Modulation of RGD sequence motifs regulates disintegrin recognition of alphaIIb beta3 and alpha5 beta1 integrin complexes. Replacement of elegantin alanine-50 with proline, N-terminal to the RGD sequence, diminishes recognition of the alpha5 beta1 complex with restoration induced by Mn2+ cation

Full sequencing and comparison of five venom metalloproteases of Trimeresurus gracilis: The PI-enzyme is most similar to okinalysin but the PIII-enzyme is most similar to Crotalus venom enzymes

The cDNAs encoding the Zn⁺²-metalloproteases (SVMPs) of Trimeresurus gracilis (abbreviated as Tgc), a pitviper endemic to Taiwan, were cloned from venom glands and sequenced. The amino-acid sequences of five novel SVMPs, including one P-III, three P-II and one P-I class enzymes, were thus deduced and subjected to BLAST-analyses. The P-III enzyme (designated as Tgc-PIII) is structurally most similar to the PIII-SVMPs of New World pitvipers but not similar to the PIII-SVMP of Ovophis okinavensis. Sequence-similarity analysis of 22 homologous PIII-SVMPs reveal three major structural subtypes of the pitviper PIII-SVMPs, which possibly have different substrate specificities. In addition, Tgc-PIII and the PI-class SVMP (named Tgc-MP) were isolated from the venom and verified by mass spectrometry. All the three deduced sequences of PII-SVMPs (Tgc-PIIs) contain an abnormal Zn⁺²-binding-site in their catalytic-domain, and an identical "long-disintegrin" domain. The predicted 85-residues disintegrin, gracilisin, bears high similarities to some long-disintegrins of the New-World pitvipers and salmosin3. By BLAST search and comparison, Tgc-MP is 96% similar to okinalysin, the hemorrhagic PI-SVMP of O. okinavensis, rather than any other PI-SVMPs in the databanks. Our results confirm the fast evolution of Tgc-SVMPs as well as their structural similarities to different SVMP-classes of the New-World pitvipers and of O. okinavensis, respectively. The implications of our findings are discussed along with our previous sequence comparisons of venom phospholipases A₂ and ten venom serine proteases of Tgc.

Structure-Function Relationship of the Disintegrin Family: Sequence Signature and Integrin Interaction

Disintegrins are small cysteine-rich proteins found in a variety of snake venom. These proteins selectively modulate integrin function, heterodimeric receptors involved in cell-cell and cell-matrix interaction that are widely studied as therapeutic targets. Snake venom disintegrins emerged from the snake venom metalloproteinase and are classified according to the sequence size and number of disulfide bonds. Evolutive structure and function diversification of disintegrin family involves a stepwise decrease in the polypeptide chain, loss of cysteine residues, and selectivity. Since the structure elucidation of echistatin, the description of the structural properties of disintegrins has allowed the investigation of the mechanisms involved in integrin-cell-extracellular matrix interaction. This review provides an analysis of the structures of all family groups enabling the description of an expanded classification of the disintegrin family in seven groups. Each group presents a particular disulfide pattern and sequence signatures, facilitating the identification of new disintegrins. The classification was based on the disintegrin-like domain of the human metalloproteinase (ADAM-10). We also present the sequence and structural signatures important for disintegrin-integrin interaction, unveiling the relationship between the structure and function of these proteins.

Recombinant and Chimeric Disintegrins in Preclinical Research

Disintegrins are a family of small cysteine-rich peptides, found in a wide variety of snake venoms of different phylogenetic origin. These peptides selectively bind to integrins, which are heterodimeric adhesion receptors that play a fundamental role in the regulation of many physiological and pathological processes, such as hemostasis and tumor metastasis. Most disintegrins interact with integrins through the RGD (Arg-Gly-Asp) sequence loop, resulting in an active site that modulates the integrin activity. Some variations in the tripeptide sequence and the variability in its neighborhood result in a different specificity or affinity toward integrin receptors from platelets, tumor cells or neutrophils. Recombinant forms of these proteins are obtained mainly through Escherichia coli, which is the most common host used for heterologous expression. Advances in the study of the structure-activity relationship and importance of some regions of the molecule, especially the hairpin loop and the C-terminus, rely on approaches such as site-directed mutagenesis and the design and expression of chimeric peptides. This review provides highlights of the biological relevance and contribution of recombinant disintegrins to the understanding of their binding specificity, biological activities and therapeutic potential. The biological and pharmacological relevance on the newest discoveries about this family of integrin-binding proteins are discussed.

Effects of the RGD loop and C-terminus of rhodostomin on regulating integrin αIIbβ3 recognition

Rhodostomin (Rho) is a medium disintegrin containing a ⁴⁸PRGDMP motif. We here showed that Rho proteins with P48A, M52W, and P53N mutations can selectively inhibit integrin αIIbβ3. To study the roles of the RGD loop and C-terminal region in disintegrins, we expressed Rho ⁴⁸PRGDMP and ⁴⁸ARGDWN mutants in Pichia pastoris containing ⁶⁵P, ⁶⁵PR, ⁶⁵PRYH, ⁶⁵PRNGLYG, and ⁶⁵PRNPWNG C-terminal sequences. The effect of C-terminal region on their integrin binding affinities was αIIbβ3 > αvβ3 ≥ α5β1, and the ⁴⁸ARGDWN-⁶⁵PRNPWNG protein was the most selective integrin αIIbβ3 mutant. The ⁴⁸ARGDWN-⁶⁵PRYH, ⁴⁸ARGDWN-⁶⁵PRNGLYG, and ⁴⁸ARGDWN-⁶⁵PRNPWNG mutants had similar activities in inhibiting platelet aggregation and the binding of fibrinogen to platelet. In contrast, ⁴⁸ARGDWN-⁶⁵PRYH and ⁴⁸ARGDWN-⁶⁵PRNGLYG exhibited 2.9- and 3.0-fold decreases in inhibiting cell adhesion in comparison with that of ⁴⁸ARGDWN-⁶⁵PRNPWNG. Based on the results of cell adhesion, platelet aggregation and the binding of fibrinogen to platelet inhibited by ARGDWN mutants, integrin αIIbβ3 bound differently to immobilized and soluble fibrinogen. NMR structural analyses of ⁴⁸ARGDWN-⁶⁵PRYH, ⁴⁸ARGDWN-⁶⁵PRNGLYG, and ⁴⁸ARGDWN-⁶⁵PRNPWNG mutants demonstrated that their C-terminal regions interacted with the RGD loop. In particular, the W52 sidechain of ⁴⁸ARGDWN interacted with H68 of ⁶⁵PRYH, L69 of ⁶⁵PRNGLYG, and N70 of ⁶⁵PRNPWNG, respectively. The docking of the ⁴⁸ARGDWN-⁶⁵PRNPWNG mutant into integrin αIIbβ3 showed that the N70 residue formed hydrogen bonds with the αIIb D159 residue, and the W69 residue formed cation-π interaction with the β3 K125 residue. These results provide the first structural evidence that the interactions between the RGD loop and C-terminus of medium disintegrins depend on their amino acid sequences, resulting in their functional differences in the binding and selectivity of integrins.

