Dynamics of mitochondria in living cells: shape changes, dislocations, fusion, and fission of mitochondria

Autophagy and mitochondrial remodelling in mouse mesenchymal stromal cells challenged with Staphylococcus epidermidis

The bone marrow stroma constitutes the marrow-blood barrier, which sustains immunochemical homoeostasis and protection of the haematopoietic tissue in sequelae of systemic bacterial infections. Under these conditions, the bone marrow stromal cells affected by circulating bacterial pathogens shall elicit the adaptive stress-response mechanisms to maintain integrity of the barrier. The objective of this communication was to demonstrate (i) that in vitro challenge of mesenchymal stromal cells, i.e. colony-forming unit fibroblasts (CFU-F), with Staphylococcus epidermidis can activate the autophagy pathway to execute antibacterial defence response, and (ii) that homoeostatic shift because of the bacteria-induced stress includes the mitochondrial remodelling and sequestration of compromised organelles via mitophagy. Implication of Drp1 and PINK1–PARK2-dependent mechanisms in the mitophagy turnover of the aberrant mitochondria in mesenchymal stromal cells is investigated and discussed.

Toward high-content screening of mitochondrial morphology and membrane potential in living cells

Mitochondria are double membrane organelles involved in various key cellular processes. Governed by dedicated protein machinery, mitochondria move and continuously fuse and divide. These "mitochondrial dynamics" are bi-directionally linked to mitochondrial and cell functional state in space and time. Due to the action of the electron transport chain (ETC), the mitochondrial inner membrane displays a inside-negative membrane potential (Δψ). The latter is considered a functional readout of mitochondrial "health" and required to sustain normal mitochondrial ATP production and mitochondrial fusion. During the last decade, live-cell microscopy strategies were developed for simultaneous quantification of Δψ and mitochondrial morphology. This revealed that ETC dysfunction, changes in Δψ and aberrations in mitochondrial structure often occur in parallel, suggesting they are linked potential targets for therapeutic intervention. Here we discuss how combining high-content and high-throughput strategies can be used for analysis of genetic and/or drug-induced effects at the level of individual organelles, cells and cell populations. This article is part of a Directed Issue entitled: Energy Metabolism Disorders and Therapies.

Membranes in motion: mitochondrial dynamics and their role in apoptosis

Mitochondrial dynamics is crucial for cell survival, development and homeostasis and impairment of these functions leads to neurologic disorders and metabolic diseases. The key components of mitochondrial dynamics have been identified. Mitofusins and OPA1 mediate mitochondrial fusion, whereas Drp1 is responsible for mitochondrial fission. In addition, an interplay between the proteins of the mitochondrial fission/fusion machinery and the Bcl-2 proteins, essential mediators in apoptosis, has been also described. Here, we review the molecular mechanisms regarding mitochondrial dynamics together with their role in apoptosis.

Quality matters: how does mitochondrial network dynamics and quality control impact on mtDNA integrity?

Mammalian mtDNA encodes for 13 core proteins of oxidative phosphorylation. Mitochondrial DNA mutations and deletions cause severe myopathies and neuromuscular diseases. Thus, the integrity of mtDNA is pivotal for cell survival and health of the organism. We here discuss the possible impact of mitochondrial fusion and fission on mtDNA maintenance as well as positive and negative selection processes. Our focus is centred on the important question of how the quality of mtDNA nucleoids can be assured when selection and mitochondrial quality control works on functional and physiological phenotypes constituted by oxidative phosphorylation proteins. The organelle control theory suggests a link between phenotype and nucleoid genotype. This is discussed in the light of new results presented here showing that mitochondrial transcription factor A/nucleoids are restricted in their intramitochondrial mobility and probably have a limited sphere of influence. Together with recent published work on mitochondrial and mtDNA heteroplasmy dynamics, these data suggest first, that single mitochondria might well be internally heterogeneous and second, that nucleoid genotypes might be linked to local phenotypes (although the link might often be leaky). We discuss how random or site-specific mitochondrial fission can isolate dysfunctional parts and enable their elimination by mitophagy, stressing the importance of fission in the process of mtDNA quality control. The role of fusion is more multifaceted and less understood in this context, but the mixing and equilibration of matrix content might be one of its important functions.

Profiling of the Tox21 chemical collection for mitochondrial function to identify compounds that acutely decrease mitochondrial membrane potential

BACKGROUND

Mitochondrial dysfunction has been implicated in the pathogenesis of a variety of disorders including cancer, diabetes, and neurodegenerative and cardiovascular diseases. Understanding whether different environmental chemicals and druglike molecules impact mitochondrial function represents an initial step in predicting exposure-related toxicity and defining a possible role for such compounds in the onset of various diseases.

OBJECTIVES

We sought to identify individual chemicals and general structural features associated with changes in mitochondrial membrane potential (MMP).

METHODS

We used a multiplexed [two end points in one screen; MMP and adenosine triphosphate (ATP) content] quantitative high throughput screening (qHTS) approach combined with informatics tools to screen the Tox21 library of 10,000 compounds (~ 8,300 unique chemicals) at 15 concentrations each in triplicate to identify chemicals and structural features that are associated with changes in MMP in HepG2 cells.

RESULTS

Approximately 11% of the compounds (913 unique compounds) decreased MMP after 1 hr of treatment without affecting cell viability (ATP content). In addition, 309 compounds decreased MMP over a concentration range that also produced measurable cytotoxicity [half maximal inhibitory concentration (IC50) in MMP assay/IC50 in viability assay ≤ 3; p < 0.05]. More than 11% of the structural clusters that constitute the Tox21 library (76 of 651 clusters) were significantly enriched for compounds that decreased the MMP.

CONCLUSIONS

Our multiplexed qHTS approach allowed us to generate a robust and reliable data set to evaluate the ability of thousands of drugs and environmental compounds to decrease MMP. The use of structure-based clustering analysis allowed us to identify molecular features that are likely responsible for the observed activity.

Mitochondrial pleomorphy in plant cells is driven by contiguous ER dynamics

Mitochondria are pleomorphic, double membrane-bound organelles involved in cellular energetics in all eukaryotes. Mitochondria in animal and yeast cells are typically tubular-reticulate structures and several micro-meters long but in green plants they are predominantly observed as 0.2-1.5 μm punctae. While fission and fusion, through the coordinated activity of several conserved proteins, shapes mitochondria, the endoplasmic reticulum (ER) has recently been identified as an additional player in this process in yeast and mammalian cells. The mitochondria-ER relationship in plant cells remains largely uncharacterized. Here, through live-imaging of the entire range of mitochondria pleomorphy we uncover the underlying basis for the predominantly punctate mitochondrial form in plants. We demonstrate that mitochondrial morphology changes in response to light and cytosolic sugar levels in an ER mediated manner. Whereas, large ER polygons and low dynamics under dark conditions favor mitochondrial fusion and elongation, small ER polygons result in increased fission and predominantly small mitochondria. Hypoxia also reduces ER dynamics and increases mitochondrial fusion to produce giant mitochondria. By observing elongated mitochondria in normal plants and fission-impaired Arabidopsis nmt1-2 and drp3a mutants we also establish that thin extensions called matrixules and a beads-on-a-string mitochondrial phenotype are direct consequences of mitochondria-ER interactions.

