The intracellular domain of mouse Notch: a constitutively activated repressor of myogenesis directed at the basic helix-loop-helix region of MyoD

Genetic and epigenetic control of skeletal muscle development

In recent years our understanding of the molecular processes underlying skeletal myogenesis has improved considerably. Overt myogenesis is preceded by a number of steps leading to the specification of muscle precursor cells. During this period, myogenic precursors express mRNAs for Muscle Regulatory Factors (MRFs) of the bHLH-family of transcription factors: MyoD, Myf5, Myogenin and MRF4. These factors are specific for developing skeletal muscle and their identification belongs to the great achievements in muscle research. Other transcriptional regulators involved in myogenesis are Pax3 and Pax7, as well as the myocyte enhancer factors (MEFs), especially MEF2. Other inhibitory transcription factors may interact with histones to render muscle-specific genes inacessible. More recently, signaling events involving the Wnt-glycoproteins and Sonic Hedgehog have been identified that lead to the induction or expansion of muscle-specific genes during embryogenesis. Sources of these signals were identified to be the neural tube, ectoderm and notochord. Interestingly, a bias towards muscle differentiation already resides in cells of the epiblast. Thus, it can be reasoned that muscle differentiation does not have to be induced, but maybe just derepressed. Apart from inductive or permissive signals involved in differentiation control, other signalling events have been described leading to the definite arrangement of muscle groups in the body. These processes involve the changes in the cytoskeleton, delay of differentiation, cell migration and target recognition.

Muscle regeneration in dystrophin-deficient mdx mice studied by gene expression profiling

Background

Duchenne muscular dystrophy (DMD), caused by mutations in the dystrophin gene, is lethal. In contrast, dystrophin-deficient mdx mice recover due to effective regeneration of affected muscle tissue. To characterize the molecular processes associated with regeneration, we compared gene expression levels in hindlimb muscle tissue of mdx and control mice at 9 timepoints, ranging from 1–20 weeks of age.

Results

Out of 7776 genes, 1735 were differentially expressed between mdx and control muscle at at least one timepoint (p < 0.05 after Bonferroni correction). We found that genes coding for components of the dystrophin-associated glycoprotein complex are generally downregulated in the mdx mouse. Based on functional characteristics such as membrane localization, signal transduction, and transcriptional activation, 166 differentially expressed genes with possible functions in regeneration were analyzed in more detail. The majority of these genes peak at the age of 8 weeks, where the regeneration activity is maximal. The following pathways are activated, as shown by upregulation of multiple members per signalling pathway: the Notch-Delta pathway that plays a role in the activation of satellite cells, and the Bmp15 and Neuregulin 3 signalling pathways that may regulate proliferation and differentiation of satellite cells. In DMD patients, only few of the identified regeneration-associated genes were found activated, indicating less efficient regeneration processes in humans.

Conclusion

Based on the observed expression profiles, we describe a model for muscle regeneration in mdx mice, which may provide new leads for development of DMD therapies based on the improvement of muscle regeneration efficacy.

Turning it up a Notch: cross-talk between TGF beta and Notch signaling

Signaling through both the transforming growth factor beta (TGF beta) superfamily of growth factors and Notch play crucial roles during embryonic pattern formation and cell fate determination. Although both pathways are able to exert similar biological responses in certain cell types, a functional interaction between these two signaling pathways has not been described. Now, three papers provide evidence of both synergy and antagonism between TGF beta and Notch signaling. These reports describe a requirement for Notch signal transducers in TGF beta- and BMP-induced expression of Notch target genes, as well as in BMP-controlled cell differentiation and migration. These papers uncover a direct link between the Notch and TGF beta pathways and suggest a critical role for Notch in some of the biological responses to TGF beta family signaling.

The NLRR gene family and mouse development: Modified differential display PCR identifies NLRR-1 as a gene expressed in early somitic myoblasts

During vertebrate embryogenesis, the somites form by segmentation of the trunk mesoderm, lateral to the neural tube, in an anterior to posterior direction. Analysis of differential gene expression during somitogenesis has been problematic due to the limited amount of tissue available from early mouse embryos. To circumvent these problems, we developed a modified differential display PCR technique that is highly sensitive and yields products that can be used directly as in situ hybridisation probes. Using this technique, we isolated NLRR-1 as a gene expressed in the myotome of developing somites but not in the presomitic mesoderm. Detailed expression analysis showed that this gene was expressed in the skeletal muscle precursors of the myotome, branchial arches and limbs as well as in the developing nervous system. Somitic expression occurs in the earliest myoblasts that originate from the dorsal lip in a pattern reminiscent of the muscle determination gene Myf5, but not at the ventral lip, indicating that NLRR-1 is expressed in a subset of myotome cells. The NLRR genes comprise a three-gene family encoding glycosylated transmembrane proteins with external leucine-rich repeats, a fibronectin domain, an immunoglobulin domain and short intracellular tails capable of mediating protein-protein interaction. Analysis of NLRR-3 expression revealed regulated expression in the neural system in developing ganglia and motor neurons. NLRR-2 expression appears to be predominately confined to the adult. The regulated embryonic expression and cellular location of these proteins suggest important roles during mouse development in the control of cell adhesion, movement or signalling.

Activation of the Rat Renin Promoter by HOXD10·PBX1b·PREP1, Ets-1, and the Intracellular Domain of Notch

Renin gene expression is subject to complex developmental and tissue-specific regulation. A comparison of the promoter sequences of the human, rat, and mouse renin genes has revealed a highly conserved sequence homologous to the DNA recognition sequence for CBF1 (CSL/RBP-Jkappa/Su(H)/LAG1/RBPSUH). Electrophoretic mobility shift assays document that As4.1 cell nuclear protein complex binding to the putative rat renin CBF1-binding site (-175 to -168 bp) contains CBF1. Transient transfection analyses in COS-7 cells further document that a CBF1-VP16 fusion protein and the intracellular domain of Notch1 robustly activate a promoter containing multiple copies of the rat renin CBF1-binding site. An Ets-binding site (-143 to -138 bp) has also been identified in the rat renin promoter by sequence comparisons and electrophoretic mobility shift assays. Transcription factor Ets-1 is capable of activating the rat renin promoter through the Ets-binding site. Mutation of the CBF-binding site significantly increases transcriptional activity of the rat renin promoter in Calu-6 and COS-7 cells but not in As4.1 cells, whereas mutation of the Ets-binding site reduces promoter activity of the rat renin gene in all three cell lines. Finally, we show that the intracellular domain of Notch1, Ets-1, and HOXD10.PBX1b.PREP1 activate the rat renin promoter cooperatively in COS-7 cells. These results strongly suggest that the renin gene is a downstream target of the Notch signaling pathway.

Side Population cells isolated from different tissues share transcriptome signatures and express tissue-specific markers

Side Population (SP) cells, isolated from murine adult bone marrow (BM) based on the exclusion of the DNA dye Hoechst 33342, exhibit potent hematopoietic stem cell (HSC) activity when compared to Main Population (MP) cells. Furthermore, SP cells derived from murine skeletal muscle exhibit both hematopoietic and myogenic potential in vivo. The multipotential capacity of SP cells isolated from variable tissues is supported by an increasing number of studies. To investigate whether the SP phenotype is associated with a unique transcriptional profile, we characterized gene expression of SP cells isolated from two biologically distinct tissues, bone marrow and muscle. Comparison of SP cells with differentiated MP cells within a tissue revealed that SP cells are in an active transcriptional and translational status and underexpress genes reflecting tissue-specific functions. Direct comparison of gene expression of SP cells isolated from different tissues identified genes common to SP cells as well as genes specific to SP cells within a particular tissue and further define a muscle and bone marrow environment. This study reports gene expression of muscle SP cells, common features and differences between SP cells isolated from muscle and bone marrow, and further identifies common signaling pathways that might regulate SP cell functions.

