- Email: [email protected]
Focus on Molecules: The OPA1 protein
Experimental Eye Research 83 (2006) 1003e1004 www.elsevier.com/locate/yexer
Focus on Molecules: The OPA1 protein Vanessa Davies, Marcela Votruba* School of Optometry and Vision Sciences, Cardiff University, Redwood Building, King Edward VII Avenue Cathays Park, Cardiff CF10 3NB, UK Available online 23 March 2006
Keywords: retinal ganglion cell; optic neuropathy; mitochondrial fusion
1. Structure The OPA1 gene (accession numbers: NM 012062, OMIM 605290, GenBank AB011139, SwissProt 060313) maps to chromosome 3q28q29. It comprises 29 exons spanning more than 100 kb genomic DNA, of which 28 are coding exons. The gene encodes a 960 amino acid mitochondrial dynamin-related guanosine triphosphatase (GTPase) protein. Alternative splicing of two additional exons, 4b and 5b, result in eight transcript variants. The protein contains a mitochondrial leader sequence within the highly basic amino-terminal, a GTPase domain, a central dynamin domain that is conserved across all dynamins, and a carboxy terminus of unknown function. The carboxy terminus differs from that of other dynamin family members in lacking a proline-rich region, a GTPase effector domain and a pleckstrin homology domain. OPA1 is widely expressed throughout the body: in heart, skeletal muscle, liver, testis, and most abundantly in, brain and retina. In the eye, OPA1 is present in the cells of the retinal ganglion cell layer, inner and outer plexiform layers, and inner nuclear layer (Aijaz et al., 2004). 2. Function The precise function of the OPA1 protein is unknown. Functional insights are gained from homologous proteins belonging to the same dynamin subfamily, the sublocalization of the OPA1 protein within the cell, patient data, and in vitro expression knockdown studies. OPA1 is the human homologue of yeast dynamin-related GTPbinding protein Mgm1, which is involved in mitochondrial genome maintenance (Griparic et al., 2004). Owing to the 33% homology shared between these two proteins, it would be surprising if they did not share similar properties. Within the cell, mitochondrial morphology is maintained through a balance of fusion and fission * Corresponding author. E-mail address: [email protected] (M. Votruba). 0014-4835/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.exer.2005.11.021
(Chen and Chan, 2005). A role for Mgm1 in mitochondrial fusion is reported. Mutations in Mgm1 are shown to disrupt fusion between mitochondria (Griparic et al., 2004); and overexpression of mutant or wildtype Mgm1 causes the mitochondria to become fragmented within the cell, clustering near the nucleus. From these data, the OPA1 protein is predicted to have a role in the maintenance of mitochondrial morphology. The presence of the mitochondrial leader sequence advocates that the OPA1 protein is transported to the mitochondria. In fact, much evidence localises the OPA1 protein to the mitochondria; specifically to the outer surface of the inner membrane facing the mitochondrial intermembrane space (Griparic et al., 2004). A functional role associated with mitochondria, therefore, appears inevitable. OPA1 mutant patient data highlight a potential role in mitochondrial maintenance (Chen and Chan, 2005). In such patients, the mitochondrial DNA content is lower, oxidative phosphorylation in the calf muscle is defective, and the structure of the mitochondrial network in monocytes is reportedly altered compared to normal control subjects. OPA1 expression knockdown studies provide much evidence to support a functional role in mitochondrial morphology maintenance. These studies utilize small interfering (si) RNA to reduce OPA1 expression. Olichon and colleagues reported that the down-regulation of OPA1 protein leads to fragmentation of the mitochondrial network, dissipation of the mitochondrial membrane potential, disorganisation of the mitochondrial cristae, and release of cytochrome c followed by caspase-dependent apoptotic nuclear events (reviewed in Chen and Chan, 2005). Griparic et al. (2004) also demonstrated that OPA1 loss leads to gross morphological changes in the mitochondrial shape before mitochondrial fragmentation arises. These data, therefore, imply a role for OPA1 in mitochondrial fusion, and cristae organisation and integrity. Recently, Arnoult et al. (2005) confirmed this pattern of results, reporting that an initial leak of OPA1, a consequence of mitochondrial outer membrane permeabilization, causes restructuring of the mitochondrial cristae, exposing and releasing the sequestered pools of OPA1 and cytochrome c. The loss of OPA1 then caused a block in mitochondrial fusion, providing an explanation for the observed mitochondrial fragmented phenotype.
