Intracellular Cholesterol Transport by Sterol Transfer Proteins at Membrane Contact Sites

Heukcha, naturally post-fermented green tea extract, ameliorates diet-induced hypercholesterolemia and NAFLD in hamster

Hypercholesterolemia, characterized by an increase in plasma low-density lipoprotein (LDL) cholesterol and total cholesterol (TC), is the leading cause of non-alcoholic fatty liver disease (NAFLD). The present study examined the effect of Heukcha extract (HCE), a naturally post-fermented green tea extract, on diet-induced hypercholesterolemia and related NAFLD in hamsters that metabolize lipids in a similar fashion to humans. The 10-week-old golden Syrian hamsters were fed a normal diet (ND) or a high cholesterol diet (HCD) containing 0.2% cholesterol and 10% lard, and some were also given HCE (200 or 500 mg/kg/day) orally for 12 weeks. The HCE did not affect the body weight gain, food intake, or the calorie intake. HCD significantly (p < 0.05) increased LDL (0.9 to 2.1 mmol/L), TC (2.7 to 7.8 mmol/L), and triglyceride (TG; 2.3 to 4.0 mmol/L), which was significantly decreased by 27.7%, 17.3%, and 60%, respectively, by HCE. HDL was significantly increased by HCD (0.6 to 1.6 mmol/L), but it was not affected by HCE administration. Furthermore, HCE suppressed HCD-induced liver oxidative stress, fibrosis, and lipid accumulation almost to control levels. Interestingly, HCE significantly increased the protein level of cholesterol 7 alpha-hydroxylase (CYP7A1), the rate-limiting enzyme for bile acid synthesis, by 1.5-fold in the liver. The present data suggest that HCE could be a functional food ingredient that can suppress the occurrence of diet-induced hypercholesterolemia and NAFLD, possibly by increasing the expression of CYP7A1.

Prostate Cancer-Focus on Cholesterol

Prostate cancer (PC) is the most common malignancy in men. Common characteristic involved in PC pathogenesis are disturbed lipid metabolism and abnormal cholesterol accumulation. Cholesterol can be further utilized for membrane or hormone synthesis while cholesterol biosynthesis intermediates are important for oncogene membrane anchoring, nucleotide synthesis and mitochondrial electron transport. Since cholesterol and its biosynthesis intermediates influence numerous cellular processes, in this review we have described cholesterol homeostasis in a normal cell. Additionally, we have illustrated how commonly deregulated signaling pathways in PC (PI3K/AKT/MTOR, MAPK, AR and p53) are linked with cholesterol homeostasis regulation.

A basic model for cell cholesterol homeostasis

Cells manage their cholesterol by negative feedback using a battery of sterol-responsive proteins. How these activities are coordinated so as to specify the abundance and distribution of the sterol is unclear. We present a simple mathematical model that addresses this question. It assumes that almost all of the cholesterol is associated with phospholipids in stoichiometric complexes. A small fraction of the sterol is uncomplexed and thermodynamically active. It equilibrates among the organelles, setting their sterol level according to the affinity of their phospholipids. The activity of the homeostatic proteins in the cytoplasmic membranes is then set by their fractional saturation with uncomplexed cholesterol in competition with the phospholipids. The high-affinity phospholipids in the plasma membrane (PM) are filled to near stoichiometric equivalence, giving it most of the cell sterol. Notably, the affinity of the phospholipids in the endomembranes (EMs) is lower by orders of magnitude than that of the phospholipids in the PM. Thus, the small amount of sterol in the EMs rests far below stoichiometric capacity. Simulations match a variety of experimental data. The model captures the essence of cell cholesterol homeostasis, makes coherent a diverse set of experimental findings, provides a surprising prediction and suggests new experiments.

The 3-beta-hydroxysteroid-Delta(8), Delta(7)-isomerase EBP inhibits cholesterylation of Smoothened

Hedgehog (Hh) pathway plays a central role in vertebrate embryonic development and carcinogenesis. The G-protein coupled receptor-like protein Smoothened (SMO) is one of the major members in Hh pathway. Covalent modification of cholesterol on the 95th asparagine (D95) of human SMO, which is regulated by Hh and PTCH1, is critical for SMO activation. However, it is not known whether SMO cholesterylation is regulated by other proteins. In this study, we identified Emopamil binding protein (EBP, also known as 3-beta-hydroxysteroid-Delta(8),Delta(7)-isomerase) as a SMO-interacting protein. Overexpression of EBP suppressed SMO cholesterylation and Hh pathway activity, whereas genetic disruption of EBP enhanced SMO cholesterylation and the downstream signaling. EBP-mediated inhibition of SMO cholesterylation was independent of its isomerase activity, but dependent on the C-terminus of EBP that was required for SMO binding. The X-linked dominant chondrodysplasia punctate 2 (CDPX2)-associated EBP mutants inhibited SMO cholesterylation too. Together, this study shows that EBP modulates SMO cholesterylation through direct binding and suggests a possible mechanism of CDPX2 pathogenesis.

GRAMD1-mediated accessible cholesterol sensing and transport

Cholesterol, an essential lipid for cell signaling and structural integrity of cellular membranes, is highly enriched in the plasma membrane (PM). However, the regulatory mechanisms that control its biosynthesis and uptake both reside in the endoplasmic reticulum (ER). Thus, the ER needs to constantly monitor the levels of PM cholesterol. This is in part mediated by regulated transport of a biochemically defined pool of cholesterol, termed "accessible" cholesterol, from the PM to the ER via evolutionarily conserved ER-anchored lipid transfer proteins, the GRAMD1s/Asters (GRAMD1a/1b/1c) (Lam/Ltc proteins in yeast). GRAMD1s possess cytosolically exposed GRAM domain and StART-like domain followed by a transmembrane ER anchor. They form homo- and hetero-meric complexes and move to the contacts formed between the ER and the PM by sensing a transient expansion of the accessible pool of cholesterol in the PM via the GRAM domain and facilitate its extraction and transport to the ER via the StART-like domain. The GRAMD1b GRAM domain possesses distinct, but synergistic sites, for recognizing accessible cholesterol and anionic lipids, including phosphatidylserine, within the PM. This property of the GRAM domain contributes to regulated tethering of the PM to ER membrane where GRAMD1s are anchored and fine-tunes StART-like domain-dependent accessible cholesterol transport. Thus, cells use GRAMD1s to sense the levels of cholesterol in the PM and regulate transport of accessible PM cholesterol to the ER in order to maintain cholesterol homeostasis.

