A unique mitochondria-associated membrane fraction from rat liver has a high capacity for lipid synthesis and contains pre-Golgi secretory proteins including nascent lipoproteins

The effect of mitochondrial-associated endoplasmic reticulum membranes (MAMs) modulation: New insights into therapeutic targets for depression

Depression is a prevalent mental disorder with high morbidity and mortality and its pathogenesis remains exactly unclarified. However, mitochondria and endoplasmic reticulum (ER) are two highly dynamic organelles that perform an indispensable role in the development of depression. Mitochondrial dysfunction and ER stress are recognized as vital pathological hallmarks in depression. The changes of intracellular activities such as mitochondrial dynamics, mitophagy, energy metabolism and ER stress are closely correlated with the progression of depression. Moreover, organelles interactions are conducive to homeostasis and cellular functions, and mitochondrial-associated endoplasmic reticulum membranes (MAMs) serve as signaling hubs of the two organelles and the coupling of the pathological progression. The main roles of MAMs are involved in metabolism, signal transduction, lipid transport, and maintenance of its structure and function. At present, accumulating studies elucidated that MAMs have gradually become a novel therapeutic target in treatment of depression. In the review, we focus on influence of mitochondria dysfunction and ER stress on depression. Furthermore, we discuss the underlying role of MAMs in depression and highlight natural products targeting MAMs as potential antidepressants to treat depression.

Lipid metabolism, remodelling and intercellular transfer in the CNS

Lipid metabolism encompasses the catabolism and anabolism of lipids, and is fundamental for the maintenance of cellular homeostasis, particularly within the lipid-rich CNS. Increasing evidence further underscores the importance of lipid remodelling and transfer within and between glial cells and neurons as key orchestrators of CNS lipid homeostasis. In this Review, we summarize and discuss the complex landscape of processes involved in lipid metabolism, remodelling and intercellular transfer in the CNS. Highlighted are key pathways, including those mediating lipid (and lipid droplet) biogenesis and breakdown, lipid oxidation and phospholipid metabolism, as well as cell-cell lipid transfer mediated via lipoproteins, extracellular vesicles and tunnelling nanotubes. We further explore how the dysregulation of these pathways contributes to the onset and progression of neurodegenerative diseases, and examine the homeostatic and pathogenic impacts of environment, diet and lifestyle on CNS lipid metabolism.

Endoplasmic Reticulum Stress and Its Role in Metabolic Reprogramming of Cancer

Background/Objectives: Endoplasmic reticulum (ER) stress occurs when ER homeostasis is disrupted, leading to the accumulation of misfolded or unfolded proteins. This condition activates the unfolded protein response (UPR), which aims to restore balance or trigger cell death if homeostasis cannot be achieved. In cancer, ER stress plays a key role due to the heightened metabolic demands of tumor cells. This review explores how metabolomics can provide insights into ER stress-related metabolic alterations and their implications for cancer therapy. Methods: A comprehensive literature review was conducted to analyze recent findings on ER stress, metabolomics, and cancer metabolism. Studies examining metabolic profiling of cancer cells under ER stress conditions were selected, with a focus on identifying potential biomarkers and therapeutic targets. Results: Metabolomic studies highlight significant shifts in lipid metabolism, protein synthesis, and oxidative stress management in response to ER stress. These metabolic alterations are crucial for tumor adaptation and survival. Additionally, targeting ER stress-related metabolic pathways has shown potential in preclinical models, suggesting new therapeutic strategies. Conclusions: Understanding the metabolic impact of ER stress in cancer provides valuable opportunities for drug development. Metabolomics-based approaches may help identify novel biomarkers and therapeutic targets, enhancing the effectiveness of antitumor therapies.

Mitochondria associated membranes in dilated cardiomyopathy: connecting pathogenesis and cellular dysfunction

Dilated cardiomyopathy (DCM) is a leading cause of heart failure, yet therapeutic options remain limited. While traditional research has focused on mechanisms such as energy deficits and calcium dysregulation, increasing evidence suggests that mitochondria-associated membranes (MAMs) could provide new insights into understanding and treating DCM. In this narrative review, we summarize the key role of MAMs, crucial endoplasmic reticulum (ER)-mitochondria interfaces, in regulating cellular processes such as calcium homeostasis, lipid metabolism, and mitochondrial dynamics. Disruption of MAMs function may initiate pathological cascades, including ER stress, inflammation, and cell death. These disruptions in MAM function lead to further destabilization of cellular homeostasis. Identifying MAMs as key modulators of cardiac health may provide novel insights for early diagnosis and targeted therapies in DCM.

Emerging interactions between mitochondria and NAD+ metabolism in cardiometabolic diseases

Nicotinamide adenine dinucleotide (NAD+) is an essential coenzyme for redox reactions and regulates cellular catabolic pathways. An intertwined relationship exists between NAD+ and mitochondria, with consequences for mitochondrial function. Dysregulation in NAD+ homeostasis can lead to impaired energetics and increased oxidative stress, contributing to the pathogenesis of cardiometabolic diseases. In this review, we explore how disruptions in NAD+ homeostasis impact mitochondrial function in various cardiometabolic diseases. We discuss emerging studies demonstrating that enhancing NAD+ synthesis or inhibiting its consumption can ameliorate complications of this family of pathological conditions. Additionally, we highlight the potential role and therapeutic promise of mitochondrial NAD+ transporters in regulating cellular and mitochondrial NAD+ homeostasis.

The roles of mitochondria in global and local intracellular calcium signalling

Activation of Ca2+ channels in Ca2+ stores in organelles and the plasma membrane generates cytoplasmic calcium ([Ca2+]c) signals that control almost every aspect of cell function, including metabolism, vesicle fusion and contraction. Mitochondria have a high capacity for Ca2+ uptake and chelation, alongside efficient Ca2+ release mechanisms. Still, mitochondria do not store Ca2+ in a prolonged manner under physiological conditions and lack the capacity to generate global [Ca2+]c signals. However, mitochondria take up Ca2+ at high local [Ca2+]c signals that originate from neighbouring organelles, and also during sustained global elevations of [Ca2+]c. Accumulated Ca2+ in the mitochondria stimulates oxidative metabolism and upon return to the cytoplasm, can produce spatially confined rises in [Ca2+]c to exert control over processes that are sensitive to Ca2+. Thus, the mitochondrial handling of [Ca2+]c is of physiological relevance. Furthermore, dysregulation of mitochondrial Ca2+ handling can contribute to debilitating diseases. We discuss the mechanisms and relevance of mitochondria in local and global calcium signals.

Mitochondria-Associated Endoplasmic Reticulum Membranes in Microglia: One Contact Site to Rule Them all

Microglia, the resident immune cells of the central nervous system (CNS), play a crucial role in maintaining tissue homeostasis by monitoring and responding to environmental changes through processes such as phagocytosis, cytokine production or synapse remodeling. Their dynamic nature and diverse functions are supported by the regulation of multiple metabolic pathways, enabling microglia to efficiently adapt to fluctuating signals. A key aspect of this regulation occurs at mitochondria-associated ER membranes (MAM), specialized contact sites between the ER and mitochondria. These structures facilitate the exchange of calcium, lipids, and metabolites and serve as metabolic and signaling hubs. This review synthesizes current research on how MAM influence microglial physiology, with an emphasis on their role in immunometabolism, offering new insights into the integration of metabolic and immune functions in the CNS and its impact in the context of neurodegeneration.

