Shedding of plasma membrane proteins

The E-Cadherin Cleavage Associated to Pathogenic Bacteria Infections Can Favor Bacterial Invasion and Transmigration, Dysregulation of the Immune Response and Cancer Induction in Humans

Once bound to the epithelium, pathogenic bacteria have to cross epithelial barriers to invade their human host. In order to achieve this goal, they have to destroy the adherens junctions insured by cell adhesion molecules (CAM), such as E-cadherin (E-cad). The invasive bacteria use more or less sophisticated mechanisms aimed to deregulate CAM genes expression or to modulate the cell-surface expression of CAM proteins, which are otherwise rigorously regulated by a molecular crosstalk essential for homeostasis. Apart from the repression of CAM genes, a drastic decrease in adhesion molecules on human epithelial cells can be obtained by induction of eukaryotic endoproteases named sheddases or through synthesis of their own (prokaryotic) sheddases. Cleavage of CAM by sheddases results in the release of soluble forms of CAM. The overexpression of soluble CAM in body fluids can trigger inflammation and pro-carcinogenic programming leading to tumor induction and metastasis. In addition, the reduction of the surface expression of E-cad on epithelia could be accompanied by an alteration of the anti-bacterial and anti-tumoral immune responses. This immune response dysfunction is likely to occur through the deregulation of immune cells homing, which is controlled at the level of E-cad interaction by surface molecules α_E integrin (CD103) and lectin receptor KLRG1. In this review, we highlight the central role of CAM cell-surface expression during pathogenic microbial invasion, with a particular focus on bacterial-induced cleavage of E-cad. We revisit herein the rapidly growing body of evidence indicating that high levels of soluble E-cad (sE-cad) in patients’ sera could serve as biomarker of bacterial-induced diseases.

Plasma membrane proteomic analysis of human Gastric Cancer tissues: revealing flotillin 1 as a marker for Gastric Cancer

Background

Gastric cancer remains the second leading cause of cancer-related deaths in the world. Successful early gastric cancer detection is hampered by lack of highly sensitive and specific biomarkers. Plasma membrane proteins participate and/or have a central role in the metastatic process of cancer cells and are potentially useful for cancer therapy due to easy accessibility of the targets.

Methods

In the present research, TMT method followed by mass spectrometry analysis was used to compare the relative expression levels of plasma membrane proteins between noncancer and gastric cancer tissues.

Results

Of a total data set that included 501 identified proteins, about 35% of the identified proteins were found to be plasma membrane and associated proteins. Among them, 82 proteins were at least 1.5-fold up- or down-regulated in gastric cancer compared with the adherent normal tissues.

Conclusions

A number of markers (e.g. annexin A6, caveolin 1, epidermal growth factor receptor, integrin beta 4) were previously reported as biomarkers of GC. Additionally, several potential biomarkers participated in endocytosis pathway and integrin signaling pathways were firstly identified as differentially expressed proteins in GC samples. Our findings also supported the notion that flotillin 1 is a potential biomarker that could be exploited for molecular imaging-based detection of gastric cancer. Together, the results show that subcellular proteomics of tumor tissue is a feasible and promising avenue for exploring oncogenesis.

Electronic supplementary material

The online version of this article (doi:10.1186/s12885-015-1343-5) contains supplementary material, which is available to authorized users.

