Phosphatidylserine on viable sperm and phagocytic machinery in oocytes regulate mammalian fertilization

doi:10.1038/s41467-019-12406-z

. 2019 Oct 1;10(1):4456.

doi: 10.1038/s41467-019-12406-z.

Phosphatidylserine on viable sperm and phagocytic machinery in oocytes regulate mammalian fertilization

Claudia M Rival^{1

2

3}, Wenhao Xu², Laura S Shankman^{1

2}, Sho Morioka^{1

2}, Sanja Arandjelovic^{1

2}, Chang Sup Lee^{1

2

4}, Karen M Wheeler³, Ryan P Smith³, Lisa B Haney^{1

2}, Brant E Isakson⁵, Scott Purcell⁶, Jeffrey J Lysiak^{7

8}, Kodi S Ravichandran^{9

10

11}

Affiliations

¹ The Center for Cell Clearance, School of Medicine, University of Virginia, 1340 Jefferson Park Avenue, Pinn Hall, Charlottesville, VA, 22903, USA.
² Department of Microbiology, Immunology, and Cancer Biology, School of Medicine, University of Virginia, 1340 Jefferson Park Avenue, Pinn Hall, Charlottesville, VA, 22903, USA.
³ Department of Urology, School of Medicine, University of Virginia, 1340 Jefferson Park Avenue, Pinn Hall, Charlottesville, VA, 22903, USA.
⁴ College of Pharmacy and Research Institute of Pharmaceutical Sciences, Gyeongsang National University, 501 Jinju-daero, Jinju, Gyeongnam, 52828, Republic of Korea.
⁵ Department of Molecular Physiology and Biological Physics, School of Medicine, University of Virginia, 1340 Jefferson Park Avenue, Pinn Hall, Charlottesville, VA, 22903, USA.
⁶ Reproductive Medicine and Surgery Center of Virginia, 595 Martha Jefferson Dr., Charlottesville, VA, 22911, USA.
⁷ The Center for Cell Clearance, School of Medicine, University of Virginia, 1340 Jefferson Park Avenue, Pinn Hall, Charlottesville, VA, 22903, USA. jl6n@virginia.edu.
⁸ Department of Urology, School of Medicine, University of Virginia, 1340 Jefferson Park Avenue, Pinn Hall, Charlottesville, VA, 22903, USA. jl6n@virginia.edu.
⁹ The Center for Cell Clearance, School of Medicine, University of Virginia, 1340 Jefferson Park Avenue, Pinn Hall, Charlottesville, VA, 22903, USA. ravi@virginia.edu.
¹⁰ Department of Microbiology, Immunology, and Cancer Biology, School of Medicine, University of Virginia, 1340 Jefferson Park Avenue, Pinn Hall, Charlottesville, VA, 22903, USA. ravi@virginia.edu.
¹¹ Department of Biomedical Molecular Biology, Ghent University, and the UGent-VIB Center for Inflammation Research, Technologiepark 71, 9052, Ghent, Belgium. ravi@virginia.edu.

PMID: 31575859
PMCID: PMC6773685
DOI: 10.1038/s41467-019-12406-z

Phosphatidylserine on viable sperm and phagocytic machinery in oocytes regulate mammalian fertilization

Claudia M Rival et al. Nat Commun. 2019.

. 2019 Oct 1;10(1):4456.

doi: 10.1038/s41467-019-12406-z.

