antioxidants Review Mitochondrial Functionality in Male Fertility: From Spermatogenesis to Fertilization Yoo-Jin Park and Myung-Geol Pang * Department of Animal Science & Technology and BET Research Institute, Chung-Ang University, Anseong 17546, Gyeonggi-do, Korea; [email protected] * Correspondence: [email protected] Abstract: Mitochondria are structurally and functionally distinct organelles that produce adenosine triphosphate (ATP) through oxidative phosphorylation (OXPHOS), to provide energy to spermatozoa. They can also produce reactive oxidation species (ROS). While a moderate concentration of ROS is critical for tyrosine phosphorylation in cholesterol efflux, sperm–egg interaction, and fertilization, excessive ROS generation is associated with male infertility. Moreover, mitochondria participate in diverse processes ranging from spermatogenesis to fertilization to regulate male fertility. This review aimed to summarize the roles of mitochondria in male fertility depending on the sperm developmen- tal stage (from male reproductive tract to female reproductive tract). Moreover, mitochondria are also involved in testosterone production, regulation of proton secretion into the lumen to maintain an acidic condition in the epididymis, and sperm DNA condensation during epididymal maturation. We also established the new signaling pathway using previous proteomic data associated with male fertility, to understand the overall role of mitochondria in male fertility. The pathway revealed that male infertility is associated with a loss of mitochondrial proteins in spermatozoa, which induces low sperm motility, reduces OXPHOS activity, and results in male infertility. Keywords: mitochondria; oxidative phosphorylation; spermatozoa; male infertility; testis; epididymis; capacitation; fertilization Citation: Park, Y.-J.; Pang, M.-G. Mitochondrial Functionality in Male Fertility: From Spermatogenesis to 1. Introduction Fertilization. Antioxidants 2021, 10, 98. https://doi.org/10.3390/ Mitochondria are structurally and functionally distinct organelles that have double antiox10010098 membranes and produce more than 90% of energy in eukaryotic cells, through oxidative phosphorylation (OXPHOS) [1,2]. The inner mitochondrial membrane, namely cristae, Received: 26 November 2020 is a highly folded and specialized compartment that increases the membrane surface for Accepted: 9 January 2021 storage of 94% of the OXPHOS complexes and adenosine triphosphate (ATP) synthase, and Published: 12 January 2021 85% of the total cytochrome c that is important for the OXPHOS system [3,4]. To produce ATP through OXPHOS, four respiratory enzyme complexes (complex I–IV) work in concert Publisher’s Note: MDPI stays neu- to create the electrochemical proton gradient in the mitochondrial inner membrane [5,6]. tral with regard to jurisdictional clai- The tricarboxylic acid (TCA) cycle, a series of enzymatic reactions that generate acetyl ms in published maps and institutio- coenzyme A (acetyl-CoA) by catabolism of carbohydrates, fats, and proteins, takes place in nal affiliations. the mitochondrial matrix. Acetyl Co-A is oxidized to generate carbon dioxide (CO2) and reducing agents, including nicotinamide adenine dinucleotide (NADH) and flavin adenine dinucleotide (FADH2). NADH and FADH2 work as fuel for the respiratory chain, through the transfer of electrons to the mitochondrial respiratory chain for the initiation of OXPHOS Copyright: © 2021 by the authors. Li- censee MDPI, Basel, Switzerland. (Figure1)[ 7]. The flux of electrons derived from the oxidation of substrates through various This article is an open access article redox carriers of the inner membrane electron transport chain (ETC) ultimately terminates distributed under the terms and con- in a four-electron reduction of molecular oxygen to water [8]. However, reactive oxidation − ditions of the Creative Commons At- species (ROS), such as superoxide (O2 ) and hydrogen peroxide (H2O2), are produced tribution (CC BY) license (https:// by the incomplete reduction of oxygen by an electron [9,10]. ROS work as important sec- creativecommons.org/licenses/by/ ondary messengers that regulate intracellular pathways through the oxidative activation of 4.0/). proteins, receptors, kinases, phosphatases, caspases, ion channels, and transcription factors Antioxidants 2021, 10, 98. https://doi.org/10.3390/antiox10010098 https://www.mdpi.com/journal/antioxidants Antioxidants 2021, 10, x FOR PEER REVIEW 2 of 27 Antioxidants 2021, 10, 98 2 of 24 [9,10]. ROS work as important secondary messengers that regulate intracellular pathways through the oxidative activation of proteins, receptors, kinases, phosphatases, caspases, ion channels, and transcription factors in normal conditions, and an imbalance between