Identification of a murine CD45-F4/80lo HSC-derived marrow endosteal cell associated with donor stem cell engraftment

Hematopoietic stem cells (HSCs) reside in specialized microenvironments within the marrow designated as stem cell niches, which function to support HSCs at homeostasis and promote HSC engraftment after radioablation. We previously identified marrow space remodeling after hematopoietic ablation, including osteoblast thickening, osteoblast proliferation, and megakaryocyte migration to the endosteum, which is critical for effective engraftment of donor HSCs. To further evaluate the impact of hematopoietic cells on marrow remodeling, we used a transgenic mouse model (CD45Cre/iDTR) to selectively deplete hematopoietic cells in situ. Depletion of hematopoietic cells immediately before radioablation and hematopoietic stem cell transplantation abrogated donor HSC engraftment and was associated with strikingly flattened endosteal osteoblasts with preserved osteoblast proliferation and megakaryocyte migration. Depletion of monocytes, macrophages, or megakaryocytes (the predominant hematopoietic cell populations that survive short-term after irradiation) did not lead to an alteration of osteoblast morphology, suggesting that a hematopoietic-derived cell outside these lineages regulates osteoblast morphologic adaptation after irradiation. Using 2 lineage-tracing strategies, we identified a novel CD45-F4/80lo HSC-derived cell that resides among osteoblasts along the endosteal marrow surface and, at least transiently, survives radioablation. This newly identified marrow cell may be an important regulator of HSC engraftment, possibly by influencing the shape and function of endosteal osteoblasts

Evaluation of transcriptional, phenotypic and functional modifications of mesenchymal stem cells in de novo acute myeloid leukemia

La contribution des Cellules Souches/Stromales Mésenchymateuses (CSM) dans le développement des Leucémies Aiguës Myéloblastiques (LAM) n’est pas encore clairement établie. L'objectif de ce travail a été de rechercher de potentielles modifications phénotypiques et fonctionnelles au sein des CSM médullaires de patients atteints de LAM de novo au diagnostic. Nous montrons que ces cellules présentent un défaut prolifératif accompagné d’une augmentation de l’apoptose et d’un déficit d’expression de certains facteurs de la niche (Ang-1, SCF, TPO et VCAM-1). De façon intéressante, ce défaut prolifératif est indépendamment associé à une évolution péjorative de la maladie. Néanmoins, ces anomalies des CSM de LAM ne semblent pas affecter leur capacité de soutien de l’hématopoïèse physiologique ou leucémique in vitro. En effet, comme les CSM normales, elles protègent les cellules leucémiques de l’apoptose, induisent leur quiescence (principalement par contact direct) et ainsi diminuent la proportion des cassures double-brin d’ADN. Ces données suggèrent que les modifications des CSM de LAM, probablement une des conséquences délétères de la prolifération tumorale, n'auraient pas un rôle spécifique dans le développement du processus leucémique.The contribution of Mesenchymal Stem/Stromal Cells (MSCs) to the development of Acute Myeloid Leukemias (AMLs) remains poorly understood. In the present study, we investigated potential functional and phenotypic modifications of Bone Marrow (BM)-derived MSCs from patients with AML de novo at diagnosis. We showed that BM-derived MSCs from most of AML patients display proliferative defect, had increased apoptosis levels and demonstrated defective expression of several niche-related factors (Ang-1, SCF, TPO and VCAM-1). Interestingly, this proliferative defect was independently associated with disease progression. Nevertheless, these abnormalities in AML MSCs did not affect their in vitro capacity to support physiological but also leukemic hematopoiesis. Indeed, as normal MSCs do, they protect blast cells from apoptosis, induce their quiescence (mainly by direct contact), and decreased yields of DNA double-strand breaks. Consequently, in AML de novo these stromal cell alterations, probably a consequence of the deleterious effect of the tumor cell growth on BM MSCs, do not appear to have a specific role in the development of the leukemic process

Menin controls the concentration of retinoblastoma protein.

International audienceMenin, the protein encoded by the Multiple Endocrine Neoplasia type 1 gene, is involved in the cell cycle control through its participation in functional dynamics of chromatin and regulation of transcription. RB, the protein of the retinoblastoma gene RB1, controls the progression of the cell cycle and is regulated in its activity by means of a feedback by phosphorylation. Studies in double heterozygous knockout mice for Men1 and the Retinoblastoma gene Rb1 have recently indicated that both genes may be implicated in the same pathways. In the course of our studies on Menin, we found that after suppression or in absence of Menin, RB1 expression was strongly reduced in a posttranscriptional manner. Under conditions of growth arrest, the hyperphosphorylated form of RB was most strongly affected, whereas its hypophosphorylated form was less or not at all reduced. Our findings confirm the hypothesis that the pathways of two tumor suppressor genes are connected

Extracellular vesicles released from mesenchymal stromal cells stimulate bone growth in osteogenesis imperfecta