Expression of Hairy/Enhancer of Split genes, Hes1 and Hes5, during murine nephron morphogenesis

Hairy/Enhancer of Split (Hes) genes encode transcriptional repressors that function as downstream targets of activated Notch receptors in cell fate decisions during tissue development. During nephrogenesis, multiple Notch pathway genes are co-expressed in multi-potent epithelial progenitors (i.e. pre-tubular aggregates), but demonstrate distinct expression patterns in early nephrons (i.e. S-shaped bodies), suggesting that Notch signaling functions in patterning epithelial cell fate during nephron morphogenesis. To define the spatial activation of the Notch pathway in developing nephrons, we analyzed the expression of Hes1 and Hes5 by mRNA in situ hybridization in cryosections of developing kidneys, and compared their spatiotemporal expression with the expression of other Notch pathway genes in nephron progenitors. Hes1, and to a lesser extent Hes5, were expressed in pre-tubular aggregates and comma-shaped bodies of embryonic day (E) 13.5 and newborn kidneys. In S-shaped bodies, Hes1 expression was detected in the middle part which gives rise to the proximal tubule, but also extended into the lower and upper parts which give rise to the glomerulus and distal tubule, respectively, and was similar to the proximal-distal expression patterns for Notch1 and Jagged1 in these nephrogenic structures. In contrast, strong Hes5 expression was restricted to the middle segment of S-shaped bodies, and resembled Delta-like 1 expression. These data show that Hes1 and Hes5 expression are independently regulated along the proximal-distal axis of the developing nephron. Consequently, the differential, spatial regulation of Hes1 and Hes5 gene expression by the Notch signaling pathway in developing nephrons may be a mechanism for patterning cell fate decisions during nephron morphogenesis.

Viral interactions with the Notch pathway

The Notch signaling pathway influences cell fate decisions, proliferation versus differentiation and cell survival. Viruses both utilize and manipulate the differentiation state of infected cells, promote or block cell cycling and employ a variety of mechanisms to evade innate cellular anti-viral responses and promote cell survival. In light of these commonalities, it is perhaps not surprising that several viruses have tapped into the Notch pathway to advance their own life cycles. This first became apparent from studies showing targeting of Epstein-Barr virus proteins to the nuclear effector of Notch signaling CSL (CBF1/RBPJk). More recently the Kaposi's sarcoma-associated herpesvirus RTA protein has been found to bind CSL. Notch pathway interactions have also been described for adenovirus SV40 and human papilloma virus. This review focuses on the herpesvirus protein interactions with the Notch pathway and the insights that these interactions have provided.

Sharp-1/DEC2 inhibits skeletal muscle differentiation through repression of myogenic transcription factors

Skeletal muscle differentiation is regulated by the basic-helix-loop-helix (bHLH) family of transcription factors. The myogenic bHLH factors form heterodimers with the ubiquitously expressed bHLH E-proteins and bind E-box (CANNTG) sites present in the promoters of several muscle-specific genes. Our previous studies have shown that the bHLH factor Sharp-1 is expressed in skeletal muscle and interacts with MyoD and E-proteins. However, its role in regulation of myogenic differentiation remains unknown. We report here that endogenous Sharp-1 is expressed in proliferating C2C12 myoblasts and is down-regulated during myogenic differentiation. Constitutive expression of Sharp-1 in C2C12 myoblasts promotes cell cycle exit causing a decrease in cyclin D1 expression but blocks terminal differentiation. Although MyoD expression is not inhibited, the induction of differentiation-specific genes such as myogenin, MEF2C, and myosin heavy chain is impaired by Sharp-1 overexpression. We demonstrate that the interaction of Sharp-1 with MyoD and E-proteins results in reduced DNA binding and transactivation from MyoD-dependent E-box sites. Re-expression of MyoD approximately E47 rescues the differentiation defect imposed by Sharp-1, suggesting that myogenic bHLH factors function downstream of Sharp-1. Our data suggest that protein-protein interactions between Sharp-1, MyoD, and E47 resulting in interference with MyoD function underlies Sharp-1-mediated repression of myogenic differentiation.

Monoubiquitination and endocytosis direct gamma-secretase cleavage of activated Notch receptor

Activation of mammalian Notch receptor by its ligands induces TNFalpha-converting enzyme-dependent ectodomain shedding, followed by intramembrane proteolysis due to presenilin (PS)-dependent gamma-secretase activity. Here, we demonstrate that a new modification, a monoubiquitination, as well as clathrin-dependent endocytosis, is required for gamma-secretase processing of a constitutively active Notch derivative, DeltaE, which mimics the TNFalpha-converting enzyme-processing product. PS interacts with this modified form of DeltaE, DeltaEu. We identified the lysine residue targeted by the monoubiquitination event and confirmed its importance for activation of Notch receptor by its ligand, Delta-like 1. We propose a new model where monoubiquitination and endocytosis of Notch are a prerequisite for its PS-dependent cleavage, and discuss its relevance for other gamma-secretase substrates.

Structure and stability of the ankyrin domain of the Drosophila Notch receptor

The Notch receptor contains a conserved ankyrin repeat domain that is required for Notch-mediated signal transduction. The ankyrin domain of Drosophila Notch contains six ankyrin sequence repeats previously identified as closely matching the ankyrin repeat consensus sequence, and a putative seventh C-terminal sequence repeat that exhibits lower similarity to the consensus sequence. To better understand the role of the Notch ankyrin domain in Notch-mediated signaling and to examine how structure is distributed among the seven ankyrin sequence repeats, we have determined the crystal structure of this domain to 2.0 angstroms resolution. The seventh, C-terminal, ankyrin sequence repeat adopts a regular ankyrin fold, but the first, N-terminal ankyrin repeat, which contains a 15-residue insertion, appears to be largely disordered. The structure reveals a substantial interface between ankyrin polypeptides, showing a high degree of shape and charge complementarity, which may be related to homotypic interactions suggested from indirect studies. However, the Notch ankyrin domain remains largely monomeric in solution, demonstrating that this interface alone is not sufficient to promote tight association. Using the structure, we have classified reported mutations within the Notch ankyrin domain that are known to disrupt signaling into those that affect buried residues and those restricted to surface residues. We show that the buried substitutions greatly decrease protein stability, whereas the surface substitutions have only a marginal affect on stability. The surface substitutions are thus likely to interfere with Notch signaling by disrupting specific Notch-effector interactions and map the sites of these interactions.

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.

Identification of two binding regions for the suppressor of hairless protein within the intracellular domain of Drosophila notch

Notch is a phylogenetically conserved transmembrane receptor that is required for many aspects of animal development. Upon ligand stimulation, a fragment of Notch is released proteolytically and enters the nucleus to form a complex with the DNA-binding protein CSL (CBF1/Suppressor of Hairless/Lag1) and activate transcription of Notch-CSL target genes. The physical structure of the Notch-CSL complex remains unclear, however, clouding the interpretation of previous efforts to correlate Notch structure and function. We have, therefore, characterized the binding of Drosophila CSL (called Suppressor of Hairless, or Su(H)) to the intracellular domain of Drosophila Notch both in vitro and in vivo. We report the identification of two Su(H) binding regions in Notch. The first is in the juxtamembrane region (the "RAM" domain). The second is just C-terminal to the Notch ankyrin repeats, overlapping or identical to two previously proposed nuclear localization sequences, in a domain we term PPD (potential phosphorylated domain). The ankyrin repeats themselves do not bind to Su(H); however, they substantially enhance binding of Su(H) to the more C-terminal region. Consistent with this picture, removal of either the Ram or PPD binding sites, separately, modestly reduces Notch activity in vivo, whereas removal of both renders Notch severely defective. These results clarify the relationship between Notch and CSL, help to explain the importance of the ankyrin repeats in Notch signaling, and reconcile many apparently contradictory results from previous Notch structure/function studies. Moreover, they suggest a second function for the Notch nuclear localization sequence elements.

