Perspective: Re-defining “Pheromone” in a Mammalian Context to Encompass Seminal Fluid

Reproductive physiology of the boar: What defines the potential fertility of an ejaculate?

Despite decades of research and handling of semen for use in artificial insemination (AI) and other assisted reproductive technologies, 5-10% of selected boar sires are still considered sub-fertile, escaping current assessment methods for sperm quality and resilience to preservation. As end-product, the ejaculate (emitted spermatozoa sequentially exposed to the composite seminal plasma, the SP) ought to define the homeostasis of the testes, the epididymis, and the accessory sexual glands. Yet, linking findings in the ejaculate to sperm production biology and fertility is suboptimal. The present essay critically reviews how the ejaculate of a fertile boar can help us to diagnose both reproductive health and resilience to semen handling, focusing on methods -available and under development- to identify suitable biomarkers for cryotolerance and fertility. Bulk SP, semen proteins and microRNAs (miRNAs) have, albeit linked to sperm function and fertility after AI, failed to enhance reproductive outcomes at commercial level, perhaps for just being components of a complex functional pathway. Hence, focus is now on the interaction sperm-SP, comparing in vivo with ex vivo, and regarding nano-sized lipid bilayer seminal extracellular vesicles (sEVs) as priority. sEVs transport fragile molecules (lipids, proteins, nucleic acids) which, shielded from degradation, mediate cell-to-cell communication with spermatozoa and the female internal genital tract. Such interaction modulates essential reproductive processes, from sperm homeostasis to immunological female tolerance. sEVs can be harvested, characterized, stored, and manipulated, e.g. can be used for andrological diagnosis, selection of breeders, and alternatively be used as additives to improve cryosurvival and fertility.

Mating modifies the expression of crucial oxidative-reductive transcripts in the pig oviductal sperm reservoir: is the female ensuring sperm survival?

Background

Mating induces large changes in the female genital tract, warranting female homeostasis and immune preparation for pregnancy, including the preservation of crucial oxidative status among its pathways. Being highly susceptible to oxidative stress, sperm survival and preserved function depend on the seminal plasma, a protection that is removed during sperm handling but also after mating when spermatozoa enter the oviduct. Therefore, it is pertinent to consider that the female sperm reservoir takes up this protection, providing a suitable environment for sperm viability. These aspects have not been explored despite the increasing strategies in modulating the female status through diet control and nutritional supplementation.

Aims

To test the hypothesis that mating modifies the expression of crucial oxidative-reductive transcripts across the entire pig female genital tract (cervix to infundibulum) and, particularly in the sperm reservoir at the utero-tubal junction, before ovulation, a period dominated by estrogen stimulation of ovarian as well as of seminal origin.

Methods

The differential expression of estrogen (ER) and progesterone (PR) receptors and of 59 oxidative-reductive transcripts were studied using a species-specific microarray platform, in specific segments of the peri-ovulatory sow reproductive tract in response to mating.

Results

Mating induced changes along the entire tract, with a conspicuous downregulation of both ER and PR and an upregulation of superoxide dismutase 1 (SOD1), glutaredoxin (GLRX3), and peroxiredoxin 1 and 3 (PRDX1, PRDX3), among other NADH Dehydrogenase Ubiquinone Flavoproteins, in the distal uterus segment. These changes perhaps helped prevent oxidative stress in the area adjacent to the sperm reservoir at the utero-tubal junction. Concomitantly, there were a downregulation of catalase (CAT) and NADH dehydrogenase (ubiquinone) oxidoreductases 1 beta subcomplex, subunit 1 (NDUFB1) in the utero-tubal junction alongside an overall downregulation of CAT, SOD1, and PRDX3 in the ampullar and infundibulum segments.

Conclusions

Natural mating is an inducer of changes in the expression of female genes commanding antioxidant enzymes relevant for sperm survival during sperm transport, under predominant estrogen influence through the bloodstream and semen. The findings could contribute to the design of new therapeutics for the female to improve oxidative-reductive balance.

Seminal plasma nerve growth factor signaling on the reproductive physiology of female llamas

The ovulation mechanism is one of the fascinating physiological processes in reproductive biology in mammals. From the reproductive point of view, the species have been classified as spontaneous or induced ovulators. Although the release of GnRH followed by the preovulatory LH surge is shared between both types of ovulation, the stimulus to initiate GnRH release varies between both categories. In spontaneous ovulators, ovulation depends on the systemic concentration of ovarian steroids, however, in induced ovulators, different stimuli such as copulation, environmental, and social cues can facilitate or induce ovulation regardless of the increases in systemic estradiol concentration. In this review, we document evidence that a male-derived protein is the main factor responsible for inducing ovulation and also modulating the ovarian function in the domestic South American camelid, the llama. The neurotrophin beta-Nerve Growth Factor (β-NGF) is the principal factor present in the semen of llamas responsible for inducing ovulation in this species. After the intrauterine deposit of semen during mating, β-NGF is absorbed through the endometrium to reach the circulatory system, where it reaches the hypothalamus and stimulates GnRH release. The potential site of action of this neurotrophin at the brain has not been elucidated, however, hypotheses are raised that the factor may cross the blood-brain barrier and stimulate upstream neuronal networks that lead to the stimulation of GnRH-secreting neurons. It is possible that β-NGF could be sensed at the median eminence without crossing the blood-brain barrier. Finally, it has been observed that this factor is not only a powerful stimulator of ovulation but also has a luteotrophic effect, resulting in the development of a corpus luteum capable of secreting more progesterone when compared to other ovulation-stimulating analogues.

