Identification of NPC1 as the target of U18666A, an inhibitor of lysosomal cholesterol export and Ebola infection

Cholesterol-dependent Nsp5-endosomes co-trafficking to lysosomes facilitates porcine reproductive and respiratory syndrome virus replication by activating autophagy

Our previous studies showed that intracellular endosomal vesicles participated in PRRS virions trafficking in the early stage of viral infection, and cholesterol retention in endosomal vesicles disturbed viral replication via blocking PRRSV-endosomal vesicles membrane fusion. However, whether endosomal vesicles were associated with PRRSV protein(s) trafficking and the role of cholesterol in this process was still unclarity. In this study, we sought to elucidate the mechanism of cholesterol in endosomal vesicles-mediated viral protein transportation. The results showed that endosomal vesicles participated in trafficking of PRRSV Nsp5 protein. After being synthesized in endoplasmic reticulum (ER) and Golgi apparatus, Nsp5 was trafficked to early endosomes (EEs), but not endocytic recycling compartments (ERCs), then to late endosomes (LEs), and eventually reached lysosomal compartments, whereas disruption of cholesterol flux or LEs function led to the inability of Nsp5 arriving at lysosomes, where Nsp5 activated cellular autophagy to promote PRRSV replication. Molecular docking predictions revealed that cholesterol could form two hydrogen bonds with 74 alanine and 78 asparagine of Nsp5. After mutating the aforementioned binding sites, the replication efficiency of PRRSV decreased. Subsequently, the role of cholesterol in PRRSV replication was explored. Blocking of cholesterol flux significantly inhibited PRRSV replication. Single virus infection cycle analysis showed that cholesterol flux disorder did not affect virus adsorption, but could inhibit virus entry into host cells and block EEs-LEs-lysosomes mediated trafficking of virions, leading to virions retention in endosomal compartments. The present studies suggest that cholesterol and endosomal vesicles synergistically participate in Nsp5 trafficking to promote PRRSV replication, which may provide new insights for the development of novel antiviral drugs targeting cholesterol metabolism pathways or the improvement of commercial vaccines.

c-Abl/TFEB Pathway Activation as a Common Pathogenic Mechanism in Lysosomal Storage Diseases: Therapeutic Potential of c-Abl Inhibitors

Lysosomal storage diseases (LSDs) are characterized by the accumulation of undegraded substrates within lysosomes, often associated with oxidative stress and impaired lysosomal function. In this study, we investigate the role of the c-Abl/TFEB pathway in different LSDs: Gaucher, Niemann-Pick type A (NPA), and Niemann-Pick type C (NPC). Our findings identify c-Abl activation (p-c-Abl) as a common pathogenic mechanism in these disorders. We demonstrate that c-Abl phosphorylates TFEB at Tyr173, leading to its cytoplasmic retention. Using pharmacological models of Gaucher, NPA and NPC in SH-SY5Y neuronal cells and HeLa cells, we assess the effects of the c-Abl inhibitors Imatinib and Neurotinib, as well as the antioxidant α-Tocopherol (α-TOH), on TFEB nuclear translocation and p-c-Abl protein levels. Additionally, we explore the effects of c-Abl inhibitors in cholesterol accumulation in LSDs neuronal models. Our results show that treatment with c-Abl inhibitors or α-TOH promotes TFEB nuclear translocation, enhances lysosomal clearance, and reduces cholesterol accumulation in all three LSD models. These findings highlight the c-Abl/TFEB pathway as a potential therapeutic target for LSDs and potentially other neurodegenerative disorders associated with lysosomal dysfunction.

Cholesterol homeostasis and lipid raft dynamics at the basis of tumor-induced immune dysfunction in chronic lymphocytic leukemia

Autologous T-cell therapies show limited efficacy in chronic lymphocytic leukemia (CLL), where acquired immune dysfunction prevails. In CLL, disturbed mitochondrial metabolism has been linked to defective T-cell activation and proliferation. Recent research suggests that lipid metabolism regulates mitochondrial function and differentiation in T cells, yet its role in CLL remains unexplored. This comprehensive study compares T-cell lipid metabolism in CLL patients and healthy donors, revealing critical dependence on exogenous cholesterol for human T-cell expansion following TCR-mediated activation. Using multi-omics and functional assays, we found that T cells present in viably frozen samples of patients with CLL (CLL T cells) showed impaired adaptation to cholesterol deprivation and inadequate upregulation of key lipid metabolism transcription factors. CLL T cells exhibited altered lipid storage, with increased triacylglycerols and decreased cholesterol, and inefficient fatty acid oxidation (FAO). Functional consequences of reduced FAO in T cells were studied using samples from patients with inherent FAO disorders. Reduced FAO was associated with lower T-cell activation but did not affect proliferation. This implicates low cholesterol levels as a primary factor limiting T-cell proliferation in CLL. CLL T cells displayed fewer and less clustered lipid rafts, potentially explaining the impaired immune synapse formation observed in these patients. Our findings highlight significant disruptions in lipid metabolism as drivers of functional deficiencies in CLL T cells, underscoring the pivotal role of cholesterol in T-cell proliferation. This study suggests that modulating cholesterol metabolism could enhance T-cell function in CLL, presenting novel immunotherapeutic approaches to improve outcome in this challenging disease.

Mechanistic insights into arimoclomol mediated effects on lysosomal function in Niemann-pick type C disease

Niemann-Pick disease type C (NPC) is an ultra-rare, fatal neurodegenerative disease. It is characterized by lysosomal dysfunction with cytotoxic accumulation of unesterified cholesterol and glycosphingolipids in lysosomes, which causes neurodegeneration and peripheral organ dysfunction. Arimoclomol, an orally available small molecule, is the first FDA-approved treatment for NPC when used in combination with miglustat. Here, we present the results of a series of in vitro studies performed to explore the pathways by which arimoclomol targets the fundamentals of NPC etiology. While the precise cellular interactions of arimoclomol remain unclear, the increased translocation of the transcription factors EB and E3 (TFEB and TFE3) from the cytosol to the nucleus is a key initial step for triggering a cascade of downstream events that can rescue cellular functions. Activation of TFEB and TFE3 raises the expression rates of coordinated lysosomal expression and regulation (CLEAR) genes including NPC1 that are essential for the regulation of lysosomal function. The subsequent upregulation of CLEAR network proteins combined with increased unfolded protein response activation was shown to enlarge the pool of matured NPC1 capable of reaching the lysosome to reduce cholesterol accumulation. By also amplifying expression of CLEAR genes associated with autophagy, arimoclomol has the potential to act on different pathways and improve cell viability independent of NPC1 protein levels and functionality. In summary, the findings presented illustrate how arimoclomol improves lysosomal function and potentially autophagy flux to decrease lipid burden in NPC patient fibroblasts.

ApoE4 requires lipidation enhancement to resolve cellular lipid and protein abnormalities following NPC1 inhibition

Apolipoprotein E (ApoE) variants are central to Alzheimer's disease (AD), Lewy body dementia (LBD) and Niemann-Pick disease type C (NPC). The ApoE4 variant elevates AD risk by 3-15-fold. ApoE's normal function in lipid transport is known. The question remains how different ApoE isoforms cause cellular pathogenesis. We determined the effects of ApoE isoforms on lipid accumulation induced by inhibiting the endo-lysosomal cholesterol transporter NPC1. In human fibroblasts and astrocytes, NPC1 inhibition caused a 4-fold cholesterol accumulation and mis-localization with altered cholesterol sensing and increased synthesis of cholesterol and triglycerides. Total APP, APP C-terminal fragments (CTF) and BACE1 levels increased 3-fold. Remarkably, the intracellular neutral lipids co-localized with APP and APP C-terminal fragments. ApoE2 and ApoE3, but not ApoE4, reduced intracellular cholesterol levels by 67% and 62%, respectively, normalized APP, BACE, CTF, and improved cell survival. ApoE4 combined with a synthetic lipopeptide, which increased the proportion of large lipidated ApoE4 particles, corrected these abnormalities. This highlights ApoE in lipid pathogenesis and targeting ApoE4 lipidation to restore ApoE4 function.

Small-molecule activation of TFEB alleviates Niemann-Pick disease type C via promoting lysosomal exocytosis and biogenesis

Niemann-Pick disease type C (NPC) is a devastating lysosomal storage disease characterized by abnormal cholesterol accumulation in lysosomes. Currently, there is no treatment for NPC. Transcription factor EB (TFEB), a member of the microphthalmia transcription factors (MiTF), has emerged as a master regulator of lysosomal function and promoted the clearance of substrates stored in cells. However, it is not known whether TFEB plays a role in cholesterol clearance in NPC disease. Here, we show that transgenic overexpression of TFEB, but not TFE3 (another member of MiTF family) facilitates cholesterol clearance in various NPC1 cell models. Pharmacological activation of TFEB by sulforaphane (SFN), a previously identified natural small-molecule TFEB agonist by us, can dramatically ameliorate cholesterol accumulation in human and mouse NPC1 cell models. In NPC1 cells, SFN induces TFEB nuclear translocation via a ROS-Ca2+-calcineurin-dependent but MTOR-independent pathway and upregulates the expression of TFEB-downstream genes, promoting lysosomal exocytosis and biogenesis. While genetic inhibition of TFEB abolishes the cholesterol clearance and exocytosis effect by SFN. In the NPC1 mouse model, SFN dephosphorylates/activates TFEB in the brain and exhibits potent efficacy of rescuing the loss of Purkinje cells and body weight. Hence, pharmacological upregulating lysosome machinery via targeting TFEB represents a promising approach to treat NPC and related lysosomal storage diseases, and provides the possibility of TFEB agonists, that is, SFN as potential NPC therapeutic candidates.

