OPA1 processing in cell death and disease - the long and short of it

PHB2 (prohibitin 2) promotes PINK1-PRKN/Parkin-dependent mitophagy by the PARL-PGAM5-PINK1 axis

Mitophagy, which is a conserved cellular process for selectively removing damaged or unwanted mitochondria, is critical for mitochondrial quality control and the maintenance of normal cellular physiology. However, the precise mechanisms underlying mitophagy remain largely unknown. Prior studies on mitophagy focused on the events in the mitochondrial outer membrane. PHB2 (prohibitin 2), which is a highly conserved membrane scaffold protein, was recently identified as a novel inner membrane mitophagy receptor that mediates mitophagy. Here, we report a new signaling pathway for PHB2-mediated mitophagy. Upon mitochondrial membrane depolarization or misfolded protein aggregation, PHB2 depletion destabilizes PINK1 in the mitochondria, which blocks the mitochondrial recruitment of PRKN/Parkin, ubiquitin and OPTN (optineurin), leading to an inhibition of mitophagy. In addition, PHB2 overexpression directly induces PRKN recruitment to the mitochondria. Moreover, PHB2-mediated mitophagy is dependent on the mitochondrial inner membrane protease PARL, which interacts with PHB2 and is activated upon PHB2 depletion. Furthermore, PGAM5, which is processed by PARL, participates in PHB2-mediated PINK1 stabilization. Finally, a ligand of PHB proteins that we synthesized, called FL3, was found to strongly inhibit PHB2-mediated mitophagy and to effectively block cancer cell growth and energy production at nanomolar concentrations. Thus, our findings reveal that the PHB2-PARL-PGAM5-PINK1 axis is a novel pathway of PHB2-mediated mitophagy and that targeting PHB2 with the chemical compound FL3 is a promising strategy for cancer therapy.Abbreviations: AIFM1: apoptosis inducing factor mitochondria associated 1; ATP5F1A/ATP5A1: ATP synthase F1 subunit alpha; BAF: bafilomycin A₁; CALCOCO2/NDP52: calcium binding and coiled-coil domain 2; CCCP: chemical reagent carbonyl cyanide m-chlorophenyl hydrazine; FL3: flavaglines compound 3; HSPD1/HSP60: heat shock protein family D (Hsp60) member 1; LC3B/MAP1LC3B: microtubule associated protein 1 light chain 3 beta; MEF: mouse embryo fibroblasts; MPP: mitochondrial-processing peptidase; MT-CO2/COX2: mitochondrially encoded cytochrome c oxidase II; MTS: mitochondrial targeting sequence; OA: oligomycin and antimycin A; OPTN: optineurin; OTC: ornithine carbamoyltransferase; PARL: presenilin associated rhomboid like; PBS: phosphate-buffered saline; PGAM5: PGAM family member 5, mitochondrial serine/threonine protein phosphatase; PHB: prohibitin; PHB2: prohibitin 2; PINK1: PTEN induced kinase 1; PRKN/Parkin: parkin RBR E3 ubiquitin protein ligase; Roc-A: rocaglamide A; TOMM20: translocase of outer mitochondrial membrane 20; TUBB: tubulin beta class I.

Linking mitochondrial dynamics, cristae remodeling and supercomplex formation: How mitochondrial structure can regulate bioenergetics

The dynamic and fluid nature of mitochondria allows for modifications in mitochondrial shape, connectivity and cristae architecture. The precise balance of mitochondrial dynamics is among the most critical features in the control of mitochondrial function. In the past few years, mitochondrial shape has emerged as a key regulatory factor in the determination of the bioenergetic capacity of cells. This is mostly due to the recent discoveries linking changes in cristae organization with supercomplex assembly of the electron transport chain (ETC), also defined as the formation of respirosomes. Here we will review the most current advances demonstrating the impact of mitochondrial dynamics and cristae shape on oxidative metabolism, respiratory efficiency, and redox state. Furthermore, we will discuss the implications of mitochondrial dynamics and supercomplex assembly under physiological and pathological conditions.

Lipidomics Reveals Triacylglycerol Accumulation Due to Impaired Fatty Acid Flux in Opa1-Disrupted Fibroblasts

OPA1 is a dynamin GTPase implicated in mitochondrial membrane fusion. Despite its involvement in lipid remodeling, the function of OPA1 has never been analyzed by whole-cell lipidomics. We used a nontargeted, reversed-phase lipidomics approach, validated for cell cultures, to investigate OPA1-inactivated mouse embryonic fibroblasts ( Opa1 ^-/- MEFs). This led to the identification of a wide range of 14 different lipid subclasses comprising 212 accurately detected lipids. Multivariate and univariate statistical analyses were then carried out to assess the differences between the Opa1 ^-/- and Opa1 ^+/+ genotypes. Of the 212 lipids identified, 69 were found to discriminate between Opa1 ^-/- MEFs and Opa1 ^+/+ MEFs. Among these lipids, 34 were triglycerides, all of which were at higher levels in Opa1 ^-/- MEFs with fold changes ranging from 3.60 to 17.93. Cell imaging with labeled fatty acids revealed a sharp alteration of the fatty acid flux with a reduced mitochondrial uptake. The other 35 discriminating lipids included phosphatidylcholines, lysophosphatidylcholines, phosphatidylethanolamine, and sphingomyelins, mainly involved in membrane remodeling, and ceramides, gangliosides, and phosphatidylinositols, mainly involved in apoptotic cell signaling. Our results show that the inactivation of OPA1 severely affects the mitochondrial uptake of fatty acids and lipids through membrane remodeling and apoptotic cell signaling.

Protective effect of hyperoside against renal ischemia-reperfusion injury via modulating mitochondrial fission, oxidative stress, and apoptosis

Ischemia/reperfusion (IR) is a common cause of acute kidney injury (AKI). However, effective therapies for IR-induced AKI are lacking. Hyperoside is an active constituent in the flowers of Abelmoschus manihot (L.) Medic, which is a traditional Chinese herbal medicine for the treatment of various ischemic brain and heart diseases. Our previous study demonstrated that hyperoside inhibited adriamycin induced podocyte injury both in vivo and in vitro. The aim of this study is to investigate the effect of hyperoside in IR-induced AKI. In mice, pretreatment of hyperoside could markedly attenuate IR-induced AKI, tubular cell apoptosis, and oxidative stress in the kidneys. Meanwhile, we found hyperoside inhibited IR-induced mitochondrial fission by suppressing OMA1 mediated proteolysis of optic atrophy 1 (OPA1). Consistently, in human proximal tubular epithelial cells, hyperoside might inhibit CoCl₂-induced mitochondrial fission, oxidative stress, and apoptosis by regulating OMA1-OPA1 axis. Taken together, our results support the idea that OMA1-OPA1 mediated mitochondrial fission can be used for the prevention of AKI. Hyperoside might have novel therapeutic potential in the treatment of AKI.

Pharmacological Inhibition of Necroptosis Protects from Dopaminergic Neuronal Cell Death in Parkinson's Disease Models

Dysfunctions in mitochondrial dynamics and metabolism are common pathological processes associated with Parkinson’s disease (PD). It was recently shown that an inherited form of PD and dementia is caused by mutations in the OPA1 gene, which encodes for a key player in mitochondrial fusion and structure. iPSC-derived neural cells from these patients exhibited severe mitochondrial fragmentation, respiration impairment, ATP deficits, and heightened oxidative stress. Reconstitution of normal levels of OPA1 in PD-derived neural cells normalized mitochondria morphology and function. OPA1-mutated neuronal cultures showed reduced survival in vitro. Intriguingly, selective inhibition of necroptosis effectively rescued this survival deficit. Additionally, dampening necroptosis in MPTP-treated mice protected from DA neuronal cell loss. This human iPSC-based model captures both early pathological events in OPA1 mutant neural cells and the beneficial effects of blocking necroptosis, highlighting this cell death process as a potential therapeutic target for PD.

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OPA1 mutant iPSC-derived NPCs contain dysfunctional mitochondria

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OPA1 mutant iPSC-derived NPCs present high levels of oxidative stress

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Nec-1s can improve survival of OPA1 mutant human neurons in vitro

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Nec-1s counteracts the dopaminergic cell loss in MPTP-treated neurons

Bif-1 Interacts with Prohibitin-2 to Regulate Mitochondrial Inner Membrane during Cell Stress and Apoptosis

BACKGROUND

Mitochondria are dynamic organelles that undergo fission and fusion. During cell stress, mitochondrial dynamics shift to fission, leading to mitochondrial fragmentation, membrane leakage, and apoptosis. Mitochondrial fragmentation requires the cleavage of both outer and inner membranes, but the mechanism of inner membrane cleavage is unclear. Bif-1 and prohibitin-2 may regulate mitochondrial dynamics.

