Mfn2 Overexpression Attenuates Cardio-Cerebrovascular Ischemia-Reperfusion Injury Through Mitochondrial Fusion and Activation of the AMPK/Sirt3 Signaling.

Mitochondria are potential targets for the treatment of cardio-cerebrovascular ischemia-reperfusion (I/R) injury. However, the role of the mitofusin 2 (Mfn2) protein in regulating mitochondrial fusion and cell survival has not been investigated. In the present study, an adenovirus-mediated Mfn2 overexpression assay was performed to understand the effects of Mfn2 on mitochondrial function and cell damage during cardio-cerebrovascular I/R injury. After exposure to I/R injury in vitro, the transcription and expression of Mfn2 were significantly downregulated, which correlated with decreased cell viability and increased apoptosis. By contrast, overexpression of Mfn2 significantly repressed I/R-mediated cell death through modulation of glucose metabolism and oxidative stress. Furthermore, Mfn2 overexpression improved mitochondrial fusion in cells, an effect that was followed by increased mitochondrial membrane potential, improved mitophagy, and inhibition of mitochondria-mediated apoptosis. Our data also demonstrated that Mfn2 overexpression was associated with activation of the AMPK/Sirt3 signaling pathway. Inhibition of the AMPK/Sirt3 pathway abolished the protective effects of Mfn2 on I/R-induced cell injury arising from mitochondrial damage. Our results indicate that Mfn2 protects against cardio-cerebrovascular I/R injury by augmenting mitochondrial fusion and activating the AMPK/Sirt3 signaling pathway.

Introduction

Cardio-cerebrovascular ischemia–reperfusion (I/R) injury primarily occurs in patients with ischemic stroke (Chen et al., 2013). It is initiated during cardio-cerebrovascular ischemia and aggravated during reperfusion (Zhang Y. et al., 2019). Stroke is currently the second leading cause of death worldwide (Gao et al., 2019). More than half of patients with ischemic stroke will suffer cardio-cerebrovascular I/R injury after reperfusion (Chen et al., 2020). Therefore, it is necessary to determine the molecular mechanisms underlying cardio-cerebrovascular I/R injury and develop new approaches to reduce I/R injury in ischemic stroke patients.

At the molecular level, cardio-cerebrovascular I/R injury causes cell death induced by excessive oxidative stress, calcium overload, and energy depletion (Lee et al., 2019; Zhai et al., 2019; Xu et al., 2020). Several studies have explored the pathological alterations of cardio-cerebrovascular I/R injury. They found that reperfusion causes reactive oxygen species (ROS) overproduction at the mitochondrial respiration complex (Wei et al., 2019). Excessive ROS induce mitochondrial membrane damage and repress mitochondria-mediated ATP generation (Xiang et al., 2020), leading to an energy crisis. The resultant decrease in the ATP supply suppresses the ability of the endoplasmic reticulum to recycle calcium, promoting baseline calcium overload (Yang W. T. et al., 2019). This effect is more severe in cardio-cerebrovascular blood vessels because intracellular calcium overload is associated with endothelial spasm and vascular restriction, resulting in reduced blood flow to brain/heart tissues. The above factors may increase the susceptibility of the cell to reperfusion and augment cell death through apoptosis or necrosis (Kanazawa et al., 2019). Given these pathological alterations, the therapeutic efficiency of anti-oxidative compounds or calcium channel activators/antagonists has been evaluated in preclinical studies (Zhang X. et al., 2019; Zheng et al., 2019; Yin et al., 2020). These studies indicate that mitochondrial dysfunction triggers cardio-cerebrovascular I/R injury.

