4 Sperm, but not seminal plasma, elicit changes in the bovine endometrial transcriptome after natural mating

Abstract

Studies in mice and pigs have shown that seminal plasma (SP) can modulate the uterine environment, leading to an increase in embryo survival and implantation. However, in cattle, conflicting evidence has resulted from studies in which SP was infused into the uterus. In contrast to mice and pigs, it is questionable whether any SP reaches the bovine uterus during natural mating. However, sperm could act as vehicle for SP proteins that bind to their plasma membrane at ejaculation. Thus, the aim of this study was to analyse the effect of exposure to SP, in the presence or absence of sperm, on the endometrial transcriptome. Beef heifers were oestrus synchronized and mated after being seen in standing oestrus with intact bulls (which ejaculate sperm and SP, n = 7) or vasectomized bulls (VX, which only ejaculate SP, n = 8) or left unmated (control, n = 7). To reduce a potential bull effect, 3 vasectomized and 2 intact Holstein bulls were used. Each heifer was mounted once to one intact or VX bull. Heifers were slaughtered 24 h (±6 h) after mating, and their reproductive tracts were recovered. Endometrial tissue from the base of the uterine horn ipsilateral to the ovulatory follicle was processed for RNA sequencing. Surprisingly, mating with a VX bull resulted in no differentially expressed genes (DEG) in the endometrium compared with unmated controls. In contrast, mating with intact bulls resulted in 22 DEG (10 down regulated and 12 upregulated) compared with unmated controls and 24 DEG (6 downregulated and 18 upregulated) compared with VX samples. The upregulated genes in intact versus VX samples included several that may have a potential role in implantation: CX3CL1, C4BPA, VCAM1-like, and IL17F. Gene CX3CL1 promotes trophoblast migration in women and has been described as a potential bovine embryokine. In addition, decreased levels of C4BPA have been correlated with implantation failure and unexplained recurrent abortions in women. Further, VCAM1 is involved in bovine conceptus implantation and is downregulated in women with unexplained infertility. In mice, SP regulates inflammation of IL17F in the uterus through the γδT/IL-17 pathway. On the other hand, the immune-related genes PRSS2 and CEBPD were downregulated in intact versus control samples, and BOLA-DQB was downregulated in intact versus VX samples. Gene PRSS2, involved in tissue remodelling, is upregulated in the endometrium of repeated breeder cows. Gene CEBPD, is a transcription factor that enhances IL6 expression, whereas BOLA-DQB is involved in antigen presentation. Interestingly, the gene coding for the α-chain of this protein is also downregulated in high fertility cows. In conclusion, these results do not support a direct effect of SP in regulating pregnancy success in cattle. However, sperm can elicit modest endometrial transcriptomic changes related to implantation and immune system regulation. Further research is needed to elucidate the role of intrinsic sperm proteins, or SP proteins transported by these cells, in the modulation of the uterine environment.

This work was supported by EU Horizon 2020 Marie Sklodowska-Curie (No. 792212).