Structural insights into selective small molecule activation of PKG1α

cGMP-dependent protein kinase I-α (PKG1α) is a target for pulmonary arterial hypertension due to its role in the regulation of smooth muscle function. While most work has focused on regulation of cGMP turnover, we recently described several small molecule tool compounds which were capable of activating PKG1α via a cGMP independent pathway. Selected molecules were crystallized in the presence of PKG1α and were found to bind to an allosteric site proximal to the low-affinity nucleotide binding domain. These molecules act to displace the switch helix and cause activation of PKG1α representing a new mechanism for the activation and control of this critical therapeutic path. The described structures are vital to understanding the function and control of this key regulatory pathway.

Discovery of IACS-52825, a Potent and Selective DLK Inhibitor for Treatment of Chemotherapy-Induced Peripheral Neuropathy

Chemotherapy-induced peripheral neuropathy (CIPN) is a major unmet medical need with limited treatment options. Despite different mechanisms of action, diverse chemotherapeutics can cause CIPN through a converged pathway─an active axon degeneration program that engages the dual leucine zipper kinase (DLK). DLK is a neuronally enriched kinase upstream in the MAPK-JNK cascade, and while it is dormant under physiological conditions, DLK mediates a core mechanism for neuronal injury response under stress conditions, making it an attractive target for treatment of neuronal injury and neurodegenerative diseases. We have developed potent, selective, brain penetrant DLK inhibitors with excellent PK and activity in mouse models of CIPN. Lead compound IACS-52825 (22) showed strongly effective reversal of mechanical allodynia in a mouse model of CIPN and was advanced into preclinical development.

Potent targeted activator of cell kill molecules eliminate cells expressing HIV-1

Antiretroviral therapy inhibits HIV-1 replication but is not curative due to establishment of a persistent reservoir after virus integration into the host genome. Reservoir reduction is therefore an important HIV-1 cure strategy. Some HIV-1 nonnucleoside reverse transcriptase inhibitors induce HIV-1 selective cytotoxicity in vitro but require concentrations far exceeding approved dosages. Focusing on this secondary activity, we found bifunctional compounds with HIV-1-infected cell kill potency at clinically achievable concentrations. These targeted activator of cell kill (TACK) molecules bind the reverse transcriptase-p66 domain of monomeric Gag-Pol and act as allosteric modulators to accelerate dimerization, resulting in HIV-1⁺ cell death through premature intracellular viral protease activation. TACK molecules retain potent antiviral activity and selectively eliminate infected CD4⁺ T cells isolated from people living with HIV-1, supporting an immune-independent clearance strategy.

Optimization and Mechanistic Investigations of Novel Allosteric Activators of PKG1α

Activation of PKG1α is a compelling strategy for the treatment of cardiovascular diseases. As the main effector of cyclic guanosine monophosphate (cGMP), activation of PKG1α induces smooth muscle relaxation in blood vessels, lowers pulmonary blood pressure, prevents platelet aggregation, and protects against cardiac stress. The development of activators has been mostly limited to cGMP mimetics and synthetic peptides. Described herein is the optimization of a piperidine series of small molecules to yield activators that demonstrate in vitro phosphorylation of vasodilator-stimulated phosphoprotein as well as antiproliferative effects in human pulmonary arterial smooth muscle cells. Hydrogen/deuterium exchange mass spectrometry experiments with the small molecule activators revealed a mechanism of action consistent with cGMP-induced activation, and an X-ray co-crystal structure with a construct encompassing the regulatory domains illustrated a binding mode in an allosteric pocket proximal to the low-affinity cyclic nucleotide-binding domain.

Development of a robust crystallization platform for immune receptor TREM2 using a crystallization chaperone strategy

TREM2 has been identified by genomic analysis as a potential and novel target for the treatment of Alzheimer's disease. To enable structure-based screening of potential small molecule therapeutics, we sought to develop a robust crystallization platform for the TREM2 Ig-like domain. A systematic set of constructs containing the structural chaperone, maltose binding protein (MBP), fused to the Ig domain of TREM2, were evaluated in parallel expression and purification, followed by crystallization studies. Using protein crystallization and high-resolution diffraction as a readout, a MBP-TREM2 Ig fusion construct was identified that generates reproducible protein crystals diffracting at 2.0 Å, which makes it suitable for soaking of potential ligands. Importantly, analysis of crystal packing interfaces indicates that most of the surface of the TREM2 Ig domain is available for small molecule binding. A proof of concept co-crystallization study with a small library of fragments validated potential utility of this system for the discovery of new TREM2 therapeutics.

