ReviewProteolytic regulation of mitochondrial dynamics
Introduction
Mitochondrial dynamics is a process that can be defined as spatiotemporal changes in mitochondrial shape, number and position within the cell. These changes occur in response to various stimuli and conditions and comprise reciprocal events known as mitochondrial fragmentation (fission) and fusion of mitochondrial network. Proper balance between fission and fusion events is necessary to maintain appropriate mitochondrial functions including, but not limited to energy conversion, cofactor and metabolite synthesis, ion homeostasis maintenance, cell signaling, differentiation and death (Scheffler, 2007; Scott and Youle, 2010; Twig and Shirihai, 2011; Zhong et al., 2018). Disruption of these dynamic events prompts mitochondria-linked neurodegeneration, neuropathies, cardiovascular disorders, myopathies, certain cancers, and metabolic disorders (Anzell et al., 2018; Bohovych et al., 2015; Levytskyy et al., 2016; Panchal and Tiwari, 2018; Scheffler, 2007; Trotta and Chipuk, 2017).
Mitochondria harbor two sets of membranes – the outer (OM) and the inner (IM) mitochondrial membranes, the fusion and division of which are coordinated but physically separate processes (Malka et al., 2005), each of which requires complex multi-component machinery. Moreover, molecular events in the IM appear to precede and overpower ones that involve the OM (Ban et al., 2017; Chakrabarti et al., 2018; Cho et al., 2017; Song et al., 2009). Although new regulatory mechanisms and molecules are still being discovered, the key factors behind mitochondrial dynamics have been well characterized (Fig. 1A). The mammalian mitochondrial dynamin-like guanosine triphosphatases (GTPases) mitofusin 1 and 2 (Mfn1/2) (Fig. 1B) direct OM fusion by forming GTP-dependent homo- and heterotypic complexes linking adjacent mitochondria (Chen et al., 2005; Chen et al., 2003; Franco et al., 2016; Huang et al., 2017; Ishihara et al., 2004; Koshiba et al., 2004; Mattie et al., 2018). Mitofusins are conserved proteins with robust homologs present in Saccharomyces cerevisiae and Schizosaccharomyces pombe (Fzo1), Caenorhabditis elegans (FZO-1), and Drosophila melanogaster (FZO and DMFN) (reviewed in Westermann, 2010a). Numerous post-translational modifications, ubiquitylation in particular, regulate mitofusins' activity and can target them for ubiquitin proteasome system (UPS)-mediated degradation (Ali and McStay, 2018; Wai and Langer, 2016). Regulation by ubiquitylation and proteasomal degradation of mitofusins is an evolutionarily conserved process.
In mammals, OM fission is completed by the dynamin-related protein GTPase Drp1 (Fig. 1). Specific cellular signals induce Drp1 to localize to the mitochondria at a future site of division (Cereghetti et al., 2008; Chang and Blackstone, 2007a; Chang and Blackstone, 2010b; Cribbs and Strack, 2007; Ji et al., 2015; Kashatus et al., 2015; Prieto et al., 2016). Upon translocation to the mitochondria, the OM fission factor self-oligomerizes and forms a ring-like structure around the organelle. Adaptor proteins (Fis1, Mff, MiD49, MiD51) are necessary to link the Drp1-ring to the mitochondria at the site of fission. GTP hydrolysis by Drp1 causes the ring-like structure to constrict until fragmentation is completed (Otera et al., 2013; Otera and Mihara, 2011; Wai and Langer, 2016). An OM fission factor structurally and mechanistically similar to Drp1 has been described in other eukaryotes including S. cerevisiae (Dnm1), C. elegans (DRP-1), and D. melanogaster (DRP1). However, the mitochondrial adaptor proteins necessary for Drp1-ring tethering are not nearly as conserved (Westermann, 2010a). Currently, known homologs for Fis1 have been identified in S. cerevisiae (Fis1), C. elegans (FIS-1/2), and D. melanogaster (FIS1), and an Mff-like homolog has been described in D. melanogaster (reviewed by Westermann, 2010a). In S. cerevisiae, additional OM fission adaptor proteins were identified as Mdv1 and Caf4 (Griffin et al., 2005; Ingerman et al., 2005; Tieu et al., 2002; Tieu and Nunnari, 2000). Mammalian Drp1 stability and activity are regulated by phosphorylation and by ubiquitylation and UPS-mediated degradation in an E3 ubiquitin ligase Parkin-dependent manner (Tang et al., 2016; Wang et al., 2011a). The mechanisms behind the turnover of Drp1 homologs remain to be clarified. It should be noted that regulation of OM fission adaptor proteins does not appear to be a conserved process. Only the yeast adaptor Mdv1 is identified as a target for ubiquitylation and UPS-mediated degradation by an unknown E3 ubiquitin ligase (Christiano et al., 2014; Swaney et al., 2013).
