Non-Cell Autonomous Expression of TNF-α-Converting Enzyme ADAM17 Is Required for Normal Lymphocyte Development

The ADAM17 sheddase complex regulator iTAP/Frmd8 modulates inflammation and tumor growth

The metalloprotease ADAM17 is a sheddase of key molecules, including TNF and epidermal growth factor receptor ligands. ADAM17 exists within an assemblage, the "sheddase complex," containing a rhomboid pseudoprotease (iRhom1 or iRhom2). iRhoms control multiple aspects of ADAM17 biology. The FERM domain-containing protein iTAP/Frmd8 is an iRhom-binding protein that prevents the precocious shunting of ADAM17 and iRhom2 to lysosomes and their consequent degradation. As pathophysiological role(s) of iTAP/Frmd8 have not been addressed, we characterized the impact of iTAP/Frmd8 loss on ADAM17-associated phenotypes in mice. We show that iTAP/Frmd8 KO mice exhibit defects in inflammatory and intestinal epithelial barrier repair functions, but not the collateral defects associated with global ADAM17 loss. Furthermore, we show that iTAP/Frmd8 regulates cancer cell growth in a cell-autonomous manner and by modulating the tumor microenvironment. Our work suggests that pharmacological intervention at the level of iTAP/Frmd8 may be beneficial to target ADAM17 activity in specific compartments during chronic inflammatory diseases or cancer, while avoiding the collateral impact on the vital functions associated with the widespread inhibition of ADAM17.

The Role of ADAM17 in Inflammation-Related Atherosclerosis

Atherosclerosis is a chronic inflammatory disease that poses a huge economic burden due to its extremely poor prognosis. Therefore, it is necessary to explore potential mechanisms to improve the prevention and treatment of atherosclerosis. A disintegrin and metalloprotease 17 (ADAM17) is a cell membrane-bound protein that performs a range of functions through membrane protein shedding and intracellular signaling. ADAM17-mediated inflammation has been identified to be an important contributor to atherosclerosis; however, the specific relationship between its multiple regulatory roles and the pathogenesis of atherosclerosis remains unclear. Here, we reviewed the activation, function, and regulation of ADAM17, described in detail the role of ADAM17-mediated inflammatory damage in atherosclerosis, and discussed several controversial points. We hope that these insights into ADAM17 biology will lead to rational management of atherosclerosis. ADAM17 promotes vascular inflammation in endothelial cells, smooth muscle cells, and macrophages, and regulates the occurrence and development of atherosclerosis.

The role of A Disintegrin and Metalloproteinase (ADAM)-10 in T helper cell biology

A Disintegrin and Metalloproteinases (ADAM)-10 is a member of a family of membrane-anchored proteinases that regulate a broad range of cellular functions with central roles within the immune system. This has spurred the interest to modulate ADAM activity therapeutically in immunological diseases. CD4 T helper (Th) cells are the key regulators of adaptive immune responses. Their development and function is strongly dependent on Notch, a key ADAM-10 substrate. However, Th cells rely on a variety of additional ADAM-10 substrates regulating their functional activity at multiple levels. The complexity of both, the ADAM substrate expression as well as the functional consequences of ADAM-mediated cleavage of the various substrates complicates the analysis of cell type specific effects. Here we provide an overview on the major ADAM-10 substrates relevant for CD4 T cell biology and discuss the potential effects of ADAM-mediated cleavage exemplified for a selection of important substrates.

ADAM17 orchestrates Interleukin-6, TNFα and EGF-R signaling in inflammation and cancer

It was realized in the 1990s that some membrane proteins such as TNFα, both TNF receptors, ligands of the EGF-R and the Interleukin-6 receptor are proteolytically cleaved and are shed from the cell membrane as soluble proteins. The major responsible protease is a metalloprotease named ADAM17. So far, close to 100 substrates, including cytokines, cytokine receptors, chemokines and adhesion molecules of ADAM17 are known. Therefore, ADAM17 orchestrates many different signaling pathways and is a central signaling hub in inflammation and carcinogenesis. ADAM17 plays an important role in the biology of Interleukin-6 (IL-6) since the generation of the soluble Interleukin-6 receptor (sIL-6R) is needed for trans-signaling, which has been identified as the pro-inflammatory activity of this cytokine. In contrast, Interleukin-6 signaling via the membrane-bound Interleukin-6 receptor is mostly regenerative and protective. Probably due to its broad substrate spectrum, ADAM17 is essential for life and most of the few human individuals identified with ADAM17 gene defects died at young age. Although the potential of ADAM17 as a therapeutic target has been recognized, specific blockade of ADAM17 is not trivial since the metalloprotease domain of ADAM17 shares high structural homology with other proteases, in particular matrix metalloproteases. Here, the critical functions of ADAM17 in IL-6, TNFα and EGF-R pathways and strategies of therapeutic interventions are discussed.