Mitochondrial dysfunction in Parkinson's disease

The exact cause of nigral cell death in Parkinson's disease (PD) is still unknown. However, research on MPTP-induced experimental parkinsonism has significantly advanced our understanding. In this model, it is widely accepted that mitochondrial respiratory failure is the primary mechanism of cell death. Studies have shown that a toxic metabolite of MPTP inhibits Complex I and alpha-ketoglutarate dehydrogenase activities in mitochondria. Since then, many research groups have focused on mitochondrial dysfunction in PD, identifying deficiencies in Complex I or III in PD patients' brains, skeletal muscle, and platelets. There is some debate about the decline in mitochondrial function in peripheral organs. However, since α-synuclein, the main component protein of Lewy bodies, accumulates in peripheral organs, it is reasonable to consider PD a systemic disease. Additionally, mutant mitochondrial DNA with a 4,977 base pair deletion has been found in the brains of PD patients, suggesting that age-related accumulation of deleted mtDNA is accelerated in the striatum and may contribute to the pathophysiology of PD. While the cause of PD remains unknown, mitochondrial dysfunction is undoubtedly a factor in cell death in PD. In addition, the causative gene for familial PD, parkin (now PRKN), and PTEN-induced putative kinase 1 (PINK1), both gene products are also involved in mitochondrial quality control. Moreover, we have successfully isolated and identified CHCHD2, which is involved in the mitochondrial electron transfer system. There is no doubt that mitochondrial dysfunction contributes to cell death in PD.

MAMs and Mitochondrial Quality Control: Overview and Their Role in Alzheimer's Disease

Alzheimer's disease (AD) represents the most widespread neurodegenerative disorder, distinguished by a gradual onset and slow progression, presenting a substantial challenge to global public health. The mitochondrial-associated membrane (MAMs) functions as a crucial center for signal transduction and material transport between mitochondria and the endoplasmic reticulum, playing a pivotal role in various pathological mechanisms of AD. The dysregulation of mitochondrial quality control systems is considered a fundamental factor in the development of AD, leading to mitochondrial dysfunction and subsequent neurodegenerative events. Recent studies have emphasized the role of MAMs in regulating mitochondrial quality control. This review will delve into the molecular mechanisms underlying the imbalance in mitochondrial quality control in AD and provide a comprehensive overview of the role of MAMs in regulating mitochondrial quality control.

Endoplasmic reticulum: Monitoring and maintaining protein and membrane homeostasis in the endoplasmic reticulum by the unfolded protein response

The endoplasmic reticulum (ER) regulates essential cellular processes, including protein folding, lipid synthesis, and calcium homeostasis. The ER homeostasis is maintained by a conserved set of signaling cascades called the Unfolded Protein Response (UPR). How the UPR senses perturbations in ER homeostasis has been the subject of active research for decades. In metazoans, the UPR consists of three ER-membrane embedded sensors: IRE1, PERK and ATF6. These sensors detect the accumulation of misfolded proteins in the ER lumen and adjust protein folding capacity according to cellular needs. Early work revealed that the ER-resident chaperone BiP binds to all three UPR sensors in higher eukaryotes and BiP binding was suggested to regulate their activity. More recent data have shown that in higher eukaryotes the interaction of the UPR sensors with a complex network of chaperones and misfolded proteins modulates their activation and deactivation dynamics. Furthermore, emerging evidence suggests that the UPR monitors ER membrane integrity beyond protein folding defects. However, the mechanistic and structural basis of UPR activation by proteotoxic and lipid bilayer stress in higher eukaryotes remains only partially understood. Here, we review the current understanding of novel protein interaction networks and the contribution of the lipid membrane environment to UPR activation.

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.

A-Syn(ful) MAM: A Fresh Perspective on a Converging Domain in Parkinson's Disease

Parkinson's disease (PD) is a disease of an unknown origin. Despite that, decades of research have provided considerable evidence that alpha-synuclein (αSyn) is central to the pathogenesis of disease. Mitochondria-associated endoplasmic reticulum (ER) membranes (MAMs) are functional domains formed at contact sites between the ER and mitochondria, with a well-established function of MAMs being the control of lipid homeostasis within the cell. Additionally, there are numerous proteins localized or enriched at MAMs that have regulatory roles in several different molecular signaling pathways required for cellular homeostasis, such as autophagy and neuroinflammation. Alterations in several of these signaling pathways that are functionally associated with MAMs are found in PD. Taken together with studies that find αSyn localized at MAMs, this has implicated MAM (dys)function as a converging domain relevant to PD. This review will highlight the many functions of MAMs and provide an overview of the literature that finds αSyn, in addition to several other PD-related proteins, localized there. This review will also detail the direct interaction of αSyn and αSyn-interacting partners with specific MAM-resident proteins. In addition, recent studies exploring new methods to investigate MAMs will be discussed, along with some of the controversies regarding αSyn, including its several conformations and subcellular localizations. The goal of this review is to highlight and provide insight on a domain that is incompletely understood and, from a PD perspective, highlight those complex interactions that may hold the key to understanding the pathomechanisms underlying PD, which may lead to the targeted development of new therapeutic strategies.

Aberrant ER-mitochondria communication is a common pathomechanism in mitochondrial disease

Genetic mutations causing primary mitochondrial disease (i.e those compromising oxidative phosphorylation [OxPhos]) resulting in reduced bioenergetic output display great variability in their clinical features, but the reason for this is unknown. We hypothesized that disruption of the communication between endoplasmic reticulum (ER) and mitochondria at mitochondria-associated ER membranes (MAM) might play a role in this variability. To test this, we assayed MAM function and ER-mitochondrial communication in OxPhos-deficient cells, including cybrids from patients with selected pathogenic mtDNA mutations. Our results show that each of the various mutations studied indeed altered MAM functions, but notably, each disorder presented with a different MAM "signature". We also found that mitochondrial membrane potential is a key driver of ER-mitochondrial connectivity. Moreover, our findings demonstrate that disruption in ER-mitochondrial communication has consequences for cell survivability that go well beyond that of reduced ATP output. The findings of a "MAM-OxPhos" axis, the role of mitochondrial membrane potential in controlling this process, and the contribution of MAM dysfunction to cell death, reveal a new relationship between mitochondria and the rest of the cell, as well as providing new insights into the diagnosis and treatment of these devastating disorders.

Ethanol-Induced Hepatic Ferroptosis Is Mediated by PERK-Dependent MAMs Formation: Preventive Role of Quercetin

SCOPE

Iron deposition is frequently observed in alcoholic liver disease (ALD), which indicates a potential role of ferroptosis in its development. This study aims to explore the effects of quercetin on ferroptosis in ALD and elucidates the underlying mechanism involving the formation of mitochondria-associated endoplasmic reticulum membranes (MAMs) mediated by protein kinase RNA-like endoplasmic reticulum kinase (PERK).

METHODS AND RESULTS

C57BL/6J mice are fed either a regular or an ethanol-containing liquid diet (with 28% energy form ethanol) with or without quercetin supplementation (100 mg kg-1 BW) for 12 weeks. Ethanol feeding or treatment induced ferroptosis in mice and AML12 cells, which is associated with increased MAMs formation and PERK expression within MAMs. Quercetin attenuates these changes and protects against ethanol-induced liver injury. The antiferroptotic effect of quercetin is abolished by ferroptosis inducers, but mimicked by ferroptosis inhibitors and PERK knockdown. The study demonstrates that PERK structure, rather than its kinase activity (transfected with the K618A site mutation that inhibits kinase activity-ΔK plasmid or protein C terminal knockout-ΔC plasmid of PERK), mediates the enhanced MAMs formation and ferroptosis during the ethanol exposure.

CONCLUSION

Quercetin ameliorates ethanol-induced liver injury by inhibiting ferroptosis via modulating PERK-dependent MAMs formation.