Recently, changes in mitochondrial dynamics have been cited as an early sign of mitochondrial dysfunction (Wang et al., 2020b). Mitochondrial dynamics include mitochondrial fission and fusion (Delmotte and Sieck, 2019; Yu et al., 2020). Physiological fission increases mitochondrial mass, whereas mitochondrial fusion is utilized to reduce the accumulation of damaged mitochondria (Lee et al., 2020). Abnormal mitochondrial fission is considered to be a primary factor in mediating mitochondrial damage through induction of mitochondrial respiration inhibition. However, the role of mitochondrial fusion in brain/heart protection has not been validated. At the molecular level, mitochondrial fusion is regulated by mitofusin 2 (Mfn2) (Dorn, 2020), a GTPase localized on the outer membrane mitochondria. Mfn2-mediated cell protection has been described in I/R injury to multiple organs, including the heart, liver, and kidney (Delmotte and Sieck, 2019; Wang et al., 2020a). In the present study, we investigate whether Mfn2 activates mitochondrial fusion and attenuates cardio-cerebrovascular I/R injury.

Materials and Methods

Cell Culture and Treatment

The N2a cell line was purchased from the American Type Culture Collection (ATCC, Rockville, MD, United States). Dulbecco’s modified Eagle’s medium (DMEM)-F12 medium (Thermo Fisher Scientific, Shanghai, China) containing 10% fetal bovine serum (FBS) and 1% penicillin and streptomycin were used for cell culture. When the cells were 80% confluent, 0.25% trypsin (Thermo Fisher HyClone, Utah, United States) was used for cell trypsinization and passage (Bao et al., 2018). Cells in the logarithmic growth phase were selected for later use. To stimulate I/R injury in vitro, cells were subjected to 3 h of hypoxia and 3 h of reoxygenation, according to a previous study (Zhao et al., 2018).

Cell Counting Kit-8 Assay

Cells in the logarithmic growth phase were trypsinized and adjusted to 2 × 103/ml, then inoculated into 96-well plates with 100 μl of cell suspension per well. Afterward, the 96-well plates were placed in an incubator for culture. After 24 h, each well was supplied with 10 μl of cell counting kit-8 (CCK8) liquor (Hubei Bios Biotechnology Co., Ltd.), and the cells were incubated for 1 h. A microplate reader was then used to measure the absorbance [optical density (OD) value] of the 96-well plates at a 450-nm wavelength. OD values of the cells were measured to evaluate cell viability (Zhou et al., 2018c).

Mitochondrial ROS Staining and Mitochondrial Membrane Potential Staining

Mitochondrial ROS (mtROS) were measured using a MitoSOX red mitocondrial superoxide indicator (Molecular Probes, United States) (Zhou et al., 2018a). Mitochondrial membrane potential was determined using a mitochondrial membrane potential assay kit with JC-1 (Beyotime, China, Cat. No. C2006). In brief, cells were subjected to mimic I/R (mI/R) injury and then washed with phosphate buffered saline (PBS). Subsequently, MitoSOX red mitocondrial superoxide indicator (5 μg/ml) and JC-1 (2 μg/ml) were added into the medium. After washing with PBS three times, cells were stained with 4′,6-diamidino-2-phenylindole (DAPI). Samples were observed under an epifluorescence microscope (DMR; Leica) using a 63× objective. Only brightness and contrast were adjusted in figures. For comparison, different sample images of the same antigen were acquired under constant acquisition settings (Guo et al., 2019).

Protein Extraction and Immunoblotting

Protein extracts were obtained from cultured cells with radioimmunoprecipitation assay (RIPA) buffer [20 mM Tris–HCl, pH 7.5, 150 mM NaCl, 1 mM EDTA, 1% NP-40, 1% sodium dodecyl sulfate (SDS), Complete Protease Inhibitor Cocktail Tablets (Roche)]. For analyzing phosphorylated proteins, PhosSTOP EASYpack (Roche) was added to the lysis buffer. Protein extracts were then mixed with SDS sample buffer. Immunoblotting was performed, and peroxidase activities were quantified using ImageJ software. β-actin or glyceraldehyde 3-phosphate dehydrogenase (GAPDH) levels were analyzed as loading controls. Results shown are representative of three to five independent experiments.

ELISA

Superoxide dismutase (SOD), glutathione peroxidase (GPX), and glutathione (GSH) activities were visualized using the enhanced chemiluminescence (ECL) kits (Bionote, Gyeonggi-do, Korea). A lactate dehydrogenase (LDH) release assay was performed using an LDH Cytotoxicity Assay Kit (Beyotime, China, Cat. No. C0016). ELISA was conducted according to the manufacturer’s instructions (Lan et al., 2019).

