Cholesterol depletion facilitates ubiquitylation of NPC1 and its association with SKD1/Vps4

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.

Targeting fungal membrane homeostasis with imidazopyrazoindoles impairs azole resistance and biofilm formation

Fungal infections cause more than 1.5 million deaths annually. With an increase in immune-deficient susceptible populations and the emergence of antifungal drug resistance, there is an urgent need for novel strategies to combat these life-threatening infections. Here, we use a combinatorial screening approach to identify an imidazopyrazoindole, NPD827, that synergizes with fluconazole against azole-sensitive and -resistant isolates of Candida albicans. NPD827 interacts with sterols, resulting in profound effects on fungal membrane homeostasis and induction of membrane-associated stress responses. The compound impairs virulence in a Caenorhabditis elegans model of candidiasis, blocks C. albicans filamentation in vitro, and prevents biofilm formation in a rat model of catheter infection by C. albicans. Collectively, this work identifies an imidazopyrazoindole scaffold with a non-protein-targeted mode of action that re-sensitizes the leading human fungal pathogen, C. albicans, to azole antifungals.

Cholesterol restricts lymphotoxin β receptor-triggered NF-κB signaling

BACKGROUND

Lymphotoxin β receptor (LTβR) plays important roles in the development of the immune system and immune response. At the cellular level, ligand-bound LTβR activates the pro-inflammatory NF-κB pathway but the detailed mechanisms regulating its signaling remain unknown. Understanding them is of high importance since LTβR and its ligands are promising therapeutic targets. Here, we studied the consequences of perturbed cellular cholesterol content on LTβR-induced NF-κB signaling.

METHODS

To modulate cholesterol availability and/or level in lung carcinoma A549 and H2228, and endothelial HUVEC cells different treatment regimens with filipin, methyl-β-cyclodextrin and simvastatin were applied. LTβR localization was studied by confocal microscopy. The activity of LTβR-induced NF-κB pathway was assessed by measuring the levels of NF-κB pathway inhibitor IκBα and phosphorylation of RelA transcription factor by Western blotting. The NF-κB transcriptional response, production of chemokines and adhesion molecules were examined by qRT-PCR, ELISA, and Western blotting, respectively. Adherence of different types of primary immune cells to epithelial A549 cells and endothelial HUVECs was measured fluorometrically. Interactions of LTβR with its protein partners were investigated by immunoprecipitation.

RESULTS

We showed that filipin-mediated sequestration of cholesterol or its depletion from the plasma membrane with methyl-β-cyclodextrin impaired LTβR internalization and potentiated LTβR-dependent activation of the canonical branch of the NF-κB pathway. The latter was manifested by enhanced degradation of IκBα inhibitor, elevated RelA phosphorylation, substantial increase in the expression of NF-κB target genes encoding, among others, cytokines and adhesion molecules known to play important roles in immune response. It was followed by robust secretion of CXCL8 and upregulation of ICAM1, that favored the adhesion of immune cells (NK and T cells, neutrophils) to A549 cells and HUVECs. Mechanistically, we showed that cholesterol depletion stabilized interactions of ligand-stimulated LTβR with modified forms of TRAF2 and NEMO proteins.

CONCLUSIONS

Our results showed that the reduction of the plasma membrane content of cholesterol or its sequestration strongly potentiated signaling outcome initiated by LTβR. Thus, drugs modulating cholesterol levels could potentially improve efficacy of LTβR-based therapies. Video abstract.

The complexity of a monogenic neurodegenerative disease: More than two decades of therapeutic driven research into Niemann-Pick type C disease

Niemann-Pick type C (NP-C) disease is a rare and fatal neurodegenerative disease typified by aberrations in intracellular lipid transport. Cholesterol and other lipids accumulate in the late endosome/lysosome of all diseased cells thereby causing neuronal and visceral atrophy. A cure for NP-C remains elusive despite the extensive molecular advances emanating from the identification of the primary genetic defect in 1997. Penetration of the blood-brain barrier and efficacy in the viscera are prerequisites for effective therapy, however the rarity of NP-C disease is the major impediment to progress. Disease diagnosis is challenging and establishment of appropriate test populations for clinical trials difficult. Fortunately, disease models that span the diversity of microbial and metazoan life have been utilized to advance the quest for a therapy. The complexity of lipid storage in this disorder and in the model systems, has led to multiple theories on the primary disease mechanism and consequently numerous and varied proposed interventions. Here, we conduct an evaluation of these studies.

