Boar seminal plasma: current insights on its potential role for assisted reproductive technologies in swine

Abstract

Seminal plasma (SP) supports not only sperm function but also the ability of spermatozoa to withstand biotechnological procedures as artificial insemination, freezing or sex sorting. Moreover, evidence has been provided that SP contains identifiable molecules which can act as fertility biomarkers, and even improve the output of assisted reproductive technologies by acting as modulators of endometrial and embryonic changes of gene expression, thus affecting embryo development and fertility beyond the sperm horizon. In this overview, we discuss current knowledge of the composition of SP, mainly proteins and cytokines, and their influence on semen basic procedures, such as liquid storage or cryopreservation. The role of SP as modulator of endometrial and embryonic molecular changes that lead to successful pregnancy will also be discussed.

Seminal Plasma: Relevant for Fertility?

“…Many of the above studies, and of many others being rather empirical in nature, however, show simple associations between the presence of SP and effects, with some of these complex in nature. The evidence gathered via association studies, at least regarding livestock fertility, show for proteins [ 11 , 33 , 35 , 36 , 43 ] or cytokines [ 41 , 70 ] either positive, negative or ambiguous statistical relations. This variation is not surprising, considering inter- or intra-male variations in composition and the relative amounts of these molecules and even the effects of handling [ 43 , 58 , 109 ].…”

“…The evidence gathered via association studies, at least regarding livestock fertility, show for proteins [ 11 , 33 , 35 , 36 , 43 ] or cytokines [ 41 , 70 ] either positive, negative or ambiguous statistical relations. This variation is not surprising, considering inter- or intra-male variations in composition and the relative amounts of these molecules and even the effects of handling [ 43 , 58 , 109 ]. Such variations, together with specific differences in experimental layout, are present among the plethora of contradictory reports [ 36 , 94 , 131 , 211 , 212 ].…”

“…Semen being processed, for any species, seems to benefit from the presence of SP for sperm survival and function, for instance when high-extension rates are applied [ 248 , 250 , 251 ]. In species with fractionated ejaculation of less concentrated semen, such as pigs and horses, SP is relevant when the semen is cooled [ 250 , 251 , 252 , 253 , 254 ], and particularly, when cryopreserved ( Figure 7 ) [ 2 , 22 , 43 , 95 , 255 ], to the extent of affecting embryo development in vitro [ 256 ]. The impact of SP on freezability enabled not only the selection of males as good or bad freezers [ 70 , 257 , 258 ] but proved that the addition of SP from good-freezer boars could improve the cryosurvival of the spermatozoa of other males [ 259 ], and even increase fertility in equines [ 17 ].…”

“…The SP is basically formed by secretions of epithelial cells and thus contains electrolytes, protein and steroid hormones, sugars and proteins/peptides, including enzymes [24]. The relative proportions of these components certainly varies among species, the year [43,58] and the type of ejaculate in question. Classical examples of the latter are when either all components are ejaculated at once (e.g., poultry and ruminants) or in relative different proportions (e.g., humans, stallion, boar, etc.).…”

“…As well, transcriptomic studies have signaled that many genes encoding the SP proteome display signs of a high rate of adaptative evolution [ 3 , 31 , 37 ]. Other components, including exosomes (epididymosomes, prostasomes and vesiculosomes) [ 38 , 39 , 40 ] and small-size proteins/peptides, such as cytokines [ 41 ] and enzymes with anti-oxidant capacity [ 42 ], are yet to be fully understood regarding sperm selection, function and fertility signaling to the female [ 43 ], as well as regarding the long-term effects on the health of the offspring, regardless of the species considered [ 44 ].…”