Notch signaling: direct or what?

The role of endothelial cell-pericyte interactions in vascularization and diseases

BACKGROUND

Endothelial cells (ECs) and pericytes (PCs) are crucial components of the vascular system, with ECs lining the inner layer of blood vessels and PCs surrounding capillaries to regulate blood flow and angiogenesis. Intercellular communication between ECs and PCs is vital for the formation, stability, and function of blood vessels. Various signaling pathways, such as the vascular endothelial growth factor/vascular endothelial growth factor receptor pathway and the platelet-derived growth factor-B/platelet-derived growth factor receptor-β pathway, play roles in communication between ECs and PCs. Dysfunctional communication between these cells is associated with various diseases, including vascular diseases, central nervous system disorders, and certain types of cancers.

AIM OF REVIEW

This review aimed to explore the diverse roles of ECs and PCs in the formation and reshaping of blood vessels. This review focused on the essential signaling pathways that facilitate communication between these cells and investigated how disruptions in these pathways may contribute to disease. Additionally, the review explored potential therapeutic targets, future research directions, and innovative approaches, such as investigating the impact of EC-PCs in novel systemic diseases, addressing resistance to antiangiogenic drugs, and developing novel antiangiogenic medications to enhance therapeutic efficacy.

KEY SCIENTIFIC CONCEPTS OF REVIEW

Disordered EC-PC intercellular signaling plays a role in abnormal blood vessel formation, thus contributing to the progression of various diseases and the development of resistance to antiangiogenic drugs. Therefore, studies on EC-PC intercellular interactions have high clinical relevance.

A Dynamic Role of Mastermind-Like 1: A Journey Through the Main (Path)ways Between Development and Cancer

Major signaling pathways, such as Notch, Hedgehog (Hh), Wnt/β-catenin and Hippo, are targeted by a plethora of physiological and pathological stimuli, ultimately resulting in the modulation of genes that act coordinately to establish specific biological processes. Many biological programs are strictly controlled by the assembly of multiprotein complexes into the nucleus, where a regulated recruitment of specific transcription factors and coactivators on gene promoter region leads to different transcriptional outcomes. MAML1 results to be a versatile coactivator, able to set up synergistic interlinking with pivotal signaling cascades and able to coordinate the network of cross-talking pathways. Accordingly, despite its original identification as a component of the Notch signaling pathway, several recent reports suggest a more articulated role for MAML1 protein, showing that it is able to sustain/empower Wnt/β-catenin, Hh and Hippo pathways, in a Notch-independent manner. For this reason, MAML1 may be associated to a molecular “switch”, with the function to control the activation of major signaling pathways, triggering in this way critical biological processes during embryonic and post-natal life. In this review, we summarize the current knowledge about the pleiotropic role played by MAML proteins, in particular MAML1, and we recapitulate how it takes part actively in physiological and pathological signaling networks. On this point, we also discuss the contribution of MAML proteins to malignant transformation. Accordingly, genetic alterations or impaired expression of MAML proteins may lead to a deregulated crosstalk among the pathways, culminating in a series of pathological disorders, including cancer development. Given their central role, a better knowledge of the molecular mechanisms that regulate the interplay of MAML proteins with several signaling pathways involved in tumorigenesis may open up novel opportunities for an attractive molecular targeted anticancer therapy.

It Takes Two: Endothelial-Perivascular Cell Cross-Talk in Vascular Development and Disease

The formation of new blood vessels is a crucial step in the development of any new tissue both during embryogenesis and in vitro models as without sufficient perfusion the tissue will be unable to grow beyond the size where nutrition and oxygenation can be managed by diffusion alone. Endothelial cells are the primary building block of blood vessels and are capable of forming tube like structures independently however they are unable to independently form functional vasculature which is capable of conducting blood flow. This requires support from other structures including supporting perivascular cells and the extracellular matrix. The crosstalk between endothelial cells and perivascular cells is vital in regulating vasculogenesis and angiogenesis and the consequences when this is disrupted can be seen in a variety of congenital and acquired disease states. This review details the mechanisms of vasculogenesis in vivo during embryogenesis and compares this to currently employed in vitro techniques. It also highlights clinical consequences of defects in the endothelial cell-pericyte cross-talk and highlights therapies which are being developed to target this pathway. Improving the understanding of the intricacies of endothelial-pericyte signaling will inform pathophysiology of multiple vascular diseases and allow the development of effective in vitro models to guide drug development and assist with approaches in tissue engineering to develop functional vasculature for regenerative medicine applications.

Low expression of NCOA5 predicts poor prognosis in human cervical cancer and promotes proliferation, migration, and invasion of cervical cancer cell lines by regulating notch3 signaling pathway

Nuclear receptor coactivator 5 (NCOA5) specifically enhances estrogen receptor α-modulated transcriptional activity. As a novel tumor suppressor, depletion of NCOA5 is associated with the development of a variety of tumors, but its function in cervical cancer is currently unclear. In this study, we addressed how expression of NCOA5 changed in the development of human cervical cancer and its association with clinicopathological features, prognosis, and biology characteristics of cervical cancer. Analysis of the microarrays in the Oncomine database indicated that NCOA5 expression was lower in human cervical squamous cell carcinoma tissues than that in normal cervical tissues. That was corroborated by our experiments using fresh tissues: the expression levels of NCOA5 messenger RNA and protein were both significantly decreased in cervical cancer tissues compared with paired adjacent nontumor tissues (P < 0.01). Low expression of NCOA5 is associated with the International Federation of Gynecology and Obstetrics stage ( P = 0.043) and histological grade ( P = 0.018) of human cervical cancer. In addition, patients possessing low NCOA5 expression had poorer prognosis. Univariate and multivariate Cox regression analyses indicated that low NCOA5 expression may be an independent prognostic factor for poorer overall survival in cervical cancer. Further, downregulation of NCOA5 expression results in a significant increase in proliferation, migration, and invasion of HeLa cells. Data of xenograft tumor on BALB/c nude mice manifested that HeLa cells with low NCOA5 expression tend to form larger tumors than negative control ones. In contrast, overexpression of NCOA5 expression leads to the opposite results. Finally, we found that NCOA5 might affect the biological function of human cervical cancer cells by mediating the notch3 signaling pathway. These findings suggest that NCOA5 acts as a tumor suppressor to inhibit tumorigenicity, migration, and invasion, and thus represents a potential novel prognostic marker for overall survival in cervical cancer.

CLDN-1 promoted the epithelial to migration and mesenchymal transition (EMT) in human bronchial epithelial cells via Notch pathway

Claudin-1 (CLDN-1) is one of main tight junction components that play an important role in epithelial-mesenchymal transition (EMT). However, the effects of CLDN-1 on the migration and EMT induced by TGF-β1 in primary normal human bronchial epithelial (NHBE) and BEAS-2B cells have not been clear. The expression of CLDN-1 was quantified by Western blotting in NHBE and BEAS-2B cells. Cell migration and invasion were detected using transwell assays. The expression level of E-cadherin, N-cadherin, α-SMA, and Vimentin was evaluated by quantitative real-time PCR and Western blotting. Here we showed that the protein expression of CLDN-1 was increased exposed to TGF-β1 in a dose- and time-dependent manner. Knockdown of CLDN-1 using small interfering CLDN-1 RNA (siCLDN-1) prevented the migration and invasion in NHBE and BEAS-2B cells. Moreover, depletion of CLDN-1 promoted the E-cadherin expression and decreased the mRNA and protein levels of N-cadherin, α-SMA, and Vimentin induced by TGF-β1. Furthermore, CLDN-1 silencing resulted in the reduction of the Notch intracellular domain (NICD) and hairy enhancer of split-1 (Hes-1) in mRNA and protein level. Jagged-1, an activator of Notch signaling pathway, abrogated the protective function of siCLDN-1 in migration and EMT. In conclusion, CLDN-1 promoted the migration and EMT through the Notch signaling pathway.

