Tracking sodium channels in live cells: confocal imaging using fluorescently labeled toxins

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.

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.

Chemical and Biological Tools for the Study of Voltage-Gated Sodium Channels in Electrogenesis and Nociception

The malfunction and misregulation of voltage-gated sodium channels (NaV s) underlie in large part the electrical hyperexcitability characteristic of chronic inflammatory and neuropathic pain. NaV s are responsible for the initiation and propagation of electrical impulses (action potentials) in cells. Tissue and nerve injury alter the expression and localization of multiple NaV isoforms, including NaV 1.1, 1.3, and 1.6-1.9, resulting in aberrant action potential firing patterns. To better understand the role of NaV regulation, localization, and trafficking in electrogenesis and pain pathogenesis, a number of chemical and biological reagents for interrogating NaV function have been advanced. The development and application of such tools for understanding NaV physiology are the focus of this review.

Controlling ion channel trafficking by targeted ubiquitination and deubiquitination

Plasma membrane-localized ion channels are essential for diverse physiological processes such as neurotransmission, muscle contraction, and osmotic homeostasis. The surface density of such ion channels is a major determinant of their function, and tuning this variable is a powerful way to regulate physiology. Dysregulation of ion channel surface density due to inherited or de novo mutations underlies many serious diseases, and molecules that can correct trafficking deficits are potential therapeutics and useful research tools. We have developed targeted ubiquitination and deubiquitination approaches that enable selective posttranslational down- or up-regulation, respectively, of desired ion channels. The method employs bivalent molecules comprised of an ion-channel-targeted nanobody fused to catalytic domains of either an E3 ubiquitin ligase or a deubiquitinase. Here, we use two examples to provide detailed protocols that illustrate the utility of the approach-rescued surface expression of a trafficking-deficient mutant K_V7.1 (KCNQ1) channel that causes long QT syndrome, and selective elimination of the Ca_V2.2 voltage-gated calcium channel from the plasma membrane using targeted ubiquitination. Important aspects of the approach include having a robust assay to measure ion channel surface density and generating nanobody binders to cytosolic domains or subunits of targeted ion channels. Accordingly, we also review available methods for determining ion channel surface density and nanobody selection.

Visualizing individual sodium channels on the move

Visualization of voltage-gated sodium channels at work is an important requirement for the understanding of rapid electrical signaling in nerve cells. In this issue of Chemistry & Biology, Ondrus and colleagues have mastered this challenge by chemical synthesis of a fluorescent antagonist and by monitoring single sodium channels in living cells with unprecedented optical resolution.

Subtype specificity interaction of bactridines with mammalian, insect and bacterial sodium channels under voltage clamp conditions

The present work demonstrates that bactridines (Bacts) possess different selectivities for neuronal and muscular voltage-dependent sodium (Na(V) ) channels, with subtle differences on channel isoforms. Bacts 2, 3, 4, 5 and 6 (100 nm) reduced the peak current of several skeletal and neuronal channel isoforms selectively. Bacts 2 and 3 were more potent on Na(V) 1.4, Bacts 4 and 6 on Na(V) 1.3 and Bact 5 on Na(V) 1.7. Bactridines (except Bacts 1 and 5) caused a hyperpolarizing shift in the V(1/2) of activation and inactivation of Na(V) 1.3, Na(V) 1.4 and Na(V) 1.6. Voltage shifts of Boltzmann curves fitted to activation and inactivation occurred with a decrease in κ. Since the slope is proportional to κ = RT/zF, changes in κ probably express changes in z, the valence, in a voltage-dependent manner. Changes in z may express toxin-induced changes in the channel ionic environment, perhaps due to surface charges of the molecules. Bact 2 induced a Na(V) 1.2 voltage shift of the activation curves but no shift of the mutant Na(V) 1.2 IFM/QQQ; peak I(N) (a) was reduced in both channel forms, suggesting that channel blockage resulted from toxin binding to a site partially distinct from the α subunit binding site 4. Bactridines emerge as potential research tools to understand sodium channel isoform structure-function relationships and also as pharmacologically interesting peptides.

Fluorescent saxitoxins for live cell imaging of single voltage-gated sodium ion channels beyond the optical diffraction limit

A desire to better understand the role of voltage-gated sodium channels (Na(V)s) in signal conduction and their dysregulation in specific disease states motivates the development of high precision tools for their study. Nature has evolved a collection of small molecule agents, including the shellfish poison (+)-saxitoxin, that bind to the extracellular pore of select Na(V) isoforms. As described in this report, de novo chemical synthesis has enabled the preparation of fluorescently labeled derivatives of (+)-saxitoxin, STX-Cy5, and STX-DCDHF, which display reversible binding to Na(V)s in live cells. Electrophysiology and confocal fluorescence microscopy studies confirm that these STX-based dyes function as potent and selective Na(V) labels. The utility of these probes is underscored in single-molecule and super-resolution imaging experiments, which reveal Na(V) distributions well beyond the optical diffraction limit in subcellular features such as neuritic spines and filopodia.

Purification and characterization of a novel short-chain insecticidal toxin with two disulfide bridges from the venom of the scorpion Liocheles australasiae

Scorpion venoms contain a variety of peptides toxic to mammals, insects and crustaceans. Most of the scorpion toxins have been isolated from the venoms of scorpions in the family Buthidae, but little interest has been paid to non-Buthidae scorpions. In this study, we isolated a short-chain insecticidal toxin (LaIT1) from the venom of the scorpion Liocheles australasiae belonging to the Hemiscorpiidae family. This toxin showed insect toxicity against crickets at a dose of 1.0 microg/insect, but no toxicity was observed against mice even after injection of 1.0 microg of LaIT1 via the intracerebroventricular route, suggesting that the effect of the toxin is insect-selective. Edman sequencing and mass spectrometric analysis revealed that the toxin is composed of 36 amino acid residues and cross-linked by only two disulfide bridges. The pattern of the disulfide bridges was assigned by LC/MS analysis after enzymatic digestion. LaIT1 shows no sequence homology to any other known toxins, suggesting that this toxin represents a novel structural motif class.