Quantitative RT-PCR

RNA isolation and reverse transcription (RT) were performed as previously described. PCR measurements were performed in triplicate using a Maxima SYBR Green quantitative PCR (qPCR) Master Mix (Thermo Fisher Scientific, Waltham, MA, United States) according to the manufacturer’s instructions, with amplification being performed using the Applied Biosystems QuantStudio Real-Time PCR System (Thermo Fisher Scientific). Cycle threshold (Ct) values were normalized with 18S RNA levels (ΔCt), and relative mRNA expression levels were calculated using the formula 2−ΔΔCt.

Immunofluorescence Microscopy

Immunofluorescence microscopy staining was performed using standard technique, as previously described (Luo et al., 2019). Briefly, cells were seeded on 0.5% cross-linked gelatin. Cells were fixed and permeabilized in ice-cold methanol at −20°C for 5 min. Fixed cells were incubated for 30 min in a blocking solution (PBS containing Ca2+ and Mg2+, 2.5% skim milk, and 0.3% Triton X-100). Cells were then incubated for 1 h at room temperature with primary antibodies diluted in blocking buffer. Alternatively, cells were fixed with 1% paraformaldehyde (PAF) in triethanolamine, pH 7.5, containing 0.1% Triton X-100 and 0.1% NP-40 for 20 min. Fixed cells were incubated for 30 min in a blocking solution [Tris-buffered saline (TBS) containing 5% bovine serum albumin (BSA) and 0.3% Triton X-100]. Cells were then incubated overnight at 4°C with primary antibodies diluted in blocking buffer. Appropriate secondary antibodies were applied to cells for 45 min at RT, and cells were then mounted with VECTASHIELD containing DAPI (Vector Biolabs). Samples were observed under an epifluorescence microscope (DMR; Leica) using a 63× objective. Only brightness and contrast were adjusted in figures. For comparison, different sample images of the same antigen were acquired under constant acquisition settings (Chang L. et al., 2019).

Mitochondrial Respiration

Mitochondrial respiration (oxygen consumption and ATP production) was measured in treated cells (Zhu et al., 2019). In brief, mitochondria isolated from cells were pooled for each experiment as previously described. Mitochondrial respiration rates were assessed by measuring oxygen consumption with a Clark-type oxygen electrode at 30°C. After depletion of endogenous substrates with 100 μmol/L ADP, state 3 respiration rates were recorded in the presence of 100 μmol/L ADP. State 4 respiration rates were recorded after ADP depletion. Electron transport chain (ETC) enzyme activities were measured spectrophotometrically as specific donor–acceptor oxidoreductase activities using permeabilized mitochondria (Aluja et al., 2019).

Apoptosis

Apoptosis was evaluated by using the terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL) Apoptosis Detection Kit (Beyotime, China) as directed by the manufacturer’s instructions (Chang H. C. et al., 2019). Briefly, cells in four-chamber slides were fixed with 4% paraformaldehyde for 20 min, washed in PBS, and incubated with 75 μl of equilibration buffer for 5 min at room temperature. The cells were then incubated with 55 μl of working strength TdT enzyme for 1 h at 37°C and washed with working strength stop buffer for 10 min and then three times with PBS. The cells were incubated with anti-digoxigenin conjugate for 30 min at room temperature and washed in PBS. Slides were mounted with medium containing DAPI (Vector Laboratories, Inc., Cat no. H1200). Experiments were repeated twice.

Mitochondrial Fusion Assay

Mitochondrial fusion was investigated using the polyethylene glycol (PEG) fusion assay, as described before with modifications. In brief, cells were transfected with mitoRFP using retrovirus infection, and 25,000 cells expressing mitoRFP were co-plated on 18-mm coverslips in six-well plates. Fusion was initiated by adding 300 μl of 50% PEG 1500 (Roche, Indianapolis, IN) for 60 s, followed by the addition of 2 ml DMEM and two additional washing steps with DMEM. Cells were incubated with 1 ml cycloheximide (30 μg/ml) for 8 h, washed with PBS, and mixed with 3.7% formaldehyde. Cells were viewed on a FluoView FV1000 confocal microscope (Olympus, Tokyo, Japan) at a magnification of 63×.

