ORP5/8 and MIB/MICOS link ER-mitochondria and intra-mitochondrial contacts for non-vesicular transport of phosphatidylserine

MTMR regulates KRAS function by controlling plasma membrane levels of phospholipids

KRAS, a small GTPase involved in cell proliferation and differentiation, frequently gains activating mutations in human cancers. For KRAS to function, it must bind the plasma membrane (PM) via interactions between its membrane anchor and phosphatidylserine (PtdSer). Therefore, depleting PM PtdSer abrogates KRAS PM binding and activity. From a genome-wide siRNA screen to identify genes regulating KRAS PM localization, we identified a set of phosphatidylinositol (PI) 3-phosphatases: myotubularin-related proteins (MTMR) 2, 3, 4, and 7. Here, we show that silencing MTMR 2/3/4/7 disrupts KRAS PM interactions by reducing PM PI 4-phosphate (PI4P) levels, thereby disrupting the localization and operation of ORP5, a lipid transfer protein maintaining PM PtdSer enrichment. Concomitantly, silencing MTMR 2/3/4/7 elevates PM PI3P levels while reducing PM and total PtdSer levels. We also observed MTMR 2/3/4/7 expression is interdependent. We propose that the PI 3-phosphatase activity of MTMR is required for generating PM PI, necessary for PM PI4P synthesis, promoting the PM localization of PtdSer and KRAS.

Molecular machineries shaping the mitochondrial inner membrane

Mitochondria display intricately shaped deep invaginations of the mitochondrial inner membrane (MIM) termed cristae. This peculiar membrane architecture is essential for diverse mitochondrial functions, such as oxidative phosphorylation or the biosynthesis of cellular building blocks. Conserved protein nano-machineries such as F1Fo-ATP synthase oligomers and the mitochondrial contact site and cristae organizing system (MICOS) act as adaptable protein-lipid scaffolds controlling MIM biogenesis and its dynamic remodelling. Signal-dependent rearrangements of cristae architecture and MIM fusion events are governed by the dynamin-like GTPase optic atrophy 1 (OPA1). Recent groundbreaking structural insights into these nano-machineries have considerably advanced our understanding of the functional architecture of mitochondria. In this Review, we discuss how the MIM-shaping machineries cooperate to control cristae and crista junction dynamics, including MIM fusion, in response to cellular signalling pathways. We also explore how mutations affecting MIM-shaping machineries compromise mitochondrial functions.

Mitochondria-Associated Membranes: A Key Point of Neurodegenerative Diseases

BACKGROUND

Neurodegenerative diseases pose significant health challenges in the 21st century, with increasing morbidity and mortality, particularly among the elderly population. One of the key factors contributing to the pathogenesis of these diseases is the disrupted crosstalk between mitochondria and the endoplasmic reticulum. Mitochondria-associated membranes (MAMs), which are regions where the ER interfaces with mitochondria, serve as crucial platforms facilitating communication between these organelles.

OBJECTIVES

This review focuses on the structural composition and functions of MAMs and highlights their roles. Additionally, in this review, we summarize the relationship between MAM dysfunction and various neurodegenerative diseases, including Alzheimer's disease, Parkinson's disease, and others. The involvement of key proteins such as Sig-1R, IP3R, and VAPB in maintaining ER-mitochondrial communication and their dysfunction in neurodegenerative diseases is emphasized.

CONCLUSION

Through analyzing the effects of MAM on neurodegenerative diseases, we provide the newest insights and potential therapeutic targets for the treatment of these debilitating conditions.

Mitochondria-membranous organelle contacts at a glance

Mitochondrial contact sites are specialized interfaces where mitochondria physically interact with other organelles. Stabilized by molecular tethers and defined by unique proteomic and lipidomic profiles, these sites enable direct interorganellar communication and functional coordination, playing crucial roles in cellular physiology and homeostasis. Recent advances have expanded our knowledge of contact site-resident proteins, illuminated the dynamic and adaptive nature of these interfaces, and clarified their contribution to pathophysiology. In this Cell Science at a Glance article and the accompanying poster, we summarize the mitochondrial contacts that have been characterized in mammals, the molecular mechanisms underlying their formation, and their principal functions.

Three's company: Membrane waltz among organelles

The study of membrane contact sites (MCS) has profoundly transformed our understanding of inter-organelle communication. These sites, where the membranes of two organelles are closely apposed, facilitate the transfer of small molecules such as lipids and ions. They are especially crucial for the maintenance of the structure and function of organelles like mitochondria and lipid droplets, which are largely excluded from vesicular trafficking. The significant advancements in imaging techniques, and molecular and cell biology research have shown that MCS are more complex than what originally thought and can involve more than two organelles. This has revealed the intricate nature and critical importance of these subcellular connections. Here, we provide an overview of newly described three-way inter-organelles associations, and the proteins involved in these MCS. We highlight the roles these contacts play in key cellular processes such as lipid droplet biogenesis and mitochondrial division. Additionally, we discuss the latest advances in super-resolution imaging that enable the study of these complex three-way interactions. Ongoing research, driven by technological innovations, promises to uncover further insights into their roles in fundamental cellular processes and their implications for health and disease.

