From servant to master: the ribosome’s instructive role in hematopoietic cell fate determination

Chahal AS1,2, Mitchell N1, Zaytseva O1, Geroge A1, Hannan RD1 and Quinn LM1

  1. JCSMR, Australian National University.
  2. University of Melbourne.

Transcriptional control of gene expression is viewed as the key instructor of stem cell fate. In contrast, dogma has relegated ribosome biogenesis and mRNA translation as a necessary, but passive, contributor to cell and tissue specification during development. Intuitively, impaired ribosome function should (and usually does) cause reduced tissue growth and stunted development. Inexplicably, cancer arises more frequently in patients with ribosomal protein (RP) loss, particularly in the blood lineage. Our recently developed Drosophila hematopoietic models may provide a rationale for the prevalence of leukemia in patients with reduced RP levels. Following tissue specific depletion of the RPs (s19 and s24) most frequently mutated in the ribosomopathy Diamond Blackfan Anemia (DBA), we observe stem cell fate defects, overproliferation and massive overgrowth of the blood compartment. Interestingly, knockdown of RPs19 and RPs24 differentially altered stem and progenitor cell differentiation, which suggests depletion of particular RPs from ribosomes might actively disrupt cell fate by modulating the classes of transcripts translated. Indeed, Mass Spec data from the RP deficient lymph glands revealed increased protein abundance for several factors previously implicated in driving developmental programs of growth and differentiation, including master transcriptional regulators and chromatin remodeling machinery. As RNA-Seq revealed mRNA transcript levels for these factors were unchanged, we hypothesise that their defective translation underlies the stem cell fate defects and blood compartment overgrowth. A combination of TEM and Bi-FC analysis revealed that RP depletion did not significantly reduce mature ribosomes in the cytoplasm, suggesting that altered protein abundance is not due to competition for ribosomes. We are currently using mass spec to determine if RPs19 or s24 depletion alters ribosomal composition in Drosophila hematopoietic cell lines, and whether mRNA species encoding stem cell fate proteins are differentially translated using RNA-seq of polysome fractions.