Unidirectional Notch signaling depends on continuous cleavage of Delta

Reversible and bidirectional signaling of notch ligands

The Notch signaling pathway is a direct cell-cell communication system involved in a wide variety of biological processes, and its disruption is observed in several pathologies. The pathway is comprised of a ligand-expressing (sender) cell and a receptor-expressing (receiver) cell. The canonical ligands are members of the Delta/Serrate/Lag-1 (DSL) family of proteins. Their binding to a Notch receptor in a neighboring cell induces a conformational change in the receptor, which will undergo regulated intramembrane proteolysis (RIP), liberating the Notch intracellular domain (NICD). The NICD is translocated to the nucleus and promotes gene transcription. It has been demonstrated that the ligands can also undergo RIP and nuclear translocation, suggesting a function for the ligands in the sender cell and possible bidirectionality of the Notch pathway. Although the complete mechanism of ligand processing is not entirely understood, and its dependence on Notch receptors has not been ruled out. Also, ligands have autonomous functions beyond Notch activation. Here we review the concepts of reverse and bidirectional signalization of DSL proteins and discuss the characteristics that make them more than just ligands of the Notch pathway.

The developmental origins of Notch-driven intrahepatic bile duct disorders

The Notch signaling pathway is an evolutionarily conserved mechanism of cell-cell communication that mediates cellular proliferation, cell fate specification, and maintenance of stem and progenitor cell populations. In the vertebrate liver, an absence of Notch signaling results in failure to form bile ducts, a complex tubular network that radiates throughout the liver, which, in healthy individuals, transports bile from the liver into the bowel. Loss of a functional biliary network through congenital malformations during development results in cholestasis and necessitates liver transplantation. Here, we examine to what extent Notch signaling is necessary throughout embryonic life to initiate the proliferation and specification of biliary cells and concentrate on the animal and human models that have been used to define how perturbations in this signaling pathway result in developmental liver disorders.

Transcriptional repression of the ectodomain sheddase ADAM10 by TBX2 and potential implication for Alzheimer's disease

BACKGROUND

The ADAM10-mediated cleavage of transmembrane proteins regulates cellular processes such as proliferation or migration. Substrate cleavage by ADAM10 has also been implicated in pathological situations such as cancer or Morbus Alzheimer. Therefore, identifying endogenous molecules, which modulate the amount and consequently the activity of ADAM10, might contribute to a deeper understanding of the enzyme's role in both, physiology and pathology.

METHOD

To elucidate the underlying cellular mechanism of the TBX2-mediated repression of ADAM10 gene expression, we performed overexpression, RNAi-mediated knockdown and pharmacological inhibition studies in the human neuroblastoma cell line SH-SY5Y. Expression analysis was conducted by e.g. real-time RT-PCR or western blot techniques. To identify the binding region of TBX2 within the ADAM10 promoter, we used luciferase reporter assay on deletion constructs and EMSA/WEMSA experiments. In addition, we analyzed a TBX2 loss-of-function Drosophila model regarding the expression of ADAM10 orthologs by qPCR. Furthermore, we quantified the mRNA level of TBX2 in post-mortem brain tissue of AD patients.

RESULTS

Here, we report TBX2 as a transcriptional repressor of ADAM10 gene expression: both, the DNA-binding domain and the repression domain of TBX2 were necessary to effect transcriptional repression of ADAM10 in neuronal SH-SY5Y cells. This regulatory mechanism required HDAC1 as a co-factor of TBX2. Transcriptional repression was mediated by two functional TBX2 binding sites within the core promoter sequence (- 315 to - 286 bp). Analysis of a TBX2 loss-of-function Drosophila model revealed that kuzbanian and kuzbanian-like, orthologs of ADAM10, were derepressed compared to wild type. Vice versa, analysis of cortical brain samples of AD-patients, which showed reduced ADAM10 mRNA levels, revealed a 2.5-fold elevation of TBX2, while TBX3 and TBX21 levels were not affected.

CONCLUSION

Our results characterize TBX2 as a repressor of ADAM10 gene expression and suggest that this regulatory interaction is conserved across tissues and species.

Intriguing Roles for Endothelial ADAM10/Notch Signaling in the Development of Organ-Specific Vascular Beds

The vasculature is a remarkably interesting, complex, and interconnected organ. It provides a conduit for oxygen and nutrients, filtration of waste products, and rapid communication between organs. Much remains to be learned about the specialized vascular beds that fulfill these diverse, yet vital functions. This review was prompted by the discovery that Notch signaling in mouse endothelial cells is crucial for the development of specialized vascular beds found in the heart, kidneys, liver, intestines, and bone. We will address the intriguing questions raised by the role of Notch signaling and that of its regulator, the metalloprotease ADAM10, in the development of specialized vascular beds. We will cover fundamentals of ADAM10/Notch signaling, the concept of Notch-dependent cell fate decisions, and how these might govern the development of organ-specific vascular beds through angiogenesis or vasculogenesis. We will also consider common features of the affected vessels, including the presence of fenestra or sinusoids and their occurrence in portal systems with two consecutive capillary beds. We hope to stimulate further discussion and study of the role of ADAM10/Notch signaling in the development of specialized vascular structures, which might help uncover new targets for the repair of vascular beds damaged in conditions like coronary artery disease and glomerulonephritis.

