Restoration of Opa1-long isoform inhibits retinal injury-induced neurodegeneration

SIRT4 Protects Müller Glial Cells Against Apoptosis by Mediating Mitochondrial Dynamics and Oxidative Stress

SIRT4 is a member of the sirtuin family, which is related to mitochondrial function and possesses antioxidant and regulatory redox effects. Currently, the roles of SIRT4 in retinal Müller glial cells, oxidative stress, and mitochondrial function are still unclear. We confirmed, by immunofluorescence staining, that SIRT4 is located mainly in the mitochondria of retinal Müller glial cells. Using flow cytometry and Western blotting, we analyzed cell apoptosis, intracellular reactive oxygen species (ROS) levels, apoptotic and proapoptotic proteins, mitochondrial dynamics-related proteins, and mitochondrial morphology and number after the overexpression and downregulation of SIRT4 in rMC-1 cells. Neither the upregulation nor the downregulation of SIRT4 alone affected apoptosis. SIRT4 overexpression reduced intracellular ROS, reduced the BAX/BCL2 protein ratio, and increased the L-OPA/S-OPA1 ratio and the levels of the mitochondrial fusion protein MFN2 and the mitochondrial cleavage protein FIS1, increasing mitochondrial fusion. SIRT4 downregulation had the opposite effect. Mitochondria tend to divide after serum starvation for 24 h, and SIRT4 downregulation increases mitochondrial fragmentation and oxidative stress, leading to aggravated cell damage. The mitochondrial division inhibitor Mdivi-1 reduced oxidative stress levels and thus reduced cell damage caused by serum starvation. The overexpression of SIRT4 in rMC-1 cells reduced mitochondrial fragmentation caused by serum starvation, leading to mitochondrial fusion and reduced expression of cleaved caspase-3, thus alleviating the cellular damage caused by oxidative stress. Thus, we speculate that SIRT4 may protect retinal Müller glial cells against apoptosis by mediating mitochondrial dynamics and oxidative stress.

Cell life-or-death events in osteoporosis: All roads lead to mitochondrial dynamics

Mitochondria exhibit heterogeneous shapes and networks within and among cell types and tissues, also in normal or osteoporotic bone tissues with complex cell types. This dynamic characteristic is determined by the high plasticity provided by mitochondrial dynamics and is stemmed from responding to the survival and functional requirements of various bone cells in a specific microenvironments. In contrast, mitochondrial dysfunction, induced by dysregulation of mitochondrial dynamics, may act as a trigger of cell death signals, including common apoptosis and other forms of programmed cell death (PCD). These PCD processes consisting of tightly structured cascade gene expression events, can further influence the bone remodeling by facilitating the death of various bone cells. Mitochondrial dynamics, therefore, drive the bone cells to stand at the crossroads of life and death by integrating external signals and altering metabolism, shape, and signal-response properties of mitochondria. This implies that targeting mitochondrial dynamics displays significant potential in treatment of osteoporosis. Considerable effort has been made in osteoporosis to emphasize the parallel roles of mitochondria in regulating energy metabolism, calcium signal transduction, oxidative stress, inflammation, and cell death. However, the emerging field of mitochondrial dynamics-related PCD is not well understood. Herein, to bridge the gap, we outline the latest knowledge on mitochondrial dynamics regulating bone cell life or death during normal bone remodeling and osteoporosis.

A review for the correlation between optic atrophy 1-dependent mitochondrial fusion and cardiovascular disorders

Mitochondrial dynamics homeostasis is sustained by continuous and balanced fission and fusion, which are determinants of morphology, abundance, biogenesis and mitophagy of mitochondria. Optic atrophy 1 (OPA1), as the only inner mitochondrial membrane fusion protein, plays a key role in stabilizing mitochondrial dynamics. The disturbance of mitochondrial dynamics contributes to the pathophysiological progress of cardiovascular disorders, which are the main cause of death worldwide in recent decades and result in tremendous social burden. In this review, we describe the latest findings regarding OPA1 and its role in mitochondrial fusion. We summarize the post-translational modifications (PTMs) for OPA1 and its regulatory role in mitochondrial dynamics. Then the diverse cell fates caused by OPA1 expression during cardiovascular disorders are discussed. Moreover, cardiovascular disorders (such as heart failure, myocardial ischemia/reperfusion injury, cardiomyopathy and cardiac hypertrophy) relevant to OPA1-dependent mitochondrial dynamics imbalance have been detailed. Finally, we highlight the potential that targeting OPA1 to impact mitochondrial fusion may be used as a novel strategy against cardiovascular disorders.

Empagliflozin targets Mfn1 and Opa1 to attenuate microglia-mediated neuroinflammation in retinal ischemia and reperfusion injury

BACKGROUND

Neuroinflammation and mitochondrial dysfunction play crucial roles in retinal ischemia and reperfusion (IR) injury. Recent studies have identified mitochondrial function as a promising target for immunomodulation. Empagliflozin (EMPA), an anti-diabetic drug, has exhibited great potential as both an anti-inflammatory agent and a protector of mitochondrial health. This study aimed to assess the therapeutic efficacy of EMPA in retinal IR injury.

METHODS

To evaluate the protective effects of EMPA, the drug was injected into the vitreous body of mice post-retinal IR. Single-cell RNA sequencing (scRNA-seq) analysis was conducted to uncover the underlying mechanisms, and the results were further validated through in vivo and in vitro experiments.

