INTRACELLULAR TRANSPORT. PI4P/phosphatidylserine countertransport at ORP5- and ORP8-mediated ER-plasma membrane contacts

Structure of calcineurin bound to PI4KA reveals dual interface in both PI4KA and FAM126A

Phosphatidylinositol 4-kinase alpha (PI4KA) maintains the phosphatidylinositol 4-phosphate (PI4P) and phosphatidylserine pools of the plasma membrane. A key regulator of PI4KA is its association into a complex with TTC7 and FAM126 proteins. This complex can be regulated by the CNAβ1 isoform of the phosphatase calcineurin. We previously identified that CNAβ1 directly binds to FAM126A. Here, we report a cryoelectron microscopic (cryo-EM) structure of a truncated PI4KA complex bound to calcineurin, revealing a unique direct interaction between PI4KA and calcineurin. Hydrogen deuterium exchange mass spectrometry (HDX-MS) and computational analysis show that calcineurin forms a complex with an evolutionarily conserved IKISVT sequence in PI4KA's horn domain. We also characterized conserved LTLT and PSISIT calcineurin binding sequences in the C terminus of FAM126A. These dual sites in PI4KA and FAM126A are both in close proximity to phosphorylation sites in the PI4KA complex, suggesting key roles of calcineurin-regulated phosphosites in PI4KA regulation. This work reveals novel insight into how calcineurin can regulate PI4KA activity.

Imaging Interorganelle Phospholipid Transport by Extended Synaptotagmins Using Bioorthogonally Tagged Lipids

The proper distribution of lipids within organelle membranes requires rapid interorganelle lipid transport, much of which occurs at membrane contact sites and is mediated by lipid transfer proteins (LTPs). Our current understanding of LTP mechanism and function is based largely on structural studies and in vitro reconstitution. Existing cellular assays for LTP function use indirect readouts, and it remains an open question as to whether substrate specificity and transport kinetics established in vitro are similar in cellular settings. Here, we harness bioorthogonal chemistry to develop tools for direct visualization of interorganelle transport of phospholipids between the plasma membrane (PM) and the endoplasmic reticulum (ER). Unnatural fluorescent phospholipid analogs generated by the transphosphatidylation activity of phospholipase D (PLD) at the PM are rapidly transported to the ER dependent in part upon extended synaptotagmins (E-Syts), a family of LTPs at ER-PM contact sites. Ectopic expression of an artificial E-Syt-based tether at ER-mitochondria contact sites results in fluorescent phospholipid accumulation in mitochondria. Finally, in vitro reconstitution assays demonstrate that the fluorescent lipids are bona fide E-Syt substrates. Thus, fluorescent lipids generated in situ via PLD activity and bioorthogonal chemical tagging can enable direct visualization of the activity of LTPs that mediate bulk phospholipid transport at ER-PM contact sites.

The ORP9-ORP11 dimer promotes sphingomyelin synthesis

Numerous lipids are heterogeneously distributed among organelles. Most lipid trafficking between organelles is achieved by a group of lipid transfer proteins (LTPs) that carry lipids using their hydrophobic cavities. The human genome encodes many intracellular LTPs responsible for lipid trafficking and the function of many LTPs in defining cellular lipid levels and distributions is unclear. Here, we created a gene knockout library targeting 90 intracellular LTPs and performed whole-cell lipidomics analysis. This analysis confirmed known lipid disturbances and identified new ones caused by the loss of LTPs. Among these, we found major sphingolipid imbalances in ORP9 and ORP11 knockout cells, two proteins of previously unknown function in sphingolipid metabolism. ORP9 and ORP11 form a heterodimer to localize at the ER-trans-Golgi membrane contact sites, where the dimer exchanges phosphatidylserine (PS) for phosphatidylinositol-4-phosphate (PI(4)P) between the two organelles. Consequently, loss of either protein causes phospholipid imbalances in the Golgi apparatus that result in lowered sphingomyelin synthesis at this organelle. Overall, our LTP knockout library toolbox identifies various proteins in control of cellular lipid levels, including the ORP9-ORP11 heterodimer, which exchanges PS and PI(4)P at the ER-Golgi membrane contact site as a critical step in sphingomyelin synthesis in the Golgi apparatus.

Metabolic channeling of lipids via the contact zones between different organelles

Various lipid transfer proteins (LTPs) mediate the inter-organelle transport of lipids. By working at membrane contact zones between donor and acceptor organelles, LTPs achieve rapid and accurate inter-organelle transfer of lipids. This article will describe the emerging paradigm that the action of LTPs at organelle contact zones generates metabolic channeling events in lipid metabolism, mainly referring to how ceramide synthesized in the endoplasmic reticulum is preferentially metabolized to sphingomyelin in the distal Golgi region, how cholesterol and phospholipids receive specific metabolic reactions in mitochondria, and how the hijacking of host LTPs by intracellular pathogens may generate new channeling-like events. In addition, the article will discuss how the function of LTPs is regulated, exemplified by a few representative LTP systems, and will briefly touch on experiments that will be necessary to establish the paradigm that LTP-mediated inter-organelle transport of lipids is one of the mechanisms of compartmentalization-based metabolic channeling events.

The conformational plasticity of structurally unrelated lipid transport proteins correlates with their mode of action

Lipid transfer proteins (LTPs) are key players in cellular homeostasis and regulation, as they coordinate the exchange of lipids between different cellular organelles. Despite their importance, our mechanistic understanding of how LTPs function at the molecular level is still in its infancy, mostly due to the large number of existing LTPs and to the low degree of conservation at the sequence and structural level. In this work, we use molecular simulations to characterize a representative dataset of lipid transport domains (LTDs) of 12 LTPs that belong to 8 distinct families. We find that despite no sequence homology nor structural conservation, the conformational landscape of LTDs displays common features, characterized by the presence of at least 2 main conformations whose populations are modulated by the presence of the bound lipid. These conformational properties correlate with their mechanistic mode of action, allowing for the interpretation and design of experimental strategies to further dissect their mechanism. Our findings indicate the existence of a conserved, fold-independent mechanism of lipid transfer across LTPs of various families and offer a general framework for understanding their functional mechanism.

