Long-sarafotoxins: characterization of a new family of endothelin-like peptides

Structural and Functional Diversity of Animal Toxins Interacting With GPCRs

Peptide toxins from venoms have undergone a long evolutionary process allowing host defense or prey capture and making them highly selective and potent for their target. This has resulted in the emergence of a large panel of toxins from a wide diversity of species, with varied structures and multiple associated biological functions. In this way, animal toxins constitute an inexhaustible reservoir of druggable molecules due to their interesting pharmacological properties. One of the most interesting classes of therapeutic targets is the G-protein coupled receptors (GPCRs). GPCRs represent the largest family of membrane receptors in mammals with approximately 800 different members. They are involved in almost all biological functions and are the target of almost 30% of drugs currently on the market. Given the interest of GPCRs in the therapeutic field, the study of toxins that can interact with and modulate their activity with the purpose of drug development is of particular importance. The present review focuses on toxins targeting GPCRs, including peptide-interacting receptors or aminergic receptors, with a particular focus on structural aspects and, when relevant, on potential medical applications. The toxins described here exhibit a great diversity in size, from 10 to 80 amino acids long, in disulfide bridges, from none to five, and belong to a large panel of structural scaffolds. Particular toxin structures developed here include inhibitory cystine knot (ICK), three-finger fold, and Kunitz-type toxins. We summarize current knowledge on the structural and functional diversity of toxins interacting with GPCRs, concerning first the agonist-mimicking toxins that act as endogenous agonists targeting the corresponding receptor, and second the toxins that differ structurally from natural agonists and which display agonist, antagonist, or allosteric properties.

Cardiovascular Effects of Snake Toxins: Cardiotoxicity and Cardioprotection

Snake venoms, as complex mixtures of peptides and proteins, affect various vital systems of the organism. One of the main targets of the toxic components from snake venoms is the cardiovascular system. Venom proteins and peptides can act in different ways, exhibiting either cardiotoxic or cardioprotective effects. The principal classes of these compounds are cobra cardiotoxins, phospholipases A2, and natriuretic, as well as bradykinin-potentiating peptides. There is another group of proteins capable of enhancing angiogenesis, which include, e.g., vascular endothelial growth factors possessing hypotensive and cardioprotective activities. Venom proteins and peptides exhibiting cardiotropic and vasoactive effects are promising candidates for the design of new drugs capable of preventing or constricting the development of pathological processes in cardiovascular diseases, which are currently the leading cause of death worldwide. For example, a bradykinin-potentiating peptide from Bothrops jararaca snake venom was the first snake venom compound used to create the widely used antihypertensive drugs captopril and enalapril. In this paper, we review the current state of research on snake venom components affecting the cardiovascular system and analyse the mechanisms of physiological action of these toxins and the prospects for their medical application.

Toxinology provides multidirectional and multidimensional opportunities: A personal perspective

In nature, toxins have evolved as weapons to capture and subdue the prey or to counter predators or competitors. When they are inadvertently injected into humans, they cause symptoms ranging from mild discomfort to debilitation and death. Toxinology is the science of studying venoms and toxins that are produced by a wide variety of organisms. In the past, the structure, function and mechanisms of most abundant and/or most toxic components were characterized to understand and to develop strategies to neutralize their toxicity. With recent technical advances, we are able to evaluate and determine the toxin profiles using transcriptomes of venom glands and proteomes of tiny amounts of venom. Enormous amounts of data from these studies have opened tremendous opportunities in many directions of basic and applied research. The lower costs for profiling venoms will further fuel the expansion of toxin database, which in turn will provide greater exciting and bright opportunities in toxin research.

Snake Venom Peptides: Tools of Biodiscovery

Nature endowed snakes with a lethal secretion known as venom, which has been fine-tuned over millions of years of evolution. Snakes utilize venom to subdue their prey and to survive in their natural habitat. Venom is known to be a very poisonous mixture, consisting of a variety of molecules, such as carbohydrates, nucleosides, amino acids, lipids, proteins and peptides. Proteins and peptides are the major constituents of the dry weight of snake venoms and are of main interest for scientific investigations as well as for various pharmacological applications. Snake venoms contain enzymatic and non-enzymatic proteins and peptides, which are grouped into different families based on their structure and function. Members of a single family display significant similarities in their primary, secondary and tertiary structures, but in many cases have distinct pharmacological functions and different bioactivities. The functional specificity of peptides belonging to the same family can be attributed to subtle variations in their amino acid sequences. Currently, complementary tools and techniques are utilized to isolate and characterize the peptides, and study their potential applications as molecular probes, and possible templates for drug discovery and design investigations.