Lysosome and endoplasmic reticulum quality control pathways in Niemann-Pick type C disease

Lysosomal storage diseases result from inherited deficiencies of lysosomal hydrolytic activities or lipid transport. Collectively, these disorders are a common cause of morbidity in the pediatric population and are often associated with severe neurodegeneration. Among this group of diseases is Niemann-Pick type C, an autosomal recessive disorder of lipid trafficking that causes cognitive impairment, ataxia and death, most often in childhood. Here, we review the current knowledge of disease pathogenesis, with particular focus on insights gleaned from genetics and the study of model systems. Critical advances in understanding mechanisms that regulate intracellular cholesterol trafficking have emerged from this work and are highlighted. We review effects of disease-causing mutations on quality control pathways involving the lysosome and endoplasmic reticulum, and discuss how they function to clear the most common mutant protein found in Niemann-Pick type C patients, NPC1-I1061T. Finally, we summarize insights into the mechanisms that degrade misfolded transmembrane proteins in the endoplasmic reticulum and how manipulating these quality control pathways may lead to the identification of novel targets for disease-modifying therapies. This article is part of a Special Issue entitled SI:Autophagy.

[Analysis of differentially expressed proteome in urine from non-small cell lung cancer patients]

背景与目的

筛查非小细胞肺癌（non-small cell lung cancer, NSCLC）患者尿液中差异表达蛋白，确定可用于NSCLC早期诊断、监测预后和治疗评估的生物标记物。

方法

分别收集40例已病理证实初诊NSCLC患者、8例肺部良性疾病患者和22例健康志愿者的尿液样本。利用0.9%一维十二烷基硫酸钠-聚丙烯酰胺凝胶电泳（sodium dode-cyl sulfate polyacrylamide gel electrophoresis, 1D SDS-PAGE）技术和MS-Thermo-Orbitrap-Velos质谱分析仪对NSCLC组和非肿瘤组尿液中蛋白质进行分离、提取及识别，鉴定出NSCLC患者尿液中的差异表达蛋白。应用SPSS 20.0软件中受试者工作特征曲线（receiver operating characteristic curve, ROC）分别对其敏感性、特异性进行分析，并进行实验验证，从而确定出与NSCLC相关的生物标记物。

结果

NSCLC患者组和非肿瘤组尿液差异性表达蛋白质集中表现在90 kDa、60 kDa和20 kDa-30 kDa凝胶条带中。在NSCLC患者尿液蛋白分析中发现了4种与NSCLC相关的差异表达蛋白，包括上调蛋白LRG1、CA1和下调蛋白VPS4B、YWHAZ。这4种差异表达蛋白作为独立的NSCLC生物标记物其敏感性较低：LRG1蛋白敏感性83.0%（25/30）、特异性90.0%（18/20）；CA1蛋白敏感性60.0%（18/30）、特异性90.0%（18/20)；VPS4B蛋白敏感性73.3%（22/30）、特异性90.0%（18/20）；YWHAZ蛋白敏感性60.0%（18/30）、特异性95.0%（19/20）。而采用蛋白质组合模式对NSCLC进行筛查、诊断，则其敏感性和特异性分别可高达96.7%（29/30）和85%（17/20）。

结论

LRG1、CA1蛋白在NSCLC患者尿液中高表达，而VPS4B、YWHAZ蛋白呈低表达，差异表达蛋白均提示有可能成为用于NSCLC早期筛查、监测预后和治疗评估的生物标记物。LRG1、CA1、VPS4B和YWHAZ尿液蛋白作为单一生物标记物应用于NSCLC筛查和诊断的敏感性较低，而采用蛋白质组合模式明显优于独立模式对NSCLC的筛查和诊断，故蛋白质组合模式在临床诊疗中将更具有良好应用价值和前景。

Endoplasmic reticulum-associated degradation of Niemann-Pick C1: evidence for the role of heat shock proteins and identification of lysine residues that accept ubiquitin

