Cholesterol transfer at endosomal-organelle membrane contact sites

The interactions of subcellular organelles in pulmonary fibrosis induced by carbon black nanoparticles: a comprehensive review

Owing to the widespread use and improper emissions of carbon black nanoparticles (CBNPs), the adverse effects of CBNPs on human health have attracted much attention. In toxicological research, carbon black is frequently utilized as a negative control because of its low toxicity and poor solubility. However, recent studies have indicated that inhalation exposure to CBNPs could be a risk factor for severe and prolonged pulmonary inflammation and fibrosis. At present, the pathogenesis of pulmonary fibrosis induced by CBNPs is still not fully elucidated, but it is known that with small particle size and large surface area, CBNPs are more easily ingested by cells, leading to organelle damage and abnormal interactions between organelles. Damaged organelle and abnormal organelles interactions lead to cell structure and function disorders, which is one of the important factors in the development and occurrence of various diseases, including pulmonary fibrosis. This review offers a comprehensive analysis of organelle structure, function, and interaction mechanisms, while also summarizing the research advancements in organelles and organelle interactions in CBNPs-induced pulmonary fibrosis.

Lysosomal Ion Channels and Lysosome-Organelle Interactions

Intracellular organelles exchange their luminal contents with each other via both vesicular and non-vesicular mechanisms. By forming membrane contact sites (MCSs) with ER and mitochondria, lysosomes mediate bidirectional transport of metabolites and ions between lysosomes and organelles that regulate lysosomal physiology, movement, membrane remodeling, and membrane repair. In this chapter, we will first summarize the current knowledge of lysosomal ion channels and then discuss the molecular and physiological mechanisms that regulate lysosome-organelle MCS formation and dynamics. We will also discuss the roles of lysosome-ER and lysosome-mitochondria MCSs in signal transduction, lipid transport, Ca ²⁺ transfer, membrane trafficking, and membrane repair, as well as their roles in lysosome-related pathologies.

Molecular machinery regulating organelle dynamics during axon growth and guidance

Axon growth and guidance in the developing nervous system rely on intracellular membrane dynamics that involve endosome maturation and transport, as well as its regulated tethering to the endoplasmic reticulum (ER). Recent studies have identified several key molecules, such as protrudin, which plays a dynamic role at membrane contact sites between the ER and endosomes/lysosomes, and myosin Va, which acts as a sensor for ER-derived Ca²⁺ that triggers peri-ER membrane export. These molecules form different types of multiprotein complexes at the interface of organelles and, in response to their surrounding microenvironments, such as Ca²⁺ concentrations and lipid contents, regulate the directional movement of endosomal vesicles in extending axons. Here, we review the molecular mechanisms underlying membrane dynamics and inter-organelle interactions during neuronal morphogenesis.

ER membrane contact sites support endosomal small GTPase conversion for exosome secretion

Exosomes are endosome-derived extracellular vesicles involved in intercellular communication. They are generated as intraluminal vesicles within endosomal compartments that fuse with the plasma membrane (PM). The molecular events that generate secretory endosomes and lead to the release of exosomes are not well understood. We identified a subclass of non-proteolytic endosomes at prelysosomal stage as the compartment of origin of CD63 positive exosomes. These compartments undergo a Rab7a/Arl8b/Rab27a GTPase cascade to fuse with the PM. Dynamic endoplasmic reticulum (ER)-late endosome (LE) membrane contact sites (MCS) through ORP1L have the distinct capacity to modulate this process by affecting LE motility, maturation state, and small GTPase association. Thus, exosome secretion is a multi-step process regulated by GTPase switching and MCS, highlighting the ER as a new player in exosome-mediated intercellular communication.

Aster la vista: Unraveling the biochemical basis of carotenoid homeostasis in the human retina

Carotenoids play pivotal roles in vision as light filters and precursor of chromophore. Many vertebrates also display the colorful pigments as ornaments in bare skin parts and feathers. Proteins involved in the transport and metabolism of these lipids have been identified including class B scavenger receptors and carotenoid cleavage dioxygenases. Recent research implicates members of the Aster protein family, also known as GRAM domain-containing (GRAMD), in carotenoid metabolism. These multi-domain proteins facilitate the intracellular movement of carotenoids from their site of cellular uptake by scavenger receptors to the site of their metabolic processing by carotenoid cleavage dioxygenases. We provide a model how the coordinated interplay of these proteins and their differential expression establishes carotenoid distribution patterns and function in tissues, with particular emphasis on the human retina.

