Conserved and breed-specific differences in the cervical transcriptome of sheep with divergent fertility at the follicular phase of a natural oestrus cycle

Lipidomic profiling of cervical mucus reveals the potential role of pro-inflammatory derived metabolites on sperm transport across the ovine cervix

Internationally, cervical artificial insemination (AI) in sheep yields low pregnancy rates when frozen-thawed semen is used. An exception to this is in Norway where vaginal AI of frozen-thawed semen to a natural oestrus yields non-return rates in excess of 60%, which has been attributed to the ewe breed used in Norway. This study used both metabolomics and an RNA-sequencing approach to assess the lipid production and composition from cervical mucus and tissue of four European ewe breeds (n = 28-30 ewes per breed) with previously reported differences in pregnancy rates following cervical AI with frozen-thawed semen. These breeds included Suffolk (exhibiting low fertility), Belclare (medium fertility) as well as Norwegian White Sheep and Fur (both with high fertility and pregnancy rates > 60%) at both a synchronised and natural oestrous cycle. The aim was to explore the differences between ewe breeds in the lipidomic profile and to identify candidate biomarkers associated with an optimal environment for cervical sperm transport. The results revealed the identification of 255 lipids, of which 170, 102 and 83 were different between ewe breeds, types of cycle and affected by their interaction, respectively (P < 0.05). Reduced levels of lipids involved in the resolution of inflammation (i.e. 14-HDoHE,17-HDoHE, 15-HETE) were identified in the low-fertility Suffolk breed compared to high-fertility ewe breeds. However, there was an up-regulation of the COX pathway accompanied by increased levels of prostaglandins in the Suffolk breed. These findings indicated a sub-optimal and pro-inflammatory environment that could have a negative effect on cervical sperm transport.

Exploring Endogenous and Exogenous Factors for Successful Artificial Insemination in Sheep: A Global Overview

Artificial insemination (AI) plays a vital role in animal breeding programs. AI is applied to enhance animal genetics and facilitate the widespread integration of desirable characteristics with a high potential for productivity. However, in sheep, this biotechnology is not commonly practicable due to multi-factorial challenges, resulting in inconsistent outcomes and unpredictable results. Thoughtful selection of semen donors and recipients based on genetic merit deeply impacts ovine AI outcomes. Additionally, endogenous factors such as breed, age, fertility traits, genetic disorders, and cervical anatomy in ewes contribute to ovine AI success. Extensive research has studied exogenous influences on sexual behavior, reproductive health, and hormonal regulation, all impacting ovine AI success. These exogenous factors include techniques like estrus induction, synchronization, semen handling methods (fresh/chilled/frozen), and insemination methods (cervical/laparoscopic), as well as nutritional factors and climatic conditions. This overview of the literature highlights the endogenous and exogenous challenges facing successful ovine AI and proposes strategies and best practices for improvement. This paper will serve as a guide for understanding and optimizing the success of ovine AI.

Metabolic signature of cervical mucus in ewe breeds with divergent cervical sperm transport: a focus on metabolites involved in amino acid metabolism

INTRODUCTION

Cervical artificial insemination (AI) with frozen-thawed semen in sheep has yielded unacceptably low pregnancy rates. The exception is in Norway where vaginal AI yields non-return rates in excess of 60%, which has been attributed to the ewe breed used.

OBJECTIVES AND METHODS

This study aimed to characterise, for the first time, the ovine follicular phase cervical mucus metabolome, with a focus on the amino acid profile. Cervical mucus was collected from four European ewe breeds with known differences in pregnancy rates following cervical AI with frozen-thawed semen. These were Suffolk (low fertility), Belclare (medium fertility), Norwegian White Sheep (NWS) and Fur (both high fertility).

RESULTS

A total of 689 metabolites were identified in the cervical mucus of all the four ewe breeds. Of these, 458 metabolites were altered by ewe breed, which had the greatest effect in the dataset (P < 0.05). We detected 194 metabolites involved in the amino acid pathway, of which 133, 56 and 63 were affected by ewe breed, type of cycle and their interaction, respectively (P < 0.05). N-methylhydantoin and N-carbamoylsarcosine (degradation products of creatinine pathway) exhibited the greatest fold change decrease in the Suffolk breed compared to Fur and NWS (P < 0.001). Oxidized metabolites were also decreased in Suffolk compared to high fertility breeds (P < 0.05). In contrast, other metabolites such as 3-indoxyl-sulfate, putrescine, cadaverine were significantly increased in Suffolk at the synchronised cycle.