Mechanisms of lipid droplet degradation

Lipid droplets (LDs) are subcellular organelles that play an integral role in lipid metabolism by regulating the storage and release of fatty acids, which are essential for energy production and various cellular processes. Lipolysis and lipophagy are the two major LD degradation pathways that mediate the utilization of lipids stored in these organelles. Recent studies have further uncovered alternative pathways, including direct lysosomal LD degradation and LD exocytosis. Here, we highlight recent findings that dissect the molecular basis of these diverse LD degradation pathways. Then, we discuss speculations on the crosstalk among these pathways and the potential unconventional roles of LD degradation.

Molecular basis for plasma membrane recruitment of PI4KA by EFR3

The lipid kinase phosphatidylinositol 4 kinase III alpha (PI4KIIIa/PI4KA) is a master regulator of the lipid composition and asymmetry of the plasma membrane. PI4KA exists primarily in a heterotrimeric complex with its regulatory proteins TTC7 and FAM126. Fundamental to PI4KA activity is its targeted recruitment to the plasma membrane by the lipidated proteins EFR3A and EFR3B. Here, we report a cryo-EM structure of the C-terminus of EFR3A bound to the PI4KA-TTC7B-FAM126A complex, with extensive validation using both hydrogen deuterium exchange mass spectrometry (HDX-MS), and mutational analysis. The EFR3A C-terminus undergoes a disorder-order transition upon binding to the PI4KA complex, with an unexpected direct interaction with both TTC7B and FAM126A. Complex disrupting mutations in TTC7B, FAM126A, and EFR3 decrease PI4KA recruitment to the plasma membrane. Multiple post-translational modifications and disease linked mutations map to this site, providing insight into how PI4KA membrane recruitment can be regulated and disrupted in human disease.

Interorganelle phospholipid communication, a house not so divided

The presence of membrane-bound organelles with specific functions is one of the main hallmarks of eukaryotic cells. Organelle membranes are composed of specific lipids that govern their function and interorganelle communication. Discoveries in cell biology using imaging and omic technologies have revealed the mechanisms that drive membrane remodeling, organelle contact sites, and metabolite exchange. The interplay between multiple organelles and their interdependence is emerging as the next frontier for discovery using 3D reconstruction of volume electron microscopy (vEM) datasets. We discuss recent findings on the links between organelles that underlie common functions and cellular pathways. Specifically, we focus on the metabolism of ether glycerophospholipids that mediate organelle dynamics and their communication with each other, and the new imaging techniques that are powering these discoveries.

Exploring host-pathogen interactions in the Dictyostelium discoideum-Mycobacterium marinum infection model of tuberculosis

Mycobacterium tuberculosis is a pathogenic mycobacterium that causes tuberculosis. Tuberculosis is a significant global health concern that poses numerous clinical challenges, particularly in terms of finding effective treatments for patients. Throughout evolution, host immune cells have developed cell-autonomous defence strategies to restrain and eliminate mycobacteria. Concurrently, mycobacteria have evolved an array of virulence factors to counteract these host defences, resulting in a dynamic interaction between host and pathogen. Here, we review recent findings, including those arising from the use of the amoeba Dictyostelium discoideum as a model to investigate key mycobacterial infection pathways. D. discoideum serves as a scalable and genetically tractable model for human phagocytes, providing valuable insights into the intricate mechanisms of host-pathogen interactions. We also highlight certain similarities between M. tuberculosis and Mycobacterium marinum, and the use of M. marinum to more safely investigate mycobacteria in D. discoideum.

In or out of the groove? Mechanisms of lipid scrambling by TMEM16 proteins

Phospholipid scramblases mediate the rapid movement of lipids between membrane leaflets, a key step in establishing and maintaining membrane homeostasis of the membranes of all eukaryotic cells and their organelles. Thus, impairment of lipid scrambling can lead to a variety of pathologies. How scramblases catalyzed the transbilayer movement of lipids remains poorly understood. Despite the availability of direct structural information on three unrelated families of scramblases, the TMEM16s, the Xkrs, and ATG-9, a unifying mechanism has failed to emerge thus far. Among these, the most extensively studied and best understood are the Ca2+ activated TMEM16s, which comprise ion channels and/or scramblases. Early work supported the view that these proteins provided a hydrophilic, membrane-exposed groove through which the lipid headgroups could permeate. However, structural, and functional experiments have since challenged this mechanism, leading to the proposal that the TMEM16s distort and thin the membrane near the groove to facilitate lipid scrambling. Here, we review our understanding of the structural and mechanistic underpinnings of lipid scrambling by the TMEM16s and discuss how the different proposals account for the various experimental observations.

Decoding Organelle Interactions: Unveiling Molecular Mechanisms and Disease Therapies

Organelles, substructures in the cytoplasm with specific morphological structures and functions, interact with each other via membrane fusion, membrane transport, and protein interactions, collectively termed organelle interaction. Organelle interaction is a complex biological process involving the interaction and regulation of several organelles, including the interaction between mitochondria-endoplasmic reticulum, endoplasmic reticulum-Golgi, mitochondria-lysosomes, and endoplasmic reticulum-peroxisomes. This interaction enables intracellular substance transport, metabolism, and signal transmission, and is closely related to the occurrence, development, and treatment of many diseases, such as cancer, neurodegenerative diseases, and metabolic diseases. Herein, the mechanisms and regulation of organelle interactions are reviewed, which are critical for understanding basic principles of cell biology and disease development mechanisms. The findings will help to facilitate the development of novel strategies for disease prevention, diagnosis, and treatment opportunities.

Linking phosphoinositide function to mitosis

Phosphoinositides (PtdIns) are a family of differentially phosphorylated lipid second messengers localized to the cytoplasmic leaflet of both plasma and intracellular membranes. Kinases and phosphatases can selectively modify the PtdIns composition of different cellular compartments, leading to the recruitment of specific binding proteins, which control cellular homeostasis and proliferation. Thus, while PtdIns affect cell growth and survival during interphase, they are also emerging as key drivers in multiple temporally defined membrane remodeling events of mitosis, like cell rounding, spindle orientation, cytokinesis, and abscission. In this review, we summarize and discuss what is known about PtdIns function during mitosis and how alterations in the production and removal of PtdIns can interfere with proper cell division.

