Proteolytic release of membrane-bound angiotensin-converting enzyme: role of the juxtamembrane stalk sequence

Cryo-EM reveals mechanisms of angiotensin I-converting enzyme allostery and dimerization

Hypertension (high blood pressure) is a major risk factor for cardiovascular disease, which is the leading cause of death worldwide. The somatic isoform of angiotensin I‐converting enzyme (sACE) plays a critical role in blood pressure regulation, and ACE inhibitors are thus widely used to treat hypertension and cardiovascular disease. Our current understanding of sACE structure, dynamics, function, and inhibition has been limited because truncated, minimally glycosylated forms of sACE are typically used for X‐ray crystallography and molecular dynamics simulations. Here, we report the first cryo‐EM structures of full‐length, glycosylated, soluble sACE (sACE^S1211). Both monomeric and dimeric forms of the highly flexible apo enzyme were reconstructed from a single dataset. The N‐ and C‐terminal domains of monomeric sACE^S1211 were resolved at 3.7 and 4.1 Å, respectively, while the interacting N‐terminal domains responsible for dimer formation were resolved at 3.8 Å. Mechanisms are proposed for intradomain hinging, cooperativity, and homodimerization. Furthermore, the observation that both domains were in the open conformation has implications for the design of sACE modulators.

Novel Therapeutic Approaches Targeting the Renin-Angiotensin System and Associated Peptides in Hypertension and Heart Failure

Despite the success of renin-angiotensin system (RAS) blockade by angiotensin-converting enzyme (ACE) inhibitors and angiotensin II type 1 receptor (AT1R) blockers, current therapies for hypertension and related cardiovascular diseases are still inadequate. Identification of additional components of the RAS and associated vasoactive pathways, as well as new structural and functional insights into established targets, have led to novel therapeutic approaches with the potential to provide improved cardiovascular protection and better blood pressure control and/or reduced adverse side effects. The simultaneous modulation of several neurohumoral mediators in key interconnected blood pressure-regulating pathways has been an attractive approach to improve treatment efficacy, and several novel approaches involve combination therapy or dual-acting agents. In addition, increased understanding of the complexity of the RAS has led to novel approaches aimed at upregulating the ACE2/angiotensin-(1-7)/Mas axis to counter-regulate the harmful effects of the ACE/angiotensin II/angiotensin III/AT1R axis. These advances have opened new avenues for the development of novel drugs targeting the RAS to better treat hypertension and heart failure. Here we focus on new therapies in preclinical and early clinical stages of development, including novel small molecule inhibitors and receptor agonists/antagonists, less conventional strategies such as gene therapy to suppress angiotensinogen at the RNA level, recombinant ACE2 protein, and novel bispecific designer peptides.

Gonadectomy prevents the increase in blood pressure and glomerular injury in angiotensin-converting enzyme 2 knockout diabetic male mice. Effects on renin-angiotensin system

BACKGROUND

Angiotensin-converting enzyme 2 (ACE2) deletion worsens kidney injury, and its amplification ameliorates diabetic nephropathy. Male sex increases the incidence, prevalence, and progression of chronic kidney disease in our environment.

METHOD

Here, we studied the effect of ACE2 deficiency and gonadectomy (GDX) on diabetic nephropathy and its relationship with fibrosis, protein kinase B (Akt) activation, and the expression of several components of the renin-angiotensin system (RAS).Mice were injected with streptozotocin to induce diabetes and followed for 19 weeks. Physiological and renal parameters were studied in wild-type and ACE2 knockout (ACE2KO) male mice with and without GDX.

RESULTS

Diabetic ACE2KO showed increased blood pressure (BP), glomerular injury, and renal fibrosis compared with diabetic wild-type. Gonadectomized diabetic ACE2KO presented a decrease in BP. In the absence of ACE2, GDX attenuated albuminuria and renal lesions, such as mesangial matrix expansion and podocyte loss. Both, α-smooth muscle actin accumulation and collagen deposition were significantly decreased in renal cortex of gonadectomized diabetic ACE2KO but not diabetic wild-type mice. GDX also reduced circulating ACE activity in ACE2KO mice. Loss of ACE2 modified the effect of GDX on cortical gene expression of RAS in diabetic mice. Akt phosphorylation in renal cortex was increased by diabetes and loss of ACE2 and decreased by GDX in control and diabetic ACE2KO but not in wild-type mice.

CONCLUSIONS

Our results suggest that GDX may exert a protective effect within the kidney under pathological conditions of diabetes and ACE2 deficiency. This renoprotection may be ascribed to different mechanisms such as decrease in BP, modulation of RAS, and downregulation of Akt-related pathways.

TSH Receptor Cleavage Into Subunits and Shedding of the A-Subunit; A Molecular and Clinical Perspective

The TSH receptor (TSHR) on the surface of thyrocytes is unique among the glycoprotein hormone receptors in comprising two subunits: an extracellular A-subunit, and a largely transmembrane and cytosolic B-subunit. Unlike its ligand TSH, whose subunits are encoded by two genes, the TSHR is expressed as a single polypeptide that subsequently undergoes intramolecular cleavage into disulfide-linked subunits. Cleavage is associated with removal of a C-peptide region, a mechanism similar in some respects to insulin cleavage into disulfide linked A- and B-subunits with loss of a C-peptide region. The potential pathophysiological importance of TSHR cleavage into A- and B-subunits is that some A-subunits are shed from the cell surface. Considerable experimental evidence supports the concept that A-subunit shedding in genetically susceptible individuals is a factor contributing to the induction and/or affinity maturation of pathogenic thyroid-stimulating autoantibodies, the direct cause of Graves' disease. The noncleaving gonadotropin receptors are not associated with autoantibodies that induce a "Graves' disease of the gonads." We also review herein current information on the location of the cleavage sites, the enzyme(s) responsible for cleavage, the mechanism by which A-subunits are shed, and the effects of cleavage on receptor signaling.

Kinetic and structural characterization of amyloid-β peptide hydrolysis by human angiotensin-1-converting enzyme

Angiotensin‐1‐converting enzyme (ACE), a zinc metallopeptidase, consists of two homologous catalytic domains (N and C) with different substrate specificities. Here we report kinetic parameters of five different forms of human ACE with various amyloid beta (Aβ) substrates together with high resolution crystal structures of the N‐domain in complex with Aβ fragments. For the physiological Aβ(1–16) peptide, a novel ACE cleavage site was found at His14‐Gln15. Furthermore, Aβ(1–16) was preferentially cleaved by the individual N‐domain; however, the presence of an inactive C‐domain in full‐length somatic ACE (sACE) greatly reduced enzyme activity and affected apparent selectivity. Two fluorogenic substrates, Aβ(4–10)Q and Aβ(4–10)Y, underwent endoproteolytic cleavage at the Asp7‐Ser8 bond with all ACE constructs showing greater catalytic efficiency for Aβ(4–10)Y. Surprisingly, in contrast to Aβ(1–16) and Aβ(4–10)Q, sACE showed positive domain cooperativity and the double C‐domain (CC‐sACE) construct no cooperativity towards Aβ(4–10)Y. The structures of the Aβ peptide–ACE complexes revealed a common mode of peptide binding for both domains which principally targets the C‐terminal P2′ position to the S2′ pocket and recognizes the main chain of the P1′ peptide. It is likely that N‐domain selectivity for the amyloid peptide is conferred through the N‐domain specific S2′ residue Thr358. Additionally, the N‐domain can accommodate larger substrates through movement of the N‐terminal helices, as suggested by the disorder of the hinge region in the crystal structures. Our findings are important for the design of domain selective inhibitors as the differences in domain selectivity are more pronounced with the truncated domains compared to the more physiological full‐length forms.

Database

The atomic coordinates and structure factors for N‐domain ACE with Aβ peptides 4–10 (5AM8), 10–16 (5AM9), 1–16 (5AMA), 35–42 (5AMB) and (4–10)Y (5AMC) complexes have been deposited in the Protein Data Bank, Research Collaboratory for Structural Bioinformatics, Rutgers University, New Brunswick, NJ, USA (http://www.rcsb.org/).