Mitochondrial protein quality control in health and disease

Progressive mitochondrial dysfunction is linked with the onset of many age-related pathologies and neurological disorders. Mitochondrial damage can come in many forms and be induced by a variety of cellular insults. To preserve organelle function during biogenesis or times of stress, multiple surveillance systems work to ensure the persistence of a functional mitochondrial network. This review provides an overview of these processes, which collectively contribute to the maintenance of a healthy mitochondrial population, which is critical for cell physiology and survival.

Senescence in adipose-derived stem cells and its implications in nerve regeneration

Adult mesenchymal stem cells, specifically adipose-derived stem cells have self-renewal and multiple differentiation potentials and have shown to be the ideal candidate for therapeutic applications in regenerative medicine, particularly in peripheral nerve regeneration. Adipose-derived stem cells are easily harvested, although they may show the effects of aging, hence their potential in nerve repair may be limited by cellular senescence or donor age. Cellular senescence is a complex process whereby stem cells grow old as consequence of intrinsic events (e.g., DNA damage) or environmental cues (e.g., stressful stimuli or diseases), which determine a permanent growth arrest. Several mechanisms are implicated in stem cell senescence, although no one is exclusive of the others. In this review we report some of the most important factors modulating the senescence process, which can influence adipose-derived stem cell morphology and function, and compromise their clinical application for peripheral nerve regenerative cell therapy.

Mitochondrial reactive oxygen species (ROS) and ROS-induced ROS release

Byproducts of normal mitochondrial metabolism and homeostasis include the buildup of potentially damaging levels of reactive oxygen species (ROS), Ca(2+), etc., which must be normalized. Evidence suggests that brief mitochondrial permeability transition pore (mPTP) openings play an important physiological role maintaining healthy mitochondria homeostasis. Adaptive and maladaptive responses to redox stress may involve mitochondrial channels such as mPTP and inner membrane anion channel (IMAC). Their activation causes intra- and intermitochondrial redox-environment changes leading to ROS release. This regenerative cycle of mitochondrial ROS formation and release was named ROS-induced ROS release (RIRR). Brief, reversible mPTP opening-associated ROS release apparently constitutes an adaptive housekeeping function by the timely release from mitochondria of accumulated potentially toxic levels of ROS (and Ca(2+)). At higher ROS levels, longer mPTP openings may release a ROS burst leading to destruction of mitochondria, and if propagated from mitochondrion to mitochondrion, of the cell itself. The destructive function of RIRR may serve a physiological role by removal of unwanted cells or damaged mitochondria, or cause the pathological elimination of vital and essential mitochondria and cells. The adaptive release of sufficient ROS into the vicinity of mitochondria may also activate local pools of redox-sensitive enzymes involved in protective signaling pathways that limit ischemic damage to mitochondria and cells in that area. Maladaptive mPTP- or IMAC-related RIRR may also be playing a role in aging. Because the mechanism of mitochondrial RIRR highlights the central role of mitochondria-formed ROS, we discuss all of the known ROS-producing sites (shown in vitro) and their relevance to the mitochondrial ROS production in vivo.

Absorption of monochromatic and narrow band radiation in the visible and near IR by both mitochondrial and non-mitochondrial photoacceptors results in photobiomodulation

In addition to the major functions performed by in the cell, mitochondria play a major role in cell-light interaction. Accordingly it is generally accepted that mitochondria are crucial in cell photobiomodulation; however a variety of biomolecules themselves proved to be targets of light irradiation. We describe whether and how mitochondria can interact with monochromatic and narrow band radiation in the red and near IR optical regions with dissection of both structural and functional effects likely leading to photobiostimulation. Moreover we also report that a variety of biomolecules localized in mitochondria and/or in other cell compartments including cytochrome c oxidase, some proteins, nucleic acids and adenine nucleotides are light sensitive with major modifications in their biochemistry. All together the reported investigations show that the elucidation of the mechanism of the light interaction with biological targets still remains to be completed, this needing further research, however the light sensitivity of a variety of molecules strongly suggests that photobiomodulation could be used in both in photomedicine and in biotechnology.

Mitochondria-targeted therapies for acute kidney injury

Acute kidney injury (AKI) is a serious clinical condition with no effective treatment. Tubular cells are key targets in AKI. Tubular cells and, specifically, proximal tubular cells are extremely rich in mitochondria and mitochondrial changes had long been known to be a feature of AKI. However, only recent advances in understanding the molecules involved in mitochondria biogenesis and dynamics and the availability of mitochondria-targeted drugs has allowed the exploration of the specific role of mitochondria in AKI. We now review the morphological and functional mitochondrial changes during AKI, as well as changes in the expression of mitochondrial genes and proteins. Finally, we summarise the current status of novel therapeutic strategies specifically targeting mitochondria such as mitochondrial permeability transition pore (MPTP) opening inhibitors (cyclosporine A (CsA)), quinone analogues (MitoQ, SkQ1 and SkQR1), superoxide dismutase (SOD) mimetics (Mito-CP), Szeto-Schiller (SS) peptides (Bendavia) and mitochondrial division inhibitors (mdivi-1). MitoQ, SkQ1, SkQR1, Mito-CP, Bendavia and mdivi-1 have improved the course of diverse experimental models of AKI. Evidence for a beneficial effect of CsA on human cardiac ischaemia-reperfusion injury derives from a clinical trial; however, CsA is nephrotoxic. MitoQ and Bendavia have been shown to be safe for humans. Ongoing clinical trials are testing the efficacy of Bendavia in AKI prevention following renal artery percutaneous transluminal angioplasty.