NPC1 promotes the progression of hepatocellular carcinoma by mediating the accumulation of neutrophils into the tumor microenvironment

Hepatocellular carcinoma remains a significant threat to human health. Recent studies have found that the intake of cellular cholesterol contributes to the development and progression of hepatocellular carcinoma, although the exact mechanisms remain unclear. Our analysis of transcriptomic and proteomic databases has identified increased mRNA and protein expression levels of NPC1, a cholesterol intracellular transporter protein, in hepatocellular carcinoma tissues. This increase is significantly associated with a worse prognosis for patients. To corroborate these findings, we performed immunohistochemical staining of NPC1 on liver tissue samples from patients, revealing significantly higher expression levels of NPC1 in hepatocellular carcinoma tissues compared to normal tissues. Subsequent investigations have revealed that NPC1 expression does not significantly influence the proliferation of hepatocellular carcinoma cells in vitro. However, it has a substantial inhibitory effect on the progression of hepatocellular carcinoma tumors when observed in vivo. Utilizing flow cytometry to monitor cellular changes within the tumor microenvironment has led us to discover that NPC1 plays a crucial role in the regulation of neutrophil recruitment within the tumor. Using further neutrophil depletion experiments, we determined that the role of NPC1 in advancing hepatocellular carcinoma progression truly relies on neutrophils. These observations are further reinforced by a comprehensive analysis of clinical databases alongside immunohistochemistry findings. In conclusion, our research suggests that NPC1's overexpression could contribute to hepatocellular carcinoma progression by promoting neutrophil recruitment, positioning NPC1 as a promising new biomarker and therapeutic target for hepatocellular carcinoma.

Cholesterol mobilization regulates dendritic cell maturation and the immunogenic response to cancer

Maturation of conventional dendritic cells (cDCs) is crucial for maintaining tolerogenic safeguards against auto-immunity and for promoting immunogenic responses to pathogens and cancer. The subcellular mechanism for cDC maturation remains poorly defined. We show that cDCs mature by leveraging an internal reservoir of cholesterol (harnessed from extracellular cell debris and generated by de novo synthesis) to assemble lipid nanodomains on cell surfaces of maturing cDCs, enhance expression of maturation markers and stabilize immune receptor signaling. This process is dependent on cholesterol transport through Niemann-Pick disease type C1 (NPC1) and mediates homeostatic and Toll-like receptor (TLR)-induced maturation. Importantly, we identified the receptor tyrosine kinase AXL as a regulator of the NPC1-dependent construction of lipid nanodomains. Deleting AXL from cDCs enhances their maturation, thus improving anti-tumor immunity. Altogether, our study presents new insights into cholesterol mobilization as a fundamental basis for cDC maturation and highlights AXL as a therapeutic target for modulating cDCs.

Identification of a Privileged Scaffold for Inhibition of Sterol Transport Proteins through the Synthesis and Ring Distortion of Diverse, Pseudo-Natural Products

Sterol transport proteins mediate intracellular sterol transport, organelle contact sites, and lipid metabolism. Despite their importance, the similarities in their sterol-binding domains have made the identification of selective modulators difficult. Herein we report a combination of different compound library synthesis strategies to prepare a cholic acid-inspired compound collection for the identification of potent and selective inhibitors of sterol transport proteins. The fusion of a primary sterol scaffold with a range of different fragments found in natural products followed by various ring distortions allowed the synthesis of diverse sterol-inspired compounds. This led to the identification of a complex and three-dimensional spirooxepinoindole as a privileged scaffold for sterol transport proteins. With careful optimization of the scaffold, the selectivity could be directed toward a single transporter, as showcased by the development of a potent and selective Aster-A inhibitor. We suggest that the combination of different design strategies is generally applicable for the identification of potent and selective bioactive compounds with drug-like properties.

NPC1 controls TGFBR1 stability in a cholesterol transport-independent manner and promotes hepatocellular carcinoma progression

Niemann-Pick disease type C protein 1 (NPC1), classically associated with cholesterol transport and viral entry, has an emerging role in cancer biology. Here, we demonstrate that knockout of Npc1 in hepatocytes attenuates hepatocellular carcinoma (HCC) progression in both DEN (diethylnitrosamine)-CCl4 induced and MYC-driven HCC mouse models. Mechanistically, NPC1 significantly promotes HCC progression by modulating the TGF-β pathway, independent of its traditional role in cholesterol transport. We identify that the 692-854 amino acid region of NPC1's transmembrane domain is critical for its interaction with TGF-β receptor type-1 (TGFBR1). This interaction prevents the binding of SMAD7 and SMAD ubiquitylation regulatory factors (SMURFs) to TGFBR1, reducing TGFBR1 ubiquitylation and degradation, thus enhancing its stability. Notably, the NPC1 (P691S) mutant, which is defective in cholesterol transport, still binds TGFBR1, underscoring a cholesterol-independent mechanism. These findings highlight a cholesterol transport-independent mechanism by which NPC1 contributes to the stability of TGFBR1 in HCC and suggest potential therapeutic strategies targeting NPC1 for HCC treatment.

Noncanonical PI(4,5)P2 coordinates lysosome positioning through cholesterol trafficking

In p53-deficient cancers, targeting cholesterol metabolism has emerged as a promising therapeutic approach, given that p53 loss dysregulates sterol regulatory element-binding protein 2 (SREBP-2) pathways, thereby enhancing cholesterol biosynthesis. While cholesterol synthesis inhibitors such as statins have shown initial success, their efficacy is often compromised by the development of acquired resistance. Consequently, new strategies are being explored to disrupt cholesterol homeostasis more comprehensively by inhibiting its synthesis and intracellular transport. In this study, we investigate a previously underexplored function of PI5P4Ks, which catalyzes the conversion of PI(5)P to PI(4,5)P2 at intracellular membranes. Our findings reveal that PI5P4Ks play a key role in facilitating lysosomal cholesterol transport, regulating lysosome positioning, and sustaining growth signaling via the mTOR pathway. While PI5P4Ks have previously been implicated in mTOR signaling and tumor proliferation in p53-deficient contexts, this work elucidates an upstream mechanism that unifies these earlier observations.

Structural and biochemical analysis of ligand binding in yeast Niemann-Pick type C1-related protein

In eukaryotes, integration of sterols into the vacuolar/lysosomal membrane is critically dependent on the Niemann-Pick type C (NPC) system. The system consists of an integral membrane protein, called NCR1 in yeast, and NPC2, a luminal soluble protein that transfers sterols to the N-terminal domain (NTD) of NCR1 before membrane integration. Both proteins have been implicated in sterol homeostasis of yeast and humans. Here, we investigate sterol and lipid binding of the NCR1/NPC2 transport system and determine crystal structures of the sterol binding NTD. The NTD binds both ergosterol and cholesterol, with nearly identical conformations of the binding pocket. Apart from sterols, the NTD can also bind fluorescent analogs of phosphatidylinositol, phosphatidylcholine, and phosphatidylserine, as well as sphingosine and ceramide. We confirm the multi-lipid scope of the NCR1/NPC2 system using photo-crosslinkable and clickable lipid analogs, namely, pac-cholesterol, pac-sphingosine, and pac-ceramide. Finally, we reconstitute the transfer of pac-sphingosine from NPC2 to the NTD in vitro. Collectively, our results support that the yeast NPC system can work as versatile machinery for vacuolar homeostasis of structurally diverse lipids, besides ergosterol.

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

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

Manipulation of Host Cholesterol by SARS-CoV-2

SARS-CoV-2 infection is associated with alterations in host lipid metabolism, including disruptions in cholesterol homeostasis. However, the specific mechanisms by which viral proteins influence cholesterol remain incompletely understood. Here, we report that SARS-CoV-2 infection induces cholesterol sequestration within lysosomes, with the viral protein ORF3a identified as the primary driver of this effect. Mechanistically, we found that ORF3a interacts directly with the HOPS complex subunit VPS39 through a hydrophobic interface formed by residues W193 and Y184. A W193A mutation in ORF3a significantly rescues cholesterol egress and corrects the mislocalization of the lysosomal cholesterol transporter NPC2, which is caused by defective trafficking of the trans-Golgi network (TGN) sorting receptor, the cation-independent mannose-6-phosphate receptor (CI-MPR). We further observed a marked reduction in bis(monoacylglycero)phosphate (BMP), a lipid essential for lysosomal cholesterol egress, in both SARS-CoV-2-infected cells and ORF3a-expressing cells, suggesting BMP reduction as an additional mechanism of SARS-CoV-2-caused cholesterol sequestration. Inhibition of lysosomal cholesterol egress using the compound U18666A significantly decreased SARS-CoV-2 infection, highlighting a potential viral strategy of manipulating lysosomal cholesterol to modulate host cell susceptibility. Our findings reveal that SARS-CoV-2 ORF3a disrupts cellular cholesterol transport by altering lysosomal protein trafficking and BMP levels, providing new insights into virus-host interactions that contribute to lipid dysregulation in infected cells.

