MIEF1/2 function as adaptors to recruit Drp1 to mitochondria and regulate the association of Drp1 with Mff

The role of mitochondrial dynamics and mitophagy in skeletal muscle atrophy: from molecular mechanisms to therapeutic insights

Skeletal muscle is the largest metabolic organ of the human body. Maintaining the best quality control and functional integrity of mitochondria is essential for the health of skeletal muscle. However, mitochondrial dysfunction characterized by mitochondrial dynamic imbalance and mitophagy disruption can lead to varying degrees of muscle atrophy, but the underlying mechanism of action is still unclear. Although mitochondrial dynamics and mitophagy are two different mitochondrial quality control mechanisms, a large amount of evidence has indicated that they are interrelated and mutually regulated. The former maintains the balance of the mitochondrial network, eliminates damaged or aged mitochondria, and enables cells to survive normally. The latter degrades damaged or aged mitochondria through the lysosomal pathway, ensuring cellular functional health and metabolic homeostasis. Skeletal muscle atrophy is considered an urgent global health issue. Understanding and gaining knowledge about muscle atrophy caused by mitochondrial dysfunction, particularly focusing on mitochondrial dynamics and mitochondrial autophagy, can greatly contribute to the prevention and treatment of muscle atrophy. In this review, we critically summarize the recent research progress on mitochondrial dynamics and mitophagy in skeletal muscle atrophy, and expound on the intrinsic molecular mechanism of skeletal muscle atrophy caused by mitochondrial dynamics and mitophagy. Importantly, we emphasize the potential of targeting mitochondrial dynamics and mitophagy as therapeutic strategies for the prevention and treatment of muscle atrophy, including pharmacological treatment and exercise therapy, and summarize effective methods for the treatment of skeletal muscle atrophy.

The role of mitochondrial dynamics in disease

Mitochondria are multifaceted and dynamic organelles regulating various important cellular processes from signal transduction to determining cell fate. As dynamic properties of mitochondria, fusion and fission accompanied with mitophagy, undergo constant changes in number and morphology to sustain mitochondrial homeostasis in response to cell context changes. Thus, the dysregulation of mitochondrial dynamics and mitophagy is unsurprisingly related with various diseases, but the unclear underlying mechanism hinders their clinical application. In this review, we summarize the recent developments in the molecular mechanism of mitochondrial dynamics and mitophagy, particularly the different roles of key components in mitochondrial dynamics in different context. We also summarize the roles of mitochondrial dynamics and target treatment in diseases related to the cardiovascular system, nervous system, respiratory system, and tumor cell metabolism demanding high-energy. In these diseases, it is common that excessive mitochondrial fission is dominant and accompanied by impaired fusion and mitophagy. But there have been many conflicting findings about them recently, which are specifically highlighted in this view. We look forward that these findings will help broaden our understanding of the roles of the mitochondrial dynamics in diseases and will be beneficial to the discovery of novel selective therapeutic targets.

The mystery of phospho-Drp1 with four adaptors in cell cycle: when mitochondrial fission couples to cell fate decisions

Recent study had deepened our knowledge of the mitochondrial dynamics to classify mitochondrial fission into two types. To further clarify the relationship between the two distinct fission machinery and the four major adaptors of Drp1, we propose a model of mechanism elucidating the multiple functions of phospho-Drp1 with its adaptors during cell cycle and providing in-depth insights into the molecular basis and evolutionary implications in depth. The model highlights not only the clustering characteristics of different phospho-Drp1 with respective subsets of mitochondrial pro-fission adaptors but also the correlation, crosstalk and shifting between different clustering of phosphorylated Drp1-adaptors during different key fission situations. Particularly, phospho-Drp1 (Ser616) couples with Mff/MiD51 to exert mitochondrial division and phospho-Drp1 (Ser637) couples with MiD49/Fis1 to execute mitophagy in M-phase. We then apply the model to address the relationship of mitochondrial dynamics to Parkinson's disease (PD) and carcinogenesis. Our proposed model is indeed compatible with current research results and pathological observations, providing promising directions for future treatment design.

Mitochondria in health, disease, and aging

Mitochondria are well known as organelles responsible for the maintenance of cellular bioenergetics through the production of ATP. Although oxidative phosphorylation may be their most important function, mitochondria are also integral for the synthesis of metabolic precursors, calcium regulation, the production of reactive oxygen species, immune signaling, and apoptosis. Considering the breadth of their responsibilities, mitochondria are fundamental for cellular metabolism and homeostasis. Appreciating this significance, translational medicine has begun to investigate how mitochondrial dysfunction can represent a harbinger of disease. In this review, we provide a detailed overview of mitochondrial metabolism, cellular bioenergetics, mitochondrial dynamics, autophagy, mitochondrial damage-associated molecular patterns, mitochondria-mediated cell death pathways, and how mitochondrial dysfunction at any of these levels is associated with disease pathogenesis. Mitochondria-dependent pathways may thereby represent an attractive therapeutic target for ameliorating human disease.

Targeting NEDD8-activating enzyme for cancer therapy: developments, clinical trials, challenges and future research directions

NEDDylation, a post-translational modification through three-step enzymatic cascades, plays crucial roles in the regulation of diverse biological processes. NEDD8-activating enzyme (NAE) as the only activation enzyme in the NEDDylation modification has become an attractive target to develop anticancer drugs. To date, numerous inhibitors or agonists targeting NAE have been developed. Among them, covalent NAE inhibitors such as MLN4924 and TAS4464 currently entered into clinical trials for cancer therapy, particularly for hematological tumors. This review explains the relationships between NEDDylation and cancers, structural characteristics of NAE and multistep mechanisms of NEDD8 activation by NAE. In addition, the potential approaches to discover NAE inhibitors and detailed pharmacological mechanisms of NAE inhibitors in the clinical stage are explored in depth. Importantly, we reasonably investigate the challenges of NAE inhibitors for cancer therapy and possible development directions of NAE-targeting drugs in the future.

Mitochondrial dysfunction in heart diseases: Potential therapeutic effects of Panax ginseng

Heart diseases have a high incidence and mortality rate, and seriously affect people's quality of life. Mitochondria provide energy for the heart to function properly. The process of various heart diseases is closely related to mitochondrial dysfunction. Panax ginseng (P. ginseng), as a traditional Chinese medicine, is widely used to treat various cardiovascular diseases. Many studies have confirmed that P. ginseng and ginsenosides can regulate and improve mitochondrial dysfunction. Therefore, the role of mitochondria in various heart diseases and the protective effect of P. ginseng on heart diseases by regulating mitochondrial function were reviewed in this paper, aiming to gain new understanding of the mechanisms, and promote the clinical application of P. ginseng.