Differential array sensing for cancer cell classification and novelty detection

A series of semi-specific peptides reported in the literature to bind various epitopes on cell surfaces were used in a differential sensing array to pattern cell line identity.

Functional analysis of four single (RGDWL, RGDWM, RGDWP, RGDMN) and two double (RGDNM, RGDMP) mutants: The importance of methionine (M) in the functional potency of recombinant mojastin (r-Moj)

We have demonstrated in previous studies that a single amino acid change can alter the activity of the recombinant disintegrin r-Moj. In this study, four r-Moj recombinants containing single mutations (r-Moj-WL, r-Moj-WM, r-Moj-WP, r-Moj-MN) and two containing double mutations (r-Moj-MP and r-Moj-NM) at the binding loop were produced, purified, and tested. All r-Moj-W_, r-Moj-M_, and r-Moj-NM mutant peptides inhibited platelet aggregation at higher potency than r-Moj-D_ mutants. Five of the seven r-Moj peptides inhibited angiogenesis at different levels. Two of the mutant peptides with a methionine at the second position carboxyl of the RGD (r-Moj-WM and r-Moj-NM) were the strongest angiogenesis inhibitors, with r-Moj-WM being the most potent. Recombinant r-Moj-MP and r-Moj-WN failed to inhibit angiogenesis. Only the r-Moj-MP mutant peptide induced apoptosis of SK-Mel-28 cells significantly (p = 0.001). This was confirmed by chromatin condensation. Proliferation of SK-Mel-28 cells was inhibited at high levels (>70%) by all r-Moj mutant peptides. Recombinant r-Moj-MN and r-Moj-WN failed to inhibit cell migration significantly (p > 0.5). Recombinant r-Moj-NM was the strongest cell migration inhibitor (98% ± 0.69), followed by r-Moj-MP (80% ± 2.87), and r-Moj-WM (61.8% ± 5.45). The lowest inhibitor was r-Moj-WL (50% ± 12.16). Our functional data suggest that the most potent r-Moj disintegrins contain a methionine in the first or the second position carboxyl to the RGD.

Joint Antiangiogenic Effect of ATN-161 and Anti-VEGF Antibody in a Rat Model of Early Wet Age-Related Macular Degeneration

The wet form of age-related macular degeneration (AMD) is a leading cause of blindness among elderly Americans and is characterized by abnormal vessel growth, termed choroidal neovascularization (CNV). Integrin α5β1 is a transmembrane receptor that binds matrix macromolecules and proteinases to stimulate angiogenesis. We recently demonstrated that integrin α5β1 plays a critical role in the development of choroidal neovascularization. In this study, we determined the role and underlying mechanisms of integrin α5β1 in angiogenesis in human choroidal endothelial cells and evaluated the antiangiogenic effects of delivering a combination therapy of ATN-161, an integrin α5β1 inhibitor, and an anti-VEGF monoclonal antibody to rats with laser-induced CNV. Vascular endothelial growth factor (VEGF) is a signaling protein that stimulates vasculogenesis and angiogenesis through a pathway that is distinct from the integrin α5β1 signaling pathway. Our results indicate that fibronectin binds to integrin α5β1 and synergizes VEGF-induced angiogenesis via two independent signaling pathways, FN/integrin α5β1/FAK/ERK1/2 and FN/integrin α5β1/FAK/AKT. Integrin α5 knockdown by shRNA inhibits endothelial cell migration, tube formation, and proliferation, while ATN-161 only partially decreases integrin α5 function. Treatment with ATN-161 combined with anti-VEGF antibody showed joint effects in attenuating angiogenesis. In summary, our results provide the first evidence for the mechanisms by which integrin α5β1 is involved in ocular pathological neovascularization in vivo, suggesting that dual inhibition of integrin α5β1 and VEGF may be a promising novel therapeutic strategy for CNV in wet AMD.

High-throughput screening of one-bead-one-compound peptide libraries using intact cells

Screening approaches based on one-bead-one-compound (OBOC) combinatorial libraries have facilitated the discovery of novel peptide ligands for cellular targeting in cancer and other diseases. Recognition of cell surface proteins is optimally achieved using live cells, yet screening intact cell populations is time-consuming and inefficient. Here, we evaluate the Complex Object Parametric Analyzer and Sorter (COPAS) large particle biosorter for high-throughput sorting of bead-bound human cell populations. When a library of RGD-containing peptides was screened against human cancer cells that express αvβ3 integrin, it was found that bead-associated cells are rapidly dissociated when sorted through the COPAS instrument. When the bound cells were reversibly cross-linked onto the beads, however, we demonstrated that cell/bead mixtures can be sorted quickly and accurately. This reversible cross-linking approach is compatible with matrix-assisted laser desorption ionization time-of-flight mass spectrometry-based peptide sequence deconvolution. This approach should allow one to rapidly screen an OBOC library and identify novel peptide ligands against cell surface targets in their native conformation.