De novo transcriptome sequencing and analysis of freshwater snail (Radix balthica) to discover genes and pathways affected by exposure to oxazepam

Pharmaceuticals are increasingly found in aquatic ecosystems due to the non-efficiency of waste water treatment plants. Therefore, aquatic organisms are frequently exposed to a broad diversity of pharmaceuticals. Freshwater snail Radix balthica has been chosen as model to study the effects of oxazepam (psychotropic drug) on developmental stages ranging from trochophore to hatching. In order to provide a global insight of these effects, a transcriptome deep sequencing has been performed on exposed embryos. Eighteen libraries were sequenced, six libraries for three conditions: control, exposed to the lowest oxazepam concentration with a phenotypic effect (delayed hatching) (TA) and exposed to oxazepam concentration found in freshwater (TB). A total of 39,759,772 filtered raw reads were assembled into 56,435 contigs having a mean length of 1579.68 bp and mean depth of 378.96 reads. 44.91% of the contigs have at least one annotation. The differential expression analysis between the control condition and the two exposure conditions revealed 146 contigs differentially expressed of which 144 for TA and two for TB. 34.0% were annotated with biological function. There were four mainly impacted processes: two cellular signalling systems (Notch and JNK) and two biosynthesis pathways (Polyamine and Catecholamine pathways). This work reports a large-scale analysis of differentially transcribed genes of R. balthica exposed to oxazepam during egg development until hatching. In addition, these results enriched the de novo database of potential ecotoxicological models.

Dpp dependent Hematopoietic stem cells give rise to Hh dependent blood progenitors in larval lymph gland of Drosophila

Drosophila hematopoiesis bears striking resemblance with that of vertebrates, both in the context of distinct phases and the signaling molecules. Even though, there has been no evidence of Hematopoietic stem cells (HSCs) in Drosophila, the larval lymph gland with its Hedgehog dependent progenitors served as an invertebrate model of progenitor biology. Employing lineage-tracing analyses, we have now identified Notch expressing HSCs in the first instar larval lymph gland. Our studies clearly establish the hierarchical relationship between Notch expressing HSCs and the previously described Domeless expressing progenitors. These HSCs require Decapentapelagic (Dpp) signal from the hematopoietic niche for their maintenance in an identical manner to vertebrate aorta-gonadal-mesonephros (AGM) HSCs. Thus, this study not only extends the conservation across these divergent taxa, but also provides a new model that can be exploited to gain better insight into the AGM related Hematopoietic stem cells (HSCs).

Notch signaling in cerebrovascular diseases (Review)

The Notch signaling pathway is a crucial regulator of numerous fundamental cellular processes. Increasing evidence suggests that Notch signaling is involved in inflammation and oxidative stress, and thus in the progress of cerebrovascular diseases. In addition, Notch signaling in cerebrovascular diseases is associated with apoptosis, angiogenesis and the function of blood-brain barrier. Despite the contradictory results obtained to date as to whether Notch signaling is harmful or beneficial, the regulation of Notch signaling may provide a novel strategy for the treatment of cerebrovascular diseases.

Postnatal dysregulation of Notch signal disrupts dendrite development of adult-born neurons in the hippocampus and contributes to memory impairment

Deficits in the Notch pathway are involved in a number of neurologic diseases associated with mental retardation or/and dementia. The mechanisms by which Notch dysregulation are associated with mental retardation and dementia are poorly understood. We found that Notch1 is highly expressed in the adult-born immature neurons in the hippocampus of mice. Retrovirus mediated knockout of notch1 in single adult-born immature neurons decreases mTOR signaling and compromises their dendrite morphogenesis. In contrast, overexpression of Notch1 intracellular domain (NICD), to constitutively activate Notch signaling in single adult-born immature neurons, promotes mTOR signaling and increases their dendrite arborization. Using a unique genetic approach to conditionally and selectively knockout notch 1 in the postnatally born immature neurons in the hippocampus decreases mTOR signaling, compromises their dendrite morphogenesis, and impairs spatial learning and memory. Conditional overexpression of NICD in the postnatally born immature neurons in the hippocampus increases mTOR signaling and promotes dendrite arborization. These data indicate that Notch signaling plays a critical role in dendrite development of immature neurons in the postnatal brain, and dysregulation of Notch signaling in the postnatally born neurons disrupts their development and thus contributes to the cognitive deficits associated with neurological diseases.

Notch signaling indirectly promotes chondrocyte hypertrophy via regulation of BMP signaling and cell cycle arrest

Cell cycle regulation is critical for chondrocyte differentiation and hypertrophy. Recently we identified the Notch signaling pathway as an important regulator of chondrocyte proliferation and differentiation during mouse cartilage development. To investigate the underlying mechanisms, we assessed the role for Notch signaling regulation of the cell cycle during chondrocyte differentiation. Real-time RT-PCR data showed that over-expression of the Notch Intracellular Domain (NICD) significantly induced the expression of p57, a cell cycle inhibitor, in chondrocytes. Flow cytometric analyses further confirmed that over-expression of NICD in chondrocytes enhances the G0/G1 cell cycle transition and cell cycle arrest. In contrast, treatment of chondrocytes with the Notch inhibitor, DAPT, decreased both endogenous and BMP2-induced SMAD 1/5/8 phosphorylation and knockdown of SMAD 1/5/8 impaired NICD-induced chondrocyte differentiation and p57 expression. Co-immunoprecipitation using p-SMAD 1/5/8 and NICD antibodies further showed a strong interaction of these proteins during chondrocyte maturation. Finally, RT-PCR and Western blot results revealed a significant reduction in the expression of the SMAD-related phosphatase, PPM1A, following NICD over-expression. Taken together, our results demonstrate that Notch signaling induces cell cycle arrest and thereby initiates chondrocyte hypertrophy via BMP/SMAD-mediated up-regulation of p57.

Tips, stalks, tubes: notch-mediated cell fate determination and mechanisms of tubulogenesis during angiogenesis

Angiogenesis is the process of developing vascular sprouts from existing blood vessels. Luminal endothelial cells convert into "tip" cells that contribute to the development of a multicellular stalk, which then undergoes lumen formation. In this review, we consider a variety of cellular and molecular pathways that mediate these transitions. We focus first on Notch signaling in cell fate determination as a mechanism to define tip and stalk cells. We next discuss the current models of lumen formation and describe new players in this process, such as chloride intracellular channel proteins. Finally, we consider the possible medical therapeutic benefits of understanding these processes and acknowledge potential obstacles in drug development.

Notch1 modulates oxidative stress induced cell death through suppression of apoptosis signal-regulating kinase 1

Notch1 genes encode receptors for a signaling pathway that regulates various aspects of cell growth and differentiation; however, the role of Notch1 signaling in p38 mitogen-activated protein kinase (MAPK) signaling pathway is still not well defined. In this study, we found that Notch1 intracellular domain (Notch1-IC) prevents oxidative stress-induced cell death through the suppression of the Apoptosis signal-regulating kinase (ASK) 1 signaling pathway. Notch1-IC inhibited H2O2-induced activation of ASK1 and the activation of downstream kinases in the p38 MAPK signaling cascade. The results of both in vivo binding and kinase studies have revealed that ASK1 is the direct target of Notch1-IC, whereas it produced no effect on either MAP kinase kinase (MKK) 3 or p38 MAPK. Notch1-IC blocked both the homooligomerization of ASK1 and inhibited ASK1 activity. Furthermore, Notch1-IC facilitated the translocation of activated ASK1 toward the nucleus. Notch1 knockdown was determined to be highly susceptible to oxidative stress-induced activation of ASK1-MKK3/MKK6-p38 MAPK signaling cascade and cell death. Taken together, our findings suggest that Notch1-IC may act as a negative regulator in ASK1 signaling cascades.

