Notch1 protects against myocardial ischaemia-reperfusion injury via regulating mitochondrial fusion and function.

Mitochondrial fusion and fission dynamic are critical to the myocardial protection against ischaemia-reperfusion injury. Notch1 signalling plays an important role in heart development, maturation and repair. However, the role of Notch1 in the myocardial mitochondrial fusion and fission dynamic remains elusive. Here, we isolated myocardial cells from rats and established myocardial ischaemia-reperfusion injury (IRI) model. We modulated Notch1, MFN1 and DRP1 expression levels in myocardial cells via infection with recombinant adenoviruses. The results showed that Notch1 improves the cell viability and mitochondrial fusion in myocardiocytes exposed to IRI. These improvements were dependent on the regulation of MFN1 and DRP1. On the mechanism, we found that MNF1 is transcriptionally activated by RBP-Jk in myocardiocytes. Notch1 also improves the mitochondrial membrane potential in myocardiocytes exposed to IRI. Moreover, we further confirmed the protection of the Notch1-MFN1/Drp1 axis on the post-ischaemic recovery of myocardial performance is associated with the preservation of the mitochondrial structure. In conclusion, this study presented a detailed mechanism by which Notch1 signalling improves mitochondrial fusion during myocardial protection.

INTRODUCTION

Acute myocardial infarction (MI) is the leading cause of death and disability worldwide.1 In patients with myocardial infarction, the treatment options for reducing acute myocardial ischaemic injury and limiting the size of myocardial infarction are timely and effective myocardial reperfusion.2, 3 However, the process of myocardial reperfusion can itself induce further cardiomyocyte death, which is known as myocardial reperfusion injury (IRI).4, 5, 6 Mitochondria are the power centre of cells, which determine whether cells survive or not.7 A large number of studies have proved that various signal pathways of myocardial protection ultimately target mitochondria.8, 9, 10, 11 Therefore, it is of great significance to explore the molecular mechanism of myocardial protection based on mitochondria.

The Notch signalling pathway is taken as an information exchange platform between neighbouring cells.12, 13 After combining the ligand of Jagged/Delta, it releases the intracellular structure domain (Notch intracellular domain, NICD) into the nucleus, and targets to the recombination signal binding protein for immunoglobulin J kappa (RBP‐Jk) transcription factors, subsequently activates the gene transcription of Hes, HRT, etc14, 15, 16 Notch signalling pathway is critical to the cell fate, development, differentiation, cell proliferation, apoptosis, adhesion and epithelial to mesenchymal transformation (EMT).17, 18

The role of Notch signalling pathway was supposed to protect the heart against the myocardial IRI.17, 19, 20 After myocardial ischaemia, key molecules of Notch signalling pathway are activated to induce angiogenesis and improve myocardial blood supply.21, 22 It also promotes the proliferation of myocardial cells to rescue the dying myocardium.23, 24 We previously reported that Notch1 acts as an endogenous myocardial protective factor through the RISK/SAFE/HIF‐1 alpha signalling, which reduces myocardial intracellular reactive oxygen species (ROS), enhance the cell vitality of myocardiocytes and significantly reduce the myocardial IRI.25 Recently, we found that Notch1 modulated the dynamic balance between mitochondrial fusion/fission and mitophagy in myocardial cells exposed to IRI and provided protection for myocardial cells.26 However, the underlying mechanism of Notch signalling on myocardial mitochondrial fusion and fission is not well illustrated.

In this study, we aimed to investigate the role and the underlying mechanism of Notch signalling in the regulation of myocardial mitochondrial fusion and fission. We found that Notch1 modulated the dynamic balance between mitochondrial fusion and fission via RBP‐Jk dependent transcriptional activation of Mfn1 and Drp1 in myocardial cells exposed to IRI and provided protection for myocardial mitochondrial.

MATERIALS AND METHOD

Reagents and antibodies

Dulbecco's modified Eagle's medium (DMEM), foetal bovine serum (FBS) and trypsin were purchased from Thermo Fisher. Notch1, Mfn1, Mfn2, OPA1, Drp1, Fis1, Cytochrome C and COX IV antibodies were purchased from Cell Signaling Technology. GAPDH antibody was purchased from Sigma. ATP detection kit was from Beyotime Biotechnology. Enhanced chemiluminescence kit was purchased from Thermo Fisher Scientific.

