Three-dimensional electron microscopy techniques for unravelling mitochondrial dysfunction in heart failure and identification of new pharmacological targets

Mitochondrial Structure and Function in Human Heart Failure

Despite clinical and scientific advancements, heart failure is the major cause of morbidity and mortality worldwide. Both mitochondrial dysfunction and inflammation contribute to the development and progression of heart failure. Although inflammation is crucial to reparative healing following acute cardiomyocyte injury, chronic inflammation damages the heart, impairs function, and decreases cardiac output. Mitochondria, which comprise one third of cardiomyocyte volume, may prove a potential therapeutic target for heart failure. Known primarily for energy production, mitochondria are also involved in other processes including calcium homeostasis and the regulation of cellular apoptosis. Mitochondrial function is closely related to morphology, which alters through mitochondrial dynamics, thus ensuring that the energy needs of the cell are met. However, in heart failure, changes in substrate use lead to mitochondrial dysfunction and impaired myocyte function. This review discusses mitochondrial and cristae dynamics, including the role of the mitochondria contact site and cristae organizing system complex in mitochondrial ultrastructure changes. Additionally, this review covers the role of mitochondria-endoplasmic reticulum contact sites, mitochondrial communication via nanotunnels, and altered metabolite production during heart failure. We highlight these often-neglected factors and promising clinical mitochondrial targets for heart failure.

Human Heart Failure Alters Mitochondria and Fiber 3D Structure Triggering Metabolic Shifts

This study, utilizing SBF-SEM, reveals structural alterations in mitochondria and myofibrils in human heart failure (HF). Mitochondria in HF show changes in structure, while myofibrils exhibit increased cross-sectional area and branching. Metabolomic and lipidomic analyses indicate concomitant dysregulation in key pathways. The findings underscore the need for personalized treatments considering individualized structural changes in HF.

Electron microscopy of cardiac 3D nanodynamics: form, function, future

The 3D nanostructure of the heart, its dynamic deformation during cycles of contraction and relaxation, and the effects of this deformation on cell function remain largely uncharted territory. Over the past decade, the first inroads have been made towards 3D reconstruction of heart cells, with a native resolution of around 1 nm³, and of individual molecules relevant to heart function at a near-atomic scale. These advances have provided access to a new generation of data and have driven the development of increasingly smart, artificial intelligence-based, deep-learning image-analysis algorithms. By high-pressure freezing of cardiomyocytes with millisecond accuracy after initiation of an action potential, pseudodynamic snapshots of contraction-induced deformation of intracellular organelles can now be captured. In combination with functional studies, such as fluorescence imaging, exciting insights into cardiac autoregulatory processes at nano-to-micro scales are starting to emerge. In this Review, we discuss the progress in this fascinating new field to highlight the fundamental scientific insight that has emerged, based on technological breakthroughs in biological sample preparation, 3D imaging and data analysis; to illustrate the potential clinical relevance of understanding 3D cardiac nanodynamics; and to predict further progress that we can reasonably expect to see over the next 10 years.

Differential remodelling of mitochondrial subpopulations and mitochondrial dysfunction are a feature of early stage diabetes

Mitochondrial dysfunction is a feature of type I and type II diabetes, but there is a lack of consistency between reports and links to disease development. We aimed to investigate if mitochondrial structure–function remodelling occurs in the early stages of diabetes by employing a mouse model (GENA348) of Maturity Onset Diabetes in the Young, exhibiting hyperglycemia, but not hyperinsulinemia, with mild left ventricular dysfunction. Employing 3-D electron microscopy (SBF-SEM) we determined that compared to wild-type, WT, the GENA348 subsarcolemma mitochondria (SSM) are ~ 2-fold larger, consistent with up-regulation of fusion proteins Mfn1, Mfn2 and Opa1. Further, in comparison, GENA348 mitochondria are more irregular in shape, have more tubular projections with SSM projections being longer and wider. Mitochondrial density is also increased in the GENA348 myocardium consistent with up-regulation of PGC1-α and stalled mitophagy (down-regulation of PINK1, Parkin and Miro1). GENA348 mitochondria have more irregular cristae arrangements but cristae dimensions and density are similar to WT. GENA348 Complex activity (I, II, IV, V) activity is decreased but the OCR is increased, potentially linked to a shift towards fatty acid oxidation due to impaired glycolysis. These novel data reveal that dysregulated mitochondrial morphology, dynamics and function develop in the early stages of diabetes.

