Embryonic stem cells (ESCs) possess the unique ability to self-renew in a naïve state while retaining the competence to differentiate into all lineages of the developing embryo. This balance relies on the fine-tuned regulation of gene expression, which operates at multiple levels, from chromatin to messenger RNA maturation. Among these levels of control, ribosome biogenesis and translation have recently emerged as major drivers of stem cell homeostasis. It is notably well established that undifferentiated ESCs exhibit lower translational efficiency than their differentiated descendants, even though messenger RNA and protein levels are often poorly correlated in these cells — underscoring the importance of translational regulation. The underlying molecular mechanisms, however, remained largely unknown.
In this context, the authors identified the factor RSL24D1 as being highly expressed in mouse and human pluripotent stem cells. To characterize its function, the team combined cell fractionation and sucrose-cushion ribosome purification approaches, polysome profiling, analyses of protein synthesis by metabolic labeling (radiolabeled methionine/cysteine and HPG), as well as RNA sequencing and bioinformatic enrichment analyses. RSL24D1 was found to associate with nuclear pre-ribosomes and to be required for the biogenesis of 60S subunits in murine ESCs. Protein conservation was further examined through sequence alignment across several species and through structural modeling based on pre-60S structures obtained by cryo-electron microscopy.
RSL24D1 depletion significantly impairs global translation, and most particularly that of key pluripotency factors as well as components of the Polycomb repressive complex 2 (PRC2). The effect on differentiation remains moderate, but ESC self-renewal and their lineage commitment choices are markedly altered. These observations outline a dual role for RSL24D1-dependent ribosome biogenesis: supporting the expression of pluripotency transcriptional programs while, at the same time, maintaining PRC2-mediated repression of developmental programs.
Taken together, this work establishes that RSL24D1-dependent ribosome biogenesis contributes in a concerted manner to embryonic stem cell homeostasis, linking the maintenance of pluripotent identity with the control of commitment toward differentiation. It thereby delineates a molecular mechanism by which the control of protein synthesis participates in cell fate decisions in stem cells.