Niemann-Pick Type C Disease and Intracellular Cholesterol Trafficking

Niemann-Pick type C (NPC) 1The abbreviations used are: NPC, Niemann-Pick type C; SSD, sterol-sensing domain; TMD, transmembrane domain; LDL, low density lipoprotein; ER, endoplasmic reticulum; PM, plasma membrane; NB-DNJ, N-butyl deoxynojirimycin; PND, postnatal day. disease is a rare neurovisceral disorder characterized by progressive hepatosplenomegaly and central nervous system neurodegeneration (reviewed in Ref. 1Patterson M.C. Vanier M.T. Suzuki K. Morris J.A. Carstea E. Neufeld E.B. Blanchette-Mackie J.E. Pentchev P.G. Scriver C.R. Beaudet A.L. Sly W.S. Valle D. 8th Ed. The Metabolic and Molecular Bases of Inherited Disease. 3. McGraw-Hill, New York2001: 3611-3633Google Scholar). The estimated prevalence is 1:150,000 individuals. The disease involves the accumulation of unesterified cholesterol, sphingolipids, and other lipids within cells of the endosomal/lysosomal system, various tissues, and the brain. The disease is autosomal recessive and is caused by mutations in one of two genetic loci, npc1 and npc2. Mutations in npc1 account for 95% of NPC cases. Affected individuals usually die before adulthood. Currently there is no cure; however, new biochemical insight has provided clues to how to slow the disease. The human NPC1 encodes a 1278-amino acid (170–190 kDa) glycoprotein with 13 putative transmembrane domains, including a conserved “sterol-sensing domain” (SSD) located between the third and seventh transmembrane domains. SSDs consist of ∼180 amino acids organized in five consecutive transmembrane domains. The SSD is found in several other polytopic membrane proteins that are involved in cellular cholesterol homeostasis (2Radhakrishnan A. Sun L.P. Kwon H.J. Brown M.S. Goldstein J.L. Mol. Cell. 2004; 15: 259-268Abstract Full Text Full Text PDF PubMed Scopus (262) Google Scholar), cell-cell signaling (3Kuwabara P.E. Labouesse M. Trends Genet. 2002; 18: 193-201Abstract Full Text Full Text PDF PubMed Scopus (223) Google Scholar), and the dietary uptake of cholesterol (4Altmann S.W. Davis H.R.J. Zhu L.J. Yao X. Hoos L.M. Tetzloff G. Iyer S.P. Maguire M. Golovko A. Zeng M. Wang L. Murgolo N. Graziano M.P. Science. 2004; 303: 1201-1204Crossref PubMed Scopus (1440) Google Scholar). SSDs are needed for NPC1 protein to function in intact cells (5Ko D.C. Gordon M.D. Jin J.Y. Scott M.P. Mol. Biol. Cell. 2001; 12: 601-614Crossref PubMed Scopus (216) Google Scholar). Binding occurs between NPC1 and a photoactivable analog of cholesterol (azocholestanol); the binding is partially blocked by cholesterol and is much diminished in NPC1 proteins that contain mutations within the SSD (6Ohgami N. Ko D.C. Thomas M. Scott M.P. Chang C.C. Chang T.Y. Proc. Natl. Acad. Sci. U. S. A. 2004; 101: 12473-12478Crossref PubMed Scopus (171) Google Scholar). Thus, one function of the SSD in NPC1 protein is to mediate sterol binding. NPC1 may work as a lipid permease (7Davies J.P. Chen F.W. Ioannou Y.A. Science. 2000; 290: 2295-2298Crossref PubMed Scopus (260) Google Scholar); however, the substrate specificity and the role of the SSD in mediating permease activity have not yet been determined. In addition to the SSD, a cysteine-rich luminal loop between TMD 8 and 9 (8Watari H. Blanchette-Mackie E.J. Dwyer N.K. Watari M. Burd C.G. Patel S. Pentchev P.G. Strauss III, J.F. Exp. Cell Res. 2000; 259: 247-256Crossref PubMed Scopus (43) Google Scholar) and the region between amino acids 1038 and 1253 are also important for NPC1 function (9Park W.D. O'Brien J.F. Lundquist P.A. Kraft D.L. Vockley C.W. Karnes P.S. Patterson M.C. Snow K. Hum. Mutat. 