Keeping mitochondria in shape: a matter of life and death

A longitudinal study of the brain structure network changes in HIV patients with ANI: combined VBM with SCN

Background

Despite the widespread adoption of combination antiretroviral therapy (cART) in managing HIV, the virus's impact on the brain structure of patients remains significant. This study aims to longitudinally explore the persistent effects of HIV on brain structure, focusing on changes in gray matter volume (GMV) and structural covariance network (SCN) among patients at the Asymptomatic Neurocognitive Impairment (ANI) stage.

Methods

This research involved 45 HIV patients diagnosed with ANI and 45 demographically matched healthy controls (HCs). The participants were observed over a 1.5-year period. Differences in GMV between groups were analyzed using voxel-based morphometry (VBM), while the graph theory model facilitated the establishment of topological metrics for assessing network indices. These differences were evaluated using two-sample t-tests and paired-sample t-tests, applying the network-based statistics method. Additionally, the study examined correlations between GMV and cognitive performance, as well as clinical variables.

Results

Compared with HCs, HIV patients demonstrated reduced GMV in the right middle temporal gyrus and left middle frontal gyrus (FWE, p < 0.05), along with decreased betweenness centrality (BC) in the left anterior cingulate and paracingulate cortex. Conversely, an increase in the clustering coefficient (Cp) was observed (FDR, p < 0.05). During the follow-up period, a decline in GMV in the right fusiform gyrus (FWE, p < 0.05) and a reduction in node efficiency (Ne) in the triangular part of the inferior frontal gyrus were noted compared with baseline measurements (FDR, p < 0.05). The SCN of HIV patients exhibited small-world properties across most sparsity levels (Sigma >1), and area under the curve (AUC) analysis revealed no significant statistical differences between groups.

Conclusion

The findings suggest that despite the administration of combination antiretroviral therapy (cART), HIV continues to exert slow and sustained damage on brain structures. However, when compared to HCs, the small-world properties of the patients' SCNs did not significantly differ, and the clustering coefficient, indicative of the overall information-processing capacity of the brain network, was slightly elevated in HIV patients. This elevation may relate to compensatory effects of brain area functions, the impact of cART, functional reorganization, or inflammatory responses.

Asiatic Acid Alleviates Lipopolysaccharide-Induced Acute Myocardial Injury by Promoting Mitophagy and Regulating Mitochondrial Dynamics in Broilers

Acute myocardial injury (AMI) induced by lipopolysaccharide (LPS) can cause cardiovascular dysfunction and lead to death in poultry. Traditional antibiotic therapy has been found to have many limitations and negative effects. Asiatic acid (AA) is a naturally occurring pentacyclic triterpenoid that is extracted from Centella asiatica and has anti-inflammatory, antioxidant, and anticancer pharmacological properties. Previously, we studied the effect of AA on LPS-induced liver and kidney injury; however, the impact of AA on LPS-induced AMI remained unclear. Sixty 1-day-old broilers were randomly divided into control group, LPS group, LPS + AA 15 mg/kg group, LPS + AA 30 mg/kg group, LPS + AA 60 mg/kg group, and control + AA 60 mg/kg group. The histopathology of cardiac tissues was detected by hematoxylin and eosin (H&E) staining. The mRNA and protein expressions related to mitochondrial dynamics and mitophagy were detected by quantitative real-time PCR, western blot, immunofluorescence, and immunohistochemistry. Disorganized myocardial cells and fractured myocardial fibers were found in the LPS group, and obvious red-blood-cell filling can be seen in the gaps between the myocardial fibers in the low-dose AA group. Nevertheless, the medium and high dose of AA obviously attenuated these changes. Our results showed that AA significantly restored the mRNA and protein expressions related to mitochondrial dynamic through further promoting mitophagy. This study revealed the effect of AA on LPS-induced AMI in broilers. Mechanically, AA regulated mitochondrial dynamic homeostasis and further promoted mitophagy. These novel findings indicate that AA may be a potential drug for LPS-induced AMI in broilers.

Apoptosis and its pathways as targets for intracellular pathogens to persist in cells

Apoptosis is a finely programmed process of cell death in which cells silently dismantle and actively participate in several operations such as immune response, differentiation, and cell growth. It can be initiated by three main pathways: the extrinsic, the perforin granzyme, and the intrinsic that culminate in the activation of several proteins in charge of tearing down the cell. On the other hand, apoptosis represents an ordeal for pathogens that live inside cells and maintain a strong dependency with them; thus, they have evolved multiple strategies to manipulate host cell apoptosis on their behalf. It has been widely documented that diverse intracellular bacteria, fungi, and parasites can interfere with most steps of the host cell apoptotic machinery to inhibit or induce apoptosis. Indeed, the inhibition of apoptosis is considered a virulence property shared by many intracellular pathogens to ensure productive replication. Some pathogens intervene at an early stage by interfering with the sensing of extracellular signals or transduction pathways. Others sense cellular stress or target the apoptosis regulator proteins of the Bcl-2 family or caspases. In many cases, the exact molecular mechanisms leading to the interference with the host cell apoptotic cascade are still unknown. However, intense research has been conducted to elucidate the strategies employed by intracellular pathogens to modulate host cell death. In this review, we summarize the main routes of activation of apoptosis and present several processes used by different bacteria, fungi, and parasites to modulate the apoptosis of their host cells.

Combination Organelle Mitochondrial Endoplasmic Reticulum Therapy (COMET) for Multidrug Resistant Breast Cancer

It is time for the story of mitochondria and intracellular communication in multidrug resistant cancer to be rewritten. Herein we characterize the extent and cellular advantages of mitochondrial network fusion in multidrug resistant (MDR) breast cancer and have designed a novel nanomedicine that disrupts mitochondrial network fusion and systematically manipulates organelle fusion and function. Combination Organelle Mitochondrial Endoplasmic reticulum Therapy (COMET) is an innovative translational nanomedicine for treating MDR triple negative breast cancer (TNBC) that has superior safety and equivalent efficacy to the current standard of care (paclitaxel). Our study has demonstrated that the increased mitochondrial networks in MDR TNBC contribute to apoptotic resistance and network fusion is mediated by mitofusin2 (MFN2) on the outer mitochondrial membrane. COMET consists of three components; Mitochondrial Network Disrupting (MiND) nanoparticles (NPs) that are loaded with an anti-MFN2 peptide, tunicamycin, and Bam7. The therapeutic rationale of COMET is to reduce the apoptotic threshold in MDR cells with MiND NPs, followed by inducing the endoplasmic reticulum mediated unfolded protein response (UPR) by stressing MDR cells with tunicamycin, and finally, directly inducing mitochondrial apoptosis with Bam7 which is a specific bcl-2 Bax activator. MiND NPs are PEGylated liposomes with the 21 amino acid (2577.98 MW) anti-MFN2 peptide compartmentalized in the aqueous core. Hypoxia (0.5% oxygen) was used to create MDR derivatives of MDA-MB-231 cells and BT-549 cells. Mitochondrial networks were quantified using 3D analysis of 60× live cell images acquired with a Keyence BZ-X710 microscope and MiND NPs effectively fragmented mitochondrial networks in drug sensitive and MDR TNBC cells. The IC50 values, combination index, and dose reduction index derived from dose response studies demonstrate that MiND NPs decrease the apoptotic threshold of both drug sensitive and MDR TNBC cells and COMET is a synergistic drug combination. Complex V (ATP synthase) extracted from bovine cardiac mitochondria was used to assess the effect of MiND NPs on OXPHOS; both MiND NPs and anti-MFN2 peptide solution significantly decrease the activity of mitochondrial complex V and decrease the capacity of OXPHOS. A BacMam viral vector based fluorescent biosensor was used to quantify the unfolded protein response (UPR) at the level of the endoplasmic reticulum and tunicamycin specifically induces the UPR in drug sensitive and MDR TNBC cells. A caspase 3 colorimetric assay demonstrated that the synergistic triple drug combination of COMET increases the ability of Bam7 to specifically induce apoptosis. Dose limiting toxicity and off target effects are a significant challenge for current chemotherapy regimens including paclitaxel. COMET has significantly lower cytotoxicity than paclitaxel in human embryonic kidney epithelial cells and has the potential to fulfill the clinical need for safer cancer therapeutics. COMET is a promising early stage translational nanomedicine for treating MDR TNBC. Manipulating intracellular communication and organelle fusion is a novel approach to treating MDR cancer. The data from this study has rewritten the story of mitochondria, organelle fusion, and intracellular communication and by targeting this intersection, COMET is an exciting new chapter in cancer therapeutics that could transform the clinical outcome of MDR TNBC.