Withdrawn: TSH Receptor Cleavage Into Subunits and Shedding of the A-Subunit; A Molecular and Clinical Perspective

The TSH receptor (TSHR) on the surface of thyrocytes is unique among the glycoprotein hormone receptors in comprising two subunits: an extracellular A-subunit, and a largely transmembrane and cytosolic B-subunit. Unlike its ligand TSH, whose subunits are encoded by two genes, the TSHR is expressed as a single polypeptide that subsequently undergoes intramolecular cleavage into disulfide-linked subunits. Cleavage is associated with removal of a C-peptide region, a mechanism similar in some respects to insulin cleavage into disulfide linked A- and B-subunits with lossofaC-peptideregion. The potential pathophysiological importance of TSHR cleavage into A-and B-subunits is that some A-subunits are shed from the cell surface. Considerable experimental evidence supports the concept that A-subunit shedding in genetically susceptible individuals is a factor contributing to the induction and/or affinity maturation of pathogenic thyroid-stimulating autoantibodies, the direct cause of Graves' disease. The noncleaving gonadotropin receptors are not associated with autoantibodies that induce a "Graves' disease of the gonads." We also review herein current information on the location of the cleavage sites, the enzyme(s) responsible for cleavage, the mechanism by which A-subunits are shed, and the effects of cleavage on receptor signaling. (Endocrine Reviews 37: 114-134, 2016).

Structural basis of Ac-SDKP hydrolysis by Angiotensin-I converting enzyme

Angiotensin-I converting enzyme (ACE) is a zinc dipeptidylcarboxypeptidase with two active domains and plays a key role in the regulation of blood pressure and electrolyte homeostasis, making it the principal target in the treatment of cardiovascular disease. More recently, the tetrapetide N-acetyl-Ser–Asp–Lys–Pro (Ac-SDKP) has emerged as a potent antifibrotic agent and negative regulator of haematopoietic stem cell differentiation which is processed exclusively by ACE. Here we provide a detailed biochemical and structural basis for the domain preference of Ac-SDKP. The high resolution crystal structures of N-domain ACE in complex with the dipeptide products of Ac-SDKP cleavage were obtained and offered a template to model the mechanism of substrate recognition of the enzyme. A comprehensive kinetic study of Ac-SDKP and domain co-operation was performed and indicated domain interactions affecting processing of the tetrapeptide substrate. Our results further illustrate the molecular basis for N-domain selectivity and should help design novel ACE inhibitors and Ac-SDKP analogues that could be used in the treatment of fibrosis disorders.

Angiotensin-I converting enzyme (ACE): structure, biological roles, and molecular basis for chloride ion dependence

Somatic angiotensin-I converting enzyme (sACE) has an essential role in the regulation of blood pressure and electrolyte fluid homeostasis. It is a zinc protease that cleaves angiotensin-I (AngI), bradykinin, and a broad range of other signalling peptides. The enzyme activity is provided by two homologous domains (N- and C-), which display clear differences in substrate specificities and chloride activation. The presence of chloride ions in sACE and its unusual role in activity was identified early on in the characterisation of the enzyme. The molecular mechanisms of chloride activation have been investigated thoroughly through mutagenesis studies and shown to be substrate-dependent. Recent results from X-ray crystallography structural analysis have provided the basis for the intricate interactions between ACE, its substrate and chloride ions. Here we describe the role of chloride ions in human ACE and its physiological consequences. Insights into the chloride activation of the N- and C-domains could impact the design of improved domain-specific ACE inhibitors.

TMPRSS2 and ADAM17 cleave ACE2 differentially and only proteolysis by TMPRSS2 augments entry driven by the severe acute respiratory syndrome coronavirus spike protein

The type II transmembrane serine proteases TMPRSS2 and HAT can cleave and activate the spike protein (S) of the severe acute respiratory syndrome coronavirus (SARS-CoV) for membrane fusion. In addition, these proteases cleave the viral receptor, the carboxypeptidase angiotensin-converting enzyme 2 (ACE2), and it was proposed that ACE2 cleavage augments viral infectivity. However, no mechanistic insights into this process were obtained and the relevance of ACE2 cleavage for SARS-CoV S protein (SARS-S) activation has not been determined. Here, we show that arginine and lysine residues within ACE2 amino acids 697 to 716 are essential for cleavage by TMPRSS2 and HAT and that ACE2 processing is required for augmentation of SARS-S-driven entry by these proteases. In contrast, ACE2 cleavage was dispensable for activation of the viral S protein. Expression of TMPRSS2 increased cellular uptake of soluble SARS-S, suggesting that protease-dependent augmentation of viral entry might be due to increased uptake of virions into target cells. Finally, TMPRSS2 was found to compete with the metalloprotease ADAM17 for ACE2 processing, but only cleavage by TMPRSS2 resulted in augmented SARS-S-driven entry. Collectively, our results in conjunction with those of previous studies indicate that TMPRSS2 and potentially related proteases promote SARS-CoV entry by two separate mechanisms: ACE2 cleavage, which might promote viral uptake, and SARS-S cleavage, which activates the S protein for membrane fusion. These observations have interesting implications for the development of novel therapeutics. In addition, they should spur efforts to determine whether receptor cleavage promotes entry of other coronaviruses, which use peptidases as entry receptors.

A modern understanding of the traditional and nontraditional biological functions of angiotensin-converting enzyme

Angiotensin-converting enzyme (ACE) is a zinc-dependent peptidase responsible for converting angiotensin I into the vasoconstrictor angiotensin II. However, ACE is a relatively nonspecific peptidase that is capable of cleaving a wide range of substrates. Because of this, ACE and its peptide substrates and products affect many physiologic processes, including blood pressure control, hematopoiesis, reproduction, renal development, renal function, and the immune response. The defining feature of ACE is that it is composed of two homologous and independently catalytic domains, the result of an ancient gene duplication, and ACE-like genes are widely distributed in nature. The two ACE catalytic domains contribute to the wide substrate diversity of ACE and, by extension, the physiologic impact of the enzyme. Several studies suggest that the two catalytic domains have different biologic functions. Recently, the X-ray crystal structure of ACE has elucidated some of the structural differences between the two ACE domains. This is important now that ACE domain-specific inhibitors have been synthesized and characterized. Once widely available, these reagents will undoubtedly be powerful tools for probing the physiologic actions of each ACE domain. In turn, this knowledge should allow clinicians to envision new therapies for diseases not currently treated with ACE inhibitors.

Neuronal brain-derived neurotrophic factor is synthesized in excess, with levels regulated by sortilin-mediated trafficking and lysosomal degradation

Brain-derived neurotrophic factor (BDNF) regulates neuronal differentiation, synaptic plasticity, and morphology, and modest changes in BDNF levels results in complex behavioral phenotypes. BDNF levels and intracellular localization in neurons are regulated by multiple mechanisms, including use of distinct promoters, mRNA and protein transport, and regulated cleavage of proBDNF to mature BDNF. Sortilin is an intracellular chaperone that binds to the prodomain of BDNF to traffic it to the regulated secretory pathway. However, sortilin binds to numerous ligands and plays a major role in mannose 6-phosphate receptor-independent transport of lysosomal hydrolases utilizing motifs in the intracellular domain that mediate trafficking from the Golgi and late endosomes. Sortilin is modified by ectodomain shedding, although the biological implications of this are not known. Here we demonstrate that ADAM10 is the preferred protease to cleave sortilin in the extracellular stalk region, to release the ligand binding sortilin ectodomain from the transmembrane and cytoplasmic domains. We identify sortilin shedding at the cell surface and in an intracellular compartment. Both sortilin and BDNF are trafficked to and degraded by the lysosome in neurons, and this is dependent upon the sortilin cytoplasmic tail. Indeed, expression of the sortilin ectodomain, which corresponds to the domain released after shedding, impairs lysosomal targeting and degradation of BDNF. These findings characterize the regulation of sortilin shedding and identify a novel mechanism by which sortilin ectodomain shedding acts as a regulatory switch for delivery of BDNF to the secretory pathway or to the lysosome, thus modulating the bioavailability of endogenous BDNF.

