Redecorating the Mitochondrial Inner Membrane: A Treatment for mtDNA Disorders
Authors: Zofia M.A. Chrzanowska-Lightowlers, Robert N. Lightowlers
Journal: Molecular Therapy (2020)

Mitochondrial diseases are progressive, often devastating disorders for which there are currently no cures. The pathogenic mutations responsible can arise in either the nuclear or the mitochondrial genome. For sequence changes in nuclear genes, a molecular therapy mediated by the delivery of a correct copy or by approaches that cause enhanced expression of a different gene that has compensatory effects represent possible therapeutic interventions. In this issue of Molecular Therapy, the article by Luna-Sanchez etal.1Luna-Sanchez M. Benincá C. Cerutti R. Brea-Calvo G. Yeates A. Scorrano L. Zeviani M. Viscomi C. Opa1 Overexpression Protects from Early-Onset Mpv17-/--Related Mouse Kidney Disease.Mol. Ther. 2020; 28 (this issue): 1918-1930Abstract Full Text Full Text PDF Scopus (4) Google Scholar evaluates the potential efficacy of treating a rare disorder caused by mutations in the nuclear-encoded MPV17 gene. MPV17 is a protein found in the inner of the two mitochondrial membranes, but its functions remain obscure. Disease caused by dysfunctional MPV17 has thus far only been reported in 100 patients and is found at the highest density in a restricted region of the southwestern US.2Umetsu S. Inui A. Kobayashi S. Shimura M. Uehara T. Uchida H. Irie R. Sogo T. Komatsu H. Yoshioka T. et al.First cases of MPV17 related mitochondrial DNA depletion syndrome with compound heterozygous mutations in p.R50Q/p.R50W: a case report.Hepatoma Res. 2020; (Published January 6, 2020)https://doi.org/10.20517/2394-5079.2019.030Crossref Google Scholar,3El-Hattab A.W. Wang J. Dai H. Almannai M. Staufner C. Alfadhel M. Gambello M.J. Prasun P. Raza S. Lyons H.J. et al.MPV17-related mitochondrial DNA maintenance defect: New cases and review of clinical, biochemical, and molecular aspects.Hum. Mutat. 2018; 39: 461-470Crossref PubMed Scopus (30) Google Scholar Survival beyond infancy or early childhood is rare and, although symptoms manifest as a broad spectrum of defects, a relatively common feature is the instability of the mitochondrial genome (mtDNA). This is particularly acute in the liver, which Luna-Sanchez etal.1Luna-Sanchez M. Benincá C. Cerutti R. Brea-Calvo G. Yeates A. Scorrano L. Zeviani M. Viscomi C. Opa1 Overexpression Protects from Early-Onset Mpv17-/--Related Mouse Kidney Disease.Mol. Ther. 2020; 28 (this issue): 1918-1930Abstract Full Text Full Text PDF Scopus (4) Google Scholar suggest is due to aberrant infolding of the inner mitochondrial membrane (IMM). They report that, in a mouse lacking Mpv17, the mtDNA levels were partially rescued, and the average life expectancy of the mouse was substantially increased following moderate overexpression of Opa1, a protein responsible for sculpting the IMM. The authors generated an Mpv17 knockout (KO) mouse model (Mpv17−/−) that recapitulated the mtDNA depletion phenotype, albeit strongly in kidney as well as liver. Through various crosses, a further mouse model was generated that still lacked Mpv17 but also overexpressed Opa1, another IMM protein. Opa1 was selected as a potential suppressor of the Mpv17 defect as it is known to play a crucial role in the architecture of the IMM to which mtDNA is anchored.4Gerhold J.M. Cansiz-Arda Ş. Lõhmus M. Engberg O. Reyes A. van Rennes H. Sanz A. Holt I.J. Cooper H.M. Spelbrink J.N. Human Mitochondrial DNA-Protein Complexes Attach to a Cholesterol-Rich Membrane Structure.Sci. Rep. 2015; 5: 15292Crossref PubMed Scopus (65) Google Scholar Previous results from this same group had revealed that overexpression of Opa1 in mouse models with mitochondrial respiratory chain defects had shown mild improvement, which they proposed resulted from an improved environment for the defective multisubunit respiratory chain complex that is normally embedded in the IMM.5Civiletto G. Varanita T. Cerutti R. Gorletta T. Barbaro S. Marchet S. Lamperti C. Viscomi C. Scorrano L. Zeviani M. Opa1 overexpression ameliorates the phenotype of two mitochondrial disease mouse models.Cell Metab. 