The role of Mfn2 in the structure and function of endoplasmic reticulum-mitochondrial tethering in vivo

Sex-related differences in SIRT3-mediated mitochondrial dynamics in renal ischemia/reperfusion injury

The prevalence of renal ischemia/reperfusion injury (IRI) in premenopausal women is considerably lower than that in age-matched men. This suggests that sex-related differences in mitochondrial function and homeostasis may contribute to sexual dimorphism in renal injury, though the mechanism remains unclear. Mouse model of unilateral left renal IRI with contralateral kidney enucleation, Ovariectomy in female mice, and a human embryonic kidney (HEK) cell model of hypoxia-reoxygenation were used to study how estrogen affects the sexual dimorphism of renal IRI through SIRT3 in vitro and in vivo, respectively. Here, we demonstrate differential expression of renal SIRT3 may induce sexual dimorphism in IRI using the renal IRI model. Higher SIRT3 level in female mice was associated with E2-induced protection of renal tubular epithelium, reduced mitochondrial reactive oxygen species (ROS), and IRI resistance. In hypoxia-reoxygenated HEK cells, SIRT3 knockdown increased oxidative stress, shifted the interconnected mitochondrial network toward fission, exacerbated hypoxia/reoxygenation-induced endoplasmic reticulum stress (ERS), and abolished the protective effects of E2 on IRI. Mechanistically, the SIRT3 level is E2-dependent and that E2 increases the SIRT3 protein level via estrogen receptor. SIRT3 targeted an i-AAA protease, yeast mitochondrial AAA metalloprotease (YME1L1), and hydrolyzed long optic atrophy 1 (L-OPA) to short-OPA1 (S-OPA1) by deacetylating YME1L1, regulating mitochondrial dynamics toward fusion to reduce oxidative stress and ERS. These findings explored the mechanism by how estrogen alleviates renal IRI and providing a basis for potential therapeutic interventions targeting SIRT3.

Ginsenoside Rb1 ameliorates lipotoxicity-induced myocardial injury in diabetes mellitus by regulating Mfn2

PURPOSE

Diabetic cardiomyopathy is a prevalent cardiovascular complication of diabetes mellitus. This study aimed to investigate the effects of ginsenoside Rb1 (GRb1) on the diabetic myocardium.

METHODS

Leptin receptor-deficient db/db mice and palmitic acid (PA)-treated cardiomyocyte models were utilized. Cardiac systolic and diastolic function, mitochondrial morphology, and respiratory chain function were determined. The expression of mitochondrial dynamics proteins was measured. Mitofusin 2 (Mfn2) overexpression and inhibition were achieved by lentiviral infection and small interfering RNA (siRNA) transfection.

RESULTS

In comparison to non-diabetic mice, db/db mice exhibited significant increases in body weight, blood glucose, blood lipids, and cardiac free fatty acid levels. This was accompanied by myocardial hypertrophy and left ventricular diastolic dysfunction, which were significantly ameliorated by GRb1 intervention. Stimulation with PA increased oxidative stress and apoptosis, and decreased viability in H9c2 cardiomyocytes. PA also reduced sarcomere contractility and relaxation in adult mice ventricular myocytes. PA-induced cellular and mitochondrial damage were reversed with GRb1 treatment. The cardiac tissue of db/db mice and PA-treated cardiomyocytes exhibited a decrease in Mfn2 expression, which was markedly improved by GRb1. Mfn2 overexpression reversed PA-induced mitochondrial fragmentation and functional damage in cardiomyocytes, while inhibition of Mfn2 expression by siRNA transfection blocked the protective effects of GRb1.

CONCLUSION

GRb1 alleviated myocardial lipid accumulation and mitochondrial injury, and attenuated ventricular diastolic dysfunction in diabetic mice. The regulation of Mfn2 was involved in the protective effects of GRb1 against lipotoxic myocardial injury.

Mfn2 regulates mitochondria and mitochondria-associated endoplasmic reticulum membrane function in neurodegeneration induced by repeated sevoflurane exposure

Repeated sevoflurane exposure in neonatal mice can leads to neuronal apoptosis and mitochondrial dysfunction. The mitochondria are responsible for energy production to maintain homeostasis in the central nervous system. The mitochondria-associated endoplasmic reticulum membrane (MAM) is located between the mitochondria and endoplasmic reticulum (ER), and it is critical for mitochondrial function and cell survival. MAM malfunction contributes to neurodegeneration, however, whether it is involved in sevoflurane-induced neurotoxicity remains unknown. Our study demonstrated that repeated sevoflurane exposure induced mitochondrial dysfunction and dampened the MAM structure. The upregulated ER-mitochondria tethering enhanced Ca2+ transition from the cytosol to the mitochondria. Overload of mitochondrial Ca2+ contributed to opening of the mitochondrial permeability transition pore (mPTP), which caused neuronal apoptosis. Mitofusin 2(Mfn2), a key regulator of ER-mitochondria contacts, was found to be suppressed after repeated sevoflurane exposure, while restoration of Mfn2 expression alleviated cognitive dysfunction due to repeated sevoflurane exposure in the adult mice. These evidences suggest that sevoflurane-induced MAM malfunction is vulnerable to Mfn2 suppression, and the enhanced ER-mitochondria contacts promotes mitochondrial Ca2+ overload, contributing to mPTP opening and neuronal apoptosis. This paper sheds light on a novel mechanism of sevoflurane-induced neurotoxicity. Furthermore, targeting Mfn2-mediated regulation of the MAM structure and mitochondrial function may provide a therapeutic advantage in sevoflurane-induced neurodegeneration.

