Alteration of the heme prosthetic group of neuronal nitric-oxide synthase during inactivation by N(G)-amino-L-arginine in vitro and in vivo

Synthesis and reactivity of an osmium(iii) aminoguanidine complex

The biological activities of aminoguanidine (GNH₂) and its derivatives have been extensively studied due to their properties as radical scavengers and antioxidants. Some of their biological activities may result from their binding to various metals present in biological systems. However, the reactivity of coordinated aminoguanidines has not been investigated. We report herein the synthesis, structure and reactivity of a novel osmium(iii) complex bearing the parent aminoguanidine, mer-[Os{NHC(NH₂)(NHNH₂)}(L)(CN)₃]^- (OsGNH₂, HL = 2-(2-hydroxyphenyl)benzoxazole). The antioxidant properties of OsGNH₂ have been investigated by reactions with various oxidants, including O₂, H₂O₂, m-chloroperbenzoic acid (m-CPBA) and Ce(iv). Various osmium products are produced, which depend on the type of oxidant used. OsGNH₂ is readily oxidized by O₂ or H₂O₂ under ambient conditions to afford an osmium(iii) formamidine complex, [Os^III(NH₂C[double bond, length as m-dash]NH)(L)(CN)₃]^- (OsFA, FA = formamidine). With m-CPBA, the nitrosyl complex, mer-[Os(NO)(L)(CN)₃]^- (OsNO), is formed instead. On the other hand, the nitrido complex mer-[Os(N)(L)(CN)₃]^- (OsN) is produced when the one-electron oxidant (NH₄)₂[Ce^IV(NO₃)₆] (Ce(iv)) is employed. The molecular structures of OsGNH₂ and OsFA have been determined by X-ray crystallography. The oxidation of OsGNH₂ to OsFA by O₂ or H₂O₂ is proposed to go through initial dehydrogenation to give a diazoamidine intermediate. In the oxidation by m-CPBA and Ce(iv), it is proposed that the initially formed OsFA is further oxidized to OsNO and OsN, respectively, via osmium(iii) hydrogen cyanamido and osmium(iv) cyanoimido intermediates.

Development of nitric oxide synthase inhibitors for neurodegeneration and neuropathic pain

Nitric oxide (NO) is an important signaling molecule in the human body, playing a crucial role in cell and neuronal communication, regulation of blood pressure, and in immune activation. However, overproduction of NO by the neuronal isoform of nitric oxide synthase (nNOS) is one of the fundamental causes underlying neurodegenerative disorders and neuropathic pain. Therefore, developing small molecules for selective inhibition of nNOS over related isoforms (eNOS and iNOS) is therapeutically desirable. The aims of this review focus on the regulation and dysregulation of NO signaling, the role of NO in neurodegeneration and pain, the structure and mechanism of nNOS, and the use of this information to design selective inhibitors of this enzyme. Structure-based drug design, the bioavailability and pharmacokinetics of these inhibitors, and extensive target validation through animal studies are addressed.

A model in which heat shock protein 90 targets protein-folding clefts: rationale for a new approach to neuroprotective treatment of protein folding diseases

In an EBM Minireview published in 2010, we proposed that the heat shock protein (Hsp)90/Hsp70-based chaperone machinery played a major role in determining the selection of proteins that have undergone oxidative or other toxic damage for ubiquitination and proteasomal degradation. The proposal was based on a model in which the Hsp90 chaperone machinery regulates signaling by modulating ligand-binding clefts. The model provides a framework for thinking about the development of neuroprotective therapies for protein-folding diseases like Alzheimer's disease (AD), Parkinson's disease (PD), and the polyglutamine expansion disorders, such as Huntington's disease (HD) and spinal and bulbar muscular atrophy (SBMA). Major aberrant proteins that misfold and accumulate in these diseases are "client" proteins of the abundant and ubiquitous stress chaperone Hsp90. These Hsp90 client proteins include tau (AD), α-synuclein (PD), huntingtin (HD), and the expanded glutamine androgen receptor (polyQ AR) (SBMA). In this Minireview, we update our model in which Hsp90 acts on protein-folding clefts and show how it forms a rational basis for developing drugs that promote the targeted elimination of these aberrant proteins.

