Sterol transfer, PI4P consumption, and control of membrane lipid order by endogenous OSBP

Repurposing oxiconazole to inhibit STING trafficking via OSBP and alleviate autoimmune pathology in Trex1-/- mice

The cGAS-STING pathway is a critical component of the innate immune response to cytosolic DNA, driving the production of type I interferons (IFNs) and pro-inflammatory cytokines. However, excessive activation of this pathway is associated with various autoimmune and inflammatory diseases. In this study, we evaluated the regulation of FDA-approved azole antifungal drugs on the cGAS-STING pathway. Among these drugs, oxiconazole, miconazole, and itraconazole demonstrate significant inhibitory effects, with oxiconazole showing the strongest activity. Our data demonstrates that oxiconazole significantly suppressed type I IFN production and downstream inflammatory responses in macrophages and fibroblasts stimulated with synthetic DNA or infected with HSV-1. Mechanistically, oxiconazole hindered STING trafficking via oxysterol-binding protein OSBP. Using the Listeria monocytogenes infection model and the Trex1-/- mouse disease model, both representing in vivo models of inflammation driven by excessive cGAS-STING activation, we demonstrate that oxiconazole enhanced bacterial clearance and reduced tissue damage in the Listeria monocytogenes infection model. Moreover, oxiconazole treatment significantly alleviated multi-organ inflammation and normalized aberrant IFN responses in the Trex1-/- autoimmune disease mouse model. These findings highlight the potential of oxiconazole as a promising therapeutic agent for STING-driven autoimmune and inflammatory diseases.

Regulation and function of PI4P at the Golgi complex

Fifty years after Bob Michell's visionary prediction, phosphatidylinositol 4-phosphate (PI4P) has emerged as a central regulator of Golgi function, influencing membrane trafficking, lipid metabolism, and signaling. PI4P homeostasis is tightly controlled by phosphatidylinositol 4-kinases (PI4Ks), phosphatidylinositol transfer proteins (PITPs), and the phosphatase SAC1, ensuring precise regulation across Golgi subdomains. Beyond its classical role in vesicular transport, PI4P orchestrates lipid exchange at membrane contact sites, enabling dynamic Golgi maturation and functional specialization. The interplay between PI4P, lipid transfer proteins, and Golgi adaptors underlies cargo sorting, glycosylation, and organelle architecture. Emerging evidence also highlights PI4P's role in oncogenesis and cellular signaling, positioning the Golgi as a critical hub beyond secretion. Yet, key questions remain regarding PI4P compartmentalization and its broader physiological impact. This review revisits PI4P's essential functions, integrating historical insights with recent discoveries to illuminate its pivotal role in Golgi biology and beyond.

Depletion of oxysterol-binding proteins by OSW-1 triggers RIP1/RIP3-independent necroptosis and sensitization to cancer immunotherapy

Oxysterol-binding proteins (OSBPs), lipid transfer proteins functioning at intracellular membrane contact sites, are recently found to be dysregulated in cancer and promote cancer cell survival. However, their role as potential targets in cancer therapy remains largely unexplored. In this study, we found OSW-1, a natural compound and OSBP inhibitor, potently and selectively kills colon cancer cells by activating a previously unknown necroptosis pathway that is independent of receptor-interacting protein 1 (RIP1) and RIP3. OSW-1 stabilizes p53 and degrades OSBPs to promote endoplasmic reticulum (ER) stress and glycogen synthase kinase 3β (GSK3β)/Tip60-mediated p53 acetylation at Lysine 120, which selectively induces its target PUMA. PUMA-mediated mitochondrial calcium influx activates calcium/calmodulin-dependent protein kinase IIδ (CamKIIδ) to promote mixed lineage kinase domain-like (MLKL) phosphorylation and necroptotic cell death. Furthermore, OSW-1-induced necroptosis is highly immunogenic and sensitizes syngeneic colorectal tumors to anti-PD-1 immunotherapy. Together, our results identified a novel RIP1/RIP3-independent necroptosis pathway underlying the extremely potent anticancer activity of OSW-1, which can be harnessed to develop new anticancer therapies by selectively stimulating antitumor immunity.

Analogues of Natural Macarangin B Display Potent Antiviral Activity and Better Metabolic Stability

The development of innovative antiviral strategies is critical to address the global health threats posed by RNA viruses, including the Zika virus (ZIKV), which can cause severe neurological complications. The lipid transporter Oxysterol Binding Protein (OSBP), essential for cholesterol and phosphatidylinositol 4-phosphate trafficking, is exploited by many positive-strand RNA viruses, making it an attractive novel antiviral target. This study investigates simplified analogues of macarangin B, a natural compound with potent OSBP-targeted antiviral activity against ZIKV, but limited stability due to its flavonol moiety. A series of analogues was synthesized, replacing the flavonol with a flavone core while retaining the essential hexahydroxanthene (HHX) motif. These compounds demonstrated improved stability (t1/2=16 hours), high OSBP binding affinity (4 - 69 nM), and low cytotoxicity (>20 μM). The most active compounds exhibited antiviral activity comparable to established OSBP inhibitors and were stable in physiologic media, highlighting their potential as leads for therapeutic development. This work advances the structure-activity relationship (SAR) understanding of macarangin B analogues and provides a foundation for designing effective antivirals targeting in ZIKV infections.

The bridge-like lipid transport protein VPS13C/PARK23 mediates ER-lysosome contacts following lysosome damage

Based on genetic studies, lysosome dysfunction is thought to play a pathogenetic role in Parkinson's disease. Here we show that VPS13C, a bridge-like lipid-transport protein and a Parkinson's disease gene, is a sensor of lysosome stress or damage. Following lysosome membrane perturbation, VPS13C rapidly relocates from the cytosol to the surface of lysosomes where it tethers their membranes to the ER. This recruitment depends on Rab7 and requires a signal at the damaged lysosome surface that releases an inhibited state of VPS13C, which hinders access of its VAB domain to lysosome-bound Rab7. Although another Parkinson's disease protein, LRRK2, is also recruited to stressed or damaged lysosomes, its recruitment occurs at much later stages and by different mechanisms. Given the role of VPS13 proteins in bulk lipid transport, these findings suggest that lipid delivery to lysosomes by VPS13C is part of an early protective response to lysosome damage.

