The mitochondrial fission protein hFis1 requires the endoplasmic reticulum gateway to induce apoptosis

The interplay between mitochondrial dynamics and autophagy: From a key homeostatic mechanism to a driver of pathology

The complex relationship between mitochondrial dynamics and autophagy illustrates how two cellular housekeeping processes are intimately linked, illuminating fundamental principles of cellular homeostasis and shedding light on disparate pathological conditions including several neurodegenerative disorders. Here we review the basic tenets of mitochondrial dynamics i.e., the concerted balance between fusion and fission of the organelle, and its interplay with macroautophagy and selective mitochondrial autophagy, also dubbed mitophagy, in the maintenance of mitochondrial quality control and ultimately in cell viability. We illustrate how conditions of altered mitochondrial dynamics reverberate on autophagy and vice versa. Finally, we illustrate how altered interplay between these two key cellular processes participates in the pathogenesis of human disorders affecting multiple organs and systems.

A preliminary study of the miRNA restitution effect on CNV-induced miRNA downregulation in CAKUT

BACKGROUND

The majority of CAKUT-associated CNVs overlap at least one miRNA gene, thus affecting the cellular levels of the corresponding miRNA. We aimed to investigate the potency of restitution of CNV-affected miRNA levels to remediate the dysregulated expression of target genes involved in kidney physiology and development in vitro.

METHODS

Heterozygous MIR484 knockout HEK293 and homozygous MIR185 knockout HEK293 cell lines were used as models depicting the deletion of the frequently affected miRNA genes by CAKUT-associated CNVs. After treatment with the corresponding miRNA mimics, the levels of the target genes have been compared to the non-targeting control treatment. For both investigated miRNAs, MDM2 and PKD1 were evaluated as common targets, while additional 3 genes were investigated as targets of each individual miRNA (NOTCH3, FIS1 and APAF1 as hsa-miR-484 targets and RHOA, ATF6 and CDC42 as hsa-miR-185-5p targets).

RESULTS

Restitution of the corresponding miRNA levels in both knockout cell lines has induced a change in the mRNA levels of certain candidate target genes, thus confirming the potential to alleviate the CNV effect on miRNA expression. Intriguingly, HEK293 WT treatment with investigated miRNA mimics has triggered a more pronounced effect, thus suggesting the importance of miRNA interplay in different genomic contexts.

CONCLUSIONS

Dysregulation of multiple mRNA targets mediated by CNV-affected miRNAs could represent the underlying mechanism behind the unresolved CAKUT occurrence and phenotypic variability observed in CAKUT patients. Characterizing miRNAs located in CNVs and their potential to become molecular targets could eventually help in understanding and improving the management of CAKUT.

Human Fis1 directly interacts with Drp1 in an evolutionarily conserved manner to promote mitochondrial fission

Mitochondrial fission protein 1 (Fis1) and dynamin-related protein 1 (Drp1) are the only two proteins evolutionarily conserved for mitochondrial fission, and directly interact in Saccharomyces cerevisiae to facilitate membrane scission. However, it remains unclear if a direct interaction is conserved in higher eukaryotes as other Drp1 recruiters, not present in yeast, are known. Using NMR, differential scanning fluorimetry, and microscale thermophoresis, we determined that human Fis1 directly interacts with human Drp1 (KD = 12-68 μM), and appears to prevent Drp1 assembly, but not GTP hydrolysis. Similar to yeast, the Fis1-Drp1 interaction appears governed by two structural features of Fis1: its N-terminal arm and a conserved surface. Alanine scanning mutagenesis of the arm identified both loss-of-function and gain-of-function alleles with mitochondrial morphologies ranging from highly elongated (N6A) to highly fragmented (E7A), demonstrating a profound ability of Fis1 to govern morphology in human cells. An integrated analysis identified a conserved Fis1 residue, Y76, that upon substitution to alanine, but not phenylalanine, also caused highly fragmented mitochondria. The similar phenotypic effects of the E7A and Y76A substitutions, along with NMR data, support that intramolecular interactions occur between the arm and a conserved surface on Fis1 to promote Drp1-mediated fission as in S. cerevisiae. These findings indicate that some aspects of Drp1-mediated fission in humans derive from direct Fis1-Drp1 interactions that are conserved across eukaryotes.

Role of Mitochondrial Dynamics in Heart Diseases

Mitochondrial dynamics, including fission and fusion processes, are essential for heart health. Mitochondria, the powerhouses of cells, maintain their integrity through continuous cycles of biogenesis, fission, fusion, and degradation. Mitochondria are relatively immobile in the adult heart, but their morphological changes due to mitochondrial morphology factors are critical for cellular functions such as energy production, organelle integrity, and stress response. Mitochondrial fusion proteins, particularly Mfn1/2 and Opa1, play multiple roles beyond their pro-fusion effects, such as endoplasmic reticulum tethering, mitophagy, cristae remodeling, and apoptosis regulation. On the other hand, the fission process, regulated by proteins such as Drp1, Fis1, Mff and MiD49/51, is essential to eliminate damaged mitochondria via mitophagy and to ensure proper cell division. In the cardiac system, dysregulation of mitochondrial dynamics has been shown to cause cardiac hypertrophy, heart failure, ischemia/reperfusion injury, and various cardiac diseases, including metabolic and inherited cardiomyopathies. In addition, mitochondrial dysfunction associated with oxidative stress has been implicated in atherosclerosis, hypertension and pulmonary hypertension. Therefore, understanding and regulating mitochondrial dynamics is a promising therapeutic tool in cardiac diseases. This review summarizes the role of mitochondrial morphology in heart diseases for each mitochondrial morphology regulatory gene, and their potential as therapeutic targets to heart diseases.

Control of mitochondria-associated endoplasmic reticulum membranes by protein S-palmitoylation: Novel therapeutic targets for neurodegenerative diseases

Mitochondria-associated endoplasmic reticulum membranes (MAMs) are dynamic coupling structures between mitochondria and the endoplasmic reticulum (ER). As a new subcellular structure, MAMs combine the two critical organelle functions. Mitochondria and the ER could regulate each other via MAMs. MAMs are involved in calcium (Ca²⁺) homeostasis, autophagy, ER stress, lipid metabolism, etc. Researchers have found that MAMs are closely related to metabolic syndrome and neurodegenerative diseases (NDs). The formation of MAMs and their functions depend on specific proteins. Numerous protein enrichments, such as the IP3R-Grp75-VDAC complex, constitute MAMs. The changes in these proteins govern the interaction between mitochondria and the ER; they also affect the biological functions of MAMs. S-palmitoylation is a reversible protein post-translational modification (PTM) that mainly occurs on protein cysteine residues. More and more studies have shown that the S-palmitoylation of proteins is closely related to their membrane localization. Here, we first briefly describe the composition and function of MAMs, reviewing the component and biological roles of MAMs mediated by S-palmitoylation, elaborating on S-palmitoylated proteins in Ca²⁺ flux, lipid rafts, and so on. We try to provide new insight into the molecular basis of MAMs-related diseases, mainly NDs. Finally, we propose potential drug compounds targeting S-palmitoylation.

