Interplay between Asters/GRAMD1s and phosphatidylserine in intermembrane transport of LDL cholesterol

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.

Molecular insights into human phosphatidylserine synthase 1 reveal its inhibition promotes LDL uptake

In mammalian cells, two phosphatidylserine (PS) synthases drive PS synthesis. Gain-of-function mutations in the Ptdss1 gene lead to heightened PS production, causing Lenz-Majewski syndrome (LMS). Recently, pharmacological inhibition of PSS1 has been shown to suppress tumorigenesis. Here, we report the cryo-EM structures of wild-type human PSS1 (PSS1WT), the LMS-causing Pro269Ser mutant (PSS1P269S), and PSS1WT in complex with its inhibitor DS55980254. PSS1 contains 10 transmembrane helices (TMs), with TMs 4-8 forming a catalytic core in the luminal leaflet. These structures revealed a working mechanism of PSS1 akin to the postulated mechanisms of the membrane-bound O-acyltransferase family. Additionally, we showed that both PS and DS55980254 can allosterically inhibit PSS1 and that inhibition by DS55980254 activates the SREBP pathways, thus enhancing the expression of LDL receptors and increasing cellular LDL uptake. This work uncovers a mechanism of mammalian PS synthesis and suggests that selective PSS1 inhibitors have the potential to lower blood cholesterol levels.

Phosphatidylserine regulates plasma membrane repair through tetraspanin-enriched macrodomains

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

Low MBOAT7 expression, a genetic risk for MASH, promotes a profibrotic pathway involving hepatocyte TAZ upregulation

BACKGROUND AND AIMS

The common genetic variant rs641738 C>T is a risk factor for metabolic dysfunction-associated steatotic liver disease and metabolic dysfunction-associated steatohepatitis (MASH), including liver fibrosis, and is associated with decreased expression of the phospholipid-remodeling enzyme MBOAT7 (LPIAT1). However, whether restoring MBOAT7 expression in established metabolic dysfunction-associated steatotic liver disease dampens the progression to liver fibrosis and, importantly, the mechanism through which decreased MBOAT7 expression exacerbates MASH fibrosis remain unclear.

APPROACH AND RESULTS

We first showed that hepatocyte MBOAT7 restoration in mice with diet-induced steatohepatitis slows the progression to liver fibrosis. Conversely, when hepatocyte-MBOAT7 was silenced in mice with established hepatosteatosis, liver fibrosis but not hepatosteatosis was exacerbated. Mechanistic studies revealed that hepatocyte-MBOAT7 restoration in MASH mice lowered hepatocyte-TAZ (WWTR1), which is known to promote MASH fibrosis. Conversely, hepatocyte-MBOAT7 silencing enhanced TAZ upregulation in MASH. Finally, we discovered that changes in hepatocyte phospholipids due to MBOAT7 loss-of-function promote a cholesterol trafficking pathway that upregulates TAZ and the TAZ-induced profibrotic factor Indian hedgehog (IHH). As evidence for relevance in humans, we found that the livers of individuals with MASH carrying the rs641738-T allele had higher hepatocyte nuclear TAZ, indicating higher TAZ activity and increased IHH mRNA.

CONCLUSIONS

This study provides evidence for a novel mechanism linking MBOAT7-LoF to MASH fibrosis, adds new insight into an established genetic locus for MASH, and, given the druggability of hepatocyte TAZ for MASH fibrosis, suggests a personalized medicine approach for subjects at increased risk for MASH fibrosis due to inheritance of variants that lower MBOAT7.

The nonvesicular sterol transporter Aster-C plays a minor role in whole body cholesterol balance

Introduction

The Aster-C protein (encoded by the Gramd1c gene) is an endoplasmic reticulum (ER) resident protein that has been reported to transport cholesterol from the plasma membrane to the ER. Although there is a clear role for the closely-related Aster-B protein in cholesterol transport and downstream esterification in the adrenal gland, the specific role for Aster-C in cholesterol homeostasis is not well understood. Here, we have examined whole body cholesterol balance in mice globally lacking Aster-C under low or high dietary cholesterol conditions.

Method

Age-matched Gramd1c +/+ and Gramd1c -/- mice were fed either low (0.02%, wt/wt) or high (0.2%, wt/wt) dietarycholesterol and levels of sterol-derived metabolites were assessed in the feces, liver, and plasma.

