Neuron-specific expression of neuroglobin in mammals

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.

Downregulation of glob1 suppresses pathogenesis of human neuronal tauopathies in Drosophila by regulating tau phosphorylation and ROS generation

Human tauopathies represent a group of neurodegenerative disorders, characterized by abnormal hyperphosphorylation and aggregation of tau protein, which ultimately cause neurodegeneration. The aberrant tau hyperphosphorylation is mostly attributed to the kinases/phosphatases imbalance, which is majorly contributed by the generation of reactive oxygen species (ROS). Globin(s) represent a well-conserved group of proteins which are involved in O₂ management, regulation of cellular ROS in different cell types. Similarly, Drosophila globin1 (a homologue of human globin) with its known roles in oxygen management and development of nervous system exhibits striking similarities with the mammalian neuroglobin. Several recent evidences support the hypothesis that neuroglobins are associated with Alzheimer's disease pathogenesis. We herein noted that targeted expression of human-tau induces the cellular level of Glob1 protein in Drosophila tauopathy models. Subsequently, RNAi mediated restored level of Glob1 restricts the pathogenic effect of human-tau by minimizing its hyperphosphorylation via GSK-3β/p-Akt and p-JNK pathways. In addition, it also activates the Nrf2-keap1-ARE cascade to stabilize the tau-mediated increased level of ROS. These two parallel cellular events provide a significant rescue against human tau-mediated neurotoxicity in the fly models. For the first time we report a direct involvement of an oxygen sensing globin gene in tau etiology. In view of the fact that human genome encodes for the multiple Globin proteins including a nervous system specific neuroglobin; and therefore, our findings may pave the way to investigate if the conserved oxygen sensing globin gene(s) can be exploited in devising novel therapeutic strategies against tauopathies.

Neuroglobin as a Novel Biomarker for Predicting Poor Outcomes in Aneurysmal Subarachnoid Hemorrhage

OBJECTIVE

Neuroglobin (Ngb) has a high affinity for oxygen and helps prevent hypoxic-ischemic brain damage. In this study we analyzed the relationship between Ngb levels and clinical outcomes of aneurysmal subarachnoid hemorrhage (aSAH).

METHODS

Serum Ngb levels were measured in 58 patients with aSAH and 27 control individuals using the enzyme-linked immunosorbent assay. To continuously assess aSAH, we measured serum Ngb levels on days 1, 2, 3, 5, and 7 after aSAH. Clinical data were collected using the Hunt and Hess Scale, the Glasgow Coma Scale (GCS), the World Federation of Neurological Surgeons (WFNS) Scale, and the modified Fisher Scale. Clinical outcomes included 6-month mortality and 6-month unfavorable outcomes (modified Rankin Scale (mRS) score of 3-6).

RESULTS

Serum Ngb levels increased after aSAH, peaked on day 2, and then gradually decreased. Serum Ngb levels on admission were higher in the patient group than in the control group (7.67 ± 2.56 ng/mL vs. 6.45 ± 0.88 ng/mL, P < 0.05). Multivariate logistic regression analysis indicated that serum Ngb levels on day 2 after aSAH were independently related to 6-month mortality (odds ratio [OR] = 0.265, 95% confidence interval [CI] = 0.094-0.747, P < 0.05) and 6-month unfavorable outcomes (OR = 1.919, 95% CI = 1.158-3.180, P < 0.05), and receiver operating characteristic curve analysis showed that serum Ngb levels on day 2 predicted 6-month mortality and 6-month unfavorable outcomes, with areas under the curve of 0.893 (P < 0.05; 95% CI, 0.812-0.974) and 0.818 (P < 0.05; 95% CI, 0.691-0.954), respectively, based on the best thresholds.

CONCLUSIONS

Serum Ngb levels on day 2 after aSAH were strongly associated with poor outcomes in aSAH, suggesting that Ngb may be a novel biomarker for predicting poor outcomes in aSAH.

Cytoglobin Promotes Cardiac Progenitor Cell Survival against Oxidative Stress via the Upregulation of the NFκB/iNOS Signal Pathway and Nitric Oxide Production

Human cardiac stem/progenitor cells (hCPCs) may serve in regenerative medicine to repair the infarcted heart. However, this approach is severely limited by the poor survival of donor cells. Recent studies suggest that the mammalian globin cytoglobin (CYGB) regulates nitric oxide (NO) metabolism and cell death. In the present study, we found that CYGB is expressed in hCPCs. Through molecular approaches aimed at increasing or decreasing CYGB expression in hCPCs, we found that CYGB functions as a pro-survival factor in response to oxidative stress. This was associated with the upregulation of primary antioxidant systems such as peroxiredoxins-1, heme oxygenase-1, and anti-apoptotic factors, including BCL2, BCL-XL, and MCL1. Most significantly, we established that CYGB increased the expression of NFкB-dependent genes including iNOS, and that iNOS-dependent NO production was required for a feedforward loop that maintains CYGB expression. Our study delineates for the first time a role for a globin in regulating hCPC survival and establishes mechanistic insights in the function of CYGB. It provides a rationale for the exploration of the CYGB pathway as a molecular target that can be used to enhance the effectiveness of cardiac stem/progenitor cell therapy for ischemic heart disease.

Impaired hypoxic tolerance in APP23 mice: a dysregulation of neuroprotective globin levels

Although neuroglobin confers neuroprotection against Alzheimer's disease (AD) pathology, its expression becomes downregulated in late-stage AD. Here, we provide evidence that indicates that this decrease is associated with the AD-linked angiopathy. While wild-type mice of different ages show upregulated cerebral neuroglobin expression upon whole-body hypoxia, APP23 mice exhibit decreased cerebral transcription of neuroglobin. Interestingly, transcription of cytoglobin, whose involvement in amyloid pathology still needs to be elucidated, follows a similar pattern. To further unravel the underlying mechanism, we examined the expression levels of the RE-1-silencing transcription factor (REST/NRSF) after identifying a recognition site for it in the regulatory region of both globins. Neuroglobin-cytoglobin-REST/NRSF expression correlations are detected mainly in the cortex. This raises the possibility of REST/NRSF being an upstream regulator of these globins.

