Coming together to define membrane contact sites

Microtubule-independent movement of the fission yeast nucleus

Movement of the cell nucleus typically involves the cytoskeleton and either polymerization-based pushing forces or motor-based pulling forces. In the fission yeast Schizosaccharomyces pombe, nuclear movement and positioning are thought to depend on microtubule polymerization-based pushing forces. Here, we describe a novel, microtubule-independent, form of nuclear movement in fission yeast. Microtubule-independent nuclear movement is directed towards growing cell tips, and it is strongest when the nucleus is close to a growing cell tip, and weakest when the nucleus is far from that tip. Microtubule-independent nuclear movement requires actin cables but does not depend on actin polymerization-based pushing or myosin V-based pulling forces. The vesicle-associated membrane protein (VAMP)-associated proteins (VAPs) Scs2 and Scs22, which are critical for endoplasmic reticulum-plasma membrane contact sites in fission yeast, are also required for microtubule-independent nuclear movement. We also find that in cells in which microtubule-based pushing forces are present, disruption of actin cables leads to increased fluctuations in interphase nuclear positioning and subsequent altered septation. Our results suggest two non-exclusive mechanisms for microtubule-independent nuclear movement, which may help illuminate aspects of nuclear positioning in other cells.

Phospholipid ebb and flow makes mitochondria go

Mitochondria, so much more than just being energy factories, also have the capacity to synthesize macromolecules including phospholipids, particularly cardiolipin (CL) and phosphatidylethanolamine (PE). Phospholipids are vital constituents of mitochondrial membranes, impacting the plethora of functions performed by this organelle. Hence, the orchestrated movement of phospholipids to and from the mitochondrion is essential for cellular integrity. In this review, we capture recent advances in the field of mitochondrial phospholipid biosynthesis and trafficking, highlighting the significance of interorganellar communication, intramitochondrial contact sites, and lipid transfer proteins in maintaining membrane homeostasis. We then discuss the physiological functions of CL and PE, specifically how they associate with protein complexes in mitochondrial membranes to support bioenergetics and maintain mitochondrial architecture.

Exploiting Connections for Viral Replication

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), the cause of the COVID-19 (coronavirus disease 2019) pandemic, is a positive strand RNA (+RNA) virus. Like other +RNA viruses, SARS-CoV-2 is dependent on host cell metabolic machinery to survive and replicate, remodeling cellular membranes to generate sites of viral replication. Viral RNA-containing double-membrane vesicles (DMVs) are a striking feature of +RNA viral replication and are abundant in SARS-CoV-2-infected cells. Their generation involves rewiring of host lipid metabolism, including lipid biosynthetic pathways. Viruses can also redirect lipids from host cell organelles; lipid exchange at membrane contact sites, where the membranes of adjacent organelles are in close apposition, has been implicated in the replication of several +RNA viruses. Here we review current understanding of DMV biogenesis. With a focus on the exploitation of contact site machinery by +RNA viruses to generate replication organelles, we discuss evidence that similar mechanisms support SARS-CoV-2 replication, protecting its RNA from the host cell immune response.

Mitochondria-rough-ER contacts in the liver regulate systemic lipid homeostasis

Contacts between organelles create microdomains that play major roles in regulating key intracellular activities and signaling pathways, but whether they also regulate systemic functions remains unknown. Here, we report the ultrastructural organization and dynamics of the inter-organellar contact established by sheets of curved rough endoplasmic reticulum closely wrapped around the mitochondria (wrappER). To elucidate the in vivo function of this contact, mouse liver fractions enriched in wrappER-associated mitochondria are analyzed by transcriptomics, proteomics, and lipidomics. The biochemical signature of the wrappER points to a role in the biogenesis of very-low-density lipoproteins (VLDL). Altering wrappER-mitochondria contacts curtails VLDL secretion and increases hepatic fatty acids, lipid droplets, and neutral lipid content. Conversely, acute liver-specific ablation of Mttp, the most upstream regulator of VLDL biogenesis, recapitulates this hepatic dyslipidemia phenotype and promotes remodeling of the wrappER-mitochondria contact. The discovery that liver wrappER-mitochondria contacts participate in VLDL biology suggests an involvement of inter-organelle contacts in systemic lipid homeostasis.

Interorganelle communication and membrane shaping in the early secretory pathway

Biomolecules in the secretory pathway use membrane trafficking for reaching their final intracellular destination or for secretion outside the cell. This highly dynamic and multipartite process involves different organelles that communicate to one another while maintaining their identity, shape, and function. Recent studies unraveled new mechanisms of interorganelle communication that help organize the early secretory pathway. We highlight how the spatial proximity between endoplasmic reticulum (ER) exit sites and early Golgi elements provides novel means of ER-Golgi communication for ER export. We also review recent findings on how membrane contact sites between the ER and the trans-Golgi membranes can sustain anterograde traffic out of the Golgi complex.

From observing to controlling: Inducible control of organelle dynamics and interactions

The dynamics and interactions of cellular organelles underlie many aspects of cellular functioning. Until recently, assessment of organelle dynamics has been primarily observational or required whole-cell perturbations to assess the implications of altered organelle motility and positioning. However, thanks to recently developed and optimized intervention strategies, we now have the ability to control organelles in their unperturbed state, altering organelle positioning, membrane trafficking pathways, as well as organelle interactions. This can be performed both globally and locally, giving fine control over the range, reversibility, and extent of organelle dynamics. Here, we describe how these tools are currently used for controlling organelles and give insight into the exciting future of this emerging field.

In the Right Place at the Right Time: Regulation of Cell Metabolism by IP3R-Mediated Inter-Organelle Ca2+ Fluxes

In the last few years, metabolism has been shown to be controlled by cross-organelle communication. The relationship between the endoplasmic reticulum and mitochondria/lysosomes is the most studied; here, inositol 1,4,5-triphosphate (IP3) receptor (IP3R)-mediated calcium (Ca²⁺) release plays a central role. Recent evidence suggests that IP3R isoforms participate in synthesis and degradation pathways. This minireview will summarize the current findings in this area, emphasizing the critical role of Ca²⁺ communication on organelle function as well as catabolism and anabolism, particularly in cancer.