MYC Enhances Cholesterol Biosynthesis and Supports Cell Proliferation Through SQLE

Oncogene c-Myc (referred in this report as MYC) promotes tumorigenesis in multiple human cancers. MYC regulates numerous cellular programs involved in cell growth and cell metabolism. Tumor cells exhibit obligatory dependence on cholesterol metabolism, which provides essential membrane components and metabolites to support cell growth. To date, how cholesterol biosynthesis is delicately regulated to promote tumorigenesis remains unclear. Here, we show that MYC enhances cholesterol biosynthesis and promotes cell proliferation. Through transcriptional upregulation of SQLE, a rate-limiting enzyme in cholesterol synthesis pathway, MYC increases cholesterol production and promotes tumor cell growth. SQLE overexpression restores the cellular cholesterol levels in MYC-knockdown cells. More importantly, in SQLE-depleted cells, enforced expression of MYC has no effect on cholesterol levels. Therefore, our findings reveal that SQLE is critical for MYC-mediated cholesterol synthesis, and further demonstrate that SQLE may be a potential therapeutic target in MYC-amplified cancers.

Molecular basis of accessible plasma membrane cholesterol recognition by the GRAM domain of GRAMD1b

Cholesterol is essential for cell physiology. Transport of the "accessible" pool of cholesterol from the plasma membrane (PM) to the endoplasmic reticulum (ER) by ER-localized GRAMD1 proteins (GRAMD1a/1b/1c) contributes to cholesterol homeostasis. However, how cells detect accessible cholesterol within the PM remains unclear. We show that the GRAM domain of GRAMD1b, a coincidence detector for anionic lipids, including phosphatidylserine (PS), and cholesterol, possesses distinct but synergistic sites for sensing accessible cholesterol and anionic lipids. We find that a mutation within the GRAM domain of GRAMD1b that is associated with intellectual disability in humans specifically impairs cholesterol sensing. In addition, we identified another point mutation within this domain that enhances cholesterol sensitivity without altering its PS sensitivity. Cell-free reconstitution and cell-based assays revealed that the ability of the GRAM domain to sense accessible cholesterol regulates membrane tethering and determines the rate of cholesterol transport by GRAMD1b. Thus, cells detect the codistribution of accessible cholesterol and anionic lipids in the PM and fine-tune the non-vesicular transport of PM cholesterol to the ER via GRAMD1s.

Endomembrane Tension and Trafficking

Eukaryotic cells employ diverse uptake mechanisms depending on their specialized functions. While such mechanisms vary widely in their defining criteria: scale, molecular machinery utilized, cargo selection, and cargo destination, to name a few, they all result in the internalization of extracellular solutes and fluid into membrane-bound endosomes. Upon scission from the plasma membrane, this compartment is immediately subjected to extensive remodeling which involves tubulation and vesiculation/budding of the limiting endomembrane. This is followed by a maturation process involving concomitant retrograde transport by microtubule-based motors and graded fusion with late endosomes and lysosomes, organelles that support the degradation of the internalized content. Here we review an important determinant for sorting and trafficking in early endosomes and in lysosomes; the control of tension on the endomembrane. Remodeling of endomembranes is opposed by high tension (caused by high hydrostatic pressure) and supported by the relief of tension. We describe how the timely and coordinated efflux of major solutes along the endocytic pathway affords the cell control over such tension. The channels and transporters that expel the smallest components of the ingested medium from the early endocytic fluid are described in detail as these systems are thought to enable endomembrane deformation by curvature-sensing/generating coat proteins. We also review similar considerations for the lysosome where resident hydrolases liberate building blocks from luminal macromolecules and transporters flux these organic solutes to orchestrate trafficking events. How the cell directs organellar trafficking based on the luminal contents of organelles of the endocytic pathway is not well-understood, however, we propose that the control over membrane tension by solute transport constitutes one means for this to ensue.

Cholesterol interaction attenuates scramblase activity of SCRM-1 in the artificial membrane

Phospholipid (PL) scramblases are single-pass transmembrane protein mediating bidirectional PL translocation. Previously in silico analysis of human PL scramblases, predicted the presence of an uncharacterized cholesterol-binding domain spanning partly in the transmembrane helix as well as in the adjacent extracellular coil. This domain was found to be universally conserved in diverse organisms like Caenorhabditis elegans. In this study, we investigated the saturable cholesterol-binding domain of SCRM-1 using fluorescence sterol binding assay, Stern-Volmer quenching, Förster resonance energy transfer, and CD spectroscopy. We observed high-affinity interaction between cholesterol and SCRM-1. Our results support a previous report, which showed that the cholesterol ordering effect reduced the scramblase activity of hPLSCR1. Considering the presence of a high-affinity binding sequence, we propose that the reduction in activity could partly be due to the cholesterol binding. To validate this, we generated a C-terminal helix (CTH) deletion construct (∆CTH SCRM-1) and a point mutation in the putative cholesterol-binding domain I273D SCRM-1. Deletion construct greatly reduced cholesterol affinity along with loss of scramblase activity. In contrast to this, I273D SCRM-1 retained scrambling activity in proteoliposomes containing ~30 mol% cholesterol but lost sterol binding ability. These results suggest that C-terminal helix is crucial for membrane insertion and in the lipid bilayer the scrambling activity of SCRM-1 is modulated through its interaction with cholesterol.

Function of the endolysosomal network in cholesterol homeostasis and metabolic-associated fatty liver disease (MAFLD)

Background

Metabolic-associated fatty liver disease (MAFLD), also known as non-alcoholic fatty liver disease, has become the leading cause of chronic liver disease worldwide. In addition to hepatic accumulation of triglycerides, dysregulated cholesterol metabolism is an important contributor to the pathogenesis of MAFLD. Maintenance of cholesterol homeostasis is highly dependent on cellular cholesterol uptake and, subsequently, cholesterol transport to other membrane compartments, such as the endoplasmic reticulum (ER).

Scope of review

The endolysosomal network is key for regulating cellular homeostasis and adaptation, and emerging evidence has shown that the endolysosomal network is crucial to maintain metabolic homeostasis. In this review, we will summarize our current understanding of the role of the endolysosomal network in cholesterol homeostasis and its implications in MAFLD pathogenesis.

Major conclusions

Although multiple endolysosomal proteins have been identified in the regulation of cholesterol uptake, intracellular transport, and degradation, their physiological role is incompletely understood. Further research should elucidate their role in controlling metabolic homeostasis and development of fatty liver disease.

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The intracellular cholesterol transport is tightly regulated by the endocytic and lysosomal network.