Anchoring notch genetics and biochemistry; structural analysis of the ankyrin domain sheds light on existing data

Notch signaling is important in development and in human disease. Notch receptors regulate transcription through direct interactions with several proteins at the promoter regions of target genes. To understand the mechanism of Notch signaling, numerous deletion and mutagenesis studies have been carried out to identify functional domains in Notch, but domain definition and their role during the assembly of the transcriptionally active complex remains controversial. Recently reported biophysical and structural studies of the Notch ANK domain permit us to reevaluate the existing domain assignments and their predicted functional role, thereby providing further insight into the mechanism of Notch signaling.

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

The Notch and transforming growth factor-beta (TGF-beta) 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-beta signaling in the regulation of Hes-1, a direct target of the Notch pathway. Activation of TGF-beta 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-beta 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-beta 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-beta signals are integrated by direct protein-protein interactions between the signal-transducing intracellular elements from both pathways.

Notch promotes epithelial-mesenchymal transition during cardiac development and oncogenic transformation

Epithelial-to-mesenchymal transition (EMT) is fundamental to both embryogenesis and tumor metastasis. The Notch intercellular signaling pathway regulates cell fate determination throughout metazoan evolution, and overexpression of activating alleles is oncogenic in mammals. Here we demonstrate that Notch activity promotes EMT during both cardiac development and oncogenic transformation via transcriptional induction of the Snail repressor, a potent and evolutionarily conserved mediator of EMT in many tissues and tumor types. In the embryonic heart, Notch functions via lateral induction to promote a selective transforming growth factor-beta (TGFbeta)-mediated EMT that leads to cellularization of developing cardiac valvular primordia. Embryos that lack Notch signaling elements exhibit severely attenuated cardiac snail expression, abnormal maintenance of intercellular endocardial adhesion complexes, and abortive endocardial EMT in vivo and in vitro. Accordingly, transient ectopic expression of activated Notch1 (N1IC) in zebrafish embryos leads to hypercellular cardiac valves, whereas Notch inhibition prevents valve development. Overexpression of N1IC in immortalized endothelial cells in vitro induces EMT accompanied by oncogenic transformation, with corresponding induction of snail and repression of VE-cadherin expression. Notch is expressed in embryonic regions where EMT occurs, suggesting an intimate and fundamental role for Notch, which may be reactivated during tumor metastasis.

Evidence that C promoter-binding factor 1 binding is required for Notch-1-mediated repression of activator protein-1

Cell fate determination in invertebrate and vertebrate systems is regulated by the Notch signaling pathway. Four mammalian Notch genes, Notch 1-4, encode differentially expressed transmembrane receptors. The canonical Notch pathway involves proteolytic liberation of the Notch-1 intracellular domain (NIC-1), which activates CSL (CBF1, Su(H), and Lag-1)-mediated transactivation. We showed previously that NIC-1 also represses activator protein-1 (AP-1)-mediated transactivation. The N-terminal RAM (RBP-Jkappa associated molecule) domain of NIC-1 was required for both activation and repression. To investigate the mechanism of AP-1 repression, we tested whether distinct sequences within the RAM domain mediate activation versus repression. We analyzed the capacity of RAM domain mutants to bind endogenous CBF1, to activate CSL-mediated transactivation, and to repress AP-1. A mutant lacking 20 amino acids of the RAM domain (Delta1759-1778) resembled the RAM domain deletion mutant in being defective in all activities. Analysis of 14 deletion and alanine substitution mutants revealed a correlation between CBF1 binding, CSL-mediated transactivation, and AP-1 repression. Stably transfected K562 cells could only tolerate very low level expression of wild-type NIC-1 and NIC-1 mutants retaining activation/repression activities. By contrast, transcriptionally compromised NIC-1 mutants accumulated at high levels. These results support a model in which the binding of NIC-1 to CBF1 is required for AP-1 repression and reveal a powerful cell-sensing mechanism that suppresses the levels of transcriptionally competent NIC-1.

Osteogenic differentiation of the mesenchymal progenitor cells, Kusa is suppressed by Notch signaling

Notch receptor plays a crucial role in proliferation and differentiation of many cell types. To elucidate the function of Notch signaling in osteogenesis, we transfected the constitutively active Notch1 (Notch intracellular domain, NICD) into two different osteoblastic mesenchymal cell lines, KusaA and KusaO, and examined the changes of their osteogenic potentials. In NICD stable transformants (KusaA(NICD) and KusaO(NICD)), osteogenic properties including alkaline phosphatase activity, expression of osteocalcin and type I collagen, and in vitro calcification were suppressed. Transient transfection of NICD attenuated the promoter activities of Cbfa1 and Ose2 element. KusaA was capable of forming trabecular bone-like tissues when injected into mouse abdomen, but this in vivo bone forming activity was significantly suppressed in KusaA(NICD). Osteoclasts were induced in the KusaA-derived bone-like tissues, but lacked in the KusaA(NICD)-derived tissues. These results suggest that Notch signaling suppresses the osteoblastic differentiation of mesenchymal progenitor cells.

Radiation-induced deletions in the 5' end region of Notch1 lead to the formation of truncated proteins and are involved in the development of mouse thymic lymphomas

Notch1 protein is a transmembrane receptor that directs various cell fate decisions. Active forms of Notch1 consisting of a transmembrane domain and an intracellular domain (Notch1TM) or only an intracellular domain (Notch1IC) function as oncoproteins. To elucidate the effect of Notch1 abnormalities in radiation-induced lymphomagenesis, we determined the structure of the Notch1 gene and examined the frequency and the sites of Notch1 rearrangements in radiation-induced mouse thymic lymphomas. The Notch1 gene consists of 37 exons, including three exons upstream of the previously reported exon 1. The transcript starting from exon 1 was the major transcript whereas the transcripts read upstream from exon 1a, in which amino acid sequences in the N-terminal region were changed, were minor. More than 50% of radiation-induced thymic lymphomas exhibited Notch1 rearrangements, suggesting that Notch1 acts as a major oncogene in radiation-induced lymphomagenesis. We identified three rearranged sites: novel sites in the 5' end region encompassing exons 1 and 2, the previously identified juxtamembrane extracellular region, and the 3' end region. The 5' deletion and the insertion of murine leukemia virus in the juxtamembrane region led to the production of abnormal transcripts starting from cryptic transcription start sites located halfway through the Notch1 gene and resulted in transcripts lacking most of the extracellular domain. As a result of these rearrangements, truncated Notch1 polypeptides resembling Notch1TM or Notch1IC were formed. In contrast, the 3' deletion led to the production of a C-terminal PEST motif-deleted transcript. The downstream target gene Hes1 was transcribed in a lymphoma with insertion of murine leukemia virus, but not in a lymphoma with a 5' deletion. These results indicate that in addition to Hes1 expression, other Notch1 pathway(s) have a role in thymic lymphomagenesis and suggest the presence of a novel mechanism for oncogenic activation of Notch1 by 5' deletion.