A multifaceted analysis reveals two distinct phases of chloroplast biogenesis during de-etiolation in Arabidopsis

Light triggers chloroplast differentiation whereby the etioplast transforms into a photosynthesizing chloroplast and the thylakoid rapidly emerges. However, the sequence of events during chloroplast differentiation remains poorly understood. Using Serial Block Face Scanning Electron Microscopy (SBF-SEM), we generated a series of chloroplast 3D reconstructions during differentiation, revealing chloroplast number and volume and the extent of envelope and thylakoid membrane surfaces. Furthermore, we used quantitative lipid and whole proteome data to complement the (ultra)structural data, providing a time-resolved, multi-dimensional description of chloroplast differentiation. This showed two distinct phases of chloroplast biogenesis: an initial photosynthesis-enabling 'Structure Establishment Phase' followed by a 'Chloroplast Proliferation Phase' during cell expansion. Moreover, these data detail thylakoid membrane expansion during de-etiolation at the seedling level and the relative contribution and differential regulation of proteins and lipids at each developmental stage. Altogether, we establish a roadmap for chloroplast differentiation, a critical process for plant photoautotrophic growth and survival.

Perspectives on Organelle Interaction, Protein Dysregulation, and Cancer Disease

In recent decades, compelling evidence has emerged showing that organelles are not static structures but rather form a highly dynamic cellular network and exchange information through membrane contact sites. Although high-throughput techniques facilitate identification of novel contact sites (e.g., organelle-organelle and organelle-vesicle interactions), little is known about their impact on cellular physiology. Moreover, even less is known about how the dysregulation of these structures impacts on cellular function and therefore, disease. Particularly, cancer cells display altered signaling pathways involving several cell organelles; however, the relevance of interorganelle communication in oncogenesis and/or cancer progression remains largely unknown. This review will focus on organelle contacts relevant to cancer pathogenesis. We will highlight specific proteins and protein families residing in these organelle-interfaces that are known to be involved in cancer-related processes. First, we will review the relevance of endoplasmic reticulum (ER)-mitochondria interactions. This section will focus on mitochondria-associated membranes (MAMs) and particularly the tethering proteins at the ER-mitochondria interphase, as well as their role in cancer disease progression. Subsequently, the role of Ca²⁺ at the ER-mitochondria interphase in cancer disease progression will be discussed. Members of the Bcl-2 protein family, key regulators of cell death, also modulate Ca²⁺ transport pathways at the ER-mitochondria interphase. Furthermore, we will review the role of ER-mitochondria communication in the regulation of proteostasis, focusing on the ER stress sensor PERK (PRKR-like ER kinase), which exerts dual roles in cancer. Second, we will review the relevance of ER and mitochondria interactions with other organelles. This section will focus on peroxisome and lysosome organelle interactions and their impact on cancer disease progression. In this context, the peroxisome biogenesis factor (PEX) gene family has been linked to cancer. Moreover, the autophagy-lysosome system is emerging as a driving force in the progression of numerous human cancers. Thus, we will summarize our current understanding of the role of each of these organelles and their communication, highlighting how alterations in organelle interfaces participate in cancer development and progression. A better understanding of specific organelle communication sites and their relevant proteins may help to identify potential pharmacological targets for novel therapies in cancer control.

An ESCRT-dependent step in fatty acid transfer from lipid droplets to mitochondria through VPS13D-TSG101 interactions

Upon starvation, cells rewire their metabolism, switching from glucose-based metabolism to mitochondrial oxidation of fatty acids, which require the transfer of FAs from lipid droplets (LDs) to mitochondria at mitochondria−LD membrane contact sites (MCSs). However, factors responsible for FA transfer at these MCSs remain uncharacterized. Here, we demonstrate that vacuolar protein sorting-associated protein 13D (VPS13D), loss-of-function mutations of which cause spastic ataxia, coordinates FA trafficking in conjunction with the endosomal sorting complex required for transport (ESCRT) protein tumor susceptibility 101 (TSG101). The VPS13 adaptor-binding domain of VPS13D and TSG101 directly remodels LD membranes in a cooperative manner. The lipid transfer domain of human VPS13D binds glycerophospholipids and FAs in vitro. Depletion of VPS13D, TSG101, or ESCRT-III proteins inhibits FA trafficking from LDs to mitochondria. Our findings suggest that VPS13D mediates the ESCRT-dependent remodeling of LD membranes to facilitate FA transfer at mitochondria-LD contacts.

Metabolic rewiring requires the mobilization of fatty acids (FA) from lipid droplets (LDs) at membrane contact sites (MCSs), although the details of FA transfer remain unclear. Here, the authors show that VPS13D and the ESCRT complex remodel LD membranes to promote FA trafficking to mitochondria.

Mind the Gap: Mitochondria and the Endoplasmic Reticulum in Neurodegenerative Diseases

The way organelles are viewed by cell biologists is quickly changing. For many years, these cellular entities were thought to be unique and singular structures that performed specific roles. However, in recent decades, researchers have discovered that organelles are dynamic and form physical contacts. In addition, organelle interactions modulate several vital biological functions, and the dysregulation of these contacts is involved in cell dysfunction and different pathologies, including neurodegenerative diseases. Mitochondria–ER contact sites (MERCS) are among the most extensively studied and understood juxtapositioned interorganelle structures. In this review, we summarise the major biological and ultrastructural dysfunctions of MERCS in neurodegeneration, with a particular focus on Alzheimer’s disease as well as Parkinson’s disease, amyotrophic lateral sclerosis and frontotemporal dementia. We also propose an updated version of the MERCS hypothesis in Alzheimer’s disease based on new findings. Finally, we discuss the possibility of MERCS being used as possible drug targets to halt cell death and neurodegeneration.

Morphological bases of phytoplankton energy management and physiological responses unveiled by 3D subcellular imaging

Eukaryotic phytoplankton have a small global biomass but play major roles in primary production and climate. Despite improved understanding of phytoplankton diversity and evolution, we largely ignore the cellular bases of their environmental plasticity. By comparative 3D morphometric analysis across seven distant phytoplankton taxa, we observe constant volume occupancy by the main organelles and preserved volumetric ratios between plastids and mitochondria. We hypothesise that phytoplankton subcellular topology is modulated by energy-management constraints. Consistent with this, shifting the diatom Phaeodactylum from low to high light enhances photosynthesis and respiration, increases cell-volume occupancy by mitochondria and the plastid CO₂-fixing pyrenoid, and boosts plastid-mitochondria contacts. Changes in organelle architectures and interactions also accompany Nannochloropsis acclimation to different trophic lifestyles, along with respiratory and photosynthetic responses. By revealing evolutionarily-conserved topologies of energy-managing organelles, and their role in phytoplankton acclimation, this work deciphers phytoplankton responses at subcellular scales.

