Paclitaxel causes degeneration of both central and peripheral axon branches of dorsal root ganglia in mice

Abstract

Background: Peripheral neuropathy is a common and dose-limiting side effect of many cancer chemotherapies. The taxane agents, including paclitaxel (Taxol(®)), are effective chemotherapeutic drugs but cause degeneration of predominantly large myelinated afferent sensory fibers of the peripheral nervous system in humans and animal models. Dorsal root ganglia (DRG) neurons are sensory neurons that have unipolar axons each with two branches: peripheral and central. While taxane agents induce degeneration of peripheral axons, whether they also cause degeneration of central nervous system axons is not clear. Using a mouse model of paclitaxel-induced neuropathy, we investigated the effects of paclitaxel on the central branches of sensory axons.

Results: We observed that in the spinal cords of paclitaxel-intoxicated mice, degenerated axons were present in the dorsal columns, where the central branches of DRG axons ascend rostrally. In the peripheral nerves, degenerated myelinated fibers were present in significantly greater numbers in distal segments than in proximal segments indicating that this model exhibits the distal-to-proximal degeneration pattern generally observed in human peripheral nerve disorders.

Conclusions: We conclude that paclitaxel causes degeneration of both the peripheral and central branches of DRG axons, a finding that has implications for the site and mode of action of chemotherapy agents on the nervous system.

Keywords: Activated macrophages; Axonal degeneration; Neuropathy.

Fig. 1

Degeneration of axons in the dorsal column of mice intoxicated with paclitaxel. Images are cross-sectioned semithin (1 µm) plastic sections of spinal cords stained for toluidine blue. a Control vehicle-treated (cremophor) spinal cord at lumbar level. No degenerated axons are observed. b Paclitaxel treated spinal cord. Degenerated myelin profiles are present in dorsal column of thoracic level where ascending central branches axons of DRG neurons reside. c Paclitaxel treated spinal cord. Degenerated myelin profiles are present in dorsal column of lumbar level where ascending central branches axons of DRG neurons reside. d High magnification of a showing no degenerated axons. e High magnification of b showing degenerated myelin profiles (arrows). f High magnification of c showing degenerated myelin profiles (arrows). Scale bar in c = 50 μm and applies to a–c. Scale bar in f = 10 μm and applies to d–f

Fig. 2

Electron microscope images of degenerated of axons in the dorsal column. A EM image showing healthy and no degenerated axons (arrows) and myelin ovoids in the ventral horn of lumbar region of mice intoxicated with paclitaxel. B Low magnification EM image showing degenerated axons (arrows) and myelin ovoids in the dorsal column of lumbar region. C High magnification EM images of lumbar level showing degenerated axons at various stages of degeneration. Arrowhead points to degenerated axoplasm filled with organelles. Arrow points to degenerated myelin profile that has already collapsed following degeneration of axoplasm

Fig. 3

Paclitaxel causes distal to proximal axonal degeneration in mice. Mice were treated with i.v. 30 mg/kg paclitaxel 3 times a week for 2 weeks. a–c are cross-sectioned semithin (1 µm) plastic sections stained for toluidine blue. a Vehicle-treated (cremophor) mice at the mid-thigh level of sciatic nerve. No degenerated axons are observed. b Paclitaxel-treated mice at the mid-thigh level of sciatic nerve. Degenerated myelin profiles were present across the nerve crossed section. Arrows point to large (black arrow) and small (white arrow) degenerated myelinated axons. c Paclitaxel-treated mice at 7–8 mm distal to the sciatic nerve segment in a. Greater numbers of degenerated myelin profiles are present across the distal segment of the nerve, indicating distal to proximal degeneration pattern. d Quantification of degenerating myelinated fibers in the whole cross-sectional area of nerve segments. N = 3 per nerve segment. Values are mean ± SEM. e Electron micrograph of distal nerve intoxicated with paclitaxel showing occasional swollen unmyelinated axon (arrow) and degeneration of myelinated axon (*). f Quantification of Remak bundles. N = 3 mice per treatment. There was no statistical difference between vehicle- and paclitaxel-treated nerves. Scale bar in c = 20 μm and applies to a–c. Scale bar in e = 500 nm

Fig. 4

Macrophages infiltration along degenerated axons in nerve intoxicated with paclitaxel. Longitudinal frozen sectioned nerve segment stained for CD68 and MBP. a MBP staining of nerve segment intoxicated with paclitaxel. The vast majority of fibers appear normal with the exception of a single fiber that show degenerated myelin (arrow). b CD68 staining. Macrophages (CD68-positive cells) infiltrate along a degenerating fiber, while not associating with other normal fibers (arrow). c Merged image of a and b. Scale bar 50 μm

Fig. 5

DRG cell bodies after paclitaxel intoxication. a Control DRG. b Paclitaxel intoxicated DRG. Minor alternations are seen such as small nucleoli in the large neurons of the paclitaxel-treated mice. Scale bar in b = 10 μm and applies to a, b