The role of phospholipid hydroperoxide glutathione peroxidase isoforms in murine embryogenesis

The Roles of Glutathione Peroxidases during Embryo Development

Embryo development relies on the complex interplay of the basic cellular processes including proliferation, differentiation, and apoptotic cell death. Precise regulation of these events is the basis for the establishment of embryonic structures and the organ development. Beginning with fertilization of the oocyte until delivery the developing embryo encounters changing environmental conditions such as varying levels of oxygen, which can give rise to reactive oxygen species (ROS). These challenges are met by the embryo with metabolic adaptations and by an array of anti-oxidative mechanisms. ROS can be deleterious by modifying biological molecules including lipids, proteins, and nucleic acids and may induce abnormal development or even embryonic lethality. On the other hand ROS are vital players of various signaling cascades that affect the balance between cell growth, differentiation, and death. An imbalance or dysregulation of these biological processes may generate cells with abnormal growth and is therefore potentially teratogenic and tumorigenic. Thus, a precise balance between processes generating ROS and those decomposing ROS is critical for normal embryo development. One tier of the cellular protective system against ROS constitutes the family of selenium-dependent glutathione peroxidases (GPx). These enzymes reduce hydroperoxides to the corresponding alcohols at the expense of reduced glutathione. Of special interest within this protein family is the moonlighting enzyme glutathione peroxidase 4 (Gpx4). This enzyme is a scavenger of lipophilic hydroperoxides on one hand, but on the other hand can be transformed into an enzymatically inactive cellular structural component. GPx4 deficiency - in contrast to all other GPx family members - leads to abnormal embryo development and finally produces a lethal phenotype in mice. This review is aimed at summarizing the current knowledge on GPx isoforms during embryo development and tumor development with an emphasis on GPx4.

Redox control in mammalian embryo development

The development of an embryo constitutes a complex choreography of regulatory events that underlies precise temporal and spatial control. Throughout this process the embryo encounters ever changing environments, which challenge its metabolism. Oxygen is required for embryogenesis but it also poses a potential hazard via formation of reactive oxygen and reactive nitrogen species (ROS/RNS). These metabolites are capable of modifying macromolecules (lipids, proteins, nucleic acids) and altering their biological functions. On one hand, such modifications may have deleterious consequences and must be counteracted by antioxidant defense systems. On the other hand, ROS/RNS function as essential signal transducers regulating the cellular phenotype. In this context the combined maternal/embryonic redox homeostasis is of major importance and dysregulations in the equilibrium of pro- and antioxidative processes retard embryo development, leading to organ malformation and embryo lethality. Silencing the in vivo expression of pro- and antioxidative enzymes provided deeper insights into the role of the embryonic redox equilibrium. Moreover, novel mechanisms linking the cellular redox homeostasis to gene expression regulation have recently been discovered (oxygen sensing DNA demethylases and protein phosphatases, redox-sensitive microRNAs and transcription factors, moonlighting enzymes of the cellular redox homeostasis) and their contribution to embryo development is critically reviewed.

Defining the immunoreactive epitope for the monoclonal anti-human glutathione peroxidase-4 antibody anti-hGPx4 Mab63-1

Glutathione peroxidases (GPx) form a heterogeneous enzyme family and GPx4-isoforms have been implicated in anti-oxidative defense, brain development, neuroinjury and sperm maturation. In humans seven GPx isoforms (GPx1-GPx7) can be separated. To selectively quantify the expression of GPx4-isoforms we have raised a monoclonal antibody (anti-hGPx4 Mab63-1) against the pure recombinant Sec46Cys mutant of human cytosolic GPx4 and used it for immunoblotting, immunoprecipitation and immunohistochemistry. The antibody recognizes human GPx4, its mouse ortholog but neither reacted with rat GPx4 nor other human GPx-isoforms. Sequence alignment of human and rat GPx4 proteins indicated three different amino acids (S18, F35, K99 in humans, A18, C35, R99 in rats) and a S18A exchange in the human enzyme completely abolished immunoreactivity. To further characterize the immunological epitope we synthesized a set of 12-mer peptides flanking S18* of human GPx4 and found that the sequence SMHEFS*AKDIDG exhibited strongest immunoreactivity. Substitution analysis and peptide length variation narrowed down the essential epitope to FS*AKDI and indicated that most mutations in this region strongly impaired immunoreactivity. In silico blast searches of public protein databases failed to identify proteins with potential immunoreactivity suggesting that the antibody exhibits a high specificity for human and mouse GPx4 and may not cross-react with unrelated proteins.

