UCP2 Mitigates the Loss of Human Spermatozoa Motility by Promoting mROS Elimination

Abstract

Background/Aims: To demonstrate the function of uncoupling protein 2 (UCP2) in the regulation of human spermatozoa motility. Methods: Semen samples were collected from donors with either normal spermatozoa motility (normospermia [NS]) or poor spermatozoa motility (asthenospermia [AS]). UCP2 protein in spermatozoawas quantified by Western blotting. The level of mitochondrial reactive oxygen species (mROS) was evaluated by MitoSOX Red. The activity of mitochondrial membrane potential (MMP) in spermatozoa was evaluated by a JC-1 assay and the ATP level was monitored by a luciferin-luciferase assay. Results: UCP2 was expressed in both NS and AS groups, with the former exhibiting a higher level than the latter. Immunofluorescence analysis shows that UCP2 is mainly located at the mid-region of human spermatozoa. The inhibition of UCP2 by a highly selective inhibitor, Genipin, results in not only impaired spermatozoa mobility (P<.05) but also an elevated level of mROS (P<.05), suggesting that UCP2 is involved in the maintenance of the spermatozoa mobility, which probably is achieved by promoting mROS elimination. Furthermore, H₂O₂ treatment of spermatozoa increases the mROS level coupled with the loss of spermatozoa mobility. Unexpectedly, this treatment also has a positive impact on the expression of UCP2 within a certain range of supplemental H₂O₂, indicating the moderate mROS level possibly serves as a feedback signal to stimulate the expression of UCP2. Finally, the treatment of spermatozoa by an ROS scavenger, N-acetyl-l-cysteine (NAC),decreases the level of mROS and increases the curvilinear velocity (VCL) of spermatozoa, but the UCP2 level is not affected. Conclusion: These results suggest an UCP2–mROS–motility regulatory system exists for maintaining spermatozoa mobility in humans. In such a system, UCP2 fulfills its function by promoting mROS elimination, and slightly over-produced mROS in turn serves as a signal to stimulates the expression of UCP2. This regulatory system represents a new potential target for the discovery of novel pharmaceuticals for the treatment of patients with low spermatozoa motility.

Mitochondrial Dynamics Markers and Related Signaling Molecules Are Important Regulators of Spermatozoa Number and Functionality

“…It has been reported that a testis-specific mouse TFAM isoform lacks the mitochondrial targeting sequence and is present only in the nucleus of spermatocytes and elongating spermatids [ 57 ]. Our observations about significant increase in the level of Ucp2 in spermatozoa from both stressed and adrenaline-stimulated spermatozoa [ 14 ] could be a possible explanation for findings of other authors showing that the presence of UCP2 mitigates the loss of human spermatozoa motility [ 32 ]. Since the transcripts for main markers ( Mfn1 , Mfn2 , Opa1 ) of mitochondrial fusion/architectures, important for homeostasis of mitochondrial network and functionality [ 35 , 36 , 40 ], were dramatically increased in spermatozoa from stressed rats, these results may explain results of other authors showing the positive relation of the expression level of MFN2 with motility and cryoprotective potential of human sperm [ 33 ], as well as mitofusin-mediated promotion of OXPHOS [ 58 ].…”

“…Human sperm motility and viability are regulated by mitophagy [ 31 ]. The loss of human spermatozoa motility is mitigated by UCP2 (uncoupling protein 2) [ 32 ], while the motility and cryoprotective potential of human sperm is connected with the MFN2 (mitofusin 2) expression level [ 33 ]. According to everything mentioned above, the mitochondria are a crucial organelle for spermatozoa functionality, and their homeostasis strongly correlates with (sub/in)fertility [ 1 ].…”

Reduced spermatozoa functionality during stress is the consequence of adrenergic-mediated disturbance of mitochondrial dynamics markers

“…This is in line with findings that TFAM is associated with the reduction in mtDNA content of human sperm 34 and that TFAM gene expression positively correlate with abnormal forms, sperm DNA fragmentation and mtDNA copy number 35 , 36 . Our results showing that combination β-ADRs-antagonist-propranolol + β-ADRs-agonist-isoproterenol significantly decreased the level of Ucp2 suggest the positive involvement of β-ADRs in Ucp2 regulation and could be possible explanation for findings that UCP2 mitigates the loss of human spermatozoa motility 38 . The effects of β-ADRs-agonist-isoproterenol were not always in parallel with the effects of adrenaline mediated through β-ADRs and β-ADRs-antagonist-propranolol was not always diminished/abolished the effects of isoproterenol.…”

“…It has been shown that TFAM is associated with the reduction in mtDNA content of human sperm 34 and that TFAM gene expression positive correlate with abnormal forms, sperm DNA fragmentation and mtDNA copy number 35,36 . Besides, mitophagy may be regulating human sperm function such as motility and viability 37 , UCP2 mitigates the loss of human spermatozoa motility 38 , while the expression level of MFN2 is related to motility and cryoprotective potentials of human sperm 39 . Accordingly, the mitochondria are a key organelle for sperm motility and strongly correlate with (in)fertility 21 .…”

Hydrogen Peroxide Has Adverse Effects on Human Sperm Quality Parameters, Induces Apoptosis, and Reduces Survival

“…Furthermore, it was reported that the loss of sperm motility due to oxidative stress can also be consequence of reduced activity of protein that functions as ROS eliminator such as uncoupling protein 2 in the human sperm. [ 15 ] This is confirmed by cervical mucus penetration assay in which H 2 O 2 -treated sperm cells were unable to penetrate cervical mucus as good as untreated control. The cervical mucus contains proteins such as catalase that are important for sperm survival that can bind to the sperm cell to eliminate oxidative attack.…”