METHODS

We used azide-induced ATP depletion to incite cell stress in mouse embryonic fibroblasts and renal proximal tubular cells, and renal ischemia-reperfusion to induce stress in mice. We also used knockout cells and mice to determine the role of Bif-1, and used multiple techniques to analyze the molecular interaction between Bif-1 and prohibitin-2.

RESULTS

Upon cell stress, Bif-1 translocated to mitochondria to bind prohibitin-2, resulting in the disruption of prohibitin complex and proteolytic inactivation of the inner membrane fusion protein OPA1. Bif-1-deficiency inhibited prohibitin complex disruption, OPA1 proteolysis, mitochondrial fragmentation, and apoptosis. Domain deletion analysis indicated that Bif-1 interacted with prohibitin-2 via its C-terminus. Notably, mutation of Bif-1 at its C-terminal tryptophan-344 not only prevented Bif-1/prohibitin-2 interaction but also reduced prohibitin complex disruption, OPA1 proteolysis, mitochondrial fragmentation, and apoptosis, supporting a pathogenic role of Bif-1/prohibitin-2 interaction. In mice, Bif-1 bound prohibitin-2 during renal ischemia/reperfusion injury, and Bif-1-deficiency protected against OPA1 proteolysis, mitochondrial fragmentation, apoptosis and kidney injury.

CONCLUSIONS

These findings suggest that during cell stress, Bif-1 regulates mitochondrial inner membrane by interacting with prohibitin-2 to disrupt prohibitin complexes and induce OPA1 proteolysis and inactivation.

Drosophila ADCK1 is critical for maintaining mitochondrial structures and functions in the muscle

The function of AarF domain-containing kinase 1 (ADCK1) has not been thoroughly revealed. Here we identified that ADCK1 utilizes YME1-like 1 ATPase (YME1L1) to control optic atrophy 1 (OPA1) and inner membrane mitochondrial protein (IMMT) in regulating mitochondrial dynamics and cristae structure. We firstly observed that a serious developmental impairment occurred in Drosophila ADCK1 (dADCK1) deletion mutant, resulting in premature death before adulthood. By using temperature sensitive ubiquitously expression driver tub-Gal80^ts/tub-Gal4 or muscle-specific expression driver mhc-Gal4, we observed severely defective locomotive activities and structural abnormality in the muscle along with increased mitochondrial fusion in the dADCK1 knockdown flies. Moreover, decreased mitochondrial membrane potential, ATP production and survival rate along with increased ROS and apoptosis in the flies further demonstrated that the structural abnormalities of mitochondria induced by dADCK1 knockdown led to their functional abnormalities. Consistent with the ADCK1 loss-of-function data in Drosophila, ADCK1 over-expression induced mitochondrial fission and clustering in addition to destruction of the cristae structure in Drosophila and mammalian cells. Interestingly, knockdown of YME1L1 rescued the phenotypes of ADCK1 over-expression. Furthermore, genetic epistasis from fly genetics and mammalian cell biology experiments led us to discover the interactions among IMMT, OPA1 and ADCK1. Collectively, these results established a mitochondrial signaling pathway composed of ADCK1, YME1L1, OPA1 and IMMT, which has essential roles in maintaining mitochondrial morphologies and functions in the muscle.

Mitochondria function as energy producing factories in the cell, and thus the malfunctioning of mitochondria becomes the causes of many diseases. Especially in muscles that continuously require a vast amount of energy, dysfunction of mitochondria leads to abnormalities in muscles. Mitochondria maintain their homeostasis and recover from stresses induced by external stimuli through a dynamic process of continuous fusion and fission. Moreover, they constantly produce ATP through their wrinkled internal structure, called the cristae. We discovered that ADCK1 is important in maintaining these mitochondrial functions. In the fruit fly model, a severe developmental anomaly was observed in ADCK1 mutant, and inhibition of ADCK1 expression led to defects in locomotive activity, along with abnormalities in mitochondrial structure and functions in muscles. Interestingly, these anomalies in mitochondria were due to OPA1 and IMMT proteins that exist downstream of ADCK1, regulated by ADCK1 through a protease called YME1L1. These results provide better molecular understanding on how mitochondria contribute to degenerative diseases in the muscular system.

Exploration of age-related mitochondrial dysfunction and the anti-aging effects of resveratrol in zebrafish retina

It is currently believed that aging is closely linked with mitochondrial dysfunction, and that resveratrol exhibits anti-aging and neuroprotective effects by improving mitochondrial function, even though the mechanisms are not well defined. This study explored mitochondrial quality (mitochondrial DNA integrity and copy number), mitochondrial function (fusion/fission, mitophagy/autophagy), antioxidant system and activity of the Akt/mTOR and Ampk/Sirt1/Pgc1α pathways, and inflammation in aging zebrafish retinas to identify the probable mechanisms of resveratrol's anti-aging and neuroprotective effects. mtDNA integrity, mtDNA copy number, mitochondrial fusion regulators, mitophagy, and antioxidant-related genes were all decreased whereas Akt/mTOR activity and inflammation was increased upon aging in zebrafish retinas. Resveratrol was shown to not only increase mitochondrial quality and function, but also to suppress Akt/mTOR activity in zebrafish retinas. These results support the notion that mitochondrial dysfunction and increased Akt/mTOR activity are major players in age-related retinal neuropathy in zebrafish, and demonstrate a trend towards mitochondrial fragmentation in the aging retina. Importantly, resveratrol promoted mitochondrial function, up-regulating Ampk/Sirt1/Pgc1α, and down-regulated Akt/mTOR pathway activity in zebrafish retinas, suggesting that it may be able to prevent age-related oculopathy.

ER-Targeted Beclin 1 Supports Autophagosome Biogenesis in the Absence of ULK1 and ULK2 Kinases

Autophagy transports cytoplasmic material and organelles to lysosomes for degradation and recycling. Beclin 1 forms a complex with several other autophagy proteins and functions in the initiation phase of autophagy, but the exact role of Beclin 1 subcellular localization in autophagy initiation is still unclear. In order to elucidate the role of Beclin 1 localization in autophagosome biogenesis, we generated constructs that target Beclin 1 to the endoplasmic reticulum (ER) or mitochondria. Our results confirmed the proper organelle-specific targeting of the engineered Beclin 1 constructs, and the proper formation of autophagy-regulatory Beclin 1 complexes. The ULK kinases are required for autophagy initiation upstream of Beclin 1, and autophagosome biogenesis is severely impaired in ULK1/ULK2 double knockout cells. We tested whether Beclin 1 targeting facilitated its ability to rescue autophagosome formation in ULK1/ULK2 double knockout cells. ER-targeted Beclin 1 was most effective in the rescue experiments, while mitochondria-targeted and non-targeted Beclin 1 also showed an ability to rescue, but with lower activity. However, none of the constructs was able to increase autophagic flux in the knockout cells. We also showed that wild type Beclin 1 was enriched on the ER during autophagy induction, and that ULK1/ULK2 facilitated the ER-enrichment of Beclin 1 under basal conditions. The results suggest that one of the functions of ULK kinases may be to enhance Beclin 1 recruitment to the ER to drive autophagosome formation.

Mitofusins: Disease Gatekeepers and Hubs in Mitochondrial Quality Control by E3 Ligases

Mitochondria are dynamic organelles engaged in quality control and aging processes. They constantly undergo fusion, fission, transport, and anchoring events, which empower mitochondria with a very interactive behavior. The membrane remodeling processes needed for fusion require conserved proteins named mitofusins, MFN1 and MFN2 in mammals and Fzo1 in yeast. They are the first determinants deciding on whether communication and content exchange between different mitochondrial populations should occur. Importantly, each cell possesses hundreds of mitochondria, with a different severity of mitochondrial mutations or dysfunctional proteins, which potentially spread damage to the entire network. Therefore, the degree of their merging capacity critically influences cellular fitness. In turn, the mitochondrial network rapidly and dramatically changes in response to metabolic and environmental cues. Notably, cancer or obesity conditions, and stress experienced by neurons and cardiomyocytes, for example, triggers the downregulation of mitofusins and thus fragmentation of mitochondria. This places mitofusins upfront in sensing and transmitting stress. In fact, mitofusins are almost entirely exposed to the cytoplasm, a topology suitable for a critical relay point in information exchange between mitochondria and their cellular environment. Consistent with their topology, mitofusins are either activated or repressed by cytosolic post-translational modifiers, mainly by ubiquitin. Ubiquitin is a ubiquitous small protein orchestrating multiple quality control pathways, which is covalently attached to lysine residues in its substrates, or in ubiquitin itself. Importantly, from a chain of events also mediated by E1 and E2 enzymes, E3 ligases perform the ultimate and determinant step in substrate choice. Here, we review the ubiquitin E3 ligases that modify mitofusins. Two mitochondrial E3 enzymes—March5 and MUL1—one ligase located to the ER—Gp78—and finally three cytosolic enzymes—MGRN1, HUWE1, and Parkin—were shown to ubiquitylate mitofusins, in response to a variety of cellular inputs. The respective outcomes on mitochondrial morphology, on contact sites to the endoplasmic reticulum and on destructive processes, like mitophagy or apoptosis, are presented. Ultimately, understanding the mechanisms by which E3 ligases and mitofusins sense and bi-directionally signal mitochondria-cytosolic dysfunctions could pave the way for therapeutic approaches in neurodegenerative, cardiovascular, and obesity-linked diseases.

