Reactivity and endogenous modification by nitrite and hydrogen peroxide: does human neuroglobin act only as a scavenger?

Molecular Interactions between Neuroglobin and Cytochrome c: Possible Mechanisms of Antiapoptotic Defense in Neuronal Cells

Neuroglobin, which is a heme protein from the globin family that is predominantly expressed in nervous tissue, can promote a neuronal survivor. However, the molecular mechanisms underlying the neuroprotective function of Ngb remain poorly understood to this day. The interactions between neuroglobin and mitochondrial cytochrome c may serve as at least one of the mechanisms of neuroglobin-mediated neuroprotection. Interestingly, neuroglobin and cytochrome c possibly can interact with or without electron transfer both in the cytoplasm and within the mitochondria. This review provides a general picture of molecular interactions between neuroglobin and cytochrome c based on the recent experimental and computational work on neuroglobin and cytochrome c interactions.

Biological Oxidations and Nitrations Promoted by the Hemin-Aβ16 Complex

Both β-amyloid (Aβ) peptides and oxidative stress conditions play key roles in Alzheimer's disease. Hemin contributes to the development of the disease as it possesses redox properties and its level increases in pathological conditions or traumatic brain injuries. The aim of this work was to deepen the investigation of the reactivity of the hemin-Aβ₁₆ complex, considering its ability to catalyze oxidation and nitration reactions. We performed kinetic studies in the presence of hydrogen peroxide and nitrite with phenolic and catechol substrates, as well as mass spectrometry studies to investigate the modifications occurring on the peptide itself. The kinetic constants were similar for oxidation and nitration reactions, and their values suggest that the hemin-Aβ₁₆ complex binds negatively charged substrates with higher affinity. Mass spectrometry studies showed that tyrosine residue is the endogenous target of nitration. Hemin degradation analysis showed that hemin bleaching is only partly prevented by the coordinated peptide. In conclusion, hemin has rich reactivity, both in oxidation and nitration reactions on aromatic substrates, that could contribute to redox equilibrium in neurons. This reactivity is modulated by the coordination of the Aβ₁₆ peptide and is only partly quenched when oxidative and nitrative conditions lead to hemin degradation.

Hydrogen peroxide induces heme degradation and protein aggregation in human neuroglobin: roles of the disulfide bridge and hydrogen-bonding in the distal heme cavity

In the present study, human neuroglobin (hNgb) was found to undergo H₂ O₂ -induced breakdown of the heme center at a much slower rate than other globins, namely in the timescale of hours against minutes. We investigated how the rate of the process is affected by the Cys46/Cys55 disulfide bond and the network of non-covalent interactions in the distal heme side involving Tyr44, Lys67, the His64 heme iron axial ligand and the heme propionate-7. The rate is increased by the Tyr44 to Ala and Phe mutations; however the rate is lowered by Lys67 to Ala swapping. The absence of the disulfide bridge slows down the reaction further. Therefore, the disulfide bond-controlled accessibility of the heme site and the residues at position 44 and 67 affect the activation barrier of the reaction. Wild-type and mutated species form β-amyloid aggregates in the presence of H₂ O₂ producing globular structures. Furthermore, the C46A/C55A, Y44A, Y44F and Y44F/C46A/C55A variants yield potentially harmful fibrils. Finally, the nucleation and growth kinetics for the aggregation of the amyloid structures can be successfully described by the Finke-Watzky model.

Redox and spectroscopic properties of mammalian nitrite reductase-like hemoproteins

Besides the canonical pathway of L-arginine oxidation to produce nitric oxide (NO) in vivo, the nitrate-nitrite-NO pathway has been widely accepted as another source for circulating NO in mammals, especially under hypoxia. To date, there have been at least ten heme-containing nitrite reductase-like proteins discovered in mammals with activities mainly identified in vitro, including four globins (hemoglobin, myoglobin, neuroglobin (Ngb), cytoglobin (Cygb)), three mitochondrial respiratory chain enzymes (cytochrome c oxidase, cytochrome bc₁, cytochrome c), and three other heme proteins (endothelial nitric oxide synthase, cytochrome P450 and indoleamine 2,3-dioxygenase 1 (IDO1)). The pathophysiological functions of these proteins are closely related to their redox and spectroscopic properties, as well as their protein structure, although the physiological roles of Ngb, Cygb and IDO1 remain unclear. So far, comprehensive summaries of the redox and spectroscopic properties of these nitrite reductase-like hemoproteins are still lacking. In this review, we have mainly summarized the published data on the application of ultraviolet-visible, electron paramagnetic resonance, circular dichroism and resonance Raman spectroscopies, and X-ray crystallography in studying nitrite reductase-like activity of these 10 proteins, in order to sort out the relationships among enzymatic function, structure and spectroscopic characterization, which might help in understanding their roles in redox biology and medicine.