2003; 22: 313-325Crossref PubMed Scopus (157) Google Scholar). NPC1 protein is predominantly located within the late endosomal membrane but is also transiently associated with lysosomes and the trans-Golgi network (10Zhang M. Dwyer N.K. Neufeld E.B. Love D.C. Cooney A. Comly M. Patel S. Watari H. Strauss III, J.F. Pentchev P.G. Hanover J.A. Blanchette-Mackie E.J. J. Biol. Chem. 2001; 276: 3417-3425Abstract Full Text Full Text PDF PubMed Scopus (97) Google Scholar). Multiple peptide sequences within the protein are responsible for targeting to the endosomal compartment (11Scott C. Higgins M.E. Davies J.P. Ioannou Y.A. J. Biol. Chem. 2004; 279: 48214-48223Abstract Full Text Full Text PDF PubMed Scopus (32) Google Scholar). Late endosomes are comprised of limiting membranes and internal membranes (12Kobayashi T. Beuchat M.-H. Chevallier J. Makino A. Mayran N. Escola J.-M. Lebrand C. Cosson P. Kobayashi T. Gruenberg J. J. Biol. Chem. 2002; 277: 32157-32164Abstract Full Text Full Text PDF PubMed Scopus (293) Google Scholar). The exact location of the NPC1 protein in the late endosomal membranes is not known. NPC2 is a soluble lysosomal protein that can be secreted from cells. It uses mannose 6-phosphate marker for targeting to the late endosome (13Naureckiene S. Sleat D.E. Lackland H. Fensom A. Vanier M.T. Wattiaux R. Jadot M. Lobel P. Science. 2000; 290: 2298-2301Crossref PubMed Scopus (710) Google Scholar) and is a high affinity cholesterol-binding protein (14Ko D.C. Binkley J. Sidow A. Scott M.P. Proc. Natl. Acad. Sci. U. S. A. 2003; 100: 2518-2525Crossref PubMed Scopus (166) Google Scholar). NPC2 also binds fatty acids in vitro but with lower affinity (14Ko D.C. Binkley J. Sidow A. Scott M.P. Proc. Natl. Acad. Sci. U. S. A. 2003; 100: 2518-2525Crossref PubMed Scopus (166) Google Scholar). A crystal structure in the ligand free state shows that the protein has three small hydrophobic cavities that form a “gate,” which may represent the incipient cholesterol-binding site that dilates to accommodate the cholesterol molecule; the gate involves tyrosine 100 and phenylalanine 66 (15Friedland N. Liu H.L. Lobel P. Stock A.M. Proc. Natl. Acad. Sci. U. S. A. 2003; 100: 2512-2517Crossref PubMed Scopus (260) Google Scholar). Low Density Lipoprotein (LDL)-derived Cholesterol—In mammalian cells, LDL, the principal cholesterol carrier in the blood, binds to the LDL receptor, internalizes, and enters the endocytic compartment. There, its main cargo, comprised of cholesteryl esters, is dissimilated by hydrolysis to cholesterol and fatty acids. Hydrolysis of cholesteryl esters requires the enzyme acid lipase. In tissue culture cells, most of the lipase is located in endocytic compartments that are distinct from the late endosomes/lysosomes; after lipase action, the liberated cholesterol appears in the late endosomes/lysosomes (16Sugii S. Reid P.C. Ohgami N. Du H. Chang T.Y. J. Biol. Chem. 2003; 278: 27180-27189Abstract Full Text Full Text PDF PubMed Scopus (78) Google Scholar). In NPC1 cells (i.e. cells affected by the NPC1 mutation), the transport of cholesterol from the late endosomes to various destinations, including the plasma membrane, is defective (17Wojtanik K.M. Liscum L. J. Biol. Chem. 2003; 278: 14850-14856Abstract Full Text Full Text PDF PubMed Scopus (128) Google Scholar). At present, it is not clear how NPC1 and NPC2 work in concert to transport cholesterol. Oxysterols play an important role in mediating cellular cholesterol