Structural Basis for Regulated Proteolysis by the α-Secretase ADAM10.

Cleavage of membrane-anchored proteins by ADAM (a disintegrin and metalloproteinase) endopeptidases plays a key role in a wide variety of biological signal transduction and protein turnover processes. Among ADAM family members, ADAM10 stands out as particularly important because it is both responsible for regulated proteolysis of Notch receptors and catalyzes the non-amyloidogenic α-secretase cleavage of the Alzheimer's precursor protein (APP). We present here the X-ray crystal structure of the ADAM10 ectodomain, which, together with biochemical and cellular studies, reveals how access to the enzyme active site is regulated. The enzyme adopts an unanticipated architecture in which the C-terminal cysteine-rich domain partially occludes the enzyme active site, preventing unfettered substrate access. Binding of a modulatory antibody to the cysteine-rich domain liberates the catalytic domain from autoinhibition, enhancing enzymatic activity toward a peptide substrate. Together, these studies reveal a mechanism for regulation of ADAM activity and offer a roadmap for its modulation.

Favorable prognostic significance of membranous ß-catenin expression and negative prognostic significance of cytoplasmic ß-catenin expression in pancreatic cancer.

The aim of this study was to investigate the prognostic significance of membranous ß-catenin and cytoplasmic ß-catenin expression in pancreatic cancer patients (pts). One hundred pts with histologically verified exocrine pancreatic ductal adenocarcinoma were retrospectively studied. The membranous ß-catenin, cytoplasmic ß-catenin, and cell nucleus ß-catenin expression were immunohistochemically evaluated. The expression of membranous ß-catenin was <5% in none of the pts, 5-25% in one patient, 26-50% in 2 pts, 51-75% in 14 pts, and >75% in 81 pts. The expression of cytoplasmic ß-catenin was <5% in 34 pts, 5-25% in 42 pts, 26-50% in 18 pts, 51-75% in 3 pts, and >75% in one patient. The expression of ß-catenin in the cell nucleus was negative in all pts. At the time of the last follow-up, 21 pts were alive and 79 pts had died. Median OS was 1.3 (0.4-2.3) years in pts with membranous ß-catenin expression ≤75% and 1.7 (1.3-2.1) years in pts with membranous ß-catenin expression >75% (p=0.045). Median OS was (1.3-2.0) 1.6 years in pts with cytoplasmic ß-catenin expression ≤25% and 0.9 (0.5-1.2) years in pts with cytoplasmic ß-catenin expression >25% (p=0.040). In the univariate Cox proportional hazard models HR (95% CI) was 0.556 (0.311-0.995) in pts with membranous ß-catenin expression >75% (p=0.048) and 2.200 (1.216-3.980) in pts with cytoplasmic ß-catenin expression >25% (p=0.009). The present results indicate a favorable prognostic significance of membranous ß-catenin expression in pancreatic cancer.

The crystal structure of DR6 in complex with the amyloid precursor protein provides insight into death receptor activation.

The amyloid precursor protein (APP) has garnered considerable attention due to its genetic links to Alzheimer's disease. Death receptor 6 (DR6) was recently shown to bind APP via the protein extracellular regions, stimulate axonal pruning, and inhibit synapse formation. Here, we report the crystal structure of the DR6 ectodomain in complex with the E2 domain of APP and show that it supports a model for APP-induced dimerization and activation of cell surface DR6.

Structural and functional analyses reveal promiscuous and species specific use of ephrin receptors by Cedar virus.

Cedar virus (CedV) is a bat-borne henipavirus related to Nipah virus (NiV) and Hendra virus (HeV), zoonotic agents of fatal human disease. CedV receptor-binding protein (G) shares only ∼30% sequence identity with those of NiV and HeV, although they can all use ephrin-B2 as an entry receptor. We demonstrate that CedV also enters cells through additional B- and A-class ephrins (ephrin-B1, ephrin-A2, and ephrin-A5) and report the crystal structure of the CedV G ectodomain alone and in complex with ephrin-B1 or ephrin-B2. The CedV G receptor-binding site is structurally distinct from other henipaviruses, underlying its capability to accommodate additional ephrin receptors. We also show that CedV can enter cells through mouse ephrin-A1 but not human ephrin-A1, which differ by 1 residue in the key contact region. This is evidence of species specific ephrin receptor usage by a henipavirus, and implicates additional ephrin receptors in potential zoonotic transmission.

