Seminal Proteins Related to Fertility
Four specific proteins have been identified in bovine semen related to fertility, and they are summarized in Table 2 below.
Research at Pennsylvania State University in the lab of Dr. Gary Killian led to the discovery that relative amounts of osteopontin
(OPN) and a lipocalin-Type D prostaglandin synthase (PGD) in semen were reflective of fertility in Holsteins (Killian et al.,
1993; Cancel et al., 1997) and Angus. The Ax lab at the University of Arizona has studied Fertility-associated antigen (FAA;
McCauley et al., 1999) and Type-2 tissue inhibitor of metalloproteinases (TIMP-2; McCauley et al., 2001; Dawson et al., 2002).
Presence of either of those proteins detected with Western blots corresponded to a 13-19% increase in fertility of bulls used
to breed cows in multiple-sire pastures (Bellin et al., 1994; 1996; 1998).
Table 1. Characteristics of four known fertility related proteins.
In the future, it may be useful to measure combinations of these proteins, or other ones not yet identified, to develop a
fertility index. Since subtle genetic changes in a gene can drastically alter the amount or structure of a protein produced,
it makes sense that genetic screening with modern molecular tools will become a powerful selection tool.
Examples of that type of approach are contained in a companion paper to this manuscript.
Is there a commercially available test to detect a marker for fertility potential in semen? Yes, a company called ReproTec
in Tucson, AZ has a lateral-flow cassette on the market that works much like a home pregnancy test (McCauley et al., 2004).
A color change indicates presence of FAA, and that color develops within 20 min after a semen sample is applied to the cassette.
Details are provided in the last section of this article and in a companion paper at this symposium.
The Relationship Between Aforementioned Proteins and Fertility
As mentioned, Killian and coworkers (1993) identified four seminal plasma proteins in Holstein bull semen that appeared to
be associated with fertility. They consisted of a 55 kDa (pI 4.1), 26 kDa (pI 6.2), 16 kDa (pI 6.7) and 16 kDa (pI 4.1) proteins
utilizing two-dimensional (2D) SDS-PAGE separation techniques. By analysis of the protein density, the first two, later characterized
as osteopontin (55 kDa; Cancel et al., 1997) and lipocalin-type prostaglandin D (PGD) synthase (26 kDa; Gerena et al., 1998)
were more prominent in higher fertility bulls while the latter two (currently uncharacterized) were prevalent in low fertility
bulls. Based on relative protein density values, OPN was positively correlated (r=0.48) with fertility (Cancel et al., 1997).
When bulls are categorized as above average or below average fertility, lipocalin-type PGD synthase is 3.5 times more prevalent
in the above average group of bulls (Killian et al., 1993 and Gerena et al., 1998; 2000). The work of Killian and others
has led to a regression model to predict the fertility of bulls based on these four fertility-associated proteins with an
R value equal to 0.89 (for formula see Killian et al., 1993).
Currently, the cellular roles of OPN and lipocalin-type PGD synthase and their influence on bull fertility remain to be elucidated.
However, OPN plays an integral part in a number of signal transduction pathways (for review see Denhardt et al., 2001) including
defense mechanisms and inflammatory conditions. In addition, OPN may serve as a functional cell attachment protein to provide
transitory stabilization of the sperm plasma membrane prior to fertilization.
Localization studies have identified lipocalin-type PGD synthase to be present within elongating spermatids, Sertoli cells,
rete testis and efferent duct epithelial cells along with epididymal epithelial cells (Gerena et al., 2000). In addition,
fluorescent microscopy revealed the presence of lipocalin-type PGD synthase on the apical ridge of the acrosome on ejaculated
bovine sperm (Gerena et al., 2000). This evidence would suggest lipocalin-type PGD synthase may play an important role in
the spermatogenic cycle, as well as maturation of spermatozoa during storage in the epididymis. Lipocalin-type PGD synthase's
multifaceted sites of localization and activity lend support to a number of potential roles that may reflect why bulls with
above average fertility have 3.5-fold higher concentrations of this protein.
These, as well as other functional roles of OPN and lipocalin-type PGD synthase need to be investigated to give us insight
on how they influence bull fertility.