Alterations in Mitochondrial Morphology and Quality Control in Primary Mouse Lung Microvascular Endothelial Cells and Human Dermal Fibroblasts under Hyperglycemic Conditions

The effect of hyperglycemia on the morphology of individual mitochondria and the state of the mitochondrial network in primary mouse lung microvascular endotheliocytes and human dermal fibroblasts has been investigated. The cells were exposed to high (30 mM) and low (5.5 mM) glucose concentrations for 36 h. In primary endotheliocytes, hyperglycemic stress induced a significant increase in the number of mitochondria and a decrease in the interconnectivity value of the mitochondrial network, which was associated with a decrease in the mean size of the mitochondria. Analysis of the mRNA level of the genes of proteins responsible for mitochondrial biogenesis and mitophagy revealed an increase in the expression level of the Ppargc1a, Pink1, and Parkin genes, indicating stimulated mitochondrial turnover in endotheliocytes under high glucose conditions. In primary fibroblasts, hyperglycemia caused a decrease in the number of mitochondria and an increase in their size. As a result, the mitochondria exhibited higher values for elongation. In parallel, the mRNA level of the Ppargc1a and Mfn2 genes in fibroblasts exposed to hyperglycemia was reduced. These findings indicate that high glucose concentrations induced cell-specific morphological rearrangements of individual mitochondria and the mitochondrial network, which may be relevant during mitochondria-targeted drug testing and therapy for hyperglycemic and diabetic conditions.

Calcineurin inhibition protects against dopamine toxicity and attenuates behavioral decline in a Parkinson's disease model

BACKGROUND

Parkinson's disease (PD), a highly prevalent neuro-motor disorder is caused due to progressive loss of dopaminergic (DAergic) neurons at substantia nigra region of brain. This leads to depleted dopamine (DA) content at striatum, thus affecting the fine tuning of basal ganglia. In patients, this imbalance is manifested by akinesia, catalepsy and tremor. PD associated behavioral dysfunctions are frequently mitigated by l-DOPA (LD) therapy, a precursor for DA synthesis. Due to progressive neurodegeneration, LD eventually loses applicability in PD. Although DA is cytotoxic, it is unclear whether LD therapy can accelerate PD progression or not. LD itself does not lead to neurodegeneration in vivo, but previous reports demonstrate that LD treatment mediated excess DA can potentiate neurotoxicity when PD associated genetic or epigenetic aberrations are involved. So, minimizing DA toxicity during the therapy is an absolute necessity to halt or slowdown PD progression. The two major contributing factors associated with DA toxicity are: degradation by Monoamine oxidase and DAquinone (DAQ) formation.

RESULTS

Here, we report that apoptotic mitochondrial fragmentation via Calcineurin (CaN)-DRP1 axis is a common downstream event for both these initial cues, inhibiting which can protect cells from DA toxicity comprehensively. No protective effect is observed, in terms of cell survival when only PxIxIT domain of CaN is obstructed, demonstrating the importance to block DRP1-CaN axis specifically. Further, evaluation of the impact of DA exposure on PD progression in a mice model reveal that LD mediated behavioral recovery diminishes with time, mostly because of continued DAergic cell death and dendritic spine loss at striatum. CaN inhibition, alone or in combination with LD, offer long term behavioral protection. This protective effect is mediated specifically by hindering CaN-DRP1 axis, whereas inhibiting interaction between CaN and other substrates, including proteins involved in neuro-inflammation, remained ineffective when LD is co-administered.

CONCLUSIONS

In this study, we conclude that DA toxicity can be circumvented by CaN inhibition and it can mitigate PD related behavioral aberrations by protecting neuronal architecture at striatum. We propose that CaN inhibitors might extend the therapeutic efficacy of LD treatment.

Taurine Protects against the Fatty Liver Hemorrhagic Syndrome in Laying Hens through the Regulation of Mitochondrial Homeostasis

Metabolic-associated fatty liver disease (MAFLD) is a chronic liver disease caused by fat deposition in the liver of humans and mammals, while fatty liver hemorrhagic syndrome (FLHS) is a fatty liver disease in laying hens which can increase the mortality and cause severe economic losses to the laying industry. Increasing evidence has shown a close relationship between the occurrence of fatty liver disease and the disruption of mitochondrial homeostasis. Studies have proven that taurine can regulate hepatic fat metabolism, reduce hepatic fatty deposition, inhibit oxidative stress, and alleviate mitochondrial dysfunction. However, the mechanisms by which taurine regulates mitochondrial homeostasis in hepatocytes need to be further studied. In this study, we determined the effects and mechanisms of taurine on high-energy low-protein diet-induced FLHS in laying hens and in cultured hepatocytes in free fatty acid (FFA)-induced steatosis. The liver function, lipid metabolism, antioxidant capacity, mitochondrial function, mitochondrial dynamics, autophagy, and biosynthesis were detected. The results showed impaired liver structure and function, mitochondrial damage and dysfunction, lipid accumulation, and imbalance between mitochondrial fusion and fission, mitochondrial autophagy, and biosynthesis in both FLHS hens and steatosis hepatocytes. Taurine administration can significantly inhibit the occurrence of FLHS, protect mitochondria in hepatocytes from disease induced by lipid accumulation and FFA, up-regulate the expression levels of Mfn1, Mfn2, Opa1, LC3I, LC3II, PINK1, PGC-1α, Nrf1, Nrf2, and Tfam, and down-regulate the expression levels of Fis1, Drp1, and p62. In conclusion, taurine can protect laying hens from FLHS through the regulation of mitochondrial homeostasis, including the regulation of mitochondrial dynamics, autophagy, and biosynthesis.

High-Throughput Microscopy Analysis of Mitochondrial Membrane Potential in 2D and 3D Models

Recent proteomic, metabolomic, and transcriptomic studies have highlighted a connection between changes in mitochondria physiology and cellular pathophysiological mechanisms. Secondary assays to assess the function of these organelles appear fundamental to validate these -omics findings. Although mitochondrial membrane potential is widely recognized as an indicator of mitochondrial activity, high-content imaging-based approaches coupled to multiparametric to measure it have not been established yet. In this paper, we describe a methodology for the unbiased high-throughput quantification of mitochondrial membrane potential in vitro, which is suitable for 2D to 3D models. We successfully used our method to analyze mitochondrial membrane potential in monolayers of human fibroblasts, neural stem cells, spheroids, and isolated muscle fibers. Moreover, by combining automated image analysis and machine learning, we were able to discriminate melanoma cells from macrophages in co-culture and to analyze the subpopulations separately. Our data demonstrated that our method is a widely applicable strategy for large-scale profiling of mitochondrial activity.

Mitochondria associated ER membrane and cerebral ischemia: molecular mechanisms and therapeutic strategies

Endoplasmic reticulum (ER) and mitochondria are two important organelles that are highly dynamic in mammalian cells. The physical connection between them is mitochondria associated ER membranes (MAM). In recent years, studies on endoplasmic reticulum and mitochondria have shifted from independent division to association and comparison, especially MAM has gradually become a research hotspot. MAM connects the two organelles, not only to maintain their independent structure and function, but also to promote metabolism and signal transduction between them. This paper reviews the morphological structure and protein localization of MAM, and briefly analyzes the functions of MAM in regulating Ca²⁺ transport, lipid synthesis, mitochondrial fusion and fission, endoplasmic reticulum stress and oxidative stress, autophagy and inflammation. Since ER stress and mitochondrial dysfunction are important pathological events in neurological diseases including ischemic stroke, MAM is likely to play an important role in cerebral ischemia by regulating the signaling of the two organelles and the crosstalk of the two pathological events.

Automatic segmentation and reconstruction of intracellular compartments in volumetric electron microscopy data

BACKGROUND AND OBJECTIVES

In recent years, electron microscopy is enabling the acquisition of volumetric data with resolving power to directly observe the ultrastructure of intracellular compartments. New insights and knowledge about cell processes that are offered by such data require a comprehensive analysis which is limited by the time-consuming manual segmentation and reconstruction methods.