RESULTS

EMPA effectively protected retinal ganglion cells (RGCs) from IR injury by attenuating local retinal inflammation. The scRNA-seq analysis revealed that EMPA downregulated the nucleotide-binding domain and leucine-rich repeat containing protein 3 (NLRP3) signaling pathway and restored mitochondrial dynamics by upregulating the expression of mitochondrial fusion-related genes, Mitofusin 1 (Mfn1) and optic atrophy 1 (Opa1). These findings were further corroborated by Western blotting. In vitro experiments provided additional insights, demonstrating that EMPA suppressed lipopolysaccharide (LPS)-induced cell inflammation and NLRP3 inflammasome activation. Moreover, EMPA enhanced mitochondrial fusion, neutralized mitochondrial reactive oxygen species (mtROS), and restored mitochondrial membrane potential (MMP) in BV2 microglia. Notably, genetic ablation of Mfn1 or Opa1 abolished the anti-inflammatory effects of EMPA.

CONCLUSIONS

Our findings highlight the positive contribution of Mfn1 and Opa1 to the anti-inflammatory therapeutic effect of EMPA. By restoring mitochondrial dynamics, EMPA effectively mitigates microglia-mediated neuroinflammation and prevents RGC loss in retinal IR injury.

Mechanistic study of optic atrophy 1 in ischemia-reperfusion disease

Mitochondria consist of the inner mitochondrial membrane and the outer mitochondrial membrane, which maintain mitochondrial homeostasis through continuous fission and fusion to ensure a healthy mitochondrial network and thus regulate normal cellular function, namely mitochondrial dynamics. The imbalance between mitochondrial fusion and fission results in abnormal mitochondrial structure and eventually mitochondrial dysfunction, which is involved in the pathological process of ischemia-reperfusion injury (IRI). Optic atrophy 1 (OPA1) is a key protein that regulates mitochondrial inner membrane fusion and ensures normal mitochondrial function by balancing mitochondrial dynamics, participating in various processes such as mitochondrial fusion, oxidative stress, and apoptosis. Ischemia-induced changes in mitochondrial dynamics may be a key factor in limiting the recanalization time window and exacerbating reperfusion injury, and the mechanisms of these changes deserve further attention. Therefore, targeting OPA1-related mitochondrial fusions, thereby balancing mitochondrial dynamics and improving mitochondrial dysfunction, is a promising therapeutic strategy for ischemia-reperfusion diseases. This review will elaborate on the structure and function of OPA1 and the role of OPA1 in IRI to provide promising therapeutic targets for the treatment of ischemia-reperfusion diseases.

Vitamin B3 Provides Neuroprotection via Antioxidative Stress in a Rat Model of Anterior Ischemic Optic Neuropathy

Supplementing with vitamin B3 has been reported to protect against retinal ganglion cell (RGC) damage events and exhibit multiple neuroprotective properties in a mouse model of optic nerve injury. In this study, a rat model of anterior ischemic optic neuropathy was used to assess the neuroprotective benefits of vitamin B3 (rAION). Vitamin B3 (500 mg/kg/day) or phosphate-buffered saline (PBS) was administered to the rAION-induced rats every day for 28 days. The vitamin B3-treated group had significantly higher first positive and second negative peak (P1-N2) amplitudes of flash visual-evoked potentials and RGC densities than the PBS-treated group (p < 0.05). A terminal deoxynucleotidyl transferase dUTP nick end labeling assay conducted on vitamin B3-treated rats revealed a significant reduction in apoptotic cells (p < 0.05). Superoxide dismutase and thiobarbituric acid reactive substance activity showed that vitamin B3 treatment decreased reactive oxygen species (p < 0.05). Therefore, vitamin B3 supplementation preserves vision in rAION-induced rats by reducing oxidative stress, neuroinflammation, and mitochondrial apoptosis.

Current advances in gene therapy of mitochondrial diseases

Mitochondrial diseases (MD) are a heterogeneous group of multisystem disorders involving metabolic errors. MD are characterized by extremely heterogeneous symptoms, ranging from organ-specific to multisystem dysfunction with different clinical courses. Most primary MD are autosomal recessive but maternal inheritance (from mtDNA), autosomal dominant, and X-linked inheritance is also known. Mitochondria are unique energy-generating cellular organelles designed to survive and contain their own unique genetic coding material, a circular mtDNA fragment of approximately 16,000 base pairs. The mitochondrial genetic system incorporates closely interacting bi-genomic factors encoded by the nuclear and mitochondrial genomes. Understanding the dynamics of mitochondrial genetics supporting mitochondrial biogenesis is especially important for the development of strategies for the treatment of rare and difficult-to-diagnose diseases. Gene therapy is one of the methods for correcting mitochondrial disorders.

