The Roles of Glutathione Peroxidases during Embryo Development

Abstract

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.

Keywords: anti-oxidative defense; reactive oxygen species; selenium; teratogenesis.

Figure 1

Oxygen and metabolic status during embryo development in mouse. Owing to the variable environmental conditions including oxygen and glucose concentrations in different regions of the female reproductive tract, the developing early embryo has to adapt its energy metabolism. Before the circulation system in the ectoplacental cone, the yolk sac, the placenta and the embryonic heart develop, early embryos transit from relative aerobic (red area, ~8% O₂) and low glucose to anaerobic (blue area, < 5% O₂) and high glucose conditions. Redox metabolism transits from oxidative phosphorylation (low glucose, high oxygen) to glycolysis (high glucose, low oxygen). Pyruvate or oxaloacetate are essential substrates during early cleavage whereas glucose and lactate prevail from the eight-cell stage embryos to peri-implanted blastocyst.

Figure 2

Coding multiplicity of the GPx4 gene. The GPx4 gene gives rise to three isoenzymes designated m-GPx4, c-GPx4, and n-GPx4. They can be distinguished by their N-terminal sequences that determine their subcellular localization [mtp, mitochondrial targeting peptide (light hatching); nls, nuclear localization sequence (dark hatching)]. The mammalian GPx4 gene consists of seven exons and contains three windows of transcriptional (arrows) and translational (5′AUG, 3′AUG, n-AUG) initiation, that are specific to the isoenzymes. Two protein factors (DJ-1, Grsf1) have been identified that affect post-transcriptional regulation of the GPx4 gene. cds, coding sequence.

Figure 3

Functional roles of GPx4 in mouse development. (Upper panel) Diagrams show spatial expression of mitochondrial GPx (m-GPx4, blue) and nuclear GPx (n-GPx4, red) in developing mouse embryos at gestational day E8.5–E10.5. Expression of m-GPx4 and n-GPx4 are overlapping in frontal forebrain (fb) and otic vesicles (ov). m-GPx4 is mainly expressed in both rostral and caudal neural tube (rnt and cnt) in early stage and at forebrain (fb), midbrain (mb), and hindbrain (hb) in later stages. n-GPx4 is expressed in the developing heart (ht). (Lower panel) Diagrams show the abnormal embryo development following isoform specific GPx4 knockdown by RNA interference in whole mouse embryo culture. m-GPx4 siRNA constructs induce minor microencephaly and abnormal hindbrain development (blue arrow) in the fifth and sixth rhombomeres (r5 and r6). n-GPx4 siRNA constructs induce growth restriction and abnormal heart formation (red arrow) of the left atrium (la).