Noncanonical regulation of phosphatidylserine metabolism by a Sec14-like protein and a lipid kinase

The yeast phosphatidylserine (PtdSer) decarboxylase Psd2 is proposed to engage in a membrane contact site (MCS) for PtdSer decarboxylation to phosphatidylethanolamine (PtdEtn). This proposed MCS harbors Psd2, the Sec14-like phosphatidylinositol transfer protein (PITP) Sfh4, the Stt4 phosphatidylinositol (PtdIns) 4-OH kinase, the Scs2 tether, and an uncharacterized protein. We report that, of these components, only Sfh4 and Stt4 regulate Psd2 activity in vivo. They do so via distinct mechanisms. Sfh4 operates via a mechanism for which its PtdIns-transfer activity is dispensable but requires an Sfh4-Psd2 physical interaction. The other requires Stt4-mediated production of PtdIns-4-phosphate (PtdIns4P), where Stt4 (along with the Sac1 PtdIns4P phosphatase and endoplasmic reticulum–plasma membrane tethers) indirectly modulate Psd2 activity via a PtdIns4P homeostatic mechanism that influences PtdSer accessibility to Psd2. These results identify an example in which the biological function of a Sec14-like PITP is cleanly uncoupled from its canonical in vitro PtdIns-transfer activity and challenge popular functional assumptions regarding lipid-transfer protein involvements in MCS function.

ER-PM Contact Sites - SNARING Actors in Emerging Functions

The compartmentalisation achieved by confining cytoplasm into membrane-enclosed organelles in eukaryotic cells is essential for maintaining vital functions including ATP production, synthetic and degradative pathways. While intracellular organelles are highly specialised in these functions, the restricting membranes also impede exchange of molecules responsible for the synchronised and responsive cellular activities. The initial identification of contact sites between the ER and plasma membrane (PM) provided a potential candidate structure for communication between organelles without mixing by fusion. Over the past decades, research has revealed a far broader picture of the events. Membrane contact sites (MCSs) have been recognized as increasingly important actors in cell differentiation, plasticity and maintenance, and, upon dysfunction, responsible for pathological conditions such as cancer and neurodegenerative diseases. Present in multiple organelles and cell types, MCSs promote transport of lipids and Ca²⁺ homoeostasis, with a range of associated protein families. Interestingly, each MCS displays a unique molecular signature, adapted to organelle functions. This review will explore the literature describing the molecular components and interactions taking place at ER-PM contact sites, their functions, and implications in eukaryotic cells, particularly neurons, with emphasis on lipid transfer proteins and emerging function of SNAREs.

Lipid Droplet Contact Sites in Health and Disease

After having been disregarded for a long time as inert fat drops, lipid droplets (LDs) are now recognized as ubiquitous cellular organelles with key functions in lipid biology and beyond. The identification of abundant LD contact sites, places at which LDs are physically attached to other organelles, has uncovered an unexpected level of communication between LDs and the rest of the cell. In recent years, many disease factors mutated in hereditary disorders have been recognized as LD contact site proteins. Furthermore, LD contact sites are dramatically rearranged in response to infections with intracellular pathogens, as well as under pathological metabolic conditions such as hepatic steatosis. Collectively, it is emerging that LD-organelle contacts are important players in development and progression of disease.

Mitochondrial control of cellular protein homeostasis

Mitochondria are involved in several vital functions of the eukaryotic cell. The majority of mitochondrial proteins are coded by nuclear DNA. Constant import of proteins from the cytosol is a prerequisite for the efficient functioning of the organelle. The protein import into mitochondria is mediated by diverse import pathways and is continuously under watch by quality control systems. However, it is often challenged by both internal and external factors, such as oxidative stress or energy shortage. The impaired protein import and biogenesis leads to the accumulation of mitochondrial precursor proteins in the cytosol and activates several stress response pathways. These defense mechanisms engage a network of processes involving transcription, translation, and protein clearance to restore cellular protein homeostasis. In this review, we provide a comprehensive analysis of various factors and processes contributing to mitochondrial stress caused by protein biogenesis failure and summarize the recovery mechanisms employed by the cell.

Emr1 regulates the number of foci of the endoplasmic reticulum-mitochondria encounter structure complex

The endoplasmic reticulum-mitochondria encounter structure (ERMES) complex creates contact sites between the endoplasmic reticulum and mitochondria, playing crucial roles in interorganelle communication, mitochondrial fission, mtDNA inheritance, lipid transfer, and autophagy. The mechanism regulating the number of ERMES foci within the cell remains unclear. Here, we demonstrate that the mitochondrial membrane protein Emr1 contributes to regulating the number of ERMES foci. We show that the absence of Emr1 significantly decreases the number of ERMES foci. Moreover, we find that Emr1 interacts with the ERMES core component Mdm12 and colocalizes with Mdm12 on mitochondria. Similar to ERMES mutant cells, cells lacking Emr1 display defective mitochondrial morphology and impaired mitochondrial segregation, which can be rescued by an artificial tether capable of linking the endoplasmic reticulum and mitochondria. We further demonstrate that the cytoplasmic region of Emr1 is required for regulating the number of ERMES foci. This work thus reveals a crucial regulatory protein necessary for ERMES functions and provides mechanistic insights into understanding the dynamic regulation of endoplasmic reticulum-mitochondria communication.

Interorganelle membrane contact sites between the endoplasmic reticulum and mitochondria can be mediated with the ER-mitochondria encounter structure (ERMES) complex, though precise regulation is unclear. Here, the authors report that the number of ERMES foci is regulated by the previously uncharacterized mitochondrial membrane protein Emr1.