Diminished OPA1 expression and impaired mitochondrial morphology and homeostasis in Aprataxin-deficient cells

Ataxia with oculomotor apraxia type 1 (AOA1) is an early onset progressive spinocerebellar ataxia caused by mutation in aprataxin (APTX). APTX removes 5'-AMP groups from DNA, a product of abortive ligation during DNA repair and replication. APTX deficiency has been suggested to compromise mitochondrial function; however, a detailed characterization of mitochondrial homeostasis in APTX-deficient cells is not available. Here, we show that cells lacking APTX undergo mitochondrial stress and display significant changes in the expression of the mitochondrial inner membrane fusion protein optic atrophy type 1, and components of the oxidative phosphorylation complexes. At the cellular level, APTX deficiency impairs mitochondrial morphology and network formation, and autophagic removal of damaged mitochondria by mitophagy. Thus, our results show that aberrant mitochondrial function is a key component of AOA1 pathology. This work corroborates the emerging evidence that impaired mitochondrial function is a characteristic of an increasing number of genetically diverse neurodegenerative disorders.

Proteolytic regulation of mitochondrial dynamics

Spatiotemporal changes in the abundance, shape, and cellular localization of the mitochondrial network, also known as mitochondrial dynamics, are now widely recognized to play a key role in mitochondrial and cellular physiology as well as disease states. This process involves coordinated remodeling of the outer and inner mitochondrial membranes by conserved dynamin-like guanosine triphosphatases and their partner molecules in response to various physiological and stress stimuli. Although the core machineries that mediate fusion and partitioning of the mitochondrial network have been extensively characterized, many aspects of their function and regulation are incompletely understood and only beginning to emerge. In the present review we briefly summarize current knowledge about how the key mitochondrial dynamics-mediating factors are regulated via selective proteolysis by mitochondrial and cellular proteolytic machineries.

PISD is a mitochondrial disease gene causing skeletal dysplasia, cataracts, and white matter changes

This work demonstrates that pathogenic variants in PISD cause mitochondrial disease and suggests a novel mechanistic link whereby impaired lipid content in the inner mitochondrial membrane alters the activity of inner mitochondrial membrane proteases.

Exome sequencing of two sisters with congenital cataracts, short stature, and white matter changes identified compound heterozygous variants in the PISD gene, encoding the phosphatidylserine decarboxylase enzyme that converts phosphatidylserine to phosphatidylethanolamine (PE) in the inner mitochondrial membrane (IMM). Decreased conversion of phosphatidylserine to PE in patient fibroblasts is consistent with impaired phosphatidylserine decarboxylase (PISD) enzyme activity. Meanwhile, as evidence for mitochondrial dysfunction, patient fibroblasts exhibited more fragmented mitochondrial networks, enlarged lysosomes, decreased maximal oxygen consumption rates, and increased sensitivity to 2-deoxyglucose. Moreover, treatment with lyso-PE, which can replenish the mitochondrial pool of PE, and genetic complementation restored mitochondrial and lysosome morphology in patient fibroblasts. Functional characterization of the PISD variants demonstrates that the maternal variant causes an alternative splice product. Meanwhile, the paternal variant impairs autocatalytic self-processing of the PISD protein required for its activity. Finally, evidence for impaired activity of mitochondrial IMM proteases suggests an explanation as to why the phenotypes of these PISD patients resemble recently described “mitochondrial chaperonopathies.” Collectively, these findings demonstrate that PISD is a novel mitochondrial disease gene.

Mitochondrial presequence import: Multiple regulatory knobs fine-tune mitochondrial biogenesis and homeostasis

Mitochondria are pivotal organelles for cellular signaling and metabolism, and their dysfunction leads to severe cellular stress. About 60-70% of the mitochondrial proteome consists of preproteins synthesized in the cytosol with an amino-terminal cleavable presequence targeting signal. The TIM23 complex transports presequence signals towards the mitochondrial matrix. Ultimately, the mature protein segments are either transported into the matrix or sorted to the inner membrane. To ensure accurate preprotein import into distinct mitochondrial sub-compartments, the TIM23 machinery adopts specific functional conformations and interacts with different partner complexes. Regulatory subunits modulate the translocase dynamics, tailoring the import reaction to the incoming preprotein. The mitochondrial membrane potential and the ATP generated via oxidative phosphorylation are key energy sources in driving the presequence import pathway. Thus, mitochondrial dysfunctions have rapid repercussions on biogenesis. Cellular mechanisms exploit the presequence import pathway to monitor mitochondrial dysfunctions and mount transcriptional and proteostatic responses to restore functionality.

Targeted OMA1 therapies for cancer

The mitochondrial inner membrane proteins OMA1 and OPA1 belong to the BAX/BAK1-dependent apoptotic signaling pathway, which can be regulated by tumor protein p53 and the prohibitins PHB and PHB2 in the context of neoplastic disease. For the most part these proteins have been studied separate from each other. Here, I argue that the OMA1 mechanism of action represents the missing link between p53 and cytochrome c release. The mitochondrial fusion protein OPA1 is cleaved by OMA1 in a stress-dependent manner generating S-OPA1. Excessive S-OPA1 can facilitate outer membrane permeabilization upon BAX/BAK1 activation through its membrane shaping properties. p53 helps outer membrane permeabilization in a 2-step process. First, cytosolic p53 activates BAX/BAK1 at the mitochondrial surface. Then, in a second step, p53 binds to prohibitin thereby releasing the restraint on OMA1. This activates OMA1, which cleaves OPA1 and promotes cytochrome c release. Clearly, OMA1 and OPA1 are not root causes for cancer. Yet many cancer cells rely on this pathway for survival, which can explain why loss of p53 function promotes tumor growth and confers resistance to chemotherapies.

Toxicity of blue led light and A2E is associated to mitochondrial dynamics impairment in ARPE-19 cells: implications for age-related macular degeneration

Age-related macular degeneration (AMD) is a multifactorial retinal disease characterized by a progressive loss of central vision. Retinal pigment epithelium (RPE) degeneration is a critical event in AMD. It has been associated to A2E accumulation, which sensitizes RPE to blue light photodamage. Mitochondrial quality control mechanisms have evolved to ensure mitochondrial integrity and preserve cellular homeostasis. Particularly, mitochondrial dynamics involve the regulation of mitochondrial fission and fusion to preserve a healthy mitochondrial network. The present study aims to clarify the cellular and molecular mechanisms underlying photodamage-induced RPE cell death with particular focus on the involvement of defective mitochondrial dynamics. Light-emitting diodes irradiation (445 ± 18 nm; 4.43 mW/cm²) significantly reduced the viability of both unloaded and A2E-loaded human ARPE-19 cells and increased reactive oxygen species production. A2E along with blue light, triggered apoptosis measured by MC540/PI-flow cytometry and activated caspase-3. Blue light induced mitochondrial fusion/fission imbalance towards mitochondrial fragmentation in both non-loaded and A2E-loaded cells which correlated with the deregulation of mitochondria-shaping proteins level (OPA1, DRP1 and OMA1). To our knowledge, this is the first work reporting that photodamage causes mitochondrial dynamics deregulation in RPE cells. This process could possibly contribute to AMD pathology. Our findings suggest that the regulation of mitochondrial dynamics may be a valuable strategy for treating retinal degeneration diseases, such as AMD.