Opa1, a GTPase anchored to the IM, functions as the mediator of IM fusion (Fig. 1). Canonically, mammalian Opa1 exists at its full length (L-Opa1) and promotes mitochondrial fusion, but upon specific homeostatic stimuli, it is cleaved producing a short form (S-Opa1). Accumulation of S-Opa1 results in mitochondrial fission transitioning Opa1 into an IM fission factor (Del Dotto et al., 2018a; Wai and Langer, 2016). The factor is highly conserved with known homologs in S. cerevisiae (Mgm1), S. pombe (msp1), C. elegans (EAT-3), and D. melanogaster (OPA1). However, maturation and regulation of these proteins via proteolysis has significantly diverged.
As such, complex and differential machineries mediate changes in the IM and OM dynamics in response to various stimuli. The IM dynamics is a subject of the proteolytic control by the enzymes intrinsic to the compartment, whereas the OM division and fusion are regulated by the UPS-mediated degradation of the OM fusion factors or relevant adaptor proteins.
Mitochondrial dynamics-mediated morphological and ultrastructural changes, as well as their role in maintaining mitochondrial homeostasis, are evolutionarily conserved. Studies in fungal, mammalian cell culture and murine models indicate that specific proteases and the ubiquitin proteasome system are imperative in regulating the proteins necessary for efficient mitochondrial fusion and fission. In the present review, we will discuss various mechanisms involved in proteolytic regulation of OM and IM fusion and division factors.
Section snippets
The OM fusion factors mitofusin/Fzo1 and Miro/Gem1
Proteolytic processing plays an important role in the regulation of the OM-localized GTPases mitofusins 1 and 2 (Mfn1/2 in mammals, Fzo1 in yeast) that are central to OM fusion (Wai and Langer, 2016). Likewise, yeast Fzo1 is central to mitochondrial fusion and normal mitochondrial morphology, and its deficiency induces mitochondrial fragmentation, mtDNA loss, and petite colony formation (Hermann et al., 1998; Rapaport et al., 1998). It is noteworthy, however, that multiple studies have unveiled
The IM fusion factor Opa1/Mgm1
The fusion and division dynamics of the IM are primarily controlled via proteolytic processing of the dynamin-like GTPase Opa1 (Baker et al., 2014; Ishihara et al., 2006; Saita et al., 2016). This pro-fusion factor was originally associated with the dominantly inherited progressive vision loss Optic atrophy type 1 (Alexander et al., 2000; Delettre et al., 2000), hence the name. Within the last two decades more than 370 mutations in the human OPA1 gene causing neurological disorders of various
Concluding remarks
A well-tuned and timely regulation of the mitochondrial network morphology is critical for responses to various stimuli and adaptation to both normal and stress conditions. Deregulated mitochondrial dynamics has been associated with severe, often detrimental, effects on cellular physiology and fate and has now emerged as an important factor behind a large number of human pathologies. Although the core machineries that mediate fusion and partitioning of mitochondrial network have been
Conflicts of interest
The authors declare no competing or financial interests.
Acknowledgments
We apologize to those authors whose work we were unable to cite due to space limitations. We thank the members of Khalimonchuk lab and Dr. Jennifer Fox (College of Charleston) for insightful comments and editorial help. We thank Carey Goddard for her expert help with figures preparation. We acknowledge support from the U.S. National Institutes of Health (R01 GM108975 to O.K. and T32 GM107001-01A1 to J.V.D.).
References (282)
- et al.
Membrane tethering and nucleotide-dependent conformational changes drive mitochondrial genome maintenance (Mgm1) protein-mediated membrane fusion
J. Biol. Chem.
(2012) - et al.
Proteolytic control of mitochondrial function and morphogenesis
Biochim. Biophys. Acta, Mol. Cell Res.
(2013) Electron microscopy morphology of the mitochondrial network in human cancer
Int. J. Biochem. Cell Biol.
(2009)- et al.
Mitofusin-2 determines mitochondrial network architecture and mitochondrial metabolism. A novel regulatory mechanism altered in obesity
J. Biol. Chem.
(2003) - et al.
Regulation of ER-mitochondria contacts by parkin via Mfn2
Pharmacol. Res.
