Distinct effects of the soluble versus membrane-bound forms of the notch ligand delta-4 on human CD34+CD38low cell expansion and differentiation

Multifactorial Contribution of Notch Signaling in Head and Neck Squamous Cell Carcinoma

Head and neck squamous cell carcinoma (HNSCC) defines a group of solid tumors originating from the mucosa of the upper aerodigestive tract, pharynx, larynx, mouth, and nasal cavity. It has a metastatic evolution and poor prognosis and is the sixth most common cancer in the world, with 600,000 new cases reported every year. HNSCC heterogeneity and complexity is reflected in a multistep progression, involving crosstalk between several molecular pathways. The Notch pathway is associated with major events supporting cancerogenic evolution: cell proliferation, self-renewal, angiogenesis, and preservation of a pro-oncogenic microenvironment. Additionally, Notch is pivotal in tumor development and plays a dual role acting as both oncogene and tumor suppressor. In this review, we summarize the role of the Notch pathway in HNSCC, with a special focus on its compelling role in major events of tumor initiation and growth.

PUMILIO/FOXP1 signaling drives expansion of hematopoietic stem/progenitor and leukemia cells

RNA-binding proteins (RBPs) have emerged as important regulators of invertebrate adult stem cells, but their activities remain poorly appreciated in mammals. Using a short hairpin RNA strategy, we demonstrate here that the 2 mammalian RBPs, PUMILIO (PUM)1 and PUM2, members of the PUF family of posttranscriptional regulators, are essential for hematopoietic stem/progenitor cell (HSPC) proliferation and survival in vitro and in vivo upon reconstitution assays. Moreover, we found that PUM1/2 sustain myeloid leukemic cell growth. Through a proteomic approach, we identified the FOXP1 transcription factor as a new target of PUM1/2. Contrary to its canonical repressive activity, PUM1/2 rather promote FOXP1 expression by a direct binding to 2 canonical PUM responsive elements present in the FOXP1-3' untranslated region (UTR). Expression of FOXP1 strongly correlates with PUM1 and PUM2 levels in primary HSPCs and myeloid leukemia cells. We demonstrate that FOXP1 by itself supports HSPC and leukemic cell growth, thus mimicking PUM activities. Mechanistically, FOXP1 represses the expression of the p21^-CIP1 and p27^-KIP1 cell cycle inhibitors. Enforced FOXP1 expression reverses shPUM antiproliferative and proapoptotic activities. Altogether, our results reveal a novel regulatory pathway, underscoring a previously unknown and interconnected key role of PUM1/2 and FOXP1 in regulating normal HSPC and leukemic cell growth.

LKB1 and Notch Pathways Interact and Control Biliary Morphogenesis

BACKGROUND

LKB1 is an evolutionary conserved kinase implicated in a wide range of cellular functions including inhibition of cell proliferation, regulation of cell polarity and metabolism. When Lkb1 is inactivated in the liver, glucose homeostasis is perturbed, cellular polarity is affected and cholestasis develops. Cholestasis occurs as a result from deficient bile duct development, yet how LKB1 impacts on biliary morphogenesis is unknown.

METHODOLOGY/PRINCIPAL FINDINGS

We characterized the phenotype of mice in which deletion of the Lkb1 gene has been specifically targeted to the hepatoblasts. Our results confirmed that lack of LKB1 in the liver results in bile duct paucity leading to cholestasis. Immunostaining analysis at a prenatal stage showed that LKB1 is not required for differentiation of hepatoblasts to cholangiocyte precursors but promotes maturation of the primitive ductal structures to mature bile ducts. This phenotype is similar to that obtained upon inactivation of Notch signaling in the liver. We tested the hypothesis of a functional overlap between the LKB1 and Notch pathways by gene expression profiling of livers deficient in Lkb1 or in the Notch mediator RbpJκ and identified a mutual cross-talk between LKB1 and Notch signaling. In vitro experiments confirmed that Notch activity was deficient upon LKB1 loss.

CONCLUSION

LKB1 and Notch share a common genetic program in the liver, and regulate bile duct morphogenesis.

The Notch Delta-4 ligand helps to maintain the quiescence and the short-term reconstitutive potential of haematopoietic progenitor cells through activation of a key gene network

Understanding the role of Notch and its ligands within the different bone marrow niches could shed light on the mechanisms regulating haematopoietic progenitor cells (HPCs) maintenance and self-renewal. Here, we report that murine bone marrow HPCs activation by the vascular Notch Delta-4 ligand maintains a significant proportion of cells specifically in the G0 state. Furthermore, Delta-4/Notch pathway limits significantly the loss of the in vivo short-term reconstitutive potential upon transplantation of Delta-4 activated HPCs into lethally irradiated recipient mice. Both effects are directly correlated with the decrease of cell cycle genes transcription such as CYCLIN-D1, -D2, and -D3, and the upregulation of stemness related genes transcription such as BMI1, GATA2, HOXB4 and C-MYC. In addition, the transcriptional screening also highlights new downstream post-transcriptional factors, named PUMILIO1 and -2, as part of the stem signature associated with the Delta-4/Notch signalling pathway.