Architecture and function of metallopeptidase catalytic domains

The cleavage of peptide bonds by metallopeptidases (MPs) is essential for life. These ubiquitous enzymes participate in all major physiological processes, and so their deregulation leads to diseases ranging from cancer and metastasis, inflammation, and microbial infection to neurological insults and cardiovascular disorders. MPs cleave their substrates without a covalent intermediate in a single-step reaction involving a solvent molecule, a general base/acid, and a mono- or dinuclear catalytic metal site. Most monometallic MPs comprise a short metal-binding motif (HEXXH), which includes two metal-binding histidines and a general base/acid glutamate, and they are grouped into the zincin tribe of MPs. The latter divides mainly into the gluzincin and metzincin clans. Metzincins consist of globular ∼ 130-270-residue catalytic domains, which are usually preceded by N-terminal pro-segments, typically required for folding and latency maintenance. The catalytic domains are often followed by C-terminal domains for substrate recognition and other protein-protein interactions, anchoring to membranes, oligomerization, and compartmentalization. Metzincin catalytic domains consist of a structurally conserved N-terminal subdomain spanning a five-stranded β-sheet, a backing helix, and an active-site helix. The latter contains most of the metal-binding motif, which is here characteristically extended to HEXXHXXGXX(H,D). Downstream C-terminal subdomains are generally shorter, differ more among metzincins, and mainly share a conserved loop--the Met-turn--and a C-terminal helix. The accumulated structural data from more than 300 deposited structures of the 12 currently characterized metzincin families reviewed here provide detailed knowledge of the molecular features of their catalytic domains, help in our understanding of their working mechanisms, and form the basis for the design of novel drugs.

Specific inhibition of ectodomain shedding of glycoprotein Ibα by targeting its juxtamembrane shedding cleavage site

BACKGROUND

Ectodomain shedding of glycoprotein Ibα (GPIbα), a proteolytic event in which metalloprotease ADAM17 cleaves the Gly464-Val465 bond and releases glycocalicin to the plasma, is considered a critical step in mediating clearance of stored platelets. Supporting evidence has largely come from studies using ADAM17 inhibitors. However, the definitive proof is lacking due to the broad substrate specificity of ADAM17.

AIM

To achieve substrate-specific inhibition of GPIbα shedding.

METHODS

Development of monoclonal antibodies that directly bind the sequence around the GPIbα shedding cleavage site and inhibit GPIbα shedding by blocking ADAM17 access to the cleavage site.

RESULTS

Six anti-GPIbα monoclonal antibodies with varying binding affinities were obtained. The prototypic clone, designated 5G6, and its monomeric Fab fragment bind specifically purified GPIb-IX complex, human platelets, and transgenic murine platelets expressing human GPIbα. The clone 5G6 showed similar inhibitory potency as a widely used shedding inhibitor GM6001 in both constitutive and induced GPIbα shedding in human platelets. It does not recognize mouse GPIbα or inhibit shedding of other platelet receptors. Finally, 5G6 binding displays no detectable effect on platelet activation and aggregation.

CONCLUSIONS

The clone 5G6 specifically inhibits GPIbα shedding with no detectable effect on platelet functions. The method of substrate-specific shedding inhibition by macromolecular binding of the shedding cleavage site can be applicable to many other transmembrane receptors undergoing ectodomain shedding.

A Lactobacillus rhamnosus GG-derived soluble protein, p40, stimulates ligand release from intestinal epithelial cells to transactivate epidermal growth factor receptor

p40, a Lactobacillus rhamnosus GG (LGG)-derived soluble protein, ameliorates intestinal injury and colitis, reduces apoptosis, and preserves barrier function by transactivation of the EGF receptor (EGFR) in intestinal epithelial cells. The aim of this study is to determine the mechanisms by which p40 transactivates the EGFR in intestinal epithelial cells. Here we show that p40-conditioned medium activates EGFR in young adult mouse colon epithelial cells and human colonic epithelial cell line, T84 cells. p40 up-regulates a disintegrin and metalloproteinase domain-containing protein 17 (ADAM17) catalytic activity, and broad spectrum metalloproteinase inhibitors block EGFR transactivation by p40 in these two cell lines. In ADAM17-deficient mouse colonic epithelial (ADAM17(-/-) MCE) cells, p40 transactivation of EGFR is blocked, but can be rescued by re-expression with WT ADAM17. Furthermore, p40 stimulates release of heparin binding (HB)-EGF, but not transforming growth factor (TGF)α or amphiregulin, in young adult mouse colon cells and ADAM17(-/-) MCE cells overexpressing WT ADAM17. Knockdown of HB-EGF expression by siRNA suppresses p40 effects on transactivating EGFR and Akt, preventing apoptosis, and preserving tight junction function. The effects of p40 on HB-EGF release and ADAM17 activation in vivo are examined after administration of p40-containing pectin/zein hydrogel beads to mice. p40 stimulates ADAM17 activity and EGFR activation in colonic epithelial cells and increases HB-EGF levels in blood from WT mice, but not from mice with intestinal epithelial cell-specific ADAM17 deletion. Thus, these data define a mechanism of a probiotic-derived soluble protein in modulating intestinal epithelial cell homeostasis through ADAM17-mediated HB-EGF release, leading to transactivation of EGFR.