Human Fis1 directly interacts with Drp1 in an evolutionarily conserved manner to promote mitochondrial fission

Mitochondrial Fission Protein 1 (Fis1) and Dynamin Related Protein 1 (Drp1) are the only two proteins evolutionarily conserved for mitochondrial fission, and directly interact in S. cerevisiae to facilitate membrane scission. However, it remains unclear if a direct interaction is conserved in higher eukaryotes as other Drp1 recruiters, not present in yeast, are known. Using NMR, differential scanning fluorimetry, and microscale thermophoresis, we determined that human Fis1 directly interacts with human Drp1 ( K D = 12-68 µM), and appears to prevent Drp1 assembly, but not GTP hydrolysis. Similar to yeast, the Fis1-Drp1 interaction appears governed by two structural features of Fis1: its N-terminal arm and a conserved surface. Alanine scanning mutagenesis of the arm identified both loss- and gain-of-function alleles with mitochondrial morphologies ranging from highly elongated (N6A) to highly fragmented (E7A) demonstrating a profound ability of Fis1 to govern morphology in human cells. An integrated analysis identified a conserved Fis1 residue, Y76, that upon substitution to alanine, but not phenylalanine, also caused highly fragmented mitochondria. The similar phenotypic effects of the E7A and Y76A substitutions, along with NMR data, support that intramolecular interactions occur between the arm and a conserved surface on Fis1 to promote Drp1-mediated fission as in S. cerevisiae . These findings indicate that some aspects of Drp1-mediated fission in humans derive from direct Fis1-Drp1 interactions that are conserved across eukaryotes.

Determinants and outcomes of mitochondrial dynamics

Mitochondria are not only central organelles in metabolism and energy conversion but are also platforms for cellular signaling cascades. Classically, the shape and ultrastructure of mitochondria were depicted as static. The discovery of morphological transitions during cell death and of conserved genes controlling mitochondrial fusion and fission contributed to establishing the concept that mitochondrial morphology and ultrastructure are dynamically regulated by mitochondria-shaping proteins. These finely tuned, dynamic changes in mitochondrial shape can in turn control mitochondrial function, and their alterations in human diseases suggest that this space can be explored for drug discovery. Here, we review the basic tenets and molecular mechanisms of mitochondrial morphology and ultrastructure, describing how they can coordinately define mitochondrial function.

PEX11β and FIS1 cooperate in peroxisome division independently of mitochondrial fission factor

Peroxisome membrane dynamics and division are essential to adapt the peroxisomal compartment to cellular needs. The peroxisomal membrane protein PEX11β (also known as PEX11B) and the tail-anchored adaptor proteins FIS1 (mitochondrial fission protein 1) and MFF (mitochondrial fission factor), which recruit the fission GTPase DRP1 (dynamin-related protein 1, also known as DNML1) to both peroxisomes and mitochondria, are key factors of peroxisomal division. The current model suggests that MFF is essential for peroxisome division, whereas the role of FIS1 is unclear. Here, we reveal that PEX11β can promote peroxisome division in the absence of MFF in a DRP1- and FIS1-dependent manner. We also demonstrate that MFF permits peroxisome division independently of PEX11β and restores peroxisome morphology in PEX11β-deficient patient cells. Moreover, targeting of PEX11β to mitochondria induces mitochondrial division, indicating the potential for PEX11β to modulate mitochondrial dynamics. Our findings suggest the existence of an alternative, MFF-independent pathway in peroxisome division and report a function for FIS1 in the division of peroxisomes. This article has an associated First Person interview with the first authors of the paper.

Mitochondria-Associated Membrane Scaffolding with Endoplasmic Reticulum: A Dynamic Pathway of Developmental Disease

Communication between intracellular organelles is essential for overall cellular function. How this communication occurs and under what circumstances alterations transpire are only the beginning to be elucidated. The pathways of calcium homeostasis, lipid transfer, mitochondrial dynamics, and mitophagy/apoptosis have been linked to the endoplasmic reticulum and tethering sites on the outer and/or inner mitochondrial membrane called mitochondria-associated endoplasmic reticulum membranes (MAM). Sensitive visualization by high-powered microscopy coupled with the advent of massive parallel sequencing has elaborated the structure, while patient’s diseases have uncovered the physiological function of these networks. Using specific patient examples from our pediatric mitochondrial center, we expand how specific genetic pathological variants in certain MAM structures induce disease. Genetic variants in MICU1, PASC-2, CYP2U1, SERAC1, and TANGO2 can induce early development abnormalities in the areas of cognition, motor, and central nervous system structures across multiple MAM pathways and implicate mitochondrial dysregulation.

The SUMO protease SENP3 regulates mitochondrial autophagy mediated by Fis1

Mitochondria are unavoidably subject to organellar stress resulting from exposure to a range of reactive molecular species. Consequently, cells operate a poorly understood quality control programme of mitophagy to facilitate elimination of dysfunctional mitochondria. Here, we used a model stressor, deferiprone (DFP), to investigate the molecular basis for stress-induced mitophagy. We show that mitochondrial fission 1 protein (Fis1) is required for DFP-induced mitophagy and that Fis1 is SUMOylated at K149, an amino acid residue critical for Fis1 mitochondrial localization. We find that DFP treatment leads to the stabilization of the SUMO protease SENP3, which is mediated by downregulation of the E3 ubiquitin (Ub) ligase CHIP. SENP3 is responsible for Fis1 deSUMOylation and depletion of SENP3 abolishes DFP-induced mitophagy. Furthermore, preventing Fis1 SUMOylation by conservative K149R mutation enhances Fis1 mitochondrial localization. Critically, expressing a Fis1 K149R mutant restores DFP-induced mitophagy in SENP3-depleted cells. Thus, we propose a model in which SENP3-mediated deSUMOylation facilitates Fis1 mitochondrial localization to underpin stress-induced mitophagy.

Long-Term Storage Does Not Affect the Expression Profiles of mRNA and Long Non-Coding RNA in Vitrified-Warmed Human Embryos

Although vitrification has been widely applied in assisted reproductive technology, it is unknown whether storage time has any impact on the mRNA and lncRNA expression profiles in human embryos. Eleven women (aged 23-35 years) who had undergone in vitro fertilization treatment were recruited for this study. The transcriptomes of 3 fresh eight-cell embryos and 8 surviving vitrified-warmed eight-cell embryos (4 embryos were cryostored for 3 years, and the others were cryostored for 8 years) were analyzed through single-cell RNA-Seq. No differentially expressed mRNAs or lncRNAs were identified between the 3-years group and 8-years group. A total of 128 mRNAs and 365 lncRNAs were differentially expressed in the 8 vitrified-warmed embryos compared with the fresh embryos. The vitrification-warming impact was moderate, and it was mainly related to the pathways of metabolism, stress response, apoptosis, cell cycle, cell adhesion, and signaling for TFG-β and Hippo. The analysis of target mRNAs suggested that lncRNAs might contribute to the regulation of mRNAs after vitrification-warming. Our findings indicated that long-term storage after vitrification does not affect the mRNA and lncRNA expression profiles in human embryos, however, the procedure of vitrification-warming would lead to minor alteration of transcriptome.

