A lipid associated with the antiphospholipid syndrome regulates endosome structure and function

ATM immunolocalization in mouse neuronal endosomes: implications for ataxia-telangiectasia

Ataxia-telangiectasia (A-T) is a human disorder with pleiotropic manifestations that include neoplasms, immune dysfunction and neurodegeneration. The disorder is due to mutations in the gene known as ATM (A-T, mutated), which causes a deficiency in its protein product (Atm in mice) that is necessary for DNA damage surveillance. This nuclear function of Atm explains in principle the propensity to cancer and immunodeficiency in A-T, but not the neurodegeneration which results in the earliest clinical manifestations and causes progressive disability. Here we report ultrastructural evidence of cytoplasmic localization of Atm-like immunoreactivity (ALI) within endosomes in murine cerebellocortical neurons, one of the principal targets of A-T. The ALI was obtained with two separate monoclonal antibodies that recognize Atm specifically. By contrast, electron-dense endosomes that could be confused with ALI occur in negligible amounts in both wild-type mice and in mice deficient in Atm ("knockout" mice). Furthermore, there was a marked preferential distribution of Atm-immunopositive endosomes in the granule cell layer - where they are present in granule neurons - with a much lower density in the Purkinje and molecular layers. These observations suggest that endosome-bound Atm may be more important for the function of certain neurons than others - or that it is processed differently among them - and that this protein may be involved in molecular sorting in the cytoplasm. This is relevant to elucidating the role of Atm deficiency in the pathobiology of neurodegeneration in A-T.

Two di-leucine-based motifs account for the different subcellular localizations of the human endothelin-converting enzyme (ECE-1) isoforms

Endothelin-converting enzyme (ECE-1) is a type II integral membrane protein which plays a key role in the biosynthetic pathway of the vasoconstricting endothelins. Three ECE-1 isoforms, differing by their N-terminal cytoplasmic tails, are generated from a single gene. When expressed in CHO cells, they display comparable enzymatic activity but whereas ECE-1a is strongly expressed at the cell surface, ECE-1b is exclusively intracellular and ECE-1c presents an intermediate distribution. In the present study these different localizations were further described at the ultrastructural level, by electron microscope immunocytochemistry. To characterize the motifs responsible for the intracellular localization of ECE-1b we constructed chimeric proteins and point mutants. Two di-leucine-based motifs, contained in the N-terminal part of ECE-1b, were thus identified. One of these motifs (LV), displayed by both ECE-1b and ECE-1c, accounts for the reduced surface expression of ECE-1c as compared to ECE-1a. Mutation of both motifs (LL and LV) induces a very strong appearance of ECE-1b at the cell surface indicating that their presence in the N-terminal extremity of ECE-1b is critical for its exclusively intracellular localization.

Broad screening test for sphingolipid-storage diseases

BACKGROUND

Lipid-storage diseases are collectively important because they cause substantial morbidity and mortality, and because they may present as dementia, major psychiatric illness, developmental delay, or cerebral palsy. At present, no single assay can be used as an initial general screen for lipid-storage diseases.

METHODS

We used a fluorescent analogue of lactosylceramide, called N-[5-(5,7-dimethylborondipyrromethenedifluoride)-1-pentanoyl]D- lactosylsphingosine (BODIPY-LacCer), the emission of which changes from green to red wavelengths with increasing concentrations in membranes, to examine the intracellular distribution of the lipid within living cells.

FINDINGS

During a brief pulse-chase experiment, the fluorescent lipid accumulated in the lysosomes of fibroblasts from patients with Fabry's disease, GM1 gangliosidosis, GM2 gangliosidosis (Tay-Sachs and Sandhoff forms), metachromatic leucodystrophy, mucolipidosis type IV, Niemann-Pick disease (types A, B, and C), and sphingolipid-activator-protein-precursor (prosaposin) deficiency. In control cells, the lipid was mainly confined to the Golgi complex. In a masked study, replicate samples of 25 of 26 unique cell lines representing ten different lipid-storage diseases, and 18 of 20 unique cell lines representing controls were correctly identified; the sensitivity was 96.2% (95% CI 80.4-99.9) and the specificity 90.0% (68.3-98.8).

INTERPRETATION

This method may be useful as an initial general screen for lipid-storage diseases, and, with modification, could be used for large-scale automated screening of drugs to abrogate lysosomal storage in various lipidoses. The unexpected accumulation of BODIPY-LacCer in several biochemically distinct diseases raises important questions about common mechanisms of cellular dysfunction in these disorders.