Isolation and culture of neonate rat myocardiocytes

All animal experiments were performed in accordance with the guidelines of NIH and under approved protocols of the Animal Care and Use Committee of Nanchang University. Neonate rat myocardial cells were isolated and using a protocol previously described.26

Construction and infection of recombinant adenoviruses

Recombinant adenoviruses expressing Rat N1ICD/MFN1/DRP1 complementary DNA (cDNA) were prepared using the pAdEasyTM vector system (Qbiogene) as described previously.26 Primary cardiac myocytes were infected with adenoviral particles at the multiplicity of infection of 50.

Hypoxia/reoxygenation protocols

Myocardiocytes were cultured in hypoxic solution in a hypoxic incubator (95% N2/5% CO2) for 3 hours; subsequently, the hypoxic solution was replaced with a reoxygenation solution, and the cells were cultured in a high oxygen incubator (95% O2‐5% CO2) for 3 hours.

Cell viability assay

The cell viability of myocardiocytes was detected with CCK‐8 assay (Dojindo) as described previously.26 Briefly, the isolated neonate rat myocardiocytes were cultured in a 96‐well plate (5 × 103 cells/well). After 24 hours, the density of each well was evaluated at 450 nm by a microplate reader (Thermo).

Mitochondrial fusion/fission detection

The mitochondrial fusion/fission detection of myocardiocytes was detected as described previously.26 Briefly, the isolated neonate rat myocardiocytes were labelled with MitoTracker Red (50 nmol/L, 20 minutes) and analysed by Zeiss LSM800 confocal microscopy. Fragmented mitochondria were characterized as punctate, shortened and rounded. Filamentous mitochondria presented a thread‐like tubular structure. They were statistically classified based on the morphology of the majority (70%) of mitochondria.

Flow cytometry analysis of apoptosis

The apoptosis was evaluated by FTIC‐Annexin V/PI kit as previously reported.25 Briefly, the isolated neonate rat myocardiocytes were suspended (1 × 106 cells/mL) and incubated with Annexin V‐fluorescein isothiocyanate/PI (BD Biosciences) for 15 minutes. After incubation, the apoptotic myocardiocytes were evaluated by a flow cytometer (FACSCalibur, BD Biosciences) and analysed by CellQuest software version 3.3.

Western blot analysis

Myocardiocytes were lysed in cell lysis buffer (Beyotime Institute of Biotechnology) at 4°C. Protein samples were separated by 8%‐10% SDS‐PAGE, then transferred to nitrocellulose membranes (Millipore). Membranes were incubated with primary antibodies overnight at 4°C, followed by incubation with secondary antibodies at room temperature for 1 hour The fluorescent signals were detected using enhanced chemiluminescence by ImageQuant LAS4000 (GE).

I/R injury model in Langendorff‐perfused rat hearts

After rats were anesthetized with sodium pentobarbital (45 mg/kg i.p.), the hearts were rapidly excised and perfused with Krebs‐Henseleit (K‐H) solution at 37°C using a Langendorff apparatus at a constant pressure of 80 mm Hg as previously described. A water‐filled latex balloon connected to a pressure transducer (Gould P23Db, AD Instrument) was inserted into the LV cavity to achieve a stable LVEDP of 5‐10 mm Hg during initial equilibration. After equilibration perfusion, the heart was subjected to 30 minutes of global no‐flow ischaemia followed by 45 minutes of reperfusion. LVDP and ±dp/dt max were evaluated with the PowerLab system (AD Instrument).

In vivo adenoviral gene delivery

The surgical procedures and adenoviral delivery were carried out as described.26

Electron microscopy

Tissues were fixed in 2.5% glutaraldehyde solution. The samples were then processed following standard protocol, including dehydration, embedding and sectioning, and then examined and photographed under a Hitachi 7500 transmission electron microscope (Hitachi).

Statistical analysis

All data were presented as a mean ± standard deviation and analysed by SPSS 11.0 statistical software. Differences among various treatment groups were compared by one‐way analysis of variance. Differences were considered significant at P < .05.

RESULTS

Notch1 improves the cell viability and mitochondrial fusion in myocardiocytes exposed to IRI

First, we confirmed that Ad‐shN1ICD decreased while Ad‐N1ICD increased the expression of N1ICD in myocardiocytes with previously constructed adenoviral vectors. We found that N1ICD overexpression remarkably increased the cell viability of myocardial cells after hypoxia/reoxygenation (H/R), whereas N1ICD knockdown significantly reduced the cell viability of myocardial cells after H/R (Figure 1A). Mitochondrial fusion/fission detection showed that N1ICD overexpression remarkably increased the number of elongated mitochondria in myocardiocytes after hypoxia/reoxygenation (H/R), whereas N1ICD knockdown significantly elevated the number of fragmented mitochondria in myocardiocytes after H/R (Figure 1B). Consistently, the apoptotic cells were obviously reduced in N1ICD overexpressed myocardiocytes, which were significantly increased in N1ICD knockdown myocardiocytes (Figure 1C). Furthermore, we found the expression level of mitochondrial fusion markers (MFN1, MFN2 and OPA1) were elevated by N1ICD overexpression, whereas the mitochondrial fission markers (DRP1 and FIS1) were impaired (Figure 1D). These results indicate that Notch1 improves the cell viability and mitochondrial fusion in myocardiocytes exposed to IRI.