Endothelium-targeted Delta-like 1 promotes hematopoietic stem cell expansion ex vivo and engraftment in hematopoietic tissues in vivo

BACKGROUND

Notch ligands enhance ex vivo expansion of hematopoietic stem cells (HSCs). But to use Notch ligands in HSC therapies of human diseases, efforts are required to improve ex vivo expansion efficiency and in vivo transplant engraftment.

DESIGN AND METHODS

We designed and produced an endothelium-targeted soluble Notch ligand, the DSL domain of Delta-like 1 fused with a RGD motif (D1R), and examined the effects of this protein on HSCs ex vivo and in vivo.

RESULTS

D1R efficiently promoted ex vivo expansion of both mouse bone marrow (BM) and human umbilical cord blood HSCs. HSCs expanded with D1R up-regulated many of the stemness-related genes, and showed high BM engraftment efficacy with long-term repopulation capacity after transplantation. Moreover, in vivo administration of D1R increased the number of BM HSCs in mice, and facilitated BM recovery of mice after irradiation. Injection of D1R significantly improved HSC engraftment and myeloid recovery after BM transplantation in irradiated mice. D1R enhanced HSC engraftment not only in BM, but also in the liver and spleen after BM transplantation in mice. D1R induced the formation of compact cell clusters containing the transplanted HSCs in close contact with endothelial cells, reminiscent of HSC niches, in the liver and spleen.

CONCLUSIONS

D1R might be applied in improving both HSC expansion ex vivo and HSC engraftment in vivo in transplantation.

Context- and cell-dependent effects of Delta-like 4 targeting in the bone marrow microenvironment

Delta-like 4 (Dll4) is a ligand of the Notch pathway family which has been widely studied in the context of tumor angiogenesis, its blockade shown to result in non-productive angiogenesis and halted tumor growth. As Dll4 inhibitors enter the clinic, there is an emerging need to understand their side effects, namely the systemic consequences of Dll4:Notch blockade in tissues other than tumors. The present study focused on the effects of systemic anti-Dll4 targeting in the bone marrow (BM) microenvironment. Here we show that Dll4 blockade with monoclonal antibodies perturbs the BM vascular niche of sub-lethally irradiated mice, resulting in increased CD31⁺, VE-Cadherin⁺ and c-kit⁺ vessel density, and also increased megakaryocytes, whereas CD105⁺, VEGFR3⁺, SMA⁺ and lectin⁺ vessel density remained unaltered. We investigated also the expression of angiocrine genes upon Dll4 treatment in vivo, and demonstrate that IGFbp2, IGFbp3, Angpt2, Dll4, DHH and VEGF-A are upregulated, while FGF1 and CSF2 are reduced. In vitro treatment of endothelial cells with anti-Dll4 reduced Akt phosphorylation while maintaining similar levels of Erk 1/2 phosphorylation. Besides its effects in the BM vascular niche, anti-Dll4 treatment perturbed hematopoiesis, as evidenced by increased myeloid (CD11b⁺), decreased B (B220⁺) and T (CD3⁺) lymphoid BM content of treated mice, with a corresponding increase in myeloid circulating cells. Moreover, anti-Dll4 treatment also increased the number of CFU-M and -G colonies in methylcellulose assays, independently of Notch1. Finally, anti-Dll4 treatment of donor BM improved the hematopoietic recovery of lethally irradiated recipients in a transplant setting. Together, our data reveals the hematopoietic (BM) effects of systemic anti-Dll4 treatment result from qualitative vascular changes and also direct hematopoietic cell modulation, which may be favorable in a transplant setting.

Polymeric biomaterials for stem cell bioengineering

This review covers the application of polymeric materials in stem cell bioengineering. Main emphasis is directed towards current material design concepts that mimic distinct exogenous signals of the stem cell microenvironment. Progress within the field of stem cell-specific biomaterials will be discussed, focusing on pluripotent, hematopoietic, mesenchymal and neural stem cells. The future role of biomaterials will be outlined with possible applications for cell reprogramming and engineering cancer cell microenvironments.

Notch signaling and neural connectivity

The cell surface receptor Notch contributes to the development of nearly every tissue in most metazoans by controlling the fates and differentiation of cells. Recent results have now established that Notch also regulates the connectivity of the nervous system, and does so at a variety of levels, including specification of neuronal identity, division, survival and migration, as well as axon guidance, morphogenesis of dendritic arbors and weighting of synapse strength. To these ends, Notch engages at least two signal transduction pathways, one that controls nuclear gene expression and another that directly targets the cytoskeleton. Coordinating the many functions of Notch to produce neural structure is thus a pivotal aspect of building and maintaining the nervous system.

