(Glyco)sphingolipids are sorted in sub-apical compartments in HepG2 cells: a role for non-Golgi-related intracellular sites in the polarized distribution of (glyco)sphingolipids

Cargo sorting at the trans-Golgi network at a glance

The Golgi functions principally in the biogenesis and trafficking of glycoproteins and lipids. It is compartmentalized into multiple flattened adherent membrane sacs termed cisternae, which each contain a distinct repertoire of resident proteins, principally enzymes that modify newly synthesized proteins and lipids sequentially as they traffic through the stack of Golgi cisternae. Upon reaching the final compartments of the Golgi, the trans cisterna and trans-Golgi network (TGN), processed glycoproteins and lipids are packaged into coated and non-coated transport carriers derived from the trans Golgi and TGN. The cargoes of clathrin-coated vesicles are chiefly residents of endo-lysosomal organelles, while uncoated carriers ferry cargo to the cell surface. There are outstanding questions regarding the mechanisms of protein and lipid sorting within the Golgi for export to different organelles. Nonetheless, conceptual advances have begun to define the key molecular features of cargo clients and the mechanisms underlying their sorting into distinct export pathways, which we have collated in this Cell Science at a Glance article and the accompanying poster.

MAL Is a Regulator of the Recruitment of Myelin Protein PLP to Membrane Microdomains

In oligodendrocytes (OLGs), an indirect, transcytotic pathway is mediating transport of de novo synthesized PLP, a major myelin specific protein, from the apical-like plasma membrane to the specialized basolateral-like myelin membrane to prevent its premature compaction. MAL is a well-known regulator of polarized trafficking in epithelial cells, and given its presence in OLGs it was therefore of interest to investigate whether MAL played a similar role in PLP transport in OLGs, taking into account its timely expression in these cells. Our data revealed that premature expression of mCherry-MAL in oligodendrocyte progenitor cells interfered with terminal OLG differentiation, although myelin membrane formation per se was not impaired. In fact, also PLP transport to myelin membranes via the cell body plasma membrane was unaffected. However, the typical shift of PLP from TX-100-insoluble membrane domains to CHAPS-resistant, but TX-100-soluble membrane domains, seen in the absence of MAL expression, is substantially reduced upon expression of the MAL protein. Interestingly, not only in vitro, but also in developing brain a strongly diminished shift from TX-100 resistant to TX-100 soluble domains was observed. Consistently, the MAL-expression mediated annihilation of the typical membrane microdomain shift of PLP is also reflected by a loss of the characteristic surface expression profile of conformation-sensitive anti-PLP antibodies. Hence, these findings suggest that MAL is not involved in vesicular PLP trafficking to either the plasma membrane and/or the myelin membrane as such. Rather, we propose that MAL may regulate PLP’s distribution into distinct membrane microdomains that allow for lateral diffusion of PLP, directly from the plasma membrane to the myelin membrane once the myelin sheath has been assembled.

Structural and functional hepatocyte polarity and liver disease

Hepatocytes form a crucially important cell layer that separates sinusoidal blood from the canalicular bile. They have a uniquely organized polarity with a basal membrane facing liver sinusoidal endothelial cells, while one or more apical poles can contribute to several bile canaliculi jointly with the directly opposing hepatocytes. Establishment and maintenance of hepatocyte polarity is essential for many functions of hepatocytes and requires carefully orchestrated cooperation between cell adhesion molecules, cell junctions, cytoskeleton, extracellular matrix and intracellular trafficking machinery. The process of hepatocyte polarization requires energy and, if abnormal, may result in severe liver disease. A number of inherited disorders affecting tight junction and intracellular trafficking proteins have been described and demonstrate clinical and pathophysiological features overlapping those of the genetic cholestatic liver diseases caused by defects in canalicular ABC transporters. Thus both structural and functional components contribute to the final hepatocyte polarity phenotype. Many acquired liver diseases target factors that determine hepatocyte polarity, such as junctional proteins. Hepatocyte depolarization frequently occurs but is rarely recognized because hematoxylin-eosin staining does not identify the bile canaliculus. However, the molecular mechanisms underlying these defects are not well understood. Here we aim to provide an update on the key factors determining hepatocyte polarity and how it is affected in inherited and acquired diseases.

The special case of hepatocytes: unique tissue architecture calls for a distinct mode of cell division

Columnar epithelia (e.g., kidney, intestine) and hepatocytes embody the two major organizational phenotypes of non-stratified epithelial cells. Columnar epithelia establish their apical and basal domains at opposing poles and organize in monolayered cysts and tubules, in which their apical surfaces form a single continuous lumen whereas hepatocytes establish their apical domains in the midst of their basolateral domains and organize a highly branched capillary luminal network, the bile canaliculi, in which a single hepatocyte can engage in lumen formation with multiple neighbors. To maintain their distinct tissue architectures, columnar epithelial cells bisect their luminal domains during symmetric cell divisions, while the cleavage furrow in dividing hepatocytes avoids bisecting the bile canalicular domains. We discuss recently discovered molecular mechanisms that underlie the different cell division phenotypes in columnar and hepatocytic model cell lines. The serine/threonine kinase Par1b determines both the epithelial lumen polarity and cell division phenotype via cell adhesion signaling that converges on the small GTPase RhoA.

Hepatocyte polarity

Hepatocytes, like other epithelia, are situated at the interface between the organism's exterior and the underlying internal milieu and organize the vectorial exchange of macromolecules between these two spaces. To mediate this function, epithelial cells, including hepatocytes, are polarized with distinct luminal domains that are separated by tight junctions from lateral domains engaged in cell-cell adhesion and from basal domains that interact with the underlying extracellular matrix. Despite these universal principles, hepatocytes distinguish themselves from other nonstriated epithelia by their multipolar organization. Each hepatocyte participates in multiple, narrow lumina, the bile canaliculi, and has multiple basal surfaces that face the endothelial lining. Hepatocytes also differ in the mechanism of luminal protein trafficking from other epithelia studied. They lack polarized protein secretion to the luminal domain and target single-spanning and glycosylphosphatidylinositol-anchored bile canalicular membrane proteins via transcytosis from the basolateral domain. We compare this unique hepatic polarity phenotype with that of the more common columnar epithelial organization and review our current knowledge of the signaling mechanisms and the organization of polarized protein trafficking that govern the establishment and maintenance of hepatic polarity. The serine/threonine kinase LKB1, which is activated by the bile acid taurocholate and, in turn, activates adenosine monophosphate kinase-related kinases including AMPK1/2 and Par1 paralogues has emerged as a key determinant of hepatic polarity. We propose that the absence of a hepatocyte basal lamina and differences in cell-cell adhesion signaling that determine the positioning of tight junctions are two crucial determinants for the distinct hepatic and columnar polarity phenotypes.