Epsin-Dependent Ligand Endocytosis Activates Notch by Force

DSL ligands activate Notch by inducing proteolytic cleavage of the receptor ectodomain, an event that requires ligand to be endocytosed in signal-sending cells by the adaptor protein Epsin. Two classes of explanation for this unusual requirement are (1) recycling models, in which the ligand must be endocytosed to be modified or repositioned before it binds Notch and (2) pulling models, in which the ligand must be endocytosed after it binds Notch to exert force that exposes an otherwise buried site for cleavage. We demonstrate in vivo that ligands that cannot enter the Epsin pathway nevertheless bind Notch but fail to activate the receptor because they cannot exert sufficient force. This argues against recycling models and in favor of pulling models. Our results also suggest that once ligand binds receptor, activation depends on a competition between Epsin-mediated ligand endocytosis, which induces cleavage, and transendocytosis of the ligand by the receptor, which aborts the incipient signal.

The alpha secretase ADAM10: A metalloprotease with multiple functions in the brain

Proteins belonging to the 'A Disintegrin And Metalloproteinase' (ADAM) family are membrane-anchored proteases that are able to cleave the extracellular domains of several membrane-bound proteins in a process known as 'ectodomain shedding'. In the central nervous system, ADAM10 has attracted the most attention, since it was described as the amyloid precursor protein α-secretase over ten years ago. Despite the excitement over the potential of ADAM10 as a novel drug target in Alzheimer disease, the physiological functions of ADAM10 in the brain are not yet well understood. This is largely because of the embryonic lethality of ADAM10-deficient mice, which results from the loss of cleavage and signaling of the Notch receptor, another ADAM10 substrate. However, the recent generation of conditional ADAM10-deficient mice and the identification of further ADAM10 substrates in the brain has revealed surprisingly numerous and fundamental functions of ADAM10 in the development of the embryonic brain and also in the homeostasis of adult neuronal networks. Mechanistically, ADAM10 controls these functions by utilizing unique postsynaptic substrates in the central nervous system, in particular synaptic cell adhesion molecules, such as neuroligin-1, N-cadherin, NCAM, Ephrin A2 and A5. Consequently, a dysregulation of ADAM10 activity is linked to psychiatric and neurological diseases, such as epilepsy, fragile X syndrome and Huntington disease. This review highlights the recent progress in understanding the substrates and function as well as the regulation and cell biology of ADAM10 in the central nervous system and discusses the value of ADAM10 as a drug target in brain diseases.

Evolution of Vertebrate Adam Genes; Duplication of Testicular Adams from Ancient Adam9/9-like Loci

Members of the disintegrin metalloproteinase (ADAM) family have important functions in regulating cell-cell and cell-matrix interactions as well as cell signaling. There are two major types of ADAMs: the somatic ADAMs (sADAMs) that have a significant presence in somatic tissues, and the testicular ADAMs (tADAMs) that are expressed predominantly in the testis. Genes encoding tADAMs can be further divided into two groups: group I (intronless) and group II (intron-containing). To date, tAdams have only been reported in placental mammals, and their evolutionary origin and relationship to sAdams remain largely unknown. Using phylogenetic and syntenic tools, we analyzed the Adam genes in various vertebrates ranging from fishes to placental mammals. Our analyses reveal duplication and loss of some sAdams in certain vertebrate species. In particular, there exists an Adam9-like gene in non-mammalian vertebrates but not mammals. We also identified putative group I and group II tAdams in all amniote species that have been examined. These tAdam homologues are more closely related to Adams 9 and 9-like than to other sAdams. In all amniote species examined, group II tAdams lie in close vicinity to Adam9 and hence likely arose from tandem duplication, whereas group I tAdams likely originated through retroposition because of their lack of introns. Clusters of multiple group I tAdams are also common, suggesting tandem duplication after retroposition. Therefore, Adam9/9-like and some of the derived tAdam loci are likely preferred targets for tandem duplication and/or retroposition. Consistent with this hypothesis, we identified a young retroposed gene that duplicated recently from Adam9 in the opossum. As a result of gene duplication, some tAdams were pseudogenized in certain species, whereas others acquired new expression patterns and functions. The rapid duplication of Adam genes has a major contribution to the diversity of ADAMs in various vertebrate species.