homeostasis. Cells produce more oxysterols when cultured in the presence of LDL, and this production is decreased in cells overexpressing NPC1 and NPC2 (18Frolov A. Zielinski S.E. Crowley J.R. Dudley-Rucker N. Schaffer J.E. Ory D.S. J. Biol. Chem. 2003; 278: 25517-25525Abstract Full Text Full Text PDF PubMed Scopus (169) Google Scholar). These results suggest that NPC1 and NPC2 may participate in delivering LDL-derived cholesterol to proper cellular site(s) for conversion to oxysterols. Sterols Synthesized from Acetate—In mammals, extrahepatic tissues synthesize as much cholesterol as the liver (19Dietschy J.M. Turley S.D. Spady D.K. J. Lipid Res. 1993; 34: 1637-1659Abstract Full Text PDF PubMed Google Scholar). In Chinese hamster ovary cells and human fibroblast cells, biosynthesis of sterols takes place at the endoplasmic reticulum (ER). After synthesis, most sterols are rapidly transported from the ER to the caveolae/lipid raft domain of the plasma membrane (PM) in an energy-dependent manner. This process does not require NPC1 (20Liscum L. Ruggiero R.M. Faust J.R. J. Cell. Biol. 1989; 108: 1625-1636Crossref PubMed Scopus (242) Google Scholar). After reaching the PM, the newly synthesized sterol may recycle rapidly (within minutes) between the PM and the recycling endosome (21Hao M. Maxfield F.R. J. Biol. Chem. 2000; 275: 15279-15286Abstract Full Text Full Text PDF PubMed Scopus (204) Google Scholar). After 8 or more hours, the endogenously synthesized sterols accumulate in the late endosomal/lysosomal compartment of NPC1 cells but not in normal cells. The recycling of these sterols from the late endosomes to the PM, and the esterification of these molecules within the ER are also partially defective in NPC1 cells (22Cruz J.C. Chang T.Y. J. Biol. Chem. 2000; 275: 41309-41316Abstract Full Text Full Text PDF PubMed Scopus (79) Google Scholar, 23Lange Y. Ye J. Rigney M. Steck T. J. Biol. Chem. 2000; 275: 17468-17475Abstract Full Text Full Text PDF PubMed Scopus (157) Google Scholar, 24Lange Y. Ye J. Steck T.L. J. Biol. Chem. 1998; 273: 18915-18922Abstract Full Text Full Text PDF PubMed Scopus (106) Google Scholar). The effect of NPC1 on trafficking of endogenously synthesized sterols is cell-type dependent: macrophages and glial cells are prominently affected by the NPC1 mutation, whereas embryonic fibroblasts are less affected (25Reid P.C. Sugii S. Chang T.Y. J. Lipid Res. 2003; 44: 1010-1019Abstract Full Text Full Text PDF PubMed Scopus (44) Google Scholar, 26.Reid, P. C., and Chang, T. Y. (2003) International Atherosclerosis Society (IAS) Commentary (http://www.athero.org)Google Scholar). Sterol Synthesis, Transport, and Secretion in Brain Cells—In mammals, the brain contains more unesterified cholesterol, most of which is acquired by endogenous synthesis, than any other organ in the body (27Dietschy J.M. Turley S.D. J. Lipid Res. 2004; 45: 1375-1397Abstract Full Text Full Text PDF PubMed Scopus (786) Google Scholar). Both neurons and astrocytes isolated from the NPC1–/– mouse exhibit trafficking defects in exogenously provided cholesterol and endogenously synthesized sterol (28Henderson L.P. Lin L. Prasad A. Paul C.A. Chang T.Y. Maue R.A. J. Biol. Chem. 2000; 275: 20179-20187Abstract Full Text Full Text PDF PubMed Scopus (76) Google Scholar, 29Karten B. Vance D.E. Campenot R.B. Vance J.E. J. Biol. Chem. 2003; 278: 4168-4175Abstract Full Text Full Text PDF PubMed Scopus (94) Google Scholar, 30Karten B. Vance D.E. Campenot R.B. Vance J.E. J. Neurochem. 