Notch signaling: simplicity in design, versatility in function

CD46: the 'multitasker' of complement proteins

Complement is undeniably quintessential for innate immunity by detecting and eliminating infectious microorganisms. Recent work, however, highlights an equally profound impact of complement on the induction and regulation of a wide range of immune cells. In particular, the complement regulator CD46 emerges as a key sensor of immune activation and a vital modulator of adaptive immunity. In this review, we summarize the current knowledge of CD46-mediated signalling events and their functional consequences on immune-competent cells with a specific focus on those in CD4(+) T cells. We will also discuss the promises and challenges that potential therapeutic modulation of CD46 may hold and pose.

The very many faces of presenilins and the γ-secretase complex

Presenilin is a central, catalytic component of the γ-secretase complex which conducts intramembrane cleavage of various protein substrates. Although identified and mainly studied through its role in the development of amyloid plaques in Alzheimer disease, γ-secretase has many other important functions. The complex seems to be evolutionary conserved throughout the Metazoa, but recent findings in plants and Dictyostelium discoideum as well as in archeons suggest that its evolution and functions might be much more diversified than previously expected. In this review, a selective survey of the multitude of functions of presenilins and the γ-secretase complex is presented. Following a brief overview of γ-secretase structure, assembly and maturation, three functional aspects are analyzed: (1) the role of γ-secretase in autophagy and phagocytosis; (2) involvement of the complex in signaling related to endocytosis; and (3) control of calcium fluxes by presenilins.

A critical role for Sox9 in notch-induced astrogliogenesis and stem cell maintenance

Notch signaling is a key regulator of cell-fate decisions and is essential for proper neuroectodermal development. There, it favors the formation of ectoderm, promotes maintenance of neural stem cells, inhibits differentiation into neurons, and commits neural progenitors to a glial fate. In this report, we explore downstream effects of Notch important for astroglial differentiation. Transient activation of Notch1 during early stages of neuroectodermal differentiation of embryonic stem cells resulted in an increase of neural stem cells, a reduction in neurons, an induction of astroglial cell differentiation, and an induction of neural crest (NC) development. Transient or continuous activation of Notch1 during neuroectodermal differentiation led to upregulation of Sox9 expression. Knockdown of the Notch1-induced Sox9 expression reversed Notch1-induced astroglial cell differentiation, increase in neural stem cells, and the decrease in neurons, whereas the Notch1 effects on NC development were hardly affected by knockdown of Sox9 expression. These findings reveal a critical role for Notch-mediated upregulation of Sox9 in a select set of neural lineage determination steps controlled by Notch.

A glycosphingolipid binding domain controls trafficking and activity of the mammalian notch ligand delta-like 1

The activity of Notch ligands is tightly regulated by trafficking events occurring both before and after ligand-receptor interaction. In particular endocytosis and recycling have been shown to be required for full signaling activity of the ligands before they encounter the Notch receptor. However little is known about the precise endocytic processes that contribute to ligand internalization. Here we demonstrate that endocytosis contributes to Dll1 signaling activity by preserving the ligand from shedding and degradation. We further show that the glycosphingolipid-binding motif originally identified in Drosophila Notch ligands is conserved in mammals and is necessary for Dll1 internalization. Mutation of its conserved tryptophan residue results in a Dll1 molecule which is rapidly inactivated by shedding and degradation, does not recycle to the cell surface and does not activate Notch signaling. Finally, silencing in the signal-sending cells of glucosylceramide synthase, the enzyme implicated in the initial phase of glycosphingolipid synthesis, down-regulates Notch activation. Our data indicate that glycosphingolipids, by interacting with Dll1, may act as functional co-factors to promote its biological activity.

HDAC1 and HDAC2 restrain the intestinal inflammatory response by regulating intestinal epithelial cell differentiation

Acetylation and deacetylation of histones and other proteins depends on histone acetyltransferases and histone deacetylases (HDACs) activities, leading to either positive or negative gene expression. HDAC inhibitors have uncovered a role for HDACs in proliferation, apoptosis and inflammation. However, little is known of the roles of specific HDACs in intestinal epithelial cells (IEC). We investigated the consequences of ablating both HDAC1 and HDAC2 in murine IECs. Floxed Hdac1 and Hdac2 homozygous mice were crossed with villin-Cre mice. Mice deficient in both IEC HDAC1 and HDAC2 weighed less and survived more than a year. Colon and small intestinal sections were stained with hematoxylin and eosin, or with Alcian blue and Periodic Acid Schiff for goblet cell identification. Tissue sections from mice injected with BrdU for 2 h, 14 h and 48 h were stained with anti-BrdU. To determine intestinal permeability, 4-kDa FITC-labeled dextran was given by gavage for 3 h. Microarray analysis was performed on total colon RNAs. Inflammatory and IEC-specific gene expression was assessed by Western blot or semi-quantitative RT-PCR and qPCR with respectively total colon protein and total colon RNAs. HDAC1 and HDAC2-deficient mice displayed: 1) increased migration and proliferation, with elevated cyclin D1 expression and phosphorylated S6 ribosomal protein, a downstream mTOR target; 2) tissue architecture defects with cell differentiation alterations, correlating with reduction of secretory Paneth and goblet cells in jejunum and goblet cells in colon, increased expression of enterocytic markers such as sucrase-isomaltase in the colon, increased expression of cleaved Notch1 and augmented intestinal permeability; 3) loss of tissue homeostasis, as evidenced by modifications of claudin 3 expression, caspase-3 cleavage and Stat3 phosphorylation; 4) chronic inflammation, as determined by inflammatory molecular expression signatures and altered inflammatory gene expression. Thus, epithelial HDAC1 and HDAC2 restrain the intestinal inflammatory response, by regulating intestinal epithelial cell proliferation and differentiation.

notch3 is essential for oligodendrocyte development and vascular integrity in zebrafish

Mutations in the human NOTCH3 gene cause CADASIL syndrome (cerebral autosomal dominant arteriopathy with subcortical infarcts and leukoencephalopathy). CADASIL is an inherited small vessel disease characterized by diverse clinical manifestations including vasculopathy, neurodegeneration and dementia. Here we report two mutations in the zebrafish notch3 gene, one identified in a previous screen for mutations with reduced expression of myelin basic protein (mbp) and another caused by a retroviral insertion. Reduced mbp expression in notch3 mutant embryos is associated with fewer oligodendrocyte precursor cells (OPCs). Despite an early neurogenic phenotype, mbp expression recovered at later developmental stages and some notch3 homozygous mutants survived to adulthood. These mutants, as well as adult zebrafish carrying both mutant alleles together, displayed a striking stress-associated accumulation of blood in the head and fins. Histological analysis of mutant vessels revealed vasculopathy, including: an enlargement (dilation) of vessels in the telencephalon and fin, disorganization of the normal stereotyped arrangement of vessels in the fin, and an apparent loss of arterial morphological structure. Expression of hey1, a well-known transcriptional target of Notch signaling, was greatly reduced in notch3 mutant fins, suggesting that Notch3 acts via a canonical Notch signaling pathway to promote normal vessel structure. Ultrastructural analysis confirmed the presence of dilated vessels in notch3 mutant fins and revealed that the vessel walls of presumed arteries showed signs of deterioration. Gaps in the arterial wall and the presence of blood cells outside of vessels in mutants indicated that compromised vessel structure led to hemorrhage. In notch3 heterozygotes, we found elevated expression of both notch3 itself and target genes, indicating that specific alterations in gene expression due to partial loss of Notch3 function might contribute to the abnormalities observed in heterozygous larvae and adults. Our analysis of zebrafish notch3 mutants indicates that Notch3 regulates OPC development and mbp gene expression in larvae, and maintains vascular integrity in adults.