Membrane Blue Dual Protects Retinal Pigment Epithelium Cells/Ganglion Cells-Like through Modulation of Mitochondria Function

Although recent data highlight the greater protective effects exerted by Membrane Blue Dual (MBD), a precise analysis of the mechanisms of action is missing. We examined the effects of MBD with/without polyethylene glycol (PEG) on both human retinal pigment epithelial cells (ARPE-19) and retinal ganglion cells-like (RGC-5) cultured in the presence/absence of ultraviolet B (UVB) treatment on mitochondria function, oxidants, and apoptosis. In ARPE-19/RGC-5 cells either treated or not with UVB, the effects of MBD with/without PEG were evaluated by specific assays for viability, mitochondrial membrane potential and mitochondrial reactive oxygen species (mitoROS) release. Annexin V was used to detect apoptosis, whereas trypan blue and the scratch assay were used for proliferation/migration. In both physiologic conditions and in the presence of UVB, MBD with/without PEG increased cell viability, mitochondrial membrane potential, proliferation and migration in both ARPE-19 and RGC-5 cells. In general, the effects of MBD with PEG were greater than those caused by MBD without PEG. Our results suggest that, in particular, MBD with PEG is a safe and effective dye for vitreoretinal surgery through the modulation of mitochondrial function.

Mitochondrial protein dysfunction in pathogenesis of neurological diseases

Mitochondria are essential organelles for neuronal function and cell survival. Besides the well-known bioenergetics, additional mitochondrial roles in calcium signaling, lipid biogenesis, regulation of reactive oxygen species, and apoptosis are pivotal in diverse cellular processes. The mitochondrial proteome encompasses about 1,500 proteins encoded by both the nuclear DNA and the maternally inherited mitochondrial DNA. Mutations in the nuclear or mitochondrial genome, or combinations of both, can result in mitochondrial protein deficiencies and mitochondrial malfunction. Therefore, mitochondrial quality control by proteins involved in various surveillance mechanisms is critical for neuronal integrity and viability. Abnormal proteins involved in mitochondrial bioenergetics, dynamics, mitophagy, import machinery, ion channels, and mitochondrial DNA maintenance have been linked to the pathogenesis of a number of neurological diseases. The goal of this review is to give an overview of these pathways and to summarize the interconnections between mitochondrial protein dysfunction and neurological diseases.

L-OPA1 deficiency aggravates necroptosis of alveolar epithelial cells through impairing mitochondrial function during ALI in mice

Necroptosis, a recently described form of programmed cell death, is the main way of alveolar epithelial cells (AECs) death in acute lung injury (ALI). While the mechanism of how to trigger necroptosis in AECs during ALI has been rarely evaluated. Long optic atrophy protein 1 (L-OPA1) is a crucial mitochondrial inner membrane fusion protein, and its deficiency impairs mitochondrial function. This study aimed to investigate the role of L-OPA1 deficiency-mediated mitochondrial dysfunction in AECs necroptosis. We comprehensively investigated the detailed contribution and molecular mechanism of L-OPA1 deficiency in AECs necroptosis by inhibiting or activating L-OPA1. Firstly, our data showed that L-OPA1 expression was down-regulated in the lungs and AECs under the lipopolysaccharide (LPS) challenge. Furthermore, inhibition of L-OPA1 aggravated the pathological injury, inflammatory response, and necroptosis in the lungs of LPS-induced ALI mice. In vitro, inhibition of L-OPA1 induced necroptosis of AECs, while activation of L-OPA1 alleviated necroptosis of AECs under the LPS challenge. Mechanistically, inhibition of L-OPA1 aggravated necroptosis of AECs by inducing mitochondrial fragmentation and reducing mitochondrial membrane potential. While activation of L-OPA1 had the opposite effects. In summary, these findings indicate for the first time that L-OPA1 deficiency mediates mitochondrial fragmentation, induces necroptosis of AECs, and exacerbates ALI in mice. This article is protected by copyright. All rights reserved.

Ischemia-induced cleavage of OPA1 at S1 site aggravates mitochondrial fragmentation and reperfusion injury in neurons

Neuronal mitochondrial dynamics are disturbed after ischemic stroke. Optic atrophy 1 (OPA1) and its GTPase activity are involved in maintaining mitochondrial cristae and inner membrane fusion. This study aimed to explore the role of OMA1-mediated OPA1 cleavage (S1-OPA1) in neurons exposed to cerebral ischemia and reperfusion. After oxygen-glucose deprivation (OGD) for 60 min, we found that mitochondrial fragmentation occurred successively in the axon and soma of neurons, accompanied by an increase in S1-OPA1. In addition, S1-OPA1 overexpression significantly aggravated mitochondrial damage in neurons exposed to OGD for 60 min and 24 h after OGD/R, characterized by mitochondrial fragmentation, decreased mitochondrial membrane potential, mitochondrial cristae ultrastructural damage, increased superoxide production, decreased ATP production and increased mitochondrial apoptosis, which was inhibited by the lysine 301 to alanine mutation (K301A). Furthermore, we performed neuron-specific overexpression of S1-OPA1 in the cerebral cortex around ischemia of middle cerebral artery occlusion/reperfusion (MCAO/R) mice. The results further demonstrated in vivo that S1-OPA1 exacerbated neuronal mitochondrial ultrastructural destruction and injury induced by cerebral ischemia-reperfusion, while S1-OPA1-K301 overexpression had no effect. In conclusion, ischemia induced neuronal OMA1-mediated cleavage of OPA1 at the S1 site. S1-OPA1 aggravated neuronal mitochondrial fragmentation and damage in a GTPase-dependent manner, and participated in neuronal ischemia-reperfusion injury.