1004
V. Davies, M. Votruba / Experimental Eye Research 83 (2006) 1003e1004
Fig. 1. Mitochondrial fusion and fission molecules (taken from Chen and Chan, 2005). (A) Fusion molecules. Mfn is a mitochondrial outer membrane protein with cytosolic GTPase domain (orange oval) and two coiled coil regions (coils). The C-terminal coiled coil mediates oligomerization between Mfn molecules on adjacent mitochondria. OPA1 (white oval) is a GTPase in the intermembrane space. Mfns and OPA1 coordinate mitochondria fusion, as shown by mixing of green and red matrix markers to produce yellow. (B) Fission molecules. Fis 1 localized uniformly to the mitochondrial outer membrane, whereas Drp1 is localized to the cytosol and punctate spots on mitochondria. Some of these spots are constriction sites that lead to mitochondrial fission.
It is increasingly apparent that additional ‘mitochondrial shaping’ proteins function with OPA1 to maintain the dynamic control of the mitochondrial structure (see Fig. 1 taken from Chen and Chan, 2005). Such proteins include pro-fusion GTPases: mitofusin (Mfn) 1 and 2; and pro-fission GTPases: dynamin-related protein 1 (Drp1) and Fis1. Specifically, Cipolat and colleagues demonstrated that OPA1 and Mfn 1 work synergistically to regulate mitochondrial fusion (reviewed in Chen and Chan, 2005). OPA1 is unable to promote mitochondrial fusion in the absence of Mfn1, and Mfn1 cannot induce mitochondrial elongation in the absence of OPA1. Furthermore, from the many studies investigating mitochondrial homeostasis, OPA1 and Mfn1 may have a protective role within the cell. Acting as anti-apoptotic GTPases, they may protect the cell from spontaneous apoptosis and the detrimental effects/consequences of apoptotic stimuli (see Fig. 1). 3. Disease involvement Mutations in the OPA1 gene cause optic atrophy type 1 (OPA1; OMIM 165500) or autosomal dominant optic atrophy (ADOA): the most common form of dominantly inherited optic neuropathy. Ninety-six mutations have been identified to date (http://lbbma. univ-angers.fr). These include missense and nonsense substitutions, deletions, insertions and complex rearrangements. The majority of these mutations arise in the GTPase and dynamin central regions, coded by exons 8e16 and in the C-terminal coding region, exons 27e28. They result mainly in protein truncation and the functional loss of one allele, thereby causing haplo-insufficiency of OPA1. ADOA affects approximately 1:50 000 individuals and usually manifests early in childhood. It is characterised by a progressive loss of visual acuity, colour vision defects, central visual field defects and temporal optic disc pallor. The fundamental pathology of the disease is a loss of retinal ganglion cells, particularly those in the macular region, followed by ascending atrophy of the optic nerve. 4. Future studies It remains unclear why ADOA manifests with an ocular phenotype; particularly, since it is ubiquitously expressed throughout the
body, most abundantly in the brain after the retina (Aijaz et al., 2004). Several possible explanations are offered that require further investigation. First, the loss of one allele decreases the amount of OPA1 protein below a critical threshold for normal functionality of the mitochondria; and thus, may compromise retinal ganglion cell survival. Neurons, in particular, owing to their high-energy demands may be particularly susceptible to changes in mitochondrial function; however, this does not explain why the central neural tissues of the brain are clinically unaffected in ADOA. Second, the OPA1 protein may have different functions in the mitochondria of different tissues, particularly as the eight mRNA splice forms are differentially expressed. Third, haplo-insufficiency may increase the tissue’s susceptibility to apoptotic stimuli; in the case of retinal ganglion cells, daily exposure to UV light and reactive oxygen species. In vitro studies have provided new insights into the function of the OPA1 protein in mitochondrial homeostasis. There remains a great need for an in vivo animal model of mutant OPA1. Since the homology between rodents and humans is high, a rodent model would provide vital information about the mechanism behind retinal ganglion cell loss, and the additional factors that increase the susceptibility of these cells to programmed cell death. Factors are either absent or less detrimental in the other OPA1 expressing mitochondrial rich tissues of the body.
References Aijaz, S., Erskine, L., Jeffrey, G., Bhattacharya, S.S., Votruba, M., 2004. Developmental expression profile of the optic atrophy gene product: OPA1 is not localized exclusively in the mammalian retinal ganglion cell layer. Investig. Ophthalmol. Vis. Sci. 45, 1667e1673. Arnoult, D., Grodet, A., Lee, Y.-L., Estaquier, J., Blackstone, C., 2005. Release of OPA1 during apoptosis participates in the rapid and complete release of cytochrome c and subsequent mitochondrial fragmentation. J. Biol. Chem. 280, 35742e35750. Chen, H., Chan, D.C., 2005. Emerging functions of mammalian mitochondrial fusion and fission. Hum. Mol. Genet. 14, R283eR289. Griparic, L., van der Wel, N.N., Orozco, I.J., Peters, P.J., van der Bliek, A.M., 2004. Loss of the intermembrane space protein Mgm1/OPA1 induces swelling and localized constrictions along the lengths of mitochondria. J. Biol. Chem. 279, 18792e18798.