Most cases with Niemann-Pick disease type C carry mutations in NPC1. Some of the mutations, including the most frequent I1061T, give rise to unstable proteins selected for endoplasmic reticulum-associated degradation. The purpose of the current study was to shed mechanistic insights into the degradation process. A proteasome inhibitor MG132 prolonged the life span of the wild-type NPC1 expressed in COS cells. The expressed protein associated with multiple chaperones including heat shock protein 90 (Hsp90), Hsp70, heat shock cognate protein 70 (Hsc70), and calnexin. Accordingly, expression of an E3 ligase CHIP (carboxyl terminus of Hsp70-interacting protein) enhanced MG132-induced accumulation of ubiquitylated NPC1. Co-expression and RNAi knockdown experiments in HEK cells indicated that Hsp70/Hsp90 stabilized NPC1, whereas Hsc70 destabilized it. In human fibroblasts carrying the I1061T mutation, adenovirus-mediated expression of Hsp70 or treatment with an HSP-inducer geranylgeranylacetone (GGA) increased the level of the mutant protein. In GGA-treated cells, the rescued protein was localized in the late endosome and ameliorated cholesterol accumulation. MALDI-TOF mass spectrometry revealed three lysine residues at amino acids 318, 792, and 1180 as potential ubiquitin-conjugation sites. Substitutions of the three residues with alanine yielded a mutant protein with a steady-state level more than three times higher than that of the wild-type. Introduction of the same substitutions to the I1061T mutant resulted in an increase in its protein level and functional restoration. These findings indicated the role of HSPs in quality control of NPC1 and revealed the role of three lysine residues as ubiquitin-conjugation sites.

Endosomal cholesterol trafficking: protein factors at a glance

The delivery of low-density lipoprotein-derived cholesterol (LDL-C) from endosomal compartments to the plasma membrane and the endoplasmic reticulum (ER) is an important yet poorly understood cellular process. Niemann-Pick C1 (NPC1), a multi-pass integral membrane protein on the limiting membranes of late endosomes (LE)/lysosomes (Ly), is known to insert lumenal LDL-C to the limiting membrane of LE/Ly. Recent progress has identified novel cytoplasmic proteins that regulate the exit of LDL-C from LE/Ly, such as ORP5, a member of the oxysterol-binding protein-related protein (ORPs) family, and Hrs/VPS27, a well-established regulator of the endosomal sorting complex required for transport pathway. Whereas ORP5/ORPs may serve as cytosolic cholesterol carriers and deliver cholesterol in a non-vesicular manner, how Hrs/VPS27 regulate endosomal cholesterol sorting remains enigmatic. We discuss the functional relationship between NPC1, Hrs, and ORP5, and formulate possible schemes on how LDL-C may be moved from endosomal compartments to other cellular organelles.

The Niemann-Pick C1 gene is downregulated in livers of C57BL/6J mice by dietary fatty acids, but not dietary cholesterol, through feedback inhibition of the SREBP pathway

The Niemann-Pick C1 (NPC1) gene is associated with human obesity. Mouse models with decreased Npc1 gene dosage are susceptible to weight gain when fed a high-fat diet, but not a low-fat diet, consistent with an Npc1 gene-diet interaction. The objectives of this study were to define regulation of the Npc1 gene and to investigate the Npc1 gene-diet interaction responsible for weight gain. The experimental design involved feeding C57BL/6J male mice a low-fat diet (with 0.00, 0.10, or 1.00% cholesterol) or a high-fat diet (with 0.02% cholesterol) until 30 wk to determine regulation of the Npc1 gene in liver. The key results showed that the Npc1 gene was downregulated by dietary fatty acids (54%, P = 0.022), but not by dietary cholesterol, through feedback inhibition of the sterol regulatory element-binding protein (SREBP) pathway. However, the dietary fatty acids secondarily increased liver cholesterol, which also inhibits the SREBP pathway. Similarly, the Npc1 gene was downregulated in peritoneal fibroblasts isolated from C57BL/6J weanling male mice not exposed to the experimental diets and incubated in media supplemented with purified oleic acid (37%, P = 0.038) but not in media supplemented with purified cholesterol. These results are important because they suggest a novel mechanism for the interaction of fatty acids with the Npc1 gene to influence energy balance and to promote weight gain. Moreover, the responsiveness of the Npc1 gene to fatty acids is consistent with studies that suggest that the encoded NPC1 protein has a physiologic role in regulating both cholesterol and fatty acid metabolism.