Alpha-lipoic acid suppresses Nε-(carboxymethyl) lysine-induced epithelial mesenchymal transition in HK-2 human renal proximal tubule cells

N^ε-(carboxymethyl) lysine (CML) plays causal roles in diabetic complications. In the present study, we investigated whether CML-induced HIF-1α accumulation and epithelial-mesenchymal transition (EMT) in HK-2 renal proximal tubular epithelial cells. Treatment with CML-BSA increased reactive oxygen species (ROS) production reduced the mitochondrial membrane potential and induced mitochondrial fragmentation. Pre-treatment of cells with antioxidant, α-lipoic acid, normalised the ROS production and restored the mitochondrial membrane potential. These changes were accompanied with morphological changes of epithelial mesenchymal transition. CML-BSA increased the protein level of hypoxia-inducible factor-1α (HIF-1α), and the EMT-associated transcription factor, TWIST. These effects were reversed by α-lipoic acid. CML-BSA increased the protein levels of mesenchymal-specific markers, including vimentin, α-smooth muscle actin, which were alleviated by pre-treatment with α-lipoic acid. Our data suggest that CML-BSA induces EMT through a ROS/HIF-1α/TWIST-dependent mechanism, and that α-lipoic acid may alleviate the CML-induced EMT in renal tubular cells.

Mitochondrial stress response triggered by defects in protein synthesis quality control

Quality control defects of mitochondrial nascent chain synthesis trigger a sequential stress response characterized by OMA1 activation and ribosome decay, determining mitochondrial form and function.

Mitochondria have a compartmentalized gene expression system dedicated to the synthesis of membrane proteins essential for oxidative phosphorylation. Responsive quality control mechanisms are needed to ensure that aberrant protein synthesis does not disrupt mitochondrial function. Pathogenic mutations that impede the function of the mitochondrial matrix quality control protease complex composed of AFG3L2 and paraplegin cause a multifaceted clinical syndrome. At the cell and molecular level, defects to this quality control complex are defined by impairment to mitochondrial form and function. Here, we establish the etiology of these phenotypes. We show how disruptions to the quality control of mitochondrial protein synthesis trigger a sequential stress response characterized first by OMA1 activation followed by loss of mitochondrial ribosomes and by remodelling of mitochondrial inner membrane ultrastructure. Inhibiting mitochondrial protein synthesis with chloramphenicol completely blocks this stress response. Together, our data establish a mechanism linking major cell biological phenotypes of AFG3L2 pathogenesis and show how modulation of mitochondrial protein synthesis can exert a beneficial effect on organelle homeostasis.

Calcineurin/P38MAPK/HSP27-dependent pathways are involved in the attenuation of postischemic mitochondrial injury afforded by sodium bicarbonate co-transporter (NBCe1) inhibition

The electrogenic sodium bicarbonate co-transporter isoform 1 (NBCe1) plays an important role in ischemia-reperfusion injury. The cardioprotective action of an antibody directed to the extracellular loop 3 (a-L3) of NBCe1 was previously demonstrated by us. However, the role of a-L3 on mitochondrial post-ischemic alterations has not yet been determined. In this study, we aimed to elucidate the effects of a-L3 on post-ischemic mitochondrial state and dynamics analysing the involved mechanisms. Isolated rat hearts were assigned to the following groups: 1) Non-ischemic control (NIC): 110 min of perfusion; 2) Ischemic control (IC): 30 min of global ischemia and 60 min of reperfusion (R); 3) a-L3: a-L3 was administered during the initial 10 min of R; 4) SB + a-L3: SB202190 (p38MAPK inhibitor) plus a-L3. Infarct size (IS) was measured by TTC staining. Developed pressure (LVDP), maximal velocities of rise and decay of LVP (+dP/dt max, -dP/dt max) and end-diastolic pressure (LVEDP) of the left ventricle were used to assess systolic and diastolic function. Mitochondrial Ca²⁺ response (CaR), Ca²⁺ retention capacity (CRC), membrane potential (ΔΨm) and MnSOD levels were measured. The expression of P-p38MAPK, calcineurin, P-HSP27, P-Drp1, Drp1, and OPA1 were determined. a-L3 decreased IS, improved post-ischemic recovery of myocardial function, increased P-p38MAPK, P-HSP27, P-Drp1, cytosolic Drp1, and OPA1 expression and decreased calcineurin. These effects were abolished by p38MAPK inhibition with SB. These data show that NBCe1 inhibition by a-L3 limits the cell death, improves myocardial post-ischemic contractility and mitochondrial state and dynamic through calcium decrease/calcineurin inhibition-mediated p38MAPK activation and p38MAPK/HSP27-dependent pathways. Thus, we demonstrated that a-L3 is a potential therapeutic strategy in post-ischemic alterations.

Dysregulated Mitochondrial Dynamics and Metabolism in Obesity, Diabetes, and Cancer

Metabolism describes the life-sustaining chemical reactions in organisms that provide both energy and building blocks for cellular survival and proliferation. Dysregulated metabolism leads to many life-threatening diseases including obesity, diabetes, and cancer. Mitochondria, subcellular organelles, contain the central energy-producing metabolic pathway, the tricarboxylic acid (TCA) cycle. Also, mitochondria exist in a dynamic network orchestrated by extracellular nutrient levels and intracellular energy needs. Upon stimulation, mitochondria undergo consistent interchange through fusion (small to big) and fission (big to small) processes. Mitochondrial fusion is primarily controlled by three GTPases, mitofusin 1 (Mfn1), Mfn2, and optic atrophy 1 (Opa1), while mitochondrial fission is primarily regulated by GTPase dynamin-related protein 1 (Drp1). Dysregulated activity of these GTPases results in disrupted mitochondrial dynamics and cellular metabolism. This review will update the metabolic roles of these GTPases in obesity, diabetes, and cancer.

ROMO1 is a constituent of the human presequence translocase required for YME1L protease import

Mitochondria are the powerhouses of eukaryotic cells and rely on protein import from the cytosol. Richter et al. found ROMO1 as a new constituent of the human mitochondrial import machinery linking protein import to quality control and mitochondrial morphology.

The mitochondrial presequence translocation machinery (TIM23 complex) is conserved between the yeast Saccharomyces cerevisiae and humans; however, functional characterization has been mainly performed in yeast. Here, we define the constituents of the human TIM23 complex using mass spectrometry and identified ROMO1 as a new translocase constituent with an exceptionally short half-life. Analyses of a ROMO1 knockout cell line revealed aberrant inner membrane structure and altered processing of the GTPase OPA1. We show that in the absence of ROMO1, mitochondria lose the inner membrane YME1L protease, which participates in OPA1 processing and ROMO1 turnover. While ROMO1 is dispensable for general protein import along the presequence pathway, we show that it participates in the dynamics of TIM21 during respiratory chain biogenesis and is specifically required for import of YME1L. This selective import defect can be linked to charge distribution in the unusually long targeting sequence of YME1L. Our analyses establish an unexpected link between mitochondrial protein import and inner membrane protein quality control.

Does Oxidation of Mitochondrial Cardiolipin Trigger a Chain of Antiapoptotic Reactions?

Oxidative stress causes selective oxidation of cardiolipin (CL), a four-tail lipid specific for the inner mitochondrial membrane. Interaction with oxidized CL transforms cytochrome c into peroxidase capable of oxidizing even more CL molecules. Ultimately, this chain of events leads to the pore formation in the outer mitochondrial membrane and release of mitochondrial proteins, including cytochrome c, into the cytoplasm. In the cytoplasm, cytochrome c promotes apoptosome assembly that triggers apoptosis (programmed cell death). Because of this amplification cascade, even an occasional oxidation of a single CL molecule by endogenously formed reactive oxygen species (ROS) might cause cell death, unless the same CL oxidation triggers a separate chain of antiapoptotic reactions that would prevent the CL-mediated apoptotic cascade. Here, we argue that the key function of CL in mitochondria and other coupling membranes is to prevent proton leak along the interface of interacting membrane proteins. Therefore, CL oxidation should increase proton permeability through the CL-rich clusters of membrane proteins (CL islands) and cause a drop in the mitochondrial membrane potential (MMP). On one hand, the MMP drop should hinder ROS generation and further CL oxidation in the entire mitochondrion. On the other hand, it is known to cause rapid fission of the mitochondrial network and formation of many small mitochondria, only some of which would contain oxidized CL islands. The fission of mitochondrial network would hinder apoptosome formation by preventing cytochrome c release from healthy mitochondria, so that slowly working protein quality control mechanisms would have enough time to eliminate mitochondria with the oxidized CL. Because of these two oppositely directed regulatory pathways, both triggered by CL oxidation, the fate of the cell appears to be determined by the balance between the CL-mediated proapoptotic and antiapoptotic reactions. Since this balance depends on the extent of CL oxidation, mitochondria-targeted antioxidants might be able to ensure cell survival in many pathologies by preventing CL oxidation.