Dynamics and functional differences between dendroaspin and rhodostomin: insights into protein scaffolds in integrin recognition

Dendroaspin (Den) and rhodostomin (Rho) are snake venom proteins containing a PRGDMP motif. Although Den and Rho have different 3D structures, they are highly potent integrin inhibitors. To study their structure, function, and dynamics relationships, we expressed Den and Rho in Pichia pastoris. The recombinant Den and Rho inhibited platelet aggregation with the K(I) values of 149.8 and 83.2 nM. Cell adhesion analysis showed that Den was 3.7 times less active than Rho when inhibiting the integrin αIIbβ3 and 2.5 times less active when inhibiting the integrin αvβ3. In contrast, Den and Rho were similarly active when inhibiting the integrin α5β1 with the IC₅₀ values of 239.8 and 256.8 nM. NMR analysis showed that recombinant Den and Rho have different 3D conformations for their arginyl-glycyl-aspartic acid (RGD) motif. However, the comparison with Rho showed that the docking of Den into integrin αvβ3 resulted in a similar number of contacts. Analysis of the dynamic properties of the RGD loop in Den and Rho showed that they also had different dynamic properties. These results demonstrate that protein scaffolds affect the function, structure, and dynamics of their RGD motif.

Exploitation of Cellular Cytoskeletons and Signaling Pathways for Cell Entry by Kaposi's Sarcoma-Associated Herpesvirus and the Closely Related Rhesus Rhadinovirus

As obligate intracellular pathogens, viruses depend on the host cell machinery to complete their life cycle. Kaposi’s sarcoma-associated herpes virus (KSHV) is an oncogenicvirus causally linked to the development of Kaposi’s sarcoma and several other lymphoproliferative malignancies. KSHV entry into cells is tightly regulated by diverse viral and cellular factors. In particular, KSHV actively engages cellular integrins and ubiquitination pathways for successful infection. Emerging evidence suggests that KSHV hijacks both actin and microtubule cytoskeletons at different phases during entry into cells. Here, we review recent findings on the early events during primary infection of KSHV and its closely related primate homolog rhesus rhadinovirus with highlights on the regulation of cellular cytoskeletons and signaling pathways that are important for this phase of virus life cycle.

Transferable cell-secreted extracellular matrices enhance osteogenic differentiation

The coating of synthetic biomaterials with cell-derived decellularized extracellular matrices (DMs) represents a promising approach to confer bioactivity to otherwise inert materials and direct cell fate of host or transplanted cells. These coatings are typically deposited on biomaterials by culturing matrix-depositing cells for a sufficient duration on the target, followed by decellularization of the substrate. We hypothesized that DMs created in monolayer culture could be collected and then transferred to a secondary substrate while retaining their instructive potential. Transferred decellularized matrices (tDMs) were created by culturing human mesenchymal stem cells (hMSCs) on tissue culture plastic (TCP) under a controlled microenvironment to deposit a highly osteogenic DM, followed by collection, mechanical homogenization and transfer to a secondary culture surface. We then investigated its capacity to accelerate naïve hMSC osteogenic differentiation by quantifying gene expression, intracellular alkaline phosphatase production, and calcium deposition when cultured on DMs or tDMs. All markers were significantly higher in hMSCs seeded on DMs or tDMs compared to cells on TCP. The osteogenic response of naïve hMSCs to tDMs was dose dependent. We observed a reduction in ERK phosphorylation in hMSCs, as well as a possible role of the cell surface integrin α2β1, when probing the mode of efficacy for tDMs. This study represents a proof-of-principle that cell-derived matrix coatings can be deposited and effectively transferred while retaining the ability to instruct cell phenotype, thus offering a novel approach toward the development of hybrid biomaterials that mimic the complex interactions between cells and the extracellular matrix.

Effect of P to A mutation of the N-terminal residue adjacent to the Rgd motif on rhodostomin: importance of dynamics in integrin recognition

Rhodostomin (Rho) is an RGD protein that specifically inhibits integrins. We found that Rho mutants with the P48A mutation 4.4-11.5 times more actively inhibited integrin α5β1. Structural analysis showed that they have a similar 3D conformation for the RGD loop. Docking analysis also showed no difference between their interactions with integrin α5β1. However, the backbone dynamics of RGD residues were different. The values of the R(2) relaxation parameter for Rho residues R49 and D51 were 39% and 54% higher than those of the P48A mutant, which caused differences in S(2), R(ex), and τ(e). The S(2) values of the P48A mutant residues R49, G50, and D51 were 29%, 14%, and 28% lower than those of Rho. The R(ex) values of Rho residues R49 and D51 were 0.91 s(-1) and 1.42 s(-1); however, no R(ex) was found for those of the P48A mutant. The τ(e) values of Rho residues R49 and D51 were 9.5 and 5.1 times lower than those of P48A mutant. Mutational study showed that integrin α5β1 prefers its ligands to contain (G/A)RGD but not PRGD sequences for binding. These results demonstrate that the N-terminal proline residue adjacent to the RGD motif affect its function and dynamics, which suggests that the dynamic properties of the RGD motif may be important in Rho's interaction with integrin α5β1.

Recombinant rubistatin (r-Rub), an MVD disintegrin, inhibits cell migration and proliferation, and is a strong apoptotic inducer of the human melanoma cell line SK-Mel-28

Disintegrins are low molecular weight peptides isolated from viper venom. These peptides bind to integrin receptors using a conserved binding motif sequence containing an RGD or similar motif. As a consequence, disintegrins can inhibit platelet aggregation and inhibit cell migration, proliferation, and initiate apoptosis in cancer cell lines. Rubistatin is a MVD disintegrin cloned from a Crotalus ruber ruber venom gland. The biological activity of MVD disintegrins is poorly understood. Recombinant rubistatin (r-Rub) was cloned into a pET32b plasmid and expressed in reductase-deficient Escherichia coli. Expression was induced with IPTG and the resulting fusion peptide was affinity purified, followed by thrombin cleavage, and removal of vector coded sequences. r-Rub peptide inhibited ADP-induced platelet aggregation by 54% ± 6.38 in whole blood. We assessed the ability of r-Rub to initiate apoptosis in three human cancer cell lines. Cultures of SK-Mel-28, HeLA, and T24 cells were grown for 24 h with 2.5 μM r-Rub followed by Hoechst staining. Chromatin fragmentation was observed in treated SK-Mel-28, but not in T24 or HeLA cells. A TUNEL assay revealed that 51.55% ± 5.28 of SK-Mel-28 cells were apoptotic after 18 h of treatment with 3.5 μM of r-Rub. Cell migration and proliferation assays were performed in order to further characterize the biological effects of r-Rub on SK-Mel-28 cells. At 3 μM, r-Rub inhibited cell migration by 44.4% ± 0.5, while at 3.5 μM it was able to inhibit cell proliferation by 83% ± 6.0.