Focus on Molecules: The OPA1 protein Vanessa Davies, Marcela Votruba* School of Optometry and Vision Sciences, Cardiff University, Redwood Building, King Edward VII Avenue Cathays Park, Cardiff CF10 3NB, UK Available online 23 March 2006
Keywords: retinal ganglion cell; optic neuropathy; mitochondrial fusion
1. Structure The OPA1 gene (accession numbers: NM 012062, OMIM 605290, GenBank AB011139, SwissProt 060313) maps to chromosome 3q28q29. It comprises 29 exons spanning more than 100 kb genomic DNA, of which 28 are coding exons. The gene encodes a 960 amino acid mitochondrial dynamin-related guanosine triphosphatase (GTPase) protein. Alternative splicing of two additional exons, 4b and 5b, result in eight transcript variants. The protein contains a mitochondrial leader sequence within the highly basic amino-terminal, a GTPase domain, a central dynamin domain that is conserved across all dynamins, and a carboxy terminus of unknown function. The carboxy terminus differs from that of other dynamin family members in lacking a proline-rich region, a GTPase effector domain and a pleckstrin homology domain. OPA1 is widely expressed throughout the body: in heart, skeletal muscle, liver, testis, and most abundantly in, brain and retina. In the eye, OPA1 is present in the cells of the retinal ganglion cell layer, inner and outer plexiform layers, and inner nuclear layer (Aijaz et al., 2004). 2. Function The precise function of the OPA1 protein is unknown. Functional insights are gained from homologous proteins belonging to the same dynamin subfamily, the sublocalization of the OPA1 protein within the cell, patient data, and in vitro expression knockdown studies. OPA1 is the human homologue of yeast dynamin-related GTPbinding protein Mgm1, which is involved in mitochondrial genome maintenance (Griparic et al., 2004). Owing to the 33% homology shared between these two proteins, it would be surprising if they did not share similar properties. Within the cell, mitochondrial morphology is maintained through a balance of fusion and fission * Corresponding author. E-mail address: [email protected] (M. Votruba). 0014-4835/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.exer.2005.11.021
(Chen and Chan, 2005). A role for Mgm1 in mitochondrial fusion is reported. Mutations in Mgm1 are shown to disrupt fusion between mitochondria (Griparic et al., 2004); and overexpression of mutant or wildtype Mgm1 causes the mitochondria to become fragmented within the cell, clustering near the nucleus. From these data, the OPA1 protein is predicted to have a role in the maintenance of mitochondrial morphology. The presence of the mitochondrial leader sequence advocates that the OPA1 protein is transported to the mitochondria. In fact, much evidence localises the OPA1 protein to the mitochondria; specifically to the outer surface of the inner membrane facing the mitochondrial intermembrane space (Griparic et al., 2004). A functional role associated with mitochondria, therefore, appears inevitable. OPA1 mutant patient data highlight a potential role in mitochondrial maintenance (Chen and Chan, 2005). In such patients, the mitochondrial DNA content is lower, oxidative phosphorylation in the calf muscle is defective, and the structure of the mitochondrial network in monocytes is reportedly altered compared to normal control subjects. OPA1 expression knockdown studies provide much evidence to support a functional role in mitochondrial morphology maintenance. These studies utilize small interfering (si) RNA to reduce OPA1 expression. Olichon and colleagues reported that the down-regulation of OPA1 protein leads to fragmentation of the mitochondrial network, dissipation of the mitochondrial membrane potential, disorganisation of the mitochondrial cristae, and release of cytochrome c followed by caspase-dependent apoptotic nuclear events (reviewed in Chen and Chan, 2005). Griparic et al. (2004) also demonstrated that OPA1 loss leads to gross morphological changes in the mitochondrial shape before mitochondrial fragmentation arises. These data, therefore, imply a role for OPA1 in mitochondrial fusion, and cristae organisation and integrity. Recently, Arnoult et al. (2005) confirmed this pattern of results, reporting that an initial leak of OPA1, a consequence of mitochondrial outer membrane permeabilization, causes restructuring of the mitochondrial cristae, exposing and releasing the sequestered pools of OPA1 and cytochrome c. The loss of OPA1 then caused a block in mitochondrial fusion, providing an explanation for the observed mitochondrial fragmented phenotype.
