Mitoflash lights single mitochondrial dynamics events in mature cardiomyocytes

Evaluation of Oxygen Consumption Rates In Situ

An analysis of the mitochondrial respiration function represented by the oxygen consumption rate is necessary to assess mitochondrial bioenergetics and redox function. This protocol describes two alternative techniques to evaluate mitochondrial respiration function in situ: (1) measure oxygen consumption rates via an electrode; (2) measure oxygen consumption rates via a seahorse instrument. These in situ approaches provide more physiological access to mitochondria to evaluate mitochondrial respiration function in a relatively integrated cellular system.

Targeting mitochondrial shape: at the heart of cardioprotection

There remains an unmet need to identify novel therapeutic strategies capable of protecting the myocardium against the detrimental effects of acute ischemia-reperfusion injury (IRI), to reduce myocardial infarct (MI) size and prevent the onset of heart failure (HF) following acute myocardial infarction (AMI). In this regard, perturbations in mitochondrial morphology with an imbalance in mitochondrial fusion and fission can disrupt mitochondrial metabolism, calcium homeostasis, and reactive oxygen species production, factors which are all known to be critical determinants of cardiomyocyte death following acute myocardial IRI. As such, therapeutic approaches directed at preserving the morphology and functionality of mitochondria may provide an important strategy for cardioprotection. In this article, we provide an overview of the alterations in mitochondrial morphology which occur in response to acute myocardial IRI, and highlight the emerging therapeutic strategies for targeting mitochondrial shape to preserve mitochondrial function which have the future therapeutic potential to improve health outcomes in patients presenting with AMI.

MFN2 deficiency promotes cardiac response to hypobaric hypoxia by reprogramming cardiomyocyte metabolism

AIM

Under hypobaric hypoxia (HH), the heart triggers various defense mechanisms including metabolic remodeling against lack of oxygen. Mitofusin 2 (MFN2), located at the mitochondrial outer membrane, is closely involved in the regulation of mitochondrial fusion and cell metabolism. To date, however, the role of MFN2 in cardiac response to HH has not been explored.

METHODS

Loss- and gain-of-function approaches were used to investigate the role of MFN2 in cardiac response to HH. In vitro, the function of MFN2 in the contraction of primary neonatal rat cardiomyocytes under hypoxia was examined. Non-targeted metabolomics and mitochondrial respiration analyses, as well as functional experiments were performed to explore underlying molecular mechanisms.

RESULTS

Our data demonstrated that, following 4 weeks of HH, cardiac-specific MFN2 knockout (MFN2 cKO) mice exhibited significantly better cardiac function than control mice. Moreover, restoring the expression of MFN2 clearly inhibited the cardiac response to HH in MFN2 cKO mice. Importantly, MFN2 knockout significantly improved cardiac metabolic reprogramming during HH, resulting in reduced capacity for fatty acid oxidation (FAO) and oxidative phosphorylation, and increased glycolysis and ATP production. In vitro data showed that down-regulation of MFN2 promoted cardiomyocyte contractility under hypoxia. Interestingly, increased FAO through palmitate treatment decreased contractility of cardiomyocyte with MFN2 knockdown under hypoxia. Furthermore, treatment with mdivi-1, an inhibitor of mitochondrial fission, disrupted HH-induced metabolic reprogramming and subsequently promoted cardiac dysfunction in MFN2-knockout hearts.

CONCLUSION

Our findings provide the first evidence that down-regulation of MFN2 preserves cardiac function in chronic HH by promoting cardiac metabolic reprogramming.

Inhibition of mitochondrial superoxide promotes the development of hiPS-CMs during differentiation

The redox state is a crucial determinant of the maturation transition of cardiomyocytes in vivo. Mitochondria, the primary site of superoxide generation, are very sensitive to various stimulations, including oxygen and nutrient supply. How mitochondrial superoxide affects the differentiation and development of induced pluripotent stem cell (iPSC)-derived cardiac myocytes (iPS-CMs) is not completely clear. To address the questions, we monitored the superoxide level during the differentiation and development of human iPS-CMs using MitoSOX. Mitochondria-targeted antioxidant Mito-TEMPO was used to treat hiPS-CMs in the differentiation period. We found that mitochondrial superoxide generation was dramatically enhanced during the differentiation and early development of iPS-CMs. Increased oxidative stress induced oxidative damage to macromolecules in iPS-CMs, such as lipids, proteins, and DNA. Mito-TEMPO protected mitochondrial functions, alleviated oxidative damage to lipids, proteins, and DNA and improved cellular structure and fatty acid utilization. Our findings confirmed that iPS-CM suffered from oxidative stress during differentiation and that mitochondrial-targeted antioxidant is beneficial for the maturation of iPS-CMs.

The Combination of Paraformaldehyde and Glutaraldehyde Is a Potential Fixative for Mitochondria

Mitochondria are highly dynamic organelles, constantly undergoing shape changes, which are controlled by mitochondrial movement, fusion, and fission. Mitochondria play a pivotal role in various cellular processes under physiological and pathological conditions, including metabolism, superoxide generation, calcium homeostasis, and apoptosis. Abnormal mitochondrial morphology and mitochondrial protein expression are always closely related to the health status of cells. Analysis of mitochondrial morphology and mitochondrial protein expression in situ is widely used to reflect the abnormality of cell function in the chemical fixed sample. Paraformaldehyde (PFA), the most commonly used fixative in cellular immunostaining, still has disadvantages, including loss of antigenicity and disruption of morphology during fixation. We tested the effect of ethanol (ETHO), PFA, and glutaraldehyde (GA) fixation on cellular mitochondria. The results showed that 3% PFA and 1.5% GA (PFA-GA) combination reserved mitochondrial morphology better than them alone in situ in cells. Mitochondrial network and protein antigenicity were well maintained, indicated by preserved MitoTracker and mitochondrial immunostaining after PFA-GA fixation. Our results suggest that the PFA-GA combination is a valuable fixative for the study of mitochondria in situ.

