Rad18 is required for long-term maintenance of spermatogenesis in mouse testes

Maintaining the integrity of spermatogenic stem cells is essential to transfer genetic information to a descendant. However, knowledge of maintenance of genetic stability in stem cells is still limited. RAD18 is critical for postreplication repair through mono- and multi-ubiquitination of proliferating cell nuclear antigen (PCNA) to maintain genomic stability. Mammalian RAD18 is highly expressed in the spermatocytes and the nuclei of a few spermatogonia in adult mice. To elucidate the physiological function of RAD18, we analyzed a phenotype of Rad18-/- mice. The mice were born and appeared to grow normally. Although the mice were fertile, fertility and testis weight decreased with age. Histological examination revealed normal spermatogenesis in almost all seminiferous tubules in Rad18-/- testes at 2 months old, and abnormal sperm could not be detected in the epididymis. However, 25% of the tubules lost almost all germ cells at 12 months. The seminiferous tubules frequently retained only late differentiated phase germ cells, suggesting that the exhaustion of spermatogonial stem cells leads to the loss of all germ cells in the seminiferous tubules. Wild-type germ cells were successfully transplanted into and colonized in the seminiferous tubules of aged Rad18-/- mice, indicating that Sertoli cells have a normal supportive function even in aged testes. We conclude that RAD18 is intrinsically required for the long-term maintenance of spermatogenesis.