Adenovirus

Adenoviruses encoding Mfn2 were generated using the pAdEasy XL adenoviral vector system (Stratagene) according to the manufacturer’s protocols. Briefly, the Mfn2 gene was subcloned into the NotI/SalI site of pShuttleIRES-hrGFP vector (Stratagene). Then, cells were transfected with Mfn2 adenovirus at an indicated multiplicity of infection (MOI) for 24 or 48 h, as previously described (Yang Q. et al., 2019). The overexpression efficiency was confirmed through Western blot.

Statistics

Normal distribution of the data was tested prior to statistical analysis. Non-parametric tests were used for data sets that were not normally distributed. Data are presented as mean ± standard error of the mean (SEM). Single comparisons were performed using unpaired Student’s t-test. Multiple comparisons were performed using one-way ANOVA, followed by Bonferroni’s post hoc test. P-values < 0.05 were considered significant and are indicated in the figures by asterisks (∗P < 0.05). Analyses were performed using SigmaPlot (version 13.0) or GraphPad Prism software (GraphPad Software Inc., Version 6.0).

Results

Mitofusin 2 Overexpression Attenuates Mimic Ischemia/Reperfusion-Mediated Cell Death by Inhibiting Apoptosis

In our study, hypoxia–reoxygenation was used to mI/R injury in vitro. Compared to control N2a cells, mI/R injury repressed the transcription (Figure 1A) and expression (Figure 1B) of Mfn2. To understand the role of Mfn2 in cardio-cerebrovascular I/R injury, adenovirus-mediated Mfn2 overexpression assay was performed. Subsequently, cell viability was measured through CCK8 assay. As shown in Figure 1C, compared to the control group, mI/R injury significantly repressed N2a cell viability. This alteration was improved by Mfn2 overexpression. We also found that Mfn2 adenovirus transfection significantly reduced the LDH release from N2a cells into the medium in the presence of mI/R injury (Figure 1D), suggesting that mI/R-mediated N2a cell death is attenuated by Mfn2 overexpression.

FIGURE 1

Mitofusin 2 (Mfn2) overexpression attenuates mitochondrial ischemia–reperfusion (mI/R)-mediated neural death through inhibition of apoptosis. (A) Hypoxia–reoxygenation was used to mimic I/R (mI/R) injury in vitro. Then, RNA was isolated to analyze the transcription of Mfn2 in N2a cells. (B) Proteins were isolated from mI/R-treated N2a cells, and the expression of Mfn2 was determined through Western blots. (C) Cell viability was measured through cell counting kit-8 (CCK8) assay. (D) A lactate dehydrogenase (LDH) release assay was used to evaluate cell death. (E) ELISA for caspase-3 activity. (F,G) Terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL) staining was used to evaluate cell apoptosis. ∗P < 0.05.

According to previous studies, cardio-cerebrovascular I/R injury is characterized by cell apoptosis that can be reversed by pharmacological intervention. To understand whether Mfn2 interrupts apoptosis in the presence of mI/R injury, caspase-3 activity was determined. As shown in Figure 1E, compared to the control group, mI/R injury elevated the activity of caspase-3, a marker of cell apoptosis. In contrast, cells transfected with Mfn2 adenovirus showed a decrease in caspase-3 activity, suggesting that apoptosis could be inhibited by Mfn2 overexpression in mI/R-treated N2a cells. TUNEL staining also revealed an increase in the number of TUNEL-positive, mI/R-treated N2a cells (Figures 1F,G); however, this alteration was inhibited by Mfn2 overexpression. Together, our results indicate that Mfn2 overexpression reduces mI/R-mediated cell death by inhibiting apoptosis.