METHOD

We present methods for automatic segmentation, reconstruction, and analysis of intracellular compartments from volumetric data obtained by the dual-beam electron microscopy. We specifically address segmentation of fusiform vesicles and the Golgi apparatus, reconstruction of mitochondria and fusiform vesicles, and morphological analysis of the reconstructed mitochondria.

RESULTS AND CONCLUSION

Evaluation on the public UroCell dataset demonstrated high accuracy of the proposed methods for segmentation of fusiform vesicles and the Golgi apparatus, as well as for reconstruction of mitochondria and analysis of their shapes, while reconstruction of fusiform vesicles proved to be more challenging. We published an extension of the UroCell dataset with all of the data used in this work, to further contribute to research on automatic analysis of the ultrastructure of intracellular compartments.

Chemotherapy Resistance: Role of Mitochondrial and Autophagic Components

Simple Summary

Chemotherapy resistance is a common occurrence during cancer treatment that cancer researchers are attempting to understand and overcome. Mitochondria are a crucial intracellular signaling core that are becoming important determinants of numerous aspects of cancer genesis and progression, such as metabolic reprogramming, metastatic capability, and chemotherapeutic resistance. Mitophagy, or selective autophagy of mitochondria, can influence both the efficacy of tumor chemotherapy and the degree of drug resistance. Regardless of the fact that mitochondria are well-known for coordinating ATP synthesis from cellular respiration in cellular bioenergetics, little is known its mitophagy regulation in chemoresistance. Recent advancements in mitochondrial research, mitophagy regulatory mechanisms, and their implications for our understanding of chemotherapy resistance are discussed in this review.

Abstract

Cancer chemotherapy resistance is one of the most critical obstacles in cancer therapy. One of the well-known mechanisms of chemotherapy resistance is the change in the mitochondrial death pathways which occur when cells are under stressful situations, such as chemotherapy. Mitophagy, or mitochondrial selective autophagy, is critical for cell quality control because it can efficiently break down, remove, and recycle defective or damaged mitochondria. As cancer cells use mitophagy to rapidly sweep away damaged mitochondria in order to mediate their own drug resistance, it influences the efficacy of tumor chemotherapy as well as the degree of drug resistance. Yet despite the importance of mitochondria and mitophagy in chemotherapy resistance, little is known about the precise mechanisms involved. As a consequence, identifying potential therapeutic targets by analyzing the signal pathways that govern mitophagy has become a vital research goal. In this paper, we review recent advances in mitochondrial research, mitophagy control mechanisms, and their implications for our understanding of chemotherapy resistance.

Mitofusin-2 Negatively Regulates Melanogenesis by Modulating Mitochondrial ROS Generation

Inter-organellar communication is emerging as one of the most crucial regulators of cellular physiology. One of the key regulators of inter-organellar communication is Mitofusin-2 (MFN2). MFN2 is also involved in mediating mitochondrial fusion–fission dynamics. Further, it facilitates mitochondrial crosstalk with the endoplasmic reticulum, lysosomes and melanosomes, which are lysosome-related organelles specialized in melanin synthesis within melanocytes. However, the role of MFN2 in regulating melanocyte-specific cellular function, i.e., melanogenesis, remains poorly understood. Here, using a B16 mouse melanoma cell line and primary human melanocytes, we report that MFN2 negatively regulates melanogenesis. Both the transient and stable knockdown of MFN2 leads to enhanced melanogenesis, which is associated with an increase in the number of mature (stage III and IV) melanosomes and the augmented expression of key melanogenic enzymes. Further, the ectopic expression of MFN2 in MFN2-silenced cells leads to the complete rescue of the phenotype at the cellular and molecular levels. Mechanistically, MFN2-silencing elevates mitochondrial reactive-oxygen-species (ROS) levels which in turn increases melanogenesis. ROS quenching with the antioxidant N-acetyl cysteine (NAC) reverses the MFN2-knockdown-mediated increase in melanogenesis. Moreover, MFN2 expression is significantly lower in the darkly pigmented primary human melanocytes in comparison to lightly pigmented melanocytes, highlighting a potential contribution of lower MFN2 levels to higher physiological pigmentation. Taken together, our work establishes MFN2 as a novel negative regulator of melanogenesis.

Time-of-Day Circadian Modulation of Grape-Seed Procyanidin Extract (GSPE) in Hepatic Mitochondrial Dynamics in Cafeteria-Diet-Induced Obese Rats

Major susceptibility to alterations in liver function (e.g., hepatic steatosis) in a prone environment due to circadian misalignments represents a common consequence of recent sociobiological behavior (i.e., food excess and sleep deprivation). Natural compounds and, more concisely, polyphenols have been shown as an interesting tool for fighting against metabolic syndrome and related consequences. Furthermore, mitochondria have been identified as an important target for mediation of the health effects of these compounds. Additionally, mitochondrial function and dynamics are strongly regulated in a circadian way. Thus, we wondered whether some of the beneficial effects of grape-seed procyanidin extract (GSPE) on metabolic syndrome could be mediated by a circadian modulation of mitochondrial homeostasis. For this purpose, rats were subjected to “standard”, “cafeteria” and “cafeteria diet + GSPE” treatments (n = 4/group) for 9 weeks (the last 4 weeks, GSPE/vehicle) of treatment, administering the extract/vehicle at diurnal or nocturnal times (ZT0 or ZT12). For circadian assessment, one hour after turning the light on (ZT1), animals were sacrificed every 6 h (ZT1, ZT7, ZT13 and ZT19). Interestingly, GSPE was able to restore the rhythm on clock hepatic genes (Bmal1, Per2, Cry1, Rorα), as this correction was more evident in nocturnal treatment. Additionally, during nocturnal treatment, an increase in hepatic fusion genes and a decrease in fission genes were observed. Regarding mitochondrial complex activity, there was a strong effect of cafeteria diet at nearly all ZTs, and GSPE was able to restore activity at discrete ZTs, mainly in the diurnal treatment (ZT0). Furthermore, a differential behavior was observed in tricarboxylic acid (TCA) metabolites between GSPE diurnal and nocturnal administration times. Therefore, GSPE may serve as a nutritional preventive strategy in the recovery of hepatic-related metabolic disease by modulating mitochondrial dynamics, which is concomitant to the restoration of the hepatic circadian machinery.

Mitochondrial nanomedicine: Subcellular organelle-specific delivery of molecular medicines

As mitochondria network together to act as the master sensors and effectors of apoptosis, ATP production, reactive oxygen species management, mitophagy/autophagy, and homeostasis; this organelle is an ideal target for pharmaceutical manipulation. Mitochondrial dysfunction contributes to many diseases, for example, β-amyloid has been shown to interfere with mitochondrial protein import and induce apoptosis in Alzheimer's Disease while some forms of Parkinson's Disease are associated with dysfunctional mitochondrial PINK1 and Parkin proteins. Mitochondrial medicine has applications in the treatment of an array of pathologies from cancer to cardiovascular disease. A challenge of mitochondrial medicine is directing therapies to a subcellular target. Nanotechnology based approaches combined with mitochondrial targeting strategies can greatly improve the clinical translation and effectiveness of mitochondrial medicine. This review discusses mitochondrial drug delivery approaches and applications of mitochondrial nanomedicines. Nanomedicine approaches have the potential to drive the success of mitochondrial therapies into the clinic.

Mitochondrial Adaptation to Diet and Swimming Activity in Gilthead Seabream: Improved Nutritional Efficiency

Sustained exercise promotes growth in different fish species, and in gilthead seabream we have demonstrated that it improves nutrient use efficiency. This study assesses for differences in growth rate, tissue composition and energy metabolism in gilthead seabream juveniles fed two diets: high-protein (HP; 54% protein, 15% lipid) or high energy (HE; 50% protein, 20% lipid), under voluntary swimming (VS) or moderate-to-low-intensity sustained swimming (SS) for 6 weeks. HE fed fish under VS conditions showed lower body weight and higher muscle lipid content than HP fed fish, but no differences between the two groups were observed under SS conditions. Irrespective of the swimming regime, the white muscle stable isotopes profile of the HE group revealed increased nitrogen and carbon turnovers. Nitrogen fractionation increased in the HP fed fish under SS, indicating enhanced dietary protein oxidation. Hepatic gene expression markers of energy metabolism and mitochondrial biogenesis showed clear differences between the two diets under VS: a significant shift in the COX/CS ratio, modifications in UCPs, and downregulation of PGC1a in the HE-fed fish. Swimming induced mitochondrial remodeling through upregulation of fusion and fission markers, and removing almost all the differences observed under VS. In the HE-fed fish, white skeletal muscle benefited from the increased energy demand, amending the oxidative uncoupling produced under the VS condition by an excess of lipids and the pro-fission state observed in mitochondria. Contrarily, red muscle revealed more tolerant to the energy content of the HE diet, even under VS conditions, with higher expression of oxidative enzymes (COX and CS) without any sign of mitochondrial stress or mitochondrial biogenesis induction. Furthermore, this tissue had enough plasticity to shift its metabolism under higher energy demand (SS), again equalizing the differences observed between diets under VS condition. Globally, the balance between dietary nutrients affects mitochondrial regulation due to their use as energy fuels, but exercise corrects imbalances allowing practical diets with lower protein and higher lipid content without detrimental effects.

