Synthesis and Biosynthetic Trafﬁcking of Membrane Lipids

Synthesis and Biosynthetic Traffickingof Membrane Lipids

Tomas Blom1, Pentti Somerharju2, and Elina Ikonen1

1Institute of Biomedicine, Department of Anatomy, University of Helsinki, FIN-00014 Finland2Institute of Biomedicine, Department of Medical Biochemistry and Developmental Biology, Universityof Helsinki, FIN-00014 Finland

Correspondence: [email protected]

Eukaryotic cells can synthesize thousands of different lipid molecules that are incorporatedinto their membranes. This involves the activity of hundreds of enzymes with the task ofcreating lipid diversity. In addition, there are several, typically redundant, mechanisms totransport lipids from their site of synthesis to other cellular membranes. Biosynthetic lipidtransport helps to ensure that each cellular compartment will have its characteristic lipidcomposition that supports the functions of the associated proteins. In this article, we providean overview of the biosynthesis of the major lipid constituents of cell membranes, that is,glycerophospholipids, sphingolipids, and sterols, and discuss the mechanisms by whichthese newly synthesized lipids are delivered to their target membranes.

The endoplasmic reticulum (ER) is the mainsite for lipid synthesis. Intracellular lipid

trafficking is necessary to maintain most otherorganelle membranes as they lack the capabili-ty to synthesize lipids de novo (van Meer et al.2008). In their target locations, lipids may bepresent as structural compounds or they mayundergo further biosynthetic modifications togenerate different lipid species, some of whichmay be transported further to new destinations.Because of their hydrophobic nature, mostlipids cannot be effectively transferred by freediffusion from one compartment to anotherand must therefore rely on active mechanismsto facilitate intercompartmental transport. Inprinciple, three basic mechanisms can be de-picted. A major form of trafficking is membrane

transport that involves the budding of vesiclesor tubules from a donor membrane and sub-sequent fusion with an acceptor membrane(Fig. 1A). The acceptor membrane may containenzymes that modify the inserted lipids, help-ing to generate a lipid composition that dif-fers from the donor membrane. In addition,cells use cytosolic carrier proteins for transfer-ring lipids between compartments (Fig. 1B).The hydrophobic lipid binding pockets of theseproteins are selective, allowing only one—ormore often, a few closely related—lipid speciesto bind. Carrier proteins may also contain pep-tide determinants that target to the donor andacceptor membranes, providing compartmen-tal specificity for transfer. Yet another possi-bility is the bringing of donor and acceptor

Editor: Kai Simons

Additional Perspectives on The Biology of Lipids available at www.cshperspectives.org

Copyright # 2011 Cold Spring Harbor Laboratory Press; all rights reserved.

Advanced Online Article. Cite this article as Cold Spring Harb Perspect Biol doi: 10.1101/cshperspect.a004713

1

on March 25, 2022 - Published by Cold Spring Harbor Laboratory Press http://cshperspectives.cshlp.org/Downloaded from

http://cshperspectives.cshlp.org/

membranes into close proximity and transfer oflipids via membrane contact sites (Fig. 1C). Invivo, combinations of these three mechanismsare likely to operate in parallel, but their dissec-tion is not simple.

It was postulated already in the late 1960sthat lipid exchange can be facilitated by intracel-lular lipid transfer proteins (Wirtz and Zilver-smit 1969). Following the initial observations,several proteins with lipid transfer activityhave been identified and cloned. Based on theirlipid binding specificity they are broadly dividedinto three classes, namely glycerophospholipid,sphingolipid, and sterol transfer proteins (Lev2010). Proteins that accelerate the exchange oflipids between donor and acceptor membranesin vitro have traditionally been regarded aslipid transfer proteins. However, the in vitrolipid transfer may be a consequence of a generalbinding activity of a lipid sensing or chaperonedomain of the protein, and is not necessarilyindicative of a direct physiological role in lipidtransfer. Consequently, a long-standing problem

has been the unequivocal identification of pro-teins that are physiologically relevant mediatorsof intermembrane lipid transfer. Most of theproteins with lipid transfer activity in vitrohave been shown to affect lipid metabolism invivo but apart from a few exceptions, it is atpresent unknown whether these effects arebecause of actual lipid transfer activity (Wirtzet al. 2006, D’Angelo et al. 2008).

In addition to targeting lipids to specific cel-lular compartments, the cell upholds a differen-tial composition of lipids over the membranebilayer leaflets. The ER membrane leaflets are,however, considered similar in lipid composi-tion because of a high degree of lipid “flip-flopping” between leaflets (Kol et al. 2004;Holthuis and Levine 2005). Because of this,the decision whether a lipid will have its headgroup oriented toward the cytosol or the extra-cellular environment is largely made in post-ERmembranes. Here, cells actively transport lipidsto enrich specific lipid types in the cytosolicleaflet, and others in the exoplasmic leaflet.

Membrane transport

A

B

C

Carrier proteins

Membrane contact sites

Figure 1. Mechanisms of intermembrane lipid transport. (A) Membrane transport moves lipids together withproteins in vesicular and tubular carriers that bud off from a donor membrane, and are transported along cyto-skeletal tracks to the acceptor membrane, in which they fuse to deliver their cargo. (B) Cytosolic carrier proteinstransfer lipids in hydrophobic pockets that show selectivity toward one or a few lipid types. Carrier proteins oftencontain domains that bind to the donor and/or acceptor membranes. (C) Lipid exchange may also occurbetween membranes that are in very close proximity. Transfer via such membrane contact sites may be facilitatedby carrier proteins (combination of models B and C).

T. Blom et al.

2 Advanced Online Article. Cite this article as Cold Spring Harb Perspect Biol doi: 10.1101/cshperspect.a004713



The compartmentalization of lipids is in-volved in regulating cellular functions. For in-stance, in polarized cells membrane proteinsare differentially transported to the apicalversus basal plasma membrane. This proteintargeting is partially dependent on lipid sortingin the Golgi complex (Simons and Ikonen 1997;Weisz and Rodriguez-Boulan 2009). Similarly,localized sphingolipid metabolism is associatedwith membrane budding and the formation ofexosomes in multivesicular bodies (Trajkovicet al. 2008). Lipids do not only modulate thelocalization and function of constitutivelymembrane associated proteins but may alsorecruit soluble proteins to membranes, by serv-ing as temporally and spatially regulated tags forlipid binding cytosolic proteins.

BIOSYNTHETIC TRAFFICKING OFGLYCEROPHOSPHOLIPIDS

Glycerophospholipids represent the most abun-dant class of lipids in mammalian cells. Thesynthesis of glycerophospholipids takes placein the ER and is initiated by acylation of glycerol3-phosphate (or acylation and subsequent re-duction of dihydroxyacetone-phosphate) toform phosphatidic acid (Fig. 2), from whichall other glycerophospholipids are formed bythe addition of a polar head group. The mostcommon head group in mammalian cells ischoline and the respective phospholipid class,phosphatidylcholines, makes up approximatelyhalf of the total cellular phospholipids. Phos-phatidylethanolamines, -serines and -inositolsare other common mammalian glycerophos-pholipid classes. Each of them consists of tensor hundreds of different molecular speciesvarying in their alkyl chain composition (vanMeer et al. 2008). Accordingly, a single cellmay contain thousands of different phospho-lipid molecules. The functional significance ofthis complexity is only starting to be unraveled.

Phosphatidylcholine

Phosphatidylcholine (PC) is formed by thetransfer of a phosphocholine group fromCDP-choline to a diacylglycerol molecule.