The Drp1-Mediated Mitochondrial Fission Protein Interactome as an Emerging Core Player in Mitochondrial Dynamics and Cardiovascular Disease Therapy

Mitochondria, the membrane-bound cell organelles that supply most of the energy needed for cell function, are highly regulated, dynamic organelles bearing the ability to alter both form and functionality rapidly to maintain normal physiological events and challenge stress to the cell. This amazingly vibrant movement and distribution of mitochondria within cells is controlled by the highly coordinated interplay between mitochondrial dynamic processes and fission and fusion events, as well as mitochondrial quality-control processes, mainly mitochondrial autophagy (also known as mitophagy). Fusion connects and unites neighboring depolarized mitochondria to derive a healthy and distinct mitochondrion. In contrast, fission segregates damaged mitochondria from intact and healthy counterparts and is followed by selective clearance of the damaged mitochondria via mitochondrial specific autophagy, i.e., mitophagy. Hence, the mitochondrial processes encompass all coordinated events of fusion, fission, mitophagy, and biogenesis for sustaining mitochondrial homeostasis. Accumulated evidence strongly suggests that mitochondrial impairment has already emerged as a core player in the pathogenesis, progression, and development of various human diseases, including cardiovascular ailments, the leading causes of death globally, which take an estimated 17.9 million lives each year. The crucial factor governing the fission process is the recruitment of dynamin-related protein 1 (Drp1), a GTPase that regulates mitochondrial fission, from the cytosol to the outer mitochondrial membrane in a guanosine triphosphate (GTP)-dependent manner, where it is oligomerized and self-assembles into spiral structures. In this review, we first aim to describe the structural elements, functionality, and regulatory mechanisms of the key mitochondrial fission protein, Drp1, and other mitochondrial fission adaptor proteins, including mitochondrial fission 1 (Fis1), mitochondrial fission factor (Mff), mitochondrial dynamics 49 (Mid49), and mitochondrial dynamics 51 (Mid51). The core area of the review focuses on the recent advances in understanding the role of the Drp1-mediated mitochondrial fission adaptor protein interactome to unravel the missing links of mitochondrial fission events. Lastly, we discuss the promising mitochondria-targeted therapeutic approaches that involve fission, as well as current evidence on Drp1-mediated fission protein interactions and their critical roles in the pathogeneses of cardiovascular diseases (CVDs).

Mitochondrial dynamics involves molecular and mechanical events in motility, fusion and fission

Mitochondria are cell organelles that play pivotal roles in maintaining cell survival, cellular metabolic homeostasis, and cell death. Mitochondria are highly dynamic entities which undergo fusion and fission, and have been shown to be very motile in vivo in neurons and in vitro in multiple cell lines. Fusion and fission are essential for maintaining mitochondrial homeostasis through control of morphology, content exchange, inheritance of mitochondria, maintenance of mitochondrial DNA, and removal of damaged mitochondria by autophagy. Mitochondrial motility occurs through mechanical and molecular mechanisms which translocate mitochondria to sites of high energy demand. Motility also plays an important role in intracellular signaling. Here, we review key features that mediate mitochondrial dynamics and explore methods to advance the study of mitochondrial motility as well as mitochondrial dynamics-related diseases and mitochondrial-targeted therapeutics.

Exploring metabolism in scleroderma reveals opportunities for pharmacological intervention for therapy in fibrosis

Background

Recent evidence has indicated that alterations in energy metabolism play a critical role in the pathogenesis of fibrotic diseases. Studies have suggested that ‘metabolic reprogramming’ involving the glycolysis and oxidative phosphorylation (OXPHOS) in cells lead to an enhanced generation of energy and biosynthesis. The aim of this study was to assess the molecular basis of changes in fibrotic metabolism in systemic sclerosis (Scleroderma; SSc) and highlight the most appropriate targets for anti-fibrotic therapies.

Materials and methods

Dermal fibroblasts were isolated from five SSc patients and five healthy donors. Cells were cultured in medium with/without TGF-β1 and with/without ALK5, pan-PIM or ATM kinase inhibitors. Extracellular flux analyses were performed to evaluate glycolytic and mitochondrial respiratory function. The mitochondrial network in TMRM-stained cells was visualized by confocal laser-scanning microscopy, followed by semi-automatic analysis on the ImageJ platform. Protein expression of ECM and fibroblast components, glycolytic enzymes, subunits of the five OXPHOS complexes, and dynamin-related GTPases and receptors involved in mitochondrial fission/fusion were assessed by western blotting.

Results

Enhanced mitochondrial respiration coupled to ATP production was observed in SSc fibroblasts at the expense of spare respiratory capacity. Although no difference was found in glycolysis when comparing SSc with healthy control fibroblasts, levels of phophofructokinase-1 isoform PFKM were significantly lower in SSc fibroblasts (P<0.05). Our results suggest that the number of respirasomes is decreased in the SSc mitochondria; however, the organelles formed a hyperfused network, which is thought to increase mitochondrial ATP production through complementation. The increased mitochondrial fusion correlated with a change in expression levels of regulators of mitochondrial morphology, including decreased levels of DRP1, increased levels of MIEF2 and changes in OPA1 isoform ratios. TGF-β1 treatment strongly stimulated glycolysis and mitochondrial respiration and induced the expression of fibrotic markers. The pan-PIM kinase inhibitor had no effect, whereas both ALK5 and ATM kinase inhibition abrogated TGF-β1-mediated fibroblast activation, and upregulation of glycolysis and respiration.

Conclusions

Our data provide evidence for a novel mechanism(s) by which SSc fibroblasts exhibit altered metabolic programs and highlight changes in respiration and dysregulated mitochondrial morphology and function, which can be selectively targeted by small molecule kinase inhibitors.

Mid51/Fis1 mitochondrial oligomerization complex drives lysosomal untethering and network dynamics

Lysosomes are highly dynamic organelles implicated in multiple diseases. Using live super-resolution microscopy, we found that lysosomal tethering events rarely undergo lysosomal fusion, but rather untether over time to reorganize the lysosomal network. Inter-lysosomal untethering events are driven by a mitochondrial Mid51/Fis1 complex that undergoes coupled oligomerization on the outer mitochondrial membrane. Importantly, Fis1 oligomerization mediates TBC1D15 (Rab7-GAP) mitochondrial recruitment to drive inter-lysosomal untethering via Rab7 GTP hydrolysis. Moreover, inhibiting Fis1 oligomerization by either mutant Fis1 or a Mid51 oligomerization mutant potentially associated with Parkinson’s disease prevents lysosomal untethering events, resulting in misregulated lysosomal network dynamics. In contrast, dominant optic atrophy–linked mutant Mid51, which does not inhibit Mid51/Fis1 coupled oligomerization, does not disrupt downstream lysosomal dynamics. As Fis1 conversely also regulates Mid51 oligomerization, our work further highlights an oligomeric Mid51/Fis1 mitochondrial complex that mechanistically couples together both Drp1 and Rab7 GTP hydrolysis machinery at mitochondria–lysosome contact sites. These findings have significant implications for organelle networks in cellular homeostasis and human disease.

The Role of a Pathological Interaction between β-amyloid and Mitochondria in the Occurrence and Development of Alzheimer's Disease

Alzheimer's disease (AD) is one of the most common neurodegenerative diseases in existence. It is characterized by an impaired cognitive function that is due to a progressive loss of neurons in the brain. Extracellular β-amyloid (Aβ) plaques are the main pathological features of the disease. In addition to abnormal protein aggregation, increased mitochondrial fragmentation, altered expression of the genes involved in mitochondrial biogenesis, disruptions in the ER-mitochondria interaction, and mitophagy are observed. Reactive oxygen species are known to affect Aβ expression and aggregation. In turn, oligomeric and aggregated Aβ cause mitochondrial disorders. In this review, we summarize available knowledge about the pathological effects of Aβ on mitochondria and the potential molecular targets associated with proteinopathy and mitochondrial dysfunction for the pharmacological treatment of Alzheimer's disease.

