Binding of nucleotide triphosphates to cardiotoxin analogue II from the Taiwan cobra venom (Naja naja atra). Elucidation of the structural interactions in the dATP-cardiotoxin analogue ii complex

NMR Methods to Characterize Protein-Ligand Interactions

Structural information pertaining to the interactions between biological macromolecules and ligands is of potential significance for understanding of molecular mechanisms in key biological processes. Recently, nuclear magnetic resonance (NMR) spectroscopic techniques has come of age and has widened its scope to characterize binding interactions of small molecules with biological macromolecules especially, proteins. NMR spectroscopy-based techniques are versatile due to their ability to examine weak binding interactions and for rapid screening the binding affinities of ligands with proteins at atomic resolution. In this review, we provide a broad overview of some of the important NMR approaches to investigate interactions of small organic molecules with proteins.

Detection of Naja atra Cardiotoxin Using Adenosine-Based Molecular Beacon

This study presents an adenosine (A)-based molecular beacon (MB) for selective detection of Naja atra cardiotoxin (CTX) that functions by utilizing the competitive binding between CTX and the poly(A) stem of MB to coralyne. The 5′- and 3′-end of MB were labeled with a reporter fluorophore and a non-fluorescent quencher, respectively. Coralyne induced formation of the stem-loop MB structure through A₂-coralyne-A₂ coordination, causing fluorescence signal turn-off due to fluorescence resonance energy transfer between the fluorophore and quencher. CTX3 could bind to coralyne. Moreover, CTX3 alone induced the folding of MB structure and quenching of MB fluorescence. Unlike that of snake venom α-neurotoxins, the fluorescence signal of coralyne-MB complexes produced a bell-shaped concentration-dependent curve in the presence of CTX3 and CTX isotoxins; a turn-on fluorescence signal was noted when CTX concentration was ≤80 nM, while a turn-off fluorescence signal was noted with a further increase in toxin concentrations. The fluorescence signal of coralyne-MB complexes yielded a bell-shaped curve in response to varying concentrations of N. atra crude venom but not those of Bungarus multicinctus and Protobothrops mucrosquamatus venoms. Moreover, N. nigricollis venom also functioned as N. atra venom to yield a bell-shaped concentration-dependent curve of MB fluorescence signal, again supporting that the hairpin-shaped MB could detect crude venoms containing CTXs. Taken together, our data validate that a platform composed of coralyne-induced stem-loop MB structure selectively detects CTXs.

Snake cytotoxins bind to membranes via interactions with phosphatidylserine head groups of lipids

The major representatives of Elapidae snake venom, cytotoxins (CTs), share similar three-fingered fold and exert diverse range of biological activities against various cell types. CT-induced cell death starts from the membrane recognition process, whose molecular details remain unclear. It is known, however, that the presence of anionic lipids in cell membranes is one of the important factors determining CT-membrane binding. In this work, we therefore investigated specific interactions between one of the most abundant of such lipids, phosphatidylserine (PS), and CT 4 of Naja kaouthia using a combined, experimental and modeling, approach. It was shown that incorporation of PS into zwitterionic liposomes greatly increased the membrane-damaging activity of CT 4 measured by the release of the liposome-entrapped calcein fluorescent dye. The CT-induced leakage rate depends on the PS concentration with a maximum at approximately 20% PS. Interestingly, the effects observed for PS were much more pronounced than those measured for another anionic lipid, sulfatide. To delineate the potential PS binding sites on CT 4 and estimate their relative affinities, a series of computer simulations was performed for the systems containing the head group of PS and different spatial models of CT 4 in aqueous solution and in an implicit membrane. This was done using an original hybrid computational protocol implementing docking, Monte Carlo and molecular dynamics simulations. As a result, at least three putative PS-binding sites with different affinities to PS molecule were delineated. Being located in different parts of the CT molecule, these anion-binding sites can potentially facilitate and modulate the multi-step process of the toxin insertion into lipid bilayers. This feature together with the diverse binding affinities of the sites to a wide variety of anionic targets on the membrane surface appears to be functionally meaningful and may adjust CT action against different types of cells.