VEGF and Notch in tip and stalk cell selection

Sprouting angiogenesis is a dynamic process in which endothelial cells collectively migrate, shape new lumenized tubes, make new connections, and remodel the nascent network into a hierarchically branched and functionally perfused vascular bed. Endothelial cells in the nascent sprout adopt two distinct cellular phenotypes--known as tip and stalk cells--with specialized functions and gene expression patterns. VEGF and Notch signaling engage in an intricate cross talk to balance tip and stalk cell formation and to regulate directed tip cell migration and stalk cell proliferation. In this article, we summarize the current knowledge and implications of the tip/stalk cell concepts and the quantitative and dynamic integration of VEGF and Notch signaling in tip and stalk cell selection.

Notch signalling pathways mediate synovial angiogenesis in response to vascular endothelial growth factor and angiopoietin 2

Objective

Notch signalling pathways are critical for angiogenesis and endothelial cell (EC) fate; however the mechanisms regulating these processes in the inflamed joint remain to be elucidated. Here, we examine whether Notch signalling mediates vascular endothelial growth factor (VEGF) and angiopoietin 2 (Ang2)-induced vascular function.

Methods

Notch-1 intracellular domain (Notch-1 IC), Notch-4 IC, Delta-like-ligand 4, Hes-related transcriptional repressors-1 and 2 (Hrt-1, Hrt-2) mRNA and/or protein expression was measured by Real-time PCR and/or western blot. VEGF/Ang2 induced EC function was assessed using transwell invasion chambers, matrigel tube formation assays and wound repair scratch assays ± Notch-1 siRNA or an γ-secretase inhibitor N-(N-(3,5-Difluorophenacetyl-L-alanly))-S-phenylglycine-t-Butyl Ester (DAPT) in RA synovial explants or human microvascular EC. Interleukin (IL)-6 and IL-8 were measured by ELISA and MMP2 and 9 by gelatine zymography.

Results

Notch-1 IC and Notch-4 IC protein expressions were demonstrated in RA and psoriatic arthritis synovial biopsies, with minimal expression observed in Osteoarthritis (OA). VEGF and Ang2 induced Notch-1 IC/ Notch-4 IC protein expression in synovial explant cultures and human microvascular EC levels were further potentiated by VEGF/Ang2 stimulation in combination. Notch-1, Delta-like-ligand 4, and Hrt-2 mRNA expression were significantly induced by VEGF and Ang2 alone and in combination. Furthermore VEGF/Ang2-induced EC invasion, angiogenesis and migration were inhibited by Notch-1 siRNA or DAPT. Conditioned media from VEGF/Ang2 stimulated RA synovial explants induced EC tube formation, an effect that was inhibited by DAPT. Finally, DAPT significantly decreased VEGF/Ang2 induced IL-6, IL-8, MMP2 and 9 expressions in RA synovial explants.

Conclusions

Notch-1 mediates VEGF/Ang2-induced angiogenesis and EC invasion in inflammatory arthritis.

Wnt5a controls Notch1 signaling through CaMKII-mediated degradation of the SMRT corepressor protein

Serine-threonine Ca(2+)/calmodulin-dependent protein kinase II (CaMKII) is the key component in noncanonical Wnt5a signaling and has been shown to regulate its signaling. In this study, we found that CaMKII induced by Wnt5a remarkably reduced the protein stability of the silencing mediator of retinoic acid and thyroid hormone receptor (SMRT), a co-repressor of Notch signaling, through proteasomal degradation. Wnt5a was found to enhance Notch1 intracellular domain (Notch1-IC) transcription activity, which could be inhibited by treatment with KN93, a CaMKII inhibitor. The kinase activity of CaMKII was essential for the activation of Notch signaling. We also determined that CaMKII could enhance the association between Notch1-IC and RBP-Jk. Furthermore, the physical association between RBP-Jk and SMRT was substantially suppressed by CaMKII. We demonstrated that CaMKII directly bound and phosphorylated SMRT at Ser-1407, thereby facilitating SMRT translocation from the nucleus to the cytoplasm and proteasome-dependent degradation. These results suggest that CaMKII down-regulated the protein stability of SMRT through proteasomal degradation.

Presenilin-2 regulates the degradation of RBP-Jk protein through p38 mitogen-activated protein kinase

Transcriptional regulation performs a central role in Notch1 signaling by recombining binding protein Suppressor of Hairless (RBP-Jk)--a signaling pathway that is widely involved in determination of cell fate. Our earlier work demonstrated the possible regulation of the Notch1-RBP-Jk pathway through protein degradation of RBP-Jk; however, the potential regulator for the degradation of RBP-Jk remains to be determined. Here, we report that the expression of endogenous and exogenous RBP-Jk was increased significantly in cells treated with proteasome- and lysosome-specific inhibitors. The effects of these inhibitors on RBP-Jk occurred in a dose- and time-dependent manner. The level of RBP-Jk protein was higher in presenilin-2 (PS2)-knockout cells than in presenilin-1 (PS1)-knockout cells. Furthermore, the level of RBP-Jk was decreased by expression of PS2 in PS1 and PS2 double-knockout cells. We also found that PS1-knockout cells treated with a specific inhibitor of p38 mitogen-activated protein kinase ∂ (MAPK) had significantly increased levels of RBP-Jk. p38 MAPK phosphorylates RBP-Jk at Thr339 by physical binding, which subsequently induces the degradation and ubiquitylation of the RBP-Jk protein. Collectively, our results indicate that PS2 modulates the degradation of RBP-Jk through phosphorylation by p38 MAPK.

Dual regulation of notch1 signaling pathway by adaptor protein fe65

Notch1 receptor functions as a critical controller of cell fate decisions and also as a key regulator of cell growth, differentiation, and proliferation in invertebrates and vertebrates. In this study, we have demonstrated that the adaptor protein Fe65 attenuates Notch1 signaling via the accelerated degradation of the membrane-tethered Notch1 in the cytoplasm. Fe65 also suppresses Notch1 transcriptional activity via the dissociation of the Notch1-IC-recombining binding protein suppressor of hairless (RBP)-Jk complex within the nucleus. Fe65 is capable of forming a trimeric complex with Itch and membrane-tethered Notch1, and Fe65 enhances the protein degradation of membrane-tethered Notch1 via an Itch-dependent proteasomal pathway. Collectively, our results demonstrate that Fe65 carries out different functions depending on its location in the regulation of Notch1 signaling.

A role for Notch signaling in trophoblast endovascular invasion and in the pathogenesis of pre-eclampsia

Placental trophoblasts (TBs) invade and remodel uterine vessels with an arterial bias. This process, which involves vascular mimicry, re-routes maternal blood to the placenta, but fails in pre-eclampsia. We investigated Notch family members in both contexts, as they play important roles in arterial differentiation/function. Immunoanalyses of tissue sections showed step-wise modulation of Notch receptors/ligands during human TB invasion. Inhibition of Notch signaling reduced invasion of cultured human TBs and expression of the arterial marker EFNB2. In mouse placentas, Notch activity was highest in endovascular TBs. Conditional deletion of Notch2, the only receptor upregulated during mouse TB invasion, reduced arterial invasion, the size of maternal blood canals by 30-40% and placental perfusion by 23%. By E11.5, there was litter-wide lethality in proportion to the number of mutant offspring. In pre-eclampsia, expression of the Notch ligand JAG1 was absent in perivascular and endovascular TBs. We conclude that Notch signaling is crucial for TB vascular invasion.