Systematic proteomic analysis identifies β-site amyloid precursor protein cleaving enzyme 2 and 1 (BACE2 and BACE1) substrates in pancreatic β-cells

Expansion of functional islet β-cell mass is a physiological process to compensate for increased insulin demand. Deficiency or pharmacological inhibition of the plasma membrane protease BACE2 enhances pancreatic β-cell function and proliferation, and therefore BACE2 is a putative target for the therapeutic intervention under conditions of β-cell loss and dysfunction. To gain a molecular understanding of BACE2 function, we performed a systematic and quantitative proteomic analysis to map the natural substrate repertoire of BACE2 and its homologue BACE1 in β-cells. Loss- and gain-of-function studies of in vitro and in vivo models identified specific and functionally heterogeneous targets. Our analysis revealed non-redundant roles of BACE1/2 in ectodomain shedding with BACE1 regulating a broader and BACE2 a more distinct set of β-cell-enriched substrates including two proteins of the seizure 6 protein family (SEZ6L and SEZ6L2). Lastly, our study provides insights into the global β-cell sheddome and secretome, an important prerequisite to uncover novel mechanisms contributing to β-cell homeostasis and a resource for therapeutic target and biomarker discoveries.

Structural basis for the sheddase function of human meprin β metalloproteinase at the plasma membrane

Ectodomain shedding at the cell surface is a major mechanism to regulate the extracellular and circulatory concentration or the activities of signaling proteins at the plasma membrane. Human meprin β is a 145-kDa disulfide-linked homodimeric multidomain type-I membrane metallopeptidase that sheds membrane-bound cytokines and growth factors, thereby contributing to inflammatory diseases, angiogenesis, and tumor progression. In addition, it cleaves amyloid precursor protein (APP) at the β-secretase site, giving rise to amyloidogenic peptides. We have solved the X-ray crystal structure of a major fragment of the meprin β ectoprotein, the first of a multidomain oligomeric transmembrane sheddase, and of its zymogen. The meprin β dimer displays a compact shape, whose catalytic domain undergoes major rearrangement upon activation, and reveals an exosite and a sugar-rich channel, both of which possibly engage in substrate binding. A plausible structure-derived working mechanism suggests that substrates such as APP are shed close to the plasma membrane surface following an "N-like" chain trace.

Colon-specific delivery of a probiotic-derived soluble protein ameliorates intestinal inflammation in mice through an EGFR-dependent mechanism

Probiotic bacteria can potentially have beneficial effects on the clinical course of several intestinal disorders, but our understanding of probiotic action is limited. We have identified a probiotic bacteria-derived soluble protein, p40, from Lactobacillus rhamnosus GG (LGG), which prevents cytokine-induced apoptosis in intestinal epithelial cells. In the current study, we analyzed the mechanisms by which p40 regulates cellular responses in intestinal epithelial cells and p40's effects on experimental colitis using mouse models. We show that the recombinant p40 protein activated EGFR, leading to Akt activation. Activation of EGFR by p40 was required for inhibition of cytokine-induced apoptosis in intestinal epithelial cells in vitro and ex vivo. Furthermore, we developed a pectin/zein hydrogel bead system to specifically deliver p40 to the mouse colon, which activated EGFR in colon epithelial cells. Administration of p40-containing beads reduced intestinal epithelial apoptosis and disruption of barrier function in the colon epithelium in an EGFR-dependent manner, thereby preventing and treating DSS-induced intestinal injury and acute colitis. Furthermore, p40 activation of EGFR was required for ameliorating colon epithelial cell apoptosis and chronic inflammation in oxazolone-induced colitis. These data define what we believe to be a previously unrecognized mechanism of probiotic-derived soluble proteins in protecting the intestine from injury and inflammation.