Human cytomegalovirus UL7, a homologue of the SLAM-family receptor CD229, impairs cytokine production

Human cytomegalovirus (HCMV), the β-herpesvirus prototype, has evolved a wide spectrum of mechanisms to counteract host immunity. Among them, HCMV uses cellular captured genes encoding molecules capable of interfering with the original host function or of fulfilling new immunomodulatory tasks. Here, we report on UL7, a novel HCMV heavily glycosylated transmembrane protein, containing an Ig-like domain that exhibits remarkable amino acid similarity to CD229, a cell-surface molecule of the signalling lymphocyte-activation molecule (SLAM) family involved in leukocyte activation. The UL7 Ig-like domain, which is well-preserved in all HCMV strains, structurally resembles the SLAM-family N-terminal Ig-variable domain responsible for the homophilic and heterophilic interactions that trigger signalling. UL7 is transcribed with early-late kinetics during the lytic infectious cycle. Using a mAb generated against the viral protein, we show that it is constitutively shed, through its mucine-like stalk, from the cell-surface. Production of soluble UL7 is enhanced by PMA and reduced by a broad-spectrum metalloproteinase inhibitor. Although UL7 does not hold the ability to interact with CD229 or other SLAM-family members, it shares with them the capacity to mediate adhesion to leukocytes, specifically to monocyte-derived DCs. Furthermore, we demonstrate that UL7 expression attenuates the production of proinflammatory cytokines TNF, IL-8 and IL-6 in DCs and myeloid cell lines. Thus, the ability of UL7 to interfere with cellular proinflammatory responses may contribute to viral persistence. These results enhance our understanding of those HCMV-encoded molecules involved in sustaining the balance between HCMV and the host immune system.

Updated biological roles for matrix metalloproteinases and new "intracellular" substrates revealed by degradomics

Shotgun proteomics techniques are conceptually unbiased, but data interpretation and follow-up experiments are often constrained by dogma, established beliefs that are accepted without question, that can dilute the power of proteomics and hinder scientific progress. Proteomics and degradomics, the characterization of all proteases, inhibitors, and protease substrates by genomic and proteomic techniques, have exponentially expanded the known substrate repertoire of the matrix metalloproteinases (MMPs), even to include intracellular proteins with newly recognized extracellular functions. Thus, the dogma that MMPs are dowdy degraders of extracellular matrix has been resolutely overturned, and the metamorphosis of MMPs into modulators of multiple signaling pathways has been facilitated. Here we review progress made in the field of degradomics and present a current view of the MMP degradome.

Cleavage of the human thyrotropin receptor by ADAM10 is regulated by thyrotropin

The thyrotropin receptor (TSHR) has a unique 50 residue (317-366) ectodomain insertion that sets it apart from other glycoprotein hormone receptors (GPHRs). Other ancient members of the leucine-rich repeat G protein-coupled receptor (GPCR) (LGR) family do exhibit ectodomain insertions of variable lengths and sequences. The TSHR-specific insert is digested, apparently spontaneously, to release the ectodomain (A-subunit) leaving the balance of the ectodomain attached to the serpentine (B-subunit). Despite concerted efforts for the last 12 years by many laboratories, the enzyme involved in TSHR cleavage has not been identified and a physiologic role for this process remains unclear. Several lines of evidence had suggested that the TSHR protease is likely a member of the a disintegrin and metalloprotease (ADAM) family of metalloproteases. We show here that the expression of ADAM10 was specific to the thyroid by specially designed DNA microarrays. We also show that TSH increases TSHR cleavage in a dose-dependent manner. To prove that ADAM10 is indeed the TSHR cleavage enzyme, we investigated the effect of TSH-induced cleavage by a peptide based on a motif (TSHR residues 334-349), shared with known ADAM10 substrates. TSH increased dose dependently TSHR ectodomain cleavage in the presence of wild-type peptide but not a scrambled control peptide. Interestingly, TSH increased the abundance of non-cleaved single chain receptor, as well higher molecular forms of the A-subunit, despite their enhancement of the appearance of the fully digested A-subunit. This TSH-related increase in TSHR digested forms was further increased by wild-type peptide. We have identified for the first time ADAM10 as the TSHR cleavage enzyme and shown that TSH regulates its activation.

Expression, purification, and physicochemical characterization of the N-terminal active site of human angiotensin-I converting enzyme

We have cloned, over expressed, and purified one of the two catalytic domains (residues Ala361 to Gly468, ACE-N) of human somatic angiotensin-I converting enzyme in Escherichia coli. This construct represents the N-catalytic domain including the two binding motifs and the 23 amino acid spacers as well as some amino acid residues before and after the motifs that might help in correct conformation. The overexpressed protein was exclusively localized to insoluble inclusion bodies. Inclusion bodies were solubilized in an 8-M urea buffer. Purification was carried out by differential centrifugation and gel filtration chromatography under denaturing conditions. About 12 mg of ACE-N peptide per liter of bacterial culture was obtained. The integrity of recombinant protein domain was confirmed by ESI/MS. Structural analysis using CD spectroscopy has shown that, in the presence of TFE, the ACE-N protein fragment has taken a conformation, which is consistent with the one found in testis ACE by X-ray crystallography. This purification procedure enables the production of an isotopically labeled protein fragment for structural studying in solution by NMR spectroscopy.

Homologous substitution of ACE C-domain regions with N-domain sequences: effect on processing, shedding, and catalytic properties

Angiotensin-converting enzyme (ACE) exists as two isoforms: somatic ACE (sACE), comprised of two homologous N and C domains, and testis ACE (tACE), comprised of the C domain only. The N and C domains are both active, but show differences in substrate and inhibitor specificity. While both isoforms are shed from the cell surface via a sheddase-mediated cleavage, tACE is shed much more efficiently than sACE. To delineate the regions of tACE that are important in catalytic activity, intracellular processing, and regulated ectodomain shedding, regions of the tACE sequence were replaced with the corresponding N-domain sequence. The resultant chimeras C1-163Ndom-ACE, C417-579Ndom-ACE, and C583-623Ndom-ACE were processed to the cell surface of transfected Chinese hamster ovary (CHO) cells, and were cleaved at the identical site as that of tACE. They also showed acquisition of N-domain-like catalytic properties. Homology modelling of the chimeric proteins revealed structural changes in regions required for tACE-specific catalytic activity. In contrast, C164-416Ndom-ACE and C191-214Ndom-ACE demonstrated defective intracellular processing and were neither enzymatically active nor shed. Therefore, critical elements within region D164-V416 and more specifically I191-T214 are required for the processing, cell-surface targeting, and enzyme activity of tACE, and cannot be substituted for by the homologous N-domain sequence.

Structure of testis ACE glycosylation mutants and evidence for conserved domain movement

Human angiotensin-converting enzyme is an important drug target for which little structural information has been available until recent years. The slow progress in obtaining a crystal structure was due to the problem of surface glycosylation, a difficulty that has thus far been overcome by the use of a glucosidase-1 inhibitor in the tissue culture medium. However, the prohibitive cost of these inhibitors and incomplete glucosidase inhibition makes alternative routes to minimizing the N-glycan heterogeneity desirable. Here, glycosylation in the testis isoform (tACE) has been reduced by Asn-Gln point mutations at N-glycosylation sites, and the crystal structures of mutants having two and four intact sites have been solved to 2.0 A and 2.8 A, respectively. Both mutants show close structural identity with the wild-type. A hinge mechanism is proposed for substrate entry into the active cleft, based on homology to human ACE2 at the levels of sequence and flexibility. This is supported by normal-mode analysis that reveals intrinsic flexibility about the active site of tACE. Subdomain II, containing bound chloride and zinc ions, is found to have greater stability than subdomain I in the structures of three ACE homologues. Crystallizable glycosylation mutants open up new possibilities for cocrystallization studies to aid the design of novel ACE inhibitors.

