Regulation and physiologic functions of GTPases in mitochondrial fusion and fission in mammals

Ion channel-mediated mitochondrial volume regulation and its relationship with mitochondrial dynamics

The mitochondrion, one of the important cellular organelles, has the major function of generating adenosine triphosphate and plays an important role in maintaining cellular homeostasis, governing signal transduction, regulating membrane potential, controlling programmed cell death and modulating cell proliferation. The dynamic balance of mitochondrial volume is an important factor required for maintaining the structural integrity of the organelle and exerting corresponding functions. Changes in the mitochondrial volume are closely reflected in a series of biological functions and pathological changes. The mitochondrial volume is controlled by the osmotic balance between the cytoplasm and the mitochondrial matrix. Thus, any disruption in the influx of the main ion, potassium, into the cells can disturb the osmotic balance between the cytoplasm and the matrix, leading to water movement between these compartments and subsequent alterations in mitochondrial volume. Recent studies have shown that mitochondrial volume homeostasis is closely implicated in a variety of diseases. In this review, we provide an overview of the main influencing factors and research progress in the field of mitochondrial volume homeostasis.

Deciphering mitochondrial dysfunction: Pathophysiological mechanisms in vascular cognitive impairment

Vascular cognitive impairment (VCI) encompasses a range of cognitive deficits arising from vascular pathology. The pathophysiological mechanisms underlying VCI remain incompletely understood; however, chronic cerebral hypoperfusion (CCH) is widely acknowledged as a principal pathological contributor. Mitochondria, crucial for cellular energy production and intracellular signaling, can lead to numerous neurological impairments when dysfunctional. Recent evidence indicates that mitochondrial dysfunction-marked by oxidative stress, disturbed calcium homeostasis, compromised mitophagy, and anomalies in mitochondrial dynamics-plays a pivotal role in VCI pathogenesis. This review offers a detailed examination of the latest insights into mitochondrial dysfunction within the VCI context, focusing on both the origins and consequences of compromised mitochondrial health. It aims to lay a robust scientific groundwork for guiding the development and refinement of mitochondrial-targeted interventions for VCI.

Biogenesis, inheritance, and 3D ultrastructure of the microsporidian mitosome

During the reductive evolution of obligate intracellular parasites called microsporidia, a tiny remnant mitochondrion (mitosome) lost its typical cristae, organellar genome, and most canonical functions. Here, we combine electron tomography, stereology, immunofluorescence microscopy, and bioinformatics to characterise mechanisms of growth, division, and inheritance of this minimal mitochondrion in two microsporidia species (grown within a mammalian RK13 culture-cell host). Mitosomes of Encephalitozoon cuniculi (2-12/cell) and Trachipleistophora hominis (14-18/nucleus) displayed incremental/non-phasic growth and division and were closely associated with an organelle identified as equivalent to the fungal microtubule-organising centre (microsporidian spindle pole body; mSPB). The mitosome-mSPB association was resistant to treatment with microtubule-depolymerising drugs nocodazole and albendazole. Dynamin inhibitors (dynasore and Mdivi-1) arrested mitosome division but not growth, whereas bioinformatics revealed putative dynamins Drp-1 and Vps-1, of which, Vps-1 rescued mitochondrial constriction in dynamin-deficient yeast (Schizosaccharomyces pombe). Thus, microsporidian mitosomes undergo incremental growth and dynamin-mediated division and are maintained through ordered inheritance, likely mediated via binding to the microsporidian centrosome (mSPB).

Oocyte Health and Quality: Implication of Mitochondria-related Organelle Interactions

Among factors like hormonal imbalance and uterine condition, oocyte quality is regarded as one of the key factors involved in age-related decline in the reproductive capacity. Here, are discussions about the functions played by organelles within the oocyte in forming the next generation that is more suitable for survival. Many insights on the adaptation to aging and maintenance of quality can be obtained from: interactions between mitochondria and other organelles that enable the long life of primordial oocytes; characteristics of organelle interactions after breaking dormancy from primary oocytes to mature oocytes; and characteristics of interactions between mitochondria and other organelles of aged oocytes collected during the ovulatory cycle from elderly individuals and animals. This information would potentially be beneficial to the development of future therapeutic methods or agents.

Restoring the infected powerhouse: Mitochondrial quality control in sepsis

Sepsis is a dysregulated host response to an infection, characterized by organ failure. The pathophysiology is complex and incompletely understood, but mitochondria appear to play a key role in the cascade of events that culminate in multiple organ failure and potentially death. In shaping immune responses, mitochondria fulfil dual roles: they not only supply energy and metabolic intermediates crucial for immune cell activation and function but also influence inflammatory and cell death pathways. Importantly, mitochondrial dysfunction has a dual impact, compromising both immune system efficiency and the metabolic stability of end organs. Dysfunctional mitochondria contribute to the development of a hyperinflammatory state and loss of cellular homeostasis, resulting in poor clinical outcomes. Already in early sepsis, signs of mitochondrial dysfunction are apparent and consequently, strategies to optimize mitochondrial function in sepsis should not only prevent the occurrence of mitochondrial dysfunction, but also cover the repair of the sustained mitochondrial damage. Here, we discuss mitochondrial quality control (mtQC) in the pathogenesis of sepsis and exemplify how mtQC could serve as therapeutic target to overcome mitochondrial dysfunction. Hence, replacing or repairing dysfunctional mitochondria may contribute to the recovery of organ function in sepsis. Mitochondrial biogenesis is a process that results in the formation of new mitochondria and is critical for maintaining a pool of healthy mitochondria. However, exacerbated biogenesis during early sepsis can result in accumulation of structurally aberrant mitochondria that fail to restore bioenergetics, produce excess reactive oxygen species (ROS) and exacerbate the disease course. Conversely, enhancing mitophagy can protect against organ damage by limiting the release of mitochondrial-derived damage-associated molecules (DAMPs). Furthermore, promoting mitophagy may facilitate the growth of healthy mitochondria by blocking the replication of damaged mitochondria and allow for post sepsis organ recovery through enabling mitophagy-coupled biogenesis. The remaining healthy mitochondria may provide an undamaged scaffold to reproduce functional mitochondria. However, the kinetics of mtQC in sepsis, specifically mitophagy, and the optimal timing for intervention remain poorly understood. This review emphasizes the importance of integrating mitophagy induction with mtQC mechanisms to prevent undesired effects associated with solely the induction of mitochondrial biogenesis.

Resveratrol ameliorates diabetic encephalopathy through PDE4D/PKA/Drp1 signaling

Diabetic encephalopathy (DE) is a central nervous complication of diabetes mellitus which is characterized by cognitive impairment and neurochemical abnormalities. However, no effective approaches are available to prevent its progression and development. PDE4D serves many functions in the pathogenesis of neurodegenerative diseases involving PKA signaling. This study illustrated the role of PDE4D in DE and investigated whether resveratrol protected against DE via inhibiting PDE4D. db/db male mice and hippocampus cell line (HT22) were used to investigate the role of PDE4D and the protective effect of resveratrol on cognitive function under high glucose (HG). PDE4D overexpression or knockdown lentivirus and PKA specific inhibitor H89 were used to further identify the indispensable role of PDE4D/PKA signaling pathway in resveratrol's amelioration effect of neurotoxicity. Resveratrol attenuated cognitive impairment in db/db mice, reduced PDE4D protein, restored the impaired mitochondrial function in db/db mice. The in vitro study also confirmed the neuroprotective effect of resveratrol on neurotoxicity. PDE4D overexpression resulted in cell injury and downregulation of cAMP, PKA and pDrp1(Ser637) under normal condition. In contrast, PDE4D knockdown improved cell injury and elevated cAMP, PKA and pDrp1(Ser637) levels caused in HG-cultured HT22 cells. PDE4D over-expression blunted the improvement effects of resveratrol on PKA, pDrp1(Ser637) and mitochondrial function. Moreover, PKA inhibitor H89 blunted the inhibitory effects of resveratrol on pDrp1(Ser637) and mitochondrial function in HG-treated HT22. These data indicated that resveratrol may improve cognitive impairment in db/db mice by modulating mitochondrial function through the PDE4D dependent pathway.