Compositionally unique mitochondria in filopodia support cellular migration

Local metabolic demand within cells varies widely, and the extent to which individual mitochondria can be specialized to meet these functional needs is unclear. We examined the subcellular distribution of the mitochondrial contact site and cristae organizing system (MICOS) complex, a spatial and functional organizer of mitochondria, and discovered that it dynamically enriches at the tip of a minor population of mitochondria in the cell periphery. Based on their appearance, we term these mitochondria "METEORs". METEORs have a unique composition, and MICOS enrichment sites are depleted of mtDNA and matrix proteins and contain high levels of the Ca2+ uniporter MCU, suggesting a functional specialization. METEORs are also enriched for the myosin MYO19, which promotes their trafficking to a small subset of filopodia. We identify a positive correlation between the length of filopodia and the presence of METEORs and show that elimination of mitochondria from filopodia impairs cellular motility. Our data reveal a novel type of mitochondrial heterogeneity and suggest compositionally specialized mitochondria support cell migration.

MTMR regulates KRAS function by controlling plasma membrane levels of phospholipids

KRAS, a small GTPase involved in cell proliferation and differentiation, frequently gains activating mutations in human cancers. For KRAS to function, it must bind the plasma membrane (PM) via interactions between its membrane anchor and phosphatidylserine (PtdSer). Therefore, depleting PM PtdSer abrogates KRAS PM binding and activity. From a genome-wide siRNA screen to identify genes regulating KRAS PM localization, we identified a set of phosphatidylinositol (PI) 3-phosphatases: myotubularin-related proteins (MTMR) 2, 3, 4, and 7. Here, we show that silencing MTMR 2/3/4/7 disrupts KRAS PM interactions by reducing PM PI 4-phosphate (PI4P) levels, thereby disrupting the localization and operation of ORP5, a lipid transfer protein maintaining PM PtdSer enrichment. Concomitantly, silencing MTMR 2/3/4/7 elevates PM PI3P levels while reducing PM and total PtdSer levels. We also observed MTMR 2/3/4/7 expression is interdependent. We propose that the PI 3-phosphatase activity of MTMR is required for generating PM PI, necessary for PM PI4P synthesis, promoting the PM localization of PtdSer and KRAS.

eTOC summary

We discovered that silencing the phosphatidylinositol (PI) 3-phosphatase, MTMR , disrupts the PM localization of PtdSer and KRAS. We propose a model, where MTMR loss depletes PM PI needed for PM PI4P synthesis, an essential phospholipid for PM PtdSer enrichment, thereby impairing KRAS PM localization.

Ultrastructure analysis of mitochondria, lipid droplet and sarcoplasmic reticulum apposition in human heart failure

Background

Cardiomyocyte structural remodeling is reported as a causal contributor to heart failure (HF) development and progression. Growing evidence highlights the role of organelle apposition in cardiomyocyte function and homeostasis. Disruptions in organelle crosstalk, such as that between the sarcoplasmic reticulum (SR) and mitochondria, are thought to impact numerous cellular processes such as calcium handling and cellular bioenergetics; two processes that are disrupted and implicated in cardiac pathophysiology. While the physical distance between organelles is thought to be essential for homeostatic cardiomyocyte function, whether the interactions and coupling of organelles are altered in human heart failure remains unclear.

Methods

Here, we utilized transmission electron microscopy and careful quantification of ultrastructure to characterize the changes in organelle apposition in cardiomyocytes isolated from the hearts of patients diagnosed with various types of HF. Subsequently we employed molecular approaches to examine the expression of proposed organelle tethers.

Results

We demonstrate that cardiomyocytes isolated from dilated cardiomyopathy, hypertrophic cardiomyopathy and ischemic cardiomyopathy hearts display smaller, more rounded mitochondria, as compared to nonfailing controls. Failing cardiomyocytes also exhibited disrupted SR-mitochondria juxtaposition and changes in the expression of proposed molecular tethers. Further analysis revealed alterations in lipid droplet dynamics including decreased lipid droplet content and less lipid droplets in association with mitochondria in failing cardiomyocytes.

Conclusion

Here we observed changes in organelle dynamics in cardiomyocytes isolated from heart failure patients diagnosed with differing etiologies. Our results suggest that organelle structure and apposition may be a ubiquitous contributor to human HF progression.

Mitochondrial Sorting and Assembly Machinery: Chaperoning a Moonlighting Role?

The mitochondrial outer membrane (OMM) β-barrel proteins link the mitochondrion with the cytosol, endoplasmic reticulum, and other cellular membranes, establishing cellular homeostasis. Their active insertion and assembly in the outer mitochondrial membrane is achieved in an energy-independent yet highly effective manner by the Sorting and Assembly Machinery (SAM) of the OMM. The core SAM constituent is the 16-stranded transmembrane β-barrel Sam50. For over two decades, the primary role of Sam50 has been linked to its function as a chaperone in the OMM, wherein it assembles all β-barrels through a lateral gating and β-barrel switching mechanism. Interestingly, recent studies have demonstrated that despite its low copy number, Sam50 performs various diverse functions beyond assembling β-barrels. This includes maintaining cristae morphology, bidirectional lipid shuttling between the ER and mitochondrial inner membrane, import of select proteins, regulation of PINK1-Parkin function, and timed trigger of cell death. Given these multifaceted critical regulatory functions of SAM across all eukaryotes, we now reason that SAM merely moonlights as the hub for β-barrel biogenesis and has indeed evolved a diverse array of primary roles in maintaining mitochondrial function and cellular homeostasis.

