Lucrecia Calvo

The presence of heparan sulfate in the mammalian oocyte provides a clue to human sperm nuclear decondensation in vivo

BACKGROUND Previous results from our laboratory have led us to propose heparan sulfate (HS) as a putative protamine acceptor during human sperm decondensation in vivo. The aim of this paper was to investigate the presence of glycosaminoglycans in the mammalian oocyte in an effort to better support this contention. METHODS Two experimental approaches are used: oocyte labeling to identify the presence of HS and analysis of sperm decondensing ability of fresh oocytes in the presence or absence of specific glycosidases. RESULTS Staining of mouse zona-intact oocytes with the fluorescent cationic dye, Rubipy, at pH 1.5 allowed for the detection of sulfate residues in the ooplasm by confocal microscopy. HS was detected in the ooplasm by immunocytochemistry. A sperm decondensation microassay using heparin and glutathione was successfully developed. The same level of sperm decondensation could be attained when heparin was replaced by mouse zona-free oocytes. Addition of heparinase to the oocyte/glutathione mixture significantly reduced sperm decondensation (P = 0.0159), while there was no effect following addition of either chondroitinase ABC or hyaluronidase. CONCLUSIONS The results presented in this paper demonstrate for the first time that HS is present in the mammalian oocyte and show that HS is necessary for fresh oocytes to express their sperm decondensing ability in vitro.

Heparin enhances protamine disulfide bond reduction during in vitro decondensation of human spermatozoa.

BACKGROUND Human sperm nuclear decondensation in vivo involves protamine disulfide bond reduction by glutathione (GSH) and protamine/histone exchange, presumably with heparan sulfate (HS) as the protamine acceptor. The aim of the present study was to test the hypothesis that these two events occur simultaneously rather than sequentially, as has been hitherto accepted, and to test for the presence of HS in the human oocyte. METHODS Spermatozoa and isolated sperm nuclei obtained from normal volunteers were exposed in vitro to heparin, the functional analogue of HS and either GSH or dithiothreitol (DTT) as the disulfide reducing agent. Decondensing reagents were added either simultaneously or sequentially. Percentage sperm nuclear decondensation was assayed by phase contrast microscopy. Thiol reduced status of isolated sperm nuclei was evaluated both indirectly [acridine orange (AO) staining of acid-denatured DNA] and directly [monobromobimane (mBBr) staining of protamine-free thiols]. The presence of HS in mature metaphase II (MII) human oocytes was analyzed by immunocytochemistry. RESULTS Sequential addition of reagents always resulted in significantly lower decondensation if GSH was used as the disulfide bond reducer (P < 0.05 for sperm and P < 0.001 for nuclei), but only when heparin was used first, when DTT was the disulfide reducing agent (P < 0.05 for sperm and P < 0.01 for nuclei). Both AO staining of DNA and mBBr staining of protamines revealed that the addition of heparin to GSH but not to DTT significantly increased the thiol reduced status of sperm chromatin. HS was detected in the ooplasm of zona-free MII human oocytes. CONCLUSIONS The results presented in this paper clearly show that heparin enhances the sperm chromatin thiol reducing activity of GSH in vitro, suggesting that in vivo thiol reduction and protamine/histone exchange could occur as simultaneous, rather than sequential, events. We also demonstrate for the first time the presence of HS in the human oocyte.

146 HEPARAN SULFATE IS INVOLVED IN NUCLEAR SPERM DECONDENSATION AFTER FERTILIZATION IN BOVINE

Reduced glutathione (GSH) is an endogenous disulfide bond reducer present in mammalian oocytes. It plays a critical role in sperm decondensation following fertilization, disrupting the protamine bonds that sustain the hypercondensed state of sperm DNA. However, disulfide bond reduction needs to be followed by protamine removal to achieve male pronuclear formation. In humans, heparan sulfate (HS) has been shown to exert this role (Romanato et al. 2008 Hum. Reprod. 23, 1145-1450). Although there are no reports in bovine, we recently demonstrated the presence of HS in cow oocytes by indirect immunofluorescence, using a specific anti-HS monoclonal antibody (Canel et al. 2015, Proc. SSR 48th Annual Meeting). Heparinases are known to cleave HS chains selectively, leading to its depolymerization. In the present work, we analysed the possible role of HS as protamine acceptor after fertilization in cattle. To this aim, we directly injected heparinase into the cytoplasm of IVF presumptive zygotes, and analysed its effect on pronuclei formation. Cumulus-oocyte complexes were collected from slaughtered cow ovaries and matured in vitro under standard conditions (Canel et al. 2012 Cell. Div. 7, 23-33). After 21h, IVF was performed following Brackett and Oliphants protocol (1975 Biol. Reprod. 12, 260-274), using frozen-thawed semen from 1 or 2 bulls at a final concentration of 15×106 spermatozoa/mL (5 replicates). After 5h of incubation, cumulus cells and sperm bound to zona pellucidae were removed from presumptive zygotes. Heparinase III solution (H8891, Sigma, St. Louis, MO, USA) was diluted in 50% (vol/vol) polyvinylpyrrolidone solution in PBS-(polyvinylpyrrolidone) at a final concentration of 50 UmL-1 and ~30 pL was mechanically injected into the cytoplasm of each IVF presumptive zygote (Hep group) using a 9-μm inner diameter injection pipette. A group of zygotes was injected with the same volume of 10% polyvinylpyrrolidone (sham), whereas others were not subjected to injection (control). All zygotes were cultured for 16h from the beginning of IVF in SOF medium (Holm et al. 1999 Theriogenology 52, 693-700). For pronuclear formation assessment, presumptive zygotes were permeabilized with 0.2% Triton X-100 for 15min at room temperature, and their DNA content was stained with 5µgmL-1 propidium iodide and observed under an epifluorescence microscope. Zygotes showing 2 pronuclei (PN) were considered as synchronically fertilized, whereas those showing one PN and one condensed sperm head were considered as asynchronically fertilized. Data were analysed by Fishers exact test (P<0.05). The rate of IVF zygotes showing 2 PN was lower for the Hep group (60.3%, n=131) than those from sham (94.1%, n=119) and control groups (98%, n=101), which did not differ between them (P<0.05). In conclusion, our results show for the first time that HS is involved in bull chromatin sperm decondensation and allow us to propose HS as a putative protamine acceptor during male pronucleus formation after IVF in cattle. Given the high frequency of sperm decondensation failure observed in bovine after intracytoplasmic sperm injection, this work provides new insights for the development of novel sperm/egg treatments that might improve intracytoplasmic sperm injection outcomes in cattle.

Decondensation of Sperm Chromatin

During sperm development, a highly condensed chromatin is formed due to the replacement of histones by other positively charged proteins: the protamines. This renders a stable yet transcriptionally inactive DNA which is carried by sperm to the fertilization site where the oocyte resides. After fertilization, chromatin decondensation must take place to form the male pronucleus. This is accomplished, in rodents and humans, by the concomitant action of glutathione and heparan sulfate, both present in the oocyte cytoplasm.