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Hematopoietic stem cells (HSCs) are the most primitive cells in the hematopoietic system and are under tight regulation for self-renewal and differentiation. Notch signals are essential for the emergence of definitive hematopoiesis in mouse embryos and are critical regulators of lymphoid lineage fate determination. However, it remains unclear how Notch regulates the balance between HSC self-renewal and differentiation in the adult bone marrow (BM). Here we report a novel mechanism that prevents HSCs from undergoing premature lymphoid differentiation in BM. Using a series of in vivo mouse models and functional HSC assays, we show that leukemia/lymphoma related factor (LRF) is necessary for HSC maintenance by functioning as an erythroid-specific repressor of Delta-like 4 (Dll4) expression. Lrf deletion in erythroblasts promoted up-regulation of Dll4 in erythroblasts, sensitizing HSCs to T-cell instructive signals in the BM. Our study reveals novel cross-talk between HSCs and erythroblasts, and sheds a new light on the regulatory mechanisms regulating the balance between HSC self-renewal and differentiation. (Blood. 2013;121(6):918-929)

Coordinated function of the innate and adaptive arms of the immune system in vertebrates is essential to promote protective immunity and to avoid immunopathology. The Notch signalling pathway, which was originally identified as a pleiotropic mediator of cell fate in invertebrates, has recently emerged as an important regulator of immune cell development and function. Notch was initially shown to be a key determinant of cell-lineage commitment in developing lymphocytes, but it is now known to control the homeostasis of several innate cell populations. Moreover, the roles of Notch in adaptive immunity have expanded to include the regulation of T cell differentiation and function. The aim of this Review is to summarize the current status of immune regulation by Notch. A better understanding of Notch function in both innate and adaptive immunity will hopefully provide multiple avenues for therapeutic intervention in disease.

Background Congenital heart disease is the most common type of birth defect, affecting ≈2% of the population. Malformations involving the cardiac outflow tract and semilunar valves account for >50% of these cases predominantly because of a bicuspid aortic valve, which has an estimated prevalence of 1% in the population. We previously reported that mutations in NOTCH 1 were a cause of bicuspid aortic valve in nonsyndromic autosomal‐dominant human pedigrees. Subsequently, we described a highly penetrant mouse model of aortic valve disease, consisting of a bicuspid aortic valve with thickened cusps and associated stenosis and regurgitation, in Notch1 ‐haploinsufficient adult mice backcrossed into a Nos3 ‐null background. Methods and Results Here, we described the congenital cardiac abnormalities in Notch1 +/− ;Nos3 −/− embryos that led to ≈65% lethality by postnatal day 10. Although expected Mendelian ratios of Notch1 +/− ;Nos3 −/− embryos were found at embryonic day 18.5, histological examination revealed thickened, malformed semilunar valve leaflets accompanied by additional anomalies of the cardiac outflow tract including ventricular septal defects and overriding aorta. The aortic valve leaflets of Notch1 +/− ;Nos3 −/− embryos at embryonic day 15.5 were significantly thicker than controls, consistent with a defect in remodeling of the semilunar valve cushions. In addition, we generated mice haploinsufficient for Notch1 specifically in endothelial and endothelial‐derived cells in a Nos3 ‐null background and found that Notch1 fl /+ ; Tie2‐Cre +/− ;Nos3 −/− mice recapitulate the congenital cardiac phenotype of Notch1 +/− ;Nos3 −/− embryos. Conclusions Our data demonstrate the role of endothelial Notch1 in the proper development of the semilunar valves and cardiac outflow tract.

Chronic inflammation is associated with a variety of pathological conditions in epithelial tissues, including cancer, metaplasia and aberrant wound healing. In relation to this, a significant body of evidence suggests that aberration of epithelial stem and progenitor cell function is a contributing factor in inflammation-related disease, although the underlying cellular and molecular mechanisms remain to be fully elucidated. In this study, we have delineated the effect of chronic inflammation on epithelial stem/progenitor cells using the corneal epithelium as a model tissue. Using a combination of mouse genetics, pharmacological approaches and in vitro assays, we demonstrate that chronic inflammation elicits aberrant mechanotransduction in the regenerating corneal epithelium. As a consequence, a YAP–TAZ/β-catenin cascade is triggered, resulting in the induction of epidermal differentiation on the ocular surface. Collectively, the results of this study demonstrate that chronic inflammation and mechanotransduction are linked and act to elicit pathological responses in regenerating epithelia.

Here we present an electrical lysis throughput of 600 microliters per minute at high cell density (108 yeast cells per ml) with 90% efficiency, thus improving the current common throughput of one microliter per minute. We also demonstrate the extraction of intracellular luciferase from mammalian cells with efficiency comparable to off-chip bulk chemical lysis. The goal of this work is to develop a sample preparation module that can act as a stand-alone device or be integrated to other functions already demonstrated in miniaturized devices, including sorting and analysis, towards a true lab-on-a-chip.

Signaling through the Notch1 receptor is essential for the control of numerous developmental processes during embryonic life as well as in adult tissue homeostasis and disease. Since the outcome of Notch1 signaling is highly context-dependent, and its precise physiological and pathological role in many organs is unclear, it is of great interest to localize and identify the cells that receive active Notch1 signals in vivo. Here, we report the generation and characterization of a BAC-transgenic mouse line, N1-Gal4VP16, that when crossed to a Gal4-responsive reporter mouse line allowed the identification of cells undergoing active Notch1 signaling in vivo. Analysis of embryonic and adult N1-Gal4VP16 mice demonstrated that the activation pattern of the transgene coincides with previously observed activation patterns of the endogenous Notch1 receptor. Thus, this novel reporter mouse line provides a unique tool to specifically investigate the spatial and temporal aspects of Notch1 signaling in vivo. genesis 50:700710, 2012. (c) 2012 Wiley Periodicals, Inc.

Nuocytes are essential in innate type 2 immunity and contribute to the exacerbation of asthma responses. Here we found that nuocytes arose in the bone marrow and differentiated from common lymphoid progenitors, which indicates they are distinct, previously unknown members of the lymphoid lineage. Nuocytes required interleukin 7 (IL-7), IL-33 and Notch signaling for development in vitro. Pro-T cell progenitors at double-negative stage 1 (DN1) and DN2 maintained nuocyte potential in vitro, although the thymus was not essential for nuocyte development. Notably, the transcription factor RORα was critical for the development of nuocytes and their role in the expulsion of parasitic worms.