Targeting endoplasmic reticulum stress as a potential therapeutic strategy for diabetic cardiomyopathy

Endoplasmic reticulum (ER) is an essential organelle involved in vesicular transport, calcium handling, protein synthesis and folding, and lipid biosynthesis and metabolism. ER stress occurs when ER homeostasis is disrupted by the accumulation of unfolded and/or misfolded proteins in the ER lumen. Adaptive pathways of the unfolded protein response (UPR) are activated to maintain ER homeostasis. In obesity and type 2 diabetes mellitus (T2DM), accumulating data indicate that persistent ER stress due to maladaptive UPR interacts with insulin/leptin signaling, which may be the potential and central mechanistic link between obesity-/T2DM-induced metabolic dysregulation (chronic hyperglycemia, dyslipidemia and lipotoxicity in cardiomyocytes), insulin/leptin resistance and the development of diabetic cardiomyopathy (DiabCM). Meanwhile, these pathological conditions further exacerbate ER stress. However, their interrelationships and the underlying molecular mechanisms are not fully understood. A deeper understanding of ER stress-mediated pathways in DiabCM is needed to develop novel therapeutic strategies. The aim of this review is to discuss the crosstalk between ER stress and leptin/insulin signaling and their involvement in the development of DiabCM focusing on mitochondria-associated ER membranes and chronic inflammation. We also present the current direction of drug development and important considerations for translational research into targeting ER stress for the treatment of DiabCM.

Measurement of Lipid Transport in Mitochondria by the MTL Complex

Membrane biogenesis requires an extensive traffic of lipids between different cell compartments. Two main pathways, the vesicular and non-vesicular pathways, are involved in such a process. Whereas the mechanisms involved in vesicular trafficking are well understood, less is known about non-vesicular lipid trafficking, particularly in plants. This pathway involves the direct exchange of lipids at membrane contact sites (MCSs) between organelles. In plants, extensive traffic of the chloroplast-synthesized digalactosyldiacylglycerol (DGDG) to mitochondria is specifically promoted during phosphate starvation. This lipid exchange likely occurs by non-vesicular trafficking pathways at MCSs between mitochondria and plastids. By a biochemical approach, a mitochondrial lipoproteic super-complex called MTL (mitochondrial transmembrane lipoprotein complex) involved in mitochondrial lipid trafficking has been identified in Arabidopsis thaliana. This protocol describes the method used to separate the MTL complex and to study the implication of a component of this complex (AtMic60) in mitochondrial lipid transport.

Unraveling the complex interplay between Mitochondria-Associated Membranes (MAMs) and cardiovascular Inflammation: Molecular mechanisms and therapeutic implications

Cardiovascular diseases (CVDs) represent a significant public health concern because of their associations with inflammation, oxidative stress, and abnormal remodeling of the heart and blood vessels. In this review, we discuss the intricate interplay between mitochondria-associated membranes (MAMs) and cardiovascular inflammation, highlighting their role in key cellular processes such as calcium homeostasis, lipid metabolism, oxidative stress management, and ERS. We explored how these functions impact the pathogenesis and progression of various CVDs, including myocardial ischemia-reperfusion injury, atherosclerosis, diabetic cardiomyopathy, cardiovascular aging, heart failure, and pulmonary hypertension. Additionally, we examined current therapeutic strategies targeting MAM-related pathways and proteins, emphasizing the potential of MAMs as therapeutic targets. Our review aims to provide new insights into the mechanisms of cardiovascular inflammation and propose novel therapeutic approaches to improve cardiovascular health outcomes.

Current insights on mitochondria-associated endoplasmic reticulum membranes (MAMs) and their significance in the pathophysiology of ocular disorders

The intricate interaction network necessary for essential physiological functions underscores the interdependence among eukaryotic cells. Mitochondria-Associated Endoplasmic Reticulum Membranes (MAMs), specialized junctions between mitochondria and the ER, were recently discovered. These junctions participate in various cellular processes, including calcium level regulation, lipid metabolism, mitochondrial integrity maintenance, autophagy, and inflammatory responses via modulating the structure and molecular composition of various cellular components. Therefore, MAMs contribute to the pathophysiology of numerous ocular disorders, including Diabetic Retinopathy (DR), Age-related Macular Degeneration (AMD) and glaucoma. In addition to providing a concise overview of the architectural and functional aspects of MAMs, this review explores the key pathogenetic pathways involving MAMs in the development of several ocular disorders.

Interplay of α-Synuclein Oligomers and Endoplasmic Reticulum Stress in Parkinson'S Disease: Insights into Cellular Dysfunctions

Oligomeric forms of α-synuclein (α-syn) are critical in the formation of α-synuclein fibrils, exhibiting neurotoxic properties that are pivotal in the pathogenesis of Parkinson's disease (PD). A salient feature of this pathology is the disruption of the protein folding capacity of the endoplasmic reticulum (ER), leading to a perturbation in the ER's protein quality control mechanisms. The accumulation of unfolded or misfolded proteins instigates ER stress. However, the onset of ER stress and the consequent activation of the Unfolded Protein Response (UPR) and Endoplasmic Reticulum-Associated Degradation (ERAD) pathways do not merely culminate in apoptosis when they fail to restore cellular homeostasis. More critically, this condition initiates a cascade of reactions involving ER-related structures and organelles, resulting in multifaceted cellular damage and, potentially, a feedback loop that precipitates neuroinflammation. In this review, we elucidate the interplay between UPR and ERAD, as well as the intricate crosstalk among the ER and other organelles such as mitochondria, lysosomes, and the Golgi apparatus, underscoring their roles in the neurodegenerative process.

