Djamaa ATAMENA – PhD defense

Defense in french

Team : MItochondria- & ExperieNce- Dependent neuronal plastIcity, NeurodeGeneration (MINDING), CRCA-CBI

Supervisor : Pr. Pascale BELENGUER (CRCA-CBI)

Committee members :

• Dr. CORRAL-DEBRINSKI Marisol, Rapporteure
• Pr. BLONDEL Marc, Rapporteur
• Dr. DUNIA Daniel, Examinateur
• Pr. BELENGUER Pascale, Directrice de thèse

Abstract :

Hereditary optic neuropathies (HON), leading to severely impaired vision, are due to the degeneration of the Retinal Ganglion Cells (RCG) whose axons form the optic nerve. One of the most common HON encountered in clinics is Dominant Optic Atrophy (DOA). DOA is mainly caused by mutations of the gene coding the mitochondrial protein OPA1. OPA1 is involved in mitochondrial fusion a mechanism that controls, together with mitochondrial fission, the morphology and the functions of mitochondria. Clinical manifestations include progressive bilateral and symmetrical vision loss beginning from early childhood. In up to 20% of cases, extra-ocular manifestations are also reported leading to syndromic forms of DOA (sensorineural deafness, ataxia, peripheral neuropathy …). The disease is marked by a highly variable inter- and intra-familial expressivity, from patients being asymptomatic, to some totally blind or suffering from multisystemic affections. Altogether, this suggests the presence of genetic and/or environmental factors that confer, along with the primary mutation, a strong influence on the disease manifestation. So far, no modifying factors that could modulate DOA expressivity have been clearly identified. To investigate the influence of genetic modifying factors on DOA expressivity, we used a previously described DOA mouse model bearing the c.1065+5G→A Opa1 mutation which we switched from the mixed C3H;C57BL/6J to pure C57BL/6J genetic background. The phenotypic consequences on the pure genetic background was less severe, without any sign of RGC and axonal degeneration, supporting the contribution of genetic secondary factors to the phenotypic variability in DOA. However, we described a negative effect on RGC connectivity suggesting that OPA1 deficiency may first impair RGC functioning through early synaptic defects and eventually lead to neuronal degeneration. This opens a way to new clinical considerations for early diagnosis together with a new therapeutic window before neurodegeneration. Currently, there is no curative treatment for DOA and innovative therapeutic solutions are still awaited. We aimed at establishing the proof of concept for two novel therapeutic approaches based on their abilities to rescue the consequences of OPA1-deficiency. The first approach aims to evaluate the effects of a neuroprotective protein, the Bornavirus X protein. Our team showed that the expression of this protein in primary cultured neurons restores the mitochondrial and neuronal defects associated with OPA1 deficiency, so I set up a preclinical trial using another DOA mouse model. For that purpose, adeno-associated viruses expressing the X protein were injected into the retina of 3-months-old mice, before the onset of the first symptoms. The consequences of the X protein expression on the alterations of the visual function as well as on RGC and optic nerve degeneration will be soon analyzed after 11 months of treatment. The second pharmacological approach is based on the repositioning of FDA-approved molecules, Hexestrol and Clomiphene. Our team showed that these molecules prevent the lethality, the mtDNA loss and the mitochondrial fragmentation induced by inactivation of the OPA1 homologue in yeast, Msp1p. I pursued this study evaluating the effect of these molecules in mammalian DOA model cells ie fibroblasts of OPA1 mutated patients and OPA1-deficient rat cortical neurons. In both models, we showed that both molecules rescued mitochondrial morphology defects by inhibiting mitochondrial fission. Further investigations are currently performed to address their effect on dendritic arborization and synapses in neurons. The encouraging results that I obtained make these two therapeutic strategies good candidates to treat DOA by focusing on the correction of the defects rather than the mutation itself. They could be thus applied to other mitochondrial HON as well as mitochondrial-related neurodegenerative diseases.