Notch1 cardioprotection in myocardial ischemia/reperfusion involves reduction of oxidative/nitrative stress

Oxidative/nitrative stress plays an important role in myocardial ischemia/reperfusion (MI/R) injury. Notch1 participates in the regulation of cardiogenesis and cardiac response to hypertrophic stress, but the function of Notch1 signaling in MI/R has not been explored. This study aims to determine the role of Notch1 in MI/R, and investigate whether Notch1 confers cardioprotection. Notch1 specific small interfering RNA (siRNA, 20 μg) or Jagged1 (a Notch ligand, 12 μg) was delivered through intramyocardial injection. 48 h after injection, mice were subjected to 30 min of myocardial ischemia followed by 3 h (for cell apoptosis and oxidative/nitrative stress), 24 h (for infarct size and cardiac function), or 2 weeks (for cardiac fibrosis and function) of reperfusion. Cardiac-specific Notch1 knockdown resulted in significantly aggravated I/R injury, as evidenced by enlarged infarct size, depressed cardiac function, increased myocardial apoptosis and cardiac fibrosis. Downregulation of Notch1 increased expression of inducible NO synthase (iNOS) and gp(91phox), enhanced the production of NO metabolites and superoxide, as well as their cytotoxic reaction product peroxynitrite. Moreover, Notch1 blockade also reduced phosphorylation of endothelial NO synthase (eNOS) and Akt, and increased expression of PTEN, a key phosphatase involved in the regulation of Akt phosphorylation. In addition, activation of Notch1 by Jagged1 or administration of peroxynitrite scavenger reduced production of peroxynitrite and attenuated MI/R injury. These data indicate that Notch1 signaling protects against MI/R injury partly though PTEN/Akt mediated anti-oxidative and anti-nitrative effects.

Regulated intramembrane proteolysis and degradation of murine epithelial cell adhesion molecule mEpCAM

Epithelial cell adhesion molecule EpCAM is a transmembrane glycoprotein, which is highly and frequently expressed in carcinomas and (cancer-)stem cells, and which plays an important role in the regulation of stem cell pluripotency. We show here that murine EpCAM (mEpCAM) is subject to regulated intramembrane proteolysis in various cells including embryonic stem cells and teratocarcinomas. As shown with ectopically expressed EpCAM variants, cleavages occur at α-, β-, γ-, and ε-sites to generate soluble ectodomains, soluble Aβ-like-, and intracellular fragments termed mEpEX, mEp-β, and mEpICD, respectively. Proteolytic sites in the extracellular part of mEpCAM were mapped using mass spectrometry and represent cleavages at the α- and β-sites by metalloproteases and the b-secretase BACE1, respectively. Resulting C-terminal fragments (CTF) are further processed to soluble Aβ-like fragments mEp-β and cytoplasmic mEpICD variants by the g-secretase complex. Noteworthy, cytoplasmic mEpICD fragments were subject to efficient degradation in a proteasome-dependent manner. In addition the γ-secretase complex dependent cleavage of EpCAM CTF liberates different EpICDs with different stabilities towards proteasomal degradation. Generation of CTF and EpICD fragments and the degradation of hEpICD via the proteasome were similarly demonstrated for the human EpCAM ortholog. Additional EpCAM orthologs have been unequivocally identified in silico in 52 species. Sequence comparisons across species disclosed highest homology of BACE1 cleavage sites and in presenilin-dependent γ-cleavage sites, whereas strongest heterogeneity was observed in metalloprotease cleavage sites. In summary, EpCAM is a highly conserved protein present in fishes, amphibians, reptiles, birds, marsupials, and placental mammals, and is subject to shedding, γ-secretase-dependent regulated intramembrane proteolysis, and proteasome-mediated degradation.

Co-ordinating Notch, BMP, and TGF-β signaling during heart valve development

Congenital heart defects affect approximately 1-5 % of human newborns each year, and of these cardiac defects 20-30 % are due to heart valve abnormalities. Recent literature indicates that the key factors and pathways that regulate valve development are also implicated in congenital heart defects and valve disease. Currently, there are limited options for treatment of valve disease, and therefore having a better understanding of valve development can contribute critical insight into congenital valve defects and disease. There are three major signaling pathways required for early specification and initiation of endothelial-to-mesenchymal transformation (EMT) in the cardiac cushions: BMP, TGF-β, and Notch signaling. BMPs secreted from the myocardium set up the environment for the overlying endocardium to become activated; Notch signaling initiates EMT; and both BMP and TGF-β signaling synergize with Notch to promote the transition of endothelia to mesenchyme and the mesenchymal cell invasiveness. Together, these three essential signaling pathways help form the cardiac cushions and populate them with mesenchyme and, consequently, set off the cascade of events required to develop mature heart valves. Furthermore, integration and cross-talk between these pathways generate highly stratified and delicate valve leaflets and septa of the heart. Here, we discuss BMP, TGF-β, and Notch signaling pathways during mouse cardiac cushion formation and how they together produce a coordinated EMT response in the developing mouse valves.

Hutchinson-Gilford progeria syndrome through the lens of transcription

Lamins are nuclear intermediate filaments. In addition to their structural roles, they are implicated in basic nuclear functions such as chromatin organization, DNA replication, transcription, DNA repair, and cell-cycle progression. Mutations in human LMNA gene cause several diseases termed laminopathies. One of the laminopathic diseases is Hutchinson-Gilford progeria syndrome (HGPS), which is caused by a spontaneous mutation and characterized by premature aging. HGPS phenotypes share certain similarities with several apparently comparable medical conditions, such as aging and atherosclerosis, with the conspicuous absence of neuronal degeneration and cancer rarity during the short lifespan of the patients. Cell lines from HGPS patients are characterized by multiple nuclear defects, which include abnormal morphology, altered histone modification patterns, and increased DNA damage. These cell lines provide insight into the molecular pathways including senescence that require lamins A and B1. Here, we review recent data on HGPS phenotypes through the lens of transcriptional deregulation caused by lack of functional lamin A, progerin accumulation, and lamin B1 silencing.