Potentiation of macrophage Piezo1 by atherogenic 7-ketocholesterol

The mechanosensitive ion channel Piezo1 present in endothelial and smooth muscle cells, as well as in macrophages, is emerging as a novel, important player in the etiology of atherosclerosis. Here, we show that myeloid-specific deficiency of Piezo1 in atherogenic Ldlr-/- mice reduces plaque formation. Moreover, chronic oxLDL, as well as its main oxysterol 7-ketocholesterol (7-KC), promotes Piezo1 opening by pressure stimulation in both mouse macrophages and transfected HEK cells. 7-KC dramatically enhances Piezo1 current amplitude and slows down inactivation and deactivation. This up-modulation involves an increase in Piezo1 expression, as well as a potentiation of mechanical gating that depends on membrane cholesterol depletion and decreased order. By contrast, Piezo1 is inhibited by the athero-protective free docosahexaenoic acid, either without or with 7-KC. Altogether, these findings indicate that macrophage Piezo1 is differentially modulated by pro- and anti-atherogenic lipids, pointing to the role of Piezo1 and its potentiation by oxysterols in atherosclerosis.

Fast and effective preparation of highly cytotoxic hybrid molecules of schweinfurthin E and OSW-1

Herein, we present the first synthesis of hybrid molecules combining the pharmacophores of two natural compounds, schweinfurthin E (SW-E) and the glycosidic moiety of OSW-1. These hybrids were designed leveraging the complementary binding of SW-E and OSW-1 to their biological target. The synthetic process highlights, in particular, one-pot functionalization and glycosylation of an L-arabinose unit using a D-xyloside donor and a CuAAC click reaction involving a polyfunctionalized prenylated stilbene derived from SW-E. The cytotoxicity of the four SW-E and OSW1 hybrids is also reported, two of them being much more cytotoxic than SW-E on a glioblastoma cancer cell line. Finally, a molecular modeling study is conducted to rationalize the biological results obtained.

ER-to-Golgi trafficking through a dynamic intermediate cis-Golgi tubular network in Arabidopsis

Endoplasmic reticulum (ER)-to-Golgi trafficking is a central process of the secretory system of eukaryotic cells that ensures proper spatiotemporal sorting of proteins and lipids. However, the nature of the ER-Golgi intermediate compartments (ERGICs) and the molecular mechanisms mediating the transition between ERGICs and the Golgi, as well as the universality of these processes among eukaryotes, remain undiscovered. Here we identify a reticulated tubulo-vesicular network, labelled by MEMBRIN proteins, that is mostly independent of the Golgi, highly dynamic at the ER-Golgi interface and crossed by ER-induced released luminal cargos. We find that plant ERGICs become stabilized by the interaction they establish with pre-existing Golgi and gradually mature into Golgi cisternae, this process being dependent on C24-ceramide sphingolipids. Our study is a major twist in the understanding of the Golgi, as it identifies that the ERGICs in plants comprise a Golgi-independent and highly dynamic tubular network from which arise more stable Golgi-associated pre-cisternae structures.

The sterol-regulating human ARV1 binds cholesterol and phospholipids through its conserved ARV1 homology domain

Evidence suggests that ARV1 regulates sterol movement within the cell. Saccharomyces cerevisiae cells lacking ScArv1 have defects in sterol trafficking, distribution, and biosynthesis. HepG2 cells treated with hARV1 antisense oligonucleotides accumulate cholesterol in the endoplasmic reticulum. Mice lacking Arv1 have a lean phenotype when fed a high fat diet and show no signs of liver triglyceride or cholesterol accumulation, suggesting a role for Arv1 in lipid transport. Here, we explored the direct lipid-binding activity of recombinant human ARV1 using in vitro lipid-binding assays. ARV1 lipid-binding activity was observed within the first N-terminal 98 amino acids containing the conserved ARV1 homology domain (AHD). The zinc-binding domain and conserved cysteine clusters within the AHD were necessary for lipid binding. Both full-length ARV1 and the AHD bound cholesterol, several phospholipids, and phosphoinositides with high affinity. The AHD showed the highest binding affinity for monophosphorylated phosphoinositides. Several conserved amino acids within the AHD were necessary for phospholipid binding. Biochemical studies suggested that ARV1 exists as a dimer in cells, with oligomerization being critical for ARV1 function, as amino acid mutations predicted to have a negative effect on dimerization caused weakened or complete loss of lipid binding. Our results show for the first time that human ARV1 can directly bind cholesterol and phospholipids. How this activity may function to regulate lipid binding and maintain proper lipid trafficking and/or transport in cells requires further studies.

Lysosome damage triggers acute formation of ER to lysosomes membrane tethers mediated by the bridge-like lipid transport protein VPS13C

Based on genetic studies, lysosome dysfunction is thought to play a pathogenetic role in Parkinson's disease (PD). Here we show that VPS13C, a bridge-like lipid transport protein and a PD gene, is a sensor of lysosome stress/damage. Upon lysosome membrane perturbation, VPS13C rapidly relocates from the cytosol to the surface of lysosomes where it tethers their membranes to the ER. This recruitment depends on Rab7 and requires a signal at the damaged lysosome surface that releases an inhibited state of VPS13C which hinders access of its VAB domain to lysosome-bound Rab7. While another PD protein, LRRK2, is also recruited to stressed/damaged lysosomes, its recruitment occurs at much later stages and by different mechanisms. Given the role of VPS13 proteins in bulk lipid transport, these findings suggest that lipid delivery to lysosomes by VPS13C is part of an early protective response to lysosome damage.