Isolation and reconstruction of cardiac mitochondria from SBEM images using a deep learning-based method

Mitochondrial morphological defects are a common feature of diseased cardiac myocytes. However, quantitative assessment of mitochondrial morphology is limited by the time-consuming manual segmentation of electron micrograph (EM) images. To advance understanding of the relation between morphological defects and dysfunction, an efficient morphological reconstruction method is desired to enable isolation and reconstruction of mitochondria from EM images. We propose a new method for isolating and reconstructing single mitochondria from serial block-face scanning EM (SBEM) images. CDeep3M, a cloud-based deep learning network for EM images, was used to segment mitochondrial interior volumes and boundaries. Post-processing was performed using both the predicted interior volume and exterior boundary to isolate and reconstruct individual mitochondria. Series of SBEM images from two separate cardiac myocytes were processed. The highest F1-score was 95% using 50 training datasets, greater than that for previously reported automated methods and comparable to manual segmentations. Accuracy of separation of individual mitochondria was 80% on a pixel basis. A total of 2315 mitochondria in the two series of SBEM images were evaluated with a mean volume of 0.78 µm³. The volume distribution was very broad and skewed; the most frequent mitochondria were 0.04-0.06 µm³, but mitochondria larger than 2.0 µm³ accounted for more than 10% of the total number. The average short-axis length was 0.47 µm. Primarily longitudinal mitochondria (0-30 degrees) were dominant (54%). This new automated segmentation and separation method can help quantitate mitochondrial morphology and improve understanding of myocyte structure-function relationships.

Characterisation of the Myocardial Mitochondria Structural and Functional Phenotype in a Murine Model of Diabetic Cardiomyopathy

People affected by diabetes are at an increased risk of developing heart failure than their non-diabetic counterparts, attributed in part to a distinct cardiac pathology termed diabetic cardiomyopathy. Mitochondrial dysfunction and excess reactive oxygen species (ROS) have been implicated in a range of diabetic complications and are a common feature of the diabetic heart. In this study, we sought to characterise impairments in mitochondrial structure and function in a recently described experimental mouse model of diabetic cardiomyopathy. Diabetes was induced in 6-week-old male FVB/N mice by the combination of three consecutive-daily injections of low-dose streptozotocin (STZ, each 55 mg/kg i.p.) and high-fat diet (42% fat from lipids) for 26 weeks. At study end, diabetic mice exhibited elevated blood glucose levels and impaired glucose tolerance, together with increases in both body weight gain and fat mass, replicating several aspects of human type 2 diabetes. The myocardial phenotype of diabetic mice included increased myocardial fibrosis and left ventricular (LV) diastolic dysfunction. Elevated LV superoxide levels were also evident. Diabetic mice exhibited a spectrum of LV mitochondrial changes, including decreased mitochondria area, increased levels of mitochondrial complex-III and complex-V protein abundance, and reduced complex-II oxygen consumption. In conclusion, these data suggest that the low-dose STZ-high fat experimental model replicates some of the mitochondrial changes seen in diabetes, and as such, this model may be useful to study treatments that target the mitochondria in diabetes.

Superiority of focused ion beam-scanning electron microscope tomography of cardiomyocytes over standard 2D analyses highlighted by unmasking mitochondrial heterogeneity

BACKGROUND

Cardioprotection by preventing or repairing mitochondrial damage is an unmet therapeutic need. To understand the role of cardiomyocyte mitochondria in physiopathology, the reliable characterization of the mitochondrial morphology and compartment is pivotal. Previous studies mostly relied on two-dimensional (2D) routine transmission electron microscopy (TEM), thereby neglecting the real three-dimensional (3D) mitochondrial organization. This study aimed to determine whether classical 2D TEM analysis of the cardiomyocyte ultrastructure is sufficient to comprehensively describe the mitochondrial compartment and to reflect mitochondrial number, size, dispersion, distribution, and morphology.