Respiratory Effects of Sarafotoxins from the Venom of Different Atractaspis Genus Snake Species

Sarafotoxins (SRTX) are endothelin-like peptides extracted from the venom of snakes belonging to the Atractaspididae family. A recent in vivo study on anesthetized and ventilated animals showed that sarafotoxin-b (SRTX-b), extracted from the venom of Atractaspis engaddensis, decreases cardiac output by inducing left ventricular dysfunction while sarafotoxin-m (SRTX-m), extracted from the venom of Atractaspis microlepidota microlepidota, induces right ventricular dysfunction with increased airway pressure. The aim of the present experimental study was to compare the respiratory effects of SRTX-m and SRTX-b. Male Wistar rats were anesthetized, tracheotomized and mechanically ventilated. They received either a 1 LD₅₀ IV bolus of SRTX-b (n = 5) or 1 LD₅₀ of SRTX-m (n = 5). The low-frequency forced oscillation technique was used to measure respiratory impedance. Airway resistance (Raw), parenchymal damping (G) and elastance (H) were determined from impedance data, before and 5 min after SRTX injection. SRTX-m and SRTX-b injections induced acute hypoxia and metabolic acidosis with an increased anion gap. Both toxins markedly increased Raw, G and H, but with a much greater effect of SRTX-b on H, which may have been due to pulmonary edema in addition to bronchoconstriction. Therefore, despite their structural analogy, these two toxins exert different effects on respiratory function. These results emphasize the role of the C-terminal extension in the in vivo effect of these toxins.

Endothelin-1: Biosynthesis, Signaling and Vasoreactivity

Endothelin-1 (ET-1) is an extremely potent vasoconstrictor peptide originally isolated from endothelial cells. Its synthesis, mainly regulated at the gene transcription level, involves processing of a precursor by a furin-type proprotein convertase to an inactive intermediate, big ET-1. The latter peptide can then be cleaved directly by an endothelin-converting enzyme (ECE) into ET-1 or reach the active metabolite through a two-step process involving chymase hydrolyzing big ET-1 to ET-1 (1-31), itself needing conversion to ET-1 by neprilysin (NEP) to exert physiological activity. ET-1 signals through two G protein-coupled receptors, endothelin receptor A (ETA) and endothelin receptor B (ETB). Both receptors induce an increase in intracellular Ca(2+), mainly from the extracellular space through voltage-independent mechanisms, the receptor-operated channels and store-operated channels. ET-1 also induces signaling through epidermal growth factor receptor transactivation, oxidative stress induction, rho-kinase, and the activation (ETA) or inhibition (ETB) of the adenylate cyclase/cyclic adenosine monophosphate pathway. Arterial vasoconstriction is mediated mainly by the ETA receptor. ET-1, via endothelium-located ETB, relaxes arteries or constricts vessels following activation of the same receptor type on the smooth muscle, where it can interact with ETA. In addition, ETB-dependent vasoconstriction seems more prominent in the venous vasculature. A better understanding of how ET-1 is synthesized and how ETA and ETB receptors interact could help design better pharmacological agents in the treatment of cardiovascular diseases where targeting the ET-1 system is indicated.

Short- versus Long-Sarafotoxins: Two Structurally Related Snake Toxins with Very Different in vivo Haemodynamic Effects

