The Sperm Protein Spaca6 is Essential for Fertilization in Zebrafish

Fertilization is a key process in all sexually reproducing species, yet the molecular mechanisms that underlie this event remain unclear. To date, only a few proteins have been shown to be essential for sperm-egg binding and fusion in mice, and only some are conserved across vertebrates. One of these conserved, testis-expressed factors is SPACA6, yet its function has not been investigated outside of mammals. Here we show that zebrafish spaca6 encodes for a sperm membrane protein which is essential for fertilization. Zebrafish spaca6 knockout males are sterile. Furthermore, Spaca6-deficient sperm have normal morphology, are motile, and can approach the egg, but fail to bind to the egg and therefore cannot complete fertilization. Interestingly, sperm lacking Spaca6 have decreased levels of another essential and conserved sperm fertility factor, Dcst2, revealing a previously unknown dependence of Dcst2 expression on Spaca6. Together, our results show that zebrafish Spaca6 regulates Dcst2 levels and is required for binding between the sperm membrane and the oolemma. This is in contrast to murine sperm lacking SPACA6, which was reported to be able to bind but unable to fuse with oocytes. These findings demonstrate that Spaca6 is essential for zebrafish fertilization and is a conserved sperm factor in vertebrate reproduction.

The sperm protein Spaca6 is essential for fertilization in zebrafish

Fertilization is a key process in all sexually reproducing species, yet the molecular mechanisms that underlie this event remain unclear. To date, only a few proteins have been shown to be essential for sperm-egg binding and fusion in mice, and only some are conserved across vertebrates. One of these conserved, testis-expressed factors is SPACA6, yet its function has not been investigated outside of mammals. Here we show that zebrafish spaca6 encodes for a sperm membrane protein which is essential for fertilization. Zebrafish spaca6 knockout males are sterile. Furthermore, Spaca6-deficient sperm have normal morphology, are motile, and can approach the egg, but fail to bind to the egg and therefore cannot complete fertilization. Interestingly, sperm lacking Spaca6 have decreased levels of another essential and conserved sperm fertility factor, Dcst2, revealing a previously unknown dependence of Dcst2 expression on Spaca6. Together, our results show that zebrafish Spaca6 regulates Dcst2 levels and is required for binding between the sperm membrane and the oolemma. This is in contrast to murine SPACA6, which was reported not to be required for sperm-egg membrane binding but necessary for fusion. These findings demonstrate that Spaca6 is essential for zebrafish fertilization and is a conserved sperm factor in vertebrate reproduction.

P–214 Effect of artificial activation of oocytes (AOA-ICSI) on the ploidy status of the resultant blastocysts. A sibling-oocytes pilot study