Maternal exposure to cetylpyridinium chloride impairs oogenesis by causing mitochondria disorder in neonates

Cetylpyridinium Chloride (CPC) is a common disinfectant with potential mitochondrial toxicity. However, the effects of CPC on female reproduction remains unclear. In the present study, pregnant mice were exposed to environmentally relevant doses of CPC for 3 days, the effects were evaluated in the female offspring. Maternal exposure to CPC caused loss of oocytes in neonatal ovaries. TEM analysis of neonatal ovaries showed CPC caused aberrant mitochondrial morphology including vacuolated and disorganized structure, reduced functional cristae. In addition, CPC decreased mitochondrial membrane potential in neonatal oocytes. Seahorse analysis showed that CPC hampered mitochondrial reserve, manifested as reduced spare respiratory capacity. Furthermore, CPC damaged mitochondrial function and impaired developmental competence of MII oocytes, suggesting a persisting impact into adulthood. In summary, this is the first known demonstration that maternal exposure to CPC caused mitochondrial disorders in neonatal ovaries and had long-term effects on fertility of the female offspring.

Discovery of Potent Allosteric DRP1 Inhibitors by Disrupting Protein-Protein Interaction with MiD49

Mitochondrial dysfunction has been attributed to many disease indications, including metabolic, cardiovascular, neoplastic, and neurodegenerative diseases. Dynamin related protein 1 (DRP1) is crucial in regulating mitochondrial fission and maintaining mitochondrial homeostasis. MiD49 is a dynamic peripheral protein receptor on the surface of the mitochondrial membrane that recruits DRP1 protein to induce mitochondrial binary fission. By targeting the protein-protein interaction of DRP1/MiD49, we have discovered a novel and potent allosteric DRP1 inhibitor that inhibits mitochondria fragmentation in vitro. X-ray cocrystal structure revealed that it locked the closed DRP1 conformation by induced dimerization.

Mycobacterial protein PE_PGRS30 induces macrophage apoptosis through prohibitin 2 mitochondrial function interference

PE_PGRS30 belongs to the PE_PGRS protein family and is characterized by a conserved Pro-Glu (PE) domain and a typically polymorphic GC-rich sequence (PGRS) domain. PE_PGRS30 is a virulence factor of Mycobacterium tuberculosis that induces macrophage cell death. We found that RAW264.7 cells and murine alveolar macrophages underwent apoptosis in response to PE_PGRS30. The host protein prohibitin 2 (PHB2) was identified as a target molecule. PE_PGRS30 and PHB2 interact via the PGRS domain and mitochondrial targeting sequence, respectively. PHB2 overexpression reduced macrophage apoptosis in response to PE_PGRS30. PE_PGRS30 co-localized with PHB2, not in mitochondria, but in lysosomes. The maintenance of mitochondrial structure by PHB2 was impaired in response to the PGRS domain. These results indicated that PE_PGRS30 reduces PHB2 in mitochondria, resulting in mitochondrial dysfunction and cellular apoptosis.

Alteration of Mitochondrial DNA Copy Number and Increased Expression Levels of Mitochondrial Dynamics-Related Genes in Sjögren's Syndrome

Sjögren's syndrome (SS) is a chronic autoimmune multifactorial disease characterized by inflammation and lymphocytic infiltration of the exocrine glands. Several studies have highlighted the involvement of oxidative stress in this pathology, suggesting that it could induce mitochondrial dysfunctions. Mitochondria could have a role in inflammatory and immune processes. Since the mitochondrial DNA (mtDNA) copy number could change in response to physiological or environmental stimuli, this study aimed to evaluate possible alterations in the mtDNA copy number in SS. We have analyzed the amount of mtDNA in the peripheral blood of 74 SS patients and 61 healthy controls by qPCR. Then, since mitochondrial fusion and fission play a crucial role in maintaining the number of mitochondria, we investigated the expression variability of the genes most commonly involved in mitochondrial dynamics in a subgroup of SS patients and healthy controls. Interestingly, we observed a highly significant decrease in mtDNA copies in the SS patients compared to healthy controls (p = 1.44 × 10^-12). Expression levels of mitochondrial fission factor (MFF), mitofusin-1 (MFN1), and mitochondrial transcription factor A (TFAM) genes were analyzed, showing a statistically significant increase in the expression of MFF (p = 0.003) and TFAM (p = 0.022) in the SS patients compared to healthy controls. These results give further insight into the possible involvement of mitochondrial dysfunctions in SS disease.

Current Studies of Mitochondrial Quality Control in the Preeclampsia

Mitochondria are cellular energy powerhouses that play important roles in regulating cellular processes. Mitochondrial quality control (mQC), including mitochondrial biogenesis, mitophagy, mitochondrial fusion and fission, maintains physiological demand and adapts to changed conditions. mQC has been widely investigated in neurodegeneration, cardiovascular disease and cancer because of the high demand for ATP in these diseases. Although placental implantation and fetal growth similarly require a large amount of energy, the investigation of mQC in placental-originated preeclampsia (PE) is limited. We elucidate mitochondrial morphology and function in different pregnancy stages, outline the role of mQC in cellular homeostasis and PE and summarize the current findings of mQC-related PE studies. This review also provides suggestions on the future investigation of mQC in PE, which will lead to the development of new prevention and therapy strategies for PE.

Mitofusin 2, a key coordinator between mitochondrial dynamics and innate immunity

Remodeling of mitochondrial dynamics and mitochondrial morphology plays a pivotal role in the maintenance of mitochondrial homeostasis in response to pathogenic attacks or stress stimuli. In addition to their role in metabolism and energy production, mitochondria participate in diverse biological functions, including innate immune responses driven by macrophages in response to infections or inflammatory stimuli. Mitofusin-2 (MFN2), a mitochondria-shaping protein regulating mitochondrial fusion and fission, plays a crucial role in linking mitochondrial function and innate immune responses. In this article, we review the role of MFN2 in the regulation of innate immune responses during viral and bacterial infections. We also summarize the current knowledge on the role of MFN2 in coordinating inflammatory, atherogenic, and fibrotic responses. MFN2-mediated crosstalk between mitochondrial dynamics and innate immune responses may determine the outcomes of pathogenic infections.

Non-alcoholic fatty liver disease in mice with hepatocyte-specific deletion of mitochondrial fission factor

AIMS/HYPOTHESIS

Mitochondria are highly dynamic organelles continuously undergoing fission and fusion, referred to as mitochondrial dynamics, to adapt to nutritional demands. Evidence suggests that impaired mitochondrial dynamics leads to metabolic abnormalities such as non-alcoholic steatohepatitis (NASH) phenotypes. However, how mitochondrial dynamics are involved in the development of NASH is poorly understood. This study aimed to elucidate the role of mitochondrial fission factor (MFF) in the development of NASH.