Sarafotoxin-m (24 amino acids) from the venom of Atractaspis microlepidota microlepidota was the first long-sarafotoxin to be identified, while sarafotoxin-b (21 aa) is a short-sarafotoxin from Atractaspis engaddensis. Despite the presence of three additional C-terminus residues in sarafotoxin-m, these two peptides display a high sequence homology and share similar three-dimensional structures. However, unlike sarafotoxin-b, sarafotoxin-m shows a very low in vitro affinity for endothelin receptors, but still has a very high in vivo toxicity in mammals, similar to that of sarafotoxin-b. We have previously demonstrated, in vitro, the crucial role of the C-terminus extension in terms of pharmacological profiles and receptor affinities of long- versus short-sarafotoxins. One possible hypothesis to explain the high in vivo toxicity of sarafotoxin-m could be that its C-terminus extension is processed in vivo, resulting in short-like sarafotoxin. To address this possibility, we investigated, in the present study, the in vivo cardiovascular effects of sarafotoxin-b, sarafotoxin-m and sarafotoxin-m−Cter (sarafotoxin-m without the C -terminus extension). Male Wistar rats were anaesthetised and mechanically ventilated. Invasive haemodynamic measurements and echocardiographic measurements of left and right ventricular function were performed. The rats were divided into four groups that respectively received intravenous injections of: saline, sarafotoxin-b (one LD₅₀), sarafotoxin-m (one LD₅₀) or sarafotoxin-m−Cter (one LD₅₀). All measurements were performed at baseline, at 1 minute (+1) and at 6 minutes (+6) after injection. Results: Sarafotoxin-b and sarafotoxin-m-Cter decreased cardiac output and impaired left ventricle systolic and diastolic function, whilst sarafotoxin-m decreased cardiac output, increased airway pressures and led to acute right ventricular dilatation associated with a decreased tricuspid annulus peak systolic velocity. Sarafotoxin-b and sarafotoxin-m−Cter appear to exert toxic effects via impairment of left ventricular function, whilst sarafotoxin-m increases airway pressures and impairs right ventricular function. These results do not support the hypothesis of an in vivo processing of long sarafotoxins.

High-throughput production of two disulphide-bridge toxins

A quick and efficient production method compatible with high-throughput screening was developed using 36 toxins belonging to four different families of two disulphide-bridge toxins. Final toxins were characterized using HPLC co-elution, CD and pharmacological studies.

Privileged frameworks from snake venom

Venom as a form of chemical prey capture is a key innovation that has underpinned the explosive radiation of the advanced snakes (Caenophidia). Small venom proteins are often rich in disulfide bonds thus facilitating stable molecular scaffolds that present key functional residues on the protein surface. New toxin types are initially developed through the venom gland over-expression of normal body proteins, their subsequent gene duplication and diversification that leads to neofunctionalisation as random mutations modify their structure and function. This process has led to preferentially selected (privileged) cysteine-rich scaffolds that enable the snake to build arrays of toxins many of which may lead to therapeutic products and research tools. This review focuses on cysteine-rich small proteins and peptides found in snake venoms spanning natriuretic peptides to phospholipase enzymes, while highlighting their three-dimensional structures and biological functions as well as their potential as therapeutic agents or research tools.

Atractaspis aterrima toxins: the first insight into the molecular evolution of venom in side-stabbers

Although snake venoms have been the subject of intense research, primarily because of their tremendous potential as a bioresource for design and development of therapeutic compounds, some specific groups of snakes, such as the genus Atractaspis, have been completely neglected. To date only limited number of toxins, such as sarafotoxins have been well characterized from this lineage. In order to investigate the molecular diversity of venom from Atractaspis aterrima—the slender burrowing asp, we utilized a high-throughput transcriptomic approach completed with an original bioinformatics analysis pipeline. Surprisingly, we found that Sarafotoxins do not constitute the major ingredient of the transcriptomic cocktail; rather a large number of previously well-characterized snake venom-components were identified. Notably, we recovered a large diversity of three-finger toxins (3FTxs), which were found to have evolved under the significant influence of positive selection. From the normalized and non-normalized transcriptome libraries, we were able to evaluate the relative abundance of the different toxin groups, uncover rare transcripts, and gain new insight into the transcriptomic machinery. In addition to previously characterized toxin families, we were able to detect numerous highly-transcribed compounds that possess all the key features of venom-components and may constitute new classes of toxins.

