Chemical synthesis of MT1 and MT7 muscarinic toxins: critical role of Arg-34 in their interaction with M1 muscarinic receptor

MT9, a natural peptide from black mamba venom antagonizes the muscarinic type 2 receptor and reverses the M2R-agonist-induced relaxation in rat and human arteries

All five muscarinic receptors have important physiological roles. The endothelial M2 and M3 subtypes regulate arterial tone through direct coupling to Gq or Gi/o proteins. Yet, we lack selective pharmacological drugs to assess the respective contribution of muscarinic receptors to a given function. We used mamba snake venoms to identify a selective M2R ligand to investigate its contribution to arterial contractions. Using a bio-guided screening binding assay, we isolated MT9 from the black mamba venom, a three-finger toxin active on the M2R subtype. After sequencing and chemical synthesis of MT9, we characterized its structure by X-ray diffraction and determined its pharmacological characteristics by binding assays, functional tests, and ex vivo experiments on rat and human arteries. Although MT9 belongs to the three-finger fold toxins family, it is phylogenetically apart from the previously discovered muscarinic toxins, suggesting that two groups of peptides evolved independently and in a convergent way to target muscarinic receptors. The affinity of MT9 for the M2R is 100 times stronger than that for the four other muscarinic receptors. It also antagonizes the M2R/G_i pathways in cell-based assays. MT9 acts as a non-competitive antagonist against acetylcholine or arecaine, with low nM potency, for the activation of isolated rat mesenteric arteries. These results were confirmed on human internal mammary arteries. In conclusion, MT9 is the first fully characterized M2R-specific natural toxin. It should provide a tool for further understanding of the effect of M2R in various arteries and may position itself as a new drug candidate in cardio-vascular diseases.

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.

Allosteric Modulator Leads Hiding in Plain Site: Developing Peptide and Peptidomimetics as GPCR Allosteric Modulators

Allosteric modulators (AMs) of G-protein coupled receptors (GPCRs) are desirable drug targets because they can produce fewer on-target side effects, improved selectivity, and better biological specificity (e.g., biased signaling or probe dependence) than orthosteric drugs. An underappreciated source for identifying AM leads are peptides and proteins-many of which were evolutionarily selected as AMs-derived from endogenous protein-protein interactions (e.g., transducer/accessory proteins), intramolecular receptor contacts (e.g., pepducins or extracellular domains), endogenous peptides, and exogenous libraries (e.g., nanobodies or conotoxins). Peptides offer distinct advantages over small molecules, including high affinity, good tolerability, and good bioactivity, and specific disadvantages, including relatively poor metabolic stability and bioavailability. Peptidomimetics are molecules that combine the advantages of both peptides and small molecules by mimicking the peptide's chemical features responsible for bioactivity while improving its druggability. This review 1) discusses sources and strategies to identify peptide/peptidomimetic AMs, 2) overviews strategies to convert a peptide lead into more drug-like "peptidomimetic," and 3) critically analyzes the advantages, disadvantages, and future directions of peptidomimetic AMs. While small molecules will and should play a vital role in AM drug discovery, peptidomimetics can complement and even exceed the advantages of small molecules, depending on the target, site, lead, and associated factors.

Interaction of Synthetic Human SLURP-1 with the Nicotinic Acetylcholine Receptors

Human SLURP-1 is a secreted protein of the Ly6/uPAR/three-finger neurotoxin family that co-localizes with nicotinic acetylcholine receptors (nAChRs) and modulates their functions. Conflicting biological activities of SLURP-1 at various nAChR subtypes have been based on heterologously produced SLURP-1 containing N- and/or C-terminal extensions. Here, we report the chemical synthesis of the 81 amino acid residue human SLURP-1 protein, characterization of its 3D structure by NMR, and its biological activity at nAChR subtypes. Radioligand assays indicated that synthetic SLURP-1 did not compete with [¹²⁵I]-α-bungarotoxin (α-Bgt) binding to human neuronal α7 and Torpedo californica muscle-type nAChRs, nor to mollusk acetylcholine binding proteins (AChBP). Inhibition of human α7-mediated currents only occurred in the presence of the allosteric modulator PNU120596. In contrast, we observed robust SLURP-1 mediated inhibition of human α3β4, α4β4, α3β2 nAChRs, as well as human and rat α9α10 nAChRs. SLURP-1 inhibition of α9α10 nAChRs was accentuated at higher ACh concentrations, indicating an allosteric binding mechanism. Our results are discussed in the context of recent studies on heterologously produced SLURP-1 and indicate that N-terminal extensions of SLURP-1 may affect its activity and selectivity on its targets. In this respect, synthetic SLURP-1 appears to be a better probe for structure-function studies.