Anticoagulant effect of Naja naja venom 5′nucleotidase: Demonstration through the use of novel specific inhibitor, vanillic acid

The snake venom proteins affect hemostasis by either advancing/delaying blood coagulation. Apart from proteases and phospholipase A(2)s (PLA(2)s), 5'nucleotidase is known to affect hemostasis by inhibiting platelet aggregation. In this study, the possible involvement of Naja naja venom 5'nucleotidase in mediating anticoagulant affect is evaluated. Vanillic acid selectively and specifically inhibited 5'nucleotidase activity among other enzymes present in N. naja venom. It is a competitive inhibitor as evident of inhibition relieving upon increased substrate concentration. Vanillic acid dose dependently inhibited the anticoagulant effect of N. naja venom up to 40%. This partial involvement of 5'nucleotidase in mediating anticoagulant effect is substantiated by concanavalin-A (Con-A) inhibition studies. Con-A, competitively inhibited in vitro protease and 5'nucleotidase activity up to 100%. However, it did not exhibit inhibitory activity on PLA(2). The complete inhibition of anticoagulant effect by Con-A upon recalcification time suggests the participation of both 5'nucleotidase and protease in mediating anticoagulant effect of N. naja venom. Vanillic acid and Con-A inhibition studies together suggest that probably 5'nucleotidase interacts with one or more factors of intrinsic pathway of blood coagulation to bring about anticoagulant effect. Thus, this study for the first time demonstrates the involvement of 5'nucleotidase in mediating N. naja venom anticoagulant effect.

Bone marrow production of lung cells: the impact of G-CSF, cardiotoxin, graded doses of irradiation, and subpopulation phenotype

OBJECTIVE

Previous studies have demonstrated the production of various types of lung cells from marrow cells under diverse experimental conditions. Our aim was to identify some of the variables that influence conversion in the lung.

METHODS

In separate experiments, mice received various doses of total-body irradiation followed by transplantation with whole bone marrow or various subpopulations of marrow cells (Lin(-/+), c-kit(-/+), Sca-1(-/+)) from GFP(+) (C57BL/6-TgN[ACTbEGFP]1Osb) mice. Some were given intramuscular cardiotoxin and/or mobilized with granulocyte colony-stimulating factor (G-CSF).

RESULTS

The production of pulmonary epithelial cells from engrafted bone marrow was established utilizing green fluorescent protein (GFP) antibody labeling to rule out autofluorescence and deconvolution microscopy to establish the colocaliztion of GFP and cytokeratin and the absence of CD45 in lung samples after transplantation. More donor-derived lung cells (GFP(+)/CD45(-)) were seen with increasing doses of radiation (5.43% of all lung cells, 1200 cGy). In the 900-cGy group, 61.43% of GFP(+)/CD45(-) cells were also cytokeratin(+). Mobilization further increased GFP(+)/CD45(-) cells to 7.88% in radiation-injured mice. Up to 1.67% of lung cells were GFP(+)/CD45(-) in radiation-injured mice transplanted with Lin(-), c-kit(+), or Sca-1(+) marrow cells. Lin(+), c-kit(-), and Sca-1(-) subpopulations did not significantly engraft the lung.

CONCLUSIONS

We have established that marrow cells are capable of producing pulmonary epithelial cells and identified radiation dose and G-CSF mobilization as variables influencing the production of lung cells from marrow cells. Furthermore, the putative lung cell-producing marrow cell has the phenotype of a hematopoietic stem cell.

