BackgroundRelaxin is a small peptide also known as pregnancy hormone in many mammals. It is synthesized by both male and female tissues, and its secretions are found in various body fluids such as plasma serum, ovarian follicular fluid, utero-oviduct secretions, and seminal plasma of many mammals, including pigs. However, the presence and effects of relaxin in porcine gametes and embryos are still not well-known. The purpose of this study was to assess the presence of relaxin and its receptors RXFP1 and RXFP2 in pig gametes and embryos.MethodsImmature cumulus-oocyte complexes (COCs) were aspirated from sows' ovaries collected at the abattoir. After in vitro-maturation, COCs were in vitro-fertilized and cultured. For studies, immature and mature COCs were separately collected, and oocytes were freed from their surrounding cumulus cells. Denuded oocytes, cumulus cells, mature boar spermatozoa, zygotes, and embryos (cleaved and blastocysts) were harvested for temporal and spatial gene expression studies. Sections of ovary, granulosa and neonatal porcine uterine cells were also collected to use as controls.ResultsUsing both semi-quantitative and quantitative PCRs, relaxin transcripts were not detected in all tested samples, while RXFP1 and RXFP2 mRNA were present. Both receptor gene products were found at higher levels in oocytes compared to cumulus cells, irrespective of the maturation time. Cleaved-embryos contained higher levels of RXFP2 mRNA, whereas, blastocysts were characterized by a higher RXFP1 mRNA content. Using western-immunoblotting or in situ immunofluorescence, relaxin and its receptor proteins were detected in all samples. Their fluorescence intensities were consistently more important in mature oocytes than immature ones. The RXFP1 and RXFP2 signal intensities were mostly located in the plasma membrane region, while the relaxin ones appeared homogeneously distributed within the oocytes and embryonic cells. Furthermore, spermatozoa displayed stronger RXFP2 signal than RXFP1 after western-immunoblotting.ConclusionAll together, our findings suggest potential roles of relaxin and its receptors during oocyte maturation, early embryo development, and beyond.

BackgroundRelaxin is detected in seminal plasma of many species and its association with sperm motility may be beneficial in some aspects of assisted reproduction. Here, we immunolocalized relaxin receptors and investigated the effects of exogenous relaxin on motility characteristics, viability, and cAMP content of boar spermatozoa after storage.MethodsCommercial doses of boar semen were obtained on the collection day (Day 0) and kept in shipping containers at room temperature for up to 4 days (Day 4). On Day 0, spermatozoa were fixed for immunofluorescence detection of relaxin receptors RXFP1 and RXFP2 (Experiment 1). Semen aliquots were taken from the same dose at Day 0, Day 1, and Day 2 (Experiment 2a), and Day 2 and Day 4 (Experiment 2b) for analyses. Alive spermatozoa were purified and incubated (1 h-37°C) with 0, 50, or 100 ng relaxin/ml (Experiment 2a) and 0, 100, or 500 ng relaxin/ml (Experiment 2b). Afterward, aliquots of each treatment group were subjected to motility (Experiments 2), viability (Experiment 3) analyses, and cAMP quantification (Experiment 4). Data (3–4 independent replicates) were statistically analyzed (ANOVA followed by pairwise comparisons) and p values less or equal to 0.05 was set for significant difference.ResultsBoth RXFP1 and RXFP2 receptors were immunolocalized on the entire spermatozoon. Relaxin concentration of 100 ng/ml significantly improved the proportions of motile, progressive, and rapid spermatozoa up to Day 2. Only 500 ng relaxin/ml provided beneficial effects on Day 4. The viability of spermatozoa was not affected by relaxin (100 ng/ml) during storage, but the extent of mitochondria membrane damages was significantly decreased. Furthermore, relaxin did not affect the cAMP contents of spermatozoa during storage, in our conditions.ConclusionsRelaxin could be a valuable motility booster of stored- or aged-spermatozoa for assisted reproduction techniques. However, the related-intracellular signaling cascades of relaxin in boar spermatozoa remain undetermined.Electronic supplementary materialThe online version of this article (doi:10.1186/s12958-015-0021-4) contains supplementary material, which is available to authorized users.

Relaxin is a polypeptide hormone secreted by male and female reproductive tissues to facilitate spermatozoa progression in the female tract and parturition. Relaxin secretions are found in the vicinity of oocytes and embryos, and exert their effects through membrane receptors, which have not yet been described in porcine embryos. Here, we determined the presence of RXFP1 and RXFP2 receptors in porcine gametes and embryo, and evaluated the developmental effects of porcine relaxin (pRLX; Yan et al. 2006 Reproduction 131, 943-950). Cumulus-oocyte complexes (COC) were aspirated from sows ovaries collected at a local abattoir. Homogeneous COC were selected for IVM (44 h) and fertilization (6 to 8 h). Presumptive zygotes were cultured in NCSU-23 + 0.4% BSA for up to 7 days. All procedures were done at 39°C, under 5% CO2 in a humidified atmosphere. Matured oocytes, BTS-diluted spermatozoa, and embryos were collected for gene expression studies. For developmental studies, COC were matured (experiment 1), or embryos cultured from the zygote stage (experiment 2) in the presence ofpRLX (0, 20, or 40 ng mL-1). In experiment 3, zygotes derived from oocytes matured in the presence of pRLX (40 ng mL-1) were cultured with pRLX (20 or 40 ng mL-1). The pRLX effects were assessed on cleaved embryos and blastocysts recorded on Days 2 and 7 postinsemination, respectively. The total cell numbers of Day-7 blastocysts were also evaluated. All data were analyzed using ANOVA. Gametes and embryos expressed RXFP1 and RXFP2 at both the mRNA and protein level. The amounts of both gene transcripts were higher in mature oocytes (metaphase II) compared with spermatozoa (P < 0.05). The RXFP1/2 mRNA ratios were in favor of RXFP2 in mature oocytes (0.9×), zygotes (0.8 ×), and cleaved embryos (0.8×), and for RXFP1 in spermatozoa (1.1 ×) and blastocysts (1.1 ×). A similar pattern during embryo development was revealed at the protein level, showing a higher RXFP2 fluorescence signal in cleaved embryos and a lower signal in blastocysts compared with RXFP1 protein. In experiment 1, COC exposed to 40 ng mL-1 pRLX resulted in fewer cleaved embryos (36 ± 4%) compared with controls (42 ± 5%, P < 0.05). Of the 40 ng mL-1 pRLX-derived cleaved embryos, a greater proportion developed to the blastocyst stage (38 ± 6%; P < 0.05) compared with control and 20 ng mL-1 pRLX-derived cleaved embryos (26 ± 4% and 17 ± 8%, respectively). In experiment 2, however, 40 ng mL-1 pRLX induced higher cleavage but lower blastocyst rates (51 ± 5% and 20 ± 4%, respectively) compared with the control group (37 ± 4% and 32 ± 7%, respectively) (P < 0.05). In experiment 3, the exposure of both oocytes and derived embryos did not affect the developmental rates (P > 0.05). Nevertheless, pRLX significantly increased the mean cell number of blastocysts in all experiments (P < 0.05). We concluded that pig embryos express RXFP1 and RXFP2 receptors, which may facilitate a role for pRLX during oocyte maturation and embryo development in the pig. This work was supported by the USDA-ARS Biophotonics Initiative project# 58-6402-3-0120 and the Mississippi Agricultural and Forestry Experiment Station (MAFES).

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