Structural elements contribute to the calcium/calmodulin dependence on enzyme activation in human endothelial nitric-oxide synthase

Protein kinase N2 connects blood flow with NO production in a double AKT

Shear stress is an important regulator of blood flow, and luminal endothelial cells (ECs) sense increases in frictional forces and respond with an appropriate release of vasoactive mediators. In this issue of the JCI, Jin et al. identified a mechanism by which ECs respond to shear stress with endothelial NOS (eNOS) activation and NO release. The authors showed that PKN2 was activated by fluid shear stress and contributed to eNOS activation via a double play - indirect phosphorylation at serine 1177 (S1177) via AKT and direct phosphorylation of the S1179 site. Phosphorylation of both sites individually increased eNOS activity, but together they had an additive effect. In sum, these findings reveal exciting details about how shear stress regulates eNOS and have important implications for blood flow and blood pressure.

The role of polymorphisms rs2070744 and rs1799983 eNOS gene in patients with POAG: a systematic review and meta-analysis

BACKGROUND

Glaucoma is a progressive disease of the optic nerve that has several underlying causes, but in most cases, the cause is unknown. Given the importance of the role of nitric oxide in the occurrence of ocular nerve damage and the effect of eNOS gene polymorphic sites on protein function, to better understand the mechanism of formation of POAG, the relationship between polymorphisms rs2070744 and rs1799983 eNOS gene with POAG risk was investigated in this study using meta-analysis.

METHODS

In this study, systematic review and meta-analysis of study data related to the study of polymorphisms rs2070744 and rs1799983 eNOS gene in patients with POAG using the keywords eNOS, NOS3, Gluuc8898, POAG, primary open-angle glaucoma. It was extracted from SID, MagIran, IranMedex, IranDoc, ScienceDirect, Embase, Scopus, PubMed, Web of Science, and Google Scholar search engines without a time limit until May 2020. To perform the analysis of qualified studies, the model of random effects was used and the inconsistency of studies with the I² index was investigated. Data analysis was performed with Comprehensive Meta-Analysis (Version 2).

RESULTS

In a review of 16 studies (9 studies on polymorphism rs2070744 and seven studies on polymorphism rs1799983) with a sample size of 1631 subjects and a control group of 2405 subjects related to polymorphism rs2070744 and a group of 1456 subjects and a control group of 2240 subjects related to polymorphism 9997 rs1, the odds ratio of TT, CT, and CC genotypes was reported to be 0.95, 1.01, and 1.14, respectively, and the odds ratio of GG, GT and TT genotypes to be 0.88, 0.97, and 1.31, respectively, was reported in patients with POAG.

CONCLUSION

The results of our systematic review and meta-analysis study show that the eNOS gene polymorphisms rs2070744 and rs1799983 may increase the risk of POAG among individuals. However, further studies are needed to confirm these findings.

Nitric oxide synthase enzymology in the 20 years after the Nobel Prize

This review briefly summarizes what was known about NOS enzymology at the time of the Nobel Prize award in 1998 and then discusses from the author's perspective some of the advances in NOS enzymology over the subsequent 20 years, focused on five aspects: the maturation process of NOS enzymes and its regulation; the mechanism of NO synthesis; the redox roles played by the 6R-tetrahydrobiopterin cofactor; the role of protein conformational behaviour in enabling NOS electron transfer and its regulation by NOS structural elements and calmodulin, and the catalytic cycling pathways of NOS enzymes and their influence on NOS activity. LINKED ARTICLES: This article is part of a themed section on Nitric Oxide 20 Years from the 1998 Nobel Prize. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v176.2/issuetoc.

Nitric oxide: what's new to NO?

