Surface expression and limited proteolysis of ADAM10 are increased by a dominant negative inhibitor of dynamin

Topical Alginate Protection against Pepsin-Mediated Esophageal Damage: E-Cadherin Proteolysis and Matrix Metalloproteinase Induction

Epithelial barrier dysfunction is a hallmark of gastroesophageal reflux disease (GERD) related to symptom origination, inflammatory remodeling and carcinogenesis. Alginate-based antireflux medications were previously shown to topically protect against peptic barrier disruption, yet the molecular mechanisms of injury and protection were unclear. Herein, Barrett's esophageal (BAR-T) cells were pretreated with buffered saline (HBSS; control), dilute alginate medications (Gaviscon Advance or Gaviscon Double Action, Reckitt Benckiser), a viscosity-matched placebo, or ADAM10 and matrix metalloproteinase (MMP) inhibitors before exposure to HBSS pH7.4 or pH4 ± 1 mg/mL pepsin for 10-60 min. Cell viability was assessed by ATP assay; mediators of epithelial integrity, E-cadherin, ADAM10, and MMPs were examined by Western blot and qPCR. Alginate rescued peptic reduction of cell viability (p < 0.0001). Pepsin-pH4 yielded E-cadherin fragments indicative of regulated intramembrane proteolysis (RIP) which was not rescued by inhibitors of known E-cadherin sheddases. Transcriptional targets of E-cadherin RIP fragments were elevated at 24 h (MMP-1,2,9,14; p < 0.01). Alginate rescued E-cadherin cleavage, ADAM10 maturation, and MMP induction (p < 0.01). Results support RIP as a novel mechanism of peptic injury during GERD. Alginate residue after wash-out to mimic physiologic esophageal clearance conferred lasting protection against pepsin-induced molecular mechanisms that may exacerbate GERD severity and promote carcinogenesis in the context of weakly acidic reflux.

Loss of the Metastasis Suppressor NME1, But Not of Its Highly Related Isoform NME2, Induces a Hybrid Epithelial-Mesenchymal State in Cancer Cells

Epithelial-mesenchymal transition (EMT) is important for the initial steps of metastasis. Although it is well accepted that the nucleoside diphosphate kinase NME1 is a metastasis suppressor, its effect on EMT remains poorly documented, as does that of its closely related isoform, NME2. Here, by using gene silencing, inactivation and overexpression strategies in a variety of cellular models of cancer, we show that NME1 is a powerful inhibitor of EMT. Genetic manipulation of NME2, by contrast, had no effect on the EMT phenotype of cancer cells, indicating a specific function of NME1 in EMT regulation. Loss of NME1 in epithelial cancer cells resulted in a hybrid phenotype intermediate between epithelial and mesenchymal cells, which is known to be associated with cells with a highly metastatic character. Conversely, overexpression of NME1 in mesenchymal cancer cells resulted in a more epithelial phenotype. We found that NME1 expression was negatively associated with EMT markers in many human cancers and was reduced in human breast tumor cell lines with the aggressive 'triple-negative' phenotype when compared to human breast tumor cell lines positive for estrogen receptor. We show that NME1, but not NME2, is an inhibitor of essential concerted intracellular signaling pathways involved in inducing EMT, including the AKT and MAPK (ERK, p38, and JNK) pathways. Additionally, NME1 depletion considerably altered the distribution of E-cadherin, a gatekeeper of the epithelial phenotype, shifting it from the plasma membrane to the cytosol and resulting in less E-cadherin on the cell surface than in control cells. Functional aggregation and dispersion assays demonstrated that inactivation of NME1 decreases E-cadherin-mediated cell-cell adhesion. We conclude that NME1, but not NME2, acts specifically to inhibit EMT and prevent the earliest stages of metastasis.

The metalloproteinase ADAM10 requires its activity to sustain surface expression

The metalloproteinase ADAM10 critically contributes to development, inflammation, and cancer and can be controlled by endogenous or synthetic inhibitors. Here, we demonstrate for the first time that loss of proteolytic activity of ADAM10 by either inhibition or loss of function mutations induces removal of the protease from the cell surface and the whole cell. This process is temperature dependent, restricted to mature ADAM10, and associated with an increased internalization, lysosomal degradation, and release of mature ADAM10 in extracellular vesicles. Recovery from this depletion requires de novo synthesis. Functionally, this is reflected by loss and recovery of ADAM10 substrate shedding. Finally, ADAM10 inhibition in mice reduces systemic ADAM10 levels in different tissues. Thus, ADAM10 activity is critically required for its surface expression in vitro and in vivo. These findings are crucial for development of therapeutic ADAM10 inhibition strategies and may showcase a novel, physiologically relevant mechanism of protease removal due to activity loss.