LRRK2 and RAB8A regulate cell death after lysosomal damage in macrophages through cholesterol-related pathways

Activating mutations in Leucine Rich Repeat Kinase 2 (LRRK2) are among the most common genetic causes of Parkinson's disease (PD). The mechanistic path from LRRK2 mutations to PD is not established, but several lines of data suggest that LRRK2 modulation of lysosomal function is involved. It has previously been shown that LRRK2 is recruited to lysosomes upon lysosomal damage leading to increased phosphorylation of its RAB GTPase substrates in macrophage-derived RAW 264.7 cells. Here, we find that LRRK2 kinase inhibition reduces cell death induced by the lysosomotropic compound LLOMe in RAW 264.7 cells showing that lysosomal damage and LRRK2 functionally interacts in both directions: lysosomal damage can lead to activation of LRRK2 signaling and LRRK2 inhibition can attenuate LLOMe-induced cell death. The effect is lysosome specific, as only lysosomal stressors and not a variety of other cell death inducers could be modulated by LRRK2 kinase inhibition. We show with timing and Lysotracker experiments that LRRK2 inhibition does not affect the immediate lysosomal permeabilization induced by LLOMe, but rather modulates the subsequent cellular response to lysosomal damage. siRNA-mediated knockdown of LRRK2 and its main substrates, the RAB GTPases, showed that LRRK2 and RAB8A knockdown could attenuate LLOMe-induced cell death, but not other RAB GTPases tested. An RNA sequencing study was done to identify downstream pathways modulated by LLOMe and LRRK2 inhibition. The most striking finding was that almost all cholesterol biosynthesis genes were strongly downregulated by LLOMe and upregulated with LRRK2 inhibition in combination with LLOMe treatment. To explore the functional relevance of the transcriptional changes, we pretreated cells with the NPC1 inhibitor U18666A that can lead to accumulation of lysosomal cholesterol. U18666A-treated cells were less sensitive to LLOMe-induced cell death, but the attenuation of cell death by LRRK2 inhibition was strongly reduced suggesting that LRRK2 inhibition and lysosomal cholesterol reduces cell death by overlapping mechanisms. Thus, our data demonstrates a LRRK2- and RAB8A-mediated attenuation of RAW 264.7 cell death induced by lysosomal damage that is modulated by lysosomal cholesterol.

NPC1 links cholesterol trafficking to microglial morphology via the gastrosome

Microglia play important roles in brain development and homeostasis by removing dying neurons through efferocytosis. Morphological changes in microglia are hallmarks of many neurodegenerative conditions, such as Niemann-Pick disease type C. Here, NPC1 loss causes microglia to shift from a branched to an ameboid form, though the cellular basis and functional impact of this change remain unclear. Using zebrafish, we show that NPC1 deficiency causes an efferocytosis-dependent expansion of the microglial gastrosome, a collection point for engulfed material. In vivo and in vitro experiments on microglia and mammalian macrophages demonstrate that NPC1 localizes to the gastrosome, and its absence leads to cholesterol accumulation in this compartment. NPC1 loss and neuronal cell death synergistically affect gastrosome size and cell shape, increasing the sensitivity of NPC1-deficient cells to neuronal cell death. Finally, we demonstrate conservation of cholesterol accumulation and gastrosome expansion in NPC patient-derived fibroblasts, offering an interesting target for further disease investigation.

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.

Why make it if you can take it: review on extracellular cholesterol uptake and its importance in breast and ovarian cancers

Cholesterol homeostasis is essential for healthy mammalian cells and dysregulation of cholesterol metabolism contributes to the pathogenesis of various diseases including cancer. Cancer cells are dependent on cholesterol. Malignant progression is associated with high cellular demand for cholesterol, and extracellular cholesterol uptake is often elevated in cancer cell to meet its metabolic needs. Tumors take up cholesterol from the blood stream through their vasculature. Breast cancer grows in, and ovarian cancer metastasizes into fatty tissue that provides them with an additional source of cholesterol. High levels of extracellular cholesterol are beneficial for tumors whose cancer cells master the uptake of extracellular cholesterol. In this review we concentrate on cholesterol uptake mechanisms, receptor-mediated endocytosis and macropinocytosis, and how these are utilized and manipulated by cancer cells to overcome their possible intrinsic or pharmacological limitations in cholesterol synthesis. We focus especially on the involvement of lysosomes in cholesterol uptake. Identifying the vulnerabilities of cholesterol metabolism and manipulating them could provide novel efficient therapeutic strategies for treatment of cancers that manifest dependency for extracellular cholesterol.

A new perspective on the regulation of glucose and cholesterol transport by mitochondria-lysosome contact sites

Mitochondria and lysosomes play a very important role in maintaining cellular homeostasis, and the dysfunction of these organelles is closely related to many diseases. Recent studies have revealed direct interactions between mitochondria and lysosomes, forming mitochondria-lysosome contact sites that regulate organelle network dynamics and mediate the transport of metabolites between them. Impaired function of these contact sites is not only linked to physiological processes such as glucose and cholesterol transport but also closely related to the pathological processes of metabolic diseases. Here, we highlight the recent progress in understanding the mitochondria-lysosome contact sites, elucidate their role in regulating metabolic homeostasis, and explore the potential implications of this pathway in metabolic disorders.

Genetic and epigenetic regulation of inflammasomes: Role in atherosclerosis

The cardiovascular complications of atherosclerosis are thought to arise from an inflammatory response to the accumulation of cholesterol-rich lipoproteins in the arterial wall. The positive outcome of CANTOS (Canakinumab Anti-inflammatory Thrombosis Outcome Study) provided key evidence to support this concept and suggested that inflammasomes and IL-1β are important inflammatory mediators in human atherosclerotic cardiovascular diseases (ACVD). In specific settings NLRP3 or AIM2 inflammasomes can induce inflammatory responses in the arterial wall and promote the formation of unstable atherosclerotic plaques. Clonal hematopoiesis (CH) has recently emerged as a major independent risk factor for ACVD. CH mutations arise during ageing and commonly involves variants in genes mediating epigenetic modifications (TET2, DNMT3A, ASXL1) or cytokine signaling (JAK2). Accumulating evidence points to the role of inflammasomes in the progression of CH-induced ACVD events and has shed light on the regulatory pathways and possible therapeutic approaches that specifically target inflammasomes in atherosclerosis. Epigenetic dynamics play a vital role in regulating the generation and activation of inflammasome components by causing changes in DNA methylation patterns and chromatin assembly. This review examines the genetic and epigenetic regulation of inflammasomes, the intersection of macrophage cholesterol accumulation with inflammasome activation and their roles in atherosclerosis. Understanding the involvement of inflammasomes in atherosclerosis pathogenesis may lead to customized treatments that reduce the burden of ACVD.

Mutant induced neurons and humanized mice enable identification of Niemann-Pick type C1 proteostatic therapies

Therapeutics that rescue folding, trafficking, and function of disease-causing missense mutants are sought for a host of human diseases, but efforts to leverage model systems to test emerging strategies have met with limited success. Such is the case for Niemann-Pick type C1 disease, a lysosomal disorder characterized by impaired intracellular cholesterol trafficking, progressive neurodegeneration, and early death. NPC1, a multipass transmembrane glycoprotein, is synthesized in the endoplasmic reticulum and traffics to late endosomes/lysosomes, but this process is often disrupted in disease. We sought to identify small molecules that promote folding and enable lysosomal localization and functional recovery of mutant NPC1. We leveraged a panel of isogenic human induced neurons expressing distinct NPC1 missense mutations. We used this panel to rescreen compounds that were reported previously to correct NPC1 folding and trafficking. We established mo56-hydroxycholesterol (mo56Hc) as a potent pharmacological chaperone for several NPC1 mutants. Furthermore, we generated mice expressing human I1061T NPC1, a common mutation in patients. We demonstrated that this model exhibited disease phenotypes and recapitulated the protein trafficking defects, lipid storage, and response to mo56Hc exhibited by human cells expressing I1061T NPC1. These tools established a paradigm for testing and validation of proteostatic therapeutics as an important step toward the development of disease-modifying therapies.