Mitochondria-lysosome membrane contacts are defective in GDAP1-related Charcot-Marie-Tooth disease

Mutations in the GDAP1 gene cause Charcot-Marie-Tooth (CMT) neuropathy. GDAP1 is an atypical glutathione S-transferase (GST) of the outer mitochondrial membrane and the mitochondrial membrane contacts with the endoplasmic reticulum (MAMs). Here, we investigate the role of this GST in the autophagic flux and the membrane contact sites (MCSs) between mitochondria and lysosomes in the cellular pathophysiology of GDAP1 deficiency. We demonstrate that GDAP1 participates in basal autophagy and that its depletion affects LC3 and PI3P biology in autophagosome biogenesis and membrane trafficking from MAMs. GDAP1 also contributes to the maturation of lysosome by interacting with PYKfyve kinase, a pH-dependent master lysosomal regulator. GDAP1 deficiency causes giant lysosomes with hydrolytic activity, a delay in the autophagic lysosome reformation, and TFEB activation. Notably, we found that GDAP1 interacts with LAMP-1, which supports that GDAP1-LAMP-1 is a new tethering pair of mitochondria and lysosome membrane contacts. We observed mitochondria-lysosome MCSs in soma and axons of cultured mouse embryonic motor neurons and human neuroblastoma cells. GDAP1 deficiency reduces the MCSs between these organelles, causes mitochondrial network abnormalities, and decreases levels of cellular glutathione (GSH). The supply of GSH-MEE suffices to rescue the lysosome membranes and the defects of the mitochondrial network, but not the interorganelle MCSs nor early autophagic events. Overall, we show that GDAP1 enables the proper function of mitochondrial MCSs in both degradative and nondegradative pathways, which could explain primary insults in GDAP1-related CMT pathophysiology, and highlights new redox-sensitive targets in axonopathies where mitochondria and lysosomes are involved.

Snd3 controls nucleus-vacuole junctions in response to glucose signaling

Membrane contact sites facilitate the exchange of metabolites between organelles to support interorganellar communication. The nucleus-vacuole junctions (NVJs) establish physical contact between the perinuclear endoplasmic reticulum (ER) and the vacuole. Although the NVJ tethers are known, how NVJ abundance and composition are controlled in response to metabolic cues remains elusive. Here, we identify the ER protein Snd3 as central factor for NVJ formation. Snd3 interacts with NVJ tethers, supports their targeting to the contacts, and is essential for NVJ formation. Upon glucose exhaustion, Snd3 relocalizes from the ER to NVJs and promotes contact expansion regulated by central glucose signaling pathways. Glucose replenishment induces the rapid dissociation of Snd3 from the NVJs, preceding the slow disassembly of the junctions. In sum, this study identifies a key factor required for formation and regulation of NVJs and provides a paradigm for metabolic control of membrane contact sites.

Relevance of Membrane Contact Sites in Cancer Progression

Membrane contact sites (MCS) are typically defined as areas of proximity between heterologous or homologous membranes characterized by specific proteins. The study of MCS is considered as an emergent field that shows how crucial organelle interactions are in cell physiology. MCS regulate a myriad of physiological processes such as apoptosis, calcium, and lipid signaling, just to name a few. The membranal interactions between the endoplasmic reticulum (ER)–mitochondria, the ER–plasma membrane, and the vesicular traffic have received special attention in recent years, particularly in cancer research, in which it has been proposed that MCS regulate tumor metabolism and fate, contributing to their progression. However, as the therapeutic or diagnostic potential of MCS has not been fully revisited, in this review, we provide recent information on MCS relevance on calcium and lipid signaling in cancer cells and on its role in tumor progression. We also describe some proteins associated with MCS, like CERT, STIM1, VDAC, and Orai, that impact on cancer progression and that could be a possible diagnostic marker. Overall, these information might contribute to the understanding of the complex biology of cancer cells.

Remodelling of Nucleus-Vacuole Junctions During Metabolic and Proteostatic Stress

Cellular adaptation to stress and metabolic cues requires a coordinated response of different intracellular compartments, separated by semipermeable membranes. One way to facilitate interorganellar communication is via membrane contact sites, physical bridges between opposing organellar membranes formed by an array of tethering machineries. These contact sites are highly dynamic and establish an interconnected organellar network able to quickly respond to external and internal stress by changing size, abundance and molecular architecture. Here, we discuss recent work on nucleus-vacuole junctions, connecting yeast vacuoles with the nucleus. Appearing as small, single foci in mitotic cells, these contacts expand into one enlarged patch upon nutrient exhaustion and entry into quiescence or can be shaped into multiple large foci essential to sustain viability upon proteostatic stress at the nuclear envelope. We highlight the remarkable plasticity and rapid remodelling of these contact sites upon metabolic or proteostatic stress and their emerging importance for cellular fitness.

One ring to bring them all and in the darkness bind them: The trafficking of heme without deliverers

Heme, as a hydrophobic iron-containing organic ring, is lipid soluble and can interact with biological membranes. The very same properties of heme that nature exploits to support life also renders heme potentially cytotoxic. In order to utilize heme, while also mitigating its toxicity, cells are challenged to tightly control the concentration and bioavailability of heme. On the bright side, it is reasonable to envision that, analogous to other transition metals, a combination of membrane-bound transporters, soluble carriers, and chaperones coordinate heme trafficking to subcellular compartments. However, given the dual properties exhibited by heme as a transition metal and lipid, it is compelling to consider the dark side: the potential role of non-proteinaceous biomolecules including lipids and nucleic acids that bind, sequester, and control heme trafficking and bioavailability. The emergence of inter-organellar membrane contact sites, as well as intracellular vesicles derived from various organelles, have raised the prospect that heme can be trafficked through hydrophobic channels. In this review, we aim to focus on heme delivery without deliverers - an alternate paradigm for the regulation of heme homeostasis through chaperone-less pathways for heme trafficking.

The Endoplasmic Reticulum-Mitochondria Encounter Structure and its Regulatory Proteins

In fungi, the endoplasmic reticulum-mitochondria encounter structure (ERMES) is present between the endoplasmic reticulon (ER) and mitochondria to promote the formation of the ER-mitochondria contact sites. Four constitutive components (Mmm1, Mdm12, Mdm34, and Mdm10) assemble to form the ERMES complex while regulator proteins are required for regulating the organization and function of the ERMES complex. Multiple regulator proteins, including Gem1, Lam6, Tom7, and Emr1, of the ERMES complex, have been identified recently. In this review, we discuss the organization of the ERMES complex and the roles of the regulator proteins of the ERMES complex.

The Fast and the Furious: Golgi Contact Sites

Contact sites are areas of close apposition between two membranes that coordinate nonvesicular communication between organelles. Such interactions serve a wide range of cellular functions from regulating metabolic pathways to executing stress responses and coordinating organelle inheritance. The past decade has seen a dramatic increase in information on certain contact sites, mostly those involving the endoplasmic reticulum. However, despite its central role in the secretory pathway, the Golgi apparatus and its contact sites remain largely unexplored. In this review, we discuss the current knowledge of Golgi contact sites and share our thoughts as to why Golgi contact sites are understudied. We also highlight what exciting future directions may exist in this emerging field.