Epigallocatechin-3-gallate downregulates lipopolysaccharide signaling in human aortic endothelial cells by inducing ectodomain shedding of TLR4

Epigallocatechin-3-gallate (EGCG), the most abundant polyphenol in green tea leaves, has anti-inflammatory effects. In this study, we investigated the mechanism by which EGCG attenuates the effects of lipopolysaccharide (LPS), an agonist of toll-like receptor 4 (TLR4), in cultured human aortic endothelial cells (HAECs). The increase in the expression of intercellular adhesion molecule-1 (ICAM-1) induced by LPS (100 ng/ml) was effectively attenuated by pretreatment with EGCG (50 μM). Importantly, EGCG treatment resulted in a rapid reduction of cellular TLR4, which was accompanied by an increase in the N-terminal fragment of TLR4 in the culture supernatant, indicating that EGCG induces ectodomain shedding of TLR4. EGCG increased cytosolic Ca²⁺ by inducing the release of intracellular stored Ca²⁺ and the influx of extracellular Ca²⁺; accordingly, EGCG-induced ectodomain shedding of TLR4 was nullified by pretreatment with BAPTA-AM (10 μM), an intracellular Ca²⁺ chelator. EGCG induced translocation of a disintegrin and metalloprotease 10 (ADAM10) to the cell surface, which was also blocked by BAPTA-AM. Treatment with ADAM10 inhibitor (GI254023X, 2 μM) and siRNA-mediated depletion of ADAM10 prevented EGCG-induced ectodomain shedding of TLR4 and abolished the inhibitory effect of EGCG on LPS-induced ICAM-1 expression. Collectively, these findings suggest that EGCG decreases cell surface TLR4 in HAECs by inducing ADAM10-mediated ectodomain shedding, and thereby attenuates the effects of LPS. This is a new mechanism of the suppressive effect of EGCG on LPS signaling.

Regulation of Fibrotic Processes in the Liver by ADAM Proteases

Fibrosis in the liver is mainly associated with the activation of hepatic stellate cells (HSCs). Both activation and clearance of HSCs can be mediated by ligand–receptor interactions. Members of the a disintegrin and metalloprotease (ADAM) family are involved in the proteolytic release of membrane-bound ligands and receptor ectodomains and the remodelling of the extracellular matrix. ADAM proteases are therefore major regulators of intercellular signalling pathways. In the present review we discuss how ADAM proteases modulate pro- and anti-fibrotic processes and how ADAM proteases might be harnessed therapeutically in the future.

Mutagenesis of the ADAM17-phosphatidylserine-binding motif leads to embryonic lethality in mice

ADAM17, prominent member of the "Disintegrin and Metalloproteinase" (ADAM) family, controls vital cellular functions through cleavage of transmembrane substrates. Several of these play central roles in oncogenesis and inflammation, yet despite its importance, the mechanism by which ADAM17 is activated is not fully understood. We recently presented evidence that surface exposure of phosphatidylserine (PS) is the penultimate event required for sheddase activation, which occurs upon binding of a membrane-proximal, cationic binding motif to the anionic phospholipid headgroup. Here, we show that mutagenesis of the 3 amino acids constituting the PS-binding motif leads to embryonic lethality in mice. Heterozygotes showed no abnormalities. Primary hepatocytes and fibroblasts were analysed and found to express the mutant protease on the cell surface. However, PMA-stimulated release of ADAM17 substrates was completely abolished. The results directly support the novel concept of transiently externalised PS as essential trigger of extracellular protease function in vivo.

Regulation of Alpha-Secretase ADAM10 In vitro and In vivo: Genetic, Epigenetic, and Protein-Based Mechanisms

ADAM10 (A Disintegrin and Metalloproteinase 10) has been identified as the major physiological alpha-secretase in neurons, responsible for cleaving APP in a non-amyloidogenic manner. This cleavage results in the production of a neuroprotective APP-derived fragment, APPs-alpha, and an attenuated production of neurotoxic A-beta peptides. An increase in ADAM10 activity shifts the balance of APP processing toward APPs-alpha and protects the brain from amyloid deposition and disease. Thus, increasing ADAM10 activity has been proposed an attractive target for the treatment of neurodegenerative diseases and it appears to be timely to investigate the physiological mechanisms regulating ADAM10 expression. Therefore, in this article, we will (1) review reports on the physiological regulation of ADAM10 at the transcriptional level, by epigenetic factors, miRNAs and/or protein interactions, (2) describe conditions, which change ADAM10 expression in vitro and in vivo, (3) report how neuronal ADAM10 expression may be regulated in humans, and (4) discuss how this knowledge on the physiological and pathophysiological regulation of ADAM10 may help to preserve or restore brain function.

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.