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Dysfunction of the endolysosomal network affects hepatic lipid homeostasis.

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The endosomal sorting of lipoprotein receptors is precisely regulated and is not a bulk process.

Regulation of Plasma Membrane Sterol Homeostasis by Nonvesicular Lipid Transport

Sterol contributes to the structural integrity of cellular membranes and plays an important role in the regulation of cell signaling in eukaryotes. It is either produced in the endoplasmic reticulum or taken up from the extracellular environment. In most eukaryotic cells, however, the majority of sterol is enriched in the plasma membrane. Thus, the transport of sterol between the plasma membrane and other organelles, including the endoplasmic reticulum, is crucial for maintaining sterol homeostasis. While vesicular transport that relies on membrane budding and fusion reactions plays an important role in bulk sterol transport, this mode of transport is slow and non-selective. Growing evidence suggests a critical role of nonvesicular transport mediated by evolutionarily conserved families of lipid transfer proteins in more rapid and selective delivery of sterol. Some lipid transfer proteins act primarily at the sites of contacts formed between the endoplasmic reticulum and other organelles or the plasma membrane without membrane fusion. In this review, we describe the similarities and differences of sterol biosynthesis and uptake in mammals and yeast and discuss the role of their lipid transfer proteins in maintaining plasma membrane sterol homeostasis.

Ceramide-1-phosphate transfer protein (CPTP) regulation by phosphoinositides

Ceramide-1-phosphate transfer proteins (CPTPs) are members of the glycolipid transfer protein (GLTP) superfamily that shuttle ceramide-1-phosphate (C1P) between membranes. CPTPs regulate cellular sphingolipid homeostasis in ways that impact programmed cell death and inflammation. CPTP downregulation specifically alters C1P levels in the plasma and trans-Golgi membranes, stimulating proinflammatory eicosanoid production and autophagy-dependent inflammasome-mediated cytokine release. However, the mechanisms used by CPTP to target the trans-Golgi and plasma membrane are not well understood. Here, we monitored C1P intervesicular transfer using fluorescence energy transfer (FRET) and showed that certain phosphoinositides (phosphatidylinositol 4,5 bisphosphate (PI-(4,5)P2) and phosphatidylinositol 4-phosphate (PI-4P)) increased CPTP transfer activity, whereas others (phosphatidylinositol 3-phosphate (PI-3P) and PI) did not. PIPs that stimulated CPTP did not stimulate GLTP, another superfamily member. Short-chain PI-(4,5)P2, which is soluble and does not remain membrane-embedded, failed to activate CPTP. CPTP stimulation by physiologically relevant PI-(4,5)P2 levels surpassed that of phosphatidylserine (PS), the only known non-PIP stimulator of CPTP, despite PI-(4,5)P2 increasing membrane equilibrium binding affinity less effectively than PS. Functional mapping of mutations that led to altered FRET lipid transfer and assessment of CPTP membrane interaction by surface plasmon resonance indicated that di-arginine motifs located in the α-6 helix and the α3-α4 helix regulatory loop of the membrane-interaction region serve as PI-(4,5)P2 headgroup-specific interaction sites. Haddock modeling revealed specific interactions involving the PI-(4,5)P2 headgroup that left the acyl chains oriented favorably for membrane embedding. We propose that PI-(4,5)P2 interaction sites enhance CPTP activity by serving as preferred membrane targeting/docking sites that favorably orient the protein for function.

Bis(monoacylglycero)phosphate, an important actor in the host endocytic machinery hijacked by SARS-CoV-2 and related viruses

Viruses, including the novel coronavirus SARS-CoV-2, redirect infected cell metabolism to their own purposes. After binding to its receptor angiotensin-converting enzyme 2 (ACE2) on the cell surface, the SARS-CoV-2 is taken up by receptor-mediated endocytosis ending in the acidic endolysosomal compartment. The virus hijacks the endosomal machinery leading to fusion of viral and endosomal membranes and release of the viral RNA into the cytosol. This mini-review specifically highlights the membrane lipid organization of the endosomal system focusing on the unconventional and late endosome/lysosome-specific phospholipid, bis(monoacylglycero)phosphate (BMP). BMP is enriched in alveolar macrophages of lung, one of the target tissue of SARS-CoV-2. This review details the BMP structure, its unsaturated fatty acid composition and fusogenic properties that are essential for the highly dynamic formation of the intraluminal vesicles inside the endosomes. Interestingly, BMP is necessary for infection and replication of enveloped RNA virus such as SARS-CoV-1 and Dengue virus. We also emphasize the role of BMP in lipid sorting and degradation, especially cholesterol transport in cooperation with Niemann Pick type C proteins (NPC 1 and 2) and with some oxysterol-binding protein (OSBP)-related proteins (ORPs) as well as in sphingolipid degradation. Interestingly, numerous virus infection required NPC1 as well as ORPs along the endocytic pathway. Furthermore, BMP content is increased during pathological endosomal lipid accumulation in various lysosomal storage disorders. This is particularly important knowing the high percentage of patients with metabolic disorders among the SARS-CoV-2 infected patients presenting severe forms of COVID-19.

Active cholesterol 20 years on

This review considers the following hypotheses, some well-supported and some speculative. Almost all of the sterol molecules in plasma membranes are associated with bilayer phospholipids in complexes of varied strength and stoichiometry. These complexes underlie many of the material properties of the bilayer. The small fraction of cholesterol molecules exceeding the binding capacity of the phospholipids is thermodynamically active and serves diverse functions. It circulates briskly among the cell membranes, particularly through contact sites linking the organelles. Active cholesterol provides the upstream feedback signal to multiple mechanisms governing plasma membrane homeostasis, pegging the sterol level to a threshold set by its phospholipids. Active cholesterol could also be the cargo for various inter-organelle transporters and the form excreted from cells by reverse transport. Furthermore, it is integral to the function of caveolae; a mediator of Hedgehog regulation; and a ligand for the binding of cytolytic toxins to membranes. Active cholesterol modulates a variety of plasma membrane proteins-receptors, channels and transporters-at least in vitro.