New Strategies for Stroke Therapy: Nanoencapsulated Neuroglobin

Stroke is a global health and socio-economic problem. However, no efficient preventive and/or palliative treatments have yet been found. Neuroglobin (Ngb) is an endogen neuroprotective protein, but it only exerts its beneficial action against stroke after increasing its basal levels. Therefore, its systemic administration appears to be an efficient therapy applicable to stroke and other neurodegenerative pathologies. Unfortunately, Ngb cannot cross the blood-brain barrier (BBB), making its direct pharmacological use unfeasible. Thus, the association of Ngb with a drug delivery system (DDS), such as nanoparticles (NPs), appears to be a good strategy for overcoming this handicap. NPs are a type of DDS which efficiently transport Ngb and increase its bioavailability in the infarcted area. Hence, we previously built hyaluronate NPS linked to Ngb (Ngb-NPs) as a therapeutic tool against stroke. This nanoformulation induced an improvement of the cerebral infarct prognosis. However, this innovative therapy is still in development, and a more in-depth study focusing on its long-lasting neuroprotectant and neuroregenerative capabilities is needed. In short, this review aims to update the state-of-the-art of stroke therapies based on Ngb, paying special attention to the use of nanotechnological drug-delivering tools.

Neuroglobin deficiency increases seizure susceptibility but does not affect basal behavior in mice

Neuroglobin (Ngb) is found in the neurones of several different brain areas and is known to bind oxygen and other gaseous molecules and reactive oxygen species (ROS) in vitro, but it does not seem to act as a respiratory molecule for neurones. Using male and female Ngb‐knockout (KO) mice, we addressed the role of Ngb in neuronal brain activity using behavioral tests but found no differences in general behaviors, memory processes, and anxiety−/depression‐like behaviors. Oxidative stress and ROS play key roles in epileptogenesis, and oxidative injury produced by an excessive production of free radicals is involved in the initiation and progression of epilepsy. The ROS binding properties led us to hypothesize that lack of Ngb could affect central coping with excitatory stimuli. We consequently explored whether exposure to the excitatory molecule kainate (KA) would increase severity of seizures in mice lacking Ngb. We found that the duration and severity of seizures were increased, while the latency time to develop seizures was shortened in Ngb‐KO compared to wildtype adult female mice. Consistently, c‐fos expression after KA was significantly increased in Ngb‐KO mice in the amygdala and piriform cortex, regions rich in Ngb and known to be centrally involved in seizure generation. Moreover, the measured c‐fos expression levels were correlated with seizure susceptibility. With these new findings combined with previous studies we propose that Ngb could constitute an intrinsic defense mechanism against neuronal hyperexcitability and oxidative stress by buffering of ROS in amygdala and other Ngb‐containing brain regions.

Molecular characterization and functional analysis of Schistosoma mekongi neuroglobin homolog

Schistosomes are blood-dwelling parasites that are constantly exposed to high-level oxidative stress arising from parasite-intrinsic and host defense mechanisms. To survive in their hosts, schistosomes require an antioxidant system to minimize with oxidative stress. Several schistosome antioxidant enzymes have been identified and have been suggested to play indispensable antioxidant roles for the parasite. In addition to antioxidant enzymes, non-enzymatic antioxidants including small molecules, peptides, and proteins have been identified and characterized. Neuroglobin (Ngb), a nervous system-specific heme-binding protein, has been classified as a non-enzymatic antioxidant and is capable of scavenging a variety of free radical species. The antioxidant activity of Ngb has been well-studied in humans. Ngb is involved in cellular oxygen homeostasis and reactive oxygen/nitrogen scavenging in the central and peripheral nervous systems, but its functions in schistosome parasites have not yet been characterized. In this study, we aimed to characterize the molecular properties and functions of Schistosoma mekongi Ngb (SmeNgb) using bioinformatic, biochemical, and molecular biology approaches. The amino acid sequence of Ngb was highly conserved among schistosomes as well as closely related trematodes. SmeNgb was abundantly localized in the gastrodermis, vitelline, and ovary of adult female S. mekongi worms as well as in the tegument of adult male worms. Assessment of antioxidant activity demonstrated that recombinant SmeNgb had Fe²⁺ chelating and hydrogen peroxide scavenging activities. Intriguingly, siRNA silencing of SmeNgb gene expression resulted in tegument pathology. Understanding the properties and functions of SmNgb will help in future development of effective treatments and vaccines against S. mekongi, other schistosome parasites, and other platyhelminths.

Structural and (Pseudo-)Enzymatic Properties of Neuroglobin: Its Possible Role in Neuroprotection

Neuroglobin (Ngb), the third member of the globin family, was discovered in human and murine brains in 2000. This monomeric globin is structurally similar to myoglobin (Mb) and hemoglobin (Hb) α and β subunits, but it hosts a bis-histidyl six-coordinated heme-Fe atom. Therefore, the heme-based reactivity of Ngb is modulated by the dissociation of the distal HisE7-heme-Fe bond, which reflects in turn the redox state of the cell. The high Ngb levels (~100–200 μM) present in the retinal ganglion cell layer and in the optic nerve facilitate the O₂ buffer and delivery. In contrast, the very low levels of Ngb (~1 μM) in most tissues and organs support (pseudo-)enzymatic properties including NO/O₂ metabolism, peroxynitrite and free radical scavenging, nitrite, hydroxylamine, hydrogen sulfide reduction, and the nitration of aromatic compounds. Here, structural and (pseudo-)enzymatic properties of Ngb, which are at the root of tissue and organ protection, are reviewed, envisaging a possible role in the protection from neuronal degeneration of the retina and the optic nerve.

