Reactive nucleolar and Cajal body responses to proteasome inhibition in sensory ganglion neurons.

The dysfunction of the ubiquitin proteasome system has been related to a broad array of neurodegenerative disorders in which the accumulation of misfolded protein aggregates causes proteotoxicity. The ability of proteasome inhibitors to induce cell cycle arrest and apoptosis has emerged as a powerful strategy for cancer therapy. Bortezomib is a proteasome inhibitor used as an antineoplastic drug, although its neurotoxicity frequently causes a severe sensory peripheral neuropathy. In this study we used a rat model of bortezomib treatment to study the nucleolar and Cajal body responses to the proteasome inhibition in sensory ganglion neurons that are major targets of bortezomib-induced neurotoxicity. Treatment with bortezomib induced dose-dependent dissociation of protein synthesis machinery (chromatolysis) and nuclear retention of poly(A) RNA granules resulting in neuronal dysfunction. However, as a compensatory response to the proteotoxic stress, both nucleoli and Cajal bodies exhibited reactive changes. These include an increase in the number and size of nucleoli, strong nucleolar incorporation of the RNA precursor 5'-fluorouridine, and increased expression of both 45S rRNA and genes encoding nucleolar proteins UBF, fibrillarin and B23. Taken together, these findings appear to reflect the activation of the nucleolar transcription in response to proteotoxic stress Furthermore, the number of Cajal bodies, a parameter related to transcriptional activity, increases upon proteasome inhibition. We propose that nucleoli and Cajal bodies are important targets in the signaling pathways that are activated by the proteotoxic stress response to proteasome inhibition. The coordinating activity of these two organelles in the production of snRNA, snoRNA and rRNA may contribute to neuronal survival after proteasome inhibition. This article is part of a Special Issue entitled: Role of the Nucleolus in Human Disease.