Porcine germinal angiotensin I-converting enzyme: isolation, characterization and molecular cloning

Germinal angiotensin I-converting enzyme (gACE) was purified to homogeneity from porcine seminal plasma. The molecular weight of the purified enzyme was calculated to be 182,000 on non-denaturing PAGE and 94,000 and 93,000 on SDS-PAGE in the absence and presence of beta-ME, respectively. These findings suggest that the enzyme is composed of two identical subunits in seminal plasma. The K(m), V(max), K(cat) and K(cat)/K(m) values of gACE at optimal pH (pH 7.2) were 680 microM, 1.0 micromol/mg/min, 33.1 s(-1) and 4.87 x 10(4) s(-1) M(-1) for Z-Val-Lys-Met-MCA, respectively. gACE was potently inhibited by EDTA, 1,10-phenanthroline, captopril and lisinopril, and it promptly released the dipeptides His-Leu and Phe-Arg from angiotensin I and bradykinin. Met- and Leu-enkephalins, neuromedine B and beta-neo-endorphin were also good natural substrates for gACE. We determined the structure of gACE cDNA from the porcine testis, and deduced the amino acid sequence of gACE. The cDNA is composed of 2508 bp of nucleotides in length and encodes 745 amino acids in the coding region. The overall homology of amino acid sequences between porcine, human, sheep and rat gACEs is 72.6 to 84.7%. Zinc-binding motif, chloride-binding site and positions of cysteine residues were well conserved.

The N domain of somatic angiotensin-converting enzyme negatively regulates ectodomain shedding and catalytic activity

sACE (somatic angiotensin-converting enzyme) consists of two homologous, N and C domains, whereas the testis isoenzyme [tACE (testis ACE)] consists of a single C domain. Both isoenzymes are shed from the cell surface by a sheddase activity, although sACE is shed much less efficiently than tACE. We hypothesize that the N domain of sACE plays a regulatory role, by occluding a recognition motif on the C domain required for ectodomain shedding and by influencing the catalytic efficiency. To test this, we constructed two mutants: CNdom-ACE and CCdom-ACE. CNdom-ACE was shed less efficiently than sACE, whereas CCdom-ACE was shed as efficiently as tACE. Notably, cleavage occurred both within the stalk and the interdomain bridge in both mutants, suggesting that a sheddase recognition motif resides within the C domain and is capable of directly cleaving at both positions. Analysis of the catalytic properties of the mutants and comparison with sACE and tACE revealed that the k(cat) for sACE and CNdom-ACE was less than or equal to the sum of the kcat values for tACE and the N-domain, suggesting negative co-operativity, whereas the kcat value for the CCdom-ACE suggested positive co-operativity between the two domains. Taken together, the results provide support for (i) the existence of a sheddase recognition motif in the C domain and (ii) molecular flexibility of the N and C domains in sACE, resulting in occlusion of the C-domain recognition motif by the N domain as well as close contact of the two domains during hydrolysis of peptide substrates.

Tumor necrosis factor-alpha convertase (ADAM17) mediates regulated ectodomain shedding of the severe-acute respiratory syndrome-coronavirus (SARS-CoV) receptor, angiotensin-converting enzyme-2 (ACE2)

Angiotensin-converting enzyme-2 (ACE2) is a critical regulator of heart function and a cellular receptor for the causative agent of severe-acute respiratory syndrome (SARS), SARS-CoV (coronavirus). ACE2 is a type I transmembrane protein, with an extracellular N-terminal domain containing the active site and a short intracellular C-terminal tail. A soluble form of ACE2, lacking its cytosolic and transmembrane domains, has been shown to block binding of the SARS-CoV spike protein to its receptor. In this study, we examined the ability of ACE2 to undergo proteolytic shedding and investigated the mechanisms responsible for this shedding event. We demonstrated that ACE2, heterologously expressed in HEK293 cells and endogenously expressed in Huh7 cells, undergoes metalloproteinase-mediated, phorbol ester-inducible ectodomain shedding. By using inhibitors with differing potency toward different members of the ADAM (a disintegrin and metalloproteinase) family of proteases, we identified ADAM17 as a candidate mediator of stimulated ACE2 shedding. Furthermore, ablation of ADAM17 expression using specific small interfering RNA duplexes reduced regulated ACE2 shedding, whereas overexpression of ADAM17 significantly increased shedding. Taken together, these data provided direct evidence for the involvement of ADAM17 in the regulated ectodomain shedding of ACE2. The identification of ADAM17 as the protease responsible for ACE2 shedding may provide new insight into the physiological roles of ACE2.

Monoclonal antibodies 1B3 and 5C8 as probes for monitoring the integrity of the C-terminal end of soluble angiotensin-converting enzyme

Angiotensin-converting enzyme (ACE) is a membrane-anchored ectoprotein that is proteolytically cleaved, yielding an enzymatically active soluble ACE. Two mouse monoclonal antibodies, MAbs 1B3 and 5C8, were generated to the C-terminal part of human soluble ACE. MAb 1B3 recognized the catalytically active ACE, as revealed by ELISA and precipitation assays, whereas Western blotting and immunohistochemisty on paraffin- embedded sections using MAb 5C8 detected denatured ACE. MAb 1B3 showed extensive cross-reactivity, recognizing 15 species out of the 16 tested. The binding of this MAb to ACE was greatly affected by conformational changes induced by adsorption on plastic, formalin fixation, and underglycosylation. Furthermore, MAb 1B3 binding to the mutated ACE (Pro1199Leu substitution in the juxtamembrane region, leading to a fivefold increase in serum ACE level) was markedly decreased. MAb 5C8 detected all the known expression sites of full-size ACE using formalin-fixed and paraffin-embedded human tissues. The sequential epitope for MAb 5C8 is formed by the last 11 amino acid residues of soluble ACE (Pro1193-Arg1203), whereas the conformational epitope for 1B3 is formed by a motif within these 11 amino acid residues and, in addition, by at least one stretch that includes Ala837-His839 located distal to the sequential epitope. Our findings demonstrated that MAbs 1B3 and 5C8 are very useful for the study of ACE shedding, for identification of mutations in stalk regions, and for studying alternatively spliced variants of ACE. In addition, binding of MAb 1B3 is a sensitive determinant of the integrity of soluble ACE.

Severity of coronary artery stenosis is associated with a polymorphism in the CXCL16/SR-PSOX gene

OBJECTIVE

Enhanced expression of CXCL16 has been demonstrated in atherosclerotic plaques and several properties have been attributed to CXCL16 that could influence the atherosclerotic process. CXCL16 exists in transmembrane and soluble forms. The transmembrane form acts as a scavenger receptor for oxidised LDL whereas the soluble form acts a chemoattractant for mainly CD8+ T cells. In addition, the soluble form of CXCL16 influences human aortic smooth muscle cell proliferation in vitro. In the present work, a human molecular genetic approach employing a common polymorphism within exon 4 of CXCL16 (181 Ala>Val) was used to investigate whether CXCL16 may be involved in the development of coronary artery disease. The polymorphism is located within the spacer region between the chemokine and transmembrane region and potentially influences an Ala/Val cleavage site, a site commonly used for the release of chemokines by tumour necrosis factor-alpha converting enzyme.

DESIGN AND SUBJECTS

We first genotyped 387 unselected survivors of a first myocardial infarction aged <60 years and 387 sex- and age-matched controls. A subset of patients (n = 236) was evaluated by quantitative coronary angiography. Secondly, a cohort of 468 patients undergoing percutaneous transluminal coronary angioplasty (PTCA) with stent implantation was genotyped.

RESULTS

No significant difference in allele frequency between patient and controls of the 181 A>V polymorphism was detected. However, the V-allele was associated with increased severity of coronary stenoses. Secondly, the V-allele was associated with smaller minimal luminal diameter in the coronary segment subjected to intervention in a second cohort of patients undergoing PTCA with stent implantation.

CONCLUSIONS

The present work provides evidence that CXCL16 is involved in processes leading to enhanced stenosis in atherosclerotic coronary arteries.