2015; 21: 845-854Abstract Full Text Full Text PDF PubMed Scopus (167) Google Scholar The current study was based on a similar premise. Their hypothesis was that a disturbance of the shape and number of infoldings of the IMM, termed cristae (see Figure 1), may potentially affect the mitochondrial genome’s stability. Consequently, restoring the natural structure of the IMM by moderate Opa1 overexpression may lead to an improvement in efficiency of mtDNA replication and transcription (discussed in Nicholls and Gustafsson6Nicholls T.J. Gustafsson C.M. Separating and Segregating the Human Mitochondrial Genome.Trends Biochem. Sci. 2018; 43: 869-881Abstract Full Text Full Text PDF PubMed Scopus (24) Google Scholar). Data indicated that overexpression caused (1) an increased cristae volume within the mitochondrion; (2) a narrowing of the cristae junction where it joins the inner boundary membrane of the IMM (see Figure 1), thus potentially restoring the biophysical separation of the cristae compartment from the intermembrane space; and (3) a subsequent elevation of the mtDNA copy number that, in some tissues, approached control levels. Most positive effects were found in kidney tissues where the mtDNA depletion and other physiological defects in the Mpv1−/− mice were pronounced, unlike the situation in MPV1 patients, where kidney defects are uncommon. Is it likely that a moderate overexpression of Opa1 will, therefore, be of therapeutic benefit to patients with disorders resulting from MPV1 mutations? What might be the caveats to this possibility? The mouse model recapitulates the mtDNA depletion, but the predominantly affected tissue is kidney. This contrasts with the human condition, where the vast majority of the patients display hepato-cerebral involvement, where liver problems are the most severe and are consistent with the major depletion of mtDNA being in the liver. Although Opa1 expression in Mpv17 KO background doubled the mtDNA levels in liver compared to the Mpv17 KO, this elevated level still only reached ∼10% of control levels, and no significant improvement in respiratory chain complexes was detected in this tissue. The other devastating aspect of this condition is the short life expectancy, mirrored in the mouse model. With Opa1 overexpression there was a significant change in the median lifespan; however, the maximal lifespan remained almost unchanged from the Mpv17−/− mice. These points do not distract from the evidence that, where there are disorders with clear defects in cristae formation and the cristae junctions, modest overexpression of Opa1 may indeed offer a therapeutic option to alleviate mtDNA depletion or defective mtDNA maintenance. The importance of developing such therapies is self-evident when one considers the ubiquity of mitochondria in all nucleated eukaryotic cells and the important roles they play in a number of different functions, including calcium homeostasis, apoptosis, iron-sulfur cluster formation, and, perhaps most widely recognized, the efficient production of ATP through oxidative phosphorylation (OXPHOS). These reticular organelles are generated and maintained through a coordinated expression of two genomes, with the nuclear DNA contributing approximately 1,200 gene products and the mitochondrial genome encoding 13 open reading frames, all of which are members of the OXPHOS machinery. The latter is positioned within the inner mitochondrial membrane, which is invaginated to form cristae that both increase the surface area of the IMM but, importantly, generates a further discrete intraorganellar subcompartment (see Figure 1). Fission and fusion events determine the reticular or punctate nature of the mitochondrial network. Such rapid adaptation allows the organelle to regulate ATP production and respond to the energetic demands of the cell. These processes of fission and fusion are strictly controlled and involve a number of proteins, including Drp1 (fission), MFN1/2 (fusion), and a further key protein that modulates the cristae architecture, namely Opa1, the potential therapeutic candidate put forward by Luna-Sanchez etal.1Luna-Sanchez M. Benincá C. Cerutti R. Brea-Calvo G. Yeates A. Scorrano L. Zeviani M. Viscomi C. Opa1 Overexpression Protects from Early-Onset Mpv17-/--Related Mouse Kidney Disease.Mol. Ther. 2020; 28 (this issue): 1918-1930Abstract Full Text Full Text PDF Scopus (4) Google Scholar What is becoming increasingly evident is that the balance between fission and fusion coupled with alterations to internal cristae structure serves to regulate and control key aspects of mitochondrial function and that disturbances to this status have implications in development7Wasilewski M. Semenzato M. Rafelski S.M. Robbins J. Bakardjiev A.I. Scorrano L. Optic atrophy 1-dependent mitochondrial remodeling controls steroidogenesis in trophoblasts.Curr. Biol. 2012; 22: 1228-1234Abstract Full Text Full Text PDF PubMed Scopus (60) Google Scholar and disease (reviewed in Ma et al.8Ma Y. Wang L. Jia R. The role of mitochondrial dynamics in human cancers.Am. J. Cancer Res. 2020; 10: 1278-1293PubMed Google Scholar and Chrzanowska-Lightowlers and Lightowlers9Chrzanowska-Lightowlers Z.M. Lightowlers R.N. How much does a disrupted mitochondrial network influence neuronal dysfunction?.EMBO Mol. Med. 2019; 11: 11Crossref Scopus (1) Google Scholar). For example, reduced levels of Drp1 in skeletal muscle have been seen in a number of muscle wasting conditions,10Favaro G. Romanello V. Varanita T. Andrea Desbats M. Morbidoni V. Tezze C. Albiero M. Canato M. Gherardi G. De Stefani D. et al.DRP1-mediated mitochondrial shape controls calcium homeostasis and muscle mass.Nat. Commun. 