METHODS

Spatial distribution of the complex mitochondrial network and morphology, number, and size heterogeneity of cardiac mitochondria in isolated adult mouse cardiomyocytes and adult wild-type left ventricular tissues (C57BL/6) were assessed using a comparative 3D imaging system based on focused ion beam-scanning electron microscopy (FIB-SEM) nanotomography. For comparison of 2D vs. 3D data sets, analytical strategies and mathematical comparative approaches were performed. To confirm the value of 3D data for mitochondrial changes, we compared the obtained values for number, coverage area, size heterogeneity, and complexity of wild-type cardiomyocyte mitochondria with data sets from mice lacking the cytosolic and mitochondrial protein BNIP3 (BCL-2/adenovirus E1B 19-kDa interacting protein 3; Bnip3^-/- ) using FIB-SEM. Mitochondrial respiration was assessed on isolated mitochondria using the Seahorse XF analyser. A cardiac biopsy was obtained from a male patient (48 years) suffering from myocarditis.

RESULTS

The FIB-SEM nanotomographic analysis revealed that no linear relationship exists for mitochondrial number (r = 0.02; P = 0.9511), dispersion (r = -0.03; P = 0.9188), and shape (roundness: r = 0.15, P = 0.6397; elongation: r = -0.09, P = 0.7804) between 3D and 2D results. Cumulative frequency distribution analysis showed a diverse abundance of mitochondria with different sizes in 3D and 2D. Qualitatively, 2D data could not reflect mitochondrial distribution and dynamics existing in 3D tissue. 3D analyses enabled the discovery that BNIP3 deletion resulted in more smaller, less complex cardiomyocyte mitochondria (number: P < 0.01; heterogeneity: C.V. wild-type 89% vs. Bnip3^-/- 68%; complexity: P < 0.001) forming large myofibril-distorting clusters, as seen in human myocarditis with disturbed mitochondrial dynamics. Bnip3^-/- mice also show a higher respiration rate (P < 0.01).

CONCLUSIONS

Here, we demonstrate the need of 3D analyses for the characterization of mitochondrial features in cardiac tissue samples. Hence, we observed that BNIP3 deletion physiologically acts as a molecular brake on mitochondrial number, suggesting a role in mitochondrial fusion/fission processes and thereby regulating the homeostasis of cardiac bioenergetics.

Mitochondrial Arrest on the Microtubule Highway-A Feature of Heart Failure and Diabetic Cardiomyopathy?

A pathophysiological consequence of both type 1 and 2 diabetes is remodelling of the myocardium leading to the loss of left ventricular pump function and ultimately heart failure (HF). Abnormal cardiac bioenergetics associated with mitochondrial dysfunction occurs in the early stages of HF. Key factors influencing mitochondrial function are the shape, size and organisation of mitochondria within cardiomyocytes, with reports identifying small, fragmented mitochondria in the myocardium of diabetic patients. Cardiac mitochondria are now known to be dynamic organelles (with various functions beyond energy production); however, the mechanisms that underpin their dynamism are complex and links to motility are yet to be fully understood, particularly within the context of HF. This review will consider how the outer mitochondrial membrane protein Miro1 (Rhot1) mediates mitochondrial movement along microtubules via crosstalk with kinesin motors and explore the evidence for molecular level changes in the setting of diabetic cardiomyopathy. As HF and diabetes are recognised inflammatory conditions, with reports of enhanced activation of the NLRP3 inflammasome, we will also consider evidence linking microtubule organisation, inflammation and the association to mitochondrial motility. Diabetes is a global pandemic but with limited treatment options for diabetic cardiomyopathy, therefore we also discuss potential therapeutic approaches to target the mitochondrial-microtubule-inflammatory axis.

