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Published on: August 2, 2024
Nucleolar disruption and apoptosis are distinct neuronal responses to etoposide-induced DNA damage
Maciej Pietrzak1, Scott C Smith, Justin T Geralds
1Kentucky Spinal Cord Injury Research Center and the Departments of Neurological Surgery, University of Louisville, Louisville, Kentucky 40292, USA.
Abstract:
Although DNA damaging topoisomerase inhibitors induce apoptosis in developing neurons, their effects on adult neurons have not yet been characterized. We report a blockage of RNA-Polymerase-1-driven transcription and nucleolar stress in neocortical neurons of adult rats after intracarotid injection of the DNA-topoisomerase-2 inhibitor, etoposide. Intracerebroventricular injection of etoposide induced a similar response in neonatal rats. In contrast, etoposide triggered neuronal apoptosis in the neonates, but not the adults. Nucleolar disruption and apoptosis were also observed in etoposide-challenged cultured cortical neurons from newborn rats. In that system, activation of the DNA double strand break signaling kinase ataxia telangiectasia-mutated protein kinase, p53 and p53-dependent apoptosis required lower etoposide concentrations than did the p53-independent induction of nucleolar stress. These distinct responses may be coupled to different forms of etoposide-induced DNA damage. Indeed, double strand breaks by the over-expressed endonuclease I-Ppo1 were sufficient to induce p53-dependent apoptosis. Moreover, nucleolar transcription was insensitive to such damage implying single strand breaks and/or topoisomerase-2-DNA adducts as triggers of nucleolar stress. Because nucleolar stress is not age-restricted, it may underlie non-apoptotic neurotoxicity of chemotherapy- or neurodegeneration-associated DNA damage by reducing ribosomal biogenesis in adult brain. Conversely, nucleolar insensitivity to double strand breaks likely contributes to mature neuron tolerance of such lesions.
Insights
Etoposide causes nucleolar stress and blocks transcription in adult neurons, unlike developing neurons which undergo apoptosis. This age-dependent difference in response to DNA damage may explain chemotherapy side effects.
Area of Science:
- Neuroscience
- Molecular Biology
- Toxicology
Background:
- DNA-damaging topoisomerase inhibitors like etoposide induce apoptosis in developing neurons.
- The effects of these agents on adult neurons remain largely uncharacterized.
Purpose of the Study:
- To investigate the differential effects of etoposide on neuronal apoptosis and nucleolar stress in adult versus developing neurons.
- To elucidate the molecular mechanisms underlying etoposide-induced neurotoxicity and neuronal tolerance to DNA damage.
Main Methods:
- Intracarotid and intracerebroventricular injections of etoposide in adult and neonatal rats.
- In vitro studies using cultured cortical neurons from newborn rats.
- Analysis of RNA-Polymerase-1 transcription, nucleolar stress, apoptosis, and DNA damage signaling pathways (ATM, p53).
Main Results:
- Etoposide induced RNA-Polymerase-1 transcriptional blockage and nucleolar stress in adult rat neocortical neurons.
- In contrast, etoposide triggered apoptosis in neonatal rats and cultured newborn neurons.
- Nucleolar stress was induced by lower etoposide concentrations than p53-dependent apoptosis, and was insensitive to double-strand breaks but sensitive to single-strand breaks or topoisomerase-2-DNA adducts.
Conclusions:
- Adult neurons exhibit tolerance to etoposide-induced double-strand breaks, characterized by nucleolar stress rather than apoptosis.
- Nucleolar stress, a p53-independent process, may contribute to non-apoptotic neurotoxicity from DNA damage in adult brains by impairing ribosomal biogenesis.
- The distinct responses highlight age-dependent mechanisms in neuronal response to DNA damage and chemotherapy.
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