Uterine receptivity implies a dialogue between the hormonally primed maternal endometrium and the free-floating blastocyst. Endometrial stromal cells proliferate, avert apoptosis, and undergo decidualization in preparation for implantation; however, the molecular mechanisms that underlie differentiation into the decidual phenotype remain largely undefined. The Notch family of transmembrane receptors transduce extracellular signals responsible for cell survival, cell-to-cell communication, and differentiation, all fundamental processes for decidualization and pregnancy. Using a murine artificial decidualization model, pharmacological inhibition of Notch signaling by gamma-secretase inhibition resulted in a significantly decreased deciduoma. Furthermore, a progesterone receptor (PR)-Cre Notch1 bigenic (Notch1d/d) confirmed a Notch1-dependent hypomorphic decidual phenotype. Microarray and pathway analysis, following Notch1 ablation, demonstrated significantly altered signaling repertoire. Concomitantly, hierarchical clustering demonstrated Notch1-dependent differences in gene expression. Uteri deprived of Notch1 signaling demonstrated decreased cellular proliferation; namely, reduced proliferation-specific antigen, Ki67, altered p21, cdk6, and cyclinD activity and an increased apoptotic-profile, cleaved caspase-3, Bad, and attenuated Bcl2. The results demonstrate that the preimplantation uterus relies on Notch signaling to inhibit apoptosis of stromal fibroblasts and regulate cell cycle progression, which together promotes successful decidualization. In summary, Notch1 signaling modulates multiple signaling mechanisms crucial for decidualization and these studies provide additional perspectives to the coordination of multiple signaling modalities required during decidualization.-Afshar, Y., Jeong, J.-W., Roqueiro, D., DeMayo, F., Lydon, J., Radtke, F., Radnor, R., Miele, L., Fazleabas, A. Notch1 mediates uterine stromal differentiation and is critical for complete decidualization in the mouse. FASEB J. 26, 282-294 (2012). www.fasebj.org

While the pro-differentiation and tumour suppressive functions of Notch signalling in keratinocytes are well established, the underlying mechanisms remain poorly understood. We report here that interferon regulatory factor 6 (IRF6), an IRF family member with an essential role in epidermal development, is induced in differentiation through a Notch-dependent mechanism and is a primary Notch target in keratinocytes and keratinocyte-derived SCC cells. Increased IRF6 expression contributes to the impact of Notch activation on growth/differentiation-related genes, while it is not required for induction of 'canonical' Notch targets like p21(WAF1/Cip1), Hes1 and Hey1. Down-modulation of IRF6 counteracts differentiation of primary human keratinocytes in vitro and in vivo, promoting ras-induced tumour formation. The clinical relevance of these findings is illustrated by the strikingly opposite pattern of expression of Notch1 and IRF6 versus epidermal growth factor receptor in a cohort of clinical SCCs, as a function of their grade of differentiation. Thus, IRF6 is a primary Notch target in keratinocytes, which contributes to the role of this pathway in differentiation and tumour suppression.

Background— The signaling mechanisms that regulate the recruitment of bone marrow (BM)–derived cells to the injured heart are not well known. Notch receptors mediate binary cell fate determination and may regulate the function of BM-derived cells. However, it is not known whether Notch1 signaling in BM-derived cells mediates cardiac repair after myocardial injury. Methods and Results— Mice with postnatal cardiac-specific deletion of Notch1 exhibit infarct size and heart function after ischemic injury that is similar to that of control mice. However, mice with global hemizygous deletion of Notch1 (N1 ± ) developed larger infarct size and worsening heart function. When the BM of N1 ± mice were transplanted into wild-type (WT) mice, infarct size and heart function were worsened and neovascularization in the infarct border area was reduced compared with WT mice transplanted with WT BM. In contrast, transplantation of WT BM into N1 ± mice lessened the myocardial injury observed in N1 ± mice. Indeed, hemizygous deletion of Notch1 in BM-derived cells leads to decreased recruitment, proliferation, and survival of mesenchymal stem cells (MSC). Compared with WT MSC, injection of N1 ± MSC into the infarcted heart leads to increased myocardial injury whereas injection of MSC overexpressing Notch intracellular domain leads to decreased infarct size and improved cardiac function. Conclusions— These findings indicate that Notch1 signaling in BM-derived cells is critical for cardiac repair and suggest that strategies that increase Notch1 signaling in BM-derived MSC could have therapeutic benefits in patients with ischemic heart disease.

Loss of Ikaros has been correlated with Notch activation in T cell acute lymphoblastic leukemia (T-ALL), however, the mechanism remains unknown. We identified promoters in Notch1 that drive expression of Notch1 proteins active in the absence of ligand. Ikaros bound to both canonical and alternative Notch1 promoters and its loss increased permissive chromatin, facilitating recruitment of transcription regulators. At early stages of leukemogenesis, increased basal expression from the canonical and 5’-alternative promoters initiated a feed-back loop, progressively augmenting Notch1 signaling. Ikaros also repressed intragenic promoters that are cryptic in wild-type, poised in pre-leukemic, and active in leukemic cells and which also produced ligand-independent Notch1 proteins. Only ligand-independent Notch1 isoforms were required for Ikaros-mediated leukemogenesis. Notch1 alternative-promoter usage was observed at stages of T cell development dependent on Notch signaling and during T-ALL progression. These studies identify a network of epigenetic and transcriptional regulators that control conventional and unconventional Notch signaling during normal development and leukemogenesis.