Figure 1

Notch1 improves the cell viability and mitochondrial fusion in myocardiocytes exposed to IRI. A, The cell viability of myocardiocytes exposed to H/R was evaluated by CCK‐8 assays. B, Confocal microscopy analysis of the mitochondria fusion and fission in myocardiocytes exposed to H/R. C, FACs analysis the apoptotic cell numbers of myocardiocytes exposed to H/R. D, The level of N1ICD and mitochondrial fusion‐fission markers (MFN1, MFN2, OPA1, DRP1 AND FIS1) were evaluated by Western blot. All data are means ± SEM *P < .05 with the control group

Notch1 improves cell viability and mitochondrial fusion through regulating MFN1

To determine the role of MFN1 in the Notch1 improved myocardiocytes viability and mitochondrial fusion, we constructed two MFN1 overexpression and knockdown adenovirus. As shown in Figure 2A, the N1ICD preserved cell viability was significantly impaired by MFN1 knockdown. However, the cell viability in the N1ICD knockdown group was increased with the MFN1 overexpression. Confocal microscopy analysis of the mitochondria showed that MFN1 knockdown obviously decreased the elongated mitochondria in the N1ICD knockdown group, while MFN1 overexpression dramatically increased the fragmented mitochondria in N1ICD knockdown group (Figure 2B). A similar tendency was observed in the apoptotic cell numbers indicated by the FAC assay (Figure 2C). The efficiency of N1ICD/MFN1 overexpression and knockdown was confirmed by Western blot (Figure 2D). We also found that the other two mitochondrial fusion markers (MFN2 and OPA1) were reduced by MFN1 knockdown, but elevated by MFN1 overexpression. However, the mitochondrial fission markers (DRP1 and FIS1) were reversely elevated by MFN1 knockdown but decreased by MFN1 overexpression (Figure 2D). These observations suggest that MFN1 was critical to the Notch1 improved cell viability and mitochondrial fusion in myocardiocytes exposed to IRI.

Figure 2

Notch1 improves cell viability and mitochondrial fusion through regulating MFN1. A, The myocardiocytes infected with indicated adenovirus were exposed to H/R and evaluated by CCK‐8 assays. B, After H/R, confocal microscopy analysis of the mitochondria fusion and fission in myocardiocytes infected with indicated adenovirus. C, FACs analysis the apoptotic cell numbers of myocardiocytes infected with indicated adenovirus. D, The level of N1ICD and mitochondrial fusion‐fission markers (MFN1, MFN2, OPA1, DRP1 AND FIS1) were evaluated by Western blot. All data are means ± SEM *P < .05 with the indicated group

DRP1 is critical to the Notch1 improved cell viability and mitochondrial fusion in myocardiocytes exposed to IRI

To further investigate the role of DRP1 in the Notch1 improved myocardiocytes viability and mitochondrial fusion, we also constructed two DRP1 overexpression and knockdown adenovirus. Conversely, with the effect of MNF1 knockdown, the cell viability in N1ICD overexpressed group was decreased with the DRP1 overexpression, whereas it was increased in both N1ICD and DRP1 knockdown group (Figure 3A). Next, confocal microscopy analysis of the mitochondria showed that DRP1 overexpression obviously decreased the elongated mitochondria in the N1ICD knockdown group, while DRP1 knockdown dramatically increased the fragmented mitochondria in N1ICD knockdown group (Figure 3B). The number of apoptotic myocardiocytes in the N1ICD knockdown or overexpressed group was seemly reversed by DRP1 overexpression or knockdown (Figure 3C). With the efficiency of N1ICD/DRP1 overexpression and knockdown was confirmed by Western blot (Figure 3D), we found that the other two mitochondrial fusion markers (MFN2 and OPA1) were reduced by DRP1 overexpression, but elevated by DRP1 knockdown. These data suggest that DRP1 was critical to the Notch1 improved cell viability and mitochondrial fusion in myocardiocytes exposed to IRI.