METHODS

We created mice with hepatocyte-specific deletion of MFF (MffLiKO). MffLiKO mice fed normal chow diet (NCD) or high-fat diet (HFD) were evaluated for metabolic variables and their livers were examined by histological analysis. To elucidate the mechanism of development of NASH, we examined the expression of genes related to endoplasmic reticulum (ER) stress and lipid metabolism, and the secretion of triacylglycerol (TG) using the liver and primary hepatocytes isolated from MffLiKO and control mice.

RESULTS

MffLiKO mice showed aberrant mitochondrial morphologies with no obvious NASH phenotypes during NCD, while they developed full-blown NASH phenotypes in response to HFD. Expression of genes related to ER stress was markedly upregulated in the liver from MffLiKO mice. In addition, expression of genes related to hepatic TG secretion was downregulated, with reduced hepatic TG secretion in MffLiKO mice in vivo and in primary cultures of MFF-deficient hepatocytes in vitro. Furthermore, thapsigargin-induced ER stress suppressed TG secretion in primary hepatocytes isolated from control mice.

CONCLUSIONS/INTERPRETATION

We demonstrated that ablation of MFF in liver provoked ER stress and reduced hepatic TG secretion in vivo and in vitro. Moreover, MffLiKO mice were more susceptible to HFD-induced NASH phenotype than control mice, partly because of ER stress-induced apoptosis of hepatocytes and suppression of TG secretion from hepatocytes. This study provides evidence for the role of mitochondrial fission in the development of NASH.

Mitochondrial Dynamics Related Genes -MFN1, MFN2 and DRP1 Polymorphisms are Associated with Risk of Lung Cancer

Purpose

This study aimed to evaluate the associations between mitochondrial dynamics related genes -MFN1, MFN2 and DRP1 polymorphisms and risk of lung cancer.

Methods

Six polymorphisms of MFN1, MFN2 and DRP1 were genotyped in 600 cases and 600 controls using a MassARRAY platform.

Results

The MFN1 rs13098637-C and DRP1 rs879255689-A alleles were associated with an increased risk of lung cancer (p_rs13098637=0.004, p_rs879255689=0.005), while MFN2 rs4240897-A and rs2236058-G were related to a decreased risk of disease (p<0.001). The rs13098637-TC/CC and rs879255689-GA/AA were determined as risk genotypes for lung cancer (p_rs13098637=0.014, p_rs879255689=0.013), whereas the rs4240897-GA/AA and rs2236058-GG were identified as protective genotypes against lung cancer risk (p<0.001). Genetic model analysis showed that rs13098637 was correlated with an elevated risk of lung cancer in dominant and log-additive models (p_dominant=0.007, p_log-additive=0.004). Moreover, rs879255689 was associated with an increased risk of disease in all three models (p_dominant=0.014, p_recessive=0.028, p_log-additive=0.005). In contrast, rs4240897 and rs2236058 were related to reduced risk of disease in all three models (rs4240897: p_all<0.001; rs2236058: p_dominant=0.008, p_recessive<0.001, p_log-additive<0.001). In addition, these associations were related to the smoking status and pathological type of lung cancer patients.

Conclusion

These results shed new light on the association between mitochondrial dynamics related genes and risk of lung cancer.

Maintenance of Skeletal Muscle to Counteract Sarcopenia in Patients with Advanced Chronic Kidney Disease and Especially Those Undergoing Hemodialysis

Life extension in modern society has introduced new concepts regarding such disorders as frailty and sarcopenia, which has been recognized in various studies. At the same time, cutting-edge technology methods, e.g., renal replacement therapy for conditions such as hemodialysis (HD), have made it possible to protect patients from advanced lethal chronic kidney disease (CKD). Loss of muscle and fat mass, termed protein energy wasting (PEW), has been recognized as prognostic factor and, along with the increasing rate of HD introduction in elderly individuals in Japan, appropriate countermeasures are necessary. Although their origins differ, frailty, sarcopenia, and PEW share common components, among which skeletal muscle plays a central role in their etiologies. The nearest concept may be sarcopenia, for which diagnosis techniques have recently been reported. The focus of this review is on maintenance of skeletal muscle against aging and CKD/HD, based on muscle physiology and pathology. Clinically relevant and topical factors related to muscle wasting including sarcopenia, such as vitamin D, myostatin, insulin (related to diabetes), insulin-like growth factor I, mitochondria, and physical inactivity, are discussed. Findings presented thus far indicate that in addition to modulation of the aforementioned factors, exercise combined with nutritional supplementation may be a useful approach to overcome muscle wasting and sarcopenia in elderly patients undergoing HD treatments.

Functions of outer mitochondrial membrane proteins: mediating the crosstalk between mitochondrial dynamics and mitophagy

Most cellular stress responses converge on the mitochondria. Consequently, the mitochondria must rapidly respond to maintain cellular homeostasis and physiological demands by fine-tuning a plethora of mitochondria-associated processes. The outer mitochondrial membrane (OMM) proteins are central to mediating mitochondrial dynamics, coupled with continuous fission and fusion. These OMM proteins also have vital roles in controlling mitochondrial quality and serving as mitophagic receptors for autophagosome enclosure during mitophagy. Mitochondrial fission segregates impaired mitochondria in smaller sizes from the mother mitochondria and may favor mitophagy for eliminating damaged mitochondria. Conversely, mitochondrial fusion mixes dysfunctional mitochondria with healthy ones to repair the damage by diluting the impaired components and consequently prevents mitochondrial clearance via mitophagy. Despite extensive research efforts into deciphering the interplay between fission-fusion and mitophagy, it is still not clear whether mitochondrial fission essentially precedes mitophagy. In this review, we summarize recent breakthroughs concerning OMM research, and dissect the functions of these proteins in mitophagy from their traditional roles in fission-fusion dynamics, in response to distinct context, at the intersection of the OMM platform. These insights into the OMM proteins in mechanistic researches would lead to new aspects of mitochondrial quality control and better understanding of mitochondrial homeostasis intimately tied to pathological impacts.

Multiple assay systems to analyze the dynamics of mitochondrial nucleoids in living mammalian cells

BACKGROUND

Mitochondria, which play a critical role in energy production by oxidative respiration, are highly dynamic organelles and their double membranes undergo frequent events of fusion and fission. Mitochondria are believed to be derived from the endosymbiosis of proteobacteria, and thus mitochondria still contain their own DNA, mitochondrial DNA (mtDNA). Several copies of mtDNA form mitochondrial nucleoid with DNA-binding proteins. Recently, the morphology and distribution of the mitochondrial membrane and nucleoid were reported to be cooperatively regulated during their dynamic movement. However, the molecular mechanism is unclear, because the involved molecules are poorly understood, and suitable techniques to analyze nucleoid have not been fully developed.

RESULTS

To solve these issues, we examined the molecular mechanism of nucleoid dynamics by two approaches. First, we constructed a new probe to perform live imaging of nucleoid dynamics using the DNA-binding domain of mitochondrial transcriptional factor A (TFAM) and the photo-convertible fluorescent protein Kikume Green-Red (KikGR). Nucleoids were visualized stably for a long period using the new probe. Second, we searched for nucleoid regulatory factors by small interfering RNA screening using HeLa cells and identified a subset of MARCH family ubiquitin ligases that affect nucleoid morphology.

CONCLUSION

The factors and probe, reported in this study, would be useful to reveal novel mechanisms of mitochondrial regulation.

GENERAL SIGNIFICANCE

The mtDNA dynamics should be concerned in the regulation of mitochondrial activity and its quality control, associated with mitochondrial membrane dynamics.