Green mamba peptide targets type-2 vasopressin receptor against polycystic kidney disease

Polycystic kidney diseases (PKDs) are genetic disorders that can cause renal failure and death in children and adults. Lowering cAMP in cystic tissues through the inhibition of the type-2 vasopressin receptor (V2R) constitutes a validated strategy to reduce disease progression. We identified a peptide from green mamba venom that exhibits nanomolar affinity for the V2R without any activity on 155 other G-protein-coupled receptors or on 15 ionic channels. Mambaquaretin-1 is a full antagonist of the V2R activation pathways studied: cAMP production, beta-arrestin interaction, and MAP kinase activity. This peptide adopts the Kunitz fold known to mostly act on potassium channels and serine proteases. Mambaquaretin-1 interacts selectively with the V2R through its first loop, in the same manner that aprotinin inhibits trypsin. Injected in mice, mambaquaretin-1 increases in a dose-dependent manner urine outflow with concomitant reduction of urine osmolality, indicating a purely aquaretic effect associated with the in vivo blockade of V2R. CD1-pcy/pcy mice, a juvenile model of PKD, daily treated with 13 [Formula: see text]g of mambaquaretin-1 for 99 d, developed less abundant (by 33%) and smaller (by 47%) cysts than control mice. Neither tachyphylaxis nor apparent toxicity has been noted. Mambaquaretin-1 represents a promising therapeutic agent against PKDs.

Ancestral protein resurrection and engineering opportunities of the mamba aminergic toxins

Mamba venoms contain a multiplicity of three-finger fold aminergic toxins known to interact with various α-adrenergic, muscarinic and dopaminergic receptors with different pharmacological profiles. In order to generate novel functions on this structural scaffold and to avoid the daunting task of producing and screening an overwhelming number of variants generated by a classical protein engineering strategy, we accepted the challenge of resurrecting ancestral proteins, likely to have possessed functional properties. This innovative approach that exploits molecular evolution models to efficiently guide protein engineering, has allowed us to generate a small library of six ancestral toxin (AncTx) variants and associate their pharmacological profiles to key functional substitutions. Among these variants, we identified AncTx1 as the most α_1A-adrenoceptor selective peptide known to date and AncTx5 as the most potent inhibitor of the three α2 adrenoceptor subtypes. Three positions in the ρ-Da1a evolutionary pathway, positions 28, 38 and 43 have been identified as key modulators of the affinities for the α₁ and α_2C adrenoceptor subtypes. Here, we present a first attempt at rational engineering of the aminergic toxins, revealing an epistasis phenomenon.

The three-finger toxin fold: a multifunctional structural scaffold able to modulate cholinergic functions

Three-finger fold toxins are miniproteins frequently found in Elapidae snake venoms. This fold is characterized by three distinct loops rich in β-strands and emerging from a dense, globular core reticulated by four highly conserved disulfide bridges. The number and diversity of receptors, channels, and enzymes identified as targets of three-finger fold toxins is increasing continuously. Such manifold diversity highlights the specific adaptability of this fold for generating pleiotropic functions. Although this toxin superfamily disturbs many biological functions by interacting with a large diversity of molecular targets, the most significant target is the cholinergic system. By blocking the activity of the nicotinic and muscarinic acetylcholine receptors or by inhibiting the enzyme acetylcholinesterase, three-finger fold toxins interfere most drastically with neuromuscular junction functioning. Several of these toxins have become powerful pharmacological tools for studying the function and structure of their molecular targets. Most importantly, since dysfunction of these receptors/enzyme is involved in many diseases, exploiting the three-finger scaffold to create novel, highly specific therapeutic agents may represent a major future endeavor. This is an article for the special issue XVth International Symposium on Cholinergic Mechanisms.

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.

Mambalgin-1 Pain-relieving Peptide, Stepwise Solid-phase Synthesis, Crystal Structure, and Functional Domain for Acid-sensing Ion Channel 1a Inhibition

Mambalgins are peptides isolated from mamba venom that specifically inhibit a set of acid-sensing ion channels (ASICs) to relieve pain. We show here the first full stepwise solid phase peptide synthesis of mambalgin-1 and confirm the biological activity of the synthetic toxin both in vitro and in vivo. We also report the determination of its three-dimensional crystal structure showing differences with previously described NMR structures. Finally, the functional domain by which the toxin inhibits ASIC1a channels was identified in its loop II and more precisely in the face containing Phe-27, Leu-32, and Leu-34 residues. Moreover, proximity between Leu-32 in mambalgin-1 and Phe-350 in rASIC1a was proposed from double mutant cycle analysis. These data provide information on the structure and on the pharmacophore for ASIC channel inhibition by mambalgins that could have therapeutic value against pain and probably other neurological disorders.

A straightforward method for automated Fmoc-based synthesis of bio-inspired peptide crypto-thioesters

A bio-inspired method for the synthesis of peptide thioester surrogates for native chemical ligation was developed. The process can be fully automated and does not require postsynthetic steps.

Despite recent advances, the direct Fmoc-based solid phase synthesis of peptide α-thioesters for the convergent synthesis of proteins via native chemical ligation (NCL) remains a challenge in the field. We herein report a simple and general methodology, enabling access to peptide thioester surrogates. A novel C-terminal N-(2-hydroxybenzyl)cysteine thioesterification device based on an amide-to-thioester rearrangement was developed, and the resulting peptide crypto-thioesters can be directly used in NCL reactions with fast N → S shift kinetics at neutral pH. These fast kinetics arise from our bio-inspired design, via intein-like intramolecular catalysis. Due to a well-positioned phenol moiety, an impressive >50 fold increase in the kinetic rate is observed compared to an O-methylated derivative. Importantly, the synthesis of this new device can be fully automated using inexpensive commercially available materials and does not require any post-synthetic steps prior to NCL. We successfully applied this new method to the synthesis of two long naturally-occurring cysteine-rich peptide sequences.

