Lipophagy contributes to long-term storage of spermatozoa in the epididymis of the Chinese soft-shelled turtle Pelodiscus sinensis

Spermatozoa are known to be stored in the epididymis of the Chinese soft-shelled turtle Pelodiscus sinensis for long periods after spermiation from the testes, but the molecular mechanisms underlying this storage are largely unknown. In this study, epididymal spermatozoa were investigated to determine the potential molecular mechanism for long-term sperm storage in P. sinensis. Transmission electron microscopy (TEM) and Oil red O staining indicated that unusually large cytoplasmic droplets containing lipid droplets (LDs) were attached to the epididymal spermatozoa. However, the content of LDs decreased gradually with the sperm storage. LDs were surrounded by autophagic vesicles and sequestered as degradative cargo within autophagosome. Immunofluorescence and western blotting demonstrated that autophagy in spermatozoa increased gradually with the storage time. In vitro studies found that spermatozoa obtained from soft-shelled turtles in January can survive more than 40 days at 4°C. Furthermore, immunofluorescence and TEM showed that autophagy was involved in the degradation of LDs with the extension of sperm incubation. Inhibition of autophagy with 3-methyladenine significantly suppressed LD degradation. Moreover, adipose triglyceride lipase was involved in the metabolism of LDs. These findings indicate that lipophagy was activated to maximise LD breakdown, which contributes to long-term sperm storage in the epididymis of P. sinensis.