Frontiers in sheep reproduction - making use of natural responses to environmental challenges to manage productivity

This review addresses advances, directions and opportunities for research on sheep reproduction in the context of the global challenges of food security and climate change, and demand for 'clean, green and ethical' (CGE) animal management. The foundation of CGE management is an understanding of the physiological processes through which the reproductive system responds to changes in the animal's environment. These days, to the main environmental factors (photoperiod, nutrition, pheromones), we need to add stress from extreme weather events. With respect to nutrition in rams, we now have a deeper understanding of the responses of the brain centres that control gonadotrophin secretion (the kisspeptin system). At testis level, we have found that nutrition affects non-coding RNAs in Sertoli cells and germ cells, thus affecting the balance between cell proliferation and apoptosis. This proliferation-apoptosis balance is also affected during prenatal development, when undernutrition or stress in pregnant ewes seems to elicit epigenetic changes in developing gonads that could affect offspring fertility in adult life. With respect to nutrition in ewes, metabolic signals act directly on ovarian follicles, and thus change ovulation rate, but the variety of signals now includes the adipokines. An early concern was that nutritional supplements that increase ovulation rate would also increase embryo mortality but we now know that embryo survival is improved under field conditions. Finally, we had always thought that the efficiency gains from early puberty in lambs could only be achieved by accelerating fat accumulation, but we now know that faster muscle growth will achieve the same goal, offering two advantages in meat production systems. With respect to pheromones ('ram effect'), we have a deeper understanding of the brain responses (kisspeptin system) but, most importantly, a realization that the response of ewes to the ram signal involves cell division in memory centres. Many opportunities remain.

On the Role of Seminal Fluid Protein and Nucleic Acid Content in Paternal Epigenetic Inheritance

The evidence supports the occurrence of environmentally-induced paternal epigenetic inheritance that shapes the offspring phenotype in the absence of direct or indirect paternal care and clearly demonstrates that sperm epigenetics is one of the major actors mediating these paternal effects. However, in most animals, while sperm makes up only a small portion of the seminal fluid, males also have a complex mixture of proteins, peptides, different types of small noncoding RNAs, and cell-free DNA fragments in their ejaculate. These seminal fluid contents (Sfcs) are in close contact with the reproductive cells, tissues, organs, and other molecules of both males and females during reproduction. Moreover, their production and use are adjusted in response to environmental conditions, making them potential markers of environmentally- and developmentally-induced paternal effects on the next generation(s). Although there is some intriguing evidence for Sfc-mediated paternal effects, the underlying molecular mechanisms remain poorly defined. In this review, the current evidence regarding the links between seminal fluid and environmental paternal effects and the potential pathways and mechanisms that seminal fluid may follow in mediating paternal epigenetic inheritance are discussed.

Extracellular vesicles in seminal plasma: A safe and relevant tool to improve fertility in livestock?

Seminal plasma (SP) is not a pre-requisite for pregnancy. Yet, this heterogeneous, composite SP has proven relevant for fertility, as mediator for cell-to-cell communication between producing cells, spermatozoa and the female internal genital tract, regulating complex reproductive processes. Bearing hormones, proteins, cytokines as well as nuclei acids in nano-sized lipid bilayer seminal extracellular vesicles (sEVs), the SP concerts signaling to the female. Signals influence timing of ovulation, sperm transport and, particularly, enable the female immune system to balance her cryptic choice to engage into pregnancy or reject an eventual fertilization. This essay, focusing on livestock in general and pigs in particular, discusses the intrinsic roles of sEVs with regards to reproductive homeostasis, while binding and internalizing their cargo in spermatozoa and female tract epithelia to regulate their functional activity. Since prior studies had inconclusive results using bulk SP or single SP-contained free molecules, argumentation is hereby provided to increase the current incipient research on livestock sEVs, where fragile molecules relevant for fertility are shielded from degradation during handling. Seminal EVs isolated from SP can be used for andrological diagnosis and perhaps to select breeders with optimal fertility. Moreover, sEVs can be laboratory-uploaded with specific molecules or even engineered as lipid nanodroplets used as additives for extenders to improve fertility after artificial insemination (AI) or reproductive biotechnologies.