This reaction can take place both in the ERand the Golgi apparatus. In the ER, the finalstep of PC synthesis is mediated by a choline/ethanolamine phosphotransferase of dual spe-cificity, whereas a choline specific phospho-transferase catalyzes this reaction in the Golgi(Henneberry and McMaster 1999; Henneberryet al. 2000, 2002). In the ER, PC and other lipidsmove readily over the bilayer, but in the Golgi,such movement is more restricted (Butonet al. 1996; van Meer et al. 2008). Because PCis the major lipid in most organelle membranes,it must be carried by membrane transport.However, as PC is efficiently transferred fromthe ER to the plasma membrane when proteinsecretion is inhibited, nonvesicular mechanismsalso play an important role in this transfer(Fig. 5) (Kaplan and Simoni 1985). There areseveral possible mechanisms of nonvesiculartransport of (phospho)lipids, including spon-taneous or protein-mediated diffusion andmembrane contact-site dependent transloca-tion (Fig. 1), but the relative contributions ofthese are not known (Voelker 2009). Potentiallyimportant phospholipid transfer proteinsinclude StARD2, StARD7, and StARD10, whichcan bind a PC molecule in a hydrophobicpocket of their steroidogenic acute regulatoryprotein related lipid transfer (START) domainand mediate intermembrane transfer of PC invitro. StARD2 and StARD7 bind PC only,whereas StARD10 binds phosphatidylethanol-amine (PE) as well (Olayioye et al. 2005; Wirtz2006; Kanno et al. 2007; Horibata and Sugi-moto 2010; Kang et al. 2010). However, it isunclear if these proteins play a relevant role inphospholipid trafficking in vivo.

Phosphatidylethanolamine

Phosphatidylethanolamine (PE) is the secondmost abundant phospholipid in mammaliancells. PE is synthesized either by transfer of phos-phoethanolamine from CDP-ethanolamine todiacylglycerol or by decarboxylation of phos-phatidylserine (PS) (Vance 2008). The formerpathway takes place in the ER and the nuclearenvelope and is catalyzed either by ethanolaminephosphotransferase or choline/ethanolamine

Lipid Biosynthetic Trafficking

Advanced Online Article. Cite this article as Cold Spring Harb Perspect Biol doi: 10.1101/cshperspect.a004713 3



Dihydroxyacetone phosphate

Dihydroxyacetone phosphateacyl ester

Diacylglycerol

Phosphatidylcholine

Phosphatidylserine

Phosphatidylserinesynthase 1


Diacylglycerolcholinephosphotransferase

Phosphatidylserinesynthase 2

Phosphatidylserinedecarboxylase

Ethanolaminephosphotransferase

Phosphatidic acid

Diacylglycerolkinase

Phosphatidatephosphatase

Lysophosphatidic acid

Glycerol-3-phosphate

Dihydroxyacetonephosphate

acyltransferase

Glycerol-3-phosphateacyltransferase

Lysophosphatidateacyltransferase

1-acylglyceronephosphate reductase

O

O

OH

OH

P O

OH

O

O

OH

R O

O

OH

RO

OH

P O

O

O

R R O O

O

OH

P OH

O

OH

O

P OH

O

HO

OH

O

O

OH

P OH

OH

OH

O

R O

O RO

O

O

OH

O

PN+

(CH3)3O

R O

O RO

O

O

OH

O

P O

NH2

R O

O RO

O

O

OH

O

PNH2

O

R O

O RO

O

O

Figure 2. See facing page for legend.

T. Blom et al.




phosphotransferase (Henneberry and McMas-ter 1999; Henneberry et al. 2002; Horibata andHirabayashi 2007). The decarboxylation reac-tion, on the other hand, takes place in the innermitochondrial membrane and much, if not all,of the mitochondrial PE derives from this reac-tion (Zborowski et al. 1983; Shiao et al. 1995;Vance 2008). Elimination of the PS decarboxy-lase in mice is lethal, probably because (1) PE isessential for the proper structure and functionof mitochondria and (2) the PE formed in theendoplasmic reticulum via the CDP-ethanol-amine pathway cannot be efficiently importedinto mitochondria (Steenbergen et al. 2005).Notably, not all of the PE formed by decarbox-ylation of PS remains in mitochondria, but istransported to the ER and other organelles.The mechanism(s) of PE transport out of mito-chondria remain to be established, but couldbe similar to that of PS transport from the ERto mitochondria (Achleitner et al. 1995, 1999)(see below). In the plasma membrane, most ofthe PE is found in the inner leaflet and thisasymmetrical distribution is maintained byP-type ATPases (Riekhof and Voelker 2009).

Phosphatidylserine

In mammalian cells, PS is synthesized byexchanging the polar head group of a PC or aPE molecule for serine. The PS synthases 1and 2 responsible for this reaction are highlyenriched in mitochondria-associated ER do-mains (Fig. 5) (Stone and Vance 2000). Fromthere, phosphatidylserine translocates to otherorganelles, such as the plasma membrane as wellas to mitochondria where it is decarboxylated toPE. In the plasma membrane, PS is abundantbut resides almost exclusively in the cytoplasmicleaflet. Such asymmetrical distribution of PS is

maintained by an ATP-driven PS translocation(catalyzed by a yet unidentified protein) fromthe outer to the inner leaflet (Daleke 2007;Smriti et al. 2007). PS interacts with and regu-lates several important enzymes, including pro-tein kinase C in the cytoplasmic leaflet of theplasma membrane (Nishizuka 1992). In addi-tion, loss of the transbilayer asymmetry of PSis one of the hallmarks of apoptosis (Leventisand Grinstein 2010).

Interestingly, the transport of PS from theER to mitochondria is a relatively slow processwith a half-time of several hours (Voelker1985; Heikinheimo and Somerharju 1998).The mechanism of this transfer process hasnot been resolved. It has been proposed thatthe close apposition of mitochondria and theER membranes is important for the transfer,and the transfer may be mediated by specificproteins located at the ER/mitochondria con-tact sites (Vance 2008). However, the relativelyslow transfer of PS in general as well as fastertransfer of the less hydrophobic PS speciesfrom ER to mitochondria implies that sponta-neous diffusion of PS monomers via the cyto-plasm may also contribute to this process(Heikinheimo and Somerharju 2002). PS maymove by lateral diffusion via membrane contactsites from the outer to the inner mitochondrialmembrane where PS decarboxylase resides(Jasinska et al. 1993).

BIOSYNTHETIC SPHINGOLIPIDTRAFFICKING

Numerous types of sphingolipid species exist ineukaryotic cells and most of them are enrichedin the plasma membrane. Sphingomyelin (SM)and glycosphingolipids (GSLs) are importantstructural lipids that, together with cholesterol,

Figure 2. An overview of the biosynthetic pathways of major glycerophospholipids. De novo synthesis of theglycerophospholipids begins in the ER by a series of reduction and acylation reactions leading to the formationof phosphatidic acid. Dephosphorylation of phosphatidic acid yields diacylglycerol, which can be turnedinto phosphatidylcholine or phosphatidylethanolamine by the addition of a phosphocholine or a phosphoetha-nolamine head group, respectively. Phosphatidylserine is formed by exchanging the head group from eitherphosphatidylcholine or phosphatidylethanolamine for serine. Mitochondrial phosphatidylethanolamine issynthesized on location by decarboxylation of phosphatidylserine.





are thought to participate in the formationof membrane domains which can affect thedistribution and function of membrane pro-teins (Coskun and Simons 2010). On the otherhand, some less abundant sphingolipids, suchas sphingosine, sphingosine-1-phosphate, andceramide, are important modulators of cell sig-naling (Hannun and Obeid 2008). The enzymesthat catalyze the different steps in the synthesisof the more complex sphingolipids (e.g., SMand GSLs) differ in their subcellular locationand membrane sidedness. Thus, the synthesisof complex sphingolipids requires both inter-organelle and transbilayer lipid movement.

The initial steps of sphingolipid synthesistake place on the cytosolic face of the ER(Fig. 3). First, 3-dehydrosphinganine is formedby condensation of serine and palmitoyl-CoAby the enzyme serine palmitoyl transferase(SPT). The 3-dehydrosphinganine is then re-duced to sphinganine by 3-dehydrosphinganinereductase (Futerman and Riezman 2005; Sabo-urdy et al. 2008). Sphinganine can be N-acylatedby one of the several ceramide synthases to formdihydroceramide (Lahiri and Futerman 2007)with an acyl chain of varying length and degreeof unsaturation. This provides diversity in themembrane behavior of the sphingolipids even-tually formed (Ewers et al. 2010; Koivusaloet al. 2007). Dihydroceramide is then convertedto ceramide by a desaturase residing in theER. Ceramide is the first lipid in the de novosynthesis pathway that contains a sphingosinebackbone, and can therefore be considered acentral compound in sphingolipid metabolism.

