Arginosuccinate synthetase, arginosuccinate lyase and NOS in canine gastrointestinal tract: immunocytochemical studies

Role of oxidative stress in oxaliplatin-induced enteric neuropathy and colonic dysmotility in mice

BACKGROUND AND PURPOSE

Oxaliplatin is a platinum-based chemotherapeutic drug used as a first-line therapy for colorectal cancer. However, its use is associated with severe gastrointestinal side-effects resulting in dose limitations and/or cessation of treatment. In this study, we tested whether oxidative stress, caused by chronic oxaliplatin treatment, induces enteric neuronal damage and colonic dysmotility.

EXPERIMENTAL APPROACH

Oxaliplatin (3 mg·kg^-1 per day) was administered in vivo to Balb/c mice intraperitoneally three times a week. The distal colon was collected at day 14 of treatment. Immunohistochemistry was performed in wholemount preparations of submucosal and myenteric ganglia. Neuromuscular transmission was studied by intracellular electrophysiology. Circular muscle tone was studied by force transducers. Colon propulsive activity studied in organ bath experiments and faeces were collected to measure water content.

KEY RESULTS

Chronic in vivo oxaliplatin treatment resulted in increased formation of reactive oxygen species (O₂ -), nitration of proteins, mitochondrial membrane depolarisation resulting in the release of cytochrome c, loss of neurons, increased inducible NOS expression and apoptosis in both the submucosal and myenteric plexuses of the colon. Oxaliplatin treatment enhanced NO-mediated inhibitory junction potentials and altered the response of circular muscles to the NO donor, sodium nitroprusside. It also reduced the frequency of colonic migrating motor complexes and decreased circular muscle tone, effects reversed by the NO synthase inhibitor, Nω-Nitro-L-arginine.

CONCLUSION AND IMPLICATIONS

Our study is the first to provide evidence that oxidative stress is a key player in enteric neuropathy and colonic dysmotility leading to symptoms of chronic constipation observed in oxaliplatin-treated mice.

Extrarenal citrulline disposal in mice with impaired renal function

The endogenous synthesis of arginine, a semiessential amino acid, relies on the production of citrulline by the gut and its conversion into arginine by the kidney in what has been called the "intestinal-renal axis" for arginine synthesis. Although the kidney is the main site for citrulline disposal, it only accounts for ~60-70% of the citrulline produced. Because the only known fate for citrulline is arginine synthesis and the enzymes that catalyze this reaction are widespread among body tissues, we hypothesized that citrulline can be utilized directly by tissues to meet, at least partially, their arginine needs. To test this hypothesis, we used stable and radioactive tracers in conscious, partially nephrectomized (½ and ⅚) and anesthetized acutely kidney-ligated mouse models. Nephrectomy increased plasma citrulline concentration but did not affect citrulline synthesis rates, thus reducing its clearance. Nephrectomy (⅚) reduced the amount of citrulline accounted for as plasma arginine from 88 to 42%. Acute kidney ligation increased the half-life and mean retention time of citrulline. Whereas the rate of citrulline conversion into plasma arginine was reduced, it was not eliminated. In addition, we observed direct utilization of citrulline for arginine synthesis and further incorporation into tissue protein in kidney-ligated mice. These observations indicate that a fraction of the citrulline produced is utilized directly by multiple tissues to meet their arginine needs and that extrarenal sites contribute to plasma arginine. Furthermore, when the interorgan synthesis of arginine is impaired, these extrarenal sites are able to increase their rate of citrulline utilization.

The role of arginine and arginine-metabolizing enzymes during Giardia - host cell interactions in vitro

BACKGROUND

Arginine is a conditionally essential amino acid important in growing individuals and under non-homeostatic conditions/disease. Many pathogens interfere with arginine-utilization in host cells, especially nitric oxide (NO) production, by changing the expression of host enzymes involved in arginine metabolism. Here we used human intestinal epithelial cells (IEC) and three different isolates of the protozoan parasite Giardia intestinalis to investigate the role of arginine and arginine-metabolizing enzymes during intestinal protozoan infections.