Optimization of murine retinal mitochondrial injury model

The retinal mitochondrial injury model in rat has been developed using the mitochondrial oxidative phosphorylation uncoupler, carbonylcyanide m-chlorophenyl hydrazine (CCCP). However, the CCCP-induced murine retinal mitochondrial injury model has not been reported. Here, the optimized conditions for the murine retinal mitochondrial injury model were established by intravitreal injection of different doses of CCCP (0, 2.5, 5, 7.5, 10, 12.5, 15 μg). Indeed, it has been reported that CCCP induces Opa1 cleavage and phosphorylation of ERK in cultured cells and rat retinas. Thus, we measured phosphorylated (p) -Erk and L/S-Opa1 following CCCP-induced retinal injury. Meanwhile, KW6002 (A_2A receptor antagonist) pretreatment inhibited retinal injury induced by CCCP at 10 and 15 μg doses differently. Intravitreal injection of 10 μg doses of CCCP can induce apoptosis of retinal ganglion cells and decrease of retinal thickness, but intravitreal injection of 15 μg doses of CCCP is the appropriate dose to study the protective effect of A_2A receptor.

(1) Dose dependent effects of intravitreal injection of CCCP on the levels of L/S-Opa1 and p-Erk;

(2) A_2A receptor antagonist (KW6002) only inhibited the apoptosis of ganglion cells, but did not affect the thickness of retina with 10µg dosage of CCCP intravitreal injection;

(3) A_2A receptor antagonist (KW6002) inhibited the apoptosis of ganglion cells and increased the thickness of retina with 15µg dosage of CCCP intravitreal injection.

Mitochondrial dynamics in the neonatal brain - a potential target following injury?

The impact of birth asphyxia and its sequelae, hypoxic–ischaemic (HI) brain injury, is long-lasting and significant, both for the infant and for their family. Treatment options are limited to therapeutic hypothermia, which is not universally successful and is unavailable in low resource settings. The energy deficits that accompany neuronal death following interruption of blood flow to the brain implicate mitochondrial dysfunction. Such HI insults trigger mitochondrial outer membrane permeabilisation leading to release of pro-apoptotic proteins into the cytosol and cell death. More recently, key players in mitochondrial fission and fusion have been identified as targets following HI brain injury. This review aims to provide an introduction to the molecular players and pathways driving mitochondrial dynamics, the regulation of these pathways and how they are altered following HI insult. Finally, we review progress on repurposing or repositioning drugs already approved for other indications, which may target mitochondrial dynamics and provide promising avenues for intervention following brain injury. Such repurposing may provide a mechanism to fast-track, low-cost treatment options to the clinic.

The adenosine A2A receptor antagonist protects against retinal mitochondrial injury in association with an altered network of competing endogenous RNAs

Blockade of adenosine A_2A receptors (A_2ARs) protects against neuronal damage caused by various brain insults including mitochondrial toxicity, but the precise neuroprotective mechanisms are unclear. Here, we studied the effects of the A_2AR antagonist KW6002 on retinal injury induced by the mitochondrial oxidative phosphorylation uncoupler, carbonylcyanide m-chlorophenyl hydrazine (CCCP) and alterations in competing endogenous RNA (ceRNA) network. We found that KW6002 treatment partially reversed CCCP-induced reduction in retinal thickness and retinal ganglia cell number by increasing mitochondrial content and reducing retinal ganglia cells apoptosis. Furthermore, we employed whole-transcriptome sequencing to explore ceRNA network changes associated with CCCP-induced retinal injury and its reversal by KW6002. This analysis revealed that A_2AR blockade reduced the number of CCCP-induced microRNAs by ∼60%, but increased the number of CCCP-induced circular RNAs by ∼50%. Among CeRNA network changes, CCCP-induced retinal injury was associated with a possible enrichment of the tumor necrosis factor signaling pathway and its related 126 microRNAs, 237 long non-coding RNAs, 58 circular RNAs competing. Moreover, the A_2AR antagonist-mediated protection against CCCP-induced retinal injury was possibly associated with the up-regulation of mature brain-derived neurotrophic factor and its related 4 microRNAs competed by 43 long non-coding RNAs and 9 circular RNAs competing. These ceRNA network alterations by CCCP treatment and its reversal by A_2AR antagonist may contribute to understanding the transcriptome mechanism for protection against CCCP-induced retinal injury by A_2AR antagonists.

Critical Roles of Calpastatin in Ischemia/Reperfusion Injury in Aged Livers

Ischemia/reperfusion (I/R) injury unavoidably occurs during hepatic resection and transplantation. Aged livers poorly tolerate I/R during surgical treatment. Although livers have a powerful endogenous inhibitor of calpains, calpastatin (CAST), I/R activates calpains, leading to impaired autophagy, mitochondrial dysfunction, and hepatocyte death. It is unknown how I/R in aged livers affects CAST. Human and mouse liver biopsies at different ages were collected during in vivo I/R. Hepatocytes were isolated from 3-month- (young) and 26-month-old (aged) mice, and challenged with short in vitro simulated I/R. Cell death, protein expression, autophagy, and mitochondrial permeability transition (MPT) between the two age groups were compared. Adenoviral vector was used to overexpress CAST. Significant cell death was observed only in reperfused aged hepatocytes. Before the commencement of ischemia, CAST expression in aged human and mouse livers and mouse hepatocytes was markedly greater than that in young counterparts. However, reperfusion substantially decreased CAST in aged human and mouse livers. In hepatocytes, reperfusion rapidly depleted aged cells of CAST, cleaved autophagy-related protein 5 (ATG5), and induced defective autophagy and MPT onset, all of which were blocked by CAST overexpression. Furthermore, mitochondrial morphology was shifted toward an elongated shape with CAST overexpression. In conclusion, CAST in aged livers is intrinsically short-lived and lost after short I/R. CAST depletion contributes to age-dependent liver injury after I/R.