B-cell receptor configuration and mutational analysis of patients with chronic lymphocytic leukaemia and trisomy 12 reveal recurrent molecular abnormalities

Trisomy 12 (+12) is the third most frequent cytogenetic aberration in chronic lymphocytic leukaemia (CLL) retrievable both as the sole chromosomal abnormality or in association with additional alterations. NOTCH1 mutations are known to be more prevalent among +12 patients, whereas mutations of FBXW7, a gene involved in NOTCH1 degradation, that lead to the constitutional activation of NOTCH1 have not been investigated in this setting. We analyzed a unicentric cohort of 44 +12 patients with CLL for mutations of TP53, NOTCH1 and FBXW7 genes, and we correlated them with B-cell receptor (BCR) configurations. FBXW7, TP53 and NOTCH1 mutations were identified in 4.5%, 6.8% and 18.2% of patients, respectively. FBXW7 and NOTCH1 mutations appeared in a mutually exclusive fashion, suggesting that both aberrations might affect the same biological pathway. We found that 44.1% of +12 CLL patients had stereotyped B-cell receptors, which is significantly higher than that observed in patients with CLL and no +12 (27%, p = 0.01). Subsets #1, #8, #10, #28 and #59 were the most represented stereotyped patterns, and IGHV4-39*01 was the gene configuration most commonly used. There was a significantly higher risk for Richter's syndrome (RS) transformation in patients with NOTCH1 or FBXW7 mutations, with four of the seven (57%) patients developing RS and characterized at least by one of the two abnormalities. These observations suggest that, similarly to the aberrations of NOTCH1, FBXW7 gene mutations may also result in cell proliferation and evasion from apoptosis in patients with +12 CLL. Together with the extremely high frequency of stereotyped BCRs and RS transformation, these abnormalities appear to cluster in these CLL patients with additional chromosome 12, suggesting a connection with the prognosis of the disease.

Transcriptional regulation of endothelial cell and vascular development

The establishment and maintenance of the vascular system is critical for embryonic development and postnatal life. Defects in endothelial cell development and vessel formation and function lead to embryonic lethality and are important in the pathogenesis of vascular diseases. Here, we review the underlying molecular mechanisms of endothelial cell differentiation, plasticity, and the development of the vasculature. This review focuses on the interplay among transcription factors and signaling molecules that specify the differentiation of vascular endothelial cells. We also discuss recent progress on reprogramming of somatic cells toward distinct endothelial cell lineages and its promise in regenerative vascular medicine.

The Drosophila importin-α3 is required for nuclear import of notch in vivo and it displays synergistic effects with notch receptor on cell proliferation

The Notch signaling pathway controls diverse cell-fate specification events throughout development. The versatility of this pathway to influence different aspects of development comes from its multiple levels of regulation. Upon ligand-induced Notch activation, the Notch intracellular domain (Notch-ICD) is released from the membrane and translocates to the nucleus, where it transduces Notch signals by regulating the transcription of downstream target genes. But the exact mechanism of translocation of Notch-ICD into the nucleus is not clear. Here, we implicate Importin-α3 (also known as karyopherin-α3) in the nuclear translocation of Notch-ICD in Drosophila. Our present analyses reveal that Importin-α3 can directly bind to Notch-ICD and loss of Importin-α3 function results in cytoplasmic accumulation of the Notch receptor. Using MARCM (Mosaic Analysis with a Repressible Cell Marker) technique, we demonstrate that Importin-α3 is required for nuclear localization of Notch-ICD. These results reveal that the nuclear transport of Notch-ICD is mediated by the canonical Importin-α3/Importin-β transport pathway. In addition, co-expression of both Notch-ICD and Importin-α3 displays synergistic effects on cell proliferation. Taken together, our results suggest that Importin-α3 mediated nuclear import of Notch-ICD may play important role in regulation of Notch signaling.

Notch3 is activated by chronic hypoxia and contributes to the progression of human prostate cancer

Prostate cancer (PC) is still the second cause of cancer-related death among men. Although patients with metastatic presentation have an ominous outcome, the vast majority of PCs are diagnosed at an early stage. Nonetheless, even among patients with clinically localized disease the outcome may vary considerably. Other than androgen sensitivity, little is known about which other signaling pathways are deranged in aggressive, localized cancers. The elucidation of such pathways may help to develop innovative therapies aimed at specific molecular targets. We report that in a hormone-sensitive PC cell line, LNCaP, Notch3 was activated by hypoxia and sustained cell proliferation and colony formation in soft agar. Hypoxia also modulated cellular cholesterol content and the number and size of lipid rafts, causing a coalescence of small rafts into bigger clusters; under this experimental condition, Notch3 migrated from the non-raft into the raft compartment where it colocalized with the γ-secretase complex. We also looked at human PC biopsies and found that expression of Notch3 positively correlated with Gleason score and with expression of carbonic anhydrase IX, a marker of hypoxia. In conclusion, hypoxia triggers the activation of Notch3, which, in turn, sustains proliferation of PC cells. Notch3 pathway represents a promising target for adjuvant therapy in patients with PC.

Notch signaling and bone remodeling

Notch signaling plays context-dependent roles in the development and maintenance of many cell types and tissues in mammals. In the skeleton, both osteoblasts and osteoclasts require Notch signaling for proper differentiation and function, and the specific roles of Notch are dependent on the differentiation status of the cell. The recent discovery of activating NOTCH2 mutations as the cause of Hajdu-Cheney syndrome has highlighted the significance of Notch signaling in human bone physiology.

Notch2 regulates BMP signaling and epithelial morphogenesis in the ciliary body of the mouse eye

The ciliary body (CB) of the mammalian eye is responsible for secreting aqueous humor to maintain intraocular pressure, which is elevated in the eyes of glaucoma patients. It contains a folded two-layered epithelial structure comprising the nonpigmented inner ciliary epithelium (ICE), the pigmented outer ciliary epithelium (OCE), and the underlying stroma. Although the CB has an important function in the eye, its morphogenesis remains poorly studied. In this study, we show that conditional inactivation of the Jagged 1 (Jag1)-Notch2 signaling pathway in the developing CB abolishes its morphogenesis. Notch2 is expressed in the OCE of the CB, whereas Jag1 is expressed in the ICE. Conditional inactivation of Jag1 in the ICE or Notch2 in the OCE disrupts CB morphogenesis, but neither affects the specification of the CB region. Notch2 signaling in the OCE is required for promoting cell proliferation and maintaining bone morphogenetic protein (BMP) signaling, both of which have been suggested to be important for CB morphogenesis. Although Notch and BMP signaling pathways are known to cross-talk via the interaction between their downstream transcriptional factors, this study suggests that Notch2 maintains BMP signaling in the OCE possibly by repressing expression of secreted BMP inhibitors. Based on our findings, we propose that Jag1-Notch2 signaling controls CB morphogenesis at least in part by regulating cell proliferation and BMP signaling.

Notch signaling maintains neural rosette polarity

Formation of the metazoan body plan requires a complex interplay of morphological changes and patterning, and central to these processes is the establishment of apical/basal cell polarity. In the developing nervous system, apical/basal cell polarity is essential for neural tube closure and maintenance of the neural stem cell population. In this report we explore how a signaling pathway important for nervous system development, Notch signaling, impacts on apical/basal cell polarity in neural differentiation. CSL(-/-) mouse embryos, which are devoid of canonical Notch signaling, demonstrated a neural tube phenotype consistent with cell polarity and convergent extension defects, including deficiencies in the restricted expression of apical polarity markers in the neuroepithelium. CSL(-/-) mouse embryonic stem (ES) cells, cultured at low density, behaved as wild-type in the establishment of neural progenitors and apical specification, though progression through rosette formation, an in vitro correlate of neurulation, required CSL for correct maintenance of rosette structure and regulation of neuronal differentiation. Similarly, acute pharmacological inhibition of Notch signaling led to the breakdown of neural rosettes and accelerated neuronal differentiation. In addition to functional Notch signaling, rosette integrity was found to require actin polymerization and Rho kinase (ROCK) activity. Disruption of rosettes through inhibition of actin polymerization or ROCK activity, however, had no effect on neuronal differentiation, indicating that rosette maintenance is not a prerequisite for normal neuronal differentiation. In conclusion, our data indicate that Notch signaling plays a role not only in differentiation, but also in organization and maintenance of polarity during development of the early nervous system.