CD40 ligand: a neo-inflammatory molecule in vascular diseases

CD40 Ligand (CD40L), a member of the TNF family, was initially thought to be solely implicated in thymus-dependent humoral responses. However, work by several groups showed that CD40L plays a more global role in various systems. Recent evidence has outlined an important role for CD40L in the physiopathology of the vascular system. Indeed, by interacting with its principal receptor, CD40, or with the recently identified receptors, namely αIIbβ3, α5β1, and Mac-1 integrins, CD40L displayed many biological functions in different types of vascular cells. In addition, the CD40L system was demonstrated a major player in the pathology of vascular diseases, such as atherosclerosis and restenosis. This review outlines the expression pattern and the functional properties of CD40L and its receptors at different cellular levels in the vascular system. In addition, we thoroughly describe evidence showing the implication of CD40L interactions in atherosclerosis, restenosis, and their associated clinical complications.

Anti-metastatic activity of the recombinant disintegrin, r-mojastin 1, from the Mohave rattlesnake

Cancer is the uncontrollable growth of cell, which may spread to other parts of the body. The interaction of cancer cells with extracellular matrix (ECM) is essential for metastasis, which is the principal cause of death in cancer patients. Disintegrins are naturally occurring low molecular weight peptides found in the venoms of many snakes. Disintegrins were first used to inhibit platelet aggregation, but more recently have been used to inhibit cancer cell growth, adhesion, migration, invasion and/or angiogenesis. The purpose of this study was to determine the anti-tumor properties of recombinant mojastin 1 (r-mojastin 1) and r-mojastin-GST, cloned from the venom glands of the Mohave rattlesnake (Crotalus scutulatus scutulatus). Human urinary bladder carcinoma (T24), human fibrosarcoma (HT-1080), human skin melanoma (SK-ML-28) and murine skin melanoma (B16F10) cell lines were used. r-Mojastin 1 inhibited SK-MEL-28 cell adhesion to fibronectin, but was not able to inhibit T24 cell adhesion to fibronectin. However, r-Mojastin-GST inhibited SK-MEL-28 and T24 cells adhesion to fibronectin. r-Mojastin-GST and r-mojastin 1 decreased the ability of SK-MEL-28 cells to migrate after 24 h of incubation but were not able to inhibit T24 cell migration. r-Mojastin 1 and r-mojastin-GST inhibited invasion of T24 and SK-MEL-28 cancer cells in vitro, and r-Mojastin 1 inhibited lung tumor colonization of B16F10 cells in mice in vivo. In conclusion, our studies suggest that r-mojastin could be a useful tool to develop novel anti-tumor agents by virtue of its ability to inhibit tumor cell adhesion, migration and invasion.

Functional analysis of a recombinant PIII-SVMP, GST-acocostatin; an apoptotic inducer of HUVEC and HeLa, but not SK-Mel-28 cells

Disintegrins and disintegrin-like peptides interact with integrins and interfere with cell-cell and cell-matrix interactions. A disintegrin-like snake venom gene, Acocostatin was cloned from the venom gland mRNA of Agkistrodon contortrix contortrix. Acocostatin belongs to the PIII-SVMP subfamily of disintegrin-like peptides. The recombinant acocostatin peptide was produced and purified as GST-fusion. The GST-acocostatin peptide, at 44 μg/mL, inhibited platelet aggregation by 30% in PRP and 18% in whole blood. In addition GST-acocostatin, at 220 μg/mL, inhibited SK-Mel-28 cell migration by 48%, but did not inhibit T24 cell migration. The GST-acocostatin peptide ability to induce apoptosis on HUVEC, HeLa, and SK-Mel-28 cells was determined using Annexin V-FITC and chromatin fragmentation assays after 24 h of treatment. At 5 μM GST-acocostatin peptide, 19.68%+/- 3.09 of treated HUVEC, and 35.86% +/- 2.05 of treated HeLa cells were in early apoptosis. The GST-acocostatin peptide also caused chromatin fragmentation of HUVEC and HeLa cells as determined by fluorescent microscopy and Hoechst staining. The GST-acocostatin peptide failed to induce apoptosis of SK-Mel-28 cells. We characterized the HUVEC, HeLa, and T24 integrin expression by flow cytometry, as the first step in determining GST-acocostatin binding specificity. Our results indicate that HUVEC express αv, αvβ3, αvβ5, α6, β1, and β3 integrin receptors. HeLa cells express α1, α2, α6, αv, αvβ5, and β1 integrin receptors. T24 cells express α1, α3, α6, αv, αvβ3, αvβ5, β1, β3, and β6 integrin receptors.

"Insularin, a disintegrin from Bothrops insularis venom: inhibition of platelet aggregation and endothelial cell adhesion by the native and recombinant GST-insularin proteins"

Insularin (INS) was obtained from Bothrops insularis venom by reversed-phase high-performance liquid chromatography using a C(18) column and characterized as a disintegrin by peptide mass fingerprint and inhibition of ADP-induced platelet aggregation. A cDNA coding for P-II a metalloproteinase/disintegrin was cloned from a cDNA library from B. insularis venom glands. The deduced protein sequence possesses 73 amino acid residues, including the N-terminal, internal peptides of native insularin, the ARGDNP-sequence and 12 cysteines in a conserved alignment. This cDNA fragment was subcloned in the pGEX-4T-1 vector and expressed in a prokaryotic expression system as a fusion protein with glutathione S-transferase (GST-INS). Both native and recombinant insularin inhibited ADP-induced platelet aggregation and endothelial cells (HUVEC) adhesion with similar activities indicating that GST-INS folded correctly and preserved the integrin-binding loop. Insularin may be a tool in studies that involve platelets and endothelial cell adhesion dependent on alphaIIbeta3 and alphavbeta3 integrins.