Ganglioside GM1-mediated transcytosis of cholera toxin bypasses the retrograde pathway and depends on the structure of the ceramide domain

Cholera toxin causes diarrheal disease by binding ganglioside GM1 on the apical membrane of polarized intestinal epithelial cells and trafficking retrograde through sorting endosomes, the trans-Golgi network (TGN), and into the endoplasmic reticulum. A fraction of toxin also moves from endosomes across the cell to the basolateral plasma membrane by transcytosis, thus breeching the intestinal barrier. Here we find that sorting of cholera toxin into this transcytotic pathway bypasses retrograde transport to the TGN. We also find that GM1 sphingolipids can traffic from apical to basolateral membranes by transcytosis in the absence of toxin binding but only if the GM1 species contain cis-unsaturated or short acyl chains in the ceramide domain. We found previously that the same GM1 species are needed to efficiently traffic retrograde into the TGN and endoplasmic reticulum and into the recycling endosome, implicating a shared mechanism of action for sorting by lipid shape among these pathways.

Hepatocytes traffic and export hepatitis B virus basolaterally by polarity-dependent mechanisms

Viruses commonly utilize the cellular trafficking machinery of polarized cells to effect viral export. Hepatocytes are polarized in vivo, but most in vitro hepatocyte models are either nonpolarized or have morphology unsuitable for the study of viral export. Here, we investigate the mechanisms of trafficking and export for the hepadnaviruses hepatitis B virus (HBV) and duck hepatitis B virus (DHBV) in polarized hepatocyte-derived cell lines and primary duck hepatocytes. DHBV export, but not replication, was dependent on the development of hepatocyte polarity, with export significantly abrogated over time as primary hepatocytes lost polarity. Using Transwell cultures of polarized N6 cells and adenovirus-based transduction, we observed that export of both HBV and DHBV was vectorially regulated and predominantly basolateral. Monitoring of polarized N6 cells and nonpolarized C11 cells during persistent, long-term DHBV infection demonstrated that newly synthesized sphingolipid and virus displayed significant colocalization and fluorescence resonance energy transfer, implying cotransportation from the Golgi complex to the plasma membrane. Notably, 15% of virus was released apically from polarized cells, corresponding to secretion into the bile duct in vivo, also in association with sphingolipids. We conclude that DHBV and, probably, HBV are reliant upon hepatocyte polarity to be efficiently exported and this export is in association with sphingolipid structures, possibly lipid rafts. This study provides novel insights regarding the mechanisms of hepadnavirus trafficking in hepatocytes, with potential relevance to pathogenesis and immune tolerance.

Interaction of PLP with GFP-MAL2 in the human oligodendroglial cell line HOG

The velocity of the nerve impulse conduction of vertebrates relies on the myelin sheath, an electrically insulating layer that surrounds axons in both the central and peripheral nervous systems, enabling saltatory conduction of the action potential. Oligodendrocytes are the myelin-producing glial cells in the central nervous system. A deeper understanding of the molecular basis of myelination and, specifically, of the transport of myelin proteins, will contribute to the search of the aetiology of many dysmyelinating and demyelinating diseases, including multiple sclerosis. Recent investigations suggest that proteolipid protein (PLP), the major myelin protein, could reach myelin sheath by an indirect transport pathway, that is, a transcytotic route via the plasma membrane of the cell body. If PLP transport relies on a transcytotic process, it is reasonable to consider that this myelin protein could be associated with MAL2, a raft protein essential for transcytosis. In this study, carried out with the human oligodendrocytic cell line HOG, we show that PLP colocalized with green fluorescent protein (GFP)-MAL2 after internalization from the plasma membrane. In addition, both immunoprecipitation and immunofluorescence assays, indicated the existence of an interaction between GFP-MAL2 and PLP. Finally, ultrastructural studies demonstrated colocalization of GFP-MAL2 and PLP in vesicles and tubulovesicular structures. Taken together, these results prove for the first time the interaction of PLP and MAL2 in oligodendrocytic cells, supporting the transcytotic model of PLP transport previously suggested.

Potential of ultraviolet wide-field imaging and multiphoton microscopy for analysis of dehydroergosterol in cellular membranes

Dehydroergosterol (DHE) is an intrinsically fluorescent sterol with absorption/emission in the ultraviolet (UV) region and biophysical properties similar to those of cholesterol. We compared the potential of UV-sensitive low-light-level wide-field (UV-WF) imaging with that of multiphoton (MP) excitation microscopy to monitor DHE in living cells. Significantly reduced photobleaching in MP microscopy of DHE enabled us to acquire three-dimensional z-stacks of DHE-stained cells and to obtain high-resolution maps of DHE in surface ruffles, nanotubes, and the apical membrane of epithelial cells. We found that the lateral resolution of MP microscopy is ∼1.5-fold higher than that of UV-WF deconvolution microscopy, allowing for improved spatiotemporal analysis of plasma membrane sterol distribution. Surface intensity patterns of DHE with a diameter of 0.2 μm persisting over several minutes could be resolved by MP time-lapse microscopy. Diffusion coefficients of 0.25-μm-diameter endocytic vesicles containing DHE were determined by MP spatiotemporal image correlation spectroscopy. The requirement of extremely high laser power for visualization of DHE by MP microscopy made this method less potent for multicolor applications with organelle markers like green fluorescent protein-tagged proteins. The signal-to-noise ratio obtainable by UV-WF imaging could be significantly improved by pixelwise bleach rate fitting and calculation of an amplitude image from the decay model and by frame averaging after pixelwise bleaching correction of the image stacks. We conclude that UV-WF imaging and MP microscopy of DHE provide complementary information regarding membrane distribution and intracellular targeting of sterols.

