Sphingolipid trafficking and protein sorting in epithelial cells

Genetic and lipidomic analyses suggest that Nostoc punctiforme, a plant-symbiotic cyanobacterium, does not produce sphingolipids

Sphingolipids, a class of amino-alcohol-based lipids, are well characterized in eukaryotes and in some anaerobic bacteria. However, the only sphingolipids so far identified in cyanobacteria are two ceramides (i.e., an acetylsphingomyelin and a cerebroside), both based on unbranched, long-chain base (LCB) sphingolipids in Scytonema julianum and Moorea producens, respectively. The first step in de novo sphingolipid biosynthesis is the condensation of l-serine with palmitoyl-CoA to produce 3-keto-diyhydrosphingosine (KDS). This reaction is catalyzed by serine palmitoyltransferase (SPT), which belongs to a small family of pyridoxal phosphate-dependent α-oxoamine synthase (AOS) enzymes. Based on sequence similarity to molecularly characterized bacterial SPT peptides, we identified a putative SPT (Npun_R3567) from the model nitrogen-fixing, plant-symbiotic cyanobacterium, Nostoc punctiforme strain PCC 73102 (ATCC 29133). Gene expression analysis revealed that Npun_R3567 is induced during late-stage diazotrophic growth in N. punctiforme. However, Npun_R3567 could not produce the SPT reaction product, 3-keto-diyhydrosphingosine (KDS), when heterologously expressed in Escherichia coli. This agreed with a sphingolipidomic analysis of N. punctiforme cells, which revealed that no LCBs or ceramides were present. To gain a better understanding of Npun_R3567, we inferred the phylogenetic position of Npun_R3567 relative to other bacterial AOS peptides. Rather than clustering with other bacterial SPTs, Npun_R3567 and the other cyanobacterial BioF homologues formed a separate, monophyletic group. Given that N. punctiforme does not appear to possess any other gene encoding an AOS enzyme, it is altogether unlikely that N. punctiforme is capable of synthesizing sphingolipids. In the context of cross-kingdom symbiosis signalling in which sphingolipids are emerging as important regulators, it appears unlikely that sphingolipids from N. punctiforme play a regulatory role during its symbiotic association with plants.

Sphingolipidomics in Translational Sepsis Research-Biomedical Considerations and Perspectives

Scientific Background: Sphingolipids are a highly diverse group of lipids with respect to physicochemical properties controlling either structure, distribution, or function, all of them regulating cellular response in health and disease. Mass spectrometry, on the other hand, is an analytical technique characterizing ionized molecules or fragments thereof by mass-to-charge ratios, which has been prosperingly developed for rapid and reliable qualitative and quantitative identification of lipid species. Parallel to best performance of in-depth chromatographical separation of lipid classes, preconditions of precise quantitation of unique molecular species by preprocessing of biological samples have to be fulfilled. As a consequence, “lipid profiles” across model systems and human individuals, esp. complex (clinical) samples, have become eminent over the last couple of years due to sensitivity, specificity, and discriminatory capability. Therefore, it is significance to consider the entire experimental strategy from sample collection and preparation, data acquisition, analysis, and interpretation.

Areas Covered: In this review, we outline considerations with clinical (i.e., human) samples with special emphasis on sample handling, specific physicochemical properties, target measurements, and resulting profiling of sphingolipids in biomedicine and translational research to maximize sensitivity and specificity as well as to provide robust and reproducible results. A brief commentary is also provided regarding new insights of “clinical sphingolipidomics” in translational sepsis research.

Expert Opinion: The role of mass spectrometry of sphingolipids and related species (“sphingolipidomics”) to investigate cellular and compartment-specific response to stress, e.g., in generalized infection and sepsis, is on the rise and the ability to integrate multiple datasets from diverse classes of biomolecules by mass spectrometry measurements and metabolomics will be crucial to fostering our understanding of human health as well as response to disease and treatment.

Effect of Erufosine on Membrane Lipid Order in Breast Cancer Cell Models

Alkylphospholipids are a novel class of antineoplastic drugs showing remarkable therapeutic potential. Among them, erufosine (EPC3) is a promising drug for the treatment of several types of tumors. While EPC3 is supposed to exert its function by interacting with lipid membranes, the exact molecular mechanisms involved are not known yet. In this work, we applied a combination of several fluorescence microscopy and analytical chemistry approaches (i.e., scanning fluorescence correlation spectroscopy, line-scan fluorescence correlation spectroscopy, generalized polarization imaging, as well as thin layer and gas chromatography) to quantify the effect of EPC3 in biophysical models of the plasma membrane, as well as in cancer cell lines. Our results indicate that EPC3 affects lipid-lipid interactions in cellular membranes by decreasing lipid packing and increasing membrane disorder and fluidity. As a consequence of these alterations in the lateral organization of lipid bilayers, the diffusive dynamics of membrane proteins are also significantly increased. Taken together, these findings suggest that the mechanism of action of EPC3 could be linked to its effects on fundamental biophysical properties of lipid membranes, as well as on lipid metabolism in cancer cells.

Miscibility of hBest1 and sphingomyelin in surface films - A prerequisite for interaction with membrane domains

Human bestrophin-1 (hBest1) is a transmembrane Ca²⁺- dependent anion channel, associated with the transport of Cl^-, HCO^3- ions, γ-aminobutiric acid (GABA), glutamate (Glu), and regulation of retinal homeostasis. Its mutant forms cause retinal degenerative diseases, defined as Bestrophinopathies. Using both physicochemical - surface pressure/mean molecular area (π/A) isotherms, hysteresis, compressibility moduli of hBest1/sphingomyelin (SM) monolayers, Brewster angle microscopy (BAM) studies, and biological approaches - detergent membrane fractionation, Laurdan (6-dodecanoyl-N,N-dimethyl-2-naphthylamine) and immunofluorescence staining of stably transfected MDCK-hBest1 and MDCK II cells, we report: 1) Ca²⁺, Glu and GABA interact with binary hBest1/SM monolayers at 35 °C, resulting in changes in hBest1 surface conformation, structure, self-organization and surface dynamics. The process of mixing in hBest1/SM monolayers is spontaneous and the effect of protein on binary films was defined as "fluidizing", hindering the phase-transition of monolayer from liquid-expanded to intermediate (LE-M) state; 2) in stably transfected MDCK-hBest1 cells, bestrophin-1 was distributed between detergent resistant (DRM) and detergent-soluble membranes (DSM) - up to 30 % and 70 %, respectively; in alive cells, hBest1 was visualized in both liquid-ordered (L_o) and liquid-disordered (L_d) fractions, quantifying protein association up to 35 % and 65 % with L_o and L_d. Our results indicate that the spontaneous miscibility of hBest1 and SM is a prerequisite to diverse protein interactions with membrane domains, different structural conformations and biological functions.