Phe28B10 Induces Channel-Forming Cytotoxic Amyloid Fibrillation in Human Neuroglobin, the Brain-Specific Hemoglobin

Since its discovery, neuroglobin (Ngb), a neuron-specific oxygen binding hemoglobin, distinct from the classical myoglobin and blood hemoglobin, has attracted attention as an endogenous neuroprotectant. Recent reports suggest that Ngb protects neurons from brain stroke, ischemic stress-induced degeneration, and other brain disorders. Proteins with a specific role in neuroprotection are often associated with neurodegeneration, as well, depending on the cellular environment or specific cellular triggers that tilt the balance one way or the other. This investigation explored the potential role of Ngb in amyloid fibril-related neuronal disorder. Ngb was capable of amyloid formation in vitro at neutral pH and ambient temperature, in both apo and holo forms, albeit at a slower rate in the holo form, unlike other hemoglobins that exhibit such behavior exclusively in the apo states. Elevated temperature enhanced the rate of fibril formation significantly. The B-helix, which is known to play a major role in Ngb ligand binding kinetics, was found to be amyloidogenic with the Phe28^B10 amino acid side chain as the key inducer of fibrillation. The Ngb amyloid fibril was also significantly cytotoxic to neuroblastoma cell lines, compared to those obtained from reference hemoglobins. The Ngb fibril probably promoted toxicity by inducing channel formation in the cell membrane, as investigated here using synthetic lipid bilayer membranes and the propidium iodide uptake assay. These findings imply that Ngb plays a role in neurodegenerative disorders in vivo, for which there seems to be indirect evidence by association. Ngb thus presents a novel prospect for understanding amyloid-related brain disorders beyond the limited set of proteins currently investigated for such diseases.

Lactucopicrin ameliorates oxidative stress mediated by scopolamine-induced neurotoxicity through activation of the NRF2 pathway

Cholinergic activity plays a vital role in cognitive function, and is reduced in individuals with neurodegenerative diseases. Scopolamine, a muscarinic cholinergic antagonist, has been employed in many studies to understand, identify, and characterize therapeutic targets for Alzheimer's disease (AD). Scopolamine-induced dementia is associated with impairments in memory and cognitive function, as seen in patients with AD. The current study aimed to investigate the molecular mechanisms underlying scopolamine-induced cholinergic neuronal dysfunction and the neuroprotective effect of lactucopicrin, an inhibitor of acetylcholine esterase (AChE). We investigated apoptotic cell death, caspase activation, generation of reactive oxygen species (ROS), mitochondrial dysfunction, and the expression levels of anti- and pro-apoptotic proteins in scopolamine-treated C6 cells. We also analyzed the expression levels of antioxidant enzymes and nuclear factor (erythroid-derived 2)-like 2 (NRF2) in C6 cells and neurite outgrowth in N2a neuroblastoma cells. Our results revealed that 1 h scopolamine pre-treatment induced cytotoxicity by increasing apoptotic cell death via oxidative stress-mediated caspase 3 activation and mitochondrial dysfunction. Scopolamine also downregulated the expression the antioxidant enzymes superoxide dismutase, glutathione peroxidase, and catalase, and the transcription factor NRF2. Lactucopicrin treatment protected C6 cells from scopolamine-induced toxicity by reversing the effects of scopolamine on those markers of toxicity. In addition, scopolamine attenuated the secretion of neurotrophic nerve growth factor (NGF) in C6 cells and neurite outgrowth in N2a cells. As expected, lactucopicrin treatment enhanced NGF secretion and neurite outgrowth. Our study is the first to show that lactucopicrin, a potential neuroprotective agent, ameliorates scopolamine-induced cholinergic dysfunction via NRF2 activation and subsequent expression of antioxidant enzymes.

Distribution of Cytoglobin in the Mouse Brain

Cytoglobin (Cygb) is a vertebrate globin with so far poorly defined function. It is expressed in the fibroblast cell-lineage but has also been found in neurons. Here we provide, using immunohistochemistry, a detailed study on the distribution of Cygb in the mouse brain. While Cygb is a cytoplasmic protein in active cells of the supportive tissue, in neurons it is located in the cytoplasm and the nucleus. We found the expression of Cygb in all brain regions, although only a fraction of the neurons was Cygb-positive. Signals were of different intensity ranging from faint to very intense. Telencephalic neurons in all laminae of the cerebral cortex (CCo), in the olfactory bulb (in particular periglomerular cells), in the hippocampal formation (strongly stained pyramidal cells with long processes), basal ganglia (scattered multipolar neurons in the dorsal striatum, dorsal and ventral pallidum (VP)), and in the amygdala (neurons with unlabeled processes) were labeled by the antibody. In the diencephalon, we observed Cygb-positive neurons of moderate intensity in various nuclei of the dorsal thalamus, in the hypothalamus, metathalamus (geniculate nuclei), epithalamus with strong labeling of habenular nucleus neurons and no labeling of pineal cells, and in the ventral thalamus. Tegmental neurons stood out by strongly stained somata with long processes in, e.g., the laterodorsal nucleus. In the tectum, faintly labeled neurons and fibers were detected in the superior colliculus (SC). The cerebellum exhibited unlabeled Purkinje-neurons but signs of strong afferent cortical innervation. Neurons in the gray matter of the spinal cord showed moderate immunofluorescence. Peripheral ganglia were not labeled by the antibody. The Meynert-fascicle and the olfactory and optic nerves/tracts were the only Cygb-immunoreactive (Cygb-IR) fiber systems. Notably, we found a remarkable level of colocalization of Cygb and neuronal nitric oxide (NO)-synthase in neurons, which supports a functional association.