Sigma 1 Receptor, Cholesterol and Endoplasmic Reticulum Contact Sites

Recent studies indicated potential importance of membrane contact sites (MCS) between the endoplasmic reticulum (ER) and other cellular organelles. These MCS have unique protein and lipid composition and serve as hubs for inter-organelle communication and signaling. Despite extensive investigation of MCS protein composition and functional roles, little is known about the process of MCS formation. In this perspective, we propose a hypothesis that MCS are formed not as a result of random interactions between membranes of ER and other organelles but on the basis of pre-existing cholesterol-enriched ER microdomains.

Mitochondria Endoplasmic Reticulum Contact Sites (MERCs): Proximity Ligation Assay as a Tool to Study Organelle Interaction

Organelles cooperate with each other to regulate vital cellular homoeostatic functions. This occurs through the formation of close connections through membrane contact sites. Mitochondria-Endoplasmic-Reticulum (ER) contact sites (MERCS) are one of such contact sites that regulate numerous biological processes by controlling calcium and metabolic homeostasis. However, the extent to which contact sites shape cellular biology and the underlying mechanisms remain to be fully elucidated. A number of biochemical and imaging approaches have been established to address these questions, resulting in the identification of a number of molecular tethers between mitochondria and the ER. Among these techniques, fluorescence-based imaging is widely used, including analysing signal overlap between two organelles and more selective techniques such as in-situ proximity ligation assay (PLA). While these two techniques allow the detection of endogenous proteins, preventing some problems associated with techniques relying on overexpression (FRET, split fluorescence probes), they come with their own issues. In addition, proper image analysis is required to minimise potential artefacts associated with these methods. In this review, we discuss the protocols and outline the limitations of fluorescence-based approaches used to assess MERCs using endogenous proteins.

Lipid Metabolism at Membrane Contacts: Dynamics and Functions Beyond Lipid Homeostasis

Membrane contact sites (MCSs), regions where the membranes of two organelles are closely apposed, play critical roles in inter-organelle communication, such as lipid trafficking, intracellular signaling, and organelle biogenesis and division. First identified as “fraction X” in the early 90s, MCSs are now widely recognized to facilitate local lipid synthesis and inter-organelle lipid transfer, which are important for maintaining cellular lipid homeostasis. In this review, we discuss lipid metabolism and related cellular and physiological functions in MCSs. We start with the characteristics of lipid synthesis and breakdown at MCSs. Then we focus on proteins involved in lipid synthesis and turnover at these sites. Lastly, we summarize the cellular function of lipid metabolism at MCSs beyond mere lipid homeostasis, including the physiological meaning and relevance of MCSs regarding systemic lipid metabolism. This article is part of an article collection entitled: Coupling and Uncoupling: Dynamic Control of Membrane Contacts.

Direct lysosome-based autophagy of lipid droplets in hepatocytes

Hepatocytes metabolize energy-rich cytoplasmic lipid droplets (LDs) in the lysosome-directed process of autophagy. An organelle-selective form of this process (macrolipophagy) results in the engulfment of LDs within double-membrane delimited structures (autophagosomes) before lysosomal fusion. Whether this is an exclusive autophagic mechanism used by hepatocytes to catabolize LDs is unclear. It is also unknown whether lysosomes alone might be sufficient to mediate LD turnover in the absence of an autophagosomal intermediate. We performed live-cell microscopy of hepatocytes to monitor the dynamic interactions between lysosomes and LDs in real-time. We additionally used a fluorescent variant of the LD-specific protein (PLIN2) that exhibits altered fluorescence in response to LD interactions with the lysosome. We find that mammalian lysosomes and LDs undergo interactions during which proteins and lipids can be transferred from LDs directly into lysosomes. Electron microscopy (EM) of primary hepatocytes or hepatocyte-derived cell lines supports the existence of these interactions. It reveals a dramatic process whereby the lipid contents of the LD can be "extruded" directly into the lysosomal lumen under nutrient-limited conditions. Significantly, these interactions are not affected by perturbations to crucial components of the canonical macroautophagy machinery and can occur in the absence of double-membrane lipoautophagosomes. These findings implicate the existence of an autophagic mechanism used by mammalian cells for the direct transfer of LD components into the lysosome for breakdown. This process further emphasizes the critical role of lysosomes in hepatic LD catabolism and provides insights into the mechanisms underlying lipid homeostasis in the liver.

Complex Interactions Between Membrane-Bound Organelles, Biomolecular Condensates and the Cytoskeleton

Membrane-bound and membraneless organelles/biomolecular condensates ensure compartmentalization into functionally distinct units enabling proper organization of cellular processes. Membrane-bound organelles form dynamic contacts with each other to enable the exchange of molecules and to regulate organelle division and positioning in coordination with the cytoskeleton. Crosstalk between the cytoskeleton and dynamic membrane-bound organelles has more recently also been found to regulate cytoskeletal organization. Interestingly, recent work has revealed that, in addition, the cytoskeleton and membrane-bound organelles interact with cytoplasmic biomolecular condensates. The extent and relevance of these complex interactions are just beginning to emerge but may be important for cytoskeletal organization and organelle transport and remodeling. In this review, we highlight these emerging functions and emphasize the complex interplay of the cytoskeleton with these organelles. The crosstalk between membrane-bound organelles, biomolecular condensates and the cytoskeleton in highly polarized cells such as neurons could play essential roles in neuronal development, function and maintenance.

Improved Split-GFP Systems for Visualizing Organelle Contact Sites in Yeast and Human Cells

Inter-organelle contact sites have attracted a lot of attention as functionally specialized regions that mediate the exchange of metabolites, including lipids and ions, between distinct organelles. However, studies on inter-organelle contact sites are at an early stage and it remains enigmatic what directly mediates the organelle-organelle interactions and how the number and degree of the contacts are regulated. As a first step to answer these questions, we previously developed split-GFP probes that could visualize and quantify multiple inter-organelle contact sites in the yeast and human cultured cells. However, the split-GFP probes possessed a disadvantage of inducing artificial connections between two different organelle membranes, especially when overexpressed. In the present study, we developed a way to express the split-GFP probes whose expressions remained at low levels, with minimal variations between different yeast cells. Besides, we constructed a HeLa cell line in which the expression of the split-GFP probes could be induced by the addition of doxycycline to minimize the artificial effects. The improved split-GFP systems may be faithful tools to quantify organelle contact sites and screen new factors involved in organelle-organelle tethering in yeast and mammalian cells.