Loss of Mitochondrial AAA Proteases AFG3L2 and YME1L Impairs Mitochondrial Structure and Respiratory Chain Biogenesis

Mitochondrial protein quality control is crucial for the maintenance of correct mitochondrial homeostasis. It is ensured by several specific mitochondrial proteases located across the various mitochondrial subcompartments. Here, we focused on characterization of functional overlap and cooperativity of proteolytic subunits AFG3L2 (AFG3 Like Matrix AAA Peptidase Subunit 2) and YME1L (YME1 like ATPase) of mitochondrial inner membrane AAA (ATPases Associated with diverse cellular Activities) complexes in the maintenance of mitochondrial structure and respiratory chain integrity. We demonstrate that loss of AFG3L2 and YME1L, both alone and in combination, results in diminished cell proliferation, fragmentation of mitochondrial reticulum, altered cristae morphogenesis, and defective respiratory chain biogenesis. The double AFG3L2/YME1L knockdown cells showed marked upregulation of OPA1 protein forms, with the most prominent increase in short OPA1 (optic atrophy 1). Loss of either protease led to marked elevation in OMA1 (OMA1 zinc metallopeptidase) (60 kDa) and severe reduction in the SPG7 (paraplegin) subunit of the m-AAA complex. Loss of the YME1L subunit led to an increased Drp1 level in mitochondrial fractions. While loss of YME1L impaired biogenesis and function of complex I, knockdown of AFG3L2 mainly affected the assembly and function of complex IV. Our results suggest cooperative and partly redundant functions of AFG3L2 and YME1L in the maintenance of mitochondrial structure and respiratory chain biogenesis and stress the importance of correct proteostasis for mitochondrial integrity.

Mice harbouring a SCA28 patient mutation in AFG3L2 develop late-onset ataxia associated with enhanced mitochondrial proteotoxicity

Spinocerebellar ataxia 28 is an autosomal dominant neurodegenerative disorder caused by missense mutations affecting the proteolytic domain of AFG3L2, a major component of the mitochondrial m-AAA protease. However, little is known of the underlying pathogenetic mechanisms or how to treat patients with SCA28. Currently available Afg3l2 mutant mice harbour deletions that lead to severe, early-onset neurological phenotypes that do not faithfully reproduce the late-onset and slowly progressing SCA28 phenotype. Here we describe production and detailed analysis of a new knock-in murine model harbouring an Afg3l2 allele carrying the p.Met665Arg patient-derived mutation. Heterozygous mutant mice developed normally but adult mice showed signs of cerebellar ataxia detectable by beam test. Although cerebellar pathology was negative, electrophysiological analysis showed a trend towards increased spontaneous firing in Purkinje cells from heterozygous mutants with respect to wild-type controls. As homozygous mutants died perinatally with evidence of cardiac atrophy, for each genotype we generated mouse embryonic fibroblasts (MEFs) to investigate mitochondrial function. MEFs from mutant mice showed altered mitochondrial bioenergetics, with decreased basal oxygen consumption rate, ATP synthesis and mitochondrial membrane potential. Mitochondrial network formation and morphology was altered, with greatly reduced expression of fusogenic Opa1 isoforms. Mitochondrial alterations were also detected in cerebella of 18-month-old heterozygous mutants and may be a hallmark of disease. Pharmacological inhibition of de novo mitochondrial protein translation with chloramphenicol caused reversal of mitochondrial morphology in homozygous mutant MEFs, supporting the relevance of mitochondrial proteotoxicity for SCA28 pathogenesis and therapy development.

Concurrent AFG3L2 and SPG7 mutations associated with syndromic parkinsonism and optic atrophy with aberrant OPA1 processing and mitochondrial network fragmentation

Mitochondrial dynamics and quality control are crucial for neuronal survival and their perturbation is a major cause of neurodegeneration. m-AAA complex is an ATP-dependent metalloprotease located in the inner mitochondrial membrane and involved in protein quality control. Mutations in the m-AAA subunits AFG3L2 and paraplegin are associated with autosomal dominant spinocerebellar ataxia (SCA28) and autosomal recessive hereditary spastic paraplegia (SPG7), respectively. We report a novel m-AAA-associated phenotype characterized by early-onset optic atrophy with spastic ataxia and L-dopa-responsive parkinsonism. The proband carried a de novo AFG3L2 heterozygous mutation (p.R468C) along with a heterozygous maternally inherited intragenic deletion of SPG7. Functional analysis in yeast demonstrated the pathogenic role of AFG3L2 p.R468C mutation shedding light on its pathogenic mechanism. Analysis of patient's fibroblasts showed an abnormal processing pattern of OPA1, a dynamin-related protein essential for mitochondrial fusion and responsible for most cases of hereditary optic atrophy. Consistently, assessment of mitochondrial morphology revealed a severe fragmentation of the mitochondrial network, not observed in SCA28 and SPG7 patients' cells. This case suggests that coincidental mutations in both components of the mitochondrial m-AAA protease may result in a complex phenotype and reveals a crucial role for OPA1 processing in the pathogenesis of neurodegenerative disease caused by m-AAA defects.

Mitochondrial Stasis Reveals p62-Mediated Ubiquitination in Parkin-Independent Mitophagy and Mitigates Nonalcoholic Fatty Liver Disease

It is unknown what occurs if both mitochondrial division and fusion are completely blocked. Here, we introduced mitochondrial stasis by deleting two dynamin-related GTPases for division (Drp1) and fusion (Opa1) in livers. Mitochondrial stasis rescues liver damage and hypotrophy caused by the single knockout (KO). At the cellular level, mitochondrial stasis re-establishes mitochondrial size and rescues mitophagy defects caused by division deficiency. Using Drp1KO livers, we found that the autophagy adaptor protein p62/sequestosome-1-which is thought to function downstream of ubiquitination-promotes mitochondrial ubiquitination. p62 recruits two subunits of a cullin-RING ubiquitin E3 ligase complex, Keap1 and Rbx1, to mitochondria. Resembling Drp1KO, diet-induced nonalcoholic fatty livers enlarge mitochondria and accumulate mitophagy intermediates. Resembling Drp1Opa1KO, Opa1KO rescues liver damage in this disease model. Our data provide a new concept that mitochondrial stasis leads the spatial dimension of mitochondria to a stationary equilibrium and a new mechanism for mitochondrial ubiquitination in mitophagy.

Pluripotent Stem Cell-Based Approaches to Explore and Treat Optic Neuropathies

Sight is a major sense for human and visual impairment profoundly affects quality of life, especially retinal degenerative diseases which are the leading cause of irreversible blindness worldwide. As for other neurodegenerative disorders, almost all retinal dystrophies are characterized by the specific loss of one or two cell types, such as retinal ganglion cells, photoreceptor cells, or retinal pigmented epithelial cells. This feature is a critical point when dealing with cell replacement strategies considering that the preservation of other cell types and retinal circuitry is a prerequisite. Retinal ganglion cells are particularly vulnerable to degenerative process and glaucoma, the most common optic neuropathy, is a frequent retinal dystrophy. Cell replacement has been proposed as a potential approach to take on the challenge of visual restoration, but its application to optic neuropathies is particularly challenging. Many obstacles need to be overcome before any clinical application. Beyond their survival and differentiation, engrafted cells have to reconnect with both upstream synaptic retinal cell partners and specific targets in the brain. To date, reconnection of retinal ganglion cells with distal central targets appears unrealistic since central nervous system is refractory to regenerative processes. Significant progress on the understanding of molecular mechanisms that prevent central nervous system regeneration offer hope to overcome this obstacle in the future. At the same time, emergence of reprogramming of human somatic cells into pluripotent stem cells has facilitated both the generation of new source of cells with therapeutic potential and the development of innovative methods for the generation of transplantable cells. In this review, we discuss the feasibility of stem cell-based strategies applied to retinal ganglion cells and optic nerve impairment. We present the different strategies for the generation, characterization and the delivery of transplantable retinal ganglion cells derived from pluripotent stem cells. The relevance of pluripotent stem cell-derived retinal organoid and retinal ganglion cells for disease modeling or drug screening will be also introduced in the context of optic neuropathies.