Ferroptosis: principles and significance in health and disease

Ferroptosis, an iron-dependent form of cell death characterized by uncontrolled lipid peroxidation, is governed by molecular networks involving diverse molecules and organelles. Since its recognition as a non-apoptotic cell death pathway in 2012, ferroptosis has emerged as a crucial mechanism in numerous physiological and pathological contexts, leading to significant therapeutic advancements across a wide range of diseases. This review summarizes the fundamental molecular mechanisms and regulatory pathways underlying ferroptosis, including both GPX4-dependent and -independent antioxidant mechanisms. Additionally, we examine the involvement of ferroptosis in various pathological conditions, including cancer, neurodegenerative diseases, sepsis, ischemia-reperfusion injury, autoimmune disorders, and metabolic disorders. Specifically, we explore the role of ferroptosis in response to chemotherapy, radiotherapy, immunotherapy, nanotherapy, and targeted therapy. Furthermore, we discuss pharmacological strategies for modulating ferroptosis and potential biomarkers for monitoring this process. Lastly, we elucidate the interplay between ferroptosis and other forms of regulated cell death. Such insights hold promise for advancing our understanding of ferroptosis in the context of human health and disease.

ORP9-PH domain-based fluorescent reporters for visualizing phosphatidylinositol 4-phosphate dynamics in living cells

Fluorescent reporters that visualize phosphatidylinositol 4-phosphate (PI4P) in living cells are indispensable to elucidate the roles of this fundamental lipid in cell physiology. However, currently available PI4P reporters have limitations, such as Golgi-biased localization and low detection sensitivity. Here, we present a series of fluorescent PI4P reporters based on the pleckstrin homology (PH) domain of oxysterol-binding protein-related protein 9 (ORP9). We show that the green fluorescent protein AcGFP1-tagged ORP9-PH domain can be used as a fluorescent PI4P reporter to detect cellular PI4P across its wide distribution at multiple cellular locations, including the plasma membrane (PM), Golgi, endosomes, and lysosomes with high specificity and contrast. We also developed blue, red, and near-infrared fluorescent PI4P reporters suitable for multicolor fluorescence imaging experiments. Finally, we demonstrate the utility of the ORP9-PH domain-based reporter to visualize dynamic changes in the PI4P distribution and level in living cells upon synthetic ER-PM membrane contact manipulation and GPCR stimulation. This work offers a new set of genetically encoded fluorescent PI4P reporters that are practically useful for the study of PI4P biology.

Phosphatidylserine regulates plasma membrane repair through tetraspanin-enriched macrodomains

The integrity of the plasma membrane is critical to cell function and survival. Cells have developed multiple mechanisms to repair damaged plasma membranes. A key process during plasma membrane repair is to limit the size of the damage, which is facilitated by the presence of tetraspanin-enriched rings surrounding damage sites. Here, we identify phosphatidylserine-enriched rings surrounding damaged sites of the plasma membrane, resembling tetraspanin-enriched rings. Importantly, the formation of both the phosphatidylserine- and tetraspanin-enriched rings requires phosphatidylserine and its transfer proteins ORP5 and ORP9. Interestingly, ORP9, but not ORP5, is recruited to the damage sites, suggesting cells acquire phosphatidylserine from multiple sources upon plasma membrane damage. We further demonstrate that ORP9 contributes to efficient plasma membrane repair. Our results thus unveil a role for phosphatidylserine and its transfer proteins in facilitating the formation of tetraspanin-enriched macrodomains and plasma membrane repair.

Lipid osmosis, membrane tension, and other mechanochemical driving forces of lipid flow

Nonvesicular lipid transport among different membranes or membrane domains plays crucial roles in lipid homeostasis and organelle biogenesis. However, the forces that drive such lipid transport are not well understood. We propose that lipids tend to flow towards the membrane area with a higher membrane protein density in a process termed lipid osmosis. This process lowers the membrane tension in the area, resulting in a membrane tension difference called osmotic membrane tension. We examine the thermodynamic basis and experimental evidence of lipid osmosis and osmotic membrane tension. We predict that lipid osmosis can drive bulk lipid flows between different membrane regions through lipid transfer proteins, scramblases, or similar barriers that selectively pass lipids but not membrane proteins. We also speculate on the biological functions of lipid osmosis. Finally, we explore other driving forces for lipid transfer and describe potential methods and systems to further test our theory.

ER-organelle contacts: A signaling hub for neurological diseases

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

Interplay of mitochondria-associated membrane proteins and autophagy: Implications in neurodegeneration

Since the discovery of membrane contact sites between ER and mitochondria called mitochondria-associated membranes (MAMs), several pieces of evidence identified their role in the regulation of different cellular processes such as Ca2+ signalling, mitochondrial transport, and dynamics, ER stress, inflammation, glucose homeostasis, and autophagy. The integrity of these membranes was found to be essential for the maintenance of these cellular functions. Accumulating pieces of evidence suggest that MAMs serve as a platform for autophagosome formation. However, the alteration within MAMs structure is associated with the progression of neurodegenerative diseases. Dysregulated autophagy is a hallmark of neurodegeneration. Here, in this review, we highlight the present knowledge on MAMs, their structural composition, and their roles in different cellular functions. We also discuss the association of MAMs proteins with impaired autophagy and their involvement in the progression of neurodegenerative diseases such as Alzheimer's disease and Parkinson's disease.

Non-vesicular phosphatidylinositol transfer plays critical roles in defining organelle lipid composition

Phosphatidylinositol (PI) is the precursor lipid for the minor phosphoinositides (PPIns), which are critical for multiple functions in all eukaryotic cells. It is poorly understood how phosphatidylinositol, which is synthesized in the ER, reaches those membranes where PPIns are formed. Here, we used VT01454, a recently identified inhibitor of class I PI transfer proteins (PITPs), to unravel their roles in lipid metabolism, and solved the structure of inhibitor-bound PITPNA to gain insight into the mode of inhibition. We found that class I PITPs not only distribute PI for PPIns production in various organelles such as the plasma membrane (PM) and late endosomes/lysosomes, but that their inhibition also significantly reduced the levels of phosphatidylserine, di- and triacylglycerols, and other lipids, and caused prominent increases in phosphatidic acid. While VT01454 did not inhibit Golgi PI4P formation nor reduce resting PM PI(4,5)P2 levels, the recovery of the PM pool of PI(4,5)P2 after receptor-mediated hydrolysis required both class I and class II PITPs. Overall, these studies show that class I PITPs differentially regulate phosphoinositide pools and affect the overall cellular lipid landscape.