Modulation of heme/substrate binding cleft of neuronal nitric-oxide synthase (nNOS) regulates binding of Hsp90 and Hsp70 proteins and nNOS ubiquitination

Like other nitric-oxide synthase (NOS) enzymes, neuronal NOS (nNOS) turnover and activity are regulated by the Hsp90/Hsp70-based chaperone machinery, which regulates signaling proteins by modulating ligand binding clefts (Pratt, W. B., Morishima, Y., and Osawa, Y. (2008) J. Biol. Chem. 283, 22885-22889). We have previously shown that nNOS turnover is due to Hsp70/CHIP-dependent ubiquitination and proteasomal degradation. In this work, we use an intracellular cross-linking approach to study both chaperone binding and nNOS ubiquitination in intact HEK293 cells. Treatment of cells with N(G)-nitro-L-arginine, a slowly reversible competitive inhibitor that stabilizes nNOS, decreases both nNOS ubiquitination and binding of Hsp90, Hsp70, and CHIP. Treatment with the calcium ionophore A23187, which increases Ca(2+)-calmodulin binding to nNOS, increases nNOS ubiquitination and binding of Hsp90, Hsp70, and CHIP in a manner that is specific for changes in the heme/substrate binding cleft. Both Hsp90 and Hsp70 are bound to the expressed nNOS oxygenase domain, which contains the heme/substrate binding cleft, but not to the reductase domain, and binding is increased to an expressed fragment containing both the oxygenase domain and the calmodulin binding site. Overexpression of Hsp70 promotes nNOS ubiquitination and decreases nNOS protein, and overexpression of Hsp90 inhibits nNOS ubiquitination and increases nNOS protein, showing the opposing effects of the two chaperones as they participate in nNOS quality control in the cell. These observations support the notion that changes in the state of the heme/substrate binding cleft affect chaperone binding and thus nNOS ubiquitination.

Proposal for a role of the Hsp90/Hsp70-based chaperone machinery in making triage decisions when proteins undergo oxidative and toxic damage

The Hsp90/Hsp70-based chaperone machinery plays a well-established role in signaling protein function, trafficking and turnover. A number of recent observations also support the notion that Hsp90 and Hsp70 play key roles in the triage of damaged and aberrant proteins for degradation via the ubiquitin-proteasome pathway. In the mid-1990s, it was discovered that Hsp70 is required for ubiquitin-dependent degradation of short-lived and abnormal proteins, and it became clear that inhibition of Hsp90 uniformly leads to the proteasomal degradation of Hsp90 client proteins. Subsequently, CHIP and parkin were shown to be Hsp70-binding ubiquitin E3 ligases that direct ubiquitin-charged E2 enzymes to the Hsp70-bound client protein. Stabilization by Hsp90 reflects the interaction of the chaperone with the ligand binding cleft of the client protein. These hydrophobic clefts must be open to allow passage of ligands to binding sites in the protein interior, and they are inherent sites of conformational instability. Hsp90 stabilizes the open state of the cleft and prevents Hsp70-dependent ubiquitination. In the model we propose here, progressive oxidative events result in cleft opening as the initial step in protein unfolding, and as long as Hsp90 can interact to stabilize the cleft, it will buffer the effect of oxidative damage. When cleft opening is such that Hsp90 can no longer interact, Hsp70-dependent ubiquitination occurs. We summarize evidence that Hsp90 interacts very dynamically with a variety of proteins that are not classic Hsp90 clients, and we show that this dynamic cycling of Hsp90 with nitric oxide synthase protects against CHIP-mediated ubiquitination. Scientific interest to date has focused on stringent regulation of the classic client proteins, which have metastable clefts and are inherently short lived. But, the recognition that Hsp90 cycles dynamically with longer lived proteins with more stable clefts may permit extension of the triage model to the quality control of damaged proteins in general.

Dynamic cycling with Hsp90 stabilizes neuronal nitric oxide synthase through calmodulin-dependent inhibition of ubiquitination

NO production by neuronal nitric oxide synthase (nNOS) requires calmodulin and is enhanced by the chaperone Hsp90, which cycles dynamically with the enzyme. The proteasomal degradation of nNOS is enhanced by suicide inactivation and by treatment with Hsp90 inhibitors, the latter suggesting that dynamic cycling with Hsp90 stabilizes nNOS. Here, we use a purified ubiquitinating system containing CHIP (carboxyl terminus of Hsp70-interacting protein) as the E3 ligase to show that Hsp90 inhibits CHIP-dependent nNOS ubiquitination. Like the established Hsp90 enhancement of NO synthesis, Hsp90 inhibition of nNOS ubiquitination is Ca2+/calmodulin-dependent, suggesting that the same interaction of Hsp90 with the enzyme is responsible for both enhancement of nNOS activity and inhibition of ubiquitination. It is established that CHIP binds to Hsp90 as well as to Hsp70, but we show here the two chaperones have opposing actions on nNOS ubiquitination, with Hsp70 stimulating and Hsp90 inhibiting. We have used two mechanism-based inactivators, guanabenz and NG-amino-L-arginine, to alter the heme/substrate binding cleft and promote nNOS ubiquitination that can be inhibited by Hsp90. We envision that, as nNOS undergoes toxic damage, the heme/substrate binding cleft opens exposing hydrophobic residues as the initial step in unfolding. As long as Hsp90 can form even transient complexes with the opening cleft, ubiquitination by Hsp70-dependent ubiquitin E3 ligases, like CHIP, is inhibited. When unfolding of the cleft progresses to a state that cannot cycle with Hsp90, Hsp70-dependent ubiquitination is unopposed. In this way, the Hsp70/Hsp90 machinery makes the quality control decision for stabilization versus degradation of nNOS.