Regulation of Notch1 signaling by Delta-like ligand 1 intracellular domain through physical interaction

Notch signaling involves the proteolytic cleavage of the transmembrane Notch receptor after binding to its transmembrane ligands. The Delta-like ligand 1 also undergoes proteolytic cleavage upon Notch binding, resulting in the production of a free intracellular domain. In this study, we have demonstrated that the Delta-like 1 intracellular domain (Dll1-IC) specifically binds to Notch1-IC in the nucleus, thereby disrupting the association of the Notch1-IC-RBP-Jk-MAM transcription activator complex. Additionally, the Notch1-mediated blockage of the induction of MyoD is abolished by the co-expression of Dll1-IC. Collectively, our results show that Dll1-IC functions as a negative regulator in Notch signaling via the disruption of the Notch1-IC-RBP-Jk complex.

Regulation of Notch1 signaling by the APP intracellular domain facilitates degradation of the Notch1 intracellular domain and RBP-Jk

The Notch1 receptor is a crucial controller of cell fate decisions, and is also a key regulator of cell growth and differentiation in a variety of contexts. In this study, we have demonstrated that the APP intracellular domain (AICD) attenuates Notch1 signaling by accelerated degradation of the Notch1 intracellular domain (Notch1-IC) and RBP-Jk, through different degradation pathways. AICD suppresses Notch1 transcriptional activity by the dissociation of the Notch1-IC-RBP-Jk complex after processing by γ-secretase. Notch1-IC is capable of forming a trimeric complex with Fbw7 and AICD, and AICD enhances the protein degradation of Notch1-IC through an Fbw7-dependent proteasomal pathway. AICD downregulates the levels of RBP-Jk protein through the lysosomal pathway. AICD-mediated degradation is involved in the preferential degradation of non-phosphorylated RBP-Jk. Collectively, our results demonstrate that AICD functions as a negative regulator in Notch1 signaling through the promotion of Notch1-IC and RBP-Jk protein degradation.

Inhibition of Notch1 signaling by Runx2 during osteoblast differentiation

Notch1 genes encode receptors for a signaling pathway that regulates cell growth and differentiation in various contexts, but the role of Notch1 signaling in osteogenesis is not well defined. Notch1 controls osteoblast differentiation by affecting Runx2, but the question arises whether normal osteoblastic differentiation can occur regardless of the presence of Notch1. In this study, we observed the downregulation of Notch1 signaling during osteoblastic differentiation. BMPR-IB/Alk6-induced Runx2 proteins reduced Notch1 activity to a marked degree. Accumulated Runx2 suppressed Notch1 transcriptional activity by dissociating the Notch1-IC-RBP-Jk complex. Using deletion mutants, we also determined that the N-terminal domain of Runx2 was crucial to the binding and inhibition of the N-terminus of the Notch1 intracellular domain. Notably, upregulation of the Runx2 protein level paralleled reduced expression of Hes1, which is a downstream target of Notch1, during osteoblast differentiation. Collectively, our data suggest that Runx2 is an inhibitor of the Notch1 signaling pathway during normal osteoblast differentiation.

Up-regulated expression of Notch1 and Jagged1 in human colon adenocarcinoma

Deregulated expression of Notch molecules is observed in many malignant tumors, however, the expression of Notch1 and Jagged1 in colon adenocarcinoma is still unknown. This study is to investigate the expression of Notch1 and Jagged1 in human colon adenocarcinoma. Sixty-five human colon adenocarcinoma and 60 adjacent nontumor colon tissue sections were detected by immunohistochemistry. Ten paired fresh surgical human colon adenocarcinoma and adjacent nontumor colon samples were analyzed by Western blot and RT-PCR. Both Notch1 and Jagged1 were expressed in the cytoplasm of neoplastic cells of colon adenocarcinoma tissue. The protein and mRNA levels of both molecules were higher in colon adenocarcinoma than in adjacent nontumor tissue. Moreover, Notch1 was positively correlated with tumor stage. This investigation demonstrates that Notch1 and Jagged1 are up-regulated in human colon adenocarcinoma and suggests that Notch1/Jagged1 signaling might play a role in the development of colon adenocarcinoma.

Serum- and glucocorticoid-inducible kinase 1 (SGK1) controls Notch1 signaling by downregulation of protein stability through Fbw7 ubiquitin ligase

Notch is a transmembrane protein that acts as a transcriptional factor in the Notch signaling pathway for cell survival, cell death and cell differentiation. Notch1 and Fbw7 mutations both lead the activation of the Notch1 pathway and are found in the majority of patients with the leukemia T-ALL. However, little is known about the mechanisms and regulators that are responsible for attenuating the Notch signaling pathway through Fbw7. Here, we report that the serum- and glucocorticoid-inducible protein kinase SGK1 remarkably reduced the protein stability of the active form of Notch1 through Fbw7. The protein level and transcriptional activity of the Notch1 intracellular domain (Notch1-IC) were higher in SGK1-deficient cells than in SGK1 wild-type cells. Notch1-IC was able to form a trimeric complex with Fbw7 and SGK1, thereby SGK1 enhanced the protein degradation of Notch1-IC via a Fbw7-dependent proteasomal pathway. Furthermore, activated SGK1 phosphorylated Fbw7 at serine 227, an effect inducing Notch1-IC protein degradation and ubiquitylation. Moreover, accumulated dexamethasone-induced SGK1 facilitated the degradation of Notch1-IC through phosphorylation of Fbw7. Together our results suggest that SGK1 inhibits the Notch1 signaling pathway via phosphorylation of Fbw7.

A potential role for notch signaling in the pathogenesis and regulation of hemangiomas

Hemangiomas are the most common benign tumor of infancy, yet its pathogenesis and the mechanisms governing proliferation and involution are not well understood. It is believed that hemangiomas arise out of clonal, abnormal hemangioma endothelial cells (HemECs). The underlying anomaly of the HemEC is not known, although studies have shown that vascular endothelial growth factor (VEGF) and VEGF signaling may influence HemECs. Moreover, there are numerous subtypes of hemangiomas, with differences in natural history, potential for morbidity, and prognosis, and little is known how this relates to HemEC. The Notch signaling pathway is a highly conserved pathway across species from worms to mammals. Notch signaling has been shown to play a role during embryogenesis in directing vascular patterning and development and arterial and venous cell fate determination. Postnatally, it has been implicated in tumor angiogenesis in multiple malignancies. Notch signaling triggers tumor angiogenesis at least in part to stimulation by VEGF, thus establishing that there is a cross talk between the VEGF and Notch pathways. Given the presence of VEGF and its receptors in hemangiomas and known VEGF-Notch cross talk in tumor angiogenesis, the authors hypothesize that Notch signaling may contribute to hemangioma proliferation and involution. Preliminary studies of resected hemangioma specimens by reverse transcription polymerase chain reaction (RT-PCR) show that all 4 Notch receptors and 2 Notch ligands, Jagged1 and Delta-like ligand 4, are expressed by hemangiomas. These findings support a role for Notch in hemangiomas, meriting further analysis of the functional relevance of Notch signaling in hemangiomas.