Compositions and Functions of Mitochondria-Associated Endoplasmic Reticulum Membranes and Their Contribution to Cardioprotection by Exercise Preconditioning

Mitochondria-associated endoplasmic reticulum membranes (MAMs) are important components of intracellular signaling and contribute to the regulation of intracellular Ca²⁺/lipid homeostasis, mitochondrial dynamics, autophagy/mitophagy, apoptosis, and inflammation. Multiple studies have shown that proteins located on MAMs mediate cardioprotection. Exercise preconditioning (EP) has been shown to protect the myocardium from adverse stimuli, but these mechanisms are still being explored. Recently, a growing body of evidence points to MAMs, suggesting that exercise or EP may be involved in cardioprotection by modulating proteins on MAMs and subsequently affecting MAMs. In this review, we summarize the latest findings on MAMs, analyzing the structure and function of MAMs and the role of MAM-related proteins in cardioprotection. We focused on the possible mechanisms by which exercise or EP can modulate the involvement of MAMs in cardioprotection. We found that EP may affect MAMs by regulating changes in MFN2, MFN1, AMPK, FUNDC1, BECN1, VDAC1, GRP75, IP3R, CYPD, GSK3β, AKT, NLRP3, GRP78, and LC3, thus playing a cardioprotective role. We also provided direction for future studies that may be of interest so that more in-depth studies can be conducted to elucidate the relationship between EP and cardioprotection.

Role of Mitochondrial Dynamics in Cocaine's Neurotoxicity

The dynamic balance of mitochondrial fission and fusion maintains mitochondrial homeostasis and optimal function. It is indispensable for cells such as neurons, which rely on the finely tuned mitochondria to carry out their normal physiological activities. The potent psychostimulant cocaine impairs mitochondria as one way it exerts its neurotoxicity, wherein the disturbances in mitochondrial dynamics have been suggested to play an essential role. In this review, we summarize the neurotoxicity of cocaine and the role of mitochondrial dynamics in cellular physiology. Subsequently, we introduce current findings that link disturbed neuronal mitochondrial dynamics with cocaine exposure. Finally, the possible role and potential therapeutic value of mitochondrial dynamics in cocaine neurotoxicity are discussed.

Mitochondrial Dysfunction Pathway Alterations Offer Potential Biomarkers and Therapeutic Targets for Ovarian Cancer

The mitochondrion is a very versatile organelle that participates in some important cancer-associated biological processes, including energy metabolism, oxidative stress, mitochondrial DNA (mtDNA) mutation, cell apoptosis, mitochondria-nuclear communication, dynamics, autophagy, calcium overload, immunity, and drug resistance in ovarian cancer. Multiomics studies have found that mitochondrial dysfunction, oxidative stress, and apoptosis signaling pathways act in human ovarian cancer, which demonstrates that mitochondria play critical roles in ovarian cancer. Many molecular targeted drugs have been developed against mitochondrial dysfunction pathways in ovarian cancer, including olive leaf extract, nilotinib, salinomycin, Sambucus nigra agglutinin, tigecycline, and eupatilin. This review article focuses on the underlying biological roles of mitochondrial dysfunction in ovarian cancer progression based on omics data, potential molecular relationship between mitochondrial dysfunction and oxidative stress, and future perspectives of promising biomarkers and therapeutic targets based on the mitochondrial dysfunction pathway for ovarian cancer.

Mitochondrial Dynamics, Mitophagy, and Mitochondria–Endoplasmic Reticulum Contact Sites Crosstalk Under Hypoxia

Mitochondria are double membrane organelles within eukaryotic cells, which act as cellular power houses, depending on the continuous availability of oxygen. Nevertheless, under hypoxia, metabolic disorders disturb the steady-state of mitochondrial network, which leads to dysfunction of mitochondria, producing a large amount of reactive oxygen species that cause further damage to cells. Compelling evidence suggests that the dysfunction of mitochondria under hypoxia is linked to a wide spectrum of human diseases, including obstructive sleep apnea, diabetes, cancer and cardiovascular disorders. The functional dichotomy of mitochondria instructs the necessity of a quality-control mechanism to ensure a requisite number of functional mitochondria that are present to fit cell needs. Mitochondrial dynamics plays a central role in monitoring the condition of mitochondrial quality. The fission–fusion cycle is regulated to attain a dynamic equilibrium under normal conditions, however, it is disrupted under hypoxia, resulting in mitochondrial fission and selective removal of impaired mitochondria by mitophagy. Current researches suggest that the molecular machinery underlying these well-orchestrated processes are coordinated at mitochondria–endoplasmic reticulum contact sites. Here, we establish a holistic understanding of how mitochondrial dynamics and mitophagy are regulated at mitochondria–endoplasmic reticulum contact sites under hypoxia.

Impact of ER Stress and ER-Mitochondrial Crosstalk in Huntington's Disease

Accumulation of misfolded proteins is a common phenomenon of several neurodegenerative diseases. The misfolding of proteins due to abnormal polyglutamine (PolyQ) expansions are linked to the development of PolyQ diseases including Huntington's disease (HD). Though the genetic basis of PolyQ repeats in HD remains prominent, the primary molecular basis mediated by PolyQ toxicity remains elusive. Accumulation of misfolded proteins in the ER or disruption of ER homeostasis causes ER stress and activates an evolutionarily conserved pathway called Unfolded protein response (UPR). Protein homeostasis disruption at organelle level involving UPR or ER stress response pathways are found to be linked to HD. Due to dynamic intricate connections between ER and mitochondria, proteins at ER-mitochondria contact sites (mitochondria associated ER membranes or MAMs) play a significant role in HD development. The current review aims at highlighting the most updated information about different UPR pathways and their involvement in HD disease progression. Moreover, the role of MAMs in HD progression has also been discussed. In the end, the review has focused on the therapeutic interventions responsible for ameliorating diseased states via modulating either ER stress response proteins or modulating the expression of ER-mitochondrial contact proteins.

The Role of Mitochondria in Optic Atrophy With Autosomal Inheritance

Optic atrophy (OA) with autosomal inheritance is a form of optic neuropathy characterized by the progressive and irreversible loss of vision. In some cases, this is accompanied by additional, typically neurological, extra-ocular symptoms. Underlying the loss of vision is the specific degeneration of the retinal ganglion cells (RGCs) which form the optic nerve. Whilst autosomal OA is genetically heterogenous, all currently identified causative genes appear to be associated with mitochondrial organization and function. However, it is unclear why RGCs are particularly vulnerable to mitochondrial aberration. Despite the relatively high prevalence of this disorder, there are currently no approved treatments. Combined with the lack of knowledge concerning the mechanisms through which aberrant mitochondrial function leads to RGC death, there remains a clear need for further research to identify the underlying mechanisms and develop treatments for this condition. This review summarizes the genes known to be causative of autosomal OA and the mitochondrial dysfunction caused by pathogenic mutations. Furthermore, we discuss the suitability of available in vivo models for autosomal OA with regards to both treatment development and furthering the understanding of autosomal OA pathology.