Pharmacological and structural characterization of long-sarafotoxins, a new family of endothelin-like peptides: Role of the C-terminus extension

Long-sarafotoxins (l-SRTXs) have recently been identified in both the venom of Atractaspis microlepidota and that of Atractaspis irregularis. They are characterized by different C-terminus extensions that follow the invariant Trp21, which plays a crucial role in endothelin-receptor binding. We initially determined the toxicity and three-dimensional structures of two chemically synthesized l-SRTXs that have different C-terminus extensions, namely SRTX-m (24 aa, including extension "D-E-P") and SRTX-i3 (25 aa, including extension "V-N-R-N"). Both peptides were shown to be highly toxic in mice and displayed the cysteine-stabilized α-helical motif that characterizes endothelins and short-SRTXs, to which a longer C-terminus with variable flexibility is added. To discern the functional and pharmacological consequences of the supplementary amino acids, different chimerical as well as truncated forms of SRTX were designed and synthesized. Thus, we either removed the extra-C-terminal residues of SRTX-m or i3, or grafted the latter onto the C-terminal extremity of a short-SRTX (s-SRTX) (ie. SRTX-b). Our competitive binding assays where SRTXs competed for iodinated endothelin-1 binding to cloned ET(A) and ET(B) receptor subtypes over-expressed in CHO cells, revealed the essential role of the C-terminus extensions for ET-receptor recognition. Indeed, l-SRTXs displayed an affinity three to four orders of magnitude lower as compared to SRTX-b for the two receptor subtypes. Moreover, grafting the C-terminus extension to SRTX-b induced a drastic decrease in affinity, while its removal (truncated l-SRTXs) yielded an affinity for ET-receptors similar to that of s-SRTXs. Furthermore, we established by intracellular Ca(2+) measurements that l-SRTXs, as well as s-SRTXs, display agonistic activities. We thus confirmed in these functional assays the major difference in potency for these two SRTX families as well as the crucial role of the C-terminus extension in their various pharmacological profiles. Finally, one of the chimeric toxin synthesized in this study appears to be one of the most potent and selective ligand of the ET(B) receptor known to date.

Pharmacological characteristics of endothelin receptors on sheep rectal blood vessels

Haemorrhoids is associated with high blood flow of the anorectal region. The question of whether pharmacological manipulation of vascular supply can relieve the symptoms of haemorrhoids has been raised. In order to undertake this type of clinical investigation, it is first essential to gain a better understanding of the properties of vascular receptors that may regulate blood flow into anal cushions and haemorrhoids. Due to the limited availability of human anorectal specimens and the good reliability of sheep tissue as an experimental model of human anorectal diseases, we studied the properties of endothelin receptors in sheep rectal artery (SRA) and vein (SRV), the vessels contributing to the blood flow of haemorrhoidal plexus, using isometric tension recordings. We found that endothelin-1 and sarafotoxin 6a were very potent constrictor agents in both SRA and SRV. The selective ET(A) receptor antagonist PD156707 (100 nM) produced a parallel rightward displacement of ET-1-induced contractions in both vessels and abolished sarafotoxin 6a-induced contractions in the SRA. PD156707 (3 μM) practically abolished contractions to ET-1 in the SRA, suggesting that the response is entirely mediated by ET(A) receptors. While, the selective ET(B) receptor antagonist BQ788 (100 nM) caused no significant change in ET-1-induced contractions in both vessels, a minor role for ET(B) receptor subtype to responses to sarafotoxin 6a in the artery was suggested.

Exploring the venom proteome of the western diamondback rattlesnake, Crotalus atrox, via snake venomics and combinatorial peptide ligand library approaches

We report the proteomic characterization of the venom of the medically important North American western diamondback rattlesnake, Crotalus atrox, using two complementary approaches: snake venomics (to gain an insight of the overall venom proteome), and two solid-phase combinatorial peptide ligand libraries (CPLL), followed by 2D electrophoresis and mass spectrometric characterization of in-gel digested protein bands (to capture and "amplify" low-abundance proteins). The venomics approach revealed approximately 24 distinct proteins belonging to 2 major protein families (snake venom metalloproteinases, SVMP, and serine proteinases), which represent 69.5% of the total venom proteins, 4 medium abundance families (medium-size disintegrin, PLA(2), cysteine-rich secretory protein, and l-amino acid oxidase) amounting to 25.8% of the venom proteins, and 3 minor protein families (vasoactive peptides, endogenous inhibitor of SVMP, and C-type lectin-like). This toxin profile potentially explains the cytotoxic, myotoxic, hemotoxic, and hemorrhagic effects evoked by C. atrox envenomation. Further, our results showing that C. atrox exhibits a similar level of venom variation as Sistrurus miliarius points to a "diversity gain" scenario in the lineage leading to the Sistrurus catenatus taxa. On the other hand, the two combinatorial hexapeptide libraries captured distinct sets of proteins. Although the CPLL-treated samples did not retain a representative venom proteome, protein spots barely, or not at all, detectable in the whole venom were enriched in the two CPLL-treated samples. The amplified low copy number C. atrox venom proteins comprised a C-type lectin-like protein, several PLA(2) molecules, PIII-SVMP isoforms, glutaminyl cyclase isoforms, and a 2-cys peroxiredoxin highly conserved across the animal kingdom. Peroxiredoxin and glutaminyl cyclase may participate, respectively, in redox processes leading to the structural/functional diversification of toxins, and in the N-terminal pyrrolidone carboxylic acid formation required in the maturation of bioactive peptides such as bradykinin-potentiating peptides and endogenous inhibitors of metalloproteases. Our findings underscore the usefulness of combinatorial peptide libraries as powerful tools for mining below the tip of the iceberg, complementing thereby the data gained using the snake venomics protocol toward a complete visualization of the venom proteome.

