Paclitaxel increases axonal localization and vesicular trafficking of Na(v)1.7

The microtubule-stabilizing chemotherapy drug paclitaxel (PTX) causes dose-limiting chemotherapy-induced peripheral neuropathy (CIPN), which is often accompanied by pain. Among the multifaceted effects of PTX is an increased expression of sodium channel Na(v)1.7 in rat and human sensory neurons, enhancing their excitability. However, the mechanisms underlying this increased Na(v)1.7 expression have not been explored, and the effects of PTX treatment on the dynamics of trafficking and localization of Na(v)1.7 channels in sensory axons have not been possible to investigate to date. In this study we used a recently developed live imaging approach that allows visualization of Na(v)1.7 surface channels and long-distance axonal vesicular transport in sensory neurons to fill this basic knowledge gap. We demonstrate concentration and time-dependent effects of PTX on vesicular trafficking and membrane localization of Na(v)1.7 in real-time in sensory axons. Low concentrations of PTX increase surface channel expression and vesicular flux (number of vesicles per axon). By contrast, treatment with a higher concentration of PTX decreases vesicular flux. Interestingly, vesicular velocity is increased for both concentrations of PTX. Treatment with PTX increased levels of endogenous Na(v)1.7 mRNA and current density in dorsal root ganglion neurons. However, the current produced by transfection of dorsal root ganglion neurons with Halo-tag Na(v)1.7 was not increased after exposure to PTX. Taken together, this suggests that the increased trafficking and surface localization of Halo-Na(v)1.7 that we observed by live imaging in transfected dorsal root ganglion neurons after treatment with PTX might be independent of an increased pool of Na(v)1.7 channels. After exposure to inflammatory mediators to mimic the inflammatory condition seen during chemotherapy, both Na(v)1.7 surface levels and vesicular transport are increased for both low and high concentrations of PTX. Overall, our results show that PTX treatment increases levels of functional endogenous Na(v)1.7 channels in dorsal root ganglion neurons and enhances trafficking and surface distribution of Na(v)1.7 in sensory axons, with outcomes that depend on the presence of an inflammatory milieu, providing a mechanistic explanation for increased excitability of primary afferents and pain in CIPN.