The Molecular Assembly State of Drp1 Controls its Association With the Mitochondrial Recruitment Receptors Mff and MIEF1/2

Drp1 is a central player in mitochondrial fission and is recruited to mitochondria by Mff and MIEFs (MIEF1 and MIEF2), but little is known about how its assembly state affects Drp1 mitochondrial recruitment and fission. Here, we used in vivo chemical crosslinking to explore the self-assembly state of Drp1 and how it regulates the association of Drp1 with MIEFs and Mff. We show that in intact mammalian cells Drp1 exists as a mixture of multiple self-assembly forms ranging from the minimal, probably tetrameric, self-assembly subunit to several higher order oligomers. Precluding mitochondria-bound Drp1 in Mff/MIEF1/2-deficient cells does not affect the oligomerization state of Drp1, while conversely forced recruitment of Drp1 to mitochondria by MIEFs or Mff facilitates Drp1 oligomerization. Mff preferentially binds to higher order oligomers of Drp1, whereas MIEFs bind to a wider-range of Drp1 assembly subunits, including both lower and higher oligomeric states. Mff only recruits active forms of Drp1, while MIEFs are less selective and recruit both active and inactive Drp1 as well as oligomerization- or GTPase-deficient Drp1 mutants to mitochondria. Moreover, all the fission-incompetent Drp1 mutants tested (except the monomeric mutant K668E) affect Drp1-driven mitochondrial dynamics via incorporation of the mutants into the native oligomers to form function-deficient Drp1 assemblies. We here confirm that MIEFs also serve as a platform facilitating the binding of Drp1 to Mff and loss of MIEFs severely impairs the interaction between Drp1 and Mff. Collectively, our findings suggest that Mff and MIEFs respond differently to the molecular assembly state of Drp1 and that the extent of Drp1 oligomerization regulates mitochondrial dynamics.

MIEF1/2 orchestrate mitochondrial dynamics through direct engagement with both the fission and fusion machineries

Background

Mitochondrial dynamics is the result of a dynamic balance between fusion and fission events, which are driven via a set of mitochondria-shaping proteins. These proteins are generally considered to be binary components of either the fission or fusion machinery, but potential crosstalk between the fission and fusion machineries remains less explored. In the present work, we analyzed the roles of mitochondrial elongation factors 1 and 2 (MIEF1/2), core components of the fission machinery in mammals.

Results

We show that MIEFs (MIEF1/2), besides their action in the fission machinery, regulate mitochondrial fusion through direct interaction with the fusion proteins Mfn1 and Mfn2, suggesting that MIEFs participate in not only fission but also fusion. Elevated levels of MIEFs enhance mitochondrial fusion in an Mfn1/2- and OPA1-dependent but Drp1-independent manner. Moreover, mitochondrial localization and self-association of MIEFs are crucial for their fusion-promoting ability. In addition, we show that MIEF1/2 can competitively decrease the interaction of hFis1 with Mfn1 and Mfn2, alleviating hFis1-induced mitochondrial fragmentation and contributing to mitochondrial fusion.

Conclusions

Our study suggests that MIEFs serve as a central hub that interacts with and regulates both the fission and fusion machineries, which uncovers a novel mechanism for balancing these opposing forces of mitochondrial dynamics in mammals.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12915-021-01161-7.

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.

Defective mitophagy and synaptic degeneration in Alzheimer's disease: Focus on aging, mitochondria and synapse

Alzheimer's disease (AD) is a progressive neurodegenerative disease characterized by memory loss and multiple cognitive impairments. AD is marked by multiple cellular changes, including deregulation of microRNAs, activation of glia and astrocytes, hormonal imbalance, defective mitophagy, synaptic degeneration, in addition to extracellular neuritic amyloid-beta (Aβ) plaques, phosphorylated tau (P-tau), and intracellular neurofibrillary tangles (NFTs). Recent research in AD revealed that defective synaptic mitophagy leads to synaptic degeneration and cognitive dysfunction in AD neurons. Our critical analyses of mitochondria and Aβ and P-tau revealed that increased levels of Aβ and P-Tau, and abnormal interactions between Aβ and Drp1, P-Tau and Drp1 induced increased mitochondrial fragmentation and proliferation of dysfunctional mitochondria in AD neurons and depleted Parkin and PINK1 levels. These events ultimately lead to impaired clearance of dead and/or dying mitochondria in AD neurons. The purpose of our article is to highlight the recent research on mitochondria and synapses in relation to Aβ and P-tau, focusing on recent developments.

Protective Effect of Salidroside on Mitochondrial Disturbances via Reducing Mitophagy and Preserving Mitochondrial Morphology in OGD-induced Neuronal Injury

Salidroside is the active ingredient extracted from Rhodiola rosea, and has been reported to show protective effects in cerebral ischemia, but the exact mechanisms of neuronal protective effects are still unrevealed. In this study, the protective effects of salidroside (1 µmol/L) in ameliorating neuronal injuries induced by oxygen-glucose deprivation (OGD), which is a classical model of cerebral ischemia, were clarified. The results showed that after 8 h of OGD, the mouse hippocampal neuronal cell line HT22 cells showed increased cell death, accompanied with mitochondrial fragmentation and augmented mitophagy. However, the cell viability of HT22 cells showed significant restoration after salidroside treatment. Mitochondrial morphology and mitochondrial function were effectively preserved by salidroside treatment. The protective effects of salidroside were further related to the prevention of mitochondrial over-fission. The results showed that mTOR could be recruited to the mitochondria after salidroside treatment, which might be responsible for inhibiting excessive mitophagy caused by OGD. Thus, salidroside was shown to play a protective role in reducing neuronal death under OGD by safeguarding mitochondrial function, which may provide evidence for further translational studies of salidroside in ischemic diseases.

Advances in the mechanism and treatment of mitochondrial quality control involved in myocardial infarction

Mitochondria are important organelles in eukaryotic cells. Normal mitochondrial homeostasis is subject to a strict mitochondrial quality control system, including the strict regulation of mitochondrial production, fission/fusion and mitophagy. The strict and accurate modulation of the mitochondrial quality control system, comprising the mitochondrial fission/fusion, mitophagy and other processes, can ameliorate the myocardial injury of myocardial ischaemia and ischaemia-reperfusion after myocardial infarction, which plays an important role in myocardial protection after myocardial infarction. Further research into the mechanism will help identify new therapeutic targets and drugs for the treatment of myocardial infarction. This article aims to summarize the recent research regarding the mitochondrial quality control system and its molecular mechanism involved in myocardial infarction, as well as the potential therapeutic targets in the future.

A link between protein acetylation and mitochondrial dynamics under energy metabolism: A comprehensive overview

Cells adjust mitochondrial morphologies to coordinate between the cellular demand for energy and the availability of resources. Mitochondrial morphology is regulated by the balance between two counteracting mitochondrial processes of fusion and fission. Fission and fusion are dynamic and reversible processes that depend on the coordination of a number of proteins and are primarily regulated by posttranslational modifications. In the mitochondria, more than 20% of proteins are acetylated in proteomic surveys, partly involved in the dynamic regulation of mitochondrial fusion and fission. This article focuses on the molecular mechanism of the mitochondrial dynamics of fusion and fission, and summarizes the related mechanisms and targets of mitochondrial protein acetylation to regulate the mitochondrial dynamics of fusion and fission in energy metabolism.

Role of Mitochondrial Dysfunction in the Pathology of Amyloid-β

Mitochondrial dysfunction has been widely reported in several neurodegenerative disorders, including in the brains of patients with Alzheimer's disease (AD), Parkinson's disease, and Huntington disease. An increasing number of studies have implicated altered glucose and energy metabolism in patients with AD. There is compelling evidence of abnormalities in some of the key mitochondrial enzymes involved in glucose metabolism, including the pyruvate dehydrogenase and α-ketoglutarate dehydrogenase complexes, which play a great significance role in the pathogenesis of AD. Changes in some of the enzyme activities of the mitochondria found in AD have been linked with the pathology of amyloid-β (Aβ). This review highlights the role of mitochondrial function in the production and clearance of Aβ and how the pathology of Aβ leads to a decrease in energy metabolism by affecting mitochondrial function.

