Degeneration mechanisms in human dopaminergic neurons

Abstract

Parkinson’s disease (PD) is the most common neurodegenerative movement disorder. The extensive loss of dopaminergic neurons in the Substantia nigra pars compacta and the resulting lack of dopamine in the target regions lead to severe motor symptoms. The single most consistent risk factor is aging. With a steadily increasing age of the world population, the prevalence of the global burden of PD will further rise in the future.So far, only symptomatic treatments that reduce motor complications are available. Neuroprotective and –restorative therapies are still lacking. To make things worse, at the time point of diagnosis around 60% of nigral dopaminergic neurons are already lost. Thus, strategies to prevent or at least to slow disease progression remain the central aim of PD-related research. We think that a deeper understanding of molecular mechanisms of the selective neurodegeneration is needed.Also, new and optimized models for the study of protective strategies are essential for further achievements in PD research. Animal models are often used, but differences in signaling pathways to humans exist. Furthermore, PD is a disease affecting solely humans. In vitro models, based on human material, represent a relevant alternative to in vivo experiments.In this thesis, the human LUHMES cell line was introduced as model system for studies on neurodevelopment and neurodegeneration. We characterized the cell line with a focus on the suitability for PD research. We optimized the differentiation protocol and demonstrated a fast, homogeneous and irreversible differentiation to post-mitotic dopaminergic neurons. A distinct sensitivity to the parkinsonian model toxin 1-methyl-4-phenylpyridinium (MPP+) was shown. This allowed mechanistic studies on parkinsonian neurodegeneration processes and the investigation of possible intervention strategies. The LUHMES/MPP+ model reproduced many molecular pathways of the complex disease. A newly established differentiation protocol, allowing the generation of dopamine-free LUHMES cells, demonstrated that dopamine contributes to MPP+-toxicity. Furthermore, different strategies using diverse classes of pharmacological inhibitors were found to protect LUHMES from MPP+-induced neurodegeneration.Major findings were that (i) the LUHMES cell line represents an advanced in vitro model for PD research, and that (ii) dopaminergic neurons can be protected from degeneration, although a strong MPP+-mediated energy depletion via inhibition of complex I of the respiratory chain occured. We showed that ATP-depletion and cell death are not inevitably coupled. Transferred to a general view on PD, with mitochondrial dysfunctions as key contributor to disease progression, novel strategies for the protection from degeneration of dopaminergic neurons might be developed. The primary effect of MPP+ (in the cell model) and of mitochondrial dysfunction (in the disease), does not necessarily have to be prevented to be able to protect cells from dying. This approach might, in combination with an earlier diagnosis of the disease, make an important contribution to the development of neuroprotective therapies for PD.