Mitochondrial Fusion Protein Mfn2 and Its Role in Heart Failure

Mitofusin 2 (Mfn2) is a transmembrane GTPase located on the mitochondrial outer membrane that contributes to mitochondrial network regulation. It is an essential multifunctional protein that participates in various biological processes under physical and pathological conditions, including mitochondrial fusion, reticulum–mitochondria contacts, mitochondrial quality control, and apoptosis. Mfn2 dysfunctions have been found to contribute to cardiovascular diseases, such as ischemia-reperfusion injury, heart failure, and dilated cardiomyopathy. Here, this review mainly focuses on what is known about the structure and function of Mfn2 and its crucial role in heart failure.

Mitochondrial Dynamics in Adult Cardiomyocytes and Heart Diseases

Mitochondria are the powerhouse organelles of cells; they participate in ATP generation, calcium homeostasis, oxidative stress response, and apoptosis. Thus, maintenance of mitochondrial function is critical for cellular functions. As highly dynamic organelles, the function of mitochondria is dynamically regulated by their fusion and fission in many cell types, which regulate mitochondrial morphology, number, distribution, metabolism, and biogenesis in cells. Mature rod-shaped cardiomyocytes contain thousands of end-to-end contacted spheroid mitochondria. The movement of mitochondria in these cells is limited, which hinders the impetus for research into mitochondrial dynamics in adult cardiomyocytes. In this review, we discuss the most recent progress in mitochondrial dynamics in mature (adult) cardiomyocytes and the relationship thereof with heart diseases.

Mitochondrial fusion mediated by fusion promotion and fission inhibition directs adult mouse heart function toward a different direction

Mitochondrial fusion and fission are essential for heart function. Abrogating mitochondrial dynamism leads to cardiomyopathy. Excessive mitochondrial fragmentation is involved in most heart diseases, thus enhancing mitochondrial fusion will be a potential therapeutic strategy. To understand the effects of promoting mitochondrial fusion in adult cardiac, we investigated mice hearts, and cultured murine embryonic fibroblasts (MEFs), in which mitofusin 2 (Mfn2) overexpressed or dynamin-related protein 1 (Drp1) was abrogated concomitantly forcing mitochondrial fusion. Parallel studies revealed that fission-defective Drp1 knockout hearts and MEFs evoked stronger mitochondrial enlargement, enhanced mitophagy with mitochondrial volume decrease and increased mitochondrial calcium uptake, superoxide production, and permeability transition pore opening, contributed to cardiomyocyte apoptosis and dilated cardiomyopathy. Mfn2 overexpression in the adult heart is comparable with the control except for slight mitochondrial enlargement and mitochondrial volume increase, but without mitophagy induction. Moreover, Mfn2 overexpression increases mitochondrial biogenesis and fusion could protect against mitochondrial fragmentation and Drp1 deletion evoking mitophagy in MEFs. Our findings indicate that mitochondrial fusion provoked by fusion promotion and fission inhibition direct the different fate of heart, Mfn2 upregulation other than Drp1 downregulation well maintains heart mitochondrial function is a more safe strategy for correcting excessive mitochondrial fragmentation in hearts.

Comparative study on isolation and mitochondrial function of adult mouse and rat cardiomyocytes

BACKGROUND

Cultured adult mouse and rat cardiomyocytes are the best and low-cost cell model for cardiac cellular physiology, pathology, drug toxicity screening, and intervention. The functions of mouse cardiomyocytes decline faster than rat cardiomyocytes in culture conditions. However, little is known about the difference of mitochondrial function between cultured mouse and rat myocytes.

METHODS AND RESULTS

A large number of adult mouse and rat cardiomyocytes were comparative isolated using a simple perfusion system. Cardiomyocytes mitochondrial functions were measured after 2 h, 1 day, 2 days, 3 days, and 4 days culture by monitoring mitoflashes. We found that the mitochondrial function of mouse myocytes was remarkedly declined on the third day. Then, we focused on the third day cultured mouse and rat myocytes, comparatively analyzing the respiration function and superoxide generation stimulated by pyruvate/malate/ADP and the mitochondrial permeability transition pore (mPTP) opening induction. Mouse myocytes showed lower respiration and mitoflash activity, but without the change of maximum uncoupled respiration when compared with rat myocytes. Although the response to superoxide production stimulated by respiration substrates was slower than rat myocytes, the basal superoxide generation is faster than the rat. The faster mitochondrial reactive oxygen species (ROS) generation of mouse myocytes upon laser stimulation triggered the faster mPTP opening compared with the rat. Finally, antioxidant MitoTEMPO pretreatment preserved the mitochondrial function of mouse myocytes on the third day.

CONCLUSIONS

The mitochondrial function and stability are different between cultured mouse and rat cardiac myocytes beyond 3 days even though they both belong to Muridae. Mitochondrial ROS impairs the mitochondrial functions of mouse cardiomyocytes on the third day. Suppressing superoxide maintained the mitochondrial function of mouse myocytes on the third day.