Regulation of lymphocyte fate by Ras/ERK signals

The Ras-ERK cascade is activated by countless external cues that stimulate diverse receptors. Therefore, the mechanisms by which distinct receptors dictate different cellular outcomes by activating the same signaling module has long fascinated many researchers. Initial clues came from observations that the duration of ERK activation is critical to cell-fate decisions. In classical experiments, PC12 cells proliferated after transient ERK activation by epidermal growth factor, but terminally differentiated after more sustained ERK activation by nerve growth factor. Subsequent studies suggested that the duration of ERK signaling is interpreted by cells through a network of immediate-early genes. Nevertheless, it remains ill-defined how the duration and strength of Ras-ERK signaling is established and what genes are differently regulated, thereby translating the response into different biological outcomes. Recent studies with lymphocytes have evoked a new idea that two types of interconnected mechanisms can contribute to the sensitivity and robustness of the ERK activity; 1) interplay between two types of Ras activators (Sos and RasGRP); 2) existence of two subcellular compartments for Ras activation (plasma membrane and Golgi). Moreover, candidate immediate early genes that regulate lymphocyte proliferation and differentiation have emerged.     

Regulation of ganglion cell production by Notch signaling during retinal development

Although progenitor cells in developing vertebrate retina are capable of producing all retinal cell types, they are competent to produce only certain cell types at a given time, and this competence changes as development progresses. We asked whether a change in progenitor cell competence is primarily responsible for ending production of a specific cell type, the retinal ganglion cell. Reducing Notch expression using an antisense oligonucleotide in vitro or in vivo increased ganglion cell genesis. The antisense treatment could reinitiate ganglion cell genesis after it had terminated in a region of the retina, but only for a brief period. The failure of the Notch antisense treatment to reinitiate ganglion cell production after this period was not due to the lack of receptor or ligand expression, as both Notch-1 and Delta-1 were still expressed. The failure of the Notch antisense treatment to reinitiate ganglion cell production is consistent with the suggestion that the intrinsic competence of progenitor cells changes as development progresses. Because reducing Notch signaling can reinitiate ganglion cell production for a brief period after ganglion cell production has normally ceased, it appears that ganglion cell production initially ends in a region of the retina because of cell-cell interactions and not because progenitor cells lose the competence to make ganglion cells. Notch signaling appears to temporarily prevent production of ganglion cells in a region, while some other signal must initiate a change in progenitor cell competence, thus permanently ending the possibility of further ganglion cell production.     

Regulation of human cytotoxic T lymphocyte development by IL-7

The effects of IL-7 on the generation of human CTL in alloantigen-, virus-, and lectin-stimulated systems were examined. Addition of IL-7 at the onset of cultures resulted in marked (up to 80-fold) augmentation of cytotoxicity accompanied by smaller (1.5- to 4-fold) increases in total lymphocyte number. Studies of CTL development in purified lectin-stimulated CD8+ T cell populations demonstrated that IL-7 could act directly on the CD8+ lymphocyte subset to augment cytotoxicity. In MLC, the IL-7-induced enhancement of cytotoxicity was found to be mediated primarily by the CD8+ subpopulation of lymphocytes. Late addition of IL-7 (day 5 of 7) resulted in an increase in cytolytic activity that was associated with little or no increase in total or activated CD8+ lymphocyte number indicating that IL-7 may act as a differentiation factor for human CTL. A role for endogenous IL-7 in CTL development was suggested by the observation that addition of neutralizing antiserum to IL-7 to MLC at initiation (or 5 days thereafter) resulted in decreased levels of cytotoxicity. These results indicate that IL-7 can exert major up-regulatory effects on human CTL development and suggest that these effects are both proliferative and differentiative.     

Regulation of Notch signaling by glycosylation

Notch receptors are approximately 300 kDa cell surface glycoproteins whose activation by Notch ligands regulates cell fate decisions in the metazoa. The extracellular domain of Notch receptors has many epidermal growth factor like repeats that are glycosylated with O-fucose and O-glucose glycans as well as N-glycans. Disruption of O-fucose glycan synthesis leads to severe Notch signaling defects in Drosophila and mammals. Removal or addition of O-fucose glycan consensus sites on Notch receptors also leads to Notch signaling defects. Ligand binding and ligand-induced Notch signaling assays have provided insights into how changes in the O-fucose glycans of Notch receptors alter Notch signaling.     

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 neuronal survival and death by extracellular signals during development

Cell death is a prominent feature of the developing vertebrate nervous system, affecting neurons, glial cells and their progenitors. The most extensively studied and best understood phase of cell death occurs in populations of neurons shortly after they begin establishing connections with other neurons and/or non-neural tissues. This phase of cell death makes appropriate adjustments to the relative sizes of interconnected groups of neurons and matches the size of neuronal populations that innervate non-neural tissues to the optimal requirements of these tissues. The fate of neurons during this period of development is regulated by a variety of secreted proteins that either promote survival or bring about cell death after binding to receptors expressed on the neurons. These proteins may be derived from the targets the neurons innervate, the afferents they receive or from associated glial cells, or they may be secreted by the neurons themselves. In this review, I will outline the established and emerging principles that modulate neuronal number in the developing nervous system.     