MAL2 selectively regulates polymeric IgA receptor delivery from the Golgi to the plasma membrane in WIF-B cells

Myelin and lymphocyte protein 2 (MAL2) has been identified as a hepatic transcytotic regulator that mediates delivery from basolateral endosomes to the subapical compartment (SAC). However, overexpression of polymeric immunoglobulin A-receptor (pIgA-R) in polarized, hepatic WIF-B cells led to the dramatic redistribution of MAL2 into the Golgi and all the transcytotic intermediates occupied by the receptor. Although overexpressed hemagglutinin and dipeptidylpeptidase IV (DPPIV) distributed to the same compartments, MAL2 distributions did not change indicating the effect is selective. Cycloheximide treatment led to decreased pIgA-R and MAL2 intracellular staining, first in the Golgi then the SAC, suggesting they were apically delivered and that MAL2 was mediating the process. This was tested in Clone 9 cells (that lack endogenous MAL2). When expressed alone, pIgA-R was restricted to the Golgi whereas when coexpressed with MAL2, it distributed to the surface, was internalized and delivered to MAL2-positive puncta. In contrast, DPPIV distributions were independent of MAL2. Surface delivery of newly synthesized pIgA-R, but not DPPIV, was enhanced greater than ninefold by MAL2 coexpression. In WIF-B cells where MAL2 expression was knocked down, pIgA-R, but not DPPIV, was retained in the Golgi and its basolateral delivery was impaired. Thus, in addition to its role in transcytosis, MAL2 also regulates pIgA-R delivery from the Golgi to the plasma membrane.

The Na+/H+ exchanger NHE6 in the endosomal recycling system is involved in the development of apical bile canalicular surface domains in HepG2 cells

This study underscores the emerging role of NHE6 as a novel regulatory protein in the apical surface development of human hepatoma HepG2 cells. A limited range of endosomal pH facilitated by NHE6.1 is suggested to be important for securing the polarized distribution of membrane lipids and proteins and maintenance of apical bile canaliculi.

Polarized epithelial cells develop and maintain distinct apical and basolateral surface domains despite a continuous flux of membranes between these domains. The Na⁺/H⁺exchanger NHE6 localizes to endosomes but its function is unknown. Here, we demonstrate that polarized hepatoma HepG2 cells express an NHE6.1 variant that localizes to recycling endosomes and colocalizes with transcytosing bulk membrane lipids. NHE6.1 knockdown or overexpression decreases or increases recycling endosome pH, respectively, and inhibits the maintenance of apical, bile canalicular plasma membranes and, concomitantly, apical lumens. NHE6.1 knockdown or overexpression has little effect on the de novo biogenesis of apical surface domains. NHE6.1 knockdown does not inhibit basolateral-to-apical transcytosis of bulk membrane lipids, but it does promote their progressive loss from the apical surface, leaving cells unable to efficiently retain bulk membrane and bile canalicular proteins at the apical surface. The data suggest that a limited range of endosome pH mediated by NHE6.1 is important for securing the polarized distribution of membrane lipids at the apical surface and maintenance of apical bile canaliculi in HepG2 cells and hence cell polarity. This study underscores the emerging role of the endosomal recycling system in apical surface development and identifies NHE6 as a novel regulatory protein in this process.

Glucosylceramide in humans

Glucosylceramide has a unique and often ambiguous role in mammalian cells. Activation of glucosylceramide synthase, the enzyme that places a glucosyl moiety onto ceramide, is the first pathway-committed step to the production of more complex glycosphingolipids such as lactosylceramide and gangliosides. Alterations in the level of glucosylceramide are noted in cells and tissues in response to cardiovascular disease, diabetes, skin disorders and cancer. Overall, upregulation of glucosylceramide offers cellular protection and primes certain cells for proliferation. However, prolonged overabundance of glucosylceramide is detrimental, as seen in Gaucher disease in humans.

On the biogenesis of myelin membranes: sorting, trafficking and cell polarity

In the central nervous system, a multilayered membrane layer known as the myelin sheath enwraps axons, and is required for optimal saltatory signal conductance. The sheath develops from membrane processes that extend from the plasma membrane of oligodendrocytes and displays a unique lipid and protein composition. Myelin biogenesis is carefully regulated, and multiple transport pathways involving a variety of endosomal compartments are involved. Here we briefly summarize how the major myelin proteins proteolipid protein and myelin basic protein reach the sheath, and highlight potential mechanisms involved, including the role of myelin specific lipids and cell polarity related transport pathways.

Kinetic imaging of NPC1L1 and sterol trafficking between plasma membrane and recycling endosomes in hepatoma cells

Niemann-Pick C1-like 1 (NPC1L1) is a recently identified protein that mediates intestinal cholesterol absorption and regulates biliary cholesterol excretion. The itineraries and kinetics of NPC1L1 trafficking remain uncertain. In this study, we have visualized movement of NPC1L1-enhanced green fluorescent protein (NPC1L1-EGFP) and cholesterol analogs in hepatoma cells. At steady state, about 42% of NPC1L1 resided in the transferrin (Tf)-positive, sterol-enriched endocytic recycling compartment (ERC), whereas time-lapse microscopy demonstrated NPC1L1 traffic between the plasma membrane and the ERC. Fluorescence recovery after photobleaching revealed rapid recovery (half-time approximately 2.5 min) of about 35% of NPC1L1 in the ERC, probably replenished from peripheral sorting endosomes. Acute cholesterol depletion blocked internalization of NPC1L1-EGFP and Tf and stimulated recycling of NPC1L1-EGFP from the ERC to the plasma membrane. NPC1L1-EGFP facilitated transport of fluorescent sterols from the plasma membrane to the ERC. Insulin induced translocation of vesicles containing NPC1L1 and fluorescent sterol from the ERC to the cell membrane. Upon polarization of hepatoma cells, NPC1L1 resided almost exclusively in the canalicular membrane, where the protein is highly mobile. Our study demonstrates dynamic trafficking of NPC1L1 between the cell surface and intracellular compartments and suggests that this transport is involved in NPC1L1-mediated cellular sterol uptake.

cAMP-dependent protein kinase A and the dynamics of epithelial cell surface domains: moving membranes to keep in shape

Cyclic adenosine monophosphate (cAMP) and cAMP-dependent protein kinase A (PKA) are evolutionary conserved molecules with a well-established position in the complex network of signal transduction pathways. cAMP/PKA-mediated signaling pathways are implicated in many biological processes that cooperate in organ development including the motility, survival, proliferation and differentiation of epithelial cells. Cell surface polarity, here defined as the anisotropic organisation of cellular membranes, is a critical parameter for most of these processes. Changes in the activity of cAMP/PKA elicit a variety of effects on intracellular membrane dynamics, including membrane sorting and trafficking. One of the most intriguing aspects of cAMP/PKA signaling is its evolutionary conserved abundance on the one hand and its precise spatial-temporal actions on the other. Here, we review recent developments with regard to the role of cAMP/PKA in the regulation of intracellular membrane trafficking in relation to the dynamics of epithelial surface domains.