Mitochondria-associated endoplasmic reticulum membrane (MAM): a dark horse for diabetic cardiomyopathy treatment

Diabetic cardiomyopathy (DCM), an important complication of diabetes mellitus (DM), is one of the most serious chronic heart diseases and has become a major cause of heart failure worldwide. At present, the pathogenesis of DCM is unclear, and there is still a lack of effective therapeutics. Previous studies have shown that the homeostasis of mitochondria and the endoplasmic reticulum (ER) play a core role in maintaining cardiovascular function, and structural and functional abnormalities in these organelles seriously impact the occurrence and development of various cardiovascular diseases, including DCM. The interplay between mitochondria and the ER is mediated by the mitochondria-associated ER membrane (MAM), which participates in regulating energy metabolism, calcium homeostasis, mitochondrial dynamics, autophagy, ER stress, inflammation, and other cellular processes. Recent studies have proven that MAM is closely related to the initiation and progression of DCM. In this study, we aim to summarize the recent research progress on MAM, elaborate on the key role of MAM in DCM, and discuss the potential of MAM as an important therapeutic target for DCM, thereby providing a theoretical reference for basic and clinical studies of DCM treatment.

Nitric oxide-induced endoplasmic reticulum stress of Schistosoma japonicum inhibits the worm development in rats

Schistosomiasis, caused by Schistosoma spp., is a zoonotic parasitic disease affecting human health. Rattus norvegicus (rats) are a non-permissive host of Schistosoma, in which the worms cannot mature and cause typical egg granuloma. We previously demonstrated that inherent high levels of nitric oxide (NO), produced by inducible NO synthase (iNOS), is a key molecule in blocking the development of S. japonicum in rats. To further explore the mechanism of NO inhibiting S. japonicum development in rats, we performed S-nitrosocysteine proteomics of S. japonicum collected from infected rats and mice. The results suggested that S. japonicum in rats may have undergone endoplasmic reticulum (ER) stress. Interestingly, we found that the ER of S. japonicum in rats showed marked damage, while the ER of the worm in iNOS-/- rats and mice were relatively normal. Moreover, the expression of ER stress markers in S. japonicum from WT rats was significantly increased, compared with S. japonicum from iNOS-/- rats and mice. Using the NO donor sodium nitroprusside in vitro, we demonstrated that NO could induce ER stress in S. japonicum in a dose-dependent manner, and the NO-induced ER stress in S. japonicum could be inhibited by ER stress inhibitor 4-Phenyl butyric acid. We further verified that inhibiting ER stress of S. japonicum in rats promoted parasite development and survival. Furthermore, we demonstrated that NO-induced ER stress of S. japonicum was related to the efflux of Ca2+ from ER and the impairment of mitochondrial function. Collectively, these findings show that high levels of NO in rats could induce ER stress in S. japonicum by promoting the efflux of Ca2+ from ER and damaging the mitochondrial function, which block the worm development. Thus, this study further clarifies the mechanism of anti-schistosome in rats and provides potential strategies for drug development against schistosomiasis and other parasitosis.

Relevance of the endoplasmic reticulum-mitochondria axis in cancer diagnosis and therapy

Dynamic interactions between organelles are responsible for a variety of intercellular functions, and the endoplasmic reticulum (ER)-mitochondrial axis is recognized as a representative interorganelle system. Several studies have confirmed that most proteins in the physically tethered sites between the ER and mitochondria, called mitochondria-associated ER membranes (MAMs), are vital for intracellular physiology. MAM proteins are involved in the regulation of calcium homeostasis, lipid metabolism, and mitochondrial dynamics and are associated with processes related to intracellular stress conditions, such as oxidative stress and unfolded protein responses. Accumulating evidence has shown that, owing to their extensive involvement in cellular homeostasis, alterations in the ER-mitochondrial axis are one of the etiological factors of tumors. An in-depth understanding of MAM proteins and their impact on cell physiology, particularly in cancers, may help elucidate their potential as diagnostic and therapeutic targets for cancers. For example, the modulation of MAM proteins is utilized not only to target diverse intracellular signaling pathways within cancer cells but also to increase the sensitivity of cancer cells to anticancer reagents and regulate immune cell activities. Therefore, the current review summarizes and discusses recent advances in research on the functional roles of MAM proteins and their characteristics in cancers from a diagnostic perspective. Additionally, this review provides insights into diverse therapeutic strategies that target MAM proteins in various cancer types.

Towards a Unitary Hypothesis of Alzheimer's Disease Pathogenesis

The "amyloid cascade" hypothesis of Alzheimer's disease (AD) pathogenesis invokes the accumulation in the brain of plaques (containing the amyloid-β protein precursor [AβPP] cleavage product amyloid-β [Aβ]) and tangles (containing hyperphosphorylated tau) as drivers of pathogenesis. However, the poor track record of clinical trials based on this hypothesis suggests that the accumulation of these peptides is not the only cause of AD. Here, an alternative hypothesis is proposed in which the AβPP cleavage product C99, not Aβ, is the main culprit, via its role as a regulator of cholesterol metabolism. C99, which is a cholesterol sensor, promotes the formation of mitochondria-associated endoplasmic reticulum (ER) membranes (MAM), a cholesterol-rich lipid raft-like subdomain of the ER that communicates, both physically and biochemically, with mitochondria. We propose that in early-onset AD (EOAD), MAM-localized C99 is elevated above normal levels, resulting in increased transport of cholesterol from the plasma membrane to membranes of intracellular organelles, such as ER/endosomes, thereby upregulating MAM function and driving pathology. By the same token, late-onset AD (LOAD) is triggered by any genetic variant that increases the accumulation of intracellular cholesterol that, in turn, boosts the levels of C99 and again upregulates MAM function. Thus, the functional cause of AD is upregulated MAM function that, in turn, causes the hallmark disease phenotypes, including the plaques and tangles. Accordingly, the MAM hypothesis invokes two key interrelated elements, C99 and cholesterol, that converge at the MAM to drive AD pathogenesis. From this perspective, AD is, at bottom, a lipid disorder.

Mitochondrial Proteomes in Neural Cells: A Systematic Review

Mitochondria are ancient endosymbiotic double membrane organelles that support a wide range of eukaryotic cell functions through energy, metabolism, and cellular control. There are over 1000 known proteins that either reside within the mitochondria or are transiently associated with it. These mitochondrial proteins represent a functional subcellular protein network (mtProteome) that is encoded by mitochondrial and nuclear genomes and significantly varies between cell types and conditions. In neurons, the high metabolic demand and differential energy requirements at the synapses are met by specific modifications to the mtProteome, resulting in alterations in the expression and functional properties of the proteins involved in energy production and quality control, including fission and fusion. The composition of mtProteomes also impacts the localization of mitochondria in axons and dendrites with a growing number of neurodegenerative diseases associated with changes in mitochondrial proteins. This review summarizes the findings on the composition and properties of mtProteomes important for mitochondrial energy production, calcium and lipid signaling, and quality control in neural cells. We highlight strategies in mass spectrometry (MS) proteomic analysis of mtProteomes from cultured cells and tissue. The research into mtProteome composition and function provides opportunities in biomarker discovery and drug development for the treatment of metabolic and neurodegenerative disease.

Lipocalin 2 regulates mitochondrial phospholipidome remodeling, dynamics, and function in brown adipose tissue in male mice

Mitochondrial function is vital for energy metabolism in thermogenic adipocytes. Impaired mitochondrial bioenergetics in brown adipocytes are linked to disrupted thermogenesis and energy balance in obesity and aging. Phospholipid cardiolipin (CL) and phosphatidic acid (PA) jointly regulate mitochondrial membrane architecture and dynamics, with mitochondria-associated endoplasmic reticulum membranes (MAMs) serving as the platform for phospholipid biosynthesis and metabolism. However, little is known about the regulators of MAM phospholipid metabolism and their connection to mitochondrial function. We discover that LCN2 is a PA binding protein recruited to the MAM during inflammation and metabolic stimulation. Lcn2 deficiency disrupts mitochondrial fusion-fission balance and alters the acyl-chain composition of mitochondrial phospholipids in brown adipose tissue (BAT) of male mice. Lcn2 KO male mice exhibit an increase in the levels of CLs containing long-chain polyunsaturated fatty acids (LC-PUFA), a decrease in CLs containing monounsaturated fatty acids, resulting in mitochondrial dysfunction. This dysfunction triggers compensatory activation of peroxisomal function and the biosynthesis of LC-PUFA-containing plasmalogens in BAT. Additionally, Lcn2 deficiency alters PA production, correlating with changes in PA-regulated phospholipid-metabolizing enzymes and the mTOR signaling pathway. In conclusion, LCN2 plays a critical role in the acyl-chain remodeling of phospholipids and mitochondrial bioenergetics by regulating PA production and its function in activating signaling pathways.

