Distinct Translation Initiation Strategies during Hematopoietic Differentiation

Abstract

Blood production occurs in a hierarchical differentiation process, where each blood cell has a unique transcriptome conferring proteomes that dictate cell-type specific functions. Although transcriptional studies have provided significant insights into blood differentiation, regulation of protein synthesis has become increasingly recognized for its importance in blood development and function, particularly in the context of cell stress. The rate limiting step of protein synthesis is translation initiation, whereby distinct mechanisms govern the loading of ribosomes onto mRNAs. Translation initiation in hematopoiesis and other regenerative system has not been well investigated, in part, due to the lack of appropriate animal models and methods to study these processes in rare populations of cells, like stem cells. In the first study presented in this dissertation, we developed and used a novel mouse model to define rates of IRES-mediated translation at a single-cell level. IRES-mediated translation is a subtype of translation initiation that is utilized by some RNA viruses and mammalian genes associated with cell survival and differentiation. Our goal was to evaluate the rate IRES-mediated translation in distinct hematopoietic cell types. We discovered that IRES-translation is least active in hematopoietic stem cells and increases with differentiation. The second study presented investigates how the bone marrow niche signals to hematopoietic progenitors in response to genotoxic and infectious stress by regulating translation initiation. We found that processed tRNAs (tiRNAs) are transferred from osteoblasts to granulocyte-monocyte progenitors via extracellular vesicles (EVs). The cargo of EVs includes specific species of tiRNAs that modulate translation initiation to enhance myeloid differentiation and improve survival after genotoxic injury and fungal sepsis. Together these findings further our understanding of how regulation of translation initiation contributes to hematopoietic differentiation and overcoming physiological stress. These findings will be important to developing therapeutics that rely on protein synthesis for efficacy.