Endogenous Protein-Protein Interaction Network of the NPC Cholesterol Transporter 1 in the Cerebral Cortex

NPC intracellular cholesterol transporter 1 (NPC1) is a multipass, transmembrane glycoprotein mostly recognized for its key role in facilitating cholesterol efflux. Mutations in the NPC1 gene result in Niemann-Pick disease, type C (NPC), a fatal, lysosomal storage disease. Due to the progressively expanding implications of NPC1-related disorders, we investigated endogenous NPC1 protein-protein interactions in the mouse cortex and human-derived iPSCs neuronal models of the disease through coimmunoprecipitation-coupled with LC-MS based proteomics. The current study investigated protein-protein interactions specific to the wild-type and the most prevalent NPC1 mutation (NPC1I1061T) while filtering out any protein interactor identified in the Npc1-/- mouse model. Additionally, the results were matched across the two species to map the parallel interactome of wild-type and mutant NPC1I1061T. Most of the identified wild-type NPC1 interactors were related to cytoskeleton organization, synaptic vesicle activity, and translation. We found many putative NPC1 interactors not previously reported, including two SCAR/WAVE complex proteins that regulate ARP 2/3 complex actin nucleation and multiple membrane proteins important for neuronal activity at synapse. Moreover, we identified proteins important in trafficking specific to wild-type and mutant NPC1I1061T. Together, the findings are essential for a comprehensive understanding of NPC1 biological functions in addition to its classical role in sterol efflux.

Cholesterol-rich lysosomes induced by respiratory syncytial virus promote viral replication by blocking autophagy flux

Respiratory syncytial virus (RSV) hijacks cholesterol or autophagy pathways to facilitate optimal replication. However, our understanding of the associated molecular mechanisms remains limited. Here, we show that RSV infection blocks cholesterol transport from lysosomes to the endoplasmic reticulum by downregulating the activity of lysosomal acid lipase, activates the SREBP2-LDLR axis, and promotes uptake and accumulation of exogenous cholesterol in lysosomes. High cholesterol levels impair the VAP-A-binding activity of ORP1L and promote the recruitment of dynein-dynactin, PLEKHM1, or HOPS VPS39 to Rab7-RILP, thereby facilitating minus-end transport of autophagosomes and autolysosome formation. Acidification inhibition and dysfunction of cholesterol-rich lysosomes impair autophagy flux by inhibiting autolysosome degradation, which promotes the accumulation of RSV fusion protein. RSV-F storage is nearly abolished after cholesterol depletion or knockdown of LDLR. Most importantly, the knockout of LDLR effectively inhibits RSV infection in vivo. These findings elucidate the molecular mechanism of how RSV co-regulates lysosomal cholesterol reprogramming and autophagy and reveal LDLR as a novel target for anti-RSV drug development.

Altered lipid homeostasis and autophagy precipitate diffuse alveolar hemorrhage in murine lupus

Abnormal autophagy regulation is implicated in lupus and other autoimmune diseases. We investigated autophagy in the murine pristane-induced lupus model. Pristane causes monocyte/macrophage-mediated endoplasmic reticulum (ER) stress in lung endothelial cells and diffuse alveolar hemorrhage (DAH) indistinguishable from DAH in lupus patients. Enlarged macrophages with abundant lipid droplets containing neutral lipid and exhibiting increased autophagosome staining were observed in the lung and peritoneal macrophages after pristane treatment. Cellular overload of neutral lipid can lead to selective autophagy (lipophagy) of lipid droplets and transport to lysosomes. The autophagy inducer rapamycin decreased neutral lipid staining but aggravated DAH, while an autophagy inhibitor (3-methyladenine) blocked the onset of DAH. Pristane-induced autophagy in macrophages was confirmed by acridine orange assay and LC3 western blot. Pristane also enlarged lysosomal volume and enhanced cathepsin S, D, and K expression while decreasing lysosomal acid lipase activity. If the capacity to degrade neutral lipid into free cholesterol and fatty acids is overwhelmed, lysosomes enlarge and can release cathepsins into the cytoplasm promoting cell death. Increasing lysosomal cholesterol content by blocking the Niemann-Pick C disease protein NPC1 protects against lysosome-dependent cell death. Treatment with NPC1 inhibitors U18666A or cepharanthine, which stabilize lysosomes, normalized lysosomal volume, reversed ER stress, and prevented DAH in pristane-treated mice. We conclude that pristane disrupts lipid homeostasis, promoting autophagy, lysosomal dysfunction, ER stress, and cell death leading to DAH. NPC1 inhibition reverses these abnormalities, preventing DAH. The findings shed light on the role of autophagy and lysosomal dysfunction in the pathogenesis of lupus.

ALOX15B controls macrophage cholesterol homeostasis via lipid peroxidation, ERK1/2 and SREBP2

Macrophage cholesterol homeostasis is crucial for health and disease and has been linked to the lipid-peroxidizing enzyme arachidonate 15-lipoxygenase type B (ALOX15B), albeit molecular mechanisms remain obscure. We performed global transcriptome and immunofluorescence analysis in ALOX15B-silenced primary human macrophages and observed a reduction of nuclear sterol regulatory element-binding protein (SREBP) 2, the master transcription factor of cellular cholesterol biosynthesis. Consequently, SREBP2-target gene expression was reduced as were the sterol biosynthetic intermediates desmosterol and lathosterol as well as 25- and 27-hydroxycholesterol. Mechanistically, suppression of ALOX15B reduced lipid peroxidation in primary human macrophages and thereby attenuated activation of mitogen-activated protein kinase ERK1/2, which lowered SREBP2 abundance and activity. Low nuclear SREBP2 rendered both, ALOX15B-silenced and ERK1/2-inhibited macrophages refractory to SREBP2 activation upon blocking the NPC intracellular cholesterol transporter 1. These studies suggest a regulatory mechanism controlling macrophage cholesterol homeostasis based on ALOX15B-mediated lipid peroxidation and concomitant ERK1/2 activation.

ORMDL mislocalization by impaired autophagy in Niemann-Pick type C disease leads to increased de novo sphingolipid biosynthesis

Niemann-Pick type C1 (NPC1) disease is a rare neurodegenerative cholesterol and sphingolipid storage disorder primarily due to mutations in the cholesterol-trafficking protein NPC1. In addition to catabolic-derived sphingolipids, NPC1 dysfunction also leads to an increase in de novo sphingolipid biosynthesis, yet little is known about the cellular mechanism involved. Although deletion of NPC1 or inhibition of the NPC1 sterol binding domain enhanced de novo sphingolipid biosynthesis, surprisingly levels of the ORMDLs, the regulatory subunits of serine palmitoyltransferase (SPT), the rate-limiting step in sphingolipid biosynthesis, were also greatly increased. Nevertheless, less ORMDL was bound in the SPT-ORMDL complex despite elevated ceramide levels. Instead, ORMDL colocalized with p62, the selective autophagy receptor, and accumulated in stalled autophagosomes due to defective autophagy in NPC1 disease cells. Restoration of autophagic flux with N-acetyl-L-leucine in NPC1 deleted cells decreased ORMDL accumulation in autophagosomes and reduced de novo sphingolipid biosynthesis and their accumulation. This study revealed a previously unknown link between de novo sphingolipid biosynthesis, ORMDL, and autophagic defects present in NCP1 disease. In addition, we provide further evidence and mechanistic insight for the beneficial role of N-acetyl-L-leucine treatment for NPC1 disease which is presently awaiting approval from the Food and Drug Administration and the European Medicines Agency.

Potential Use of the Cholesterol Transfer Inhibitor U18666A as a Potent Research Tool for the Study of Cholesterol Mechanisms in Neurodegenerative Disorders

Cholesterol is an essential component of mammalian cell membranes and a precursor for crucial signaling molecules. The brain contains the highest level of cholesterol in the body, and abnormal cholesterol metabolism links to many neurodegenerative disorders. The results indicate that faulty cholesterol metabolism is a common feature among people living with neurodegenerative conditions. The researchers suggest that restoring cholesterol levels may become a beneficial new strategy in treating certain neurodegenerative conditions. Several neurodegenerative disorders, such as Alzheimer's disease (AD), Niemann-Pick type C (NPC) disease, and Parkinson's disease (PD), have been connected to abnormalities in brain cholesterol metabolism. Consequently, using a lipid research tool is vital to study further and understand the effect of lipids in neurodegenerative disorders such as NPC, AD, PD, and Huntington's disease (HD). U18666A, also known as 3-(2-(diethylamino) ethoxy) androst-5-en-17-one, is a pharmaceutical drug that suppresses cholesterol trafficking and is a well-known class-2 amphiphile. U18666A has performed many functions, allowing for essential discoveries in lipid studies and shedding light on the pathophysiology of neurodegenerative disorders. Additionally, U18666A prevented the downregulation of low-density lipoprotein (LDL) receptors that are induced by LDL and led to the buildup of cholesterol in lysosomes. Numerous studies show that U18666A impacts the function of cholesterol trafficking to control the metabolism and transport of amyloid precursor proteins (APPs). Treating cortical neurons with U18666A may provide a new in vitro model system for studying the underlying molecular process of NPC, AD, HD, and PD. In this article, we review the mechanism and function of U18666A as a vital tool for studying cholesterol mechanisms in neurological diseases related to abnormal cholesterol metabolism, such as AD, NPC, HD, and PD.