Role of ephrinB2 in nonproductive angiogenesis induced by Delta-like 4 blockade

Delta-like 4 (DLL4) is one of the Notch ligands and plays an important role in vascular development. DLL4 blockade inhibits tumor growth by promoting nonproductive angiogenesis, which is characterized by an increase in vascular density and decrease in tissue perfusion. However, a detailed mechanism remains unclear. In this study, newly developed neutralizing antibodies against mouse and human DLL4 were used to investigate the possible involvement of VEGF-DLL4-ephrinB2 cascade in nonproductive angiogenesis caused by DLL4 blockade. DLL4 blockade and soluble ephrinB2 treatment suppressed tumor growth and induced nonproductive angiogenesis. DLL4 was expressed in subcutaneous tumors, and DLL4 blockade suppressed ephrinB2 expression in the tumors. DLL4 blockade significantly promoted human umbilical vein endothelial cell (HUVEC) proliferation in vitro, and the effect was additive to that of VEGF. Both DLL4 blockade and VEGF significantly increased cord length and branch points in a tubular formation assay. Expression of ephrinB2 in HUVECs was enhanced by VEGF alone, and the enhancement was inhibited by DLL4 blockade. Moreover, when we studied the effect of ephrinB2 RNA interference on HUVEC tubular formation, knockdown of ephrinB2 mimicked the effect of DLL4. These results suggest that ephrinB2 plays a crucial role in nonproductive angiogenesis caused by DLL4 blockade.

Notch alters VEGF responsiveness in human and murine endothelial cells by direct regulation of VEGFR-3 expression

The Notch family of cell surface receptors and its ligands are highly conserved proteins that regulate cell fate determination, including those involved in mammalian vascular development. We report that Notch induces VEGFR-3 expression in vitro in human endothelial cells and in vivo in mice. In vitro, Notch in complex with the DNA-binding protein CBF-1/suppressor of hairless/Lag1 (CSL) bound the VEGFR-3 promoter and transactivated VEGFR-3 specifically in endothelial cells. Through induction of VEGFR-3, Notch increased endothelial cell responsiveness to VEGF-C, promoting endothelial cell survival and morphological changes. In vivo, VEGFR-3 was upregulated in endothelial cells with active Notch signaling. Mice heterozygous for null alleles of both Notch1 and VEGFR-3 had significantly reduced viability and displayed midgestational vascular patterning defects analogous to Notch1 nullizygous embryos. We found that Notch1 and Notch4 were expressed in normal and tumor lymphatic endothelial cells and that Notch1 was activated in lymphatic endothelium of invasive mammary micropapillary carcinomas. These results demonstrate that Notch1 and VEGFR-3 interact genetically, that Notch directly induces VEGFR-3 in blood endothelial cells to regulate vascular development, and that Notch may function in tumor lymphangiogenesis.

Deregulated expression of Notch receptors in human hepatocellular carcinoma

BACKGROUND AND AIM

Notch signaling controls cellular differentiation and proliferation. Deregulated expression of Notch receptors is observed in a growing number of malignant tumours, however, the role of Notch signaling in hepatocellular carcinoma is still unknown. To address this, the expression of Notch receptors in human hepatocellular carcinoma was examined in both protein and ribonucleic acid levels.

PATIENTS AND METHODS

Fifty-three hepatocellular carcinoma tissue sections were detected by immunohistochemistry. Three paired fresh surgical hepatocellular carcinoma and adjacent nontumour liver samples were analyzed by Western blot and reverse transcriptase polymerase chain reaction. Immunohistochemistry, Western blot and reverse transcriptase polymerase chain reaction are reliable methods to examine the expression of protein and RNA.

RESULTS

All of the four Notch receptors were expressed in the neoplastic cells of hepatocellular carcinoma tissues with different intensity and extensity. Notch1 and Notch4 were expressed in both cytoplasm and nucleus, and all of the nuclear staining showed up in the cytoplasm-positive cases. Cytoplasmic and nuclear Notch1 was detected in 88.7% (47/53) and 9.4% (5/53) of hepatocellular carcinoma tissues, respectively; positive rates of Notch4 were 67.9% (36/53) in cytoplasm and 52.8% (31/53) in nucleus. Notch2 and Notch3 were only in cytoplasm, with positive rates of 26.4% (14/53) and 52.8% (28/53), respectively. Compared with adjacent nontumour liver, Notch1 (cytoplasmic) and Notch4 (nuclear) were up-regulated (P<0.05, P<0.05), Notch2 was down-regulated (P<0.05), while Notch1 (nuclear), Notch3 and Notch4 (cytoplasmic) showed no difference between hepatocellular carcinoma and adjacent nontumour liver. Western blot and reverse transcriptase polymerase chain reaction analysis showed a consistent result.

CONCLUSION

Our findings indicate that the expression of Notch receptors was deregulated and Notch signaling might be involved in the development of hepatocellular carcinoma.

Strategies to regenerate hair cells: identification of progenitors and critical genes

Deafness commonly results from a lesion of the sensory cells and/or of the neurons of the auditory part of the inner ear. There are currently no treatments designed to halt or reverse the progression of hearing loss. A key goal in developing therapy for sensorineural deafness is the identification of strategies to replace lost hair cells. In amphibians and birds, a spontaneous post-injury regeneration of all inner ear sensory hair cells occurs. In contrast, in the mammalian cochlea, hair cells are only produced during embryogenesis. Many studies have been carried out in order to demonstrate the persistence of endogenous progenitors. The present review is first focused on the occurrence of spontaneous supernumerary hair cells and on nestin positive precursors found in the organ of Corti. A second approach to regenerating hair cells would be to find genes essential for their differentiation. This review will also focus on critical genes for embryonic hair cell formation such as the cell cycle related proteins, the Atoh1 gene and the Notch signaling pathway. Understanding mechanisms that underlie hair cell production is an essential prerequisite to defining therapeutic strategies to regenerate hair cells in the mature inner ear.

Signaling pathways and preimplantation development of mammalian embryos

The mammalian preimplantation embryo is a critical and unique stage in embryonic development. This stage includes a series of crucial events: the transition from oocyte to embryo, the first cell divisions, and the establishment of cellular contacts. These events are regulated by multiple signal-transduction pathways. In this article we describe patterns of stage-specific expression in several signal-transduction pathways and try to give a profile of the signaling transduction network in preimplantation development of mammalian embryo.

Tip60 histone acetyltransferase acts as a negative regulator of Notch1 signaling by means of acetylation

The Notch signaling pathway appears to perform an important function in a wide variety of organisms and cell types. In our present study, we provide evidence that UV irradiation-induced Tip60 proteins reduced Notch1 activity to a marked degree. Accumulated UV irradiation-induced Tip60 suppresses Notch1 transcriptional activity via the dissociation of the Notch1-IC-CSL complex. The binding between endogenous Tip60 and Notch1-IC in UV radiation-exposed cells was verified in this study by coimmunoprecipitation. Interestingly, the physical interaction of Tip60 with Notch1-IC occurs to a more profound degree in the presence of CSL but does not exist in a trimeric complex. Using Notch1-IC and Tip60 deletion mutants, we also determined that the N terminus, which harbors the RAM domain and seven ankyrin repeats of Notch1-IC, interacts with the zinc finger and acetyl coenzyme A domains of Tip60. Furthermore, here we report that Notch1-IC is a direct target of the acetyltransferase activity of Tip60. Collectively, our data suggest that Tip60 is an inhibitor of the Notch1 signaling pathway and that Tip60-dependent acetylation of Notch1-IC may be relevant to the mechanism by which Tip60 suppresses Notch1 signaling.

Up-regulation of endothelial delta-like 4 expression correlates with vessel maturation in bladder cancer

PURPOSE

Angiogenesis and vascular endothelial growth factor (VEGF) expression are associated with a poor outcome in bladder cancer. To understand more about the mechanisms, we studied the role of delta-like 4 (DLL4), an endothelial-specific ligand of the Notch signaling pathway, in bladder cancer angiogenesis.