A new perspective on liver diseases: Focusing on the mitochondria-associated endoplasmic reticulum membranes

The pathogenesis of liver diseases is multifaceted and intricate, posing a persistent global public health challenge with limited therapeutic options. Therefore, further research into liver diseases is imperative for better comprehension and advancement in treatment strategies. Numerous studies have confirmed the endoplasmic reticulum (ER) and mitochondria as key organelles driving liver diseases. Notably, the mitochondrial-associated ER membranes (MAMs) establish a physical and functional connection between the ER and mitochondria, highlighting the importance of inter-organelle communication in maintaining their functional homeostasis. This review delves into the intricate architecture and regulative mechanism of the integrated MAM that facilitate the physiological transfer of signals and substances between organelles. Additionally, we also provide a detailed overview regarding the varied pathogenic roles of malfunctioning MAM in liver diseases, focusing on its involvement in the progression of ER stress and mitochondrial dysfunction, the regulation of mitochondrial dynamics and Ca2+ transfer, as well as the disruption of lipid and glucose homeostasis. Furthermore, the current challenges and prospects associated with MAM in liver disease research are thoroughly discussed. In conclusion, elucidating the specific structure and function of MAM in different liver diseases may pave the way for novel therapeutic strategies.

Remodelling of mitochondrial function by import of specific lipids at multiple membrane-contact sites

Organelles form physical and functional contact between each other to exchange information, metabolic intermediates, and signaling molecules. Tethering factors and contact site complexes bring partnering organelles into close spatial proximity to establish membrane contact sites (MCSs), which specialize in unique functions like lipid transport or Ca2+ signaling. Here, we discuss how MCSs form dynamic platforms that are important for lipid metabolism. We provide a perspective on how import of specific lipids from the ER and other organelles may contribute to remodeling of mitochondria during nutrient starvation. We speculate that mitochondrial adaptation is achieved by connecting several compartments into a highly dynamic organelle network. The lipid droplet appears to be a central hub in coordinating the function of these organelle neighborhoods.

Functional interplay of lipid droplets and mitochondria

Our body stores energy mostly in form of fatty acids (FAs) in lipid droplets (LDs). From there the FAs can be mobilized and transferred to peroxisomes and mitochondria. This transfer is dependent on close opposition of LDs and mitochondria and peroxisomes and happens at membrane contact sites. However, the composition and the dynamics of these contact sites is not well understood, which is in part due to the dependence on the metabolic state of the cell and on the cell- and tissue-type. Here, we summarize the current knowledge on the contacts between lipid droplets and mitochondria both in mammals and in the yeast Saccharomyces cerevisiae, in which various contact sites are well studied. We discuss possible functions of the contact site and their implication in disease.

Emerging functions of the mitochondria-ER-lipid droplet three-way junction in coordinating lipid transfer, metabolism, and storage in cells

Over the past two decades, we have witnessed a growing appreciation for the importance of membrane contact sites (CS) in facilitating direct communication between organelles. CS are tiny regions where the membranes of two organelles meet but do not fuse and allow the transfer of metabolites between organelles, playing crucial roles in the coordination of cellular metabolic activities. The significant advancements in imaging techniques and molecular and cell biology research have revealed that CS are more complex than what originally thought, and as they are extremely dynamic, they can remodel their shape, composition, and functions in accordance with metabolic and environmental changes and can occur between more than two organelles. Here, we describe how recent studies led to the identification of a three-way mitochondria-ER-lipid droplet CS and discuss the emerging functions of these contacts in maintaining lipid storage, homeostasis, and balance. We also summarize the properties and functions of key protein components localized at the mitochondria-ER-lipid droplet interface, with a special focus on lipid transfer proteins. Understanding tripartite CS is essential for unraveling the complexities of inter-organelle communication and cooperation within cells.

Mitochondrial phospholipid transport: Role of contact sites and lipid transport proteins

One of the major constituents of mitochondrial membranes is the phospholipids, which play a key role in maintaining the structure and the functions of the mitochondria. However, mitochondria do not synthesize most of the phospholipids in situ, necessitating the presence of phospholipid import pathways. Even for the phospholipids, which are synthesized within the inner mitochondrial membrane (IMM), the phospholipid precursors must be imported from outside the mitochondria. Therefore, the mitochondria heavily rely on the phospholipid transport pathways for its proper functioning. Since, mitochondria are not part of a vesicular trafficking network, the molecular mechanisms of how mitochondria receive its phospholipids remain a relevant question. One of the major ways that hydrophobic phospholipids can cross the aqueous barrier of inter or intraorganellar spaces is by apposing membranes, thereby decreasing the distance of transport, or by being sequestered by lipid transport proteins (LTPs). Therefore, with the discovery of LTPs and membrane contact sites (MCSs), we are beginning to understand the molecular mechanisms of phospholipid transport pathways in the mitochondria. In this review, we will present a brief overview of the recent findings on the molecular architecture and the importance of the MCSs, both the intraorganellar and interorganellar contact sites, in facilitating the mitochondrial phospholipid transport. In addition, we will also discuss the role of LTPs for trafficking phospholipids through the intermembrane space (IMS) of the mitochondria. Mechanistic insights into different phospholipid transport pathways of mitochondria could be exploited to vary the composition of membrane phospholipids and gain a better understanding of their precise role in membrane homeostasis and mitochondrial bioenergetics.