Age-Dependent Surface Receptor Expression Patterns in Immature Versus Mature Platelets in Mouse Models of Regenerative Thrombocytopenia

Aging is a multifaceted process that unfolds at both the individual and cellular levels, resulting in changes in platelet count and platelet reactivity. These alterations are influenced by shifts in platelet production, as well as by various environmental factors that affect circulating platelets. Aging also triggers functional changes in platelets, including a reduction in RNA content and protein production capacity. Older individuals and RNA-rich immature platelets often exhibit hyperactivity, contributing significantly to pathologic conditions such as cardiovascular diseases, sepsis, and thrombosis. However, the impact of aging on surface receptor expression of circulating platelets, particularly whether these effects vary between immature and mature platelets, remains largely unexplored. Thus, we investigated the expression of certain surface and activation receptors on platelets from young and old mice as well as on immature and mature platelets from mouse models of regenerative thrombocytopenia by flow cytometry. Our findings indicate that aged mice show an upregulated expression of the platelet endothelial cell adhesion molecule-1 (CD31), tetraspanin-29 (CD9), and Toll-like receptor 2 (TLR2) compared to their younger counterparts. Interestingly, when comparing immature and mature platelets in both young and old mice, no differences were observed in mature platelets. However, immature platelets from young mice displayed higher surface expression compared to immature platelets from old mice. Additionally, in mouse models of regenerative thrombocytopenia, the majority of receptors were upregulated in immature platelets. These results suggest that distinct surface receptor expressions are increased on platelets from old mice and immature platelets, which may partially explain their heightened activity and contribute to an increased thrombotic risk.

Glycosphingolipids within membrane contact sites influence their function as signaling hubs in neurodegenerative diseases

Intracellular organelles carry out many of their functions by engaging in extensive interorganellar communication through specialized membrane contact sites (MCSs) formed where two organelles tether to each other or to the plasma membrane (PM) without fusing. In recent years, these ubiquitous membrane structures have emerged as central signaling hubs that control a multitude of cellular pathways, ranging from lipid metabolism/transport to the exchange of metabolites and ions (i.e., Ca2+ ), and general organellar biogenesis. The functional crosstalk between juxtaposed membranes at MCSs relies on a defined composite of proteins and lipids that populate these microdomains in a dynamic fashion. This is particularly important in the nervous system, where alterations in the composition of MCSs have been shown to affect their functions and have been implicated in the pathogenesis of neurodegenerative diseases. In this review, we focus on the MCSs that are formed by the tethering of the endoplasmic reticulum (ER) to the mitochondria, the ER to the endo-lysosomes and the mitochondria to the lysosomes. We highlight how glycosphingolipids that are aberrantly processed/degraded and accumulate ectopically in intracellular membranes and the PM change the topology of MCSs, disrupting signaling pathways that lead to neuronal demise and neurodegeneration. In particular, we focus on neurodegenerative lysosomal storage diseases linked to altered glycosphingolipid catabolism.

Role of Mitochondria-ER Contact Sites in Mitophagy

Mitochondria are often referred to as the "powerhouse" of the cell. However, this organelle has many more functions than simply satisfying the cells' metabolic needs. Mitochondria are involved in calcium homeostasis and lipid metabolism, and they also regulate apoptotic processes. Many of these functions require contact with the ER, which is mediated by several tether proteins located on the respective organellar surfaces, enabling the formation of mitochondria-ER contact sites (MERCS). Upon damage, mitochondria produce reactive oxygen species (ROS) that can harm the surrounding cell. To circumvent toxicity and to maintain a functional pool of healthy organelles, damaged and excess mitochondria can be targeted for degradation via mitophagy, a form of selective autophagy. Defects in mitochondria-ER tethers and the accumulation of damaged mitochondria are found in several neurodegenerative diseases, including Parkinson's disease and amyotrophic lateral sclerosis, which argues that the interplay between the two organelles is vital for neuronal health. This review provides an overview of the different mechanisms of mitochondrial quality control that are implicated with the different mitochondria-ER tether proteins, and also provides a novel perspective on how MERCS are involved in mediating mitophagy upon mitochondrial damage.

The Role of Mitochondrial Dynamics and Mitotic Fission in Regulating the Cell Cycle in Cancer and Pulmonary Arterial Hypertension: Implications for Dynamin-Related Protein 1 and Mitofusin2 in Hyperproliferative Diseases

Mitochondria, which generate ATP through aerobic respiration, also have important noncanonical functions. Mitochondria are dynamic organelles, that engage in fission (division), fusion (joining) and translocation. They also regulate intracellular calcium homeostasis, serve as oxygen-sensors, regulate inflammation, participate in cellular and organellar quality control and regulate the cell cycle. Mitochondrial fission is mediated by the large GTPase, dynamin-related protein 1 (Drp1) which, when activated, translocates to the outer mitochondrial membrane (OMM) where it interacts with binding proteins (Fis1, MFF, MiD49 and MiD51). At a site demarcated by the endoplasmic reticulum, fission proteins create a macromolecular ring that divides the organelle. The functional consequence of fission is contextual. Physiological fission in healthy, nonproliferating cells mediates organellar quality control, eliminating dysfunctional portions of the mitochondria via mitophagy. Pathological fission in somatic cells generates reactive oxygen species and triggers cell death. In dividing cells, Drp1-mediated mitotic fission is critical to cell cycle progression, ensuring that daughter cells receive equitable distribution of mitochondria. Mitochondrial fusion is regulated by the large GTPases mitofusin-1 (Mfn1) and mitofusin-2 (Mfn2), which fuse the OMM, and optic atrophy 1 (OPA-1), which fuses the inner mitochondrial membrane. Mitochondrial fusion mediates complementation, an important mitochondrial quality control mechanism. Fusion also favors oxidative metabolism, intracellular calcium homeostasis and inhibits cell proliferation. Mitochondrial lipids, cardiolipin and phosphatidic acid, also regulate fission and fusion, respectively. Here we review the role of mitochondrial dynamics in health and disease and discuss emerging concepts in the field, such as the role of central versus peripheral fission and the potential role of dynamin 2 (DNM2) as a fission mediator. In hyperproliferative diseases, such as pulmonary arterial hypertension and cancer, Drp1 and its binding partners are upregulated and activated, positing mitochondrial fission as an emerging therapeutic target.

Altered GM1 catabolism affects NMDAR-mediated Ca 2+ signaling at ER-PM junctions and increases synaptic spine formation

Endoplasmic reticulum-plasma membrane (ER-PM) junctions mediate Ca 2+ flux across neuronal membranes. The properties of these membrane contact sites are defined by their lipid content, but little attention has been given to glycosphingolipids (GSLs). Here, we show that GM1-ganglioside, an abundant GSL in neuronal membranes, is integral to ER-PM junctions; it interacts with synaptic proteins/receptors and regulates Ca 2+ signaling. In a model of the neurodegenerative lysosomal storage disease, GM1-gangliosidosis, pathogenic accumulation of GM1 at ER-PM junctions due to β-galactosidase deficiency drastically alters neuronal Ca 2+ homeostasis. Mechanistically, we show that GM1 interacts with the phosphorylated NMDAR Ca 2+ channel, thereby increasing Ca 2+ flux, activating ERK signaling, and increasing the number of synaptic spines without increasing synaptic connectivity. Thus, GM1 clustering at ER-PM junctions alters synaptic plasticity and exacerbates the generalized neuronal cell death characteristic of GM1-gangliosidosis.