Engineered sarafotoxins as tissue inhibitor of metalloproteinases-like matrix metalloproteinase inhibitors

The sarafotoxins and endothelins are approximately 25-residue peptides that spontaneously fold into a defined tertiary structure with specific pairing of four cysteines into two disulfide bonds. Their structures show an interesting topological similarity to the core of the metalloproteinase interaction sites of the tissue inhibitors of metalloproteinases. Previous work indicates that sarafotoxins and endothelins can be engineered to eliminate or greatly reduce their vasopressive action and that their structural framework can withstand multiple sequence changes. When sarafotoxin 6b, which possesses modest matrix metalloproteinase inhibitory activity, was C-terminally truncated to remove its toxic vasopressive activity, the metalloproteinase inhibitory activity was essentially abolished. However, further changes, based on the sequences of peptides selected from libraries of sarafotoxin variants or suggested by analogy with tissue inhibitors of metalloproteinases, progressively enhanced the matrix metalloproteinase inhibitory activity. Peptide variants with multiple substitutions folded correctly and formed native disulfide bonds. Improvements in matrix metalloproteinase affinity have generated a peptide with micromolar K(i) values for matrix metalloproteinase-1 and -9 that are selective inhibitors of different metalloproteinases. Characterization of its solution structure indicates a close similarity to sarafotoxin but with a more extended C-terminal helix. The effects of N-acetylation and other changes, as well as docking studies, support the hypothesis that the engineered sarafotoxins bind to matrix metalloproteinases in a manner analogous to the tissue inhibitors of metalloproteinases.

Characteristics of neural and humoral systems involved in the regulation of blood pressure in snakes

Cardiovascular function is affected by many mechanisms, including the autonomic system, the kallikrein-kinin system (KKS), the renin-angiotensin system (RAS) and the endothelin system. The function of these systems seems to be fairly well preserved throughout the vertebrate scale, but evolution required several adaptations. Snakes are particularly interesting for studies related to the cardiovascular function because of their elongated shape, their wide variation in size and length, and because they had to adapt to extremely different habitats and gravitational influences. To keep the normal cardiovascular control the snakes developed anatomical and functional adaptations and interesting structural peculiarities are found in their autonomic, KKS, RAS and endothelin systems. Our laboratory has characterized some biochemical, pharmacological and physiological properties of these systems in South American snakes. This review compares the components and function of these systems in snakes and other vertebrates, and focuses on differences found in snakes, related with receptor or ligand structure and/or function in autonomic system, RAS and KKS, absence of components in KKS and the intriguing identity between a venom and a plasma component in the endothelin system.

Proteomic and peptidomic analysis of the venom from Chinese tarantulaChilobrachys jingzhao

Chinese tarantula, Chilobrachys jingzhao is one of the most venomous spiders in southern China and its venom is a mixture of various compounds with diversified biological activities. The proteome of C. jingzhao venom was analyzed by proteomic techniques. Proteins with molecular weight of over 10 kDa, indicated by gel-filtration and SDS-PAGE, were analyzed using 2-DE and MALDI-TOF/TOF and LC/ESI-Q-TOF MS. More than 90 proteins were detected, with 47 confirmed by sequence similarity search using mass spectrum driven basic local alignment search tool (MS BLAST). On the other hand, peptides with MW lower than 10 kDa were separated by HPLC and identified by MALDI-TOF MS and Edman degradation sequencing. About 120 peptides were detected, 60 of which were fully or partially sequenced. Our results indicate that peptides with MW lower than 10 kDa are the major components in the crude venom of C. jingzhao. Like those of other tarantulas, these peptides are very likely to act on various ion channels. These results pave a way for further detailed structure-function correlation analysis of the individual toxins present in the venom of C. jingzhao.