Ceramide can be further metabolized toother sphingolipids by the addition of a polarhead group or by removal or exchange of theacyl chain in the recycling pathway. Most ofthe newly synthesized ceramide is used in theGolgi for the synthesis of SM and glycosphingo-lipids (Tafesse et al. 2006). In oligodendrocytesceramide can be glycosylated in the ER to formgalactosylceramide which is an important con-stituent of the myelin sheath (Raff et al. 1978;Stoffel and Bosio 1997; Sprong et al. 2003).In mammalian cells, the sphingoid backboneis almost invariably composed of 18 carbonunits (C18). Diversity—and alterations in the

membrane behavior of the sphingolipid—isachieved by linking acyl chains with varyinglength and degree of saturation to the sphingo-sine backbone. In addition to its impact onmembrane fluidity, the variations in the acylchain moiety has functional implications,such as affecting, for example, endocytosis ofvirus particles (Ewers et al. 2010) and the organ-ization of myelin sheaths (Ben-David andFuterman 2010).

Sphingomyelin

The synthesis of SM, the major sphingolipid inthe plasma membrane, takes place in the Golgiand requires transfer of the ceramide precursorfrom the ER. Ceramide can be transferred fromthe ER either in transport vesicles or by solubletransfer proteins. In mammalian cells the latterroute dominates and is mediated by the cera-mide transport protein (CERT) that takes cera-mide from the cytosolic leaflet of the ER andtransfers it to the trans-Golgi cisternae whereit is converted to SM by sphingomyelin synthase1 (SMS1) (Hanada et al. 2003, 2009; Yamajiet al. 2008). CERT is a cytosolic protein withfour domains that are needed for transfer ofceramide from the ER to the Golgi. The amino-terminal pleckstrin homology (PH) domainspecifically binds to phosphatidyl inositol-4-phosphate, a phospholipid that is abundant inGolgi membranes. The PH domain is crucialfor targeting of CERT to the Golgi apparatus,because mutations in the PH domain canimpair binding of CERT to the Golgi, thusinhibiting the synthesis of SM (Hanada et al.2003). The START domain in the carboxyl ter-minus of CERT is responsible for extractingceramide from the cytosolic leaflet of the ER.The START domain of CERT has a high speci-ficity for ceramide as it does not transfer othersphingolipids or cholesterol in vitro (Kumagaiet al. 2005). Molecular modeling studies haveshown that the START domain binds oneceramide in its hydrophobic pocket, suggestingthat CERT carries a single ceramide at a timefrom the ER to the Golgi (Kudo et al. 2008,2010). CERT also contains a FFAT motif thatmediates binding to the ER-resident protein

T. Blom et al.




Serinepalmitoyltransferase

Ceramide synthase

Dihydroceramidedesaturase

Sphingomyelin synthase

Sphingomyelinase

Ceramideglucosyltransferase

Ceramidase Sphingosine kinase

S1P phosphataseCeramide synthase

β-galactosidase

Lactosylceramide synthase

Palmitoyl-CoA3-dehydrosphinganine Sphinganine HexadecanalSphinganine 1-phosphate

3-dehydrosphinganinereductase S1P phosphatase

Sphingosine kinase

O

HO OH

O HO

OHOH

NH2 NH2HO O

NH2

NH2

OH

HO

Dihydroceramide

Sphingomyelin

OO P OH

HO

HO

HOHO

O

OO

Glucosylceramide Lactosylceramide

Complex glycosphingolipids

Glucosyl-ceramidase

Ceramide Sphingosine Sphingosine 1-phosphate

HO O

OH

NH

RHO HO

OH

OH

NH2

H2NOHO Phosphoethanolamine

+Hexadecenal

P

OH

OOHO P

OH

S1P lyase

NH2NH

N+

(CH3)3

OR

HONH

OR

L-serine+S-CoA

O

O

OH

O

OHO P

HONH

OR

HO

HOHO

O

O

HO

HOHO

OHO

O

HONH

OR

+

Figure 3. Overview of the biosynthetic pathways of sphingolipids. The sphingoid backbone is formed by thecondensation of serine and palmitoyl-CoA. Three further synthetic steps are needed to produce ceramide, whichthe first compound with a bona fide sphingosine backbone. After its synthesis in the ER, ceramide can be metab-olized into sphingomyelin or glycosphingolipids in the lumenal leaflet of the Golgi. The postceramide metabolicsteps are reversible and ceramide can also be formed by the sequential degradation of more complex sphingo-lipids. Deacylation of ceramide yields sphingosine that can be phosphorylated to sphingosine-1-phosphate.The irreversible degradation of the sphingoid backbone is catalyzed by a lyase that acts on either sphingosine-1-phosphate or sphinganine-1-phosphate.





VAP and thus allows CERT to target to the ER.Next to the PH domain there is a serine repeatmotif that regulates the activity of CERT. Hyper-phosphorylation of the serines in this motifimpairs binding of CERT to the ER and Golgimembranes, thus inhibiting ceramide transferfrom the ER to the Golgi (Yamaji et al. 2008;Hanada et al. 2009; Voelker 2009).

Glycosphingolipids

The synthesis of glycosphingolipids fromceramide takes place mainly in the Golgi ap-paratus. Similarly to SM, efficient de novosynthesis of glycosphingolipids is dependenton CERT (Hanada et al. 2003; Halter et al.2007). In contrast, the synthesis of glyco-sphingolipids from sphingosine is largelyindependent of CERT (D’Angelo et al. 2007;Giussani et al. 2008), and thus probably relieson vesicular trafficking of ceramide from theER to the Golgi. The synthesis of most glyco-sphingolipids begins by the addition of a gluco-syl moiety to a ceramide on the cytosolic sideof Golgi membranes. Glucosylceramide thenneeds to be translocated to the lumenal sideof the Golgi to be converted to more complexglycosphingolipids. This process requires theinvolvement of the four-phosphate-adaptorprotein 2 (FAPP2). FAPP2 contains a carboxy-terminal domain that is homologous to the gly-colipid transfer protein (GLTP) domain andbinds glucosyl ceramide with high specificity.The amino-terminal region of FAPP2 containsa PH-domain that binds phosphatidyl inosi-tol-4-phosphate and ARF1, which tether theprotein to the Golgi (D’Angelo et al. 2007; Hal-ter et al. 2007; Mattjus 2009). Glucosylceramideis converted into lactosylceramide which canthen be further glycosylated to more complexglycosphingolipids in the lumenal side of trans-Golgi membranes. Because of their large andvery polar head groups, glycosphingolipids donot readily flip over the membrane and thusremain on the lumenal side. Most of the SMand GSLs synthesized move in transport vesiclesfrom the trans-Golgi network to the plasmamembrane where they end up in the extracellu-lar leaflet (van Meer et al. 2008).

Sphingolipid recycling

Sphingolipids are actively metabolized and re-cycled via salvage pathways. Some of the meta-bolic steps can occur at the plasma membrane.For instance, SM can be degraded therein toceramide by sphingomyelinases, and ceramidecan be converted back to SM by the sphingo-myelin synthase 2 (SMS2) located at the plasmamembrane (Milhas et al. 2010). However, mostof the sphingolipid metabolism takes place onintracellular membranes. Some of the endocy-tosed sphingolipids are transported back tothe plasma membrane, whereas others are tar-geted to lysosomes for degradation. Whetheran endocytosed sphingolipid is degraded orrecycled back to the plasma membrane is inpart dependent on its acyl chain length, withlonger chain sphingolipids preferentially tar-geted for degradation (Koivusalo et al. 2007).

