OPA1 loss of function affects in vitro neuronal maturation

Abstract

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.

Alterations of mitochondrial dynamics allow retrograde propagation of locally initiated axonal insults

“…Prior to seeding in the microfluidic devices, CGN neurons were electroporated with constructs encoding either wild type OPA1, or OPA1 G330E , a dominant negative mutant of OPA141, or wild type DRP1. Co-transfection with the MitoDsRed expression vector allowed for direct visualization of mitochondrial morphology35. Quantitative analysis of axonal mitochondrial morphology 10 days after transfection showed that in control condition, small fissionned mitochondria (≤2 μm long), represented approximately 40% of the mitochondrial population, while the remaining ≈60% consisted of filamentous (2–4 μm) and hyper-filamentous (≥4 μm) mitochondria (Fig.…”

“…Consequently, several studies have shown that an increase of mitochondrial fission, through DRP1 overexpression or by Mitofusins and/or OPA1 inhibition, enhances cell vulnerability toward apoptosis, or sensitizes cells to stress24272829, particularly neurons3031. Moreover, MFN2 is necessary for axonal mitochondrial transport and positioning3233, and alteration of mitochondrial fission through DRP1 inactivation or OPA1 inactivation leads to abnormal mitochondria positioning in neurites3435. Alteration of mitochondrial dynamics has been associated to the early stage of neurodegenerative conditions203637; and inhibiting DRP1 proved to be a promising strategy in order to delay neuronal demise3138.…”

OPA1-related disorders: Diversity of clinical expression, modes of inheritance and pathophysiology

“…An RGC dendritopathy has also been reported in another Opa1 +/− mouse model (Williams et al, 2010). Mitochondrial inner membrane remodeling with its link to mitochondrial bioenergetics (Mishra et al, 2014;Varanita et al, 2015), altered calcium homeostasis, compromised synaptic maturation and dendritogenesis (Bertholet et al, 2013) were also recently reported, illustrating the deep complexity of the pathophysiological mechanisms responsible for neurological suffering.…”

Metabolic regulation of adult stem cell-derived neurons

“…Mitochondrial fusion and fission impact on mitochondrial membrane potential, thereby modulating reactive oxygen species (ROS) generation, the sequestration of cytosolic calcium, and electron transport chain activity. In vitro studies have not only implied mitochondria distribution, but also mitochondrial bioenergetics and calcium buffering in the regulation of neuronal morphogenesis and synaptogenesis (Li et al, 2004;Dietrich et al, 2008;Macaskill et al, 2009;Wang and Schwarz, 2009;MacAskill et al, 2010;Li et al, 2010;Dickey and Strack, 2011;Cheng et al, 2012;Steketee et al, 2012;Bertholet et al, 2013;Courchet et al, 2013). Regulation of dendritic Ca 2+ buffering capacity in adult-generated hippocampal granule cells was proposed to contribute to their activity-dependent dendritic growth and synaptogenesis (Stocca et al, 2008).…”