ADAM17 (TACE) regulates TGFβ signaling through the cleavage of vasorin

The activity of a variety of extracellular signaling factors is tightly regulated by proteins containing A Disintegrin And a Metalloprotease domain (ADAM) metalloproteases through limited proteolysis. Thus, the identification of ADAM substrates may unveil novel components and mechanisms of cell signaling pathways. We report the identification of the transmembrane protein vasorin (VASN), a transforming growth factor-β (TGFβ) trap, as a substrate of ADAM17. The metalloprotease efficiently generates a soluble fragment encompassing the extracellular domain of VASN. Despite the importance of TGFβ in normal development and tumor progression, the regulation of VASN is completely unknown. Here, we show that only the soluble form of VASN inhibits TGFβ and that the secretion of VASN is tightly controlled by ADAM17. Hence, inhibition of ADAM17 leads to the upregulation of TGFβ signaling. Adding a new level of complexity to the function of ADAM17, we finally show that, through the cleavage of VASN, the metalloprotease controls TGFβ-mediated epithelial-to-mesenchymal transition.

Metastasis-associated C4.4A, a GPI-anchored protein cleaved by ADAM10 and ADAM17

Metalloproteases play a complex role in tumor progression. While the activity of some ADAM, ADAMTS and matrix metalloproteases (MMPs) seems to be protumorigenic, the activity of others seems to prevent tumor progression. The identification of the array of substrates of a given metalloprotease (degradome) seems an adequate approach to predict the effect of the inhibition of a metalloprotease in tumors. Here, we present the proteomic identification of a novel substrate for ADAM10 and -17. We used SILAC (Stable Isotope Labeling by Amino acids in Cell culture), a proteomic technique based on the differential metabolic labeling of cells in different conditions. This was applied to MCF7 cells derived from an invasive mammary tumor, and the same cells expressing shRNAs that knock down ADAM10 or -17. Following this approach, we have identified C4.4A as a substrate to both metalloproteases. Since C4.4A is likely involved in tumor invasion, these results indicate that the cleavage of C4.4A by ADAM10 and ADAM17 contributes to tumor progression.

Gamma-secretase and metalloproteinase activity regulate the distribution of endoplasmic reticulum to hippocampal neuron dendritic spines

The neuronal endoplasmic reticulum (ER) contributes to many physiological and pathological processes in the brain. A subset of dendritic spines on hippocampal neurons contains ER that may contribute to synapse-specific intracellular signaling. Distribution of ER to spines is dynamic, but knowledge of the regulatory mechanisms is lacking. In live cell imaging experiments we now show that cultured hippocampal neurons rapidly lost ER from spines after phorbol ester treatment. ER loss was reduced by inhibiting gamma-secretase (DAPT at 2 microM) and metalloproteinase (TAPI-0 and GM6001 at 4 microM) activity. Inhibition of protein kinase C also diminished loss of ER by preventing exit of ER from spines. Furthermore, gamma-secretase and metalloproteinase inhibition, in the absence of phorbol ester, triggered a dramatic increase in spine ER content. Metalloproteinases and gamma-secretase cleave several transmembrane proteins. Many of these substrates are known to localize to adherens junctions, a structural specialization with which spine ER interacts. One interesting possibility is thus that ER content within spines may be regulated by proteolytic activity affecting adherens junctions. Our data demonstrate a hitherto unknown role for these two proteolytic activities in regulating dynamic aspects of cellular ultrastructure, which is potentially important for cellular calcium homeostasis and several intracellular signaling pathways.