Inhibition of ACAT as a Therapeutic Target for Alzheimer's Disease Is Independent of ApoE4 Lipidation

APOE4, encoding apolipoprotein E4 (apoE4), is the greatest genetic risk factor for Alzheimer's disease (AD), compared to the common APOE3. While the mechanism(s) underlying APOE4-induced AD risk remains unclear, increasing the lipidation of apoE4 is an important therapeutic target as apoE4-lipoproteins are poorly lipidated compared to apoE3-lipoproteins. ACAT (acyl-CoA: cholesterol-acyltransferase) catalyzes the formation of intracellular cholesteryl-ester droplets, reducing the intracellular free cholesterol (FC) pool. Thus, inhibiting ACAT increases the FC pool and facilitates lipid secretion to extracellular apoE-containing lipoproteins. Previous studies using commercial ACAT inhibitors, including avasimibe (AVAS), as well as ACAT-knock out (KO) mice, exhibit reduced AD-like pathology and amyloid precursor protein (APP) processing in familial AD (FAD)-transgenic (Tg) mice. However, the effects of AVAS with human apoE4 remain unknown. In vitro, AVAS induced apoE efflux at concentrations of AVAS measured in the brains of treated mice. AVAS treatment of male E4FAD-Tg mice (5xFAD+/-APOE4+/+) at 6-8 months had no effect on plasma cholesterol levels or distribution, the original mechanism for AVAS treatment of CVD. In the CNS, AVAS reduced intracellular lipid droplets, indirectly demonstrating target engagement. Surrogate efficacy was demonstrated by an increase in Morris water maze measures of memory and postsynaptic protein levels. Amyloid-beta peptide (Aβ) solubility/deposition and neuroinflammation were reduced, critical components of APOE4-modulated pathology. However, there was no increase in apoE4 levels or apoE4 lipidation, while amyloidogenic and non-amyloidogenic processing of APP were significantly reduced. This suggests that the AVAS-induced reduction in Aβ via reduced APP processing was sufficient to reduce AD pathology, as apoE4-lipoproteins remained poorly lipidated.

Organelle interactions compartmentalize hepatic fatty acid trafficking and metabolism

Organelle interactions play a significant role in compartmentalizing metabolism and signaling. Lipid droplets (LDs) interact with numerous organelles, including mitochondria, which is largely assumed to facilitate lipid transfer and catabolism. However, quantitative proteomics of hepatic peridroplet mitochondria (PDM) and cytosolic mitochondria (CM) reveals that CM are enriched in proteins comprising various oxidative metabolism pathways, whereas PDM are enriched in proteins involved in lipid anabolism. Isotope tracing and super-resolution imaging confirms that fatty acids (FAs) are selectively trafficked to and oxidized in CM during fasting. In contrast, PDM facilitate FA esterification and LD expansion in nutrient-replete medium. Additionally, mitochondrion-associated membranes (MAM) around PDM and CM differ in their proteomes and ability to support distinct lipid metabolic pathways. We conclude that CM and CM-MAM support lipid catabolic pathways, whereas PDM and PDM-MAM allow hepatocytes to efficiently store excess lipids in LDs to prevent lipotoxicity.

Hakuna MAM-Tata: Investigating the role of mitochondrial-associated membranes in ALS

Amyotrophic lateral sclerosis (ALS) is a devastating neurodegenerative disease leading to selective and progressive motor neuron (MN) death. Despite significant heterogeneity in pathogenic and clinical terms, MN demise ultimately unifies patients. Across the many disturbances in neuronal biology present in the disease and its models, two common trends are loss of calcium homeostasis and dysregulations in lipid metabolism. Since both mitochondria and endoplasmic reticulum (ER) are essential in these functions, their intertwin through the so-called mitochondrial-associated membranes (MAMs) should be relevant in this disease. In this review, we present a short overview of MAMs functional aspects and how its dysfunction could explain a substantial part of the cellular disarrangements in ALS's natural history. MAMs are hubs for lipid synthesis, integrating glycerophospholipids, sphingolipids, and cholesteryl ester metabolism. These lipids are essential for membrane biology, so there should be a close coupling to cellular energy demands, a role that MAMs may partially fulfill. Not surprisingly, MAMs are also host part of calcium signaling to mitochondria, so their impairment could lead to mitochondrial dysfunction, affecting oxidative phosphorylation and enhancing the vulnerability of MNs. We present data supporting that MAMs' maladaptation could be essential to MNs' vulnerability in ALS.

Acute ACAT1/SOAT1 Blockade Increases MAM Cholesterol and Strengthens ER-Mitochondria Connectivity

Cholesterol is a key component of all mammalian cell membranes. Disruptions in cholesterol metabolism have been observed in the context of various diseases, including neurodegenerative disorders such as Alzheimer's disease (AD). The genetic and pharmacological blockade of acyl-CoA:cholesterol acyltransferase 1/sterol O-acyltransferase 1 (ACAT1/SOAT1), a cholesterol storage enzyme found on the endoplasmic reticulum (ER) and enriched at the mitochondria-associated ER membrane (MAM), has been shown to reduce amyloid pathology and rescue cognitive deficits in mouse models of AD. Additionally, blocking ACAT1/SOAT1 activity stimulates autophagy and lysosomal biogenesis; however, the exact molecular connection between the ACAT1/SOAT1 blockade and these observed benefits remain unknown. Here, using biochemical fractionation techniques, we observe cholesterol accumulation at the MAM which leads to ACAT1/SOAT1 enrichment in this domain. MAM proteomics data suggests that ACAT1/SOAT1 inhibition strengthens the ER-mitochondria connection. Confocal and electron microscopy confirms that ACAT1/SOAT1 inhibition increases the number of ER-mitochondria contact sites and strengthens this connection by shortening the distance between these two organelles. This work demonstrates how directly manipulating local cholesterol levels at the MAM can alter inter-organellar contact sites and suggests that cholesterol buildup at the MAM is the impetus behind the therapeutic benefits of ACAT1/SOAT1 inhibition.

Inhibition of ferroptosis through regulating neuronal calcium homeostasis: An emerging therapeutic target for Alzheimer's disease

Alzheimer's disease (AD), a chronic and progressive neurodegenerative disease, generates a serious threat to the health of the elderly. The AD brain is microscopically characterized by amyloid plaques and neurofibrillary tangles. There are still no effective therapeutic drugs to restrain the progression of AD though much attention has been paid to exploit AD treatments. Ferroptosis, a type of programmed cell death, has been reported to promote the pathological occurrence and development of AD, and inhibition of neuronal ferroptosis can effectively improve the cognitive impairment of AD. Studies have shown that calcium (Ca²⁺) dyshomeostasis is closely related to the pathology of AD, and can drive the occurrence of ferroptosis through several pathways, such as interacting with iron, and regulating the crosstalk between endoplasmic reticulum (ER) and mitochondria. This paper mainly reviews the roles of ferroptosis and Ca²⁺ in the pathology of AD, and highlights that restraining ferroptosis through maintaining the homeostasis of Ca²⁺ may be an innovative target for the treatment of AD.

Comparison among Neuroblastoma Stages Suggests the Involvement of Mitochondria in Tumor Progression

Neuroblastoma (NB) is the most common extracranial tumor of early childhood and accounts for 15% of all pediatric cancer mortalities. However, the precise pathways and genes underlying its progression are unknown. Therefore, we performed a differential gene expression analysis of neuroblastoma stage 1 and stage 4 + 4S to discover biological processes associated with NB progression. From this preliminary analysis, we found that NB samples (stage 4 + 4S) are characterized by altered expression of some proteins involved in mitochondria function and mitochondria-ER contact sites (MERCS). Although further analyses remain necessary, this review may provide new hints to better understand NB molecular etiopathogenesis, by suggesting that MERCS alterations could be involved in the progression of NB.