Plasmodesmata act as unconventional membrane contact sites regulating intercellular molecular exchange in plants

Membrane contact sites (MCSs) are fundamental for intracellular communication, but their role in intercellular communication remains unexplored. We show that in plants, plasmodesmata communication bridges function as atypical endoplasmic reticulum (ER)-plasma membrane (PM) tubular MCSs, operating at cell-cell interfaces. Similar to other MCSs, ER-PM apposition is controlled by a protein-lipid tethering complex, but uniquely, this serves intercellular communication. Combining high-resolution microscopy, molecular dynamics, and pharmacological and genetic approaches, we show that cell-cell trafficking is modulated through the combined action of multiple C2 domains transmembrane domain proteins (MCTPs) 3, 4, and 6 ER-PM tethers and phosphatidylinositol-4-phosphate (PI4P) lipid. Graded PI4P amounts regulate MCTP docking to the PM, their plasmodesmata localization, and cell-cell permeability. SAC7, an ER-localized PI4P-phosphatase, regulates MCTP4 accumulation at plasmodesmata and modulates cell-cell trafficking capacity in a cell-type-specific manner. Our findings expand MCS functions in information transmission from intracellular to intercellular cellular activities.

Orpinolide disrupts a leukemic dependency on cholesterol transport by inhibiting OSBP

Metabolic alterations in cancer precipitate in associated dependencies that can be therapeutically exploited. To meet this goal, natural product-inspired small molecules can provide a resource of invaluable chemotypes. Here, we identify orpinolide, a synthetic withanolide analog with pronounced antileukemic properties, via orthogonal chemical screening. Through multiomics profiling and genome-scale CRISPR-Cas9 screens, we identify that orpinolide disrupts Golgi homeostasis via a mechanism that requires active phosphatidylinositol 4-phosphate signaling at the endoplasmic reticulum-Golgi membrane interface. Thermal proteome profiling and genetic validation studies reveal the oxysterol-binding protein OSBP as the direct and phenotypically relevant target of orpinolide. Collectively, these data reaffirm sterol transport as a therapeutically actionable dependency in leukemia and motivate ensuing translational investigation via the probe-like compound orpinolide.

Inhibition of OSBP blocks retrograde trafficking by inducing partial Golgi degradation

Sterol-binding proteins are important regulators of lipid homeostasis and membrane integrity; however, the discovery of selective modulators can be challenging due to structural similarities in the sterol-binding domains. We report the discovery of potent and selective inhibitors of oxysterol-binding protein (OSBP), which we term oxybipins. Sterol-containing chemical chimeras aimed at identifying new sterol-binding proteins by targeted degradation, led to a significant reduction in levels of Golgi-associated proteins. The degradation occurred in lysosomes, concomitant with changes in protein glycosylation, indicating that the degradation of Golgi proteins was a downstream effect. By establishing a sterol transport protein biophysical assay panel, we discovered that the oxybipins potently inhibited OSBP, resulting in blockage of retrograde trafficking and attenuating Shiga toxin toxicity. As the oxybipins do not target other sterol transporters and only stabilized OSBP in intact cells, we advocate their use as tools to study OSBP function and therapeutic relevance.

Minimalist Natural ORPphilin Macarangin B Delineates OSBP Biological Function

OSBP ligands from the ORPphilin family are chemically complex natural products with promising anticancer properties. Here, we describe macarangin B, a natural racemic flavonoid selective for OSBP, which stands out from other ORPphilins due to its structural simplicity and distinct biological activity. Using a bioinspired strategy, we synthesized both (R,R,R) and (S,S,S)-macarangin B enantiomers, enabling us to study their interaction with OSBP based on their unique optical properties. Experimental and computational analyzes revealed that (R,R,R)-macarangin B has the highest affinity for OSBP. Importantly, both enantiomers showed significantly decreased cytotoxicity compared to other ORPphilins, suggesting OSBP is not the primary target in ORPphilin-induced cell death. Yet, OSBP is an attractive antiviral target, as it is hijacked by many positive-strand RNA viruses. Remarkably, (R,R,R)-macarangin B significantly inhibited Zika virus replication in human cells, highlighting its potential as a lead compound for antiviral drug development.

Effect of Linker Length on the Function of Biotinylated OSW-1 Probes

The biotinylated probes based on anticancer saponin OSW-1 with varied linker lengths were synthesized and their cell growth inhibitory activity and affinity pulldown efficiency were evaluated. All probes demonstrated comparable cytotoxicity to the parent natural product, highlighting that the linker moiety had a minimal impact on cell uptake or target engagement. In contrast, when evaluated against the known target proteins, OSBP and ORP4, the biotinylated probe 3 with PEG5 linker enabled most effective enrichment of target proteins in the affinity pulldown assay, suggesting that the cytotoxicity and pulldown efficiency did not correlate among the probes studied. Our data provided the first evidence that OSW-1 specifically binds to endogenously expressed OSBP and ORP4. The selectivity of affinity pulldown using probe 3 was also validated by facile identification of the enriched protein by silver staining and LC/MS analysis. Therefore, probe 3 with PEG5 linker comprising of 25 atoms (28 Å) was found as an optimal biotinylated probe for isolating OSW-1 binding proteins from cell lysate.