A Comprehensive High-Resolution Targeted Workflow for the Deep Profiling of Sphingolipids

Sphingolipids make up a highly diverse group of biomolecules that not only are membrane components but also are involved in various cellular functions such as signaling and protein sorting. To obtain a quantitative view of the sphingolipidome, sensitive, accurate, and comprehensive methods are needed. Here, we present a targeted reversed-phase liquid chromatography-high-resolution mass spectrometry-based workflow that significantly increases the accuracy of measured sphingolipids by resolving nearly isobaric and isobaric species; this is accomplished by a use of (i) an optimized extraction procedure, (ii) a segmented gradient, and (iii) parallel reaction monitoring of a sphingolipid specific fragmentation pattern. The workflow was benchmarked against an accepted sphingolipid model system, the RAW 264.7 cell line, and 61 sphingolipids were quantified over a dynamic range of 7 orders of magnitude, with detection limits in the low femtomole per milligram of protein level, making this workflow an extremely versatile tool for high-throughput sphingolipidomics.

LDL switches the LRP6 internalization route from flotillin dependent to clathrin dependent in hepatic cells

Low-density lipoprotein (LDL) receptor-related protein 6 (LRP6) was originally identified as a co-receptor of the Wnt signalling pathway and has been shown to be involved in LDL transport. In polarized hepatocytes, many apical proteins are sorted to the basolateral membrane and then internalized and transported to the apical bile canalicular membrane - a process known as transcytosis. We show that LRP6 is transcytosed to the apical membrane of polarized hepatic HepG2 cells via a flotillin-dependent manner in the absence of LDL. LRP6 formed a complex with Niemann-Pick type C1-like 1 (NPC1L1), which is localized to the bile canalicular membrane of the liver and is involved in cholesterol absorption from the bile. LRP6 was required for apical membrane localization of NPC1L1 in the absence of LDL. Clathrin-dependent LRP6 internalization occurred in the presence of LDL, which resulted in trafficking of LRP6 to the lysosome, thereby reducing apical sorting of LRP6 and NPC1L1. These results suggest that LRP6 endocytosis proceeds by two routes, depending on the presence of LDL, and that LRP6 controls the intracellular destination of NPC1L1 in hepatocytes.

The major myelin-resident protein PLP is transported to myelin membranes via a transcytotic mechanism: involvement of sulfatide

Myelin membranes are sheet-like extensions of oligodendrocytes that can be considered membrane domains distinct from the cell's plasma membrane. Consistent with the polarized nature of oligodendrocytes, we demonstrate that transcytotic transport of the major myelin-resident protein proteolipid protein (PLP) is a key element in the mechanism of myelin assembly. Upon biosynthesis, PLP traffics to myelin membranes via syntaxin 3-mediated docking at the apical-surface-like cell body plasma membrane, which is followed by subsequent internalization and transport to the basolateral-surface-like myelin sheet. Pulse-chase experiments, in conjunction with surface biotinylation and organelle fractionation, reveal that following biosynthesis, PLP is transported to the cell body surface in Triton X-100 (TX-100)-resistant microdomains. At the plasma membrane, PLP transiently resides within these microdomains and its lateral dissipation is followed by segregation into 3-[(3-cholamidopropyl)-dimethylammonio]-1-propanesulfonate (CHAPS)-resistant domains, internalization, and subsequent transport toward the myelin membrane. Sulfatide triggers PLP's reallocation from TX-100- into CHAPS-resistant membrane domains, while inhibition of sulfatide biosynthesis inhibits transcytotic PLP transport. Taking these findings together, we propose a model in which PLP transport to the myelin membrane proceeds via a transcytotic mechanism mediated by sulfatide and characterized by a conformational alteration and dynamic, i.e., transient, partitioning of PLP into distinct membrane microdomains involved in biosynthetic and transcytotic transport.

A novel hypothesis for an alkaline phosphatase 'rescue' mechanism in the hepatic acute phase immune response

The liver isoform of the enzyme alkaline phosphatase (AP) has been used classically as a serum biomarker for hepatic disease states such as hepatitis, steatosis, cirrhosis, drug-induced liver injury, and hepatocellular carcinoma. Recent studies have demonstrated a more general anti-inflammatory role for AP, as it is capable of dephosphorylating potentially deleterious molecules such as nucleotide phosphates, the pathogenic endotoxin lipopolysaccharide (LPS), and the contact clotting pathway activator polyphosphate (polyP), thereby reducing inflammation and coagulopathy systemically. Yet the mechanism underlying the observed increase in liver AP levels in circulation during inflammatory insults is largely unknown. This paper hypothesizes an immunological role for AP in the liver and the potential of this system for damping generalized inflammation along with a wide range of ancillary pathologies. Based on the provided framework, a mechanism is proposed in which AP undergoes transcytosis in hepatocytes from the canalicular membrane to the sinusoidal membrane during inflammation and the enzyme's expression is upregulated as a result. Through a tightly controlled, nucleotide-stimulated negative feedback process, AP is transported in this model as an immune complex with immunoglobulin G by the asialoglycoprotein receptor through the cell and secreted into the serum, likely using the receptor's State 1 pathway. The subsequent dephosphorylation of inflammatory stimuli by AP and uptake of the circulating immune complex by endothelial cells and macrophages may lead to decreased inflammation and coagulopathy while providing an early upstream signal for the induction of a number of anti-inflammatory gene products, including AP itself.

Role of membrane traffic in the generation of epithelial cell asymmetry

Epithelial cells have an apical-basolateral axis of polarity, which is required for epithelial functions including barrier formation, vectorial ion transport and sensory perception. Here we review what is known about the sorting signals, machineries and pathways that maintain this asymmetry, and how polarity proteins interface with membrane-trafficking pathways to generate membrane domains de novo. It is becoming apparent that membrane traffic does not simply reinforce polarity, but is critical for the generation of cortical epithelial cell asymmetry.

