Medecine

The Polycomb group gene Bmi1 is essential for the proliferation of neural and hematopoietic stem cells. Much remains to be learned about the pathways involved in the severe hematopoietic phenotype observed in Bmi1 homozygous mutant mice except for the fact that loss of p53 or concomitant loss of p16 Ink4a and p19 Arf functions achieves only a partial rescue. Here we report the identification of E4F1, an inhibitor of cellular proliferation, as a novel BMI1-interacting partner in hematopoietic cells. We provide evidence that Bmi1 and E4f1 genetically interact in the hematopoietic compartment to regulate cellular proliferation. Most importantly, we demonstrate that reduction of E4f1 levels through RNA interference mediated knockdown is sufficient to rescue the clonogenic and repopulating ability of Bmi1 −/− hematopoietic cells up to 3 mo post-transplantation. Using cell lines and MEF, we also demonstrate that INK4A/ARF and p53 are not essential for functional interaction between Bmi1 and E4f1. Together, these findings identify E4F1 as a key modulator of BMI1 activity in primitive hematopoietic cells. [Keywords: Polycomb; Bmi1; hematopoietic stem cell self-renewal; E4F1; senescence] Supplemental material is available at http://www.genesdev.org. Corresponding author. E-MAIL guy.sauvageau@umontreal.ca; FAX (514) 343-7379. Article is online at http://www.genesdev.org/cgi/

Chromosomal deletions, as a genetic tool for functional genomics, remain underexploited for vertebrate stem cells mostly because presently available methods are too laborintensive. To address this, we developed and validated a set of complementary retroviruses that creates a wide range of nested chromosomal deletions. When applied to mouse embryonic stem cells (ESCs), this retrovirus-based method yielded deletions ranging from 6 kb to 23 Mb (average 2.9 Mb), with an efficiency of 64% for drug-selected clones. Notably, several of the engineered ESC clones, mostly those with large deletions, showed major alteration in cell fate. In comparison to other methods that have also exploited retroviruses for chromosomal engineering, this modified strategy is more efficient and versatile because it bypasses the need for homologous recombination, and thus can be exploited for rapid and extensive functional screens in embryonic and adult stem cells.

Understanding the function of important DNA elements in mammalian stem cell genomes would be enhanced by the availability of deletion collections in which segmental haploidies are precisely characterized. Using a modified Cre-loxPbased system, we now report the creation and characterization of a collection of ,1,300 independent embryonic stem cell (ESC) clones enriched for nested chromosomal deletions. Mapping experiments indicate that this collection spans over 25% of the mouse genome with good representative coverage of protein-coding genes, regulatory RNAs, and other non-coding sequences. This collection of clones was screened for in vitro defects in differentiation of ESC into embryoid bodies (EB). Several putative novel haploinsufficient regions, critical for EB development, were identified. Functional characterization of one of these regions, through BAC complementation, identified the ribosomal gene Rps14 as a novel haploinsufficient determinant of embryoid body formation. This new library of chromosomal deletions in ESC (DelES: http://bioinfo.iric.ca/ deles) will serve as a unique resource for elucidation of novel protein-coding and non-coding regulators of ESC activity.

It is believed that hemopoietic stem cells (HSC), which colonize the fetal liver (FL) rapidly, expand to establish a supply of HSCs adequate for maintenance of hemopoiesis throughout life. Accordingly, FL HSCs are actively cycling as opposed to their predominantly quiescent bone marrow counterparts, suggesting that the FL microenvironment provides unique signals that support HSC proliferation and self-renewal. We now report the genera-tion and characterization of mice with a mutant allele of Baf250a lacking exons 2 and 3. Baf250a E2E3/E2E3 mice are viable until E19.5, but do not survive beyond birth. Most interestingly, FL HSC numbers are markedly higher in these mice than in control littermates, thus raising the possibility that Baf250a determines the HSC pool size in vivo. Limit dilution experiments indicate that the activity of Baf250a E2E3/E2E3 HSC is equivalent to that of the wild-type counterparts. The Baf250a E2E3/E2E3 FL-derived stroma, in contrast, exhibits a hemopoiesis-supporting potential superior to the developmentally matched controls. To our knowledge, this demonstration is the first that a mechanism operating in a cell nonautonomous manner canexpand the pool size of the fetal HSC populations. (Blood. 2010;116(10): 1678-1684)