2002; 83: 1154-1163Crossref PubMed Scopus (131) Google Scholar, 25Reid P.C. Sugii S. Chang T.Y. J. Lipid Res. 2003; 44: 1010-1019Abstract Full Text Full Text PDF PubMed Scopus (44) Google Scholar). Despite these defects, NPC1–/– mouse astrocytes synthesize and secrete the NPC2 and apolipoprotein E proteins (31Mutka A.L. Lusa S. Linder M.D. Jokitalo E. Kopra O. Jauhiainen M. Ikonen E. J. Biol. Chem. 2004; 279: 48654-48662Abstract Full Text Full Text PDF PubMed Scopus (42) Google Scholar, 32Karten B. Hayashi H. Francis G.A. Campenot R.B. Vance D.E. Vance J.E. Biochem. J. 2004; 387: 779-788Crossref Scopus (36) Google Scholar). Glycosphingolipids and Other Lipids—In addition to cholesterol, various other lipids, such as sphingomyelin, glucosylceramide, certain gangliosides (especially GM2 and GM3), and lysobisphosphatidic acid, also accumulate in NPC1 cells (1Patterson M.C. Vanier M.T. Suzuki K. Morris J.A. Carstea E. Neufeld E.B. Blanchette-Mackie J.E. Pentchev P.G. Scriver C.R. Beaudet A.L. Sly W.S. Valle D. 8th Ed. The Metabolic and Molecular Bases of Inherited Disease. 3. McGraw-Hill, New York2001: 3611-3633Google Scholar). Gangliosides are acidic glycosphingolipids that are normally present in cell membranes at high levels. Mutations in genes encoding enzymes or proteins involved in the catabolism of glycosphingolipids cause various glycosphingolipids to accumulate within lysosomes, leading to secondary cholesterol accumulation (33Puri V. Watanabe R. Dominguez M. Sun X. Wheatley C.L. Marks D.L. Pagano R.E. Nat. Cell. Biol. 1999; 1: 386-388Crossref PubMed Scopus (255) Google Scholar, 34Puri V. Jefferson J.R. Singh R.D. Wheatley C.L. Marks D.L. Pagano R.E. J. Biol. Chem. 2003; 278: 20961-20970Abstract Full Text Full Text PDF PubMed Scopus (80) Google Scholar). N-Butyl deoxynojirimycin (NB-DNJ) is an inhibitor of the enzyme glucosylceramide synthetase, a key enzyme involved in the biosynthesis of gangliosides in animal cells. In NPC1 cells, some of the endosomal malfunction can be corrected by treating cells with NB-DNJ (35te Vruchte D. Lloyd-Evans E. Veldman R.J. Neville D.C. Dwek R.A. Platt F.M. van Blitterswijk W.J. Sillence D.J. J. Biol. Chem. 2004; 279: 26167-26175Abstract Full Text Full Text PDF PubMed Scopus (159) Google Scholar); however, the drug has little effect on reversing the cholesterol trafficking defect (22Cruz J.C. Chang T.Y. J. Biol. Chem. 2000; 275: 41309-41316Abstract Full Text Full Text PDF PubMed Scopus (79) Google Scholar, 35te Vruchte D. Lloyd-Evans E. Veldman R.J. Neville D.C. Dwek R.A. Platt F.M. van Blitterswijk W.J. Sillence D.J. J. Biol. Chem. 2004; 279: 26167-26175Abstract Full Text Full Text PDF PubMed Scopus (159) Google Scholar). Thus, it is unlikely that the cholesterol trafficking defects observed in NPC1 cells are due to secondary consequence of glycosphingolipid accumulation. The accumulation of glycosphingolipid in NPC1 cells may be explained by the high affinity between cholesterol and sphingolipids, which are the major components of lipid microdomains or “rafts.” Accumulation of one raft lipid in late endosomes/lysosomes may lead to the trapping and accumulation of another raft lipid (36Simons K. Gruenberg J. Trends Cell Biol. 2000; 10: 459-462Abstract Full Text Full Text PDF PubMed Scopus (210) Google Scholar). In addition, it has been shown that in NPC1 cells endosomal/lysosomal cholesterol accumulation causes inhibition of lysosomal sphingomyelinase (37Reagan Jr., J.W. Hubbert M.L. Shelness G.S. J. Biol. Chem. 2000; 275: 38104-38110Abstract Full Text Full Text PDF PubMed Scopus (46) Google Scholar) and lysosomal