Evidence for the presence of functional lipid rafts in immune cells of ectothermic organisms

The role of lipid rafts in non-mammalian leukocytes has been scarcely investigated. We performed biochemical and functional analysis of lipid rafts in fish leukocytes. Fish Flotillin-1 and a fish GM1-like molecule (fGM1-L) were found in low density detergent-resistant membranes (LD-DRM) in goldfish macrophages and catfish B lymphocytes, similarly to mammals. The presence of flotillin-1 and fGM1-L in LD-DRM was sensitive to increased detergent concentrations, and cholesterol extraction. Confocal microscopy analysis of flotillin-1 and fGM1-L in fish leukocytes showed a distinctive punctuated staining pattern, suggestive of pre-existing rafts. Confocal microscopy analysis of macrophages showed that the membrane of phagosomes containing serum-opsonized zymosan was enriched in fGM1-L, and zymosan phagocytosis was reduced after cholesterol extraction. The presence of flotillin-1 and fGM1-L in LD-DRM, the microscopic evidence of flotillin-1 and fGM1-L on fish macrophages and B-cells, and the sensitivity of phagocytosis to cholesterol extraction, indicate that lipid rafts are biochemically and functionally similar in leukocytes from fish and mammals.

Nitric oxide produced by endothelial nitric oxide synthase promotes diuresis

Extracellular fluid volume is highly regulated, at least in part, by peripheral resistance and renal function. Nitric oxide (NO) produced by NO synthase type 3 (NOS 3) in the nonrenal vasculature may promote fluid retention by reducing systemic vascular resistance and arterial pressure. In contrast, NO produced by renal NOS 3 promotes water excretion by reducing renal vascular resistance, increasing glomerular filtration, and inhibiting reabsorption along the nephron. Thus, the net effect of NO from NOS 3 on urinary volume (UV) is unclear. We hypothesized that NO produced by NOS 3 promotes water excretion primarily due to renal tubular effects. We gave conscious wild-type and NOS 3 -/- mice an acute volume load and measured UV, blood pressure, plasma renin concentration (PRC), Na(+), vasopressin, and urinary Na(+) and creatinine concentrations. To give the acute volume load, we trained mice to drink a large volume of water while in metabolic cages. On the day of the experiment, water was replaced with 1% sucrose, and mice had access to it for 1 h. Volume intake was similar in both groups. Over 3 h, wild-type mice excreted 62 +/- 10% of the volume load, but NOS 3 -/- excreted only 42 +/- 5% (P < 0.05). Blood pressure in NOS 3 -/- was 118 +/- 3 compared with 110 +/- 2 mmHg in wild-type mice (P < 0.05), but it did not change following volume load in either strain. PRC, vasopressin, and glomerular filtration rate were similar between groups. Urinary Na(+) excretion was 49.3 +/- 7.0 in wild-type vs. 37.8 +/- 6.4 mumol/3 h in NOS 3 -/- mice (P < 0.05). Bumetanide administration eliminated the difference in volume excretion between wild-type and NOS 3 -/- mice. We conclude that 1) NO produced by NOS 3 promotes water and Na(+) excretion and 2) the renal epithelial actions of NO produced by NOS 3 supersede the systemic and renal vascular actions.

Fatty acid-dependent globotriaosyl ceramide receptor function in detergent resistant model membranes

Glycosphingolipid (GSL) fatty acid strictly regulates verotoxin 1 (VT1) and the HIV adhesin, gp120 binding to globotriaosyl ceramide within Gb(3)/cholesterol detergent resistant membrane (DRM) vesicle constructs and in Gb(3) water-air interface monolayers in a similar manner. VT2 bound Gb(3)/cholesterol vesicles irrespective of fatty acid composition, but VT1 bound neither C18 nor C20Gb(3)vesicles. C18/C20Gb(3) were dominant negative in mixed Gb(3) fatty acid isoform vesicles, but including C24:1Gb(3) gave maximal binding. VT1 bound C18Gb(3) vesicles after cholesterol removal, but C20Gb(3)vesicles required sphingomyelin in addition for binding. HIV-1gp120 also bound C16, C22, and C24, but neither C18 nor C20Gb(3) vesicles. C18 and C20Gb(3) were, in mixtures without C24:1Gb(3), dominant negative for gp120 vesicle binding. Gp120/VT1bound C18 and C24:1Gb(3) mixtures, although neither isoform bound alone. Monolayer surface pressure measurement showed VT1, but not VT2, bound Gb(3) at cellular DRM surface pressures, and confirmed loss of VT1 and gp120 (but not VT2) specific C18Gb(3) binding. We conclude fatty-acid mediated fluidity within simple model GSL/cholesterol DRM can selectively regulate GSL carbohydrate-ligand binding.

Basolateral internalization of GPI-anchored proteins occurs via a clathrin-independent flotillin-dependent pathway in polarized hepatic cells

In polarized hepatocytes, the predominant route for apical resident proteins to reach the apical bile canalicular membrane is transcytosis. Apical proteins are first sorted to the basolateral membrane from which they are internalized and transported to the opposite surface. We have noted previously that transmembrane proteins and GPI-anchored proteins reach the apical bile canaliculi at very different rates. Here, we investigated whether these differences may be explained by the use of distinct endocytic mechanisms. We show that endocytosis of both classes of proteins at the basolateral membrane of polarized hepatic cells is dynamin dependent. However, internalization of transmembrane proteins is clathrin mediated, whereas endocytosis of GPI-anchored proteins does not require clathrin. Further analysis of basolateral endocytosis of GPI-anchored proteins showed that caveolin, as well as the small GTPase cdc42 were dispensable. Alternatively, internalized GPI-anchored proteins colocalized with flotillin-2-positive vesicles, and down-expression of flotillin-2 inhibited endocytosis of GPI-anchored proteins. These results show that basolateral endocytosis of GPI-anchored proteins in hepatic cells occurs via a clathrin-independent flotillin-dependent pathway. The use of distinct endocytic pathways may explain, at least in part, the different rates of transcytosis between transmembrane and GPI-anchored proteins.