Anchoring of protein kinase A-regulatory subunit IIalpha to subapically positioned centrosomes mediates apical bile canalicular lumen development in response to oncostatin M but not cAMP

Oncostatin M and cAMP signaling stimulate apical surface-directed membrane trafficking and apical lumen development in hepatocytes, both in a protein kinase A (PKA)-dependent manner. Here, we show that oncostatin M, but not cAMP, promotes the A-kinase anchoring protein (AKAP)-dependent anchoring of the PKA regulatory subunit (R)IIalpha to subapical centrosomes and that this requires extracellular signal-regulated kinase 2 activation. Stable expression of the RII-displacing peptide AKAP-IS, but not a scrambled peptide, inhibits the association of RIIalpha with centrosomal AKAPs and results in the repositioning of the centrosome from a subapical to a perinuclear location. Concomitantly, common endosomes, but not apical recycling endosomes, are repositioned from a subapical to a perinuclear location, without significant effects on constitutive or oncostatin M-stimulated basolateral-to-apical transcytosis. Importantly, however, the expression of the AKAP-IS peptide completely blocks oncostatin M-, but not cAMP-stimulated apical lumen development. Together, the data suggest that centrosomal anchoring of RIIalpha and the interrelated subapical positioning of these centrosomes is required for oncostatin M-, but not cAMP-mediated, bile canalicular lumen development in a manner that is uncoupled from oncostatin M-stimulated apical lumen-directed membrane trafficking. The results also imply that multiple PKA-mediated signaling pathways control apical lumen development and that subapical centrosome positioning is important in some of these pathways.

Cell polarity development and protein trafficking in hepatocytes lacking E-cadherin/beta-catenin-based adherens junctions

Using a mutant hepatocyte cell line in which E-cadherin and beta-catenin are completely depleted from the cell surface, and, consequently, fail to form adherens junctions, we have investigated adherens junction requirement for apical-basolateral polarity development and polarized membrane trafficking. It is shown that these hepatocytes retain the capacity to form functional tight junctions, develop full apical-basolateral cell polarity, and assemble a subapical cortical F-actin network, although with a noted delay and a defect in subsequent apical lumen remodeling. Interestingly, whereas hepatocytes typically target the plasma membrane protein dipeptidyl peptidase IV first to the basolateral surface, followed by its transcytosis to the apical domain, hepatocytes lacking E-cadherin-based adherens junctions target dipeptidyl peptidase IV directly to the apical surface. Basolateral surface-directed transport of other proteins or lipids tested was not visibly affected in hepatocytes lacking E-cadherin-based adherens junctions. Together, our data show that E-cadherin/beta-catenin-based adherens junctions are dispensable for tight junction formation and apical lumen biogenesis but not for apical lumen remodeling. In addition, we suggest a possible requirement for E-cadherin/beta-catenin-based adherens junctions with regard to the indirect apical trafficking of specific proteins in hepatocytes.

Differences in the domain forming properties of N-palmitoylated neutral glycosphingolipids in bilayer membranes

We have compared the domain forming properties of three neutral acyl chain defined glycosphingolipids differing in their head group structures. The aim of the study was to explore if glycosphingolipids and sterols exist in the same lateral domains in bilayer membranes and how the structure of the head group influences the capacity of the glycosphingolipids to colocalize with cholesterol. The glycosphingolipids used in the study were galactosyl-, glucosyl- and lactosylceramides with a palmitic acid in the N-linked position. Domain formation in mixed bilayer vesicles was examined using fluorescent reporter molecules associating with ordered domains, together with a fluorescence quencher lipid in the disordered membrane phase. Our results show that the glycosphingolipids studied were poor in forming sterol-enriched domains compared to palmitoyl-sphingomyelin as detected by cholestatrienol quenching. However, the tendency to associate with cholesterol was clearly dependent on the carbohydrate structure of the glycosphingolipids, also when two glycosphingolipids with different head groups were mixed in the bilayer. All palmitoylated glycosphingolipids associated with palmitoyl-sphingomyelin/cholesterol domains. Our results show that the head group structures of neutral glycosphingolipids markedly affect their domain forming properties in bilayers both with and without cholesterol. The most striking observation being that large differences in domain forming properties were seen even between glucosylceramide and galactosylceramide, which differ only in the stereochemistry of one hydroxyl group in the carbohydrate head group.

Quantification of polarized trafficking of transferrin and comparison with bulk membrane transport in hepatic cells

Transport of the recycling marker transferrin was analysed in polarized hepatic HepG2 cells using quantitative fluorescence microscopy and mathematical modelling. A detailed map and kinetic model for transport of transferrin in hepatic cells was developed. Fluorescent transferrin was found to be transported sequentially through basolateral SE (sorting endosomes) to a SAC/ARC (subapical compartment/apical recycling compartment). DiI (di-indocarbocyanine) lipid probes of different acyl chain length (DiIC12 and DiIC16) co-localized with transferrin in basolateral SE and in the SAC/ARC. By kinetic comparison of hepatic transport of transferrin and labelled HDL (high-density lipoprotein), it is shown that transport of transferrin from SE to the SAC/ARC follows a default pathway together with HDL. Kinetic modelling of fluorescence data provides an identical half-time for SE-to-SAC/ARC transport of transferrin and fluorescent HDL (t(1/2)=4.2 min). Fluorescent transferrin was found to recycle with a half-time of t(1/2)=12.9 min from the SAC/ARC to the basolateral cell surface of HepG2 cells. In contrast with HDL, targeting of labelled transferrin from the SAC/ARC to the apical biliary canaliculus was negligible. The results indicate that transport from basolateral hepatic SE to the SAC/ARC represents a bulk flow process and that polarized sorting occurs mainly at the level of the SAC/ARC.

Rho kinase, myosin-II, and p42/44 MAPK control extracellular matrix-mediated apical bile canalicular lumen morphogenesis in HepG2 cells

The molecular mechanisms that regulate multicellular architecture and the development of extended apical bile canalicular lumens in hepatocytes are poorly understood. Here, we show that hepatic HepG2 cells cultured on glass coverslips first develop intercellular apical lumens typically formed by a pair of cells. Prolonged cell culture results in extensive organizational changes, including cell clustering, multilayering, and apical lumen morphogenesis. The latter includes the development of large acinar structures and subsequent elongated canalicular lumens that span multiple cells. These morphological changes closely resemble the early organizational pattern during development, regeneration, and neoplasia of the liver and are rapidly induced when cells are cultured on predeposited extracellular matrix (ECM). Inhibition of Rho kinase or its target myosin-II ATPase in cells cultured on glass coverslips mimics the morphogenic response to ECM. Consistently, stimulation of Rho kinase and subsequent myosin-II ATPase activity by lipoxygenase-controlled eicosatetranoic acid metabolism inhibits ECM-mediated cell multilayering and apical lumen morphogenesis but not initial apical lumen formation. Furthermore, apical lumen remodeling but not cell multilayering requires basal p42/44 MAPK activity. Together, the data suggest a role for hepatocyte-derived ECM in the spatial organization of hepatocytes and apical lumen morphogenesis and identify Rho kinase, myosin-II, and MAPK as potentially important players in different aspects of bile canalicular lumen morphogenesis.