Activity of ulilysin, an archaeal PAPP-A-related gelatinase and IGFBP protease

Human growth and development are conditioned by insulin-like growth factors (IGFs), which have also implications in pathology. Most IGF molecules are sequestered by IGF-binding proteins (IGFBPs) so that exertion of IGF activity requires disturbance of these complexes. This is achieved by proteolysis mediated by IGFBP proteases, among which the best characterised is human PAPP-A, the first member of the pappalysin family of metzincins. We have previously identified and studied the only archaeal homologue found to date, Methanosarcina acetivorans ulilysin. This is a proteolytically functional enzyme encompassing a pappalysin catalytic domain and a pro-domain involved in maintenance of latency of the zymogen, proulilysin. Once activated, the protein hydrolyses IGFBP-2 to -6 and insulin chain beta in vitro. We report here that ulilysin is also active against several other substrates, viz (azo)casein, azoalbumin, and extracellular matrix components. Ulilysin has gelatinolytic but not collagenolytic activity. Moreover, the proteolysis-resistant skeletal proteins actin and elastin are also cleaved, as is fibrinogen, but not plasmin and alpha1-antitrypsin from the blood coagulation cascade. Ulilysin develops optimal activity at pH 7.5 and strictly requires peptide bonds preceding an arginine residue, as determined by means of a novel fluorescence resonance energy transfer assay, thus pointing to biotechnological applications as an enzyme complementary to trypsin.

Regulated proteolytic processing of LRP6 results in release of its intracellular domain

Low-density lipoprotein receptor-related protein 6 (LRP6) is a member of low-density lipoprotein receptor (LDLR) family which cooperates with Frizzled receptors to transduce the canonical Wnt signal. As a critical component of the canonical Wnt pathway, LRP6 is essential for appropriate brain development, however, the mechanism by which LRP6 facilitates Wnt canonical signaling has not been fully elucidated. Interestingly, LRP6 which lacks its extracellular domain can constitutively activate TCF/LEF and potentiate the Wnt signal. Further, the free cytosolic tail of LRP6 interacts directly with glycogen synthase kinase (GSK3) and inhibits GSK3's activity in the Wnt canonical pathway which results in increased TCF/LEF activation. However, whether these truncated forms of LRP6 are physiologically relevant is unclear. Recent studies have shown that other members of the LDLR family undergo gamma-secretase dependent regulated intramembrane proteolysis (RIP). Using independent experimental approaches, we show that LRP6 also undergoes RIP. The extracellular domain of LRP6 is shed and released into the surrounding milieu and the cytoplasmic tail is cleaved by gamma-secretase-like activity to release the intracellular domain. Furthermore, protein kinase C, Wnt 3a and Dickkopf-1 modulate this process. These findings suggest a novel mechanism for LRP6 in Wnt signaling: induction of ectodomain shedding of LRP6, followed by the gamma-secretase involved proteolytic releasing its intracellular domain (ICD) which then binds to GSK3 inhibiting its activity and thus activates the canonical Wnt signaling pathway.

Post-transcriptional up-regulation of ADAM17 upon epidermal growth factor receptor activation and in breast tumors

ADAM17 is a transmembrane metalloprotease involved in the proteolytic release of the extracellular domain of many cell surface molecules, a process known as ectodomain shedding. Despite its likely participation in tumor progression and its current consideration as a therapeutic target, very little is known about the regulation of the expression of ADAM17. Here we show that long term treatment with epidermal growth factor (EGF) leads to a marked increase in the levels of ADAM17. EGF receptor activation does not affect the levels of the mRNA that encodes for, or the rate of synthesis of, ADAM17 but increases its half-life. The effect of EGF is biologically relevant because it increases the shedding of several substrates of ADAM17, including the desmosomal cadherin Dsg-2. Analysis of protein and mRNA levels in mammary tumor samples shows that in vivo the levels of ADAM17 can also be controlled post-transcriptionally. Finally, we show that both the shed extracellular domains of Dsg-2 and ADAM17 are frequently expressed in tumors, further supporting the participation of the metalloprotease in malignant progression.

MT1-MMP shedding involves an ADAM and is independent of its localization in lipid rafts

The membrane type 1-matrix metalloproteinase (MT1-MMP) is a membrane-anchored protease that its entire ectodomain is shed from the cell surface. Here we show that in HT1080 cells MT1-MMP is shed as two soluble forms of approximately 52 and approximately 50kDa. Analyses in purified HT1080 plasma membranes show that release of these species is a two-step time-dependent process that is mediated by integral membrane metalloprotease(s). Differential sensitivity to TIMP-3 inhibition of the shedding process suggests that the second cleavage step leading to the formation of the 50-kDa soluble species is mediated by an ADAM. We also show that shedding of MT1-MMP is independent of its partition into lipid rafts because both wild type and glycosylphosphatidylinositol (GPI)-anchored MT1-MMP are shed. These studies provide new insights into the process of MT1-MMP ectodomain shedding, which may regulate pericellular proteolysis.