Results

Compared to wild type controls (Gramd1c +/+) mice, mice lackingGramd1c (Gramd1c -/-) have no significant alterations in fecal, liver, or plasma cholesterol. Given the potential role for Aster C in modulating cholesterol metabolism in diverse tissues, we quantified levels of cholesterol metabolites such as bile acids, oxysterols, and steroid hormones. Compared to Gramd1c +/+ controls, Gramd1c -/- mice had modestly reduced levels of select bile acid species and elevated cortisol levels, only under low dietary cholesterol conditions. However, the vast majority of bile acids, oxysterols, and steroid hormones were unaltered in Gramd1c -/- mice. Bulk RNA sequencing in the liver showed that Gramd1c -/- mice did not exhibit alterations in sterol-sensitive genes, but instead showed altered expression of genes in major urinary protein and cytochrome P450 (CYP) families only under low dietary cholesterol conditions.

Discussion

Collectively, these data indicate nominal effects of Aster-C on whole body cholesterol transport and metabolism under divergent dietary cholesterol conditions. These results strongly suggest that Aster-C alone is not sufficient to control whole body cholesterol balance, but can modestly impact circulating cortisol and bile acid levels when dietary cholesterol is limited.

Bis(2-ethylhexyl)-tetrabromophthalate Poses a Higher Exposure Risk and Induces Gender-Specific Metabolic Disruptions in Zebrafish Liver

Bis(2-ethylhexyl)-tetrabromophthalate (TBPH), a typical novel brominated flame retardant, has been ubiquitously identified in various environmental and biotic media. Consequently, there is an urgent need for precise risk assessment based on a comprehensive understanding of internal exposure and the corresponding toxic effects on specific tissues. In this study, we first investigated the toxicokinetic characteristics of TBPH in different tissues using the classical pseudo-first-order toxicokinetic model. We found that TBPH was prone to accumulate in the liver rather than in the gonad, brain, and muscle of both female and male zebrafish, highlighting a higher internal exposure risk for the liver. Furthermore, long-term exposure to TBPH at environmentally relevant concentrations led to increased visceral fat accumulation, signaling potential abnormal liver function. Hepatic transcriptome analysis predominantly implicated glycolipid metabolism pathways. However, alterations in the profile of associated genes and biochemical indicators revealed gender-specific responses following TBPH exposure. Besides, histopathological observations as well as the inflammatory response in the liver confirmed the development of nonalcoholic fatty liver disease, particularly in male zebrafish. Altogether, our findings highlight a higher internal exposure risk for the liver, enhancing our understanding of the gender-specific metabolic-disrupting potential associated with TBPH exposure.

T cell cholesterol transport is a metabolic checkpoint that links intestinal immune responses to dietary lipid absorption

The intrinsic pathways that control membrane organization in immune cells and the impact of such pathways on cellular function are not well defined. Here we report that the non-vesicular cholesterol transporter Aster-A links plasma membrane (PM) cholesterol availability in T cells to immune signaling and systemic metabolism. Aster-A is recruited to the PM during T-cell receptor (TCR) activation, where it facilitates the removal of newly generated "accessible" membrane cholesterol. Loss of Aster-A leads to excess PM cholesterol accumulation, resulting in enhanced TCR nano-clustering and signaling, and Th17 cytokine production. Finally, we show that the mucosal Th17 response is restrained by PM cholesterol remodeling. Ablation of Aster-A in T cells leads to enhanced IL-22 production, reduced intestinal fatty acid absorption, and resistance to diet-induced obesity. These findings delineate a multi-tiered regulatory scheme linking immune cell lipid flux to nutrient absorption and systemic physiology.

