Neuroglobin Expression Models as a Tool to Study Its Function

An Investigation into the Effects of Chemical, Pharmaceutical, and Herbal Compounds on Neuroglobin: A Literature Review

Neuroglobin (Ngb) is an oxygen-binding globin protein that is mainly expressed in the neurons of the central and peripheral nervous system. However, moderate levels of Ngb have also been detected in non-neural tissues. Ngb and Ngb modulating factors have been increasingly studied over the last decade due to their neuroprotective role in neurological disorders and hypoxia. Studies have shown that a number of chemicals, pharmaceuticals, and herbal compounds can modulate the expression of Ngb at different dose levels, indicating a protective role against neurodegenerative diseases. Iron chelators, hormones, antidiabetic drugs, anticoagulants, antidepressants, plant derivatives and short-chain fatty acids are among these compounds. Therefore, this study aimed to review the literature focused on the possible effects and mechanisms of chemical, pharmaceutical, and herbal compounds on Ngbs.

Isoquercitrin Played a Neuroprotective Role in Rats After Cerebral Ischemia/Reperfusion Through Up-Regulating Neuroglobin and Anti-Oxidative Stress

BACKGROUND

This study aims to investigate whether isoquercitrin (Iso) exerts a neuroprotective role effect after cerebral ischemia-reperfusion (CIR) via up-regulating neuroglobin (Ngb) or reducing oxidative stress.

METHODS

The middle cerebral artery occlusion/reperfusion (MCAO/R) model was constructed using Sprague Dawley rats. First, we divided 40 mice into 5 groups (n = 8): sham, MCAO/R, Low-dosed Iso (5 mg/kg Iso), Mid-dosed Iso (10 mg/kg Iso), and High-dosed Iso (20 mg/kg Iso). Then, 48 rats were separated into 6 groups (n = 8): sham, MCAO/R, Iso, artificial cerebrospinal fluid, Ngb antisense oligodeoxynucleotides (AS-ODNs), and AS-ODNs ± Iso. The effects of Iso on brain tissue injury and oxidative stress were evaluated using hematoxylin-eosin staining, terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling assay, immunofluorescence, western blotting, and real-time quantitative polymerase chain reaction, enzyme-linked immunosorbent assay, and reactive oxygen species (ROS) detection.

RESULTS

The neurologic score, infarct volume, histopathology, apoptosis rate, and ROS production were reduced in Iso dose-dependent. The Ngb expression enhanced in Iso dose-dependent. The oxidative stress-related factors SOD, GSH, CAT, Nrf2, HO-1, and HIF-1α levels also increased in Iso dose-dependent, whereas the MDA levels decreased. However, related regulation of Iso on brain tissue damage and oxidative stress were reversed after low expression of Ngb.

CONCLUSION

Isoquercitrin played a neuroprotective role after CIR through up-regulating of Ngb and anti-oxidative stress.

Neuronal Cytoglobin in the Auditory Brainstem of Rat and Mouse: Distribution, Cochlear Projection, and Nitric Oxide Production

Cytoglobin (Cygb), a hemoprotein of the globin family, is expressed in the supportive tissue cells of the fibroblast lineage and in distinct neuronal cell populations. The expression pattern and regulatory parameters of fibroblasts and related cells were studied in organs such as the kidney and liver in a variety of animal models. In contrast, knowledge about cytoglobin-expressing neurons is sparse. Only a few papers described the distribution in the brain as ubiquitous with a restricted number of neurons in focal regions. Although there is evidence for cytoglobin involvement in neuronal hypoxia tolerance, its presence in the auditory system was not studied despite high metabolism rates and oxygen demands of the cochlea and related brainstem centers. In a continuation of a previous study demonstrating Cygb-neurons in, inter alia, auditory regions of the mouse brain, we concentrated on the superior olivary complex (SOC) in the present study. We sought to investigate the distribution, projection pattern and neurochemistry of Cygb-neurons in the SOC. We conducted immunohistochemistry using a Cygb antibody and found that this brainstem region, functionally competent for bilateral hearing and providing cochlear hair cell innervation, contains a considerable number of Cygb-expressing neurons (averaging 2067 ± 211 making up 10 ±1% percent of total neuron number) in rats, and 514 ± 138 (6 ± 1%) in mice. They were observed in all regions of the SOC. Retrograde neuronal tract tracing with Fluorogold injected into the cochlea demonstrated that 1243 ± 100 (6 ± 1% of total neuron number in rat SOC)) were olivocochlear neurons. Approximately 56% of total Cygb neurons were retrogradely labelled, while the majority of olivocochlear neurons of both lateral and medial systems were Cygb-immunoreactive. We also conducted double immunofluorescence staining for Cygb and neuronal nitric oxide synthase (nNOS), the enzyme responsible for nitric oxide production, and observed that cytoglobin in the SOC frequently co-localized with nNOS. Our findings suggest that cytoglobin plays an important physiologic role in the oxygen homeostasis of the peripheral and central auditory nervous system. Further studies, also including transgenic animal models, are required to shed more light on the function(s) of Cygb in neurons, in particular of the auditory system.