RESULTS

RNA expression analyses of major arginine-metabolizing enzymes revealed the arginine-utilizing pathways in human IECs (differentiated Caco-2 cells) grown in vitro. Most genes were constant or down-regulated (e.g. arginase 1 and 2) upon interaction with Giardia, whereas inducible NO synthase (iNOS) and ornithine decarboxylase (ODC) were up-regulated within 6 h of infection. Giardia was shown to suppress cytokine-induced iNOS expression, thus the parasite has both iNOS inducing and suppressive activities. Giardial arginine consumption suppresses NO production and the NO-degrading parasite protein flavohemoglobin is up-regulated in response to host NO. In addition, the secreted, arginine-consuming giardial enzyme arginine deiminase (GiADI) actively reduces T-cell proliferation in vitro. Interestingly, the effects on NO production and T cell proliferation could be reversed by addition of external arginine or citrulline.

CONCLUSIONS

Giardia affects the host's arginine metabolism on many different levels. Many of the effects can be reversed by addition of arginine or citrulline, which could be a beneficial supplement in oral rehydration therapy.

Arginine homeostasis in allergic asthma

Allergic asthma is a chronic disease characterized by early and late asthmatic reactions, airway hyperresponsiveness, airway inflammation and airway remodelling. Changes in l-arginine homeostasis may contribute to all these features of asthma by decreased nitric oxide (NO) production and increased formation of peroxynitrite, polyamines and l-proline. Intracellular l-arginine levels are regulated by at least three distinct mechanisms: (i) cellular uptake by cationic amino acid (CAT) transporters, (ii) metabolism by NO-synthase (NOS) and arginase, and (iii) recycling from l-citrulline. Ex vivo studies using animal models of allergic asthma have indicated that attenuated l-arginine bioavailability to NOS causes deficiency of bronchodilating NO and increased production of procontractile peroxynitrite, which importantly contribute to allergen-induced airway hyperresponsiveness after the early and late asthmatic reaction, respectively. Decreased cellular uptake of l-arginine, due to (eosinophil-derived) polycations inhibiting CATs, as well as increased consumption by increased arginase activity are major causes of substrate limitation to NOS. Increasing substrate availability to NOS by administration of l-arginine, l-citrulline, the polycation scavenger heparin, or an arginase inhibitor alleviates allergen-induced airway hyperresponsiveness by restoring the production of bronchodilating NO. In addition, reduced l-arginine levels may contribute to the airway inflammation associated with the development of airway hyperresponsiveness, which similarly may involve decreased NO synthesis and increased peroxynitrite formation. Increased arginase activity could also contribute to airway remodelling and persistent airway hyperresponsiveness in chronic asthma via increased synthesis of l-ornithine, the precursor of polyamines and l-proline. Drugs that increase the bioavailability of l-arginine in the airways - particularly arginase inhibitors - may have therapeutic potential in allergic asthma.

Ubiquitous presence of argininosuccinate at millimolar levels in the central nervous system of Aplysia californica

Endogenous nitric oxide (NO) is generated by nitric oxide synthases (NOSs), which convert arginine (Arg) and oxygen to citrulline (Cit) and NO. Cit can be enzymatically transformed back to Arg by argininosuccinate synthetase (ASS) and argininosuccinate lyase (ASL) via a pathway involving argininosuccinate (ArgSuc). Arg, Cit, and ArgSuc levels have been measured in single neurons, neuronal clusters, and neuropil from the nervous system of the common neurobiological model Aplysia californica. Using capillary electrophoresis with laser-induced fluorescence detection, ArgSuc was found to be present in the nervous system in millimolar concentrations at levels significantly exceeding Cit levels (p<0.01). ArgSuc levels are proportional to Arg concentrations in single neurons, whereas they have no clear correlation to the Cit or Arg/Cit ratio. NOS-expressing neurons often exhibit fixative-resistant nicotinamide adenine dinucleotide phosphate-diaphorase (NADPH-d) staining. Incubation of ganglia with Arg results in an increase in Cit and ArgSuc levels in the NADPH-d-positive neuropil with no effect on ArgSuc levels in NADPH-d-negative neurons, suggesting NOS activity in the neuropil. Similar incubation with Cit leads to decreased ArgSuc levels in NADPH-d-negative neurons. These results can be explained by localization of NOS and ASS in different neurons; therefore, the complete Arg-Cit-NO cycle may not be present in the same neuron. The surprisingly high intracellular ArgSuc concentration suggests alternative sources of ArgSuc and that at least a portion may be formed by the reverse reaction of ASL (catalyzing the conversion of Arg to ArgSuc), which can be inhibited by Cit.