Dysregulation of PI4P in the trans Golgi regions activates the mammalian Golgi stress response

The Golgi stress response is an important cytoprotective system that enhances Golgi function in response to cellular demand, while cells damaged by prolonged Golgi stress undergo cell death. OSW-1, a natural compound with anticancer activity, potently inhibits OSBP that transports cholesterol and phosphatidylinositol-4-phosphate (PI4P) at contact sites between the endoplasmic reticulum and the Golgi apparatus. Previously, we reported that OSW-1 induces the Golgi stress response, resulting in Golgi stress-induced transcription and cell death. However, the underlying molecular mechanism has been unknown. To reveal the mechanism of a novel pathway of the Golgi stress response regulating transcriptional induction and cell death (the PI4P pathway), we performed a genome-wide KO screen and found that transcriptional induction as well as cell death induced by OSW-1 was repressed by the loss of regulators of PI4P synthesis, such as PITPNB and PI4KB. Our data indicate that OSW-1 induces Golgi stress-dependent transcriptional induction and cell death through dysregulation of the PI4P metabolism in the Golgi.

A budding yeast-centric view of oxysterol binding protein family function

The Trans Golgi Network (TGN)/endosomal system is a sorting center for cargo brought via the anterograde secretory pathway and the endocytic pathway that internalizes material from the plasma membrane. As many of the cargo that transit this central trafficking hub are components of key homeostatic signaling pathways, TGN/endosomes define a critical signaling hub for cellular growth control. A particularly interesting yet incompletely understood aspect of regulation of TGN/endosome function is control of this system by two families of lipid exchange/lipid transfer proteins. The phosphatidylinositol transfer proteins promote pro-trafficking phosphoinositide (i.e. phosphatidylinositol-4-phosphate) signaling pathways whereas proteins of the oxysterol binding protein family play reciprocal roles in antagonizing those arms of phosphoinositide signaling. The precise mechanisms for how these lipid binding proteins execute their functions remain to be resolved. Moreover, information regarding the coupling of individual members of the oxysterol binding protein family to specific biological activities is particularly sparse. Herein, we review what is being learned regarding functions of the oxysterol binding protein family in the yeast model system. Focus is primarily directed at a discussion of the Kes1/Osh4 protein for which the most information is available.

Salmonella-induced cholesterol accumulation in infected macrophages suppresses autophagy via mTORC1 activation

Salmonella enterica serovar Typhimurium is a Gram-negative bacillus that infects the host intestinal epithelium and resident macrophages. Many intracellular pathogens induce an autophagic response in host cells but have evolved mechanisms to subvert that response. Autophagy is closely linked to cellular cholesterol levels; mTORC1 senses increased cholesterol in lysosomal membranes, leading to its hyperactivity and suppression of autophagy. Previous studies indicate that Salmonella infection induces dramatic accumulation of cholesterol in macrophages, a fraction of which localizes to Salmonella containing vacuoles (SCVs). We previously reported that the bacterial effector protein SseJ triggers cholesterol accumulation through a signaling cascade involving focal adhesion kinase (FAK) and Akt. Here we show that mTORC1 is recruited to SCVs and is hyperactivated in a cholesterol-dependent manner. If cholesterol accumulation is prevented pharmacologically or through mutation of sseJ, autophagy is induced and bacterial survival is attenuated. Notably, the host lipid transfer protein OSBP (oxysterol binding protein 1) is also recruited to SCVs and its activity is necessary for both cholesterol transfer to SCVs and mTORC1 activation during infection. Finally, lipidomic analysis of Salmonella-infected macrophages revealed new insights into how Salmonella may manipulate lipid homeostasis to benefit its survival. We propose that S. Typhimurium induces cholesterol accumulation through SseJ to activate mTORC1, preventing autophagic clearance of bacteria.

Functional omics of ORP7 in primary endothelial cells

BACKGROUND

Many members of the oxysterol-binding protein-related protein (ORP) family have been characterized in detail over the past decades, but the lipid transport and other functions of ORP7 still remain elusive. What is known about ORP7 points toward an endoplasmic reticulum and plasma membrane-localized protein, which also interacts with GABA type A receptor-associated protein like 2 (GABARAPL2) and unlipidated Microtubule-associated proteins 1A/1B light chain 3B (LC3B), suggesting a further autophagosomal/lysosomal association. Functional roles of ORP7 have been suggested in cholesterol efflux, hypercholesterolemia, and macroautophagy. We performed a hypothesis-free multi-omics analysis of chemical ORP7 inhibition utilizing transcriptomics and lipidomics as well as proximity biotinylation interactomics to characterize ORP7 functions in a primary cell type, human umbilical vein endothelial cells (HUVECs). Moreover, assays on angiogenesis, cholesterol efflux, and lipid droplet quantification were conducted.

RESULTS

Pharmacological inhibition of ORP7 leads to an increase in gene expression related to lipid metabolism and inflammation, while genes associated with cell cycle and cell division were downregulated. Lipidomic analysis revealed increases in ceramides and lysophosphatidylcholines as well as saturated and monounsaturated triacylglycerols. Significant decreases were seen in all cholesteryl ester and in some unsaturated triacylglycerol species, compatible with the detected decrease of mean lipid droplet area. Along with the reduced lipid stores, ATP-binding cassette subfamily G member 1 (ABCG1)-mediated cholesterol efflux and angiogenesis decreased. Interactomics revealed an interaction of ORP7 with AKT1, a central metabolic regulator.

CONCLUSIONS

The transcriptomics results suggest an increase in prostanoid as well as oxysterol synthesis, which could be related to the observed upregulation of proinflammatory genes. We envision that the defective angiogenesis in HUVECs subjected to ORP7 inhibition could be the result of an unfavorable plasma membrane lipid composition and/or reduced potential for cell division. To conclude, the present study suggests multifaceted functions of ORP7 in lipid homeostasis, angiogenic tube formation, and gene expression of lipid metabolism, inflammation, and cell cycle in primary endothelial cells.