Analysis of Mitochondrial Membrane Fusion GTPase OPA1 Expressed by the Silkworm Expression System

Mitochondria are highly dynamic organelles, which move and fuse to regulate their shape, size, and fundamental function. The dynamin-related GTPases play a critical role in mitochondrial membrane fusion. In vitro reconstitution of membrane fusion using recombinant proteins and model membranes is quite useful in elucidating the molecular mechanisms underlying membrane fusion and to identify the essential elements involved in fusion. However, only a few reconstituting approaches have been reported for mitochondrial fusion machinery due to the difficulty of preparing active recombinant mitochondrial fusion GTPases. Recently, we succeeded in preparing a sufficient amount of recombinant OPA1 involved in mitochondrial inner membrane fusion using a BmNPV bacmid-silkworm expression system. In this chapter, we describe the method for the expression and purification of a membrane-anchored form of OPA1 and liposome-based in vitro reconstitution of membrane fusion.

Effects of power frequency electric field exposure on kidney

With the rapid development of ultra high voltage alternating current (UHV AC) transmission, the intensity of environmental power frequency electric field (PFEF) near UHV AC transmission lines increased continuously, which has attracted considerable public attention on the potential health effects of PFEF. In this study, the effect of PFEF exposure on the kidney was explored. Institute of Cancer Research (ICR) mice were exposed to 35 kV/m PFEF (50 Hz). Two indicators relating to renal function (urea nitrogen and creatinine) were tested after the exposure of 7d, 14d, 21d, 35d and 49d. The pathological morphology and cellular ultrastructure of kidney were observed respectively by light microscopy and electron microscopy after the exposure of 25d and 52d. Results showed that compared with that of the control group, the concentration of urea nitrogen of 35 kV/m PFEF exposure group significantly increased on the 21st and 35th days, and the concentration of creatinine significantly increased on the 14th, 21st and 35th days. However, the concentrations of creatinine and urea nitrogen both returned to normal levels on the 49th day. Furthermore, an enlarged Bowman's space, the vacuolation of renal tubular epithelial cells and the foot process effacement of podocyte were found after 25d exposure, but no abnormality was observed after 52d exposure. Obviously, a short-term (35d) exposure of 35 kV/m PFEF could cause kidney injury, which could be recovered after a longer-term (52d) exposure. Based on this study and relevant literatures, one explanation for this two-way effect is as follows. Kidney injury was caused by the disequilibrium of mitochondrial dynamics under 35 kV/m PFEF exposure. PFEF could also activate Wnt/β-catenin signal to promote the recovery of renal tubular epithelial cells and glomerular podocytes, so kidney injury could be repaired.

Mitochondrial functions and rare diseases

Mitochondria are dynamic cellular organelles responsible for a large variety of biochemical processes as energy transduction, REDOX signaling, the biosynthesis of hormones and vitamins, inflammation or cell death execution. Cell biology studies established that 1158 human genes encode proteins localized to mitochondria, as registered in MITOCARTA. Clinical studies showed that a large number of these mitochondrial proteins can be altered in expression and function through genetic, epigenetic or biochemical mechanisms including the interaction with environmental toxics or iatrogenic medicine. As a result, pathogenic mitochondrial genetic and functional defects participate to the onset and the progression of a growing number of rare diseases. In this review we provide an exhaustive survey of the biochemical, genetic and clinical studies that demonstrated the implication of mitochondrial dysfunction in human rare diseases. We discuss the striking diversity of the symptoms caused by mitochondrial dysfunction and the strategies proposed for mitochondrial therapy, including a survey of ongoing clinical trials.

Tourette Syndrome Risk Genes Regulate Mitochondrial Dynamics, Structure, and Function

Gilles de la Tourette syndrome (GTS) is a neurodevelopmental disorder characterized by motor and vocal tics with an estimated prevalence of 1% in children and adolescents. GTS has high rates of inheritance with many rare mutations identified. Apart from the role of the neurexin trans-synaptic connexus (NTSC) little has been confirmed regarding the molecular basis of GTS. The NTSC pathway regulates neuronal circuitry development, synaptic connectivity and neurotransmission. In this study we integrate GTS mutations into mitochondrial pathways that also regulate neuronal circuitry development, synaptic connectivity and neurotransmission. Many deleterious mutations in GTS occur in genes with complementary and consecutive roles in mitochondrial dynamics, structure and function (MDSF) pathways. These genes include those involved in mitochondrial transport (NDE1, DISC1, OPA1), mitochondrial fusion (OPA1), fission (ADCY2, DGKB, AMPK/PKA, RCAN1, PKC), mitochondrial metabolic and bio-energetic optimization (IMMP2L, MPV17, MRPL3, MRPL44). This study is the first to develop and describe an integrated mitochondrial pathway in the pathogenesis of GTS. The evidence from this study and our earlier modeling of GTS molecular pathways provides compounding support for a GTS deficit in mitochondrial supply affecting neurotransmission.

Augmenter of Liver Regeneration Alleviates Renal Hypoxia-Reoxygenation Injury by Regulating Mitochondrial Dynamics in Renal Tubular Epithelial Cells

Mitochondria are highly dynamic organelles that constantly undergo fission and fusion processes that closely related to their function. Disruption of mitochondrial dynamics has been demonstrated in acute kidney injury (AKI), which could eventually result in cell injury and death. Previously, we reported that augmenter of liver regeneration (ALR) alleviates renal tubular epithelial cell injury. Here, we gained further insights into whether the renoprotective roles of ALR are associated with mitochondrial dynamics. Changes in mitochondrial dynamics were examined in experimental models of renal ischemia-reperfusion (IR). In a model of hypoxia-reoxygenation (HR) injury in vitro , dynamin-related protein 1 (Drp1) and mitochondrial fission process protein 1 (MTFP1), two key proteins of mitochondrial fission, were downregulated in the Lv-ALR + HR group. ALR overexpression additionally had an impact on phosphorylation of Drp1 Ser637 during AKI. The inner membrane fusion protein, Optic Atrophy 1 (OPA1), was significantly increased whereas levels of outer membrane fusion proteins Mitofusin-1 and -2 (Mfn1, Mfn2) were not affected in the Lv-ALR + HR group, compared with the control group. Furthermore, the mTOR/4E-BP1 signaling pathway was highly activated in the Lv-ALR + HR group. ALR overexpression led to suppression of HR-induced apoptosis. Our collective findings indicate that ALR gene transfection alleviates mitochondrial injury, possibly through inhibiting fission and promoting fusion of the mitochondrial inner membrane, both of which contribute to reduction of HK-2 cell apoptosis. Additionally, fission processes are potentially mediated by promoting tubular cell survival through activating the mTOR/4E-BP1 signaling pathway.

Mitochondrial dynamics and interorganellar communication in the development and dysmorphism of mammalian oocytes

Mitochondria play many critical roles in cells, not only by supplying energy, but also by supplying metabolites, buffering Ca2+ levels and regulating apoptosis. During oocyte maturation and subsequent embryo development, mitochondria change their morphology by membrane fusion and fission, and coordinately undergo multiple cellular events with the endoplasmic reticulum (ER) closely apposed. Mitochondrial fusion and fission, known as mitochondrial dynamics, are regulated by family members of dynamin GTPases. Oocytes in animal models with these regulators artificially altered exhibit morphological abnormalities in nearby mitochondria and at the ER interface that are reminiscent of major cytoplasmic dysmorphisms in human assisted reproductive technology, in which a portion of mature oocytes retrieved from patients contain cytoplasmic dysmorphisms associated with mitochondria and ER abnormal morphologies. Understanding organelle morpho-homeostasis in oocytes obtained from animal models will contribute to the development of novel methods for determining oocyte health and for how to deal with dysmorphic oocytes.