Authors

Affiliations

¹ The Center for Cell Clearance, School of Medicine, University of Virginia, 1340 Jefferson Park Avenue, Pinn Hall, Charlottesville, VA, 22903, USA.
² Department of Microbiology, Immunology, and Cancer Biology, School of Medicine, University of Virginia, 1340 Jefferson Park Avenue, Pinn Hall, Charlottesville, VA, 22903, USA.
³ Department of Urology, School of Medicine, University of Virginia, 1340 Jefferson Park Avenue, Pinn Hall, Charlottesville, VA, 22903, USA.
⁴ College of Pharmacy and Research Institute of Pharmaceutical Sciences, Gyeongsang National University, 501 Jinju-daero, Jinju, Gyeongnam, 52828, Republic of Korea.
⁵ Department of Molecular Physiology and Biological Physics, School of Medicine, University of Virginia, 1340 Jefferson Park Avenue, Pinn Hall, Charlottesville, VA, 22903, USA.
⁶ Reproductive Medicine and Surgery Center of Virginia, 595 Martha Jefferson Dr., Charlottesville, VA, 22911, USA.
⁷ The Center for Cell Clearance, School of Medicine, University of Virginia, 1340 Jefferson Park Avenue, Pinn Hall, Charlottesville, VA, 22903, USA. jl6n@virginia.edu.
⁸ Department of Urology, School of Medicine, University of Virginia, 1340 Jefferson Park Avenue, Pinn Hall, Charlottesville, VA, 22903, USA. jl6n@virginia.edu.
⁹ The Center for Cell Clearance, School of Medicine, University of Virginia, 1340 Jefferson Park Avenue, Pinn Hall, Charlottesville, VA, 22903, USA. ravi@virginia.edu.
¹⁰ Department of Microbiology, Immunology, and Cancer Biology, School of Medicine, University of Virginia, 1340 Jefferson Park Avenue, Pinn Hall, Charlottesville, VA, 22903, USA. ravi@virginia.edu.
¹¹ Department of Biomedical Molecular Biology, Ghent University, and the UGent-VIB Center for Inflammation Research, Technologiepark 71, 9052, Ghent, Belgium. ravi@virginia.edu.

PMID: 31575859
PMCID: PMC6773685
DOI: 10.1038/s41467-019-12406-z

Abstract

Fertilization is essential for species survival. Although Izumo1 and Juno are critical for initial interaction between gametes, additional molecules necessary for sperm:egg fusion on both the sperm and the oocyte remain to be defined. Here, we show that phosphatidylserine (PtdSer) is exposed on the head region of viable and motile sperm, with PtdSer exposure progressively increasing during sperm transit through the epididymis. Functionally, masking phosphatidylserine on sperm via three different approaches inhibits fertilization. On the oocyte, phosphatidylserine recognition receptors BAI1, CD36, Tim-4, and Mer-TK contribute to fertilization. Further, oocytes lacking the cytoplasmic ELMO1, or functional disruption of RAC1 (both of which signal downstream of BAI1/BAI3), also affect sperm entry into oocytes. Intriguingly, mammalian sperm could fuse with skeletal myoblasts, requiring PtdSer on sperm and BAI1/3, ELMO2, RAC1 in myoblasts. Collectively, these data identify phosphatidylserine on viable sperm and PtdSer recognition receptors on oocytes as key players in sperm:egg fusion.

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Conflict of interest statement

The authors declare no competing interests.

Figures

**Fig. 1**
Phosphatidylserine on live sperm is important for in vitro fertilization. a Depiction of the mouse testis and epididymis. b Sperm from different regions of the epididymis were allowed to swim/disperse in TYH + BSA medium, stained with Annexin V and Hoechst, and evaluated by microscopy. c, d Annexin V staining of sperm. Asterisks denote sperm heads, and arrows midpiece. Scale bar, 20 μm. e Percentage of Annexin V + sperm from the caput (n = 9 mice), corpus (n = 8 mice), and cauda (n = 15 mice) epididymis, with each dot representing one mouse (six independent experiments). *p < 0.05, ***p < 0.001 (one-way non parametric ANOVA was followed by Kruskal–Wallis test for multiple comparisons). f, g, Sperm from cauda epididymis were incubated with 50 μg/ml GST only f or GST-BAI1-TSR g, washed, fixed and visualized by GST immunofluorescence. Scale bar, 20 μm. h Snap shots of a movie depicting motility of live Annexin V + (green) sperm (t: time in min). The trajectory of a single sperm is traced by a white dotted line. Scale bar, 30 μm. i Schematic of the in vitro fertilization assay: cumulus oocyte complexes isolated from wt super-ovulated females were incubated with caudal sperm previously capacitated, in the presence or absence of 10 μg/ml Annexin V, 50 μg/ml GST, or 50 μg/ml GST-BAI1-TSR. The percentage of fertilized eggs (two-cell embryos) was evaluated after 24 h. j Multiple two-cell embryos fertilization with control sperm (left panel, arrows), whereas fewer fertilized eggs were observed with Annexin V (right panel, arrows). Scale bar, 100 μm. k, l Annexin V masking of PtdSer on sperm (k, n = 4 experiments) or GST-BAI1-TSR (l, n = 4 experiments) reduces fertilization. The total number of eggs analyzed is shown in parentheses. Each line represents one experiment and the matching experiments are shown (shape and color). Error bars are s.e.m. *p < 0.05, **p < 0.01 (Two-tailed unpaired Student’s t test). m, n Greater unfertilized oocytes (asterisks) seen after competition with O-Phospho-l-Serine compared with O-Phospho-d-Serine (top). Scale bar, 100 μm. Each line represents one experiment and matching experiments are shown with the same shape and color (n = 4 experiments). Error bars are s.e.m. **p < 0.01 (Two-tailed unpaired Student’s t test). Source Data are provided in the Source Data File