Endocytosed sphingolipids reaching thelate endosomal/lysosomal compartments aredegraded to simpler sphingolipids in a stepwisemanner. This process is accomplished by theconcerted action of sphingolipid activatorproteins (SAPs) and hydrolytic enzymes. SAPsbind to and lift sphingolipids up from themembrane and then hand over to a hydrolase.However, the acid sphingomyelinase containsan internal saposin-homology domain andcan function without the assistance of saposinsproper (Linke et al. 2001; Kolter and Sandhoff2005). As endosomes mature into late endo-somes/lysosomes, their lipid composition andstructure changes and multivesicular bodiesare formed. The vesicles inside these bodiesare enriched in an acidic lipid with an uncon-ventional stereochemistry, namely bis-(mono-acylglycero)-phosphate (BMP) (Kobayashiet al. 1999; Mobius et al. 2003). SM and thecomplex sphingolipids in the internal vesiclesare oriented so that they face the lumen of themultivesicular bodies/lysosomes. They are thusexposed to the SAPs and acid hydrolases, whichallows for their facile degradation (Kolter andSandhoff 2010).

Eventually, GSLs and SM are degraded totheir building blocks, (e.g., monosaccharides),fatty acids and sphingosine. Sphingosine can

T. Blom et al.




then leave the lysosomes and is either degradedor reused for building new sphingolipids viasalvage pathways (Kolter and Sandhoff 2005;Schulze et al. 2009). Sphingosine can reenterthe salvage pathway by being acylated in theER to form ceramide. The reutilization ofsphingosine for the synthesis of GSLs is largelyindependent of CERT, suggesting that this path-way relies on vesicular trafficking of ceramidefrom the ER to the Golgi apparatus (D’Angeloet al. 2007; Giussani et al. 2008). Sphingosinedestined for degradation must first be phos-phorylated to sphingosine-1-phosphate, whichis then hydrolyzed to phosphoethanolamineand hexadecenal by a sphingosine-1-phosphatelyase in the ER (Ikeda et al. 2004).

BIOSYNTHETIC TRAFFICKING OF STEROLS

Cholesterol is a major structural lipid in mam-malian cell membranes and is enriched in theplasma membrane as well as intracellular mem-branes that actively communicate with theplasma membrane, such as recycling endo-somes and the trans-Golgi network (Ikonen2008). Cholesterol has a rigid four-ring struc-ture with 17 carbon atoms, to which two methylgroups and an iso-octyl side chain are attached(Fig. 4). All nucleated cells are capable of syn-thesizing cholesterol by using acetate as thesole carbon source. The rate-limiting enzymein the pathway, hydroxymethylglutaryl CoAreductase, catalyzes the synthesis of mevalonate.Six enzyme reactions then convert mevalonateto squalene. The mevalonate pathway is alsoused for the synthesis of other molecules, suchas isoprenoids, dolichol, and ubiquinone. Thefirst enzyme in the mevalonate pathway com-mitted to cholesterol synthesis is squalene oxi-dase. This reaction requires molecular oxygenand the product, lanosterol, is the first cyclicintermediate in the pathway. The steps postlanosterol involve �20 enzymatic reactionsand there are two alternative pathways depend-ing on whether or not there is a double bondbetween sterol carbons 24 and 25; the Blochpathway is followed when the double bond ispresent, whereas Kandutsch-Russell pathway isfollowed when it is absent (Fig. 4) (Bloch 1992).

Of the individual sterol biosynthetic steps,the rate-limiting enzyme, HMG CoA reductase(which is the target of the cholesterol loweringstatin drugs), has been extensively characterized.HMG CoA reductase is an ER integral membraneprotein and is stringently regulated both at thetranscriptional and posttranslational levels (seeYe and DeBose-Boyd 2011). Based on enzymeactivity measurements, also the later steps ofsterol biosynthesis take place in the ER. Sterolsynthesis by microsomal enzymes is highly effi-cient to the extent that organization via multien-zyme complex(es) have been suggested (Gaylorand Delwiche 1973). However, several of thecholesterol biosynthetic enzymes are also foundin other compartments, such as peroxisomes(presqualene steps [Kovacs et al. 2007]), nucleus(Wu et al. 2004), or nuclear envelope (Zwergeret al. 2010) (sterol 24-reductase and lamin Breceptor, respectively), lipid droplets (NAD(P)Hsteroid dehydrogenase like protein) (Ohashi et al.2003), and the Golgi complex (Cotman et al.2004) (lanosterol 14a-demethylase). What func-tions this compartmentalization of the enzymesmight serve, is not known.

Although the ER is the main site of choles-terol synthesis, cholesterol concentration inthe ER is low. This is because newly synthesizedsterols are rapidly transported to other cellularmembranes (Fig. 5). The fast transport of theintermediates may also explain the high effi-ciency of sterol biosynthesis despite the diver-gent localizations of the enzymes involved.Many of the newly synthesized postlanosterolintermediates can rapidly reach the plasmamembrane. They can also be secreted out of cellsto physiological (lipoprotein) or pharmacolog-ical (e.g., cyclodextrin) acceptors, or return tothe ER for conversion into cholesterol (Johnsonet al. 1995; Lusa et al. 2003). There is evidencethat the efflux of individual precursor sterolsfrom cells to extracellular acceptors varies,with the more polar zymosterol being moreavidly effluxed than lathosterol and bothexceeding that of newly synthesized cholesterol(Lange et al. 1991; Lusa et al. 2003). Indeed, cir-culating lathosterol levels can be used as a surro-gate marker of cholesterol biosynthetic activityin humans (Gylling et al. 1989).





O O+

Acetyl-CoA Acetoacetyl-CoA

HMG-CoA synthase

HMG-CoA reductase

(6 steps)

(2 steps)

(~8 steps)

H3C S-CoA

O

H3C S-CoA

OH3COHO

HMG-CoA

Mevalonate

Squalene

Lanosterol

Δ24-sterol reductase

HO S-CoA

(~9 steps) (2 steps)

H3C

H3C

CH3 H3C

OHO

HO

HO

OH

H3C

H3C

H3C

H3C

H3C

H3CCH3

CH3

CH3

CH3

CH3

H3C H3C

H

H

H

Lathosterol

H3C

H3C

HO

H3CCH3

H

CH3

H

H

H

Desmosterol

Cytochrome P450scc Sterol 27-hydroxylase

HO

H3C

H3C

H3CCH3

H

CH3H

H H

H

Cholesterol

H3C

H3C

HO

H3CCH3

CH3

H H H

H

Pregnenolone(steroid hormones)

HO

H3C

H3C

H3C

H

H H

HO

27-Hydroxycholesterol(oxysterols)

H3C

H3C

HO

H3CCH3

OH

H H H

H

Figure 4. Overview of the biosynthetic pathways of sterols. The four-ring sterol backbone derives from reductivepolymerizations of acetate to generate squalene, which is cyclized to form lanosterol, the first sterol in the path-way. The rate-limiting enzyme of cholesterol biosynthesis is HMG-CoA reductase. The postlanosterol pathwayinvolves roughly 20 steps, with some of the enzymes capable of acting on multiple substrates. If the carbon-24double bond is reduced early on, the pathway procedes via lathosterol (and 7-dehydrocholesterol, not shown) tocholesterol, whereas reduction of carbon-24 only in the last step results in the generation of desmosterol as thepenultimate cholesterol cholesterol precursor. Cholesterol serves as a precursor for other bioactive sterols, suchas steroid hormones and oxysterols.