Role of the l-citrulline/l-arginine cycle in iNANC nerve-mediated nitric oxide production and airway smooth muscle relaxation in allergic asthma

Nitric oxide synthase (NOS) converts L-arginine into nitric oxide (NO) and L-citrulline. In NO-producing cells, L-citrulline can be recycled to L-arginine in a two-step reaction involving argininosuccinate synthase (ASS) and -lyase (ASL). In guinea pig trachea, L-arginine is a limiting factor in neuronal nNOS-mediated airway smooth muscle relaxation upon inhibitory nonadrenergic noncholinergic (iNANC) nerve stimulation. Moreover, in a guinea pig model of asthma iNANC nerve-induced NO production and airway smooth muscle relaxation are impaired after the allergen-induced early asthmatic reaction, due to limitation of L-arginine. Using guinea pig tracheal preparations, we now investigated whether (i) the L-citrulline/L-arginine cycle is active in airway iNANC nerves and (ii) the NO deficiency after the early asthmatic reaction involves impaired L-citrulline recycling. Electrical field stimulation-induced relaxation was measured in tracheal open-rings precontracted with histamine. L-citrulline as well as the ASL inhibitor succinate did not affect electrical field stimulation-induced relaxation under basal conditions. However, reduced relaxation induced by a submaximal concentration of the NOS inhibitor N(omega)-nitro-L-arginine was restored by L-citrulline, which was prevented by the additional presence of succinate or the ASS inhibitor alpha-methyl-D,L-aspartate. Remarkably, the impaired iNANC relaxation after the early asthmatic reaction was restored by L-citrulline. In conclusion, the L-citrulline/L-arginine cycle is operative in guinea pig iNANC nerves in the airways and may be effective under conditions of low L-arginine utilization by nNOS (caused by NOS inhibitors), and during reduced L-arginine availability after allergen challenge. Enzymatic dysfunction in the L-citrulline/L-arginine cycle appears not to be involved in the L-arginine limitation and reduced iNANC activity after the early asthmatic reaction.

Caveolae and calcium handling, a review and a hypothesis

Caveolae are associated with molecules crucial for calcium handling. This review considers the roles of caveolae in calcium handling for smooth muscle and interstitial cells of Cajal (ICC). Structural studies showed that the plasma membrane calcium pump (PMCA), a sodium-calcium exchanger (NCX1), and a myogenic nNOS appear to be colocalized with caveolin I, the main constituent of these caveolae. Voltage dependent calcium channels (VDCC) are associated but not co-localized with caveolin 1, as are proteins of the peripheral sarcoplasmic reticulum (SR) such as calreticulin. Only the nNOS is absent from caveolin 1 knockout animals. Functional studies in calcium free media sugest that a source of calcium in tonic smooth muscles exists, partly sequestered from extracellular EGTA. This source supported sustained contractions to carbachol using VDCC and dependent on activity of the SERCA pump. This source is postulated to be caveolae, near peripheral SR. New evidence, presented here, suggests that a similar source exists in phasic smooth muscle of the intestine and its ICC. These results suggest that caveolae and peripheral SR are a functional unit recycling calcium through VDCC and controlling its local concentration. Calcium handling molecules associated with caveolae in smooth muscle and ICC were identified and their possible functions also reviewed.

The effects of exogenous amino acids on the relaxant responses of pig urethral smooth muscle evoked by stimulation of the inhibitory nitrergic nerves

Inhibitory innervation of urethral smooth muscle is mediated partly through release of NO. We investigated the mechanisms involved in the supply of the substrate L: -arginine to NO synthase by examining the relaxant response of the muscle to electrical field stimulation (EFS) and the effects of addition of amino acids to the bathing medium. Relaxant responses persisted during hours of repetitive stimulation but were enhanced rapidly by addition of L: -arginine (the "arginine paradox"). Addition of L: -lysine (competes with L: -arginine for transport on the y(+) carrier) and L: -glutamine (competing on the y(+)L carrier) attenuated the enhancement. Enhancement persisted after washing but was reversed by application of L: -lysine, suggesting that exogenous L: -arginine fills an intracellular pool and that L: -lysine can trans-stimulate its efflux from the pool. After prolonged depolarization in high-K(+), Na(+)-free solution the relaxant response became purely nitrergic. Addition of L: -arginine during the exposure continued to enhance the subsequent responses but L: -glutamine added with L: -arginine, could no longer reduce this enhancement. The results show the arginine paradox in inhibitory nerves and suggest the involvement of y(+) and y(+)L carriers in the transport of L: -arginine.