Cancer cell injury by cytotoxins from cobra venom is mediated through lysosomal damage

Cytotoxins from cobra venom are known to manifest cytotoxicity in various cell types. It is widely accepted that the plasma membrane is a target of cytotoxins, but the mechanism of their action remains obscure. Using the confocal spectral imaging technique, we show for the first time that cytotoxins from cobra venom penetrate readily into living cancer cells and accumulate markedly in lysosomes. Cytotoxins CT1 and CT2 from Naja oxiana, CT3 from Naja kaouthia and CT1 from Naja haje are demonstrated to possess this property with respect to human lung adenocarcinoma A549 and promyelocytic leukaemia HL60 cells. Immobilized plasma membrane binding accompanies the internalization of CT3 from Naja kaouthia in the HL60 cells, but it is very weak for other cytotoxins. Detectable membrane binding is not a property of any of the cytotoxins tested in A549 cells. The kinetics and concentration-dependence of cytotoxin accumulation in lysosomes correlate well with their cytotoxic effects. On the basis of the results obtained, we propose that lysosomes are a primary target of the lytic action of cytotoxins. Plasma membrane permeabilization seems to be a downstream event relative to lysosome rupture. Direct damage to the plasma membrane may be a complementary mechanism, but its relative contribution to the cytotoxic action depends on the cytotoxin structure and cell type.

Amphiphilic β-sheet cobra cardiotoxin targets mitochondria and disrupts its network

Recent advance in understanding the role of toxin proteins in controlling cell death has revealed that pro-apoptotic viral proteins targeting mitochondria contain amphiphilic alpha-helices with pore-forming properties. Herein, we describe that the pore-forming amphiphilic beta-sheet cardiotoxins (or cytotoxins, CTXs) from Taiwan cobra (Naja atra) also target mitochondrial membrane after internalization and act synergistically with CTX-induced cytosolic calcium increase to disrupt mitochondria network. It is suggested that CTX-induced fragmentation of mitochondria play a role in controlling CTX-induced necrosis of myocytes and cause severe tissue necrosis in the victims.

Tumor suppressor p53 inhibits transcriptional activation of invasion gene thromboxane synthase mediated by the proto-oncogenic factor ets-1

Cancer formation and progression is a complex process determined by several mechanisms that promote cell growth, invasiveness, neo-angiogenesis, and render neoplastic cells resistant to apoptosis. The tumor suppressor p53 and the proto-oncogenic factor ets-1 are important regulators of such mechanisms. While it is well established that p53 and ets-1 influence various aspects of cell behavior by regulating the transcription of specific genes, little is known about the functional relationship between these transcription factors. We found that the gene encoding thromboxane synthase (TXSA), which we recently identified as a factor promoting invasion and resistance to apoptosis in gliomas, is a novel target gene for both p53 and ets-1. We demonstrate that p53 and ets-1 coregulate TXSA in an antagonistic and inter-related manner, with ets-1 being a potent transcriptional activator and p53 inhibiting ets-1-dependent transcription. Negative interference with ets-1 transcription requires functional p53 and is lost in mutant p53 proteins. We show that ets-1 and p53 associate physically in vitro and in vivo and that their interaction, rather than a direct binding of p53 to the TXSA promoter, is required for transcriptional repression of TXSA by wild-type p53. An important implication of our findings is that the loss of p53-mediated negative control over ets-1-dependent transcription may lead to the acquisition of an invasive phenotype in tumor cells.

Cytotoxic potency of cardiotoxin from Naja sputatrix: development of a new cytolytic assay

The possible involvement of specific regions/loops of cardiotoxin from Naja sputatrix venom in mediating its cytolytic activity is evaluated using a new cytolytic assay. In this assay, the amount of chloramphenicol acetyltransferase (CAT) that is released upon lysis of the cellular membranes by the cytotoxin has been measured as an index of cytolysis. This newly developed CAT system is more sensitive than the traditional haemolysis method utilizing red blood cells or the lactate dehydrogenase assay for cytolysis. Series of chimaeric toxin molecules have been constructed by swapping the loops between highly hydrophilic neurotoxin and highly hydrophobic cardiotoxin molecules from Naja sputatrix, which are known to exhibit structural similarity (three-finger conformation) but to have different functional properties. Comparison of the cytolytic activities of the recombinant chimaeric toxins demonstrated the possible involvement of all three loops of cardiotoxin in its cytolytic potency. However, the first two loops of the protein appear to make the major contribution to its lytic activity. cDNAs encoding cardiotoxin and the chimaeric toxins, when expressed in transfected cultured Chinese hamster ovary cells, resulted in cell lysis, indicating that these cDNAs can be developed as useful cytolytic agents.