Mitochondria-endoplasmic reticulum contacts in sepsis-induced myocardial dysfunction

Mitochondrial and endoplasmic reticulum (ER) are important intracellular organelles. The sites that mitochondrial and ER are closely related in structure and function are called Mitochondria-ER contacts (MERCs). MERCs are involved in a variety of biological processes, including calcium signaling, lipid synthesis and transport, autophagy, mitochondrial dynamics, ER stress, and inflammation. Sepsis-induced myocardial dysfunction (SIMD) is a vital organ damage caused by sepsis, which is closely associated with mitochondrial and ER dysfunction. Growing evidence strongly supports the role of MERCs in the pathogenesis of SIMD. In this review, we summarize the biological functions of MERCs and the roles of MERCs proteins in SIMD.

Palmitate and thapsigargin have contrasting effects on ER membrane lipid composition and ER proteostasis in neuronal cells

The endoplasmic reticulum (ER) is an organelle that performs several key functions such as protein synthesis and folding, lipid metabolism and calcium homeostasis. When these functions are disrupted, such as upon protein misfolding, ER stress occurs. ER stress can trigger adaptive responses to restore proper functioning such as activation of the unfolded protein response (UPR). In certain cells, the free fatty acid palmitate has been shown to induce the UPR. Here, we examined the effects of palmitate on UPR gene expression in a human neuronal cell line and compared it with thapsigargin, a known depletor of ER calcium and trigger of the UPR. We used a Gaussia luciferase-based reporter to assess how palmitate treatment affects ER proteostasis and calcium homeostasis in the cells. We also investigated how ER calcium depletion by thapsigargin affects lipid membrane composition by performing mass spectrometry on subcellular fractions and compared this to palmitate. Surprisingly, palmitate treatment did not activate UPR despite prominent changes to membrane phospholipids. Conversely, thapsigargin induced a strong UPR, but did not significantly change the membrane lipid composition in subcellular fractions. In summary, our data demonstrate that changes in membrane lipid composition and disturbances in ER calcium homeostasis have a minimal influence on each other in neuronal cells. These data provide new insight into the adaptive interplay of lipid homeostasis and proteostasis in the cell.

Balancing life and death: BCL-2 family members at diverse ER-mitochondrial contact sites

The outer mitochondrial membrane is a busy place. One essential activity for cellular survival is the regulation of membrane integrity by the BCL-2 family of proteins. Another critical facet of the outer mitochondrial membrane is its close approximation with the endoplasmic reticulum. These mitochondrial-associated membranes (MAMs) occupy a significant fraction of the mitochondrial surface and serve as key signaling hubs for multiple cellular processes. Each of these pathways may be considered as forming their own specialized MAM subtype. Interestingly, like membrane permeabilization, most of these pathways play critical roles in regulating cellular survival and death. Recently, the pro-apoptotic BCL-2 family member BOK has been found within MAMs where it plays important roles in their structure and function. This has led to a greater appreciation that multiple BCL-2 family proteins, which are known to participate in numerous functions throughout the cell, also have roles within MAMs. In this review, we evaluate several MAM subsets, their role in cellular homeostasis, and the contribution of BCL-2 family members to their functions.

Proteomic Analysis of Retinal Mitochondria-Associated ER Membranes Identified Novel Proteins of Retinal Degeneration in Long-Term Diabetes

The mitochondria-associated endoplasmic reticulum (ER) membrane (MAM) is the physical contact site between the ER and the mitochondria and plays a vital role in the regulation of calcium signaling, bioenergetics, and inflammation. Disturbances in these processes and dysregulation of the ER and mitochondrial homeostasis contribute to the pathogenesis of diabetic retinopathy (DR). However, few studies have examined the impact of diabetes on the retinal MAM and its implication in DR pathogenesis. In the present study, we investigated the proteomic changes in retinal MAM from Long Evans rats with streptozotocin-induced long-term Type 1 diabetes. Furthermore, we performed in-depth bioinformatic analysis to identify key MAM proteins and pathways that are potentially implicated in retinal inflammation, angiogenesis, and neurodegeneration. A total of 2664 unique proteins were quantified using IonStar proteomics-pipeline in rat retinal MAM, among which 179 proteins showed significant changes in diabetes. Functional annotation revealed that the 179 proteins are involved in important biological processes such as cell survival, inflammatory response, and cellular maintenance, as well as multiple disease-relevant signaling pathways, e.g., integrin signaling, leukocyte extravasation, PPAR, PTEN, and RhoGDI signaling. Our study provides comprehensive information on MAM protein changes in diabetic retinas, which is helpful for understanding the mechanisms of metabolic dysfunction and retinal cell injury in DR.

The Adhesion GPCR VLGR1/ADGRV1 Regulates the Ca2+ Homeostasis at Mitochondria-Associated ER Membranes

The very large G protein-coupled receptor (VLGR1, ADGRV1) is the largest member of the adhesion GPCR family. Mutations in VLGR1 have been associated with the human Usher syndrome (USH), the most common form of inherited deaf-blindness as well as childhood absence epilepsy. VLGR1 was previously found as membrane–membrane adhesion complexes and focal adhesions. Affinity proteomics revealed that in the interactome of VLGR1, molecules are enriched that are associated with both the ER and mitochondria, as well as mitochondria-associated ER membranes (MAMs), a compartment at the contact sites of both organelles. We confirmed the interaction of VLGR1 with key proteins of MAMs by pull-down assays in vitro complemented by in situ proximity ligation assays in cells. Immunocytochemistry by light and electron microscopy demonstrated the localization of VLGR1 in MAMs. The absence of VLGR1 in tissues and cells derived from VLGR1-deficient mouse models resulted in alterations in the MAM architecture and in the dysregulation of the Ca²⁺ transient from ER to mitochondria. Our data demonstrate the molecular and functional interaction of VLGR1 with components in MAMs and point to an essential role of VLGR1 in the regulation of Ca²⁺ homeostasis, one of the key functions of MAMs.

ER-mitochondria contact sites; a multifaceted factory for Ca2+ signaling and lipid transport

Membrane contact sites (MCS) between organelles of eukaryotic cells provide structural integrity and promote organelle homeostasis by facilitating intracellular signaling, exchange of ions, metabolites and lipids and membrane dynamics. Cataloguing MCS revolutionized our understanding of the structural organization of a eukaryotic cell, but the functional role of MSCs and their role in complex diseases, such as cancer, are only gradually emerging. In particular, the endoplasmic reticulum (ER)-mitochondria contacts (EMCS) are key effectors of non-vesicular lipid trafficking, thereby regulating the lipid composition of cellular membranes and organelles, their physiological functions and lipid-mediated signaling pathways both in physiological and diseased conditions. In this short review, we discuss key aspects of the functional complexity of EMCS in mammalian cells, with particular emphasis on their role as central hubs for lipid transport between these organelles and how perturbations of these pathways may favor key traits of cancer cells.

Chronic social stress disrupts the intracellular redistribution of brain hexokinase 3 induced by shifts in peripheral glucose levels

Abstract

Chronic stress has the potential to impair health and may increase the vulnerability for psychiatric disorders. Emerging evidence suggests that specific neurometabolic dysfunctions play a role herein. In mice, chronic social defeat (CSD) stress reduces cerebral glucose uptake despite hyperglycemia. We hypothesized that this metabolic decoupling would be reflected by changes in contact sites between mitochondria and the endoplasmic reticulum, important intracellular nutrient sensors, and signaling hubs. We thus analyzed the proteome of their biochemical counterparts, mitochondria-associated membranes (MAMs) from whole brain tissue obtained from CSD and control mice. This revealed a lack of the glucose-metabolizing enzyme hexokinase 3 (HK3) in MAMs from CSD mice. In controls, HK3 protein abundance in MAMs and also in striatal synaptosomes correlated positively with peripheral blood glucose levels, but this connection was lost in CSD. We conclude that the ability of HK3 to traffic to sites of need, such as MAMs or synapses, is abolished upon CSD and surmise that this contributes to a cellular dysfunction instigated by chronic stress.