**Fig. 2**
BAI1/3 and CD36 are expressed on oocytes and contribute to fertilization. a Schematic for the assay to determine PtdSer recognition moieties on mouse oocytes. b Oocytes-bound multiple untreated carboxylate beads within the microvillar region (left), but not the BAI1-TSR-treated beads (right). Scale bar, 20 μm. c Summary of three experiments using carboxylate binding oocytes. ***p < 0.001 (Two-tailed unpaired Student’s t-test). d, PtdSer expression (qPCR) on cumulus-free Metaphase II oocytes from wild-type female mice (n = 4 experiments). e Concanavalin A (ConA) staining (marking microvilli) overlap with staining for BAI1/3 and CD36 on ZP-free oocytes. Scale bar, 20 μm. f Immunofluorescence (top) and immunohistochemistry (bottom) staining of mouse ovaries with BAI1/3 antibodies. Connexin-43 (green) marks follicular cells, ovarian follicles (dotted lines) are indicated. Arrows: oocytes in secondary or tertiary follicles; arrowheads: oocytes in primary follicles. Scale bar, 50 μm. g, Schematic of fertilization assays assessing the role of BAI1/3 and CD36. h, i Multiple two-cell embryos are observed in the control group (left, arrows) but reduced after CD36 and BAI1/3 antibody treatment (right). Scale bar, 100 μm. The compilation of data from 7 independent experiments is shown in i. Each dot represents one experiment, with matching experiments shown (shape and color). sIgG, sheep IgG, mIgA, mouse IgA. **p < 0.05 (two-tailed unpaired Student’s t test). j Schematic of in vitro fertilization assays to test BAI1/3 and CD36 in sperm entry. Untreated or antibody-treated oocytes were loaded with DAPI, and mixed with sperm. The percentage of oocytes with decondensed sperm DNA was evaluated after 3 h. k, l A representative image of a fertilized oocyte with one decondensed sperm DNA k. Scale bar: 20 μm. Quantitation of oocytes with decondensed sperm DNA after blocking with CD36 and BAI1/3 antibodies l. Each dot represents one experiment (n = 3 independent experiments) and the matching experiments are shown (shape and color). **p < 0.01 (two-tailed unpaired Student t test). Data are presented as mean ± s.e.m. Source Data are provided in the Source Data File