Aster Proteins Regulate the Accessible Cholesterol Pool in the Plasma Membrane

Recent studies have demonstrated the existence of a discrete pool of cholesterol in the plasma membranes (PM) of mammalian cells-referred to as the accessible cholesterol pool-that can be detected by the binding of modified versions of bacterial cytolysins (e.g., anthrolysin O). When the amount of accessible cholesterol in the PM exceeds a threshold level, the excess cholesterol moves to the endoplasmic reticulum (ER), where it regulates the SREBP2 pathway and undergoes esterification. We reported previously that the Aster/Gramd1 family of sterol transporters mediates nonvesicular movement of cholesterol from the PM to the ER in multiple mammalian cell types. Here, we investigated the PM pool of accessible cholesterol in cholesterol-loaded fibroblasts with a knockdown of Aster-A and in mouse macrophages from Aster-B and Aster-A/B-deficient mice. Nanoscale secondary ion mass spectrometry (NanoSIMS) analyses revealed expansion of the accessible cholesterol pool in cells lacking Aster expression. The increased accessible cholesterol pool in the PM was accompanied by reduced cholesterol movement to the ER, evidenced by increased expression of SREBP2-regulated genes. Cosedimentation experiments with liposomes revealed that the Aster-B GRAM domain binds to membranes in a cholesterol concentration-dependent manner and that the binding is facilitated by the presence of phosphatidylserine. These studies revealed that the Aster-mediated nonvesicular cholesterol transport pathway controls levels of accessible cholesterol in the PM, as well as the activity of the SREBP pathway.

POST1/C12ORF49 regulates the SREBP pathway by promoting site-1 protease maturation

Sterol-regulatory element binding proteins (SREBPs) are the key transcriptional regulators of lipid metabolism. The activation of SREBP requires translocation of the SREBP precursor from the endoplasmic reticulum to the Golgi, where it is sequentially cleaved by site-1 protease (S1P) and site-2 protease and releases a nuclear form to modulate gene expression. To search for new genes regulating cholesterol metabolism, we perform a genome-wide CRISPR/Cas9 knockout screen and find that partner of site-1 protease (POST1), encoded by C12ORF49, is critically involved in the SREBP signaling. Ablation of POST1 decreases the generation of nuclear SREBP and reduces the expression of SREBP target genes. POST1 binds S1P, which is synthesized as an inactive protease (form A) and becomes fully mature via a two-step autocatalytic process involving forms B'/B and C'/C. POST1 promotes the generation of the functional S1P-C'/C from S1P-B'/B (canonical cleavage) and, notably, from S1P-A directly (non-canonical cleavage) as well. This POST1-mediated S1P activation is also essential for the cleavages of other S1P substrates including ATF6, CREB3 family members and the α/β-subunit precursor of N-acetylglucosamine-1-phosphotransferase. Together, we demonstrate that POST1 is a cofactor controlling S1P maturation and plays important roles in lipid homeostasis, unfolded protein response, lipoprotein metabolism and lysosome biogenesis.

Annexin A6 modulates TBC1D15/Rab7/StARD3 axis to control endosomal cholesterol export in NPC1 cells

Cholesterol accumulation in late endosomes is a prevailing phenotype of Niemann-Pick type C1 (NPC1) mutant cells. Likewise, annexin A6 (AnxA6) overexpression induces a phenotype reminiscent of NPC1 mutant cells. Here, we demonstrate that this cellular cholesterol imbalance is due to AnxA6 promoting Rab7 inactivation via TBC1D15, a Rab7-GAP. In NPC1 mutant cells, AnxA6 depletion and eventual Rab7 activation was associated with peripheral distribution and increased mobility of late endosomes. This was accompanied by an enhanced lipid accumulation in lipid droplets in an acyl-CoA:cholesterol acyltransferase (ACAT)-dependent manner. Moreover, in AnxA6-deficient NPC1 mutant cells, Rab7-mediated rescue of late endosome-cholesterol export required the StAR-related lipid transfer domain-3 (StARD3) protein. Electron microscopy revealed a significant increase of membrane contact sites (MCS) between late endosomes and ER in NPC1 mutant cells lacking AnxA6, suggesting late endosome-cholesterol transfer to the ER via Rab7 and StARD3-dependent MCS formation. This study identifies AnxA6 as a novel gatekeeper that controls cellular distribution of late endosome-cholesterol via regulation of a Rab7-GAP and MCS formation.

Sterol biosensor reveals LAM-family Ltc1-dependent sterol flow to endosomes upon Arp2/3 inhibition

Sterols are crucial components of biological membranes, which are synthetized in the ER and accumulate in the plasma membrane (PM). Here, by applying a genetically encoded sterol biosensor (D4H), we visualize a sterol flow between PM and endosomes in the fission yeast Schizosaccharomyces pombe. Using time-lapse and correlative light-electron microscopy, we found that inhibition of Arp2/3-dependent F-actin assembly promotes the reversible relocalization of D4H from the PM to internal sterol-rich compartments (STRIC) labeled by synaptobrevin Syb1. Retrograde sterol internalization to STRIC is independent of endocytosis or an intact Golgi, but depends on Ltc1, a LAM/StARkin-family protein localized to ER-PM contact sites. The PM in ltc1Δ cells over-accumulates sterols and upon Arp2/3 inhibition forms extended ER-interacting invaginations, indicating that sterol transfer contributes to PM size homeostasis. Anterograde sterol movement from STRIC is independent of canonical vesicular trafficking but requires Arp2/3, suggesting a novel role for this complex. Thus, transfer routes orthogonal to vesicular trafficking govern the flow of sterols in the cell.

Lipid Dynamics at Contact Sites Between the Endoplasmic Reticulum and Other Organelles

Phospholipids are synthesized primarily within the endoplasmic reticulum and are subsequently distributed to various subcellular membranes to maintain the unique lipid composition of specific organelles. As a result, in most cases, the steady-state localization of membrane phospholipids does not match their site of synthesis. This raises the question of how diverse lipid species reach their final membrane destinations and what molecular processes provide the energy to maintain the lipid gradients that exist between various membrane compartments. Recent studies have highlighted the role of inositol phospholipids in the nonvesicular transport of lipids at membrane contact sites. This review attempts to summarize our current understanding of these complex lipid dynamics and highlights their implications for defining future research directions.