Arginine metabolic enzymes, nitric oxide and infection

Nitric oxide (NO) is synthesized from arginine by NO synthase (NOS), and the availability of arginine is one of the rate-limiting factors in cellular NO production. Citrulline that is formed as a by-product of the NOS reaction can be recycled to arginine by successive actions of argininosuccinate synthetase (AS) and argininosuccinate lyase (AL), forming the citrulline-NO cycle. AS and sometimes AL have been shown to be coinduced with inducible NOS (iNOS) in various cell types including activated macrophages, microglia, vascular smooth muscle cells, glial cells, neuronal PC12 cells, retinal pigment epithelial cells, and pancreatic beta-cells. Coinduction of endothelial NOS (eNOS), AS, and AL are observed in human umbilical vein endothelial cells. In contrast, arginase can downregulate NO production by decreasing intracellular arginine concentrations. iNOS and arginase activities are regulated reciprocally in macrophages by cytokines, and this may guarantee the efficient production of NO. In contrast, iNOS and arginase isoforms (type I and/or II) are coinduced in immunostimulated macrophages, but not in PC12 cells and glial cells. These results indicate that NO production is modulated by the recycling and degradation of arginine. Arginase also plays an important role in regulation of polyamine and proline synthesis.

Argininosuccinate synthase expression is required to maintain nitric oxide production and cell viability in aortic endothelial cells

Although cellular levels of arginine greatly exceed the apparent K(m) for endothelial nitric-oxide synthase, current evidence suggests that the bulk of this arginine may not be available for nitric oxide (NO) production. We propose that arginine regeneration, that is the recycling of citrulline back to arginine, defines the essential source of arginine for NO production. To support this proposal, RNA interference analysis was used to selectively reduce the expression of argininosuccinate synthase (AS), because the only known metabolic role for AS in endothelial cells is in the regeneration of l-arginine from l-citrulline. Western blot analysis demonstrated a significant and dose-dependent reduction of AS protein as a result of AS small interfering RNA treatment with a corresponding diminished capacity to produce basal or stimulated levels of NO, despite saturating levels of arginine in the medium. Unanticipated, however, was the finding that the viability of AS small interfering RNA-treated endothelial cells was significantly decreased when compared with control cells. Trypan blue exclusion analysis suggested that the loss of viability was not because of necrosis. Two indicators, reduced expression of Bcl-2 and an increase in caspase activity, which correlated directly with reduced expression of AS, suggested that the loss of viability was because of apoptosis. The exposure of cells to an NO donor prevented apoptosis associated with reduced AS expression. Overall, these results demonstrate the essential role of AS for endothelial NO production and cell viability.

L-citrulline recycling by argininosuccinate synthetase and lyase in rat gastric fundus

The aim of this study was to investigate in rat gastric fundus whether L-citrulline, the co-product in the nitric oxide (NO) biosynthesis catalyzed by neuronal nitric oxide synthase (nNOS), can be converted back to the nNOS substrate L-arginine. Immunohistochemistry showed that argininosuccinate synthetase and argininosuccinate lyase, that mediate transformation of L-citrulline to L-arginine in the ureum cycle in hepatocytes, co-localize with nNOS. In longitudinal smooth muscle strips, L-arginine as well as L-citrulline (10(-3) M) was capable of completely respectively partially preventing the N(G)-nitro-L-arginine methyl ester (L-NAME) (3 x 10(-5) M)-induced inhibition of electrically induced nitrergic relaxations, whereas D-citrulline (10(-3) M) was not. The L-citrulline-mediated prevention of the L-NAME-induced inhibition was reduced by L-glutamine (3 x 10(-3) M), the putative L-citrulline uptake inhibitor, and by succinate, an argininosuccinate lyase inhibitor. The results demonstrate that the L-citrulline recycling mechanism is active in rat gastric fundus. Recycling of L-citrulline might play a role in providing sufficient amounts of nNOS substrate during long-lasting relaxations in gastric fundus after food intake.