Long-term neuroglobin expression of human astrocytes following brain trauma

Neuroglobin (Ngb), a 17 kDa monomeric protein, was initially described as a vertebrate oxygen-binding heme protein in 2000 and detected in metabolically active organs or cells, like the brain, peripheral nervous system as well as certain endocrine cells. A large array of initial experimental work reported that Ngb displayed a neuron restricted expression pattern in mammalian brains. However, growing evidence indicated astrocytes may also express Ngb under pathological conditions. To address the question whether human astrocytes express Ngb under traumatic insults, we investigated Ngb immuno-reactivity in post-mortem human brain tissues that died of acute, sub-acute and chronic brain trauma, respectively. We observed astrocytic Ngb expression in sub-acute and chronic traumatic brains rather than acute traumatic brains. Strikingly, the Ngb immuno-reactive astrocytes were still strongly detectable in groups that died 12 months after brain trauma. Our findings may imply an unexplored role of Ngb in astrocytes and the involved mechanisms were suggested to be further characterized. Also, therapeutic application of Ngb or Ngb-inducible chemical compounds in neuro-genesis or astrocytic scar forming can be expected.

Neuroglobin Expression in the Mammalian Auditory System

The energy-yielding pathways that provide the large amounts of metabolic energy required by inner ear sensorineural cells are poorly understood. Neuroglobin (Ngb) is a neuron-specific hemoprotein of the globin family, which is suggested to be involved in oxidative energy metabolism. Here, we present quantitative real-time reverse transcription PCR, in situ hybridization, immunohistochemical, and Western blot evidence that neuroglobin is highly expressed in the mouse and rat cochlea. For primary cochlea neurons, Ngb expression is limited to the subpopulation of type I spiral ganglion cells, those which innervate inner hair cells, while the subpopulation of type II spiral ganglion cells which innervate the outer hair cells do not express Ngb. We further investigated Ngb distribution in rat, mouse, and human auditory brainstem centers, and found that the cochlear nuclei and superior olivary complex (SOC) also express considerable amounts of Ngb. Notably, the majority of olivocochlear neurons, those which provide efferent innervation of outer hair cells as identified by neuronal tract tracing, were Ngb-immunoreactive. We also observed that neuroglobin in the SOC frequently co-localized with neuronal nitric oxide synthase, the enzyme responsible for nitric oxide production. Our findings suggest that neuroglobin is well positioned to play an important physiologic role in the oxygen homeostasis of the peripheral and central auditory nervous system, and provides the first evidence that Ngb signal differentiates the central projections of the inner and outer hair cells.

Hematopoietic stem/progenitor cells express myoglobin and neuroglobin: adaptation to hypoxia or prevention from oxidative stress?

Oxidative metabolism and redox signaling prove to play a decisional role in controlling adult hematopoietic stem/progenitor cells (HSPCs) biology. However, HSPCs reside in a hypoxic bone marrow microenvironment raising the question of how oxygen metabolism might be ensued. In this study, we provide for the first time novel functional and molecular evidences that human HSPCs express myoglobin (Mb) at level comparable with that of a muscle-derived cell line. Optical spectroscopy and oxymetry enabled to estimate an O2-sensitive heme-containing protein content of approximately 180 ng globin per 10(6) HSPC and a P50 of approximately 3 µM O2. Noticeably, expression of Mb mainly occurs through a HIF-1-induced alternative transcript (Mb-V/Mb-N = 35 ± 15, p < .01). A search for other Mb-related globins unveiled significant expression of neuroglobin (Ngb) but not of cytoglobin. Confocal microscopy immune detection of Mb in HSPCs strikingly revealed nuclear localization in cell subsets expressing high level of CD34 (nuclear/cytoplasmic Mb ratios 1.40 ± 0.02 vs. 0.85 ± 0.05, p < .01) whereas Ngb was homogeneously distributed in all the HSPC population. Dual-color fluorescence flow cytometry indicated that while the Mb content was homogeneously distributed in all the HSPC subsets that of Ngb was twofold higher in more immature HSPC. Moreover, we show that HSPCs exhibit a hypoxic nitrite reductase activity releasing NO consistent with described noncanonical functions of globins. Our finding extends the notion that Mb and Ngb can be expressed in nonmuscle and non-neural contexts, respectively, and is suggestive of a differential role of Mb in HSPC in controlling oxidative metabolism at different stages of commitment.

Function and evolution of vertebrate globins

Globins are haem-proteins that bind O2 and thus play an important role in the animal's respiration and oxidative energy production. However, globins may also have other functions such as the decomposition or production of NO, the detoxification of reactive oxygen species or intracellular signalling. In addition to the well-investigated haemoglobins and myoglobins, genome sequence analyses have led to the identification of six further globin types in vertebrates: androglobin, cytoglobin, globin E, globin X, globin Y and neuroglobin. Here, we review the present state of knowledge on the functions, the taxonomic distribution and evolution of vertebrate globins, drawing conclusions about the functional changes underlying present-day globin diversity.

Effect of neuroglobin genetically modified bone marrow mesenchymal stem cells transplantation on spinal cord injury in rabbits

Objective

This study aims to investigate the potentially protective effect of neuroglobin (Ngb) gene-modified bone marrow mesenchymal stem cells (BMSCs) on traumatic spinal cord injury (SCI) in rabbits.

Methods

A lentiviral vector containing an Ngb gene was constructed and used to deliver Ngb to BMSCs. Ngb gene-modified BMSCs were then injected at the SCI sites 24 hours after SCI. The motor functions of the rabbits were evaluated by the Basso–Beattie–Bresnahan rating scale. Fluorescence microscopy, quantitative real-time PCRs, Western blots, malondialdehyde (MDA) tests, and terminal deoxynucleotidyltransferase-mediated UTP end labeling assays were also performed.

Results

Ngb expression in the Ngb-BMSC group increased significantly. A more significant functional improvement was observed in the Ngb-BMSC group compared with those in the other groups. Traumatic SCI seemingly led to an increase in MDA level and number of apoptotic cells, which can be prevented by Ngb-BMSC treatment.

Conclusion

This study demonstrates that Ngb gene-modified BMSCs can strengthen the therapeutic benefits of BMSCs in reducing secondary damage and improving the neurological outcome after traumatic SCI. Therefore, the combined strategy of BMSC transplantation and Ngb gene therapy can be used to treat traumatic SCI.