BMI1 is a key component of multiprotein Polycomb repression complex 1 (PRC1), and its disruption in mice induces severe aplastic anemia by early adulthood. The contributing mechanisms responsible for this phenotype remain elusive. Here we show that transformed human cell lines as well as primitive hematopoietic cells exhibit a high frequency of spontaneous chromosome breaks upon BMI1 depletion and are hypersensitive to genotoxic agents. Consistent with these observations, we found that BMI1 is recruited rapidly to DNA damage foci where it blocks transcriptional elongation. We also show that BMI1 contributes to homologous recombination DNA repair and is required for checkpoint recovery. Taken together, our results suggest that BMI1 is critical for the maintenance of chromosome integrity in both normal and transformed cells.

The ubiquitin-associated protein 2-like (UBAP2L) gene remains poorly studied in human and mouse development. UBAP2L interacts with the Polycomb group protein B lymphoma Mo-MLV insertion region 1 homolog (BMI1) and determines the activity of mouse hematopoietic stem cells in vivo Here we show that loss of Ubap2l leads to disorganized respiratory epithelium of mutant neonates, which die of respiratory failure. We also show that UBAP2L overexpression leads to epithelial-mesenchymal transition-like phenotype in a non-small cell lung carcinoma (NSCLC) cell line. UBAP2L is amplified in 15% of human primary lung adenocarcinoma specimens. Such patients express higher levels of UBAP2L and show a reduction in survival when compared with those who do not have this gene amplification. Supporting a possible role for UBAP2L in lung tumor progression, NSCLC cells engineered to express low levels of this gene produce much smaller tumors in vivo than wild-type control cells. Together, these results ...

Elucidation of the molecular cues required to balance adult stem cell self-renewal and differentiation is critical for advancing cellular therapies. Herein, we report that the hematopoietic stem cell (HSC) self-renewal agonist UM171 triggers a balanced pro-and anti-inflammatory/detoxification network that relies on NFKB activation and protein C receptor-dependent ROS detoxification, respectively. We demonstrate that within this network, EPCR serves as a critical protective component as its deletion hypersensitizes primitive hematopoietic cells to pro-inflammatory signals and ROS accumulation resulting in compromised stem cell function. Conversely, abrogation of the pro-inflammatory activity of UM171 through treatment with dexamethasone, cAMP elevating agents or NFkB inhibitors abolishes EPCR upregulation and HSC expansion. Together, these results show that UM171 stimulates ex vivo HSC expansion by establishing a critical balance between key pro-and anti-inflammatory mediators of self-renewal.

Afin d'eclaicir les proprietes fonctionnelles du recepteur Ly49C des cellules NK murines, nous avons evalue, dans un premier temps, la concentration des ARNm specifiques au TNF-α et a l'IFN-y apres stimulation cellulaire avec l' AcM anti-Ly49C 4L03311. Ainsi, nous avons observe par RT-PCR quantitatif et par buvardage de type Northern que la stimulation des cellules LAK de souris BALB/c par l' AcM 4L03311 induisait une nette augmentation de la concentration de ces ARNm et ce, dans de nombreuses experiences. Cependant, des experiences plus poussees utilisant des cellules triees sur la base de leur expression du recepteur Ly49C ont revele que l'activation cellulaire ne semblait pas etre induite de facon specifique par la stimulation avec l' AcM 4L03311. Nous en avons conclu que l'augmentation des ARNm specifiques observee dans les cellules stimulees avec cet AcM etait due a la presence d'un element activateur inconnu dans les preparations d'AcM. Para...