Crystallization of ectonucleotide phosphodiesterase/pyrophosphatase-3 and orientation of the SMB domains in the full-length ectodomain

Ectonucleotide phosphodiesterase/pyrophosphatase-3 (NPP3, ENPP3) is an ATP-hydrolyzing glycoprotein that is located in the extracellular space. The full-length ectodomain of rat NPP3 was expressed in HEK293S GntI- cells, purified using two chromatographic steps and crystallized. Its structure at 2.77 Å resolution reveals that the active-site zinc ions are missing and a large part of the active site and the surrounding residues are flexible. The SMB-like domains have the same orientation in all four molecules in the asymmetric unit. The SMB2 domain is oriented as in NPP2, but the SMB1 domain does not interact with the PDE domain but extends further away from the PDE domain. Deletion of the SMB domains resulted in crystals that diffracted to 2.4 Å resolution and are suitable for substrate-binding studies.

Structure of the Dual-Mode Wnt Regulator Kremen1 and Insight into Ternary Complex Formation with LRP6 and Dickkopf

Kremen 1 and 2 have been identified as co-receptors for Dickkopf (Dkk) proteins, hallmark secreted antagonists of canonical Wnt signaling. We present here three crystal structures of the ectodomain of human Kremen1 (KRM1ECD) at resolutions between 1.9 and 3.2 Å. KRM1ECD emerges as a rigid molecule with tight interactions stabilizing a triangular arrangement of its Kringle, WSC, and CUB structural domains. The structures reveal an unpredicted homology of the WSC domain to hepatocyte growth factor. We further report the general architecture of the ternary complex formed by the Wnt co-receptor Lrp5/6, Dkk, and Krm, determined from a low-resolution complex crystal structure between β-propeller/EGF repeats (PE) 3 and 4 of the Wnt co-receptor LRP6 (LRP6PE3PE4), the cysteine-rich domain 2 (CRD2) of DKK1, and KRM1ECD. DKK1CRD2 is sandwiched between LRP6PE3 and KRM1Kringle-WSC. Modeling studies supported by surface plasmon resonance suggest a direct interaction site between Krm1CUB and Lrp6PE2.

ZNRF3/RNF43--A direct linkage of extracellular recognition and E3 ligase activity to modulate cell surface signalling

The interactions of extracellular ligands with single membrane spanning receptors, such as kinases, typically serve to agonise or antagonise the intracellular activation of signalling pathways. Within the cell, E3 ligases can act to alter the localisation and activity of proteins involved in signalling systems. Structural and functional characterisation of two closely related single membrane spanning molecules, RNF43 and ZNRF3, has recently revealed the receptor-like functionalities of a ligand-binding ectodomain combined with the intracellular architecture and activity of an E3 ligase. This direct link provides a hereto novel mechanism for extracellular control of ubiquitin ligase activity that is used for the modulation of Wnt signalling, a pathway of major importance in embryogenesis, stem cell biology and cancer. In this review we discuss recent findings for the structure and interactions of the extracellular region of RNF43/ZNRF3 and draw parallels with the properties and function of signalling receptor ectodomains.

Crystal structure of R-spondin 2 in complex with the ectodomains of its receptors LGR5 and ZNRF3

The four secreted R-spondin (Rspo1-4) proteins of vertebrates function as stem cell growth factors and potentiate canonical Wnt signalling. Rspo proteins act by cross-linking members of two cell surface receptor families, complexing the stem cell markers LGR4-6 with the Frizzled-specific E3 ubiquitin ligases ZNRF3/RNF43. The consequent internalisation of the ternary LGR-Rspo-E3 complex removes the E3 ligase activity, which otherwise targets the Wnt receptor Frizzled for degradation, and thus enhances Wnt signalling. Multiple combinations of LGR4-6, Rspo1-4 and ZNRF3/RNF43 are possible, implying the existence of generic interaction determinants, but also of specific differences in complex architecture and activity. We present here a high resolution crystal structure of an ectodomain variant of human LGR5 (hLGR5ecto) complexed with a signalling competent fragment of mouse Rspo2 (mRspo2Fu1-Fu2). The structure shows that the particularly potent Rspo2 ligand engages LGR5 in a fashion almost identical to that reported for hRSPO1. Comparison of our hLGR5ecto structure with previously published structures highlights a surprising plasticity of the LGR ectodomains, characterised by a nearly 9° or larger rotation of the N-terminal half of the horseshoe-like fold relative to the C-terminal half. We also report a low resolution hLGR5-mRspo2Fu1-Fu2-mZNRF3ecto ternary complex structure. This crystal structure confirms our previously suggested hypothesis, showing that Rspo proteins cross-link LGRs and ZNRF3 into a 2:2:2 complex, whereas a 1:1:1 complex is formed with RNF43.