Quantitative proteomic analysis of Niemann-Pick disease, type C1 cerebellum identifies protein biomarkers and provides pathological insight

Niemann-Pick disease, type C1 (NPC1) is a fatal, neurodegenerative disorder for which there is no definitive therapy. In NPC1, a pathological cascade including neuroinflammation, oxidative stress and neuronal apoptosis likely contribute to the clinical phenotype. While the genetic cause of NPC1 is known, we sought to gain a further understanding into the pathophysiology by identifying differentially expressed proteins in Npc1 mutant mouse cerebella. Using two-dimensional gel electrophoresis and mass spectrometry, 77 differentially expressed proteins were identified in Npc1 mutant mice cerebella compared to controls. These include proteins involved in glucose metabolism, detoxification/oxidative stress and Alzheimer disease-related proteins. Furthermore, members of the fatty acid binding protein family, including FABP3, FABP5 and FABP7, were found to have altered expression in the Npc1 mutant cerebellum relative to control. Translating our findings from the murine model to patients, we confirm altered expression of glutathione s-transferase α, superoxide dismutase, and FABP3 in cerebrospinal fluid of NPC1 patients relative to pediatric controls. A subset of NPC1 patients on miglustat, a glycosphingolipid synthesis inhibitor, showed significantly decreased levels of FABP3 compared to patients not on miglustat therapy. This study provides an initial report of dysregulated proteins in NPC1 which will assist with further investigation of NPC1 pathology and facilitate implementation of therapeutic trials.

A role for oxysterol-binding protein-related protein 5 in endosomal cholesterol trafficking

ORP5 works together with Niemann Pick C-1 to facilitate exit of cholesterol from endosomes and lysosomes.

Oxysterol-binding protein (OSBP) and its related proteins (ORPs) constitute a large and evolutionarily conserved family of lipid-binding proteins that target organelle membranes to mediate sterol signaling and/or transport. Here we characterize ORP5, a tail-anchored ORP protein that localizes to the endoplasmic reticulum. Knocking down ORP5 causes cholesterol accumulation in late endosomes and lysosomes, which is reminiscent of the cholesterol trafficking defect in Niemann Pick C (NPC) fibroblasts. Cholesterol appears to accumulate in the limiting membranes of endosomal compartments in ORP5-depleted cells, whereas depletion of NPC1 or both ORP5 and NPC1 results in luminal accumulation of cholesterol. Moreover, trans-Golgi resident proteins mislocalize to endosomal compartments upon ORP5 depletion, which depends on a functional NPC1. Our results establish the first link between NPC1 and a cytoplasmic sterol carrier, and suggest that ORP5 may cooperate with NPC1 to mediate the exit of cholesterol from endosomes/lysosomes.

Cholesterol in Niemann-Pick Type C disease

Niemann-Pick Type C (NPC) disease is associated with accumulation of cholesterol and other lipids in late endosomes/lysosomes in virtually every organ; however, neurodegeneration represents the fatal cause for the disease. Genetic analysis has identified loss-of-function mutations in NPC1 and NPC2 genes as the molecular triggers for the disease. Although the precise function of these proteins has not yet been clarified, recent research suggests that they orchestrate cholesterol efflux from late endosomes/lysosomes. NPC protein deficits result in impairment in intracellular cholesterol trafficking and dysregulation of cholesterol biosynthesis. Disruption of cholesterol homeostasis is also associated with deregulation of autophagic activity and early-onset neuroinflammation, which may contribute to the pathogenesis of NPC disease. This chapter reviews recent achievements in the investigation of disruption of cholesterol homeostasis-induced neurodegeneration in NPC disease, and provides new insight for developing a potential therapeutic strategy for this disorder.

Spreading the wealth: Niemann-Pick type C proteins bind and transport cholesterol

Endocytosed cholesterol must be transferred from the environment (e.g., low-density lipoproteins) via the lysosomal system to the rest of the cell. In Niemann-Pick type C disease, this process fails. In a recent issue of Cell, Kwon et al. (2009) suggest how this transpires mechanistically by crystallizing a domain of a protein defective in this syndrome.