Mitochondrial Uncoupling Proteins: Subtle Regulators of Cellular Redox Signaling

SIGNIFICANCE

Mitochondria are the energetic, metabolic, redox, and information signaling centers of the cell. Substrate pressure, mitochondrial network dynamics, and cristae morphology state are integrated by the protonmotive force Δp or its potential component, ΔΨ, which are attenuated by proton backflux into the matrix, termed uncoupling. The mitochondrial uncoupling proteins (UCP1-5) play an eminent role in the regulation of each of the mentioned aspects, being involved in numerous physiological events including redox signaling. Recent Advances: UCP2 structure, including purine nucleotide and fatty acid (FA) binding sites, strongly support the FA cycling mechanism: UCP2 expels FA anions, whereas uncoupling is achieved by the membrane backflux of protonated FA. Nascent FAs, cleaved by phospholipases, are preferential. The resulting Δp dissipation decreases superoxide formation dependent on Δp. UCP-mediated antioxidant protection and its impairment are expected to play a major role in cell physiology and pathology. Moreover, UCP2-mediated aspartate, oxaloacetate, and malate antiport with phosphate is expected to alter metabolism of cancer cells.

CRITICAL ISSUES

A wide range of UCP antioxidant effects and participations in redox signaling have been reported; however, mechanisms of UCP activation are still debated. Switching off/on the UCP2 protonophoretic function might serve as redox signaling either by employing/releasing the extra capacity of cell antioxidant systems or by directly increasing/decreasing mitochondrial superoxide sources. Rapid UCP2 degradation, FA levels, elevation of purine nucleotides, decreased Mg²⁺, or increased pyruvate accumulation may initiate UCP-mediated redox signaling.

FUTURE DIRECTIONS

Issues such as UCP2 participation in glucose sensing, neuronal (synaptic) function, and immune cell activation should be elucidated. Antioxid. Redox Signal. 29, 667-714.

Mitochondrial dysfunction in iPSC-derived neurons of subjects with chronic mountain sickness

Patients with chronic mountain sickness (CMS) suffer from hypoxemia, erythrocytosis, and numerous neurologic deficits. Here we used induced pluripotent stem cell (iPSC)-derived neurons from both CMS and non-CMS subjects to study CMS neuropathology. Using transmission electron microscopy, we report that CMS neurons have a decreased mitochondrial volume density, length, and less cristae membrane surface area. Real-time PCR confirmed a decreased mitochondrial fusion gene optic atrophy 1 (OPA1) expression. Immunoblot analysis showed an accumulation of the short isoform of OPA1 (S-OPA1) in CMS neurons, which have reduced ATP levels under normoxia and increased lactate dehydrogenase (LDH) release and caspase 3 activation after hypoxia. Improving the balance between the long isoform of OPA1 and S-OPA1 in CMS neurons increased the ATP levels and attenuated LDH release under hypoxia. Our data provide initial evidence for altered mitochondrial morphology and function in CMS neurons, and reveal increased cell death under hypoxia due in part to altered mitochondrial dynamics. NEW & NOTEWORTHY Induced pluripotent stem cell-derived neurons from chronic mountain sickness (CMS) subjects have altered mitochondrial morphology and dynamics, and increased sensitivity to hypoxic stress. Modification of OPA1 can attenuate cell death after hypoxic treatment, providing evidence that altered mitochondrial dynamics play an important role in increased vulnerability under stress in CMS neurons.

Deciphering OPA1 mutations pathogenicity by combined analysis of human, mouse and yeast cell models

OPA1 is the major gene responsible for Dominant Optic Atrophy (DOA) and the syndromic form DOA "plus". Over 370 OPA1 mutations have been identified so far, although their pathogenicity is not always clear. We have analyzed one novel and a set of known OPA1 mutations to investigate their impact on protein functions in primary skin fibroblasts and in two "ad hoc" generated cell systems: the MGM1/OPA1 chimera yeast model and the Opa1-/- MEFs model expressing the mutated human OPA1 isoform 1. The yeast model allowed us to confirm the deleterious effects of these mutations and to gain information on their dominance/recessivity. The MEFs model enhanced the phenotypic alteration caused by mutations, nicely correlating with the clinical severity observed in patients, and suggested that the DOA "plus" phenotype could be induced by the combinatorial effect of mitochondrial network fragmentation with variable degrees of mtDNA depletion. Overall, the two models proved to be valuable tools to functionally assess and define the deleterious mechanism and the pathogenicity of novel OPA1 mutations, and useful to testing new therapeutic interventions.

Selective killing of human T-ALL cells: an integrated approach targeting redox homeostasis and the OMA1/OPA1 axis

Approximately 20% of pediatric T-cell acute lymphoblastic leukemia (T-ALL) patients are currently incurable due to primary or secondary resistance to glucocorticoid-based therapies. Here we employed an integrated approach to selectively kill T-ALL cells by increasing mitochondrial reactive oxygen species (ROS) using NS1619, a benzimidazolone that activates the K⁺ (BK) channel, and dehydroepiandrosterone (DHEA), which blunts ROS scavenging through inhibition of the pentose phosphate pathway. These compounds selectively killed T-ALL cell lines, patient-derived xenografts and primary cells from patients with refractory T-ALL, but did not kill normal human thymocytes. T-ALL cells treated with NS1619 and DHEA showed activation of the ROS-responsive transcription factor NRF2, indicating engagement of antioxidant pathways, as well as increased cleavage of OPA1, a mitochondrial protein that promotes mitochondrial fusion and regulates apoptosis. Consistent with these observations, transmission electron microscopy analysis indicated that NS1619 and DHEA increased mitochondrial fission. OPA1 cleavage and cell death were inhibited by ROS scavengers and by siRNA-mediated knockdown of the mitochondrial protease OMA1, indicating the engagement of a ROS-OMA1-OPA1 axis in T-ALL cells. Furthermore, NS1619 and DHEA sensitized T-ALL cells to TRAIL-induced apoptosis. In vivo, the combination of dexamethasone and NS1619 significantly reduced the growth of a glucocorticoid-resistant patient-derived T-ALL xenograft. Taken together, our findings provide proof-of-principle for an integrated ROS-based pharmacological approach to target refractory T-ALL.

Mitochondrial dynamics: overview of molecular mechanisms

Mitochondria are highly dynamic organelles undergoing coordinated cycles of fission and fusion, referred as ‘mitochondrial dynamics’, in order to maintain their shape, distribution and size. Their transient and rapid morphological adaptations are crucial for many cellular processes such as cell cycle, immunity, apoptosis and mitochondrial quality control. Mutations in the core machinery components and defects in mitochondrial dynamics have been associated with numerous human diseases. These dynamic transitions are mainly ensured by large GTPases belonging to the Dynamin family. Mitochondrial fission is a multi-step process allowing the division of one mitochondrion in two daughter mitochondria. It is regulated by the recruitment of the GTPase Dynamin-related protein 1 (Drp1) by adaptors at actin- and endoplasmic reticulum-mediated mitochondrial constriction sites. Drp1 oligomerization followed by mitochondrial constriction leads to the recruitment of Dynamin 2 to terminate membrane scission. Inner mitochondrial membrane constriction has been proposed to be an independent process regulated by calcium influx. Mitochondrial fusion is driven by a two-step process with the outer mitochondrial membrane fusion mediated by mitofusins 1 and 2 followed by inner membrane fusion, mediated by optic atrophy 1. In addition to the role of membrane lipid composition, several members of the machinery can undergo post-translational modifications modulating these processes. Understanding the molecular mechanisms controlling mitochondrial dynamics is crucial to decipher how mitochondrial shape meets the function and to increase the knowledge on the molecular basis of diseases associated with morphology defects. This article will describe an overview of the molecular mechanisms that govern mitochondrial fission and fusion in mammals.

How Mitochondrial Metabolism Contributes to Macrophage Phenotype and Functions

Metabolic reprogramming of cells from the innate immune system is one of the most noteworthy topics in immunological research nowadays. Upon infection or tissue damage, innate immune cells, such as macrophages, mobilize various immune and metabolic signals to mount a response best suited to eradicate the threat. Current data indicate that both the immune and metabolic responses are closely interconnected. On account of its peculiar position in regulating both of these processes, the mitochondrion has emerged as a critical organelle that orchestrates the coordinated metabolic and immune adaptations in macrophages. Significant effort is now underway to understand how metabolic features of differentiated macrophages regulate their immune specificities with the eventual goal to manipulate cellular metabolism to control immunity. In this review, we highlight some of the recent work that place cellular and mitochondrial metabolism in a central position in the macrophage differentiation program.