Structure and functional properties of Norrin mimic Wnt for signalling with Frizzled4, Lrp5/6, and proteoglycan

Wnt signalling regulates multiple processes including angiogenesis, inflammation, and tumorigenesis. Norrin (Norrie Disease Protein) is a cystine-knot like growth factor. Although unrelated to Wnt, Norrin activates the Wnt/β-catenin pathway. Signal complex formation involves Frizzled4 (Fz4), low-density lipoprotein receptor related protein 5/6 (Lrp5/6), Tetraspanin-12 and glycosaminoglycans (GAGs). Here, we report crystallographic and small-angle X-ray scattering analyses of Norrin in complex with Fz4 cysteine-rich domain (Fz4CRD), of this complex bound with GAG analogues, and of unliganded Norrin and Fz4CRD. Our structural, biophysical and cellular data, map Fz4 and putative Lrp5/6 binding sites to distinct patches on Norrin, and reveal a GAG binding site spanning Norrin and Fz4CRD. These results explain numerous disease-associated mutations. Comparison with the Xenopus Wnt8-mouse Fz8CRD complex reveals Norrin mimics Wnt for Frizzled recognition. The production and characterization of wild-type and mutant Norrins reported here open new avenues for the development of therapeutics to combat abnormal Norrin/Wnt signalling.

The biochemical properties of the Arabidopsis ecto-nucleoside triphosphate diphosphohydrolase AtAPY1 contradict a direct role in purinergic signaling

The Arabidopsis E-NTPDase (ecto-nucleoside triphosphate diphosphohydrolase) AtAPY1 was previously shown to be involved in growth and development, pollen germination and stress responses. It was proposed to perform these functions through regulation of extracellular ATP signals. However, a GFP-tagged version was localized exclusively in the Golgi and did not hydrolyze ATP. In this study, AtAPY1 without the bulky GFP-tag was biochemically characterized with regard to its suggested role in purinergic signaling. Both the full-length protein and a soluble form without the transmembrane domain near the N-terminus were produced in HEK293 cells. Of the twelve nucleotide substrates tested, only three--GDP, IDP and UDP--were hydrolyzed, confirming that ATP was not a substrate of AtAPY1. In addition, the effects of pH, divalent metal ions, known E-NTPDase inhibitors and calmodulin on AtAPY1 activity were analyzed. AtAPY1-GFP extracted from transgenic Arabidopsis seedlings was included in the analyses. All three AtAPY1 versions exhibited very similar biochemical properties. Activity was detectable in a broad pH range, and Ca(2+), Mg(2+) and Mn(2+) were the three most efficient cofactors. Of the inhibitors tested, vanadate was the most potent one. Surprisingly, sulfonamide-based inhibitors shown to inhibit other E-NTPDases and presumed to inhibit AtAPY1 as well were not effective. Calmodulin stimulated the activity of the GFP-tagless membranous and soluble AtAPY1 forms about five-fold, but did not alter their substrate specificities. The apparent Km values obtained with AtAPY1-GFP indicate that AtAPY1 is primarily a GDPase. A putative three-dimensional structural model of the ecto-domain is presented, explaining the potent inhibitory potential of vanadate and predicting the binding mode of GDP. The found substrate specificity classifies AtAPY1 as a nucleoside diphosphatase typical of N-terminally anchored Golgi E-NTPDases and negates a direct function in purinergic signaling.