NOTCH signalling is a key development pathway whose aberrant activation is known to play a role in multiple human cancers. In human tumors the NOTCH pathway can be activated by various genetic lesions such over expression of ligands/receptors, GOF mutations in NOTCH receptors, including protein stabilizing mutations in the PEST domain of NOTCH, chromosomal translocations, or loss-of-function mutations in the E3 ubiquitin ligase FBXW7 and other negative regulators of the pathway (SPEN, NUMB). Activation of signalling due to above mentioned mechanisms can be addressed in part using blocking antibodies against NOTCH ligands/receptors or small molecule inhibitors of the gamma secretase enzyme (GSIs). However, in human tumors where NOTCH signalling is constitutively activated due to chromosomal translocations in the NOTCH receptors (NOTCH1 and 2), none of the above-mentioned strategies will be effective. Moreover, due to on-target and off-target toxicities associated with blocking antibodies and GSIs, these anti-NOTCH agents failed to advance in clinical trials, although some of them showed signs of clinical efficacy. Given the role of NOTCH signalling in human tumors, there is a need to identify novel targets in the NOTCH pathway and develop new and more selective anti-NOTCH agents. To inhibit pan-NOTCH signalling in human tumors independently of the mechanisms of NOTCH activation, and in the most downstream part of the pathway, we have previously reported the discovery of a new class of small molecules able to target the NOTCH transcription complex enabling the specific inhibition of NOTCH target gene expression (e.g. cMYC, HES1, DTX1, CCND1). These small molecules act as protein-protein interaction inhibitors, and thereby compromise the assembly of functional NOTCH transcription complex. Here we present further in vitro and in vivo characterization of the lead molecule CB-103. The anti-cancer activity of CB-103 was extensively profiled in several human cancer cell lines representing NOTCH positive solid tumors, leukemias and lymphomas. Moreover, CB-103 responsiveness of these cell lines correlates with a downregulation of the NOTCH signature following treatment with CB-103. Specifically, we will present data outlining the in vivo pharmacokinetic and pharmacodynamic properties of CB-103. Citation Format: Rajwinder Lehal, Charlotte Urech, Michele Vigolo, Maximilien Murone, Freddy Radtke. Characterization and profiling of CB-103, a novel small-molecule protein-protein interaction inhibitor targeting the NOTCH transcription complex [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 5799.

Hyperplasia associated with a loss of tissue homeostasis can induce DNA replication stress, leading to precancerous dysregulation. Epidermal {gamma}{delta} T cells reside in the primary barrier that protects against diverse environmental insults; however, the functions of these T cells in tissue surveillance are not completely understood. In mice with inducible Notch1 inactivation in keratinocytes that causes epidermal hyperplasia, epidermal {gamma}{delta} T cells sensed stressed keratinocytes and migrated into the cutaneous draining lymph nodes. Simultaneous induction of β-galactosidase (β-Gal) as a putative antigen expressed in the process of precancerous dysregulation and Notch1 ablation in the epidermis resulted in elevated β-Gal-specific IgG2a production. Epidermal {gamma}{delta} T cells were found to have the capacity to express chemokine (C-C motif) receptor 7 and migrate into the lymph nodes. Cutaneous draining lymph node cells in Notch1-inactivated mice expressed high levels of IFN-{gamma} upon anti-CD3 plus anti-CD28 stimulation. Furthermore, induced expression of β-Gal in mice that lacked epidermal {gamma}{delta} T cells failed to induce anti-β-Gal IgG. These results suggest that epidermal {gamma}{delta} T cells play an essential role in the initiation process of epidermal antigen-specific humoral immune responses and demonstrate the importance of epidermal {gamma}{delta} T cells in sensing precancerous dysregulation and activating adaptive immunity.

Background: NOTCH pathway is recurrently activated in human cancers, including lymphomas, and overactivation of the pathway has been correlated to a worse prognosis in patients with solid tumors, leukemias, and lymphomas, making it an attractive target for treatment (Rossi et al, JEM 2012; Rossi et al, Blood 2012; Kridel et al, Blood 2012). However, so far, pharmacologic approaches to inhibit its activity are limited to the use of nonspecific gamma secretase inhibitors and blocking antibodies with limited application. None of these therapeutic agents have been approved yet for cancer treatment. CB-103 is a novel first-in-class pan-NOTCH inhibitor that blocks signaling by directly and specifically targeting the NOTCH transcription complex, and it has shown preclinical activity in a variety of solid tumor and leukemia models (Lehal et al, AACR 2017). Here, we present the very first results with CB-103 in a large panel of B and T cell lymphoma cell lines. Methods: 58 human lymphoma cell lines [27 diffuse large B cell lymphomas (DLBCL); 9 mantle cell lymphomas; 6 marginal zone lymphomas; 9 T-cell lymphomas; 7 others], 2 murine and 1 canine, were exposed to increasing doses of CB-103 for 72h. Cell proliferation was measured by using the MTT assay. Transcriptome analysis was performed using the Illumina HumanHT 12 Expression BeadChips and with the HTG EdgeSeq Oncology Biomarker Panel, a targeted RNA-Seq investigating over 2500 genes, followed by a limma moderate t-test, and GSEA. Results: CB-103 presented a median IC50 above 20 μM across the whole panel of 61 lymphoma cell lines (range from 400 nM to > 20 μM), without significant differences among lymphoma subtypes. Importantly, 13/58 cell lines had an IC50 10 μM, presented a gene expression signature significantly enriched with genes involved in the epithelial-mesenchymal transition (FDR 0.0014), a Notch-related process. Indeed, the transcripts higher in the sensitive cell lines included, among others, DLL1 (P 0.010; Log Fold Change 2.39), BAMBI (P 0.046; Log Fold Change 0.92), MCM7 (P 0.015; Log Fold Change 1), and ULK1 (P 0.045; Log Fold Change 2.39), all connected to NOTCH pathway. Finally, 10 of the CB-103 sensitive human cell lines were also exposed to another NOTCH inhibitor, the gamma secretase inhibitor LY-3039187, which resulted active only in one MCL cell line, while it had IC50 >20 μM in the other cell lines tested. Conclusions: CB-103 presented in vitro antilymphoma activity in a restricted number of B cell lymphoma cell lines, which were characterized by a NOTCH-related gene expression signature. Further studies with the compound are ongoing and a phase I is planned to start in Q3 2017 (EUDRACT Number 2017-001491-35). Citation Format: Filippo Spriano, Chiara Tarantelli, Alberto Arribas, Eugenio Gaudio, Luciano Cascione, Luca Aresu, Emanuele Zucca, Davide Rossi, Anastasios Stathis, Maximilien Murone, Dirk Weber, Rajwinder Lehal, Freddy Radtke, Francesco Bertoni. Targeting lymphomas with the novel first-in-class pan-NOTCH transcription inhibitor CB-103 [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2017 Oct 26-30; Philadelphia, PA. Philadelphia (PA): AACR; Mol Cancer Ther 2018;17(1 Suppl):Abstract nr B061.