Neuroglobin involvement in visual pathways through the optic nerve

Neuroglobin is a member of the globin superfamily proposed to be only expressed in neurons and involved in neuronal protection from hypoxia or oxidative stress. A significant fraction of the protein localizes within the mitochondria and is directly associated with mitochondrial metabolism and integrity. The retina is the site of the highest concentration for neuroglobin and has been reported to be up to 100-fold higher than in the brain. Since neuroglobin was especially abundant in retinal ganglion cell layer, we investigated its abundance in optic nerves. Remarkably in optic nerves, neuroglobin is observed, as expected, in retinal ganglion cell axon profiles but also astrocyte processes, in physiological conditions, possess high levels of the protein. Neuroglobin mRNA and protein levels are ~10-fold higher in optic nerves than in retinas, indicating an important accumulation of neuroglobin in these support cells. Additionally, neuroglobin levels increase in Müller cells during reactive gliosis in response to eye injury. This suggests the pivotal role of neuroglobin in retinal glia involved in neuronal support and/or healing. This article is part of a Special Issue entitled: Oxygen Binding and Sensing Proteins.

The anti-apoptotic role of neuroglobin

The small heme-protein neuroglobin is expressed at high concentrations in certain brain neurons and in the rod cells of the retina. This paper reviews the many studies which have recently identified a protective role for neuroglobin, in a wide range of situations involving apoptotic cell death. The origins of this protective mechanism are discussed in terms of both experimental results and computational modeling of the intrinsic pathway of apoptosis, which shows that neuroglobin can intervene in this process by a reaction with released mitochondrial cytochrome c. An integrated model, based on the various molecular actions of both neuroglobin and cytochrome c, is developed, which accounts for the cellular distribution of neuroglobin.

Comparative genomics of neuroglobin reveals its early origins

BACKGROUND

Neuroglobin (Ngb) is a hexacoordinated globin expressed mainly in the central and peripheral nervous system of vertebrates. Although several hypotheses have been put forward regarding the role of neuroglobin, its definite function remains uncertain. Ngb appears to have a neuro-protective role enhancing cell viability under hypoxia and other types of oxidative stress. Ngb is phylogenetically ancient and has a substitution rate nearly four times lower than that of other vertebrate globins, e.g. hemoglobin. Despite its high sequence conservation among vertebrates Ngb seems to be elusive in invertebrates.

PRINCIPAL FINDINGS

We determined candidate orthologs in invertebrates and identified a globin of the placozoan Trichoplax adhaerens that is most likely orthologous to vertebrate Ngb and confirmed the orthologous relationship of the polymeric globin of the sea urchin Strongylocentrotus purpuratus to Ngb. The putative orthologous globin genes are located next to genes orthologous to vertebrate POMT2 similarly to localization of vertebrate Ngb. The shared syntenic position of the globins from Trichoplax, the sea urchin and of vertebrate Ngb strongly suggests that they are orthologous. A search for conserved transcription factor binding sites (TFBSs) in the promoter regions of the Ngb genes of different vertebrates via phylogenetic footprinting revealed several TFBSs, which may contribute to the specific expression of Ngb, whereas a comparative analysis with myoglobin revealed several common TFBSs, suggestive of regulatory mechanisms common to globin genes.

SIGNIFICANCE

Identification of the placozoan and echinoderm genes orthologous to vertebrate neuroglobin strongly supports the hypothesis of the early evolutionary origin of this globin, as it shows that neuroglobin was already present in the placozoan-bilaterian last common ancestor. Computational determination of the transcription factor binding sites repertoire provides on the one hand a set of transcriptional factors that are responsible for the specific expression of the Ngb genes and on the other hand a set of factors potentially controlling expression of a couple of different globin genes.

Neuroglobin of seals and whales: evidence for a divergent role in the diving brain

Although many physiological adaptations of diving mammals have been reported, little is known about how their brains sustain the high demands for metabolic energy and thus O(2) when submerged. A recent study revealed in the deep-diving hooded seal (Cystophora cristata) a unique shift of the oxidative energy metabolism and neuroglobin, a respiratory protein that is involved in neuronal hypoxia tolerance, from neurons to astrocytes. Here we have investigated neuroglobin in another pinniped species, the harp seal (Pagophilus groenlandicus), and in two cetaceans, the harbor porpoise (Phocoena phocoena) and the minke whale (Balaenoptera acutorostrata). Neuroglobin sequences, expression levels and patterns were compared with those of terrestrial relatives, the ferret (Mustela putorius furo) and the cattle (Bos taurus), respectively. Neuroglobin sequences of whales and seals only differ in two or three amino acids from those of cattle and ferret, and are unlikely to confer functional differences, e.g. in O(2) affinity. Neuroglobin is expressed in the astrocytes also of P. groenlandicus, suggesting that the shift of neuroglobin and oxidative metabolism is a common adaptation in the brains of deep-diving phocid seals. In the cetacean brain neuroglobin resides in neurons, like in terrestrial mammals. However, neuroglobin mRNA expression levels were 4-15 times higher in the brains of harbor porpoises and minke whales than in terrestrial mammals or in seals. Thus neuroglobin appears to play a specific role in diving mammals, but seals and whales have evolved divergent strategies to cope with cerebral hypoxia. The specific function of neuroglobin that conveys hypoxia tolerance may either relate to oxygen supply or protection from reactive oxygen species. The different strategies in seals and whales resulted from a divergent evolution and an independent adaptation to diving.

The expression of neuroglobin in astrocytoma

Neuroglobin (NGB) is a characterized heme globin that is widely expressed in the vertebrate central and peripheral nervous system as well as retina and endocrine tissues. However, to date, it is not determined whether functional NGB is expressed in cells of glia origin. In this study, we aimed to explore the detailed expression of NGB in a rat astrocytoma cell line (C6) and human astrocytoma cell line (U251) by reverse transcription-polymerase chain reaction, immunofluorescence, and Western blotting, and to detect the expression of NGB in human astrocytoma tissues by an immunohistochemical method. We found that NGB was present in a rat astrocytoma cell line (C6), human astrocytoma cell line (U251), and human astrocytoma tissues. The expression and potential roles of NGB in astrocytomas may provide insight into the mechanisms of tumor cells to adapt and survive in hypoxic microenvironments and also represent a novel therapeutic approach to astrocytomas.