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.

Structural determinants for the interactions between muscarinic toxin 7 and muscarinic acetylcholine receptors

Muscarinic acetylcholine receptors (mAChRs) have five subtypes and play crucial roles in various physiological functions and pathophysiological processes. Poor subtype specificity of mAChR modulators has been an obstacle to discover new therapeutic agents. Muscarinic toxin 7 (MT7) is a natural peptide toxin with high selectivity for the M1 receptor. With three to five residues substituted, M3, M4, and M5 receptor mutants could bind to MT7 at nanomolar concentration as the M1 receptor. However, the structural mechanisms explaining MT7-mAChRs binding are still largely unknown. In this study, we constructed 10 complex models of MT7 and each mAChR subtype or its mutant, performed molecular dynamics simulations, and calculated the binding energies to investigate the mechanisms. Our results suggested that the structural determinants for the interactions on mAChRs were composed of some critical residues located separately in the extracellular loops of mAChRs, such as Glu4.56, Leu4.60, Glu/Gln4.63, Tyr4.65, Glu/Asp6.67, and Trp7.35. The subtype specificity of MT7 was attributed to the non-conserved residues at positions 4.56 and 6.67. These structural mechanisms could facilitate the discovery of novel mAChR modulators with high subtype specificity and enhance the understanding of the interactions between ligands and G-protein-coupled receptors.

Orthosteric binding of ρ-Da1a, a natural peptide of snake venom interacting selectively with the α1A-adrenoceptor

ρ-Da1a is a three-finger fold toxin from green mamba venom that is highly selective for the α_1A-adrenoceptor. This toxin has atypical pharmacological properties, including incomplete inhibition of ³H-prazosin or ¹²⁵I-HEAT binding and insurmountable antagonist action. We aimed to clarify its mode of action at the α_1A-adrenoceptor. The affinity (pKi 9.26) and selectivity of ρ-Da1a for the α_1A-adrenoceptor were confirmed by comparing binding to human adrenoceptors expressed in eukaryotic cells. Equilibrium and kinetic binding experiments were used to demonstrate that ρ-Da1a, prazosin and HEAT compete at the α_1A-adrenoceptor. ρ-Da1a did not affect the dissociation kinetics of ³H-prazosin or ¹²⁵I-HEAT, and the IC₅₀ of ρ-Da1a, determined by competition experiments, increased linearly with the concentration of radioligands used, while the residual binding by ρ-Da1a remained stable. The effect of ρ-Da1a on agonist-stimulated Ca²⁺ release was insurmountable in the presence of phenethylamine- or imidazoline-type agonists. Ten mutations in the orthosteric binding pocket of the α_1A-adrenoceptor were evaluated for alterations in ρ-Da1a affinity. The D106^3.32A and the S188^5.42A/S192^5.46A receptor mutations reduced toxin affinity moderately (6 and 7.6 times, respectively), while the F86^2.64A, F288^6.51A and F312^7.39A mutations diminished it dramatically by 18- to 93-fold. In addition, residue F86^2.64 was identified as a key interaction point for ¹²⁵I-HEAT, as the variant F86^2.64A induced a 23-fold reduction in HEAT affinity. Unlike the M1 muscarinic acetylcholine receptor toxin MT7, ρ-Da1a interacts with the human α_1A-adrenoceptor orthosteric pocket and shares receptor interaction points with antagonist (F86^2.64, F288^6.51 and F312^7.39) and agonist (F288^6.51 and F312^7.39) ligands. Its selectivity for the α_1A-adrenoceptor may result, at least partly, from its interaction with the residue F86^2.64, which appears to be important also for HEAT binding.

New α-adrenergic property for synthetic MTβ and CM-3 three-finger fold toxins from black mamba

Despite their isolation more than fifteen years ago from the venom of the African mamba Dendroaspis polylepis, very few data are known on the functional activity of MTβ and CM-3 toxins. MTβ was initially classified as a muscarinic toxin interacting non-selectively and with low affinity with the five muscarinic receptor subtypes while no biological function was determined for CM-3. Recent results highlight the multifunctional activity of three-finger fold toxins for muscarinic and adrenergic receptors and reveal some discrepancies in the pharmacological profiles of their venom-purified and synthetic forms. Here, we report the pharmacological characterization of chemically-synthesized MTβ and CM-3 toxins on nine subtypes of muscarinic and adrenergic receptors and demonstrate their high potency for α-adrenoceptors and in particular a sub-nanomolar affinity for the α1A-subtype. Strikingly, no or very weak affinity were found for muscarinic receptors, highlighting that pharmacological characterizations of venom-purified peptides may be risky due to possible contaminations. The biological profile of these two homologous toxins looks like that one previously reported for the Dendroaspis angusticeps ρ-Da1a toxin. Nevertheless, MTβ and CM-3 interact more potently than ρ-Da1a with α1B- and α1D-AR subtypes. A computational analysis of the stability of the MTβ structure suggests that mutation S38I, could be involved in this gain in function.