Neuroglobin in Retinal Neurodegeneration: A Potential Target in Therapeutic Approaches

Retinal neurodegeneration affects an increasing number of people worldwide causing vision impairments and blindness, reducing quality of life, and generating a great economic challenge. Due to the complexity of the tissue, and the diversity of retinal neurodegenerative diseases in terms of etiology and clinical presentation, so far, there are no cures and only a few early pathological markers have been identified. Increasing efforts have been made to identify and potentiate endogenous protective mechanisms or to abolish detrimental stress responses to preserve retinal structure and function. The discovering of the intracellular monomeric globin neuroglobin (NGB), found at high concentration in the retina, has opened new possibilities for the treatment of retinal disease. Indeed, the NGB capability to reversibly bind oxygen and its neuroprotective function against several types of insults including oxidative stress, ischemia, and neurodegenerative conditions have raised the interest in the possible role of the globin as oxygen supplier in the retina and as a target for retinal neurodegeneration. Here, we provide the undercurrent knowledge on NGB distribution in retinal layers and the evidence about the connection between NGB level modulation and the functional outcome in terms of retinal neuroprotection to provide a novel therapeutic/preventive target for visual pathway degenerative disease.

Emerging perspectives on cytoglobin, beyond NO dioxygenase and peroxidase

Cytoglobin is an evolutionary ancient hemoglobin with poor functional annotation. Rather than constrained to penta coordination, cytoglobin's heme iron may exist either as a penta or hexacoordinated arrangement when exposed to different intracellular environments. Two cysteine residues at the surface of the protein form an intramolecular disulfide bond that regulates iron coordination, ligand binding, and peroxidase activity. Overall, biochemical results do not support a role for cytoglobin as a direct antioxidant enzyme that scavenges hydrogen peroxide because the rate of the reaction of cytoglobin with hydrogen peroxide is several orders of magnitude slower than metal and thiol-based peroxidases. Thus, alternative substrates such as fatty acids have been suggested and regulation of nitric oxide bioavailability through nitric oxide dioxygenase and nitrite reductase activities has received experimental support. Cytoglobin is broadly expressed in connective, muscle, and nervous tissues. Rational for differential cellular distribution is poorly understood but inducibility in response to hypoxia is one of the most established features of cytoglobin expression with regulation through the transcription factor hypoxia-inducible factor (HIF). Phenotypic characterization of cytoglobin deletion in the mouse have indicated broad changes that include a heightened inflammatory response and fibrosis, increase tumor burden, cardiovascular dysfunction, and hallmarks of senescence. Some of these changes might be reversed upon inhibition of nitric oxide synthase. However, subcellular and molecular interactions have been seldom characterized. In addition, specific molecular mechanisms of action are still lacking. We speculate that cytoglobin functionality will extend beyond nitric oxide handling and will have to encompass indirect regulatory antioxidant and redox sensing functions.

Neuroglobin: A Novel Player in the Oxidative Stress Response of Cancer Cells

Reactive oxygen species (ROS) result from intracellular aerobic metabolism and/or extracellular stimuli. Although endogenous antioxidant systems exquisitely balance ROS production, an excess of ROS production, commonly found in diverse human degenerative pathologies including cancer, gives rise to the oxidative stress. Increased oxidative stress in cancer is related to the sustained proliferation and metabolism of cancer cells. However, cancer cells show an intrinsic higher antioxidant capacity with respect to the normal counterpart as well as an ability to cope with oxidative stress-induced cell death by establishing mechanisms of adaptation, which define a selective advantage against the adverse oxidative stress environment. The identification of survival factors and adaptive pathways, set up by cancer cells against oxidative stress, provides multiple targets for the therapeutic intervention against cancer. Neuroglobin (NGB), a globin primarily described in neurons as an oxidative stress sensor and cytoprotective factor against redox imbalance, has been recently recognized as a novel tumor-associated protein. In this review, the involvement of NGB in the cancer cell adaptation and resistance to oxidative stress will be discussed highlighting the globin role in the regulation of both the stress-induced apoptotic pathway and antioxidant systems activated by cancer cells.