Ectodomain shedding of the glycoprotein GP of Ebola virus

In this study, release of abundant amounts of the Ebola virus (EBOV) surface glycoprotein GP in a soluble form from virus-infected cells was investigated. We demonstrate that the mechanism responsible for the release of GP is ectodomain shedding mediated by cellular sheddases. Proteolytic cleavage taking place at amino-acid position D637 removes the transmembrane anchor and liberates complexes consisting of GP1 and truncated GP2 (GP(2delta)) subunits from the cell surface. We show that tumor necrosis factor alpha-converting enzyme (TACE), a member of the ADAM family of zinc-dependent metalloproteases, is involved in EBOV GP shedding. This finding shows for the first time that virus-encoded surface glycoproteins are substrates for ADAMs. Furthermore, we provide evidence that shed GP is present in significant amounts in the blood of virus-infected animals and that it may play an important role in the pathogenesis of infection by efficiently blocking the activity of virus-neutralizing antibodies.

Deletion of the cytoplasmic domain increases basal shedding of angiotensin-converting enzyme

Ectodomain shedding generates soluble isoforms of cell-surface proteins, including angiotensin-converting enzyme (ACE). Increasing evidence suggests that the juxtamembrane stalk of ACE, where proteolytic cleavage-release occurs, is not the major site of sheddase recognition. The role of the cytoplasmic domain has not been completely defined. We deleted the cytoplasmic domain of human testis ACE and found that this truncation mutant (ACE-DeltaCYT) was shed constitutively from the surface of transfected CHO-K1 cells. Phorbol ester treatment produced only a slight increase in shedding of ACE-DeltaCYT, unlike the marked stimulation seen with wild-type ACE. However, for both wild-type ACE and ACE-DeltaCYT, shedding was inhibited by the peptide hydroxamate TAPI and the major cleavage site was identical, indicating the involvement of similar or identical sheddases. Cytochalasin D markedly increased the basal shedding of wild-type ACE but had little effect on the shedding of ACE-DeltaCYT. These data suggest that the cytoplasmic domain of ACE interacts with the actin cytoskeleton and that this interaction is a negative regulator of ectodomain shedding.

Ace revisited: a new target for structure-based drug design

Angiotensin-converting enzyme (ACE) is a chloride-dependent metalloenzyme that catalyses the hydrolytic cleavage of dipeptides from the carboxyl terminus of many regulatory oligopeptides. ACE is central to the renin–angiotensin system that regulates blood pressure, fluid homeostasis, and renal and vascular function. It is therefore a major target for cardiovascular therapies.

ACE inhibitors (for example, captopril, enalaprilat and lisinopril) have been on the market for more than 20 years. Side effects of treatment with ACE inhibitors include cough and angioedema.

ACE comprises an N- and a C-domain, each containing an active site with distinct substrates and activation properties. The design of domain-selective inhibitors might produce new drugs with improved safety and efficacy — this endeavour will be facilitated by the recent determination of the three-dimensional structure of ACE.

The C-domain seems to be primarily responsible for the regulation of blood pressure. Data indicate that C-domain-selective inhibitors will have less severe side effects than current-generation inhibitors, which generally target both the N- and C-domains.

In contrast to the C-domain, the N-domain seems to have relatively low affinity for the peptides that control blood pressure. It preferentially hydrolyses at least three other physiologically important peptides, so targeted inhibition of the N-domain might have novel therapeutic applications.

Current-generation angiotensin-converting enzyme (ACE) inhibitors are widely used for cardiovascular diseases, including high blood pressure, heart failure, heart attack and kidney failure, and have combined annual sales in excess of US $6 billion. However, the use of these ACE inhibitors, which were developed in the late 1970s and early 1980s, is hampered by common side effects. Moreover, we now know that ACE actually consists of two parts (called the N- and C-domains) that have different functions. Therefore, the design of specific domain-selective ACE inhibitors is expected to produce next-generation drugs that might be safer and more effective. Here we discuss the structural features of current inhibitors and outline how next-generation ACE inhibitors could be designed by using the three-dimensional molecular structure of human testis ACE. The ACE structure provides a unique opportunity for rational drug design, based on a combination of in silico modelling using existing inhibitors as scaffolds and iterative lead optimization to drive the synthetic chemistry.

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

Protein ectodomain shedding, the proteolytic release of the extracellullar domain of membrane-tethered proteins, can dramatically affect the function of cell surface receptors, growth factors, cytokines, and other proteins. In this study, we evaluated the activities involved in ectodomain shedding of p75NTR, a neurotrophin receptor with critical roles in neuronal differentiation and survival. p75NTR is shed in a variety of cell types, including dorsal root ganglia cells and PC12 cells. In Chinese hamster ovary cells, inhibitors of the MEK/ERK and p38 MAP kinase pathways uncovered distinct signaling pathways required for the constitutive and stimulated shedding of p75NTR. Stimulated p75NTR shedding is abrogated in M2 mutant Chinese hamster ovary cells that lack functional tumor necrosis factor-alpha converting enzyme (TACE, also referred to as ADAM17) and in cells isolated from adam17-/- mice, but not in cells from adam9/12/15-/- or adam10-/- mice. Stimulated p75(NTR) shedding is strongly reduced by deletion of 15 amino acid residues in its extracellular membrane-proximal stalk domain. However, similar to other shed proteins, point mutations and overlapping shorter deletions within this region have little or no effect on shedding. Because ectodomain shedding of p75NTR releases a soluble ectodomain and could also be a prerequisite for its regulated intramembrane proteolysis, these findings may have important implications for the functional regulation of p75NTR.

The ectodomain shedding of angiotensin-converting enzyme is independent of its localisation in lipid rafts

Angiotensin-converting enzyme (ACE), a type I integral membrane protein that plays a major role in vasoactive peptide metabolism, is shed from the plasma membrane by proteolytic cleavage within the juxtamembrane stalk. To investigate whether this shedding is regulated by lateral segregation in cholesterol-rich lipid rafts, Chinese hamster ovary cells and human neuroblastoma SH-SY5Y cells were transfected with either wild-type ACE (WT-ACE) or a construct with a glycosylphosphatidylinositol (GPI) anchor attachment signal replacing the transmembrane and cytosolic domains (GPI-ACE). In both cell types, GPI-ACE, but not WT-ACE, was sequestered in caveolin or flotillin-enriched lipid rafts and was released from the cell surface by treatment with phosphatidylinositol-specific phospholipase C. When cells were treated with activators of the protein kinase C signalling cascade (phorbol myristate acetate or carbachol) the shedding of GPI-ACE was stimulated to a similar extent to that of WT-ACE. The release of WT-ACE and GPI-ACE from the cells was inhibited in an identical manner by a range of hydroxamate-based zinc metalloprotease inhibitors. Disruption of lipid rafts by filipin treatment did not alter the shedding of GPI-ACE, and phorbol ester treatment did not alter the distribution of WT-ACE or GPI-ACE between raft and non-raft membrane compartments. These data clearly show that the protein kinase C-stimulated shedding of ACE does not require the transmembrane or cytosolic regions of the protein, and that sequestration in lipid rafts does not regulate the shedding of the protein.

Deglycosylation, processing and crystallization of human testis angiotensin-converting enzyme

Angiotensin I-converting enzyme (ACE) is a highly glycosylated type I integral membrane protein. A series of underglycosylated testicular ACE (tACE) glycoforms, lacking between one and five N-linked glycosylation sites, were used to assess the role of glycosylation in tACE processing, crystallization and enzyme activity. Whereas underglycosylated glycoforms showed differences in expression and processing, their kinetic parameters were similar to that of native tACE. N-glycosylation of Asn-72 or Asn-109 was necessary and sufficient for the production of enzymically active tACE but glycosylation of Asn-90 alone resulted in rapid intracellular degradation. All mutants showed similar levels of phorbol ester stimulation and were solubilized at the same juxtamembrane cleavage site as the native enzyme. Two mutants, tACEDelta36-g1234 and -g13, were successfully crystallized, diffracting to 2.8 and 3.0 A resolution respectively. Furthermore, a truncated, soluble tACE (tACEDelta36NJ), expressed in the presence of the glucosidase-I inhibitor N -butyldeoxynojirimycin, retained the activity of the native enzyme and yielded crystals belonging to the orthorhombic P2(1)2(1)2(1) space group (cell dimensions, a=56.47 A, b=84.90 A, c=133.99 A, alpha=90 degrees, beta=90 degrees and gamma=90 degrees ). These crystals diffracted to 2.0 A resolution. Thus underglycosylated human tACE mutants, lacking O-linked oligosaccharides and most N-linked oligosaccharides or with only simple N-linked oligosaccharides attached throughout the molecule, are suitable for X-ray diffraction studies.