Mitochondrial dynamics regulators: implications for therapeutic intervention in cancer

Regardless of the recent advances in therapeutic developments, cancer is still among the primary causes of death globally, indicating the need for alternative therapeutic strategies. Mitochondria, a dynamic organelle, continuously undergo the fusion and fission processes to meet cell requirements. The balanced fission and fusion processes, referred to as mitochondrial dynamics, coordinate mitochondrial shape, size, number, energy metabolism, cell cycle, mitophagy, and apoptosis. An imbalance between these opposing events alters mitochondWangrial dynamics, affects the overall mitochondrial shape, and deregulates mitochondrial function. Emerging evidence indicates that alteration of mitochondrial dynamics contributes to various aspects of tumorigenesis and cancer progression. Therefore, targeting the mitochondrial dynamics regulator could be a potential therapeutic approach for cancer treatment. This review will address the role of imbalanced mitochondrial dynamics in mitochondrial dysfunction during cancer progression. We will outline the clinical significance of mitochondrial dynamics regulators in various cancer types with recent updates in cancer stemness and chemoresistance and its therapeutic potential and clinical utility as a predictive biomarker.

BAP31: Physiological functions and roles in disease

B-cell receptor-associated protein 31 (BAP31 or BCAP31) is a ubiquitously expressed transmembrane protein found mainly in the endoplasmic reticulum (ER), including in mitochondria-associated membranes (MAMs). It acts as a broad-specificity membrane protein chaperone and quality control factor, which can promote different fates for its clients, including ER retention, ER export, ER-associated degradation (ERAD), or evasion of degradation, and it also acts as a MAM tetherer and regulatory protein. It is involved in several cellular processes - it supports ER and mitochondrial homeostasis, promotes proliferation and migration, plays several roles in metabolism and the immune system, and regulates autophagy and apoptosis. Full-length BAP31 can be anti-apoptotic, but can also mediate activation of caspase-8, and itself be cleaved by caspase-8 into p20-BAP31, which promotes apoptosis by mobilizing ER calcium stores at MAMs. BAP31 loss-of-function mutations is the cause of 'deafness, dystonia, and central hypomyelination' (DDCH) syndrome, characterized by severe neurological symptoms and early death. BAP31 is furthermore implicated in a growing number of cancers and other diseases, and several viruses have been found to target it to promote their survival or life cycle progression. The purpose of this review is to provide an overview and examination of the basic properties, functions, mechanisms, and roles in disease of BAP31.

Apoptotic signals at the endoplasmic reticulum-mitochondria interface

The maintenance of cellular homeostasis involves the participation of multiple organelles, such as the endoplasmic reticulum (ER) and mitochondria. Specifically, ER plays a key role in calcium (Ca²⁺) storage, lipid synthesis, protein folding, and assembly, while mitochondria are the "energy factories" and provide energy to drive intracellular processes. Hence, alteration in ER or mitochondrial homeostasis has detrimental effects on cell survival, being linked to the triggering of apoptosis, a programmed form of cell death. Besides, ER stress conditions affect mitochondria functionality and vice-versa, as ER and mitochondria communicate via mitochondria-associated ER membranes (MAMs) to carry out a number of fundamental cellular functions. It is not surprising, thus, that also MAMs perturbations are involved in the regulation of apoptosis. This chapter intends to accurately discuss the involvement of MAMs in apoptosis, highlighting their crucial role in controlling this delicate cellular process.

Opa1 relies on cristae preservation and ATP synthase to curtail reactive oxygen species accumulation in mitochondria

Reactive oxygen species (ROS) are a common product of active mitochondrial respiration carried in mitochondrial cristae, but whether cristae shape influences ROS levels is unclear. Here we report that the mitochondrial fusion and cristae shape protein Opa1 requires mitochondrial ATP synthase oligomers to reduce ROS accumulation. In cells fueled with galactose to force ATP production by mitochondria, cristae are enlarged, ATP synthase oligomers destabilized, and ROS accumulate. Opa1 prevents both cristae remodeling and ROS generation, without impinging on levels of mitochondrial antioxidant defense enzymes that are unaffected by Opa1 overexpression. Genetic and pharmacologic experiments indicate that Opa1 requires ATP synthase oligomerization and activity to reduce ROS levels upon a blockage of the electron transport chain. Our results indicate that the converging effect of Opa1 and mitochondrial ATP synthase on mitochondrial ultrastructure regulate ROS abundance to sustain cell viability.

The MAMs Structure and Its Role in Cell Death

The maintenance of cellular homeostasis involves the participation of multiple organelles. These organelles are associated in space and time, and either cooperate or antagonize each other with regards to cell function. Crosstalk between organelles has become a significant topic in research over recent decades. We believe that signal transduction between organelles, especially the endoplasmic reticulum (ER) and mitochondria, is a factor that can influence the cell fate. As the cellular center for protein folding and modification, the endoplasmic reticulum can influence a range of physiological processes by regulating the quantity and quality of proteins. Mitochondria, as the cellular "energy factory," are also involved in cell death processes. Some researchers regard the ER as the sensor of cellular stress and the mitochondria as an important actuator of the stress response. The scientific community now believe that bidirectional communication between the ER and the mitochondria can influence cell death. Recent studies revealed that the death signals can shuttle between the two organelles. Mitochondria-associated membranes (MAMs) play a vital role in the complex crosstalk between the ER and mitochondria. MAMs are known to play an important role in lipid synthesis, the regulation of Ca²⁺ homeostasis, the coordination of ER-mitochondrial function, and the transduction of death signals between the ER and the mitochondria. Clarifying the structure and function of MAMs will provide new concepts for studying the pathological mechanisms associated with neurodegenerative diseases, aging, and cancers. Here, we review the recent studies of the structure and function of MAMs and its roles involved in cell death, especially in apoptosis.

Mitochondria Associated Membranes (MAMs): Architecture and physiopathological role

In the last decades, the communication between the Endoplasmic reticulum (ER) and mitochondria has obtained great attention: mitochondria-associated membranes (MAMs), which represent the contact sites between the two organelles, have indeed emerged as central hub involved in different fundamental cell processes, such as calcium signalling, apoptosis, autophagy and lipid biosynthesis. Consistently, dysregulation of ER-mitochondria crosstalk has been associated with different pathological conditions, ranging from diabetes to cancer and neurodegenerative diseases. In this review, we will try to summarize the current knowledge on MAMs' structure and functions in health and their relevance for human diseases.

Mitochondrial Fission Protein 1: Emerging Roles in Organellar Form and Function in Health and Disease

Mitochondrial fission protein 1 (Fis1) was identified in yeast as being essential for mitochondrial division or fission and subsequently determined to mediate human mitochondrial and peroxisomal fission. Yet, its exact functions in humans, especially in regard to mitochondrial fission, remains an enigma as genetic deletion of Fis1 elongates mitochondria in some cell types, but not others. Fis1 has also been identified as an important component of apoptotic and mitophagic pathways suggesting the protein may have multiple, essential roles. This review presents current perspectives on the emerging functions of Fis1 and their implications in human health and diseases, with an emphasis on Fis1's role in both endocrine and neurological disorders.

ER-Mitochondria Contact Sites Reporters: Strengths and Weaknesses of the Available Approaches

Organelle intercommunication represents a wide area of interest. Over the last few decades, increasing evidence has highlighted the importance of organelle contact sites in many biological processes including Ca²⁺ signaling, lipid biosynthesis, apoptosis, and autophagy but also their involvement in pathological conditions. ER–mitochondria tethering is one of the most investigated inter-organelle communications and it is differently modulated in response to several cellular conditions including, but not limited to, starvation, Endoplasmic Reticulum (ER) stress, and mitochondrial shape modifications. Despite many studies aiming to understand their functions and how they are perturbed under different conditions, approaches to assess organelle proximity are still limited. Indeed, better visualization and characterization of contact sites remain a fascinating challenge. The aim of this review is to summarize strengths and weaknesses of the available methods to detect and quantify contact sites, with a main focus on ER–mitochondria tethering.