Old proteins - new locations: myoglobin, haemoglobin, neuroglobin and cytoglobin in solid tumours and cancer cells

AIM

The unexpected identification of myoglobin (MB) in breast cancer prompted us to evaluate the clinico-pathological value of MB, haemoglobin (HB) and cytoglobin (CYGB) in human breast carcinoma cases. We further screened for the presence of neuroglobin (NGB) and CYGB in tumours of diverse origin, and assessed the O(2) -response of HB, MB and CYGB mRNAs in cancer cell lines, to better elicit the links between this ectopic globin expression and tumour hypoxia.

METHODS

Breast tumours were analysed by immunohistochemistry for HB, MB and CYGB and correlated with clinico-pathological parameters. Screening for CYGB and NGB mRNA expression in tumour entities was performed by hybridization, quantitative PCR (qPCR) and bioinformatics. Hypoxic or anoxic responses of HB, MB and CYGB mRNAs was analysed by qPCR in human Hep3B, MCF7, HeLa and RCC4 cancer cell lines.

RESULTS

78.8% of breast cancer cases were positive for MB, 77.9% were positive for HB and 55.4% expressed CYGB. The closest correlation with markers of hypoxia was observed for CYGB. Compared to the weakly positive status of MB in healthy breast tissues, invasive tumours either lost or up-regulated MB. Breast carcinomas showed the tendency to silence CYGB. HB was not seen in normal tissues and up-regulated in tumours. Beyond breast malignancies, expression levels of NGB and CYGB mRNAs were extremely low in brain tumours (glioblastoma, astrocytoma). NGB was not observed in non-brain tumours. CYGB mRNA, readily detectable in breast cancer and other tumours, is down-regulated in lung adenocarcinomas. Alpha1 globin (α1 globin) and Mb were co-expressed in MCF7 and HeLa cells; CYGB transcription was anoxia-inducible in Hep3B and RCC4 cells.

CONCLUSIONS

This is the first time that HB and CYGB are reported in breast cancer. Neither NGB nor CYGB are systematically up-regulated in tumours. The down-regulated CYGB expression in breast and lung tumours is in line with a tumour-suppressor role. Each of the screened cancer cells expresses at least one globin (i.e. main globin species: CYGB in Hep3B; α1 globin + MB in MCF7 and HeLa). Thus, globins exist in a wide variety of solid tumours. However, the generally weak expression of the endogenous proteins in the cancer argues against a significant contribution to tumour oxygenation. Future studies should consider that cancer-expressed globins might function in ways not directly linked to the binding and transport of oxygen.

Examination of glucose transporter-1, transforming growth factor-β and neuroglobin immunoreactivity in the orbitofrontal cortex in late-life depression

AIMS

  This study immunohistochemically examined the orbitofrontal cortex for three possible candidates in hypoxic/ischemic signaling: the cytokine transforming growth factor-β, the glucose transporter-1 and the neuron-specific oxygen-binding protein neuroglobin.

METHODS

  Post-mortem tissue from 20 depressed and 20 non-depressed individuals was obtained and the expression of the three proteins was analyzed using image analysis software.

RESULTS

  No significant changes were found in transforming growth factor-β or neuroglobin in the orbitofrontal cortex between depressed and non-depressed individuals. There was, however, a trend towards a reduction in glucose transporter-1 in the depressed group.

CONCLUSIONS

  This study does not clearly support the hypothesis that hypoxic/ischemic processes are behind the pathological deficits in the frontal-subcortical circuitry associated with depression and therefore does not provide evidence to support the 'vascular depression' hypothesis.

Neuroglobin, cytoglobin, and myoglobin contribute to hypoxia adaptation of the subterranean mole rat Spalax

The subterranean mole rat Spalax is an excellent model for studying adaptation of a mammal toward chronic environmental hypoxia. Neuroglobin (Ngb) and cytoglobin (Cygb) are O(2)-binding respiratory proteins and thus candidates for being involved in molecular hypoxia adaptations of Spalax. Ngb is expressed primarily in vertebrate nerves, whereas Cygb is found in extracellular matrix-producing cells and in some neurons. The physiological functions of both proteins are not fully understood but discussed with regard to O(2) supply, the detoxification of reactive oxygen or nitrogen species, and apoptosis protection. Spalax Ngb and Cygb coding sequences are strongly conserved. However, mRNA and protein levels of Ngb in Spalax brain are 3-fold higher than in Rattus norvegicus under normoxia. Importantly, Spalax expresses Ngb in neurons and additionally in glia, whereas in hypoxia-sensitive rodents Ngb expression is limited to neurons. Hypoxia causes an approximately 2-fold down-regulation of Ngb mRNA in brain of rat and mole rat. A parallel regulatory response was found for myoglobin (Mb) in Spalax and rat muscle, suggesting similar functions of Mb and Ngb. Cygb also revealed an augmented normoxic expression in Spalax vs. rat brain, but not in heart or liver, indicating distinct tissue-specific functions. Hypoxia induced Cygb transcription in heart and liver of both mammals, with the most prominent mRNA up-regulation (12-fold) in Spalax heart. Our data suggest that tissue globins contribute to the remarkable tolerance of Spalax toward environmental hypoxia. This is consistent with the proposed cytoprotective effect of Ngb and Cygb under pathological hypoxic/ischemic conditions in mammals.

Mutations in FLVCR1 cause posterior column ataxia and retinitis pigmentosa

The study of inherited retinal diseases has advanced our knowledge of the cellular and molecular mechanisms involved in sensory neural signaling. Dysfunction of two specific sensory modalities, vision and proprioception, characterizes the phenotype of the rare, autosomal-recessive disorder posterior column ataxia and retinitis pigmentosa (PCARP). Using targeted DNA capture and high-throughput sequencing, we analyzed the entire 4.2 Mb candidate sequence on chromosome 1q32 to find the gene mutated in PCARP in a single family. Employing comprehensive bioinformatic analysis and filtering, we identified a single-nucleotide coding variant in the feline leukemia virus subgroup C cellular receptor 1 (FLVCR1), a gene encoding a heme-transporter protein. Sanger sequencing confirmed the FLVCR1 mutation in this family and identified different homozygous missense mutations located within the protein's transmembrane channel segment in two other unrelated families with PCARP. To determine whether the selective pathologic features of PCARP correlated with FLVCR1 expression, we examined wild-type mouse Flvcr1 mRNA levels in the posterior column of the spinal cord and the retina via quantitative real-time reverse-transcriptase PCR. The Flvcr1 mRNA levels were most abundant in the retina, followed by the posterior column of the spinal cord and other brain regions. These results suggest that aberrant FLVCR1 causes a selective degeneration of a subpopulation of neurons in the retina and the posterior columns of the spinal cord via dysregulation of heme or iron homeostasis. This finding broadens the molecular basis of sensory neural signaling to include common mechanisms that involve proprioception and vision.