The Notch signaling pathway regulates many aspects of embryonic development, as well as differentiation processes and tissue homeostasis in multiple adult organ systems. Disregulation of Notch signaling is associated with several human disorders, including cancer. In the last decade, it became evident that Notch signaling plays important roles within the hematopoietic and immune systems. Notch plays an essential role in the development of embryonic hematopoietic stem cells and influences multiple lineage decisions of developing lymphoid and myeloid cells. Moreover, recent evidence suggests that Notch is an important modulator of T cell-mediated immune responses. In this review, we discuss Notch signaling in hematopoiesis, lymphocyte development, and function as well as in T cell acute lymphoblastic leukemia.

Hox genes encode transcription factors that play a central role in the specification of regional identities along the anterior to posterior body axis. In the developing mouse embryo, Hox genes from all four genomic clusters are involved in range of developmental processes, including the patterning of skeletal structures and the formation of several organs. However, the functional redundancy observed either between paralogous genes, or among neighboring genes from the same cluster, has hampered functional analyses, in particular when synergistic, cluster-specific functions are considered. Here, we report that mutant mice lacking the entire HoxA cluster in mesodermal lineages display the expected spectrum of postnatal respiratory, cardiac and urogenital defects, previously reported for single gene mutations. Likewise, mild phenotypes are observed in both appendicular and axial skeleton. However, a striking effect was uncovered in the hematopoietic system, much stronger than that seen for Hoxa9 inactivation alone, which involves stem cells (HSCs) as well as the erythroid lineage, indicating that several Hoxa genes are necessary for normal hematopoiesis to occur. Finally, the combined deletions of Hoxa and Hoxd genes reveal abnormalities in axial elongation as well as skin morphogenesis that are likely the results of defects in epithelial–mesenchymal interactions.

Background— Notch1 regulates binary cell fate determination and is critical for angiogenesis and cardiovascular development. However, the pathophysiological role of Notch1 in the postnatal period is not known. We hypothesize that Notch1 signaling in vascular smooth muscle cells (SMCs) may contribute to neointimal formation after vascular injury. Methods and Results— We performed carotid artery ligation in wild-type, control (SMC-specific Cre recombinase transgenic [smCre-Tg]), general Notch1 heterozygous deficient (N1 +/− ), SMC-specific Notch1 heterozygous deficient (smN1 +/− ), and general Notch3 homozygous deficient (N3 −/− ) mice. Compared with wild-type or control mice, N1 +/− and smN1 +/− mice showed a 70% decrease in neointimal formation after carotid artery ligation. However, neointimal formation was similar between wild-type and N3 −/− mice. Indeed, SMCs derived from explanted aortas of either N1 +/− - or smN1 +/− mice showed decreased chemotaxis and proliferation and increased apoptosis compared with control or N3 −/− mice. This correlated with decreased staining of proliferating cell nuclear antigen–positive cells and increased staining of cleaved caspase-3 in the intima of N1 +/− - or smN1 +/− mice. In SMCs derived from CHF1/Hey2 −/− mice, activation of Notch signaling did not lead to increased SMC proliferation or migration. Conclusions— These findings indicate that Notch1, rather than Notch3, mediates SMC proliferation and neointimal formation after vascular injury through CHF1/Hey2 and suggest that therapies that target Notch1/CHF1/Hey2 in SMCs may be beneficial in preventing vascular proliferative diseases.

Hematopietic stem cells (HSCs) maintain life-long hematopoiesis in the bone marrow via their ability to self-renew and to differentiate into all blood lineages. Although a central role for the canonical wnt signaling pathway has been suggested in HSC self-renewal as well as in the development of B and T cells, conditional deletion of beta-catenin (which is considered to be essential for Wnt signaling) has no effect on hematopoiesis or lymphopoiesis. Here, we address whether this discrepancy can be explained by a redundant and compensatory function of gamma-catenin, a close homolog of beta-catenin. Unexpectedly, we find that combined deficiency of beta- and gamma-catenin in hematopoietic progenitors does not impair their ability to self-renew and to reconstitute all myeloid, erythroid, and lymphoid lineages, even in competitive mixed chimeras and serial transplantations. These results exclude an essential role for canonical Wnt signaling (as mediated by beta- and/or gamma-catenin) during hematopoiesis and lymphopoiesis.

T cell development in the thymus depends on continuous colonization by hematopoietic precursors. Several distinct T cell precursors have been identified, but whether one or several independent precursor cell types maintain thymopoiesis is unclear. We have used thymus transplantation and an inducible lineage-tracing system to identify the intrathymic precursor cells among previously described thymus-homing progenitors that give rise to the T cell lineage in the thymus. Extrathymic precursors were not investigated in these studies. Both approaches show that the stream of T cell lineage precursor cells, when entering the thymus, selectively passes through the early T lineage precursor (ETP) stage. Immigrating precursor cells do not exhibit characteristics of double-negative (DN) 1c, DN1d, or DN1e stages, or of populations containing the common lymphoid precursor 2 (CLP-2) or the thymic equivalent of circulating T cell progenitors (CTPs). It remains possible that an unknown hematopoietic precursor cell or previously described extrathymic precursors with a CLP, CLP-2, or CTP phenotype feed into T cell development by circumventing known intrathymic T cell lineage progenitor cells. However, it is clear that of the known intrathymic precursors, only the ETP population contributes significant numbers of T lineage precursors to T cell development.

Notch signaling regulates B and T lymphocyte development and T cell effector class decision. In this work, we tested whether Notch activity affects mature B cell activation and differentiation to antibody-secreting cells (ASC). We show increased frequency of ASC in cultures of splenic B cells activated with LPS or anti-CD40 when provided exogenous Notch ligand Delta-like-1 (Dll1). Our results indicate that Notch–Dll1 interaction releases a default pathway that otherwise inhibits Ig secretion upon B cell activation. Thus, Dll1 enhanced spontaneous Ig secretion by naturally activated marginal zone B and B1 cells and reversed the inhibition of ASC differentiation mediated by B cell receptor crosslinking during LPS. Moreover, suppression of Notch signaling in B cell expression of either a dominant-negative mutant form of Mastermind-like 1 or a null mutation of Notch1 not only prevented Dll1-mediated enhancement of ASC differentiation but also reduced dramatically LPS-induced Ig secretion. Finally, we show that Dll1 and Jagged-1 are differentially expressed in discrete areas of the spleen, and that the effect of Notch engagement on Ig secretion is ligand-specific. These results indicate that Notch ligands participate in the definition of the mature B cell microenvironment that influences their terminal differentiation.