Isolation and characterization of two human monoclonal anti-phospholipid IgG from patients with autoimmune disease

The antigenic specificity of anti-phospholipid antibodies (APA) is a matter of intensive investigation. To further characterize these antibodies, we attempted to isolate human monoclonal APA. B-cells of patients with at least one positive test for antibodies against cardiolipin, phosphatidylserine, beta2-glycoprotein I (beta2-GPI) or the lupus anti-coagulant were immortalized by transformation with Epstein-Barr virus and screened for production of specific IgG. Positive pools were fused with a heteromyeloma cell line and APA-secreting clones were isolated by standard procedures. Two monoclonal APA, HL-5B from a 51-year-old man with primary anti-phospholipid syndrome and recurrent cerebral microinfarctions, and RR-7F from a 48-year-old women with systemic lupus erythematosus but no evidence for thrombotic events were obtained. HL-5B is of the IgG2 subtype with lambda light chains, while RR-7F is IgG2 with kappa light chains. Both monoclonals show reactivity against cardiolipin and phosphatidylserine but lack reactivity against beta2-GPI or lupus anti-coagulant activity. To yield the same OD in the cardiolipin and phosphatidylserine ELISAs RR-7F must be used in an approximately 10-fold higher concentration than HL-5B, indicating a lower affinity towards these antigens. Interestingly, both mAPA can bind to cardiolipin in the absence of beta2-GPI. They do not cross-react with dsDNA but show reactivity against oxidized low-density lipoproteins. Analysis of the heavy chain mRNA of HL-5B and RR-7F showed that both are members of the VH3 family. While HL-5B shows extensive somatic mutations in the CDR1 and 2 regions, indicating that it was derived by a T cell-dependent antigen driven process, RR-7F is apparently germline encoded. The two monoclonal APA can be used as tools in further structural and functional analyses.

Electrospray ionization tandem mass spectrometry (ESI-MS/MS) analysis of the lipid molecular species composition of yeast subcellular membranes reveals acyl chain-based sorting/remodeling of distinct molecular species en route to the plasma membrane

Nano-electrospray ionization tandem mass spectrometry (nano-ESI-MS/MS) was employed to determine qualitative differences in the lipid molecular species composition of a comprehensive set of organellar membranes, isolated from a single culture of Saccharomyces cerevisiae cells. Remarkable differences in the acyl chain composition of biosynthetically related phospholipid classes were observed. Acyl chain saturation was lowest in phosphatidylcholine (15.4%) and phosphatidylethanolamine (PE; 16.2%), followed by phosphatidylserine (PS; 29.4%), and highest in phosphatidylinositol (53.1%). The lipid molecular species profiles of the various membranes were generally similar, with a deviation from a calculated average profile of approximately +/- 20%. Nevertheless, clear distinctions between the molecular species profiles of different membranes were observed, suggesting that lipid sorting mechanisms are operating at the level of individual molecular species to maintain the specific lipid composition of a given membrane. Most notably, the plasma membrane is enriched in saturated species of PS and PE. The nature of the sorting mechanism that determines the lipid composition of the plasma membrane was investigated further. The accumulation of monounsaturated species of PS at the expense of diunsaturated species in the plasma membrane of wild-type cells was reversed in elo3Delta mutant cells, which synthesize C24 fatty acid-substituted sphingolipids instead of the normal C26 fatty acid-substituted species. This observation suggests that acyl chain-based sorting and/or remodeling mechanisms are operating to maintain the specific lipid molecular species composition of the yeast plasma membrane.

Intracellular distribution of a biotin-labeled ganglioside, GM1, by immunoelectron microscopy after endocytosis in fibroblasts

A radioactive and biotin-labeled analogue of GM1 (biotin-GM1) was synthesized which enabled us to analyze its intracellular distribution in the compartments of the endocytic route by electron microscopic immunocytochemistry using thin sections of human skin fibroblasts labeled with gold-conjugated antibiotin antibodies. Metabolic studies with the biotin-GM1 showed its partial degradation to the corresponding GM2 and GM3 derivatives. Further degradation was inhibited by the biotin residue. The distribution of biotin-GM1 after uptake by cells was studied by postembedding labeling techniques. On the plasma membrane the biotin-GM1 was detectable in the form of patches (0.1 micrometer in diameter), in caveola-like structures and, to a much lesser extent, in coated pits or vesicles. During endocytic uptake, the biotin-GM1 became detectable in organelles identified as late endosomes and lysosomes. The intracellular distribution of the biotin-GM1 was compared to the localization of the EGF receptor in EGF-stimulated fibroblasts. Both the biotin-GM1 and the EGF receptor were transported to intraendosomal and intralysosomal membranes, indicating that both membrane constituents follow the same pathway of endocytosis. Our observations show that biotin-GM1 can be successfully incorporated into the plasma membrane and be used as a tool for morphological detection of its pathway to lysosomes.

The electrostatics of lipid surfaces

Charged lipids constitute a substantial fraction of all membrane lipids. Their charges vary in quantity and distribution within their headgroup regions. In long range interactions, their charges' value and electrostatic potential in the vicinity of the membrane surface can be approximated by the Guy-Chapman theory. This theory treats the interface as a charged structureless plain surrounded by uniform environments. However, if one considers intermolecular interactions, such assumptions need to be revised. The interface is in reality a thick region containing the residual charges of lipid headgroups. Their arrangement depends on the type of lipid present in the membrane. The variety of lipids and their biological functions suggests that charge distribution determines the extent and type of interaction with surface associated molecules. Numerous examples show that protein behavior at the lipid bilayer surface is determined by the type of lipid present, indicating protein specificity towards certain surface locations and local properties (determined by lipid composition) of a particular type. Such specificity is achieved by a combination of electrostatic, hydrophobic and enthropic effects. Comparing lipid biological activity, it can be stated that residual charge distribution is one of the factors of intermolecular recognition leading to the specific interaction of lipid molecules and selected proteins in various processes, particularly those involved with signal transduction pathways. Such specificity enables a variety of processes occurring simultaneously on the same membrane surface to function without cross-reaction interference.