Perineurial glia require Notch signaling during motor nerve development but not regeneration

Motor nerves play the critical role of shunting information out of the CNS to targets in the periphery. Their formation requires the coordinated development of distinct cellular components, including motor axons and the Schwann cells and perineurial glia that ensheath them. During nervous system assembly, these glial cells must migrate long distances and terminally differentiate, ensuring the efficient propagation of action potentials. Although we know quite a bit about the mechanisms that control Schwann cell development during this process, nothing is known about the mechanisms that mediate the migration and differentiation of perineurial glia. Using in vivo imaging in zebrafish, we demonstrate that Notch signaling is required for both perineurial migration and differentiation during nerve formation, but not regeneration. Interestingly, loss of Notch signaling in perineurial cells also causes a failure of Schwann cell differentiation, demonstrating that Schwann cells require perineurial glia for aspects of their own development. These studies describe a novel mechanism that mediates multiple aspects of perineurial development and reveal the critical importance of perineurial glia for Schwann cell maturation and nerve formation.

Notch1 functions as a negative regulator of lymphatic endothelial cell differentiation in the venous endothelium

In development, lymphatic endothelial cells originate within veins and differentiate via a process requiring Prox1. Notch signaling regulates cell-fate decisions, and expression studies suggested that Jag1/Notch1 signaling functions in veins during lymphatic endothelial specification. Using an inducible lymphatic endothelial Prox1CreER(T2) driver, Notch signaling was suppressed by deleting Notch1 or expressing dominant-negative Mastermind-like in Prox1+ endothelial cells. Either loss of Notch1 or reduced Notch signaling increased Prox1+ lymphatic endothelial progenitor cell numbers in the veins, leading to incomplete separation of venous and lymphatic vessels. Notch loss of function resulted in excessive Prox1+ lymphatic cells emerging from the cardinal vein and significant lymphatic overgrowth. Moreover, loss of one allele of Notch1 in Prox1 heterozygous mice rescued embryonic lethality due to Prox1 haploinsufficiency and significantly increased Prox1+ lymphatic endothelial progenitor cell numbers. Expression of a constitutively active Notch1 protein in Prox1+ cells suppressed endothelial Prox1 from E9.75 to E13.5, resulting in misspecified lymphatic endothelial cells based upon reduced expression of podoplanin, LYVE1 and VEGFR3. Notch activation resulted in the appearance of blood endothelial cells in peripheral lymphatic vessels. Activation of Notch signaling in the venous endothelium at E10.5 did not arterialize the cardinal vein, suggesting that Notch can no longer promote arterialization in the cardinal vein during this developmental stage. We report a novel role for Notch1 in limiting the number of lymphatic endothelial cells that differentiate from the veins to assure proper lymphatic specification.

Dll4 and PDGF-BB convert committed skeletal myoblasts to pericytes without erasing their myogenic memory

Pericytes are endothelial-associated cells that contribute to vessel wall. Here, we report that pericytes may derive from direct conversion of committed skeletal myoblasts. When exposed to Dll4 and PDGF-BB, but not Dll1, skeletal myoblasts downregulate myogenic genes, except Myf5, and upregulate pericyte markers, whereas inhibition of Notch signaling restores myogenesis. Moreover, when cocultured with endothelial cells, skeletal myoblasts, previously treated with Dll4 and PDGF-BB, adopt a perithelial position stabilizing newly formed vessel-like networks in vitro and in vivo. In a transgenic mouse model in which cells expressing MyoD activate Notch, skeletal myogenesis is abolished and pericyte genes are activated. Even if overexpressed, Myf5 does not trigger myogenesis because Notch induces Id3, partially sequestering Myf5 and inhibiting MEF2 expression. Myf5-expressing cells adopt a perithelial position, as occasionally also observed in wild-type (WT) embryos. These data indicate that endothelium, via Dll4 and PDGF-BB, induces a fate switch in adjacent skeletal myoblasts.

LIN-12/Notch signaling instructs postsynaptic muscle arm development by regulating UNC-40/DCC and MADD-2 in Caenorhabditis elegans

The diverse cell types and the precise synaptic connectivity between them are the cardinal features of the nervous system. Little is known about how cell fate diversification is linked to synaptic target choices. Here we investigate how presynaptic neurons select one type of muscles, vm2, as a synaptic target and form synapses on its dendritic spine-like muscle arms. We found that the Notch-Delta pathway was required to distinguish target from non-target muscles. APX-1/Delta acts in surrounding cells including the non-target vm1 to activate LIN-12/Notch in the target vm2. LIN-12 functions cell-autonomously to up-regulate the expression of UNC-40/DCC and MADD-2 in vm2, which in turn function together to promote muscle arm formation and guidance. Ectopic expression of UNC-40/DCC in non-target vm1 muscle is sufficient to induce muscle arm extension from these cells. Therefore, the LIN-12/Notch signaling specifies target selection by selectively up-regulating guidance molecules and forming muscle arms in target cells.

DOI:http://dx.doi.org/10.7554/eLife.00378.001

The development of the nervous system involves the formation of complex networks of connections between diverse cell types, such as motor neurons, interneurons and pyramidal cells. However, the mechanisms by which individual cells are programmed to acquire particular identities, and how they are instructed to form connections with other specific cells, remain unclear.

In many species, the Notch signaling pathway has a role in setting up these networks. Notch is a transmembrane protein, which means that it has one component inside the cell and another outside. When a ligand binds to the extracellular part of Notch, this causes the receptor to break in two. The intracellular domain then travels to the nucleus where it can influence gene expression.

The nematode worm (C. elegans), which has two Notch receptors, is often used to study the formation of neuronal networks because each worm has only around 300 neurons, and they are connected in roughly the same way in each worm. C. elegans relies on two types of cell that are very similar to each other—type-1 and type-2 vulval muscle cells—to lay eggs, and the neurons that trigger egg-laying form synaptic connections on specialized structures called muscle arms. However, these structures are found only in type-2 vulval muscle.

To investigate the mechanisms underlying the formation of the egg-laying circuit, Li et al. screened large numbers of mutant worms to find animals that lacked muscle arms. They identified a number of such mutants, which laid fewer eggs compared to wild-type worms, and found that they all had mutations in genes that encode for proteins or ligands that are involved in the LIN-12/Notch pathway. This pathway mediates cell–cell interactions that help to specify cell fates.

Li et al. showed that type-2 vulval muscle cells develop muscle arms when their neighbors—type-1 vulval muscle cells and vulval epithelial cells—produce enough ligand to activate the LIN-12 Notch receptor on the type-2 vulval muscle cells. They also identified two of the downstream targets of LIN-12, and found that artificially expressing one of these in type-1 vulval muscle cells is sufficient to trigger the formation of muscle arms.

The work of Li et al. provides further evidence that the Notch signalling pathway, which is well known for its role in early development, also acts at later developmental stages to determine cell fate and patterns of connectivity.

