High Affinity Nav Channel Binder With Specific Conjugation Site Derived From Scorpion Toxin With Attenuated Toxicity

Voltage-gated sodium channels (Nav) play a key role in generating action potential which leads to physiological signaling in excitable cells. However, the availability of probes for functional study of mammalian Nav is limited. Based on the beta-scorpion toxin Ts1, we have chemically synthesized a novel biologically friendly binder [S14R, W50Pra]Ts1 (Ts1-S14R) for Nav with high affinity and extremely low dissociation rate. This Ts1 analog has a S14R point mutation, corresponding to E15R in Css4, another beta-scorpion toxin, and also has a propargylglycine (Pra) mutation from W50 for bioconjugation. We identified the pharmacological properties of Ts1-S14R on rat skeletal muscle Nav channel (Nav1.4) using the cut-open oocyte voltage clamp technique and confirmed that its toxicity has been dramatically reduced. Chemical synthesis of this binder enabled us to conjugate with molecules of interest for various studies. Using the fluorescent-dye, BODIPY, conjugated binder, we examined its binding location in Nav1.4 using Lanthanide-based Resonance Energy Transfer technique (LRET). Our LRET study showed Ts1-S14R-BODIPY binds to Nav1.4 at virtually identical location as that of Ts1-BODIPY. In addition, LRET data showed that, once Ts1-S14R-BODIPY binds to Nav1.4, it stays in place for at least 40 min in toxin-free solution, indicating that Ts1-S14R-BODIPY can be employed as a stable binder for Nav1.4. Moreover, we generated Ts1-S14R conjugated with gold nanoparticle (Ts1-S14R-AuNP). We demonstrated this conjugate can induce action potentials by visible light stimulation through the opto-thermal-capacitive effect in rat dorsal root ganglia (DRG) neurons similarly to that previously reported (Carvalho-de-Souza JL et al. Neuron 2015). Ts1-S14R is therefore a novel probe with great potential for wider applications in Nav-related neuroscience research. Support: U54GM087519 and GM030376.