Specificity of lipid transfer proteins: An in vitro story

Lipids, which are highly diverse, are finely distributed between organelle membranes and the plasma membrane (PM) of eukaryotic cells. As a result, each compartment has its own lipid composition and molecular identity, which is essential for the functional fate of many proteins. This distribution of lipids depends on two main processes: lipid synthesis, which takes place in different subcellular regions, and the transfer of these lipids between and across membranes. This review will discuss the proteins that carry lipids throughout the cytosol, called LTPs (Lipid Transfer Proteins). More than the modes of action or biological roles of these proteins, we will focus on the in vitro strategies employed during the last 60 years to address a critical question: What are the lipid ligands of these LTPs? We will describe the extent to which these strategies, combined with structural data and investigations in cells, have made it possible to discover proteins, namely ORPs, Sec14, PITPs, STARDs, Ups/PRELIs, START-like, SMP-domain containing proteins, and bridge-like LTPs, which compose some of the main eukaryotic LTP families, and their lipid ligands. We will see how these approaches have played a central role in cell biology, showing that LTPs can connect distant metabolic branches, modulate the composition of cell membranes, and even create new subcellular compartments.

HCMV strain- and cell type-specific alterations in membrane contact sites point to the convergent regulation of organelle remodeling

Viruses are ubiquitous entities that infect organisms across the kingdoms of life. While viruses can infect a range of cells, tissues, and organisms, this aspect is often not explored in cell culture analyses. There is limited information about which infection-induced changes are shared or distinct in different cellular environments. The prevalent pathogen human cytomegalovirus (HCMV) remodels the structure and function of subcellular organelles and their interconnected networks formed by membrane contact sites (MCSs). A large portion of this knowledge has been derived from fibroblasts infected with a lab-adapted HCMV strain. Here, we assess strain- and cell type-specific alterations in MCSs and organelle remodeling induced by HCMV. Integrating quantitative mass spectrometry, super-resolution microscopy, and molecular virology assays, we compare infections with lab-adapted and low-passage HCMV strains in fibroblast and epithelial cells. We determine that, despite baseline proteome disparities between uninfected fibroblast and epithelial cells, infection induces convergent changes and is remarkably similar. We show that hallmarks of HCMV infection in fibroblasts, mitochondria-endoplasmic reticulum (ER) encapsulations and peroxisome proliferation, are also conserved in infected epithelial and macrophage-like cells. Exploring cell type-specific differences, we demonstrate that fibroblasts rely on endosomal cholesterol transport while epithelial cells rely on cholesterol from the Golgi. Despite these mechanistic differences, infections in both cell types result in phenotypically similar cholesterol accumulation at the viral assembly complex. Our findings highlight the adaptability of HCMV, in that infections can be tailored to the initial cell state by inducing both shared and unique proteome alterations, ultimately promoting a unified pro-viral environment.IMPORTANCEHuman cytomegalovirus (HCMV) establishes infections in diverse cell types throughout the body and is connected to a litany of diseases associated with each of these tissues. However, it is still not fully understood how HCMV replication varies in distinct cell types. Here, we compare HCMV replication with lab-adapted and low-passage strains in two primary sites of infection, lung fibroblasts and retinal epithelial cells. We discover that, despite displaying disparate protein compositions prior to infection, these cell types undergo convergent alterations upon HCMV infection, reaching a more similar cellular state late in infection. We find that remodeling of the subcellular landscape is a pervasive feature of HCMV infection, through alterations to both organelle structure-function and the interconnected networks they form via membrane contact sites. Our findings show how HCMV infection in different cell types induces both shared and divergent changes to cellular processes, ultimately leading to a more unified state.

Regulation of cellular and systemic sphingolipid homeostasis

One hundred and fifty years ago, Johann Thudichum described sphingolipids as unusual "Sphinx-like" lipids from the brain. Today, we know that thousands of sphingolipid molecules mediate many essential functions in embryonic development and normal physiology. In addition, sphingolipid metabolism and signalling pathways are dysregulated in a wide range of pathologies, and therapeutic agents that target sphingolipids are now used to treat several human diseases. However, our understanding of sphingolipid regulation at cellular and organismal levels and their functions in developmental, physiological and pathological settings is rudimentary. In this Review, we discuss recent advances in sphingolipid pathways in different organelles, how secreted sphingolipid mediators modulate physiology and disease, progress in sphingolipid-targeted therapeutic and diagnostic research, and the trans-cellular sphingolipid metabolic networks between microbiota and mammals. Advances in sphingolipid biology have led to a deeper understanding of mammalian physiology and may lead to progress in the management of many diseases.

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.

The journey of STING: Guiding immune signaling through membrane trafficking

Stimulator of Interferon Genes (STING) serves as a pivotal mediator in the innate immune signaling pathway, transducing signals from various DNA receptors and playing a crucial role in natural immune processes. During cellular quiescence, STING protein resides in the endoplasmic reticulum (ER), and its activation typically occurs through the cGAS-STING signaling pathway. Upon activation, STING protein is transported to the Golgi apparatus, thereby initiating downstream signaling cascades. Vesicular transport serves as the primary mechanism for STING protein trafficking between the ER and Golgi apparatus, with COPII mediating anterograde transport from the ER to Golgi apparatus, while COPI is responsible for retrograde transport. Numerous factors influence these transport processes, thereby exerting either promoting or inhibitory effects on STING protein expression. Upon reaching the Golgi apparatus, to prevent over-activation, STING protein is transported to post-Golgi compartments for degradation. In addition to the conventional lysosomal degradation pathway, ESCRT has also been identified as one of the degradation pathways for STING protein. This review summarizes the recent findings on the membrane trafficking pathways of STING, highlighting their contributions to the regulation of cytokine production, the activation of immune cells, and the coordination of immune signaling pathways.