Myelin, DIGs, and membrane rafts in the central nervous system

Over the past 40 years our understanding of the organization of cell membranes has changed dramatically. Membranes are no longer viewed as a homogenous sea of phospholipids studded with randomly positioned islands of proteins. Our current view of the membrane involves the formation of small lipid clusters, comprised mainly of cholesterol and sphingolipids, known as membrane rafts. These lipid clusters apparently include and exclude specific proteins leading to the hypothesis that these domains (1) regulate cellular polarity and compartmentalization through trafficking and sorting, (2) provide platforms for cellular signaling and adhesion, and (3) function as cellular gate keepers. Tremendous controversy surrounds the concept of membrane rafts primarily because these small, highly dynamic entities are too small to be observed with traditional microscopic methods and the most utilized approach for raft analysis relies on poorly quantified, inconsistent biochemical extractions. New analytical approaches are being developed and applied to the study of membrane rafts and these techniques provide great promise for furthering our understanding of these enigmatic domains. In this review we will provide a brief summary of the current understanding of membrane rafts, utilizing the CNS myelin literature for illustrative purposes, and present caveats that should be considered when studying these domains.

Complex gangliosides are apically sorted in polarized MDCK cells and internalized by clathrin-independent endocytosis

Gangliosides are glycosphingolipids mainly present at the outer leaflet of the plasma membrane of eukaryotic cells, where they participate in recognition and signalling activities. The synthesis of gangliosides is carried out in the lumen of the Golgi apparatus by a complex system of glycosyltransferases. After synthesis, gangliosides leave the Golgi apparatus via the lumenal surface of transport vesicles destined to the plasma membrane. In this study, we analysed the synthesis and membrane distribution of GD3 and GM1 gangliosides endogenously synthesized by Madin-Darby canine kidney (MDCK) cell lines genetically modified to express appropriate ganglioside glycosyltransferases. Using biochemical techniques and confocal laser scanning microscopy analysis, we demonstrated that GD3 and GM1, after being synthesized at the Golgi apparatus, were transported and accumulated mainly at the plasma membrane of nonpolarized MDCK cell lines. More interestingly, both complex gangliosides were found to be enriched mainly at the apical domain when these cell lines were induced to polarize. In addition, we demonstrated that, after arrival at the plasma membrane, GD3 and GM1 gangliosides were endocytosed using a clathrin-independent pathway. Then, internalized GD3, in association with a specific monoclonal antibody, was accumulated in endosomal compartments and transported back to the plasma membrane. In contrast, endocytosed GM1, in association with cholera toxin, was transported to endosomal compartments en route to the Golgi apparatus. In conclusion, our results demonstrate that complex gangliosides are apically sorted in polarized MDCK cells, and that GD3 and GM1 gangliosides are internalized by clathrin-independent endocytosis to follow different intracellular destinations.

Physiology, structure, and regulation of the cloned organic anion transporters

1. The transport of negatively charged drugs, xenobiotics, and metabolites by epithelial tissues, particularly the kidney, plays critical roles in controlling their distribution, concentration, and retention in the body. Thus, organic anion transporters (OATs) impact both their therapeutic efficacy and potential toxicity. 2. This review summarizes current knowledge of the properties and functional roles of the cloned OATs, the relationships between transporter structure and function, and those factors that determine the efficacy of transport. Such factors include plasma protein binding of substrates, genetic polymorphisms among the transporters, and regulation of transporter expression. 3. Clearly, much progress has been made in the decade since the first OAT was cloned. However, unresolved questions remain. Several of these issues--drug-drug interactions, functional characterization of newly cloned OATs, tissue differences in expression and function, and details of the nature and consequences of transporter regulation at genomic and intracellular sites--are discussed in the concluding Perspectives section.

The chlamydial inclusion preferentially intercepts basolaterally directed sphingomyelin-containing exocytic vacuoles

Chlamydiae replicate intracellularly within a unique vacuole termed the inclusion. The inclusion circumvents classical endosomal/lysosomal pathways but actively intercepts a subset of Golgi-derived exocytic vesicles containing sphingomyelin (SM) and cholesterol. To further examine this interaction, we developed a polarized epithelial cell model to study vectoral trafficking of lipids and proteins to the inclusion. We examined seven epithelial cell lines for their ability to form single monolayers of polarized cells and support chlamydial development. Of these cell lines, polarized colonic mucosal C2BBe1 cells were readily infected with Chlamydia trachomatis and remained polarized throughout infection. Trafficking of (6-((N-(7-nitrobenz-2-oxa-1, 3-diazol-4-yl) amino)hexanoyl)sphingosine) (NBD-C(6)-ceramide) and its metabolic derivatives, NBD-glucosylceramide (GlcCer) and NBD-SM, was analyzed. SM was retained within L2-infected cells relative to mock-infected cells, correlating with a disruption of basolateral SM trafficking. There was no net retention of GlcCer within L2-infected cells and purification of C. trachomatis elementary bodies from polarized C2BBe1 cells confirmed that bacteria retained only SM. The chlamydial inclusion thus appears to preferentially intercept basolaterally-directed SM-containing exocytic vesicles, suggesting a divergence in SM and GlcCer trafficking. The observed changes in lipid trafficking were a chlamydia-specific effect because Coxiella burnetii-infected cells revealed no changes in GlcCer or SM polarized trafficking.

Different glycoforms of prostate-specific membrane antigen are intracellularly transported through their association with distinct detergent-resistant membranes

Hormone-refractory prostate carcinomas as well as the neovasculature of different tumours express high levels of PSMA (prostate-specific membrane antigen). PSMA is a type II-transmembrane glycoprotein and a potential tumour marker for both diagnosis and passive immunotherapy. Here, we report on the association of PSMA with DRMs (detergent-resistant membranes) at different stages of the protein maturation pathway in human prostate carcinoma LNCaP cells. At least three PSMA glycoforms were biochemically identified based on their extractability behaviour in different non-ionic detergents. In particular, one precursor glycoform of PSMA is associated with Tween 20-insoluble DRMs, whereas the complex glycosylated protein segregates into membrane structures that are insoluble in Lubrol WX and display a different lipid composition. Association of PSMA with these membranes occurs in the Golgi compartment together with the acquisition of a native conformation. PSMA homodimers reach the plasma membrane of LNCaP cells in Lubrol WX-insoluble lipid/protein complexes. At the steady state, the majority of PSMA remains within these membrane microdomains at the cell surface. We conclude that the intracellular transport of PSMA occurs through populations of DRMs distinct for each biosynthetic form and cellular compartment.