Metabolic Syndrome Impairs 3D Mitochondrial Structure, Dynamics, and Function in Swine Mesenchymal Stem Cells

Transplantation of autologous mesenchymal stem cells (MSCs) is an effective therapy for several diseases. Mitochondria modulate several important aspects of MSC function, but might be damaged by comorbidities and cardiovascular risk factors. We hypothesized that metabolic syndrome (MetS) compromises 3D mitochondrial structure, dynamics, and function in swine adipose tissue-derived MSCs. Domestic pigs were fed a Lean or MetS diet (n = 6 each) for 16 weeks. MSCs were collected from subcutaneous abdominal fat and their mitochondria analyzed using state-of-the-art Serial Block Face Electron Microscopy and 3D reconstruction. Mitochondrial dynamics (fusion/fission) were assessed by mRNA sequencing and Western blotting, and bioenergetics by membrane potential (TMRE), cytochrome-c oxidase (COX)-IV activity, and Seahorse Analyzer. Expression of mitochondria-associated microRNAs (mitomiRs) was measured by quantitative polymerase chain reaction (qPCR). MetS pigs developed obesity, hypertension, insulin resistance, and hyperlipidemia. Mitochondrial density was similar between the groups, but 3D mitochondrial and matrix volumes were lower in MetS-MSCs versus Lean-MSCs. Mitochondrial fission was higher, but fusion lower in MetS-MSCs versus Lean-MSCs, as were membrane potential, COX-IV activity, and ATP production. Contrarily, expression of the mitomiRs miR15a, miR-137, and miR-181c, which target mitochondrial genes that support mitochondrial structure, energy pathways, and dynamics, was higher in MetS-MSCs compared to Lean-MSCs, suggesting a potential to modulate their expression. MetS damages MSC 3D mitochondrial structure, dynamics, and function, and may modulate genes encoding for mitochondrial proteins. These observations support development of mitoprotective strategies to preserve the regenerative potency of MSCs and their suitability for autologous transplantation in patients with MetS.

Mitochondrial structural alterations in ovarian cancer patient-derived xenografts resistant to cisplatin

Mitochondria have attracted attention in cancer research as organelles associated with tumor development and response to therapy. We recently reported acquisition of resistance to cisplatin (DDP) associated with a metabolic rewiring in ovarian cancer patient-derived xenografts (PDXs) models. DDP-resistant PDXs models were obtained mimicking the clinical setting, treating mice bearing sensitive-DDP tumors with multiple cycles of DDP until the development of resistance. To further characterize the metabolic rewiring, the present study focused on tumor mitochondria. We analysed by transmission electron microscopy the mitochondria structure in two models of DDP-resistant and the corresponding DDP-sensitive PDXs and evaluated tumor mDNA content, the expression of genes and proteins involved in mitochondria functionality, and mitochondria fitness-related processes, such as autophagy. We observed a decrease in the number of mitochondria paralleled by an increased volume in DDP-resistant versus DDP-sensitive PDXs. DDP-resistant PDXs presented a higher percentage of damaged mitochondria, in particular of type 2 (concave-shape), and type 3 (cristolysis) damage. We found no difference in the mDNA content, and the expression of genes involved in mitochondrial biogenesis was similar between the sensitive and resistant PDXs. An upregulation of some genes involved in mitochondrial fitness in DDP-R versus DDP-S PDXs was observed. At protein level, no difference in the expression of proteins involved in mitochondrial function and biogenesis, and in autophagy/mitophagy was found. We here reported that the acquisition of DDP resistance is associated with morphological alterations in mitochondria, even if we couldn't find any dysregulation in the studied genes/proteins that could explain the observed differences.

mtDNA Heteroplasmy at the Core of Aging-Associated Heart Failure. An Integrative View of OXPHOS and Mitochondrial Life Cycle in Cardiac Mitochondrial Physiology