DOI:http://dx.doi.org/10.7554/eLife.00378.002

Regulatory microRNA network identification in bovine blastocyst development

Mammalian blastocyst formation is characterized by two lineage segregations resulting in the formation of the trophectoderm, the hypoblast, and the epiblast cell lineages. Cell fate determination during these early lineage segregations is associated with changes in the expression of specific transcription factors. In addition to the transcription factor-based control, it has become clear that also microRNAs (miRNAs) play an important role in the post-transcriptional regulation of pluripotency and differentiation. To elucidate the role of miRNAs in early lineage segregation, we compared the miRNA expression in early bovine blastocysts with the more advanced stage of hatched blastocysts. Reverse transcription-quantitative PCR-based miRNA expression profiling revealed eight upregulated miRNAs (miR-127, miR-130a, miR-155, miR-196a, miR-203, miR-28, miR-29c, and miR-376a) and four downregulated miRNAs (miR-135a, miR-218, miR-335, and miR-449b) in hatched blastocysts. Through an integrative analysis of matching miRNA and mRNA expression data, candidate miRNA-mRNA interaction pairs were prioritized for validation. Using an in vitro luciferase reporter assay, we confirmed a direct interaction between miR-218 and CDH2, miR-218 and NANOG, and miR-449b and NOTCH1. By interfering with the FGF signaling pathway, we found functional evidence that miR-218, mainly expressed in the inner cell mass, regulates the NANOG expression in the bovine blastocyst in response to FGF signaling. The results of this study expand our knowledge about the miRNA signature of the bovine blastocyst and of the interactions between miRNAs and cell fate regulating transcription factors.

Interactions between VEGFR and Notch signaling pathways in endothelial and neural cells

Notch cell interaction mechanism governs cell fate decisions in many different cell contexts throughout the lifetime of all Metazoan species. It links the fate of one cell to that of its neighbors through cell-to-cell contacts, and binding of Notch receptors expressed on one cell to their membrane bound ligands on an adjacent cell. Environmental cues, such as growth factors and extracellular matrix molecules, superimpose a dynamic regulation on this canonical Notch signaling pathway. In this review, we will focus on Notch signaling in the vertebrate vascular and nervous systems and examine its role in angiogenesis, neurogenesis, and neurovascular interactions. We will also highlight the molecular relationships of the Notch pathway with vascular endothelial growth factors (VEGFs) and their high-affinity tyrosine kinase VEGF receptors, key regulators of both angiogenesis and neurogenesis.

Mastermind-like transcriptional co-activator-mediated Notch signaling is indispensable for maintaining conjunctival epithelial identity

Conjunctival goblet cells primarily synthesize mucins to lubricate the ocular surface, which is essential for normal vision. Notch signaling has been known to associate with goblet cell differentiation in intestinal and respiratory tracts, but its function in ocular surface has yet to be fully characterized. Herein, we demonstrate that conditional inhibition of canonical Notch signaling by expressing dominant negative mastermind-like 1 (dnMaml1) in ocular surface epithelia resulted in complete suppression of goblet cell differentiation during and subsequent to development. When compared with the ocular surface of wild-type mice (OS(Wt)), expression of dnMaml1 at the ocular surface (OS(dnMaml1)) caused conjunctival epithelial hyperplasia, aberrant desquamation, failure of Mucin 5ac (Muc5ac) synthesis, subconjunctival inflammation and epidermal metaplasia in cornea. In addition, conditional deletion of Notch1 from the ocular surface epithelia partially recapitulated OS(dnMaml1) phenotypes. We have demonstrated that N1-ICD (Notch1 intracellular domain) transactivated the mouse Krüppel-like factor 4 (Klf) promoter and that Klf4 directly bound to and significantly potentiated the Muc5ac promoter. By contrast, OS(dnMaml1) dampened Klf4 and Klf5 expression, and diminished Muc5ac synthesis. Collectively, these findings indicated that Maml-mediated Notch signaling plays a pivotal role in the initiation and maintenance of goblet cell differentiation for normal ocular surface morphogenesis and homeostasis through regulation of Klf4 and Klf5.

Notch2 is required in somatic cells for breakdown of ovarian germ-cell nests and formation of primordial follicles

Background

In the mouse ovary, oocytes initially develop in clusters termed germ-cell nests. Shortly after birth, these germ-cell nests break apart, and the oocytes individually become surrounded by somatic granulosa cells to form primordial follicles. Notch signaling plays essential roles during oogenesis in Drosophila, and recent studies have suggested that Notch signaling also plays an essential role during oogenesis and ovary development in mammals. However, no in vivo loss-of-function studies have been performed to establish whether Notch family receptors have an essential physiological role during normal ovarian development in mutant mice.

Results

Female mice with conditional deletion of the Notch2 gene in somatic granulosa cells of the ovary exhibited reduced fertility, accompanied by the formation of multi-oocyte follicles, which became hemorrhagic by 7 weeks of age. Formation of multi-oocyte follicles resulted from defects in breakdown of the primordial germ-cell nests. The ovaries of the Notch2 conditional mutant mice had increased numbers of oocytes, but decreased numbers of primordial follicles. Oocyte numbers in the Notch2 conditional mutants were increased not by excess or extended cellular proliferation, but as a result of decreased oocyte apoptosis.

Conclusions

Our work demonstrates that Notch2-mediated signaling in the somatic-cell lineage of the mouse ovary regulates oocyte apoptosis non-cell autonomously, and is essential for regulating breakdown of germ-cell nests and formation of primordial follicles. This model provides a new resource for studying the developmental and physiological roles of Notch signaling during mammalian reproductive biology.

Hypoxia-induced arterial differentiation requires adrenomedullin and notch signaling

Hypoxia (low oxygen) and Notch signaling are 2 important regulators of vascular development, but how they interact in controlling the choice between arterial and venous fates for endothelial cells during vasculogenesis is less well understood. In this report, we show that hypoxia and Notch signaling intersect in promotion of arterial differentiation. Hypoxia upregulated expression of the Notch ligand Dll4 and increased Notch signaling in a process requiring the vasoactive hormone adrenomedullin. Notch signaling also upregulated Dll4 expression, leading to a positive feedback loop sustaining Dll4 expression and Notch signaling. In addition, hypoxia-mediated upregulation of the arterial marker genes Depp, connexin40 (Gja5), Cxcr4, and Hey1 required Notch signaling. In conclusion, the data reveal an intricate interaction between hypoxia and Notch signaling in the control of endothelial cell differentiation, including a hypoxia/adrenomedullin/Dll4 axis that initiates Notch signaling and a requirement for Notch signaling to effectuate hypoxia-mediated induction of the arterial differentiation program.

Combining Notch inhibition with current therapies for breast cancer treatment

Over recent years, there has been an increasing interest in targeting Notch signalling for the treatment of breast cancer. This has stemmed from the realization that many Notch pathway components display altered expression in breast cancer, and that Notch signalling impacts on many of the cellular properties associated with tumour initiation and progression. Consequently, Notch pathway inhibitors are now entering the initial stages of clinical trials. However, there is a definite need to consider how best to use these inhibitors and therefore which treatment strategies are likely to yield the most promising results. In particular, recent studies suggest that the greatest success will come from combining Notch pathway inhibitors with current breast cancer therapies.