A PKD-MFF signaling axis couples mitochondrial fission to mitotic progression

Mitochondria are highly dynamic organelles subjected to fission and fusion events. During mitosis, mitochondrial fission ensures equal distribution of mitochondria to daughter cells. If and how this process can actively drive mitotic progression remains largely unknown. Here, we discover a pathway linking mitochondrial fission to mitotic progression in mammalian cells. The mitochondrial fission factor (MFF), the main mitochondrial receptor for the Dynamin-related protein 1 (DRP1), is directly phosphorylated by Protein Kinase D (PKD) specifically during mitosis. PKD-dependent MFF phosphorylation is required and sufficient for mitochondrial fission in mitotic but not in interphasic cells. Phosphorylation of MFF is crucial for chromosome segregation and promotes cell survival by inhibiting adaptation of the mitotic checkpoint. Thus, PKD/MFF-dependent mitochondrial fission is critical for the maintenance of genome integrity during cell division.

Drp1-dependent mitochondrial fission in cardiovascular disease

Mitochondria are highly dynamic organelles undergoing cycles of fusion and fission to modulate their morphology, distribution, and function, which are referred as 'mitochondrial dynamics'. Dynamin-related protein 1 (Drp1) is known as the major pro-fission protein whose activity is tightly regulated to clear the damaged mitochondria via mitophagy, ensuring a strict control over the intricate process of cellular and organ dynamics in heart. Various posttranslational modifications (PTMs) of Drp1 have been identified including phosphorylation, SUMOylation, palmitoylation, ubiquitination, S-nitrosylation, and O-GlcNAcylation, which implicate a role in the regulation of mitochondrial dynamics. An intact mitochondrial homeostasis is critical for heart to fuel contractile function and cardiomyocyte metabolism, while defects in mitochondrial dynamics constitute an essential part of the pathophysiology underlying various cardiovascular diseases (CVDs). In this review, we summarize current knowledge on the critical role of Drp1 in the pathogenesis of CVDs including endothelial dysfunction, smooth muscle remodeling, cardiac hypertrophy, pulmonary arterial hypertension, myocardial ischemia-reperfusion, and myocardial infarction. We also highlight how the targeting of Drp1 could potentially contribute to CVDs treatments.

Mitochondrial dynamics: Shaping and remodeling an organelle network

Mitochondria form networks that continually remodel and adapt to carry out their cellular function. The mitochondrial network is remodeled through changes in mitochondrial morphology, number, and distribution within the cell. Mitochondrial dynamics depend directly on fission, fusion, shape transition, and transport or tethering along the cytoskeleton. Over the past several years, many of the mechanisms underlying these processes have been uncovered. It has become clear that each process is precisely and contextually regulated within the cell. Here, we discuss the mechanisms regulating each aspect of mitochondrial dynamics, which together shape the network as a whole.

ER Stress-Sensor Proteins and ER-Mitochondrial Crosstalk-Signaling Beyond (ER) Stress Response

Recent studies undoubtedly show the importance of inter organellar connections to maintain cellular homeostasis. In normal physiological conditions or in the presence of cellular and environmental stress, each organelle responds alone or in coordination to maintain cellular function. The Endoplasmic reticulum (ER) and mitochondria are two important organelles with very specialized structural and functional properties. These two organelles are physically connected through very specialized proteins in the region called the mitochondria-associated ER membrane (MAM). The molecular foundation of this relationship is complex and involves not only ion homeostasis through the shuttling of calcium but also many structural and apoptotic proteins. IRE1alpha and PERK are known for their canonical function as an ER stress sensor controlling unfolded protein response during ER stress. The presence of these transmembrane proteins at the MAM indicates its potential involvement in other biological functions beyond ER stress signaling. Many recent studies have now focused on the non-canonical function of these sensors. In this review, we will focus on ER mitochondrial interdependence with special emphasis on the non-canonical role of ER stress sensors beyond ER stress.

MIEF2 reprograms lipid metabolism to drive progression of ovarian cancer through ROS/AKT/mTOR signaling pathway

MIEF2 (mitochondrial elongation factor 2) is one of the key regulators of mitochondrial fission. Bioinformatics analysis indicated that high expression of MIEF2 predicted a poor prognosis in ovarian cancer patients. However, the relationship between MIEF2 and aberrant lipid metabolism in OC remains elusive. In this study, we demonstrated that MIEF2 significantly promoted lipid synthesis, while has no significant effect on fatty acid uptake and oxidation in OC cells. MIEF2 enhanced de novo fatty acid synthesis through up-regulating the expression of sterol regulatory element binding protein 1 (SREBP1) and its transcriptional target lipogenic genes ACC1, FASN and SCD1. Meanwhile, MIEF2-promoted cholesterol biosynthesis through up-regulating the expression of sterol regulatory element binding protein 2 (SREBP2) and its transcriptional target cholesterol biosynthesis genes HMGCS1 and HMGCR. Mechanistically, increased mitochondrial reactive oxygen species (ROS) production and subsequently activation of AKT/mTOR signaling pathway was found to be involved in the up-regulation of SREBP1 and SREBP2 in OC cells. Moreover, cell growth and metastasis assays indicated that MIEF2-regulated fatty acid synthesis and cholesterol biosynthesis played a critical role in the progression of OC. Taken together, our findings indicate that MIEF2 is a critical regulator of lipid synthesis in OC, which provides a strong line of evidence for this molecule to serve as a drug target in the treatment of this malignancy.

Mitochondrial Membrane Intracellular Communication in Healthy and Diseased Myocardium

Research efforts in the twenty-first century have been paramount to the discovery and development of novel pharmacological treatments in a variety of diseases resulting in improved life expectancy. Yet, cardiac disease remains a leading cause of morbidity and mortality worldwide. Over time, there has been an expansion in conditions such as atrial fibrillation (AF) and heart failure (HF). Although past research has elucidated specific pathways that participate in the development of distinct cardiac pathologies, the exact mechanisms of action leading to disease remain to be fully characterized. Protein turnover and cellular bioenergetics are integral components of cardiac diseases, highlighting the importance of mitochondria and endoplasmic reticulum (ER) in driving cellular homeostasis. More specifically, the interactions between mitochondria and ER are crucial to calcium signaling, apoptosis induction, autophagy, and lipid biosynthesis. Here, we summarize mitochondrial and ER functions and physical interactions in healthy physiological states. We then transition to perturbations that occur in response to pathophysiological challenges and how this alters mitochondrial–ER and other intracellular organelle interactions. Finally, we discuss lifestyle interventions and innovative therapeutic targets that may be used to restore beneficial mitochondrial and ER interactions, thereby improving cardiac function.

Defective mitophagy in Alzheimer's disease

Alzheimer's disease (AD) is a progressive, mental illness without cure. Several years of intense research on postmortem AD brains, cell and mouse models of AD have revealed that multiple cellular changes are involved in the disease process, including mitochondrial abnormalities, synaptic damage, and glial/astrocytic activation, in addition to age-dependent accumulation of amyloid beta (Aβ) and hyperphosphorylated tau (p-tau). Synaptic damage and mitochondrial dysfunction are early cellular changes in the disease process. Healthy and functionally active mitochondria are essential for cellular functioning. Dysfunctional mitochondria play a central role in aging and AD. Mitophagy is a cellular process whereby damaged mitochondria are selectively removed from cell and mitochondrial quality and biogenesis. Mitophagy impairments cause the progressive accumulation of defective organelle and damaged mitochondria in cells. In AD, increased levels of Aβ and p-tau can induce reactive oxygen species (ROS) production, causing excessive fragmentation of mitochondria and promoting defective mitophagy. The current article discusses the latest developments of mitochondrial research and also highlights multiple types of mitophagy, including Aβ and p-tau-induced mitophagy, stress-induced mitophagy, receptor-mediated mitophagy, ubiquitin mediated mitophagy and basal mitophagy. This article also discusses the physiological states of mitochondria, including fission-fusion balance, Ca²⁺ transport, and mitochondrial transport in normal and diseased conditions. Our article summarizes current therapeutic interventions, like chemical or natural mitophagy enhancers, that influence mitophagy in AD. Our article discusses whether a partial reduction of Drp1 can be a mitophagy enhancer and a therapeutic target for mitophagy in AD and other neurological diseases.