Engineering of three-finger fold toxins creates ligands with original pharmacological profiles for muscarinic and adrenergic receptors

Protein engineering approaches are often a combination of rational design and directed evolution using display technologies. Here, we test “loop grafting,” a rational design method, on three-finger fold proteins. These small reticulated proteins have exceptional affinity and specificity for their diverse molecular targets, display protease-resistance, and are highly stable and poorly immunogenic. The wealth of structural knowledge makes them good candidates for protein engineering of new functionality. Our goal is to enhance the efficacy of these mini-proteins by modifying their pharmacological properties in order to extend their use in imaging, diagnostics and therapeutic applications. Using the interaction of three-finger fold toxins with muscarinic and adrenergic receptors as a model, chimeric toxins have been engineered by substituting loops on toxin MT7 by those from toxin MT1. The pharmacological impact of these grafts was examined using binding experiments on muscarinic receptors M1 and M4 and on the α_1A-adrenoceptor. Some of the designed chimeric proteins have impressive gain of function on certain receptor subtypes achieving an original selectivity profile with high affinity for muscarinic receptor M1 and α_1A-adrenoceptor. Structure-function analysis supported by crystallographic data for MT1 and two chimeras permits a molecular based interpretation of these gains and details the merits of this protein engineering technique. The results obtained shed light on how loop permutation can be used to design new three-finger proteins with original pharmacological profiles.

Adrenoceptor activity of muscarinic toxins identified from mamba venoms

BACKGROUND AND PURPOSE

Muscarinic toxins (MTs) are snake venom peptides named for their ability to interfere with ligand binding to muscarinic acetylcholine receptors (mAChRs). Recent data infer that these toxins may have other G-protein-coupled receptor targets than the mAChRs. The purpose of this study was to systematically investigate the interactions of MTs with the adrenoceptor family members.

EXPERIMENTAL APPROACH

We studied the interaction of four common MTs, MT1, MT3, MT7 and MTα, with cloned receptors expressed in insect cells by radioligand binding. Toxins showing modest to high-affinity interactions with adrenoceptors were additionally tested for effects on functional receptor responses by way of inhibition of agonist-induced Ca²⁺ increases.

KEY RESULTS

All MTs behaved non-competitively in radioligand displacement binding. MT1 displayed higher binding affinity for the human α(2B)-adrenoceptor (IC₅₀ = 2.3 nM) as compared with muscarinic receptors (IC₅₀ ≥ 100 nM). MT3 appeared to have a broad spectrum of targets showing high-affinity binding (IC₅₀ = 1-10 nM) to M₄ mAChR, α(1A)-, α(1D)- and α(2A)-adrenoceptors and lower affinity binding (IC₅₀ ≥ 25 nM) to α(1B)- and α(2C)-adrenoceptors and M₁ mAChR. MT7 did not detectably bind to other receptors than M₁, and MTα was specific for the α(2B)-adrenoceptor. None of the toxins showed effects on β₁- or β₂-adrenoceptors.

CONCLUSIONS AND IMPLICATIONS

Some of the MTs previously found to interact predominantly with mAChRs were shown to bind with high affinity to selected adrenoceptor subtypes. This renders these peptide toxins useful for engineering selective ligands to target various adrenoceptors.

Selective targeting of G-protein-coupled receptor subtypes with venom peptides

The G-protein-coupled receptor (GPCR) family is one of the largest gene superfamilies with approx. 370 members responding to endogenous ligands in humans and a roughly equal amount of receptors sensitive to external stimuli from the surrounding. A number of receptors from this superfamily are well recognized targets for medical treatment of various disease conditions, whereas for many others the potential medical benefit of interference is still obscure. A general problem associated with GPCR research and therapeutics is the insufficient specificity of available ligands to differentiate between closely homologous receptor subtypes. In this context, venom peptides could make a significant contribution to the development of more specific drugs. Venoms from certain animals specialized in biochemical hunting contain a mixture of molecules that are directed towards a variety of membrane proteins. Peptide toxins isolated from these mixtures usually exhibit high specificity for their targets. Muscarinic toxins found from mamba snakes attracted much attention during the 1990s. These are 65-66 amino acid long peptides with a structural three-finger folding similar to the α-neurotoxins and they target the muscarinic acetylcholine receptors in a subtype-selective manner. Recently, several members of the three-finger toxins from mamba snakes as well as conotoxins from marine cone snails have been shown to selectively interact with subtypes of adrenergic receptors. In this review, we will discuss the GPCR-directed peptide toxins found from different venoms and how some of these can be useful in exploring specific roles of receptor subtypes.

Glycosylphosphatidylinositol (GPI)-anchoring of mamba toxins enables cell-restricted receptor silencing

Muscarinic toxins (MTs) are snake venom peptides found to selectively target specific subtypes of G-protein-coupled receptors. In here, we have attached a glycosylphosphatidylinositol (GPI) tail to three different toxin molecules and evaluated their receptor-blocking effects in a heterologous expression system. MT7-GPI remained anchored to the cell surface and selectively inhibited M(1) muscarinic receptor signaling expressed in the same cell. To further demonstrate the utility of the GPI tail, we generated MT3- and MTα-like gene sequences and fused these to the signal sequence for GPI attachment. Functional assessment of these membrane-anchored toxins on coexpressed target receptors indicated a prominent antagonistic effect. In ligand binding experiments the GPI-anchored toxins were found to exhibit similar selection profiles among receptor subtypes as the soluble toxins. The results indicate that GPI attachment of MTs and related receptor toxins could be used to assess the role of receptor subtypes in specific organs or even cells in vivo by transgenic approaches.