ADAM metalloproteases promote a developmental switch in responsiveness to the axonal repellant Sema3A

During embryonic development, axons can gain and lose sensitivity to guidance cues, and this flexibility is essential for the correct wiring of the nervous system. Yet, the underlying molecular mechanisms are largely unknown. Here we show that receptor cleavage by ADAM (A Disintegrin And Metalloprotease) metalloproteases promotes murine sensory axons loss of responsiveness to the chemorepellant Sema3A. Genetic ablation of ADAM10 and ADAM17 disrupts the developmental downregulation of Neuropilin-1 (Nrp1), the receptor for Sema3A, in sensory axons. Moreover, this is correlated with gain of repulsive response to Sema3A. Overexpression of Nrp1 in neurons reverses axonal desensitization to Sema3A, but this is hampered in a mutant Nrp1 with high susceptibility to cleavage. Lastly, we detect guidance errors of proprioceptive axons in ADAM knockouts that are consistent with enhanced response to Sema3A. Our results provide the first evidence for involvement of ADAMs in regulating developmental switch in responsiveness to axonal guidance cues.

Identification and characterization of ADAM41, a novel ADAM metalloproteinase in Xenopus

The ADAM family of transmembrane metalloproteinases has important functions in fertilization, development and disease, and is widely distributed throughout the Metazoa. In this study, we identified a novel ADAM protein in Xenopus tropicalis (X. tropicalis) with closest overall sequence similarity to the Xenopus ADAM10 protein. Based on comparisons of available sequence information, putative orthologs of this ADAM (which we designate ADAM41) are identified in several other vertebrate species including non-placental mammals, but absent from placental mammals and aves. ADAM41 mRNA is maternally deposited in X. tropicalis with subsequent zygotic expression detected in the neural plate at neurula stages. Antisense morpholino knockdown of ADAM41 results in a delay in early neuronal marker expression, which can be rescued by a non-targeted ADAM41 transcript. Thus, ADAM41 is likely required for maintaining proper timing of neurogenesis in X. tropicalis.

Amyloid precursor proteins are protective in Drosophila models of progressive neurodegeneration

The processing of Amyloid Precursor Proteins (APPs) results in several fragments, including soluble N-terminal ectodomains (sAPPs) and C-terminal intracellular domains (AICD). sAPPs have been ascribed neurotrophic or neuroprotective functions in cell culture, although β-cleaved sAPPs can have deleterious effects and trigger neuronal cell death. Here we describe a neuroproprotective function of APP and fly APPL (Amyloid Precursor Protein-like) in vivo in several Drosophila mutants with progressive neurodegeneration. We show that expression of the N-terminal ectodomain is sufficient to suppress the progressive degeneration in these mutants and that the secretion of the ectodomain is required for this function. In addition, a protective effect is achieved by expressing kuzbanian (which has α-secretase activity) whereas expression of fly and human BACE aggravates the phenotypes, suggesting that the protective function is specifically mediated by the α-cleaved ectodomain. Furthermore, genetic and molecular studies suggest that the N-terminal fragments interact with full-length APPL activating a downstream signaling pathway via the AICD. Because we show protective effects in mutants that affect different genes (AMP-activated protein kinase, MAP1b, rasGAP), we propose that the protective effect is not due to a genetic interaction between APPL and these genes but a more general aspect of APP proteins. The result that APP proteins and specifically their soluble α-cleaved ectodomains can protect against progressive neurodegeneration in vivo provides support for the hypothesis that a disruption of the physiological function of APP could play a role in the pathogenesis of Alzheimer's Disease.

The role of Adams in Notch signaling

Regulated intramembrane proteolysis (RIP) is a highly conserved signaling paradigm whereby membrane-bound signaling proteins are cleaved in their transmembrane region and then released into the cytoplasm to act as signaling molecules. In most if not all cases intramembrane cleavage is preceded and regulated by a membrane proximal cleavage step called 'ectodomain shedding'. Here we will review the role of ectodomain shedding in RIP of the NOTCH signaling pathway, a highly conserved cell-cell communication pathway that mediates cell fate decisions during development and in adult tissues.

Roles of ADAM13-regulated Wnt activity in early Xenopus eye development

Pericellular proteolysis by ADAM family metalloproteinases has been widely implicated in cell signaling and development. We recently found that Xenopus ADAM13, an ADAM metalloproteinase, is required for activation of canonical Wnt signaling during cranial neural crest (CNC) induction by regulating a novel crosstalk between Wnt and ephrin B (EfnB) signaling pathways (Wei et al., 2010b). In the present study we show that the metalloproteinase activity of ADAM13 also plays important roles in eye development in Xenopus tropicalis. Knockdown of ADAM13 results in reduced expression of eye field markers pax6 and rx1, as well as that of the pan-neural marker sox2. Activation of canonical Wnt signaling or inhibition of forward EfnB signaling rescues the eye defects caused by loss of ADAM13, suggesting that ADAM13 functions through regulation of the EfnB-Wnt pathway interaction. Downstream of Wnt, the head inducer Cerberus was identified as an effector that mediates ADAM13 function in early eye field formation. Furthermore, ectopic expression of the Wnt target gene snail2 restores cerberus expression and rescues the eye defects caused by ADAM13 knockdown. Together these data suggest an important role of ADAM13-regulated Wnt activity in eye development in Xenopus.