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.

Navigating the landscape of mitochondrial-ER communication in health and disease

Intracellular organelle communication enables the maintenance of tissue homeostasis and health through synchronized adaptive processes triggered by environmental cues. Mitochondrial-Endoplasmic Reticulum (ER) communication sustains cellular fitness by adjusting protein synthesis and degradation, and metabolite and protein trafficking through organelle membranes. Mitochondrial-ER communication is bidirectional and requires that the ER-components of the Integrated Stress Response signal to mitochondria upon activation and, likewise, mitochondria signal to the ER under conditions of metabolite and protein overload to maintain proper functionality and ensure cellular survival. Declines in the mitochondrial-ER communication occur upon ageing and correlate with the onset of a myriad of heterogeneous age-related diseases such as obesity, type 2 diabetes, cancer, or neurodegenerative pathologies. Thus, the exploration of the molecular mechanisms of mitochondrial-ER signaling and regulation will provide insights into the most fundamental cellular adaptive processes with important therapeutical opportunities. In this review, we will discuss the pathways and mechanisms of mitochondrial-ER communication at the mitochondrial-ER interface and their implications in health and disease.

Mic19 depletion impairs endoplasmic reticulum-mitochondrial contacts and mitochondrial lipid metabolism and triggers liver disease

Endoplasmic reticulum (ER)-mitochondria contacts are critical for the regulation of lipid transport, synthesis, and metabolism. However, the molecular mechanism and physiological function of endoplasmic reticulum-mitochondrial contacts remain unclear. Here, we show that Mic19, a key subunit of MICOS (mitochondrial contact site and cristae organizing system) complex, regulates ER-mitochondria contacts by the EMC2-SLC25A46-Mic19 axis. Mic19 liver specific knockout (LKO) leads to the reduction of ER-mitochondrial contacts, mitochondrial lipid metabolism disorder, disorganization of mitochondrial cristae and mitochondrial unfolded protein stress response in mouse hepatocytes, impairing liver mitochondrial fatty acid β-oxidation and lipid metabolism, which may spontaneously trigger nonalcoholic steatohepatitis (NASH) and liver fibrosis in mice. Whereas, the re-expression of Mic19 in Mic19 LKO hepatocytes blocks the development of liver disease in mice. In addition, Mic19 overexpression suppresses MCD-induced fatty liver disease. Thus, our findings uncover the EMC2-SLC25A46-Mic19 axis as a pathway regulating ER-mitochondria contacts, and reveal that impairment of ER-mitochondria contacts may be a mechanism associated with the development of NASH and liver fibrosis.

Not So Rare: Diseases Based on Mutant Proteins Controlling Endoplasmic Reticulum-Mitochondria Contact (MERC) Tethering

Mitochondria-endoplasmic reticulum contacts (MERCs), also called endoplasmic reticulum (ER)-mitochondria contact sites (ERMCS), are the membrane domains, where these two organelles exchange lipids, Ca2+ ions, and reactive oxygen species. This crosstalk is a major determinant of cell metabolism, since it allows the ER to control mitochondrial oxidative phosphorylation and the Krebs cycle, while conversely, it allows the mitochondria to provide sufficient ATP to control ER proteostasis. MERC metabolic signaling is under the control of tethers and a multitude of regulatory proteins. Many of these proteins have recently been discovered to give rise to rare diseases if their genes are mutated. Surprisingly, these diseases share important hallmarks and cause neurological defects, sometimes paired with, or replaced by skeletal muscle deficiency. Typical symptoms include developmental delay, intellectual disability, facial dysmorphism and ophthalmologic defects. Seizures, epilepsy, deafness, ataxia, or peripheral neuropathy can also occur upon mutation of a MERC protein. Given that most MERC tethers and regulatory proteins have secondary functions, some MERC protein-based diseases do not fit into this categorization. Typically, however, the proteins affected in those diseases have dominant functions unrelated to their roles in MERCs tethering or their regulation. We are discussing avenues to pharmacologically target genetic diseases leading to MERC defects, based on our novel insight that MERC defects lead to common characteristics in rare diseases. These shared characteristics of MERCs disorders raise the hope that they may allow for similar treatment options.

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.

ESYT1 tethers the ER to mitochondria and is required for mitochondrial lipid and calcium homeostasis

Mitochondria interact with the ER at structurally and functionally specialized membrane contact sites known as mitochondria-ER contact sites (MERCs). Combining proximity labelling (BioID), co-immunoprecipitation, confocal microscopy and subcellular fractionation, we found that the ER resident SMP-domain protein ESYT1 was enriched at MERCs, where it forms a complex with the outer mitochondrial membrane protein SYNJ2BP. BioID analyses using ER-targeted, outer mitochondrial membrane-targeted, and MERC-targeted baits, confirmed the presence of this complex at MERCs and the specificity of the interaction. Deletion of ESYT1 or SYNJ2BP reduced the number and length of MERCs. Loss of the ESYT1-SYNJ2BP complex impaired ER to mitochondria calcium flux and provoked a significant alteration of the mitochondrial lipidome, most prominently a reduction of cardiolipins and phosphatidylethanolamines. Both phenotypes were rescued by reexpression of WT ESYT1 and an artificial mitochondria-ER tether. Together, these results reveal a novel function for ESYT1 in mitochondrial and cellular homeostasis through its role in the regulation of MERCs.

