Oocyte activation deficiency (OAD) remains the predominant cause of total/low fertilization rate in assisted reproductive technology. Phospholipase C zeta (PLCZ1) is the dominant sperm-specific factor responsible for triggering oocyte activation in mammals. OAD has been linked to numerous PLCZ1 abnormalities in patients experiencing failed in vitro fertilization or intracytoplasmic sperm injection cycles. While significant efforts have enhanced our understanding of the clinical relevance of PLCZ1, and the potential effects of genetic variants upon functionality, our ability to apply PLCZ1 in a diagnostic or therapeutic role remains limited. Artificial oocyte activation is the only option for patients experiencing OAD but lacks a reliable diagnostic approach. Immunofluorescence analysis has revealed that the levels and localization patterns of PLCZ1 within sperm can help us to indirectly diagnose a patient’s ability to induce oocyte activation. Screening of the gene encoding PLCZ1 protein is also critical if we are to fully determine the extent to which genetic factors might play a role in the aberrant expression and/or localization patterns observed in infertile patients. Collectively, these findings highlight the clinical potential of PLCZ1, both as a prognostic indicator of OAD and eventually as a therapeutic agent. In this review, we focus on our understanding of the association between OAD and PLCZ1 by discussing the localization and expression of this key protein in human sperm, the potential genetic causes of OAD, and the diagnostic tools that are currently available to us to identify PLCZ1 deficiency and select patients that would benefit from targeted therapy.
In 2002, a report suggested that oocyte activation is induced by Plcz1 in mouse oocytes, which prompted great interest in exploring the role of sperm PLCZ1. Thus, PLCZ1 loss-of-function experiments became a crucial tool for addressing this subject. Although the only option to completely delete a target protein in fully functional spermatozoa is to use gene-deficient animals, Plcz1-deficient mice were not reported until 2017. Challenges to obtain suitable in vivo models have been related to altered expression of Capza3, a neighbor gene to Plcz1 locus in mammalian genomes that is required for spermatogenesis. With the advancement of genome-editing technologies, two groups independently and simultaneously produced Plcz1 mutant mouse lines, which were the first animal models to be artificially and reliably deficient for sperm PLCZ1. All Plcz1 mutant mouse lines display normal spermatogenesis and, surprisingly, subfertility rather than complete infertility. Moreover, analysis of oocyte Ca2+ dynamics indicates that mouse PLCζ1 is an essential sperm-derived oocyte activation factor via intracytoplasmic sperm injection, as PLCZ1 deficiency causes a complete lack of Ca2+ oscillations. This seemingly contradictory phenotype can be explained by atypical Ca2+ oscillations that are provoked slowly and less frequently in the case of fertilization accompanied by physiological sperm–egg fusion. These findings not only raise new questions concerning the sperm basic biology, by clearly demonstrating the existence of a PLCZ1-independent oocyte activation mechanism in mice, but also have implications for the treatment and phenotypic interpretation of patients presenting oocyte activation failure.
Angelos Thanassoulas, Karl Swann, F Anthony Lai, and Michail Nomikos
In 2002, sperm-specific phospholipase C zeta1 (PLCZ1) was discovered and through these 20 years, it has been established as the predominant sperm oocyte-activating factor. PLCZ1 cRNA expression or direct protein microinjection into mammalian oocytes triggers calcium (Ca2+) oscillations indistinguishable from those observed at fertilization. The imperative role of PLCZ1 in oocyte activation is revealed by the vast number of human mutations throughout the PLCZ1 gene that have been identified and directly linked with certain forms of male infertility due to oocyte activation deficiency. PLCZ1 is the smallest PLC in size, comprising four N-terminal EF-hand domains, followed by X and Y catalytic domains, which are separated by the XY-linker, and ending with a C-terminal C2 domain. The EF hands are responsible for the high Ca2+ sensitivity of PLCZ1. The X and Y catalytic domains are responsible for the catalysis of the phosphatidylinositol-4,5-bisphosphate [PI(4,5)P2] substrate to produce the Ca2+-mobilising messenger, inositol 1,4,5-trisphosphate (IP3), while the XY-linker plays multiple roles in the unique mode of PLCZ1 action. Finally, the C2 domain has been proposed to facilitate the anchoring of PLCZ1 to intracellular vesicles through its direct interactions with specific phosphoinositides. This review discusses recent advances in the structure and function relationship of PLCZ1 and the potential binding partners of this important sperm-specific protein in the sperm and oocyte. The unravelling of all the remaining hidden secrets of sperm PLCZ1 should help us to understand the precise mechanism of fertilization, as well as enabling the diagnosis and treatment of currently unknown forms of PLCZ1 -linked human infertility.
Yuta Kanazawa, Takuya Omotehara, Hiroki Nakata, Tsuyoshi Hirashima, and Masahiro Itoh
Spermatozoa are released from Sertoli cells and flow in the seminiferous tubule to the rete testis. Our results suggest that the luminal flow in the tubules is repeatedly reversed and that this physical force helps spermatozoa release from the Sertoli cells.