Cloning, characterization, and expression analysis of goat (Capra hircus) phospholipid hydroperoxide glutathione peroxidase (PHGPx)

Phospholipid hydroperoxide glutathione peroxidase (PHGPx), as a ubiquitous antioxidant enzyme in the glutathione peroxidases (GPx) family, plays multiple roles in organisms. However, there is very little information on PHGPx in goats (Capra hircus). In this study, a full-length cDNA was cloned and characterized from Taihang black goat testes. The 844 bp cDNA contains an open reading frame (ORF) of 597 bp. The goat PHGPx nucleotide sequence contains a selenocysteine (sec) codon TGA^244-246, two potential start codons ATG^20-22 and ATG^108-110, a polyadenylation signal AATAAA^813-818 and selenocysteine insertion sequence (SECIS) motif AUGA^688-691, UGA^729-731 and AAA^703-705. As a selenoprotein, the active-site motifs and GPx family signature motifs LAFPCNQF^101-108 and WNFEK^165-170 were also found. The order of PHGPx mRNA expression levels was: testes >> heart > brain > epididymis > kidney > liver > lung > spleen > muscle. Real-time PCR and immunohistochemistry results revealed similar expression differences in different age testes, with high expression levels during adolescence. Immunofluorescence results suggested that PHGPx mainly expressed in Leydig cells and spermatids in mature goat testes.

Structure-function relations, physiological roles, and evolution of mammalian ER-resident selenoproteins

Selenium is an essential trace element in mammals. The major biological form of this micronutrient is the amino acid selenocysteine, which is present in the active sites of selenoenzymes. Seven of 25 mammalian selenoproteins have been identified as residents of the endoplasmic reticulum, including the 15-kDa selenoprotein, type 2 iodothyronine deiodinase and selenoproteins K, M, N, S, and T. Most of these proteins are poorly characterized. However, recent studies implicate some of them in quality control of protein folding in the ER, retrotranslocation of misfolded proteins from the ER to the cytosol, metabolism of the thyroid hormone, and regulation of calcium homeostasis. In addition, some of these proteins are involved in regulation of glucose metabolism and inflammation. This review discusses evolution and structure-function relations of the ER-resident selenoproteins and summarizes recent findings on these proteins, which reveal the emerging important role of selenium and selenoproteins in ER function.

Short form glutathione peroxidase 4 is the essential isoform required for survival and somatic mitochondrial functions

Glutathione peroxidase 4 (Gpx4) is an essential antioxidant enzyme having multiple functions. A long form Gpx4 protein and a short form Gpx4 protein, which are distinguishable by the presence or lack of a mitochondrial signal peptide at the N terminus, are generated from the Gpx4 gene. In this study, we generated transgenic mice using mutated GPX4 genes encoding either the long form Gpx4 (lGPX4 gene) or the short form Gpx4 (sGPX4 gene). Our results showed that transgenic mice with the sGPX4 gene had increased Gpx4 protein in all tissues and were protected against diquat-induced apoptosis in liver. Moreover, the sGPX4 gene was able to rescue the lethal phenotype of the mouse Gpx4-null mutation. In contrast, transgenic mice with the lGPX4 gene had increased Gpx4 protein only in the testes, and the lGPX4 gene failed to rescue the lethal phenotype of the mouse Gpx4-null mutation. In Gpx4-null mice rescued by the sGPX4 gene, the Gpx4 protein was present in mitochondria isolated from somatic tissues, and the submitochondrial distribution pattern of the Gpx4 protein in these mice was identical to that in wild-type mice. Interestingly, the male Gpx4-null mice rescued by the sGPX4 gene were infertile and exhibited sperm malformation. Together, our results demonstrated for the first time that the short form Gpx4 protein is present in somatic tissue mitochondria and is essential for survival and protection against apoptosis in mice, whereas the long form Gpx4 protein is important for male fertility.