ERLIN1/2 scaffolds bridge TMUB1 and RNF170 and restrict cholesterol esterification to regulate the secretory pathway

Complexes of ERLIN1 and ERLIN2 (ER lipid raft-associated 1 and 2) form large ring-like cup-shaped structures on the endoplasmic reticulum (ER) membrane and serve as platforms to bind cholesterol and E3 ubiquitin ligases, potentially defining functional nanodomains. Here, we show that ERLIN scaffolds mediate the interaction between the full-length isoform of TMUB1 (transmembrane and ubiquitin-like domain-containing 1) and RNF170 (RING finger protein 170). We identify a luminal N-terminal conserved region in TMUB1 and RNF170, which is required for this interaction. Three-dimensional modelling shows that this conserved motif binds the stomatin/prohibitin/flotillin/HflKC domain of two adjacent ERLIN subunits at different interfaces. Protein variants that preclude these interactions have been previously linked to hereditary spastic paraplegia. Using omics-based approaches in combination with phenotypic characterization of HeLa cells lacking both ERLINs, we demonstrate a role of ERLIN scaffolds in limiting cholesterol esterification, thereby favouring cholesterol transport from the ER to the Golgi apparatus and regulating Golgi morphology and the secretory pathway.

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.

OSBP-mediated PI(4)P-cholesterol exchange at endoplasmic reticulum-secretory granule contact sites controls insulin secretion

Insulin is packaged into secretory granules that depart the Golgi and undergo a maturation process that involves changes in the protein and lipid composition of the granules. Here, we show that insulin secretory granules form physical contacts with the endoplasmic reticulum and that the lipid exchange protein oxysterol-binding protein (OSBP) is recruited to these sites in a Ca2+-dependent manner. OSBP binding to insulin granules is positively regulated by phosphatidylinositol-4 (PI4)-kinases and negatively regulated by the PI4 phosphate (PI(4)P) phosphatase Sac2. Loss of Sac2 results in excess accumulation of cholesterol on insulin granules that is normalized when OSBP expression is reduced, and both acute inhibition and small interfering RNA (siRNA)-mediated knockdown of OSBP suppress glucose-stimulated insulin secretion without affecting insulin production or intracellular Ca2+ signaling. In conclusion, we show that lipid exchange at endoplasmic reticulum (ER)-granule contact sites is involved in the exocytic process and propose that these contacts act as reaction centers with multimodal functions during insulin granule maturation.

The function of sphingolipids in membrane trafficking and cell signaling in plants, in comparison with yeast and animal cells

Sphingolipids are essential membrane components involved in a wide range of cellular, developmental and signaling processes. Sphingolipids are so essential that knock-out mutation often leads to lethality. In recent years, conditional or weak allele mutants as well as the broadening of the pharmacological catalog allowed to decipher sphingolipid function more precisely in a less invasive way. This review intends to provide a discussion and point of view on the function of sphingolipids with a main focus on endomembrane trafficking, Golgi-mediated protein sorting, cell polarity, cell-to-cell communication and cell signaling at the plasma membrane. While our main angle is the plant field research, we will constantly refer to and compare with the advances made in the yeast and animal field. In this review, we will emphasize the role of sphingolipids not only as a membrane component, but also as a key player at a center of homeostatic regulatory networks involving direct or indirect interaction with other lipids, proteins and ion fluxes.

Calcium flow at ER-TGN contact sites facilitates secretory cargo export

Ca2+ influx into the trans-Golgi Network (TGN) promotes secretory cargo sorting by the Ca2+-ATPase SPCA1 and the luminal Ca2+ binding protein Cab45. Cab45 oligomerizes upon local Ca2+ influx, and Cab45 oligomers sequester and separate soluble secretory cargo from the bulk flow of proteins in the TGN. However, how this Ca2+ flux into the lumen of the TGN is achieved remains mysterious, as the cytosol has a nanomolar steady-state Ca2+ concentration. The TGN forms membrane contact sites (MCS) with the Endoplasmic Reticulum (ER), allowing protein-mediated exchange of molecular species such as lipids. Here, we show that the TGN export of secretory proteins requires the integrity of ER-TGN MCS and inositol 3 phosphate receptor (IP3R)-dependent Ca2+ fluxes in the MCS, suggesting Ca2+ transfer between these organelles. Using an MCS-targeted Ca2+ FRET sensor module, we measure the Ca2+ flow in these sites in real time. These data show that ER-TGN MCS facilitates the Ca2+ transfer required for Ca2+-dependent cargo sorting and export from the TGN, thus solving a fundamental question in cell biology.

Cholesterol-binding motifs in STING that control endoplasmic reticulum retention mediate anti-tumoral activity of cholesterol-lowering compounds

The cGAS-STING pathway plays a crucial role in anti-tumoral responses by activating inflammation and reprogramming the tumour microenvironment. Upon activation, STING traffics from the endoplasmic reticulum (ER) to Golgi, allowing signalling complex assembly and induction of interferon and inflammatory cytokines. Here we report that cGAMP stimulation leads to a transient decline in ER cholesterol levels, mediated by Sterol O-Acyltransferase 1-dependent cholesterol esterification. This facilitates ER membrane curvature and STING trafficking to Golgi. Notably, we identify two cholesterol-binding motifs in STING and confirm their contribution to ER-retention of STING. Consequently, depletion of intracellular cholesterol levels enhances STING pathway activation upon cGAMP stimulation. In a preclinical tumour model, intratumorally administered cholesterol depletion therapy potentiated STING-dependent anti-tumoral responses, which, in combination with anti-PD-1 antibodies, promoted tumour remission. Collectively, we demonstrate that ER cholesterol sets a threshold for STING signalling through cholesterol-binding motifs in STING and we propose that this could be exploited for cancer immunotherapy.