The cell biology of mitochondrial membrane dynamics

Owing to their ability to efficiently generate ATP required to sustain normal cell function, mitochondria are often considered the 'powerhouses of the cell'. However, our understanding of the role of mitochondria in cell biology recently expanded when we recognized that they are key platforms for a plethora of cell signalling cascades. This functional versatility is tightly coupled to constant reshaping of the cellular mitochondrial network in a series of processes, collectively referred to as mitochondrial membrane dynamics and involving organelle fusion and fission (division) as well as ultrastructural remodelling of the membrane. Accordingly, mitochondrial dynamics influence and often orchestrate not only metabolism but also complex cell signalling events, such as those involved in regulating cell pluripotency, division, differentiation, senescence and death. Reciprocally, mitochondrial membrane dynamics are extensively regulated by post-translational modifications of its machinery and by the formation of membrane contact sites between mitochondria and other organelles, both of which have the capacity to integrate inputs from various pathways. Here, we discuss mitochondrial membrane dynamics and their regulation and describe how bioenergetics and cellular signalling are linked to these dynamic changes of mitochondrial morphology.

ER-Mitochondria Calcium Transfer, Organelle Contacts and Neurodegenerative Diseases

It is generally accepted that interorganellar contacts are central to the control of cellular physiology. Virtually, any intracellular organelle can come into proximity with each other and, by establishing physical protein-mediated contacts within a selected fraction of the membrane surface, novel specific functions are acquired. Endoplasmic reticulum (ER) contacts with mitochondria are among the best studied and have a major role in Ca²⁺ and lipid transfer, signaling, and membrane dynamics.Their functional (and structural) diversity, their dynamic nature as well as the growing number of new players involved in the tethering concurred to make their monitoring difficult especially in living cells. This review focuses on the most established examples of tethers/modulators of the ER-mitochondria interface and on the roles of these contacts in health and disease by specifically dissecting how Ca²⁺ transfer occurs and how mishandling eventually leads to disease. Additional functions of the ER-mitochondria interface and an overview of the currently available methods to measure/quantify the ER-mitochondria interface will also be discussed.

Human Fis1 regulates mitochondrial dynamics through inhibition of the fusion machinery

Mitochondrial dynamics is important for life. At center stage for mitochondrial dynamics, the balance between mitochondrial fission and fusion is a set of dynamin-related GTPases that drive mitochondrial fission and fusion. Fission is executed by the GTPases Drp1 and Dyn2, whereas the GTPases Mfn1, Mfn2, and OPA1 promote fusion. Recruitment of Drp1 to mitochondria is a critical step in fission. In yeast, Fis1p recruits the Drp1 homolog Dnm1p to mitochondria through Mdv1p and Caf4p, but whether human Fis1 (hFis1) promotes fission through a similar mechanism as in yeast is not established. Here, we show that hFis1-mediated mitochondrial fragmentation occurs in the absence of Drp1 and Dyn2, suggesting that they are dispensable for hFis1 function. hFis1 instead binds to Mfn1, Mfn2, and OPA1 and inhibits their GTPase activity, thus blocking the fusion machinery. Consistent with this, disruption of the fusion machinery in Drp1-/- cells phenocopies the fragmentation phenotype induced by hFis1 overexpression. In sum, our data suggest a novel role for hFis1 as an inhibitor of the fusion machinery, revealing an important functional evolutionary divergence between yeast and mammalian Fis1 proteins.

LETM1 is required for mitochondrial homeostasis and cellular viability (Review)

Leucine zipper/EF-hand-containing transmembrane protein 1 (LETM1) has been identified as the gene responsible for Wolf-Hirschhorn syndrome (WHS), which is characterized by intellectual disability, epilepsy, growth delay and craniofacial dysgenesis. LETM1 is a mitochondrial inner membrane protein that encodes a homolog of the yeast protein Mdm38, which is involved in mitochondrial morphology. In the present review, the importance of LETM1 in WHS and its role within the mitochondrion was explored. LETM1 governs the mitochondrion ion channel and is involved in mitochondrial respiration. Recent studies have reported that LETM1 acts as a mitochondrial Ca²⁺/H⁺ antiporter. LETM1 has also been identified as a K⁺/H⁺ exchanger, and serves a role in Mg²⁺ homeostasis. The function of LETM1 in mitochondria regulation is regulated by its binding partners, carboxyl-terminal modulator protein and mitochondrial ribosomal protein L36. Therefore, we describe the remarkable role of LETM1 in mitochondrial network physiology and its function in mitochondrion-mediated cell death. In the context of these findings, we suggest that the participation of LETM1 in tumorigenesis through the alteration of cancer metabolism should be investigated. This review provides a comprehensive description of LETM1 function, which is required for mitochondrial homeostasis and cellular viability.

Mitophagy, a potential therapeutic target for stroke

Mitochondria autophagy, termed as mitophagy, is a mechanism of specific autophagic elimination of mitochondria. Mitophagy controls the quality and the number of mitochondria, eliminating dysfunctional or excessive mitochondria that can generate reactive oxygen species (ROS) and cause cell death. Mitochondria are centrally implicated in neuron and tissue injury after stroke, due to the function of supplying adenosine triphosphate (ATP) to the tissue, regulating oxidative metabolism during the pathologic process, and contribution to apoptotic cell death after stroke. As a catabolic mechanism, mitophagy links numbers of a complex network of mitochondria, and affects mitochondrial dynamic process, fusion and fission, reducing mitochondrial production of ROS, mediated by the mitochondrial permeability transition pore (MPTP). The precise nature of mitophagy’s involvement in stroke, and its underlying molecular mechanisms, have yet to be fully clarified. This review aims to provide a comprehensive overview of the integration of mitochondria with mitophagy, also to introduce and discuss recent advances in the understanding of the potential role, and possible signaling pathway, of mitophagy in the pathological processes of both hemorrhagic and ischemic stroke. The author also provides evidence to explain the dual role of mitophagy in stroke.

The cristae modulator Optic atrophy 1 requires mitochondrial ATP synthase oligomers to safeguard mitochondrial function

It is unclear how the mitochondrial fusion protein Optic atrophy 1 (OPA1), which inhibits cristae remodeling, protects from mitochondrial dysfunction. Here we identify the mitochondrial F₁F_o-ATP synthase as the effector of OPA1 in mitochondrial protection. In OPA1 overexpressing cells, the loss of proton electrochemical gradient caused by respiratory chain complex III inhibition is blunted and this protection is abolished by the ATP synthase inhibitor oligomycin. Mechanistically, OPA1 and ATP synthase can interact, but recombinant OPA1 fails to promote oligomerization of purified ATP synthase reconstituted in liposomes, suggesting that OPA1 favors ATP synthase oligomerization and reversal activity by modulating cristae shape. When ATP synthase oligomers are genetically destabilized by silencing the key dimerization subunit e, OPA1 is no longer able to preserve mitochondrial function and cell viability upon complex III inhibition. Thus, OPA1 protects mitochondria from respiratory chain inhibition by stabilizing cristae shape and favoring ATP synthase oligomerization.