Nitric oxide (NO) is one of the critical components of the vasculature, regulating key signaling pathways in health. In macrovessels, NO functions to suppress cell inflammation as well as adhesion. In this way, it inhibits thrombosis and promotes blood flow. It also functions to limit vessel constriction and vessel wall remodeling. In microvessels and particularly capillaries, NO, along with growth factors, is important in promoting new vessel formation, a process termed angiogenesis. With age and cardiovascular disease, animal and human studies confirm that NO is dysregulated at multiple levels including decreased production, decreased tissue half-life, and decreased potency. NO has also been implicated in diseases that are related to neurotransmission and cancer although it is likely that these processes involve NO at higher concentrations and from nonvascular cell sources. Conversely, NO and drugs that directly or indirectly increase NO signaling have found clinical applications in both age-related diseases and in younger individuals. This focused review considers recently reported advances being made in the field of NO signaling regulation at several levels including enzymatic production, receptor function, interacting partners, localization of signaling, matrix-cellular and cell-to-cell cross talk, as well as the possible impact these newly described mechanisms have on health and disease.

Phosphorylation Controls Endothelial Nitric-oxide Synthase by Regulating Its Conformational Dynamics

The activity of endothelial NO synthase (eNOS) is triggered by calmodulin (CaM) binding and is often further regulated by phosphorylation at several positions in the enzyme. Phosphorylation at Ser¹¹⁷⁹ occurs in response to diverse physiologic stimuli and increases the NO synthesis and cytochrome c reductase activities of eNOS, thereby enhancing its participation in biological signal cascades. Despite its importance, the mechanism by which Ser¹¹⁷⁹ phosphorylation increases eNOS activity is not understood. To address this, we used stopped-flow spectroscopy and computer modeling approaches to determine how the phosphomimetic mutation (S1179D) may impact electron flux through eNOS and the conformational behaviors of its reductase domain, both in the absence and presence of bound CaM. We found that S1179D substitution in CaM-free eNOS had multiple effects; it increased the rate of flavin reduction, altered the conformational equilibrium of the reductase domain, and increased the rate of its conformational transitions. We found these changes were equivalent in degree to those caused by CaM binding to wild-type eNOS, and the S1179D substitution together with CaM binding caused even greater changes in these parameters. The modeling indicated that the changes caused by the S1179D substitution, despite being restricted to the reductase domain, are sufficient to explain the stimulation of both the cytochrome c reductase and NO synthase activities of eNOS. This helps clarify how Ser¹¹⁷⁹ phosphorylation regulates eNOS and provides a foundation to compare its regulation by other phosphorylation events.

Transcriptional and Posttranslational Regulation of eNOS in the Endothelium

Nitric oxide (NO) is a highly reactive free radical gas and these unique properties have been adapted for a surprising number of biological roles. In neurons, NO functions as a neurotransmitter; in immune cells, NO contributes to host defense; and in endothelial cells, NO is a major regulator of blood vessel homeostasis. In the vasculature, NO is synthesized on demand by a specific enzyme, endothelial nitric oxide synthase (eNOS) that is uniquely expressed in the endothelial cells that form the interface between the circulating blood and the various tissues of the body. NO regulates endothelial and blood vessel function via two distinct pathways, the activation of soluble guanylate cyclase and cGMP-dependent signaling and the S-nitrosylation of proteins with reactive thiols (S-nitrosylation). The chemical properties of NO also serve to reduce oxidation and regulate mitochondrial function. Reduced synthesis and/or compromised biological activity of NO precede the development of cardiovascular disease and this has generated a high level of interest in the mechanisms controlling the synthesis and fate of NO in the endothelium. The amount of NO produced results from the expression level of eNOS, which is regulated at the transcriptional and posttranscriptional levels as well as the acute posttranslational regulation of eNOS. The goal of this chapter is to highlight and integrate past and current knowledge of the mechanisms regulating eNOS expression in the endothelium and the posttranslational mechanisms regulating eNOS activity in both health and disease.

