OPA1 alternate splicing uncouples an evolutionary conserved function in mitochondrial fusion from a vertebrate restricted function in apoptosis

Abstract

In most eucaryote cells, release of apoptotic proteins from mitochondria involves fission of the mitochondrial network and drastic remodelling of the cristae structures. The intramitochondrial dynamin OPA1, as a potential central actor of these processes, exists as eight isoforms resulting from the alternate splicing combinations of exons (Ex) 4, 4b and 5b, which functions remain undetermined. Here, we show that Ex4 that is conserved throughout evolution confers functions to OPA1 involved in the maintenance of the DW m and in the fusion of the mitochondrial network. Conversely, Ex4b and Ex5b, which are vertebrate specific, define a function involved in cytochrome c release, an apoptotic process also restricted to vertebrates. The drastic changes of OPA1 variant abundance in different organs suggest that nuclear splicing can control mitochondrial dynamic fate and susceptibility to apoptosis and pathologies. Mitochondrial membrane dynamics, involving both the fusionfission of the mitochondrial network and remodelling of cristae structures, are essential processes in cellular adaptation to changes of growth condition and programmed cell death. [1][2][3] The outer and inner mitochondrial membrane (OMM and IMM) dynamics crosstalk and involve large GTPases: respectively, the mitofusins Mfn1 and Mfn2 4,5 and the dynamin-related DRP1 6 on the OMM, and the dynaminrelated OPA1, localized in the inter membrane space (IMS). 7 Mitochondrial fission is tightly associated to the apoptotic process, and physical and functional interactions between apoptotic proteins including Bcl2-homologue domains and DRP1 and mitofusins have been identified. 8 In addition, BH3-only proapoptotic proteins, exemplified by tBid and Bik, might act directly on the IMM structure, although their intramitochondrial target remains unknown. 3,9 Further crosstalk between the OMM and the IMM is exemplified by the OMM fission and fusion regulation by the mitochondrial membrane potential (DC m ) 10 and structural integrity 11 and by interaction between OPA1 and Mfn1. 12 The IMM forms, an architecturally complex structure, that has been reconsidered recently in the light of tomography analysis associated to transmission electron microscopy (TEM). Indeed, tubular inner membrane junctions, called cristae junctions, have been evidenced between the IMM facing the OMM and the vesicular-shaped cristae. 13 In vertebrate cells, during the apoptotic process, the cristae junction topology is drastically modified to mobilize cytochrome c (cyt c) from the intracristae compartment to the IMS, before its extensive release in the cytoplasm. 3,9 How this process is physically controlled and related to the OMM dynamic remains unknown. We and others have shown that OPA1 is a good candidate for the regulation of the cristae structure in the mitochondria. 11,14 Indeed, downregulation of OPA1 affect the IMM structure and integrity, leading to respiration defects and DC m loss, fission of the mitochondrial network, impairment of growth and apoptosis 11,14-18 and OPA1 overexpressio...

Impaired quality control of mitochondria: Aging from a new perspective

“…We recently showed that after dissipation of the mitochondrial membrane potential as well as in several in vivo model systems of mitochondrial dysfunction proteolytic processing of the fusion factor OPA1 is induced and that this is a key step in inducing fragmentation of dysfunctional mitochondria (Duvezin-Caubet et al, 2006). This is in line with other studies showing that dissipation of the mitochondrial membrane potential and inducing apoptosis also led to proteolytic processing of OPA1 (Ishihara et al, 2006;Olichon et al, 2007). OPA1 processing and mitochondrial fragmentation was observed in mouse embryonic fibroblasts derived from a knock-in mouse expressing an error-prone mitochondrial DNA Polymerase  (Trifunovic et al, 2004), the so-called "mutator mouse" (DuvezinCaubet et al, 2006).…”

Loss of Drosophila i-AAA protease, dYME1L, causes abnormal mitochondria and apoptotic degeneration

“…58,59 However, the S2 cleavage site of i-AAA is not present in Drosophila OPA1. 60 Ectopic overexpression of another mitochondrial protease, Rhomboid-7, could process dOPA1. 61 Our western blot analysis revealed similar patterns of dOPA1 isoforms in wild-type and mutant flies (Supplementary Figure S5d), further substantiating that dYME1L is unlikely to be required for OPA1 processing.…”

Quality Control of Mitochondrial Proteostasis

“…Under conditions of mitochondrial dysfunction, including lowered ATP levels, loss of mtDNA and dissipation of the mitochondrial membrane potential across the IM, the short forms of OPA1 accumulate at the expense of the long OPA1 isoforms (Ishihara et al 2004;Duvezin-Caubet et al 2006;Baricault et al 2007;Olichon et al 2007;Guillery et al 2008). Stress-induced degradation of long OPA1 isoforms was found to be mediated by OMA1 (Ehses et al 2009;Head et al 2009).…”