Cytotoxic sigma-2 ligands trigger cancer cell death via cholesterol-induced-ER-stress

Sigma-2-ligands (S2L) are characterized by high binding affinities to their cognate sigma-2 receptor, overexpressed in rapidly proliferating tumor cells. As such, S2L were developed as imaging probes (ISO1) or as cancer therapeutics, alone (SV119 [C6], SW43 [C10]) and as delivery vehicles for cytotoxic drug cargoes (C6-Erastin, C10-SMAC). However, the exact mechanism of S2L-induced cytotoxicity remains to be fully elucidated. A series of high-affinity S2L were evaluated regarding their cytotoxicity profiles across cancer cell lines. While C6 and C10 displayed distinct cytotoxicities, C0 and ISO1 were essentially non-toxic. Confocal microscopy and lipidomics analysis in cellular and mouse models revealed that C10 induced increases in intralysosomal free cholesterol and in cholesterol esters, suggestive of unaltered intracellular cholesterol trafficking. Cytotoxicity was caused by cholesterol excess, a phenomenon that contrasts the effects of NPC1 inhibition. RNA-sequencing revealed gene clusters involved in cholesterol homeostasis and ER stress response exclusively by cytotoxic S2L. ER stress markers were confirmed by qPCR and their targeted modulation inhibited or enhanced cytotoxicity of C10 in a predicted manner. Moreover, C10 increased sterol regulatory element-binding protein 2 (SREBP2) and low-density lipoprotein receptor (LDLR), both found to be pro-survival factors activated by ER stress. Furthermore, inhibition of downstream processes of the adaptive response to S2L with simvastatin resulted in synergistic treatment outcomes in combination with C10. Of note, the S2L conjugates retained the ER stress response of the parental ligands, indicative of cholesterol homeostasis being involved in the overall cytotoxicity of the drug conjugates. Based on these findings, we conclude that S2L-mediated cell death is due to free cholesterol accumulation that leads to ER stress. Consequently, the cytotoxic profiles of S2L drug conjugates are proposed to be enhanced via concurrent ER stress inducers or simvastatin, strategies that could be instrumental on the path toward tumor eradication.

Evoked Weibel-Palade Body Exocytosis Modifies the Endothelial Cell Surface by Releasing a Substrate-Selective Phosphodiesterase

Weibel Palade bodies (WPB) are lysosome-related secretory organelles of endothelial cells. Commonly known for their main cargo, the platelet and leukocyte receptors von-Willebrand factor (VWF) and P-selectin, WPB play a crucial role in hemostasis and inflammation. Here, the authors identify the glycerophosphodiester phosphodiesterase domain-containing protein 5 (GDPD5) as a WPB cargo protein and show that GDPD5 is transported to WPB following uptake from the plasma membrane via an unique endocytic transport route. GDPD5 cleaves GPI-anchored, plasma membrane-resident proteins within their GPI-motif, thereby regulating their local activity. The authors identify a novel target of GDPD5 , the complement regulator CD59, and show that it is released from the endothelial surface by GDPD5 following WPB exocytosis. This results in increased deposition of complement components and can enhance local inflammatory and thrombogenic responses. Thus, stimulus-induced WPB exocytosis can modify the endothelial cell surface by GDPD5-mediated selective release of a subset of GPI-anchored proteins.

Cholesterol trafficking to the ER leads to the activation of CaMKII/JNK/NLRP3 and promotes atherosclerosis

The deposition of cholesterol-rich lipoproteins in the arterial wall triggers macrophage inflammatory responses, which promote atherosclerosis. The NLRP3 inflammasome aggravates atherosclerosis; however, cellular mechanisms connecting macrophage cholesterol accumulation to inflammasome activation are poorly understood. We investigated the mechanisms of NLRP3 inflammasome activation in cholesterol-loaded macrophages and in atherosclerosis-prone Ldlr-/- mice with defects in macrophage cholesterol efflux. We found that accumulation of cholesterol in macrophages treated with modified LDL or cholesterol crystals, or in macrophages defective in the cholesterol efflux promoting transporters ABCA1 and ABCG1, leads to activation of NLRP3 inflammasomes as a result of increased cholesterol trafficking from the plasma membrane to the ER, via Aster-B. In turn, the accumulation of cholesterol in the ER activates the inositol triphosphate-3 receptor, CaMKII/JNK, and induces NLRP3 deubiquitylation by BRCC3. An NLRP3 deubiquitylation inhibitor or deficiency of Abro1, an essential scaffolding protein in the BRCC3-containing cytosolic complex, suppressed inflammasome activation, neutrophil extracellular trap formation (NETosis), and atherosclerosis in vivo. These results identify a link between the trafficking of cholesterol to the ER, NLRP3 deubiquitylation, inflammasome activation, and atherosclerosis.

FLEX: genetically encodable enzymatic fluorescence signal amplification using engineered peroxidase

Fluorescent tagging of biomolecules enables their sensitive detection during separation and determining their subcellular location. In this context, peroxidase-based reactions are actively utilized for signal amplification. To harness this potential, we developed a genetically encodable enzymatic fluorescence signal amplification method using APEX (FLEX). We synthesized a fluorescent probe, Jenfluor triazole (JFT1), which effectively amplifies and restricts fluorescence signals under fixed conditions, enabling fluorescence-based detection of subcellularly localized electron-rich metabolites. Moreover, JFT1 exhibited stable fluorescence signals even under osmium-treated and polymer-embedded conditions, which supported findings from correlative light and electron microscopy (CLEM) using APEX. Using various APEX-conjugated proteins of interest (POIs) targeted to different organelles, we successfully visualized their localization through FLEX imaging while effectively preserving organelle ultrastructures. FLEX provides insights into dynamic lysosome-mitochondria interactions upon exposure to chemical stressors. Overall, FLEX holds significant promise as a sensitive and versatile system for fluorescently detecting APEX2-POIs in multiscale biological samples.

The paracaspase MALT1 controls cholesterol homeostasis in glioblastoma stem-like cells through lysosome proteome shaping

Glioblastoma stem-like cells (GSCs) compose a tumor-initiating and -propagating population remarkably vulnerable to variation in the stability and integrity of the lysosomal compartment. Previous work has shown that the expression and activity of the paracaspase MALT1 control GSC viability via lysosome abundance. However, the underlying mechanisms remain elusive. By combining RNA sequencing (RNA-seq) with proteome-wide label-free quantification, we now report that MALT1 repression in patient-derived GSCs alters the homeostasis of cholesterol, which accumulates in late endosomes (LEs)-lysosomes. This failure in cholesterol supply culminates in cell death and autophagy defects, which can be partially reverted by providing exogenous membrane-permeable cholesterol to GSCs. From a molecular standpoint, a targeted lysosome proteome analysis unraveled that Niemann-Pick type C (NPC) lysosomal cholesterol transporters are diluted when MALT1 is impaired. Accordingly, we found that NPC1/2 inhibition and silencing partially mirror MALT1 loss-of-function phenotypes. This supports the notion that GSC fitness relies on lysosomal cholesterol homeostasis.

Tubeimosides are pan-coronavirus and filovirus inhibitors that can block their fusion protein binding to Niemann-Pick C1

SARS-CoV-2 and filovirus enter cells via the cell surface angiotensin-converting enzyme 2 (ACE2) or the late-endosome Niemann-Pick C1 (NPC1) as a receptor. Here, we screened 974 natural compounds and identified Tubeimosides I, II, and III as pan-coronavirus and filovirus entry inhibitors that target NPC1. Using in-silico, biochemical, and genomic approaches, we provide evidence that NPC1 also binds SARS-CoV-2 spike (S) protein on the receptor-binding domain (RBD), which is blocked by Tubeimosides. Importantly, NPC1 strongly promotes productive SARS-CoV-2 entry, which we propose is due to its influence on fusion in late endosomes. The Tubeimosides' antiviral activity and NPC1 function are further confirmed by infection with SARS-CoV-2 variants of concern (VOC), SARS-CoV, and MERS-CoV. Thus, NPC1 is a critical entry co-factor for highly pathogenic human coronaviruses (HCoVs) in the late endosomes, and Tubeimosides hold promise as a new countermeasure for these HCoVs and filoviruses.

Lysosomal BK channels facilitate silica-induced inflammation in macrophages

BACKGROUND

Lysosomal ion channels are proposed therapeutic targets for a number of diseases, including those driven by NLRP3 inflammasome-mediated inflammation. Here, the specific role of the lysosomal big conductance Ca2+-activated K+ (BK) channel was evaluated in a silica model of inflammation in murine macrophages. A specific-inhibitor of BK channel function, paxilline (PAX), and activators NS11021 and NS1619 were utilized to evaluate the role of lysosomal BK channel activity in silica-induced lysosomal membrane permeabilization (LMP) and NLRP3 inflammasome activation resulting in IL-1β release.

METHODS

Murine macrophages were exposed in vitro to crystalline silica following pretreatment with BK channel inhibitors or activators and LMP, cell death, and IL-1β release were assessed. In addition, the effect of PAX treatment on silica-induced cytosolic K+ decrease was measured. Finally, the effects of BK channel modifiers on lysosomal pH, proteolytic activity, and cholesterol transport were also evaluated.