Molecular conversion of muscarinic acetylcholine receptor M(5) to muscarinic toxin 7 (MT7)-binding protein

Muscarinic toxin 7 (MT7) is a mamba venom peptide that binds selectively to the M(1) muscarinic acetylcholine receptor. We have previously shown that the second (ECL2) and third (ECL3) extracellular loops of the M(1) receptor are critically involved in binding the peptide. In this study we used a mutagenesis approach on the M(5) subtype of the receptor family to find out if this possesses a similar structural architecture in terms of toxin binding as the M(1) receptor. An M(5) receptor construct (M(5)-E(175)Y(184)E(474)), mutated at the formerly deciphered critical residues on ECL2 and 3, gained the ability to bind MT7, but with rather low affinity as determined in a functional assay (apparent K(i) = 24 nM; apparent K(i) for M(1) = 0.5 nM). After screening for different domains and residues, we found a specific residue (P(179) to L in M(5)) in the middle portion of ECL2 that was necessary for high affinity binding of MT7 (M(5)-EL(179)YE, apparent K(i) = 0.5 nM). Mutation of P(179) to A confirmed a role for the leucine side chain in the binding of MT7. Together the results reveal new binding interactions between receptors and the MT7 peptide and strengthen the hypothesis that ECL2 sequence is of utmost importance for MT binding to muscarinic receptors.

Identification of a novel snake peptide toxin displaying high affinity and antagonist behaviour for the α2-adrenoceptors

BACKGROUND AND PURPOSE Muscarinic and adrenergic G protein-coupled receptors (GPCRs) are the targets of rare peptide toxins isolated from snake or cone snail venoms. We used a screen to identify novel toxins from Dendroaspis angusticeps targeting aminergic GPCRs. These toxins may offer new candidates for the development of new tools and drugs. EXPERIMENTAL APPROACH In binding experiments with (3) H-rauwolscine, we studied the interactions of green mamba venom fractions with α(2) -adrenoceptors from rat brain synaptosomes. We isolated, sequenced and chemically synthesized a novel peptide, ρ-Da1b. This peptide was pharmacologically characterized using binding experiments and functional tests on human α(2)-adrenoceptors expressed in mammalian cells. KEY RESULTS ρ-Da1b, a 66-amino acid peptide stabilized by four disulphide bridges, belongs to the three-finger-fold peptide family. Its synthetic homologue inhibited 80% of (3) H-rauwolscine binding to the three α(2)-adrenoceptor subtypes, with an affinity between 14 and 73 nM and Hill slopes close to unity. Functional experiments on α(2A) -adrenoceptor demonstrated that ρ-Da1b is an antagonist, shifting adrenaline activation curves to the right. Schild regression revealed slopes of 0.97 and 0.67 and pA(2) values of 5.93 and 5.32 for yohimbine and ρ-Da1b, respectively. CONCLUSIONS AND IMPLICATIONS ρ-Da1b is the first toxin identified to specifically interact with α(2)-adrenoceptors, extending the list of class A GPCRs sensitive to toxins. Additionally, its affinity and atypical mode of interaction open up the possibility of its use as a new pharmacological tool, in the study of the physiological roles of α(2)-adrenoceptor subtypes.

Muscarinic toxins

Muscarinic toxins isolated from the venom of Dendroaspis snakes may interact with a high affinity, large selectivity and various functional properties with muscarinic receptors. Therefore, these toxins are invaluable tools for studying the physiological role, molecular functioning and structural organization of the five subtypes of these G-Protein Coupled Receptors. We review the data on the most relevant results dealing with the isolation/identification, mode of action, structure/function and exploitation of these toxins and finally highlight the unresolved issues related to their pharmacological studies.

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.

Structural model of ligand-G protein-coupled receptor (GPCR) complex based on experimental double mutant cycle data: MT7 snake toxin bound to dimeric hM1 muscarinic receptor

The snake toxin MT7 is a potent and specific allosteric modulator of the human M1 muscarinic receptor (hM1). We previously characterized by mutagenesis experiments the functional determinants of the MT7-hM1 receptor interaction (Fruchart-Gaillard, C., Mourier, G., Marquer, C., Stura, E., Birdsall, N. J., and Servent, D. (2008) Mol. Pharmacol. 74, 1554-1563) and more recently collected evidence indicating that MT7 may bind to a dimeric form of hM1 (Marquer, C., Fruchart-Gaillard, C., Mourier, G., Grandjean, O., Girard, E., le Maire, M., Brown, S., and Servent, D. (2010) Biol. Cell 102, 409-420). To structurally characterize the MT7-hM1 complex, we adopted a strategy combining double mutant cycle experiments and molecular modeling calculations. First, thirty-three ligand-receptor proximities were identified from the analysis of sixty-one double mutant binding affinities. Several toxin residues that are more than 25 Å apart still contact the same residues on the receptor. As a consequence, attempts to satisfy all the restraints by docking the toxin onto a single receptor failed. The toxin was then positioned onto two receptors during five independent flexible docking simulations. The different possible ligand and receptor extracellular loop conformations were described by performing simulations in explicit solvent. All the docking calculations converged to the same conformation of the MT7-hM1 dimer complex, satisfying the experimental restraints and in which (i) the toxin interacts with the extracellular side of the receptor, (ii) the tips of MT7 loops II and III contact one hM1 protomer, whereas the tip of loop I binds to the other protomer, and (iii) the hM1 dimeric interface involves the transmembrane helices TM6 and TM7. These results structurally support the high affinity and selectivity of the MT7-hM1 interaction and highlight the atypical mode of interaction of this allosteric ligand on its G protein-coupled receptor target.

