The sterol-sensing domain of the Niemann-Pick C1 (NPC1) protein regulates trafficking of low density lipoprotein cholesterol

The Niemann-Pick C1 (NPC1) protein is a key participant in intracellular sterol trafficking and regulation of cholesterol homeostasis. NPC1 contains a pentahelical region that is evolutionarily related to sterol-sensing domains found in other polytopic proteins involved in sterol interactions or sterol metabolism, including sterol regulatory element-binding protein cleavage-activating protein and hydroxymethylglutaryl-CoA reductase. To gain insight into the role of the sterol-sensing domain of NPC1, we examined the effect of point mutations in the NPC1 sterol-sensing domain on the trafficking of low density lipoprotein-derived cholesterol and sphingolipids. We show that an NPC1 P692S loss of function mutation results in decreased cholesterol delivery to the plasma membrane and endoplasmic reticulum. By contrast, NPC1 proteins carrying a L657F or D787N point mutation, which correspond to the activating SCAP L315F and D443N mutations, respectively, exhibit a gain of function phenotype. Specifically, cell lines expressing the NPC1 L657F or D787N mutations show a nearly 2-fold increase in the rates of low density lipoprotein cholesterol trafficking to the plasma membrane and to the endoplasmic reticulum, and more rapid suppression of sterol regulatory element-binding protein-dependent gene expression. Trafficking of sphingolipids is intact in the D787N and L657F cell lines. Our finding that D787N and L657F are activating NPC1 mutations provide evidence for a conserved mechanism for the sterol-sensing domain among cholesterol homeostatic proteins.     

Cholesterol homeostasis: not until the SCAP lady INSIGs

Three new studies have wide implications for cholesterol homeostasis, identifying a novel mechanism by which a sterol-sensing domain functions in the regulated activation of sterol regulatory element binding proteins.     

The inhibition of degradation of 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase by sterol regulatory element binding protein cleavage-activating protein requires four phenylalanine residues in span 6 of HMG-CoA reductase transmembrane domain

3-hydroxy-3-methylglutaryl coenzyme A reductase (HMGR) is the rate-limiting enzyme in the cholesterol biosynthetic pathway. This endoplasmic reticulum membrane protein contains a cytosolic catalytic domain and a transmembrane domain with eight membrane spans that are necessary for sterol-accelerated degradation. Competition experiments showed that wild-type transmembrane domains of HMGR and sterol regulatory element binding protein cleavage-activating protein (SCAP) blocked sterol-accelerated degradation of intact HMGR and HMGal, a model protein containing the membrane domain of HMGR linked to Escherichia coli beta-galactosidase. However, mutant transmembrane domains of HMGR and SCAP whose sterol-sensing functions were abolished did not inhibit sterol-accelerated degradation of HMGR and HMGal. In addition, our mutagenesis studies on HMGal indicated that four Phe residues conserved in span 6 of HMGR and the sterol-sensing domains of other sterol-related proteins are required for the regulated degradation of HMGR. These results suggest that HMGR and SCAP compete for binding to a sterol-regulated regulator protein, and this binding may need the four Phe residues.     

Accelerated degradation of HMG CoA reductase mediated by binding of insig-1 to its sterol-sensing domain

Sterols accelerate degradation of the ER enzyme 3-hydroxy-3-methylglutaryl CoA reductase (HMG CoA reductase), which catalyzes a rate-controlling step in cholesterol biosynthesis. This degradation contributes to feedback inhibition of synthesis of cholesterol and nonsterol isoprenoids. Here, we show that degradation of HMG CoA reductase is accelerated by the sterol-induced binding of its sterol-sensing domain to the ER protein insig-1. Accelerated degradation is inhibited by overexpression of the sterol-sensing domain of SREBP cleavage-activating protein (SCAP), suggesting that both proteins bind to the same site on insig-1. Whereas insig-1 binding to SCAP leads to ER retention, insig-1 binding to HMG CoA reductase leads to accelerated degradation that is blocked by proteasome inhibitors. Insig-1 appears to play an essential role in the sterol-mediated trafficking of two proteins with sterol-sensing domains, HMG CoA reductase and SCAP.     

The sterol-sensing domain: multiple families, a unique role?

The "sterol-sensing domain" (SSD) is conserved across phyla and is present in several membrane proteins, such as Patched (a Hedgehog receptor) and NPC-1 (the protein defective in Niemann-Pick type C1 disease). The role of the SSD is perhaps best understood from the standpoint of its involvement in cholesterol homeostasis. This article discusses how the SSD appears to function as a regulatory domain involved in linking vesicle trafficking and protein localization with such varied processes as cholesterol homeostasis, cell signalling and cytokinesis.     