Phosphoinositide-dependent kinase l (PDK1) phosphorylates and activates multiple AGC serine kinases, including protein kinase B (PKB), p70Ribosomal S6 kinase (S6K) and p90Ribosomal S6 kinase (RSK). PDK1 is required for thymocyte differentiation and proliferation, and herein, we explore the molecular basis for these essential functions of PDK1 in T lymphocyte development. A key finding is that PDK1 is required for the expression of key nutrient receptors in T cell progenitors: CD71 the transferrin receptor and CD98 a subunit of L-amino acid transporters. PDK1 is also essential for Notch-mediated trophic and proliferative responses in thymocytes. A PDK1 mutant PDK1 L155E, which supports activation of PKB but no other AGC kinases, can restore CD71 and CD98 expression in pre-T cells and restore thymocyte differentiation. However, PDK1 L155E is insufficient for thymocyte proliferation. The role of PDK1 in thymus development thus extends beyond its ability to regulate PKB. In addition, PDK1 phosphorylation of AGC kinases such as S6K and RSK is also necessary for thymocyte development.

Although Notch signaling plays important roles in lineage commitment and differentiation of multiple cell types including conventional T cells, nothing is currently known concerning Notch function in innate-like T cells. We have found that the homeostasis of several well-characterized populations of innate-like T cells including invariant NKT cells (iNKT), CD8ααTCRαβ small intestinal intraepithelial lymphocytes, and innate memory phenotype CD8 T cells is controlled by Notch. Notch selectively regulates hepatic iNKT cell survival via tissue-restricted control of B cell lymphoma 2 and IL-7Rα expression. More generally, Notch regulation of innate-like T cell homeostasis involves both cell-intrinsic and -extrinsic mechanisms and relies upon context-dependent interactions with Notch ligand-expressing fibroblastic stromal cells. Collectively, using conditional ablation of Notch receptors on peripheral T cells or Notch ligands on putative fibroblastic stromal cells, we show that Notch signaling is indispensable for the homeostasis of three tissue-restricted populations of innate-like T cells: hepatic iNKT, CD8ααTCRαβ small intestinal intraepithelial lymphocytes, and innate memory phenotype CD8 T cells, thus supporting a generalized role for Notch in innate T cell homeostasis.

The mammalian intestine has one of the highest turnover rates in the body. The intestinal epithelium is completely renewed in less than a week. It is divided into spatially distinct compartments in the form of finger-like projections and invaginations that are dedicated to specific functions. Intestinal cells are constantly produced from a stem cell reservoir that gives rise to proliferating transient amplifying cells, which subsequently differentiate and migrate to the correct compartment before dying after having fulfilled their physiological function. In recent years, a substantial body of evidence has accumulated to support the concept that signaling pathways known to be crucial for embryonic development of multiple organisms play a critical role in tightly regulating and controlling the self-renewing process of the intestine. Moreover, the same pathways appear to be deregulated in several hereditary and sporadic colorectal cancer syndromes due to activating and/or inactivating mutations of key components of such pathways. In this review we discuss recent findings demonstrating that differentiation and homeostasis of the intestine are controlled by developmental pathways such as Wnt, Notch, TGF-beta and Hedgehog, and illustrate how their deregulation contributes to intestinal neoplasia.

The Notch signaling pathway is involved in diverse biological processes such as cell fate decisions or stem cell maintenance. In this study, we assessed the role of this pathway for melanocyte development and hair pigmentation using RBP-J kappa, Notch1, and Notch2 conditional knockout mice. Disruption of the Notch pathway by inactivating RBP-J kappa in the melanocyte lineage using Tyr::Cre mice led to a severe coat color dilution. Similarly, hair graying was observed when Notch1 and/or Notch2 receptors were ablated in melanocytes. This phenotype was proportional to the number of floxed Notch alleles, with the most pronounced effect seen in Tyr::Cre/degrees; Notch1(flox/flox); Notch2(flox/flox) mice. Deletion of Notch1 and/or Notch2 in melanoblasts did not induce a congenital defect. The number of Dct-expressing cells at embryonic stages was not affected, but melanocytes located within the hair matrix progressively disappeared during the first regeneration of the hair follicle. In contrast, non-follicular melanocytes and pigmentation in the dermis and in the choroid were not affected. We suggest that both Notch1 and Notch2 receptors contribute to the maintenance of melanoblasts and melanocyte stem cells, and are essential for proper hair pigmentation.

The Notch family of receptors is involved in a wide variety of developmental processes, including cell fate specification, cell proliferation, and cell survival decisions during cell differentiation and tissue morphogenesis. Notch1 and Notch ligands are expressed in the developing limbs, and Notch signalling has been implicated in the formation of a variety of tissues that comprise the limb, such as the skeleton, musculature, and vasculature. Notch signalling has also been implicated in regulating overall limb size. We have used a conditional allele of Notch1 in combination with two different Cre transgenic lines to delete Notch1 function either in the limb mesenchyme or in the apical ectodermal ridge (AER) and limb ectoderm. We demonstrate that Notch signalling, involving Notch1 and Jagged2, is required to regulate the number of Fgf8-expressing cells that comprise the AER and that regulation of the levels of fibroblast growth factor signalling is important for the freeing of the digits during normal limb formation. Regulation of the extent of the AER is achieved by Notch signalling positively regulating apoptosis in the AER. We also demonstrate that Notch1 is not required for proper formation of all the derivatives of the limb mesenchyme.

The self-renewing epithelium of the small intestine is ordered into stem/progenitor crypt compartments and differentiated villus compartments. Recent evidence indicates that the Wnt cascade is the dominant force in controlling cell fate along the crypt-villus axis. Here we show a rapid, massive conversion of proliferative crypt cells into post-mitotic goblet cells after conditional removal of the common Notch pathway transcription factor CSL/RBP-J. We obtained a similar phenotype by blocking the Notch cascade with a gamma-secretase inhibitor. The inhibitor also induced goblet cell differentiation in adenomas in mice carrying a mutation of the Apc tumour suppressor gene. Thus, maintenance of undifferentiated, proliferative cells in crypts and adenomas requires the concerted activation of the Notch and Wnt cascades. Our data indicate that gamma-secretase inhibitors, developed for Alzheimer's disease, might be of therapeutic benefit in colorectal neoplastic disease.