Disease models provide insights into post-golgi protein trafficking, localization and processing

Over the past two years, the use of in vitro systems and the identification of autoantibodies to Golgi proteins have provided important new tools for analyzing vesicle and cargo trafficking in the distal secretory pathway. In addition, the phenotypic characterization of mice with knockouts of various prohormone convertases has led to significant progress in understanding the biological relevance of prohormone processing in post-Golgi compartments.

Mammalian phospholipase D structure and regulation

The recent identification of cDNA clones for phospholipase D1 and 2 has opened the door to new studies on its structure and regulation. PLD activity is encoded by at least two different genes that contain catalytic domains that relate their mechanism of action to phosphodiesterases. In vivo roles for PLD suggest that it may be important for multiple specialized steps in receptor dependent and constitutive processes of secretion, endocytosis, and membrane biogenesis.

Enzyme distributions in subcellular fractions of BHK cells infected with Semliki forest virus: evidence for a major fraction of sphingomyelin synthase in the trans-golgi network

BHK cells either untreated or infected with Semliki Forest virus have been fractionated on sucrose density gradients. Virus infection caused an increase in density of a membrane fraction enriched in sphingomyelin (SM), cholesterol, SM synthase and sialyltransferase activity. This increase in density was related to incorporation of viral proteins into this fraction, which is likely to contain trans-Golgi network (TGN) membranes. In contrast, glucosylceramide synthase and galactosyltransferase activities (markers for cis/medial and trans-Golgi respectively) underwent no density shift and alkaline phosphodiesterase, a plasma membrane marker, was only slightly density-shifted in infected cells. When cells were incubated with NBD-ceramide to enable them to synthesise NBD-SM and then washed with albumin to remove surface label, fluorescence in untreated cells was concentrated in a single juxtanuclear spot but in infected cells this region of bright fluorescence was larger and extended around the nucleus. After fractionation of these cells, NBD-SM (but only a small proportion of the NBD-ceramide) was found to be shifted into the higher density fraction in infected cells. This work provides further evidence that SM synthase is not mainly localised in the early Golgi cisternae as previously thought, but is associated more with a cholesterol-rich compartment which could be the TGN.

Structural and membrane-binding properties of saposin D

Saposin D is generated together with three similar proteins, saposins A, B and C, from a common precursor, called prosaposin, in acidic organelles such as late endosomes and lysosomes. Although saposin D has been reported to stimulate the enzymatic hydrolysis of sphingomyelin and ceramide, its physiological role has not yet been clearly established. In the present study we examined structural and membrane-binding properties of saposin D. At acidic pH, saposin D showed a great affinity for phospholipid membranes containing an anionic phospholipid such as phosphatidylserine or phosphatidic acid. The binding of saposin D caused destabilization of the lipid surface and, conversely, the association with the membrane markedly affected the fluorescence properties of saposin D. The presence of phosphatidylserine-containing vesicles greatly enhanced the intrinsic tyrosine fluorescence of saposin D, which contains tyrosines but not tryptophan residues. The structural properties of saposin D were investigated in detail using advanced MS analysis. It was found that the main form of saposin D consists of 80 amino acid residues and that the six cysteine residues are linked in the following order: Cys5-Cys78, Cys8-Cys72 and Cys36-Cys47. The disulfide pattern of saposin D is identical with that previously established for two other saposins, B and C, which also exhibit a strong affinity for lipids. The common disulfide structure probably has an important role in the interaction of these proteins with membranes. The analysis of the sugar moiety of saposin D revealed that the single N-glycosylation site present in the molecule is mainly modified by high-mannose-type structures varying from two to six hexose residues. Deglycosylation had no effect on the interaction of saposin D with phospholipid membranes, indicating that the glycosylation site is not related to the lipid-binding site. The association of saposin D with membranes was highly dependent on the composition of the bilayer. Neither ceramide nor sphingomyelin, sphingolipids whose hydrolysis is favoured by saposin D, promoted its binding, while the presence of an acidic phospholipid such as phosphatidylserine or phosphatidic acid greatly favoured the interaction of saposin D with vesicles at low pH. These results suggest that, in the acidic organelles where saposins are localized, anionic phospholipids may be determinants of the saposin D topology and, conversely, saposin D may affect the lipid organization of anionic phospholipid-containing membranes.

Storage of tissue-type plasminogen activator in Weibel-Palade bodies of human endothelial cells