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.

Lipid osmosis, membrane tension, and other mechanochemical driving forces of lipid flow

Nonvesicular lipid transport among different membranes or membrane domains plays crucial roles in lipid homeostasis and organelle biogenesis. However, the forces that drive such lipid transport are not well understood. We propose that lipids tend to flow towards the membrane area with a higher membrane protein density in a process termed lipid osmosis. This process lowers the membrane tension in the area, resulting in a membrane tension difference called osmotic membrane tension. We examine the thermodynamic basis and experimental evidence of lipid osmosis and osmotic membrane tension. We predict that lipid osmosis can drive bulk lipid flows between different membrane regions through lipid transfer proteins, scramblases, or other similar barriers that selectively pass lipids but not membrane proteins. We also speculate on the biological functions of lipid osmosis. Finally, we explore other driving forces for lipid transfer and describe potential methods and systems to further test our theory.

Coordinated regulation of phosphatidylinositol 4-phosphate and phosphatidylserine levels by Osh4p and Osh5p is an essential regulatory mechanism in autophagy

Macroautophagy (hereafter autophagy) is an intracellular degradative pathway in budding yeast cells. Certain lipid types play essential roles in autophagy; yet the precise mechanisms regulating lipid composition during autophagy remain unknown. Here, we explored the role of the Osh family proteins in the modulating lipid composition during autophagy in budding yeast. Our results showed that osh1-osh7∆ deletions lead to autophagic dysfunction, with impaired GFP-Atg8 processing and the absence of autophagosomes and autophagic bodies in the cytosol and vacuole, respectively. Freeze-fracture electron microscopy (EM) revealed elevated phosphatidylinositol 4-phosphate (PtdIns(4)P) levels in cytoplasmic and luminal leaflets of autophagic bodies and vacuolar membranes in all deletion mutants. Phosphatidylserine (PtdSer) levels were significantly decreased in the autophagic bodies and vacuolar membranes in osh4∆ and osh5∆ mutants, whereas no significant changes were observed in other osh deletion mutants. Furthermore, we identified defects in autophagic processes in the osh4∆ and osh5∆ mutants, including rare autophagosome formation in the osh5∆ mutant and accumulation of autophagic bodies in the vacuole in the osh4∆ mutant, even in the absence of the proteinase inhibitor PMSF. These findings suggest that Osh4p and Osh5p play crucial roles in the transport of PtdSer to autophagic bodies and autophagosome membranes, respectively. The precise control of lipid composition in the membranes of autophagosomes and autophagic bodies by Osh4p and Osh5p represents an important regulatory mechanism in autophagy.

Reconstitution of ORP-mediated lipid exchange coupled to PI4P metabolism

Oxysterol-binding protein-related proteins (ORPs) play key roles in the distribution of lipids in eukaryotic cells by exchanging sterol or phosphatidylserine for PI4P between the endoplasmic reticulum (ER) and other cell regions. However, it is unclear how their exchange capacity is coupled to PI4P metabolism. To address this question quantitatively, we analyze the activity of a representative ORP, Osh4p, in an ER/Golgi interface reconstituted with ER- and Golgi-mimetic membranes functionalized with PI4P phosphatase Sac1p and phosphatidylinositol (PI) 4-kinase, respectively. Using real-time assays, we demonstrate that upon adenosine triphosphate (ATP) addition, Osh4p creates a sterol gradient between these membranes, relying on the spatially distant synthesis and hydrolysis of PI4P, and quantify how much PI4P is needed for this process. Then, we develop a quantitatively accurate kinetic model, validated by our data, and extrapolate this to estimate to what extent PI4P metabolism can drive ORP-mediated sterol transfer in cells. Finally, we show that Sec14p can support PI4P metabolism and Osh4p activity by transferring PI between membranes. This study establishes that PI4P synthesis drives ORP-mediated lipid exchange and that ATP energy is needed to generate intermembrane lipid gradients. Furthermore, it defines to what extent ORPs can distribute lipids in the cell and reassesses the role of PI-transfer proteins in PI4P metabolism.

Phosphatidylserine turns the gears of phospholipids in B cell lymphoma

Phosphatidylserine levels and distribution are tightly controlled by dedicated enzymes at the ER and plasma membrane. Nakatsu and Kawasaki discuss new work by Aoki and colleagues (https://doi.org/10.1083/jcb.202212074), which reveals an acute reliance on phosphatidylserine synthesis in B cell lymphomas needed to prevent aberrant B cell receptor activation and ensuing apoptosis.

Phosphatidylserine synthesis controls oncogenic B cell receptor signaling in B cell lymphoma

Cancer cells harness lipid metabolism to promote their own survival. We screened 47 cancer cell lines for survival dependency on phosphatidylserine (PS) synthesis using a PS synthase 1 (PTDSS1) inhibitor and found that B cell lymphoma is highly dependent on PS. Inhibition of PTDSS1 in B cell lymphoma cells caused a reduction of PS and phosphatidylethanolamine levels and an increase of phosphoinositide levels. The resulting imbalance of the membrane phospholipidome lowered the activation threshold for B cell receptor (BCR), a B cell-specific survival mechanism. BCR hyperactivation led to aberrant elevation of downstream Ca2+ signaling and subsequent apoptotic cell death. In a mouse xenograft model, PTDSS1 inhibition efficiently suppressed tumor growth and prolonged survival. Our findings suggest that PS synthesis may be a critical vulnerability of malignant B cell lymphomas that can be targeted pharmacologically.

