A Naturally Occurring Peptide with an Elementary Single Disulfide-Directed β-Hairpin Fold

A toxin-based approach to neuropeptide and peptide hormone discovery

Peptide hormones and neuropeptides form a diverse class of bioactive secreted molecules that control essential processes in animals. Despite breakthroughs in peptide discovery, many signaling peptides remain undiscovered. Recently, we demonstrated the use of somatostatin-mimicking toxins from cone snails to identify the invertebrate ortholog of somatostatin. Here, we show that this toxin-based approach can be systematically applied to discover other unknown secretory peptides that are likely to have signaling function. Using large sequencing datasets, we searched for homologies between cone snail toxins and secreted proteins from the snails' prey. We identified and confirmed expression of five toxin families that share strong similarities with unknown secretory peptides from mollusks and annelids and in one case also from ecdysozoans. Based on several lines of evidence we propose that these peptides likely act as signaling peptides that serve important physiological functions. Indeed, we confirmed that one of the identified peptides belongs to the family of crustacean hyperglycemic hormone, a peptide not previously observed in Spiralia. We propose that this discovery pipeline can be broadly applied to other systems in which one organism has evolved molecules to manipulate the physiology of another.

A previously unrecognized superfamily of macro-conotoxins includes an inhibitor of the sensory neuron calcium channel Cav2.3

Animal venom peptides represent valuable compounds for biomedical exploration. The venoms of marine cone snails constitute a particularly rich source of peptide toxins, known as conotoxins. Here, we identify the sequence of an unusually large conotoxin, Mu8.1, which defines a new class of conotoxins evolutionarily related to the well-known con-ikot-ikots and 2 additional conotoxin classes not previously described. The crystal structure of recombinant Mu8.1 displays a saposin-like fold and shows structural similarity with con-ikot-ikot. Functional studies demonstrate that Mu8.1 curtails calcium influx in defined classes of murine somatosensory dorsal root ganglion (DRG) neurons. When tested on a variety of recombinantly expressed voltage-gated ion channels, Mu8.1 displayed the highest potency against the R-type (Cav2.3) calcium channel. Ca2+ signals from Mu8.1-sensitive DRG neurons were also inhibited by SNX-482, a known spider peptide modulator of Cav2.3 and voltage-gated K+ (Kv4) channels. Our findings highlight the potential of Mu8.1 as a molecular tool to identify and study neuronal subclasses expressing Cav2.3. Importantly, this multidisciplinary study showcases the potential of uncovering novel structures and bioactivities within the largely unexplored group of macro-conotoxins.

An evolution-inspired strategy to design disulfide-rich peptides tolerant to extensive sequence manipulation

Natural disulfide-rich peptides (DRPs) are valuable scaffolds for the development of new bioactive molecules and therapeutics. However, there are only a limited number of topologically distinct DRP folds in nature, and most of them suffer from the problem of in vitro oxidative folding. Thus, strategies to design DRPs with new constrained topologies beyond the scope of natural folds are desired. Herein we report a general evolution-inspired strategy to design new DRPs with diverse disulfide frameworks, which relies on the incorporation of two cysteine residues and a random peptide sequence into a precursor disulfide-stabilized fold. These peptides can spontaneously fold in redox buffers to the expected tricyclic topologies with high yields. Moreover, we demonstrated that these DRPs can be used as templates for the construction of phage-displayed peptide libraries, enabling the discovery of new DRP ligands from fully randomized sequences. This study thus paves the way for the development of new DRP ligands and therapeutics with structures not derived from natural DRPs.

The Single Disulfide-Directed β-Hairpin Fold: Role of Disulfide Bond in Folding and Effect of an Additional Disulfide Bond on Stability

Disulfide bonds play a key role in the oxidative folding, conformational stability, and functional activity of many peptides. A few disulfide-rich peptides with privileged architecture such as the inhibitor cystine knot motif have garnered attention as templates in drug design. The single disulfide-directed β-hairpin (SDH), a novel fold identified more recently in contryphan-Vc1, has been shown to possess remarkable thermal, conformational, and chemical stability and can accept a short bioactive epitope without compromising the core structure of the peptide. In this study, we demonstrated that the single disulfide bond is critical in maintaining the native fold by replacing both cysteine residues with serine. We also designed an analogue with an additional, non-native disulfide bridge by replacing Gln1 and Tyr9 with Cys. Contryphan-Vc11–22[Q1C, Y9C] was synthesised utilising orthogonal cysteine protection and its solution structure determined using solution NMR spectroscopy. This analogue maintained the overall fold of native contryphan-Vc1. Previous studies had shown that the β-hairpin core of contryphan-Vc1 was resistant to proteolysis by trypsin and α-chymotrypsin but susceptible to cleavage by pepsin. Contryphan-Vc11–22[Q1C, Y9C] proved to be completely resistant to pepsin, thus confirming our design strategy. These results highlight the role of the disulfide bond in maintaining the SDH fold and provide a basis for the design of more stable analogues for peptide epitope grafting.