Studies in the antiviral molecular mechanisms of 25-hydroxycholesterol: Disturbing cholesterol homeostasis and post-translational modification of proteins

Efficient antiviral drug discovery has been a pressing issue of global public health concern since the outbreak of coronavirus disease 2019. In recent years, numerous in vitro and in vivo studies have shown that 25-hydroxycholesterol (25HC), a reactive oxysterol catalyzed by cholesterol-25-hydroxylase, exerts broad-spectrum antiviral activity with high efficiency and low toxicity. 25HC restricts viral internalization and disturbs the maturity of viral proteins using multiple mechanisms. First, 25HC reduces lipid rafts and cholesterol in the cytomembrane by inhibiting sterol-regulatory element binding proteins-2, stimulating liver X receptor, and activating Acyl-coenzyme A: cholesterol acyl-transferase. Second, 25HC impairs endosomal pathways by restricting the function of oxysterol-binding protein or Niemann-pick protein C1, causing the virus to fail to release nucleic acid. Third, 25HC disturbs the prenylation of viral proteins by suppressing the sterol-regulatory element binding protein pathway and glycosylation by increasing the sensitivity of glycans to endoglycosidase. This paper reviews previous studies on the antiviral activity of 25HC in order to fully understand its role in innate immunity and how it may contribute to the development of urgently needed broad-spectrum antiviral drugs.

Linking Late Endosomal Cholesterol with Cancer Progression and Anticancer Drug Resistance

Cancer cells undergo drastic metabolic adaptions to cover increased bioenergetic needs, contributing to resistance to therapies. This includes a higher demand for cholesterol, which often coincides with elevated cholesterol uptake from low-density lipoproteins (LDL) and overexpression of the LDL receptor in many cancers. This implies the need for cancer cells to accommodate an increased delivery of LDL along the endocytic pathway to late endosomes/lysosomes (LE/Lys), providing a rapid and effective distribution of LDL-derived cholesterol from LE/Lys to other organelles for cholesterol to foster cancer growth and spread. LDL-cholesterol exported from LE/Lys is facilitated by Niemann–Pick Type C1/2 (NPC1/2) proteins, members of the steroidogenic acute regulatory-related lipid transfer domain (StARD) and oxysterol-binding protein (OSBP) families. In addition, lysosomal membrane proteins, small Rab GTPases as well as scaffolding proteins, including annexin A6 (AnxA6), contribute to regulating cholesterol egress from LE/Lys. Here, we summarize current knowledge that links upregulated activity and expression of cholesterol transporters and related proteins in LE/Lys with cancer growth, progression and treatment outcomes. Several mechanisms on how cellular distribution of LDL-derived cholesterol from LE/Lys influences cancer cell behavior are reviewed, some of those providing opportunities for treatment strategies to reduce cancer progression and anticancer drug resistance.

Ligand Recognition by the Lipid Transfer Domain of Human OSBP Is Important for Enterovirus Replication

Oxysterol-binding protein (OSBP), which transports cholesterol and phosphatidylinositol 4-monophosphate (PtdIns[4]P) between different organelles, serves as a conserved host factor for the replication of various viruses, and OSBP inhibitors exhibit antiviral effects. Here, we determined the crystal structure of the lipid transfer domain of human OSBP in complex with endogenous cholesterol. The hydrocarbon tail and tetracyclic ring of cholesterol interact with the hydrophobic tunnel of OSBP, and the hydroxyl group of cholesterol forms a hydrogen bond network at the bottom of the tunnel. Systematic mutagenesis of the ligand-binding region revealed that M446W and L590W substitutions confer functional tolerance to an OSBP inhibitor, T-00127-HEV2. Employing the M446W variant as a functional replacement for the endogenous OSBP in the presence of T-00127-HEV2, we have identified previously unappreciated amino acid residues required for viral replication. The combined use of the inhibitor and the OSBP variant will be useful in elucidating the enigmatic in vivo functions of OSBP.