Inhibition of oxidative phosphorylation (OXPHOS) is a promising therapeutic strategy in Acute Myeloid Leukemia (AML), but patients respond heterogeneously. Through chemically interrogation of 200 sequenced specimens, we identified Mubritinib as a strong in vitro and in vivo anti-leukemic compound, acting through ubiquinone-dependent inhibition of Electron Transport Chain complex I (ETC1). ETC1 targeting showed selective toxicity against a subgroup of chemotherapy-resistant leukemias exhibiting OXPHOS hyperactivity, high expression of mitochondrial activity-related genes, and mutations affecting NPM1, FLT3 and DNMT3A. Altogether, our work thus identifies a novel ETC1 inhibitor with high clinical potential and reveals the landscape of OXPHOS dependency in AML.

Multipotent long-term repopulating hematopoietic stem cells (LT-HSCs) can self-renew or differentiate into the less primitive short-term repopulating stem cells (ST-HSCs), which themselves produce progenitors that ensure the daily supply of all essential blood components. The Polycomb group (PcG) protein BMI1 is essential for the activity of both HSCs and progenitor cells. Although BMI1 operates by suppressing the Ink4a/Arf locus in progenitors and ST-HSCs, the mechanisms through which this gene regulates the activity of LT-HSCs remain poorly understood. Toward this goal, we isolated BMI1-containing protein complexes and identified UBAP2L as a novel BMI1-interacting protein. We also showed that UBAP2L is preferentially expressed in mouse and human HSC-enriched populations when compared with more mature cell types, and that this gene is essential for the activity of LT-HSCs. In contrast to what is observed for Bmi1 knockdown, we found that UBAP2L depletion does not affect the Ink4a/A...

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Intimal hyperplasia within the body of the stent is the primary mechanism for in-stent restenosis; however, stent edge restenosis has been described after brachytherapy. Our current understanding about the magnitude of in vivo intimal hyperplasia and edge restenosis is limited to data obtained primarily from select, symptomatic patients requiring repeat angiography. The purpose of this study was to determine the extent and distribution of intimal hyperplasia both within the stent and along the stent edge in relatively nonselect, asymptomatic patients scheduled for 6-month intravascular ultrasound (IVUS) as part of a multicenter trial: Heparin Infusion Prior to Stenting. Planar IVUS measurements 1 mm apart were obtained throughout the stent and over a length of 10 mm proximal and distal to the stent at index and follow-up. Of the 179 patients enrolled, 140 returned for repeat angiography and IVUS at 6.4 ؎ 1.9 months and had IVUS images adequate for analysis.

We investigated whether, in primary prevention patients with metabolic syndrome, statins affect the platelet protease-activated receptor-1 (PAR-1) thrombin receptor by performing serial measurements of its activity and the antigen expression level by flow cytometry before and during treatment. Recent data from randomized trials of statins are compatible with the possibility of clinically relevant pleiotropic effects. The use of statins is associated with a reduced thrombosis burden and diminished platelet activity, as shown in animal models and in vitro studies. Seventy patients with the metabolic syndrome who were not taking antiplatelet agents were assigned consecutively at starting doses at the discretion of the responsible clinician to 1 of 6 statins (atorvastatin, fluvastatin, lovastatin, pravastatin, rosuvastatin, or simvastatin) or to a no-statin group for 6 weeks. Platelet expression of intact (SPAN12 antibody) and cleaved (WEDE15) PAR-1 thrombin receptors were assessed by flow cytometry at baseline and at weeks 4 and 6 of treatment. At baseline, no difference was found in receptor expression. However, after 4 weeks of treatment, all statins had significantly inhibited (46% to 55%) the activated epitope of PAR-1 expression. After 6 weeks, inhibition remained, despite a slight rebound (22% to 37%). Also, a delayed pattern of inhibition of the intact PAR-1 receptor epitope was found. In conclusion, all statins inhibited the activity and antigen level of the platelet PAR-1 thrombin receptor, which has a major role in regulating platelet activity and thrombin formation. These observational data offer a plausible mechanism for the recently demonstrated pleiotropic effects of statins that may contribute to early clinical benefit.

