SAT-032 Follistatin-Like 3 (FSTL3) and Early Postnatal Testicular Development

Activin, a TGFβ family ligand, induces Sertoli cell (SC) proliferation during the early postnatal stage in mice. Overexpression of activin, however, leads to the disruption of spermatogenesis. FSTL3, a glycoprotein inhibitor of activin, is highly expressed in the testis and its expression is induced by activin creating an inhbitory feedback loop. FSTL3 deletion in mice is therefore expected to produce a mouse model of increased activin action. Contrary to overexpression of activin, however, as we have shown before, global deletion of FSTL3 in mice results in increased numbers of SC and increased sperm production in older males. Stereological analyses show that although the overall number of SC increases with age in both genotypes during pre-pubertal stages, increase in SC numbers is significantly higher in FSTL3 KO males. Here we show our transcriptomic analyses of WT and FSTL3 KO mice at 3d and 8wk. mRNA sequencing data showed that more than 1000 genes are diffferentially regulated between WT and FSTL3 KO at 3d. There is a much lower number of genes differentially expressed at 8wk. Among several canonical pathways that are altered at 3d in FSTL3 KO mice compared to WT we investigated the “Sertoli-to-Sertoli cell Communication” pathway. We found increased expression of junction proteins, including those that are involved in the blood testis barrier (BTB) as well as earlier establishment of BTB in FSTL3 KO testis. Without the BTB, preleptotene spermatocytes cannot progress through the spermatogenic programme. Importantly, we found accelerated SYCP3 organisation, indicative of early entry into meiosis, concomitant with early establishment of BTB. To directly address whether FSTL3 deletion can induce SC proliferation we knocked down FSTL3 expression in the TM4 SC line. We found increased PCNA expression with FSTL3 siRNA compared to control siRNA transfection demonstrating FSTL3 impairment does induce SC proliferation. We are currently investigating the mechanisms for early establishment of BTB in FSTL3 KO testes and additional canonical pathways. The results of the study presented suggests that loss of FSTL3 promotes SC proliferation which likely allows establishment of SC-SC interactions including BTB formation earlier than in WT. Given that the difference in size between WT and FSTL3 KO testis persists at older ages, it is possible that the timing of increase in SC number and establishment of BTB has a sustained effect on testicular function later in life.