Wnt signaling induces the myogenic specification of resident CD45+ adult stem cells during muscle regeneration

The observation that CD45(+) stem cells injected into the circulation participate in muscle regeneration raised the question of whether CD45(+) stem cells resident in muscle play a physiological role during regeneration. We found that CD45(+) cells cultured from uninjured muscle were uniformly nonmyogenic. However, CD45(+) cells purified from regenerating muscle readily gave rise to determined myoblasts. The number of CD45(+) cells in muscle rapidly expanded following injury, and a high proportion entered the cell cycle. Investigation of candidate pathways involved in embryonic myogenesis revealed that Wnt signaling was sufficient to induce the myogenic specification of muscle-derived CD45(+) stem cells. Moreover, injection of the Wnt antagonists sFRP2/3 into regenerating muscle markedly reduced CD45(+) stem cell proliferation and myogenic specification. Our data therefore suggest that mobilization of resident CD45(+) stem cells is an important factor in regeneration after injury and highlight the Wnt pathway as a potential therapeutic target for degenerative neuromuscular disease.

HES and HERP families: multiple effectors of the Notch signaling pathway

Notch signaling dictates cell fate and critically influences cell proliferation, differentiation, and apoptosis in metazoans. Multiple factors at each step-ligands, receptors, signal transducers and effectors-play critical roles in executing the pleiotropic effects of Notch signaling. Ligand-binding results in proteolytic cleavage of Notch receptors to release the signal-transducing Notch intracellular domain (NICD). NICD migrates into the nucleus and associates with the nuclear proteins of the RBP-Jkappa family (also known as CSL or CBF1/Su(H)/Lag-1). RBP-Jkappa, when complexed with NICD, acts as a transcriptional activator, and the RBP-Jkappa-NICD complex activates expression of primary target genes of Notch signaling such as the HES and enhancer of split [E(spl)] families. HES/E(spl) is a basic helix-loop-helix (bHLH) type of transcriptional repressor, and suppresses expression of downstream target genes such as tissue-specific transcriptional activators. Thus, HES/E(spl) directly affects cell fate decisions as a primary Notch effector. HES/E(spl) had been the only known effector of Notch signaling until a recent discovery of a related but distinct bHLH protein family, termed HERP (HES-related repressor protein, also called Hey/Hesr/HRT/CHF/gridlock). In this review, we summarize the recent data supporting the idea of HERP being a new Notch effector, and provide an overview of the similarities and differences between HES and HERP in their biochemical properties as well as their tissue distribution. One key observation derived from identification of HERP is that HES and HERP form a heterodimer and cooperate for transcriptional repression. The identification of the HERP family as a Notch effector that cooperates with HES/E(spl) family has opened a new avenue to our understanding of the Notch signaling pathway.

HPV16 E6 and E7 oncoproteins regulate Notch-1 expression and cooperate to induce transformation

Notch receptor signaling has been implicated in cellular transformation. Notch-1 receptor expression is increased during the progression from cervical intraepithelial lesions (CIN) to invasive cervical carcinoma. Moreover, the main cellular localization of Notch-1 protein changes from cytoplasmic to nuclear with the transition from CIN III to microinvasive carcinoma. Since the E6 and E7 proteins encoded by human papilloma virus (HPV) are a causative agent of cervical carcinoma, this study determined whether E6 and E7 protein expression causes the observed upregulation in Notch-1 expression. Mouse and human primary cell lines were transfected with HPV16 E6 and E7 and Notch-1 expression and activity were analyzed. We show that Notch-1 expression and activity are upregulated by E6 and E7 independently. This was due to both transcriptional and post-transcriptional mechanisms. A protein involved in Notch processing, Presenilin-1 (PS-1), was also upregulated by E6 and E7. In the presence of E6 and E7, Notch-1 protein expression is localized in the cytoplasm. Downregulation of Notch-1 expression in a human cervical carcinoma cell line expressing E6/E7 caused striking inhibition of proliferation in vitro and tumorigenicity in vivo. These data suggest that E6- and E7-mediated upregulation of Notch signaling may contribute to disruption of regular cell growth in cervical cancer.

IkappaBalpha and p65 regulate the cytoplasmic shuttling of nuclear corepressors: cross-talk between Notch and NFkappaB pathways

Notch and NFkappaB pathways are key regulators of numerous cellular events such as proliferation, differentiation, or apoptosis. In both pathways, association of effector proteins with nuclear corepressors is responsible for their negative regulation. We have previously described that expression of a p65-NFkappaB mutant that lacks the transactivation domain (p65DeltaTA) induces cytoplasmic translocation of N-CoR leading to a positive regulation of different promoters. Now, we show that cytoplasmic sequestration of p65 by IkappaBalpha is sufficient to both translocate nuclear corepressors SMRT/N-CoR to the cytoplasm and upregulate transcription of Notch-dependent genes. Moreover, p65 and IkappaBalpha are able to directly bind SMRT, and this interaction can be inhibited in a dose-dependent manner by the CREB binding protein (CBP) coactivator and after TNF-alpha treatment, suggesting that p65 acetylation is modulating this interaction. In agreement with this, TNF-alpha treatment results in downregulation of the Hes1 gene. Finally, we present evidence on how this mechanism may influence cell differentiation in the 32D myeloid progenitor system.

Regulation of marginal zone B cell development by MINT, a suppressor of Notch/RBP-J signaling pathway

We found that Msx2-interacting nuclear target protein (MINT) competed with the intracellular region of Notch for binding to a DNA binding protein RBP-J and suppressed the transactivation activity of Notch signaling. Although MINT null mutant mice were embryonic lethal, MINT-deficient splenic B cells differentiated about three times more efficiently into marginal zone B cells with a concomitant reduction of follicular B cells. MINT is expressed in a cell-specific manner: high in follicular B cells and low in marginal zone B cells. Since Notch signaling directs differentiation of marginal zone B lymphocytes and suppresses that of follicular B lymphocytes in mouse spleen, the results indicate that high levels of MINT negatively regulate Notch signaling and block differentiation of precursor B cells into marginal zone B cells. MINT may serve as a functional homolog of Drosophila Hairless.

Inhibition of skeletal muscle development: less differentiation gives more muscle

The fact that stem cells have to be protected from premature differentiation is true for many organs in the developing embryo and the adult organism. However, there are several arguments that this is particularly important for (skeletal) muscle. There are some evolutionary arguments that muscle is a "default" pathway for mesodermal cells, which has to be actively prevented in order to allow cells to differentiate into other tissues. Myogenic cells originate from very small areas of the embryo where only a minor portion of these cells is supposed to differentiate. Differentiated muscle fibres are unconditionally post-mitotic, leaving undifferentiated stem cells as the only source of regeneration. The mechanical usage of muscle and its superficial location in the vertebrate body makes regeneration a frequently used mechanism. Looking at the different inhibitory mechanisms that have been found within the past 10 or so years, it appears as if evolution has taken this issue very serious. At all possible levels we find regulatory mechanisms that help to fine tune the differentiation of myogenic cells. Secreted molecules specifying different populations of somitic cells, diffusing or membrane-bound signals among fellow myoblasts, modulating molecules within the extracellular matrix and last, but not least, a changing set of activating and repressing cofactors. We have come a long way from the simple model of MyoD just to be turned on at the right time in the right cell.

