p53 activation during ribosome biogenesis regulates normal erythroid differentiation.

The role of ribosome biogenesis in erythroid development is supported by the recognition of erythroid defects in ribosomopathies in both Diamond-Blackfan anemia and 5q- syndrome. Whether ribosome biogenesis exerts a regulatory function on normal erythroid development is still unknown. In the present study, a detailed characterization of ribosome biogenesis dynamics during human and murine erythropoiesis showed that ribosome biogenesis is abruptly interrupted by the decline in ribosomal DNA transcription and the collapse of ribosomal protein neosynthesis. Its premature arrest by the RNA Pol I inhibitor CX-5461 targeted the proliferation of immature erythroblasts. p53 was activated spontaneously or in response to CX-5461, concomitant to ribosome biogenesis arrest, and drove a transcriptional program in which genes involved in cell cycle-arrested, negative regulation of apoptosis, and DNA damage response were upregulated. RNA Pol I transcriptional stress resulted in nucleolar disruption and activation of the ATR-CHK1-p53 pathway. Our results imply that the timing of ribosome biogenesis extinction and p53 activation is crucial for erythroid development. In ribosomopathies in which ribosome availability is altered by unbalanced production of ribosomal proteins, the threshold downregulation of ribosome biogenesis could be prematurely reached and, together with pathological p53 activation, prevents a normal expansion of erythroid progenitors.

Loss of hematopoietic progenitors heterogeneity is an adverse prognostic factor in lower-risk myelodysplastic neoplasms.

Myelodysplastic neoplasms (MDS) are characterized by clonal evolution starting from the compartment of hematopoietic stem and progenitors cells (HSPCs), leading in some cases to leukemic transformation. We hypothesized that deciphering the diversity of the HSPCs compartment may allow for the early detection of an emergent sub-clone that drives disease progression. Deep analysis of HSPCs repartition by multiparametric flow cytometry revealed a strong disorder of the hematopoietic branching system in most patients at diagnosis with different phenotypic signatures closely related to specific MDS features. In two independent cohorts of 131 and 584 MDS, the HSPCs heterogeneity quantified through entropy calculation was decreased in 47% and 46% of cases, reflecting a more advanced state of the disease with deeper cytopenias, higher IPSS-R risk and accumulation of somatic mutations. We demonstrated that patients with lower-risk MDS and low CD34 + CD38+HSPCs entropy had an adverse outcome and that this parameter is as an independent predictive biomarker for progression free survival, leukemia free survival and overall survival. Analysis of HSPCs repartition at diagnosis represents therefore a very powerful tool to identify lower-risk MDS patients with a worse outcome and valuable for clinical decision-making, which could be fully integrated in the MDS diagnostic workflow.

Mitochondrial dynamics and reactive oxygen species initiate thrombopoiesis from mature megakaryocytes.

Blood platelets are essential for controlling hemostasis. They are released by megakaryocytes (MKs) located in the bone marrow, upon extension of cytoplasmic protrusions into the lumen of bone marrow sinusoids. Their number increases in postpulmonary capillaries, suggesting a role for oxygen gradient in thrombopoiesis (ie, platelet biogenesis). In this study, we show that initiation of thrombopoiesis from human mature MKs was enhanced under hyperoxia or during pro-oxidant treatments, whereas antioxidants dampened it. Quenching mitochondrial reactive oxygen species (mtROS) with MitoTEMPO decreased thrombopoiesis, whereas genetically enhancing mtROS by deacetylation-null sirtuin-3 expression increased it. Blocking cytosolic ROS production by NOX inhibitors had no impact. Classification according to the cell roundness index delineated 3 stages of thrombopoiesis in mature MKs. Early-stage round MKs exhibited the highest index, which correlated with low mtROS levels, a mitochondrial tubular network, and the mitochondrial recruitment of the fission activator Drp1. Intermediate MKs at the onset of thrombopoiesis showed high mtROS levels and small, well-delineated mitochondria. Terminal MKs showed the lowest roundness index and long proplatelet extensions. Inhibiting Drp1-dependent mitochondrial fission of mature MKs by Mdivi-1 favored a tubular mitochondrial network and lowered both mtROS levels and intermediate MKs proportion, whereas enhancing Drp1 activity genetically had opposite effects. Reciprocally, quenching mtROS limited mitochondrial fission in round MKs. These data demonstrate a functional coupling between ROS and mitochondrial fission in MKs, which is crucial for the onset of thrombopoiesis. They provide new molecular cues that control initiation of platelet biogenesis and may help elucidate some unexplained thrombocytopenia.