Mitochondria form contact sites with the nucleus to couple prosurvival retrograde response

Mitochondria drive cellular adaptation to stress by retro-communicating with the nucleus. This process is known as mitochondrial retrograde response (MRR) and is induced by mitochondrial dysfunction. MRR results in the nuclear stabilization of prosurvival transcription factors such as the nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB). Here, we demonstrate that MRR is facilitated by contact sites between mitochondria and the nucleus. The translocator protein (TSPO) by preventing the mitophagy-mediated segregation o mitochonria is required for this interaction. The complex formed by TSPO with the protein kinase A (PKA), via the A-kinase anchoring protein acyl-CoA binding domain containing 3 (ACBD3), established the tethering. The latter allows for cholesterol redistribution of cholesterol in the nucleus to sustain the prosurvival response by blocking NF-κB deacetylation. This work proposes a previously unidentified paradigm in MRR: the formation of contact sites between mitochondria and nucleus to aid communication.

Relevance of endoplasmic reticulum and mitochondria interactions in age-associated diseases

Although the elixir of youth remains in the darkness, medical and scientific advances have succeeded in increasing human longevity; however, the predisposition to disease and its high economic cost are raising. Different strategies (e.g., antioxidants) and signaling pathways (e.g., Nrf2) have been identified to help regulate disease progression, nevertheless, there are still missing links that we need to understand. Contact sites called mitochondria-associated membranes (MAM) allow bi-directional communication between organelles as part of the essential functions in the cell to maintain its homeostasis. Different groups have deeply studied the role of MAM in aging; however, it's necessary to analyze their involvement in the progression of age-related diseases. In this review, we highlight the role of contact sites in these conditions, as well as the morphological and functional changes of mitochondria and ER in aging. We emphasize the intimate relationship between both organelles as a reflection of the biological processes that take place in the cell to try to regulate the deterioration characteristic of the aging process; proposing MAM as a potential target to help limit the disease progression with age.

A central role of the endoplasmic reticulum in the cell emerges from its functional contact sites with multiple organelles

Early eukaryotic cells emerged from the compartmentalization of metabolic processes into specific organelles through the development of an endomembrane system (ES), a precursor of the endoplasmic reticulum (ER), which was essential for their survival. Recently, substantial evidence emerged on how organelles communicate among themselves and with the plasma membrane (PM) through contact sites (CSs). From these studies, the ER-the largest single structure in eukaryotic cells-emerges as a central player communicating with all organelles to coordinate cell functions and respond to external stimuli to maintain cellular homeostasis. Herein we review the functional insights into the ER-CSs with other organelles in a physiological perspective. We hypothesize that, in addition to the primitive role by the ES in the appearance of proto-eukaryotes, its successor-the ER-emerges as the key coordinator of inter-organelle/PM communication. The ER thus appears to be the 'maestro' driving eukaryotic cell evolution by incorporating new functions/organelles, while remaining the real coordinator overarching cellular functions and orchestrating them with the external milieu.

An expanded palette of improved SPLICS reporters detects multiple organelle contacts in vitro and in vivo

Membrane contact sites between virtually any known organelle have been documented and, in the last decades, their study received momentum due to their importance for fundamental activities of the cell and for the subtle comprehension of many human diseases. The lack of tools to finely image inter-organelle proximity hindered our understanding on how these subcellular communication hubs mediate and regulate cell homeostasis. We develop an improved and expanded palette of split-GFP-based contact site sensors (SPLICS) for the detection of single and multiple organelle contact sites within a scalable distance range. We demonstrate their flexibility under physiological conditions and in living organisms.

Cerebellar and hepatic alterations in ACBD5-deficient mice are associated with unexpected, distinct alterations in cellular lipid homeostasis

ACBD5 deficiency is a novel peroxisome disorder with a largely uncharacterized pathology. ACBD5 was recently identified in a tethering complex mediating membrane contacts between peroxisomes and the endoplasmic reticulum (ER). An ACBD5-deficient mouse was analyzed to correlate ACBD5 tethering functions with the disease phenotype. ACBD5-deficient mice exhibit elevated very long-chain fatty acid levels and a progressive cerebellar pathology. Liver did not exhibit pathologic changes but increased peroxisome abundance and drastically reduced peroxisome-ER contacts. Lipidomics of liver and cerebellum revealed tissue-specific alterations in distinct lipid classes and subspecies. In line with the neurological pathology, unusual ultra-long chain fatty acids (C > 32) were elevated in phosphocholines from cerebelli but not liver indicating an organ-specific imbalance in fatty acid degradation and elongation pathways. By contrast, ether lipid formation was perturbed in liver towards an accumulation of alkyldiacylglycerols. The alterations in several lipid classes suggest that ACBD5, in addition to its acyl-CoA binding function, might maintain peroxisome-ER contacts in order to contribute to the regulation of anabolic and catabolic cellular lipid pathways.

Darwisch, von Spangenberg et al. show that ACBD5‐deficient mice exhibit elevated levels of very long‐chain fatty acids and a progressive cerebellar pathology. A complex metabolic phenotype suggests that ACBD5 with its acyl‐CoA binding and peroxisome‐ER tethering functions might contribute to the regulation of anabolic and catabolic cellular lipid pathways.

TORC2-Dependent Ypk1-Mediated Phosphorylation of Lam2/Ltc4 Disrupts Its Association with the β-Propeller Protein Laf1 at Endoplasmic Reticulum-Plasma Membrane Contact Sites in the Yeast Saccharomyces cerevisiae