T. Blom et al.




Glycerophospholipids Sphingolipids Sterols

DiacylglycerolSphinganine

Sphingosine

Dihydroceramide

Ceramide

Sphingomyelin

Glucosylceramide

Glycosphingolipids

Golgi

ER

PM

Mitochondria

Px

Squalene

LanosterolLn

Lt Lathosterol

Desmosterol

Cholesterol

Cholesterylester

Pregnenolone

Phosphatidic acid

Phosphatidylserine


PhosphatidylcholineS

S

E

E S

E

D

C

C

E

Pn

LnLt

LD

DC

C

C

E

Pn

SE

FAPP2

CERT

ORPs

StAR

Figure 5. Biosynthetic trafficking of major membrane lipids. (Left) Glycerophospholipids are synthesized in theER, with phosphatidylserine synthases enriched in mitochondria associated membrane fractions. Glycerophos-pholipids are transported from the ER both along exocytic membrane transport and by nonvesicular, yet poorlycharacterized mechanisms. (Middle) De novo sphingolipid synthesis is initiated in the ER. The ER to Golgitransport of ceramide for the assembly of more complex sphingolipids is mediated by CERT, and to a lesserextent by membrane transport. The post-Golgi transport of complex sphingolipids is dependent on membranetransport. Sphingosine that stems from lysosomally degraded complex sphingolipids can be acylated to formceramide in the ER and recycled to sphingomyelin and glycosphingolipids. (Right) Cholesterol biosyntheticenzymes reside in the ER, with some presqualene enzymes also localized in peroxisomes (Px). Sterols are trans-ported to the plasma membrane (PM) largely via Golgi bypass route(s) and the ORP proteins play a role in thisprocess, as well as in the reverse transport from the plasma membrane to the ER. Sterols imported into mito-chondria by StAR can be used for steroid hormone synthesis. Excess cholesterol can be esterified in the ERby acyl-CoA cholesterol acyltransferase and stored in lipid droplets (LD). Arrows indicate the direction of lipidtransport. Please note that arrows do not necessarily reflect the transport distance as membranes move con-stantly and lipid transfer may be facilitated by close apposition of membranes. Carrier proteins are indicatedby black circles.





The techniques available to study the mech-anisms of biosynthetic sterol trafficking fromthe ER to the more sterol-enriched membranesare indirect. Typically, cells are labeled with theradioactive precursors acetate or mevalonate,chased, and the incorporation of the radiolabelinto cholesterol is measured, after subcellularfractionation, lipid extraction, and chromato-graphic separation of the radiolabeled lipophiliccompounds. A simpler method for analyzingthe plasma membrane arrival of sterols is rapidextraction of sterols to the efficient extracellularacceptor, methyl b cyclodextrin. This should beperformed at reduced temperature (Heino et al.2000), because at 37oC cyclodextrin is endocy-tosed and no longer assesses only the plasmamembrane sterol pool (Rosenbaum et al. 2010).

Most of the newly synthesized cholesterolcan reach the plasma membrane by route(s)that bypass the Golgi complex as assessed bypharmacological disassembly of the Golgi com-plex by using Brefeldin A treatment (Urbaniand Simoni 1990; Heino et al. 2000) or bygenetic perturbation of ER-Golgi membranetransport in yeast (Baumann et al. 2005). Inter-estingly, in yeast steroids and sterols can be ace-tylated by a sterol acetyltransferase bound tothe ER membrane. This acetylation controlsthe export of steroids and sterols from cellsvia the secretory pathway, and acts as a lipiddetoxification mechanism (Tiwari et al. 2007).Acetylation renders the lipid more hydropho-bic, conferring higher affinity for membranes,and apparently therefore preferential deliveryvia membrane transport. Instead, mammalsuse a different strategy for sterol excretion: cho-lesterol is rendered increasingly hydrophilic viaoxidation to generate oxysterols (Fig. 4) (fol-lowed by further oxidation to bile acids in theliver) (Russell 2003) to increase water solubilityand excretion into the aqueous extracellularmilieu.

There is increasing evidence for the role ofsoluble sterol carrier proteins in facilitatingsterol trafficking between cellular membranes.Particularly two sterol-binding protein families,the START (steroidogenic acute regulatory pro-tein related lipid transfer) and ORP (oxysterol-binding protein related protein) families are

important in this process (Lavigne et al. 2010;Ridgway 2010). These proteins are encoded by27 genes comprising almost 0.1% of the entirehuman genome (Ngo et al. 2010). The STARTand ORP proteins have verified sterol-bindingdomains (several of which have been crystal-lized) and adjacent regulatory and membrane-targeting motifs.

The steroidogenic acute regulatory protein(StAR) is the prototype of the START familyproteins. StAR regulates the rate-limiting stepin steroid hormone biosynthesis, that is, thedelivery of cholesterol into the mitochondrialinner membrane (Manna et al. 2009). Thismembrane harbors the P450 side chain cleavageenzyme that catalyzes pregnenolone produc-tion from cholesterol. StAR is a labile phospho-protein whose synthesis correlates tightly withsteroid synthesis. Its amino-terminal mito-chondrial leader sequence is cleaved during orafter translocation into mitochondria, leavingintact the START domain that is responsiblefor inducing cholesterol transfer. However, therole of mitochondrial targeting and processingof StAR on steroidogenesisis is controversial,as the START domain can function at the outermitochondrial membrane (Arakane et al. 1996),and internalization of StAR into mitochondriadoes not appear necessary for the sterol transfer(Bose et al. 2002).

In yeast, ORP homologs (Osh proteins)have been implicated in ER-plasma membranesterol delivery. Moreover, recent data implicateOsh and ORP proteins in bidirectional deliveryof sterols between the plasma membrane andthe ER (Raychaudhuri et al. 2006; Jansen et al.2011). Several of the ORP proteins carry aFFAT (phenylalanine in an acidic tract) motifthat interacts with the ER protein VAMP associ-ated membrane protein (VAP) (Loewen et al.2003). This may display a regulatory functionin sterol transfer analogously to CERT, in whichthis motif regulates ER interaction and cera-mide transfer activity (Kawano et al. 2006).The plextrin homology (PH) domains presentin several ORPs may provide additional bindingspecifities between donor and acceptor mem-branes. PIP interactions may also take place inthe absence of a PH domain, as in Osh4, where

T. Blom et al.




a membrane-binding surface of the proteincan generate a phosphoinositide binding site(Schulz et al. 2009). In mammalian cells, ORP2was shown to enhance biosynthetic sterol traf-ficking from the ER to the plasma membrane(Hynynen et al. 2005). It can also facilitate thedelivery of sterols in the reverse direction,from the plasma membrane to the ER and lipiddroplets (Jansen et al. 2011).

CONCLUDING REMARKS AND FUTUREPROSPECTS

The enzymes involved in the biosynthesis ofglycerophospholipids, sphingolipids, and ster-ols have been characterized, and there is ampleinformation on the regulation of the rate-limit-ing enzymes of the pathways. Instead, insightinto the mechanisms of delivery of newly syn-thesized lipids to their target membranes in cellsis less comprehensive.

Although the basic biophysical principles oflipid mobility have been extensively studied inmodel membranes, the unequivocal identifica-tion of physiologically relevant mediators oflipid transfer has not been easy. A major chal-lenge is the redundancy of lipid trafficking sys-tems in cells. This calls for manipulations thatcan target an entire protein family, for instanceby pharmacologic compounds, or by geneticstrategies in multicellular organisms, as hasbeen pioneered in yeast. Moreover, analyticaltools need to be further developed. Mass spec-trometry based lipidomics has only recentlyopened the possibility to obtain comprehensiveand precise structural information on individ-ual lipid molecules. This technology should beextended and complemented with improvedimaging systems to visualizing lipids andlipid–protein interactions in cells and tissues,to obtain improved spatial and temporal infor-mation on lipid delivery.

Finally, it is important to acknowledge thatdespite the redundancy in lipid biosynthetictrafficking systems, not all delivery systemsare backed up in all cells. This is exemplifiedby human diseases because of defects in thisprocess that manifest as complex tissue levelmalfunction. For instance, loss of function

mutations in the mitochondrial cholesteroltransporter StAR cause a severe impairment inthe synthesis of all adrenal and gonadal steroidhormones, resulting in lipoid congenital adre-nal hyperplasia (Lin et al. 1995), whereas muta-tions in the keratinocyte lipid transporterABCA12 cause a skin disorder, harlequin ich-tyosis, that results from the loss of long-chainceramide esters in the skin and disruption ofthe epidermal permeability barrier (Zuo et al.2008). Such examples with Mendelian inheri-tance patterns represent the severe end of thespectrum, the tip of the iceberg, in lipid synthe-sis and delivery problems in human disease.With improved analytical technologies, thenumber and variety of such disturbances isbound to increase.

ACKNOWLEDGMENTS

This study was supported by the Academy ofFinland grant nos. 123261 (T.B.), 131429, and218066 (E.I.).