The expression of glucose transporters and mitochondrial division and fusion proteins in rats exposed to hypoxic preconditioning to attenuate propofol neurotoxicity

Purpose: Evidence has shown that propofol may cause widespread apoptotic neurodegeneration. Hypoxic preconditioning has been demonstrated to provide neuroprotection and brain recovery from both acute and chronic neurodegeneration in several cellular and animal models. However, the mechanism has not been well elucidated. Therefore, the present study was designed to investigate the expression of glucose transporters (GLUT1 and GLUT3) and mitochondrial division and fusion (Drp1 and Mfn2) proteins in rats exposed to hypoxic preconditioning to attenuate propofol neurotoxicity.Methods: Propofol (100 mg/kg) was given to 7-day-old Sprague-Dawley rats; in some rats, hypoxic preconditioning was administered before intraperitoneal propofol injection by subjecting rats to five cycles of 10 min of hypoxia (8% O₂₎ and 10 min of normoxia (21% O₂). Then, the rats were allowed to breathe room air for 2 h. Neuronal mitochondrial morphology was observed by transmission electron microscopy. ATP content was detected using an ATP assay kit. The expression levels of GLUT1, GLUT3, pDrp1, Drp1 and Mfn2 were detected by Western blot, and the expression levels of GLUT1 and GLUT3 were further examined by immunohistochemistry.Results: Propofol damaged mitochondria, and decreased ATP content and GLUT3 and pDrp1 protein expression. However, our results suggested that hypoxic preconditioning could attenuate propofol neurotoxicity by reducing mitochondrial damage and increasing ATP content and pDrp1, GLUT1 and GLUT3 protein expression.Conclusion: Hypoxic preconditioning reduced propofol-induced damage in the hippocampus of neonatal rats by attenuating the increase in mitochondrial division and decrease in GLUT3 expression.

ROCK1 induces dopaminergic nerve cell apoptosis via the activation of Drp1-mediated aberrant mitochondrial fission in Parkinson's disease

Dopamine deficiency is mainly caused by apoptosis of dopaminergic nerve cells in the substantia nigra of the midbrain and the striatum and is an important pathologic basis of Parkinson’s disease (PD). Recent research has shown that dynamin-related protein 1 (Drp1)-mediated aberrant mitochondrial fission plays a crucial role in dopaminergic nerve cell apoptosis. However, the upstream regulatory mechanism remains unclear. Our study showed that Drp1 knockdown inhibited aberrant mitochondrial fission and apoptosis. Importantly, we found that ROCK1 was activated in an MPP⁺-induced PD cell model and that ROCK1 knockdown and the specific ROCK1 activation inhibitor Y-27632 blocked Drp1-mediated aberrant mitochondrial fission and apoptosis of dopaminergic nerve cells by suppressing Drp1 dephosphorylation/activation. Our in vivo study confirmed that Y-27632 significantly improved symptoms in a PD mouse model by inhibiting Drp1-mediated aberrant mitochondrial fission and apoptosis. Collectively, our findings suggest an important molecular mechanism of PD pathogenesis involving ROCK1-regulated dopaminergic nerve cell apoptosis via the activation of Drp1-induced aberrant mitochondrial fission.

Researchers in China have revealed how a protein molecule plays an early part in the molecular steps that can lead to Parkinson’s disease, which is caused by the death of nerve cells that make the neurotransmitter dopamine. Disruption of mitochondria, the energy-generating bodies inside cells, was already known to lead to the death of dopamine-producing cells. Rong Zhang, Guobing Li and colleagues at The Second Affiliated Hospital of Army Medical University in Chongqing, China traced the chain of cause and effect back to a protein called ROCK-1. Using a mouse model of Parkinson’s disease, they found that ROCK-1 activates another protein previously shown to trigger the disruption of mitochondria. ROCK-1’s early role in the sequence might make it a suitable target for treatment using drugs that inhibit its activity.

Mitochondrial Stasis Reveals p62-Mediated Ubiquitination in Parkin-Independent Mitophagy and Mitigates Nonalcoholic Fatty Liver Disease

It is unknown what occurs if both mitochondrial division and fusion are completely blocked. Here, we introduced mitochondrial stasis by deleting two dynamin-related GTPases for division (Drp1) and fusion (Opa1) in livers. Mitochondrial stasis rescues liver damage and hypotrophy caused by the single knockout (KO). At the cellular level, mitochondrial stasis re-establishes mitochondrial size and rescues mitophagy defects caused by division deficiency. Using Drp1KO livers, we found that the autophagy adaptor protein p62/sequestosome-1-which is thought to function downstream of ubiquitination-promotes mitochondrial ubiquitination. p62 recruits two subunits of a cullin-RING ubiquitin E3 ligase complex, Keap1 and Rbx1, to mitochondria. Resembling Drp1KO, diet-induced nonalcoholic fatty livers enlarge mitochondria and accumulate mitophagy intermediates. Resembling Drp1Opa1KO, Opa1KO rescues liver damage in this disease model. Our data provide a new concept that mitochondrial stasis leads the spatial dimension of mitochondria to a stationary equilibrium and a new mechanism for mitochondrial ubiquitination in mitophagy.

Adhesion Regulating Molecule 1 Mediates HAP40 Overexpression-Induced Mitochondrial Defects

Striatal neuron death in Huntington's disease is associated with abnormal mitochondrial dynamics and functions. However, the mechanisms for this mitochondrial dysregulation remain elusive. Increased accumulation of Huntingtin-associated protein 40 (HAP40) has been shown to be associated with Huntington's disease. However, the link between increased HAP40 and Huntington's disease remains largely unknown. Here we show that HAP40 overexpression causes mitochondrial dysfunction and reduces cell viability in the immortalized mouse striatal neurons. HAP40-associated mitochondrial dysfunction is associated with reduction of adhesion regulating molecule 1 (ADRM1) protein. Consistently, depletion of ADRM1 by shRNAs impaired mitochondrial functions and increased mitochondrial fragmentation in mouse striatal cells. Moreover, reducing ADRM1 levels enhanced activity of fission factor dynamin-related GTPase protein 1 (Drp1) via increased phosphorylation at serine 616 of Drp1 (Drp1^Ser616). Restoring ADRM1 protein levels was able to reduce HAP40-induced ROS levels and mitochondrial fragmentation and improved mitochondrial functions and cell viability. Moreover, reducing Drp1 activity by Drp1 inhibitor, Mdivi-1, ameliorates both HAP40 overexpression- and ADRM1 depletion-induced mitochondrial dysfunction. Taken together, our studies suggest that HAP40-mediated reduction of ADRM1 alters the mitochondrial fission activity and results in mitochondrial fragmentation and mitochondrial dysfunction.

An emerging role for mitochondrial dynamics in schizophrenia

Abnormal brain development has long been thought to contribute to the pathophysiology of schizophrenia. Impaired dendritic arborization, synaptogenesis, and long term potentiation and memory have been demonstrated in animal models of schizophrenia. In addition to aberrant nervous system development, altered brain metabolism and mitochondrial function has long been observed in schizophrenic patients. Single nucleotide polymorphisms in the mitochondrial genome as well as impaired mitochondrial function have both been associated with increased risk for developing schizophrenia. Mitochondrial function in neurons is highly dependent on fission, fusion, and transport of the organelle, collectively referred to as mitochondrial dynamics. Indeed, there is mounting evidence that mitochondrial dynamics strongly influences neuron development and synaptic transmission. While there are a few studies describing altered mitochondrial shape in schizophrenic patients, as well as in animal and in vitro models of schizophrenia, the precise role of mitochondrial dynamics in the pathophysiology of schizophrenia is all but unexplored. Here we discuss the influence of mitochondrial dynamics and mitochondrial function on nervous system development, and highlight recent work suggesting a link between aberrant mitochondrial dynamics and schizophrenia.