Functions and evolution of selenoprotein methionine sulfoxide reductases

Methionine sulfoxide reductases (Msrs) are thiol-dependent enzymes which catalyze conversion of methionine sulfoxide to methionine. Three Msr families, MsrA, MsrB, and fRMsr, are known. MsrA and MsrB are responsible for the reduction of methionine-S-sulfoxide and methionine-R-sulfoxide residues in proteins, respectively, whereas fRMsr reduces free methionine-R-sulfoxide. Besides acting on proteins, MsrA can additionally reduce free methionine-S-sulfoxide. Some MsrAs and MsrBs evolved to utilize catalytic selenocysteine. This includes MsrB1, which is a major MsrB in cytosol and nucleus in mammalian cells. Specialized machinery is used for insertion of selenocysteine into MsrB1 and other selenoproteins at in-frame UGA codons. Selenocysteine offers catalytic advantage to the protein repair function of Msrs, but also makes these proteins dependent on the supply of selenium and requires adjustments in their strategies for regeneration of active enzymes. Msrs have roles in protecting cellular proteins from oxidative stress and through this function they may regulate lifespan in several model organisms.

Redox regulation and its emerging roles in stem cells and stem-like cancer cells

The existence of cancer stem cells has impelled the pursuit to understanding and characterizing this subset of cells, which are thought to be responsible for tumor recurrence and to contribute to therapy resistance. Recent studies suggest that cancer stem cells seem to possess properties similar to those of normal stem cells, revealing a possible therapeutic strategy/target. For this to be feasible, it is imperative to understand the relation between cancer cells, cancer stem cells, and normal stem cells. Cancer cells have been found to be in a state of redox imbalance, an alteration in the homeostasis between oxidants and antioxidants, resulting in increased oxidants within the cell. Studies have shown redox balance plays an important role in the maintenance of stem cell self-renewal and in differentiation. Very little is known about the redox status in cancer stem cells. In this review, we focus on the sites of oxidant generation and the regulation of redox status in cancer cells and stem cells. In addition, evidence that supports the involvement of redox homeostasis for stem cell self-renewal, differentiation, and survival are reviewed. Given the significance of redox in stem cells, we also discuss the possibility of exploiting the redox status in cancer stem cells as a novel therapeutic strategy.

Glutathione peroxidase-1 regulates mitochondrial function to modulate redox-dependent cellular responses

Glutathione peroxidase-1 (GPx-1) is a selenocysteine-containing enzyme that plays a major role in the reductive detoxification of peroxides in cells. In permanently transfected cells with approximate 2-fold overexpression of GPx-1, we found that intracellular accumulation of oxidants in response to exogenous hydrogen peroxide was diminished, as was epidermal growth factor receptor (EGFR)-mediated Akt activation in response to hydrogen peroxide or EGF stimulation. Knockdown of GPx-1 augmented EGFR-mediated Akt activation, whereas overexpression of catalase decreased Akt activation, suggesting that EGFR signaling is regulated by redox mechanisms. To determine whether mitochondrial oxidants played a role in these processes, cells were pretreated with a mitochondrial uncoupler prior to EGF stimulation. Inhibition of mitochondrial function attenuated EGF-mediated activation of Akt in control cells but had no additional effect in GPx-1-overexpressing cells, suggesting that GPx-1 overexpression decreased EGFR signaling by decreasing mitochondrial oxidants. Consistent with this finding, GPx-1 overexpression decreased global protein disulfide bond formation, which is dependent on mitochondrially produced oxidants. GPx-1 overexpression, in permanently transfected or adenovirus-treated cells, also caused overall mitochondrial dysfunction with a decrease in mitochondrial potential and a decrease in ATP production. GPx-1 overexpression also decreased EGF- and serum-mediated [(3)H]thymidine incorporation, indicating that alterations in GPx-1 can attenuate cell proliferation. Taken together, these data suggest that GPx-1 can modulate redox-dependent cellular responses by regulating mitochondrial function.