CONCLUSION

The suboptimal amino acid profile in the cervical mucus of the low fertility Suffolk breed may have negative consequences for sperm transport.

Review: Non-surgical artificial insemination and embryo recovery as safe tools for genetic preservation in small ruminants

Artificial insemination (AI) and in vivo embryo production (or multiple ovulation and embryo transfer, MOET) programs are both instrumental in accelerating the propagation of genetically and economically superior goats and sheep. The aim of this review was to present the current gestalt of non-surgical AI and embryo recovery (NSER) procedures in small ruminants. Small body size, precluding rectal palpation, and highly limited penetrability of the uterine cervix in ewes are the major reasons for the scarce use of non-surgical assisted reproduction techniques in this species. As a result, AI and embryo recovery techniques in sheep mainly involve laparoscopy or laparotomy (LAP). In does, however, the Embrapa method of AI allows for successful intrauterine deposition of semen, resulting in pregnancy rates from 50 to 80% under field conditions (>3 000 goats inseminated) when frozen-thawed semen is used. After the administration of prostaglandin F2α (PGF2α), non-surgical (transcervical) embryo recovery is also feasible in goats, with the cervical penetration rate approaching 100%. There is a paucity of information on the efficacy of non-surgical AI using frozen semen in sheep, but the results are satisfactory with fresh, cooled, or chilled ram semen. An application of the NSER technique in ewes has greatly improved over the last decade, and cervical penetration rates of ∼90% can be achieved when a hormonal cervical dilation protocol using PGF2α, oxytocin, and/or estradiol ester (e.g., estradiol benzoate) is applied. In some genotypes of sheep, sufficient cervical dilation can be induced without estradiol ester included in the protocol. Several studies indicated that recovery of transferable quality ovine embryos using NSER is comparable to that employing a ventral midline laparotomy, and NSER is evidently a method of choice when animal welfare is concerned. Considering both the number of retrievable embryos and animal well-being, the NSER is a viable alternative for surgical procedures. With further developments, it has the makings of a primary, if not exclusive, embryo recovery technique in small ruminants worldwide.

Molecular insights to the sperm-cervix interaction and the consequences for cryopreserved sperm

Cryopreserved ram spermatozoa are limited in their capacity to traverse the ovine cervix and achieve fertilization. This altered interaction may be related to modified molecular communication between frozen-thawed ram spermatozoa, seminal plasma, and the female tract. As such, this review aims to identify the biological processes which underpin sperm maturation and transport throughout the female reproductive tract to elucidate factors which may alter this natural process in cryopreserved ram spermatozoa. We also assess critical barriers to ram spermatozoa specific to the ovine cervix and the role of seminal plasma in mitigating these barriers. Transcriptomics is explored as a new approach to understand the sperm-cervix interaction. Recent studies have demonstrated that both spermatozoa and seminal plasma contain a complex profile of coding and non-coding RNAs. These molecular species have clear links with functional fertility, and mounting evidence suggests they may be altered by cryopreservation. Emerging in vitro cell culture models are also investigated as a "next step" in studying this interaction, utilizing transcriptomics to identify subtle changes in female tract gene expression in response to spermatozoa. The application of such models is proposed as an exciting opportunity to investigate the unique challenges faced by cryopreserved spermatozoa traversing the ovine cervix prior to fertilization.

Cervical immune activation during the luteal phase may compromise subsequent trans-cervical ram sperm transport