Dynamin-related protein-1 as potential therapeutic target in various diseases

Mitochondria can interchange morphology due to their dynamic nature. It can exist in either fragmented disconnected arrangement or elongated interconnected mitochondrial networks due to fission and fusion, respectively. The recent studies have revealed the remarkable and unexpected insights into the physiological impact and molecular regulation of mitochondrial morphology. The balance between fission and fusion governs the faith of the cell. The active targeting of DRP 1 to the outer mitochondrial membrane (OMM) is done by non-GTPase receptor proteins such as mitochondrial fission factor, mitochondrial fission protein 1 and mitochondrial elongation factor 1. The active targeting of DRP 1 to OMM leads to the fission of mitochondria. However, the imbalance of DRP 1-dependent mitochondrial fission and modulation of equilibrium of fission and fusion has been documented to be involved in several cardiovascular and neurodegenerative disorders. In this review, we are focusing on the active participation of DRP 1 in various diseases and also the factors responsible for the activation of DRP 1 for its action.

Mechanistic Role of Kinases in the Regulation of Mitochondrial Fitness

Mounting evidence indicates that mitochondria contain multiple phosphorylation substrates and that protein kinases translocate into mitochondria, suggesting that protein phosphorylation in this organelle could be fundamental for the regulation of its own function. Here we examine the mechanistic role of cellular kinases in the fine regulation of key mitochondrial activities, including mitochondrial quality control, fission/fusion processes, metabolism, and mitophagy.

Mitochondrial Dynamics in Mitochondrial Diseases

Mitochondria are very versatile organelles in continuous fusion and fission processes in response to various cellular signals. Mitochondrial dynamics, including mitochondrial fission/fusion, movements and turnover, are essential for the mitochondrial network quality control. Alterations in mitochondrial dynamics can cause neuropathies such as Charcot-Marie-Tooth disease in which mitochondrial fusion and transport are impaired, or dominant optic atrophy which is caused by a reduced mitochondrial fusion. On the other hand, mitochondrial dysfunction in primary mitochondrial diseases promotes reactive oxygen species production that impairs its own function and dynamics, causing a continuous vicious cycle that aggravates the pathological phenotype. Mitochondrial dynamics provides a new way to understand the pathophysiology of mitochondrial disorders and other diseases related to mitochondria dysfunction such as diabetes, heart failure, or Hungtinton’s disease. The knowledge about mitochondrial dynamics also offers new therapeutics targets in mitochondrial diseases.

Mitochondrial dynamics and cell death in heart failure

The highly regulated processes of mitochondrial fusion (joining), fission (division) and trafficking, collectively called mitochondrial dynamics, determine cell-type specific morphology, intracellular distribution and activity of these critical organelles. Mitochondria are critical for cardiac function, while their structural and functional abnormalities contribute to several common cardiovascular diseases, including heart failure (HF). The tightly balanced mitochondrial fusion and fission determine number, morphology and activity of these multifunctional organelles. Although the intracellular architecture of mature cardiomyocytes greatly restricts mitochondrial dynamics, this process occurs in the adult human heart. Fusion and fission modulate multiple mitochondrial functions, ranging from energy and reactive oxygen species production to Ca(2+) homeostasis and cell death, allowing the heart to respond properly to body demands. Tightly controlled balance between fusion and fission is of utmost importance in the high energy-demanding cardiomyocytes. A shift toward fission leads to mitochondrial fragmentation, while a shift toward fusion results in the formation of enlarged mitochondria and in the fusion of damaged mitochondria with healthy organelles. Mfn1, Mfn2 and OPA1 constitute the core machinery promoting mitochondrial fusion, whereas Drp1, Fis1, Mff and MiD49/51 are the core components of fission machinery. Growing evidence suggests that fusion/fission factors in adult cardiomyocytes play essential noncanonical roles in cardiac development, Ca(2+) signaling, mitochondrial quality control and cell death. Impairment of this complex circuit causes cardiomyocyte dysfunction and death contributing to heart injury culminating in HF. Pharmacological targeting of components of this intricate network may be a novel therapeutic modality for HF treatment.

An evidence based hypothesis on the existence of two pathways of mitochondrial crista formation

Metabolic function and architecture of mitochondria are intimately linked. More than 60 years ago, cristae were discovered as characteristic elements of mitochondria that harbor the protein complexes of oxidative phosphorylation, but how cristae are formed, remained an open question. Here we present experimental results obtained with yeast that support a novel hypothesis on the existence of two molecular pathways that lead to the generation of lamellar and tubular cristae. Formation of lamellar cristae depends on the mitochondrial fusion machinery through a pathway that is required also for homeostasis of mitochondria and mitochondrial DNA. Tubular cristae are formed via invaginations of the inner boundary membrane by a pathway independent of the fusion machinery. Dimerization of the F₁F_O-ATP synthase and the presence of the MICOS complex are necessary for both pathways. The proposed hypothesis is suggested to apply also to higher eukaryotes, since the key components are conserved in structure and function throughout evolution.

DOI:http://dx.doi.org/10.7554/eLife.18853.001

Cells contain compartments called mitochondria, which are often called the powerhouses of the cell because they provide energy that drives vital cellular processes. Mitochondria have two membranes: an outer and an inner membrane. The outer membrane separates the mitochondria from the rest of the cell. The inner membrane is elaborately folded and the folds – called cristae – create a larger space to accommodate all of the protein machinery involved in producing energy. The cristae can be shaped as flat sac-like structures called lamellar cristae or as tubes known as tubular cristae.

Mitochondria are dynamic and are constantly fusing with other mitochondria and splitting up. Even though the internal architecture of mitochondria was first revealed around 60 years ago, it is still not clear how the cristae form. Harner et al. now address this question in yeast cells by combining imaging, biochemistry and genetic approaches.

The experiments show that lamellar cristae form when two mitochondria fuse with each other. The outer membranes merge and then the inner membranes start to fuse around their edges to generate the sac-like structure of lamellar cristae. A yeast protein called Mgm1 (known as Opa1 in mammals) drives the fusion of the inner membranes, but this process only takes place when enzymes called F₁F_O-ATP synthases on the inner membrane form pairs with one another. These F₁F_O-ATP synthase pairs stabilize the cristae membranes as they curve to form the sac-like structure. Later on, the formation of a group of proteins called the MICOS complex halts the fusion process to prevent the lamellar cristae from completely separating from the rest of the inner membrane.

Harner et al. also found that tubular cristae form using a different mechanism when the inner membrane of the mitochondria grows inwards. This process also requires pairs of F₁F_O-ATP synthases and the MICOS complex, but does not involve Mgm1/Opa1. Together, these findings show that lamellar and tubular cristae in yeast form using two different mechanisms. Since the key components of these mechanisms are also found in virtually all other eukaryotes, the findings of Harner et al. are also likely to apply to many other organisms including animals.

DOI:http://dx.doi.org/10.7554/eLife.18853.002

The reduction of temporal optic nerve head microcirculation in autosomal dominant optic atrophy

PURPOSE

To evaluate the optic nerve head (ONH) microcirculation in autosomal dominant optic atrophy (ADOA) patients.