Membrane-tethered sterol-binding Lam/Ltc proteins localize at junctions between the endoplasmic reticulum (ER) membrane and other organelles. Two of the six family members-Lam2/Ltc4 (initially Ysp2) and paralog Lam4/Ltc3-localize to ER-plasma membrane (PM) contact sites (CSs) and mediate retrograde ergosterol transport from the PM to the ER. Our prior work demonstrated that Lam2 and Lam4 are substrates of TORC2-regulated protein kinase Ypk1, that Ypk1-mediated phosphorylation inhibits their function in retrograde sterol transport, and that PM sterol retention bolsters cell survival under stressful conditions. At ER-PM CSs, Lam2 and Lam4 associate with Laf1/Ymr102c and Dgr2/Ykl121w (paralogous WD40 repeat-containing proteins) that reportedly bind sterol. Using fluorescent tags, we found that Lam2 and Lam4 remain at ER-PM CSs when Laf1 and Dgr2 are absent, whereas neither Laf1 nor Dgr2 remain at ER-PM CSs when Lam2 and Lam4 are absent. Loss of Laf1 (but not Dgr2) impedes retrograde ergosterol transport, and a laf1∆ mutation does not exacerbate the transport defect of lam2∆ lam4∆ cells, indicating a shared function. Lam2 and Lam4 bind Laf1 and Dgr2 in vitro in a pull-down assay, and the PH domain in Lam2 hinders its interaction with Laf1. Lam2 phosphorylated by Ypk1, and Lam2 with phosphomimetic (Glu) replacements at its Ypk1 sites, exhibited a marked reduction in Laf1 binding. Thus, phosphorylation prevents Lam2 interaction with Laf1 at ER-PM CSs, providing a mechanism by which Ypk1 action inhibits retrograde sterol transport.

Mitochondria-ER Tethering in Neurodegenerative Diseases

Organelles juxtaposition has been detected for decades, although only recently gained importance due to a pivotal role in the regulation of cellular processes dependent on membrane contact sites. Endoplasmic reticulum (ER) and mitochondria interaction is a prime example of organelles contact sites. Mitochondria-associated membranes (MAM) are proposed to harbor ER-mitochondria tether complexes, mainly when these organelles are less than 30 nm apart. Dysfunctions of proteins located at the MAM are associated with neurodegenerative diseases such as Parkinson's, Alzheimer's and amyotrophic lateral sclerosis, as well as neurodevelopmental disorders; hence any malfunction in MAM can potentially trigger cell death. This review will focus on the role of ER-mitochondria contact sites, regarding calcium homeostasis, lipid metabolism, autophagy, morphology and dynamics of mitochondria, mainly in the context of neurodegenerative diseases. Approaches that have been employed so far to study organelles contact sites, as well as methods that were not used in neurosciences yet, but are promising and accurate ways to unveil the functions of MAM during neurodegeneration, is also discussed in the present review.

Inter-organelle membrane contact sites: implications for lipid metabolism

This article supplements a recent Perspective by Scorrano et al. in Nature Communications [10 [ (1)]:1287] in which the properties and functions of inter-organelle membrane contact sites were summarized. It is now clear that inter-organelle membrane contact sites are widespread in eukaryotic cells and that diverse pairs of organelles can be linked via unique protein tethers. An appropriate definition of what constitutes an inter-organelle membrane contact site was proposed in the Perspective. In addition, the various experimental approaches that are frequently used to study these organelle associations, as well as the advantages and disadvantages of each of these methods, were considered. The nature of the tethers that link the pairs of organelles at the contact sites was discussed in detail and some biological functions that have been ascribed to specific membrane contact sites were highlighted. Nevertheless, the functions of most types of organelle contact sites remain unclear. In the current article I have considered some of the points raised in the Perspective but have omitted detailed information on the roles of membrane contact sites in biological functions such as apoptosis, autophagy, calcium homeostasis and mitochondrial fusion. Instead, I have provided some background on the initial discovery of mitochondria-endoplasmic reticulum membrane contact sites, and have focussed on the known roles of membrane contact sites in inter-organelle lipid transport. In addition, potential roles for membrane contact sites in human diseases are briefly discussed.

Drp1 Tubulates the ER in a GTPase-Independent Manner

Mitochondria are highly dynamic organelles that continuously grow, divide, and fuse. The division of mitochondria is crucial for human health. During mitochondrial division, the mechano-guanosine triphosphatase (GTPase) dynamin-related protein (Drp1) severs mitochondria at endoplasmic reticulum (ER)-mitochondria contact sites, where peripheral ER tubules interact with mitochondria. Here, we report that Drp1 directly shapes peripheral ER tubules in human and mouse cells. This ER-shaping activity is independent of GTP hydrolysis and located in a highly conserved peptide of 18 amino acids (termed D-octadecapeptide), which is predicted to form an amphipathic α helix. Synthetic D-octadecapeptide tubulates liposomes in vitro and the ER in cells. ER tubules formed by Drp1 promote mitochondrial division by facilitating ER-mitochondria interactions. Thus, Drp1 functions as a two-in-one protein during mitochondrial division, with ER tubulation and mechano-GTPase activities.

Friend or Foe: Lipid Droplets as Organelles for Protein and Lipid Storage in Cellular Stress Response, Aging and Disease

Lipid droplets (LDs) were considered as a mere lipid storage organelle for a long time. Recent evidence suggests that LDs are in fact distinct and dynamic organelles with a specialized proteome and functions in many cellular roles. As such, LDs contribute to cellular signaling, protein and lipid homeostasis, metabolic diseases and inflammation. In line with the multitude of functions, LDs interact with many cellular organelles including mitochondria, peroxisomes, lysosomes, the endoplasmic reticulum and the nucleus. LDs are highly mobile and dynamic organelles and impaired motility disrupts the interaction with other organelles. The reduction of interorganelle contacts results in a multitude of pathophysiologies and frequently in neurodegenerative diseases. Contacts not only supply lipids for β-oxidation in mitochondria and peroxisomes, but also may include the transfer of toxic lipids as well as misfolded and harmful proteins to LDs. Furthermore, LDs assist in the removal of protein aggregates when severe proteotoxic stress overwhelms the proteasomal system. During imbalance of cellular lipid homeostasis, LDs also support cellular detoxification. Fine-tuning of LD function is of crucial importance and many diseases are associated with dysfunctional LDs. We summarize the current understanding of LDs and their interactions with organelles, providing a storage site for harmful proteins and lipids during cellular stress, aging inflammation and various disease states.

Interorganelle Tethering to Endocytic Organelles Determines Directional Cytokine Transport in CD4+ T Cells

Delivery of vesicles to their desired destinations plays a central role in maintaining proper cell functionality. In certain scenarios, depending on loaded cargos, the vesicles have spatially distinct destinations. For example, in T cells, some cytokines (e.g., IL-2) are polarized to the T cell-target cell interface, whereas the other cytokines are delivered multidirectionally (e.g., TNF-α). In this study, we show that in primary human CD4⁺ T cells, both TNF-α⁺ and IL-2⁺ vesicles can tether with endocytic organelles (lysosomes/late endosomes) by forming membrane contact sites. Tethered cytokine-containing vesicle (CytV)-endocytic organelle pairs are released sequentially. Only endocytic organelle-tethered CytVs are preferentially transported to their desired destination. Mathematical models suggest that endocytic organelle tethering could regulate the direction of cytokine transport by selectively attaching different microtubule motor proteins (such as kinesin and dynein) to the corresponding CytVs. These findings establish the previously unknown interorganelle tethering to endocytic organelles as a universal solution for directional cytokine transport in CD4⁺ T cells. Modulating tethering to endocytic organelles can, therefore, coordinately control directionally distinct cytokine transport.