Translational regulation of glutathione peroxidase 4 expression through guanine-rich sequence-binding factor 1 is essential for embryonic brain development

Phospholipid hydroperoxide glutathione peroxidase (GPx4) is a moonlighting selenoprotein, which has been implicated in basic cell functions such as anti-oxidative defense, apoptosis, and gene expression regulation. GPx4-null mice die in utero at midgestation, and developmental retardation of the brain appears to play a major role. We investigated post-transcriptional mechanisms of GPx4 expression regulation and found that the guanine-rich sequence-binding factor 1 (Grsf1) up-regulates GPx4 expression. Grsf1 binds to a defined target sequence in the 5'-untranslated region (UTR) of the mitochondrial GPx4 (m-GPx4) mRNA, up-regulates UTR-dependent reporter gene expression, recruits m-GPx4 mRNA to translationally active polysome fractions, and coimmunoprecipitates with GPx4 mRNA. During embryonic brain development, Grsf1 and m-GPx4 are coexpressed, and functional knockdown (siRNA) of Grsf1 prevents embryonic GPx4 expression. When compared with mock controls, Grsf1 knockdown embryos showed significant signs of developmental retardations that are paralleled by apoptotic alterations (TUNEL staining) and massive lipid peroxidation (isoprostane formation). Overexpression of m-GPx4 prevented the apoptotic alterations in Grsf1-deficient embryos and rescued them from developmental retardation. These data indicate that Grsf1 up-regulates translation of GPx4 mRNA and implicate the two proteins in embryonic brain development.

Molecular biology of glutathione peroxidase 4: from genomic structure to developmental expression and neural function

Selenoproteins have been recognized as modulators of brain function and signaling. Phospholipid hydroperoxide glutathione peroxidase (GPx4/PHGPx) is a unique member of the selenium-dependent glutathione peroxidases in mammals with a pivotal role in brain development and function. GPx4 exists as a cytosolic, mitochondrial, and nuclear isoform derived from a single gene. In mice, the GPx4 gene is located on chromosome 10 in close proximity to a functional retrotransposome that is expressed under the control of captured regulatory elements. Elucidation of crystallographic data uncovered structural peculiarities of GPx4 that provide the molecular basis for its unique enzymatic properties and substrate specificity. Monomeric GPx4 is multifunctional: it acts as a reducing enzyme of peroxidized phospholipids and thiols and as a structural protein. Transcriptional regulation of the different GPx4 isoforms requires several isoform-specific cis-regulatory sequences and trans-activating factors. Cytosolic and mitochondrial GPx4 are the major isoforms exclusively expressed by neurons in the developing brain. In stark contrast, following brain trauma, GPx4 is specifically upregulated in non-neuronal cells, i.e., reactive astrocytes. Molecular approaches to genetic modification in mice have revealed an essential and isoform-specific function for GPx4 in development and disease. Here we review recent findings on GPx4 with emphasis on its molecular structure and function and consider potential mechanisms that underlie neural development and neuropathological conditions.

