of BH4 and scavenging of .O2(-), respectively (b). Activation of PPARδ also increases expression of catalase thereby inactivating H2O2. Generation of H2O2 by uncoupled eNOS in cerebral microvessels of Tg2576 mice is hypothetical.

Arginase: an old enzyme with new tricks

Arginase has roots in early life-forms. It converts L-arginine to urea and ornithine. The former provides protection against NH3; the latter serves to stimulate cell growth and other physiological functions. Excessive arginase activity in mammals has been associated with cardiovascular and nervous system dysfunction and disease. Two relevant aspects of this elevated activity may be involved in these disease states. First, excessive arginase activity reduces the supply of L-arginine needed by nitric oxide (NO) synthase to produce NO. Second, excessive production of ornithine leads to vascular structural problems and neural toxicity. Recent research has identified inflammatory agents and reactive oxygen species (ROS) as drivers of this pathologic elevation of arginase activity and expression. We review the involvement of arginase in cardiovascular and nervous system dysfunction, and discuss potential therapeutic interventions targeting excess arginase.

Long-term safety and efficacy of sapropterin: the PKUDOS registry experience

The Phenylketonuria (PKU) Demographics, Outcomes and Safety (PKUDOS) registry is designed to provide longitudinal safety and efficacy data on subjects with PKU who are (or have been) treated with sapropterin dihydrochloride. The PKUDOS population consists of 1189 subjects with PKU: N = 504 who were continuously exposed to sapropterin from date of registry enrollment, N = 211 who had intermittent exposure to the drug, and N = 474 with some other duration of exposure. Subjects continuously exposed to sapropterin showed an average 34% decrease in blood phenylalanine (Phe)--from 591 ± 382 μmol/L at baseline to 392 ± 239 μmol/L (p = 0.0009) after 5 years. This drop in blood Phe was associated with an increase in dietary Phe tolerance [from 1000 ± 959 mg/day (pre-sapropterin baseline) to 1539 ± 840 mg/day after 6 years]. Drug-related adverse events (AEs) were reported in 6% of subjects, were mostly considered non-serious, and were identified in the gastrointestinal, respiratory, and nervous systems. Serious drug-related AEs were reported in ≤ 1% of subjects. Similar safety and efficacy data were observed for children<4 years. Long-term data from the PKUDOS registry suggest that sapropterin has a tolerable safety profile and that continuous use is associated with a significant and persistent decrease in blood Phe and improvements in dietary Phe tolerance.

A requirement for Gch1 and tetrahydrobiopterin in embryonic development

INTRODUCTION

GTP cyclohydrolase I (GTPCH) catalyses the first and rate-limiting reaction in the synthesis of the enzymatic cofactor, tetrahydrobiopterin (BH4). Loss of function mutations in the GCH1 gene lead to congenital neurological diseases such as DOPA-responsive dystonia and hyperphenylalaninemia. However, little is known about how GTPCH and BH4 affects embryonic development in utero, and in particular whether metabolic replacement or supplementation in pregnancy is sufficient to rescue genetic GTPCH deficiency in the developing embryo.

METHODS AND RESULTS

Gch1 deficient mice were generated by the insertion of loxP sites flanking exons 2-3 of the Gch1 gene. Gch1(fl/fl) mice were bred with Sox2cre mice to generate mice with global Gch1 deficiency. Genetic ablation of Gch1 caused embryonic lethality by E13.5. Despite loss of Gch1 mRNA and GTPCH enzymatic activity, whole embryo BH4 levels were maintained until E11.5, indicating sufficient maternal transfer of BH4 to reach this stage of development. After E11.5, Gch1(-/-) embryos were deficient in BH4, but an unbiased metabolomic screen indicated that the lethality was not due to a gross disturbance in metabolic profile. Embryonic lethality in Gch1(-/-) embryos was not caused by structural abnormalities, but was associated with significant bradycardia at E11.5. Embryonic lethality was not rescued by maternal supplementation of BH4, but was partially rescued, up to E15.5, by maternal supplementation of BH4 and l-DOPA.

CONCLUSION

These findings demonstrate a requirement for Gch1 in embryonic development and have important implications for the understanding of pathogenesis and treatment of genetic BH4 deficiencies, as well as the identification of new potential roles for BH4.

Inflammation and oxidative stress caused by nitric oxide synthase uncoupling might lead to left ventricular diastolic and systolic dysfunction in patients with hypertension

OBJECTIVE

To investigate the role of oxidative stress, inflammation, hypercoagulability and neuroendocrine activation in the transition of hypertensive heart disease to heart failure with preserved ejection fraction (HFPEF).