Mitofusin 2 Overexpression Promotes Energy Metabolism and Reduces Oxidative Stress

Energy crisis and oxidative stress, which have been observed in reperfused brain tissue, are the main upstream signals that induce cell apoptosis in cardio-cerebrovascular I/R injury. Thus, we wanted to determine whether Mfn2 overexpression regulates energy metabolism and oxidative stress in mI/R-treated N2a cells. We found that compared to the control group, ATP generation was reduced in mI/R-treated N2a cells, whereas Mfn2 overexpression promoted ATP production (Figure 2A). We also found a significant increase in the amount of glucose remaining in the medium in response to mI/R injury (Figure 2B). However, cells transfected with Mfn2 adenovirus had less glucose in the medium (Figure 2B). These findings suggest that Mfn2 overexpression promotes glucose consumption.

FIGURE 2

Mitofusin 2 (Mfn2) overexpression promotes energy metabolism and reduces oxidative stress. (A) ATP production was determined through ELISA. (B) The levels of glucose in the medium were measured through ELISA. (C,D) Immunofluorescence assay for mitochondrial reactive oxygen species (mtROS). (E–G) ELISAs for the activities of anti-oxidative factors such as glutathione (GSH), superoxide dismutase (SOD), and glutathione peroxidase (GPX). ∗P < 0.05.

At the molecular level, oxidative stress is primarily regulated by mtROS. Immunofluorescence was used to quantify the levels of mtROS in N2a cells. Compared to the control group, mI/R injury increased mtROS generation, whereas Mfn2 overexpression prevented mtROS accumulation in N2a cells (Figures 2C,D). In addition, we found that the activities of mitochondrial anti-oxidative factors (including SOD, GPX, and GSH) were significantly reduced in response to mI/R injury (Figures 2E–G). Interestingly, Mfn2 overexpression could normalize the levels of SOD, GSH, and GPX in N2a cells under mI/R injury. These results indicate that Mfn2 overexpression improves energy metabolism and represses oxidative stress in N2a cells in the presence of mI/R injury.

Overexpression of Mitofusin 2 Sustains Mitochondrial Function

Considering the necessary role played by mitochondrial damage in inducing cardio-cerebrovascular I/R injury, we asked whether Mfn2 overexpression could protect mitochondrial against I/R injury. We found that after exposure to mI/R injury, mitochondrial membrane potential was reduced (Figures 3A,B). Interestingly, Mfn2 overexpression sustained mitochondrial potential (Figures 3A,B). Mitophagy, as a mitochondrial repairing system, was also inhibited by mI/R injury, whereas Mfn2 overexpression reversed mitophagy activity as evidenced by increased levels of Parkin, Atg5, and Beclin1 (Figures 3C,E). Mitochondria-initiated death has been found to play a role in inducing cell apoptosis in the I/R-injured brain/heart. ELISA demonstrated that caspase-9 activity was augmented by mI/R injury, whereas Mfn2 overexpression prevented caspase-9 activation (Figure 3F). We also found that the mitochondrial permeability transition pore (mPTP) opening rate, a marker of mitochondrial apoptosis, was increased (Figure 3G). However, in cells transfected with Mfn2 adenovirus, mPTP opening was inhibited (Figure 3G), suggesting that mitochondrial apoptosis could be repressed by Mfn2. These results indicate that Mfn2 overexpression sustains mitochondrial fusion in the presence of mI/R injury.

FIGURE 3

Overexpression of mitofusin 2 (Mfn2) sustains mitochondrial function. (A,B) Mitochondrial membrane potential was observed using a JC-1 probe. (C–E) RNA was isolated from cells, and the transcription of Atg5, Parkin, and Beclin1 was determined through quantitative PCR (qPCR). (F) ELISA for caspase-9 activity. (G) The mitochondrial permeability transition pore (mPTP) opening rate was determined through ELISA. ∗P < 0.05.

Mitofusin 2 Is Required for Mitochondrial Fusion Activation

To understand whether Mfn2 corrected mitochondrial fusion, we observed the morphological alterations of mitochondria under mI/R injury. As shown in Figures 4A–C, compared to the control group, mI/R injury promoted mitochondrial fragmentation, a result that may be caused by decreased mitochondrial fusion. qPCR analysis demonstrated that the transcription of mitochondrial fusion-related genes, such as Mfn1 and Opa1, was significantly downregulated in mI/R-treated N2a cells (Figures 4D,E). Interestingly, Mfn2 overexpression inhibited mitochondrial fragmentation (Figures 4A–C), and this effect was accompanied by an increase in the expression of Mfn1 and Opa1 (Figures 4D,E). Our results indicate that Mfn2 is involved in mitochondrial fusion in mI/R-treated N2a cells.