Mitochondria autophagy is induced after hypoxic/ischemic stress in a Drp1 dependent manner: the role of inhibition of Drp1 in ischemic brain damage

Mitochondria dysfunction is implicated in diverse conditions, including metabolic and neurodegenerative disorders. Mitochondrial dynamics has attracted increasing attention as to its relationship with mitochondria autophagy, also known as mitophagy, which is critical for degradation of dysfunctional mitochondria maintaining mitochondrial homeostasis. Mitochondrial fission and its role in clearance of injured mitochondria in acute ischemic injury, however, have not been elucidated yet. Here we showed that hypoxic/ischemic conditions led to fragmentation of mitochondria and induction of mitophagy in permanent middle cerebral artery occlusion (pMCAO) rats and oxygen-glucose deprivation (OGD) PC12 cells. Inhibition of Drp1 by pharmacologic inhibitor or siRNA resulted in accumulation of damaged mitochondria mainly through selectively blocking mitophagy without affecting mitochondrial biogenesis and non-selective autophagy. Drp1 inhibitors increased the infarct volume and aggravated the neurological deficits in a rat model of pMCAO. We demonstrated that the devastating role of disturbed mitochondrial fission by inhibiting Drp1 contributed to the damaged mitochondria-mediated injury such as ROS generation, cyt-c release and activation of caspase-3. Taken together, we proved that under hypoxic/ischemic stress a Drp1-dependent mitophagy was triggered which was involved in the removal of damaged mitochondria and cellular survival at the early stage of hypoxic/ischemic injury. Thus, Drp1 related pathway involved in selective removal of dysfunctional mitochondria is proposed as an efficient target for treatment of cerebral ischemia.

A proteomic screen with Drosophila Opa1-like identifies Hsc70-5/Mortalin as a regulator of mitochondrial morphology and cellular homeostasis

Mitochondrial morphology is regulated by conserved proteins involved in fusion and fission processes. The mammalian Optic atrophy 1 (OPA1) that functions in mitochondrial fusion is associated with Optic Atrophy and has been implicated in inner membrane cristae remodeling during cell death. Here, we show Drosophila Optic atrophy 1-like (Opa1-like) influences mitochondrial morphology through interaction with 'mitochondria-shaping' proteins like Mitochondrial assembly regulatory factor (Marf) and Drosophila Mitofilin (dMitofilin). To gain an insight into Opa1-like's network, we delineated bonafide interactors like dMitofilin, Marf, Serine protease High temperature requirement protein A2 (HTRA2), Rhomboid-7 (Rho-7) along with novel interactors such as Mortalin ortholog (Hsc70-5) from Drosophila mitochondrial extract. Interestingly, RNAi mediated down-regulation of hsc70-5 in Drosophila wing imaginal disc's peripodial cells resulted in fragmented mitochondria with reduced membrane potential leading to proteolysis of Opa1-like. Increased ecdysone activity induced dysfunctional fragmented mitochondria for clearance through lysosomes, an effect enhanced in hsc70-5 RNAi leading to increased cell death. Over-expression of Opa1-like rescues mitochondrial morphology and cell death in prepupal tissues expressing hsc70-5 RNAi. Taken together, we have identified a novel interaction between Hsc70-5/Mortalin and Opa1-like that influences cellular homeostasis through mitochondrial fusion.

Mitochondrial fission is required for cardiomyocyte hypertrophy mediated by a Ca2+-calcineurin signaling pathway

Cardiomyocyte hypertrophy has been associated with diminished mitochondrial metabolism. Mitochondria are crucial organelles for the production of ATP, and their morphology and function are regulated by the dynamic processes of fusion and fission. The relationship between mitochondrial dynamics and cardiomyocyte hypertrophy is still poorly understood. Here, we show that treatment of cultured neonatal rat cardiomyocytes with the hypertrophic agonist norepinephrine promotes mitochondrial fission (characterized by a decrease in mitochondrial mean volume and an increase in the relative number of mitochondria per cell) and a decrease in mitochondrial function. We demonstrate that norepinephrine acts through α1-adrenergic receptors to increase cytoplasmic Ca(2+), activating calcineurin and promoting migration of the fission protein Drp1 (encoded by Dnml1) to mitochondria. Dominant-negative Drp1 (K38A) not only prevented mitochondrial fission, it also blocked hypertrophic growth of cardiomyocytes in response to norepinephrine. Remarkably, an antisense adenovirus against the fusion protein Mfn2 (AsMfn2) was sufficient to increase mitochondrial fission and stimulate a hypertrophic response without agonist treatment. Collectively, these results demonstrate the importance of mitochondrial dynamics in the development of cardiomyocyte hypertrophy and metabolic remodeling.

Noninvasive assessment of mitochondrial organization in three-dimensional tissues reveals changes associated with cancer development

Mitochondrial organization is often altered to accommodate cellular bioenergetic and biosynthetic demands. Changes in metabolism are a hallmark of a number of diseases, including cancer; however, the interdependence between mitochondrial metabolic function and organization is not well understood. Here, we present a noninvasive, automated and quantitative method to assess mitochondrial organization in three-dimensional (3D) tissues using exclusively endogenous two-photon excited fluorescence (TPEF) and show that mitochondrial organization reflects alterations in metabolic activities. Specifically, we examine the organization of mitochondria within live, engineered epithelial tissue equivalents that mimic normal and precancerous human squamous epithelial tissues. We identify unique patterns of mitochondrial organization in the different tissue models we examine, and we attribute these to differences in the metabolic profiles of these tissues. We find that mitochondria are clustered in tissues with high levels of glycolysis and are more highly networked in tissues where oxidative phosphorylation is more dominant. The most highly networked organization is observed within cells with high levels of glutamine consumption. Furthermore, we demonstrate that mitochondrial organization provides complementary information to traditional morphological hallmarks of cancer development, including variations in nuclear size. Finally, we present evidence that this automated quantitative analysis of endogenous TPEF images can identify differences in the mitochondrial organization of freshly excised normal and pre-cancerous human cervical tissue specimens. Thus, this method could be a promising new modality to assess the role of mitochondrial organization in the metabolic activity of 3D tissues and could be further developed to serve as an early cancer clinical diagnostic biomarker.

Mitochondria dynamism: of shape, transport and cell migration

Mitochondria are highly dynamic and functionally versatile organelles that continuously fragment and fuse in response to different physiological needs of the cell. The list of proteins that strictly regulate the morphology of these organelles is constantly growing, adding new players every day and new pieces to the comprehension and elucidation of this complex machinery. The structural complexity of mitochondria is only paralled by their functional versatility. Indeed, changes in mitochondria shape play critical roles in vertebrate development programmed cell death and in various processes of normal cell physiology, such as calcium signaling, reactive oxygen species production, and lifespan. Here, we present the latest findings on the regulation of mitochondrial dynamics and some of their physiological roles, focusing on cell migration. In cells where migration represents a crucial function in their physiology, such as T and tumoral metastatic cells, mitochondria need to be fragmented and recruited to specific subcellular regions to make movement possible. In depth analysis of this role of mitochondrial dynamics should help in identifying potential targeted therapy against cancer or in improving the immune system's efficiency.