Distinct conformational behaviors of four mammalian dual-flavin reductases (cytochrome P450 reductase, methionine synthase reductase, neuronal nitric oxide synthase, endothelial nitric oxide synthase) determine their unique catalytic profiles

Multidomain enzymes often rely on large conformational motions to function. However, the conformational setpoints, rates of domain motions and relationships between these parameters and catalytic activity are not well understood. To address this, we determined and compared the conformational setpoints and the rates of conformational switching between closed unreactive and open reactive states in four mammalian diflavin NADPH oxidoreductases that catalyze important biological electron transfer reactions: cytochrome P450 reductase, methionine synthase reductase and endothelial and neuronal nitric oxide synthase. We used stopped-flow spectroscopy, single turnover methods and a kinetic model that relates electron flux through each enzyme to its conformational setpoint and its rates of conformational switching. The results show that the four flavoproteins, when fully-reduced, have a broad range of conformational setpoints (from 12% to 72% open state) and also vary 100-fold with respect to their rates of conformational switching between unreactive closed and reactive open states (cytochrome P450 reductase > neuronal nitric oxide synthase > methionine synthase reductase > endothelial nitric oxide synthase). Furthermore, simulations of the kinetic model could explain how each flavoprotein can support its given rate of electron flux (cytochrome c reductase activity) based on its unique conformational setpoint and switching rates. The present study is the first to quantify these conformational parameters among the diflavin enzymes and suggests how the parameters might be manipulated to speed or slow biological electron flux.

The N-terminal portion of autoinhibitory element modulates human endothelial nitric-oxide synthase activity through coordinated controls of phosphorylation at Thr495 and Ser1177

NO production catalysed by eNOS (endothelial nitric-oxide synthase) plays an important role in the cardiovascular system. A variety of agonists activate eNOS through the Ser1177 phosphorylation concomitant with Thr495 dephosphorylation, resulting in increased ·NO production with a basal level of calcium. To date, the underlying mechanism remains unclear. We have previously demonstrated that perturbation of the AIE (autoinhibitory element) in the FMN-binding subdomain can also lead to eNOS activation with a basal level of calcium, implying that the AIE might regulate eNOS activation through modulating phosphorylation at Thr495 and Ser1177. Here we generated stable clones in HEK-293 (human embryonic kidney 293) cells with a series of deletion mutants in both the AIE (Δ594-604, Δ605-612 and Δ626-634) and the C-terminal tail (Δ14; deletion of 1164-1177). The expression of Δ594-604 and Δ605-612 mutants in non-stimulated HEK-293 cells substantially increased nitrate/nitrite release into the culture medium; the other two mutants, Δ626-634 and Δ1164-1177, displayed no significant difference when compared with WTeNOS (wild-type eNOS). Intriguingly, mutant Δ594-604 showed close correlation between Ser1177 phosphorylation and Thr495 dephosphorylation, and NO production. Our results have indicated that N-terminal portion of AIE (residues 594-604) regulates eNOS activity through coordinated phosphorylation on Ser1177 and Thr495.

Insights into the arginine paradox: evidence against the importance of subcellular location of arginase and eNOS

Reduced production of nitric oxide (NO) is one of the first indications of endothelial dysfunction and precedes overt cardiovascular disease. Increased expression of Arginase has been proposed as a mechanism to account for diminished NO production. Arginases consume l-arginine, the substrate for endothelial nitric oxide synthase (eNOS), and l-arginine depletion is thought to competitively reduce eNOS-derived NO. However, this simple relationship is complicated by the paradox that l-arginine concentrations in endothelial cells remain sufficiently high to support NO synthesis. One mechanism proposed to explain this is compartmentalization of intracellular l-arginine into distinct, poorly interchangeable pools. In the current study, we investigated this concept by targeting eNOS and Arginase to different intracellular locations within COS-7 cells and also BAEC. We found that supplemental l-arginine and l-citrulline dose-dependently increased NO production in a manner independent of the intracellular location of eNOS. Cytosolic arginase I and mitochondrial arginase II reduced eNOS activity equally regardless of where in the cell eNOS was expressed. Similarly, targeting arginase I to disparate regions of the cell did not differentially modify eNOS activity. Arginase-dependent suppression of eNOS activity was reversed by pharmacological inhibitors and absent in a catalytically inactive mutant. Arginase did not directly interact with eNOS, and the metabolic products of arginase or downstream enzymes did not contribute to eNOS inhibition. Cells expressing arginase had significantly lower levels of intracellular l-arginine and higher levels of ornithine. These results suggest that arginases inhibit eNOS activity by depletion of substrate and that the compartmentalization of l-arginine does not play a major role.