Dietary α-Ketoglutarate Alleviates Escherichia coli LPS-Induced Intestinal Barrier Injury by Modulating the Endoplasmic Reticulum-Mitochondrial System Pathway in Piglets

BACKGROUND

α-Ketoglutarate (AKG) plays a pivotal role in mitigating inflammation and enhancing intestinal health.

OBJECTIVES

This study aimed to investigate whether AKG could protect against lipopolysaccharide (LPS)-induced intestinal injury by alleviating disorders in mitochondria-associated endoplasmic reticulum (MAM) membranes, dysfunctional mitochondrial dynamics, and endoplasmic reticulum (ER) stress in a piglet model.

METHODS

Twenty-four piglets were subjected to a 2 × 2 factorial design with dietary factors (basal diet or 1% AKG diet) and LPS treatment (LPS or saline). After 21 d of consuming either the basal diet or AKG diet, piglets received injections of LPS or saline. The experiment was divided into 4 treatment groups [control (CON) group: basal diet + saline; LPS group: basal diet +LPS; AKG group: AKG diet + saline; and AKG_LPS group: AKG + LPS], each consisting of 6 piglets.

RESULTS

The results demonstrated that compared with the CON group, AKG enhanced jejunal morphology, antioxidant capacity, and the messenger RNA and protein expression of tight junction proteins. Moreover, it has shown a reduction in serum diamine oxidase activity and D-lactic acid content in piglets. In addition, fewer disorders in the ER-mitochondrial system were reflected by AKG, as evidenced by AKG regulating the expression of key molecules of mitochondrial dynamics (mitochondrial calcium uniporter, optic atrophy 1, fission 1, and dynamin-related protein 1), ER stress [activating transcription factor (ATF) 4, ATF 6, CCAAT/enhancer binding protein homologous protein, eukaryotic initiation factor 2α, glucose-regulated protein (GRP) 78, and protein kinase R-like ER kinase], and MAM membranes [mitofusin (Mfn)-1, Mfn-2, GRP 75, and voltage-dependent anion channel-1].

CONCLUSIONS

Dietary AKG can prevent mitochondrial dynamic dysfunction, ER stress, and MAM membrane disorder, ultimately alleviating LPS-induced intestinal damage in piglets.

Modulation of ER-mitochondria tethering complex VAPB-PTPIP51: Novel therapeutic targets for aging-associated diseases

Aging is a gradual and irreversible natural process. With aging, the body experiences a functional decline, and the effects amplify the vulnerability to a range of age-related diseases, including neurodegenerative, cardiovascular, and metabolic diseases. Within the aging process, the morphology and function of mitochondria and the endoplasmic reticulum (ER) undergo alterations, particularly in the structure connecting these organelles known as mitochondria-associated membranes (MAMs). MAMs serve as vital intracellular signaling hubs, facilitating communication between the ER and mitochondria when regulating various cellular events, including calcium homeostasis, lipid metabolism, mitochondrial function, and apoptosis. The formation of MAMs is partly dependent on the interaction between the vesicle-associated membrane protein-associated protein-B (VAPB) and protein tyrosine phosphatase-interacting protein-51 (PTPIP51). Accumulating evidence has begun to elucidate the pivotal role of the VAPB-PTPIP51 tether in the initiation and progression of age-related diseases. In this study, we delineate the intricate structure and multifunctional role of the VAPB-PTPIP51 tether and discuss its profound implications in aging-associated diseases. Moreover, we provide a comprehensive overview of potential therapeutic interventions and pharmacological agents targeting the VAPB-PTPIP51-mediated MAMs, thereby offering a glimmer of hope in mitigating aging processes and treating age-related disorders.

MFN2 coordinates mitochondria motility with α-tubulin acetylation and this regulation is disrupted in CMT2A

Mitofusin-2 (MFN2), a large GTPase residing in the mitochondrial outer membrane and mutated in Charcot-Marie-Tooth type 2 disease (CMT2A), is a regulator of mitochondrial fusion and tethering with the ER. The role of MFN2 in mitochondrial transport has however remained elusive. Like MFN2, acetylated microtubules play key roles in mitochondria dynamics. Nevertheless, it is unknown if the α-tubulin acetylation cycle functionally interacts with MFN2. Here, we show that mitochondrial contacts with microtubules are sites of α-tubulin acetylation, which occurs through MFN2-mediated recruitment of α-tubulin acetyltransferase 1 (ATAT1). This activity is critical for MFN2-dependent regulation of mitochondria transport, and axonal degeneration caused by CMT2A MFN2 associated R94W and T105M mutations may depend on the inability to release ATAT1 at sites of mitochondrial contacts with microtubules. Our findings reveal a function for mitochondria in α-tubulin acetylation and suggest that disruption of this activity plays a role in the onset of MFN2-dependent CMT2A.