The activation of the adaptor protein STING depends on its interactions with the phospholipid PI4P

Activation of the endoplasmic reticulum (ER)-resident adaptor protein STING, a component of a cytosolic DNA-sensing pathway, induces the transcription of genes encoding type I interferons (IFNs) and other proinflammatory factors. Because STING is activated at the Golgi apparatus, control of the localization and activation of STING is important in stimulating antiviral and antitumor immune responses. Through a genome-wide CRISPR interference screen, we found that STING activation required the Golgi-resident protein ACBD3, which promotes the generation of phosphatidylinositol 4-phosphate (PI4P) at the trans-Golgi network, as well as other PI4P-associated proteins. Appropriate localization and activation of STING at the Golgi apparatus required ACBD3 and the PI4P-generating kinase PI4KB. In contrast, STING activation was enhanced when the lipid-shuttling protein OSBP, which removes PI4P from the Golgi apparatus, was inhibited by the US Food and Drug Administration-approved antifungal itraconazole. The increase in the abundance of STING-activating phospholipids at the trans-Golgi network resulted in the increased production of IFN-β and other cytokines in THP-1 cells. Furthermore, a mutant STING that could not bind to PI4P failed to traffic from the ER to the Golgi apparatus in response to a STING agonist, whereas forced relocalization of STING to PI4P-enriched areas elicited STING activation in the absence of stimulation with a STING agonist. Thus, PI4P is critical for STING activation, and manipulating PI4P abundance may therapeutically modulate STING-dependent immune responses.

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.

The expanding roles of PI4P and PI(4,5)P2 at the plasma membrane: Role of phosphatidylinositol transfer proteins

Phosphoinositides are phosphorylated derivatives of phosphatidylinositol, a phospholipid that is synthesised at the endoplasmic reticulum. The plasma membrane contains the enzymes to phosphorylate phosphatidylinositol and is therefore rich in the phosphorylated derivatives, PI4P and PI(4,5)P2. PI(4,5)P2 is a substrate for phospholipase C and during cell signaling, PI(4,5)P2 levels are reduced. Here I discuss a family of proteins, phosphatidylinositol transfer proteins (PITPs) that can restore PI(4,5)P2 levels.

Viral regulation of organelle membrane contact sites

At the core of organelle functions lies their ability and need to form dynamic organelle-organelle networks that drive intracellular communication and coordination of cellular pathways. These networks are facilitated by membrane contact sites (MCSs) that promote both intra-organelle and inter-organelle communication. Given their multiple functions, MCSs and the proteins that form them are commonly co-opted by viruses during infection to promote viral replication. This Essay discusses mechanisms acquired by diverse human viruses to regulate MCS functions in either proviral processes or host defense. It also examines techniques used for examining MCSs in the context of viral infections.

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.

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.

Single-molecule localization microscopy reveals STING clustering at the trans-Golgi network through palmitoylation-dependent accumulation of cholesterol

Stimulator of interferon genes (STING) is critical for the type I interferon response to pathogen- or self-derived DNA in the cytosol. STING may function as a scaffold to activate TANK-binding kinase 1 (TBK1), but direct cellular evidence remains lacking. Here we show, using single-molecule imaging of STING with enhanced time resolutions down to 5 ms, that STING becomes clustered at the trans-Golgi network (about 20 STING molecules per cluster). The clustering requires STING palmitoylation and the Golgi lipid order defined by cholesterol. Single-molecule imaging of TBK1 reveals that STING clustering enhances the association with TBK1. We thus provide quantitative proof-of-principle for the signaling STING scaffold, reveal the mechanistic role of STING palmitoylation in the STING activation, and resolve the long-standing question of the requirement of STING translocation for triggering the innate immune signaling.

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.

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.

Lipid exchange at ER-trans-Golgi contact sites governs polarized cargo sorting

Oxysterol binding protein (OSBP) extracts cholesterol from the ER to deliver it to the TGN via counter exchange and subsequent hydrolysis of the phosphoinositide PI(4)P. Here, we show that this pathway is essential in polarized epithelial cells where it contributes not only to the proper subcellular distribution of cholesterol but also to the trans-Golgi sorting and trafficking of numerous plasma membrane cargo proteins with apical or basolateral localization. Reducing the expression of OSBP, blocking its activity, or inhibiting a PI4Kinase that fuels OSBP with PI(4)P abolishes the epithelial phenotype. Waves of cargo enrichment in the TGN in phase with OSBP and PI(4)P dynamics suggest that OSBP promotes the formation of lipid gradients along the TGN, which helps cargo sorting. During their transient passage through the trans-Golgi, polarized plasma membrane proteins get close to OSBP but fail to be sorted when OSBP is silenced. Thus, OSBP lipid exchange activity is decisive for polarized cargo sorting and distribution in epithelial cells.

Transcriptomic and lipid profiling analysis reveals a functional interplay between testosterone and growth hormone in hypothyroid liver

Preclinical and clinical studies suggest that hypothyroidism might cause hepatic endocrine and metabolic disturbances with features that mimic deficiencies of testosterone and/or GH. The absence of physiological interactions between testosterone and GH can be linked to male differentiated liver diseases. Testosterone plays relevant physiological effects on somatotropic-liver axis and liver composition and the liver is a primary organ of interactions between testosterone and GH. However, testosterone exerts many effects on liver through complex and poorly understood mechanisms. Testosterone impacts liver functions by binding to the Androgen Receptor, and, indirectly, through its conversion to estradiol, and cooperation with GH. However, the role of testosterone, and its interaction with GH, in the hypothyroid liver, remains unclear. In the present work, the effects of testosterone, and how they impact on GH-regulated whole transcriptome and lipid composition in the liver, were studied in the context of adult hypothyroid-orchiectomized rats. Testosterone replacement positively modulated somatotropic-liver axis and impacted liver transcriptome involved in lipid and glucose metabolism. In addition, testosterone enhanced the effects of GH on the transcriptome linked to lipid biosynthesis, oxidation-reduction, and metabolism of unsaturated and long-chain fatty acids (FA). However, testosterone decreased the hepatic content of cholesterol esters and triacylglycerols and increased fatty acids whereas GH increased neutral lipids and decreased polar lipids. Biological network analysis of the effects of testosterone on GH-regulated transcriptome confirmed a close connection with crucial proteins involved in steroid and fatty acid metabolism. Taken together, this comprehensive analysis of gene expression and lipid profiling in hypothyroid male liver reveals a functional interplay between testosterone and pulsed GH administration.