Role of extrinsic innervation in modulating nitrergic transmission in the canine ileocolonic region

The human colon can dilate, often to life-threatening proportions. Our aim was to explore nitrergic mechanisms underlying colonic dilation in conscious dogs with enterically isolated ileocolonic loops either extrinsically innervated (n = 4) or extrinsically denervated (n = 4). We recorded phasic pressures in ileum and ileocolonic sphincter (ICS), colonic tone, compliance, and relaxation during ileal distension. By NADPH-diaphorase histochemistry, we assessed effects of extrinsic denervation and enteric isolation on nitrergic fibers. Extrinsic denervation increased phasic pressures in ileum, ICS, and colon and abolished ICS and colonic relaxation in response to ileal distension. The nitric oxide synthase (NOS) inhibitor N(omega)-nitro-L-arginine (L-NNA) increased phasic pressures at all sites and ICS tone but did not abolish colonic relaxation during ileal distension in innervated loops. L-NNA reduced compliance and induced colonic high-amplitude propagated contractions in denervated loops. The NOS substrate donor L-arginine reversed effects of L-NNA. The number of NADPH-diaphorase fibers increased in both enterically isolated preparations. Nonnitrergic extrinsic nerve pathways mediate reflex colonic relaxation during ileal distension. Enteric isolation augments the number of NOS fibers, an effect not modified by extrinsic denervation.

Physiology and pathophysiology of the interstitial cell of Cajal: from bench to bedside. III. Interaction of interstitial cells of Cajal with neuromediators: an interim assessment

Interstitial cells of Cajal (ICC) control gastrointestinal motility; some pace slow waves and others act in enteric neurotransmission. This review asks the question, does either class of ICC receive and respond to messages carried by neuromediators from these nerves? Relevant evidence includes the presence of receptors or responses to exogenous neuromediators and responses to endogenous neuromediators. Some pacemaking ICC networks have receptors for or respond to some exogenous neuromediators. None is known to respond to endogenous neuromediators. Intramuscular ICC have receptors for and respond to some neuromediators and are required in mice for responses to the exogenous and endogenous neuromediators nitric oxide and acetylcholine. The mechanisms underlying this requirement remain unclear. ICC pathologies exist, but their origins are unknown.

nNOS in canine lower esophageal sphincter: colocalized with Cav-1 and Ca2+-handling proteins?

Immunochemical studies with light microscopy, confocal microscopy, and electron microscopy were used to examine proteins associated with caveolin (Cav) in canine lower esophageal sphincter. The main Cav was Cav-1. It appeared to be colocalized at the cell periphery, in punctate sites, with immunoreactivity to antibodies against different COOH- and NH2-terminal epitopes of neuronal nitric oxide (NO) synthase (nNOS). One COOH-terminal-directed antibody, made in guinea pig, was used to colocalize other immunoreactivities. Those that apparently colocalized with nNOS were L-Ca2+ channels, the PM Ca2+ pump, and, in part, calreticulin and calsequestrin. The large-conductance Ca2+-activated K+ (BK(Ca)) channels were located in discrete peripheral sites, some with Cav. Immunoreactivities not fully colocalized with nNOS were to the sarcoplasmic reticulum Ca2+ pump, connexins 43, 40, and 45, and vinculin. In patch-clamp studies, NO-driven outward currents, mainly through BK(Ca) channels, were inhibited by antibodies to Cav-1 and not by calmodulin and were restored by an NO donor. Several Ca2+-handling molecules are localized at the PM with and/or near Cav. This may allow intracellular calcium concentration levels to be controlled differently than those in the cytosol near caveolae.

Endothelium-derived relaxing factors: A focus on endothelium-derived hyperpolarizing factor(s)

Endothelium-derived hyperpolarizing factor (EDHF) is defined as the non-nitric oxide (NO) and non-prostacyclin (PGI2) substance that mediates endothelium-dependent hyperpolarization (EDH) of vascular smooth muscle cells (VSMC). Although both NO and PGI2 have been demonstrated to hyperpolarize VSMC by cGMP- and cAMP-dependent mechanisms, respectively, and in the case of NO by cGMP-independent mechanisms, a considerable body of evidence suggests that an additional cellular mechanism must exist that mediates EDH. Despite intensive investigation, there is no agreement as to the nature of the cellular processes that mediates the non-NO/PGI2 mediated hyperpolarization. Epoxyeicosatrienoic acids (EET), an endogenous anandamide, a small increase in the extracellular concentration of K+, and electronic coupling via myoendothelial cell gap junctions have all been hypothesized as contributors to EDH. An attractive hypothesis is that EDH is mediated via both chemical and electrical transmissions, however, the contribution from chemical mediators versus electrical transmission varies in a tissue- and species-dependent manner, suggesting vessel-specific specialization. If this hypothesis proves to be correct then the potential exists for the development of vessel and organ-selective vasodilators. Because endothelium-dependent vasodilatation is dysfunctional in disease states (i.e., atherosclerosis), selective vasodilators may prove to be important therapeutic agents.Key words: endothelium, nitric oxide, potassium channels, hyperpolarization, gap junctions.