Endothelial nitric-oxide synthase activation generates an inducible nitric-oxide synthase-like output of nitric oxide in inflamed endothelium

High levels of NO generated in the vasculature under inflammatory conditions are usually attributed to inducible nitric-oxide synthase (iNOS), but the role of the constitutively expressed endothelial NOS (eNOS) is unclear. In normal human lung microvascular endothelial cells (HLMVEC), bradykinin (BK) activates kinin B2 receptor (B2R) signaling that results in Ca(2+)-dependent activation of eNOS and transient NO. In inflamed HLMVEC (pretreated with interleukin-1β and interferon-γ), we found enhanced binding of eNOS to calcium-calmodulin at basal Ca(2+) levels, thereby increasing its basal activity that was dependent on extracellular l-Arg. Furthermore, B2R stimulation generated prolonged high output eNOS-derived NO that is independent of increased intracellular Ca(2+) and is mediated by a novel Gα(i)-, MEK1/2-, and JNK1/2-dependent pathway. This high output NO stimulated with BK was blocked with a B2R antagonist, eNOS siRNA, or eNOS inhibitor but not iNOS inhibitor. Moreover, B2R-mediated NO production and JNK phosphorylation were inhibited with MEK1/2 and JNK inhibitors or MEK1/2 and JNK1/2 siRNA but not with ERK1/2 inhibitor. BK induced Ca(2+)-dependent eNOS phosphorylation at Ser(1177), Thr(495), and Ser(114) in cytokine-treated HLMVEC, but these modifications were not dependent on JNK1/2 activation and were not responsible for prolonged NO output. Cytokine treatment did not alter the expression of B2R, Gα(q/11), Gα(i1,2), JNK, or eNOS. B2R activation in control endothelial cells enhanced migration, but in cytokine-treated HLMVEC it reduced migration. Both responses were NO-dependent. Understanding how JNK regulates prolonged eNOS-derived NO may provide new therapeutic targets for the treatment of disorders involving vascular inflammation.

Nitric oxide and guanylate cyclase signalling are differentially involved in gonadotrophin (LH) release responses to two endogenous GnRHs from goldfish pituitary cells

Nitric oxide synthase (NOS) immunoreactivity is present in goldfish gonadotrophs. The present study investigated whether two native goldfish gonadotrophin-releasing hormones (GnRHs), salmon (s)GnRH and chicken (c)GnRH-II, use NOS/nitric oxide (NO) and soluble guanylate cyclase (sGC)/cyclic (c)GMP/protein kinase G (PKG) signalling to stimulate maturational gonadotrophin [teleost gonadotrophin-II, luteinising hormone (LH)] release. In cell column perifusion experiments with dispersed goldfish pituitary cells, the application of three NOS inhibitors (aminoguanidine hemisulphate, 1400W and 7-nitroindazole) and two NO scavengers [2-phenyl-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (PTIO) and rutin hydrate] reduced sGnRH-elicited, but not cGnRH-II-induced, LH increases. The NO donor sodium nitroprusside (SNP) increased NO production in goldfish pituitary cells in static incubation. SNP-stimulated LH release in column perifusion was attenuated by PTIO and the sGC inhibitor 1H-(1,2,4)oxadiazolo[4,3-a]quinoxalin-1-oneon (ODQ), and additive to responses elicited by cGnRH-II, but not sGnRH. ODQ and the PKG inhibitor KT5823 decreased sGnRH- and cGnRH-II-stimulated LH release. Similarly, the LH response to dibutyryl cGMP was reduced by KT5823. These results indicate that, although only sGnRH uses the NOS/NO pathway to stimulate LH release, both GnRHs utilise sGC/PKG to increase LH secretion.

eNOS activation and NO function: structural motifs responsible for the posttranslational control of endothelial nitric oxide synthase activity

Rather than being a constitutive enzyme as was first suggested, endothelial nitric oxide synthase (eNOS) is dynamically regulated at the transcriptional, posttranscriptional, and posttranslational levels. This review will focus on how changes in eNOS function are conferred by various posttranslational modifications. The latest knowledge regarding eNOS targeting to the plasma membrane will be discussed as the role of protein phosphorylation as a modulator of catalytic activity. Furthermore, new data are presented that provide novel insights into how disruption of the eNOS dimer prevents eNOS uncoupling and the production of superoxide under conditions of elevated oxidative stress and identifies a novel regulatory region we have termed the 'flexible arm'.

