Urban Lendahl

Identification of a Neural Stem Cell in the Adult Mammalian Central Nervous System

New neurons are continuously added in specific regions of the adult mammalian central nervous system. These neurons are derived from multipotent stem cells whose identity has been enigmatic. In this work, we present evidence that ependymal cells are neural stem cells. Ependymal cells give rise to a rapidly proliferating cell type that generates neurons that migrate to the olfactory bulb. In response to spinal cord injury, ependymal cell proliferation increases dramatically to generate migratory cells that differentiate to astrocytes and participate in scar formation. These data demonstrate that ependymal cells are neural stem cells and identify a novel process in the response to central nervous system injury.

Notch signalling controls pancreatic cell differentiation

The pancreas contains both exocrine and endocrine cells, but the molecular mechanisms controlling the differentiation of these cell types are largely unknown. Despite their endodermal origin, pancreatic endocrine cells share several molecular characteristics with neurons, and, like neurons in the central nervous system,, differentiating endocrine cells in the pancreas appear in a scattered fashion within a field of progenitor cells,. This indicates that they may be generated by lateral specification through Notch signalling,. Here, to test this idea, we analysed pancreas development in mice genetically altered at several steps in the Notch signalling pathway. Mice deficient for Delta-like gene 1 (Dll1) or the intracellular mediator RBP-Jκ showed accelerated differentiation of pancreatic endocrine cells. A similar phenotype was observed in mice over-expressing neurogenin 3(ngn 3) or the intracellular form of Notch3 (ref. 13) (a repressor of Notch signalling). These data provide evidence that ngn3 acts as pro-endocrine gene and that Notch signalling is critical for the decision between theendocrine and progenitor/exocrine fates in the developing pancreas.

Notch signaling mediates hypoxia-induced tumor cell migration and invasion

Tumor hypoxia is linked to increased metastatic potential, but the molecular mechanisms coupling hypoxia to metastasis are poorly understood. Here, we show that Notch signaling is required to convert the hypoxic stimulus into epithelial–mesenchymal transition (EMT), increased motility, and invasiveness. Inhibition of Notch signaling abrogated hypoxia-induced EMT and invasion, and, conversely, an activated form of Notch could substitute for hypoxia to induce these processes. Notch signaling deploys two distinct mechanisms that act in synergy to control the expression of Snail-1, a critical regulator of EMT. First, Notch directly up-regulated Snail-1 expression by recruitment of the Notch intracellular domain to the Snail-1 promoter, and second, Notch potentiated hypoxia-inducible factor 1α (HIF-1α) recruitment to the lysyl oxidase (LOX) promoter and elevated the hypoxia-induced up-regulation of LOX, which stabilizes the Snail-1 protein. In sum, these data demonstrate a complex integration of the hypoxia and Notch signaling pathways in regulation of EMT and open up perspectives for pharmacological intervention with hypoxiainduced EMT and cell invasiveness in tumors.

Nestin mRNA expression correlates with the central nervous system progenitor cell state in many, but not all, regions of developing central nervous system

Nestin is a recently discovered intermediate filament (IF) gene. Nestin expression has been extensively used as a marker for central nervous system (CNS) progenitor cells in different contexts, based on observations indicating a correlation between nestin expression and this cell type in vivo. To evaluate this correlation in more detail nestin mRNA expression in developing and adult mouse CNS was analysed by in situ hybridization. We find that nestin is expressed from embryonic day (E) 7.75 and that expression is detected in many proliferating CNS regions: at E10.5 nestin is expressed in cells of both the rostral and caudal neural tube, including the radial glial cells; at E15.5 and postnatal day (P) 0 expression is observed largely in the developing cerebellum and in the ventricular and subventricular areas of the developing telencephalon. Furthermore, the transition from a proliferating to a post-mitotic cell state is accompanied by a rapid decrease in nestin mRNA for motor neurons in the ventral spinal cord and for neurons in the marginal layer of developing telencephalon. In contrast to these data we observe two proliferating areas, the olfactory epithelium and the precursor cells of the hippocampal granule neurons, which do not express nestin at detectable levels. Thus, nestin mRNA expression correlates with many, but not all, regions of proliferating CNS progenitor cells. In addition to its temporal and spatial regulation nestin expression also appears to be regulated at the level of subcellular mRNA localization: in columnar neuroepithelial and radial glial cells nestin mRNA is predominantly localized to the pial endfeet.