Mitochondrial cristae shape influences apoptosis and respiration. Here the authors show that the mitochondrial fusion protein OPA1 protects mitochondria from dysfunction by promoting ATP synthase oligomerization and reversal activity.

The role of the mitochondria and the endoplasmic reticulum contact sites in the development of the immune responses

Mitochondria and endoplasmic reticulum (ER) contact sites (MERCs) are dynamic modules enriched in subset of lipids and specialized proteins that determine their structure and functions. The MERCs regulate lipid transfer, autophagosome formation, mitochondrial fission, Ca2+ homeostasis and apoptosis. Since these functions are essential for cell biology, it is therefore not surprising that MERCs also play a critical role in organ physiology among which the immune system stands by its critical host defense function. This defense system must discriminate and tolerate host cells and beneficial commensal microorganisms while eliminating pathogenic ones in order to preserve normal homeostasis. To meet this goal, the immune system has two lines of defense. First, the fast acting but unspecific innate immune system relies on anatomical physical barriers and subsets of hematopoietically derived cells expressing germline-encoded receptors called pattern recognition receptors (PRR) recognizing conserved motifs on the pathogens. Second, the slower but very specific adaptive immune response is added to complement innate immunity. Adaptive immunity relies on another set of specialized cells, the lymphocytes, harboring receptors requiring somatic recombination to be expressed. Both innate and adaptive immune cells must be activated to phagocytose and process pathogens, migrate, proliferate, release soluble factors and destroy infected cells. Some of these functions are strongly dependent on lipid transfer, autophagosome formation, mitochondrial fission, and Ca2+ flux; this indicates that MERCs could regulate immunity.

Alpha 5/6 helix domains together with N-terminus determine the apoptotic potency of the Bcl-2 family proteins

A critical process in apoptosis is the permeabilization of the mitochondrial outer membrane (MOM). This process is known to be regulated by the multi-domain Bcl-2 family proteins. For example, the pro-apoptotic proteins Bax and Bak are responsible for forming pores at MOM. The anti-apoptotic proteins (including Bcl-2, Mcl-1 and Bcl-xL), on the other hand, can inhibit this pore-forming process. Interestingly, although these two subgroups of proteins perform opposite apoptotic functions, their structures are very similar. This raises two highly interesting questions: (1) Why do these structurally similar proteins play opposite roles in apoptosis? (2) What are the roles of different functional domains of a Bcl-2 family protein in determining its apoptotic property? In this study, we generated a series of deletion mutants and substitution chimera, and used a combination of molecular biology, bio-informatics and living cell imaging techniques to answer these questions. Our major findings are: (1) All of the Bcl-2 family proteins appear to possess an intrinsic pro-apoptotic property. (2) The N-termini of these proteins play an active role in suppressing their pro-apoptotic function. (3) The apoptotic potency is positively correlated with membrane affinity of the alpha 5/6 helix domains. (4) Charge distribution flanking the alpha 5/6 helices is also important for the apoptotic potency. These findings explain why different members of Bcl-2 family proteins with similar domain composition can function oppositely in the apoptotic process.

Tumor-selective mitochondrial network collapse induced by atmospheric gas plasma-activated medium

Non-thermal atmospheric gas plasma (AGP) exhibits cytotoxicity against malignant cells with minimal cytotoxicity toward normal cells. However, the mechanisms of its tumor-selective cytotoxicity remain unclear. Here we report that AGP-activated medium increases caspase-independent cell death and mitochondrial network collapse in a panel of human cancer cells, but not in non-transformed cells. AGP irradiation stimulated reactive oxygen species (ROS) generation in AGP-activated medium, and in turn the resulting stable ROS, most likely hydrogen peroxide (H₂O₂), activated intracellular ROS generation and mitochondrial ROS (mROS) accumulation. Culture in AGP-activated medium resulted in cell death and excessive mitochondrial fragmentation and clustering, and these responses were inhibited by ROS scavengers. AGP-activated medium also increased dynamin-related protein 1-dependent mitochondrial fission in a tumor-specific manner, and H₂O₂ administration showed similar effects. Moreover, the vulnerability of tumor cells to mitochondrial network collapse appeared to result from their higher sensitivity to mROS accumulation induced by AGP-activated medium or H₂O₂. The present findings expand our previous observations on death receptor-mediated tumor-selective cell killing and reinforce the importance of mitochondrial network remodeling as a powerful target for tumor-selective cancer treatment.

The role of Drp1 adaptor proteins MiD49 and MiD51 in mitochondrial fission: implications for human disease

Mitochondrial morphology is governed by the balance of mitochondrial fusion, mediated by mitofusins and optic atrophy 1 (OPA1), and fission, mediated by dynamin-related protein 1 (Drp1). Disordered mitochondrial dynamics alters metabolism, proliferation, apoptosis and mitophagy, contributing to human diseases, including neurodegenerative syndromes, pulmonary arterial hypertension (PAH), cancer and ischemia/reperfusion injury. Post-translational regulation of Drp1 (by phosphorylation and SUMOylation) is an established means of modulating Drp1 activation and translocation to the outer mitochondrial membrane (OMM). This review focuses on Drp1 adaptor proteins that also regulate fission. The proteins include fission 1 (Fis1), mitochondrial fission factor (Mff) and mitochondrial dynamics proteins of 49 kDa and 51 kDa (MiD49, MiD51). Heterologous MiD overexpression sequesters inactive Drp1 on the OMM, promoting fusion; conversely, increased endogenous MiD creates focused Drp1 multimers that optimize OMM scission. The triggers that activate MiD-bound Drp1 in disease states are unknown; however, MiD51 has a unique capacity for ADP binding at its nucleotidyltransferase domain. Without ADP, MiD51 inhibits Drp1, whereas ADP promotes MiD51-mediated fission, suggesting a link between metabolism and fission. Confusion over whether MiDs mediate fusion (by sequestering inactive Drp1) or fission (by guiding Drp1 assembly) relates to a failure to consider cell types used and to distinguish endogenous compared with heterologous changes in expression. We speculate that endogenous MiDs serve as Drp1-binding partners that are dysregulated in disease states and may be important targets for inhibiting cell proliferation and ischemia/reperfusion injury. Moreover, it appears that the composition of the fission apparatus varies between disease states and amongst individuals. MiDs may be important targets for inhibiting cell proliferation and attenuating ischemia/reperfusion injury.

The Concerted Action of Mitochondrial Dynamics and Positioning: New Characters in Cancer Onset and Progression

Mitochondria are dynamic organelles whose morphology and activity are extremely variable, depending on the metabolic state of the cell. In particular, their shape and movements within the cell are finely regulated by an increasing number of proteins, which take part in the process of mitochondrial fission/fusion and connect the organelles to the cytoskeleton. As to their activities, mitochondria are considered to be at the crossroad between cell life and death since, on the one hand, they are essential in ATP production and in multiple metabolic pathways but, on the other, they are involved in the intrinsic apoptotic cascade, triggered by different stress conditions. Importantly, the process of mitochondrial Ca²⁺ uptake, as well as the morphology and the dynamics of these organelles, is known to deeply impact on both pro-survival and pro-death mitochondrial activities. Recently, increasing evidence has accrued on a central role of deregulated mitochondrial functionalities in the onset and progression of different pathologies, ranging from neurodegenerative diseases to cancer. In this contribution, we will present the latest findings connecting alterations in the machineries that control mitochondrial dynamics and localization to specific cancer hallmarks, highlighting the importance of mitochondria for the viability of cancer cells and discussing their role as promising targets for the development of novel anticancer therapies.