3D Mitochondrial Structure in Aging Human Skeletal Muscle: Insights into MFN-2 Mediated Changes

Age-related atrophy of skeletal muscle, is characterized by loss of mass, strength, endurance, and oxidative capacity during aging. Notably, bioenergetics and protein turnover studies have shown that mitochondria mediate this decline in function. Although exercise has been the only therapy to mitigate sarcopenia, the mechanisms that govern how exercise serves to promote healthy muscle aging are unclear. Mitochondrial aging is associated with decreased mitochondrial capacity, so we sought to investigate how aging affects mitochondrial structure and potential age-related regulators. Specifically, the three-dimensional (3D) mitochondrial structure associated with morphological changes in skeletal muscle during aging requires further elucidation. We hypothesized that aging causes structural remodeling of mitochondrial 3D architecture representative of dysfunction, and this effect is mitigated by exercise. We used serial block-face scanning electron microscopy to image human skeletal tissue samples, followed by manual contour tracing using Amira software for 3D reconstruction and subsequent analysis of mitochondria. We then applied a rigorous in vitro and in vivo exercise regimen during aging. Across 5 human cohorts, we correlate differences in magnetic resonance imaging, mitochondria 3D structure, exercise parameters, and plasma immune markers between young (under 50 years) and old (over 50 years) individuals. We found that mitochondria we less spherical and more complex, indicating age-related declines in contact site capacity. Additionally, aged samples showed a larger volume phenotype in both female and male humans, indicating potential mitochondrial swelling. Concomitantly, muscle area, exercise capacity, and mitochondrial dynamic proteins showed age-related losses. Exercise stimulation restored mitofusin 2 (MFN2), one such of these mitochondrial dynamic proteins, which we show is required for the integrity of mitochondrial structure. Furthermore, we show that this pathway is evolutionarily conserved as Marf, the MFN2 ortholog in Drosophila, knockdown alters mitochondrial morphology and leads to the downregulation of genes regulating mitochondrial processes. Our results define age-related structural changes in mitochondria and further suggest that exercise may mitigate age-related structural decline through modulation of mitofusin 2.

ER-organelle contacts: A signaling hub for neurological diseases

Neuronal health is closely linked to the homeostasis of intracellular organelles, and organelle dysfunction affects the pathological progression of neurological diseases. In contrast to isolated cellular compartments, a growing number of studies have found that organelles are largely interdependent structures capable of communicating through membrane contact sites (MCSs). MCSs have been identified as key pathways mediating inter-organelle communication crosstalk in neurons, and their alterations have been linked to neurological disease pathology. The endoplasmic reticulum (ER) is a membrane-bound organelle capable of forming an extensive network of pools and tubules with important physiological functions within neurons. There are multiple MCSs between the ER and other organelles and the plasma membrane (PM), which regulate a variety of cellular processes. In this review, we focus on ER-organelle MCSs and their role in a variety of neurological diseases. We compared the biological effects between different tethering proteins and the effects of their respective disease counterparts. We also discuss how altered ER-organelle contacts may affect disease pathogenesis. Therefore, understanding the molecular mechanisms of ER-organelle MCSs in neuronal homeostasis will lay the foundation for the development of new therapies targeting ER-organelle contacts.

Phthalates Induce Neurotoxicity by Disrupting the Mfn2-PERK Axis-Mediated Endoplasmic Reticulum-Mitochondria Interaction

Di-(2-ethylhexyl) phthalate (DEHP), as the most common phthalate, has been extensively used as a plasticizer to improve the plasticity of agricultural products, which pose severe harm to human health. Mitochondrial dynamics and endoplasmic reticulum (ER) homeostasis are indispensable for maintaining mitochondria-associated ER membrane (MAM) integrity. In this study, we aimed to explore the effect of DEHP on the nervous system and its association with the ER-mitochondria interaction. Here, we showed that DEHP caused morphological changes, motor deficits, cognitive impairments, and blood-brain barrier disruption in the brain. DEHP triggered ER stress, which is mainly mediated by protein kinase R-like endoplasmic reticulum kinase (PERK) signaling. Moreover, DEHP-induced mitofusin-2 (Mfn2) downregulation results in imbalance of the mitochondrial dynamics. Interestingly, DEHP exposure impaired MAMs by inhibiting the Mfn2-PERK interaction. Above all, this study elucidates the disruption of the Mfn2-PERK axis-mediated ER-mitochondria interaction as a phthalate-induced neurotoxicity that could be potentially developed as a novel therapy for neurological diseases.