Enhancement of protein stability by an additional disulfide bond designed in human neuroglobin

Human neuroglobin (Ngb) forms an intramolecular disulfide bond between Cys46 and Cys55, with a third Cys120 near the protein surface, which is a promising protein model for heme protein design. In order to protect the free Cys120 and to enhance the protein stability, we herein developed a strategy by designing an additional disulfide bond between Cys120 and Cys15 via A15C mutation. The design was supported by molecular modeling, and the formation of Cys15–Cys120 disulfide bond was confirmed experimentally by ESI-MS analysis. Molecular modeling, UV-Vis and CD spectroscopy showed that the additional disulfide bond caused minimal structural alterations of Ngb. Meanwhile, the disulfide bond of Cys15–Cys120 was found to enhance both Gdn·HCl-induced unfolding stability (increased by ∼0.64 M) and pH-induced unfolding stability (decreased by ∼0.69 pH unit), as compared to those of WT Ngb with a single native disulfide bond of Cys46–Cys55. Moreover, the half denaturation temperature (T_m) of A15C Ngb was determined to be higher than 100 °C. In addition, the disulfide bond of Cys15–Cys120 has slight effects on protein function, such as an increase in the rate of O₂ release by ∼1.4-fold. This study not only suggests a crucial role of the artificial disulfide in protein stabilization, but also lays the groundwork for further investigation of the structure and function of Ngb, as well as for the design of other functional heme proteins, based on the scaffold of A15C Ngb with an enhanced stability.

A disulfide bond of Cys120 and Cys15 was rationally designed in human neuroglobin (Ngb) by A15C mutation, which caused minimal structural alterations, whereas enhanced both chemical and pH stability, with a thermal stability higher than 100 °C.

Nitrosyl Myoglobins and Their Nitrite Precursors: Crystal Structural and Quantum Mechanics and Molecular Mechanics Theoretical Investigations of Preferred Fe -NO Ligand Orientations in Myoglobin Distal Pockets

The globular dioxygen binding heme protein myoglobin (Mb) is present in several species. Its interactions with the simple nitrogen oxides, namely, nitric oxide (NO) and nitrite, have been known for decades, but the physiological relevance has only recently become more fully appreciated. We previously reported the O-nitrito mode of binding of nitrite to ferric horse heart wild-type (wt) Mb^III and human hemoglobin. We have expanded on this work and report the interactions of nitrite with wt sperm whale (sw) Mb^III and its H64A, H64Q, and V68A/I107Y mutants whose dissociation constants increase in the following order: H64Q < wt < V68A/I107Y < H64A. We also report their X-ray crystal structures that reveal the O-nitrito mode of binding of nitrite to these derivatives. The Mb^II-mediated reductions of nitrite to NO and structural data for the wt and mutant Mb^II-NOs are described. We show that their FeNO orientations vary with distal pocket identity, with the FeNO moieties pointing toward the hydrophobic interiors when the His64 residue is present but toward the hydrophilic exterior when this His64 residue is absent in this set of mutants. This correlates with the nature of H-bonding to the bound NO ligand (nitrosyl O vs N atom). Quantum mechanics and hybrid quantum mechanics and molecular mechanics calculations help elucidate the origin of the experimentally preferred NO orientations. In a few cases, the calculations reproduce the experimentally observed orientations only when the whole protein is taken into consideration.

Compensatory role of Neuroglobin in nervous and non-nervous cancer cells in response to the nutrient deprivation

Environmental factors or adverse growth conditions that may reduce cell function or viability are considered stress. The cell ability to sense and respond to environmental stresses determine its function and survival destiny. We recently defined Neuroglobin (NGB), a heme-protein, as a compensatory protein in the 17β-Estradiol (E2) anti-apoptotic activity and as a sensor of oxidative stress in both neurons and breast cancer cells. Here, the possibility that NGB levels could represent a pivotal regulator of integrated response of cancer cells to stress has been evaluated. Data obtained in neuroblastoma and in breast cancer cell lines evidence that nutrient deprivation significantly up-regulated NGB levels at different time points. However, the analysis of autophagy activation led to exclude any possible role of stress- or E2-induced NGB in the upstream regulation of general autophagy. However, the over-expression of Flag-NGB in ERα stable transfected HEK-293 cells completely affects nutrient deprivation-induced decrease in cell number. In addition, reported results indicate that modulation of the anti-apoptotic Bcl-2 level may play a key role in the protective NGB function against energetic stress. Overall, these data define a role of NGB as compensatory protein in the cell machinery activated in response to stress and as general stress adaptation marker of cancer cells susceptible to oxidative stress, oxygen and, as demonstrated here for the first time, even to nutrient willingness. Despite the lacking of any direct NGB role on autophagic flux activated by energetic stress, NGB upregulation appears functional in delaying stress-related cell death allowing an appropriate cell response and adaptation to the changing extracellular conditions.

Redox and Peroxidase Activities of the Hemoglobin Superfamily: Relevance to Health and Disease

SIGNIFICANCE

Erythrocyte hemoglobin (Hb) and myocyte myoglobin, although primarily oxygen-carrying proteins, have the capacity to do redox chemistry. Such redox activity in the wider family of globins now appears to have important associations with the mechanisms of cell stress response. In turn, an understanding of such mechanisms in vivo may have a potential in the understanding of cancer therapy resistance and neurodegenerative disorders such as Alzheimer's. Recent Advances: There has been an enhanced understanding of the redox chemistry of the globin superfamily in recent years, leading to advances in development of Hb-based blood substitutes and in hypotheses relating to specific disease mechanisms. Neuroglobin (Ngb) and cytoglobin (Cygb) have been linked to cell protection mechanisms against hypoxia and oxidative stress, with implications in the onset and progression of neurodegenerative diseases for Ngb and cancer for Cygb.

