Endocytic Regulation of Notch Signalling During Development

Delta/Notch signalling is of major importance for embryonic development and adult life. While endocytosis is often viewed as a way to down-regulate biological signals, ligand and receptor internalization are essential for Notch activation. The development of Drosophila mecanosensory bristles is a powerful model to study Delta/Notch signalling. Following the asymmetric division of bristle precursor cells, Delta ligands and Notch receptors traffic differently in the two daughter cells, leading to directional signal activation. Recent evidence suggests that in addition to differential ligand endocytosis after division, a subpopulation of multivesicular endosomes ensures the directional transport of Delta/Notch already during asymmetric cell division. Biochemical analysis suggests that different phases of endocytic Delta trafficking exert complementary but distinct actions required for ligand recycling, ligand/receptor interaction and ligand-mediated receptor activation, respectively. Finally, novel data suggest that different endosomal compartments may act as Delta/Notch signalling platforms. In this review, we discuss the implications of these novel findings for our cell biological understanding of Delta/Notch signalling.     

The multifaceted role of Notch in cardiac development and disease

Notch receptors and their cognate ligands transduce crucial signals between cells in various tissues, and have been conserved across millions of years of evolution. Mutations in Notch signalling components result in congenital heart defects in humans and mice, demonstrating an essential role for Notch in cardiovascular development. The results of recent experiments implicate this signalling pathway in many stages of heart development, and provide mechanistic insight into the vital functions of Notch in the aetiology of several common forms of paediatric and adult cardiac disease.     

The abruptex mutations of notch disrupt the establishment of proneural clusters in Drosophila

The receptor encoded by the Notch gene plays a central role in preventing cells from making decisions about their fates until appropriate signals are present. This function of Notch requires the product of the Suppressor of Hairless gene. Loss of either Notch or Suppressor of Hairless function results in cells making premature and incorrect cell fate decisions, whilst increases in Notch signalling prevent cells from making these decisions. Here we find that the proneural clusters are not established correctly in certain Abruptex mutations of Notch and this failure to establish proneural clusters correctly is not due to increased Notch signalling during lateral inhibition. In addition we show that the overexpression of certain dominant negative Notch molecules can disrupt the initiation of proneural cluster development in a manner similar to the Abruptex mutants.     

Notch signalling in B cells

Notch signalling is likely to regulate multiple aspects of lymphoid development and function. During T cell development, Notch signalling is required for commitment of the earliest progenitor, and may also function during other developmental stages. T cell commitment from a common lymphoid progenitor occurs at the expense of B cell development, suggesting that Notch signalling inhibits the earliest stage of B lymphopoiesis. In contrast, recent evidence suggests that Notch promotes the development of marginal zone lymphocytes. Not only is Notch required for later stages of B cell development, but several viral proteins appear to utilize Notch signalling in B cells to mediate their functions. In this review, we will focus on potential roles of Notch signalling in B lymphopoiesis and also consider how viral proteins may utilize Notch signalling in B cells.     

Autophagy is involved in mouse kidney development and podocyte differentiation regulated by Notch signalling

Podocyte dysfunction results in glomerular diseases accounted for 90% of end‐stage kidney disease. The evolutionarily conserved Notch signalling makes a crucial contribution in podocyte development and function. However, the underlying mechanism of Notch pathway modulating podocyte differentiation remains less obvious. Autophagy, reported to be related with Notch signalling pathways in different animal models, is regarded as a possible participant during podocyte differentiation. Here, we found the dynamic changes of Notch1 were coincided with autophagy: they both increased during kidney development and podocyte differentiation. Intriguingly, when Notch signalling was down‐regulated by DAPT, autophagy was greatly diminished, and differentiation was also impaired. Further, to better understand the relationship between Notch signalling and autophagy in podocyte differentiation, rapamycin was added to enhance autophagy levels in DAPT‐treated cells, and as a result, nephrin was recovered and DAPT‐induced injury was ameliorated. Therefore, we put forward that autophagy is involved in kidney development and podocyte differentiation regulated by Notch signalling.     

Notch signalling in the regulation of peripheral T-cell function

The Notch signalling pathway plays a highly-conserved role in regulating the cellular differentiation and proliferation events that characterise pattern formation in the embryo. As cells in the embryo respond to environmental signals, similarly T-cells in the peripheral immune system must monitor their environment for antigens and respond accordingly by entering one of several potential differentiation pathways. Recent studies have identified a role for the Notch pathway in regulating the responses of T-cells in the periphery. In this review, we discuss these findings in the context of the Notch signalling pathway's role as an orchestrator of cellular differentiation, and propose a central role for Notch as a regulator of immune system function.     

