SUN-218 The Role Of Fstl3 In Spermatogenesis: A Transcriptome Analysis

Spermatogenesis in mammals depends on the function of Sertoli cells (SC). The total number of SC present in the testes is determined in the pre-pubertal stages of development. Activin, a TGFβ family ligand, regulates SC proliferation in the early postnatal phase in mice. During this phase immature SC are unable to assemble tight-junctions which are required to form the blood-testis-barrier (BTB) and consequently do not support spermatogenesis. Overexpression of activin beyond pre-pubertal stages results in the disruption of spermatogenesis and results in infertility. Follistatin Like-3 (Fstl3) is a glycoprotein known to bind and inhibit activin. Contrary to overexpression of activin, however, global deletion of Fstl3 in mice results in increased numbers of SC and increased sperm production in older males. Based on this striking observation, in this study, using the global deleted Fstl3 mouse model, we sought to elucidate the function of Fstl3 in testicular development during the transition from pre-pubertal to post-pubertal stages. In testis sections from BrdU treated Fstl3 KO mice, we found that SC number per tubule is maintained from pre-pubertal to post-pubertal stages without a concomitant increase in BrdU positive SC. In contrast, SC numbers in similarly treated WT mice are depleted over the same time period. Stereological analyses further show that although the overall number of SC increases in both genotypes during pre-pubertal stages, increase in SC numbers is significantly higher in Fstl3 KO males. We also found by immunofluorescence that Sycp3 organization appears early (postnatal day 7) in Fstl3 KO mouse testes indicating early meiotic entry. To identify signalling pathways associated with SC proliferation and maturation that are affected by Fstl3 deletion we performed total mRNA sequencing and compared the findings between WT and Fstl3 KO testes of 3 day and 8 weeks mice. Our results show that more than 1000 genes are differentially expressed between WT and KO testes. Pathway analyses reveal that the “Sertoli-to-Sertoli cell Communication” pathway is one of the most relevant enriched canonical pathways in 3 day old Fstl3 KO mice compared to WT. We are currently investigating the mechanisms for early establishment of BTB in Fstl3 KO testes. The results of the study presented suggests that loss of Fstl3 promotes the establishment of SC-SC interactions that support spermatogenesis during the proliferative phase of these cells.