Background— Notch signaling influences binary cell fate decisions in a variety of tissues. The Notch1 receptor is widely expressed during embryogenesis and is essential for embryonic development. Loss of global Notch1 function results in early embryonic lethality, but the cell type responsible for this defect is not known. Here, we identify the endothelium as the primary target tissue affected by Notch1 signaling. Methods and Results— We generated an endothelium-specific deletion of Notch1 using Tie2Cre and conditional Notch1 flox/flox mice. Mutant embryos lacking endothelial Notch1 died at approximately embryonic day 10.5 with profound vascular defects in placenta, yolk sac, and embryo proper, whereas heterozygous deletion had no effect. In yolk sacs of mutant embryos, endothelial cells formed a primary vascular plexus indicative of intact vasculogenesis but failed to induce the secondary vascular remodeling required to form a mature network of well-organized large and small blood vessels, which demonstrates a defect in angiogenesis. These vascular defects were also evident in the placenta, where blood vessels failed to invade the placental labyrinth, and in the embryo proper, where defective blood vessel maturation led to pericardial and intersomitic hemorrhage. Enhanced activation of caspase-3 was detected in endothelial and neural cells of mutant mice, which resulted in enhanced apoptotic degeneration of somites and the neural tube. Conclusions— These findings recapitulate the vascular phenotype of global Notch1 −/− mutants and indicate an essential cell-autonomous role of Notch1 signaling in the endothelium during vascular development. These results may have important clinical implications with regard to Notch1 signaling in adult angiogenesis.

The discovery that the human Jagged1 gene (JAG1) is the Alagille syndrome disease gene indicated that Notch signaling has an important role in bile duct homeostasis. The functional study of this signaling pathway has been difficult because mice with targeted mutations in Jagged1, Notch1, or Notch2 have an embryonic lethal phenotype. We have previously generated mice with inducible Notch1 disruption using an interferon-inducible Cre-recombinase transgene in combination with the loxP flanked Notch1 gene. We used this conditional Notch1 knockout mouse model to investigate the role of Notch1 signaling in liver cell proliferation and differentiation. Deletion of Notch1 did not result in bile duct paucity, but, surprisingly, resulted in a continuous proliferation of hepatocytes. In conclusion, within weeks after Notch1 inactivation, the mice developed nodular regenerative hyperplasia without vascular changes in the liver.

The evolutionarily conserved Notch signaling pathway regulates a broad spectrum of cell fate decisions and differentiation processes during fetal and postnatal life. It is involved in embryonic organogenesis as well as in the maintenance of homeostasis of self-renewing systems. In this article, we review the role of Notch signaling in the hematopoietic system with particular emphasis on lymphocyte development and highlight the similarities in Notch function between Drosophila and mammalian differentiation processes. Recent studies indicating that aberrant NOTCH signaling is frequently linked to the induction of T leukemia in humans will also be discussed.

The Notch and fibroblast growth factor (FGF) pathways both regulate cell fate specification during mammalian neural development. We have shown previously that Notch1 activation in the murine forebrain promotes radial glial identity. This result, together with recent evidence that radial glia can be progenitors, suggested that Notch1 signaling might promote progenitor and radial glial character simultaneously. Consistent with this idea, we found that in addition to promoting radial glial characterin vivo, activated Notch1 (ActN1) increased the frequency of embryonic day 14.5 (E14.5) ganglionic eminence (GE) progenitors that grew into neurospheres in FGF2. Constitutive activation of C-promoter binding factor (CBF1), a Notch pathway effector, also increased neurosphere frequency in FGF2, suggesting that the effect of Notch1 on FGF responsiveness is mediated by CBF1.The observation that ActN1 promoted FGF responsiveness in telencephalic progenitors prompted us to examine the effect of FGF pathway activationin vivo. We focused on FGFR2 because it is expressed in radial glia in the GEs where ActN1 increases FGF2 neurosphere frequency, but not in the septum where it does not. Like ActN1, activated FGFR2 (ActFGFR2) promoted radial glial characterin vivo. However, unlike ActN1, ActFGFR2 did not enhance neurosphere frequency at E14.5. Additional analysis demonstrated that, unexpectedly, neither ActFGFR2 nor ActFGFR1 could replace the need for ligand in promoting neurosphere proliferation. This study suggests that telencephalic progenitors with radial glial morphology are maintained by interactions between the Notch and FGF pathways, and that the mechanisms by which FGF signaling promotes radial glial characterin vivoand progenitor proliferationin vitrocan be uncoupled.

Signaling through the Notch1 receptor is essential for T cell development in the thymus. Stromal OP9 cells ectopically expressing the Notch ligand Delta-like1 mimic the thymic environment by inducing hemopoietic stem cells to undergo in vitro T cell development. Notch1 is also expressed on Pax5−/− pro-B cells, which are clonable lymphoid progenitors with a latent myeloid potential. In this study, we demonstrate that Pax5−/− progenitors efficiently differentiate in vitro into CD4+CD8+ αβ and γδ T cells upon coculture with OP9-Delta-like1 cells. In vitro T cell development of Pax5−/− progenitors strictly depends on Notch1 function and progresses through normal developmental stages by expressing T cell markers and rearranging TCRβ, γ, and δ loci in the correct temporal sequence. Notch-stimulated Pax5−/− progenitors efficiently down-regulate the expression of B cell-specific genes, consistent with a role of Notch1 in preventing B lymphopoiesis in the thymus. At the same time, Notch signaling rapidly induces cell surface expression of the c-Kit receptor and transcription of the target genes Deltex1 and pre-Tα concomitant with the activation of TCR Vβ germline transcription and the regulatory genes GATA3 and Tcf1. These data suggest that Notch1 acts upstream of GATA3 and Tcf1 in early T cell development and regulates Vβ-DJβ rearrangements by controlling the chromatin accessibility of Vβ genes at the TCRβ locus.

β-catenin–mediated Wnt signaling has been suggested to be critically involved in hematopoietic stem cell maintenance and development of T and B cells in the immune system. Unexpectedly, here we report that inducible Cre-loxP–mediated inactivation of the β-catenin gene in bone marrow progenitors does not impair their ability to self-renew and reconstitute all hematopoietic lineages (myeloid, erythroid, and lymphoid), even in competitive mixed chimeras. In addition, both thymocyte survival and antigen-induced proliferation of peripheral T cells is β-catenin independent. In contrast to earlier reports, these data exclude an essential role for β-catenin during hematopoiesis and lymphopoiesis.