METHODS

This study comprised 22 eyes of 12 ADOA patients, diagnosed according to clinical findings including family history and the presence of mutations in the OPA1 gene. Twenty-four normal eyes of 24 age-matched subjects, with either the right or left eye randomly selected for use, served as controls. Circumpapillary retinal nerve fibre layer thickness (cpRNFLT) and mean blur rate (MBR) in the ONH were determined with optical coherence tomography (OCT) and laser speckle flowgraphy (LSFG), respectively. For each ONH quadrant (superior, temporal, inferior and nasal), the MBR and cpRNFLT ratio was also calculated by dividing tissue MBR in that quadrant by tissue MBR in the entire ONH and by dividing cpRNFLT in that quadrant by cpRNFLT in the entire ONH respectively.

RESULTS

Mean blur rate (MBR) in all quadrants was significantly lower in the ADOA patients than in the controls (p < 0.001 in each). The MBR ratio was significantly lower in the ADOA patients only in the temporal quadrant (p < 0.001). Similarly, cpRNFLT was lower in the ADOA patients in all quadrants (p < 0.001 in each), and the cpRNFLT ratio was lower in the temporal quadrant (p < 0.001).

CONCLUSION

Reduced blood flow in the temporal optic disc in ADOA patients is associated with reduced temporal cpRNFLT, suggesting that both are caused by damage to the papillomacular bundle. The anatomical characteristics of the papillomacular bundle may make it especially susceptible to mitochondrial dysfunction-induced damage, which occurs in ADOA.

Dynamin-Related Protein 1 Promotes Mitochondrial Fission and Contributes to The Hippocampal Neuronal Cell Death Following Experimental Status Epilepticus

AIMS

Prolonged seizure activity may result in mitochondrial dysfunction and lead to cell death in the hippocampus. Mitochondrial fission may occur in an early stage of neuronal cell death. This study examined the role of the mitochondrial fission protein dynamin-related protein 1 (Drp1) in the hippocampus following status epilepticus.

METHODS

Kainic acid (KA) was microinjected unilaterally into the hippocampal CA3 area in Sprague Dawley rats to induce prolonged seizure activity. Biochemical analysis, electron microscopy, and immunofluorescence staining were performed to evaluate the subsequent molecular and cellular events. The effects of pretreatment with a mitochondrial fission protein inhibitor, Mdivi-1 (2 nmol), were also evaluated.

RESULTS

Phosphorylation of Drp1 at serine 616 (p-Drp1(Ser616)) was elevated from 1 to 24 h after the elicited seizure activity. Pretreatment with Mdivi-1 decreased the Drp1 phosphorylation at Ser616 and limited the mitochondrial fission. Mdivi-1 rescued the Complex I dysfunction, decreased the levels of oxidized proteins, decreased the activation of cytochrome c/caspase-3 signaling, and blunted cell death in CA3 neurons.

CONCLUSION

Our findings suggest that activation of p-Drp1(Ser616) is related to seizure-induced neuronal damage. Modulation of p-Drp1(Ser616) expression is accompanied by decreases in mitochondrial fission, mitochondrial dysfunction, and oxidation, providing a neuroprotective effect against seizure-induced hippocampal neuronal damage.

Distinct types of protease systems are involved in homeostasis regulation of mitochondrial morphology via balanced fusion and fission

Mitochondrial morphology is dynamically regulated by fusion and fission. Several GTPase proteins control fusion and fission, and posttranslational modifications of these proteins are important for the regulation. However, it has not been clarified how the fusion and fission is balanced. Here, we report the molecular mechanism to regulate mitochondrial morphology in mammalian cells. Ablation of the mitochondrial fission, by repression of Drp1 or Mff, or by over-expression of MiD49 or MiD51, results in a reduction in the fusion GTPase mitofusins (Mfn1 and Mfn2) in outer membrane and long form of OPA1 (L-OPA1) in inner membrane. RNAi- or CRISPR-induced ablation of Drp1 in HeLa cells enhanced the degradation of Mfns via the ubiquitin-proteasome system (UPS). We further found that UPS-related protein BAT3/BAG6, here we identified as Mfn2-interacting protein, was implicated in the turnover of Mfns in the absence of mitochondrial fission. Ablation of the mitochondrial fission also enhanced the proteolytic cleavage of L-OPA1 to soluble S-OPA1, and the OPA1 processing was reversed by inhibition of the inner membrane protease OMA1 independent on the mitochondrial membrane potential. Our findings showed that the distinct degradation systems of the mitochondrial fusion proteins in different locations are enhanced in response to the mitochondrial morphology.

Mitochondrial dysfunction in inflammatory bowel disease

Inflammatory Bowel Disease (IBD) represents a group of idiopathic disorders characterized by chronic or recurring inflammation of the gastrointestinal tract. While the exact etiology of disease is unknown, IBD is recognized to be a complex, multifactorial disease that results from an intricate interplay of genetic predisposition, an altered immune response, changes in the intestinal microbiota, and environmental factors. Together, these contribute to a destruction of the intestinal epithelial barrier, increased gut permeability, and an influx of immune cells. Given that most cellular functions as well as maintenance of the epithelial barrier is energy-dependent, it is logical to assume that mitochondrial dysfunction may play a key role in both the onset and recurrence of disease. Indeed several studies have demonstrated evidence of mitochondrial stress and alterations in mitochondrial function within the intestinal epithelium of patients with IBD and mice undergoing experimental colitis. Although the hallmarks of mitochondrial dysfunction, including oxidative stress and impaired ATP production are known to be evident in the intestines of patients with IBD, it is as yet unclear whether these processes occur as a cause of consequence of disease. We provide a current review of mitochondrial function in the setting of intestinal inflammation during IBD.

Disruption of mitochondrial fission in the liver protects mice from diet-induced obesity and metabolic deterioration

AIM/HYPOTHESIS

Mitochondria and the endoplasmic reticulum (ER) physically interact by close structural juxtaposition, via the mitochondria-associated ER membrane. Inter-organelle communication between the ER and mitochondria has been shown to regulate energy metabolism and to be central to the modulation of various key processes such as ER stress. We aimed to clarify the role of mitochondrial fission in this communication.

METHODS

We generated mice lacking the mitochondrial fission protein dynamin-related protein 1 (DRP1) in the liver (Drp1LiKO mice).

RESULTS

Drp1LiKO mice showed decreased fat mass and were protected from high-fat diet (HFD)-induced obesity. Analysis of liver gene expression profiles demonstrated marked elevation of ER stress markers. In addition, we observed increased expression of the fibroblast growth factor 21 (FGF21) gene through induction of activating transcription factor 4, master regulator of the integrated stress response.

CONCLUSIONS/INTERPRETATION

Disruption of mitochondrial fission in the liver provoked ER stress, while inducing the expression of FGF21 to increase energy expenditure and protect against HFD-induced obesity.

Loss of Drp1 in the liver leads to an alteration in expression of the genes involved in the immune system

Dynamin-related protein 1 (Drp1) is a member of the dynamin family of large GTPase, which cycles between the cytosol and the mitochondrial outer membrane, and mediates mitochondrial fission. Using microarray analysis of gene expression in the livers of wild-type and Drp1 knockout mice, we have previously identified that endoplasmic reticulum (ER) stress marker genes are significantly increased by the absence of Drp1 [1]. Here, we provide methodological and analytical details of the microarray data, which have been deposited in the Gene Expression Omnibus as data set GSE64222. We have performed further gene ontology analysis of the data and found the differential expression of a subset of genes that are involved in the immune response in the livers of Drp1 knockout mice versus wild-type controls.

