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Role of platelet-activating factor in reproduction: sperm function

William E. Roudebush

Reproductive Biology Associates, Atlanta, Georgia  30342, USA

Asian J Androl  2001 Jun; 3:  81-85

Keywords:  platelet-activating factor; receptor; sperm; fertility
Abstract
Since its discovery nearly thirty years ago, platelet-activating factor has emerged as one of the more important lipid mediators known. Plateletactivating factor (PAF; 1-O-alkyl-2-O-acetyl-sn-glycero-3-phosphorylcholine) exists endogenously as a mixture of molecular species with structural variants of the alkyl moiety. PAF is a novel potent signaling phospholipid that has unique pleiotropic biological properties in addition to platelet activation. PAF also plays a significant role in reproduction. PAF content in squirrel monkey sperm is significantly higher during the breeding season than the non-breeding season. PAF content in human sperm has a positive correlation with seminal parameters and pregnancy outcomes. High-fertility boars have significantly more PAF in their sperm than low-fertility boars. The enzymes (lyso-PAF-acetyltransferase and PAF-acetylhydrolase) necessary for PAF activation and deactivation are present in sperm. PAF-acetylhydrolase may act as a decapacitation factor. Removal of this enzyme during capacitation may promote PAF synthesis increasing motility and fertilization. PAF also plays a significant role in the fertilization process, enhancing the fertilization rates of oocytes. Enhanced embryo development has also been reported in oocytes fertilized with PAF-treated sperm. PAF antagonists inhibit sperm motility, acrosome reaction, and fertilization, thus suggesting the presence of receptors for PAF. The PAF-receptor is present on sperm, with altered transcript levels and distribution patterns on abnormal cells. Whereas the exact mechanism of PAF in sperm function and reproduction is uncertain, its importance in normal fertility is substantial. The reproductive significance of PAF activity in sperm and fertility plus the role of PAF in the establishment of pregnancy requires further study.
1 Introduction
Platelet-activating factor (PAF) is a unique signaling phospholipid that is produced by a variety of cell types. Since its discovery in the early 1970's this novel compound has been implicated in a wide variety of reproductive functions. The exact mechanism is uncertain, yet its importance in normal fertility is significant. Basic understanding and knowledge of platelet-activating factor in sperm plus the factors, which influence its levels and mechanism of action are important for many reasons. For example, development of a biochemical test for a marker of sperm function may allow more precise definition of sperm functional abnormalities, and, or utilization of exogenous PAF to enhance sperm functions thus facilitating fertility.
2 Discovery and role of platelet-activating factor in reproduction
Since its discovery nearly thirty years ago[1], platelet-activating factor has emerged as one of the more important lipid mediators known. Platelet-activating factor (PAF) or 1-O-alkyl-2-O-acetyl-sn-glycero-3-phosphorylcholine exists endogenously as a mixture of molecular species with structural variants of the alkyl moiety. PAF is a novel potent signaling phospholipid that has unique pleiotropic biological properties in addition to platelet activation[2,3].  PAF was first described by Benveniste et al in 1972[1], who documented rabbit platelet aggregation triggered by Ig E stimulated basophils. But, in addition, PAF has a significant role in reproduction, influencing ovulation, fertilization, preimplantation embryo development, implantation and parturition[4].

3 Sperm-derived platelet-activating factor

This unique molecule is found in the sperm of many laboratory and livestock species including the rabbit, mouse, bull and boar[5-8].  PAF is also present in human sperm[9] and is principally of the C-16 molecular species[10]. PAF content in bull sperm has a positive and significant relationship with motility (all sperm motion parameters) as measured by computer assisted semen analysis[11]. Additionally, a significant relationship exists between sperm PAF content and blood serum insulin-like growth factor I levels in the bull[12]. Insulin-like growth factor I (IGF-I) is a mitotic polypeptide that stimulates glucose and sulfate uptake. IGF-I will attenuate the intracellular calcium response to PAF in cultured rat mesengial cells[13]. Additionally, PAF will induce production of IGF-I binding proteins in human adenocarcinoma cells[14]. Whereas IGF-I's effects on female reproductive functions have been studied, little information is known concerning its effects on male reproductive functions. Scrotal circumference and percentage of normal sperm cells are related to blood serum IGF-I concentration in yearling Angus bulls[15]. IGF-I, an important factor for germ cell development and maturation of sperm[16], may promote PAF synthesis in sperm thus enhancing cell motility[11].