Subcellular localization and activation of ADAM proteases in the context of FasL shedding in T lymphocytes

The "A Disintegrin And Metalloproteinases" (ADAMs) form a subgroup of the metzincin endopeptidases. Proteolytically active members of this protein family act as sheddases and govern key processes in development and inflammation by regulating cell surface expression and release of cytokines, growth factors, adhesion molecules and their receptors. In T lymphocytes, ADAM10 sheds the death factor Fas Ligand (FasL) and thereby regulates T cell activation, death and effector function. Although FasL shedding by ADAM10 was confirmed in several studies, its regulation is still poorly defined. We recently reported that ADAM10 is highly abundant on T cells whereas its close relative ADAM17 is expressed at low levels and transiently appears at the cell surface upon stimulation. Since FasL is also stored intracellularly and brought to the plasma membrane upon stimulation, we addressed where the death factor gets exposed to ADAM proteases. We report for the first time that both ADAM10 and ADAM17 are associated with FasL-containing secretory lysosomes. Moreover, we demonstrate that TCR/CD3/CD28-stimulation induces a partial positioning of both proteases and FasL to lipid rafts and only the activation-induced raft-positioning results in FasL processing. TCR/CD3/CD28-induced FasL proteolysis is markedly affected by reducing both ADAM10 and ADAM17 protein levels, indicating that in human T cells also ADAM17 is implicated in FasL processing. Since FasL shedding is affected by cholesterol depletion and by inhibition of Src kinases or palmitoylation, we conclude that it requires mobilization and co-positioning of ADAM proteases in lipid raft-like platforms associated with an activation of raft-associated Src-family kinases.

Crucial role of the Rap G protein signal in Notch activation and leukemogenicity of T-cell acute lymphoblastic leukemia

The Rap G protein signal regulates Notch activation in early thymic progenitor cells, and deregulated Rap activation (Rap(high)) results in the development of Notch-dependent T-cell acute lymphoblastic leukemia (T-ALL). We demonstrate that the Rap signal is required for the proliferation and leukemogenesis of established Notch-dependent T-ALL cell lines. Attenuation of the Rap signal by the expression of a dominant-negative Rap1A17 or Rap1GAP, Sipa1, in a T-ALL cell line resulted in the reduced Notch processing at site 2 due to impaired maturation of Adam10. Inhibition of the Rap1 prenylation with a geranylgeranyl transferase inhibitor abrogated its membrane-anchoring to Golgi-network and caused reduced proprotein convertase activity required for Adam10 maturation. Exogenous expression of a mature form of Adam10 overcame the Sipa1-induced inhibition of T-ALL cell proliferation. T-ALL cell lines expressed Notch ligands in a Notch-signal dependent manner, which contributed to the cell-autonomous Notch activation. Although the initial thymic blast cells barely expressed Notch ligands during the T-ALL development from Rap(high) hematopoietic progenitors in vivo, the ligands were clearly expressed in the T-ALL cells invading extrathymic vital organs. These results reveal a crucial role of the Rap signal in the Notch-dependent T-ALL development and the progression.

Adrenergic regulation of IgE involves modulation of CD23 and ADAM10 expression on exosomes

Soluble CD23 plays a role in the positive regulation of an IgE response. Engagement of the β2 adrenergic receptor (β2AR) on a B cell is known to enhance the level of both soluble CD23 and IgE, although the mechanism by which this occurs is not completely understood. In this study, we report that, in comparison with a CD40 ligand/IL-4-primed murine B cell alone, β2AR engagement on a primed B cell increased gene expression of a disintegrin and metalloproteinase (ADAM)10, which is the primary sheddase of CD23, as well as protein expression of both CD23 and ADAM10, in a protein kinase A- and p38 MAPK-dependent manner, and promoted the localization of these proteins to exosomes as early as 2 d after priming, as determined by both Western blot and flow cytometry and confirmed by electron microscopy. In comparison with isolated exosomes released from primed B cells alone, the transfer of exosomes released from β2AR agonist-exposed primed B cells to cultures of recipient primed B cells resulted in an increase in the level of IgE produced per cell, without affecting the number of cells producing IgE, as determined by ELISPOT. These effects still occurred when a β2AR antagonist was added along with the transfer to block residual agonist, and they failed to occur when exosomes were isolated from β2AR-deficient B cells. These findings suggest that the mechanism responsible for mediating the β2AR-induced increase in IgE involves a shuttling of the β2AR-induced increase in CD23 and ADAM10 proteins to exosomes that subsequently mediate an increase in IgE.

