Notch signaling: the core pathway and its posttranslational regulation

Delta-Notch signaling is involved in the segregation of the three germ layers in Xenopus laevis

In vertebrates, the induction of the three germ layers (ectoderm, mesoderm and endoderm) has been extensively studied, but less is known about how they segregate. Here, we investigated whether Delta-Notch signaling is involved in this process. Activating the pathway in the marginal zone with Notch(ICD) resulted in an expansion of endodermal and neural ectoderm precursors, leaving a thinner mesodermal ring around the blastopore at gastrula stage, when germ layers are segregated. On the other hand, when the pathway was blocked with Delta-1(STU) or with an antisense morpholino oligonucleotide against Notch, the pan-mesodermal brachyury (bra) domain was expanded and the neural border was moved animalwards. Strikingly, the suprablastoporal endoderm was either expanded when Delta-1 signaling was blocked, or reduced after the general knock-down of Notch. In addition, either activating or blocking the pathway delays the blastopore closure. We conclude that the process of delimiting the three germ layers requires Notch signaling, which may be finely regulated by ligands and/or involve non-canonical components of the pathway. Moreover, Notch activity must be modulated at appropriate levels during this process in order to keep normal morphogenetic movements during gastrulation.

Signalling pathways in vasculogenic mimicry

Solid tumour growth is dependent on the development of an adequate blood supply. For years, sprouting angiogenesis has been considered an exclusive mechanism of tumour vascularization. However, over the last years, several other mechanisms have been identified, including vessel-co-option, intussusception, recruitment of endothelial precursor cells (EPCs) and even mechanisms that do not involve endothelial cells, a process called vasculogenic mimicry (VM). The latter describes a mechanism by which highly aggressive tumour cells can form vessel-like structures themselves, by virtue of their high plasticity. VM has been observed in several tumour types and its occurrence is strongly associated with a poor prognosis. This review will focus on signalling molecules and cascades involved in VM. In addition, we will discuss the presence of VM in relation to ongoing cancer research. Finally, we describe the clinical significance of VM regarding anti-angiogenesis treatment modalities.

Presentation counts: microenvironmental regulation of stem cells by biophysical and material cues

Stem cells reside in adult and embryonic tissues in a broad spectrum of developmental stages and lineages, and they are thus naturally exposed to diverse microenvironments or niches that modulate their hallmark behaviors of self-renewal and differentiation into one or more mature lineages. Within each such microenvironment, stem cells sense and process multiple biochemical and biophysical cues, which can exert redundant, competing, or orthogonal influences to collectively regulate cell fate and function. The proper presentation of these myriad regulatory signals is required for tissue development and homeostasis, and their improper appearance can potentially lead to disease. Whereas these complex regulatory cues can be challenging to dissect using traditional cell culture paradigms, recently developed engineered material systems offer advantages for investigating biochemical and biophysical cues, both static and dynamic, in a controlled, modular, and quantitative fashion. Advances in the development and use of such systems have helped elucidate novel regulatory mechanisms controlling stem cell behavior, particularly the importance of solid-phase mechanical and immobilized biochemical microenvironmental signals, with implications for basic stem cell biology, disease, and therapeutics.

Regulation of notch signaling via O-glucosylation insights from Drosophila studies

Recent work using Drosophila melanogaster has shown that a protein O-glucosyltransferase called Rumi regulates Notch signaling. Studies on several alleles of rumi identified in a forward genetic screen indicated that Rumi is a temperature-sensitive regulator of Notch signaling in flies. Further genetic and rescue experiments demonstrated that Rumi is a general regulator of Drosophila Notch signaling. Biochemical analyses showed that Rumi adds glucose to specific EGF repeats in the extracellular domain of Notch receptor in the Drosophila S2 cell line. Furthermore, RNAi-mediated knockdown of Rumi in this cell line resulted in a severe decrease in the level of O-linked glucose on Notch. In this chapter, we discuss the genetic and biochemical methods used to determine the role of Rumi in the regulation of Notch signaling in flies.

Genome-wide association study identifies five loci associated with lung function

Pulmonary function measures are heritable traits that predict morbidity and mortality and define chronic obstructive pulmonary disease (COPD). We tested genome-wide association with forced expiratory volume in 1 s (FEV(1)) and the ratio of FEV(1) to forced vital capacity (FVC) in the SpiroMeta consortium (n = 20,288 individuals of European ancestry). We conducted a meta-analysis of top signals with data from direct genotyping (n < or = 32,184 additional individuals) and in silico summary association data from the CHARGE Consortium (n = 21,209) and the Health 2000 survey (n < or = 883). We confirmed the reported locus at 4q31 and identified associations with FEV(1) or FEV(1)/FVC and common variants at five additional loci: 2q35 in TNS1 (P = 1.11 x 10(-12)), 4q24 in GSTCD (2.18 x 10(-23)), 5q33 in HTR4 (P = 4.29 x 10(-9)), 6p21 in AGER (P = 3.07 x 10(-15)) and 15q23 in THSD4 (P = 7.24 x 10(-15)). mRNA analyses showed expression of TNS1, GSTCD, AGER, HTR4 and THSD4 in human lung tissue. These associations offer mechanistic insight into pulmonary function regulation and indicate potential targets for interventions to alleviate respiratory disease.