Notum is required for neural and head induction via Wnt deacylation, oxidation, and inactivation

Secreted Wnt morphogens are essential for embryogenesis and homeostasis and require a lipid/palmitoleoylate modification for receptor binding and activity. Notum is a secreted Wnt antagonist that belongs to the α/β hydrolase superfamily, but its mechanism of action and roles in vertebrate embryogenesis are not fully understood. Here, we report that Notum hydrolyzes the Wnt palmitoleoylate adduct extracellularly, resulting in inactivated Wnt proteins that form oxidized oligomers incapable of receptor binding. Thus, Notum is a Wnt deacylase, and palmitoleoylation is obligatory for the Wnt structure that maintains its active monomeric conformation. Notum is expressed in naive ectoderm and neural plate in Xenopus and is required for neural and head induction. These findings suggest that Notum is a prerequisite for the "default" neural fate and that distinct mechanisms of Wnt inactivation by the Tiki protease in the Organizer and the Notum deacylase in presumptive neuroectoderm orchestrate vertebrate brain development.

Notum deacylates Wnt proteins to suppress signalling activity

Signalling by Wnt proteins is finely balanced to ensure normal development and tissue homeostasis while avoiding diseases such as cancer. This is achieved in part by Notum, a highly conserved secreted feedback antagonist. Notum has been thought to act as a phospholipase, shedding glypicans and associated Wnt proteins from the cell surface. However, this view fails to explain specificity, as glypicans bind many extracellular ligands. Here we provide genetic evidence in Drosophila that Notum requires glypicans to suppress Wnt signalling, but does not cleave their glycophosphatidylinositol anchor. Structural analyses reveal glycosaminoglycan binding sites on Notum, which probably help Notum to co-localize with Wnt proteins. They also identify, at the active site of human and Drosophila Notum, a large hydrophobic pocket that accommodates palmitoleate. Kinetic and mass spectrometric analyses of human proteins show that Notum is a carboxylesterase that removes an essential palmitoleate moiety from Wnt proteins and thus constitutes the first known extracellular protein deacylase.

Structures of Legionella pneumophila NTPDase1 in complex with polyoxometallates

Nucleoside triphosphate diphosphohydrolases (NTPDases) are secreted or membrane-bound ectonucleotidases that hydrolyze the anhydride bonds of nucleoside triphosphates and nucleoside diphosphates. Mammalian cell-surface NTPDase enzymes are inhibited by various polyoxometallates. Here, the structures of NTPDase1 from the bacterium Legionella pneumophila (LpNTPDase1) in complex with the dodecatungstate POM-1, decavanadate and octamolybdate/heptamolybdate are described. The metal clusters are bound at different sites but always in a highly ordered fashion via electrostatic interactions and hydrogen bonds. For octamolybdate, covalent interactions after oxygen ligand exchange by a serine and histidine side chain are also observed. The potential inhibitory mechanism and the use of the metal clusters as phasing tools for new NTPDase structures are discussed. The binding mode of a tartrate ion at the catalytic centre suggests novel strategies for the structure-based design of NTPDase inhibitors, and the observation of the enzyme in an intermediate open state contributes to our understanding of NTPDase enzyme dynamics.

Detection and characterisation of disseminated tumour cells in bone marrow of breast cancer patients by immunostaining of Her-2 and MUC-1 in combination with Thomsen-Friedenreich (CD176)

Disseminated tumour cells (DTCs) in the bone marrow derive from many primary tumours, such as breast cancer. Their mere existence hints to present or future metastasis and implicates a worse prognosis for the patient. DTCs may possess different characteristics in comparison to the primary tumour due to events like Epithelial-Mesenchymal-Transition. Therefore, these cells might be able to survive chemotherapy and cause relapses of the disease at a later point. We aimed to detect and further characterise DTCs by an immunostaining approach with three different antigen markers (Her-2, MUC-1 and TF, also known as CD 176). For that reason, bone marrow of 41 breast cancer patients was obtained during surgery; DTCs were enriched by density gradient centrifugation and cytospins were prepared. After fixation, immunofluorescent double-stainings were carried out with antibodies against CD176 in combination with HER-2 or MUC-1. Cells co-expressing two antigens were found in all staining combinations (Her-2 and CD176: 46.14%; MUC-1 and CD176: 18.15% of all cases). Cells that stained for a single antigen only were also found (Her-2: 36.86%; MUC-1: 34.45%; CD176: 29.65% of all cases). Significant correlations between the stainings of all markers could be shown (p<0,001). In conclusion, Thomsen-Friedenreich Antigen (TF, CD176) is a promising marker in combination with the established marker Her-2 and other markers like MUC-1. These results may serve as a basis for future DTC detection routines and help to individualize medical treatment, reducing side effects and increasing the efficiency of the therapy.

