The Supply of Proteins to the Testicles Could One Day Contribute to the Treatment of Male Infertility

According to the Mayo Clinic, approximately 15% of all couples are infertile, with male infertility playing a role in more than a third of these cases. Problems with sperm development are often responsible for this. Researchers have found a way to transport a protein important for sperm production directly into the testicles of mice, where it restored normal sperm development and enabled previously infertile mice to produce offspring.

What is the Significance of the Testicles?

The testicles play a central role in male fertility, as they are responsible for both sperm production and the production of the hormone testosterone. Spermatogenesis takes place in the fine seminiferous tubules of the testicles – a process lasting several weeks in which immature germ cells develop into fully mature sperm. So-called Sertoli cells support the development of sperm, while Leydig cells produce testosterone, which is essential for the maturation of sperm and for maintaining overall testicular function.

Disorders of these structures, such as genetic changes, infections like mumps, untreated undescended testicles, or varicocele, can significantly impair sperm production and lead to infertility. Another important function of the testicles is temperature regulation: they are located outside the abdominal cavity in the scrotum because the production of functional sperm requires a temperature 2–3 °C lower than body temperature. Increased heat exposure—for example, due to tight clothing, frequent sauna visits, or varicocele—can negatively affect sperm quality and count. Overall, the testicles are therefore the central organ for male fertility, and disorders of their structure or function are among the most common causes of male infertility.

What is the Blood-Testis Barrier?

The blood-testis barrier is one of the most important protective structures in the male reproductive system. It is located in the seminiferous tubules of the testicles and is formed by Sertoli cells, which are linked together by very tight junctions. It has several important functions.

1. Protection of Germ Cells

During puberty, mature sperm develop in the testicles for the first time. These differ genetically from the body’s cells, which is why the immune system could recognize them as foreign. The blood-testis barrier prevents immune cells or antibodies from entering the area where the sperm develop. It therefore protects the germ cells from autoimmune reactions.

2. Controlled Exchange of Substances

The BTB regulates which substances from the blood are allowed to enter the area of spermatogenesis. It retains harmful substances, toxins, and pathogens while allowing only the nutrients and hormones necessary for sperm development to pass through.

3. Separation of Two Compartments

The barrier creates two separate spaces in the seminiferous tubules:

• the basal area, where early germ cells are located, and
• the adluminal area, where late sperm maturation takes place. This spatial separation is crucial for the correct sequence of spermatogenesis.

If the blood-testis barrier is damaged by inflammation, injury, heat, toxins, or infections, immune reactions against the body’s own sperm and disturbances in sperm maturation can occur. This often leads to a reduced sperm count or even infertility.

PIN1 is an Important Key Enzyme

Male infertility often occurs due to a lack of sperm cells in the semen, which can be caused by damage to the blood-testis barrier (BTB). The blood-testis barrier protects reproductive cells from harmful toxins and drugs, and a protein called PIN1 is important for its function. PIN1 is a key enzyme that acts as a molecular switch, controlling numerous important processes by altering the structure of phosphorylated proteins. It is important for cell division, signal transduction, and protein stability — and is therefore centrally involved in many biological processes, from cancer to spermatogenesis. PIN1 plays an important role in the stability of the blood-testis barrier by regulating the proteins that form the tight cell contacts between Sertoli cells.

These cell contacts — such as tight junctions and adherens junctions — consist of proteins such as claudin-11, occludin, N-cadherin, and β-catenin, whose correct folding and function depend largely on their phosphorylation and thus on PIN1. PIN1 alters the spatial structure of such phosphorylated proteins so that they remain stable, are correctly embedded in the cell membrane, and can interact effectively with each other. This keeps the blood-testis barrier tight and functional. If PIN1 is damaged or missing, these contact proteins become unstable and are broken down more quickly in some cases, making the barrier more permeable. This can cause immune cells from the blood to enter the sensitive area of developing sperm, where they trigger inflammation or autoimmune reactions. As a result, spermatogenesis is disrupted, which can lead to a reduced sperm count or even infertility. PIN1 is a key enzyme for the integrity of the blood-testis barrier and thus for male fertility.

