Elucidating the role of FBLL1 in ribosome function during neuronal differentiation

PROJECT SUMMARY Chemical modifications of RNA have been seen to have numerous biological consequences including regulation of development. Ribosomal RNA is heavily modified by 2’O methylation (2’O-Me). There is increasing evidence that regulation of rRNA modification impacts developmental programs including neuronal differentiation. Fibrillarin (FBL) is known to be the canonical 2’O methyltransferase that applies 2’O-Me to ribosomal RNA. While total loss of FBL is lethal as it is critical to ribosome biogenesis, modulation of FBL expression can regulate stem cell differentiation. Overexpression of FBL extends mouse embryonic stem cell pluripotency, while partial knockdown induces neural stem cells marker expression. Additionally, altering FBL expression can cause changes in 2’O-Me of rRNA. Changes in 2’O-Me of rRNA have been seen to alter translational levels of select mRNAs and modulate cap-independent translation. The role of additional methyltransferases in shaping ribosomal function in neurogenesis is unknown. FBL has a mammalian-specific paralog FBLL1. The function of FBLL1 is unknown, though its structural similarity to FBL suggests that it is also a methyltransferase. Its role in neuron differentiation has not been studied. Unlike FBL which is expressed throughout all adult tissue types, FBLL1 is specifically expressed in the brain and testes. Within the brain, FBLL1 is expressed exclusively in neurons. While FBL expression decreases, FBLL1 expression increases through neuronal differentiation. We hypothesize that FBLL1 may act as an additional RNA methyltransferase that applies distinct 2’O-Me to ribosomes to shape translation through neuronal differentiation. Indeed, our preliminary results show that, when ectopically expressed in HEK293 cells, FBLL1 binds 18S rRNA and localizes to the nucleolus, the site of rRNA modification. In neurons, FBLL1 localizes to the nucleolus. Additionally, we observed reduction in 2’O-Me of specific sites on rRNA with genetic loss of FBLL1 in neurons. The goal of this project is to understand the function of FBLL1 and its impact on translational regulation through neuronal differentiation through two Specific Aims. Aim 1 is to characterize the protein and RNA binding partners of FBLL1 that it may use to function in neurons. Aim 2 is to determine the role of FBLL1 in regulation of rRNA 2’O methylation and effects on ribosome function. We will examine how translational changes induced by FBLL1 promote gene expression needed for neurogenesis. This work may reveal an enzyme that creates cell type-specific rRNA modifications to fine tune translation in neuronal development.