Dominant Optic Atrophy (DOA): Modeling the Kaleidoscopic Roles of OPA1 in Mitochondrial Homeostasis

In the year 2000, the discovery of OPA1 mutations as causative for dominant optic atrophy (DOA) was pivotal to rapidly expand the field of mitochondrial dynamics and describe the complex machinery governing this pathway, with a multitude of other genes and encoded proteins involved in neurodegenerative disorders of the optic nerve. OPA1 turned out to be a much more complex protein than initially envisaged, connecting multiple pathways beyond its strict role in mitochondrial fusion, such as sensing of OXPHOS needs and mitochondrial DNA maintenance. As a consequence, an increasing need to investigate OPA1 functions at multiple levels has imposed the development of multiple tools and models that are here reviewed. Translational mitochondrial medicine, with the ultimate objective of translating basic science necessary to understand pathogenic mechanisms into therapeutic strategies, requires disease modeling at multiple levels: from the simplest, like in yeast, to cell models, including the increasing use of reprogrammed stem cells (iPSCs) from patients, to animal models. In the present review, we thoroughly examine and provide the state of the art of all these approaches.

Optimized OPA1 Isoforms 1 and 7 Provide Therapeutic Benefit in Models of Mitochondrial Dysfunction

Optic Atrophy 1 (OPA1) is a mitochondrially targeted GTPase that plays a pivotal role in mitochondrial health, with mutations causing severe mitochondrial dysfunction and typically associated with Dominant Optic Atrophy (DOA), a progressive blinding disease involving retinal ganglion cell loss and optic nerve damage. In the current study, we investigate the use of codon-optimized versions of OPA1 isoform 1 and 7 as potential therapeutic interventions in a range of in vitro and in vivo models of mitochondrial dysfunction. We demonstrate that both isoforms perform equally well in ameliorating mitochondrial dysfunction in OPA1 knockout mouse embryonic fibroblast cells but that OPA1 expression levels require tight regulation for optimal benefit. Of note, we demonstrate for the first time that both OPA1 isoform 1 and 7 can be used independently to protect spatial visual function in a murine model of retinal ganglion cell degeneration caused by mitochondrial dysfunction, as well as providing benefit to mitochondrial bioenergetics in DOA patient derived fibroblast cells. These results highlight the potential value of OPA1-based gene therapy interventions.

Resveratrol protects retinal ganglion cells against ischemia induced damage by increasing Opa1 expression

Loss of idiopathic retinal ganglion cells (RGCs) leads to irreversible vision defects and is considered the primary characteristic of glaucoma. However, effective treatment strategies in terms of RGC neuroprotection remain elusive. In the present study, the protective effects of resveratrol on RGC apoptosis, and the mechanisms underlying its effects were investigated, with a particular emphasis on the function of optic atrophy 1 (Opa1). In an ischemia/reperfusion (I/R) injury model, the notable thinning of the retina, significant apoptosis of RGCs, reduction in Opa1 expression and long Opa1 isoform to short Opa1 isoform ratios (L-Opa1/S-Opa1 ratio) were observed, all of which were reversed by resveratrol administration. Serum deprivation resulted in reductions in R28 cell viability, superoxide dismutase (SOD) activity, Opa1 expression and induced apoptosis, which were also partially reversed by resveratrol treatment. To conclude, results from the present study suggest that resveratrol treatment significantly reduced retinal damage and RGC apoptosis in I/R injury and serum deprivation models. In addition, resveratrol reversed the downregulated expression of Opa1 and reduced SOD activity. Mechanistically, resveratrol influenced mitochondrial dynamics by regulating the L-Opa1/S-Opa1 ratio. Therefore, these observations suggest that resveratrol may exhibit potential as a therapeutic agent for RGC damage in the future.

Effects of Diabetes on Mitochondrial Morphology and Its Implications in Diabetic Retinopathy

Purpose

To determine whether high glucose (HG) or diabetes alters mitochondrial morphology and promotes mitochondrial fragmentation in retinal vascular cells and thereby triggers apoptosis associated with diabetic retinopathy.

Methods

To assess whether diabetes promotes mitochondrial fragmentation and thereby triggers apoptosis, retinas from nondiabetic and diabetic rats were analyzed using electron microscopy (EM) and in parallel, wild-type, diabetic, and OPA1+/- mice were analyzed for optic atrophy gene 1 (OPA1) and cytochrome c levels using Western blot (WB) analysis. To assess the relationship between mitochondrial fragmentation and OPA1 levels, rat retinal endothelial cells (RRECs) were grown in normal (N; 5 mmol/L) medium, HG (30 mmol/L) medium, or in N medium transfected with OPA1 siRNA for seven days. Cells were examined for OPA1 expression and cytochrome c release by WB. In parallel, cells were stained with MitoTracker Red and assessed for mitochondrial fragmentation in live cells using confocal microscopy.