Efficient trafficking of MDR1/P-glycoprotein to apical canalicular plasma membranes in HepG2 cells requires PKA-RIIalpha anchoring and glucosylceramide

In hepatocytes, cAMP/PKA activity stimulates the exocytic insertion of apical proteins and lipids and the biogenesis of bile canalicular plasma membranes. Here, we show that the displacement of PKA-RIIalpha from the Golgi apparatus severely delays the trafficking of the bile canalicular protein MDR1 (P-glycoprotein), but not that of MRP2 (cMOAT), DPP IV and 5'NT, to newly formed apical surfaces. In addition, the direct trafficking of de novo synthesized glycosphingolipid analogues from the Golgi apparatus to the apical surface is inhibited. Instead, newly synthesized glucosylceramide analogues are rerouted to the basolateral surface via a vesicular pathway, from where they are subsequently endocytosed and delivered to the apical surface via transcytosis. Treatment of HepG2 cells with the glucosylceramide synthase inhibitor PDMP delays the appearance of MDR1, but not MRP2, DPP IV, and 5'NT at newly formed apical surfaces, implicating glucosylceramide synthesis as an important parameter for the efficient Golgi-to-apical surface transport of MDR1. Neither PKA-RIIalpha displacement nor PDMP inhibited (cAMP-stimulated) apical plasma membrane biogenesis per se, suggesting that other cAMP effectors may play a role in canalicular development. Taken together, our data implicate the involvement of PKA-RIIalpha anchoring in the efficient direct apical targeting of distinct proteins and glycosphingolipids to newly formed apical plasma membrane domains and suggest that rerouting of Golgi-derived glycosphingolipids may underlie the delayed Golgi-to-apical surface transport of MDR1.

Dynamics of MAL2 During Glycosylphosphatidylinositol-Anchored Protein Transcytotic Transport to the Apical Surface of Hepatoma HepG2 Cells

Delivery of glycosylphosphatidylinositol (GPI)-anchored proteins to the apical surface takes place by transcytosis in hepatocytes and also probably in epithelial Madin-Darby canine cells. The integral protein MAL2 was demonstrated to be essential for basolateral-to-apical transcytosis in hepatoma HepG2 cells. Reduction of endogenous MAL2 levels impedes cargo delivery to the apical membrane, but, paradoxically, cargo does not accumulate in the subapical compartment where MAL2 predominantly resides but in distant endosome elements. To understand how transcytosis can be apparently mediated at a distance, we have analyzed the dynamics of machinery and cargo by live-cell imaging of MAL2 and transcytosing CD59, a GPI-anchored protein, in HepG2 cells. MAL2 was revealed as being a highly dynamic protein. Soon after basolateral endocytosis of CD59, a fraction of MAL2 redistributed into peripheral vesicular clusters that concentrated CD59 and that were accessible to transferrin (Tf) receptor, a basolateral recycling protein. Following Tf receptor segregation, the clusters fused in a MAL2(+)globular structure and moved toward the apical surface for CD59 delivery. All these processes were impaired in cells with reduced MAL2 content. Other GPI-anchored proteins examined behave similarly. As MAL2 is expressed by many types of epithelia, the sorting events described herein are probably of quite general utility.

Improved visualization and quantitative analysis of fluorescent membrane sterol in polarized hepatic cells

Dehydroergosterol is a natural yeast sterol which has recently been employed for direct observation of intracellular sterol transport by UV microscopy. Here, methods are described for improved visualization and quantification of dehydroergosterol in the membranes of polarized HepG2 cells. Using a new online assay, it is shown that dehydroergosterol derived from a cyclodextrin complex inserted into the plasma membrane with a half time of t(1/2) approximately 34 s. Based on a detailed bleaching analysis of dehydroergosterol, slightly different bleaching rates for dehydroergosterol in the basolateral and canalicular membrane were found, indicating different fluorophore environments. Bleaching correction in concert with 3D imaging allows for detection of dehydroergosterol enrichment in microvilli of the canalicular membrane forming the biliary canaliculus. Evidence is provided that some dehydroergosterol accumulating in a subapical compartment or apical recycling compartment can rapidly (t(1/2) approximately 2 min) exchange in vesicles towards the biliary canaliculus while the majority of dehydroergosterol does not redistribute from this compartment. The rapidly exchanging pool resembles only a small portion of the total subapical compartment or apical recycling compartment-associated dehydroergosterol (about 15-30%). Kinetic modelling supports the theory that the subapical compartment or apical recycling compartment to biliary canaliculus transport pathway for sterol is unidirectional. This pathway might be important for rapid biliary transport of free sterol produced by hydrolysis of cholesteryl esters derived from high density lipoprotein.

Calmodulin modulates hepatic membrane polarity by protein kinase C-sensitive steps in the basolateral endocytic pathway

Membrane polarity is maintained by a complex intermingling of various trafficking pathways, including basolateral and apical endocytosis. The present work was undertaken to better define the role of basolateral endocytic transport in apical membrane homeostasis. When polarized HepG2 hepatoma cells were incubated with calmodulin antagonists, the cells lost their polarity, as reflected by an inhibition of lipid transport of a fluorescent sphingomyelin to the apical membrane and an impediment of its recycling to the basolateral membrane. Instead, an accumulation of the lipid in dilated early endosomal compartments was observed, presumably due to a frustration of vesiculation. Interestingly, lipid transport to the apical pole, lipid recycling to the basolateral membrane and cell polarity were reestablished, while dilated compartments disappeared, when the cells were simultaneously treated with specific inhibitors of protein kinase C (PKC). Consistently, following activation of PKC, extensive dilation/vacuolation of early sorting endosomes was observed, very similar as seen upon treatment with calmodulin antagonists. Thus, the results indicate that membrane trafficking at early steps of the basolateral endocytic pathway in HepG2 cells is regulated by an intricate interplay between calmodulin and PKC. This interference, although not affecting endocytosis as such, compromises cell polarity by impeding membrane trafficking from early endosomes to the apical membrane.