Mitochondrial Dynamics as a Therapeutic Target for Treating Cardiac Diseases

Mitochondria are dynamic in nature and are able to shift their morphology between elongated interconnected mitochondrial networks and a fragmented disconnected arrangement by the processes of mitochondrial fusion and fission, respectively. Changes in mitochondrial morphology are regulated by the mitochondrial fusion proteins - mitofusins 1 and 2 (Mfn1 and 2), and optic atrophy 1 (Opa1) as well as the mitochondrial fission proteins - dynamin-related peptide 1 (Drp1) and fission protein 1 (Fis1). Despite having a unique spatial arrangement, cardiac mitochondria have been implicated in a variety of disorders including ischemia-reperfusion injury (IRI), heart failure, diabetes, and pulmonary hypertension. In this chapter, we review the influence of mitochondrial dynamics in these cardiac disorders as well as their potential as therapeutic targets in tackling cardiovascular disease.

PM2.5, SO2 and NO2 co-exposure impairs neurobehavior and induces mitochondrial injuries in the mouse brain

Air pollution is a serious environmental health problem that has been previously associated with neuropathological disorders. However, current experimental evidence mainly focuses on the adverse effects of a single air pollutant, ignoring the biological responses to the co-existence of these pollutants. In the present study, we co-exposed C57BL/6 J mice to PM2.5, SO2 and NO2 and explored their neurobehavior, histopathologic abnormalities, apoptosis-related protein expression and mitochondrial dysfunction. The results indicate that co-exposure to PM2.5, SO2 and NO2 impaired spatial learning and memory and caused abnormal expression of apoptosis-related genes (p53, bax and bcl-2). Additionally, these alterations were related to morphological changes in mitochondria, a reduction of ATP, the elevation of mitochondrial fission proteins and the downregulation of fusion proteins. These findings provide a basis for the understanding of mitochondrial abnormality-related neuropathological dysfunction in response to co-exposure to ambient air pollutants, which suggests an adaptive response to the frangibility of the central nerve system.

Mitochondrial Fission Increases Apoptosis and Decreases Autophagy in Renal Proximal Tubular Epithelial Cells Treated with High Glucose

The aim of this study was to examine the effect of mitochondrial morphogenesis changes on apoptosis and autophagy of high-glucose-treated proximal tubular epithelial cells (HK2). Cell viability, apoptosis, and mitochondrial morphogenesis were examined using crystal violet, terminal deoxynucleotidyl transferase-mediated dUTP-biotin nick end labeling (TUNEL), and mitotracker staining, respectively. High glucose inhibited cell viability and induced mitochondrial fission in HK2 cells. After depleting mitofusin 1 (MFN1), the MFN1(-) HK2 cells (fission type) became more susceptible to high-glucose-induced apoptosis and mitochondrial fragmentation observed by TUNEL and mitotracker assays. In siMFN2 HK2 cells (fission type), mitochondria were highly fragmented (>80% fission rate) with or without high-glucose treatment; however, siFIS1 (mitochondrial fission protein 1) HK2 cells (fusion type) exhibited little fragmentation (<13%). High-glucose treatment induced autophagy, characterized by the formation of autophagosome and microtubule-associated protein light chain 3 (LC3) B-II, as observed by transmission electron microscopy and western blotting, respectively. LC3B-II levels decreased in both MFN1(-) and siMFN2 HK2 cells, but increased in siFIS1 HK2 cells. Moreover, autophagy displays a protective role against high-glucose-induced cell death based on cotreatment with autophagy inhibitors (3-methyladenine and chloroquine). Mitochondrial fission may increase apoptosis and decrease autophagy of high-glucose-treated HK2 cells.

Distinct effects of TRAIL on the mitochondrial network in human cancer cells and normal cells: role of plasma membrane depolarization

Apo2 ligand/tumor necrosis factor-related apoptosis-inducing ligand (Apo2L/TRAIL) is a promising anticancer drug due to its tumor-selective cytotoxicity. Here we report that TRAIL exhibits distinct effects on the mitochondrial networks in malignant cells and normal cells. Live-cell imaging revealed that multiple human cancer cell lines and normal cells exhibited two different modes of mitochondrial responses in response to TRAIL and death receptor agonists. Mitochondria within tumor cells became fragmented into punctate and clustered in response to toxic stimuli. The mitochondrial fragmentation was observed at 4 h, then became more pronounced over time, and associated with apoptotic cell death. In contrast, mitochondria within normal cells such as melanocytes and fibroblasts became only modestly truncated, even when they were treated with toxic stimuli. Although TRAIL activated dynamin-related protein 1 (Drp1)-dependent mitochondrial fission, inhibition of this process by Drp1 knockdown or with the Drp1 inhibitor mdivi-1, potentiated TRAIL-induced apoptosis, mitochondrial fragmentation, and clustering. Moreover, mitochondrial reactive oxygen species (ROS)-mediated depolarization accelerated mitochondrial network abnormalities in tumor cells, but not in normal cells, and TRAIL caused higher levels of mitochondrial ROS accumulation and depolarization in malignant cells than in normal cells. Our findings suggest that tumor cells are more prone than normal cells to oxidative stress and depolarization, thereby being more vulnerable to mitochondrial network abnormalities and that this vulnerability may be relevant to the tumor-targeting killing by TRAIL.

Mitochondria-associated membranes as hubs for neurodegeneration

There is a growing appreciation that membrane-bound organelles in eukaryotic cells communicate directly with one another through direct membrane contact sites. Mitochondria-associated membranes are specialized subdomains of the endoplasmic reticulum that function as membrane contact sites between the endoplasmic reticulum and mitochondria. These sites have emerged as major players in lipid metabolism and calcium signaling. More recently also autophagy and mitochondrial dynamics have been found to be regulated at ER-mitochondria contact sites. Neurons critically depend on mitochondria-associated membranes as a means to exchange metabolites and signaling molecules between these organelles. This is underscored by the fact that genes affecting mitochondrial and endoplasmic reticulum homeostasis are clearly overrepresented in several hereditary neurodegenerative disorders. Conversely, the processes affected by the contact sites between the endoplasmic reticulum and mitochondria are widely implicated in neurodegeneration. This review will focus on the most recent data addressing the structural composition and function of the mitochondria-associated membranes. In addition, the 3D morphology of the contact sites as observed using volume electron microscopy is discussed. Finally, it will highlight the role of several key proteins associated with these contact sites that are involved not only in dementias, amyotrophic lateral sclerosis and Parkinson's disease, but also in axonopathies such as hereditary spastic paraplegia and Charcot-Marie-Tooth disease.

