RNA-binding proteins (RBPs) are important post-transcriptional regulators of gene expression. However, the role of these proteins in influencing the fate decisions in stem cells are not well understood. Now, Xin Wu, Jon Oatley, John Zhong Li and colleagues use a technique called mRBPome capture to identify novel RBPs involved in spermatogonial stem cell maintenance. By crosslinking RBP-mRNA complexes, followed by polyA-pulldown and mass spectrometric analysis of mRNA-bound proteins, they identify 473 putative RBPs in cultured undifferentiated spermatogonia. They compare the expression patterns of these RBPs in mouse embryonic stem cells, fibroblasts, Sertoli cells and the spermatogenic lineage and thereby identify three candidate proteins, TRIM71, ESRP1 and MEX3A, with previously uncharacterised roles in the mammalian germline. Knockdown of TRIM71 or MEX3A decreases the expansion rate of undifferentiated spermatogonia, while TRIM71 knockdown additionally induces increased apoptosis. The authors subsequently generate mice with a conditional knockout of TRIM71 and find that transit-amplifying progenitors in these animals fail to both proliferate and differentiate. Interestingly, they also show that TRIM71 interacts with EWSR1 protein, a known regulator of cellular senescence in haematopoietic stem cells. Thus, this study provides a first description of the RNA-binding proteome in undifferentiated spermatogonia and shows that TRIM71 regulates spermatogonial differentiation, possibly by controlling the function of EWSR1.