A bridging interaction allows calmodulin to activate NO synthase through a bi-modal mechanism

Calmodulin (CaM) activates the nitric-oxide synthases (NOS) by a mechanism that is not completely understood. A recent crystal structure showed that bound CaM engages in a bridging interaction with the NOS FMN subdomain. We investigated its importance in neuronal NOS (nNOS) by mutating the two residues that primarily create the bridging interaction (Arg(752) in the FMN subdomain and Glu(47) in CaM). Mutations designed to completely destroy the bridging interaction prevented bound CaM from increasing electron flux through the FMN subdomain and diminished the FMN-to-heme electron transfer by 90%, whereas mutations that partly preserve the interaction had intermediate effects. The bridging interaction appeared to control FMN subdomain interactions with both its electron donor (NADPH-FAD subdomain) and electron acceptor (heme domain) partner subdomains in nNOS. We conclude that the Arg(752)-Glu(47) bridging interaction is the main feature that enables CaM to activate nNOS. The mechanism is bi-modal and links a single structural aspect of CaM binding to specific changes in nNOS protein conformational and electron transfer properties that are essential for catalysis.

Beta-actin association with endothelial nitric-oxide synthase modulates nitric oxide and superoxide generation from the enzyme

Protein-protein interactions represent an important post-translational mechanism for endothelial nitric-oxide synthase (eNOS) regulation. We have previously reported that beta-actin is associated with eNOS oxygenase domain and that association of eNOS with beta-actin increases eNOS activity and nitric oxide (NO) production. In the present study, we found that beta-actin-induced increase in NO production was accompanied by decrease in superoxide formation. A synthetic actin-binding sequence (ABS) peptide 326 with amino acid sequence corresponding to residues 326-333 of human eNOS, one of the putative ABSs, specifically bound to beta-actin and prevented eNOS association with beta-actin in vitro. Peptide 326 also prevented beta-actin-induced decrease in superoxide formation and increase in NO and L-citrulline production. A modified peptide 326 replacing hydrophobic amino acids leucine and tryptophan with neutral alanine was unable to interfere with eNOS-beta-actin binding and to prevent beta-actin-induced changes in NO and superoxide formation. Site-directed mutagenesis of the actin-binding domain of eNOS replacing leucine and tryptophan with alanine yielded an eNOS mutant that exhibited reduced eNOS-beta-actin association, decreased NO production, and increased superoxide formation in COS-7 cells. Disruption of eNOS-beta-actin interaction in endothelial cells using ABS peptide 326 resulted in decreased NO production, increased superoxide formation, and decreased endothelial monolayer wound repair, which was prevented by PEG-SOD and NO donor NOC-18. Taken together, this novel finding indicates that beta-actin binding to eNOS through residues 326-333 in the eNOS protein results in shifting the enzymatic activity from superoxide formation toward NO production. Modulation of NO and superoxide formation from eNOS by beta-actin plays an important role in endothelial function.