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.

Protective role of Cytoglobin and Neuroglobin against the Lipopolysaccharide (LPS)-induced inflammation in Leydig cells ex vivo

Leydig cells are responsible for testosterone production in male testis upon stimulation by luteinizing hormone. Inflammation and oxidative stress related Leydig cell dysfunction is one of the major causes of male infertility. Cytoglobin (CYGB) and Neuroglobin (NGB) are two globin family member proteins which protect cells against oxidative stress. In the current study, we established a Lipopolysaccharide (LPS)-induced inflammation model in TM3 Leydig cell culture to study the function of CYGB and NGB proteins under inflammatory conditions. CYGB and NGB were downregulated using siRNA and shRNA based experimental strategies. Overexpression was conducted using lentiviral pLenti-III-CYGB-2A-GFP, and pLenti-III-NGB-2A-GFP vector systems. As testicular macrophages regulate immune function upon inflammation and steroidogenesis of Leydig cells, we generated direct/indirect co-culture systems of TM3 and mouse macrophage (RAW264.7) cells ex vivo. Downregulation of CYGB and NGB induced nitride oxide (NO) release, blocked cell cycle progression, reduced testosterone production and increased inflammatory and apoptotic pathway gene expression in the presence and absence of LPS. On the other hand, CYGB and NGB overexpression reduced TNFα and COX-2 protein expressions and increased the expression of testosterone biogenesis pathway genes upon LPS stimulation. In addition, CYGB and NGB overexpression upregulated testosterone production. The present study successfully established an inflammatory interaction model of TM3 and RAW264.7 cells. Suppression of CYGB and NGB in TM3 cells changed macrophage morphology, enhanced macrophage cell number and NO release in co-culture experiments upon LPS exposure. In summary, these results demonstrate that globin family members might control LPS induced inflammation by regulating apoptotic mechanisms and macrophage response.

Ordered Motions in the Nitric-Oxide Dioxygenase Mechanism of Flavohemoglobin and Assorted Globins with Tightly Coupled Reductases

Nitric-oxide dioxygenases (NODs) activate and combine O₂ with NO to form nitrate. A variety of oxygen-binding hemoglobins with associated partner reductases or electron donors function as enzymatic NODs. Kinetic and structural investigations of the archetypal two-domain microbial flavohemoglobin-NOD have illuminated an allosteric mechanism that employs selective tunnels for O₂ and NO, gates for NO and nitrate, transient O₂ association with ferric heme, and an O₂ and NO-triggered, ferric heme spin crossover-driven, motion-controlled, and dipole-regulated electron-transfer switch. The proposed mechanism facilitates radical-radical coupling of ferric-superoxide with NO to form nitrate while preventing suicidal ferrous-NO formation. Diverse globins display the structural and functional motifs necessary for a similar allosteric NOD mechanism. In silico docking simulations reveal monomeric erythrocyte hemoglobin alpha-chain and beta-chain intrinsically matched and tightly coupled with NADH-cytochrome b₅ oxidoreductase and NADPH-cytochrome P450 oxidoreductase, respectively, forming membrane-bound flavohemoglobin-like mammalian NODs. The neuroprotective neuroglobin manifests a potential NOD role in a close-fitting ternary complex with membrane-bound NADH-cytochrome b₅ oxidoreductase and cytochrome b₅. Cytoglobin interfaces weakly with cytochrome b₅ for O₂ and NO-regulated electron-transfer and coupled NOD activity. The mechanistic model also provides insight into the evolution of O₂ binding cooperativity in hemoglobin and a basis for the discovery of allosteric NOD inhibitors.