ATF4-dependent increase in mitochondrial-endoplasmic reticulum tethering following OPA1 deletion in skeletal muscle

Mitochondria and endoplasmic reticulum (ER) contact sites (MERCs) are protein- and lipid-enriched hubs that mediate interorganellar communication by contributing to the dynamic transfer of Ca2+, lipid, and other metabolites between these organelles. Defective MERCs are associated with cellular oxidative stress, neurodegenerative disease, and cardiac and skeletal muscle pathology via mechanisms that are poorly understood. We previously demonstrated that skeletal muscle-specific knockdown (KD) of the mitochondrial fusion mediator optic atrophy 1 (OPA1) induced ER stress and correlated with an induction of Mitofusin-2, a known MERC protein. In the present study, we tested the hypothesis that Opa1 downregulation in skeletal muscle cells alters MERC formation by evaluating multiple myocyte systems, including from mice and Drosophila, and in primary myotubes. Our results revealed that OPA1 deficiency induced tighter and more frequent MERCs in concert with a greater abundance of MERC proteins involved in calcium exchange. Additionally, loss of OPA1 increased the expression of activating transcription factor 4 (ATF4), an integrated stress response (ISR) pathway effector. Reducing Atf4 expression prevented the OPA1-loss-induced tightening of MERC structures. OPA1 reduction was associated with decreased mitochondrial and sarcoplasmic reticulum, a specialized form of ER, calcium, which was reversed following ATF4 repression. These data suggest that mitochondrial stress, induced by OPA1 deficiency, regulates skeletal muscle MERC formation in an ATF4-dependent manner.

COPI-regulated mitochondria-ER contact site formation maintains axonal integrity

Coat protein complex I (COPI) is best known for its role in Golgi-endoplasmic reticulum (ER) trafficking, responsible for the retrograde transport of ER-resident proteins. The ER is crucial to neuronal function, regulating Ca2+ homeostasis and the distribution and function of other organelles such as endosomes, peroxisomes, and mitochondria via functional contact sites. Here we demonstrate that disruption of COPI results in mitochondrial dysfunction in Drosophila axons and human cells. The ER network is also disrupted, and the neurons undergo rapid degeneration. We demonstrate that mitochondria-ER contact sites (MERCS) are decreased in COPI-deficient axons, leading to Ca2+ dysregulation, heightened mitophagy, and a decrease in respiratory capacity. Reintroducing MERCS is sufficient to rescue not only mitochondrial distribution and Ca2+ uptake but also ER morphology, dramatically delaying neurodegeneration. This work demonstrates an important role for COPI-mediated trafficking in MERC formation, which is an essential process for maintaining axonal integrity.

Single-cell transcriptional profiling of hearts during cardiac hypertrophy reveals the role of MAMs in cardiomyocyte subtype switching

Pathological cardiac hypertrophy is the main predecessor of heart failure. Its pathology is sophisticated, and its progression is associated with multiple cellular processes. To explore new therapeutic approaches, more precise examination of cardiomyocyte subtypes and involved biological processes is required in response to hypertrophic stimuli. Mitochondria and the endoplasmic reticulum (ER) are two crucial organelles associated with the progression of cardiac hypertrophy and are connected through junctions known as mitochondria-associated endoplasmic reticulum membranes (MAMs). Although MAM genes are altered in cardiac hypertrophy, the importance of MAMs in cardiac hypertrophy and the expression pattern of MAMs in certain cardiac cell types require a comprehensive analysis. In this study, we analyzed the temporal expression of MAM proteins in the process of cardiac hypertrophy and observed that MAM-related proteins preferentially accumulated in cardiomyocytes at the initial stage of cardiac hypertrophy and underwent a gradual decline, which was synchronized with the proportion of two cardiomyocyte subtypes (CM2 and CM3). Meanwhile, these subtypes went through a functional switch during cardiac hypertrophy. Trajectory analysis suggested that there was a differentiation trajectory of cardiomyocyte subtypes from high to low MAM protein expression. Distinct regulon modules across different cardiomyocyte cell types were revealed by transcriptional regulatory network analysis. Furthermore, scWGCNA revealed that MAM-related genes were clustered into a module that correlated with diabetic cardiomyopathy. Altogether, we identified cardiomyocyte subtype transformation and the potential critical transcription factors involved, which may serve as therapeutic targets in combating cardiac hypertrophy.