Hypoxic regulation of cytoglobin and neuroglobin expression in human normal and tumor tissues

Background

Cytoglobin (Cygb) and neuroglobin (Ngb) are recently identified globin molecules that are expressed in vertebrate tissues. Upregulation of Cygb and Ngb under hypoxic and/or ischemic conditions in vitro and in vivo increases cell survival, suggesting possible protective roles through prevention of oxidative damage. We have previously shown that Ngb is expressed in human glioblastoma multiforme (GBM) cell lines, and that expression of its transcript and protein can be significantly increased after exposure to physiologically relevant levels of hypoxia. In this study, we extended this work to determine whether Cygb is also expressed in GBM cells, and whether its expression is enhanced under hypoxic conditions. We also compared Cygb and Ngb expression in human primary tumor specimens, including brain tumors, as well as in human normal tissues. Immunoreactivity of carbonic anhydrase IX (CA IX), a hypoxia-inducible metalloenzyme that catalyzes the hydration of CO₂ to bicarbonate, was used as an endogenous marker of hypoxia.

Results

Cygb transcript and protein were expressed in human GBM cells, and this expression was significantly increased in most cells following 48 h incubation under hypoxia. We also showed that Cygb and Ngb are expressed in both normal tissues and human primary cancers, including GBM. Among normal tissues, Cygb and Ngb expression was restricted to distinct cell types and was especially prominent in ductal cells. Additionally, certain normal organs (e.g. stomach fundus, small bowel) showed distinct regional co-localization of Ngb, Cygb and CA IX. In most tumors, Ngb immunoreactivity was significantly greater than that of Cygb. In keeping with previous in vitro results, tumor regions that were positively stained for CA IX were also positive for Ngb and Cygb, suggesting that hypoxic upregulation of Ngb and Cygb also occurs in vivo.

Conclusions

Our finding of hypoxic up-regulation of Cygb/Ngb in GBM cell lines and human tumor tissues suggests that these globin molecules may be part of the repertoire of defense mechanisms that allow cancer cells to survive in hypoxic microenvironments.

Regulation of the NRSF/REST gene by methylation and CREB affects the cellular phenotype of small-cell lung cancer

The neuron-restrictive silencer factor/RE1-silencing transcription factor (NRSF/REST) is a negative regulator of gene expression restricting the expression of neuronal genes to the nervous system. NRSF/REST is highly expressed in non-neuronal tissues like the lung. In previous work, we identified small-cell lung cancer (SCLC) cell lines with no detectable NRSF/REST expression that, as a consequence, expressed neuronal markers like L1-cell adhesion molecule (L1-CAM) and neural cell adhesion molecule (NCAM). The loss of NRSF/REST expression was linked to malignant progression; however, its mechanistic role remained elusive. Here, we show that NRSF/REST itself, rather than one of its regulated genes, acts like a classic tumour suppressor, being in part regulated by methylation. In SCLCs, NRSF/REST is positively regulated by CREB, with an NRSF/REST promoter fragment showing cell type specificity. Downstream, NRSF/REST directly regulates AKT2, in which NRSF/REST loss leads to an epidermal growth factor-mediated de-regulation of AKT-Serine473 phosphorylation, important for cellular proliferation and survival. Assaying anchorage-independent growth, we observed that with reduced NRSF/REST expression, proliferation was significantly enhanced, whereas NRSF/REST rescue decreased the potential of cells to grow anchorage independently. Our observations support the fact that NRSF/REST may act as an important modulator of malignant progression in SCLC.

When the brain goes diving: glial oxidative metabolism may confer hypoxia tolerance to the seal brain

Deep diving mammals have developed strategies to cope with limited oxygen availability when submerged. These adaptations are associated with an increased neuronal hypoxia tolerance. Brain neurons of the hooded seal Cystophora cristata remain much longer active in hypoxic conditions than those of mice. To understand the cellular basis of neuronal hypoxia tolerance, we studied neuroglobin and cytochrome c in C. cristata brain. Neuroglobin, a respiratory protein typically found in vertebrate neurons, displays three unique amino acid substitutions in hooded seal. However, these substitutions unlikely contribute to a modulation of O(2) affinity. Moreover, there is no significant difference in total neuroglobin protein levels in mouse, rat and seal brains. However, in terrestrial mammals neuroglobin resided exclusively in neurons, whereas in seals neuroglobin is mainly located in astrocytes. This unusual localization of neuroglobin is accompanied by a shift in the distribution of cytochrome c. In seals, this marker for oxidative metabolism is mainly localized in astrocytes, whereas in terrestrial mammals it is essentially found in neurons. Our results indicate that in seals aerobic ATP production depends significantly on astrocytes, while neurons rely less on aerobic energy metabolism. This adaptation may imbue seal neurons with an increased tolerance to hypoxia and potentially also to reactive oxygen species, and may explain in part the ability of deep diving mammals to sustain neuronal activity during prolonged dives.

Neuroprotective roles and mechanisms of neuroglobin

OBJECTIVE

The objectives of this work were to update and summarize recent experimental works on neuroglobin, mainly focus on its neuroprotective effects and the mechanisms.

METHODS

The literature was reviewed using PubMed database, and some of the recent findings from our laboratory were included.

RESULTS

Neuroglobin is a recently discovered tissue globin with a high affinity for oxygen and is widely and specifically expressed in neurons of vertebrate's central and peripheral nervous systems. Investigations in the past several years have advanced our knowledge on the functions and mechanisms of neuroglobin, but many issues remain unclear. Emerging reports have shown that overexpression of neuroglobin confers neuroprotection against neuronal hypoxia or ischemia-induced damage in cultured neurons and in cerebral ischemic animal models. Accumulating findings suggest several possible neuroprotective roles and mechanisms including ligand binding and oxygen sensing, modulation of cell signaling pathways and maintenance of mitochondria function.