Minhas et al[9] found that PAF content in human sperm was dependent upon isolation technique. Sperm isolated by the swim-up technique had more PAF than sperm obtained by a simple washing technique. Whereas it is apparent that PAF content is higher in the motile fraction, they did not compare levels between normal and abnormal (e.g. non-motile) specimens. The concentration of PAF in human sperm was originally found to be inversely related to sperm quality[17]. Whereas Angle et al[17] did look at PAF content between normal and abnormal specimens, they did not process the samples to look at PAF content in different subpopulations (i.e., motile vs. non-motile) of the ejaculates. We recently reported that non-motile sperm do indeed have significantly higher levels of PAF than motile sperm[18]. This may be due to the non-motile sperm being unable to utilize their endogenous PAF due to poor or ineffective receptor activity[18]. In contrast, we recently reported that PAF content in human sperm (motile population) has a positive correlation with seminal parameters and pregnancy rates[19].

Since research in human biology is also founded on a comparative knowledge of our closest relatives we have studied the importance of PAF in non-human primates. PAF is present in squirrel monkey sperm[20]. This research documented PAF in squirrel monkey sperm with levels significantly higher during the breeding (fertile period) season than the non-breeding (non- or subfertile period) season. Thus demonstrating for the first time that PAF content in sperm has a positive relationship with fertility. We have also found that PAF content in boar sperm is also affected by fertility status of the male. High-fertility males have a significantly higher PAF content in their sperm than low-fertility males[8]. PAF metabolism is affected by hormones, testosterone, estrogen and progesterone[21,22]. Androgen hormones (e.g. testosterone), which have an important role in male fertility, are significantly depressed during the non-breeding season. Therefore, seasonal differences in hormone levels may affect sperm-derived PAF levels. We have also discovered that stress may impact PAF content in sperm. PAF levels in rhesus sperm are significantly lower in stressed males (individually caged) than non-stressed males (housed in free-ranging corrals)[23]. PAF content in sperm may be affected by circulating androgens and, or cortisol.

4 Platelet-activating factor synthesis

The calcium dependent phospholipase A2 (PL-A2) present in human, mouse, and hamster sperm[24], catalyses the formation of lyso-PAF (1-alkyl-2-lyso-sn-glycero-3-phosphocholine) from alkyl-acyl-glycerophosphocholine (alkyl-acyl-GPC), an inert structural cellular membrane component. This lyso-PAF can either be acetylated by lyso-PAF-acetyl transferase (with acetyl-CoA as the acetate donor) to form PAF or acylated by a CoA-independent arachidonyl transacylase to reform the precursor alkyl-acyl-GPC. PAF-acetyl hydrolase is the enzyme responsible for removal of the acetate group from the sn-2 position of the PAF molecule and inactivates PAF, resulting in the reformation of lyso-PAF. Both lyso-PAF-acetyl transferase and PAF-acetyl hydrolase are present in human semen[25]. PAF-acetyl hydrolase may act as a decapacitation factor[26]. Removal of this enzyme during normal capacitation promotes PAF synthesis that results in increased sperm motility, sperm-egg interactions and fertilization[27-30].

5 Effect of exogenous platelet-activating factor on sperm and fertilization

Treatment with synthetic PAF increases motility of fresh or frozen-thawed sperm[27,31]. PAF's effect on sperm motility appears to be mediated by cAMP. However, in other cell types PAF also affects inositol triphosphate and intracellular calcium[32-34]. Additionally, PAF also increases sperm capacitation and the acrosome reaction and is calcium dependent[35,36]. PAF may affect intracellular cAMP, inositol triphosphate and calcium levels.