The ATP/ADP substrate specificity switch between Toxoplasma gondii NTPDase1 and NTPDase3 is caused by an altered mode of binding of the substrate base

Two nucleoside triphosphate diphosphohydrolase isoforms (NTPDase1 and NTPDase3) are responsible for the hydrolysis of nucleotides by the intracellular protozoan Toxoplasma gondii. They constitute about 3 % of the total parasite protein. Despite sharing 97 % sequence identity they exhibit opposite ATP versus ADP substrate discrimination ratios. Here we show by mutagenesis that the residues G492/G493 in NTPDase3 and R492/E493 in NTPDase1 are predominantly responsible for the differences in substrate specificity. Crystal structures of NTPDase1 in complexation with analogues of ATP and ADP reveal that the inverted substrate specificity of NTPDase1 relative to NTPDase3 is achieved by switching from the canonical substrate binding mode to a very different alternative one. Instead of being stacked on top of a helix of the C-terminal domain the nucleotide base is positioned in the interdomain space between the side chains of R108 and R492, recruited from both domains. Furthermore, we show that the NTPDase1 substrate specificity is mainly dependent on the presence of the side chain of E493, which causes repositioning of the ribose component of the nucleotide. All in all, binding by the flexible side chains in the alternative binding mode in NTPDase1 allows for equally good positioning of ATP and ADP with increased activity toward ADP relative to what is seen in the case of NTPDase3.

Crystallographic snapshots along the reaction pathway of nucleoside triphosphate diphosphohydrolases

In vertebrates, membrane-bound ecto-nucleoside triphosphate diphosphohydrolases (NTPDases) on the cell surface are responsible for signal conversion and termination in purinergic signaling by extracellular nucleotides. Here we present apo and complex structures of the rat NTPDase2 extracellular domain and Legionella pneumophila NTPDase1, including a high-resolution structure with a transition-state analog. Comparison of ATP and ADP binding modes shows how NTPDases engage the same catalytic site for hydrolysis of nucleoside triphosphates and diphosphates. We find that this dual specificity is achieved at the expense of base specificity. Structural and mutational studies indicate that a conserved active-site water is replaced by the phosphate product immediately after phosphoryl transfer. Partial base specificity for purines in LpNTPDase1 is based on a different intersubunit base binding site for pyrimidine bases. A comparison of the bacterial enzyme in six independent crystal forms shows that NTPDases can undergo a domain closure motion of at least 17°.

New crystal forms of NTPDase1 from the bacterium Legionella pneumophila

Nucleoside triphosphate diphosphohydrolases (NTPDases) are a large class of nucleotidases that hydrolyze the (γ/β)- and (β/α)-anhydride bonds of nucleoside triphosphates and diphosphates, respectively. NTPDases are found throughout the eukaryotic domain. In addition, a very small number of members can be found in bacteria, most of which live as parasites of eukaryotic hosts. NTPDases of intracellular and extracellular parasites are emerging as important regulators for the survival of the parasite. To deepen the knowledge of the structure and function of this enzyme class, recombinant production of the NTPDase1 from the bacterium Legionella pneumophila has been established. The protein could be crystallized in six crystal forms, of which one has been described previously. The crystals diffracted to resolutions of between 1.4 and 2.5 Å. Experimental phases determined by a sulfur SAD experiment using an orthorhombic crystal form produced an interpretable electron-density map.

Abstract P2-01-11: Detection and characterization of circulating and disseminated tumour cells in blood and bone marrow of breast cancer patients by two different biochemical methods

Background: The main reason for breast cancer associated death are remote metastases, that are due to cells, that dissolve from the primary tumour, and are found in blood (circulating tumour cells, CTCs) and bone marrow (disseminated tumour cells, DTCs). The detection of these cells in breast cancer patients is mostly linked to a worse prognosis.

We present an immunohistochemical staining method for the detection of DTCs from bone marrow by using the cancer-associated Thomsen-Friedenreich antigen (TF) in combination with Her-2, and stem cell markers (Muc-1, ALDH1A1), and a highly sensitive RT-PCR based approach for CTC-detection from peripheral blood of adjuvant breast cancer patients using the established cytokeratin markers CK8, 18 and 19.

Materials and Methods: 20ml blood and 2–3ml of bone marrow were withdrawn from 20 respectively 40 primary breast cancer patients during surgery. For this investigation we used only samples of patients with at least one CTC already detected with the FDA approved VERIDEX CellSearch System.

