Rbfox1 Regulates mRNA Translation to Promote Germ Cell Differentiation

Germ cells are the only cells that can give rise to an embryo. During differentiation, female germ cells that will give rise to oocytes form a syncytium called a germline cyst. The mechanisms that regulate germline cyst development remain poorly understood. In Drosophila, germline stem cells (GSCs) undergo an asymmetric division, giving rise to a stem cell and a cystoblast that then divides four times to produce a 16-cell germline cyst. This 16-cell cyst will then continue differentiation until it forms a mature oocyte. Drosophila RNA-binding Fox-1 (Rbfox1), also known as Ataxin-2 Binding Protein 1 (A2BP1), mutant females exhibit a germ cell differentiation defect that results in germline cystic tumors. The Rbfox genes encode several isoforms, many of which contain a highly conserved RNA recognition motif (RRM). Disruption of human RBFOX homologs have been linked with a number of different neurological disorders and cancers. Some of these isoforms localize to the nucleus while others localize to the cytoplasm. Nuclear forms have well-established roles in regulating alternative splicing. However the function of Rbfox in the cytoplasm remains unclear. Here, we demonstrate that cytoplasmic Drosophila Rbfox1 regulates germline cyst development. We further show that Rbfox1 represses the translation of mRNAs that contain (U)GCAUG elements within their 3’ UTRs. We have identified pumilio (pum) as a critical Rbfox1 target gene. Pum is an RNA-binding protein essential for germline maintenance across species. During germline cyst differentiation, Rbfox1 silences pum mRNA translation thereby promoting germ cell development. Mis-expression of pum results in the formation of germline cystic tumors that resemble Rbfox1 mutant phenotype. In addition, these cysts breakdown and dedifferentiate back to single, mitotically active cells. Together these results reveal that cytoplasmic Rbfox family members regulate the translation of specific target mRNAs to promote differentiation. In the Drosophila ovary, this activity provides a genetic barrier that prevents germ cells from reverting back to an earlier developmental state. These findings have thus advanced our understanding of germline development and the molecular function of Rbfox proteins, with implications in cellular differentiation and Rbfox-related disorders.