Wingless modulates the effects of dominant negative notch molecules in the developing wing of Drosophila

The development and patterning of the wing in Drosophila relies on a sequence of cell interactions molecularly driven by a number of ligands and receptors. Genetic analysis indicates that a receptor encoded by the Notch gene and a signal encoded by the wingless gene play a number of interdependent roles in this process and display very strong functional interactions. At certain times and places, during wing development, the expression of wingless requires Notch activity and that of its ligands Delta and Serrate. This has led to the proposal that all the interactions between Notch and wingless can be understood in terms of this regulatory relationship. Here we have tested this proposal by analysing interactions between Delta- and Serrate-activated Notch signalling and Wingless signalling during wing development and patterning. We find that the cell death caused by expressing dominant negative Notch molecules during wing development cannot be rescued by coexpressing Nintra. This suggests that the dominant negative Notch molecules cannot only disrupt Delta and Serrate signalling but can also disrupt signalling through another pathway. One possibility is the Wingless signalling pathway as the cell death caused by expressing dominant negative Notch molecules can be rescued by activating Wingless signalling. Furthermore, we observe that the outcome of the interactions between Notch and Wingless signalling differs when we activate Wingless signalling by expressing either Wingless itself or an activated form of the Armadillo. For example, the effect of expressing the activated form of Armadillo with a dominant negative Notch on the patterning of sense organ precursors in the wing resembles the effects of expressing Wingless alone. This result suggests that signalling activated by Wingless leads to two effects, a reduction of Notch signalling and an activation of Armadillo.     

Mutations in rugose promote cell type-specific apoptosis in the Drosophila eye

RUGOSE (RG): encodes an A kinase anchor protein and was isolated as a genetic interactor of the Notch and epidermal growth factor receptor (EGFR) pathways during eye development in Drosophila. rg mutants display a small, rough eye phenotype primarily caused by the loss of cone cells. Here we show that the basis of this phenotype is cell type-specific apoptosis rather than transformation and hence can be rescued by reduction of proapoptotic signals. Moreover, a nearly complete rescue is observed by an increased Notch signal suggesting an antiapoptotic function of Notch in this developmental context. Cone cell loss in rg mutants is accompanied by enhanced Jun N-terminal kinase activity and, concomitantly, by a reduction of EGFR signalling activity. Together, these findings support the idea that rg plays an important role in the integration of different signals required for the exact regulation of cone cell development and survival.     

Notch signalling in healthy and diseased vasculature

Notch signalling is an evolutionarily highly conserved signalling mechanism governing differentiation and regulating homeostasis in many tissues. In this review, we discuss recent advances in our understanding of the roles that Notch signalling plays in the vasculature. We describe how Notch signalling regulates different steps during the genesis and remodelling of blood vessels (vasculogenesis and angiogenesis), including critical roles in assigning arterial and venous identities to the emerging blood vessels and regulation of their branching. We then proceed to discuss how experimental perturbation of Notch signalling in the vasculature later in development affects vascular homeostasis. In this review, we also describe how dysregulated Notch signalling, as a consequence of direct mutations of genes in the Notch pathway or aberrant Notch signalling output, contributes to various types of vascular disease, including CADASIL, Snedden syndrome and pulmonary arterial hypertension. Finally, we point out some of the current knowledge gaps and identify remaining challenges in understanding the role of Notch in the vasculature, which need to be addressed to pave the way for Notch-based therapies to cure or ameliorate vascular disease.     

Notch signalling pathway in tooth development and adult dental cells

Notch signalling is a highly conserved intercellular signal transfer mechanism that includes canonical and non-canonical pathways. It regulates differentiation and proliferation of stem/progenitor cells by means of para-inducing effects. Expression and activation of Notch signalling factors (receptors and ligands) are critical not only for development of the dental germ but also for regeneration of injured tissue associated with mature teeth. Notch signalling plays key roles in differentiation of odontoblasts and osteoblasts, calcification of tooth hard tissue, formation of cusp patterns and generation of tooth roots. After tooth eruption, Notch signalling can also be triggered in dental stem cells of the pulp, where it induces them to differentiate into odontoblasts, thus generating fresh dentine tissue. Other signalling pathways, such as TGFβ, NF-κB, Wnt, Fgf and Shh also interact with Notch signalling during tooth development.     

