Loss of OPA1 disturbs cellular calcium homeostasis and sensitizes for excitotoxicity

Abstract

Optic atrophy 1 (OPA1) mutations cause dominant optic atrophy (DOA) with retinal ganglion cell (RGC) and optic nerve degeneration. The mechanism for the selective degeneration of RGCs in DOA remains elusive. To address the mechanism, we reduced OPA1 protein expression in cell lines and RGCs by RNA interference. OPA1 loss results in mitochondrial fragmentation, deficiency in oxidative phosphorylation, decreased ATP levels, decreased mitochondrial Ca 2 þ retention capacity, reduced mtDNA copy numbers, and sensitization to apoptotic insults. We demonstrate profound cristae depletion and loss of crista junctions in OPA1 knockdown cells, whereas the remaining crista junctions preserve their normal size. OPA1-depleted cells exhibit decreased agonist-evoked mitochondrial Ca 2 þ transients and corresponding reduction of NAD þ to NADH, but the impairment in NADH oxidation leads to an overall more reduced mitochondrial NADH pool. Although in our model OPA1 loss in RGCs has no apparent impact on mitochondrial morphology, it decreases buffering of cytosolic Ca 2 þ and sensitizes RGCs to excitotoxic injury. Exposure to glutamate triggers delayed calcium deregulation (DCD), often in a reversible manner, indicating partial resistance of RGCs to this injury. However, when OPA1 is depleted, DCD becomes irreversible. Thus, our data show that whereas OPA1 is required for mitochondrial fusion, maintenance of crista morphology and oxidative phosphorylation, loss of OPA1 also results in defective Ca 2 þ homeostasis.

C11orf83, a Mitochondrial Cardiolipin-Binding Protein Involved in bc1 Complex Assembly and Supercomplex Stabilization

“…8A). This observation is strikingly similar to what was reported with cells depleted for OPA1, a GTPase with the same localization and topology as C11orf83 (61)(62)(63). In addition, C11orf83-deficient cells present several features which are similar to those observed in OPA1-downregulating cells, including impaired respiration, decreased ETC enzymatic activities, and high apoptosis sensitivity (62,63) (Fig.…”

“…These two observations are probably linked, since crista morphology maintenance and SC formation have been shown to be connected (60). These mitochondrial ultrastructure defects may also explain the reduced enzymatic activities of all the ETC complexes, as reported for OPA1-depleted cells (63). A recent study highlighted that OPA1 oligomerization was required both to modulate crista ultrastructure and to regulate complex V assembly by stabilizing the expression of the F 0 subunit MT-ATP6 (70).…”

Mitochondrial Ca2+ uptake correlates with the severity of the symptoms in autosomal dominant optic atrophy

“…The biological significance of the enhanced Ca 2+ uptake in OPA1-silenced H295R cells was indicated by enhanced aldosterone production [28]. Although our data in HeLa cells were partly at variance with those of Kushnareva et al [55] (in this respect see [56]), studies on the Ca 2+ retention capacity of mitochondria in OPA1-silenced murine retinal ganglion cells [16] and the Ca 2+ sensitivity of mitochondrial permeability transition pore in OPA1-silenced HeLa cells [55] are in harmony with our observations. The report that knock-down of Opa1 increased the amplitude of Ca 2+ uptake and the Ca 2+ retention capacity in murine cardiac mitochondria [57] also supports the conclusion that an impairment of OPA1 function may lead to enhanced mitochondrial Ca 2+ signalling.…”

Strategic Positioning and Biased Activity of the Mitochondrial Calcium Uniporter in Cardiac Muscle

“…Both proteins displayed preference for the contact points and inner boundary membrane as opposed to being deep in the cristae folding, as resolved via membrane fractionation and super-resolution approaches in both isolated mitochondria and freshly isolated primary cardiomyocytes. mtCU in some non-muscle cells has been proposed to also operate inside the cristae so that opening and tightening cristae junctions would regulate mtCU exposure to Ca 2ϩ signals (30,31). Such topology in adult myocardial mitochondria would require a lot of mtCU to populate all of the numerous cristae for an effective local Ca 2ϩ signal reception.…”