FIGURE 4

Mitofusin 2 (Mfn2) is required for mitochondrial fusion activation. (A–C) Electron microscope for mitochondrial morphology. (D,E) RNA was isolated from cells, and the transcription of Mfn1 and Opa1 was determined through quantitative PCR (qPCR). (F–H) Immunofluorescence assay for AMPK and Sirt3 in cells. ∗P < 0.05.

According to our previous studies (Huang et al., 2019), the AMPK/Sirt3 pathway is a protective signal in neural inflammation disease caused by modulation of mitochondrial function. We want to determine whether the AMPK/Sirt3 pathway functions downstream of Mfn2-mediated mitochondrial fusion in mI/R-treated N2a cells. Immunofluorescence assay demonstrated that both AMPK and Sirt3 were significantly repressed by mI/R injury in N2a cells (Figures 4F–H). However, in cells transfected with Mfn2 adenovirus, the expression of AMPK and Sirt3 was significantly elevated (Figures 4F–H), suggesting that Mfn2-mediated mitochondrial fusion activates the AMPK/Sirt3 signaling pathway.

Inhibition of the AMPK/Sirt3 Signaling Pathway Abolishes Mitofusin 2-Mediated Protection

To understand whether Mfn2 mediates mitochondrial protection through the AMPK/Sirt3 signaling pathway, compound c (CC) was used to inhibit the activation of AMPK. Cell viability and mitochondrial function were then measured. As shown in Figure 5A, although Mfn2 overexpression sustained N2a cell viability in the presence of mI/R injury, this pro-survival effect was nullified by CC administration. LDH release assay also demonstrated that Mfn2 overexpression prevented mI/R-mediated LDH release in N2a cells, whereas this effect was dependent on the AMPK/Sirt3 signaling pathway (Figure 5B). mtROS production was normalized in N2a cells transfected with Mfn2 adenovirus; however, this effect was abolished by CC (Figures 5C,D). mI/R-mediated caspase-9 activation could be inhibited by Mfn2 overexpression, but this action was completely abrogated by CC (Figures 5C,D). Our results indicate that Mfn2 protects N2a cells and mitochondria against mI/R injury through activation of the AMPK/Sirt3 signaling pathway.

FIGURE 5

Inhibition of the AMPK/Sirt3 signaling pathway abolishes mitofusin 2 (Mfn2)-mediated protection. (A) Cell viability was determined through a cell counting kit-8 (CCK8) assay. (B) Lactate dehydrogenase (LDH) release into the medium was measured through ELISA. (C,D) Immunofluorescence assay for mitochondrial reactive oxygen species (mtROS). ∗P < 0.05.