PINK1: A Bridge between Mitochondria and Parkinson's Disease

Mitochondria are known as highly dynamic organelles essential for energy production. Intriguingly, in the recent years, mitochondria have revealed the ability to maintain cell homeostasis and ultimately regulate cell fate. This regulation is achieved by evoking mitochondrial quality control pathways that are capable of sensing the overall status of the cellular environment. In a first instance, actions to maintain a robust pool of mitochondria take place; however, if unsuccessful, measures that lead to overall cell death occur. One of the central key players of these mitochondrial quality control pathways is PINK1 (PTEN-induce putative kinase), a mitochondrial targeted kinase. PINK1 is known to interact with several substrates to regulate mitochondrial functions, and not only is responsible for triggering mitochondrial clearance via mitophagy, but also participates in maintenance of mitochondrial functions and homeostasis, under healthy conditions. Moreover, PINK1 has been associated with the familial form of Parkinson’s disease (PD). Growing evidence has strongly linked mitochondrial homeostasis to the central nervous system (CNS), a system that is replenished with high energy demanding long-lasting neuronal cells. Moreover, sporadic cases of PD have also revealed mitochondrial impairments. Thus, one could speculate that mitochondrial homeostasis is the common denominator in these two forms of the disease, and PINK1 may play a central role in maintaining mitochondrial homeostasis. In this review, we will discuss the role of PINK1 in the mitochondrial physiology and scrutinize its role in the cascade of PD pathology.

Non-invasive in vivo human model of post-ischaemic skin preconditioning by measurement of flow-mediated 460-nm autofluorescence

AIMS

Transient ischaemia and reperfusion (TIAR) induce early ischaemic preconditioning (IPC) in different tissues and organs, including the skin. IPC protects tissues by modifying the mitochondrial function and decreasing the amount of the reduced form of nicotinamide adenine dinucleotide (NADH). Skin 460-nm autofluorescence is proportional to the NADH content and can be non-invasively measured during TIAR. We propose a non-invasive in vivo human model of skin IPC for studying the effects of repeated TIARs on the NADH content.

METHODS

Fifty-one apparently healthy volunteers (36 women) underwent three 100-second forearm ischaemia episodes induced by inflation of brachial pressure cuff to the pressure of 60 mmHg above systolic blood pressure, followed by 500-second long reperfusion episodes. Changes in skin NADH content were measured using 460-nm fluorescence before and during each of the three TIARs.

RESULTS

The first two TIARs caused a significant reduction in the skin NADH content before (P = .0065) and during the third ischaemia (P = .0011) and reperfusion (P = .0003) up to 3.0%. During the third TIAR, the increase in skin NADH was 20% lower than during the first ischaemia (P = .0474).

CONCLUSIONS

The measurement of the 460-nm fluorescence during repeated TIARs allows for a non-invasive in vivo investigation of human skin IPC. Although IPC reduces the overall NADH skin content, the most noticeable NADH reduction appears during ischaemia after earlier TIARs. Studying the skin model of IPC may provide new avenues for in vivo physiological, clinical and pharmacological research on mitochondrial metabolism.

Mitochondria: Potential Targets for Osteoarthritis

Osteoarthritis (OA) is a common and disabling joint disorder that is mainly characterized by cartilage degeneration and narrow joint spaces. The role of mitochondrial dysfunction in promoting the development of OA has gained much attention. Targeting endogenous molecules to improve mitochondrial function is a potential treatment for OA. Moreover, research on exogenous drugs to improve mitochondrial function in OA based on endogenous molecular targets has been accomplished. In addition, stem cells and exosomes have been deeply researched in the context of cartilage regeneration, and these factors both reverse mitochondrial dysfunctions. Thus, we hypothesize that biomedical approaches will be applied to the treatment of OA. Furthermore, we have summarized the global status of mitochondria and osteoarthritis research in the past two decades, which will contribute to the research field and the development of novel treatment strategies for OA.

Mitochondrial Metabolism, Contact Sites and Cellular Calcium Signaling: Implications for Tumorigenesis

Mitochondria are organelles that are mainly involved in the generation of ATP by cellular respiration. In addition, they modulate several intracellular functions, ranging from cell proliferation and differentiation to cell death. Importantly, mitochondria are social and can interact with other organelles, such as the Endoplasmic Reticulum, lysosomes and peroxisomes. This symbiotic relationship gives advantages to both partners in regulating some of their functions related to several aspects of cell survival, metabolism, sensitivity to cell death and metastasis, which can all finally contribute to tumorigenesis. Moreover, growing evidence indicates that modulation of the length and/or numbers of these contacts, as well as of the distance between the two engaged organelles, impacts both on their function as well as on cellular signaling. In this review, we discuss recent advances in the field of contacts and communication between mitochondria and other intracellular organelles, focusing on how the tuning of mitochondrial function might impact on both the interaction with other organelles as well as on intracellular signaling in cancer development and progression, with a special focus on calcium signaling.

Mitochondria Associated Membranes (MAMs): Emerging Drug Targets for Diabetes

MAMs, the physical association between the Endoplasmic Reticulum (ER) and mitochondria are, functional domains performing a significant role in the maintenance of cellular homeostasis. It is evolving as an important signaling center that coordinates nutrient and hormonal signaling for the proper regulation of hepatic insulin action and glucose homeostasis. Moreover, MAMs can be considered as hot spots for the transmission of stress signals from ER to mitochondria. The altered interaction between ER and mitochondria results in the amendment of several insulin-sensitive tissues, revealing the role of MAMs in glucose homeostasis. The development of mitochondrial dysfunction, ER stress, altered lipid and Ca2+ homeostasis are typically co-related with insulin resistance and β cell dysfunction. But little facts are known about the role played by these stresses in the development of metabolic disorders. In this review, we highlight the mechanisms involved in maintaining the contact site with new avenues of investigations for the development of novel preventive and therapeutic targets for T2DM.

Mitochondrial Function in Enamel Development

Enamel is the most calcified tissue in vertebrates. Enamel formation and mineralization is a two-step process that is mediated by ameloblast cells during their secretory and maturation stages. In these two stages, ameloblasts are characterized by different morphology and function, which is fundamental for proper mineral growth in the extracellular space. Ultrastructural studies have shown that the mitochondria in these cells localize to different subcellular regions in both stages. However, limited knowledge is available on the role/s of mitochondria in enamel formation. To address this issue, we analyzed mitochondrial biogenesis and respiration, as well as the redox status of rat primary enamel cells isolated from the secretory and maturation stages. We show that maturation stage cells have an increased expression of PGC1α, a marker of mitochondrial biogenesis, and of components of the electron transport chain. Oxygen consumption rate (OCR), a proxy for mitochondrial function, showed a significant increase in oxidative phosphorylation during the maturation stage, promoting ATP production. The GSH/GSSG ratio was lower in the maturation stage, indicative of increased oxidation. Because higher oxidative phosphorylation can lead to higher ROS production, we tested if ROS affected the expression of AmelX and Enam genes that are essential for enamel formation. The ameloblast cell line LS8 treated with H₂O₂ to promote ROS elicited significant expression changes in AmelX and Enam. Our data highlight important metabolic and physiological differences across the two enamel stages, with higher ATP levels in the maturation stage indicative of a higher energy demand. Besides these metabolic shifts, it is likely that the enhanced ETC function results in ROS-mediated transcriptional changes.

Defective nucleotide-dependent assembly and membrane fusion in Mfn2 CMT2A variants improved by Bax

Mfn2 CMT2A–associated variants located proximal to the hinge connecting its two extended helical bundles have impaired GTP-dependent assembly and mitochondrial fusion activity, which are both improved by cytosolic Bax.

