Oxidative Stress in Reproduction: A Mitochondrial Perspective

Abstract

Mitochondria are fundamental organelles in eukaryotic cells that provide ATP through oxidative phosphorylation. During this process, reactive oxygen species (ROS) are produced, and an imbalance in their concentrations can induce oxidative stress (OS), causing cellular damage. However, mitochondria and ROS play also an important role in cellular homeostasis through a variety of other signaling pathways not related to metabolic rates, highlighting the physiological relevance of mitochondria-ROS interactions. In reproduction, mitochondria follow a peculiar pattern of activation, especially in gametes, where they are relatively inactive during the initial phases of development, and become more active towards the final maturation stages. The reasons for the lower metabolic rates are attributed to the evolutionary advantage of keeping ROS levels low, thus avoiding cellular damage and apoptosis. In this review, we provide an overview on the interplay between mitochondrial metabolism and ROS during gametogenesis and embryogenesis, and how OS can influence these physiological processes. We also present the possible effects of assisted reproduction procedures on the levels of OS, and the latest techniques developed to select gametes and embryos based on their redox state. Finally, we evaluate the treatments developed to manage OS in assisted reproduction to improve the chances of pregnancy.

Keywords: assisted reproduction; gametogenesis; mitochondria; oxidative stress; reactive oxygen species.

Figure 1

Structure, metabolic activity and reactive oxygen species (ROS) scavenging system in mitochondria. Mitochondria present in glycolytic cells with low oxidative phosphorylation (OXPHOS) rates are defined as “orthodox mitochondria” (colored as green), and are characterized by an ovoid form, large matrix volume, small intracristal volume and lamellar cristae. Mitochondria colored as red, the so called “condensed mitochondria”, are formed to provide cells with adenosine triphosphate (ATP) generated by OXPHOS. Condensed mitochondria are characterized by relatively small matrix volume and an expanded intracristae space with cristae shaped as a crescent. Both orthodox and condensed mitochondria share two enzymatic systems to counteract ROS that are produced in both glycolytic and OXPHOS metabolism. The first antioxidant reaction consists of the reduction of two superoxide anions to hydrogen peroxide catalyzed by a superoxide dismutase (SOD2 or MnSOD). The glutathione peroxidase catalyzes the reduction of the hydrogen peroxide to water oxidizing a glutathione molecule. Glutathione reductase catalyzes the reduction of the oxidized glutathione molecule, thus renewing reduced glutathione reservoir (ATP, adenosine triphosphate; ETC, electronic transport chain; GPx, glutathione peroxidase; GR, glutathione reductase; NAD, nicotinamide adenine dinucleotide; OXPHOS, oxidative phosphorylation; PDH, pyruvate dehydrogenase; ROS, reactive oxygen species; SOD, superoxide dismutase; TCA, tricarboxylic acid).

Figure 2

Mitochondrial requirements during fetal gonad development. (A) After migration to the genital ridge, primordial germ cells (PGCs in white) colonize the gonadal ridges, and start proliferating and specifying with the help of gonadal somatic cells (in pale green). At that point, mitochondria present at PGCs present a condensed form that corresponds to their metabolic requirement of ATP provided by OXPHOS (colored as red). Sexual commitment depends on the gonadal environment and PGCs commit to spermatogenesis (B) in a fetal testis or oogenesis (C) in a fetal ovary. (B) In a fetal testis, germ cells proliferate to form spermatogonia, which are glycolytic cells presenting orthodox mitochondria (colored as green). At this moment, signals provided from somatic cells of the developing testis block the meiotic entrance of spermatogonia. (C) In a fetal ovary, germ cells commit to oogenesis in the absence of inhibiting signals. PGCs proliferate and differentiate to oogonia and undergo meiosis to form primary oocytes. At that moment, the metabolic requirements are low and mitochondria present in those primary oocytes are transcriptionally and bioenergetically silent (quiescent state colored as brown). ATP is almost entirely provided by somatic cells surrounding oogonia and primary oocytes.

Figure 3

Mitochondrial requirements during spermatogenesis. The spermatogenic process is mainly driven by Sertoli cells (big green cell engulfing the spermatogenic cascade), glycolytic cells carrying orthodox mitochondria, providing and controlling nutritional support to germ cells throughout their development. (i) Spermatogonia, proliferating and self-renewing cells, are glycolytic cells presenting orthodox mitochondria and allocated at the basal part of the seminiferous tubule, close to Leydig cells where vascularization is low. Every 60 days, differentiation is engaged by retinoic acid (RA) produced by Sertoli cells, to spermatocyte precursors (ii) and up to primary spermatocytes (iii), when OXPHOS metabolism becomes the main mechanism to produce ATP (condensed mitochondria, in red). After two meiotic divisions at the spermatocyte stage (iii,iv) producing early spermatids (v), which need again RA signaling from Sertoli cells to differentiates to spermatozoa (vi), with few mitochondria rearranged in the midpiece. Once spermatozoa are produced, they will receive signals from physiological levels of ROS in the epididymis to ensure proper sperm maturation, and in the oviduct to promote hyperactivation and acrosome reaction to allow sperm–oocyte fusion during fertilization.

Figure 4

Mitochondrial requirements during oogenesis. After birth and up to puberty, primary follicles (i) and immature oocytes are stored in a quiescent state characterized by immature mitochondria. After puberty, folliculogenesis starts producing secondary follicles (ii), antral follicles (iii) and preovulatory follicles (iv). The low energetic requirements of growing oocytes are provided by cumulus cells (theca and granulosa cells), providing ATP and pyruvate by metabolizing glucose via glycolysis. In preovulatory follicles, granulosa cells increase ROS levels to favor apoptosis of cumulus cells to induce ovulation. During final meiotic maturation, just before ovulation (v), functional mitochondria are located in a layer in the subcortical region.