Neuroglobin, clues to function and mechanism

Neuroglobin is expressed in vertebrate brain and belongs to a branch of the globin family that diverged early in evolution. Sequence conservation and presence in nervous cells of several taxa suggests a relevant role in the nervous system, with tight structural restraints. Twenty years after its discovery, a rich scientific literature provides convincing evidence of the involvement of neuroglobin in sustaining neuron viability in physiological and pathological conditions however, a full and conclusive picture of its specific function, or set of functions is still lacking. The difficulty of unambiguously assigning a precise mechanism and biochemical role to neuroglobin might arise from the participation to one or more cell mechanism that redundantly guarantee the functioning of the highly specialized and metabolically demanding central nervous system of vertebrates. Here we collect findings and hypotheses arising from recent biochemical, biophysical, structural, in cell and in vivo experimental work on neuroglobin, aiming at providing an overview of the most recent literature. Proteins are said to have jobs and hobbies, it is possible that, in the case of neuroglobin, evolution has selected for it more than one job, and support to cover for its occasional failings. Disentangling the mechanisms and roles of neuroglobin is thus a challenging task that might be achieved by considering data from different disciplines and experimental approaches.

Role of Neuroglobin in the Neuroprotective Actions of Estradiol and Estrogenic Compounds

Estradiol exerts neuroprotective actions that are mediated by the regulation of a variety of signaling pathways and homeostatic molecules. Among these is neuroglobin, which is upregulated by estradiol and translocated to the mitochondria to sustain neuronal and glial cell adaptation to injury. In this paper, we will discuss the role of neuroglobin in the neuroprotective mechanisms elicited by estradiol acting on neurons, astrocytes and microglia. We will also consider the role of neuroglobin in the neuroprotective actions of clinically relevant synthetic steroids, such as tibolone. Finally, the possible contribution of the estrogenic regulation of neuroglobin to the generation of sex differences in brain pathology and the potential application of neuroglobin as therapy against neurological diseases will be examined.

Proteomic and Bioinformatic Investigation of Altered Pathways in Neuroglobin-Deficient Breast Cancer Cells

Neuroglobin (NGB) is a myoglobin-like monomeric globin that is involved in several processes, displaying a pivotal redox-dependent protective role in neuronal and extra-neuronal cells. NGB remarkably exerts its function upon upregulation by NGB inducers, such as 17β-estradiol (E2) and H₂O₂. However, the molecular bases of NGB’s functions remain undefined, mainly in non-neuronal cancer cells. Human MCF-7 breast cancer cells with a knocked-out (KO) NGB gene obtained using CRISPR/Cas9 technology were analyzed using shotgun label-free quantitative proteomics in comparison with control cells. The differential proteomics experiments were also performed after treatment with E2, H₂O₂, and E2 + H₂O₂. All the runs acquired using liquid chromatography–tandem mass spectrometry were elaborated within the same MaxQuant analysis, leading to the quantification of 1872 proteins in the global proteomic dataset. Then, a differentially regulated protein dataset was obtained for each specific treatment. After the proteomic study, multiple bioinformatics analyses were performed to highlight unbalanced pathways and processes. Here, we report the proteomic and bioinformatic investigations concerning the effects on cellular processes of NGB deficiency and cell treatments. Globally, the main processes that were affected were related to the response to stress, cytoskeleton dynamics, apoptosis, and mitochondria-driven pathways.

Neuroglobin and neuroprotection: the role of natural and synthetic compounds in neuroglobin pharmacological induction

Neuroglobin (Ngb) is a 17 kDa monomeric hexa-coordinated heme protein belonging to the globin family. Ngb is mainly expressed in neurons of the central and peripheral nervous system, although moderate levels of Ngb have been detected in non-nervous tissues. In the past decade, Ngb has been studied for its neuroprotective role in a large number of neurological disorders such as Alzheimer's disease, Huntington's disease, brain ischemia and hypoxia. This review discusses and summarizes the natural compounds and the small synthetic molecules capable of modulating Ngb expression that exhibits a protective role against various neurodegenerative diseases.

Extracellular Neuroglobin as a Stress-Induced Factor Activating Pre-Adaptation Mechanisms against Oxidative Stress and Chemotherapy-Induced Cell Death in Breast Cancer

Components of tumor microenvironment, including tumor and/or stromal cells-derived factors, exert a critical role in breast cancer (BC) progression. Here we evaluated the possible role of neuroglobin (NGB), a monomeric globin that acts as a compensatory protein against oxidative and apoptotic processes, as part of BC microenvironment. The extracellular NGB levels were evaluated by immunofluorescence of BC tissue sections and by Western blot of the culture media of BC cell lines. Moreover, reactive oxygen species (ROS) generation, cell apoptosis, and cell migration were evaluated in different BC cells and non-tumorigenic epithelial mammary cells treated with BC cells (i.e., Michigan Cancer Foundation-7, MCF-7) conditioned culture media and extracellular NGB. Results demonstrate that NGB is a component of BC microenvironment. NGB is released in tumor microenvironment by BC cells only under oxidative stress conditions where it can act as autocrine/paracrine factor able to communicate cell resilience against oxidative stress and chemotherapeutic treatment.