The ER-mitochondria interface, where Ca2+ and cell death meet

Endoplasmic reticulum (ER)-mitochondria contact sites are crucial to allow Ca²⁺ flux between them and a plethora of proteins participate in tethering both organelles together. Inositol 1,4,5-trisphosphate receptors (IP₃Rs) play a pivotal role at such contact sites, participating in both ER-mitochondria tethering and as Ca²⁺-transport system that delivers Ca²⁺ from the ER towards mitochondria. At the ER-mitochondria contact sites, the IP₃Rs function as a multi-protein complex linked to the voltage-dependent anion channel 1 (VDAC1) in the outer mitochondrial membrane, via the chaperone glucose-regulated protein 75 (GRP75). This IP₃R-GRP75-VDAC1 complex supports the efficient transfer of Ca²⁺ from the ER into the mitochondrial intermembrane space, from which the Ca²⁺ ions can reach the mitochondrial matrix through the mitochondrial calcium uniporter. Under physiological conditions, basal Ca²⁺ oscillations deliver Ca²⁺ to the mitochondrial matrix, thereby stimulating mitochondrial oxidative metabolism. However, when mitochondrial Ca²⁺ overload occurs, the increase in [Ca²⁺] will induce the opening of the mitochondrial permeability transition pore, thereby provoking cell death. The IP₃R-GRP75-VDAC1 complex forms a hub for several other proteins that stabilize the complex and/or regulate the complex's ability to channel Ca²⁺ into the mitochondria. These proteins and their mechanisms of action are discussed in the present review with special attention for their role in pathological conditions and potential implication for therapeutic strategies.

Role of Microbiota-Modified Bile Acids in the Regulation of Intracellular Organelles and Neurodegenerative Diseases

Bile acids (BAs) are amphiphilic steroidal molecules generated from cholesterol in the liver and facilitate the digestion and absorption of fat-soluble substances in the gut. Some BAs in the intestine are modified by the gut microbiota. Because BAs are modified in a variety of ways by different types of bacteria present in the gut microbiota, changes in the gut microbiota can affect the metabolism of BAs in the host. Although most BAs absorbed from the gut are transferred to the liver, some are transferred to the systemic circulation. Furthermore, BAs have also been detected in the brain and are thought to migrate into the brain through the systemic circulation. Although BAs are known to affect a variety of physiological functions by acting as ligands for various nuclear and cell-surface receptors, BAs have also been found to act on mitochondria and autophagy in the cell. This review focuses on the BAs modified by the gut microbiota and their roles in intracellular organelles and neurodegenerative diseases.

Acquired disorders of mitochondrial metabolism and dynamics in pulmonary arterial hypertension

Pulmonary arterial hypertension (PAH) is an orphan disease of the cardiopulmonary unit that reflects an obstructive pulmonary vasculopathy and presents with hypertrophy, inflammation, fibrosis, and ultimately failure of the right ventricle (RVF). Despite treatment using pulmonary hypertension (PH)-targeted therapies, persistent functional impairment reduces the quality of life for people with PAH and death from RVF occurs in approximately 40% of patients within 5 years of diagnosis. PH-targeted therapeutics are primarily vasodilators and none, alone or in combination, are curative. This highlights a need to therapeutically explore molecular targets in other pathways that are involved in the pathogenesis of PAH. Several candidate pathways in PAH involve acquired mitochondrial dysfunction. These mitochondrial disorders include: 1) a shift in metabolism related to increased expression of pyruvate dehydrogenase kinase and pyruvate kinase, which together increase uncoupled glycolysis (Warburg metabolism); 2) disruption of oxygen-sensing related to increased expression of hypoxia-inducible factor 1α, resulting in a state of pseudohypoxia; 3) altered mitochondrial calcium homeostasis related to impaired function of the mitochondrial calcium uniporter complex, which elevates cytosolic calcium and reduces intramitochondrial calcium; and 4) abnormal mitochondrial dynamics related to increased expression of dynamin-related protein 1 and its binding partners, such as mitochondrial dynamics proteins of 49 kDa and 51 kDa, and depressed expression of mitofusin 2, resulting in increased mitotic fission. These acquired mitochondrial abnormalities increase proliferation and impair apoptosis in most pulmonary vascular cells (including endothelial cells, smooth muscle cells and fibroblasts). In the RV, Warburg metabolism and induction of glutaminolysis impairs bioenergetics and promotes hypokinesis, hypertrophy, and fibrosis. This review will explore our current knowledge of the causes and consequences of disordered mitochondrial function in PAH.

Role of mitochondria-associated endoplasmic reticulum membranes in insulin sensitivity, energy metabolism, and contraction of skeletal muscle

Skeletal muscle has a critical role in the regulation of the energy balance of the organism, particularly as the principal tissue responsible for insulin-stimulated glucose disposal and as the major site of peripheral insulin resistance (IR), which has been related to accumulation of lipid intermediates, reduced oxidative capacity of mitochondria and endoplasmic reticulum (ER) stress. These organelles form contact sites, known as mitochondria-associated ER membranes (MAMs). This interconnection seems to be involved in various cellular processes, including Ca²⁺ transport and energy metabolism; therefore, MAMs could play an important role in maintaining cellular homeostasis. Evidence suggests that alterations in MAMs may contribute to IR. However, the evidence does not refer to a specific subcellular location, which is of interest due to the fact that skeletal muscle is constituted by oxidative and glycolytic fibers as well as different mitochondrial populations that appear to respond differently to stimuli and pathological conditions. In this review, we show the available evidence of possible differential responses in the formation of MAMs in skeletal muscle as well as its role in insulin signaling and the beneficial effect it could have in the regulation of energetic metabolism and muscular contraction.