Study question Will artificial activation of oocytes alter the ploidy status of the resultant blastocysts? A sibling-oocytes pilot study Summary answer AOA-ICSI does not increase the risk of having aneuploidy blastocysts and can improve the fertilization rate in patients with sperm factor deficiency. What is known already Despite introducing ICSI as an aid to improve chances of fertilization, fertilization failure can still occur in 2–3% of ICSI cycles. Fertilization is a complex process triggered by a cascade of events following calcium (Ca2+) oscillations. Evidence suggests that the deficiency, localization or altered structure of the sperm-derived protein PLCζ in oocyte activation may be a reason for meiotic II arrest in the oocyte. Artificial oocyte activation has been proposed to compensate for the lack of calcium oscillation and resumes meiotic progression. There are however insufficient studies to determine its effect on the chromosomal status of the resultant blastocysts. Study design, size, duration This is a prospective, randomized study conducted at our Center from August-October 2020. A total of 20 couples intended for ICSI + Preimplatation Genetic Testing for Aneuploidy (PGT-A) cycles were recruited based on fulfilling one of the following criteria: 1) previous total fertilization failure (TFF), 2) history of low fertilization rate (<30%), 3) more than 2 cycles of failed IVF cycles (no implantation) 4) poor embryo development (no blastocysts formed) and 5) severe male factor. Participants/materials, setting, methods A total of 231 MII oocytes underwent randomization in a 1:1 ratio between AOA-ICSI and control group. All oocytes are subjected to ICSI treatment. Oocytes in the AOA-ICSI group are treated in 25μl droplets 10μM ready to use bicarbonate buffered calcium ionophore (Kitazato, Japan) for 15 minutes post-ICSI. The blastocysts were biopsied and subjected to PGT-A. Primary outcome was the aneuploidy rate and secondary outcomes were fertilization rate and blastocyst rate. Main results and the role of chance There were 11 out of 40 (27.5%) aneuploid blastocysts in the AOA-ICSI group and 7 out of 23 aneuploid blastocysts (30.4%) in the control group [odds ratio (OR) = 0.87; 95% confidence interval (CI) 0.28–2.68, p = 0.8040). There was no statistically significant difference between both groups. However, fertilization rate of the AOA- ICSI group was significantly higher than the fertilization rate in the control group (68.6% vs 49.6% respectively, OR = 2.22; 95% CI, 1.31–3.81, p = 0.0034). There were 40 blastocysts formed in the AOA-ICSI group and 23 blastocysts formed in the control group. It was found that the AOA-ICSI group yielded a higher blastocyst rate (49.4%) compared to the control group (41.1%) (OR = 1.40; 95% CI, 0.71 to 2.78, p = 0.3379) but the difference was not statistically significant. Limitations, reasons for caution The possibility of TE cells biopsied may not be representative of the whole blastocyst makes it possible to have false clinical data. The dosage and time were also not evaluated in this study as exposure time was found to be a critical factor of fertilization rate in a previous study. Wider implications of the findings: This study showed that AOA-ICSI does not increase the risk of having aneuploidy blastocysts and can improve the fertilization rate in patients with sperm factor deficiency. Additional studies involving a larger number of patients with more specific indication can further justify the benefits of AOA as a therapeutic application. Trial registration number NA

Evolutionarily conserved sperm factors, DCST1 and DCST2, are required for gamete fusion

To trigger gamete fusion, spermatozoa need to activate the molecular machinery in which sperm IZUMO1 and oocyte JUNO (IZUMO1R) interaction plays a critical role in mammals. Although a set of factors involved in this process has recently been identified, no common factor that can function in both vertebrates and invertebrates has yet been reported. Here, we first demonstrate that the evolutionarily conserved factors dendrocyte expressed seven transmembrane protein domain-containing 1 (DCST1) and dendrocyte expressed seven transmembrane protein domain-containing 2 (DCST2) are essential for sperm–egg fusion in mice, as proven by gene disruption and complementation experiments. We also found that the protein stability of another gamete fusion-related sperm factor, SPACA6, is differently regulated by DCST1/2 and IZUMO1. Thus, we suggest that spermatozoa ensure proper fertilization in mammals by integrating various molecular pathways, including an evolutionarily conserved system that has developed as a result of nearly one billion years of evolution.

Diagnosis and Treatment of Male Infertility-Related Fertilization Failure

Infertility affects approximately 15% of reproductive-aged couples worldwide, of which up to 30% of the cases are caused by male factors alone. The origin of male infertility is mostly attributed to sperm abnormalities, of which many are caused by genetic defects. The development of intracytoplasmic sperm injection (ICSI) has helped to circumvent most male infertility conditions. However, there is still a challenging group of infertile males whose sperm, although having normal sperm parameters, are unable to activate the oocyte, even after ICSI treatment. While ICSI generally allows fertilization rates of 70 to 80%, total fertilization failure (FF) still occurs in 1 to 3% of ICSI cycles. Phospholipase C zeta (PLCζ) has been demonstrated to be a critical sperm oocyte activating factor (SOAF) and the absence, reduced, or altered forms of PLCζ have been shown to cause male infertility-related FF. The purpose of this review is to (i) summarize the current knowledge on PLCζ as the critical sperm factor for successful fertilization, as well as to discuss the existence of alternative sperm-induced oocyte activation mechanisms, (ii) describe the diagnostic tests available to determine the cause of FF, and (iii) summarize the beneficial effect of assisted oocyte activation (AOA) to overcome FF.