The three-finger toxin MTalpha is a selective alpha(2B)-adrenoceptor antagonist

Muscarinic toxins (MTs) are three-finger folded peptides isolated from mamba snake venoms. In this report we describe a selective antagonistic interaction of MTalpha with the human alpha(2B)-adrenoceptor. In a functional assay, measuring the alpha(2B)-adrenoceptor-induced increase in intracellular [Ca(2+)], we found that both venomous MTalpha and synthetic MTalpha inhibited the response in a concentration-dependent way. MTalpha did not affect the responses of alpha(2A)-, alpha(2C)-, alpha(1A)- or alpha(1B)-adrenoceptors. To further explore the binding of MTalpha to the alpha(2B)-adrenoceptor, we performed ligand binding experiments on Sf9 cell homogenates with [(3)H]RX821002 as reporter ligand. MTalpha bound to the receptor rather slowly requiring about 60 min to reach equilibrium. In equilibrium binding experiments, MTalpha displaced the radioligand with an IC(50) of 3.2 nM, but was not able to displace all bound radioligand. Using a saturation binding protocol, we found that MTalpha suppressed the maximum binding without any greater impact on the affinity of the radioligand, indicating a non-competitive mode of inhibition. The toxin bound reversibly to alpha(2B)-adrenoceptor, but extensive washing was needed for full recovery of binding sites at high toxin concentrations. Surprisingly, MTalpha did not affect [(3)H]-N-methylscopolamine binding to the muscarinic receptor subtypes at concentrations found to fully block alpha(2B)-adrenoceptors, showing that the toxin is a more potent antagonist for the alpha(2B)-adrenoceptor than for muscarinic receptors. These findings should open up new views in terms of selective adrenoceptor drug design as well as in elucidation of alpha(2)-adrenoceptor physiology.

Isolation and pharmacological characterization of AdTx1, a natural peptide displaying specific insurmountable antagonism of the alpha1A-adrenoceptor

BACKGROUND AND PURPOSE

Venoms are a rich source of ligands for ion channels, but very little is known about their capacity to modulate G-protein coupled receptor (GPCR) activity. We developed a strategy to identify novel toxins targeting GPCRs.

EXPERIMENTAL APPROACH

We studied the interactions of mamba venom fractions with alpha(1)-adrenoceptors in binding experiments with (3)H-prazosin. The active peptide (AdTx1) was sequenced by Edman degradation and mass spectrometry fragmentation. Its synthetic homologue was pharmacologically characterized by binding experiments using cloned receptors and by functional experiments on rabbit isolated prostatic smooth muscle.

KEY RESULTS

AdTx1, a 65 amino-acid peptide stabilized by four disulphide bridges, belongs to the three-finger-fold peptide family. It has subnanomolar affinity (K(i)= 0.35 nM) and high specificity for the human alpha(1A)-adrenoceptor subtype. We showed high selectivity and affinity (K(d)= 0.6 nM) of radio-labelled AdTx1 in direct binding experiments and revealed a slow association constant (k(on)= 6 x 10(6).M(-1).min(-1)) with an unusually stable alpha(1A)-adrenoceptor/AdTx1 complex (t(1/2diss)= 3.6 h). AdTx1 displayed potent insurmountable antagonism of phenylephrine's actions in vitro (rabbit isolated prostatic muscle) at concentrations of 10 to 100 nM.

CONCLUSIONS AND IMPLICATIONS

AdTx1 is the most specific and selective peptide inhibitor for the alpha(1A)-adrenoceptor identified to date. It displays insurmountable antagonism, acting as a potent relaxant of smooth muscle. Its peptidic nature can be exploited to develop new tools, as a radio-labelled-AdTx1 or a fluoro-labelled-AdTx1. Identification of AdTx1 thus offers new perspectives for developing new drugs for treating benign prostatic hyperplasia.

Muscarinic toxins: tools for the study of the pharmacological and functional properties of muscarinic receptors

Muscarinic receptors mediate metabotropic actions of acetylcholine in the CNS and PNS and autocrine functions of acetylcholine in non-neuronal systems. Because of the lack of highly selective muscarinic ligands, the precise location, functional role, and roles in various diseases of the five muscarinic receptor subtypes remain unclear. Muscarinic toxins isolated from the venom of Dendroaspis snakes have a natural high affinity and selectivity, associated with roles as competitive antagonists, allosteric modulators, and potential agonists. These toxins may therefore be invaluable tools for studying muscarinic receptors. We review data on the structural and pharmacological characterization of the muscarinic toxins, focusing on recent structure-function studies on toxin-receptor interactions. We discuss the potential benefits of using these toxins for investigating muscarinic function in vivo.