Proteolytic cleavage of Notch: "HIT and RUN"

The Notch pathway is a highly conserved signaling pathway in multicellular eukaryotes essential in controlling spatial patterning, morphogenesis and homeostasis in embryonic and adult tissues. Notch proteins coordinate cell-cell communication through receptor-ligand interactions between adjacent cells. Notch signaling is frequently deregulated by oncogenic mutation or overexpression in many cancer types. Notch activity is controlled by three sequential cleavage steps leading to ectodomain shedding and transcriptional activation. Here we review the key regulatory steps in the activation of Notch, from receptor maturation to receptor activation (HIT) via a rate-limiting proteolytic cascade (RUN) in the context of species-specific differences.

Loss of PTB or negative regulation of Notch mRNA reveals distinct zones of Notch and actin protein accumulation in Drosophila embryo

Polypyrimidine Tract Binding (PTB) protein is a regulator of mRNA processing and translation. Genetic screens and studies of wing and bristle development during the post-embryonic stages of Drosophila suggest that it is a negative regulator of the Notch pathway. How PTB regulates the Notch pathway is unknown. Our studies of Drosophila embryogenesis indicate that (1) the Notch mRNA is a potential target of PTB, (2) PTB and Notch functions in the dorso-lateral regions of the Drosophila embryo are linked to actin regulation but not their functions in the ventral region, and (3) the actin-related Notch activity in the dorso-lateral regions might require a Notch activity at or near the cell surface that is different from the nuclear Notch activity involved in cell fate specification in the ventral region. These data raise the possibility that the Drosophila embryo is divided into zones of different PTB and Notch activities based on whether or not they are linked to actin regulation. They also provide clues to the almost forgotten role of Notch in cell adhesion and reveal a role for the Notch pathway in cell fusions.

Notch and delta mRNAs in early-stage and mid-stage drosophila embryos exhibit complementary patterns of protein-producing potentials

Notch and Delta proteins generate Notch signaling that specifies cell fates during animal development. There is an intriguing phenomenon in Drosophila embryogenesis that has not received much attention and whose significance to embryogenesis is unknown. Notch and Delta mRNAs expressed in early-stage embryos are shorter than their counterparts in mid-stage embryos. We show here that the difference in sizes is due to mRNA 3' processing at alternate polyadenylation sites. While the early-stage Notch mRNA has a lower protein-producing potential than the mid-stage Notch mRNA, the early-stage Delta mRNA has a higher protein-producing potential than the mid-stage Delta mRNA. Our data can explain the complementary patterns of Notch and Delta protein levels in early- and mid-stage embryos. Our data also raise the possibility that the manner and regulation of Notch signaling change in the course of embryogenesis and that this change is effected by 3' UTR and mRNA 3' processing factors.

Conservation and divergence of ADAM family proteins in the Xenopus genome

BACKGROUND

Members of the disintegrin metalloproteinase (ADAM) family play important roles in cellular and developmental processes through their functions as proteases and/or binding partners for other proteins. The amphibian Xenopus has long been used as a model for early vertebrate development, but genome-wide analyses for large gene families were not possible until the recent completion of the X. tropicalis genome sequence and the availability of large scale expression sequence tag (EST) databases. In this study we carried out a systematic analysis of the X. tropicalis genome and uncovered several interesting features of ADAM genes in this species.

RESULTS

Based on the X. tropicalis genome sequence and EST databases, we identified Xenopus orthologues of mammalian ADAMs and obtained full-length cDNA clones for these genes. The deduced protein sequences, synteny and exon-intron boundaries are conserved between most human and X. tropicalis orthologues. The alternative splicing patterns of certain Xenopus ADAM genes, such as adams 22 and 28, are similar to those of their mammalian orthologues. However, we were unable to identify an orthologue for ADAM7 or 8. The Xenopus orthologue of ADAM15, an active metalloproteinase in mammals, does not contain the conserved zinc-binding motif and is hence considered proteolytically inactive. We also found evidence for gain of ADAM genes in Xenopus as compared to other species. There is a homologue of ADAM10 in Xenopus that is missing in most mammals. Furthermore, a single scaffold of X. tropicalis genome contains four genes encoding ADAM28 homologues, suggesting genome duplication in this region.

CONCLUSIONS

Our genome-wide analysis of ADAM genes in X. tropicalis revealed both conservation and evolutionary divergence of these genes in this amphibian species. On the one hand, all ADAMs implicated in normal development and health in other species are conserved in X. tropicalis. On the other hand, some ADAM genes and ADAM protease activities are absent, while other novel ADAM proteins in this species are predicted by this study. The conservation and unique divergence of ADAM genes in Xenopus probably reflect the particular selective pressures these amphibian species faced during evolution.