Downregulation of Keap1 Confers Features of a Fasted Metabolic State

Transcription factor nuclear factor erythroid 2 p45-related factor 2 (Nrf2) and its main negative regulator, Kelch-like ECH-associated protein 1 (Keap1), are at the interface between redox and intermediary metabolism, allowing adaptation and survival under conditions of oxidative, inflammatory, and metabolic stress. Nrf2 is the principal determinant of redox homeostasis, and contributes to mitochondrial function and integrity and cellular bioenergetics. Using proteomics and lipidomics, we show that genetic downregulation of Keap1 in mice, and the consequent Nrf2 activation to pharmacologically relevant levels, leads to upregulation of carboxylesterase 1 (Ces1) and acyl-CoA oxidase 2 (Acox2), decreases triglyceride levels, and alters the lipidome. This is accompanied by downregulation of hepatic ATP-citrate lyase (Acly) and decreased levels of acetyl-CoA, a trigger for autophagy. These findings suggest that downregulation of Keap1 confers features of a fasted metabolic state, which is an important consideration in the drug development of Keap1-targeting pharmacologic Nrf2 activators.

Mitochondrial Surveillance by Cdc48/p97: MAD vs. Membrane Fusion

Cdc48/p97 is a ring-shaped, ATP-driven hexameric motor, essential for cellular viability. It specifically unfolds and extracts ubiquitylated proteins from membranes or protein complexes, mostly targeting them for proteolytic degradation by the proteasome. Cdc48/p97 is involved in a multitude of cellular processes, reaching from cell cycle regulation to signal transduction, also participating in growth or death decisions. The role of Cdc48/p97 in endoplasmic reticulum-associated degradation (ERAD), where it extracts proteins targeted for degradation from the ER membrane, has been extensively described. Here, we present the roles of Cdc48/p97 in mitochondrial regulation. We discuss mitochondrial quality control surveillance by Cdc48/p97 in mitochondrial-associated degradation (MAD), highlighting the potential pathologic significance thereof. Furthermore, we present the current knowledge of how Cdc48/p97 regulates mitofusin activity in outer membrane fusion and how this may impact on neurodegeneration.

Protrudin and PDZD8 contribute to neuronal integrity by promoting lipid extraction required for endosome maturation

Endosome maturation depends on membrane contact sites (MCSs) formed between endoplasmic reticulum (ER) and endolysosomes (LyLEs). The mechanism underlying lipid supply for this process and its pathophysiological relevance remains unclear, however. Here, we identify PDZD8-the mammalian ortholog of a yeast ERMES subunit-as a protein that interacts with protrudin, which is located at ER-LyLE MCSs. Protrudin and PDZD8 promote the formation of ER-LyLE MCSs, and PDZD8 shows the ability to extract various lipids from the ER. Overexpression of both protrudin and PDZD8 in HeLa cells, as well as their depletion in mouse primary neurons, impairs endosomal homeostasis by inducing the formation of abnormal large vacuoles reminiscent of those apparent in spastin- or REEP1-deficient neurons. The protrudin-PDZD8 system is also essential for the establishment of neuronal polarity. Our results suggest that protrudin and PDZD8 cooperatively promote endosome maturation by mediating ER-LyLE tethering and lipid extraction at MCSs, thereby maintaining neuronal polarity and integrity.

Dynamic organization of intracellular organelle networks

Intracellular organelles are membrane-bound and biochemically distinct compartments constructed to serve specialized functions in eukaryotic cells. Through extensive interactions, they form networks to coordinate and integrate their specialized functions for cell physiology. A fundamental property of these organelle networks is that they constantly undergo dynamic organization via membrane fusion and fission to remodel their internal connections and to mediate direct material exchange between compartments. The dynamic organization not only enables them to serve critical physiological functions adaptively but also differentiates them from many other biological networks such as gene regulatory networks and cell signaling networks. This review examines this fundamental property of the organelle networks from a systems point of view. The focus is exclusively on homotypic networks formed by mitochondria, lysosomes, endosomes, and the endoplasmic reticulum, respectively. First, key mechanisms that drive the dynamic organization of these networks are summarized. Then, several distinct organizational properties of these networks are highlighted. Next, spatial properties of the dynamic organization of these networks are emphasized, and their functional implications are examined. Finally, some representative molecular machineries that mediate the dynamic organization of these networks are surveyed. Overall, the dynamic organization of intracellular organelle networks is emerging as a fundamental and unifying paradigm in the internal organization of eukaryotic cells. This article is categorized under: Metabolic Diseases > Molecular and Cellular Physiology.

The silence of the fats: A MAM's story about Alzheimer

The discovery of contact sites was a breakthrough in cell biology. We have learned that an organelle cannot function in isolation, and that many cellular functions depend on communication between two or more organelles. One such contact site results from the close apposition of the endoplasmic reticulum (ER) and mitochondria, known as mitochondria-associated ER membranes (MAMs). These intracellular lipid rafts serve as hubs for the regulation of cellular lipid and calcium homeostasis, and a growing body of evidence indicates that MAM domains modulate cellular function in both health and disease. Indeed, MAM dysfunction has been described as a key event in Alzheimer disease (AD) pathogenesis. Our most recent work shows that, by means of its affinity for cholesterol, APP-C99 accumulates in MAM domains of the ER and induces the uptake of extracellular cholesterol as well as its trafficking from the plasma membrane to the ER. As a result, MAM functionality becomes chronically upregulated while undergoing continual turnover. The goal of this review is to discuss the consequences of C99 elevation in AD, specifically the upregulation of cholesterol trafficking and MAM activity, which abrogate cellular lipid homeostasis and disrupt the lipid composition of cellular membranes. Overall, we present a novel framework for AD pathogenesis that can be linked to the many complex alterations that occur during disease progression, and that may open a door to new therapeutic strategies.