The Ephb2 Receptor Uses Homotypic, Head-to-Tail Interactions within Its Ectodomain as an Autoinhibitory Control Mechanism.

The Eph receptor tyrosine kinases and their ephrin ligands direct axon pathfinding and neuronal cell migration, as well as mediate many other cell-cell communication events. Their dysfunctional signaling has been shown to lead to various diseases, including cancer. The Ephs and ephrins both localize to the plasma membrane and, upon cell-cell contact, form extensive signaling assemblies at the contact sites. The Ephs and the ephrins are divided into A and B subclasses based on their sequence conservation and affinities for each other. The molecular details of Eph-ephrin recognition have been previously revealed and it has been documented that ephrin binding induces higher-order Eph assemblies, which are essential for full biological activity, via multiple, distinct Eph-Eph interfaces. One Eph-Eph interface type is characterized by a homotypic, head-to-tail interaction between the ligand-binding and the fibronectin domains of two adjacent Eph molecules. While the previous Eph ectodomain structural studies were focused on A class receptors, we now report the crystal structure of the full ectodomain of EphB2, revealing distinct and unique head-to-tail receptor-receptor interactions. The EphB2 structure and structure-based mutagenesis document that EphB2 uses the head-to-tail interactions as a novel autoinhibitory control mechanism for regulating downstream signaling and that these interactions can be modulated by posttranslational modifications.

Tie1-Tie2 interactions mediate functional differences between angiopoietin ligands.

The Tie family of endothelial-specific receptor tyrosine kinases is essential for cell proliferation, migration, and survival during angiogenesis. Despite considerable similarity, experiments with Tie1- or Tie2-deficient mice highlight distinct functions for these receptors in vivo. The Tie2 receptor is further unique with respect to its structurally homologous ligands. Angiopoietin-2 and -3 can function as agonists or antagonists; angiopoietin-1 and -4 are constitutive agonists. To address the role of Tie1 in angiopoietin-mediated Tie2 signaling and determine the basis for the behavior of the individual angiopoietins, we used an in vivo FRET-based proximity assay to monitor Tie1 and -2 localization and association. We provide evidence for Tie1-Tie2 complex formation on the cell surface and identify molecular surface areas essential for receptor-receptor recognition. We further demonstrate that the Tie1-Tie2 interactions are dynamic, inhibitory, and differentially modulated by angiopoietin-1 and -2. Based on the available data, we propose a unified model for angiopoietin-induced Tie2 signaling.

Crystal Structure of the Pre-fusion Nipah Virus Fusion Glycoprotein Reveals a Novel Hexamer-of-Trimers Assembly.

Nipah virus (NiV) is a paramyxovirus that infects host cells through the coordinated efforts of two envelope glycoproteins. The G glycoprotein attaches to cell receptors, triggering the fusion (F) glycoprotein to execute membrane fusion. Here we report the first crystal structure of the pre-fusion form of the NiV-F glycoprotein ectodomain. Interestingly this structure also revealed a hexamer-of-trimers encircling a central axis. Electron tomography of Nipah virus-like particles supported the hexameric pre-fusion model, and biochemical analyses supported the hexamer-of-trimers F assembly in solution. Importantly, structure-assisted site-directed mutagenesis of the interfaces between F trimers highlighted the functional relevance of the hexameric assembly. Shown here, in both cell-cell fusion and virus-cell fusion systems, our results suggested that this hexamer-of-trimers assembly was important during fusion pore formation. We propose that this assembly would stabilize the pre-fusion F conformation prior to cell attachment and facilitate the coordinated transition to a post-fusion conformation of all six F trimers upon triggering of a single trimer. Together, our data reveal a novel and functional pre-fusion architecture of a paramyxoviral fusion glycoprotein.

Association of the combined parameters including the frequency of primary cilia, CD8+ tumor infiltrating lymphocytes and PD-1 expression with the outcome in intestinal cancer.