Astaxanthin prevents pulmonary fibrosis by promoting myofibroblast apoptosis dependent on Drp1-mediated mitochondrial fission

Promotion of myofibroblast apoptosis is a potential therapeutic strategy for pulmonary fibrosis. This study investigated the antifibrotic effect of astaxanthin on the promotion of myofibroblast apoptosis based on dynamin-related protein-1 (Drp1)-mediated mitochondrial fission in vivo and in vitro. Results showed that astaxanthin can inhibit lung parenchymal distortion and collagen deposition, as well as promote myofibroblast apoptosis. Astaxanthin demonstrated pro-apoptotic function in myofibroblasts by contributing to mitochondrial fission, thereby leading to apoptosis by increasing the Drp1 expression and enhancing Drp1 translocation into the mitochondria. Two specific siRNAs were used to demonstrate that Drp1 is necessary to promote astaxanthin-induced mitochondrial fission and apoptosis in myofibroblasts. Drp1-associated genes, such as Bcl-2-associated X protein, cytochrome c, tumour suppressor gene p53 and p53-up-regulated modulator of apoptosis, were highly up-regulated in the astaxanthin group compared with those in the sham group. This study revealed that astaxanthin can prevent pulmonary fibrosis by promoting myofibroblast apoptosis through a Drp1-dependent molecular pathway. Furthermore, astaxanthin provides a potential therapeutic value in pulmonary fibrosis treatment.

Translocase of inner mitochondrial membrane 44 alters the mitochondrial fusion and fission dynamics and protects from type 2 diabetes

OBJECTIVE

In obesity and type 2 diabetes, the impairment of mitochondrial function in white adipose tissue (WAT) is linked to a reduction in whole body insulin sensitivity. Timm44 is upregulated in the kidneys of streptozotocin-induced diabetic mice. In the inner mitochondrial membrane, Timm44 anchors mitochondrial heat-shock protein 70 (mtHsp70) to the translocase of inner mitochondrial membrane 23 (TIM23) complex and facilitates the import of mitochondria-targeted preproteins into the mitochondrial matrix dependent on the inner membrane potential and ATP hydrolysis on ATPase domain of mtHsp70.

METHODS

We generated the aP2-promoter driven Timm44 transgenic (Tg) mouse model and investigated whether Timm44 Tg mice fed high-fat/high-sucrose (HFHS) chow are protected from type 2 diabetes and obesity.

RESULTS

The body weight of aP2-promoter driven Timm44 Tg mice was lower than that of wild type mice, and insulin sensitivity was greater in Timm44 Tg mice than in wild type mice. Although WAT weight was not altered in Timm44 Tg mice fed HFHS chow, adipocyte size was reduced, and mitochondrial fusion associated with decreased expression of fission genes, such as Dnm1l and Fis1, was observed. In addition, when fed standard (STD) chow, the expressions of the fusion genes Opa1, Mfn1 and Mfn2, and Mfn1 were significantly increased in Timm44 Tg mice compared to wild type mice, and fused mitochondria were also observed in Timm44 Tg mice fed STD chow.

CONCLUSIONS

The Timm44 gene may be a new target for the treatment of type 2 diabetes.

Impaired mitochondrial energy metabolism in Alzheimer's disease: Impact on pathogenesis via disturbed epigenetic regulation of chromatin landscape

The amyloid cascade hypothesis for the pathogenesis of Alzheimer's disease (AD) was proposed over twenty years ago. However, the mechanisms of neurodegeneration and synaptic loss have remained elusive delaying the effective drug discovery. Recent studies have revealed that amyloid-β peptides as well as phosphorylated and fragmented tau proteins accumulate within mitochondria. This process triggers mitochondrial fission (fragmentation) and disturbs Krebs cycle function e.g. by inhibiting the activity of 2-oxoglutarate dehydrogenase. Oxidative stress, hypoxia and calcium imbalance also disrupt the function of Krebs cycle in AD brains. Recent studies on epigenetic regulation have revealed that Krebs cycle intermediates control DNA and histone methylation as well as histone acetylation and thus they have fundamental roles in gene expression. DNA demethylases (TET1-3) and histone lysine demethylases (KDM2-7) are included in the family of 2-oxoglutarate-dependent oxygenases (2-OGDO). Interestingly, 2-oxoglutarate is the obligatory substrate of 2-OGDO enzymes, whereas succinate and fumarate are the inhibitors of these enzymes. Moreover, citrate can stimulate histone acetylation via acetyl-CoA production. Epigenetic studies have revealed that AD is associated with changes in DNA methylation and histone acetylation patterns. However, the epigenetic results of different studies are inconsistent but one possibility is that they represent both coordinated adaptive responses and uncontrolled stochastic changes, which provoke pathogenesis in affected neurons. Here, we will review the changes observed in mitochondrial dynamics and Krebs cycle function associated with AD, and then clarify the mechanisms through which mitochondrial metabolites can control the epigenetic landscape of chromatin and induce pathological changes in AD.

Mitochondria hyperfusion and elevated autophagic activity are key mechanisms for cellular bioenergetic preservation in centenarians

Mitochondria have been considered for long time as important determinants of cell aging because of their role in the production of reactive oxygen species. In this study we investigated the impact of mitochondrial metabolism and biology as determinants of successful aging in primary cultures of fibroblasts isolated from the skin of long living individuals (LLI) (about 100 years old) compared with those from young (about 27 years old) and old (about 75 years old) subjects. We observed that fibroblasts from LLI displayed significantly lower complex I-driven ATP synthesis and higher production of H₂O₂ in comparison with old subjects. Despite these changes, bioenergetics of these cells appeared to operate normally. This lack of functional consequences was likely due to a compensatory phenomenon at the level of mitochondria, which displayed a maintained supercomplexes organization and an increased mass. This appears to be due to a decreased mitophagy, induced by hyperfused, elongated mitochondria. The overall data indicate that longevity is characterized by a preserved bioenergetic function likely attained by a successful mitochondria remodeling that can compensate for functional defects through an increase in mass, i.e. a sort of mitochondrial “hypertrophy”.

Dynamics of mitochondrial DNA nucleoids regulated by mitochondrial fission is essential for maintenance of homogeneously active mitochondria during neonatal heart development

Mitochondria are dynamic organelles, and their fusion and fission regulate cellular signaling, development, and mitochondrial homeostasis, including mitochondrial DNA (mtDNA) distribution. Cardiac myocytes have a specialized cytoplasmic structure where large mitochondria are aligned into tightly packed myofibril bundles; however, recent studies have revealed that mitochondrial dynamics also plays an important role in the formation and maintenance of cardiomyocytes. Here, we precisely analyzed the role of mitochondrial fission in vivo. The mitochondrial fission GTPase, Drp1, is highly expressed in the developing neonatal heart, and muscle-specific Drp1 knockout (Drp1-KO) mice showed neonatal lethality due to dilated cardiomyopathy. The Drp1 ablation in heart and primary cultured cardiomyocytes resulted in severe mtDNA nucleoid clustering and led to mosaic deficiency of mitochondrial respiration. The functional and structural alteration of mitochondria also led to immature myofibril assembly and defective cardiomyocyte hypertrophy. Thus, the dynamics of mtDNA nucleoids regulated by mitochondrial fission is required for neonatal cardiomyocyte development by promoting homogeneous distribution of active mitochondria throughout the cardiomyocytes.