(2018) - et al.
Lysine 27 ubiquitination of the mitochondrial transport protein miro is dependent on serine 65 of the parkin ubiquitin ligase
J. Biol. Chem.
(2014) - et al.
Stress-triggered activation of the metalloprotease Oma1 involves its C-terminal region and is important for mitochondrial stress protection in yeast
J. Biol. Chem.
(2014) - et al.
GRIF-1 and OIP106, members of a novel gene family of coiled-coil domain proteins
J. Biol. Chem.
(2005) - et al.
Cyclic AMP-dependent protein kinase phosphorylation of Drp1 regulates its GTPase activity and mitochondrial morphology
J. Biol. Chem.
(2007) - et al.
Disruption of fusion results in mitochondrial heterogeneity and dysfunction
J. Biol. Chem.
(2005)
Global proteome turnover analyses of the yeasts S. cerevisiae and S. pombe HHS public access
Cell Rep.
Apoptosis inducing factor deficiency causes reduced mitofusion 1 expression and patterned Purkinje cell degeneration
Neurobiol. Dis.
Mitochondrial cristae shape determines respiratory chain supercomplexes assembly and respiratory efficiency
Cell
OPA1 isoforms in the hierarchical organization of mitochondrial functions
Cell Rep.
Deciphering OPA1 mutations pathogenicity by combined analysis of human, mouse and yeast cell models
Biochim. Biophys. Acta Mol. basis Dis.
A novel Drosophila model of nerve injury reveals an essential role of endogenous Nmnat in maintaining axonal integrity
Curr. Biol.
Atypical rho GTPases have roles in mitochondrial homeostasis and apoptosis
J. Biol. Chem.
The atypical rho GTPases Miro-1 and Miro-2 have essential roles in mitochondrial trafficking
Biochem. Biophys. Res. Commun.
OPA1 controls apoptotic cristae remodeling independently from mitochondrial fusion
Cell
Domain interactions within Fzo1 oligomers are essential for mitochondrial fusion
J. Biol. Chem.
The GTPase dMiro is required for axonal transport of mitochondria to Drosophila synapses
Neuron
Sumo1 conjugates mitochondrial substrates and participates in mitochondrial fission
Curr. Biol.
Processing of Mgm1 by the rhomboid-type protease Pcp1 is required for maintenance of mitochondrial morphology and of mitochondrial DNA
J. Biol. Chem.
Functional impairment in miro degradation and mitophagy is a shared feature in familial and sporadic Parkinson's disease
Cell Stem Cell
Ablation of the stress protease OMA1 protects against heart failure in mice
Sci. Transl. Med.
Characterization of OPA1 isoforms isolated from mouse tissues
J. Neurochem.
OPA1, encoding a dynamin-related GTPase, is mutated in autosomal dominant optic atrophy linked to chromosome 3q28
Nat. Genet.
Regulation of mitochondrial dynamics by proteolytic processing and protein turnover
Antioxidants
OPA1-associated disorders: phenotypes and pathophysiology
Int. J. Biochem. Cell Biol.
A mutation associated with CMT2A neuropathy causes defects in Fzo1 GTP hydrolysis, ubiquitylation, and protein turnover
Mol. Biol. Cell
The i-AAA protease YME1L and OMA1 cleave OPA1 to balance mitochondrial fusion and fission
J. Cell Biol.
Two deubiquitylases act on Mitofusin and regulate mitochondrial fusion along independent pathways
Mol. Cell
Mitochondrial quality control and disease: insights into ischemia-reperfusion injury
Mol. Neurobiol.
Cellular/molecular miro's N-terminal GTPase domain is required for transport of mitochondria into axons and dendrites
J. Neurosci.
Inhibition of Drp1 ameliorates synaptic depression, Aβ deposition, and cognitive impairment in an Alzheimer's disease model
J. Neurosci.
Stress-induced OMA1 activation and autocatalytic turnover regulate OPA1-dependent mitochondrial dynamics
EMBO J.
Molecular basis of selective mitochondrial fusion by heterotypic action between OPA1 and cardiolipin
Nat. Cell Biol.
Molecular mechanism of DRP1 assembly studied in vitro by cryo-electron microscopy
PLoS One
OPA1, associated with autosomal dominant optic atrophy, is widely expressed in the human brain
Acta Neuropathol.
The mitochondrial deubiquitinase USP30 opposes parkin-mediated mitophagy
Nature
The dynamin-related GTPase Dnm1 regulates mitochondrial fission in yeast
Nat. Cell Biol.