ADAM28: Another ambivalent protease in cancer

Emergence of novel therapeutic options in a perspective of personalized therapy of cancer relies on the discovery of precise molecular mechanisms involved in the switch from a localized tumor to invasive metastasis spread. Pro-tumor functions have been mostly ascribed to proteolytic enzymes from the metalloproteinase family including A Disintegrin And Metalloproteinases (ADAMs). Particularly, when expressed by cancer cells, ADAM28 protease supports cancer cell proliferation, survival and migration as well as metastatic progression. In sharp contrast, ADAM28 derived from the tumor microenvironment has shown to exert strong protective effects against deleterious metastasis dissemination. Indeed, depletion of host-derived ADAM28 (ADAM28 KO mice) accelerates colonization lung tissues, increases tumor foci implantation, and impairs T cell immune response. In this review, we outline specific ADAM28 functions when specifically expressed by carcinoma cells or by tumor microenvironment. Finally, we discuss about future research strategies that could be pursued to highlight new functions of this protease in cancer.

Activation of ADAM17 (A Disintegrin and Metalloprotease 17) on Glutamatergic Neurons Selectively Promotes Sympathoexcitation

Chronic activation of the brain renin-angiotensin system contributes to the development of hypertension by altering autonomic balance. Beyond the essential role of Ang II (angiotensin II) type 1 receptors, ADAM17 (A disintegrin and metalloprotease 17) is also found to promote brain renin-angiotensin system overactivation. ADAM17 is robustly expressed in various cell types within the central nervous system. The aim of this study was to determine whether ADAM17 modulates presympathetic neuronal activity to promote autonomic dysregulation in salt-sensitive hypertension. To test our hypothesis, ADAM17 was selectively knocked down in glutamatergic neurons using Cre-loxP technology. In mice lacking ADAM17 in glutamatergic neurons, the blood pressure increase induced by deoxycorticosterone acetate-salt treatment was blunted. Deoxycorticosterone acetate-salt significantly elevated cardiac and vascular sympathetic drive in control mice, while such effects were reduced in mice with ADAM17 knockdown. This blunted sympathoexcitation was extended to the spleen, with a lesser activation of the peripheral immune system, translating into a sequestration of circulating T cells within this organ, compared with controls. Within the paraventricular nucleus, Ang II-induced activation of kidney-related presympathetic glutamatergic neurons was reduced in ADAM17 knockdown mice, with the majority of cells no longer responding to Ang II stimulation, confirming the supportive role of ADAM17 in increasing presympathetic neuronal activity. Overall, our data highlight the pivotal role of neuronal ADAM17 in regulating sympathetic activity and demonstrate that activation of ADAM17 in glutamatergic neurons leads to a selective increase of sympathetic output, but not vagal tone, to specific organs, ultimately contributing to dysautonomia and salt-sensitive hypertension.

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.

Ectodomain Shedding by ADAM17: Its Role in Neutrophil Recruitment and the Impairment of This Process during Sepsis

Neutrophils are specialized at killing bacteria and are recruited from the blood in a rapid and robust manner during infection. A cascade of adhesion events direct their attachment to the vascular endothelium and migration into the underlying tissue. A disintegrin and metalloproteinase 17 (ADAM17) functions in the cell membrane of neutrophils and endothelial cells by cleaving its substrates, typically in a cis manner, at an extracellular site proximal to the cell membrane. This process is referred to as ectodomain shedding and it results in the downregulation of various adhesion molecules and receptors, and the release of immune regulating factors. ADAM17 sheddase activity is induced upon cell activation and rapidly modulates intravascular adhesion events in response to diverse environmental stimuli. During sepsis, an excessive systemic inflammatory response against infection, neutrophil migration becomes severely impaired. This involves ADAM17 as indicated by increased levels of its cleaved substrates in the blood of septic patients, and that ADAM17 inactivation improves neutrophil recruitment and bacterial clearance in animal models of sepsis. Excessive ADAM17 sheddase activity during sepsis thus appears to undermine in a direct and indirect manner the necessary balance between intravascular adhesion and de-adhesion events that regulate neutrophil migration into sites of infection. This review provides an overview of ADAM17 function and regulation and its potential contribution to neutrophil dysfunction during sepsis.