Stay in touch with the endoplasmic reticulum

The endoplasmic reticulum (ER), which is composed of a continuous network of tubules and sheets, forms the most widely distributed membrane system in eukaryotic cells. As a result, it engages a variety of organelles by establishing membrane contact sites (MCSs). These contacts regulate organelle positioning and remodeling, including fusion and fission, facilitate precise lipid exchange, and couple vital signaling events. Here, we systematically review recent advances and converging themes on ER-involved organellar contact. The molecular basis, cellular influence, and potential physiological functions for ER/nuclear envelope contacts with mitochondria, Golgi, endosomes, lysosomes, lipid droplets, autophagosomes, and plasma membrane are summarized.

OSBP is a major determinant of Golgi phosphatidylinositol 4-phosphate homeostasis

The lipid phosphatidylinositol 4-phosphate (PI4P) plays a master regulatory role at Golgi membranes, orchestrating membrane budding, non-vesicular lipid transport and membrane organization. It follows that harmonious Golgi function requires strictly maintained PI4P homeostasis. One of the most abundant PI4P effector proteins is the oxysterol binding protein (OSBP), a lipid transfer protein that exchanges trans Golgi PI4P for ER cholesterol. Although this protein consumes PI4P as part of its lipid anti-porter function, whether it actively contributes to Golgi PI4P homeostasis has been questioned. Here, we employed a series of acute and chronic genetic manipulations, together with orthogonal targeting of OSBP, to interrogate its control over Golgi PI4P abundance. Modulating OSBP levels at ER:Golgi membrane contact sites produces reciprocal changes in PI4P levels. Additionally, we observe that OSBP has a high capacity for PI4P turnover, even at orthogonal organelle membranes. However, despite also visiting the plasma membrane, endogenous OSBP makes no impact on PI4P levels in this compartment. We conclude that OSBP is a major determinant of Golgi PI4P homeostasis.

Myotubularin-related proteins regulate KRAS function by controlling plasma membrane levels of polyphosphoinositides and phosphatidylserine

KRAS is a small GTPase, ubiquitously expressed in mammalian cells, that functions as a molecular switch to regulate cell proliferation and differentiation. Oncogenic mutations that render KRAS constitutively active occur frequently in human cancers. KRAS must localize to the plasma membrane (PM) for biological activity. KRAS PM binding is mediated by interactions of the KRAS membrane anchor with phosphatidylserine (PtdSer), therefore, depleting PM PtdSer content abrogates KRAS PM binding and oncogenic function. From a genome-wide siRNA screen to search for genes that regulate KRAS PM localization, we identified a set of phosphatidylinositol (PI) 3-phosphatase family members: myotubularin-related (MTMR) proteins 2, 3, 4 and 7. Here we show that knockdown of MTMR 2/3/4/7 expression disrupts KRAS PM interactions. The molecular mechanism involves depletion of PM PI 4-phosphate (PI4P) levels, which in turn disrupts the subcellular localization and operation of oxysterol-binding protein related protein (ORP) 5, a PtdSer lipid transfer protein that maintains PM PtdSer content. Concomitantly, silencing MTMR 2/3/4/7 expression elevates PM levels of PI3P and reduces PM and total cellular levels of PtdSer. In summary we propose that the PI 3-phosphatase activity provided by MTMR proteins is required to generate PM PI for the synthesis of PM PI4P, which in turn, promotes the PM localization of PtdSer and KRAS.

Making the connection: How membrane contact sites have changed our view of organelle biology

The view of organelles and how they operate together has changed dramatically over the last two decades. The textbook view of organelles was that they operated largely independently and were connected by vesicular trafficking and the diffusion of signals through the cytoplasm. We now know that all organelles make functional close contacts with one another, often called membrane contact sites. The study of these sites has moved to center stage in cell biology as it has become clear that they play critical roles in healthy and developing cells and during cell stress and disease states. Contact sites have important roles in intracellular signaling, lipid metabolism, motor-protein-mediated membrane dynamics, organelle division, and organelle biogenesis. Here, we summarize the major conceptual changes that have occurred in cell biology as we have come to appreciate how contact sites integrate the activities of organelles.

Orthogonal targeting of SAC1 to mitochondria implicates ORP2 as a major player in PM PI4P turnover

Oxysterol binding protein (OSBP)-related proteins (ORPs) 5 and 8 have been shown to deplete the lipid phosphatidylinositol 4-phosphate (PI4P) at sites of membrane contact between the endoplasmic reticulum (ER) and plasma membrane (PM). This is believed to be caused by transport of PI4P from the PM to the ER, where PI4P is degraded by an ER-localized SAC1 phosphatase. This is proposed to power the anti-port of phosphatidylserine (PS) lipids from ER to PM, up their concentration gradient. Alternatively, ORPs have been proposed to sequester PI4P, dependent on the concentration of their alternative lipid ligand. Here, we aimed to distinguish these possibilities in living cells by orthogonal targeting of PI4P transfer and degradation to PM-mitochondria contact sites. Surprisingly, we found that orthogonal targeting of SAC1 to mitochondria enhanced PM PI4P turnover independent of targeting to contact sites with the PM. This turnover could be slowed by knock-down of soluble ORP2, which also has a major impact on PM PI4P levels even without SAC1 over-expression. The data reveal a role for contact site-independent modulation of PM PI4P levels and lipid antiport.

Orthogonal Targeting of SAC1 to Mitochondria Implicates ORP2 as a Major Player in PM PI4P Turnover

Oxysterol-binding protein (OSBP)-related proteins (ORPs) 5 and 8 have been shown to deplete the lipid phosphatidylinositol 4-phosphate (PI4P) at sites of membrane contact between the endoplasmic reticulum (ER) and plasma membrane (PM). This is believed to be caused by transport of PI4P from the PM to the ER, where PI4P is degraded by an ER-localized SAC1 phosphatase. This is proposed to power the anti-port of phosphatidylserine (PS) lipids from ER to PM, up their concentration gradient. Alternatively, ORPs have been proposed to sequester PI4P, dependent on the concentration of their alternative lipid ligand. Here, we aimed to distinguish these possibilities in living cells by orthogonal targeting of PI4P transfer and degradation to PM-mitochondria contact sites. Surprisingly, we found that orthogonal targeting of SAC1 to mitochondria enhanced PM PI4P turnover independent of targeting to contact sites with the PM. This turnover could be slowed by knock-down of soluble ORP2, which also has a major impact on PM PI4P levels even without SAC1 over-expression. The data reveal a role for contact site-independent modulation of PM PI4P levels and lipid antiport.