Mitofusins are members of the dynamin-related protein family of large GTPases that harness the energy from nucleotide hydrolysis to remodel membranes. Mitofusins possess four structural domains, including a GTPase domain, two extended helical bundles (HB1 and HB2), and a transmembrane region. We have characterized four Charcot-Marie-Tooth type 2A–associated variants with amino acid substitutions in Mfn2 that are proximal to the hinge that connects HB1 and HB2. A functional defect was not apparent in cells as the mitochondrial morphology of Mfn2-null cells was restored by expression of any of these variants. However, a significant fusion deficiency was observed in vitro, which was improved by the addition of crude cytosol extract or soluble Bax. All four variants had reduced nucleotide-dependent assembly in cis, but not trans, and this was also improved by the addition of Bax. Together, our data demonstrate an important role for this region in Mfn2 GTP-dependent oligomerization and membrane fusion and is consistent with a model where cytosolic factors such as Bax are masking molecular defects associated with Mfn2 disease variants in cells.

Mitochondrial fission and fusion: A dynamic role in aging and potential target for age-related disease

The mitochondria is the major hub to convert energy for cellular processes. Dysregulation of mitochondrial function is one of the classical hallmarks of aging, and mitochondrial interventions have repeatedly been shown to improve outcomes in age-related diseases. Crucial to mitochondrial regulation is the dynamic nature of their network structure. Mitochondria separate and merge using fission and fusion processes in response to changes in energy and stress status. While many mitochondrial processes are already characterized in relation to aging, specific evidence in multicellular organisms causally linking mitochondrial dynamics to the regulation of lifespan is limited. There does exist, however, a large body of evidence connecting mitochondrial dynamics to other aging-related cellular processes and implicates them in a number of human diseases. Here, we discuss the mechanisms of mitochondrial fission and fusion, the current evidence of their role in aging of multicellular organisms, and how these connect to cell cycle regulation, quality control, and transmission of energy status. Finally, we discuss the current evidence implicating these processes in age-related human pathologies, such as neurodegenerative or cardio-metabolic diseases. We suggest that deeper understanding of the regulatory mechanisms within this system and downstream implications could benefit in understanding and intervention of these conditions.

Interplay Between Mitochondrial Peroxiredoxins and ROS in Cancer Development and Progression

Mitochondria are multifunctional cellular organelles that are major producers of reactive oxygen species (ROS) in eukaryotes; to maintain the redox balance, they are supplemented with different ROS scavengers, including mitochondrial peroxiredoxins (Prdxs). Mitochondrial Prdxs have physiological and pathological significance and are associated with the initiation and progression of various cancer types. In this review, we have focused on signaling involving ROS and mitochondrial Prdxs that is associated with cancer development and progression. An upregulated expression of Prdx3 and Prdx5 has been reported in different cancer types, such as breast, ovarian, endometrial, and lung cancers, as well as in Hodgkin's lymphoma and hepatocellular carcinoma. The expression of Prdx3 and Prdx5 in different types of malignancies involves their association with different factors, such as transcription factors, micro RNAs, tumor suppressors, response elements, and oncogenic genes. The microenvironment of mitochondrial Prdxs plays an important role in cancer development, as cancerous cells are equipped with a high level of antioxidants to overcome excessive ROS production. However, an increased production of Prdx3 and Prdx5 is associated with the development of chemoresistance in certain types of cancers and it leads to further complications in cancer treatment. Understanding the interplay between mitochondrial Prdxs and ROS in carcinogenesis can be useful in the development of anticancer drugs with better proficiency and decreased resistance. However, more targeted studies are required for exploring the tumor microenvironment in association with mitochondrial Prdxs to improve the existing cancer therapies and drug development.

Mitochondrial Dynamics in Stem Cells and Differentiation

Mitochondria are highly dynamic organelles that continuously change their shape. Their main function is adenosine triphosphate (ATP) production; however, they are additionally involved in a variety of cellular phenomena, such as apoptosis, cell cycle, proliferation, differentiation, reprogramming, and aging. The change in mitochondrial morphology is closely related to the functionality of mitochondria. Normal mitochondrial dynamics are critical for cellular function, embryonic development, and tissue formation. Thus, defects in proteins involved in mitochondrial dynamics that control mitochondrial fusion and fission can affect cellular differentiation, proliferation, cellular reprogramming, and aging. Here, we review the processes and proteins involved in mitochondrial dynamics and their various associated cellular phenomena.

MeHg Causes Ultrastructural Changes in Mitochondria and Autophagy in the Spinal Cord Cells of Chicken Embryo

Methylmercury (MeHg) is a known neurodevelopmental toxicant, which causes changes in various structures of the central nervous system (CNS). However, ultrastructural studies of its effects on the developing CNS are still scarce. Here, we investigated the effect of MeHg on the ultrastructure of the cells in spinal cord layers. Chicken embryos at E3 were treated in ovo with 0.1 μg MeHg/50 μL saline solution and analyzed at E10. Then, we used transmission electron microscopy (TEM) to identify possible damage caused by MeHg to the structures and organelles of the spinal cord cells. After MeHg treatment, we observed, in the spinal cord mantle layer, a significant number of altered mitochondria with external membrane disruptions, crest disorganization, swelling in the mitochondrial matrix, and vacuole formation between the internal and external mitochondrial membranes. We also observed dilations in the Golgi complex and endoplasmic reticulum cisterns and the appearance of myelin-like cytoplasmic inclusions. We observed no difference in the total mitochondria number between the control and MeHg-treated groups. However, the MeHg-treated embryos showed an increased number of altered mitochondria and a decreased number of mitochondrial fusion profiles. Additionally, unusual mitochondrial shapes were found in MeHg-treated embryos as well as autophagic vacuoles similar to mitophagic profiles. In addition, we observed autophagic vacuoles with amorphous, homogeneous, and electron-dense contents, similar to the autophagy. Our results showed, for the first time, the neurotoxic effect of MeHg on the ultrastructure of the developing spinal cord. Using TEM we demonstrate that changes in the endomembrane system, mitochondrial damage, disturbance in mitochondrial dynamics, and increase in mitophagy were caused by MeHg exposure.

Mitochondrial function - gatekeeper of intestinal epithelial cell homeostasis

The intestinal epithelium is a multicellular interface in close proximity to a dense microbial milieu that is completely renewed every 3-5 days. Pluripotent stem cells reside at the crypt, giving rise to transient amplifying cells that go through continuous steps of proliferation, differentiation and finally anoikis (a form of programmed cell death) while migrating upwards to the villus tip. During these cellular transitions, intestinal epithelial cells (IECs) possess distinct metabolic identities reflected by changes in mitochondrial activity. Mitochondrial function emerges as a key player in cell fate decisions and in coordinating cellular metabolism, immunity, stress responses and apoptosis. Mediators of mitochondrial signalling include molecules such as ATP and reactive oxygen species and interrelate with pathways such as the mitochondrial unfolded protein response (MT-UPR) and AMP kinase signalling, in turn affecting cell cycle progression and stemness. Alterations in mitochondrial function and MT-UPR activation are integral aspects of pathologies, including IBD and cancer. Mitochondrial signalling and concomitant changes in metabolism contribute to intestinal homeostasis and regulate IEC dedifferentiation-differentiation programmes in the context of diseases, suggesting that mitochondrial function as a cellular checkpoint critically contributes to disease outcome. This Review highlights mitochondrial function and MT-UPR signalling in epithelial cell stemness, differentiation and lineage commitment and illustrates mitochondrial function in intestinal diseases.

The regulation of tumor cell physiology by mitochondrial dynamics

Mitochondrial dynamics are increasingly recognized to play an important role in regulating mitochondrial function in response to diverse stimuli. Given the overlap in the physiological processes influenced by mitochondria and the physiological processes disrupted in tumor cells, we speculate that tumor cells alter mitochondrial shape to promote the tumorigenic phenotype. Here, we briefly review the evidence linking changes in mitochondrial fusion and fission to a number of key tumorigenic processes, including metabolic rewiring, inhibition of cell death, cell migration, cell proliferation and self-renewal capacity. The role of mitochondrial dynamics in tumor growth is an important emerging area of research, a better understanding of which may lead to promising new therapeutic options for the treatment of cancer.