Endocytosis of synaptic ADAM10 in neuronal plasticity and Alzheimer's disease

A disintegrin and metalloproteinase 10 (ADAM10), a disintegrin and metalloproteinase that resides in the postsynaptic densities (PSDs) of excitatory synapses, has previously been shown to limit β-amyloid peptide (Aβ) formation in Alzheimer's disease (AD). ADAM10 also plays a critical role in regulating functional membrane proteins at the synapse. Using human hippocampal homogenates, we found that ADAM10 removal from the plasma membrane was mediated by clathrin-dependent endocytosis. Additionally, we identified the clathrin adaptor AP2 as an interacting partner of a previously uncharacterized atypical binding motif in the ADAM10 C-terminal domain. This domain was required for ADAM10 endocytosis and modulation of its plasma membrane levels. We found that the ADAM10/AP2 association was increased in the hippocampi of AD patients compared with healthy controls. Long-term potentiation (LTP) in hippocampal neuronal cultures induced ADAM10 endocytosis through AP2 association and decreased surface ADAM10 levels and activity. Conversely, long-term depression (LTD) promoted ADAM10 synaptic membrane insertion and stimulated its activity. ADAM10 interaction with the synapse-associated protein-97 (SAP97) was necessary for LTD-induced ADAM10 trafficking and required for LTD maintenance and LTD-induced changes in spine morphogenesis. These data identify and characterize a mechanism controlling ADAM10 localization and activity at excitatory synapses that is relevant to AD pathogenesis.

Who decides when to cleave an ectodomain?

Many life-essential molecules such as growth factors, cytokines, ectoenzymes, and decoy receptors are produced by ectodomain cleavage of transmembrane precursor molecules. Not surprisingly, misregulation of such essential functions is linked to numerous diseases. Ectodomain cleavage is the function of transmembrane ADAMs (a disintegrin and metalloprotease) and other membrane-bound metalloproteases, which have an extracellular catalytic domain. Almost all work on ectodomain cleavage regulation has focused on the control of enzyme activity determined by substrate cleavage as surrogate. However, the number of substrates far exceeds the number of enzymes. Specificity can therefore not be achieved by solely modulating enzyme activity. Here, we argue that specific regulatory pathways must exist to control the availability and susceptibility of substrates.

Calsyntenin-1 shelters APP from proteolytic processing during anterograde axonal transport

Endocytosis of amyloid-β precursor protein (APP) is thought to represent the major source of substrate for the production of the amyloidogenic Aβ peptide by the β-secretase BACE1. The irreversible nature of proteolytic cleavage implies the existence of an efficient replenishment route for APP from its sites of synthesis to the cell surface. We recently found that APP exits the trans-Golgi network in intimate association with calsyntenin-1, a transmembrane cargo-docking protein for Kinesin-1-mediated vesicular transport. Here we characterized the function of calsyntenin-1 in neuronal APP transport using selective immunoisolation of intracellular trafficking organelles, immunocytochemistry, live-imaging, and RNAi. We found that APP is co-transported with calsyntenin-1 along axons to early endosomes in the central region of growth cones in carriers that exclude the α-secretase ADAM10. Intriguingly, calsyntenin-1/APP organelles contained BACE1, suggesting premature cleavage of APP along its anterograde path. However, we found that APP contained in calsyntenin-1/APP organelles was stable. We further analyzed vesicular trafficking of APP in cultured hippocampal neurons, in which calsyntenin-1 was reduced by RNAi. We found a markedly increased co-localization of APP and ADAM10 in axons and growth cones, along with increased proteolytic processing of APP and Aβ secretion in these neurons. This suggested that the reduced capacity for calsyntenin-1-dependent APP transport resulted in mis-sorting of APP into additional axonal carriers and, therefore, the premature encounter of unprotected APP with its ectodomain proteases. In combination, our results characterize calsyntenin-1/APP organelles as carriers for sheltered anterograde axonal transport of APP.

Identification and biology of α-secretase

'Secretase' is a generic term coined more than 20 years ago to refer to a group of proteases responsible for the cleavage of a vast number of membrane proteins. These endoproteolytic events result in the extracellular or intracellular release of soluble metabolites associated with a broad range of intrinsic physiological functions. α-Secretase refers to the activity targeting the amyloid precursor protein (APP) and generating sAPPα, a soluble extracellular fragment potentially associated with neurotrophic and neuroprotective functions. Several proteases from the a disintegrin and metalloproteinase (ADAM) family, including ADAM10 and ADAM17, have been directly or indirectly associated with the constitutive and regulated α-secretase activities. Recent evidence in primary neuronal cultures indicates that ADAM10 may represent the genuine constitutive α-secretase. Mainly because α-secretase cleaves APP within the sequence of Aβ, the core component of the cerebral amyloid plaques in Alzheimer's disease, α-secretase activation is considered to be of therapeutic value. In this article, we will provide a historical perspective on the characterization of α-secretase and review the recent literature on the identification and biology of the current α-secretase candidates.