METHODS

We performed echocardiography for 112 patients (≥ 60 years old) with normal EF (18 controls and 94 with hypertension), and determined protein carbonylation (PC), and tetrahydrobiopterin (BH4), C-reactive protein (CRP), interleukin-6 (IL-6), tumor necrosis factor-α (TNF-α), fibrinogen, plasminogen activator inhibitor type-I (PAI-I), von Willebrand factor, chromogranin A (cGA) and B-type natriuretic peptide (BNP) levels from their blood samples.

RESULTS

We found that 40% (38/94) of the patients with hypertension (HT) had no diastolic dysfunction (HTDD-), and 60% (56/94) had diastolic dysfunction (HTDD+). Compared to the controls, both patient groups had increased PC and BH4, TNF-α, PAI-I and BNP levels, while the HTDD+ group had elevated cGA and CRP levels. Decreased atrial and longitudinal left ventricular (LV) systolic and diastolic myocardial deformation (strain and strain rate) was demonstrated in both patient groups versus the control. Patients whose LV diastolic function deteriorated during the follow-up had elevated PC and IL-6 level compared to their own baseline values, and to the respective values of patients whose LV diastolic function remained unchanged. Oxidative stress, inflammation, BNP and PAI-I levels inversely correlated with LV systolic, diastolic and atrial function.

CONCLUSIONS

In patients with HT and normal EF, the most common HFPEF precursor condition, oxidative stress and inflammation may be responsible for LV systolic, diastolic and atrial dysfunction, which are important determinants of the transition of HT to HFPEF.

Erythropoietin prevents endothelial dysfunction in GTP-cyclohydrolase I-deficient hph1 mice

: In this study, we used the mutant hph1 mouse model, which has deficiency in GTP-cyclohydrolase I (GTPCH I) activity, to test the hypothesis that erythropoietin (EPO) protects aortic wall from oxidative stress induced by uncoupling of endothelial nitric oxide synthase (eNOS). Both GTPCH I activity and tetrahydrobiopterin (BH4) levels were reduced in hph1 mice, whereas 7,8-dihydrobiopterin (7,8-BH2) levels were significantly increased. Furthermore, BH4 deficiency caused increased production of superoxide anion and hydrogen peroxide in the aorta thus resulting in impairment of endothelium-dependent relaxations to acetylcholine. Treatment of hph1 mice with recombinant human EPO (1000 U/kg, subcutaneously for 3 days) significantly decreased superoxide anion production by eNOS and improved BH4 to 7,8-BH2 ratio in aortas. EPO also significantly decreased production of hydrogen peroxide and improved endothelium-dependent relaxations in aortas of hph1 mice. In addition, EPO treatment increased protein expressions of copper-/zinc-superoxide dismutase, manganese-superoxide dismutase, and catalase in the aorta of hph1 mice. Our findings demonstrate that treatment with EPO prevented oxidative stress and endothelial dysfunction caused by eNOS uncoupling. Increased vascular expressions of antioxidants seem to be an important molecular mechanism underlying vascular protection by EPO during chronic BH4 deficiency.

Crucial role for neuronal nitric oxide synthase in early microcirculatory derangement and recipient survival following murine pancreas transplantation

OBJECTIVE

Aim of this study was to identify the nitric oxide synthase (NOS) isoform involved in early microcirculatory derangements following solid organ transplantation.

BACKGROUND

Tetrahydrobiopterin donor treatment has been shown to specifically attenuate these derangements following pancreas transplantation, and tetrahydrobiopterin-mediated protective effects to rely on its NOS-cofactor activity, rather than on its antioxidant capacity. However, the NOS-isoform mainly involved in this process has still to be defined.

METHODS

Using a murine pancreas transplantation model, grafts lacking one of the three NOS-isoforms were compared to grafts from wild-type controls. Donors were treated with either tetrahydrobiopterin or remained untreated. All grafts were subjected to 16 h cold ischemia time and transplanted into wild-type recipients. Following 4 h graft reperfusion, microcirculation was analysed by confocal intravital fluorescence microscopy. Recipient survival was monitored for 50 days.

RESULTS

Transplantation of the pancreas from untreated wild-type donor mice resulted in microcirculatory damage of the transplanted graft and no recipient survived more than 72 h. Transplanting grafts from untreated donor mice lacking either endothelial or inducible NOS led to similar outcomes. In contrast, donor treatment with tetrahydrobiopterin prevented microcirculatory breakdown enabling long-term survival. Sole exception was transplantation of grafts from untreated donor mice lacking neuronal NOS. It resulted in intact microvascular structure and long-term recipient survival, either if donor mice were untreated or treated with tetrahydrobiopterin.