Novel Mitochondria-Targeted Furocoumarin Derivatives as Possible Anti-Cancer Agents

Targeting small molecules to appropriate subcellular compartments is a way to increase their selectivity and effectiveness while minimizing side effects. This can be accomplished either by stably incorporating specific "homing" properties into the structure of the active principle, or by attaching to it a targeting moiety via a labile linker, i.e., by producing a "targeting pro-drug." Mitochondria are a recognized therapeutic target in oncology, and blocking the population of the potassium channel Kv1.3 residing in the inner mitochondrial membrane (mtKv1.3) has been shown to cause apoptosis of cancerous cells expressing it. These concepts have led us to devise novel, mitochondria-targeted, membrane-permeant drug candidates containing the furocoumarin (psoralenic) ring system and the triphenylphosphonium (TPP) lipophilic cation. The strategy has proven effective in various cancer models, including pancreatic ductal adenocarcinoma, melanoma, and glioblastoma, stimulating us to devise further novel molecules to extend and diversify the range of available drugs of this type. New compounds were synthesized and tested in vitro; one of them-a prodrug in which the coumarinic moiety and the TPP group are linked by a bridge comprising a labile carbonate bond system-proved quite effective in in vitro cytotoxicity assays. Selective death induction is attributed to inhibition of mtKv1.3. This results in oxidative stress, which is fatal for the already-stressed malignant cells. This compound may thus be a candidate drug for the mtKv1.3-targeting therapeutic approach.

A reciprocal relationship between reactive oxygen species and mitochondrial dynamics in neurodegeneration

Mitochondrial fragmentation due to fission/fusion imbalance has often been linked to mitochondrial dysfunction and apoptosis in neurodegeneration. Conventionally, it is believed that once mitochondrial morphology shifts away from its physiological tubular form, mitochondria become defective and downstream apoptotic signaling pathways are triggered. However, our study shows that beta-amyloid (Aβ) induces morphological changes in mitochondria where they become granular-shaped and are distinct from fragmented mitochondria in terms of both morphology and functions. Accumulation of mitochondrial reactive oxygen species triggers granular mitochondria formation, while mitoTEMPO (a mitochondria-targeted superoxide scavenger) restores tubular mitochondrial morphology within Aβ-treated neurons. Interestingly, modulations of mitochondria fission and fusion by genetic and pharmacological tools attenuated not only the induction of granular mitochondria, but also mitochondrial superoxide levels in Aβ-treated neurons. Our study shows a reciprocal relationship between mitochondrial dynamics and reactive oxygen species and provides a new potential therapeutic target at early stages of neurodegenerative disease pathogenesis.

Mitochondrial fragmentation affects neither the sensitivity to TNFα-induced apoptosis of Brucella-infected cells nor the intracellular replication of the bacteria

Mitochondria are complex organelles that participate in many cellular functions, ranging from ATP production to immune responses against viruses and bacteria. This integration of a plethora of functions within a single organelle makes mitochondria a very attractive target to manipulate for intracellular pathogens. We characterised the crosstalk that exists between Brucella abortus, the causative agent of brucellosis, and the mitochondria of infected cells. Brucella replicates in a compartment derived from the endoplasmic reticulum (ER) and modulates ER functionality by activating the unfolded protein response. However, the impact of Brucella on the mitochondrial population of infected cells still requires a systematic study. We observed physical contacts between Brucella containing vacuoles and mitochondria. We also found that B. abortus replication is independent of mitochondrial oxidative phosphorylation and that mitochondrial reactive oxygen species do not participate to the control of B. abortus infection in vitro. We demonstrated that B. abortus and B. melitensis induce a drastic mitochondrial fragmentation at 48 hours post-infection in different cell types, including myeloid and non-myeloid cells. This fragmentation is DRP1-independent and might be caused by a deficit of mitochondrial fusion. However, mitochondrial fragmentation does not change neither Brucella replication efficiency, nor the susceptibility of infected cells to TNFα-induced apoptosis.

Mitochondria and Sex-Specific Cardiac Function

The focus of this chapter is the gender differences in mitochondria in cardiovascular disease. There is broad evidence suggesting that some of the gender differences in cardiovascular outcome may be partially related to differences in mitochondrial biology (Ventura-Clapier R, Moulin M, Piquereau J, Lemaire C, Mericskay M, Veksler V, Garnier A, Clin Sci (Lond) 131(9):803-822, 2017)). Mitochondrial disorders are causally affected by mutations in either nuclear or mitochondrial genes involved in the synthesis of respiratory chain subunits or in their posttranslational control. This can be due to mutations of the mtDNA which are transmitted by the mother or mutations in the nuclear DNA. Because natural selection on mitochondria operates only in females, mutations may have had more deleterious effects in males than in females (Ventura-Clapier R, Moulin M, Piquereau J, Lemaire C, Mericskay M, Veksler V, Garnier A, Clin Sci (Lond) 131(9):803-822, 2017; Camara AK, Lesnefsky EJ, Stowe DF. Antioxid Redox Signal 13(3):279-347, 2010). As mitochondrial mutations can affect all tissues, they are responsible for a large panel of pathologies including neuromuscular disorders, encephalopathies, metabolic disorders, cardiomyopathies, neuropathies, renal dysfunction, etc. Many of these pathologies present sex/gender specificity. Thus, alleviating or preventing mitochondrial dysfunction will contribute to mitigating the severity or progression of the development of diseases. Here, we present evidence for the involvement of mitochondria in the sex specificity of cardiovascular disorders.

Energetic mitochondrial failing in vitiligo and possible rescue by cardiolipin

Vitiligo is characterized by death or functional defects of epidermal melanocytes through still controversial pathogenic process. Previously, we showed that mitochondria-driven pre-senescent phenotype diminishes the capability of vitiligo melanocytes to cope with stressful stimuli. In the current study, we investigated markers of mitochondrial energy metabolism including the PGC1a axis, and then we determined the index of mitochondrial impairment using a cytomic approach. We found in cultured epidermal vitiligo melanocytes, compared to healthy ones, low ATP, increased proton leakage, and altered expression of several glycolytic enzymes (hexokinase II, pyruvic dehydrogenase kinase 1 and pyruvic kinase M2), We suggest that the low ATP production may be sufficient in steady-state conditions but it is unable to cover further needs. We also found in vitiligo melanocyrtes hyper-activation of the PGC1α axis, finalized to counteract the energy defect. Cytomic analysis, supported by MitoTracker Red pattern and ex-vivo immunohistochemistry, suggested an increased mitochondrial mass, possibly useful to ensure the essential ATP level. Finally, pharmacological cardiolipin stabilization reverted the energetic impairment, confirming the initial mitochondrial role. In conclusion, we report new insight in the pathogenetic mechanism of viitligo and indicate that the mitochondrial failure rescue by cardiolipin manipulation may be a new intriguing target in treatment development.

Optogenetic control of mitochondrial metabolism and Ca2+ signaling by mitochondria-targeted opsins

Key mitochondrial functions such as ATP production, Ca²⁺ uptake and release, and substrate accumulation depend on the proton electrochemical gradient (ΔμH⁺) across the inner membrane. Although several drugs can modulate ΔμH⁺, their effects are hardly reversible, and lack cellular specificity and spatial resolution. Although channelrhodopsins are widely used to modulate the plasma membrane potential of excitable cells, mitochondria have thus far eluded optogenetic control. Here we describe a toolkit of optometabolic constructs based on selective targeting of channelrhodopsins with distinct functional properties to the inner mitochondrial membrane of intact cells. We show that our strategy enables a light-dependent control of the mitochondrial membrane potential (Δψ_m) and coupled mitochondrial functions such as ATP synthesis by oxidative phosphorylation, Ca²⁺ dynamics, and respiratory metabolism. By directly modulating Δψ_m, the mitochondria-targeted opsins were used to control complex physiological processes such as spontaneous beats in cardiac myocytes and glucose-dependent ATP increase in pancreatic β-cells. Furthermore, our optometabolic tools allow modulation of mitochondrial functions in single cells and defined cell regions.