Defining the boundaries of the testis angiotensin I-converting enzyme ectodomain

Numerous cytokines, receptors, and ectoenzymes, including angiotensin I-converting enzyme (ACE), are shed from the cell surface by limited proteolysis at the juxtamembrane stalk region. The membrane-proximal C domain of ACE has been implicated in sheddase-substrate recognition. We mapped the functional boundaries of the testis ACE ectodomain (identical to the C domain of somatic ACE) by progressive deletions from the N- and C-termini and analysing the effects on catalytic activity, stability, and shedding in transfected cells. We found that deletions extending beyond Leu37 at the N-terminus and Trp616 at the C-terminus abolished catalytic activity and shedding, either by disturbing the ectodomain conformation or by inhibiting maturation and surface expression. Based on these data and on sequence alignments, we propose that the boundaries of the ACE ectodomain are Asp40 at the N-terminus and Gly615 at the C-terminus.

Transmodulation of cell surface regulatory molecules via ectodomain shedding

Cell responses to exogenous stimuli often result in a rapid decrease of cell surface density of a wide range of diverse regulatory proteins, receptor and adhesion molecules in particular. This decrease may occur in a ligand-dependent fashion (down-regulation), following endocytosis and degradation by lysosomal proteases, or by down-modulation, where molecules are targeted by endoproteases directly on cell surface. These proteases are recruited by trans-modulating agents, different from ligand, which act via their own receptors and the related intracellularly-generated signals. Endoproteolytic activity determines the release of large portions (shedding) of substrate proteins, called ectodomains, which are usually not ligand-bound, and therefore represent biologically-active molecules. Ectodomain shedding is involved in a number of pathophysiological processes, such as inflammation, cell degeneration and apoptosis, and oncogenesis. Common features of the process, such as the involvement of protein kinase C and of transmembrane metalloproteases, have been identified. In this review, we summarize basic concepts on down-modulation and ectodomain shedding, and provide an update of the issue with respect to: (i) new entries to the list of molecules found involved in the process; (ii) current views about the upstream control of shedding, i.e. the pathways linking the signals triggered by the trans-modulating agents to the activation of endoproteolytic activity on the cell surface.

Point mutation in the stalk of angiotensin-converting enzyme causes a dramatic increase in serum angiotensin-converting enzyme but no cardiovascular disease

BACKGROUND

Angiotensin-converting enzyme (ACE) metabolizes many small peptides and plays a key role in blood pressure regulation. Elevated serum ACE is claimed to be associated with an increased risk for cardiovascular disease. Previously, two families with dramatically increased serum ACE were described, but no systematic survey of affected individuals was performed, and the molecular background of this trait is unknown.

METHODS AND RESULTS

Eight families were identified with autosomal dominant inheritance of a dramatic (5-fold) increase of serum ACE activity. Strikingly, no clinical abnormalities were apparent in the affected subjects. Isolated blood cells were used for genetic and biochemical analysis. The level of ACE expression on the blood leukocytes and dendritic cells and total cell-associated ACE of the affected individuals was similar to that in nonaffected relatives; however membrane-bound mutant ACE was much more efficiently clipped from the cell surface compared with its wild-type counterpart. A point mutation causing Pro1199Leu in the stalk region of the ACE molecule cosegregates with the increase in serum ACE (LOD score, 6.63).

CONCLUSIONS

A point mutation in the stalk region of the ACE protein causes increased shedding, leading to increased serum ACE, whereas cell-bound ACE is unaltered, and affected individuals exhibit no clinical abnormalities. These findings qualify the importance of serum ACE and establish a new determinant of ACE solubilization.

Roles of the juxtamembrane and extracellular domains of angiotensin-converting enzyme in ectodomain shedding

Angiotensin-converting enzyme (ACE) is one of a growing number of integral membrane proteins that is shed from the cell surface through proteolytic cleavage by a secretase. To investigate the requirements for ectodomain shedding, we replaced the glycosylphosphatidylinositol addition sequence in membrane dipeptidase (MDP) - a membrane protein that is not shed - with the juxtamembrane stalk, transmembrane (TM) and cytosolic domains of ACE. The resulting construct, MDP-STM(ACE), was targeted to the cell surface in a glycosylated and enzymically active form, and was shed into the medium. The site of cleavage in MDP-STM(ACE) was identified by MS as the Arg(374)-Ser(375) bond, corresponding to the Arg(1203)-Ser(1204) secretase cleavage site in somatic ACE. The release of MDP-STM(ACE) and ACE from the cells was inhibited in an identical manner by batimastat and two other hydroxamic acid-based zinc metallosecretase inhibitors. In contrast, a construct lacking the juxtamembrane stalk, MDP-TM(ACE), although expressed at the cell surface in an enzymically active form, was not shed, implying that the juxtamembrane stalk is the critical determinant of shedding. However, an additional construct, ACEDeltaC, in which the N-terminal domain of somatic ACE was fused to the stalk, TM and cytosolic domains, was also not shed, despite the presence of a cleavable stalk, implying that in contrast with the C-terminal domain, the N-terminal domain lacks a signal required for shedding. These data are discussed in the context of two classes of secretases that differ in their requirements for recognition of substrate proteins.

A point mutation in the juxtamembrane stalk of human angiotensin I-converting enzyme invokes the action of a distinct secretase

Angiotensin I-converting enzyme (ACE) is one of a number of integral membrane proteins that is proteolytically shed from the cell surface by a zinc metallosecretase. Mutagenesis of Asn(631) to Gln in the juxtamembrane stalk region of ACE resulted in more efficient secretion of the mutant protein (ACE(NQ)) as determined by pulse-chase analysis. In contrast to the wild-type ACE, the cleavage of ACE(NQ) was not blocked by the metallosecretase inhibitor batimastat but by the serine protease inhibitor, 1,3-dichloroisocoumarin. Incubation of the cells at 15 degrees C revealed that ACE(NQ) was cleaved in the endoplasmic reticulum, and mass spectrometric analysis of the secreted form of the protein indicated that it had been cleaved at the Asn(635)-Ser(636) bond, three residues N-terminal to the normal secretase cleavage site at Arg(638)-Ser(639). These data clearly show that a point mutation in the juxtamembrane region of an integral membrane protein can invoke the action of a mechanistically and spatially distinct secretase. In light of this observation, previous data on the effect of mutations in the juxtamembrane stalk of shed proteins being accommodated by a single secretase having a relaxed specificity need to be re-evaluated.

Increased shedding of angiotensin-converting enzyme by a mutation identified in the stalk region

Angiotensin-converting enzyme (ACE), an enzyme that plays a major role in vasoactive peptide metabolism, is a type 1 ectoprotein, which is released from the plasma membrane by a proteolytic cleavage occurring in the stalk sequence adjacent to the membrane anchor. In this study, we have discovered the molecular mechanism underlying the marked increase of plasma ACE levels observed in three unrelated individuals. We have identified a Pro(1199) --> Leu mutation in the juxtamembrane stalk region. In vitro analysis revealed that the shedding of [Leu(1199)]ACE was enhanced compared with wild-type ACE. The solubilization process of [Leu(1199)]ACE was stimulated by phorbol esters and inhibited by compound 3, an inhibitor of ACE-secretase. The results of Western blot analysis were consistent with a cleavage at the major described site (Arg(1203)/Ser(1204)). Two-dimensional structural analysis of ACE showed that the mutated residue was critical for the positioning of a specific loop containing the cleavage site. We therefore propose that a local conformational modification caused by the Pro(1199) --> Leu mutation leads to more accessibility at the stalk region for ACE secretase and is responsible for the enhancement of the cleavage-secretion process. Our results show that different molecular mechanisms are responsible for the common genetic variation of plasma ACE and for its more rare familial elevation.