CONCLUSION

Emerging experimental works suggest that neuroglobin is neuroprotective against hypoxic/ischemic insults, probably via ligand binding and oxygen sensing, modulation of cell signaling pathways and maintenance of mitochondria function.

Neuroglobin and prion cellular localization: investigation of a potential interaction

Neuroglobin (Ngb) and the cellular prion protein (PrP(c)), proteins of unknown function in the nervous system, are known to be expressed in the retina and have been observed in different rat retinal cells. The retina is the site of the highest concentration for Ngb, a heme protein of similar size and conformation to myoglobin. In this study, we demonstrated by immunohistochemical analysis of retinal colocalization of Ngb and PrP(c) in the ganglion cell layer. Considering for these two a common protective role in relation to oxidative stress and a possible transient contact during migration of PrP(c) through the eye or upon neuronal degradation, we undertook in vitro studies of the interaction of the purified proteins. Mixing these two proteins leads to rapid aggregation, even at submicromolar concentrations. As observed with the use of dynamic light scattering, particles comprising both proteins evolve to hundreds of nanometers within several seconds, a first report showing that PrP(c) is able to form aggregates without major structural changes. The main effect would then appear to be a protein-protein interaction specific to the surface charge of the Ngb protein with PrP(c) N-terminal sequence. A dominant parameter is the solvent ionic force, which can significantly modify the final state of aggregation. PrP(c), normally anchored to the cell membrane, is toxic in the cytoplasm, where Ngb is present; this could suggest an Ngb function of scavenging proteins capable of forming deleterious aggregates considering a charge complementarity in the complex.

Expression and hypoxic up-regulation of neuroglobin in human glioblastoma cells

Neuroglobin is a recently identified globin molecule that is expressed predominantly in the vertebrate brain. Neuroglobin expression increases in oxygen-deprived neurons, suggesting it protects neurons from ischemic cell death. We report that neuroglobin transcript and protein are expressed in human glioblastoma cells, and that this expression increases in hypoxia in vitro. We also show that neuroglobin is up-regulated in hypoxic microregions of glioblastoma tumor xenografts. Our finding of hypoxic up-regulation of neuroglobin in human glioblastoma cells may provide insight into how tumor cells adapt to and survive in hypoxic microenvironments.

Hypoxia differentially regulates the expression of neuroglobin and cytoglobin in rat brain

Neuroglobin (Ngb) and Cytoglobin (Cygb) are new members of the globin family and display heterotopic expression patterns. To examine the effect of different hypoxia profiles on expression of Ngb and Cygb in rodent brain, rats were exposed to either sustained hypoxia (SH; 10% O(2)) or intermittent hypoxia (IH; 10% and 21% O(2) alternating every 90 s) for 1, 3, 7 and 14 days, and mRNA and protein expression of Ngb and Cygb were assessed in brain cortex. SH increased Ngb mRNA and protein expression throughout the exposure, while IH only elicited slight increases in Ngb expression at day 1. Neither SH nor IH elicited increases in Cygb expression. Thus, hypoxic stimulus presentation is a major determinant of the regulation of hypoxic sensitive genes such as Ngb. Furthermore, disparities between Ngb and Cygb responses to hypoxia further suggest that these two globins may play divergent roles in brain.

Protective effect of a synthetic anti-oxidant on neuronal cell apoptosis resulting from experimental hypoxia re-oxygenation injury

Oxidative stress is associated with the pathology of acute and chronic neurodegenerative disease. Cultured neuronal cells exposed to hypoxia-reoxygenation (H/R) injury, as a model for stroke, yield a burst of reactive oxygen species (ROS) as measured with electron paramagnetic resonance (EPR) spectroscopy in combination with spin trapping. Added superoxide dismutase inhibited spin-adduct formation verifying that superoxide radical anion was formed in neuronal cells following H/R injury. The intracellular ADP/ATP ratio increased rapidly over the first 5 h following injury and this was due primarily to the decreased cellular pools of ATP, consistent with the notion that H/R promotes mitochondrial dysfunction leading to decreased ATP reserve and increased ROS formation. As an early response to the enhanced oxidative stress, genes encoding for hypoxia-inducible factor 1-alpha (HIF1-alpha), inducible haemoxygenase-1 (HO-1), and the oxygen-sensor neuroglobin increased significantly. Up-regulation of the HO-1 gene was paralleled by increased HO protein expression and activity. Despite this cellular response, apoptosis increased significantly following H/R injury indicating that the endogenous anti-oxidant defenses were unable to protect the cells. In contrast, addition of a phenolic anti-oxidant, bisphenol (BP), prior to H/R injury, inhibited ROS production and gene regulation and significantly decreased neuronal cell apoptosis. Added BP was converted stoichiometrically to the corresponding diphenoquinone indicating the synthetic anti-oxidant effectively decreased oxidative stress through a radical scavenging mechanism. Together, these data indicate that BP has the potential to act as a neuro-protective drug.

Increased neuroglobin levels in the cerebral cortex and serum after ischemia–reperfusion insults

Neuroglobin (NGB) is a newly discovered protein localized in neurons of the central and peripheral nervous systems in vertebrates. It functions to bind, store, and facilitate the utilization of oxygen in neuronal cells. Recent studies suggest that it may modulate hypoxic and ischemic injury. The major goal of the present study is to characterize the dynamic changes of NGB protein in the brain and serum in a global forebrain ischemia-reperfusion model using gerbils. The sensitivity and validity of serum NGB as a potential biomarker for brain injury were further evaluated. Global cerebral ischemia-reperfusion models were induced by bilateral carotid occlusion for 20 min followed with 2-, 8-, 16-, 24-, 48-, or 72-h reperfusion in forty-six Mongolian gerbils. Sham-operated and operated animals were sacrificed at the designated time after reperfusion. Brains were fixed for immunocytochemical study to evaluate the time-dependent expression of NGB, and the concentration of NGB in serum was measured by enzyme-linked immunosorbent assay. Our results showed that the expression of NGB was upregulated in the cerebral cortex but significantly downregulated in the hippocampus from 2 to 72 h of reperfusion after 20 min of bilateral common carotid arteries occlusion. The concentration of NGB in serum was significantly increased at 8 h and reached a peak at 48 h of reperfusion. There is a significant correlation between NGB levels in the serum and severity of neuronal damage in the gerbil brain. In summary, the upregulation of NGB in cerebral cortex and downregulation in hippocampus after reperfusion insults in the gerbil brain are consistent with the fact that cerebral cortex is more tolerant to hypoxic or ischemic injury than the hippocampus. Moreover, the changes of NGB levels in serum may be used to monitor the extent of brain damage in ischemic brain diseases.