A Disintegrin and Metalloprotease 17 in the Cardiovascular and Central Nervous Systems

ADAM17 is a metalloprotease and disintegrin that lodges in the plasmatic membrane of several cell types and is able to cleave a wide variety of cell surface proteins. It is somatically expressed in mammalian organisms and its proteolytic action influences several physiological and pathological processes. This review focuses on the structure of ADAM17, its signaling in the cardiovascular system and its participation in certain disorders involving the heart, blood vessels, and neural regulation of autonomic and cardiovascular modulation.

ADAM Proteases and Gastrointestinal Function

A disintegrin and metalloproteinases (ADAMs) are a family of cell surface proteases that regulate diverse cellular functions, including cell adhesion, migration, cellular signaling, and proteolysis. Proteolytically active ADAMs are responsible for ectodomain shedding of membrane-associated proteins. ADAMs rapidly modulate key cell signaling pathways in response to changes in the extracellular environment (e.g., inflammation) and play a central role in coordinating intercellular communication within the local microenvironment. ADAM10 and ADAM17 are the most studied members of the ADAM family in the gastrointestinal tract. ADAMs regulate many cellular processes associated with intestinal development, cell fate specification, and the maintenance of intestinal stem cell/progenitor populations. Several signaling pathway molecules that undergo ectodomain shedding by ADAMs [e.g., ligands and receptors from epidermal growth factor receptor (EGFR)/ErbB and tumor necrosis factor α (TNFα) receptor (TNFR) families] help drive and control intestinal inflammation and injury/repair responses. Dysregulation of these processes through aberrant ADAM expression or sustained ADAM activity is linked to chronic inflammation, inflammation-associated cancer, and tumorigenesis.

Loss of ADAM17-Mediated Tumor Necrosis Factor Alpha Signaling in Intestinal Cells Attenuates Mucosal Atrophy in a Mouse Model of Parenteral Nutrition

Total parenteral nutrition (TPN) is commonly used clinically to sustain patients; however, TPN is associated with profound mucosal atrophy, which may adversely affect clinical outcomes. Using a mouse TPN model, removing enteral nutrition leads to decreased crypt proliferation, increased intestinal epithelial cell (IEC) apoptosis and increased mucosal tumor necrosis factor alpha (TNF-α) expression that ultimately produces mucosal atrophy. Upregulation of TNF-α signaling plays a central role in mediating TPN-induced mucosal atrophy without intact epidermal growth factor receptor (EGFR) signaling. Currently, the mechanism and the tissue-specific contributions of TNF-α signaling to TPN-induced mucosal atrophy remain unclear. ADAM17 is an ectodomain sheddase that can modulate the signaling activity of several cytokine/growth factor receptor families, including the TNF-α/TNF receptor and ErbB ligand/EGFR pathways. Using TPN-treated IEC-specific ADAM17-deficient mice, the present study demonstrates that a loss of soluble TNF-α signaling from IECs attenuates TPN-induced mucosal atrophy. Importantly, this response remains dependent on the maintenance of functional EGFR signaling in IECs. TNF-α blockade in wild-type mice receiving TPN confirmed that soluble TNF-α signaling is responsible for downregulation of EGFR signaling in IECs. These results demonstrate that ADAM17-mediated TNF-α signaling from IECs has a significant role in the development of the proinflammatory state and mucosal atrophy observed in TPN-treated mice.

The role of metalloproteinase ADAM17 in regulating ICOS ligand-mediated humoral immune responses

Immune cells regulate cell surface receptor expression during their maturation, activation, and motility. Although many of these receptors are regulated largely at the level of expression, protease-mediated ectodomain shedding represents an alternative means of refashioning the surface of immune cells. Shedding is largely attributed to a family of a disintegrin and metalloprotease domain (ADAM) metalloproteases, including ADAM17. Although ADAM17 is well known to contribute to the innate immune response, mainly by releasing TNF-α, much less is known about whether/how this metalloprotease regulates adaptive immunity. To determine whether ADAM17 contributes to regulating adaptive immune responses, we took advantage of ADAM17 hypomorphic (ADAM17(ex/ex)) mice, in which ADAM17 expression is reduced by 90-95% compared with wild-type littermates. In this study, we show that that ADAM17 deficiency results in spleen and lymph node enlargement, as well as increased levels of Ag-specific class-switched Ig production following immunization with OVA together with anti-CD40 mAbs and polyinosinic-polycytidylic acid. Moreover, we demonstrate that the costimulatory ligand ICOS ligand (ICOSL) is selectively downregulated on the surface of B cells in an ADAM17-specific manner, although it is not proteolitically processed by recombinant ADAM17 in vitro. Finally, we show that higher cell surface levels of ICOSL in ADAM17(ex/ex) mice may contribute to the development of excessive Ab responses. Therefore, our data suggest a functional link between ADAM17 and ICOSL in controlling adaptive immune responses.

