Selective Recruitment of Germ Cell mRNAs by eIF4 Factors

Abstract

During germ line development critical steps in the proliferation and differentiation of stem cells into oocytes and sperm are regulated by mRNA translational control. This regulation changes the subset of proteins being synthesized, which ultimately alters cell fate. In this dissertation I focused on the role of translation initiation machinery, which we have shown selects discrete subsets of mRNAs to be recruited to ribosomes, as a means to drive cell fate decisions in gametogenesis. The rate-limiting step for mRNA recruitment is the formation of the 48S translation initiation complex. Eukaryotic translation initiation factor eIF4E binds the 7-methylguanosine 5' cap of mRNAs and recruits them to the 48S by binding to the scaffolding protein eIF4G to form a productive initiation complex. In mammalian cells, increased eIF4E-eIF4G complex leads to up-regulation of cap-dependent translation, which is required for growth and cell division. Cap-dependent mRNA translation has also been shown to be essential for sperm and oocytes development, as well as in the early embryonic cell cycle. In this body of work, I used C. elegans as a model for studying the contributions of eIF4 isoforms to germ cell fate decisions. Specifically my studies focused on an eIF4E isoform expressed in the germ line, IFE-1, and the cap-dependent eIF4G isoform, IFG-1 p170, and their regulation of translation initiation during sperm and egg development. Initial studies from our lab indicated that IFE-1 was required to promote maturation of spermatocytes and oocytes. Using a polysomal bioinformatics approach, I identified 77 mRNAs preferentially translated via IFE-1, several of which have critical known roles germ cell differentiation and maturation (gld-1, vab-1, vpr-1, rab-7, ran-1, rnp-3). Using live worm gonad imaging techniques, I further showed IFE-1-dependent selectivity in situ for de-repressed mRNAs in both early and late-stage oocytes. These studies outline how IFE-1, in a coordinated effort with IFG-1 p170, exerts positive mRNA translation control toward the selective expression of germ cell proteins that influence cell fate decisions.