Results

EM images revealed significant mitochondrial fragmentation in vascular cells of retinal capillaries of diabetic rats compared with that of nondiabetic rats. WB analysis showed significant OPA1 downregulation concomitant with increased levels of proapoptotic cytochrome c levels in cells grown in HG and in cells transfected with OPA1 siRNA alone. Similarly, OPA1 level was significantly reduced in diabetic retinas compared with that of nondiabetic retinas. Interestingly, OPA1+/- animals exhibited elevated cytochrome c release similar to those of diabetic mice.

Conclusions

Findings indicate that diabetes promotes mitochondrial fragmentation in retinal vascular cells, which are driven, at least in part, by decreased OPA1 levels leading to apoptosis in diabetic retinopathy.

Hypoxia-induced NAD+ interventions promote tumor survival and metastasis by regulating mitochondrial dynamics

Hypoxia, an important feature of the tumor microenvironment, is responsible for the chemo-resistance and metastasis of malignant solid tumors. Recent studies indicated that mitochondria undergo morphological transitions as an adaptive response to maintain self-stability and connectivity under hypoxic conditions. NAD⁺ may not only provide reducing equivalents for biosynthetic reactions and in determining energy production, but also functions as a signaling molecule in mitochondrial dynamics regulation. In this review, we describe the upregulated KDAC deacetylase expression in the mitochondria and cytoplasm of tumor cells that results from sensing the changes in NAD⁺ to control mitochondrial dynamics and distribution, which is responsible for survival and metastasis in hypoxia.

Restoration of L-OPA1 alleviates acute ischemic stroke injury in rats via inhibiting neuronal apoptosis and preserving mitochondrial function

BACKGROUND

Ischemic stroke can induce changes in mitochondrial morphology and function. As a regulatory gene in mitochondria, optic atrophy 1 (OPA1) plays a pivotal role in the regulation of mitochondrial dynamics and other related functions. However, its roles in cerebral ischemia-related conditions are barely understood.

METHODS

Cultured rat primary cortical neurons were respectively transfected with OPA1-v1ΔS1-encoding and OPA1-v1-encoding lentivirus before exposure to 2-h oxygen-glucose deprivation (OGD) and subsequent reoxygenation (OGD/R). Adult male SD rats received an intracranial injection of AAV-OPA1-v1ΔS1 and were subjected to 90 min of transient middle cerebral artery occlusion (tMCAO) followed by reperfusion. OPA1 expression and function were detected by in vitro and in vivo assays.

RESULTS

OPA1 was excessively cleaved after cerebral ischemia/reperfusion injury, both in vitro and in vivo. Under OGD/R condition, compared with that of the LV-OPA1-v1-treated group, the expression of OPA1-v1ΔS1 efficiently restored L-OPA1 level and alleviated neuronal death and mitochondrial morphological damage. Meanwhile, the expression of OPA1-v1ΔS1 markedly improved cerebral ischemia/reperfusion-induced motor function damage, attenuated brain infarct volume, neuronal apoptosis, mitochondrial bioenergetics deficits, oxidative stress, and restored the morphology of mitochondrial cristae and mitochondrial length. It also preserved the mitochondrial integrity and reinforced the mtDNA content and expression of mitochondrial biogenesis factors in ischemic rats.

INTERPRETATION

Our results demonstrate that the stabilization of L-OPA1 protects ischemic brains by reducing neuronal apoptosis and preserving mitochondrial function, suggesting its significance as a promising therapeutic target for stroke prevention and treatment.

The short variant of optic atrophy 1 (OPA1) improves cell survival under oxidative stress

Optic atrophy 1 (OPA1) is a dynamin protein that mediates mitochondrial fusion at the inner membrane. OPA1 is also necessary for maintaining the cristae and thus essential for supporting cellular energetics. OPA1 exists as membrane-anchored long form (L-OPA1) and short form (S-OPA1) that lacks the transmembrane region and is generated by cleavage of L-OPA1. Mitochondrial dysfunction and cellular stresses activate the inner membrane-associated zinc metallopeptidase OMA1 that cleaves L-OPA1, causing S-OPA1 accumulation. The prevailing notion has been that L-OPA1 is the functional form, whereas S-OPA1 is an inactive cleavage product in mammals, and that stress-induced OPA1 cleavage causes mitochondrial fragmentation and sensitizes cells to death. However, S-OPA1 contains all functional domains of dynamin proteins, suggesting that it has a physiological role. Indeed, we recently demonstrated that S-OPA1 can maintain cristae and energetics through its GTPase activity, despite lacking fusion activity. Here, applying oxidant insult that induces OPA1 cleavage, we show that cells unable to generate S-OPA1 are more sensitive to this stress under obligatory respiratory conditions, leading to necrotic death. These findings indicate that L-OPA1 and S-OPA1 differ in maintaining mitochondrial function. Mechanistically, we found that cells that exclusively express L-OPA1 generate more superoxide and are more sensitive to Ca²⁺-induced mitochondrial permeability transition, suggesting that S-OPA1, and not L-OPA1, protects against cellular stress. Importantly, silencing of OMA1 expression increased oxidant-induced cell death, indicating that stress-induced OPA1 cleavage supports cell survival. Our findings suggest that S-OPA1 generation by OPA1 cleavage is a survival mechanism in stressed cells.