Notch signaling: simplicity in design, versatility in function

Notch signaling is evolutionarily conserved and operates in many cell types and at various stages during development. Notch signaling must therefore be able to generate appropriate signaling outputs in a variety of cellular contexts. This need for versatility in Notch signaling is in apparent contrast to the simple molecular design of the core pathway. Here, we review recent studies in nematodes, Drosophila and vertebrate systems that begin to shed light on how versatility in Notch signaling output is generated, how signal strength is modulated, and how cross-talk between the Notch pathway and other intracellular signaling systems, such as the Wnt, hypoxia and BMP pathways, contributes to signaling diversity.

Constitutive activation of NF‐κB and T‐cell leukemia/lymphoma in Notch3 transgenic mice

The multiplicity of Notch receptors raises the question of the contribution of specific isoforms to T‐cell development. Notch3 is expressed in CD4−8− thymocytes and is down‐regulated across the CD4−8− to CD4+8+ transition, controlled by pre‐T‐cell receptor signaling. To determine the effects of Notch3 on thymocyte development, transgenic mice were generated, expressing lck promoter‐driven intracellular Notch3. Thymuses of young transgenics showed an increased number of thymocytes, particularly late CD4−8− cells, a failure to down‐regulate CD25 in post‐CD4−8− subsets and sustained activity of NF‐κB. Subsequently, aggressive multicentric T‐cell lymphomas developed with high penetrance. Tumors sustained characteristics of immature thymocytes, including expression of CD25, pTα and activated NF‐κB via IKKα‐dependent degradation of IκBα and enhancement of NF‐κB‐dependent anti‐apoptotic and proliferative pathways. Together, these data identify activated Notch3 as a link between signals leading to NF‐κB activation and T‐cell tumorigenesis. The phenotypes of pre‐malignant thymocytes and of lymphomas indicate a novel and particular role for Notch3 in co‐ordinating growth and differentiation of thymocytes, across the pre‐T/T cell transition, consistent with the normal expression pattern of Notch3.

The novel Notch homologue mouse Notch 3 lacks specific epidermal growth factor-repeats and is expressed in proliferating neuroepithelium

In Drosophila, the Notch gene is pivotal for cell fate decisions at many stages of development and, in particular, during the formation of the nervous system. Absence of Notch results in the generation of excessive numbers of neural cells at the expense of epidermal cells. Two previously identified mammalian Notch homologous encode all the principal features of the Drosophila gene, e.g. 36 EGF-repeats and 3 Notch/lin-12 repeats extracellularly and 6 intracellular cdc10/SWI6 repeats. We report here the characterisation of a third mammalian homologue, mouse Notch 3, which shares the same remarkable conservation relative to the Drosophila gene as the two previously identified homologues, but with three important distinctions. First, Notch 3 specifically lacks the equivalent of EGF-repeat 21; second, it lacks an EGF-repeat-sized region comprising parts of EGF-repeats 2 and 3; and third, it encodes a considerably shorter intracellular domain. The Notch 3 gene is expressed at high levels in proliferating neuroepithelium and expression is downregulated at later stages. The expression patterns of the Notch 1, 2 and 3 genes are quite distinct during central nervous system (CNS) development, and all possible combinations of expression, i.e. none, one, two, or all three genes, are seen, suggesting a combinatorial code of Notch function in mammals. Considering the predominantly early expression in CNS and its distinct structural features, the Notch 3 gene is likely to contribute significantly to vertebrate Notch function during CNS development.