A novel sterol-binding protein reveals heterogeneous cholesterol distribution in neurite outgrowth and in late endosomes/lysosomes

We identified a mushroom-derived protein, maistero-2 that specifically binds 3-hydroxy sterol including cholesterol (Chol). Maistero-2 bound lipid mixture in Chol-dependent manner with a binding threshold of around 30%. Changing lipid composition did not significantly affect the threshold concentration. EGFP-maistero-2 labeled cell surface and intracellular organelle Chol with higher sensitivity than that of well-established Chol probe, D4 fragment of perfringolysin O. EGFP-maistero-2 revealed increase of cell surface Chol during neurite outgrowth and heterogeneous Chol distribution between CD63-positive and LAMP1-positive late endosomes/lysosomes. The absence of strictly conserved Thr-Leu pair present in Chol-dependent cytolysins suggests a distinct Chol-binding mechanism for maistero-2.

Stable Integration of Inducible SPLICS Reporters Enables Spatio-Temporal Analysis of Multiple Organelle Contact Sites upon Modulation of Cholesterol Traffic

The study of organelle contact sites has received a great impulse due to increased interest in the understanding of their involvement in many disease conditions. Split-GFP-based contact sites (SPLICS) reporters emerged as essential tools to easily detect changes in a wide range of organelle contact sites in cultured cells and in vivo, e.g., in zebrafish larvae. We report here on the generation of a new vector library of SPLICS cloned into a piggyBac system for stable and inducible expression of the reporters in a cell line of interest to overcome any potential weakness due to variable protein expression in transient transfection studies. Stable HeLa cell lines expressing SPLICS between the endoplasmic reticulum (ER) and mitochondria (MT), the ER and plasma membrane (PM), peroxisomes (PO) and ER, and PO and MT, were generated and tested for their ability to express the reporters upon treatment with doxycycline. Moreover, to take advantage of these cellular models, we decided to follow the behavior of different membrane contact sites upon modulating cholesterol traffic. Interestingly, we found that the acute pharmacological inhibition of the intracellular cholesterol transporter 1 (NPC1) differently affects membrane contact sites, highlighting the importance of different interfaces for cholesterol sensing and distribution within the cell.

Intracellular Cholesterol Synthesis and Transport

Cholesterol homeostasis is related to multiple diseases in humans, including cardiovascular disease, cancer, and neurodegenerative and hepatic diseases. The cholesterol levels in cells are balanced dynamically by uptake, biosynthesis, transport, distribution, esterification, and export. In this review, we focus on de novo cholesterol synthesis, cholesterol synthesis regulation, and intracellular cholesterol trafficking. In addition, the progression of lipid transfer proteins (LTPs) at multiple contact sites between organelles is considered.

Cholesterol transport in the late endocytic pathway: Roles of ORP family proteins

Oxysterol-binding protein (OSBP) homologues, designated ORP or OSBPL proteins, constitute one of the largest families of intracellular lipid-binding/transfer proteins (LTP). This review summarizes the mounting evidence that several members of this family participate in the machinery facilitating cholesterol trafficking in the late endocytic pathway. There are indications that OSBP, besides acting as a cholesterol/phosphatidylinositol 4-phosphate (PI4P) exchanger at the endoplasmic reticulum (ER)-trans-Golgi network (TGN) membrane contact sites (MCS), also exchanges these lipids at ER-lysosome (Lys) contacts, increasing Lys cholesterol content. The long isoform of ORP1 (ORP1L), which also targets ER-late endosome (LE)/Lys MCS, has the capacity to mediate cholesterol transport either from ER to LE or in the opposite direction. Moreover, it regulates the motility, positioning and fusion of LE as well as autophagic flux. ORP2, the closest relative of ORP1, is mainly cytosolic, but also targets PI(4,5)P₂-rich endosomal compartments. Our latest data suggest that ORP2 transfers cholesterol from LE to recycling endosomes (RE) in exchange for PI(4,5)P₂, thus stimulating the recruitment of focal adhesion kinase (FAK) on the RE and cell adhesion. FAK activates phosphoinositide kinase on the RE to enhance PI(4,5)P₂ synthesis. ORP2 in turn transfers PI(4,5)P₂ from RE to LE, thus regulating LE tubule formation and transport activity.