RT PCR investigations blood samples from healthy donors were taken and used as control group.

For both methods an enrichment of mononuclear cell fraction, containing DTCs and CTCs, was carried out by density gradient centrifugation.

For immunohistochemical staining cells were spun down on coverslips fixed and stained with antibodies against Thomsen-Friedenreich-Antigen and Her-2, MUC1 or ALDH1A1.

For Real-Time PCR RNA was isolated from the cell pellet obtained by density gradient centrifugation, reversely transcribed to cDNA and RT-PCR was run with Taq-Man Primers against Cytokeratin (CK) 8, 18 and 19. The housekeeping gene 18S was used as internal reference.

Results: For the staining of Her-2 and TF, 78,01% of all samples showed at least one stained cell. The biggest subgroup within these samples was the double stained one with 46,14%.

In the other two experimental settings respectively the biggest cohorts were only single stained with TF (47,4% for TF-MUC1) or ALDH1A1 (59,65% for TF-ALDH1A1).

Within the PCR trials, the investigation between the negative control group and adjuvant breast cancer blood samples showed significant correlations for the expressions of CK8 (p &lt; 0,047) and CK18 (p &lt; 0,041). CK19 showed borderline significance (P &lt; 0,057) of correlation.

Conclusion: The immunostaining-combination of diverse cell surface antigens for detection and characterization of DTCs, especially the TF antigen, in combination with Her-2, Muc-1 and ALDH1A1 is an excellent way to detect DTC's in bone marrow due to its significant correlations (p &lt; 0.001).

The verification of CTC's by using RT-PCR is possible by using epithelial marker genes like the cytokeratins, especially CK8 and 18.

Both methods can be a step towards modern personalized medicine and lead to new ways of early tumour detection. Additionally an improved tumour characterization might ease the decision on specific medical treatment (eg against Her-2).

Citation Information: Cancer Res 2012;72(24 Suppl):Abstract nr P2-01-11.

Crystallization and preliminary X-ray analysis of the open form of human ecto-5'-nucleotidase (CD73)

Eukaryotic ecto-5'-nucleotidase (e5NT) catalyses the hydrolysis of extracellular AMP to adenosine and plays a pivotal role in switching on adenosine signalling via the P1 receptors of the purinergic signalling pathway. With such an important regulatory role, e5NT has become an appealing new drug target, with potential applications in the treatment of inflammation, chronic pain, hypoxia and cancer. In order to gain insight into the structure and function of the eukaryotic e5NT enzymes and to assist in structure-based drug design, the crystal structure of human e5NT has been solved. Recombinant human e5NT comprising four asparagine-to-aspartate surface mutations targeting potential glycosylation sites was refolded from bacterial inclusion bodies. Refolded and purified human e5NT crystallized in space group P4(3)32 and a data set to 1.85 Å resolution was obtained. The structure could be solved by molecular replacement using a polyalanine model generated from Thermus thermophilus 5'-nucleotidase (5NT). An anomalous data set revealed the presence of a metal-ion binding site, as well as calcium and chloride ion-binding sites. Structural comparisons with bacterial 5NT homologues showed that the human e5NT crystal structure has an open conformation in which the metal- and substrate-binding sites are distant from each other. Here, the crystallization and preliminary X-ray crystallographic analysis of an open structural conformation of human e5NT are described.

Crystal structure of the human ecto-5'-nucleotidase (CD73): insights into the regulation of purinergic signaling

In vertebrates ecto-5'-nucleotidase (e5NT) catalyzes the hydrolysis of extracellular AMP to adenosine and represents the major control point for extracellular adenosine levels. Due to its pivotal role for activation of P1 adenosine receptors, e5NT has emerged as an appealing drug target for treatment of inflammation, chronic pain, hypoxia, and cancer. Crystal structures of the dimeric human e5NT reveal an extensive 114° conformational switch between the open and closed forms of the enzyme. The dimerization interface is formed by the C-terminal domains and exhibits interchain motions of up to 13°. Complex structures with adenosine and AMPCP indicate that structural control of the domain movement determines the selectivity for monophosphate nucleotides. Binding modes of nucleotide-derived and flavonoid-based compounds complexed to the C-terminal domain in the open form reveal an additional binding pocket of ∼210 Å(3) that might be explored to design more potent inhibitors.