REFERENCES

Achleitner G, Gaigg B, Krasser A, Kainersdorfer E, KohlweinSD, Perktold A, Zellnig G, Daum G. 1999. Associationbetween the endoplasmic reticulum and mitochondriaof yeast facilitates interorganelle transport of phospho-lipids through membrane contact. Eur J Biochem 264:545–553.

Achleitner G, Zweytick D, Trotter PJ, Voelker DR, Daum G.1995. Synthesis and intracellular transport of aminogly-cerophospholipids in permeabilized cells of the yeast,Saccharomyces cerevisiae. J Biol Chem 270: 29836–29842.

Arakane F, Sugawara T, Nishino H, Liu Z, Holt JA, Pain D,Stocco DM, Miller WL, Strauss JF III. 1996. Steroido-genic acute regulatory protein (StAR) retains activity inthe absence of its mitochondrial import sequence: Impli-cations for the mechanism of StAR action. Proc Natl AcadSci 93: 13731–13736.

Baumann NA, Sullivan DP, Ohvo-Rekila H, Simonot C, Pot-teka A, Klaassen Z, Beh CT, Menon AK. 2005. Transportof newly synthesized sterol to the sterol-enriched plasmamembrane occurs via nonvesicular equilibration. Bio-chemistry 44: 5816–5826.

Ben-David O, Futerman AH. 2010 The role of the ceramideacyl chain length in neurodegeneration: Involvement ofceramide synthases. Neuromolecular Med 12: 341–350.

Bloch K. 1992. Sterol molecule: Structure, biosynthesis, andfunction. Steroids 57: 378–383.

Bose HS, Lingappa VR, Miller WL. 2002. Rapid regulationof steroidogenesis by mitochondrial protein import.Nature 417: 87–91.





Buton X, Morrot G, Fellmann P, Seigneuret M. 1996. Ultra-fast glycerophospholipid-selective transbilayer motionmediated by a protein in the endoplasmic reticulummembrane. J Biol Chem 271: 6651–6657.

Coskun U, Simons K. 2010. Membrane rafting: From apicalsorting to phase segregation. FEBS Lett 584: 1685–1693.

Cotman M, Jezek D, Fon Tacer K, Frangez R, Rozman D.2004. A functional cytochrome P450 lanosterol 14a-demethylase CYP51 enzyme in the acrosome: Trans-port through the Golgi and synthesis of meiosis-activat-ing sterols. Endocrinology 145: 1419–1426.

D’Angelo G, Vicinanza M, DeMatteis MA. 2008. Lipid-transfer proteins in biosynthetic pathways. Curr OpinCell Biol 20: 360–370.

D’Angelo G, Polishchuk E, Di Tullio G, Santoro M, Di Cam-pli A, Godi A, West G, Bielawski J, Chuang CC, van derSpoel AC, et al. 2007. Glycosphingolipid synthesisrequires FAPP2 transfer of glucosylceramide. Nature449: 62–67.

Daleke DL. 2007. Phospholipid flippases. J Biol Chem 282:821–825.

Ewers H, Romer W, Smith AE, Bacia K, Dmitrieff S, Chai W,Mancini R, Kartenbeck J, Chambon V, Berland L, et al.2010. GM1 structure determines SV40-induced mem-brane invagination and infection. Nat Cell Biol 12:11–12.

Futerman AH, Riezman H. 2005. The ins and outs of sphin-golipid synthesis. Trends Cell Biol 15: 312–318.

Gaylor JL, Delwiche CV. 1973. Investigation of the multien-zymic system of microsomal cholesterol biosynthesis.Ann N Y Acad Sci 212: 122–138.

Giussani P, Colleoni T, Brioschi L, Bassi R, Hanada K, Tetta-manti G, Riboni L, Viani P. 2008. Ceramide traffic in C6glioma cells: Evidence for CERT-dependent and inde-pendent transport from ER to the Golgi apparatus. Bio-chim Biophys Acta 1781: 40–51.

Gylling H, Kuusi T, Vanhanen H, Miettinen TA. 1989. Apo-lipoprotein E phenotype and cholesterol metabolism infamilial hypercholesterolemia. Atherosclerosis 80: 27–32.

Halter D, Neumann S, van Dijk SM, Wolthoorn J, deMaziere AM, Vieira OV, Mattjus P, Klumperman J, vanMeer G, Sprong H. 2007. Pre- and post-Golgi transloca-tion of glucosylceramide in glycosphingolipid synthesis.J Cell Biol 179: 101–115.

Hanada K, Kumagai K, Tomishige N, Yamaji T. 2009. CERT-mediated trafficking of ceramide. Biochim Biophys Acta1791: 684–691.

Hanada K, Kumagai K, Yasuda S, Miura Y, Kawano M, Fuka-sawa M, Nishijima M. 2003. Molecular machinery fornon-vesicular trafficking of ceramide. Nature 426:803–809.

Hannun YA, Obeid LM. 2008. Principles of bioactive lipidsignalling: Lessons from sphingolipids. Nat Rev MolCell Biol 9: 139–150.

Heikinheimo L, Somerharju P. 1998. Preferential decarbox-ylation of hydrophilic phosphatidylserine species in cul-tured cells. Implications on the mechanism of transportto mitochondria and cellular aminophospholipid speciescompositions. J Biol Chem 273: 3327–3335.

Heikinheimo L, Somerharju P. 2002. Translocation ofphosphatidylthreonine and -serine to mitochondria

diminishes exponentially with increasing molecularhydrophobicity. Traffic 3: 367–377.

Heino S, Lusa S, Somerharju P, Ehnholm C, Olkkonen VM,Ikonen E. 2000. Dissecting the role of the golgi complexand lipid rafts in biosynthetic transport of cholesterolto the cell surface. Proc Natl Acad Sci 97: 8375–8380.

Henneberry AL, McMaster CR. 1999. Cloning andexpression of a human choline/ethanolaminephospho-transferase: Synthesis of phosphatidylcholine and phos-phatidylethanolamine. Biochem J 339: 291–298.

Henneberry AL, Wistow G, McMaster CR. 2000. Cloning,genomic organization, and characterization of a humancholinephosphotransferase. J Biol Chem 275: 29808–29815.

Henneberry AL, Wright MM, McMaster CR. 2002. Themajor sites of cellular phospholipid synthesis and molec-ular determinants of fatty acid and lipid head group spe-cificity. Mol Biol Cell 13: 3148–3161.

Holthuis JC, Levine TP. 2005. Lipid traffic: Floppy drivesand a superhighway. Nat Rev Mol Cell Biol 6: 209–220.

Horibata Y, Hirabayashi Y. 2007. Identification and charac-terization of human ethanolaminephosphotransferase1.J Lipid Res 48: 503–508.

Horibata Y, Sugimoto H. 2010 StarD7 mediates the intracel-lular trafficking of phosphatidylcholine to mitochondria.J Biol Chem 285: 7358–7365.

Hynynen R, Laitinen S, Kakela R, Tanhuanpaa K, Lusa S,Ehnholm C, Somerharju P, Ikonen E, Olkkonen VM.2005. Overexpression of OSBP-related protein 2 (ORP2)induces changes in cellular cholesterol metabolism andenhances endocytosis. Biochem J 390: 273–283.

Ikeda M, Kihara A, Igarashi Y. 2004. Sphingosine-1-phosphate lyase SPL is an endoplasmic reticulum-resident, integral membrane protein with the pyridoxal50-phosphate binding domain exposed to the cytosol.Biochem Biophys Res Commun 325: 338–343.

Ikonen E. 2008. Cellular cholesterol trafficking and com-partmentalization. Nat Rev Mol Cell Biol 9: 125–138.

Jansen M, Ohsaki Y, Rega LR, Bittman R, Olkkonen VM,Ikonen E. 2010. Role of ORPs in sterol transportfrom plasma membrane to ER and lipid dropletsin mammalian cells. Traffic 12: 218–231.

Jasinska R, Zborowski J, Somerharju P. 1993. Intramitochon-drial distribution and transport of phosphatidylserine andits decarboxylation product, phosphatidylethanolamine.Application of pyrene-labeled species. Biochim BiophysActa 1152: 161–170.