Spermatozoa released from Sertoli cells must be transported to the epididymis. However, the mechanism of the luminal flow in seminiferous tubules has remained unclear to date. Therefore, in this study, we investigated luminal flow and movements in the seminiferous tubules by three-dimensional analysis and in vivo imaging. Serial 5-μm-thick mouse testicular sections at 50-µm-intervals were prepared and stained by Periodic Acid-Schiff-hematoxylin. After three-dimensional reconstruction of the seminiferous tubules, the localization of the released spermatozoa and the stages observed in the sections were recorded in each reconstructed tubule. Luminal movements in the seminiferous tubules were observed by in vivo imaging using a fluorescent-reporter mouse and two-photon excitation microscopy system. Spermatozoa without contact to the seminiferous epithelium were not accumulated toward the rete testis. Additionally, such spermatozoa were found on their way not only to the most proximal rete testis but also a more distant rete testis from any stage VIII seminiferous epithelia. In vivo imaging demonstrated that the direction of the flagella of spermatozoa attached to the seminiferous epithelium was repeatedly reversed. The epithelium at the inner curve of the seminiferous tubule was shaken more actively and had fewer spermatozoa attached compared with the epithelium at the outer curve. Our results hence suggest that the luminal flow in the seminiferous tubules is repeatedly reversed and that this physical force helps spermatozoa to be released from Sertoli cells.
Ashlee Jade Medica, Zamira Gibb, Alecia Sheridan, Natasha Harrison, and Robert John Aitken
MTT is a commonly used cell vitality probe, due to its ability to form insoluble formazan deposits at cellular locations of intense oxidoreductase activity. Although this response is considered a reflection of mitochondrial redox activity, extra-mitochondrial sites of MTT reduction have been recognized within the spermatozoa of several mammalian species. Therefore, the aim of this study was to determine the major sites and causative mechanisms of MTT reduction in stallion spermatozoa. Our results show that stallion spermatozoa displayed substantial mitochondrial formazan deposition, as well as a single extra-mitochondrial formazan deposit in various locations on the sperm head in approximately 20% of cells. The quality and capacitation status of stallion spermatozoa were positively correlated with the presence of an extra-mitochondrial formazan granule. Additionally, extra-mitochondrial formazan deposition was suppressed by the presence of an NADPH oxidase (NOX) inhibitor (VAS2870; active against NOX2, NOX4 and NOX5), MnTMPyP (SOD mimetic) and zinc (NOX5 inhibitor) suggesting that extra-mitochondrial MTT reduction may be facilitated by NOX-mediated ROS generating activity, conceivably NOX5 or NOX2. When comparing MTT to resazurin, another well-known probe used to detect metabolically active cells, MTT reduction had a higher correlation with sperm concentration and motility parameters (R2= 0.91), than resazurin reduction (R2 = 0.76). We conclude that MTT reduction in stallion spermatozoa follows a species-specific pattern due to a high dependence on oxidative phosphorylation and a degree of NOX activity. As such, MTT reduction is a useful diagnostic tool to assess extra-mitochondrial redox activity, and therefore, the functional qualities of stallion spermatozoa.
Yue Zhao, Haoran Liu, Yang Yang, Wenqian Huang, and Lan Chao
Abnormal sperm parameters such as oligospermia, asthenospermia, and teratozoospermia result in male factor infertility. Previous studies have shown that mitochondria play an important role in human spermatozoa motility. But the related pathogenesis is far from elucidated. The aim of this study was to investigate the association between gene associated with retinoid-interferon-induced mortality 19 (GRIM19) and asthenospermia. In this study, Grim19 knockout model (Grim19+/− mouse) was created through genome engineering. We showed that compared with WT mice, the sperm count and motility of Grim19+/− mice were significantly reduced. Grim19 may contribute to sperm count and vitality by influencing the mitochondrial membrane potential, intracellular reactive oxygen species production, and increasing cell apoptosis. The spermatogenic cells of all levels in the lumen of the seminiferous tubules were sparsely arranged, and the intercellular space became larger in the testis tissue of Grim19+/− mice. The serum testosterone concentration is significantly reduced in Grim19+/− mice. The expression of steroid synthesis-related proteins STAR, CYP11A1, and HSD3B was decreased in Grim19+/− mice. To further confirm whether changes in testosterone biosynthesis were due to Grim19 downregulation, we validated this result using Leydig cells and TM3 cells. We also found that Notch signaling pathway was involved in Grim19-mediated testosterone synthesis to some extent. In conclusion, we revealed a mechanism underlying Grim19 mediated spermatozoa motility and suggested that Grim19 affected the synthesis of testosterone and steroid hormones in male mouse partly through regulating Notch signal pathways.