Recognition sequences and structural elements contribute to shedding susceptibility of membrane proteins

Although regulated ectodomain shedding affects a large panel of structurally and functionally unrelated proteins, little is known about the mechanisms controlling this process. Despite a lack of sequence similarities around cleavage sites, most proteins are shed in response to the stimulation of protein kinase C by phorbol esters. The signal-transducing receptor subunit gp130 is not a substrate of the regulated shedding machinery. We generated several chimaeric proteins of gp130 and the proteins tumour necrosis factor alpha (TNF-alpha), transforming growth factor alpha (TGF-alpha) and interleukin 6 receptor (IL-6R), which are known to be subject to shedding. By exchanging small peptide sequences of gp130 for cleavage-site peptides of TNF-alpha, TGF-alpha and IL-6R we showed that these short sequences conferred susceptibility to spontaneous and phorbol-ester-induced shedding of gp130. Importantly, these chimaeric gp130 proteins were functional, as shown by the phosphorylation of gp130 and the activation of signal transduction and activators of transcription 3 ('STAT3') on stimulation with cytokine. To investigate minimal requirements for shedding, truncated cleavage-site peptides of IL-6R were inserted into gp130. The resulting chimaeras were susceptible to shedding and showed the same cleavage pattern as observed in the chimaeras containing the complete IL-6R cleavage site. Surprisingly, we could also generate cleavable chimaeras by exchanging the juxtamembrane sequence of gp130 for the corresponding region of leukaemia inhibitory factor ('LIF') receptor, a protein that like gp130 is not subject to regulated or spontaneous shedding. Thus it seems that there is no minimal consensus shedding sequence. We speculate that structural changes allow the access of the protease to a membrane-proximal region, leading to shedding of the protein.

Shedding of interleukin-6 receptor and tumor necrosis factor alpha. Contribution of the stalk sequence to the cleavage pattern of transmembrane proteins

A functionally and structurally diverse group of transmembrane proteins including transmembrane forms of mediators or receptors can be proteolytically cleaved to form soluble growth factors or receptors. Recently, the proteolytic activity responsible for pro-tumor necrosis factor alpha (proTNFalpha) processing has been identified and named TACE (TNFalpha converting enzyme). In experiments with TACE deficient (TACE-/-) fibroblasts we found that 4beta-phorbol 12-myristate 13-acetate (PMA)-induced shedding of the interleukin-6 receptor (IL-6R) is strongly reduced. A basal hydroxamate sensitive release of IL-6R, however, could still be detected. This result demonstrates that TACE plays a role in IL-6R processing and that additional metalloproteases might be involved. PMA-induced shedding of IL-6R in TACE deficient mouse fibroblasts could be restored by stable transfection of a TACE cDNA. To characterize differences between shedding of IL-6R and proTNFalpha we generated chimeric IL-6R and proTNFalpha proteins wherein the endogenous cleavage sites (CS) had been replaced by the corresponding region of proTNFalpha and IL-6R, respectively. Interestingly, proTNFalpha chimeric proteins showed only minimal shedding. In contrast, IL-6R chimeras containing the proTNFalpha CS were shed spontaneously, processing was not further induced by PMA. Thus, the cleavage pattern transferred by the introduction of the proTNFalpha CS is similar to that of proTNFalpha itself. We conclude that the amino-acid sequence at the proteolytic CS contributes to the cleavage characteristics of a protein. However, this information alone is not sufficient to transfer cleavability as seen with proTNFalpha chimeras containing the IL-6R CS and which were resistant to shedding.

The release of acetylcholine receptor inducing activity (ARIA) from its transmembrane precursor in transfected fibroblasts

Acetylcholine receptor inducing activity (ARIA) is made by motoneurons and is released at the neuromuscular synapse to stimulate the synthesis of acetylcholine receptors by skeletal muscle. ARIA is derived from a transmembrane precursor (pro-ARIA) via proteolytic cleavage of the ectodomain. We studied requirements in the amino acid sequence at the cleavage site with various substitution and deletion mutations. Wild type (WT) and mutant proteins were transiently expressed in COS cells, and release of ARIA into the conditioned medium was measured by tyrosine phosphorylation of its receptor, p185, in L6 cells. Removal of all potential cleavage sites between the extracellular epidermal growth factor domain and the transmembrane domain by substitution and small deletions (<11 amino acid residues out of 21) did not significantly reduce ARIA release, whereas larger deletions abolished it. We propose that cleavage occurs independently of amino acid sequence at a short distance from the epidermal growth factor domain, unless sterically hindered by the nearby secondary structure. A mutant with shorter cytoplasmic domain ("c" isoform) released significantly less ARIA than the WT ("a" isoform), suggesting that the c isoform may be suitable for signaling through direct cell-cell contact. Alternatively, proteolytic conversion of the a isoform to the c isoform may rapidly down-regulate release of ARIA.

Constitutive and regulated alpha-secretase cleavage of Alzheimer's amyloid precursor protein by a disintegrin metalloprotease

Amyloid beta peptide (Abeta), the principal proteinaceous component of amyloid plaques in brains of Alzheimer's disease patients, is derived by proteolytic cleavage of the amyloid precursor protein (APP). Proteolytic cleavage of APP by a putative alpha-secretase within the Abeta sequence precludes the formation of the amyloidogenic peptides and leads to the release of soluble APPsalpha into the medium. By overexpression of a disintegrin and metalloprotease (ADAM), classified as ADAM 10, in HEK 293 cells, basal and protein kinase C-stimulated alpha-secretase activity was increased severalfold. The proteolytically activated form of ADAM 10 was localized by cell surface biotinylation in the plasma membrane, but the majority of the proenzyme was found in the Golgi. These results support the view that APP is cleaved both at the cell surface and along the secretory pathway. Endogenous alpha-secretase activity was inhibited by a dominant negative form of ADAM 10 with a point mutation in the zinc binding site. Studies with purified ADAM 10 and Abeta fragments confirm the correct alpha-secretase cleavage site and demonstrate a dependence on the substrate's conformation. Our results provide evidence that ADAM 10 has alpha-secretase activity and many properties expected for the proteolytic processing of APP. Increases of its expression and activity might be beneficial for the treatment of Alzheimer's disease.

Inflammatory Cytokines and Vascular Endothelial Growth Factor Stimulate the Release of Soluble Tie Receptor From Human Endothelial Cells Via Metalloprotease Activation

Activation of endothelial cells, important in processes such as angiogenesis, is regulated by cell surface receptors, including those in the tyrosine kinase (RTK) family. Receptor activity, in turn, can be modulated by phosphorylation, turnover, or proteolytic release of a soluble extracellular domain. Previously, we demonstrated that release of soluble tie-1 receptor from endothelial cells by phorbol myristate acetate (PMA) is mediated through protein kinase C and a Ca2+-dependent protease. In this study, the release of soluble tie-1 was shown to be stimulated by inflammatory cytokines and vascular endothelial growth factor (VEGF), but not by growth factors such as basic fibroblast growth factor (bFGF) or transforming growth factor  (TGF). Release of soluble tie by tumor necrosis factor  (TNF) or VEGF occurred within 10 minutes of stimulation and reached maximal levels within 60 minutes. Specificity was shown by fluorescence-activated cell sorting (FACS) analysis; endothelial cells exhibited a significant decrease in cell surface tie-1 expression in response to TNF, whereas expression of epidermal growth factor receptor (EGF-R) and CD31 was stable. In contrast, tie-1 expression on megakaryoblastic UT-7 cells was unaffected by PMA or TNF. Sequence analysis of the cleaved receptor indicated that tie-1 was proteolyzed at the E749/S750 peptide bond in the proximal transmembrane domain. Moreover, the hydroxamic acid derivative BB-24 demonstrated dose-dependent inhibition of cytokine-, PMA-, and VEGF-stimulated shedding, suggesting that the tie-1 protease was a metalloprotease. Protease activity in a tie-1 peptide cleavage assay was (1) associated with endothelial cell membranes, (2) specifically activated in TNF-treated cells, and (3) inhibited by BB-24. Additionally, proliferation of endothelial cells in response to VEGF, but not bFGF, was inhibited by BB-24, suggesting that the release of soluble tie-1 receptor plays a role in VEGF-mediated proliferation. This study demonstrated that the release of soluble tie-1 from endothelial cells is stimulated by inflammatory cytokines and VEGF through the activation of an endothelial membrane-associated metalloprotease.

