Major Proteins of Bovine Seminal Plasma Bind to the Low-Density Lipoprotein Fraction of Hen's Egg Yolk1

Abstract

Over the past 60 years, egg yolk (EY) has been routinely used in both liquid semen extenders and those used to cryopreserve sperm. However, the mechanism by which EY protects sperm during liquid storage or from freezing damage is unknown. Bovine seminal plasma contains a family of proteins designated BSP-A1/-A2, BSP-A3, and BSP-30-kDa (collectively called BSP proteins). These proteins are secretory products of seminal vesicles that are acquired by sperm at ejaculation, modifying the sperm membrane by inducing cholesterol efflux. Because cholesterol efflux is time and concentration dependent, continuous exposure to seminal plasma (SP) that contains BSP proteins may be detrimental to the sperm membrane, which may adversely affect the ability of sperm to be preserved. In this article, we show that the BSP proteins bind to the low-density fraction (LDF), a lipoprotein component of the EY extender. The binding is rapid, specific, saturable, and stable even after freeze-thawing of semen. Furthermore, LDF has a very high capacity for BSP protein binding. The binding of BSP proteins to LDF may prevent their detrimental effect on sperm membrane, and this may be crucial for sperm storage. Thus, we propose that the sequestration of BSP proteins of SP by LDF may represent the major mechanism of sperm protection by EY.

Ejaculados individuais e pools de sêmen: diferenças em condições experimentais

“…Especula-se que algumas proteínas componentes do plasma seminal, como as proteínas com afinidade pela heparina, têm papel fundamental no processo de capacitação espermática e também influenciam a resistência dos espermatozoides à criopreservação, podendo estar relacionadas com as diferenças individuais na criorresistência do sêmen (Manjunath et al, 2002;Moore et al, 2005;Souza et al, 2007). A variabilidade na qualidade do sêmen e na resistência à criopreservação é uma característica inerente à espécie canina (Threlfall, 2003;Kutzler, 2005).…”

Interactions of egg yolk lipoprotein fraction with boar spermatozoa assessed with a fluorescent membrane probe.

“…Moreover, the intensity of ANS fluorescence showed that the action of sperm coating by LPFo was more obvious after storage, suggesting that the lipoprotein fraction confers protection to the sperm membrane. It has been postulated that the major protective factor from hen egg yolk is the low-density lipoprotein fraction (LDL), which is largely responsible for protecting spermatozoa during semen preservation [24], probably by the sequestration of detrimental seminal plasma proteins and binding to spermatozoa [10][11]. Moreover, it has been reported that the phospholipids, particularly phosphatidylcholine, improve membrane fluidity and protect spermatozoa by interacting with their plasma membrane [25].…”

“…Little is known about the mechanism underlying the egg yolk lipoprotein interactions with sperm membrane architecture. However, many hypotheses have been suggested to explain the mechanisms by which lipoproteins of hen egg yolk protect spermatozoa during semen preservation [10][11].…”

Sperm surface changes and physiological consequences induced by sperm handling and storage

“…common use of egg yolk and skim milk in cryoprotective diluents appears to have unintentionally masked this detrimental effect, as low-density lipoproteins and casein micelles present in these diluents preferentially bind BSP proteins, reducing the number available to bind to the sperm membrane (Manjunath et al 2002). In the boar, the effect of heparin-binding proteins is not as well documented and has been reported to have both positive and negative effects on sperm function, particularly in relation to the inhibition or stimulation of capacitation.…”