Proteins in the Testicles for the Treatment of Male Infertility

Mice that lack PIN1 due to genetic manipulation are infertile, have small testicles, a lack of sperm stem cells, and a low sperm count. Although scientists have considered gene therapies to treat male infertility, these procedures are risky because they could cause unwanted genetic changes in reproductive cells that could be passed on to offspring. Hyun-Mo Ryoo and his colleagues wanted to develop a system to deliver proteins (such as PIN1) to the testes instead of genes, but first they had to find a way to get the proteins through the complex channels of the testes into the cells.

The researchers developed a delivery system called Fibroplex, which consisted of spherical nanoparticles made of silk fibroin and a lipid coating. They loaded PIN1 into Fibroplex and showed that the particles were safe and showed no signs of toxicity or testicular damage in mice. When the team injected PIN1-loaded Fibroplex into the testes of young mice with PIN1 deletions, the treatment restored near-normal PIN1 levels and sperm stem cell numbers and repaired the BTB. The treated mice had normal testicular weight and size and about 50% of the sperm count of wild-type mice. Until about 5 months after treatment, when the protein was degraded, the mice treated with PIN1 Fibroplex sired a similar number of offspring as wild-type mice, while untreated mice with PIN1 deletions remained infertile. According to the researchers, this is the first evidence of direct delivery of proteins to the testes for the treatment of male infertility.

What Can Men Themselves Do to Effectively Protect their Testicles?

Men can take a number of measures to protect their testicles and thus support their fertility, hormone production, and overall health. The most important measures relate to lifestyle, temperature, protection from injury, and avoidance of harmful influences.

1. Keep the Temperature Low

The testicles need to be about 2–3 °C cooler than body temperature to produce sperm optimally.

Therefore, it is advisable to:

• Avoid tight pants and synthetic underwear
• Do not use your laptop on your lap for long periods of time
• Use seat heaters sparingly
• Reduce frequent sauna visits and hot baths
• Seek medical advice if you have varicose veins in the scrotum (varicocele)

2. Avoid Overheating in Everyday Life

Everyday factors can also increase the temperature, e.g.:

• Prolonged sitting
• Being overweight (increases local heat)

Regular exercise and a healthy body weight help to maintain testicular function.

3. Protection Against Injury

Mechanical damage can directly damage the blood-testis barrier or tissue.

• Wear protective equipment when playing sports (soccer, martial arts, cycling)
• Avoid prolonged cycling with poor saddles or use ergonomic saddles

4. Avoid Pollutants and Toxins

Many environmental substances are toxic to the testicles and can damage sperm.

• Do not smoke
• Drink alcohol only in moderation
• Minimize contact with solvents, pesticides, paints, and heavy metals
• Only work with chemicals while wearing protective clothing

5. Critically Examine Medications and Drugs

Some substances disrupt hormones or damage the testicles:

• Anabolic steroids/testosterone preparations (can almost completely stop sperm production)
• Some painkillers and medications when taken long-term
• Cannabis and other drugs can reduce sperm quality

If in doubt: always consult a doctor.

6. Avoid Infections

Infections such as mumps orchitis, STIs (e.g., chlamydia), or chronic inflammation can cause permanent damage.

• Mumps vaccination (MMR) protects against testicular damage
• Use condoms
• Treat urinary tract infections early
• Seek immediate medical attention for testicular or epididymal pain

7. Support Hormone and Metabolic Health

Metabolic disorders can affect the testicles.

• Maintain a normal body weight
• Exercise and eat a healthy diet (especially vegetables, omega-3, zinc, selenium)
• Reduce stress – chronic stress lowers testosterone and damages testicular function
• Get enough sleep (poor sleep → poorer sperm quality)

8. Regular Self-Examination

Once a month in a warm shower:

• Check the size, shape, and structure of both testicles
• Look for lumps, swelling, or hardness

Early detection protects against testicular tumors and inflammation