Mutations in the leucine zipper motif and sterol-sensing domain inactivate the Niemann-Pick C1 glycoprotein.

Niemann-Pick type C (NPC) disease, characterized by accumulation of low density lipoprotein-derived free cholesterol in lysosomes, is caused by mutations in the NPC1 gene. We examined the ability of wild-type NPC1 and NPC1 mutants to correct the NPC sterol trafficking defect and their subcellular localization in CT60 cells. Cells transfected with wild-type NPC1 expressed 170- and 190-kDa proteins. Tunicamycin treatment resulted in a 140-kDa protein, the deduced size of NPC1, suggesting that NPC1 is N-glycosylated. Mutation of all four asparagines in potential N-terminal N-glycosylation sites to glutamines resulted in a 20-kDa reduction of the expressed protein. Proteins with a single N-glycosylation site mutation localized to late endosome/lysosomal compartments, as did wild-type NPC1, and each corrected the cholesterol trafficking defect. However, mutation of all four potential N-glycosylation sites reduced ability to correct the NPC phenotype commensurate with reduced expression of the protein. Mutations in the putative sterol-sensing domain resulted in inactive proteins targeted to lysosomal membranes encircling cholesterol-laden cores. N-terminal leucine zipper motif mutants could not correct the NPC defect, although they accumulated in lysosomal membranes. We conclude that NPC1 is a glycoprotein that must have an intact sterol-sensing domain and leucine zipper motif for cholesterol-mobilizing activity.     

Mutations of the central tyrosines of putative cholesterol recognition amino acid consensus (CRAC) sequences modify folding,activity, and sterol-sensing of the human ABCG2 multidrug transporter

Human ABCG2 is a plasma membrane glycoprotein causing multidrug resistance in cancer. Membrane cholesterol and bile acids are efficient regulators of ABCG2 function, while the molecular nature of the sterol-sensing sites has not been elucidated. The cholesterol recognition amino acid consensus (CRAC, L/V-(X)_(1–5)-Y-(X)_(1–5)-R/K) sequence is one of the conserved motifs involved in cholesterol binding in several proteins. We have identified five potential CRAC motifs in the transmembrane domain of the human ABCG2 protein. In order to define their roles in sterol-sensing, the central tyrosines of these CRACs (Y413, 459, 469, 570 and 645) were mutated to S or F and the mutants were expressed both in insect and mammalian cells. We found that mutation in Y459 prevented protein expression; the Y469S and Y645S mutants lost their activity; while the Y570S, Y469F, and Y645F mutants retained function as well as cholesterol and bile acid sensitivity. We found that in the case of the Y413S mutant, drug transport was efficient, while modulation of the ATPase activity by cholesterol and bile acids was significantly altered. We suggest that the Y413 residue within a putative CRAC motif has a role in sterol-sensing and the ATPase/drug transport coupling in the ABCG2 multidrug transporter.     

Sterols and intracellular vesicular trafficking: lessons from the study of NPC1

Cholesterol is an important structural component of membranes as well as a precursor for steroid hormone, bile acid and regulatory oxysterol biosynthesis. Recent observations revealed that cholesterol plays an important role in signaling and the regulation of intracellular vesicular trafficking. Studies on Niemann-Pick type C disease, a fatal neuro-visceral cholesterol storage disorder, led to the elucidation of a sterol-modulated vesicular trafficking pathway. Mutations in the NPC1 gene, which cause the majority of cases of Niemann-Pick type C disease, result in the accumulation of free cholesterol in lysosomes and associated defects in glycolipid sorting. NPC1 has a sterol-sensing domain that presumably recognizes free sterols in the protein's environment and participates in the movement of cholesterol out of lysosomes. The compartment containing NPC1 is a subset of late endosomes; it is highly mobile, travels along microtubules, emitting flexible tubules. The movements of this compartment require an intact NPC1 sterol-sensing domain and are dramatically suppressed when free cholesterol accumulates in the late endosomes. Two other proteins involved in sterol trafficking enter into the NPC1 compartment, NPC2 also known as HE1, a secreted sterol-binding glycoprotein, and MLN64, a StAR-related lipid transfer (START) domain protein, which can bind cholesterol and promote its movement from donor to acceptor membranes. Mutations in NPC2 cause a rarer form of Niemann-Pick type C disease, establishing its importance in intracellular sterol movement. NPC2, NPC1 and MLN64 may act in an ordered sequence to sense cholesterol, effect sterol movement, and consequently, influence the process of vesicular trafficking.     

