Notch信号途径的调节

Notch信号途径是通过局部细胞间相互作用,实现细胞间通讯、胞浆内的信号转导以及核内转录,从而控制细胞的增殖、分化、凋亡、迁移及粘附等细胞的命运的途径。它在进化中非常保守,在机体的整个生长发育过程的调控中发挥着重要的作用。Notch信号途径作用过程受其他多种分子和途径的调节。本文从细胞外水平、细胞浆水平和细胞核水平分别讨论了Notch信号途径的调节。对进一步了解Notch信号途径,解释生理病理现象、控制和治疗疾病提供基础。     

Drosophila S2 Cells Secrete Wingless on Exosome‐Like Vesicles but the Wingless Gradient Forms Independently of Exosomes

Wingless acts as a morphogen in Drosophila wing discs, where it specifies cell fates and controls growth several cell diameters away from its site of expression. Thus, despite being acylated and membrane associated, Wingless spreads in the extracellular space. Recent studies have focussed on identifying the route that Wingless follows in the secretory pathway and determining how it is packaged for release. We have found that, in medium conditioned by Wingless‐expressing Drosophila S2 cells, Wingless is present on exosome‐like vesicles and that this fraction activates signal transduction. Proteomic analysis shows that Wingless‐containing exosome‐like structures contain many Drosophila proteins that are homologous to mammalian exosome proteins. In addition, Evi, a multipass transmembrane protein, is also present on exosome‐like vesicles. Using these exosome markers and a cell‐based RNAi assay, we found that the small GTPase Rab11 contributes significantly to exosome production. This finding allows us to conclude from in vivo Rab11 knockdown experiments, that exosomes are unlikely to contribute to Wingless secretion and gradient formation in wing discs. Consistent with this conclusion, extracellularly tagged Evi expressed from a Bacterial Artificial Chromosome is not released from imaginal disc Wingless‐expressing cells.     

Notch and Wingless modulate the response of cells to Hedgehog signalling in the Drosophila wing

During Drosophila wing development, Hedgehog (Hh) signalling is required to pattern the imaginal disc epithelium along the anterior-posterior (AP) axis. The Notch (N) and Wingless (Wg) signalling pathways organise the dorsal-ventral (DV) axis, including patterning along the presumptive wing margin. Here, we describe a functional hierarchy of these signalling pathways that highlights the importance of competing influences of Hh, N, and Wg in establishing gene expression domains. Investigation of the modulation of Hh target gene expression along the DV axis of the wing disc revealed that collier/knot (col/kn), patched (ptc), and decapentaplegic (dpp) are repressed at the DV boundary by N signalling. Attenuation of Hh signalling activity caused by loss of fused function results in a striking down-regulation of col, ptc, and engrailed (en) symmetrically about the DV boundary. We show that this down-regulation depends on activity of the canonical Wg signalling pathway. We propose that modulation of the response of cells to Hh along the future proximodistal (PD) axis is necessary for generation of the correctly patterned three-dimensional adult wing. Our findings suggest a paradigm of repression of the Hh response by N and/or Wnt signalling that may be applicable to signal integration in vertebrate appendages.     

Regulation of skeletal myogenesis by Notch

Notch signaling has emerged as a key player in skeletal muscle development and regeneration. Simply stated, Notch signaling inhibits differentiation. Accordingly, fine-tuning the pathway is essential for proper muscle homeostasis. This review will address various aspects of Notch signaling, including our current views of the core pathway, its effects in muscle, its interactions with other signaling pathways, and its relationship with ageing.     

Regulation of Notch signaling and endocytosis by the Lgl neoplastic tumor suppressor

The evolutionarily conserved neoplastic tumor suppressor protein, Lethal (2) giant larvae (Lgl), plays roles in cell polarity and tissue growth via regulation of the Hippo pathway. In our recent study, we showed that in the developing Drosophila eye epithelium, depletion of Lgl leads to increased ligand-dependent Notch signaling. lgl mutant tissue also exhibits an accumulation of early endosomes, recycling endosomes, early-multivesicular body markers and acidic vesicles. We showed that elevated Notch signaling in lgl⁻ tissue can be rescued by feeding larvae the vesicle de-acidifying drug chloroquine, revealing that Lgl attenuates Notch signaling by limiting vesicle acidification. Strikingly, chloroquine also rescued the lgl⁻ overgrowth phenotype, suggesting that the Hippo pathway defects were also rescued. In this extraview, we provide additional data on the regulation of Notch signaling and endocytosis by Lgl, and discuss possible mechanisms by which Lgl depletion contributes to signaling pathway defects and tumorigenesis.     