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.

Mathematical analysis of hepatic high density lipoprotein transport based on quantitative imaging data

Hepatocytes internalize high density lipoprotein (HDL) at the basolateral membrane. Most HDL is recycled while some is shuttled to the canalicular membrane by transcytosis. Here, transport of HDL was analyzed by mathematical modeling based on measurements in polarized hepatic HepG2 cells. Recycling of HDL from basolateral sorting endosomes was modeled by applying the rapid equilibrium approach. Analytical expressions were derived, which describe in one model the transport of HDL to the subapical compartment/apical recycling compartment, the biliary canaliculus (BC), and to late endosomes and lysosomes (LE/LYS). Apical endocytosis of HDL predicted by the model was confirmed for rhodamine-dextran and fluorescent asialoorosomucoid, markers for LE/LYS in living HepG2 cells. Budding of endocytic vesicles from the BC was directly observed by time lapse imaging of a fluorescent lipid probe. Based on fitted kinetic parameters and their covariance matrix a Monte Carlo simulation of HDL transport in hepatocytes was performed. The model was used to quantitatively assess release of HDL-associated free cholesterol by scavenger receptor BI. It is shown that only 6% of HDL-associated sterol reaches the BC as a constituent of the HDL particles, whereas the remaining sterol is rapidly released from HDL and shuttled to the BC by non-vesicular transport.

Oncostatin M-stimulated apical plasma membrane biogenesis requires p27(Kip1)-regulated cell cycle dynamics

Oncostatin M regulates membrane traffic and stimulates apicalization of the cell surface in hepatoma cells in a protein kinase A-dependent manner. Here, we show that oncostatin M enhances the expression of the cyclin-dependent kinase (cdk)2 inhibitor p27(Kip1), which inhibits G(1)-S phase progression. Forced G(1)-S-phase transition effectively renders presynchronized cells insensitive to the apicalization-stimulating effect of oncostatin M. G(1)-S-phase transition prevents oncostatin M-mediated recruitment of protein kinase A to the centrosomal region and precludes the oncostatin M-mediated activation of a protein kinase A-dependent transport route to the apical surface, which exits the subapical compartment (SAC). This transport route has previously been shown to be crucial for apical plasma membrane biogenesis. Together, our data indicate that oncostatin M-stimulated apicalization of the cell surface is critically dependent on the ability of oncostatin M to control p27(Kip1)/cdk2-mediated G(1)-S-phase progression and suggest that the regulation of apical plasma membrane-directed traffic from SAC is coupled to centrosome-associated signaling pathways.

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.

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.

Transcytosis: crossing cellular barriers

Transcytosis, the vesicular transport of macromolecules from one side of a cell to the other, is a strategy used by multicellular organisms to selectively move material between two environments without altering the unique compositions of those environments. In this review, we summarize our knowledge of the different cell types using transcytosis in vivo, the variety of cargo moved, and the diverse pathways for delivering that cargo. We evaluate in vitro models that are currently being used to study transcytosis. Caveolae-mediated transcytosis by endothelial cells that line the microvasculature and carry circulating plasma proteins to the interstitium is explained in more detail, as is clathrin-mediated transcytosis of IgA by epithelial cells of the digestive tract. The molecular basis of vesicle traffic is discussed, with emphasis on the gaps and uncertainties in our understanding of the molecules and mechanisms that regulate transcytosis. In our view there is still much to be learned about this fundamental process.

Membrane dynamics and cell polarity: the role of sphingolipids

In recent years, glycosphingolipids (GSLs) have attracted widespread attention due to the appreciation that this class of lipids has a major impact on biological life. Inhibition of the synthesis of glucosylceramide, which serves as a precursor for the generation of complex glycosphinglipids, is embryonic lethal. GSLs play a major role in growth and development. Metabolites of sphingolipids, such as ceramide, sphinganine, and sphingosine, may function as second messengers or regulators of signal transduction that affect events ranging from apoptosis to the (co)regulation of the cell cycle. In addition, GSLs can provide a molecular platform for clustering of signal transducers. The ability of sphingolipids, with or without cholesterol, to form microdomains or rafts is critical in sorting and membrane transport that underlies the biogenesis of polarized membrane domains. Here, a brief summary is presented of some recent developments in this field, with a particular emphasis on raft assembly and membrane transport in the establishment of membrane polarity.

Trans-Golgi network and subapical compartment of HepG2 cells display different properties in sorting and exiting of sphingolipids

In HepG2 cells, the subapical compartment (SAC) is involved in the biogenesis of membrane polarity. By contrast, direct apical transport originating from the trans-Golgi network (TGN), which may contribute to polarity establishment, has been poorly defined in these cells. Thus, although newly synthesized sphingolipids can be directly transported from the TGN to the apical membrane, numerous apical resident proteins are traveling via the transcytotic route. Here, we developed an in vitro transport assay and compared the molecular sorting of 6-[N-(7-nitrobenz-2-oxa-1,3 diazol-4-yl)amino] hexanoyl-sphingomyelin (C(6)NBD-SM) and C(6)NBD-glucosylceramide (C(6)NBD-GlcCer) in TGN and SAC. SM is released from both TGN and SAC in the lumenal leaflet of transport vesicles. This holds also for GlcCer released from the SAC but not for a substantial fraction that departed from the Golgi. Distinct transport vesicles, enriched in either SM or GlcCer are released from SAC, consistent with their rigid sorting in this compartment. Different vesicle populations could not be recovered from TGN, although in situ experiments reveal that GlcCer is preferentially transported to the apical membrane, reflecting different transport mechanisms. The results indicate that in HepG2 cells sphingolipids are mainly sorted in the SAC membrane and that the release of SM from SAC and TGN is differentially regulated.

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.

Oncostatin M regulates membrane traffic and stimulates bile canalicular membrane biogenesis in HepG2 cells

Hepatocytes are the major epithelial cells of the liver and they display membrane polarity: the sinusoidal membrane representing the basolateral surface, while the bile canalicular membrane is typical of the apical membrane. In polarized HepG2 cells an endosomal organelle, SAC, fulfills a prominent role in the biogenesis of the canalicular membrane, reflected by its ability to sort and redistribute apical and basolateral sphingolipids. Here we show that SAC appears to be a crucial target for a cytokine-induced signal transduction pathway, which stimulates membrane transport exiting from this compartment promoting apical membrane biogenesis. Thus, oncostatin M, an IL-6-type cytokine, stimulates membrane polarity development in HepG2 cells via the gp130 receptor unit, which activates a protein kinase A-dependent and sphingomyelin-marked membrane transport pathway from SAC to the apical membrane. To exert its signal transducing function, gp130 is recruited into detergent-resistant membrane microdomains at the basolateral membrane. These data provide a clue for a molecular mechanism that couples the biogenesis of an apical plasma membrane domain to the regulation of intracellular transport in response to an extracellular, basolaterally localized stimulus.