Conotoxins: Chemistry and Biology

The venom of the marine predatory cone snails (genus Conus) has evolved for prey capture and defense, providing the basis for survival and rapid diversification of the now estimated 750+ species. A typical Conus venom contains hundreds to thousands of bioactive peptides known as conotoxins. These mostly disulfide-rich and well-structured peptides act on a wide range of targets such as ion channels, G protein-coupled receptors, transporters, and enzymes. Conotoxins are of interest to neuroscientists as well as drug developers due to their exquisite potency and selectivity, not just against prey but also mammalian targets, thereby providing a rich source of molecular probes and therapeutic leads. The rise of integrated venomics has accelerated conotoxin discovery with now well over 10,000 conotoxin sequences published. However, their structural and pharmacological characterization lags considerably behind. In this review, we highlight the diversity of new conotoxins uncovered since 2014, their three-dimensional structures and folds, novel chemical approaches to their syntheses, and their value as pharmacological tools to unravel complex biology. Additionally, we discuss challenges and future directions for the field.

Design and Synthesis of Cross-Link-Dense Peptides by Manipulating Regioselective Bisthioether Cross-Linking and Orthogonal Disulfide Pairing

Existing disulfide-rich peptides, both naturally occurring and de novo designed, only represent a tiny amount of the possible sequence space because natural evolution and de novo design only keep sequences that are structurally approachable by correct disulfide pairings. To bypass this limitation for designing new peptide scaffolds beyond the natural sequence space, we dedicate to developing novel disulfide-rich peptides with predefined disulfide pairing patterns irrelevant to primary sequences. However, most of these designed peptides still suffer from disulfide rearrangements to at least one to three possible isomers. Here, we report a general and reliable strategy for the design and synthesis of a range of structurally diverse cross-link-dense peptide (CDP) scaffolds with two orthogonal disulfide bonds and a bisthioether bridge that are not subject to disulfide isomerizations. Altering the pattern of cysteine and penicillamine generates hundreds of different CDP scaffolds tolerant to extensive sequence manipulations. This work thus provides many useful scaffolds for the design of functional molecules such as protein binders with improved proteolytic stability (e.g., designed by epitope grafting).

New techniques, applications and perspectives in neuropeptide research

Neuropeptides are one of the most diverse classes of signaling molecules and have attracted great interest over the years owing to their roles in regulation of a wide range of physiological processes. However, there are unique challenges associated with neuropeptide studies stemming from the highly variable molecular sizes of the peptides, low in vivo concentrations, high degree of structural diversity and large number of isoforms. As a result, much effort has been focused on developing new techniques for studying neuropeptides, as well as novel applications directed towards learning more about these endogenous peptides. The areas of importance for neuropeptide studies include structure, localization within tissues, interaction with their receptors, including ion channels, and physiological function. Here, we discuss these aspects and the associated techniques, focusing on technologies that have demonstrated potential in advancing the field in recent years. Most identification and structural information has been gained by mass spectrometry, either alone or with confirmations from other techniques, such as nuclear magnetic resonance spectroscopy and other spectroscopic tools. While mass spectrometry and bioinformatic tools have proven to be the most powerful for large-scale analyses, they still rely heavily on complementary methods for confirmation. Localization within tissues, for example, can be probed by mass spectrometry imaging, immunohistochemistry and radioimmunoassays. Functional information has been gained primarily from behavioral studies coupled with tissue-specific assays, electrophysiology, mass spectrometry and optogenetic tools. Concerning the receptors for neuropeptides, the discovery of ion channels that are directly gated by neuropeptides opens up the possibility of developing a new generation of tools for neuroscience, which could be used to monitor neuropeptide release or to specifically change the membrane potential of neurons. It is expected that future neuropeptide research will involve the integration of complementary bioanalytical technologies and functional assays.

The Single Disulfide-Directed β-Hairpin Fold. Dynamics, Stability, and Engineering

Grafting bioactive peptide sequences onto small cysteine-rich scaffolds is a promising strategy for enhancing their stability and value as novel peptide-based therapeutics. However, correctly folded disulfide-rich peptides can be challenging to produce by either recombinant or synthetic means. The single disulfide-directed β-hairpin (SDH) fold, first observed in contryphan-Vc1, provides a potential alternative to complex disulfide-rich scaffolds. We have undertaken recombinant production of full-length contryphan-Vc1 (rCon-Vc1[Z1Q]) and a truncated analogue (rCon-Vc1_1-22[Z1Q]), analyzed the backbone dynamics of rCon-Vc1[Z1Q], and probed the conformational and proteolytic stability of these peptides to evaluate the potential of contryphan-Vc1 as a molecular scaffold. Backbone ¹⁵N relaxation measurements for rCon-Vc1[Z1Q] indicate that the N-terminal domain of the peptide is ordered up to Thr19, whereas the remainder of the C-terminal region is highly flexible. The solution structure of truncated rCon-Vc1_1-22[Z1Q] was similar to that of the full-length peptide, indicating that the flexible C-terminus does not have any effect on the structured domain of the peptide. Contryphan-Vc1 exhibited excellent proteolytic stability against trypsin and chymotrypsin but was susceptible to pepsin digestion. We have investigated whether contryphan-Vc1 can accept a bioactive epitope while maintaining the structure of the peptide by introducing peptide sequences based on the DINNN motif of inducible nitric oxide synthase. We show that sCon-Vc1_1-22[NNN_12-14] binds to the iNOS-binding protein SPSB2 with an affinity of 1.3 μM while maintaining the SDH fold. This study serves as a starting point in utilizing the SDH fold as a peptide scaffold.