N(G)-aminoguanidines from primary amines and the preparation of nitric oxide synthase inhibitors

A concise, general, and high-yielding method for the preparation of N(G)-aminoguanidines from primary amines is reported. Using available and readily prepared materials, primary amines are converted to protected N(G)-aminoguanidines in a one-pot procedure. The method has been successfully applied to a number of examples including the syntheses of four nitric oxide synthase (NOS) inhibitors. The inhibitors prepared were investigated as competitive inhibitors and as mechanistic inactivators of the inducible isoform of NOS (iNOS). In addition, one of the four inhibitors prepared, N(G)-amino-N(G)-2,2,2-trifluoroethyl-L-arginine 19, displays the unique ability to both inhibit NO formation and prevent NADPH consumption by iNOS without irreversible inactivation of the enzyme.

Metabolism of aminoguanidine, diaminoguanidine, and NG-amino-L-arginine by neuronal NO-synthase and covalent alteration of the heme prosthetic group

It is established that aminoguanidine (AG), diaminoguanidine (DAG), and NG-amino-l-arginine (NAA) are metabolism-based inactivators of the three major isoforms of nitric oxide synthase (NOS). In the case of neuronal NOS (nNOS), heme alteration is known to be a major cause of inactivation, although the exact mechanism by which this occurs is not well-understood. We show here by the use of LC/MS/MS techniques that AG, DAG, and NAA are metabolized by nNOS to products with corresponding mass ions at m/z of 45.2, 60.2, and 160.0, respectively. These results are consistent with the loss of a hydrazine moiety from each inactivator. These findings are confirmed by exact mass measurements and comparison to authentic standards in the case of the products for NAA and AG, respectively. Moreover, the major dissociable heme product that was formed during inactivation of nNOS by AG, DAG, and NAA had molecular ions at m/z 660.2, 675.2, and 775.3, respectively. These results are consistent with an adduct of heme and inactivator minus a hydrazine moiety. In support of this, MS/MS studies reveal a fragment ion of heme in each case. With the use of 14C-labeled heme, we also show that in the case of AG, the dissociable heme adduct accounts for approximately one-half of the heme that is altered. In addition, we employ a software-based differential metabolic profiling method by subtracting LC/MS data sets derived from samples that contained nNOS from those that did not contain the enzyme to search for products and substrates in complex reaction mixtures. The metabolic profiling method established in this study can be used as a general tool to search for substrates and products of enzyme systems, including the drug-metabolizing liver microsomal P450 cytochromes. We propose that the metabolism-based inactivation of nNOS by AG, DAG, and NAA occurs through oxidative removal of the hydrazine group and the formation of a radical intermediate that forms stable products after H-atom abstraction or reacts with the heme prosthetic moiety and inactivates nNOS.

Tetrahydrobiopterin depletion and ubiquitylation of neuronal nitric oxide synthase

Tetrahydrobiopterin is a necessary cofactor for the synthesis of nitric oxide by the hemeprotein enzyme, NO-synthase (NOS). It is widely thought that inadequate levels of tetrahydrobiopterin lead to tissue injury and organ dysfunction due, in part, to formation of superoxide from pterin-deficient NOS. In the course of studies on the ubiquitylation of neuronal NOS (nNOS), we have found that certain substrate analogs, such as N(G)-nitro-L-arginine, stabilize the dimeric form of nNOS and protect the enzyme from ubiquitylation. Since tetrahydrobiopterin is known to bind near heme and confers stability to the active dimeric structure of nNOS, we wondered if the loss of tetrahydrobiopterin could be an endogenous signal for nNOS ubiquitylation and degradation. We show here in HEK293 cells stably transfected with nNOS that depletion of tetrahydrobiopterin by treatment with 2,4-diamino-6-hydroxypyrimidine leads to destabilization of the dimeric form and enhances ubiquitylation of nNOS. Sepiapterin, a precursor to tetrahydrobiopterin in the salvage pathway, completely reverses the effect of 2,4-diamino-6-hydroxypyrimidine on nNOS ubiquitylation. Consistent with that found in cells, the in vitro ubiquitylation of nNOS by reticulocyte proteins decreases when tetrahydrobiopterin is present. Thus, inadequate amounts of tetrahydrobiopterin may lead to a sustained decrease in the steady state level of nNOS that is not readily reversed.

