Evidence for a critical role of the tumor necrosis factor alpha convertase (TACE) in ectodomain shedding of the p75 neurotrophin receptor (p75NTR)

L1 is sequentially processed by two differently activated metalloproteases and presenilin/gamma-secretase and regulates neural cell adhesion, cell migration, and neurite outgrowth

The immunoglobulin superfamily recognition molecule L1 plays important functional roles in the developing and adult nervous system. Metalloprotease-mediated cleavage of this adhesion molecule has been shown to stimulate cellular migration and neurite outgrowth. We demonstrate here that L1 cleavage is mediated by two distinct members of the disintegrin and metalloprotease family, ADAM10 and ADAM17. This cleavage is differently regulated and leads to the generation of a membrane bound C-terminal fragment, which is further processed through gamma-secretase activity. Pharmacological approaches with two hydroxamate-based inhibitors with different preferences in blocking ADAM10 and ADAM17, as well as loss of function and gain of function studies in murine embryonic fibroblasts, showed that constitutive shedding of L1 is mediated by ADAM10 while phorbol ester stimulation or cholesterol depletion led to ADAM17-mediated L1 cleavage. In contrast, N-methyl-d-aspartate treatment of primary neurons stimulated ADAM10-mediated L1 shedding. Both proteases were able to affect L1-mediated adhesion and haptotactic migration of neuronal cells. In particular, both proteases were involved in L1-dependent neurite outgrowth of cerebellar neurons. Thus, our data identify ADAM10 and ADAM17 as differentially regulated L1 membrane sheddases, both critically affecting the physiological functions of this adhesion protein.

ERK-mediated phosphorylation of Thr735 in TNFalpha-converting enzyme and its potential role in TACE protein trafficking

Tumor necrosis factor alpha-converting enzyme (TACE/ADAM17) has been implicated in the inducible shedding of numerous cell surface proteins. In light of this, the regulation of TACE catalytic activation and protein maturation has recently received considerable interest in the context of extracellular signal-regulated kinase activation and the subsequent phosphorylation of TACE at residue Thr735. In this study, we analysed the subcellular localisation of TACE derivatives using laser-scanning confocal microscopy and cell surface biotinylation. Whereas WT.TACE- and T735A.TACE-enhanced green fluorescent protein (-eGFP) fusion derivatives of TACE were both found to localise with the endoplasmic reticulum, a phosphomimicking form of TACE (T735E.TACE-eGFP) was found to colocalise predominantly with components of the protein secretory pathway (COPII vesicles and trans-Golgi network). Additionally, upon analysis of biotinylated ectopic TACE, we observed that inducible trafficking of TACE to the cell surface was dependent upon extracellular signal-regulated kinase activation and phosphorylation of Thr735. We conclude from our observations that phosphorylation of TACE at Thr735 highlights a key step in inducible TACE protein trafficking and maturation.

Nuclear translocation of the p75 neurotrophin receptor cytoplasmic domain in response to neurotrophin binding

The intracellular domain of the p75 neurotrophin receptor (p75ICD) can be released by gamma-secretase in response to the previous activation of alpha-secretase by phorbol esters. However, ligand-dependent release of p75ICD has yet to be described. We show here that nerve growth factor can induce the release of p75ICD and facilitate its translocation to the nucleus in a gamma-secretase-dependent manner. This effect was observed in RN22 schwannoma cells cultured under serum-free conditions, as well as in Schwann cells, and it was mimicked by other neurotrophins, such as brain-derived neurotrophic factor or neurotrophin-3. Unlike other known examples of regulated intramembrane proteolysis, ligand-dependent release of p75ICD did not need the previous activation of alpha-secretase. These results suggest that nuclear translocation of p75ICD may represent a novel neurotrophin-mediated signaling pathway.

L-selectin: mechanisms and physiological significance of ectodomain cleavage

L-selectin is a cell adhesion molecule consisting of a large, highly glycosylated, extracellular domain, a single spanning transmembrane domain and a small cytoplasmic tail. It is expressed on most leukocytes and is involved in their rolling on inflamed vascular endothelium prior to firm adhesion and transmigration. It is also required for the constitutive trafficking of lymphocytes through secondary lymphoid organs. Like most adhesion molecules, L-selectin function is regulated by a variety of mechanisms including gene transcription, post-translational modifications, association with the actin cytoskeleton, and topographic distribution. In addition, it is rapidly downregulated by proteolytic cleavage near the cell surface by ADAM-17 (TACE) and at least one other "sheddase". This process of "ectodomain shedding" results in the release of most of the extracellular portion of L-selectin from the cell surface while retaining the cytoplasmic, transmembrane, and eleven amino acids of the extracellular domain on the cell. This review will examine the mechanism(s) of L-selectin ectodomain shedding and discuss the physiological implications.