Inhibition of proteolysis of Delta-like-1 does not promote or reduce T-cell developmental potential

Notch signaling is critical for T-cell generation in the thymus. Notch signaling is linear in nature and is highly regulated through differential gene expression and post-translational modification. Upon ligand binding, the Notch receptor is sequentially cleaved, first via extracellular ADAM protease-mediated cleavage, followed by an intracellular presenilin-dependent cleavage to release the Notch intracellular domain and activate transcription. Delta-like-1 (Dll1) is a Notch ligand that positively regulates T-cell development. Dll1 is proteolytically processed in a similar manner to the Notch receptor, and it has been speculated to participate in bidirectional signaling. We hypothesized that inhibition of Dll1 processing in Notch signal sending cells would lead to changes in their ability to support thymopoiesis. We used the OP9 in vitro co-culture system, and transduced OP9s with full length, cleavable Dll1 or a non-cleavable mutant (NC-Dll1) lacking the ADAM protease cleavage site. OP9-NC-Dll1 cells were able to support T-cell development with similar efficacy to OP9-Dll1 cells. Interestingly, expression of the Notch target gene Hes5 was more highly induced in T-cell progenitors by NC-Dll1, whereas expression of Hes1, Deltex1, and pre-Tα were similar to controls. Furthermore, a reduced ability of hematopoietic progenitors to assume the granulocyte cell fate in OP9-NC-Dll1 cultures was noted. Taken together, these findings show that proteolytic cleavage of Dll1 in Notch signal sending cells is dispensable for murine T-cell development, differentially affects expression of Notch target genes, and might be a mechanism that regulates myelopoiesis.

Canonical and non-canonical Notch ligands

Notch signaling induced by canonical Notch ligands is critical for normal embryonic development and tissue homeostasis through the regulation of a variety of cell fate decisions and cellular processes. Activation of Notch signaling is normally tightly controlled by direct interactions with ligand-expressing cells, and dysregulated Notch signaling is associated with developmental abnormalities and cancer. While canonical Notch ligands are responsible for the majority of Notch signaling, a diverse group of structurally unrelated noncanonical ligands has also been identified that activate Notch and likely contribute to the pleiotropic effects of Notch signaling. Soluble forms of both canonical and noncanonical ligands have been isolated, some of which block Notch signaling and could serve as natural inhibitors of this pathway. Ligand activity can also be indirectly regulated by other signaling pathways at the level of ligand expression, serving to spatiotemporally compartmentalize Notch signaling activity and integrate Notch signaling into a molecular network that orchestrates developmental events. Here, we review the molecular mechanisms underlying the dual role of Notch ligands as activators and inhibitors of Notch signaling. Additionally, evidence that Notch ligands function independent of Notch is presented. We also discuss how ligand posttranslational modification, endocytosis, proteolysis, and spatiotemporal expression regulate their signaling activity.

Notch mRNA expression in Drosophila embryos is negatively regulated at the level of mRNA 3' processing

Notch receptor regulates differentiation of almost all tissues and organs during animal development. Many mechanisms function at the protein level to finely regulate Notch activity. Here we provide evidence for Notch regulation at an earlier step - mRNA 3′ processing. Processing at the Notch consensus polyadenylation site appears by default to be suppressed in Drosophila embryos. Interference with this suppression, by a mutation, results in increased levels of polyadenylated Notch mRNA, excess Notch signaling, and severe developmental defects. We propose that Notch mRNA 3′ processing is negatively regulated to limit the production of Notch protein and render it a controlling factor in the generation of Notch signaling.

Metalloprotease ADAM10 is required for Notch1 site 2 cleavage

Notch signaling is controlled by ligand binding, which unfolds a negative control region to induce proteolytic cleavage of the receptor. First, a membrane-proximal cleavage is executed by a metalloprotease, removing the extracellular domain. This allows gamma-secretase to execute a second cleavage within the Notch transmembrane domain, which releases the intracellular domain to enter the nucleus. Here we show that the ADAM10 metalloprotease Kuzbanian, but not ADAM17/tumor necrosis factor alpha-converting enzyme, plays an essential role in executing ligand-induced extracellular cleavage at site 2 (S2) in cells and localizes this step to the plasma membrane. Importantly, genetic or pharmacological inhibition of metalloproteases still allowed extracellular cleavage of Notch, indicating the presence of unknown proteases with the ability to cleave at S2. Gain of function mutations identified in human cancers and in model organisms that map to the negative control region alleviate the requirement for ligand binding for extracellular cleavage to occur. Because cancer-causing Notch1 mutations also depend on (rate-limiting) S2 proteolysis, the identity of these alternative proteases has important implications for understanding Notch activation in normal and cancer cells.

Selective use of ADAM10 and ADAM17 in activation of Notch1 signaling

Notch signaling requires a series of proteolytic cleavage events to release the Notch intracellular domain (NICD) that functions directly in signal transduction. The Notch receptor is locked down in a protease-resistant state by a negative regulatory region (NRR) that protects an ADAM (a disintegrin and metalloprotease) cleavage site. Engagement with ligand-bearing cells induces global conformational movements in Notch that unfold the NRR structure to expose the ADAM cleavage site and initiate proteolytic activation. Although both ADAM10 and ADAM17 have been reported to cleave Notch to facilitate NICD release by gamma-secretase, the relevant ADAM has remained controversial. Our study provides new insight into this conflict, as we find that although Notch1 (N1) is a substrate for both ADAM10 and ADAM17, the particular ADAM required for receptor activation is context dependent. Specifically, ADAM10 was absolutely required for N1 signaling induced by ligands, while signaling independent of ligands required ADAM17. In contrast to the strict and differential use of ADAM10 and ADAM17 in normal and dysregulated signaling, respectively, both proteases participated in signaling intrinsic to N1 mutations associated with leukemia. We propose that in addition to exposing the ADAM cleavage site, activating N1 conformational changes facilitate selective cleavage by specific proteases.