Ubiquitination and degradation of neuronal nitric-oxide synthase in vitro: dimer stabilization protects the enzyme from proteolysis

It is established that neuronal NO synthase (nNOS) is ubiquitinated and proteasomally degraded. The metabolism-based inactivation of nNOS and the inhibition of heat shock protein 90 (hsp90)-based chaperones, which are known to regulate nNOS, both lead to enhanced proteasomal degradation of nNOS. The mechanism of this selective proteolytic degradation, or in essence how the nNOS becomes labilized and recognized for ubiquitination and subsequent degradation, has not been determined. In the current study, we used a crude preparation of reticulocyte proteins, which contains ubiquitin-conjugating enzymes and the proteasome, to determine how nNOS is labilized. We found that the inactive monomeric heme-deficient nNOS (apo-nNOS) is rapidly degraded in vitro, consistent with the finding that both metabolism-based inactivation and inhibition of hsp90-based chaperones cause the formation of apo-nNOS and enhance its degradation in vivo. In the current study, we discovered that destabilization of the dimeric nNOS, as determined by measuring the SDS-resistant dimer, is sufficient to trigger ubiquitin-proteasomal degradation. Treatment of nNOS with NG-nitro-L-arginine or 7-nitroindazole led to stabilization of the dimeric nNOS and decreased proteasomal degradation of the enzyme, consistent with that observed in cells. Thus, it seems that the dimeric structure is a major determinant of nNOS stability and proteolysis.

Metabolism-based inactivation of neuronal nitric-oxide synthase by components of cigarette and cigarette smoke

It has been shown that administration of cigarette smoke to rats leads to loss of neuronal nitric-oxide synthase (nNOS) activity and nNOS protein in penile tissue. The exact mechanism for this loss of activity and protein is not known. In the current study, we investigated whether extracts prepared from cigarette smoke or from the cigarette itself could directly inhibit nNOS activity. We discovered that the cigarette smoke extract and the cigarette extract cause a time-, concentration-, and calmodulin-dependent inactivation of nNOS in an in vitro system containing the purified enzyme. L-Arginine, but not D-arginine, protects nNOS from this time-dependent inactivation, suggesting an active site directed event. The kinetics of inactivation are consistent with the metabolism-based or suicide inactivation of nNOS. Based on studies with other metabolism-based inactivators, this cigarette-mediated inactivation may render nNOS more susceptible to proteasomal degradation and thereby may explain the loss of nNOS protein in vivo. The component(s) responsible for nNOS inactivation is not volatile, is not retained by a 3,000 molecular weight cut-off membrane, binds to activated charcoal, and is highly water-soluble under both acidic and basic conditions. The discovery of a direct inactivation of nNOS by an organic, cationic compound(s) present in tobacco and tobacco smoke provides a basis for further study of not only the mechanisms responsible for the biological effects of tobacco but also a search for a potentially novel inactivator of nNOS.

Proteolytic degradation of nitric oxide synthase: effect of inhibitors and role of hsp90-based chaperones

Nitric oxide synthase (NOS) is a highly regulated enzyme that produces nitric oxide, a critical messenger in many physiological processes. In this perspective, we explore the role of proteolytic degradation of NOS, in particular the inducible and neuronal isoforms of NOS, as a mechanism of regulation of the enzyme. The ubiquitin-proteasome and calpain pathways are the major proteolytic systems identified to date that are responsible for this regulated degradation. The degradation of NOS is affected by diverse agents, including glucocorticoids, caveolin, neurotoxic compounds, and certain NOS inhibitors. Some irreversible inactivators of NOS enhance the proteolytic degradation of the enzyme, and this property may be of great importance in understanding the biological effects of these inhibitors, some of which are being developed for clinical use. Analogies with the regulated degradation of liver microsomal cytochromes P450, which are related to NOS, provide a framework for understanding these processes. Finally, a new perspective on the regulation of NOS by hsp90-based chaperones is presented that involves facilitated heme insertion into the enzyme.