New insights into the interplay between autophagy and oxidative and endoplasmic reticulum stress in neuronal cell death and survival

Autophagy is a dynamic process that maintains the normal homeostasis of cells by digesting and degrading aging proteins and damaged organelles. The effect of autophagy on neural tissue is still a matter of debate. Some authors suggest that autophagy has a protective effect on nerve cells, whereas others suggest that autophagy also induces the death of nerve cells and aggravates nerve injury. In mammals, oxidative stress, autophagy and endoplasmic reticulum stress (ERS) constitute important defense mechanisms to help cells adapt to and survive the stress conditions caused by physiological and pathological stimuli. Under many pathophysiological conditions, oxidative stress, autophagy and ERS are integrated and amplified in cells to promote the progress of diseases. Over the past few decades, oxidative stress, autophagy and ERS and their interactions have been a hot topic in biomedical research. In this review, we summarize recent advances in understanding the interactions between oxidative stress, autophagy and ERS in neuronal cell death and survival.

Transfer of H2O2 from Mitochondria to the endoplasmic reticulum via Aquaporin-11

Some aquaporins (AQPs) can transport H2O2 across membranes, allowing redox signals to proceed in and between cells. Unlike other peroxiporins, human AQP11 is an endoplasmic reticulum (ER)-resident that can conduit H2O2 to the cytosol. Here, we show that silencing Ero1α, an ER flavoenzyme that generates abundant H2O2 during oxidative folding, causes a paradoxical increase in luminal H2O2 levels. The simultaneous AQP11 downregulation prevents this increase, implying that H2O2 reaches the ER from an external source(s). Pharmacological inhibition of the electron transport chain reveals that Ero1α downregulation activates superoxide production by complex III. In the intermembrane space, superoxide dismutase 1 generates H2O2 that enters the ER channeled by AQP11. Meanwhile, the number of ER-mitochondria contact sites increases as well, irrespective of AQP11 expression. Taken together, our findings identify a novel interorganellar redox response that is activated upon Ero1α downregulation and transfers H2O2 from mitochondria to the ER via AQP11.

Neurohormonal Connections with Mitochondria in Cardiomyopathy and Other Diseases

Neurohormonal signaling and mitochondrial dynamism are seemingly distinct processes that are almost ubiquitous among multicellular organisms. Both of these processes are regulated by GTPases, and disturbances in either can provoke disease. Here, inconspicuous pathophysiological connectivity between neurohormonal signaling and mitochondrial dynamism is reviewed in the context of cardiac and neurological syndromes. For both processes, greater understanding of basic mechanisms has evoked a reversal of conventional pathophysiological concepts. Thus, neurohormonal systems induced in, and previously thought to be critical for, cardiac functioning in heart failure are now pharmaceutically interrupted as modern standard of care. And, mitochondrial abnormalities in neuropathies that were originally attributed to an imbalance between mitochondrial fusion and fission are increasingly recognized as an interruption of axonal mitochondrial transport. The data are presented in a historical context to provided insight into how scientific thought has evolved and to foster an appreciation for how seemingly different areas of investigation can converge. Finally, some theoretical notions are presented to explain how different molecular and functional defects can evoke tissue-specific disease.

Cell death regulation by MAMs: from molecular mechanisms to therapeutic implications in cardiovascular diseases

The endoplasmic reticulum (ER) and mitochondria are interconnected intracellular organelles with vital roles in the regulation of cell signaling and function. While the ER participates in a number of biological processes including lipid biosynthesis, Ca²⁺ storage and protein folding and processing, mitochondria are highly dynamic organelles governing ATP synthesis, free radical production, innate immunity and apoptosis. Interplay between the ER and mitochondria plays a crucial role in regulating energy metabolism and cell fate control under stress. The mitochondria-associated membranes (MAMs) denote physical contact sites between ER and mitochondria that mediate bidirectional communications between the two organelles. Although Ca²⁺ transport from ER to mitochondria is vital for mitochondrial homeostasis and energy metabolism, unrestrained Ca²⁺ transfer may result in mitochondrial Ca²⁺ overload, mitochondrial damage and cell death. Here we summarize the roles of MAMs in cell physiology and its impact in pathological conditions with a focus on cardiovascular disease. The possibility of manipulating ER-mitochondria contacts as potential therapeutic approaches is also discussed.

Mitochondria-associated membranes: A hub for neurodegenerative diseases

In eukaryotic cells, organelles could coordinate complex mechanisms of signaling transduction metabolism and gene expression through their functional interactions. The functional domain between ER and mitochondria, called mitochondria-associated membranes (MAM), is closely associated with various physiological functions including intracellular lipid transport, Ca²⁺ transfer, mitochondria function maintenance, and autophagosome formation. In addition, more evidence suggests that MAM modulate cellular functions in health and disease. Studies have also demonstrated the association of MAM with numerous diseases, including neurodegenerative diseases, cancer, viral infection, obesity, and diabetes. In fact, recent evidence revealed a close relationship of MAM with Alzheimer's disease (AD), Parkinson's disease (PD), Huntington's disease (HD), amyotrophic lateral sclerosis (ALS), and other neurodegenerative diseases. In this view, elucidating the role of MAM in neurodegenerative diseases is particularly important. This review will focus the main tethering protein complexes of MAM and functions of MAM. Besides, the role of MAM in the regulation of neurodegenerative diseases and the potential molecular mechanisms is introduced to provide a new understanding of the pathogenesis of these diseases.

