Function and regulation of mitofusin 2 in cardiovascular physiology and pathology

Moderate mechanical strain and exercise reduce inflammation and excessive autophagy in osteoarthritis by downregulating mitofusin 2

AIMS

The major pathological mechanisms of osteoarthritis (OA) progression include inflammation, autophagy, and apoptosis, etc. Moderate mechanical strain and exercise effectively improve chondrocyte degeneration by reducing these adverse factors. Mitofusin 2 (MFN2) is a crucial regulatory factor associated with inflammation, autophagy and apoptosis, and its expression is regulated by exercise. This study aims to elucidate the effects of moderate mechanical strain and exercise on MFN2 expression and its influence on OA progression.

MAIN METHODS

Destabilization of the medial meniscus (DMM) surgery was performed on rats to induce an OA rat model. Subsequently, adeno-associated virus (overexpression/knockdown) intra-articular injection or moderate treadmill exercise was administered to evaluate the effects of these treatments on MFN2 expression and OA progression. Overexpressed plasmids and siRNA vectors were used to regulate MFN2 expression in chondrocytes. An inflammatory degeneration cell model was generated by IL-1β stimulation. Moderate mechanical strain was applied to MFN2-overexpressing cells to explore their interactions.

KEY FINDINGS

MFN2 overexpression aggravated inflammation by activating the NF-κB and P38 pathways and induced excessive autophagy by inhibiting the PI3K/AKT/mTOR pathway, thereby causing chondrocyte apoptosis and metabolic disorder. Moderate mechanical strain partially reversed these adverse effects. In the DMM rat model, MFN2 overexpression in articular cartilage exacerbated OA progression, whereas MFN2 knockdown and treadmill exercise alleviated cartilage degeneration, inflammation, and mechanical pain.

SIGNIFICANCE

MFN2 is a critical factor mediating the association between inflammation and excessive autophagy in OA progression. Moderate mechanical strain and treadmill exercise may improve OA through downregulating MFN2 expression. This study may provide a theoretical basis for exercise therapy in OA treatment.

Identification of mitochondria-related key gene and association with immune cells infiltration in intervertebral disc degeneration

Intervertebral disc (IVD) degeneration and its inflammatory microenvironment can result in discogenic pain, which has been shown to stem from the nucleus pulposus (NP). Increasing evidence suggests that mitochondrial related genes are strictly connected to cell functionality and, importantly, it can regulate cell immune activity in response to damaged associated signals. Therefore, identification of mitochondria related genes might offer new diagnostic markers and therapeutic targets for IVD degeneration. In this study, we identified key genes involved in NP tissue immune cell infiltration during IVD degeneration by bioinformatic analysis. The key modules were screened by weighted gene co-expression network analysis (WCGNA). Characteristic genes were identified by random forest analysis. Then gene set enrichment analysis (GSEA) was used to explore the signaling pathways associated with the signature genes. Subsequently, CIBERSORT was used to classify the infiltration of immune cells. Function of the hub gene was confirmed by PCR, Western blotting and ELISA. Finally, we identified MFN2 as a crucial molecule in the process of NP cell pyroptosis and NLRP3 inflammasome activation. We speculate that the increased MFN2 expression in NP tissue along with the infiltration of CD8⁺ T cells, NK cell and neutrophils play important roles in the pathogenesis of IVD degeneration.

Compositions and Functions of Mitochondria-Associated Endoplasmic Reticulum Membranes and Their Contribution to Cardioprotection by Exercise Preconditioning

Mitochondria-associated endoplasmic reticulum membranes (MAMs) are important components of intracellular signaling and contribute to the regulation of intracellular Ca²⁺/lipid homeostasis, mitochondrial dynamics, autophagy/mitophagy, apoptosis, and inflammation. Multiple studies have shown that proteins located on MAMs mediate cardioprotection. Exercise preconditioning (EP) has been shown to protect the myocardium from adverse stimuli, but these mechanisms are still being explored. Recently, a growing body of evidence points to MAMs, suggesting that exercise or EP may be involved in cardioprotection by modulating proteins on MAMs and subsequently affecting MAMs. In this review, we summarize the latest findings on MAMs, analyzing the structure and function of MAMs and the role of MAM-related proteins in cardioprotection. We focused on the possible mechanisms by which exercise or EP can modulate the involvement of MAMs in cardioprotection. We found that EP may affect MAMs by regulating changes in MFN2, MFN1, AMPK, FUNDC1, BECN1, VDAC1, GRP75, IP3R, CYPD, GSK3β, AKT, NLRP3, GRP78, and LC3, thus playing a cardioprotective role. We also provided direction for future studies that may be of interest so that more in-depth studies can be conducted to elucidate the relationship between EP and cardioprotection.