CONCLUSION

We demonstrate for the first time the crucial involvement of neuronal NOS in early microcirculatory derangements following solid organ transplantation. In this model, protective effects of tetrahydrobiopterin are mediated by targeting this isoform.

Acute tetrahydrobiopterin supplementation attenuates sympathetic vasoconstrictor responsiveness in resting and contracting skeletal muscle of healthy rats

Tetrahydrobiopterin (BH₄) is an essential cofactor for the production of nitric oxide (NO) and supplementation with BH₄ improves NO‐dependent vasodilation. NO also reduces sympathetic vasoconstrictor responsiveness in resting and contracting skeletal muscle. Thus, we hypothesized that supplementation with BH₄ would blunt sympathetic vasoconstrictor responsiveness in resting and contracting skeletal muscle. Sprague‐Dawley rats (n = 15, 399 ± 57 g) were anesthetized and instrumented with an indwelling brachial artery catheter, femoral artery flow probe, and a stimulating electrode on the lumbar sympathetic chain. Triceps surae muscles were stimulated to contract rhythmically at 30% and 60% of maximal contractile force (MCF). The percentage change of femoral vascular conductance (%FVC) in response to sympathetic stimulations delivered at 2 and 5 Hz was determined at rest and during muscle contraction in control and acute BH₄ supplementation (20 mg·kg⁻¹ + 10 mg·kg⁻¹·h⁻¹, IA) conditions. BH₄ reduced (P < 0.05) the vasoconstrictor response to sympathetic stimulation (i.e., decrease in FVC) at rest (Control: 2 Hz: −28 ± 5%FVC; 5 Hz: −45 ± 5%; BH₄: 2 Hz: −17 ± 4%FVC; 5 Hz: −34 ± 7%FVC) and during muscular contraction at 30% MCF (Control: 2 Hz: −14 ± 6%FVC; 5 Hz: −28 ± 11%; BH₄: 2 Hz: −6 ± 6%FVC; 5 Hz: −16 ± 10%) and 60% MCF (Control: 2 Hz: −7 ± 3%FVC; 5 Hz: −16 ± 6%FVC; BH₄: 2 Hz: −2 ± 3%FVC; 5 Hz: −11 ± 6%FVC). These data are consistent with our hypothesis that acute BH₄ supplementation decreases sympathetic vasoconstrictor responsiveness in resting and contracting skeletal muscle.

Tetrahydrobiopterin (BH4) is an essential cofactor for the production of nitric oxide (NO) and NO reduces sympathetic vasoconstrictor responsiveness in the skeletal muscle vascular bed. Thus, we hypothesized that supplementation with BH4 would blunt sympathetic vasoconstrictor responsiveness. The data demonstrate that acute BH4 supplementation decreases sympathetic vasoconstrictor responsiveness in resting and contracting skeletal muscle.

Effects of sapropterin on endothelium-dependent vasodilation in patients with CADASIL: a randomized controlled trial

BACKGROUND AND PURPOSE

Cerebral autosomal-dominant arteriopathy with subcortical infarcts and leukoencephalopathy (CADASIL), a rare autosomal dominant disorder caused by NOTCH3 mutations, is characterized by vascular smooth muscle and endothelial cells abnormalities, altered vasoreactivity, and recurrent lacunar infarcts. Vasomotor function may represent a key factor for disease progression. Tetrahydrobiopterin, essential cofactor for nitric oxide synthesis in endothelial cells, ameliorates endothelial function. We assessed whether supplementation with sapropterin, a synthetic tetrahydrobiopterin analog, improves endothelium-dependent vasodilation in CADASIL patients.

METHODS

In a 24-month, multicenter randomized, double-blind, placebo-controlled trial, CADASIL patients aged 30 to 65 years were randomly assigned to receive placebo or sapropterin 200 to 400 mg BID. The primary end point was change in the reactive hyperemia index by peripheral arterial tonometry at 24 months. We also assessed the safety and tolerability of sapropterin. Analysis was done by intention-to-treat.