Molecular strategies for targeting antioxidants to mitochondria: therapeutic implications

Mitochondrial function and specifically its implication in cellular redox/oxidative balance is fundamental in controlling the life and death of cells, and has been implicated in a wide range of human pathologies. In this context, mitochondrial therapeutics, particularly those involving mitochondria-targeted antioxidants, have attracted increasing interest as potentially effective therapies for several human diseases. For the past 10 years, great progress has been made in the development and functional testing of molecules that specifically target mitochondria, and there has been special focus on compounds with antioxidant properties. In this review, we will discuss several such strategies, including molecules conjugated with lipophilic cations (e.g., triphenylphosphonium) or rhodamine, conjugates of plant alkaloids, amino-acid- and peptide-based compounds, and liposomes. This area has several major challenges that need to be confronted. Apart from antioxidants and other redox active molecules, current research aims at developing compounds that are capable of modulating other mitochondria-controlled processes, such as apoptosis and autophagy. Multiple chemically different molecular strategies have been developed as delivery tools that offer broad opportunities for mitochondrial manipulation. Additional studies, and particularly in vivo approaches under physiologically relevant conditions, are necessary to confirm the clinical usefulness of these molecules.

Discovery of the membrane receptor for mitochondrial fission GTPase Drp1

Mitochondria frequently change their morphology by fusion and fission, and these dynamic morphologic changes are essential for maintaining both mitochondrial and cellular functions. The cytoplasmic dynamin-related guanosine triphosphatase (GTPase) Drp1 (Dnm1 in yeast) is recruited to mitochondrial fission sites and severs mitochondria. Although the mitochondrial outer membrane (MOM) protein Fis1 functions as a membrane receptor for Dnm1 in yeast, it is not yet known whether the human homolog of yeast Fis1 (hFis1) is a membrane receptor for Drp1 in mammals. We recently identified the C-tail anchored MOM protein Mff as the bona fide receptor essential for recruiting Drp1 to mitochondrial fission sites. Here, we focus on this key molecule for mitochondrial fission after a brief description of the proteins involved in mitochondrial fission and fusion reactions. Finally, we discuss the expected role of hFis1 for regulating the mitochondrial dynamics in mammals.

Mitochondria: from cell death executioners to regulators of cell differentiation

Most, if not all mitochondrial functions, including adenosine-5'-triphosphate (ATP) production and regulation of apoptosis and Ca(2+) homeostasis, are inextricably linked to mitochondrial morphology and dynamics, a process controlled by a family of GTP-dependent dynamin related 'mitochondria-shaping' proteins. Mitochondrial fusion and fission directly influence mitochondrial metabolism, apoptotic and necrotic cell death, autophagy, muscular atrophy and cell migration. In this review, we discuss the recent evidence indicating that mitochondrial dynamics influence complex signaling pathways, affect gene expression and define cell differentiation. These findings extend the importance of mitochondria to developmental biology, far beyond their mere bioenergetic role.

Metabolic and transcriptional responses of gilthead sea bream (Sparus aurata L.) to environmental stress: new insights in fish mitochondrial phenotyping

The aim of the current study was to phenotype fish metabolism and the transcriptionally-mediated response of hepatic mitochondria of gilthead sea bream to intermittent and repetitive environmental stressors: (i) changes in water temperature (T-ST), (ii) changes in water level and chasing (C-ST) and (iii) multiple sensory perception stressors (M-ST). Gene expression profiling was done using a quantitative PCR array of 60 mitochondria-related genes, selected as markers of transcriptional regulation, oxidative metabolism, respiration uncoupling, antioxidant defense, protein import/folding/assembly, and mitochondrial dynamics and apoptosis. The mitochondrial phenotype mirrored changes in fish performance, haematology and lactate production. T-ST especially up-regulated transcriptional factors (PGC1α, NRF1, NRF2), rate limiting enzymes of fatty acid β-oxidation (CPT1A) and tricarboxylic acid cycle (CS), membrane translocases (Tim/TOM complex) and molecular chaperones (mtHsp10, mtHsp60, mtHsp70) to improve the oxidative capacity in a milieu of a reduced feed intake and impaired haematology. The lack of mitochondrial response, increased production of lactate and negligible effects on growth performance in C-ST fish were mostly considered as a switch from aerobic to anaerobic metabolism. A strong down-regulation of PGC1α, NRF1, NRF2, CPT1A, CS and markers of mitochondrial dynamics and apoptosis (BAX, BCLX, MFN2, MIRO2) occurred in M-ST fish in association with the greatest circulating cortisol concentration and a reduced lactate production and feed efficiency, which represents a metabolic condition with the highest allostatic load score. These findings evidence a high mitochondrial plasticity against stress stimuli, providing new insights to define the threshold level of stress condition in fish.

Mitochondrial division inhibitor-1 induces mitochondrial hyperfusion and sensitizes human cancer cells to TRAIL-induced apoptosis

Tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) is a promising candidate for cancer treatment, but some cancer cell types are resistant to TRAIL cytotoxicity. Therefore, overcoming this resistance is necessary for effective TRAIL therapy. Mitochondrial morphology is important for the maintenance of cell function and survival, and is regulated by the delicate balance between fission and fusion. However, the role of mitochondrial morphology dynamics in TRAIL-induced apoptosis is unknown. Here we show that mitochondrial division inhibitor-1 (mdivi-1), an inhibitor of dynamin-related protein1 (Drp1), modulates mitochondrial morphology and TRAIL-induced apoptosis in human cancer cells. mdivi-1 treatment (≥12.5 µM) caused dose- and time‑dependent cell death in malignant melanoma, lung cancer and osteosarcoma cells, while sparing normal cells. mdivi-1 also sensitized cancer cells to TRAIL-induced apoptosis. This potentiation of apoptosis occurred through a caspase-depependent mechanism including the mitochondrial and endoplasmic reticulum (ER) stress pathways. Mdivi-1 potentiated mitochondrial oxidative stress, a major cause of mitochondrial and ER stresses, as evidenced by increases in mitochondrial reactive oxygen species levels, mitochondrial mass, and cardiolipin oxidation. Live cell fluorescence imaging using MitoTracker Red CMXRos revealed that Mdivi-1 caused substantial mitochondrial hyperfusion. Moreover, silencing of Drp1 expression also caused mitochondrial hyperfusion and sensitized cancer cells to TRAIL-induced apoptosis. Our results suggest that cancer cells are more vulnerable than normal cells to a perturbation in mitochondrial morphology dynamics and that this higher susceptibility can be exploited to selectively kill cancer cells and sensitize to TRAIL.

At the right distance: ER-mitochondria juxtaposition in cell life and death

The interface between mitochondria and the endoplasmic reticulum is emerging as a crucial hub for calcium signalling, apoptosis, autophagy and lipid biosynthesis, with far reaching implications in cell life and death and in the regulation of mitochondrial and endoplasmic reticulum function. Here we review our current knowledge on the structural and functional aspects of this interorganellar juxtaposition. This article is part of a Special Issue entitled: Calcium Signaling In Health and Disease. Guest Editors: Geert Bultynck, Jacques Haiech, Claus W. Heizmann, Joachim Krebs, and Marc Moreau.

Mitochondrial alterations in apoptosis

Besides their conventional role as energy suppliers for the cell, mitochondria in vertebrates are active regulators of apoptosis. They release apoptotic factors from the intermembrane space into the cytosol through a mechanism that involves the Bcl-2 protein family, mediating permeabilization of the outer mitochondrial membrane. Associated with this event, a number of additional changes affect mitochondria during apoptosis. They include loss of important mitochondrial functions, such as the ability to maintain calcium homeostasis and to generate ATP, as well as mitochondrial fragmentation and cristae remodeling. Moreover, the lipidic component of mitochondrial membranes undergoes important alterations in composition and distribution, which have turned out to be relevant regulatory events for the proteins involved in apoptotic mitochondrial damage.