Towards a Unitary Hypothesis of Alzheimer's Disease Pathogenesis

The "amyloid cascade" hypothesis of Alzheimer's disease (AD) pathogenesis invokes the accumulation in the brain of plaques (containing the amyloid-β protein precursor [AβPP] cleavage product amyloid-β [Aβ]) and tangles (containing hyperphosphorylated tau) as drivers of pathogenesis. However, the poor track record of clinical trials based on this hypothesis suggests that the accumulation of these peptides is not the only cause of AD. Here, an alternative hypothesis is proposed in which the AβPP cleavage product C99, not Aβ, is the main culprit, via its role as a regulator of cholesterol metabolism. C99, which is a cholesterol sensor, promotes the formation of mitochondria-associated endoplasmic reticulum (ER) membranes (MAM), a cholesterol-rich lipid raft-like subdomain of the ER that communicates, both physically and biochemically, with mitochondria. We propose that in early-onset AD (EOAD), MAM-localized C99 is elevated above normal levels, resulting in increased transport of cholesterol from the plasma membrane to membranes of intracellular organelles, such as ER/endosomes, thereby upregulating MAM function and driving pathology. By the same token, late-onset AD (LOAD) is triggered by any genetic variant that increases the accumulation of intracellular cholesterol that, in turn, boosts the levels of C99 and again upregulates MAM function. Thus, the functional cause of AD is upregulated MAM function that, in turn, causes the hallmark disease phenotypes, including the plaques and tangles. Accordingly, the MAM hypothesis invokes two key interrelated elements, C99 and cholesterol, that converge at the MAM to drive AD pathogenesis. From this perspective, AD is, at bottom, a lipid disorder.

Reversible translocation of acyl-CoA:cholesterol acyltransferase (ACAT) between the endoplasmic reticulum and vesicular structures

The enzyme acyl-CoA:cholesterol acyltransferase (ACAT) is normally localized in the endoplasmic reticulum (ER) where it can esterify cholesterol for storage in lipid droplets and/or the formation of lipoproteins. Here, we report that ACAT can translocate from the ER into vesicular structures in response to different ACAT inhibitors. The translocation was fast (within minutes), reversible and occurred in different cell types. Interestingly, oleic acid was able to fasten the re-translocation from vesicles back into the reticular ER network. The process of ACAT translocation could also be induced by cyclodextrins, cholesterol, lanosterol (but not 4-cholestene-3 one), 25-hydroxycholesterol, and by certain stress stimuli such as hyperosmolarity (sucrose treatment), temperature change, or high-density cultivation. In vitro esterification showed that ACAT remains fully active after it has been translocated to vesicles in response to hyperosmotic sucrose treatment of the cells. The translocation process was not accompanied by changes in the electrophoretic mobility of ACAT, even after chemical crosslinking. Interestingly, the protein synthesis inhibitor cycloheximide showed a stimulating effect on ACAT activity and prevented the translocation of ACAT from the ER into vesicles.

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.

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.

Cholesterol Transport to the Endoplasmic Reticulum

Most cholesterol in mammalian cells is stored in the plasma membrane (PM). Cholesterol transport from the PM to low-sterol regulatory regions of the endoplasmic reticulum (ER) controls cholesterol synthesis and uptake, and thereby influences the rates of cholesterol flux between tissues of complex organisms. Cholesterol transfer to the ER is also required for steroidogenesis, oxysterol and bile acid synthesis, and cholesterol esterification. The ER-resident Aster proteins (Aster-A, -B, and -C) form contacts with the PM to move cholesterol to the ER in mammals. Mice lacking Aster-B have low adrenal cholesteryl ester stores and impaired steroidogenesis because of a defect in cholesterol transport from high-density lipoprotein (HDL) to the ER. This work reviews the molecular characteristics of Asters, their role in HDL- and low-density lipoprotein (LDL)-cholesterol movement, and how cholesterol transferred to the ER is utilized by cells. The roles of other lipid transporters and of membrane lipid organization in maintaining aspects of cholesterol homeostasis are also highlighted.

GRAMD1/ASTER-mediated cholesterol transport promotes Smoothened cholesterylation at the endoplasmic reticulum