Niemann-Pick C2 (NPC2) and intracellular cholesterol trafficking

Cholesterol is an important precursor for numerous biologically active molecules, and it plays a major role in membrane structure and function. Cholesterol can be endogenously synthesized or exogenously taken up via the endocytic vesicle system and subsequently delivered to post-endo/lysosomal sites including the plasma membrane and the endoplasmic reticulum. Niemann-Pick C (NPC) disease results in the accumulation of exogenously-derived cholesterol, as well as other lipids, in late endosomes and lysosomes (LE/LY). Identification of the two genes that underlie NPC disease, NPC1 and NPC2, has focused attention on the mechanisms by which lipids, in particular cholesterol, are transported out of the LE/LY compartment. This review discusses the role of the NPC2 protein in cholesterol transport, and the potential for concerted action of NPC1 and NPC2 in regulating normal intracellular cholesterol homeostasis.

Liver X receptor agonist T0901317 reduces atherosclerotic lesions in apoE-/- mice by up-regulating NPC1 expression

In this study, we studied the effect of liver X receptor (LXR) agonist T0901317 on Niemann-Pick C1 protein (NPC1) expression in apoE-/- mice. Male apoE-/- mice were randomized into 4 groups, baseline group (n=10), control group (n = 14), treatment group (n = 14) and prevention group (n = 14). All of the mice were fed with a high-fat/high-cholesterol (HFHC) diet containing 15% fat and 0.25% cholesterol. The baseline group treated with vehicle was sacrificed after 8 weeks of the diet. The control group and the prevention group were treated with either vehicle or T0901317 daily by oral gavage for 14 weeks. The treatment group was treated with vehicle for 8 weeks, and then was treated with the agonist T0901317 for additional 6 weeks. Gene and protein expression was analyzed by real-time quantitative PCR, immunohistochemistry and Western blotting, respectively. Plasma lipid concentrations were measured by commercially enzymatic methods. We used RNA interference technology to silence NPC1 gene expression in THP-1 macrophage-derived foam cells and then detected the effect of LXR agonist T0901317 on cholesterol efflux. Plasma triglyceride (TG), total cholesterol (TC), high density lipoprotein cholesterol (HDL-C) and apoA-I concentrations were markedly increased in T0901317-treated groups. T0901317 treatment reduced the aortic atherosclerotic lesion area by 64.2% in the prevention group and 58.3% in the treatment group. LXR agonist treatment increased NPC1 mRNA expression and protein levels in the small intestine, liver and aorta of apoE-/- mice. Compared with the normal cells, cholesterol efflux of siRNA THP-1 macrophage-derived foam cells was significantly decreased, whereas cholesterol efflux of LXR agonist T0901317-treated THP-1 macrophage-derived foam cells was significantly increased. Our results suggest that LXR agonist T0901317 inhibits atherosclerosis development in apoE-/- mice, which is related to up-regulating NPC1 expression.

The Niemann-Pick C1 gene is downregulated by feedback inhibition of the SREBP pathway in human fibroblasts

The Niemann-Pick C1 (NPC1) protein regulates the transport of cholesterol from late endosomes/lysosomes to other compartments responsible for maintaining intracellular cholesterol homeostasis. The present study examined the expression of the NPC1 gene and the distribution of the NPC1 protein that resulted from the transport of LDL-derived cholesterol through normal human fibroblasts. A key finding was that the transport of cholesterol from late endosomes/lysosomes to the sterol-regulatory pool at the endoplasmic reticulum, as determined by feedback inhibition of the sterol-regulatory element binding protein (SREBP) pathway, was associated with the downregulation of the NPC1 gene. Consistent with these results, fibroblasts incubated with LDL had decreased amounts of SREBP protein that interacted with sterol-regulatory element (SRE) sequences positioned within the NPC1 gene promoter region. Finally, partial colocalization of the NPC1 protein with late endosomes/lysosomes and distinct regions of the endoplasmic reticulum suggested that the NPC1 protein may facilitate the transport of cholesterol directly between these two compartments. Together, these results indicate that the transport of LDL-derived cholesterol from late endosomes/lysosomes to the sterol-regulatory pool, known to be regulated by the NPC1 protein, is responsible for promoting feedback inhibition of the SREBP pathway and downregulation of the NPC1 gene.