Coordination of inter-organelle communication and lipid fluxes by OSBP-related proteins

Oxysterol-binding protein (OSBP) and OSBP-related proteins (ORPs) constitute one of the largest families of lipid-binding/transfer proteins (LTPs) in eukaryotes. The current view is that many of them mediate inter-organelle lipid transfer over membrane contact sites (MCS). The transfer occurs in several cases in a 'counter-current' fashion: A lipid such as cholesterol or phosphatidylserine (PS) is transferred against its concentration gradient driven by transport of a phosphoinositide in the opposite direction. In this way ORPs are envisioned to maintain the distinct organelle lipid compositions, with impacts on multiple organelle functions. However, the functions of ORPs extend beyond lipid homeostasis to regulation of processes such as cell survival, proliferation and migration. Important expanding areas of mammalian ORP research include their roles in viral and bacterial infections, cancers, and neuronal function. The yeast OSBP homologue (Osh) proteins execute multifaceted functions in sterol and glycerophospholipid homeostasis, post-Golgi vesicle transport, phosphatidylinositol-4-phosphate, sphingolipid and target of rapamycin (TOR) signalling, and cell cycle control. These observations identify ORPs as lipid transporters and coordinators of signals with an unforeseen variety of cellular processes. Understanding their activities not only enlightens the biology of the living cell but also allows their employment as targets of new therapeutic approaches for disease.

Lessons in self-defence: inhibition of virus entry by intrinsic immunity

Virus entry, consisting of attachment to and penetration into the host target cell, is the first step of the virus life cycle and is a critical 'do or die' event that governs virus emergence in host populations. Most antiviral vaccines induce neutralizing antibodies that prevent virus entry into cells. However, while the prevention of virus invasion by humoral immunity is well appreciated, considerably less is known about the immune defences present within cells (known as intrinsic immunity) that interfere with virus entry. The interferon-induced transmembrane (IFITM) proteins, known for inhibiting fusion between viral and cellular membranes, were once the only factors known to restrict virus entry. However, the progressive development of genetic and pharmacological screening platforms and the onset of the COVID-19 pandemic have galvanized interest in how viruses infiltrate cells and how cells defend against it. Several host factors with antiviral potential are now implicated in the regulation of virus entry, including cholesterol 25-hydroxylase (CH25H), lymphocyte antigen 6E (LY6E), nuclear receptor co-activator protein 7 (NCOA7), interferon-γ-inducible lysosomal thiol reductase (GILT), CD74 and ARFGAP with dual pleckstrin homology domain-containing protein 2 (ADAP2). This Review summarizes what is known and what remains to be understood about the intrinsic factors that form the first line of defence against virus infection.

High-Order Epistasis and Functional Coupling of Infection Steps Drive Virus Evolution toward Independence from a Host Pathway

The phosphatidylinositol-4 kinase IIIβ (PI4KB)/oxysterol-binding protein (OSBP) family I pathway serves as an essential host pathway for the formation of viral replication complex for viral plus-strand RNA synthesis; however, poliovirus (PV) could evolve toward substantial independence from this host pathway with four mutations. Recessive epistasis of the two mutations (3A-R54W and 2B-F17L) is essential for viral RNA replication. Quantitative analysis of effects of the other two mutations (2B-Q20H and 2C-M187V) on each step of infection reveals functional couplings between viral replication, growth, and spread conferred by the 2B-Q20H mutation, while no enhancing effect was conferred by the 2C-M187V mutation. The effects of the 2B-Q20H mutation occur only via another recessive epistasis between the 3A-R54W/2B-F17L mutations. These mutations confer enhanced replication in PI4KB/OSBP-independent infection concomitantly with an increased ratio of viral plus-strand RNA to the minus-strand RNA. This work reveals the essential roles of the functional coupling and high-order, multi-tiered recessive epistasis in viral evolution toward independence from an obligatory host pathway.

IMPORTANCE Each virus has a different strategy for its replication, which requires different host factors. Enterovirus, a model RNA virus, requires host factors PI4KB and OSBP, which form an obligatory functional axis to support viral replication. In an experimental evolution system in vitro, virus mutants that do not depend on these host factors could arise only with four mutations. The two mutations (3A-R54W and 2B-F17L) are required for the replication but are not sufficient to support efficient infection. Another mutation (2B-Q20H) is essential for efficient spread of the virus. The order of introduction of the mutations in the viral genome is essential (known as “epistasis”), and functional couplings of infection steps (i.e., viral replication, growth, and spread) have substantial roles to show the effects of the 2B-Q20H mutation. These observations would provide novel insights into an evolutionary pathway of the virus to require host factors for infection.