The Role of Impaired Mitochondrial Dynamics in MFN2-Mediated Pathology

The Mitofusin 2 protein (MFN2), encoded by the MFN2 gene, was first described for its role in mediating mitochondrial fusion. However, MFN2 is now recognized to play additional roles in mitochondrial autophagy (mitophagy), mitochondrial motility, lipid transfer, and as a tether to other organelles including the endoplasmic reticulum (ER) and lipid droplets. The tethering role of MFN2 is an important mediator of mitochondrial-ER contact sites (MERCs), which themselves have many important functions that regulate mitochondria, including calcium homeostasis and lipid metabolism. Exemplifying the importance of MFN2, pathogenic variants in MFN2 are established to cause the peripheral neuropathy Charcot-Marie-Tooth Disease Subtype 2A (CMT2A). However, the mechanistic basis for disease is not clear. Moreover, additional pathogenic phenotypes such as lipomatosis, distal myopathy, optic atrophy, and hearing loss, can also sometimes be present in patients with CMT2A. Given these variable patient phenotypes, and the many cellular roles played by MFN2, the mechanistic underpinnings of the cellular impairments by which MFN2 dysfunction leads to disease are likely to be complex. Here, we will review what is known about the various functions of MFN2 that are impaired by pathogenic variants causing CMT2A, with a specific emphasis on the ties between MFN2 variants and MERCs.

Regulation of Mitochondrial Function by the Actin Cytoskeleton

The regulatory role of actin cytoskeleton on mitochondrial function is a growing research field, but the underlying molecular mechanisms remain poorly understood. Specific actin-binding proteins (ABPs), such as Gelsolin, have also been shown to participate in the pathophysiology of mitochondrial OXPHOS disorders through yet to be defined mechanisms. In this mini-review, we will summarize the experimental evidence supporting the fundamental roles of actin cytoskeleton and ABPs on mitochondrial trafficking, dynamics, biogenesis, metabolism and apoptosis, with a particular focus on Gelsolin involvement in mitochondrial disorders. The functional interplay between the actin cytoskeleton, ABPs and mitochondrial membranes for the regulation of cellular homeostasis thus emerges as a new exciting field for future research and therapeutic approaches.

Mfn2 Regulates High Glucose-Induced MAMs Dysfunction and Apoptosis in Podocytes via PERK Pathway

The endoplasmic reticulum (ER) stress and mitochondrial dysfunction in high glucose (HG)-induced podocyte injury have been demonstrated to the progression of diabetic kidney disease (DKD). However, the pathological mechanisms remain equivocal. Mitofusin2 (Mfn2) was initially identified as a dynamin-like protein involved in fusing the outer mitochondrial membrane (OMM). More recently, Mfn2 has been reported to be located at the ER membranes that contact OMM. Mitochondria-associated ER membranes (MAMs) is the intercellular membrane subdomain, which connects the mitochondria and ER through a proteinaceous tether. Here, we observed the suppression of Mfn2 expression in the glomeruli and glomerular podocytes of patients with DKD. Streptozotocin (STZ)-induced diabetic rats exhibited abnormal mitochondrial morphology and MAMs reduction in podocytes, accompanied by decreased expression of Mfn2 and activation of all three unfolded protein response (UPR) pathways (IRE1, ATF6, and PERK). The HG-induced mitochondrial dysfunction, MAMs reduction, and increased apoptosis in vitro were accompanied by the downregulation of Mfn2 and activation of the PERK pathway. Mfn2 physically interacts with PERK, and HG promotes a decrease in Mfn2-PERK interaction. In addition, Mfn2-silenced podocytes showed mitochondrial dysfunction, MAMs reduction, activation of PERK pathway, and increased apoptosis. Conversely, all these effects of HG stimulation were alleviated significantly by Mfn2 overexpression. Furthermore, the inhibition of PERK phosphorylation protected mitochondrial functions but did not affect the expression of Mfn2 in HG-treated podocytes. Therefore, this study confirmed that Mfn2 regulates the morphology and functions of MAMs and mitochondria, and exerts anti-apoptotic effects on podocytes by inhibiting the PERK pathway. Hence, the Mfn2-PERK signaling pathway may be a new therapeutic target for preventing podocyte injury in DKD.