CRITICAL ISSUES

Despite advances in the understanding of redox chemistry of globins, the physiological roles of many of these proteins still remain ambiguous at best. Confusion over potential physiological roles may relate to multifunctional roles for globins, which may be modulated by surface-exposed cysteine pairs in some globins. Such roles may be critical in deciphering the relationships of these globins in human diseases.

FUTURE DIRECTIONS

Further studies are required to connect the considerable knowledge on the mechanisms of globin redox chemistry in vitro with the physiological and pathological roles of globins in vivo. In doing so, new therapies for neurodegenerative disorders and cancer therapy resistance may be targeted. Antioxid. Redox Signal. 26, 763-776.

A molecule for all seasons: The heme

If life without heme-Fe were at all possible, it would definitely be different. Indeed this complex and versatile iron-porphyrin macrocycle upon binding to different “globins” yields hemeproteins crucial to sustain a variety of vital functions, generally classified, for convenience, in a limited number of functional families. Over-and-above the array of functions briefly outlined below, the spectacular progress in molecular genetics seen over the last 30 years led to the discovery of many hitherto unknown novel hemeproteins in prokaryotes and eukaryotes. Here, we highlight a few basic aspects of the chemistry of the hemeprotein universe, in particular those that are relevant to the control of heme-Fe reactivity and specialization, as sculpted by a variety of interactions with the protein moiety.

Oxidation and nitration of mononitrophenols by a DyP-type peroxidase

Substantial conversion of nitrophenols, typical high-redox potential phenolic substrates, by heme peroxidases has only been reported for lignin peroxidase (LiP) so far. But also a dye-decolorizing peroxidase of Auricularia auricula-judae (AauDyP) was found to be capable of acting on (i) ortho-nitrophenol (oNP), (ii) meta-nitrophenol (mNP) and (iii) para-nitrophenol (pNP). The pH dependency for pNP oxidation showed an optimum at pH 4.5, which is typical for phenol conversion by DyPs and other heme peroxidases. In the case of oNP and pNP conversion, dinitrophenols (2,4-DNP and 2,6-DNP) were identified as products and for pNP additionally p-benzoquinone. Moreover, indications were found for the formation of random polymerization products originating from initially formed phenoxy radical intermediates. Nitration was examined using (15)N-labeled pNP and Na(14)NO2 as an additional source of nitro-groups. Products were identified by HPLC-MS, and mass-to-charge ratios were evaluated to clarify the origin of nitro-groups. The additional nitrogen in DNPs formed during enzymatic conversion was found to originate both from (15)N-pNP and (14)NO2Na. Based on these results, a hypothetical reaction scheme and a catalytically responsible confine of the enzyme's active site are postulated.

A possible mechanism for redox control of human neuroglobin activity

Neuroglobin (Ngb) promotes neuron survival under hypoxic/ischemic conditions. In vivo and in vitro assays provide evidence for redox-regulated functioning of Ngb. On the basis of X-ray crystal structures and our MD simulations, a mechanism for redox control of human Ngb (hNgb) activity via the influence of the CD loop on the active site is proposed. We provide evidence that the CD loop undergoes a strand-to-helix transition when the external environment becomes sufficiently oxidizing, and that this CD loop conformational transition causes critical restructuring of the active site. We postulate that the strand-to-helix mechanics of the CD loop allows hNgb to utilize the lability of Cys46/Cys55 disulfide bonding and of the Tyr44/His64/heme propionate interaction network for redox-controlled functioning of hNgb.

Nitrite reduction and cardiovascular protection

Inorganic nitrite, a metabolite of endogenously produced nitric oxide (NO) from NO synthases (NOS), provides the largest endocrine source of directly bioavailable NO. The conversion of nitrite to NO occurs mainly through enzymatic reduction, mediated by a range of proteins, including haem-globins, molybdo-flavoproteins, mitochondrial proteins, cytochrome P450 enzymes, and NOS. Such nitrite reduction is particularly favoured under hypoxia, when endogenous formation of NO from NOS is impaired. Under normoxic conditions, the majority of these nitrite reductases also scavenge NO, or diminish its bioavailability via reactive oxygen species (ROS) production, suggesting an intricate balance. Moreover, nitrite, whether produced endogenously, or derived from exogenous nitrite or nitrate administration (including dietary sources via the Nitrate-Nitrite-NO pathway) beneficially modulates many key cardiovascular pathological processes. In this review, we highlight the landmark studies which revealed nitrite's function in biological systems, and inspect its evolving role in cardiovascular protection. Whilst these effects have mainly been ascribed to the activity of one or more nitrite reductases, we also discuss newly-identified mechanisms, including nitrite anhydration, the involvement of s-nitrosothiols, nitro-fatty acids, and direct nitrite normoxic signalling, involving modification of mitochondrial structure and function, and ROS production. This article is part of a Special Issue entitled "Redox Signalling in the Cardiovascular System".