Getting to Grips with the Oxysterol-Binding Protein Family - a Forty Year Perspective

This review discusses how research around the oxysterol-binding protein family has evolved. We briefly summarize how this protein family, designated OSBP-related (ORP) or OSBP-like (OSBPL) proteins, was discovered, how protein domains highly conserved among family members between taxa paved the way for understanding their mechanisms of action, and how insights into protein structural and functional features help to understand their versatility as lipid transporters. We also discuss questions and future avenues of research opened by these findings. The investigations on oxysterol-binding protein family serve as a real-life example of the notion that science often advances as a collective effort of multiple lines of enquiry, including serendipitous routes. While original articles invariably explain the motivation of the research undertaken in rational terms, the actual paths to findings may be less intentional. Fortunately, this does not reduce the impact of the discoveries made. Besides hopefully providing a useful account of ORP family proteins, we aim to convey this message.

Role of Lipids and Divalent Cations in Membrane Fusion Mediated by the Heptad Repeat Domain 1 of Mitofusin

Mitochondria are highly dynamic organelles that constantly undergo fusion and fission events to maintain their shape, distribution and cellular function. Mitofusin 1 and 2 proteins are two dynamin-like GTPases involved in the fusion of outer mitochondrial membranes (OMM). Mitofusins are anchored to the OMM through their transmembrane domain and possess two heptad repeat domains (HR1 and HR2) in addition to their N-terminal GTPase domain. The HR1 domain was found to induce fusion via its amphipathic helix, which interacts with the lipid bilayer structure. The lipid composition of mitochondrial membranes can also impact fusion. However, the precise mode of action of lipids in mitochondrial fusion is not fully understood. In this study, we examined the role of the mitochondrial lipids phosphatidylethanolamine (PE), cardiolipin (CL) and phosphatidic acid (PA) in membrane fusion induced by the HR1 domain, both in the presence and absence of divalent cations (Ca2+ or Mg2+). Our results showed that PE, as well as PA in the presence of Ca2+, effectively stimulated HR1-mediated fusion, while CL had a slight inhibitory effect. By considering the biophysical properties of these lipids in the absence or presence of divalent cations, we inferred that the interplay between divalent cations and specific cone-shaped lipids creates regions with packing defects in the membrane, which provides a favorable environment for the amphipathic helix of HR1 to bind to the membrane and initiate fusion.

Mitochondrial phospholipid metabolism in health and disease

Studies of rare human genetic disorders of mitochondrial phospholipid metabolism have highlighted the crucial role that membrane phospholipids play in mitochondrial bioenergetics and human health. The phospholipid composition of mitochondrial membranes is highly conserved from yeast to humans, with each class of phospholipid performing a specific function in the assembly and activity of various mitochondrial membrane proteins, including the oxidative phosphorylation complexes. Recent studies have uncovered novel roles of cardiolipin and phosphatidylethanolamine, two crucial mitochondrial phospholipids, in organismal physiology. Studies on inter-organellar and intramitochondrial phospholipid transport have significantly advanced our understanding of the mechanisms that maintain mitochondrial phospholipid homeostasis. Here, we discuss these recent advances in the function and transport of mitochondrial phospholipids while describing their biochemical and biophysical properties and biosynthetic pathways. Additionally, we highlight the roles of mitochondrial phospholipids in human health by describing the various genetic diseases caused by disruptions in their biosynthesis and discuss advances in therapeutic strategies for Barth syndrome, the best-studied disorder of mitochondrial phospholipid metabolism.

Mechanisms of Modulation of Mitochondrial Architecture

Mitochondrial network architecture plays a critical role in cellular physiology. Indeed, alterations in the shape of mitochondria upon exposure to cellular stress can cause the dysfunction of these organelles. In this scenario, mitochondrial dynamics proteins and the phospholipid composition of the mitochondrial membrane are key for fine-tuning the modulation of mitochondrial architecture. In addition, several factors including post-translational modifications such as the phosphorylation, acetylation, SUMOylation, and o-GlcNAcylation of mitochondrial dynamics proteins contribute to shaping the plasticity of this architecture. In this regard, several studies have evidenced that, upon metabolic stress, mitochondrial dynamics proteins are post-translationally modified, leading to the alteration of mitochondrial architecture. Interestingly, several proteins that sustain the mitochondrial lipid composition also modulate mitochondrial morphology and organelle communication. In this context, pharmacological studies have revealed that the modulation of mitochondrial shape and function emerges as a potential therapeutic strategy for metabolic diseases. Here, we review the factors that modulate mitochondrial architecture.