RESULTS

PAX pretreatment significantly attenuated silica-induced cell death and IL-1β release. PAX caused an increase in lysosomal pH and decrease in lysosomal proteolytic activity. PAX also caused a significant accumulation of lysosomal cholesterol. BK channel activators NS11021 and NS1619 increased silica-induced cell death and IL-1β release. BK channel activation also caused a decrease in lysosomal pH and increase in lysosomal proteolytic function as well as a decrease in cholesterol accumulation.

CONCLUSION

Taken together, these results demonstrate that inhibiting lysosomal BK channel activity with PAX effectively reduced silica-induced cell death and IL-1β release. Blocking cytosolic K+ entry into the lysosome prevented LMP through the decrease of lysosomal acidification and proteolytic function and increase in lysosomal cholesterol.

TMEM241 is a UDP-N-acetylglucosamine transporter required for M6P modification of NPC2 and cholesterol transport

Accurate intracellular cholesterol traffic plays crucial roles. Niemann Pick type C (NPC) proteins NPC1 and NPC2, are two lysosomal cholesterol transporters that mediate the cholesterol exit from lysosomes. However, other proteins involved in this process remain poorly defined. Here, we find that the previously unannotated protein TMEM241 is required for cholesterol egressing from lysosomes through amphotericin B-based genome-wide CRISPR-Cas9 KO screening. Ablation of TMEM241 caused impaired sorting of NPC2, a protein utilizes the mannose-6-phosphate (M6P) modification for lysosomal targeting, resulting in cholesterol accumulation in the lysosomes. TMEM241 is a member of solute transporters 35 nucleotide sugar transporters family and localizes on the cis-Golgi network. Our data indicate that TMEM241 transports UDP-N-acetylglucosamine (UDP-GlcNAc) into Golgi lumen and UDP-GlcNAc is used for the M6P modification of proteins including NPC2. Furthermore, Tmem241-deficient mice display cholesterol accumulation in pulmonary cells and behave pulmonary injury and hypokinesia. Taken together, we demonstrate that TMEM241 is a Golgi-localized UDP-GlcNAc transporter and loss of TMEM241 causes cholesterol accumulation in lysosomes because of the impaired M6P-dependent lysosomal targeting of NPC2.

Imipramine Treatment Alters Sphingomyelin, Cholesterol, and Glycerophospholipid Metabolism in Isolated Macrophage Lysosomes

Lysosomes are degradative organelles that facilitate the removal and recycling of potentially cytotoxic materials and mediate a variety of other cellular processes, such as nutrient sensing, intracellular signaling, and lipid metabolism. Due to these central roles, lysosome dysfunction can lead to deleterious outcomes, including the accumulation of cytotoxic material, inflammation, and cell death. We previously reported that cationic amphiphilic drugs, such as imipramine, alter pH and lipid metabolism within macrophage lysosomes. Therefore, the ability for imipramine to induce changes to the lipid content of isolated macrophage lysosomes was investigated, focusing on sphingomyelin, cholesterol, and glycerophospholipid metabolism as these lipid classes have important roles in inflammation and disease. The lysosomes were isolated from control and imipramine-treated macrophages using density gradient ultracentrifugation, and mass spectrometry was used to measure the changes in their lipid composition. An unsupervised hierarchical cluster analysis revealed a clear differentiation between the imipramine-treated and control lysosomes. There was a significant overall increase in the abundance of specific lipids mostly composed of cholesterol esters, sphingomyelins, and phosphatidylcholines, while lysophosphatidylcholines and ceramides were overall decreased. These results support the conclusion that imipramine's ability to change the lysosomal pH inhibits multiple pH-sensitive enzymes in macrophage lysosomes.

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.

Caveolin-3 loss linked with the P104L LGMD-1C mutation modulates skeletal muscle mTORC1 signalling and cholesterol homeostasis

BACKGROUND

Caveolins are the principal structural components of plasma membrane caveolae. Dominant pathogenic mutations in the muscle-specific caveolin-3 (Cav3) gene isoform, such as the limb girdle muscular dystrophy type 1C (LGMD-1C) P104L mutation, result in dramatic loss of the Cav3 protein and pathophysiological muscle weakness/wasting. We hypothesize that such muscle degeneration may be linked to disturbances in signalling events that impact protein turnover. Herein, we report studies assessing the effects of Cav3 deficiency on mammalian or mechanistic target of rapamycin complex 1 (mTORC1) signalling in skeletal muscle cells.

METHODS

L6 myoblasts were stably transfected with Cav3P104L or expression of native Cav3 was abolished by CRISPR/Cas9 genome editing (Cav3 knockout [Cav3KO]) prior to performing subcellular fractionation and immunoblotting, analysis of real-time mitochondrial respiration or fixed cell immunocytochemistry. Skeletal muscle from wild-type and Cav3-/- mice was processed for immunoblot analysis of downstream mTORC1 substrate phosphorylation.

RESULTS

Cav3 was detected in lysosomal-enriched membranes isolated from L6 myoblasts and observed by confocal microscopy to co-localize with lysosomal-specific markers. Cav3P104L expression, which results in significant (~95%) loss of native Cav3, or CRISPR/Cas9-mediated Cav3KO, reduced amino acid-dependent mTORC1 activation. The decline in mTORC1-directed signalling was detected by immunoblot analysis of L6 muscle cells and gastrocnemius Cav3-/- mouse muscle as judged by reduced phosphorylation of mTORC1 substrates that play key roles in the initiation of protein synthesis (4EBP1S65 and S6K1T389 ). S6K1T389 and 4EBP1S65 phosphorylation reduced by over 75% and 80% in Cav3KO muscle cells and by over 90% and 30% in Cav3-/- mouse skeletal muscle, respectively. The reduction in protein synthetic capacity in L6 muscle cells was confirmed by analysis of puromycylated peptides using the SUnSET assay. Cav3 loss was also associated with a 26% increase in lysosomal cholesterol, and pharmacological manipulation of lysosomal cholesterol was effective in replicating the reduction in mTORC1 activity observed in Cav3KO cells. Notably, re-expression of Cav3 in Cav3KO myoblasts normalized lysosomal cholesterol content, which coincided with a recovery in protein translation and an associated increase in mTORC1-directed phosphorylation of downstream targets.

CONCLUSIONS

Our findings indicate that Cav3 can localize on lysosomal membranes and is a novel regulator of mTORC1 signalling in muscle. Cav3 deficiency associated with the Cav3P104L mutation impairs mTORC1 activation and protein synthetic capacity in skeletal muscle cells, which may be linked to disturbances in lysosomal cholesterol trafficking and contribute to the pathology of LGMD-1C.

Cyclodextrin-Mediated Cholesterol Depletion Induces Adiponectin Secretion in 3T3-L1 Adipocytes

Adipocytes store a significant amount of cholesterol and triglycerides. However, whether cholesterol modulates adipocyte function remains largely unknown. We modulated the cholesterol level in adipocytes to examine its effect on the secretion of adiponectin, an important hormone specifically secreted by adipocytes. Treating differentiated 3T3-L1 adipocytes with 4 mM methyl-β-cyclodextrin (MβCD), a molecule with a high affinity for cholesterol, rapidly depleted cholesterol in adipocytes. Interestingly, MβCD treatment increased adiponectin in the medium without affecting its intracellular level, suggesting a modulation of secretion. By contrast, cholesterol addition did not affect adiponectin secretion, suggesting that cholesterol-depletion-induced intracellular cholesterol trafficking, but not reduced cholesterol level, accounted for MβCD-induced adiponectin secretion. MβCD-induced adiponectin secretion was reduced after 10 μg/mL U18666A treatment that suppressed cholesterol transport out of late endosomes/lysosomes. Depleting Niemann-Pick type C1 (NPC1) or NPC2 proteins, which mediate endosomal/lysosomal cholesterol export, consistently reduced MβCD-induced adiponectin secretion. Furthermore, treatment with 1 μM bafilomycin A1, which neutralized acidic endosomes/lysosomes, also attenuated MβCD-induced adiponectin secretion. Finally, MβCD treatment redistributed cellular adiponectin to lower-density fractions in sucrose gradient fractionation. Our results show that MβCD-mediated cholesterol depletion elevates the secretion of adiponectin, highlighting the involvement of endosomes and lysosomes in adiponectin secretion in adipocytes.

Functional significance of cholesterol metabolism in cancer: from threat to treatment

Cholesterol is an essential structural component of membranes that contributes to membrane integrity and fluidity. Cholesterol homeostasis plays a critical role in the maintenance of cellular activities. Recently, increasing evidence has indicated that cholesterol is a major determinant by modulating cell signaling events governing the hallmarks of cancer. Numerous studies have shown the functional significance of cholesterol metabolism in tumorigenesis, cancer progression and metastasis through its regulatory effects on the immune response, ferroptosis, autophagy, cell stemness, and the DNA damage response. Here, we summarize recent literature describing cholesterol metabolism in cancer cells, including the cholesterol metabolism pathways and the mutual regulatory mechanisms involved in cancer progression and cholesterol metabolism. We also discuss various drugs targeting cholesterol metabolism to suggest new strategies for cancer treatment.