Background Systemic infusion of mesenchymal stromal cells (MSCs) has been shown to induce acute acceleration of growth velocity in children with osteogenesis imperfecta (OI) despite minimal engraftment of infused MSCs in bones. Using an animal model of OI we have previously shown that MSC infusion stimulates chondrocyte proliferation in the growth plate and that this enhanced proliferation is also observed with infusion of MSC conditioned medium in lieu of MSCs, suggesting that bone growth is due to trophic effects of MSCs. Here we sought to identify the trophic factor secreted by MSCs that mediates this therapeutic activity. Methods To examine whether extracellular vesicles (EVs) released from MSCs have therapeutic activity, EVs were isolated from MSC conditioned medium by ultracentrifugation. To further characterize the trophic factor, RNA or microRNA (miRNA) within EVs was depleted by either ribonuclease (RNase) treatment or suppressing miRNA biogenesis in MSCs. The functional activity of these modified EVs was evaluated using an in vitro chondrocyte proliferation assay. Finally, bone growth was evaluated in an animal model of OI treated with EVs. Results We found that infusion of MSC-derived EVs stimulated chondrocyte proliferation in the growth plate, resulting in improved bone growth in a mouse model of OI. However, infusion of neither RNase-treated EVs nor miRNA-depleted EVs enhanced chondrocyte proliferation. Conclusion MSCs exert therapeutic effects in OI by secreting EVs containing miRNA, and EV therapy has the potential to become a novel cell-free therapy for OI that will overcome some of the current limitations in MSC therapy

Oncogenic STAT5 signaling promotes oxidative stress in chronic myeloid leukemia cells by repressing antioxidant defenses

International audienceSTAT5 transcription factors are frequently activated in hematopoietic neoplasms and are targets of various tyrosine kinase oncogenes. Evidences for a crosstalk between STAT5 and reactive oxygen species (ROS) metabolism have recently emerged but mechanisms involved in STAT5-mediated regulation of ROS still remain elusive. We demonstrate that sustained activation of STAT5 induced by Bcr-Abl in chronic myeloid leukemia (CML) cells promotes ROS production by repressing expression of two antioxidant enzymes, catalase and glutaredoxin-1(Glrx1). Downregulation of catalase and Glrx1 expression was also observed in primary cells from CML patients. Catalase was shown not only to reduce ROS levels but also, to induce quiescence in Bcr-Abl-positive leukemia cells. Furthermore, reduction of STAT5 phosphorylation and upregulation of catalase and Glrx1 were also evidenced in leukemia cells co-cultured with bone marrow stromal cells to mimic a leukemic niche. This caused downregulation of ROS levels and enhancement of leukemic cell quiescence. These data support a role of persistent STAT5 signaling in the regulation of ROS production in myeloid leukemias and highlight the repression of antioxidant defenses as an important regulatory mechanism

Alteration Analysis of Bone Marrow Mesenchymal Stromal Cells from De Novo Acute Myeloid Leukemia Patients at Diagnosis

International audienceBone marrow (BM)-derived mesenchymal stromal cells (MSCs) frequently display alterations in several hematologic disorders, such as acute lymphoid leukemia, acute myeloid leukemia (AML), and myelodysplastic syndromes. In acute leukemias, it is not clear whether MSC alterations contribute to the development of the malignant clone or whether they are simply the effect of tumor expansion on the microenvironment. We extensively investigated the characteristics of MSCs isolated from the BM of patients with de novo AML at diagnosis (L-MSCs) in terms of phenotype (gene and protein expression, apoptosis and senescence levels, DNA double-strand break formation) and functions (proliferation and clonogenic potentials, normal and leukemic hematopoiesis-supporting activity). We found that L-MSCs show reduced proliferation capacity and increased apoptosis levels compared with MSCs from healthy controls. Longer population doubling time in L-MSCs was not related to the AML characteristics at diagnosis (French-American-British type, cytogenetics, or tumor burden), but was related to patient age and independently associated with poorer patient outcome, as was cytogenetic prognostic feature. Analyzing, among others, the expression of 93 genes, we found that proliferative deficiency of L-MSCs was associated with a perivascular feature at the expense of the osteo-chondroblastic lineage with lower expression of several niche factors, such as KITLG, THPO, and ANGPT1 genes, the cell adhesion molecule VCAM1, and the developmental/embryonic genes, BMI1 and DICER1. L-MSC proliferative capacity was correlated positively with CXCL12, THPO, and ANGPT1 expression and negatively with JAG1 expression. Anyway, these changes did not affect their in vitro capacity to support normal hematopoiesis and to modify leukemic cell behavior (protection from apoptosis and quiescence induction). Our findings indicate that BM-derived MSCs from patients with newly diagnosed AML display phenotypic and functional alterations such as proliferative deficiency that could be attributed to tumor progression, but does not seem to play a special role in the leukemic process