Sirtuins in mitochondrial stress: Indispensable helpers behind the scenes

Mitochondria play an essential part in guaranteeing normal cellular physiological functions through providing ATP and participating in diverse processes and signaling pathways. Recently, more and more studies have revealed the vital roles of mitochondria in coping with stressors in the aging process, metabolic disturbances and neurological disorders. Mitochondrial stress responses, including the mitochondrial unfolded protein response (UPR^mt), antioxidant defense, mitochondrial fission, mitochondrial fusion and mitophagy, are induced to maintain cellular integrity in response to stress. The sirtuin family, a group of NAD⁺-dependent deacetylases, has been the focus of much attention in recent years for their multiple regulatory functions, especially in aging and metabolism. Recent reports validated the significant link between mitochondrial stress responses and the sirtuin family, which may help to elucidate the pathogenesis and therapies for diseases such as Alzheimer's disease or Parkinson's disease. This review will summarize recent related studies and illuminate the interplay between sirtuins and mitochondrial stress.

TLR4 Activation Promotes the Progression of Experimental Autoimmune Myocarditis to Dilated Cardiomyopathy by Inducing Mitochondrial Dynamic Imbalance

Mitochondrial dynamic imbalance associates with several cardiovascular diseases. However, the role of mitochondrial dynamics in TLR4 activation-mediated dilated cardiomyopathy (DCM) progress remains unknown. A model of experimental autoimmune myocarditis (EAM) was established in BALB/c mice on which TLR4 activation by LPS-EB or TLR4 inhibition by LPS-RS was performed to induce chronic inflammation for 5 weeks. TLR4 activation promoted the transition of EAM to DCM as demonstrated by increased cardiomyocyte apoptosis, myocardial fibrosis, ventricular dilatation, and declined heart function. TLR4 inhibition mitigated the above DCM changes. Transmission electron microscope study showed that mitochondria became fragmented, also with damaged crista in ultrastructure in EAM mice. TLR4 activation aggravated the above mitochondrial aberration, and TLR4 inhibition alleviated it. The mitochondrial dynamic imbalance and damage in DCM development were mainly associated with OPA1 downregulation, which may be caused by elevated TNF-α level and ROS stress after TLR4 activation. Furthermore, OMA1/YME1L abnormal degradation was involved in the OPA1 dysfunction, and intervening OMA1/YME1L in H9C2 significantly alleviated mitochondrial fission, ultrastructure damage, and cell apoptosis induced by TNF-α and ROS. These data indicate that TLR4 activation resulted in OPA1 dysfunction, promoting mitochondrial dynamic imbalance and damage, which may involve in the progress of EAM to DCM.

SIRT4 interacts with OPA1 and regulates mitochondrial quality control and mitophagy

The stress-responsive mitochondrial sirtuin SIRT4 controls cellular energy metabolism in a NAD⁺-dependent manner and is implicated in cellular senescence and aging. Here we reveal a novel function of SIRT4 in mitochondrial morphology/quality control and regulation of mitophagy. We report that moderate overexpression of SIRT4, but not its enzymatically inactive mutant H161Y, sensitized cells to mitochondrial stress. CCCP-triggered dissipation of the mitochondrial membrane potential resulted in increased mitochondrial ROS levels and autophagic flux, but surprisingly led to increased mitochondrial mass and decreased Parkin-regulated mitophagy. The anti-respiratory effect of elevated SIRT4 was accompanied by increased levels of the inner-membrane bound long form of the GTPase OPA1 (L-OPA1) that promotes mitochondrial fusion and thereby counteracts fission and mitophagy. Consistent with this, upregulation of endogenous SIRT4 expression in fibroblast models of senescence either by transfection with miR-15b inhibitors or by ionizing radiation increased L-OPA1 levels and mitochondrial fusion in a SIRT4-dependent manner. We further demonstrate that SIRT4 interacts physically with OPA1 in co-immunoprecipitation experiments. Overall, we propose that the SIRT4-OPA1 axis is causally linked to mitochondrial dysfunction and altered mitochondrial dynamics that translates into aging-associated decreased mitophagy based on an unbalanced mitochondrial fusion/fission cycle.

Mitophagy is required for brown adipose tissue mitochondrial homeostasis during cold challenge

Brown adipose tissue (BAT) is a specialized thermogenic organ in mammals. The ability of BAT mitochondria to generate heat in response to cold-challenge to maintain core body temperature is essential for organismal survival. While cold activated BAT mitochondrial biogenesis is recognized as critical for thermogenic adaptation, the contribution of mitochondrial quality control to this process remains unclear. Here, we show mitophagy is required for brown adipocyte mitochondrial homeostasis during thermogenic adaptation. Mitophagy is significantly increased in BAT from cold-challenged mice (4 °C) and in β-agonist treated brown adipocytes. Blockade of mitophagy compromises brown adipocytes mitochondrial oxidative phosphorylation (OX-PHOS) capacity, as well as BAT mitochondrial integrity. Mechanistically, cold-challenge induction of BAT mitophagy is UCP1-dependent. Furthermore, our results indicate that mitophagy coordinates with mitochondrial biogenesis, maintaining activated BAT mitochondrial homeostasis. Collectively, our in vivo and in vitro findings identify mitophagy as critical for brown adipocyte mitochondrial homeostasis during cold adaptation.

The regulation of tumor cell physiology by mitochondrial dynamics

Mitochondrial dynamics are increasingly recognized to play an important role in regulating mitochondrial function in response to diverse stimuli. Given the overlap in the physiological processes influenced by mitochondria and the physiological processes disrupted in tumor cells, we speculate that tumor cells alter mitochondrial shape to promote the tumorigenic phenotype. Here, we briefly review the evidence linking changes in mitochondrial fusion and fission to a number of key tumorigenic processes, including metabolic rewiring, inhibition of cell death, cell migration, cell proliferation and self-renewal capacity. The role of mitochondrial dynamics in tumor growth is an important emerging area of research, a better understanding of which may lead to promising new therapeutic options for the treatment of cancer.

WBSCR16 Is a Guanine Nucleotide Exchange Factor Important for Mitochondrial Fusion

Regulated inter-mitochondrial fusion/fission is essential for maintaining optimal mitochondrial respiration and control of apoptosis and autophagy. In mammals, mitochondrial fusion is controlled by outer membrane GTPases MFN1 and MFN2 and by inner membrane (IM) GTPase OPA1. Disordered mitochondrial fusion/fission contributes to various pathologies, and MFN2 or OPA1 mutations underlie neurodegenerative diseases. Here, we show that the WBSCR16 protein is primarily associated with the outer face of the inner mitochondrial membrane and is important for mitochondrial fusion. We provide evidence of a WBSCR16/OPA1 physical interaction in the intact cell and of a WBSCR16 function as an OPA1-specific guanine nucleotide exchange factor (GEF). Homozygosity for a Wbscr16 mutation causes early embryonic lethality, whereas neurons of mice heterozygous for the mutation have mitochondria with reduced membrane potential and increased susceptibility to fragmentation upon exposure to stress, suggesting roles for WBSCR16 deficits in neuronal pathologies.

Hepatic steatosis associated with decreased β-oxidation and mitochondrial function contributes to cell damage in obese mice after thermal injury

Severely burned patients who are morbidly obese have poor clinical outcomes with aggravated metabolic consequences, a higher incidence of multiple organ dysfunction/failure, and significantly increased morbidity and mortality. The underlying mechanisms of these adverse outcomes are essentially unknown. Since the liver is one of the central metabolic organs, we hypothesized that thermal injury in obese patients leads to substantially increased lipolysis, hepatic fat infiltration, resulting in profound hepatic cellular and organellar alterations, consequently causing liver damage and severely augmented metabolic dysfunction. We tested this hypothesis using an obese mouse model subjected to a 20% total body surface area burn injury. C57BL/6 mice were randomly divided into low-fat diet (LFD) and high-fat diet (HFD) sham and burn groups (n = 6 per group) and fed for 16 weeks. 7 days after the thermal injury portal and cardiac blood were taken separately and liver tissue was collected for western blotting and immunohistochemical analysis. Gross examination of the liver showed apparent lipid infiltration in HFD fed and burned mice. We confirmed that augmented ER stress and inhibition of Akt-mTOR signaling dysregulated calcium homeostasis, contributed to the decrease of ER-mitochondria contact, and reduced mitochondrial β-oxidation in HFD fed and burned mice, leading to profound hepatic fat infiltration and substantial liver damage, hence increased morbidity and mortality. We conclude that obesity contributes to hepatic fat infiltration by suppressing β-oxidation, inducing cell damage and subsequent organ dysfunction after injury.