Mitochondrial Fusion and Fission in Neuronal Death Induced by Cerebral Ischemia-Reperfusion and Its Clinical Application: A Mini-Review

Mitochondria are highly dynamic organelles which are joined by mitochondrial fusion and divided by mitochondrial fission. The balance of mitochondrial fusion and fission plays a critical role in maintaining the normal function of neurons, of which the processes are both mediated by several proteins activated by external stimulation. Cerebral ischemia-reperfusion (I/R) injury can disrupt the balance of mitochondrial fusion and fission through regulating the expression and post-translation modification of fusion- and fission-related proteins, thereby destroying homeostasis of the intracellular environment and causing neuronal death. Furthermore, human intervention in fusion- and fission-related proteins can influence the function of neurons and change the outcomes of cerebral I/R injury. In recent years, researchers have found that mitochondrial dysfunction was one of the main factors involved in I/R, and mitochondria is an attractive target in I/R neuroprotection. Therefore, mitochondrial-targeted therapy of the nervous system for I/R gradually started from basic study to clinical application. In the present review, we highlight recent progress in mitochondria fusion and fission in neuronal death induced by cerebral I/R to help understanding the regulatory factors and signaling networks of aberrant mitochondrial fusion and fission contributing to neuronal death during I/R, as well as the potential neuroprotective therapeutics targeting mitochondrial dynamics, which may help clinical treatment and development of relevant dugs.

Control by Ca2+ of mitochondrial structure and function in pancreatic β-cells

Mitochondria play a central role in glucose metabolism and the stimulation of insulin secretion from pancreatic β-cells. In this review, we discuss firstly the regulation and roles of mitochondrial Ca2+ transport in glucose-regulated insulin secretion, and the molecular machinery involved. Next, we discuss the evidence that mitochondrial dysfunction in β-cells is associated with type 2 diabetes, from a genetic, functional and structural point of view, and then the possibility that these changes may in part be mediated by dysregulation of cytosolic Ca2+. Finally, we review the importance of preserved mitochondrial structure and dynamics for mitochondrial gene expression and their possible relevance to the pathogenesis of type 2 diabetes.

Overexpression of wild type or a Q311E mutant MB21D2 promotes a pro-oncogenic phenotype in HNSCC

Cadherin‐mediated cell–cell contacts regulated by intracellular binders play critical roles in tissue homeostasis and tumorigenesis. Here, we screened mutational profiles of 312 annotated genes involved in cadherin binding in human squamous cell carcinomas and found MB21D2 to carry a unique recurrent Q311E mutation. MB21D2 overexpression was also frequently found in head and neck cancer (HNSCC) and was associated with poor clinical outcomes. Cell‐based characterizations revealed pro‐oncogenic roles for MB21D2 wild‐type (WT) and its Q311E mutant (Q311E) in cell proliferation, colony formation, sphere growth, and migration/invasion by promoting epithelial–mesenchymal transition. Conversely, MB21D2 knockdown in MB21D2‐overexpressing cells resulted in cell growth arrest and apoptosis. Xenograft tumor models with Q311E‐expressing cells formed larger and more aggressive lesions, compared to models with WT‐MB21D2‐expressing cells or an empty vector. Transcriptome and protein interactome analyses revealed enrichment of KRAS signaling by MB21D2 expression. Immunoblotting confirmed RAS elevation, along with upregulation/phosphorylation of PI3K, AKT, and CREB. Blocking RAS signaling in MB21D2‐expressing cells by manumycin significantly reduced cell growth and survival. Our study thus defined RAS signaling‐dependent pro‐oncogenic roles for MB21D2 overexpression and Q311E MB21D2 expression in HNSCC development.

Drp1/Mff signaling pathway is involved in fluoride-induced abnormal fission of hepatocyte mitochondria in mice

Fluoride, a toxic substance, is widely distributed in the environment and causes serious damage to the body. This study was performed to investigate the effects of fluoride on mitochondrial fission in mouse hepatocytes. A total of 48 mice were equally divided into four groups and admisnistered with NaF in drinking water at fluorine ion concentrations of 0, 25, 50 and 100 mg/L for 70 days. The pathomorphology and ultrastructurre of hepatocytes were then observed. The mitochondrial lesion parameters (number, length, width and vacuolization area) are evaluated. The expression of Drp1, Mff, Fis1, MiD49, MiD51 and Dyn2, which are associated with mitochondrial fission, was determined by quantitative real-time PCR and Western blot analysis. Apoptosis was detected by using TUNEL assay. Results showed that fluoride causes notable changes in the pathological morphology of liver tissues and severely damages the ultrastructure of hepatocytes. Damage manifested as nuclear condensation, nuclear membrane breakdown, mitochondrial vacuolation, increased fragmentation, and mitochondrial fission. Moreover, mRNA and protein expression levels were significantly upregulated in the Drp1/Mff signaling pathway. The mRNA expression levels of Cyt c, caspase 9 and 3 markedly increased in the fluoride treated groups in a dose-dependent manner. The percentage of TUNEL-positive nuclei in the liver remarkably increased after fluoride treatment. Overall, the results indicate that excessive fluoride exposure can increase mitochondrial fission via the Drp1/Mff signaling pathway, severely damage the mitochondrial structure, and lead to apoptosis of hepatocytes.

Mechanisms and roles of mitochondrial localisation and dynamics in neuronal function

Neurons are highly polarised, complex and incredibly energy intensive cells, and their demand for ATP during neuronal transmission is primarily met by oxidative phosphorylation by mitochondria. Thus, maintaining the health and efficient function of mitochondria is vital for neuronal integrity, viability and synaptic activity. Mitochondria do not exist in isolation, but constantly undergo cycles of fusion and fission, and are actively transported around the neuron to sites of high energy demand. Intriguingly, axonal and dendritic mitochondria exhibit different morphologies. In axons mitochondria are small and sparse whereas in dendrites they are larger and more densely packed. The transport mechanisms and mitochondrial dynamics that underlie these differences, and their functional implications, have been the focus of concerted investigation. Moreover, it is now clear that deficiencies in mitochondrial dynamics can be a primary factor in many neurodegenerative diseases. Here, we review the role that mitochondrial dynamics play in neuronal function, how these processes support synaptic transmission and how mitochondrial dysfunction is implicated in neurodegenerative disease.

Pervasive functional translation of noncanonical human open reading frames

Ribosome profiling has revealed pervasive but largely uncharacterized translation outside of canonical coding sequences (CDSs). In this work, we exploit a systematic CRISPR-based screening strategy to identify hundreds of noncanonical CDSs that are essential for cellular growth and whose disruption elicits specific, robust transcriptomic and phenotypic changes in human cells. Functional characterization of the encoded microproteins reveals distinct cellular localizations, specific protein binding partners, and hundreds of microproteins that are presented by the human leukocyte antigen system. We find multiple microproteins encoded in upstream open reading frames, which form stable complexes with the main, canonical protein encoded on the same messenger RNA, thereby revealing the use of functional bicistronic operons in mammals. Together, our results point to a family of functional human microproteins that play critical and diverse cellular roles.