The mojastin mutant Moj-DM induces apoptosis of the human melanoma SK-Mel-28, but not the mutant Moj-NN nor the non-mutated recombinant Moj-WN

In this study, three recombinant mojastin peptides (Moj-WN, Moj-NN, and Moj-DM) were produced and compared functionally. Recombinant Moj peptides were purified as GST-fusions. GST-Moj-WN and GST-Moj-NN inhibited ADP-induced platelet aggregation in platelet rich plasma. The GST-Moj-WN had an IC(50) of 160nM, while the GST-Moj-NN had an IC(50) of 493nM. The GST-Moj-DM did not inhibit platelet aggregation. All three GST-Moj peptides inhibited SK-Mel-28 cell adhesion to fibronectin. The GST-Moj-WN inhibited the binding of SK-Mel-28 cells to fibronectin with an IC(50) of 11nM, followed by the GST-Moj-NN (IC(50) of 28nM), and the GST-Moj-DM (IC(50) of 46nM). The GST-Moj peptides' ability to induce apoptosis on SK-Mel-28 cells was determined using Annexin-V-FITC and nuclear fragmentation assays. Cells were incubated with 5muM GST-Moj peptides for 24h. At 5microM GST-Moj-DM peptide, 13.56%+/-2.08 of treated SK-Mel-28 cells were in early apoptosis. The GST-Moj-DM peptide also caused nuclear fragmentation as determined by fluorescent microscopy and Hoechst staining. The GST-Moj-WN and GST-Moj-NN peptides failed to induce apoptosis. We characterized the SK-Mel-28 integrin expression, as the first step in determining r-Moj binding specificity. Our results indicate that SK-Mel-28 cells express alphavbeta3, alphav, alpha6, beta1, and beta3 integrin receptors.

Integrin inhibitors from snake venom: exploring the relationship between the structure and activity of RGD-peptides

alphaIIbbeta3 is an integrin that is involved in platelet adhesion and aggregation. This receptor may be inhibited by cysteine-rich peptides known as disintegrins. We isolated two disintegrins from Bothrops jararaca venom called jarastatin and jararacin. We evaluated the structural characteristics and the effects on human platelet aggregation of these disintegrins. Inhibitory profiles were compared to six distinct peptides synthesized based on their RGD hairpin loop primary sequences. Both jarastatin and jararacin inhibited ADP and thrombin induction. Conversely, none of the cyclic peptides showed high-quality activity in assays induced by ADP or thrombin. We constructed homology models for all of these molecules, and theoretically evaluated their interaction with the alphaIIbbeta3 crystal structure using a molecular modeling approach. These results support the observations that the cyclic peptides had little effects, and also reinforce the observation that residues outside the disintegrin RGD sequence are required for interactions with receptor.

Differences in binding of (99m)Tc-disintegrins to integrin alphavbeta3 on tumor and vascular cells

Disintegrins, which contain an Arg-Gly-Asp sequence in their binding domains are antagonists of integrins such as alphavbeta3. The purpose of this study was to compare a range of disintegrins with different integrin selectivities for their binding behavior in vitro to vascular endothelial cells bearing alphavbeta3 and to cultured tumor cells which express alphavbeta3.

METHODS

Five disintegrins (bitistatin, kistrin, flavoridin, VLO4 and echistatin) and a cyclic pentapeptide, c[RGDyK], were radiolabeled with (99m)Tc and tested for binding to cells in vitro.

RESULTS

(99m)Tc-Kistrin, flavoridin and VLO4 had the highest binding, (99m)Tc-echistatin had moderate binding, and (99m)Tc-bitistatin and (99m)Tc-c[RGDyK] had low binding to cells. The observed binding was attributed to alphavbeta3 to various extents: echistatin, bitistatin>kistrin>flavoridin>VLO4. Cancer cells internalized bound disintegrins after binding, but endothelial cells did not. After binding to endothelial cells, (99m)Tc-kistrin was not displaced by competing peptide or plasma proteins.

CONCLUSIONS

These data suggest that radiolabeled kistrin, flavoridin and VLO4 may have advantages over labeled bitistatin and small cyclic peptides for targeting alphavbeta3 in vivo. Since receptor-bound radioligand is not internalized by endothelial cells, disintegrins may provide an advantage for targeting alphavbeta3 on vasculature because they bind strongly to surface receptors and are not readily displaced.

The “Linker” Region (Amino Acids 38-47) of the Disintegrin Elegantin Is a Novel Inhibitory Domain of Integrin α5β1-Dependent Cell Adhesion on Fibronectin

Disintegrins are a family of potent inhibitors of cell-cell and cell-matrix adhesion. In this study we have identified a region of the disintegrin elegantin, termed the "linker domain" (amino acids 38-47), with inhibitory activity toward alpha(5)beta(1)-mediated cell adhesion on fibronectin (Fn). Using a chimeric structure-function approach in which sequences of the functionally distinct disintegrin kistrin were introduced into the elegantin template at targeted sites, a loss of inhibitory function toward alpha(5)beta(1)-mediated adhesion on Fn was observed when the elegantin linker domain was substituted. Subsequent analysis comparing the inhibitory efficacies of the panel of elegantin-kistrin chimeras toward CHO alpha(5) cell adhesion on recombinant Fn III(6-10) fragments showed that the loss of inhibitory activity associated with the disruption of the elegantin linker domain was dependent upon the presence of a functional Fn III(9) synergy site within the Fn III(6-10) substrate. This suggested that the elegantin linker domain inhibits primarily the activity of the Fn synergy domain in promoting alpha(5)beta(1) integrin-mediated cell adhesion. Construction of a cyclic peptide corresponding to the entire region of the elegantin linker domain showed that this domain has intrinsic alpha(5)beta(1) inhibitory activity comparable with the activity of the RGDS peptide. These data demonstrate a novel biological function for a disintegrin domain that antagonizes integrin-mediated cell adhesion.

Activation of human T lymphocytes via integrin signaling induced by RGD-disintegrins

Adhesive interactions play important roles in coordinating T cell migration and activation, which are mediated by binding of integrins to RGD motif found on extracellular matrix proteins. Disintegrins, isolated from snake venoms, contain the RGD sequence that confers selectivity to integrin interaction. We have investigated the ability of three RGD-disintegrins, ligands of alpha(5)beta(1) and alpha(v)beta(3), Flavoridin (Fl), Kistrin (Kr) and Echistatin (Ech), in modulating the activation of human T lymphocyte. The disintegrins induced T cell proliferation and CD69 expression. This activation parallels with actin cytoskeleton reorganization and tyrosine phosphorylation. Furthermore, the peptides induced focal adhesion kinase (FAK) and phosphoinositide 3-kinase (PI3K) activation. Finally, RGD-disintegrins were capable of driving NF-kappaB nuclear translocation and c-Fos expression, in a PI3K and ERK1/2 activities dependent manner. This report is the first to show that RGD-disintegrins interact with integrins on human T lymphocyte surface, modulating cell proliferation and activation of specific pathways coupled to integrin receptor.