The mitochondrial nucleoside diphosphate kinase (NDPK-D/NME4), a moonlighting protein for cell homeostasis

Mitochondrial nucleoside diphosphate kinase (NDPK-D; synonyms: NME4, NM23-H4) represents the major mitochondrial NDP kinase. The homohexameric complex emerged as a protein with multiple functions in bioenergetics and phospholipid signaling. It occurs at different but precise mitochondrial locations and can affect among other mitochondrial shapes and dynamics, as well as the specific elimination of defective mitochondria or cells via mitophagy or apoptosis. With these various functions in cell homeostasis, NDPK-D/NME4 adds to the group of so-called moonlighting (or gene sharing) proteins.

m-AAA and i-AAA complexes coordinate to regulate OMA1, the stress-activated supervisor of mitochondrial dynamics

The proteolytic processing of dynamin-like GTPase OPA1, mediated by the activity of both YME1L1 [intermembrane (i)-AAA protease complex] and OMA1, is a crucial step in the regulation of mitochondrial dynamics. OMA1 is a zinc metallopeptidase of the inner mitochondrial membrane that undergoes pre-activating proteolytic and auto-proteolytic cleavage after mitochondrial import. Here, we identify AFG3L2 [matrix (m)-AAA complex] as the major protease mediating this event, which acts by maturing the 60 kDa pre-pro-OMA1 to the 40 kDa pro-OMA1 form by severing the N-terminal portion without recognizing a specific consensus sequence. Therefore, m-AAA and i-AAA complexes coordinately regulate OMA1 processing and turnover, and consequently control which OPA1 isoforms are present, thus adding new information on the molecular mechanisms of mitochondrial dynamics and neurodegenerative diseases affected by these phenomena.This article has an associated First Person interview with the first author of the paper.

Mitochondrial Quality Control and Disease: Insights into Ischemia-Reperfusion Injury

Mitochondria are key regulators of cell fate during disease. They control cell survival via the production of ATP that fuels cellular processes and, conversely, cell death via the induction of apoptosis through release of pro-apoptotic factors such as cytochrome C. Therefore, it is essential to have stringent quality control mechanisms to ensure a healthy mitochondrial network. Quality control mechanisms are largely regulated by mitochondrial dynamics and mitophagy. The processes of mitochondrial fission (division) and fusion allow for damaged mitochondria to be segregated and facilitate the equilibration of mitochondrial components such as DNA, proteins, and metabolites. The process of mitophagy are responsible for the degradation and recycling of damaged mitochondria. These mitochondrial quality control mechanisms have been well studied in chronic and acute pathologies such as Parkinson's disease, Alzheimer's disease, stroke, and acute myocardial infarction, but less is known about how these two processes interact and contribute to specific pathophysiologic states. To date, evidence for the role of mitochondrial quality control in acute and chronic disease is divergent and suggests that mitochondrial quality control processes can serve both survival and death functions depending on the disease state. This review aims to provide a synopsis of the molecular mechanisms involved in mitochondrial quality control, to summarize our current understanding of the complex role that mitochondrial quality control plays in the progression of acute vs chronic diseases and, finally, to speculate on the possibility that targeted manipulation of mitochondrial quality control mechanisms may be exploited for the rationale design of novel therapeutic interventions.

The mycotoxin phomoxanthone A disturbs the form and function of the inner mitochondrial membrane

Mitochondria are cellular organelles with crucial functions in the generation and distribution of ATP, the buffering of cytosolic Ca²⁺ and the initiation of apoptosis. Compounds that interfere with these functions are termed mitochondrial toxins, many of which are derived from microbes, such as antimycin A, oligomycin A, and ionomycin. Here, we identify the mycotoxin phomoxanthone A (PXA), derived from the endophytic fungus Phomopsis longicolla, as a mitochondrial toxin. We show that PXA elicits a strong release of Ca²⁺ from the mitochondria but not from the ER. In addition, PXA depolarises the mitochondria similarly to protonophoric uncouplers such as CCCP, yet unlike these, it does not increase but rather inhibits cellular respiration and electron transport chain activity. The respiration-dependent mitochondrial network structure rapidly collapses into fragments upon PXA treatment. Surprisingly, this fragmentation is independent from the canonical mitochondrial fission and fusion mediators DRP1 and OPA1, and exclusively affects the inner mitochondrial membrane, leading to cristae disruption, release of pro-apoptotic proteins, and apoptosis. Taken together, our results suggest that PXA is a mitochondrial toxin with a novel mode of action that might prove a useful tool for the study of mitochondrial ion homoeostasis and membrane dynamics.

m-AAA proteases, mitochondrial calcium homeostasis and neurodegeneration

The function of mitochondria depends on ubiquitously expressed and evolutionary conserved m-AAA proteases in the inner membrane. These ATP-dependent peptidases form hexameric complexes built up of homologous subunits. AFG3L2 subunits assemble either into homo-oligomeric isoenzymes or with SPG7 (paraplegin) subunits into hetero-oligomeric proteolytic complexes. Mutations in AFG3L2 are associated with dominant spinocerebellar ataxia (SCA28) characterized by the loss of Purkinje cells, whereas mutations in SPG7 cause a recessive form of hereditary spastic paraplegia (HSP7) with motor neurons of the cortico-spinal tract being predominantly affected. Pleiotropic functions have been assigned to m-AAA proteases, which act as quality control and regulatory enzymes in mitochondria. Loss of m-AAA proteases affects mitochondrial protein synthesis and respiration and leads to mitochondrial fragmentation and deficiencies in the axonal transport of mitochondria. Moreover m-AAA proteases regulate the assembly of the mitochondrial calcium uniporter (MCU) complex. Impaired degradation of the MCU subunit EMRE in AFG3L2-deficient mitochondria results in the formation of deregulated MCU complexes, increased mitochondrial calcium uptake and increased vulnerability of neurons for calcium-induced cell death. A reduction of calcium influx into the cytosol of Purkinje cells rescues ataxia in an AFG3L2-deficient mouse model. In this review, we discuss the relationship between the m-AAA protease and mitochondrial calcium homeostasis and its relevance for neurodegeneration and describe a novel mouse model lacking MCU specifically in Purkinje cells. Our results pledge for a novel view on m-AAA proteases that integrates their pleiotropic functions in mitochondria to explain the pathogenesis of associated neurodegenerative disorders.

OPA1: How much do we know to approach therapy?

OPA1 is a GTPase that controls several functions, such as mitochondrial dynamics and energetics, mtDNA maintenance and cristae integrity. In the last years, there have been described other cellular pathways and mechanisms involving OPA1 directly or through its interaction. All this new information, by implementing our knowledge on OPA1 is instrumental to elucidating the pathogenic mechanisms of OPA1 mutations. Indeed, these are associated with dominant optic atrophy (DOA), one of the most common inherited optic neuropathies, and with an increasing number of heterogeneous neurodegenerative disorders. In this review, we overview all recent findings on OPA1 protein functions, on its dysfunction and related clinical phenotypes, focusing on the current therapeutic options and future perspectives to treat DOA and the other associated neurological disorders due to OPA1 mutations.

The ever-growing complexity of the mitochondrial fission machinery

The mitochondrial network constantly changes and remodels its shape to face the cellular energy demand. In human cells, mitochondrial fusion is regulated by the large, evolutionarily conserved GTPases Mfn1 and Mfn2, which are embedded in the mitochondrial outer membrane, and by OPA1, embedded in the mitochondrial inner membrane. In contrast, the soluble dynamin-related GTPase Drp1 is recruited from the cytosol to mitochondria and is key to mitochondrial fission. A number of new players have been recently involved in Drp1-dependent mitochondrial fission, ranging from large cellular structures such as the ER and the cytoskeleton to the surprising involvement of the endocytic dynamin 2 in the terminal abscission step. Here we review the recent findings that have expanded the mechanistic model for the mitochondrial fission process in human cells and highlight open questions.

Eight human OPA1 isoforms, long and short: What are they for?

OPA1 is a dynamin-related GTPase that controls mitochondrial dynamics, cristae integrity, energetics and mtDNA maintenance. The exceptional complexity of this protein is determined by the presence, in humans, of eight different isoforms that, in turn, are proteolytically cleaved into combinations of membrane-anchored long forms and soluble short forms. Recent advances highlight how each OPA1 isoform is able to fulfill "essential" mitochondrial functions, whereas only some variants carry out "specialized" features. Long forms determine fusion, long or short forms alone build cristae, whereas long and short forms together tune mitochondrial morphology. These findings offer novel challenging therapeutic potential to gene therapy.