A new role for phosphoinositides in regulating mitochondrial dynamics

Phosphoinositides are a minor group of membrane-associated phospholipids that are transiently generated on the cytoplasmic leaflet of many organelle membranes and the plasma membrane. There are seven functionally distinct phosphoinositides, each derived via the reversible phosphorylation of phosphatidylinositol in various combinations on the inositol ring. Their generation and termination is tightly regulated by phosphatidylinositol-kinases and -phosphatases. These enzymes can function together in an integrated and coordinated manner, whereby the phosphoinositide product of one enzyme may subsequently serve as a substrate for another to generate a different phosphoinositide species. This regulatory mechanism not only enables the transient generation of phosphoinositides on membranes, but also more complex sequential or bidirectional conversion pathways, and phosphoinositides can also be transferred between organelles via membrane contacts. It is this capacity to fine-tune phosphoinositide signals that makes them ideal regulators of membrane organization and dynamics, through their recruitment of signalling, membrane altering and lipid transfer proteins. Research spanning several decades has provided extensive evidence that phosphoinositides are major gatekeepers of membrane organization, with roles in endocytosis, exocytosis, autophagy, lysosome dynamics, vesicular transport and secretion, cilia, inter-organelle membrane contact, endosome maturation and nuclear function. By contrast, there has been remarkably little known about the role of phosphoinositides at mitochondria - an enigmatic and major knowledge gap, with challenges in reliably detecting phosphoinositides at this site. Here we review recent significant breakthroughs in understanding the role of phosphoinositides in regulating mitochondrial dynamics and metabolic function.

OSBP is a Major Determinant of Golgi Phosphatidylinositol 4-Phosphate Homeostasis

The lipid phosphatidylinositol 4-phosphate (PI4P) plays a master regulatory role at Golgi membranes, orchestrating membrane budding, non-vesicular lipid transport and membrane organization. It follows that harmonious Golgi function requires strictly maintained PI4P homeostasis. One of the most abundant PI4P effector proteins is the oxysterol binding protein (OSBP), a lipid transfer protein that exchanges trans-Golgi PI4P for ER cholesterol. Although this protein consumes PI4P as part of its lipid anti-porter function, whether it actively contributes to Golgi PI4P homeostasis has been questioned. Here, we employed a series of acute and chronic genetic manipulations, together with orthogonal targeting of OSBP, to interrogate its control over Golgi PI4P abundance. Modulating OSBP levels at ER:Golgi membrane contact sites produces reciprocal changes in PI4P levels. Additionally, we observe that OSBP has a high capacity for PI4P turnover, even at orthogonal organelle membranes. However, despite also visiting the plasma membrane, endogenous OSBP makes no impact on PI4P levels in this compartment. We conclude that OSBP is a major determinant of Golgi PI4P homeostasis.

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.

Organelles are miscommunicating: Membrane contact sites getting hijacked by pathogens

Membrane Contact Sites (MCS) are areas of close apposition of organelles that serve as hotspots for crosstalk and direct transport of lipids, proteins and metabolites. Contact sites play an important role in Ca2+ signalling, phospholipid synthesis, and micro autophagy. Initially, altered regulation of vesicular trafficking was regarded as the key mechanism for intracellular pathogen survival. However, emerging studies indicate that pathogens hijack MCS elements - a novel strategy for survival and replication in an intracellular environment. Several pathogens exploit MCS to establish direct contact between organelles and replication inclusion bodies, which are essential for their survival within the cell. By establishing this direct control, pathogens gain access to cytosolic compounds necessary for replication, maintenance, escaping endocytic maturation and circumventing lysosome fusion. MCS components such as VAP A/B, OSBP, and STIM1 are targeted by pathogens through their effectors and secretion systems. In this review, we delve into the mechanisms which operate in the evasion of the host immune system when intracellular pathogens hostage MCS. We explore targeting MCS components as a novel therapeutic approach, modifying molecular pathways and signalling to address the disease's mechanisms and offer more effective, tailored treatments for affected individuals.

Structural basis for the conserved roles of PI4KA and its regulatory partners and their misregulation in disease

The type III Phosphatidylinositol 4-kinase alpha (PI4KA) is an essential lipid kinase that is a master regulator of phosphoinositide signalling at the plasma membrane (PM). It produces the predominant pool of phosphatidylinositol 4-phosphate (PI4P) at the PM, with this being essential in lipid transport and in regulating the PLC and PI3K signalling pathways. PI4KA is essential and is highly conserved in all eukaryotes. In yeast, the PI4KA ortholog stt4 predominantly exists as a heterodimer with its regulatory partner ypp1. In higher eukaryotes, PI4KA instead primarily forms a heterotrimer with a TTC7 subunit (ortholog of ypp1) and a FAM126 subunit. In all eukaryotes PI4KA is recruited to the plasma membrane by the protein EFR3, which does not directly bind PI4KA, but instead binds to the TTC7/ypp1 regulatory partner. Misregulation in PI4KA or its regulatory partners is involved in myriad human diseases, including loss of function mutations in neurodevelopmental and inflammatory intestinal disorders and gain of function in human cancers. This review describes an in-depth analysis of the structure function of PI4KA and its regulatory partners, with a major focus on comparing and contrasting the differences in regulation of PI4KA throughout evolution.