Cytokine and growth factor receptors in the nucleus: what's up with that?

Signaling via cell surface receptors that are anchored by a single transmembrane domain is a well-established paradigm. Ligand binding to the extracellular domain of the receptor facilitates receptor dimerization, which juxtaposes the intracellular domains, typically activating intrinsic or associated kinases. Two large families of tyrosine kinase activating receptors have been particularly well characterized: the receptor-type protein tyrosine kinases and the receptors for the alpha-helical cytokines, which activate non-covalently bound JAK family tyrosine kinases. Despite the well-established function of these receptors at the cell surface, both intact and cleaved receptors belonging to these families have been repeatedly detected in the nucleus. Furthermore, there is evidence that some of these receptors or receptor fragments function directly in modulating gene transcription. In this essay, I examine how close we are to demonstrating that direct translocation of receptors, or receptor fragments, from the cell surface to the nucleus is a physiologically relevant means of intracellular signaling that can supplant or complement canonical signaling cascades.

Differential endocytic sorting of p75NTR and TrkA in response to NGF: a role for late endosomes in TrkA trafficking

NGF binds to two receptors, p75NTR and TrkA. The endosomal trafficking of receptors is of emerging importance for the understanding of their signaling. We compared the endocytic trafficking of the two NGF receptors in PC12 cells. Both p75NTR and TrkA were internalized in response to NGF and colocalized with early endosomes. However, surprisingly, the subsequent endosomal trafficking paths of both NGF receptors diverged: whereas p75NTR recycled back to the surface, TrkA moved to late endosomes and underwent lysosomal degradation. By performing subcellular fractionations of NGF stimulated PC12 cells, tyrosine-phosphorylated TrkA was recovered in fractions corresponding to late endosomes. This implicates these organelles as novel endosomal NGF signaling platforms. Furthermore, the trafficking of NGF receptors could be manipulated by pharmacological means. Disrupting p75NTR recycling diminished TrkA activation in response to low concentrations of NGF, demonstrating a functional role for the recycling of p75NTR.

Shedding light on ADAM metalloproteinases

ADAM metalloproteinase disintegrins have emerged as the major proteinase family that mediates ectodomain shedding, the proteolytic release of extracellular domains from their membrane-bound precursors. Recent gene-manipulation studies have established the role of ADAM-mediated shedding in mammalian physiology and, in addition, raised the issue of functional redundancy among ADAM sheddases. ADAM sheddases activate, for example, growth factors and cytokines, thus regulating signalling pathways that are important in development and pathological processes such as cancer. The recent studies have also begun to elucidate the substrate specificity and the mechanisms that control ADAM-mediated shedding events that regulate, for example, growth-factor and Notch signalling, and the processing of the amyloid precursor protein.

Metalloprotease-disintegrin ADAM8: expression analysis and targeted deletion in mice

ADAM8 (a disintegrin and metalloprotease 8, also referred to as MS2/CD156a) is a membrane-anchored metalloprotease that was first identified in a macrophage cell line and has been implicated in neurodegenerative diseases. Here, we evaluated the expression of ADAM8 during mouse development and generated mice lacking ADAM8 (Adam8-/- mice). During early mouse development, ADAM8 is expressed by maternal cells in the decidua and by trophoblast derivatives of the embryo but not in the derivatives of the inner cell mass. At later stages, prominent expression of ADAM8 is seen in the embryo proper, in the gonadal ridge, thymus, developing cartilage and bone, brain and spinal cord, and in the mesenchyme in close proximity to the branch point between the jugular vein and developing lymphatic vessels. Examination of Adam8-/- mice, however, revealed no major defects in these or other structures during development or in adult tissues and no evident pathological phenotypes.