Key messages

Chronic social defeat (CSD) alters brain glucose metabolism

CSD depletes hexokinase 3 (HK3) from mitochondria-associated membranes (MAMs)

CSD results in loss of positive correlation between blood glucose and HK3 in MAMs and synaptosomes

The mitochondrial associated endoplasmic reticulum membranes: A platform for the pathogenesis of inflammation-mediated metabolic diseases

Mitochondria-associated endoplasmic reticulum membranes (MAM) are specialized subcellular compartments that are shaped by endoplasmic reticulum (ER) subdomains placed side by side to the outer membrane of mitochondria (OMM) being connected by tethering proteins in mammalian cells. Studies showed that MAM has multiple physiological functions. These include regulation of lipid synthesis and transport, Ca2+ transport and signaling, mitochondrial dynamics, apoptosis, autophagy, and formation and activation of an inflammasome. However, alterations of MAM integrity lead to deleterious effects due to an increased generation of mitochondrial reactive oxygen species (ROS) via increased Ca2+ transfer from the ER to mitochondria. This, in turn, causes mitochondrial damage and release of mitochondrial components into the cytosol as damage-associated molecular patterns which rapidly activate MAM-resident Nod-like receptor protein-3 (NLRP3) inflammasome components. This complex induces the release of pro-inflammatory cytokines that initiate low-grade chronic inflammation that subsequently causes the development of metabolic diseases. But, the mechanisms of how MAM is involved in the pathogenesis of these diseases are not exhaustively reviewed. Therefore, this review was aimed to highlight the contribution of MAM to a variety of cellular functions and consider its significance pertaining to the pathogenesis of inflammation-mediated metabolic diseases.

Emerging role of LETM1/GRP78 axis in lung cancer

The selective autophagy of damaged mitochondria is called mitophagy. Mitochondrial dysfunction, mitophagy, and apoptosis have been suggested to be interrelated in various human lung carcinomas. Leucine zipper EF-hand-containing transmembrane protein-1 (LETM1) was cloned in an attempt to identify candidate genes for Wolf-Hirschhorn syndrome. LETM1 plays a role in mitochondrial morphology, ion homeostasis, and cell viability. LETM1 has also been shown to be overexpressed in different human cancer tissues, including lung cancer. In the current study, we have provided clear evidence that LETM1 acts as an anchoring protein for the mitochondria-associated ER membrane (MAM). Fragmented mitochondria have been found in lung cancer cells with LETM1 overexpression. In addition, a reduction of mitochondrial membrane potential and significant accumulation of microtubule-associated protein 1 A/1B-light chain 3 punctate, which localizes with Red-Mito, was found in LETM1-overexpressed cells, suggesting that mitophagy is upregulated in these cells. Interestingly, glucose-regulated protein 78 kDa (GRP78; an ER chaperon protein) and glucose-regulated protein 75 kDa (GRP75) were posited to interact with LETM1 in the immunoprecipitated LETM1 of H460 cells. This interaction was enhanced in cells treated with carbonyl cyanide m-chlorophenylhydrazone, a chemical mitophagy inducer. Treatment of cells with honokiol (a GRP78 inhibitor) blocked LETM1-mediated mitophagy, and CRISPR/Cas9-mediated GRP75 knockout inhibited LETM1-induced autophagy. Thus, GRP78 interacts with LETM1. Taken together, these observations support the notion that the complex formation of LETM1/GRP75/GRP78 might be an important step in MAM formation and mitophagy, thus regulating mitochondrial quality control in lung cancer.

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.

Hidden Agenda - The Involvement of Endoplasmic Reticulum Stress and Unfolded Protein Response in Inflammation-Induced Muscle Wasting

Critically ill patients at the intensive care unit (ICU) often develop a generalized weakness, called ICU-acquired weakness (ICUAW). A major contributor to ICUAW is muscle atrophy, a loss of skeletal muscle mass and function. Skeletal muscle assures almost all of the vital functions of our body. It adapts rapidly in response to physiological as well as pathological stress, such as inactivity, immobilization, and inflammation. In response to a reduced workload or inflammation muscle atrophy develops. Recent work suggests that adaptive or maladaptive processes in the endoplasmic reticulum (ER), also known as sarcoplasmic reticulum, contributes to this process. In muscle cells, the ER is a highly specialized cellular organelle that assures calcium homeostasis and therefore muscle contraction. The ER also assures correct folding of proteins that are secreted or localized to the cell membrane. Protein folding is a highly error prone process and accumulation of misfolded or unfolded proteins can cause ER stress, which is counteracted by the activation of a signaling network known as the unfolded protein response (UPR). Three ER membrane residing molecules, protein kinase R-like endoplasmic reticulum kinase (PERK), inositol requiring protein 1a (IRE1a), and activating transcription factor 6 (ATF6) initiate the UPR. The UPR aims to restore ER homeostasis by reducing overall protein synthesis and increasing gene expression of various ER chaperone proteins. If ER stress persists or cannot be resolved cell death pathways are activated. Although, ER stress-induced UPR pathways are known to be important for regulation of skeletal muscle mass and function as well as for inflammation and immune response its function in ICUAW is still elusive. Given recent advances in the development of ER stress modifying molecules for neurodegenerative diseases and cancer, it is important to know whether or not therapeutic interventions in ER stress pathways have favorable effects and these compounds can be used to prevent or treat ICUAW. In this review, we focus on the role of ER stress-induced UPR in skeletal muscle during critical illness and in response to predisposing risk factors such as immobilization, starvation and inflammation as well as ICUAW treatment to foster research for this devastating clinical problem.

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.

In Vivo Pravastatin Treatment Reverses Hypercholesterolemia Induced Mitochondria-Associated Membranes Contact Sites, Foam Cell Formation, and Phagocytosis in Macrophages

Statins are successful drugs used to treat hypercholesterolemia, a primary cause of atherosclerosis. In this work, we investigated how hypercholesterolemia and pravastatin treatment impact macrophage and mitochondria functions, the key cell involved in atherogenesis. By comparing bone marrow-derived macrophages (BMDM) of wild-type (WT) and LDL receptor knockout (LDLr^−/−) mice, we observed hypercholesterolemia increased the number of contact sites at mitochondria-associated endoplasmic reticulum (ER) membranes (MAMs), enhanced mitochondrial hydrogen peroxide release, altered the gene expression of inflammatory markers, and increased oxidized LDL (ox-LDL) uptake and phagocytic activity. Three months of in vivo pravastatin treatment of LDLr^−/− mice reversed the number of contact sites at the MAM, ox-LDL uptake, and phagocytosis in LDLr^−/− BMDM. Additionally, pravastatin increased BMDM mitochondrial network branching. In peritoneal macrophages (PMs), hypercholesterolemia did not change MAM stability, but stimulated hydrogen peroxide production and modulated gene expression of pro- and anti-inflammatory markers. It also increased mitochondrial branching degree and had no effects on ox-LDL uptake and phagocytosis in PM. Pravastatin treatment increased superoxide anion production and changed inflammation-related gene expression in LDLr^−/− PM. In addition, pravastatin increased markedly the expression of the mitochondrial dynamics-related genes Mfn2 and Fis1 in both macrophages. In summary, our results show that hypercholesterolemia and pravastatin treatment affect macrophage mitochondria network structure as well as their interaction with the endoplasmic reticulum (ER). These effects impact on macrophage conversion rates to foam cell and macrophage phagocytic capacity. These findings associate MAM stability changes with known mechanisms involved in atherosclerosis progression and resolution.