Stem cells: balancing resistance and sensitivity to DNA damage

Embryonic stem cells (ESCs) are known to be very sensitive to DNA damage and undergo rapid apoptosis even after low-damage doses. By contrast, adult stem cells show variable sensitivity to damage. Here we describe the multiple pathways that have been proposed to affect the sensitivity of stem cells to damage, including proximity to the apoptotic threshold (mitochondrial priming) and the p53 signaling pathway, through activation of transcription or direct interaction with proapoptotic proteins in the cytoplasm. We also discuss which cellular factors might connect mitochondrial priming with pluripotency and the potential therapeutic advances that can be achieved by better understanding of the molecular mechanisms leading to sensitivity or resistance of embryonic or adult stem cells from different tissues.

Membrane depolarization activates the mitochondrial protease OMA1 by stimulating self-cleavage

Mitochondrial inner membrane fusion depends on the dynamin-related GTPase OPA1 and the function of OPA1 is regulated by proteolytic cleavage. The mitochondrial proteases Yme1L and OMA1 cleave OPA1 at S2 and S1 sites, respectively. Here, we show that OMA1 is cleaved to a short form (S-OMA1) by itself upon mitochondrial membrane depolarization; S-OMA1 is degraded quickly but could be stabilized by CCCP treatment or Prohibitin knockdown in cells. In addition, OMA1 processing is positively correlated with OPA1 cleavage at the S1 site and the regulation of mitochondrial morphology. Thus, our results reveal the molecular mechanism for OMA1 activation toward OPA1 processing.

Pre-spermiogenic initiation of flagellar growth and correlative ultrastructural observations on nuage, nuclear and mitochondrial developmental morphology in the zebrafish Danio rerio

The microstructural and ultrastructural changes of germ cells during spermatogenesis of zebrafish (Danio rerio) were examined using light microscopy (LM) and transmission electron microscopy (TEM). Generally the process of spermatogenesis in zebrafish is similar to that of other teleosts, however, here we describe some peculiar features of zebrafish spermatogenic cells which have a limited report in this species. (1) The basic events of spermiogenesis are asynchronous, location of flagellum finished in initial stage, while chromatin condensation sharply occurred in intermediate stage and elimination of excess cytoplasm mainly taken place in final stages. (2) Surprisingly, the cilia or initial flagellae are created in spermatocytes, approach toward the nucleus of early stage spermatids, and then the centrioles depress into nuclear fossa and change their orientation to each other from right angle to obtuse angle about 125°. (3) During spermatogenesis, the chromatin compaction performs in a distinctive pattern, condensed heterogeneously from granular into chromatin clumps with central electron-lucent areas, round or long, which diminished to small nuclear vacuoles in spermatozoa. This finding demonstrates the origin of nuclear vacuoles in zebrafish spermatozoa for the first time. (4) Nuages are observed in both spermatogonia and spermatocytes. They are connected with the mitochondria and nuclear membrane, and are even located in the perinuclear spaces of spermatogonia nuclei. (5) Mitochondrial morphology and distribution shows diversity in different germ cells. The condensed mitochondria appear in pachytene spermatocytes, and mitochondria including membrane conglomerate exist in both spermatocytes and spermatids. This study was undertaken in order to disclose specific spermatogenic cells features in zebrafish that could be helpful for understanding the correlative function in this model species.

Mutations in the SPTLC1 protein cause mitochondrial structural abnormalities and endoplasmic reticulum stress in lymphoblasts

Mutations in serine palmitoyltransferase long chain subunit 1 (SPTLC1) cause the typical length-dependent axonal degeneration hereditary sensory neuropathy type 1 (HSN1). Transmission electron microscopy studies on SPTLC1 mutant lymphoblasts derived from patients revealed specific structural abnormalities of mitochondria. Swollen mitochondria with abnormal cristae were clustered around the nucleus, with some mitochondria being wrapped in rough endoplasmic reticulum (ER) membranes. Total mitochondrial counts revealed a significant change in mitochondrial numbers between healthy and diseased lymphocytes but did not reveal any change in length to width ratios nor were there any changes to cellular function. However, there was a notable change in ER homeostasis, as assessed using key ER stress markers, BiP and ERO1-Lα, displaying reduced protein expression. The observations suggest that SPTLC1 mutations cause mitochondrial abnormalities and ER stress in HSN1 cells.

Mitofusin 1 degradation is induced by a disruptor of mitochondrial calcium homeostasis, CGP37157: a role in apoptosis in prostate cancer cells

Mitochondria constantly divide (mitochondrial fission) and fuse (mitochondrial fusion) in a normal cell. Disturbances in the balance between these two physiological processes may lead to cell dysfunction or to cell death. Induction of cell death is the prime goal of prostate cancer chemotherapy. Our previous study demonstrated that androgens increase the expression of a mitochondrial protein involved in fission and facilitate an apoptotic response to CGP37157 (CGP), an inhibitor of mitochondrial calcium efflux, in prostate cancer cells. However, the regulation and role of mitochondrial fusion proteins in the death of these cells have not been examined. Therefore, our objective was to investigate the effect of CGP on a key mitochondrial fusion protein, mitofusin 1 (Mfn1), and the role of Mfn1 in prostate cancer cell apoptosis. We used various prostate cancer cell lines and western blot analysis, qRT-PCR, siRNA, M30 apoptosis assay and immunoprecipitation techniques to determine mechanisms regulating Mfn1. Treatment of prostate cancer cells with CGP resulted in selective degradation of Mfn1. Mfn1 ubiquitination was detected following immunoprecipitation of overexpressed Myc-tagged Mfn1 protein from CGP-treated cells, and treatment with the proteasomal inhibitor lactacystin, as well as siRNA-mediated knockdown of the E3 ubiquitin ligase March5, protected Mfn1 from CGP-induced degradation. These data indicate the involvement of the ubiquitin-proteasome pathway in CGP-induced degradation of Mfn1. We also demonstrated that downregulation of Mfn1 by siRNA enhanced the apoptotic response of LNCaP cells to CGP, suggesting a likely pro-survival role for Mfn1 in these cells. Our results suggest that manipulation of mitofusins may provide a novel therapeutic advantage in treating prostate cancer.