Nectin-4, a new serological breast cancer marker, is a substrate for tumor necrosis factor-alpha-converting enzyme (TACE)/ADAM-17

Serum markers are extensively used in diagnostic and follow-up of cancer patients. We recently described Nectin-4, a 66-kDa adhesion molecule of the Nectin family, which is a valuable new histological and serological marker for breast carcinoma. In vivo, Nectin-4 is re-expressed in breast carcinoma, and a circulating form of Nectin-4 is detected in the sera of patients with metastatic breast cancer. In vitro, a soluble form of Nectin-4 is produced in the supernatant of breast tumor cell lines (S. Fabre-Lafay, C. Ginestier, S. Garrido-Urbani, C. Berruyer, R. Sauvan, N. Reymond, J. Adelaide, J. Geneix, P. Dubreuil, J. Jacquemier, D. Birnbaum, and M. Lopez, manuscript in preparation). We have investigated the mechanisms that regulate the production of this soluble form. It was found that the soluble form of Nectin-4 detected in the sera of patients and the supernatant of breast tumor cell lines share similar biochemical and immunological features. The soluble Nectin-4 form (43 kDa) is formed by the entire Nectin-4 ectodomain. Nectin-4 shedding is constitutive, strongly enhanced by 12-O-tetradecanoylphorbol-13-acetate activation, and reduced tumor necrosis factor-alpha protease inhibitor TAPI-1 or by the tissue inhibitor of metalloproteinase-3 (TIMP-3). TAPI-1 and TIMP-3 are inhibitors of the endoprotease tumor necrosis factor-alpha-converting enzyme (TACE)/ADAM-17. Overexpression or small interfering RNA-mediated silencing of TACE enhanced or reduced Nectin-4 shedding, respectively. Nectin-4 is not shed when expressed in TACE-deficient fibroblasts. Interestingly, the active form of TACE is overexpressed in breast tumors and may indicate that TACE is responsible for Nectin-4 shedding not only in vitro but also in vivo.

Cleavage of p75 Neurotrophin Receptor by α-Secretase and γ-Secretase Requires Specific Receptor Domains

The p75 neurotrophin receptor (p75(NTR)), a member of the tumor necrosis factor superfamily of receptors, undergoes multiple proteolytic cleavage events. These events are initiated by an alpha-secretase-mediated release of the extracellular domain followed by a gamma-secretase-mediated intramembrane cleavage. However, the specific determinants of p75(NTR) cleavage events are unknown. Many other substrates of gamma-secretase cleavage have been identified, including Notch, amyloid precursor protein, and ErbB4, indicating there is broad substrate recognition by gamma-secretase. Using a series of deletion mutations and chimeric receptors of p75(NTR) and the related Fas receptor, we have identified domains that are essential for p75(NTR) proteolysis. The initial alpha-secretase cleavage was extracellular to the transmembrane domain. Unfortunately, deletion mutants were not capable of defining the requirements of ectodomain shedding. Although this cleavage is promiscuous with respect to amino acid sequence, its position with respect to the transmembrane domain is invariant. The generation of chimeric receptors exchanging different domains of noncleavable Fas receptor with p75(NTR), however, revealed that a discrete domain above the membrane is sufficient for efficient cleavage of p75(NTR). Mass spectrometric analysis confirmed the cleavage can occur with a truncated p75(NTR) displaying only 15 extracellular amino acids in the stalk region.

Zinc metalloproteinase-mediated cleavage of the human Nogo-66 receptor

The central nervous system myelin components oligodendrocyte-myelin glycoprotein, myelin-associated glycoprotein and the Nogo-66 domain of Nogo-A inhibit neurite outgrowth by binding the neuronal glycosyl-phosphatidylinositol-anchored Nogo-66 receptor (NgR) that transduces the inhibitory signal to the cell interior via a transmembrane co-receptor, p75NTR. Here, we demonstrate that human NgR expressed in human neuroblastoma cells is constitutively cleaved in a post-ER compartment to generate a lipid-raft associated C-terminal fragment that is present on the cell surface and a soluble N-terminal fragment that is released into the medium. Mass spectrometric analysis demonstrated that the N-terminal fragment terminated just after the C-terminus of the ligand-binding domain of NgR. In common with other shedding mechanisms, the release of this fragment was blocked by a hydroxamate-based inhibitor of zinc metalloproteinases, but not by inhibitors of other protease classes and up-regulated by treatment with the cellular cholesterol depleting agent methyl-beta-cyclodextrin. The N-terminal fragment bound Nogo-66 and blocked Nogo-66 binding to cell surface NgR but failed to associate with p75NTR, indicative of a role as a Nogo-66 antagonist. Furthermore, the N- and C-terminal fragments of NgR were detectable in human brain cortex and the N-terminal fragment was also present in human cerebrospinal fluid, demonstrating that NgR proteolysis occurs within the human nervous system. Our findings thus identify a potential cellular mechanism for the regulation of NgR function at the level of the receptor.