Up-Regulation of Heme Biosynthesis during Differentiation of Neuro2a Cells

Heme is an iron-containing tetrapyrrole molecule that functions as a prosthetic group for proteins such as mitochondrial respiratory enzymes. Several studies have suggested that heme has essential functions in the construction and maintenance of the nervous system. In this study, the contents of three biologically important forms of heme (types a, b, and c) and the expression of heme biosynthetic enzymes were examined in differentiating Neuro2a cells. During neuronal differentiation, there were increases in the cellular heme levels and increases in the mRNA levels for the rate-limiting enzymes of heme biosynthesis, such as aminolevulinic acid synthase (ALAS; EC 2.3.1.37) and coproporphyrinogen oxidase (EC 1.3.3.3). With respect to heme contents, heme b increased in the late phase of differentiation, but no apparent increase in heme a or b was observed in the early phase. In contrast, heme c (cytochrome c) markedly increased during the early phase of differentiation. This change preceded the increase in heme b and the up-regulation of the mRNA levels for heme biosynthetic enzymes. This study suggests the up-regulation of heme biosynthesis and differential regulation of the heme a, b, and c levels during neuronal differentiation.

The nerve hemoglobin of the bivalve mollusc Spisula solidissima: molecular cloning, ligand binding studies, and phylogenetic analysis

Members of the hemoglobin (Hb) superfamily are present in nerve tissue of several vertebrate and invertebrate species. In vertebrates they display hexacoordinate heme iron atoms and are typically expressed at low levels (microM). Their function is still a matter of debate. In invertebrates they have a hexa- or pentacoordinate heme iron, are mostly expressed at high levels (mM), and have been suggested to have a myoglobin-like function. The native Hb of the surf clam, Spisula solidissima, composed of 162 amino acids, does not show specific deviations from the globin templates. UV-visible and resonance Raman spectroscopy demonstrate a hexacoordinate heme iron. Based on the sequence analogy, the histidine E7 is proposed as a sixth ligand. Kinetic and equilibrium measurements show a moderate oxygen affinity (P(50) approximately 0.6 torr) and no cooperativity. The histidine binding affinity is 100-fold lower than in neuroglobin. Phylogenetic analysis demonstrates a clustering of the S. solidissima nerve Hb with mollusc Hbs and myoglobins, but not with the vertebrate neuroglobins. We conclude that invertebrate nerve Hbs expressed at high levels are, despite the hexacoordinate nature of their heme iron, not essentially different from other intracellular Hbs. They most likely fulfill a myoglobin-like function and enhance oxygen supply to the neurons.

Divergent Distribution in Vascular and Avascular Mammalian Retinae Links Neuroglobin to Cellular Respiration

The visual function of the vertebrate retina relies on sufficient supply with oxygen. Neuroglobin is a respiratory protein thought to play an essential role in oxygen homeostasis of neuronal cells. For further understanding of its function, we compared the distribution of neuroglobin and mitochondria in both vascular and avascular mammalian retinae. In the vascular retinae of mouse and rat, oxygen is supplied by the outer choroidal, deep retinal, and inner capillaries. We show that in this type of retina, mitochondria are concentrated in the inner segments of photoreceptor cells, the outer and the inner plexiform layers, and the ganglion cell layer. These are the same regions in which oxygen consumption takes place and in which neuroglobin is present at high levels. In the avascular retina of guinea pig the deep retinal and inner capillaries are absent. Therefore, only the inner segments of the photoreceptors adjacent to choroidal capillaries display an oxidative metabolism. We demonstrate that in the retina of guinea pigs both neuroglobin and mitochondria are restricted to this layer. Our results clearly demonstrate an association of neuroglobin and mitochondria, thus supporting the hypothesis that neuroglobin is a respiratory protein that supplies oxygen to the respiratory chain.

Neuroglobin and cytoglobin: Two new entries in the hemoglobin superfamily*

Neuroglobin (Ngb) and cytoglobin (Cygb) are two newly discovered intracellular members of the vertebrate hemoglobin (Hb) family. Ngb, predominantly expressed in nerve cells, is of ancient evolutionary origin and is homologous to nerve-globins of invertebrates. Cygb, present in many different tissues, shares common ancestry with myoglobin (Mb) and can be traced to early vertebrate evolution. Ngb and Cygb display the classical three-on-three α-helical globin fold and are endowed with a hexa-coordinate heme Fe atom, in both their ferrous and ferric forms, having the heme distal HisE7 residue as the endogenous sixth ligand. Reversible intramolecular hexa- to penta-coordination of the heme Fe atom modulates Ngb and Cygb ligand-binding properties. In Ngb and Cygb, ligand migration to/from the heme distal site may be assisted by protein/matrix tunnel cavity systems. The physiological roles of Ngb and Cygb are poorly understood. Ngb may protect neuronal cells from hypoxic-ischemic insults, may act as oxidative stress-responsive sensor protein, and may sustain NO/O(2) scavenging and/or reactive oxygen species (ROS) detoxification. Cygb, located in the cytoplasm of fibroblasts, chondroblasts, osteoblasts, and hepatic stellate cells, has been hypothesized to be involved in collagen synthesis. In neurons, Cygb, located in both cytoplasm and nucleus, may provide O(2) for enzymatic reactions, and may be involved in a ROS (NO)-signaling pathway(s). Here, we review current knowledge on Ngb and Cygb in terms of their structure, function, and evolutionary links to the well-known human HbA and Mb.