PURPOSE: Primary cilium (PC) is considered to be a functional homologue of the immune synapse. Microtubule structures, PC of cancer associated fibroblasts and immune synapses between cytotoxic CD8+ tumor infiltrating lymphocytes (TILs) and cancer cells, are regularly found in varying amounts in the microenvironment of solid tumors. The purpose of this study was to find out the potential association and combined prognostic significance of the frequency of PC, PD-1 and CD8+ TILs in patients with intestinal cancer. METHODS: The frequency of PC, programmed cell death protein-1 receptor (PD-1) expression and the frequency of stromal and intraepithelial CD8+TILs were evaluated in samples of colorectal adenocarcinoma (32 patiens) and small bowel cancer (8 patients). RESULTS: The median frequency of PC was 0.25%. The expression of PD1 was <5% in 34 patients, 5-25% in 5 patients and 26-50% in 1 patient. The frequency of stromal CD8+ TILs was negative in 3 patients, <25% in 26, 26-50% in 10 and >50% in 1 patient, respectively. Intraepithelial CD8+ TILs were not detectable in 14, <25% in 24 and 26-50% in 2 patients, respectively. Statistically, the frequency of PC and PD-1 positivity were significantly associated (p=0.004). An association between the PC frequency and intraepithelial CD8+ TILs was of borderline statistical significance (p=0.059). An index combining the frequency of PC and stromal CD8+ TILs, but not the combination of frequency of PC and intraepithelial CD8+ TILs, was of borderline prognostic significance (p=0.067). CONCLUSIONS: The present study provides the first data on the potential association and combined prognostic significance of frequency of PC, PD-1 and CD8+ TILs in patients with intestinal cancer.

Neoadjuvant chemoradiotherapy of rectal carcinoma : Baseline hematologic parameters influencing outcomes.

BACKGROUND: The link between the blood count and a systemic inflammatory response (SIR) is indisputable and well described. Pretreatment hematological parameters may predict the overall clinical outcomes in many types of cancer. Thus, this study aims to systematically evaluate the relationship between baseline blood count levels and treatment response in rectal cancer patients treated with neoadjuvant chemoradiotherapy. PATIENTS AND METHODS: From 2009-2015, 173 patients with locally advanced rectal cancer were retrospectively enrolled in the study and analyzed. The baseline blood count was recorded in all patients 1 week before chemoradiation. Tumor response was evaluated through pathologic findings. Blood count levels which included RBC (red blood cells), Hb (hemoglobin), PLT (platelet count), neutrophil count, WBC (white blood cells), NLR (neutrophil-to-lymphocyte ratio), and PLR (platelet-to-lymphocyte ratio) were analyzed in relation to tumor downstaging, pCR (pathologic complete response), OS (overall survival), and DFS (disease-free survival). RESULTS: Hb levels were associated with a response in logistic regression analysis: pCR (p = 0.05; OR 1.04, 95 % CI 1.00-1.07); T downstaging (p = 0.006; OR 1.03, 95 % CI 1.01-1.05); N downstaging (p = 0.09; OR 1.02, 95 % CI 1.00-1.04); T or N downstaging (p = 0.007; OR 1.04, 95 % CI 1.01-1.07); T and N downstaging (p = 0.02; OR 1.02, 95 % CI 1.00-1.04); Hb and RBC were the most significant parameters influencing OS; PLT was a negative prognostic factor for OS and DFS (p = 0.008 for OS); an NLR value of 2.8 was associated with the greatest significance for OS (p = 0.03) and primary tumor downstaging (p = 0.02). CONCLUSION: Knowledge of pretreatment hematological parameters appears to be an important prognostic factor in patients with rectal carcinoma.

Insights into Eph receptor tyrosine kinase activation from crystal structures of the EphA4 ectodomain and its complex with ephrin-A5.

Eph receptor tyrosine kinases and their ephrin ligands mediate cell signaling during normal and oncogenic development. Eph signaling is initiated in a multistep process leading to the assembly of higher-order Eph/ephrin clusters that set off bidirectional signaling in interacting cells. Eph and ephrins are divided in two subclasses based on their abilities to bind and activate each other and on sequence conservation. EphA4 is an exception to the general rule because it can be activated by both A- and B-class ephrin ligands. Here we present high-resolution structures of the complete EphA4 ectodomain and its complexes with ephrin-A5. The structures reveal how ligand binding promotes conformational changes in the EphA4 ligand-binding domain allowing the formation of signaling clusters at the sites of cell-cell contact. In addition, the structural data, combined with structure-based mutagenesis, reveal a previously undescribed receptor-receptor interaction between the EphA4 ligand-binding and membrane-proximal fibronectin domains, which is functionally important for efficient receptor activation.