The most common aging-associated diseases are cardiovascular diseases which affect 40% of elderly people. Elderly people are prone to suffer aging-associated diseases which are not only related to health and medical cost but also to labor, household productivity and mortality cost. Aging is becoming a world problem and it is estimated that 21.8% of global population will be older than 65 years old in 2050; and for the first time in human history, there will be more elderly people than children. It is well accepted that the origin of aging-associated cardiovascular diseases is mitochondrial dysfunction. Mitochondria have their own genome (mtDNA) that is circular, double-stranded, and 16,569 bp long in humans. There are between 500 to 6000 mtDNA copies per cell which are tissue-specific. As a by-product of ATP production, reactive oxygen species (ROS) are generated which damage proteins, lipids, and mtDNA. ROS-mutated mtDNA co-existing with wild type mtDNA is called mtDNA heteroplasmy. The progressive increase in mtDNA heteroplasmy causes progressive mitochondrial dysfunction leading to a loss in their bioenergetic capacity, disruption in the balance of mitochondrial fusion and fission events (mitochondrial dynamics, MtDy) and decreased mitophagy. This failure in mitochondrial physiology leads to the accumulation of depolarized and ROS-generating mitochondria. Thus, besides attenuated ATP production, dysfunctional mitochondria interfere with proper cellular metabolism and signaling pathways in cardiac cells, contributing to the development of aging-associated cardiovascular diseases. In this context, there is a growing interest to enhance mitochondrial function by decreasing mtDNA heteroplasmy. Reduction in mtDNA heteroplasmy is associated with increased mitophagy, proper MtDy balance and mitochondrial biogenesis; and those processes can delay the onset or progression of cardiovascular diseases. This has led to the development of mitochondrial therapies based on the application of nutritional, pharmacological and genetic treatments. Those seeking to have a positive impact on mtDNA integrity, mitochondrial biogenesis, dynamics and mitophagy in old and sick hearts. This review covers the current knowledge of mitochondrial physiopathology in aging, how disruption of OXPHOS or mitochondrial life cycle alter mtDNA and cardiac cell function; and novel mitochondrial therapies to protect and rescue our heart from cardiovascular diseases.

Comparison of electron microscopic findings and clinical presentation in three patients with mitochondrial cardiomyopathy caused by the mitochondrial DNA mutation m.3243A > G

Mitochondrial cardiomyopathy can be described as a condition characterized by abnormal heart-muscle structure and/or function, secondary to mutations in nuclear or mitochondrial DNA. Its severity can range from subclinical to critical conditions. We presented three cases of mitochondrial cardiomyopathy with m.3243A > G mutation and compared the clinical manifestations with the histological findings for each of these cases. All cases showed cardiac hypertrophy, juvenile-onset diabetes mellitus, and hearing loss. Case 1 (43-year-old male) showed less cardiac involvement and shorter duration of mitochondrial disease-related symptoms than case 2 (67-year-old female) and case 3 (51-year-old male), who showed the most advanced cardiac condition and longest duration from the manifestation of heart failure. The histological findings revealed that cardiomyocytes from case 1 showed no hypertrophy and mitochondrial degeneration in electron microscopy. Alternatively, cases 2 and 3 showed hypertrophy in their cardiomyocytes, and mitochondrial degeneration (e.g. onion-like lesions, swollen cristae, and lamellar bodies) was most apparent in case 3. These results suggested that mitochondrial degeneration, as evaluated by electron microscopy, might be correlated with impaired heart function in patients with mitochondrial cardiomyopathy.

Elephantopinolide A-P, germacrane-type sesquiterpene lactones from Elephantopus scaber induce apoptosis, autophagy and G2/M phase arrest in hepatocellular carcinoma cells

Chromatographic purification of Elephantopus scaber led to 16 new germacrane-type sesquiterpene lactones (1-16), named elephantopinolide A-P, along with a known analogue (17). Their structures were confirmed by comprehensive spectroscopic analyses, single-crystal X-ray diffraction, and comparison between the experimental and calculated ECD spectra. Their hepatocellular inhibition activities against Hep3B and HepG2 cells were screened by MTT assay, and the structure-activity relationships were examined. The results revealed that 10 (IC50 value of 2.83 μM and 1.98 μM) is more potent than sorafenib. The underlying mechanism study demonstrated that 10 could markedly induce apoptosis accompanied by increased ROS production and decreased mitochondrial membrane potential, resulting in the autophagy and G2/M phase cell arrest in Hep3B and HepG2 cells. Furthermore, signal pathways including MAPKs and AKT may play important roles in 10-induced hepatocellular carcinoma cells death.