Label-Free Quantification of Intracellular Mitochondrial Dynamics Using Dielectrophoresis

Mitochondrial dynamics play an important role within several pathological conditions, including cancer and neurological diseases. For the purpose of identifying therapies that target aberrant regulation of the mitochondrial dynamics machinery and characterizing the regulating signaling pathways, there is a need for label-free means to detect the dynamic alterations in mitochondrial morphology. We present the use of dielectrophoresis for label-free quantification of intracellular mitochondrial modifications that alter cytoplasmic conductivity, and these changes are benchmarked against label-based image analysis of the mitochondrial network. This is validated by quantifying the mitochondrial alterations that are carried out by entirely independent means on two different cell lines: human embryonic kidney cells and mouse embryonic fibroblasts. In both cell lines, the inhibition of mitochondrial fission that leads to a mitochondrial structure of higher connectivity is shown to substantially enhance conductivity of the cell interior, as apparent from the significantly higher positive dielectrophoresis levels in the 0.5-15 MHz range. Using single-cell velocity tracking, we show ∼10-fold higher positive dielectrophoresis levels at 0.5 MHz for cells with a highly connected versus those with a highly fragmented mitochondrial structure, suggesting the feasibility for frequency-selective dielectrophoretic isolation of cells to aid the discovery process for development of therapeutics targeting the mitochondrial machinery.

Mitochondria-Associated Membranes and ER Stress

The endoplasmic reticulum (ER) is a crucial organelle for coordinating cellular Ca²⁺ signaling and protein synthesis and folding. Moreover, the dynamic and complex membranous structures constituting the ER allow the formation of contact sites with other organelles and structures, including among others the mitochondria and the plasma membrane (PM). The contact sites that the ER form with mitochondria is a hot topic in research, and the nature of the so-called mitochondria-associated membranes (MAMs) is continuously evolving. The MAMs consist of a proteinaceous tether that physically connects the ER with mitochondria. The MAMs harness the main functions of both organelles to form a specialized subcompartment at the interface of the ER and mitochondria. Under homeostatic conditions, MAMs are crucial for the efficient transfer of Ca²⁺ from the ER to mitochondria, and for proper mitochondria bioenergetics and lipid synthesis. MAMs are also believed to be the master regulators of mitochondrial shape and motility, and to form a crucial site for autophagosome assembly. Not surprisingly, MAMs have been shown to be a hot spot for the transfer of stress signals from the ER to mitochondria, most notably under the conditions of loss of ER proteostasis, by engaging the unfolded protein response (UPR). In this chapter after an introduction on ER biology and ER stress, we will review the emerging and key signaling roles of the MAMs, which have a root in cellular processes and signaling cascades coordinated by the ER.

Dysfunctional mitochondrial fission impairs cell reprogramming

We have recently shown that mitochondrial fission is induced early in reprogramming in a Drp1-dependent manner; however, the identity of the factors controlling Drp1 recruitment to mitochondria was unexplored. To investigate this, we used a panel of RNAi targeting factors involved in the regulation of mitochondrial dynamics and we observed that MiD51, Gdap1 and, to a lesser extent, Mff were found to play key roles in this process. Cells derived from Gdap1-null mice were used to further explore the role of this factor in cell reprogramming. Microarray data revealed a prominent down-regulation of cell cycle pathways in Gdap1-null cells early in reprogramming and cell cycle profiling uncovered a G2/M growth arrest in Gdap1-null cells undergoing reprogramming. High-Content analysis showed that this growth arrest was DNA damage-independent. We propose that lack of efficient mitochondrial fission impairs cell reprogramming by interfering with cell cycle progression in a DNA damage-independent manner.

Milestones and recent discoveries on cell death mediated by mitochondria and their interactions with biologically active amines

Mitochondria represent cell "powerhouses," being involved in energy transduction from the electrochemical gradient to ATP synthesis. The morphology of their cell types may change, according to various metabolic processes or osmotic pressure. A new morphology of the inner membrane and mitochondrial cristae, significantly different from the previous one, has been proposed for the inner membrane and mitochondrial cristae, based on the technique of electron tomography. Mitochondrial Ca(2+) transport (the transporter has been isolated) generates reactive oxygen species and induces the mitochondrial permeability transition of both inner and outer mitochondrial membranes, leading to induction of necrosis and apoptosis. In the mitochondria of several cell types (liver, kidney, and heart), mitochondrial oxidative stress is an essential step in the induction of cell death, although not in brain, in which the phenomenon is caused by a different mechanism. Mitochondrial permeability transition drives both apoptosis and necrosis, whereas mitochondrial outer membrane permeability is characteristic of apoptosis. Adenine nucleotide translocase remains the most important component involved in membrane permeability, with the opening of the transition pore, although other proteins, such as ATP synthase or phosphate carriers, have been proposed. Intrinsic cell death is triggered by the release from mitochondria of proteic factors, such as cytochrome c, apoptosis inducing factor, and Smac/DIABLO, with the activation of caspases upon mitochondrial permeability transition or mitochondrial outer membrane permeability induction. Mitochondrial permeability transition induces the permeability of the inner membrane in sites in contact with the outer membrane; mitochondrial outer membrane permeability forms channels on the outer membrane by means of various stimuli involving Bcl-2 family proteins. The biologically active amines, spermine, and agmatine, have specific functions on mitochondria which distinguish them from other amines. Enzymatic oxidative deamination of spermine by amine oxidases in tumor cells may produce reactive oxygen species, leading to transition pore opening and apoptosis. This process could be exploited as a new therapeutic strategy to combat cancer.

Syntaxin-17 delivers PINK1/parkin-dependent mitochondrial vesicles to the endolysosomal system

Vesicular transport from mitochondria to lysosomes is an emerging mitochondrial quality control mechanism. Here, McLelland et al. identify how mitochondrial vesicles are targeted for degradation, showing that syntaxin-17 is recruited to these structures to govern their SNARE-dependent fusion with endolysosomes.

Mitochondria are considered autonomous organelles, physically separated from endocytic and biosynthetic pathways. However, recent work uncovered a PINK1/parkin-dependent vesicle transport pathway wherein oxidized or damaged mitochondrial content are selectively delivered to the late endosome/lysosome for degradation, providing evidence that mitochondria are indeed integrated within the endomembrane system. Given that mitochondria have not been shown to use canonical soluble NSF attachment protein receptor (SNARE) machinery for fusion, the mechanism by which mitochondrial-derived vesicles (MDVs) are targeted to the endosomal compartment has remained unclear. In this study, we identify syntaxin-17 as a core mitochondrial SNARE required for the delivery of stress-induced PINK1/parkin-dependent MDVs to the late endosome/lysosome. Syntaxin-17 remains associated with mature MDVs and forms a ternary SNARE complex with SNAP29 and VAMP7 to mediate MDV–endolysosome fusion in a manner dependent on the homotypic fusion and vacuole protein sorting (HOPS) tethering complex. Syntaxin-17 can be traced to the last eukaryotic common ancestor, hinting that the removal of damaged mitochondrial content may represent one of the earliest vesicle transport routes in the cell.

Mitofusins, from Mitochondria to Metabolism

Mitochondrial architecture is involved in several functions crucial for cell viability, proliferation, senescence, and signaling. In particular, mitochondrial dynamics, through the balance between fusion and fission events, represents a central mechanism for bioenergetic adaptation to metabolic needs of the cell. As key regulators of mitochondrial dynamics, the fusogenic mitofusins have recently been linked to mitochondrial biogenesis and respiratory functions, impacting on cell fate and organism homeostasis. Here we review the implication of mitofusins in the regulation of mitochondrial metabolism, and their consequence on energy homeostasis at the cellular and physiological level, highlighting their crucial role in metabolic disorders, cancer, and aging.

Orphan proteins of unknown function in the mitochondrial intermembrane space proteome: New pathways and metabolic cross-talk

The mitochondrial intermembrane space (IMS) is involved in protein transport, lipid homeostasis and metal ion exchange, while further acting in signalling pathways such as apoptosis. Regulation of these processes involves protein modifications, as well as stress-induced import or release of proteins and other signalling molecules. Even though the IMS is the smallest sub-compartment of mitochondria, its redox state seems to be tightly regulated. However, the way in which this compartment participates in the cross-talk between the multiple organelles and the cytosol is far from understood. Here we focus on newly identified IMS proteins that may represent future challenges in mitochondrial research. We present an overview of the import pathways, the recently discovered new components of the IMS proteome and how these relate to key aspects of cell signalling and progress made in stem cell and cancer research.