Hedgehog (Hh) signaling pathway plays a pivotal role in embryonic development. Hh binding to Patched1 (PTCH1) derepresses Smoothened (SMO), thereby activating the downstream signal transduction. Covalent SMO modification by cholesterol in its cysteine-rich domain (CRD) is essential for SMO function. SMO cholesterylation is a calcium-accelerated autoprocessing reaction, and STIM1-ORAI1-mediated store-operated calcium entry promotes cholesterylation and activation of endosome-localized SMO. However, it is unknown whether the Hh-PTCH1 interplay regulates the activity of the endoplasmic reticulum (ER)-localized SMO. Here, we found that PTCH1 inhibited the COPII-dependent export of SMO from the ER, whereas Hh promoted this process. The RRxWxR amino acid motif in the cytosolic tail of SMO was essential for COPII recognition, ciliary localization, and signal transduction activity. Hh and PTCH1 regulated cholesterol modification of the ER-localized SMO, and SMO cholesterylation accelerated its exit from ER. The GRAMD1/ASTER sterol transport proteins facilitated cholesterol transfer to ER from PM, resulting in increased SMO cholesterylation and enhanced Hh signaling. Collectively, we reveal a regulatory role of GRAMD-mediated cholesterol transport in ER-resident SMO maturation and Hh signaling.

Identifying the Transcriptional Drivers of Metastasis Embedded within Localized Melanoma

In melanoma, predicting which tumors will ultimately metastasize guides treatment decisions. Transcriptional signatures of primary tumors have been utilized to predict metastasis, but which among these are driver or passenger events remains unclear. We used data from the adjuvant AVAST-M trial to identify a predictive gene signature in localized tumors that ultimately metastasized. Using a zebrafish model of primary melanoma, we interrogated the top genes from the AVAST-M signature in vivo. This identified GRAMD1B, a cholesterol transfer protein, as a bona fide metastasis suppressor, with a majority of knockout animals rapidly developing metastasis. Mechanistically, excess free cholesterol or its metabolite 27-hydroxycholesterol promotes invasiveness via activation of an AP-1 program, which is associated with increased metastasis in humans. Our data demonstrate that the transcriptional seeds of metastasis are embedded within localized tumors, suggesting that early targeting of these programs can be used to prevent metastatic relapse.

SIGNIFICANCE

We analyzed human melanoma transcriptomics data to identify a gene signature predictive of metastasis. To rapidly test clinical signatures, we built a genetic metastasis platform in adult zebrafish and identified GRAMD1B as a suppressor of melanoma metastasis. GRAMD1B-associated cholesterol overload activates an AP-1 program to promote melanoma invasion. This article is highlighted in the In This Issue feature, p. 1.

Roles of Phosphatidylinositol 4-Phosphorylation in Non-vesicular Cholesterol Trafficking

Cholesterol (Chol) is an essential component of all eukaryotic cell membranes that affects the function of numerous peripheral as well as integral membrane proteins. Chol is synthesized in the ER, but it is selectively enriched within the plasma membrane (PM) and other endomembranes, which requires Chol to cross the aqueous phase of the cytoplasm. In addition to the classical vesicular trafficking pathways that are known to facilitate the bulk transport of membrane intermediates, Chol is also transported via non-vesicular lipid transfer proteins that work primarily within specialized membrane contact sites. Some of these transport pathways work against established concentration gradients and hence require energy. Recent studies highlight the unique role of phosphoinositides (PPIns), and phosphatidylinositol 4-phosphate (PI4P) in particular, for the control of non-vesicular Chol transport. In this chapter, we will review the emerging connection between Chol, PPIns, and lipid transfer proteins that include the important family of oxysterol-binding protein related proteins, or ORPs.

Hepatic nonvesicular cholesterol transport is critical for systemic lipid homeostasis

In cell models, changes in the 'accessible' pool of plasma membrane (PM) cholesterol are linked with the regulation of endoplasmic reticulum sterol synthesis and metabolism by the Aster family of nonvesicular transporters; however, the relevance of such nonvesicular transport mechanisms for lipid homeostasis in vivo has not been defined. Here we reveal two physiological contexts that generate accessible PM cholesterol and engage the Aster pathway in the liver: fasting and reverse cholesterol transport. During fasting, adipose-tissue-derived fatty acids activate hepatocyte sphingomyelinase to liberate sequestered PM cholesterol. Aster-dependent cholesterol transport during fasting facilitates cholesteryl ester formation, cholesterol movement into bile and very low-density lipoprotein production. During reverse cholesterol transport, high-density lipoprotein delivers excess cholesterol to the hepatocyte PM through scavenger receptor class B member 1. Loss of hepatic Asters impairs cholesterol movement into feces, raises plasma cholesterol levels and causes cholesterol accumulation in peripheral tissues. These results reveal fundamental mechanisms by which Aster cholesterol flux contributes to hepatic and systemic lipid homeostasis.