Johnson WJ, Fischer RT, Phillips MC, Rothblat GH. 1995.Efflux of newly synthesized cholesterol and biosyntheticsterol intermediates from cells. Dependence on acceptortype and on enrichment of cells with cholesterol. J BiolChem 270: 25037–25046.

Kang HW, Wei J, Cohen DE. 2010 PC-TP/StARD2: Ofmembranes and metabolism. Trends Endocrinol Metab21: 449–456.

Kanno K, Wu MK, Scapa EF, Roderick SL, Cohen DE. 2007.Structure and function of phosphatidylcholine transferprotein (PC-TP)/StarD2. Biochim Biophys Acta 1771:654–662.

T. Blom et al.




Kaplan MR, Simoni RD. 1985. Intracellular transport ofphosphatidylcholine to the plasma membrane. J CellBiol 101: 441–445.

Kawano M, Kumagai K, Nishijima M, Hanada K. 2006. Effi-cient trafficking of ceramide from the endoplasmic retic-ulum to the Golgi apparatus requires a VAMP-associatedprotein-interacting FFATmotif of CERT. J Biol Chem 281:30279–30288.

Kobayashi T, Beuchat MH, Lindsay M, Frias S, Palmiter RD,Sakuraba H, Parton RG, Gruenberg J. 1999. Late endoso-mal membranes rich in lysobisphosphatidic acid regulatecholesterol transport. Nat Cell Biol 1: 113–118.

Koivusalo M, Jansen M, Somerharju P, Ikonen E. 2007.Endocytic trafficking of sphingomyelin depends on itsacyl chain length. Mol Biol Cell 18: 5113–5123.

Kol MA, de Kroon AI, Killian JA, de Kruijff B. 2004. Trans-bilayer movement of phospholipids in biogenic mem-branes. Biochemistry 43: 2673–2681.

Kolter T, Sandhoff K. 2005. Principles of lysosomal mem-brane digestion: Stimulation of sphingolipid degradationby sphingolipid activator proteins and anionic lysosomallipids. Annu Rev Cell Dev Biol 21: 81–103.

Kolter T, Sandhoff K. 2010 Lysosomal degradation of mem-brane lipids. FEBS Lett 584: 1700–1712.

Kovacs WJ, Tape KN, Shackelford JE, Duan X, Kasumov T,Kelleher JK, Brunengraber H, Krisans SK. 2007. Localiza-tion of the pre-squalene segment of the isoprenoidbiosynthetic pathway in mammalian peroxisomes. Histo-chem Cell Biol 127: 273–290.

Kudo N, Kumagai K, Matsubara R, Kobayashi S, Hanada K,Wakatsuki S, Kato R. 2010. Crystal structures of the CERTSTART domain with inhibitors provide insights intothe mechanism of ceramide transfer. J Mol Biol 396:245–251.

Kudo N, Kumagai K, Tomishige N, Yamaji T, Wakatsuki S,Nishijima M, Hanada K, Kato R. 2008. Structural basisfor specific lipid recognition by CERT responsible fornonvesicular trafficking of ceramide. Proc Natl Acad Sci105: 488–493.

Kumagai K, Yasuda S, Okemoto K, Nishijima M, KobayashiS, Hanada K. 2005. CERT mediates intermembranetransfer of various molecular species of ceramides. JBiol Chem 280: 6488–6495.

Lahiri S, Futerman AH. 2007. The metabolism and functionof sphingolipids and glycosphingolipids. Cell Mol Life Sci64: 2270–2284.

Lange Y, Echevarria F, Steck TL. 1991. Movement ofzymosterol, a precursor of cholesterol, among threemembranes in human fibroblasts. J Biol Chem 266:21439–21443.

Lavigne P, Najmanivich R, Lehoux JG. 2010. MammalianStAR-related lipid transfer (START) domains with specif-icity for cholesterol: Structural conservation and mecha-nism of reversible binding. Subcell Biochem 51: 425–437.

Lev S. 2010. Non-vesicular lipid transport by lipid transferproteins and beyond. Nat Rev Mol Cell Biol 11: 739–750.

Leventis PA, Grinstein S. 2010. The distribution and func-tion of phosphatidylserine in cellular membranes. AnnuRev Biophys 39: 407–427.

Lin D, Sugawara T, Strauss JF III, Clark BJ, Stocco DM,Saenger P, Rogol A, Miller WL. 1995. Role of

steroidogenic acute regulatory protein in adrenal andgonadal steroidogenesis. Science 267: 1828–1831.

Linke T, Wilkening G, Lansmann S, Moczall H, Bartelsen O,Weisgerber J, Sandhoff K. 2001. Stimulation of acidsphingomyelinase activity by lysosomal lipids and sphin-golipid activator proteins. Biol Chem 382: 283–290.

Loewen CJ, Roy A, Levine TP. 2003. A conserved ER target-ing motif in three families of lipid binding proteins and inOpi1p binds VAP. Embo J 22: 2025–2035.

Lusa S, Heino S, Ikonen E. 2003. Differential mobilizationof newly synthesized cholesterol and biosynthetic sterolprecursors from cells. J Biol Chem 278: 19844–19851.

Manna PR, Dyson MT, Stocco DM. 2009. Regulation of thesteroidogenic acute regulatory protein gene expression:Present and future perspectives. Mol Hum Reprod 15:321–333.

Mattjus P. 2009. Glycolipid transfer proteins and membraneinteraction. Biochim Biophys Acta 1788: 267–272.

Milhas D, Clarke CJ, Hannun YA. 2010. Sphingomyelinmetabolism at the plasma membrane: Implications forbioactive sphingolipids. FEBS Lett 584: 1887–1894.

Mobius W, van Donselaar E, Ohno-Iwashita Y, Shimada Y,Heijnen HF, Slot JW, Geuze HJ. 2003. Recycling compart-ments and the internal vesicles of multivesicular bodiesharbor most of the cholesterol found in the endocyticpathway. Traffic 4: 222–231.

Ngo MH, Colbourne TR, Ridgway ND. 2010. Functionalimplications of sterol transport by the oxysterol-bindingprotein gene family. Biochem J 429: 13–24.

Nishizuka Y. 1992. Intracellular signaling by hydrolysis ofphospholipids and activation of protein kinase C. Science258: 607–614.

Ohashi M, Mizushima N, Kabeya Y, Yoshimori T. 2003.Localization of mammalian NAD(P)H steroid dehydro-genase-like protein on lipid droplets. J Biol Chem 278:36819–36829.

Olayioye MA, Vehring S, Muller P, Herrmann A, Schiller J,Thiele C, Lindeman GJ, Visvader JE, Pomorski T. 2005.StarD10, a START domain protein overexpressed inbreast cancer, functions as a phospholipid transferprotein. J Biol Chem 280: 27436–27442.

Raff MC, Mirsky R, Fields KL, Lisak RP, Dorfman SH, Sil-berberg DH, Gregson NA, Leibowitz S, Kennedy MC.1978. Galactocerebroside is a specific cell-surface anti-genic marker for oligodendrocytes in culture. Nature274: 813–816.

Raychaudhuri S, Im YJ, Hurley JH, Prinz WA. 2006. Nonve-sicular sterol movement from plasma membrane to ERrequires oxysterol-binding protein-related proteins andphosphoinositides. J Cell Biol 173: 107–119.

Ridgway ND. 2010. Oxysterol-binding proteins. Subcell Bio-chem 51: 159–182.

Riekhof WR, Voelker DR. 2009. The yeast plasma membraneP4-ATPases are major transporters for lysophospho-lipids. Biochim Biophys Acta 1791: 620–627.

Rosenbaum AI, Zhang G, Warren JD, Maxfield FR. 2010.Endocytosis of b-cyclodextrins is responsible for choles-terol reduction in Niemann-Pick type C mutant cells.Proc Natl Acad Sci 107: 5477–5482.

Russell DW. 2003. The enzymes, regulation, and genetics ofbile acid synthesis. Annu Rev Biochem 72: 137–174.





Sabourdy F, Kedjouar B, Sorli SC, Colie S, Milhas D, Salma Y,Levade T. 2008. Functions of sphingolipid metabolism inmammals–lessons from genetic defects. Biochim BiophysActa 1781: 145–183.