The role of an intracellular cysteine stretch in the sorting of the type II Na/phosphate cotransporter

The type II Na/phosphate cotransporters (NaPi-II) are critical for the control of plasma phosphate levels in vertebrates. NaPi-IIb mediates phosphate uptake from the small intestine followed by glomerular filtration and selective reabsorption from the renal proximal tubule by NaPi-IIa and NaPi-IIc. A C-terminal stretch of cysteine residues represents the hallmark of the NaPi-IIb isoforms. This motif is well conserved among NaPi-IIb type transporters but not found in other membrane proteins. To investigate the role of this motif we analyzed NaPi-II constructs in transiently and stably transfected MDCK cells. This cell line targets the NaPi-IIb isoforms from flounder and mouse to the apical membrane whereas the mouse IIa isoform shows no plasma membrane preference. Different parts of mouse NaPi-IIa and NaPi-IIb C-termini were fused to GFP-tagged flounder NaPi-II. The constructs showed strong staining of the plasma membrane with NaPi-IIb related constructs sorted predominantly apically, the IIa constructs localized apically and basolaterally with slight intracellular retention. When the cysteine stretch was inserted into the NaPi-IIa C-terminus, the construct was retained in a cytoplasmic compartment. 2-bromopalmitate, a specific palmitoylation inhibitor, released the transporter to apical and basolateral membranes. The drug also leads to a redistribution of the NaPi-IIb construct to both plasma membrane compartments. Immunoprecipitation of tagged NaPi-II constructs from [(3)H]-palmitate labeled MDCK cells indicated that the cysteine stretch is palmitoylated. Our results suggest that the modified cysteine motif prevents the constructs from basolateral sorting. Additional sorting determinants located downstream of the cysteine stretch may release the cargo to the apical compartment.

Changes in GM1 ganglioside content and localization in cholestatic rat liver

(Glyco)sphingolipids (GSL) are believed to protect the cell against harmful environmental factors by increasing the rigidity of plasma membrane. Marked decrease of membrane fluidity in cholestatic hepatocytes was described but the role of GSL therein has not been investigated so far. In this study, localization in hepatocytes of a representative of GSL, the GM1 ganglioside, was compared between of rats with cholestasis induced by 17alpha-ethinylestradiol (EE) and vehicle propanediol treated or untreated animals. GM1 was monitored by histochemical reaction employing cholera toxin B-subunit. Our findings in normal rat liver tissue showed that GM1 was localized in sinusoidal and canalicular hepatocyte membranes in both peripheral and intermediate zones of the hepatic lobules, and was nearly absent in central zones. On the contrary, in EE-treated animals GM1 was also expressed in central lobular zones. Moreover, detailed densitometry analysis at high magnification showed greater difference of GM1 expression between sinusoidal surface areas and areas of adjacent cytoplasm, caused as well by increased sinusoidal staining in central lobular zone as by decreased staining in cytoplasm in peripheral zone. These differences correlated with serum bile acids as documented by linear regression analyses. Both GM1 content and mRNA corresponding to GM1-synthase remained unchanged in livers; the enhanced expression of GM1 at sinusoidal membrane thus seems to be due to re-distribution of cellular GM1 at limited biosynthesis and could be responsible for protection of hepatocytes against harmful effects of bile acids accumulated during cholestasis.

Role of lipid rafts in membrane delivery of renal epithelial Na+-K+-ATPase, thick ascending limb

Lipid rafts are cholesterol- and shingolipid-enriched membrane microdomains implicated in membrane signaling and trafficking. To assess renal epithelial raft functions through the characterization of their associated membrane proteins, we have isolated lipid rafts from rat kidney by sucrose gradient fractionation after detergent treatment. The low-density fraction was enriched in cholesterol, sphingolipid, and flotillin-1 known as lipid raft markers. Based on proteomic analysis of the low-density fraction, the protein with the highest significance score was the alpha-subunit of Na(+)-K(+)-ATPase (NKA), whose raft association was validated by simultaneous immunoblotting. The beta-subunit of NKA was copurified from the low-density fraction. To test the role of lipid rafts in sorting and membrane delivery of renal-transporting epithelia, we have chosen to study thick ascending limb (TAL) epithelium for its high NKA activity and the property to be stimulated by antidiuretic hormone (ADH). Cultured rabbit TAL cells were studied. Cholesterol depletion and detergent extraction at warmth caused a shift of NKA to the higher-density fractions. Comparative preparations from blood monocytes revealed the absence of NKA from rafts in these nonpolarized cells. Short-term exposure of rabbit TAL cells to ADH (1 h) caused translocation and enhanced raft association of NKA via cAMP activation. Preceding cholesterol depletion prevented this effect. TAL-specific, glycosylphosphatidylinositol-anchored Tamm Horsfall protein was copurified with NKA in the same raft fraction, suggesting functional interference between these products. These results may have functional implications regarding the turnover, trafficking, and regulated surface expression of NKA as the major basolateral ion transporter of TAL.

Ceramides and other bioactive sphingolipid backbones in health and disease: lipidomic analysis, metabolism and roles in membrane structure, dynamics, signaling and autophagy

Sphingolipids are comprised of a backbone sphingoid base that may be phosphorylated, acylated, glycosylated, bridged to various headgroups through phosphodiester linkages, or otherwise modified. Organisms usually contain large numbers of sphingolipid subspecies and knowledge about the types and amounts is imperative because they influence membrane structure, interactions with the extracellular matrix and neighboring cells, vesicular traffic and the formation of specialized structures such as phagosomes and autophagosomes, as well as participate in intracellular and extracellular signaling. Fortunately, "sphingolipidomic" analysis is becoming feasible (at least for important subsets such as all of the backbone "signaling" subspecies: ceramides, ceramide 1-phosphates, sphingoid bases, sphingoid base 1-phosphates, inter alia) using mass spectrometry, and these profiles are revealing many surprises, such as that under certain conditions cells contain significant amounts of "unusual" species: N-mono-, di-, and tri-methyl-sphingoid bases (including N,N-dimethylsphingosine); 3-ketodihydroceramides; N-acetyl-sphingoid bases (C2-ceramides); and dihydroceramides, in the latter case, in very high proportions when cells are treated with the anticancer drug fenretinide (4-hydroxyphenylretinamide). The elevation of DHceramides by fenretinide is befuddling because the 4,5-trans-double bond of ceramide has been thought to be required for biological activity; however, DHceramides induce autophagy and may be important in the regulation of this important cellular process. The complexity of the sphingolipidome is hard to imagine, but one hopes that, when partnered with other systems biology approaches, the causes and consequences of the complexity will explain how these intriguing compounds are involved in almost every aspect of cell behavior and the malfunctions of many diseases.