4-Methyl sterols regulate fission yeast SREBP-Scap under low oxygen and cell stress

In fission yeast, orthologs of mammalian SREBP and Scap, called Sre1 and Scp1, monitor oxygen-dependent sterol synthesis as a measure of cellular oxygen supply. Under low oxygen conditions, sterol synthesis is inhibited, and Sre1 cleavage is activated. However, the sterol signal for Sre1 activation is unknown. In this study, we characterized the sterol signal for Sre1 activation using a combination of Sre1 cleavage assays and gas chromatography sterol analysis. We find that Sre1 activation is regulated by levels of the 4-methyl sterols 24-methylene lanosterol and 4,4-dimethylfecosterol under conditions of low oxygen and cell stress. Both increases and decreases in the level of these ergosterol pathway intermediates induce Sre1 proteolysis in a Scp1-dependent manner. The SREBP ortholog in the pathogenic fungus Cryptococcus neoformans is also activated by high levels of 4-methyl sterols, suggesting that this signal for SREBP activation is conserved among unicellular eukaryotes. Finally, we provide evidence that the sterol-sensing domain of Scp1 is important for regulating Sre1 proteolysis. The conserved mutations Y247C, L264F, and D392N in Scp1 that render Scap insensitive to sterols cause constitutive Sre1 activation. These findings indicate that unlike Scap, fission yeast Scp1 responds to 4-methyl sterols and thus shares properties with mammalian HMG-CoA reductase, a sterol-sensing domain protein whose degradation is regulated by the 4-methyl sterol lanosterol.     

Dispatched, a novel sterol-sensing domain protein dedicated to the release of cholesterol-modified hedgehog from signaling cells

Members of the Hedgehog (Hh) family of secreted signaling proteins function as potent short-range organizers in animal development. Their range of action is limited by a C-terminal cholesterol tether and the upregulation of Patched (Ptc) receptor levels. Here we identify a novel segment-polarity gene in Drosophila, dispatched (disp), and demonstrate that its product is required in sending cells for normal Hh function. In the absence of Disp, cholesterol-modified but not cholesterol-free Hh is retained in these cells, indicating that Disp functions to release cholesterol-anchored Hh. Despite their opposite roles, Disp and Ptc share structural homology in the form of a sterol-sensing domain, suggesting that release and sequestration of cholesterol-modified Hh may be based on related molecular pathways.     

Spatial and temporal distribution of Patched-related protein in the Drosophila embryo

Patched-related (Ptr) encodes a protein with 12 potential transmembrane domains and a sterol-sensing domain that is closely related in predicted topology and domain organization to Patched, the canonical receptor of the Hedgehog pathway. Here we describe the production of an antibody specific for Drosophila Ptr and analyse its spatial and temporal distribution in the embryo. We find that at early developmental stages Ptr is predominantly localized at cell periphery but later on it becomes strongly and almost exclusively expressed in hemocytes. Interestingly Ptr null mutant embryos died without hatching. Our findings suggest that Ptr plays an essential function in Drosophila development, perhaps as a new receptor of embryonic hemocytes.     

Direct binding of cholesterol to the purified membrane region of SCAP: mechanism for a sterol-sensing domain

Mammalian cells control their membrane composition by regulating the vesicular transport of membrane-bound sterol regulatory element binding proteins (SREBPs) from endoplasmic reticulum (ER) to Golgi. Transport is blocked by cholesterol, which triggers SCAP, the SREBP escort protein, to bind to Insigs, which are ER retention proteins. The cholesterol trigger mechanism is unknown. Using recombinant SCAP purified in detergent, we show that cholesterol acts by binding with high affinity and specificity to the 767 amino acid octahelical membrane region of SCAP. This octahelical region contains a conserved pentahelical sterol-sensing domain found in six other polytopic membrane proteins. We show that the membrane domain of SCAP is a tetramer and that cholesterol binding is inhibited by cationic amphiphiles, raising the possibility of allosteric regulation by positively charged phospholipids. The current studies show that cells control their cholesterol content through receptor-ligand interactions and not through changes in the physical properties of the membrane.     