MiD51 Is Important for Maintaining Mitochondrial Health in Pancreatic Islet and MIN6 Cells

Background: Mitochondrial dynamics are important for glucose-stimulated insulin secretion in pancreatic beta cells. The mitochondrial elongation factor MiD51 has been proposed to act as an anchor that recruits Drp1 from the cytosol to the outer mitochondrial membrane. Whether MiD51 promotes mitochondrial fusion by inactivation of Drp1 is a controversial issue. Since both the underlying mechanism and the effects on mitochondrial function remain unknown, this study was conducted to investigate the role of MiD51 in beta cells. Methods: Overexpression and downregulation of MiD51 in mouse insulinoma 6 (MIN6) and mouse islet cells was achieved using the pcDNA expression vector and specific siRNA, respectively. Expression of genes regulating mitochondrial dynamics and autophagy was analyzed by quantitative Real-Time PCR, glucose-stimulated insulin secretion by ELISA, and cellular oxygen consumption rate by optode sensor technology. Mitochondrial membrane potential and morphology were visualized after TMRE and MitoTracker Green staining, respectively. Immunofluorescence analyses were examined by confocal microscopy. Results: MiD51 is expressed in insulin-positive mouse and human pancreatic islet and MIN6 cells. Overexpression of MiD51 resulted in mitochondrial fragmentation and cluster formation in MIN6 cells. Mitochondrial membrane potential, glucose-induced oxygen consumption rate and glucose-stimulated insulin secretion were reduced in MIN6 cells with high MiD51 expression. LC3 expression remained unchanged. Downregulation of MiD51 resulted in inhomogeneity of the mitochondrial network in MIN6 cells with hyperelongated and fragmented mitochondria. Mitochondrial membrane potential, maximal and glucose-induced oxygen consumption rate and insulin secretion were diminished in MIN6 cells with low MiD51 expression. Furthermore, reduced Mfn2 and Parkin expression was observed. Based on MiD51 overexpression and downregulation, changes in the mitochondrial network structure similar to those in MIN6 cells were also observed in mouse islet cells. Conclusion: We have demonstrated that MiD51 plays a pivotal role in regulating mitochondrial function and hence insulin secretion in MIN6 cells. We propose that this anchor protein of Drp1 is important to maintain a homogeneous mitochondrial network and to avoid morphologies such as hyperelongation and clustering which are inaccessible for degradation by autophagy. Assuming that insulin granule degradation frequently suppresses autophagy in beta cells, MiD51 could be a key element maintaining mitochondrial health.

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

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

Regulation of Mammalian Mitochondrial Dynamics: Opportunities and Challenges

Mitochondria are highly dynamic organelles and important for a variety of cellular functions. They constantly undergo fission and fusion events, referred to as mitochondrial dynamics, which affects the shape, size, and number of mitochondria in the cell, as well as mitochondrial subcellular transport, mitochondrial quality control (mitophagy), and programmed cell death (apoptosis). Dysfunctional mitochondrial dynamics is associated with various human diseases. Mitochondrial dynamics is mediated by a set of mitochondria-shaping proteins in both yeast and mammals. In this review, we describe recent insights into the potential molecular mechanisms underlying mitochondrial fusion and fission, particularly highlighting the coordinating roles of different mitochondria-shaping proteins in the processes, as well as the roles of the endoplasmic reticulum (ER), the actin cytoskeleton and membrane phospholipids in the regulation of mitochondrial dynamics. We particularly focus on emerging roles for the mammalian mitochondrial proteins Fis1, Mff, and MIEFs (MIEF1 and MIEF2) in regulating the recruitment of the cytosolic Drp1 to the surface of mitochondria and how these proteins, especially Fis1, mediate crosstalk between the mitochondrial fission and fusion machineries. In summary, this review provides novel insights into the molecular mechanisms of mammalian mitochondrial dynamics and the involvement of these mechanisms in apoptosis and autophagy.

Roles of glycogen synthase kinase 3 alpha and calcineurin in regulating the ability of sperm to fertilize eggs

Glycogen synthase kinase 3 (GSK3) was identified as an enzyme regulating sperm protein phosphatase. The GSK3α paralog, but not GSK3β, is essential for sperm function. Sperm lacking GSK3α display altered motility and are unable to undergo hyperactivation, which is essential for fertilization. Male mice lacking sperm-specific calcineurin (PP2B), a calcium regulated phosphatase, in testis and sperm, are also infertile. Loss of PP2B results in impaired epididymal sperm maturation and motility. The phenotypes of GSK3α and PP2B knockout mice are similar, prompting us to examine the interrelationship between these two enzymes in sperm. High calcium levels must exist to permit catalytically active calcineurin to function during epididymal sperm maturation. Total and free calcium levels are high in immotile compared to motile epididymal sperm. Inhibition of calcineurin by FK506 results in an increase in the net phosphorylation and a consequent decrease in catalytic activity of sperm GSK3. The inhibitor FK506 and an isoform-selective inhibitor of GSK3α, BRD0705, also inhibited fertilization of eggs in vitro. Interrelated functions of GSK3α and sperm PP2B are essential during epididymal sperm maturation and during fertilization. Our results should enable the development of male contraceptives targeting one or both enzymes.

Mitochondrial Dynamics and Its Involvement in Disease

The dynamic properties of mitochondria-including their fusion, fission, and degradation-are critical for their optimal function in energy generation. The interplay of fusion and fission confers widespread benefits on mitochondria, including efficient transport, increased homogenization of the mitochondrial population, and efficient oxidative phosphorylation. These benefits arise through control of morphology, content exchange, equitable inheritance of mitochondria, maintenance of high-quality mitochondrial DNA, and segregation of damaged mitochondria for degradation. The key components of the machinery mediating mitochondrial fusion and fission belong to the dynamin family of GTPases that utilize GTP hydrolysis to drive mechanical work on biological membranes. Defects in this machinery cause a range of diseases that especially affect the nervous system. In addition, several common diseases, including neurodegenerative diseases and cancer, strongly affect mitochondrial dynamics.

The phosphorylation status of Ser-637 in dynamin-related protein 1 (Drp1) does not determine Drp1 recruitment to mitochondria

Recruitment of the GTPase dynamin-related protein 1 (Drp1) to mitochondria is a central step required for mitochondrial fission. Reversible Drp1 phosphorylation has been implicated in the regulation of this process, but whether Drp1 phosphorylation at Ser-637 determines its subcellular localization and fission activity remains to be fully elucidated. Here, using HEK 293T cells and immunofluorescence, immunoblotting, RNAi, subcellular fractionation, co-immunoprecipitation assays, and CRISPR/Cas9 genome editing, we show that Drp1 phosphorylated at Ser-637 (Drp1^pS637) resides both in the cytosol and on mitochondria. We found that the receptors mitochondrial fission factor (Mff) and mitochondrial elongation factor 1/2 (MIEF1/2) interact with and recruit Drp1^pS637 to mitochondria and that elevated Mff or MIEF levels promote Drp1^pS637 accumulation on mitochondria. We also noted that protein kinase A (PKA), which mediates phosphorylation of Drp1 on Ser-637, is partially present on mitochondria and interacts with both MIEFs and Mff. PKA knockdown did not affect the Drp1-Mff interaction, but slightly enhanced the interaction between Drp1 and MIEFs. In Drp1-deficient HEK 293T cells, both phosphomimetic Drp1-S637D and phospho-deficient Drp1-S637A variants, like wild-type Drp1, located to the cytosol and to mitochondria and rescued a Drp1 deficiency-induced mitochondrial hyperfusion phenotype. However, Drp1-S637D was less efficient than Drp1-WT and Drp1-S637A in inducing mitochondrial fission. In conclusion, the Ser-637 phosphorylation status in Drp1 is not a determinant that controls Drp1 recruitment to mitochondria.