Phosphatidylserine transport in cell life and death

Phosphatidylserine (PS) is a negatively charged glycerophospholipid found mainly in the plasma membrane (PM) and in the late secretory/endocytic compartments, where it regulates cellular activity and can mediate apoptosis. Export of PS from the endoplasmic reticulum, its site of synthesis, to other compartments, and its transbilayer asymmetry must therefore be precisely regulated. We review recent findings on nonvesicular transport of PS by lipid transfer proteins (LTPs) at membrane contact sites, on PS flip-flop between membrane leaflets by flippases and scramblases, and on PS nanoclustering at the PM. We also discuss emerging data on cooperation between scramblases and LTPs, how perturbation of PS distribution can lead to disease, and the specific role of PS in viral infection.

SIRT3 ameliorates diabetes-associated cognitive dysfunction via regulating mitochondria-associated ER membranes

BACKGROUND

Diabetes is associated with an increased risk of cognitive decline and dementia. These diseases are linked with mitochondrial dysfunction, most likely as a consequence of excessive formation of mitochondria-associated membranes (MAMs). Sirtuin3 (SIRT3), a key mitochondrial NAD⁺-dependent deacetylase, is critical responsible for mitochondrial functional homeostasis and is highly associated with neuropathology. However, the role of SIRT3 in regulating MAM coupling remains unknown.

METHODS

Streptozotocin-injected diabetic mice and high glucose-treated SH-SY5Y cells were established as the animal and cellular models, respectively. SIRT3 expression was up-regulated in vivo using an adeno-associated virus in mouse hippocampus and in vitro using a recombinant lentivirus vector. Cognitive function was evaluated using behavioural tests. Hippocampus injury was assessed using Golgi and Nissl staining. Apoptosis was analysed using western blotting and TUNEL assay. Mitochondrial function was detected using flow cytometry and confocal fluorescence microscopy. The mechanisms were investigated using co-immunoprecipitation of VDAC1-GRP75-IP3R complex, fluorescence imaging of ER and mitochondrial co-localisation and transmission electron microscopy of structural analysis of MAMs.

RESULTS

Our results demonstrated that SIRT3 expression was significantly reduced in high glucose-treated SH-SY5Y cells and hippocampal tissues from diabetic mice. Further, up-regulating SIRT3 alleviated hippocampus injuries and cognitive impairment in diabetic mice and mitigated mitochondrial Ca²⁺ overload-induced mitochondrial dysfunction and apoptosis. Mechanistically, MAM formation was enhanced under high glucose conditions, which was reversed by genetic up-regulation of SIRT3 via reduced interaction of the VDAC1-GRP75-IP3R complex in vitro and in vivo. Furthermore, we investigated the therapeutic effects of pharmacological activation of SIRT3 in diabetic mice via honokiol treatment, which exhibited similar effects to our genetic interventions.

CONCLUSIONS

In summary, our findings suggest that SIRT3 ameliorates cognitive impairment in diabetic mice by limiting aberrant MAM formation. Furthermore, targeting the activation of SIRT3 by honokiol provides a promising therapeutic candidate for diabetes-associated cognitive dysfunction. Overall, our study suggests a novel role of SIRT3 in regulating MAM coupling and indicates that SIRT3-targeted therapies are promising for diabetic dementia patients.

Novel tumor therapy strategies targeting endoplasmic reticulum-mitochondria signal pathways

Organelles form tight connections through membrane contact sites, thereby cooperating to regulate homeostasis and cell function. Among them, the contact between endoplasmic reticulum (ER), the main intracellular calcium storage organelles, and mitochondria has been recognized for decades, and its main roles in the ion and lipid transport, ROS signaling, membrane dynamic changes and cellular metabolism are basically determined. At present, many tumor chemotherapeutic drugs rely on ER-mitochondrial calcium signal to function, but the mechanism of targeting resident molecules at the mitochondria-associated endoplasmic reticulum membranes (MAM) to sensitize traditional chemotherapy and the new tumor therapeutic targets identified based on the signal pathways on the MAM have not been thoroughly discussed. In this review, we highlight the key roles of various signaling pathways at the ER-mitochondria contact site in tumorigenesis and focus on novel anticancer therapy strategies targeting potential targets at this contact site.

Phospholipid metabolism in the liver - Implications for phosphatidylserine in non-alcoholic fatty liver disease

Mammalian cells contain more than a thousand different glycerophospholipid species that are essential membrane components and signalling molecules, with phosphatidylserine (PS) giving membranes their negative surface charge. Depending on the tissue, PS is important in apoptosis, blood clotting, cancer pathogenesis, as well as muscle and brain function, processes that are dependent on the asymmetrical distribution of PS on the plasma membrane and/or the capacity of PS to act as anchorage for various signalling proteins. Recent studies have implicated hepatic PS in the progression of non-alcoholic fatty liver disease (NAFLD), either as beneficial in the context of suppressing hepatic steatosis and fibrosis, or on the other hand as a potential contributor to the progression of liver cancer. This review provides an extensive overview of hepatic phospholipid metabolism, including its biosynthetic pathways, intracellular trafficking and roles in health and disease, further taking a deeper dive into PS metabolism, including associate and causative evidence of the role of PS in advanced liver disease.