**Fig. 3**
Genetic testing of PtdSer receptors and cytoplasmic signaling in oocytes. a Schematic of PtdSer receptors BAI1/3, the downstream ELMO-DOCK-RAC1 signaling pathway, and other receptors on oocytes. b, c The PtdSer receptors Tim-4, BAI1 and Mer-TK participate in fertilization. ZP-intact oocytes from wt or Tim-4^−/− mice b *Mer-tk* + / + Bai1 + / + , *Mer-tk*^−/− *Bai1* + / + , or *Mer-tk*^−/− *Bai1* + /- or *Mer-tk*^−/− *Bai1*^−/− c were mixed with wt sperm, and two-cell embryos evaluated at 24 h. Fertilization index is the percentage of fertilized eggs from the experimental group divided by percentage of fertilized eggs from the control group (wt mice). Each dot represents one mouse (b, n = 6 experiments including 17 wt mice and 9 Tim-4^−/− mice, c: n = 5 experiments including 15 wt mice, 6 *Mer-tk*^−/− *Bai1* + / + and 6 *Mer-tk*^−/− *Bai1*^−/−), total number of eggs (parentheses). *p < 0.05 (b, two-tailed unpaired Student t test), **p < 0.01 (c one-way ANOVA followed by Dunnet’s multiple comparisons test). d *Elmo1* and *Elmo2* mRNA on cumulus-free Metaphase II oocytes. e Intracellular ELMO1 in isolated Metaphase II (MII) ZP-free oocytes. Scale bar, 20 μm. f Schematic for generation of oocyte-specific *Elmo1*-deficient mice. g Percentage of fertilized eggs after incubation of control (*Elmo1*^fl/fl) or *Elmo1*-deficient (*Ddx4-Cre/Elmo1*^fl/fl) oocytes with wt sperm (n = 6 independent experiments including 15 *Elmo1*^fl/fl mice and 12 *Ddx4-Cre/Elmo1*^fl/fl mice). Each dot represents 1 or 2 pooled mice. *p < 0.05 (Two-tailed unpaired Student’s t test). h Schematic for the evaluation early sperm entry into oocytes via DAPI staining of decondensed sperm DNA. ZP-free wt oocytes were incubated with RAC1 inhibitor (EHT-1864, 80 μm) and loaded with DAPI. After several washes, sperm were added, and the presence of internalized sperm with decondensed nuclei was evaluated after 1 h. i Control oocytes displaying the decondensation of the sperm DNA incorporated into the oocyte (arrow), whereas the sperm tail has not yet been internalized. DAPI also highlights the oocyte chromosomes in anaphase II indicating the resumption of meiosis II. Scale bar, 20 μm. j, Decreased percentage of oocytes with decondensed sperm DNA after RAC1 inhibition (n = 4 independent experiments). Numbers in parentheses reflect total number of eggs scored. **p < 0.01 (Two-tailed unpaired Student’s t test). k Sperm motility was not affected by RAC1 inhibition p > 0.05 (two-tailed unpaired Student’s t test). Data are presented as mean ± s.e.m. Source Data are provided in the Source Data File

**Fig. 4**
Sperm:myoblast fusion via PtdSer and the BAI3-ELMO2-RAC1 signaling axis. a Oocytes and myoblasts express similar molecules. Juno expression is readily detected on oocytes but not myoblasts. Bars represent mean ± s.e.m. c, Schematic of the sperm:myoblast fusion assay. Caudal epididymal sperm were labeled with Calcein-AM (red) and co-cultured with murine C2C12 myoblasts. After 4 h, myoblasts were washed, stained with Hoechst dye (to stain nuclei) and the percentage of Calcein-AM⁺ myoblasts was evaluated by microscopy. d Representative images depicting myoblasts that fuse with sperm to become Calcein-AM⁺ (3–10% per field) under control conditions (left panel), whereas this is greatly reduced when the sperm was pretreated with BAI1-TSR to mask PtdSer (middle panel) or after treatment of the myoblasts with the RAC1 inhibitor (right panel). Scale bar, 50 μm. e Detection of sperm inside the myoblasts. YFP⁺ sperm were co-incubated with C2C12 myoblasts for 4 h. Myoblasts were washed, fixed, and stained by immunofluorescence with antibodies to YFP/GFP (green) and Izumo1 (pink). Phalloidin (red) and Hoechst (blue) were used to stain the actin cytoskeleton and DNA, respectively. On the left panel, the dense sperm nucleus contained within the phalloidin⁺ cytoplasm is shown on the cross-sectional plane. White arrows: sperm nucleus; green arrow: sperm tail; dotted line: outline of the sperm head. Scale bar: 5 μm. f Reduced percentage of Calcein-AM⁺ myoblasts after masking of PtdSer on sperm using BAI1-TSR, antibody to BAI3, shRNA-mediated knockdown of *Elmo2* in myoblasts, treatment of myoblasts with the RAC1 inhibitor EHT-1864 or after pretreatment of myoblasts with cytoskeletal disruption via cytochalasin D (Cyto. D) or paraformaldehyde (PFA) fixation. (n = 3 independent experiments for each condition except for RAC1 inhibitor, n = 4 independent experiments). Bar charts show mean ± s.e.m. *p < 0.05, **p < 0.001, ***p < 0.0001 (one-way ANOVA followed by Dunnet’s multiple comparisons test and two-tailed unpaired Student’s t test). Each dot represents one experiment. Source Data are provided in the Source Data File