Figure 3

DRP1 is critical to the Notch1 improved cell viability and mitochondrial fusion in myocardiocytes exposed to IRI. A, The myocardiocytes infected with indicated adenovirus were exposed to H/R and evaluated by CCK‐8 assays. B, After H/R, confocal microscopy analysis of the mitochondria fusion and fission in myocardiocytes infected with indicated adenovirus. C, FACs analysis the apoptotic cell numbers of myocardiocytes infected with indicated adenovirus. D, The level of N1ICD and mitochondrial fusion‐fission markers (MFN1, MFN2, OPA1, DRP1, AND FIS1) were evaluated by Western blot. All data are means ± SEM *P < .05 with the indicated group

MNF1 is transcriptionally activated by RBP‐Jk

To identify the regulatory mechanism of MNF1 expression, we generated a wild‐type and mutant MNF1 promoter report vectors. The results suggested that RBP‐Jk overexpression significantly induced the promoter activity of wild‐type MNF1, but failed to induce the activity of mutant MFN1 promoter (Figure 4A). Next, chromatin immunoprecipitation assay was used to evaluate the direct interaction between RBP‐Jk and the promoter region of MFN1. The results showed that the promoter region of MFN1 was dramatically enriched in the RBP‐Jk immunoprecipitation component (Figure 4B). These results were further confirmed by ChIP‐PCR (Figure 4C) and electrophoretic mobility shift assay (Figure 4D). These results suggest that MNF1 was transcriptionally activated by RBP‐Jk.

Figure 4

MNF1 is transcriptionally activated by RBP‐Jk. A, A dual‐luciferase report assay was used to detect the effect of RBP‐Jk on the promoter activity of MNF1. B, Chromatin immunoprecipitation (ChIP) assay and ChIP‐PCR (C) were used to evaluate the direct interaction between RBP‐Jk and the promoter region of MFN1. D, Electrophoretic mobility shift assay was used to confirm the direct interaction between RBP‐Jk and the promoter region of MFN1. All data are means ± SEM *P < .05 with the indicated group

Notch1 improves the mitochondrial membrane potential in myocardiocytes exposed to IRI

Mitochondrial asymmetric fission often occurs when mitochondrial membrane potential (△Ψm) decreases due to cell damage. The damaged mitochondria with mild decreased △Ψm can be fused with other mitochondria and return to normal, but mitochondria with severely reduced △Ψm resulted in an elevated release of ROS and lead to activate the mitochondrial‐dependent apoptosis. To evaluate the protective effect of Notch1 on the mitochondrial membrane potential, FACs analysis of the JC1 labelled myocardiocytes. The results showed that N1ICD overexpression dramatically improved the △Ψm in myocardiocytes exposed to IRI (Figure 5A). However, the N1ICD knockdown did not affect the △Ψm. Moreover, the mitochondrial released cytochrome c in the cytoplasm was detected by Western blot (Figure 5B). As shown in Figure 5B, the level of cytoplasmic cytochrome c was obviously reduced in N1ICD overexpressed myocardiocytes exposed to IRI. Furthermore, it is found that the level of mitochondrial permeability transition pore subunits (ANT, Cyp‐D, PiC and VDAC) was consistently reduced in N1ICD knockdown myocardiocytes exposed to IRI (Figure 5C), which indicated the mitochondrial permeability transition pore opening and damage.

Figure 5

Notch1 improves the mitochondrial membrane potential in myocardiocytes exposed to IRI. A, The mitochondrial membrane potential was analysed by FACs of the JC1 labelled myocardiocytes. B, The level of cytochrome c in mitochondria and cytoplasm were evaluated by Western blot. C, the level of mitochondrial permeability transition pore subunits (ANT, Cyp‐D, PiC and VDAC) was detected by Western blot. All data are means ± SEM *P < .05 with the indicated group

Protection of Notch1‐MFN1/Drp1 axis on the post‐ischaemic recovery of myocardial performance is associated with the preservation of the mitochondrial structure