Effects of amino acid substitutions in and around the arginine-glycine-aspartic acid (RGD) sequence on fertilization and parthenogenetic development in mature bovine oocytes

Integrins have been shown to be involved in the process of fertilization and many integrin-ligand interactions are mediated through the recognition of an arginine-glycine-aspartic acid (RGD) sequence. Despite the fact the RGD domain is a principal player in determining the functional characteristics of an adhesive protein, increasing evidence has accumulated implicating the amino acids flanking the RGD sequence in determining the functional properties of the RGD-containing protein. A set of linear peptides in which the amino acid sequence in and around the RGD tri-peptide was modified was synthesized to better understand the specificity of the RGD-receptor interaction. Mature oocytes were fertilized in vitro in the presence of RGD-containing and RGD-modified peptides. Both the RGD-containing and RGD-modified peptides impaired the ability of sperm to fertilize bovine oocytes, illustrated by a reduction in cleavage. The linear modified RGD containing peptides were also examined for their ability to induce parthenogenetic development with the objective of providing a linear RGD peptide with greater biological activity than the one (GRGDSPK) used previously (Campbell et al., 2000). The data demonstrate the specificity of the receptor for the RGD sequence, further implicate the involvement of integrins in the process of bovine fertilization, and illustrate the importance of the amino acids surrounding the RGD sequence in determining the binding and functional properties of RGD-containing peptides. The data support the findings that a linear RGD peptide can block fertilization and that amino acids around the RGD sequence have an impact on the biological activity of the receptor.

Solution structure of gamma-bungarotoxin: the functional significance of amino acid residues flanking the RGD motif in integrin binding

Gamma-bungarotoxin, a snake venom protein isolated from Bungarus multicinctus, contains 68 amino acids, including 10 cysteine residues and a TAVRGDGP sequence at positions 30-37. The solution structure of gamma-bungarotoxin has been determined by nuclear magnetic resonance (NMR) spectroscopy. The structure is similar to that of the short-chain neurotoxins that contain three loops extending from a disulfide-bridged core. The tripeptide Arg-Gly-Asp (RGD) sequence is located at the apex of the flexible loop and is similar to that of other RGD-containing proteins. However, gamma-bungarotoxin only inhibits platelet aggregations with an IC50 of 34 microM. To understand its weak activity in inhibiting platelet aggregation, we mutated the RGD loop sequences of rhodostomin, a potent platelet aggregation inhibitor, from RIPRGDMP to TAVRGDGP, resulting in a 196-fold decrease in activity. In addition, the average Calpha-to-Calpha distance between R33 and G36 of gamma-bungarotoxin is 6.02 A, i.e., shorter than that of other RGD-containing proteins that range from 6.55 to 7.46 A. These results suggested that the amino acid residues flanking the RGD motif might control the width of the RGD loop. This structural difference may be responsible for its decrease in platelet aggregation inhibition compared with other RGD-containing proteins.

Snake venom metalloproteases--structure and function of catalytic and disintegrin domains

Snake venoms are relevant sources of toxins that have evolved towards the engineering of highly active compounds. In the last years, research efforts have produced great advance in their understanding and uses. Metalloproteases with disintegrin domains are among the most abundant toxins in many Viperidae snake venoms. This review will focus on the structure, function and possible applications of the metalloprotease and disintegrin domains.

Molecular engineering of an EGFP/disintegrin-based integrin marker

Disintegrins are viper venom peptides, which bind integrins with high affinity (10(-8) M) and selectivity. Among them, eristostatin (Er) selectively binds and inhibits alphaIIbbeta3 integrin function. In this work we have engineered an enhanced green fluorescence protein (EGFP)-tagged Er as an alphaIIbbeta3 biomarker to be used in bioassays involving fluorescence detectors. For this, we have first constructed an EGFP bacterial expression vector, which resulted in a 6xHis tag-coding region followed by the EGFP gene and a 3' multiple cloning site (MCS) comprising nine restriction sites. This vector, termed pAZ, was used to clone the Er gene, resulting in a 32 kDa EGFP-Er fusion protein when expressed as characterized by SDS-PAGE and Western blot. Both EGFP-Er and EGFP (expressed from the empty pAZ vector) were purified by immobilized metal affinity chromatography (IMAC) and their fluorescence was measured showing similar values, thus suggesting that the Er portion is not affecting the EGFP activity. EGFP-Er, but not EGFP selectively bound to immobilized platelets as detected by confocal microscopy indicating the preservation of Er disintegrin activity and its potential use as a marker for alphaIIbbeta3 integrin. Our data suggest the use of the pAZ vector for expressing soluble EGFP-labeled proteins and the use of EGFP-fused disintegrins as markers for integrins.

Snake venom disintegrins: evolution of structure and function

Disintegrins represent a family of polypeptides present in the venoms of various vipers that selectively block the function of integrin receptors. Here, we review our current view and hypothesis on the emergence and the structural and functional diversification of disintegrins by accelerated evolution and the selective loss of disulfide bonds of duplicated genes. Research on disintegrins is relevant for understanding the biology of viper venom toxins, but also provides information on new structural determinants involved in integrin recognition that may be useful in basic and clinical research. The role of the composition, conformation, and dynamics of the integrin inhibitory loop acting in concert with the C-terminal tail in determining the selective inhibition of integrin receptors is discussed.

Crystal Structure of Schistatin, a Disintegrin Homodimer from Saw-scaled Viper (Echis carinatus) at 2.5Å Resolution

This is the first structure of a biological homodimer of disintegrin. Disintegrins are a class of small (4-14 kDa) proteins that bind to transmembrane integrins selectively. The present molecule is the first homodimer that has been isolated from the venom of Echis carinatus. The monomeric chain contains 64 amino acid residues. The three-dimensional structure of schistatin has been determined by the multiple isomorphous replacement method. It has been refined to an R-factor of 0.190 using all the data to 2.5 A resolution. The two subunits of the disintegrin homodimer are related by a 2-fold crystallographic symmetry. Thus, the crystallographic asymmetric unit contains a monomer of disintegrin. The monomer folds into an up-down topology with three sets of antiparallel beta-strands. The structure is well ordered with four intramolecular disulfide bonds. the two monomers are firmly linked to each other through two intermolecular disulfide bridges at their N termini together with several other interactions. This structure has corrected the error in the disulfide bond pattern of the two intermolecular disulfide bridges that was reported earlier using chemical methods. Unique sequence and structural features of the schistatin monomers suggest that they have the ability to bind well with both alphaIIb beta3 and alphav beta3 integrins. The N termini anchored two chains of the dimer diverge away at their C termini exposing the Arg-Gly-Asp motif into opposite directions thus enhancing their binding efficiency to integrins. This is one of the unique features of the present disintegrin homodimer and seems to be responsible for the clustering of integrin molecules. The homodimer binds to integrins apparently with a higher affinity than the monomers and also plays a role in the signaling pathway.