ROSin and normal the antioxidant conditions, defense and an imbalancesystem induces between oxidative ROS and stress the [11,12]. antioxidant Oxidative defense stress system frominduces ROS generation oxidative can stress cause [11, 12various]. Oxidative diseas stresses, including from ROS type generation II diabetes, can chronic cause various in- flammation,diseases, ischemia, including neurodegenerative type II diabetes, chronic disease, inflammation, and male infertility ischemia, [11,13–15]. neurodegenerative disease, and male infertility [11,13–15]. FigureFigure 1. Schematic 1. Schematic image imageof OXPHOS of OXPHOS and glycolysis and glycolysis in spermatozoa. in spermatozoa. Pyruvate is Pyruvate produced is by produced glycolysisby glycolysis and is then and converted is then converted to lactate in to th lactatee principal in the piece principal of spermatozoa piece of spermatozoaby lactate dehy- by lactate drogenase.dehydrogenase. Then, pyruvate Then, in pyruvate the principal in the piece principal is transported piece is transported inside into insidethe mitochondria into the mitochondria and it is usedand as it fuel is used for asTCA fuel cycles, for TCA which cycles, produce which the produce NADH. the OXPHOS NADH. OXPHOStake place take in mitochondria place in mitochondria of spermof midpiece. sperm midpiece. Oxidation Oxidation of NADH ofNADH in the elec in thetron electron transport transport chain pr chainoduce produce the ATP the molecules ATP molecules by OXPHOS.by OXPHOS. In spermatozoa,In spermatozoa, approximately approximately 80 mitochondr 80 mitochondriaia are present are present in the midpiece in the midpiece [16]. The [16]. mitochondria’sThe mitochondria’s roles are rolesnot limited are not to limitedproviding to providingenergy in sperm energy cells; in sperm they have cells; diverse they have functions,diverse including functions, the including production the productionof steroid hormones of steroid in hormones the testis, in and the testis,regulation and regula-of cell proliferationtion of cell proliferation as well as cell as death well asfor cell maintaining death for male maintaining fertility [17–20]. male fertility ROS generated [17–20]. ROS fromgenerated mitochondria from are mitochondria a double-edged are asword: double-edged they are crucial sword: for they tyrosine are crucial phosphoryla- for tyrosine tion phosphorylationin spermatozoa, cholesterol in spermatozoa, efflux, cholesterol and sperm–egg efflux, interaction, and sperm–egg but also interaction, play a role but in also pathologicalplay a role processes in pathological through processes oxidative through stress [21]. oxidative Approximately stress [21]. 30–80% Approximately of male 30–80%in- fertilityof male cases infertility are associated cases arewith associated ROS-mediated with ROS-mediated damage to spermatozoa damage to [22]. spermatozoa Exposure [22]. to ROSExposure induces to damage ROS induces of structural damage and of structural functional and components functional of components cells such as of proteins, cells such as membrane,proteins, and membrane, DNA in spermatozoa, and DNA in spermatozoa,which affects sperm which motility, affects sperm its ability motility, to penetrate its ability to oocytes,penetrate and embryonic oocytes, and development embryonic [22–24]. development A previous [22–24 study]. A previous reported study that lower reported mi- that tochondriallower mitochondrial respiratory activities respiratory is closely activities associated is closely with associated not only with the not reduced only the sperm reduced motility,sperm i.e., motility, asthenozoospermia, i.e., asthenozoospermia, but also lower but also viability lower and viability concentration and concentration of spermato- of sper- zoa matozoa[25]. Moreover, [25]. Moreover, lower mitochondrial lower mitochondrial membrane membrane potential potentialdue to the due abnormal to the abnormal mor- phologymorphology of the axoneme of the axoneme is a major is acause major of cause low sperm of low motility sperm motilityand severe and asthenozoo- severe astheno- spermiazoospermia [26]. Recently, [26]. Recently, comparative comparative proteomi proteomicc studies studiesbetween between sperm samples sperm samples from in- from fertileinfertile patients patients with asthenozoospermia with asthenozoospermia and fe andrtile fertile men menrevealed revealed that thatproteins proteins associated associated withwith mitochondrial mitochondrial OXPHOS, OXPHOS, and andTCA TCA cycles, cycles, and andmetabolism metabolism of pyruvate of pyruvate are downreg- are downreg- ulatedulated in spermatozoa in spermatozoa from from asthenozoospermia asthenozoospermia patients patients [27,28]. [27, 28These]. These previous previous reports reports