Phagocytosis Of Apoptotic Spermatogenic Cells By Sertoli Cells: Mechanism And Significance

About 75%of spermatogenic cells are estimated to undergo apoptosis during mammalian spermatogenesis,and the cytoplasmic portions of the rest germ cells are shed and form residual bodies before they mature into spermatozoa.The apoptotic spermatogenic cells and residual bodies are phagocytosed and degraded by Sertoli cells. The mechanisms and significance underlying these phenomena remain to be clarified.In this study,we investigate these issues with an effort on the essential role of Tyro3 receptor tyrosine kinase subfamily in regulating phagocytic ability of Sertoli cells,and the meaning of phagocytosis of apoptotic germ cell by Sertoli cells.We established an in vitro method to evaluate the phagocytic ability of Sertoli cells through detecting lipid droplet formation.By using this model,we observed that Tyro3 subfamily of receptor tyrosine kinases(RTKs)—Tyro3,Axl,Mer,and their ligand Gas6 regulate phagocytic ability of Sertoli cells.The presence of Gas6 in serum-free medium increased phagocytic ability of Sertoli cells by 5 times.By analysis of the Tyro3 RTKs deficient Sertoli cells,we demonstrated that the three members of Tyro3 RTKs act cooperatively in regulating the phagocytic ability of Sertoli cells.Mer is responsible for triggering phagocytosis of apoptotic spermatogenic cells by Sertoli cells,Tyro3 and Axl enhance cooperatively the action of Mer by recognizing and binding apoptotic cells.Furthermore,we demonstrated that apoptotic spermatogenic cells and residual bodies can be energy sources for Sertoli cells.In vitro assay showed that engulfment of apoptotic spermatogenic cells increases ATP production by Sertoli cells.The ATP production in seminiferous tubules is closely related to the phagocytosis of residual bodies by Sertoli cells.Induced apoptosis of spermatogenic cells in vivo increased ATP level in seminiferous tubules.The ATP production both in vivo and in vitro associated with the lipid formation in Sertoli cells after phagocytosis of apoptotic spermatogenic cells.We therefore conclude that after phagocytosis by Sertoli cells,apoptotic spermatogenic cells are degraded to form lipids,which can be used to produce ATP.The results define a novel meaning of spermatogenic cell death.Finally,we found that lysates of apoptotic germ cells can trigger inflammatory responses by Sertoli cells.The lysates induced expression of various inflammatory cytokines in Sertoli cells.This process depends on the activation of Toll-like receptor 3 and 4(TLR3,4).The activation of Toll-like receptors results in the translocation of NF-κB from cytoplasm to nuclei,and subsequently induces the expression of inflammatory factors.By contrast,the apoptotic germ cells can not trigger the expression of cytokine genes in Sertoli cells.These results suggest that lysis of apoptotic germ cells release potential endogenous agonists of TLR3 and 4,and sequentially provoke inflammatory responses through activation of TLR3 and 4 in Sertoli cells.These results may provide noval insight into the meanings of clearance of apoptotic germ cells by Sertoli cells.The effect of this phenomenon on spermatogenesis would be an interesting issue that is worthile to be investigated.