To confirm the cardioprotective effects of Notch1‐MFN1/Drp1 in contractile function and mitochondrial function during I/R, we analysed the post‐ischaemic contractile function and mitochondrial function in Langendorff‐perfused rat hearts. The pre‐ischaemic contractile parameters were similar between all the groups, while the I/R (30 min/45 min)‐suppressed LV contractile function, characterized by LV development pressure (LVDP), LV end‐diastolic pressure (LVEDP), and maximal speed of LV pressure development and decline (±dp/dt), was markedly alleviated by N1ICD overexpression group (Figure 6A). Notably, the pre‐ischaemic contractile parameters in N1ICD overexpression rats were impaired by MFN1 knockdown or DRP1 overexpression, whereas those in N1ICD knockdown rats were conversely improved by MFN1 overexpression or DRP1 knockdown. Consistently, the infarct size after 2 hours of reperfusion was inhibited by N1ICD overexpression (Figure 6B), similar to the groups of N1ICD knockdown but MFN1 overexpression or DRP1 knockdown (Figure 6B). The efficiency of N1ICD/MFN1/DRP1 overexpression and knockdown was confirmed by Western blot (Figure 6C). We next confirmed the other two mitochondrial fusion markers (MFN2 and OPA1) were reduced by MFN1 knockdown, but elevated by MFN1 overexpression or DRP1 knockdown. However, the mitochondrial fission marker (FIS1) in the N1ICD overexpression group was reversely elevated by MFN1 knockdown or DRP1 knockdown but decreased by MFN1 overexpression (Figure 6C). Observation of the structure of mitochondria by electron microscopy showed that lots of mitochondrial lesions in I/R hearts had marked alterations including breaks in mitochondrial crests and matrix swelling which indicates the damage of mitochondrial structure (Figure 6D). However, these damages were attenuated in the N1ICD overexpression group and the groups of N1ICD knockdown but MFN1 overexpression or DRP1 knockdown (Figure 6D). All these data suggest that the protection of the Notch1‐MFN1/Drp1 axis on the post‐ischaemic recovery of myocardial performance is associated with the preservation of mitochondrial structure and function.

Figure 6

Protection of the Notch1‐MFN1/Drp1 axis on the post‐ischaemic recovery of myocardial performance is associated with the preservation of the mitochondrial structure. A, Representative traces and summarized data of LV pressure (LVP) during ischaemia‐reperfusion (I/R) in isolated rat hearts from indicated groups. B, Representative images and analysis of the infarct size in isolated I/R (30 min/2 h) hearts. C, The mitochondrial fusion and fission markers were analysed by Western blot. D, The structure of mitochondria was observed by electron microscopy. All data are means ± SEM *P < .05, **P < .01 and ***P < .001, with the indicated group

DISCUSSIONS

The ramifications of myocardial I/R injury are widespread, imparting damage to multiple loci within the cell and its constituents. Many studies have shown that N1ICD induces cardiac protection from I/R injury,17, 23 while the underlying mechanisms are poorly understood. We previously reported that Notch1 acts as an endogenous myocardial protective factor through the RISK/SAFE/HIF‐1 alpha signalling, which reduces myocardial intracellular reactive oxygen species (ROS), enhance the cell vitality of myocardiocytes and significantly reduce the myocardial IRI.25 In this study, we found that Notch1 modulated the dynamic balance between mitochondrial fusion and fission via RBP‐Jk dependent transcriptional activation of Mfn1 and Drp1 in myocardial cells exposed to IRI.

Notch signalling was reported to regulate the expression of key proteins of mitochondrial oxidative phosphorylation and modulates the dynamic balance of mitochondrial fusion/fission.27 Mitochondrial morphology and quality are controlled by a dynamic equilibrium between fusion and fission. The major fusion proteins of mammalian mitochondria are MFN1/2 and OPA1.28, 29, 30 MFN proteins are localized to the outer mitochondrial membrane and aid the tethering of two discrete mitochondria at the early stages of fusion.30 The two major proteins involved in mammalian mitochondrial fission are Fis1 and Drp1.31 Fis1 is an adaptor protein located in the outer mitochondrial membrane, whereas Drp1 is localized in the cytosol, from where it shuttles back and forth to the outer mitochondrial membrane during fission events.32, 33 Notch was reported to up‐regulate MFN2 expression through the cascade‐up effect of NICD/Akt/Mfn and promote mitochondrial fission.26, 27 On the other hand, it down‐regulates Drp1 expression and inhibits mitochondrial fission.34 Abnormal fusion and fission caused mitochondrial disorders to contribute to myocardial IRI.11, 35 Our study further showed Notch1 can reduce mitochondrial lysis, reduce myocardial infarct size and inhibit ventricular remodelling, which plays a role in myocardial protection. This cardio‐protection was almost reversed by MFN1 knockdown or DRP1 overexpression, which indicates that the protective effect of Notch1 signalling was dependent on the MFN1/DRP1 regulated mitochondrial fusion‐fission dynamic.

In conclusion, this study presented a detailed mechanism by which Notch1 signalling improves mitochondrial quality control during myocardial protection.

CONFLICTS OF INTEREST

The authors declare that they have no conflicts of interest.

AUTHORS’ CONTRIBUTIONS

SH Dai and XL Zhou conceived of the study and participated in its design and co‐ordination. SH Dai performed all experiments. QC Wu, RR Zhu and XM Wan analysed and interpreted the data. The draft was improved through discussion and editing by all the authors, who read and approved the final manuscript.