Schulze H, Kolter T, Sandhoff K. 2009. Principles of lyso-somal membrane degradation: Cellular topology andbiochemistry of lysosomal lipid degradation. BiochimBiophys Acta 1793: 674–683.

Schulz TA, Choi MG, Raychaudhuri S, Mears JA, GhirlandoR, Hinshaw JE, Prinz WA. 2009. Lipid-regulated steroltransfer between closely apposed membranes byoxysterol-binding protein homologues. J Cell Biol 187:889–903.

Shiao YJ, Lupo G, Vance JE. 1995. Evidence that phos-phatidylserine is imported into mitochondria via amitochondria-associated membrane and that the major-ity of mitochondrial phosphatidylethanolamine isderived from decarboxylation of phosphatidylserine.J Biol Chem 270: 11190–11198.

Simons K, Ikonen E. 1997. Functional rafts in cell mem-branes. Nature 387: 569–572.

Smriti, Nemergut EC, Daleke DL. 2007. ATP-dependenttransport of phosphatidylserine analogues in humanerythrocytes. Biochemistry 46: 2249–2259.

Sprong H, Degroote S, Nilsson T, Kawakita M, Ishida N,van der Sluijs P, van Meer G. 2003. Associationof the Golgi UDP-galactose transporter with UDP-galactose:ceramide galactosyltransferase allows UDP-galactose import in the endoplasmic reticulum. MolBiol Cell 14: 3482–3493.

Steenbergen R, Nanowski TS, Beigneux A, Kulinski A,Young SG, Vance JE. 2005. Disruption of the phosphati-dylserine decarboxylase gene in mice causes embryoniclethality and mitochondrial defects. J Biol Chem 280:40032–40040.

Stoffel W, Bosio A. 1997. Myelin glycolipids and their func-tions. Curr Opin Neurobiol 7: 654–661.

Stone SJ, Vance JE. 2000. Phosphatidylserine synthase-1 and-2 are localized to mitochondria-associated membranes.J Biol Chem 275: 34534–34540.

Tafesse FG, Ternes P, Holthuis JC. 2006. The multigenicsphingomyelin synthase family. J Biol Chem 281:29421–29425.

Tiwari R, Koffel R, Schneiter R. 2007. An acetylation/deace-tylation cycle controls the export of sterols and steroidsfrom S. cerevisiae. Embo J 26: 5109–5119.

Trajkovic K, Hsu C, Chiantia S, Rajendran L, Wenzel D, Wie-land F, Schwille P, Brugger B, Simons M. 2008. Ceramidetriggers budding of exosome vesicles into multivesicularendosomes. Science 319: 1244–1247.

Urbani L, Simoni RD. 1990. Cholesterol and vesicular sto-matitis virus G protein take separate routes from theendoplasmic reticulum to the plasma membrane. J BiolChem 265: 1919–1923.

van Meer G, Voelker DR, Feigenson GW. 2008. Membranelipids: Where they are and how they behave. Nat RevMol Cell Biol 9: 112–124.

Vance JE. 2008. Phosphatidylserine and phosphatidyletha-nolamine in mammalian cells: Two metabolically relatedaminophospholipids. J Lipid Res 49: 1377–1387.

Voelker DR. 1985. Disruption of phosphatidylserine trans-location to the mitochondria in baby hamster kidneycells. J Biol Chem 260: 14671–14676.

Voelker DR. 2009. Genetic and biochemical analysis of non-vesicular lipid traffic. Annu Rev Biochem 78: 827–856.

Weisz OA, Rodriguez-Boulan E. 2009. Apical trafficking inepithelial cells: Signals, clusters and motors. J Cell Sci122: 4253–4266.

Wirtz KW. 2006. Phospholipid transfer proteins in perspec-tive. FEBS Lett 580: 5436–5441.

Wirtz KW, Zilversmit DB. 1969. Participation of solubleliver proteins in the exchange of membrane phospho-lipids. Biochim Biophys Acta 193: 105–116.

Wirtz KW, Schouten A, Gros P. 2006. Phosphatidylinositoltransfer proteins: From closed for transport to open forexchange. Advan Enzyme Regul 46: 301–311.

Wu C, Miloslavskaya I, Demontis S, Maestro R, GalaktionovK. 2004. Regulation of cellular response to oncogenic andoxidative stress by Seladin-1. Nature 432: 640–645.

Yamaji T, Kumagai K, Tomishige N, Hanada K. 2008. Twosphingolipid transfer proteins, CERT and FAPP2: Theirroles in sphingolipid metabolism. IUBMB Life 60:511–518.

Ye J, DeBose-Boyd A. 2011. Regulation of cholesterol andfatty acid synthesis. Cold Spring Harb Perspect Bioldoi.10.1101/cshperspect.a004754.

Zborowski J, Dygas A, Wojtczak L. 1983. Phosphatidylserinedecarboxylase is located on the external side of the innermitochondrial membrane. FEBS Lett 157: 179–182.

Zuo Y, Zhuang DZ, Han R, Isaac G, Tobin JJ, McKee M, WeltiR, Brissette JL, Fitzgerald ML, Freeman MW. 2008.ABCA12 maintains the epidermal lipid permeabilitybarrier by facilitating formation of ceramide linoleicesters. J Biol Chem 283: 36624–36635.

Zwerger M, Kolb T, Richter K, Karakesisoglou I, HerrmannH. 2010. Induction of a massive endoplasmic reticulumand perinuclear space expansion by expression of laminB receptor mutants and the related sterol reductasesTM7SF2 and DHCR7. Mol Biol Cell 21: 354–368.

T. Blom et al.




published online April 11, 2011Cold Spring Harb Perspect Biol Tomas Blom, Pentti Somerharju and Elina Ikonen Synthesis and Biosynthetic Trafficking of Membrane Lipids

Subject Collection The Biology of Lipids

Role of Lipids in Virus ReplicationMaier Lorizate and Hans-Georg Kräusslich

Membrane Organization and Lipid RaftsKai Simons and Julio L. Sampaio

Model Answers to Lipid Membrane QuestionsOle G. Mouritsen Spectrometers

Shotgun Lipidomics on High Resolution Mass

Herzog, et al.Dominik Schwudke, Kai Schuhmann, Ronny

Glycosphingolipid FunctionsClifford A. Lingwood

Glycosphingolipid FunctionsClifford A. Lingwood

SynthesisRegulation of Cholesterol and Fatty Acid

Jin Ye and Russell A. DeBose-Boyd

Phosphoinositides in Cell Architecture

MostovAnnette Shewan, Dennis J. Eastburn and Keith

Lipid-Mediated EndocytosisHelge Ewers and Ari Helenius Membrane Lipids

Synthesis and Biosynthetic Trafficking of

Tomas Blom, Pentti Somerharju and Elina Ikonen

DynamicsFluorescence Techniques to Study Lipid

Erdinc Sezgin and Petra Schwille

Lipid Polymorphisms and Membrane Shape

ZimmerbergVadim A. Frolov, Anna V. Shnyrova and Joshua

Lysosomal Lipid Storage DiseasesHeike Schulze and Konrad Sandhoff Interactions

Lipid−Specificity of Intramembrane Protein

Felix Wieland, et al.Francesc-Xabier Contreras, Andreas Max Ernst,

SphingolipidsDistribution and Functions of Sterols and

Howard RiezmanJ. Thomas Hannich, Kyohei Umebayashi and

Dynamic Transbilayer Lipid AsymmetryGerrit van Meer

http://cshperspectives.cshlp.org/cgi/collection/ For additional articles in this collection, see

Copyright © 2011 Cold Spring Harbor Laboratory Press; all rights reserved


http://cshperspectives.cshlp.org/cgi/collection/

http://cshperspectives.cshlp.org/cgi/collection/

http://cshperspectives.cshlp.org/cgi/adclick/?ad=55917&adclick=true&url=http%3A%2F%2Fwww.genelink.com


Documents

Synthesis and Biosynthetic Trafﬁcking of Membrane Lipids