Non-covalent and covalent modifications modulate the reactivity of monomeric mammalian globins

Multimeric globins (e.g., hemoglobin) are considered to be the prototypes of allosteric enzymes, whereas monomeric globins (e.g., myoglobin; Mb) usually are assumed to be non-allosteric. However, the modulation of the functional properties of monomeric globins by non-covalent (or allosteric) and covalent modifications casts doubts on this general assumption. Here, we report examples referable to these two extreme mechanisms modulating the reactivity of three mammalian monomeric globins. Sperm whale Mb, which acts as a reserve supply of O2 and facilitates the O2 flux within a myocyte, displays the allosteric modulation of the O2 affinity on lactate, an obligatory product of glycolysis under anaerobic conditions, thus facilitating O2 diffusion to the mitochondria in supporting oxidative phosphorylation. Human neuroglobin (NGB), which appears to protect neurons from hypoxia in vitro and in vivo, undergoes hypoxia-dependent phosphorylation (i.e., covalent modulation) affecting the coordination equilibrium of the heme-Fe atom and, in turn, the heme-protein reactivity. This facilitates heme-Fe-ligand binding and enhances the rate of anaerobic nitrite reduction to form NO, thus contributing to cellular adaptation to hypoxia. The reactivity of human cytoglobin (CYGB), which has been postulated to protect cells against oxidative stress, depends on both non-covalent and covalent mechanisms. In fact, the heme reactivity of CYGB depends on the lipid, such as oleate, binding which stabilizes the penta-coordination geometry of the heme-Fe atom. Lastly, the reactivity of NGB and CYGB is modulated by the redox state of the intramolecular CysCD7/CysD5 and CysB2/CysE9 residue pairs, respectively, affecting the heme-Fe atom coordination state. In conclusion, the modulation of monomeric globins reactivity by non-covalent and covalent modifications appears a very widespread phenomenon, opening new perspectives in cell survival and protection. This article is part of a Special Issue entitled: Oxygen Binding and Sensing Proteins.

Hemoglobin: a nitric-oxide dioxygenase

Members of the hemoglobin superfamily efficiently catalyze nitric-oxide dioxygenation, and when paired with native electron donors, function as NO dioxygenases (NODs). Indeed, the NOD function has emerged as a more common and ancient function than the well-known role in O2 transport-storage. Novel hemoglobins possessing a NOD function continue to be discovered in diverse life forms. Unique hemoglobin structures evolved, in part, for catalysis with different electron donors. The mechanism of NOD catalysis by representative single domain hemoglobins and multidomain flavohemoglobin occurs through a multistep mechanism involving O2 migration to the heme pocket, O2 binding-reduction, NO migration, radical-radical coupling, O-atom rearrangement, nitrate release, and heme iron re-reduction. Unraveling the physiological functions of multiple NODs with varying expression in organisms and the complexity of NO as both a poison and signaling molecule remain grand challenges for the NO field. NOD knockout organisms and cells expressing recombinant NODs are helping to advance our understanding of NO actions in microbial infection, plant senescence, cancer, mitochondrial function, iron metabolism, and tissue O2 homeostasis. NOD inhibitors are being pursued for therapeutic applications as antibiotics and antitumor agents. Transgenic NOD-expressing plants, fish, algae, and microbes are being developed for agriculture, aquaculture, and industry.

14-3-3 binding and phosphorylation of neuroglobin during hypoxia modulate six-to-five heme pocket coordination and rate of nitrite reduction to nitric oxide

Neuroglobin protects neurons from hypoxia in vitro and in vivo; however, the underlying mechanisms for this effect remain poorly understood. Most of the neuroglobin is present in a hexacoordinate state with proximal and distal histidines in the heme pocket directly bound to the heme iron. At equilibrium, the concentration of the five-coordinate neuroglobin remains very low (0.1-5%). Recent studies have shown that post-translational redox regulation of neuroglobin surface thiol disulfide formation increases the open probability of the heme pocket and allows nitrite binding and reaction to form NO. We hypothesized that the equilibrium between the six- and five-coordinate states and secondary reactions with nitrite to form NO could be regulated by other hypoxia-dependent post-translational modification(s). Protein sequence models identified candidate sites for both 14-3-3 binding and phosphorylation. In both in vitro experiments and human SH-SY5Y neuronal cells exposed to hypoxia and glucose deprivation, we observed that 1) neuroglobin phosphorylation and protein-protein interactions with 14-3-3 increase during hypoxic and metabolic stress; 2) neuroglobin binding to 14-3-3 stabilizes and increases the half-life of phosphorylation; and 3) phosphorylation increases the open probability of the heme pocket, which increases ligand binding (CO and nitrite) and accelerates the rate of anaerobic nitrite reduction to form NO. These data reveal a series of hypoxia-dependent post-translational modifications to neuroglobin that regulate the six-to-five heme pocket equilibrium and heme access to ligands. Hypoxia-regulated reactions of nitrite and neuroglobin may contribute to the cellular adaptation to hypoxia.