TACE in perinatal mouse lung epithelial cells promotes lung saccular formation

Tumor necrosis factor-α converting enzyme (TACE) is a cell membrane sheddase, expressed in both developmental lung epithelia and mesenchyme. Global abrogation of TACE results in neonatal lethality and multiple organ developmental abnormalities, including dysplastic lung. To further define the roles of TACE in regulating lung development, lung epithelial and/or mesenchymal specific TACE conditional knockout mice were generated. Blockade of TACE function in developing lung epithelial cells caused reduced saccular formation, decreased cell proliferation, and reduced mid-distal lung epithelial cell differentiation. In contrast, mesenchymal TACE knockout did not have any phenotypic change in developing lung. Simultaneous abrogation of TACE in both lung epithelial and mesenchymal cells did not result in a more severe lung abnormality. Interestingly, these lung-specific TACE conditional knockout mice were not neonatal lethal, and their lung structures were essentially normal after alveolarization. In addition, TACE conditional knockout in developing cardiomyocytes resulted in noncompaction of ventricular myocardium, as seen in TACE conventional knockout mice. However, these mice were also not neonatal lethal. In conclusion, lung epithelial TACE is essential for promoting fetal lung saccular formation, but not postnatal lung alveolarization in mice. Because the developmental abnormality of either lung or heart induced by TACE deficiency does not directly lead to neonatal lethality, the neonatal death of TACE conventional knockout mice is likely a result of synergistic effects of multiple organ abnormalities.

Increased levels of plasma soluble Sema4D in patients with heart failure

Semaphorin 4D (Sema4D/CD100) is a 150-kDa transmembrane glycoprotein expressed by platelets and T-cells. When these cells are activated, Sema4D is cleaved proteolytically, generating a biologically active 120-kDa fragment (soluble Sema4D) capable of targeting receptors on platelets, B-cells, endothelial cells and tumor cells. However, its plasma levels and significance in heart failure (HF) have not been reported. In this study, we established an ELISA and detected soluble Sema4D in human plasma. In healthy controls, plasma Sema4D levels were higher in men than women (5.15±3.30 ng/mL, n = 63, vs. 4.19±2.39 ng/mL, n = 63, P<0.05). In HF patients, plasma Sema4D levels were significantly higher than those in healthy controls (8.94±5.89 ng/mL, n = 157 vs. 4.67±2.99 ng/mL, n = 126, P<0.0001) with the highest levels being in HF patients with diabetes mellitus (DM) (10.45±5.76 ng/mL, n = 40). We also found that there was a higher percentage of Sema4D(high) CD3(+) (P<0.01), CD4(+) (P<0.001), and CD8(+) (P<0.01) T-cells in samples from HF patients, but no changes in Sema4D expression levels in B cells and platelets. Therefore, our investigation shows that plasma Sema4D levels are increased in HF patients, especially in those who also have diabetes. There was an accompanying increase in the Sema4D(high) population of T-cells, suggesting a potential role of these T-cells in heart failure.

Notch activation by the metalloproteinase ADAM17 regulates myeloproliferation and atopic barrier immunity by suppressing epithelial cytokine synthesis

Epithelial cells of mucosal tissues provide a barrier against environmental stress, and keratinocytes are key decision makers for immune cell function in the skin. Currently, epithelial signaling networks that instruct barrier immunity remain uncharacterized. Here we have shown that keratinocyte-specific deletion of a disintegrin and metalloproteinase 17 (Adam17) triggers T helper 2 and/or T helper 17 (Th2 and/or Th17) cell-driven atopic dermatitis and myeloproliferative disease. In vivo and in vitro deficiency of ADAM17 dampened Notch signaling, increasing production of the Th2 cell-polarizing cytokine TSLP and myeloid growth factor G-CSF. Ligand-independent Notch activation was identified as a regulator of AP-1 transcriptional activity, with Notch antagonizing c-Fos recruitment to the promoters of Tslp and Csf3 (G-CSF). Further, skin inflammation was rescued and myeloproliferation ameliorated by delivery of active Notch to Adam17(-)(/-) epidermis. Our findings uncover an essential role of ADAM17 in the adult epidermis, demonstrating a gatekeeper function of the ADAM17-Notch-c-Fos triad in barrier immunity.

