Cryopreservation induces changes in the expression of noncoding RNAs in semen: a mini-review

Semen cryopreservation is an important approach for preserving male fertility, and thawed semen is often used in in vitro fertilization/intracytoplasmic sperm injection (IVF/ICSI) procedures. This process has been used for decades and seems both safe and efficient, despite causing cryodamage to spermatozoa, including the impairment of sperm functions, with decreased viability and motility being the main indicators of poor sperm quality post-thawing. Although an increase in sperm DNA fragmentation is common during cryopreservation, data on how cryopreservation affects sperm at the molecular level are scarce. There are especially limited data on the influence of sperm cryopreservation on spermatozoa non-coding RNA (ncRNA) stability and expression. To date, only three publications have explored this issue in human sperm samples, and there have been only a few more studies in animals, including mouse, bull, ram, boar and giant panda. The results of studies on human and animal semen indicate that cryopreservation affects ncRNA expression, which could crucially affect fertilization and embryonic development. Moreover, a study employing boar spermatozoa further showed that alterations in ncRNA expression also depend on the sperm cryopreservation protocol used, and a study employing human spermatozoa showed that microRNA (miRNA) expression changes during cryostorage. Therefore, the effects of cryopreservation on ncRNA expression in spermatozoa should be studied more thoroughly, mostly because sperm cryopreservation is important for medically assisted reproduction. Furthermore, despite the wide usage of sperm cryopreservation, how cryopreservation changes sperm at the molecular level and whether these changes have any effect on future generations have not been determined.