Current perspectives and challenges in understanding the role of nitrite as an integral player in nitric oxide biology and therapy

Beyond an inert oxidation product of nitric oxide (NO) metabolism, current thinking posits a key role for nitrite as a mediator of NO signaling, especially during hypoxia. This concept has been discussed in the context of nitrite serving a role as an endogenous modulator of NO homeostasis, but also from a novel clinical perspective whereby nitrite therapy may replenish NO signaling and prevent ischemic tissue injury. Indeed, the relatively rapid translation of studies delineating mechanisms of action to ongoing and planned clinical trials has been critical in fuelling interest in nitrite biology, and several excellent reviews have been written on this topic. In this article we limit our discussions to current concepts and what we feel are questions that remain unanswered within the paradigm of nitrite being a mediator of NO biology.

Protection by neuroglobin and cell-penetrating peptide-mediated delivery in vivo: a decade of research. Comment on Cai et al: TAT-mediated delivery of neuroglobin protects against focal cerebral ischemia in mice. Exp Neurol. 2011; 227(1): 224-31

Over the last decade, numerous studies have suggested that neuroglobin is able to protect against the effects of ischemia. However, such results have mostly been based on models using transgenic overexpression or viral delivery. As a therapy, new technology would need to be applied to enable delivery of high concentrations of neuroglobin shortly after the patient suffers the stroke. An approach to deliver proteins in ischemia in vivo in a timely manner is the use of cell-penetrating peptides (CPP). CPP have been used in animal models for brain diseases for about a decade as well. In a recent issue of Experimental Neurology, Cai and colleagues test the effect of CPP-coupled neuroglobin in an in vivo stroke model. They find that the fusion protein protects the brain against the effect of ischemia when applied before stroke onset. Here, a concise review of neuroglobin research and the application of CPP peptides in hypoxia and ischemia is provided.

Human neuroglobin functions as a redox-regulated nitrite reductase

Neuroglobin is a highly conserved hemoprotein of uncertain physiological function that evolved from a common ancestor to hemoglobin and myoglobin. It possesses a six-coordinate heme geometry with proximal and distal histidines directly bound to the heme iron, although coordination of the sixth ligand is reversible. We show that deoxygenated human neuroglobin reacts with nitrite to form nitric oxide (NO). This reaction is regulated by redox-sensitive surface thiols, cysteine 55 and 46, which regulate the fraction of the five-coordinated heme, nitrite binding, and NO formation. Replacement of the distal histidine by leucine or glutamine leads to a stable five-coordinated geometry; these neuroglobin mutants reduce nitrite to NO ∼2000 times faster than the wild type, whereas mutation of either Cys-55 or Cys-46 to alanine stabilizes the six-coordinate structure and slows the reaction. Using lentivirus expression systems, we show that the nitrite reductase activity of neuroglobin inhibits cellular respiration via NO binding to cytochrome c oxidase and confirm that the six-to-five-coordinate status of neuroglobin regulates intracellular hypoxic NO-signaling pathways. These studies suggest that neuroglobin may function as a physiological oxidative stress sensor and a post-translationally redox-regulated nitrite reductase that generates NO under six-to-five-coordinate heme pocket control. We hypothesize that the six-coordinate heme globin superfamily may subserve a function as primordial hypoxic and redox-regulated NO-signaling proteins.

Neuroglobin protects neurons against oxidative stress in global ischemia

Neuroglobin (Ngb) is a recently discovered globin that affords protection against hypoxic/ischemic-induced cell injury in brain. Hypoxic/ischemic injury is associated with accumulation of reactive oxygen species (ROS) and/or reactive nitrogen species (RNS). In previous studies, we found that Ngb has antioxidative properties, and protects PC-12 cells against hypoxia- and β-amyloid-induced cell death. To further delineate the potential role of Ngb in protection against cerebral ischemia-reperfusion injury in vivo, we developed a transgenic mouse line that overexpresses Ngb. Hippocampal ischemia-reperfusion injury was induced by a 10-minute bilateral occlusion of the common carotid arteries, and the animal brains were assessed 3 days later. CA1 neural injury was determined by cresyl violet staining. Lipid peroxidation was assessed using a malonyldialdehyde assay kit, whereas ROS/RNS accumulation was determined by Het staining in the CA1 hippocampal region. Hippocampal Ngb mRNA and protein expressions were assessed by reverse transcriptase-PCR and western blotting, respectively. Neuroglobin was successfully overexpressed in the hippocampus of Ngb transgenic mice. After ischemia-reperfusion, CA1 ROS/RNS production and lipid peroxidation were markedly decreased in Ngb transgenic mice compared with wild-type mice. Furthermore, CA1 neuronal injury was also markedly reduced. Thus, Ngb may confer protection against ischemia-reperfusion injury in the brain through its intrinsic antioxidant properties.