Endocytic regulation of Notch activation and down-regulation (Review)

Notch receptor signalling plays a central role in development and its misfunction has been linked to a number of diseases. In the cannonical Notch signalling pathway, ligand binding to Notch activates a series of proteolytic cleavages that release the Notch intracellular domain for trafficking to the nucleus, where it activates the transcription factor, Suppressor of Hairless (Su(H)). A number of recent papers have demonstrated the importance of endocytic trafficking of Notch and its ligands for both the activation and the down-regulation of the Notch receptor. These reports highlight uncertainty regarding the whereabouts in the cell where Notch activation occurs, and the form of the ligand that can induce signalling. In this review we speculate that, decision points between alternative trafficking pathways represent important regulatory nodes that may allow Notch signalling levels to be modulated by other developmental signals, providing context-dependency to Notch activation. We also review data that suggest that key proteolytic events, associated with Notch activation, may occur within the endocytic pathway or require prior endocytosis and recycling of Notch and its ligands to the cell surface. Sorting within the endocytic pathway, regulated by several different ubiquitin ligase proteins, may be involved in ensuring whether ligand and receptor are competent to signal. Furthermore, the utilisation of an alternative mechanism of Notch signalling, independent of Su(H), may depend on driving endocytic Notch into a specific compartment, in response to the activity of the ring finger domain protein, Deltex.

Lkb1 deficiency alters goblet and paneth cell differentiation in the small intestine

The Lkb1 tumour suppressor is a multitasking kinase participating in a range of physiological processes. We have determined the impact of Lkb1 deficiency on intestinal homeostasis, particularly focussing on secretory cell differentiation and development since we observe strong expression of Lkb1 in normal small intestine Paneth and goblet cells. We crossed mice bearing an Lkb1 allele flanked with LoxP sites with those carrying a Cyp1a1-specific inducible Cre recombinase. Lkb1 was efficiently deleted from the epithelial cells of the mouse intestine after intraperitoneal injection of the inducing agent beta-naphthoflavone. Bi-allelic loss of Lkb1 led to the perturbed development of Paneth and goblet cell lineages. These changes were characterised by the lack of Delta ligand expression in Lkb1-deficient secretory cells and a significant increase in the levels of the downstream Notch signalling effector Hes5 but not Hes1. Our data show that Lkb1 is required for the normal differentiation of secretory cell lineages within the intestine, and that Lkb1 deficiency modulates Notch signalling modulation in post-mitotic cells.

The role of Delta-like 1 shedding in muscle cell self-renewal and differentiation

Myogenic cells have the ability to adopt two divergent fates upon exit from the cell cycle: differentiation or self-renewal. The Notch signaling pathway is a well-known negative regulator of myogenic differentiation. Using mouse primary myoblasts cultured in vitro or C2C12 myogenic cells, we found that Notch activity is essential for maintaining the expression of Pax7, a transcription factor associated with the self-renewal lineage, in quiescent undifferentiated myoblasts after they exit the cell cycle. Stimulation of the Notch pathway by expression of a constitutively active Notch-1, or co-culture of myogenic cells with CHO cells transfected with Delta like-1 (DLL1), increases the level of Pax7. DLL1, a ligand for Notch receptor, is shed by ADAM metalloproteases in a pool of Pax7+ C2C12 reserve cells, but it remains intact in differentiated myotubes. DLL1 shedding changes the receptor/ligand ratio and modulates the level of Notch signaling. Inhibition of DLL1 cleavage by a soluble, dominant-negative mutant form of ADAM12 leads to elevation of Notch signaling, inhibition of differentiation, and expansion of the pool of self-renewing Pax7+/MyoD- cells. These results suggest that ADAM-mediated shedding of DLL1 in a subset of cells during myogenic differentiation in vitro contributes to downregulation of Notch signaling in neighboring cells and facilitates their progression into differentiation. We propose that the proteolytic processing of DLL1 helps achieve an asymmetry in Notch signaling in initially equivalent myogenic cells and helps sustain the balance between differentiation and self-renewal.

ADAM function in embryogenesis

Cleavage of proteins inserted into the plasma membrane (shedding) is an essential process controlling many biological functions including cell signaling, cell adhesion and migration as well as proliferation and differentiation. ADAM surface metalloproteases have been shown to play an essential role in these processes. Gene inactivation during embryonic development have provided evidence of the central role of ADAM proteins in nematodes, flies, frogs, birds and mammals. The relative contribution of four subfamilies of ADAM proteins to developmental processes is the focus of this review.