Tissue-type plasminogen activator (t-PA) is acutely released by endothelial cells. Although its endothelial storage compartment is still not well defined, t-PA release is often accompanied by release of von Willebrand factor (vWf), a protein stored in Weibel-Palade bodies. We investigated, therefore, whether t-PA is stored in these secretory organelles. Under basal culture conditions, a minority of human umbilical vein endothelial cells (HUVEC) exhibited immunofluorescent staining for t-PA, which was observed only in Weibel-Palade bodies. To increase t-PA expression, HUVEC were infected with a t-PA recombinant adenovirus (AdCMVt-PA). Overexpressed t-PA was detected in Weibel-Palade bodies and acutely released together with endogenous vWf by thrombin or calcium ionophore stimulation. In contrast, plasminogen activator inhibitor type 1 and urokinase were not detected in Weibel-Palade bodies after adenovirus-mediated overexpression. Infection of HUVEC with proinsulin recombinant adenovirus resulted in the storage of insulin in Weibel-Palade bodies, indicating that these organelles can also store nonendothelial proteins that show regulated secretion. Infection of AtT-20 pituitary cells, a cell type with regulated secretion, with AdCMVt-PA resulted in the localization of t-PA in adrenocorticotropic hormone-containing granules, indicating that t-PA can be diverted to secretory granules independently of vWf. Coinfection of AtT-20 cells with AdCMVt-PA and proinsulin recombinant adenovirus resulted in the colocalization of t-PA and insulin in the same granules. Taken together, these results suggest that HUVEC have protein sorting mechanisms similar to those of other regulated secretory cells. Although the results did not exclude an alternative storage site for t-PA in HUVEC, they established that t-PA can be stored in Weibel-Palade bodies. This finding may explain the acute coordinate secretion of t-PA and vWf.

Molecular characterization of caveolin association with the Golgi complex: identification of a cis-Golgi targeting domain in the caveolin molecule

Caveolins are integral membrane proteins which are a major component of caveolae. In addition, caveolins have been proposed to cycle between intracellular compartments and the cell surface but the exact trafficking route and targeting information in the caveolin molecule have not been defined. We show that antibodies against the caveolin scaffolding domain or against the COOH terminus of caveolin-1 show a striking specificity for the Golgi pool of caveolin and do not recognize surface caveolin by immunofluorescence. To analyze the Golgi targeting of caveolin in more detail, caveolin mutants were expressed in fibroblasts. Specific mutants lacking the NH2 terminus were targeted to the cis Golgi but were not detectable in surface caveolae. Moreover, a 32-amino acid segment of the putative COOH-terminal cytoplasmic domain of caveolin-3 was targeted specifically and exclusively to the Golgi complex and could target a soluble heterologous protein, green fluorescent protein, to this compartment. Palmitoylation-deficient COOH-terminal mutants showed negligible association with the Golgi complex. This study defines unique Golgi targeting information in the caveolin molecule and identifies the cis Golgi complex as an intermediate compartment on the caveolin cycling pathway.

Biogenesis of transport intermediates in the endocytic pathway

Evidence is accumulating that membrane traffic between organelles can be achieved by different types of intermediates. Small (< 100 nm) and short-lived vesicles mediate transport from the plasma membrane or the trans-Golgi network to endosomes, and formation of these vesicles depends on specific adapter complexes. In contrast, transport from early to late endosomes is achieved by relatively large (approximately 0.5 microm), long-lived and multivesicular intermediates, and their biogenesis depends on endosomal COP-I proteins. Here, we review recent work on the formation of these different transport intermediates, and we discuss, in particular, coat proteins, sorting signals contained in cargo molecules and the emerging role of lipid in vesicle biogenesis.

Dominant-negative caveolin inhibits H-Ras function by disrupting cholesterol-rich plasma membrane domains

The plasma membrane pits known as caveolae have been implicated both in cholesterol homeostasis and in signal transduction. CavDGV and CavKSY, two dominant-negative amino-terminal truncation mutants of caveolin, the major structural protein of caveolae, significantly inhibited caveola-mediated SV40 infection, and were assayed for effects on Ras function. We find that CavDGV completely blocked Raf activation mediated by H-Ras, but not that mediated by K-Ras. Strikingly, the inhibitory effect of CavDGV on H-Ras signalling was completely reversed by replenishing cell membranes with cholesterol and was mimicked by cyclodextrin treatment, which depletes membrane cholesterol. These results provide a crucial link between the cholesterol-trafficking role of caveolin and its postulated role in signal transduction through cholesterol-rich surface domains. They also provide direct evidence that H-Ras and K-Ras, which are targeted to the plasma membrane by different carboxy-terminal anchors, operate in functionally distinct microdomains of the plasma membrane.

Late endosomal membranes rich in lysobisphosphatidic acid regulate cholesterol transport

The fate of free cholesterol released after endocytosis of low-density lipoproteins remains obscure. Here we report that late endosomes have a pivotal role in intracellular cholesterol transport. We find that in the genetic disease Niemann-Pick type C (NPC), and in drug-treated cells that mimic NPC, cholesterol accumulates in late endosomes and sorting of the lysosomal enzyme receptor is impaired. Our results show that the characteristic network of lysobisphosphatidic acid-rich membranes contained within multivesicular late endosomes regulates cholesterol transport, presumably by acting as a collection and distribution device. The results also suggest that similar endosomal defects accompany the anti-phospholipid syndrome and NPC.