EXPERIMENTAL DESIGN

The expression of DLL4, CD34, and VEGF were studied in a cohort of 60 bladder tumors and 10 normal samples using quantitative PCR. In situ hybridization was used to study the pattern of DLL4 expression in 22 tumor and 9 normal samples. Serial sections were also stained for CD34 and alpha-smooth muscle actin (alpha-SMA) using conventional immunohistochemistry.

RESULTS

The expression of DLL4 was significantly up-regulated in superficial (P < 0.01) and invasive (P < 0.05) bladder cancers. DLL4 expression significantly correlated with CD34 (P < 0.001) and VEGF (P < 0.001) expression. The in situ hybridization studies showed that DLL4 was highly expressed within bladder tumor vasculature. Additionally, DLL4 expression significantly correlated with vessel maturation as judged by periendothelial cell expression of alpha-SMA, 98.7% of DLL4-positive tumor vessels coexpressed alpha-SMA, compared with 64.5% of DLL4-negative tumor vessels (P < 0.001). High DLL4 expression may have prognostic value in superficial and invasive bladder.

CONCLUSION

DLL4 expression is associated with vascular differentiation in bladder cancer; thus, targeting DLL4 may be a novel antiangiogenic therapy.

Notch1 signaling influences v2 interneuron and motor neuron development in the spinal cord

The Notch signaling pathway plays a variety of roles in cell fate decisions during development. Previous studies have shown that reduced Notch signaling results in premature differentiation of neural progenitor cells, while increased Notch activities promote apoptotic death of neural progenitor cells in the developing brain. Whether Notch signaling is involved in the specification of neuronal subtypes is unclear. Here we examine the role of Notch1 in the development of neuronal subtypes in the spinal cord using conditional knockout (cKO) mice lacking Notch1 specifically in neural progenitor cells. Notch1 inactivation results in accelerated neuronal differentiation in the ventral spinal cord and gradual disappearance of the ventral central canal. These changes are accompanied by reduced expression of Hes1 and Hes5 and increased expression of Mash1 and Neurogenin 1 and 2. Using markers (Nkx2.2, Nkx6.1, Olig2, Pax6 and Dbx1) for one or multiple progenitor cell types, we found reductions of all subtypes of progenitor cells in the ventral spinal cord of Notch1 cKO mice. Similarly, using markers (Islet1/2, Lim3, Sim1, Chox10, En1 and Evx1/2) specific for motor neurons and distinct classes of interneurons, we found increases in the number of V0-2 interneurons in the ventral spinal cord of Notch1 cKO mice. Specifically, the number of Lim3+/Chox10+ V2 interneurons is markedly increased while the number of Lim3+/Islet+motor neurons is decreased in the Notch1 cKO spinal cord, suggesting that V2 interneurons are generated at the expense of motor neurons in the absence of Notch1. These results provide support for a role of Notch1 in neuronal subtype specification in the ventral spinal cord.

Up-regulation of delta-like 4 ligand in human tumor vasculature and the role of basal expression in endothelial cell function

The Notch signaling pathway and the delta-like 4 ligand (DLL4) play key roles in embryonic vascular development. Many of the pathways involved in embryonic vascular development also play important roles in tumor angiogenesis. In this study, we assessed the expression of DLL4 in primary renal cancer and investigated the biological function of DLL4 in primary endothelial cells. Using real-time quantitative PCR and in situ hybridization, we showed that the expression of DLL4 was up-regulated within the vasculature of clear cell-renal cell carcinoma almost 9-fold more than normal kidney and was correlated with the expression of vascular endothelial growth factor (VEGF). The expression of DLL4 in endothelial cells was up-regulated by VEGF and basic fibroblast growth factor synergistically, and by hypoxia through hypoxia-inducible factor 1alpha. Down-regulation of DLL4 expression with RNA interference led to decreased expression of HEY1 and EphrinB2, and the inhibition of endothelial cell proliferation, migration, and network formation, all of which are important processes in tumor angiogenesis. The inhibition of proliferation occurred via the induction of cell cycle arrest in G0-G1 by increased expression of p21 and decreased phosphorylation of retinoblastoma. We conclude that an optimal window of the DLL4 expression is essential for tumor angiogenesis and that selective modulation of the DLL4 expression within human tumors may represent a potential novel antiangiogenic therapy.

Subdividing the embryo: a role for Notch signaling during germ layer patterning in Xenopus laevis

The development of all vertebrate embryos requires the establishment of a three-dimensional coordinate system in order to pattern embryonic structures and create the complex shape of the adult organism. During the process of gastrulation, the three primary germ layers are created under the guidance of numerous signaling pathways, allowing cells to communicate during development. Cell-cell communication, mediated by receptors of the Notch family, has been shown to be involved in mediating diverse cellular behaviors during development and has been implicated in the regulation of cell fate decisions in both vertebrate and invertebrate organisms. In order to investigate a role for Notch signaling during boundary formation between the mesoderm and endoderm during gastrulation, we manipulated Notch signaling in gastrula stage embryos and examined gene expression in resultant tissues and organs. Our findings demonstrate a much broader role for Notch signaling during germ layer determination than previously reported in a vertebrate organism. Activation of the Notch pathway, specifically in gastrula stage embryos, results in a dramatic decrease in the expression of genes necessary to create many different types of mesodermal tissues while causing a dramatic expansion of endodermal tissue markers. Conversely, temporally controlled suppression of this pathway results in a loss of endodermal cell types and an expansion of molecular markers of mesoderm. Thus, our data are consistent with and significantly extend the implications of prior observations suggesting roles for Notch signaling during germ layer formation and establish an evolutionarily conserved role for Notch signaling in mediating mesoderm-endoderm boundaries during early vertebrate development.

Notch signaling in vascular morphogenesis

PURPOSE OF REVIEW

This review highlights recent developments in the role of the Notch signaling pathway during vascular morphogenesis, angiogenesis, and vessel homeostasis.

RECENT FINDINGS

Studies conducted over the past 4 years have significantly advanced the understanding of the effect of Notch signaling on vascular development. Major breakthroughs have elucidated the role of Notch in arterial versus venular specification and have placed this pathway downstream of vascular endothelial growth factor.

SUMMARY

An emerging hallmark of the Notch signaling pathway is its nearly ubiquitous participation in cell fate decisions that affect several tissues, including epithelial, neuronal, hematopoietic, and muscle. The vascular compartment has been the latest addition to the list of tissues known to be regulated by Notch. Unraveling the contribution of Notch signaling to blood vessel formation has resulted principally from gain-of-function and loss-of-function experiments in mouse and zebrafish. During the past 4 years, these mechanistic studies have revealed that Notch is required for the successful completion of several steps during vascular morphogenesis and differentiation. In addition, the findings that Notch mutations are linked to some late-onset hereditary vascular pathologic conditions suggest the added contribution of this signaling pathway to vascular homeostasis.