Lipid rafts: dream or reality for cholesterol transporters?

As a key constituent of the cell membranes, cholesterol is an endogenous component of mammalian cells of primary importance, and is thus subjected to highly regulated homeostasis at the cellular level as well as at the level of the whole body. This regulation requires adapted mechanisms favoring the handling of cholesterol in aqueous compartments, as well as its transfer into or out of membranes, involving membrane proteins. A membrane exhibits functional properties largely depending on its lipid composition and on its structural organization, which very often involves cholesterol-rich microdomains. Then there is the appealing possibility that cholesterol may regulate its own transmembrane transport at a purely functional level, independently of any transcriptional regulation based on cholesterol-sensitive nuclear factors controling the expression level of lipid transport proteins. Indeed, the main cholesterol "transporters" presently believed to mediate for instance the intestinal absorption of cholesterol, that are SR-BI, NPC1L1, ABCA1, ABCG1, ABCG5/G8 and even P-glycoprotein, all present privileged functional relationships with membrane cholesterol-containing microdomains. In particular, they all more or less clearly induce membrane disorganization, supposed to facilitate cholesterol exchanges with the close aqueous medium. The actual lipid substrates handled by these transporters are not yet unambiguously determined, but they likely concern the components of membrane microdomains. Conversely, raft alterations may provide specific modulations of the transporter activities, as well as they can induce indirect effects via local perturbations of the membrane. Finally, these cholesterol transporters undergo regulated intracellular trafficking, with presumably some relationships to rafts which remain to be clarified.

Sterol transport in yeast and the oxysterol binding protein homologue (OSH) family

Sterols such as cholesterol are a significant component of eukaryotic cellular membranes, and their unique physical properties influence a wide variety of membrane processes. It is known that the concentration of sterol within the membrane varies widely between organelles, and that the cell actively maintains this distribution through various transport processes. Vesicular pathways such as secretion or endocytosis may account for this traffic, but increasing evidence highlights the importance of nonvesicular routes as well. The structure of an oxysterol-binding protein homologue (OSH) in yeast (Osh4p/Kes1p) has recently been solved, identifying it as a sterol binding protein, and there is evidence consistent with the role of a cytoplasmic, nonvesicular sterol transporter. Yeast have seven such proteins, which appear to have distinct but overlapping functions with regard to maintaining intracellular sterol distribution and homeostasis. Control of sterol distribution can have far-reaching effects on membrane-related functions, and Osh proteins have been implicated in a variety of processes such as secretory vesicle budding from the Golgi and establishment of cell polarity. This review summarizes the current body of knowledge regarding this family and its potential functions, placing it in the context of known and hypothesized pathways of sterol transport in yeast.

Misassembled mutant DeltaF508 CFTR in the distal secretory pathway alters cellular lipid trafficking

Most patients with cystic fibrosis (CF) have a single codon deletion (DeltaF508) in the gene encoding the cystic fibrosis transmembrane conductance regulator (CFTR) that impairs assembly of the multidomain glycoprotein. The mutant protein escapes endoplasmic reticulum (ER) quality control at low temperature, but is rapidly cleared from the distal secretory pathway and degraded in lysosomes. CF cells accumulate free cholesterol similar to Niemann-Pick disease type C cells. We show that this lipid alteration is caused by the presence of misassembled mutant CFTR proteins, including DeltaF508, in the distal secretory pathway rather than the absence of functional CFTR. By contrast, cholesterol distribution is not changed by either D572N CFTR, which does not mature even at low temperature, or G551D, which is processed normally but is inactive. On expression of the DeltaF508 mutant, cholesterol and glycosphingolipids accumulate in punctate endosomal structures and cholesterol esters are reduced, indicating a block in the translocation of cholesterol to the ER for esterification. This is overcome by Rab9 overexpression, resulting in clearance of accumulating intracellular cholesterol. Similar but less pronounced alterations in intracellular cholesterol distribution are observed on expression of a temperature-rescued mutant variant of the related ATP-binding cassette (ABC) protein multidrug resistance-associated protein 1 (MRP1). Thus, on escape from ER quality control, misassembled mutants of CFTR and MRP1 impair lipid homeostasis in endocytic compartments.