NPC1-dependent alterations in KV2.1-CaV1.2 nanodomains drive neuronal death in models of Niemann-Pick Type C disease

Lysosomes communicate through cholesterol transfer at endoplasmic reticulum (ER) contact sites. At these sites, the Niemann Pick C1 cholesterol transporter (NPC1) facilitates the removal of cholesterol from lysosomes, which is then transferred to the ER for distribution to other cell membranes. Mutations in NPC1 result in cholesterol buildup within lysosomes, leading to Niemann-Pick Type C (NPC) disease, a progressive and fatal neurodegenerative disorder. The molecular mechanisms connecting NPC1 loss to NPC-associated neuropathology remain unknown. Here we show both in vitro and in an animal model of NPC disease that the loss of NPC1 function alters the distribution and activity of voltage-gated calcium channels (CaV). Underlying alterations in calcium channel localization and function are KV2.1 channels whose interactions drive calcium channel clustering to enhance calcium entry and fuel neurotoxic elevations in mitochondrial calcium. Targeted disruption of KV2-CaV interactions rescues aberrant CaV1.2 clustering, elevated mitochondrial calcium, and neurotoxicity in vitro. Our findings provide evidence that NPC is a nanostructural ion channel clustering disease, characterized by altered distribution and activity of ion channels at membrane contacts, which contribute to neurodegeneration.

Regulation of cholesterol metabolism during high fatty acid-induced lipid deposition in calf hepatocytes

Cholesterol in the circulation is partly driven by changes in feed intake, but aspects of cholesterol metabolism during development of fatty liver are not well known. The objective of this study was to investigate mechanisms of cholesterol metabolism in calf hepatocytes challenged with high concentrations of fatty acids (FA). To address mechanistic insights regarding cholesterol metabolism, liver samples were collected from healthy control dairy cows (n = 6; 7-13 d in milk) and cows with fatty liver (n = 6; 7-11 d in milk). In vitro, hepatocytes isolated from 3 healthy female calves (1 d old) were challenged with or without a mix of 1.2 mM FA to induce metabolic stress. In addition, hepatocytes were processed with 10 µmol/L of the cholesterol synthesis inhibitor simvastatin or 6 µmol/L of the cholesterol intracellular transport inhibitor U18666A with or without the 1.2 mM FA mix. To evaluate the role of cholesterol addition, hepatocytes were treated with 0.147 mg/mL methyl-β-cyclodextrin (MβCD + FA) or 0.147 mg/mL MβCD with or without 10 and 100 µmol/L cholesterol before incubation with FA (CHO10 + FA and CHO100 + FA). In vivo data from liver biopsies were analyzed by 2-tailed unpaired Student's t-test. Data from in vitro calf hepatocytes were analyzed by one-way ANOVA. Compared with healthy cows, blood plasma total cholesterol and plasma low-density lipoprotein cholesterol content in cows with fatty liver was markedly lower, whereas the hepatic total cholesterol content did not differ. In contrast, compared with healthy controls, the triacylglycerol content in the liver and the content of FA, β-hydroxybutyrate, and aspartate aminotransferase in the plasma of cows with fatty liver were greater. The results revealed that both fatty liver in vivo and challenge of calf hepatocytes with 1.2 mM FA in vitro led to greater mRNA and protein abundance of sterol regulatory element binding transcription factor 1 (SREBF1) and fatty acid synthase (FASN). In contrast, mRNA and protein abundance of sterol regulatory element binding transcription factor 2 (SREBF2), acyl coenzyme A-cholesterol acyltransferase, and ATP-binding cassette subfamily A member 1 (ABCA1) were lower. Compared with the FA group, the cholesterol synthesis inhibitor simvastatin led to greater protein abundance of microsomal triglyceride transfer protein and mRNA abundance of SREBF2, 3-hydroxy-3-methylglutaryl-CoA reductase (HMGCR), ACAT2, and lower ABCA1 and FASN protein abundance. In contrast, compared with the FA group, the cholesterol intracellular transport inhibitor U18666A + FA led to greater total cholesterol concentration and greater protein and mRNA abundance of FASN. Compared with the MβCD + FA group, the addition of 10 µmol/L cholesterol led to greater concentration of cholesteryl ester and excretion of apolipoprotein B100, and greater protein and mRNA abundance of ABCA1 and microsomal triglyceride transfer protein, and lower concentration of malondialdehyde. Overall, a reduction in cholesterol synthesis promoted FA metabolism in hepatocytes likely to relieve the oxidative stress caused by the high FA load. The data suggest that maintenance of normal cholesterol synthesis promotes very low-density lipoprotein excretion and can reduce lipid accumulation and oxidative stress in dairy cows that experience fatty liver.

Cholesterol-dependent molecular mechanisms contribute to cationic amphiphilic drugs' prevention of silica-induced inflammation

Silicosis is considered an irreversible chronic inflammatory disease caused by the inhalation of crystalline silica (cSiO2). The cycle of inflammation that drives silicosis and other particle-caused respiratory diseases is mediated by NLRP3 inflammasome activity in macrophages resulting in the release of IL-1β. Lysosomal membrane permeability (LMP) initiated by inhaled particles is the key regulatory step in leading to NLRP3 activity. In addition to its role in LMP, the lysosome is crucial to cellular cholesterol trafficking. Lysosomal cholesterol has been demonstrated to regulate LMP while cationic amphiphilic drugs (CADs) reduce cholesterol trafficking from lysosomes and promote endolysosomal cholesterol accumulation as seen in Niemann Pick disease. Using a bone marrow derived macrophage (BMdM) model, four CADs were examined for their potential to reduce cSiO2-induced inflammation. Here we found that FDA-approved CAD drugs imipramine, hydroxychloroquine, fluvoxamine, and fluoxetine contributed to reduced LMP and IL-1β release in cSiO2 treated BMdM. These drugs inhibited lysosomal enzymatic activity of acid sphingomyelinase, decreased lysosomal proteolytic function, and increased lysosomal pH. CADs also demonstrated a significant increase in lysosomal-associated free cholesterol. Increased lysosomal cholesterol was associated with a significant reduction in cSiO2 induced LMP and IL-1β release. In contrast, reduced lysosomal cholesterol significantly exacerbated cSiO2-induced IL-1β release and reduced the protective effect of CADs on IL-1β release following cSiO2 exposure. Taken together, these results suggest that CAD modification of lysosomal cholesterol may be used to reduce LMP and cSiO2-induced inflammation and could prove an effective therapeutic for silicosis and other particle-caused respiratory diseases.

Novel Mutation in the Feline NPC2 Gene in Cats with Niemann-Pick Disease

Niemann-Pick disease (NP) type C is an autosomal, recessive, and inherited neurovisceral genetic disorder characterized by the accumulation of unesterified cholesterol and glycolipids in cellular lysosomes and late endosomes, with a wide spectrum of clinical phenotypes. This study aimed to determine the molecular genetic alterations in two cases of felines with NP in Japan, a Siamese cat in 1989 and a Japanese domestic (JD) cat in 1998. Sanger sequencing was performed on 25 exons of the feline NPC1 gene and 4 exons of the feline NPC2 gene, using genomic DNA extracted from paraffin-embedded tissue specimens. The sequenced exons were compared with reference sequences retrieved from the GenBank database. The identified mutations and alterations were then analyzed using different prediction algorithms. No pathogenic mutations were found in feline NPC1; however, c.376G>A (p.V126M) was identified as a pathogenic mutation in the NPC2 gene. The Siamese cat was found to be homozygous for this mutation. The JD cat was heterozygous for the same mutation, but no other exonic NPC2 mutation was found. Furthermore, the JD cat had a homozygous splice variant (c.364-4C>T) in the NPC2 gene, which is not known to be associated with this disease. The NPC2:c.376G>A (p.V126M) mutation is the second reported pathogenic mutation in the feline NPC2 gene that may be present in the Japanese cat population.

Exploring Receptor Binding Affinities and Hepatic Cell Association of N-Acetyl-d-Galactosamine-Modified β-Cyclodextrin-Based Polyrotaxanes for Liver-Targeted Therapies

Acid-degradable polyrotaxanes (PRXs) containing threading β-cyclodextrins (β-CDs) are promising candidates for therapeutic applications of β-CDs in metabolic diseases with cholesterol overload or imbalance. To improve cellular uptake specificity and efficiency of PRXs in hepatocytes, N-acetyl-d-galactosamine (GalNAc)-modified PRXs were developed to facilitate asialoglycoprotein receptor (ASGR)-mediated endocytosis. Binding affinity studies revealed that the dissociation constant (K_D) values between recombinant ASGR and GalNAc-PRXs decreased with an increase in the number of modified GalNAc units. Additionally, the K_D values for GalNAc-PRXs were smaller than those for GalNAc-modified β-CD and amylose, suggesting that the PRX backbone structure improves the binding affinity with ASGR. However, the intracellular uptake levels of GalNAc-PRXs in HepG2 cells increased with a decrease in the number of modified GalNAc units, which was opposite to the trend observed in the binding affinity study. We found that GalNAc-PRXs had a large number of GalNAc units localized in recycling endosomes, resulting in the low intracellular uptake. The cholesterol-reducing abilities of GalNAc-PRXs were assessed using cholesterol-overloaded HepG2 cells. GalNAc-PRXs with a small number of GalNAc units were demonstrated to show superior cholesterol-reducing effects compared to previously designed acid-degradable PRX and clinically tested β-CD derivatives. Thus, we conclude that GalNAc modification is a promising molecular design for the therapeutic application of β-CD-threaded PRXs in various metabolic diseases with cholesterol overload or imbalance in the liver.