N-desmethylclozapine an M1 receptor agonist enhances nitric oxide's cardiac vagal facilitation in the isolated innervated rat right atrium

We have previously determined that neuronal nitric oxide (NO) may partly mediate its established cholinergic effect via activation of muscarinic type 1 (M1) receptors located at the preganglionic/postganglionic synapse. In this series of experiments we set out to confirm this finding using an M1 agonist. Experiments were carried out on the isolated vagally innervated right atrium in the presence of atenolol (4 microM). The right vagus was stimulated at 4, 8, 16, 32 Hz; pulse duration 1 ms at 20 V for 20 s and the effect on cardiac interval (ms) assessed. N-desmethylclozapine (100 nM), a potent M1 agonist, enhanced the vagally induced increase in cardiac interval, a lower concentration of 50 nM had no significant effect on cardiac interval. This effect was prevented by pre-treatment of the atria with the neuronal NO synthase inhibitor 1 (2-trifluoromethylphenyl)imidazole (TRIM) at 0.14 mM. The vagal stimulation protocol was repeated in order to rule out a reduction in vagal effectiveness which may have been due to the experimental stimulation protocol used in this study. TRIM (0.14 mM) alone causes a small but significant attenuation of the vagally induced increase in cardiac interval. These results show that agonism of M1 receptors on cardiac vagal preganglionic fibres enhances vagal cardiac effects which can be prevented by a neuronal NO inhibitor.

Muscarinic type 1 receptors mediate part of nitric oxide’s vagal facilitatory effect in the isolated innervated rat right atrium

We investigated whether vagal cardiac cholinergic facilitation by nitric oxide (NO) is mediated by cardiac muscarinic receptor subtypes in the vagally innervated rat right atrium in vitro. Experiments were carried out in the presence of atenolol (4 microM). The right vagus was stimulated at 4, 8, 16, 32 Hz; pulse duration 1 ms at 20 V for 20s; vagal postganglionic activation was achieved using nicotine (0.1, 0.3, 0.5, 1mM) and the effect on cardiac interval (ms) assessed. Pirenzepine (1 microM), a M1 antagonist, attenuated vagally induced increase in cardiac interval. L-Arginine (0.34 mM) superfused with pirenzepine failed to reverse this attenuation, however, L-arginine applied alone reversed the reduction vagal cardiac slowing. Similarly, sodium nitroprusside (10 microM) applied alone, and not together with pirenzepine, was able to reverse the attenuation of vagal effects caused by pirenzepine. Synthetic MT7 (1 nM) toxin, a selective M1 antagonist confirmed these results. M3 antagonism using para-fluorohexahydrosiladifenidol (p-F-HHSiD) (300 nM) and M4 antagonism with PD 102807 (200 nM) did not affect the vagally induced increase in cardiac interval. Nicotine induced increase in cardiac interval was not altered by pirenzepine. These results show that antagonism of M1 receptors on cardiac vagal preganglionic fibres reduces vagal efficacy which can be recovered by either a nitric oxide synthase substrate or a NO donor.

Denmotoxin, a three-finger toxin from the colubrid snake Boiga dendrophila (Mangrove Catsnake) with bird-specific activity

Boiga dendrophila (mangrove catsnake) is a colubrid snake that lives in Southeast Asian lowland rainforests and mangrove swamps and that preys primarily on birds. We have isolated, purified, and sequenced a novel toxin from its venom, which we named denmotoxin. It is a monomeric polypeptide of 77 amino acid residues with five disulfide bridges. In organ bath experiments, it displayed potent postsynaptic neuromuscular activity and irreversibly inhibited indirectly stimulated twitches in chick biventer cervicis nerve-muscle preparations. In contrast, it induced much smaller and readily reversible inhibition of electrically induced twitches in mouse hemidiaphragm nerve-muscle preparations. More precisely, the chick muscle alpha(1)betagammadelta-nicotinic acetylcholine receptor was 100-fold more susceptible compared with the mouse receptor. These data indicate that denmotoxin has a bird-specific postsynaptic activity. We chemically synthesized denmotoxin, crystallized it, and solved its crystal structure at 1.9 A by the molecular replacement method. The toxin structure adopts a non-conventional three-finger fold with an additional (fifth) disulfide bond in the first loop and seven additional residues at its N terminus, which is blocked by a pyroglutamic acid residue. This is the first crystal structure of a three-finger toxin from colubrid snake venom and the first fully characterized bird-specific toxin. Denmotoxin illustrates the relationship between toxin specificity and the primary prey type that constitutes the snake's diet.

Identification of Various Allosteric Interaction Sites on M1Muscarinic Receptor Using125I-Met35-Oxidized Muscarinic Toxin 7

Monoiodinated, Met35-oxidized muscarinic toxin 7 (MT7ox) was synthesized, and its affinity constants for free or N-methyl scopolamine (NMS)-occupied hM1 receptor were measured directly by equilibrium and kinetic binding experiments. Identical values were obtained with the two types of assay methods, 14 pM and 0.9 nM in free or NMS-liganded receptor states, respectively, highlighting a strong negative cooperativity between this allosteric toxin and NMS. Identical results were obtained with indirect binding experiments with [3H]NMS using the ternary complex model, clearly demonstrating the reciprocal nature of this cooperativity. Furthermore, the effects of various orthosteric and allosteric agents on the dissociation kinetic of 125I-MT7ox were measured and show that, except for the MT1 toxin, all of the ligands studied [NMS, atropine, gallamine, brucine, tacrine, staurosporine, and (9S,10S,12R)-2,3,9,10,11-hexahydro-10-hydroxy-9-methyl-1-oxo-9,12-epoxy-1H-diindolo[1,2,3-fg:3',2',1'-kl]pyrrolo[3,4-i][1,6]benzodiazocine-10-carboxylic acid hexyl ester (KT5720)] interact allosterically with muscarinic toxin 7. Equilibrium binding experiments with 125I-MT7ox and [3H]NMS were conducted to reveal the effects of these ligands on the free receptor, and affinity constants (pKx values) were calculated using the allosteric ternary complex model. Our results suggest that MT7 toxin interacts with hM1 receptor at a specific allosteric site, which may partially overlap those identified previously for "classic" or "atypical" allosteric agents and highlight the potential of this new allosteric tracer in studying allosterism at muscarinic receptors.