A three-residue cyclic scaffold of non-RGD containing peptide analogues as platelet aggregation inhibitors: design, synthesis, and structure--function relationships

Antagonists of fibrinogen at the GPIIb/IIIa receptor, which is the most abundant membrane protein on the platelet surface, are under active investigation as potential antithrombotics. The critical interaction between GPIIb/IIIa and fibrinogen can be inhibited by either linear or cyclic RGDS-containing peptides, which have been proved as lead compounds in the design of platelet aggregation inhibitors. In this study we present the design and construction of a new class of cyclic (S,S) non-RGD containing peptide sequences, using two Cys as a structural scaffold for the development of antiaggregatory agents. The (S,S)-CDC- sequence was incorporated as a conformational constraint, in molecules bearing at least one positive charge with the general formula (S,S)XCDCZ, where X = Ac-Arg, Pro-Arg, Pro-Ser-Lys, and Pro-Ser-Arg, and Z = -NH(2) and Arg-NH(2). Investigation of the structure-function relationships was performed on the basis of (a) the local conformation induced by the (S,S)-CDC motif, (b) the distance of the positively (R-C(zeta) or K-N(zeta)) and negatively (D-C(gamma)) charged centers, (c) the presence of a second positive or negative charge on the molecule, and (d) the orientation of the basic and acidic side chains defined by the pseudo dihedral angle (Pdo), which is formed by the R-C(zeta), R-C(alpha), D-C(alpha), and D-C(gamma) atoms in the case of (S,S)-RCDC and by the K-N(zeta), K-C(alpha), D-C(alpha), and D-C(gamma) atoms in the case of (S,S)-KCDC.

Thy-1 binds to integrin beta(3) on astrocytes and triggers formation of focal contact sites

BACKGROUND

Thy-1 is an abundant neuronal glycoprotein in mammals. Despite such prevalence, Thy-1 function remains largely obscure in the absence of a defined ligand. Astrocytes, ubiquitous cells of the brain, express a putative Thy-1 ligand that prevents neurite outgrowth. In this paper, a ligand molecule for Thy-1 was identified, and the consequences of Thy-1 binding for astrocyte function were investigated.

RESULTS

Thy-1 has been implicated in cell adhesion and, indeed, all known Thy-1 sequences were found to contain an integrin binding, RGD-like sequence. Thy-1 interaction with beta3 integrin on astrocytes was demonstrated in an adhesion assay using a thymoma line (EL-4) expressing high levels of Thy-1. EL-4 cells bound to astrocytes five times more readily than EL-4(-f), control cells lacking Thy-1. Binding was blocked by either anti-Thy-1 or anti-beta3 antibodies, by RGD-related peptides, or by soluble Thy-1-Fc chimeras. However, neither RGE/RLE peptides nor Thy-1(RLE)-Fc fusion protein inhibited the interaction. Immobilized Thy-1-Fc, but not Thy-1(RLE)-Fc fusion protein supported the attachment and spreading of astrocytes in a Mn(2+)-dependent manner. Binding to Thy-1-Fc was inhibited by RGD peptides. Moreover, vitronectin, fibrinogen, denatured collagen (dcollagen), and a kistrin-derived peptide, but not fibronectin, also mediated Mn(2+)-dependent adhesion, suggesting the involvement of beta3 integrin. The addition of Thy-1 to matrix-bound astrocytes induced recruitment of paxillin, vinculin, and focal adhesion kinase (FAK) to focal contacts and increased tyrosine phosphorylation of proteins such as p130(Cas) and FAK. Furthermore, astrocyte binding to immobilized Thy-1-Fc alone was sufficient to promote focal adhesion formation and phosphorylation on tyrosine.

CONCLUSIONS

Thy-1 binds to beta3 integrin and triggers tyrosine phosphorylation of focal adhesion proteins in astrocytes, thereby promoting focal adhesion formation, cell attachment, and spreading.

Positional importance of Pro53 adjacent to the Arg49-Gly50-Asp51 sequence of rhodostomin in binding to integrin alphaIIbbeta3

Rhodostomin (RHO), a disintegrin isolated from snake venom, has been demonstrated to inhibit platelet aggregation through interaction with integrin alphaIIbbeta3, but there is a lack of direct evidence for RHO-integrin alphaIIbbeta3 binding. In addition, no study on the length of Arg(49)-Gly(50)-Asp(51) (RGD) loop of RHO influencing on its binding to integrin alphaIIbbeta3 has been reported. In the present study we have developed a highly sensitive dot-blot and glutathione S-transferase-RHO pull-down assays; the latter was coupled with a biotin-avidin-horseradish peroxidase enhanced-chemiluminescence detection system. These were able to demonstrate the direct binding of RHO to integrin alphaIIbbeta3. The pull-down assay further showed that four alanine-insertion mutants upstream of the RGD motif and three insertions downstream of the RGD were able to decrease integrin alphaIIbbeta3 binding activity to only a limited extent. By contrast, two insertions immediately next to RGD and one insertion in front of the Cys(57) caused almost complete loss of binding activity to alphaIIbbeta3. The results of the platelet-aggregation-inhibition assay and platelet-adhesion assay for the insertion mutants were consistent with results of the pull-down assay. It is thus concluded that, although an insertion of a single alanine residue in many positions of the RGD loop has only minor effects on RHO binding to integrin alphaIIbbeta3, the specific position of Pro(53) residue adjacent to the RGD sequence is important for RHO binding to platelet integrin alphaIIbbeta3.