Composition, structure and absorption of milk lipids: a source of energy, fat-soluble nutrients and bioactive molecules

Milkfat is a remarkable source of energy, fat-soluble nutrients and bioactive lipids for mammals. The composition and content of lipids in milkfat vary widely among mammalian species. Milkfat is not only a source of bioactive lipid components, it also serves as an important delivery medium for nutrients, including the fat-soluble vitamins. Bioactive lipids in milk include triacylglycerides, diacylglycerides, saturated and polyunsaturated fatty acids, and phospholipids. Beneficial activities of milk lipids include anticancer, antimicrobial, anti-inflammatory, and immunosuppression properties. The major mammalian milk that is consumed by humans as a food commodity is that from bovine whose milkfat composition is distinct due to their diet and the presence of a rumen. As a result of these factors bovine milkfat is lower in polyunsaturated fatty acids and higher in saturated fatty acids than human milk, and the consequences of these differences are still being researched. The physical properties of bovine milkfat that result from its composition including its plasticity, make it a highly desirable commodity (butter) and food ingredient. Among the 12 major milk fatty acids, only three (lauric, myristic, and palmitic) have been associated with raising total cholesterol levels in plasma, but their individual effects are variable-both towards raising low-density lipoproteins and raising the level of beneficial high-density lipoproteins. The cholesterol-modifying response of individuals to consuming saturated fats is also variable, and therefore the composition, functions and biological properties of milkfat will need to be re-evaluated as the food marketplace moves increasingly towards more personalized diets.

Identification of a basolateral sorting signal within the cytoplasmic domain of the interleukin-6 signal transducer gp130

Interleukin-6-type cytokine receptors are expressed in polarized cells such as hepatocytes and intestinal cells. For the interleukin-6-receptor gp80 and its signal transducer gp130, a preferential basolateral localization was demonstrated in Madin-Darby canine kidney (MDCK) cells and two basolateral sorting signals were identified within the cytoplasmic domain of gp80. The cytoplasmic tail of gp130 is responsible for signaling via the Janus kinase/signal transducer and activator of transcription pathway. In addition, it mediates the internalization of the receptor complex which is dependent on a di-leucine motif. Truncated gp130 lacking the cytoplasmic domain is sorted apically in MDCK cells. For identification of the basolateral sorting signal(s) of gp130, a series of deletion mutants in the cytoplasmic domain of gp130 have been generated and stably expressed in MDCK cells. Biotinylation analyses of these mutants show that a ten amino acids sequence between amino acids 782 and 792 which contains the di-leucine internalization motif is also essential for a basolateral sorting. Accordingly, we detect apical delivery of a gp130 mutant in which the di-leucine motif has been exchanged by two alanines (gp130LL/AA). These findings indicate that the di-leucine motif which directs the internalization of the IL-6 receptor complex also mediates the basolateral sorting of the signal transducer gp130.

Multiple lipid transport pathways to the plasma membrane in yeast

The plasma membrane of the yeast Saccharomyces cerevisiae is devoid of lipid-synthesizing enzymes, but contains all classes of bilayer-forming lipids. As the lipid composition of the plasma membrane does not match any of the intracellular membranes, specific trafficking of lipids from internal membranes, especially the endoplasmic reticulum and the Golgi, to the cell periphery is required. Although the secretory pathway is an obvious route to translocate glycerophospholipids, sphingolipids and sterols to the plasma membrane, experimental evidence for the role of this pathway in lipid transport is rare. Addressing this issue in a systematic way, we labeled temperature-sensitive secretory yeast mutants (sec mutants) with appropriate lipid precursors, isolated the plasma membranes at high purity and quantified labeled lipids of this compartment. Shifting sec mutants to the restrictive temperature reduced transport of both proteins and lipids to the plasma membrane, indicating that the latter compounds are also trafficked to the cell periphery through the protein secretory pathway. However, efficient sec blocks did not abrogate protein and lipid transport, suggesting that parallel pathway(s) for the translocation of membrane components to the plasma membrane of yeast must exist.

Regulation of membrane trafficking by a novel Cdc42-related protein in Caenorhabditis elegans epithelial cells

Rho GTPases are mainly known for their implication in cytoskeleton remodeling. They have also been recently shown to regulate various aspects of membrane trafficking. Here, we report the identification and the characterization of a novel Caenorhabditis elegans Cdc42-related protein, CRP-1, that shows atypical enzymatic characteristics in vitro. Expression in mouse fibroblasts revealed that, in contrast with CDC-42, CRP-1 was unable to reorganize the actin cytoskeleton and mainly localized to trans-Golgi network and recycling endosomes. This subcellular localization, as well as its expression profile restricted to a subset of epithelial-like cells in C. elegans, suggested a potential function for this protein in polarized membrane trafficking. Consistent with this hypothesis, alteration of CRP-1 expression affected the apical trafficking of CHE-14 in vulval and rectal epithelial cells and sphingolipids (C(6)-NBD-ceramide) uptake and/or trafficking in intestinal cells. However, it did not affect basolateral trafficking of myotactin in the pharynx and the targeting of IFB-2 and AJM-1, two cytosolic apical markers of intestine epithelial cells. Hence, our data demonstrate a function for CRP-1 in the regulation of membrane trafficking in a subset of cells with epithelial characteristics.

The subapical compartment: a traffic center in membrane polarity development

Spatially separated apical and basolateral plasma membrane domains that have distinct functions and molecular compositions are a characteristic feature of epithelial cell polarity. The subapical compartment (SAC), also known as the common endosome (CE), where endocytic pathways from both surfaces merge, plays a crucial role in the maintenance and probably the biogenesis of these distinct membrane domains. Although differences in morphology are apparent, the same principal features of a SAC can be distinguished in different types of epithelial cells. As polarity develops, the compartment acquires several distinct machineries that, in conjunction with the cytoskeleton, are necessary for polarized trafficking. Disrupting trafficking via the SAC and hence bypassing its sorting machinery, as occurs upon actin depolymerization, leads to mis-sorting of apical and basolateral molecules, thereby compromising the development of polarity. The structural and functional integrity of the compartment in part depends on microtubules. Moreover, the acquisition of a particular set of Rab proteins, including Rab11 and Rab3, appears to be crucial in regulating molecular sorting and vesicular transport relevant both to recycling to either plasma membrane domain and to de novo assembly of the apical domain. Furthermore, subcompartmentalization of the SAC appears to be key to its various functions.

