Caractérisation de la vulnérabilité sélective des neurones dopaminergiques dans le contexte de la maladie de Parkinson

Parkinson's disease (PD) is a neurodegenerative disease whose characteristic motor symptoms are caused by the death of midbrain dopaminergic neurons of the subtantia nigra pars compacta (SNc). In about 15% of cases, mutations in gene products such as Parkin, Pink1, DJ-1, α-synuclein, LRRK2 or GBA are responsible for the onset of the disease. These genes are involved in physiological processes such as mitophagy, lysosomal function, oxidative stress response, mitochondrial antigen presentation or axonal transport. Interestingly, these processes are all directly or indirectly related to the ability of neurons to produce the energy they need for survival and to their level of basal oxidative stress. One of the central questions in the study of PD is why alterations in such ubiquitous processes lead to the selective death of small groups of neurons in the brain, such as the dopaminergic neurons of the SNc. The comparison of these neurons with their neighbours of the ventral tegmental area (VTA), which are also dopaminergic but much less affected in PD, is one of the most striking examples of this selective vulnerability.  One recent hypothesis suggests that certain characteristics specific to SNc dopaminergic neurons induce a tremendous pressure on their bioenergetic capacities and increase basal oxidative stress, which could make them selectively vulnerable to any additional stress. For example, these neurons are thought to form toxic derivatives of dopamine under stress conditions, which could negatively impact their ability to efficiently produce their energy. They also have a pacemaking firing pattern implicating energy intensive calcium currents. Finally, they are thought to have a particularly elaborated axonal arborization, requiring a phenomenal amount of energy to propagate the action potentials and induce release of neurotransmitters. One of our hypotheses is that these characteristics make SNc dopaminergic neurons particularly active bioenergetically and push them to the limit of their energetic capacities. Any subsequent cellular stress, such as those associated with the previously mentioned genetic alterations, exposure to environmental toxins or simply aging, could then lead to their death.  Interestingly, the development of an ultra-complex axonal arborization is a potentially shared feature of neuronal populations affected in PD, including dopaminergic neurons of the SNc, cholinergic neurons of the pedunculopontine nucleus or noradrenergic neurons of the locus coeruleus. It is therefore possible that this characteristic is central to the selective vulnerability of certain neuronal populations in PD. Unfortunately, no direct comparison of the axonal arborization size of these neurons with spared neuronal populations is currently available, neither is the impact of the modulation of the axonal arborization size on energy requirements and neuronal vulnerability.  To begin to evaluate this possibility, we compared the dopaminergic neurons of the SNc to those of the VTA in culture and showed that they are particularly bioenergetically active, with little spare capacity, and have a much larger axonal arborization size. Using semaphorin 7A, an axon guidance factor, we reduced the size of this axon and at the same time reduced the energy requirements and vulnerability of these neurons. In a second step, we evaluated these same parameters in SNc neurons from Parkin, Pink1 or DJ-1 KO mice and showed that SNc neurons from Parkin KO mice are more vulnerable to culture conditions and are less efficient in producing their energy. Lastly, we demonstrated, in vivo this time, that dopaminergic neurons of the SNc have a larger axonal arborization than those of the VTA and used the conditional KO of the D2 receptor to further increase the size of this axon. In this model, SNc neurons were more vulnerable to 6-OHDA lesions, but not to overexpression of α-synuclein.  Overall, this thesis provides for the first-time direct evidence that axonal arborization size is a major factor in the selective vulnerability of dopaminergic neurons of the SNc in the context of Parkinson's disease.

Keyphrases: Arborization Axonale, Parkin, Parkinson's Disease, Dopaminergic Neurons, Axonal Arborization, Mitochondria, Selective Vulnerability