Keywords: inflammation ; stem cells ; mechanotransduction ; tissue repair ; cancer Reference EPFL-ARTICLE-221997doi:10.1080/15384101.2016.1171645View record in Web of Science Record created on 2016-10-18, modified on 2017-05-12

Notch signaling regulates many cell fate decisions during development of multi-cellular organisms. Signals initiated by Notch influence a wide variety of processes that include lineage specification, cell survival and proliferation, and border formation. During development of the immune system, Notch has been shown to influence the fate of both hematopoietic stem cells (HSCs) and committed progenitors. Notch appears to play an especially important role in the development of cells that mediate acquired immunity where Notch influences multiple aspects of T and B cell development. In this review, we will focus on the potential functions of Notch signaling during lymphoid development.

The developmental origin of dendritic cells (DCs) is controversial. In the mouse CD8alpha(+) and CD8alpha(-) DC subsets are often considered to be of lymphoid and myeloid origin respectively, although evidence on this point is conflicting. Very recently a novel CD11c(+) B220(+) DC subset has been identified that appears to be the murine counterpart to interferon alpha (IFNalpha)-producing human plasmacytoid DCs (PDCs). We show here that CD11c(+) B220(+) mouse PDCs, like human PDCs, are present in the thymus and express T lineage markers such as CD8alpha and CD4. However, the intrathymic development of PDCs can be completely dissociated from immature T lineage cells in mixed chimeras established with bone marrow cells from mice deficient for either Notch-1 or T-cell factor 1, two independent mutations that severely block early T-cell development. Our data indicate that thymic PDCs do not arise from a bipotential T/DC precursor.

Over the last few years a vast amount of progress has been made in identifying mechanisms controlling lineage commitment and plasticity of hematopoietic precursors to different lymphoid or myeloid lineages. This has been due largely to the ability to identify and isolate rare cell populations in order to investigate their developmental potential, together with the development of inducible and/or tissue specific targeting technology. One family of proteins that has been postulated to be involved in hematopoietic stem cell maintenance as well as in multiple commitment processes during T cell development is the Notch receptors and their ligands. In this review we will summarize recent findings and controversies regarding the role of Notch signaling in the myeloid and lymphoid systems.

Notch proteins influence cell fate decisions in many developmental systems. During lymphoid development, Notch1 signaling is essential to direct a bipotent T/B precursor toward the T cell fate, but the role of Notch1 at later stages of T cell development remains controversial. We have recently reported that tissue-specific inactivation of Notch1 in immature (CD44(-) CD25(+)) thymocytes does not affect subsequent T cell development. Here, we demonstrate that loss of Notch1 signaling at an earlier (CD44(+)CD25(+)) developmental stage results in severe perturbation of alpha beta but not gamma delta lineage development. Immature Notch1(-/-) thymocytes show impaired VDJ beta rearrangement and aberrant pre-TCR-independent survival. Collectively, our data demonstrate that Notch1 controls several nonredundant functions necessary for alpha beta lineage development.

Reference EPFL-CONF-232358View record in Web of Science Record created on 2017-11-08, modified on 2017-11-20

NOTCH signaling is a developmental pathway known to play critical roles during embryonic development as well as for the regulation of self-renewing tissues. Aberrant activation of NOTCH signaling leads to deregulation of the self-renewal process resulting in sustained proliferation, evasion of cell death, loss of differentiation capacity, invasion and metastasis, all of which are hallmarks of cancer. Over activation of NOTCH in human cancers can be a consequence of over expression of NOTCH ligands/receptors, GOF mutations in NOTCH receptors as well as chromosomal translocations leading to constitutive activation of the pathway by producing truncated version of NOTCH1 or NOTCH2. Over activation of NOTCH and its oncogenic role in various cancers is prognostically relevant with shorter survival seen in patients harbouring these genetic alterations. Given the importance of Notch signaling in human cancers, several therapeutic approaches have been utilized to block NOTCH signaling. Two of these strategies are; a) the use of monoclonal blocking antibodies (mAbs) against NOTCH ligands and receptors and b) the use of small molecule γ-secretase inhibitors (GSIs). However, these approaches can only be effective if tumor cells express full-length ligand or receptor molecules. On the contrary, in human cancers harbouring NOTCH gene fusion due to chromosomal translocations, the use of mAbs and GSIs will have very limited clinical benefits. A third, yet not fully explored approach would be the blockage of NOTCH signalling by targeting the most downstream complex in the NOTCH signal transduction cascade, the NOTCH transcriptional activation complex, using small molecule inhibitors. Previously we have reported the discovery and characterization of CB-103, a novel first-in-class orally-active small molecule inhibitor of the NOTCH pathway. CB-103 inhibits NOTCH signaling by targeting the NOTCH transcriptional activation complex in the nucleus. CB-103 has shown the ability to block NOTCH signalling in various human cancer cell lines with active NOTCH pathway. CB-103 has also been tested in in vivo models. In a GSI/mAb resistant model of human triple negative breast cancer, CB-103 has demonstrated excellent anti-tumor efficacy. In this study we present additional pharmacology, pharmacokinetics data and provide an overview of the good safety and tolerability seen with CB-103 in the non-clinical studies. Based on these findings CB-103 has been selected as clinical candidate to be investigated in a FIM, phase I/IIA clinical study in advanced cancer patients. Citation Format: Rajwinder Lehal, Viktoras Frismantas, Charlotte Urech, Maximilien Murone, Dirk Weber, Michael Bauer, Jean-Pierre Bourquin, Freddy Radtke. Non clinical pharmacology, pharmacokinetics and safety profiling of CB-103: A novel first-in-class small molecule inhibitor of the NOTCH pathway [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 1163. doi:10.1158/1538-7445.AM2017-1163

Nature Communications 7: Article number: 12597 (2016); Published: 31 August 2016; Updated: 3 May 2017 The authors inadvertently omitted Christine Häger, who was involved in the initial characterization of Notch mutant mice presented in this Article, from the author list and Author contributions statement.