Endocytosis and intracellular trafficking of Notch and its ligands

Notch signaling occurs through direct interaction between Notch, the receptor, and its ligands, presented on the surface of neighboring cells. Endocytosis has been shown to be essential for Notch signal activation in both signal-sending and signal-receiving cells, and numerous genes involved in vesicle trafficking have recently been shown to act as key regulators of the pathway. Defects in vesicle trafficking can lead to gain- or loss-of-function defects in a context-dependent manner. Here, we discuss how endocytosis and vesicle trafficking regulate Notch signaling in both signal-sending and signal-receiving cells. We will introduce the key players in different trafficking steps, and further illustrate how they impact the signal outcome. Some of these players act as general factors and modulate Notch signaling in all contexts, whereas others modulate signaling in a context-specific fashion. We also discuss Notch signaling during mechanosensory organ development in the fly to exemplify how endocytosis and vesicle trafficking are effectively used to determine correct cell fates. In summary, endocytosis plays an essential role in Notch signaling, whereas intracellular vesicle trafficking often plays a context-dependent or regulatory role, leading to divergent outcomes in different developmental contexts.

Notch signalling and haematopoietic stem cell formation during embryogenesis

The Notch signalling pathway is repeatedly employed during embryonic development and adult homeostasis of a variety of tissues. In particular, its frequent involvement in the regulation of stem and progenitor cell maintenance and proliferation, as well as its role in binary fate decisions in cells that are destined to differentiate, is remarkable. Here, we review its role in the development of haematopoietic stem cells during vertebrate embryogenesis and put it into the context of Notch's functions in arterial specification, angiogenic vessel sprouting and vessel maintenance. We further discuss interactions with other signalling cascades, and pinpoint open questions and some of the challenges that lie ahead.

Wide-scale use of Notch signaling factor CSL/RBP-Jkappa in RTA-mediated activation of Kaposi's sarcoma-associated herpesvirus lytic genes

For Kaposi's sarcoma-associated herpesvirus (KSHV; also called human herpesvirus 8 [HHV8]), the switch from latency to active lytic replication requires RTA, the product of open reading frame 50 (ORF50). RTA activates transcription from nearly 40 early and delayed-early viral promoters, mainly through interactions with cellular DNA binding proteins, such as CSL/RBP-Jkappa, Oct-1, C/EBPalpha, and c-Jun. Reliance on cellular coregulators may allow KSHV to adjust its lytic program to suit different cellular contexts or interpret signals from the outside. CSL is a key component of the Notch signaling pathway and is targeted by several viruses. A search with known CSL binding sequences from cellular genes found at least 260 matches in the KSHV genome, many from regions containing known or suspected lytic promoters. Analysis of clustered sites located immediately upstream of ORF70 (thymidylate synthase), ORF19 (tegument protein), and ORF47 (glycoprotein L) uncovered RTA-responsive promoters that were validated using mRNAs isolated from KSHV-infected cells undergoing lytic reactivation. Notably, ORF19 behaves as a true late gene, indicating that RTA regulates all three phases of the lytic program. For each new promoter, the response to RTA was dependent on CSL, and 5 of the 10 candidate sites were shown to bind CSL in vitro. Analysis of individual sites highlighted the importance of a cytosine residue flanking the core CSL binding sequence. These findings broaden the role for CSL in coordinating the KSHV lytic gene expression program and help to define a signature motif for functional CSL sites within the viral genome.

Lunatic Fringe-mediated Notch signaling is required for lung alveogenesis

Distal lung development occurs through coordinated induction of myofibroblasts, epithelial cells, and capillaries. Lunatic Fringe (Lfng) is a beta(1-3) N-acetylglucosamine transferase that modifies Notch receptors to facilitate their activation by Delta-like (Dll1/4) ligands. Lfng is expressed in the distal lung during saccular development, and deletion of this gene impairs myofibroblast differentiation and alveogenesis in this context. A similar defect was observed in Notch2(beta-geo/+)Notch3(beta-geo/beta-geo) compound mutant mice but not in Notch2(beta-geo/+) or Notch3(beta-geo/beta-geo) single mutants. Finally, to directly test for the role of Notch signaling in myofibroblast differentiation in vivo, we used ROSA26-rtTA(/+);tetO-CRE(/+);RBPJkappa(flox/flox) inducible mutant mice to show that disruption of canonical Notch signaling during late embryonic development prevents induction of smooth muscle actin in mesenchymal cells of the distal lung. In sum, these results demonstrate that Lfng functions to enhance Notch signaling in myofibroblast precursor cells and thereby to coordinate differentiation and mobilization of myofibroblasts required for alveolar septation.