Intertwined and Finely Balanced: Endoplasmic Reticulum Morphology, Dynamics, Function, and Diseases

The endoplasmic reticulum (ER) is an organelle that is responsible for many essential subcellular processes. Interconnected narrow tubules at the periphery and thicker sheet-like regions in the perinuclear region are linked to the nuclear envelope. It is becoming apparent that the complex morphology and dynamics of the ER are linked to its function. Mutations in the proteins involved in regulating ER structure and movement are implicated in many diseases including neurodegenerative diseases such as Alzheimer’s, Parkinson’s, and amyotrophic lateral sclerosis (ALS). The ER is also hijacked by pathogens to promote their replication. Bacteria such as Legionella pneumophila and Chlamydia trachomatis, as well as the Zika virus, bind to ER morphology and dynamics-regulating proteins to exploit the functions of the ER to their advantage. This review covers our understanding of ER morphology, including the functional subdomains and membrane contact sites that the organelle forms. We also focus on ER dynamics and the current efforts to quantify ER motion and discuss the diseases related to ER morphology and dynamics.

Life in the lumen: The multivesicular endosome

The late endosomes/endo‐lysosomes of vertebrates contain an atypical phospholipid, lysobisphosphatidic acid (LBPA) (also termed bis[monoacylglycero]phosphate [BMP]), which is not detected elsewhere in the cell. LBPA is abundant in the membrane system present in the lumen of this compartment, including intralumenal vesicles (ILVs). In this review, the current knowledge on LBPA and LBPA‐containing membranes will be summarized, and their role in the control of endosomal cholesterol will be outlined. Some speculations will also be made on how this system may be overwhelmed in the cholesterol storage disorder Niemann‐Pick C. Then, the roles of intralumenal membranes in endo‐lysosomal dynamics and functions will be discussed in broader terms. Likewise, the mechanisms that drive the biogenesis of intralumenal membranes, including ESCRTs, will also be discussed, as well as their diverse composition and fate, including degradation in lysosomes and secretion as exosomes. This review will also discuss how intralumenal membranes are hijacked by pathogenic agents during intoxication and infection, and what is the biochemical composition and function of the intra‐endosomal lumenal milieu. Finally, this review will allude to the size limitations imposed on intralumenal vesicle functions and speculate on the possible role of LBPA as calcium chelator in the acidic calcium stores of endo‐lysosomes.

On the possible existence of a liver LDL-ostat, and its malfunctioning in familial hypercholesterolemia

The investigation of familial hypercholesterolemia (FH) and its relationship to atherosclerosis has led to enormous scientific and medical progress, including the identification of genetic defects underlying FH, the elucidation of molecular mechanisms crucial for cellular cholesterol homeostasis and the development of current pharmaceutical tools for FH treatment (which are directed at increasing LDL uptake). These successes also led to the establishment of a model centered on cellular rather than whole organism processes, and a view of FH as resulting from a deficiency in LDL uptake. On the other hand, whole organism fluxes of cholesterol (like those of other nutrients) are centered on the liver, LDL (ultimately derived from the liver) is the main cholesterol transporter in plasma, and there is evidence of evolutionary pressure favoring mechanisms to maintain LDL plasma concentrations. Furthermore, the alterations in cellular metabolism observed in FH are consistent with a coordinated response by the liver to increase the levels of plasma LDL, suggesting that a signaling defect (rather than an uptake deficiency) is the fundamental problem underlying hypercholesterolemia - an hypothesis that explains the occurrence of hypercholesterolemia in CESD, despite normal LDL binding and uptake. I therefore propose that the liver contains a mechanism to assess and regulate plasma levels of LDL (an "LDL-ostat"), and that hypercholesterolemia is caused by defects in it. This model has implications for future research directions, and suggests alternative therapeutic approaches, particularly centered on efforts to restore LDL measurement/signaling (rather than its uptake), some of which are in stark contrast to those currently in use.

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.

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.

