ERK1 Regulates the Hematopoietic Stem Cell Niches

The mitogen-activated protein kinases (MAPK) ERK1 and ERK2 are among the major signal transduction molecules but little is known about their specific functions in vivo. ERK activity is provided by two isoforms, ERK1 and ERK2, which are ubiquitously expressed and share activators and substrates. However, there are not in vivo studies which have reported a role for ERK1 or ERK2 in HSCs and the bone marrow microenvironment. The present study shows that the ERK1-deficient mice present a mild osteopetrosis phenotype. The lodging and the homing abilities of the ERK1−/− HSC are impaired, suggesting that the ERK1−/−-defective environment may affect the engrafment of HSCs. Serial transplantations demonstrate that ERK1 is involved in the maintenance of an appropriate medullar microenvironment, but that the intrinsic properties of HSCs are not altered by the ERK1−/− defective microenvironment. Deletion of ERK1 impaired in vitro and in vivo osteoclastogenesis while osteoblasts were unaffected. As osteoclasts derive from precursors of the monocyte/macrophage lineage, investigation of the monocytic compartment was performed. In vivo analysis of the myeloid lineage progenitors revealed that the frequency of CMPs increased by approximately 1.3-fold, while the frequency of GMPs significantly decreased by almost 2-fold, compared with the respective WT compartments. The overall mononuclear-phagocyte lineage development was compromised in these mice due to a reduced expression of the M-CSF receptor on myeloid progenitors. These results show that the cellular targets of ERK1 are M-CSFR-responsive cells, upstream to osteoclasts. While ERK1 is well known to be activated by M-CSF, the present results are the first to point out an ERK1-dependent M-CSFR regulation on hematopoietic progenitors. This study reinforces the hypothesis of an active cross-talk between HSCs, their progeny and bone cells in the maintenance of the homeostasis of these compartments

Etude des mécanismes moléculaires de la régulation transcriptionnelle de IEX-1 (Immediate-Early Gene X-1) par la thrombopoïétine

LE KREMLIN-B.- PARIS 11-BU Méd (940432101) / SudocPARIS-BIUP (751062107) / SudocSudocFranceF

Rôles et partenaires de la protéine IEX-1 (une nouvelle cible des MAPK ERK)

PARIS7-Bibliothèque centrale (751132105) / SudocSudocFranceF

Etude des mécanismes d'activation (rôle et cibles de MAPK de type ERK au cours de la différenciation mégacaryocytaire induite par la TPO)

La thrombopoïétine et son récepteur, Mpl, jouent un rôle majeur dans la survie et la prolifération des progéniteurs hématopoïétiques multipotentiels, l'expansion de la lignée mégacaryocytaire et la formation de plaquettes. L'étude de lignées cellulaires exprimant Mpl établies au laboratoire (UT7-Mpl), ainsi que celle de mégacaryocytes normaux, ont permis de montrer que ce sont des différences quantitatives dans l'intensité et la durée du signal, et non l'activation en soi des MAPK (Mitogen Activated Protein Kinases) de type ERK qui jouent un rôle primordial dans la décision de la cellule UT7-Mpl à différencier vers une voie donnée ou à proliférer en réponse à une cytokine. L'objectif principal de mon projet de thèse était de comprendre 1) comment les MAP kinases sont activées par la TPO et 2) comment elles participent à la différenciation et/ou prolifération des précurseurs mégacaryocytaires en cherchant leurs substrats spécifiques dans ces cellules. En ce qui concerne le mécanisme d'activation de ERK par la TPO, nos travaux préalables ont montré que le mutant de Mpl, MplDelta3, n'est plus capable d'induire la maturation mégacaryocytaire et active le MAPK de façon transitoire. Nous avons donc cherché à comprendre comment la région Delta3 de Mpl participe au signal MAPK, en tentant d'identifier les voies de signalisation qu'elle active. L'identification de ces voies de signalisation constitue une étape vers la compréhension des mécanismes moléculaires régulant la mégacaryopoïèse. Nous avons montré que le signal MAPK induit par la TPO est le produit de deux vagues d'activation, l'une précoce et transitoire dépendante de Ras/Raf-1 et l'autre, ayant lieu tardivement mais persistant à long terme, impliquant Rap1/B-Raf. C'est pour cette dernière vague de signalisation que la présence de la région Delta3 semble cruciale...Thrombopoietin and its receptor play a major role in survival and proliferation of multipotent hematopoietic progenitors, in the expansion of megakaryocytic lignage and in platelet formation. The study of megakaryocytic cell lines expressing the TPO receptor established in the laboratory (UT7-Mpl) and of normal human megakaryocytes, have shown that : it is the quantitative differences in the intensity and duration of the signal and not the actual activation of ERK MAPK (Mitogen Activated Protein Kinases) that play an essential role in the decision by the cell to differentiate into a given pathway or to proliferate in reponse to a cytokine. The main project of this thesis was to understand 1) how are MAPKs activated by TPO and 2) how do they participate in the differentiation and/or proliferation of megakaryocytic precursors by searching their specific substrates in these cells...PARIS5-BU Saints-Pères (751062109) / SudocSudocFranceF