Down-regulated expression of Notch signaling molecules in human endometrial cancer

Notch signaling pathway is a highly conserved developmental pathway, which plays an important role in the regulation of cellular proliferation, differentiation and apoptosis. Deregulation of Notch pathway has been connected with the carcinogenesis in a variety of cancers. In this study, we investigated the expression of Notch receptors (NOTCH1, NOTCH2, NOTCH3 and NOTCH4), ligands (JAG1, JAG2 and DLL1) and target gene HES1. Fifty paired samples of endometrial cancer and adjacent nontumor endometrial tissue from endometrial cancer patients were analyzed by quantitative PCR. The mRNA levels of all investigated molecules were lower in endometrial cancer compared to adjacent nontumor tissue. The expression of NOTCH1, NOTCH4 and DLL1 in IB stage adenocarcinoma was significantly lower (P < 0.05) than the expression in IA stage adenocarcinoma. Significant correlations were found between mRNA expression levels of Notch target gene HES1 and several Notch signaling molecules: NOTCH1, NOTCH3, DLL1 (P < 0.001) and NOTCH2, JAG2 (P < 0.05). This supports the notion that Notch pathway can function as tumor suppressor in human endometrial cancer.

Transcriptional dynamics elicited by a short pulse of notch activation involves feed-forward regulation by E(spl)/Hes genes

Dynamic activity of signaling pathways, such as Notch, is vital to achieve correct development and homeostasis. However, most studies assess output many hours or days after initiation of signaling, once the outcome has been consolidated. Here we analyze genome-wide changes in transcript levels, binding of the Notch pathway transcription factor, CSL [Suppressor of Hairless, Su(H), in Drosophila], and RNA Polymerase II (Pol II) immediately following a short pulse of Notch stimulation. A total of 154 genes showed significant differential expression (DE) over time, and their expression profiles stratified into 14 clusters based on the timing, magnitude, and direction of DE. E(spl) genes were the most rapidly upregulated, with Su(H), Pol II, and transcript levels increasing within 5–10 minutes. Other genes had a more delayed response, the timing of which was largely unaffected by more prolonged Notch activation. Neither Su(H) binding nor poised Pol II could fully explain the differences between profiles. Instead, our data indicate that regulatory interactions, driven by the early-responding E(spl)bHLH genes, are required. Proposed cross-regulatory relationships were validated in vivo and in cell culture, supporting the view that feed-forward repression by E(spl)bHLH/Hes shapes the response of late-responding genes. Based on these data, we propose a model in which Hes genes are responsible for co-ordinating the Notch response of a wide spectrum of other targets, explaining the critical functions these key regulators play in many developmental and disease contexts.

Signaling via the Notch pathway conveys important information that helps to shape tissues and, when misused, contributes to diseases. Cells respond to the Notch signal by changing which genes are transcribed. Most previous studies have looked at changes in gene activity at a single time point, long after the start of signaling. By looking at carefully timed intervals immediately after Notch pathway activation, we have been able to follow the dynamic changes in transcription of all the genes and have found that they exhibit different patterns of activity. For example, activity of some genes, especially a previously characterised family called the E(spl) genes, starts very early, whereas others show more delayed upregulation. Our investigations into the underlying mechanisms reveal that cross-regulatory interactions driven by the early genes are required to shape the timing of the delayed response. This feed-forward mechanism is important because it explains why the E(spl)/Hes genes can play such a pivotal role in the Notch response, despite the fact that many other genes are regulated by the signal, a finding that will be valuable for understanding the contribution of E(spl)/Hes genes in diseases associated with altered Notch.

Stem cell proliferation versus meiotic fate decision in Caenorhabditis elegans

The C. elegans germ line has emerged as an important model for -understanding how a stem cell population is maintained throughout the life of the animal while still producing the gametes necessary for propagation of the species. The stem cell population in the adult hermaphrodite is relatively large, with stem cells giving rise to daughters that appear intrinsically equivalent; however, some of the daughters retain the proliferative fate while others enter meiotic prophase. While machinery exists for cells to progress through the mitotic cell cycle and machinery exists for cells to progress through meiotic prophase, central to understanding germ line development is identifying the genes and regulatory processes that determine whether the mitotic cell cycle or meiotic prophase machinery will be utilized; in other words, the genes that regulate the switch of germ cells from the proliferative stem cell fate to the meiotic development fate. Whether a germ cell self-renews or enters meiotic prophase is largely determined by its proximity to the distal tip cell (DTC), which is the somatic niche cell that caps the distal end of the gonad. Germ cells close to the DTC have high levels of GLP-1 Notch signaling, which promotes the proliferative fate, while cells further from the DTC have high activity levels of the GLD-1 and GLD-2 redundant RNA regulatory pathways, as well as a third uncharacterized pathway, each of which direct cells to enter meiotic prophase. Other factors and pathways modulate this core genetic pathway, or work in parallel to it, presumably to ensure that a tight balance is maintained between proliferation and meiotic entry.

Polycomb controls gliogenesis by regulating the transient expression of the Gcm/Glide fate determinant

The Gcm/Glide transcription factor is transiently expressed and required in the Drosophila nervous system. Threshold Gcm/Glide levels control the glial versus neuronal fate choice, and its perdurance triggers excessive gliogenesis, showing that its tight and dynamic regulation ensures the proper balance between neurons and glia. Here, we present a genetic screen for potential gcm/glide interactors and identify genes encoding chromatin factors of the Trithorax and of the Polycomb groups. These proteins maintain the heritable epigenetic state, among others, of HOX genes throughout development, but their regulatory role on transiently expressed genes remains elusive. Here we show that Polycomb negatively affects Gcm/Glide autoregulation, a positive feedback loop that allows timely accumulation of Gcm/Glide threshold levels. Such temporal fine-tuning of gene expression tightly controls gliogenesis. This work performed at the levels of individual cells reveals an undescribed mode of Polycomb action in the modulation of transiently expressed fate determinants and hence in the acquisition of specific cell identity in the nervous system.

Morphogen pathways as molecular targets for the treatment of fibrosis in systemic sclerosis

Wnt-, Hedgehog- and Notch-signaling cascades are morphogen pathways that play crucial roles in development and tissue homeostasis. While morphogen pathways are tightly regulated at multiple levels, inappropriate activation of Wnt, Hedgehog and Notch signaling has been implicated into the pathogenesis of various diseases. In particular, Wnt, Hedgehog and Notch signaling have emerged as central players in the pathogenesis of fibrotic diseases. Here, we will review the pro-fibrotic effects of Wnt, Hedgehog and Notch signaling in systemic sclerosis (SSc), prototypical systemic fibrotic disease. Wnt, Hedgehog and Notch pathways are activated in SSc. They potently stimulate fibroblasts to differentiate into myofibroblasts and to release collagen and other extracellular matrix components. Genetic or pharmacological inhibition of morphogen pathways effectively prevents experimental fibrosis in different preclinical models and induces regression of pre-established fibrosis. As several inhibitors of Wnt, Hedgehog and Notch have recently been developed with first ones being already approved for clinical trials, morphogen pathways maybe a novel approach for the treatment of fibrosis.