Long Non-Coding RNAs in Cardiac Hypertrophy

Heart disease represents one of the main challenges in modern medicine with insufficient treatment options. Whole genome sequencing allowed for the discovery of several classes of non-coding RNA (ncRNA) and widened our understanding of disease regulatory circuits. The intrinsic ability of long ncRNAs (lncRNAs) and circular RNAs (circRNAs) to regulate gene expression by a plethora of mechanisms make them candidates for conceptually new treatment options. However, important questions remain to be addressed before we can fully exploit the therapeutic potential of these molecules. Increasing our knowledge of their mechanisms of action and refining the approaches for modulating lncRNAs expression are just a few of the challenges we face. The accurate identification of novel lncRNAs is hampered by their relatively poor cross-species sequence conservation and their low and context-dependent expression pattern. Nevertheless, progress has been made in their annotation in recent years, while a few experimental studies have confirmed the value of lncRNAs as new mechanisms in the development of cardiac hypertrophy and other cardiovascular diseases. Here, we explore cardiac lncRNA biology and the evidence that this class of molecules has therapeutic benefit to treat cardiac hypertrophy.

Mst1 Knockout Alleviates Mitochondrial Fission and Mitigates Left Ventricular Remodeling in the Development of Diabetic Cardiomyopathy

The disruption of mitochondrial dynamics is responsible for the development of diabetic cardiomyopathy (DCM). However, the mechanisms that regulate the balance of mitochondrial fission and fusion are not well-understood. Wild-type, Mst1 transgenic and Mst1 knockout mice were induced with experimental diabetes by streptozotocin injection. In addition, primary neonatal cardiomyocytes were isolated and cultured to simulate diabetes to explore the mechanisms. Echocardiograms and hemodynamic measurements revealed that Mst1 knockout alleviated left ventricular remodeling and cardiac dysfunction in diabetic mice. Mst1 knockdown significantly decreased the number of TUNEL-positive cardiomyocytes subjected to high-glucose (HG) medium culture. Immunofluorescence study indicated that Mst1 overexpression enhanced, while Mst1 knockdown mitigated mitochondrial fission in DCM. Mst1 participated in the regulation of mitochondrial fission by upregulating the expression of Drp1, activating Drp1^S616 phosphorylation and Drp1^S637 dephosphorylation, as well as promoting Drp1 recruitment to the mitochondria. Furthermore, Drp1 knockdown abolished the effects of Mst1 on mitochondrial fission, mitochondrial membrane potential and mitochondrial dysfunction in cardiomyocytes subjected to HG treatment. These results indicated that Mst1 knockout inhibits mitochondrial fission and alleviates left ventricular remodeling thus prevents the development of DCM.

MFN2 contributes to metabolic disorders and inflammation in the aging of rat chondrocytes and osteoarthritis

OBJECTIVE

Metabolic disorders and inflammation of chondrocytes are major pathological changes in aging cells and osteoarthritis (OA). Recent studies demonstrated age-related mitochondrial dysfunction may be a key contributing factor in the development of OA. Mitofusin 2 (MFN2) is a key regulator of mitochondrial fusion, cell metabolism, autophagy and apoptosis. This study was performed to ascertain whether MFN2 was involved in the aging of chondrocytes and OA.

METHODS

Metabolic measurements were taken in rat chondrocytes between different ages (3-week, 5-month, 12-month). MFN2 activity was detected in both human and rat chondrocytes during aging and OA. Then, knockdown of MFN2 with small interfering RNA (siRNA) was performed to confirm whether MFN2 contributes to metabolic changes. Lentiviruses were used to establish MFN2-overexpression/knockdown OA models both in vivo and in vitro to confirm whether MFN2 contributes to OA progress. Further, regulatory mechanism of MFN2 was assessed and interaction between MFN2 and PARKIN was performed.

RESULTS

A metabolic shift to mitochondrial respiration was confirmed in rat chondrocytes during aging. MFN2 expression was elevated in both human and rat chondrocytes during aging and OA. Knockdown of MFN2 with siRNA reversed the age-related metabolic changes in rat chondrocytes. Overexpression of MFN2 exacerbated inflammation and OA progress, while knockdown of MFN2 ameliorated inflammation and OA progress. Further, MFN2 could be ubiquitinated by PARKIN, declined PARKIN expression during aging and OA might result in elevated MFN2 expression.

CONCLUSIONS

Elevated MFN2 contributes to metabolic changes and inflammation during aging of rat chondrocytes and osteoarthritis.