Neuronal expression and interaction with the synaptic protein CASK suggest a role for Neph1 and Neph2 in synaptogenesis

Formation, differentiation, and plasticity of synapses require interactions between pre- and postsynaptic partners. Recently, it was shown that the transmembrane immunoglobulin superfamily protein SYG-1 is required for providing synaptic specificity in C. elegans. However, it is unclear whether the mammalian orthologs of SYG-1 are also involved in local cell interactions to determine specificity during synapse formation. We used in situ hybridization, immunohistochemistry, and immunogold electron microscopy to study the temporal and spatial expression of Neph1 and Neph2 in the developing and adult mouse brain. Both proteins show similar patterns with neuronal expression starting around embryonic days 12 and 11, respectively. Expression is strongest in areas of high migratory activity. In the adult brain, Neph1 and Neph2 are predominantly seen in the olfactory nerve layer and the glomerular layer of the olfactory bulb, in the hippocampus, and in Purkinje cells of the cerebellum. At the ultrastructural level, Neph1 and Neph2 are detectable within the dendritic shafts of pyramidal neurons. To a lesser extent, there is also synaptic localization of Neph1 within the stratum pyramidale of the hippocampal CA1 and CA3 region on both pre- and postsynaptic sites. Here it colocalizes with the synaptic scaffolder calmodulin-associated serin/threonin kinase (CASK), and both Neph1 and Neph2 interact with the PDZ domain of CASK via their cytoplasmic tail. Our results show that Neph proteins are expressed in the developing nervous system of mammals and suggest that these proteins may have a conserved function in synapse formation or neurogenesis.

Substrate specificity of a metalloprotease of the pappalysin family revealed by an inhibitor and a product complex

Human pappalysin-1 is a multi-domain metalloprotease engaged in the homeostasis of insulin-like growth factors and the founding member of the pappalysin family within the metzincin clan of metalloproteases. We have recently identified an archaeal relative, ulilysin, encompassing only the protease domain. It is a 262-residue active protease with a novel 3D structure with two subdomains separated by an active-site cleft. Despite negligible overall sequence similarity, noticeable similarity is found with other metzincin prototypes, adamalysins/ADAMs and matrix metalloproteinases. Ulilysin has been crystallised in a product complex with an arginine-valine dipeptide occupying the active-site S(1') and S(2') positions and in a complex with the broad-spectrum hydroxamic acid-based metalloprotease inhibitor, batimastat. This molecule inhibits mature ulilysin with an IC(50) value of 61 microM under the conditions assayed. The binding of batimastat to ulilysin evokes binding to vertebrate matrix metalloproteases but is much weaker. These data give insight into substrate specificity and mechanism of action and inhibition of the novel pappalysin family.

Redox regulation of CD21 shedding involves signaling via PKC and indicates the formation of a juxtamembrane stalk

Soluble CD21 (sCD21), released from the plasma membrane by proteolytic cleavage (shedding) of its extracellular domain (ectodomain) blocks B cell/follicular dendritic cell interaction and activates monocytes. We show here that both serine- and metalloproteases are involved in CD21 shedding. Using the oxidant pervanadate to mimic B cell receptor activation and thiol antioxidants such as N-acetylcysteine (NAC) and glutathione (GSH) we show that CD21 shedding is a redox-regulated process inducible by oxidation presumably through activation of a tyrosine kinase-mediated signal pathway involving protein kinase C (PKC), and by reducing agents that either directly activate the metalloprotease and/or modify intramolecular disulfide bridges within CD21 and thereby facilitate access to the cleavage site. Lack of short consensus repeat 16 (SCR16) abolishes CD21 shedding, and opening of the disulfide bridge between cys-2 (Cys941) and cys-4 (Cys968) of SCR16 is a prerequisite for CD21 shedding. Replacing these cysteines with selenocysteines (thereby changing the redox potential from -180 to -381 mV) results in a loss of inducible CD21 shedding, and removing this bridge by exchanging these cysteines with methionines increases CD21 shedding.