Ectodomain shedding and ADAMs in development

Proteolytic enzymes belonging to the A Disintegin And Metalloproteinase (ADAM) family are able to cleave transmembrane proteins close to the cell surface, in a process referred to as ectodomain shedding. Substrates for ADAMs include growth factors, cytokines, chemokines and adhesion molecules, and, as such, many ADAM proteins play crucial roles in cell-cell adhesion, extracellular and intracellular signaling, cell differentiation and cell proliferation. In this Review, we summarize the fascinating roles of ADAMs in embryonic and adult tissue development in both vertebrates and invertebrates.

A targeted in vivo RNAi screen reveals deubiquitinases as new regulators of Notch signaling

Notch signaling is highly conserved in all metazoan animals and plays critical roles in cell fate specification, cell proliferation, apoptosis, and stem cell maintenance. Although core components of the Notch signaling cascade have been identified, many gaps in the understanding of the Notch signaling pathway remain to be filled. One form of posttranslational regulation, which is controlled by the ubiquitin-proteasome system, is known to modulate Notch signaling. The ubiquitination pathway is a highly coordinated process in which the ubiquitin moiety is either conjugated to or removed from target proteins by opposing E3 ubiquitin ligases and deubiquitinases (DUBs). Several E3 ubiquitin ligases have been implicated in ubiquitin conjugation to the receptors and the ligands of the Notch signaling cascade. In contrast, little is known about a direct role of DUBs in Notch signaling in vivo. Here, we report an in vivo RNA interference screen in Drosophila melanogaster targeting all 45 DUBs that we annotated in the fly genome. We show that at least four DUBs function specifically in the formation of the fly wing margin and/or the specification of the scutellar sensory organ precursors, two processes that are strictly dependent on the balanced Notch signaling activity. Furthermore, we provide genetic evidence suggesting that these DUBs are necessary to positively modulate Notch signaling activity. Our study reveals a conserved molecular mechanism by which protein deubiquitination process contributes to the complex posttranslational regulation of Notch signaling in vivo.

Non-canonical Notch signaling activates IL-6/JAK/STAT signaling in breast tumor cells and is controlled by p53 and IKKα/IKKβ

Notch signaling is frequently hyperactivated in breast cancer, but how the enhanced signaling contributes to the tumor process is less well understood. In this report, we identify the proinflammatory cytokine interleukin-6 (IL-6) as a novel Notch target in breast tumor cells. Enhanced Notch signaling upregulated IL-6 expression, leading to activation of autocrine and paracrine Janus kinase/signal transducers and activators of transcription signaling. IL-6 upregulation was mediated by non-canonical Notch signaling, as it could be effectuated by a cytoplasmically localized Notch intracellular domain and was independent of the DNA-binding protein CSL. Instead, Notch-mediated IL-6 upregulation was controlled by two proteins in the nuclear factor (NF)-κB signaling cascade, IKKα and IKKβ (inhibitor of nuclear factor kappa-B kinase subunit alpha and beta, respectively), as well as by p53. Activation of IL-6 by Notch required IKKα/IKKβ function, but interestingly, did not engage canonical NF-κB signaling, in contrast to IL-6 activation by inflammatory agents such as lipopolysaccharide. With regard to p53 status, IL-6 expression was upregulated by Notch when p53 was mutated or lost, and restoring wild-type p53 into p53-mutated or -deficient cells abrogated the IL-6 upregulation. Furthermore, Notch-induced transcriptomes from p53 wild-type and -mutated breast tumor cell lines differed extensively, and for a subset of genes upregulated by Notch in a p53-mutant cell line, this upregulation was reduced by wild-type p53. In conclusion, we identify IL-6 as a novel non-canonical Notch target gene, and reveal roles for p53 and IKKα/IKKβ in non-canonical Notch signaling in breast cancer and in the generation of cell context-dependent diversity in the Notch signaling output.

Notch signaling in pediatric soft tissue sarcomas

Pediatric soft tissue sarcomas are rare tumors of childhood, frequently characterized by specific chromosome translocations. Despite improvements in treatment, their clinical management is often challenging due to the low responsiveness of metastatic forms and aggressive variants to conventional therapeutic approaches, which leads to poor overall survival. It is widely thought that soft tissue sarcomas derive from mesenchymal progenitor cells that, during embryonic life, have developed chromosomal aberrations with de-regulation of the main pathways governing tissue morphogenesis. The Notch signaling pathway is one of the most important molecular networks involved in differentiation processes. Emerging evidence highlights the role of Notch signaling de-regulation in the biology of these pediatric sarcomas. In this review, we present an outline of recently gathered evidence on the role of Notch signaling in soft tissue sarcomas, highlighting its importance in tumor cell biology. The potential challenges and opportunities of targeting Notch signaling in the treatment of pediatric soft tissue sarcomas are also discussed.

Fe65 matters: new light on an old molecule

The discovery that the main constituents of amyloid deposits, characteristic of Alzheimer neuropathology, derive from the proteolytic processing of the membrane precursor amyloid precursor protein (APP) is one of the milestones of the research history of this disease. Despite years of intense studies, the functions of APP and of its amyloidogenic processing are still under debate. One focus of these studies was the complex network of protein-protein interactions centered at the cytosolic domain of APP, which suggests the involvement of APP in a lively signaling pathway. Fe65 was the first protein to be demonstrated to interact with the APP cytodomain. Starting from this observation, a large body of data has been gathered, indicating that Fe65 is an adaptor protein, which binds numerous proteins, further than APP. Among these proteins, the crosstalk with Mena, mDab, and Abl suggested the involvement of the Fe65-APP complex in the regulation of cell motility, with a relevant role in differentiation and development. Other partners, like the histone acetyltransferase Tip60, indicated the possibility that the nuclear fraction of Fe65 could be involved in gene regulation and/or DNA repair.

Transcriptome analysis for Notch3 target genes identifies Grip2 as a novel regulator of myogenic response in the cerebrovasculature

OBJECTIVE

Notch3 is critically important for the structure and myogenic response of distal arteries, particularly of cerebral arteries. However, signaling pathways acting downstream of Notch3 remain largely unknown.

METHODS AND RESULTS

Transcriptome analysis using tail arteries of Notch3-null mice identified a core set of 17 novel Notch3-regulated genes confirmed in tail or brain arteries. Postnatal deletion of RBP-Jκ in smooth muscle cells recapitulated the structural, functional, and molecular defects of brain arteries induced by Notch3 deficiency. Transient in vivo blockade of the Notch pathway with γ-secretase inhibitors uncovered, in addition to Notch3, 6 immediate responders, including the voltage-gated potassium channel Kv1.5, which opposes to myogenic constriction of brain arteries, and the glutamate receptor-interacting protein 2 (Grip2) with no previously established role in the cerebrovasculature. We identified a vascular smooth muscle cell isoform of Grip2. We showed that Notch3-RBP-Jκ specifically regulates this isoform. Finally, we found that cerebral arteries of Grip2 mutant mice, which express an N-terminally truncated Grip2 protein, exhibited selective attenuation of pressure-induced contraction.

CONCLUSIONS

Our data provide insight into how Notch3 signals in the brain arteries, establishing the postnatal requirement of smooth muscle RBP-Jκ in this context. Notch3-regulated transcriptome provides potential for modulating myogenic response in the cerebrovasculature.