Down-regulation of an ankyrin repeat-containing protein, V-1, during skeletal muscle differentiation and its re-expression in the regenerative process of muscular dystrophy

Using Western blot analysis and immunohistochemical methods, we examined the expression of V-1, a member of the ankyrin repeat-containing protein family, during differentiation and regeneration of skeletal muscle. The expression of V-1 was high in cultured myoblasts and decreased during their differentiation into myotubes, while high expression was maintained when muscle differentiation was inhibited by treatment with basic fibroblast growth factor. Down-regulation of V-1 also occurred during in vivo muscle differentiation from embryonic to postnatal stages, reaching an undetectable level in mature skeletal muscle. In contrast, strong V-1 immunoreactivity was detected again in myoblasts and regenerating muscle fibers with a small diameter, which were observed in Duchenne muscular dystrophy and its animal model, mdx mouse. Thus, it seems that V-1 is a good marker for early stage of muscle regeneration and changes of its expression suggest that V-1 plays a role in prenatal muscle differentiation and postnatal muscle regeneration.

Activation of Notch-1 signaling maintains the neoplastic phenotype in human Ras-transformed cells

Truncated Notch receptors have transforming activity in vitro and in vivo. However, the role of wild-type Notch signaling in neoplastic transformation remains unclear. Ras signaling is deregulated in a large fraction of human malignancies and is a major target for the development of novel cancer treatments. We show that oncogenic Ras activates Notch signaling and that wild-type Notch-1 is necessary to maintain the neoplastic phenotype in Ras-transformed human cells in vitro and in vivo. Oncogenic Ras increases levels and activity of the intracellular form of wild-type Notch-1, and upregulates Notch ligand Delta-1 and also presenilin-1, a protein involved in Notch processing, through a p38-mediated pathway. These observations place Notch signaling among key downstream effectors of oncogenic Ras and suggest that it might be a novel therapeutic target.

The notch pathway: modulation of cell fate decisions in hematopoiesis

The hematopoietic system is maintained by a rare population of hematopoietic stem cells (HSC) that are thought to undergo self-renewal as well as continuously produce progeny that differentiate into the various hematopoietic lineages. However, the mechanisms regulating cell fate choices by HSC and their progeny have not been understood. Results of most studies support a stochastic model of cell fate determination in which growth factors support only the survival or proliferation of the progeny specified along a particular lineage. In other developmental systems, however, Notch signaling has been shown to play a central role in regulating fate decisions of numerous types of precursors, often inhibiting a particular (default) pathway while permitting self-renewal or differentiation along an alternative pathway. There is also accumulating evidence that the Notch pathway affects survival, proliferation, and cell fate choices at various stages of hematopoietic cell development, including the decisions of HSC to self-renew or differentiate and of common lymphoid precursors to undergo T- or B-cell differentiation. These data suggest that the Notch pathway plays a fundamental role in the development and maintenance of the hematopoietic system.

Activated Notch4 inhibits angiogenesis: role of beta 1-integrin activation

Notch4 is a member of the Notch family of transmembrane receptors that is expressed primarily on endothelial cells. Activation of Notch in various cell systems has been shown to regulate cell fate decisions. The sprouting of endothelial cells from microvessels, or angiogenesis, involves the modulation of the endothelial cell phenotype. Based on the function of other Notch family members and the expression pattern of Notch4, we postulated that Notch4 activation would modulate angiogenesis. Using an in vitro endothelial-sprouting assay, we show that expression of constitutively active Notch4 in human dermal microvascular endothelial cells (HMEC-1) inhibits endothelial sprouting. We also show that activated Notch4 inhibits vascular endothelial growth factor (VEGF)-induced angiogenesis in the chick chorioallantoic membrane in vivo. Activated Notch4 does not inhibit HMEC-1 proliferation or migration through fibrinogen. However, migration through collagen is inhibited. Our data show that Notch4 cells exhibit increased beta1-integrin-mediated adhesion to collagen. HMEC-1 expressing activated Notch4 do not have increased surface expression of beta 1-integrins. Rather, we demonstrate that Notch4-expressing cells display beta1-integrin in an active, high-affinity conformation. Furthermore, using function-activating beta 1-integrin antibodies, we demonstrate that activation of beta1-integrins is sufficient to inhibit VEGF-induced endothelial sprouting in vitro and angiogenesis in vivo. Our findings suggest that constitutive Notch4 activation in endothelial cells inhibits angiogenesis in part by promoting beta 1-integrin-mediated adhesion to the underlying matrix.

Identification and characterization of presenilin-independent Notch signaling

Presenilin (PS) proteins control the proteolytic cleavage that precedes nuclear access of the Notch intracellular domain. Here we observe that a partial activation of the HES1 promoter can be detected in PS1/PS2 (PS1/2) double null cells using Notch1 Delta E constructs or following Delta 1 stimulation, despite an apparent abolition of the production and nuclear accumulation of the Notch intracellular domain. PS1/2-independent Notch activation is sensitive to Numblike, a physiological inhibitor of Notch. PS1/2-independent Notch signaling is also inhibited by an active gamma-secretase inhibitor in the low micromolar range and is not inhibited by an inactive analogue, similar to PS-dependent Notch signaling. However, experiments using a Notch1-Gal4-VP16 fusion protein indicate that the PS1/2-independent activity does not release Gal4-VP16 and is therefore unlikely to proceed via an intramembranous cleavage. These data reveal that a novel PS1/2-independent mechanism plays a partial role in Notch signal transduction.

c-Cbl binding and ubiquitin-dependent lysosomal degradation of membrane-associated Notch1

Regulation of Notch1 activity is critical for cell fate decisions and differentiation of skeletal myoblasts. We have employed the skeletal myoblast cell line C2C12 to study posttranslational regulation of Notch1 protein levels during myogenesis. Although the major degradation pathway of the activated intracellular Notch1 fragment appears to involve ubiquitination and degradation by the 26 S proteasome, we provide evidence for an alternative catalytic pathway where the endogenous, transmembrane form of Notch1 is targeted to the lysosomal compartment. Immunoprecipitation analysis revealed ubiquitin-dependent accumulation of transmembrane Notch1 protein after treatment with the lysosomal inhibitor chloroquine but not after treatment with various proteasome inhibitors. This finding was supported by the observation that the transmembrane form of Notch1 was tyrosine-phosphorylated and specifically coprecipitated with the ubiquitin ligase c-Cbl. Our data suggest a regulatory mechanism down-regulating Notch1 protein levels already at the cellular surface, possibly with consequences for Notch-dependent signal transduction during terminal differentiation processes.

Intracellular cell-autonomous association of Notch and its ligands: a novel mechanism of Notch signal modification

Notch (N) and its ligands, Delta (Dl) and Serrate (Ser), are membrane-spanning proteins with EGF repeats. They play an essential role in mediating proliferation and segregated differentiation of stem cells. One of the prominent features of N signal system is that its ligands are anchored to the plasma membrane, which allows the ligand/receptor association only between the neighboring cells. Various lines of evidences have verified this intercellular signal transmission, but there also have been implications that expression of Dl or Ser interferes cell-autonomously with the ability of the cell to receive N signal, implying that N and its ligands may interact in the same cell. Here, we demonstrate that N, Dl, and Ser cell-autonomously form homomeric or heteromeric complexes. The cell-autonomous heteromeric complexes are not present on the cell surface, implying that the association occurs in the endoreticulum or Golgi apparatus. Expression of Dl or Ser cell-autonomously reduces the N-mediated HES-5 promoter activity, indicating that the cell-autonomous association alters the N signal receptivity. Intracellular deletion of Dl shows elevated activity of this dominant-negative effect. In vivo overexpression study suggests that the cell-autonomous function of Dl and Ser is independent of the ligand specificity and may be modulated by Fringe (Fg), which inhibits the formation of the cell-autonomous Dl/N or Ser/N complex.