Mycobacterium smegmatis MSMEG_0129 is a nutrition-associated regulator that interacts with CarD and ClpP2

Mycobacterium smegmatis MSMEG_0129 and Rv0164, its homologue in Mycobacterium tuberculosis, are single START-domain proteins essential for bacterial growth and survival, but their biochemical activities and biological roles remain undetermined. Here, we probed the possible functions of MSMEG_0129 and its underlying mechanisms by determining its cellular location, searching for its interaction partners and monitoring its transcription profile. MSMEG_0129, and Rv0164 by extension, were found to be cytosolic proteins rather than secreted components as previously understood. Increases in MSMEG_0129 expression at physiological levels accelerated bacterial growth in a proportional manner, but additional growth acceleration was not observed when MSMEG_0129 was overexpressed up to 20 fold. MSMEG_0129 is a short-lived protein, unstable at both the mRNA and protein levels. Co-IP and GST pull-down assays showed that MSMEG_0129 interacts with the ClpP2 protease and a global transcription factor, CarD, their expression being correlated with that of MSMEG_0129. Nutrient deficiency led to the downregulation of MSMEG_0129 but upregulation of CarD. However, in the context of constitutive MSMEG_0129 overexpression under nutrient-rich or starvation conditions, the mRNA level of CarD was reduced 3 fold. Conversely, expression of ClpP2 decreased with MSMEG_0129 downregulation under starvation conditions, but increased 4-8 fold when MSMEG_0129 was overexpressed. Our data suggest that MSMEG_0129, and Rv0164 by analogy, are likely to be nutrition sensing factors that regulate mycobacterial growth and may be involved in signal transfer under nutrient deficiency, possibly via physical and regulatory interactions with CarD and ClpP2.

Modeling the Sterol-Binding Domain of Aster-A Provides Insight into Its Multiligand Specificity

Intracellular transport of cholesterol and related sterols relies to a large degree on nonvesicular mechanisms, which are only partly understood at the molecular level. Aster proteins belonging to the Lam family of sterol transfer proteins have recently been identified as important catalysts of nonvesicular sterol exchange between the plasma membrane (PM) and endoplasmic reticulum (ER). Here, we used a range of computational tools to study the molecular mechanisms underlying sterol binding as well as multisterol ligand specificity of Aster-A. Our study focused primarily on gaining atomistic insight into the bound ligand-protein complex and was, on this basis, performed in the absence of any membrane. Molecular mechanics Poisson-Boltzmann surface area (MM-PBSA) calculations provide a rationale for the experimentally found ranking of binding affinities of various sterols to Aster-A. In particular, the polarity of the sterols and the length of their alkyl chain could be identified as being critical determinants of ligand affinity. A Gibbs free energy decomposition identified a charged residue, Glu444, at the base of the binding pose as an important control point for sterol binding. Removing its net charge via protonation was found to cause significant changes to the environment surrounding this residue. In addition, the protonation of Glu444 was found to be paralleled by a large redistribution of molecular flexibility in the Aster domain. This finding was supplemented by multiple branched adaptive steered molecular dynamics (MB-ASMD) simulations by which we defined a possible molecular path for sterol release and demonstrated the importance of Glu444 in this process.

Emerging roles for human glycolipid transfer protein superfamily members in the regulation of autophagy, inflammation, and cell death

Glycolipid transfer proteins (GLTPs) were first identified over three decades ago as ~24kDa, soluble, amphitropic proteins that specifically accelerate the intermembrane transfer of glycolipids. Upon discovery that GLTPs use a unique, all-α-helical, two-layer 'sandwich' architecture (GLTP-fold) to bind glycosphingolipids (GSLs), a new protein superfamily was born. Structure/function studies have provided exquisite insights defining features responsible for lipid headgroup selectivity and hydrophobic 'pocket' adaptability for accommodating hydrocarbon chains of differing length and unsaturation. In humans, evolutionarily-modified GLTP-folds have been identified with altered sphingolipid specificity, e. g. ceramide-1-phosphate transfer protein (CPTP), phosphatidylinositol 4-phosphate adaptor protein-2 (FAPP2) which harbors a GLTP-domain and GLTPD2. Despite the wealth of structural data (>40 Protein Data Bank deposits), insights into the in vivo functional roles of GLTP superfamily members have emerged slowly. In this review, recent advances are presented and discussed implicating human GLTP superfamily members as important regulators of: i) pro-inflammatory eicosanoid production associated with Group-IV cytoplasmic phospholipase A₂; ii) autophagy and inflammasome assembly that drive surveillance cell release of interleukin-1β and interleukin-18 inflammatory cytokines; iii) cell cycle arrest and necroptosis induction in certain colon cancer cell lines. The effects exerted by GLTP superfamily members appear linked to their ability to regulate sphingolipid homeostasis by acting in either transporter and/or sensor capacities. These timely findings are opening new avenues for future cross-disciplinary, translational medical research involving GLTP-fold proteins in human health and disease. Such avenues include targeted regulation of specific GLTP superfamily members to alter sphingolipid levels as a therapeutic means for combating viral infection, neurodegenerative conditions and circumventing chemo-resistance during cancer treatment.

Interactions of OSW-1 with Lipid Bilayers in Comparison with Digitonin and Soyasaponin

OSW-1, a unique steroidal saponin isolated from the bulbs of Ornithogalum saundersiae, has potent cell-growth inhibition activity. In this study, we conducted fluorescence measurements and microscopic observations using palmitoyloleoylphosphatidylcholine (POPC)-cholesterol (Chol) bilayers to evaluate the membrane-binding affinity of OSW-1 in comparison with another steroidal saponin, digitonin, and the triterpenoid saponin, soyasaponin Bb(I). The membrane activities of these saponins were evaluated using calcein leakage assays and fitted to the binding isotherm by changing the ratios of saponin-lipids. Digitonin showed the highest binding affinity for the POPC-Chol membrane (K_app = 0.38 μM^-1) and the strongest membrane disruptivity in the bound saponin-lipid ratio at the point of 50% calcein leakage (r₅₀ = 0.47) occurrence. OSW-1 showed slightly lower activity (K_app = 0.31 μM^-1; r₅₀ = 0.78), and the soyasaponin was the lowest in the membrane affinity and the calcein leakage activity (K_app = 0.017 μM^-1; r₅₀ = 1.66). The effect of OSW-1 was further assessed using confocal microscopy in an experiment utilizing DiI and rhodamine 6G as the fluorescence probes. The addition of 30 μM OSW-1 induced inward membrane curvature in some giant unilamellar vesicles (GUVs). At the higher OSW-1 concentration (58 μM, r₅₀ = 0.78) where the 50% calcein leakage was observed, the morphology of some GUVs became elongated. With digitonin at the corresponding concentration (35 μM, r₅₀ = 0.47), membrane disruption and formation of large aggregates in aqueous solution were observed, probably due to a detergent-type mechanism. These saponins, including OSW-1, required Chol to exhibit their potent membrane activity although their mechanisms are thought to be different. At the effective concentration, OSW-1 preferably binds to the bilayers without prominent disruption of vesicles and exerts its activity through the formation of saponin-Chol complexes, probably resulting in membrane permeabilization.