OPA1 overexpression ameliorates mitochondrial cristae remodeling, mitochondrial dysfunction, and neuronal apoptosis in prion diseases

Prion diseases caused by the cellular prion protein (PrP^C) conversion into a misfolded isoform (PrP^Sc) are associated with multiple mitochondrial damages. We previously reported mitochondrial dynamic abnormalities and cell death in prion diseases via modulation of a variety of factors. Optic atrophy 1 (OPA1) is one of the factors that control mitochondrial fusion, mitochondrial DNA (mtDNA) maintenance, bioenergetics, and cristae integrity. In this study, we observed downregulation of OPA1 in prion disease models in vitro and in vivo, mitochondria structure damage and dysfunction, loss of mtDNA, and neuronal apoptosis. Similar mitochondria findings were seen in OPA1-silenced un-infected primary neurons. Overexpression of OPA1 not only alleviated prion-induced mitochondrial network fragmentation and mtDNA loss, decrease in intracellular ATP, increase in ADP/ATP ratio, and decrease in mitochondrial membrane potential but also protected neurons from apoptosis by suppressing the release of cytochrome c from mitochondria to cytosol and activation of the apoptotic factor, caspase 3. Our results demonstrated that overexpression of OPA1 alleviates prion-associated mitochondrial network fragmentation and cristae remodeling, mitochondrial dysfunction, mtDNA depletion, and neuronal apoptosis, suggesting that OPA1 may be a novel and effective therapeutic target for prion diseases.

The Good and the Bad of Mitochondrial Breakups

Mitochondrial morphology is a crucial determinant of mitochondrial and cellular function. Opposing fusion and fission events shape the tubular mitochondrial reticulum and ensure mitochondrial transport within cells. Cellular stress and pathophysiological conditions can lead to fragmentation of the mitochondrial network, which facilitates mitophagy and is associated with cell death. However, mitochondrial shape changes are also intertwined with the cellular metabolism, and metabolic switches can induce but also result from alterations in mitochondrial morphology. Here, we discuss recent advances in the field of mitochondrial dynamics, demonstrating cell- and tissue-specific effects of mitochondrial fragmentation on cellular metabolism, cell survival, and mitochondrial quality control.

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.

OPA1 Isoforms in the Hierarchical Organization of Mitochondrial Functions

OPA1 is a GTPase that controls mitochondrial fusion, cristae integrity, and mtDNA maintenance. In humans, eight isoforms are expressed as combinations of long and short forms, but it is unclear whether OPA1 functions are associated with specific isoforms and/or domains. To address this, we expressed each of the eight isoforms or different constructs of isoform 1 in Opa1^-/- MEFs. We observed that any isoform could restore cristae structure, mtDNA abundance, and energetic efficiency independently of mitochondrial network morphology. Long forms supported mitochondrial fusion; short forms were better able to restore energetic efficiency. The complete rescue of mitochondrial network morphology required a balance of long and short forms of at least two isoforms, as shown by combinatorial isoform silencing and co-expression experiments. Thus, multiple OPA1 isoforms are required for mitochondrial dynamics, while any single isoform can support all other functions. These findings will be useful in designing gene therapies for patients with OPA1 haploinsufficiency.

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.

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.

Mitochondrial dynamics, mitophagy and biogenesis in neonatal hypoxic-ischaemic brain injury

Hypoxic-ischaemic encephalopathy, resulting from asphyxia during birth, affects 2-3 in every 1000 term infants and depending on severity, brings about life-changing neurological consequences or death. This hypoxic-ischaemia (HI) results in a delayed neural energy failure during which the majority of brain injury occurs. Currently, there are limited treatment options and additional therapies are urgently required. Mitochondrial dysfunction acts as a focal point in injury development in the immature brain. Not only do mitochondria become permeabilised, but recent findings implicate perturbations in mitochondrial dynamics (fission, fusion), mitophagy and biogenesis. Mitoprotective therapies may therefore offer a new avenue of intervention for babies who suffer lifelong disabilities due to birth asphyxia.

Autophagy controls the pathogenicity of OPA1 mutations in dominant optic atrophy

Optic Atrophy 1 (OPA1) gene mutations cause diseases ranging from isolated dominant optic atrophy (DOA) to various multisystemic disorders. OPA1, a large GTPase belonging to the dynamin family, is involved in mitochondrial network dynamics. The majority of OPA1 mutations encodes truncated forms of the protein and causes DOA through haploinsufficiency, whereas missense OPA1 mutations are predicted to cause disease through deleterious dominant-negative mechanisms. We used 3D imaging and biochemical analysis to explore autophagy and mitophagy in fibroblasts from seven patients harbouring OPA1 mutations. We report new genotype-phenotype correlations between various types of OPA1 mutation and mitophagy. Fibroblasts bearing dominant-negative OPA1 mutations showed increased autophagy and mitophagy in response to uncoupled oxidative phosphorylation. In contrast, OPA1 haploinsufficiency was correlated with a substantial reduction in mitochondrial turnover and autophagy, unless subjected to experimental mitochondrial injury. Our results indicate distinct alterations of mitochondrial physiology and turnover in cells with OPA1 mutations, suggesting that the level and profile of OPA1 may regulate the rate of mitophagy.