Developmental changes of protein profiles in the embryonic Sanhuang chicken liver

Understanding embryonic liver development bears the potential to provide important insights into treatments and preventative strategies for paediatric liver disease. Using Sanhuang (SH) chicken as a model system, we sought to identify the proteomic changes associated with embryonic liver development using differential display of proteins with two-dimensional (2-D) polyacrylamide gel electrophoresis and matrix-assisted laser desorption/ionization-time of flight (MALDI-TOF) mass spectrometry analysis. Embryonic livers from 200 SH chicken embryos were isolated on days 9, 14 and 19 during incubation and also immediately after hatching. Six hundred and two protein spots were displayed on 2-D gels stained with colloidal Coomassie brilliant blue, of which, 25 protein spots were found to have changes up to threefold in abundance during development. We identified these spots using MALDI-TOF mass spectrometry and found 23 of 25 proteins to be associated with carbohydrate metabolism, cell division, lipid metabolism and signal transduction. Our results provide insight into the biochemical events taking place during the development of SH chicken embryonic liver and highlight the value of proteomics in characterizing complex biochemical processes. Furthermore, the proteome maps may facilitate future studies addressing the effects of genetic and environmental factors and related studies on the development and quality of chicken embryonic liver.

Sperm nuclei glutathione peroxidases and their occurrence in animal species with cysteine-containing protamines

The selenoenzyme sperm nuclei glutathione peroxidase (snGPx), also called the nuclear form of phospholipid hydroperoxide glutathione peroxidase (n-PHGPx), was found to be involved in the stabilization of condensed sperm chromatin, most likely by thiol to disulfide oxidation of the cysteine residues of the mammalian protamines, small nuclear basic proteins in the nuclei of sperm cells. By applying Acidic Urea-PAGE in combination with SDS-PAGE, snGPx with an apparent molecular mass of 34 kDa and a 24-kDa protein were purified from rat sperm nuclei. The 24-kDa protein was identified by means of mass spectrometry as a truncated form of snGPx produced by cleavage at the N-terminal end. After defined processing of spermatozoa and detergent treatment of the sperm nuclei fraction, snGPx and its truncated form were shown to be the only selenoproteins present in mature mammalian sperm nuclei. Both forms were found in mature rat and horse sperm nuclei but in man only snGPx was detected. In trout and chicken, species with sperm cells which likewise undergo chromatin condensation but do not contain cysteine in their protamines, the snGPx proteins were missing. This can be taken as an indirect proof of the function of snGPx to act as protamine cysteine thiol peroxidase in the mammalian species with cysteine-containing protamines.

Tea epigallocatechin-3-gallate increases 8-isoprostane level and induces caudal regression in developing rat embryos

Tea is the most common beverage after water. Concerns have been raised about the safety of tea during pregnancy, especially for embryo development. We aimed at studying the effects of active tea components on developing embryos by in vitro rat embryo culture. Rat embryos during early organogenesis were cultivated in serum supplemented with one of the tea catechins. Developmental hallmarks and malformations (Mal) in the developing embryos were compared and evaluated by a standard morphological scoring system. The embryotoxicity of each tea catechin was classified according to the European Center for the Validation of Alternative Methods. Cell viability was assessed by supervital dye staining, apoptosis by TUNEL assay, and peroxidation by the 8-isoprostane EIA method. We found that (+)-catechin had the least effect on developing embryos (Mal(50)=715.1 mg/L; IC50(Mal)=435 mg/L), whereas (-)-epigallocatechin gallate had the most adverse effect (Mal(50)=54.2 mg/L; IC50(Mal)=45.8 mg/L). The major malformation in affected embryos included caudal retardation with abnormal axial flexion and delayed hind-limb formation. All catechins were classified as nonembryotoxic except (-)-epigallocatechin gallate, which was classified as weakly embryotoxic. With (-)-epigallocatechin gallate, increased numbers of nonviable and apoptotic cells in the malformed embryos were associated with increased embryo 8-isoprostane.