The role of mitochondrial fusion and fission in the process of cardiac oxidative stress

Mitochondria are the energy suppliers in the cell and undergo constant fusion and fission to meet metabolic demand during the cell life cycle. Well-balanced mitochondrial dynamics are extremely important and necessary for cell survival as well as for tissue homeostasis. Cardiomyocytes contain large numbers of mitochondria to satisfy the high energy demand. It has been established that deregulated processes of mitochondrial dynamics play a major role in myocardial cell death. Currently, cardiac mitochondrial cell death pathways attract great attention in the cell biology and regenerative medicine fields. Importantly, mitochondrial dynamics are tightly linked to oxidative stress-induced cardiac damage. This review summarizes molecular mechanisms of mitochondrial fusion and fission processes and their potential roles in myocardial cell death triggered by oxidative stress. Advances in understanding the effect of both normal and abnormal mitochondrial dynamics on heart protection will lead to significant improvement of therapeutic discoveries.

Mfn2 is involved in intervertebral disc degeneration through autophagy modulation

OBJECTIVE

To explore whether Mitofusin 2 (Mfn2) is implicated in the pathogenesis of intervertebral disc degeneration (IVDD).

METHODS

We detected the protein content of Mfn2 in degenerated human nucleus pulposus (NP) tissues and investigated the effects of Mfn2 knockdown and Mfn2 overexpression on rat nucleus pulposus cells (NPCs) under oxidative stress by using a range of biological techniques. Afterwards, we confirmed the effects of Mfn2 overexpression on NPCs in vivo and further evaluated the therapeutic action of adenovirus (AV)-Mfn2 injection in a rodent IVDD model.

RESULTS

Mfn2 expression was decreased in human NP tissues during IVDD. Mfn2 knockdown aggravated the impairment of autophagic flux, mitochondrial dysfunction and cellular apoptosis in rat NPCs after Tert-Butyl hydroperoxide (TBHP) treatment, while Mfn2 overexpression significantly reversed these alterations. Besides, Mfn2 overexpression promoted an ROS (reactive oxygen species)-dependent mitophagy via PINK1 (PTEN-induced putative kinase 1)/Parkin pathway in TBHP-treated NPCs. Inhibition of autophagy with chloroquine (CQ) disordered the protective effects of Mfn2 overexpression on NPCs. Furthermore, Mfn2 overexpression in discs by AV-Mfn2 injection ameliorated the development of IVDD in rats.

CONCLUSION

Mfn2 repression is deeply involved in the pathogenesis of IVDD with its impairment on autophagy, leading to the aggravation of mitochondrial dysfunction and apoptotic cell death, which ought to be a promising therapeutic target for IVDD.

MFN2 silencing promotes neural differentiation of embryonic stem cells via the Akt signaling pathway

Mitofusin 2 (MFN2) is a regulatory protein participating in mitochondria dynamics, cell proliferation, death, differentiation, and so on. This study aims at revealing the functional role of MFN2 in the pluripotency maintenance and primitive differetiation of embryonic stem cell (ESCs). A dox inducible silencing and routine overexpressing approach was used to downregulate and upregulate MFN2 expression, respectively. We have compared the morphology, cell proliferation, and expression level of pluripotent genes in various groups. We also used directed differentiation methods to test the differentiation capacity of various groups. The Akt signaling pathway was explored by the western blot assay. MFN2 upregulation in ESCs exhibited a typical cell morphology and similar cell proliferation, but decreased pluripotent gene markers. In addition, MFN2 overexpression inhibited ESCs differentiation into the mesendoderm, while MFN2 silencing ESCs exhibited a normal cell morphology, slower cell proliferation and elevated pluripotency markers. For differentiation, MFN2 silencing ESCs exhibited enhanced three germs' differentiation ability. Moreover, the protein levels of phosphorylated Akt308 and Akt473 decreased in MFN2 silenced ESCs, and recovered in the neural differentiation process. When treated with the Akt inhibitor, the neural differentiation capacity of the MFN2 silenced ESCs can reverse to a normal level. Taken together, the data indicated that the appropriate level of MFN2 expression is essential for pluripotency and differentiation capacity in ESCs. The increased neural differentiation ability by MFN2 silencing is strongly related to the Akt signaling pathway.

The enigma of an interconnected mitochondrial reticulum: new insights into mitochondrial fusion

It has been over 20 years since the identification of the first GTPases that regulate mitochondrial fusion in drosophila, yeast, and mammalian cells. While the molecular identification of these players solidified the new field of mitochondrial dynamics, cell imaging had established the dynamic properties of mitochondria over a century before. The genetic dissection of mitochondrial fusion, fission, and positioning within cells cemented our understanding of the essential nature of this plasticity in health and disease. Loss of either mitochondrial fusion or fission causes embryonic lethality in mice, and mutations in a number of the core fusion/fission machines were identified in patients with neurodegenerative disease. From these early studies, there has been a rapid expansion of research into mitochondrial dynamics within diverse fields of interest, in various model systems. This review will focus on recent work investigating the mechanisms of mitochondrial fusion, where new findings are challenging some longstanding assumptions. We hope to highlight some essential remaining questions and generate a framework for future studies.