ELABELA and an ELABELA Fragment Protect against AKI

Renal ischemia-reperfusion (I/R) injury is the most common cause of AKI, which associates with high mortality and has no effective therapy. ELABELA (ELA) is a newly identified 32-residue hormone peptide highly expressed in adult kidney. To investigate whether ELA has protective effects on renal I/R injury, we administered the mature peptide (ELA32) or the 11-residue furin-cleaved fragment (ELA11) to hypoxia-reperfusion (H/R)-injured or adriamycin-treated renal tubular cells in vitro ELA32 and ELA11 significantly inhibited the elevation of the DNA damage response, apoptosis, and inflammation in H/R-injured renal tubular cells and suppressed adriamycin-induced DNA damage response. Similarly, overexpression of ELA32 or ELA11 significantly inhibited H/R-induced cell death, DNA damage response, and inflammation. Notably, treatment of mice with ELA32 or ELA11 but not an ELA11 mutant with a cysteine to alanine substitution at the N terminus (AE11C) inhibited I/R injury-induced renal fibrosis, inflammation, apoptosis, and the DNA damage response and markedly reduced the renal tubular lesions and renal dysfunction. Together, our results suggest that ELA32 and ELA11 may be therapeutic candidates for treating AKI.

Histone HIST1H1C/H1.2 regulates autophagy in the development of diabetic retinopathy

Autophagy plays critical and complex roles in many human diseases, including diabetes and its complications. However, the role of autophagy in the development of diabetic retinopathy remains uncertain. Core histone modifications have been reported involved in the development of diabetic retinopathy, but little is known about the histone variants. Here, we observed increased autophagy and histone HIST1H1C/H1.2, an important variant of the linker histone H1, in the retinas of type 1 diabetic rodents. Overexpression of histone HIST1H1C upregulates SIRT1 and HDAC1 to maintain the deacetylation status of H4K16, leads to upregulation of ATG proteins, then promotes autophagy in cultured retinal cell line. Histone HIST1H1C overexpression also promotes inflammation and cell toxicity in vitro. Knockdown of histone HIST1H1C reduces both the basal and stresses (including high glucose)-induced autophagy, and inhibits high glucose induced inflammation and cell toxicity. Importantly, AAV-mediated histone HIST1H1C overexpression in the retinas leads to increased autophagy, inflammation, glial activation and neuron loss, similar to the pathological changes identified in the early stage of diabetic retinopathy. Furthermore, knockdown of histone Hist1h1c by siRNA in the retinas of diabetic mice significantly attenuated the diabetes-induced autophagy, inflammation, glial activation and neuron loss. These results indicate that histone HIST1H1C may offer a novel therapeutic target for preventing diabetic retinopathy.

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

The regulation of mitochondrial dynamics by the GTPase OPA1, which is located at the inner mitochondrial membrane, is crucial for adapting mitochondrial function and preserving cellular health. OPA1 governs the delicate balance between fusion and fission in the dynamic mitochondrial network. A disturbance of this balance, often observed under stress and pathologic conditions, causes mitochondrial fragmentation and can ultimately result in cell death. As discussed in this Commentary, these morphological changes are regulated by proteolytic processing of OPA1 by the inner-membrane peptidases YME1L (also known as YME1L1) and OMA1. Long, membrane-bound forms of OPA1 are required for mitochondrial fusion, but their processing to short, soluble forms limits fusion and can facilitate mitochondrial fission. Excessive OPA1 processing by the stress-activated protease OMA1 promotes mitochondrial fragmentation and, if persistent, triggers cell death and tissue degeneration in vivo The prevention of OMA1-mediated OPA1 processing and mitochondrial fragmentation might thus offer exciting therapeutic potential for human diseases associated with mitochondrial dysfunction.

Loss of functional OPA1 unbalances redox state: implications in dominant optic atrophy pathogenesis

Objective

OPA1 mutations cause protein haploinsufficiency leading to dominant optic atrophy (DOA), an incurable retinopathy with variable severity. Up to 20% of patients also develop extraocular neurological complications. The mechanisms that cause this optic atrophy or its syndromic forms are still unknown. After identifying oxidative stress in a mouse model of the pathology, we sought to determine the consequences of OPA1 dysfunction on redox homeostasis.

Methods

Mitochondrial respiration, reactive oxygen species levels, antioxidant defenses, and cell death were characterized by biochemical and in situ approaches in both in vitro and in vivo models of OPA1 haploinsufficiency.

Results

A decrease in aconitase activity suggesting an increase in reactive oxygene species and an induction of antioxidant defenses was observed in cortices of a murine model as well as in OPA1 downregulated cortical neurons. This increase is associated with a decline in mitochondrial respiration in vitro. Upon exogenous oxidative stress, OPA1‐depleted neurons did not further exhibit upregulated antioxidant defenses but were more sensitive to cell death. Finally, low levels of antioxidant enzymes were found in fibroblasts from patients supporting their role as modifier factors.

Interpretation

Our study suggests that the pro‐oxidative state induced by OPA1 loss may contribute to DOA pathogenesis and that differences in antioxidant defenses can explain the variability in expressivity. Furthermore, antioxidants may be used as therapy as they could prevent or delay DOA symptoms in patients.