The many facets of Notch ligands

The Notch signaling pathway regulates a diverse array of cell types and cellular processes and is tightly regulated by ligand binding. Both canonical and noncanonical Notch ligands have been identified that may account for some of the pleiotropic nature associated with Notch signaling. This review focuses on the molecular mechanisms by which Notch ligands function as signaling agonists and antagonists, and discusses different modes of activating ligands as well as findings that support intrinsic ligand signaling activity independent of Notch. Post-translational modification, proteolytic processing, endocytosis and membrane trafficking, as well as interactions with the actin cytoskeleton may contribute to the recently appreciated multifunctionality of Notch ligands. The regulation of Notch ligand expression by other signaling pathways provides a mechanism to coordinate Notch signaling with multiple cellular and developmental cues. The association of Notch ligands with inherited human disorders and cancer highlights the importance of understanding the molecular nature and activities intrinsic to Notch ligands. Oncogene (2008) 27, 5148-5167; doi:10.1038/onc.2008.229.

ADAM proteases: ligand processing and modulation of the Notch pathway

ADAM metalloproteases play important roles in development and disease. One of the key functions of ADAMs is the proteolytic processing of Notch receptors and their ligands. ADAM-mediated cleavage of Notch represents the first step in regulated intramembrane proteolysis of the receptor, leading to activation of the Notch pathway. Recent reports indicate that the transmembrane Notch ligands also undergo ADAM-mediated processing in cultured cells and in vivo. The proteolytic processing of Notch ligands modulates the strength and duration of Notch signals, leads to generation of soluble intracellular domains of the ligands, and may support a bi-directional signaling between cells.

Kuz and TACE can activate Notch independent of ligand

A central mechanism in activation of the Notch signaling pathway is cleavage of the Notch receptor by ADAM metalloproteases. ADAMs also cleave Delta, the ligand for Notch, thereby downregulating Notch signals. Two ADAMs, Kuzbanian (Kuz) and TNF-alpha converting enzyme (TACE), are known to process both Delta and Notch, yet the role of these cleavages in signal propagation has remained controversial. Using an in vitro model, we show that Kuz regulates Notch signaling primarily by activating the receptor and has little overall effect on signaling via disabling Delta. We confirm that Kuz-dependent activation of Notch requires stimulation of Notch by Delta. However, over-expression of Kuz gives ligand-independent Notch activation. In contrast, TACE, which is elevated in expression in the developing Drosophila nervous system, can efficiently activate Notch in a ligand-independent manner. Altogether, these data demonstrate the potential for Kuz and TACE to participate in context- and mechanism-specific modes of Notch activation.

Signal integration during development: mechanisms of EGFR and Notch pathway function and cross-talk

Metazoan development relies on a highly regulated network of interactions between conserved signal transduction pathways to coordinate all aspects of cell fate specification, differentiation, and growth. In this review, we discuss the intricate interplay between the epidermal growth factor receptor (EGFR; Drosophila EGFR/DER) and the Notch signaling pathways as a paradigm for signal integration during development. First, we describe the current state of understanding of the molecular architecture of the EGFR and Notch signaling pathways that has resulted from synergistic studies in vertebrate, invertebrate, and cultured cell model systems. Then, focusing specifically on the Drosophila eye, we discuss how cooperative, sequential, and antagonistic relationships between these pathways mediate the spatially and temporally regulated processes that generate this sensory organ. The common themes underlying the coordination of the EGFR and Notch pathways appear to be broadly conserved and should, therefore, be directly applicable to elucidating mechanisms of information integration and signaling specificity in vertebrate systems.

Proteolytic processing of delta-like 1 by ADAM proteases

Delta-like 1 (Dll1) is a mammalian ligand for Notch receptors. Interactions between Dll1 and Notch in trans activate the Notch pathway, whereas Dll1 binding to Notch in cis inhibits Notch signaling. Dll1 undergoes proteolytic processing in its extracellular domain by ADAM10. In this work we demonstrate that Dll1 represents a substrate for several other members of the ADAM family. In co-transfected cells, Dll1 is constitutively cleaved by ADAM12, and the N-terminal fragment of Dll1 is released to medium. ADAM12-mediated cleavage of Dll1 is cell density-dependent, takes place in cis orientation, and does not require the presence of the cytoplasmic domain of ADAM12. Full-length Dll1, but not its N- or C-terminal proteolytic fragment, co-immunoprecipitates with ADAM12. By using a Notch reporter construct, we show that Dll1 processing by ADAM12 increases Notch signaling in a cell-autonomous manner. Furthermore, ADAM9 and ADAM17 have the ability to process Dll1. In contrast, ADAM15 does not cleave Dll1, although the two proteins still co-immunoprecipitate with each other. Asn-353 present in the catalytic motif of ADAM12 and other Dll1-processing ADAMs, but absent in ADAM15, is necessary for Dll1 cleavage. Dll1 cleavage is reduced in ADAM9/12/15(-/-) mouse embryonic fibroblasts (MEFs), suggesting that the endogenous ADAM9 and/or ADAM12 present in wild type MEFs contribute to Dll1 processing. Finally, the endogenous Dll1 present in primary mouse myoblasts undergoes cleavage in confluent, differentiating myoblast cultures, and this cleavage is decreased by ADAM12 small interfering RNAs. Our findings expand the role of ADAM proteins in the regulation of Notch signaling.