Lys842 in neuronal nitric-oxide synthase enables the autoinhibitory insert to antagonize calmodulin binding, increase FMN shielding, and suppress interflavin electron transfer

Neuronal nitric-oxide synthase (nNOS) contains a unique autoinhibitory insert (AI) in its FMN subdomain that represses nNOS reductase activities and controls the calcium sensitivity of calmodulin (CaM) binding to nNOS. How the AI does this is unclear. A conserved charged residue (Lys(842)) lies within a putative CaM binding helix in the middle of the AI. We investigated its role by substituting residues that neutralize (Ala) or reverse (Glu) the charge at Lys(842). Compared with wild type nNOS, the mutant enzymes had greater cytochrome c reductase and NADPH oxidase activities in the CaM-free state, were able to bind CaM at lower calcium concentration, and had lower rates of heme reduction and NO synthesis in one case (K842A). Moreover, stopped-flow spectrophotometric experiments with the nNOS reductase domain indicate that the CaM-free mutants had faster flavin reduction kinetics and had less shielding of their FMN subdomains compared with wild type and no longer increased their level of FMN shielding in response to NADPH binding. Thus, Lys(842) is critical for the known functions of the AI and also enables two additional functions of the AI as newly identified here: suppression of electron transfer to FMN and control of the conformational equilibrium of the nNOS reductase domain. Its effect on the conformational equilibrium probably explains suppression of catalysis by the AI.

Structural and mechanistic aspects of flavoproteins: electron transfer through the nitric oxide synthase flavoprotein domain

Nitric oxide synthases belong to a family of dual-flavin enzymes that transfer electrons from NAD(P)H to a variety of heme protein acceptors. During catalysis, their FMN subdomain plays a central role by acting as both an electron acceptor (receiving electrons from FAD) and an electron donor, and is thought to undergo large conformational movements and engage in two distinct protein-protein interactions in the process. This minireview summarizes what we know about the many factors regulating nitric oxide synthase flavoprotein domain function, primarily from the viewpoint of how they impact electron input/output and conformational behaviors of the FMN subdomain.

NO formation by neuronal NO-synthase can be controlled by ultrafast electron injection from a nanotrigger

Nitric oxide synthases (NOSs) are unique flavohemoproteins with various roles in mammalian physiology. Constitutive NOS catalysis is initiated by fast hydride transfer from NADPH, followed by slower structural rearrangements. We used a photoactive nanotrigger (NT) to study the initial electron transfer to FAD in native neuronal NOS (nNOS) catalysis. Molecular modeling and fluorescence spectroscopy showed that selective NT binding to NADPH sites close to FAD is able to override Phe1395 regulation. Ultrafast injection of electrons into the protein electron pathway by NT photoactivation through the use of a femtosecond laser pulse is thus possible. We show that calmodulin, required for NO synthesis by constitutive NOS, strongly promotes intramolecular electron flow (6.2-fold stimulation) by a mechanism involving proton transfer to the reduced FAD(-) site. Site-directed mutagenesis using the S1176A and S1176T mutants of nNOS supports this hypothesis. The NT synchronized the initiation of flavoenzyme catalysis, leading to the formation of NO, as detected by EPR. This NT is thus promising for time-resolved X-ray and other cellular applications.

Phosphorylation of endothelial nitric-oxide synthase regulates superoxide generation from the enzyme

In the vasculature, nitric oxide (NO) is generated by endothelial NO synthase (eNOS) in a calcium/calmodulin-dependent reaction. With oxidative stress, the critical cofactor BH(4) is depleted, and NADPH oxidation is uncoupled from NO generation, leading to production of (O(2)*). Although phosphorylation of eNOS regulates in vivo NO generation, the effects of phosphorylation on eNOS coupling and O(2)* generation are unknown. Therefore, we phosphorylated recombinant BH(4)-free eNOS in vitro using native kinases and determined O(2)* generation using EPR spin trapping. Phosphorylation of Ser-1177 by Akt led to an increase (>50%) in maximal O(2)* generation from eNOS. Moreover, Ser-1177 phosphorylation greatly altered the Ca(2+) sensitivity of eNOS, such that O(2)* generation became largely Ca(2+)-independent. In contrast, phosphorylation of eNOS at Thr-495 by protein kinase Calpha (PKCalpha) had no effect on maximum activity or calcium sensitivity but decreased calmodulin binding and increased association with caveolin. In endothelial cells, eNOS-dependent O(2)* generation was stimulated by vascular endothelial growth factor that induced phosphorylation of Ser-1177. With PKC activation that led to phosphorylation of Thr-495, no inhibition of O(2)* generation occurred. As such, phosphorylation of eNOS at Ser-1177 is pivotal in the direct regulation of O(2)* and NO generation, altering both the Ca(2+) sensitivity of the enzyme and rate of product formation, whereas phosphorylation of Thr-495 indirectly affects this process through regulation of the calmodulin and caveolin interaction. Thus, Akt-mediated phosphorylation modulates eNOS uncoupling and greatly increases O(2)* generation from the enzyme at low Ca(2+) concentrations, and PKCalpha-mediated phosphorylation alters the sensitivity of the enzyme to other negative regulatory signals.