MAL2, a novel raft protein of the MAL family, is an essential component of the machinery for transcytosis in hepatoma HepG2 cells

Transcytosis is used alone (e.g., hepatoma HepG2 cells) or in combination with a direct pathway from the Golgi (e.g., epithelial MDCK cells) as an indirect route for targeting proteins to the apical surface. The raft-associated MAL protein is an essential element of the machinery for the direct route in MDCK cells. Herein, we present the functional characterization of MAL2, a member of the MAL protein family, in polarized HepG2 cells. MAL2 resided selectively in rafts and is predominantly distributed in a compartment localized beneath the subapical F-actin cytoskeleton. MAL2 greatly colocalized in subapical endosome structures with transcytosing molecules en route to the apical surface. Depletion of endogenous MAL2 drastically blocked transcytotic transport of exogenous polymeric immunoglobulin receptor and endogenous glycosylphosphatidylinositol-anchored protein CD59 to the apical membrane. MAL2 depletion did not affect the internalization of these molecules but produced their accumulation in perinuclear endosome elements that were accessible to transferrin. Normal transcytosis persisted in cells that expressed exogenous MAL2 designed to resist the depletion treatment. MAL2 is therefore essential for transcytosis in HepG2 cells.

Sphingolipid trafficking and protein sorting in epithelial cells

Sphingolipids represent a minor, but highly dynamic subclass of lipids in all eukaryotic cells. They are involved in functions that range from structural protection to signal transduction and protein sorting, and participate in lipid raft assembly. In polarized epithelial cells, which display an asymmetric apical and basolateral membrane surface, rafts have been proposed as a sorting principle for apical resident proteins, following their biosynthesis. However, raft-mediated trafficking is ubiquitous in cells. Also, sphingolipids per se, which are strongly enriched in the apical domain, are subject to sorting in polarity development. Next to the trans Golgi network, a subapical compartment called SAC or common endosome appears instrumental in regulating these sorting events.

Nonpolarized cells selectively sort apical proteins from cell surface to a novel compartment, but lack apical retention mechanisms

Membrane trafficking is central to establishing and maintaining epithelial cell polarity. One open question is to what extent the mechanisms regulating membrane trafficking are conserved between nonpolarized and polarized cells. To answer this question, we examined the dynamics of domain-specific plasma membrane (PM) proteins in three classes of hepatic cells: polarized and differentiated WIF-B cells, nonpolarized and differentiated Fao cells, and nonpolarized and nondifferentiated Clone 9 cells. In nonpolarized cells, mature apical proteins were uniformly distributed in the PM. Surprisingly, they were also in an intracellular compartment. Double labeling revealed that the compartment contained only apical proteins. By monitoring the dynamics of antibody-labeled molecules in nonpolarized cells, we further found that apical proteins rapidly recycled between the compartment and PM. In contrast, the apical PM residents in polarized cells showed neither internalization nor return to the basolateral PM from which they had originally come. Cytochalasin D treatment of these polarized cells revealed that the retention mechanisms are actin dependent. We conclude from these data that both polarized and nonpolarized cells selectively sort apical proteins from the PM and transport them to specific, but different cellular locations. We propose that the intracellular recycling compartment in nonpolarized cells is an intermediate in apical surface formation.

Rapid nonvesicular transport of sterol between the plasma membrane domains of polarized hepatic cells

We studied the transport of the fluorescent cholesterol analog dehydroergosterol (DHE) in polarized HepG2 human hepatoma cells. DHE delivered via methyl-beta-cyclodextrin was delivered to both the apical and basolateral membranes and became concentrated in the apical membrane within 1 min. Intracellular DHE was targeted mainly to vesicles of the subapical compartment or apical recycling compartment (SAC/ARC), where it colocalized with fluorescent transferrin and fluorescent analogs of phosphatidylcholine and sphingomyelin. In contrast, transport of DHE from the plasma membrane to the trans-Golgi network was found to be very low. Vesicles containing DHE traversed the cells in both directions, but vesicular export of DHE from the SAC/ARC to the plasma membrane domains was low. Disruption of the microtubule cytoskeleton disturbed vesicular transport of DHE but not its enrichment in the apical (canalicular) membrane. Transport of DHE to the canalicular membrane after photobleaching was very rapid (t(12) = 1.6 min) and was largely ATP-independent in contrast to enrichment of DHE in the SAC/ARC. Release of DHE from the canalicular membrane was also ATP-independent but slower than the enrichment of sterol in the biliary canaliculus (t(12) = 5.4 min). Canalicular DHE could completely redistribute to the basolateral plasma membrane but could not transfer from one cell to the other cell of an HepG2 couplet. We conclude that sterol shuttles rapidly among the plasma membrane domains and other membrane organelles and that this nonvesicular pathway includes fast transbilayer migration.

Preferential transduction of human hepatocytes with lentiviral vectors pseudotyped by Sendai virus F protein

One of the major challenges facing gene therapy is the development of vectors targeting specific cell types. Restricting gene delivery to the relevant cell type leads to reduced T-cell responses to transgene products and prolonged gene expression. In this study, we demonstrate that vectors derived from human immunodeficiency virus (HIV) can be pseudotyped with Sendai virus fusion protein F. Such vectors transduced human hepatoma cells and primary human hepatocytes efficiently, but not non-liver cells. Several different approaches were also taken to significantly increase the titer of the pseudotyped vector. These studies may facilitate HIV vector-mediated gene delivery into liver in vivo.

Fluorescent lipid probes: some properties and applications (a review)

Odd as it may seem, experimental challenges in lipid research are often hampered by the simplicity of the lipid structure. Since, as in protein research, mutants or overexpression of lipids are not realistic, a considerable amount of lipid research relies on the use of tagged lipid analogues. However, given the size of an average lipid molecule, special care is needed for the selection of probes, since if the size and intramolecular localization of the probe is not specifically taken into account, it may dramatically affect the properties of the lipids. The latter is particularly important in cell biological studies of lipid trafficking and sorting, where the probed lipid should resemble its natural counterpart as closely as possible. On the other hand, for biophysical applications, these considerations may be less critical. Here we provide a brief overview of the application of several lipid probes in cell biological and biophysical research, and critically analyze their validity in the various fields.