An essential role for ectodomain shedding in mammalian development

The ectodomains of numerous proteins are released from cells by proteolysis to yield soluble intercellular regulators. The responsible protease, tumor necrosis factor-alpha converting enzyme (TACE), has been identified only in the case when tumor necrosis factor-alpha (TNFalpha) is released. Analyses of cells lacking this metalloproteinase-disintegrin revealed an expanded role for TACE in the processing of other cell surface proteins, including a TNF receptor, the L-selectin adhesion molecule, and transforming growth factor-alpha (TGFalpha). The phenotype of mice lacking TACE suggests an essential role for soluble TGFalpha in normal development and emphasizes the importance of protein ectodomain shedding in vivo.

ADAMs: focus on the protease domain

ADAMs are proteins containing a disintegrin and metalloproteinase domain. Several important insights were provided in the past year regarding ADAM metalloproteinases. ADAM 10 was implicated in the Notch signaling pathway. ADAM 17 was shown to be the long sought after tumor necrosis factor-alpha convertase and the crystal structure of its metalloproteinase domain was determined.

CD23 (FcepsilonRII) release from cell membranes is mediated by a membrane-bound metalloprotease

CD23 (low-affinity IgE receptor, FcepsilonRII) is expressed as a Type II extracellular protein on a variety of cells such as B-cells, monocytes and macrophages and is cleaved from the cell surface to generate several distinct fragments. The expression of CD23 on the cell surface as well as the generation of soluble fragments of CD23 has been shown to be involved in the regulation of IgE synthesis. Here we report that the release of CD23 from the cell surface is mediated by a metalloprotease. An assay utilizing purified CD23 and an neo-epitope antibody specific for one of the known cleavage products is described and used to demonstrate unambiguously the cleavage of CD23 by a distinct protease. Characterization of the mechanism of CD23 processing shows that the protease exists as an integral membrane protein with a functional molecular mass of approx. 63 kDa as determined by gel-filtration chromatography. The CD23-cleaving activity found in enriched plasma membranes from RPMI 8866 cells is inhibited by the metalloprotease inhibitors 1, 10-phenanthroline and imidazole and by the matrix metalloprotease inhibitor batimastat, but not by inhibitors of cysteine proteases, serine proteases or acid proteases. The same or a similar activity that cleaves CD23 to the known 33 kDa fragment and is inhibited by batimastat is present in diverse cell types such as unstimulated fibroblasts and monocytic cell lines not expressing CD23, as well as in the Epstein-Barr virus-transformed B-cell line, RPMI 8866, which constitutively expresses CD23.

Cell surface ectodomain cleavage of human amphiregulin precursor is sensitive to a metalloprotease inhibitor. Release of a predominant N-glycosylated 43-kDa soluble form

Biosynthesis and processing of amphiregulin (AR) have been investigated in human colorectal (HCA-7, Caco-2) and mammary (MCF-7) cancer cell lines, as well as in Madin-Darby canine kidney cells stably expressing various human AR precursor (pro-AR) forms. Both cells expressing endogenous and transfected AR produce multiple cellular and soluble forms of AR with an N-glycosylated 50-kDa pro-AR form being predominant. Our results demonstrate that sequential proteolytic cleavage within the ectodomain of the 50-kDa pro-AR form leads to release of a predominant N-glycosylated 43-kDa soluble AR, as well as the appearance of other cellular and soluble AR forms. Cell surface biotinylation studies using a C-terminal epitope-tagged pro-AR indicate that all cell surface forms are membrane-anchored and support that AR is released by ectodomain cleavage of pro-AR at the plasma membrane. We also show that pro-AR ectodomain cleavage is a regulated process, which can be stimulated by phorbol 12-myristate 13-acetate and inhibited by the metalloprotease inhibitor, batimastat. In addition, we provide evidence that high molecular mass AR forms may retain the full-length N-terminal pro-region, which may influence the biological activities of these forms.

Angiotensin-converting enzyme and male fertility

The angiotensin-converting enzyme (ACE; EC 3.4.15.1) gene (Ace) encodes both a somatic isozyme found in blood and several other tissues, including the epididymis, and a testis-specific isozyme (testis ACE) found only in developing spermatids and mature sperm. We recently used gene targeting to disrupt the gene coding for both ACE isozymes in mice and reported that male homozygous mutants mate normally but have reduced fertility; the mutant females are fertile. Here we explore the male fertility defect. We demonstrate that ACE is important for achieving in vivo fertilization and that sperm from mice lacking both ACE isozymes show defects in transport within the oviducts and in binding to zonae pellucidae. Males generated by gene targeting that lack somatic ACE but retain testis ACE are normally fertile, establishing that somatic ACE in males is not essential for their fertility. Furthermore, male and female mice lacking angiotensinogen have normal fertility, indicating that angiotensin I is not a necessary substrate for testis ACE. Males heterozygous for the mutation inactivating both ACE isozymes sire wild-type and heterozygous offspring at an indistinguishable frequency, indicating no selection against sperm carrying the mutation.

The distal ectodomain of angiotensin-converting enzyme regulates its cleavage-secretion from the cell surface

Angiotensin-converting enzyme (ACE) is a type I ectoprotein that is cleaved off the cell surface by a plasma membrane-bound metalloprotease. However, CD4, another type I ectoprotein does not undergo such cleavage-secretion. In this study, we investigated the structural determinants of the ACE protein that regulate the cleavage-secretion process. Substitution and deletion mutations revealed that the cytoplasmic domain, the transmembrane domain, and the juxtamembrane region encompassing the major and the minor cleavage sites of ACE do not regulate its cleavage. Moreover, a chimeric protein containing the distal extracellular domain of CD4 and the juxtamembrane, transmembrane, and the cytoplasmic domains of ACE, although transported to the cell surface, was not cleavage-secreted. In contrast, the distal extracellular domain of ACE was shown to be the important determinant: a protein containing the distal extracellular domain of ACE and the juxtamembrane, transmembrane, and cytoplasmic domain of CD4 was efficiently cleaved off the cell surface. The chimeric protein was cleaved within the CD4 sequence and the responsible enzymatic activity was inhibited by Compound 3, a relatively specific inhibitor of the ACE secretase activity. These results demonstrate that, in a chimeric protein, the distal extracellular domain of a cleavable protein, such as ACE, can induce a proteolytic cleavage within the juxtamembrane domain of an uncleaved protein such as CD4.

Limited proteolysis of human kidney angiotensin-converting enzyme and generation of catalytically active N- and C-terminal domains

The somatic form of angiotensin converting enzyme is a class I ectoenzyme that is bound to the surface of endothelial calls. It consists of two homologous, catalytic domains of approximately 600 residues each; a juxtamembrane "stalk" region; a transmembrane, hydrophobic sequence; and a 30 residue, C-terminal cytosolic domain. We have used limited proteolysis to probe the structural and functional properties of the enzyme. Endoproteinase Asp-N cleaves both the Thr615-Asp616 and the Leu1219-Asp1220 peptide bonds to generate the two catalytic domains which were isolated by a combination of immunoaffinity and lisinopril Sepharose affinity chromatography. The enzymatic characteristics of the N and C fragments were examined with angiotensin I, hippuryl-His-Leu, and luteinizing hormone-releasing hormone and indicate that both fragments contain catalytically active sites that retain their individual functional integrity.