Lessons from the post-genomic era: Globin diversity beyond oxygen binding and transport

Vertebrate hemoglobin (Hb) and myoglobin (Mb) were among the first proteins whose structures and sequences were determined over 50 years ago. In the subsequent pregenomic period, numerous related proteins came to light in plants, invertebrates and bacteria, that shared the myoglobin fold, a signature sequence motif characteristic of a 3-on-3 α-helical sandwich. Concomitantly, eukaryote and bacterial globins with a truncated 2-on-2 α-helical fold were discovered. Genomic information over the last 20 years has dramatically expanded the list of known globins, demonstrating their existence in a limited number of archaeal genomes, a majority of bacterial genomes and an overwhelming majority of eukaryote genomes. In vertebrates, 6 additional globin types were identified, namely neuroglobin (Ngb), cytoglobin (Cygb), globin E (GbE), globin X (GbX), globin Y (GbY) and androglobin (Adgb). Furthermore, functions beyond the familiar oxygen transport and storage have been discovered within the vertebrate globin family, including NO metabolism, peroxidase activity, scavenging of free radicals, and signaling functions. The extension of the knowledge on globin functions suggests that the original roles of bacterial globins must have been enzymatic, involved in defense against NO toxicity, and perhaps also as sensors of O₂, regulating taxis away or towards high O₂ concentrations. In this review, we aimed to discuss the evolution and remarkable functional diversity of vertebrate globins with particular focus on the variety of non-canonical expression sites of mammalian globins and their according impressive variability of atypical functions.

The Modulatory Effect of Metformin on Ethanol-Induced Anxiety, Redox Imbalance, and Extracellular Matrix Levels in the Brains of Wistar Rats

The study investigated the potential neuroprotective effects of metformin (MET) on alcohol-induced neurotoxicity in adult Wistar rats. The animals were randomized in four groups (n = 10): control, alcohol (ALC), ALC + MET, and MET. ALC (2 g/kg b.w.) and MET (200 mg/kg b.w.) were orally administered for 21 days, once daily. For the ALC + MET group, MET was administered 2 h after ALC treatment. On day 22, the open field test (OFT) and elevated plus maze (EPM) were performed. MET improved global activity and increased the time spent in unprotected open arms, decreased oxidative stress, both in the frontal lobe and in the hippocampus, and increased neuroglobin expression in the frontal cortex. Histopathologically, an increased neurosecretory activity in the frontal cortex in the ALC + MET group was noticed. Thus, our findings suggest that metformin has antioxidant and anxiolytic effects and may partially reverse the neurotoxic effects induced by ethanol.

Negative surface charges in neuroglobin modulate the interaction with cytochrome c

Neuroglobin is a heme protein present in the nervous system cells of mammals and other organisms. Although cytoprotective effects of neuroglobin on neuronal damage have been reported, the physiological mechanisms of neuroglobin function remain unknown. In recent years, a role for neuroglobin as a reductant for extramitochondrial cytochrome c has been proposed. According to this hypothesis, cytoplasmic neuroglobin can interact with cytochrome c released from the mitochondria and reduce its heme group to the ferrous state, thus preventing cytochrome c-dependent assembly of the apoptosome. The interaction of neuroglobin and cytochrome c has been studied by surface plasmon resonance techniques and molecular dynamics, however the empirical evidence on the specific residues of neuroglobin and cytochrome c involved in the interaction is scarce and indirect. This study analyzes the role of five negatively charged residues in the neuroglobin surface putatively involved in the interaction with cytochrome c - Glu60, Asp63, Asp73, Glu 87 and Glu151 - by site-directed mutagenesis. Characterization of the electron transfer between neuroglobin mutants and cytochrome c indicates that Asp73 is critical for the interaction, and Glu60, Asp63 and Glu87 also contribute to the neuroglobin-cytochrome c interaction. Based on the results, structures and binding surfaces for the neuroglobin-cytochrome c complex compatible with the experimental observations are proposed. These data can guide further studies on neuroglobin function and its involvement in cytochrome c signaling cascades.