The molecular role of Sigmar1 in regulating mitochondrial function through mitochondrial localization in cardiomyocytes

Sigmar1 is a widely expressed molecular chaperone protein in mammalian cell systems. Accumulating research demonstrated the cardioprotective roles of pharmacologic Sigmar1 activation by ligands in preclinical rodent models of cardiac injury. Extensive biochemical and immuno-electron microscopic research demonstrated Sigmar1's sub-cellular localization largely depends on cell and organ types. Despite comprehensive studies, Sigmar1's direct molecular role in cardiomyocytes remains elusive. In the present study, we determined Sigmar1's subcellular localization, transmembrane topology, and function using complementary microscopy, biochemical, and functional assays in cardiomyocytes. Quantum dots in transmission electron microscopy showed Sigmar1 labeled quantum dots on the mitochondrial membranes, lysosomes, and sarcoplasmic reticulum-mitochondrial interface. Subcellular fractionation of heart cell lysates confirmed Sigmar1's localization in purified mitochondria fraction and lysosome fraction. Immunocytochemistry confirmed Sigmar1 colocalization with mitochondrial proteins in isolated adult mouse cardiomyocytes. Sigmar1's mitochondrial localization was further confirmed by Sigmar1 colocalization with Mito-Tracker in isolated mouse heart mitochondria. A series of biochemical experiments, including alkaline extraction and proteinase K treatment of purified heart mitochondria, demonstrated Sigmar1 as an integral mitochondrial membrane protein. Sigmar1's structural requirement for mitochondrial localization was determined by expressing FLAG-tagged Sigmar1 fragments in cells. Full-length Sigmar1 and Sigmar1's C terminal-deletion fragments were able to localize to the mitochondrial membrane, whereas N-terminal deletion fragment was unable to incorporate into the mitochondria. Finally, functional assays using extracellular flux analyzer and high-resolution respirometry showed Sigmar1 siRNA knockdown significantly altered mitochondrial respiration in cardiomyocytes. Overall, we found that Sigmar1 localizes to mitochondrial membranes and is indispensable for maintaining mitochondrial respiratory homeostasis in cardiomyocytes.

MITOL regulates phosphatidic acid-binding activity of RMDN3/PTPIP51

The transfer of phospholipids from the endoplasmic reticulum to mitochondria via the mitochondria-endoplasmic reticulum (ER) contact site (MERCS) is essential for maintaining mitochondrial function and integrity. Here, we identified RMDN3/PTPIP51, possessing phosphatidic acid (PA)-transfer activity, as a neighboring protein of the mitochondrial E3 ubiquitin ligase MITOL/MARCH5 by proximity-dependent biotin labeling using APEX2. We found that MITOL interacts with and ubiquitinates RMDN3. Mutational analysis identified lysine residue 89 in RMDN3 as a site of ubiquitination by MITOL. Loss of MITOL or the substitution of lysine 89 to arginine in RMDN3 significantly reduced the PA-binding activity of RMDN3, suggesting that MITOL regulates the transport of PA to mitochondria by activating RMDN3. Our findings imply that ubiquitin signaling regulates phospholipid transport at the MERCS.

High Glucose Activated Cardiac Fibroblasts by a Disruption of Mitochondria-Associated Membranes

Cardiac fibrosis is evident even in the situation without a significant cardiomyocyte loss in diabetic cardiomyopathy and a high glucose (HG) level independently activates the cardiac fibroblasts (CFs) and promotes cell proliferation. Mitochondrial respiration and glycolysis, which are key for cell proliferation and the mitochondria-associated membranes (MAMs), are critically involved in this process. However, the roles and the underlying mechanism of MAMs in the proliferation of HG-induced CFs are largely unknown. The proliferation and apoptosis of CFs responding to HG treatment were evaluated. The MAMs were quantified, and the mitochondrial respiration and cellular glycolytic levels were determined using the Seahorse XF analyzer. The changes of signal transducer and activator of transcription 3 (STAT3) and mitofusin-2 (MFN2) in responding to HG were also determined, the effects of which on cell proliferation, MAMs, and mitochondrial respiration were assessed. The effects of STAT3 on MFN2 transcription was determined by the dual-luciferase reporter assay (DLRA) and chromatin immunoprecipitation (CHIP). HG-induced CFs proliferation increased the glycolytic levels and adenosine triphosphate (ATP) production, while mitochondrial respiration was inhibited. The MAMs and MFN2 expressions were significantly reduced on the HG treatment, and the restoration of MFN2 expression counteracted the effects of HG on cell proliferation, mitochondrial respiration of the MAMs, glycolytic levels, and ATP production. The mitochondrial STAT3 contents were not changed by HG, but the levels of phosphorylated STAT3 and nuclear STAT3 were increased. The inhibition of STAT3 reversed the reduction of MFN2 levels induced by HG. The DLRA and CHIP directly demonstrated the negative regulation of MFN2 by STAT3 at the transcription levels via interacting with the sequences in the MFN2 promoter region locating at about −400 bp counting from the start site of transcription. The present study demonstrated that the HG independently induced CFs proliferation via promoting STAT3 translocation to the nucleus, which switched the mitochondrial respiration to glycolysis to produce ATP by inhibiting MAMs in an MFN2-depression manner.