The role of lipid microdomains in virus biology

Many of the highly pathogenic viruses including influenza virus, HIV and others of world wide epidemiological importance are enveloped and possess a membrane around the nucleocapsid containing the viral genome. Viral membrane is required to protect the viral genome and provide important functions for attachment, morphogenesis and transmission. Viral membrane is essentially composed of lipids and proteins. While the proteins on the viral envelope are almost exclusively virally encoded, lipids, on the other hand, are all of host origin and recruited from host membrane. However, lipids on the viral membrane are not incorporated randomly and do not represent average lipid composition of the host membrane. Recent studies support that specific lipid microdomains such as lipid rafts play critical roles in many aspects of the virus infectious cycle including attachment, entry, uncoating, protein transport and sorting as well as viral morphogenesis and budding. Lipid microdomains aid in bringing and concentrating viral components to the budding site. Similarly, specific viral protein plays an important role in organizing lipid microdomains in and around the assembly and budding site of the virus. This review deals with the specific role of lipid microdomains in different aspects of the virus life cycle and the role of specific viral proteins in organizing the lipid microdomains.

Lysophosphatidic acid triggers calcium entry through a non-store-operated pathway in human neutrophils

Lysophosphatidic acid (LPA) is a bioactive lipid, which is structurally similar to sphingosine 1-phosphate (S1P) and which can mobilize Ca2+ in multiple cell types. We recently showed that S1P induces Ca2+ entry directly through store-operated Ca2+ entry (SOCE) channels in human polymorphonuclear neutrophils (PMN). We therefore examined the mechanisms by which LPA induces intracellular Ca2+ mobilization in PMN. External application of low micromolar LPA caused dose-dependent Ca2+ influx without releasing Ca2+ stores, whereas G-protein-coupled (GPC) LPA receptors respond to nanomolar LPA. Additive Ca2+ influx by LPA compared with 100 nM ionomycin-induced Ca2+ influx suggests that LPA-induced Ca2+ influx does not pass through SOCE channels. Ca2+ influx was resistant to inhibition of Gi/o by pertussis toxin, of phospholipase C by U73122, and of G12/13/Rho by Y27632, all demonstrating GPC receptor independence. This Ca2+ influx was inhibited by Gd3+, La3+, Zn2+, or MRS1845 but not by Ni2+ or the sphingosine kinase inhibitor dimethylsphingosine. In addition, we found that LPA has no effect on neutrophil chemotaxis; however, it has stimulatory effects on neutrophil respiratory burst in a dose-response manner. These findings suggest that LPA-induced Ca2+ influx in PMN occurs through a mechanism other than SOCE channels, independent of Ca2+ store-depletion and S1P synthesis, and that the characteristics of LPA-induced Ca2+ influx are similar to those of S1P-induced influx in terms of sensitivity to inorganic inhibitors. Unlike S1P, LPA has stimulatory effects on neutrophil respiratory burst.

Ganglioside GD3 Traffics from the trans-Golgi Network to Plasma Membrane by a Rab11-independent and Brefeldin A-insensitive Exocytic Pathway

Gangliosides, complex glycosphingolipids containing sialic acids, have been found to reside in glycosphingolipid-enriched microdomains (GEM) at the plasma membrane. They are synthesized in the lumen of the Golgi complex and appear unable to translocate from the lumenal toward the cytosolic surface of Golgi membrane to access the monomeric lipid transport. As a consequence, they can only leave the Golgi complex via the lumenal surface of transport vesicles. In this work we analyzed the exocytic transport of the disialo ganglioside GD3 from trans-Golgi network (TGN) to plasma membrane in CHO-K1 cells by immunodetection of endogenously synthesized GD3. We found that ganglioside GD3, unlike another luminal membrane-bounded lipid (glycosylphosphatidylinositol-anchored protein), did not partition into GEM domains in the Golgi complex and trafficked from TGN to plasma membrane by a brefeldin A-insensitive exocytic pathway. Moreover, a dominant negative form of Rab11, which prevents exit of vesicular stomatitis virus glycoprotein from the Golgi complex, did not influence the capacity of GD3 to reach the cell surface. Our results strongly support the notion that most ganglioside GD3 traffics from the TGN to the plasma membrane by a non-conventional vesicular pathway where lateral membrane segregation of vesicular stomatitis virus glycoprotein (non-GEM resident) and glycosylphosphatidylinositol-anchored proteins (GEM resident) from GD3 is required before exiting TGN.

Polarized membrane traffic and cell polarity development is dependent on dihydroceramide synthase-regulated sphinganine turnover

Sphingoid bases have been implicated in various cellular processes including cell growth, apoptosis and cell differentiation. Here, we show that the regulated turnover of sphingoid bases is crucial for cell polarity development, i.e., the biogenesis of apical plasma membrane domains, in well-differentiated hepatic cells. Thus, inhibition of dihydroceramide synthase or sphinganine kinase activity with fumonisin B1 or N,N-dimethylsphingosine, respectively, dramatically perturbs cell polarity development, which is due to increased levels of sphinganine. Consistently, reduction of free sphinganine levels stimulates cell polarity development. Moreover, dihydroceramide synthase, the predominant enzyme responsible for sphinganine turnover, is a target for cell polarity stimulating cAMP/protein kinase A (PKA) signaling cascades. Indeed, electrospray ionization tandem mass spectrometry analyses revealed a significant reduction in sphinganine levels in cAMP/PKA-stimulated cells. These data suggest that sphinganine turnover is critical for and is actively regulated during HepG2 cell polarity development. Previously, we have identified an apical plasma membrane-directed trafficking pathway from the subapical compartment. This transport pathway, which is part of the basolateral-to-apical transcytotic itinerary, plays a crucial role in apical plasma membrane biogenesis. Here, we show that, as a part of the underlying mechanism, the inhibition of dihydroceramide synthase activity and ensuing increased sphinganine levels specifically perturb the activation of this particular pathway in the de novo apical membrane biogenesis.

An outer segment localization signal at the C terminus of the photoreceptor-specific retinol dehydrogenase

Photoreceptor retinol dehydrogenase (prRDH) is a membrane-associated cytosolic protein that localizes to the outer segments (OS) of rods and cones. Here, we demonstrate that the C-terminal 16 amino acids of prRDH confer membrane association as well as cone and rod OS targeting on a linked green fluorescent protein. Membrane association in transfected 293 cells and in transgenic Xenopus photoreceptors is mediated by fatty acylation at one or more evolutionarily conserved cysteines within the prRDH C-terminal tail. In bovine OS, native prRDH is similarly acylated, and hydrolysis of this linkage releases the protein from the membrane. Efficient OS localization requires both membrane association and the prRDH sequence. (V/I)XPX at the extreme C terminus, which closely resembles the C-terminal sequence that targets opsin/rhodopsin to the OS. Taken together, these data imply that the C-terminal. (V/I)XPX sequence is a general OS localization signal that can function in the context of both integral and peripheral membrane proteins. This strategy for OS localization resembles those used for protein localization to mitochondria, peroxisomes, endosomes, and endoplasmic reticulum; in each case, a short N- or C-terminal sequence is shared among structurally diverse proteins that are targeted to the same subcellular destination.

