The impact of L-carnitine on human semen quality upon cryopreservation, its association with sperm fatty acids, and assessment of a novel analytical tool for DNA fragmentation in mammals

Abstract

Reduced male fertility is associated with reduced semen quality and can result from medical conditions, such as infections or blockages in the reproductive organs, and lifestyle factors like obesity. However, the cause of reduced fertility remains unexplained in about 50% of affected males. Even though semen analysis constitutes a central and initial part of male infertility investigation, relying on it alone is insufficient for a comprehensive diagnosis of male infertility. Recent studies suggest that oxidative stress may be a contributing factor in male infertility. Oxidative stress arises from an imbalance between reactive oxygen species (ROS) production and the body's antioxidant defence, leading to sperm DNA damage at various stages of sperm development. While sperm fatty acids are vital for sperm function, L-carnitine is equally important, facilitating the transport of long-chain fatty acids for β-oxidation and supporting antioxidant defence. High levels of L-carnitine from the epididymis are found in seminal plasma and positively correlate with sperm concentration and motility. Additionally, L-carnitine supplementation in cryopreservation media improves post-thaw semen quality by reducing ROS levels and DNA fragmentation. The primary objective of this project was to assess the effect of endogenous seminal L-carnitine on human sperm parameters in fresh and post-thaw semen samples. Additionally, we explored alternative approaches for evaluating DNA fragmentation, using bull spermatozoa as a model system. In the first article, we revealed that free seminal L-carnitine levels positively correlate with palmitic acid, docosahexaenoic acid, and total n-3 PUFAs in sperm. Seminal L-carnitine levels were not linked to BMI, indicating that the reduced semen quality associated with obesity is unrelated to L-carnitine. These results support the role of the epididymis in sperm maturation. In the second paper, we showed that both endogenous L-carnitine and L-carnitine supplemented to the cryopreservation medium improve sperm motility and mitigate the oxidative stress caused by freezing. In addition, this study evaluated alternative methods to the Sperm Chromatin Structure Assay (SCSA) for detecting DNA damage in bovine sperm. Our study found that SCSA and Sperm Chromatin Dispersion (SCD) tests both effectively identified increased DNA damage in bull spermatozoa exposed to high H₂O₂ concentrations, whereas Sensitive Recognition of Individual DNA Ends (dSTRIDE) test did not. These results suggest that SCD is a promising alternative to SCSA for assessing DNA damage, while dSTRIDE needs further refinement.