1004
V. Davies, M. Votruba / Experimental Eye Research 83 (2006) 1003e1004
Fig. 1. Mitochondrial fusion and fission molecules (taken from Chen and Chan, 2005). (A) Fusion molecules. Mfn is a mitochondrial outer membrane protein with cytosolic GTPase domain (orange oval) and two coiled coil regions (coils). The C-terminal coiled coil mediates oligomerization between Mfn molecules on adjacent mitochondria. OPA1 (white oval) is a GTPase in the intermembrane space. Mfns and OPA1 coordinate mitochondria fusion, as shown by mixing of green and red matrix markers to produce yellow. (B) Fission molecules. Fis 1 localized uniformly to the mitochondrial outer membrane, whereas Drp1 is localized to the cytosol and punctate spots on mitochondria. Some of these spots are constriction sites that lead to mitochondrial fission.
It is increasingly apparent that additional ‘mitochondrial shaping’ proteins function with OPA1 to maintain the dynamic control of the mitochondrial structure (see Fig. 1 taken from Chen and Chan, 2005). Such proteins include pro-fusion GTPases: mitofusin (Mfn) 1 and 2; and pro-fission GTPases: dynamin-related protein 1 (Drp1) and Fis1. Specifically, Cipolat and colleagues demonstrated that OPA1 and Mfn 1 work synergistically to regulate mitochondrial fusion (reviewed in Chen and Chan, 2005). OPA1 is unable to promote mitochondrial fusion in the absence of Mfn1, and Mfn1 cannot induce mitochondrial elongation in the absence of OPA1. Furthermore, from the many studies investigating mitochondrial homeostasis, OPA1 and Mfn1 may have a protective role within the cell. Acting as anti-apoptotic GTPases, they may protect the cell from spontaneous apoptosis and the detrimental effects/consequences of apoptotic stimuli (see Fig. 1). 3. Disease involvement Mutations in the OPA1 gene cause optic atrophy type 1 (OPA1; OMIM 165500) or autosomal dominant optic atrophy (ADOA): the most common form of dominantly inherited optic neuropathy. Ninety-six mutations have been identified to date (http://lbbma. univ-angers.fr). These include missense and nonsense substitutions, deletions, insertions and complex rearrangements. The majority of these mutations arise in the GTPase and dynamin central regions, coded by exons 8e16 and in the C-terminal coding region, exons 27e28. They result mainly in protein truncation and the functional loss of one allele, thereby causing haplo-insufficiency of OPA1. ADOA affects approximately 1:50 000 individuals and usually manifests early in childhood. It is characterised by a progressive loss of visual acuity, colour vision defects, central visual field defects and temporal optic disc pallor. The fundamental pathology of the disease is a loss of retinal ganglion cells, particularly those in the macular region, followed by ascending atrophy of the optic nerve. 4. Future studies It remains unclear why ADOA manifests with an ocular phenotype; particularly, since it is ubiquitously expressed throughout the
body, most abundantly in the brain after the retina (Aijaz et al., 2004). Several possible explanations are offered that require further investigation. First, the loss of one allele decreases the amount of OPA1 protein below a critical threshold for normal functionality of the mitochondria; and thus, may compromise retinal ganglion cell survival. Neurons, in particular, owing to their high-energy demands may be particularly susceptible to changes in mitochondrial function; however, this does not explain why the central neural tissues of the brain are clinically unaffected in ADOA. Second, the OPA1 protein may have different functions in the mitochondria of different tissues, particularly as the eight mRNA splice forms are differentially expressed. Third, haplo-insufficiency may increase the tissue’s susceptibility to apoptotic stimuli; in the case of retinal ganglion cells, daily exposure to UV light and reactive oxygen species. In vitro studies have provided new insights into the function of the OPA1 protein in mitochondrial homeostasis. There remains a great need for an in vivo animal model of mutant OPA1. Since the homology between rodents and humans is high, a rodent model would provide vital information about the mechanism behind retinal ganglion cell loss, and the additional factors that increase the susceptibility of these cells to programmed cell death. Factors are either absent or less detrimental in the other OPA1 expressing mitochondrial rich tissues of the body.
References Aijaz, S., Erskine, L., Jeffrey, G., Bhattacharya, S.S., Votruba, M., 2004. Developmental expression profile of the optic atrophy gene product: OPA1 is not localized exclusively in the mammalian retinal ganglion cell layer. Investig. Ophthalmol. Vis. Sci. 45, 1667e1673. Arnoult, D., Grodet, A., Lee, Y.-L., Estaquier, J., Blackstone, C., 2005. Release of OPA1 during apoptosis participates in the rapid and complete release of cytochrome c and subsequent mitochondrial fragmentation. J. Biol. Chem. 280, 35742e35750. Chen, H., Chan, D.C., 2005. Emerging functions of mammalian mitochondrial fusion and fission. Hum. Mol. Genet. 14, R283eR289. Griparic, L., van der Wel, N.N., Orozco, I.J., Peters, P.J., van der Bliek, A.M., 2004. Loss of the intermembrane space protein Mgm1/OPA1 induces swelling and localized constrictions along the lengths of mitochondria. J. Biol. Chem. 279, 18792e18798.