Photoactivatable green fluorescent protein-based visualization and quantification of mitochondrial fusion and mitochondrial network complexity in living cells

Technological improvements in microscopy and the development of mitochondria-specific imaging molecular tools have illuminated the dynamic rearrangements of these essential organelles. These rearrangements are mainly the result of two opposing processes: mitochondrial fusion and mitochondrial fission. Consistent with this, in addition to mitochondrial motility, these two processes are major factors determining the overall degree of continuity of the mitochondrial network, as well as the average size of mitochondria within the cell. In this chapter, we detail the use of advanced confocal microscopy and mitochondrial matrix-targeted photoactivatable green fluorescent protein (mito-PAGFP) for the investigation of mitochondrial dynamics. We focus on direct visualization and quantification of mitochondrial fusion and mitochondrial network complexity in living mammalian cells. These assays were instrumental in important recent discoveries within the field of mitochondrial biology, including the role of mitochondrial fusion in the activation of mitochondrial steps in apoptosis, participation of Bcl-2 family proteins in mitochondrial morphogenesis, and stress-induced mitochondrial hyperfusion. We present some basic directions that should be helpful in designing mito-PAGFP-based experiments. Furthermore, since analyses of mitochondrial fusion using mito-PAGFP-based assays rely on time-lapse imaging, critical parameters of time-lapse microscopy and cell preparation are also discussed.

Do we age because we have mitochondria?

The process of aging remains a great riddle. Production of reactive oxygen species (ROS) by mitochondria is an inevitable by-product of respiration, which has led to a hypothesis proposing the oxidative impairment of mitochondrial components (e.g., mtDNA, proteins, lipids) that initiates a vicious cycle of dysfunctional respiratory complexes producing more ROS, which again impairs function. This does not exclude other processes acting in parallel or targets for ROS action in other organelles than mitochondria. Given that aging is defined as the process leading to death, the role of mitochondria-based impairments in those organ systems responsible for human death (e.g., the cardiovascular system, cerebral dysfunction, and cancer) is described within the context of "garbage" accumulation and increasing insulin resistance, type 2 diabetes, and glycation of proteins. Mitochondrial mass, fusion, and fission are important factors in coping with impaired function. Both biogenesis of mitochondria and their degradation are important regulatory mechanisms stimulated by physical exercise and contribute to healthy aging. The hypothesis of mitochondria-related aging should be revised to account for the limitations of the degradative capacity of the lysosomal system. The processes involved in mitochondria-based impairments are very similar across a large range of organisms. Therefore, studies on model organisms from yeast, fungi, nematodes, flies to vertebrates, and from cells to organisms also add considerably to the understanding of human aging.

Effect of preeclampsia on placental function: influence of sexual dimorphism, microRNA's and mitochondria

In pregnancy fetal growth and development occur in a sexually dimorphic manner. Male and female fetuses respond differently to the intrauterine environment with males disproportionately suffering from perinatal morbidity and mortality. We have demonstrated placental dysfunction and sexually dimorphic responses in pregnancies complicated by severe preeclampsia. Production of cytokines and apoptosis in the male placenta is heightened relative to that of the female placenta. We also find increased expression and stabilization and a sexual dimorphism in expression of the transcription factor HIF-1α, but a defect in binding to the hypoxia response element with corresponding reduced expression of HIF-1α target genes including VEGF and Glut-1. HIF-1α is involved in crosstalk with the redox sensitive transcription factor NFκB in regulation by cytokines, reactive oxygen species and expression of inflammatory genes. We find increased placental expression and DNA binding of NFκB and a sexually dimorphic response suggesting a role for NFκB in placental dysfunction with preeclampsia. Placental mitochondrial complex III activity and complex I and IV expression are reduced and alterations in mitochondrial morphology are found in preeclampsia and are linked to the hypoxamir miR-210. We propose that with severe PE placental HIF-1α is stabilized by excessive ROS, inflammation and relative hypoxia. This increases the expression of miR-210 in the placenta causing repression of mitochondria-associated target genes, potentially leading to mitochondrial and placental dysfunction. This placental dysfunction may lead to a fetal programming effect that results in disease in later life.

Fluorescence staining of mitochondria for morphology analysis in Saccharomyces cerevisiae

Mitochondria are highly dynamic organelles in all eukaryotic cells. Most of our insights regarding the mechanisms that determine the morphogenesis and motility of mitochondria have been identified and analyzed first in the model organism Saccharomyces cerevisiae. To this end high-resolution microscopic methods were applied that rely on fluorescence labeling of the organelle. A comprehensive overview of fluorescence staining approaches that were successfully applied to study the behavior of mitochondria in vivo but also in fixed cells is provided. Slightly modified versions of the methods described here can also be used to analyze other compartments of the yeast cell. Microscopic setups and imaging methods will only be shortly discussed since these are highly dependent on each laboratory's basic infrastructure.

Mitochondrial dynamics in aging and disease

Mitochondria are self-replicating organelles but nevertheless strongly depend on supply coded in nuclear genes. They serve many physiological demands in living cells. Supply of the cytoplasm with ATP and engagement in Ca(2+) regulation belong to the main functions of mitochondria. In large eukaryotic cells, in particular in neurons, with their long dendrites and axons, mitochondria have to move to the sites of their action. This trafficking involves several motor molecules and mechanisms to sense the sites of requirements of mitochondria. With aging and as a consequence of some diseases, mitochondrial components may be rendered dysfunctional, and mtDNA mutations arise during the course of replication and by the action of reactive oxygen species. Mutants in motor molecules engaged in trafficking and in the machinery of fusion and fission are causing severe deficiencies on the cellular level; they support neurodegeneration and, thus, cause many diseases. Frequent fusion and fission events mediate the elimination of impaired parts from mitochondria which finally will be degraded by autophagosomes. Extensive fusion provides a basis for functional complementation. Mobility of proteins and small molecules within the mitochondria is necessary to reach the functional goals of fusion and fission, although cristae and a large fraction of proteins of the respiratory complexes proved to be stable for hours after fusion and perform slow exchange of material.

Morphological characterization of testicular cells, spermatogenesis and formation of spermatophores in a fish ectoparasite Argulus bengalensis Ramakrishna, 1951 (Crustacea: Branchiura)

The present study has been carried out to describe the cell morphology of the developing male gametes in a fish ectoparasite, Argulus bengalensis Ramakrishna, 1951. With respect to cell volume and nucleoplasmic index, spermatogonia are the smallest and primary spermatocytes are the largest in this lineage. The spermatogonia and the differentiating spermatogenic cells are in separate niches and confined to different enclaves within each testicular lobe. Spermiogenesis occurs within the inner enclave of each testicular lobe. During this process the nucleus becomes streamlined; an acrosome is formed, axoneme is originated, and residual cytoplasm is discarded through the flagellum. The sperm cell morphology displays a general pattern comprising head, mid-piece, and a full length flagellum. In the axoneme 9+2 arrangement of the microtubule is conserved. In addition to the axoneme, some more singlet microtubules are found surrounding a fiber sheath and around one of the mitochondria adjacent to the axoneme. This arrangement indicates a close phylogenetic relationship with pentastomida. In the present study, structure and formation of spermatophore are described in this branchiuran parasite.