Notch receptor-ligand interactions during T cell development, a ligand endocytosis-driven mechanism

Notch signaling plays an important role during the development of different cell types and tissues. The role of Notch signaling in lymphocyte development, in particular in the development and commitment to the T cell lineage, has been the focus of research for many years. Notch signaling is absolutely required during the commitment and early stages of T cell development. Activation of the Notch signaling pathway is initiated by ligand-receptor interactions and appears to require active endocytosis of Notch ligands. Studies addressing the mechanism underlying endocytosis of Notch ligands have helped to identify the main players important and necessary for this process. Here, we review the Notch ligands, and the proposed models of Notch activation by Notch ligand endocytosis, highlighting key molecules involved. In particular, we discuss recent studies on Notch ligands involved in T cell development, current studies aimed at elucidating the relevance of Notch ligand endocytosis during T cell development and the identification of key players necessary for ligand endocytosis in the thymus and during T cell development.

ADAM17: a molecular switch to control inflammation and tissue regeneration

A disintegrin and metalloproteinase 17 (ADAM17), also known as tumor necrosis factor-α converting enzyme (TACE), is a membrane-bound enzyme that cleaves cell surface proteins, such as cytokines (e.g. TNFα), cytokine receptors (e.g. IL-6R and TNF-R), ligands of ErbB (e.g. TGFα and amphiregulin) and adhesion proteins (e.g. L-selectin and ICAM-1). Here we examine how ectodomain shedding of these molecules can alter their biology and impact on immune and inflammatory responses and cancer development. Gene targeting of Adam17 is embryonic lethal, highlighting the importance of ectodomain shedding during development. Tissue-specific deletion, or hypomorphic knock-in, of Adam17 demonstrates an in vivo role for ADAM17 in controlling inflammation and tissue regeneration. The potential of ADAM17 as therapeutic target is also discussed.

The emergence of ADAM10 as a regulator of lymphocyte development and autoimmunity

Proteolytic processing of transmembrane receptors and ligands can have a dramatic impact on cell signaling processes and subsequent cellular responses, including activation and differentiation. A member of the disintegrin and metalloproteinase family, ADAM10, has emerged as a prominent regulator of numerous receptors and ligands, including Notch and CD23. Here, we review studies resulting from the recent generation of ADAM10 conditional knockout mice which revealed a critical role for ADAM10 in Notch-dependent lymphocyte development. Additionally, we discuss results of numerous in vitro and ex vivo studies indicating that ADAM10 regulates the production of multiple secreted factors that contribute to autoimmune reactions.

Hes1 is a critical but context-dependent mediator of canonical Notch signaling in lymphocyte development and transformation

Although canonical Notch signaling regulates multiple hematopoietic lineage decisions including T cell and marginal zone B cell fate specification, the downstream molecular mediators of Notch function are largely unknown. We showed here that conditional inactivation of Hes1, a well-characterized Notch target gene, in adult murine bone marrow (BM) cells severely impaired T cell development without affecting other Notch-dependent hematopoietic lineages such as marginal zone B cells. Competitive mixed BM chimeras, intrathymic transfer experiments, and in vitro culture of BM progenitors on Delta-like-expressing stromal cells further demonstrated that Hes1 is required for T cell lineage commitment, but dispensable for Notch-dependent thymocyte maturation through and beyond the beta selection checkpoint. Furthermore, our data strongly suggest that Hes1 is essential for the development and maintenance of Notch-induced T cell acute lymphoblastic leukemia. Collectively, our studies identify Hes1 as a critical but context-dependent mediator of canonical Notch signaling in the hematopoietic system.

ADAM17 deletion in thymic epithelial cells alters aire expression without affecting T cell developmental progression

Background

Cellular interactions between thymocytes and thymic stromal cells are critical for normal T cell development. Thymic epithelial cells (TECs) are important stromal niche cells that provide essential growth factors, cytokines, and present self-antigens to developing thymocytes. The identification of genes that mediate cellular crosstalk in the thymus is ongoing. One candidate gene, Adam17, encodes a metalloprotease that functions by cleaving the ectodomain of several transmembrane proteins and regulates various developmental processes. In conventional Adam17 knockout mice, a non-cell autonomous role for ADAM17 in adult T cell development was reported, which strongly suggested that expression of ADAM17 in TECs was required for normal T cell development. However, knockdown of Adam17 results in multisystem developmental defects and perinatal lethality, which has made study of the role of Adam17 in specific cell types difficult. Here, we examined T cell and thymic epithelial cell development using a conditional knockout approach.

Methodology/Principal Findings

We generated an Adam17 conditional knockout mouse in which floxed Adam17 is deleted specifically in TECs by Cre recombinase under the control of the Foxn1 promoter. Normal T cell lineage choice and development through the canonical αβ T cell stages was observed. Interestingly, Adam17 deficiency in TECs resulted in reduced expression of the transcription factor Aire. However, no alterations in the patterns of TEC phenotypic marker expression and thymus morphology were noted.