Developmental expression of the Notch signaling pathway genes during mouse preimplantation development

Notch signaling is an evolutionary conserved pathway involved in intercellular signaling and essential for proper cell fate choices during development. Thus, it could be involved in mouse preimplantation development where intercellular signaling plays a crucial role, particularly between the inner cell mass and the trophectoderm of the blastocyst. At their face value, the phenotypes observed when disrupting each of the four Notch genes known in the mouse do not support this view as none of them involves perturbation of preimplantation development. However this could be due to functional redundancy and/or maternal expression. As a first step to address this issue, we decided to examine the expression in early development of various genes known to participate in Notch signaling. Here, we report on the expression pattern of Notch1-4, Jagged1 (Jag1), Jag2, Delta-like1 (Dll-1), Dll-3, Dll-4, Rbpsuh, Deltex1(Dtx1)and Dtx2 genes during preimplantation development from unfertilized eggs until late blastocyst stage using a RT-PCR strategy. We show that Notch1, 2, Jag1-2, Dll-3, Rbpsuh and Dtx2 transcripts are expressed at all stages. Notch4 and Dll-4 mRNAs are synthesized from the 2-cell through to the hatched blastocyst stage. Notch3, Dll-1 and Dtx1exhibit a stage dependent expression as their mRNAs are detected in 2-cell embryos and in hatched blastocysts, but are absent or weakly detected at the morula stage. Finally, we show that all the above genes are expressed both in Embryonic and Trophoblast Stem cells (ES and TS cells, respectively). Our results suggest that the Notch pathway may be active during mouse preimplantation development.

The intracellular domain of X-Serrate-1 is cleaved and suppresses primary neurogenesis in Xenopus laevis

The Notch ligands, Delta/Serrate/Lag-2 (DSL) proteins, mediate the Notch signaling pathway in a numerous developmental processes in multicellular organisms. Although the ligands induce the activation of the Notch receptor, the intracellular domain-deleted forms of the ligands cause dominant-negative phenotypes, implying that the intracellular domain is necessary for the Notch signal transduction. Here we examined the role of the intracellular domain of Xenopus Serrate (XSICD) in Xenopus embryos. X-Serrate-1 has the putative nuclear localization sequence (NLS) in downstream of the transmembrane domain. Biochemical analysis revealed that XSICD fragments are cleaved from the C-terminus side of X-Serrate-1. Fluorescence microscopic analysis showed that the nuclear localization of XSICD occurs in the neuroectoderm of the embryo injected with the full-length X-Serrate-1/GFP. Overexpression of XSICD showed the inhibitory effect on primary neurogenesis. However, a point mutation in the NLSs of XSICD inhibited the nuclear localization of XSICD, which caused the induction of a neurogenic phenotype. The animal cap assay revealed that X-Serrate-1 suppresses primary neurogenesis in neuralized animal cap, but X-Delta-1 does not. Moreover, XSICD could not activate the expression of the canonical Notch target gene, XESR-1 in contrast to the case of full-length X-Serrate-1. These results suggest that the combination of XSICD-mediated intracellular signaling and the extracellular domain of Notch ligands-mediated activation of Notch receptor is involved in the primary neurogenesis. Moreover, we propose a bi-directional signaling pathway mediated by X-Serrate-1 in Notch signaling.

Notch function in the vasculature: insights from zebrafish, mouse and man

Vascular development entails multiple cell-fate decisions to specify a diverse array of vascular structures. Notch proteins are signaling receptors that regulate cell-fate determination in a variety of cell types. The finding that Notch genes are robustly expressed in the vasculature suggests roles for Notch in guiding endothelial and associated mural cells through the myriad of cell-fate decisions needed to form the vasculature. In fact, mice with defects in genes encoding Notch, Notch ligands, and components of the Notch signaling cascade invariably display vascular defects. Human Notch genes are linked to Alagille's Syndrome, a developmental disorder with vascular defects, and CADASIL, a cerebral arteriopathy. Studies in zebrafish, mice and humans indicate that Notch works in conjunction with other angiogenic pathways to pattern and stabilize the vasculature. Here, we will focus on established functions for Notch in vascular remodeling and arterial/venous specification and more speculative roles in vascular homeostasis and organ-specific angiogenesis.

Notch activation induces apoptosis in neural progenitor cells through a p53-dependent pathway

Notch signaling is involved in a variety of cell-fate decisions during development. Here we investigate the role of Notch signaling in apoptotic cell death of neural progenitors through the generation and analysis of cell type-specific conditional transgenic and knockout mice. We show that conditional expression of a constitutively active form of Notch1 in early neural progenitor cells, but not postmitotic neurons, selectively induces extensive apoptosis, resulting in a markedly reduced progenitor population. Conversely, attenuation of Notch signaling in Notch1 conditional knockout or Presenilin-1-/- mice results in reduced apoptosis of early neural progenitor cells. Furthermore, Notch activation in neural progenitor cells leads to elevated levels of nuclear p53 and transcriptional upregulation of the target genes Bax and Noxa, and the promotion of apoptotic cell death by Notch activation is completely suppressed by p53 deficiency. Together, these complementary gain-of-function and loss-of-function studies reveal a previously unappreciated role of Notch signaling in the regulation of apoptotic cell death during early mammalian neural development.

The non-transmembrane form of Delta1, but not of Jagged1, induces normal migratory behavior accompanied by fibroblast growth factor receptor 1-dependent transformation

The interactions between Notch (N) receptors and their transmembrane ligands, Jagged1 (JI) and Delta1 (Dl1), mediate signaling events between neighboring cells that are crucial during embryonal development and in adults. Since the non-transmembrane extracellular form of J1 acts as an antagonist of N activation in NIH 3T3 mouse fibroblast cells and induces fibroblast growth factor 1 (FGF1)-dependent transformation (Small, D., Kovalenko, D., Soldi, R., Mandinova, A., Kolev, V., Trifonova, R., Bagala, C., Kacer, D., Battelli, C., Liaw, L., Prudovsky, I., and Maciag, T. (2003) J. Biol. Chem. 278, 16405-16413), we examined the potential redundant functions of the two subfamilies of Notch ligands and report that while the soluble (s) forms of both Dl1 and J1 act as N signaling antagonists in NIH 3T3 cells, they do display disparate functions. While sJ1 induced an attenuation of cell motility which is accompanied by a decrease in actin stress fibers and an increase in adherence junctions, sDl1 does not. However, sJ1, like sDl1, induces a NIH 3T3 cell tranformed phenotype mediated by FGF signaling. Because the inhibition of N signaling by sJ1 and sDl1 is rescued by dominant-negative Src expression, we suggest that there may be cooperation between the Notch and Src signaling pathways.

Active form of Notch members can enforce T lymphopoiesis on lymphoid progenitors in the monolayer culture specific for B cell development

The in vitro induction of T lymphopoiesis needs the precise stereoscopic structure of thymus tissues as seen in fetal thymus organ culture. In this study, we demonstrated for the first time that the introduction of the intracellular region of Notch1 can induce T cells expressing TCR without any thymic environment. In the coculture on the monolayer of OP-9, which was originally known to support B cell specific development, hemopoietic progenitors developed into Thy-1(+)CD25(+) T lineage cells if the progenitor cells were infected with the retrovirus containing Notch1 intracellular domains. The Thy-1(+) cells progressed to a further developmental stage, CD4 and CD8 double-positive cells expressing TCR on the cell surface, if they were further cultured on OP-9 or in the thymus. However, T cell induction by intracellular Notch1 failed unless both OP-9 and IL-7 were present. It is notable that Notch2 and Notch3 showed an effect on T lymphopoiesis similar to that of Notch1. These results indicate that in vitro T lymphopoiesis is inducible by signaling via Notch family members in a lineage-specific manner but shares other stroma-derived factors including IL-7 with B lymphopoiesis.

Sequential activation of Notch family receptors during mouse spermatogenesis

The expression pattern of Notch family receptors during mouse spermatogenesis was examined by immunohistochemistry. The entire cytoplasm of spermatogonia, spermatocytes and spermatids showed staining with antibodies against extracellular domains of Notch1, 2 and 4. In contrast, the nuclei of spermatogonia showed staining with an antibody against the intracellular domain of Notch3, and the nuclei of spermatocytes and spermatids showed staining with antibodies against the intracellular domains of Notch1 and 4. During regeneration of spermatogonia in busulfan-treated mice, the nuclei of all proliferating cells showed staining for the intracellular domain of Notch3. Western blot analysis showed that the molecular weights of the intracellular domains of Notch1 and 3 localizing in the nuclear fraction were smaller than those in the cytoplasmic fraction. This was consistent with the theory that the intracellular domain of Notch was cleaved in the cytoplasm and translocated to the nucleus. These results suggest that different Notch signals are sequentially activated during mouse spermatogenesis and control the proliferation and differentiation of spermatogenic stem cells.