Structural basis for angiopoietin-1-mediated signaling initiation.

Angiogenesis is a complex cellular process involving multiple regulatory growth factors and growth factor receptors. Among them, the ligands for the endothelial-specific tunica intima endothelial receptor tyrosine kinase 2 (Tie2) receptor kinase, angiopoietin-1 (Ang1) and Ang2, play essential roles in balancing vessel stability and regression during both developmental and tumor-induced angiogenesis. Despite possessing a high degree of sequence identity, Ang1 and Ang2 have distinct functional roles and cell-signaling characteristics. Here, we present the crystal structures of Ang1 both unbound and in complex with the Tie2 ectodomain. Comparison of the Ang1-containing structures with their Ang2-containing counterparts provide insight into the mechanism of receptor activation and reveal molecular surfaces important for interactions with Tie2 coreceptors and associated signaling proteins. Using structure-based mutagenesis, we identify a loop within the angiopoietin P domain, adjacent to the receptor-binding interface, which confers the specific agonist/antagonist properties of the molecule. We demonstrate using cell-based assays that an Ang2 chimera containing the Ang1 loop sequence behaves functionally similarly to Ang1 as a constitutive Tie2 agonist, able to efficiently dissociate the inhibitory Tie1/Tie2 complex and elicit Tie2 clustering and downstream signaling.

Dermatofibroma-like granular cell tumour: a potential diagnostic pitfall.

Dermatofibroma-like granular cell tumour (GCT) is a rare entity, with only two cases having been described so far. We report another case in a 62-year-old woman, discuss histopathological features, and review other tumours in which granular changes have been observed. Our tumour was composed predominantly of oval-to-spindle granular cells with prominent nucleoli, arranged in short fascicles and storiform pattern, infiltrating around collagen bundles. Immunohistochemical analysis with antibodies against CD31, CD56, CD68, CD117, S-100 protein, inhibin, calretinin, EMA, p53 and MIB-1 was performed, showing expression of CD56, CD68, S-100 protein, inhibin and calretinin. The diagnosis of atypical dermatofibroma-like GCT was made.

Gastric and Colorectal Metastases of Lobular Breast Carcinoma: A Case Report.

BACKGROUND: Occurrence of gastric metastasis as the first symptom of breast carcinoma with a long period of latency before presentation of the primary breast carcinoma is rare. CASE REPORT: A patient with gastric metastasis as the first symptom of lobular breast carcinoma, treated by neoadjuvant preoperative chemoradiotherapy and total gastrectomy, with complete local control. Fourteen months after presentation of the gastric metastasis a primary lobular breast carcinoma was discovered, treated by radiotherapy, chemotherapy and hormonal treatment with complete local response. Twenty-three months after diagnosis of breast cancer multiple colorectal metastases from the breast cancer occurred, which were treated by chemotherapy and hormonal treatment. Eighty-six months after diagnosis of gastric metastasis the patient died due to progression of cancer. CONCLUSIONS: Metastases to gastrointestinal or gynaecological tracts are more likely in invasive lobular carcinoma than invasive ductal cancer. The pathologist should determine whether or not they check estrogen and progesterone receptor status not simply by signet ring cell morphology but also by consideration of clinic-pathological correlation of the patient, such as the presence of a past history of breast cancer, or the colorectal localization of poorly differentiated carcinoma, which may occur less frequently than in the stomach.

Prognostic significance of the frequency of primary cilia in cells of small bowel and colorectal adenocarcinoma.