Tumour necrosis factor alpha-converting enzyme mediates ectodomain shedding of Vps10p-domain receptor family members

Several transmembrane molecules are cleaved at juxtamembrane extracellular sites leading to shedding of ectodomains. We analysed shedding of members of the Vps10p-D (Vps10p domain; where Vps is vacuolar protein sorting) family of neuronal type-I receptors with partially overlapping functions, and additional proteolytic events initiated by the shedding. When transfected into CHO (Chinese-hamster ovary) cells (CHO-K1), sorCS1a-sorCS1c isoforms were shed at high rates (approximately 0.61% x min(-1)) that were increased approx. 3-fold upon stimulation with phorbol ester. sorCS1c identified in the cultured neuroblastoma cell line SH-SY5Y was shed similarly. In CHO-K1 transfectants, constitutive and stimulated shedding of sorCS3 also occurred at high rates (0.29% and 1.03% x min(-1)). By comparison, constitutive and stimulated shedding of sorLA occurred at somewhat lower rates (0.07% and 0.48% x min(-1)), whereas sorCS2 and sortilin were shed at very low rates even when stimulated (approximately 0.01% x min(-1)). Except for sorCS2, shedding of the receptors was dramatically reduced in mutant CHO cells (CHO-M2) devoid of active TACE (tumour necrosis factor alpha-converting enzyme), demonstrating that this enzyme accounts for most sheddase activity. The release of sorCS1 and sorLA ectodomains initiated rapid cleavage of the membrane-tethered C-terminal stubs that accumulated only in the presence of gamma-secretase inhibitors. Purified shed sorLA bound several ligands similarly to the entire luminal domain of the receptor, including PDGF-BB (platelet-derived growth factor-BB) and amyloid-beta precursor protein. In addition, PDGF-BB also bound to the luminal domains of sorCS1 and sorCS3. The results suggest that ectodomains shed from a subset of Vps10p-D receptors can function as carrier proteins.

Phagocytosis stimulates mobilization and shedding of intracellular CD16A in human monocytes and macrophages: inhibition by HIV-1 infection

Surface and intracellular staining coupled with flow cytometric analysis was used to show for the first time that human macrophages and a minor subset of peripheral blood monocytes have an internal pool of CD16A, which is mobilized and shed during Fc receptor for immunoglobulin G-mediated phagocytosis. Human immunodeficiency virus type 1 (HIV-1) infection of monocyte-derived macrophages in vitro led to a reduction in the phagocytosis-induced up-regulation in CD16A shedding. These results suggest that monocytes and macrophages may be a source of soluble CD16A, which is elevated in the serum of patients in a variety of disease states and that the mobilization and shedding of CD16A in response to phagocytosis are disrupted by HIV-1 infection.

Calmodulin binds to the cytoplasmic domain of angiotensin-converting enzyme and regulates its phosphorylation and cleavage secretion

The rate of cleavage secretion of the enzymatically active ectodomain of angiotensin-converting enzyme (ACE) is regulated by tyrosine phosphorylation of the protein and by the phorbol ester, phorbol 12-myristate 13-acetate (PMA), an activator of protein kinase C. Here, we report that both calmodulin inhibitor (CaMI) and calmodulin kinase inhibitor could also enhance cleavage secretion of ACE. This effect was accompanied by the dissociation of calmodulin from a specific region within the cytoplasmic domain of ACE to which it had been bound. The same domain of ACE was phosphorylated, and both CaMI and PMA caused dephosphorylation of ACE as well. Mass spectrometric and mutational analyses identified Ser730 as the only phosphorylated residue in the cytoplasmic domain of ACE. The Ser730 --> Ala mutant of ACE was not phosphorylated, but it still bound calmodulin, and its cleavage secretion was enhanced by both CaMI and PMA. Similarly, when Ser730 was replaced by the phosphoserine mimetic, Asp, cleavage secretion of the resultant mutant remained susceptible to the enhancing effect of CaMI and PMA. These results demonstrate that, although CaMI and PMA can enhance both cleavage secretion of ACE and its dephosphorylation, the two effects are not mutually interdependent.

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.