Mitochondrial behaviour, morphology, and animal performance

We have a limited understanding of the proximate mechanisms that are responsible for the development of variation in animal performance and life-history strategies. Provided that components of an organism's successful life history - for example, mate competition, gestation, lactation, etc. - are energetically demanding, increased energy production within mitochondria is likely the foundation from which organisms are able to perform these tasks. Mitochondrial behaviour (positioning within the cell and communication between mitochondria) and morphology affect variation in energy production at the molecular, cellular, and organismal levels. Therefore, adaptations in mitochondrial behaviour and morphology that favour efficient energy production likely influence variation in animal performance. Previous work has linked greater proportions of inter-mitochondrial junctions and density of the inner mitochondrial membrane, among other traits, with increased energetic demand. Future research should focus on how inter-mitochondrial junctions and morphology of the inner mitochondrial membrane, in particular, influence animal performance in accordance with mitochondrial density, fission, and fusion.

Rosuvastatin protects isolated hearts against ischemia-reperfusion injury: role of Akt-GSK-3β, metabolic environment, and mitochondrial permeability transition pore

The cardioprotective activity of rosuvastatin (R) is yet to be known. The objective of this study was to research whether R perfusion before global ischemia can mitigate myocardial ischemia-reperfusion damage, considering the metabolic condition in which these effects occur, and to contemplate potential mitochondrial benefits. Protein kinase B (Akt)/glycogen synthase kinase-3β (GSK-3β) and mitochondrial permeability transition pore (MPTP) are key elements in myocardial injury produced by ischemia-reperfusion. Isolated rat hearts were subjected to 25-min ischemia and 1-h reperfusion in the presence or absence of R, with or without Wortmannin (W), a phosphatidylinositol 3-kinase (PI3K)/Akt inhibitor. Akt and GSK-3β were measured by Western blot analysis; lactate, glycogen, and G6PDH were determined; and Ca²⁺-induced MPTP opening was evaluated using a spectrophotometric method. Contractility was assessed by left ventricular developed pressure (LVDP), and rate-pressure product (RPP), peak rate of contraction and peak rate of relaxation (± dP/dt), and left ventricular end-diastolic pressure (LVEDP) were determined. Tissue samples were extracted to evaluate mitochondrial damage by electron microscopy and to assess infarct size. Statistical analysis employed ANOVA (n = 6/per group). Myocardial infarct size was significantly reduced by R, which also improved cardiac function. MPTP opening was delayed to 300 μM CaCl₂, while use of W resulted in MPTP opening at 200 μM CaCl₂. Electron microscopy showed better mitochondrial preservation with R, which reduced lactic acid production, increased glycogen consumption and G6PDH activity, as well as phosphorylation of Akt and GSK-3β. R before ischemia is cardioprotective against ischemic and reperfusion damage, activating Akt and regulating GSK-3β negatively and attenuating the MPTP opening.

Mitochondrial pharmacology: featured mechanisms and approaches for therapy translation

LINKED ARTICLES

This article is part of a themed section on Mitochondrial Pharmacology: Featured Mechanisms and Approaches for Therapy Translation. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v176.22/issuetoc.

Three-dimensional electron microscopy techniques for unravelling mitochondrial dysfunction in heart failure and identification of new pharmacological targets

A hallmark of heart failure is mitochondrial dysfunction leading to a bioenergetics imbalance in the myocardium. Consequently, there is much interest in targeting mitochondrial abnormalities to attenuate the pathogenesis of heart failure. This review discusses (i) how electron microscopy (EM) techniques have been fundamental for the current understanding of mitochondrial structure–function, (ii) the paradigm shift in resolutions now achievable by 3‐D EM techniques due to the introduction of direct detection devices and phase plate technology, and (iii) the application of EM for unravelling mitochondrial pathological remodelling in heart failure. We further consider the tremendous potential of multi‐scale EM techniques for the development of therapeutics, structure‐based ligand design and for delineating how a drug elicits nanostructural effects at the molecular, organelle and cellular levels. In conclusion, 3‐D EM techniques have entered a new era of structural biology and are poised to play a pivotal role in discovering new therapies targeting mitochondria for treating heart failure.

Linked Articles

This article is part of a themed section on Mitochondrial Pharmacology: Featured Mechanisms and Approaches for Therapy Translation. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v176.22/issuetoc