Conclusions/Significance

In contrast to expectation, our data clearly shows that absence of Adam17 in TECs is dispensable for normal T cell development. Differentiation of TECs is also unaffected by loss of Adam17 based on phenotypic markers. Surprisingly, we have uncovered a novel genetic link between Adam17and Aire expression in vivo. The cell type in which ADAM17 mediates its non-cell autonomous impact and the mechanisms by which it regulates intrathymic T cell development remain to be identified.

Deficiency of the metalloproteinase-disintegrin ADAM8 is associated with thymic hyper-cellularity

BACKGROUND

Thymopoiesis requires thymocyte-stroma interactions and proteases that promote cell migration by degrading extracellular matrix and releasing essential cytokines and chemokines. A role for several members of the A Disintegrin and Metalloprotease (ADAM) family in T cell development has been reported in the past.

METHODOLOGY/PRINCIPAL FINDINGS

Here, we present data indicating that the family member ADAM8 plays a role in thymic T cell development. We used qrtPCR on FACS sorted thymic subsets together with immunofluorescence to analyze thymic ADAM8 expression. We found that ADAM8 was expressed in murine thymic stromal cells and at lower levels in thymocytes where its expression increased as cell matured, suggesting involvement of ADAM8 in thymopoiesis. Further flow cytometry analysis revealed that ADAM8 deficient mice showed normal development and expansion of immature thymocyte subsets. There was however an intrathymic accumulation of single positive CD4 and CD8 T cells which was most noticeable in the late mature T cell subsets. Accumulation of single positive T cells coincided with changes in the thymic architecture manifest in a decreased cortex/medulla ratio and an increase in medullary epithelial cells as determined by histology and flow cytometry. The increase in single positive T cells was thymus-intrinsic, independent of progenitor homing to the thymus or thymic exit rate of mature T cells. Chemotaxis assays revealed that ADAM8 deficiency was associated with reduced migration of single positive thymocytes towards CCL21.

CONCLUSIONS/SIGNIFICANCE

Our results show that ADAM8 is involved in T cell maturation in the medulla and suggest a role for this protease in fine-tuning maturation of thymocytes in the medulla. In contrast to ADAM10 and ADAM17 lack of ADAM8 appears to have a relatively minor impact on T cell development, which was unexpected given that maturation of thymocytes is dependent on proper localization and timing of migration.

ADAM-17: the enzyme that does it all

This review focuses on the role of ADAM-17 in disease. Since its debut as the tumor necrosis factor converting enzyme (TACE), ADAM-17 has been reported to be an indispensible regulator of almost every cellular event from proliferation to migration. The central role of ADAM-17 in cell regulation is rooted in its diverse array of substrates: cytokines, growth factors, and their receptors as well as adhesion molecules are activated or inactivated by their cleavage with ADAM-17. It is therefore not surprising that ADAM-17 is implicated in numerous human diseases including cancer, heart disease, diabetes, rheumatoid arthritis, kidney fibrosis, Alzheimer's disease, and is a promising target for future treatments. The specific role of ADAM-17 in the pathophysiology of these diseases is very complex and depends on the cellular context. To exploit the therapeutic potential of ADAM-17, it is important to understand how its activity is regulated and how specific organs and cells can be targeted to inactivate or activate the enzyme.

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.

ADAM17 activity and other mechanisms of soluble L-selectin production during death receptor-induced leukocyte apoptosis

L-selectin is an adhesion molecule expressed by neutrophils that broadly directs their infiltration in to sites of inflammation. It is also present at relatively high levels in the serum of normal individuals. It is well established that L-selectin is efficiently shed from the surface of neutrophils upon their activation, a process that regulates its density and binding activity. Neutrophil programmed cell death is critical for the resolution of inflammation, and L-selectin downregulation is induced during this process as well. The mechanisms underpinning this latter process are much less understood, and were investigated in this study. Using a disintegrin and metalloprotease (ADAM)-17 radiation chimeric mice, we demonstrate for the first time that during early events of death receptor-mediated neutrophil apoptosis, L-selectin downregulation occurs primarily by ADAM17-mediated shedding. This was observed as well upon using shRNA to knock down ADAM17 expression in Jurkat cells, a well-studied cell line in terms of the molecular processes involved in the induction of apoptosis. These findings directly reveal that ADAM17 activity occurs during programmed cell death. Hence, the cleavage of particular ADAM17 substrates may be an additional component of the anti-inflammatory program initiated by apoptotic neutrophils. Of interest was that during later stages of induced leukocyte apoptosis, soluble L-selectin production occurred independent of ADAM17, as well as membrane events, such as blebbing and microparticle production. This process may provide an explanation for the lack of diminished serum L-selectin levels in ADAM17-null mice, and suggests a mechanism for the homeostatic maintenance of soluble L-selectin levels in the blood of healthy individuals.

