ILP4 and InR regulate paclitaxel-induced hypersensitivity differently in <i>Drosophila</i> larvae

Paclitaxel (PTX), a chemotherapeutic that stabilizes microtubules, induces nociceptive hypersensitivity and sensory neuron damage in humans, mice, and flies. To enhance our basic understanding of PTX-induced effects, we undertook a molecular/genetic dissection of PTX-induced nociceptive hypersensitivity. Larvae fed viable doses of PTX exhibited dose-dependent hypersensitivity to subnoxious thermal stimuli. Hypersensitivity developed rapidly and did not completely resolve at the larval stage. Live imaging of peripheral thermal nociceptors showed that lower doses of PTX (< 10 µM) caused hyper-sprouting of tertiary dendritic branches. At 10 µM and above, dendritic beading was observed. PTX-induced hypersensitivity does not depend on signaling pathways previously implicated in acute injury-induced nociceptive sensitization. However, the insulin-like peptide 4 (ILP4) was required for PTX-induced thermal hypersensitivity at 10 µM PTX. Surprisingly, RNAi targeting the insulin receptor (InR) in nociceptors increased PTX-induced hypersensitivity, suggesting that ILP4 does not activate InR in this context. The salivary gland is likely the primary tissue source of functional ILP4. ILP4 mutant larvae did not exhibit PTX-induced beading (10 µM) but did exhibit hypersprouting at lower PTX concentrations. In summary, our model of PTX-induced hypersensitivity reveals a disconnect between hypersensitivity and neuronal morphology and a genetic separation of ILP4 and InR in PTX-induced hypersensitivity.