RESULTS

The intention-to-treat population included 61 patients. We found no significant difference between sapropterin (n=32) and placebo (n=29) in the primary end point (mean difference in reactive hyperemia index by peripheral arterial tonometry changes 0.19 [95% confidence interval, -0.18, 0.56]). Reactive hyperemia index by peripheral arterial tonometry increased after 24 months in 37% of patients on sapropterin and in 28% on placebo; however, after adjustment for age, sex, and clinical characteristics, improvement was not associated with treatment arm. The proportion of patients with adverse events was similar on sapropterin and on placebo (50% versus 48.3%); serious adverse events occurred in 6.3% versus 13.8%, respectively.

CONCLUSIONS

Sapropterin was safe and well-tolerated at the average dose of 5 mg/kg/day, but did not affect endothelium-dependent vasodilation in CADASIL patients.

CLINICAL TRIAL REGISTRATION URL

https://www.clinicaltrialsregister.eu. Unique identifier: 2007-004370-55.

Mechanisms of vascular dysfunction in mice with endothelium-specific deletion of the PPAR-δ gene

Peroxisome proliferator-activated receptor (PPAR)-δ is a nuclear hormone receptor that is mainly involved in lipid metabolism. Recent studies have suggested that PPAR-δ agonists exert vascular protective effects. The present study was designed to characterize vascular function in mice with genetic inactivation of PPAR-δ in the endothelium. Mice with vascular endothelial cell-specific deletion of the PPAR-δ gene (ePPARδ(-/-) mice) were generated using loxP/Cre technology. ePPARδ(-/-) mice were normotensive and did not display any sign of metabolic syndrome. Endothelium-dependent relaxations to ACh and endothelium-independent relaxations to the nitric oxide (NO) donor diethylammonium (Z)-1-(N,N-diethylamino)diazen-1-ium-1,2-diolate were both significantly impaired in the aorta and carotid arteries of ePPARδ(-/-) mice (P < 0.05). In ePPARδ(-/-) mouse aortas, phosphorylation of endothelial NO synthase at Ser(1177) was significantly decreased (P < 0.05). However, basal levels of cGMP were unexpectedly increased (P < 0.05). Enzymatic activity of GTP-cyclohydrolase I and tetrahydrobiopterin levels were also enhanced in ePPARδ(-/-) mice (P < 0.05). Most notably, endothelium-specific deletion of the PPAR-δ gene significantly decreased protein expressions of catalase and glutathione peroxidase 1 and resulted in increased levels of H2O2 in the aorta (P < 0.05). In contrast, superoxide anion production was unaltered. Moreover, treatment with catalase prevented the endothelial dysfunction and elevation of cGMP detected in aortas of ePPARδ(-/-) mice. The findings suggest that increased levels of cGMP caused by H2O2 impair vasodilator reactivity to endogenous and exogenous NO. We speculate that chronic elevation of H2O2 predisposes PPAR-δ-deficient arteries to oxidative stress and vascular dysfunction.

Correlation between biopterin levels and intimal-media thickness in type-2 diabetic hypertensive patients

Background

Biopterins have a crucial role in the function of nitric oxide synthase, uncoupling of the enzyme leads to endothelial dysfunction and vascular damage, The aim of this study was to evaluate the relationship between the levels of biopterins with carotid intima-media thickness (CIMT) in hypertensive type-2 diabetic patients.

Methods

We studied 30 hypertensive type-2 diabetic patients and 30 normotensive non-diabetic age-matched subjects, in whom biopterins levels were measured by reverse phase high performance liquid chromatography with fluorescence detection. Additionally, the CIMT of both the common and internal carotid arteries was measured. The levels of biopterins and CIMT were correlated using the Pearson correlation coefficient test.

Results

We did not find a significantly correlation between biopterins levels and CIMT. However, we found a significantly inverse correlation between the BH4/BH2 ratio and the CIMT in patients (r = -0.54, p < 0.01). A multiple regression analysis revealed that the CIMT correlated significantly and independently with the BH4/BH2 ratio.

Conclusion

Our results suggest that the BH4/BH2 ratio seems to be a better marker of vascular disease than biopterin levels.

Arginine and nitric oxide synthase: regulatory mechanisms and cardiovascular aspects

L-Arginine (L-Arg) is a conditionally essential amino acid in the human diet. The most common dietary sources of L-Arg are meat, poultry and fish. L-Arg is the precursor for the synthesis of nitric oxide (NO); a key signaling molecule via NO synthase (NOS). Endogenous NOS inhibitors such as asymmetric-dimethyl-L-Arg inhibit NO synthesis in vivo by competing with L-Arg at the active site of NOS. In addition, NOS possesses the ability to be "uncoupled" to produce superoxide anion instead of NO. Reduced NO bioavailability may play an essential role in cardiovascular pathologies and metabolic diseases. L-Arg deficiency syndromes in humans involve endothelial inflammation and immune dysfunctions. Exogenous administration of L-Arg restores NO bioavailability, but it has not been possible to demonstrate, that L-Arg supplementation improved endothelial function in cardiovascular disease such as heart failure or hypertension. L-Arg supplementation may be a novel therapy for obesity and metabolic syndrome. The utility of l-Arg supplementation in the treatment of L-Arg deficiency syndromes remains to be established. Clinical trials need to continue to determine the optimal concentrations and combinations of L-Arg, with other protective compounds such as tetrahydrobiopterin (BH4 ), and antioxidants to combat oxidative stress that drives down NO production in humans.

