Chronic exposure to paclitaxel diminishes phosphoinositide signaling by calpain-mediated neuronal calcium sensor-1 degradation

Paclitaxel (Taxol) is a well established chemotherapeutic agent for the treatment of solid tumors, but it is limited in its usefulness by the frequent induction of peripheral neuropathy. We found that prolonged exposure of a neuroblastoma cell line and primary rat dorsal root ganglia with therapeutic concentrations of Taxol leads to a reduction in inositol trisphosphate (InsP(3))-mediated Ca2+ signaling. We also observed a Taxol-specific reduction in neuronal calcium sensor 1 (NCS-1) protein levels, a known modulator of InsP3 receptor (InsP(3)R) activity. This reduction was also found in peripheral neuronal tissue from Taxol treated animals. We further observed that short hairpin RNA-mediated NCS-1 knockdown had a similar effect on phosphoinositide-mediated Ca2+ signaling. When NCS-1 protein levels recovered, so did InsP(3)-mediated Ca2+ signaling. Inhibition of the Ca2+-activated protease mu-calpain prevented alterations in phosphoinositide-mediated Ca2+ signaling and NCS-1 protein levels. We also found that NCS-1 is readily degraded by mu-calpain in vitro and that mu-calpain activity is increased in Taxol but not vehicle-treated cells. From these results, we conclude that prolonged exposure to Taxol activates g-calpain, which leads to the degradation of NCS-1, which, in turn, attenuates InsP(3)-mediated Ca2+ signaling. These findings provide a previously undescribed approach to understanding and treating Taxol-induced peripheral neuropathy.