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References

1. Okabe Masaru. Sperm–egg interaction and fertilization: past, present, and future. Biology of Reproduction. 2018;99(1):134–146. doi: 10.1093/biolre/ioy028. - DOI - PubMed
1. Bianchi E, Wright GJ. Sperm meets egg: the genetics of mammalian fertilization. Annu. Rev. Genet. 2016;50:93–111. doi: 10.1146/annurev-genet-121415-121834. - DOI - PubMed
1. Bianchi E, Doe B, Goulding D, Wright GJ. Juno is the egg Izumo receptor and is essential for mammalian fertilization. Nature. 2014;508:483–487. doi: 10.1038/nature13203. - DOI - PMC - PubMed
1. Inoue N, Ikawa M, Isotani A, Okabe M. The immunoglobulin superfamily protein Izumo is required for sperm to fuse with eggs. Nature. 2005;434:234–238. doi: 10.1038/nature03362. - DOI - PubMed
1. Ohto U, et al. Structure of IZUMO1-JUNO reveals sperm-oocyte recognition during mammalian fertilization. Nature. 2016;534:566–569. doi: 10.1038/nature18596. - DOI - PubMed

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[1] Okabe Masaru. Sperm–egg interaction and fertilization: past, present, and future. Biology of Reproduction. 2018;99(1):134–146. doi: 10.1093/biolre/ioy028. - DOI - PubMed

[2] Okabe Masaru. Sperm–egg interaction and fertilization: past, present, and future. Biology of Reproduction. 2018;99(1):134–146. doi: 10.1093/biolre/ioy028. - DOI - PubMed

[3] Bianchi E, Wright GJ. Sperm meets egg: the genetics of mammalian fertilization. Annu. Rev. Genet. 2016;50:93–111. doi: 10.1146/annurev-genet-121415-121834. - DOI - PubMed

[4] Bianchi E, Wright GJ. Sperm meets egg: the genetics of mammalian fertilization. Annu. Rev. Genet. 2016;50:93–111. doi: 10.1146/annurev-genet-121415-121834. - DOI - PubMed

[5] Bianchi E, Doe B, Goulding D, Wright GJ. Juno is the egg Izumo receptor and is essential for mammalian fertilization. Nature. 2014;508:483–487. doi: 10.1038/nature13203. - DOI - PMC - PubMed

[6] Bianchi E, Doe B, Goulding D, Wright GJ. Juno is the egg Izumo receptor and is essential for mammalian fertilization. Nature. 2014;508:483–487. doi: 10.1038/nature13203. - DOI - PMC - PubMed

[7] Inoue N, Ikawa M, Isotani A, Okabe M. The immunoglobulin superfamily protein Izumo is required for sperm to fuse with eggs. Nature. 2005;434:234–238. doi: 10.1038/nature03362. - DOI - PubMed

[8] Inoue N, Ikawa M, Isotani A, Okabe M. The immunoglobulin superfamily protein Izumo is required for sperm to fuse with eggs. Nature. 2005;434:234–238. doi: 10.1038/nature03362. - DOI - PubMed

[9] Ohto U, et al. Structure of IZUMO1-JUNO reveals sperm-oocyte recognition during mammalian fertilization. Nature. 2016;534:566–569. doi: 10.1038/nature18596. - DOI - PubMed

[10] Ohto U, et al. Structure of IZUMO1-JUNO reveals sperm-oocyte recognition during mammalian fertilization. Nature. 2016;534:566–569. doi: 10.1038/nature18596. - DOI - PubMed

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Phosphatidylserine on viable sperm and phagocytic machinery in oocytes regulate mammalian fertilization

Affiliations

Phosphatidylserine on viable sperm and phagocytic machinery in oocytes regulate mammalian fertilization

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Abstract

Conflict of interest statement

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