Mitochondrial protein transport: Versatility of translocases and mechanisms

Biogenesis of mitochondria requires the import of approximately 1,000 different precursor proteins into and across the mitochondrial membranes. Mitochondria exhibit a wide variety of mechanisms and machineries for the translocation and sorting of precursor proteins. Five major import pathways that transport proteins to their functional intramitochondrial destination have been elucidated; these pathways range from the classical amino-terminal presequence-directed pathway to pathways using internal or even carboxy-terminal targeting signals in the precursors. Recent studies have provided important insights into the structural organization of membrane-embedded preprotein translocases of mitochondria. A comparison of the different translocases reveals the existence of at least three fundamentally different mechanisms: two-pore-translocase, β-barrel switching, and transport cavities open to the lipid bilayer. In addition, translocases are physically engaged in dynamic interactions with respiratory chain complexes, metabolite transporters, quality control factors, and machineries controlling membrane morphology. Thus, mitochondrial preprotein translocases are integrated into multi-functional networks of mitochondrial and cellular machineries.

New discoveries in ER-mitochondria communication

The study of endoplasmic reticulum (ER)-mitochondria communication is a vast and expanding field with many novel developments in the past few years. In this mini-review, we focus on several recent publications that identify novel functions of tether complexes, in particular autophagy regulation and lipid droplet biogenesis. We review novel findings that shed light on the role of triple contacts between ER and mitochondria with peroxisomes or lipid droplets as the third player. We also summarize recent findings on the role of ER-mitochondria contacts in human neurodegenerative diseases, which implicate either enhanced or reduced ER-mitochondria contacts in neurodegeneration. Taken together, the discussed studies highlight the need for further research into the role of triple organelle contacts, as well as into the exact mechanisms of increased and decreased ER-mitochondria contacts in neurodegeneration.

ORP9 and ORP10 form a heterocomplex to transfer phosphatidylinositol 4-phosphate at ER-TGN contact sites

Oxysterol-binding protein (OSBP) and its related proteins (ORPs) are a family of lipid transfer proteins (LTPs) that mediate non-vesicular lipid transport. ORP9 and ORP10, members of the OSBP/ORPs family, are located at the endoplasmic reticulum (ER)-trans-Golgi network (TGN) membrane contact sites (MCSs). It remained unclear how they mediate lipid transport. In this work, we discovered that ORP9 and ORP10 form a binary complex through intermolecular coiled-coil (CC) domain-CC domain interaction. The PH domains of ORP9 and ORP10 specially interact with phosphatidylinositol 4-phosphate (PI4P), mediating the TGN targeting. The ORP9-ORP10 complex plays a critical role in regulating PI4P levels at the TGN. Using in vitro reconstitution assays, we observed that while full-length ORP9 efficiently transferred PI4P between two apposed membranes, the lipid transfer kinetics was further accelerated by ORP10. Interestingly, our data showed that the PH domains of ORP9 and ORP10 participate in membrane tethering simultaneously, whereas ORDs of both ORP9 and ORP10 are required for lipid transport. Furthermore, our data showed that the depletion of ORP9 and ORP10 led to increased vesicle transport to the plasma membrane (PM). These findings demonstrate that ORP9 and ORP10 form a binary complex through the CC domains, maintaining PI4P homeostasis at ER-TGN MCSs and regulating vesicle trafficking.

Ubiquitin-mediated mitochondrial regulation by MITOL/MARCHF5 at a glance

Mitochondria are involved in various cellular processes, such as energy production, inflammatory responses and cell death. Mitochondrial dysfunction is associated with many age-related diseases, including neurological disorders and heart failure. Mitochondrial quality is strictly maintained by mitochondrial dynamics linked to an adequate supply of phospholipids and other substances from the endoplasmic reticulum (ER). The outer mitochondrial membrane-localized E3 ubiquitin ligase MITOL/MARCHF5 is responsible for mitochondrial quality control through the regulation of mitochondrial dynamics, formation of mitochondria-ER contacts and mitophagy. MITOL deficiency has been shown to impair mitochondrial function, cause an excessive inflammatory response and increase vulnerability to stress, resulting in the exacerbation of the disease. In this study, we overview the ubiquitin-mediated regulation of mitochondrial function by MITOL and the relationship between MITOL and diseases.

Fundamental roles for inter-organelle communication in aging

Advances in public health have nearly doubled life expectancy over the last century, but this demographic shift has also changed the landscape of human illness. Today, chronic and age-dependent diseases dominate the leading causes of morbidity and mortality worldwide. Targeting the underlying molecular, genetic and cell biological drivers of the aging process itself appears to be an increasingly viable strategy for developing therapeutics against these diseases of aging. Towards this end, one of the most exciting developments in cell biology over the last decade is the explosion of research into organelle contact sites and related mechanisms of inter-organelle communication. Identification of the molecular mediators of inter-organelle tethering and signaling is now allowing the field to investigate the consequences of aberrant organelle interactions, which frequently seem to correlate with age-onset pathophysiology. This review introduces the major cellular roles for inter-organelle interactions, including the regulation of organelle morphology, the transfer of ions, lipids and other metabolites, and the formation of hubs for nutrient and stress signaling. We explore how these interactions are disrupted in aging and present findings that modulation of inter-organelle communication is a promising avenue for promoting longevity. Through this review, we propose that the maintenance of inter-organelle interactions is a pillar of healthy aging. Learning how to target the cellular mechanisms for sensing and controlling inter-organelle communication is a key next hurdle for geroscience.