Staying in touch with the endocytic network: The importance of contacts for cholesterol transport

Cholesterol homeostasis is critical for cell function and human health. Cholesterol is heterogeneously distributed among cellular membranes, with the redistribution of endocytosed dietary cholesterol playing a pivotal role in the regulation of cholesterol homeostasis. While gaps remain in our understanding of intracellular dietary cholesterol transport, a highly complex network of pathways is starting to emerge, often involving inter‐dependent vesicular and non‐vesicular transport mechanisms. The last decade has seen a surge in interest in non‐vesicular transport and inter‐organellar communication at membrane contact sites. By providing platforms for protein interactions, signalling events, lipid exchange and calcium flux, membrane contact sites (MCS) are now appreciated as controlling the fate of large amounts of lipid and play central roles in the regulation and co‐ordination of endocytic trafficking. Here, we review the role of MCS in multiple pathways for cholesterol export from the endocytic pathway and highlight the intriguing interplay between vesicular and non‐vesicular transport mechanisms and relationship with neurodegenerative disease.

Cholesterol Handling in Lysosomes and Beyond

Lysosomes are of major importance for the regulation of cellular cholesterol homeostasis. Food-derived cholesterol and cholesterol esters contained within lipoproteins are delivered to lysosomes by endocytosis. From the lysosomal lumen, cholesterol is transported to the inner surface of the lysosomal membrane through the glycocalyx; this shuttling requires Niemann-Pick C (NPC) 1 and NPC2 proteins. The lysosomal membrane proteins lysosomal-associated membrane protein (LAMP)-2 and lysosomal integral membrane protein (LIMP)-2/SCARB2 also bind cholesterol. LAMP-2 may serve as a cholesterol reservoir, whereas LIMP-2, like NPC1, is able to transport cholesterol through a transglycocalyx tunnel. Contact sites and fusion events between lysosomes and other organelles mediate the distribution of cholesterol. Lysosomal cholesterol content is sensed thereby regulating mammalian target of rapamycin complex (mTORC)-dependent signaling. This review summarizes our understanding of the major steps in cholesterol handling from the moment it enters the lysosome until it leaves this compartment.

The START-domain proteins in intracellular lipid transport and beyond

The Steroidogenic Acute Regulatory Protein-related Lipid Transfer (START) domain is a ~210 amino acid sequence that folds into an α/β helix-grip structure forming a hydrophobic pocket for lipid binding. The helix-grip fold structure defines a large superfamily of proteins, and this review focuses on the mammalian START domain family members that include single START domain proteins with identified ligands, and larger multi-domain proteins that may have novel roles in metabolism. Much of our understanding of the mammalian START domain proteins in lipid transport and changes in metabolism has advanced through studies using knockout mouse models, although for some of these proteins the identity and/or physiological role of ligand binding remains unknown. The findings that helped define START domain lipid-binding specificity, lipid transport, and changes in metabolism are presented to highlight that fundamental questions remain regarding the biological function(s) for START domain-containing proteins.

[A lipid exchange market : vectorial cholesterol transport by the protein OSBP]

Cholesterol is synthesized in the endoplasmic reticulum (RE) and then transported to cellular compartments whose functions require high cholesterol levels. Here, we describe the mechanism by which cholesterol is transported from the RE to the trans-Golgi network (TGN) by the protein OSBP (Oxysterol-Binding Protein). OSBP has two complementary activities. First, it tethers the RE to the TGN by forming a contact site where the two membranes are about twenty nanometers away. Then, it exchanges RE cholesterol for a TGN lipid, phosphatidylinositol 4-phosphate (PI4P). Eventually, PI4P is hydrolyzed at the RE, making the exchange cycle irreversible. Thus, OSBP is at the center of a lipid exchange market where a transported cholesterol "costs" a PI4P. Antiviral or anti-cancer molecules target OSBP, suggesting the importance of the OSBP cycle in different physiopathological contexts. The general principles of this cycle are shared by other lipid-transfer proteins.

Kinobead/LC-MS Phosphokinome Profiling Enables Rapid Analyses of Kinase-Dependent Cell Signaling Networks

Kinase-catalyzed protein phosphorylation is fundamental to eukaryotic signal transduction, regulating most cellular processes. Kinases are frequently dysregulated in cancer, inflammation, and degenerative diseases, and because they can be inhibited with small molecules, they became important drug targets. Accordingly, analytical approaches that determine kinase activation states are critically important to understand kinase-dependent signal transduction and to identify novel drug targets and predictive biomarkers. Multiplexed inhibitor beads (MIBs or kinobeads) efficiently enrich kinases from cell lysates for liquid chromatography-mass spectrometry (LC-MS) analysis. When combined with phosphopeptide enrichment, kinobead/LC-MS can also quantify the phosphorylation state of kinases, which determines their activation state. However, an efficient kinobead/LC-MS kinase phospho-profiling protocol that allows routine analyses of cell lines and tissues has not yet been developed. Here, we present a facile workflow that quantifies the global phosphorylation state of kinases with unprecedented sensitivity. We also found that our kinobead/LC-MS protocol can measure changes in kinase complex composition and show how these changes can indicate kinase activity. We demonstrate the utility of our approach in specifying kinase signaling pathways that control the acute steroidogenic response in Leydig cells; this analysis establishes the first comprehensive framework for the post-translational control of steroid biosynthesis.

Cholesterol metabolism in cancer: mechanisms and therapeutic opportunities

Cholesterol metabolism produces essential membrane components as well as metabolites with a variety of biological functions. In the tumour microenvironment, cell-intrinsic and cell-extrinsic cues reprogram cholesterol metabolism and consequently promote tumourigenesis. Cholesterol-derived metabolites play complex roles in supporting cancer progression and suppressing immune responses. Preclinical and clinical studies have shown that manipulating cholesterol metabolism inhibits tumour growth, reshapes the immunological landscape and reinvigorates anti-tumour immunity. Here, we review cholesterol metabolism in cancer cells, its role in cancer progression and the mechanisms through which cholesterol metabolites affect immune cells in the tumour microenvironment. We also discuss therapeutic strategies aimed at interfering with cholesterol metabolism, and how the combination of such approaches with existing anti-cancer therapies can have synergistic effects, thus offering new therapeutic opportunities.