2019; 10: 2576Crossref PubMed Scopus (180) Google Scholar and its expression levels and phosphorylation status have been shown to correlate with the severity of progression in various cancers.11Tsuyoshi H. Orisaka M. Fujita Y. Asare-Werehene M. Tsang B.K. Yoshida Y. Prognostic impact of Dynamin related protein 1 (Drp1) in epithelial ovarian cancer.BMC Cancer. 2020; 20: 467Crossref PubMed Scopus (14) Google Scholar Defects in MFN1/2 can cause a form of Charcot Marie tooth disease (OMIM: 609260)12Züchner S. Mersiyanova I.V. Muglia M. Bissar-Tadmouri N. Rochelle J. Dadali E.L. Zappia M. Nelis E. Patitucci A. Senderek J. et al.Mutations in the mitochondrial GTPase mitofusin 2 cause Charcot-Marie-Tooth neuropathy type 2A.Nat. Genet. 2004; 36: 449-451Crossref PubMed Scopus (1238) Google Scholar and Opa1, as its name implies (optic atrophy 1), has also been linked with neurological disorders and disease. It exists in multiple active and inactive forms that need to be correctly balanced to retain a healthy mitochondrial network.13Caglayan S. Hashim A. Cieslar-Pobuda A. Jensen V. Behringer S. Talug B. Chu D.T. Pecquet C. Rogne M. Brech A. et al.Optic Atrophy 1 Controls Human Neuronal Development by Preventing Aberrant Nuclear DNA Methylation.iScience. 2020; 23: 101154Abstract Full Text Full Text PDF PubMed Scopus (15) Google Scholar These data emphasize that, to ensure correct development and avoid disease, these key proteins that modulate the mitochondrial reticulum need to be maintained at both the appropriate physiological level and modification status. This understanding has led to investigations that have used different in vitro and in vivo approaches to determine whether, by controlling the fission/fusion status of the network, it is possible to correct any imbalances and implement this as a way to suppress dysfunction and prevent disease.14Humphries B.A. Cutter A.C. Buschhaus J.M. Chen Y.C. Qyli T. Palagama D.S.W. Eckley S. Robison T.H. Bevoor A. Chiang B. et al.Enhanced mitochondrial fission suppresses signaling and metastasis in triple-negative breast cancer.Breast Cancer Res. 2020; 22: 60Crossref PubMed Scopus (27) Google Scholar Opa1’s role in mitochondrial architecture made it a prime candidate for such manipulation. Mouse models generated from crosses of Opa1-overexpressing transgenic mice with KO strains displaying dysfunction in either complex I or complex IV of the respiratory chain showed that increased levels of Opa1 could ameliorate the defect in mitochondrial bioenergetics.5Civiletto G. Varanita T. Cerutti R. Gorletta T. Barbaro S. Marchet S. Lamperti C. Viscomi C. Scorrano L. Zeviani M. Opa1 overexpression ameliorates the phenotype of two mitochondrial disease mouse models.Cell Metab. 2015; 21: 845-854Abstract Full Text Full Text PDF PubMed Scopus (167) Google Scholar When taken all together, these data support the potential of targeting Opa1 expression as a mitochondrial staple gun to restore defective cristae junction structures in disorders where such architectural aberrations affect either mtDNA replication/transcription or the integration of OXPHOS complexes into the inner mitochondrial membranes. The devastating and progressive nature of mitochondrial diseases is a potent reason to consider evaluating all novel therapeutic approaches that could ameliorate these conditions. R.N.L. and Z.M.A.C.-L. were responsible for writing the original draft, review, and editing of this manuscript. The authors declare that there are no conflicts of interest. This work was supported by The Wellcome Trust (203105/Z/16/Z to R.N.L. and Z.M.A.C.-L.) and EU Horizon2020 MCS (ITN REMIX 721757). Opa1 Overexpression Protects from Early-Onset Mpv17−/−-Related Mouse Kidney DiseaseLuna-Sanchez et al.Molecular TherapyJune 11, 2020In BriefNo treatment is currently available to syndromes associated to mitochondrial DNA instability. We investigated the possibility to correct the kidney disease typical of Mpv17−/− mice by transgenically overexpressing Opa1, the master regulator of mitochondrial cristae shape. Mpv17−/−::Opa1 double recombinant mice showed a marked prolongation of the lifespan, with correction of the proteinuria and of the focal segmental glomerulosclerosis, by reducing apoptosis in podocytes. Accordingly, mitochondrial cristae morphology, largely disrupted in Mpv17−/− glomeruli, was improved in double recombinant animals. Full-Text PDF Open Access