Notch signaling is involved in the regulation of Id3 gene transcription during Xenopus embryogenesis

During Xenopus embryogenesis, XId3, a member of the Id helix-loop-helix protein family, is expressed in a large variety of differentiating tissues including epidermis, cement gland, brain, neural tube, neural crest cell derivatives, somites, and tailbud. Transcription of XId3 is mediated by several cis-regulatory elements including an enhancer of 440 bp located 870 bp upstream from the transcription initiation site. The enhancer activity in embryos was studied using transgenic methodology. A galactosidase reporter gene, driven by a regulatory element composed of the enhancer and a minimal promoter derived from the XId3 gene, was expressed in transgenic embryos with a profile that faithfully reproduced that of the endogenous XId3 gene. The pattern resulted from a synergistic effect between the enhancer and the promoter, and in vitro transactivation assays showed that transcription can be stimulated by Notch signaling. The presence of potential Su(H) binding sites, in both the enhancer and the promoter, suggests that these represent candidates for in vivo cis-regulatory elements. The data presented here suggest that Notch control of differentiation may involve activation of transcription of Id, a negative regulator of bHLH transcription factors.

Vertebrate hairy and Enhancer of split related proteins: transcriptional repressors regulating cellular differentiation and embryonic patterning

The basic-helix-loop-helix (bHLH) proteins are a superfamily of DNA-binding transcription factors that regulate numerous biological processes in both invertebrates and vertebrates. One family of bHLH transcriptional repressors is related to the Drosophila hairy and Enhancer-of-split proteins. These repressors contain a tandem arrangement of the bHLH domain and an adjacent sequence known as the Orange domain, so we refer to these proteins as bHLH-Orange or bHLH-O proteins. Phylogenetic analysis reveals the existence of four bHLH-O subfamilies, with distinct, evolutionarily conserved features. A principal function of bHLH-O proteins is to bind to specific DNA sequences and recruit transcriptional corepressors to inhibit target gene expression. However, it is likely that bHLH-O proteins repress transcription by additional mechanisms as well. Many vertebrate bHLH-O proteins are effectors of the Notch signaling pathway, and bHLH-O proteins are involved in regulating neurogenesis, vasculogenesis, mesoderm segmentation, myogenesis, and T lymphocyte development. In this review, we discuss mechanisms of action and biological roles for the vertebrate bHLH-O proteins, as well as some of the unresolved questions about the functions and regulation of these proteins during development and in human disease.

Notch1 inhibits differentiation of hematopoietic cells by sustaining GATA-2 expression

Notch signaling is involved in cell fate decisions in many systems including hematopoiesis. It has been shown that expression of an activated form of Notch1 (aNotch1) in 32D mouse myeloid progenitor cells inhibits the granulocytic differentiation induced by granulocyte colony-stimulating factor (G-CSF). Results of the current study show that aNotch1, when expressed in F5-5 mouse erythroleukemia cells, also inhibits erythroid differentiation. Comparison of the expression levels of several transcription factors after stimulation for myeloid and erythroid differentiation, in the presence or absence of aNotch1, revealed that aNotch1 did not change its regulation pattern with any of the transcription factors examined, except for GATA-2, despite its inhibitory effect on differentiation. GATA-2 was down-regulated when the parental 32D and F5-5 were induced to differentiate into granulocytic and erythroid lineages, respectively. In these induction procedures, however, the level of GATA-2 expression was sustained when aNotch1 was expressed. To ascertain whether maintenance of GATA-2 is required for the Notch-induced inhibition of differentiation, the dominant-negative form of GATA-3 (DN-GATA), which acted also against GATA-2, or transcription factor PU.1, which was recently shown to be the repressor of GATA-2, was introduced into aNotch1-expressing 32D (32D/aNotch1) cells that do not express GATA family proteins other than GATA2. Both DN-GATA and PU.1 reversed the phenotype of 32D/aNotch1 inducing its differentiation when G-CSF was added. Furthermore, enforced expression of HES-1, which is involved in Notch signaling, delayed differentiation of 32D, and again this phenotype was neutralized by DN-GATA. These results indicate that GATA-2 activity is necessary for the Notch signaling in hematopoietic cells.

Nicastrin is required for Presenilin-mediated transmembrane cleavage in Drosophila

The transmembrane glycoprotein Nicastrin was identified in a complex with the multipass membrane protein Presenilin. Presenilin mediates transmembrane cleavage of single-pass transmembrane proteins with short extracellular domains, including the ligand-activated form of the receptor Notch and beta-amyloid precursor protein (beta-APP). Transmembrane cleavage of Notch is essential for signal transduction, and transmembrane cleavage of beta-APP generates pathogenic amyloid peptides implicated in Alzheimer's disease. Here, we investigate the requirement for Nicastrin in Presenilin-mediated transmembrane cleavage. We show that, in Drosophila, loss of Nicastrin activity blocks the accumulation of Presenilin associated with the apical plasma membrane, abolishes Presenilin-dependent cleavage of the transmembrane domains of Notch and beta-APP, and abrogates Notch signal transduction.

EBNA2 and Notch signalling in Epstein-Barr virus mediated immortalization of B lymphocytes

Epstein-Barr virus (EBV) has the ability to immortalize B cells. A viral key protein for immortalization is the transactivator EBNA2 that controls expression of several viral and cellular genes. EBNA2 is tethered to promoters by interacting with the cellular repressor RBP-J. This resembles the physiological activation of RBP-J-repressed promoters by activated Notch receptors (Notch-IC). Since EBNA2 and Notch-IC have been shown to be partially interchangeable in regard to activation of target genes in B cell lines and modulation of differentiation processes it is conceivable that EBNA2 is a biological equivalent of an activated Notch receptor.

Phosphorylation of Ser2078 modulates the Notch2 function in 32D cell differentiation

Notch signaling is involved in the regulation of many cell fate determination events in both embryonic development and adult tissue homeostasis. We previously demonstrated that Notch1 and Notch2 molecules inhibit myeloid differentiation in a cytokine-specific manner and that the Notch cytokine response domain is necessary for this functional specificity. We have now investigated the putative role of phosphorylation in the activity of Notch in response to cytokine signals. Our results show that the granulocyte colony-stimulating factor (G-CSF) stimulation of 32D cells expressing the intracellular Notch2 protein induces phosphorylation at specific sites of this molecule, rendering the molecule inactive and permitting differentiation of these cells. In contrast, when cells are stimulated with granulocyte macrophage colony-stimulating factor (GM-CSF), intracellular notch2 is not phosphorylated at these residues and differentiation is inhibited. We also show that deletion of the Ser/Thr-rich region between amino acids 2067 and 2099 abrogates G-CSF-induced phosphorylation and results in a molecule that inhibits differentiation in response to either G-CSF or GM-CSF. Our results further indicate that Ser(2078) is a critical residue for phosphorylation and modulation of Notch2 activity in the context of G-CSF-induced differentiation of 32D cells.

p300 acts as a transcriptional coactivator for mammalian Notch-1

Notch-1 belongs to a family of transmembrane receptor proteins that direct the decisions as to various cell fates. After ligand binding, a proteolytic cleavage step occurs and the intracellular part of Notch-1, Notch-1-IC, translocates into the nucleus, where it targets the DNA binding protein RBP-J kappa/CBF1. RBP-J kappa mediates repression through recruitment of a histone deacetylase-containing complex. The Notch-1-IC/RBP-J kappa complex overcomes repression and activates the transcription of Notch target genes. We have identified a novel domain in Notch-1-IC, the EP domain, which is indispensable for full transcriptional activation. This transactivation domain is localized adjacent to the ankyrin repeats of Notch-1-IC. In cotransfection experiments, Notch-1-IC-mediated transcriptional activation was inhibited by E1A12S and p53, two proteins, which interfere with the function of the common coactivator p300. Protein-protein interaction assays demonstrated the association of Notch-1-IC and the CH3 region of p300. In addition, the interaction of mammalian Notch-1-IC with p300 was destabilized after deletion of the EP domain of Notch-1-IC. Based on physical interaction with Notch-1-IC and coactivator functions of p300, we propose a model for Notch-1-mediated gene regulation via p300.