Interaction of the GM2-activator protein with phospholipid-ganglioside bilayer membranes and with monolayers at the air-water interface

Differential scanning calorimetry (DSC) and film balance measurements were performed to study the interactions of the GalNAcbeta1-->4(NeuAcalpha2-->3)Galbeta1-->4Glc1 -->1'Cer (GM2)-activator protein with phospholipid/ganglioside vesicles and monolayers. The nonglycosylated form of the GM2-activator protein, added to unilamellar lipid vesicles of different composition, causes differential effects on the gel to liquid-crystalline phase transition peaks. The phase transition temperature (Tm) of pure dimyristoylglycerophosphocholine (DMPC) bilayer is slightly decreased. When lipids which specifically bind the GM2-activator protein are incorporated into the vesicles (e.g. a sulfatide or gangliosides) a shoulder in the thermograms at higher temperatures is observed, indicating an increase of the stability of the gel phase in relation to the liquid-crystalline phase. We also studied the surface activity of a glycosylated and a nonglycosylated GM2-activator protein at the air-water interface. The glycosylated form showed a slightly lower surface activity than the GM2-activator protein without oligosaccharide moiety. When the GM2-activator protein is added to the sub-phase of a surface covered with a lipid monolayer, it can only insert into the monolayer and reach the air-water interface below a monolayer pressure of 25 mN.m-1, depending on the lipid composition, and not when the monolayers are at the bilayer equivalence pressure of 30-35 mN.m-1. Particularly for Galbeta1-->3GalNAcbeta1-->4(NeuAcalpha2-->3)Galbeta 1-->4Glc1-->1'Cer (GM1) and GM2 containing films, the critical pressures (picrit) when no additional increase in surface pressure is observed after addition of the protein into the subphase, are much lower. This leads to the conclusion that binding of the GM2 activator protein to the ganglioside headgroups prevents the protein from reaching the air-water interface. The protein is then located preferentially at the lipid-water interface and cannot penetrate into the chain region.

Nef-induced CD4 degradation: a diacidic-based motif in Nef functions as a lysosomal targeting signal through the binding of beta-COP in endosomes

The Nef protein of primate lentiviruses downregulates the cell surface expression of CD4 through a two-step process. First, Nef connects the cytoplasmic tail of CD4 with adaptor protein complexes (AP), thereby inducing the formation of CD4-specific clathrin-coated pits that rapidly endocytose the viral receptor. Second, Nef targets internalized CD4 molecules for degradation. Here we show that Nef accomplishes this second task by acting as a connector between CD4 and the beta subunit of COPI coatomers in endosomes. A sequence encompassing a critical acidic dipeptide, located nearby but distinct from the AP-binding determinant of HIV-1 Nef, is responsible for beta-COP recruitment and for routing to lysosomes. A novel class of endosomal sorting motif, based on acidic residues, is thus revealed, and beta-COP is identified as its downstream partner.

Transacylase formation of bis(monoacylglycerol)phosphate

Recent work within our laboratory has focused on the enzymes we hypothesize are involved in the biosynthesis of bis(monoacylglycerol)phosphate from phosphatidylglycerol. Here we describe a transacylase, active at acidic pH values, isolated from a macrophage-like cell line, RAW 264.7. This enzyme acylates the head group glycerol of sn-3:sn-1' lysophosphatidylglycerol to form sn-3:sn-1' bis(monoacylglycerol)phosphate. Here we demonstrate that this enzyme uses two lysophosphatidylglycerol molecules, one as an acyl donor and another as an acyl acceptor, and that the acyl contributions from all other lipids tested are comparatively minor. This enzyme prefers saturated acyl chains to monounsaturates, 16 and 18 carbon fatty acids over 14 carbon fatty acids, and saturated acyl chains at the sn-1 position to monounsaturated acyl chains on the sn-2 carbon of lysophosphatidylglycerol. We present data which show the transacylase activity depends on the presence of a lipid-water interface and the lipid polymorphic state.

Endocytic sorting of lipid analogues differing solely in the chemistry of their hydrophobic tails

To understand the mechanisms for endocytic sorting of lipids, we investigated the trafficking of three lipid-mimetic dialkylindocarbocyanine (DiI) derivatives, DiIC16(3) (1,1'-dihexadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate), DiIC12(3) (1,1'- didodecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate), and FAST DiI (1,1'-dilinoleyl-3,3,3', 3'-tetramethylindocarbocyanine perchlorate), in CHO cells by quantitative fluorescence microscopy. All three DiIs have the same head group, but differ in their alkyl tail length or unsaturation; these differences are expected to affect their distribution in membrane domains of varying fluidity or curvature. All three DiIs initially enter sorting endosomes containing endocytosed transferrin. DiIC16(3), with two long 16-carbon saturated tails is then delivered to late endosomes, whereas FAST DiI, with two cis double bonds in each tail, and DiIC12(3), with saturated but shorter (12-carbon) tails, are mainly found in the endocytic recycling compartment. We also find that DiOC16(3) (3,3'- dihexadecyloxacarbocyanine perchlorate) and FAST DiO (3, 3'-dilinoleyloxacarbocyanine perchlorate) behave similarly to their DiI counterparts. Furthermore, whereas a phosphatidylcholine analogue with a BODIPY (4,4-difluoro-4-bora-3a,4a-diaza-s-indacene) fluorophore attached at the end of a 5-carbon acyl chain is delivered efficiently to the endocytic recycling compartment, a significant fraction of another derivative with BODIPY attached to a 12-carbon acyl chain entered late endosomes. Our results thus suggest that endocytic organelles can sort membrane components efficiently based on their preference for association with domains of varying characteristics.