Metabolic implications of organelle-mitochondria communication

Cellular organelles are not static but show dynamism-a property that is likely relevant for their function. In addition, they interact with other organelles in a highly dynamic manner. In this review, we analyze the proteins involved in the interaction between mitochondria and other cellular organelles, especially the endoplasmic reticulum, lipid droplets, and lysosomes. Recent results indicate that, on one hand, metabolic alterations perturb the interaction between mitochondria and other organelles, and, on the other hand, that deficiency in proteins involved in the tethering between mitochondria and the ER or in specific functions of the interaction leads to metabolic alterations in a variety of tissues. The interaction between organelles is an emerging field that will permit to identify key proteins, to delineate novel modulation pathways, and to elucidate their implications in human disease.

Mitochondrial Morphofunction in Mammalian Cells

Significance: In addition to their classical role in cellular ATP production, mitochondria are of key relevance in various (patho)physiological mechanisms including second messenger signaling, neuro-transduction, immune responses and death induction. Recent Advances: Within cells, mitochondria are motile and display temporal changes in internal and external structure ("mitochondrial dynamics"). During the last decade, substantial empirical and in silico evidence was presented demonstrating that mitochondrial dynamics impacts on mitochondrial function and vice versa. Critical Issues: However, a comprehensive and quantitative understanding of the bidirectional links between mitochondrial external shape, internal structure and function ("morphofunction") is still lacking. The latter particularly hampers our understanding of the functional properties and behavior of individual mitochondrial within single living cells. Future Directions: In this review we discuss the concept of mitochondrial morphofunction in mammalian cells, primarily using experimental evidence obtained within the last decade. The topic is introduced by briefly presenting the central role of mitochondria in cell physiology and the importance of the mitochondrial electron transport chain (ETC) therein. Next, we summarize in detail how mitochondrial (ultra)structure is controlled and discuss empirical evidence regarding the equivalence of mitochondrial (ultra)structure and function. Finally, we provide a brief summary of how mitochondrial morphofunction can be quantified at the level of single cells and mitochondria, how mitochondrial ultrastructure/volume impacts on mitochondrial bioreactions and intramitochondrial protein diffusion, and how mitochondrial morphofunction can be targeted by small molecules.

Role of GTPases in the regulation of mitochondrial dynamics in Parkinson's disease

Mitochondria are highly dynamic organelle that undergo frequent fusion and division, and the balance of these opposing processes regulates mitochondrial morphology, distribution, and function. Mitochondrial fission facilitates the replication and distribution of mitochondria during cell division, whereas the fusion process including inner and outer mitochondrial membrane fusion allows the exchange of intramitochondrial material between adjacent mitochondria. Despite several GTPase family proteins have been implicated as key modulators of mitochondrial dynamics, the mechanisms by which these proteins regulate mitochondrial homeostasis and function remain not clearly understood. Neuronal function and survival are closely related to mitochondria dynamics, and disturbed mitochondrial fission/fusion may influence neurotransmission, synaptic maintenance, neuronal survival and function. Recent studies have shown that mitochondrial dysfunction caused by aberrant mitochondrial dynamics plays an essential role in the pathogenesis of both sporadic and familial Parkinson's disease (PD). Collectively, we review the molecular mechanism of known GTPase proteins in regulating mitochondrial fission and fusion, but also highlight the causal role for mitochondrial dynamics in PD pathogenesis.

Mst1 overexpression combined with Yap knockdown augments thyroid carcinoma apoptosis via promoting MIEF1-related mitochondrial fission and activating the JNK pathway

Background

Cancer cell viability is strongly modulated by the Hippo pathway, which includes mammalian STE20-like protein kinase 1 (Mst1) and yes-associated protein (Yap). Although the roles of Mst1 and Yap in thyroid carcinoma cell death have been fully addressed, no study has determined whether differential modification of Mst1 and Yap could further suppress thyroid carcinoma progression. The aim of our study was to explore the antiapoptotic effects exerted by combined Mst1 overexpression and Yap knockdown in thyroid carcinoma MDA-T32 cells in vitro.

Methods

Mst1 adenovirus and Yap shRNA were transfected into MDA-T32 cells to overexpress Mst1 and inhibit Yap, respectively. Cell viability and death were determined via an MTT assay, a TUNEL assay and western blotting. Mitochondrial function, mitochondrial fission and pathway studies were performed via western blotting and immunofluorescence.

Results

The results of our study showed that combined Mst1 overexpression and Yap knockdown further augmented MDA-T32 cell death by mediating mitochondrial damage. In addition, cancer cell migration and proliferation were suppressed by combined Mst1 overexpression and Yap knockdown. At the molecular level, mitochondrial membrane potential, ATP production, respiratory function, and caspase-9-related apoptosis were activated by combined Mst1 overexpression and Yap knockdown. Further, we found that fatal mitochondrial fission was augmented by combined Mst1 overexpression and Yap knockdown in a manner dependent on the JNK-MIEF1 pathway. Inhibition of JNK-MIEF1 pathway activity abolished the proapoptotic effects exerted by Mst1/Yap on MDA-T32 cells.

Conclusions

Taken together, our data suggest that Mst1 activation and Yap inhibition coordinate to augment thyroid cancer cell death by controlling the JNK-MIEF1-mitochondria pathway, suggesting that differential regulation of the core Hippo pathway components is potentially a novel therapeutic tool for the treatment of thyroid cancer.

Electronic supplementary material

The online version of this article (10.1186/s12935-019-0860-8) contains supplementary material, which is available to authorized users.

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.

Yap-Hippo promotes A549 lung cancer cell death via modulating MIEF1-related mitochondrial stress and activating JNK pathway

Although the role of Yes-associated protein (Yap) has been described in the progression of lung cancer, the downstream effector of the Yap-Hippo pathway has not been identified. Accordingly, the aim of our study is to explore whether Yap modulates the activity of lung cancer by controlling mitochondrial elongation factor 1 (MIEF1)-related mitochondrial stress in a manner dependent on the JNK pathway. Cell viability was determined via MTT, LDH release and immunofluorescence assays. ATP production, the mitochondrial membrane potential, and caspase-9 activity were investigated to assess mitochondrial function. siRNA transfection and pathway blockers were used to observe the roles of MIEF1 and JNK in Yap-modulated cell viability in lung cancer cells in vitro. Yap deletion reduced cell viability in A549 and H358 lung cancer cells. At the molecular level, Yap deletion promoted mitochondrial dysfunction, as evidenced by the decreased mitochondrial potential, increased mitochondrial oxidative stress, augmented mitochondrial pro-apoptotic factor leakage and elevated caspase-9 activity. In addition, we found that Yap modulated mitochondrial stress via MIEF1 and that loss of MIEF1 abolished the regulatory actions of Yap on mitochondrial stress and cell viability. Besides, we provided evidence to support the necessary role of JNK in Yap-mediated MIEF1 upregulation. Inhibition of JNK abolished the promotive effect of Yap deletion on MIEF1 activation. Taken together, our results identified the JNK-MIEF1 pathway and mitochondrial stress as downstream effectors of Yap in lung cancer. This finding suggests a novel approach for the treatment of lung cancer in clinical practice.