Surface charge interactions of the FMN module govern catalysis by nitric-oxide synthase

The FMN module of nitric-oxide synthase (NOS) plays a pivotal role by transferring NADPH-derived electrons to the enzyme heme for use in oxygen activation. The process may involve a swinging mechanism in which the same face of the FMN module accepts and provides electrons during catalysis. Crystal structure shows that this face of the FMN module is electronegative, whereas the complementary interacting surface is electropositive, implying that charge interactions enable function. We used site-directed mutagenesis to investigate the roles of six electronegative surface residues of the FMN module in electron transfer and catalysis in neuronal NOS. Results are interpreted in light of crystal structures of NOS and related flavoproteins. Neutralizing or reversing the negative charge of each residue altered the NO synthesis, NADPH oxidase, and cytochrome c reductase activities of neuronal NOS and also altered heme reduction. The largest effects occurred at the NOS-specific charged residue Glu(762). Together, the results suggest that electrostatic interactions of the FMN module help to regulate electron transfer and to minimize flavin autoxidation and the generation of reactive oxygen species during NOS catalysis.

Functional relevance of Golgi- and plasma membrane-localized endothelial NO synthase in reconstituted endothelial cells

OBJECTIVE

We have previously shown in COS-7 cells that targeting of endothelial NO synthase (eNOS) to the Golgi or plasma membrane (PM) regulates the mechanism and degree of eNOS activation. However, little is known about the functional significance of eNOS targeting in endothelial cells (ECs). The goal of the current study was to isolate these 2 pools of enzyme in ECs and determine their functional significance in response to agonist stimulation and manipulation of membrane cholesterol levels.

METHODS AND RESULTS

Using an RNA interference strategy, we generated stable populations of ECs that had >90% inhibition of eNOS expression and lacked the ability to produce NO. Reconstitution of these eNOS "knockdown" ECs with Golgi- and PM-targeted eNOS restored the ability of ECs to produce NO. Calcium-dependent agonists were the more efficient stimulus for the PM-restricted eNOS in ECs. In contrast, Golgi eNOS was less responsive to both calcium- and Akt-dependent agonists. eNOS restricted to the PM was more sensitive to manipulation of membrane cholesterol levels and was significantly attenuated by modified low-density lipoprotein.

CONCLUSIONS

Within ECs, the PM is the most efficient location to produce NO but is more vulnerable to cholesterol levels and modified low-density lipoprotein.

Differences in eNOS Activity Because of Subcellular Localization Are Dictated by Phosphorylation State Rather than the Local Calcium Environment

Nitric oxide (NO) produced in the endothelium via the enzyme endothelial nitric-oxide synthase (eNOS) is an important vasoactive compound. Wild-type (WT) eNOS is localized to the plasma membrane and perinuclear/Golgi region by virtue of N-terminal myristoylation and palmitoylation. Acylation-deficient mutants (G2AeNOS) remain cytosolic and release less NO in response to Ca2+-elevating agonists; a disparity that we hypothesized was attributed to the greater distance between G2AeNOS and plasma membrane Ca2+ influx channels. The reduced activity of G2AeNOS versus WT was reversed upon disruption of cellular integrity with detergents or sonication. NO production from both constructs relied almost exclusively on the influx of extracellular Ca2+, and elevating intracellular Ca2+ to saturating levels with 10 microM ionomycin in the presence of 10 mM extracellular Ca2+ equalized NO production. To identify the contribution of calcium to the differences in activity between these enzymes, we created Ca2+/CaM-independent eNOS mutants by deleting the two putative autoinhibitory domains of eNOS. There was no difference in NO production between WT and G2A-targeted Ca2+-independent eNOS, suggesting that Ca2+ was the factor responsible. When eNOS constructs were fused in-frame to the bioluminescent probe aequorin, membrane-bound probes were exposed to higher [Ca2+] in unstimulated cells but upon ionomycin stimulation, the probes experienced equal amounts of Ca2+. The WT and G2A enzymes displayed significant differences in the phosphorylation state of Ser617, Ser635, and Ser1179, and mutating all three sites to alanine or restoring phosphorylation with the phosphatase inhibitor calyculin abolished the differences in activity. We therefore conclude that the disparity in NO production between WTeNOS and G2AeNOS is not caused by different localized [Ca2+] upon stimulation with ionomycin, but rather differences in phosphorylation state between the two constructs.

