Voltage-gated sodium ion channels (Nav¬¬¬¬¬¬) are transmembrane proteins responsible for initiating electrical signals in excitable cells. Because Nav channels play a crucial role in neuromuscular coordination, they are targeted by a diverse array of neurotoxins produced across the animal kingdom. Arizona bark scorpions (Centruroides sculpturatus) produce toxins that disrupt Nav channel function, causing pain, muscle paralysis and respiratory failure. Southern grasshopper mice (Onychomys torridus) hunt bark scorpions. In response to selection by scorpion venom, grasshopper mice have evolved physiological resistance to toxins that cause pain and death. Although previous work identified modifications in one grasshopper mouse Nav channel (Nav1.8) that provide resistance to venom pain, mechanisms underlying resistance to muscle paralysis remain unknown. In skeletal muscle, Nav1.4 channels regulate muscle contraction. Previous studies showed that toxins in C. elegans and C. vittatus venoms disrupt Nav1.4 gating mechanisms. Thus, I tested the hypothesis that C. sculpturatus venom contains toxins that target Nav1.4, and that grasshopper mice are resistant to the effects of these toxins via molecular changes to their Nav1.4. Using molecular and electrophysiological analyses, I compared the structural and functional properties of grasshopper mice Nav1.4 channels to those of house mice and rats (rodents that are both sensitive to scorpion venom) and found that grasshopper mice Nav1.4 has evolved reduced sensitivity to C. sculpturatus venom. Further, I identified amino acid changes in the grasshopper mice Nav1.4 protein that contribute to reduced toxin sensitivity. Finally, I show that the beta subunits (accessory proteins that modulate Nav1.4 channel gating kinetics) of grasshopper mice do not have species-specific effects on channel function in the presence or absence of venom. My results demonstrate that highly conserved proteins can be evolutionarily modified with minimal effects to their baseline functional properties.
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