glucosylceramidase (the enzymes responsible for degrading sphingomyelin and glucosylceramides) (38Salvioli R. Scarpa S. Ciaffoni F. Tatti M. Ramoni C. Vanier M.T. Vaccaro A.M. J. Biol. Chem. 2004; 279: 17674-17680Abstract Full Text Full Text PDF PubMed Scopus (36) Google Scholar). The lower glucosylceramidase activity in NPC1 cells has been attributed to mislocalization of the enzyme due to cholesterol loading. It is also possible that, in addition to cholesterol trafficking, NPC1 may also be involved in sphingolipid recycling. Studies in yeast show that a mutation in the sterol-sensing domain of NPC1 results in defective recycling, localization, and increased quantities of complex glycosphingolipids, without obvious changes in sterol metabolism (39Krishnamurthy M. Higaki K. Tinkelenberg A.H. Balderes D.A. Almanzar-Paramio D. Wilcox L. Erdeniz N. Redican F. Padamsee M. Liu Y. Khan S. Alcantara F. Carstea E.D. Morris J.A. Sturley S.L. J. Cell Biol. 2004; 164: 547-556Crossref PubMed Scopus (112) Google Scholar). Endosomal Cholesterol and Rab Proteins—Various abnormalities can cause endosomal cholesterol to accumulate and perturb the functions of Rab7 and Rab4 proteins. Late endosomes and lysosomes exhibit bidirectional motility, moving back and forth between the periphery and the pericentriolar region of cells. Endosomal motility is controlled in part by Rab proteins, small GTPases that are intimately involved in various membrane trafficking events. Rab7 and the related Rab9 are located in the late endosomes. Rab7 interacts more with earlier endosomes and lysosomes, whereas Rab9 interacts more with the trans-Golgi (40Pfeffer S. Aivazian D. Nat. Rev. Mol. Cell. Biol. 2004; 5: 886-896Crossref PubMed Scopus (376) Google Scholar). Rab4 is located in early endosomes. Mammalian cells treated with the hydrophobic amine “U-drug” or cells doubly deficient in the major late endosomal/lysosomal membrane proteins Lamp1/Lamp2, exhibit significantly reduced motility of the late endosomes, accumulate endosomal cholesterol, and exhibit NPC-like phenotypes (41Eskelinen E.L. Schmidt C.K. Neu S. Willenborg M. Fuertes G. Salvador N. Tanaka Y. Lullmann-Rauch R. Hartmann D. Heeren J. von Figura K. Knecht E. Saftig P. Mol. Biol. Cell. 2004; 15: 3132-3145Crossref PubMed Scopus (209) Google Scholar, 42Lebrand C. Corti M. Goodson H. Cosson P. Cavalli V. Mayran N. Faure J. Gruenberg J. EMBO J. 2002; 21: 1289-1300Crossref PubMed Scopus (278) Google Scholar). The above observations may be explained by cholesterol accumulation due to various endosomal abnormalities, leading in turn to the inhibition of Rab7 and Rab4 (42Lebrand C. Corti M. Goodson H. Cosson P. Cavalli V. Mayran N. Faure J. Gruenberg J. EMBO J. 2002; 21: 1289-1300Crossref PubMed Scopus (278) Google Scholar, 43Choudhury A. Sharma D.K. Marks D.L. Pagano R.E. Mol. Biol. Cell. 2004; 15: 4500-4511Crossref PubMed Scopus (120) Google Scholar). The inhibition of Rab7 reduces the motility of the late endosomes (42Lebrand C. Corti M. Goodson H. Cosson P. Cavalli V. Mayran N. Faure J. Gruenberg J. EMBO J. 2002; 21: 1289-1300Crossref PubMed Scopus (278) Google Scholar). Strikingly, overexpressing Rab9 corrects the lipid trafficking defects in NPC1 cells (44Choudhury A. Dominguez M. Puri V. Sharma D.K. Narita K. Wheatley C.L. Marks D.L. Pagano R.E. J. Clin. Invest. 