The cell giveth and the cell taketh away: an overview of Notch pathway activation by endocytic trafficking of ligands and receptors

Notch signaling is firmly established as a form of cell-to-cell communication that is critical throughout development. Dysregulation of Notch has been linked to cancer and developmental disorders, making it an important target for therapeutic intervention. One aspect of this pathway that sets it apart from others is its apparent reliance on endocytosis by signal-sending and signal-receiving cells. The subtle details of endocytosis-mediated molecular processing within both ligand- and receptor-presenting cells that are required for the Notch signal to maintain fidelity remain unclear. The endosomal system has long been known to play an important role in terminating signal transduction by directing lysosomal trafficking and degradation of cell surface receptors. More recently, endocytic trafficking has also been shown to be critical for activation of signaling. This review highlights four models of endocytic processing in the context of the Notch pathway. In ligand-presenting cells, endocytosis may be required for pre-processing of ligands to make them competent for interaction with Notch receptors and/or for exerting a pulling force on the ligand/Notch complex. In receptor-presenting cells, endocytosis may be a prerequisite for Notch cleavage and thus activation and/or it could be a means of limiting ligand-independent Notch activation. Recent advances in our understanding of how and why endocytosis of Notch receptors and ligands is required for activation and termination of signaling during normal development and in disease states are discussed.

Non-canonical activation of Notch signaling/target genes in vertebrates

Evolutionarily conserved Notch signaling orchestrates diverse physiological mechanisms during metazoan development and homeostasis. Classically, ligand-activated Notch receptors transduce the signaling cascade through the interaction of DNA-bound CBF1-co-repressor complex. However, recent reports have demonstrated execution of a CBF1-independent Notch pathway through signaling cross-talks in various cells/tissues. Here, we have tried to congregate the reports that describe the non-canonical/CBF1-independent Notch signaling and target gene activation in vertebrates with specific emphasis on their functional relevance.

Effect of Notch activation on the regenerative response to acute renal failure

Episodes of acute renal failure (ARF) are not always fully reversible and may lead to chronic disease, due in part to an inadequate regenerative response. The Notch signaling pathway is involved in determining cell fate during development, and tissue maintenance and repair in adult organs. The purpose of this study was to examine the role of the Notch pathway in renal regeneration following ARF. Kidney injury, induced by ischemia-reperfusion, resulted in early activation of the Notch pathway, as evidenced by increased expression of Notch1 and Notch2 intracellular domain (cleaved Notch). The effect of exogenous administration of the Notch ligand Delta-like-4 (DLL4) on recovery from ARF was then studied. Rats were pretreated by intraperitoneal injection of DLL4 or vehicle control. Two days following the last DLL4 dose, ARF was induced by bilateral renal artery clamping for 45 min followed by reperfusion. The severity of renal injury was similar in DLL4 and control rats. Renal recovery was facilitated by DLL4 treatment, as evidenced by faster return of serum creatinine to baseline by 48 h in DLL4-treated rats as against 5 days in vehicle-treated control rats. Cell proliferation was higher in the DLL4-treated group. In conclusion, activation of the Notch pathway occurs following ARF. Pretreatment with the Notch ligand DLL4 enhanced recovery from ARF and represents a potential novel therapeutic option for regenerating the injured kidney.

The OTT-MAL fusion oncogene activates RBPJ-mediated transcription and induces acute megakaryoblastic leukemia in a knockin mouse model

Acute megakaryoblastic leukemia (AMKL) is a form of acute myeloid leukemia (AML) associated with a poor prognosis. The genetics and pathophysiology of AMKL are not well understood. We generated a knockin mouse model of the one twenty-two-megakaryocytic acute leukemia (OTT-MAL) fusion oncogene that results from the t(1;22)(p13;q13) translocation specifically associated with a subtype of pediatric AMKL. We report here that OTT-MAL expression deregulated transcriptional activity of the canonical Notch signaling pathway transcription factor recombination signal binding protein for immunoglobulin kappa J region (RBPJ) and caused abnormal fetal megakaryopoiesis. Furthermore, cooperation between OTT-MAL and an activating mutation of the thrombopoietin receptor myeloproliferative leukemia virus oncogene (MPL) efficiently induced a short-latency AMKL that recapitulated all the features of human AMKL, including megakaryoblast hyperproliferation and maturation block, thrombocytopenia, organomegaly, and extensive fibrosis. Our results establish that concomitant activation of RBPJ (Notch signaling) and MPL (cytokine signaling) transforms cells of the megakaryocytic lineage and suggest that specific targeting of these pathways could be of therapeutic value for human AMKL.