OPA1 haploinsufficiency induces a BNIP3-dependent decrease in mitophagy in neurons: relevance to Dominant Optic Atrophy.

Dominant optic atrophy (DOA) is because of mutations in the mitochondrial protein OPA1. The disease principally affects retinal ganglion cells, whose axons degenerate leading to vision impairments, and sometimes other neuronal phenotypes. The exact mechanisms underlying DOA pathogenesis are not known. We previously demonstrated that the main role of OPA1, as a mitochondrial fusogenic and anti-apoptotic protein, are inhibited by interaction with the stress inducible pro-apoptotic BNIP3 protein. Because BNIP3 was recently reported to participate in autophagy and mitophagy, we tested the involvement of these processes in DOA pathogenesis. Using an in vitro neuronal model of DOA, we identified a BNIP3 down-regulation that reduced autophagy and mitophagy. Restoring BNIP3 had a biphasic effect, first rescuing autophagy and mitophagy levels but later leading to cell death. Similarly, in an in vivo mouse model of DOA, we showed that BNIP3 levels are decreased in young adult mice and increase to normal levels upon aging, paralleling disease progression. Altogether, our results indicate that the relationship between OPA1 and BNIP3 may have important bearings on DOA pathogenesis.

OPA1 loss of function affects in vitro neuronal maturation.

Mitochondrial dynamics control the organelle's morphology, with fusion leading to the formation of elongated tubules and fission leading to isolated puncta, as well as mitochondrial functions. Recent reports have shown that disruptions of mitochondrial dynamics contribute to neurodegenerative diseases. Mutations of the inner membrane GTPase OPA1 are responsible for type 1 dominant optic atrophy, by mechanisms not fully understood. We show here that in rodent cortical primary neurons, downregulation of the OPA1 protein leads to fragmented mitochondria that become less abundant along the dendrites. Furthermore, this inhibition results in reduced expression of mitochondrial respiratory complexes as well as mitochondrial DNA, decreased mitochondrial membrane potential, and diminished reactive oxygen species levels. The onset of synaptogenesis was markedly impaired through reductions in pre- and postsynaptic structural protein expression and synapse numbers without first affecting the dendritic arborization. With longer time in culture, OPA1 extinction led to a major restriction of dendritic growth, together with reduction of synaptic proteins. Furthermore, in maturing neurons we observed a transitory increase in mitochondrial filament length, associated with marked changes in the expression levels of OPA1, which occurred at the onset of synaptogenesis simultaneously with transitory increase in reactive oxygen species levels and NRF2/NFE2L2 nuclear translocation. This observation suggests that mitochondrial hyperfilamentation acts upstream of a reactive oxygen species-dependent NRF2 transcriptional activity, possibly impacting neuronal maturation, such a process being impaired by insufficient amount of OPA1. Our findings suggest a new role for OPA1 in synaptic maturation and dendritic growth through maintenance of proper mitochondrial oxidative metabolism and distribution, highlighting the role of mitochondrial dynamics in neuronal functioning and providing insights into dominant optic atrophy pathogenesis, as OPA1 loss affecting neuronal maturation could lead to early synaptic dysfunction.

OPA1 deficiency impairs oxidative metabolism in cycling cells, underlining a translational approach for degenerative diseases.

Dominant optic atrophy is an optic neuropathy with varying clinical symptoms and progression. A severe disorder is associated with certain OPA1 mutations and includes additional symptoms for >20% of patients. This underscores the consequences of OPA1 mutations in different cellular populations, not only retinal ganglionic cells. We assessed the effects of OPA1 loss of function on oxidative metabolism and antioxidant defences using an RNA-silencing strategy in a human epithelial cell line. We observed a decrease in the mitochondrial respiratory chain complexes, associated with a reduction in aconitase activity related to an increase in reactive oxygen species (ROS) production. In response, the NRF2 (also known as NFE2L2) transcription factor was translocated into the nucleus and upregulated SOD1 and GSTP1. This study highlights the effects of OPA1 deficiency on oxidative metabolism in replicative cells, as already shown in neurons. It underlines a translational process to use cycling cells to circumvent and describe oxidative metabolism. Moreover, it paves the way to predict the evolution of dominant optic atrophy using mathematical models that consider mitochondrial ROS production and their detoxifying pathways.

Loss of functional OPA1 unbalances redox state: implications in dominant optic atrophy pathogenesis.

OBJECTIVE: OPA1 mutations cause protein haploinsufficiency leading to dominant optic atrophy (DOA), an incurable retinopathy with variable severity. Up to 20% of patients also develop extraocular neurological complications. The mechanisms that cause this optic atrophy or its syndromic forms are still unknown. After identifying oxidative stress in a mouse model of the pathology, we sought to determine the consequences of OPA1 dysfunction on redox homeostasis. METHODS: Mitochondrial respiration, reactive oxygen species levels, antioxidant defenses, and cell death were characterized by biochemical and in situ approaches in both in vitro and in vivo models of OPA1 haploinsufficiency. RESULTS: A decrease in aconitase activity suggesting an increase in reactive oxygene species and an induction of antioxidant defenses was observed in cortices of a murine model as well as in OPA1 downregulated cortical neurons. This increase is associated with a decline in mitochondrial respiration in vitro. Upon exogenous oxidative stress, OPA1-depleted neurons did not further exhibit upregulated antioxidant defenses but were more sensitive to cell death. Finally, low levels of antioxidant enzymes were found in fibroblasts from patients supporting their role as modifier factors. INTERPRETATION: Our study suggests that the pro-oxidative state induced by OPA1 loss may contribute to DOA pathogenesis and that differences in antioxidant defenses can explain the variability in expressivity. Furthermore, antioxidants may be used as therapy as they could prevent or delay DOA symptoms in patients.