Membrane dynamics and the regulation of epithelial cell polarity

Plasma membranes of epithelial cells consist of two domains, an apical and a basolateral domain, the surfaces of which differ in composition. The separation of these domains by a tight junction and the fact that specific transport pathways exist for intracellular communication between these domains and distinct intracellular compartments relevant to cell polarity development, have triggered extensive research on issues that focus on how the polarity is generated and maintained. Apart from proper assembly of tight junctions, their potential functioning as landmark for the transport machinery, cell-cell adhesion is obviously instrumental in barrier formation. In recent years, distinct endocytic compartments, defined as subapical compartment or common endosome, were shown to play a prominent role in regulating membrane trafficking to and from polarized membrane domains. Sorting devices remain to be determined but likely include distinct rab proteins, and evidence is accumulating to indicate that signaling events may direct intracellular membrane transport, intimately involved in the biogenesis and maintenance of polarized membrane domains and hence the development of cell polarity.

The Hippocampal Cholinergic Neurostimulating Peptide, the N-terminal Fragment of the Secreted Phosphatidylethanolamine-binding Protein, Possesses a New Biological Activity on Cardiac Physiology

Phosphatidylethanolamine-binding protein (PEBP), alternatively named Raf-1 kinase inhibitor protein, is the precursor of the hippocampal cholinergic neurostimulating peptide (HCNP) corresponding to its natural N-terminal fragment, previously described to be released by hippocampal neurons. PEBP is a soluble cytoplasmic protein, also associated with plasma and reticulum membranes of numerous cell types. In the present report, using biochemistry and cell biology techniques, we report for the first time the presence of PEBP in bovine chromaffin cell, a well described secretion model. We have examined its presence at the subcellular level and characterized this protein on both secretory granule membranes and intragranular matrix. In addition, its presence in bovine chromaffin cell and platelet exocytotic medium, as well as in serum, was reported showing that it is secreted. Like many other proteins that lack signal sequence, PEBP may be secreted through non-classic signal secretory mechanisms, which could be due to interactions with granule membrane lipids and lipid rafts. By two-dimensional liquid chromatography-tandem mass spectrometry, HCNP was detected among the intragranular matrix components. The observation that PEBP and HCNP were secreted with catecholamines into the circulation prompted us to investigate endocrine effects of this peptide on cardiovascular system. By using as bioassay an isolated and perfused frog (Rana esculenta) heart preparation, we show here that HCNP acts on the cardiac mechanical performance exerting a negative inotropism and counteracting the adrenergic stimulation of isoproterenol. All together, these data suggest that PEBP and HCNP might be considered as new endocrine factors involved in cardiac physiology.

Ceramidase expression facilitates membrane turnover and endocytosis of rhodopsin in photoreceptors

Transgenic expression of ceramidase suppresses retinal degeneration in Drosophila arrestin and phospholipase C mutants. Here, we show that expression of ceramidase facilitates the dissolution of incompletely formed and inappropriately located elements of rhabdomeric membranes in ninaE(I17) mutants lacking the G protein receptor Rh1 in R1-R6 photoreceptor cells. Ceramidase expression facilitates the endocytic turnover of Rh1. Although ceramidase expression aids the removal of internalized rhodopsin, it does not affect the turnover of Rh1 in photoreceptors maintained in dark, where Rh1 is not activated and thus has a slower turnover and a long half-life. Therefore, the phenotypic consequence of ceramidase expression in photoreceptors is caused by facilitation of endocytosis. This study provides mechanistic insight into the sphingolipid biosynthetic pathway-mediated modulation of endocytosis and suppression of retinal degeneration.

Spatial control of exocytosis

During many key biological processes, exocytosis is confined to distinct regions of the plasma membrane. Spatial control of exocytosis correlates with altered membrane skeleton dynamics and assembly of local membrane microdomains. These domains act as local stages for the assembly and the regulation of molecular complexes (targeting patches) that mediate vesicle-membrane fusion. Furthermore, local activation of signaling pathways reinforces formation of these patches and might effect global repositioning of the secretory pathway toward sites of localized exocytosis.

Sphingosine 1-phosphate, a diffusible calcium influx factor mediating store-operated calcium entry

Store-operated calcium entry (SOCE) is a fundamental mechanism of calcium signaling. The mechanisms linking store depletion to SOCE remain controversial, hypothetically involving both diffusible messengers and conformational coupling of stores to channels. Sphingosine 1-phosphate (S1P) is a bioactive sphingolipid that can signal via cell surface G-protein-coupled receptors, but S1P can also act as a second messenger, mobilizing calcium directly via unknown mechanisms. We show here that S1P opens calcium entry channels in human neutrophils (PMNs) and HL60 cells without prior store depletion, independent of G-proteins and of phospholipase C. S1P-mediated entry has the typical divalent cation permeability profile and inhibitor profile of SOCE in PMNs, is fully inhibited by 1 microm Gd3+, and is independent of [Ca2+]i. Depletion of PMN calcium stores by thapsigargin induces S1P synthesis. Inhibition of S1P synthesis by dimethylsphingosine blocks thapsigargin-, ionomycin-, and platelet-activating factor-mediated SOCE despite normal store depletion. We propose that S1P is a "calcium influx factor," linking calcium store depletion to downstream SOCE.

Role of Rab Proteins in Epithelial Membrane Traffic

Small GTPase rab proteins play an important role in various aspects of membrane traffic, including cargo selection, vesicle budding, vesicle motility, tethering, docking, and fusion. Recent data suggest also that rabs, and their divalent effector proteins, organize organelle subdomains and as such may define functional organelle identity. Most rabs are ubiquitously expressed. However, some rabs are preferentially expressed in epithelial cells where they appear intimately associated with the epithelial-specific transcytotic pathway and/or tight junctions. This review discusses the role of rabs in epithelial membrane transport.