PAF plays a significant role in the fertilization process, enhancing the in vitro fertilization rates of mouse and rabbit oocytes[28,30,35,37]. Conversely, PAF antagonists inhibit fertilization[6,37]. Endogenous PAF of sperml origin will increase mouse in vitro fertilization[6]. Enhanced embryo development has also been reported in rabbit oocytes fertilized in vitro with PAF-treated sperm[30]. Exposure of sperm to exogenous PAF prior to intrauterine insemination has resulted in a significant increase in human pregnancy outcomes[38]. Further PAF-IUI trials are currently in progress.

6 The platelet-activating factor receptor

PAF antagonists will inhibit the motility, acrosome reaction and hamster oocyte penetration in exposed sperm thus suggesting the presence of receptors for PAF[6,29,35,39]. We have recently reported on the presence and distribution of the PAF-receptor in human and baboon sperm[40,41] and our preliminary data demonstrates that distribution of the receptor is significantly altered in abnormal sperm[42]. Additionally, we have discovered that PAF-receptor mRNA expression differs significantly between motile (high content) and non-motile (low content) sperm[18].

The cDNA for the PAF-receptor cloned from a mouse macrophage cDNA library is a guanine-protein-linked receptor with seven transmembrane loops and has an open reading frame encoding 341 amino acid residues. The PAF-receptor was mapped to a region of the D2.2 and 1p35p34.3 bands of chromosomes 4 (mouse)[43]and 1 (human)[44], respectively. PAF receptors are functionally linked to inositol lipid hydrolysis, the mobilization of intracellular-free calcium and activation of a tyrosine kinase pathway[45]. In most cells, PAF binds to surface receptors inducing the formation of inositol triphosphate (IP3) and diacylglycerol (DAG) and increasing intracellular calcium[32,46]. Exogenous PAF affects intracellular calcium levels in mouse preimplantation two-cell embryos[33,34]. The receptor-activated calcium mobilization by the inositol-calcium signaling system involves two phases: (a) calcium release from an intracellular store; and (b) extracellular calcium entry by channels[47,48]. Intracellular calcium regulates NAD kinase activity, protein function, and protein synthesis[49,50]. PAF appears to bind to cell surface receptors initiating the formation of IP3 and DAG, and increasing intracellular calcium. The resulting elevated calcium signal depolymerizes the inter-membrane actin network and activates phospholipases, leading to an acrosome reaction[51]. PAF may affect sperm motility and fertilization potential via a receptor mediated control of intracellular calcium (Figure 1).

Figure 1. Proposed mechanism of PAF's action in sperm.

7 Summary

In summary, sperm produce PAF and this unique and novel ligand binds to its membrane bound receptor, which can be blocked by PAF-antagonists. This PAF binding results in increased sperm motility, acrosome reaction, fertilization and subsequent preimplantation embryo development rates. Endogenous PAF and PAF-receptor activity levels may have a direct role in the fertilizing capacity of sperm. The collective data provides further evidence that PAF's effect on fertilization and development is receptor-mediated and may involve the inositol triphosphate system. PAF may affect cellular activity (motility and fertilization potential) by modulating intracellular calcium levels. PAF-induced signaling events play a key role in sperm motility and fertilization. Knowledge of these signals is essential to our understanding of the physiology of normal reproductive events, as well as disorders of reproduction (e.g. male factor infertility). Lipid composition of the sperm membrane has a significant effect upon the functional characteristics of sperm. Additional studies will determine the significance of PAF (ligand and receptor content) and the receptor related signaling events in sperm as it relates to fertility and pregnancy outcome.

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Correspondence to: William E. Roudebush, Ph.D., Reproductive Biology Associates, 1150 Lake Hearn Drive, Suite 400, Atlanta, Georgia, 30342 USA.
Tel: +1-404-843 3064    Fax: +1-404-256 1528
E-mail: roudebush@rba-online.com
Received 2001-05-09     Accepted 2001-05-22