Arg-Tyr-Asp (RYD) and Arg-Cys-Asp (RCD) motifs in dendroaspin promote selective inhibition of beta1 and beta3 integrins

Arg-Gly-Asp (RGD) is a unique minimal integrin-binding sequence that is found within several glycoprotein ligands. This sequence has also been found in snake-venom anti-platelet proteins, including the disintegrins and dendroaspin, a natural variant of short-chain neurotoxins isolated from the venom of Dendroaspis jamesonii. In the present study, the motifs RYD and RCD were introduced into the dendroaspin scaffold to replace RGD. Both motifs in dendroaspin caused inhibition of ADP-induced platelet aggregation with IC(50) values of 200 and 300 nM respectively, similar to that of the wild-type RGD motif (170 nM). In comparison with wild-type dendroaspin, both RYD- and RCD-containing dendroaspins were more selective in the inhibition of the adhesion of K562 cells to laminin rather than to fibrinogen and fibronectin, even though they were 10-30-fold less potent at inhibiting K562 cell (containing alpha(5)beta(1) integrin) adhesion to laminin compared with wild-type. Interestingly, the RYD motif produced a similar IC(50) value to the RGD motif at inhibiting A375-SM cell (beta(3) integrin) adhesion to collagen, whereas the RCD motif was approx. 2-6-fold less potent compared with either RGD or RYD. These findings show that the selectivity of dendroaspin binding to beta(1) and beta(3) integrins can be modulated by the introduction of alternative cell recognition sequences.

Evaluation of the role of proline residues flanking the RGD motif of dendroaspin, an inhibitior of platelet aggregation and cell adhesion

The effect of a panel of proline mutants of dendroaspin, an inhibitor of platelet aggregation and cell adhesion, including A(42)-dendroaspin, A(47)-dendroaspin, A(49)-dendroaspin, A(42,47)-dendroaspin and A(47,49)-dendroaspin, was investigated using platelet-aggregation and cell-adhesion assays. Here we show that a single alanine-for-proline substitution did not affect potency when measured as the ability either to inhibit platelet aggregation induced by ADP (IC(50) approximately 170 nM) or to block transfected A375-SM cell adhesion to fibrinogen in the presence of Mn(2+) as compared with wild-type dendroaspin. By comparison, double proline substitution with alanines significantly reduced the potency in both assays by approx. 5-8-fold. These observations, therefore, suggest that proline residues flanking the RGD motif in dendroaspin and other RGD-containing venom proteins, e.g. disintegrins, may contribute to maintaining a favourable conformation for the solvent-exposed RGD site for its recognition by integrin receptors.

Differential recognition of snake venom proteins expressing specific Arg-Gly-Asp (RGD) sequence motifs by wild-type and variant integrin alphaIIbbeta3: further evidence for distinct sites of RGD ligand recognition exhibiting negative allostery

Several studies have demonstrated that the amino acid residues flanking the Arg-Gly-Asp (RGD) sequence of high-affinity ligands modulate their specificity of interaction with integrin complexes. Because of the absence of structural data for integrin complexes with bound ligand, the molecular basis for this specificity modulation remains obscure. In a previous paper [Rahman, Lu, Kakkar and Authi (1995) Biochem. J. 312, 223-232] we demonstrated that two genetically distinct venom-derived RGD proteins, kistrin and dendroaspin (both containing the sequence PRGDMP), were simple competitors, indicating the recognition of an identical binding site on the alpha(IIb)beta(3) complex. Furthermore, both kistrin and dendroaspin inhibited the binding of the disintegrin elegantin (containing the sequence ARGDNP) via a non-competitive mechanism, suggesting that the binding of elegantin to the alpha(IIb)beta(3) complex was at a remote site and down-regulated via an allosteric mechanism. Here we present further evidence for distinct RGD ligand recognition sites on the alpha(IIb)beta(3) complex that exhibit a negative allosteric relationship. A panel of well-characterized recombinant dendroaspin and elegantin derivatives were employed for this study. These recombinant molecules were constructed as glutathione S-transferase fusion proteins with either an Ala or Pro residue N-terminal to the RGD sequence in combination with either a Met or an Asn residue immediately C-terminal. Equilibrium competition experiments showed that elegantin binding to ADP-treated platelets was inhibited by derivatives Eleg. AM (ARGDMP) and Eleg. PM (PRGDMP) via an allosteric competitive mechanism, providing direct evidence that modulation of the RGD motif can alter competitive behaviour. In addition, recombinant kistrin and dendroaspin both inhibited elegantin binding via a non-competitive mechanism, confirming our previous observations. Further evidence for distinct binding sites employing an independent approach was obtained by analysing the binding of the panel of venom proteins to the functionally defective heterodimer alpha(IIb)beta(3) Ser(123)-->Ala expressed on Chinese hamster ovary cells. These studies demonstrated that simple competitors kistrin and dendroaspin bound with high affinity to the variant integrin complex. In contrast, the binding of elegantin and most significantly, recombinant Dendro. PN (PRGDNP) and Dendro. AN (ARGDNP) were abolished. These observations, taken together, are consistent with a model depicting the presence of distinct sites of RGD ligand recognition on the alpha(IIb)beta(3) complex that show the preferential recognition of specific RGD motifs. Competition experiments demonstrate a negative allosteric relationship between these RGD recognition sites.

Disulphide-bond pattern and molecular modelling of the dimeric disintegrin EMF-10, a potent and selective integrin alpha5beta1 antagonist from Eristocophis macmahoni venom

The disulphide-bond pattern of the heterodimeric disintegrin EMF-10, a potent and selective integrin alpha(5)beta(1) antagonist from Eristocophis macmahoni venom, was established by combination of amino-acid analysis, N-terminal sequencing and collision-induced dissociation by nanoelectrospray ionization quadrupole ion-trap MS of fragments isolated by reversed-phase HPLC after degradation of EMF-10 with oxalic acid. Each EMF-10 subunit contains four intrachain disulphide bonds. Two interchain cystine residues join the EMF-10 polypeptides. The intrachain linkages are conserved in monomeric disintegrins. A molecular model of EMF-10 was built using averaged NMR co-ordinates of flavoridin as a template. The active hairpin loops of the EMF-10 subunits occupy opposite locations at the ends of an elongated disulphide-bond ladder. In the EMF-10 model the N-terminal polypeptide of EMF-10B is close to the RGD-loop of the EMF-10A subunit, suggesting that the N-terminal region of the B-subunit could potentially influence the biological activity of the A-subunit.