Orchestrating vesicular and nonvesicular membrane dynamics by intrinsically disordered proteins

Compartmentalization by membranes is a common feature of eukaryotic cells and serves to spatiotemporally confine biochemical reactions to control physiology. Membrane-bound organelles such as the endoplasmic reticulum (ER), the Golgi complex, endosomes and lysosomes, and the plasma membrane, continuously exchange material via vesicular carriers. In addition to vesicular trafficking entailing budding, fission, and fusion processes, organelles can form membrane contact sites (MCSs) that enable the nonvesicular exchange of lipids, ions, and metabolites, or the secretion of neurotransmitters via subsequent membrane fusion. Recent data suggest that biomolecule and information transfer via vesicular carriers and via MCSs share common organizational principles and are often mediated by proteins with intrinsically disordered regions (IDRs). Intrinsically disordered proteins (IDPs) can assemble via low-affinity, multivalent interactions to facilitate membrane tethering, deformation, fission, or fusion. Here, we review our current understanding of how IDPs drive the formation of multivalent protein assemblies and protein condensates to orchestrate vesicular and nonvesicular transport with a special focus on presynaptic neurotransmission. We further discuss how dysfunction of IDPs causes disease and outline perspectives for future research.

ER-dependent membrane repair of mycobacteria-induced vacuole damage

IMPORTANCE

Tuberculosis still remains a global burden and is one of the top infectious diseases from a single pathogen. Mycobacterium tuberculosis, the causative agent, has perfected many ways to replicate and persist within its host. While mycobacteria induce vacuole damage to evade the toxic environment and eventually escape into the cytosol, the host recruits repair machineries to restore the MCV membrane. However, how lipids are delivered for membrane repair is poorly understood. Using advanced fluorescence imaging and volumetric correlative approaches, we demonstrate that this involves the recruitment of the endoplasmic reticulum (ER)-Golgi lipid transfer protein OSBP8 in the Dictyostelium discoideum/Mycobacterium marinum system. Strikingly, depletion of OSBP8 affects lysosomal function accelerating mycobacterial growth. This indicates that an ER-dependent repair pathway constitutes a host defense mechanism against intracellular pathogens such as M. tuberculosis.

RBG Motif Bridge-Like Lipid Transport Proteins: Structure, Functions, and Open Questions

The life of eukaryotic cells requires the transport of lipids between membranes, which are separated by the aqueous environment of the cytosol. Vesicle-mediated traffic along the secretory and endocytic pathways and lipid transfer proteins (LTPs) cooperate in this transport. Until recently, known LTPs were shown to carry one or a few lipids at a time and were thought to mediate transport by shuttle-like mechanisms. Over the last few years, a new family of LTPs has been discovered that is defined by a repeating β-groove (RBG) rod-like structure with a hydrophobic channel running along their entire length. This structure and the localization of these proteins at membrane contact sites suggest a bridge-like mechanism of lipid transport. Mutations in some of these proteins result in neurodegenerative and developmental disorders. Here we review the known properties and well-established or putative physiological roles of these proteins, and we highlight the many questions that remain open about their functions.

Sec22b regulates phagosome maturation by promoting ORP8-mediated lipid exchange at endoplasmic reticulum-phagosome contact sites

Phagosome maturation is critical for immune defense, defining whether ingested material is destroyed or converted into antigens. Sec22b regulates phagosome maturation, yet how has remained unclear. Here we show Sec22b tethers endoplasmic reticulum-phagosome membrane contact sites (MCS) independently of the known tether STIM1. Sec22b knockdown increases calcium signaling, phagolysosome fusion and antigen degradation and alters phagosomal phospholipids PI(3)P, PS and PI(4)P. Levels of PI(4)P, a lysosome docking lipid, are rescued by Sec22b re-expression and by expression of the artificial tether MAPPER but not the MCS-disrupting mutant Sec22b-P33. Moreover, Sec22b co-precipitates with the PS/PI(4)P exchange protein ORP8. Wild-type, but not mutant ORP8 rescues phagosomal PI(4)P and reduces antigen degradation. Sec22b, MAPPER and ORP8 but not P33 or mutant-ORP8 restores phagolysosome fusion in knockdown cells. These findings clarify an alternative mechanism through which Sec22b controls phagosome maturation and beg a reassessment of the relative contribution of Sec22b-mediated fusion versus tethering to phagosome biology.

Lipid Sorting and Organelle Identity

The sorting and trafficking of lipids between organelles gives rise to a dichotomy of bulk membrane properties between organelles of the secretory and endolysosome networks, giving rise to two "membrane territories" based on differences in lipid-packing density, net membrane charge, and bilayer leaflet asymmetries. The cellular organelle membrane dichotomy emerges from ER-to-PM anterograde membrane trafficking and the synthesis of sphingolipids and cholesterol flux at the trans-Golgi network, which constitutes the interface between the two membrane territories. Organelle homeostasis is maintained by vesicle-mediated retrieval of bulk membrane from the distal organelles of each territory to the endoplasmic reticulum or plasma membrane and by soluble lipid transfer proteins that traffic particular lipids. The concept of cellular membrane territories emphasizes the contrasting features of organelle membranes of the secretory and endolysosome networks and the essential roles of lipid-sorting pathways that maintain organelle function.

The sterol transporter STARD3 transports sphingosine at ER-lysosome contact sites

Sphingolipids are important structural components of membranes. Additionally, simple sphingolipids such as sphingosine are highly bioactive and participate in complex subcellular signaling. Sphingolipid deregulation is associated with many severe diseases including diabetes, Parkinson's and cancer. Here, we focus on how sphingosine, generated from sphingolipid catabolism in late endosomes/lysosomes, is reintegrated into the biosynthetic machinery at the endoplasmic reticulum (ER). We characterized the sterol transporter STARD3 as a sphingosine transporter acting at lysosome-ER contact sites. Experiments featuring crosslinkable sphingosine probes, supported by unbiased molecular dynamics simulations, exposed how sphingosine binds to the lipid-binding domain of STARD3. Following the metabolic fate of pre-localized lysosomal sphingosine showed the importance of STARD3 and its actions at contact sites for the integration of sphingosine into ceramide in a cellular context. Our findings provide the first example of interorganellar sphingosine transfer and pave the way for a better understanding of sphingolipid - sterol co-regulation.