Regulation of notch signaling activity

Cell-cell signalling mediated by the receptor Notch is used widely across the metazoans to determine cell fate and regulate pattern formation. Notch signals via a conserved regulated intramembrane proteolysis. Recent analyses of the cell biology of the Notch receptor have identified several fundamental mechanisms that contribute to regulate Notch signaling activity in space and time.     

Not(ch) just development: Notch signalling in the adult brain

The Notch pathway is often regarded as a developmental pathway, but components of Notch signalling are expressed and active in the adult brain. With the advent of more sophisticated genetic manipulations, evidence has emerged that suggests both conserved and novel roles for Notch signalling in the adult brain. Not surprisingly, Notch is a key regulator of adult neural stem cells, but it is increasingly clear that Notch signalling also has roles in the regulation of migration, morphology, synaptic plasticity and survival of immature and mature neurons. Understanding the many functions of Notch signalling in the adult brain, and its dysfunction in neurodegenerative disease and malignancy, is crucial to the development of new therapeutics that are centred around this pathway.     

Distinct contextual roles for Notch signalling in skeletal muscle stem cells

Notch signalling acts in virtually every tissue during the lifetime of metazoans. Recent studies have pointed to multiple roles for Notch in stem cells during quiescence, proliferation, temporal specification, and maintenance of the niche architecture. Skeletal muscle has served as an excellent paradigm to examine these diverse roles as embryonic, foetal, and adult skeletal muscle stem cells have different molecular signatures and functional properties, reflecting their developmental specification during ontology. Notably, Notch signalling has emerged as a major regulator of all muscle stem cells. This review will provide an overview of Notch signalling during myogenic development and postnatally, and underscore the seemingly opposing contextual activities of Notch that have lead to a reassessment of its role in myogenesis.     

Role of Notch signalling pathway in cancer and its association with DNA methylation

The Notch signalling pathway is an evolutionarily conserved cell signalling pathway involved in the development of organisms as diverse as humans and fruit flies. It plays a pivotal role in cell fate determination. Dysregulated Notch signalling is oncogenic, inhibits apoptosis and promotes cell survival. Abnormal Notch signalling is seen in many cancers like T-cell acute lymphoblastic leukaemia, acute myeloid leukaemia and cancers of the breast, cervix, colon, pancreas, skin and brain. Inhibition of Notch signalling leads to growth arrest and differentiation in those cells in which Notch pathway is activated and this represents a new target for cancer therapy. Cancer develops from genome defects, including both genetic and epigenetic alterations. Epigenetics deals with heritable changes in gene function that occur without a change in the DNA sequence. Among various epigenetic alterations such as acetylation, phosphorylation, ubiquitylation and sumoylation, promoter region methylation is considered as an important component in cancer development. Epigenetic alterations can be used as biomarkers in screening, detection, diagnosis, staging and risk stratification of various cancers. DNA methylation can be therapeutically reversed and demethylating drugs have proven to be promising in cancer treatment. This review focusses on the methylation status of genes in Notch signalling pathway from various cancers and how this epigenetic alteration can be used as a biomarker for cancer diagnosis and subsequent treatment.     

Notch signalling stabilises boundary formation at the midbrain-hindbrain organiser

The midbrain-hindbrain interface gives rise to a boundary of particular importance in CNS development as it forms a local signalling centre, the proper functioning of which is essential for the formation of tectum and cerebellum. Positioning of the mid-hindbrain boundary (MHB) within the neuroepithelium is dependent on the interface of Otx2 and Gbx2 expression domains, yet in the absence of either or both of these genes, organiser genes are still expressed, suggesting that other, as yet unknown mechanisms are also involved in MHB establishment. Here, we present evidence for a role for Notch signalling in stabilising cell lineage restriction and regulating organiser gene expression at the MHB. Experimental interference with Notch signalling in the chick embryo disrupts MHB formation, including downregulation of the organiser signal Fgf8. Ectopic activation of Notch signalling in cells of the anterior hindbrain results in an exclusion of those cells from rhombomeres 1 and 2, and in a simultaneous clustering along the anterior and posterior boundaries of this area, suggesting that Notch signalling influences cell sorting. These cells ectopically express the boundary marker Fgf3. In agreement with a role for Notch signalling in cell sorting, anterior hindbrain cells with activated Notch signalling segregate from normal cells in an aggregation assay. Finally, misexpression of the Notch modulator Lfng or the Notch ligand Ser1 across the MHB leads to a shift in boundary position and loss of restriction of Fgf8 to the MHB. We propose that differential Notch signalling stabilises the MHB through regulating cell sorting and specifying boundary cell fate.