The redox activity of hemoglobins: from physiologic functions to pathologic mechanisms

Pentacoordinate respiratory hemoproteins such as hemoglobin and myoglobin have evolved to supply cells with oxygen. However, these respiratory heme proteins are also known to function as redox enzymes, reacting with compounds such as nitric oxide and peroxides. The recent discoveries of hexacoordinate hemoglobins in vertebrates and nonsymbiotic plants suggest that the redox activity of globins is inherent to the molecule. The uncontrolled formation of radical species resulting from such redox chemistry on respiratory hemoproteins can lead to oxidative damage and cellular toxicity. In this review, we examine the functions of various globins and the mechanisms by which these globins act as redox enzymes under physiologic conditions. Evidence that redox reactions also occur under disease conditions, leading to pathologic complications, also is examined, focusing on recent discoveries showing that the ferryl oxidation state of these hemoproteins is present in these disease states in vivo. In addition, we review the latest advances in the understanding of globin redox mechanisms and how they might affect cellular signaling pathways and how they might be controlled therapeutically or, in the case of hemoglobin-based blood substitutes, through rational design.

Identification of protein radicals formed in the human neuroglobin-H2O2 reaction using immuno-spin trapping and mass spectrometry

Neuroglobin (Ngb) is a recently discovered protein that shows only minor sequence similarity with myoglobin and hemoglobin but conforms to the typical 3-over-3 alpha-helical fold characteristic of vertebrate globins. An intriguing feature of Ngb is its heme hexacoordination in the absence of external ligands, observed both in the ferrous and in the ferric (met) forms. In Ngb, the imidazole of a histidine residue (His-64) in the distal position, above the heme plane, provides the sixth coordination bond. In this work, a valine residue was introduced at position 64 (H64V variant) to clarify the possible role(s) of the distal residue in protecting the heme iron of Ngb from attack by strong oxidants. SDS-PAGE analyses revealed that the oxidation of the H64V variant of metNgb by H 2O 2 resulted in the formation of dimeric and trimeric products in contrast to the native protein. Dityrosine cross-links were shown by their fluorescence to be present in the oligomeric products. When the spin trap 5,5-dimethyl-1-pyrroline N-oxide (DMPO) was included in the reaction mixture, nitrone adducts were detected by immuno-spin trapping. The specific location of the DMPO adducts on the H64V variant protein was determined by a mass spectrometry method that combines off-line immuno-spin trapping and chromatographic procedures. This method revealed Tyr-88 to be the site of modification by DMPO. The presence of His-64 in the wild-type protein results in the nearly complete loss of detectable radical adducts. Together, the data support the argument that wild-type Ngb is protected from attack by H 2O 2 by the coordinated distal His.

Neuroglobin protects PC12 cells against oxidative stress

Neuroglobin (Ngb) is a newly discovered globin in the vertebrate brain that exhibits neuroprotection against hypoxic/ischemic injury. Hypoxic/ischemic brain injury is associated with accumulation of reactive oxygen species (ROS) and/or reactive nitrogen species (RNS), and antioxidants or ROS scavengers promote cell survival. Therefore, Ngb may serve as a scavenger of toxic reactive species, such as hydrogen peroxide. To examine the anti-oxidative role of neuroglobin, PC12 cells were transfected with wild type and mutant (H64 V/H96A) Ngb for 48 h and then treated with H2O2 (0.1, 0.2 and 0.4 mM) for 6 h. Ngb siRNA decreased the H2O2-induced Ngb expression and exacerbated H2O2-induced cell injury. Transient transfection of Ngb induced dose-dependent increases in Ngb protein expression and did not alter SOD, GPX, and catalase activities. Overexpression of wild type Ngb, but not of mutant Ngb, significantly attenuated H2O2-induced ROS/RNS accumulation and lipid peroxidation, decreased H2O2-induced mitochondrial dysfunction and apoptosis, and promoted overall cell survival. Thus, Ngb plays a protective role against oxidative stress, which appears to be primarily mediated by intrinsic Ngb antioxidant properties.

Neuroglobin protects against nitric oxide toxicity

Neuroglobin (Ngb) is a novel vertebrate globin expressed principally in neurons. Ngb expression is induced by hypoxia and ischemia, and Ngb protects neurons against these insults. The mechanism of Ngb's protective action is unknown, but its ability to bind NO suggests that NO scavenging might be involved. To test this hypothesis, we treated wild type and Ngb-transfected HN33 (mouse hippocampal neuronxN18TG2 neuroblastoma) cells with NO donors and compared their sensitivity to NO-induced cell death. Ngb overexpression shifted concentration-toxicity curves to the right, indicating reduced susceptibility to NO or is metabolites. The results suggest that the ability of Ngb to neutralize the neurotoxic effects of reactive nitrogen species may be an important contributor to its neuroprotective properties.