ADAM10 is essential for Notch2-dependent marginal zone B cell development and CD23 cleavage in vivo

The proteolytic activity of a disintegrin and metalloproteinase 10 (ADAM10) regulates cell-fate decisions in Drosophila and mouse embryos. However, in utero lethality of ADAM10^−/− mice has prevented examination of ADAM10 cleavage events in lymphocytes. To investigate their role in B cell development, we generated B cell–specific ADAM10 knockout mice. Intriguingly, deletion of ADAM10 prevented development of the entire marginal zone B cell (MZB) lineage. Additionally, cleavage of the low affinity IgE receptor, CD23, was profoundly impaired, but subsequent experiments demonstrated that ADAM10 regulates CD23 cleavage and MZB development by independent mechanisms. Development of MZBs is dependent on Notch2 signaling, which requires proteolysis of the Notch2 receptor by a previously unidentified proteinase. Further experiments revealed that Notch2 signaling is severely impaired in ADAM10-null B cells. Thus, ADAM10 critically regulates MZB development by initiating Notch2 signaling. This study identifies ADAM10 as the in vivo CD23 sheddase and an important regulator of B cell development. Moreover, it has important implications for the treatment of numerous CD23- and Notch-mediated pathologies, ranging from allergy to cancer.

Conditional inactivation of TACE by a Sox9 promoter leads to osteoporosis and increased granulopoiesis via dysregulation of IL-17 and G-CSF

The TNF-alpha converting enzyme (TACE/ADAM17) is involved in the proteolytic release of the ectodomain of diverse cell surface proteins with critical roles in development, immunity, and hematopoiesis. As the perinatal lethality of TACE-deficient mice has prevented an analysis of the roles of TACE in adult animals, we generated mice in which floxed Tace alleles were deleted by Cre recombinase driven by a Sox9 promoter. These mutant mice survived up to 9-10 mo, but exhibited severe growth retardation as well as skin defects and infertility. The analysis of the skeletal system revealed shorter long bones and prominent bone loss, characterized by an increase in osteoclast and osteoblast activity. In addition, these mice exhibited hypercellularity in the bone marrow and extramedullary hematopoiesis in the spleen and liver. Flow cytometric analysis of the bone marrow cells showed a sharp increase in granulopoiesis and in the population of c-Kit-1(+) Sca-1(+) lineage(-) cells, and a decrease in lymphopoiesis. Moreover, we found that serum levels of IL-17 and G-CSF were significantly elevated compared with control littermates. These findings indicate that TACE is associated with a regulation of IL-17 and G-CSF expression in vivo, and that the dysregulation in G-CSF production is causally related to both the osteoporosis-like phenotype and the defects in the hematopoietic system.

[The two sides of ADAM17 in inflammation: implications in atherosclerosis and obesity]

ADAM17 was initially characterized as the TNF Alpha Converting Enzyme (TACE) and, until now, has been the most studied member of the ADAM family. It is a type I transmembrane metalloproteinase involved in the shedding of the extracellular domain of several transmembrane proteins (at least 40) such as cytokines, growth factors, receptors or adhesion molecules. As a consequence, depending on the transmembrane molecule cleaved, one may expect possible opposite effects of ADAM17 activity on inflammation (e.g. TNF and its receptors). The role of ADAM17 in regulating inflammatory cellular processes is clearly demonstrated in cells deficient in active ADAM17 or expressing substrates mutated for the ADAM17 cleavage site. As ADAM17-deficient mice died at birth, mice overexpressing the mutated uncleavable form of some substrates and recently conditional knock-out of ADAM17 are used to approach in vivo the role of this metalloprotease in regulating inflammation. Arguments are provided that ADAM17 plays a role in atherosclerosis, in adipose tissue metabolism, insulin resistance and diabetes. The multitude of substrates cleaved by ADAM17 makes this enzyme an attractive candidate to study its role in inflammation-driven pathologies.

Deficiency of TNFalpha converting enzyme (TACE/ADAM17) causes a lean, hypermetabolic phenotype in mice

Energy homeostasis involves central nervous system integration of afferent inputs that coordinately regulate food intake and energy expenditure. Here, we report that adult homozygous TNFalpha converting enzyme (TACE)-deficient mice exhibit one of the most dramatic examples of hypermetabolism yet reported in a rodent system. Because this effect is not matched by increased food intake, mice lacking TACE exhibit a lean phenotype. In the hypothalamus of these mice, neurons in the arcuate nucleus exhibit intact responses to reduced fat mass and low circulating leptin levels, suggesting that defects in other components of the energy homeostasis system explain the phenotype of Tace(DeltaZn/DeltaZn) mice. Elevated levels of uncoupling protein-1 in brown adipose tissue from Tace(DeltaZn/DeltaZn) mice when compared with weight-matched controls suggest that deficient TACE activity is linked to increased sympathetic outflow. These findings collectively identify a novel and potentially important role for TACE in energy homeostasis.

