Differential IRES-initiated translation during homeostatic stem cell differentiation and stress

Cell stress commonly induces preferentially reduced m7G-cap-dependent translation initiation while cap-indendepent, IRES-mediated initiation persists. We investigated differential translation initiation in the highly regulated protein production of somatic stem cells by generating a reporter mouse quantifying the ratio of IRES-mediated to cap-dependent translation in vivo. Both hematopoietic and epithelial stem cells exhibited minimal IRES utilization that progressively increased with differentiation. Notably, even within defined differentiation states, lower IRES/Cap was associated with increased clonogenicity regardless of cell cycle and among HSCs, with greater long-term multi-lineage regeneration. The RNA binding protein PTBP1 is a key modulator of IRES utilization during differentiation by CRISPR/Cas9i screening. Even within IRES-low immature stem/progenitor cells, nutrient stress upregulated IRES utilization, pointing to distinct roles of IRES in cell behavior depending on physiologic state.