ABCA1 and cholesterol transfer protein Aster-A promote an asymmetric cholesterol distribution in the plasma membrane

Cholesterol is a major and essential component of the mammalian cell plasma membrane (PM), and the loss of cholesterol homeostasis leads to various pathologies. Cellular cholesterol uptake and synthesis are regulated by a cholesterol sensor in the endoplasmic reticulum (ER). However, it remains unclear how changes in the cholesterol level of the PM are recognized. Here, we show that the sensing of cholesterol in the PM depends on ABCA1 and the cholesterol transfer protein Aster-A, which cooperatively maintain the asymmetric transbilayer cholesterol distribution in the PM. We demonstrate that ABCA1 translocates (flops) cholesterol from the inner leaflet of the PM to the outer leaflet of the PM to maintain a low inner leaflet cholesterol level. We also found that when inner cholesterol levels were increased, Aster-A was recruited to the PM-ER contact site to transfer cholesterol to the ER. These results suggest that ABCA1 could promote an asymmetric cholesterol distribution to suppress Aster-A recruitment to the PM-ER contact site to maintain intracellular cholesterol homeostasis.

Creating and sensing asymmetric lipid distributions throughout the cell

A key feature of eukaryotic cells is the asymmetric distribution of lipids along their secretory pathway. Because of the biological significance of these asymmetries, it is crucial to define the mechanisms which create them. Extensive studies have led to the identification of lipid transfer proteins (LTPs) that work with lipid-synthesizing enzymes to carry lipids between two distinct membranes in a directional manner, and are thus able to create asymmetries in lipid distribution throughout the cell. These networks are often in contact sites where two organelle membranes are in close proximity for reasons we have only recently started to understand. A question is whether these networks transfer lipids en masse within the cells or adjust the lipid composition of organelle membranes. Finally, recent data have confirmed that some networks organized around LTPs do not generate lipid asymmetries between membranes but sense them and rectify the lipid content of the cell.

Synthesis, function, and regulation of sterol and nonsterol isoprenoids

Cholesterol, the bulk end-product of the mevalonate pathway, is a key component of cellular membranes and lipoproteins that transport lipids throughout the body. It is also a precursor of steroid hormones, vitamin D, and bile acids. In addition to cholesterol, the mevalonate pathway yields a variety of nonsterol isoprenoids that are essential to cell survival. Flux through the mevalonate pathway is tightly controlled to ensure cells continuously synthesize nonsterol isoprenoids but avoid overproducing cholesterol and other sterols. Endoplasmic reticulum (ER)-localized 3-hydroxy-3-methylglutaryl coenzyme A (HMG CoA) reductase (HMGCR), the rate limiting enzyme in the mevalonate pathway, is the focus of a complex feedback regulatory system governed by sterol and nonsterol isoprenoids. This review highlights transcriptional and post-translational regulation of HMGCR. Transcriptional regulation of HMGCR is mediated by the Scap-SREBP pathway. Post-translational control is initiated by the intracellular accumulation of sterols, which causes HMGCR to become ubiquitinated and subjected to proteasome-mediated ER-associated degradation (ERAD). Sterols also cause a subfraction of HMGCR molecules to bind the vitamin K₂ synthetic enzyme, UbiA prenyltransferase domain-containing protein-1 (UBIAD1). This binding inhibits ERAD of HMGCR, which allows cells to continuously synthesize nonsterol isoprenoids such as geranylgeranyl pyrophosphate (GGPP), even when sterols are abundant. Recent studies reveal that UBIAD1 is a GGPP sensor, dissociating from HMGCR when GGPP thresholds are met to allow maximal ERAD. Animal studies using genetically manipulated mice disclose the physiological significance of the HMGCR regulatory system and we describe how dysregulation of these pathways contributes to disease.