A microfluidic device for immuno-affinity-based separation of mitochondria from cell culture

In this work, we present a method to isolate mitochondria of mammalian cells after cell disruption on microscale. The device is composed of linear microchannels cast in PDMS (polydimethylsiloxane). Specific antibodies against the translocase outer membrane protein of the mitochondria are immobilized on the surface of the substrate using an avidin-biotin sandwich construct. The mitochondria can be captured in the channel, whereas the remains of the cell lysate flow out the chip unhindered. The captured mitochondria can be observed directly on chip. A successful immobilization of pre-isolated mitochondria was shown at a flow rate between 0 and 5 μl min(-1) (≈0-2.5 mm s(-1)). After fluorescence staining, we demonstrated that the mitochondria covered around 3% of the channel surface. The mitochondria appeared in a distinct spherical shape with a diameter of around 0.8-1.2 μm. Further validation of the microfluidic device using non-treated cell lysate was done at 2 μl min(-1). The immobilized mitochondria were smaller with a diameter of around ≈490 nm. We observed a surface coverage of around 4%. The immobilized mitochondria were active and stable for over 2 h without cooling and were shown to be able to produce ATP under stage 3 respiration on chip.

Loss of Yme1L perturbates mitochondrial dynamics

Yme1L is an AAA protease that is embedded in the mitochondrial inner membrane with its catalytic domain facing the mitochondrial inner-membrane space. However, how Yme1L regulates mammalian mitochondrial function is still obscure. We find that endogenous Yme1L locates at punctate structures of mitochondria, and that loss of Yme1L in mouse embryonic fibroblast (MEF) cells results in mitochondrial fragmentation and leads to significant increased ‘kiss-and-run' type of mitochondrial fusion; however, Yme1L knockdown (shYme1L (short hairpin-mediated RNA interference of Yme1L)) cells still remain normal mitochondrial fusion although shYme1L mitochondria have a little bit less fusion and fission rates, and the shYme1L-induced fragmentation is due to a little bit more mitochondrial fission than fusion in cells. Furthermore, shYme1L-induced mitochondrial fragmentation is independent on optic atrophy 1 (OPA1) S1 or S2 processing, and shYme1L results in the stabilization of OPA1 long form (L-OPA1); in addition, the exogenous expression of OPA1 or L-OPA1 facilitates the shYme1L-induced mitochondrial fragmentation, thus this fragmentation induced by shYme1L appears to be associated with L-OPA1's stability. ShYme1L also causes a slight increase of mitochondrial dynamics proteins of 49 kDa and mitochondrial fission factor (Mff), which recruit mitochondrial key fission factor dynamin-related protein 1 (Drp1) into mitochondria in MEF cells, and loss of Drp1 or Mff inhibits the shYme1L-induced mitochondrial fragmentation. In addition, there is interaction between SLP-2 with Yme1L and shYme1L cells retain stress-induced mitochondrial hyperfusion. Taken together, our results clarify how Yme1L regulates mitochondrial morphology.

Mitochondria and mammalian reproduction

Mitochondria are cellular organelles with crucial roles in ATP synthesis, metabolic integration, reactive oxygen species (ROS) synthesis and management, the regulation of apoptosis (namely via the intrinsic pathway), among many others. Additionally, mitochondria in different organs or cell types may have distinct properties that can decisively influence functional analysis. In terms of the importance of mitochondria in mammalian reproduction, and although there are species-specific differences, these aspects involve both energetic considerations for gametogenesis and fertilization, control of apoptosis to ensure the proper production of viable gametes, and ROS signaling, as well as other emerging aspects. Crucially, mitochondria are the starting point for steroid hormone biosynthesis, given that the conversion of cholesterol to pregnenolone (a common precursor for all steroid hormones) takes place via the activity of the cytochrome P450 side-chain cleavage enzyme (P450scc) on the inner mitochondrial membrane. Furthermore, mitochondrial activity in reproduction has to be considered in accordance with the very distinct strategies for gamete production in the male and female. These include distinct gonad morpho-physiologies, different types of steroids that are more prevalent (testosterone, estrogens, progesterone), and, importantly, the very particular timings of gametogenesis. While spermatogenesis is complete and continuous since puberty, producing a seemingly inexhaustible pool of gametes in a fixed environment; oogenesis involves the episodic production of very few gametes in an environment that changes cyclically. These aspects have always to be taken into account when considering the roles of any common element in mammalian reproduction.

Localization of exogenous DNA to mitochondria in skeletal muscle following hydrodynamic limb vein injection

Mitochondrial genetic disorders are a major cause of mitochondrial diseases. It is therefore likely that mitochondrial gene therapy will be useful for the treatment of such diseases. Here, we report on the possibility of mitochondrial gene delivery in skeletal muscle using hydrodynamic limb vein (HLV) injection. The HLV injection procedure, a useful method for transgene expression in skeletal muscle, involves the rapid injection of a large volume of naked plasmid DNA (pDNA) into the distal vein of a limb. We hypothesized that the technique could be used to deliver pDNA not only to nuclei but also to mitochondria, since cytosolic pDNA that is internalized by the method may be able to overcome mitochondrial membrane. We determined if pDNA could be delivered to myofibrillar mitochondria by HLV injection by PCR analysis. Mitochondrial toxicity assays showed that the HLV injection had no influence on mitochondrial function. These findings indicate that HLV injection promises to be a useful technique for in vivo mitochondrial gene delivery.

Highly expressed MT-ND3 positively associated with histological severity of hepatic steatosis

Hepatic steatosis is the accumulation of an excess amount of triglycerides and other fats inside liver cells resulting from abnormal hepatic lipid metabolism. Mitochondrial structural and molecular defects are involved in the progression of hepatic steatosis pathogenesis. Hepatic methylation and transcriptional activity of the mitochondrial-encoded NADH dehydrogenase (MT-ND) play a critical role in the progression of non-alcoholic fatty liver disease (NAFLD). However, the expression of MT-ND3 in hepatic steatosis has not been extensively studied. In this study, liver specimens were collected from different patients, and were subjected to immunohistochemistry. Primary hepatocytes were treated with oxidative stress, hypoxia, and lipotoxicity to investigate the respective roles of these factors on MT-ND3 expression and cell apoptosis by western blotting and flow cytometry, respectively. We found that increased MT-ND3 expression in human hepatic steatosis was positively associated with histological severity of hepatic steatosis. Hypoxia, H2O2 , and saturated fatty acid treatment induced cell apoptosis mediated by mitochondria. These three factors all had effects on MT-ND3 expression in cultured hepatocytes. Taken together, MT-ND3 may play important roles in hepatic steatosis progress. Hypoxia, oxidative stress, and lipotoxicity could all influence expression of MT-ND3 and thus may play a role in the progression of hepatic steatosis.