Endothelial nitric oxide: protector of a healthy mind

Endothelial nitric oxide (NO) is generated by constitutively active endothelial nitric oxide synthase (eNOS), an essential enzyme responsible for cardiovascular homeostasis. Historically, endothelial NO was first recognized as a major vasodilator involved in control of vasomotor function and local blood flow. In this review, our attention is focused on the emerging role of endothelial NO in linking cerebrovascular function with cognition. We will discuss the recognized ability of endothelial NO to modulate processing of amyloid precursor protein (APP), influence functional status of microglia, and affect cognitive function. Existing evidence suggests that the loss of NO in cultured human cerebrovascular endothelium causes increased expression of APP and β-site APP-cleaving enzyme 1 (BACE1) thereby resulting in increased secretion of amyloid β peptides (Aβ1-40 and Aβ1-42). Furthermore, increased expression of APP and BACE1 as well as increased production of Aβ peptides was detected in the cerebral microvasculature and brain tissue of eNOS-deficient mice. Since Aβ peptides are considered major cytotoxic molecules responsible for the pathogenesis of Alzheimer's disease, these observations support the concept that a loss of endothelial NO might significantly contribute to the initiation and progression of cognitive decline. In addition, genetic inactivation of eNOS causes activation of microglia and promotes a pro-inflammatory phenotype in the brain. Behavioural analysis revealed that eNOS-deficient mice exhibit impaired cognitive performance thereby indicating that selective loss of endothelial NO has a detrimental effect on the function of neuronal cells. Together with findings from prior studies demonstrating the ability of endothelial NO to affect synaptic plasticity, mitochondrial biogenesis, and function of neuronal progenitor cells, it is becoming apparent that the role of endothelial NO in the control of central nervous system function is very complex. We propose that endothelial NO represents the key molecule linking cerebrovascular and neuronal function.

Biopterin metabolism and eNOS expression during hypoxic pulmonary hypertension in mice

Tetrahydrobiopterin (BH4), which fosters the formation of and stabilizes endothelial NO synthase (eNOS) as an active dimer, tightly regulates eNOS coupling / uncoupling. Moreover, studies conducted in genetically-modified models demonstrate that BH4 pulmonary deficiency is a key determinant in the pathogenesis of pulmonary hypertension. The present study thus investigates biopterin metabolism and eNOS expression, as well as the effect of sepiapterin (a precursor of BH4) and eNOS gene deletion, in a mice model of hypoxic pulmonary hypertension. In lungs, chronic hypoxia increased BH4 levels and eNOS expression, without modifying dihydrobiopterin (BH2, the oxidation product of BH4) levels, GTP cyclohydrolase-1 or dihydrofolate reductase expression (two key enzymes regulating BH4 availability). In intrapulmonary arteries, chronic hypoxia also increased expression of eNOS, but did not induce destabilisation of eNOS dimers into monomers. In hypoxic mice, sepiapterin prevented increase in right ventricular systolic pressure and right ventricular hypertrophy, whereas it modified neither remodelling nor alteration in vasomotor responses (hyper-responsiveness to phenylephrine, decrease in endothelium-dependent relaxation to acetylcholine) in intrapulmonary arteries. Finally, deletion of eNOS gene partially prevented hypoxia-induced increase in right ventricular systolic pressure, right ventricular hypertrophy and remodelling of intrapulmonary arteries. Collectively, these data demonstrate the absence of BH4/BH2 changes and eNOS dimer destabilisation, which may induce eNOS uncoupling during hypoxia-induced pulmonary hypertension. Thus, even though eNOS gene deletion and sepiapterin treatment exert protective effects on hypoxia-induced pulmonary vascular remodelling, increase on right ventricular pressure and / or right ventricular hypertrophy, these effects appear unrelated to biopterin-dependent eNOS uncoupling within pulmonary vasculature of hypoxic wild-type mice.