Worldwide, cervical artificial insemination using frozen–thawed semen yields low pregnancy rates. The only exception to this is in Norway, where vaginal insemination with frozen–thawed semen yields pregnancy rates in excess of 60% and which has been attributed to the specific ewe breed used. Our previous work demonstrated differences in cervical gene expression at the follicular phase of the estrous cycle in ewe breeds with known differences in pregnancy rates. In this study, we characterized the cervical transcriptome of the same ewe breeds [Suffolk, Belclare, Fur, and Norwegian White Sheep (NWS)] during the luteal phase, as an optimal environment at the luteal phase could better prepare the cervix for sperm migration through the cervix at the subsequent follicular phase. High-quality RNA extracted from postmortem cervical tissue was analyzed by RNA sequencing. After stringent filtering, 1051, 1924, and 611 differentially expressed genes (DEGs) were detected in the low-fertility Suffolk breed compared with Belclare, Fur, and NWS, respectively. Gene ontology analysis identified increased humoral adaptive immune response pathways in Suffolk. Increased expression of multiple immune genes supports the presence of an active immune response in the cervix of Suffolk ewes, which differentiates them significantly from the other three ewe breeds. Inflammatory pathways were upregulated in the Suffolk, resulting in higher expression of the potent pro-inflammatory cytokines. Therefore, higher levels of pro-inflammatory cytokines indicate unresolved inflammation in the cervix of the low-fertility Suffolk breed that could contribute to reduced cervical sperm transport in the next follicular phase.

Ewe breed differences in the cervical transcriptome at the follicular phase of a synchronised oestrous cycle

Background

Cervical artificial insemination (AI) with frozen-thawed semen results in unacceptably low pregnancy rates internationally. The exception is in Norway, where vaginal deposition of frozen-thawed semen to a natural oestrous routinely yields pregnancy rates in excess of 70%. Previous studies by our group has demonstrated that this is due to differences in cervical sperm transport. However, a potentially important contributory factor is that ewes are inseminated to a natural oestrous in Norway but to a synchronised oestrous across most of the rest of the world. In this study, we interrogated the gene expression of the sheep cervix of four ewe breeds with known differences in pregnancy rates following cervical AI using frozen-thawed semen under the effect of exogenous hormones to synchronise the oestrous cycle. These four ewe breeds (n = 8 to 11 ewes per breed) are from two countries: Ireland (Belclare and Suffolk; medium and low fertility, respectively) and Norway (Norwegian White Sheep (NWS) and Fur; both with high fertility compared to the Irish ewe breeds).

Results

RNA extracted from cervical biopsies collected from these breeds was analysed by RNA-sequencing and differential gene expression analysis. Using the low-fertility Suffolk breed as a reference level; 27, 1827 and 2641 genes were differentially expressed in Belclare, Fur and NWS ewes, respectively (P <  0.05 and FC > 1.5). Gene ontology (GO) analysis revealed that Fur and NWS had an up-regulation of enriched pathways involved in muscle contraction and development compared to Suffolk. However, there was a down-regulation of the immune response pathway in NWS compared to Suffolk. In addition, GO analysis showed similar expression patterns involved in muscle contraction, extracellular matrix (ECM) development and cell-cell junction in both Norwegian ewe breeds, which differed to the Irish ewe breeds.

Conclusions

This novel study has identified a number of conserved and breed-specific biological processes under the effect of oestrous synchronisation that may impact cervical sperm transport during the follicular phase of the reproductive cycle.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12864-022-08603-8.

Biochemical and molecular characterisation of sialylated cervical mucins in sheep

Sialic acid occupies terminal positions on O-glycans of cervical mucins, where they contribute to the increased viscosity of mucin thereby regulating sperm transport. This study characterised the sialylated cervical mucins from follicular phase mucus of six European ewe breeds with known differences in pregnancy rates following cervical artificial insemination using frozen-thawed semen at both synchronised and natural oestrus cycles. These were Suffolk (low fertility) and Belclare (medium fertility) in Ireland, Ile de France and Romanov (both with medium fertility) in France and Norwegian White Sheep (NWS) and Fur (both with high fertility) in Norway. Expression of mucin and sialic acid related genes was quantified using RNA-sequencing in cervical tissue from Suffolk, Belclare, Fur and NWS only. Cervical tissue was also assessed for the percentage of cervical epithelial populated by mucin secreting goblet cells in the same four ewe breeds. Biochemical analysis showed that there was an effect of ewe breed on sialic acid species, which was represented by Suffolk having higher levels of Neu5,9Ac2 compared to NWS (P < 0.05). Suffolk ewes had a lower percentage of goblet cells than Fur and NWS (P < 0.05). Gene expression analysis identified higher expression of MUC5AC, MUC5B, ST6GAL1, ST6GAL2 and lower expression of ST3GAL3, ST3GAL4 and SIGLEC10 in Suffolk compared to high fertility ewe breeds (P < 0.05). Our results indicate that specific alterations in sialylated mucin composition may be related to impaired cervical sperm transport.