Mitochondrial morphology in metabolic diseases

SIGNIFICANCE

Mitochondria are the cellular energy-producing organelles and are at the crossroad of determining cell life and death. As such, the function of mitochondria has been intensely studied in metabolic disorders, including diabetes and associated maladies commonly grouped under all-inclusive pathological condition of metabolic syndrome. More recently, the altered metabolic profiles and function of mitochondria in these ailments have been correlated with their aberrant morphologies. This review describes an overview of mitochondrial fission and fusion machineries, and discusses implications of mitochondrial morphology and function in these metabolic maladies.

RECENT ADVANCES

Mitochondria undergo frequent morphological changes, altering the mitochondrial network organization in response to environmental cues, termed mitochondrial dynamics. Mitochondrial fission and fusion mediate morphological plasticity of mitochondria and are controlled by membrane-remodeling mechanochemical enzymes and accessory proteins. Growing evidence suggests that mitochondrial dynamics play an important role in diabetes establishment and progression as well as associated ailments, including, but not limited to, metabolism-secretion coupling in the pancreas, nonalcoholic fatty liver disease progression, and diabetic cardiomyopathy.

CRITICAL ISSUES

While mitochondrial dynamics are intimately associated with mitochondrial bioenergetics, their cause-and-effect correlation remains undefined in metabolic diseases.

FUTURE DIRECTIONS

The involvement of mitochondrial dynamics in metabolic diseases is in its relatively early stages. Elucidating the role of mitochondrial dynamics in pathological metabolic conditions will aid in defining the intricate form-function correlation of mitochondria in metabolic pathologies and should provide not only important clues to metabolic disease progression, but also new therapeutic targets.

Keeping mitochondria in shape: a matter of life and death

BACKGROUND

Mitochondria are dynamic organelles that participate in energy conversion, metabolism, signaling and apoptosis. To fulfill their multiple tasks, mitochondria come in varied morphologies and ultrastructures resulting from the equilibrium between fusion and fission, Furthermore, they are in close contact with the endoplasmic reticulum generating an essential interface in cell physiology and death.

MATERIALS AND METHODS

In this review we will discuss the impact of our findings on the molecular mechanisms of mitochondrial morphology and tethering to the endoplasmic reticulum on cell death.

RESULTS AND CONCLUSIONS

The work reviewed here supports a key role for mitochondrial dynamics in cell life and death and in disease.

Why mitochondria must fuse to maintain their genome integrity

SIGNIFICANCE

The maintenance of mitochondrial genome integrity is a major challenge for cells to sustain energy production by respiration.

RECENT ADVANCES

Recently, mitochondrial membrane dynamics emerged as a key process contributing to prevent mitochondrial DNA (mtDNA) alterations. Indeed, both fundamental and clinical data suggest that disruption of mitochondrial fusion, related to mutations in the OPA1, MFN2, PINK1, and PARK2 genes, leads to the accumulation of mutations in the mitochondrial genome.

CRITICAL ISSUES

We discuss here the possibility that mitochondrial fusion acts as a direct mechanism to prevent the generation of altered mtDNA and to eliminate mutated deleterious genomes either by trans-complementation or by mitophagy.

FUTURE DIRECTIONS

Finally, we conclude this review with a short evolutionary comparison between the mechanisms involved in mitochondrial and bacterial modes of genome distribution and plasticity, highlighting possible common conserved processes required for the maintenance of their genome integrity, which should inspire our future investigations.

Retinal ganglion cells: Energetics, compartmentation, axonal transport, cytoskeletons and vulnerability

Retinal ganglion cells (RGCs) are specialized projection neurons that relay an immense amount of visual information from the retina to the brain. RGC signal inputs are collected by dendrites and output is distributed from the cell body via very thin (0.5-1 μm) and long (∼50 mm) axons. The RGC cell body is larger than other retinal neurons, but is still only a very small fraction (one ten thousandths) of the length and total surface area of the axon. The total distance traversed by RGCs extends from the retina, starting from synapses with bipolar and amacrine cells, to the brain, to synapses with neurons in the lateral geniculate nucleus. This review will focus on the energy demands of RGCs and the relevant tissues that surround them. RGC survival and function unexceptionally depends upon free energy, predominantly adenosine triphosphate (ATP). RGC energy metabolism is vastly different when compared to that of the photoreceptors. Each subcellular component of the RGC is remarkably different in terms of structure, function and extracellular environment. The energy demands and distribution of each component are also distinct as evidenced by the uneven distribution of mitochondria and ATP within the RGC - signifying the presence of intracellular energy gradients. In this review we will describe RGCs as having four subcellular components, (1) Dendrites, (2) Cell body, (3) Non-myelinated axon, including intraocular and optic nerve head portions, and (4) Myelinated axon, including the intra-orbital and intracranial portions. We will also describe how RGCs integrate information from each subcellular component in order achieve intracellular homeostatic stability as well as respond to perturbations in the extracellular environment. The possible cellular mechanisms such as axonal transport and axonal cytoskeleton proteins that are involved in maintaining RGC energy homeostasis during normal and disease conditions will also be discussed in depth. The emphasis of this review will be on energetic mechanisms within RGC components that have the most relevance to clinical ophthalmology.

Viral degradasome hijacks mitochondria to suppress innate immunity

The balance between the innate immunity of the host and the ability of a pathogen to evade it strongly influences pathogenesis and virulence. The two nonstructural (NS) proteins, NS1 and NS2, of respiratory syncytial virus (RSV) are critically required for RSV virulence. Together, they strongly suppress the type I interferon (IFN)-mediated innate immunity of the host cells by degrading or inhibiting multiple cellular factors required for either IFN induction or response pathways, including RIG-I, IRF3, IRF7, TBK1 and STAT2. Here, we provide evidence for the existence of a large and heterogeneous degradative complex assembled by the NS proteins, which we named "NS-degradasome" (NSD). The NSD is roughly ∼300-750 kD in size, and its degradative activity was enhanced by the addition of purified mitochondria in vitro. Inside the cell, the majority of the NS proteins and the substrates of the NSD translocated to the mitochondria upon RSV infection. Genetic and pharmacological evidence shows that optimal suppression of innate immunity requires mitochondrial MAVS and mitochondrial motility. Together, we propose a novel paradigm in which the mitochondria, known to be important for the innate immune activation of the host, are also important for viral suppression of the innate immunity.