2002; 109: 1541-1550Crossref PubMed Scopus (383) Google Scholar, M. Davies J.P. Ioannou Y.A. J. Lipid Res. 2003; 44: Full Text Full Text PDF PubMed Scopus Google Scholar). Despite the effect of overexpressing certain Rab proteins, this may of NPC which is and has no Mutations in Other an NPC-like other proteins in late including and F. C. M.C. C. J. Biol. Chem. 2002; 277: Full Text Full Text PDF PubMed Scopus Google Scholar), may also be involved in endosomal cholesterol The role of in sterol trafficking is not clear with mutation of are and in sterol T. F. Liu P. Dwyer N.K. Blanchette-Mackie E.J. Strauss III, J.F. J. Biol. Chem. 2004; 279: Full Text Full Text PDF PubMed Scopus Google Scholar). A cell without the NPC1 mutation but with defects in late endosomal cholesterol trafficking has been isolated A. K. D. L.M. Schaffer J.E. Ory D.S. J. Biol. Chem. 2001; 276: Full Text Full Text PDF PubMed Scopus (42) Google Scholar). is a key protein that sterol In cells cholesterol and sphingomyelin accumulate in late endocytic and these lipids exhibit E.B. J.A. Jr., C.L. C.A. Cooney A. Comly M. Dwyer N. Blanchette-Mackie J. S. Jr., J. Biol. Chem. 2004; 279: Full Text Full Text PDF PubMed Scopus Google Scholar). cell for the disease D.A. J. Cell Mol. Biol. 2004; PubMed Scopus (32) Google Scholar), and fibroblast cells with mutations in the C.A. D. M. R.D. Mol. Genet. 2002; PubMed Scopus (42) Google Scholar), also accumulate cholesterol in a to NPC1 cells. The mouse for NPC the mouse as the NPC1 has a mutation in the npc1 and phenotypes to of human NPC disease (1Patterson M.C. Vanier M.T. Suzuki K. Morris J.A. Carstea E. Neufeld E.B. Blanchette-Mackie J.E. Pentchev P.G. Scriver C.R. Beaudet A.L. Sly W.S. Valle D. 8th Ed. The Metabolic and Molecular Bases of Inherited Disease. 3. McGraw-Hill, New York2001: 3611-3633Google Scholar). In NPC1 neurons of the a of NPC disease. other abnormalities have been in NPC1 at abnormalities in the and U. A. G. Patel Exp. Brain Res. 2001; PubMed Scopus (97) Google Scholar). cholesterol accumulation occurs in various of the brain P.C. N. Sugii S. Y. Y. Chang T.Y. J. Lipid Res. 2004; 45: Full Text Full Text PDF PubMed Scopus Google Scholar) At and of in the and are observed S. T. T. Suzuki K. J. Exp. 2004; PubMed Scopus Google Scholar). astrocytes in these cells also accumulate cholesterol P.C. N. Sugii S. Y. Y. Chang T.Y. J. Lipid Res. 2004; 45: Full Text Full Text PDF PubMed Scopus Google Scholar). At this GM2 and also accumulate in various cells M. K. J. Exp. 2001; PubMed Scopus (223) Google Scholar); cells in and cells in the cell and D.C. C.L. T. U. C. J.M. 2002; 109: PubMed Scopus Google Scholar). and At the in protein and cholesterol C. D.K. Turley S.D. J.M. J. Exp. 2000; PubMed Scopus Google Scholar). occurs between the and The neurodegeneration in the NPC1 can be by of a NPC1 that the NPC1 protein in the central nervous system A. R.A. W.J. Hum. Mol. Genet. 2002; PubMed Google Scholar). A mouse for NPC2 with much lower of the NPC2 protein of has been D.E. J.A. M. L. G.S. Vanier M.T. Lobel P. Proc. Natl. Acad. Sci. U. S. A. 2004; 101: PubMed Scopus Google Scholar). In of disease the phenotypes of NPC1 NPC2 and are or In of the two drug in NPC1 have by animal NB-DNJ in NPC1 and the of of NPC1 by and reduces cellular in the M. Biol. 2001; Full Text Full Text PDF PubMed Scopus Google Scholar). which are by brain cells, and and (reviewed in Ref. 2000; 21: PubMed Scopus Google Scholar). NPC1 at contain than of in early postnatal NPC1 the of and cell reduces GM2 and and the of the treated NPC1 a at the most to NPC1 L. Nat. 2004; 10: PubMed Scopus Google Scholar). is yet to be lipid trafficking in in endosomal lipid transport have in the brain. insight cellular lipid trafficking and the of the disease.