Border of Notch activity establishes a boundary between the two dorsal appendage tube cell types

Boundaries establish and maintain separate populations of cells critical for organ formation. We show that Notch signaling establishes the boundary between two types of post-mitotic epithelial cells, the Rhomboid- and the Broad-positive cells. These cells will undergo morphogenetic movements to generate the two sides of a simple organ, the dorsal appendage tube of the Drosophila egg chamber. The boundary forms due to a difference in Notch levels in adjacent cells. The Notch expression pattern mimics the boundary; Notch levels are high in Rhomboid cells and low in Broad cells. Notch(-) mutant clones generate an ectopic boundary: ectopic Rhomboid cells arise in Notch(+) cells adjacent to the Notch(-) mutant cells but not further away from the clonal border. Pangolin, a component of the Wingless pathway, is required for Broad expression and for rhomboid repression. We further show that Broad represses rhomboid cell autonomously. Our data provide a foundation for understanding how a single row of Rhomboid cells arises adjacent to the Broad cells in the dorsal appendage primordia. Generating a boundary by the Notch pathway might constitute an evolutionarily conserved first step during organ formation in many tissues.     

Maternal almondex,a neurogenic gene,is required for proper subcellular Notch distribution in early Drosophila embryogenesis

Notch signaling plays crucial roles in the control of cell fate and physiology through local cell–cell interactions. The core processes of Notch signal transduction are well established, but the mechanisms that fine-tune the pathway in various developmental and post-developmental contexts are less clear. Drosophila almondex, which encodes an evolutionarily conserved double-pass transmembrane protein, was identified in the 1970s as a maternal-effect gene that regulates Notch signaling in certain contexts, but its mechanistic function remains obscure. In this study, we examined the role of almondex in Notch signaling during early Drosophila embryogenesis. We found that in addition to being required for lateral inhibition in the neuroectoderm, almondex is also partially required for Notch signaling-dependent single-minded expression in the mesectoderm. Furthermore, we found that almondex is required for proper subcellular Notch receptor distribution in the neuroectoderm, specifically during mid-stage 5 development. The absence of maternal almondex during this critical window of time caused Notch to accumulate abnormally in cells in a mesh-like pattern. This phenotype did not include any obvious change in subcellular Delta ligand distribution, suggesting that it does not result from a general vesicular-trafficking defect. Considering that dynamic Notch trafficking regulates signal output to fit the specific context, we speculate that almondex may facilitate Notch activation by regulating intracellular Notch receptor distribution during early embryogenesis.     

miR‐124 interacts with the Notch1 signalling pathway and has therapeutic potential against gastric cancer

Aberrant Notch signalling plays an important role in cancer progression. However, little is known about the interaction between miRNA and the Notch signalling pathway and its role in gastric cancer (GC). In this study, we found that miR‐124 was down‐regulated in GC compared with adjacent normal tissue. Forced expression of miR‐124 inhibited GC cell growth, migration and invasion, and induced cell cycle arrest. miR‐124 negatively regulated Notch1 signalling by targeting JAG1. miR‐124 levels were also shown to be inversely correlated with JAG1 expression in GC. Furthermore, we found that the overexpression of the intracellular domain of Notch1 repressed miR‐124 expression, promoted GC cell growth, migration and invasion. Conversely, blocking Notch1 using a γ‐secretase inhibitor up‐regulated miR‐124 expression, inhibited GC cell growth, migration and invasion. In conclusion, our data demonstrates a regulatory feedback loop between miR‐124 and Notch1 signalling in GC cells, suggesting that the miR‐124/Notch axis may be a potential therapeutic target against GC.     

Role of Notch pathway in terminal follicle cell differentiation during Drosophila oogenesis

 During Drosophila oogenesis the body axes are determined by signaling between the oocyte and the somatic follicle cells that surround the egg chamber. A key event in the establishment of oocyte anterior-posterior polarity is the differential patterning of the follicle cell epithelium along the anterior-posterior axis. Both the Notch and epithelial growth factor (EGF) receptor pathways are required for this patterning. To understand how these pathways act in the process we have analyzed markers for anterior and posterior follicle cells accompanying constitutive activation of the EGF receptor, loss of Notch function, and ectopic expression of Delta. We find that a constitutively active EGF receptor can induce posterior fate in anterior but not in lateral follicle cells, showing that the EGF receptor pathway can act only on predetermined terminal cells. Furthermore, Notch function is required at both termini for appropriate expression of anterior and posterior markers, while loss of both the EGF receptor and Notch pathways mimic the Notch loss-of-function phenotype. Ectopic expression of the Notch ligand, Delta, disturbs EGF receptor dependent posterior follicle cell differentiation and anterior-posterior polarity of the oocyte. Our data are consistent with a model in which the Notch pathway is required for early follicle cell differentiation at both termini, but is then repressed at the posterior for proper determination of the posterior follicle cells by the EGF receptor pathway. Received: 5 November 1998 / Accepted: 14 December 1998