Recruitment of governing elements for electron transfer in the nitric oxide synthase family

At least three building blocks are responsible for the molecular basis of the modulation of electron transfer in nitric oxide synthase (NOS) isoforms: the calmodulin-binding sequence, the C-terminal extension, and the autoregulatory loop in the reductase domain. We have attempted to impart the control conferred by the C termini of NOS to cytochrome P450 oxidoreductase (CYPOR), which contains none of these regulatory elements. The effect of these C termini on the properties of CYPOR sheds light on the possible evolutionary origin of NOS and addresses the recruitment of new peptides on the development of new functions for CYPOR. The C termini of NOSs modulate flavoprotein-mediated electron transfer to various electron acceptors. The reduction of the artificial electron acceptors cytochrome c, 2,6-dichlorophenolindophenol, and ferricyanide was inhibited by the addition of any of these C termini to CYPOR, whereas the reduction of molecular O(2) was increased. This suggests a shift in the rate-limiting step, indicating that the NOS C termini interrupt electron flux between flavin mononucleotide (FMN) and flavin adenine dinucleotide (FAD) and/or the electron acceptors. The modulation of CYPOR by the addition of the NOS C termini is also supported by flavin reoxidation and fluorescence-quenching studies and antibody recognition of the C-terminal extension. These experiments support the origin of the NOS enzymes from modules consisting of a heme domain and CYPOR or ferredoxin-NADP(+) reductase- and flavodoxin-like subdomains that constitute CYPOR, followed by further recruitment of smaller modulating elements into the flavin-binding domains.

A crustacean nitric oxide synthase expressed in nerve ganglia, Y-organ,gill and gonad of the tropical land crab,Gecarcinus lateralis

NO signaling is involved in many physiological processes in invertebrates. In crustaceans, it plays a role in the regulation of the nervous system and muscle contraction. Nested reverse transcription-polymerase chain reaction(RT-PCR) and 5′ and 3′ rapid amplification of cDNA ends (RACE) PCR generated a full-length cDNA sequence (3982 bp) of land crab NO synthase(Gl-NOS) from molting gland (Y-organ) and thoracic ganglion mRNA. The open reading frame encoded a protein of 1199 amino acids with an estimated mass of 135 624 Da. Gl-NOS had the highest sequence identity with insect NOS. The amino acid sequences for binding heme and tetrahydrobiopterin in the oxygenase domain, binding calmodulin and binding FMN, FAD and NADPH in the reductase domain were highly conserved. Gl-NOS had single amino acid differences in all three highly conserved FAD-binding sequences, which distinguished it from other NOS sequences. RT-PCR showed that the Gl-NOS mRNA was present in testis,ovary, gill, eyestalk neural ganglia, thoracic ganglion and Y-organ. NOS mRNA varied between preparations of Y-organ, thoracic ganglion and gill, while NOS mRNA was at consistently high levels in the ovary, testis and eyestalk ganglia. Immunohistochemistry confirmed that the Gl-NOS protein was expressed in Y-organ, ovary and gill. These results suggest that NOS has functions in addition to neuromodulation in adults, such as regulating or modulating ecdysteroid synthesis in the Y-organ.