Immunometabolic function of cholesterol in cardiovascular disease and beyond

Inflammation represents the driving feature of many diseases, including atherosclerosis, cancer, autoimmunity and infections. It is now established that metabolic processes shape a proper immune response and within this context the alteration in cellular cholesterol homeostasis has emerged as a culprit of many metabolic abnormalities observed in chronic inflammatory diseases. Cholesterol accumulation supports the inflammatory response of myeloid cells (i.e. augmentation of toll-like receptor signalling, inflammasome activation, and production of monocytes and neutrophils) which is beneficial in the response to infections, but worsens diseases associated with chronic metabolic inflammation including atherosclerosis. In addition to the innate immune system, cells of adaptive immunity, upon activation, have also been shown to undergo a reprogramming of cellular cholesterol metabolism, which results in the amplification of inflammatory responses. Aim of this review is to discuss (i) the molecular mechanisms linking cellular cholesterol metabolism to specific immune functions; (ii) how cellular cholesterol accumulation sustains chronic inflammatory diseases such as atherosclerosis; (iii) the immunometabolic profile of patients with defects of genes affecting cholesterol metabolism including familial hypercholesterolaemia, cholesteryl ester storage disease, Niemann-Pick type C, and immunoglobulin D syndrome/mevalonate kinase deficiency. Available data indicate that cholesterol immunometabolism plays a key role in directing immune cells function and set the stage for investigating the repurposing of existing 'metabolic' drugs to modulate the immune response.

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.

Poliovirus Evolution toward Independence from the Phosphatidylinositol-4 Kinase III β/Oxysterol-Binding Protein Family I Pathway

Phosphatidylinositol-4 kinase III β (PI4KB) and oxysterol-binding protein (OSBP) family I provide a conserved host pathway required for enterovirus replication. Here, we analyze the role and essentiality of this pathway in enterovirus replication. Phosphatidylinositol 4-phosphate (PI4P) production and cholesterol accumulation in the replication organelle (RO) are severely suppressed in cells infected with a poliovirus (PV) mutant isolated from a PI4KB-knockout cell line (RD[Δ PI4KB]). Major determinants of the mutant for infectivity in RD(Δ PI4KB) cells map to the A5270U(3A-R54W) and U3881C(2B-F17L) mutations. The 3A mutation is required for PI4KB-independent development of RO. The 2B mutation rather sensitizes PV to PI4KB/OSBP inhibitors by itself but confers substantially complete resistance to the inhibitors with the 3A mutation. The 2B mutation also confers hypersensitivity to interferon alpha treatment on PV. These suggest that the PI4KB/OSBP pathway is not necessarily essential for enterovirus replication in vitro. This work supports a two-step resistance model of enterovirus to PI4KB/OSBP inhibitors involving unique recessive epistasis of 3A and 2B and offers insights into a potential evolutionary pathway of enterovirus toward independence from the PI4KB/OSBP pathway.

Lysosomal storage disorders - challenges, concepts and avenues for therapy: beyond rare diseases

The pivotal role of lysosomes in cellular processes is increasingly appreciated. An understanding of the balanced interplay between the activity of acidic hydrolases, lysosomal membrane proteins and cytosolic proteins is required. Lysosomal storage diseases (LSDs) are characterized by disturbances in this network and by intralysosomal accumulation of substrates, often only in certain cell types. Even though our knowledge of these diseases has increased and therapies have been established, many aspects of the molecular pathology of LSDs remain obscure. This Review aims to discuss how lysosomal storage affects functions linked to lysosomes, such as membrane repair, autophagy, exocytosis, lipid homeostasis, signalling cascades and cell viability. Therapies must aim to correct lysosomal storage not only morphologically, but reverse its (patho)biochemical consequences. As different LSDs have different molecular causes, this requires custom tailoring of therapies. We will discuss the major advantages and drawbacks of current and possible future therapies for LSDs. Study of the pathological molecular mechanisms underlying these 'experiments of nature' often yields information that is relevant for other conditions found in the general population. Therefore, more common diseases may profit from a correction of impaired lysosomal function.

Cholesterol Overload: Contact Sites to the Rescue!

Delivery of low-density lipoprotein-derived cholesterol to the endoplasmic reticulum (ER) is essential for cholesterol homeostasis, yet the mechanism of this transport has largely remained elusive. Two recent reports shed some light on this process, uncovering a role for Niemann Pick type-C1 protein (NPC1) in the formation of membrane contact sites (MCS) between late endosomes (LE)/lysosomes (Lys) and the ER. Both studies identified a loss of MCS in cells lacking functional NPC1, where cholesterol accumulates in late endocytic organelles. Remarkably, and taking different approaches, both studies have made a striking observation that expansion of LE/Lys-ER MCS can rescue the cholesterol accumulation phenotype in NPC1 mutant or deficient cells. In both cases, the cholesterol was shown to be transported to the ER, demonstrating the importance of ER-LE/Lys contact sites in the direct transport of low-density lipoprotein-derived cholesterol to the ER.