Regulation of phosphoinositide metabolism in Apicomplexan parasites

Phosphoinositides are a biologically essential class of phospholipids that contribute to organelle membrane identity, modulate membrane trafficking pathways, and are central components of major signal transduction pathways that operate on the cytosolic face of intracellular membranes in eukaryotes. Apicomplexans (such as Toxoplasma gondii and Plasmodium spp.) are obligate intracellular parasites that are important causative agents of disease in animals and humans. Recent advances in molecular and cell biology of Apicomplexan parasites reveal important roles for phosphoinositide signaling in key aspects of parasitosis. These include invasion of host cells, intracellular survival and replication, egress from host cells, and extracellular motility. As Apicomplexans have adapted to the organization of essential signaling pathways to accommodate their complex parasitic lifestyle, these organisms offer experimentally tractable systems for studying the evolution, conservation, and repurposing of phosphoinositide signaling. In this review, we describe the regulatory mechanisms that control the spatial and temporal regulation of phosphoinositides in the Apicomplexan parasites Plasmodium and T. gondii. We further discuss the similarities and differences presented by Apicomplexan phosphoinositide signaling relative to how these pathways are regulated in other eukaryotic organisms.

ORP8 acts as a lipophagy receptor to mediate lipid droplet turnover

Lipophagy, the selective engulfment of lipid droplets (LDs) by autophagosomes for lysosomal degradation, is critical to lipid and energy homeostasis. Here we show that the lipid transfer protein ORP8 is located on LDs and mediates the encapsulation of LDs by autophagosomal membranes. This function of ORP8 is independent of its lipid transporter activity and is achieved through direct interaction with phagophore-anchored LC3/GABARAPs. Upon lipophagy induction, ORP8 has increased localization on LDs and is phosphorylated by AMPK, thereby enhancing its affinity for LC3/GABARAPs. Deletion of ORP8 or interruption of ORP8-LC3/GABARAP interaction results in accumulation of LDs and increased intracellular triglyceride. Overexpression of ORP8 alleviates LD and triglyceride deposition in the liver of ob/ob mice, and Osbpl8-/- mice exhibit liver lipid clearance defects. Our results suggest that ORP8 is a lipophagy receptor that plays a key role in cellular lipid metabolism.

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

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

Increased Phospholipid Flux Bypasses Overlapping Essential Requirements for the Yeast Sac1p Phosphoinositide Phosphatase and ER-PM Membrane Contact Sites

In budding yeast cells, much of the inner surface of the plasma membrane (PM) is covered with the endoplasmic reticulum (ER). This association is mediated by seven ER membrane proteins that confer cortical ER-PM association at membrane contact sites (MCSs). Several of these membrane "tether" proteins are known to physically interact with the phosphoinositide phosphatase Sac1p. However, it is unclear how or if these interactions are necessary for their interdependent functions. We find that SAC1 inactivation in cells lacking the homologous synaptojanin-like genes INP52 and INP53 results in a significant increase in cortical ER-PM MCSs. We show in sac1Δ, sac1tsinp52Δ inp53Δ, or Δ-super-tether (Δ-s-tether) cells lacking all seven ER-PM tethering genes that phospholipid biosynthesis is disrupted and phosphoinositide distribution is altered. Furthermore, SAC1 deletion in Δ-s-tether cells results in lethality, indicating a functional overlap between SAC1 and ER-PM tethering genes. Transcriptomic profiling indicates that SAC1 inactivation in either Δ-s-tether or inp52Δ inp53Δ cells induces an ER membrane stress response and elicits phosphoinositide-dependent changes in expression of autophagy genes. In addition, by isolating high-copy suppressors that rescue sac1Δ Δ-s-tether lethality, we find that key phospholipid biosynthesis genes bypass the overlapping function of SAC1 and ER-PM tethers and that overexpression of the phosphatidylserine/phosphatidylinositol-4-phosphate transfer protein Osh6 also provides limited suppression. Combined with lipidomic analysis and determinations of intracellular phospholipid distributions, these results suggest that Sac1p and ER phospholipid flux controls lipid distribution to drive Osh6p-dependent phosphatidylserine/phosphatidylinositol-4-phosphate counter-exchange at ER-PM MCSs.

Phosphoinositides in New Spaces

Phosphoinositides (PIs) are phospholipids derived from phosphatidylinositol. PIs are regulated via reversible phosphorylation, which is directed by the opposing actions of PI kinases and phosphatases. PIs constitute a minor fraction of the total cellular lipid pool but play pleiotropic roles in multiple aspects of cell biology. Genetic mutations of PI regulatory enzymes have been identified in rare congenital developmental syndromes, including ciliopathies, and in numerous human diseases, such as cancer and metabolic and neurological disorders. Accordingly, PI regulatory enzymes have been targeted in the design of potential therapeutic interventions for human diseases. Recent advances place PIs as central regulators of membrane dynamics within functionally distinct subcellular compartments. This brief review focuses on the emerging role PIs play in regulating cell signaling within the primary cilium and in directing transfer of molecules at interorganelle membrane contact sites and identifies new roles for PIs in subcellular spaces.

The unfolded protein response links ER stress to cancer-associated thrombosis

Thrombosis is a common complication of advanced cancer, yet the cellular mechanisms linking malignancy to thrombosis are poorly understood. The unfolded protein response (UPR) is an ER stress response associated with advanced cancers. A proteomic evaluation of plasma from patients with gastric and non-small cell lung cancer who were monitored prospectively for venous thromboembolism demonstrated increased levels of UPR-related markers in plasma of patients who developed clots compared with those who did not. Release of procoagulant activity into supernatants of gastric, lung, and pancreatic cancer cells was enhanced by UPR induction and blocked by antagonists of the UPR receptors inositol-requiring enzyme 1α (IRE1α) and protein kinase RNA-like endoplasmic reticulum kinase (PERK). Release of extracellular vesicles bearing tissue factor (EVTFs) from pancreatic cancer cells was inhibited by siRNA-mediated knockdown of IRE1α/XBP1 or PERK pathways. Induction of UPR did not increase tissue factor (TF) synthesis, but rather stimulated localization of TF to the cell surface. UPR-induced TF delivery to EVTFs was inhibited by ADP-ribosylation factor 1 knockdown or GBF1 antagonism, verifying the role of vesicular trafficking. Our findings show that UPR activation resulted in increased vesicular trafficking leading to release of prothrombotic EVTFs, thus providing a mechanistic link between ER stress and cancer-associated thrombosis.