Isoproterenol inhibits fluid-phase endocytosis from early to late endosomes

We have shown recently that isoproterenol affects both the cellular location and the morphology of late endosomes in a pH-dependent manner [Marjomäki et al., Eur. J. Cell Biol. 65, 1-13 (1994)]. In this study, using fluorescence and quantitative electron microscopy, we wanted to examine further what is the fate of internalized markers during their translocation from early to late endosomes under isoproterenol treatment. Fluorescein dextran internalized for 30 min (10-min pulse followed by a 20-min chase) showed accumulation in the cellular periphery during isoproterenol treatment in contrast to the control cells, which accumulated dextran in the perinuclear region. Quantitative electron microscopy showed that the markers accumulated in the early endosomes and putative carrier vesicles. In addition, different particulate markers that were internalized sequentially accumulated in similar structures due to the isoproterenol treatment, altogether suggesting that isoproterenol retards the translocation of markers to the later structures. Prelabelling of the late endosomes with fluorescent dextran or BSA-coated gold particles showed that isoproterenol causes a reduction of the mean size of the prelabelled late endosomes as well as a shift of these vesicles to the cellular periphery. Isoproterenol had no apparent effect on the morphology nor on the location of lysosomes. Percoll fractionation showed that the changes in late endosomal location and morphology did not change their characteristic density. Furthermore, electron microscopy showed that, in the cellular periphery, these late endosomal elements did not fuse with early endosomal structures, which is in agreement with the results of biochemical in vitro cell-free assays carried out by others. In conclusion, the results show that isoproterenol inhibits transport from early to late endosomes in a manner that may be pH- and/or Ca(2+)-dependent. Simultaneously, isoproterenol causes fragmentation of the late endosomal compartment and the shift of these fragments to the cellular periphery, where they have a restricted ability to fuse with earlier endosomal structures.

Parasitophorous vacuoles of Leishmania mexicana acquire macromolecules from the host cell cytosol via two independent routes

The intracellular parasite Leishmania survives and proliferates in host macrophages. In this study we show that parasitophorous vacuoles of L. mexicana gain access to cytosolic material via two different routes. (1) Small anionic molecules such as Lucifer Yellow are rapidly transported into the vacuoles by an active transport mechanism that is sensitive to inhibitors of the host cell's organic anion transporter. (2) Larger molecules such as fluorescent dextrans introduced into the host cell cytosol are also delivered to parasitophorous vacuoles. This transport is slower and sensitive to modulators of autophagy. Infected macrophages were examined by two novel assays to visualize and quantify this process. Immunoelectron microscopy of cells loaded with digoxigenin-dextran revealed label in multivesicular endosomes, which appeared to fuse with parasitophorous vacuoles. The inner membranes of the multivesicular vesicles label strongly with antibodies against lysobisphosphatidic acid, suggesting that they represent a point of confluence between the endosomal and autophagosomal pathways. Although the rate of autophagous transfer was comparable in infected and uninfected cells, infected cells retained hydrolyzed cysteine proteinase substrate to a greater degree. These data suggest that L. mexicana-containing vacuoles have access to potential nutrients in the host cell cytosol via at least two independent mechanisms.

Involvement of MIIC-Like Late Endosomes in B Cell Receptor-Mediated Antigen Processing in Murine B Cells

Currently, the involvement of classical vs novel endocytic compartments in the phenomenon of B cell receptor (BCR)-mediated Ag processing is a matter of considerable debate. In murine B cells, class II vesicles (CIIV) represent a novel endocytic compartment involved in BCR-mediated Ag processing and class II peptide loading. Alternatively, in human B cells, the MHC class II-enriched compartment (MIIC) represents a lysosome (L)-like endocytic compartment that appears to be involved in this process. Presently, the relationship between CIIV, MIIC, and classical endosomes and L remains to be determined. Using density gradient centrifugation, a subcellular compartment morphologically and immunologically similar to human MIIC has been identified, isolated, and characterized in murine B cells. These MIIC-like vesicles represent a population of class II-positive late endosomes (LE) and are distinct from CIIV. MIIC-like LE are uniquely marked by the thiol protease cathepsin B, and along with mature L, appear to be the major repository of DM molecules in these cells. Importantly, both MIIC-like LE and CIIV isolated from Ag-pulsed B cells contain BCR-internalized Ag as well as antigenic peptide-class II complexes.

Multivesicular body morphogenesis requires phosphatidyl-inositol 3-kinase activity

Multivesicular bodies are endocytic compartments containing multiple small vesicles that originate from the invagination and 'pinching off' of the limiting membrane into the luminal space [1] [2] [3]. The molecular mechanisms responsible for the formation of these compartments are unknown. In the human melanoma cell line Mel JuSo, newly synthesised major histocompatibility complex (MHC) class II molecules accumulate in multivesicular early lysosomes [4]. The phosphatidylinositol (PI) 3-kinase inhibitor wortmannin induced the transient vacuolation of early MHC class II compartments, but also of early and late endosomes. We demonstrate that endocytic membrane influx is required for the wortmannin-induced swelling of vesicles. The wortmannin-induced vacuoles contained a reduced number of intraluminal vesicles that were linked to the limiting membrane by membraneous connections. These data suggest that wortmannin inhibits the invagination and/or pinching off of intraluminal vesicles and provide evidence of a role for PI 3-kinase in multivesicular body morphogenesis. We propose that the wortmannin-induced vacuolation occurs as a result of the inability of multivesicular bodies to store endocytosed membranes as intraluminal vesicles thereby causing the formation of large 'empty' vacuoles.