The pharmacological action of MT-7

The mamba toxin MT-7 is the most selective ligand currently available for the muscarinic M1 receptor subtype. The toxin binds stably to the receptor and blocks the agonist-induced activation non-competitively. Although its mode of action on M1 receptors is not yet fully understood, some of the toxin properties support an allosteric mechanism. Thus, the toxin fails to elicit a complete inhibition of the binding of either the muscarinic antagonist [3H]N-methyl-scopolamine ([3H]NMS) or the agonist [3H]acetylcholine ([3H]ACh). When added to ligand-occupied M1 receptors, the toxin slows the dissociation rate of [3H]NMS and increases that of [3H]ACh. Site-directed mutagenesis studies have provided important information about the toxin amino acid residues which are critical for the stable binding to the receptor and for the allosteric modulation of antagonist dissociation. In vivo studies have shown that the intracerebral injection of MT-7 causes a long-lasting blockade of M1 receptor, thus providing a tool for the characterization of the functional role of this receptor subtype in discrete brain areas.

Muscarinic Toxin 7 Selectivity Is Dictated by Extracellular Receptor Loops

Muscarinic toxin 7 (MT7) is a mamba venom protein antagonist with extremely high selectivity for the M1 muscarinic acetylcholine receptor. To map the sites for the interaction of MT7 with muscarinic receptors we have used chimeric M1:M3 receptors and site-directed mutagenesis of the M3 and M4 receptor subtypes. Two Glu residues in M1, one in extracellular loop 2 and one in extracellular loop 3, were found to be important for the high affinity binding of MT7. Substitution of the corresponding Lys residues in the M3 receptor with Glu converted the M3 mutant to an MT7 binding receptor, albeit with lower affinity compared with M1. A Phe --> Tyr substitution in extracellular loop 2 of M3 together with the 2 Glu mutations generated a receptor with an increased MT7 affinity (apparent Ki = 0.26 nM in a functional assay) compared with the M1 receptor (apparent Ki = 1.31 nM). The importance of the identified amino acid residues was confirmed with a mutated M4 receptor constructs. The results indicate that the high selectivity of MT7 for the M1 receptor depends on very few residues, thus providing good prospects for future design and synthesis of muscarinic receptor-selective ligands.

Action of the muscarinic toxin MT7 on agonist-bound muscarinic M1 receptors

The muscarinic toxin MT7 is the most selective ligand for the muscarinic M(1) receptors. Previous studies have shown that the toxin interacts with the antagonist-receptor complex and slows the antagonist dissociation rate, possibly by binding to an allosteric site and impeding the access to and egress from the orthosteric binding pocket. In the present study, we investigated the action of MT7 on agonist-occupied receptors in functional and radioligand binding assays of the cloned human muscarinic M(1) receptor expressed in Chinese hamster ovary cells. In time-course experiments, the addition of MT7 rapidly blocked the acetylcholine-stimulated guanosine-5'-O-(3-[(35)S]thio)triphosphate binding to membrane G proteins. Similarly, in acetylcholine-treated cells MT7 completely stopped the agonist-stimulated [(3)H]inositol phosphate accumulation. In dissociation experiments using membranes pre-equilibrated with [(3)H]acetylcholine, the addition of MT7 increased the rate of radioligand dissociation. The data indicate that MT7, while partially stabilizing the antagonist-receptor complex, effectively destabilizes the agonist-occupied muscarinic M(1) receptors.

Functional M3 muscarinic acetylcholine receptors in mammalian hearts

In contrast to most peripheral tissues where multiple subtypes of muscarinic acetylcholine receptor (mAChR) coexist, with each of them playing its part in the orchestra of parasympathetic innervation, the myocardium has been traditionally considered to possess a single mAChR subtype. Although there is much evidence to support the notion that one receptor subtype (M2) orchestrates myocardial muscarinic transduction, there is emerging evidence that M1 and M3 receptors are also expressed and are of potential physiological, pathophysiological and pharmacological relevance. Clarifying this issue has a profound impact on our thinking about the cholinergic control of the heart function and disease and approaches to new drug development for the treatment of heart disease associated with parasympathetic dysfunction. This review article presents evidence for the presence of the M3 receptor subtype in the heart, and analyzes the controversial data from published pharmacological, functional and molecular studies. The potential roles of the M3 receptors, in parasympathetic control of heart function under normal physiological conditions and in heart failure, myocardial ischemia and arrhythmias, are discussed. On the basis of these considerations, we have made some proposals concerning the future of myocardial M3 receptor research.