Nutrient-regulated control of lysosome function by signaling lipid conversion

Lysosomes serve dual antagonistic functions in cells by mediating anabolic growth signaling and the catabolic turnover of macromolecules. How these janus-faced activities are regulated in response to cellular nutrient status is poorly understood. We show here that lysosome morphology and function are reversibly controlled by a nutrient-regulated signaling lipid switch that triggers the conversion between peripheral motile mTOR complex 1 (mTORC1) signaling-active and static mTORC1-inactive degradative lysosomes clustered at the cell center. Starvation-triggered relocalization of phosphatidylinositol 4-phosphate (PI(4)P)-metabolizing enzymes reshapes the lysosomal surface proteome to facilitate lysosomal proteolysis and to repress mTORC1 signaling. Concomitantly, lysosomal phosphatidylinositol 3-phosphate (PI(3)P), which marks motile signaling-active lysosomes in the cell periphery, is erased. Interference with this PI(3)P/PI(4)P lipid switch module impairs the adaptive response of cells to altering nutrient supply. Our data unravel a key function for lysosomal phosphoinositide metabolism in rewiring organellar membrane dynamics in response to cellular nutrient status.

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.

Structure-Based Design of a Lead Compound Derived from Natural Schweinfurthins with Antitumor Properties That Target Oxysterol-Binding Protein

Schweinfurthins (SWs) are naturally occurring prenylated stilbenes with promising anticancer properties. They act through a novel mechanism of action similar to that of other families of natural compounds. Their known target, oxysterol-binding protein (OSBP), plays a crucial role in controlling the intracellular distribution of cholesterol. We synthesized 15 analogues of SWs and demonstrated for the first time that their cytotoxicity as well as that of natural derivatives correlates with their affinity for OSBP. Through this extensive SAR study, we selected one synthetic analogue obtained in one step from SW-G. Using its fluorescence properties, we showed that this compound recapitulates the effect of natural SW-G in cells and confirmed that it leads to cell death via the same mechanism. Finally, after pilot PK experiments, we provided the first evidence of its in vivo efficacy in combination with temozolomide in a patient-derived glioblastoma xenograft model.

Visualization of accessible cholesterol using a GRAM domain-based biosensor

Cholesterol is important for membrane integrity and cell signaling, and dysregulation of the distribution of cellular cholesterol is associated with numerous diseases, including neurodegenerative disorders. While regulated transport of a specific pool of cholesterol, known as "accessible cholesterol", contributes to the maintenance of cellular cholesterol distribution and homeostasis, tools to monitor accessible cholesterol in live cells remain limited. Here, we engineer a highly sensitive accessible cholesterol biosensor by taking advantage of the cholesterol-sensing element (the GRAM domain) of an evolutionarily conserved lipid transfer protein, GRAMD1b. Using this cholesterol biosensor, which we call GRAM-W, we successfully visualize in real time the distribution of accessible cholesterol in many different cell types, including human keratinocytes and iPSC-derived neurons, and show differential dependencies on cholesterol biosynthesis and uptake for maintaining levels of accessible cholesterol. Furthermore, we combine GRAM-W with a dimerization-dependent fluorescent protein (ddFP) and establish a strategy for the ultrasensitive detection of accessible plasma membrane cholesterol. These tools will allow us to obtain important insights into the molecular mechanisms by which the distribution of cellular cholesterol is regulated.

Structure of the ceramide-bound SPOTS complex

Sphingolipids are structural membrane components that also function in cellular stress responses. The serine palmitoyltransferase (SPT) catalyzes the rate-limiting step in sphingolipid biogenesis. Its activity is tightly regulated through multiple binding partners, including Tsc3, Orm proteins, ceramides, and the phosphatidylinositol-4-phosphate (PI4P) phosphatase Sac1. The structural organization and regulatory mechanisms of this complex are not yet understood. Here, we report the high-resolution cryo-EM structures of the yeast SPT in complex with Tsc3 and Orm1 (SPOT) as dimers and monomers and a monomeric complex further carrying Sac1 (SPOTS). In all complexes, the tight interaction of the downstream metabolite ceramide and Orm1 reveals the ceramide-dependent inhibition. Additionally, observation of ceramide and ergosterol binding suggests a co-regulation of sphingolipid biogenesis and sterol metabolism within the SPOTS complex.

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.

Regulation of cellular cholesterol distribution via non-vesicular lipid transport at ER-Golgi contact sites

Abnormal distribution of cellular cholesterol is associated with numerous diseases, including cardiovascular and neurodegenerative diseases. Regulated transport of cholesterol is critical for maintaining its proper distribution in the cell, yet the underlying mechanisms remain unclear. Here, we show that lipid transfer proteins, namely ORP9, OSBP, and GRAMD1s/Asters (GRAMD1a/GRAMD1b/GRAMD1c), control non-vesicular cholesterol transport at points of contact between the ER and the trans-Golgi network (TGN), thereby maintaining cellular cholesterol distribution. ORP9 localizes to the TGN via interaction between its tandem α-helices and ORP10/ORP11. ORP9 extracts PI4P from the TGN to prevent its overaccumulation and suppresses OSBP-mediated PI4P-driven cholesterol transport to the Golgi. By contrast, GRAMD1s transport excess cholesterol from the Golgi to the ER, thereby preventing its build-up. Cells lacking ORP9 exhibit accumulation of cholesterol at the Golgi, which is further enhanced by additional depletion of GRAMD1s with major accumulation in the plasma membrane. This is accompanied by chronic activation of the SREBP-2 signalling pathway. Our findings reveal the importance of regulated lipid transport at ER-Golgi contacts for maintaining cellular cholesterol distribution and homeostasis.

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.