Repression of XMyoD expression and myogenesis by Xhairy-1 in Xenopus early embryo

Activated Notch-Delta signalling was shown to inhibit myogenesis, but whether and how it regulates myogenic gene expression is not clear. We analyzed the implication of Xenopus hairy-1 (Xhairy-1), a member of the hairy and enhancer-of-split (E(spl)) family that may function as nuclear effector of Notch signalling pathway, in regulating XMyoD gene expression at the initial step of myogenesis. Xhairy-1 transcripts are expressed soon after mid-blastula transition and exhibits overlapping expression with Notch pathway genes such as Delta-1 in the posterior somitic mesoderm. We show that overexpression of Xhairy-1 blocks the expression of XMyoD in early gastrula ectodermal cells treated with the mesoderm-inducing factor activin, and in the mesoderm tissues of early embryos. It inhibits myogenesis and produces trunk defects at later stages. Xhairy-1 also inhibits the expression of the pan-mesodermal marker Xbra, but expression of other early mesoderm markers such as goosecoid and chordin is not affected. These effects require the basic helix-loop-helix (bHLH) domain, as well as a synergy between the central Orange domain and the C-terminus WRPW-Groucho-interacting domain. Furthermore, overexpression in ectodermal cells of Xhairy-1/VP16, in which Xhairy-1 repressor domain is replaced by the activator domain of the viral protein VP16, induces the expression of XMyoD in the absence of protein synthesis. Interestingly, Xhairy-1/VP16 does not induce the expression of Xbra and XMyf5 in the same condition. During neurulation, the expression of XMyoD induced by Xhairy-1/VP16 declines and the expression of muscle actin gene was never detected. These results suggest that Notch signalling through hairy-related genes may specifically regulate XMyoD expression at the initial step of myogenesis in vertebrates.

Functional interaction between SEL-10, an F-box protein, and the nuclear form of activated Notch1 receptor

The Notch signaling pathway is essential in many cell fate decisions in invertebrates as well as in vertebrates. After ligand binding, a two-step proteolytic cleavage releases the intracellular part of the receptor which translocates to the nucleus and acts as a transcriptional activator. Although Notch-induced transcription of genes has been reported extensively, its endogenous nuclear form has been seldom visualized. We report that the nuclear intracellular domain of Notch1 is stabilized by proteasome inhibitors and is a substrate for polyubiquitination in vitro. SEL-10, an F-box protein of the Cdc4 family, was isolated in a genetic screen for Lin12/Notch-negative regulators in Caenorhabditis elegans. We isolated human and murine counterparts of SEL-10 and investigated the role of a dominant-negative form of this protein, deleted of the F-box, on Notch1 stability and activity. This molecule could stabilize intracellular Notch1 and enhance its transcriptional activity but had no effect on inactive membrane-anchored forms of the receptor. We then demonstrated that SEL-10 specifically interacts with nuclear forms of Notch1 and that this interaction requires a phosphorylation event. Taken together, these data suggest that SEL-10 is involved in shutting off Notch signaling by ubiquitin-proteasome-mediated degradation of the active transcriptional factor after a nuclear phosphorylation event.

Nuclear localization of CBF1 is regulated by interactions with the SMRT corepressor complex

The CSL family protein CBF1 is a nuclear mediator of Notch signaling and has been predicted to contain an N-terminal nuclear localization signal in exon 4. Surprisingly, we found that CBF1 carrying mutations at codon 233 or 249 within exon 7 was restricted to the cytoplasm. In mammalian and yeast two-hybrid assays, these mutations were also associated with a loss of CBF1-mediated transcriptional repression and a severely impaired interaction with the corepressors SMRT and CIR. Overexpression of SMRT rescued the ability of mutant CBF1 to target to the nucleus of transfected cells and similarly rescued nuclear targeting of enhanced green fluorescent protein (EGFP)-CBF1 exons 6 to 9 CBF1(6-9)carrying the codon 233 or 249 mutations. Carboxy-terminally truncated SMRT with amino acids (aa) 1291 to 1495 deleted was unable to rescue the nuclear targeting of mutant EGFP-CBF1(6-9). In yeast two-hybrid assays, the SMRT aa 1291 to 1495 domain interacted with SKIP and SMRT aa 1291 to 1495 colocalized with SKIP within the nuclei of cotransfected cells. Comparison of the intracellular localization of CBF1(6-9) with that of CBF1(5-9) further supported the suggestion that nuclear targeting of CBF1 is dependent on the formation of a CBF1-SMRT-SKIP corepressor complex. These observations suggest that nuclear targeting of CBF1 is itself a component of CBF1-mediated gene regulation and that in the absence of signaling, CBF1 enters the nucleus precommitted to a transcriptional repression function. The activators NotchIC (the intracellular domain of Notch) and Epstein-Barr virus EBNA2 also mediated nuclear targeting of mutant CBF1, consistent with the competition model for activator versus corepressor binding to CBF1.

Hyperphosphorylation and association with RBP of the intracellular domain of Notch1

Although the intracellular domain of Notch1 is phosphorylated and it associates with members of the CSL family, the relationship of these events is poorly understood. Using in vivo [(32)P]orthophosphate labeling of cells expressing transfected Notch1, we observed that the furin cleaved Notch1 (TMIC) and the soluble intracellular forms (NICD), but not the full-length molecule were phosphorylated. Furthermore, transfected NICD molecules showed a significantly greater specific activity of phosphorylation, or hyperphosphorylation, compared to TMIC molecules. Hyperphosphorylation of NICD was also observed when NICD was generated by an endogenous intramembraneous cleavage of TMIC. However, TMIC molecules bearing a mutation that reduces intramembraneous cleavage (V1744K) did not show an enhanced incorporation of phosphate, suggesting that cleavage is required for hyperphosphorylation. Using deletion constructs to map the sites of phosphorylation, we observed that a domain of 93 amino acids downstream of the ankyrin repeats incorporated the majority of (32)P in vivo. This sequence was also required for activation of the HES-1 promoter. In addition, we observed that hyperphosphorylated forms of the intracellular domain were more likely to interact with the transcriptional coactivator RBP. However, dephosphorylation experiments showed that the interaction between RBP and the intracellular domain of Notch was not dependent upon Notch1IC phosphorylation. These studies reveal that phosphorylation of the intracellular domain of the Notch receptor is a dynamic process during the events of Notch signal transduction.

Nrarp is a novel intracellular component of the Notch signaling pathway

The Lin12/Notch receptors regulate cell fate during embryogenesis by activating the expression of downstream target genes. These receptors signal via their intracellular domain (ICD), which is released from the plasma membrane by proteolytic processing and associates in the nucleus with the CSL family of DNA-binding proteins to form a transcriptional activator. How the CSL/ICD complex activates transcription and how this complex is regulated during development remains poorly understood. Here we describe Nrarp as a new intracellular component of the Notch signaling pathway in Xenopus embryos. Nrarp is a member of the Delta-Notch synexpression group and encodes a small protein containing two ankyrin repeats. Nrarp expression is activated in Xenopus embryos by the CSL-dependent Notch pathway. Conversely, overexpression of Nrarp in embryos blocks Notch signaling and inhibits the activation of Notch target genes by ICD. We show that Nrarp forms a ternary complex with the ICD of XNotch1 and the CSL protein XSu(H) and that in embryos Nrarp promotes the loss of ICD. By down-regulating ICD levels, Nrarp could function as a negative feedback regulator of Notch signaling that attenuates ICD-mediated transcription.