Identification of a family of mastermind-like transcriptional coactivators for mammalian notch receptors

The molecular mechanisms by which Notch receptors induce diverse biological responses are not fully understood. We recently cloned a mammalian homologue of the Mastermind gene of Drosophila melanogaster, MAML1 (Mastermind-like-1 molecule) and determined that it functions as a transcriptional coactivator for Notch receptors. In this report, we characterize two additional genes in this Mastermind-like gene family: MAML2 and MAML3. The three MAML genes are widely expressed in adult tissues but exhibit distinct expression patterns in mouse early spinal cord development. All MAML proteins localize to nuclear bodies, share a conserved basic domain in their N termini that binds to the ankyrin repeat domain of Notch, and contain a transcriptional activation domain in their C termini. Moreover, as determined by using coimmunoprecipitation assays, each MAML protein was found to be capable of forming a multiprotein complex with the intracellular domain of each Notch receptor (ICN1 to -4) and CSL in vivo. However, MAML3 bound less efficiently to the ankyrin repeat domain of Notch1. Also, in U20S cells, whereas MAML1 and MAML2 functioned efficiently as coactivators with each of the Notch receptors to transactivate a Notch target HES1 promoter construct, MAML3 functioned more efficiently with ICN4 than with other forms of ICN. Similarly, MAML1 and MAML2 amplified Notch ligand (both Jagged2 and Delta1)-induced transcription of the HES-1 gene, whereas MAML3 displayed little effect. Thus, MAML proteins may modify Notch signaling in different cell types based on their own expression levels and differential activities and thereby contribute to the diversity of the biological effects resulting from Notch activation.

Down-regulation of Delta by proteolytic processing

Notch signaling regulates cell fate decisions during development through local cell interactions. Signaling is triggered by the interaction of the Notch receptor with its transmembrane ligands expressed on adjacent cells. Recent studies suggest that Delta is cleaved to release an extracellular fragment, DlEC, by a mechanism that involves the activity of the metalloprotease Kuzbanian; however, the functional significance of that cleavage remains controversial. Using independent functional assays in vitro and in vivo, we examined the biological activity of purified soluble Delta forms and conclude that Delta cleavage is an important down-regulating event in Notch signaling. The data support a model whereby Delta inactivation is essential for providing the critical ligand/receptor expression differential between neighboring cells in order to distinguish the signaling versus the receiving partner.

The regulation of Notch signaling controls satellite cell activation and cell fate determination in postnatal myogenesis

We have studied the role of Notch-1 and its antagonist Numb in the activation of satellite cells during postnatal myogenesis. Activation of Notch-1 promoted the proliferation of myogenic precursor cells expressing the premyoblast marker Pax3. Attenuation of Notch signaling by increases in Numb expression led to the commitment of progenitor cells to the myoblast cell fate and the expression of myogenic regulatory factors, desmin, and Pax7. In many intermediate progenitor cells, Numb was localized asymmetrically in actively dividing cells, suggesting an asymmetric cell division and divergent cell fates of daughter cells. The results indicate that satellite cell activation results in a heterogeneous population of precursor cells with respect to Notch-1 activity and that the balance between Notch-1 and Numb controls cellular homeostasis and cell fate determination.

Notch1 control of oligodendrocyte differentiation in the spinal cord

We have selectively inhibited Notch1 signaling in oligodendrocyte precursors (OPCs) using the Cre/loxP system in transgenic mice to investigate the role of Notch1 in oligodendrocyte (OL) development and differentiation. Early development of OPCs appeared normal in the spinal cord. However, at embryonic day 17.5, premature OL differentiation was observed and ectopic immature OLs were present in the gray matter. At birth, OL apoptosis was strongly increased in Notch1 mutant animals. Premature OL differentiation was also observed in the cerebrum, indicating that Notch1 is required for the correct spatial and temporal regulation of OL differentiation in various regions of the central nervous system. These findings establish a widespread function of Notch1 in the late steps of mammalian OPC development in vivo.

Notch/Notch ligands and Math1 expression patterns in the organ of Corti of wild-type and Hes1 and Hes5 mutant mice

The sensory epithelium of the mammalian cochlea (the organ of Corti) represents an excellent developmental system. The organ of Corti contains two main cell types: the sensory hair cells and the supporting cells which are organized in a defined mosaic pattern. Previous results have demonstrated the participation of Notch signaling in the regulation of the pattern of hair cell differentiation within this sensory mosaic. It has also been shown that the basic helix-loop-helix (bHLH) transcription factor Math1 is a positive regulator of hair cell differentiation. We demonstrated that Hes1 and Hes5, two members of the inhibitory bHLH transcription factors, act as negative regulators of hair cell differentiation. Loss-of-function studies implicating the neurogenic genes Notch1, Jag2, Hes1 and Hes5 generated a significant increase in the number of hair cells. However, their functional interplay within the organ of Corti has not been determined. To clarify the mechanisms that regulate hair cell differentiation, we examined the expression of Notch/Notch ligand system and Math1 in the developing organ of Corti of Hes1- and Hes5-deficient mice. Our study suggests complex specific relationships between Notch signaling, Math1 and Hes1/Hes5 in the control of hair cell differentiation in the developing organ of Corti.

HERP1 is a cell type-specific primary target of Notch

Notch signaling is involved in many cell fate determination events in metazoans. Ligand binding results in proteolytic cleavage to release the signal-transducing Notch intracellular domain (NICD). The nuclear protein RBP-J kappa, when complexed with NICD, acts as a transcriptional activator which, in turn, induces a target gene of Notch such as the repressors HES/E(spl) and HERP2. Under physiological stimulation using co-culture with Notch ligand-expressing cells and target cells expressing Notch receptors, the HES1 gene and the HERP2 gene have been shown to be directly up-regulated by Notch ligand binding. However, expression of another member of the HERP family, HERP1, was not induced by ligand stimulation in any cells tested, leading to the suggestion that HERP1 may not be an immediate target of Notch or that Notch pathways can be cell type-specific. Because HERP1 appears to play a central role in the development of the aorta (Zhong, T. P., Rosenberg, M., Mohideen, M. A., Weinstein, B., and Fishman, M. C. (2000) Science 287, 1820-1824), we re-addressed the issue of its relationship with the Notch pathway by examining its expression in A10 smooth muscle cells derived from thoracic aorta. We show that in these specific cells HERP1 is also a direct target gene of Notch. NICD activates the HERP1 promoter in an RBP-J kappa-dependent manner, and induces expression of endogenous HERP1 mRNA as well as HERP1 protein in A10 cells. Co-culture with Notch ligand-bearing cells induces endogenous HERP1 mRNA expression in A10 cells, and these events occur even in the absence of de novo protein synthesis. In addition, RBP-J kappa proved essential for induction of HERP1 mRNA in Notch signaling because exogenous RBP-J kappa was sufficient to rescue HERP1 mRNA expression in RBP-J kappa-deficient cells. These findings provide the first solid evidence that HERP1 is a novel primary target of Notch and underscores the cell-specific complexity of the Notch regulatory pathway. Given that Notch signaling plays a crucial role in vascular development, Notch may derive its function via HERP family members.