Structural basis for catalytic activity and enzyme polymerization of phospholipid hydroperoxide glutathione peroxidase-4 (GPx4)

Phospholipid hydroperoxide glutathione peroxidase (GPx4) is a moonlighting selenoprotein, which has been implicated in anti-oxidative defense, sperm development, and cerebral embryogenesis. Among GPx-isoforms, GPx4 is unique because of its capability to reduce complex lipid hydroperoxides and its tendency toward polymerization, but the structural basis for these properties remained unclear. To address this, we solved the crystal structure of the catalytically active U46C mutant of human GPx4 to 1.55 A resolution. X-ray data indicated a monomeric protein consisting of four alpha-helices and seven beta-strands. GPx4 lacks a surface exposed loop domain, which appears to limit the accessibility of the active site of other GPx-isoforms, and these data may explain the broad substrate specificity of GPx4. The catalytic triad (C46, Q81, and W136) is localized at a flat impression of the protein surface extending into a surface exposed patch of basic amino acids (K48, K135, and R152) that also contains polar T139. Multiple mutations of the catalytic triad indicated its functional importance. Like the wild-type enzyme, the U46C mutant exhibits a strong tendency toward protein polymerization, which was prevented by reductants. Site-directed mutagenesis suggested involvement of the catalytic C46 and surface exposed C10 and C66 in polymer formation. In GPx4 crystals, these residues contact adjacent protein monomers.

Role for glutathione peroxidase-4 in brain development and neuronal apoptosis: specific induction of enzyme expression in reactive astrocytes following brain injury

Glutathione peroxidase-4 (GPx4) is a multifunctional selenoprotein expressed as mitochondrial, cytosolic, or nuclear isoforms. As a catalytically active enzyme it has been implicated in antioxidative defense, but during sperm development it functions as a structural protein. GPx4 null mice die in utero at midgestation and knockdown of GPx4 during embryogenesis disturbs brain development. To explore the cerebral function of GPx4 we profiled cell-specific enzyme expression at various stages of perinatal brain maturation and investigated its regulation following brain injury by immunohistochemistry, in situ hybridization, and quantitative RT-PCR. Large amounts of GPx4 mRNA were detected in all neuronal layers during perinatal brain development but expression became restricted during postnatal maturation. In adult brain mitochondrial and cytosolic GPx4 isoforms were detected in neurons of cerebral cortex, hippocampus, and cerebellum whereas glial cells were devoid of GPx4. Following selective brain injury expression of the enzyme was upregulated in reactive astrocytes of lesioned areas and deafferented regions but not in neurons. Selective knockdown of GPx4 by small interfering RNA induced depletion of phosphatidylinositol-(4,5)-bisphosphate in the neuronal plasma membrane and subsequently apoptosis as indicated by caspase-3 activation. We hypothesize that astrocytic upregulation of GPx4 in response to injury is part of a protective cascade counteracting further cell damage.

Gpx4 protects mitochondrial ATP generation against oxidative damage

Mitochondrial ATP production can be impaired by oxidative stress. Glutathione peroxidase 4 (Gpx4) is an antioxidant defense enzyme found in mitochondria as well as other subcellular organelles that directly detoxifies membrane lipid hydroperoxides. To determine if Gpx4 protects ATP production in vivo, we compared mitochondrial ATP production between wild-type mice and Gpx4 transgenic mice using a diquat model. Diquat (50 mg/kg) significantly decreased mitochondrial ATP synthesis in livers of wild-type mice; however, no decrease in mitochondrial ATP synthesis was detected in Gpx4 transgenic mice after diquat. We observed no differences in activities of mitochondrial respiratory chain complexes between Gpx4 transgenic mice and wild-type mice. However, compared to wild-type mice, diquat-induced loss of mitochondrial membrane potential was attenuated in Gpx4 transgenic mice. Therefore, our results indicate that decreased ATP production under oxidative stress is primarily due to reduced mitochondrial membrane potential and overexpression of Gpx4 maintains mitochondrial membrane potential under oxidative stress.

Peroxiredoxins in gametogenesis and embryo development

Reactive oxygen species have been implicated in gametogenesis and embryo development in animals. As peroxiredoxins are now recognized as important protective antioxidant enzymes as well as modulators of hydrogen peroxide-mediated signaling, we addressed here the putative role of this novel family of peroxidases in gamete maturation and during embryogenesis in mammals and insects.