Glycosphingolipids are required for sorting melanosomal proteins in the Golgi complex

Although glycosphingolipids are ubiquitously expressed and essential for multicellular organisms, surprisingly little is known about their intracellular functions. To explore the role of glycosphingolipids in membrane transport, we used the glycosphingolipid-deficient GM95 mouse melanoma cell line. We found that GM95 cells do not make melanin pigment because tyrosinase, the first and rate-limiting enzyme in melanin synthesis, was not targeted to melanosomes but accumulated in the Golgi complex. However, tyrosinase-related protein 1 still reached melanosomal structures via the plasma membrane instead of the direct pathway from the Golgi. Delivery of lysosomal enzymes from the Golgi complex to endosomes was normal, suggesting that this pathway is not affected by the absence of glycosphingolipids. Loss of pigmentation was due to tyrosinase mislocalization, since transfection of tyrosinase with an extended transmembrane domain, which bypassed the transport block, restored pigmentation. Transfection of ceramide glucosyltransferase or addition of glucosylsphingosine restored tyrosinase transport and pigmentation. We conclude that protein transport from Golgi to melanosomes via the direct pathway requires glycosphingolipids.

The organizing potential of sphingolipids in intracellular membrane transport

Eukaryotes are characterized by endomembranes that are connected by vesicular transport along secretory and endocytic pathways. The compositional differences between the various cellular membranes are maintained by sorting events, and it has long been believed that sorting is based solely on protein-protein interactions. However, the central sorting station along the secretory pathway is the Golgi apparatus, and this is the site of synthesis of the sphingolipids. Sphingolipids are essential for eukaryotic life, and this review ascribes the sorting power of the Golgi to its capability to act as a distillation apparatus for sphingolipids and cholesterol. As Golgi cisternae mature, ongoing sphingolipid synthesis attracts endoplasmic reticulum-derived cholesterol and drives a fluid-fluid lipid phase separation that segregates sphingolipids and sterols from unsaturated glycerolipids into lateral domains. While sphingolipid domains move forward, unsaturated glycerolipids are retrieved by recycling vesicles budding from the sphingolipid-poor environment. We hypothesize that by this mechanism, the composition of the sphingolipid domains, and the surrounding membrane changes along the cis-trans axis. At the same time the membrane thickens. These features are recognized by a number of membrane proteins that as a consequence of partitioning between domain and environment follow the domains but can enter recycling vesicles at any stage of the pathway. The interplay between protein- and lipid-mediated sorting is discussed.

How proteins move lipids and lipids move proteins

Cells determine the bilayer characteristics of different membranes by tightly controlling their lipid composition. Local changes in the physical properties of bilayers, in turn, allow membrane deformation, and facilitate vesicle budding and fusion. Moreover, specific lipids at specific locations recruit cytosolic proteins involved in structural functions or signal transduction. We describe here how the distribution of lipids is directed by proteins, and, conversely, how lipids influence the distribution and function of proteins.

Secretory IgA specific for a conserved epitope on gp41 envelope glycoprotein inhibits epithelial transcytosis of HIV-1

As one of the initial mucosal transmission pathways of HIV (HIV-1), epithelial cells translocate HIV-1 from apical to basolateral surface by nondegradative transcytosis. Transcytosis is initiated when HIV-1 envelope glycoproteins bind to the epithelial cell membrane. Here we show that the transmembrane gp41 subunit of the viral envelope binds to the epithelial glycosphingolipid galactosyl ceramide (Gal Cer), an alternative receptor for HIV-1, at a site involving the conserved ELDKWA epitope. Disrupting the raft organization of the Gal Cer-containing microdomains at the apical surface inhibited HIV-1 transcytosis. Immunological studies confirmed the critical role of the conserved ELDKWA hexapeptide in HIV-1 transcytosis. Mucosal IgA, but not IgG, from seropositive subjects targeted the conserved peptide, neutralized gp41 binding to Gal Cer, and blocked HIV-1 transcytosis. These results underscore the important role of secretory IgA in designing strategies for mucosal protection against HIV-1 infection.

Vesicular and nonvesicular transport of phosphatidylcholine in polarized HepG2 cells

We have investigated the transport and canalicular enrichment of fluorescent phosphatidylcholine (PC) in HepG2 cells using the fluorescent analogs of PC C6-NBD-PC and beta-BODIPY-PC. Fluorescent PC was efficiently transported to the biliary canaliculus (BC) and became enriched on the lumenal side of the canalicular membrane as shown for C6-NBD-PC. Some fluorescent PC was transported in vesicles to a subapical compartment (SAC) or apical recycling compartment (ARC) in polarized HepG2 cells as shown by colocalization with fluorescent sphingomyelin (C6-NBD-SM) and fluorescent transferrin, respectively. Extensive trafficking of vesicles containing fluorescent PC between the basolateral domain, the SAC/ARC and the BC as well as endocytosis of PC analogs from the canalicular membrane were found. Evidence for nonvesicular transport included enrichment of the PC-analog beta-BODIPY-PC in the BC (t1/2 = 3.54 min) prior to its accumulation in the SAC/ARC (t1/2 = 18.5 min) at 37 degrees C. Transport of fluorescent PC to the canalicular membrane also continued after disruption of the actin or microtubule cytoskeleton and at 2 degrees C. These results indicate that: (i) a nonvesicular transport pathway significantly contributes to the canalicular enrichment of PC in hepatocytic cells, and (ii) vesicular transport of fluorescent PC occurs from both membrane domains via the SAC/ARC.

Membrane domains and polarized trafficking of sphingolipids

The plasma membrane of polarized cells consists of distinct domains, the apical and basolateral membrane, that are characterized by a distinct lipid and protein content. Apical protein transport is largely mediated by (glyco)sphingolipid--cholesterol enriched membrane microdomains, so called rafts. In addition changes in the direction of polarized sphingolipid transport appear instrumental in cell polarity development. Knowledge is therefore required of the mechanisms that mediate sphingolipid sorting and the complexity of the trafficking pathways that are involved in polarized transport of both sphingolipids and proteins. Here we summarize specific biophysical properties that underly mechanisms relevant to sphingolipid sorting, cargo recruitment and polarized trafficking, and discuss the central role of a subapical compartment, SAC or common endosome (CE), as a major intracellular site involved in polarized sorting of sphingolipids, and in development and maintenance of membrane polarity.