Bone marrow niches in myeloid and lymphoid malignancies

International audienc

Le substrat de ERK IEX-1 est un inhibiteur général des phosphatases PP2A de la famille B56 impliqué dans la signalisation TPO

PARIS7-Bibliothèque centrale (751132105) / SudocSudocFranceF

The MAPK ERK1 is a negative regulator of the adult steady-state splenic erythropoiesis.

International audienceThe mitogen-activated protein kinases (MAPKs) extracellular signal-regulated kinase 1 (ERK1) and ERK2 are among the main signal transduction molecules, but little is known about their isoform-specific functions in vivo. We have examined the role of ERK1 in adult hematopoiesis with ERK1(-/-) mice. Loss of ERK1 resulted in an enhanced splenic erythropoiesis, characterized by an accumulation of erythroid progenitors in the spleen, without any effect on the other lineages or on bone marrow erythropoiesis. This result suggests that the ablation of ERK1 induces a splenic stress erythropoiesis phenotype. However, the mice display no anemia. Deletion of ERK1 did not affect erythropoietin (EPO) serum levels or EPO/EPO receptor signaling and was not compensated by ERK2. Splenic stress erythropoiesis response has been shown to require bone morphogenetic protein 4 (BMP4)-dependent signaling in vivo and to rely on the expansion of a resident specialized population of erythroid progenitors, termed stress erythroid burst-forming units (BFU-Es). A great expansion of stress BFU-Es and increased levels of BMP4 mRNA were found in ERK1(-/-) spleens. The ERK1(-/-) phenotype can be transferred by bone marrow cells. These findings show that ERK1 controls a BMP4-dependent step, regulating the steady state of splenic erythropoiesis

Thrombopoietin protects hematopoietic stem cells from retrotransposon-mediated damage by promoting an antiviral response.

Maintenance of genomic integrity is crucial for the preservation of hematopoietic stem cell (HSC) potential. Retrotransposons, spreading in the genome through an RNA intermediate, have been associated with loss of self-renewal, aging, and DNA damage. However, their role in HSCs has not been addressed. Here, we show that mouse HSCs express various retroelements (REs), including long interspersed element-1 (L1) recent family members that further increase upon irradiation. Using mice expressing an engineered human L1 retrotransposition reporter cassette and reverse transcription inhibitors, we demonstrate that L1 retransposition occurs in vivo and is involved in irradiation-induced persistent γH2AX foci and HSC loss of function. Thus, RE represents an important intrinsic HSC threat. Furthermore, we show that RE activity is restrained by thrombopoietin, a critical HSC maintenance factor, through its ability to promote a potent interferon-like, antiviral gene response in HSCs. This uncovers a novel mechanism allowing HSCs to minimize irradiation-induced injury and reinforces the links between DNA damage, REs, and antiviral immunity