Investigation of 2-Hydroxypropyl-β-Cyclodextrin Treatment in a Neuronal-Like Cell Model of Niemann-Pick Type C Using Quantitative Proteomics

Niemann-Pick, type C (NPC) is a fatal, neurovisceral lysosomal storage disorder with progressive neurodegeneration and no FDA-approved therapy. Significant efforts have been focused on the development of therapeutic options, and 2-hydroxypropyl-β-cyclodextrin (HP-b-CD) has emerged as a promising candidate. In cell culture, HP-b-CD ameliorates cholesterol storage in endo/lysosomes, a hallmark of the disorder. Furthermore, in animal studies, treatment with HP-b-CD delays neurodegeneration and extends lifespan. While HP-b-CD has been promising in vitro and in vivo, a clear understanding of the mechanism(s) of action is lacking. Utilizing a neuron-like cell culture model of SH-SY5Y differentiated cells and U18666A to induce the NPC phenotype, we report here a large-scale mass-spectrometry-based proteomic study to evaluate proteome changes upon treatment with these small molecules. In this study, we show that differentiated SH-SY5Y cells display morphological changes representative of neuronal-like cells along with increased levels of proliferation markers. Inhibition of the NPC cholesterol transporter 1 protein by U18666A resulted in increased levels of known NPC markers including SCARB2/LIMP2 and LAMP2. Finally, investigation of HP-b-CD treatment was performed where we observe that, although HP-b-CD reduces cholesterol storage, levels of NPC1 and NPC2 are not normalized to control levels. This finding further supports the need for a proteostasis strategy for NPC drug development. Moreover, proteins that were dysregulated in the U18666A model of NPC and normalized to control levels suggest that HP-b-CD promotes exocytosis in this neuron-like model. Utilizing state of the art mass spectrometry analysis, these data demonstrate newly reported changes with pharmacological perturbations related to NPC disease and provide insight into the mechanisms of HP-b-CD as a potential therapeutic.

Defective iron homeostasis and hematological abnormalities in Niemann-Pick disease type C1

Background: Niemann-Pick disease type C1 (NPC1) is a neurodegenerative lysosomal storage disorder characterized by the accumulation of multiple lipids in the late endosome/lysosomal system and reduced acidic store calcium. The lysosomal system regulates key aspects of iron homeostasis, which prompted us to investigate whether there are hematological abnormalities and iron metabolism defects in NPC1. Methods: Iron-related hematological parameters, systemic and tissue metal ion and relevant hormonal and proteins levels, expression of specific pro-inflammatory mediators and erythrophagocytosis were evaluated in an authentic mouse model and in a large cohort of NPC patients. Results: Significant changes in mean corpuscular volume and corpuscular hemoglobin were detected in Npc1 -/- mice from an early age. Hematocrit, red cell distribution width and hemoglobin changes were observed in late-stage disease animals. Systemic iron deficiency, increased circulating hepcidin, decreased ferritin and abnormal pro-inflammatory cytokine levels were also found. Furthermore, there is evidence of defective erythrophagocytosis in Npc1 -/- mice and in an in vitro NPC1 cellular model. Comparable hematological changes, including low normal serum iron and transferrin saturation and low cerebrospinal fluid ferritin were confirmed in NPC1 patients. Conclusions: These data suggest loss of iron homeostasis and hematological abnormalities in NPC1 may contribute to the pathophysiology of this disease.

Rab7a-mTORC1 signaling-mediated cholesterol trafficking from the lysosome to mitochondria ameliorates hepatic lipotoxicity induced by aflatoxin B1 exposure

Aflatoxin B1 (AFB1) is a common contaminant in many foodstuffs and is considered a public health concern worldwide due to its hepatotoxicity caused by lipid metabolism disorders. However, the molecular mechanism underlying AFB1-induced lipotoxicity-dependent liver injury via regulating cholesterol metabolism remains unclear. We established a cholesterol trafficking disorder-mediated hepatic lipotoxicity model with AFB1 mixture exposure in vitro (HepaRG and HepG2 cells, 1.6 μM for 36 h) and in vivo (C57BL/6 mice, 3 mg kg^-1, i.g., every other day for 6 weeks). In vitro, the interaction between lysosomal Niemann-Pick type C1 (NPC1) protein and mitochondrial translocator protein (TSPO) regulated lipotoxicity induced by AFB1 mixture exposure, including lysosomal membrane permeabilization and mitochondria-dependent necroptosis. Moreover, the downregulation of lysosomal Ras-associated protein 7a (Rab7a) enhanced the mammalian target of rapamycin complex 1 (mTORC1)-mediated disorders of cholesterol trafficking from the lysosome to mitochondria. Furthermore, cholesterol trafficking disorder-mediated hepatic lipotoxicity induced by the low-dose level of AFB1 exposure was relieved by genetic or pharmaceutic activation of Rab7a to inhibit mTORC1 in vitro and ex vivo. In vivo, mTORC1 inhibitor (Torin1, 4 mg kg^-1, i.p., every other day for 3 weeks) alleviated the cholesterol trafficking disorder-mediated hepatic lipotoxicity via upregulating the molecular machinery of lysosomes and mitochondria contact mediated by NPC1 and TSPO interaction in the low dose of AFB1 exposure. Altogether, our data suggested a novel mechanism that lysosomal Rab7a-mTORC1 signaling determined the cholesterol trafficking regulated by NPC1-TSPO from the lysosome to mitochondria, which promoted hepatic lipotoxicity via lysosomal quality control and mitochondria-dependent necroptosis signaling pathways in chemical mixture exposure.

Potential use of the cholesterol transfer inhibitor U18666A as an antiviral drug for research on various viral infections

Cholesterol plays critical functions in arranging the biophysical attributes of proteins and lipids in the plasma membrane. For various viruses, an association with cholesterol for virus entrance and/or morphogenesis has been demonstrated. Therefore, the lipid metabolic pathways and the combination of membranes could be targeted to selectively suppress the virus replication steps as a basis for antiviral treatment. U18666A is a cationic amphiphilic drug (CAD) that affects intracellular transport and cholesterol production. A robust tool for investigating lysosomal cholesterol transfer and Ebola virus infection is an androstenolone derived termed U18666A that suppresses three enzymes in the cholesterol biosynthesis mechanism. In addition, U18666A inhibited low-density lipoprotein (LDL)-induced downregulation of LDL receptor and triggered lysosomal aggregation of cholesterol. According to reports, U18666A inhibits the reproduction of baculoviruses, filoviruses, hepatitis, coronaviruses, pseudorabies, HIV, influenza, and flaviviruses, as well as chikungunya and flaviviruses. U18666A-treated viral infections may act as a novel in vitro model system to elucidate the cholesterol mechanism of several viral infections. In this article, we discuss the mechanism and function of U18666A as a potent tool for studying cholesterol mechanisms in various viral infections.

Acute ACAT1/SOAT1 Blockade Increases MAM Cholesterol and Strengthens ER-Mitochondria Connectivity

Cholesterol is a key component of all mammalian cell membranes. Disruptions in cholesterol metabolism have been observed in the context of various diseases, including neurodegenerative disorders such as Alzheimer's disease (AD). The genetic and pharmacological blockade of acyl-CoA:cholesterol acyltransferase 1/sterol O-acyltransferase 1 (ACAT1/SOAT1), a cholesterol storage enzyme found on the endoplasmic reticulum (ER) and enriched at the mitochondria-associated ER membrane (MAM), has been shown to reduce amyloid pathology and rescue cognitive deficits in mouse models of AD. Additionally, blocking ACAT1/SOAT1 activity stimulates autophagy and lysosomal biogenesis; however, the exact molecular connection between the ACAT1/SOAT1 blockade and these observed benefits remain unknown. Here, using biochemical fractionation techniques, we observe cholesterol accumulation at the MAM which leads to ACAT1/SOAT1 enrichment in this domain. MAM proteomics data suggests that ACAT1/SOAT1 inhibition strengthens the ER-mitochondria connection. Confocal and electron microscopy confirms that ACAT1/SOAT1 inhibition increases the number of ER-mitochondria contact sites and strengthens this connection by shortening the distance between these two organelles. This work demonstrates how directly manipulating local cholesterol levels at the MAM can alter inter-organellar contact sites and suggests that cholesterol buildup at the MAM is the impetus behind the therapeutic benefits of ACAT1/SOAT1 inhibition.