Cross-talk between the Notch and TGF-β signaling pathways mediated by interaction of the Notch intracellular domain with Smad3

The Notch and transforming growth factor-β (TGF-β) signaling pathways play critical roles in the control of cell fate during metazoan development. However, mechanisms of cross-talk and signal integration between the two systems are unknown. Here, we demonstrate a functional synergism between Notch and TGF-β signaling in the regulation of Hes-1, a direct target of the Notch pathway. Activation of TGF-β signaling up-regulated Hes-1 expression in vitro and in vivo. This effect was abrogated in myogenic cells by a dominant-negative form of CSL, an essential DNA-binding component of the Notch pathway. TGF-β regulated transcription from the Hes-1 promoter in a Notch-dependent manner, and the intracellular domain of Notch1 (NICD) cooperated synergistically with Smad3, an intracellular transducer of TGF-β signals, to induce the activation of synthetic promoters containing multimerized CSL- or Smad3-binding sites. NICD and Smad3 were shown to interact directly, both in vitro and in cells, in a ligand-dependent manner, and Smad3 could be recruited to CSL-binding sites on DNA in the presence of CSL and NICD. These findings indicate that Notch and TGF-β signals are integrated by direct protein–protein interactions between the signal-transducing intracellular elements from both pathways.

Cross-talk between Notch and the Estrogen Receptor in Breast Cancer Suggests Novel Therapeutic Approaches

High expression of Notch-1 and Jagged-1 mRNA correlates with poor prognosis in breast cancer. Elucidating the cross-talk between Notch and other major breast cancer pathways is necessary to determine which patients may benefit from Notch inhibitors, which agents should be combined with them, and which biomarkers indicate Notch activity in vivo. We explored expression of Notch receptors and ligands in clinical specimens, as well as activity, regulation, and effectors of Notch signaling using cell lines and xenografts. Ductal and lobular carcinomas commonly expressed Notch-1, Notch-4, and Jagged-1 at variable levels. However, in breast cancer cell lines, Notch-induced transcriptional activity did not correlate with Notch receptor levels and was highest in estrogen receptor alpha-negative (ERalpha(-)), Her2/Neu nonoverexpressing cells. In ERalpha(+) cells, estradiol inhibited Notch activity and Notch-1(IC) nuclear levels and affected Notch-1 cellular distribution. Tamoxifen and raloxifene blocked this effect, reactivating Notch. Notch-1 induced Notch-4. Notch-4 expression in clinical specimens correlated with proliferation (Ki67). In MDA-MB231 (ERalpha(-)) cells, Notch-1 knockdown or gamma-secretase inhibition decreased cyclins A and B1, causing G(2) arrest, p53-independent induction of NOXA, and death. In T47D:A18 (ERalpha(+)) cells, the same targets were affected, and Notch inhibition potentiated the effects of tamoxifen. In vivo, gamma-secretase inhibitor treatment arrested the growth of MDA-MB231 tumors and, in combination with tamoxifen, caused regression of T47D:A18 tumors. Our data indicate that combinations of antiestrogens and Notch inhibitors may be effective in ERalpha(+) breast cancers and that Notch signaling is a potential therapeutic target in ERalpha(-) breast cancers.

Functional Notch signaling is required for BMP4-induced inhibition of myogenic differentiation

The bone morphogenetic protein (BMP) and Notch signaling pathways are crucial for cellular differentiation. In many cases, the two pathways act similarly; for example, to inhibit myogenic differentiation. It is not known whether this inhibition is caused by distinct mechanisms or by an interplay between Notch and BMP signaling. Here we demonstrate that functional Notch signaling is required for BMP4-mediated block of differentiation of muscle stem cells, i.e. satellite cells and the myogenic cell line C2C12. Addition of BMP4 during induction of differentiation dramatically reduced the number of differentiated satellite and C2C12 cells. Differentiation was substantially restored in BMP4-treated cultures by blocking Notch signaling using either theγ -secretase inhibitor L-685,458 or by introduction of a dominant-negative version of the Notch signal mediator CSL. BMP4 addition to C2C12 cells increased transcription of two immediate Notch responsive genes, Hes1 and Hey1, an effect that was abrogated by L-685,458. A 3 kb Hey1-promoter reporter construct was synergistically activated by the Notch 1 intracellular domain (Notch 1 ICD) and BMP4. The BMP4 mediator SMAD1 mimicked BMP activation of the Hey1 promoter. A synthetic Notch-responsive promoter containing no SMAD1 binding sites responded to SMAD1, indicating that DNA-binding activity of SMAD1 is not required for activation. Accordingly, Notch 1 ICD and SMAD1 interacted in binding experiments in vitro. Thus, the data presented here provide evidence for a direct interaction between the Notch and BMP signaling pathways, and indicate that Notch has a crucial role in the execution of certain aspects of BMP-mediated differentiation control.