Sphingosine kinases regulate ER contacts with late endocytic organelles and cholesterol trafficking

Membrane contact sites (MCS), close membrane apposition between organelles, are platforms for interorganellar transfer of lipids including cholesterol, regulation of lipid homeostasis, and co-ordination of endocytic trafficking. Sphingosine kinases (SphKs), two isoenzymes that phosphorylate sphingosine to the bioactive sphingosine-1-phosphate (S1P), have been implicated in endocytic trafficking. However, the physiological functions of SphKs in regulation of membrane dynamics, lipid trafficking and MCS are not known. Here, we report that deletion of SphKs decreased S1P with concomitant increases in its precursors sphingosine and ceramide, and markedly reduced endoplasmic reticulum (ER) contacts with late endocytic organelles. Expression of enzymatically active SphK1, but not catalytically inactive, rescued the deficit of these MCS. Although free cholesterol accumulated in late endocytic organelles in SphK null cells, surprisingly however, cholesterol transport to the ER was not reduced. Importantly, deletion of SphKs promoted recruitment of the ER-resident cholesterol transfer protein Aster-B (also called GRAMD1B) to the plasma membrane (PM), consistent with higher accessible cholesterol and ceramide at the PM, to facilitate cholesterol transfer from the PM to the ER. In addition, ceramide enhanced in vitro binding of the Aster-B GRAM domain to phosphatidylserine and cholesterol liposomes. Our study revealed a previously unknown role for SphKs and sphingolipid metabolites in governing diverse MCS between the ER network and late endocytic organelles versus the PM to control the movement of cholesterol between distinct cell membranes.

Endolysosomal cholesterol export: More than just NPC1

NPC1 plays a central role in cholesterol egress from endolysosomes, a critical step for maintaining intracellular cholesterol homeostasis. Despite recent advances in the field, the full repertoire of molecules and pathways involved in this process remains unknown. Emerging evidence suggests the existence of NPC1-independent, alternative routes. These may involve vesicular and non-vesicular mechanisms, as well as release of extracellular vesicles. Understanding the underlying molecular mechanisms that bypass NPC1 function could have important implications for the development of therapies for lysosomal storage disorders. Here we discuss how cholesterol may be exported from lysosomes in which NPC1 function is impaired.

Molecular and Signaling Mechanisms for Docosahexaenoic Acid-Derived Neurodevelopment and Neuroprotection

The neurodevelopmental and neuroprotective actions of docosahexaenoic acid (DHA) are mediated by mechanisms involving membrane- and metabolite-related signal transduction. A key characteristic in the membrane-mediated action of DHA results from the stimulated synthesis of neuronal phosphatidylserine (PS). The resulting DHA-PS-rich membrane domains facilitate the translocation and activation of kinases such as Raf-1, protein kinase C (PKC), and Akt. The activation of these signaling pathways promotes neuronal development and survival. DHA is also metabolized in neural tissues to bioactive mediators. Neuroprotectin D1, a docosatriene synthesized by the lipoxygenase activity, has an anti-inflammatory property, and elovanoids formed from DHA elongation products exhibit antioxidant effects in the retina. Synaptamide, an endocannabinoid-like lipid mediator synthesized from DHA in the brain, promotes neurogenesis and synaptogenesis and exerts anti-inflammatory effects. It binds to the GAIN domain of the GPR110 (ADGRF1) receptor, triggers the cAMP/protein kinase A (PKA) signaling pathway, and activates the cAMP-response element binding protein (CREB). The DHA status in the brain influences not only the PS-dependent signal transduction but also the metabolite formation and expression of pre- and post-synaptic proteins that are downstream of the CREB and affect neurotransmission. The combined actions of these processes contribute to the neurodevelopmental and neuroprotective effects of DHA.

Sorting (Nexin-13) out Novel Insights into Endolysosomal Cholesterol Export

Transport in and out of the endolysosomal compartment represents a key step in the regulation of cellular cholesterol homeostasis. Despite important recent advances, how LDL-derived, free cholesterol is exported from the lumen of endolysosomes to other organelles is still a matter of debate. We recently devised a CRISPR/Cas9 genome-scale strategy to uncover genes involved in the regulation of endolysosomal cholesterol homeostasis and the functionally linked phospholipid, bis(monoacylglycerol)-phosphate. This approach confirmed known genes and pathways involved in this process, and more importantly revealed previously unrecognized roles for new players, such as Sorting Nexin-13 (SNX13). Here we discuss the unexpected regulatory role of SNX13 in endolysosomal cholesterol export.