ATAD3A oligomerization promotes neuropathology and cognitive deficits in Alzheimer's disease models

Predisposition to Alzheimer's disease (AD) may arise from lipid metabolism perturbation, however, the underlying mechanism remains elusive. Here, we identify ATPase family AAA-domain containing protein 3A (ATAD3A), a mitochondrial AAA-ATPase, as a molecular switch that links cholesterol metabolism impairment to AD phenotypes. In neuronal models of AD, the 5XFAD mouse model and post-mortem AD brains, ATAD3A is oligomerized and accumulated at the mitochondria-associated ER membranes (MAMs), where it induces cholesterol accumulation by inhibiting gene expression of CYP46A1, an enzyme governing brain cholesterol clearance. ATAD3A and CYP46A1 cooperate to promote APP processing and synaptic loss. Suppressing ATAD3A oligomerization by heterozygous ATAD3A knockout or pharmacological inhibition with DA1 restores neuronal CYP46A1 levels, normalizes brain cholesterol turnover and MAM integrity, suppresses APP processing and synaptic loss, and consequently reduces AD neuropathology and cognitive deficits in AD transgenic mice. These findings reveal a role for ATAD3A oligomerization in AD pathogenesis and suggest ATAD3A as a potential therapeutic target for AD.

Imeglimin Ameliorates β-Cell Apoptosis by Modulating the Endoplasmic Reticulum Homeostasis Pathway

The effects of imeglimin, a novel antidiabetes agent, on β-cell function remain unclear. Here, we unveiled the impact of imeglimin on β-cell survival. Treatment with imeglimin augmented mitochondrial function, enhanced insulin secretion, promoted β-cell proliferation, and improved β-cell survival in mouse islets. Imeglimin upregulated the expression of endoplasmic reticulum (ER)-related molecules, including Chop (Ddit3), Gadd34 (Ppp1r15a), Atf3, and Sdf2l1, and decreased eIF2α phosphorylation after treatment with thapsigargin and restored global protein synthesis in β-cells under ER stress. Imeglimin failed to protect against ER stress-induced β-cell apoptosis in CHOP-deficient islets or in the presence of GADD34 inhibitor. Treatment with imeglimin showed a significant decrease in the number of apoptotic β-cells and increased β-cell mass in Akita mice. Imeglimin also protected against β-cell apoptosis in both human islets and human pluripotent stem cell-derived β-like cells. Taken together, imeglimin modulates the ER homeostasis pathway, which results in the prevention of β-cell apoptosis both in vitro and in vivo.

Endoplasmic reticulum-mitochondria signaling in neurons and neurodegenerative diseases

Recent advances have revealed common pathological changes in neurodegenerative diseases, such as Alzheimer's disease, Parkinson's disease and amyotrophic lateral sclerosis with related frontotemporal dementia (ALS/FTD). Many of these changes can be linked to alterations in endoplasmic reticulum (ER)-mitochondria signaling, including dysregulation of Ca2+ signaling, autophagy, lipid metabolism, ATP production, axonal transport, ER stress responses and synaptic dysfunction. ER-mitochondria signaling involves specialized regions of ER, called mitochondria-associated membranes (MAMs). Owing to their role in neurodegenerative processes, MAMs have gained attention as they appear to be associated with all the major neurodegenerative diseases. Furthermore, their specific role within neuronal maintenance is being revealed as mutant genes linked to major neurodegenerative diseases have been associated with damage to these specialized contacts. Several studies have now demonstrated that these specialized contacts regulate neuronal health and synaptic transmission, and that MAMs are damaged in patients with neurodegenerative diseases. This Review will focus on the role of MAMs and ER-mitochondria signaling within neurons and how damage of the ER-mitochondria axis leads to a disruption of vital processes causing eventual neurodegeneration.

Get Closer to the World of Contact Sites: A Beginner's Guide to Proximity-Driven Fluorescent Probes

To maintain cellular homeostasis and to coordinate the proper response to a specific stimulus, information must be integrated throughout the cell in a well-organized network, in which organelles are the crucial nodes and membrane contact sites are the main edges. Membrane contact sites are the cellular subdomains where two or more organelles come into close apposition and interact with each other. Even though many inter-organelle contacts have been identified, most of them are still not fully characterized, therefore their study is an appealing and expanding field of research. Thanks to significant technological progress, many tools are now available or are in rapid development, making it difficult to choose which one is the most suitable for answering a specific biological question. Here we distinguish two different experimental approaches for studying inter-organelle contact sites. The first one aims to morphologically characterize the sites of membrane contact and to identify the molecular players involved, relying mainly on the application of biochemical and electron microscopy (EM)-related methods. The second approach aims to understand the functional importance of a specific contact, focusing on spatio-temporal details. For this purpose, proximity-driven fluorescent probes are the experimental tools of choice, since they allow the monitoring and quantification of membrane contact sites and their dynamics in living cells under different cellular conditions or upon different stimuli. In this review, we focus on these tools with the purpose of highlighting their great versatility and how they can be applied in the study of membrane contacts. We will extensively describe all the different types of proximity-driven fluorescent tools, discussing their benefits and drawbacks, ultimately providing some suggestions to choose and apply the appropriate methods on a case-to-case basis and to obtain the best experimental outcomes.

Nuclear Interactions: A Spotlight on Nuclear Mitochondrial Membrane Contact Sites

Membrane contact sites (MCS) are critical for cellular functions of eukaryotes, as they enable communication and exchange between organelles. Research over the last decade unravelled the function and composition of MCS between a variety of organelles including mitochondria, ER, plasma membrane, lysosomes, lipid droplets, peroxisome and endosome, to name a few. In fact, MCS are found between any pair of organelles studied to date, with common functions including lipid exchange, calcium signalling and organelle positioning in the cell. Work in the past year has started addressing the composition and function of nuclear-mitochondrial MCS. Tether components mediating these contacts in yeast have been identified via comprehensive phenotypic screens, which also revealed a possible link between this contact and phosphatidylcholine metabolism. In human cells, and in the protozoan parasites causing malaria, proximity between these organelles is proposed to promote cell survival via a mitochondrial retrograde response. These pioneering studies should inspire the field to explore what cellular processes depend on the exchange between the nucleus and the mitochondrion, given that they play such central roles in cell biology.

Adaptive immune cells shape obesity-associated type 2 diabetes mellitus and less prominent comorbidities

Obesity and type 2 diabetes mellitus (T2DM) are increasing in prevalence owing to decreases in physical activity levels and a shift to diets that include addictive and/or high-calorie foods. These changes are associated with the adoption of modern lifestyles and the presence of an obesogenic environment, which have resulted in alterations to metabolism, adaptive immunity and endocrine regulation. The size and quality of adipose tissue depots in obesity, including the adipose tissue immune compartment, are critical determinants of overall health. In obesity, chronic low-grade inflammation can occur in adipose tissue that can progress to systemic inflammation; this inflammation contributes to the development of insulin resistance, T2DM and other comorbidities. An improved understanding of adaptive immune cell dysregulation that occurs during obesity and its associated metabolic comorbidities, with an appreciation of sex differences, will be critical for repurposing or developing immunomodulatory therapies to treat obesity and/or T2DM-associated inflammation. This Review critically discusses how activation and metabolic reprogramming of lymphocytes, that is, T cells and B cells, triggers the onset, development and progression of obesity and T2DM. We also consider the role of immunity in under-appreciated comorbidities of obesity and/or T2DM, such as oral cavity inflammation, neuroinflammation in Alzheimer disease and gut microbiome dysbiosis. Finally, we discuss previous clinical trials of anti-inflammatory medications in T2DM and consider the path forward.