Site-Specific Fluorescent Labeling of the Cysteine-Rich Toxin, DkTx, for TRPV1 Ion Channel Imaging and Membrane Binding Studies

Peptide toxins as tools in ion channel biology

Animal venom contains ion channel-targeting peptide toxins that inflict paralysis or pain. The high specificity and potency of these toxins for their target ion channels provides enticing opportunities for their deployment as tools in channel biology. Mechanistic studies on toxin-mediated ion channel modulation have yielded landmark breakthroughs in our understanding of channel architectures and gating mechanisms. Toxins have been recently repurposed as powerful structural biology probes to obtain structures of ion channels in elusive toxin-stabilized conformations providing unprecedented insights into channel gating. Insightful glimpses of protein-lipid interactions provided by some of these structures have served as blueprints for electrophysiology-based studies aimed at elucidating the functional roles of these interactions. Moreover, toxins appended with fluorophores have been used for clinical, biophysical, and cell biology-based studies. Herein, we summarize the contributions of ion channel-targeting toxins as tools in voltage-gated ion channel and transient receptor potential channel biology.

Shining a Light on Venom-Peptide Receptors: Venom Peptides as Targeted Agents for In Vivo Molecular Imaging

Molecular imaging has revolutionised the field of biomedical research by providing a non-invasive means to visualise and understand biochemical processes within living organisms. Optical fluorescent imaging in particular allows researchers to gain valuable insights into the dynamic behaviour of a target of interest in real time. Ion channels play a fundamental role in cellular signalling, and they are implicated in diverse pathological conditions, making them an attractive target in the field of molecular imaging. Many venom peptides exhibit exquisite selectivity and potency towards ion channels, rendering them ideal agents for molecular imaging applications. In this review, we illustrate the use of fluorescently-labelled venom peptides for disease diagnostics and intraoperative imaging of brain tumours and peripheral nerves. Finally, we address challenges for the development and clinical translation of venom peptides as nerve-targeted imaging agents.

Sortase-mediated labeling: Expanding frontiers in site-specific protein functionalization opens new research avenues

New applications for biomolecules demand novel approaches for their synthesis and modification. Traditional methods for modifying proteins and cells using non-specific labeling chemistry are insufficiently precise to rigorously interrogate the mechanistic biological and physiological questions at the forefront of biomedical science. Site-specific catalytic modification of proteins promises to meet these challenges. Here, we describe recent applications of the enzyme sortase A in facilitating precise biomolecule labeling. We focus on describing new chemistries to broaden the scope of sortase-mediated labeling (sortagging), the development of new probes for imaging via enzymatic labeling, and the modulation of biological systems using probes and reactions mediated by sortase.

Synthesis, Activity, and Application of Fluorescent Analogs of [D1G, Δ14Q]LvIC Targeting α6β4 Nicotinic Acetylcholine Receptor

α6β4* nicotinic acetylcholine receptor (nAChR) (* represents the possible presence of additional subunits) is mainly distributed in the central and peripheral nervous system and is associated with neurological diseases, such as neuropathic pain; however, the ability to explore its function and distribution is limited due to the lack of pharmacological tools. As one of the analogs of α-conotoxin (α-CTx) LvIC from Conus lividus, [D1G, Δ14Q]LvIC (Lv) selectively and potently blocks α6/α3β4 nAChR (α6/α3 represents a chimera). Here, we synthesized three fluorescent analogs of Lv by connecting fluorescent molecules 6-carboxytetramethylrhodamine succinimidyl ester (6-TAMRA-SE, R), Cy3 NHS ester (Cy3, C) and BODIPY-FL NHS ester (BDP, B) to the N-terminus of the peptide and obtained Lv-R, Lv-C, and Lv-B, respectively. The potency and selectivity of three fluorescent peptides were evaluated using two-electrode voltage-clamp recording on nAChR subtypes expressed in Xenopus laevis oocytes, and the potency and selectivity of Lv-B were almost maintained with the half-maximal inhibition (IC50) of 64 nM. Then, we explored the stability of Lv-B in artificial cerebrospinal fluid and stained rat brain slices with Lv-B. The results indicated that the stability of Lv-B was slightly improved compared to that of native Lv. Additionally, we detected the distribution of the α6β4* nAChR subtype in the cerebral cortex using green fluorescently labeled peptide and fluorescence microscopy. Our findings not only provide a visualized pharmacological tool for exploring the distribution of the α6β4* nAChR subtype in various situ tissues and organs but also extend the application of α-CTx [D1G, Δ14Q]LvIC to demonstrate the involvement of α6β4 nAChR function in pathophysiology and pharmacology.

Neurotoxin-Derived Optical Probes for Biological and Medical Imaging

The superb specificity and potency of biological toxins targeting various ion channels and receptors are of major interest for the delivery of therapeutics to distinct cell types and subcellular compartments. Fused with reporter proteins or labelled with fluorophores and nanocomposites, animal toxins and their detoxified variants also offer expanding opportunities for visualisation of a range of molecular processes and functions in preclinical models, as well as clinical studies. This article presents state-of-the-art optical probes derived from neurotoxins targeting ion channels, with discussions of their applications in basic and translational biomedical research. It describes the design and production of probes and reviews their applications with advantages and limitations, with prospects for future improvements. Given the advances in imaging tools and expanding research areas benefiting from the use of optical probes, described here resources should assist the discovery process and facilitate high-precision interrogation and therapeutic interventions.

Dissecting the contributions of membrane affinity and bivalency of the spider venom protein DkTx to its sustained mode of TRPV1 activation

The spider venom protein, double-knot toxin (DkTx), partitions into the cellular membrane and binds bivalently to the pain-sensing ion channel, TRPV1, triggering long-lasting channel activation. In contrast, its monovalent single knots membrane partition poorly and invoke rapidly reversible TRPV1 activation. To discern the contributions of the bivalency and membrane affinity of DkTx to its sustained mode of action, here, we developed diverse toxin variants including those containing truncated linkers between individual knots, precluding bivalent binding. Additionally, by appending the single-knot domains to the Kv2.1 channel-targeting toxin, SGTx, we created monovalent double-knot proteins that demonstrated higher membrane affinity and more sustained TRPV1 activation than the single-knots. We also produced hyper-membrane affinity-possessing tetra-knot proteins, (DkTx)2 and DkTx-(SGTx)2, that demonstrated longer-lasting TRPV1 activation than DkTx, establishing the central role of the membrane affinity of DkTx in endowing it with its sustained TRPV1 activation properties. These results suggest that high membrane affinity-possessing TRPV1 agonists can potentially serve as long-acting analgesics.