Nick Warr, Pam Siggers, Joel May, Nicolas Chalon, Madeleine Pope, Sara Wells, Marie-Christine Chaboissier, and Andy Greenfield
Sex determination in mammals is controlled by the dominance of either pro-testis (SRY-SOX9-FGF9) or pro-ovary (RSPO1-WNT4-FOXL2) genetic pathways during early gonad development in XY and XX embryos, respectively. We have previously shown that early, robust expression of mouse Sry is dependent on the nuclear protein GADD45g. In the absence of GADD45g, XY gonadal sex reversal occurs, associated with a major reduction of Sry levels at 11.5 dpc. Here, we probe the relationship between Gadd45g and Sry further, using gain- and loss-of-function genetics. First, we show that transgenic Gadd45g overexpression can elevate Sry expression levels at 11.5 dpc in the B6.YPOS model of sex reversal, resulting in phenotypic rescue. We then show that the zygosity of pro-ovarian Rspo1 is critical for the degree of gonadal sex reversal observed in both B6.YPOS and Gadd45g-deficient XY gonads, in contrast to that of Foxl2. Phenotypic rescue of sex reversal is observed in XY gonads lacking both Gadd45g and Rspo1, but this is not associated with rescue of Sry expression levels at 11.5 dpc. Instead, Sox9 levels are rescued by around 12.5 dpc. We conclude that Gadd45g is absolutely required for timely expression of Sry in XY gonads, independently of RSPO1-mediated WNT signalling, and discuss these data in light of our understanding of antagonistic interactions between the pro-testis and pro-ovary pathways.
Yali Hao, Yan Li, Jianlei Wu, Na Hao, Yuzhen Qin, Haibo zhang, Wei Zhao, and Shan Kang
Abnormal gene expression caused by epigenetic changes, including DNA methylation, is associated with the development and progression of endometriosis. Grainyhead-like 2 gene (GRHL2), a suppressor of epithelial–mesenchymal transition, has been suggested to be associated with the occurrence, progression and poor survival of a variety of cancers. Although endometriosis is a benign disease, it has the biological behaviour of migration and invasion as malignant tumor. This study aims to determine whether the abnormal expression of the GRHL2 caused by aberrant methylation of its promoter is associated with the pathogenesis of ovarian endometriosis. Our results demonstrated that GRHL2 promoter region was significantly hypermethylated in the ectopic endometrium of patients with ovarian endometriosis compared with the normal endometrium of control patients. In contrast, the levels of GRHL2 mRNA and protein were significantly lower in the ectopic endometrium than in the control endometrium. Correlation analysis showed the methylation levels of GRHL2 were significantly negatively correlated with the mRNA expression of GRHL2. Moreover, the in vitro results suggested that the knockdown of GRHL2 could significantly increase the invasion and migration ability of EECs and may promote ZEB1 and vimentin expression while decreasing the expression of E-cadherin in EECs. Taken together, these results suggest that the low expression of GRHL2 caused by hypermethylation of the GRHL2 promoter is associated with ovarian endometriosis. The knockdown of GRHL2 may be involved in the occurrence of endometriosis by increasing EEC migration and invasion. This study provides more evidence for the hypothesis that endometriosis may be an epigenetic regulatory disorder.
Ludmila Volozonoka, Anna Miskova, Liene Kornejeva, Inga Kempa, Veronika Bargatina, and Linda Gailite
Genetic testing is becoming increasingly required at almost every stage of failed female reproduction/infertility. Nonetheless, clinical evidence for the majority of identified gene–disease relationships is ill-defined, thus leading to difficult gene variant interpretation and poor translation of existing knowledge into clinics. We aimed to identify the genes that have ever been implicated in monogenic female reproductive failure in humans and to classify the identified gene–disease relationship pairs using a standardized clinical validity assessment. A PubMed search following PRISMA guidelines was conducted on 20 September 2021 aiming to identify studies pertaining to genetic causes of phenotypes of female reproductive failure. The clinical validity of identified gene–disease pairs was assessed using standardized criteria, counting whether sufficient genetic and experimental evidence has been accumulated to consider a single gene ‘characterized’ for a single Mendelian disease. In total, 1256 articles were selected for the data extraction; 183 unique gene–disease pairs were classified spanning the following phenotypes: hypogonadotropic hypogonadism, ovarian dysgenesis, premature ovarian failure/insufficiency, ovarian hyperstimulation syndrome, empty follicle syndrome, oocyte maturation defect, fertilization failure, early embryonic arrest, recurrent hydatidiform mole, adrenal disfunction and Mullerian aplasia. Twenty-four gene–disease pairs showed definitive evidence, 36 – strong, 19 – moderate, 81 – limited and 23 – showed no evidence. Here, we provide comprehensive, systematic and timely information on the genetic causes of female infertility. Our classification of genetic causes of female reproductive failure will facilitate the composition of up-to-date guidelines on genetic testing in female reproduction, the development of diagnostic gene panels and the advancement of reproductive decision-making.