Comparative study of preimplantation development following distinct assisted oocyte activation protocols in a PLC-zeta knockout mouse model

Mammalian fertilization encompasses a series of Ca2+ oscillations initiated by the sperm factor phospholipase C zeta (PLCζ). Some studies have shown that altering the Ca2+ oscillatory regime at fertilization affects preimplantation blastocyst development. However, assisted oocyte activation (AOA) protocols can induce oocyte activation in a manner that diverges profoundly from the physiological Ca2+ profiling. In our study, we used the newly developed PLCζ-null sperm to investigate the independent effect of AOA on mouse preimplantation embryogenesis. Based on previous findings, we hypothesized that AOA protocols with Ca2+ oscillatory responses might improve blastocyst formation rates and differing Ca2+ profiles might alter blastocyst transcriptomes. A total of 326 MII B6D2F1-oocytes were used to describe Ca2+ profiles and to compare embryonic development and individual blastocyst transcriptomes between four control conditions: C1 (in-vivo fertilization), C2 (ICSI control sperm), C3 (parthenogenesis) and C4 (ICSI-PLCζ-KO sperm) and four AOA groups: AOA1 (human recombinant PLCζ), AOA2 (Sr2+), AOA3 (ionomycin) and AOA4 (TPEN). All groups revealed remarkable variations in their Ca2+ profiles; however, oocyte activation rates were comparable between the controls (91.1% ± 13.8%) and AOA (86.9% ± 11.1%) groups. AOA methods which enable Ca2+ oscillatory responses (AOA1: 41% and AOA2: 75%) or single Ca2+ transients (AOA3: 50%) showed no significantly different blastocyst rates compared to ICSI control group (C2: 70%). In contrast, we observed a significant decrease in compaction (53% vs. 83%) and blastocyst rates (41% vs. 70%) in the absence of an initial Ca2+ trigger (AOA4) compared with the C2 group. Transcription profiles did not identify significant differences in gene expression levels between the ICSI control group (C2) and the four AOA groups.

The soluble sperm factor that activates the egg: PLCzeta and beyond

PLCzeta(ζ) initiates Ca2+ oscillations and egg activation at fertilization in mammals, but studies in mouse eggs fertilized by PLCζ knockout (KO) sperm imply that there is another slow acting factor causing Ca2+ release. Here, I propose a hypothesis for how this second sperm factor might cause Ca2+ oscillations in mouse eggs.

Extra-mitochondrial citrate synthase initiates calcium oscillation and suppresses age-dependent sperm dysfunction

Men and women become infertile with age, but the mechanism of declining male fertility, more specifically, the decrease in in sperm quality, is not well known. Citrate synthase (CS) is a core enzyme of the mitochondrial tricarboxylic acid (TCA) cycle, which directly controls cellular function. Extra-mitochondrial CS (eCS) is produced and abundant in the sperm head; however, its role in male fertility is unknown. We investigated the role of eCS in male fertility by producing eCs-deficient (eCs-KO) mice. The initiation of the first spike of Ca2+ oscillation was substantially delayed in egg fused with eCs-KO sperm, despite normal expression of sperm factor phospholipase C zeta 1. The eCs-KO male mice were initially fertile, but the fertility dropped with age. Metabolomic analysis of aged sperm revealed that the loss of eCS enhances TCA cycle in the mitochondria with age, presumably leading to depletion of extra-mitochondrial citrate. The data suggest that eCS suppresses age-dependent male infertility, providing insights into the decline of male fertility with age.