The venom of the snake genus Atheris contains a new class of peptides with clusters of histidine and glycine residues

We investigated venoms from members of the genus Atheris (Serpentes, Viperidae), namely the rough scale bush viper (Atheris squamigera), the green bush viper (A. chlorechis) and the great lakes bush viper (A. nitschei), using mass spectrometry-based strategies, relying on matrix-assisted laser desorption/ionisation time-of-flight mass spectrometry (MALDI-TOF-MS) and electrospray ionisation tandem mass spectrometry (ESI-MS/MS) with de novo peptide sequencing. We discovered a set of novel peptides with masses in the 2-3 kDa range and containing poly-His and poly-Gly segments (pHpG). Complete primary structural elucidation and confirmation of two sequences by Edman degradation indicated the consensus sequence EDDH(9)GVG(10). Bioinformatic investigations in protein sequence databanks did not show relevant homology with known peptides or proteins. However, a more extensive investigation of data in nucleic acid databases revealed some similarities to the precursor sequences of bradykinin potentiating peptides (BPP) and C-type natriuretic peptides (CNP), agents that are known to affect the cardiovascular system by acting on specific metalloproteases and receptors. The novel pHpG peptides found in Atheris venoms might also act on the cardiovascular system by inhibiting particular metalloproteases, which however remain to be identified.

Mass spectrometry strategies for venom mapping and peptide sequencing from crude venoms: case applications with single arthropod specimen

Due to their complexity and diversity, animal venoms represent an extensive source of bioactive compounds such as peptides and proteins. Conventional approaches for their characterization often require large quantities of biological material. Current mass spectrometry (MS) techniques now give access to a wealth of information in a short working time frame with minute amounts of sample. Such MS approaches may now be used for the discovery of novel compounds, and once their structure has been determined they may be synthesized and tested for functional activity. Molecular mass fingerprints of venoms allow the rapid identification of known toxins as well as preliminary structural characterization of new compounds. De novo peptide sequencing by tandem mass spectrometry (MS/MS) offers rapid access to partial or total primary peptide structures. This article, written as a tutorial, also contains new material: molecular mass fingerprint analysis of Orthochirus innesi scorpion venom, and identification of components from bumblebee Bombus lapidarius venom, both collected from one single specimen. The structure of the three major peptides detected in the Bombus venom was fully characterized in one working day by de novo sequencing using an electrospray ionization hybrid quadrupole time-of-flight instrument (ESI-QqTOF) and a matrix-assisted laser desorption ionization time-of-flight instrument (MALDI-LIFT-TOF-TOF). After presenting the MS-based sequence elucidation, perspectives in using MS and MS/MS techniques in toxinology are discussed.

Identification and functional analysis of a novel bradykinin inhibitory peptide in the venoms of New World Crotalinae pit vipers

A novel undecapeptide has been isolated and structurally characterized from the venoms of three species of New World pit vipers from the subfamily, Crotalinae. These include the Mexican moccasin (Agkistrodon bilineatus), the prairie rattlesnake (Crotalus viridis viridis), and the South American bushmaster (Lachesis muta). The peptide was purified from all three venoms using a combination of gel permeation chromatography and reverse-phase HPLC. Automated Edman degradation sequencing and MALDI-TOF mass spectrometry established its peptide primary structure as: Thr-Pro-Pro-Ala-Gly-Pro-Asp-Val-Gly-Pro-Arg-OH, with a non-protonated molecular mass of 1063.18 Da. A synthetic replicate of the peptide was found to be an antagonist of bradykinin action at the rat vascular B2 receptor. This is the first bradykinin inhibitory peptide isolated from snake venom. Database searching revealed the peptide to be highly structurally related (10/11 residues) with a domain residing between the bradykinin-potentiating peptide and C-type natriuretic peptide domains of a recently cloned precursor from tropical rattlesnake (Crotalus durissus terrificus) venom gland. BIP thus represents a novel biological entity from snake venom.