Fluorescent ion-selective nanosensors for intracellular analysis with improved lifetime and size

We describe the synthesis and characterization of sodium-selective polymeric nanosensors that improves upon the lifetime and size of previous fiberless nanosensors. Sonication is used to form the polymer nanospheres that contain all the components needed for ion sensing. Even though the size is small (approximately 120 nm), the lifetime of these sensors in solution is on the order of a week. The surface coating has also been optimized for stability, biocompatibility, and ease of chemical modification.

Subtype specificity of scorpion beta-toxin Tz1 interaction with voltage-gated sodium channels is determined by the pore loop of domain 3

Voltage-gated sodium (Nav) channels are modulated by a variety of specific neurotoxins. Scorpion beta-toxins affect the voltage-dependence of channel gating: In their presence, Nav channels activate at subthreshold membrane voltages. Previous mutagenesis studies have revealed that the beta-toxin Css4 interacts with the extracellular linker between segments 3 and 4 in domain 2 of Nav channels with the effect to trap this voltage sensor in an open position (Neuron 21: 919-931, 1998 ). The voltage sensor of domain 2 was thus identified to constitute a major part of neurotoxin receptor site 4. In this work, we studied the effects of the beta-toxin Tz1 from the Venezuelan scorpion Tityus zulianus on various mammalian Nav channel types expressed in HEK 293 cells. Although skeletal muscle channels (Nav1.4) were strongly affected by Tz1, the neuronal channels Nav1.6 and Nav1.2 were less sensitive, and the cardiac Nav1.5 and the peripheral nerve channel Nav1.7 were essentially insensitive. Analysis of channel chimeras in which whole domains of Nav1.2 were inserted into a Nav1.4 background revealed that the Nav1.2 phenotype was not conferred to Nav1.4 by domain 2 but by domain 3. The interaction epitope could be narrowed down to residues Glu1251, Lys1252, and His1257 located in the C-terminal pore loop in domain 3. The receptor site for beta-toxin interaction with Nav channels thus spans domains 2 and 3, where the pore loop in domain 3 specifies the pharmacological properties of individual neuronal Nav channel types.

Age effects on the pharmacokinetics of tityustoxin from Tityus serrulatus scorpion venom in rats

The pharmacokinetics of scorpion venom and its toxins has been investigated in experimental models using adult animals, although, severe scorpion accidents are associated more frequently with children. We compared the effect of age on the pharmacokinetics of tityustoxin, one of the most active principles of Tityus serrulatus venom, in young male/female rats (21-22 days old, N=5-8) and in adult male rats (150-160 days old, N=5-8). Tityustoxin (6 microg) labeled with 99mTechnetium was administered subcutaneously to young and adult rats. The plasma concentration vs time data were subjected to non-compartmental pharmacokinetic analysis to obtain estimates of various pharmacokinetic parameters such as total body clearance (CL/F), distribution volume (Vd/F), area under the curve (AUC), and mean residence time. The data were analyzed with and without considering body weight. The data without correction for body weight showed a higher Cmax (62.30 +/- 7.07 vs 12.71 +/- 2.11 ng/ml, P<0.05) and AUC (296.49 +/- 21.09 vs 55.96 +/- 5.41 ng h(-1) ml(-1), P<0.05) and lower Tmax (0.64 +/- 0.19 vs 2.44 +/- 0.49 h, P<0.05) in young rats. Furthermore, Vd/F (0.15 vs 0.42 l/kg) and CL/F (0.02 +/- 0.001 vs 0.11 +/- 0.01 l h(-1) kg(-1), P<0.05) were lower in young rats. However, when the data were reanalyzed taking body weight into consideration, the Cmax (40.43 +/- 3.25 vs 78.21 +/- 11.23 ng kg(-1) ml(-1), P<0.05) and AUC (182.27 +/- 11.74 vs 344.62 +/- 32.11 ng h(-1) ml(-1), P<0.05) were lower in young rats. The clearance (0.03 +/- 0.002 vs 0.02 +/- 0.002 l h(-1) kg(-1), P<0.05) and Vd/F (0.210 vs 0.067 l/kg) were higher in young rats. The raw data (not adjusted for body weight) strongly suggest that age plays a pivotal role in the disposition of tityustoxin. Furthermore, our results also indicate that the differences in the severity of symptoms observed in children and adults after scorpion envenomation can be explained in part by differences in the pharmacokinetics of the toxin.

Correction of image instability in confocal microscopy using image realignment. Effects on the analysis of intracellular calcium

Using confocal microscopy, we have examined the increases in [Ca(2+)](i) evoked by sodium channel toxins in cells labelled with the fluorescent dye INDO-1. We describe a new image analysis method that improves the detection of region-specific, toxin-induced patterns of change of intracellular calcium. This method is based on correction of global image motion followed by calculation of the strength of correlation between calcium changes in "seed" or reference pixels chosen to represent different regions of cells and those in other regions of the image. When the selected "seed" pixel was chosen to be in either varicosities or neurites, correlations were detected in the same regions of other cells as well as in the soma, indicating specific but spatially distinct patterns of behaviour. Control images (without changes in [Ca(2+)](i)) did not reveal significant interpixel correlations. The ability to recognize correlated patterns of calcium change in different regions of cells was greatly improved by correction for global motion.