Urokinase-type plasminogen activator receptor is internalized by different mechanisms in polarized and nonpolarized Madin-Darby canine kidney epithelial cells

Accumulated data indicate that endocytosis of the glycosylphosphatidyl-inositol-anchored protein urokinase plasminogen activator receptor (uPAR) depends on binding of the ligand uPA:plasminogen activator inhibitor-1 (PAI-1) and subsequent interaction with internalization receptors of the low-density lipoprotein receptor family, which are internalized through clathrin-coated pits. This interaction is inhibited by receptor-associated protein (RAP). We show that uPAR with bound uPA:PAI-1 is capable of entering cells in a clathrin-independent process. First, HeLaK44A cells expressing mutant dynamin efficiently internalized uPA:PAI-1 under conditions in which transferrin endocytosis was blocked. Second, in polarized Madin-Darby canine kidney (MDCK) cells, which expressed human uPAR apically, the low basal rate of uPAR ligand endocytosis, which could not be inhibited by RAP, was increased by forskolin or phorbol ester (phorbol 12-myristate 13-acetate), which selectively up-regulate clathrin-independent endocytosis from the apical domain of epithelial cells. Third, in subconfluent nonpolarized MDCK cells, endocytosis of uPA:PAI-1 was only decreased marginally by RAP. At the ultrastructural level uPAR was largely excluded from clathrin-coated pits in these cells and localized in invaginated caveolae only in the presence of cross-linking antibodies. Interestingly, a larger fraction of uPAR in nonpolarized relative to polarized MDCK cells was insoluble in Triton X-100 at 0 degreesC, and by surface labeling with biotin we also show that internalized uPAR was mainly detergent insoluble, suggesting a correlation between association with detergent-resistant membrane microdomains and higher degree of clathrin-independent endocytosis. Furthermore, by cryoimmunogold labeling we show that 5-10% of internalized uPAR in nonpolarized, but not polarized, MDCK cells is targeted to lysosomes by a mechanism that is regulated by ligand occupancy.

Fab1p PtdIns(3)P 5-kinase function essential for protein sorting in the multivesicular body

Sorting of signal-transducing cell surface receptors within multivesicular bodies (MVBs) is required for their rapid down-regulation and degradation within lysosomes. Yeast mutants defective in late stages of transport to the vacuole/lysosome accumulate MVBs. We demonstrate that the membrane glycoprotein carboxypeptidase S and the G protein-coupled receptor Ste2p are targeted into the vacuole lumen, and this process requires a subset of VPS gene products essential for normal endosome function. The PtdIns(3)P 5-kinase activity of Fab1p, which converts the product of the Vps34p PtdIns 3-kinase PtdIns(3)P into PtdIns(3,5)P2, also is required for cargo-selective sorting into the vacuole lumen. These findings demonstrate a role for phosphoinositide signaling at distinct stages of vacuolar/lysosomal protein transport and couple PtdIns(3,5)P2 synthesis to regulation of MVB sorting.

Molecular aspects of the endocytic pathway

Observation of the flow of material along the endocytic pathway has lead to the description of the basic architecture of the pathway and provided insight into the relationship between compartments. Significant advances have been made in the study of endocytic transport steps at the molecular level, of which studies of cargo selection, vesicle budding and membrane fusion events comprise the major part. Progress in this area has been driven by two approaches, yeast genetics and in vitro or cell-free assays, which reconstitute particular transport steps and allow biochemical manipulation. The complex protein machineries that control vesicle budding and fusion are significantly conserved between the secretory and endocytic pathways such that proteins that regulate particular steps are often part of a larger family of proteins which exercise a conserved function at other locations within the cell. Well characterized examples include vesicle coat proteins, rabs (small GTPases) and soluble N-ethylmaleimide-sensitive fusion protein (NSF) attachment protein (SNAP) receptors (SNAREs). Intracompartmental pH, lipid composition and cytoskeletal organization have also been identified as important determinants of the orderly flow of material within the endocytic pathway.

Lupus anticoagulants and antiphospholipid antibodies

In recent years, clinical syndromes involving lupus anticoagulants and antiphospholipid antibodies have come into increasing clinical prominence. Since the discovery that most antiphospholipid antibodies require the presence of anionic phospholipid-binding proteins such as B2-glycoprotein I and prothrombin, a large number of studies have attempted to delineate the specificity of these antibodies. Several mechanisms have been proposed to explain the hypercoagulable state associated with these antibodies. This review attempts to summarize these data and the challenges that confront efforts to delineate the pathogenesis of the prothrombotic state associated with the presence of these antibodies.