Mitochondrial protein quality control: the mechanisms guarding mitochondrial health
Antioxid. Redox Signal.
MAPL is a new mitochondrial SUMO E3 ligase that regulates mitochondrial fission
EMBO Rep.
Syntabulin-mediated anterograde transport of mitochondria along neuronal processes
J. Cell Biol.
Impaired mitochondrial biogenesis, defective axonal transport of mitochondria, abnormal mitochondrial dynamics and synaptic degeneration in a mouse model of Alzheimer's disease
Hum. Mol. Genet.
MFN1 structures reveal nucleotide-triggered dimerization critical for mitochondrial fusion
Nature
Dephosphorylation by calcineurin regulates translocation of Drp1 to mitochondria
Proc. Natl. Acad. Sci.
INF2-mediated actin polymerization at the ER stimulates mitochondrial calcium uptake, inner membrane constriction, and division
J. Cell Biol.
Broad activation of the ubiquitin-proteasome system by parkin is critical for mitophagy
Hum. Mol. Genet.
Structure, function, and regulation of mitofusin-2 in health and disease
Biol. Rev.
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Metabolomics analysis reveals the effects of copper on mitochondria-mediated apoptosis in kidney of broiler chicken (Gallus gallus)
2021, Journal of Inorganic BiochemistryCitation Excerpt :Recent studies have pointed out that numbers of components related to fission and fusion machinery implicated in mitochondrial shape changes including dynamin-related protein 1 (Drp1), mitofusin 1 (Mfn1), mitofusin 2 (Mfn2), and optic atrophy 1 (OPA1) are indispensable for apoptosis [18,19]. In addition, some of findings revealed that excessive mitochondrial fission can trigger the increasing open degree of mitochondria permeability transition pore (MPTP) and promote the release of CytC from mitochondria, leading to mitochondrial-mediated apoptosis [18,20]. Cellular metabolism and the pathways of cell death are regarded as the important factors in regulating cell fate.
Stomatin-Like Protein-2: A Potential Target to Treat Mitochondrial Cardiomyopathy
2021, Heart Lung and CirculationCitation Excerpt :SLP-2 overexpression increases cardiolipin synthesis, which translates into increased mitochondrial membrane formation and biogenesis, and binds to cardiolipin and PHBs (PHB-1 and PHB-2) to promote the formation of cardiolipin-rich microdomains on the IMM [38], thus promoting the assembly of respiratory supercomplexes [43,44]. SLP-2 can form SPY complexes with rhomboid protease PARL and i-AAA protease YME1L to regulate the function of mitochondria [37,45] and can reduce myocardial ischaemia-reperfusion injury, apoptosis and mitochondrial injury [46]. Cardiomyocyte apoptosis is closely related to the progression of MCM [21].
OMA1—An integral membrane protease?
2021, Biochimica et Biophysica Acta - Proteins and ProteomicsCitation Excerpt :OPA1 is a membrane-shaping dynamin-related GTPase, which exists in a number of isoforms encoded by alternative splice-variants and/or generated by selective proteolysis of its amino-terminal membrane anchor [26,27]. The ratios of the isoforms are critical determinants of OPA1's function and are regulated by the i-AAA protease under physiological conditions [28–32]. OMA1 on the other hand can cleave all OPA1 species upon its activation, which is functionally connected to mitochondrial outer membrane permeabilization [33,34].
Mitochondria (cross)talk with proteostatic mechanisms: Focusing on ageing and neurodegenerative diseases
2020, Mechanisms of Ageing and DevelopmentCitation Excerpt :Similarly, the dynamin-related protein GTPase Drp1 that mediates fission of the outer mitochondrial membrane is also degraded by the UPS in mammals. Miro1/2 that facilitate mitochondrial motility are also subjected to UPS degradation (Dietz et al., 2019). Components of the mitochondrial metabolism have also been shown to be subjected to UPS-dependent regulation.
Mitochondrial fission and fusion: A dynamic role in aging and potential target for age-related disease
2020, Mechanisms of Ageing and DevelopmentCitation Excerpt :OPA1 is regulated post-transcriptionally and post-translationally (Wai and Langer, 2016). Specifically, proteolytic processing plays a large role in regulation of mitochondrial dynamics, as reviewed by (Dietz et al., 2019). A deficiency or loss of fusion proteins leads to mitochondrial fragmentation (Ichishita et al., 2008; Kanazawa et al., 2008).
Mitochondrial Dynamics in Signaling and Disease
2019, Mitochondrion
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These authors contributed equally