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Mitochondrial (dys)function and Sperm Biology
The dogma of mitochondria as the major source of energy in supporting sperm motility should be critically reconsidered in the light of several experimental data pointing to a major role of glycolysis in mammalian spermatozoa. In this light, the reported positive correlation between the mitochondrial membrane potential (ΔΨm) and motility of ejaculated spermatozoa cannot be explained convincingly by an impaired mitochondrial ATP generation only. Evidence has been produced suggesting that, in human sperm, dysfunctional mitochondria represent the main site of generation of reactive oxygen species (ROS). Furthermore, in these organelles, a complex bidirectional relationship could exist between ROS generation and apoptosis-like events that synergize with oxidative stress in impairing sperm biological integrity and functions. Despite the activity of enzymatic and non-enzymatic antioxidant factors, human spermatozoa are particularly vulnerable to oxidative stress, which plays a major role in male factor infertility.
metabolism and oxidative/apoptotic events.
2. Are Mitochondria Really the Energetic Motor of Mammalian Sperm?
The role of mitochondria in the energetic support of sperm motility is a matter of debate [11,12,13]. Two pathways can account for the generation of ATP in mammalian spermatozoa, glycolysis and mitochondrial respiration. As mitochondrial OXPHOS is much more efficient than glycolysis in generating ATP, it has been widely accepted that the ATP needed for sperm motility is synthesized by mitochondrial respiration.
In mammalian spermatozoa, mitochondria rearrange in tubular structures that are helically distributed around the anterior portion of the axoneme, constituting the midpiece [14,15]. As the sperm flagellum is long and thin and mitochondria are confined in its proximal end, the question has been raised as to whether OXPHOS-derived ATP can passively diffuse through the entire flagellum to efficiently support axoneme activity. In sea urchin sperm, a shuttle mechanism to facilitate the ATP diffusion along the flagellum is provided by the creatine phosphate (CrP) that buffers the ATP/adenosine diphosphate (ADP) ratio at the expense of CrP/creatine . However, mammalian spermatozoa lack or contain only low amounts of CrP or other phosphagens [17,18], making it unlikely that the CrP shuttle plays a major role in providing ATP from mitochondria to the axoneme. Indeed, spermatozoa from knockout mouse models where the gene for the mitochondrial isotype of creatine kinase had been inactivated exhibited similar motility patterns to the wild-type controls . These legitimate considerations shifted the focus from OXPHOS to glycolysis.
Although mitochondrial respiration is more efficient than glycolysis in generating ATP molecules, key enzymes of glycolysis are tethered to the fibrous sheath of the principal piece [20,21,22,23], and hence they might assure an efficient production of ATP for dynein ATPase locally in the entire length of the flagellum. Consistent with this view, in mouse , bovine  and human spermatozoa [5,6,7], motility was not affected by mitochondrial inhibition when glucose was available in the extracell ular medium. We previously demonstrated that in a medium lacking glycolysable sugars, the presence of substrates for OXPHOS such as pyruvate and lactate fully supported the motility of human spermatozoa .
Interestingly, under such experimental conditions, the addition of 2-Deoxy-D-glucose (DOG), which inhibits glycolysis by competing with glucose for key enzymes, significantly decreased sperm motility . This evidence was incompatible with the hypothesis that ATP is synthesized in mitochondria and then provided to the entire axoneme by diffusion.
On the contrary, these findings supported the notion that ATP produced by OXPHOS is used to drive gluconeogenesis and thus to supply glucose to glycolytic enzymes for ATP production in the principal piece.
In this light, glycolysis would compensate for any lack of ATP production by mitochondria in maintaining sperm motility, and mitochondrial OXPHOS inhibition could depress motility only under experimental conditions of concomitant glycolysis blockage. However, differences among the species exist, as stallion spermatozoa rely primarily on mitochondrial respiration to generate energy required for motility . Overall, it is conceivable that both glycolysis and OXPHOS contribute to ATP production, depending on each other in controlling sperm functions according to the different availability of energetic substrates in the environment . Of note, in female genital tract fluids, the concentrations of lactate are higher than those of glycolysable substrates [26,27,28,29], suggesting a possible major role of mitochondrial respiration in supporting sperm motility. This hypothesis could explain why spermatozoa retain a high number of mitochondria during their differentiation, despite the dramatical decrease in the cellular volume resulting from the removal of any unnecessary structure. Anyway, an obligatory role for glycolysis seems to be confirmed by the loss of progressive motility in spermatozoa of mouse models where the gene for sperm-specific glyceraldehyde-3-phosphate dehydrogenases had been knocked out . In this view, the reported correlation of the mitochondrial membrane potential (∆Ψm)  or mitochondrial morphologic integrity  with the motility of ejaculated spermatozoa cannot be explained convincingly by an impaired mitochondrial ATP generation only.
Noteworthy, in human spermatozoa, a mitochondrial dysfunction could affect motility when it is accompanied by an intrinsic generation of ROS. Oxidative stress, indeed, is responsible for membrane lipid peroxidation [5,32] and promotes the activation of mitochondrial pathways resulting in apoptosis-like changes.
3. Biochemistry of Reactive Oxygen Species: An Overview
4. Origin of ROS in Semen
5. Mitochondria as an Interplay Center between Oxidative Stress and Apoptotic Events
6. Pathophysiology of Oxidative Stress in Human Spermatozoa
This entry is adapted from 10.3390/antiox10050695
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