Toll-like receptor 7 regulates pancreatic carcinogenesis in mice and humans

Pancreatic ductal adenocarcinoma is an aggressive cancer that interacts with stromal cells to produce a highly inflammatory tumor microenvironment that promotes tumor growth and invasiveness. The precise interplay between tumor and stroma remains poorly understood. TLRs mediate interactions between environmental stimuli and innate immunity and trigger proinflammatory signaling cascades. Our finding that TLR7 expression is upregulated in both epithelial and stromal compartments in human and murine pancreatic cancer led us to postulate that carcinogenesis is dependent on TLR7 signaling. In a mouse model of pancreatic cancer, TLR7 ligation vigorously accelerated tumor progression and induced loss of expression of PTEN, p16, and cyclin D1 and upregulation of p21, p27, p53, c-Myc, SHPTP1, TGF-β, PPARγ, and cyclin B1. Furthermore, TLR7 ligation induced STAT3 activation and interfaced with Notch as well as canonical NF-κB and MAP kinase pathways, but downregulated expression of Notch target genes. Moreover, blockade of TLR7 protected against carcinogenesis. Since pancreatic tumorigenesis requires stromal expansion, we proposed that TLR7 ligation modulates pancreatic cancer by driving stromal inflammation. Accordingly, we found that mice lacking TLR7 exclusively within their inflammatory cells were protected from neoplasia. These data suggest that targeting TLR7 holds promise for treatment of human pancreatic cancer.

Experiments, measurements, and mathematical modeling to decipher time signals in development

Good progress has been made in identifying key signaling molecules and explaining how they are used to generate spatial patterns during embryonic development. In contrast, little is known about the control of timing or how cells use time signals in the developing embryo. In this review, I describe how direct measurements from the embryo combined with mathematical modeling could bring new insights. To illustrate this point, I discuss three examples: the Dpp gradient during growth of the Drosophila wing imaginal disc; the Polycomb-based epigenetic silencing during vernalization in plants; and the Notch-dependent somite segmentation clock.

Developmental pathways in colon cancer: crosstalk between WNT, BMP, Hedgehog and Notch

A hallmark of cancer is reactivation/alteration of pathways that control cellular differentiation during developmental processes. Evidence indicates that WNT, Notch, BMP and Hedgehog pathways have a role in normal epithelial cell differentiation, and that alterations in these pathways accompany establishment of the tumorigenic state. Interestingly, there is recent evidence that these pathways are intertwined at the molecular level, and these nodes of intersection may provide opportunities for effective targeted therapies. This review will highlight the role of the WNT, Notch, BMP and Hedgehog pathways in colon cancer.

NOTCH1 nuclear interactome reveals key regulators of its transcriptional activity and oncogenic function

Activating mutations in NOTCH1, an essential regulator of T cell development, are frequently found in human T cell acute lymphoblastic leukemia (T-ALL). Despite important advances in our understanding of Notch signal transduction, the regulation of Notch functions in the nucleus remains unclear. Using immunoaffinity purification, we identified NOTCH1 nuclear partners in T-ALL cells and showed that, beyond the well-characterized core activation complex (ICN1-CSL-MAML1), NOTCH1 assembles a multifunctional complex containing the transcription coactivator AF4p12, the PBAF nucleosome remodeling complex, and the histone demethylases LSD1 and PHF8 acting through their demethylase activity to promote epigenetic modifications at Notch-target genes. Remarkably, LSD1 functions as a corepressor when associated with CSL-repressor complex and as a NOTCH1 coactivator upon Notch activation. Our work provides new insights into the molecular mechanisms that govern Notch transcriptional activity and represents glimpse into NOTCH1 interaction landscape, which will help in deciphering mechanisms of NOTCH1 functions and regulation.

Direct observation of proteolytic cleavage at the S2 site upon forced unfolding of the Notch negative regulatory region

The conserved Notch signaling pathway plays crucial roles in developing and self-renewing tissues. Notch is activated upon ligand-induced conformation change of the Notch negative regulatory region (NRR) unmasking a key proteolytic site (S2) and facilitating downstream events. Thus far, the molecular mechanism of this signal activation is not defined. However, strong indirect evidence favors a model whereby transendocytosis of the Notch extracellular domain, in tight association with ligand into the ligand-bearing cell, exerts a force on the NRR to drive the required structure change. Here, we demonstrate that force applied to the human Notch2 NRR can indeed expose the S2 site and, crucially, allow cleavage by the metalloprotease TACE (TNF-alpha-converting enzyme). Molecular insight into this process is achieved using atomic force microscopy and molecular dynamics simulations on the human Notch2 NRR. The data show near-sequential unfolding of its constituent LNR (Lin12-Notch repeat) and HD (heterodimerization) domains, at forces similar to those observed for other protein domains with a load-bearing role. Exposure of the S2 site is the first force "barrier" on the unfolding pathway, occurring prior to unfolding of any domain, and achieved via removal of the LNRAB linker region from the HD domain. Metal ions increase the resistance of the Notch2 NRR to forced unfolding, their removal clearly facilitating unfolding at lower forces. The results provide direct demonstration of force-mediated exposure and cleavage of the Notch S2 site and thus firmly establish the feasibility of a mechanotransduction mechanism for ligand-induced Notch activation.

Genome-wide association study of age-related macular degeneration identifies associated variants in the TNXB-FKBPL-NOTCH4 region of chromosome 6p21.3

Age-related macular degeneration (AMD) is a leading cause of visual loss in Western populations. Susceptibility is influenced by age, environmental and genetic factors. Known genetic risk loci do not account for all the heritability. We therefore carried out a genome-wide association study of AMD in the UK population with 893 cases of advanced AMD and 2199 controls. This showed an association with the well-established AMD risk loci ARMS2 (age-related maculopathy susceptibility 2)-HTRA1 (HtrA serine peptidase 1) (P =2.7 × 10(-72)), CFH (complement factor H) (P =2.3 × 10(-47)), C2 (complement component 2)-CFB (complement factor B) (P =5.2 × 10(-9)), C3 (complement component 3) (P =2.2 × 10(-3)) and CFI (P =3.6 × 10(-3)) and with more recently reported risk loci at VEGFA (P =1.2 × 10(-3)) and LIPC (hepatic lipase) (P =0.04). Using a replication sample of 1411 advanced AMD cases and 1431 examined controls, we confirmed a novel association between AMD and single-nucleotide polymorphisms on chromosome 6p21.3 at TNXB (tenascin XB)-FKBPL (FK506 binding protein like) [rs12153855/rs9391734; discovery P =4.3 × 10(-7), replication P =3.0 × 10(-4), combined P =1.3 × 10(-9), odds ratio (OR) = 1.4, 95% confidence interval (CI) = 1.3-1.6] and the neighbouring gene NOTCH4 (Notch 4) (rs2071277; discovery P =3.2 × 10(-8), replication P =3.8 × 10(-5), combined P =2.0 × 10(-11), OR = 1.3, 95% CI = 1.2-1.4). These associations remained significant in conditional analyses which included the adjacent C2-CFB locus. TNXB, FKBPL and NOTCH4 are all plausible AMD susceptibility genes, but further research will be needed to identify the causal variants and determine whether any of these genes are involved in the pathogenesis of AMD.