CHE-14, a protein with a sterol-sensing domain, is required for apical sorting in C. elegans ectodermal epithelial cells

BACKGROUND: Polarised trafficking of proteins is critical for normal expression of the epithelial phenotype, but its genetic control is not understood. The regulatory gene lin-26 is essential for normal epithelial differentiation in the nematode Caenorhabditis elegans. To identify potential effectors of lin-26, we characterised mutations that result in lin-26-like phenotypes. Here, we report the phenotypic and molecular analysis of one such mutant line, che-14. RESULTS: Mutations in che-14 resulted in several partially penetrant phenotypes affecting the function of most epithelial or epithelial-like cells of the ectoderm, including the hypodermis, excretory canal, vulva, rectum and several support cells. The defects were generally linked to the accumulation of vesicles or amorphous material near the apical surface, suggesting that secretion was defective. The CHE-14 protein showed similarity to proteins containing sterol-sensing domains, including Dispatched, Patched and NPC1. A fusion protein between full-length CHE-14 and the green fluorescent protein became localised to the apical surface of epithelial cells that require che-14 function. Deletions that removed the predicted transmembrane domains or extracellular loops of CHE-14 abolished apical localisation and function of the protein. CONCLUSIONS: We propose that CHE-14 is involved in a novel secretory pathway dedicated to the exocytosis of lipid-modified proteins at the apical surface of certain epithelial cells. Our data raise the possibility that the primordial function of proteins containing a sterol-sensing domain is to control vesicle trafficking: CHE-14 and Dispatched in exocytosis, Patched and NPC1 in endocytosis.     

Targeting of NPC1 to late endosomes involves multiple signals, including one residing within the putative sterol-sensing domain

The NPC1 protein is a multipass transmembrane protein whose deficiency causes the autosomal recessive lipid storage disorder Niemann-Pick type C1. NPC1 localizes predominantly to late endosomes and has a dileucine motif located within a small cytoplasmic tail thought to target the protein to this location. Our data have suggested previously that the protein can reach its correct location in the absence of its cytoplasmic tail, suggesting that other signals contribute to NPC1 targeting. By using various FLAG-tagged and CD32-NPC1 chimeric fusion constructs, we show that multiple signals are responsible for the trafficking of NPC1 to the endosomal compartment, including the dileucine motif and a previously unidentified signal residing within the putative sterol-sensing domain transmembrane domain 3. Neither region alone was capable of directing heterologous CD32 fusions to late endosomes exclusively via the trans-Golgi network to the late endosome route taken by wild-type NPC1; transmembrane domain 3 was unable to maintain CD32 in late endosomes, indicating that two or more signals work in concert to target and retain NPC1 in this compartment. In addition we confirm that the tail dileucine motif is not essential for NPC1 targeting to late endosomes, and we discuss the implications of this finding along with the previously unappreciated role for transmembrane domain 3 in NPC1 localization and function.     

Cholesterol addition to ER membranes alters conformation of SCAP,the SREBP escort protein that regulates cholesterol metabolism

Sterol accumulation in membranes blocks the exit of SCAP from the ER, preventing SREBP cleavage and reducing cholesterol synthesis. Sterols act through SCAP's sterol-sensing domain by an obscure mechanism. Here, we show that addition of cholesterol to ER membranes in vitro causes a conformational change in SCAP, detected by the unmasking of closely spaced trypsin cleavage sites. Two mutant forms of SCAP (Y298C and D443N) that are refractory to sterol regulation in vivo are also refractory to sterol-induced conformational change in vitro. 25-hydroxycholesterol, a potent regulator of SCAP in vivo, fails to change SCAP's conformation in vitro, suggesting that oxysterols act in intact cells by translocating cholesterol from plasma membrane to ER. These studies demonstrate an in vitro effect of cholesterol on the sterol regulatory machinery.     

SCAP ligands are potent new lipid-lowering drugs.

Upregulation of low-density lipoprotein receptor (LDLr) is a key mechanism to control elevated plasma LDL-cholesterol levels. Here we identify a new class of compounds that directly binds to the sterol regulatory element-binding protein (SREBP) cleavage-activating protein (SCAP). We show that a 14C-labeled, photo-activatable analog specifically labeled both SCAP and a truncated form of SCAP containing the sterol-sensing domain. When administered to hyperlipidemic hamsters, SCAP ligands reduced both LDL cholesterol and triglycerides levels by up to 80% with a three-fold increase in LDLr mRNA in the livers. Using human hepatoma cells, we show that these compounds act through the sterol-responsive element of the LDLr promoter and activate the SCAP/SREBP pathway, leading to increased LDLr expression and activity, even in presence of excess of sterols. These findings have led to the identification of a class of compounds that represent a promising new class of hypolipidemic drugs.