Jordi Guiu

The Notch/Hes1 Pathway Sustains NF-κB Activation through CYLD Repression in T Cell Leukemia

It was previously shown that the NF-κB pathway is downstream of oncogenic Notch1 in T cell acute lymphoblastic leukemia (T-ALL). Here, we visualize Notch-induced NF-κB activation using both human T-ALL cell lines and animal models. We demonstrate that Hes1, a canonical Notch target and transcriptional repressor, is responsible for sustaining IKK activation in T-ALL. Hes1 exerts its effects by repressing the deubiquitinase CYLD, a negative IKK complex regulator. CYLD expression was found to be significantly suppressed in primary T-ALL. Finally, we demonstrate that IKK inhibition is a promising option for the targeted therapy of T-ALL as specific suppression of IKK expression and function affected both the survival of human T-ALL cells and the maintenance of the disease in vivo.

Impaired embryonic haematopoiesis yet normal arterial development in the absence of the Notch ligand Jagged1.

Specific deletion of Notch1 and RBPjκ in the mouse results in abrogation of definitive haematopoiesis concomitant with the loss of arterial identity at embryonic stage. As prior arterial determination is likely to be required for the generation of embryonic haematopoiesis, it is difficult to establish the specific haematopoietic role of Notch in these mutants. By analysing different Notch‐ligand‐null embryos, we now show that Jagged1 is not required for the establishment of the arterial fate but it is required for the correct execution of the definitive haematopoietic programme, including expression of GATA2 in the dorsal aorta. Moreover, successful haematopoietic rescue of the Jagged1‐null AGM cells was obtained by culturing them with Jagged1‐expressing stromal cells or by lentiviral‐mediated transduction of the GATA2 gene. Taken together, our results indicate that Jagged1‐mediated activation of Notch1 is responsible for regulating GATA2 expression in the AGM, which in turn is essential for definitive haematopoiesis in the mouse.

Hematopoietic stem cell development requires transient Wnt/β-catenin activity

Deletion of β-catenin from mouse embryonic endothelium, but not embryonic hematopoietic cells, prevents hematopoietic differentiation; thus Wnt/β-catenin signaling is needed for emergence but not maintenance of HSCs.

Notch signal strength controls cell fate in the haemogenic endothelium.

Acquisition of the arterial and haemogenic endothelium fates concurrently occur in the aorta–gonad–mesonephros (AGM) region prior to haematopoietic stem cell (HSC) generation. The arterial programme depends on Dll4 and the haemogenic endothelium/HSC on Jag1-mediated Notch1 signalling. How Notch1 distinguishes and executes these different programmes in response to particular ligands is poorly understood. By using two Notch1 activation trap mouse models with different sensitivity, here we show that arterial endothelial cells and HSCs originate from distinct precursors, characterized by different Notch1 signal strengths. Microarray analysis on AGM subpopulations demonstrates that the Jag1 ligand stimulates low Notch strength, inhibits the endothelial programme and is permissive for HSC specification. In the absence of Jag1, endothelial cells experience high Dll4-induced Notch activity and select the endothelial programme, thus precluding HSC formation. Interference with the Dll4 signal by ligand-specific blocking antibodies is sufficient to inhibit the endothelial programme and favour specification of the haematopoietic lineage.

Bmi1 regulates murine intestinal stem cell proliferation and self-renewal downstream of Notch

Genetic data indicate that abrogation of Notch-Rbpj or Wnt-β-catenin pathways results in the loss of the intestinal stem cells (ISCs). However, whether the effect of Notch is direct or due to the aberrant differentiation of the transit-amplifying cells into post-mitotic goblet cells is unknown. To address this issue, we have generated composite tamoxifen-inducible intestine-specific genetic mouse models and analyzed the expression of intestinal differentiation markers. Importantly, we found that activation of β-catenin partially rescues the differentiation phenotype of Rbpj deletion mutants, but not the loss of the ISC compartment. Moreover, we identified Bmi1, which is expressed in the ISC and progenitor compartments, as a gene that is co-regulated by Notch and β-catenin. Loss of Bmi1 resulted in reduced proliferation in the ISC compartment accompanied by p16INK4a and p19ARF (splice variants of Cdkn2a) accumulation, and increased differentiation to the post-mitotic goblet cell lineage that partially mimics Notch loss-of-function defects. Finally, we provide evidence that Bmi1 contributes to ISC self-renewal. Summary: The polycomb complex protein Bmi1 is regulated by Notch and is required to maintain stem cell function in the mouse intestine.

Notch and Wnt signaling in the emergence of hematopoietic stem cells.

Hematopoietic stem cells (HSC), which reside in the marrow of adult mammals and sustain hematopoiesis for the lifetime of the organism, are specified and generated during embryonic development. We are just beginning to understand how HSC develop from more primitive cells and the complexity of the signaling pathways involved. In this work, we review the role of two crucial pathways, Notch and Wnt, in the specification and development of HSC and their nascent microenvironment, the arterial vessels.

Notch1 acts via Foxc2 to promote definitive hematopoiesis via effects on hemogenic endothelium

Hematopoietic and vascular development share many common features, including cell surface markers and sites of origin. Recent lineage-tracing studies have established that definitive hematopoietic stem and progenitor cells arise from vascular endothelial-cadherin(+) hemogenic endothelial cells of the aorta-gonad-mesonephros region, but the genetic programs underlying the specification of hemogenic endothelial cells remain poorly defined. Here, we discovered that Notch induction enhances hematopoietic potential and promotes the specification of hemogenic endothelium in differentiating cultures of mouse embryonic stem cells, and we identified Foxc2 as a highly upregulated transcript in the hemogenic endothelial population. Studies in zebrafish and mouse embryos revealed that Foxc2 and its orthologs are required for the proper development of definitive hematopoiesis and function downstream of Notch signaling in the hemogenic endothelium. These data establish a pathway linking Notch signaling to Foxc2 in hemogenic endothelial cells to promote definitive hematopoiesis.

Corrigendum: Notch signal strength controls cell fate in the haemogenic endothelium

Nature Communications 6: Article number: 8510 10.1038/ncomms9510 (2015); Published: October142015; Updated: March042016 The original version of this Article contained a typographical error in the spelling of the author Zhenyi Liu, which was incorrectly given as Zenhy Liu. This has now been corrected in both the PDF and HTML versions of the Article.