Cellular function and molecular structure of ecto-nucleotidases

Ecto-nucleotidases play a pivotal role in purinergic signal transmission. They hydrolyze extracellular nucleotides and thus can control their availability at purinergic P2 receptors. They generate extracellular nucleosides for cellular reuptake and salvage via nucleoside transporters of the plasma membrane. The extracellular adenosine formed acts as an agonist of purinergic P1 receptors. They also can produce and hydrolyze extracellular inorganic pyrophosphate that is of major relevance in the control of bone mineralization. This review discusses and compares four major groups of ecto-nucleotidases: the ecto-nucleoside triphosphate diphosphohydrolases, ecto-5'-nucleotidase, ecto-nucleotide pyrophosphatase/phosphodiesterases, and alkaline phosphatases. Only recently and based on crystal structures, detailed information regarding the spatial structures and catalytic mechanisms has become available for members of these four ecto-nucleotidase families. This permits detailed predictions of their catalytic mechanisms and a comparison between the individual enzyme groups. The review focuses on the principal biochemical, cell biological, catalytic, and structural properties of the enzymes and provides brief reference to tissue distribution, and physiological and pathophysiological functions.

P1-07-24: Quantitative and Immunohistochemical Detection of Breast Cancer Cells in Blood Samples

Background Disseminated tumor cells (DTCs) in bone marrow and circulating tumor cells (CTCs) in blood are found in patients with epithelial carcinomas (breast cancer) and are often correlated with poor prognosis of the disease. In current models circulating tumor cells (CTCs) dissolved from the primary tumor are thought to be responsible for the occurrence of metastases. However, the detection of CTCs is still a technical challenge. In this study, two methods for tumor cell detection of patients’ samples are presented (Real-Time-PCR and immunohisto-chemical staining). Both are simple and known methods with a high sensitivity and a spread marker panel.

Materials and Methods For the implementation of both methods, different breast cancer cell lines have been used (Cama-1, MCF-7; ZR-75-1). For Real-Time-PCR, blood samples of a healthy donors were spiked with different cell counts (0, 10, 100, 1000, 10.000 and 100.000) per ml blood sample. Total RNA was isolated from the samples, reversely transcribed into cDNA and used for TaqMan Real-Time-PCR reaction with probes against CK8, 18 and 19, while 18S was used as reference. Relative Quantification Curves were drawn by Microsoft™ Excel®. For immunohistochemical staining, cytospins were prepared from spiked blood samples, fixed with acetone, air dried and stained with antibodies against Her2- and Thomsen-Friedenreich-Antigen (CD176). In a second staining step fluorescently labelled secondary antibodies were applied. Nuclei were counterstained with DAPI, TF-Antigen was stained by Cy2 and Her2-Antigen by Cy3. The staining was controlled and documented by an epifluorescence microscope.

Results The curve of Relative Quantification for MCF-7 and ZR-75-1 cells shows an increasing slope starting from 1000 cells. For the Cama-1 cell line this trend is already seen from 10–100 cells. In ZR-75-1 all three genes analysed reveal this trend, whereas in Cama-1 and MCF-7 cells a strong increase in Relative Quantification is especially seen for CK8 and 18. In the immunohistochemical staining, the cells were considered as tumour cells if they showed staining with the antibody-combinations used. Stained cells were counted and recovery rates were determined. For ZR-75-1, 17 of 30 cells which were spiked in the blood samples were recovered. For MCF-7, 18 cells were found in average, and for Cama-1 23 cells were located per slide. The recovery rates calculated from these numbers are 56,6% and 60,0% for ZR-75-1 and MCF-7, for Cama-1 the recovery rate reaches 76,6%.

Conclusion It seems that Cama-1 cells are a better model than MCF-7 and ZR-75-1 for Real-Time PCR quantification of mamma carcinoma tumor cells in blood samples. MCF-7 and ZR-75-1 cells tend to react more likely immunologically with blood cells of the donor (agglutination between blood cells and cancer cells). The Cama-1 cell line shows also advantages in the detection of tumor cells using immunohistochemical staining. Therefore it will be necessary to test both methods on patient samples to proof their benefit.

Citation Information: Cancer Res 2011;71(24 Suppl):Abstract nr P1-07-24.