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A brief overview of the classic mitochondrial import pathways is featured

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Recent studies assigning a number of new proteins to the mitochondrial IMS are discussed

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Analysis of the expanded IMS proteomes can provide insights into organelle cross-talk and signalling pathways

Neuroprotective effects of the immunomodulatory drug FK506 in a model of HIV1-gp120 neurotoxicity

Background

HIV-associated neurocognitive disorders (HAND) continue to be a common morbidity associated with chronic HIV infection. It has been shown that HIV proteins (e.g., gp120) released from infected microglial/macrophage cells can cause neuronal damage by triggering inflammation and oxidative stress, activating aberrant kinase pathways, and by disrupting mitochondrial function and biogenesis. Previous studies have shown that FK506, an immunophilin ligand that modulates inflammation and mitochondrial function and inhibits calcineurin, is capable of rescuing the neurodegenerative pathology in models of Parkinson’s disease, Alzheimer’s disease, and Huntington’s disease. In this context, the main objective of this study was to evaluate if FK506 could rescue the neuronal degeneration and mitochondrial alterations in a transgenic (tg) animal model of HIV1-gp120 neurotoxicity.

Methods

GFAP-gp120 tg mice were treated with FK506 and analyzed for neuropathology, behavior, mitochondrial markers, and calcium flux by two-photon microscopy.

Results

We found that FK506 reduced the neuronal cell loss and neuro-inflammation in the gp120 tg mice. Moreover, while vehicle-treated gp120 tg mice displayed damaged mitochondria and increased neuro-inflammatory markers, FK506 rescued the morphological mitochondrial alterations and neuro-inflammation while increasing levels of optic atrophy 1 and mitofusin 1. By two-photon microscopy, calcium levels were not affected in the gp120 tg mice and no effects of FK506 were detected. However, at a functional level, FK506 ameliorated the gp120 tg mice hyperactivity in the open field.

Conclusions

Together, these results suggest that FK506 might be potentially neuroprotective in patients with HAND by mitigating inflammation and mitochondrial alterations.

DRP1-dependent apoptotic mitochondrial fission occurs independently of BAX, BAK and APAF1 to amplify cell death by BID and oxidative stress

During apoptosis mitochondria undergo cristae remodeling and fragmentation, but how the latter relates to outer membrane permeabilization and downstream caspase activation is unclear. Here we show that the mitochondrial fission protein Dynamin Related Protein (Drp) 1 participates in cytochrome c release by selected intrinsic death stimuli. While Bax, Bak double deficient (DKO) and Apaf1(-/-) mouse embryonic fibroblasts (MEFs) were less susceptible to apoptosis by Bcl-2 family member BID, H(2)O(2), staurosporine and thapsigargin, Drp1(-/-) MEFs were protected only from BID and H(2)O(2). Resistance to cell death of Drp1(-/-) and DKO MEFs correlated with blunted cytochrome c release, whereas mitochondrial fragmentation occurred in all cell lines in response to all tested stimuli, indicating that other mechanisms accounted for the reduced cytochrome c release. Indeed, cristae remodeling was reduced in Drp1(-/-) cells, potentially explaining their resistance to apoptosis. Our results indicate that caspase-independent mitochondrial fission and Drp1-dependent cristae remodeling amplify apoptosis. This article is part of a Special Issue entitled 'EBEC 2016: 19th European Bioenergetics Conference, Riva del Garda, Italy, July 2-6, 2016', edited by Prof. Paolo Bernardi.

Exercise mitigates mitochondrial permeability transition pore and quality control mechanisms alterations in nonalcoholic steatohepatitis

Mitochondrial quality control and apoptosis have been described as key components in the pathogenesis of nonalcoholic steatohepatitis (NASH); exercise is recognized as a nonpharmacological strategy to counteract NASH-associated consequences. We aimed to analyze the effect of voluntary physical activity (VPA) and endurance training (ET) against NASH-induced mitochondrial permeability transition pore (mPTP) opening and mitochondrial and cellular quality control deleterious alterations. Forty-eight male Sprague–Dawley rats were divided into standard-diet sedentary (SS, n = 16), standard-diet VPA (n = 8), high-fat diet sedentary (HS, n = 16), and high-fat diet VPA (n = 8). After 9 weeks of diet treatment, half of the SS and HS groups were engaged in an ET program for 8 weeks, 5 days/week, 1 h/day. Liver mPTP susceptibility through osmotic swelling, mPTP-related proteins (cyclophilin D, Sirtuin3, Cofilin-1), markers of mitochondrial biogenesis ((mitochondrial transcription factor A (Tfam) and peroxisome proliferator-activated receptor gamma co-activator protein (PGC-1α)), dynamics (Mitofusin 1 (Mfn1), Mitofusin 2 (Mfn2), Dynamin related protein 1, and Optic atrophy 1)), auto/mitophagy (Beclin-1, microtubule-associated protein 1 light chain 3, p62, PINK1, and Parkin), and apoptotic signaling (Bax, Bcl-2) and caspases-like activities were assessed. HS animals showed an increased susceptibility to mPTP, compromised expression of Tfam, Mfn1, PINK1, and Parkin and an increase in Bax content (HS vs. SS). ET and VPA improved biogenesis-related proteins (PGC-1α) and autophagy signaling (Beclin-1 and Beclin-1/Bcl-2 ratio) and decreased apoptotic signaling (caspases 8 activity, Bax content, and Bax/Bcl-2 ratio). However, only ET decreased mPTP susceptibility and positively modulated Bcl-2, Tfam, Mfn1, Mfn2, PINK1, and Parkin content. In conclusion, exercise reduces the increased susceptibility to mPTP induced by NASH and promotes the increase of auto/mitophagy and mitochondrial fusion towards a protective phenotype.

HIV alters neuronal mitochondrial fission/fusion in the brain during HIV-associated neurocognitive disorders

HIV-associated neurocognitive disorders (HAND) still occur in approximately 50% of HIV patients, and therapies to combat HAND progression are urgently needed. HIV proteins are released from infected cells and cause neuronal damage, possibly through mitochondrial abnormalities. Altered mitochondrial fission and fusion is implicated in several neurodegenerative disorders. Here, we hypothesized that mitochondrial fission/fusion may be dysregulated in neurons during HAND. We have identified decreased mitochondrial fission protein (dynamin 1-like; DNM1L) in frontal cortex tissues of HAND donors, along with enlarged and elongated mitochondria localized to the soma of damaged neurons. Similar pathology was observed in the brains of GFAP-gp120 tg mice. In vitro, recombinant gp120 decreased total and active DNM1L levels, reduced the level of Mitotracker staining, and increased extracellular acidification rate (ECAR) in primary neurons. DNM1L knockdown enhanced the effects of gp120 as measured by reduced Mitotracker signal in the treated cells. Interestingly, overexpression of DNM1L increased the level of Mitotracker staining in primary rat neurons and reduced neuroinflammation and neurodegeneration in the GFAP-gp120-tg mice. These data suggest that mitochondrial biogenesis dynamics are shifted towards mitochondrial fusion in brains of HAND patients and this may be due to gp120-induced reduction in DNM1L activity. Promoting mitochondrial fission during HIV infection of the CNS may restore mitochondrial biogenesis and prevent neurodegeneration.

Mitochondrial resetting and metabolic reprogramming in induced pluripotent stem cells and mitochondrial disease modeling

BACKGROUND

Nuclear reprogramming with pluripotency factors enables somatic cells to gain the properties of embryonic stem cells. Mitochondrial resetting and metabolic reprogramming are suggested to be key early events in the induction of human skin fibroblasts to induced pluripotent stem cells (iPSCs).

SCOPE OF REVIEW

We review recent advances in the study of the molecular basis for mitochondrial resetting and metabolic reprogramming in the regulation of the formation of iPSCs. In particular, the recent progress in using iPSCs for mitochondrial disease modeling was discussed.

MAJOR CONCLUSIONS

iPSCs rely on glycolysis rather than oxidative phosphorylation as a major supply of energy. Mitochondrial resetting and metabolic reprogramming thus play crucial roles in the process of generation of iPSCs from somatic cells.

GENERAL SIGNIFICANCE

Neurons, myocytes, and cardiomyocytes are cells containing abundant mitochondria in the human body, which can be differentiated from iPSCs or trans-differentiated from fibroblasts. Generating these cells from iPSCs derived from skin fibroblasts of patients with mitochondrial diseases or by trans-differentiation with cell-specific transcription factors will provide valuable insights into the role of mitochondrial DNA heteroplasmy in mitochondrial disease modeling and serves as a novel platform for screening of drugs to treat patients with mitochondrial diseases.