Notch signalling: a simple pathway becomes complex

A small number of signalling pathways are used iteratively to regulate cell fates, cell proliferation and cell death in development. Notch is the receptor in one such pathway, and is unusual in that most of its ligands are also transmembrane proteins; therefore signalling is restricted to neighbouring cells. Although the intracellular transduction of the Notch signal is remarkably simple, with no secondary messengers, this pathway functions in an enormous diversity of developmental processes and its dysfunction is implicated in many cancers.

The ADAMs family: coordinators of nervous system development, plasticity and repair

A disintegrin and metalloprotease (ADAM) transmembrane proteins have metalloprotease, integrin-binding, intracellular signaling and cell adhesion activities. In contrast to other metalloproteases, ADAMs are particularly important for cleavage-dependent activation of proteins such as Notch, amyloid precursor protein (APP) and transforming growth factor alpha (TGFalpha), and can bind integrins. Not surprisingly, ADAMs have been shown or suggested to play important roles in the development of the nervous system, where they regulate proliferation, migration, differentiation and survival of various cells, as well as axonal growth and myelination. On the eleventh anniversary of the naming of this family of proteins, the relatively unknown ADAMs are emerging as potential therapeutic targets for neural repair. For example, over-expression of ADAM10, one of the alpha-secretases for APP, can prevent amyloid formation and hippocampal defects in an Alzheimer mouse model. Another example of this potential neural repair role is the finding that ADAM21 is uniquely associated with neurogenesis and growing axons of the adult brain. This comprehensive review will discuss the growing literature about the roles of ADAMs in the developing and adult nervous system, and their potential roles in neurological disorders. Most excitingly, the expanding understanding of their normal roles suggests that they can be manipulated to promote neural repair in the degenerating and injured adult nervous system.

The ADAM metalloprotease Kuzbanian is crucial for proper heart formation in Drosophila melanogaster

We have screened a collection of EMS mutagenized fly lines in order to identify genes involved in cardiogenesis. In the present work, we have studied a group of alleles exhibiting a hypertrophic heart. Our analysis revealed that the ADAM protein (A Disintegrin And Metalloprotease) Kuzbanian, which is the functional homologue of the vertebrate ADAM10, is crucial for proper heart formation. ADAMs are a family of transmembrane proteins that play a critical role during the proteolytic conversion (shedding) of membrane bound proteins to soluble forms. Enzymes harboring a sheddase function recently became candidates for causing several congenital diseases, like distinct forms of the Alzheimer disease. ADAMs play also a pivotal role during heart formation and vascularisation in vertebrates, therefore mutations in ADAM genes potentially could cause congenital heart defects in humans. In Drosophila, the zygotic loss of an active form of the Kuzbanian protein results in a dramatic excess of cardiomyocytes, accompanied by a loss of pericardial cells. Our data presented herein suggest that Kuzbanian acts during lateral inhibition within the cardiac primordium. Furthermore we discuss a second function of Kuzbanian in heart cell morphogenesis.

Abundance of ADAM-8, -9, -10, -12, -15 and -17 and ADAMTS-1 in mouse uterus during the oestrous cycle

The aim of the present study was to determine whether a disintegrin and metalloproteinase (ADAM)-8, -9, -10, -12, -15 and -17 and a disintegrin and metalloproteinase with thrombospondin motifs (ADAMTS)-1 are involved in the remodelling process of the mouse uterus during the oestrous cycle. The mRNA expression of ADAM was observed in all uterine tissues throughout the entire cycle. The levels of ADAM-8 mRNA were maximal at pro-oestrus, whereas the expression of ADAM-9 and ADAMTS-1 mRNA was maximal at oestrus. The minimum mRNA level of all ADAM genes always occurred at dioestrus. The mRNA levels of ADAM-10, -12, -15 and -17 did not vary significantly, regardless of the stage of the oestrous cycle. Immunoblot analyses demonstrated the presence of all ADAM proteins throughout the cycle. In terms of protein intensities, ADAM-8, -12 and -17 were maximal at pro-oestrus, whereas ADAM-10 and ADAMTS-1 were maximal at metoestrus and ADAM-9 was maximal at oestrus. Regardless of the ADAM species, minimal protein expression always occurred at dioestrus. Immunohistochemical studies showed ADAM protein expression in luminal and glandular epithelial layers, but not in the stromal layer. Moreover, ADAM proteins were found to be heterogeneously localised and their individual localisations depended on the stage of the oestrous cycle. From these observations, we suggest that the ADAM genes play an important role in mouse uterine tissue remodelling during the oestrous cycle.