PURPOSE: The primary cilium is a solitary, sensory, immotile microtubule-based structure that arises from the centrosome and is projected from the surface of most human cell types. It has been hypothesized that primary cilia could serve as a tumor suppressor organelle. The objective of this pilot study was to investigate the presence and frequency of primary cilia in cells of small bowel and colorectal adenocarcinoma and to evaluate the prognostic significance of their frequency. METHODS: The presence of primary cilia in cells in samples of small bowel (8 patients) and colorectal adenocarcinoma (32 patients) was evaluated. The primary cilia of cells were immunofluorescently labeled using primary monoclonal anti-acetylated agr;-tubulin antibody and cell nuclei were labeled using DAPI. RESULTS: Primary cilia were identified in all examined specimens. The median frequency of primary cilia was 0.49% in cells of small bowel cancer and 0.22% in cells in colorectal cancer. Overall survival according to frequency of primary cilia in all intestinal adenocarcinomas was significantly longer in patients with higher frequency (≥ 0.187) than in patients with lower frequency of primary cilia (< 0.187) in univariate analysis (p=0.007) and also in the Cox proportional hazard model (p=0.032). Overall survival according to frequency of primary cilia in colorectal adenocarcinoma was significantly longer in patients with higher frequency (≥ 0.187) than in patients with lower frequency of primary cilia (< 0.187) (p=0.028). CONCLUSIONS: The present pilot study provides the first evidence of the prognostic significance of the frequency of primary cilia in small bowel and colorectal adenocarcinoma. Because of significantly higher median frequency of primary cilia in the rare small bowel adenocarcinoma than in the frequent colorectal adenocarcinoma (p<0.001), the results of this study support a potential role for primary cilia as a biomarker in these types of cancer.

Crystal structure of the Hendra virus attachment G glycoprotein bound to a potent cross-reactive neutralizing human monoclonal antibody.

The henipaviruses, represented by Hendra (HeV) and Nipah (NiV) viruses are highly pathogenic zoonotic paramyxoviruses with uniquely broad host tropisms responsible for repeated outbreaks in Australia, Southeast Asia, India and Bangladesh. The high morbidity and mortality rates associated with infection and lack of licensed antiviral therapies make the henipaviruses a potential biological threat to humans and livestock. Henipavirus entry is initiated by the attachment of the G envelope glycoprotein to host cell membrane receptors. Previously, henipavirus-neutralizing human monoclonal antibodies (hmAb) have been isolated using the HeV-G glycoprotein and a human naïve antibody library. One cross-reactive and receptor-blocking hmAb (m102.4) was recently demonstrated to be an effective post-exposure therapy in two animal models of NiV and HeV infection, has been used in several people on a compassionate use basis, and is currently in development for use in humans. Here, we report the crystal structure of the complex of HeV-G with m102.3, an m102.4 derivative, and describe NiV and HeV escape mutants. This structure provides detailed insight into the mechanism of HeV and NiV neutralization by m102.4, and serves as a blueprint for further optimization of m102.4 as a therapeutic agent and for the development of entry inhibitors and vaccines.

Ephrin-B2 and ephrin-B3 as functional henipavirus receptors.

Members of the ephrin cell-surface protein family interact with the Eph receptors, the largest family of receptor tyrosine kinases, mediating bi-directional signaling during tumorogenesis and various developmental events. Surprisingly, ephrin-B2 and -B3 were recently identified as entry receptors for henipaviruses, emerging zoonotic paramyxoviruses responsible for repeated outbreaks in humans and animals in Australia, Southeast Asia, India and Bangladesh. Nipah virus (NiV) and Hendra virus (HeV) are the only two identified members in the henipavirus genus. While the initial human infection cases came from contact with infected pigs (NiV) or horses (HeV), in the more recent outbreaks of NiV both food-borne and human-to-human transmission were reported. These characteristics, together with high mortality and morbidity rates and lack of effective anti-viral therapies, make the henipaviruses a potential biological-agent threat. Viral entry is an important target for the development of anti-viral drugs. The entry of henipavirus is initiated by the attachment of the viral G envelope glycoprotein to the host cell receptors ephrin-B2 and/or -B3, followed by activation of the F fusion protein, which triggers fusion between the viral envelop and the host membrane. We review recent progress in the study of henipavirus entry, particularly the identification of ephrins as their entry receptors, and the structural characterization of the ephrin/Henipa-G interactions.

Biological and structural characterization of glycosylation on ephrin-A1, a preferred ligand for EphA2 receptor tyrosine kinase.