Lysosomal degradation on vesicular membrane surfaces. Enhanced glucosylceramide degradation by lysosomal anionic lipids and activators

According to a recent hypothesis (Sandhoff, K., and Kolter, T. (1996) Trends Cell Biol. 6, 98-103), glycolipids, which originate from the plasma membrane, are exposed to lysosomal degradation on the surface of intralysosomal vesicles. Taking the interaction of membrane-bound lipid substrates and lysosomal hydrolases as an experimental model, we studied the degradation of glucosylceramides with different acyl chain lengths by purified glucocerebrosidase in a detergent-free liposomal assay system. Our investigation focused on the stimulating effect induced by lysosomal components such as sphingolipid activator protein C (SAP-C or saposin C), anionic lysosomal lipids, bis(monoacylglycero)phosphate, and dolichol phosphate, as well as degradation products of lysosomal lipids, e.g. dolichols and free fatty acids. The size of the substrate-containing liposomal vesicles was varied in the study. Enzymatic hydrolysis of glucosylceramide carried by liposomes made of phosphatidylcholine and cholesterol was rather slow and only weakly accelerated by the addition of SAP-C. However, the incorporation of anionic lipids such as bis(monoacylglycero)phosphate, dolichol phosphate, and phosphatidylinositol into the substrate carrying liposomes stimulated glucosylceramide hydrolysis up to 30-fold. Dolichol was less effective. SAP-C activated glucosylceramide hydrolysis under a variety of experimental conditions and was especially effective for the increase of enzyme activity when anionic lipids were inserted into the liposomes. Glucosylceramides with short acyl chains were found to be degraded much faster than the natural substrates. Dilution experiments indicated that the added enzyme molecules associate at least partially with the membranes and act there. Surface plasmon resonance experiments demonstrated binding of SAP-C at concentrations up to 1 microM to liposomes. At higher concentrations (2.5 microM SAP-C), liposomal lipids were released from the liposome coated chip. A model for lysosomal glucosylceramide hydrolysis is discussed.

Phosphoinositide signaling and turnover: PtdIns(3)P, a regulator of membrane traffic, is transported to the vacuole and degraded by a process that requires lumenal vacuolar hydrolase activities

The Golgi/endosome-associated Vps34 phosphatidylinositol 3-kinase is essential for the sorting of hydrolases from the Golgi to the vacuole/lysosome. Upon inactivation of a temperature-conditional Vps34 kinase, cellular levels of PtdIns(3)P rapidly decrease and it has been proposed that this decrease is due to the continued turnover of PtdIns(3)P by cytoplasmic phosphatases. Here we show that mutations in VAM3 (vacuolar t-SNARE) and YPT7 (rab GTPase), which are required to direct protein and membrane delivery from prevacuolar endosomal compartments to the vacuole, dramatically increase/stabilize PtdIns(3)P levels in vivo by disrupting its turnover. We find that the majority of the total pool of PtdIns(3)P which has been synthesized, but not PtdIns(4)P, requires transport to the vacuole in order to be turned over. Unexpectedly, strains with impaired vacuolar hydrolase activity accumulate 4- to 5-fold higher PtdIns(3)P levels than wild-type cells, suggesting that lumenal vacuolar lipase and/or phosphatase activities degrade PtdIns(3)P. Because vacuolar hydrolases act in the lumen, PtdIns(3)P is likely to be transferred from the cytoplasmic membrane leaflet where it is synthesized, to the lumen of the vacuole. Interestingly, mutants that stabilize PtdIns(3)P accumulate small uniformly-sized vesicles (40-50 nm) within prevacuolar endosomes (multivesicular bodies) or the vacuole lumen. Based on these and other observations, we propose that PtdIns(3)P is degraded by an unexpected mechanism which involves the sorting of PtdIns(3)P into vesicles generated by invagination of the limiting membrane of the endosome or vacuole, ultimately delivering the phosphoinositide into the lumen of the compartment where it can be degraded by the resident hydrolases.

Protein toxins and membrane transport

Recently, protein toxins have provided novel information on the anatomy of the machinery that mediates vesicle docking and fusion with target membranes within the cell. Their use is being extended to the study of the physiology of these processes in different cells and tissues, as well as to the intracellular pathways of membrane transport.

A pore-forming toxin interacts with a GPI-anchored protein and causes vacuolation of the endoplasmic reticulum

In this paper, we have investigated the effects of the pore-forming toxin aerolysin, produced by Aeromonas hydrophila, on mammalian cells. Our data indicate that the protoxin binds to an 80-kD glycosyl-phosphatidylinositol (GPI)-anchored protein on BHK cells, and that the bound toxin is associated with specialized plasma membrane domains, described as detergent-insoluble microdomains, or cholesterol-glycolipid "rafts." We show that the protoxin is then processed to its mature form by host cell proteases. We propose that the preferential association of the toxin with rafts, through binding to GPI-anchored proteins, is likely to increase the local toxin concentration and thereby promote oligomerization, a step that it is a prerequisite for channel formation. We show that channel formation does not lead to disruption of the plasma membrane but to the selective permeabilization to small ions such as potassium, which causes plasma membrane depolarization. Next we studied the consequences of channel formation on the organization and dynamics of intracellular membranes. Strikingly, we found that the toxin causes dramatic vacuolation of the ER, but does not affect other intracellular compartments. Concomitantly we find that the COPI coat is released from biosynthetic membranes and that biosynthetic transport of newly synthesized transmembrane G protein of vesicular stomatitis virus is inhibited. Our data indicate that binding of proaerolysin to GPI-anchored proteins and processing of the toxin lead to oligomerization and channel formation in the plasma membrane, which in turn causes selective disorganization of early biosynthetic membrane dynamics.