Regulation of mammalian Notch signaling and embryonic development by the protein O-glucosyltransferase Rumi

Protein O-glucosylation is a conserved post-translational modification that occurs on epidermal growth factor-like (EGF) repeats harboring the C(1)-X-S-X-P-C(2) consensus sequence. The Drosophila protein O-glucosyltransferase (Poglut) Rumi regulates Notch signaling, but the contribution of protein O-glucosylation to mammalian Notch signaling and embryonic development is not known. Here, we show that mouse Rumi encodes a Poglut, and that Rumi(-/-) mouse embryos die before embryonic day 9.5 with posterior axis truncation and severe defects in neural tube development, somitogenesis, cardiogenesis and vascular remodeling. Rumi knockdown in mouse cell lines results in cellular and molecular phenotypes of loss of Notch signaling without affecting Notch ligand binding. Biochemical, cell culture and cross-species transgenic experiments indicate that a decrease in Rumi levels results in reduced O-glucosylation of Notch EGF repeats, and that the enzymatic activity of Rumi is key to its regulatory role in the Notch pathway. Genetic interaction studies show that removing one copy of Rumi in a Jag1(+/-) (jagged 1) background results in severe bile duct morphogenesis defects. Altogether, our data indicate that addition of O-glucose to EGF repeats is essential for mouse embryonic development and Notch signaling, and that Jag1-induced signaling is sensitive to the gene dosage of the protein O-glucosyltransferase Rumi. Given that Rumi(-/-) embryos show more severe phenotypes compared to those displayed by other global regulators of canonical Notch signaling, Rumi is likely to have additional important targets during mammalian development.     

Negative Regulation of Notch Signaling by Xylose

The Notch signaling pathway controls a large number of processes during animal development and adult homeostasis. One of the conserved post-translational modifications of the Notch receptors is the addition of an O-linked glucose to epidermal growth factor-like (EGF) repeats with a C-X-S-X-(P/A)-C motif by Protein O-glucosyltransferase 1 (POGLUT1; Rumi in Drosophila). Genetic experiments in flies and mice, and in vivo structure-function analysis in flies indicate that O-glucose residues promote Notch signaling. The O-glucose residues on mammalian Notch1 and Notch2 proteins are efficiently extended by the addition of one or two xylose residues through the function of specific mammalian xylosyltransferases. However, the contribution of xylosylation to Notch signaling is not known. Here, we identify the Drosophila enzyme Shams responsible for the addition of xylose to O-glucose on EGF repeats. Surprisingly, loss- and gain-of-function experiments strongly suggest that xylose negatively regulates Notch signaling, opposite to the role played by glucose residues. Mass spectrometric analysis of Drosophila Notch indicates that addition of xylose to O-glucosylated Notch EGF repeats is limited to EGF14–20. A Notch transgene with mutations in the O-glucosylation sites of Notch EGF16–20 recapitulates the shams loss-of-function phenotypes, and suppresses the phenotypes caused by the overexpression of human xylosyltransferases. Antibody staining in animals with decreased Notch xylosylation indicates that xylose residues on EGF16–20 negatively regulate the surface expression of the Notch receptor. Our studies uncover a specific role for xylose in the regulation of the Drosophila Notch signaling, and suggest a previously unrecognized regulatory role for EGF16–20 of Notch.     

Regulation of Notch signalling by non-visual beta-arrestin

Signalling activity of the Notch receptor, which plays a fundamental role in metazoan cell fate determination, is controlled at multiple levels. We uncovered a Notch signal-controlling mechanism that depends on the ability of the non-visual beta-arrestin, Kurtz (Krz), to influence the degradation and, consequently, the function of the Notch receptor. We identified Krz as a binding partner of a known Notch-pathway modulator, Deltex (Dx), and demonstrated the existence of a trimeric Notch-Dx-Krz protein complex. This complex mediates the degradation of the Notch receptor through a ubiquitination-dependent pathway. Our results establish a novel mode of regulation of Notch signalling and define a new function for non-visual beta-arrestins.     

Regulation of cellular signals by G-proteins

Extracellular signals are transduced across the cell by the cell surface receptors, with the aid of G-proteins, which act at a critical point of signal transduction and cellular regulation. Structurally, G-proteins are heterotrimeric consisting α, β and γ subunits but in functionally active state they dissociate into α subunit coupled to GTP and as βγ dimer. G-proteins can be broadly divided into two classes based on their sensitivity to pertussis toxin and cholera toxin. Existence of various forms of each of the subunit allows molecular diversity in the subunit species of G-proteins. These subunits interact with a wide range of receptors and effectors, facilitated by post translational modification of their subunits. Different types of G-proteins mediate several signalling events in different parts of the body. This review summarizes the features of (i) structural and functional heterogenity among different subunits of G-proteins, (ii) interaction of G-proteins and their subunits with effectors with specific cases of G-protein mediated signalling in olfaction, phototransduction in the retina, ras andras related transduction and (iii) disease conditions associated with malfunctioning of G-proteins.     

Regulation of Notch signalling by endocytosis and endosomal sorting

Cell-cell signalling is an essential process in the formation of multicellular organisms. Notch is the receptor of an evolutionarily conserved signalling pathway regulating numerous developmental decisions. Indeed, its misregulation is linked to multiple developmental and physiological disorders. Notch and its ligands are distributed widely throughout development, yet Notch activity is highly controlled and restricted in time and space. Recent advances have highlighted that endocytosis followed by endosomal sorting of both the Notch receptor and its ligands is an essential mechanism by which Notch-mediated signalling is developmentally controlled.     

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。