Paclitaxel inhibits mRNA transport in axons☆
Introduction
Peripheral neuropathy is the most important adverse effect of the antineoplastic agent paclitaxel. Current pathogenetic concepts assume that paclitaxel impairs neuronal function due to polymerization and hence irreversible assembly of axonal microtubules that impairs axoplasmic transport (Flatters and Bennett, 2006, Komiya, 1992, Lapointe et al., 2013, Shemesh and Spira, 2010). In addition, dysfunction of neuronal mitochondria has been implicated to be an essential event that incites neuronal damage (Lin and Beal, 2006, Xiao et al., 2011, Zheng et al., 2011). However, the precise pathomechanisms that underlie microtubule stabilization, axonal transport disruption and mitochondrial dysfunction are poorly understood (Shemesh and Spira, 2010). Moreover, it is not clear which axonal cargo is primarily affected and whether impaired mitochondrial or mRNA/protein transport is involved in this process (Goshima et al., 2012).
In neurons most mitochondria are localized within the axons, where they provide large quantities of ATP, which is required for maintenance of ionic gradients and axonal transport (Lin and Beal, 2006, Navarro and Boveris, 2007, Russell et al., 2002). There is increasing evidence that the function of mitochondria is highly dependent on dynamic processes that allow the exchange of metabolites and mitochondrial DNA. These mitochondrial dynamics are controlled by a group of GTPases that direct fusion, such as mitofusin-1 (MFN1), mitofusin-2 (MFN2) and optic atrophy 1 (Opa1), or fission of mitochondria, such as dynamin related protein 1 (Drp1) and fission 1 (Fis1). Lack or malfunction of the fusion and fission machinery results in alteration of mitochondrial morphology and energetic failure (Chen et al., 2005, Reddy et al., 2011, Wang et al., 2012). Notably, those mitochondrial fusion and fission factors are encoded by nuclear genes, synthesized in the axoplasm and subsequently imported into mitochondria. It implicates that in polarized peripheral neurons, their function critically depends on adaptive axonal transport mechanisms that ensure rapid and dynamic movement of mRNA from the perikaryon to the local protein synthesis machinery in distal axonal compartments.
In this study, we examined the role of mitochondria in a murine model of paclitaxel-induced peripheral neuropathy focusing on axonal transport of mitochondria and nuclear encoded mitochondrial mRNA. We provide significant evidence that axonal transport of neuronal encoded transcripts, such as the mitochondrial fusion/fission machinery, is severely impaired, which leads to increased mitochondrial morphology alteration and hence dysfunction with subsequent development of axonal damage.
Section snippets
Animals
Female (4–5 weeks old) wild-type (C57BL/6) and heterozygous B6.Cg-Tg(Thy1-CFP/COX8A)S2Lich/J (abbr.: Mito-CFP/COX8A) mice were used for the study. Wild-type (http://jaxmice.jax.org/strain/000664.html) and Mito-CFP/COX8A (http://jaxmice.jax.org/strain/007967.html) mice were purchased from Jackson Laboratory. All mice were housed on sawdust bedding in plastic cages and maintained on a 12 h light/dark cycle with food and water provided ad libitum. The care and husbandry of animals were in conformity
Paclitaxel induces a predominant small fiber sensory neuropathy in mice
Consistent with previous reports (Carozzi et al., 2010, Mimura et al., 2000, Mo et al., 2012), we observed that repeated intravenous injections of paclitaxel provoked a predominant sensory neuropathy in mice. As shown in Fig. 1a, motor nerve conduction remained normal at the beginning of treatment and displayed only modest decrease in CMAP amplitudes after three weeks of paclitaxel treatment. In contrast, sensory nerve conductions revealed markedly prolonged latencies already seven days after
Discussion
The main findings of our study suggest that paclitaxel treatment disrupts the axonal transport of nuclear encoded transcripts such as mitochondrial fusion and fission factors, thereby affecting the dynamics, structure and function of axonal mitochondria resulting in a predominant sensory neuropathy.
Previously, mitochondrial dysfunction has been assumed to be a critical event in paclitaxel-induced neurotoxicity, based on in vivo and in vitro studies that reported increased frequency of elongated
Conclusions
Our data provide evidence that disrupted axonal transport of nuclear derived mRNA plays a crucial role in the pathogenesis of paclitaxel-induced sensory neuropathy. As proof of principle to this concept we found spatial differences in the expression and translation of mitochondrial fusion and fission factors in axons. These alterations affect the dynamics, structure and function of axonal mitochondria resulting in a predominant sensory neuropathy. The altered mRNA transport thereby correlates
Acknowledgments
Helmar Lehmann and Ilja Bobylev were supported by the Deutsche Krebshilfe (German Cancer Aid). The technical assistance of Claudia Drapatz (Department of Neurology, University Hospital of Cologne, Germany) and Petra Müller (Department of Anatomy I, Medical Faculty, University of Cologne, Germany) is greatly acknowledged.
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Conflict of interest: The authors declare no competing financial interests.