Objective: Although the application of cold energy, cryotherapy, has been shown to cause selective damage to cellular components with preservation of matrix structure resulting in less fibrosis in a variety of tissues, the effects of intravascular cryotherapy on vessel wall repair after balloon angioplasty are unknown. We sought to characterize the effects of cryotherapy application on vessel wall repair after balloon angioplasty and study the relationship between collagen accumulation in the vessel wall and late lumen loss as assessed by serial intravascular ultrasound. Methods: The immediate, early (72 h) and late (10 weeks) effects of three intravascular cryotherapy application time periods (60, 120 and 240 s) after iliac artery balloon angioplasty ('cryotherapy') were compared with balloon angioplasty alone ('control') in 59 rabbits. Arterial lumen area was measured by intravascular ultrasound immediately after the procedure, at 72 h and at 10 weeks. Collagen content was calculated separately for intima and media / adventitia layers and correlated with late lumen loss. Results: Cryotherapy produced average vessel wall temperature of 226 8C (range, 220 to 245 8C) and resulted in significantly larger lumen 2 cross-sectional area (CSA) immediately after application (5.7461.18 vs. 4.1460.75 mm , P50.008) but was not different than control arteries at 10 weeks. At 72 h, there was extensive cell loss in the medial and adventitial layers accompanied by increased macrophage infiltration in cryotherapy treated arteries compared to control. At 10 weeks, intimal hyperplasia was increased 2-fold in cryotherapy treated arteries. Collagen content was increased 2-fold in the medial / adventitial layers, and nearly 3-fold in the intima of cryotherapy treated arteries. Collagen content in arterial intima (P50.01) as well as media / adventitia (P50.005) positively correlated with late lumen loss. Foci of chondro-and osseous metaplasia and calcification were evident at the medial-adventitial junction in cryotherapy treated arteries at 10 weeks. Conclusion: Intravascular cryotherapy induced early arterial wall cell loss and late intimal hyperplasia, vascular fibrosis and chondro-and osseous metaplastic changes with no late beneficial effects on lumen area compared to balloon angioplasty alone. Collagen accumulation in all three layers of the vessel wall contributes to the development of late inward remodeling after balloon angioplasty.

Background-Radiofrequency (RF) catheter ablation is limited by thromboembolic complications. The objective of this study was to compare the incidence and characteristics of thrombi complicating RF and cryoenergy ablation, a novel technology for the catheter-based treatment of arrhythmias. Methods and Results-Ablation lesions (nϭ197) were performed in 22 mongrel dogs at right atrial, right ventricular, and left ventricular sites preselected by a randomized factorial design devised to compare RF ablation with cryocatheter configurations of varying sizes (7F and 9F), cooling rates (Ϫ1°C/s, Ϫ5°C/s, and Ϫ20°C/s) and target temperatures (Ϫ55°C and Ϫ75°C). Animals were pretreated with acetylsalicylic acid and received intraprocedural intravenous unfractionated heparin. Seven days after ablation, the incidence of thrombus formation was significantly higher with RF than with cryoablation (75.8% versus 30.1%, Pϭ0.0005). In a multiple regression model, RF energy remained an independent predictor of thrombus formation compared with cryoenergy (OR, 5.6; 95% CI, 1.7, 18.1; Pϭ0.0042). Thrombus volume was also significantly greater with RF than with cryoablation (median, 2.8 versus 0.0 mm 3 ; PϽ0.0001). More voluminous thrombi were associated with larger RF lesions, but cryolesion dimensions were not predictive of thrombus size. Conclusions-RF energy is significantly more thrombogenic than cryoenergy, with a higher incidence of thrombus formation and larger thrombus volumes. The extent of hyperthermic tissue injury is positively correlated with thrombus bulk, whereas cryoenergy lesion size does not predict thrombus volume, most likely reflecting intact tissue ultrastructure with endothelial cell preservation.