X-linked adrenoleukodystrophy: Pathology, pathophysiology, diagnostic testing, newborn screening and therapies

Adrenoleukodystrophy (ALD) is a rare X-linked disease caused by a mutation of the peroxisomal ABCD1 gene. This review summarizes our current understanding of the pathogenic cell- and tissue-specific roles of lipid species in the context of experimental therapeutic strategies and provides an overview of critical historical developments, therapeutic trials and the advent of newborn screening in the USA. In ALD, very long-chain fatty acid (VLCFA) chain length-dependent dysregulation of endoplasmic reticulum stress and mitochondrial radical generating systems inducing cell death pathways has been shown, providing the rationale for therapeutic moiety-specific VLCFA reduction and antioxidant strategies. The continuing increase in newborn screening programs and promising results from ongoing and recent therapeutic investigations provide hope for ALD.

A Combinatorial Reporter Set to Visualize the Membrane Contact Sites Between Endoplasmic Reticulum and Other Organelles in Plant Cell

The membrane contact sites (MCSs) enable interorganelle communication by associating organelles at distances of tens of nanometers over extended membrane surfaces and serve to maintain cellular homeostasis through efficient exchange of metabolites, lipid, and calcium between organelles, organelle fission, and movement. Most MCSs and a growing number of tethering proteins especially those involved in mediating the junctions between endoplasmic reticulum (ER) and other organelles have been extensively characterized in mammal and yeast. However, the studies of plant MCSs are still at stages of infancy, at least one reason might be due to the lack of bona fide markers for visualizing these membrane junctions in plant cells. In this study, a series of genetically encoded reporters using split super-folder GFP protein were designed to detect the possible MCSs between ER and three other cellular compartments including chloroplast, mitochondria and plasma membrane (PM) in plant cell. By expressing these genetically encoded reporter in Arabidopsis protoplasts as well as Nicotiana benthamiana leaf, we could intuitively observe the punctate signal surrounding chloroplast upon expression of ER-chloroplast MCS reporter, punctate signal of ER-mitochondria MCS reporter and punctate signal close to the PM upon expression of ER-PM MCS reporter. We also showed that the ER-chloroplast MCSs were dynamic structures that undergo active remodeling with concomitant occurrence of chloroplast dysfunction inside plant cells. This study demonstrates that ER associates with various organelles in close proximity in plant cells and provides tools that might be applicable for visualizing MCSs in plants.

Organelle contacts: Sub-organelle zones to facilitate rapid and accurate inter-organelle trafficking of lipids

When one person wants to communicate securely with another, he/she should contact the other person directly. This rule applies not only to human society, but also to the intracellular micro-society. In the past two decades, it has become increasingly clear that the sub-organelle regions called membrane contact sites (MCSs) are pivotal for inter-organelle transport of lipids in cells, as highlighted in the thematic review series "Interorganelle trafficking of lipids" held in Traffic in 2014-2015. In this commentary, we will describe how the currently prevailing model for lipid trafficking at MCSs was generated, and comment on three important issues that have not been explored: (a1) the principles guiding the generation of an asymmetrical inter-organelle flow of lipids in cells, (b2) the advantages in lipid trafficking at organelle contacts, and (c3) the dynamic network of inter-organelle lipid trafficking.

X-linked Adrenoleukodystrophy: Pathology, Pathophysiology, Diagnostic Testing, Newborn Screening, and Therapies

Adrenoleukodystrophy (ALD) is a rare X-linked disease caused by a mutation of the peroxisomal ABCD1 gene. This review summarizes our current understanding of the pathogenic cell- and tissue-specific role of lipid species in the context of experimental therapeutic strategies and provides an overview of critical historical developments, therapeutic trials, and the advent of newborn screening in the United States. In ALD, very long chain fatty acid (VLCFA) chain-length-dependent dysregulation of endoplasmic reticulum stress and mitochondrial radical generating systems inducing cell death pathways has been shown, providing the rationale for therapeutic moiety-specific VLCFA reduction and antioxidant strategies. The continuing increase in newborn screening programs and promising results from ongoing and recent therapeutic investigations provide hope for ALD.

Movement of accessible plasma membrane cholesterol by the GRAMD1 lipid transfer protein complex

Cholesterol is a major structural component of the plasma membrane (PM). The majority of PM cholesterol forms complexes with other PM lipids, making it inaccessible for intracellular transport. Transition of PM cholesterol between accessible and inaccessible pools maintains cellular homeostasis, but how cells monitor the accessibility of PM cholesterol remains unclear. We show that endoplasmic reticulum (ER)-anchored lipid transfer proteins, the GRAMD1s, sense and transport accessible PM cholesterol to the ER. GRAMD1s bind to one another and populate ER-PM contacts by sensing a transient expansion of the accessible pool of PM cholesterol via their GRAM domains. They then facilitate the transport of this cholesterol via their StART-like domains. Cells that lack all three GRAMD1s exhibit striking expansion of the accessible pool of PM cholesterol as a result of less efficient PM to ER transport of accessible cholesterol. Thus, GRAMD1s facilitate the movement of accessible PM cholesterol to the ER in order to counteract an acute increase of PM cholesterol, thereby activating non-vesicular cholesterol transport.

The human body contains trillions of cells. At the outer edge of each cell is the plasma membrane, which protects the cell from the external environment. This membrane is mostly made of fatty molecules known as lipids and about half of these lipids are specifically cholesterol. Human cells can either take up cholesterol that were obtained via the diet or produce it within a compartment of the cell called the endoplasmic reticulum.

Cells need to monitor the cholesterol levels in both the endoplasmic reticulum and the plasma membrane in order to regulate the uptake or production of this lipid. For example, if there is too much of cholesterol in the plasma membrane, then the cell transports some to the endoplasmic reticulum to tell it to shut down cholesterol production. However, how these different areas of the cell communicate with each other, and transport cholesterol, has remained unclear.

Naito et al. set out to look for key regulators of cholesterol transport and identified a group of endoplasmic reticulum proteins called GRAMD1 proteins. Cholesterol in the plasma membrane is either accessible or inaccessible, meaning it either can or cannot be moved back into the cell. The GRAMD1 proteins sense accessible cholesterol, and experiments with human cells grown in the laboratory showed that, specifically, the GRAMD1 proteins work together in a complex to sense accessible cholesterol at or near the plasma membrane. One particular part of the protein senses when the amount of accessible cholesterol reaches a certain level at the plasma membrane; when this threshold is reached, the complex flips a switch to start the transport of cholesterol to the endoplasmic reticulum and tell it to shut down cholesterol production.

This coupling of sensing and transporting lipids by one protein complex also helps maintain the right ratio of accessible and inaccessible cholesterol in the plasma membrane to prevent cells from activating unwanted cell-signaling events. Getting rid of the GRAMD1 proteins in cells, or removing sensing part of these proteins, leads to inefficient transport of cholesterol. A better understanding of how GRAMD1 proteins sense the accessibility of cholesterol could potentially help identify new approaches to control cholesterol transport inside cells. This may in turn eventually lead to new treatments that counteract the defects in cholesterol metabolism seen in some forms of neurodegenerative diseases such as Alzheimer’s disease and Parkinson’s disease.