Isolation of Toxoplasma gondii development mutants identifies a potential proteophosphogylcan that enhances cyst wall formation

Within warm-blooded animals, Toxoplasma gondii switches from an actively replicating form called a tachyzoite into a slow growing encysted form called a bradyzoite. To uncover the genes involved in bradyzoite development, we screened over 8000 T. gondii insertional mutants by immunofluorescence microscopy. We identified nine bradyzoite development mutants that were defective in both cyst wall formation and expression of a bradyzoite specific heat shock protein. One of these mutants, named 42F5, contained an insertion into the predicted gene TGME49_097520. The disrupted protein is serine/proline-rich with homology to proteophosphoglycans from Leishmania. T. gondii proteophosphoglycan (GU182879) expressed from the native promoter was undetectable in tachyzoites, but bradyzoites show punctate spots within the parasite and staining around the parasitophorous vacuole. Complementation of the 42F5 mutant with GU182879 expressed from either the alpha-tubulin or native promoter restores cyst wall formation. Overall, GU182879 is upregulated in bradyzoites and enhances cyst wall component expression and assembly.

Rbpj cell autonomous regulation of retinal ganglion cell and cone photoreceptor fates in the mouse retina

Vertebrate retinal progenitor cells (RPCs) are pluripotent, but pass through competence states that progressively restrict their developmental potential (Cepko et al., 1996; Livesey and Cepko, 2001; Cayouette et al., 2006). In the rodent eye, seven retinal cell classes differentiate in overlapping waves, with RGCs, cone photoreceptors, horizontals, and amacrines forming predominantly before birth, and rod photoreceptors, bipolars, and Müller glia differentiating postnatally. Both intrinsic and extrinsic factors regulate each retinal cell type (for review, see Livesey and Cepko, 2001). Here, we conditionally deleted the transcription factor Rbpj, a critical integrator of multiple Notch signals (Jarriault et al., 1995; Honjo, 1996; Kato et al., 1997; Han et al., 2002), during prenatal mouse retinal neurogenesis. Removal of Rbpj caused reduced proliferation, premature neuronal differentiation, apoptosis, and profound mispatterning. To determine the cell autonomous requirements for Rbpj during RGC and cone formation, we marked Cre-generated retinal lineages with GFP expression, which showed that Rbpj autonomously promotes RPC mitotic activity, and suppresses RGC and cone fates. In addition, the progressive loss of Rbpj-/- RPCs resulted in a diminished progenitor pool available for rod photoreceptor formation. This circumstance, along with the overproduction of Rbpj-/- cones, revealed that photoreceptor development is under homeostatic regulation. Finally, to understand how the Notch pathway regulates the simultaneous formation of multiple cell types, we compared the RGC and cone phenotypes of Rbpj to Notch1 (Jadhav et al., 2006b; Yaron et al., 2006), Notch3, and Hes1 mutants. We found particular combinations of Notch pathway genes regulate the development of each retinal cell type.

Development and function of IL-10 IFN-gamma-secreting CD4(+) T cells

IL-10 IFN-gamma-secreting CD4(+) T cells were first found in the early 1990s. They are suppressive T cells able to inhibit cytotoxic T lymphocytes. These cells (Foxp3-T bet(+)) have a similar function but are distinct from conventional Tregs. The production of IL-10 in these cells requires IL-27 and TGF-beta and was regulated by several signal pathways including Notch, STAT, and NF-kappaB. The crosstalk among these pathways is critical for the generation and function of these cells. IL-10 IFN-gamma-secreting CD4(+) T cells are activated in chronic infection and are responsible for prolonged infection. Thus, their modulation has therapeutic implications for the treatment of infectious diseases. However, it is complicated, and fine-tuning of IFN-gamma and IL-10 secretion by these cells is needed for disease management, as inhibition of these cells will also lead to overimmune responses. On the other hand, increasing their numbers in autoimmune diseases may have beneficial effects.

Neuralized-2: expression in human and rodents and interaction with Delta-like ligands

Delta-Notch signaling is a universal cell-cell communication pathway crucial for numerous developmental and physiological processes. Several proteins interact with and regulate the Notch pathway, including the E3 ubiquitin ligase Neuralized (Neur) that influences the stability and activity of Notch ligands. In mammals there are two homologs of Neur, Neur1 and Neur2, that both can interact with Notch ligands Delta-like1 and Jagged1. Here, we show that Neur2, in contrast to Neur1, is highly expressed during embryonic development of the brain and several non-neural tissues and its mRNA levels subside postnatally. In the hippocampal neurons of the adult brain Neur2 transcripts, in contrast to Neur1, are excluded from the dendrites. Neur2 protein has a predominantly cytoplasmic localization. We also show that in addition to Delta-like1, Neur1 and Neur2 interact with another Notch ligand, Delta-like4.

In vivo analysis of the Notch receptor S1 cleavage

A ligand-independent cleavage (S1) in the extracellular domain of the mammalian Notch receptor results in what is considered to be the canonical heterodimeric form of Notch on the cell surface. The in vivo consequences and significance of this cleavage on Drosophila Notch signaling remain unclear and contradictory. We determined the cleavage site in Drosophila and examined its in vivo function by a transgenic analysis of receptors that cannot be cleaved. Our results demonstrate a correlation between loss of cleavage and loss of in vivo function of the Notch receptor, supporting the notion that S1 cleavage is an in vivo mechanism of Notch signal control.