Maintenance of mitochondrial morphology by autophagy and its role in high glucose effects on chronological lifespan of Saccharomyces cerevisiae

In Saccharomyces cerevisiae, mitochondrial morphology changes when cells are shifted between nonfermentative and fermentative carbon sources. Here, we show that cells of S. cerevisiae grown in different glucose concentrations display different mitochondrial morphologies. The morphology of mitochondria in the cells growing in 0.5% glucose was similar to that of mitochondria in respiring cells. However, the mitochondria of cells growing in higher glucose concentrations (2% and 4%) became fragmented after growth in these media, due to the production of acetic acid; however, the fragmentation was not due to intracellular acidification. From a screen of mutants involved in sensing and utilizing nutrients, cells lacking TOR1 had reduced mitochondrial fragmentation, and autophagy was found to be essential for this reduction. Mitochondrial fragmentation in cells grown in high glucose was reversible by transferring them into conditioned medium from a culture grown on 0.5% glucose. Similarly, the chronological lifespan of cells grown in high glucose medium was reduced, and this phenotype could be reversed when cells were transferred to low glucose conditioned medium. These data indicate that chronological lifespan seems correlated with mitochondrial morphology of yeast cells and that both phenotypes can be influenced by factors from conditioned medium of cultures grown in low glucose medium.

Abnormal mitochondrial dynamics in the pathogenesis of Alzheimer's disease

Mitochondrial dysfunction is one of the most early and prominent features in vulnerable neurons in the brain of Alzheimer's disease (AD) patients. Recent studies suggest that mitochondria are highly dynamic organelles characterized by a delicate balance of fission and fusion, a concept that has revolutionized our basic understanding of the regulation of mitochondrial structure and function which has far-reaching significance in studies of health and disease. Tremendous progress has been made in studying changes in mitochondrial dynamics in AD brain and models and the potential underlying mechanisms. This review highlights the recent work demonstrating abnormal mitochondrial dynamics and distribution in AD models and discusses how these abnormalities may contribute to various aspects of mitochondrial dysfunction and the pathogenesis of AD.

Mechanisms of mitochondrial fission and fusion

Mitochondria continually change shape through the combined actions of fission, fusion, and movement along cytoskeletal tracks. The lengths of mitochondria and the degree to which they form closed networks are determined by the balance between fission and fusion rates. These rates are influenced by metabolic and pathogenic conditions inside mitochondria and by their cellular environment. Fission and fusion are important for growth, for mitochondrial redistribution, and for maintenance of a healthy mitochondrial network. In addition, mitochondrial fission and fusion play prominent roles in disease-related processes such as apoptosis and mitophagy. Three members of the Dynamin family are key components of the fission and fusion machineries. Their functions are controlled by different sets of adaptor proteins on the surface of mitochondria and by a range of regulatory processes. Here, we review what is known about these proteins and the processes that regulate their actions.

Mitochondria as a source of reactive oxygen and nitrogen species: from molecular mechanisms to human health

Mitochondrially generated reactive oxygen species are involved in a myriad of signaling and damaging pathways in different tissues. In addition, mitochondria are an important target of reactive oxygen and nitrogen species. Here, we discuss basic mechanisms of mitochondrial oxidant generation and removal and the main factors affecting mitochondrial redox balance. We also discuss the interaction between mitochondrial reactive oxygen and nitrogen species, and the involvement of these oxidants in mitochondrial diseases, cancer, neurological, and cardiovascular disorders.

Mitochondrial fusion proteins and human diseases

Mitochondria are highly dynamic, complex organelles that continuously alter their shape, ranging between two opposite processes, fission and fusion, in response to several stimuli and the metabolic demands of the cell. Alterations in mitochondrial dynamics due to mutations in proteins involved in the fusion-fission machinery represent an important pathogenic mechanism of human diseases. The most relevant proteins involved in the mitochondrial fusion process are three GTPase dynamin-like proteins: mitofusin 1 (MFN1) and 2 (MFN2), located in the outer mitochondrial membrane, and optic atrophy protein 1 (OPA1), in the inner membrane. An expanding number of degenerative disorders are associated with mutations in the genes encoding MFN2 and OPA1, including Charcot-Marie-Tooth disease type 2A and autosomal dominant optic atrophy. While these disorders can still be considered rare, defective mitochondrial dynamics seem to play a significant role in the molecular and cellular pathogenesis of more common neurodegenerative diseases, for example, Alzheimer's and Parkinson's diseases. This review provides an overview of the basic molecular mechanisms involved in mitochondrial fusion and focuses on the alteration in mitochondrial DNA amount resulting from impairment of mitochondrial dynamics. We also review the literature describing the main disorders associated with the disruption of mitochondrial fusion.

Tissue specific structural variations of mitochondria of fish ectoparasite Argulus bengalensis Ramakrishna, 1951 (Crustacea: Branchiura): Functional implications

We studied the fine structure of some classical and six variant mitochondria from different tissues viz. proboscis gland, spinal gland, ovary, testis, and muscle of a fish ectoparasite, Argulus bengalensis. In the proboscis gland and spinal gland, mitochondria are protected within vesicle to preserve their structure and activity from exposure to glandular synthesis for its parasitic mode of feeding. In the oocytes, mitochondria are larger and cylindrical in appearance. Oocyte mitochondria are highly dynamic and exhibit frequent fission and fusion. Those are clustered in the cytoplasm of previtellogenic oocytes which prepare for different synthetic activities for successful reproductive investment. In contrast, mitochondrial abundance is less in the male gametic lineage. The spermatocytes and the nurse cells in the testis have an unusual type of mitochondria, nebenkern which is formed by the fusions of number of mitochondria. A completely different type of mitochondrion is discovered in the flagellum of the spermatozoa. It is provided with fifteen numbers of singlet microtubules at its outer periphery which is a salient feature of the flagellum of this Branchiuran genus. This unique mitochondrion uses the microtubule tract for its movement to distribute energy efficiently along the axoneme. Such mitochondrion and microtubular association provide evidence in favor of phylogenetic relationship between Argulus and pentastomid Raillietiella. In striated muscle of thoracic appendages, mitochondria maintain tight junctions with the endoplasmic reticulum and remain in close apposition of the myofibrils which helps in Ca²⁺ uptake for stimulating continuous muscular activity required for ventilation of respiratory structures of the parasites.