Ostreolysin A and anthrolysin O use different mechanisms to control movement of cholesterol from the plasma membrane to the endoplasmic reticulum

Recent studies using two cholesterol-binding bacterial toxin proteins, perfringolysin O (PFO) and domain 4 of anthrolysin O (ALOD4), have shown that cholesterol in the plasma membranes (PMs) of animal cells resides in three distinct pools. The first pool comprises mobile cholesterol, accessible to both PFO and ALOD4, that is rapidly transported to the endoplasmic reticulum (ER) to signal cholesterol excess and maintain cholesterol homeostasis. The second is a sphingomyelin (SM)-sequestered pool inaccessible to PFO and ALOD4 but that becomes accessible by treatment with SM-degrading sphingomyelinase (SMase). The third is an essential pool also inaccessible to PFO and ALOD4 that cannot be liberated by SMase treatment. The accessible cholesterol pool can be trapped on PMs of live cells by nonlytic ALOD4, blocking its transport to the ER. However, studies of the two other pools have been hampered by a lack of available tools. Here, we used ostreolysin A (OlyA), which specifically binds SM/cholesterol complexes in membranes, to study the SM-sequestered cholesterol pool. Binding of nonlytic OlyA to SM/cholesterol complexes in PMs of live cells depleted the accessible PM cholesterol pool detectable by ALOD4. Consequently, transport of accessible cholesterol from PM to ER ceased, thereby activating SREBP transcription factors and increasing cholesterol synthesis. Thus, OlyA and ALOD4 both control movement of PM cholesterol, but through different lipid-binding mechanisms. We also found that PM-bound OlyA was rapidly internalized into cells, whereas PM-bound ALOD4 remained on the cell surface. Our findings establish OlyA and ALOD4 as complementary tools to investigate cellular cholesterol transport.

Endogenous sterol intermediates of the mevalonate pathway regulate HMGCR degradation and SREBP-2 processing

Sterol-regulated HMG-CoA reductase (HMGCR) degradation and SREBP-2 cleavage are two major feedback regulatory mechanisms governing cholesterol biosynthesis. Reportedly, lanosterol selectively stimulates HMGCR degradation, and cholesterol is a specific regulator of SREBP-2 cleavage. However, it is unclear whether other endogenously generated sterols regulate these events. Here, we investigated the sterol intermediates from the mevalonate pathway of cholesterol biosynthesis using a CRISPR/Cas9-mediated genetic engineering approach. With a constructed HeLa cell line expressing the mevalonate transporter, we individually deleted genes encoding major enzymes in the mevalonate pathway, used lipidomics to measure sterol intermediates, and examined HMGCR and SREBP-2 statuses. We found that the C4-dimethylated sterol intermediates, including lanosterol, 24,25-dihydrolanosterol, follicular fluid meiosis activating sterol, testis meiosis activating sterol, and dihydro-testis meiosis activating sterol, were significantly upregulated upon mevalonate loading. These intermediates augmented both degradation of HMGCR and inhibition of SREBP-2 cleavage. The accumulated lanosterol induced rapid degradation of HMGCR, but did not inhibit SREBP-2 cleavage. The newly synthesized cholesterol from the mevalonate pathway is dispensable for inhibiting SREBP-2 cleavage. Together, these results suggest that lanosterol is a bona fide endogenous regulator that specifically promotes HMGCR degradation, and that other C4-dimethylated sterol intermediates may regulate both HMGCR degradation and SREBP-2 cleavage.

The hepatic WASH complex is required for efficient plasma LDL and HDL cholesterol clearance

The evolutionary conserved Wiskott-Aldrich syndrome protein and SCAR homolog (WASH) complex is one of the crucial multiprotein complexes that facilitates endosomal recycling of transmembrane proteins. Defects in WASH components have been associated with inherited developmental and neurological disorders in humans. Here, we show that hepatic ablation of the WASH component Washc1 in chow-fed mice increases plasma concentrations of cholesterol in both LDLs and HDLs, without affecting hepatic cholesterol content, hepatic cholesterol synthesis, biliary cholesterol excretion, or hepatic bile acid metabolism. Elevated plasma LDL cholesterol was related to reduced hepatocytic surface levels of the LDL receptor (LDLR) and the LDLR-related protein LRP1. Hepatic WASH ablation also reduced the surface levels of scavenger receptor class B type I and, concomitantly, selective uptake of HDL cholesterol into the liver. Furthermore, we found that WASHC1 deficiency increases LDLR proteolysis by the inducible degrader of LDLR, but does not affect proprotein convertase subtilisin/kexin type 9-mediated LDLR degradation. Remarkably, however, loss of hepatic WASHC1 may sensitize LDLR for proprotein convertase subtilisin/kexin type 9-induced degradation. Altogether, these findings identify the WASH complex as a regulator of LDL as well as HDL metabolism and provide in vivo evidence for endosomal trafficking of scavenger receptor class B type I in hepatocytes.

Cholesterol transport through the peroxisome-ER membrane contacts tethered by PI(4,5)P2 and extended synaptotagmins

Most mammalian cells take up cholesterol from low-density lipoproteins (LDLs) via receptor-mediated endocytosis. After reaching lysosomes, LDL-derived cholesterol continues to transport to downstream organelles including the ER for specific structural and functional needs. Peroxisomes are recently found to receive cholesterol from lysosomes through lysosome-peroxisome membrane contacts. However, whether and how cholesterol is conveyed from peroxisomes to the ER remain unknown. Here, by combining high-resolution microscopic analyses and in vitro reconstitution of highly purified organelles or artificial liposomes, we demonstrate that peroxisomes form membrane contacts with the ER through the interaction between peroxisomal PI(4,5)P₂ and ER-resident extended synaptotagmin-1, 2 and 3 (E-Syts). Depletion of peroxisomal PI(4,5)P₂ or E-Syts markedly decreases peroxisome-ER membrane contacts and induces cholesterol accumulation in lysosomes. Furthermore, we show that cholesterol is delivered from ³H-labeled peroxisomes or PI(4,5)P₂-containing liposomes to the ER in vitro, and that the presence of peroxisomes augments cholesterol transfer from lysosomes to the ER. Together, our study reveals a new cholesterol transport pathway along the lysosome-peroxisome-ER membrane contacts in the cell.