Capacitation-associated Protein Tyrosine Phosphorylation As a Possible Biomarker of Sperm Selection

The human male deposits millions of sperm cells in the woman's vagina at the time of ejaculation. However, only a few sperm cells are able to reach the fallopian tubes. In fact, only between 100-1000 sperm will reach the cumulus-oocyte complex and only one will be able to fertilize the oocyte.

The sperm cells from the ejaculate of all mammals are unable to fertilize the oocyte. In order to acquire the fertilization competence, sperm must undergo a process called capacitation. Capacitation has been defined as a complex phenomenon that involves biochemical and physiological changes in the sperm.

These changes are such as:

• Loss of proteins or replacement by others of lower molecular weight
• Transformation of phospholipids, decreased cholesterol/phospholipid ratio
• Changes in the carbohydrate fraction of glycoproteins
• Lipid and protein mobility

Not all the events involved in the complex capacitation process are known.

The following stages can be differentiated:

• Cell membrane fluidification
• Increased intracellular Ca2+
• Generation of controlled amounts of reactive oxygen species
• Increased intracellular pH and hyperpolarization of sperm membrane potential
• Protein phosphorylation (serine, threonine and tyrosine residues)

Under in vivo conditions, motile sperm actively migrate through the cervical mucus and are separated from the rest of the ejaculate. In vitro, sperm capacitation can be achieved if they are subjected to particular culture conditions during the necessary period of time.

After capacitation, sperm acquires three characteristics:

• Hyperactivation.
• Acrosomic reaction.
• Sperm-oocyte interaction.

The capacitation process is necessary for the sperm to be able to cross the layer of cells that surround the oocyte and to undergo the acrosome reaction. Only a fraction of sperm are known to reach the capacitation state at some point.

The capacitation process depends mainly on post-translational modifications of proteins. One of the most important modifications that these proteins undergo is phosphorylation. During sperm capacitation, serine, threonine and tyrosine residues are phosphorylated. However, phosphorylation of tyrosine residues is the best indicator of sperm capacitation.

The evaluation of the phosphorylation of the tyrosine residues of the sperm proteins by flow cytometry makes it possible to assess the degree of capacitation of the sperm in vitro and to determine how many of them have carried out this process adequately . As previously mentioned, not all sperm cells of a sample undergo the capacitation process and not all of them present the same percentage of phosphorylated proteins in tyrosine residues.

Certain events, such as the cryopreservation process and the incubation times of the samples until their use in fertility treatments (IA or IVF / ICSI), can affect the capacitation process. Therefore, the measurement of the levels of tyrosine phosphorylation of the sperm by flow cytometry can allow the investigators to know the level of capacitation of the sample. There are studies that report changes in success rates when samples are incubated different period of times. The incubation period affects the rate of acrosomal reaction of sperm and may even affect DNA fragmentation. Furthermore, the temperature at which such incubation is carried out also seems to affect sperm capacitation.

Based on the above, this project aims to analyze the possible correlation between the levels of tyrosine phosphorylation in sperm and the functional quality of the seminal sample, as well as the reproductive success of the cycle. In parallel, the investigators will analyze whether the sperm capacitation process is affected by the incubation times of the sample once it has been capacitated or whether it depends exclusively on the quality of the seminal sample and is intrinsic to the male.

Inclusion criteria

• To have signed the written informed consent before starting any procedure related to the study.
• Sexual abstinence (2 to 5 days).
• To be a male patient who delivers a fresh seminal sample for diagnosis or artificial insemination treatment.
• To be a male donor who delivers a fresh seminal sample for donation or for previous tests to include him in the donation programme.
• To be a female patient undergoing artificial insemination.

Exclusion criteria

• Use of sperm of testicular or epididymal origin.
• Cryptozoospermia or oligozoospermia with sperm concentration <1 mill/ml.
• Ejaculates obtained with more than 5 days of sexual abstinence.
• Frozen semen samples for artificial insemination cycles with partner's semen.
• Female patients who present uterine malformations that compromise the viability of the pregnancy (intramural or submucosal fibroids > 3 cm, polyps, adenomyosis or congenital or acquired malformations). Female patients with hydrosalpinx.
• Patients who have suffered two or more previous abortions (repeated abortions).
• Patients with a body mass index greater than 30 Kg / m2.