Notch1IC partially replaces EBNA2 function in B cells immortalized by Epstein-Barr virus

Immortalization of B cells by Epstein-Barr virus (EBV) depends on the virally encoded EBNA2 protein. Although not related by sequence, the cellular Notch protein and EBNA2 share several biochemical and functional properties, such as interaction with CBF1 and the ability to activate transcription of a number of cellular and viral genes. Whether these similarities are coincidental or exemplify EBNA2 mimicry of evolutionarily conserved cellular signaling pathways is unclear. We therefore investigated whether activated forms of Notch could substitute for EBNA2 in maintaining the immortalized phenotype of EBV-infected B cells. To address this question, we devised a transcomplementation system using EREB2.5 cells. EREB2.5 cells are immortalized by EBV expressing a conditional estrogen receptor EBNA2 fusion protein (EREBNA2), and cellular proliferation is dependent on the availability of estrogen. Withdrawal of estrogen results in inactivation of EREBNA2, leading to growth arrest and eventually to cell death. Transduction of EREB2.5 cells with a lentiviral vector expressing wild-type EBNA2 rescued EREB2.5 cells from the growth-inhibitory effects of estrogen deprivation, in contrast to transduction with the lentivirus vector alone. EREB2.5 cells were also rescued by enforced expression of human Notch1IC after estrogen starvation, but this effect was restricted to cells expressing high levels of the transcription factor. Compared to wild-type EBNA2-expressing EREB2.5 cells, the Notch-expressing cells expanded more slowly after estrogen starvation, and once established, they continued to display a lower proliferation rate. Analysis of viral and cellular gene expression from transduced EREB2.5 cells after estrogen withdrawal indicated that both wild-type EBNA2- and Notch1IC-positive cells expressed c-Myc at levels similar to those found in parental EREB2.5 cells. However, the latter cells expressed LMP-1 far less efficiently than cells transduced with the wild-type EBNA2 gene. Cells rescued by either wild-type EBNA2 or Notch1IC expressed surface CD21 and CD23 proteins, but not CD10, indicating that induction of relevant type III latency markers was maintained. The data imply that both Notch and EBNA2 activate an important subset of cellular genes associated with type III latency and B-cell growth, while EBNA2 more efficiently induces important viral genes, such as LMP-1. Thus, exploitation of conserved Notch-related signaling pathways may represent a key mechanism by which EBNA2 contributes to EBV-induced cell immortalization.

Gamma -secretase inhibitors repress thymocyte development

A major therapeutic target in the search for a cure to the devastating Alzheimer's disease is gamma-secretase. This activity resides in a multiprotein enzyme complex responsible for the generation of Abeta42 peptides, precipitates of which are thought to cause the disease. Gamma-secretase is also a critical component of the Notch signal transduction pathway; Notch signals regulate development and differentiation of adult self-renewing cells. This has led to the hypothesis that therapeutic inhibition of gamma-secretase may interfere with Notch-related processes in adults, most alarmingly in hematopoiesis. Here, we show that application of gamma-secretase inhibitors to fetal thymus organ cultures interferes with T cell development in a manner consistent with loss or reduction of Notch1 function. Progression from an immature CD4-/CD8- state to an intermediate CD4+/CD8+ double-positive state was repressed. Furthermore, treatment beginning later at the double-positive stage specifically inhibited CD8+ single-positive maturation but did not affect CD4+ single-positive cells. These results demonstrate that pharmacological gamma-secretase inhibition recapitulates Notch1 loss in a vertebrate tissue and present a system in which rapid evaluation of gamma-secretase-targeted pharmaceuticals for their ability to inhibit Notch activity can be performed in a relevant context.

Myf-5 is transiently expressed in nonmuscle mesoderm and exhibits dynamic regional changes within the presegmented mesoderm and somites I-IV

Myf-5 is one of four myogenic regulatory factors that play important roles in skeletal muscle development. This study provides detailed analysis of Myf-5 expression during early chick development using an in situ hybridization technique that has been optimized to detect low level Myf-5 transcripts. This facilitated detection of heretofore unrecognized dynamic changes in Myf-5 expression patterns. Myf-5 expression is first detected at stage 3 in the primitive streak and exhibits transient low-level expression in nonmyogenic mesoderm. Myf-5 is later expressed in the presegmented mesoderm (psm) in a reiterating pattern that is coordinated with somitogenesis and also colocalizes with the Notch ligand C-Delta-1. In somites (S) I-IV, Myf-5 expression exhibits dynamic regional changes, and in somites rostral to S IV, Myf-5 is expressed at higher levels in muscle precursors in the dorsomedial somite. Semiquantitative comparison of Myf-5 mRNA levels in the psm and in myotome-containing somites indicates about a 10-fold difference. The expression pattern of Myf-5 differs from that of MyoD, which we find is expressed only in the dorsomedial somite. These data reveal that Myf-5 is expressed at low levels several stages before muscle differentiation occurs and suggest that only a subset of cells that initially express Myf-5 will upregulate its expression and differentiate as muscle.

Epstein-Barr virus BamHi-a rightward transcript-encoded RPMS protein interacts with the CBF1-associated corepressor CIR to negatively regulate the activity of EBNA2 and NotchIC

The Epstein-Barr virus (EBV) BamHI-A rightward transcripts (BARTs) are expressed in all EBV-associated tumors as well as in latently infected B cells in vivo and cultured B-cell lines. One of the BART family transcripts contains an open reading frame, RPMS1, that encodes a nuclear protein termed RPMS. Reverse transcription-PCR analysis revealed that BART transcripts with the splicing pattern that generates the RPMS1 open reading frame are commonly expressed in EBV-positive lymphoblastoid cell lines and are also detected in Hodgkin's disease tissues. Experiments undertaken to determine the function of RPMS revealed that RPMS interacts with both CBF1 and components of the CBF1-associated corepressor complex. RPMS interaction with CBF1 was demonstrated in a glutathione S-transferase (GST) affinity assay and by the ability of RPMS to alter the intracellular localization of a mutant CBF1. A Gal4-RPMS fusion protein mediated transcriptional repression, suggesting an additional interaction between RPMS and corepressor proteins. GST affinity assays revealed interaction between RPMS and the corepressor Sin3A and CIR. The RPMS-CIR interaction was further substantiated in mammalian two-hybrid, coimmunoprecipitation, and colocalization experiments. RPMS has been shown to interfere with NotchIC and EBNA2 activation of CBF1-containing promoters in reporter assays. Consistent with this function, immunofluorescence assays performed on cotransfected cells showed that there was colocalization of RPMS with NotchIC and with EBNA2 in intranuclear punctate speckles. The effect of RPMS on NotchIC function was further examined in a muscle cell differentiation assay where RPMS was found to partially reverse NotchIC-mediated inhibition of differentiation. The mechanism of RPMS action was examined in cotransfection and mammalian two-hybrid assays. The results revealed that RPMS blocked relief of CBF1-mediated repression and interfered with SKIP-CIR interactions. We conclude that RPMS acts as a negative regulator of EBNA2 and Notch activity through its interactions with the CBF1-associated corepressor complex.