The corneal epithelium acts as a protective barrier on the anterior ocular surface and is essential for maintaining transparency of the cornea and thus visual acuity. During both homeostasis and repair, the corneal epithelium is maintained by self-renewing stem cells, which persist throughout the lifetime of the organism. Importantly, as in other self-renewing tissues, the functional activity of corneal epithelial stem cells (CSCEs) is tightly regulated by the surrounding microenvironment, or niche, which provides a range of cues that maintain the stem cell population. This Cell Science at a Glance article and the accompanying poster will therefore aim to summarise our current understanding of the corneal epithelial stem cell niche and its role in regulating stem cell activity during homeostasis, repair and disease.

The development of T cells from pluripotent stem cells involves a coordinated series of lineage-commitment steps. Common lymphoid precursors in the fetal liver or adult bone marrow must first choose between a T, B or NK cell fate. Committed T cell precursors in the thymus then differentiate into cells committed to the alphabeta or gammadelta lineages. Recent advances have been made in our understanding of the mechanisms underlying T cell fate specification and alphabeta/gammadelta lineage divergence.

We have recently reported that Notch 1, a member of the Notch multigene family, is essential for the development of murine T cells. Using a mouse model in which Notch 1 is inactivated in bone marrow (BM) precursors we have shown that B cells instead of T cells are found in the thymus of BM chimeras. However, it is not clear whether these B cells develop by default from a common lymphoid precursor due to the absence of Notch 1 signaling, or whether they arise as a result of perturbed migration of BM-derived B cells and/or altered homeostasis of normal resident thymic B cells. In this report we show that Notch 1–deficient thymic B cells resemble BM B cells in phenotype and turnover kinetics and are located predominantly in the medulla and corticomedullary junction. Peripheral blood lymphocyte analysis shows no evidence of recirculating Notch1−/− BM B cells. Furthermore, lack of T cell development is not due to a failure of Notch1−/− precursors to home to the thymus, as even after intrathymic reconstitution with BM cells, B cells instead of T cells develop from Notch 1–deficient precursors. Taken together, these results provide evidence for de novo ectopic B cell development in the thymus, and support the hypothesis that in the absence of Notch 1 common lymphoid precursors adopt the default cell fate and develop into B cells instead.

The development and maintenance of the skin are dependent on myriad signaling pathways that regulate a variety of cellular processes. In cutaneous epithelial cells, the Notch cascade plays a central role in ensuring that proliferation and differentiation are coordinated appropriately, a function that it imparts during both ontogeny and homeostasis. Aberrations of the Notch signaling pathway result in severe abnormalities in the epidermis and its appendages and cause functional defects such as perturbed barrier function. In addition, impaired Notch signaling is associated with diseases of the skin such as atopy and cancer. The pathology associated with aberrant cutaneous Notch signaling reflects the complex mechanisms underpinning its function in this tissue and involves both cell-autonomous and nonautonomous mechanisms. This review summarizes our current knowledge of the role of Notch signaling in the skin during health and disease.

Whereas most T cells arise in the thymus, a distinct lineage of extrathymically derived T cells is present in the gut mucosa. The developmental origin of extrathymic T cells is poorly understood. We show here that Notch-1, a transmembrane receptor involved in T cell fate specification of bipotential T/B precursors in the thymus, is absolutely required for the development of extrathymic (as well as thymus-derived) mature T cells in the intestinal epithelium. In the absence of Notch-1, CD117+ T cell precursors are relatively more abundant in the gut than the thymus, whereas immature B cells accumulate in the thymus but not the gut. Collectively, these data demonstrate that Notch-1 is essential for both thymic and extrathymic T cell fate specification and further suggest that bipotential T/B precursors that do not receive a Notch-1 signal adopt a B cell fate in the thymus but become developmentally arrested in the gut.

Recent studies have begun to unravel some of the molecular pathways that appear to control the processes of T cell determination in the earliest thymocyte precursors. In addition, the analyses of mouse mutants with an entirely alymphoid thymus have shed light on the developmental relationship of pro-T cells and thymic dendritic cells, revealing that development of thymocytes and thymic dendritic cells can be dissociated.

Thymic dendritic cells (DCs) form a discrete subset of bone marrow (BM)-derived cells, the function of which is to mediate negative selection of autoreactive thymocytes. The developmental origin of thymic DCs remains controversial. Although cell transfer studies support a model in which T cells and thymic DCs develop from the same intrathymic pluripotential precursor, it remains possible that these two types of cells develop from independent intrathymic precursors. Notch proteins are cell surface receptors involved in the regulation of cell fate specification. We have recently reported that T cell development in inducible Notch1-deficient mice is severely impaired at an early stage, before the expression of T cell lineage markers. To investigate whether development of thymic DCs also depends on Notch1, we have constructed mixed BM chimeric mice. We report here that thymic DC development from Notch1−/− BM precursors is absolutely normal (in terms of absolute number and phenotype) in this competitive situation, despite the absence of Notch1−/− T cells. Furthermore, we find that peripheral DCs and Langerhans cells are also not affected by Notch1 deficiency. Our results demonstrate that the development of DCs is totally independent of Notch1 function, and strongly suggest a dissociation between intrathymic T cell and DC precursors.

Keywords: Melanoblasts ; Homeostasis Reference EPFL-ARTICLE-142206doi:10.1111/j.1755-148X.2009.00651.xView record in PubMedView record in Web of Science Record created on 2009-11-06, modified on 2017-05-12

We employed two independent approaches to inactivate the angiogenic protein VEGF in newborn mice: inducible, Cre-loxP-mediated gene targeting, or administration of mFlt(1-3)-IgG, a soluble VEGF receptor chimeric protein. Partial inhibition of VEGF achieved by inducible gene targeting resulted in increased mortality, stunted body growth and impaired organ development, most notably of the liver. Administration of mFlt(1-3)-IgG, which achieves a higher degree of VEGF inhibition, resulted in nearly complete growth arrest and lethality. Ultrastructural analysis documented alterations in endothelial and other cell types. Histological and biochemical changes consistent with liver and renal failure were observed. Endothelial cells isolated from the liver of mFlt(1-3)-IgG-treated neonates demonstrated an increased apoptotic index, indicating that VEGF is required not only for proliferation but also for survival of endothelial cells. However, such treatment resulted in less significant alterations as the animal matured, and the dependence on VEGF was eventually lost some time after the fourth postnatal week. Administration of mFlt(1-3)-IgG to juvenile mice failed to induce apoptosis in liver endothelial cells. Thus, VEGF is essential for growth and survival in early postnatal life. However, in the fully developed animal, VEGF is likely to be involved primarily in active angiogenesis processes such as corpus luteum development.