Cigarette smoke-induced mitochondrial fragmentation and dysfunction in human airway smooth muscle

The balance between mitochondrial fission and fusion is crucial for mitochondria to perform its normal cellular functions. We hypothesized that cigarette smoke (CS) disrupts this balance and enhances mitochondrial dysfunction in the airway. In nonasthmatic human airway smooth muscle (ASM) cells, CS extract (CSE) induced mitochondrial fragmentation and damages their networked morphology in a concentration-dependent fashion, via increased expression of mitochondrial fission protein dynamin-related protein 1 (Drp1) and decreased fusion protein mitofusin (Mfn) 2. CSE effects on Drp1 vs. Mfn2 and mitochondrial network morphology involved reactive oxygen species (ROS), activation of extracellular signal-regulated kinase (ERK), phosphatidylinositol 3-kinase (PI3K)/protein kinase B (Akt), protein kinase C (PKC) and proteasome pathways, as well as transcriptional regulation via factors such as NF-κB and nuclear erythroid 2-related factor 2. Inhibiting Drp1 prevented CSE effects on mitochondrial networks and ROS generation, whereas blocking Mfn2 had the opposite, detrimental effect. In ASM from asmatic patients, mitochondria exhibited substantial morphological defects at baseline and showed increased Drp1 but decreased Mfn2 expression, with exacerbating effects of CSE. Overall, these results highlight the importance of mitochondrial networks and their regulation in the context of cellular changes induced by insults such as inflammation (as in asthma) or CS. Altered mitochondrial fission/fusion proteins have a further potential to influence parameters such as ROS and cell proliferation and apoptosis relevant to airway diseases.

Mutations in Fis1 disrupt orderly disposal of defective mitochondria

The mitochondrial fission protein Drp1 binds to Mff on mitochondria, followed by entry into a complex with Fis1 at the ER–mitochondrial interface. Mutations in Fis1 disrupt disposal of defective mitochondria when fission is induced by stress. Fis1 thus acts in sequence with Mff to couple mitochondrial fission with downstream degradation processes.

Mitochondrial fission is mediated by the dynamin-related protein Drp1 in metazoans. Drp1 is recruited from the cytosol to mitochondria by the mitochondrial outer membrane protein Mff. A second mitochondrial outer membrane protein, named Fis1, was previously proposed as recruitment factor, but Fis1^−/− cells have mild or no mitochondrial fission defects. Here we show that Fis1 is nevertheless part of the mitochondrial fission complex in metazoan cells. During the fission cycle, Drp1 first binds to Mff on the surface of mitochondria, followed by entry into a complex that includes Fis1 and endoplasmic reticulum (ER) proteins at the ER–mitochondrial interface. Mutations in Fis1 do not normally affect fission, but they can disrupt downstream degradation events when specific mitochondrial toxins are used to induce fission. The disruptions caused by mutations in Fis1 lead to an accumulation of large LC3 aggregates. We conclude that Fis1 can act in sequence with Mff at the ER–mitochondrial interface to couple stress-induced mitochondrial fission with downstream degradation processes.

Effects of low intensity static magnetic field on FTIR spectra and ROS production in SH-SY5Y neuronal-like cells

Biological effects of man-made electromagnetic fields (EMFs) have been studied so far by experimental approaches exposing animals and cell cultures to EMFs. However, the evidence for cell toxicity induced by static magnetic field (SMF) is still uncertain. We investigated the effects produced by the exposure of human SH-SY5Y neuronal-like cells to a uniform magnetic field at intensities of 2.2 mT, which is less than the recommended public exposure limits set by the International Commission on Non-Ionizing Radiation Protection (ICNIRP). A decrease of membrane mitochondrial potential up to 30% was measured after 24 h of exposure to SMF in SH-SY5Y cells, and this effect was associated with reactive oxygen species production increase. Fourier transform infrared spectroscopy (FTIR) analysis showed that exposure to a static magnetic intensity around 2.2 mT changed the secondary structure of cellular proteins and lipid components. The vibration bands relative to the methylene group increased significantly after 4 h of exposure, whereas further exposure up to 24 h produced evident shifts of amide I and II modes and a relative increase in β-sheet contents with respect to α-helix components. Our study demonstrated that a moderate SMF causes alteration in cell homeostasis, as indicated by FTIR spectroscopy observations of changes in protein structures that are part of cell response to magnetic field exposure.

Functional and morphological impact of ER stress on mitochondria

Over the past years, knowledge and evidence about the existence of crosstalks between cellular organelles and their potential effects on survival or cell death have been constantly growing. More recently, evidence accumulated showing an intimate relationship between endoplasmic reticulum (ER) and mitochondria. These close contacts not only establish extensive physical links allowing exchange of lipids and calcium but they can also coordinate pathways involved in cell life and death. It is now obvious that ER dysfunction/stress and unfolded protein response (UPR) as well as mitochondria play major roles in apoptosis. However, while the effects of major ER stress on cell death have been largely studied and reviewed, it becomes more and more evident that cells might regularly deal with sublethal ER stress, a condition that does not necessarily lead to cell death but might affect the function/activity of other organelles such as mitochondria. In this review, we will particularly focus on these new, interesting and intriguing metabolic and morphological events that occur during the early adaptative phase of the ER stress, before the onset of cell death, and that remain largely unknown. Relevance and implication of these mitochondrial changes in response to ER stress conditions for human diseases such as type II diabetes and Alzheimer's disease will also be considered.

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.

Mitochondrial dynamics in cardiovascular health and disease

SIGNIFICANCE

Mitochondria are dynamic organelles capable of changing their shape and distribution by undergoing either fission or fusion. Changes in mitochondrial dynamics, which is under the control of specific mitochondrial fission and fusion proteins, have been implicated in cell division, embryonic development, apoptosis, autophagy, and metabolism. Although the machinery for modulating mitochondrial dynamics is present in the cardiovascular system, its function there has only recently been investigated. In this article, we review the emerging role of mitochondrial dynamics in cardiovascular health and disease.

RECENT ADVANCES

Changes in mitochondrial dynamics have been implicated in vascular smooth cell proliferation, cardiac development and differentiation, cardiomyocyte hypertrophy, myocardial ischemia-reperfusion injury, cardioprotection, and heart failure.

CRITICAL ISSUES

Many of the experimental studies investigating mitochondrial dynamics in the cardiovascular system have been confined to cardiac cell lines, vascular cells, or neonatal cardiomyocytes, in which mitochondria are distributed throughout the cytoplasm and are free to move. However, in the adult heart where mitochondrial movements are restricted by their tightly-packed distribution along myofibrils or beneath the subsarcolemma, the relevance of mitochondrial dynamics is less obvious. The investigation of transgenic mice deficient in cardiac mitochondrial fission or fusion proteins should help elucidate the role of mitochondrial dynamics in the adult heart.

FUTURE DIRECTIONS

Investigating the role of mitochondrial dynamics in cardiovascular health and disease should result in the identification of novel therapeutic targets for treating patients with cardiovascular disease, the leading cause of death and disability globally.