The EphA2 receptor tyrosine kinase is overexpressed in a number of malignancies and is activated by ephrin ligands, most commonly by ephrin-A1. The crystal structure of the ligand-receptor complex revealed a glycosylation on the Asn-26 of ephrin-A1. Here we report for the first time the significance of the glycosylation in the biology of EphA2 and ephrin-A1. Ephrin-A1 was enzymatically deglycosylated, and its activity was evaluated in several assays using glioblastoma (GBM) cells and recombinant EphA2. We found that deglycosylated ephrin-A1 does not efficiently induce EphA2 receptor internalization and degradation, and does not activate the downstream signaling pathways involved in cell migration and proliferation. Data obtained by surface plasmon resonance confirms that deglycosylated ephrin-A1 does not bind EphA2 with high affinity. Mutations in the glycosylation site on ephrin-A1 result in protein aggregation and mislocalization. Analysis of Eph/ephrin crystal structures reveals an interaction between the ligand's carbohydrates and two residues of EphA2: Asp-78 and Lys-136. These findings suggest that the glycosylation on ephrin-A1 plays a critical role in the binding and activation of the EphA2 receptor.

Eph/ephrin recognition and the role of Eph/ephrin clusters in signaling initiation.

The Eph receptors and their ephrin ligands play crucial roles in a large number of cell-cell interaction events, including those associated with axon pathfinding, neuronal cell migration and vasculogenesis. They are also involved in the patterning of most tissues and overall cell positioning in the development of the vertebrate body plan. The Eph/ephrin signaling system manifests several unique features that differentiate it from other receptor tyrosine kinases, including initiation of bi-directional signaling cascades and the existence of ligand and receptor subclasses displaying promiscuous intra-subclass interactions, but very rare inter-subclass interactions. In this review we briefly discuss these features and focus on recent studies of the unique and expansive high-affinity Eph/ephrin assemblies that form at the sites of cell-cell contact and are required for Eph signaling initiation. This article is part of a Special Issue entitled: Emerging recognition and activation mechanisms of receptor tyrosine kinases.

Generation and characterization of a single-chain anti-EphA2 antibody.

Recombinant antibody phage library technology provides multiple advantages, including that human antibodies can be generated against proteins that are highly conserved between species. We used this technology to isolate and characterize an anti-EphA2 single-chain antibody. We show that the antibody binds the antigen with 1:1 stoichiometry and has high specificity for EphA2. The crystal structure of the complex reveals that the antibody targets the same receptor surface cavity as the ephrin ligand. Specifically, a lengthy CDR-H3 loop protrudes deep into the ligand-binding cavity, with several hydrophobic residues at its tip forming an anchor-like structure buried within the hydrophobic Eph pocket, in a way similar to the ephrin receptor-binding loop in the Eph/ephrin structures. Consequently, the antibody blocks ephrin binding to EphA2. Furthermore, it induces apoptosis and reduces cell proliferation in lymphoma cells lines. Since Ephs are important mediators of tumorigenesis, such antibodies could have applications both in research and therapy.

Antibodies binding the ADAM10 substrate recognition domain inhibit Eph function.

The ADAM10 transmembrane metalloprotease cleaves a variety of cell surface proteins that are important in disease, including ligands for receptor tyrosine kinases of the erbB and Eph families. ADAM10-mediated cleavage of ephrins, the ligands for Eph receptors, is suggested to control Eph/ephrin-mediated cell-cell adhesion and segregation, important during normal developmental processes, and implicated in tumour neo-angiogenesis and metastasis. We previously identified a substrate-binding pocket in the ADAM10 C domain that binds the EphA/ephrin-A complex thereby regulating ephrin cleavage. We have now generated monoclonal antibodies specifically recognising this region of ADAM10, which inhibit ephrin cleavage and Eph/ephrin-mediated cell function, including ephrin-induced Eph receptor internalisation, phosphorylation and Eph-mediated cell segregation. Our studies confirm the important role of ADAM10 in cell-cell interactions mediated by both A- and B-type Eph receptors, and suggest antibodies against the ADAM10 substrate-recognition pocket as promising therapeutic agents, acting by inhibiting cleavage of ephrins and potentially other ADAM10 substrates.