Discussion

In the present study, we described a novel role played by Mfn2 in regulating mitochondrial homeostasis after cardio-cerebrovascular I/R injury. We found that Mfn2 expression was reduced under conditions of I/R stress. Decreased Mfn2 correlated with a loss of cell viability in N2a cells due to apoptosis activation. In contrast, overexpression of Mfn2 significantly attenuated I/R-mediated death in N2a cells through normalization of energy metabolism and neutralization of oxidative stress. Furthermore, Mfn2 overexpression improved mitochondrial membrane potential, enhanced mitophagy, and abolished mitochondrial apoptosis. Mechanistically, Mfn2 overexpression augmented mitochondrial fusion and thus activated the AMPK/Sirt3 signaling pathway. Inhibition of the AMPK/Sirt3 signaling pathway abolished Mfn2-mediated protection on I/R-treated N2a cells through induction of mitochondrial damage. This finding defines a Mfn2/AMPK/Sirt3 axis as a new regulator of mitochondrial homeostasis and cell survival in the context of cardio-cerebrovascular I/R injury (Chen et al., 2019; He et al., 2019). Several protective approaches have been developed to regulate mitochondrial function and attenuate brain/heart damage after I/R injury. For example, regulation of the mitochondrial Kv1.3 channel effectively reduces the inflammation response and therefore alleviates I/R injury (Ma et al., 2020). Plasma exosomes seem to repress mtROS generation and attenuate I/R-mediated oxidative stress damage (Jiang et al., 2020). Improvement of mitochondrial biogenesis and promotion of glucose metabolism through activation of Rac2 has been found to sustain brain function after I/R injury (Xia et al., 2020). Parkin-dependent mitophagy is significantly depressed in reperfused brain, and activation of mitophagy sends pro-survival signals to neurons in the context of I/R injury (He et al., 2019). Although extracellular signal-regulated kinase (ERK) has been found to be a regulator of cancer proliferation, inhibition of the ERK signaling pathway through administration of PD98059 significantly protects the brain against mitochondria-mediated apoptosis in I/R injury (Zheng et al., 2019). In accordance with these findings, our data show that Mfn2 is a protective factor that sustains mitochondrial function. Improved mitochondrial homeostasis correlates with an elevated survival rate in cells under I/R injury.

Mitochondrial fusion is a process involving two mitochondria communicating or cooperating with each other. In contrast to mitochondrial fission (Zhou et al., 2018b; Wang et al., 2020c), fusion is associated with a drop in the number of mitochondria but an increase in the quality of mitochondria. It must take place before damaged mitochondria can be repaired by a healthy mitochondrion (Kang, 2020; Wang et al., 2020, 2020d). Fusion is also necessary for the exchange of metabolic substrates between two mitochondria. Thus, mitochondrial fusion has been regarded as a protective mechanism to sustain mitochondrial function and structure. This idea has been validated in various disease models, including models of cardiac I/R injury, fatty liver disease, acute kidney damage, and obesity (Wang et al., 2019; Whitley et al., 2019; Zeng et al., 2019). In the present study, we found that mitochondrial fusion sustained mitochondrial membrane potential, reduced oxidative stress, promoted glucose metabolism, and improved mitophagy. These effects may work together to block mitochondria-mediated apoptosis through the inhibition of caspase-9 activation and mPTP opening. Based on our results, mitochondrial fusion appears to have an antiapoptotic action through normalization of mitochondrial function.

We described the AMPK/Sirt3 signaling pathway in our previous study of a model of neural inflammation disease (Huang et al., 2019). Both AMPK and Sirt3 were markedly downregulated under inflammatory conditions. By contrast, the activated AMPK/Sirt3 signaling pathway abolishes inflammation-mediated mitochondrial damage and promotes neural cell survival. This finding has been validated in other studies. For example, activation of the AMPK/Sirt3 pathway attenuates mtROS overproduction in vascular endothelium through augmentation of SOD activity (Han et al., 2020). Live fibrosis was attenuated by celastrol through the AMPK/Sirt3 signaling pathway (Wang et al., 2020). Angiotensin II-induced cardiomyocyte hypertrophy is linked to a drop in the activity of the AMPK/Sirt3 signaling pathway (Xiong et al., 2019). Diabetic nephropathy progression could be interrupted by an inhibitor of the AMPK/Sirt3 signaling pathway (Liu et al., 2019). In the present study, we identified Mfn2-mediated mitochondrial fusion upstream of the AMPK/Sirt3 signaling pathway, although the underlying mechanism is not yet fully understood.

In summary, we observed that cardio-cerebrovascular I/R injury is associated with a drop in Mfn2 expression. Overexpression of Mfn2 significantly reduces I/R-mediated mitochondrial damage by augmenting glucose metabolism, inhibiting oxidative stress, improving mitophagy, and repressing mitochondrial apoptosis in a manner dependent on the AMPK/Sirt3 signaling pathway.

Data Availability Statement

All datasets presented in this study are included in the article/supplementary material.

Author Contributions

DH and ML designed and performed parts of the experiments. ML and XL collected all the data and prepared the figures. XL and DH wrote the manuscript. All the authors approved this submission. All authors contributed to the article and approved the submitted version.

Conflict of Interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.