Structural insight into activation mechanism of Toxoplasma gondii nucleoside triphosphate diphosphohydrolases by disulfide reduction

The intracellular parasite Toxoplasma gondii produces two nucleoside triphosphate diphosphohydrolases (NTPDase1 and -3). These tetrameric, cysteine-rich enzymes require activation by reductive cleavage of a hitherto unknown disulfide bond. Despite a 97% sequence identity, both isozymes differ largely in their ability to hydrolyze ATP and ADP. Here, we present crystal structures of inactive NTPDase3 as an apo form and in complex with the product AMP to resolutions of 2.0 and 2.2 Å, respectively. We find that the enzyme is present in an open conformation that precludes productive substrate binding and catalysis. The cysteine bridge 258-268 is identified to be responsible for locking of activity. Crystal structures of constitutively active variants of NTPDase1 and -3 generated by mutation of Cys(258)-Cys(268) show that opening of the regulatory cysteine bridge induces a pronounced contraction of the whole tetramer. This is accompanied by a 12° domain closure motion resulting in the correct arrangement of all active site residues. A complex structure of activated NTPDase3 with a non-hydrolyzable ATP analog and the cofactor Mg(2+) to a resolution of 2.85 Å indicates that catalytic differences between the NTPDases are primarily dictated by differences in positioning of the adenine base caused by substitution of Arg(492) and Glu(493) in NTPDase1 by glycines in NTPDase3.

The pro-form of BMP-2 interferes with BMP-2 signalling by competing with BMP-2 for IA receptor binding

Pro-forms of growth factors have received increasing attention since it was shown that they can affect both the maturation and functions of mature growth factors. Here, we assessed the biological function of the pro-form of bone morphogenetic protein-2 (BMP-2), a member of the transforming growth factor beta (TGFbeta)/BMP superfamily. The role of the 263 amino acids of the pro-peptide is currently unclear. In order to obtain an insight into the function of the pro-form (proBMP-2), the ability of proBMP-2 to induce alkaline phosphatase (AP), a marker enzyme for cells differentiating into osteoblasts, was tested. Interestingly, in contrast to mature BMP-2, proBMP-2 did not lead to induction of AP. Instead, proBMP-2 inhibited the induction of AP by BMP-2. This result raised the question of whether proBMP-2 may compete with mature BMP-2 for receptor binding. ProBMP-2 was found to bind to the purified extracellular ligand binding domain (ECD) of BMPR-IA, a high-affinity receptor for mature BMP-2, with a similar affinity as mature BMP-2. Binding of proBMP-2 to BMPR-IA was confirmed in cell culture by cross-linking proBMP-2 to BMPR-IA presented on the cell surface. In contrast to this finding, proBMP-2 did not bind to the ECD of BMPR-II. ProBMP-2 also differed from BMP-2 in its capacity to induce p38 and Smad phosphorylation. The data presented here suggest that the pro-domain of BMP-2 can alter the signalling properties of the growth factor by modulating the ability of the mature part to interact with the receptors.

Characterization of Rat NTPDase1, -2, and -3 ectodomains refolded from bacterial inclusion bodies

The ecto-nucleoside triphosphate diphosphohydrolases or NTPDases are a family of membrane-bound enzymes that catalyze the sequential removal of gamma- and beta-phosphate from ATP, ADP, and other nucleotides. NTPDase1, -2, -3, and -8 are the enzymes responsible for signal conversion and termination in purinergic signaling. They are anchored to the cytoplasmic membrane by two transmembrane helices with a large catalytic domain pointing toward the extracellular space. Here we report the first successful expression and purification of the soluble extracellular domains of rat NTPDase1, -2, and -3 from bacterial inclusion bodies. The refolded proteins show characteristics similar to the wild type enzymes, for example in that they are dependent on divalent metal ions for catalysis and hydrolyze a wide variety of nucleoside tri- and diphosphates, whereas the monophosphate AMP is not further degraded. Nucleoside triphosphates are hydrolyzed at a higher rate than the corresponding diphosphates. Other characteristics of the recombinant enzymes however reflect the absence of transmembrane regions and side chain glycosylation. For example all three enzymes are monomeric and only subtly activated by Mg2+ ions as compared to Ca2+ ions. Although having a considerably higher specificity constant kcat/Km for ADP as for ATP, the bacterially expressed variant of NTPDase1 in contrast to its wild type counterpart releases intermediate ADP to a substantial amount. The presented expression system will allow large scale production of active protein suitable for structural studies, development of inhibitors, and even clinical application.