Got RIP? Presenilin-dependent intramembrane proteolysis in growth factor receptor signaling

A number of cell surface growth factor receptors are subject to presenilin-dependent regulated intramembrane proteolysis (PS-RIP) after ligand binding and/or ectodomain cleavage. PS-RIP is mediated by a highly conserved multi-component membrane-bound protease, termed gamma-secretase, responsible for generating Alzheimer's disease (AD)-associated Abeta peptide from its membrane-bound beta-amyloid precursor protein (APP), as well as for cleaving a number of other type-I membrane receptors. PS-RIP is a conserved cellular process by which cells transmit signals from one compartment to another, including the liberation of membrane-bound transcription factors. Recent studies indicate that PS-RIP also mediates the proteolytic inactivation of heteromeric receptor complexes by removing the transmembrane domains required for receptor-receptor interaction. Thus, PS-RIP appears to regulate diverse cellular pathways either by generating soluble effectors from membrane-bound precursors, or by removing the transmembrane domain of a membrane-tethered signaling component.

Multi-tasking by the p75 neurotrophin receptor: sortilin things out?

Signalling by the p75 neurotrophin receptor has been implicated in diverse neuronal responses, including increased differentiation or survival, inhibition of regeneration, and initiation of apoptotic cell death. These numerous roles are matched by, but are not yet correlated with, a multiplicity of extracellular ligands and intracellular interactors. Membrane proteins such as sortilin, a member of the Vps10p family of sorting receptors, and the glycosylphosphatidylinositol-linked Nogo receptor (NgR) and the associated adaptor lingo 1 have recently been added to the list of p75-interacting modulators. Other studies have described intramembranal cleavage of p75 and the potential nuclear targeting of cleavage fragments or of the complete receptor after it has been internalized into a putative signalling endosome. These findings suggest that some of the diversity in p75 activities might be due to differential subcellular localization and transport of p75 receptor complexes. We therefore argue that cell-biology-driven approaches are now required to make sense of p75 signalling.

ADAMs: key components in EGFR signalling and development

ADAM (a disintegrin and metalloprotease) proteins are membrane-anchored metalloproteases that process and shed the ectodomains of membrane-anchored growth factors, cytokines and receptors. ADAMs also have essential roles in fertilization, angiogenesis, neurogenesis, heart development and cancer. Research on ADAMs and their role in protein ectodomain shedding is emerging as a fertile ground for gathering new insights into the functional regulation of membrane proteins.

p75NTR – live or let die

During neuronal development, neurotrophins are essential factors that promote survival, differentiation and myelination of neurons. The trophic signals are relayed to the cells via binding to Trk receptor tyrosine kinases and the p75 neurotrophin receptor. Paradoxically, the p75 neurotrophin receptor also ensures rapid and appropriate apoptosis of neonatal neurons not reaching their proper targets and transmits death signals to injured neurons. Until recently, the mechanisms by which the p75 neurotrophin receptor governs these opposing functions have remained elusive. By the identification of new ligands and cytosolic interacting partners, receptor cleavage products and coreceptors, some of these mechanisms are now being unraveled. Here, we review recent progress in delineating the molecular networks that enable p75(NTR) to dictate life and death.

Mechanisms of soluble cytokine receptor generation

Soluble cytokine receptors regulate inflammatory and immune events by functioning as agonists or antagonists of cytokine signaling. As such, they act within complex receptor systems that include signaling receptors, nonsignaling decoy receptors, receptor-associated proteins, and soluble receptor antagonists. Soluble cytokine receptors can be generated by several mechanisms, which include proteolytic cleavage of receptor ectodomains, alternative splicing of mRNA transcripts, transcription of distinct genes that encode soluble cytokine-binding proteins, release of full-length receptors within the context of exosome-like vesicles, and cleavage of GPI-anchored receptors. Furthermore, the important role of soluble cytokine receptors in regulating host defense mechanisms is evidenced by viruses that encode soluble homologues of mammalian receptors and thereby evade innate host immune responses via the sequestration of essential cytokines.