Cutting edge: soluble IL-6R is produced by IL-6R ectodomain shedding in activated CD4 T cells

IL-6 trans-signaling via the soluble IL-6R (sIL-6R) plays an important role in the progression of several autoimmune diseases and cancer by providing IL-6-responsiveness to cells lacking IL-6R. However, the potential sources of sIL-6R are less understood. In this study we show that sIL-6R is produced by both naive and memory CD4 T cells upon TCR activation. The production of sIL-6R by activated CD4 T cells is mediated by shedding of the membrane-bound IL-6R, and this process correlates with the expression of the metalloproteinase ADAM17 in these cells. In contrast to CD4 T cells, CD8 T cells do not express ADAM17 and their production of sIL-6R is negligible. Thus, during an immune response CD4 T cells are an important source of sIL-6R. Production of sIL-6R by autoreactive CD4 T cells may contribute to their role in the development of autoimmune disease by conferring IL-6-responsiveness to cells lacking IL-6R such as synoviocytes.

ADAM10 is essential for proteolytic activation of Notch during thymocyte development

Notch signaling pathway has been shown to play essential roles in T lymphocyte development. Activation of Notch requires a sequential proteolytic cleavage, which converts Notch from the full-length membrane-bound form to a transcriptionally active intracellular fragment. Studies in Drosophila showed that Kuzbanian (Kuz) is responsible for the enzymatic cleavage of extracellular S2 site upon Notch binding to its ligand Delta. Both a disintegrin and metalloprotease (ADAM) 10 and ADAM17, members of the ADAM family metalloproteases, have been indicated as the mammalian counterpart of Kuz in activating Notch in mammals. Here, we investigated functions of ADAM10 in Notch signaling during thymocyte development. We show that conditional disruption of the Adam10 gene in mouse thymocytes results in a developmental defect similar to the phenotypes previously described for T lineage-specific disruption of Notch1. We further show that the activation of Notch1 and its downstream target genes Deltex-1 and Pre-Ta are impaired in Adam10-deficient thymocytes. Our study demonstrates a T cell intrinsic role for Adam10 in activation of Notch1 during thymocyte development.

Drug insight: tumor necrosis factor-converting enzyme as a pharmaceutical target for rheumatoid arthritis

The success of agents that inhibit tumor necrosis factor (TNF), such as infliximab, adalimumab and etanercept, has led to a desire for orally available small molecules that have a better safety profile and are less costly to produce than current agents. One target for anti-TNF therapy that is currently under investigation is TNF-converting enzyme, which promotes the release of soluble TNF from its membrane-bound precursor. Inhibitors of this enzyme with drug-like properties have been made and tested in the clinic. These inhibitors include TMI-005 and BMS-561392, both of which have entered into phase II clinical trials. This article summarizes preclinical and clinical findings regarding the use of inhibitors of TNF-converting enzyme for the treatment of rheumatoid arthritis.

Follicular dendritic cell networks of primary follicles and germinal centers: phenotype and function

Follicular dendritic cells (FDCs) were identified decades ago by their ability to retain immune complexes and more recent findings indicate that they are a source of B cell attractants and trophic factors. New imaging studies have shown that B cells closely associate with their dendritic processes during migration. Here we will review the properties of these specialized follicular stromal cells and provide an update on the requirements for their maturation into phenotypically distinct cells within germinal center light and dark zones. We will then discuss current understanding of how they help support the B cell immune response.

Clipping, shedding and RIPping keep immunity on cue

Exposure to infectious agents elicits defense mechanisms that necessitate a timely immune response. The immediate delivery of essential cues for immune activation is provided, in part, by proteolytic processing. A large repertoire of molecules orchestrates the activation, migration, and effector function of immune cells. The diversity of this repertoire matches well with the broad array of substrates that can be cleaved by proteinases, and many of these substrates are proving to be essential for proper immune-cell function. Here, we discuss how two specific classes of metal-dependent proteinases, the matrix metalloproteinases and the disintegrin metalloproteinases, have consequences well beyond classical cell-matrix and cell-cell interactions and motility, and we review their roles in immune-cell maturation, clonal expansion, and cytotoxic functions.