Regulated proteolysis of the IFNaR2 subunit of the interferon-alpha receptor

The type I interferons (IFNs) bind surface receptors, induce JAK kinases and activate STAT transcription factors to stimulate the transcription of genes downstream of IFN-stimulated response elements (ISREs). In this study, we demonstrate that IFNaR2, a subunit of the type I IFN receptor, is proteolytically cleaved in a regulated manner. Immunoblotting shows that multi-step cleavage occurs in response to phorbol ester (PMA) and IFN-alpha, generating both a transmembrane 'stub' and the intracellular domain (ICD), similar to Notch proteolysis. Isolated membrane fractions process IFNaR2 to release the ICD. A chimeric receptor construct is utilized to show that cleavage requires the presenilins and occurs in response to epidermal growth factor and protein kinase C-delta overexpression, as well as PMA and type I IFNs. Fluorescence microscopy demonstrates that a green fluorescent protein-ICD fusion localizes predominantly to the nucleus. A fusion between the ICD and the Gal4 DNA-binding domain represses transcription, in a histone deacetylase-dependent manner, of a Gal4 upstream activating sequence-regulated reporter, while overexpression of the ICD alone represses transcription of a reporter linked to an ISRE. Proteolytic cleavage events may facilitate receptor turnover or, more likely, function as a mechanism for signaling similar to that employed by Notch and the Alzheimer's precursor protein.

Evaluation of the contribution of different ADAMs to tumor necrosis factor alpha (TNFalpha) shedding and of the function of the TNFalpha ectodomain in ensuring selective stimulated shedding by the TNFalpha convertase (TACE/ADAM17)

Tumor necrosis factor-alpha (TNFalpha), a potent pro-inflammatory cytokine, is released from cells by proteolytic cleavage of a membrane-anchored precursor. The TNF-alpha converting enzyme (TACE; a disintegrin and metalloprotease17; ADAM17) is known to have a key role in the ectodomain shedding of TNFalpha in several cell types. However, because purified ADAMs 9, 10, and 19 can also cleave a peptide corresponding to the TNFalpha cleavage site in vitro, these enzymes are considered to be candidate TNFalpha sheddases as well. In this study we used cells lacking ADAMs 9, 10, 17 (TACE), or 19 to address the relative contribution of these ADAMs to TNFalpha shedding in cell-based assays. Our results corroborate that ADAM17, but not ADAM9, -10, or -19, is critical for phorbol ester- and pervanadate-stimulated release of TNFalpha in mouse embryonic fibroblasts. However, overexpression of ADAM19 increased the constitutive release of TNFalpha, whereas overexpression of ADAM9 or ADAM10 did not. This suggests that ADAM19 may contribute to TNFalpha shedding, especially in cells or tissues where it is highly expressed. Furthermore, we used mutagenesis of TNFalpha to explore which domains are important for its stimulated processing by ADAM17. We found that the cleavage site of TNFalpha is necessary and sufficient for cleavage by ADAM17. In addition, the ectodomain of TNFalpha makes an unexpected contribution to the selective cleavage of TNFalpha by ADAM17: it prevents one or more other enzymes from cleaving TNFalpha following PMA stimulation. Thus, selective stimulated processing of TNFalpha by ADAM17 in cells depends on the presence of an appropriate cleavage site as well as the inhibitory role of the TNF ectodomain toward other enzymes that can process this site.

Loss of ectodomain shedding due to mutations in the metalloprotease and cysteine-rich/disintegrin domains of the tumor necrosis factor-alpha converting enzyme (TACE)

Tumor necrosis factor-alpha converting enzyme (TACE), a multidomain protease essential for development and disease, releases the ectodomains from many transmembrane proteins in a regulated fashion. To understand the mechanism underlying the regulation of TACE activity, we sought to identify the cause of ectodomain shedding deficiencies in two mutated CHO sublines designated M1 and M2. Transfection of expression vectors for human and mouse TACE restored ectodomain shedding of TNF-alpha and TGF-alpha, suggesting that defects in the TACE gene contribute to the phenotype of M1 and M2 cells. The overall levels of endogenous TACE forms in M1 cells were significantly lower than those found in their parental cells, whereas only TACE zymogen, but not its mature form, was detectable in M2 cells. Molecular analyses suggested that M1 cells contained only one expressible TACE allele encoding an M435I point mutation in the catalytic center of the protease, and M2 cells produced two TACE variants with distinct point mutations in the catalytic domain (C225Y) and the cysteinerich/disintegrin domain (C600Y). Overexpression of the C225Y and C600Y TACE by transient transfection largely compensated for maturation defects in the variants but failed to restore TNF-alpha and TGF-alpha release in the shedding-defective CHO cell lines and fibroblasts derived from TACE-null mouse embryo. Further mutagenesis and functional analyses demonstrated that Cys(600) was absolutely essential for ectodomain shedding, suggesting that Cys(600), similar to Cys(225), participates in disulfide bonding, which is critical for both the processing and catalysis of TACE.